DEP ARTNIENT OF THE INTERIOR 

UNITED STATES GEOLOGICAL SUHVEY 

GEORGE OTIS S~IITH, DIRECTOR 

PROFESSIONAL PAPEH 63 

ECONOMIC GEOLOGY 

OF THE 

GEORGETOWN QlJADRANGLE 

(TOGETHER 'VITH THE EMPIRE DISTRICT) 

COLORADo· 

BY 

JOSIAH E. SPURR AND GEORGE H. GARREY 

WITH 

GENERAL GEOLOGY I 

BY 

SYDNEY H. BALL 

WASHINGTON 

GOVERNMENT PRINTING OFFICE 

1908 

· CONTENTS. 

· Page. 

OUTLINE OF RESULTS, by J. E. Spurr .......................... _ .................... _ ... _.................. 23 

PART I.-GENERAL GEOLOGY, by S. H. BalL ........................................ _ ... _................... 29 

Introduction ........................................ _ ............... _ ............... _............... 29 

Topography and geography of the Georgetown quadrangle ..................................... , . . . . . . . . 30 

Relief. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 

Drainage. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 

Climate ................................................................... _..... . . . . . . . . . . . . . . . . 30 

Vegetation.... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 

Culture and industries........................................................................... 31 

Evolution of the topographic forms ............................................................... . 

Mountainous upland ............................. _ ............ _ ............ _._ ..... _ ... : .... . 

Incised valleys ........................ ,. ................................................ -.... . 

Stream_~ilj1l_Sti!lent during the dissection of_ the mountainous upland ........................... . 

Glacial forms ............................................ _ .. _ ... __ ................. ____ . _ .. __ _ 

Postglacial topographic changes ..................................... · .......................... . 

Topography in its relations to rock formations ...................... _ .......................... . 

General geology ..................................... __ ......................... _ .. ____ ....... ___ .... . 

Preliminary outline .............................................................................. . 

Ida~~ :r:::~~~~~ -~~r-~-~~i~-~~~ ._ ._._-_- _- ~::::::::::::.::::::::::::::::::::::: :·::::::::::::::::::::::::::::: 

Distribution and general structure ..................................................... ___ ... . 

Biotite-sillimanite schist ...................................................... · .............. . 

Biotite schist ....................... · .................. _·_ .................................... . 

Quartz gneiss ...................................... _ .................. _ .......... _ .. _ ... _ .... _ 

Ellipsoidal masses .............................................................. _ ...... __ .... . 

Lime-silicate rocks .......................................................................... . 

Origin ....................................... : .......................................... _ ... . 

Age ..................................................................•...................... 

Hornblende gneiss .................... , ....................... : .................................. . 

Quartz monzonite gneiss (adamellite gneiss) ................................................ __ .... . 

Gneissoid granite (mashed biotite granite) ........................................... _ ....... __ ... . 

Quartz monzonite (adamellite) ... ~ ................... : .......................................... . 

Quartz-bearing diorite and associated hornblendite .......................................... ___ ... . 

Rosalie granite (biotite granite) ............................................................ _ ... _._ 

Silver Plume granite (porphyritic biotite granite) ..................................... __ .......... . 

Pegmatite, granite-pegmatite and associated granites, and granite porphyry .......................... . 

Definition .. : ........................ _ ....... _ ................... _ .......... _ ................ _ 

Distribution and general structure ... ~ ................................................... _ ... . 

Pegmatites ...................................................... _ ............................ _ 

Petrography .................................. __ .... _ .................. __ ... __ . _ .. _____ .. ·. _ 

Character of the pegmatitic magmas .................. ~ ..................... _. _ ... ______ ... . 

Granites and granite porphyry ................................................ _____ ... _____ .. . 

Similarity of the pre-Cambrian granites ............... : ..................... _._._ .................. _ 

Carboniferous (?)sandstone.-....................................................... · ......... _· ... . 

Intrusive porphyries ............................................... _ .................... _. _ ...... . 

Introduction ..................................................................... _ .......... . 

Distribution in Colorado .................................................................... . 

Age .......................................................................................... . 

Distribution in the Georgetown quadrangle ..................................... ____ ........... _ 

Forms of intrusion ............................. · ...................... ______ . __ . __ . _________ .. 

Quartz monzonite porphyry and associated quartz monzonite ..................... _. __ ._.: ____ .. _ .. 

Dacite ..................................................................................... . 

Granite porphyry .............. ~ ............................... • .... _ ........ _ .............. . 

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4 CONTENTS. 

PART I.-GENERAL GEOLOGY, by S. H. Ball-Continued. Page. 

General geology-Continued. 

Intrusive porphyries-Continued. 

Alaskite porphyry (ferromagnesian-poor granite porphyry) .............. __ . _ .... -..... __ .. _ .. . . .. . 77 

Bostonite and bostonite porphyry ........... _ ........ _ . _ ......... _ ....... _ ..... _ ... _ . ___ . . . . . . 78 

Alaskitic quartz monzonite porphyry (ferromagnesian-poor quartz monzonite porphyry)........... 80 

Biotite latite ...................... _ .... _ ....... _ .... _____ ........................ ________ . _. . 81 

Alkali syenite porphyry ..... __ .................... _ .. _ . ___ ........................ ________ . . . . 81 

Quaternary deposits .............. ~ .............. _ ........... _._._._ .................. _____ ._.... 83 

General outline. __ ............... ___ ........... _ .. _ ...... __ ........ _ .. _ ... _ ... _ ... _... .. . . . . . . 83 

Terrace gravels ................................... _ .. _ ........ _ .. _ ... : ........ _ .... _ .. _ . . . . . . 83 

Glaciation ................. _ .... ___ .... _______ ........ _: __ ... : ....... ' ...... _._ ........ _ .. _... 84 

Earlier glacial epoch ................ _ .. _. __ . ___ ........ _ . _ .... _ ................... _. __ . 0. • 84 

Glaciers ........... _ ..... _ .. _ .... __ ... _._ 0 0. _. _ •••••••••••••••••• _. _. ___ •••••• 84 

Character and material of the deposits .............. _.. . . .. .. . . . . . . . . . . . . . . . . . . . . . . . . . . . 85 

Later glacial epoch .......... _ ........ ___ . _ : _ ... _ . _ .. _ . __ . _ . . . . . . .. . . . . . . . . . . . . . .. .. . . . .. . 85 

Glaciers ................... _·_ ........ _. __ ._._ ... _ .... _._ ... _ .. ___ ._._................. 85 

Erosional forms ......................... _ ... _. _ .................... _. __ ..... . . . . . . . . . . 85 

Character and material of the deposits ................. _ .. _._ ... :....................... 85 

Relative ages of the glacial epochs ......... _ .. _ ....... __ ._._ ........ _._._ ....... _._._ ... _.. 86 

High basin debris sheets ................... _. __ ... _ .......... __ . _. _ .... : . .. _ . _ . _ .......... - . . . . 87 

Alluvium .......................... _ .... _ .. _. _ .. _I ___ ............ ___ . ____ . _ . _ ...... _ . . . . . . 88 

Landslides ............................. _ ...... _ ..... ____ ............ 0 ••••• ·_. _. _ ••••••• :...... 88 

Talus .....••........... _. __ .......... ____ ._._ .......... _ .. _ ................ 0 •••••• .-......... 88 

Travertine (calcareous spring deposits) ...................... _ .... _. _____ ................ _...... 88 

· Structural geology .. _ ... _ .............. 0 • _ • _ •• _ •••••••••• _ •• _ • _ •• _ ••••••••• _ • _ 0 ___ • __ • __ • • • • • • • • • • • • • • 89 

Folding ................. _ ....... _ . __ .. _ . _ . ___ .. _ .......... _ .. _ ...... _ . _ . . . . . . . . . . . . . 89 

Schistosity and gneissic structure ...... _ ....... _.__ . ___ .......... _. ____ . _...... . . . . . . . . . . . . . 89 

Faults ............. _ .... _ .............. ____ .... __ ._ .... __ .................... _._. __ .......... _.. 90 

Joints ........................ 00 .. _ ........... _ 00 •• 0 ........... 00 •• 00 00 ................. 00 • .. .. .. • 90 

Igneous intru~ion ............... _ .. _. ___ _ ......... : _ .. _ .. _ .. __ . _. __ . _. _ .. _. _ .. , ............... _. _.. 91 

Building stone ..... _ .......... _. __ . _ ... ·- _ .. _. __ . ____ .......... ____ . _ ...... _. _ .. _. _. _. _ .. __ . 91 

Geologic history ....................... _ . __ .... _ . _ ..... _ .... _ .......... _ . _ . _ ....................... _ . . 92 

Minerals and textures of metamorphic and igneous rocks ... · ....... _ . _. _ .......................... _ . . . . . . 93 

Origin of microcline. _ .. _ .. _ . _ ............ _ . _ .. _ . _ .. 0 •• _ •••• _ • _ • __ • _ • _ • • • • • • • • • • • • • • • • • • • • • • • • • • • 93 

Secondary enlargement and related phenomena ....... __ . _ .... _ .. _ .. : _ 0 • _ 0 _. _ • __ • _. _. • • • • • • • • • • • • • • 94 

Origin of the phenocrysts .. 00 ..... ' •• _ ...................... _. __ •• _. _. _. _ ••••••••••• _.............. 94 

Micropegmatitic and related textures .......... _ .............. _ ........... _ . 0 • • • • • • • • • • • • • • • • • • • • • • 95 

Relative age of titanite ...................... 00 •••• _ •• _. _ •• : •• : • ••• _. _. ___ ••••• , ••••••••••••• _.... 96 

Origin of fluorite ........... _ ........ .' ............ _ ....... 0 __ •••••••••••••••••••••••• 00 ........... 96 

PARTII.-EcoNOMICGEoLOGY, byJ. E. Spurr and G.-H. Garreyoo .. oo ..... :. ____ . _. _ ..................... ,. .. 97 

Chapter I.-Discussion of principles ... 00 ........................ __ •••• _. _ ••••••••••••• : ••••••••• •••••• 97 

Gold-bearing and silver-bearing groups of veins ................... _ ... _ ...... _. _. _ . __ . _ ............ _' 97 

Relative distribution of silver-bearing and of predominantly gold-bearing veins ..... 0.............. 97 

Recognition of distinct silver--bearing and gold-bearing areas ................. _ ..... 0. __ ••••• 97 

Silver Plume argimtiferous region .......... · .... _ ........... :-_. __ ._ ....... : ............ _.... 97 

Gold belt near Georgetown.· .................. _ . __ ........... , .. _ .. _ . . . . . . . . . . . . . . . . . . . . . . . . 97 

Idaho Springs auriferous area. __ ............. _ .. _ . _ .... _ . _ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98 

Character of veins between Georgetown and Empire. 00 ..... _. _ •••••••••••••• ,.............. 98 

The Georgetown quadrangle a part of a larger mineral-bearing area ............. 00..... ... • • • • • • .. • 98 

Division of quadrangle into two principal mineral-bearing areas ............... _. _ . _.. . . . . . . . . . . . . 99 

Relative age of gold veins and silver veiJls ............... _. _ ........ _ ........... 00 .. 00......... 99 

Mineralogical characteristics .................. _ · __ ............ _. _ . _ .... _-. . . . . . . . . . . . . . . . . . . 99 

Mineralogical groups._._ .................... _ . ______ ..... _.. . . . . . . . . . . . . . . . . . . . . . . . . . 99 

.Both types of vein filling in the same vein zone ......... ______ ......................... 99 

Intersection and faulting of silver veins by golrl veins ..... __ ._ .. ____ ._._._._._............. 101 

Northeast-southwest mineral belt ....... 00 .. 00.00 •••••• ____ • _ ••••••• _. ___ •••• _. ___ 0 _.............. 101 

Existence of a mineral belt in the area of the Georgetown quadrangle ............ _ . _ .......... _ . . 101 

Limits .............................. _ .......... _ ... _ . __ .. _ ... _ . _ ... __ . _ .............. _ . 0 101 

Relations of the vein belt to the porphyry belt ........... ______ . _ . . . . . . . . . . . . . . . . . . . . . . . . . 101 

Extension of the mineral belt of the Georgetown quadrangle ..... _ ... _,_........................ 102 

Extension from Boulder beyond Leadville .. , ...... _._._................................... 102 

Belt including Aspen and the Gunnison region ... _____ . __ . _ . _. _ ............. _ ....... 0 • • • • • 102 

Extent of the mineral belt ................ __ .... _____ . __ .. _ ... _. _________ ....... -.-- 102 
CONTENTS. 

PART H.-EcoNOMIC GEOLOGY, by J. E. Spurr and G. H. Garrey-Continued. 

Chapter I.-Discussion of principles-Continued. 

Northeast-southwest mineral belt-Continued. 

Occurrence of ores in the mineral belt ......................................................... . 

Form of ore deposits .................................................................... . 

Ores in Boulder County.............. . . . . . . . . . . . . . . . . . . . . . ....... . 

Ores in Gilpin County ............................ . 

Orcs in Clear Creek County ............................ . 

Ores from the Argentine to the Tenmile district .. : ............... . 

Ores of the Tenmile 'district ....................................................... . 

Ores of the Leadville district ................... . 

Ores of the Redcliff district ........................ . 

Ores of the Aspen and neighboring districts .................... . 

Ores from Aspen through the Gunnison. district. .......................... . 

Ores of Sa watch Range, south of Leadville .................................. . 

Coincidence of a belt of intrusive porphyries with the entire belt of mineralization ........ ~ .... . 

Nature of belt marked by dikes and veins .................................................... . 

Dynamic origin of the belt ....................................................... ~ ...... . 

Characteristic faulting along the belt in the Georgetown quadrangle ......................... . 

Similar features of other portions of the belt.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ........ . 

Dynamic movements along belt in Boulder and Gilpin counties .. . 

( Intersection and faulting ................. : .................... . 

Amount of displacement along vein faults ...... · ............................ . 

Faulting in the Tenmile, Lead ville, and Aspen districts· .............................. . 

Summary of dynamic history of belt ......... o ...•.•.•.••••••• · •••••.••.•.••..•••••....... 

Age of porphyry intrusions............................................. . ........... . 

Lack of evidence in pre-Cambrian area........... . . . . . . . . . . . . . . . . . ........ . 

Evidence outside of pre-Cambrian area ............................ . 

Ore deposits southwest of the belt described ......................... . 

Vulcan mining district ......................................................... . 

San Juan region ........................................................................ . 

General geol-ogy ....... · ........... · ............................................. _ ...... . 

Ores of the Silverton region................................... . ............... . 

Orcs of the Telluride quadrangle.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ............... . 

Ores of the Rico quadrangle ........... , . . .. . .. .. .. . .. .. .. . .. . . . .. . . .. 

Ores of the La Plata quadrangle ............. - .. - . --t-·----- · · · · · · · · · · · · · · · · 

Ores of the Needle Mountains quadrangle ................................ . 

Ores of other San Juan districts ................ , ............................. . 

Summary of conditions of ~re deposition in the San J nan region ........................ . 

Similarity of San J nan conditions to those of the Boulder-Gunnison belt.. .............. . 

Origin of the zone of weakness....... . . . . . . . . . . . . . . . . . . . . . . . . . . . . .................... . 

Duration of eruption and mineralization in the regions described ............................... . 

Criteria in the San Juan region ............................................................ . 

Criteria in the Leadville, Aspen, and Gunnison regions ..................................... · 

General Tertiary age of volcanism ..... ~ ............................... · ................... . 

Different periods of intrusion within the general volcanic epoch ........................... . 

Different periods of ore deposition within the general mineralizing epoch ....... _ ............. . 

Continuity of volcanic and mineralizing activity between different Tertiary periods ......... . 

General time range of ore deposition ........................ : ............................. . 

Mining districts definitely outside of the belt described ............................................ . 

Centers of monzonitic eruptions .............................................................. . 

Spanish Peaks mining district ............................................................ . 

Centers of alkaline lava eruptions ........................................................... . 

Silver Cliff and the Rosita Hills ......................................................... . 

General geology ..................................................................... . 

Description of the ore deposits ....................................................... . 

Mine waters ....................................................................... . 

Resemblance to Cripple Creek ...................................... ~ ................ . 

Origin or ores ....................................................................... . 

Cripple Creek mining district .............................. _· ............................ . 

General geology ......................................................... _ .......... . 

Ore deposits . . . . . . . . . . . . . . .................................. _ ........... _ ....... . 

Origin of ores............. . .................. _ .............. _ .. _ ... . 

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6 CONTENTS. 

PART II.-EcoNOMIC GEOLOGY,· by J. E. Spurr and G. H. Garrey-Continued. Page. 

Chapter I.-Discussion of principles-Continued. 

Mining districts definitely outside of the belt described-Continued. 

Centers of alkaline lava eruptions-Continued. 

Remarks on the Cripple Creek and Rosita districts...... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 128 

Features common to the two districts................................................. 128 

Age of eruption and mineralization................................................... 128 

Resume of the principal ore deposition in Colorado.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 120 

Relation of dikes to veins in the Georgetown quadrangle........................................... 130 

Definition of the two chief mineral districts.................................................... 130 

General relations of porphyry dikes to veins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 130 

Relation of the silver-bearing and the predominantly gold-bearing veins to different groups of dikes. 131 

Relative age of different dike sets ........................................... :................. 131 

Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132 

Description and correlation of the dikes in the Georgetown silver region ......... _"................... 132 

Features of the petrography.................................................................. 132 

Correlation with intrusives of other Colorado districts.......................................... 132 

Relative age of the dikes.................................................................... 133 

Description and correlation of the dikes in the Idaho Springs gold-bearing region..................... 133 

Features of the petrography.................................................................. 133 

Comparison with similar rocks in other Colorado districts... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 134 

Relative abundance of different dikes .............................. :.......................... 135 

Relative age of Idaho Springs dikes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 135 

Compound veins .................................................... : ................. ·.. . . . . . . . . . 135 

General featmes of the ore deposits............................................................... 136 

Features of the silver-lead deposits... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 136 

Composition of veins..................................................................... 136 

Metallic minerals.......... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 136 

Occurrence...................................................................... 136 

l'aragenesi,.;. . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 136 

Earthy minerals.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 137 

Summary ........................................ : . ........ : . . . . . . . . . . . . . . . . . . . . . . . . . 137 

Action of present underground waters... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 138 

General discus>ion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 138 

Summary............................................................................. 139 

Derivation of vein materials .......... : •. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 140 

Derivation of cert~in vmetallic minerals................................................. 140 

Derivation of certain earthy minerals................................................. 141 

Derivation qf fluorite .............. " ............... _................................. 142 

Summary............................... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 142 

Alteration of wall rocks............. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143 

Changes of ore in depth..................................................................... 143 

Zone of oxidation ......................................... ·...... . . . . . . . . . . . . . . . . . . . . . . . . . 143 

Superficial secondary argentiferous sulphides........................ . . . . . . . . . . . . . . . . . . . . 143 

Deeper secondary argentiferous sulphides.............................................. 144 

Secondary pyrite................. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 144 

Persistence of veins of Mendota type in depth........ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 144 

Partly or wholly secondary veins.......................................................... 144 

Summary ..................................... , .... ,................................. 145 

Resume of history of vein formation.......... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 146 

Features of the auriferous pyritic deposits..... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 147 

CompoRition of veins ................................................................... -'-.. 147 

Paragenesis of vein materialR............................................................. 148" 

Changes of value in depth................................................................ 148 

Postmineral movement............. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 149 

Alteration of wall rockR............................ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 150 

General discussion ................... ·. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 150 

Summary............................................................................ 152 

Derivation of certain vein materials....... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 152 

Present work of descending waters......................... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 154 

Summary of formation of auriferous veins ....................... ·........... . . . . . . . . . . . . . . . . . . 154 

Intrusion and age of feldspathic dikes..................... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 154 

Mineralization consequent on the intrusion............................................ 155 

General conditionR .. :. . . . . .. ... . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 155 

Nature of mineralizing solutions.................................................... 155 
CONTENTS. 7 

PART II.-EcoNOMIC GEOLOGY, by J. E." Spurr and G. II.·Garrey-Continued. Page. 

Chapter I.-Discussion of principles-Continued. 

General features of the ore deposits-Continued. 

Features of the auriferous pyritic deposits-Continued. 

Summary of formation of auriferous veins-Continued. 

Mineralization consequent on the intrusion-Continued. 

Effects of mineralizing solutions ................................. ,.................. 155 

Sequence of fumarolic activity around volcanoes................................... 156 

Comparison of volcanic fu'marolic sequence with sequence of mineralizing solutions.... 156 

Magmatic origin of mineralizing waters................. . . . . . . . . . . . . .. . . . . . . . . . . . . . . 156 

Intrusions subsequent to mineralization .............................................. ·. 157 

Magmatic origin of metals in both groups of veins.......... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . 157 

Zones of ore deposition........................................................................... 158 

Zone of pressure relief........................................................................ 158 

Relative elevation of Georgetown and Idaho Springs ore-deposition zones......................... 158 

Comparison of effects of descending waters in the two districts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 158 

Nonoutcropping zones o.f ore deposition....................................................... 158 

Localization of ores............................... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 159 

Initial conditions of vein formation............................................................. 159 

Ore deposition by mingling of solutions .............................................. :........ 159 

Ore deposition in fields of intersecting fractures................................................ 159 

Ore deposition by replacement of rock........................................................... 160 

Importance of relative area of reaction faces ................ , .... -...................... .,.. 160 

Localization of ore bodies in crushed portions of fault zones.............................. . . . 160 

Nature of fault movements ..................................................................... .'... 161 

Displacement along vein channels of the argentiferous group............... . . . . . . . . . . . . . . . . . . . . . . . 162 

Displacement along vein channels of the auriferous group.................. . . . . . . . . . . . . . . . . . . . . . . . 163 

Summary and application..................................................................... 163 

Idaho Springs waters............................................................. . . . . . . . . . . . . . . . . . 163 

Active and extinct carbonated springs ....................................................... ·. 163 

Composition of Idaho Springs waters........................................................... 164 

Effect of waters on wall rock ................................... ·.............................. 165 

Comparison of different spring waters ...... ·................................................... 165 

Distinct origin of acids and bases in spring waters .................. _........................... 165 

Origin of waters ..... ~..... .. .. .. .. .. . . . .. . .. .. . .. .. . . .. . . .. . . .. .. .. . . .. .. . . . .. . . . .. . . .. . . .. .. 166 

Character of waters in depth......... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 167 

Comparison with other hot springs .................................... ·.......................... 167 

Deductions from comparisons............. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 168 

Different types of wall-rock alteration by Rimilar waters........................................ 168 

Extension of conclusions outside of the quadrangle ................... .'........................... 168 

Progress of the magmatic theory............. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 169 

Great importance of waters of magmatic origin in the deposition of ores. . . . . . . . . . . . . . . . . . . . . . . . . . . . 171 

Chapter H.-History and production of mines ...................................... ,................... 172 

History of mining development................................................................... 172 

Statistics of production............................................................................ 175 

Chapter III.-The Silver Plume mining district......................................................... 176 

Geologic formations ................... ,........................................................... )76 

Gneisses and schists (Idaho Springs formation)................................................. 176 

Origin..................................................................................... 176 

Pebble-bearing gneiss..................................................................... 177 

Gneiss without pebble-like inclusions...................................................... 177 

Injection of gneiss by pegmatite........................................................... 178 

Dioritic rocks............................................................................... 179 

Hornblende gneiss or sheared diorite............................................................ 179 

Massive dioritic rocks...................................................................... 179 

Distribution and age.................................................................. 179 

Results of differentiation .......................................... ·.................... 179 

Order of crystallization of rock-making minerals.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 181 

Pegmatites.. .... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 181 

Early pegmatite injection............... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . .. . .. . . . .. . . 181 

Pegmatitic phases of massive dioritic rocks ................................. :.............. 181 

Pegmatitic phase of _S~lver Plume granite ..... ,............................................. 181 

General compos1t10n.................................................................. 181 

Occurrence and relative age of magnetite, muscovite, and zircon......................... 182 
·s CONTENTS. 

PART H.-EcoNOMIC GEo'LOGY, by J. E. Spurr and G. H. Garrey-Continued. 

Chapter III.-The Silver Plume mining district-Continued. · 

Geologic formations-Continued. 

Silver Plume granite ................................................................... _ ..... . 

Porphyries ........................................................................ _ ......... . 

Displacement of rocks ............................................................. _ ......... . 

Preporphyry movements.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .............. _ ..... _ . _ . _ . 

Postporphyry movements ........................................................... _ .. _ . 

Postmineral movements .............................................................. _ .. . 

Mines ........................................................................................... . 

Pelican-Bismarck vein system ........................................................... _ .... . 

History ........................................................................ _ ........ . 

Description .............................................................................. . 

Pelican-Bismarck lode ............................................... · ................... .. 

Nature of vein ...................................................................... . 

Structure of vein ............... _ ....... _ ........ _ ....... __ ....... _ ... _ ... __ .... ____ . 

Composition of vein ............................... _ ...... _ .......................... . 

Location of ore bodies ......................................... · ....................... . 

Relation of ores to depth from surface ................................................ . 

Nature of wall rock .................................................................. . 

Alteration of wall rock ............................................................... . 

Postmineral faulting ................................................................. . 

Rubble zone in gneiss ............................................................... . 

Dives-Dunkirk lode ..................................................................... . 

Description ......................................................................... . 

Nature of vein ... · .... , . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ............ . 

Nature of wall rocks ................................................................. . 

History and production ............................................................. . 

Baxter lode ............................................................................ . 

Description ......................................................................... . 

Production ........................................................................... . 

Wisconsin-Corry City -lode............. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .......... . 

Relations to Pelican-Bismarck trunk vein ............................................. . 

Nature of vein ......... · ........... : .................... , ............................ . 

Structure of vein .................................................... --- ............. . 

Composition of· vein .............................. _,_ .................................. . 

Nature of wall rock ... : ........................................................ · ...... . 

Alteration of wall rock ............................................................... . 

Influence of wal,l rock........ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .......... . 

Changes of ore in depth........ .. . .. .. .. . .. . . .. . .................................. . 

Seven-Thirty lode....... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .............. . 

Location and history ................... _ ................ _ ............................. . 

Development ....................................................................... . 

Production ......................................................................... . 

Nature of wall rock .................................................................. . 

-Nature of vein ............................. _ ....... _ ..... _ .......... _ ............... . 

Structure of vein ................................................................... . 

Composition of vein ................................................................. . 

Influence of different wall rocks ..................................................... . 

Granite ......................................................................... . 

Gneiss ....................................... _ .................................. . 

Porphyry •............................................................... , ..... . 

Alteration of wall rock ..................... _ ...... _ . _ ................................ . 

Changes of ore in depth ...................................... _ ...................... .. 

Denver lode .. : ........................................................ , ..................... . 

Pay Rock lode .............................................................................. . 

History and production ......... _._ ............ _ ............. _ ... _ .... _ ............ , ..... . 

Description ................................................ _ .......... _ ................. . 

Nature of inclosing rock ................................................................... . 

Nature of vein .............. _ ....... _ .. _ ........ _ .... _ .. _ .............................. .. 

Location of ore bodies ........... _ ... _ ............ _ ......... -... _ ........ _ ................. . 

Relation of ores to depth .......................................... __ .................... . 

Present action of descending waters ............................................ --.-- ...... . 

Vulcan lode .................... _ ... _ .. ___ ..... _ .. _ .. _. ____ ....... _: .......... _ ...... . 

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CONTENTS. 9 

PART II.-EcONOMIC GEOLOGY, by J. E. Spurr and G. H. Garrey-Continued. Page. 

Chapter III.-'fhe Silver Plume mining district-Continued. 

Mines-Continued. 

Snowdrift lode .................. · .......................... : .. -- . -- .. -.. -- -- -------------- --- 210 

Description ...... _ ....... ,_ ...... _ ..... _ ... __ ....... __ ....... ____ . __ .. _ .. _ ....... _ .... _ . . 210 

Nature of vein .............................. , ....... , .......... :......................... 211 

Production .. ~ .... -....................... _ .................. _............................ 212 

Vice-President lode ................ _ . _ ....... _., _ .... _. _ . _ ... _ .... _ .......... _ . . . . . . . . . . . . . . . 212 

Location and development .............. _ ........... _ ........ _ .. __ . _ ..... _ ........ _ .... _ . . 212 

Country rock ............. _ ......... : ......... _ ... : ................. _ ............ _........ 212 

Description of vein ...... _ ....................................... _ ....................... · 212 

Ore and gangue .......................... _............................................... 213 

Occurrence of ore ...................... _ .... _ ............... · .............. _ . . . . . . . . . . . . . . 21:3 

Montreal lode ......... ." .............................................. _ ... _ . . . . . . . . . . . . . . . . . . . 213 

J,ocation and development ............................ __ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 213 

Country rock ................................................. _ . __ . ' ... _ . . . . . . . . . . . . . . . . . . 213 

Description of vein ............................... _._ .......... __ .. __ ._.................. 213 

Shively lode .................................... _ ... _._ ...... _ ... _ .... _._._ ... _._ ......... __ .. 213 

Lebanon group of lodes ............................................. _. . . . . . . . . . . . . . . . . . . . . . . . 214 

Description ................................ __ ........ _ ........ _.......................... 214 

Characteristics of group ............................. _ .. _ ..... _ ........ _ ... _ ... __ . _ . . . . . . . . 214 

Morning Star lode....................................................................... 214 

Description ............... -................................................ ,.......... 214 

Relation to porphyritic granite dikes......... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 215 

Everett lode............................................................................ 216 

Alhambra lode ............................. _ ...... _ . . ............... _ . . . . . . . . . . . . . . . . . . . 216 

Description .................................................................... :..... 216 

Locus and character of ore ..................................... _ ...... _ . . . . . . . . . . . . . . . 216 

Paragenesis of ores............... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 216 

Northeastern extension of Alhambra lode .............................. _................... 216 

Peru lode............. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 217 

Elijah Rise lode ............... .- ...................................... _................... 217 

Edinburghlode ..... : ........ : ....................................................... _____ 217 

Barite-bearing lode .................................................. _ . . . . . . . . . . . . . . . . . . . 218 

Changes in vein-depositing waters........... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 218 

Deposition from present underground waters ............................. _ ......... _ . . . . . . . . 218 

Terrible vein group .................................. _-_ .............................. _........ 219 

Definition and general descriP.tion........................................................ 219 

Fenton-Mammoth-Terrible vein system ................................ _._ ... _ .. _ ....... _._. 219 

Veins included .................. ~ ......................... _ ................... _... . . 219 

Relations of different lodes ................................... _ . _ ... __ ... _ .... _ . . . . . . . 219 

Fenton-Mammoth lode ..................................... _ .... · .. _............... 219 

Fenton lode ..................... · .......................... _.................. 219 

Relations of Fenton, Brown, and Glasgow lodes ............... _ ........ _ . . . . . . . . 220 

Mammoth lode ........................................... _ ....... ~ _ . :. . . . . . . . . 221 

Terrible lode ................................................. _ ... _ ......... _ . . . . 222 

Branches of main lode in and west of the Terrible mine workings.... . . . . . . . . . . . 222 

Dunderberg and Frost berg lodes .... -.- ................................ _........ 223 

Maine lode ............................................... __ . . . . . . . . . . . . . . . . . . 224 

Cascade lode and branches .............................. _ ......... _ . . . . . . . . . . 225 

Mendota vein system .............. ~ ................................ _ . . . . . . . . . . . . . . . . . . . . . 225 

Composition of veins ................................................. _ . . . . . . . . . . . . . . . . . . . 227 

Nature of veins .................................................. _ .......... · ......... -... . 228 

Structure of veins ............................................. ." _ ............ '. . . . . . . . . . . . 229 

Characteristic fissure filling .................................. _ . _ ... _ . . . . . . . . . . . . . . . . . . 229 

Filling of cavities of dissolution ............................. _ ....... _ .. _ ......... -.-.... 230 

Renewed movements since beginning-of ore deposition .......... _...................... 231 

Nature of wall rocks ............................................. _ ................ _ . . . . . . . 232 

Alteration of wall rocks ........................................ ·. __ ............ _ . . . . . . . . . . . 233 

Influence of wall rock on veins ................................ _ ... ____ . . . . . . . . . . . . . . . . . . . 234 

Relative influence of granite and gneiss ..................... ___ . _ .. _ ... _ ... _ .... _.. . . . 234 

Effect of character and attitude of gneiss ...................... _ ... _ ...... _ . _ . . . . . . . . . . 236 

Effect of porphyry ............................. _ ........... _ .. _. _ .- .. _ .... _ ... _.. . . . . . 23.7 

• 
'10 CONTENTS. 

PART II.-EcoNOMIC GEOLOGY, by J. E. Spurr and G. H. Garrey-Continued. 

Chapter III.-The Silver Plume_ mining district-Continued. 

Mines-Continued. 

Terrible vein group-Continued. 

Location of ore bodies ............................ _ ..... ·'' ..... _ ........... ____ ...... _____ . 

Influence of depth on veins .................................................. __ ......... _. 

Post mineral faulting ............. __ . _ ...... _ . _ .. _ ...... _ ......... __ . _- .. _ .............. ___ _ 

Present effect of surface waters ........................................................... . 

General description .................................................................. . 

Summary .......................... _ ........................................... _ ...... _ 

Chapter IV.-Mines of the Georgetown quadrangle outside ·of the Silver Plume district. ................. . 

Mines near Georgetown ........................................................................... . 

Mines of .Leavenworth Mountain ............................. : ................................ . 

Colorado Central veins ................................................................... . 

Location and development ........................................................... . 

Description of veins .................................................................. . 

Relation of ores to depth .............................................................. . 

Nature of wall rocks ................................................................. . 

Extent of porphyry dikes ..................................................... _ ... _ .. . 

Effect of wall rocks on veins .................. _ ..................................... __ . 

Alteration of wall rock .............. _ ........... _ ... _ ..... _ .... _ ....... _ .... _ .... _ .. _. 

Composition of vein ............... _ .. _ ..................... _ .. __ ................... __ 

Development and cementation of vein channels ...................... _ ............... _ 

Distribution of ore in veins ................................................... __ . _ .. _. 

Paragenesis of vein minerals ... _ ......... _ .... _ . _ .. __ ...... _ . __ . _ ... _ ............ _ ... . 

Description of specimens._ .. _ ... ~ ...... __ ....... ___ .......... _ ... _ . ___ . _ ... _. _ ... . 

Summary statements concerning paragenesis of minerals ...... _ ....... _. _ .... __ . _ .. . 

Quartz .... _ .. ___ ._._ ... ___ . ___ . __ ._. _______ ..... __ . __ . __ .. ______ .. _ ... _____ _ 

Barite ............................... _ ........ __ ................. __ ........ __ 

Siderite or ankerite. ____ .... ___ ·_. ___ .. _._ .. _ ...... _ .. _ .. _._ ... ,_ .. ___ . __ . ___ _ 

Calcite ................... __ .. _ .............................. _ ....... _. ___ ... . 

Kaolinite ................ __ ..... __ .... ~ ......... _. ... _ .......... _ ...... _ ... _ .. 

Galena and blende ...................... ·_ .............................. _ .... . 

Pyrite ............................ _ ................... _ ......... _ ...... _ ... . 

Hematite ..................................... _ ............................. . 

Polybasite, tetrahedrite, pyrargyrite, proust.ite, argentite, and native silver ..... . 

General conclusions ..... _ ................. _ ..................... _ ................ . 

Derivation of certain vein materials ......... _ .... _ ............................... __ .. . 

Deposition from mine waters ................... _ .. _ ........................... _ ...... . 

Transverse postmineral faults ........... :._ ........ _ ......... __ ... _: ....... · .... _ ... _ .. 

Kirtley group of veins .................. _ ...................... _ .......... _ ... _ .. _ ...... . 

General description ................................... _ ............ _ .... _ ............ . 

Kirtley vein .. _ ........ __ ................. _ .......... _ ............................. . 

Stranger vein .... _ ........................... , .......... _ .... _. ... _ .. ·_ ........ _ ...... . 

0. K. vein ....... __ ............ _ .......... _ ... _ .... _ ......................... _._._ .. 

Tilden vein ............................ _ ........ _ ............. _ ........... __ ........ . 

Argentine vein ..................... _ .... _ ................................... _ ...... __ 

Creole veins .......... _ ............................................................. . 

Gates vein ...................... _ .... _ .... _ .............................. _ ......... . 

Other veins of the group ............................................ _ ................ . 

Country rock .............. _ ............... _ . _ ........................... _ ...... _ . __ .. 

Structure of veins ......... _ ................................ _ ... _ ................... . 

Composition.of veins ...................... _ .... _ ............. _ ... _ ........ __ ... _ .... . 

Location of ore ......... _ ............. _ ... _ .............. _ .................. _ ....... . 

Production .......... _ ............ _ .......... · ............. _ .............. _ .......... . 

Wide West lode .......... _ ........ _ ......... _ .......................... _ ............... . 

Location and history ...................... _ ............... _ ...... _ .................. . 

Development ..... · ............. _ ........................................ _ ........... . 

Nature of wall rocks ..... · ............................................................ . 

Nature of lode and location of orcs ..... _ ............................................... . 

Structure of lode ......... _ .... _ ............................................ :, .... : .. . 

Com position of lode .... , ......................................... _ ............ _ ..... . 

Changes or ore in depth, ........... _ ................................................. . 

I 

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CONTENTS. 

PART H.-EcoNOMIC GEOLOGY, by J. E. Spurr and G. H~ Garrey-Continued. 

Chapter IV.-Mines of the Georgetown quadrangle outside of the Silver Plume district-Continued. 

Mines near Georgetown-Continued. 

Mines of Leavenworth Mountain-Continued. 

Central Colorado lode ................... · .......................... : ...................... . 

Location and development ......................................................... . 

Nature of walls .......................... _ ........... _ .... _ ... _ ..................... . 

Nature of vein .................................. _ ................................... . 

Ore ................................................................................. . 

Welch lode ................................................................ -.- .... -.- .• .... . 

Centennial group of.veins ............................................................... . 

General description .................................................................. . 

Origin of vein ............................................ _ ..... _ ..................... . 

Structure of vein .............. : . ................................................... . 

Nature of wall rocks ................................................................. . 

Composition of veins ................ 0 •••••••••••• ••• •••••••••••••••••••••••••••••••••• 

Distribution of ore ................................................................... . 

Paragenesis of ores ....................................................... ·_ ........... . 

Changes of ore in depth ....................... : . ..................................... . 

Present work of underground waters ................................................. . 

Mines of Griffith and Saxon mountains ................................... o o •••••••••••••••••••• 

Comet-1Etna lode ...................................... : ................................ ·. 

Comet mine .................................... , ....... , , ........................... . 

Location, development, and production .......................................... . 

Nature of wall rocks ................................................ , ............ . 

Nature of vein .................................................................. . 

Structure of vein ....................... _ ....................................... . Ore .................. _ .. _ .. _._ .. ___________ . _____ . ___________________ . __ . ______ _ 

1Etna mine ......................................................................... . 

Sporting Times mine ..................................................................... . 

Magnet lode ............................................................................ . 

Location and development ........................................................... . 

Nature of wall rocks .......................................................... -_ ...... . 

Nature of veins ............................ , ........................................ . 

Composition of veins ................ 0 

•••••••••••••••••••••••••• _ •••••••••••• _ ••••••••• 

Occurrence of ore ......................................................... : ......... . 

Production ........................................................................ :. 

Griffith lode .. : ......................................................................... . 

General description .................................................................. . 

Nature of vein ............................................ __ .. __ ..... _ ......... _ .... _ . 

Nature of wall ro.ck .......... : ................................. _ .. _ ....... _ ..... __ ... . 

Influence of wall rock............. . ................................................ . 

Composition of vein ....... _ ............. _ ........................................... . 

Structure of vein ................................................................... . 

Paragenesis of minerals .............................................................. . 

Influence of depth .................................................................. . 

Postmineral faulting ................................................................. . 

Present underground waters ........ , ................................................ . 

Anglo-Saxon lode ........................................................................ . 

Anglo-Saxon mine .................................................................. . 

· History, development, and production ............................................ . 

Nature of rocks ....................................................... · ........... . 

Vein and ore .................................................................... . 

Anglo-Saxon extension vein ......................................................... . 

Location and development .... · ................................................... . 

Vein and ore .................................................................... . 

Production ..................................................................... . 

Mines of Republican, Democrat, and Columbia mountains ..................................... . 

Republican .Mountain group ............. , ............................................... . 

Existence of a zone of mineralization and intrusion ................................... ·. 

~~i;~~:~i~~~~s ~f:~: ~~:~:::.·_-_-_:: :::::-:::::::::::::::::::::::::::::::::::::::::::: 

Kelly tunnel. .......................... : ........................... __ .............. . 

11 

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12 CONTENTS. 

PART H.-ECONOMIC GEOLOGY, by J. E. Spurr and G. H.'Garrey-Continued. 

Chapter IV.-Mines of the Georgetown quadrangle outside of the Silver Plume district-Continued. 

Mines near Georgetown-Continued. 

Mines of Republican, Democrat, and Columbia mountains-Continued. 

Republican Mountain group-Continued. · 

Boston and Beecher veins ............ , ............................................... . 

Description ..................................................................... . 

Origin and structure of veins .................................................... . 

Character of wall rocks ........................................................... . 

Alteration of wall rocks .......................................................... . 

Relation to wall rock ........................................................... . 

Composition of vein ........................................................ : .... . 

Location of ore bodies ............................................................ . 

Influence of depth .............................................................. . 

Muscovite vein ...................................................................... . 

Mineral Chief vein ................................................................... . 

Spartan vein ........................................................................ . 

Minor veins of the New Boston-Mineral Chief region ................................... . 

Sceptre-Sunburst vein ..... · .......................................................... . 

Sceptre mine ................................................................... . 

Location and development ................................... : ............... . 

Country rock ...................................................... _. ......... . 

Description of vein ......................................................... . 

Minor veins ....................................... · .......................... . 

Ore ........................................................................ . 

Sequence of ore deposition .................................................. . 

Location of ore bodies ...................................................... . 

Sunburst mine ................ -- ................................................ . 

Location and development ................................................... . 

~~~~:::t;:~:f·~~i~---_·_·_-.-.-:::: :::::::::::::::::::::::::::::::::::::::::::::::: 

Nature of ore ................................................................. . 

Location of ore bodies ........................... · ........................... . 

Queen City lode .............. · ......... · ............................................... . 

Location and development .. ' .................................................... . 

Country rock ........................................................... , ........ . 

Description of vein· ............ · ....... · ........... , .............................. . 

Astor mine ................................................... : ........ , .............. . 

Democrat Mountain group of veins ................... , ................................... . 

General description ................................................................... . 

Nature of veins ........................................ · ......... : ................... . 

Nature of wall rocks .................................................................. . 

Alteration of wall rocks ............................................................... . 

Influence of wall rock on veins ...................................................... . 

co{nposition of veins ................................................................ . 

Paragenesis of vein materials ................................................. : ...... . 

Location of ore bodies .............................................................. . 

Origin of ores ...................................................................... . 

Descriptions of separate veins ....... _- ............................................... . 

Mines of Lincoln Mountain within the Georgetown quadrangle .•................................. 

Ramsdale tunnel. ........................................................ · .............. . 

Location ...........•................... : .............................. · .... , ... · ..... . 

Nature of wall rocks ................................................................. . 

Nature of vein ....................... , .............................................. . 

Virginia City mine ........................................... : .......................... . 

Location and development ............................................................ . 

Production .•......................................................................... 

Country rock ........................................................................ . 

Description of vein ................................................................. . 

Character of ore .................................................................... . 

Mines near Idaho Springs ......................................................................... . 

Placer deposits ................................................................................ . 

History ..........•.•......... ,_ ........ : ..................................... : ............ . 

Occurrence and nature of the deposits ............................... : ---. --...... · --... --

Production ............................................................... · -........ · ...... . 

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3l4 
CONTENTS. 

PART H.-EcONOMIC GEOLOGY, by J. E. Spurr and G. H. Garrey-Continued. 

Chapter IV,_:_Mines of the Georgetown quadrangle outside of the Silver Plume district-Continued. 

Mines near Idaho Springs-Continued. Mines. _______________ , __________ . __ .. ____ . _____ .. _______ . ____ . ___ . _ . ____ . ___ . _ .... __ . __ . ____ . 

Lamartine-Trail Creek group ____________ . ____ .. ____________ ._._. ___ .... _ .. __ .... _. __ ._ .. __ 

Lamartine and connecting veins. ____ . ______ . ___ .. __ ... __ .. __ .. ____ . ____ .... ___ . _ .... _ 

History and production ___ . __ .. _________ .. ____ . _ . ____ ......... __ . __ ......... _ .. __ 

Composition of vein. __ ... ___________ . ____ ... __ .... ___ . _ ..... _ .. _. __ . _ .... __ .. ___ . 

Nature of wall rock. __________________ ._ ... __ . __ . __ ... ___ .. _____ . ___ .... __ .. _._ .. _ 

Nature of veins. ___ . ___ . ___________ ... ___ .. _._._ .. __ .... __ .. __ . ___ .. __ . ____ .. __ .. 

Location of ores ... ____ . ___ · ________ .. ____ .. _._._ ...... __ .... _ .. _ ... _ .. _____ . ___ .. _ 

Brazil mine._._ .... __ ... ______ . _. _ ... ~ _ . _. __ .. _____ ... ___ ... __ .. _ .... _. _______ .. ___ : __ 

Poor Man mine .......... _ .... _._ ... _ ... _____ .. _ .. _._. ____ ... _ .... ___ . __ . ___ . ________ .. 

New Era mine ________________ ....... __ ._._._. __ .... __ ._. ___ ... ___ .. _ ... _ .. _________ _ 

General description of veins. ________ . ______ .. ___ .. _ ... _ ... ____ . __ ... ____________ _ 

Underground development. ______ . ____ . __ . ___ ... __ . _. ____ .. __ . _ ~ .. ____________ ... 

Nature of wall rock. _____________ .. __ . _____ . _____ .. ___ . .- ... _ .... _. __ . ______ .. _._ .. 

New Era vein ....... _______ .... __ . __ .. __ ... __ .. _ .... _ .. _ ...... __ . ___ .. __ ..... _ .. - _ 

Great Western vein ...... _ ........ _ .. _. __ ....................................... _ 

Cross leads. ___________ ._ ....... _ .. __ . ___ ... __ ... · .. _ ............. _ ............ _ .. . 

Nature of ore .. _. ___ ... _ ....................................................... · .. . 

Falu mine ..... _ ................................................................... . 

General description .. ____ .: ............................ _ .. _ ..... __ ...... _ ... _ ... _ .. 

Nature of vein .............. _ .. __ .... __ ... _ ... ~ .... , ... __ ........ __ ....... _ ..... . 

Teller mine ........................... __ .......... _ ...... _._ ...... _ .... _ .. _ ........ _ 

Freeland vein and connected veins .......... _ ............ _ ..................... ___ .. . 

General description of veins ................................... _ ...... _ .. __ .... __ .. 

Freeland vein .... _ ........... _ ... _ ............ _ ... _ .. ___ . __ ...... _ ... __ ..... . 

Toledo, Gum tree, and· Anchor veins ............ _ .... _ . __ ...... _ .. ___ .. __ . _·_ .. . 

Brighton and Harrisburg veins ........ _ ....... _ ...... _ ..... ·- _ ... _ ......... _. _ . 

St. Patrick lode .. _ ... _ ........... __ ..... ___ .. __ .,_ .... ___ .. _ ... __ ...... _ .... _ 

Production .... _ ......... __ .......... _ ..... ___ .. __ . __ .... __ ... ~ _ .. __ ..... __ .. _ ... _ 

Extent of mine workings ............... ___ .. __ . ___ ..... __ .. ___ . __ .. _ .. _ ... _ .. __ .. 

Nature of veins ..... _ ............. _ .... _ ............ ___ .. __ . __ .... ___ .. __ .. __ ... . 

Nature of.wall rocks ............................... ___ ._ .. __ .... ____ . ___ ._ ....... _ 

Composition of veins ......... , ...................... _ ... _ ...... _ ....... _ ..... __ .. 

Location of ores ..... _ .... _._·_ ........ __ ... ___ ............ _._ .......... __ ........ . 

Renewal of movement along veins .... ~ ................. .- ................ _ ........ . 

Changes of ore in depth .. __ ... : ........ _ ............ __ ......... _ ... ___ ....... · .... . 

Paragenesis of minerals ............................. __ ....... _ ....... _ ........... . 

Present action of underground waters ............ ___ .... _ .......... _ ...... _ .... _ .. . 

Invincible mine ........................................... _ ...... __ ................ __ 

Location ........................................... _ ..... __ .................... _ 

Character of rocks and vein .......... · .............. , ........ __ ... _ ........... _ ... . 

Alpine mine ................................................ _ .... _ ................. . 

Description ....................................... __ ... _ ... _ ......... _ .. _ ....... . 

Character of ore .............................. _ ......... _ ................... _ ... . 

Lanagan mine ................ __ · ............ __ ............ __ ........ ____ .. ____ .. __ .. __ 

Location and development.. .... : . .................. · ____ ........................ __ 

Description of veins ........................... ____ .. __ ...... ____ .. __ .. __ .... __ __ 

Character of ore. __ · ............................. ______ ...... __ .. ______ ...... ____ __ 

Location and production of ore .................................. __ .......... __ .. .. 

Miller mine ... _ ...... __ ..... _ ... · .......... _ ..................................... ! ... . Location and development ....................................................... __ 

\Vall rocks ........................................... · ........ _ .................. _ 

Description of vein ...................... _ ........... _ ............ .- ............ _. 

Character of ore ... ___ ............................ -............................... . 

Relation of vein to porphyry dike ...... __ ..................... _ .. _ .............. . 

Crazy Girl mine ...... _ .. _ .. _ ........................................ _ ........ _ ...... . 

Location ....................................................................... . 

Character of vein materiaL ................ ____ .... · ____ .... __ .. ____ .... __ ...... .. 

Donaldson-Champion Dirt group .... -- .... _ .......... ____ ...... ________ .. ____ ........ .. 

Description of veins ..................................... ____ ....... ·. __ , ....... __ 

Mine development ....................... , ............ __ .... __ ........ __ .... __ .. .. 

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14 CONTENTS. 

PART !I.-EcoNOMIC GEOLOGY, by J. E. Spurr and G. H. Garrey-Continued. 

Chapter IV.-Mines of the Georgetown quadrangle outside of the Silver Plume district-Continued. 

Mines near Idaho Springs-Continued. 

Mines-Continued. 

Lamartine-Trail Creek group-Continued. 

Donaldson-Champion Dirt group-Continued. 

Nature of wall rocks ......................................... _ . _ .. __ .. __ ... _ .. __ .. 

Composition of veins. ___ .... __ ... _______ . ______ . ____ . ____ . ___ . _ . ____________ . _ .. _ 

Production. ___ . _____ ... ___ ..... _ . _________ . ____ .. _____________ ... __ ·_ . __ . __ . __ . __ 

Donaldson mine. _______ .. _____ . ____ .. ____ . ___ . __________ . _ . _________ . __ .. __ . __ . _ 

Kelly mine ... __ ... ______ .. _ . _ . ___ . ____ . ______ . _______________________ ... __ . __ . _ 

Little A !bert mine._. _______ .... _____ . ' .. __ . ___ . _____ .... ____ .. _ . __ . _______ . _ .. _ .. 

Champion Dirt or Centurion mine ___ .. ____ .. ______ . _ .. _____ .. __ . _ . _______ .. ___ .. . 

Champion Dirt vein ____________ .. ____________________ .... __ .. ________ .. · ..... . 

Postmineral movement. .. _ .. __ ... ___ ... _ .. ___________ ... ___ . ___________ ..... . 

Occurrence of ore bodies ... ____ .. ___ .. __________ ... ____ ... ____ . ____ . _ . _ ..... . 

Harrison and Union mine and Morgan mine. ________ . ____ . ____ .. _ . ______ . ___ . _. _ .. . 

Old Settler mine ............ __ . _ .. _. ________ ..... ____ ._ . ____ . __ . ___ . ___ .. _. ____ ..... . 

History ... ____ ...................................... - -...... - -- -. -... -- -.... - -- . - Description of vein._ ... __ . _. ____ . ___ .. _____ .. ________________ .. _________ . _ . ___ .. 

Character of ore .... _ ... ___ . _ ...... _ .... __ .... ______ .... _ . __ .... __ .. _______ ..... . 

Stanley-Little Mattie group._ ...... __ ..... ___ ... ___ .. ____ , __ ...... __ .... _ . _ . __ . ___ ...... . Stanley mine .... _ .. ___ ... ________ . ___ ... ______ . ______________ . ______ . _____ : ____ ..... . 

History and production. ____ ... __ .. ___ . _______ . ____ . ____ . __ . __ .. _________ ... -___ .. 

General description of vein. ____ . ____ . ___ .. ____ ... _______ . __ . ________ .- . ______ .... _ 

Mine workings _________ .... __ .. __________________________________ ........... _____ _ 

Physical nature of vein zone ... ______ .... ____ .. ________________ .. __ ............. . 

Nature of wall rocks._ ..... __ .. ______ . _. ____ .. _________________ . __ . ______ .. _ . ____ _ 

Com position of vein._. ___ .. _. ____ .. ___ . _ .. ___________________ . __ .. ____________ . __ 

Study of the development of the lode. ___ .. _. _. __ ... __________ .. _ . _ .. _______ . ____ . _ 

Older wall rocks ........ __________ .......... ________ .. ____ .. __ .... ___________ _ 

First porphyry intrusion .... _. ____ .. __ . _ . _ . ______ .... ___ .. _____ . ____ . _ . _ ..... . 

~:~~~ris ~r~;::~~:~~~~:::: :::::::::::::::: ::·:: :::::::::::::::::::::::::::::: Formati~n of friction conglomerate. __ .... _ ..... _ . __ .. ____ . ______ . _______ ... _ . _ 

Mineralization subsequent to formation of friction conglomerate. ___ ............. . 

Alteration of rocks ...... __ .... ___ ... ______ ...... _ .............. _ ............. . 

Location of ore. _ ... _ . _____ .. ____ .... ___ . _ . __________ .. _ . __ . _ ... ____ . ____ . __ .... :. 

Summary of geologic history __ .. ___ . __ .. ______________ .. : ... __ ..... __ ... _ .. _ ...... . 

Niagara mine ............. __ .. ________ .: ____ .. ____ .. ____________ .. ______ .. __ .... _____ _ 

James mine. ___ .......... _ . _ .. _. _______ .. ___ ... __ ... __ . __ . _____ ..... __ .... ____ ... _ .. 

Lord Byron mine ..... _. _. _ .. __ . _. ________ .... _, __ .. _____ .. ______ ....... _ ............ . 

Fraction and Kitty Clyde mines. __ ... _________ .. _____ . _____ . _ .. ________ ..... ___ ..... . 

General description . .' ..... ________ .. __________ .... ______ .. ____ .... ________ .... __ .. 

N atnre of wall rocks._ ....... _ .... __ .. ____ .. __ ..... _ ................ _ ............ . 

Character of vein filling ..... _ ... ____ ... ______ .............................. --.- .. 

Development and production ........ __ ...... ________ .... __ ...... __ .... ________ ... . 

Gomer or IIeddensburg mine. ___ ._ .. ____ . _____ . _ . ________ ...... __ ...... _ ..... _ ....... . 

General description .... _______ .... ____ .... __ . _ .. _____ ............................ . 

Nature of vein .. ___ .. ___________ . _ .. ______ .. __ .... _ . ______ . ___ ... ____ .... ___ ... __ . 

Character of ore .... __ .. _ . _ . ________ .... _____ ... _____ .. __ . __ . ______ .. ______ . _ .... . 

. Banty mine ....... ____ .. ___ .. ___ .. _. _______ ._ .. __ . __ ._ .. __ . ____ ._. _____ ............ . 

Metropolitan mine. _______ .. ____ .. __ -.- __________ . _ .. _____ . ______ .. _______ ....... -.... . 

Phrenix mine ... ___ . __ . ___ ...... _ .... ____ ........ -....... -. -- -- -- . - -. -- -- -- . - -- ---- .. 

England mine ...... __ . ______ .. ___ . ____ . ________ . ___ . _ .. ____ . _ . _ . ___ ... ___ . ____ ...... . 

Location ..... _ .... _______ .. ____ .. _______ . _ . ____ . ___ ... ____ ... ______ ............ . 

\Vall rocks ........... ________ .... __ .. ______ .. __________ .... ____ ................. . 

Vein and vein filling ........ ____ .... __ ...... __ .. ____________ .... ______ ........... . 

Cardigan mine .... ____ .... ___ . _____ . ___ .... _. __ .. _. __ ... ________ ........ - ... -- -. -. -- . -

Gladstone tunnel. ... ______ . ___ . _. _ .. ___ ....... _ . _ ... __ ...... ____ .......... - -- .... -- -. 

Salish ury mine. _____ .... _ .... _ ..... _____ .............. _ . -- -.... - -- -.. - -- .. -- .. -- -- --

Lincoln group ... ____ ...... __ .... __ .... __ .... --·-- .... -- ...... ---- ...... --------······ 

General description. ___ ..... ___ .... ________ . _ ... __ . _ .... ____ .... ___ ... - .. -- . - ..... . 

Lincoln vein .... ___ .... __ ... ___ . _____ ..... ___ ..... __ .. - ..... - -- -. -.. - -. -- . - -- -- --

South Linc~ln vein (postmineral breccia) ................ __ ........ ____ -·---------- .. 

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CON'l'ENTS. 

PART H.-EcoNOMIC GEOLOGY, by J. E. Spurr and G. H. Garrey-Continued. 

Chapter IV.-Mines of the Georgetown quadrangle outside of the Silver Plume 'district-Continued. 

~lines near Idaho Springs-Continued. 

Mines-Continued. 

Stanley-Little Mattie group-Continued. 

Lincoln group-Continued. 

Josephine mine ...... __ .............. _ .. _ ...... _ ... __ .... _. ___ . _. __ . ___ .. _ .... ____ . 

Barber and Elliott mine ............ ____ ........ __ .......... __ .. __ ...... __ ....... . 

Little Mattie-N ewton vein system ........ __ ......................................... . 

General description o[ vein ....................................................... . 

~ature of wall rocks ............................................................. . 

Nature of vein .................................................................. . 

Structure of vein ......... ~ ..................................................... . 

Composition of vein ........................................... _ .......... : ...... . 

Relation of ore deposition to depth ............................................... . 

Faults ................ · ......................................................... . 

Arthur mine ....................................................................... . 

Dorritt and King Solomon mines ..................................................... . 

Wild Rose mine ..................................................................... . 

Red Lyon mine ............ · ......................................................... . 

Star mine .................................................... ~ ..................... . 

Bullion King mine ... · ...................................................... , .... _ .. . 

Mayflower mine ................................... _ ................................ . 

Star t)lnnel. ...............................•......................................... 

Columbia mine ..................................................................... __ · 

Black Eagle-Bismarck veins ......................................................... . 

Development ....................................................... _ ........... . 

Description of veins ........................ : .................................. _. 

\Vall rocks ................... : ..... _ ...... _ ..... __ ............ ·. _ ... ____ . __ . ____ . _ 

Location of ore .............................. _ .. _ .......... __ .. _._._ ............ . 

Character of ore ................................................................ . 

Ute Creek-Chicago Creek group ........................................................... . 

Cecil-Argo group .............................................. _ ..................... . 

Description of group ............................................................. . 

·cecil mine ...................................................................... . 

Argo mine ..................................................................... . 

Argonaut and Wallace claims ..................................................... . 

Onida vein ........................... _ ............. _ ...... __ ..... __ ............. . 

Sappho vein ........................... : ............... _ ... _ ... _ ............... . 

Gold Coin vein ................................................................. . 

·Humboldt-Mary Foster vein ......................................................... . 

Description of vein ............................................................. . 

Character of ore .............................................. _ ........ : . ....... . 

Alexander lode ...................................................................... . 

Silver Horn or Golden Hecla mine .................................................... . 

Location and development ....................................................... . 

Silver Horn vein ............................................. _ .................. . 

~:~:a:e~:;~-- -_ ·_ ~::: .· _· .·. ·_ ·.: : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : :. 

"M and M" tunnel. ............................................ : ... _ ... _ ............. . 

Ella McKinney mine .............................................. · .................. . 

Charter Oak mine .................................................................. . 

Kitty Emmet mine ................................................................. . 

Mary Matilda and Little Flat mines ....................................... _ .......... . 

Burns-Moore tunnel .................................................... _ ............ . 

Dum bard in mine ........... · ................................... · ..................... . 

Muscovite-Silverine lode ................................. : . ......................... . 

General description ................................... _· .......... _ ............... . 

Character of vein ................................ : ............................... . 

Character of ore and production .................................................. . 

Rocky Cliff lode .................................................................... . 

Buckeye tunnel (on Alps Mountain) ................................................. . 

Eclipse mine ........................................................................ . 

Description .................................................... , ........ _ ....... . 

Character of the ore ......................................... _·_ ......... _ ......... . 

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16 CO N'I'ENTS. 

PART H.-EcoNOMIC GEOLOGY, by J. E. Spurr and G. H. Garrey-Continued. 

Chapter IV.-Mines of the Georgetown quadrangle outside of the Silver Plume district-Continl!ed .. 

Mines near Idaho Springs-Continued. 

Mines-Continued. 

Ute Creek-Chicago Creek group-Continued. 

Big Forty mine ... _ .. _ ........ _ ........ _......... . . . . . . . . . . . . . . . . .............. _. __ _ 

Golden Glen mine ............ _ ................ _ ................................ _. __ . 

Beaver mine .. __ ................... _ ............................................ __ .. 

Location and development ...... _ ...... -.......................................... . 

Wall rocks_ .............. · ........................................................ . 

Rolling character of main vein ................................ .- .................. . 

Description of other veins ...................................................... .. 

Character of ore ............... _ ................................................. . 

Location of ores ................................................................ . 

Black Swan mine ................................................................... .. 

P. T. mine .................................. : ....................................... . 

General description............... . . . . . . . . . . . . . . . . . . . . . . . . .............. __ . ___ . __ 

Alteration of porphyry ...................................... __ · _ ... __ .... __ .... __ _ 

Relative age of dike and vein ................................................... . 

Cross lead ............................... : ....................................... . 

Character of ore ................................................................ . 

Quito mine._ ....................................................................... . 

Torpedo mine .................................................................. _ .... . 

Waltham mine .................................................... _ .... __ . ________ . __ 

Soda Creek group ......................... · __ . . . . .. .. . .. . . . . .. . . .. .. .. .. . .. ......... .. 

Soda Creek mines ........................................... _ .......... __ .. ___ . _ .. __ _ 

Barbour Fork veins ........................................ __ ... ___ .. _ . _ .. __ .. _____ . __ 

Lexington mine ......... , ............................................ _. _ .... ___ .... _ 

Little Richard mine .................. : ............................................. . 

Blizzard lode.................. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ............ . 

International lode ....................... , .......................................... . 

Chapter V.-The Empire mining district ............................................................ .. 

General geology, by S. H. Ball. ................. · ................................................ .. 

Location .............................................................................. _ .... _ 

Topography......................... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ___ ..... _ .. _ . _ 

Description of formations ...................... ·................ . . . . . . . . . . . . ............... . 

Pre-Cambrian rocks ..................................................................... . 

Intrusive porphyries .... _,................................ . . . . . . . ...................... . 

Quartz monzonite and its differentiation facies ......................................... . 

Quartz monzonite porphyry .......................................................... _ 

Dostonite and bostonite porphyry ...................................... : . ....... _ .... . 

Granite porphyry .... __ .. ____ ........................................ _ .... __ ........ . 

Alaskitic quartz monzonite porphyry ................................................ . 

Alaskite porphyry ................................................................... . 

Biotite latite ........................................................................ . 

Pleistocene deposits .................................................................... . 

Economicge9logy, byJ. E. Spurr and G. H. Garrey ............................................... . 

Empire tunnel. ............................................................................ .. 

Nature of rocks .................................................... : .................... .. 

General description .................................................................. . 

Pyrite in the rocks._ ............................................................... .. 

Nature of veins ......................................................................... . 

Alteration of rocks .......................... _. _ .................. _ ....... _ .............. _. 

Harrison mine ........................ : ....................... . 

Location and development. .................................. . 

Relation of different veins ..................................... . 

History of vein formation ....................................... . 

Upper Harrison tunneL ........ , ................ . 

Features of the ores ................................. . 

Hecla tunneL .............................. _ ._ ............................................... . 

Mint and Cashier mines ..... · ................................................................ . 

Location· and development............. . . . . . . . ........................................ . 

Nature of wall rocks .............................. __ ..................................... . 

Nature of veins .................................................. --. -. -. --------- ------- . 

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Composition of veins ............................................... , ................ -.... 394 
CONTENTS. 

PART !I.-EcoNOMIC GEOLOGY, by J. E. Spurr and G. H. Garrey-Continued. 

Chapter V.-The Empire mining district-Continued. 

Economic geology, by J. E. Spurr and G. H. Garrey-Continued. 

Empire City-Gold Bug group ...................... : ............................. ___ ..... _. __ . 

Location and development ................... _ ................ __ . _ ....................... _ 

Nature of wall rocks ............... _. ___ ................. . .... _ .................. . 

Nature of veins .................... : . . . . . . ....... _ . _ . _ . ___ .. _ ..................... _ . __ . _ 

Veins in Empire City min~ .................... _ ..... _ ................................. . 

Veins in Golden Chariot mine ....................................................... _ 

Veins in Gold Bug mine ..................................................... _ ...... . 

Vein in Jackpot mine ................................................................ . 

Vein in Oro Cash mine .............................. _ ............................... . 

Nature of ore ....................... '..................................................... . 

Silver Mountain ore zone ................................................. _. _ . ____ . _. _____ ... __ 

Nature of ore zone ............................ _ ............................... _._ ....... . 

Character of lodes within the ore zone ......................... _ ...... _ .................. . 

Process of mineralization ................... ___ ...... _ ....... _._ ...... _. 

Postmineral movements ............... _ ....................... _ ...... _ .................. . 

Or~gi~ and d_evelop~ent of ore zone ...................................... _ .............. . 

Prmc1pal vems w1thm the ore zone .................................. _ .................... . 

Mine workings .............................................................. _ . _ ......... . 

Composition of veins .................... ~ ............................................... . 

Location of ores ................................................ -........................ . 

Nature of wall rocks ............................. _ ............ _ .......................... . 

Alteration of wall rocks ........................ : . ........................................ . 

Relation of ores to depth ........................................................ : ....... . 

Present work of surface waters ............................................................ . 

Conqueror 'mine .................................................... _ . _ ..................... _ 

Location and development ................... _ ........................................... . 

Nature of wall rocks ..................................................................... . 

Nature of veins ................................ _ ....................... , ................ . 

Nature of ore ............................................................................ . 

Faulting ....... ~ ........................................ -_ ............................. ~ .. 

Rosecrans mine ............................................. _._ .. _ .. __ ._._ .................. . 

Location and development ........................................ -........ -_ .............. . 

Nature of wall rocks .................................................................. , .. . 

Description of veins .................................................................... . 

Character of ores ...................................... . 

, Union tunnel. ........................................ _ 

Crown Prince-Atlantic group .................................................... . 

General description ................................................................. . 

Crown Prince mine.............. . . . . . . . . . . . . ... __ ........................ . 

Comet Extension mine........... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .................. . 

Atlantic mine ......................................................... : ................. . 

Cleopatra mine .................................. , ...................................... . 

Mines of Lincoln Mountain within the area shown on the Empire map .......................... . 

Headlight mine ......................................................................... · 

Hidden Treasura mine ......................... · .......................................... . 

Allen mine ............................................................................. . 

General description ............................................................. · ..... . 

Structure of vein ................................................................... . 

Evidences of two periods of mineralization ............ _ ............................... . 

INDEX •• - •. - .... -.--- .. --- .... - .. - ................... -- .... - .. - .............••...... - .. - .. - .••.•...••••• 

31401-No. 63---D8-2 

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ILLUSTRATIONS. 

Page. 

PLATE I. Topographic map of tho Georgetown quadrangle ....................................... In pocket. 

II. Geologic map of the Georgetown quadrangle ... : ....................................... 'In pocket. 

III. Map of the Georgetown quadrangle and portions of the adjacent Central City quadrangle, 

showing the old mountaino).ls upland restored. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 

IV. Map of the Georgetown quadrangle and portions of the adjacent Central City quadrangle, 

'showing the glaciers of the earlier glacial epoch ................. _ ... .'. . .. . . . .. . . . .. . . . .. . 32 

V. Map of the Georgetown quadrangle and adjacent portions of the Central City quadrangle, 

showing the glaciers of the later glacier epoch. .. . . . .. . .. . . . .. . .. .. . . . .. .. . . .. .. . .. .. .. . .. 34 

VI. Map of the Georgetown quadrangle and aajacent portions of the Central City quadrangle, 

showing present topography.............................................................. 36 

VII. A, Conglomeratic type of Idaho Springs formation, showing pebbles; B, Conglomeratic type 

of Idaho Springs formation, showing pebbles somewhat elongated near the plane of schistosity.... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38 

VIII. A, Conglomeratic type of Idaho Springs formation, showing pebbles elongated into bands; 

B, Quartz-magnetite pegmatite ... :....................................................... · 40 

IX. A, Garnet-quartz rock of Idaho Springs formation; B, Epidote-qu,artz rock of Idaho Springs 

formation ............................................... ; ................... _ ...... _._. 42 

X. A, Feldspar-magnetite pegmatite, showing crystal formation of magnetite; B, Gneissic type 

of quartz-magnetite pegmatite ................................................ ·........... 60 

XI. Map of central Colorado, showing belt of intrusive porphyries....... . . . . . . . . . . . . . . . . . . . . . . . . . 70 

XII. Sketches illustrating intrusive phenomena................................................. 92 

XIII. A, Georgetown. and Clear Creek valley, looking north toward Empire from slope of Leavenworth Mountain; B, Idaho Springs and the mines of Virginia Canyon, from the north slope 

of Flirtation Peak .................................................................. _ . . . 96 

XIV. Geological plan of Annette level, Griffith mine, showing two periods of ore deposition ....... _. 98 

XV. Specimens showing history of ore deposition................................................ 98 

XVI. Plan of the Harrison mine, Empire.. .. . . .. . . . .. . . . . . . . . . .. . . . .. . . .. . . . . . . . . .. . . . . . .. .. . . . . . 100 

XVII. Map of northern part of Georgetown quadrangle, showing dikes, veins, and mines.............. 102 

XVIII. Map showing approximate distribution of principal silver, lead, and gold regions in Colorado.... 128 

XIX. Specimen from the Waldorf mine at Argentine, showing crystals of galena with some blende 

and quartz, which have formed on the walls of vugs .................................. _... 136 

XX. Geological features in the Colorado Central mine: A, Cross section of pipe, showing precipitate of calcite formed in twelve years .by cold mine waters; B, Specimen from Aliunde 

fourth level, showing nearly horizontal grooving of wall rock . ~ ..... _ ............... :. . . . . . 138 

XXI. Geologic and topographic map of the Silver Plume mining district ........................ ~. 176 

XXII. Views showing geological relations: A, Dioritic dike (of hornblende gneiss) cutting across the 

the banding of granitic gneiss belonging to the Idaho Springs formation; B, Schist injected by 

by pegmatite ........................... ·............... . .. .. . . . .. .. . . .. .. . . . . . .. . . . .. .. · 178 

XXIII. A, Sherman and Brown mountains, from Cemetery Hill; B, Mines north of Silver Plume, 

from slope of Leavenworth Mountain...................................................... 184 

XXIV. Map of Silver Plume mining district, showing skeleton of principal underground workings..... 184 

XXV. Horizontal plan of workings of the Seven-Thirty, Bismarck, Dives-Pelican, Wisconsin, Corry 

City, and Dunkirk mines, also the Burleigh and Diamond tunnels ......... _ .......... In pocket. 

XXVI. Geological plans of Pelican. and Zero levels, Dives-Pelican mine .. ·... .. . . .. . . . . . . . . . . . . . . . . . . 186 

XXVII. Vertical longitudinal projection of Pelican-Bismarck vein ...... ·.............................. 188 

XXVIII. Geological plan of the Diamond and Ashby tunnels ............................ .-.. . . . . . . . . . . 192 

XXIX. Map showing veins and porphyry dikes in the Silver Plume region............................ 194 

XXX. A, Seven-Thirty mine in Brown Gulch, looking north-northeast from Griffin monument; 

B, Burleigh tunnel and Dives-Pelican mill................................................ 196 

XXXI. Vertical projection showing workings on the Seven-Thirty vein .......................... _... 200 

XXXII. Geological plans of separate levels, Seven-Thirty mine .. _ ..... _ ................... _.......... 202 

XXXIII. Horizontal plan of workings in the Pay Rock and .Snowdrift mines, including Ashby tunnel 

. workings ............... __ ...................... ::....................................... 204 

18 
iLL USTRA 'l'IONS. 

PLATE XXXIV. Geological plans of Silver Bank and Ennis tunnels ..................................... . 

XXXV. Map showing underground workings of the Lebanon-Everett group of mines .............. . 

XXXVI. Prindpal unJergwund workings of the Terrible group of lodes ............................ . 

XXXVII. Geological plan of the Burleigh tunnel workings ......................................... . 

XXXVIII. Geological plans of B, No.4, and East Dunderberg levels ............................... . 

XXXIX. Vertical longitudinal projection of the chief workings on the Mendota vein ................ . 

XL. A, Mendota mine and mill; B, Terrible and Smuggler mines ............................ . 

XLI. A, Specimen showing stalactitic crust of zinc carbonate which has formed on the walls of 

a drift on the Mendota vein within twenty years; B, Specimen from the Mendo.ta vein, 

Victoria tunnel level. ..................... ·_ ....................... : ................ . 

XLII. A, Leavenworth Mountain and Alpine Mountain, from the dump of Vice-President mine 

in Brown Gulch; B, View showing relation of ridge separating Clear and Leavenworth 

19 

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creeks to the continental divide.................. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 246 

XLIII. Map of Colorado Central, Kirtley,, and Argentine mines. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 246 

XLIV. Geological plans of separate levels,_ Colorado Central mine ............................ In pocket. 

XLV. A, Specimen from Colorado Central mine showing pre-Cambrian pegmatitic quartz and 

pegmatite, ctush.ed and cemented by Tertiary greenish jaspery quartz containing. metallic sulphides; B, Polished specimen from Colorado Central mine showing quartz of 

. pre-Cambrian pegmatitic quartz vein which has been fractured and partly invaded by 

Tertiary polybasite, galena, and blende . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 250 

XLVI. A, Colorado Central and Kirtley mines; B, Wide West mine............................... 264 

XLVII. Horizontal plan of workings of-Wide West mine ............................... -........... 270 

XLVIII. Geological plans of separate levels, Wide·West mine..................................... 272 

XLIX. Vertical and horizontal plans of Centennial mine workings............................... 274 

L. Geological plans of separate levels, Centennial mine ....... _............. . . . . . . . . . . . . . . . . 276 

LI. A, View of Georgetown, looking northeast from Cemetery Hill, near Silver Plume; B, Griffith and Saxon Mountain and mines east of Georgetown, from the mouth of Clear Creek 

canyon ............... , ...................... _ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 278 

I,II.' Geological plans of Comet tunnel and Anglo-Saxon Extension lowest tunnel..... . . . . . . . . . . 280 

LIII. A, Magnet mine, on Saxon Mountain, looking southeast; B, Annette-Griffith vein and mine 

openings, looking east from slope of Republican Mountain............................. 282 

LIV. Horizontal plan of Magnet mine workings: ................................. , . . . . . . . . . . . . . 284 

LV. Geological plans of separate levels, Magnet mine......................................... 284 

LVI. Vertical and horizontal plans of workings on the Griffith vein covered by Griff1th and Annette 

mines .......................... : . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 286 

LVII. A, Georgetown and Republican Mountain, looking northward in Silverdale Canyon to South 

Clear Creek; B, Republican, Democrat, and Columbia mountain mines, looking west 

.from dump of Magnet mine........................................................... 292 

LVIII. Vertical and horizontal plans of Kelly tunnel and associated mine workings . . . . . . . . ........ 294 

LIX. Map of mine workings and geological plans of separate levels, Sunburst-Sceptre group of 

mines ........................... _ .. _ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 298 

LX. Map showing mine workings of the Democrat !\fountain di8trict............................ 304 

LXI. A, Old plaeer diggings on Chicago Creek near junction with Clear Creek; B, Old placer diggings near point where Jackson discovered gold in 1859 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 312 

LXII. A, Idaho Springs and some of the mines and mills in the immediate vicinity; B, Old placer 

diggings in bench gravels southeast of the junction of Soda and Clear creeks, Idaho Springs. 312 

LXIII. A, Lamartine mine shaft; B, Village of Lamartine and Lamartine mine workings . . . . . . . . . . 314 

LXIV. Horizontal and vertical plans of Lamartine mine workings . . . .... .. . . . . . . . . . . . . . . . . . . . .. . . 314 

LXV. Geological plans of separate levels, bamartine mine . .. .................................... 316 

LXVI. Geological plans of separate levels, New Era mine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 320 

LXVII. Horizontal plan of Freeland mine workings.............................................. 324 

LXVIII. Geological plan of Freeland, Toledo, Gum Tree, and Shakespeare tunnels . . . . . . . . . . . . . . . . . 326 

LXIX. Geologica.! plans of separate levels, Brighton mine ............. _......................... 328 

LXX. Horizontal plan of Donaldson-Champion Dirt mine workings .......................... '... 336 

LXXI. Geological plan of Donaldson-Kelly mine workings .. __ .................................. 338 

LXXII. Geological plans of separate leveis, Champion Dirt vein ......................... :......... 340 

LXJ\..'11I. Horizontal and vertical plans of Stanley mine workings................................... 342 

LXXIV. "Geological plans of separate levels, Stanley mine.......................................... 344 

LXXV. Horizontal and vertical plans of Fraction mine workings.................................. 350 

LXXVI. A, Mines of Lincoln group of veins; B, Waltham mine tunnels........................... 354 

LXXVII. Vertical and horizontal plans of Little .Mattie workings................................... 358 

LXXVIII. Geological plans of separate levels, Little Mattie mine................................... 360 

LXXIX. A, Town of Empire and mines immediately east; B, Town of Empire and Gold Bug group 

of veins .............................. · ................................ _ .. _ . . . . . . . . . . . 384 
20 ILLUSTRATIONS. 

PLATE LXXX. Topographic and geologic map of the Empire mining district .......... ___ ._. ______ .. _ .... _ .. 

LXXXI. Map of mining claims in Empire district.. .................................. ______ .. __ .. __ .. 

LXXXII. Geological plans of tunnels of Empire City, Golden Chariot, Jackpot, Gold Bug, and Oro 

Cash mines ............................................... ___ . _ ..... _ . _ . ___ . _ . _ ... __ ... _ 

LXXXIII. A, Generalized horizontal plan showing relations of developed veins in the Silver Mountain 

ore zone; B, Geological plan of Aorta tunnel. ........................ __ .... ___ . ___ ... ____ __ 

LXXXIV. Sketch map showing a portion of the mine workings along the Silver Mountain ore zone .. _. __ _ 

LXXXV. A, Valley of Clear Creek from Empire Pass road; B, .. Aorta tunnel, Empi~e, and surface 

workings on the Silver Mountain and Benton lodes ................ __ ... · ...... _ .......... .. 

LXXXVI. Geological plans of separate levels, Silver Mountain ore zone ......... _ ............... ___ ..... . 

LXXXVII. Geological plans of tunnels in Conqueror and Rosecrans mines, Empire ............. _._ .. _ .... . 

FIG. 1. Index map of Colorado, showing position of Georgetown quadrangle .......... _._ ... _ ............... . 

2. Sketch showing domal structure of Rosalie granite ............................ _. __ .. __ ... ____ .. __ .. 

3. Biotite-sillimanite schist of Idaho Springs formation injected by dikes and eyes of pegmatite. _______ _ 

4. Hornblende-diopside gneiss of Idaho Springs formation altered to quartz-epidote-garnet rock ... _____ _ 

5. Hornblende gneiss (mashed metadolerite) intruding hornblende-diopside gneiss of Idaho Springs 

formation ................................. _ .......... _ ...... ___ . _ . __ .. ·. ______ ..... _____ .. ____ · __ _ 

6. GnPissoid granite with inclusions of biotite-sillimanite schist of Idaho Springs formation ........... _. _ 

7. Recrystallized microclin e in fractured quartz (of gneissoid granite) ........................... _ ..... . 

8. Secondary microline filling fractures or deposited along them in fractured and altered oligoclase (in 

gneissoid granite) ........... _ ............................ ____ . ______ . __ ... ______ . __ ... _ .. _ ... . 

9. Orthoclase with the rim recrystallized in junction with secondary microline (in gneissoid granite) .... . 

10. Quartz monzonite with inclusions of biotite-sillimanite schist, etc ................................... . 

11. Titanite and hornblende intergrown and contemporaneous ....... ." ...................... : .............. . 

12. Plagioclase cut by vein let of secondary quartz and microcline ............. _. _ .. ___ .. __ . ____ . ____ ... . 

13. Flow orientation of phenocrysts of Silver Plume granite at contact with biotite-sillimanite schist of 

Idaho Springs formation and pegmatite ............................. _ ... __ ....... _. _____ .. ____ __ 

14. Sketch showing ii1tense injection of hornblende gneiss by granite pegmatite .............. _ ... _. _. _. _ 

15. Sketch showing intense injection of quartz monzonite by granite pegmatite ............... _._._._ ... . 

-Hi. Quartz-feldspar pegmatite containing crystals of magnetite ................. _._._._ .. ___ ._._._ ... _ .. 

17. Banded pegmatite dike ................ ~ .. _._ . _ ... _ ... -... __ ................. _ . ___ . _. _____ . __ .. __ .. 

18. Diagram illustrating the lag in the introduction: of muscovite into the quartz-feldspar pegmatite which 

is characteristic of the quartz monzonite pegmatite on entering the Idaho Springs formation ... _. __ . 

19. Porphyry dikes dying out with depth ................................ __ . _. ________ . _. __ : ______ .... . 

20. Porphyry dikes dying out before reaching surface .................. _ ............. _. _ . _ ... ________ .. . 

21. Dacite dike, showing inclusions of Silver Plume granite ................ ___ . _ ... _. ___ . ______ .. _____ _ 

22. Sketch showing intense magmatic corrosion of an orthoclase phenocryst in alaskite porphyry .. __ .... _ 

23. Cross sections showing erosion of streams in drift of earlier and later glacial epochs. _____ ... __ . ____ ... . 

24. Titanite molded upon a plagioclase crystal and approximately contemporaneous with orthoclase .... . 

25. Titanite inclosing hornblende_ ........................ __ .... _______________ . ___ . _________ . ___ .. _ .. 

26. Fluorite, apparently original. ........... _ ..... __ ................ __ .. _____ . _. _____ . __ .. ____ .. ,. ____ _ 

27. Sketch of specim~n from Colorado Central mine .............. _ . ___ . __ . ___ . _____ . ___ . _ . _____ . _ .... . 

28. Sketch of actual plane of a portion of the Seven-Thirty vein ................. _. ___ .... _ .. ____ . __ . __ ._ 

29. Stereogrammatic figures sh6wing nature of displacement along faults which have later become vein 

channels in the Georgetown quadrangle ..................... _ ....... _ ... ___ . __ . __ . ___ . __________ _ 

30. Sketch of fragment of gneiss with pebble-like inclusions ............ __ . ____ . _ ... _ .... _ .... _____ .. __ . 

31. Sketch of vertical outcrop showing schist intruded by crosscutting pegmatite dike ....... _____ .. _ .... . 

32. Pegmatite vein in granite ............................... _._ ................ , ........... __________ _ 

33. Sketch of Pelican tunnel showing strong portion of Pelican lead .. _ ..... _._. _______ . ____________ . ____ ,. 

34. Vertical cross section of Bismarck lode ...................... _.· ... ___ .. _. __ . ___ ._ .... _ .. ____ . ___ . __ _ 

35. Horizontal sketch of branch of Pelican vein ......... _. _. _ ........ _ .. ____ . ______ . __ . __ . ___________ _ 

36. Sketch of Pelican vein, Pelican tunnel level. ................ __ .................... _____ .... __ .. _. 

37. Sketch of wall of Pelican vein, Pelican tunnel level.. .......... __ .... _____ . _____ . _ . _ . _ . ____ ..... __ . 

38. Vertical cross section of Dives vein and Diamond tunnel level near tunnel. ........... _._ .. _ ... __ .. __ 

39. Vertical section of Dives vein in Diamond tunnel level near tunnel, showing vein as two parallel seams 

of blende and pyrite in granite ........................... __ .. ________ . __ . _ .. __ ..... ____ .. _ .. __ . 

40. Vertical cross section of veinlet in gneiss, Diamond tunnel near Dives vein .. _ .. _ ... _._ ........ ____ _ 

41. Detail of portion of vein shown in fig. 40 ............................................ _ . _ .......... ___ . _ 

42. Vertical longitudinal projection of the Wisconsin-Corry City mine workings ............... , ...... __ .. _ 

43. Sketch of barren lead on Hercules tunnel level, Seven-Thirty mine .................. _ ... _ ......... _. 

44. Plan of portion of Seven-Thirty vein, SO-foot level. ............ __ ........... ____ ._ ............. ____ _ 

45. Vertical sketch on wall of stope, Hercules level, Seven-Thirty mine .......... _ ......... _ .......... __ 

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ILLUSTRATIONS. 

FIG. 46. Sketch of breast of drift, Hercules level, Seven-Thirty mine ...................................... . 

47. Vertical section of wall of stope, Hercules level, Seven-Thirty mine ............ : ................... . 

48. Horizontal plan of portion of SO-foot level, Seven-Thirty mine .................................... . 

49. Horizontal plan of portion of 275-foot level, Seven-Thirty mine ................................... . 

50. Plan of tunnel on Denver lode .................................................................. . 

51. Vertical longitudinal projection of Pay Rock mine ...................................... ." ........ . 

52. Vertical projection of Snowdrift mine ............................................................ . 

53. Geological plan of Snowdrift tunnel. ............................................................ . 

54. Plan of tunnel of Vice-President mine .......................................................... . 

55. Plan of tunnel of Montreal mine......... . . . . . . . . .................. ·.· ...................... , ... . 

56. Geological plan of Lebanon and Everett tunnel workings ......................................... . 

57. Sketch showing successive deposits in fissure, Peru lode, Iris tunnel ............................... . 

58. Geological plan of Baltimore tunnel workings ..... , ...... : ........................................ . 

59. Geological plan of Union tunnel, Terrible mine ................................................... . 

60. Vertical section showing levels of Terrible mine ... · ................................. , ..... : ...... . 

61. Geological plan of Silver Ore tunnel. ........................................................ _____ _ 

62. Geological plan of upper Frost berg tunnel. ....... _ ...... ;_ ... ___ . ___ ........... _ ..... _ .......... . 

63. Geological plan of Bush tunnel. ...................... __ .. _ ..... _ ....... __ . _ .......... _. • ....... _ .. 

64. Geological plan of Mendota tunnel workings ....... _ ........ ___ ..... __ .... _. _ .. _ .. __ ..... __ .... _ .. 

65. llorizontal sketch showing Mendota vein as exposed in Mendota mine._ ......... _._ ....... _ ....... . 

66. Sketch of zone of Mendota vein, third level below tunnel level. ......... __ ......... ___ ......... _ .. 

(i'l. Sketch of portion of a vein let shown in fig. 66 .•. ~ ~ ..•....... _ ... · .................. _ ............. ·_ 

68. Horizontal section of roof of drift, Mendota mine, showing deposition of blonde by impregnation pf_ 

gouge zone ............. ~ ... - -..... -............... -.... -............ " -...... - - -... - -.......... . 

69. Horizontal section of roof of drift, Mendota mine, showing fissure filling characteristic of Mendota 

vein ....................................................... : ........................... -···--· 

70. Horizontal section of roof of drift, Mendota mine, showing jog in Mendota vein. __ ._ .. _ ........ _ .. _. 

71. Horizontal section of roof of drift, Mendota mine, showing exceptional and remarkable structure of 

Mendota vein due to postmineral movement. .... _ ........ __ ....... ___ . _ .. __ .... __ ..... ____ ... . 

72. Cross section of Phillips vein, Burleigh tunnel leveL ..... _. ........ __ .. __ .............. _____ ..... . 

73. Cross section of Phillips vein, west end of Burleigh tunnel level. ...... _ ...... , .... _____ .. _ ....... . 

74. Sketch of Frostberg vein near shaft on upper tunnel level. ...... _ ................. _._ ..... __ ..... . 

75. Cross section of Frostberg vein, showing probable reopening.of blende vein and deposition of galena. 

76. Cross section of Frostberg vein, showing probable reopening of blende vein and cementation by 

pyrite ........... · ..................................... _ ... _ ........... ,_ ................... _ .. 

77. Sketch of outcrop on hill north of Brown vein, showing path chosen in homogeneous rock. _____ .. __ .. 

78. Sketch. of outcrops, same locality as fig.·77, showing deflection and displacement of lode following a 

joint in granite on encountering a bunch of pegmatite .......... __ ................. ____ .... _ ·... _ .. 

79. Plan of northeastern most portion of No. 4 Dunderberg level on Terrible lode. _____ .. __ ....... _ .... _. 

80. Horizontal plan of .vein in one of the tunnels in the Maine mine ....... _____ ._. ___ .... _ .... __ ... ___ . 

81. Geological plan of tunnel on Brown vein ... _. _ ............... __ ...... __ .. __ .... __ .. _ . __ ....... __ .. 

82. Plan of tunnel on branches of Mendota vein ................... ___ . __ .... _ ..... __ ........ ____ .. __ . 

83. Plan of portion of No. 4 Dunderberg tunnel level on Terrible lode .. _ .... __ ..... ___ ... _ .. __ .. _ ... __ _ 

84. Horizontal plan of No. 1 Dunderberg level. ..................................... _. _ ..... _ .... __ .. _ 

85. Cross section of Mendota vein, Petersen shaft .................... _ ....... _ ... _ ...... _ .. _ ....... __ _ 

86. Sketch of portion of veinlet near Mendota vein, Mendota mine .... __________ .. _._ .... _._ ........ _ .. 

87. Sketch showing postmineral fault affecting upper Frost berg vein ..... ,. __ ... __ .... _ ..... _. __ .... __ . 

88. Sketch of lower tunnel on Maine lode (near Pelican tunnel) .. ____ .................. __ ._. ____ .. __ .. _ 

89. Geological plan of Hall tunnel. .......................... _. __ ............... __ . _ .. ____ .... _ ..... . 

90. Vertical cross section along line A'-B', Pl. XLIII. ........ _ ..... __ ............ _ ............. _._ .. 

91. Vertical section of face of drift, 350-foot level, Colorado Central mine ......... .' ........ __ ....... ____ . 

92. Vertical cross section of Colorado Central vein on 1,000-foot level. ... _ .. ___ ....... __ .... _ ..... __ .. . 

93. Cross section of drift, 600-foot level, Colorado Central mine ..... _. _____ ._ .. _ ... _ .. _ ........... _ .... . 

94. Vertical section at shaft, 850-foot level, Colorado Central D). inc._ ... _._ ..... _ ... _ .. _ . _ . _ .... ____ ... _ . 

95. Sketch of veinlet on 850-foot level, Colorado Central mine ................ _ ... __ . _. __ . _. ____ . _ ... .. 

96. Vertical section showing minor postmineral faulting of sulphide veinlets in pegmatite, Intermediate 

vein, 850-foot level, Colorado Central mine ............................... __ ....... _ ..... __ .. _ .. . 

97. Vertical section showing minor faulting of galena veinlet, 850-foot level, Coloradl) Central mine .. _ .. . 

98. Geological plan of Kirtley tunnel workings ................ __ ....... __ ... _ .... J ......... _ ... _. _ ... 99. Geological plans of levels in Argentine mine .................. _~_ .. _ .... _ ....... _ .. __ ...... : ..... .. 

100. Plan of Kirtley vein as shown in Kirtley tunnel. ............ _ ........ __ ...... __ ..... _. _. _ ....... . 

101. Geological plan of tunnel workings, Central Colorado vein.' ...... _ ...... _ . __ .. __ ..... _ ... _. _ .. _. __ .. 

102. Geological plan of tunnel on Welch lode ........................ _ .... _._._. __ ............ __ . ___ ... 

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22 ILLUSTRATIONS. 

Page. 

FIG. 103. Cross section of Comet vein on level 270 feet above Comet tunnel level. ...... ___ ._ ... _ .... _.... . . . . 280 

104. Ge9logical plan of Sporting Times lower tunneL...... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 282 

105. Sketch of specimen from Griffith vein.............................. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 288 

106. Vertical cross section of Griffith vein, Griffith No. 1 leveL................. . . . . . . . . . . . . . . . . . . . . . . . . 288 

107. Vertical sketch section of Griffith vein, Annette No. 1leveL ................... _.. . . . . . . . . . . . . . 289 

108. Vertical cross section of New Boston vein .......................... _ ...................... _ ... _ . . . . 295 

109. Geological plan of Muscovite tunneL ................................... _ . . . . . . . . . . . . . . . . . . . . . . . 296 

110. Geological plan of Mineral Chief tunneL.......................................................... 297 

111. Cross section of Sceptre vein ......... : ........ _................................................. 298 

112. Cross section of veinlet running into Sceptre vein.................. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 299 

113. Cross section of Sunburst-Sceptre vein in stope 80 feet above lower Sunburst tunnel.................. 301 

114. Horizontal sketch plan of developed veins (so far as known) in Democrat Mountain group........... 304 

115. Geological plan of tunnel on Buckeye vein ... :................................................... 307 

116. Plan of workings of Cliff mine.................... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 308 

117. Geological-plan of fourth level, Cliff mine ..................... _ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 309 

118. Geological plan of tunnel of Ramsdale mine .................. _ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 310 

119. Generalized horizontal plan of veins exposed in Lamartine workings ~............ . . . . . . . . . . . . . . . . . . 316 

120. Geological plan of Old Stag tunneL. ............................. ·............................... 317 

121. Geological plan of lower Brazil tunnel.......... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 319 

122. Geological plan of lower Teller tunneL .................. _ ..... _.................................. 323 

123. General plan of Freeland vein and some of its branches .......... _ ....................... _ . . . . . . . . 324 

124. Vertical and horizontal plans of Toledo mine ....... ~ .................................... _.... . . . . 325 

125. Map of Brighton mine workings .. _ .............. ·. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 326 

126. Vertical section and plan of two upper levels of Gum Tree mine .................. ~ ..... :. . . . . . . . . . . 328 

127. Geological plan of second level, Gum Tree mine................................................... 329 

128. Vertical longitudinal projection of workings on Freeland-Toledo vein.............................. 331 

129. Geological plan of Lanagan tunneL ........................................................... .-. 333 

130. Geological plan of Miller tmineL ..................... , . . . . . . . . . . . . . . .. . . . . . . . . .. .. . . . . . . . . .. . .. . 335 

131. Biotite latite porphyry from the Stanley mine, containing angular blocks of pegmatite with galenapyrite ore seams .................................. · ........................ __ ................ . . . 344 

132. Sketch of specimen of biotite latite from Stanley mine.............................................. 347 

133. Sk-etch of first level of Gomer mine............................................................... 351 

134. Geological plan of tunnel on England vein........................................................ 353 

135. Geological plan of Salisbury tunnel. ..................... .'.. . . . . . . . . . . . . . . . . . . . ... . . . . . . . . . . . . . . . . . 354 

136. Geologica'! plans of separate levels, Lincoln veins .............. _.... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 355 

137. Geological plans of separate levels, Josephine vein ....... • ... · ....... _ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 356 

138. Geological plan of Intermediate tunnel, Barber and Elliott lode ........... _........................ 357 

139. Geological plan of Mayflower tunneL............. . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . .. . . . .. .. . . .. .. . 365 

140. Geological plans of separate levels, Black Eagle-Bismarck veins......... . . . . . . . . . . . . . . . . . . . . . . . . . . 367 

141. Geological plan of Silver Horn (Golden Hecla) tunnel... . . . . . . . . .. . . . . . . . . . . . . . . . .. . . . . .. . .. .. .. . 372 

142. Horizontal plan of portion of 50-foot level, Silver Horn mine ...................................... · 373 

143. Geological plan of lower Muscovite tunneL............. . . . . . . . . . . . .. . .. . . . .. . . . . . . . . . . . . .. . . . . . .. 375 

144. Geological plan of Beaver mine ............................. .'.................................... 378 

145. Sketch of portion of vertical wall of Empire tunnel, showing barren veins of replacement quartz..... 388 

146. Sketch of portion of vertical wall of Empire tunnel, showing pyritiferous pegmatite vein faulted by 

later fracture................. . . . . . . .. . . . . . . . . . . . . .. . . . . . .. . . . . . . . . . . . . . . . . .. . .. . . . . . . .. .. .. . . . 389 

147. Horizontal plan of portion .of Harrison tunnel. ........................ · ......... ~..... . . . . . .. . . . . . . . 391 

148. Geological plan of Hecla tunneL........ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 392 

149: Geological plans of Mint and Cashier tunnels ...................... _............................... 393 

150. Geological plan of Union tunneL .............................. _ ............. ~................... 405 

151. Geological plan of i.1pper Crown Prince tunnel.................. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 406 

152. Geological plan of Comet. Extension tunneL .......................... _ ...... _.. . . . . . . . . . . . . . . . . . . . 407 

153. Geological plans of upper and lower Cleopatra tunnels .............. _.............................. 408 

154. Geological plan of Headlight tunneL. ............ _._._ ................... _ _..................... 408 

155. Geological plan of lower Hidden Treasure tunneL .................................. _ . . . . . . . . . . . . . 409 
OUTLINE OF RESULTS. 

By J. K SPURR. 

Placer gold was discovered in 1859 at Iuaho Springs, Colo., and this discovery marks 

the beginning of mining work in Clear Creek County. In 1.859, also, several lodes were discovered both witl~in the Georgetown quadrangle near Georgetown and in the adjacent Gilpin 

County region, but they were at first worked only for gold, the surface portions being sluiced 

and washed. Subsequently, however, the difficulties involved in "quartz mining" were overcome, and Clear Creek County became a steady and considerable producer. The total production of the county up to the present time is estimated at about $84,000,000, principa1ly 

from silver, but including $16,0001 000 of gold, nearly $4,000,000 of lead, and $500,000 of 

copper. Within the last few years some zinc ore has been profitably mined, and there appears 

to be a good outlook for zinc production in the future. 

The steep mountain slopes and picturesque scenery of the area covered by the Georgetown 

q~adrangle have resulted chiefly from the down-cutting in Quaternary time of gentler Tertiary valleys. The Tertiary surface still exists, but little changed, on the mountain uplands, 

and a complete restoration of the entire topography previous to the down-cutting is not difficult. During the Quaternary and within the period of the formation of the deep valleys there 

was a period of glaciation, divi'lible into two chief portions. The extent of the glaciers belonging to the two epochs.has been mapped and their effects have been determined. 

The rocks of the area, with the exception of porphyritic dike rocks, which are probably 

Tertiary, are all, so far as can be determined, of pre-Cambrian age. The oldest rocks are represented by a formation of gneisses and schists which probably have resulted from the meta"' 

morphism of a great sedimentary series consisting of conglomerates, sandstones, shales, and 

calcareous beds. These metamorphic schists have been folded, sheared, and intensely injected 

by a series of holocrystalline pre-Cambrian igneous rocks. The earlier of these igneous intrusives have been considerably mashed by the stresses which produced the schist, but the later 

have not been much affected. 

The oldest igneous rocks are siliceous pegmatites and hornblende gneisses, the pegmatites 

being the more widely distriputed of the two. Next came an intrusion of quartz monzonite, 

which has changed by subsequent pressure to the condition of a gneiss. Afterwards granite 

was injected. This was also considerably mashed, so that it is now given the name of gneissoid 

granite. Later came the intrus.ion of a quartz monzonite, a granular rock showing comparatively little mashing and covering about one-~1alf of the area. Still later another dioritic r;ock 

was intruded, less sheared than the early hornblende gneiss, which was also originally a clioritic 

intrusive. This later massive diorite is classed as quartz-bear~ing diorite and associated hornblendite. · A biotite granite now called the Rosalie g·ranite was the next rock to be intruded, 

in the form of batholiths, stocks, and dikes. This is confined to the southern portion of the . 

quadrangle. It was followed by another biotite granite of a distinct type, the Silver Plume 

granite, which covers considerable areas and is widely distributed throughout the quadrangle. 

Nearly every one of these granitic intrusions, certainly much of the granites; was accompanied or followed by the intrusion of pegmatite. very abundant pegmatitic rocks, grading 

into alaskite and muscovite granite, were also intruded earJier than any of the massive granites; 

and later than these granites there were abundant intrusions of pegmatite, with associated 

23 
24 GEOLOGY OF THE GEORGETOWN QUADRANGLE, COLORADO. 

granite pegmatite and granite porphyry. Taken as a whole, many of the pegmatites contain 

magnetite, which in some places constitutes more than one-third of the whole mass. Other 

minerals are muscovite, ~ircon, quartz, tourmaline, feldspar, garnet, allanite, apatite, and beryl. 

These pegmatites have crystallized from a fluid magma, but their composition has been influenced by absorption from the wall rocks, so that characteristic minerals are developed in certain 

formations. 

Residual bowlders of silicified sandstone arid quartzite containing pebbles of pre-Cambrian 

rocks were found in several places. They are similar to the rocks of the Fountain formation 

found along the foothills of the Colorado Range. The Fountain formation is Carboniferous. 

If the residuals of Georgetown are of the same age, they represent the only rocks within the 

quadrangle between the pre-Cambrian and the post-Laramie. 

Beginning in very late Cretaceous or early Tertiary time and continuing for an indefinite 

period in the Tertiary, dikes of porphyry were injected into the pre-Cambrian rocks. Such 

porphyries have a wide distribution in Colorado, and in the Georgetown area form part of a 

rough belt striking northeast and southwest, which has been traced from Boulder at least 100 

miles southwestward past Leadville and Aspen. The dikes within the Georgetown quadrangle 

have been classed as dacite, granite porphyry, alaskite porphyry, bostonite, alaskitic quartz 

monzonite porphyry, biotite latite, and alkali syenite. 

In general later than the intrusion of the porphyries occurred the formation of veins by 

deposition along fault fissures, which were also later than the porphyry in origin. 

The mineral veins within the quadrangle lie in distinct areas or belts. The greatest contrast in these veins is offered by those that are silver bearing and those that are predominantly 

or largely gold bearing. The fo~mer class is represented around Georgetown; the latter in 

the neighborhood of Idaho Springs. However, a belt of gold-bearing veins runs through the 

silver area of Georgetown. The auriferous area of Idaho Springs is continuous on the north 

beyond the quadrangle boundary, where there also occur other areas of argentiferous veins 

similar to that of Georgetown. North of Georgetown the argentiferous veins extend nearly 

to Empire, but at Empire there is an area of gold-bearing veins with the silver-bearing veins 

on both sides. · 

From the phenomena of reopening, renewed cementation of veins, and the crossing and 

displacement of an older vein by a younger one, it has been. determined that the gold-bearing 

veins are younger than those that carry silver. 

The area within the Georgetown quadrangle does not include a complete mining district, but 

is an arbitrarily taken section of the northeastward-trending mineral belt of considerable extent. 

Investigation of the literature, combined with the personal knowledge of the writers, establishes 

the continuity of this belt in a northeast-southwest direction from Boulder to a point beyond 

Leadville. Within this belt the ores occur as fissure veins, where they lie in the pre~Cambrian 

crystalline rocks, but in many places where the belt intersects sedimentary rocks the ores occur 

as more irregular deposits, especially in limestone. Southwest of Leadville, but within the same 

general zone, there is a mineral belt trending northeast and southwest which extends from the 

vicinity of Aspen into the Gunnison country. Veins also occur. along the Sawatch Range south 

of Leadville for a considerable distance. All these regions are within a broad northeastwardtrending belt in which the area of mineralization coincides with the belt of intrusive 'rertiary 

porphyries. The origin of this belt is due to dynamic movement. It is a zone that has been 

characterized by yielding to regional stress, which has produced fault fissures along which 

molten material, forming dikes, and mineralizing solutions, forming veins, have ascended. The 

development of fractures and faults seems to have been continuous from the late Cretaceous or 

early Tertiary down to the present day .. 

When the trend of the ore zone described is followed farther southwest, it is found that 

other ore deposits are notably massed along its projection. About 15 miles south of Gunnison 

is the Vulcan mining district, and a short distance south and west of the Vulcan district is the 

northern portion of the San Juan region. The rocks of the San Juan region are pre-Cambrian 

granites, gneisses, and schists, overlain by Paleozoic and Mesozoic sediments. During the post-
~~~- --------- -- -----~~---- ---~ -- ~ ~---- -- --

OUTLINE OF RESULTS. 25 

Laramie uplift this mountain region originated as a broad quaquaversal fold, and subsequently 

smaller domal uplifts have been superimposed upon various portions of it. After the postLaramie uplift and ensuing erosion Tertiary formations consisting of a basal conglomerate overlain by thick volcanic beds were deposited. Later monzonitic rocks were intruded in the form 

of laccoliths, stocks, sheets, and dikes, following which a complex fault system was developed; 

and these faults, with attendant and largely subsequent fault fissures, have become mineralized 

and form important veins. The ore deposits in different portions of the San Juan region have 

many characteristics in common, such as the prevalence of two systems of trend, one northeastward and the other northwestward. The essential features of the San Juan history are similar 

to those recorded in the mineral belt which stretches from Gunnison to Boulder. It is suggested, 

therefore, that the intrusions, faulting, fissuring, and vein formation in the San Juan are due to 

the same general cause which has been found to be responsible in the Gunnison-Boulder belt, 

namely, the existence of a zone of regional weakness. This zone is shown to have originated 

contemporaneously with the compression of the Rocky Mountain strata into north-south folds. 

The approximately horizontal pressure acting in an east-west direction developed two lines of 

fractures diagonal to the stress. These fractures are represented by the northeast and northwest 

·faults and veins. Subsequent stresses selected especially the northeast direction, and the faulting became localized in this remarkable diagonal belt. . 

·Certain other mining districts in Colorado arc definitely outside of this belt, and most of 

them are connected with volcanic eruptions from a single isolated center. An example of this is 

at the Spanish Peaks. More important economically arc the districts of Silver Cliff and Rosita 

and of Cripple Creek. These two districts have much in common. Both are situated in an area 

of pre-Cambrian gneisses and granites and have furnished a succession of various volcanic prod-. 

ucts. The lavas emitted by each volcano arc chemically and mineralogically related to one 

·another, and in each place the magma was marked by an unusual proportion of alkalies. 

As the result of the investigation into the important gold, silver, and lead deposits in Colorado two chief natural groups appear-those whose formation followed injections of a monzonitic 

magma, including a large portion of the northeastward-trending mineral zone running diagonally 

across the State, and those whose formation followed the outburst of isolated volcanic centers, 

the varied magmas of which had an alkaline or phonolitic tendency. Both of the consequent 

periods of ore deposition arc of Tertiary age. There is no evidence that the periods covered by 

the eruption of the t\vo magmas did not overlap, but it appears a better hypothesis to consider 

the Cripple Creek and Hosita volcanic eruptions as later than the monzonitic eruptions and to 

conceive of the latter as lasting from the earliest to the middle or later Tertiary and of the former 

as lasting from the middle to the later Tertiary. 

By the aid of the conclusions gained from the study qf the principal Colorado ore· deposits 

the special features of the Georgetown quadrangle, which, as already stated, constitutes an 

arbitrary section of the great northeastward-trending mineral belt, ·arc again taken up. The 

gold-bearing ores of the Georgetown quadrangle are typically pyritic ores; the argentiferous 

ores are typically galena-blende ores. Areas covered by these veins coincide with areas of the 

intrusion of porphyry dikes, and it is demonstrated that the mineralization followed the intrusion immediately, so that a genetic relation between the two is unmistakable. Comparison of 

the two classes of veins with the different varieties of dikes shows also unmistakable relations. 

The two mineral districts coincide with distinct petrographic provinces. In the Georgetown district, characterized by argentiferous veins, the dikes consist of alaskite porphyry, granite 

porphyry, quartz monzonite porphyry, and dacite. In the Idaho Springs district, characterized 

by auriferous veins, the dikes are bostonite, alaskitic quartz mozonite, biotite latite, and alkali 

syenite. A study of the dikes near Georgetown shows that they belong to the early Tertiary 

monzonitic magma, which stands in vital relations to ore deposits of the great mineral belt; the 

rocks of the Idaho Springs district arc referable to the alkaline magma best known and described 

at the volcanic centers of Rosita and Cripple Creek. Similarly, the two classes of ore deposits 

in the quadrangle are susceptible of subdivision into groups corresponding -\vith the orcs which 

have in general followed the eruption of these difl'erent magmas. 
26 GEOLOGY OF THE GEORGETOWN QUADRANGLE, COLORADO. 

The general features of the argentiferous silver-lead deposits attendant on the monzonitic injection are next examined. These veins contain chiefly galena, blende, and pyrite, 

with quartz as gangue. Cold descending surface waters arc very active along these veins in 

dissolving and redepositing, and have brought about a considerable change of composition and 

concentration. A study of the paragenesis of the vein minerals makes it appear probable that 

the gangue materials are derived from the wall rocks, and that a large part of the material in 

the veins is immediately due to the precipitation from ordinary atmospheric waters, though 

the original derivation of the metals is not shown by this study. The wall rocks of the veins 

have been altered to quartz, sericite, carbonates, and kaolin. The latter is probaoly the result 

of descending waters. As a result of the study of many of these veins it is concluded that the 

original ore deposit consisted of predominant blcnde with galena and pyrite; that these ore 

minerals were laid down throughout a considerable vertical range, and that the veins have been 

actively eroded, while at the same time descending waters have rearranged and concentrated 

the ores, the effect being to make the concentrated upper portions more valuable. Thus the 

more persistent veins have an enriched upper zone a few hundred feet in depth, while the veins 

whose bottoms were at first relatively high have been eroded downward to their roots or even 

below, so that along such vein fissures there are now exposed on the surface ore deposits which 

either represent enriched roots or are secondary veins formed below the original root by the 

transference of materials from the eroded portion to what was formerly the underlying barren 

fracture. 

In like manner the main features of the auriferous pyritic deposits have been studied. 

Their essential constituent is pyrite, with a quartz gangue. The pyrite contains gold, with 

a little copper and silver. Galena and blende are present in varying quantity. The wall rocks 

have been altered to quartz, sericite, carbonates, pyrite, lwd kaolin; and adularia has also been 

observed. The deposition of the auriferous veins followed closely the intrusion of certain dikes 

which contain fluorite, representing the pneumatolytic stage of consolidation. The mineralizing agents can have nothing else than ascending hot waters, and by comparison of the chemical 

composition of the mineralizing solutions, which has been reasoned out from the alteration 

of the wall rocks, and of the nature of the vein materials, with the known sequence of volcanic 

emanations, it appears probable that the mineralizing waters, as well as the contained gases·, 

were of magmatic origin. In the Idaho Springs district the intrusion of· a phonolitic magma 

was followed by the deposition of a certain type of veins, whereas in the Georgetown district 

the intrusion of a monzonitic magma resulted in the formation of another distinct type. These 

facts can be explained only by considering the metals also to .have been given off by the respective magmas, since the rocks which have been traversed by the ascending solutions are similar. 

SincP- thP- auriferous vP-i'ns have bP-en ~hown to bP- probably younger than the argentiferous veins, 

it is probable that the alkaline igneous intrusion was more recent than the momionitic intrusio.n. 

It also results from this that the argentiferous veins have been exposed to erosion and concentration longer than the auriferous veins, so that the effects of descending waters are much more 

important in the former than in the latter. 

Along the veins the ore has been for the most part especially deposited at the intersection 

· of different branches. Where no such branches are plainly visible it is probable that many ore 

shoots have formed at the intersection of different fracture fields. Although in these places 

the ores have been deposited by the reaction of one solution on the other, 'in others it has ~een 

deposited by the reaction of the mineralizing solutions on the solid wall rock, bringing about 

replacement. In the process of replacement the fineness of the material replaced, which controls the area of reaction faces, appears to• be an important factor. It is probable that the 

crushed material or gouge along faults, if permeable by solutions, is replaced more rapidly 

than the coarse material, and that such areas of gouge produce ore shoots or relatively more 

highly mineralized portions of the vein. · 

The nature of the movements along the faults which have become the seats of ore deposition has been carefully studied. On the walls of the veins of the argentiferous type near 

Georgetown the strire indicate a general nearly horizontal movement along the steeply dipping 
OUTLINE OF RESULTS. 27 

or vertical fault planes. In the veins of the auriferous group the strire clip steeply, recording in 

general the movement of one wall past the other with a strong downward plunge. These 

observations support further the conclusions as to the essentially different age of the silverbearing and the gold-bearing veins, and it is likely that the fault fissures which they occupy 

were formed at different periods. 

In connection with the ore deposition the hot spi1ngs at Idaho Springs are described. 

These occur near the contact of intrusive alkali syenite belonging to the group of alkaline Idaho 

Springs dikes. These springs dissolve portions of the rock completely. They also precipitate 

barite, quartz, and iron carbonates that on exposure to the air become iron oxides. It appears 

probable that the bases contained in the spring solutions have been derived form dissolution 

of the wall rock. The acids, however, can not have had this origin, and are regarded as having 

been given off from portions of the alkali syenite magma which are now cooling in depth. Moreover, the water is also regarded as of magmatic origin. These springs are believed to occupy 

the same relation to the alkali syenite intrusion that the hot mineralizing waters which 

deposited the auriferous veins did t~ the older dike intrusions. 

For the purpose of comparison, the hot spring waters at Glenwood, Colo., and at Steamboat Springs, Nev., are cited. The comparison of these t;vo springs with those of Idaho Springs 

shows that the waters of Steamboat Springs, which arc the hottest, contain a large proportion 

of potash, a substance present in much smaller quantity in the Glenwood Springs waters and · 

lacking in those of Idaho Springs. Moreover, the Steamboat Springs waters· contain various 

metals and other elements, including gold, silver, copper, and lead, and the proportions of 

constituents in solution approach those which have been inferred for the hot ;vaters· which 

produced the mineralization at Tonopah, Nev.a Comparisons between the composition of the 

waters of these different springs which emerge at the surface emphasize the conclusion that. 

many of the mineral bases present in such emerging hot waters are more the result of reactions 

with the wall rock than of original composition or of magmatic emanations, and that the acids 

. and the rarer metals, with very likely some other bases, such as potash, may be original and 

magmatic. Another conclusion which follows this is that with the same or different solutions 

different wall rocks have produced, by reactions, different minerals. Thus a limestone wall 

rock at Aspen has been changed to dolomite and quartz, with some iron, by solutions similar 

to those which have altered igneous wall rocks to quartz, sericite, siderite, and pyrite. . 

The conclusion as to the origin of the ores within the Georgetown quadrangle applies also 

to the districts with \vhich the Georgetown and Idaho Springs districts have been correlated, 

and the greater portion, at least, of the gold, silver, and lead deposits of Colorado are concluded 

to be probablyof magmatic origin. 

The studies of recent years make it probable that ores which are due directly to emanations 

from heated igneous magmas are not of slight importance, as was supposed a short time ago, 

but on the contrary are of the very first importance. It appears that as compared with these 

magmatic processes the other modes of formation of th!'l ores of all save the most common 

metals shrink into insignificance. ----------------------------- --------- a Spurr, J. E., Prof. Paper U.S. Geol. Survey No. 42, 1905, p. 22. 

GEOLOGY OF THE GEORGETOWN QUADRANGLE (TOGETHER WITH 

THE EMPIRE DISTRICT), COLORADO. 

~y J. E. SPURR, G. H. GARREY, and S. H. BALL. 

PART I.-GENERAL GEOLOGY. 

By SYDNEY H. BALL. 

INTRODUCTION .a 

The Georgetown quadrangle lies in the center of the north half of Colorado (see fig. 1), 

between the meridians 105° 30' and 105° 45' west longitude and the parallels 39° 30' and 

eo o::sc:Ei··==="'="""'""75C:::===::J'oo miles 

FIG. 1.-Index map of Colorado, showing position of Georgetown quadrangle. 

39° 45' north latitude. Denver is 26.5 miles east of the northeast corner. The quadrangle is 

about 17.4 miles l~mg (north and south) and 13.5 miies wide (east ·and west) and contains 

approximately 230 square miles. The northern three-fourths is situated in Clear Creek 

County, and the southern portion is in Park County. The quadrangle lies on the east slope of the 

Colorado Range. It is named from Georgetown, the county seat of Clear Creek County. (See 

Pl. I, in pocket.) 

a The writer of this part wishes to acknowledge his indebtedness to Mr. 0. H. Hershey, to whose keenness in the field. is due much of any 

value which this report may possess. 29 
30 GF.OLOGY OF THE GEORGETOWN QUADRANGLE, COLORADO. 

'l'OPOGRAPHY AND GEOGRAPHY OF THE GEORGETOWN QUADRANGLE. 

RELIEF. 

The Georgetown quadrangle is a rugged and mountainous region, with an average elevation 

of about 10,500 feet above sea level. Massive Mount Evans, its most striking topographic 

feature, is 14,260 feet high and is the highest peak of the Colorado Range not situated on the 

Continental Divide. No summit in the range overtowers it more than 200 feet. A saddle, 

cut by the h~adwatets of Geneva and South Clear creeks, deeply indents the east:.west ridge 

which connects Mount Evans with the crest line of the Colorado Range. This peak is the 

center. from which the main ridges of the quadrangle radiate, and streams which drain over 

one-half of the quadrangle rise from the straight-walled cirques sapped in its mass. An 

unnamed peak 1 ~ miles west-southwest of Mount Evans, nearly cut by oirques into three sister 

peaks, is 14,046 feet high. The flowing contour of Chief Mountain (11,710 feet high) is typical 

of the nonglaciated peaks. Clear Creek flows across the eastern border of the quadrangle at 

an elevation of 7,4.50 feet, and this is the lowest point in the quadrangle. 

DRAINAGE. 

I 

The Georgetown quadrangle is situated in the southwest corner of the drainage basin of 

.Missouri River, its streams being tributaries of the South Platte and of North Fork of the 

South Platte. The divide between. the South Platte and North Fork of the South Platte 

follows the crest of the ridge from Meridian Hill westward to Rosalie Peak, thence northwestward to a· point 2 miles beyond Mount Evans, and thence, in a sinuous line, southsouthwestward to the western border 0f the quadrangle. The principal streams of the quadrangle include Clear Creek and its affiuents, Chicago and Soda creeks, and .Bear Creek, tribu- · 

· taries of the South Platte, and Geneva, Deer, and Elk creeks, tributaries of North Fork of the 

South Platte. The streams are all mountain torrents of steep grade, and with the exception 

of Clear Creek even the larger ones fall 300 or more feet per mile. Clear Cree~, a shallow, 

rapid stream from 30 to 60 feet wide, has an averagl! fall of about 85 feet per mile. from the 

junction of South and West forks to the eastern border of the Georgetown quadrangle. Two 

distinct types of stream profile, present in the quadrangle, are well illustrated by Soda and 

Chicago creeks.· Soda Creek heads at a comparatively low altitude and has a normal compoundcurved profile. · Chicago Creek heads in glacial cirques and in consequence has a bench and 

scarp profile in its upper reaches. 

CLIMATE. 

The Georgetown quadrangle, in consequence of the great range in elevation, has a varied 

climate. Complete meteorological data are lacking, but Georgetown and Idaho Springs have 

a mean annual temperature of about 41° F. The temperature seldom rises above 80° in summer 

or falls lower than 5° below zero in winter. The climate in the higher country is more rigorous 

and the annual range in temperature greater. 

The v.alley towns have an annual recorded precipitation of 17.5 to 24.8 inches, 19.5 inches 

being an average. The precipitation on the higher peaks is doubtless ·considerably greater, 

and Mount Evans is one of the storm centers of the Colorado Range. April, May, June, July, 

and August are the months of greatest and January that of least precipitation. During the 

summer there are showers almost every afternoon. 

The heaviest snowfall of the year often occurs about the middle of October. The lower 

valleys are usually free from snow much of the winter, but snow lies on the llighest peaks almost 

all the year. 

VEGETATION. 

When first settled much of the quadrangle was heavily timbered below timber line, which 

ranges from 11,250 to 12,000 feet above sea level. In the southeast corner trees arc rather 

widely spaced and grassy glades are characteristic. Western yellow piric and Douglas fir 
TOPOGRAPHY 'AND GEOGRAPHY. 31 

grow on dry areas to altitudes somewhat over 8,500 feet. These trees are replaced on higher 

and moister ground by lodgepole pine and Engelmann spruce, which in turn are superseded 

by dwarf spruce and near timber line by mountain white pine. Larger areas have been burnt 

()Ver and are now covered with second-growth lodgepole pine and aspen. In summer the 

aspen's light green and in autumn its yellows and reds contrast strongly with the dark green 

of the conifers and add much to the beattty of the mountain sides. Above timber line there 

is an alpine .flora comprising many species. 

CULTURE AND INDUSTRIES. 

The population of the Georgetown quadrangle is largely confined to the mining districts in 

its northern portion, though a few stockmen and lumbermen live in the southern and eastcentral portions. Idaho Springs, whose population in 1900 was 2,502, is situated on Clear Creek 

in the northeast corner of the quadrangle. Georgetown, the county seat of Clear Creek County, 

had in 1900 a population of 1,419. Each town is. a mining camp, a distributing point fOT the 

surrounding mines, and also a summer resort of some note. Silver Plume, Freeland, and 

Lamartine are mining camps. 

The Georgetown branch of the Colorado and Southern Railroad follows Clear Creek canyon, 

terminating above Silver Plume, and connects the northern portion of the quadrangle with 

Denver. Numerous wagon roads, which follow the lower reaches of the main streams and 

traverse the more level ridges, lead to the more accessible mines. Of the many trails, some are 

practically impassable through disuse: 

· Mining is by far the most important industry of the quadrangle. For forty years the 

mines of Clear Creek County have been continuous producers of silver and gold. Small sawmills furnish lumber for mining and other local needs. A few cattle pasture on the hills, and 

the alluvial flats and glacial meadows furnish pasturage that is luxuriant but not easily accessible. 

The alluvial flats are suitable for farming,.but only hardy vegetables, such as potatoes, turnips, 

radishes, and lettuce, thrive. The rigorous climate with its killing frosts, which sometimes 

occur m the valleys in June, and the small area of tillable soil are hindrances to extensive 

farming. 

EVOLUTION OF 'THE TOPOGRAPHIC FORMS. 

From a point commanding a wide view of this portion of the Colorado Range three distinct 

topographic forms are recognizable-first, an old, mature mountainous upland; second, younger 

V-shaped valleys incised in this upland; and, third, glacial cirques developed at the heads of 

some of the streams, passing downstream into U-shaped valleys . 

. MOUNTAINOUS UPLANf>. 

Over considerable areas in the east:..central and southeastern portions of the quadrangle 

the old upland surface is little modified by recent erosion, and throughout the quadrangle 

remnants are preserved on the crests of ridges. From a study of these and other remnants 

in adjoining areas a topographic map of this old land surface, with 500-foot contours, has been 

constructed (Pl. III) and may be compared with the map on the same scale showing the 

present 'topography (Pl. VI). 

The mountainous upland was an ancient land surface with about the same relief as the 

present surface. Dome-shaped mountains and broad, smoothly contoured ridges, however, 

stood where sharp peaks and rugged ridges now are, and the valleys between were broader and 

less steep than those of the present streams. Maps of renmants of the mountainous upland 

contrast with those of the surrounding· younger areas in the greater distances between the 

contour lines. The surface was well adjusted to the structure of the underlying rocks and to 

the varying resistances to erosion of the different formations. Precipitous slopes, except those 

dependent on differential resistances to weathering of hard and soft rocks, were largely lacking. 

The precipice in the southeast corner of the quadrangle, at the contact of the quartz monzonite 

and the Rosalie granite, and the one southeast of Alps Mountain, at the contact of the Silver 
32 GEOLOGY OF THE GEORGETOWN QUADRANGLE, COLORADO. 

Plume granite and the Idaho Springs formation, were as well developed as now. The dip 

slope of the area north of the divide between Soda and Bear creeks was more prominent than 

the present slope. Mount Evans then, as now, was the striking topographic feature. 

The drainage was dendritic and mature. One example of the shallow valley heads which 

led down gently from the dome-shaped mountains of the old upland is preserved at the head of 

Bear Creek, from the lower end of Summit Lake t<'l the 11,500-foot contour. As the glacier 

which formed in the cirque on the north side of Mount Evans cut a deep notch northward and 

joined the Chicago Creek glacier, this portion of Bear Creek was unglaciatecl and rer~ains to-day 

practically as it was prior to the dissection of the mountainous upland. The old valleys 

throughout their courses were wider and gentler than those of the present day, and at the 

quadrangle borders those of Clear, Bear, Deer, and Elk creeks were broaJ and basin-like. In 

unglaciated areas the difference in the profiles of the former and p~esent streams decreases 

upstream until the greater grade of the present stream carries its valley up onto a remnant of 

the mountainous upland and to the head of the former stream. The old streams had normal 

profiles. Lakes did not then exist. 

The remnants of this old topographic surface' are covered by rock residuals and in many 

places by deep soil. Doubtless this sheet of residual soil and talus covered a large portion of 

the old surface, and rock exposures were comparatively uncommon, except on the steepest 

slopes. 

The extreme differences of relief between this old topography and that of the present day, 

amounting to about 1,000 feet, occur where glacial cirques have been gnawed into the broad valley 

heads of the old upland. Of the streams Clear Creek luis cut its way deepest into the upland, 

and in consequence the present stream at Idaho Springs is sunk beneath the old creek bed 

about 400 or 500 feet. The dissection of the ancient surface is also most intense along Clear 

Creek, the largest stream within the quadrangle. This dissection seems greater than is warranted by the size of the stream alone, and it may indicate that the tilting which rejuvenated 

the streams had a southern as well as an eastern component. 

The period at which tlus mountainous upland was formed is not definitely known, but it 

was probably in late Tertiary time. Cross" has shown that near the end of Mesozoic time andesitic lavas probably covered the Colorado Range in this latitude. Prior to the formation of 

this upland surface these lavas were completely removed, indicating that the surface was not 

developed very early in Tertiary time. Further, the size of the canyons cut into the mountainous upland pi·ior to the earlier glaciation indicates that the land surface is presumably prePleistocene. Similar mature uplands have been noted in the vicinity of Pikes Peak by Crossb 

and in the Sierra Madre neat Encampment, Wyo., by Spenccr.c Examination of topographic 

maps of Colorado d shows remnants of a similar mature and ancient land surface widely distributed over Colorado. It is probable that these are all approximately contemporaneous and 

of late Tertiary age. Perhaps in a, broad way to be correlated with this land surface is that 

described by Lawson," which developed in the Sierra Nevada at the close of Tertiary time, 

as well as similar land surfaces remaining on the yrests of some of the basin ranges in southwestern Nevada and eastern California recently studied by the writer/ The latter surfaces are 

without much doubt of late Pliocene age. It is probable, then, that in late Pliocene time, over 

considerable areas from the Great Plains of Colorado to the Sierra Nevada, the mountains were 

comparatively low and characterized by topography in old age. 

INCISED VALLEYS. 

After the mountainous upland had developed in the Georgetown quadrangle the gradient 

of the streams was increased, presumably by an eastward tilting of its surface, and in consequence the streams carved and are still carving valleys in this upland. The valleys are of vary- -- -------------- aCross, Whitman, Mon. U.S. Geol. Survey, vol. 27, 1890, p. 202. 

b Cross, Whitman, Geologic Atlas U. 8., folio 7, U. S. Geol. Su_rvey, 1894, p. 1. 

c Spencer, A. C., Prof. Paper U. S. Geol. Survey No. 25, 1904, p. 12. 

d Ilayden, F. V., U.S. Geol. and Geog. Survey Terr., Atlas of Colorado, 1877. • 

e Lawson, Andrew C., The geomorphogeny of the upper Kern basin: Univ. California Pub!., Bull. Dept. Geol., vol. 3, no. 15, 1904, p. 302. 

{Bull, S. H., Bull. U.S. Geoi..Survey No. 308, 1907. 
TOPOGRAPHY AND GEOGRAPHY. 33 

ing width, from the canyon of Clear Creek, which is in places 2~ miles wide, to the gorges along 

Deer, Elk, and Bear creeks, which are for the most part not over one-half mile wide. Upstream 

the valley walls approach each other, and the valleys become narrow, V-shapcd notches in the 

old upland surface. The young valleys are steep, and perpendicular elements in their walls are 

common. Rugged ribs of rock project from the valley sides, and huge bowlders, wedged from 

the cliffs, dam the. rushing waters of the streams. · 

STltEAM ADJUSTMENT DURING THE DISSECTION OF THE MOUNTAINOUS UPLAND. 

Evidence that important stream adjustment has occurred since the mou"ntainous upland 

developed is lacking, and the drainage lines of the upland, except for minor lateral shifting and · 

a few unimportant changes, persist to-day. Bard Creek, however, in early preglacial time 

appears to have flowed eastward into Clear Creek through the low gap known as Empire Pass 

instead of turning abruptly to the north and flowing into West Clear Creek, as it does now. 

This change probably occurred in late preglacia,l time. The restoration of the mountainous 

upland (Pl. III, p. 30) and the presence of a few river cobbles on the low gap in the divide 

between Chicago Creek and Barbour Fork of Soda Creek indicate that Barbour Fork was formerly a tributary of Chicago Creek. The most southerly affluent of Deer Creek in "the quad-. 

tangle has captured the upper 2 miles of its course from an eastward-flowing stream which 

empties into Deer Creek immediately south of the quadrangle boundary. 

GLACIAL FORMS. 

After the rejuvenated streams had cut back their courses in the Georgetown quadrangle 

the higher valley heads were occ11pied by alpine glaciers of two distinct epochs of glaciation. 

The changes wrought in the surface of the quadrangle by the earler glaciers (Pl. IV) have been 

largely obliterated by later glacial and interglacial erosion. The glaciers modified the topography in two ways, by erosion and by deposition, the former process being particularly effective 

in the upper portions of the glaciated areas and the latter in the lower portions. 

The principal glaciers of the second epoch occupied the valleys radiating from Mount 

Evans, and other glaciers, heading in the Continental Divide to the west, descended the eastwardflowing streams and pushed into the quadrangle (Pl. V). 

Cirques are the most striking feature of glacial erosion in the quadrangle. Glaciers which 

headed below an elevation of 12,000 feet did not form well-developed cirques, although those 

with compound cirque heads, such as that above the lower Chicago Lak{\, formed as low as 

11,700 feet. The deepest cirques are in the vicinity of Mount Evans, where the upper Chicago 

Lake, for example, lies at the base of a slope which rises 1,700 feet in less than a mjle. The 

steeper cirques have bare, rocky, impassable walls, in many places with postglacial talus accumulations along their bases, while less steep cirques are. covered in summer hy luxuriant grasses, 

well watered by melting snows. The compound character of the cirques of many of the larger 

glaciers is well exemplified by the glacier which forined Summit and Chicago lakes. Its profile 

shows three well-developed cirques, situated at different eleva;tions on the "same stream. In 

such glacial valleys streams, some of them sunk in shallow canyons, fall from cirque to cirque 

like silvery ribbons. 

Where two cirques headed opposite each other the mou,ntainous upland was eroded away 

by mutual head ward retreat, and a knife-edge ridge or arete was ·formed. The best example 

of this is 1 mile west of Mount Evans, where the precipitous walls of two cirques unite, forming 

a narrow, rugged, and impassable crest. Similar topographic forms were produced by the 

lateral abrasion of parallel glaciers. Hanging valleys formed by abrasion will be mentioned in 

another place (p. 34). 

Of the lakes of the quadrangle three types. originated during the later glacial epoch, and a 

fourth is of more recent origin. The glacial lakes are either (1) glacier hewn, lying in a basin 

scooped out by the ice, or glacier dammed, (2) by the irregular deposition of ground moraine, 

31401-No. 63....,-08-3 
34 GEOLOGY OF THE GEORGETOWN QUADRANGLE, COLORADO. 

or (3) by a lateral moraine. The fourth type may be called talus-dammed lakes. The first 

two kinds of lakes, which reach a maximum length of one-half mile, are situated upon the 

floors of cirques. Cirque walls as a rule rise steeply on three sides of the lake; the fourth side 

is a low barrier of rock (glacier-hewn lakes) or of morainal material (glacier-dammed lakes). 

Of these the latter is the more common type in the quadrangle. Several of the most elevated 

lakes are covered by ice the greater portion of the summer. The third type of glacial lakes, 

located in the lower reaches of the glaciated valleys, is represented by Edith and Eeho lakes, 

small bodies of water occupying preglacial valleys which have been dammed by lateral moraines. 

The upstream shores are smooth and genUy sloping, and a morainal wall rises at the lower end 

of the lake. Two small lakes on the border of the quadrangle northeast of Meridian Hill and a 

larger lake just without the quadrangle are examples of the fourth type. These lakes lie in a 

broad, shallow valley, are surrounded by an alluvial flat, and appear to be formed by broad 

dams of talus from the hills to the southwest and northeast. 

From the cirque downstream to the lower limit of glaciation the valleys in cross section 

present a lower glaciated U-shaped portion and an upper portion due to rock weathering. 

Near the cirques the latter component is subordinate or lacking, while near the lower limit of 

glaciation. the U-shaped portion is poorly developed. 

Lateral moraines which form prominent narrow ridges of till on either side of the valley 

walls are the most striking of the glacial deposits. These moraines are in some places double, 

the outer strand containing debris of more local origin. · 

The floors of cirques and glaciated valleys are very irregular, through unequal deposition 

of material in ground and recessional terminal moraines, although the latter deposits are unimportant. Striated and smooth rock ledges protrude from the till here and there. Stream grades 

are variable, and the streams themselves divide and reunite or spread out into marshes and 

ponds. Most of the streams, however, are sunk slightly below the valley floor, and have 

bowlder beds which are in many places bordered by a narrow strip of marsh. 

The terminal moraines have an irregular surface characterized by hillocks, ridges, and 

depressions. Many of these moraines have very steep fronts, over which the streams descend 

in a succession of waterfalls, forming a hanging valley. A second type of hanging valley is 

formed by a moraine crossing the channel of another stream. The moraine of East Geneva 

Creek glacier crosses the valley of Geneva Creek, and from its gently graded course in Geneva 

Park the latter stream falls 300 feet in 0.15 mile. Differential abrasion formed a third type 

of hanging. valley. The Clear Creek glacier eroded more powerfully than the South Clear 

Creek glacier, and in consequence at their junction the bed of the united glacier was sunk 250 

feet below that of 'the South Clear Creek glacier. South Clear Creek has since trenched the 

cliff faces above the junction. 

POSTGLACIAL TOPOGRAPHIC CHANGES. 

Since the retreat of the glaciers of the later glacial epoch a few minor topographic changes 

have occurred. Small tributaries of streams in the glaciated area were in places deflected 

downstream by lateral moraines, whose outer border they follow. The streams have eut into 

the glacial till and somewhat deepened their channels in bed rock. Alluvial fans have been 

formed at the mouths of some of the gulches. Alluvial flats, through which streams now 

meander, have been filled in behind terminal and recessional moraines. Of these the flat on 

Clear Creek between Georgetown and Empire station is a good example. Alluvial flats like 

Geneva Park have also been formed behind moraines which dam other channels, and irregular 

patches of alluvium have aecumulated in the small stream valleys which abut against lateral 

moraines. Glacial lakes have been partially or completely filled. Glacier-smoothed surfaces 

have been roughened by we:tthering and rock ribs and pinnacles formed upon them. In comparatively recent time the upper one-fourth of the east fork cf Lost Creek wandered over a 

level basin of debris from the surrounding hills and was turned into the west fork of Indian 

Creek. (See Pl. VI.) 
U.S.GEOLOGICAL SURVEY PROFESSIONAL PAPER NO. 63 PL. III 

THE OLD MOUNTAINOUS UPLAND RESTORED 

Contour interval500 :feet. 

1906 
U .S .GEOLOGICAL SURVEY PROFESSIONAL PAPER NO. 63 PL.IV 

GLACIERS OF :EARLIER GLACIAL P ERlOD 

If otlter glaciers existed O:uring tlris stage all evidence ofthem_has disapp eared 

Contour intervaJ 500 feet 

1906 
U .S .GEOLOGICAL SURVEY PROFESSIONAL PAPER NO. 63 PL.Y 

GLACIERS OF LATER GLACIAL PERIOD 

Contolll' interval500 feet 

1906 
U.S .GEOLOGlCAL SURVEY PROFESSIONAL PAPER NO. 63 PL.VI 

PRE SENT TOPOGRAPHY 

Contour interval500 feet 

1906 
U. S. GEOLOGICAL SURVEY PROFESSIONAL PAPER NO. 63 PL. VII 

A. CONGLOMERATIC TYPE OF IDAHO SPRINGS FORMATION, SHOWING PEBBLES. 

B. CONGLOMERATIC TYPE OF IDAHO SPRINGS FORMATION, SHOWING PEBBLES 

SOMEWHAT ELONGATED NEAR THE PLANE OF SCHISTOSITY. 

Note cross fractures. 
TOPOGRAPHY AND GEOGRAPHY. 35 

TOPOGRAPHY IN ITS ltELATIONS TO ROCK FORMATIONS. 

The formations of the Georgetown quadrangle are crystalline rocks, which for the most 

part resemble one another closely in mineralogical composition. In con~equence they offer to 

erosion approximately equal resistance, and no single formation persistently occupies either 

mountain tops or valleys. N evcrtheless, two scarps are determined by the differential resistance to erosion of adjacent rock formations. One of these is on the south slope of Alps .Mountain, 

the Silver Plume granite being elevated and the Idaho Springs formation being depressed; the 

other is south of Meridian Hill, quartz monzonite forming the scarp and the Rosalie granite the 

lo\vland. 

To weathering and stream corrasion the rocks, with the exception nf the biotite-sillimanite 

schist of the Idaho Springs formation and the hornblende gneiss, appear to offer approximately 

equal resistance. Of the two rocks excepted each breaks down readily, the first physically, the 

second chemically, some of its bowlders being rotten to the core. Above timber line exfoliation 

by the flaking off of platy ellipsoidal fragments produces a peculiar uneven surface on most of 

the rocks. Where these flakes are opposite. one anoth-er, holes are formed in many of the 

thinner slabs. 

It will be convenient in describing the topographic effect of the various formations to group 

them into (1) schists, including the biotite-sillimanite and biotite schists of the Idaho Springs 

formation, together with locally schistose varieties of the quartz monzonite gneiss; (2) gneisses, 

including the quartz gneiss and the lime-silicate rocks of the Idaho Springs forination, the hornblende gneiss, the quartz monzonite gneiss, and the gneissoid granite; (3) massive granular 

rocks, including the less gneissoid varieties of the gneissoid granite, the quartz monzonite, the 

quartz diorite, the Rosalie and Silver Plume granites, and the granite-pegmatite; and (4) the 

intrusive porphyries. Each of these rock groups produces a characteristic topography, which, 

however, is restricted by the small areal extent of each of the rocks. 

The country underlain by schists is one of rather uninterrupted curves. Dip slopes, best 

exemplified by the rather gentle slope on the north side of Chief Mountain, ~end to form along 

the well-developed schistosity. Outcrops, which are comparatively rare upon the old mountainous upland, are characteristically depressed heaps of bo-wlders, with slabs between, where 

the dip of the schistosity is approximately flat. Where the schistosity dips steeply, slabs 

protrude 4 or 5 feet from the soil, and the surface resembles a cemetery, long deserted, with 

many of its closely crowded gravestones o-yerturned. Along streams schists form less prominent and continuous outcrops than the other rocks. ·where the dip of schistosity is low, undulating, smoothly contoured exposures occur, and where the schistosity is approximately vertical, 

craggy and serrated forms are common. 

The gneisses weather into bedlike exposures which from a distance resemble those of 

sedimentary rocks. With the prominent development of joint systems the gneisses produce 

forms similar to those of the granites. 

On the old mountainous upland the massive granular rocks underlie rounded ridges and 

hills covered by large and small fragments, with here and there a conical heap of bowlders. 

Along stream valleys almost continuous exposures of rugged crags, whose minor details are 

controllsd by the rectangular joint systems, are characteristic. Here masonry-like piles, turrets, 

and buttresses arc characteristic and are particularly well seen in the granite-pegmatite, 

which seems the most resistant of these rocks, while quartz diorite is the least resistant. 

The well-lmown domal weathering of granite is developed in but two of the six massive 

granular rock formations, and in these two but locally. Below timber line (approximately 

l1,500 feet) the old mountainous uplancl, underlain by quartz monzonite, is beset with many 

domes from 200 to 300 feet high !tnd from 300 to 400 feet in diameter. These are unusually 

well developed upon the Captain Mountain ridge, where the bare gray rocky hemispheres 

appear to occupy about one-fifth the surface of the dark-green, forest-clad hills. Where the 

original gneissic structure is rather highly developed the domes are low; where joints are highly 

developed the domes are represented by huge hemispherical piles of bowlders. Above timber 
36 GEOLOGY OF THE GEORGETOWN QUADRANGLE, COLORADO. 

line rock disintegration, due to temperature changes, docs not permit the formation of these 

domes, and the surface is covered by huge residual masses of monzonite. 

The comparatively low-lying area of Rosalie granite in the southeast corner of the quadrangle is a topographic unit, bounded on the north by a scarp of quartz monzonite and on the 

west and northwest by country underlain by a complex of gneisses and granular rocks. It is a 

tract of rounded divides with mammillary hills and gently sloping stream valleys with narrow 

flood-plains. Cliffs are present only where the wandering streams under-cut the hillsides. 

Small inclosed basins mark areas of more rapid disintegration from which the wind has swept a 

portion of the detritus. Sparse trees of unusual size and luxuriant grass give a parklike aspect 

to the lower lands, and the higher areas are thickly covered with trees of fair size. The granite 

outcrops as flat circular bosses, low domes, or depressed piles of rounded residual bowlders. 

The ground between is covered with coarse yellow. or yellowish-pink granite soil. In disintegration to soil. wavy cracks first rim the large feldspar areas of the granite; biotite is partially 

removed in solution, forming cavities, and then the mass crumbles into angular pieces from 

half an inch to 3 inches long. Later these blocks break down into the constituent minerals 

or into cleavage fragments of them. The weathered surface of the granite is slightly rough 

through the almost complete removal of biotite, the partial removal. of feldspar, and the relief 

of quartz. Certain spheroida~ planes in the granite are resistant to weathering, and these limit 

both the depressed domes and the round residual bowlders. The domes, which range from 20 

to several hundred feet in diameter, are either intact or represented by heaps of re;idual fr~gments. In the destruction of the domes some force, probably frost, first cracks the outer and 

more resistant granite and, on exposure, the interior rapidly disintegrates. The hard crust is 

undermined and the solid granite shell 

partially breaks down, resulting in the 

formation of partial domes with overhanging caps, somewhat resembling 

mushrooms. (See fig. 2.) Finally the 

crumbling pedestals collapse and granFIG. 2.-Sketch showing domal structure of Rosa!ie granite, southeastern itic soil, with here and there a fragment 

part of quadrangle. a, Hard crusts; b, readily disintegrating centers. of the hard crust, remains. There are 

other resistant layers parallel to the 

outer crust, and by disintegration partial mushroom forms may be superimposed upon the 

domes. These resistant crusts, from the variety of surface forms which they assume, arc of 

secondary origin and are due to the deposition, upon surfaces exposed to the sun, of cementing material held in solution by water in the rock mass. Other areas underlain by the Rosalie 

granite are not characterized by well-developed domes. 

It has already been said that domal weathering in the Georgetown quadrangle is confined 

to the quartz monzonite and the Rosalie granite, and only in certain areas of these formations 

is it well developed. In consequence it is inferred that in this region such weathering is primarily determined by inherent qualities of the rocks themselves, and only secondarily by topographic and climatic conditions. Of the inherent qualities, the most evident is the lack of 

closely spaced joints, and a mean resistance to mechanical and chemical disintegration is probably 

also favorable. The domal weathering is, however, dependent to a certain degree on topographic 

conditions, since in a general way the domes are rudely concentric to the surface and, furthermore, they characterize mature surfaces away from streams. Climatic conditions also partly 

control the position of the domes, for they are poorly developed above timber line, where temperature changes are greatest. 

The intrusive porphyries are characterized by few outcrops, as they break down readily 

into angular blocks, most of which are less than a foot in diameter. Dikes as a rule weather 

flush with the surface of the pre-Cambrian rocks and only in a few places do ruined walls mark 

their course. 
GEOLOGY OF THE GEORGETOWN QUADRANGLE, COLORADO, 37 

GENERAL GEOLOGY. 

PRELIMINARY OUTLINE: 

The rocks of the Georgetown quadrangle, with the exception of igneous rocks, probably 

of late Cretaceous age, and of Quaternary deposits, belong to the pre-Cambrian complex of the 

Colorado Range. The pre-Cambrian age of these rocks, which can not be demonstrated in the 

quadrangle, is inferred from relations elsewhere in these mountains. Thus Peale a describes a 

series of crystalline rocks at Glen Eyrie, Colo., on the east front of the Colorado Range, that 

are in a broad way comparable with those of the Georgetown quadrangle. Upper Cambrian 

sandstone overlies these rocks unconformably, and the pebbles of the older rock in the sandstone are metamorphosed similarly to the corresponding rocks now in place at Glen Eyrie. 

The oldest pre-Cambrian rocks of the Georgetown quadrangle, constituting the Idaho Springs 

formation, are crystallines, presumably of· sedimentary origin. This formation, while deeply 

buried, was subjected to mountain-building forces which folded it in a· complex manner and 

produced in it a regional schistosity. Later, but undoubtedly also in pre-Cambrian time, it 

was most intensely intruded by a series of holocrystalline igneous rocks. The earlier of these 

rocks were considerably affected by the mountain-making forces which produced the regional 

schistosity in the Idaho Springs formation, while the later are mashed only along lines of intense 

local movement. From the different degrees of schistosit-y developed in the different preCambrian rocks,· the period between the deposition. of the Idaho Springs formation and the 

intrusion of the latest pre-Cambrian granite must have been of vast length, and during. the long 

period of intrusion, to judge from the granitoid habit of the igneous rocks, the surfa~e as we 

now see it must have been buried beneath a great thickness of overlying rocks. 

During early and middle Paleozoic time the area was presumably subjected to enormous 

erosion and fragmental material derived from the Colorado Range was deposited in the sea to 

the east of the present front of the range. · 

It is probable that during at least a portion of late Carboniferous or early Mesozoic time 

the quadrangle or a part of it was submerged beneath an arm of this sea. 

Probably 'in very late Cretaceous and Tertiary time dikes, sheets, and stocks of siliceous 

and intermediate igneous rocks in~ruded the pre-Cambrian complex. 

Later a rugged mountain surface was reduced. to a mature mountainous upland, and then 

after an uplift subjected to two periods of Pleistocene alpine glaciatimi and to unimportant 

postglacial erosion. · 

IDAHO SPRINGS FORMATION. 

NAME. 

The name Idaho Springs formation is applied to a series of interbedded metamorphic 

crystalline rocks, presumably of sedimentary origin, which are typically exposed in the vicinity 

of Idaho Springs. 

DISTRIRUTION AND GENERAL STRUCTURE. 

· The Idaho Springs formation is widely distributed over the quadrangle and there is scarcely 

a square m,ile in which it does not occur intruded by igneous rocks or as shattered fragments 

included in them. It is the groundwork through which the igneous rocks have been injected 

and probably at one time covered the whole plane which is now the surface. The surface 

exposure, about 53 square miles, is a trifle ·less. than one-fourth of the area of the quadrangle. 

In the northeast corner the formation occurs as a rad~ly triangular body covering about 25 

square miles. The next largest area forms the crest of the ridge from Griffith Mountain southward almost to the divide between South Clear and East Geneva creeks. Other large areas 

lie on the western and southern borders of the quadrangle, east of Geneva Creek. The 

a Peale, A. C., Seventh Ann. Rept. U.S. Geol. and Geog. Survey Terr., pt. 1, 18i4, p. 202. 
38 GEOLOGY O.F THE GEORGETOWN QUADRA:NGLE, COLORADO. 

biotite-sillimanite schist and the biotite schist members are highly sehistose, the planes of 

schistosity in some places corresponding with the supposed bedding planes, but more commonly cutting them at high angles. Peculiar features of the distribution of this formation 

are certain dikelike forms northwest of Georgetown and the narrow lenses of schist 4 miles 

east of Lincoln Lake, between the Rosalie granite and the quartz ,nonzqnite. In explanation 

of the first it may be stated that the Idaho Springs formation had a highly developed sehistosity prior to the intrusion of the granite near Georgetown and so perfect was tho consequent 

parting that the inclusions of tho schist in the granite affect a clikelike form. The cause of 

the lenses between the monzonite and granite is less certainly known, although the intrusion 

of the monzonite probably hat"dened the adjacent schist and cemented the contact firmly. In 

consequence, when the younger granite was intruded a shell of schist clung to the monzonite, 

fracture occurring between the intensely and less intensely metamorphosecl schist rather than 

between the schist and monzonite. The wavy lines in the color pattern of the Idaho Springs 

formation on the map (Pl. II, in pocket) show the general strike of the planes of schistosity. 

The metamorphism of the formation is so intense that in most places the determination of the 

planes of stratification is impossible and in consequence the thickness of the formation is 

unknown and its structure can not be unraveled. 

The Idaho Springs formation includes four intensely metamoqJhosed crystalline members, 

three of which, the biotite-sillimanite schist, the biotite schist, and the quartz gneiss, arc interbedded with and grade into one another, while the fourth, consisting of the lime-silicate rocks, 

although interbedded with the others, appears to grade into the quartz. gneiss only. The 

unity of" the formation is further indicated by the equal or approximately eq¥al degree of metamorphism to which each member has been subjected and by their common structural relations 

to all other formations. · 

The terms schist and gneiss as here used, in accordance with the interpretation of Van 

Rise, a are wholly structural. Schists, then, are "cleavable rocks the cleavage pieces of which 

are like one another and the mineral particles of which are for the most part so large as to be 

visible to the naked eye." A gneiss, on the other hand, is "a banded rock the bands of which 

are petrographically unlike one another and consist of interlocking mineral particles." 

BIOTITE-SILLIMANITE SCHIST. 

PETROGRAPHY. 

The biotite-sillimanite schist is a foliated and in many places intensely crenulated, normally fine-grained rock. In color it is dark gray or black, but on weathered outcrops it has a 

rusty appearance. Biotite, clear quartz, white, semitransparent feldspar, and sillimanite are 

everywhere recognizable mcgascopically in approximately equal amounts, and in some of the 

rock muscovite, garnet, tourmaline, and corundum are prominent. White or greenish-white 

sillimanite occurs in single rods and bundles of rods, elongated in the plane of schistosity, 

which are cut by transverse fractures whose interstices are filled by biotite flakes and magnetite 

or pyrite grains. In places the component rods of the aggregates cross one another, forming 

confused meshes. 

The schistosity is due to the parallel orientation and great elongation of biotite and muscovite blades and sillimanite rods and aggregates, to the less perfect orientation and smaller 

elongation. of the quartz granules, and to the still less perfect alignment of feldspar and the 

accessory minerals. Biotite forms folia of interlocking blades along the planes of parting. 

The schistosity is emphasized by two lit-par-lit injections of pegmatitic material; one very 

ancient and the other the youngest of the probable pre-Cambrian formations. Isolated pencils 

of pegmatite also lie in the planes of schistosity,_ and in cross section appear as quartz or feldspar 

"eyes" around which the biotite plates curve (fig. 3). These are locally abundant near pegmatite bands and resemble phenocrysts rounded by granulation. The deformed condition of ---· ------- a VanHise, C. lL, Mon. U.S. Geol. Survey, vol. 47, 1904, pp. 779, 782. 

I. 
GENERAL GEOLOGY. 39 

their constituent minerals indicates that they are not the segments of pegmatite dikes separated 

by shearing, but that their form is original and due to the power of crystal growth. (Seep. 63.) 

The microscope shows that the rock has the highly linear interlocking texture of a typical 

schist in which all original sedimentary characters have been destroyed. The larger constituents characteristically inclose crystals and blebs of one another. Biotite occurs in blades 

with partially developed basal faces and frayed ends. In certain places the elongation of the 

biotite is at right angles to its cleavage, indicating extreme recrystallization in the production 

of the present schistosity, a characteristic also of some muscovite folia. Biotite is mainly 

fresh, but is more or less altered in some thin sections to chlorite and magnetite with or without 

muscovite or sagenitic webs of rutile. The quartz ellipsoids are characterized by abundant 

lines of fluid inclusions which. pass through several quartz grains and even into feldspar grains. 

These arc evidently secondary and are distributed along minute cracks. Of the feldspar 

ellipsoids orthoclase is predominant, an acidic plagioclase (albite-oligoclase or oligoclase, rarely 

andesine) common, and microcline rare. Both the albite and pericline tw!nning of plagioclase 

are notably discontinuous and of varying width. Quartz and orthoclase are micropegmatitically 

intergrown in much of the rock. · 

Sillimanite has the transverse parting well developed and in many places aligned segments, 

broken parallel to the basal pinacoid, are embedded in the other essential constituents. By slight 

displacement of segments at the parting the columns are bent. Macropinacoidal cleavage is 

~----------------------------18m.------------------------------~ 

FIG. 3.-Biotite-sillimanite schist of Idaho Springs formation injected by dikes and eyes of pegmatite. a, Pegm~tite; 

b, gneiss. The large number of the eyes shows how completely the schist was saturated with the pegmatitic fluid.· 

North side of Chicago Creek at the mouth of Spring Gulch. 

visible only in the larger rods. In the crystallization of the schist sillimanite and feldspar appear 

to have been formed from the same material, and in consequence sillimanite-rich laminre are poor 

in feldspar. Rude pseudomorphs of sillimanite after feldspar grains also occur, while as certainly 

sillimanite here and there replaces biotite. Since sillimanite is also a product of contact metamorphism (seep. 40), this mineral appears to have crystallized at three distinct periods. It is for 

the most part fresh, but in places sericite is pseudomorphic after it. Muscovite is in part intergrown with and contemporaneous with biotite and in part secondary to it. The constant though 

nowhere abundant accessories are zircon in grains and crystals, apatite in grains and short ptisms, 

and magnetite in grains and crystals. The last two in particular show locally a distinct orientation parallel to the plane of schistosity. Fluid inclusions elongated parallel to the prism axis are 

common in some apatite individuals. Blebs and sievelike crystals of pink garnet are in no way 

peculiar. The more uncommon accessories include pyrite intergrown with magnetite and tiny 

tourmaline crystals; rutile crystals and andalusite grains are very rare. 

Since the schist was formed mass mechanical action has bent and fractured biotite and muscovite blades, broken sillimanite rods, and crushed quartz and more particularly feldspar grains, 

or produced in them undulatory extinction. Here and there this wavy extinction in orthoclase 

appears to pass into the crosshatching of microcline. These phenomena are most prominent in 

the quartz gneiss, somewhat less prominent in the lime-silicate rocks and the biotite schist, and 

least prominent in the biotite-sillimanite schist. 
40 GEOLOGY 0.1!' THE GEORGETOWN QUADRANGLE, COLORADO. 

TYPE LOCALITIES. 

Biotite-sillimanite schist is the most widely distributed member of the formation. It occurs 

interbedded with comparatively few bands of the other members on Chicago Creek and in the 

vicinity of Idaho Springs and Freeland. 

CONTACT METAMORPHISM. 

in this regionally metamorphosed rock certain minerals are developed near igneous masses 

apparently through contact metamorphism. Some of these minerals are formed by recrystallization of constituents of the schist through the agency of heat; others, in part, at least, are 

emanations from the intrusive magma. 

On the ridge between Devils Canyon and Barbour Fork of Soda Creek, 1 mile east of the 

mouth of Eclipse Gulch, residual bowlders of schist contain from 30 to 50 per cent of corundum, 

which occurs in barrel-shaped crystals from one-half to three-fourths of an inch long and from onefourth to three-eighths of an inch wide. Under the microscope the colorless corundum shows welldeveloped cleavage and polysynthetic twinning parallel to 1011. The crystals are mere skeletons 

filled with biotite and muscovite shreds and quartz and feldspar grains. Fine opaque needles 

similar to those in quartz are characteristic inclusions. Large plates of muscovite, evidently contemporaneous with the corundum, inclose grains of the latter mineral, and all of these have the 

same orientation. The structure is then allied to that of micropegmatite, From the abundance 

of sillimanite in this member of the Idaho Springs formation it is evident that it is rich in alumina. 

Sillimanite formed during the recrystallization of the gneiss, a process long antedating the 

intrusion of the quartz monzonite. Since the corundum-bearing rock, however, contains .no 

sillimanite, it is probable that the corundum was produced at the expense of sillimanite, quartz 

being simulta~1eously formed. 

In addition to tho tourmaline in: microscropic grains already mentioned as an original acces;. 

sory of the schist, large prisms of black tourmaline are embedded at right angles to the schistosity 

near pegmatite dikes containing similar tourmaline. Pink garnets up to 2 inches in diameter, 

some of them surrounded by white halos from which they have abstracted all the ferromagnesian 

materials, are developed by regional metamorphism,· while garnets without such halos are very 

abundant near garnetiferous pegmatite dikes. It is probable that tourmaline and garnet near 

pegmatite dikes were derived from solutions originating from the pegmatitic magma. Garnet, 

probably of contact-metamorphic origin, is relatively abundant in small masses of schist caught 

up in pre-Cambrian plutonic rocks. 

Unusually large plates of biotite and muscovite and aggregates of sillimanite rods, some of the 

latter being as large as· a man's hand, cut the schistosity at high angles near igneous masses and 

appear to be statically developed through contact metamorphism. Such plates of unoriented 

muscovite spangle the s~hist east and north of Chief Mountain and in the northwest and southwest corners of the quadrangle. Microscopic examination shows that the schist near the musco- · 

vite is poor in feldspar and it is probable that feldspar was recrystallized into muscovite. Large 

sillimanite aggregates of'static origin occur m garnctiferous schist inc1psions 4 ,miles cast of 

Rosalie Peak. 

BIOTITE SCHIST. 

PETROGRAPHY .. 

The biotite schist is differentiated from the type just described by a finer grain, by a medium 

or light-gray color, and by a less perfect schistosity, due to the smaller biotite content and the 

almost total absence of muscovite and sillimanite. Minute plates of biotite, well aligned, but 

not forming continuous films, set in granules of quartz, and feldspar with only a suggestion of a 

linear arrangement, give a pepper-and-salt effect to the rock. Small segregations of magnetite 

surrounded by white halos from which they have abstracted the ferric materials are mther common. Fine-grained facies in the southwest corner of the quadrangle are friable, perhaps through 

mashing. Pegmatitic bands in the biotite schist are comparatively rare, since this rock offered 
U. S. GEOLOGICAL SURVEY I'ROFESSIONAL PAPER NO. 63 PL. VIII 

A. CONGLOMERATIC TYPE OF IDAHO SPRINGS FORMATION, SHOWING PEBBLES ELONGATED 

INTO BANDS. 

B. QUARTZ-MAGNETITE PEGMATITE. 
GENERAL GEOLOGY. 41 

to lit-par-lit injection le.ss highly developed planes of weakness than the more schistose biotitesillimanite schist. 

Under the microscope the biotite schist shows approximately the same minerals as the 

biotite-sillimanite schist, although they are present in different proportions. Sillimanite and 

muscovite are relatively rare and quartz and feldspar arc more abundant than in the biotitesillimanite schist, and of the feldspars microcline is most abundant. Epidote, not observed as 

an alteration product of biotite in the biotite-sillimanite schist, is associated with secondary 

chlorite. 

TYPE LOCALITIES. 

The biotite schist is less widely distributed than the biotite-sillimanite schist, but occurs 

typically exposed near the head of West Chicago Creek and in the southwestern portion of the 

quadrangle. Small inclusions arc abundant in the quartz monzonite. (See p. 52.) 

QUARTZ GNEISS. 

PETROGRAPHY. 

The quartz gneiss is a well-banded rock formed of fine to broad laminre, varying in color 

from gray to brown, red, or black. The texture is, as a rule, dense and vitreous, although in 

places it is saccharoidal. Lit-par-:lit pegmatitic injections are rare, as is natural in a rock with 

imperfect gneissic banding. Under the microscope quartz is shown to be the essential con'-

stituent. The accessory minerals include all the minerals present in biotite-sillimanite schist, . 

with the addition of shreds of green hornblewle. The gneissic parting is due to the elongation 

of intricately interlocking quartz lenses, to the variation in granularity of the quartz of adjacent bands, and to the linear arrangement of discontinuous sheets of biotite blades and magnetite cubes. The darker color of certain bands of the gneiss is due to an unusual abundance 

of this biotite 'and magnetite. 

TYPE LOCALI'l'IES. 

The quartz gneiss is less widely distributed than the two facies previously described. It 

is exposed for a distance of half a mile on Sugarloaf Peak. 

ELLIPSOIDAL MASSES. 

Distributed in bands parallel to the lithologic boundaries of the members of the Idaho 

Springs. formation already described, but more characteristic of the biotite schist, are white or 

gray ellipsoidal masses which range in length .from one-half inch to 4 inches. The masses are 

flattened in the plane of schistosity, in some places into paper-thin sheets. (See Pls. VII, 

VIII, A.) The thickness, width, and length range in proportion from 1:2:2 in the less mashed 

ellipsoids to 1:8:100 or more in those which are more mashed. The latter are cut by sharp 

fractures that cross the elongation at angles ranging from 60° to 90°. In each ,band the masses 

are remarkably even, not only in size, but also in spacing, both parallel to and across the schistosity. They are sharply bounded and the schistosity of the matrix curves around them. The 

ellipsoids are composed of predominant quartz and subordinat·e sillimanite. The other minerals 

of the series are present only in small amounts, feldspar being absent in the thin sections examined. These masses are widely distributed in the quadrangle, but they arc particularly well 

developed on Chief and Pendleton mountains. (For further description see "Pebble-bearing 

gneiss," p. 177.) 

LilliE-SILICATE ROCKS. 

The rocks called here for convenience the lime-silicate member include several intergracling 

facies of varying cmnposition and texture-quartz-magnetite gneiss, hornblencle-cliopsicle gneiss, 

and two massive rocks, the one composed essentially of quartz, epidote, and garnet, the other 

of calcite and lime silicates. 
42 GEOLOGY OF THE GEORGETOWN QUADRANGLE, COLORADO. 

Quartz-magnetite gneiss is a finely banded steel-gray heavy rock of fine grain, which on 

weathered outcrops is heavily iron stained. Imperfect parting occurs along laminre of the 

diverse minerals. Under the microscope irregular grains of magnetite form from 20 to 40 per 

cent of the area of the thin section, quartz, brown or pink garnet, and hornblende being the 

other essential constituents. Biotite, epidote, chlorite, and zircon are accessory minerals. 

The quartz-magnetite gneiss is associated with other facies of the Idaho Springs formation near 

the head of Little Bear Creek and on Trail Creek. 

Hornblende-diopsicle g;10iss is a well-banded, flne-grained rock composed of. alternating 

bands of white quartz and feldspar and black or greenish-black hornb!ende and pyroxene, and 

as tho latter increase the rock changes in color from dark gray to black Under the microscope 

the straight or gently curved gneissic strpcture and consequent parting is seen to be due to the 

segregation of the dark and light minerals in bands and to the elongation and dimensional 

arrangement of hornblende and pyroxene parallel to the schistosity. The pyroxene is a greenish diopside and occurs as rude columns which are in places partially altered to hornblende. 

Hornblende, however, in greater part is an original constituent of the gneiss, and in some thin 

sections is present to the exclusion of cliopside. Epidote occurs here and there as an original 

constituent of the gneiss and, with or without chlorite, as an alteration product of hornblende. 

Epidote, zoisite, sericite, and calcite are products of altering hornblende and feldspar. Quartz 

and the feldspars, predominant oligoclase or a related plagioclase, and subordinate orthoclase, 

with rarely a little microcline, present no unusual features, although micropegmatitic intergrowths of the two are locally present. In some places tubular areas of these minerals also 

extend into epidote, the texture closely resembli_ng a micropegmatitic inter9rowth. Titanite 

is an exceedingly abundant accessory mineral, occurring in well-developed crystals or in irregular grains that have mutually interpenetrating borders with the essential constituents, which 

are evidently contemporaneous with them. Polysynthetic twinning is present in some of the 

titanite. Magnetite, also an abundant accessory, is as a rule clearly an original constituent of 

the gneiss. In a few places, however, it forms rude crystals with abundant inclusions of the 

other constituents of the gneiss, and these may be of later origin. Of the other original accessories, apatite is constant, calcite common and in places abundant, pyrite common, and zircon 

rare. Hornblende-diopside gneiss is widely distribut~d in the quadrangle and occurs in most 

of the areas mapped as the lime-silicate member of the Idaho Springs formation. 

This gneiss grades into lenses or bands of quartz, epidote, and garnet, parallel to or cutting 

its schistosity at low angles. The bands, clearly an alteration of the gneiss, are from 3 to 4 . 

inches wide, but expand and contract irregularly. In many places they occur in series from a 

foot to several feet apart. Numerous stringers of gneiss remain in the altered rock, the ferromagnesian minerals being as a rule rudely pseudomorphed by epidote. Isolated crystals of 

garnet and grains of magnetite in interrupted. sheets appear to form more freely in the quartzrich portions of the rock. In some places the partially epidotized hornblende columns sweep 

around the garnet crystals; and although this may be due to the fact that garnet acted as a 

buttress· by which the orogenic movements were concentrated, it is believed that the columns 

may have been pushed away from the garnet by the pressure exerted by the growth of crystals, 

some of which arc 6 inches in diameter (fig. 4). 

These rocks in which the gneissic structure is only partially destroyed grade into massive 

aggregates of large or small, mutually penetrating individuals of epidote, garnet, quartz, and 

magnetite. In many places the coarse-grained rocks with component minerals 6 inches in 

diameter have the texture of a miarolitic pegmatite with crystals of brown garnet and green 

epidote lining its cavities. There are facies in which the rock is all quartz, all epidote, quartz 

and epidote, epidote and· garnet, and quartz and garnet. (Pl. IX.) The rocks vary in color 

from white to brown or green, according as the colorless or snow-white quartz, the cinnamonbrown garnet, or the epidote predominates. Although in certain places these rocks are the 

hornblende-diopsicle gneiss recrystallized, in others they form the whole outcrop and may 

possibly be contemporaneous in origin with that rock. Microscopic examination shows that 

the rock is massive and in a general way the constituents interpenetrate one another and are 
U. S. GEOLOGICAL SURVEY PROFESSIONAL PAPER NO. 63 PL. IX 

A. GARNET-QUARTZ ROCK OF LIME-SILICATE MEMBER OF IDAHO SPRINGS 

FORMATION. 

Showing partial crystals of garnet. The garnet is dark, the quartz white. 

B. EPIDOTE-QUARTZ ROCK OF LIME-SILICATE MEMBER OF IDAHO SPRINGS FORMATION. 

Showing pegmatoid structure. The epidote is dark, the quartz white. 
GENERAL GEOLOGY. 43 

all contemporaneous, although quartz and calcite, the latter rarely prominent macroscopically 

but constantly seen in thin sections, are in general younger than the other components. Brown 

garnet occurs in large sievelike partial crystals or SlUall and more homogeneous grains. vV aldemar T. Schaller, who has made qualitative analyses of tllis garnet, finds that it contains 

much aluminum and calcium oxides and considerable ferric iron, the garnet in consequenee 

being between grossularite and andradite. Epidote occurs in sievelike grains or homogeneous 

crystals and is also intcrgrown with zoisite, and these minerals have under crossed nicols a 

moire appearance. Greenish diopside, the presence of which was not suspected in the field, 

occurs in rather abundant grains and short prisms in every thin section. It is mainly fresh, 

but in places is slightly altered to epidote or hornblende. Tremolite in partial columns was 

observed in a single section, and green )'lornblencle is also in a few section~ an original member 

of the series. Chlorite is secondary to hornblende. Of the accessory minerals titanite, polysynthetically twinned in the larger crystals, and apatite are invariably abundant; pyrite and 

magnetite are less uniformly present. All these accessory minerals tend to form well-defined 

crystals, with the exception 

of apatite, whose borders interpenetrate those of the 

essential constituents. Apatite in consequence was evidently one of- the last 

minerals to recrystallize. 

Perhaps the most striking 

textural peculiarity of the 

quartz-garnet-epidote rock is 

the presence of intergrowths 

closely allied to the micropegmatitic. Epidote and 

zoisite, epidote and garnet, 

epidote and hornblende, and 

calcite and garnet each acts 

for the other the part of 

host, all areas of the included 

mineral being in similar optical orientation. Fluiu in-

~------------ 4-f'eet ------------~ 

llomblende-diopside ~ gneiss Epidote 

~ 

~ Quartz Gamet 

FIG. 4.-Hornblende-diopside gneiss of Idaho Springs formation altered to quartz-epidotegarnet rock in a band parallel to the schistosity. Shows displacement of hornblende columns 

by the power of crystal growth of the garnet. One-third mile south of Edith Lake. 

elusions are also particularly characteristic, being present m quartz, calcite, garnet, epidote, 

and diopside. 

The extensive wandering and intense segregation of the chenlical constituents of the 

hornblende-diopsicle gneiss in its alteration to the quartz-garnet-epidote rock are suggestive in 

inuicating the manner in wllich certain lenslike ore bodies, particularly magnetite, may develop 

in gneissic metamorphic rocks. 

The quartz-garnet-epidote rock is well developed south of Freeland, on Democrat Mountain, and in many other places in the quadrangle. 

Associated with the hornblende-cliopside gneiss one-half mile west of south of Alpine 

Mountain is a massive rock composed of predominent calcite grains with some partial crystals of 

bottle-green pyroxene and brown garnet. The weathered surface of this rock, which resembles 

a metamorphosed limestone, is roughened by protruding bunches of silicates. As seen under 

the nlicroscope the rock is a rather finely granular aggregate, composed of calcite, scapolite, 

diopside, grossular garnet, quartz, tremolite, titanite, and ilmenite, named in the order of 

abundance. The twinning planes of the calcite arc in places slightly bent by deformation. 

The scapolite grains, many of which are inclosed by rims of grossular garnet, are somewhat 

altered to muscovite, calcite, and quartz, and epidote is formed at the junction of scapolite 

with calci-te. 
44 GEOLOGY OF THE GEORGETOWN QUADRANGLE, COLORADO. 

ORIGIN. 

The Idaho Springs formation has been so greatly metamorphosed that all original textures 

have been destroyed. The lithologic variation across apparently bedded bands, some mere 

laminm, others several hundred feet thick, suggests a sedimentary series, while the bands 

containing the ellipsoidal masses are most naturally regarded as conglomerates. (See Pis. VII, 

VIII, A.) The quartr. gneiss is present in masses so thick that it can scarcely be of pegmatitic 

or vein origin and probably represents intensely metamorphosed sandstone. The lime-silicate 

rocks would in this interpretation of the origin of the formation represent calcareous sandstones 

and impure limestones, and the biotite-sillimanite schist and biotite schist would be metamorphosed shales and arkoses. The abundance of aluminum silicates in the series and the similarity 

to metamorphic rocks of known sedimentary origin sup'port the view of the supposed sedimentary origin of the Idaho Springs formation. This formation may be regarded, then, as an 

intensely metamorphosed series of shales, arkoses, sandstones, conglomerates, and impure 

limestones. 

No regular stratigraphic sequence of the various members of the formation can now be 

worked out. This lack of regularity is perhaps partially original, the members having been 

laid down in lenses-an inference strengthened by the erratic distribution of the ellipsoidal 

masses-but it is doubtless in the ma!n due to the intense folding, faulting, shearing, and 

injection to which the formation has been subjected. Solution may in part account for the 

discontinuity of the bands of lime-silicate rocks. 

AGE. 

The Ida.ho Springs formation is the oldest member of the pre-Cambrian in the Georgetown • 

quadrangle, since it is cut by all the other formations. 

The investigation of the pre-Cambrian rocks of Colorado has not progressed far enough 

to correlate with certainty formations described by various authors; nevertheless suggested 

correlations may have some value. In the vicinity of Pikes Peak, Cross" found schists included 

in pre-Cambrian granites. He says: "These seem to represent earthy Algonkian quartzites 

metamorphosed by a great development of mica or of fibrolite." In the northwest corner of 

the Cripple Creek quadrangle is an area of schist which widens to the northwest toward the 

Georgetown quadrangle, 40 miles distant. A ridge near by "consists of fibrolitic quartzite 

with a small and variable amount of feldspar. This is also cut by granite dikes and is entirely 

surroun~ed by granite." b Careful examination of these schists shows some to be similar to 

the quartz gneiss and others to the biotite schist of the Idaho Springs formation. Tentatively 

these ancient rocks in the Cripple Creek and Georgetown quadrangles arc correlated with one 

another. In the Crested Butte quadrangle Cross c also found schists among the Archean (or 

pre-Cambrian) rocks. He writes: "The quartzose mica schists are sometimes fibrolitic." 

Notwithstanding the fact th\).t the- Crested Butte area lies 75 miles southwest of that under 

discussion, these mica schists are identical with gradational facies between the biotite-sillimanite 

schist and the quartz gneiss, and it is possible that the two are approximately contemporaneous. 

The pre-Cambrian quartzite of South Boulder Creek, described by Van Hise,d lies unconformably upon a granite which in the amount of mashing suffered and in lithologic character 

somewhat resembles the Silver Plume granite. (See p. 58.) The gap of 18 miles between 

Idaho Springs and South Boulder Creek has not been traversed, but it is believed that the 

Idaho Springs formation is vastly older than Van Hise's pre-Cambrian quartzite. 

a Cross, Whitman, Geologic Atlas U.S., folio 7, U.S. Geol. Survey, 1894, p. 1. Sixteenth Ann. Rept. U.S. Geol. Survey, pt. 2, 189.5, p. 26. 

b Cross, Whitman, Geologic Atlas U.S., folio 7, U. S. Geol. Survey, 1894, p. 1. 

cldcm., folio 9, U. S. Gcol. Survey, 1894, p. 6. 

dVan Rise, C. R., Bull. U.S. Geol. Survey No. 86, 1892, p. 325. 
GENERAL GEOLOGY. 45 

HORNBLENDE GNEISS. 

DISTRIBUTION AND GENERAL STRUCTURE. 

The hornblende gneiss has .. a wide clistribuiton, especially in the southern portion of the 

quadrangle. It injects the Idaho Springs formation in sheets parallel to the supposed bedding 

planeR or it cuts across them as dikes. Dikes, well exposed on the west side of Soda Creek 

near the mouth of Little Bear Creek, cut not only the biotite-sillimanite schist, but the associated 

lit-par-lit pegmatitic sheets. On the southeast slopes of the hill northwest of Geneva Park 

(elevation 10,193 feet) a dike sharply cuts the hornblende-diopside gneiss of the Idaho Springs 

formation and sends out short, jagged arms along the planes of schistosity. The smaller dikes 

and sheets occupy clean-cut fractures and rarely include fragments of older rock .. They are 

further characterized by comparatively few apophyses in contradistinction to the masses of 

more acidic igneous rocks. Portions of the basic igneous rock, now a gneiss, may have been a 

surface :flow, although there is no positive evidence that such was the case. The originallenslike 

.form of the .sheets and dikes arid the intense folding, faulting, and later intrusion suffered by 

· the igneous rock explain the small extent of most of the hornblende-gneiss areas. 

PETROGRAPHY. 

The hornblende gneiss is a fine-grained, well-banded rock in which black or greenish-black 

layers of hornblende alternate with white laminre of quartr. and feldspar. Some of the exposures 

in the southern portion of the quadrangle are formed of a slightly coarser, though still finegrained gneissic aggregate that is medium gray, of lighter shade because quartz and feldspar 

are more abundant. Biotite is developed at the expense of hornblende where maximum 

movement has occurred, and along shear planes the rock passes to a biotite-hornblende schist. 

Rounded aggregates of hornblende or biotite, or both, give a porphyritic aspect to certain 

facies. Dark massive rocks grading into normal hornblende gneiss and closely resembling 

dolerite form the central portions of the Soda Creek dikes already mentiomid. 

As seen under the microscope hornblende and biotite together form approxirhately one-half 

of the volume of the rock, and either may be present to the almost total exclusion. of the other. 

Besides the replacement of hornblende by biotite through mashing, hornblende alters locally 

to epidote, to chlorite, or to epidote and some chlorite, calcite, and magnetite. Biotite is in 

the main fresh, its most common alteration product being epidote with or without chlorite 

and magnetite. Of the feldspars a plagioclase, as a rule labradorite, generally predominates 

over orthoclase. Microclinc is present in some thin sections. The feldspars are notably fresh, 

although alteration to sericite and less commonly to sericite, zoisite, quartr., and calcite occurs. 

The original basic rock contained quartz, but in the· gneisses a portion probably represents 

the excess of silica produced in the recrysthllization of tlie constituents of the igneous rock, 

and a further portion is clearly a by-product of feldspar alteration. Micropegmatitic intergrowths of orthoclase and quartz occur on the borders of some of the grains of these minerals. 

· A pale-greenish augite in short columns, sparingly present in some thin sections, is evidently 

contemporaneous with hornblende, and some compact hornblende is secondary to it. Apatite, 

magnetite, in places titaniferous, ·and titanite are constant and rather abundant accessory 

minerals, and titanite is also derived from the alteration of titaniferous magnetite. Pyrite 

is a less common accessory, while rutile is rare. Besides the secondary production of epidote 

and zoisite already mentioned, granular aggregates of -epidote and zoisite are derived from the 

interaction of altering biotite or hornblende and feldspar, and both minerals form along shearing 

planes. 

The foliation of the gneiss is in general strongly developed. Under the microscope the 

straight or gently curved parting planes arc found to be situated along alternating bands of 

light and dark minerals. The longest axes of the rude columns of hornblende, and to a less 

degree its cleavage, n.re aligned in the plane of schistosity, although the mean axef? of the columns 

are not oriented in it. Ragged rods of biotite are less perfectly oriented, while quartz !\nd 
46 GEOLOGY OF THE GEORGETOWN QUADRANGLE, COLORADO. 

feldspar ellipsoids and the iron ores arc oriented but slightly. The texture is in consequence 

that of a typical schistose rock, and the interpenetration and inclosure of each of the essential 

constituents by the others, characteristic of many metamorphic rocks, is highly developed. 

The banded aspect is increased by lit-par-lit pegmatitic injections. These resemble the 

older pegmatite which oecurs in the Idaho Springs formation, but, since in. some places the 

laminre of the latter stop abruptly on encountering the hornblende gneiss, the intrusion of this 

siliceous rock must have begun before the solidification of the basic igneous rock that is now 

the hornblende gneiss and continued for some time after its solidification. 

· .Since the recrystallization of the gneiss mas~ mechanical action has deformed the mineral 

constituents in like manner and degree to those of the Idaho Springs formation. 

Under the microscope the massive rock from the Soda Creek dikes proves to be a metadolerite or· diabase of ophitic texture, and the hornblende gneiss prior to its recrystallization 

was probably all dolerite or a related rock. The abundant lath-shaped feldspar crystals, seen 

in the hand specimen, are labradorite. Green hornblende, like that of the gneiss, a little biotite, 

and wedges of orthoelase and quartz, in places micropegmatitically intergrown, lie between 

the plagioclase laths. Tubules of quartz also extend from the quartz wedges into 'hornblende. 

Apatite, titanite, pyrite, and titaniferous magnetite are present as accessones. 

AGE. 

The hornblende gneiss cuts the Idah<> Springs formation, .and is therefore younger. The 

gneiss itself is cut by the· other igneous rocks of the quadrangle. A well marked schistosity 

is developed in the hornblende gneiss over 

wide areas, indicating that it is a very 

ancient rock, although the Idaho Springs 

formation, prior to the injection of this 

basic igneous rock, had locally at least a 

well-developed schistosity. (See fig. 5.) 

Spencer" has described in the Sierra Madre 

near Encampment, Wyo., hornblende schists 

somewhat similar lithologically to the hornblende gneiss. The two formations have 

suffered broadly equal deformation and each 

is much older than a series of acidic and 

basic granular igneous rocks. The intrusion and extrusion of basic igneous rocks were 

evidently important events in the early pre-Cambrian history of the Colorado and Wyoming 

mountains. 

QUARTZ MONZONITE GNEISS (ADAMELLITE GNEISS). 

DEFINITION. 

Brogger b defines monzonite as a granular plutonic rock chemically and mineralogically 

between the syenites and diorites. It is characterized by the presence of nearly equal amounts 

of orthoclase and plagioclase, together with hornblende, biotite, or augite. Where a considerable amount of quartz is present the rock is a quartz monzonite, which is very closely 

related to both granite and granodiorite. ·where such a rock has a gneissic structure it is 

designated a quartz monzonite gneiss. 

DISTIUBUTION AND GENF.ltAL STitUCTURE. 

Quartz monzonite gneiss· occurs at many places, especially in the central and southern 

portioJJ.s of the quadrangle. The largest area is on Paines Mountain, and the other areas lie 

a Spencer, A. C., Prof. Paper U. S. Gcol. Survey No. 25, 1904, pp. 21-24. 

bBrogger, w. C., Die Eruptionsfolgc der triadischen Eruptivgesteine bei Predazzo, etc.: Die Eruptivgestcinc des Kristianiagebietes, 

pt. 2, Kristiania, 1895, pp. 21-23. 
GENERAL GEOLOGY . 47 

. on the north slope of the ridge between Rosalie Peak and Meridian Hill, 0.8 mile east of the 

lower Chicago Lake, and on Mount Evans. 

The igneous rock, now a gneiss, was intruded as sheets, dikes, and irregular masses into 

the Idaho Springs formation and the hornblende gneiss, and the schistosity of these rocks, 

which even then was well developed, controlled in no small degree the form of the monzonitic 

m'asses. The lack of continuity and symmetry of the quart:>; monzonite gneiss masses is to no 

considerable extent due to the intrusion of younger rocks. Although in some exposures the 

age relations of the gneiss, the Idaho Springs formation, and the hornblende gneiss are clear, 

the contacts between these formations are usually sharp lines with parallel gneissic structure, 

it being evident that the monzonite when intruded did not eat its way into the· older rocks. · 

Southeast of Naylor Lake the monzonite gneiss and the Idaho Springs formation seem to grade 

into each ~ther, apparently as a result of absorption of the schist by the quartz monzonite 

when injected. The granular texture of practically all portions of the pre-Cambrian igneous 

rocks, ir-respective of the size of the intrusive masses, probably indicates that the intruded rocks 

were themselv()s highly heated at the time of intrusion. 

The close resemblance in mineral composition of the quartz monzonite gneiss to the younger 

quart:>; monzonite, described on page 74, is striking, these rocks furnishing a good example 

of two similar monzonitic intrusions, separated by a granitic intrusion. 

Slight differentiation of the monzonite magma occurred prior to its solidification, and finer 

grained, more basic segregations are characteristic of some exposures. 

PETROGRAPHY. 

The quartz monzonite gneiss is a medium-grained gneissic rock, normally porphyritic, of 

,gray and rarely pinkish color. Many weathered exposures are stained reddish or brown by 

iron oxides. Black mica, smoky quartz, white, striated, and pinkisi1-white unstriated feldspar, and less constantly greenish-black hornblende, are the prominent macroscopic constituents. Crystals of pyrite and magnetite and wedges of reddish-brown titanite are the only 

accessories visible to the naked eye. 

The gneissic structure varies considerably in the perfection of its development, and the 

rock is massive only under very exceptional circumstances. The gneissic structure is due to 

the segregation in alternating straight or gently curved bands of quart:>; and feldspar and of 

biotite and hornblende. Under the microscope the parting parallel to the gneissic structure 

is seen to be due to the parallel orientation and elongation of biotite blades through recrystallization and to a slight elongation of the hornblende columnar grains parallel to the schistosity. 

Apatite, zircon, and titanite in certian sections show marked parallelism to the gneissic structure, indicating the extreme· recrystallization to which the rock has locally been subjected. 

Rudely ellipsoidal white striated feldspar phenocrysts from 0.15 to 1 inch in length lie 

embedded in the groundmass. These feldspars are aligned parallel to the gneissic structure 

and have suffered the same deformation as the smaller constituents. The rock in consequence 

is a mashed porphyritic quartz monzonite. 

The quartz monzonite gneiss near Geneva Park and inclusions of this gneiss in the quartz 

monzonite batholith, described on page 89, contain also pink feldspar phenocrysts, some of 

them 2 inches long and many of them twinned according to the Carlsbad law. Biotite blades, 

quartz globules, and the accessory minerals present as inclusions are in places zonally arranged. 

These phenocrysts have perfect crystal outlines, with their longest axis at right angles to the 

gneissic structure, even in the most meta,morphosed groundmass. .Microscopic examination 

shows that these microcline phenocrysts, chiefly microperthitic, arc uncracked, whereas the 

feldspars of the groundmass are fractured. The phenocrysts are probably of metamorphic 

origin and were formed after the rock became a gneiss. In certain exposures they have been 

mashed by later deformatic,m into ellip~oids and even into ribbon-like masses. 

Microscopic study shows that the rock was a porphyritic quartz monzonite of hypidio·-

morphic granular texture which was characterized by unusually abundant accessory minerals. 

In some facies, however, there is so little alkali feldspar that it passes into a granodiorite. 
48 GEOLOGY OF THE GEORGETOWN QUADRANGLE, COLORADO. 

Plagioclase is more abundant than the other essential constituents-quartz, microcline, orthoclase, and biotite-and of these latter any one may predominate. Hornblende, however, 

although in general abundant, is not present in all thin sections. The plagioclase phenocrysts 

and the smaller plagioclase individuals vary in composition from calcic oligoclase to andesine. 

The plagioclase phenocrysts in particular are characterized by 11onally arranged tiny magnetite 

cubes and by minute rods, hexagonal plates, and clots which are typically opaque and black, 

although translucent and reel on the thinnest edges. They are probably ilmenite or hematite. 

Similar interpositions occur locally in orthoclase and microcline. Orthoclase, in some places 

with a few microperthitic bands, forms small grains showing slight zonal growth. The micro-

. cline of the groundmass is in part a product of the recrystallization of the rock into a gneiss, a 

phenomenon better developed in the gneissoicl granite. (See p. 50.) The feldspars are somewhat altered to sericite and kaolin, and epidote is also developed at the expense of plagioclase, 

particularly adjoining biotite or hornblende. The fluid inclusions in quartz are mainly secondary 

and lie in distinct planes. The opaque hairlike interpositions characteristic of much granitic 

quartz are developed in this rock only in secondary quartz. Micropegmatitic intergrowths of 

quartz and each of the three feldspars occur along the borders of the grains, and tubules of 

quartz protrude into the hornblende grains. Disks of micropegmatitic orthoclase and quartz 

lie between the other constituents. Some of the biotite in almost every thin section is partially altered, the initial step being either the partial loss of color or a change from brown to 

green. By further alteration epidote is formed if hornblende is present; in its absence chlorite 

and magnetite or these minerals with either muscovite or rutile needles are formed. 

In many sections hornblende and biotite are intcrgrown. Inclusions of the other rock 

constituents are so abundant in the hornblende columnar granules that they are in places 

mere skeletons. Hornblende alters to either biotite or epidote, the former having developed 

apparently during the gneissic recrystallization of the rock, the latter alteration being a later 

process. The uncommonly abundant and larg.e accessory minerals are magnetite,' titanite, 

apatite, zircon, and pyrite. Magnetite and titanite locally form skeleton growths around · 

quartz and feldspar, probably a result of the gneissic recrystallization. Magne~ite in part is 

titaniferous, and titanite is secondary to it, although ilmenite may also be present. Both 

primary and,secondary titanite are in general altered, and even where the other constituents 

are fresh a whitish or brownish semitransparent flocculent aggregate partially replaces it. 

Short, stocky columns of apatite are common, and long rods arc ~haracteristic inclusions in 

feldspar. In several sections segments of once continuous apatite crystals arc embedded in 

quartz grains, apparently indicating movement in the magma between the crystallization of 

the apatite and quartz. Pyrite is generally surrounded by or intergrown with magnetite. 

Allanite in crystals is not an uncommon accessory mineral. · 

The effects of deformation to which the gneiss has been subjected since its recrystallization are similar in every way to those already described in the discussion of the Idaho Springs 

formation. · 

The quartz monzonite gneiss in the vicinity of Georgetown and on Leavenworth Creek is 

unusually rich in biotite and lacks plagioclase phenocrysts. Microscopically these rocks are 

without hornblende and titanite and contain.a little muscovite intergrown with biotite. 

Rather massive varieties of the gneiss, which to a remarkable degree show the porphyritic 

habit, are included in later plutonic rocks at two places, one 0.4 mile west of the summit of the 

trail from Elk Creek to Indian Creek and the other 1.1 miles N. 55° W. of the same point. The 

very fine granular grounclmass of biotite and feldspar contains small phenocrysts of biotite, 

feldspar, and hornblimde. Under the microscope the allotriomorphic granular grounclmass is 

proved to <;onsist of calcic oligoclase, hornblende· or faintly yellowish-green augite, orthoclase, 

and quartz, named in the order of abundance. The plagioclase phenocrysts are perfectly 

formed, but those of hornblende, augite, and biotite are mere skeleton growths in which the 

groundmass individuals ~re inclosed·. Augite, in particular, occurs as small disconnected areas 

of similar optical orientation,· between which the groundmass mosaic lies. The accessories are 

similar to those of the granitoid facies. 
GENERAL GEOLOGY. 49 

AGE. 

The quartz monzonite gneiss intrudes the Idaho Springs formation and the hornblende 

gneiss and includes fragments of both. It is in turn cut by the gneissoid granite and the 

igneous rocks which succeed it. 

GNEISSOID GRANITE (MASHED BIOTITE GRANITE). 

DEFINITION. 

Granite is a wholly crystalline, granular igneous rock consisting of orthoclase (or some 

other alkali feldspar), quartz, and either muscovite, biotite, or amphibole. "Where biotite is 

the predominant mineral of the three last named the rock is designated biotite granite. The 

prefix "gneissoid" indicates that the granite has suffered considerable mashing. 

DISTIUBUTION AND GENERAL STRUCTURE. 

Gneissoid granite covers considerable areas in the southwest corner of the quadrangle and 

smaller masses are widely distributed, being particularly abundant east of Georgetown and 

west of Idaho Springs. Dikes, sheets, and irregular stocklikc masses cut the Idaho Springs 

formation1 the hornblende 

gneiss, and the quartz monzonite gneiss. Planes of 

weakness parallel to the 

schistosity of the older formations controlled in a 

marked degree the form 

assumed by the injected 

magma. The continuity 

and symmetry of the gneissoid granite masses have 

been considerably modified 

by intrusions of younger 

igneous rocks. 

The granite magma 

when injected must have 

been very fluid, for it inserted itself between the fo-

~--------------~------------20ree~-------------------------~ 

FIG. 6.-Gneissoid granite (a) with inclusions of biotite-sillimanite schist (c) of Idaho Springs 

formation, one of which is cut hy hornblende gneiss (b), partly eaten and shredded by rna,grna .. 

One-half mile west of mouth of Spring Gulch, on north side of Clear Creek, Central City quadrangle. 

lia of the earlier gneisses and replaced in a notable degree inclusions of the biotite-sillimanite 

schist of the Idaho Springs formation. In proof of this, dark bands in scroll-like patterns of 

more basic granite preserve the outlines of schist inclusions, whilo in many places lacelike 

shreds of biotite suggest the forms of original inclusions now almost totally absorbed (fig~ 6). 

Similar phenomena, referred to at greater length on page 60, characterize the granite-pegmatite. 

PETROGRAPHY. 

The gneissoid granite is a fine-grained rock of even texture, which is light to medium 

gray when fresh and flesh pink to yellowish brown when weathered. Some of the gneissoid 

granite in the southwestern portion of the quadrangle is of rather uneven, medium grain. 

White to pinkish-white fel.dspar, slightly smoky quartz, and black mica are visible to the 

naked eye, although the mica is in some of the rock practically'lacking. Facies poor in biotite' 

belong in mineralogical composition with the siliceous granites for which Spurr has proposed 

the name alaskite.a Muscovite plates which reach a maximum dia~neter of an inch and inclose 

a Spurr, J. E., Am. G~ologist, vol. 25, 1900, p. 230. 

31401-No. 63-08-.-4 
50 GEOLOGY OF THE GEORGETOWN QUADRANGLE, COLORADO. 

the other constituents poikilitically are locally prominent. Magnetite, pink garnet in small 

sporadic grains, and sillimanite in matted fibrous aggregates are visible here and there. The 

sillimanite aggregates, which are one-fourth of an inch in diameter, are associated with 

muscovite. 

At several exposures west of Idaho .Springs the normal rock grades into a coarse granite, 

which in turn passes into a pegmatite characterized by the presence of both biotite and 

magnetite. 

The microscope shows the texture of the ·more massive facies to be allotriomorphic granular. The quartz is of the normal granitic type, except for the abundant inclusion in many 

Fra. 7.-Recrystallized microcline in fractured quartz (of 

gneissoid granite). q, Quartz 

'\Vith undulose extinction; o, 

orthoclase, highly altered and 

of the gra'ins of curved, opaque, threadlike microlites. The plentiful 

fluid inclusions are either in sporadic bunches and original or in 

planes crossing from one quartz graJ.n to another and secondary. 

The secondary inclusions are smaller than the original ones, and are 

ellipsoids elongated parallel to the fracture on which they are 

situated. Orthoclase and microcline greatly predominate over oligoclase or a closely related acidic plagioclase. Micropegmatitic intergrowths of each of the feldspar species with quart7; occur, and quartz 

and the feldspars inclose blebs of one another micropoikilitically. The 

content of microcline, much of it 

microperthitic, increases proportionately with the amount of recrystallization to which the rock has 

fractured; m, rnieroclinc, perfectly fresh and unfractured; 

p, plagioclase. X 12. been subjected, and in consequence 

this mineral is absent in some slides and is the predominant 

feldspar in others. In still others it is coufined to one 

portion of the thin section. Although some microcline -is 

probably original, the larger portion is certainly of secondary origin. l':a~r~;~;c~::~~fte~iZ~~~~n;h;:\~ ~~~~t~;;c~ It occurs in wedges and hooklikc anct altered oligoclase col Cin gneissoid granmasses which in places separate· ite). x24· 

quartz or orthoclase fragments of similar orientation (fig. 7); in 

other places elongated areas are arranged end to end as if forced 

into planes of weakness (fig. 8). In still other places altered plagioclase or orthoclase grains are dotted by fresh areas of microcline 

similarly oriented and elongated parallel to the cleavage of the orthoclase or the albite twinning of the plagioclase. In many sections the 

host is fractured and unshattered microcline bridges the crack. 

Microclinc incloses globules of the other feldspars, which have a 

narrow fresh rim adjacent to the microcline, in contradistinction to the 

Fra. 9.-0rthoclase with the rim altered feldspar of the main mass (fig. 9). Such fresh rims of the 

· older feldspars stop sharply where the bordering mineral changes from 

microcline to quartz. This microcline is clearly later than the other 

feldspars and formed after the solidification of the magma and prob- wn.s formed. o, Orthoclase; ably prior to the mashing of the rock, since here and there oriented q quartz; m, rnicrocline. X 24. 

recrystallized in junction with 

secondary microcline (.in gneissoid granite). A narrow rin1 

of orthoclase was recrystallized when the luicr rnicrocline 

muscovite blades replace it. It may have formed, however, in the early 

stages of mashing, the formation of the muscovite perhaps being a late phenomenon of that 

process. Orthoclase and oligoclase are considerably altered to sericite, granules of zoisite 

are locally associated with these minerals derived from oligoclase, but kaolin is less abundant. 

In some thin sections biotite and muscovite are intergrown. Biotite is in general considerably 

altered, its· most common change ·being to chlorite and magnetite, in many places with the 

separation of needles of rutile. The alteration to epidote with o1: without chlorite is more 

uncommon. In some thin sections muscovite is an original accessory, in others it is 

secondary to and replaces biotite, and in still otlwrs it is a product of recrystallization during 

mashing. 
GENERAL GEOLOGY. 51 

The poikilitic ml)scovite plates, already mentioned, are so associated with and grade 

into sericite shreds, the alteration products of feldspar, that they are themselves doubtless 

secondary. Zircon, apatite, and magnetite are abundant accessory minerals. Magnetite, 

which in some. thin sections appears to· be titaniferous, rarely incloses zircon and apatite. 

Ilmenite is also probably present in some of the rock. Original titanite is confined to the 

medium-grained gneissoid granite fmm the southwest corner of the quadrangle. The mineral 

is also secondary to titaniferous magnetite. Pyrite, seen in a. single thin section, may be 

original. The rounded grains of garnet.contain no inclusions. In. the granites of the Georgetown quadrangle garnet is confined to this granite and to the granite-pegmatite and associated granites, each the solidified product of a rather siliceous magma. 

The aggregates of interwoven sillimanite rods are rimmed by narrow bands of muscovite. 

The two minerals are approximately contemporaneous in age, and although it is probable 

that the muscovit~ is a product of m~shing, both m_ay be original. · 

The granite varies not only from place to place, but in many single exposures, ranging 

from an almost massive rock to a banded rock in which quartz and feldspar layers are separated by discontinuous sheets of aligned biotite plates. The gneissoid bands are straight 

or gently curved, being sharply folded only near later eruptive masses. Microscopic examination shows that the gneissoid structure is due largely to recrystallization and partially to 

granulation. During recrystallization biotite and muscovite blades formed parallel to the 

present gneissic structure, and quartz and feldspar were in some places slightly oriented. 

The more gncissoicl facies occur about the bGrder of large areas of granite or around gneiss 

inclusions. The banding is broadly parallel to that of the neighboring masses of gneiss and 

schist. 

Since the partial recrystalli:;r,ation of the granite the constituents have been deformed 

by later mass mechanical action. 

AGE. 

The gneissoid granite injects and incloses portions of the Idaho Springs formation, the 

hornblende gneiss, and the quartz monzonite gneiss, and in turn is included in and cut by 

the quartz monzonite and the succeeding igneous formations. 

QUARTZ MONZONITE (ADAMELLITE). 

DISTRIBUTION AND GENERAL STRUCTURE. 

Quartz monzonite covers about one-half of the area of the quadrangle. Minor intrusive 

masses of this rock are rather widely distributed, and a single body 10 miles square occupies 

the central, north-central, and east-central portions of the quadrangle. This large body is a 

portion of a batholith which extends far beyond the eastern boundary. The contact of this 

batholith and the older gneisses and schists on the north side of the monzonite dips to the 

north at an angle of 10° to 30°; on the west and south sides it is steeper and in some places 

dips toward the batholith. In the northwestern portion of the batholith considerable areas of 

the Idaho Springs formation upon ridge crests probably represent remnants of the now eroded 

covering of older rocks. Many exposures of rock wholly massive in hand specimens have in 

the fielcl a gneissic structure, which near the center of the batholith is horizontal and near the 

edges dips parallel to the contact. This gneissic structure, apparently original (see p. 45), is 

probably concentric with the roof of the batholith, and its curved plane indicates that the 

batholit.h was originally asymmetrical, for the western and southern po~tions extended much 

farther upwanl into the covering of older rocks than the central and northern p~rtions. 

The concentric strike of the schistosity of the older rocks around the batholith shows that 

the igneous mass made room for itself by thrusting aside these older rocks. This process was 

aided to an unknown extent by that of "overhead stoping," concerning which. our ideas have 

been much clarified through recent articles by Daly.a According to this hypothesis the covering of older rocks is shattered by the expansion clue to the heat of the adjacent igneous mass, ~~--~--~----~~~~~~----~~--~------~~~------

a Daly, R. A., Am . .Jour. Sci., 4th ser ., vol. 15,1903, pp. 269-298; idem, vol.l6, 1903, pp. ,108-126; Bull U .S. Geol. Survey N o. 209, 1903, pp. 93~100. 
52 GEOLOGY OF THE GEORGETOWN QUADRANGLE, COLORADO. 

and the partially loosened fragments are broken off and engulfed in themagma, in which they 

sink to abyssal depths and by which they prol)ably are eventually assimilated. Two facts of 

observation argue for the application of "overhead stoping" as a process by which the space 

now represented by the batholith was partially formed. These are, first, thfl presence of an 

unusual number of apophyses, many of which are too small to map, extending from the main 

mass into the older rocks, indicating that the surrounding rocks were profoundly shattered, 

and, second, the presence in the monzonite of myriad inclusions of the older rocks, varying 

from small fragments to masses half a mile in diameter. These inclusions, the larger of which 

alone are shown on the map, are so abundant that scarcely any large exposure is free from them 

and many exposures are literally filled with them (fig. 10). The contact between the monwnite 

and the inclusions is sharp, although the corners have been somewhat rounded by the corrosive 

action of the magma. As a rule the inclusions are somewhat more abundant near the periphery 

of the batholith than in its center, although in this area such an arrangement is not essential 

to the a.pplication of the hypothesis, since it is probable that the present land surface is nowhere 

at great depth beneath the original roof of the batholith. 

Waldemar T .. Schaller determined the specific gravity of the biotite schist of the Idaho 

Springs formation to be 2.737, that of the biotite-sillimanite schist of the same formation 2.844, 

.and that of the quartz monzonite 2.830. These figures probably account for the relatively 

greater proportion of the biotite schist inclusions in the quartz monzonite than of biotite-sillimanite schist, notwithstanding the 

fact that the former, lighter rock is 

subordinate in bulk to the latter, 

heavier rock, as far as indicated by 

surrounding areas of the Idaho 

Springs formation. This follows from 

the fact that in the process of "overhead stoping" blocks of biotite-sillimanite schist, being heavier than the 

monzonite magma, would 'sink to 

great depths, while those of biotite 

schist would be buoyed up by the 

magma. In consequence, the blocks 

of the heavier rock included in the 

FIG. 10.-Quartz monzonite (a) with inclusions of biotite-sillimanite schist (b) and 

conglomeratic biotite schist (c) of Idaho Springs formation, hornblende gneiss 

(d), and quartz monzonite gneiss (e). 'l'he last-named inclusions are cut by an 

ellipsoidal dike of granite-pegmatite (f). West side of Chicago Creek. 

monzonite are only those fragments 

that ~ere broken from the covering rocks during the last stages of "overhead stoping" and 

held in the almost viscous magma; whereas the blocks of the lighter rock include a considerable 

proportion of all the biotite schist fragments which were wedged from the roof. It must be 

admitted that the alignment of the longer axis of some of the biotite-sillimanite schist fragments 

parallel to the hypothetical roof is against this assumption, since theoretically all the fragments 

of the heavier roek should. occupy a vertical position. The strong tendency, however, of the 

biotite schist fragments to be everywhere aligned and of those of the biotite-sillimanite schist to 

be in comparatively few places aligned is on the whole corroborative evidence of the application 

of "overhead stoping" as an important process in forming the space now occupied by the 

batholith. That this space was probably not enlarged to an important degree by the corrosive 

action of the magma is indicated not only by the sharp contact between the older rocks and 

the monzonite, but also by the absence of important differentiation of the monwnitc in contact 

with gneisses 'and schists of varying composition. The rounded corners of the. inclusions, 

however, show that some assimilation has occurred. 

PETROGRAPHY. 

The quartz monzonite is a gray to bluish-gray, moderately coarse-grained, granular rock, 

much of it more or less porphyritic in habit. Megascopically, white and pinkish-white feldspar, 

slightly smoky quartz, black mica, and greenish-black hornblende are essential constituents, 
GENERAL GEOLOGY. 53 

and magnetite grains, reaching one-half inch in diameter, and titanite wedges with a maximum 

length of one-fourth inch are abundant accessories. · In the porphyritic facies rude crystals 

of pink microcline, rarely orthoclase, mainly Carlsbad twins, reach a maximum length of 1 

inch. In many places black mica flakes are present as inclusions. A matted coating of tiny 

epidote crystals, with quartz crystals locally associated, occurs along many joint fractures. 

The quartz monzonite grades into irregular equidimensional masses and more rarely is cut 

by dikes of a genetically related pegmatite. Where Bear Creek crosses the east border of the 

quadrangle one and the same mass of pegmatite grades into monzonite in one part of an exposure and sharply cuts it at another. The pegmatite is composed of predominant pink orthoclase 

and subordinate semitransparent quartz, which is milky white on account of being cut by 

myriad lines of fluid inclusions. Biotite and magnetite, one or both, are present in places. 

The rounded octahedrons of magnetite reach a maximum diameter of one-half inch and locally 

form one-third of the rock. The pegmatite is as a rule coarsely granular, although some masses 

are slightly banded with quartz in the middle. 

On microscopic examination the texture of the quartz monzonite is observed to be hypidiomorphic granular, rather even grained in the nonporphyritic and uneven in the porphyritic 

facies. With variation in relative amounts of the lime-soda and alkali feldspars, the rock 

ranges from an acidic quartz monzonite with granitic affinities to a basic quartz monzonite 

with gtanodioritic affinities. The essential minerals, in the order of their abundance, are 

oligoclase or andesine, microcline, much of it microperthitic, quaftz, and biotite. Orthoclase and hornblende, which are present in approximately half of the thin sections examined, 

are in such sections equal in amount to quartz or biotite. The rock is characterized by the 

abundance.of the accessory minerals, magnetite, titanite, apatite, pyrite, and zircon. Titanite, 

magnetite, and biotite are equal in bulk in some thin sections. The order of the solidification 

is normal to a granite, although the periods of separation of biotite and titanite, of biotite and 

plagioclase, and of quartz and the feldspars overlapped. The accessory minerals are in many 

places in contact with one another and they separated from the magma in the following order: 

Zircon, apatite and pyrite, magnetite, and titanite. 

The quartz is characterized by the presence of abundant opaque threa.dlike microlites, 

which also occur, although sparingly, in the feldspar. Rarely intersecting cracks indicate a 

poor cleavage parallel to the rhombohedron. The irregular grains and rude rectangular areas 

of oligoclase and andesine inclose abundant interpositions, in general centrically arranged. 

These include tiny magnetite cubes, blood-reel or yellow semitransparent hexagonal plates, 

probably hematite, and opaque black clots and minute rods of undetermined character. Orthoclase is uniformly original, but microcline, as in the two formations previously described, is in 

places a product of recrystallization. Quartz is micropegmatitically intergrown with each of 

the feldspars, and small disks of quartz and orthoclase micropegmatite lie between the other 

constituents. Biotite occurs in irregular areas and blades. In a number of thin sections 

minute flocculent areas of biotite in optical continuity with an adjacent biotite mass are embedded 

in plagiocfase, indicating an overlap in the separation of the two minerals. Biotite, which is for 

the most part fresh, in altering first becomes green, then bleaches, and finally is altered to 

chlorite, magnetite, and rutile. Epidote is a less common product of its alteration, and muscovite in parallel position with chlorite is very rare. Green hornblende occurs in partial columns 

many of which are intergrown with biotite, the cleavage of the two minerals being in parallel 

position. Epidote granules form in numerous places at the junction of altering biotite and 

hornblende individuals. Magnetite grains/ and partial crystals are abundant, and skeleton 

growths of magnetite are here and there included in biotite. The magnetite seems to be somewhat titaniferous. Coffee-brown titanite, much of it polysynthetically twinned, forms abundant 

partial crystals and irregular grains. This titanite, like that of the quartz-monzonite gneiss, 

alters readily. Apatite occurs in short or long columns which were in places broken and displaced before inclosure in younger minerals. Rarely the apatite crystals are deeply embayed 

as if corroded magmatically prior to the solidification of the essential constituents. Pyrite 

is rimmed by or intergrown with magnetite. Allanite crystals of chestnut-brown color occur 
54 GEOLOGY OF THE GEORGETOWN QUADRANGLE, COLORADO. 

rather abundantly in a number of thin sections, and some zircon is also present. Small shreds 

of original muscovite are intergrown with biotite in a few thin sections. 

The quartz monzonite, as already stated, has in a broad way a poorly developed original 

gneissic structure parallel to the assumed roof of the batholith. Under the microscope this 

proves to be due to the flow orientation of the feldspar phenocrysts and biotite blades in a 

common plane. On the borders of the large batholith and near large inclusions of older rocks 

a secondary gneissic structure, nowhere strongly developed, has been induced by recrystallization and granulation. The secondary gneissic planes, marked by discontinuous films of biotite, 

are as a rule straight or gently curved, being highly flexed only along faults or near later igneous 

intrusions. Orogenic movements have fractured some constituents and bent others. · 

AGE. 

The quartz monzonite is younger than the gneissoid granite and older than the Rosalie 

granite. Its relation to the quartz-bearing diorite is <;liscussed in the description of that formation in the next section. 

QUARTZ-BEARING DIORITE AND ASSOCIATED HORNBLENDITE. 

DEFINITION. 

By diorite is meant a granular igneous rock composed essentially of plagioclase (in general 

andesine) and either hornblende or biotite, with or without subordinate pyroxene. Most of the 

diorites of the Georgetown quadrangle contain a little quartz, and hence may be· designated 

quartz-bearing diorites. Such diorites, with the introduction of orthoclase, grade into 

granodiorites. 

DISTRIBUTION AND GENERAL STRUCTURE. 

Quartz-bearing diorite occurs in small stocks and dikes, which are confined largely to the 

northern quarter of the quadrangle and are particularly abundant south of Idaho Springs and 

northeast of Georgetown. Contacts with older formations are poorly exposed and the dynamics 

of intrusion of the diorite are not clear. The formation is poor in inclusions of older rocks, 

and the magma on the whole appears to have been comparatively dry. In these respects it 

resembles the other basic magma, that of the hornblende gneiss. 

PETIWGRAPHY. 

The quartz-bearing diorite is a medium- to coarse..:grained, rather uneven, granular rock, 

composed essentially of grayish-white striated feldspar and greenish-black hornblende. Tiny 

wedges of quartz between these minerals are visible in some hand specimens. In m.any places 

bronze-brown mica blades may be seen macroscopically, and at the exposures at the mouth 

of Spring Gulch and 1. 7 miles N. 75° W. of Lamartine mgged plates, which reach a maximum 

diameter of 2 inches, inclose poikilitically the other constituents. Epidote occurs in matted 

films on joint faces and in rude pseudomorphs after hornblende and biotite in the rock mass. 

Although the texture is mainly granular, here and there globular masses of hornblende lie in 

a ramifying web of feldspar and quartz. The most abundant facies, a mottled black and white 

rock, contains approximately equal amounts of hornblende and feldspar, but the series varies 

from a practically pure plagioclase rock (anorthosite) of medium-gray color to a pure greenishblack hornblende rock (hornblendite). Many steps in this gradation may be present in a single 

exposure. Small acidic or basic segregations, examples in miniature of the differentiation of 

the magma, are not uncommon. Such phenomena make it impossible to regard these rocks 

as anything but local modifications or variants of one and the same mass. However much the 

types vary among themselves in mineralogical composition, together they form but a single 

geologic unit. 
GENERAL GEOLOGY. 55 

Under the microscope the texture proves to be allotriomorphic or, with the partial development of plagioclase laths, hypidiomorphic granular. The order of crystallization of the minerals 

was usually first the accessories, then plagioclase or hornblende and pyroxene, followed by 

biotite, orthoclase, and quartz. Biotite, however, in some places solidifiecl simultaneously with 

orthoclase and quartz. Of the accessory minerals, rutile, zircon, apatite, and pyrite are older, 

and magnetite, ilmenite, and titanite are younger. Plagioclase, which occurs in irregular grains 

or partially developed laths, ranges from oligolase andesine to bytownite, a basic labradorite 

being the most common species. It is characterized, as are quartz, orthoclase, and apatite in a 

less degree, by a vast number of centrally arranged minute tabular and circular interpositions 

. tl;tat are black and opaque or clove-brown and translucent. Many of the round inclusions are 

arranged .in lines which cross one another at angles of 60°. Some of these inclusions appear to 

be closely associated with pink fluid inclusions, and the coloring matter in the translucent 

ones is irregularly distributed in globular areas as if by the drying up of a solution. In part, at 

least, they probably represent dried-up fluid inclusions. Plagioclaf:?e is altered more or less 

compl~tely to sericite, zoisite, calcite, and quartz.' Tremolite is a rare alteration product. 

Small irregular areas of orthoclase are present in most of the slides, and some of the larger of 

these contain the other constituents poikilitically. Sericite is the common alteration product. 

Quartz, which forms wedges between hornblende and plagioclase in most of the thin sections 

examined, is also micropegmatitically intergrown with bio-tite, hornblende, and each species of 

feldspar. Fluid inclusions are comparatively rare. Green-hornblende, in grains or rude prisms, 

many of them twinned parallel to 100, with the twinning in a few repeated in thin laminre, is as a 

rule filled with blebs of the other constituents. In places hornblende and plagioclase occur in 

micropegmatitic intergrowth. By deep-seated metamorphism hornblende is locally altered to 

biotite; epidote and more rarely chlorite are products of later alteration. Epidote occurs as the 

common product of altering hornblende or biotite and plagioclase. 

Diallage, in grains and irregular columns, filled with the opaque inclusions characteristic of 

the species (probably iron oxides) and surrounded by secondary rims of hornblende, is present in 

a few slides. The interpositions which here and there completely replace the dl.allage disappear 

in the alteration to hornblende, their substance apparently being absorbed in the production of 

that mineral. Although small masses of the diorite are gabbros, properly so called, it is not 

probable that the distribution of diallage was originally much more extensive than now. Since 

the hornblende has its own original form, the hypothesis that the diorites are in reality metagabbros would assume complete recrystallization of the rock, a view incompatible with the 

small amount of demonstrable recrystallization. Biotite is a more unco.mmon alteration. product 

of diallage. In a single thin section grains and partial columns of pale-green augite partly 

altered to hornblende are present. L~rge poikilitic plates of biotite have already been mentioned as characterizing some outcrops, and small biotite blades occur rather constantly. The 

large plates, which are either disconnected or joined to one another by narrow necks, are embayed 

on their borders by tubules of quartz and orthoclase, and flocculent aggregates. of biotite in 

common orientation with the larger areas are embedded in adjacent quartz and feldspar. Biotite alters to chlorite, in part at least pennine, and rutile needles, these secondary minerals being 

in places accompanied by epidote. The common accessory minerals include ilmenite, pyrite, 

titanite, magnetite, and large and abundan~ crystals of apatite. Zircon and rutile are less 

common. Titanite is not only an original accessory, but also an alteration product of ih~enite. 

Many of the pyrite crystals are rimmed by ilmenite or magnetite. The short columns of apatite 

are in some places embayed,as if by magmatic corrosion, and in others the columns were broken 

into segments prior to the solidification of the inclosing essential constituents. 

Associated with the diorite and linked to it by gradational facies are fine- to medium-grained 

allotriomorphic granular rocks of greenish-black or clark olive-green color, which are perhaps 

best styled hornblendites. The gradation from diorite to hornblendite may occur in a single 

thin section. The microscope shows that some of these rocks, which contain minor quantities 

of plagioclase, orthoclase; biotite, quartz, and accessory minerals, are formed in the main of 

green hornblende; others are n~ad-3 up of large irregular green hornblende individuals inclosing 
56 GEOLOGY OF THE GEORGETOWN QUADRANGLE, COLORADO. 

poikilitically laths of biotite, grains of enstatite showing schillerization, and partial columns of 

a colorless orthorhombic amphibole, anthophyllite. Enstatite is completely or partially altered 

to serpentine. In slides of the olive-green facies anthophyllite, which alters to talc, is as abundant as hornblende, with which it is locally in parallel intergrowth. It occurs in raggedly terminated columns with the prism and ~linopinacoidal faces well developed. lt is in some slides 

secondary to common hornblende and in others to enstatite. Anthophyllite has been noted as 

an alteration product of bronzite and hypersthene a and of olivine.b In this series anthophyllite 

is secondary to enstatite, a pyroxene closely related to bronzite and hypersthene, while the 

presence of olivine in the original rock is by no means impossible. Still another of these rocks is 

in one portion of the slide composed entirely of hornblende and in another of a white monoclinic 

pyroxene near malascolite inclosing hornblende grains poikilitically. Titanite, an abundant 

accessory mineral, is in some places intergrown with hornblende and crystallized contemporaneously with it (fig. 11). These more basic members of the formation appear to have suffered 

considerable recrystallization and their original character is in doubt. They were in part at 

least pyroxenites. 

The diorite on its borders is in places mashed, partly through recrystallization and partly 

through granulation, to a rudely banded gneissoid rock. Microscopic examination shows that 

hornblende, feldspar, and quartz are segregated into lenses rudely 

elongated parallel to the parting, and blades of biotite and columns of 

anthophyllite are developed parallel to the gneissic planes. The albite 

twins of the plagioclase are locally aligned parallel to the gneissic 

structure, and the undulose extinction of orthoclase passes into the 

grating structure of microcline, the latter mineral being clearly a 

derivative product of the former, probably through recrystallization, 

and occurring solely in mashed facies of the diorite. 

Mass mechanical action has slightly granulated and deformed the 

constituents of the quartz-bearing diorite, but the resulting phenomena 

are not unusual. 

AGE. 

The quartz-bearing diorite bears the same structural relations to the 

other formations of the Georgetown quadrangle as does the quartz 

Fw.11.-Titaniteandhornblende monzonite. The contact of these two formations is nowhere well intergrown and contemporaneous (a), in hornblendite or exposed in the quadrangle. At the mouth of Spring Gulch, however, 

quartz diorite (b). c, Zoisite poor exposures of quartz monzonite on the east bank of Chicago Creek after feldspar. X 12. 

appear to be genetically related to quartz-bearing diorite on the west 

side of the stream. Rocks which are intermediate petrographically between the two in composition, and hence difficult to assign definitely to either formation, indicate that the two are 

variants of the same magma. Near St. Marys Lake, north of the Georgetown quadrangle, the 

two appear at some places to grade into each other, and in other exposures the diorite clearly 

cuts the quartz monzonite. The two are believed to be differentiation products of the same 

magma, the quartz diorite being, on the whole, slightly younger. Such being the case, th~ 

small areal development of diorite indicates that by far the greater portion of the magma first 

solidified as monzonite, and then the basic residuum formed the quartz diorite and its associated rocks. It is probable that considerable time intervals may have intervened between the 

separation of the quartz diorite and the more basic hornblendites. Spencer c has described 

from the vicinity of Encampment, Wyo., a somewhat similar series of basic igneous rocks, but 

this series is probably younger than that of the Georgetown quadrangle. 

a Van Rise, C. R., Mon. U.S. Geo!. Survey, vol. 47, 1904, p. 282. 

b Becke, F., Tschermaks Min. u. petr. Mitthei!., new ser., vol. 4, 1882, pp. 450-452; Belowsky, Max, Zeitschr. Deutsch. geol. Gesell., 

1905, p. 47. 

c Spencer, A. C., Prof. Paper U. S. Geol. Survey No. 25, 1904, pp. 28-36. 
GENERAL GEOLOGY. 57 

In the Empire mining district (see p. 385) certain facies of a monzonite which is younget 

than the Silver Plume granite closely resemble the pre-Cambrian quartz diorite. These rocks at 

Empire appear, however, to be of late Cretaceous age. 

ROSALIE GRANITE (BIOTITE GRANITE). 

DISTRIBUTION AND GENERAL STRUCTURE. 

Batholiths, stocks, and dikes of the Rosalie granite are confined to the southern portion of 

the quadrangle, and the largest batholith, extending for at least 15 miles beyond the quadrangle, 

covers the southeast corner. An irregular area forms the ridge from .Rosalie Peak, whence the 

granite's name, almost to Mount Evans and extends beneath glacial drift west of Lake Fork.of 

East Geneva Creek. A third important area lies northwest of the junction of Bear and Truesdale creeks. The granite is comparatively free from inclusions of older formations and, except 

for dikes of the pegmatite and the associated granite and granite porphyry, from intrusive 

masses of later rocks. 

PETROGRAPHY. 

The Rosalie granite is a pink, coarse-grained, massive, granular rock whose predominant 

constituent is a salmon-pink, rarely gray microcline, much of which shows Carlsbad twinning. 

The feldspar masses, which vary in length from half an inch to 2! inches, are rudely tabular in 

form and inclose blebs of quartz and flakes of black mica. They are separated from one another 

by ramifying bands of quartz, feldspar, biotite, and magnetite, all of medium size. The rock 

resembles a mosaic of-salmon-pink rectangular or ellipsoidal blocks embedded in a black or gray 

cement. The granite 1.5 miles N. 70° E. of the mouth of East Geneva Creek is a porphyritic 

granite of gray color, with large pink feldspars of rectangular or ovoid form. 

The Rosalie granite grades into and is cut by small pegmatitic dikes, which in turn grade 

into quartz veins. The pegmatite is formed of light brick-red feldspar and white and semitranslucent quartz. A little biotite is locahy present, but ferromagnesian minerals are absent 

in most places. 

Under the microscope the texture of the granite is .seen to be uneven and hypidiomorphic 

granular. The essential constituents of the rock began to solidify from the magma in the order 

usual in granites, but the periods of separation of all overlapped, and in consequence each constituent incloses blebs and partial crystals of the others. Magnetite sometimes separated 

simultaneously with biotite, quartz, and feldspar and occurs in skeleton growths with them. 

The minerals named in the order of their abundance are microcline, quartz, orthoclase, alkali 

oligoclase, and biotite. Microcline, much of it microperthitic, contains hexagonal plates, dots, 

and rods of blood-red hematite. These inclusions also occur abundantly in plagioclase and in 

less amount in quartz, and in association with them in quartz and rarely in microcline are opaque 

hairlike inclusions. In many places plagioclase, an alkali oligoclase, has partial crystal faces 

against quartz and the alkali feldspars. Kaolin and sericite are the alteration products of the 

feldspars, calcite accompanying them in plagioclase. Qu~rtz at its contact with the various 

feldspar species and with biotite forms micropegmatitic intergrowths. Biotite occurs in blades, 

idiomorphic against the feldspars and quartz, and in irregular plates which lie between quartz 

and feldspar granules and are apparently a contemporaneous or later product of solidification. 

It is in the main fresh, but in a few thin sections it is altered to chlorite, with the simultaneous 

separation of magnetite and a granular substance resembling leucoxene. Zircon, apatite, and 

magnetite are constant and rather ·abundant accessory minerals. The first two are locally 

included in the last. Tiny shreds of muscovite are intergrown with biotite in some thin sections. 

Titanite and pyrite occur as accessory minerals in a single slide. 

The Rosalie granite has locally been subjected to slight strain, but apparently in no place 

has' this process been accompanied by changes in the relative positions of the mineral constituents. 
58 GEOLOGY OF THE GEORGETOWN QUADRANGLE, COLORADO. 

AGE. 

The Rosalie granite cuts the quartz monzonite. The contacts between the Rosalie and 

Silver Plume granites are few and poorly exposed, but at three places-namely, 2~ miles e~st 

of Lincoln Lake, 1.7 miles northwest of Rosalie Peak, and 2! miles east of the same peakdikes, apparently of Silver Plume granite, cut the Rosalie granite. Locally, the Silver Plume 

granite has been mashed, but the Rosalie granite wherever seen is massive. It is in consequence possible, although not probable, that better exposures may prove the Rosalie to be 

younger than the Silver Plume granite. 

One of the granites of Pikes Peak, Colorado, described by Mathewsa and designated by 

him the Pikes Peak granite, is strikingly similar to the Rosalie granite in mineralogical composition, in deformation suffered, and in mode of weathering. Since the Pikes Peak granite 

is one of the older of the granites of that vicinity, it and the Rosalie granite may well be 

conterriporaneous. 

SILVER PLUME GRANITE (PORPHYRITIC BIOTITE GRANITE). 

DISTRIBUTION AND GENEHAL STRUCTURE. 

Stocks, dikes, and irregular intrusive masses of the Silver Plume granite cover considerable areas north of Meridian Hill, on Alps Mountain, and in the vicinity of Georgetown and 

Silver Plume, and from the latter mining camp the granite is named. It is, indeed, widely 

distributed throughout the quadrangle except in the extreme northeast and southeast corners. 

The older rocks do not have a concentric schistosity around masses of the Silver Plume 

granite, and in consequence the space now occupied by the stocks was probably not alone 

forll.led by forcing aside the older rocks. The straight element bounding many of the masses 

indicates that faults partially determined the form of the stocks. In many places older rocks 

are included in the Silver Plume granite and'' overhead stoping" may also have been an auxiliary process. The magma appears to have been, on the whole, relatively dry, and in consequence assimilation was probably of no importance in determining the form assumed by the 

cooling mass. 

PETROGRAPHY. 

The Silver Plume granite is a medium-grained, porphyritic, granular rock composed of 

grains of white or pinkish feldspar, smoky quartz, and blades of black mica. Muscovite, locally 

in half-inch plates poikilitically inclosing the other constituents, is also visible to the naked eye 

in many exposures. Magnetite is less common. The granite, where rich in biotite, is dark 

gray mottled by pink porphyritic feldspars; where the biotite content is lower it is pinkish 

gray in color. At a distance exposures appear terra-cotta red or brown. 

A porphyritic variety of the Silver Plume granite occurs in the northwestern portion of 

the quadrangle, with phenocrysts of pinkish-white feldspar, chiefly microcline, less con'tmonly 

orthoclase. Many of the phenocrysts are Carlsbad twins. These rudely tabular crystals vary 

from 0.1 to 1 inch in length. The rock becomes a granite porphyry, the "corn rock" of the 

miners, as these phenocrysts incre~se in size and the encircling constituents become finer grained. 

It is the border facies of some of the larger stocks and alone forms some of the smaller dikes. 

'Since these phenocrysts are aligned near the border in flow orientation, it is evident that they 

solidified before the magma came to final rest. To the south and west of the divide, between 

Soda Creek and South Fork of Clear Creek, the phenocrysts become smaller and less abundant 

and are associated with irregular equidimensional feldspar areas from one-fourth to one-half 

inch across. Even such areas are wanting north of ~1:eridian Hill. 

The Silver Plume granite grades into a pegmatite with predominant pink feldspar, some 

quart;>;, and less biotite, which occurs characteristically in long ·blades. The biotite content 

decreases with the increase in the si7.e of the pegmatitic dikes. Magnetite, which is an accesa Mathews, E. il., Jour. Geol., vol. 8, 1900, pp. 217-225. 
' 

GENERAL GEOLOGY. 59 

sory, is in general rare, although locally abundant. Most of the dikes are rudely banded, biotite being segregated near the center of the mass and either feldspar or quartz next the wall. 

Under the microscope the texture of the granite is proved to be hypidiomorphic granular. 

Essential constituents of the rock are alkali feldspars, quartz, biotite, and oligoclase and oligoclase albite, named in the order of abundance. The order of the solidification of these minerals 

from the magma is normal, although the periods of separation all somewhat overlapped one 

another. The accessory minerals separated out in the following 

order, the first named being the oldest: Rutile, zircon, titanite and 

pyrite, and apatite and magnetite. Quartz is of interest on account 

of the opaque threadlike inclusions which it uniformly contains and 

which are present, though less abundantly, in feldspar. Of the 

alkali feldspars microcline, ·much of it with microperthitic bands, 

predominates in some sli<les and is lacking in others. Some of the 

microcline is a product of recrystallization, like that of the gneissoid 

granite (see fig. 12), but it is in the main an original constituent. The 

alkali-feldspar phenocrysts contain many rounded blebs of quartz 

and feldspar. Micropegmatitic intergrowths of quartz and the three 

species of feldspar occur along the borders of feldspar grains and in 

disklike areas between the essential constituents.· Of the feldspars, 

orthoclase is as a rule considerably altered to sericite and kaolin; 

plagioclase is similarly altered, bu~ in general to a less extent; and 

microcline is 'still less altered or even wholly unaltered. Calcite is 

also locally developed from plagioclase. Biotite for the most part 

forms well-developed basal faces against feldspar; but in some thin 

sections it appears to have solidified after the feldspars. Its alterations to chlorite with or without magnetite and rutile, to chlorite 

and epidote, to epidote alone, and to muscovite are in no way peculiar. 

Shreds of original muscovite are intergrown with biotite in many of 

the thin sections examined. The poikilitic plates of muscovite 

FIG. 1~.- Plagioclase (a) cnt by 

veinlet of secondary quartz (c) 

and microcline (b). Various areas 

of microcline in other portions of 

slide have similar optical orientation to that of veinlet. Plagioclase fractured and altered to 

kaolin and sericite (d). Microcline 

nnfractured and fresh. Silver 

Plume granite. X 12. 

already mentioned grade into and are so associated with sericite shreds secondary to feldspar that they themselves are doubtless secondary. Of the accessory minerals zircon and 

apatite are abundant, :magnetite rather common, and ilmenite, ,pyrite, titanite, and rutil~ 

· rare. Magnetite a:lters to hematite or limonite, 

i 1 '1 ,'t;, ilmenite to titanite and hematite, and titanite 'I I .... I - I I ~~-\I/ '._! \ ,../' ._-'- - a, ~· '"' ~,,_,,, ,,;-,-'..:-; 1 -',' , 1 , ;:: ~ 1

.;- 1 -,', ,:;_ -':_ to a flocculent dark-brown · semitransparent !...- ,- ~ .,_.;.: .:'-' '-' '' 1-;'...: -';. ,_ .... ,-:;'' 1 _ ...... :::. ""'-=-=.-:.::_ substance. · .... ',: ........ , ..... -,\-',.~' _,,,_,-, .... , .... ,.,,,..,, ~-::::::...h 

-;,~~·,--'.~·-~-;·~;'",_;;.:-t;:-~~':::.>--'~'-..=;;=~ The phenocrysts in the more porphyritic ;/ \ '~·-,-~--; ~ \ /~'"; ::.-, -;."/ .... ~ - ', . .':; :-\ ..... _ / t-';.~;{ ~';~~<.;.,'-',_"'.:::..JL'i"~r: c :::_-= .::::-=_ - 1 _- _ . ~ cz_ -=- b facies near the borders of granite masses arc 

arranged in well-defined planes which follow 

the sinuous contacts of the older rocks (fig. 13): ~-------6 f'eet---------;>1 

:FIG. 13.-Flow orientation of phenocrysts of Silver Plume granite 

(a) at contact with biotite-sillimanite schist (b) of Idaho Springs 

formation and pegmatit.; (c). Shows development of phenocrysts in a granitic magma prior to cessation of movement in 

magma. Clear Creek. · 

The granite has not been recrystallized and the 

parallel orientation and the consequent poorly 

developed cleavage are original structures clue 

to movements in the magma prior to final 

solidification. The granite, though normally massive except for this original gneissic structure, 

has been locally mashed along planes of maximum movement, the mashing being accompanied 

by a rude alignment of biotite blades through recrystallization. Thin sections show the usual 

deformation phenomena. 

AGE. 

The Silver Plume granite cuts the formations previously described and '\Vith the exception 

of the pegmatite and associated granite and granite porphyry . is the youngest of the preCambrian rocks. Petrographically this granite resembles the Cripple Creek type" of the Pikes 

a Mathews, E. B., :Jour. Geol., vol. 8, 1~00, pp. 22\l--232. 
60 GEOLOGY OF THE GEORGETOWN QUADRANGLE, COLORADO. 

Peak quadrangle, a granite cutting the Pikes Peak type, which is considered the equivalent 

of the Rosalie granite. It is not improbable that the Silver Plume and Cripple Creek granites 

are contemporaneous. 

PEGMATITE, GRANITE-PEGMATITE AND ASSOCIATED GRANITES, AND GRANITE PORPHYRY. 

DEFINITION. 

The term pegmatite, :first applied to the intimate interpenetration of quartz and orthoclase 

individuals now more often designated graphic granite, is here employed for igneous rocks of 

-------- z%f'eet -----~ 

Fw. 14.-Sketch showing intense injection of hornblende 

gneiss (a) by granite-pegmatite (b). On trail midway 

between Maximilian ami Eclipse gnlches. 

unusually coarse texture. Granite-pegmatite is, 

then, a coarse rock of granitic composition. Granite 

porphyry is a rock of granitic composition consisting of a holocrystalline groundmass, generally 

of orthoclase and quartz, which contains larger, 

more or less perfectly bounded crystals, called 

phenocrysts, ·of orthoclase, quartz, and biotite or 

hornblende. 

DISTRIBUTION AND GENERAL STRUCTURE. 

The granite-pegmatite and associated granites 

and granite porphyry are present as small bodies 

throughout the quadrangle, although the scale of the 

map used permits only the larger masses to be 

shown. The largest area, approximately 1! miles 

long, is east of south of Duck Lake .. The rocks 

occur as dikes and intrusive masses, many of which are of exceedingly irregular form. The 

dikes were injected along planes of schistosity of the gneisses and schists and along joints and 

faults and planes between the contacts of the older rocks. The intrusion is in many places 

exceedingly intricate (figs. 14 and 15). 

In a single exposure the pegmatite, 

granite, and granite porphyry may 

grade into one another at one point 

and at another one rock may cut the 

others, the pegmatite as a . rule being 

younger than either the granite or 

the granite porphyry. The pegmatitic 

magma was accordingly in most places 

later than that of the particular granite 

with which it is connected and related, 

and. was probably introduced from 

below; in other places it was merely the 

residuum of that of the granite, the 

pegmatite forming in place. 

Just as there have been repeated 

intrusions of granite in this region, there 

have also been repeated intrusions of 

pegmatite, and as the different granites 

are of very similar composition the 

pegmatites are also, so that in many 

places the pegmatitic intrusions of 

+----------15-f'eet 

FIG. 1.5.-Sketch showin;s intense injection of quartz rnonzonitc (b) containing 

inclusions of quartz monzonite gneiss (c) by granite-pegmatite (a). Mouth 

of Cascade Gulch. 

different ages can not be distinguished and separated from one another. It is demonstrated, 

however, that pegmatites were freely injected into the ancient schists among the earliest 

igneous intrusions, and perhaps formed the very earliest. Such ancient pegmatites are very 

abundant, for example, in the area shown on the Silver Plume special map (Pl. XXI, p. 176). 

I 
U. S. GEOLOGICAL SURVEY PROFESSIONAL PAPER NO. 63 PL. X 

A. FELDSPAR-MAGNETITE PEGMATITE. B. GNEISSIC TYPE OF QUARTZ-MAGNETITE PEGMATITE. 

Showing crysta l formati on of magnetit e. Showing magnetite (met al lic gray), quart z (li ght gray), and feldspar (w hite). 
GENERAL GEOLOGY. 61 

Pegmatite intrusions accompanying and following granitic intrusions have been well observed 

and studied, as, for example, that consequent on the intrusion of the Silver Plume granite at 

Silver Plume and Empire. (Seep. 181.) Within the area shown on the Silver Plume special 

map three distinct periods of pegmatitic intrusion are recorded, the first and the most abundant 

being the oldest igneous intrusion, or in general contemporaneous with the hornblende gneiss 

(p.178), which also represents one of the first intrusion~; the second a differentiation phase of 

the massive quartz diorite intrusion, and the third representing the final product of consolidation of the Silver Plume granite magma. That pegmatitic phases were associated with 

later eruptives there is no doubt. Finally great quantities of granite-pegmatites and 

associated granites crystallized from a magma which was injected into the rocks of the region 

subsequent to all of the igneous intrusions above recorded. These pegmatites therefore are 

the youngest of the probably pre-Cambrian rocks of the quadra~gle, and it is to them that the 

followiDg discussion particularly relates. · 

It is, however, impossible in many places to distinguish between the pegmatitic intrusions 

of one period and those of another, and in consequence all the masses of pegmatite large enough 

to be shown have been represented by tlie same pattern on the map. 

PlWMATlTES. 

PETROGRAPHY. 

Salmon-pink feldspar which bleaches on weathering is as a rule the predominant mineral 

of 1 the pegmatite. On microscopic exn.mination this proves to be either orthoclase or microperthitic microcline. A light-gray acidic plagioclase is restricted in its distribution to pegmatite dikes in quartz monzonite. Slightly smoky quartz, which microscopic examination shows to contain abundant opaque threadlike interpositions and myriad fluid inclusions, is invariably present, and in the 

siliceous end product of the pegmatite, the tiny banded quartz apophyses that arc given off from the large dikes, it is the only constituent. 

Plates of biotite are more common than those of muscovite, and the two 

micas characteristically occur in separate bodies of pegmatite, although 

they are locally associated in the same masses and in rar~ :instances a core 

of biotite is surrounded by a muscovite border. An interesting form 

of pegmatite is one composed of diamond-shaped muscovite plates up 

to 1 inch in diameter embedded in quartz, each mineral being present 

in nearly equal amount. The rock is closely allied to beresite from the 

Ural Mountains and to a rock described by Spurr from Belmont, Nev.a < . In Warren Gulch near the Chemsker mine and in small areas 

between that point and Chief Mountain is a pegmatite that contains 

in places crystal masses of muscovite, many of which are 2 inches 

thick and 1 to 1!i feet in diameter. A large proportion of these muscovite crystals, however, show evidences of strain and contain considerable iron, so that they will probably not prove of economic value for 

sheet mica. 

Magnetite is a widely distributed constituent, and in some places 

Fru. 16.-Q,uartz-feldspar pegmatite containing crystals of 

magnetite. Crest of ridge 

three-fourths mile cast of upper Chicago Lake. a, Magnetite; b, quartz; ·c, feldspar; 

d, quartz monzonite. 

forms over one-third of the pegmatite .mass, which in consequence becomes a lean iron ore. 

Magnetite occurs in crystals which solidified prior to the other constituents of the pegmatite 

(Pls. VIII, B; X, A,· fig. 16), or in irregular aggregates which solidified practically at the same 

time as quartz and orthoclase (Pl. X, B). The crystals, which are octahedrons or octahedrons 

modified by the faces of the cube, reach a maximum diameter of .4 inches, and some of the aggregates are 6 inches across. Although magnetite is in general associated with biotite and less 

commonly with muscovite, it is in some places the only ferromagnesian mineral of the pegmatite, 

and locally it occurs in pegmatitic quartz dikes. Magnetite protrudes on weathering, the corners 

of the crystals being somewhat rounded by solution. Magnetite concentrated by the dismtea Spurr, J. ~~., Am. Jour. Sci., 4th ser., vol. 10, 1900, p. 351. 

,· 
62 GEOLOGY OF THE GEORGETOWN QUADRANGLE, COLORADO. 

gration of the pegmatite is saitl to be effective sling-shot ammunition for the boys of Idaho 

Springs. A sample of the magnetite was partially analyzed by Waldemar T. Schaller, who 

found in it 2.07 per cent of titanic acid and no manganese. 

Southeast of benchmark 972~, on the road which crosses the ridge between Trail Creek 

and Spring Gulch southwest of Freeland, Mr. Spurr collected specimens of magnetite pegmatite and associatetl magnetite-bearing alaskite. On microscopic examination these proved of 

great theoretic interest. The following description is based on Mr. Spurr's notes: A portion 

of one thin section is an allotriomorphic granular alaskite composed of quartz, fresh microcline, 

and aggregates of fine sericite with a little quartl'l. Other thin sections show that without 

much doubt the sericite and quartz form a pseudomorph after an acidic soda-lime feldspar, 

probably oligocla~e. An octahedron of magnetite is included in the quartz. In one direction 

the alaskite becomes more quartwse until 90 per cent is quartz, with some fresh microcline, 

muscovite, and magnetite in crystals. Movement has occurred in this quartz, producing fractures, and following these are stringers or broad lenses of magnetite, with intercrystallized and 

contemporaneous biotite antl muscovite. Many of the magnetite lenses and the semidetached 

offshoots show crystal boundaries, and the magnetite crystals clustered around the lenses and 

those isolated in the alaskite near by an~ identical. 

The events in the genesis of this rock were, first, the crystallization of oligoclase; second, 

the alteration of oligoclase to n~uscovite and quartz and the solidification of microcline and 

quartz, the quartz deposition clearly outlasting that of microcline; third, the fracturing of the 

rigid alaskite; and fourth, the deposition of magnetite, biotite, and muscovite in the fractures, 

and of magnetite in crystals, visible channels of access being absent, throughout the rock. In 

this rock, therefore, magnetite is oi1e of the last minerals to solidify; and, moreover, in the inclosure 

of one mineral by another, the criterion most often used to show that magnetite is one of the 

first minerals to crystallize from a magma, fails. The presence of magnetite, then, in idiomorphic crystals in other constituents of a rock does not always indicate that it separated from 

the magma prior to the consolidation of the inclosing minerals. A second thin section from 

this locality shows similar magnetite lenses and crystals in quartz, associated with which are 

biotite, m~scovite, and zircon in large crystals. Zircon is practically contemporaneous with 

pegmatitic magnetite, and with quartz and magnetite was one of the last minerals to crystallize. How far the zircon of the rocks of the area under discussion is of this late generation can 

not be stated. (See p. 183.) . 

Black tourmaline is widely distributed, but nowhere abundant. It occurs either in crystals embedded in quartl'l or orthoclase, in micropegmatitie intergrowths with quartz and feldspar, in feldspar mctasomatically replacing it, or in felts along cracks in the pegmatite. Hence 

while tourmaline usually solidified prior to quartz and feldspar it sometimes solidified contemporaneously with them, and more rarely after them or replaced feldspar. Microscopic examination in the latter case shows that muscovite simultaneously replaced feldspar, sericite shreds 

wreathing the tourmaline grains, and the presence of contemporaneous fluorite in irregular tiny 

areas elongated along_ a plane further indicates that magmatic gases. were the agents of this alteration. · 'I'he ferromagnesian mineral commonly associated with tourmaline is muscovite, and 

both of these minerals from other localities as a mle contain some fluorine. 

Small crystals of red garnet are locally very abundant in the more siliceous pegmatites, 

and the association of this mineral with siliceous gr~nite magmas in the Georgetown quadrangle appears to be constant. Crystals of black glassy allanite up to 3 inches in length are 

particularly well developed at the head of Maximilian Gulch. This mineral is confined to magnetite pegmatite and biotite-magnetite pegmatite cutting quartz monzonite, a restriction of 

some interest, since the monzonite contains allanite as an original constituent. At several 

!ocalities pegmatite in hornblende gneiss and quartl'l diorite carry hornblende crystals. About 

2.1 miles south of Mount Evans greenish-gray columnar aggregates of scapolite Winches in length 

occur in biotite pegmatite. 'fhe microscope shows that the scapolite, which is without question an 

original constituent of the pegmatite, includes apatite, allanite, and magnetite crystals. Scapo-

·, 
GENERAL GEOLOGY. 63 

lite occurs in volcanic rocks on Mount Somma,a but to the writer's ·knowledge has not been 

previously reported fr~m pegmatite. One mile southwest of Naylor Lake irregular masses of 

green apatite, containing manganese (probably mangan-apatite), are a constituent of muscovite pegmatite. Both apatite and zircon are abundant microscopic constituents of the 

granite. Specimens of beryl from Saxon Mountain in the possession of J. S. Randall, of Georgetown, probably come from pegmatitic masses. ' 

The individuals of the coarsest pegmatites reach a diameter of 1 foot or more. Such rocks, 

by loss of ferromagnesian minerals and by the simultaneous crystallization of quartz and feldspar, pass into graphic granite, the individuals of which range in diameter in cross section from 

a £raction of an inch to 1 inch. On the other hand, granular pegmati.tes grade along their strike 

into rudely banded dikes, quartz being commonly segregated in the center (fig. 17). Miarolitic 

cavities in the pegmatite are very rare. Here and there t~1e various textures of the pegmatites 

and the granular texture of the granite grade into one another in a single outcrop 100 feet in 

diameter, and in consequence the temperature and pressure which in some places determined 

the production of one texture could have varied little from those which produced others. 

Although typically massive, the pegmatites are locally mashed into gneisses, partly by 

granulation and pai'tly by recrystallization.. Quartz-magnetite pegmatite was particularly 

subject to· recrystallization, producing a quartz-magnetite gneiss 

in which eyes and thin bands of magnetite lie in a mosaic of 

elongated quartz lenses (Pl. X, B). The larger magnetite individuals a~e much less deformed than the smaller. Futtererb in 

particular has emphasized the ease with which quartz is recrystallized, and in many of the other schists and gneisses of the Georgetown quadrangle magnetite appears to be one of the more readily 

recrystallized minerals. The pseudomorphous gneissic stmcture 

locally present in pegmatite surrounding gneiss is described in 

another place (p. 90). 

CHARACTER OF THE PEGl\iATITIC MAGMAS. 

The magmas in general appear to have varied from a comparatively dry form, from which the granites solidified, through 

molten masses rich in water from which some of the pegmatites 

solidified, to a body so saturated with water that the banded quartz 

veins deposited by it resemble water-deposited veins. The extreme 

fluidity of the magma which deposited the pegmatite is indicated 

by a number of the characteristics of that rock. The pegmatitic 

a h a. 

.FIG.17.-Bandedpegmatitedike. South 

side of Clear Creek, one-half mile above 

Georgetown. n, Silver .Plume gra.nite; 

b, feldspnr; c, biotite; d·, quartz. 

magma or fluid sought out the smallest cracks and crevices and deposited its material therein, 

and the presence of pegmatite in small masses in nearly every outcrop in the quadrangle indicates 

that the older rock masses were thoroughly saturated with the pegmatitic material. That 

the process was one of saturation rather than of ordinary igneous injection, is shown by 

the presence, particularly in the biotite-sillimanite schist of the Idaho Springs formation, of 

isolated lenticular "eyes" of pegmatite (see p. 38) similar to the rock of the larger pegmatite 

masses (fig. 3, p. 39). The constituents of these "eyes" are absolutely uncrushed and can 

not be considered segments of a sheared pegmatite dike. The pegmatites further absorbed 

a considerable quantity of this schist and from the center of the larger inclusions gradations 

occur, in certain places, from pure schist to schist containing thin bands of pegmatite along 

its folia, thence to pegmatite, in which only a few scroll-like figures of darker pegmatite faintly 

suggest the crenulations of the almost totally absorbed schist, and lastly to pure pegmatite, 

in which a fair parting is here and there retained, indicating that the structure of the schist 

is partially preserved by metasomatic replacement, so that the pegmatite parting may be considered a pseuclomorph of that of the schist. 

aDana, .T. D., System of Mineralogy, New York, 1896, p. 467. 

b Futterer, Karl, Die" Ganggranite" von Grosssachsen und die Quartzporphyre von Thai im Thuringerwald; Heidelberg, 1890, pp. 27-47. 
64 . GEOLOGY. OF THE GEORGETOWN QUADRANGLE, COLORADO. 

A further proof of the extreme fluidity of the pegmatite magma is afforded by the influence on the mineral composition of the pegmatite exerte1 by the rock in which the pegmatite 

was injected. Biotite is the characteristic ferromagnesian mineral of pegmatites which inject 

granites older than themselves, the quartz monzonite gneiss, the quartz diorite, and the hornblende· gneiss, although in many places hornblende is the typical dark mineral of pegmatites 

in the last two formations. Muscovite is the typical mineral of pegmatite dikes in the Idaho 

Springs formation, which contains 98 per cent of the occurrences of muscovite pegmatite. The 

pegmatite dikes in quartz monzonite areas are characteristically without ferromagnesian minerals and many of them contain, instead of an alkali feldspar, a soda-lime feldspar. Furthermore, the presence of allanite in pegmatite cutting quartz monzonite which has allanite as an 

accessory is perhaps significant. Magnetite was evidently an original constituent of the pegmatite magma, since it alone of the ferromagnesian constituen~s of the pegmatites is not influenced by the rock intruded. In areas of unusually complex injection, where the Idaho Springs 

formation and the granular igneous rocks are of approximately equal areal extent, both biotite 

and muscovite are associated with one another at numerous localities. Where the pegmatite 

passes from a fairly large area of granite to an area of the Idaho Springs formation, the subo;:__ ___ _c.9_._00'------.:.'8""'00 feet 

FIG.18.-Diagram illustrating the Jag in the introduction ol muscovite in to the 

quartz-feldspar pegmntitc which is characteristic ol the quartz monzonitepegmatite on entering the Idaho Springs formation. a, Quartz monzonite; 

b, Idaho Springs fonnation; c, pegmatite contnining quartz and feldspar; 

d, pegmatite containing muscovite; '!'he rock intruded h;~s ;~direct in!luencc 

on the mineral composition of the pegmatite. A short distance frorn the 

contact of the Idaho Springs schist fornuttion the pegmatite, which has 

schist for both walls, characteristically contains muscovite, which is lacking 

where the wull rocks are of quartz monzonite. 

stitution of muscovite in place of biotite 

is as a rule coextensive with the boundaries of the two formations. Where, on 

the other hand, the pegmatite has traversed a single formation for a long distance and thence passes to a second formation, there is a distinct" lag" in the 

introduction of the mineral characteristic 

of the pegmatite in the second formation. 

This is well seen on the north side of 

the quartz monzonite batholith, which 

passes beneath the schist at a low angle 

(fig. 18). Here the quartz-feldspar pegmatite extends several hundred yards 

into the biotite-sillimanite schist of the 

Idaho Springs formation, before muscovite, the characteristic mineral of pegmatites in that 

formation, is present. 

A chemical discussion of these observations is inexpedient, since the composition of the 

pegmatitic magma is unknown. It is indeed probable that the pegmatitic substances left 

the cooling mass of granite at widely varying times and that at the period of their origin they 

varied in chemical composition. Field evidence, however, indicates that the pegmatitic fluid 

abstracted prior to its solidification sufficient material from the inclosing rocks to modify 

in considerable degree the chemical composition of the pegmatite. In alumina-rich schists 

of the Idaho Springs formation a portion of the feldspar molecules of the magma were 

changed to muscovite by the addition of alumina from the schists. The lag already mentioned indicates that the change in composition did not occur immediately after the pegmatitic magma entered a formation, but only after it had penetrated for some distance. 

GRANITES AND GRANITE PORPHYRY. 

The granites associated with the latest pegmatitcs are massive, fine to medium-grained, 

granular rocks, which in the main contains the same ferromttgnesian minerals as the immediately 

associated pegmatite. Although the granites intergrade, not only with pegmatite but with 

one another, certain types are rather well defined. 

In the northern part of the quadrangle a white, medium-grained, siliceous muscovite 

granite, which in many places has pink garnet us an abundant accessory, grades into muscovite 

pegmatite. Biotite is here and there subordinately present, and such a rock on Ute Creek 
GENERAL GEOLOGY. 65 

contains tabular feldspar phenocrysts 4 inches long. North of. Georgetown the granitic rock 

is a gray muscovite alaskite in which quartz predominates over feldspar. In the central or 

southern parts of the quadrangle a pink or pinkish-gray, fine-grained biotite granite or biotitemagnetite granite predominates. A similar granite widely distributed in the southeast corner 

of the quadrangle is medium grained, and weathered surfaces are porous by the almost complete removal of biotite. 

Under the microscope the texture of these granites is seen to be allotriomorphic granular. 

The order of crystallization is normal, except that in some thin sections titanite in the pink 

biotite granite lies in irregular grains between quartz and feldspar and therefore separated 

simultaneously with them. The accessories solidified in the following order: Zircon, apatite, 

. magnetite, and titanite. Microcline, as a rule with microperthitic bands, is the predominant 

constituent of most of the granites. It is in the main original, but locally it is a product of 

recrystallization after the solidification of the granite, as it is in the gneissoid granite. (See 

p. 50.) Orthoclase, only rarely with microperthitic bands of albite or oligoclase-albite, and an 

acidic plagioclase (chiefly oligoclase-albite) are commonly also present. Sericite and kaolin 

partially replace feldspar, the former occurring locally in large secondary muscovite plates. 

Quartz, which occurs in irregular areas, also forms micropegmatitic intergrowths with the three 

species of feldspars, and is further characterized by abundant opaque threadlike inclusions. 

Fluid inclusions, with or without a tiny bubble, are very abundant. Biotite is an essential 

constituent of the pink biotite granite mentioned above, and is rare or absent in· the other 

granites. It is in general partially or completely altered to chlorite,. with the simultaneous 

separation of magnetite and either rutile or anatase. Muscovite or epidote is more rarely 

associated .with chlorite. Original mus.covite is more widely distributed in these granites than 

in any others of the quadrangle. In a single thin section blades of sillimanite are inclosed in 

muscovite plates, as in the gneissoid granite (seep. 50), and from the massive character of the 

granite sillimanite may well be an original constituent. Zircon, magnetite (some of it titaniferous), and apatite are common and abundant accessory minerals. Garnet is confined to the 

alaskitic muscovite granite and titanite and allanite to the pink biotite granite. 

Fluorite occurs in several thin sections of biotite granite of the series south of Meridian 

Hill. It is colorless or spotted with irregular purple dots, and occurs as wedges and irregular 

masses between the essential constituents and as irregular masses in feldspar. Quartz in contact 

with fluorite is generally rounded, as if the irregular protuberances characteristic of quartz in 

contact with feldspar had been eaten away by the fluorite. Feldspar has an interlocking contact with fluorite, as if the latter mineral had partially replaced the former, a process that has 

certainly taken place in the small isolated areas tJf fluorite in feldspar. As in a few places 

miarolitic cavities are present in the pegmatite into which the granite grades, and as many of 

the fluid inclusions in quartz are full, these granites probably solidified under comparatively low 

pressure, and it is probable that minute cavities existed in them after solidification. It is believed 

that fluorite, brought in by pneumatolitic agencies, partially filled such tiny interstices between 

the essential constituents and partially replaced feldspar. In the vicinity of Silver Plume 

fluorite is clearly deposited in veins by aqueous solutions; but mineralized veins are not known 

to occur within a radius of 10 miles of the fluorite-bearing granite. The constituents of the 

granite have been deformed and in places fractured by mass mechanical processes. Here and 

there unclulose extinction in quartz is highly developed, the feldspars are fractured, and the 

biotite plates and the plagioclase twinning laminre are bent. 

At several points south of Meridian Hill pegmatite and the assoc!ated granite grade into 

pink or grayish-pink granite porphyry. In the fine-grained groundmass, predominantly of 

feldspar and quartz, are many pink feldspar phenocrysts, with a maximum length of one-half 

inch; some rounded, slightly smoky quartz phenocrysts, one-fourth inch in diameter, and 

small phcnocrystic areas of either biotite, magnetite, or hornblende. Since the gramte porphyry occl!rs as irregular masses grading into nonporphyritic granite or pegmatite, the phenocrysts doubtless formed in place and are not intratelluric. Under the microscope the ground31401-No. 63-08-5 
66 . GEOLOGY OF THE G~ORGETOWN QUADRANGLE, COLOR;\-DO. 

mass appears as a :tine-grained, microgranitic aggregate of microcline and quartz, with orthoclase 

and magnetite in some thin sections and biotite in others. Biotite, on the one .hand, and 

orthoclase and magnetite, on the other, are clearly complementary. Quartz and orthoclase or 

microcline locally form micropegmatitic intergrowths. The feldspar phenocrysts, of microcline 

with narrow microperthitic bands, have interlocking borders with the constituents of the 

groundmass and are characterized by many micropoikilitic inclusions of the groundmass constituents. The less perfectly bounded quartz phenocrysts are similarly characteri:~;ed. Neither 

biotite nor hornblende forms good crystals, the latter in particular commonly occurring as 

skeleton aggregates of small hornblende areas, similarly oriented, and in many places connected 

by tiny hornblende stringers. Titanite, magnetite, zircon, and apatite are also present. Since 

small bodies of groundmass lie between the hornblende areas, it is evident that these phenocrysts solidified at approximately the same time. as the groundmass, and it is probable that 

none of the phenocrysts crystallized much before the components of the groundmass. The 

occurrence of fluorite in granite porphyry is similar to that already described in granite. 

SIMILARITY OF THE PRE-CAMBRIAN GRANITES. 

Although no chemical analyses of the rocks of the Georgetown quadrangle have been 

made, microscopic examination shows a marked mineralogical similarity in the granites, the 

intrusions of which were separated in every case by considerable time intervals and in one 

case by the intrusion of more basic rocks. The granites are characteristically biotite granites 

in which microcline, undoubtedly in some places a product of recrystallization and in other of 

deformation, is as a rule the predominant feldspar. Orthoclase in turn predominates over 

oligoclase or oligoclase albite. Microcline throughout the series is characterized by microperthitic bands which are less common in orthoclase. Micropegmatitic intergr~wths of quartz 

and orthoclase are widespread, the simultaneous separation of the two being further indicated 

by the rarity with which orthoclase forms crystal faces against the quartz. The presence in 

quartz of abundant threadlike inclusions of undetermined nature is also characteristic. Biotite 

appears to contain titaniuni, since rutile or some other titanium mineral is a by-product of its 

alteration. Original muscovite, except in certain of the pegmatitic granites, is present in 

negligible amounts. Of the accessory minerals, apatite, zircon, and magnetite are constantly 

and rather abundantly present. The granites of the Georgetown quadrangle present an interesting example of the repeated injection of a granitic magma or magmas of fairly constant composition. Mathewsa emphasized the close chemical and mineralogical similarity of the granites 

of the Pikes Peak quadrangle, which lies at the south end of the Colorado Range. The granites 

of this region and those of the Georgetown quadrangle possess many characteristics in ·common 

and in some of the intrusions were probably contemporaneom:. 

CARBONIFEROUS (?) SANDSTONE. 

Residual bowlders of silicified sandstone and quartzite occur at three points, namely, 1 

mileS. 60° W. of the junction of Cascade and Chicago creeks (altitude 10,100 feet); on Cascade 

Creek 0.1 mile west of bench mark 9592 (altitude 9,650 feet); and between West Chicago Creek 

road and the road leading toward Griffith Mountain (altitude 9,200 feet). The residuals on 

Cascade Creek are so abundant that the original exposure of the sandstone could not have been 

much above the present surface. The medium-grained, well-silicified sandstone and quartzite 

is yellowish brown, pink, rod, or purple in color. The semiporous character of certain bowlders 

indicates that some lime carbonate as cement may have been associated with silica. ~Wdlrounded pebbles of quartz monzonite, hornblende gneiss, and pegmatite are embedded here and 

there in the finer matrix. Under the microscope the silicified sandstone proves to be composed 

of well-rou.nded grains of quartz with one or more grains of plagioclase and rot)ndecl rock 

fmgments in most of the thin sections. The quartz, which contains zircon granules, ':"as derived 

partly from the gneisses and partly from the plutonic rocks of the pre-Cambrian formations. 

a Mathews, E. B., Jour. Geology, vol. 8, 1900, pp. 214-240. 
GENERAL GEOLOGY. 67 

Quartz in optical continuity with the rounded grains, from which it is separated by a film of 

limonite, is the chief cement, although a little hematite and chlorite also oceur. 

The well-sorted character of the sandstone indicates lacustrine or marine origin, while 

from the thorough cem~ntation and the almost complete removal by erosion of what was once 

perhaps an extensive formation it may be inferred that the rock is of considerable age. It is, 

however, much younger than the youngest pre-Cambrian pegmatite, since before the deposition 

of the pebbles in the sandstone the pegmatite must have cooled beneath a considerable cap of 

~wcrlying rock, which was removed by erosion prior to the depression of the land beneath the 

water in which the sand was deposited. Five miles south of the quadrangle, on North Fork 

of the South Platte about 1 mile above Shawnee, a lithologically similar sandstone rests unconformably upon the Idaho Springs formation at an elevation of approximately 8,200 feet. 

Similar sandstone is exposed on the north bank of the same stream at Pine post-office, 15 miles 

south of east of the southeast corner of the quadrangle. This sandstone belt strikes N. 60° W. 

and dips 70° to 80° S,V., and is probably a block dropped by parallel faults into the Rosalie 

(pre-Cambrian) granite. The sandstone, which includes small pebbles of the Rosalie granite, is 

similar to that of the residuals in the Georgetown quadrangle, although the red and white 

coloring matter is somewhat more unevenly distributed. Lithologically this rock is very like 

the Fountain sandstone (Carboniferous) of the foothills of the Colorado Range. This portion 

of the range was at· some time covered by the sea, which extended over the whole or a part of 

the Georgetown quadrangle, and if the above correlation be correct the submergence was in 

late Carboniferous time, although a different age is quite possible. 

INTRUSIVE PORPHYRIES. 

INTRODUCTION. 

The intrusive porphyries comprise a variety of rocks, including alaskite porphyry, granite 

porphyry, quartz monzonite porphyry, dacite, alkali syenite porphyry, bostonite, biotite 

latite, and alaskitic q1iartz monzonite porphyry. Notwithstanding their rather wide mineralogical range, these rocks have many common characteristics. All are younger than the preCambrian pegmatite and most of them arc older than the important ore deposits. All are 

massive and have been subjected ~o practically no regional ·mashing. They are confined to 

the northern portion of the quadrangle. · 

l>ISTIUBUTION IN COLOUADO. 

A belt of late Cretaceous intrusive porphyries at least 95 miles long extends from Boulder 

to Aspen and Crested Butte across the Colorado, Park, and Sawatch ranges. It courses from 

northeast to southwest and broadens to the southwest, being but 10 miles wide at Boulder and 

at least 75 miles wide near its west encl. West of longitude 106° 30' the northern and southern 

limits of the belt have not as yet been definitely located, although probably the map (Pl. XI), 

which has been compiled from all available data, is approximately correct. 

These intrusive porphyries of siliceous or intermediate composition bear a general resemblance to one another and in a broad way are probably ·contemporaneous. In sedimentary 

rocks they characteristically occur as sheets; in the pre-Cambrian complex, as dikes. Concerning their form Emmons a says: 

Normal dikes in the sedimentary strata were rarely observed, and wherever the sheets cross the strata transversely 

it is usually at a very low angle. In the Archean formations, on the other hand, the older rocks [the intrusive porphyries] were almost invariably found in the form of narrow and rather irregular dikes as a rule not over 50 feet in thickness, the principal eruptions being two large bodies of diorite whose. outlines were not very accurately determined. 

The Archean exposures studied in this region were once covered by portions of the same sedimentary $Cries in 

which these intrusive sheets are now found. It may therefore be assumed that the form of the channels through which· 

the fused masses were forced up into the overlying beds is fairly represented by the average outline of these dikes. 

According to this reasoning it is evident that the channels in the Archean were extremely small as compared with 

• a Emmons, S. F., Mon. U.S. Geol. Survey, YO!. 12, 188<i, p. 296. 
'68 GEOLOGY OF THE GEORGETOWN QUADRANGLE, COLORADO. 

the extent of the sheets themselves, and were rather in the form of the narrow fissure, which has been supposed to be 

the source of so-called massive eruptions, than of the rounded "necks," which have sometimes been observed as the 

actual vents of volcanic eruptions. 

Emmonsa notes the same conditions in the southwestern portion of Summit County, and 

Fenneman b found sheets to be characteristic of the sedimentary rocks o{ the foothills in Boulder 

County. 

Within this porphyry.belt are situated many of the most important mining camps in Col6-

rado, including Jamestown, Sugarloaf, Sunset, Gold Hill, Sunshine, Ward, Nederland, and 

·Caribou, in Boulder County; Central City, Blackhawk, Russell Gulch, and Nevadaville, in 

Gilpin County; Idaho Springs, Freeland, Lamartine, Empire, Alice, Yankee, Lawson, Georgetown, and Silver Plume, in Clear Creek County; Argentine, Montezuma, Breckenridge, Kokomo, 

and Robinson, in Summit County; Fairplay and the camps northwest of it, in Park County; 

Leadville, in Lake County; a number of comparatively unimportant camps in Chaffee County, 

and Aspen and less important camps in Pitkin County. The close association of the ore deposits 

and these igneous rocks appears not only from the economic portion of this paper, by Messrs. 

Spurr and Garrey, but also from the reports on Leadvillec and the Tenmile district a by Emmons 

and on Aspen d by Spurr. 

In commenting on the ore deposits of th~ headwaters of Eagle River, Emmons says:e 

"In this [Summit] county there is a marked connection between the prevalence of eruptive 

rocks of secondary or Mesozoic age and the richness and magnitude of the ore deposits." He 

further states/ that the ore bodies at the head of Mosquito Gulch, in Park County, are in 

Paleozoic limestones in contact with sheets of white porphyry. "Southward the masses of 

intrusive porphyry diminish in extent, as do also the number of developed mineral deposits." 

Concerning the ore· bodies of Boulder County, Foster u states that there is a close connection 

between the ore deposits and dikes at Gold Hill, the eruptive rock closely resembling the 

Elvan [quartz porphyry or granite porphyry] of Cornwall." Palmer" says qf the dike rocks in 

the vicinity of Mount Sugarloaf, which he calls porphyries and porphyrites: "They are probably concerned to a large degree with the mineralization of the county [Boulder].:' The ore 

deposits of Gilpin County have been described by Rickard,i who remarks: "The andesite dikes 

appear to have a very important influence on the vein system of this district." In short, the 

intrusive porphyries seem throughout this belt to have a direct connection with the ore bodies, 

whether they are considered as the source either of the ore or of the waters or gases which 

deposited the ore, or whether their influence is pur~ly physical. 

The intrusive porphyries in the sedimentary rocks of the foothills near Boulder have been 

mapped and described by Fennei:nan.J He describes the rock as having a "felsitic groundmass" containing "phenocrysts of quartz, feldspar, and mica." In the spring of HJ05, in company with Prof. R. D. George, of Colorado University, the writer examined these sheets and 

found them lithologically similar to the alaskite porphyry and granite porphyry of the Georgetown quadrangle. The intrusive porphyries of the pre-Cambrian complex of Boulder County 

have been mapped and described by Marvinek and Van Diest.z These maps show that the 

dikes trend in general parallel to the belt of the intrusive porphyries, that is, somewhat north 

of east. The porphyries of BoulderCounty have been described by Silliman,"" Smith,n Foster,O 

a Emmons, S. F., Geologic Atlas U.S., folio 48, U.S. Geol. Survey, 1898. 

b Fennem11n, N. M., Bull. U.S. Geol. Survey No. 265, 1905, p. 36. 

cEmmons, S. F., Mon. U.S. Geol. Survey, vol. 12, 1886, pp. 541,509-584. 

dSpurr, J. E., Mon. U.S. Geol. Survey, vol. 31, 1898, pp. 235-236: 

<Emmons, S. F., Tenth Census, vol. 13, 1885, p. 72. 

!Idem, p. 75. 

g Foster, E. Le Neve, Rept. State Gcolo;>ist of Colorado lor 1883-84, 1885, pp. 21-22. 

h Palmer, Chas. S .. Proc. Colorado Sci. Soe., vol. 3, 1830, p. 2:1:1. 

i Rickard, Forbes, Trans. Am. Inst. Min. Eng., vol. 28, 1899, p. 112. 

1 Fenneman, N. M., Bull. U. S. Geol. Survey No. 265, 1905, 1'1. II, pp. 36-40. 

kMarvine, A. H., Am. Jour. Sci., 3d ser., vol. 8, 1874, pp. 29-33. 

ZVan Diest, P. H., Proc. Colomdo Sci. Soc., vol. 2,1886, p. 51. 

til SilliTnan, B., Am. Jour. Sci., 3d scr., vol. R, 1874, p. 2U. 

"Smith, J. Alden, Rept. Colorado Gcol. Snrvey, 1879-80, pp. 19-20. 

o Foster, E. Le Neve, Rept. State Geologist of Colorado for 1883-84, 1885, pp. 21-22. 
GENERAL GEOLOGY. 69 

Emmons, a Van Diest, b Palmer, c Farish, d Blake,e Hogarty/ and Breed.g Although these men 

have styled the rocks by various names, it is evident that with a single exception they are all 

porphyries of siliceous or intermediate composition comparable with those of the Georgetown 

quadrangle, alaskite porphyry, granite porphyry, quartz monzonite porphyry, dacites, and 

siliceous andesites being most widely distributed. The exception consists of the small intrusive masses of diabase at Sugarloaf described by Emmons. a 

The intrusive porphyries along a portion of the southern border of Gilpin County were 

examined during the course of the field work for this report and were found to be similar in 

every way to those of the Georgetown quadrangle. While the dikes are especially clustered 

in the vicinity of Central City and the other mining camps of Gilpin County, the \\rriter also 

observed dikes along the Denver, Northwestern and Pacific Railway west of Rollinsville. Maps 

of Central City and vicinity compiled by Endlichh and by Underhill i show that the majority 

of the dikes strike from northeast to east-northeast. Underhilli states that they "vary from 

the composition of andesite to that of trachyte, but in the area specially studied they are of 

intermediate composition and may be called latite porphyry." From his descriptions it is 

probable that granite porphyry k and q~artz monzonite porphyry also occur. Rickard! states 

that a Gilpin County porphyry was determined. by Professor Judd, of the Royal School of 

Mines, to be a dacite. 

The boundaries of the belt in Clear Creek County were traced during the field work 

incident to the present report. 

Our knowledge of the intrusive porphyries of Summit County is clue Iitrgely to Emmonsr 

who saysm that on the upper part of Blue River there is a "considerable development of secondary eruptive rocks." The Director of the Mint states that in the Union clistrictn northeast of Breckenridge and on Negro Hill 0 east of the same town there is porphyry. The intrusive porphyries of southwestern Summit County, mapped and described by Emmons,P include 

three types of diorite porphyry which have many features in common with those of Leadville, 

and facies of which in turn are similar to the quartz monzonite porphyry of the Georgetown 

quadrangle. Mr. Spurrq found no intrusive porphyries at Argentine and that they exist at 

Montezuma is by no means certain, although the distribution of the porphyry to the south and 

the character of the Montezuma ore deposits indicate that they probably occur there. 

The country around the headwaters of Middle Fork of the South Platte in Park County 

has been mapped geologically by Emmonsr and the intrusive porphyries have been described 

petrographically by Cross.8 In·another place Emmons 1 says: "Southward [from Mosquito 

Gulch] the masses of intrusive porphyry diminish in extent, as do also the number of developed 

mineral deposits." PeaJeu states that "phonolitic trachyte occurs near the headwaters of 

Michigan Creek," a tributary of Tarryall Creek, and this rock he compares to the intrusive 

rock of Mount Silverheels, which Emmons considers one of the late Cretaceous intrusives. 

a Emmons S. F., Tenth Census, voL 13, 1885, p. 65. 

b Van Diest, 1'. H., Proc. Colorado Sci. Soc., voL 2, 1886, pp. 50-55. 

c Palmer, C. S., Proc. Colorado Sci. Soc., voL 3, 1890, pp. 233-234; vol. 5, 1808. pp. 159-164. 

d Farish, J. ll., Trans. Am. Inst. Min. Eng., voL 10, 1801, p. 547. 

c Blake, J. C., Proc. Colorado Sci. Soc., vol. 7, 1901, pp. 1-16. 

I Hogarty, Barry, Proe. Colorado Sci. Soc., vol. 6, 1902, pp. 173-185. 

g Breed, R. 8., Proc. Colorado Sci. Soc., vol. 7, 1902, pp. 216-230. 

h Endli~h, F. M., Seventh Ann. Rept. U.S. Geol. and Geog. Survey Terr., 1874, p. 280. 

• Underhill, James, Proc. Colorado Sci. Soc., vol. 8, 190G, pp. 118-121. 

i Idem, p. 119. 

k Idem, p. 120. 

'Rickard, Forbes, 'l'rans. Am. Inst. Min. Eng., vol. 28, 1899, p. 111. 

m Emmons, S. F., Tenth Census, vol. 13, LSS5, p. 7..2. 

"Rept. Director of the Mint on Production of I'recious Metals in U. s., 1882, p. 557. 

o Idem, 1883, p. 429. 

p Emmons, S. F., Geologic Atlas U. 8., folio 48, U.S. GeoL Survey, 1898. 

q Oral communication. 

r Emmons, S. F., Mon. U. S. Geol. Survey, voL 12, 1886, atlas sheet 6. 

•Cross, Whitman, Mon. U.S. Geol. Survey, vol. 12, 1886, pp. 316-358. 

t Emmons, S. F., Tenth Census, vol. 13, 1885, p. 75. 

u Peale, A. C., Seventh Ann. Rept. U.S. Geol. and Geog. Survey Terr., 187:3, p. 215. 
70 GEOLOGY OF THE GEORGETOWN QUADRANGLE_, COLORADO. 

Peale also mentions similar rocks at the head of Trout and Fourmile creeks a and near Fairplay.b 

It is evident that the distribution of these porphyries in Park County is approximately as shown 

on the map (Pl. XI). 

Our information concerning the intrusive porphyries of Chaffee County is fragmentary, 

although it is certain that practically the whole county is to be included in the belt. Foster 

remarks:c "The principal mining districts are in metamorphosed and granitic rocks through 

which dikes of porphyritic and feldspathic rocks are frequently found; limestone is also found 

in some parts of the county." Emmons d says concerning Chaffee County: "Its mountain 

slopes are composed of Archean rocks, mainly granite, traversed by porphyry dikes." Dikes 

of porphyry in the vicinity of Granite are mentioned by Emmons ;e ledges of porphyry on Clear 

and Pine creeks by the Director of the Mint/ and dikes of quartz porphyry in the Chalk Creek 

mining district by Emmons. d The Director of the Minto mentions porphyry at Summit', and 

Foster h describes "vertical sheets" of white porphyry at Monarch, in the southwestern portion 

of the county. Cross i says: "Dikes of a dense white quartz porphyry much like the Leadville 

'white porphyry' cut the schists at several places opposite Salida and the head of Bear Creek, 

in the Sangre de Cristo Mountains." 

The intrusive porphyries of Lake County in the vicinity of Leadville have been mapped 

and described, by Emmonsi and studied petrographically by Cross.k The rocks, like those 

characteristic of other portions of the porphyry belt, are siliceous and intermediate igneous 

rocks, including granite porphyry and diorite porphyry. Comparison with the rocks of the 

Georgetown quadrangle establishes the approximate identity of Cross's "white porphyry" 

with the alaskite porphyry of this report, and of the Lincoln porphyry with the quartz monzonite 

porphyry. Hayden 1 mentions the presence of dikes of "trachyte" about 2 miles above the 

falls of Lake Creek near Upper Twin Lake, and Peale m describes dikes of volcanic rock in 

granite on Tennessee Pass. It is probable that all of Lake County should be included in the 

porphyry belt. 

The Tenmile quadrangle,n an area including the southwestern portion of Summit County 

and extending into the southeast corner of Eagle County, contains both sheets a,nd dikes of 

intrusive porphyries. Emmons, 0 in describing the mines at Red Cliff, states that the lower 

Carboniferous or "blue'' limestone is cut by an eruptive sheet of quartz porphyry. From the 

Hayden reports it is evident that the porphyry belt includes only the southern portion of 

Eagle County. 

SpurrP divided the late Cretaceous or early Tertiary eruptive rocks of Aspen ·into quartz 

porphyry and diorite porphyry. He compares the former to the white porphyry of Leadville, 

and its finer-grained facies are praCtically identical with the alaskite porphyry of the Georgetown 

quadrangle. The diorite porphyry, on the other hand, has the mineralogical composition of 

the quartz monzonite porphyry o-f this report. Fosterq states that the ore deposits at Ashcroft arc in granite cut by "eruptive rocks." It is probable that eastern Pitkin County is 

largely within the porphyry belt. 

nPeaJe, A. C., Seventh Ann. Rept. U.S. Geol. and Geog. Survey Terr., 1Si3, p. 220. 

b Idem, p. 303. 

c Foster, E. Le Neve, Rept. State Geologist of Colorado for 1883-84, 1RS.1, p. 30. 

a Emmons, S. F .. •renth Census, vol. 13, 188.\ p. i9. 

e Emmons, S. F., Geological Guide Book to the. Rocky Mountains, 1894, p. 423. 

f Hcpt. Director of the Mint for 1884, 1885, p. 408. 

g Rcpt. Director of the Mint for 1901, 1902, p. IOi. 

h Foster, E. Le Neve, Rept. State Geologist of Colomdo for 1883-84, 188.5, pp. 30-32. 

i Cross, Whitman, Proc. Colorado Sci. Soc., vol. 4, 1893, p. 280. 

,i Emmons, S. F., Mon. U. S. Goo!. Survey, vol. 12, 1886, atlas sheets 1:3, 14, pp. 74-86. 

k Cross, Whitman, Mon. U. S. Geol. Survey, vol. 12, 1~86, pp. 31G-358. 

l Hayden, F. V., Seventh Ann. llcpt. U. S. Gcol. and Gcog. Survey Terr., 18i4, p .. 14. 

"'l'ealc, A. C., Seventh Ann. He pt. U. S. Goo!. and Geog. Survey Terr., 1874, p. 242. 

n Emmons, S. F., Geologic Atlas U. S., folio 48, U. S. Gcol. Survey, 1898. 

o Emmons, S. F., Geological Guide-Book to the Rocky Mountains, 1894, p. 416. 

p Spurr, J. E., Mon. U. S. Geol. Survey, vol. 31, 1898, pp. 45-53. 

q Foster, E. Lc Neve, Rcpt. State Geologist of Colorado for 1883-84, 1885, p. 53. 
U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER NO. 63 PL. X I 

MAP OF CENTRAL COLORADO, SHOWING BELT OF INTRUSIVE PORPHYRIES 

IOB::::Ea::Eis=sc:Ecio=========="'IO:======'z:Eo==:========'3"30, mil es 

D - Pre-Cam brian rocks Undifferentiat ed igneous rocks Paleou,ic and ~ lat er sedimenta ry rocks Sheet of intru

~ sive porphyry 

(from Hayden map) 

EJ Dikes of intrusive pol'phy ry 

(direction g'~ fi\ 11 ~ unknown I 

t Hr Noi< s Pn£R:-:: <- o .. w.o s''' " G' oN . I' , 
GENERAL GEOLOGY. 71 

AGE. 

In the Georgetown quadrangle the only determination of the age of the porphyries that 

can be made is that they are younger than the pre-Cambrian rocks, which they cut, and mainly 

older than the important ore deposits. In the west end of the porphyry belt already described, 

however, Emmons a found data indicating that the granite and diorite porphyries of Leadville 

were intruded in late Cretaceous time. He assigns a similar age to the intrusive porphyries 

of southwestern Summit County.b Spurrc believed the intrusive porphyries of Aspen to be 

contemporaneous with those of Leadville. 

The sheets in the vicinity of Boulder, 23 miles from Idaho Springs, were examined by Prof. 

R. D. George, of Colorado University, and the writer. These sheets, which are petrographically 

similar to the alaskite porphyry and granite porphyry of the Georgetown quadrangle, cut 

respectively the Niobrara and Pierre (Cretaceous) and the ''Red Beds." These dikes at 

Boulder are.an extension of the belt of porphyries in this quadrangle, and the later dikes are 

in consequence probably of late Cretaceous age or younger, a conclusion in accord with the views 

of Fenneman.a 

Andesitic pebbles with glassy groundmass in the late Mesozoic Denver formation of the 

Denver Basin were derived from flows that Crosse believes to have covered considerable areas 

in the Colorado Range during the deposition of that formation, which though post-Laramie is 

considered still probably Cretaceous. As Cross states, these andesitic pebbles have affinities 

with dikes near Empire, to the north of the Georgetown quadrangle. Other pebbles are rather 

similar to the dacite and biotite latite described elsewhere in this paper (pp. 75, 81). 

DISTIUBUTION IN TilE GEORGETOWN QUADRANGLE. 

The Georgetown quadrangle lies on the southern edge of the belt of intrusive dikes, sheets, 

and stocks already described. Within the quadrangle masses of porphyry are confined to an 

area north of the divide between Leavenworth Creek and the Naylor Lake drainage, northeast 

of West Chicago Creek and north of a line connecting the junction of East and West Chicago 

creeks and a point 0. 7 mile north of Chief Mountain. The porphyry is coextensive with the 

region in which active mining is carried on in the quadrangle. 

Within the area specified the porphyry masses are unevenly distributed. Dikes are particularly abundant northeast of a line running from the 9,500-foot contour on Soda Creek 

northwestward through Alps Mountain to the north border of the quadrangle. Within this 

area, in turn, dikes are concentrated on the northwest side of Chicago Creek from the mouth 

of Spring Gulch to Eclipse Gulch. A group of dikes centers around the west end of Silver 

Plume, a second group 1 mile up Leavenworth Creek, a third at the mouth of Silver Creek, and 

a fourth on Lincoln Mountaip.. Active mining operations are carried on ·at each of these 

localities. 

Dikes are particularly abundant.in the biotite-sillimanite schist of the Idaho Springs formation, partly perhaps because of the coincident location of schist areas and the centers of profound igneous intrusion, but largely because of the numerous planes of weakness offered to 

injection by the highly developed schistosity of the schist. Many adjacent masses are of a 

similar variety of porphyry, but each rock has a rather well-defined distribution. (Seep. 39.) 

FORMS OF INTRUSION. 

The intrusive porphyries occur as a rule in dikes and more rarely in sheets and stocks. 

The dikes are typically rather short, narrow, and simple. They vary in length from 50 feet to 

over 1! miles, with 1,000 feet as an average. Dikes in the pre-Cambrian plutonic rocks are on 

a Emmons, S. F., Mon. U.S. Geol. Survey, vol. 12, 1886, p. 294. 

b Emmons, S. F., Geologic Atlas U.S., folio 48, U.S. Geol. Survey, 1898. 

cSpurr, J. E., Mon. U.S. Geol. Survey, vol. 31, 1898, p. 53. 

dFenneman, N. M., Bull. U.S. Gcol. Survey No. 265, 1905, pp. 35-40. 

•Cross, Whitman, Mon. U.S. Geol. Survey, vol. 27, 1896, pp. 34,202. 
72 GEOLOGY OF THE GEORGETOWN QUADRANGLE, COLORADO. 

the whole shorter than those in schists and gneisses. In some places, where a dike dies out, 

a second and even a third segment of similar porphyry is encountered along its strike (fig. 19). 

(See p. 67.) On the north side of Silver Creek, 0.3 mile above its junction with Clear Creek, 

two small porphyry dikes intrude the hornblende~augite gneiss and the biotite-sillimanite 

schist of the Idaho Springs formation at the bottom of the canyon. Ten feet above the stream 

these dikes pinch out and are replaced by a shear zone in the older rocks (fig. 20). Further, 

~----------3~et----------~ 

FIG. 19.-l'orphyry dikes dying out with depth. 

a, Porphyry; b, pegmatite; c, inclusions of schist of 

Idaho Springs formation; d, sheared pegmatite. 

Spring Gulch, 6 miles north of mouth, Central City 

quadrangle. 

many dikes of porphyry encountered in the mines can 

not be found on the surface, and in consequence it is 

probable that dikes are more numerous at considerable 

depths. The dikes vary in width from a fraction of an 

inch to 500 feet, although a width of 50 feet is exceptional and 35 feet is the· average. Short dikes are as 

wide as long ones, and in consequence the width of a 

dike is not a function of its length. Abrupt variations 

in width occur in places, but the dike width is in general 

rather constant. 

The average strike of the porphyry dikes is slightly 

north of east and south of west, practically paraliel to the 

average strike of the regional schistosity. Dikes striking 

N. 40° to 80° E. are particularly abundant. In schist 

neighboring dikes are in the main parallel, a feature not 

characteristic of dikes in the pre-Cambrian plutonic rocks. 

As a rule the dikes are straight, although some are gently curved and others sharply change 

their direction. Some dikes intruded in schist have a sinuous course that is in many places 

due to changes in the schistosity of the older rocks, along the planes of which the dikes were 

intruded. Sharp bends may accompany the passage of a dike from one structure of the older 

rock to another, as from a plane of schistosity to a joint, or from one set of joints to another. 

Although the dikes are as a rule simple, minor parallel injections and apophyses are by no 

means uncommon. The dikes were intruded along planes of weakness in the pre-Cambrian 

rocks, such as joints, faults, and schistosity planes. The preponderating 

occurrence of porphyry dikes in schist 

and ·the control of the dike direction 

by the schistosity of the intruded rock 

are indicated by the coincidence of the 

strike of the dikes with the average 

schistosity. Dikes across· the strike 

of schistosity are exceptional, whereas 

dikes parallel to the strike of the 

gneiss, but of different dip, are more 

common. In the igneous rocks many 

of the dikes occupy joints and faults 

and, like these structures, are variable 

in direction over small areas. 

Where the porphyry was intruded 

~---------------career 

FIG. 20.-Porphyry dikes dying out before reaching surface. a, Biotite-sillimanite 

scbist of Idaho Springs formation; b, hornblende-<liopside gneiss of Idaho Springs 

formation; c, ale.skite porphyry; d, pegmatite. One-third mile west of mouth of 

Silver Creek, near Georgetown. 

along fiat gneissic or schistose planes, sheets occur. On Barbour Fork of Soda Creek, 1 mile 

above its mouth, such a sheet has been cut by erosion into a number of small bodies. 

Stocks are few in number. Radiating dikes, characteristi~ of many stocks in other regions, 

are not connected with those of the Georgetownquadrangle, and in the vicinity of stocks dikes 

are not particularly abundant. On the west bank of Chicago Creek 0.2 miJe northeast of the 

mouth of King Solomon Gulch erosion has exposed the western half of a stock of bostonite 

porphyry 115 feet long, which extends 20 feet above the stream bed. The schistosity of the 
GENERAL GEOLOGY. 73 

biotite-sillimanite .schist bows around this mass. Porphyry is not exposed on ·the east bank 

of the creek, and it is evident that the mass never connected with a surface flow, since it has 

been uncovered only by recent erosion. 

There is no evidence within the quadrangle that the porphyries were ever connected with 

spxface flows, although from the abundant debris of effusive rocks in the Denver formation 

in the Denver Basin it is probable, as Cross states," that the Colorado Range was largely covered 

by lava flows about the time when these rocks were intruded. From the fact, however, that 

some dikes and stocks have reached the present surface only through recent erosion, it is probable 

that a considerable number of dikes now exposed clicl not at the time of their intrusion reach 

the surface .and form flows. 

QUARTZ MONZONITE PORPHYRY AND ASSOCIATED QUAltTZ :MONZONITE. 

DEFINITION. 

A rock of porphyritic habit with the composition of a quartz monzonite (see pp. 51-54) is 

called quartz monzonite porphyry. 

DISTRIBUTION AND OCCURRENCE. 

A few dikes of quartz monzonite porphyry occur south of Silver Plume and Idaho Springs 

and 2 miles northeast of ColuBbia Mountain. A stock on Lincoln Mountain, approximately 1 

mile in diameter, is also composed predominantly of this porphyry, which apparently grades 

into a granular rock, quartz monzonite, although a lack of exposures renders proof of this 

impossible. 

PETROGRAPHY. 

The abundant medium-sized phenocrysts of this rock equal or exceed in bulk the finely 

crystalline gray groundmass. Of the phenocrysts the considerably predominating feldspars 

are stout lath-shaped plagioclases and fewer tabular orthoclases. Hexagonal plates of black 

mica, locally resorbed, are visible in most exposures, and some outcrops show clark hornblende 

columns or rounded quartz individuals. Octahedrons of iron ore and wedges of titanite are 

here and there noted by the naked eye. The more altered facies a.re yellowish white in color, 

the feldspars being changed to a soft, white greasy substance and black mica to a silvery white 

variety, and hornblende being in many places completely removed. 

Under the microscope the groundmass proves to be in general micropoikilitic; in some 

portions spongy irregular areas of quartz contain tiny oligoclase laths and orthoclase tablets, 

and in others orthoclase similarly contains plagioclase laths. Rarely the groundmass has a 

hypidiomorphic, microgranitic texture in which plagioclase laths lie between irregular grains 

of orthoclase apd less quartz. 

The phenocrysts, named in the order of their abundance, are plagioclase, biotite, and 

orthoclase, and hornblende and quartz, if present. Plagioclase, which varies from oligoclase 

to andesine, occurs in stocky laths that where large are as a rule compound and where small 

are simple. The most characteristic feature of orthoclase is the excellent zonal growth commonly present. Several uncrackecl orthoclase phenocrysts inclose fractured crystals of plagioclase, and these cracks appear to be of intratelluric origin and older than the solidification of 

the orthoclase host. The feldspars are more or less altered to kaolin or sericite, quartz, and 

·calcite. Biotite plates, many of them considerably eaten by the magma, are in general altered 

to chlorite, muscovite, calcite, and rutile or an ill-defined leucoxenc-like substance. The 

biotite plates were in some places bent prior to the final solidification of the magma. Sparse 

phenocrysts of a brown hornblende, zonally grown, are present in a few thin sections, and 

green hornblende, either fresh or partially or wholly altered to epidote, is rather abundant in 

another. Although hornblende thus varies in color, the rocks are identical except for the 

association of titanite with the green hornblende. Mosaics of calcite and magnetite or limonite, 

its decomposition product, possess typical hornblende outlines in still other thin sections. 

a Cross, Whitman, Mon. U. S. Gcol. Survey, vol. 27, 1896, pp. 34, 202. 
74 GEOLOGY OF THE GEORGETOWN QUADR~NGLE, COLORADO, 

Quartz where present occurs as rounded grains or in ragged individuals deeply embayed by 

magmatic corrosion. Prior to the solidification of the groundmass the quartz phenocrysts 

were strained and either undulatory ·extinction or fractures were developed in them. During 

the solidification of the groundmass the older quartz of the phenocrysts controlled the crystal 

orientation of the younger quartz of the groundmass, which joined itself to the older quartz 

in optical continuity. The perfection of the orientation is beautifully shown in some specimens. 

A sharp line, coextensive with the older quartz individuals, separates the areas of differently 

oriented younger quartz. In consequence of this process an aureole of groundmass around the 

phenocrysts is unusually siliceous. Short columns of apatite and crystals and grains of magnetite, in part magmatically resorbed, are constant accessories which are locally included in 

the phenocrysts. Some of the magnetite is probably titaniferous. Titanite, commonly 

twinned, much of it polysynthetically, is an abundant accessory in about one-half of the thin 

sections and here and there is as large as any of the phenocrysts. It is rarely fresh and in some 

places is pseudomorphed by calcite, rutile, and chalcedony. Zircon, in tiny crystals, in one 

specimen inclosed in apatite, is present in several thin sections. 

Thin sections cut from the contact of the biotite-sillimanite schist and this porphyry 

show that the older quartz of the schist controlled the orientation of the quartz of the porphyry 

during its consolidation. In consequence many small grains of the porphyry groundmass are 

in optical orientation with a single older and larger quartz grain. This phenomenon, analogous 

to the quartz aureole around the embayed phenocrysts, is also comparable to the secondary 

enlargement of quartz grains in quartzites, although in the former case the younger quartz 

solidified from magmatic and in the latter from aqueous solutions. 

The quartz monzonite associated with the porphyry is a fine-grained granular rock of 

brownish-gray color, composed predominantly of feldspar individuals with tiny interstitial 

areas or columns of a black mineral. The rather smooth weathered surface, on which some 

colorless quartz is visible, is white flecked with black. Cutting the granular rock with rather 

sharp borders are dikes 1 inch thick, which in the field appear to be aplitic. · 

The microscope shows that the texture of the monzonite is allotriomorphic and rather 

uneven grained. The feldspars, orthoclase and oligoclase, greatly predominate over augite, 

hornblende, and quartz. Orthoclase slightly predominates over. plagioclase and occurs in 

rudely tabular and irregular masses, many of which are twinned according to the Carlsbad 

law. Associated with it is apparently some anorthoclase. Oligoclase forms elongated tabular 

masses, which in many places exhibit zonal growth. Some feldspar individuals are slightly 

altered to kaolin and sericite. Augite occurs in grains and rude prisms which are twinned 

parallel to 100. Hornblende, green or brown, is closely associated or intergrown with augite. 

Quartz occurs in wedgelike forms between the other constituents. The accessory minerals 

include biotite, titaniferous magnetite or ilmenite, titanite, apatite, and zircon. The composition of this rock is such that it might well be a granular facies of the quartz monzonite 

porphyry. 

The aplitic rock one-eighth inch from the monzonite is a fine-grained allotriomorphic 

mosaic of granitic composition formed of quartz and orthoclase with a little oligoclase. Magnetite, titanite, and zircon are present as sparse accessory minerals. Toward the monzonite 

orthoclase gradually increases in granularity and incloses the quartz micropoikilitically. This 

zone grades along a rather indefinite plane into one in which quartz ramifies micropegmatitically through ·orthoclase, and these quartz tabules extend well into the monzonite, which 

gradually takes on hornblende and augite and grains of plagioclase. It appears that the aplite 

and the monzonite in part were contemporaneous. The monzonite appears to have solidified 

except on certain planes along which th~ residual magma collected· or into which residual 

magma was intruded. This residuum of the magma was rich in quartz and orthoclase and 

poor in plagioclase and ferromagnesian minerals. Solidification started from the monzonite, 

then apparently continued without interruption until both the monzonite and the aplitic rock 

were consolidated. 
GENERAL GEOLOGY. 75 

DACITE. 

DEFINITION. 

By the term dacite is meant a porphyritic igneous rock composed of soda-lime feldspar 

and quartz and containing in inany places one or more of the minerals biotite, hornblende, and 

augite. It is in consequence the porphyritic equivalent of quartz diorite and occurs either as 

dikes, sheets, or flows. 

DISTRIBUTION AND OCCURRENCE. 

Dacite forms an east-west dike 50 feet wide and 0.4 mile long, located 0.4 mile south of 

Georgetown. The dike is practically vertical and has a very irregular contact with the preCambrian rocks on the walls of the canyon of South Clear Creek (fig. 21). Large and irregular 

fragments of the pre-Cambrian rocks are included in the dacite near its borders, while the 

inclusions near the center of the dike are small and round, indicating resorption by the dacitic 

magma. ·The dacite shows wavy flow lines near its contact with the pre-Cambrian rocks, and 

there the phenocrysts have common flow orientation. Near the border of the dike hexagonal 

fractures at right angles to the border were produced during the solidification of the. rock. The 

· dike is also traversed by many joints of later origin. The dacite, while apparently poorer in 

alkali feldspar than the quartz monzonite porphyry, may well be a variant of that magma. 

PETROGRAPHY. 

The dacite is bluish gray, dense, and aphanitic, and has a conchoidal fracture. In the 

groundmass are abundant small laths 

of feldspar, columns of black hornblende, and tiny rounded grains of 

black quartz. 

Under the microscope the groundmass is seen to be either felsitic or 

pilotaxitic. Feldspars are the most 

abundant phenocrysts, but they are 

too much replaced by sericite, calcite, 

and kaolin to determine their exact 

composition, although from their lathlike shape they are without doubt 

largely plagioclase. A single phenocryst-like area of microcline is present 

in one thin section, but this is prob-

..,_------IOOf'eet -------"* 

FIG. 21.-Dacite dike (a), showing inclusions of Silver Plume granite (b). East of 

South Clear Creek canyon, one-half mile south of Georgetown. 

ably a tiny fragment torn from the older rocks. Much corroded quartz occurs in every thin 

section examined. Green hornblende has a narrow rim of magnetite. It is partially altered 

to epidote, zoisite, chlorite, and calcite, or is pseudomorphed by serpentine and calcite or by 

chlorite, serpentine, and calcite. Biotite, titaniferous magnetite, and apatite are accessory 

minerals. 

GHANITE POHPIJYRY. 

DISTRIBUTION AND OCCURRENCE. 

Intrusive bodies of granite porphyry and the associated rhyolite and aporhyolite occur in 

three areas in the northwest corner of the quadrangle. One of these, a stock 0.3 mile in diameter, is situated 1! miles east of south of Paines Mountain and is the most southerly porphyry 

mass in the quadrangle; a second is on Pendleton Mountain near the mouth of Leavenworth 

Creek, where there are a number of dikes; and a third is in the vicinity of Lincoln .Mountain, 

where a number of dikes also occur. 
76 GEOLOGY OF THE GEORGETOWN QUADRANGLE, COLORADO, 

PETROGRAPHY. 

The granite porphyry when fresh is gray and in general has a dense and aphanitic, although 

rarely :finely granular groundmass. The medium-sized to rather large phenocrysts. are very 

abundant and equal or surpass the groundmass in bulk. Of th~ phenocrysts feldspar is more 

abundant than biotite, which in turn predominates over quartz. Few of the feldspar phenocrysts 

are over one-fourth inch long and as a rule they are considerably altered. Rounded grains or 

crystals of slightly to deeply smoky quartz are commonly somewhat larger than the feldspar 

phenocrysts. Black hexagonal plates of biotite, here and there rounded by magmatic corrosion, 

are abundant, although in the main less than 0.1 inch in diameter. 

The weathered specimens are chalky, pitted by feldspar casts, roughened by the protusion 

of quartz phenocrysts, and heavily stained by reddish or brownish iron oxides. The biotite 

phe.nocrysts are altered to muscovite. 

Microscopic examination shows that the groundmass of the granite porphyry is allotriomorphic and microgranitic, quartz and orthoclase grains in equal bulk forming a fine-grained 

mosaic. In places laths of plagioclase occur with the quartz and orthoclase grains. The 

feldspars of. the groundmass are considerably altered to sericite, kaolin, chalcedony, and calcite, 

and flocculent stains of limonite are almost constantly present. 

Orthoclase, the predominant phenocryst, occurs as simple crystals and Carlsbad twins. 

Plagioclase,. mostly oligoclase but in part oligoclase-albite, is more commonly in compound 

intergrowths than orthoclase. Pericline twins are usual; Carlsbad twins are less common and 

in places zonal growth is slightly developed. Both orthoclase and plagioclase phenocrysts are 

much altered; in some thin sections they arc pscudomorphed by sericite and in others by 

chalcedony and calcite. Kaolin is a less common alteration product. Quartz phenocrysts are 

in general magmatically corroded. Fluid inclusions are present in many specimens. During 

the final solidification of the rock the quartz was fractured and an undulatory extinction 

developed in some phenocrysts. The quartz phenocrysts control the orientation of a thin band 

of the quartz grains of the groundmass which arc in optical continuity with it, and further 

drew from the surrounding magma an unusually large amount of siliceous matter, in consequence of which the groundmass around the phenocrysts is more acidic than that at a distance 

from them. 

Biotite phenocrysts arc either hexagonal plates or rounded areas, deeply embaycd by magmatic corrosion, which inclose irregular masses of groundmass. In many places biotite was 

bent prior to the final solidification of the mass. Biotite is in general pscudomorphed by 

muscovite accompanied by rutile, limonite, and locally calcite, and is unaltered only where 

calcite and chttlcedony unaccompanied by sericite are the alteration products of the feldspars. 

Hexagonal columns and rounded grains of apatite, the largest of which arc 0.25 mm. long, 

are constantly present, and are in some places rather abundant. Zircon in small crystals is 

also a constant accessory mineral, while magnetite crystals and rounded grains, magmatically 

absorbed, are less commonly present. The magnetite is probably somewhat titanifcrous. 

Rhyolites and· aporhyolites arc closely associated with granite porphyry and in places 

grade into them. In the Marshall tunnel of the Colorado Central mine the greater portion of 

the main dike is greenish-gray granite porphyry, but the border facies is a glass of black, 

greenish-gray, or rich yellowish-brown color. The rhyolite grades rather sharply into the 

granite porphyry and occurs in lenses from 10 to 100 feet long and from 1 inch to 4 feet wide. 

Locally it shows well-developed cooling joints, closely spaced, at i·ight angles to the dike. The 

groundmass of this rock is a fresh colorless glass, in which the gas cavities are arranged in 

flow lines. In other rhyolites of the series the groundmass is devitrified and is composed of a 

mosaic of irregular or more or less perfectly hexagonal units with shadowy extinction. Glass 

inclusions replace the fiuid inclusions of the quartz of the granite porphyry, and in a single 

thin section the orthoclase phenocrysts possess the sanidine parting. Quartz in one thin 

section shows a rather distinct cleavage parallel to a pyramid. A tabular crystal of orthoclase 

is surrounded by a crown of irregular orthoclase individuals of different orientation. In each 

orthoclase individual are micrographic characters and ramifying tubclets of quartz which in a 
GENERAL GEOLOGY. 77 

single individual are of common orientation. Similar micrographic phenocrysts in rhyolite 

have been described by Iddings from the Yellowstone National Park" and from Eureka, Nev.b 

In the aporhyolite from a dike on the Pendleton Mountain ridges several microcline phenocrysts 

are associated with those of orthoclase. 

ALASKITE PORPHYRY (FERROMAGNgSIAN-POOR GRANITE POltPHYRY). 

DEFINITION. 

Alaskite porphyry is a porphyritic holocrystalline rock composed essentially of quartz 

and orthoclase, being the porphyritic equivalent of alaskite, a type of siliceous granitoid rocks 

first differentiated by Spurr." 

DISTRIBUTION AND OCCURRENCE. 

Alaskite porphyry occurs as dikes 'in the vicinity of Silver Plume and on either side of 

Clear Creek north of Georgetown. Two stocks, the largest of which is approximately 0.3 mile 

in diameter, outcrop on the south side of Leavenworth Creek ncar the west border of the 

quadrangle. 

PETROGRAPHY. 

The alaskite porphyry when fresh is dull white or light gray, with dense, aphanitic, rarely 

flinty grounclmass. The large phenocrysts are scarce to fairly abundant, but are uniformly very 

subordinate to the grounclmass in bulk. Of these, feldspars predominate over quartz, which in turn predominates 

over muscovite and much altered biotite, locally present. 

The last three minerals are absent in some of the rock. 

The orthoclase and plagioclase phenocrysts, which range 

in length from one-sixteenth to three-eighths of an inch, 

are in the main opaque through alteration. Quartz of like 

s~ze forms light-gray transparent phenocrysts with dulled 

vitreous surfaces. Muscovite, present in some specimens, 

occurs in hexagonal plates or areas rounded by resorption 

which range from one-sixteenth to one-fourth inch in 

FIG. 22.~Sketch showing intense magmatic corrosion of an orthoclase phenocryst (a) in alaskite 

prophyry (b). X 20. 

diameter. The muscovite is generally silvery white or gray in color, but some of it is almost 

black, and resembles biotite in the hand specimen. Ill-defined, much-altered biotite phenocrysts are also present here and there. 

Messrs. Spurr and Gan·ey report that in the Baltimore tunnel flow banding and consequently ready parting is well developed in an alaskite porphyry. Discontinuous wavy bands 

of green, gray, brown, and white rock, varying in texture from dense to scmivitreous, open, 

'curve, and close around the quartz, feldspar, and biotite phenocrysts. 

The weathered surface of the alaskite porphyry is as a rule slightly glazed, pitted by the 

removal of the feldspar phenocrysts, and roughened by the protrusion of the quartz phenocrysts. On advanced alteration the rock is reduced to a chalky mass. Limonite stains on 

joint surfaces are apparently derived from the surrounding formations. Dendrites of manganese 

dioxide also occur along joints and in some places penetrate the rock near them. 

As seen under the microscope the groundmass is in the main allotriomorphic and microgranitic, being composed of a mosaic of orthoclase and quartz grains which vary in diameter 

from 0.02 to 0.05 mm .. Sericite is a constant and in some places a very abundant alteration 

product of the feldspar grains; caJcite and chalcedony arc less constantly.present. 

Orthoclase phenocrysts are here and there intensely corroded magmatically (fig. 22), 

detached areas of orthoclase lying in the groundmass and islands of groundmass being inclosed 

in orthoclase. During the flow of the magma prior to solidification some of the phenocrysts 

were broken and cracks and undulose extinction locally present were probably produced at 

a Iddings, J. P., Seventh Ann. Rcpt. U. S. Geol. Survey, 1888, pp. 274-2iG. 

b Mon. U. S. Geol. Survey, vol. 20, 1892, p. 37.5. 

c Spurr, J. E., Am. Geologist, \"ol. 2.'>, 1900, p. 229. 
78 GEOLOGY OF THE GEORGETOWN QUADRANGLE, COLORADO. 

the same time. Plagioclase (albite-oligoclase) forms short lath-shaped phenocrysts \rhich are 

less abundant than those of orthoclase. The feldspar phenocrysts are in many places wholly 

and everwhere partially sericitized. Much of the sericite is accompanied by chalcedony and 

some chalcedony pseudomorphs of feldspar are present. 

Quartz phenocrysts, many of which are intensely corroded, are as a rule formed of a single 

grain of quartz, although some of them consist of an aggregate of several grains diversely 

oriented. Partially filled fluid inclusions are locally present. A narrow aureole of the quartz 

of the groundmass is ordinarily in optical continuity with the quartz phenocrysts. 'During 

the flow prior to the final solidification of the magma the quartz phenocrysts suffered the same 

deformation as the orthoclase. 

The rarity of muscovi,te in granite porphyries makes its presence as an original phenocryst 

in a number of the rocks of considerable interest. Hexagonal plates with good crystal boundaries are rare, but ellipsoidal area.s rounded by magmatic corrosion are characteristic. Some 

of the plates were slightly bent prior to the final solidification of the magma. In a few slides 

small pseudoinorphs of muscovite and limonite, with or without rutile, occur, and these are 

evidently much-altered. biotite phenocrysts. Zircon granules, locally inclosed in the phenocrysts are present in the majority of the slides. 

BOSTONITE AND BOSTONITE PORPHYRY. 

DEFINITION. 

Bostonite is a light-colored holocrystalline rock of trachytic texture which is composed 

essentially of sodium-rich feldspars and is characteristically poor in ferromagnesian minerals. 

If such a rock contains phenocrysts, it is called a bostonite porphyry. 

DISTRIBUTION AND GENERAL STRUCTURE 

Bostonite and bostonite porphyry form numerous dikes in the northern part of the quadrangle east of Saxon Mountain. It is the most widely distributed of the intrusive porphyries, 

and it forms the longest and most persistent dikes. A small stock already mentioned (see 

p. 72) occurs on Chicago Creek 0.2 mile northeast of the mouth of King Solomon Gulch. 

Bostonite rarely contains inclusions of older rock. 

PETROGRAPHY. 

The bostonite is, when fresh, a lilac or pinkish-gray, dense, finely granular rock, which to 

the naked eye appears to be composed wholly of feldspar laths. A number of the dikes near 

the mouth of Little Bear Creek are of this phenocryst-lacking variety. From bostonite this 

rock grades into bostonite porphyry, in which the phenocrysts in some specimens arc equal· 

in volume to the groundmass. The phenocrysts, chiefly alkali feldspar and plagioclase, are in 

some places accompanied by pyroxene pseudomorphs. The feldspars are in general much 

altered, the alkali feldspars having compound tabular or rhombic forms and the plagioclase 

being lath-shaped. They range in size from tiny crystals to those 1 inch in diameter. In the 

alkali feldspars weathering brings out a zonal growth. In many bostonite porphyries the 

feldspar phenocrysts are small or absent within 4 inches of the border· of the dikes. The 

smaller size is in part due to the growth of the phenocrysts near the center of the dikes, where 

cooling was slower than on the borders, by addition of feldspar from the groundmass. (See pp. 

94-95.) The lack of phenocrysts is, however, not easily explained, for they are plainly of 

int.ratelluric origin, as is shown by the fact that in some places both pyroxene and feldspar 

phenocrysts have an orientation on the border of the dike parallel to its extension. Pseudomorphs resemble in form pyroxene and vary in length from one-fourth to 1 inch. Penetration 

twins, in whicl{ the individuals join one another at an angle of about 60°, are common. 

These columns are a light to blackish green, slightly fibrous substance resembling serpentine. 
GENERAL GEOLOGY. 79 

Under the mieroscope bostonite proves to have a coarse or. fine trachytic pr trachytoid 

texture. Anorthoclase laths which vary in length from 0.1 to 1.25 mm. have in the trachytic 

groundmass a highly developed flow arrangement. Between the anorthoclase crystals lie 

wedges and irregular equidimensional masses of orthoclase and quartz, locally with magnetite, 

and each of these minerals clearly solidified after the anorthoclase laths. Some of the wedges, 

particularly quartz, have common orientation over considerable areas, in which case the texture is both trachytic and ophitic. The irregular masses of quartz an(i orthoclase, on the 

other hand, micropoikilitically inclose anorthoclase laths, the texture being trachytic and 

micropoikilitic. In the trachytoid structure flow orientation of the anorthoclase laths is 

poorly developed, and they penetrate one another, many of them forming six-armed crosses. 

Here and there pyrite metasomatically replaces the feldspars. 

The textures of the bostonite porphyry are those of the bostonites, except that quartz, 

micropoikilitically inclosing anorthoclase and orthoclase, is in many places particularly prominent 

in the center of areas of groundmass between the phenocrysts. The orthoclase laths are dammed 

against the phenocrysts and then sweep around them in flowing curves. The predominant 

phenocrysts are orthoclase, which where small are simple crystals and where large are, as a 

rule, complex. The borders are in general slightly roughened by the addition of orthoclase 

from the groundmass in similar optical orientation. In some places a few anorthoclase laths 

a~;e inclosed in this younger border. Anorthoclase and oligoclase phenocrysts are also sparingly 

present. The feldspar phenocrysts are rounded oy magmatic corrosion, a:qd rarely areas of 

groundmass are included in the phenocrysts. 

Prior to solidification of the groundmass some of the feldspar phenocrysts were broken or 

cracked and an undulose extinction developed in them. Locally feldspar is completely altered 

to kaolin and to sericite, which is very commonly accompanied by quartz or chalcedony and 

only rarely by calcite. The forms resembling pyroxene are composed of either serpentine or 

talc, with or without quartz, magnetite, orthoclase, epidote, and zoisite. A leucoxene-like 

substance is also present in some specimens, indicating that the pyroxene was titaniferous or 

that magnetite, which in some places rimmed it, contained some titanium. A few sparse 

titaniferous magnetite cubes are of intratelluric origin, as are also stocky columns of apatite, 

which are confined to the porphyritic bostonites. Pseudomorphs of titanite occur only in 

the pyroxene-bearing varieties. These are composed of chalcedony or quartz with a rather 

indefinite leucoxene-like material that in a single thin section is certainly rutile. Small 

crystals of zircon have been observed in a single thin section. Fluorite occurs in bostonite 

dikes 0.2 mile S. 15° E. of the mouth of Little Bear Creek, 1.5 miles east of Freeland, and at 

other places. In thin sections from the Freeland locality it is very abundant in wedges between 

the other constituents of the groundmass and in irregular patches replacing feldspar. The 

latter occurrence is certainly of secondary origin, but the former has every appearance of 

being original, although the scanty occurrence of this mineral in the series is rather against 

this view. The purple coloring is either in clots or in bands roughly parallel to the boundary 

of the areas. 

By various processes of alteration the bostonite and bostonite porphyry undergo .a variety 

of color changes, usually accompanied by loss of density. Through sericitization the rock 

becomes a greenish white; through calcitization and kaolinization it bleaches; and through 

the hydration of its i:ron content and the introduction from without of iron oxides it takes 

on various shades of yellow, pink, red, and purple. Black stains of manganese dioxide occur 

particularly on the feldspar phenocrysts, a phenomenon also noted in the alaskitic quartz 

monzonite porphyry. In many places weathered surfaces of the bostonite porphyry are porous 

through the removal of the phenocrysts. 
80 GEOLOGY OF THE GEORGETOWN QUADRANGLE, COLORADO. 

ALASKITIC QUARTZ MONZONITE PORPHYRY (FERROMAGNESIAN-POOR QUARTZ MONZONITE 

POUPHYRY). 

DEFINITION. 

Where biotite, amphibole, and pyroxene are only sparingly present in a quartz monzonite 

porphyry (see pp. 73-74) the rock may be called an alaskitic quartz monzonite porphyry. 

DIS'l'RIBUTION AND OCCURRENCE. 

This rock occurs in dikes which are confined to the northeastern portion of the quadrangle 

and are particularly abundant near Freeland, on the tributaries of Soda Creek, and south of 

Alps Mountain. The dikes of the latter locality have a rude east-west alignment and may 

well be segments of a single dike which at some distance beneath the surface are connected 

with ohe another. 

PETROGRAPIIY. 

The alaskitic quartz monzonite porphyry is a white or light-gray rock with dense and 

flinty or finely granular groundmass. The more flinty facies have a conchoidal fracture. 

Phenocrysts of feldspar as a rule are sparse, but rarely are so abundant that they equal the 

groundmass in bulk. In general both orthoclase and plagioclase are altered. The phenocrysts, most of which are one-eighth of an inch long, reach a maximum length of three-eighths 

of an inch. Sparse quartz phenocrysts are present in the rock of some dikes. (See p. 184.) 

Most of the weathered outcrops have a smooth surface pitted by the removal of the feldspar phenocrysts. Since few, if any, iron-bearing minerals are present in the rock, weathered 

exposures are stained only a lemon yellow by iron salts. Manganese-dioxide stains are in many 

places localized on the feldspar phenocrysts, probably because feldspar is favorable either 

physically or chemically to the precipitation of manganese dioxide from aqueous solutions. 

The residual bowlders are as a rule blocks sharply bounded by joints. 

In the majority of thin sections examined the groundmass, composed of orthoclase, quartz, 

and somewhat less oligoclase, is holocrystalline. Oligoclase forms laths, orthoclase forms tablets and irregular areas, and quartz forms irregular areas, many of which are of considerable 

size, inclosing the other constituents poikilitically, a feature less characteristic of the irregular 

orthoclase areas. The groundmass in some thin sections, however, is seen to be formed of 

irregular or hexagonal areas of low birefringence, possessing slightly wavy extinction. This 

groundmass suggests a devitrified glass, and some members of the series may originally have 

had a glassy groundmass and, in consequence, be altered latites. Phenocrysts of orthoclase 

and oligoclase are in general equally abundant. The orthoclase phenocrysts, most of which 

are compound, possess in many specimens beautifully developed zonal growth. Oligoclase 

phenocrysts are in the main smaller and more commonly simple than are those of orthoclase. 

Twinning according to the Carlsbad and pericline laws is locally present. The feldspars of the 

rock vary from those that are practically fresh to others completely altered to kaolin or sericite. 

Chalcedony and quartz with sericite form feldspar psuedomorphs in several thin sections. Quartz 

where present is similar to that of the alaskite porphyry. In two thin sections small shreds 

of muscovite heavily stained by limonite probably represent original biotite phenocrysts. 

Titanite cryst:iJs reaching a maximum length of 2 mm. and showing in many places reentrant 

angles due to twinning are present in most thin sections. As a rule titanite is altered to rutile 

and chalcedony, the former mineral occurring in stocky columns and geniculated twins 

bordering the latter. This alteration is present in a rock otherwise so fresh that it is perhaps 

more logically to be considered a change which took place prior to the final solidification 

of the groundmass. Rather large apatite columns are present in the majority of thin sections. 

Magnetite, although fairly abundant in certain sections, is not characteristic of the series. 

Zirco:1 is more rare. 
GENERAL G EO LOGY. 81 

BIOTITE LATITE. 

DEFINITION. 

Latite is the lava equivalent of granular monzonite. The prefix biotite indicates that 

biotite is the characteristic ferromagnesian mineral present. 

DISTRIBUTION AND OCCURRENCE. 

Biotite latite dikes are confined to the valleys of Chicago and West Chicago creeks, from 

King Solomon Gulch southwestward to a point 1.3 miles above the junction of the creeks. 

The dikes are short and from 5 to 10 feet wide, and with one probable exception have a strike 

approximately northeast and southwest. This, together with the linear arrangement of the 

dikes, suggests that they may be segments of a single dike continuous at depths beneath the 

surface. The latite at the junction of the Ohicago creeks contains a few rounded inclusions 

of quartz monzonite from 4 to 6 inches in diameter. 

PETROGRAPHY. 

The biotite latite is a yellowish-brown or gray rock with slightly greenish or pinkish tinge. 

The groundmass is in general dense and aphanitic, although contact phases are locally a black 

glass with g1:eenish tinge. 'l'hin flakes of this phase, forrning a band several inches wide on the 

northwest side of the dike at the mouth of Eclipse Gulch, are semitransparent. · Rarely weathering reveals in this phase indistinct lines of flow. The groundmass predominates largely over 

the phenocrysts in bulk, although small phenocrysts of biotite and feldspar in equal abundance 

are very numerous. Most of the feldspar phenocrysts, both tabular orthoclase and short lathlike 

plagioclase are 0"05 inch in length, although a few reach 0.2 inch. Biotite occurs in hexagonal 

plates from 0.05 to 0.4 inch in diameter, 0.1 inch being the average size. As a.rule the crystals, 

which are locally rounded and embayed by magmatic corrosion, are very thin, being composed 

of but a few cleavage folia. 

The latite is well jointed and on weathering breaks clown into small joint blocks. Dendrites 

of manganese dioxide and ·concentric rings of limonite are present here and there on joint 

surfaces. The weathered surface which, when unstained by iron salts, is bluish gray or greenish 

white in color, is rather smooth, although slightly pittci:l by the removal of feldspar phenocrysts. 

The latite readily disintegrates to a bluish-gray clayey substance. 

Under the microscope the hyalopilitic groundmass, with flow structure in some thin sections 

and without it in others, is a clear light-brown glass, in places slightly devitrified. In it are 

microlites of lath-shaped plagioclase (labradorite), tabular orthoclase, and in some thin sections 

shrcdlike biotite. A thin section from the border of the dike shows chalcedony and a little 

chlorite filling vesicular cavities. The phenocrysts, named in the ·order of abundance, are 

orthoclase, biotite, and plagiocla~e, with brown hornblende in some slides. Most of the orthoclase phenocrysts are simple, but a few are complex aggregates. Some of them show slight 

magmatic corrosion and zonal growth is rather characteristic of ooth generations of orthoclase 

crystals. Some of the phenocrysts are twinned according to the Carlsbaclla,v. The plagioclase 

phenocrysts (oligoclase) are lath-shaped and are locally twinned according to the Carlsbad law. 

Here and there biotite in crystals and in embayed individuals is partly or completely inclosed 

in the feldspars. Flexing fracture of biotite occurred prior to the final solidification of the 

rock. The accessory minerals present are magnetite, apatite, and zircon, abundant in the 

order named.· All the constituents of the rock are remarkably fresh. 

ALKALI SYENITE PORPHYRY. 

DEFINITION. 

Alkali syenite is a granular igneous rock composed essentially of an alkali feldspar or 

feldspars and i~ general either an alkali pyroxene or amphibole with or without other alkali-

:tich minerals. Aporphyritic rock ofthis composition i~ called an alkali syenite porphyry. 

31401-No. 63-08-6 
82 GEOLOGY OF THE GEORGETOWN QUADRANGLE, COLORADO. 

DISTRIBUTION AND OCCURRENCE. 

On the south side of Clear Creek, cast of the mouth of Soda Creek, a small mass, presumably a stock, of alkali syenite porphyry, with a surface exposure 0.15 mile in diameter, intrudes 

the pre-Cambrian rocks. The alkali syenite porphyry at its contact with the pre-Cambrian 

rocks contains many inclusions of the Idaho Springs formation. A dike 0.2 mile long and 8 

feet wide of rather similar rock occurs southeast of the main mass. It courses northwest and 

dips 60° NE. The mineral springs of Idaho Springs come either from this rock or from its 

contact with the pre-Cambrian rocks, and it is believed that the water derives its high sodium 

content from the sodium-rich porphyry. The heat of the .water of some of the springs is 

perhaps genetically connected with the after effects of volcanism. 

PBTROGRAPHY. 

The alkali syenite porphyry varies considerably in texture throughout the main mass 

described above. The most widely distributed facies is a light bluish-gray porphyritic rock 

in which the groundmass is subordinate to the abundant a.ncl rather large phenocrysts. The 

grounclmass is made up of tiny individuals of feldspar. The most striking phenocrysts are 

velvet or greenish-black octagonal prisms, presumably pyroxene, and of these the largest are 

one-eighth inch in length. Equally striking but somewhat less abundant are brownish-black 

garnet crystals with metallic luster, which in some exposures are three-eighths of an inch 

in diameter. Less striking but more abundant than either pyroxene or garnet are glassy, 

whitish-gray, unstriated feldspar phenocrysts. These tabular crystals vary in length from oneeighth to five-eighths of an inch. Small wedges of clove-brown titanite and octahedrons of 

magnetite are also visible to the naked eye. The distribution of pyroxene and garnet is uneven 

throughout the mass and both may be scarce or abundant or either may be present to the 

almost t.otal exelusion of the other. This type appears to grade into a ro.ck with light greenishgray holocrystalline grounclmass in which are embedded many large pink feldspar crystals 

reaching a maximum length of 4 inches. On weathering the rock first bleaches and then is 

stained yellow, pink, brown, or purple by iron oxides. The weathered surface is smooth but 

pitted by the partial or complete removal of pyroxene and garnet. 

As seen under the microscope, in some thin sections the microgranitic groundmass almost 

equals the phenocrysts in bulk, while in others it consists of small areas between the greatly 

predominant phenocrysts. The groundmass is composed chiefly of irregular masses of anorthoclase and some orthoclase, quartz also being present in some thin sections. The pyroxene 

phenocrysts are mgirite-augite, many of them being zonally built, with regirite predominant 

on the border. Although some, as already mentioned, are partial crystals, others are little 

more than rounded grains, some of which are inelosed in anorthoclase and melanitc. Twinning 

parallel to 100, often repeated, is common. The regirite-augite is in general fresh, but in a 

single thin section is altered to serpentine and an iron ore.. The garnet, a dark-brown species 

with faint zonal growth, is apparently mclanite. Some of the larger crystals contain other 

phenocrysts as inclusions, and the smaller, in particular, are slightly cmbayed by magmatic 

corrosion. The anorthoclase and fewer orthoclase phenocrysts arc locally twinned according 

to the Carlsbad law. Rarely these crystals have as centers laths of oligoclase. The feldspars 

are slightly altered to sericite and kaolin. Wedge-shaped crystals and irregular grains of titanite, 

octahedrons of magnetite, and short crystals of apatite are present as abundant accessories. 

In many specimens titanite is twinned, some of it pol:ysynthetically. Very small crystals, 

probably zircon, and biotite shreds are present in a single slide. · 

Fluorite is present in irregular ramifying areas in the groundmass and in small areas flecking feldspar or regirite-augite. Fluorite was probably introduc~d after the solidification of the 

rock, possibly by gases, and in part fills cavities between the minerals and in part replaces 

them. Analcite occurs in similar small areas, which are, however, more largely confined to 

the groundmass. Since it occurs in thin sections of very fresh rocks, it -is perhaps an original 
GENERAL GEOLOGY. 83 

mineral, although it may be of secondary origin. The small percentage of S03 in the chemical 

analysis indicates the presence of haiiynite, or nosean, although neither mineral was recognized 

in the thin sections. 

George Steiger made the following analysis of a specimen of the type rock: 

Analysis of alkali syenite porphyry Jrmn south side of Clear Creek, 11ear Soda Creek. 

Si02 ••••••.•.••••••••••.••••••••••••••••.••..•• 

Al2 0 3 •.•••....•••••••••••••.•.•••...••••••••.•• 

Fe20 0 ••.•.••••.•••.•.•••••••••••••••••••••••.•• 

60. 30 Zr02 ••••.•••••••.••••••••• • . • • • • • • • • • • • • • • • • • • • • 0. 01 

18.12 C02 .••••••••••.•..••• _ .•••••••••.••.•••••••••• None. 

2. 45 P20 5 ••.••....•••••••.•••••••••.•.. :............. • 25 

FeO ...... : ....... --- .......................... . 1. 25 S03 .. __ . _________ . __ .. _ ..•• _ ••.•.••.••••••• _ _ _ _ • o6 

MgO ............. ---------············-········ . 28 S .............................................. None. 

CaO .......................................... . 3. 89 MnO .......... _................................. .12 

Na20 ......................................... . 5. 83 BaO............................................ . 26 

K 20 ........................................... . 5. 01 SrO. . . .. .. .. .. .. .. . .. . . . . . .. . . . . . . . . . . . . Not looked for. 

£20 ......................................... · .. . . 75 F.............................................. None. 

n;o+------------------------·-----------------

Tio" ____ ............... -.----- -----------------

.77 

. 55 99.90 

According to the elassification of igneous rocks proposed by Cross, Iddings, Pirsson, and 

Washington,a the alkali-soda syenite is pulaskose. It is analogous to a bostonite from Burlington, described by Kemp and Marsters; b a pulaskite from Fourche Mountain, Arkansas, 

described by Washington;c a quartz syenite from tlie Highwood Mountains, Montana, described 

by vVeed and Pirsson,d and a trachyte from the Rosita Hills, Colorado, described by Cross.e 

Mineralogieally the quartz syenite from the Highwood Mountains is very similar to the alkali 

syenite porphyry, although richer in quartz and without melanite. The trachyte described by 

Cross from the Rosita Hills, Custer County, 115 miles from Idaho Springs, is of very similar 

chemical, although of widely variant mineral composition. Cross I considers the trachyte of 

Eocene or Miocene age. · 

QUATERNARY DEPOSITS. 

GENERAL OUTLINE. 

The Pleistocene history of the Georgetown quadrangle has heen complex arid has included 

the deposition of river-terrace gravels, glacial drift, and alluvium. The earliest event recognized was the deposition of the 180-foot and 55-foot terraces. Then followed the earlier glacial 

epoch, after which. the 25-foot terrace was formed. 'fhe later period of glaciation, acc'ompanied by the deposition of debris sheets in the nonglaciated valley heads, followed, and in 

comparatively recent time travertine, alluvium, landslide material, and talus ha~e been 

deposited. 

TEURACE GUA VELS. 

In the vicinity of Idaho Springs three sets of terrace grave:ls, now eroded into a number 

of separate ·areas, mark three distinct stages in the downeutting of the valleys of Clear Creek 

and its tributary, Chicago Creek. These terraces are striking topographic features. (See Pl. 

XIII.) The gravels are predominantly granitic, since the pre-Camhrian schists are not resistant 

to stream corrasion. The gravels contain gold and were formerly actively·worked, in fact, 

they are now being mined just east of Idaho Springs. 

n Classification of Igneous Rocks, Chicago, 190~-

b Kemp, J, F .. nnd Marsters, V. F., BulL U.S. GcoL Survey No, lOi, 1893, p. 20. 

<Washington, n. S., Jour. Cleo!., VOL 9,1901, p. 609. 

dWeed, W. H., and Pirsson, L. V., Am. Jour. Sci., 4th ser., voL 1,1896, p. 295. 

<Cross, Whitman, Seventeenth Ann. Rcpt. U. S. GcoL Survey, pt. 2, 1896, p. 324, 

/Idem, p. 273. 
84 GEOLOGY OF THE GEORGETOWN QUADRANGLE, COLORADO. 

The following table gives the elevations of the terrace surface and its bed-rock floor above 

Clear Creek and Idaho. Springs, the average thickness of the gravel upon it, the number of 

remnants, and the approximate total area of each terrace deposit: 

Heigl:t, thickness, and area of Pleistocene river terraces in Georgetown quadrangle. 

----· ·--------- --------------,----

Height of A';,i\~~J-~~~~ht Average Number of 

surface above noor above thickness of remnants. Total urea. Clear Creek. Clear Creek. gravel. 

1-----1--------- ----------- - Feet. r~g Feet. r~g I' . Feet. ~ Acres. 

'------2-5 -'----~20_ ~- ~gJ ___ 4 ___ ~r~ _ There are four remnants of the 180-foot terrace, the floor of which lies from 150 to 160 

feet above Clear Creek. The surface, now gently .r:~urided by erosion, is 180 feet above ,the 

stream. The deposit consists of well-rounded pebbles, cobbles, and bowlders, the largest of 

which are .5 feet in diameter, with some lenses of coarse sand. The bowlders are predominantly granitic and arc comparatively fresh, the few hornblende-gneiss bowlders present being 

considerably decomposed. Portions of the mass are stained red or reddish brown. 

Since the formation of this terrace Clear Creek has sunk a canyon in solid rock 600 feet 

wide and at least 160 feet deep. The size of this canyon, much greater than that of any interglacial canyon, proves that this terrace is preglacial. 

Two remnants of the 55-foot terrace occur on Clear Creek, :mel of these .one extends a 

short distance up Chicago Creek. The bed-rock floor is from 30 to 35 feet _above· the creek, 

and the terrace surface is in most places 55 feet 1\.igh. The deposit is composed of sand and 

bowlders, the largest of which are 4 feet in diameter. This terrace may be either interglacial 

or preglacial. 

The bed-rock floor of the 25-foot terrace, whose remnants are confined to the lower 2.2 

miles of Chicago Creek, is about 20 feet above the creek level at the southern limit of the 

terrace and only 15 feet at the mouth of the stream. The old channel floor is covered by 5 to 

10 feet of gravel. At the cemetery, three-fourths of a mile above the mouth of Chicago Creek, 

the deposits consist of 3 to 5 feet of moderately coarse river gravel grading upward into a bed 

of brown silty sand from 2 to 5 feet thick. The valley train of the later Clear Creek glacier is 

18 feet above Clear Creek, and in consequence this terrace is somewhat older than the later 

glacial epoch. 

Less conspicuous old channels and terrace remnants occur at several elevations between 

the 25-foot and 180-foot terraces. 

GLACIATION. 

Alpine glaciers occupied the heads and upper reach!:)s of the valleys of the Georgetown 

quadrangle during two distinct glacial epochs, separated by a considerable interval. 

EARLIER GLACIAL ~;POCII. 

Glaciers.-The glaciers of the earlier glacial epoch headed in the Continental Divide to the 

west of the Georgetown quadrangle and flowed eastward down Clear Creek and West Fork .of 

Clear Creek (in the Central City quadrangle, to the north) to Empire station. From ·this 

point the united glaciers extended to Dumont, where there are remu"ants of the terminal 

moraine. The presencl'l of patches of till-,on Bard Creek and on South Clear Creek, opposite 

the mouth of Leavenworth Creek, leads to the belief that small lobes of tl).e main glacier 

extended a short distance up these streams. At the west border of the Central City quadrangle 

the glacier extended approximately to the 11 ,200-foot contour. The terminal moraine at 

Dumont is at an altitude of 7,900 feet, showing that the upper surface of the glacier descended 

about 400 feet per mile. Glaciers in the earlier epoch apparently did not head from Mount 

Evans, since if such. were the case earlier till should be found in some of the valleys extending 

to the east and south from that mountain. It is possibl-e, however, that small glaciers existed, 
GENERAL GEOLOGY. 85 

although they must have been less extensive than those of the later glacial epoch. Although 

the valley of South Clear Creek has steep walls 200 to 1,000 feet above the upper limit of later 

glacial erosion, this is believed to be due to sapping by the later glacier and the subsequent 

wedging off of great masses from the cliffs. 

Character and material Qjthe deposits.-The till of the earlier glacial epoch occurs in irregular 

remnants whose glacial topographic features have been in large part destroyed by erosion. In 

a broad way the distribution of these remnants suggests a glacier-like form, and at Dumont the 

terminal moraine, with a minor recessional moraine within, is rather clearly outlined. The 

deposits of the earlier epoch are well drained and without marshes and springs. 

The till is a light-yellowish sandy clay with many bowlders and cobbles of a great variety 

of rocks. The bowlders, the larger of which are 10 feet in diameter, are angular, subangular, 

or less commonly rounded. Bowlders frorn ·freshly exposed till show glacial smoothing and 

suggestions of strire, porphyry bowlders alone being well striated. The rarity of striations is 

partially due to the small development of ground-morainal till. The bowlders lying upon the 

surface of the till are roughened by weathering and somewhat decomposed, particularly those 

of hornblende gneiss. The till is oxidized to a reddish-brown color to depths varying from 3 

to 15 feet. LATER GLACIAL EPOCH. 

Glaciers.-The distribution. of the glaciers of the later glacial epoch has already been · 

described (p. 33). The lowest point (elevation 8,175 feet) reached by a glacier of the second 

epoch is marked by the terminal moraine of the Clear Creek glacier near Empire station, threefourths of 'a mile north of this quadrangle. The following table gives the length and maximum 

width of the glaciers, the altitude of the upper and lower limits of glaciation, the approximate 

maximum depth of the ice, and the· approximate rate of descent of the later glaciers: 

Dimensions, altitude, and slope of later glaciers in Georgetown quadrangle. 

Name. Length. Maximum 

width. 

Present 

altitude of 

upper limit 

of 

glaciation. 

Present 

altitude of 

lower limit 

of 

glaciation. 

Approximate maxi- mum depth 

of ice. 

Approximate rate of descent. 

--~------,----~~----1------ ------·---------1----

Miles. Miles. Feet. Feet. 

Lincoln Lake. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. .. . .. . . . . . . . . 1. 4.1 0. 45 12, 500 10, 900 

South Fork of Deer Creek.. .. . . . . .. .. . . . .. . . . . . . .. . . . . .. . 1. 80 . 70 a. 11, 500 10, 200 

Naylor Lake .............................................. ' 1. 90 . 05 a 12,200 11,200 

West GenevaCreek....................................... 2.10 .90 al0,500 9,675 Deer Creek ......................................... ·....... 2.15 . .40 12,000 10,075 

HeartrackCreek ....................... oo ............ oo ... 2.90 .55 13,200 10,250 

Bard Creek .................... 00 ........ 00.00 .. 00........ 3. 40 . 70 I 12,100 8, 925 

LeavenworthCreek ...... oo .......... oo.oo•Oooooo--···oo·· 4.00 .65 a11,400 9,700 Bear Creek ...... 00........................................ 4. 90 1. 30 13, 400 9, 250 

West Chicago Creek ............... 00 .. 00 ............. :... 5. 35 1. 15 13,400 9, 2751 

UpperEastGenevaCreek ................................ 5.70 1.451 bl3,900 10,850 

Duck Creek............................................... 7.10 .851 b 13.900 9,800 East Chicago Creek ............... 00 .. .. . . . . . . . . . . . . .. . . . 7. 15 1. 30 14, 100 8, 900 

Lake Fork of East Geneva Creek........................ 7. 50 1. 25 I 14,000 9, 300 1 ClearCreek .............................. oo............... 8.00 1.151 a11,500 8,175

1 

Feet. 

400 

700 

700 

600 

400 

GOO 

700 

500 

900 

1,ooo· 

1, 000 

700 

1,200 

1. 500 z; 200 

700 

Feet per mile. 

1,100 

720 

530 

390 

620 

1,020 

930 

42() 

850 

77o 

530 

580 

730 

G3() 

410 

South Clear Creek ............................. -- .......... 480 ·1 8. 90 . GO I b 13; 900 9, 050 

-------------------'-------'----------·-----·---'-----'------

a For those glaciers which headed beyond the limits of the quadrangle, the highest point ot glaciation on the quadrangle border is given as the nominal head of the glacier. Although not accurate, this course is adopted for the reason that the heads of some of the ghwiers have not been located. . bThe glaciers of South Clear Creek, Duck Creek, and Upper East Geneva Creek had a common head. 

From this table it is evident that greater length is, as a rule, accompanied by greater width. 

and maximum depth of ice and a less rate of descent. · 

Erosionalforms.-The topographic forms produced by the glaciers of the later period have 

already been described (p. 33). Glacial groovings and smoothings, in places partially roughened 

by postglacial weathering, are common. Glacial strire, in general parallel to the length of the 

glacier, are particularly abundant near Silver Plume. 

Character and material of the deposits.-The glacial deposits have been but little eroded, 

and except along Clear Creek, where stream erosion has been especially active, the continuity 

of the deposits has not been destroyed. Lateral moraines a.re strikingly developed, but terminal, 

recessional, and ground moraines are incohspicuous. Although the presence here and there 
86 GEOLOGY OF THE GEORGETOWN QUADRANGLE, COLORADO. 

of double lateral moraines indicates a distinct check in the shrinkage of the retreating glaciers, 

the rarity of recessional terminal moraines shows that such interruptions of retreat were not 

frequent. Kames occur to the north of the quadrangle, near Empire station. 

The morainal material is a light-yellowish sandy clay containing a wide variety of subangular 

cobbles and bowlders and fewer of rounded form. The finer material of the ground moraine is 

more clayey and the bowlders are smaller and less abundant than in lateral and terminal moraines. 

Striated bowlders are rare. The bowlders upon the surface of the till are not weathered, and 

the clay is unoxidized. 

The valley trains and outwash fans of coarse bowlders and sand are in most places of small 

development, although a valley train extends from the terminal moraine of the later- Clear 

Creek glacier at Empire station clown Clear Creek beyond Idaho Springs, and Idaho· Springs 

itself is mainly built upon it. The gravels are auritcrous and have yielded a considerable 

amount of placer gold. The postglacial canyon of Clear Creek, from 10 to 30 feet deep, separates 

this valley train into a number of disconnected areas. 

Southeast of Naylor Lake and south of Duck Lake are two peculiar deposits, partially 

surrounded by glacial till, consisting of huge angular masses of rock, many of them 20 to 100 

feet in diameter, all of material from the overhanging cliffs. The rock masses m:e unglaciated, 

although in some places patches of till are associated with them. The topography is that of 

a moraine, with kettle holes, hillocks, and ridges, but the material is similar to that of a landslide, 

and the Duck Lake deposit grades into an undoubted landslide higher up on the hill. These 

deposits were evidently landslides which lodged upon the decaying glacier and which, as the 

ice melted, took on a morainic topography. During the decline of glaciation the conditions 

for the formation of landslides are ideal, since cliffs are undercut and the rocks are saturated 

with water. It is probable that during each glacial epoch many landslides lodged upon the 

gliwiers, but the material of all except those formed immediately prior to the final retreat of 

the ice became so involved with drift that it is impossible to recognize them as landslides. 

RELA'l'IVE AGES OF THE GLACIAL E70CHS. 

The two glacial epochs, as shown by the glacial deposits and erosional features, were 

separated from each other by a considerable interval of time. The glacial deposits of the 

earlier stage are deeply oxidized, and the surface bowlders are considerably decomposed; the 

deposits of the later epoch are unoxidized, and the surface bowlders are fresh. Where lodgment 

was possible the later glacial deposits are continuous, and their topographic form is preserved. 

The earli~r glacial deposits, on the other hand, have been carved by erosion into many irregular 

patches that lack ghcial topography. Valleys that were glaciated only in the earlier stage 

are rugged and set with numerous rock pinnacles; those glaciated in the later epoch are smooth 

and in many places striated. Douglas Mountain was covered by ice in the earlier glacial stage, 

but in the later it was only partly surrounded by ice. In consequence, its base is smooth, and 

its upper slopes are very rugged. Since the earlier glacial epoch granite bowlders have weathered out, and a considerable depth of residual soil has formed on its summit. In the Central 

City quadrangle, to the north, the smaller of the two small streams named Silver Creek, a 

tributary of Clear Creek, has cut a canyon in the earlier glacial deposits 3,000 feet long with, an 

average depth of 100 feet and a width at the top of 250 feet. At one place the upper 85 feet 

is cut in glacial till and the lower 15 feet in the biotite-sillimanite schist of the Idaho Springs 

formation and in muscovite pegmatite. Farther downstream the canyon is sunk 150 feet in 

resistant pre-Cambrian rocks. The other somewhat larger stream named Silver Creek, a 

tributary of Fall River, has cut a trench in the later till 20 to 30 feet deep and 50 to 75 feet 

wide. Of like significance are the phenomena at Dumont, where Clear Creek broke through 

the terminal moraine of the earlier epoch, partially washed it away, and then deepened its 

canyon. Subsequently the canyon was par~ly filled by the valley train of the later Clear Creek 

glacier, in which a small postglacial canyon has been cut. 

The extent of interglacial erosion is unknown. A portion of the inner canyon of South 

Clear Creek, southeast of Georgetow.O:, is probably interglacial, since it is considerably larger than 
GENERAL GEOLOGY. 87 

the canyon of Clear Creek southwest of Georgetown, notwithstanding the fact that the latter 

stream is the larger of the two. 

The later glacial stage was doubtless comparatively recent and in a minor way continues 

to the present day, the snow fields and small glaciers of the Colorado Range, a short distance 

to the north, being probably remnants of the later glaciers. 

Postglacial erosion of even the larger streams has not been great. Southwest of Georgetown, 

·clear Creek, the major stream of the quadrangle, has cut a canyon 1,000 feet long and 30 feet 

deep in pre-Cambrian schists and granites. The upper 200 feet of the canyon is from 10 to 50 

feet wide; the lower 800 feet averages 60 feet in width. 

Spurr, a in describing the glacial deposits of the Aspen mining district, which were probably 

in a broad way contemporaneous- with those of the Georgetown quadrangle, considers the vall~y 

glaciers to be the shrunken remnants of the older glacier which covered a large area in the 

vicinity of Aspen. Although it is recognized that this interpretation may apply to the glaciers 

of the Front Range, ~tis believed that the two glacial epochs here described•were separated from 

each other by so long a period of interglacial erosion that their treatment as two distinct stages 

is warranted. In the interglacial time, however, as at present, snow fields and even small 

glaciers may have existed. 

HIGH BASIN mJmRIS SHEETS. 

The upper drainage areas of many streams which were not occup-ied by glaciers are covered 

by basin-like sheets of debris. The accompanying table gives the average elevation and the 

size of these debris sheets: 

Elevation and area of high basin debris sheets in Georgetown quadrangle. 

A vera eeleva-1 . Situation. tion aEovc sea ·Approximate 

-------------------~----~~----------1--Ie_v_ei_. _I area. 

Basin of Silver Creek, south of Lawson and west of Lamartine ................................................. .. Head of Tmil Creek ............................................................................................... . 

· ~~~~~~~v:r~~n.- ouici1:::::: ::::::::::: ::: :: : ::::::::: :::: :: : :: ::::::::::::: :::: : :: : :: : : :: ::::: ::: :::: ::::: ::::: :: : 

Head of Soda Creek ............................................................................................... . South of Devils Nose ............................................................................................. . 2k miles north of Chicago Lakes .................................................................................. . 

lla~i ~f 8~:;:'~ftera~e~~~~~-~-~an_s._._ ·_:::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::: Head of Lost Creek .................................................. __ , .......................................... . 1 mile northwest of Meridian Hill ....................................................................... . ! mile east of Meridian HilL ................................................................................. . J. mile southeast of Rosalie Peak............................................... . ............. . Head of Elk Creek ................................................................... _ ........................... _ 

Avemgc .................... :··················································· 

Feet. 

10,200 

10,400 

10,800 

9,600 

9,500 

10,500 I 11,000 

12,800 

12,500 

10,700 

11,100 

10,700 

11,100 

11,000 

10,8[10 

Acres. 200 

130 

320 

45 

150 

125 

400 

175 

700 

350 

100 

3ii0 

350 

140 

250 

The surface of these debris sheets rises gently from the center toward the head and the 

sides of the valley. In a -broad way it is smooth, but in the minor details it is uneven and in 

places reminds one of landslide topography. Swampy areas and springs are not uncommon, 

although true glacial forms are absent. The streams have cut small canyons from 20 to 100 feet 

deep. 

The debris sheets are composed of angular and badly weathered rock fragments, sand, and 

silt, all local material clearly derived from the surrounding ridges. The deposits are at a 

minimum 20 feet and in some places over 100 feet thick, and few rock exposures protrude from 

them. 

Three distinct stages in the history of the headwaters of the streams are recorded by these 

sheets-first, the development by erosion of broad valleys from 20 to more than 100 feet below 

the present valleys; second, the filling of these valleys approximately to their present height 

by the in washing of detritus from the surrounding ridges;- and third, t'9-e incision of small 

canyons; 

a Spurr, J. E., Mon. U. S. Geol. Survey, vol. 31, 1898, p. 250. 
88 GEOLOGY OF, THE GEORGETOWN QUADRANGLE; COLORADO. 

The debris sheet at the head of Silver Creek, west of Lamartine, lies behind the lat_eral moraine 

of the earlier Clear Creek glacier, showing that the erosion of the valley now filled with debris 

occurred in preglacial time and that the filling was deposited ·after the moraine was formed. 

The material of these debris sheets contains less clay and is probably more rapidly eroded than 

glacial till. The canyons, which equal in size or are slightly larger than the postglacial canyons 

in till, are therefore probably of postglacial age. It is probable that the wide valleys occupied 

by the debris sheets were cut in preglacial time and filled with debris mainly in late glacial 

time, and that the canyons were incised in postglacial time. While the climatic conditions were 

favorable to the formation of glaciers in the highest valleys, they were also particularly favorable 

to the existence of mud streams and sheet erosion, recently clearly discussed by Anclerson,a 

in the lower valleys. Debris would accumulate in the heads of the lower valleys, since it was 

deposited more rapidly than it could be removed by the mountain streams. 

ALLUVIUM. 

The "alluvium" of the map includes a variety of postglacial stream "deposits. On Clear 

Creek in the vicinity of Idaho Springs and on Deer and Elk creeks and the tributaries of Bear 

Creek the alluvium consists of flood-plain deposits of fine silt a,nd gravel bars. 

The alluvial flat north of Georgetown, through which Clear Creek meanders, is clue to 

stream infilling behind the. terminal moraine of the later Clear Creek glacier. Geneva Park 

was formed by the filling of a preglacial valley dammed by the lateral moraine of East Geneva 

Creek. Small patches of alluvium occur in all the minor stream valleys obstructed by lateral 

moraines, but many of these are too small to map. The larger areas of alluvium within the 

glacial deposits are mapped, but many small areas, partly filled lakelets and swamps, arc so 

intricately associated with ground moraine that the scale of the map is inadequate for their 

delimitation. Some of the glacial lakes are rimmed by alluvium, a measure of their former 

greater size. , 

The alluvium at the head of the east branch of Lost Creek is an alluvial fan built by a streamlet, now a tributary of Indian Creek. The lakes on the border of the quadrangle northeast of 

Meridian Hill, popularly known as Mud Lakes, are surrounded by mud flats and talus, the 

former infilling by a small stream, the latter slide from the neighboring hills. 

Small alluvial-fan deposits near Georgetown and Idaho Springs are also included in the 

alluvium. The extension of numerous alluvial fans upon the flood plain of Clear Creek .partially 

accounts for the sinuous course of that stream. These fans rest on later glacial valley trains 

or on postglacial alluvium, and are hence Recent in age. 

LANDSLIDES. 

A number of small lan.dslides are mapped. Their material is angular and of local origin 

and their surface is characteristically uneven and hummocky. Some (see p. 86) are glacial in 

age; others are probably postglacial and hence Recent. 

TALUS. 

The valley slopes are more or less covered by debris which has fallen from the slopes above. 

In cirques much of the postglacial talus is composed of angular blocks, 50 feet in diameter, with 

interspaces large enough to shelter a man. In many places the upper limit of later glaciation 

is marked by talus which lodges on narrow, glaciated benches or behind lateral moraines. 

TRAVERTINE (CALCAREOUS SPRING DEPOSITS). 

One of the springs on the east side of Soda Creek, near Idaho Springs, flows from a small 

dome of yellowish-white travertine 20 feet in diameter. It is said that a thickness of 0.1 to 0.2 

inch of travertine is now being added to the cone annually by the spring water. Smaller areas 

of this substance near by mark the sites of extinct springs. The travertine is of Recent and 

probably in part also of Pleistocene age.· 

a Anderson, J. J., Jour. Geol., vol. 14, 1905, pp. 91-112. 
GEOLOGY OF THE GEORGETOWN QUADHANGLE, COLORADO. 89 

STRUCTURAL GEOLOGY. 

The processes which have produced ·the structure of the rocks of the Georgetown quadrangle are folding, mashing that has caused schistosity, faulting, jointing, and igneous intrusion. 

As a result of these processes the rocks have been profoundly metamorphosed. 

FOLDING. 

If the theory that the Idaho Springs formation is of sedimentary ongm is correct, its 

members, the schists, gneisses, and massive crystalline rocks, represent beds which were deposited 

under water in a horizontal or approximately horizontal position. At the present time the 

planes separating the various members of the Idaho Springs formatio~ are, where determinable, 

in the main highly inclined, and it is surmised that the formation has been profoundly folded in 

a complex manner. The lack of any persistent horizon, recognizable over large areas, and the 

intense metamorphism suffered by the formation, however, make the unraveling of the structure 

impossible. The history of the folding of the rocks has doubtless been long and complex, 

beginning far back in pre-Cambrian time and continuing even after the intrusion of the 

porphyries, which in a few places have been slightlyflexecl. 

SCHISTOSITY AND GNEISSIC STRUCTURE. 

The earlier pre-Cambrian rocks of the Georgetown quadrangle, while deeply buried, were 

subjected to great stresses, and in consequence either schistosity or a gneissic structure 

has been developed in them. The schistosity of the biotite-sillimanite schist of the Idaho 

Springs formation is strikingly well developed and that of the associated biotite schist is good. 

The gneissic structure of the quartz schist and certain facies of the silicate rocks of the Idaho 

Springs formation, the hornblende gneiss, and the quartz monzonite gneiss, is also highly 

developed. In some places the gneissoid granite has a pronounced gneissic structure, and the 

later pr:e-Cambrian rocks are locally gneissic. The Tertiary igneous rocks, on the contrary, 

are massive. 

Abrupt variations in the strike of parting planes and intense crenulation are characteristic 

of the schistose rocks, particularly of the biotite-sillimanite schist. In this rock minute crenulations are superimposed on larger crenulations, these on minor schistosity folds, and these on 

larger folds, and still the superimposition' is much more complex. The planes of the gneissic 

structure are as a rule straight or gently curved; only in a very few places are they sharply 

crenulated. 

The planes of schistosity and gneissic structure· are in places horizontal or dip at small 

angles, but more commonly they approach a vertical position. The strike of the gneissic and 

schistose planes is shown by the wavy lines on the map. Prior to the intrusion of the preCambrian granitoid rocks the schistosity of this portion of the Colorado Range probably had a 

prevailing north-south strike parallel to the present trend of the range. Even in early preCambrian time the trend seems to have been along an important orogenic axis. The intrusion 

of the pre-Cambrian granitoid rocks has greatly disturbed the original strike of the schistosity 

and produced a rudely concentric schistosity around the larger igneous masses, a phenomenon 

well exemplified by the quartz monzonite batholith. The development of the schistosity and 

gneissic structure was a process, sometimes interrupted, which extended over an immense 

period, beginning very ea~ly and, since the youngest pre-Cambrian formations are comparatively 

massive; ceasing as an important process before Cambrian time. The relations of the rocks as 

shown by the contacts indipate that the periods of deposition of the various formations were 

separated by long time intervals, during which the rocks were successively subjected to dynamic 

forces producing schistosity or gneissic structure. 

In consequence secondary structure of these two types is highly developed in the oldest 

rocks, which were subjected to all of the deformation, and poorly developed or lacking in the 

youngest rock, which has been deformed only to an unimportant degree. The Idaho Springs 
90 GEOLOGY OF THE GEORGETOWN QUADRANGLE, COLORADO. 

formation accordingly, which has· been wholly recrystallized, is in a few places cut by rath~er 

massive facies of hornblende gneiss, a formation which under ordinary circumstances has also 

been thoroughly recrystallized. The quartz monzonite gneiss had a good gneissic structure 

prior to the intrusion of the gneissoid granite, which contains inclusions of this gneiss with the 

gneissic planes of the two rocks at right angles. Again, inclusions of gneissoid granite in quartz 

monzonite appear as gneissic as the more mashed facies now in place in the quadrangle. The 

deformation between the intrusion of the two rocks last mentioned was the last one to form 

gneissoid rocks over large areas, and the quartz monzonite and younger formations are, in a broad 

way, massive. The inclusion, however, of gneissoicl" quartz monzonite in Silver Plume granite 

and the presence of mashed fragments of this granite in massive pegmatite show that several 

periods of lesser differential movements followed the intrusion of the quartz monzonite. 

The present direction and attitude of the schistosity is due to three processes-first, the 

production of the schistosity by recrystallization and mashing caused by compressive stresses; 

second, the modification of this structure by the intrusion of igneous bodies; and, third, faulting. 

In the Colorado Range the first process is predominant, although subordinate to the second in 

the vicinity of large intrusive bodies in the Georgetown quadrangle. The third process is of 

local importance only. 

FAULTS. 

The determination of the presence of faults in the pre-Cambrian rocks is difficult, owing to 

the fact that persistent, recognizable horizons are not present in the Idaho Springs formation 

and that the other formations, being igneous, solidified as masses of irregular shape. Faults of 

large throw are probably not developed in the pre-Cambrian rocks, but faults of small throw 

are common and, in the aggregate, important structures. The magnificent exposures in the 

cirque walls show no large faults, although faults displacing igneous dikes from 2 to 10 feet 

are common. In mine workings small faults are numerous along zones of maximum stress. 

Faults are ~ore common in the massive crystalline rocks and the gneisses than in the schist 

members of the Idaho Springs formation, for the ·deformation which in the former brittle rocks 

produced displacement simply intensified the crenulation of the latter more flexible rocks. 

During recrystallization the Idaho Springs formation and the hornblende gneiss were 

buried so deeply that if faulting accompanied the folding of the series it was subordinate in 

amount. Faulting occurred, however, prior to the intrusion of the quartz monzonite, for this 

rock contains inclusions of the Idaho Springs formation cut by faults the fractures of which 

were filled by quartz prior to the monzonite intrusion. The intrusions of all the succeeding 

pre-Cambrian formations were accompanied by more or less faulting. 

Faulting also occurred between the solidification of the pre-Cambrian pegmatite and that 

of the intrusive porphyries, for the latter rocks in many localities displaced the former. The 

Tertiary dikes themselves have been faulted. A di~e 0.35 mile above the mouth of Golden 

Glen Gulch is cut by a north-south fault which offsets the dike 30 feet. Shear planes in the 

Tertiary dikes and at their contact with the pre-Cambrian rocks are common. The latest 

recognized faults are those which affect the ore bodies. 

JOINTS. 

Although joints are well developed in the rocks of the quadrangle, their directions are not 

constant over considerable areas, a fact due in part to the complexity of the stresses to whieh 

the rocks have been subjected and in part to the heterogeneous character of the rock complex 

of the quadrangle. 

The schists of the Idaho Springs formation split readily parallel to the schistosity and 

are cleanly cut by a set of joints, approximately at right angles to the strike of the schistosity. 

The other members, being more brittle, are cut by three closely spaced joint systems approximately at right angles to one another. Similar systems of joints, less highly developed, occur 

in the hornblende gneiss. The granites as a rule have several systems of well-developed joints, 
BUILDING STONE. 91 

one of which is in many places· closely spaced, and in consequence is a sheeting. The quartz 

monzonite is characterized by an approximately horizontal rift which is parallel to the gneissic 

structure, and this is cut by highly inclined systems ~f joints. The intrusive porphyries are 

brittle, and in consequence are cut by several systems of joints which are in places less than 1 

inch apart. Sheeting parallel to the length of the dike is alswcharacteristic. 

The pre-Cambrian pegmatites and the intrusive porphyries in particular have been intruded 

along ancient joints, and here and there joints which cleanly cut the gneisses and older pl_utonic 

rocks stop at the border of -pegmatite and porphyry dikes and are continued beyond. As 

these rocks are also cut by other joint systems, it is evident that joints have been produced at 

more than one period. ' . 

Joints do not control the major topographic forms, but where weathering and erosion are· 

vigorous they have played an important part in shaping the minor features.. This is particularly true along streams and in higher altitudes, where freezing water and changes of temperature are important agents of degradation. 

IGNEOUS INTRUSION. ' 

The most important structure of the rocks of the quadrangle is intense and complex intrusion. Many of the gneiss areas are aggregates of shattered blocks embedded in plutonic masses, 

which in turn are most intricately injected by later igneous rocks. Inclusions within inclusions 

are common. So intense is the intrusion that the rocks of the quadrangle may be considered 

an immense igneous breccia, exposure after exposure being crwountercd, concerning which it is 

difficult to decide whether to map it as an older rock intruded by a younger or as a younger 

rock containing inclusions of an older. Some idea of the complexity of the intrusion may be 

gained from the fact that in a distance of 1 mile south frol)l the northern boundary of the 

quadrangle, on the ridge between Silver Creek and Clear Creek, six formations alternate seventysix times, or at the rate of one alternation to 70 ·feet. This is exclusive of a number of minor 

intrusions and inclusions. The complexity of intrusion is shown to a slight degree on the map 

(Pl. II, in pocket), but can not be adequately depicted on a scale so small as that here used. 

The forms and the dynamics of intrusion have already been briefly described in the diseussion of each igneous rock. Secondary structure in the older rock controlled to a considerable 

extent the intrusion of the smaller igneous masses. In the more intensely mashed rocks rupture 

occurred readily along the planes of schistosity and of gneissic structure, and in consequence 

the smaller masses of igneous rocks predominantly followed these planes. In the more massive 

granular rocks joints. and faults were important planes of weakness. In many places where a 

gneissoid structure and joints and faults form planes of equal weakness dikes of later rock 

pass from one plane of weakness to another. 

Many of th~ various igneous intrusions were separated by long time intervals-an inference 

founded on the differing degree of metamorphism of the various formations and strengthened 

by the fact that in igneous breccias the igneous rock containing inclusions was thoroughly 

solidified and the two so firmly cemented one to the other prior to their mutual intrusion by a 

third rock that rupture and consequent intrusion occurred through the inclusion rather than 

around it. 

BUILDING STONE. 

The later pre-Cambrian granular igneous rocks, particularly the quartz monzonite and the 

Silver Plume and Rosalie granites, are suitable for building purposes. Care should be exercised, 

however, to locate quarries in areas comparatively free from inclusions of older and dikes of 

younger rocks. The quadrangle has not the advantage of cheap transportation rates to Denver, 

the natural market. A very pleasing building stone which is shipped to Denver is quarried 

from the Silver Plume granite at the mining camp of that name. , The Rosalie granite is a 

strikingly beautiful rock, but can be more advantageously shipped from areas outside of .the 

quadrangle. 
92 GEOLOGY OF THE GEORGETOWN QUADRANGLE, COLORADO. 

GEOLOGIC HISTORY. 

B~t a chapter here and there of the geologic history of the Georgetown quadrangle can be 

req,d from its rocks, since most of Paleozoic and Mesozoic time is blank. It may be well, however, to summarize the principal ev;ents. 

Early in pre-Cambrian time the area of the quadrangle was, as recorded by the older 

rocks, a sea bottom, probably adjacent to a land mass, from which sand, elay, and at times 

pebbles were carried to the sea. From the eharacter of the Idaho Springs formation, the deposit 

of this period, it is inferred that the sea was comparatively shallow, that conditions favoring 

the deposition of pure limestone rarely held sway, and that very likely the adjacent land was 

largely or in part composed of granite. After its deposition the Idaho Springs formation was 

deeply buried, folded, and pegmatized, and in it a parting was developed. This early pcgmatiza...: 

tion ceased before the end of the intrusion of dikes and sheets of diabase which followed. It 

is probable that the greater part of the intense recrystallization of the Idaho Springs formation 

occurred prior to the intrusion of the diabase, although the proportion is unknown. -Mashing, 

however, succeeded the solidification of the diabase, which \vas changed into a hornblende 

gneiss. Then followed in order intrusions of quartz monzonite (quartz monzonite gneiss) and 

siliceous granite (gneissoid granite). These are the youngest formations to which a gncissoid 

structure has been imparted over large areas, and at about this time the oldest faults Qf which· 

FIG. 23.-Cross sections showing erosion (a) of Silver Creek (a tributary 

of Clear Creek), sunk in the drift of the earlier glacial epoch; (b) of Silver Crock (a tributary of Fall River), in drift of the later glacial 

epoch. The streams are of equal size and hence the cross sections 

indicate the relative ages of the two glacial epochs. 

we have record were formed. Next the huge 

batholith of quartz monzonite cut and forced 

its way into the older roeks, and, probably as 

a later manifestation of the same intrusion, 

the quartz diorite and hornblendites solidified. 

From this time on faulting became a more 

important process and mashing, producing 

gneissoid rocks, less important. Then followed 

in order the intrusion of the Rosalie and Silver 

Plume granites and the granite-pegmatite. 

During the latter event the schistose and 

gneissic rocks were completely saturated by 

liquids emanating from the pegmatite magma 

(Sec Pl. XII.) 

·Of the greater portion of Paleozoic history we can ascertain nothing from the quadrangle 

itself beyond the fact that it was probably a land mass, although from the rocks of other portions 

of Colorado it is inferred that it was subjeded to profound erosion: It was, in faet, from the 

rocks of this and contiguous areas in ·the Colorado Range that material was derived for the 

Paleozoic rocks which underlie the Mesozoic sediments of the foothills. At some period after 

the pre-Cambrian rocks had been deeply eroded, whieh may provisionally be assigned to late 

Carboniferous time, the sea overflowed a portion at least of the Georgetown quadrangle, the 

supposed Carboniferous sandstone bowlders admitting of no other conclusion. 

Erosion accompanied by uplift persisted throughout the Mesozoic era, but toward the end 

of Cretaceous time or the beginning of the Tertiary a belt stretching from Boulder on the 

northeast to Aspen on the southwest was broken by a vast number of fractures, and into these 

porphyries were intruded. As Crossa has shown, it is probable that surface flows which 

largely or completely covered this portion of the Colorado Range were connected with these 

or related intrusive masses. It is probable that the intrusion of some of the porphyries extended 

into late Tertiary time. Since the solidifieation of the earlier intrusions, at least, these rocks 

have been fractured, faulted, and slightly flexed. Erosion continued through the Tertiary 

period, and by Pliocene time the Georgetown quadrangle was a mature, mountainous upland, 

with broad valleys, and high, rounded summits. It is evident that prior to the formation of 

this upland the late Cretaceous or early Tertiary lava flows had been completely removed, 

and that the streams were working again on the pre-Cam brian complex. 

a Cross, Whitman, Mon. U. S. Geol. Survey, vol. 27, 1896, pp. 34, 202. 
A. 

B. 

c. 

D. 

E. 

F. 

U.S. GEOLOG ICAL SURVEY 

I ,' .',-I ,-1 ·,- '. 

0 I - I-- I 

I' II, 

' .- , . - .. ~ 

fc-

• 

12Ft. 

A 

16Ft. 

c 

30Tt. 

E 

Hornblende Quartz monzonite 

gneiss gneiss 

-, I '" 1 _"' \ ' -

' -. 

L3~ 

~~ 

Quartz diorite Quartz 

monzonite 

PROFESSIONAL PAPER NO. 63 PL. XII 

fOrt. 

B 

6Ft. 

IJ 

60f't . 

.F 

Granite of Granite-pegmatite 

granite-pegmatite 

SKETCHES ILLUSTRATING INTRUSIVE PHENOMENA. 

Quartz diorite cut by dike of Silver Plume granite, which in turn is cut and faulted by a granite-pegmatite dike. 

Near Empire station. Central City quadrangle. 

Shreds of hornblende gneiss included in gneissoid granite, which in turn is inc luded in granit e of gran itepegmatite series. Newhouse tunnel . 

Quartz monzonite gneiss with par a llel intrusions of hornblende gneiss cut and faulted by dike of Silver Plume 

granite, which in turn is complexly intru ded by pegmatite. North side of Leavenwo rth Creek, three-fourths 

mile above mouth. at Atlantic tunnel. 

Inclusions of quartz monzonite gneiss in quartz monzonit e cut by a dike of Si!ver f1ume granit e, which in turn 

is cut and faulted by a dike of pegmatite. One-haif mile south of Upper Chicago Lake . 

Granite-pegmatite grading into granite, with intrusions of quartz monzonite gneiss. Northwest side of East 

Geneva Creek, 2 miles above its mouth . 

Gneissoid granite with hornblend e gne iss intru sions cut by pegmatite-granite with intru sions of gneissoid granite. 
MINERALS AND TEXTURES OF ROCKS. 93 

In early Pleistocene time the area was uplifted, and then carved by streams, which later 

were aided by glaciers oi two distinct epochs (:fig. 23). These and the more recent changes 

have, however, been already described (p. 33). · 

MINERALS AND TEXTURES OF METAMORPHIC AND IGNEOUS ROCKS. 

It is· the intention in this section to discuss briefly some of the more interesting characteristics of the metamorphic and igneous rocks of the Georgetown quadrangle. 

ORIGIN OF MICROCLINE. 

Microcline is a triclinic feldspar near orthoclase in habit and composition. As a rule, 

however, it contains somewhat more sodium than orthoclase, and when this clement is prominent it is called soda microcline. In many of the granular igneous rocks it takes the place of 

orthoclase, and is especially characteristic of graphic granite. It occurs less abundantly in 

porphyritic rocks and is very rare in those with glassy groundmass. a In many siliceous gneisses 

and schists it is the predominant feldspar. Unlike orthoclase, b microcline has not, so far as 

the writers know, been found as a. gangue mineral in ore-hearing veins, although its presence 

is to be expected, from its deposition in schists by waters analogous to those forming veins. 

Brush an.d Danae report it as an alter~tion product of spodumene by alkali solutions. 

Rosenbuschd states that it has not been produced artificially. 

In the rocks of the Georgetown quadrangle microcline either crystallized from the magma 

or formed subseql).ent to the consolidation of the rock. As a mineral separating from the 

magma it occurs in most of the pre-Cambrian granular rocks and as phenocrysts in a single 

thin section of the late Cretaceous aporhyolite. (See p. 66.) Its occurrence in the latter 

rock is extraordinary. 

In the gneissoid granite, the Silver Plume granite, the quartz monzonite, and the quartz 

monzonite gneiss much of the microcline was formed subsequent to the solidification of the 

magma. Its amount in these formations thus varies directly with the recrystallization which 

the rock has suffered. This micro cline further differs from an original Inineral in that its 

relative abundance is very uneven throughout the slide. It occurs in hooks and wedges forced 

into fractures of orthoclase, plagioclase, and· quartz or in veinlets cutting these older minerals . 

.Fresh microcline occupies fractures in altered orthoclase and plagioclase, and in many specimens these areas scattered in a single host are in similar optical orientation. Some sections 

show a narrow rim of fresh orthoclase and plagioclase against areas of fresh secondary microcline. In such places, where the adjoining mineral changes from micrcoline to quartz, orthoclase, or plagioclase, the fresh rim disappears and the feldspar is completely altered. It is 

evident iwt only that m.icrocline is a product of recrystallization, but also that during its deposition a narrow rim of the neighboring older feldspars was regenerated. This secondary microcline was formed after the solidification of the magma, and probably in the early stages of the 

processes determining the formation of the gneissic structure. That its formation ceased prior 

to the end of these processes is indicated by the fact that plates of muscovite arranged in the 

gneissic planes in rare instances replace it. That it is not secondary to orthoclase through 

strain alone is ~hown by the sharp contacts with which it invariably joins the other feldspars, 

as well as by the remarkably and uniformly fresh condition of the microeline, in contradistinction to the altered condition of plagioclase and orthoclase. If microcliilc in these instances 

was orthoclase under strain it would naturally be more altered than orthoclase, as minerals 

under strain are less resistant to alteration than unstrained minerals.e 

The microcline of the gneissoid types of quartz diorite and that of the large pink phenocrysts of the quartz monzonite gneiss are probably of similar origin. Secondary microcline in 

many ways resembling this type of microcline in the Georgetown quadrangle has been described 

a Rosen busch, H., Mikroskopische Physiographic, Stuttgart, 1892, p. 652. 

b. Spurr, J. E.; Prof. l'apcr U.S. Geol. Survey No. 42, 1905, p. 22a. 

c Brush, G. J., and Dana, E. S., Am. Jour. Sci., 3d ser., vol. 20, 1880, pp. 257-285. 

<I Op. cit., p. G52. 

eVan Hise, C. R., Mon. U. S. Gcol. Survey, vol. 47, 1904, pp. 96-97. 
94 GEOLOGY OF THE GEORGETOWN QUADRANGLE, COLORADO. 

by Bayley a from the gneissic granite of the Marquette Range, Michigan, and by Hall b from 

the gneisses of Minnesota. 

In the Idaho Springs formation near planes of movement wavy extinction of orthoclase 

grades into the grating structure of microcline. Here the common explanation of the origin 

of microcline from orthoclase as a result of pressure appears to hold . 

. SECONDARY ENLARGEMENT AND RELATED PHENOMENA. 

It has long been recognized by petrographers that in sedimentary rocks clastic grains 

retain their mineral potency and that when mineral-bearing solutions pass through the rocks 

each grain selects from the solutions material like itself. This new growth is added to the 

old grain in optical continuity, new crystal faces often being formed. The Carboniferous sandstone illustrates this process. An analogous case is that of the control of numerous tiny quartz 

individuals of the groundmass of the late Cretaceous quartz monzonite porphyry by large 

quartz areas of the biotite-sillimanite schist of the Idaho Springs formation, where these two 

formations are in contact. Notwithstanding the fact that the latter formation is the oldest 

of the pre-Cambrian rocks and the former is of late Cretaceous age, the old quartz areas were 

able to attach to themselves in similar optical orientation silica from the molten magma. 

A related phenomenon is the control exerted by phenocrysts of porphyritic rocks on like 

material of the groundmass. This phase of mineral control has been lucidly presented by 

Haworth.c In consequence of the process the phenocrysts, those of quartz in particular, are 

surrounded by rims of tiny areas of similar material, each of which is in similar optical orientation 

with the phenocryst. Such mineral control is well exemplified by the quartz aureoles surrounding the quartz phenocrysts of the quartz monzonite porphyry and the alaskite porphyry, each 

of late Cretaceous age. In these rocks not only did the phenocrysts have the power to orient 

like material in continuity with themselves, but they drew from the surrounding m·agma 

considerable quantities of quartz, increasing the percentage of silica of the groundmass in 

their vicinity. Many of the alkali feldspar phenocrysts of the bostonite porphyry, on the 

other hand,· are ragged through the addition of feldspar areas from the groundmass. The 

groundmass surrounding the phenocrysts is also enriched in feldspar, and in consequence the 

excess silica of the magma was concentrated as groundmass quartz in the center of areas 

between the feldspar phenocrysts. 

ORIGIN OF .THE PHENOCRYSTS. 

It is not the intention here to treat iii detail the much-discussed question of the origin of 

phenocrysts. It may be of value, however, to summarize the occurre~ccs of phenocrysts in 

the rocks of the Georgetown quadrangle. . 

Phenocrysts clearly of much earlier origin than the groundmass characterize the intrusive 

porphyries. The greater age of many of these phenocrysts is conclusively proved, on the one 

hand, by the intense magmatic corrosion which some of them have suffered, and, on the other 

hand, by the magmatic alterations shown by others. As examples of magmatic corrosion, the 

quartz phenocrysts of the quartz monzonite porphyry and the orthoclase phenocrysts of the 

alaskite porphyry may be cited. Magmatic alteration is typically shown in the hornblende 

phenocrysts of the andesite of the Empire mining district, which are altered to aggregates of 

hornblende, biotite, and epidote. Since the phenocrysts of the intrusive porphyries originated, 

however, considerable material has been added to the old form of some of them. This is well 

shown by the quartz phenocrysts of the quartz monzonite porphyry and the alaskite porphyry 

and by the alkali-feldspar phenocrysts of the bostonite porphyry. The amount of growth of 

the phenocrysts of the bostonitc porphyry, however, varies with their position in the dike, 

those near the center of the dike having received greater additions than those along its borders. 

This is to be expected, since the magma in the center of the dike must have cooled somewhat 

less rapidly than that along the edges, and in consequence the older phenocrysts had more time 

a Bayley, W. S., Mon. U.S. Geol. Survey, vol. 28, 1895, p. 172. c Haworth, Erasmus, Missouri Geol. Survey, vol. 8, 189.1, pp. 143-151. 

b Hall, C. W., null. U. S. Geol. Survey No. 157, 1899, p. 52. 
MINEHALS AND TEXTURES OF ROCKS. 95 

in which to control the younger constituents of the groundmass. The almost total lack of 

phenocrysts near the borders of some dikes of bostonite porphyry is, however, less readily 

explained. The feldspar phenocrysts of the Silver Plume granite arc types of phenocrysts of 

granular rocks which developed considerably earlier than the· solidification of the rest of the 

rock mass. Near contacts with the older rocks these phenocrysts show a marked flow orientation parallel to the contact. This shows that prior to the solidification of the granite these 

phenocrysts were dragged into parallel position by the movement of the still molten magma. 

In the porphyritic facies of the granite-pegmatite and of the quartz monzonite gneiss some of 

the larger crystals clearly solidified before the groundmass, and others contemporaneously with 

'it. In the quartz monzonite gneiss the larger crystals of augite and biotite are skeleton growths in 

which individuals of the groundmass are inclosed. The augite in particular occurs as small 

areas disconnected in the plane of observation, with similar optical orientation, and between 

these the groundmass mosaic lies. Similar forms characterize the biotite and hornblende of 

the granite porphyry of the granite-pegmatite series. In such rocks, without much question, 

these ferromagnesian minerals, which in the hand specimen have the appearance of phenocrysts, 

solidified contemporaneously with the crystallization of the constituents of the groundmass. 

As phenocrysts of metamorphic ·origin the garnet, corundum, and tourmaline of the Idaho 

Springs formation and the microcline of the quartz monzonite gneiss may be cited. The tourmaline and garnet, in many places at least, have been derived from adjacent dikes of pegmatite, 

and have therefore been introduced from without. The corundum, on the other hand, .developed during contact metamorphism of the biotite-sillimanite schist of the Idaho Springs formation, clue to the intrusion of the quartz monzonite batholith. The large pink microlinc phenocrysts of the quartz monzonite gneiss appear to have developed after the gneissic structure 

was produced and are believed to have been formed by recrystallization through solutions. 

MICROPEGMATITIC AND RELATED TEXTURES. 

The term micropegmatite is applied to an intimate intergrowth of two minerals, such as 

orthoclase and quartz, which is visible only under the microscope. As a rule one of the minerals 

sends cylindrical arms out into the other. The texture is very common in the pre-Cambrian 

rocks of the Georgetown quadrangle, and as an original texture indicating simultaneous separation of the minerals from the magma it is characteristic of a rather wide range of minerals. 

In the quartz monzonite gneiss, the gneissoid granite, the quartz monzonite, the quartz diorite, 

and the Rosalie and Silver Plume granites quartz is micropegmatitically intergrown with orthoclase, microcline, and plagioclase. In the Rosalie granite and the quartz monzonite biotite 

and quartz form micropegmatite; and in the quartz monzonite gneiss quartz and hornblende 

are similarly intergrown. In quartz diorite quartz is intergrown micropegmatitically with 

biotite and. with hornblende, plagioclase with hornblende, and orthoclase with biotite. In the 

granite-pegmatite tourmaline forms graphic intergrowths with both quartz and microcline. 

In the contemporaneous aplitic dike in the late Cretaceous quartz monzonite of Lincoln 

Mountain the gradation from the allotriomorphic texture through the micropoikilitic to the 

micropegmatitic is well seen. 

The micropegmatitic texture also arises through recrystallization and in the hornblendediopside gneiss of the Idaho Springs formation orthoclase, microcline, and plagioclase are each 

intergrown with quartz. In the same formation epidote and plagioclase feldspar are similarly 

intergrown, tubules of feldspar extending well into epidote. The epidote, which has grown at 

the expense of the feldspar, has attacked it very irregularly, giving rise to a typical micropegmatitic intergrowth. Identical growths were found by ·williams a in the mica diorite of 

Stony Point, N. Y., and by Adams_b in some Alaska granites. 

In numerous specimens the secondary microcline of the pre-Cambrian granites (see p. 66) 

is similarly oriented in many small areas distributed in the older orthoclase and plagioclase 

hosts. In like manner single plates of secondary muscovite inclosed numerous small areas of 

orthoclase, which ·as yet has not been altere(l. 

a Williams, G. H., Am. Jour. Sci., 3d ser., vol. 35, 1888, p. 445. h Adams, F. D., Canadian H.cc. Sci., voL 4, 1891, p. 348. 
96 GEOLOGY OF THE GEORGETOWN QUADRANGLE, COLORADO. 

In the hornblende-diopsidc gneiss of the Idaho Springs formation epidote and zoisite, 

epidot~ and garnet, epidote and hornblende, and calcite and garnet occur in large areas of 

one member of the couple inclosing smaller areas of the other, the smaller areas all being 

in similar optical orientation. This structure, a result of simultaneous solidification of the 

FIG. 24.-Titanite (t) molded 

upon a plagioclase crystal 

(p) and approximately contemporaneous with orthoclase (o). X 4. From 

monzonite, in the Central 

City quadrangle. 

mine·rals of the rock, is by no means uncommon in hornblende gneisses 

from other areas. a 

RELATIVE AGE OF TITANITE. 

Titanite or sphene is usually held to be one of the oldest minerals of 

the rock in which it occurs. In the rocks of the Georgetown quadrangle 

it appears that this is not necessarily the case. Figs. 24 and 25 illustrate 

titanite that is later than plagioclase and hornblende, and approximately 

contemporaneous with orthoclase-hence it belongs to the later period 

of the rock's consolidation. 

ORIGIN OF FLUORITE. 

The deposition of fluorite in the Georgetown quadrangle is genetically 

connected with the crystallization of three formations-the pre-Cambrian 

granite pegmatite and the associated granite 

porphyries, the bostonite, and the alkali syenite 

porphyry of the late Cretaceous rocks. In the Empire mining district it occurs in the late Cretaceous monzonite. In the. granite 

pegmatite it occurs in a disconnected veinlet replacing feldspar. In 

association with it and of contemporaneous origin is tourmaline, 

while the sericitization of feldspar was a simultaneous process. In 

the granite pegmatite, then, fluorite clearly originated after the 

solidification of the rock and was probably an effect of the process 

usual~y referred to as pncumatolitic action.. The simultaneous alteration of feldspar to sericite was undoubtedly clue to the gases or solutions which deposited the tourmaline and fluorite: In the granite 

porphyry associated with the granite-pegmatite, fluorite (sec p. 65) 

probably fills miarolitic interstices between the other constituents. 

hole 

FIG. 26.-Fluorite (b), apparently original. a, Quartz; 

c, plagioclase; d, orthoclase. X 12. From monzonite, 

in the Central City quadrangle. 

It has apparently eaten 

away the irregularities characteristic of the quartz individualsin the granite porphyry 

and has certainly replaced 

the feldspars to a limited 

extent. Its form and even 

FIG. 25.-Titanite inclosing hornblende, hence a later product of 

crystallization. t, Titanite; f, feldspar; h, hornblende. X 3;!. From 

monzonite, in the Central City 

quadrangle. 

distribution in the rocks of this series show however that ' ' it was probably deposited by vapors directly derived from 

the cooling graniteporphyry magma. 

In bostonite fluorite either lies· in wedges between 

the other constituents of the groundmass or replaces 

feldsp11r. Although the first occurrence of the mineral 

is sirnilar to that of quartz in other thin sections and 

may be original, it is probable that all of the fluorite was 

deposited after the rock solidified, by gases or solutions 

genetically related to the bostonite magma. Fluorite was also deposited by emanations from 

the alkali syenite porphyry. Some of the fluorite of the monzonite of "the Empire mining 

district, on the other hand, has every appearance of being an original mineral (fig. 26), while 

the rest was probably deposited after the final consolidation of the rock. 

a Rosen busch, H., Elemente der Gesteinslehre, Stuttgart, 1901, p. 533. 
U. S. GEOLOGICAL SURV EY PROFESSIONAL PAPER NO. 63 PL. X III 

.1.1. . GEORGETOWN AND CLEAR CREEK VALLEY, WARD EMPIRE FROM SLOPE OF LEAVENWORTH MOUNTAIN. 

1 , Ke lly t unnel; 2, Empire mine; 3· Empi ; 4• Griffit h mine; 5• Cent ennia l dump. 

B . IDAHO SPRINGS AND THE MINES OF VIRGINIA , FROM NORTH SLOPE OF FLIRTATION PEAK. 
PART II.-ECONOMIC GEOLOGY. 

By JosiAH E. SPURR and GEoRGE H. GARREY. 

CHAPTER I. 

DISCUSSION OF PRINCIPLES. 

GOLD-BEARING AND SILVER-BEARING GROUPS OF VEINS. 

' 

RELATIVE DISTRIBUTION OF SILVER-BEARING AND OF PREDOMINANTLY GOLD-BEARING 

VEINS. 

RECOGNITION OF DISTINCT SILVER-BEARING AND GOLD-BEARING. AREAS. 

The existence of areas or "belts" characterized by different vein materials within the 

region of the Georgetown quadrangle and the adjacent mineral districts to the northeast has 

. been recognized since the early period of development. The stretch of mineralized country 

between Georgetown and Boulder has long been noted for the variety or ores which it has 

yielded. The greatest contrast perhaps has been between the silver-bearing ores, containing 

little or no gold, and those which contain gold in constant and important amounts. Certain· 

veins and certain groups of veins are characterized by lead-silver ores; other veins and groups 

of veins are characterized by auriferous ores, with or without important quantities of lead and 

silver. Although in many places the silver-bearing veins and the gold-bearing veins occupy 

relatively distinct areas, or belts, yet in other places veins belonging to the two different types 

are found close together. 

SILVER PLUME ARGENTIFEROUS REGION. 

The famous and productive Silver Plume region,· near Georgetown (see Pl. XIII, A), 

contains veins producing silver and lead to the almost total exclusion of gold, and the same 

is true of the Republican and Democrat Mountain veins; so that all the veins lying in a rough 

quadrangle bounded by Clear Creek on the south and east, by Bard Creek on the north, and 

by meridian 105° 45' on the west come within this group. South of Georgetown is another 

smaller but famous silver region-that of the Colorado Central group, whose abundant ores, 

in large part of high grade, are substantially similar to those of the Silver Plume region. The 

northeastward-trending Colorado Central silver belt is continued still farther northeast by the 

Comet-JEtna and Magnet veins . 

. GOLD BELT NEAR GEORGETOWN. 

On Leavenworth Mountain, immediately south of Georgetown, the width of the area lying 

between the two silver belts-that of the Silver Plume-Republican Mountain groups of veins 

on the northwest and that of the Colorado Central-Comet group on the southeast-is about 

three-fourths of a mile. In this area are gold-bearing veins, .whi.ch contain also as a rule some 

silver. Of tl).ese veins perhaps the most typical is the Centennial, which has had a considerable 

31401-No. 63-08-7 97 
98 GEOI;OGY OF THE GEORGETOWN QUADRANGLE, COLORADO. 

production. This mine is situated in the southern part of Georgetown, and its northeastwardtrending veins run under the north slope of Leavenworth Mountain. Southwest of the Centennial is the Central Colorado, another gold-bearing vein; and the further extension of this 

zone, along the north side of Pendleton and Leavenworth mountains, can be traced to the west 

border of the Georgetown quadrangle. Northeast of the Centennial mine the continuation 

of the gold belt is shown in the Griffith mine, which has a strong northeast vein running into 

Saxon Mountain. The ore here contains a.fair amount of gold, although the silver content is 

more valuable. About a mile north of the Griffith is the parallel Anglo-Saxon lode, aiso running 

into Saxon Mountain; its ore contains gold and silver of approximately equal importance. 

IDAHO SPRINGS AURIFEROUS AREA. 

Along the northeastern extension of the mineral belt, that portion which lies within the 

Georgetown quadrangle embraces the mining region southwest of and tributary to the town 

of Idaho Springs. (See Pl. XIII, B.) Principally silver-bearing veins are present in this 

section; they are illustrated by the veins of the Cecil-Argo group (p. 368) lying south of Lamartine, in which. the value of the gold is only a small percentage of that of the silver." In the 

larger proportion of the veins, however, including some of the strongest veins of the regionfor example, the Stanley, Freeland, Little. Mattie, and Lamartine-the gold values are equal to 

or in excess of those of silver. 

Farther northeast the mineral belt passes into the areas of the Central City and Blackhawk 

quadrangles, whicli have not been geologically surveyed, and from Clear Creek County into 

Gilpin County. Here: between Idaho Springs and Central City, the chief ores are those of 

gold, though veins whose principal values are in silver are present, as, for example, on Seaton 

Mountain; and a definite silver belt has been recognized, running north and south, near the 

town of Blackhawk. In ·the gold-bearing veins of Gilpin County silver is associated with the 

gold in varying proportions, but the ores of the silver belt show little or no gold.a 

CHARACTER OF VEINS BETWEEN GEORGETOWN AND EMPIRE. 

That portion of the mineral belt within the Georgetown quadrangle which lies around 

Silver Plume and on Republican and Democrat mountains and consists entirely of practically 

nonauriferous veins is continued on the north by the veins of Lincoln Mountain. Sorne of. these 

Lincoln Mountain veins, such as the Virginia City, are, like those of the group above mentioned,. 

chiefly silver bearing, but others, such as the Ramsdale, contain ore showing predominant 

values in gold. North of Lincoln Mountain, and across West Fork of Clear Creek, is the Empire 

district, described on pages 383-410 of this report, where the typical veins are auriferous, with very 

little silver. On the eastern edge of the Empire district, however, there arc silver-bearing 

veins that carry very little gold. These appear to form the eastern portion of a definite area 

or belt of silver-bearing veins, many of which outcrop on Red Elephant Hill.b 

THE GEORGETOWN QUADRANGLE A PART OF A LARGE MINERAL-BEARING AREA. 

The above sketch outlines the distribution of the two principal types of veins (those which 

are auriferous to an important extent and those which are not) within the Georgetown quadrangle. Southwest of Georgetown, along the minera,l belt, the writers' investigations have 

also carried them as far as the Argentine district, 9 or 10 miles from Georgetown, where the 

numerous veins, so far as seen, are all of the silver-bearing, nonauriferous type. 

From this it will be seen that the area of Georgetown quadrangle is not held to include 

a complete mining di~trict, or a complete group of veins; it is a rectangular area arbitrarily 

taken, whose northern portion includes a segment of a northeastward-trending mineral belt 

that is' evidently of considerable extent. From the facts accumulated in the studied segment 

conclusions will be drawn, and these will be adjusted by the consideration of the rather meager 

information available concerning the portions of the belt that lie to the northeast and to the 

a Rickard, Forbes, Trans. Am. lust. Min. Eng., vol. 28, 1899, p. 111. b Fossett, Frank, Colorado, 1880, p. 353. 
U. S. GEOLOGICA L SURVEY 

[_j 

Vem of first penod 

of deposition 

N 

I+++H++I EJ Vem of second of dcposlti:Oenod Unmmerahzed lead 

0 

GEOLOGICAL PLAN OF ANNE Sho · TTE LEVE wing t wo periods of L , GRIFFITH ore deposition. 

100 200 300 400 feet 

MINE. 
U. S. GEOLOGICAL SURVEY 

.11. SPECIMEN FROM GRIFFITH MINE, SHOWING TWO PERIODS OF VEIN 

DEPOSITION. 

The wa ll rock is gneiss, which was split open by a fissure Subsequently the walls of t he 

ftssure were coated by comb quartz (q). Late r t he fissure was cemented shut by blende, 

with some galena and chalcopyrite. This is th e vein of the first period. Subsequently this 

vein was split open and a new fissure wa> filled by the vein of the second period, consisting 

chiefl y of manganese-iron carbonat es and FYrite, with inclosed broken fragments of the earlier 

ve in . gn, Gneiss wal l rock; q, quartz (comb structu re j ; o, blende; p , pyrite; c, chalcopyrite; g, galena. Natura l size. 

PROFESSION AL PAPER NO. 63 PL. XV 

B. THIN SECTION FROM STANLEY MINE, SHOWING FRAGMENTS INCLUDED 

IN GLASSY OR FINELY TRACHYTIC BIOTITE LATITE ( a ) . 

/,, Bosto nite ( earlier d ike rock ) ; c, c lear feldspar ( usually microcline ); d, sericitized and 

calcitized feldspar ; e, vein quartz; J, pyrite. Enl arged 11 diameters. 

SPECIMENS SHOWING HISTORY OF ORE DEPOSITION. 
GOLD-BEARING AND SILVER-BEARING VEINS. 99 

southwest of that segment. It is evident that satisfactory conclusions would depend on a 

close study of the whole belt, and that the conclusions herein reached, from a study of part of 

it, will need enlargements and modifications whon all the facts become known. 

DIVISION OF QUADRANGLE INTO TWO PRINCIPAL MINERAL-BEARING AREAS .. 

As a result of the classification of the veins into the two chief groups., (1) silver-bearing 

veins without important amounts of gold, and (2) gold-bearing veins with or without silver, the 

veins within the· quadrangle could be roughly grouped in two chief areas-the Georgetown 

region, with greatly preponderating veins of the first class, those of the second class being 

exceptional, and the region southwest and west of Idaho Springs, where veins of the second 

class predominate and those of the first class are exceptional. Between the two . areas the 

continuity of the belt is interrupted by a region of comparatively little mineralization, embracing 

the upper portions of Griffith and Saxon mountains. 

RELATIVE AGE OF GOLD VEINS AND SILVER VEINS. 

MINERALOGICAL CHARACTERISTICS. 

MINERALOGICAL GROUPS. 

:Mineralogically the lodes of the quadrangle are roughly divisible into two groups, corresponding to the two chemical groups (of predominantly gold ores and of silver ores) above 

described. These mineralogical groups are (1) galena-blende ores with more or less pyrite, 

chalcopyrite, gray copper, and polybasite; and (2) pyritic ores containing chiefly pyrite, 

cupriferous pyrite, or chalcopyrite, and a subordinate amount of galena and blende, with 

tetrahedrite and other less abundant minerals. 

BOTH TYPES OF VEIN FILLING IN THE SA~1E VEIN ZONE. 

In the Griffith mine, which is near Georgetown, within the narrow gold belt that has been 

described as lying between silver belts on each side, certain phenomena of great importance 

were first noted, though afterwards they were observed elsewhere.· These phenomena, which 

are described more thoroughly in the special account of the mine on ·page ?85, will be briefly 

referred to here. The vein, which is a strong one, shows two distinct vein periods of fissuring 

and subsequent cementation (Pl. XIV). The result of the first· dislocation of the rocks along 

the line of the· present vein was a strong fissure, which was in part opened for a considerable 

width, as is shown by the angular fragments of wall rock that occur in and are cemented by 

the vein materials deposited subsequent to this dislocation. These angular fragments of wall 

rockindicate an original rubble-filled fissure like that characteristic of the Mendota and similar 

mines of the Silver Plume district, whose veins belong to the group of typical silver veins. 

The filling of this original fissure of the Griffith vein was also similar to that of the silver 

veins mentioned. The first deposition was a coating of comb quartz, after which the inte1ior 

of the fissure was completely filled with solid sulphides consisting of galena, blende, pyrite, 

and chalcopyrite, with practically no intermtxcd gangue, a feature which is also characteristic 

of the typical silver veins. Subsequent to the formation of this vein there was another dislocation .and reopening of the· old fissure, resulting in larger openings than the first disturbance 

produced. The new fissure in general followed along the line of the old one, splitting the old 

vein and breaking it up in various ways. On account of the greater brittleness of the original 

sulphide filling as compared with that of the wall rocks, the new fissure at many places split 

the vein in the middle, leaving a layer of sulphides clinging to each wall, a condition which, 

after the new fissure was cemented with the ore of the second period, presented a false appearance. of crustification. (See Pl. XV, A.) In numerous other places the older vein was broken 

into angular fragments, which rested in the newly formed fissure in the shape of rubble, precisely 

as the fragments of country rock had filled the original fissure. The angular fragments of galenablende ore now appear as inclusions in the later ore. · This later ore, which completely filled 
100 GEOLOGY OF 1'HE GEORGETOWN QUADRANGLE, COLORADO. 

the openings of the second period of dislocation, is of quite different character from that of the 

first deposition, consisting principally of pyrite and brown carbonates of iron, manganese, 

and magnesia, with more or less quartz and a little galena, blende, chalcopyrite, and barite. 

In some places the quartz exceeds the brown carbonate in amount. 

The ores of the later period are stated to be generally of low grade, containing 10 to 12 

ounces of silver and $2 to $3 in gold to the ton, though one stope is said to have yielded 20 to 

25 ounces of silver and $6 to $10 in gold. The valuable ores are those formed during the first 

period and containing galena. The values in these ores consist chiefly in lead, next in silver; 

they contain also gold. They are reported to carry an average of 30 to 40 ounces in silver 

and $8 to $9 in gold. In general, however, owing to the brecciation of the original vein and 

the mixture of its ores with those of the later vein, the exact proportion of the metals in the 

ores of the two periods is rather uncertain. 

The composition of the older vein was practically identical with that of the typical silverbearing lodes, such as those of Silver Plume and of the Colorado Central mines; the younger 

vein formation is similar in composition to the Centennial vein, which lies almost in line with 

the Griffith and on the opposite side of the village of Georgetown. The two ores, however, 

are not identical, as the Griffith ore contains much more carbonates. 

The Centennial vein has a gangue of quartz carrying pyrite, chalcopyrite, cupriferous pyrite, 

and secondary tetrahedrite, with very little galena and blende, so that the ores show only 

traces of lead and zinc. The values are in· gold, silver, and copper, the silver running from 

20 to 30 ounces, gold from 1 to It ounces, and copper from 4 to 8 per cent. Both gold and 

silver seem to be chiefly associated with copper. 

The Centennial lies southwest of the Griflith. Southwest of the Centennial, in turn, and 

in line with it and the Griffith, is the Central Colorado vein, which runs northeastward along 

the westward-sloping face of Leavenworth Mountain. This vein is mainly gold bearing, the 

ore carrying as much as 2 to 3 ounces in gold, with practically no silver. The gangue is quartz, 

with some siderite and ferruginous rhodochrosite. The metallic minerals consist chiefly of 

pyrite and cupriferous pyrite. Seams of zinc blonde occur between the vein of cupriferous 

pyrite and the wall rock, as if the blende was of earlier origin than the pyritic vein. Probably 

the same two periods of vein formation are indicated here as in the Griffith, the old vein represented by the blende having been split open and cemented by later and much more abundant 

pyritic ores. 

Phenomena similar to those above described were repeatedly noted elsewhere independently 

before their recurrence suggested that they marked an important distinction between the 

veins of different type. These phenomena were characteristically found on the borders between 

an area of predominantly gold-hearing veins and an area of silver-bearing veins. In the cases 

just described they characterize the wedge of territory containing gold-bearing veins which 

runs southwestward through Georgetown, with silver-bearing country on both sides. Phenomena 

of this kind were noted also on the borders of the Empire district. This is a district of goldbearing veins adjoining on the north the district of silver-bearing veins that includes the Silver 

Plume, Republican Mountain, and Democrat Mountain areas, the two districts coming together 

on Lincoln Mountain. On the east the Empire gold-bearing district is adjoined by the silver 

belt of Red Elephant Hill, the two coming together just east of Empire. 

On Lincoln Mountain the chief veins examined were the Virginia City and the Ramsdale, 

on the south slope, toward Democrat Mountain, and the Allen and the Hidden Treasure, on the 

north slope, toward Empire. The Virginia City lode is a typical silver-bearing lode of the Democrat Mountain-Silver Plume type. The ores consist of galena, blende, and pyrite, carrying 

several hundred ounces of silver and from 0.2 to 0.5 ounce of gold. The Ramsdale vein shows 

in places a vein of quartz 2 feet wide, containing some pyrite, copper pyrite, and a little blende. 

In some portions of the mine the earlier ore has been brecciated and cemented by pyritiferous 

quartz. The chief values of the vein are in the gold, some of which is considered to be associated with copper and runs as high as several ounces to the ton. A sample taken assayed 3-} 

ounces of gold and 12 ounces of silver. This vein is probably a composite vein like the Griffith. 
NORTHEAST-SOUTHWEST MINERAL BELT. 101 

TheAll en mine is located north of the Virginia City, on the same general strike. The vein here 

has an interesting structure. It contains in places next to the foot wall a thin seam of blende 

and galena an inch or less in thickness adjoining the main material of the vein, which consists 

of gray jaspery ·quartz and pyrite. A similar but discontinuous seam of blende and galena 

lies on the opposite wall of the vein. The fact that these two seams of blende and galena are 

separated by a seam of pyrite-bearing quart"' suggests that there were at least two periods of 

·cementation similar to those observed in the Griffith mine, and that the zinc-lead ore represents 

an earlier ore deposit, which was reopened by a fissure in which pyritiferous quartz was subsequently deposited. 

I-NTERSECTION AND FAULTING OF SILVER VEINS BY GOLD VEINS. t. 

On the east side of the Empire district silver-bearing lodes containing galena occur close by 

pyritiferous lodes carrying gold. Phenomena of great interest were noted in the Harrison 

mine (Pl. XVI). 

The main tunnel in this mine follows a vein of jaspery quartz containing pyrite. This lead 

strikes N. 10° E. and faults two veins of east-west strike that contain sphalerite, quartz, and 

pyrite. The displacement of the faulted vein is slight, amounting to a horizontal offset having 

a maximum of only a fsw feet. Between the faulted ends of the blende-bearing veins and 

along the wall of the pyritiferous vein is a breccia containing angular fragments of sphalerite 

ore and wall rock, representing the drag a.long the fault plane. These faulted veins by their 

composition plainly belong with the silver-bearing lodes of the vicinity, but the younger pyritiferous vein is· a mineralogically typical gold-bearing lode. Another phenomenon, however, complicates the history. The older blende-bearing vein has been split open, in some places along the 

center, in others along the sides; in still other portions the ore has been brecciated. All these 

openings have been cemented by pyrite and quartz, and this cementation seems to have taken 

place somewhat earlier than the faulting of the veins and the subsequent deposition of the 

pyritic vein along the fault. 

These different observations indicate that in general the silver-bearing veins, characterized 

by galena and blende, are older than the gold-bearing veins, characterized especially by pyrite 

and cupriferous pyrite. 

NORTHEAST-SOUTHWEST MINERAL BELT. 

EXISTENCE OF A MINERAL BELT IN THE AREA OF THE GEORGETOWN QUADRANGLE. 

LIMITS. 

The mineralized area of the quadrangle is confined to the northern portion and extends 

in a general cast-northeast· direction. ·within this area there are many strong and persistent 

veins having a predominant northeast or east-northeast trend, though a smaller number of 

veins, some of them very strong, have a northwest trend. The predominant northeast trend 

is parallel to the trend of the mineralized zone. The edge of this zone, as shown on the map 

(Pl. II, in pocket), is fairly definite and strongly marked, and the remainder and greater portion 

of the quadrangle, which has been carefully surveyed, is quite barren of mineral indications. 

RELATIONS OF THE VEIN BELT TO TilE PORPHYRY BELT. 

The pre-Cambrian rocks of the mineral belt do not differ essentially from those of the rest 

of the quadrangle. On plotting the dikes of porphyry, however, which are characteristic of a 

portion of this region, it was found that the belt of porphyry dikes coincides closely with the belt 

of mineral veins. In detail it has been found that the relation between the veins and the 

porphyry dikes is in some places close, l>ut that in many other places important veins have no 

connection with individual dikes. 
102 GEOLOGY OF THE GEORGETOWN QUADRANGLE, COLORADO. 

EXTENSION OF THE MINERAL BELT OF THE GEORGETOWN QUADRANGLE. 

EXTENSION FROM BOULDER BEYOND LEADVILLE. 

The fact that the mineraf belt which passes through the northern part of the Georgetown 

quadrangle (Pl. XVII) extends farther to the northeast and southwest is fairly well kn9wn, and 

the continuity of the belt is sufficiently patent to have provoked attention from the earlier 

geologists and explorers. In general, it may be stated that this.zone of ore deposits is practically 

continuous from the foothills of the Rockies near Bouldflr on the northeast at least as far as 

Leadville on the southwest. 

At the northeast end of the belt are the Boulder County ~ines, which have long been known 

to be very productive. They have been developed in numerous districts.. Southwest of 

Boulder County the mineral zone passes with no geologic break into Gilpin County, which also 

has long been a famous producing area. The Gilpin County belt in turn passes unbroken on the 

southwest into Clear Creek County, which embraces the portion of the belt included within the 

Georgetown quadrangle. Still farther southwest the mineral belt may be followed to the 

Argentine distriet and the neighboring Hall Creek, Geneva Creek, Montezuma, and Snake River 

districts, which. are close to one another, but are partly in Clear Creek and partly in Park and. 

Summit counties. From all available descriptions the mineral belt is continuously traceable 

from this region southwestward through the Breckenridge and Tenmile districts into Leadville. 

The Leadville region seems to mark the southwest end of the most productive portion of this 

practically continuous belt. 'l'here are indications, however, that a mineral zone extends beyond 

Leadville for a certain distance to the south, but it is broader and less strongly marked than the 

belt northeast of Leadville. This zoneis in evidence on both the east and west sides of Arkansas 

River, along the Park and Sa watch ranges. In the Park Range veins are reported in the vicinity 

of Granite, and in the Sawatch there is a continuous north-south belt that embraces the Tincup 

and Monarch districts. · 

BELT INCLUDING ASPEN AND THE GUNNISON UEGION. 

A northeastward-trending mineral belt slightly offset from the main belt, which runs 

between Leadville and Boulder, includes the Aspen mining district. On the northeast this belt 

passes through Lcnado and into the Frying Pan district. Some distance northeast of the Frying 

Pan region is the Redcliff district, which is only slightly northwest of the main Leadville-Boulder 

belt at Leadville. On the southwest the Aspen belt is traceable through Ashcroft and other 

mining localities across the Elk Mountain range into Gunnison County, where it includes the 

Ruby, Irwin, and neighboring mining districts. 

EXTENT OF THE MINERAL BELT. 

The main belt, which runs from Boulder to Leadville in a direction nearly due southwest, is 

about 85 miles long and5 to 12 miles wide. The belt which passes through Aspen, if considered 

to extend from the Frying Pan country to the Gunnison region, is about 45 miles long, and if 

considered to extend to Redcliff, about 65 miles long. This belt also has a northeast strike and is 

5 or 6"miles wide. The total extent of this whole general belt as measured from Boulder to Ruby 

and Lrwin is 130 miles. 

OCCURRENCE OF ORES IN THE MINERAL BELT. 

FORM 01<' OHE DEPOSITS. 

Along this mineral belt from its northeast end near Boulder as far southwest as Breckenridge 

the ores occur in fissure veins in pre-Cambrian crystalline rocks. These veins have a steep clip 

and predominant northeast trend parallel to that of the belt, though many of them are oblique or 

transverse to it. In the Breckenridge district or farther southwest, in the Tenmile and Leadville 

districts, the mineral belt runs into Paleozoic and Cretaceous rocks, which overlie the preCambrian. In these later rocks the ore deposits are still in part fissure veins, having the same 

predominant northeast trend; in part they take the form of morz irregular deposits, especially 
U .S. GEOL OGICAL SURVEY 

0 

~0$~Gnei~ 

LL__j ~ Drifts 

Sphalerite lod 

(dash r e un . mes show ' mmeralized lead) 

[][] (d

Pyrite lode otted r 

50 

unm· m~f-:1 show mera!tzed lead) 

THE NORRIS ,.ETERS CO., WAS/1/N GTOtl, 0. C 
INDEX TO LOCALITIES 

1 AJiunde 

2 Marsl!all tunnel )j Equator tunnel 4 Ocean Wan~ tunn~J 

5 Kirtley 

t) Dunawav tnnnel 

7 A rgentiJje tunn~l 

H Wide W<>st n ~rting 'rilnes 

~~ c~~:.,r:,<;~ltmmel l2 l:lurrel 1~ Big Indian 14 Nonpareil tunnel 15 Welch 16 Ceutt-al Colorado 17 Helmick 

Hl Hall tunnel 

1\l Gambetta 

20 Prudential tunnel 21 Kelly tunnel 22 Moline tunnel 

23 New Boston 2-1 Beechet· 

~ ~~~!.!1 t~~!~~fe' 27 Mw .. co,·ite 

2ti Scf•ptr(' 2'.1 Queen a!) 1.b•tor tunt•el :H Astor ~haft 

:12 Runbur·st uppeL' tHIIttel 

HH Hunbur:->t lnwer· t.unne) 

~ Bn('k ~yt:. 

>l.'i CliJI :~1) LOWt:'l' ,Jtllh."ti(Hl 

:)7 Jmlt'tion 

:~ Edgar tunnel HH Roger~ ::-:haft 

.fO Siln~r <-tJanee 

.n Bona117-S tnnnel 

.t2 ·Matthews 

.:a ~ut>en of the \Y l'~t 

-1-1 Viqdnia City 1;; Ram>«lale ~6 A.J1gJo-Raxon Extt'n~iou 

47 l>orie tH (-i-ritlhh-A nne tte tuttrwl 

Ill Capitol ;;o !Etna 51 Comet shafts az Comet tunnel ;;.~ il'lascott.e 5-I Woodley 5.'i ~lagnet 

:36 Summit 57 Poor ~I an 5H Brazil 5\l Old Htag 60 Lamartine tunnel r.J Oneida shaft tl~ Lamartine ~hart 

68 In vincible &l Alpine tt, Lanagan 

tlti Miller tii' Ariadne 

tl~ CraZ\T Hirl H\1 Brighton 70 Freeland Exten"ion 

~~ ~~}~~s~b~ft T.l TPller tunnel 74 Lone Tt·ee i5 .New Era 76 J<'alu 77 Turner 1M Shak<>speare tunnt'l 711 l<'reeland shaft 

HO reefand tunnel 81 Toledo ttmnel 

82 G lllU rrr~ shaft H.~ Old SPttler Ht New ~~t·a o:i\1 x::; )1organ 

86 Harrison t\nd U niou 

fq Uhampiou Dirt tumwl No. MH Centurion tutmel 8!1 Little Albt'rt tunnel No. 00 Little Albert tunne l No. 9l Kelly tuuuel No. 4 9'J Donaldson No. 3 93 Lincoln W Sout.h Lincoln 95 Josephine . 96 l£lliott & Barber !l7 Salisbury 

98 Hukill 911 Crockett 100 Stanley (tunnel and shaftl 101 Golden Link shaft 

10'2 Old Stanley shaft 10.3 Maytlowet· 104 Bullion King 105 Star 106 Red Lyou 107 James 10K Phoenix 10!1 Eugland 110 Niagarn 111 Lord Bnon 112 Banty 113 Metl'Opolitan 1 H }'raction 115 Kitty Clydt> 116 Uomer 117 Torpedo Jlll Quito 

11\l I'. T. 

120 Black !:'wan 121 Beave r 12"J Blaek Eagle 1~ Bismarck 124 Newton 

12.5 Decator 126 Columbia 121 Sbu· tunne l lllil Little Mattie 121l ~ilvt-r CHance 

]~~ ~~~~'l, lomon-Dort itt tun. 

1~2 Golden Glen 1!-lll Silvet"ine la.l Muscovite 13.'\ J<cclipse 

g~ g~~bi.';.%u li!R Burns-Mom·e 13\l IWa McKinney 140 Silver Horn 1~1 lU. and M. 

142 Cecil 143 Wallace 

~~ ~~fJ' Coin 

~!~ ~~RR:o 148 Humbolt lowe r tunnel mill 14~ Alexander 

150 Charter Oak 1nl Mat·y Foster 152 Kitty Emmet 15:~ American Boy 15-1 Little Flat and Mat·y Matilda 155 Gold Leaf 156 Lexington 157 Uherusker 1i\8 Blizzard 15\l Little Richard 160 International 161 Gomez 162 Coming Nation 168 Columbia 164 Hot Soda Spring loo Cold Sulphur Spring Hlll Xewhouse tunnel ltl7 l:llue Ribbon Spring 168 Waltham l~fi :Rig Five tunnel 

U. 5 . GEOLOGICAL SURVEY 

45' 

E.M . Oouglas,Geographer in charge. 

Topography by Frank Tweedy. 

Triangulat-ion by H.L.Baldwin,Jr. 

Surveyed in 1903. 

MAP OF NORTHERN PART OF GEORGETO'WN QUADRANGLE, COLORADO , SHOVliNG DlKF.S ,\TEINS, 

cale~ 

Contour intervallOO feet 

10 

Dikes by S.H.Ball,assisted 

by O.Hershey. 

Veins byJ.E.Spurr and 

G.H.Garrey. 

Mines by G.H.Garrey. 

Surveyed in 1904-5. 

Alks 

Biot 

AlE 

lll.O 

.A

Bo 

[ 

Gra 

Qua 

po 

Area o

map 

b
NORTHEAST-SOUTHWEST MINERAL BELT. 103 

where they occur in limestones, in which many large ore bodies are formed by replacement.· 

These ore bodies, however, have been noted in a number of cases to have a general northeast 

trend, showing that they resulted from fracturing in that direction. 

OHES IN BOULDEH COUNTY. 

In Boulder County, at the northeast end of the mineral belt, there are gold-bearing veins 

and silver-bearing veins, both of great economic importance. The silver-bearing group includes 

such veins as that of the Caribou mine, which contains galena, chalcopyrite, blende, gray copper, 

stephanite, ruby silver, and native silver. The.golcl-bearing veins form two distinct groups, 

those containing free gold and those containing tellurides .. These veins also occupy fairly 

distinct belts. The telluride veins contain sylvanite, petzite, calaverite, ruby silver, and free gold. 

The gold veins contain free gold, pyrite, and copper pyrite, with small quantities of blende and 

galena. J. B. Farish a has described a free-gold vein cut through by a younger telluride vein. 

The former is called the Golden Age, the latter the Sentinel. The ore of the Golden Age is a hard 

white quartz, much of it vitreous, containing free gold, with but a small amount of sulphides. 

The Sentinel vein, which cuts through the Golden Age, has the bluish quartz common in the 

tellurium veins of Boulder County, with the characteristic chalcedony quartz crystals and finely 

disseminated pyrites. The value is in metallic gold, petzite, and sylvanite. From this Mr. 

}farish concludes that the gold mines of Boulder County belong to at least two distinct periods of 

vein formation. . 

ORES IN GILPIN COUNTY. 

In Gilpin County, also, definite belts characterized by gold-bearing veins or by silverbearing veins have been recognized.b The veins in this county occur along lines of slight faulting 

in metamorphic rocks, granite, and pegmatite. The ores of the silver belt carry little or no gold, 

but those of the gold belt carry silver in various proportions. Two sets of veins are present in 

the mineralized area, one having an east-west course, the other a northeast-southwest course. 

The main features of one group are similar to those of the other, and there is no reason to believe 

that they have originated at distinct periods. In the mineralized region are intrusive dikes, in 

part, at least, andesitic, wl10se trenus correspond with both the principal trends followed by the 

veins. These dikes are earlier than the veins. In the veins of the gold belt the principal gold 

carrier is pyrite, commonly associated with chalcopyrite, blende, galena, and gray copper, 

including both tetraheurite and tennantite. Carbonates of iron, manganese, lead, and copper 

occur in small proportion. Tellurium in appreciable quantities has been found in association 

with the ores at many of the mines and seems to play an important part as a gold carrier.c 

Bismuth and arsenic also occur, the former with the gray copper ores, the latter in small proportion in nearly all of the pyrite. Uraninite, or pitchblende, has been found in considerable 

·qtiantities. Barite and calcite are of rare occurrence. The ores of the silver belt carry galena, 

zinc blonde, and pyTite, with subordinate amounts of polybasite, stephanite, and horn silver. 

The gangue of th~ silver veins does not materially differ from that of the gold veins. Rickard 

notes in his description of the San Juan mine in this county the occurrence of large TOunded 

bowlders in a vein at a distance of several hundred feet from the surface. These bowlders are 

very likely the same as the friction conglomerate described in the present report (p. 311) 

from many of the mines in Clear Creek County near Idaho Springs, and which is due to Tenewed 

movement along the vein zone subsequent to oTe deposition. 

ORES IN CLEAR CREEK COUNTY. 

In the northeastern part of Clear Creek County the gold-bearing veins represent the southwesterly extension of the great copper-iron-gold belt of Gilpin County, which farther southwest 

becomes more argentiferous.a Still faTther southwest, as already stated, lies the principal silverproducing district around Georgetown, and in the same direction, along a series of less important 

a Proc. Colorado Sci. Soc., Yo!. ·3, 1891, pp. 3Hi-322. 

b Rickard, Forbes, Trans. Am. Inst. Min. Eng., vol. 28, 1899, p. 108. 

c Pearce, Richard, Trans. Am. Inst. Min. Eng., voL 18, 1890, p. 451. 

d Fossett, Frank, Colorado, 1880, p. 307. 
104 GEOLOGY OF THE GEORGETOWN QUADRANGLE, COLORADO. 

veins, the Argentine district, which is ncar the summit of the Front Range. McClellan Mountain, in the northeastern portion of the Argentine district, contains a number of fairly strong · 

lodes, which have formed along zones of slight faulting in granite and gneiss. Most of these 

veins trend northeastward, but some have a northwesterly direction. The ores consist of 

galena, blende, some cupriferous pyrite and pyrite, and some tetrahedrite. The sulphide 

ores are said to carry 25 to 30 ounces in silver and $2 to $3 in gold to the ton. The gangue 

is chiefly quartz, with considerable calcite and in many places much fluorite. There is a notable 

tendency for ore deposits to occur at the junction of branches. 

ORES FROM THE ARGENTINE TO THE TENMILE DISTRICT. 

In the immediate vicinity of the Argentine district are other mining districts in Clear 

Creek, Park, and Summit counties. These include the districts at the head of Geneva and Hall 

valleys. The main mineral zone runs through the crest of the range. In general the ores in 

this cluster of mining districts are stateu to be argentiferous galena, gray copper, iron pyrites, 

ruby silver, brittle silver, and native silver. Argentiferous bismuth ores are reported to occur, 

but are not usually very plentiful. . Most of the lodes of tllis section carry some gold. a 

This belt of veins is described as extending southwestward along the mountain divide past 

the headwaters of Swan Creek. b The veins in this belt carry lead and silver. Near Breckenridge veins of argentiferous galena and zinc blende occur in sedimentary roeks and associated 

with intrusive porphyries. Some lodes contain auriferous pyrite, and others are free-gold 

lodes. Breckenridge is at present most widely remembered by the fine specimens of leaf and 

wire gold wlllch occur in the shales of Fa,rncornbe Hill. This hill is traversed by a dyke of quartz 

porphyry, and the gold occurs in the dike as well as in the shales. c Well-marked quantities 

of bismuth and tellurium have been noted in the Ontario nline in an ore consisting of native 

golu crystals embedded in a matrix of hydrated oxides of iron and manganese. a 

OHES OF THE TENMILE DISTRICT. 

Directly southwest of Breckenridge is the Tenmile district, where the ores occur chiefly in 

an area of upper Carboniferous limestones and sandstones (Maroon formation), which is intruded 

by many sheets and dikes of porphyry. The ores consist chiefly of argentiferous galena, pyrite, 

and blende, and the gangue is quartz with some barite. The deposits have in part the form 

of fissure veins and in part that of irregular bodies replacing limestone. Three distinct types 

of deposits have been described by Emmons.e The Robinson mine constitutes one type. The 

ore lies along vertical faults of slight displacement, having a general strike of N. 60° E. 

The unalt.ered ore contains galena, blendc, and pyrite, with rhodochrosite, calcite, barite, and 

quartz as gangue minerals. In the deposits of the type exemplified by the ·white Quail mine 

the ore is mainly pyrite or marcasite, with small 11mounts of zinc blende and argentiferous 

galena. These materials occur as replacement deposits along well-defined shoots, in which the 

existence of an earlier system of northeast-southwest fractures may be detected. In the deposits 

of the type exemplified by the Queen of the West mine the ores consist of galena, blende, and 

· pyrite, with some sulphides and antimonides of silver, and calcite as the chief gangue. They 

occur in fissure veins which have formed in parallel fault fissures striking N. 60° to 70° E. In 

this district there is evidence of two periods of faulting, one previous to the ore deposition and 

one subsequent. . 

ORES OF' THE LEADVILLE DISTitiCT. 

Southwest of the Tenmile district and connected with it by a belt of mineral-bearing 

country is the famous Leadville district. Here the ores are chiefly replacement deposits in 

limestone. They follow in general the stratification of Paleozoic limestones, especially the 

a Fossett, Frank, Colorado; 1880, p. 381. 

b Idem, p. 481. 

c Lakes, Arthur, Geology of Western Ore Deposits, new cd., 1905, p. 225. Fossett, Frank, Colorado, 1880, p. 489. 

d Pearce, Richard, Trans. Am. Inst. Min. Eng., vol. "18, 1890, p. 452. 

e Emmons, S. F., Description of the Tenmile district: Geologic Atlas U. S., folio 48, U.s. Geol. Survey, 1898: 
NORTHEAST-SOUTHWEST MINERAL BELT. 105 

lower Carboniferous, or Leadville, blue limestone. vVithin these. sedimentary formations they 

have formed mainly beneath sheets of intrusive porphyry (the principal varieties of which are. 

called locally white porphyry and gray porphyry). The valuable ores include lead, silver, and 

gold. The gold belt is in general separated from the nonauriferous silver-bearing area and 

covers an extent of several square miles. It is marked by ores carrying more or less gold in 

addition to silver and the baser metals. According to Emmons, a the ores containing gold 

and· copper are more frequently found in siliceous beds, in porphyries, or in crystalline rocks 

than in limestones. The silver ores consist of galena, blende, and pyrite, and contain also 

arsenic, antimony, molybdenum, bismuth, etc. The gangue is jasperoid or quartz, with some 

barite. The gold ores are more highly pyritiferous. Emmons recognizes the intimate relation 

of the ores to the porphyry, subsequent to whose intrusion they were deposited. He concludes 

that they were derived from the porphyry by circulating waters of original metepric origin. 

A. A. Blowb has shown that on Iron Hill in Leadville the ore shoots, which are replacement 

deposits in limestone, follow northeastward-trending zones which are parallel with, but not 

everywhere in contact with, cross-cutting sheets of gray porphyry. Blow concludes that the 

ores :had been deposited by hot alkaline metal-bearing waters anqt vapors accompanying the 

period of solfatarism which followed and was consequent on the injection of the porphyries. 

Richard Pearcec discovered tellurium in Leadville pyrites; containing silver and a small 

value in gold. 

OHES OF THE REDCLIFF DISTRICT. 

A few miles northwest of the Tenmile district is the Eagle or Redcliff oistrict. The geology 

here is described as somewhat similar to that at Leadville. Above the fundamental granite lie 

in succession the Cambrian quartzite, Silurian and Carboniferous limestones, and the upper 

Carboniferous Weber quartzite. All these formations are intruded by porphyries. Argentiferous galena ores and gold orcs are reported. The gold deposits occur in Cambrian 

quartzites; the silver-lead deposits in Silurian and Carboniferous limestones,. intruded by 

porphyry sheets. The telluride of silver, hessite, has been found in large quantities in one or 

two mines.d 

ORES OF THE ASPEN AND NEIGHBOlUNG DISTRICTS. 

Southwest of Redcliff, on Frying Pan Creek, the geologic formations in the mineralized 

area arc described as including granite, quartzite, and limestone, and as being similar to those 

at Aspen. 

About 10 miles southwest of the Frying Pan district lies the Lenado district, and near 

this the Aspen district.e In these districts the sedimentary strata have been folded against 

the pre-Cambrian mass of the Sawatch Range and have been profoundly faulted. The folding 

and faulting include the Cretaceous rocks of the district,· and from what is known as to the 

history of the Rocky Mountains outside of this area the beginning of the vigorous dynamic 

disturbance is assigned to the general period embracing the close of the Cretaceous and the 

beginning of the Tertiary. Two varieties of igneous rock were intruded in the Paleozoic strata, 

one being a diorite porphyry similar to the rock of the porphyry laccoliths of the Elk Mountains 

and probably continuous with one of these. The other is a quartz porphyry, identical in composition and structure with the "white porphyry" of Leadville. These porphyry intrusives 

have 'been involved in most if not in all of the folding and faulting, but the period of their 

intrusion is believed to have coincided ·with the beginning of physical disturbances. The 

district is remarkable for complicated faults, whose development has proceeded from an early 

date in the history of the deformation clown to the present day, the faults becoming more 

numerous and complicated, but having less throw, as erosion removed the overlying load of 

strata. Some faults are postglacial, and in many places fault movement is going on at present. 

a Emmons, S. }'.,Geology and Mining Industry oi Leadville, Colo.: Mon. U.s. Geol. Survey, vol. 12,1880, p. 377. 

b Trans. Am. Jnst. Min. Eng., Yo!. 18, 1890, p. 145. 

c Idem, p. 451. 

d Pearce, Hichard, Trans. Am. Inst. Min. Eng., vol. 18, 1890, p. 451. 

e Spurr, J. E., Mou. U. S. Gcol. Survey, vol. 31, 1898. 
106 GEOLOGY OF THE GEORGETOWN QUADRANGLE, CObORADO. 

The ores of this district are lead-silver ores, co:q.taining no gold. The chief minerals are galena 

and blonde, with polybasite, tetrahedrite, and tennantite, and with quartz, barite, and dolomite 

as gangue. The ores were deposited. subsequent to the igneous intrusion and during the long 

period of faulting. They have been deposited along some of these faults and have been 

displaced by others. The mineralizing agent is believed to have been ascending hot waters. 

ORES FROM ASPEN THROUGH THE GUNNISON DISTRICT. 

From Asp~n southwestward a mineralized belt is traceable almost continuously from 

Ashcroft across the Elk Mountains to the Ruby, Irwin, and other districts of the AnthraciteCrested Butte region. In this region the ore deposits occur near the contacts of bodies of 

igneous rock intri.1sive into Cretaceous strata, one of the principal igneous-rock varieties being 

similar to one· of the intrusions at Aspen. Near many of these intrusions there are networks 

of small faults, usually with a verticnJ displacement of less than 100 feet and registering a 

dynamic movement since the deposition of latest Cretaceous strata. Along these faults the ore 

has formed in the shape of fissure veins, with strike mostly northeast and southwest or northwest ~nd southeast, though some of them trend north and south or east and west. The dip is 

usually vertical. As a rule, each district in this region has two principal systems of veins, 

intersecting at angles of 40° to 60°. The principal ores are galena, blonde, and pyrite, and are 

in the main of low grade in both gold and silver. In the Ruby district, however, arsenopyrite 

and ruby silver (both pyrargyrite and proustite) are present, with a gangue of quartz and 

calcite. Barite, siderite, and rarely fluorite also occur. a In the Ruby Chief mine some rhodochrosite is reported. b The best ores carry silver and locally a little gold. 

ORES OF THE SAWATCH RANGE, SOUTH OF LEADVILLE. 

The foregoing descriptions indicate the character and occurrence of the ore deposits along 

the principal mineral belt from Boulder to Leadville and along the parallel and slightly offset 

belt which extends from Redcliff to the Gunnison country. Besides these, there appears to be 

a belt forming a part of the general northeast-southwest zone of mineralization and extending 

in a direction somewhat west of north along the Sawatch Range. In the principal belt, running 

from Boulder to Leadville, the mineralization ceases to be conspicuous a short distance south 

of Leadville, though gold-bearing veins are reported in the vicinity of Granite and Buenavista.c 

·The chief belt south of Leadville, however, lies on both sides of the Sawatch Range, along the 

nearly north-south line between Chaffee and Gunnison counties. This belt includes the Tincup, 

Alpine, Monarch, and other districts. The Tincup district is in Gunnison County, near Alpine 

Pass, in the Sawatch. The Gold Cup mine is in limestone, reported to be of Carl.>Oniferous age. 

The ores contain lead and silver. The Golden Queen mine is a fissure vein containing mostly 

gold.d In the Alpine region of Chaffee County, not far from the Tincup region, the ores are 

reported to be silver sulphides and galena, carrying silver, some gold, and a little copper.e 

At the head of Tomichi and Quartz creeks, in the Sawatch Range south of Tincup, the 

country rocks are reported to include granite, quartzite, limestone, and porphyry. The ores 

are reported to include galena, pyrite, gray copP,er, and silver glance, and to occur as veins in 

granite, quartzite, and limestone/ · 

In this same belt is situated the Monarch district, at the headwaters of Arkansas River, 

in Chaffee County. The country rocks include granites and porphyries. The ores consist of 

carbonate of lead or galena, and are of low grade in silver. Copper and antimony occur 

in places.b This district has been relatively productive. 

a Emmons, S. F., Anthracite-Crested Butte folio: Geologic Atlas U. S., folio 9, U. S. Geol. Survey, 1894, p. 2. 

b Rickard, T. A., Across the San Juan Mountains, 1903, p. 95. 

c Kemp, J. F., Ore Deposits of the United States and Canada, 3d ed., 1900, p. 295. 

d Fossett, Frank, Colorado, 1880, p. 570. 

t. Idem, 5t:i9. 

I Idem, p. 5GO. 
NORTHEAST-SOUTHWEST MINERAL BELT. 107 

COINCIDENCE OF A BELT OF INTRUSIVE PORPHYRIES WITH THE ENTIRE BELT OF 

MINERALIZATION. 

The question of the distribution of intrusive porphyries in this portion of Colorado has 

been investigated by Mr. Ball; the distribution of veins has been looked into by the writers. 

Mr. Ball finds that there is a belt of porphyries having the form of dikes and sheets intrusive 

into older rocks and extending with unimportant interruptions from Boulder southwestward 

past Leadville to Aspen and Crested Butte, across the Colorado, Park, and Sawatch ranges. 

According to Mr. Ball, the belt widens to the southwest, being but 10 miles wide at Boulder 

and at least 75 miles wide near the west end of the designated stretch. These intrusive 

porphyries are mainly of siliceous or intermediate composition, resemble one another in a broad 

way, and probably belong to a single general period of intrusion. The porphyry belt coincides 

more or less exactly with the mineral belt which has been described, and the fact noted in the 

detailed study of the Georgetown quadrangle, that the porphyry dikes and mineral veins occupy 

the same definite and restricted belt, is therefore char.acteristic also, though perhaps less exactly, 

of the whole mineral belt from Boulder to Crested Butte. 

NATURE OF BELT MARKED BY DIKES AND VEINS •. 

DYNAMIC ORIGIN OF THE BELT. 

Near the southwest end of this belt of igneous intrusion and mineralization, where the 

belt passes through alternating pre-Cambrian, Paleo:wic, and Cretaceous rocks,· both the igneous 

rocks and the ore bodies tend to spread out in the form of sheets or irregular masses, many of 

them occupying relatively wide and irregular areas. In this southwestern region the true 

nature of the ore :wne is not so evident as farther northeast. For 60 miles, however, from the 

vicinity of Breckenridge to the foothills of the Rockies near Boulder, the belt of intrusion and 

mineralization runs through a broad area of pre-Cambrian crystalline rocks with practically 

no Paleozoic, Cretaceous, or Tertiary deposits. In this area the true nature of the belt becomes 

evident, and is seen to be due to dynamic movement, which has acted irrespective of geologic 

formations. In the study of the Georgetown quadrangle the exact nature of this belt has been 

made apparent as a zone of yielding to regional stress, characterized by fracturing, faulting, 

and fissuring. 

CHARACTEHISTIC FAULTING ALONG THE BELT IN THE GEORGETOWN QUADRANGLE. 

In the Georgetown region these disturbanc~s began before or at the period of the intrusion 

of the earliest dikes, and the earliest fractures, faults, and fissures were occupied by the intrusions. Taken as a whole, the porphyry dikes were intruded, as will be shown, at varioue epochs 

of a general period of volcanism, during which movements were renewed repeatedly along the 

old planes or along new planes more or less parallel to the old ones. A study of individual 

cases within the Georgetown quadrangle shows clearly that a given fault zone or fissure has 

been repeatedly opened up and repeatedly cemented shut again, either by injections of molten 

rock, forming dikes, or by deposition of minerals from solution, forming veins, or by both 

processes. l 

It was by no means uncommon for dikes of different material to be thus successively 

injected along the same line of fissuring (see description of Stanley mine, p. 341), and in some 

places as many _as three different dikes were thus successively injected (see description of 

Gol_den Chariot mine at Empire, p. 396). Between the periods of dike injection, and in general 

after all injection had ceased, the reopened fissures were cemented by vein material. A complicated succession of reopening~ and repeated depositions of vein material has been studied 

in a number of individual veins. In some cases where a vein was split open, the new fissure 

was as large or larger than the original fissure and was cemented by a later vein. The general 

result of the writers' investigations shows that repeated movements, associated with repeated 

dike injections, and with a continual process of healing by deposition from waters, have been 
108 GEOLOGY OF THE GEORGETOWN QUADRANGLE, COLORADO. 

going on, though with irregular and spasmodic intensity, since a time before the period of dike 

injection. There is evidence of the deposition of vein material very recently, and this, together 

with the existence of uncemented openings and fissures, accompanied in many localities by 

displacement, shows that the processes of faulting, fissuring, and cementation are still going on. 

Such more recent movements, later than the dikes and later also than the bulk of the mineralization, in general follow the old planes of weakness and are marked by open fissu~es along veins, 

b} seams of gouge or triturated material, and (especially in the neighborhood of Idaho Springs) 

by characteristic zones of friction breccia.in which the larger fragments are rounded by attrition, 

due to movement under eomparatively slight pressure and comparatively near the surface. 

SIMILAR FEATURES OF OTHER PORTIONS OF THE BELT. 

The, true character of this mineral belt as a zone of weakness, affording relief to expansile 

and contractile strains in a comparatively wide area of crust, is thus well shown in the area of 

the Georgetown quadrangle. Thls character is substantiated by the data at hand from other 

portions of the belt. Throughout that part of the belt which passes through the pre-Cambrian 

crystalli:he rocks the ore deposits have almost exclusively the form of fissure veins, and both 

veins and porphyry dikes have a predominant northeast trend and steep dip. In the more 

complicated portion of the belt, where it passes on the southwest into areas containing preCambrian, Paleozoic, and Mesozoic rocks, similar northeastward-trending :fissure veins are 

associated with replacement deposits in limestone, many of which have been found to fall into 

definitely aligned shoots with a northeast trend. 

DYNAMIC MOV~>M.F;NTS ALONG BELT IN BOULDER AND GILPIN COUNTIES. 

Intersection and jaulting.-The intersection and local faulting of one Yein by another in the 

Boulder region has been described," showing different periods of movement and of mineralization. In Gilpin County Forbes Rickard has recognized the fact that the ore zone is marked by 

weakness and fissuring and has noted that the veins occur along faults which are, as a rule, of 

slight displacement.b 

Amount of displacement along veinjaults.-The above observation concerning the amount 

of displacement coincides with the observations in the Georgetown area, although in some 

localities the aggregate of all faulting movements which have successively taken place along a 

siugle line, plane, or zone is more considerable. In this pre-Cambrian region markers of displacement are usually wanting or difficult to find, but the dragging apart of separate fragments of what was once a probably continuous dike by strike faulting due to renewed movement 

along the zone of the dike has been observed at many places. 

Similar strike faulting along veins has been frequently recorded. Where renewal of movement has thus taken place along old fault planes, if the plane of the new movement coincides 

in every detail with that of the old, it results only in parallel fissures or zones of gouge material 

or of breccia within or on the walls of the preexisting dike or vein, with no displacement of these 

older formations, and where the new fissures and zones of fracturing becorne ceme~ted by vein 

material the amount and in many veins the existence of the strike faulting are diffteult to 

decipher. H, however, the renewed movement takes plac~ along a zone instead of a plane, and 

includes moveme'nt along minor oblique as well as along major parallel planes, the dike or the 

vein may be dragged apart and form separate shreds in the manner mentioned. Here and 

there dikes and veins are cut by later movements along transverse planes, the result being a 

horizontal displacement or offset. The amount of such offsetting in the Georgetown quadrangle 

is usually insignificant, the most extensive displacement observed being that of the large porphyry dike in the area of the Silver Plume special map by transverse faults, the offset amounting 

in some places to about 200 feet. It is probable that this displacement is very much less than 

that which has been accomplished here and there by strike faulting, though along other strike 

faults the aggregate of displacement since the movement began has been only a few feet. 

a Lakes, Arthur, Geology of Western Ore Deposits, new ed., 1905, p. 209. Farish, J. B., Proc. Colorado Sci. Soc., vol. 3, 1891, PP· 316-322. 

b Trans. Am. Inst. Min. Eng., vol. 28, 1899; p. 108. 
NORTHEAST-SOUTHWEST MINERAL BELT.' 109 

FAULTING IN THE TENMILE, LEADVILLE, AND ASPEN DISTRICTS. 

In the Tenmile district Emmons finds two periods of faulting, one previous to the ore 

deposition and one subsequent to it. In this region faulting took place at different pe,riods 

along different planes, instead of being confined practically to the same plane, as in the Georgetown region. At Leadville, where the ores are later than the porphyry intrusions, both porphyry and ore deposits have been displaced by numerous faults, maw of them heavy. This 

faulting movemen~ was long lived, was even kept up in post-Glacial time, and is very probably 

still going on. a · In the Aspen district, where, as in Leadville, the ores occur chiefly in Paleozoic 

limestone, the intrusion of porphyry dikes was followed by the development of a complicated 

series of faults. The movement along these faults has lasted from the period of intrusion 

through the Glacial period to the present day. In tlus district, although so!ne systems of faults 

having a different strike and dip were developed later than others, there has also been a general 

movement renewed at intervals along the same fault planes. The main ore deposition began 

after the beginning of the main period of faulting and practically terminated before its close, so 

that the ore was deposited along the earlier faults. These ore deposits, however, have been 

broken up by strike faults clue to renewal of movement along the fault planes where the ore 

deposition originally took place and also by t~ansvcrse faults. 

SUMMARY OF DYNAMIC IJISTORY OF BELT. 

Thus, in general, a similar history is recorded along the whole belt of mineralization. The 

whole belt has been subject to repeated and continued faulting from the time of the first intrusion of dikes. The phenomena of dike intrusion and vein formation have been more restricted 

in point of time, the former probably more so than the latter. 

AGE OF PORPHYRY INTRUSIONS. 

LACK OF EVIDENCE IN PRE-CAMBRIAN AREA. 

Throughout the belt described, from Boulder to Leadville, and from Reclcliff past Aspen 

to the Ruby and Irwin districts in the Crested Butte region, the ores are, as a rule, later than 

the intrusions of porphyry, which consequently afford the first evidence of the existel).cc of the 

zone of regional weakness and fracturing. Where this zone lies in pre-Cambrian .rocks, as it 

does throughout that region which includes the Georgetown quadrangle, we have no further 

evidence as to its age and that of the porphyries other than that these dikes cut all the preCambrian rocks, and that the intrusions took place successively and probably occupied a considerable period of time. · · 

EVIDENCE OUTSIDE OF PRE-CAMBRIAN AREA. 

Around Breckenridge, Tenmile, and Leadville, however, intrusions of the same kinds of 

rocks cut not only the pre-Cambrian rocks but sediments of Paleozoic and Cretaceous age. In 

the Leadville district Emmons distinguished two groups of post-Paleozoic rocks, the younger 

consisting of extensive Tertiary rhyolites and andesites, with one occurrence of quartziferous 

trachyte,b the older inclurling the intrusive porphyries which accompany the ore deposition 

and which are similar to the porphyries characterizing the mineral belt ncar Georgetown. Concerning the age of this older group, Emmo:ris states that no exact definition is possible.c The 

period of chief orogenic movement, including folding and faulting, is placed hy the consensus of 

geologists at the close of the Cretaceous, since Tertiary beds (not represented in the Leadville 

district) overlie the folded Cretaceous strata unconformably. The porphyries are found only 

in Paleozoic formations within the limits of the area shown on the Leadville map, but in other 

parts of Colorado the same· rocks cut the latest Cretaceous beds. Since, however, both t~e 

porphyry sheets and the subsequently formed ore deposits were displaced by folding and faulting, 

which Emmons regards as accompanying the post-Cretaceous uplift, he regards the date of 

a Emmons, S. F., Mon. U.S. Geol. Survey, val. 12. 1886, p. 33. b Idem, p. 86. c Idem, p. 293. 
110 GEOLOGY OF THE GEORGETOWN QUADRANGLE, COLORADO. 

intrusion as not later than the end of the Cretaceous. "It seems probable, therefore, that, 

though the eruption of this type of rock may have commenced much earlier, it lasted in this 

region till near the close of the Cretaceous." a · 

In the near-by Tenmile district arc intrusive porphyries of which one at least (the Lincoln 

porphyry) is also represented at Leadville.. Later than these porphyries and cutting through 

them are rhyolites. In this case Emmons b is inclined to think that the porphyry intrusions 

were pre-Cretaceous ~or Jurassic), basing this supposition on the mapping by the Hayden 

Survey of the Mosquito fault, .which in the vicinity of Leadville has been proved to be later than 

the intrusions. The Hayden map shows the Dakota (Cretaceous) overlying and unbroken 

by the ~osquito fault.c Cross/ however, points out that this mapping lacks .confirmation, 

and presents arguments opposed to tllis conclusion as to the age of the porphyries, .the principal 

one being that almost identical rocks are intruded in the Cretaceous strata of Middle Park, at 

Blue River Butte. Cross has described the laccolithic form of the principal porphyry intrusions, 

both in the Mosquito range, including Leadville, and in the Tenmile district. He .considers 

that the same class of magmas, whlch in these districts have been injected into the older, more 

or less disturbed strata of the mountains, and are found· as dikes in the fundamental Archean 

complex, were also in other districts injected, at probably the same period, into nearly horizontal strata.e These other districts include in Utah the Henry Mountains, in the southern 

part, and the Abajo Mountains, in the eastern part of the State; in Colorado the West Elk 

Mountains, the San Miguel and La Plata, and other mountains in the San Juan region, and the El 

Late Mountains, in the southwest corner; and in Arizona the Carriso Mountains, in the northeast 

corner. In all of these localities intrusive rock occurs in the form of laccoliths. The Spanish 

Peaks and adjacent mountains in southern Colorado are made up of probably similar rocks. 

Cross finds that although the rocks forming these intrusive masses show a considerable range in 

composition, yet the great majority of them belong to one well-marked structural type, with 

but slight variation in mineralogicaJ constitution. "This is plainly the result of the similarity 

of conditions of consolidation upon magmas which are much alike in their controlling elements."/ 

Cross concludes that these porphyry intrusions are of Tertiary age.u · 

ORE DEPOSITS SOUTHWEST OF THE BELT DESCRIBED . 

. The above-described belt of ore deposits has been considered separately from other Colorado 

ore deposits on account of their forming a connected chain and having characteristics so definite 

in common. If the trend of this belt is followed farther southwest, however, it is found that 

there are ore deposits notably massed along its projection. 

VULCAN MINING DISTRICT. 

The Vulcan mining district is encountered about 15 miles south of Gunnison, ncar the 

boundary between Gunnison and Saguache counties. In this district are white pre-Cambrian 

mica schists veined with feldspar and quartz and cut by basic dikes. These are overlain by 

nearly horizontal beds of Cretaceous (Dakota) sandstone. Lying up on the Dakota sandstone 

are bodies of coarse andesitic breccia 400 to 500 feet thick. Above this is a porous tuff, and, 

still higher, basaltic flows. The principal mine of the district, the Good Hope, lies in the schist. 

The vein strikes east and west and dips steeply to the north. The gangue is quartz, much of it 

opalescent and high in free gold. There are large bodies of low-grade pyrites. Specimens of 

native tellurium, petzite, and telluride of copper have been found. There is evidence tQat the 

a Emmons, S. F., Mon. U.S. GeoL Survey, vol. 12, 188G, p. 294. 

b Emmons, S. F., Description of the Tenmile district: Geologic Atlas U.S., folio 48, U.S. GeoL Survey, 1898, p. 3. 

cAn additional reason given in the Tenmile folio for assuming a pre-Cretaceous age for the Mosquito fault, was the transgression of the 

Gunnison-Dakota formations directly on the Archean across the range from Tenmile, in the Bluo River valley. 

dCross;Whitman, Fourteenth Ann. Hept. U.S. GeoL Survey, pt. 2,1894, p. 224. 

e Idem, p. 219. 

fIdem, p 225. 

g Idem, p. 238. 
NORTHEAST-SOUTHWEST MINERAL BELT. 111 . . 

vein has been shattered and reopened at a comparatively recent date, and has then served as a 

vent for hot springs. The older vein of opalescent silica has been split open and the fissures 

have been filled with siliceous sinter inclosing fragments of the vein.a 

SAN JUAN REGION. 

A short distance south and west of the Vulcan district is the ilor.thern portion of the San 

Juan, region. This is a rugged, mountajnous country containing many mining districts, 

including those of Lake City, Ouray, Telluride, Silverton, Rico", and Creede. During the last 

ten years this region has been considerably studied by the geologists of the United States 

Geological Survey, including Cross, Purington, Ransome, and Spencer. The following general 

description is taken largely from the Surv.ey reports. · 

GENERAL GEOLOGY. 

The term San Juan region b is applied to a large tract of mountainous land in southwestern 

Colorado. The chief mountain mass is known as the San Juan Mountains. The Needle Mountains are almost continuous with the San Juan proper. The La Plata Mountains form an 

isolated group on the edge of the San Juan Mountains, and the Rico Mountains constitute an 

outlying group having certain genetic features in common with the La Plata Mountains and 

distinct from the main range. 

On the borders of the San Juan region are ancient pre-Cambrian granites, gneisses, and 

schists, above which Cambrian, Devonian, Carboniferous, Triassic, Jurassic, and Cretaceous 

strata were laid down. Cretace.ous beds from the Dakota to the Laramie are represented. The 

long Upper Cretaceous sedimentation was terminated by a continental uplift of unknown but 

possibly great extent. In the San Juan region this was marked by folding. The San Juan 

Mountains represent a broad anticlinal or quaquaversal fold c which was formed during the postLaramie uplift. 

Local ifregularities are shown by abrupt folds and faults.d Several domal uplifts, such 

as those of the Rico, La Plata, and Needle mountains, are probably of later origin and have 

been superimposed upon or added to the larger San Juan structure, in part obliterating it. 

During and subsequent to the uplift erosion was very active and produced a plane of moderate 

relief cutting across the obliquely upturned edges of the entire series of Mesozoic and Paleozoic 

formations. Upon this plane surface the succeeding formations were laid down, being separated 

from the Laramie by a. striking unconformity which is one of the most important known in 

the Rocky Mountains. The first of the unconformable overlying formations is a conglomerate 

(the Telluride conglomemte), which has been derived from .the erosion of the older quartzites, 

schists, granites, gneisses, sandstones, and limestones. This was a terrestrial deposit, which 

is exposed over certain portions of the San Juan region. It has been interpreted as the wash 

fr.om a mountainous tract upon a plain stretched out at its base.e It occurs in various thicknesses up to 1,000 feet. After several hundred feet of this formation had been deposited· a 

great epoch of volcanic activity began and a vast amount of material was emitted from volcanoes 

and fissure conduits. The earliest volcanic formation was a thick series of bedded tuffs, which 

were laid down immediately after the deposition of the Telluride conglomerate. They are 

intermixed with the conglomerates in their upper portionJ and overlie them with an aggregate 

thickness of as much as 2,500 feet. · This lower tuff series is called the San Juan tuff. Subsequent to its deposition was a period when rhyolitic and andesitic lavas were emitted as alternating flows, with accompanying deposits of tuff. The lavas and tuffs make' up the Silverton 

volcanic series, which reaches a thickness of 4,000 feet or more. Above this series is a further 

accumulation of volcanic .material, chiefly rhyolitic, which is called th~ Potosi volcanic series 

a Lakes, Arthur, Geology of Western Ore Deposits, new ed., 1905, p. 264. Rickard, T. A., Across the San Juan Mountains, 1903, p. 84. 

b Cross, .Whitman, and Howe, Ernest, Description of the Silverton quadrangle: Geologic Atlas U.S., folio 120, U.S. Geol. Survey, 1905, p. 1 

c Ransome, F. L., Rico !olio, No. 130, U.S. Geol. Survey, 1905, p. 7. 

d Silverton folio, No. 120, U.S. Geol. Survey, 1905, p. 21. 

. e Cross, Whitman, and Ilowe, Ernest, Silverton folio, No.120, U.S Geol. Survey, 1905, p. 21. 
. r 

112 GEOLOGY OF THE GEORGETOWN QUADRANGLE, COLORADO. 

and has a thickness of 1,000 feet or more. The above-named three series of bedded volcanic 

deposits make up the greater part of the western San Juan Mountains. East of the Silverton 

region their thickness gradually lessens, and they disappear under later lavas, not fully investigated, but known to include rhyolite, andesite, and basalt. The source of most of the San 

Juan lavas is unknown, but it is recognized that the Silverton volcanic series was in part emitted 

through fissures now filled by dikes. 

Subsequent to the formation of these surface volcanics occurred the intmsion of massive 

bodies of rocks such as diorite, gabbro, and monzonite, that were in many places co~rsely 

crystalline. These intmsions were in the form of stocks and penetrated, and were therefore 

later than, all the bedded lavas described. Other igneous bodies, consisting of diorite porphyry 

and allied rocks, which occur as sheets and laccoliths in the sedimentary beds adjacent to the 

San Juan Mountains, are probably also of later elate than many of the surface lavas. The 

laccoliths are thought to be earlier than the stock intmsions, since in the Rico and La Plata 

mountains the diorite and monzonite porphyries which occur elsewhere as sheets and laccoliths 

have been cut by diorite stocks. 

The three different productions of the San Juan volcanic emption, the surface lavas, the 

stocks, and the s)leets of laccoliths, do not differ essentially in chemical composition. It is 

doubtful whether the reservoirs represented by the stocks and laccoliths ever furnished material 

for surface flows of any consequence. The laccoliths are similar to tho~e of porphyry masses 

scattered through Colorado and adjacent parts of Utah, Arizona, and New Mexico. The latest 

of all the volcanic rocks were certain dike rocks, usually of basic character and probably connected with the stock and laccolith intmsions. They are regarded as probably the products of 

differentiation from the monzonitic magmas of the larger intrusions: In the Telluride quadrangle they comprise pyroxene andesite, plagioclase basaJt, augite minette, vogesite, and augite 

camptonite." In the La Plata quadrangle they are classified as pyroxenic, monzonitic, and 

dioritic lamprophyres. The basic dikes are made up of the same minerals as are present in 

the more abundant rocks with which they arc connected, but in them the dark silicates attain 

more or less decided predominance. 

Volcanic emptions in the San Juan region probably continued at intervals until late in 

the Tertiary period, although 9nly the products of the earlier outbursts are known.b Little 

evidence is available by which the sequence of events can be correlated with established divisions of Tertiary time. Certain calcareous tuffs of the Silverton quadrangle contain scanty 

plant and invertebrate remains, indicating that they were formed in Oligocene or early Miocene 

timc.c The basal member of the nearly horizontal Tertiary series, the Telluride conglomerate, 

contains no fossils, but has provisionally been referred to the Eocene. Cross has called attention to the analogy in composition and general relations between the Telluride conglomerate 

and the Arapahqe formation of the Denver Basin. d Such a comparison carries with it the 

further correlation of the San Juan tuff with the Denver formation, which overlies the Arapahoe 

formation and which is composed largely of andesitic debris. The Arapahoe and Denver formations are fossiliferous, and although they are separated from the Laramie by the great postLaramie uplift and subsequent erosion, the fossil evidence is still considered by paleontologists 

as indicating Cretaceous age. 

While the principal quaquaversal uplift of the San ,Juan Mountains was pre-Tertiary, the 

smaller domal uplifts which have been added to this larger structure took place at a later 

period. In the Rico and the La Plata mountains are quaquaversal domes whose uplift took 

place in the Tertiary period, and possibly in the Eocene epoch, soon after the formation of the 

San Juan tufl'.e These uplifts are regarded as having probably a genetic relation to the older 

San Juan structure. Some observed deformation was undoubtedly due to intmsions of igneous 

rock, but it appears certain that on the whole the Rico dome and probably the La Plata dome 

a Telluride folio, No. 57, U. S. Geol. Survey 1899, p. 7. 

b Cross, Whitman, and Howe, Ernest, Silverton folio, No. 120, U.S. Geol. Survey, 1905, p. 2. 

c Cross and Howe, Joe. cit. 

d Telluride folio, No. 57, U. S. Geol. Survey, 1899, p. 13. 

e Rico folio, No. 130, U. S. Geol. Survey, 1905, p. 11. 
NORTHEAST-SOUTHWEST MINERAL. BELT. 113 

were formed by a single uplifting force, acting independent of any actual observed intrusion 

of liquid rock material. The San Juan structure, which is believed to have been an earlier 

and larger manifestation of the same force, was also probably accomplished by a force pressing 

upward from below and not represented by any known intrusives. 

Subsequent to the eruption of all the volcanic rocks a complex fault system was developed 

in the San Juan Mountains. Many of these faults and attendant and largely subsequent 

fissures have become mineralized and forri1 the veins of the region. 

ORES 0~' THE SILV~JRTON REGION. 

In the Silverton region it has been noted by Ransome a that, although the veins strike 

in nearly every direction, the most conspicuous set has a northeast trend and a steep dip, 

while a second set, slightly less marked, strikes northwest and also dips steeply. One very 

noticeable fault zone has been traced from the center of the Silverton area northeast for a 

distance of 7 miles, and is known to extend farther for a distance yet undetermined. This 

,zone is 3 miles wide and within it there is complex faulting. The vein fissures, which have 

different strikes, intersect at numerous points, and Ransome regards it as possible that many 

of these intersecting fissures have been produced by a single stress and have been filled by one 

process of vein deposition. In some places, however, the intersecting fissures are shown to 

have been of different age, since they are filled by different material. In many such cases 

the older vein is crossed by the younger one without recognizable faulting. In a few places 

the older lodes have been faulted by the later transverse vein-filled fissures. In these cases 

the older lode, running more nearly north and south, has been faulted by barren or low-grade 

lodes trending more nearly east and west. Fissuring subsequent to the formation of the veins, 

and resulting in the reopening of the same fissures instead of the creation of new transverse 

fissures, has taken place in very many places, and the new openings have been subsequently 

filled with generally barren or low-grade quartz. Renewals of movement subsequent to all 

vein deposition are also shown by unfilled fissures and seams of gouge. The ores have a gangue 

of quartz, pyrite, calcite, dolomite, rhodochrosite, rhodonite, kaolin, and fluorite. The metallic 

minerals include pyrite, tetrahedrite, sphalerite, chalcopyrite, galena, polybasite, ruby silver, 

bismuthinite, argentite, and here and there tellurides of gold and silver in small amounts only. 

The mineralizing agents are believed to have been hot ascending waters, which derived their 

heat from volcanic rocks and which collected their mineral contents from the rocks through 

which they percolated on their downward journey from the surface to the heated regions and 

their ascent as hot springs. Since the lodes cut all the volcanic rocks of the region, their formation is regarded as not earlier than late Tertiary and as possibly having extended into the 

Pleistocene. ORES OF THE TELLURIDE QUADRANGLE. 

The veins of the Telluride quadrangle, which adjoins the Silverton quadrangle on the 

west, have been described by C. W. Purington.b His conclusions as to the relative age of the 

fJSsures agree closely with the later work of Ransome in the Silverton district. The :fissures of 

the Telluride quadrangle, though of different strike, ~re probably mainly contemporaneous. 

To judge from the geologic map accompanying the' report, the principal direction ·of the veins 

appears to be northwest, a smaller number having an east-west or northeast-southwest strike. 

There are in this region gold lodes and silver lodes, many of them quite distinct from one 

another, although there is no evidence of more than one distinct widespread period of ore 

infiltration. Only one place was noted where intersecting veins have been deposited during a 

different period. In this case a number of northwest veins are crossed and faulted by the 

Pandora vein, which runs east and west. The ore in the Pandora is of a different character 

from that in the veins which it faults, indicating that the east-west fissuring was later than the 

filling of the northwest veins by ore. The Pandora is regarded as probably the same as the 

Camp Bird vein in the Silverton quadrangle." 

a Ransome, F. L., Bull. U.S. Geol. Survey, No.182, 1901, p. 46. 

b Eighteenth Ann. Rcpt. U.S. Geol. Survey, pt. 3, 1898, pp. 751-848. 

~Ore Deposits, A Discussion (reprinted from Eng. and Min. Jour.), 1903, p. 88, 

31401-No. 63-08-8 
114 GEOLOGY OF THE GEORGETOWN QUADRANGLE, COLORADO. 

Evidences of recent strain and movement were noted along the veins, and in general the 

rock fracturing of the quadrangle is considered to be due to an extensive, far-reaching c:.tuse.a. 

The gangue minerals include quartz, barite, fluorite, calcite; rhodochrosite, and siderite, and 

. the metallic minerals pyrite, galena, blende, polybasite, ruby silver, chalcopyrite, and sphalerite. No tellurides are described from the region. The ores are believed to have been deposited from surface waters heated by volcanism and deriving more or less of their constituents 

from the exhalations of the magmas. The occurrence of fluorite in the gangue makes it probable that the ore was of deep origin. The ore deposition is assigned to a comparatively recent 

date, since it is believed that the last eruptions occurred in late Tertiary time, and thus were 

older than both the fissuring and the ore filling. 

ORES OF THE RICO QUADRANGLE. 

In the Rico Mountains, according to Cross and Spencer/ the ore deposits, together with 

those of the La Plata Mountains, belong to a great epoch of ore formation which succeeded 

the early Tertiary igneous intrusions and more typical volcanic eruptions in many parts of 

the Rocky Mountains. Ransome,C however, basing his estimate on the same facts, considers 

that the ore deposits are probably late Tertiary and possibly extend into the Pleistocene. 

Ransome assigns the same origin to these lodes as in the Silverton district, considering that 

they were deposited by waters of atmospheric origin which became heated in the neighbor-: 

hood of volcanic rocks and ascended along the lode fissures. The veins may be grouped into 

two sets, of which the principal one strikes northeast and contains profitable amounts of ore. 

The next most numerous set of fissures strikes northwest and is not ore bearing. It is regarded 

that both the northeast and northwest sets of fissures were initiated at the same time, but 

that the northeast fissures were the first to open and to permit vein deposition. The northwest fractures probably existed while the ore was being deposited in the more open northeast 

:fissures, but the principal movement of the northwest lodes was later than the ore deposition 

in the northeast lodes. At a period subsequent to this ore deposition many of the. northeast 

fissures were reopened and the new fissures were filled with barren quartz and pyrite, and 

the opening and filling of the northwest fissures is believed to have been synchronous with 

this period of reopening. The northwest lodes have been planes of movement up to recent 

times, as is shown by' the shattered condition of their filling at many places and by the occurrence of strips of gouge marking movement planes. As a rule, the lode fissures were clean, 

open fractures of slight tangential displacement and were later than the major faults of the 

region. The gangue minerals include quartz, rhodochrosite, calcite, fluorite, gypsum, and 

barite; the metallic minerals are pyrite, galena, sphalerite, chalcopyrite, tetrahedrite, ruby 

silver, and argentite. The bulk of the Rico ores may be divided into (1) pyritic ores, usually 

of low grade, and (2) argentiferous galena ores, some of which contain rich silver minerals. 

The two kinds are not capable of sharp distinction and are not necessarily of different age.d . 

In some veins of the district the rich silver ores (proustite, argentite, and polybasite) formed 

subsequent to the galena, sphalerite, and rhodochrosite; and also subsequent to these minerals, 

and, as it is supposed, to the rich silver ores, practically barren quartz containing a little pyrite 

was deposited. e 

ORES OF 'l'HB LA PLATA QUADRANGL~J. 

The La Plata quadrangle adjoins the Rico quadmngle on the south. Here the Mesozoic 

strata have been uplifted by numerous masses of molten magma which consolidated as sheets 

and small laccoliths of porphyry. Later, stocks of igneous rock were injected. At a period 

subsequent to the igneous intrusion ores of iron, copper, lead, and zinc were deposited, with 

·variable amounts of gold and silver. The La Plata uplift, which is in part contempomneous 

a Eighteenth Ann. Rept. U. S. Geol. Survey, pt. 3, 1898, p. 781. 

b Cross, Whitman, and Spencer, A. C., Twenty-first Ann. Rept. U.S. Geol. Survey, pt. 2, 1900, p. 34. 

<Hansomc, F. L., Twenty-second Ann Hept. U.S. Geol. Survey, pt. 2, 1901, p. 304. 

d Idem, p. 248. 

eldern, p. 253. · 
NORTHEAST-SOUTHWEST MINERAL BELT. 115 

with the igneous intrusions, was probably Tertiary, and presumably early Tertiary.a This 

uplift, however, together with the neighboring and similar Rjco Mountains uplift, was not 

due chiefly to the igneous rocks now exposed at the surface, since the existing porphyry masses 

are not capable of having produced the full domal structure at the center of the uplift. The 

intrusions are believed to be due to the same earth stresses which have produced t,he principal 

structure. 

The veins in the La Plata district may be grouped into three sets, according to their strike. 

One of the sets strikes east and west, and the two others strike northeast and northwest, respectively. The northeast and northwest fissures seem to have been formed latest. All the evidence points to the fissures having been formed by a force acting from the north, beyond the 

limits of the field.b The strongest veins follow the northeast-southwest and east-west fissure 

systems. The metalliferous minerals include gold and silver tellurides (sylvanite, petzite, 

and probably calaverite), argentiferous tetrahedrite, other sulphantimonides and sulpharsenides. 

of silver, pyrite, chalcopyrite, galena, and zinc blende. The gangue minerals include quartz, 

calcite, rhodochrosite, dolomite, barite, and fluorite. The orcs are of two kinds-telluride 

ores and ores in which the gold occurs free or associated with pyrite. The most important 

ores are believed to have been derived from the basic constituents of igneous rocks, from which 

they have been abstracted by waters that, having penetrated downward and become heated 

by the proximity of the volcanic intrusions, subsequently ascended as hot springs and deposited 

the ores. The ore deposition is believed to have followed closely the igneous intrusion, which, 

as stated above, was regarded by Spencer as presumably early Tertiary. 

ORES OF THE NEEDLE MOUNTAINS QUADRANGLE. 

The Needle Mountains quadrangle adjoins the Silverton quadrangle on the south. The 

general geology has been described by Whitman Cross and Ernest Howe, and the economic 

geology by J. D. Irving and W. H. Emmons.c The oldest rocks in tlus quadrangle are preCambrian sclllsts and gneisses, probably of igneous origin, and greenstones, with a great tlllckness of overlying quartzites, slates, and conglomerates, which have been named the Needle 

Mountains group. These pre-Cambrian sediments are possibly equivalent to the. schists of 

·. sedimentary origin (Idaho Springs formation) which form the oldest known rock in the Georgetown quadrangle, though in theN eedlc Mountains the strata referred to are not metamorphosed 

to anything like the extent of the Schists of the Georgetown district. Overlying these preCambrian rocks in the Needle Mountains quadrangle is a succession of Paleozoic sediments, 

including Cambrian, Devonian, and Carboniferous. strata. The Paleozoic rocks are overlain by 

the Tertiary (Eocene~) conglomerate, which in earlier reports was named the Telluride conglomerate. The older ·sclllsts of probable igneous origin and locally the sedimentary rocks of 

the Needle Mountains group are cut through by a variety of pre-Cambrian granite intrusions, 

which have been separately mapped. These granitic intrusions are accompanied by dikes of 

aplite and pegmatite. They may correspond in general with the intrusive granites which cut 

through the Idaho Springs formation in the Georgetown quadrangle. In the pegmatites·connected with some phases of the granites of the Needle Mountains magnetite occurs regularly 

though not abundantly, a fact wlllch supports (so far as it goes) the suggested correlation 

with the Georgetown rocks, since magnetite is common in the pegmatites connected with certain 

of .the granites of the Georgetown district, and in places, as described elsewhere, becomes an 

abundant and striking constituent. 

Tertiary volcanic rocks make up the same formations as described elsewhere in the San 

Juan region, comprising the San Juan tuff, composed of andesitic material, and the overlying 

Potosi volcanic series, consisting of andesitic and rhyolitic lavas and tuffs. There is also some 

intrusive rhyolite. The Needle Mountains are due to a local domal uplift superimposed upon 

the great San Juan dome and probably caused by an early Tertiary movement. The structure 

is sinlilar to that of the Rico and La Plata mountains. 

a Spencer, A. C., La Plata folio, No. 60, U.S. Geol. Survey, 1899, p. 10. 

b Purington, C. W., La Plata folio, No. 60, U.S. Geol. Survey, 1899, p. 12. 

cGeologic Atlas U.S., folio 131, U.S. Geol. Survey, 1905. 
116 GEOLOGY OF THE GEORGETOWN QUADRANGLE, COLORADO. 

The ore deposits are in, .the form of veins, which in most places have filled open fissures, 

while in others they are due to replacement of rock along fractures or fracture zones. The 

veins may be grouped into two chief classes, according to the dominant direction. The stronger 

·system consists of veins striking N. 10° W. and dipping usually 60°, though they vary from 

that angle to vertical. The other vein system has a strike of N. 80° E., exactly at right a:~gles 

to the first. At many points the veins belonging to the two systems cross each other without 

displacement, or a vein belonging to one set may terminate on intersecting the other. The 

formation of the fissures appears to have been accomplished at a single period. 

The original ore materials of the veins comprise pyrite, chalcopyrite, and galena, with 

a little sphalerite. The gangue consists of quartz, rhodochrosite, fluorite, chalcedonic silica, 

and a little barite and calcite. .Many of the veins are well crustified. There is evidence of two 

distinct periods of fissuring and subsequent vein deposition, the later fissuring having reopened 

the older veins. The filling of the original fissures consists of a fine-grained gray siliceous 

gangue containing cuprifcrous pyrite, which carries both gold and silver, the latter mineral 

forming a slightly greater proportion of the actual value of the ore. This earlier vein filling 

was shattered so that angular fragments of the vein remain, partly filling the new :fissure. The 

resulting spaces were cemented with white quartz, rhodochrosite, and fluorite, with some galena 

and barite. The layers of quartz and fluorite were several times repeated, at some points the 

fluorite filling being last, while at others the final deposition was of white quartz. The values 

in the ore, except where enriched near the surface, are low, averaging $19, of which $4 to $6 

is in gold, $5 to $9 in silver, and the rest in copper. The pay values are contained chiefly in the 

pyrite of the first period of mineralization. Richer secondary ores due to the alteration of the 

original ores by superficial waters contain brittle silver, argentite, and native silver. The 

veins are of small vertical as well as horizontal extent. From this circumstance it is assumed 

that they are the remnants of more extended fissures whose upper parts have been eroded. 

It is therefore supposed, in view of the extensive assumed erosion, that· the veins were 

not formed at a recent period. This deduction, however, appears to the writers as not following fairly from the fact on which it is founded. The relatively superficia1 character of 

the veins is on the whole characteristic of the San Juan region, as well as of many other Colorado regions. Ransome has noted this fact in the Silverton area, and from the further circumstance that the veins are limited in downward extent to about the same degree, whether they 

are located high up on mountains or far down on the valley sides, he has inferred a relation 

between the downward limit of the veins in different parts of the district and the present 

surface. He considers that such a relation can hardly be accidental, and concludes that at the 

time of the vein formation the surface configuration could not have been very. different from 

what it is now. As a corollary of this conclusion, he considers that the veins were formed in 

comparatively recent time. 

OR~JS OF OTHER SAN JUAN DISTRICTS. 

The other districts of the San Juan region are very similar in character to those described, 

although they have not been so thoroughly investigated. Near Ouray arc silver ores COJ.ltaining galena, ·gray copper, ruby silver, and brittle silver, the gangue vein minerals including 

quartz 1md barite. There are also gold ores consisting of copper and iron pyrites, galena and 

blende, tellurides of gold and bismuth, and in some places free gold. 

The general geology is similar to that in other parts of the San Juan.a Near Lake City are 

silver lodes containing galena, copper pyrites, gray copper, ruby silver, and brittle silver.b Near 

Creede the rocks are all volcanic, consisting mainly of rhyolitic and andesitic breccia and quartz 

p~rphyry. The veins are fissure veins, with a gangue of quartz and barite, and are traceable 

long distances. This is essentially a lead-silver camp." In Rio Grande County, especially 

around Summitville, are important gold-bearing veins. 

a Fossett, Frank, Colorado, 1880, p. 543. Lakes, Arthur, Geology of Western Ore Deposits, new ed. 1905, p. 2i7. 

b Fossett, Frank, op. cit., p. 517. 

c Lakes, Arthur, op. cit., p. 283. 
NORTHEAST-SOUTHWEST MINERAL BELT. 117 

SUMMARY OF CONDITIONS OF ORE DEPOSITION IN 'l'IIE SAN JUAN REGION. 

The ore deposits in different portions of the San Juan region exhibit many characteristics 

in common. They are typically fissure veins along fracture zones or fault fissures having slight 

displacement, whose formation has followed the formation of fau!ts of larger magnitude which 

have accompanied or followed domal uplifts considered to have been produced by an impulse 

exerted from below upward. These fissure veins have a predominant northeast strike and 

an approximately vertical dip. The predominance of a northeast system-is especially evident 

in the Silverton quadrangle, where there exists a broad zone of complex faulting that is 3 

miles wide and has been followed along its strike northeastward for 7 miles, beyond which it 

has an unknown further extent. 

The next most common system with regard to strike is the northwest system. Other 

veins, numerous but on the whole of subordinate importance and of less ntnnber, run in various 

other directions, especially north-south and east-west. The two chief sets of vein fissures, the 

northeast and the northwest as well as the subordinate sets, are considered to have been formed 

at the same time. Evidence of an extensive and far-reaching stress which has caused them 

has been noted in individual fissures and becomes plain when the close relationship of the 

strikes of the veins in all the districts examined is considered. Following the well-known laws 

concerning the attitude of fractures formed under a given pressure, several writers have postulated the existence of a more or less horizontal force acting in a direction which bisects the 

two chief vein systems. It has been sometimes further postulated that this stress came from 

the north, though the physical requirements are answered as well by supposing that it carne from 

the east or west. The more extensive faults, which in general are· older than the vein fissures, 

follow the same ·systems of trend, and, like the vein fissures, have a predominant steep or vertical 

dip. 

The domal uplifts began with and were part of the widespread post-Laramie disturbances, 

whose first conspicuous results were the formation of the San Juan dome and the accompanying 

but more local folding and faulting. After this event, and also after the considerable period 

of erosion which followed it, a basal series of conglomerates and a great thickness of later 

volcanic flows and tuffs were laid down unconformably upon the pre-Laramie rocks. Subsequently all these beds were intruded by sheets, dikes, stocks, and laccoliths of porphyritic and 

granular rocks. 

During the deposition of the bedded post-Laramie formations the doming movement which 

is manifested in the San Juan dome, formed previous to this deposition, continued and resulted 

in local domal structures that were superimposed upon the older and larger San Juan dome. 

These later upthrusting or doming movements were accompanied by and were largely accomplished by complex faulting, which has affected all the rocks mentioned. Along these faults, 

especially along the later and minor faults and fractures, the ores were deposited by ascending 

hot waters. Hence they followed and were probably consequent upon volcanic activity and 

seem to be more definitely connected with the latest volcanic event, the intrusion of the porphyries, since, so far as reported, veins older than the porphyries do not exist. The various 

sets of vein fissures, as stated above, were mainly formed by a single general stress and at the 

same period, and filled during a single period of mineral infiltration. In detail, however, there 

is evidence that some fissures were developed or opened later than others, and accordingly 

some veins were :filled somewhat later than other veins which they may in places cross or even 

fault. The formation of fractures and fissures, with or without faulting, did not cease with the 

period of main vein deposition, but has continued up to the present <;lay. The postmineral 

fractures and fissures usually act along the planes of the old vein fissures; showing a continuous 

or recurrence of similar stress. The results have l;>een the reopening of veins, and some of 

these new fissures have been recemented by later vein material, whereas other fissures, probably 

comparatively recent, still remain open and uncementecl. 
118 GEOLOGY OF THE GEORGETOWN QUADRANGLE, COLORADO. 

SJMILARITY OF SAN JUAN CONDITIONS TO THOSE OF THE BOULDBit-GUNNiSON BELT. 

It will be seen that the essential features of this history-namely, the post-~aramie dis-· 

turbances, continuing at intervals, though with decreasing intensity, throughout the Tertiary, 

manifested in the earlier stages more by flexing of the formations and in later stages as the 

overlying load of rocks was gradually removed by erosion, more by faulting; the intrusion of 

igneous rocks during the earlier stages of folding and faulting, and the subsequent flexing, tilting, and displacement of these igneous rocks by later warping and faulting-are essentially 

similar to those recorded in the mineral belt which stretches from Gunnison to Boulder and 

has been discussed in preceding pages. Moreover, the San Juan region is approximately in 

the path of the Boulder-Gunnison belt projected toward the southwest. It is suggested, therefore, that the intrusions, faulting, fissuring, and vein formation in the San Juan are duo to the 

same general causes as have been found to be responsible in the belt described, and that they 

.occur in especially close association and are localized along this northeast-southwest belt on 

account of the existence in the earth's crust of a northeastward-trending zone of weakness, 

which was formed at the time of the post-Laramie disturbances and has existed as such a zone 

of weakness throughout the Tertiary to the present day. Along this zone of weakness there 

have been repeated almost continuous openings and rcopenings of fault and fracture zones, 

which at intervals have afforded access to intrusive rocks and circulating waters. 

ORIGIN OF THE ZONE OF WEAKNESS. 

Assuming, therefore, that this general belt of initial weakness and igneous intrusion, faulting, 

and ore deposition extends, though with very irregular strength, through from. Boulder to the 

San Juan region, we find that it is characterized throughout by the persistent predominance of 

northeastward and northwestward trending fissures and faults. Of these the northeast system 

is invariably predominant if any extensive region is considered, though the northwest fissures 

are everywhere striking and important. These faults and fissures, moreover, are typically of 

steep· dip or vertical, and in most of the districts which have been carefully examined it has 

been concluded by different geologists independently that in general both sets of fissures were 

. formed at the same time. These facts imply the existence of a stress acting in a direction 

bisecting the chief directions of fissuring and faulting, namely, an east-west stress or a northsouth stress. That the direction was east and west and not north and sou,th is shown by the 

folding, which was also accomplished by the post-Laramie disturbance, and therefore was contemporaneous with the formation of the fault and fracture systems. These folds, as shown on 

the Hayden map, have a strong and definite trend west of north or north-northwest. Emmons a 

states that the post-Laramie dynamic movements may be most simply conceived as a pushing 

together from the east and from the west of the stratified rocks against the relatively rigid and 

already existing pre-Cambrian land masses. This compressing fo:rce was part of that great 

mountain-building pressure which, uplifting the whole Rocky Mountain region in post-Laramie 

time, upthrust the strata and compressed them into north-south folds. Simultaneous with 

the folding along north and south or north-northwest and south-southeast axes (the· direction 

of these axes being normal to the pressure from the west) the two lines of fractures were 

developed diagonally to the stress. Subsequent stresses, resulting in displacement along 

already formed fractures, selected especially the northeast direction, and the faulting resulting 

from these stresses became localized in this remarkable belt which cuts diagonally across the 

folds and mountain ranges and traverses the most of Colorado, though it is marked by only 

minor folding. The repeated fault movements recorded along this belt since its inception 

suggect that it has performed the function of accommodating and relieving major and minor 

strains which have rep.eatedly accumulated in a great mass of rigid crust on either side, thus 

perfonning the same function as the sliding joints employed by engineers to accommodate 

expansion and contraction in large structures of rigid material. 

a Emmons, S. 1<'., Mon. U.S. Geol. Survey, vol.12, 1886, p. 32. 
NORTHEAST-SOUTHWEST MINERAL BELT. 119 

DURATION OF ERUPTION AND MINERALIZATION IN THE REGIONS DESCRIBED. 

CltlTERIA IN THE SAN JUAN REGION. 

In the San Juan region the intrusive monzonitic porphyries have cut through older Tertiary 

volcanics, and hence are themselves of Tertiary age. No Cretaceous porphyries are known in 

this region. The only possible reference to such pre-Tertiary porphyries is contained in the 

description of the Telluride conglomerate, which is the basal·member of the Tertiary bedded 

series in the San Juan. Among the pebbles and bowlders of granite, gneiss, and schist .in this 

formation. are numerous pebbles of" igneous rock having a prominent porphyritic structure. a 

Cross observes that this rock, which he does not further: describe, is unlike any of the Tertiary 

porphyries and is probably of much greater age. It is possible that the rocks referred to are 

the frequently observed porphyritic phases of the pre-Cambrian granular intrusives. 

CRITERIA IN THE LEADVILLE, ASPEN, AND GUNNISON REGIONS. 

In the Leadville region are abundant porphyries belonging to varieties which, as Emmons 

mentions, b cut through the latest Cretaceous rock in other parts of Colorado, though since, at 

Leadville, they have been included in the post-Laramie uplift Emmons regards the eruption 

as having begun much earlier than the close of the"Cretaeeous. Inasmuch as the ores whose 

deposition followed the porphyry intruswn have also heel). much disturbed by folding and 

faulting, he regards the igneous intrusions as having lasted until a time near the close of the 

Cretaceous, though they may have begun much earlier. The fact that the porphyries cut the 

latest Cretaceous strata, taken by Emmons to signify that the igneous intrusion lasted nearly 

to· the close of the Cretaceous, is taken by Cross c as proof of the post-Cretaceous or Tertiary 

age of these Leadville porphyries and other intrusive rocks having similar relations to the 

Cretaceous strata. 

Moreover, the.evidences in the San Juan Mountains show that the strongly marked dynamic 

disturbance which began and was most· active immediately after the deposition of the Laramie 

(Cretaceous) did not cease at this period, but continued into the Tertiary, and that most of the 

frequently complicated faulting immediately. followed the major portion of the folding and was 

of Tertiary age. The continuity of the development of flexing and faulting since the beginning 

of the disturbance at the close of the Laramie even down to the present day has been also 

plainly shown by Bpurr a at Aspen, and the same continuity of faulting has been shown by 

Emmons, both at Leadville and in the Tenmile district. Therefore the fact that the porphyries 

and subsequent ores at Leadville have been included in folding and faulting movements does 

not prove that the porphyries were earlier than the post-Laramie uplift, though they may have 

been contemporaneous with its earlier stages and may have participated in most of its subsequent disturbances. The latter view was the conClusion 'drawn by Spurr e at Aspen, where 

intrusive porphyry similar to and probably contemporaneous with the white porphyry of Leadville occurs. The strong probability of the correctness of this view is further shown by the 

presence at Aspen not only of the white porphyry of Leadville, .but of intrusive diorite porpyhry 

in the form of 11 sheet, which extends with increasing thickness toward the Elk Mountains and 

is similar to the diorite porphyries that in the Elk Mountains intrude the Ruby formation. 

This formation is supposed to be the uppermost of the Cretaceous rocks in that region and is 

of post-Laramie age/ The diorite of the Elk Mountains has therefore been considered Tertiary. 

The diorite porphyry intrusive sheet at Aspen has participated in the folding and faulting to 

the same extent as has the white porphyry of Leadville in the same district, so that there is no 

evidence of a distinctly different age for the two rocks. 

a Telluride folio, No. 57, U.S. Oeol. Survey, 1899, pp. 4, 13. 

b Mon. U.S. Oeol. Survey, vol. 12, 1886, p. 294. 

cFourteenth Ann. Rept., U.S. Oeol. Survey, pt. 2, 1894, pp. 219-238. 

d Spurr, J. E., Mon. U.S. Oeol. Survey, vol. 31, 1898, pp. 143-150. 

e Idem, p. 53. 

f Anthracite-Crested llutte folio, No.9, U.S. Oeol. Survey, 1894 pp 5, 7. 
120 GEOLOGY OF THE GEORGETOWN QUADRANGLE, COLORADO. 

GENERAL TERTIARY AGE OF VOLCANISM. 

The known period of volcanism can therefore be stated with a strong degree of probable 

truth as having begun with the post-Laramie upheaval and continued throughout a great portion 

of the Tertiary. Taken all together, it may be conveniently designated as Tertiary, though it 

probably began in the late Cretaceous. 

DIFFEitENT PERIODS OF INTRUSION WITHIN THE GENEUAL VOLCANIC EPOCH. 

The diorite and diorite porphyry, which form laccoliths in the Elk Mountains, are considered 

by Cross to be approximately of the same age as simil,ar porphyries of variable composition, but 

on the average representing a monzonitic magma, which occurs as dikes, sheets, and laccoliths 

in the San Juan and other regions. ·while this is probably true in a large sense, as Cross states, a 

and also the statement that all are Tertiary, yet it is probable that in different localities the 

intrusion of these similar magmas took place at quite different Tertiary epochs. Probably very 

early Tertiary or late Cretaceous porphyries of this group, including the porphyries belonging to 

the Mosquito Range and Elk Mountains types found in the mining districts of Leadville, Tenmile, 

and Aspen, have been involved in most, if not all, of the post-Laramie folding and faulting. 

In the Leadville region above the folded porphyries and sedimentary rocks lie unconformable 

Tertiary volcanics, including rhyolitic and andesitic lavas, which, according to Emmons/ are 

younger than the ore deposition of the district. In the San Juan, however, the prophyries of 

this intrusive monzonitic group, instead of being involved in a post-Laramie uplift and having 

been upturned before the deposition of the overlying nearly horizontal Tertiary beds, are intrusive 

into these beds and into a thick series of nearly horizonal bedded Tertiary volcanics, which 

form the latest bedded rocks exposed. The age of these intrusive porphyries in the San Juan 

has been variously assigned by geologists as early Tertiary and late Tertiary,C but it is probable 

that a very considerable period of Tertiary time separated this intrusion from that of the Mosquito Range and Elk Mountains porphyries and that the group of monzonitic intrusives in 

Colorado, though forming a connected whol<;~ and erupted during the same general period of 

igneous activity, is made up of individuals which ~re of quite different age. 

DIFI<'ERENT PERIODS OJ<' ORE DEPOSITION WITHIN THE GENERAL MINERALIZING EPOCH. 

Similarly it appears certain that the mineralization in different places did not take place 

simultaneously. In various districts, such as the Boulder, Empire, Georgetown, and San Juan, 

veins of different age occurring close together have been described, though the various geologists 

who have made important contributions to the geology of the San Juan regimi are agreed that 

its veins are referable to a single general period. There is probably often a greater difference in 

age between the ore deposits in different regions than between veins of different age which, 

as already stated, have been described as occurring in the same district. For example, the 

probably distinct age of the porphyries of Leadville, Tenmile, Redclifi, and Aspen from that of 

the porphyries of the San Juan region, which has just been pointed out, carries with it the 

probably distinct age of the ores of these districts (and probably also of the ores of the Gunnison 

region, including the Ruby and Irwin districts) from that of the mineral deposits of the San Juan . 

. In every one of the districts mentioned all the United States geologists who have examined 

the geology have considered that the mineralization is i~timately connected with the porphyry 

intrusion and has followed it closely. If this is so, then the Leadville and Aspen ores, and 

others of this group, began to be formed in early post-Laramie time, which may mean very late 

Cretaceous or very early Tertiary, according to the placing by paleontologists of post-Laramie 

strata, such as those of the Ruby and Arl_tpahoe formations. 

In the San Juan region geologists have observed that the ores are later than the porphyry 

intrusions, which in turn are younger than the surface volcanics. Therefore a Tertiary age has 

a Rico folio, No, 130, U.S Geol. Survey, 1905, p. 11. 

b Mon. U.S. Gcol. Survey, vol. 12, 1S86, p. 39. 

c Ransome, F. L., Twenty-second Ann. Rcpt. U.S. Geol. Survey, pt. 2, 1901, p. 304. Cross, Whitman, Rico folio, No.130, U.S. Geol. Survey, 

1905, p. 11; La Plata fol!o, No. 60, U.S. Geol. Survey, 1899, p. 10. · 
NORTHEAST-SOUTHWEST MINERAL BELT. 1~1 

been assigned to the mineralization, which more definitely has been variously called probable 

early Tertiary and probable late Tertiary; and it is stated that in some places the ore deposition 

probably continued to Pleistocene time. There seems, however, little warrant for assigning 

an exact age to these deposits further than that they are probably later in general than the 

probably very early Tertiary group· at Leadville. It is perhaps inadvisable to assign a too 

recent date to the San Juan ores, since they are considered to have followed, probably directly, 

after the intrusions of the monzonitic porphyries in the form of sheets, laccoliths, and dikes. 

The crystalline condition of the more massive bodies of igneous rock presupposes several thousand feet of strata above them at the time of intrusion. a The time required for the erosion of 

this amount of overburden is considerable, though it is true that erosion and mineralization 

probably went on simultaneously and that at the end of the mineralization the surface may 

have been much lower than at the beginning. 

CONTINUITY OF VOLCANIC AND MINERALIZING ACTIVITY BETWEEN DIFFERENT TEltTIARY PERIODS. 

Some conclusions reached in the Aspen district w~aken to a certain extent the reasons given 

above for determining a sharp age distinction between the ore deposits of the Leadville district 

and those of the San Juan district. The results of the study of the geology in this districtb show 

that the post-Laramie folding resulted in the uplifting of the Sawatch Range as a comparatively 

gentle, broad anticline, on whose western flanks the Aspen district is situated. During this 

same post-Laramie epoch of mountain uplift there were intruded into the Aspen strata, certainly 

within a single general period, porphyries representing the white porphyry of the Mosquito 

Range and the diorite porphyry of the Elk Mountains. From the study of the Elk Mountains 

geology we know that tlus period of intrusion was not only post-Laramie but post-Ruby, and 

therefore probably Tertiary. These igneous intrusions were followed, probably very closely, 

by the locally intense folding characteristic of the Aspen district. This deformation seems to 

have been due to a lateral thrust, which pushed the sedimentary beds against the hard, resisting 

mass of the Sawatch Mountains. The cause of this lateral thrust was probably the uplifting 

of the Elk Mountains, which lie to the west, and this uplift was probably due to the intrusion 

in the Elk Mountains of large masses of molt~n material forced upward into the sedimentary 

beds. Continuation of the same force, therefore, which thrust the intrusive rocks into the 

sedimentary beds brought about the folding and breaking of these intrusive sheets along with 

the inclosing strata. The disturbance arising from the lateral thrust is restricted to a comparatively narrow zone running parallel to the main axis of the Sawatch. Its most important result 

·was a steep local overthrnflt anticlinal fold, the compression of which terminated in a fault plane 

passing through its axis. 

Immediately after the formation of this great fault, or perhaps synchronously with it, 

there began an uplift such as might arise from a vertically exerted force, that resulted in the 

formation of a local domal structure near Aspen. A similar though less strongly marked dome 

was also formed at Lenado. With the beginning of the formation of this local dome at Aspen 

there also began a system of local faulting which has continued from that time to the present, 

and by the displacement afforded by these faults the final uplift of the dome was largely 

accomplished. 

Immediately following the faulting and fracturing at the beginning of the uplift came an 

epoch of ore deposition. This epoch was short compared with that of faulting, for we 

know that the faulting has been continuous and can trace several distinct systems that have 

developed successively from the earliest period to the present. The systems first formed are 

ore bearing, showing that they existed at the time of the presence of the nlineral-bearing solutions. Later systems are, however, barren, and evidently no ore-bearing waters have circulated 

along them. 

The domal uplift at Aspen is purely local and has no apparent connection with the structure in the surrounding rocks. It docs not seem probable that it has been formed by regional 

aCross, Whitman, Fourteenth Ann. Rept. U.S. Geol. Survey, pt. 2,1894, p. 238. bMon. U. S.,Gcol. Survey, vel. 31,1898, p.l43. 

.. 
122 GEOLOGY OF THE GEORGETOWN QUADRANGLE, COLORADO. 

stress or by lateral thrust, but might have been •formed by a vertical push from below by 

some restricted force. The period at which its formation began was one of intense eruptive 

activity, and immediately previous to its formation the intrusive porphyries were injected 

into the strata. The explanation advanced a for the cause of the uplift was that molten rock, 

probably derived from the same reservoir as the previously intruded porphyry, accumulated 

in a restricted area, that the upward propulsion of this rock elevated the overlying rigid formations, and that this uplift caused the fracturing and faulting. 

These domal structures at Aspen are similar to the larger domal uplifts which have been 

described as present in the San Juan region, as, for example, those of the Rico and La Plata 

mountains; and the cause of these Sa:n Juan domes has been assigned independently by other 

students to a similar vertically acting uplifting force. Inasmuch as the uplift at Aspen probably 

began with the period of porphyry intrusion and of lateral folding that was synchronous with 

the intrusion of the Elk Mountains dioritic masses, and continued and was chiefly accomplished 

subsequently, much of the defonp.ation which has affected the porphyry sheets and dikes and 

the ·ore deposits was not contemporaneous with the immediately post-Laramie folding; and 

it the later deformation, as is. probable, was clue to the slow upward intrusion of an igneous 

mass that did not reach the surface, certainly at least a portion of the gap which occurred 

between the intrusion of the porphyries of the Elk Mountains and Mosquito Range and that 

of the porphyries in the San Juan was bridged over by this period of probable upward migration, 

though not of actual expulsion, of the monzonitic magma in the Aspen district. 

In this district the ore deposition has been estimated to have occupied a restricted period 

in comparison with that taken up by the faulting, but it is judged that this mineralizing period 

lasted through two-thirds or even more of the interval between the beginning of the uplift and 

the present day-in other words, two-thirds of the Tertiary; which, being roughly though 

perhaps unwarrantedly ex:pressecl in more tangible form, would place the ore deposition as 

lasting through the Eocene and Miocene. 

GENERAL TIME RANGE OF ORE DEPOSITION. 

In the light of these conclusions it is by no means certain that the period of ore deposition 

at Aspen and other districts of this group and that in the San Juan mineral districts, where it 

is consequent on the intrusion of monzonitic domes, do not overlap. Indeed, this conclusion 

seems probable, though in at least some of the San Juan districts the mineralization may have 

continued long after it became extinct in the Aspen and neighboring districts. The best broad 

conception appears to be that there was a general Tertiary period of ore deposition, which 

ensued locally aft~;Jr local intrusions of monzonitic magmas that were injected at many different 

epochs in different localities throughout the earlier half or two-thirds of the Tertiary. Thus 

the epoch of mineralization, ~hich began later than t?e earliest intrusion and considerably 

outlasted the latest intrusion~ ranges from early Tertiary to later Tertiary in age. 

MINING DISTRICTS DEFINITELY OUTSIDE OF THE BELT DESCRIBED. 

There are in Colorado certain mining districts, some of which are important, that evidently 

have no connection in origin with the northeast-southwest zone of fissuring, intrusion, and 

mineralization already described. Most of them are connected with volcanic eruptiol).s from 

single isolated centers, where an initial volcanic explosion has blown out a channel in the crust 

in a region where there are no well-defined preexisting zones of weakness. In many of these 

districts, also, the mineral-bearing veins are limited' in distribution and cluster around the 

volcanic center or centers. Moreover, their attitude does not indicate that the fissures which 

they occupy were due to any extensive or regional force, as is the case in the principal mineral 

belt described. In many places they have a radial arrangement with reference to the volcanic 

center around which they occur, showing that the fissures were due tci quite local causes connected 

with the volcanic eruption. 

•Mon. U.S. Geol. Survey, vol. 31, 1898, p. 148. 
SILVER CLIFF AND ROSITA HILLS. 123 

CENTERS OF MONZONITIC ERUPTIONS. 

SPANISH PEAKS MINING DISTRICT. 

One of these isolated centers of eruption and mineralization, of no great importance commercially, is situated in the Spanish Peaks, in Huerfano County. This has been briefly described 

by R. C. I-Iills. a The structure of the region is referable to two distinct periods. In the first 

period-that after the close of the Laramie-a long synclinal trough was formed, which has a 

north-south axis diverging toward the northwest. The Spanish Peaks country occupies the 

southern half of this trough. In the second period, which was more striking in this particular 

region, deformation was due to local lava eruptions. The effect of these eruptions was to 

produce in the very bottom of the previously formed trough a great upward bulge, "rent and 

faulted at the summit, warped by the intrusion of huge masses of igneous rock, and ribbed by 

a magnificent system of dikes." 

The igneous rocks, which represent a long series of eruptions centering in the Spanish Peaks, 

consist of stocks, dikes, and sheets. The dikes are arranged radially around a large igneous stock 

which is of later age than the dikes and probably occupies the focus of the eruptions. The 

igneous-rock varieties include monzonite porphyry, augite granite, augite diorite, granite porphyry, granite, felsophyre, lamprophyre, and basalt. The earliest eruptions were subsequent 

to the close of the Bridger (Eocene) epoch, and later eruptions have taken place intermittently 

nearly to the present time. The general magma is regarded as monzonitic, from which the 

more siliceous and the more basic rocks, including granite porphyry, lamprophyre, and basalt 

are differentiation products. The lodes are grouped around the great stock of augite diorite, 

around which the dikes are radially arranged and which, as already stated, probably occupies 

the old volcanic neck. They contain galena, gray copper, chalcopyrite, sphalerite, and siderite. 

The gangue is quartz, with calcite and locally barite. 'Vhile ore-bearing veinlcts have been 

found in the eruptive parts of the mountains, the most promising deposits are confined to the 

surrounding zone of metamorphosed Eocene-sediments. 

To judge from Hills's description, the igneous rocks and the consequent ores of the Spanish 

Peaks district are similar to those of the San Juan and other correlated regions. 

CENTERS OF ALKALINE LAVA ERUPTIONS. 

The volcanic rocks found in connection with ores around other isolated volcanic centers are 

quite_ different from the abundant class which has been described, and the ore deposits also have 

corresponding peculiarities. This statement applies particularly to two districts-that of 

Silver Cliff and Rosita and that of Cripple Creek. 

SILVER CLIFF AND THE ROSITA HILLS. 

GENERAL GEOLOGY. 

The connected mining districts of Silver Cliff and the Rosita Hills are situated in Custer 

County, about 40 miles south of the Cripple Creek district. The whole mining region has a 

length in a northeast-southwest direction of 9 or 10 miles, and a width in its shorter northeast 

diameter of 3 or 4 miles. The general geology has been described by Whitman Cross, and the 

economic geology by S. F. Emmons. b The. area includes pre-Cambrian granite and gneiss and 

Tertiary volcanics. The volcanic history began with the outbreak of an active volcano somewhere near the present site of Rosita, which emitted lavas, ashes, and fragmental material. The 

first two eruptions were of andesite, followed by dacite and rhyolite. Later there came another 

andesite eruption, followed by one of trachyte. Subsequently ores were deposited by hot 

ascending waters and possibly by volcanic (fumarolic) vapors. Later than the ore deposition 

was the intrusion of a dike of basic basalt or limburgite, whose fresh condition, where it intrudes 

andesitic agglomerate altered and silicified by the mineralizing agents which deposited the ores, . 

a Spanish Peaks folio, No. 71, U.S. Geol. Survey, 1901. b Seventeenth Ann. Rept. u: S. Geol. Survey, pt. 2, 1896, pp. 269-472. 
124 GEOLOGY OF THE GEORGETOWN QUADRANGLE, COLORADO. 

shows that it is of more recent date than the ore formation. According to Cross a the products 

of the volcano accumulated on a surface very similar in character to that of to-day. The age 

of the volcanic activity, however, is considered to be fixed as early Eocene by fossil leaves found 

in the rhyolitic flows. The lavas "\Vere erupted in the following order: (1) Andesite, (2) andesite, 

(3) diorite, (4) dacite, (5) rhyolite, (6) andesite, (7) trachyte, (8) agglomerate (Bassick agglomerate), (9) miscellaneous rocks (including limburgite). The fossil leaves above referred to were 

associated with No. 5 of this series, and their age, if correctly determined, is therefore somewhere ncar that of the middle portion of volcanic activity. The last member of the series which 

was erupted in any considerable amount and whose age is _well determined was the trachyte, 

which forms dikes and surface flows. 

The different rocks are considered to be clearly a related group, characterized by the high 

percentage of combined alkalies, potash in particular maintaining a prominent place throughout. 

The interesting formation known as the Bassick agglomerate outcrops in a roughly square 

area between 3,000 and 4,000 feet in diameter, surrounded chiefly by Archean granite and gneiss, 

with considerable andesite, rhyolite, and trachyte. The agglomerate which fills this area 

extends almost vertically downward and evidently represents the filling of a volcanic neck. The 

material consists of unsorted angular and rounded fragments and bowlders up to several feet in 

diameter, with the interstices between the larger fragments filled with smaller ones down to the 

size of fine dust. The rocks making up these fragments consist mainly of andesites, whose mineralogical characteristics enable their reference to several of the distinct andesitic eruptions 

above mentioned. Granite and gneiss also make up a large proportion of the fragments. 

Trachyte occurs in the lower workings as bowlders or dikes. Charcoal is occasionally found 

down to a depth of several hundred feet. The agglomerate is supposed to be the result of an 

explosive eruption and to consist of material which was thrown out of a volcano and fell back 

again, filling up the open pit. The charcoal is supposed to represent the remains of trees which 

covered the surface previous to the eruptions. 

DESCRIPTION OF THE ORE DEPOSITS. 

Through the region as a whole the ore-bearing veins cut the trachyte, proving them to be 

younger, while. the Bassick volcanic neck above described, which is considered probably later 

than the trachyte, contains a pipe of ore that was formed probably soon after the volcanic 

explosion that produced the agglomerate, since a subsequent dike of limburgite which traverses 

the agglomerate is later than the period of mineralization. 

Most of the veins of the region are fissure veins, many of them occurring along faults or 

fault zones. Friction breccias containing bowlders of volcanic and pre-Cambrian rocks rounded 

by attrition are found along some of these faults. In one of these fissure veins, the Humboldt, 

the ore is described as principally tetrahedrite, with a gangue of barite and sul;>Ordinate calcite, 

striking S. 50° W. and dipping 60° to 72° SW. 

The ore shoot in the Bassick volcanic agglomerate neck has the form of a pipe, with elliptical horizontal cross section, whose dimensions vary from 20 to 30 feet on the shorter diameter 

to nearly 100 feet on the longer. It extends downward vertically nearly 800 feet, without walls 

or distinct boundaries between it and the barren agglomerate, the ore being richer in the center 

of the pipe and becoming scarcer and poorer toward the periphery. The metallic minerals 

include sphalerite, galena, jamesonitc, tetrahedritc, smithsonite, calamine, free gold, chalcopyrite, and telluride minerals, probably petzite. .Some quartz is present, but no barite or 

calcite. The mineral has been deposited in successive scales or shells around bowlders in the 

agglomerate. Usually three, in some places four, distinct layers can be distinguished. The 

first layer consists of sulphides of zinc, antimony, and lead, carrying about 60 ounces of silver 

and 1 to 3 ounces of gold to the ton. The second layer is lighter in color and contains more 

lead, silver, and gold, running as high as 100 ounces of gold and 150 to 200 ounces of silver. 

The third layer is made up of zinc blende, containlng considerable iron and some copper. This 

constitutes the principal source of value in the ore, containing 60 to 120 ounces of silver and 

a Seventeenth Ann. Rept. U. S. Geol. Survey, pt. 2, 1896, p. 272. 
SILVEl't CLIFF AND ROSITA HILLS. 125 

15 to 50 ounces of gold to the ton. This is i'n many places the outermost coating, but locally 

there is a fourth layer formed of chalcopyrite which carries up to 50 or 100 ounces of gold and 

silver.· Outside of all there occurs in places a thin sprinkling of pyrite. · 

The ore body at the Bull-Domingo mine, which is several miles distant from the Bassick 

mine, occurs in Archean granite and gneiss. The ore consists of concentric shells of galena and 

spar around bowlders of country rock. These bowlders consist of gneiss and granite, with no 

volcanic rocks. The minerals which have been deposited around them embrace galena, zinc 

blende, and pyrite, with calcite, dolomite, ankerite or siderite, and quartz (usually in the form 

of chalcedony). The first band around the bowlder is either galena alone or galena and zinc 

"blende. Around this band there is in some pJaces a coating of galena, which may be associatecl 

with pyrite. Outside ·of the metallic bands come the various spars, which invariably form the 

interior linings of vugs. The order of deposition of these. is (1) white dolomite, (2) ankerite or 

siderite, (3) calcite, (4) chalcedony. The galena contains silver. There is no sharp line 

between pay ore and the barren bowlder mass. The deposit has been frequently supposed to 

represent an old volcanic chimney like the Bassick, but Emmons believed that the bowlders 

represented a. friction breccia or conglomerate somewhat rounded by attrition and further 

rounded by percolating solutions, and that the ore channel or chimney was formed primarily 

by the complicated intersection of a number of fracture planes. 

Near Silver Cliff is the Geyser mine, the main shaft of which is sunk through rhyolite and 

rhyolite tuff clown to the older granite beneath and had reached a depth of 2,100 feet at. the 

time of the report. The main vein was cut at a depth of 1,800 feet, and though very thin was 

remarkably productive. The ore minerals are galena, blendc, chalcopyrite,. argentite, tetraheclrite, and ruby silver. The telluride of silver (hessite) and free gold are reported. The 

earthy minerals include barite, calcite, and quartz, generally chalcedonie. 

Fluorite and barite, with chloride of silver, are said to occur in the Silver Bar mine. 

MINE WATERS. 

Interesting analyses of the water in the Geyser mine were made. In the upper part of the 

mine considerable water was encountered at depths of 340 to 420 feet. This was undoubtedly 

surface water. It decreased in amount downward, and practically ceased at 1,000 feet. On 

the 1,450-foot level abundant subterranean waters slightly charged with gas were encountered, 

and from this level to the bottom, at 2,100 feet, there is a constant flow of highly carbonated 

water, apparently ascending along fissures parallel with the ore-bearing fissure. On access to 

the air these waters deposit sinter, which consists mainly of carbonates of iron, lime, and magnesia. A comparison of the analyses of the vadose waters and of the deep waters shows that, 

though the latter contain about twenty times as much dissolved matter as the former, the 

relative proportions of the principal constituents are not radically different. Emmons eonsiuers that this circumstance allows the assumption that these dissolveu materials have been 

derived from a similar source, which in the ease of the surface waters must have been the material of the rocks through which they had passed. The greatest discrepancy in the analyses is 

the tenfold greater proportion of silica in the vadose waters thaq in the deep waters. The 

alk11lies have the same relative proportion in each class. In the vadose waters iron and manganese arc of nearly equal amounts, while in the deep waters iron is in one hundred fold. greater 

relative amount. In the deep waters chlorine and sulphuric acid are in about equal proportions, and carbonic acid is greatly in excess of both combined. In the vadose waters, while 

carbonic acid is still in excess, sulphuric acid is in relatively greater and chlorine in relatively 

sma1ler proportion than in the deep waters. Emmons concludes that the deep waters have 

obtained their constituents, at least in great part, from the surrounding rocks, while the great 

excess of carbonic acid in these waters, combined with the presence ofn. considerable amount 

of fluorine, boric acid, and chlorine, points to a source where ehe!-llical decomposition is actively 

going on. This source might be supposed to be a body of still uncooled igneous rock which 

surface waters had reached and from which they were sent back toward the surface again along 

the present lines of fissuring. 
126 GEOLOGY OF THE GEORGETOWN QUADRANGLE, COLORADO. 

Although they contain most of the metals that are found in the vein deposit of the .Geyser mine, it is not easy to 

conc~ive how the metallic sulphides of that deposit could have been derived from waters of such a chciuical composition as these have, and it seems more reasonable to assume that these vein materials were formed by earlier waters 

of somewhat different composition, that carried more barium and silica and were characterized by sulphureted hydrogen 

rather than by carbQnie acid. a 

RESEMBL.(I.NCE TO CRIPPLE CREEK. 

Emmons points out the various points of resemblance betwemi this region and the Cripple 

Creek district. In each the main ore deposit has taken place around a central volcanic focus 

where comparatively recent volcanics have broken through pre-Cambrian rocks, and in each 

the principal deposition has taken place along a _system of fracture planes, more abundant iri 

the eruptions than in the older rocks. Mineralogically, the contrast is greater. In Cripple 

Creek the important metalliferous gold is mainly in the form of telluride, and the prominent 

gangue mineral fluorite. In the district under consideration gold telluride occurs in certain 

portions and fluorite is sparingly found, but the greater part of the minerals· are the silver 

minerals, associated with sulphides of lea,d, zinc, and iron, and with barite as the prominent 

gangue. 

ORIGIN OF ORES. 

Emmons calls attention to the peculiar features of the Bassick ore body from the point of 

view of its origin-first, that its form is that of a long, slender, nearly vertical chimney in an 

agglomerate which appears to fill an old volcanic neck; second, that in the composition of the 

ore the earthy minerals, such ·as barite, calcite, apd qua:rtz, which are common in the other ore 

deposits of the region, do not form an essential part of the vein material, but occur, if at all, in 

subordinate amount and are apparently later than the deposition of the other minerals. He 

adds that the conclusion from these facts seems unavoidable that the ore deposition Was here 

a phase of the volcanic eruption after all explosive action had ceased. The metallic minerals 

were evidently deposited as sulphides, and to a limited extent as tellurides. It seems therefore 

reasonable to assume that they were formed during the closing phases of fumarolic activity 

when H2S and S02 were the prevailing gases, if, indeed, it was inn gaseous form that they were 

introduced. It is not impossible, however, that the aqueous vnpors carrying the sulphides and 

tellurides were under so great a pressure that they were condensed into liquid form.b 

In the cnse of the Bull-Domingo mine the conclusion reached is that the deposition of the 

ore seems to have been distinctly accomplished by aqueous solutions.c ·. 

In treating of .the whole district, embracing the different t:ypes of ore deposits mentioned, 

under the heading "Source of the metallic minerals," Emmons remarks that it seems probable 

that not only the recent eruptives but the older granites through which the ascending solutions 

must have passed contnin enough of the precious metals, and presumably, also, of the other 

vein materials, to furnish material for the formation of the existing ore bodies. The annlysis 

of the vadose waters in the Geyser mine demonstrated the capability possessed by cold surfnce 

wnters of taking up such materials in their passage through surface rocks. The subterranean 

waters that were circulating here at the time of the ore formation must have been more energetic 

solvents, being heated by contact with the cooling masses of igneous rock nnd probably deriving 

a certain amount of active nnd energetic mineralizing ngents, such ns fluorine, chlorine, etc., 

from these igneous masses at the time of contact. Emmons concludes that it is fair to assume 

tha.t the vein materials in this region were originally derived from both recent and ancient eruptive rocks. d 

As the writers understand the foregoing conclusions, Emmons considers the ores nnd the 

vein materials to have been leached out of the rocks of the region, both Tertiary and pre-Cambrian, by circulating wnters which, descending from the surface, became heated Vv proximity 

to the volcanic rocks and ascended toward the surface, depositing ore on their way. He regards 

it as probable that mineralizing agents more active thnn water, such as fluorine, chlorine, etc., ---- ~-------

a Emmons, S. F., Seventeenth Ann. Rept. U. S Geol. Survey, pt. 2, 1896~ p. 465. b Idem, p. 437. c ldem, p. 446. d Idem, p. 472. 
CRIPPLE CREEK MINING DISTRICT. 127 

which were contained in the ore-depositing solutions, may have been derived directly from the 

volcanic accumulations, as in the Bassick mine, where such gaseous volcanic emanations were 

the chief rriincra1izing agents. 

CRIPPLE CREEK MINING DISTRICT. 

The ore deposits of Cripple Creek have been described by Cross and Penrose,a and later by 

Lindgren and Ransome. b 

·. GENERAL GEOLOGY. 

The prevailing rocks in the region around Cripple Creek are pre-Cambrian granites and 

schists, which have been broken through at Cripple Creek by Tertiary volcanic eruptions. The 

main products of the Cripple Creek volcano now occupy a.rudely elliptical area about 5 miles 

. long in a northeast-southwest direction and about 3 miles wide. 

The lavas which were sueccssively erupted from the Cripple Creek volcano are phonolitic 

and comprisephonolit~, latite-phonolite, alkali syenite, trachydoleritc, vogcsitc, and monchiquite. 

All these volcanic rocks have unquestionably been derived from the same magma, the averag~ 

composition of which is elosc to that of phonolite. The age of the eruptions is considered to be 

Miocene, since phonolite is found cutting Miocene or at least post-Oligocene rhyolite. The 

greater portion of the rocks which now remain arc intrusive. The first rock was latite-phonolite, 

with syenite, followed by phonolite and the basaltic dike rocks which have been designated as 

trachydolerite, vogesite, and monchiquite. The dikes of these various rocks are grouped around 

the volcanic neck with a radial arrangement. · 

This region is considered an excellent example of a petrographic province, the sequence 

beginning with a rock of intermediate composition, passing to one extreme, the phonolites, and 

thence finally to the other extreme, the basic dike rocks. 

ORE DE_POSITS. 

The veins' are in fissures which have a genetic connection with the local volcanic center, 

occurring mainly within and near the volcanic neck, with a rough radial arrangement. They 

are as a rule nearly vertical. 

The fissures have little or no displacement and were formed under light or moderate load, 

and their evident relation to the volcanic neck suggests that they were.formed by the settling 

of the material in this neck after the cessation of eruption. The chief mineralogical feature of 

the veins is the occurrence of ore combined with tellurium, mainly as calaverite, partly as more 

argcntiferous sylvanite,_ and probably to a minor extent as other gold, silver, and lead tellurides: 

The metallic associates of the tellurides comprise pyrite, blende, and galena in varying proportions; with some stibnite and tetrahedritc and rare chalcopyrite. The chief gangue materials 

are quartz, fluorite, and dolomite, with celestite, orthoclase, roscoelite, chalcedony, opal, rhodochrosite, calcite, and barite. In some of the veins which have a quartz gangue, galena, or blonde 

and galena, are the prominent metallic minerals. There is no sharp line between these veins 

and the telluride veins. Silver is frequently found in varying amounts. The wall rocks have 

· . been altered principally to sericite and adularia, involving chemically a loss of soda and a gain 

of potash. As a rule, the alteration is remarkably slight. · 

ORIGIN OF ORES. 

The fluorite in the veins and in the wall rocks is regarded as having been given off by the 

phonolitic magmas on consolidation. Likewise, the carbonic acid shown by the presence of 

carbonates was probably an emanation of the crystallizing magma, as well as the sulphur content 

in the sulphides. The mineralizing solutions contained very little iron, that in the pyrite 

having probably been derived from the country rock. The lime and magnesia in the solutions 

aCross, Whitman, and Penrose, R. A. F., jr., Sixteenth Ann. Rept. U.S. Geol. Survey, pt. 2, 1895, pp.13-209. 

b Lindgren, w·aldemar, and Ransome, F. L., Prof. Paper U.S. Geol. Survey No. 54,1906. 
128 GEOLOGY OF THE GEORGETOWN QUADRANGLE, COLORADO. 

had probably a similar origin. It is considered that the gold, tellurium, and perhaps the rarer 

sulphides, including blonde, galena, stibnite, tetrahedrite, and molybdenite, were given off 

from the magma. The ores are regarded as having been formed at depths varying from a few 

hundred feet to 3,000 feet. The ore deposition followed closely the intrusion of the basic dikes 

and is believed to have taken place at the close of the Tertiary period. 

REMARKS ON THE ClUPPLE CREEK AND ROSITA DISTRICTS. 

FEATURES COMMON '1'0 'rHE TWO DISTRICTS. 

The two districts which have just been summarized have much in common. In both 

places there has been a single volcanic center, situated in an area of. pre-Cambrian gneisses 

and granites and furnishing a succession of various lavas, tuffs, and breccias, with basic dikes 

as the last phase of extrusion. The lavas emitted by each volcano, though differing considerably in detail, were chemically and mineralogically related to one another. Those from 

the Cripple Creek volcano were marked by the large proportion of alkalies, and have been 

classed all together as products of a phonolitic magma. The lavas of the Rosita volcano also 

contain an unusual proportion of alkalies, especially in certain members, such as the trachyte, 

and in the mica dacite, a rock which is intermediate between rhyolite, trachyte, and dacite. a 

AGE OF ERUPTION AND MINERALIZATION. 

Fossil leaves in rhyolitic material at Rosita show that the most plausible date of this 

rhyolitic eruption was early Eocene, and these rhyolites occur near the center of the series of 

eruptions. Cross regards the Rosita Hills and the Cripple Creek volcano as small outlying 

vents connected with much larger eruptions in the San Juan and South Park region, which 

were in part also of Eocene age, though probably continuing into the .Miocene.b In an earlier 

publication c the volcanic phenomena of the region including Cripple Creek are described as 

Eocene, possibly continuing into the Miocene'. This determination is based primarily on 

evidence aiiorded by fossil plants and insects in a small ancie·nt lake basin at Florissant, which 

had been determined to be Oligocene. These lake beds are made up chiefly of volcanic ash 

and are overlain by volcanic breccia and by the remnants of rhyolite flows. In High Park, 

5 miles from Cripple Creek, identical rhyolite flows are overlain by a local lake deposit (inferred, 

from its position above the Florissant lake beds, to be Miocene), which is older than a large 

intrusive mass of phonolite. In the Cripple Creek district there are certain roughly bedded 

arkoses or grits which Cross has correlated with the High Park probably Miocene lake deposits, 

since· both are considered to be older than the phonolite. According to this, the age of the 

Cripple Creek volcano is Miocene and probably late Miocene. a 

Later, however, L. C. Graton e showed that at Cripple Creek the rhyolite is intrusive into 

the grits and so is younger than them, instead of being older, as Cross believed. This destroys 

the correlation of the grits with the High Park lake heels, and with it the foundation for the 

specific determination of the age of the Cripple Creek volcano. Lindgren and Ransomef 

discuss the age of these grits and note the possibility that they are the remnants of the Fountain 

formation, which is of Carboniferous age, but agree with Cross's suggestion that it seems more 

likely that they are formed of detritus blown out of the Cripple Creek volcano at an early stage 

of the volcanic activity. If this is true, since in the Cripple Creek district both grits and rhyolite 

are intruded by the phonolite, it follows that the Miocene, or at least post-Oligocene, rhyolite 

was erupted within the period during which the Cripple Creek volcano was active. 

Similar residual bodies of grits, apparently waterlaid but of uncertain age and origin, 

occur in the Georgetown quadrangle. It is possible that these are to be correlated with those 

at Cripple Cre~k. Since the locality where they occur is about 70 miles from Cripple Creek, 

the materials undoubtedly do not represent the results of the explosion of the Cripple Creek 

a Cross, Whitman, Seventeenth Ann. Rept. U.S. Geol. St1rvey, pt. 2, 1896, p. 326. d Prof. Paper U.S. Geol. Survey No. 54,1906, pp. 2, 23. 

bScvcnteenth Ann. }{ept. U.S. Gcol. Survey, pt. 2, 1896, p. 273. eidern, p.lOO. 

c Sixteenth Ann. Rept. U.S. Gcol. Survey, pt. 2 1895, p. 18. J Idem, p. 22. 
MAP SHOWING APPROXIMATE DISTRIBUTION OF PRINCIPAL SILVER, LEAD, AND GOLD REGIONS IN COLORADO. 
RESUME OF PRINCIPAL ORE DEPOSITION. 129 

volcano, and are probably not of volcanic origin. If the possible correlation with .the sandstones of the Cripple Creek district, as above suggested, is true, the latter may, as mentioned 

by Lindgren and Ransome, correspond with the Fountain formation or may represent the 

sediments of a distinct period. If this is the case, the fact that the phonolite intrudes the 

rhyolite shows only that the former is younger than the latter. If the rhyolite is Miocene or 

post-Oligocene, the Cripple Creek volcano i~ of similar or later age and may be considerably 

younger. 

It will be seen that the data, principally founded on the paleobotanists' determinations of 

fossil leaves which assign an early Eocene age to the Rosita volcano and inferentially and by 

correlation a Miocene or later age to the Cripple Creek volcano, are at variance .with the evidence 

derived from the study of volcanic action and the comparison of magmas and ore deposition, 

which led Cross to infer a similar age for the two centers. a Of the two bases of age determinations, the senior writer is inclined to regard the latter, though imperfect, as rather ~he more 

reliable and to assume in general a like age for the two volcanoes. He questions, however, 

whether these two volcanoes arc to be ·correlated with the extensive volcanic eruptions which 

have been described in the United States Geological Survey folios covering the San Juan, as 

Cross has supposccl.a According to this supposed correlation, the San Juan ore deposits, which 

followed porphyry intrusions of later age than the surface volcanics, would be younger than 

the ore deposits of Rosita and Cripple Creek, which were formed in the closing stages of eruption 

of surface volcanoes. The writers have no convincing evidence to offer on this question, the chief 

consideration to be presented involving the relative ages of the different magmas an<l their 

associated veins within the Georgetown quadrangle. In general, however, they arc inclined to 

frame a hypothesis that the reverse of the age relations above mentioned for the San Juan and 

the Rosita and Cripple Creek ores is true, and that the alkaline volcanics represented by Rosita 

and Cripple Creek were erupted subsequent to the monzonitic porphyry intrusions. As the 

monzonitic intrusions have been roughly considered (see p. 121) to represent chiefly the 

earlier Tertiary period, say the Eocene 'and Miocene, the latest (~ate (late Miocene) set to the 

age of the Cripple Creek volcano in the above discussion accords more with the writers' conception than the earlier dates which have been assigned, and it appears that there is no 

evidence that the eruption did not last into the Pliocene. That the vents which were formed 

in the closing stages of eruption, however, are not of recent age is shown by the extensive erosion 

which has formed surface placers and by the considerable depth of oxidation. On the whole 

the following characterization by Lindgren and Ransome as to the period of ore deposition . 

conforms with the writers' conception: "If we assign a late Tertiary age to the close of direct 

volcanic activity, there is some reason for believing that the ore-forming epoch belonged to 

the close of that period."b 

RESU~1E OJ<' 'l'HE PRINCIPAL ORE DEPOSITION IN COLORADO. 

As a result of the above investigation we have found that most of the important gold, 

silver, and lead deposits in Colorado (see Pl. XVIII) belong to two chief groups-those whose 

formation followed and appears consequent on the intrusion of porphyritic igneous rocks which 

are of varied character but in general represent a monzonitic magma, and ·which were injected 

in the form of dikes, stocks, laccoliths, and sheets; and those which followed the outburst of 

isolated volcanic centers whose varied magmas had an alkaline or phonolitic tendency. Both 

of the consequent periods of ore deposition are of Tertiary age. Practically all the data available for fixing the age determinations niore closely are of a vague and unsatisfactory character. 

It seems probable, however, that the monzonitic intrusion began early in post-Laramie time, 

though it probably continued for a very long period throughout the earlier poi·tion of the 

Tertiary and very likely lasted through the Eocene and Miocene epochs. In later Tertiary 

time the intrusion of these monzonitic porphyries and related igneous rocks derived from the 

· same magma was probably not nearly so active, though there is some evidence of igneous action 

a Seventeenth Ann. Rcpt. U.S. Geol. Survey, pt. 2, 1896, p. 2i3 .. 

31401-No. 63-08-9 

b Prof. Paper U.S. Gcol. Survey No. 54, !906, pp. 226-227. 
130 GEOLOGY OF THE GEORGETOWN QUADRANGLE, COLORADO. 

connected with the monzonitic centers and lasting practically throughout the Tertiary. It also 

seems probable that the volcanic eruptions of Cripple Creek and· Rosita did not begin so early 

as did the intrusions of rnonzonitic porphyry and that they may have continued later than the 

end of the main period of monzonitic intrusion. There is no evidence, however, that the 

periods covered by the eruption of the two magmas did not overlap; though from what rather 

unsatisfactory evidence there is it appears a better hypothesis to consider that the Cripple 

Creek and Rosita volcanic eruptions are later than the monzonitic eruptions and to conceive 

of the latter as covering a period lasting from the earliest to the middle or later Tertiary and 

of the former as enduring from the middle to the later Tertiary. 

RELATION OF DIKES TO VEINS IN.THE GEORGETOWN QUADRANGLE. 

DEFINITION OF THE TWO CHIEF MINERAL DISTRICTS. 

As already stated, there are evidently within the Georgetown quadrangle certain distinct 

mineral districts. The roughest division makes of these two separate districts, one characterized by silver-lead ores containing little or no gold, and the other characterized by ores which 

may or may not contain silver but which in general contain gold as the most profitable constituent. Mineralogically the ores of the first group are typical galena-blende ores; those of the 

second group are pyritic ores, usually cupriferous. The territory covered by veins containing 

auriferous pyritic ores embraces the region southwest of and tributary to Idaho Springs. That 

containing argentiferous galena-blonde veins includes the territory around Georgetown, with 

the exception of a nall'OW northwestward-trending gold belt which runs through the silver 

district and includes the Central Colorado, Centennial, Griffith, and Anglo-Saxon veins. In 

the Empire district are pyritic gold-bearing ores, bordered on the south and east by argentiferous veins. 

GENERAL RELATIONS OF PORPHYRY DIKES TO VEINS. 

The orcs of both these groups lie throughout in similar pre-Cambrian gneissic and massive 

crystalline rocks, and ·they have the same general relations to the accompanying porphyry 

dikes. As a rule, they are later than these dikes, having formed since the dikes cooled, and 

certainly in some places after the porphyries were decomposed by percolating waters. Many 

of them follow the dikes, being formed along strike faults which have originated subsequent to 

the dikes' consolidation; others occur far from any porphyry body. In fact, the circumstance 

that many of the veins follow along the walls of dikes or lie within the dikes depends simply on 

the fact that these dikes offer lines of weakness in the rock fabric and have afforded less resistance than the surrounding rock to later faults and fissures, which have become the sites of ore 

deposition. While this is true of tho detailed relation between dikes and veins, there is a larger 

relation, already noted, in that the belt of porphyry dikes coincides with the belt of mineral 

veins, indicating a genetic connection between the two. 

It was at first regarded as possible that this association was due to both having been localized in the same zone, because the presence of preexisting planes of weakness gave rise to 

repeated fissuring and faulting which afforded access to molten rock and to circulating waters 

alike, and that therefore the dikes and the veins had no necessary interconnection; yet the fact 

that the veins were invariably subsequent to the dikes led finally to the idea that the mineralization was directly dependent on and consequent to the porphyritic intrusion. There was for 

a long time, however, no evidence that there was not a fairly well-defined interval between the 

period of dike intrusion and that of vein formation, so that this idea remained more or less a 

plain hypothesis. , 

More definite evidence was afterwards procured by a study of the Stanley mine, where it 

was proved that along a single fault fissure two dikes had been successively injected and that 

an important and rich vein had formed in the interval between the two intrusions. The 

deposition of this vein, one of the typical veins of the Idaho Springs region, was thus fixed as 

having occurred in a comparatively restricted time and having followed closely and vigorously 
RELATION OF DIKES TO VEINS. • 131 

on the intrusion of the earlier dike, which was of a different type from the later one. A direct 

genetic connection between the first dike and the vein seemed certain, and a consideration of 

the mineralization as directly consequent on the intrusion became unavoidable. The same 

conclusion, moreover, was thus extended to other veins of the district. 

RELATION OF THE SILVER-BEARING AND THE PREDOMINANTLY GOLD-BEARING VEINS TO 

DIFFERENT GROUPS OF DIKES. 

After the intimate relation between the igneous intrusion and the vein formation in the 

area of the Georgetown quadrangle had been determined, and the existence of two principal 

classes of veins-the argentiferous galena-blende veins and the auriferous pyritic veins-had 

been recognized, as well as the fact that the porphyry dikes were of different kinds of rock, the 

question as to a possible relation between different kinds of veins and different kinds of dikes 

was looked into. A comparison of the two classes of veins as mapped by the writers with the 

different varieties of dikes as determined and mapped by Mr. Ball (all the work having been 

clone before the possibi.lity of any relation was considered) showed unmistakably broad relations. 

The hiatus in the vein zone between the Georgetown and Idaho Springs districts, occupied 

by the upper portions of Saxon and Griffith mountains, is marked by a corresponding hiatus 

in the dike zone. Moreover, the two districts, marked by ores mineralogically and chemically 

different, also contain totally distinct groups of dike rocks. In each district there is a considable variety of rock types, but the types of one district do not occur in the other. Mr. Ball has 

determined the dikes in the Georgetown district to be alaskite porphyry, granite porphyry, 

quartz monzonite porphyry, and dacite, the first two being most closely connected with the ore 

deposition, which, however, was later than the intrusion of all four. The dikes in the Idaho 

Springs region, in the northeast corner of the quadrangle, have been determined to be bostonite, 

alaskitic quartz monzonite, biotite latite, and alkali syenite porphyry, the first two being the 

most abundant. · · 

In the Silver Plume area no bostonite, alaskitie quartz monzonite, biotite latite, or soda 

syenite were found, and in the Idaho Springs area no alaskite porphyry, granite porphyry, 

quartz monzonite porphyry, or dacite. Therefore the two mineral districts appear to represent 

distinct petrographic provinces. The bostonites of the Idaho Springs area have been traced 

southwestward at least as far as the top of Saxon Mountain, while the alaskite porphyry of 

the Georgetown area has been found as far northeast as the slopes of Saxon Mountain near 

Georgetown. Thus the boundary between the two provinces runs north and south, on the 

east side .of Clear Creek valley. ' 

The Empire district shows predominant bostonite, with alaskite porphyry, quartz monzonite 

porphyry, alaskitic quartz monzonite porphyry, and andesite. These dikes represent a mingling 

of the igneous materials of the two provinces. There is also on the borders of the Empire 

district a mingling of the argentiferous galena-blende veins and the auriferous pyritic veins, 

. the former being cut by the latter, instead of the two groups being separated by a barren area 

as in the case of the two principal and larger districts above mentioned. 

RELATIVE AGE OF DIFFERENT DIKE SETS. 

No relations have been observed that determine beyond question the relative age of the 

different dike sets. The two come together only iri the Empire district, and here no completely 

convincing relations were observed. Although in several places dikes belonging to the different 

provinces have been successively injected along the same fissure, the age relations were not 

definitely determinable. In this district, however, the gold-bearing. veins ch~racteristic of 

the Idaho Springs province have been injected into fissures which have followed and reopened 

the silver-lead veins characteristic of the Georgetown province, showing that the pyritiferous 

or gold-bearing veins are distinctly the younger of the two classes, a relation which is also 

shown in several other clear-cut cases. Therefore, it is probable that the dike sets have the 

. same relative age as ·their associated vein sets, and that the Idaho Springs dikes were in general 

intruded subsequent to those of the Georgetown region. 
132 GEOLOGY OF"THE GEORGETOWN QUADRANGLE, COLORADO. 

SUMMARY. 

The two distinct petrographic districts exhibited by the Tertiary dike rocks within the 

area of the Georgetown quadrangle and covering the region adjacent to Idaho Springs and 

that near Georgetown are referable to two types of magmas, which have been described as having 

been associated with at least the greater portion of the deposition of ores containing silver, 

gold, and lead in the State of Colorado. The Georgetown dikes are referable to the monzonitic 

magma, with which are definitely known to be associated the ores occurring in many portions 

of the great northeast-southwest mineral belt which runs across the State and includes the 

Leadville-Aspen-Gunnison group and the group of the San Juan deposits; while the rocks 

of the Idaho Springs district are referable to the alkaline magma best known and described 

where it has been extruded at the volcanic centers of Rosita and Cripple Creek 

The writers' idea of the relative age of these two groups in Colorado, as already stated, 

is that the eruption of the alkaline magma was in general later than the intrusion of the monzonitic magma. This idea rests largely, though not entirely, on the probable age relations 

of the products of these magmas in the area of the Georgetown quadrangle. 

It likewise follows that the ores attending the eruption of' these separate magmas in the 

area of the Georgetown quadrangle belong, respectively, to the two great divisions of ores mentioned, which we niay conveniently designate, selecting a characteristic example from each 

division to furnish a name, as the Aspen ore group and the Cripple Creek ore group. The 

Georgetown silver ores, according to this classification, represent the former group and ·the 

Idaho Springs gold ores or gold-silver orcs the latter. 

DESCRIPTION AND CORRELATION OF THE DIKES IN THE GEORGETOWN 

SILVER REGION. 

FEATURES OF THE PETROGRAPHY. 

The alaskite porphyry of the Georgetown silver region has as mineral constituents mainly 

quartz and orthoclase, with a little albite-oligoclase and sparse phenocrysts of biotite and 

original muscovite. The granite porphyry contains orthoclase, biotite, and quartz, with a 

little oligoclase-albite, together with accessory zircon and magnetite. The quartz monzonite · 

porphyry, which occurs in greater bulk than any of the other dike-rock types in this district, 

contains orthoclase, oligoclase-andesine, quartz, biotite, hornblende, and a little augite. The 

dacite contains chiefly the soda-lime feldspars, quartz, green hornblende, and biotite, with 

accessory titaniferous magnetite and apatite. 

It is evident that these four different rock types are closely related and are evidently 

variations of a granitic or monzonitic magma. 

CORRELATION WITH INTRUSIVES OF OTHER COLORADO DISTRICTS. 

The alaskite porphyry has been compared by Mr. Ball to the white porphyry at Leadville 

and found to be practically identical, and the quartz monzonite porphyry of Georgetown was 

also found to be practically the same as the Lincoln porphyry of Leadville, of which the gray 

porphyry is a variety. The presence of muscovite phenocrysts in the porphyries is very remarkable. Muscovite is not a volcanic mineral, though it occurs in isolated quartz porphyries.a 

Muscovite was found by Cross in the white porphyry of Leadville as an original constituent, 

forming dark hexagonal phenocrysts at first supposed to be biotite, but proved by optical and 

chemical tests to be muscovite.b Similar original muscovite was found by Mr. Ball in the 

alaskite porphyry of Georgetown, thus confirming his correlation of the two rocks. In the 

'fenmile district,c also, the Lincoln porphyry occurs, preceding, as at Leadville, the ore deposition; 

and at Aspen porphyry similar to the white porphyry at Leadville bears the same relation to the 

ore deposits.d In this porphyry at Aspen, the occurrence of original muscovite phenocrystse 

aRosenbusch, H., and Iddings, J.P., Microscopic Physiography of the Rock-Making Minerals, 2d ed., p. 264. 

b Mon. U. S. Geol. Survey, vol. 12, 18~0, p. 77. 

cTenmile folio, No. 48, U.S. Geol. Survey, 1898. 

d Mon. U. S. Geol. Survey, vol. 31, 1898, p. 48. 

e Idem, p. 50. 
DESCRIPTION AND CORRELATION OF DIKES. 133 

resembling biotite in the hand specimen has been described by Spurr. The dacite of Georgetown also is of very similar composition to the diorite porphyry of Aspen, which is similar to 

the Elk Mountains injections. It also corresponds in composition to one of the porphyries 

(Quail porphyry) at Tenmile.a 

Without further comparison, it is plain that these Georgetown dikes belong to well-defined 

and characteristic types derived from the early Tertiary monzonitic magma which stands in 

such vital relations to the ore deposits of this great mineral belt. 

RELATIVE AGES OF THE DIKES. 

In the Baltimore tunnel near Georgetown a dike of alaskite porphyry cuts one of quartz 

monzonite porphyry, and a surface exposure in the same general region shows granite porphyry 

also cutting quartz monzonite porphyry. These are the only observed relations, for in general 

the dikes do not intersect. The more monzonitic magma Tepresented by the quartz monzonite 

porphyry was evidently injected earlier than the more siliceous magma represented by the 

granite porphyry and the alaskite porphyry. All, however, were earlier than the ore deposition, since subsequent veins have formed along dikes belonging to each of these types. 

DESCRIPTION AND CORRELATION OF THE DIKES IN THE IDAHO SPRINGS 

GOLD-BEARING REGION. 

FEATURES OF THE PETROGRAPHY. 

The alkali syenite porphyry, or pulaskite porphyry, which occurs near Idaho Springs, is in 

many places a very fresh rock, with a groundmass made up of anorthoclase with some orthoclase 

and a little quartz. The phenocrysts consist of regirine-augite, garnet (melanite), and anorthoclase, with titanite, magnetite, and apatite as abundant accessories. Here and there are small 

crystals of zircon and biotite. Fluorite occurs in irregular masses, probably belonging to the 

pneumatolytic period of consolidation and later than the bulk of the rock. Small areas of analcite 

having similar relations occur. 

. The bostonite porphyry is a very abundant type, and is almost invariably more or less 

altered. It is characterized by sodium-rich feldspars and is poor in the ferromagnesian minerals. 

The groundmass, which has a trachytic texture, is composed of anorthoclase laths having a flow 

arrangement, together with some orthoclase, quartz, and magnetite, with predominant phenocrysts of orthoclase with some anorthoclase and oligoclase. There are pseudomorphs after 

pyroxene, which has never been found fresh. Accessories include titaniferous magnetite, 

apatite, and a little zircon. Fluorite is very common in the bostonitc and much of it is certainly 

subsequent to the consolidation of the rock, while some of it may very well be an original constituent belonging to the latest period of consolidation. 

The biotite latite is usually a remarkably fresh dike rock, which occurs much less abundantly 

than the bostonite, but more so than the alkali syenite porphyry. The dikes of this rock occur 

in a long chain or string, running northeast and southwest for several miles along the valley of 

Chicago and West Chicago creeks and extending to Clear Creek (in the Stanley mine). The 

groundmass of the rock is ordinarily dense and glassy, although in many places where the 

crystallization is better developed there are numerous lath-shaped feldspars determined in part 

as labradorite and orthoclase, which have a flow structure and locally resemble on a finer scale 

the trachytic structure of the bostonite groundmass. Small biotite crystals are very sparsely 

disseminated in the groundmass. The phenocrysts consist of abundant orthoclase, biotite, 

and plagioclase, with some brown hornblende. The accessory minerals are magnetite, apatite, 

and zircon. 

The alaskitic quartz monzonite porphyry is a monzonite porpliyry poor or lacking in 

biotite, amphibole, and pyroxene, and containing quartz. The dikes which in this region have 

been thus designated are white or light-gray rocks, having a holocryst~lline groundmass 

consisting of orthoclase, quartz, and oligoclase. The phenocrysts consist of orthoclase and 

a Tenmile fol!o, No. 48, G. S. Geol. Survey, 1898, p. 3. / 
.( 

134 GEOLOGY OF THE GEORGETOWN QUADRANGLE, COLORADO. 

oligoclase. In one or two specimens of these dike rocks probable crystals of biotite, now entirely 

altered, were observed. Titanite and apatite are common; magnetite and zircon rare. 

Evidently all these described types are closely related rocks, representing allied feldspathic 

magmas high in alkalies and with unusually low amounts of lime, iron, and manganese. 

Quartz is present in all the types in variable amount, being highest in the alaskitic quartz 

monzonite. 

COMPARISON WITH SIMILAR ROCKS IN OTHER COLORADO DISTRICTS. 

Mr. Ball has already pointed out the chemical resemblance of the alkali syenite of Idaho 

Spri,ngs to the trachyte of the Rosita volcano (p. 83). There is a still closer resemblance between 

this Idaho Springs rock and some of the phonolitic rocks of the Cripple Creek volcano. This is 

shown by the comparison of analyses 1 to 3 in the subjoined table. The analysis given from 

the Rosita Hills is that of the Pringle andesite, which is closely related to the trachyte of that 

district and is rather closer to the Idaho Springs rock than is the trachyte. The relation between 

the Idaho Springs rock and the latite-phonolites of Cripple Creek, illustrated in analyses 1 and 2, 

is very intimate, and the two represent practically the same class of magma. 

The chemical composition of the biotite latite and of the bostonite of the Idaho Springs 

region, as given in analyses 4 to 7, shows the strong relationship of these rocks to the alkali 

syenite, the characteristics of the group being the very low amount of iron and magnesium, 

the subordinate amount of lime, and the relatively large amount of the alkalies. Of all the rock 

analyses contained in Washington's publication a those which correspond most closely to the 

figures of the Idaho Springs analyses are those of the rocks of the Rosita volcano. This relationship is expressed in the table. 

Analyses of Colorado igneous rocks. 

1. 2. 3. 4. .5. 6. 7. 

---------~------------- ------ ~--------------

Si02 .. • • .. • • .. .. .. .. . • . .. .. • • • • .. .. .. • • • . .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. • 60. 30 .18. 05 63. 49 66. 44 

AI,o,................................................................... 1s.12 17.66 18.40 14.98 

Fe,Oa. . .. .. .. .. .. .. .. .. .. .. .. .. . . .. .. . . . . . . .. .. . .. . . . . . .. .. .. . .. .. .. . . .. 2. 45 3. 51 2. 44 1. 57 

}'eO.................................................................... 1.25 1.65 1.09 .43 MgO. . . . . . . . .. .. . . .. .. .. .. .. .. . . .. . . . . .. . . .. . .. .. .. . .. .. .. . . . . . .. . .. .. . . 28 1. 55 • 66 . 18 CaO.................................................................... 3.89 4.48 2.30 2.47 

Na,o................................................................... 5.83 5.so 5.70 1.12 

K.o.................................................................... s.o1 4.06 4.62 3.32 

H,O-.................................................................. .75 _. 3~ } 1.04 { 4.60 

H20+ .. ........................... ..................................... . 77 4.06 

TiO,. .. . . . . . .. . . .. .. . .. . .. .. . . .. .. . . . . . . . . . . . .. .. .. .. . .. .. . .. . . . . .. . . . . . . 55 • 91 Trace. . 20 zro,.................................................................... .01 .o2 .01 

co,..................................................................... None. None. 'frace. .67 

r,o,.................................................................... .25 .40 Trace. .11 

SOa. ... . .. .. .. . .. . . .. . . . .. .. . .. .. . . . .. . . .. . . .. . .. .. .. . . .. . . .. . . .. .. . . . .. . 06 . 04 None. 

Cl...................................................................... . . .. . . . .. . Trace. Trace? 

S...................................................... ................ None. Nuue. .02 

Cr,Os.. ................................................................. .......... None .................... . MoO................................................................... .12 .13 .16 .13 

BaO.................................................................... .26 .19 .11 

SrO.................................................................... .......... .08 .......... Trace? 

Li,O..................................................................... .......... .......... .......... None. 

1. Alkali syenite porphyry, Idaho Springs. 

2. Latite-phonolite, Anaconda mine, Cripple Creek. Prof. Paper U.S. Geol. Survey No. 54, 1906, p. 104. 

3. Pringle andesite, Rosita Hills. Seventeenth Ann. Rept. U.S. Gcol. Survey, pt. 2, 1896, p. 324. 

4. Biotite latite, Chicago Creek, near Idaho Springs. W. T. Schaller, analyst. 5. Pitchstone, Silver Cliff. Cross, Whitman, and Eakins, L. G., Am. Jour. Sci., 3d scr. vol. 44, 1892, p. 101 6. Bostonite, Red Lyon lode, Idaho Springs. W. T. Schaller, analyst. 

7. Trachyte, Silver Cliff. Seventeenth Ann. Ucpt. U. S. Geol. Survey, pt. 2, 1896, p. 324. 

65.67 

13.48 

1. 51 

n.d. 

. 31 2. 41 

1. 52 2. 42 

12.27 

Trace. 

67.41 

16.23 

.85 

1.14 

.15 

.14 

3. 95 

7.19 

.67 

.88 

.16 

.11 

.. 16 

.05 

None. 

None. 

Trace. 

65.41 

18.78 

.94 

. 72 

.16 

1.58 

5. 91 

5.41 

1.38 

Truce. 

Trace. .. · .. · .'i6 · .... Trace: . 32 Trace ....... .' .. . None ......... .. None. . ........ . 

The characteristic minerals of the Cripple Creek group of phonolites are alkali feldspars, 

chiefly soda orthoclase, nepheline, sodalite, nosean, analcite, and mgirinc-augite. Accessory 

minerals include apatite, magnetite, and here and there small amounts of blue amphibole, biotite, 

and zircon. Fluorite ana biotite occur as secondary minerals near veins. The description of the 

«Washington, H. S., Chemical analyses of igneous rocks: Prof. Paper U.S. Geol. Survey No. 14,1903. 
DESCRIPTION AND CORRELATION OF DIKES. 135 

mineral composition of the Idaho Springs alkali syenite shows its phonolitic character. The 

trachytic rocks of the Rosita volcano, as shown above, correspond closely with the bostonite and 

biotite latite of Idaho Springs; and the alaskitic quartz monzonite of Idaho Springs is another 

variant of this feldspathic group. 

RELATIVE ABUNDANCE OF DIFFERENT DIKES. 

By far the greater proportion of d.ike rocks in the Idaho Springs distrwt consists of bostonite 

and alaskitic quartz monzonite, types which are about equally abundant, while the amount of 

biotite latite and alkali syenite is relatively very small. 

RELATIVE AGE OF IDAHO SPRINGS DIKES. 

In the Stanley mine bostonite and biotite latite have been successively injected along the 

same fissure zone, the iatite being younger, and between the two periods of dike injection the 

chief period of vein deposition has intervened. In this case the biotite latite is characteristically 

much fresher than the bostonite, a distinction which is also common for the whole district. In 

general, the biotite latite dikes are not so closely associated with mineral veins as the bostonite 

~ dikes, and where tllis is the case the age relations may be the same as in the Stanley. 

The alaskitic quartz monzonite, like the bostonite, is characteristically associated with the 

ore deposits. 

The alkali symlite has been found only in two small areas near Idaho Springs, and these areas 

. are not associated with ore deposits. The rock is typically very fresh. Very near or at the contact of these syenite intrusions carbonated springs issue, some of which are hot and others cold. 

From the fresh character of the rocks and their lack of connection with ore deposits and 

association with the only hot springs in the region, it is very likely that the alkali syenite is the 

youngest of the Tertiary intrusives. 

According to these facts, the relatively more abundant bostonite and the alaskitic quartz 

monzonite are earlier than the ore deposition, while the relatively very subordinate biotite latite 

and alkali syenite are younger than the other two and_are also later than the main period of 

mineralization. 

COMPOUND VEINS. 

. . , The relative age of the veins of the two main periods has been discussed, and it has been 

shown that in a number of localities a vein belonging to the later or Idaho Springs period has 

filled a fissure which has been opened along a preexisting vein belonging to the earlier or Georgetown period. The result is a compound vein, the joint work of both periods of mineralization,· 

and partaking in its mineralogical and chemical peculiarities of the nature of both. Outside of 

the specific cases where this has been determined and described, it is not known to how large an 

extent this addition of one vein to another has taken plac~. The presence within the Idaho 

Springs mineral district of a subordinate number of silver-lead veins similar to those near Georgetown, and the fact that in the auriferous veins near Idaho 'springs the relative amount of silver 

and gold is very variable, combined with the abundant evidence that these veins have been 

repeatedly reopened, suggest that some of these may be composite veins, the net result of the two 

periods of mineralization, even where this fact has not been determined by local evidence. 

How far this is the case can hardly be determined, since it is positively known that a variable 

though usually small amount of lead, silver, and zinc occurs in the purest and most typical 

auriferous pyritic veins of the later period. Thus the exact nature of the gold-silver veins, as to 

whether they have originated as c·omposites of the two periods or simply represent argentiferous 

variations of the ores of the second period, must be usually open to some doubt; while in the 

case of the auriferous pyrite veins containing little or no silver and lead, and in the case of the 

argentiferous galena-blende veins containing little or no gold, the simple nature is evident. 
136 GEOLQGY OF THE GEORGETOWN QUADRANGLE, COLORADO. 

GENERAL FEATURES OF THE ORE DEPOSITS. 

FEATURES OF THE SILVER-LEAD DEPOSITS. 

COMPOSITION OF VEINS. 

MllTALLIC MINERALS. 

Occurrence.-The principal metallic minerals of the silver-lead deposits are blende and 

galena, with locally considerable pyrite, which is in many places more or less cupriferous and 

grades here and there into chalcopyrite. The blende occurs in two distinct varieties, one being 

light yellow (resin blende) and the other dark colored (blackjack). The galena and to a less 

extent the blende are argentiferous, the resin blende more so than the dark-colored variety. The 

silver content varies in amount. from very little up to several hundred ounces per ton. Typically, 

the ores contain a little gold, the usual quantity being one-tenth of an ounce to the ton, while 

in places it is practically absent. The main silver values are contained in the minerals polybasite, argentiferous tetrahedrite, argentite, pyrargyrite, and proustite. Stephanite has also 

been reported, and locally there is considerable native silver. Mercurial tetrahedrite has been 

determined in the Colorado Central mine. 

Changes in the relative proportions of the above metallic minerals bring about the principal 

variations in the veins from an economic standpoint, so that ores may be high" in silver or in lead 

or both, or may be unfit for mining, in porportion as galena and the rich silver minerals or blende 

an~l pyrite predominate. 

A quite exceptional occurrence of hematite was observed in the Colorado Central mine, 

where this mineral occurs in small crystals at a depth l ,050 feet. 

The chief gangue mineral is crystalline quartz, with brown carbonates, barite, calcite, cherty 

or chalcedonic silica, and local kaolinite. The brown carbonates include those of iron, manganese, 

magnesia, and lime in varying proportions, forming the species siderite, ankerite, rhodochrosite, 

magnesite, calcite, dolomite, and mesitite. Fluorite occurs as a rare gangue mineral in the 

Georgetown district, although at McClellan Mountain, in the Argentine district, where the ores 

are of the Georgetown type, it is relatively abundant. 

Paragenesis.-Galena and blende usually occur intercrystallized in all proportions to form . 

the main mass of the veins. They are in general associated with more or less pyrite, much 

of which is .. cupriferous to a varying degree. As a rule the lead and zinc sulphides are contemporaneous, but a number of different veins were noted in which the galena in the massive ore 

was of later introduction than the blende. It is also frequently found to be the case that 

the pyrite in this class of ore is later than both the lead and the zinc sulphides. 

Besides the galena and blende occurring in the massive portion of the vein and representing the earlier-formed minerals, both these minerals occur at many places in a second generation, being among the last minerals to crystallize and forming free crystals on the walls of 

geodes. 

Pyrite occurs in veins everywhere, but not in great abundance. Its period of formation 

has lasted from the beginning to the end of mineral deposition. It is frequently found in the 

quartz that has been the first deposit in the typical fissure vein of the region. As noted above, 

it is closely associated with galena and blende in the main body of the ore, and also occurs 

among the latest-formed minerals, lining vugs. These generalizations apply also to the cupriferous varieties and to chalcopyrite. 

Polybasite, argentiferous tetrahedrite, pyrargyrite, proustite, argentite, and native silver 

form the chief silver values in the ores. Polybasite and tetrahedrite are the most common 

and abundant. All these minerals are closely associated and are among the last minerals 

which have been deposited. They form on the walls of fractures in the older and baser ore, 

or cut these older deposits veinwise. They also very commonly form free crystals on the 

walls of druses. 
U. S. GEOLOGiCAL SURVEY PROFESSIONAL PAPER NO. 63 PL. XIX 

SPECIMEN FROM WALDORF MINE AT ARGENTINE, SHOWING CRYSTALS OF GALENA WITH SOME 

BLENDE AND QUARTZ, WHICH HAVE FORMED ON THE WALLS OF VUGS. 

a, Galena crysta ls ( with some blende crystals); b, quartz crystals. One-fifth natur a l size. 
FEAT.URES OF THE SILVER-LEAD DEPOSITS. 137 

EARTHY MINERALS. 

Quartz, like pyrite, has been deposited during the whole period of mineral formation. It 

is very characteristic of the veins which have filled fissures (and this includes a large proportion of the veins of the district) that the first lining of the fissure has been of comb quartz, 

much of it containing a little pyrite. It also occurs as a usually not very abundant gangue, 

associated with lead, zinc, and iron sulphides of the main period of ore deposition, and is very 

commonly also among the lastest-formed minerals, occurring as free crystals lining vugs. 

The brown carbonates, including those of lime, iron, magnesia, and manganese, occur 

rather commonly and in some places are relatively abundant. 'l'heir period of formation 

resembles that of quartz, as they have been formed at all stages of the vein deposition. Here 

and there these carbonates take the place of the quartz, that usually lines the walls of fissure 

veins, forming, like the quartz, a barren preliminary deposit which was followed by a deposit 

of the lead and zinc sulphides. In many places they are among the latest or are the very 

last minerals to crystallize, growing as the final deposit in empty cavities. 

Chalcedony or chalcedonic quartz is found locally, though not very common. Its occurrence bears no close relation to that of the crystalline quartz. It characteristically belongs 

among the later deposits of the vein filling, and very commonly forms the latest crust lining 

the walls of geodes. In some veins that have been reopened and recemented the jaspery or 

cherty quartz has cemented broken fragments of white crystalline quartz. 

Barite is found in small quantities in most of the veins, though it nowhere becomes even 

a slightly important gangue. Its place is characteristically among the latest-formed minerals, 

and it is in many veins the last in the sequence. It has no close relation to the metallic minerals, and its favorite mode of occurrence is in fine crystals lining druses. A vein on Republican Mountain (p. 218) carries a considerable amount of barite with some calcite and chalcedonic quartz, but contains no metallic sulphides whatever. 

Fluorite is rare in the Georgetown group of veins, though, as already noted, it is abundant 

in veins of the Georgetown type on McClellan Mountain, in the Argentine district. Wherever 

it has been observed it is among the last minerals which have crystallized. In the McClellan 

Mountain mines it was observed to be partly contemporaneous with and partly later than the 

crystalline vein quartz. 

Calcite has not been noted among the earlier-formed constituents of the veins, and it is 

in many places the last mineral which has crystallized. It is not especially associated with 

metallic minerals, but occurs rather commonly as the last deposit on the walls of cavities, 

where it is usually associated with the brown carbonates. 

Kaolinite occurs as a mineral which has been deposited front solution like the other minerals mentioned, and is not a product of decomposition in place. It is not abundant, but is 

frequently found forming veinlets and occurring on the walls of geodes. It is plainly one of 

the later-formed minerals. 

SUMMARY. 

To. sum up the foregoing observations, it appears that galena, blende, and pyrite (the 

pyrite being locally cupriferous and verging to chalcopyrite), with quartz as gangue, constitute 

the oldest minerals, and that these minerals, especially the first two, comprise the chief vein 

filling. Typically, as shown in the filling of fissure veins, the deposition of the main metallic 

filling, consisting of massive galena and blende, was preceded by a deposit of barren comb 

quartz, much of it containing a little pyrite. Much more rarely the place of this quartz lining 

was taken by a lining of brown carbonate, probably siderite. The quartz, and in much less 

quantity the siderite, forms universal gangue minerals which are among the earliest and the 

latest depositions. Characteristic minerals of the later stages of ore deposition include the 

rich silver sulphides and sulphantimonides, and rather common but not abundant gangue 

minerals, including chalcedonic quartz, kaolinite, calcite, and barite. The last three do not 

seem to have any definite or close relationship to metallic sulphides. Fluorite is rare, and is 
138 GEOLOGY OF THE GEORGETOWN QUADRANGLE,. COLORADO. 

generally one of the last-formed minerals. Besides the typical or invariably late-formed 

minerals, all tP.e older minerals, including quartz, siderite, galena, blende, and pyrite, are 

repeated in many places as later generations and are frequently found in close association 

with the characteristically last-formed minerals. 

ACTION OJr PRESENT UNDERGROUND WATERS. 

GENERA!, DISCUSSION. 

The upper surface of the vadose underground waters generally stands close to the topographic surface in this region. At the time of the examination, in the summer season, water 

was standing in many shafts at a depth of 50 or 60 feet below the surface, even on the mountain 

tops. A study of the underground workings show~ that these waters are most abundant near 

the surface, occurring in considerable quantity there, but lessening with depth, so that in many 

mines at depths of 500 to 1,000 feet the rocks become comparatively dry. However, along 

strong fracture zones the surface waters penetrate much deeper, probably to a depth of several 

thousand feet. They have been encountered in places in the deepest workings. The relatively 

dry deeper rocks, moreover, so far as seen, are not entirely without water, a good deal being 

· contained in their interstices. 

The effects of ascending surface waters in altering rocks at some distance away from veins 

have been observed at various depths from the surface. Wherever descending waters which 

have passed through a few hundred feet of roc.k (usually alkaline granite and granitic gneiss) 

are encountered in the numerous tunnel workings these waters are found to have acquired a 

surprising amount of calcite and iron oxide, which t~ey precipitate when they come into contact 

with the air in the tunnels. A pipe taken from the Colorado Central mine, which had been used 

for conveying ordinary cold underground waters for twelve or thirteen years, was lined with a 

deposit 0.4inch thick, made up of calcite with alittleiron and traces of manganese. (See Pl. XX, A.) 

The maximum alteration takes place along water courses which follow along fracture zones, 

and is not directl.v dependent on distance from the surface. 

Granitic quartz gneiss, 600 feet below the surface in the Baltimore tunnel and not near any 

vein, was found to contain fresh quartz, biotite, microcline, and orthoclase, with another feldspar 

which has been completely altered to kaolin with some carbonates and which inferentially was a 

soda feldspar. The effect of carbonated descending waters at this depth seems to hiwe been a 

solution of soda and some lime. 

· In the Lebanon tunnel, at a depth of 700 feet, calcite is formed wherever water drips down. 

The heaviest deposit is in a drift which has been open about twenty-three year~. The waters 

have formed on the floor a crust of calcite from 3 to 4 inches thick, and a beautiful deposit on 

the walls. From the roof depend fine stalactites upward of 6 inches long, and there are various 

other calcareous formations. It has been estimated that this whole drift, 8 feet high, would 

be cemented solid with calcite in five hundred or six hundred years. The calcite is in many 

places stained with iron and manganese, and locally with a little copper. 

In the Silver Ore tunnel, at a depth of 800 to 900 feet, the granite is decomposed and disintegrated along a zone where abundant water seeps through the rocks and accumulates in the 

bottom of the drifts. On the walls of the drift the waters have left soft stalactites of manganese 

peroxide, and where the water forms pools on the floor a plentiful surface scum of calcite is 

formed. 

In the Baltimore tunnel, at a depth of 1,100 feet, alaskite porphyry, which is nowhere near 

any known vein, has suffered alteration from surface waters. The feldspars have been mainly 

altered to kaolin, with a subordinate amount of quartz and sericite and a little pyrite. The 

waters which accomplished the alteration were evidently slightly sulphureted and have 

abstracted the alkalies and lime from the rock. At the same locality is a dike of quartz 

monz@ite porphyry. In this rock the biotite has been partly altered to secondary quartz, 

calcite, and siderite. The principal feldspar, andesine, has been largely or entirely altered to 

abundant carbonates of iron, lime, and magnesium, and to kaolin. 
U.S. GEOLOGICAL SURVEY PROFESSIONAL PA PER NO. 63 PL. XX 

A. CROSS SECTION OF PIPE, SHOWING PRECIPITATE OF CALCITE 

(WITH A LITTLE IRON AND A TRACE OF MANGANESE) FORMED 

IN TWELVE YEARS BY COLD MINE WATERS. 

Nat ural size. 

B. SPECIMEN FROM ALIUNDE FOURTH LEVEL, SHOWING NEARLY HORIZONTAL GROOVING OF 

WALL ROCK PITCHING 12° SW. 

T his indicates the nature of movement along the fault, which has later become cement ed by the vein. Nat ural size. 

GEOLOGICAL FEATURES IN COLORADO CENTRAL MINE. 
FEATURES OF THE SILVER-LEAD DEPOSITS. 139 

In the Ashby tunnel every stream of water that enters along a fracture zone has coated 

the rocks of the tunnel floor with copious deposits of calcite, hydrated iron oxide, and manganese 

dioxide. The metallic oxides are in many places distinct from one another and from the calcite, 

though in other places the deposits are mixed. At the breast of the tunnel, where as elsewhere 

the process has been active, the depth is 1,200 feet. Here the granite is bleached along joints 

to a light color, stained with red iron oxide. The fresher and darker granite c~ntains feldspar, 

quartz, biotite, and muscovite, with apatite and zircon and a little pyrite. The pyrite is fairly 

fresh. The soda-lime feldspars, albite and oligoclase-albite, are partly sericitized, but other 

feldspars, including nucrocline, microcline-orthoclase, and microcline-anorthoclase, are clear 

and unaltered. The bleached granite adjoining the fractures shows the biotite completely 

altered and bleached, with the deposition of carbonates. The fresh microcline feldspar of the 

darker granite remains fresh in this bleached rock, but the soda-lime feldspars, albite and 

oligoclase-albite, are further altered, being almost entirely decomposed to sericite, carbonates, 

and some kaolin. In this process of alteration the biotite has yielded to the waters abundant 

iron, which went into solution in the form of bicarbonate and a little manganese. The albiteoligoclase has yielded soda and some lime, which were taken into solution by the carbonated 

surface waters in the form of carbonates. The universal importance of this process is shown 

by the surprising abundance of calcite, limonite, and manganese dioxide precipitated from 

waters in underground workings, and by the universal alteration of the granites near the surface. 

The cause of the precipitation in underground workings is the access of air to the waters. Under 

normal conditions, at the underground depths mentioned, the solutions would normally deposit 

iron and manganese along the channel in the form of carbonates. 

Descending waters are active along the veins also and accomplish much solution and 

precipitation. In the upper Maine tunnel, 300 feet below the surface, hair-like crystals of sodium 

sulphate have formed in the drift. These contain traces of calcium, magnesium, and chlorine. 

In the Frostberg vein, in the Burleigh tunnel, 750 feet below the surface, needle-like crystals 

which also may be sodium sulphate were found. Along the Mendota vein in the Victoria tunnel 

workings, at a depth of 450 feet below the surface, the walls of the drift are covered withstalactites and incrustations of zinc carbonate, some of them as much as half an inch thick. In one 

place a white carbonate was covered by a turquoise-blue mineral, which is zinc carbonate containing basic copper sulphate, with traces of lime and magnesia. Tills drift has stood open for 

twenty years. Probable zinc sulphate was also noted on the Frostberg vein, in the Burleigh 

tunnel, at a depth of 750 feet below the surface. At a point where the descending waters enter 

the drift was an abundant precipitation of yellow iron oxide. At this point, a]<;;o, water dripping 

from the roof of the drift has precipitated basic sulphate of copper and lime carbonate, with 

possibly some copper carbonate. 

In a number of mines wet places or watercourses were found to be lined by black, soft, 

pulverulent material which consists of sulphides of lead and zinc, with sulphantimonides (prob- ~ 

ably tetrahedrite and polybasite) containing silver. These are rich silver ores, carrying several 

hundred ounces. 

, In the Snowdrift mine chalceclonic silica was found lining water-filled vugs, associated 

with limonite, and in the Colorado Central mine barite was noted in separate crystals embedded 

in a soft clay zone traversed by present surface waters. 

SUMMARY. 

To sum up the effects of descending waters at the present clay, it appears that they do 

an enormous work in dissolving, removing, and precipitating material in the rocks of the veins. 

These waters attack granites or similar rocks, leaving the microcline fresh, while the sodalime feldspars, albite-oligoclase, oligoclase-albite, and to a less extent the orthoclase, are vigorously attacked, the alteration products being kaolin, quartz, carbonates, sericite, and pyrite. 

The waters are slightly carbonated and contain a little sulphur, and by their reactions with 

the wall rocks, in the course of which the above-described alterations take place, they abstract 

the alkalies, especially soda, with iron, lime, and magnesia, and to a minor extent probably 
140 GEOLOGY OF THE GEORGETOWN QUADRANGLE, COLORADO. 

most of the other rock constituents. The simultaneous formation of kaolin and sericite by 

descending surface waters, the easy alteration of the soda-lime feldspar and the freshness of 

the microcline, and the abstraction from alkaline granite of so much lime, iron, and manganese that copious precipitates of these materials are thrown down when the waters are 

oxygenated, form the most interesting features of the investigation. 

In veins the metallic minerals, including zinc, copper, iron, lead, and silver, a~e dissolved 

by these descending waters and may be precipitated at a greater depth. In the mine workings 

zinc has been found deposited as carbonate; copper as basic sulphate, and possibly as carbonate; iron and manganese as oxides, rarely as carbonates. Along wet water courses, where 

the oxidizing conditions which prevail in the mine workings are not present, the waters 

precipitate lead, zinc, and silver in the form of sulphides and sulphantimonides. From these 

descending waters also chalcedonic quartz is deposited, and, in one place at least, barite. 

DERIVATION OF VEIN MATERIALS. 

DERIVATION OF CERTAIN METALLIC MINERALS. 

The highly argentiferous minerals, including tetrahedritc, polybasite, ruby silver, and 

argentite, on whose presence the richness of the ores invariably depends, have been found to 

0 inches.· 

~----~--~~----~ 

FIG. 27.-Sketch of specimen from Colorado Central 

mine, 1,050-foot level, showing primarily veinlet of 

polybasite subsequent to other sulphides. a, Polybasite; b, blende; c, granite; d, pyrite; e, quartz; 

f, galena. The following general order of deposition 

in the older vein fissure is shown: (1) Quartz and 

pyrite, (2) blende, (3) galena. 

be mainly subsequent to massive galena-blende orcs (fig. 

27) containing a relatively small quantity of silver, say 

8 to 30 ounces. These rich silver minerals occur as fresh, 

clean crystals lining the walls of geodes and of watercourses which are full of circulating water. Associated 

with them and forming part of the later generation of 

crystals is also usually some well-crystallized blende, 

pyrite, and galena. In the Waldorf mine, in the Argentine district, where this second generation of galena forms 

very large and abundant crystals in cavities (Pl. XIX), 

this ore is reported very low in silver. It seems probable that the rich silver ores have been derived from the 

older vein ores by circulating waters subsequent to the 

first deposition. Certainly, however, the silver has not 

been concentrated from the present superficial portion 

of the vein and carried down to lower levels. The 

uppermost portions of these veins are almost invariably the richest, the amount of silver diminishing from 

the surface downward. Evidently, therefore, it has 

been concentrated in part from the eroded portions of 

the veins and in part from the locally dissolved and 

reprecipitated primary ore beneath the present surface. 

The galena and blende of the second generation may 

well represent the galena and blonde taken into sohition from the upper portions of the veins· which are now 

being removed, and also to a certain extent from the 

minerals of the present vein, and precipitated with less silver values than originally. The 

deposition. of the rich silver-bearing sulphides has taken place to a greater extent in some 

localities than in others, producing the rich ore bodies. 

Following this reasoning and considering the amount of silver concentrated in the rich 

shoots and the quantity of original blende and galena which must have been taken into 

solution to furnish this silver, it seems certain that a very large amount of the original ore 

has been worked over by this process. Indeed, so abundant is the evidence of the continued 

work of solutions since the formation of the original vein opening that we can hardly be sure 

in many places as to what is really primary and not rearranged ore. Near the surface, for 
FEATURES OF THE SILVER-LEAD DEPOSITS. 141 

example, where the lead and zinc belonging to the second generation increase in amount, a 

comparatively massive ore may consist of secondary galena and blende intercrystallized with 

contemporaneous polybasite, which then has the appearance of being a primary mineral. 

DERIVATION OF CERTAIN EARTHY MINERALS. 

The wall rocks of the veins are everywhere altered, a change which has been accomplished 

both by the mineralizing solutions and by the ground waters. The alteration products include 

fine quartz, sericite, kaolin, carbonates of lime, magnesia, manganese, and iron and pyrite. 

In the fissure veins, which are very common in this d}strict, the most persistent feature 

is the presence next to the walls of a band of comb quartz, locally containing a little pyrite 

and barren of values. This quartz represents the first deposition in an open fissure and has 

no evident connection with the ore deposit. Subsequently the remaining and greater portion 

of the fissures was filled with massive galena and blende, with very little quartz. Therefore, 

throughout the district the fissure stood open for a considerable time during the deposition of 

the comb quartz before there was any ore deposition. This quartz has grown out perpendic·· 

ularly from the walls of altered rock, which contain fine secondary quartz, and the universal 

presence of this lining and its occurrence in virtual continuity with the quartz of undoubted 

local origin points strongly to the conclusion that the source of the quartz was the wall rock, 

and that the lining originated by lateral secretion in its narrowest sense. 

The quartz which was deposited later within the vein may well have had a like origin. 

A similar consideration applies to the scanty pyrite that is found at many places inclosed in 

this comb-quartz lining, the iron for which seems to have been indigenous in the rock, though 

probably the sulphur was not. In many places pyrite is more characteristic of the altered 

rock than of the veins. \Vithin the· rock siderite is more abundant than pyrite and both are 

contemporaneous, showing that; the waters which altered the rock were carbonated as well as 

sulphuretcd. As seen under the microscope, this indigenous siderite, derived in part from 

the alteration of the biotite, forms tiny veinlets. Some of the larger fissures in the rock have 

been lined with siderite, which has taken the place of the ordinary quartz lining, and, as in 

the case of the quartz, its derivation from the wall rock seems certain. It is likewise probable 

that a similar origin can be assigned to much of the subsequently formed siderite, though some 

of this has probably not a direct derivation from the wall rock, but comes from a more remote 

. source, being immediately due to the oxidation of the pyrite in the veins. This subsequent 

siderite is especially abundant near the surface, indicating the work of atmospheric waters. 

The fact that probably 99 per cent of the veins which have filled fissures have a lining 

between the ore and the wall rock, consisting of quartz with some siderite and pyrite which 

were plainly derived from the exudation of the adjacent rock, .shows that these fissures were 

not filled with circulating solutions for a considerable period after their formation. 

Afterwards ore deposition set in, and the larger portion of the fissures was filled with coarse 

interlocking crystals of galena and blende, associated with very little gangue, a fact which 

suggests rapid deposition. Since the active precipitation of galena and blende is by no means 

a universal process, it is probable that it took place in all the veins at the same time and was 

due to the same cause, and since all of these veins are lined with approximately the same thickness of laterally secreted material, it follows that the fissures were opened simultaneously, a 

catastrophic phenomenon like the fissuring which attends an earthquake shock. 

Calcite is formed in the altered rocks as the result of alteration of the soda-lime feldspars. 

Under the microscope calcite veinlets are found which are beyond any question whatever due 

to the local segregation of the indigenous mineral. These veinlets increase in size to macroscopic dimensions. Many altered rocks are cut by barren calcite veinlets, and these have no 

significance as to the proximity of an ore deposit. Where calcite does occur in the ores it is in 

small amounts and is among the latest minerals which have crystallized. In the mine workings 

it is abundantly deposited from underground waters, both near the veins and remote from them. 

All the evidence, therefore, points strongly to the derivation of the calcite in the veins from the 

wall rock, and this is probably a comparatively recent phenomenon due to descending atmos- · 

pheric waters. 
142 GEOLOGY OF THE GEORGETOWN QUADRANGLE, COLORADO. 

Kaolinite is an alteration product of feldspar and is associated with sericite and calcite. 

In the veins it occurs as a chemically precipitated mineral associated with carbonates and 

sulphides, which represent. the latest depositions. It is often found on the walls of cavities, 

forming both microscopic and macroscopic veinlets. This mineral, like the calcite, is probably 

in all cases derived from the wall rock and is mainly at least the result of descending waters. 

Barite, also one of the last-formed minerals, has been noted as contemporaneous with 

calcite and kaolinite, and is not associated with the metallic minerals in a close way. It has 

been noted in separate crystals embedded in the clay of a watercourse. The largest amounts of 

barite occur in scattered veins, which so far as observed are free from metallic minerals. Barite 

is undoubtedly present in the feldspar of the granites in small quantity, and probably passes 

into solution when the rocks arc altered. Sulphuric acid, derived from the oxidation of sulphides 

by waters, precipitates the barium in solution as barite. 

DERIVATION OF FLUORITB. 

' The sericitization of the wall rocks of the silver-bearing veins argues the presence of a little 

fluorine in the mineralizing waters, although not necessarily so much that fluorite should have 

crystallized from these waters. The occurrence described in the Seven-Thirty mine (p. 202), 

where fluorite in small crystals and veinlets was detected in the Bericitized ·wall rocks of the 

Seven-Thirty vein though no fluorite was found in the vein itself, is of interest in this connection 

and indicates that here the fluorite was really precipitated by the mineralizing waters. Elsewhere the evidence suggests that if the mineral was originally deposited by the primary mineralizing solutions it has since been reworked and deposited in a concentrated form by surface 

waters. 

The derivation of the fluorine in the original mineralizing solutions is also a matter open 

to doubt. The veins under discussion occur partly or wholly in granite. Granite is known to 

contain fluorine, which is present in mica, hornblende, and apatite, as well as in other minerals, 

and granitic rocks very commonly contain fluorite-bearing veins, in which the fluorite is either 

due to segregation from the granite or has been formed during the final process of consolidation. 

At many places vadose underground waters contain fluorine, derived from the rocks which they 

have traversed. Within the area of the Georgetown quadrangle the granites not only contain 

fluorine in small quantities in the silicates, but more abundantly in the .form of fluorite. Mr. 

Ball has found fluorite especially abundant in the granite porphyry in the southeastern portion 

of the quadrangle, where this mineral forms a rather constant accessory. The place of this 

occurrence is 10 miles distant from any known ore-bearing veins, and it is believed that the 

mineral is due to the after processes of pre-Cambrian granitic consolidation. In the Pikes Peak 

region, around Cripple Creek, fluorite is present as a rare constituent of the granites, and this 

mineral is distributed in remote and widely separated parts of the Pikes Peak quadrangle in 

localities far removed from vein mineralization. a 

No fluorite has been observed in the Silver Plume variety of granite, which forms the 

oldest rock of the veins in question. Yet since tllis mineral is of rare occurrence in the veins, 

its total amount might have easily been furnished by the fluorine in the granite. The present 

descending surface waters which attack and decompose the biotite of the granite must contain 

fluorine, and the sericitization which, together with kaolinization, is accomplished by these 

waters, shows that this is probably the case. · 

On the whole, therefore, the amount of fluorine indicated both in the original mineralizing 

waters and in the later descending waters that have altered the veins is not more than the 

amount that must have been derived from the altered wall rocks, though it is possible that a 

small quantity had a deep source. · SUMMARY. 

It is therefore probable that the gangue minerals are derived from the wall rocks. Moreover, a large part of the materials in the veins, including metallic minerals and earthy gangue 

minerals, are probably immediately due to the precipitation of ordinary atmospheric ground 

waters. The original derivation of the metals-zinc, lead, silver, copper, arsenic, antimony, 

mercury, cadmium, gold, etc.-has not been decided by this inquiry. 

a Prof. Paper U. S. Geol. Survey No. 54, 1906, p. 47. 
FEATURES OF THE SILVER-LEAD DEPOSITS. i43 

ALTERATION OF WALL ROCKs: 

The alteration of the wall rocks near the veins has resulted in the formation of quartz, 

sericite, various carbonates, and kaolin. Of these products the sericite remains practically 

insoluble, while the quartz, carbonates, and kaolin have been taken into solution and deposited 

in tiny cavities and veinlets. Fluorite was observed in one place among the secondary products, and probably occurs in another place in the form of small grains in a microscopic veinlet 

of translucent kaolinite. Kaolin has been found at various depths up to. at least a thousand 

feet. Its formation, however, in the wall rocks was undoubtedly contemporaneous with that 

in the veins, which has been found to have taken place during the last periods of deposition and 

to be probably the work of descending waters. It is therefore probable that the kaolin in the 

wall rock is an alteration product which is later than the sericite that accompanied the original 

ore deposition. The waters which accomplished this original alteration must then have contained some carbon dioxide, sulphur, and fluorine, since fluorine is inferred to be necessary for 

the formation of sericite.a 

Both the original mineralizing waters and the subsequent atmospheric waters have exercised a very remarkable preference for some varieties of feldspar over others. Microcline has 

been notably immune from all alteration, remaining fresh and clear in the very wall of the veins 

where the other feldspars and biotite have been completely altered. These other feldspars are 

principally orthoclase and oligoclase-albite. In many places one of these has been much more 

vigorously altered than the other, and as a rule it is the orthoclase which has withstood the 

attack best, although here and there the situation is reversed. Even where the orthoclase has 

been entirely altered the microcline, though strained and cracked near the veins, remains quite 

clear. In cases of extreme alteration of the rock a little sericite occurs along cracks, but does 

not penetrate the rest of the mineral, and even in the rock actually being replaced by ore the 

microcline 'is undecomposed. Under such conditions it has been observed as being replaced by 

metallic minerals without alteration.b 

Microcline has the same composition as orthoclase, and the cause of its crystallographic 

differences has been much discussed. It has been somewhat widely held that the microeline 

structure, consisting of two sets of striations, intersecting at right angles and visible under the 

microscope, with crossed nicols, had been developed in orthoclase by pressure. There is much 

microcline, however, of which this can not hav~ been true (seep. 93); and the above-described 

immunity of microcline from alteration by the same waters that attack orthoclase points 

unmistakably to a molecular difference between the two minerals. 

Epidote. and marcasite were observed as alteration products in one thin section of the wall 

rock of the Seven-Thirty vein. , 

Certain of the lodes of that portion of Republican Mountain beneath which the Kelly 

tunnel passes show a characteristic alteration of the gneiss wall rock. This rock has acquired a 

peculiar bright-green color. Under the microscope it shows a thorough alteration, the products 

being quartz and a pale-green mineral, evidently mica, which in this case has replaced the 

sericite, the usual micaceous alteration product. Analyses by W. T. Schaller show the color 

to be due to chromium, so that the mineral is a chrome-bearing mica or fuchsite. Abundant 

clouded carbonates were also present. 

CHANGES OF ORE IN DEPTH. 

ZONE OF OXIDATION, 

The zone of complete oxidation in these veins usually extends fr9m the surface to a depth 

of 5 to 40 feet. The oxidized material is a brown clay, much of it stained by various other 

colors. It is generally rich ore and contains several hundred ounces of silver. . 

SUPERFICIAL SECONDARY ARGENTIFEROUS SULPHIDES. 

' Below this oxidized material and mixed with the lower portion of it come friable, locally 

powdery black sulphides and bunches or nuggets of pure secondary galena. These pulverulent 

a Prof. Paper U.S. Geol. Survey No. 42, 1905, p. 233. 

b Compare the replacement of microcline by copper, in Doctor Sullivan's experiments (p. \60). 
144 GEOLOGY 0.1!' THE GEORGETOWN QUADRANGLE, COLORADO. 

sulphides are also rich ores containing relatively high amounts of silver and lead and typically 

more gold than occurs at greater depth. The soft sulphides arc found especially along cracks 

and water courses and are of secondary origin, having evidently been concentrated from the 

leaner ore by descending waters. They occur up to considerable depths from the surface, but 

in decreasing quantity. The older and typically more massive ores in which they have formed 

contain usually very much less silver and also less gold. For example, primary ore which contains 20 to 30 ounces of silver has yielded· by this concentration process secondary sulphides 

which carry 200 to 300 ounces. 

DEEPER sgcONDARY ARGENTH'EROUS SULPHIDES. 

Below the zone where soft secondary sulphides occur and irregularly overlapping the lower 

portion of tlus zone the rich ores contain polybasite, argentiferous tetrahedrite, and ruby silver, 

better crystallized and more massive than the pulverulent sulphides, but also subsequent in 

origin to the massive galena-blende ore. These richer ores diminish in quantity as depth 

increases, though gradually and irregularly, so that the lower portion of the veins contains 

relatively less silver and lead. The best ore in most veins has been found in the uppermost 

500 feet, although good ore extends locally down to 700 or 800 feet, and in the Colorado Central, 

and to a minor extent in other veins, down to a thousand feet or more. 

SECONDARY PYRI'rg. 

The superficial portions of many of the lodes of this district are characterized by an unusually large amount of pyrite, in many places associated with and contemporaneous with 

abundant carbonate. The repeated evidence of the dependence of this p:yrite and siderite on 

proximity to the surface shows that these minerals are probably due immediately to the action 

of descending surface .waters. This deposition of pyrite by descending waters has probably 

taken place to a less extent up to considerable depths, since many specimens from these veins 

show that the pyrite, much of which on casual observation appears intergrown with the galena 

and blonde, is really later than the sulphides and fills spaces which have been created by their 

having been broken or crushed. 

PERSISTENCE OF VEINS OF MENDOTA TYPE IN DEP'l'H. 

With the exception of the gradual decrease in depth of silver and lead, due to concentration effected by surface waters, and the greater proportion of pyrite and siderite close to the 

surface shown at many places, the primary low-grade blende-galena ores have in many of the 

strongest and most extensive veins proved to be fairly constant in amount and character to a 

depth of at least 1,250 feet, and probably much deeper. These veins, persistent in depth as 

well as in horizontal extent, are best represented in the group of which the Terrible and Mendota 

lodes arc members. PARTLY OR WHOLLY SECONDARY VEINS. 

In contrast to the type just Jescribed arc other veins which have usually a smaller horizontal extent and whose downward vertical extent is much less than that of the veins of the 

Terrible type. In the Pay Rock mine the best ore, as is usually the case with all the veins of 

the district, occurs within the uppermost few hundred feet, though good ore was found locally 

down to about 1,200 feet. The surface ores are described as having been very rich, containing 

large bodies of pyrite which carried gold in considerable quantity, some of it as much as 2 or 3 

ounces. '\Vithin this profitable upper zone the vein is a strong one, containing much quartz, 

galena, blende, pyrite, calcite, and siderite, all intercrystallized and contemporaneous. Polybasite was also probably present. The calcite and siderite seem ·to have been more abundant 

near the surfiwe than at a greater depth. The fact that calcite occurs as a gangue mineral 

contemporaneous with the metallic minerals, and its evident relation to the surface, together 

with the abundance of quartz in the gangue, marks the vein as probably belonging chiefly or 

entirely to the secondary period of ore deposition, during which the primary vein materials 

have been reworked by descending waters. On the lowest level of the mine, in the Ashby 
FEATURES OF THE SILVER-LEAD DEPOSITS. 145 

tunnel, at a depth varying from 1,000 to 1,500 feet, the zone of fractures and slips along which 

the ore has· formed in the upper levels still persists, but this zone is usually barren and the 

existence of an important vein could not be suspected. There is no resemblance between the 

strong silicified vein above and the slightly mineralized or barren slips in the rock below. 

In this case, therefore, the bottom of the enriched or reworked ore is also the bottom of 

the vein, and there appears to be little or no primary unworked portion below. The fact that 

practically the whole vein has been deposited by the descending waters is shown by its relation 

to a porphyry dike, which has a strike transverse to that of the vein and a dip of about 50°. 

The deposition of the ore has taken place most vigorously along the upper surface of this 

decomposed dike, and though the vein fracture passes through the dike and is reported on the 

other side, it was there entirely unmineralized. 

There are numerous other examples of this type of silver-bearing lode which passes from 

the superficial region, where the character of the vein is similar to that of the superficial reworked 

'portions of the deeper and more persistent veins, to almost or absolutely barren fracture or 

fault zones below. In many of these lodes the fault along which the vein has formed passes 

downward probably to a considerable d~pth, but is entirely unmineralized. Examples of this 

include the Boston, Beecher, and other neighboring lodes on Republic-an Mountain, and, so 

far as known, the lodes of the Democrat Mountain group. In the latter group the known ore 

·bodies are confined to a depth of 200 to 400 feet below the surface, and the pitch of the most 

important ones follows that of the surface sla.pes. 

SUMMARY. 

The original ore deposition consisted of predominant blende, with galena and pyrite. The 

ore~ were deposited a,ong fracture and :fissure zones of slight displacement. Many of these 

veins were deposited throughout a great vertical as well as horizontal range. Some of them are 

known to extend at least 1,800 feet below the present surface and are probably much deeper, 

and considering the amount which must have been eroded from their tops it is evident that they 

~ originally ·occupied a vertical extent of several thousand feet. Some of the original vein 

deposits, however, have a very much less horizontal or vertical range, or both. Vein deposition 

took place probably at various depths within the general zone of mineralization, so that the 

upper portions of some veins were considerably higher than the tops .of others, and likewise 

the bottoms of some were above the lower portions of others. Since the period of primary 

vein deposition the original surface has been lowered, so that so far as known all the veins have 

been exposed at the surface and have been actively attacked by erosion, while circulating 

ground waters, principally those which penetrated downward from the surface, have been very 

active in rearranging the primary ores. Probably some thousands of feet of rock, including a 

large part of the zone of mineralization, have been removed from the upper portion of the 

crust since the ores \vere :first deposited.a 

During this erosion the atmospheric wa.ters have brought about a more or less complete 

reworking and rearrangement of the metallic minerals which were close to the surface, while 

a It is impossible to determine accurately the thickness of superincumbent rock which has been removed by erosion since the intrusion of 

the porphyries. It is known, however, to be very great, and the order of the amount can be estimated. Whitman Cross (Mon. U. S. Geol. 

Survey, vol. 27, 1896, p. 202) has already shown from the character of the Denver (post-Laramie) formation in the Denver Basin that while Its 

lower portion was being deposited the streams from the Colorado Range cut at no point into the pre-Cambrian rocks. The whole range was 

almost without doubt completely covered by lavas, surface llows of the intrusive porphyries. Formations similar to the Denver cover areas 

in Middle Park and in tbe northeastern portion of South Park. (Op. cit., p. 34.) The area of these formations flanking the Colorado Range 

equals that of the pre-Cambrian rocks of the range In this latitude, and since the lower half of the Denver formation (approximately 725 feet) 

contains no pre-Cambrian material and the upper portion considerable amounts of volcanic fragments, it may be assumed that the lava flows 

had an average thickness upon the range of 800 to 900 feet. It Is, however, probable that the lava was thicker upon the llanks of the range 

than near Its crest line, and in consequence the thickness of the lava in the vicinity of Silver Plume is tentatively placed at 500 feet. During 

the erosion which produced the old mountainous upland this lava was completely stripped off, as well as an unknown thickness of the preCambrian rocks. This old surface was, however, mountainous, Mount Evans, for instance, rising 3,500 feet above the ancient valley of Clear 

Creek at Silver Plume. No lava remains on any of the old summits, and In consequence unless the lava was deposited upon a rugged surface 

with its mountains invariably where valleys now are, it Is more than probable that at Silver Plume 3,500 feet or more of pre-Cambrian rocks 

were eroded after the land was bared of its lava flows. Since early Pleistocene time, when tbe streams were revived, Clear Creek has sunk its 

valley at Silver Plume 1,250 feet, while the present 11,000-loot contour on Brown Mountain Is about 500 feet lower than the surface of the old 

mountainous upland. The total erosion since the intrusion of the porphyries is estimated at a minimum to be 4,500 feet and as a maximum 

5,2.'>0 or more feet.-S. H. B. 

31401-No. 63-08-10 
146 GEOLOGY OF THE GEORGETOWN QUADRANGLE, COLORADO. 

their effects diminish gradually in importance with increasing depth. Much of the material 

originally contained in the eroded portions of veins has been carried down along the vein 

channels, which offered open fissures, for fracturing and fissuring resulting in reopening of the 

veins was contin'uous tproughout this period. The erosion has been too active to permit the 

surface waters to form any impoverished zone near the surface, out of which the minerals 

should be leached and carried downward. In all these veins the surface portion is the richest, 

the result of the reworking process having been concentration ·of the valuable metals, lead, 

silver, and gold, in the superficial zone, and to a constantly less extent, dependent on the 

irregular distribution and strength of water courses as depth increases. These enriching 

effects are most marked in veins which originally contained most silver and lead and which have 

been laid open to the thorough searching of the waters by fissuring subsequent to the primary 

deposition. It is noticeable, however, to a more or less striking degree in all the veins. The 

result is that the upper few hundred feet of the veins has generally yielded rich lead-silver ore, 

especially in places, and on the very surface the gold, which in the original vein is a rare constituent, has locally been concentrated, so as to be of economic value. This zone of pay ore 

extends as a rule down to a depth of 300 feet, in many places to 500, in some to 700 or 800, 

and here and there to 1,000 feet or more, but as depth increases the amount of this ore becomes 

continually less and the original filling of the vein predominates more and more. This original 

ore encountered in the lower levels is a blende ore containing considerable galena and some 

pyrite, and is of low grade in respect to silver. 

Many of the smaller veins, which originally had less considerable dimensions both horizontally 

and vertically, and especially those which formed in the upper portion of -the zone of mineralization, have been eroded down so that only roots are left, as is proved by workings which pass· 

below the vein to a practically or entirely unmineralized fracture or fault zone. The solution 

and downward transfer of at least a portion of the metallic minerals of the vein, which took 

place constantly as erosion progressed, continued until the surface was worn down almost or 

quite to the vein roots, with the result that all that is left of the vein is the concentrated zone, 

enriched from the eroded portion with respect to lead and silver and locally gold. When the 

bottom of the pay ore is reached, the bottom of the vein is also encountered. In some places 

the evidence of deposition by descending waters of the whole mass of metallic minerals and 

accompanying earthy minerals is so strong that it appears probable that no portion of the ore 

bodies in their present position is due to the primary deposition, but that the bottom of the 

origin~l vein stood above the present surface. The descending waters continued to transfer 

the values downward as erosion progressed and finally brought the rearranged ores to what 

was originally the unmineralized channel, below the bottom of the original vein. These are 

therefore secondary veins, the barren lower portions of the fracture channels having inherited 

the older ores which were deposited higher up·. The abundance at various altitudes of veins 

whose profitable surface portions pass below within a few hundred feet to barren fractures, 

combined with local and minor evidence that the ores are largely due to descending waters, 

makes it probable that these secondary veins constitute a rather common type. 

UESUME OF HISTORY OF VEIN FORMATION. 

In the early Tertiary period planes of fracturing or faulting were developed in a northeastward-trending zone of weakness in the pre-Cambrian crystalline rocks. These were followed 

by the intrusion of monzonitic dikes, of which there were several varieties, intruded at distinct 

periods. Subsequent to this intrusion there was a renewal of movement within this zone of 

weakness, producing faulting, though usually the displacement of the faults was slight. Proba:.. 

bly at a certain definite time many fissures were opened simultaneously in different parts of the 

district. For some time these fissures remained relatively dry, and gradually the walls became 

covered with crusts of crystalline quartz and siderite derived from the walls. At a somewhat 

later period the fissures became filled with circulating waters, probably hot and ascending and 

deriving their heat and their metallic constituents (with the exception of iron) from the subjacent monzonitic magma. These waters began active mineral deposition along faults and in 
FEATURES OF THE AURIFEROUS PYRITIC DEPOSITS. 147 

all the fissures simultaneously, and deposited blende with some galena. With these minerals 

crystallized small amounts of silver, copper, antimony, arsenic, and a number of other metals. 

Pyrite and quartz crystallized at the same time, the iron and silica being probably derived 

from the rocks. These waters contained carbon dioxide and sulphur. The ore deposition 

extended over a vertical range of several thousand feet, probably at least 5,000, and some of 

.the veins formed were of great vertical extent while others were of less, the extent depending 

largely on the strength of the circulation channels. 

Since this primary mineralization several 'thousand feet of superficial rocks have been 

removed, exposing the veins and removing large portions of them. Descending surface waters 

have enriched the silver and lead in the upper portions of the veins by addition of the metals 

carried downward from the eroded portions to those already there, and by the removal of other 

materials to lower levels to make room for this addition. Many other important changes took 

place, so that the old veins were reworked more or less thoroughly near the surface, the relative 

amount of reworking depending largely on the postmineral fracturing of the primary vein. 

Thus the more persistent veins have an enriched upper zone a few hundred feet in depth, while 

the veins whose bottom was at first relatively high have been eroded downward to their roots, 

or even below; so that along such vein fissures there ·are now exposed on the surface ore deposits 

which either represent enriched roots or are secondary veins formed below the original roots 

by the transference of materials from the eroded portion to what was originally the underlying 

barren fracture. 

FEATURES OF THE AURIFEROUS PYRITIC DEPOSITS. 

COMPOSITION OF VEINS. 

The characteristic and essential constituent of the auriferous pyritic deposits is pyrite, 

with a gangue of quartz. This pyrite contains gold, with a little copper and silver. Galena 

and blende may be absent, but typically are present in relatively small quantity, and some 

veins contain these minerals, especially the former, in considerable amounts. Where galena is 

an important feature of the ore this mineral usually contains considerable gold and silver. 

. The pyrite is cupriferous to a varying degree and locally grades into copper pyrite. Tetra~ 

hedrite is of common but rather sparing occurrence, and this is true to a greater degree of 

chalcocite, which usually occurs in the soft black form. In th~ Griffith mine small quantities 

of the gold and silver telluride, hessite, were found mixecl with argentite and other minerals. 

:Magyagite, a sulphotelluride of gold and silver, has been deposited at the Coming Nation mine 

on Soda Creek. It is reported that in one locality in the Centennial mine the ore contained 

platinum and iridium, having a value of $3 to the ton. 

It has been demonstrated -that gold occurs in the ores in native form, and free gold visible 

to the naked eye is sometimes encountered. Native silver is more rare. 

Earthy minerals besides quartz include brown carbonates of iron, manganese, magnesia, 

and lime, usually to a slight extent. In the Griffith vein, however, the iron and manganese 

carbonates exceed in amount the contemporaneous pyrite. Barite is present in most veins in 

small quantity. Fluorite has been noted in a number of places in the decomposed dikes along 

which the veins run, but is not common as a gangue mineral. It occurs in considerable quantity 

in the Big Chief vein, near Georgetown, which is probably a branch of the Centennial, and also 

in other veins along the auriferous belt which stretches from Georgetown along the north slope 

of Leavenworth .Mountain. 

The valuable minerals in the ores consists of gold, silver, copper, and in some veins lead. 

Copper is usually present, but in many veins does not occur in economically important amount. 

A small amount of silver is uniformly present,. and typically the amount is sufficient to be of 

importance. The relative proportion of gold and silver varies considerably. In a great many of 

the mines, including the Freeland, Stanley, New Era, and Centennial, the proportion of gold 

to silver is much the same, the ore averaging from 1 to 2 ounces of gold and 20 to 40 ounces of 

silver to the ton. From these ores there is a transition on the one hand to those in which the 

silver is contained in less proportio.n, as, for example, in the mines of the Empire district, where 
148 GEOLOGY OF THE GEORGETOWN QUADRANGLE, COLO:!lADO. 

the silver is usually of slight commercial importance, and on the other hand to ores which contain relatively more silver, for example, in the Lamartine mine, where the proportion of gold 

to silver has been 1 to 70 (by weight), and in some other mines in which the relative value of 

silver is still higher.· In those of the Cecil-Argo group the silver and lead form the most important 

metals, the value of the silver being several times that of the gold. 

On the whole the auriferous gold ores of the Idaho Springs and Em.pire districts are to be 

defined mineralogically as essentially pyritic, the pyrite containing some silver and copper. 

Thus they are distinct from the blende-galena orcs of the argentiferous veins of the Georgetown district. In these auriferous veins there is a greater range of composition than in the 

Georgetown argcntiferous veins, which is due to the varying amounts of galena and blende. 

PARAGENESIS OF VEIN MATEIUALS. 

In the Griffith mine the' following order of deposition was noted in that portion of the vein 

belonging to the second or pyritic period: (1) Pyrite, brown carbonates, and quartz; (2) chalcopyrite, quartz, galena, and blende; (3) barite and kaolinite. In many places the more 

cupriferous pyrite and chalcopyrite which occur associated with pyrite in the mines do not 

show clear age relations. In the Centennial mine and those of the Empire district, however, 

the amount of copper in the pyrites decreases in the lower levels, suggesting that the more 

cupriferous pyrite and chalcopyrite are later than the relatively low grade cupriferous original 

pyrite and are derived from it by concentration. This is certainly true of the tetrahedrite, 

which was noted in both of these districts and in other mines forming along cracks in the chalcopyrite and plainly derived from it. This evident derivation makes it probable that small 

amounts of arsenic, antimony, etc., as well as copper, silver, and gold, arc contained in the 

original pyrites. Chalcocite, which fonns in small quantity, especially in the soft black form, 

near the surface and along fractures, is also plainly a later formation than the cupriferous 

pyrite. 

Fluorite, where it occurs in the Big Chief mine near Georgetown, is later than the main 

period of mineralization, since it occurs coating cavitcs in the older sulphides, iron, lead, and 

zinc. 

CHANGES OF VALUE IN DEPTH. 

These veins arc oxidized near tl;J.e surface to a small extent, the amount of oxidation corresponding with that of the argcntiferous veins. The zone of oxidation is especially ~ell marked 

in the Donaldson-Champion Dirt group of veins, where there is a definite oxidized zone 30 to 

70 feet deep, marked by the more or less complete alteration of pyrite to limonite. This zone 

follows the irregularities of the topography. At the Centennial partial oxidation is said to 

extend 15 or 20 feet down, though locally pyrite occurs at the surface. The oxidized ore of the 

Donaldson-Champion Dirt vein is reported to have been of unusually high grade, much of it 

running 6 ounces of gold. The veins of the Silver Mountain ore zone at Empire, which are 

oxidized completely to a depth of 40 feet or more, contain gold in a free state in amounts which 

seem to have been as great or greater than that in the sulphide material below. 

In the Centennial mine, where the gold and silver values are associated with chalcopyrite 

or cupriferous pyrite, and the pyrite that contains very little copper is also of low grade with 

respect to gold and silver, .the ores occur between the surface and a depth of about 550 feet, 

beneath which there is an abrupt falling off in values along a roughly horizontal line. The 

waters in this mine contain copper sulphate, showing that waters which follow down along 

the vein from the surface are actually dissolving copper from the ores. The presence of tetrahedrite, which has formed along cracks in the ore and is subsequent to the chalcopyrite, 

suggests that this chalcopyrite and the more cupriferous pyrites may have been altered from 

relatively barren pyrites by the addition of copper contained in the surface waters, in 

accordance with the process so frequently observed in veins containing copper. 

In the mines along the Silver Mountain ore zone, at Empire, the same conditions were noted 

in the same class of ore. The sulphides immediately beneath the oxidized zone not only contain 
FEATURES OF THE AURIFEROUS PYRITIC DEPOSITS. 149 

a large amount of copper pyrite, but along fractures in the ore black sulphides of copper, probably, in part at least, chalcocite, have formed, and these .are undoubtedly of secondary origin. 

The best ore is said to have been of this character, and it extended from a depth of 25 feet to 150 

feet below the surface. Much of this ore was high grade in gold, some of it assaying 10 ounces 

to the ton. The impression received by the writers was that the several veins in these mines 

were decidedly stronger in the upper levels than in the lowest level. In the highest workings 

the lodes are represented in many places by definite strong deposits of quartz and cupriferous 

pyrite, while in the lowest level they are characteristically only mere slips in the altered and 

pyritized rock, and many of these arc marked by very little or no especial mineralization. It 

also seems probable that the proportion of chalcopyrite to pyrite is much larger on the upper 

than on the lower levels, and that the .lowest level, that of the Aorta tunnel, is mostly belo\v 

the rich cupriferous and auriferous zone. The actual difference of elevation between the 

outcrop of the veins and the lowest level is about 500 feet, so that the enriched sulphide 

zone appears to correspond in depth with that which seems to occur in the Centennial vein. 

In both these localities the copper is closely associated with gold and silver, so that the 

enriched copper pyrites are aJso better-grade gold and silver ores, while the slightly cupriferous 

varieties below are of low grade in respect to these metals. 

In the mines mentioned a portion of the copper which has contributed to the enrichment 

of the original sulphides has been derived from the oxidized zone, but it seems unlikely that 

this has been the case with the gold and silver, which, like the enriched superficial portions of 

the argentiferous veins, must have been derived from the overlying portions of the lodes which 

are now eroded. 

In the veins of the Freeland group it was noted that galena was more abundant in the 

uppermost few hundred feet, while in the deeper workings only a little of this mineral was 

contained in the characteristic pyritic ore.. The upper workings also show more brown carbonates, consisting of siderite and rhodochrosite, and some barite. In one place observed it 

seems evident that the brown carbonate is secondary and has been derived from older pyrite in 

the ore. In these relatively superficial ores, also, tetrahedrite has formed along cracks in the 

chalcopyrite, and, as in the Centennial mine, is of secondary origin, and has been concentrated 

in this form by surface waters. The suggestion therefore arises that the relatively abundant 

galena near the surface may have been concentrated from the scanty galena in the original ore 

by the same agents. 

On the whole, the strongest evidence of the reworking of the 'ores by surface waters is 

afforded by markedly cupriferous ores. Copper is a mineral which is readily attacked by 

descending waters, and the presence of copper sulphate in the mine waters shows that the 

. process of solution is actually going on at present. This copper-sulphate solution seems to 

contain gold and silver also. Apart from this, however, and from the probable partial concentration of galena near the surface in some mines, the evidence of rearrangement of the ores by 

descending waters is in general not nearly so great as in the Georgetown district, and such 

reworking has probably taken place to a considerably less extent. The Lamartine mine, where 

the ore body does not reach the surface by 100 to 400 feet, affords an instance of ore which 

can not have been reworked by descending waters to any important extent. Yet this mine 

has had a very large and profitable production. Although the principal ore appears to extend 

less than 1,000 feet from the surface, it is probable that this has no relation to the effects of 

surface waters, but· that the bottom of the original shoot has been reached. In the- Little 

Mattie mine, also, the best ore is reported between the fourth and seventh levels, and although 

the ore in some places reached the surface, the amount was not so great as below. 

POSTMINEitAL MOVEMENT. 

In a number of different mines in the Idaho Springs district there has been a po<;tmineral 

movement along the veins, resulting in a characteristic band of breccia whose larger fragments 

have been rounded by attrition. This formation is in many places several fee't thick and 

typically follows along the vein. Examples arc seen in the Stanley, Freeland, and a number 
150 GEOLOGY OF THE ~EORGETOWN QUADRANGLE, COLORADO. 

of other mines. Some veins are cut .across by a similar postmineral breccia zone. In the 

Champion Dirt mine a soft breccia lode 21 to 4 feet thick enters the vein at an angle, turns and 

runs with it for several hundred feet, then cuts through it, faulting it so that it is offset 3 feet 

and disappearing in the foot wall. In many places where these zones occur parallel to the 

veins, the fact that their formation has been accompanied by displacement is also shown by 

strite. In the Stanley mine striations on the walls of the breccia zone were observed dipping 

from· 34° to 60° SW. The Little Mattie is faulted by strong but slightly mineralized cross 

veins, probably belonging to this period, and in places a zone of fricti~n breccia follows along 

the vein itself. On one of the eross veins the strire were observed to dip 10° from horizontal, 

indicating the direction of movement. · 

These breccia zones have usually been silicified, so that they are hard and firm. Here 

and there a small amount of metallic-sulphide deposition has taken place in them. They have 

evidently originated at a comparatively recent date, however, since the formation along a fault 

zone of a friction conglomerate instead of the more ordinary mass of pasty rock or gouge is a 

consequence of light pressure at the time of formation, such as could hardly exist at a considemble depth. Moreover, some of the detrital deposits which constitute a part of the breccia 

were evidently deposited in open fissures formed at the time of the fault movement. 

In the Empire district movements in the rock still more recent than those which gave rise 

to the friction conglomerates have resulted in the formation of irregular belts of broken rock, 

in which the fragments are large and rather angular. These belts were noted in at least two 

places in the Silver Mountain ore zone, extending down to ·a depth of nearly 300 feet, where 

they are quite unconsolidated and uncemented and serve as channels for underground waters. 

The recent age of these belts is evident. Further movement along them and further induration 

would result in a formation like that of the friction breccias. 

ALTERATION OF WALL ROCKS. 

GENERAL DISCUSSION. 

The principal products of alteration of the feldspar and biotite in the wall rocks are quartz, 

sericite, and various carbonates, with pyrite. In the Empire district the granitic and gneissic 

rocks have been altered on both sides of the vein channels for considerable though irregular 

distances. In many places a zone 30 to 60 feet wide on each side of the channel has been altered 

to quartz, sericite, and pyrite. This alteration was accomplished by the ore-depositing waters, 

since the pyrite is auriferous and much of it· forms a low-grade ore. In the Rosecrans mine, 

for example, this altered granite on both sides of the main fracture is said to average $4.50 a 

ton in gold. 

· A specimen of granitic gneiss from the wall rock of one of the veins in the Silver Mountain 

ore zone, at Empire, was found to contain in place of the sericite a pale-brown pleochroic mica 

of sericitic habit, which with quartz made up the bulk of the altered rock. The quartz, zircon, 

and apatite of these rocks usually remain unaltered. Where the alteration is not complete 

some of the feldspars have ·been selected, while others have not been touched. A specimen 

from the wall rock of the Gold Dirt, at Empire, shows some of the feldspars entirely altered to 

sericite, while others arc unaltered except for being slightly turbid from kaolinization. It is 

likely that this formation of kaolin is a separate and distinct process from that which has produced· the alteration above described. In some cases the kaolin is cloudy and renders the 

feldspar turbid; in others it is a clear translucent aggregate which may entirely replace the 

feldspar. Interesting examples ilh,1.strating the history of rock alteration were studied in the 

Stanley mine. 

In this mine the intrusion of a dike of bostonite porphyry has been followed by the formation of a mineral vein, subsequent to which a dike of latite porphyry was intruded along 

the same plane. Since this latite intrusion there has been only a trifling amount of mineralization. Besides the porphyries above mentioned, the wall rocks of the vein include gneiss 

and pegmatite. Fragments of the gneiss wall rock close to the vein show the feldspar entirely 
FEATURES OF THE AURIFEROUS PYRITIC DEPOSITS. 151 

altered to sericite. Specimens of the bostonite show the feldspar, which includes orthoclase 

and anorthoclase, partly altered to kaolin, in one case to kaolin and sericite. The original 

pyroxene phenocrysts are completely altered to calcite, magnetite, or hematite, with abundant 

purple fluorite, which occurs also in cracks and in the groundmass closely associated with 

calcite and evidently of secondary origin. The latite as a rule is unusually hard and fresh, 

with fresh biotite phenocrysts. The feldspar, which includes orthoclase and calcic feldspar, 

is locally clear and fresh, but may be more or less altered, the calcic feldspars to calcite or 

here and there to brown carbonates, the orthoclase to kaolin. In one place, near subsequent 

veinlets of pyritiferous quartz, the feldsuar is unusually altered and contains secondary quartz 

with some sericite and kaolin. 

Some of the specimens studied show within a single slide the result of the alteration processes of different periods. A specimen of latite porphyry shows included fragments of older 

ore, with the adjacent gneiss wall rock. The gneiss contains relatively fresh orthoclase, singly 

striated feldspar which is partly sericitized, and blocks of entirely sericitized feldspar whose 

original nature is undeterminable. This alteration is similar to that observed in the wall 

rocks of the argentiferous group of veins. The inclosed porphyry in the same thin sections 

contains fresh biotite and feldspar which is partly fresh and partly altered to calcite. The 

difference in the method of alteration in the older and younger rock is striking, and it is 

·shown that alteration of the gneiss wall rock of the veins took place before the intrusion of 

the porphyry. 

Another specimen of latite from the same mine contains intrusive veins of arkose belonging to the period of the friction conglomerate. The arkose consists of detrital material which 

has been injected into fissures in the porphyry opened by the postmineral movement described 

elsewhere. The porphyry contains fresh biotite and orthoclase which is almost entirely altered 

to kaolir. The arkose is made up of granitic grains, among which is fresh or only very slightly 

kaolinized orthoclase. There is a striking difference in the slide .between the freshness of the 

granitic orthoclase and the entirely altered condition of that in the porphyry. This shows 

that the alteration of the latite to calcite and kaolin took place before the postmineral movement that produced the friction conglom~rate, and before the injection of the pOFphyries. 

The alteration, however, was probably accomplished by surface waters, since it is, as shown 

above, strikingly distinct from the sericitic alteration which is shown in the gneissic wall rock 

and which was undoubtedly accomplished by the waters which deposited the vein. The 

. kaolinization described in the feldspar of the bostonite was probably also accomplished by 

the later vadose waters, while the rest of the alteration was attendant on mineralization. 

Adularia was observed as an alteration product in two mines, and it is probably of relatively common occurrence. At Miller's mine, on the Lamartine road, the wall rocks are 

granite, which has been considerably altered near the vein, and in which the feldspar, including 

orthoclase, has been kaolinized and somewhat sericitized. In this rock are nests and veinlets 

of quartz, adularia, pyrite, and some sericite. Adularia also forms rims around the altered 

feldspar, including the orthoclase, or forms nests in it. It is probable that this involved the 

elimination of some soda, which was present in the orthoclase, adularia being practically pure 

potash feldspar. This rock also contains an area of Clear microcline occupying an area inclosed 

by altered orthoclase crystals. Its clear and unaltered character suggests that it is secondary, 

like the adularia, but it is not closely associated with the adularia at any point, occurring 

only in this one locality, and the fact that irr the Georgetown district the microcline, which 

undoubtedly belongs to the consolidation period of the granite, has remained fresh and clear 

where the rest of the rock was entirely altered, makes it more probable that we have here a 

similar case. · 

At the P. T. mine, on the spur separating Spring Gulch and Clear Creek, the vein has 

for the foot wall a dike of bostonite porphyry, and lenses of porphyry also occur surrounded 

by vein material. A specimen from one of the lenses shows the feldspars partly altered to 

kaolin and sericite. This rock .contains secondary adularia, which is associated with pyrite 

and quartz and occurs as veinlets cutting the groundmass and as small patches in it. Pyrite 
152 GEOLOGY OF THE GEORGETOWN QUADRANGLE, COLORADO, 

also occurs in the rock, some of which may be original, while another portion is very likely · 

secondary. The values in this mine are mostly gold, but in the other mine where adularia 

was found they are mostly silver, with only $1 to $2 in gold to the ton. 

SUMMARY. 

In summing up the effect on the wall rocks of the mineralizing waters which produced 

the primary vein deposition, it may be stated that the principal minerals which they formed 

were quartz, sericite, various carbonates, pale-brown sericitic mica, adularia, pyrite, and 

fluorite. This alteration corresponds with that accomplished in the walls of the argentiferous 

group of veins, except for the adularia, the 'brown sericitic mica,· and the greater amount 

of fluorite in the auriferous veins. The alteration products of descending waters subsequent to the vein deposition are principally kaolin and calcite, with some sericite, and in the 

hornblende-bearing rocks a little epidote. 

DERIVATION OF CERTAIN VEIN MATERIALS. 

DERIVATION OF PYRITE. 

The altered wall rocks in the vicinity of the veins typically contain disseminated pyrite, 

which the microscope shows to have been derived probably from the iron in the original biotite 

and other ferruginous minerals. The iron has passed into the condition of sulphide under the 

influence of percolating sulphureted waters. In the Aorta tunnel, on both sides of the central 

fissure or vein channel of the Silver Mountain ore zone, the granitic rocks have been bleached 

and pyritized, in places for a distance of many feet, and the disseminated pyrite is auriferous. 

Here the mineralizing waters have evidently contributed not only the sulphur, but also gold and 

a little silver to the combination with the indigenous iron of the rock. This auriferous pyrite 

increases in amount locally along the central vein chaimels, but the transition to the disseminated auriferous pyrite in the wall rock shows that all has probably a similar origin. · 

In the Empire ·tunnel further evidence was found of the derivation from the wall rock of 

the iron in the pyrite and even of some at least of the sulphur. In this tunnel numerous veins 

of pegmatite and pegmatitic quartz are associated with the large intrusive granite masses, and 

these pegmatitic veins contain original magnetite and contemporaneous pyrite. The granite 

also contains disseminated pyrite, which appears to be original and to date at least as far back 

as the pegmatitic stages of the granite's consolidation. These granites and associated pegmatites show on fractured surfaces secondary pyrite, which has the aspect of having been derived 

from re-solution and reprecipitation of that originally contained within the rock. What 

appears to be a further step in the segregation is the presence of small veinlets of jaspery gray 

quartz and pyrite, which have altered and replaced the granite along a narrow zone on either 

side of the central fracture. The inspection of many of these fracture zones containing quartz 

and pyrite and running through the fresh granite makes it seem very clear that the vein filling 

is derived from the adjacent rock. This example, therefore, makes it appear probable that 

not only the iron in the pyrite, and to a certain extent the sulphur, but also the quartz of the 

veins, have been, in part at least, derived from the surrounding rocks. The veinlets described 

are similar in appearance to the typically auriferous vein of the Empire district, although on a 

small scale. 

Since, therefore, the pyrite, and to a certain extent the quartz, are of local origin, only 

the remaining metallic vein constituents, chiefly gold and copper, with a little silver, remain 

to be accounted for. Possi~ly a small proportion of these also may be derived from the neighboring rocks, since some of the pegmatitic quartz veins are reported to contain free gold. 

These veins are of pre-Cambrian age and represent the last stage of the consolidation of the 

intrusive granites. In the Grand Encampment district of Wyoming, near the northern border 

of Colorado and somewhat over 100 miles northwest of Empire, similar pre-Cambrian pegmatitic quartz veins contain a little gold, whose accumulation in placers has led to some prospecting and excitement. It is believed, however, that nearly aU of the gold, silver, and copper 

has a quite extraneous origin, as is described on page 153. 
FEATURES OF THE AURIFEROUS PYRITIC DEPOSITS. 153 

DERIVATION OF BARITE, CHERTY SILICA, AND CARBONATES. 

The prevailing occurrence of barite, cherty silica, and brown carbonates as crystals lining 

druses and representing the last frtage of mineral deposition indicates that these minerals have 

the same origin as has been ascribed to them in the case of the argentiferous veins, where they 

have been regarded as having been derived from the wall rock, probably by atmospheric waters 

subsequent to the primary ore deposition. In some places siderite may be derived from original 

pyrite, as is probably the case in one instance noted in the Harrisburg mine, where a postmineral fissure had been opened in pyritic altered rock and had become lined with siderite that 

was evidently due to lateral secretion. Siderite may also occur as one of the primary vein 

minerals, in which case, as also where it occurs among the last-formed minerals, it is usually 

mingled more or less with other carbonates, especially that of manganese. Where it thus has 

been deposited as a primary mineral it is contemporaneous with the primary pyrite, and its 

iron, like that of the pyrite, has probably been derived from the rocks traversed by the mineralizing solutions. This conclusion applies also to the manganese in the brown carbonates. The 

sulphur in these primary sulphide bodies, however, is characteristically too abundant to be 

readily ascribed to concentration from the rocks, and its introduction evidently accompanied· 

that of the rare metals, such as gold, sil;ver, copper, lead, and zinc. The abundant carbon 

dioxide which combined with iron and manganese to form the primary carbonates that were 

deposited at the same time as the sulphides is therefore probably also derived from an outside 

source. DERIVATION OF RICHER CUPRIFEROUS MINERALS. 

The alteration of chalcopyrite to tetrahedrite along cracks has been repeatedly noted in 

the different mines. This alteration is most common near the surface, where the descending 

waters are active, and the derivation of the tetrahedrite from the chalcopyrite is very evident. 

Since this is the case and there are no other known minerals from which the constituents of 

these secondary minerals can have been derived, the original m'ore or less cupriferous pyritic 

ore and chalcopyrite must contain a small amount of arsenic and antimony. In some places 

at least the chalcopyrite itself is not primary, but has been derived by the concentration of 

copper from a lean primary cupriferous pyrite, the alteration being likewise the result of surface waters and being characteristic of the superficial zone. The subject of the derivation of 

both chalcopyrite and tetrahedrite from cupriferous pyrite has been considered in the description of the Centennial mine (p. 277) and that of the Silver Mountain ore zone (p. 402). 

DERIVATION OF FLUORITE. 

Fluorite has frequently been observed in dikes of bostonite porphyry, and along many 

such dike~ veins have formed. Although the deposition of this min~ral in the dikes appears. 

later than the consolidation of the rock and it seems certain that it has produced rock alteration, including the destruction of the original pyroxene and the probable sericitization of some 

of the feldspars, it yet appears to have had a genetic relation to the intrusion, since fluorite is 

characteristic of this especial type of dike rock and occurs in dikes along which no vein has 

formed .. It .seems, therefore, to be evidently a product of the after action during the pneumatolytic period, when gases of magmatic origin rose along the channel of recently consolidated rock. This mineral occurs in comparatively few places as a gangue in the ore veins. 

Nevertheless, it is rather more common in the auriferous veins than in the silver-bearing veins 

of the Georgetown district, though it is present in relative abundance in the typically argentiferous veins at Argentine. Near Georgetown, however, the narrow belt of auriferous veins, 

which passes with a northeast-southwest trend through the town, with silver-bearing vein districts on both sides, shows fluorite in a number of places in quantity markedly greater than in 

the neighboring veins of the argentiferous period. Fluorite has been observed along this belt 

in the Big Chief mine, which is a branch of the Centennial and lies on the slope of Leavenworth 

Mountain overlooking Georgetown. On the north side of Clear Creek at Georgetown fluorite 

has been encountered in the Prudential tunnel. Here it is not associated with metallic minerals, 

but has formed along the fracture zone in pegmatite. 
154 GEOLOGY OF THE GEORGETOWN QUADRANGLE, COLORADO. 

West of the Big Chief· mine, pear the west edge of the area mapped, on the north side of 

Leavenworth Mountain, considerable fluorite was found in a weak but definite lode in granite 

at an elevation of about 9, 750 feet. This vein is reported as containing gold values but no 

silver. It carries a very little pyrite, and the fluorite is associated with j aspery quartz. Veinlets 

which are seen under the microscope contain quartz, pyrite, siderite, and fluorite. 'fhe silica 

and iron appear very likely of local origin. The fluorite has been characteristically the last of 

these minerals to be deposited. Between this vein and the Big Chief vein, also on Leavenworth 

Mountain and within the gold belt, is the Welsh mine, whose chief metallic mineral is pyrite. 

The values are reported to be gold, with very little silver. Some of the quartz from this mine 

is purplish and probably contains some fluorite. 

To judge from the relations of tho fluorite to the quartz, siderite, and pyrite, whose silica 

and iron may be derived from the granitic rocks through which the vein-forming solutions have · 

circulated, the fluorite might well be believed to be of similar origin, since, a·s has been shown, 

these granites contain fluorine both in small quantities in the silicates and also in tho form of 

fluorite. To this source a part, at least, of the fluorite found here and there in the argentiferous 

veins has been ascribed. The fact, however, that this belt of auriferous lodes contains fluorite 

to a markedly greater extent than the argentiferou~ lodes on each side argues for the exotic 

nature of at least a large part of the fluorine in this belt, and it prohably was the result of the 

pneumatolytic period of intrusion of the feldspathic dikes. On the other hand; it is highly 

probable that the lime in the fluorite may have been derived from the wall rocks; and it is 

possible that the fluorite which forms the latest mineral in veins, as above described, may have 

been leached from its first position in or near the feldspathic dikes and reprecipitated. 

PRESENT WORK OF DESCENDING W ATRRS. 

In the long tunnels which run into Griffith Mountain on the Griffith vein considerable water 

was observed to a point about 650 feet vertically below the surface. From this point to the 

end of the tunnel, which is at an actual depth of about 1,050 feet, the rocks appear quite dry. 

The active action of atmospheric waters therefore appears in this mine to be restricted to a zone 

of about 650 feet below the surface; and with considerable variation, depending on the size and 

abundance of fractures, this general condition probably holds throughout the district. It has 

been observed also in several of the other long tunnels. Where these waters circulate through 

the copper-bearing ores the copper appears to go into solution with comparative ease. In the 

Centennial mine, at Georgetown, and along the similar veins of the Silver Mountain ore zone, at 

Empire, the superficial water encountered in the drifts contains sulphate of copper. Deposits 

of amorphous copper minerals were noted at several places along the Silver Mountain ore zone 

as having been formed since the opening of the drift. In both of these mines scales of metallic 

·copper are sometimes deposited on iron objects, such as mine rails. In the Centennial the rust 

scale on tho inside of a water pipe is reported to have yielded 11 per cent of copper on assay. 

Hydrous cupric sulphate, or chalcanthite, is reported in the old upper workings of this mine. 

Iron also passes into solution in these superficial waters. A zone of unconsolidated, 

c'oarsely brecciated diorite, due to a comparatively recent movement in the rocks, has been 

described as occurring in the Aorta tunnel of the Silver Mountain ore zone, about 700:feet from. 

the mouth (p, 398). The underground waters which circulated along this zone deposited 

considerable iron oxide on encountering the air of the tunnel, and pools of water which stand in 

the drift at this point ai·e covered with a film of blood-red iron oxide. The waters in general 

also take up carbonate of lime, which is locally precipitated and has been noted in the Freeland 

- mine, where sulphates of copper and iron and native copper have also been precipitated from 

the mine waters. · . 

SUMMAHY OF FORMATION OF AURIFEROUS VEINS. 

INTRUSION AND AGE 0~' FELDSPATHIC DIKES. 

Within the region which adjoin& Idaho Springs on the south and west and also at Empire 

there occurred at a certain general period intrusions of numerous dikes representing different 

phases of a magma high in feldspathic ingredients but poor in the silicates of lime, magnesia, 
FEATURES OF TilE AURIFEROUS PYlUTIC DEPOSITS. 155 

3:nd iron. The dikes have a predominant 11ortheast strike, the less important set trending m a 

northwesterly direction. Their directions were probably controlled by preexisting faults, which 

the dikes followed. This conclusion is indicated by the study of the Silver Mountain ore zone, 

a broad northeastward-trending belt of crushing, several hundred feet wide. The zone of fracturing and crushing contains a porphyry dike which follows the general trend of the zone but is 

only a few feet thick. Its coincidence in trend with that of the fracture zone suggests that it 

found within the zone inviting lines of weakness already existing at the time of its intrusion. 

Most of these dikes are composed of bostonite and alaskitic quartz monzonite; a much 

smaller number consist of biotite latite with a core of l;tlkali syenite. These are closely related 

rock types representing evidently a single general magma, whiGh was distinct in its nature from 

the essentially monzonitic magma of the Georgetown district. From the resemblance of the 

Idaho Springs magmatic type to those at Cripple Creek and at the Rosita Hills, and considering 

. the unusual character of such types, it is correlated with the magmas represented at these two 

volcanic centers, and the Idaho Springs dike intrusion is regarded as probably roughly contemporaneous with the volcanic and dike eruptions at these other localities, which are believed to 

have occurred in late Tertiary time. 

MINERALIZATION CONSEQUENT ON THE INTRUSION. 

General conditions.-Many of the numerous bostonite dikes contain fluorite, whose deposition followed closely the intrusion of the dikes and apparently brought about at least part of ~ 

the alteration which we now find in the rock, including the destruction of the original pyroxene 

phenocrysts. Therefore it has been concluded that the fluorine thus represented had a genetic 

connection with the intrusion, and probably represented the gaseous emanations given out 

during the last stages of consolidation. After the intrusion of the bostonite dikes and those of 

alaskitic quartz monzonite, mineral veins were deposited within the same district by circulating 

waters. In point of time the mineralizing waters followed the intrusions closely. In many 

places the vein channel or fissure along which the mineralizing waters ascended, and which was 

due to a renewal of faulting and fracturing subsequent to the intrusion, followed along the 

dikes, which, occupying preexisting lines of weakness, still offered localities of least resistance. 

These mineralizing agents appear to have belonged to very much the same period as the fluorine 

emanations from which the fluorite in the bostonite dikes was formed (probably by combination with the lime liberated from the decomposed calcareous silicates), since the fact that their · 

activity closely followed the dike intrusion has been demonstrated in the Stanley mine (p. 344). 

Nature of mineralizing solutions.-The fact just mentioned shows also that the mineralizing 

waters were active for a restricted period, during which, however, the effects produced were 

very great. Therefore these agents can have been nothing else than hot waters, which were of 

course ascending, ,and the minerals deposited by them within the veins and wall· rocks testify to 

the presence of gases such as sulphureted hydrogen, carbon dioxide, and fluorine, and metals 

such as gold, silver, copper, lea'd, and zinc, with small amounts of rarer metals, including probably bismuth and tellurium. . They also contained potassium in unusually large proportion, a 

as is indicated by the formation of adularia, which has been observed in the wall rocks; and 

sodium was also probably present. 

Effects of mineralizing solutions.-The solutions altered the wall rocks to quartz, sericite, 

pyrite, carbonates (especially those of iron and lime), and adularia, one of the net results being 

'the precipitation of potash and the abstraction of soda and probably of lime, a process similar 

to that described by the writer at Tonopah and by Lindgren and Ransome at Cripple Creek. 

The biotites and other ferruginous minerals of the wall rocks were decomposed and the contained iron was reprecipitatcd in the form of contemporaneous sulphide and carbonate, partly 

in or near its original position in the rock, so that the pyrite and siderite remained disseminated, 

and partly in concentrated form along circulation channels, in which case these minerals assumed 

the form of a vein. Quartz was likewise precipitated al~ng the circulation channels and a portion of it was probably derived from the wall rocks, though another portion, whose relation to 

a See Prof. Paper U.S. Geol. Survey No. 42, 1905, p. 213. 
156 GEOLOGY OF THE GEOHGETOWN QUADRANGLE, COLORADO. 

the locally derived silica is uncertain, must have been contained in the hot ascendincr solutions. b . 

Together with the pyrite, gold, silver, and copper were precipitated, with a variable amount of 

lead, zinc, and other metals. These valuable metals were dep,osited mainly in the pyrite along 

the vein channels, while that disseminated in the rock contained little or none of them. In 

some places, however, where the wall rock was unusually porous, the deposition of .the disseminated pyrite, whose iron was derived from the ferruginous silicates, was accompanied also by 

that of gold, solutions bearing carbon dioxide, sulphureted hydrogen, and gold having penetrated 

the rock and having deposited the gold throughout, a more or less compensating amount of 

original rock materials being extracted. 

Although siderite is very common in the altered wall rocks, and is in many places equal or 

superior to the pyrite in amount, the pyrite is generally greatly predominant along the main 

veins, which represent the principal circulation channels. Therefore sulphureted hydrogen 

seems to have been predominant over carbon dioxide at the time of the principal mineralization. 

In some veins, however, the amount of carbonate increases in proportion to pyrite, and in the 

Griffith mine the siderite (with rhodochrosite) equals or exceeds the contemporaneous or intercrystallized pyrite. This seems to indicate a differing condition, when the carbon dioxide 

equaled or predominated over the sulphureted hydrogen. It probably represents a later stage 

of the volcanic hot springs or solfataric history, when the waters were cooler, the surface was 

nearer, and the source of gases more remote. 

Sequence offumarolic activity around volcanoes.-The gases given off by cooling lavas around 

active volcanoes have been studied by geologists and have been found to differ according to 

the degree of cooling attained by the lava. In the first stage, when the lava is very hot, being 

about 500° C., the gases are 'dry, and consist of chlorides of sodium and potassium with small 

amounts of metallic chlorides and also fluorine, boron, and phosphorus. At a later stage, when 

the temperature of the lava is from 300° to 400° C., the vapors given off consist chiefly of water 

containing sulphurous and hydrochloric acids. At about 100° C. these water vapors contain 

ammonia and a little sulphureted hydrogen. Below 100° the water vapors contain some sulphureted hydrogen and carbon dioxide; while after the cessation of the expulsion of water 

vapor, carbon dioxide is still given off. 

Comparison of volcanic fumarolic sequence with sequence of mineralizing solutions.-The 

mineralizing agents which have deposited the veins in the district under discussion, since their 

action followed so closely the intrusion of the dikes, and attended or immediately succeeded 

the alteration of these dikes by magmatic fluorine, bear the same relation to the igneous intrusion that fumaroles do to surface lavas. 

The first period of gaseous emanation, which closely followed the intrusion and consolidation of the dikes and very likr,ly preceded the ::>re deposition, is represented by the fluorine, 

which ascended along the dikes, partially altered the igneous rock, and combined with the 

' calcite thus liberated to form fluorite. The fluorite found here and there in the veins is perhaps 

due t,o a solution and reprecipitation of this early fluorite. The main period of mineralization, . 

accomplished by ascending waters having an excess of sulphureted hydrogen over carbon 

dioxide, and containing metals, answers to the later stages of fumarolic activity near volcanoes, 

when great quantities of water vapor containing considerable amounts of sulphurous gases are 

expelled. Finally those veins which contain more carbonates and less pyrite appear to represent a still later period, when carbon dioxide as well as sulphureted hydrogen is given off in 

water and gradually becomes predom~nant in amount. It is natural, if these conclusions are 

true, that the relative amounts of gold and silver should be comparatively less in these later 

and comparatively less efficient stages of mineralization. The Griffith vein corresponds to this 

conception. The carbonate-pyrite material in this vein contains less silver and gold than 

is usual in the auriferous veins, so that the ore is of low grade and is only in places fit for 

exploitation. 

Magmatic origin of mineralizing waters.-The relations above outlined indicate that the 

waters which have accomplished the. mineralization, as well as the gases contained therein, 

correspond with those which are expelled from surface lavas and were of magmatic origin. 
MAGMATIC ORIGIN OF METALS IN BOTH GROUPS OF VEINS. 157 

If such heated waters, speeding upward toward the surface, should have encountered other 

water or gases permeating the rocks or flowing along circulation channels, these materials 

would become mixed with the volcanic supply; but the usual comparatively dry condition 

obgerved in the rocks of this region below a depth of 500 or 600 feet (see p. 138), which is 

similar to that observed in many other mining regions, makes it apparent that this factor was 

of small account. 

INTRUSIONS SUBSEQUENT TO MINERALIZATION. 

The less numerous dikes of the Idaho Springs magma include biotite latite and alkali 

syenite. In one mine (the Stanley) it has been shown that a latite dike was distinctly subsequent to the vein deposition which followed the intrusion of a bostonite dike. The latite 

dikes occur almost exclusively along a single narrow zone, in which the occurrence in the Stanley 

mine may also probably be included. The rock is typically fairly fresh, and it is likely that 

all of these dikes may be younger than the main mineralization. Similarly the alkali syenite 

which occurs in small quantities at Idaho Springs is not associated with veins. Along the 

contacts of this intrusive rock carbonated springs, some of them very hot, ascend. They have 

probably a genetic connection with the syenite intrusion, and this and the fresh condition of 

the rock suggest that it is not only subsequent to the main period of mineralization but is the 

latest of the intrusions. 

MAGMATIC ORIGIN OF METALS IN BOTH GROUPS OF VEINS. 

In the foregoing discussion of the process of ore deposition the origin of the mineralizing 

waters and gases which have deposited the auriferous group of veins has been referred to magmatic exhalation, while most of the earthy gangue, as well as the more common metals, including 

iron and manganese, has been referred to the wall rock. There remain the rare and more 

valuable metals-gold, silver, copper, lead, zinc, arsenic, antimony, etc. These metals are the 

distinctive features of the veins and they occur in these auriferous veins in quite different 

pro,portions from those shown in the argentiferous veins of the Georgetown district. If the 

evidence afforded by either of these vein groups was taken separately, the hypothesis that these 

rarer metals have been derived by the hot ascending magmatic waters from the rocks through 

which these waters ascended is as fully justified as that which supposes the metals to have 

been given off from the magma along with the mineralizing agents, since the crystiiJline igneous 

rocks through which the waters passed contain some, if not all, of these metals in small quantities. But additional evidence is supplied by the comparison of the two districts. 

In the Georgetown district the intrusion of igneous rocks, representing variations of a 

monzonitic magma, was followed by the formation of argentiferous blende-galena ores containing 

very little gold and copper; the wall rocks were altered mainly to quartz, sericite, siderite, 

and pyrite, \vhile chrome-bearing mica, or fuchsite, occurs locally in great abundance. 

In the Idaho Springs district, at a later period, there were intrusions of igneous rocks 

representing an alkaline magma distinct from the monzonitic magma, and these. intrusions 

were followed closely by waters and gases (probably of magmatic origin) which deposited ores 

containing large amounts of gold, copper, and silver, with valuable quantities of lead and zinc. 

There is also some evidence, though not complete, of the presence of more tellurium and fluorite 

in these veins than in the Georgetown veins, while the wall rocks contain adularia as well as 

the more common alteration products (quartz, sericite, siderite, and pyrite) of the Georgetown 

period. These different metallic combinations in the two districts, representing in part the 

different composition of aqueous solutions which followed the intrusion of different magmas, 

can be explained only by considering the metals to have been given oil' from the respective 

magmas, since the rocks which have been traversed by the ascending solutions are similar. 

This line of reasoning is the same as that which was available at Tonopah,a where different 

lavas were successively erupted and were followed by veins of different mineral composition. 

a Prof. Paper U. S. Geol. Survey No. 42, 1905, pp. 253-262. 
158 GEOLOGY OF THE GEORGETOWN QUADRANGLE, COLORADO. 

ZONES OF ORE DEPOSITION. 

ZONE OF PRESSURE RELIEF. 

The ascending mineralizing waters must at some point in their progress have encountered 

a zone where relief of pressure and heat permitted a reaction between the solutions and -the 

wall rock, including the union of the magmatic sulphur with the indigenous iron, which resulted 

in the deposition of pyrite and the attendant metals, including gold and silver. It is probable 

that this zone of deposition had definite upper and lower limits, so that the ore shoot formed 

had a top and bottom, above and below which the fissured or fractured zone that formed the 

channel for the ascending solutions still continued. ·If the ore bodies thus f~rmed were 

deposited along a strong and persistent fissure or fracture zone, they may have more horizontal than vertical extent (as, for example, in the Lamartine mine); or where the horizontal 

extent of the circulation channel was slight the depth of the resultant mineralization may be 

relatively greater than the length. 

RELATIVE ELEVATION OF GEORGETOWN AND IDAHO SPRINGS ORE-DEPOSITION ZONES. 

From the conclusion that the auriferous veins of the Idaho Springs district are younger 

than the argentiferous veins of the Georgetown district, it follows that the former were deposited 

at a period when the topographic surface had been brought by erosion to a considembly lower 

position than it occupied at the time that the silver veins were deposited. Therefore the 

present surface is considerably nearer to the old surface which existed at the time of the auriferous deposition than to that which existed when the Georgetown veins were formed. If in 

each district the vertical zone of ore deposition occupied the same relative position with respect 

to the existing surface, then the argentiferous veins of Georgetown were deposited at a higher 

horizon than the Idaho Springs veins, and at the present day erosion has removed more of the 

former than of the latter. The tops of the Georgetown metalliferous veins or ore bodies were 

therefore exposed earlier than those of the Idaho Springs veins, by a time approximately equal 

to the difference in time between the two mineralizing periods, and by so much longer a time, 

to speak roughly, the descending surface waters have been reworking the superficial portions 

of the Georgetown veins. 

COMPARISON OF EFFECTS OF DESCENDING WATERS IN THE TWO DISTRICTS. 

The observed facts seem to bear out the inferences just stated. The effects of descending 

waters in reworking the primary minerals in the Idaho Springs veins seem to have been far less 

than in "the veiris of the Georgetown area, except perhaps in the veins containing cupriferous 

pyrite, where the easy solubility of the copper has perniitted unusually rapid solution and 

precipitation. Therefore, while in the Georgetown district the secondary effects produced 

by solution and reprecipitation are of the utmost importance economically, in the Idaho Springs 

district they are certainly in places, of very slight importance. 

NONOUTCROPPING ZONES OF ORE DEPOSITION. 

Another fact indicating the probaBle relatively low position of the Idaho Springs veins as 

compared with the Georgetown veins is the occurrence of ore in the Lamartine mine, where 

the surface is barren and the top of the ore shoot is from 100 to 400 feet beneath it. In this 

case the mineralized portion of the vein had not yet been reached by erosion. No similar 

occurrence has been noted in the Georgetown region, where, on the other hand, evidence that 

the roots of veins are exposed 9r even that the original roots have already been removed is 

abundant. 

Conditions like thooe described at theLamartine are present at the fam01ls mining camp of 

Guanajuato, Mexico, where they have been studied by the writers. In this district the veta 

madre, or mother lode, occurs along a great fault, which can be traced for several miles. The 

bonanzas which occur from place to place along this vein seem to be cut by the present surface 
LOCALIZATION OF ORES. 159 

near their original tops, so· that much or all of the original depth of ore has been left. The 

largest mine in this district, the wonderfully productive Valenciana, had a broad outcrop of 

barren or very low-grade quartz which, though near active mines, remained untouched for· two 

hundred years. When exploration was at last begun, the top of the ore body was encountered 

at a depth of about 300 feet and it reached from this point to a depth of about 1,300. feet. In 

this district also there is the even more striking Carmen-Pinguico vein, which is near the veta 

madre. This vein appears at the surface mainly as a crushed zone in rhyolite, or in many 

places only as the smooth single wall of a fault slip, with no mineralization whatever except that 

in one or two spots a little comb quartz outcrops. In depth this vein becomes mineralized, 

and a few hundred feet below the surface it becomes a well-marked and strong quartz lode, up to 

8 feet in thickness, which has produced several milli'ons in silver and gold. 

In the Cripple Creek district also some shafts have been sunk for a few hundred feet without 

any indications of ore and have ultimately developed productive mines. a· 

LOCALIZATION OF ORES. 

INITIAL CONDITIONS OF VEIN FORMATION. 

The initial conditions for the formation of an important mineral vein, aside from the 

existence of a source whence metals may be derived and the presence of a vehicle, such as water, 

. depend on the existence of a suitable circulation channel. Such a channel is most commonly 

supplied by a fault or fracture zone. The strength and width of the resulting vein depend 

largely on the relative openness .of the zone at different places, the vein being large where the 

channel was open and small and "tight" in the narrow places. · 

ORE DEPOSITION BY MINGLING OF SOLUTIONS. 

In the Georgetown quadrangle a great proportion of the important ore bodies are plainly 

due to the intersection of different branch veins. This is especially well shown in the Silver 

Plume region, where there are a number of good illustrations. In such a lode mineralization may begin at the junction of the two branches anQ. extend from the junction along the 

united vein, or along the united vein and back along one of the branches, or along both of the 

branches. In these cases the precipitation of ore is plainly the result of mingling solutions. 

According to this explanation, waters of slightly different composition circulated in these 

different branches, a conclusion which is corroborated by the fact that each branch now usually 

shows a more or less distinct character in its metallic filling, a fact which is well known to the 

miners in this district. For these local differences in the composition of the mineralizing 

solutions, if, as is probable, the solutions were all originally derived from the same source, we 

must look to the results of reactions with the different wall rocks ep.countered. Wherever such 

slightly different solutions have met at the junction of two branches, precipitation has taken 

place until the equilibrium of the mixture was restored. 

ORE DEPOSITION IN FIELDS OF INTERSECTING FRACTURES. 

There are, however, many ore shoots along the veins where no intersecting branches have 

been noted, and these shoots are also of great importance. It has been shown that the great 

ore bodies or bonanzas of the veins at Tonopah, b as well as of the similar veins in the Comstock, 

Pachuca, and other districts, have been caused by the intersection with the mainfracture zone, 

along which the vein has formed, of cross fractures that have not been mineralized, but whose 

intersection with the main-vein zone has produced channels of maximum circulation, which 

approach a definite shoot-like form only where the various cross fractures become dominated by 

some stronger set. Inasmuch as in the Georgetown quadrangle ore bodies have been observed 

which have been formed not only at th~ intersection of two stronger branches, but also at. the 

intersection with the main vein of some strong barren fracture or fractures, it is probable that 

the explanation given above applies to many of the more irregular shoo.ts where no such intersection has actually been observed. · 

a Prof. Paper U. S. Geol. Survey No. 54, 1006, p. 255. b Pro!. Paper U.s. Geol. Survey No. 42, 1905, p. 276. 
160 GEOLOGY OF THE GEORGETOWN QUADRANGLE, COLORADO. 

ORE DEPOSITION BY REPLACEMENT OF ROCK. 

While the ore which has been deposited in open fissures has probably been precipitated largely 

by the mingling of solutions involving the reaction of one solution on another, and partly by 

the lowering of temperature and pressure at a critical depth, the important and abundant type 

of ore which has been formed apparently by replacement probably owes its precipitation partly, 

at least, to the reaction of the mineralizing solutions on the solid wall rock. Such solutions have 

brought about replacement of the granites, gneisses, and porphyries, in the same way that the 

solutions containing magnesia and silica react on limestone to form dolomite and quartz, as at 

Aspen. 

IMPOHTANCE OF RELATIVE AREA OF REACTION FACES. 

A microscopic study of limestone in the process of replacement by dolomite, silica, and 

sulphides in the Aspen district shows that the rock was at first profoundly strained and crushed 

in the vicinity of faults, so that many tiny passages were opened to solutions, which finally 

worked through and through the strained material, replacing it. The amount of straining, 

which regulated the area of surface offered to the solutions, usually determined whether there 

would be little or much replacement. 

Recent experiments of E. C. Sullivan indicate the importance of keeping in mind the 

consideration of how much area has been exposed to solutions in a given case. Doctor 

FIG. 28.-Sketch of actual plane of a portion of the Seven-Thirty vein, showing (1) relative concentration of ore (indicated 

by dots) along different portions of the vein and (2) the greater proportion of ore in the more rigid granite wall 

rocks than in the more yielding porphyry walls. The gneiss (of varying degrees of resistance) seems to be intermediate between the granite and the porphyry in point of fitness for fracturing and consequent ore deposition. 

Sullivan treated various mineral materials, including kaolin, orthoclase, microcline, pyrite, and 

biotite, which had been ground to a fine powder, with a solution of cupric sulphate.a When 

the solution was kept in contact with the powder a few days, it was found that a reaction took 

place, involving chiefly a change of bases between the solution and the solids, the copper undergoing precipitation and an equivalent quantity of other bases (chiefly alkali and alkaline earth 

bases) entering the solution. The amount of precipitation was found to be dependent on 

grinding. The finer the powder the more copper was precipitated. Here, then, we have a 

good illustration of the influence of a large area of reaction faces in a reaction between a solid 

and a liquid. 

LOCALIZATION OF ORE BODIES IN CRUSHED PORTIONS OF FAULT ZONES. 

It was suggested to the writers from the above-described experiment that where the circulation zone along a vein is wide and is filled largely with finely ground material which is not so 

pasty as to prohibit free circulation through it, the replacement of earthy materials by metallic 

minerals as well as their accompanying gangue would go on very much more rapidly than elsewhere along the vein, where the rock is hard and is only sliced by fracture planes, and that an 

ore shoot would form under the former conditions, whereas during the same period the mineralization accomplished under the latter conditions would be very slight. Along vmhs such as 

those of the Georgetown quadrangle, which are all due to slight fault movement, such physical 

differences in different parts of the channel must exist, and being usually of fairly irregular 

form might give rise to irregular shoots (fig. 28). 

a Econc<nic Geology, vol. 1, 1905, pp. 68, 69. 
NATURE OF FAULT MOVEMENTS. 161 

The finely ground material which is found along the zone of a fault is commonly called 

gouge, and is abundantly encountered along the faults of the Georgetown quadrangle. With a 

view of testing the idea outlined above, a specimen of soft-clay gouge from the Bismarck vein 

was obtained and Doctor Sullivan kindly made some further experiments. This gouge contained 

3.52 per cent K 20, 0.18 per cent Na20, and 3.92 per cent C02, largely as FeC03• The gouge 

was already in a finely powdered state but was further ground in a mortar and allowed to 

· stand for three days, with occasional shaking, in contact with twice its weight of a cupricsulphate solution containing 2.978 grams of copper and 50 cm3 of water. At the end of this 

period the solution contained only 2.614 grams of copper, so that 0.364 grani had been 

precipitated, the gouge having taken up about 1.4 per cent of its own _weight in copper. 

The materials dissolved from the gouge and replaced by the precipitated copper are as follows: 

Material~ dissolved by cupric-sulphate soltttionfrorn soft-clay gouge. Gram. 

Si02 •••••• . . • . . . . . . ........................•...••........•••.•...•..•..••..••..• 0. 0075 

Al20 3 , etc ...... ·......... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0780 

CaO.............................. ... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0185 

MgO .................................. ;................................................ .0275 

K 20 .................. : . ............................................. ·................... .1320 

Na20 .......... ,....................................................................... . 0075 

. 2710 

More than 1 per cent of the constituents of the gouge had gone into solution. 

Another sample of the same gouge was kept four days in contact with twice its weight of 

a silver-sulphate solution containing about 0.1 gram of silver and 50 cm3 of water. At the 

end of this time the silver was practically entirely removed from solution, and the solution 

contained the following materials, chiefly in the form of sulphates: 

Materials dissol1>ed by .~ilver-sulphate solt~tion frorn soft-clay gouge. 

Si02 ••••••.•.••••••••.••...•.•••••••...•..••••..•.••.•••••....•.••••••••••••••••••••••• 

Al 0:~----- .............................................................. -... , ........ --. 

CaO .................................................................................. . 

MgO .................................................................................. . 

K20 ................................................................................... . 

Na20 ............................ . : ................................................... . 

Gram. 

0.0024 

.0004 

. 0026 

.0052 

.0444 

.0032 

This total was derived from 25 grams of gouge. . 0582 

Doctor Sullivan remarks that the ferrous carbonate contained in the gouge appears to take 

little or no part in precipitating the metals under the conditions of these experiments, which 

were made under ordinary temperature and pressure and were of brief duration. From the 

anlayses given it appears that the alkalies are about equally active in preci_pitation. In the 

gouge the proportion of N a20 to K 20 is about 1 to 22. In -the material removed from the 

gouge by the copper solution the proportion is 1 to 17 ~' and in that removed by the silver 

solution 1 to 14. If there is any difference, the soda has been more active than the potash. 

The actual replacement of alumina, which passes into solution, is highly interesting.a 

NATURE OF FAULT MOVEMENTS. 

The veins of the Georgetown quadrangle have been formed along faults of usu?-llY slight 

displacement. On the walls of veins movement strire have been observed and recorded at 

many different localities, and the detailed notes are given in the separate mine descriptions. 

These striations vary in direction locally, but taken altogether are consistent and beyond doubt 

indicate a general direction of fault movement. Most of the veins of this region have a steep 

dip, but the direction of movement along these steeply dipping veins varies locally from 

horizontal to vertical. 

a In these experiments the copper seems to be precipitated chiefly either as a basic cupric sulphate having the composition of brochantite 

or as cupric hydroxide, depending on whether the copper solution or the precipitating silicate is in excess; some cupric silicate undoubtedly 

forms also. The composition of the silver precipitate has not been investigated, but it is perhaps safe to assume that it is-principally silver 

oxide.-E. C. S. 

31401-No. 63-08-ll 
162 GEOLOGY OF .THE GEORGETOWN QUADRANGLE, COLORADO. 

DISPLACEMENT ALONG VEIN CHANNELS OF THE ARGENTIFEROUS GROUP. 

Many striations were observed on the walls of the veins of the argeiltiferous type, which 

are especially abundant near Georgetown. In the Silver Plume district, including the Lebanon 

group of veins, the majority of the striro are horizontal or dip at angles ranging up to 5°, the 

1 

3 4 

FIG. 29.-Stereograromatic figures showing nature of displacement along faults which have later l;ecome vein channels in 

the Georgetown quadrangle. 1, Unfaulted block of rock (upper side horizontal), with fracture running through it; 

2, horizontal displacement along vertical northeast fault, illustrating general type of movement in silver veins belong· 

ing to the Georgetown group; 3, movement at an angle of about 20° from the horizontal, illustrating displacement 

of the Colorado Central veins; 4, movement at an angle of 60° or so from the horizontttl, illustrating typical displacement along auriferous veins of the Idaho Springs type. 

angle in a smaller number of cases increasing to 30° or more and the dip in different places being 

in opposite directions, both to the northeast and southwest on the northe~_tstward striking 

veins and to the northwest and southeast on the northwestward striking veins. On the whole 

the average movement in the Silver Plume group of mineralized faults has been approximately 

horizontal along the fault planes. 
IDAHO SPRINGS WATERS. 163 

The striations observed along the Colorado Central vein, which is nearly vertical, show 

that one of the walls has moved southwestward past the other at a relatively small angle from 

the horizontal (Pl. XX, B). This angle has considerable local variation hut averages about Z0°. 

The maximum displacement is estimated to have been 100 feet. 

In the Kelly tunnel group of veins, including the Boston and Beecher, the movement is 

shown to have been to the northeast along the northeastward striking vertical fault plane, at 

angles varying between 10° and Z0° from the horizontal. 

The Magnet, which is also a northeast vein, shows strim indicating movements to the 

southwest at angles of 30° and 63° from the horizontal. 

DISPLACEMENT ALONG VEIN CHANNELS OF THE" AURIFEROUS GROUP. 

Similar· observations were recorded concerning the auriferous group of veins. The 

Centennial vein at Georgetown, which has a northeast strike, shows strim dipping to the southwest at an angle of 32° from the horizontal. The Anglo-Saxon mine, which also has a northeast 

strike, records movement to the northeast at an angle of 65° from the horizontal. The auriferous 

veins at Empire show strire dipping steeply at angles var}i-ing from 45° to goo and inclined both 

to the northeast and to the southwest on the predominant northeastward striking lodes, the 

average motion recorded being to the southwest at an angle of about 65° from the horizontal. 

The Lamartine and Guni Tree veins, which strike nqrtheast, show movement to the southwest 

at angles varying from 45° to goo. The Stanley, also a northeast vein, shows movement to.the 

southwest at angles of 34° and 60° from the horizontal. 

The Little Mattie, a northeast vein, shows striations horiwntal and dipping 5° in a northeasterly direction and 15° in a southwesterly direction, the average displacement indicated 

being in a direction nearly horizontal. 

SUMMARY AND APPLICATION. 

From the above statement it appears that in general the faults along which the veins of 

the argentiferous group subsequently formed, while steeply clipping or vertical, wcr~ marked by 

a nearly horizontal movement of ohe wall past the other; but in the veins of the gold group, as 

a rule, while the strike and dip in many cases are similar to those of the silver veins, one wall 

moved past the other with a strong downward plunge (fig. zg). To each of thes.e generalizations 

there is a single observed exception. The Magnet, which is one of the argentiferous veins, 

shows movement in a direction ranging from 30° to 60° from the horizontal, more like the usual 

movement of the gold veins; and the Little 1\fattie, which contains considerable gold and 

belongs in the group of auriferous veins, registers a horizontal movement like that typical of the 

silver veins. 

These observations lend further support to the conclusions already reached that the 

auriferous veins belong to a different period from the silver-bearing veins. It is likely that 

the fault fissures along which the auriferous veins were formed were opened at a date subsequent 

to the faulting that preceded the mineralization in the Georgetown district, and took place 

as a result of different stresses. Thus the horizontal fault movements were produced under 

a considerably greater load of overlying rocks than the plunging movements which afforded 

channels for the gold ores. N cvertheless, the direction of faulting is evidently not a question of 

load, but of local stress, since postmineral movements, roughly contemporaneous in both 

districts, have followed the same direc_tions as the antemineral faults. 

IDAHO SPRINGS WATERS. 

ACTIVE AND EXTINCT CARBONATED SPRINGS. 

The town of Idaho Springs derives its name from a number of' carbonated mineral springs 

which are situated there. Some of these springs are hot, while others have very nearly the 

normal temperature for underground waters. Extinct springs are indicated in many places by 

mounds of yellowish-white travertine or calcareous sinter, showing that the not-spring action 

dates from 11 considerable period hack. · 
164 GEOLOGY OF THE GEORG~TOWN QUADRANGLE, COLORADO. 

\ 

About a mile below Idaho Springs, on the north side of Clear ·Creek valley, ancient rher 

conglomerates adhere in patches to the gneiss that forms the valley slopes. These conglomerates 

correspond in age and position to the conglomerates which cover the old river benches at Idaho 

Springs and which have in the past formed productive placers. They are of preglacial age and 

have been referred to the late Tertiary and provisionally called Pliocene. The remnants of 

these conglomerates described as adhering to the valley slopes have been cemented with banded 

calcite, which has filled numerous fractures and which· is easily recognizable as the result of 

local hot-spring action. It is reported th11t this calcite contains some silver, but the assay of 

a small sample taken by the writers showed only a trace. 

It therefore appears that the springs near Idaho Springs, while shifting their position 

repeatedly, have been active since the late Tertiary. 

:Mr. Waldemar Lindgren has furnished to the writers information concerning a deposit of 

caleareous tufa or sinter whieh oceurs in Clear Creek valley 1 mile below Georgetown, on the 

lower slopes of Columbia :Mountain. At this point there are rather extensive calcareous surface 

deposits in three places, the highest of which is about 400 feet above the valley. The largest 

mass eovers the slope for a spaec perhaps 100 feet square and has peen deposited upon the 

sloping and deeomposed granitic roek. These deposits are evidently due to spring action. 

:Mr. Lindgren remarks that the springs evidently issued 400 feet above the valley and gradually 

descended, keeping pace with erosion. ~ number of small tunnels have been driven in along 

the-zone of veins, and low values in gold are reporte(l con1mon. Vestiges of thermal eonditions 

in the form of notieeable heat are reported to have been observed in one of these tunnels. 

COMPOSITION OF IDAHO SPRINGS Wfi.TERS. 

The present springs near the town may he divided into three groups-the Hot Springs, 

whieh are several in number but oecur close together; the Blue Ribbon Springs, which are of 

cold water and lie a short distance farther west; and the Cold Sulphur Springs, whieh are 

situated at a distance of several hundred yards from the other two groups and somewhat 

farther clown Clear Creek valley. The Hot Springs issue ncar the contact of an intrusive body 

of alkali syenite with pre-Cambrian gneiss, and the Blue Ribbon Springs are situated at the 

contact of a smaller body of the same intrusive roek, while the Cold Sulphur Springs rise from 

the gravel which forms the bottom of the valley. The temperature of the hot springs varies 

from 98° to 108° F .. or more. The water is used for bathing and also for medicinal purposes. 

That of the Blue Ribbon Springs is bottled and sold as a table water. Analyses of these waters 

follow: 

Analysrs of Idaho Sprinys waters. 

I 4.a 5.a 1. -;_ 1~.~, 

------~--- ---------,-----'------------1------.---

Grains Grain.e 1 Parts per I' arts per Parts p~r 

Sodium cnrhonate.... .. . .. .. .. . . .. ~~~~~~~ ~0. 80 3. 85 5. 725 SiO,.......... ~~~~~~ 56. 5 65 69.4 

Magnesium carbonate.............. 2.88 .36 .864 SO,.... 317.7 352.1 301.6 Calcium carbonate.................... n. 52 1.19 . 231 : HCO,.... . 985.3 1,320. 4 1,302. 7 · J<'errons carbonate..................... 4. 12 . 52 . O(il B,O,b .. :.... .. .. .. .. .. . Lost. 7.8 11.8 Sodium chloride................ 4. 16 . 52 . 709 Cl......... 45. 7 58.3 61. 4 Magnesium chloride................... Trace. '!'rrwe. .. .. .... .. . Fe,o,.. ... .... .... .. . 3 7. 6 3. 7 Calcium chloride ......... ·.............. '!'race. •rrace. .. .. .. . . . . . AI,O,....... .. . . . . . .. . . 1. 4 Trace? . 9 Sodium sulphate.................. .. .

1 

29. 36 3. 67 Ca........ .. .. .. .. .. . . .. .. .. 109. 8 128. 5 145.1 Magnesiumsulphate............ 18.72 2.34 ........... Mg.... .................. 29.2 35.1 36.0 Calcium sulphate....... .. .......... i 3. 44 . 43 3. 441 K.................... 54. U 59 66.1 

~fJ1~~-·-i~i~~te_._·_::::: :::::::::::::::::I .... :-.~~- .... _·_'11 _

1

· .. ·::: 47~-~ ~f·.·_·_·::::. .. ....... :::::::::: ...... · T~!~e~ '1'~~~:0~ T~!~e~ 

A

TAthia .•••• ____ . __ . _ __ __ _ ___ .... _ .... _ 1 • _. __ • 1 JNrace. ~r........ .. .. .. . . .. . . j:;:~: 1J~~ee.. 'l'Nroancce··. mmonia ........ ____ -----·····-- ... 1

... ...... one. n............ ... --·--······- Nitrates .............. ·......... .. ...... 1

... ...... None., Zn.. ...... . Tmce(?) None. None. Organicmatter........... .. ......... !........ . None. 1 

I ' Residue at 1H0° ................... .. 

1, 046. 7 1, 446 1, 525. 5 28.3 

2,491.1 

1,820.1 

1,809.1 18.3 

2,536. 5 1,874.5 

1, 955.4 

4'01.0 

Total.... .. .. ...... .. .. . --~lo7--J----ru9,--l-l.-50D- il As normn.l carbonates .... _'_ ...... . 

I ; i Free co, __ .......... .. ........ .. 

-----~----~-- --~----~----~----

a Ba, Au, Ag, As, Sb, Pb, absent or not present in appreciable amounts in qnu.ntity of water examined. b Approximate. 

1. Hot Springs. J. G. Pohle, New York, analyst. U. S. Geog. and Gcol. Surveys W. lOOth Mer., vol. 3, 1875, PP· 485, 486. 

2. Hot Springs. J. Ohly, Denver, analyst, June 6, 1905. 

3. Blue Rihhon Springs. J. Ohly, Denver, analyst. 4. Cold Sulphur Springs. W. T. Schaller, nnalyst. 5. Blue Ribbon Springs. W. '1'. Shal!er, nnalyst. G. Hot Springs. W. T. Schaller, analyst. 
IDAHO SPRINGS WAT.ERS. 165 

EFFECT OF WATERS ON WALL ROCK. 

Specimens of the intrusive alkali syenite collected by Mr. Lindgren near the Hot Springs 

and forwarded to the writers sho'v the rock riddled by small cavities which have been formed 

by the dissolving effect of hot waters. Many of the cavities are of such size that it is evident 

that their formation has been accomplished by the dissolution and removal of mi~erals representing all the mineral constituents of the rock, so that all the chemical constituents of the 

rock have passed into solution. In many of these cavities and also along cracks Mr. Lindgren 

found abundant crystals of barite, undoubtedly deposited by the spring waters. This occurrence is interesting, inasmuch as the alkali syenite (see p. 83) contains considerable barium, 

while the analyses of the waters show none. A reaction between the barium and the decomposed ror.k and the sulphates in the spring waters is cvideut, and the indigenous origin of this 

barite confirms the conclusions already drawn as to the origin of the barite in mineral veins.· 

Tiny and relatively sparse quartz crystals, not so large nor abundant as the barite crystals, 

grow out from the sides of these cavities. Limonite has also been deposited, lining the walls. 

The microscope shows that the alteration of this rock has resulted largely in the turbidity 

of the minerals, especially the feldspar, this clouding being caused by a very fine-grained 

alteration product varying in color from white to brown or yellowish brown. On the walls 

of small cavities limonite has been deposited. It seems probable from the inspection of the 

mineral which has caused the turbidity that it is chiefly carbonate, partly lim~ carbonate, 

and in part also probably iron carbonate. The limonite 11ppears to have been formed by the 

oxidatio.n of this carbonato in the cavities. 

COMPARISON OF DIFFERENT SPRING WATERS. 

A comparison of these waters is interesting for the reason that, while they differ somewhat in composition, they are evidently, as shown by the analyses, of allied nature. Both 

the Hot Springs and the Blue Ribbon Springs are carbonated and sulphureted soda springs 

occurring under the same geologie conditions within a few hundred yards of each other. The 

cold Blue Ribbon Springs, however, are evidently considerably farther removed from the 

focus of heat than the Hot Springs. They occur along the contact of a dike which is similar 

in composition to that near the Hot Springs, but whose intrusion may be of earlier date and 

whose cooling in its deep-seated portions may have reached a more advanced stage than in 

the rock whence the Hot Springs derive their heat. 

As the Cold Sulphur Springs are situated at a distance of several hundred yards from 

the other two groups and emerge from the alluvium which floors the valley of Clear Creek, 

their relation to the bed-rock geology is unknown. 

The analyses of the spring waters show that they now contain lime, magnesium, sodium, 

and _potassium, chiefly in the form of carbonates an_d sulphates and some chlorides, and also 

some silica. Soda is decidedly the predominant hase; lime, potash, and magnesia come next. 

DISTINCT ORIGIN OF ACIDS AND BASES IN SPRING WATERS. 

Since in the formation of the larger cavities mentioned above the whole rock fabric has 

been dissolved, the hot waters have at .one time or another contained in solution all the rockmaking elements, and part at least of their constituent bases at present must be derived from 

the rocks. The analysis of the alkali syenite, however, shows that S03 is present in small 

amounts, but that there is no C02 nor Cl nor compounds of boron. In the waters there are 

large amounts of free C02 and sulphur in the form of H 2S and also in the form of sulphates. 

A great excess of carbonic and sulphurous acids is thus indicated, aTl<l hydrochloric acid in 

combination as chlorides is also present. The waters also contain a notable proportion of 

boron. It is impossible that more than an extremely small proportion of these acids could 

have been derived from the adjoining rocks. . 

The alkali syenite along whose contact the springs ascend was probably a late Tertiary 

intrusion. 1Nithin this, rock fluorite occurs in irregular ramifying veinlets in the grounclmass 
166 GEOLOGY OF THE GEORGETOWN QUADRANGLE, COLORADO. 

or in small areas flecking the feldspar or the mgirine-augite. This mineral appears to represent 

the final or pneumatolytic period of consolidation. In part it may be prirnary, since it fills 

cavities between older minerals, while in part it may be considered as subsequent to the practically complete consolidation. Similar areas of analcite occur in the fresh rock. They were 

probably formed at the same period as the fluorite and, like it, may be considered as either 

original or secondary. These minerals doubtless represent not only the final stages of consolidation, but the first stages of volcanic emanations, while the great amount of sulphur and 

carbonic acid in the spring waters, which appear to have derived their heat directly from 

the volcanic rocks and which followed the intrusion, may plausibly be ascribed to the gaseous 

exhalations of the later stages of cooling. The boron in the waters may also be ascribed, with 

a high degree of probability, to the same process of gaseous exhalation.a 

·while in th9 rocks through which these waters pass potash is as abundant as soda, it 

appears that its proportion in the waters themselves is small, compared with that of soda; 

therefore, most of the potash dissolved is, apparently, not held in solution in the waters, but is 

probably directly deposited as sericite. The water analyses show that silica also is not 

retained in any amount at all corresponding to the proportion in the rocks, and the growth of 

quartz crystals on the walls of the cavities suggests that it is precipitated, after solution, practically in place; the presence of these crystals also shows that cementation of cavities and 

fissures by quartz is now probably going on at the proper depths. It is likely that the shifting 

position of the springs, recorded by the mounds of travertine in localities where no water now 

issues, is due to the repeated cementation of the conduits, forcing the waters to break out 

through new channels. 

The iron shown in the analyses is likewise small in proportion to that which is contained in 

the rocks, and it appears that it also is precipitated soon after being taken into solution. This 

is indicated by the limonite mentioned above as coating the cavities near the surface and by 

the probable iron carbonate which occurs as an alteration product (p. 165); and it is suggested 

that farther below the orifice siderite and pyrite are being deposited, corresponding with the 

formation of these minerals which has been studie(l and described in the wall rocks of the 

mineral veins and in the vein channels, and which has also been accomplished at an earlier 

period by ascending hot waters. 

The chief bases which are contained in the rocks an(l which remain in solution in the spring 

waters are soda, magnesia, .lime, and potash, the soda being predominant in both the hot and 

the cold waters, while in both also the lime predominates over the potash and the potash over 

the magnesia. . 

Although some portion of these materials may be of magmatic origin and may have been 

given off from the cooling magma along with the gaseous or combined acids which accompany 

them in the waters, yet a large portion, and possibly the greater part or all of them, must have 

been supplied by the solution of the wall rock. The relatively insoluble,rock-making materials 

have been deposited not far from their place of origin, the solution and precipitation involving 

a rearrangement, concentration, and freer crystallization of these materials, while the more 

soluble portions have been retained in solution. Where the spring waters come into contact 

with the air on emerging at the surface most of the contained lime is deposited as sinter, while 

the soda, 'magnesia, and potash are mainly carried off. 

ORIGIN OF WATERS. 

The conclusion has therefore been reached that the bases now contained in the spring 

waters, together with those deposited by them near the surface, arc the result of rock leaching, 

and that the acids, both gaseous and combined, have been derived from the emanations of a 

cooling igneous magma. There remains to be considered the source of the water. The numerous 

deep tunnels in this region do not reveal the presence of any important amount of atmospherie 

water passing down~arcl to the subterranean regions. Many of such tunnels have passed from 

wet into comparatively dry rock at tt vertical distance of several hundred feet below the surface. -------------------------------- a Spurr, J. E., Prof. Paper U.S. Gcol. Survey No. 55, 1906, p. 162. 
IDAHO SPRINGS WATERS. 167 

Therefore the ~ater is ascribed to the same source as that ·of the hot springs which formed the 

mineral veins of the region, and which are regarded a,s exhalations from the more deeply seated 

portion of the magma. 

These active carbonated springs probably occupy the same relation to the alkali syenite 

intrusion that the hot mineralizing waters did to the bostonite and alaskitic quartz monzonite 

intrusions which probably antedate that of the syenite. 

It is highly probable that at a certain depth, within a few thousand feet of the surface, 

the waters which emerge at the hot springs are depositing mineral veins at the present clay. 

CHARACTER OF WATERS IN DEPTH. 

At a depth where ore deposition is going on the solutions have probably not the same composition as those which reach the surface, since the composition when they emerge has resulted 

from the interchange of some of the original chemical constituents with· those of the wall rock. 

This view was presented in the report on the _Tonopah district in Nevada. a A study of 

the different stages of alteration in the wall rocks of the mineral veins at Tonopah resulted in 

the conclusion that the waters which produced the mineralization contained much silica and 

potash, with some carbon dioxide and sulphur, silver and gold, and relatively small amounts 

of other metal~. After these waters had altered the wall rocks by interchange of their 

__ dissolved substances for those originally contained in the rocks, their· composition became very 

different. It was reasoned out that at this stage they must have contained soda largely in 

excess of potash, important amounts of lime and magnesia, some iron, a little silica, and a very 

little alumina, and at hest only traces of the rarer metals. These changes, it was concluded, 

had taken place in the waters as they penetrated laterally, from the circulation channels 

where the veins were deposited, farther and farther into the wall rock. It was, however, 

pointed outb that similar changes must also take place vertically along the channels of 

ascending waters, so that the waters which deposited mineral veins at a certain depth would 

emerge at the surface with the final composition stated above as the result of reactions with 

the wall rocks. 

The composition of the spring waters at Idaho Springs corresponds to that deduced from 

the study of rock alteration near the mineral veins at Tonopah as the final result of these 

reactions. 

COMPARISON WITH OTHER HOT SPRINGS. 

Two other examples of hot springs in the neighborhood of mining districts and intrusive 

igneous rocks may be taken at random and discussed from the point of view of the foregoing 

conclusions. The hot springs at Glenwood, not far from the Aspen mining clistrict,c contain 

soda as the predominant constituent, but it is in the form of chloride, no carbonate or other 

salt being recorded. The next most abundant element is lime, chiefly as sulphate, with a less 

amount of carbonate. Magnesia occurs as carbonate and chloride, the two salts being in about· 

equal proportions. Potash is here present in the form of chloride and is roughly equal in 

amount to the lime and also to the n1agnesia. There is a small amount of bromine and traces 

of fluorine, iodine, and phosphorus. The temperature here is fro111 120° to 132° F., and therefore the waters are hotter than at Idaho Springs. The presence of the halogen elements above 

mentioned suggests that possibly these waters are being given off at an earlier stage of cooling 

or arc nearer to the source of emanation than the Idaho Springs waters, and to this conclusion 

the predominance of chlorides over sulphates and carbonates at Glenwood as compared with 

the predominance of carbonates and sulphates at Idaho Springs corresponds. 

A much earlier stage of cooling (or a nmch greater proximity to the volcanic source, or 

both) than that indicated at Glenwood Springs is apparently represented by the waters at 

Steamboat Springs, Nev. These springs have a temperature of 167° to 184° F.d The waters 

contain potash in much greater proportion relative to the soda than at Glenwood Springs, and 

a Spurr, J. E., Prof. Paper U.S. Geol. Survey No. 42,1905. 

b Idem, pp. 237, 238, c Mon. U. S. Geol. Survey, vol. 31, 1898, p. 213. 

dMon. U.S. Geol. Survey, vol. 13,1888, p. 347. 
168 GEOLOGY OF THE GEORGETOWN QUADRANGLE, COLORADO. 

in h1any times larger amount· than the lime or magnesia. The Steamboat Spri~gs waters also 

contain a vastly greater proportion of silica than those at Glenwood. The proportions of the 

constituents in solution therefore approach those which have been inferred for the mineralizing 

waters at Tonopah. 

At Steamboat Springs also the soda is mainly in the form of chloride, as at Glenwood; 

and the presence of abundant borates with antimony, arsenic, and mercury, probably in the 

form of sodium sulphosalts, indicates a comparatively early stage of the emanations. Moreover, the sinter deposited by these springs contains various metals and other elements, including mercury, gold, silver, copper, lead, arsenic, antimony, iron, alumina, magnesia, lithia, soda, 

potash, silica, and sulphur."' 

DEDUCTIONS FROM COMPARISONS. 

These comparisons emphasize the conclusion already pointed out that in the case of many 

hot springs which emerge at the surface the mineral bases present are more the result of reactions with the wall rock than of original composition or of magmatic emanations, while the 

acids (both combined and free), silica, and the rarer metals, with very likely some other bases, 

such as potash, where this is present, may be original and magmatic. 

DIFFERENT TYPES OF WALL-ROCK ALTERATION BY SIMILAR WATERS. 

I 

This fact that the formation .of gangue minerals and the alteration of wall rocks by hot 

springs has originated largely from the reaction of the mineralizing solutions with the rocks 

leads to the deduction that with the same or similar solutions different wall rocks have produeea by reaction different minerals. Thus in the Aspen district, if our conclusions are true, 

the mineralization took place at approximately the same period as in the Georgetown silver 

district and from similar solutions. In Georgetown the chief gangue mineral is quartz, with 

small amounts of other minerals, including siderite and barite. The wall rock is altered to 

quartz, carbonates of iron, lime, magnesia and manganese, and sericite. In Aspen the gangue 

minerals are chiefly quartz, crystallized dolomite, ·and barite, and the wall rocks are altered 

mainly to dolomite, and also to quartz with some iron. The hot springs at Glenwood, near 

Aspen, above cited, which contain a variety of constituents corresponding in a broad way to 

those in the Idaho Springs waters, are changing the limestones through which they rise to 

dolomite, silica, and some iron, the alteration being similar to that of .the wall rocks in the 

Aspen ore bodies. Therefore these waters are probably more or less similar to those which 

deposited the ores. 

EXTENSION OF CONCLUSIONS OUTSIDE OF THE QUADRANGLE. 

If the conclusion is true that the more valuable metals in the ore deposits of the Georgetown quadrangle are of direct magmatic origin and that the mineralizing agents emanated 

directly from cooling magmas, it also probably applies to the districts with which the Georgetown and Idaho Springs districts have been correlated, namely, those which lie on the great 

northeastward-trending_ mineral belt and in which, as at Georgetown, the mineralization closely 

followed the intrusion of monzonitic rocks, and also to the veins of the isolated volcanic centers 

of Rosita and Cripple Creek, whence alkaline magmas have been erupted. Concerning Cripple 

Creek, Lindgren and Ransome in their report 'have concluded that the chief valuable minerals, 

as well as the mineralizing waters and gases, were of magmatic origin. At Rosita there is 

strong evidence of a similar origin for the veins. (See p. 126.) A magmatic origin for some of 

the mineralizing gases in that district has already been argued by Elllhlons, though the ultimate origin of the water was apparently considered to have been atmospheric, and it was 

believed that the metals and the vein materials were derived from the rocks traversed by the 

ascending hot springs, a justifiable conclusion when considered in the light of local evidence 

alone, though placed in doubt by the wider field of evidence presented in this paper. 

a Mon. U.S. Geol. Surv~y, vol. 13, 1888, p. 340. 
i 

IDAHO SPRINGS WATERS. 169 

With reference to the whole group of Colorado ores whose formation followed monzonitic 

intrusions, the fact that the ore deposition followed closely and was dependent on the eruptions 

has been emphasized by separate investigators in many different localities, and the conclu~ 

sion that the ores were deposited by hot ascending waters which derived their heat and dissolving power from the volcanic rocks has been reached in nearly every case. The only exception 

to this last statement is the Leadville district, where the early 'studies of Emmons led him to 

conclude that the ores were leached from the eruptives by circulating waters of original metcorie 

origin. However, an origin by hot ascending waters in that district has been advocated by a 

later writer (seep. 105), and in view of the developments made since the first investigation this 

seems probable, though the result of the subsequent work of descending atmospheric waters 

may be very important, as it is in the Georgetown district. The writers see no reason for separating the origin of the ores of the Leadville district from those of similar and neighboring 

districts. . 

While the explanation of deposition by hot ascending waters has been all but universally 

adopted by geologists, the source of these waters, in accordance with the formerly commonly 

accepted explanation of the origin of hot springs, has been supposed to be atmospheric, and 

the metals have been supposed to have been leached from the consolidated rocks, especially the 

igneous rocks traversed by the waters. This explanation has almost invariably.been offered by 

different geologists for the various San Juan districts and also by Spurr for the Aspen district. a 

Purington's views concerning the ores of the Telluride quadrangle (p. 114), however, included 

the belief that a portion of the constituents in the mineralizing solutions had been derived from 

magmatic exhalations, although he conceived of the waters themselves as of atmospheric 

derivation. 

PROGRESS OF THE MAGMATIC THEORY.b 

In the study of the Mercur district,c made iri the year previous to that of the Aspen district, 

Spurr found within a relative1y small area evidence of two distinct but not widely separated 

periods of mineralization ·which were marked by quite different metallic and gangue 

combinations. 

His conclusions were that in both these periods the mineralizing agents, including waters, 

gases, metals, and gangue materials, were of direct magmatic origin,d and the difference between 

the two types of mineral deposits was ascribed to different stages of fumarolic activity.e In 

this paper he dismissed the theory that the ores were deposited by hot springs whose waters 

were of atmospheric origin, but inasmuch as the conclusion was out of line with current geologic 

beliefs he hesitated even in his own mind to extend it further than local evidence warmnted. 

In 1896 the same writer studied the gold-quartz veins of the Yukon district in Alaska! and 

concluded that the metallic and earthy minerals contained in these veins, as well as the mineralizing waters and vapors, were of direct magmatic origin, and were so slightly removed in nature 

and location from tho main magma that the resulting veins were best considered as a consolidation phase of the granitic rock; and he suggested that the same explanation ";.ould apply to 

many of the gold-quartz veins in other parts of the world. This theory he has further supported 

by the published results of his study of the Silver Peak district, Nevada,g and in this publication 

he has argued for a world-wide distribution and eminent importance of this typeJt 

In 1901 Lindgren published an article.i in which he collected certain literature concerning 

the occurrence of copper deposits in zones of contact metamorphism around intrusive bodies of 

monzonitic rocks. The cases cited were chiefly from the regions of the Pacific Cordilleras. 

These contact deposits contain chalcopyrite or bornite, with magnetite or hematite and usually 

a small quantity of galena and blende·, with the characteristic contact-metamorphic minerals, 

such as garnet, pyroxene, and epidote forming a· gangue. He concluded that the mineralizing 

a Mon. U.S. Gcol. Survey, vol. 31, 1898, pp. 235,236. 

b No attempt is made to make this outline complete. 

<Sixteenth Ann. Rept. U.S. Geol. Survey, pt. 2, 1895, pp. 343-455. 

d Idem, p. 397. 

<Idem, pp. 449, 451, 453. 

f Eighteenth Ann. He pt. U. S. Geol. Survey, pt. 2, 1898, p. 312. 

· u Prof. Paper U.S. Geol. Survey No. 55, 1906. 

h See also Trans. Am. Inst. Min. Eng., vol. 36, pp. 372-402. 

•Trans. Am. Inst. Min. Eng., vol. 31, 1902, p. 226. 
170 GEOLOGY OF THE GEORGETOWN QUADRANGLE, COLORADO. 

agents, together with the ores and much of the gangue material, were of direct and immediate 

magmatic origin. This view he has elaborated and supported in his study of the Clifton-Morenci 

district, Arizona, a where he shows that the deposits described are due to emanations of magmatic 

water gas and other gases, with metallic and earthy materials, from monzonitic intrusions of 

late Cretaceous or early Tertiary age. These emanations replace the intruded limestone near 

the contact. 

Spurr published in 1905 the result of studies in the Tonopah district, Nevada, a region of 

gold and silver bearing quartz veins in Tertiary volcanics. He came to the conclusion that 

these veins had been deposited by magmatic waters and gases given off from cooling volcanic 

rock; that the valuable metals, together with silica and some other gangue materials, had been 

derived from the magma, while the iron contained in the pyrites, as well as a large amount of 

the gangue material, had been derived from the wall rocks. He further correlated these Tonopah 

deposits, in point of age and origin/ with numerous ore deposits in Nevada, Idaho, and Mexico, 

and argued for the ·existence of a metallographic province characterized by similar veins and 

extending along the borders of the Pacific portion of North America; and he also showed the 

probability that this province is continuous further in both directions, bordering the Pacific on 

all sides, and so nearly encircling the worhtc 

Since the publication of the Tonopah report, its author has had opportunity to make 

critical studies of important ore deposits in Mexico, such as he had correlated with the Tonopah 

deposits, especially a.t El Oro, in the State of El Oro, and at Guanajuato, both of which have 

been very productive districts. A striking similarity in the age and composition of the volcanic 

rocks and of the consequent ores in these districts to the Tonopah type bears out in every 

respect his previous conclusions derived solely from literature. At El Oro the eruption and 

profound faulting of an earlier andesite has been followed by the formation of veins along the 

faults, and this vein formation has been succeeded by erosion. Later the veins have been 

followed by another flow of andesite, entirely subsequent to the mineralization, though it is of 

no very recent date, but is evidently Tertiary. The age relations of the rocks and ores here are 

strikingly similar to those at Tonopah. At Guanajuato the ores appear to be similar in age to 

those at El Oro, and the vein material is also similar. The principal vein (the veta madre) . 

has formed along a heavy fault in Tertiary andesitic volcanics, corresponding to the earlier 

andesite at El Oro. The later andesite was not recognized at Gunnajuato, although it may be 

present as dikes or as flows in other portions of the district. The same writer has also recently 

studied at several places in Mexico-for example, at Matehuala, in the State of San Luis Potositypical copper-bearing contact-metamorphic deposits which have formed in limestone on the 

borders of monzonitic intrusions, and the evidence in these places indicated that the origin of 

the ore was certainly magmatic, as shown by Lindgren for the similar Arizona deposits. The 

monzonitic magma which has produced all these contact deposits in limestone is apparently 

similar in composition and age to that which has been described in this report as responsible for 

great quantities of ores of silver, lead, and gold in Colorado. Moreover, the petrographic 

province seems to continue southward from Colorado through Arizona and New Mexico into 

Mexico, as well as westward into Utah, so that it is suggested that the Colorado silver-lead-gold 

veins are dependent on the same magma, intruded at the same period, as are the copper-bearing 

contact deposits in limestone described in Arimna, Mexico, and elsewhere. 

Lindgren and Ransome,a in discussing the origin of the ores of Cripple Creek, in Colorado, 

have reached a conclusion similar to· that of the senior writer concerning the Tonopah ores and 

believe that the waters and other mineralizing agents, with the more valuable metallic minerals, 

were of direct magmatic origin, while some at least of the iron nnd earthy gangue minerals 

were derived from the consolidated volcanic rocks by leaching. 

a Prof. Paper U. S. Geol. Survey No. 43, 1905, p. 219. 

~Prof. Paper U.S. Geol. Survey No. 42, 1905, p. 273. 

c Idem, p. 279. 

d Prof. Paper U. S. Geol. Survey No. 64, 1906, p. 225. 
IDAHO SPRINGS WATERS. 171 

GREAT IMPORTANCE OF WATERS OF MAGMATIC ORIGIN IN THE DEPOSITION' OF ORES. 

In view of these considerations it appears that the ores that are due directly to emanations 

from heated igneous magmas and have been deposited from a vehicle of magmatic water and 

gases-whether segregated within the cooling magma during the closing stages of solidification 

and forming, according to the best conception, a part of the resultant igneous rock (the goldquartz veins of Silver Peak, in Nevada); or formed by the reactions of magmntic mnterinls with 

immediately adjacent limestone into which the magma has been intruded (contact-metamorphic 

deposits); or formed in circulation channels, especially nlong zones of faulting, fissuring, or 

fracturing, at a more or less remote distance from the cooling body (the veins of Georgetown)-

!tre not of slight importance, as was supposed a few years ago. On the contrary, these ores 

form a class of the very first importance. 

' 
CHAPTER II. 

HISTORY AND PHODUOTION OF MINES. 

HISTORY OF MINING DEVELOPMENT. 

The early history of the area here considered is also the early history of Colorado and 

antedates the act of Congress of February 26, 1861, which organized the Territory of Colorado. 

A small amount of gold, claimed to have been obtained in the mountains of Colorado, was collected and exhibited by a party of civilized Cherokee Indians in 1857, on, their return to the 

Middle West. The following year \V. J. Russell headed a party of nine Georgians to what was 

then called the Pikes Peak country. After prospecting Platte River, Boulder and Cherry 

creeks, and other streams with little success, they at last found some gold in a small gulch 

leading into the Platte about 7 miles below the present site of Denver. The reports carried 

back to the States in the fall by members of this party caused many t'o leave for the promising 

new fields. The financial panic of 18.57 probably had much to do with influencing many people 

to try their fortunes in this then little-known region. 

The pioneers of the fall of 1858 founded the towns of Denver and Bmilder, and also several 

minor settlements. 

With the spring of 1859 came an influx of prospectors who not only explored the streams 

along the foothills of the Rockies, but also penetrated deeply into the mountains. Toward the 

end of January, 1859, B. F. Langley discovered some placer deposits or bar diggings on South 

Boulder Creek, which afterwards went under the name of the "Deadwood diggings." Before 

the end of Mareh these deposits gave employment to a number of men and were producing 

considerable gold. Some gold had been found also on Ralston Creek, a small stream running 

into Clear Creek several miles below Golden. 

The discovery of pay placer gold early in April, 1859, by George A. Jackson was, however, 

the beginning of mining work in Clear Creek County. This discovery, which was made on 

Chicago Creek only a short distance above its junction with Clear Creek, ncar the site of .the 

present town of Idaho Springs, was also about the first really important discovery of gold made 

in what is now the Stnte of Colorado, and had much to do with causing the stnmpede toward 

the new western gold fields during 1859. The discovery and locntion of the first gold-bearing 

lode on May 6, 1859, by John H. Gregory caused fully as much or possibly more excitement. 

This find was made on what is now claim No. 5 on the Gregory lode in Gilpin County, near the 

present town of Central City. Numerous other mineralized veins were found in the same 

vicinity shortly afterwards. 

Near Georgetown, which at that time was called Elizabethtown, several lodes were discovered at about the same time, and were then worked for gold, although most of them have -

since been found to produce more silver than gold. Among the most prominent of the veins 

discovered in the neighborhood of Georgetown was one found on August 1, 1859, by George 

Griffith. 

The Upper Union or Empire district, in which is situated the town of Empire, was located 

in 1860 by a number of prospectors from the old "Gregory diggings" near Central City and was 

the seat of great excitement for a few years during the time the surface ores of the lodes were 

being sluiced. 

On February 26, 1861, Congress passed a bill organizing the Territory of Colorado, and 

Col. William Gilpin was almost immediately thereafter appointed as the first governor. 

172 
HISTORY, OF MINING DEVELOPMENT. 173 

Although indications of mineral wealth had been found in Clear Creek County in 1859, 

the developments made during the first few years were not extensive, and most of the production of the Territory previous to 1865 was derived from the gold obtained from the placer 

deposits along Clear Creek for several miles above and below Idaho Springs and from the sluicing 

or stamping of the decomposed surface ores of the veins, mainly those in the vicinity of Empire. 

After about a year of successful mining of surface ores, many of the Empire miners, as their 

neighbors at Central City had previously been, were confronted by a serious difficulty in amalgamating the heavy sulphide ores which were encountered below the soft material at a depth 

of about 40 feet below the surface. In the treatment of these heavy sulphides the gold appeared 

to wash away, while some of the constituents of the sulphides formed a heavy but worthless 

amalgam with the mercury. 

In 1864 Governor R. W. Steele, James Huff, and Robert Layton started out from Empire 

to search for silver, with the result that on September 14, 1864, the "Belmont lode," on McClellan Mountain, was discovered. This was the first important discovery of a silver-bearing 

lode in Colorado. The "float" and ore from the vein both gave high assays in silver. In 

July, previous to this discovery, silver had been found in the Cooley lode, on Glacier Mountain, 

in Summit County, but ore from that vein carried much lead and only small amounts of silver. 

In 1865 and 1866 there was a rush of people to the silver district from all the other counties 

of Colorado and from other States, and as a result a large number of good silver-bearing 

veins were discovered on the hills about Georgetown. Little but prospecting was clone during 

these two years, however, and it was not until 1867 that the active development of the principal lodes was commenced and the mining industry of the region began to assume a prominent 

position. Since the earliest clays the stamp mills were the sole arbitrators of the gold product, 

and the returns from these were usually very unsatisfactory. In 1867, however, the first 

really successful attempt at smelting the ores of the district was made at Blackhawk, although 

several smelters had previously been rurming with little if any profit. The advent of smelting, 

by opening a new market for the ores, had a very beneficial ,influence. 

Until 1870 great difficulties also confronted the. miners from excessive transportation 

charges for supplies received, as well as for ore or bullion shipped, owing to the lack of railway 

transportation. However, on June 22, 1870, the Denver Pacific Railroad was completed from 

Denver to its connection with the Union Pacific at Cheyenne. On August 15 of the same year 

the Kansas Pacific reached Denver, thus making two through standard-gage lines to the Mississippi Valley. Moreover, by f'eptember 24, 1870, the Colorado Central Railway had been 

built between Denver and Golden and was opened for business, thus bringing communication 

with the East to the very entrance to the mountain valley of Clear Creek and affording an 

outlet for the products of the mines at Georgetown, Idaho Springs, and Empire, in Clear Creek 

County, and also of the Gilpin County mines. 

The·yield of ore increased wonderfullyfrom1870 to 1873, owing mainly to discovery after discovery of silver-bearing lodes on the hills about Georgetown and to the development of some of 

. these lodes into extraordinarily rich mines. , The development of argentiferous mines in the vicinity of Idaho Springs also helped to swell the output, so that in 1872, according to Raymond,a 

Clear Creek County "surpassed in the production of the precious metals its earlier settled and 

exploited neighbor, Gilpin." It was about this time also that Hall, Marture & Marshall began 

to buy high-grade ores and ship them to Europe, principally to Germany. Thus was estab-· 

lished a competing market which worked much good to the miners who had ore to sell. A 

pronounced beneficial effect was also exerted on the mines of the area by the construction in 

1872 of a narrow-gage railroad from Golden to Blackhawk, with a branch extending up Clear 

Creek from the mouth of North Clear Creek to the western base of Floyd Hill, 4 miles above. 

In the east end of the county near Idaho Springs, and also to a small extent near Georgetown, · 

many of the mines which were formerly worked entirely for gold had been found during the 

preceding few years to contain much silver, and in some the main value of the ore was found to 

a Raymond, R. W., Mineral Resources West of the Rocky Mountains, 1873, p. 267. 
174 GEOLOGY OF THE GEORGETOWN QUADRANGLE, COLORADO. 

be in the silver constituent. ·This change of gold mines to silver mines was mainly due to the 

starting of the smelters and the obtaining of silver in the matte from ores supposed to be gold 

ores. 

The fmancial panic of 1873 was a decided catastrophe to the mining industry of Clear 

Creek Colmty, which consisted mainly of silver mining, for the market for ore mined became 

very irregular, and prices for ore, which had formerly been fair, became excessively low and 

only certain grades and kinds of ore were at all salable. In spite of all these drawbacks, which 

affected the output of the precious metals for some years, the production for 1873 fell only 

about $250,000 short of the yield for 1872, and that for 1874 was far greater. The reason for 

so slight a falling off in the output, however, lay in the amount of exceptionally high grade 

ore taken from the large ore bodies on a few of the newly developed lodes, which to some extent 

compensated for the nonproduction of some of the mines that were forced to shut clown on 

account of the great financial disturbance. 

As a result of this panic, which affected mainly the values of silver, many of the miners 

near Georgetown began to prospect for gold with more or less success, and since that time 

there has been a gradual increase in the production of gold from the county, clue in part to the 

development of gold-bearing lodes about Georgetown as well as about Idaho Springs. 

In .1872 and 1873, also, began the fierce quarrels between mine owners and the almost 

endless amount of litiga~ion which for years hampered the development of many of the best 

properties of the district. 

The change of the Territory of Colorado to a State and her admission to the Union on 

July 4, 1876, seemed to have a quickening influence on the mining industry; and the completion of the narrow-gage railroad from Floyd Hill to Georgetown in 1877 had a very stimulating effect, which was reflected in the great increase in production both for that same year · 

and for 1878. 

From this time until 1894, when the records show that Clear Creek County reached the 

climax of its production, $3,560,669, there appears to have been a fairly gradual increase in 

its output of precious metals. The remarkable fact is that, in spite of the panic of 1893 and 

the decline in the price of silver which followed, the total production for both 1893 and 1894 

showed a marked increase over both the years which preceded and those which followed. In 

part this increase was undoubtedly due to the greater development of gold-bearing lodes in 

the vicinity of Idaho Springs and to some extent around Georgetown, notwithstanding the fact 

that the decline in silver for a time also seriously affected th~ production of gold, on account 

of the intimate association of the two metals in the ores. However, the fact remains that 

there was also a great increase during these same years in the yield of silver. 

Although Clear Creek County is, next to Gilpin County, the oldest producer of gold in 

Colorado, it still has a production varying between $1,500,000 and $2,000,000 per year, and is 

maintaining its record as one of the most steady and reliable mining districts in the country. 
GEOLOGY OF THE GEORGETOWN QUADRANGLE, COLORADO. 175 

STATISTICS OF PRODUCTION. 

The subjoined table shows the production of the valuable metals m Clear Creek County 

from 1859 to 1904, inclusive. 

Production of valuable metals in Clear Creek County, 1859-1901." 

Year. Gold. SiJyer. 

1859-1868. ·············· ············· $2,000,000.00 ............... . 186(}-67.......................... . . . . . . . . . . . . . . . . . . $40,500.00 1868...... .. . . . .. .. . . . .. . . .. . .. . . . . . ..................... . 1869... .. . . . . . . . . . . .. . .................................. . 1870. .. . . . . . . . . . . . . .. . . . . . . . . . . . . . ............................ . 1871........................... ... . . ......... . 18i2 ..................................................... . 1873...................... .. . .............................. . 

1874. ····· .. .. ··················. ········· .. ·············· 1875 ...... -.. ---.- ........ --- .... -.- .. -.... ---

1876 ............... ················· 95,161.00 1,837,387.31 

1877......... ················· 96,500.00 1,984,077.91 1878................................. 134,000.00 2, 275, IOii. 85 

1879 ........... ······················ 175,000.00 1,695,000.00 1880. . . ...................... ·_ 158,000.00 2, 230,000. 00 1881. . ·_ ............................. ·. 200, 000. 00 2, 000, 000. 00 1882 ........ :. . . . . . . . . . . . . . . . . . . . . . . 220, 000. 00 1, 680, 000. 00 1883.. . . . . . . . . . . .. . . . . . . . . . . . . . . . . . .. 250,000.00 1, 580,000.00 . 1884...... . . . . . . . . . . . .. .. . . . . . . . . . .. 688,864.44 1, 729,189.55 1885 ............................................................... .. 

1886. . . . . . . . . . . . . . . . . . . . . 599,_ 069. 591 2, 044,753.94 1887. . . . . . . . . . . . . .. .. .. . . . . . . . . . . . . . . 383, 939. 00 1, 733, 305. 00 1888. ................................ 419,820.80 1,896,517.67 1889................... ............. 488.631.60 2,559,588.17 1890 .. ___ . __ . _ ........ ____ .. _. __ ..... 427,991.00 I 2, 496,143.00 

1!!L:,.:-:::::~ .. -~~::":~-: .. ::J ~f~:~~ u~~:~u~ 

1900............... 

i~~LH·:::::::::.:_/~~~.>J ................. ~;n~H~ 466,193.00 l~H~H~ 1,737,261.00 1901. ................ 540,969.00 1,630.882.00 1902. ····- ................... ........ 931,671.00 1,649,996.00 1903............ ............ 470,011.00 1,098,622."00 1904.................... . . . . .. . . .. . . 644,558.00 1, 047,347.00 

Total gold ancl c I d z silver. oppcr. Total. 

1 

Lea . .inc. 

1~--1 I ___ -- -------- $2,ooo,ooo.oo ....... s2,ooo,ooo.oo 

40, 500. 00 . .. .. .. .. .. .. . . 40, 500. 00 141,820.35 ....... ..... 141,820.35 

400,354.00 ............. .............. 400,354.00 

481,354.08 .... .......... ... ..... ........... 481,3.54.08 

869,046.34 . ............. ...... ........ .............. 869,046.34 

~~~i~m:~~ 1- ·-- .: :::::::-·.::::: ~ ::::::·I:::::::::.~::. ~~~~~:~~rff 1,932,548.31 .............. $50,000.00 !.............. 1,982,548.31 

2, 080, 577.91 $3, 000. 00 123,000. 00 I_............. 2, 206,577.91 

2,409,105.85 4,000.00 98,000.00 1.............. 2,511,105.85 

1,870,000.00 17,000.00 80,000.00 .............. 1,967,000.00 

2, 388,000.00 .. . . .. . . . . . .. . . . . .. . . . . . . . .. .. . .. .. . . . .. .. 2, 388,000.00 

i:~88:888:88 ... :::::::::: ::::::::::::: r: :::::: ... .. i:~88:888:88 

u~~:gg~:gg ::::::::::::::::::::::::::-T ::::::::::::: u~~:gg~:88 2,000,000,00 .............. .............. ............. 2,000,000.00 

2, 643,823.53 $81,230.00 2, 725,053.53 

2,117,244.00 4,325.00 99,728.00 .............. 2,221,297.00 

2, 316, 338. 47 28,673.02 205,242.36 2, 550,253.85 

3, 048,219.77 28,535.66 206,718.84 . . . . .. . . . . . . . 3, 283, 474.27 2, 924, 134.00 10,066. 00 550,535.00 .. .. .. .. . .. . .. 3, 484, 735.00 2, 739,125.00 2, 792.00 226,437.00 .............. 2,968,354.00 

2, 546,920.00 . . . . . . .. . .. .. . 277,564.00 . .. .. .. . . . . . .. 2, 824,484. 00 3, 109,773.00 .. .. .. .. .. .. .. .. . .. .. .. . .. . . .. .. . .. .. . .. . 3, 100,773.00 

3, 560,669.00 ...... ------ ....... -- .. --... 3, 560,669.00 

2, 786, 442. oo 6, 885. oo 148, 083. oo 2, 941, 410. o0 2, 781,046.00 21,208.00 130,050.00 ........... ... 2,932,304.00 

2,592,926.00 53,901.00 175,900.00 .............. 2,822,727.00 

2,629,552.00 35,931.00 212,057.0~ 2,877,540.00 

2, 196,291.00 50,999.00 107, 131.00 .. . .. .. .. . .. . 2, 444,421.00 

2, 203,454.00 38,031.00 204, 113.00 . . . . . . .. .. . . . . 2, 445,598. 00 2,180,851.00 62,004.00 168,567.00 2,411,422.00 

2,581,667.00 56,282.00 133,588.00 $5,697.00 2, 777,234.00 

1,5fJ8,633.00 40,003.00 147,946.00 35,621.00 1,792,203.00 

1,691,905.00 50,110.00 173,314.00 .............. 1,915,329.00 

a From reports of the Director of the Mint. 

The total production for the years 1859 to 1904, inclusive, is as follows: 

Gold," $16,110,326.48; silver,<' $63,622,310.32; copper," $530,174.68; lead," $3,765,775.20; 

zinc, $41,318. Total, $84,069,904.68. 

These results are reached from the figures in the above table, together with approximations of the product in years for which the record is faulty. The county's yield of the valuable 

metals prior to January, 1880, is reported to be as follows: Silver, $15,761,907.99; gold, 

$3,015,661.05; lead, $431,000; copper, $37,000. 

u Approximate. 
CHAPTER III. 

THE SILVER PLUME MINING DISTRICT. 

GEOLOGIC FORMATIONS. 

GNEISSES AND SCHISTS (IDAHO SPRINGS FORMATION). 

ORIGIN. 

The earliest formation in the area covered by the Silver Plume special map (Pl. XXI) consists of a series of crystalline gneisses and schists, having an irregular strike and a variable but 

generally steep dip. These gneisses occupy about one-half of the area mapped. A study of 

o~-----'V~z----~•foot 

FIG. 30.-Sketch of frn.gment of gneiss with pebble-like inclusions, from Sherman 

Mountain. a, Gneiss with pebbles included; b, granitic gneiss, massive. no pebbles; c, pegmatite; d, sheared gneiss. Shows (1) arrangement of alternating 

pebble-bearing bands and pebble-free bands, thus indicating a possible original 

stratification; (2) stretching out of pebbles in certain areas in all degrees; (3) 

evidence of a schistosity across the original stratification only exhibited on one 

side of the fragment. 

them has suggested that they may be 

divided into three classes on the basis 

of lithologic differences. The rocks of 

one class (termed on the map" granitic 

gneiss with pebble-like inclusions") 

C<?ntain inclusions which are more 

siliceous than the matrix and which 

are held to represent original pebbles. 

It is probable, therefore, that this form 

of schist was originally a conglomerate. 

A second class (termed on the map 

"granitic gneiss") consists of rocks 

that arc siniilar to the matrix of the 

first class, but contain no pebbles; 

many of them are highly siliceous. To 

follow up the ideas obtained as to the 

origin of the gneiss of the first class, 

this second gneiss may be the result of 

metamorphism of an impure sandstone. 'l'he rocks of a third class 

(termed on the map "black .biotitic 

gneiss'') differ from the siliceous gneiss 

of the second class in containing a 

very large proportion of biotite. This 

present mineralogical composition may be due to the rocks' having formed originally a _more 

shaly member of the sedimentary series. The biotitic gneiss occurs at many, places interbanded 

with the light-colored quartzose gneiss. 

Some gneiss of the first class, containing probable altered pebbles, shows what are very likely 

the traces of original stratification, in the alternation of pt>bble-bearing and pebble-free bands. 

The gneissic structure cuts across the probable stratification without any regard to it (fig. 30). 

This shows that the schistosity and gneissic structure are no aids in working out the original 

structure, and also that the inclusions which have been interpreted as pebbles are indepe;nclent of 

the schistosity, and originated previously to it. 

176 
LEGEND 

( oon:I:Uuted) 

Strike of gneissic or scbistose structure 

Metalliferous ve:ins 

( Terti.a:ry) 

[ZJ 

Fault 

CJ 

Shaft 

Tunnel 

U . S . GEOLOGIC.<ItL SURVEY 

GEORGE OTIS SMITH . DIRECTOR 

E. M. Douglas, Geographer. 

Frank Tweedy, in charge of section. 

Topography by Pearson Chapman. 

Triangulation by Frank Tweedy. 

Surveyed i n 1904. 

GEOLOGIC MAP OF THE SILVER PLUME DISTRICT,COLORADO 

Scale~ 

0~========~~8========~~~======~==========;!====================~;==================~1DOOUe 1000 "00 0 zooo 3000 ~oofuet 

Con.touri:n.terval50 £eet. 

1906 

PROFESSIONAL PAPER NO. 63 PL. XXI 

Geology by J E.Spurr and G.H.Garrey 

LEGEND 

SEDIMENTARY 

.Alluvial B :morainal ~ ~ 

and s1rea:m.-wo:rked < 

@a.cial deposits · 5 

METAMORPHIC 

bably meta:m.orphosed sediJn.ents) 

Granitic 2:n.eiss (7TU!!ixun0rphos~ sand.aton.& or arioos<- ? ) 

Granitic £n,eiss 

with pebblelike :inclusions 

(mA<a7norplwsed oonglo""""""' 1) 

BlaCk bioti:tic 

gneiss 

(m.et=n.orplwl>9d, $~ ) 

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IGNEOUS INTRUSIVES ~ 

Pci:rph.yry 

~ dike 

rocKs 

llioritic roCks 

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U. S. GEOLOGICAL SURVEY PROFESSIONAL PAPER NO. 63 PL. XX II 

d. DIORITIC DIKE (OF HORNBLENDE GNEISS) CUTTING ACROSS THE BANDING OF 

GRANITIC GNEISS BELONGING TO THE IDAHO SPRINGS FORMATION . 

The dioritic dike a lso cuts across pegmatite, which has been inj ect ed into th e Idaho Sprin gs g neiss. 

From the road side just east of Pe li ca n t unnel. 

B. SCHIST INJECTED BY PEGMATITE, ON ROADSIDE NEAR EMPIRE PASS, 

SOUTHWEST SIDE OF DOUGLAS MOUNTAIN. 
ROOKS OF THE SILVER PLUME DISTRICT., li7 

PEBBLE-BEARING GNEISS. 

DESCRIPTION. 

In the gneiss t~at has been interpreted as metam.orphosed conglomerate, the matrix, which 

is invariably biotitic, contains nonbiotitic white flat lenses, bands, or streaks, arranged like the 

pel;lbles of a conglomerate. A study of many examples shows that the original form of these was 

rbund or oval, like that of pebbles. (See Pl. VII, p. 38.) These probable pebbles, with the 

inclosing matrix, have been elongated by stretching, apparently while in a substantially plastic 

condition, so that the pebble-like forms become long, flat, and thin, and in extreme stages (see 

Pl. VIII, A, p. '10) are represented only by thin, flat, siliceous streaks, which would be no longer 

noteworthy or suggestive were it not for the observed transition. Such elongated pebbles are in 

places contorted with the inclosing matrix. 

The less deformed pebble-like forms are easily detachable from the granitic gneiss matrix and 

ca:Q. hardly be explained save as the pebbles of a conglomerate. Originally they must have consisted of impure quartzite. The conglomerate may have contained other pebbles, but only these. 

would easily survive metamorphism and be detectable in an alteretl rock. 

As seen under the microscope, the line between pebbles and matrix is usually sharp, though 

locally there is some slight overlapping of material. The. granitic gneiss matrix has a finegrained, allotriomorphic granular structure. The principal constituents are quartz, microcline·, 

and biotite. In portions of some specimens quartz is predominant, with very little feldspar; in 

~ther specimens feldspar is subordinate and the rock consists mostly of quartz and biotite. 

Granules of zircon are usually abundant. Sillimanite is common, but much less so than in the 

pebbles: Altogether there is a decided chemical difference between the pebbles anti the matrix. 

The pebbles are composed almost entirely of quartz and dense fibrous sillimanite or fibrolite, 

theproportion of the two minerals varying, but quartz being in general tlecideclly preponderant. 

The two minerals are intergrown, or the fibrolite occurs as inclusions in quartz. ·Here and there 

some muscovite is present, apparently in process of alteration to fibrolite. Crystals of biotite, 

zircon, and micaceous hematite appear in some of the quartz-sillimanite areas; tiny green hornblende crystals have also been observed in the quartz. 

A few of the pebble-like inclusions· contain feldspar, which is disseminated throughout the 

pebble or forms a fringe around a more quartzose nucleus. In the locality where these feldsparbearing pebbles were observed they are cut by stringers of pegmatite, showing that they themselves are not of pegmatitic origin. Under the microscope the feldspathic pebbles appear otherwise substantially like the nonfeldspathic ones, consisting chiefly of quartz and fibrolite. The 

feldspar that is associated with these minerals is fresh and comprises orthoclase and some microcline. Biotite is locally present in varying quantity, as is also muscovite. 

DISTRIBU'l'ION. 

The chief occurrence of this metamorphosed conglomerate is in the northwest corner of the 

area represented on the Silver Plume special map (Pl. XXI). Farther south, on the west edge of 

this area, just north of Clear Creek, similar rock is present, though extremely stretched and 

intricately injected by pegmatite. The principal localities of this phase of the gneiss, within the 

area shown on the special map, occupy a rough belt trending northeast and southwest across the 

western part of the area. Farther southwest, along the extension of this belt, other occurrences 

of this variety of the gneiss have been noted by Mr. Ball. It is probable that this belt represents 

the strike of an original heavy conglomerate series. The schistosity does not conform with this 

strike, and indeed has no relation to it. In the northwest corner of the area the schistosity is perpendicular to the strike and in other localities cuts across it at a lesser angle. -

GNEISS WITHOUT PEBBLE-LIKE INCLUSIONS. 

The second and third varieties of gneiss, which have been interpreted respectively as 

altered impure sandstone and altered shaly sediments, are intimately mingled within the area 

here discussed, but the dark-colored, highly biotitic gneiss is most abundant in th«;~ southeastern 

31401-No. 63-08-12 
178 GEOLOGY OF THE GEORGETOWN QUADRANGLE, COLORADO. 

portion, covering a rough northeast-southwest trending zone, which is parallel to the zone of 

, conglomeratic gneiss. 

Under the microscope the gneiss of the second class, which does not contain pebble-like 

inclusions, is shown to have a general composition varying from that of granite to that approaching an jmpure quartzite. Quartz is usually predominant, in some cases making up four-fifths 

of the entire section. The feldspar is microcline and oligoclase (including oligoclase-andesiue 

and oligoclase-albite). Biotite, muscovite, magnetite, and zircon occur in much less quantity. 

The dark-colored biotitic gneiss which represents the third class difiers from the lastdescribed variety in the much greater proportion of biotite and the subordi:r:.ate amount of 

feldspar and quartz. 

INJECTION OF GNEISS BY PEGMATITE. 

The gneisses and schists of the region have been intricately intruded by a siliceous pegmatite, which has taken the form of pegmatite, alaskite, muscovite granite, and pegmatitic quartz, 

FIG. 3L-Sketch of vertical outcrop at extreme northwest corner oflarea shown in PI, XXI, showing schist intruded by 

crosscutting pegmatite dike (a) which sends out tongues and lenses between the laminrn of gneiss (b). 

.. 

one variety passing into another within a very short distance (fig. 31). As a rule the injections 

of pegmatite have found their way along the laminre of the schistosity, but in numerous places 

there are also crosscutting dikes: Many of the intercalations of pegmatite between the laminre 

of the schist are convoluted, and on superficial observation appear to have been folded, and 

thus to be older than the crosscutting dikes. In places, however, suoh dikes send out tongues 

between the laminre, showing that the folding of the schist was in the main previous to the 

injection, and that the injected pegmatitic material has simply followed the curves of the 

folding. This pegmatitic material has been injected into the schist in all proportions, so that 

in different parts of a single outcrop the schist and the pegmatite are alternately predominant. 

An extreme phase shows pegmatite with only shreds of gneiss remaining. 

In one place noted in the northwest corner of the Silver Plume special area the gneiss, which 

is intruded by the pegmatite, proves under the microscope to be of the ordinary variety, consisting principally of quartz with numerous small grains of biotite, microcline, and oligoclase, all 

sparse in relation to the quartz. Another specimen of the same rock appears to have been 

altered by the pegmatitic injection; the biotite is partly bleached and the feldspar is entirely 

altered to what at first seems a sericitic product. The highest power of the microscope, how-
ROCKS OF THE SILVER PLUME DISTRICT. 179 

ever, shows for this alteration product a pale-green color, faint pleochroism, and complete 

extinction, indicating probably a fibrous variety of amphibole. In this rock there are some 

small pink garnets and considerable epidote. 

DIORITIC ROCKS. 

HORI"BLENDE GNEISS OR SHEARED DIORITE. 

Hornblende gneiss or sheared diorite occurs only in a few narrow bands within the area 

shown on the Silvf:)r Plume special map. The localities are also widely separated. This rock 

is younger than the gneisses and schists which have been described, whose schistosity they 

traverse at an angle in many places, although in others they conform to it. In one locality the 

sheared dioritic gneiss was noted as cutting across and through the pegmatite intercalated in 

the gneiss as well as the gneiss itself (Pl. XXII, B). At another locality, however, pegmatite 

which was believed to be of the same age as that just noted was observed cutting through th~ 

hornblende gneiss (Pl. XXII, A). These two occurrences, indicating that the pegmatite is 

found locally older, as well as younger, than the hornblende gneiss, would suggest that both 

rocks are in general contemporaneous, and this accords with the studies made over the whole 

Georgetown quadrangle by Mr. Ball. 

Under the microscope the hornblende gneiss shows a fine allotriomorphic granular gneissic 

structure. The constituents are feldspar, hornblende, quartz, and biotite, in some sp.ecimens 

all nearly equally abundant. The feldspar is chiefly oligoclase and andesine, with labradorite 

here and there. Ilmenite and zoisite are also abundant, the latter in long prisms in quartz, 

feldspar, biotite, etc. Titanite is present, and in much of the rock abundant, usually surrounding ilmenite.. In one specimen original pyrite takes the place of the ilmenite. 

The feldspar in these rocks is in places partly or completely altered to sericite or .coarser 

muscovite, with a little chlorite and secondary biotite, or to sericite and quartz. 

MASSIVE DIOIUTIC ROCKS. 

DISTRIBUTION AND AGE. 

Massive dioritic rocks occur within the territory covered by the Silver Plume special map 

chiefly in one or two areas of small extent in the vicinity of the ·wisconsin and Maine workings. 

The amount of these rocks is considerably greater than that of the hornblende gneiss just 

described. This massive diorite appears to be equivalent to a fairly regular coarse diorite 

studied by Mr. Ball in other portions of the quadrangle, where much of it shows a tendency 

toward segregation of its constituent minerals. The rocks within the Silver Plume area show 

this tendency carried to an extreme. 

This massive diorite is certainly older than the porphyritic granite, which cuts and includes 

it. Within this special area its age relation to the hornblende gneiss is uncertain, both being 

younger than the ordinary gneiss and schist forlllation and older than the porphyritic granite. 

Mr. Ball, however, has collected from other parts of the quadrangle evidence which shows a 

quite distinct age of the two formations, the sheared gneiss being the older of the two. 

UESUJ,TS OF DIFFERENTIATION. 

Large outcrops of the rr1assive diorite formation near the Wisconsin and upper Maine 

workings are streaked and blotched in many places with light-colored acidic material, and 

contain nests and seams of pegmatite, commonly consisting of quart11, feldspar, and mica:. 

This pegmatite locally changes to nearly pure feldspar or to pure pegmatitic quartz. The 

dark-colored rock in which the pegmatitic streaks lie is an irregular coarse diorite which varies 

to a hornblendite. Close observation and study show that the pegmatite, diorite, and hornblendite are contemporaneous, the result of a thorough differentiation of the material in place. 

Every transition is observed between pegmatite, quartz, and hornblendite, at many places 

within a few inches or feet. The smaller pegmatitic blotches or seams may contain hornblende, 

but the larger ones contain mica instead, and the hornblenclite itself near the point of passing 
180 GEOLOGY OF THE GEORGETOWN QUADRANGLE, COLORADO. 

into pegmatite contn,ins mica., Characteristically the pegmatite in large bodies is slightly 

yc1unger than the black hornblendite and cuts across it, showing that the acidic materials of 

the magma consolidated later than the hornblende, according to the well-known rule. Many 

large bunches of pegmatite and even pegmatitic quartz, however, arc simply nests segregated 

in the hornblendite. and the smaller seams almost invariably show contemporaneity and the 

. continuation of crystallization from the basic to the acidic extremes. The process of differentiation has been singularly perfect and complete within a small rock mass, and all the different 

varieties must have formed practically in place during a very long period of consolidation. 

The banding and streaking which the outcrop shows is not a structure induced jn the consolidated 

rock. The internal structure of the rock in these streaks is the same as where the rock is massive. The banding is clue to flow while the material was still plitstic and may have been brought 

about by pressure. 

Within a distance of 50 to 75 feet specimens were collected which show numerous stages 

of transition from hornblendite to pegmatite, with the intervening clioritic phases. All these 

phases arc equally common, the normal diorite being not more abundant than the extremes. 

Microscopic examination of these specimens shows the constitu!'lnt minerals mixed together 

in all proportions, indicating a very irregular segregation. The essential constituents of the 

rock are andesine-oligoclase; hornblende, augite, ilmenite, titanite, and probably rutile .and 

zircon. Certain specimens illustrating extreme segregation are of pure feldspar rock; others 

are of pure hornblende rock with pr without augite. In some rocks augite is predominant. In 

one section the usually very scanty apatite was found to be present in numerous large grains, 

making up about 5 per cent of the whole field. 

The augite is green, and much of it is intergrown with green hornblende. It is altered in 

many places along the margins and locally along the cleavages to green hornblende, and the 

two relations are often found to occur in the same slide. The alteration of augite to hornblende is in some specimens accompanied by its partial alteration to golden-brown serpentine, 

with a little calcite. In other specimens the continuation of this process is sho·wn by pseudomorphs after pyroxene, consisting of serpentine, calcite, and green hornblende. 

Locally the hornblende is considerably altered to chlorite. Sometimes it is found included 

in tremolite. An intermediate rim of glaucophane has been noted around the hornblende, 

separating it from the inclosing tremolite. Colorless diopside is in places associated with the 

hornblende of the hornblcndite. Brown biotite is an essential constituent in some phases. 

Quartz is rare and in irregular grains; Calcite is usually in small grains and is of uncertain 

derivation. One specimen examined consists of about 60 per cent hornblende, 20 per cent 

ilmenite, and 20 per cent calcite, all crystallized and apparently contemporaneous. 

The feldspar is andesine-oligoclase. Some of the crystals of feldspar include small crystals 

of idiomorphic augite and hornblende. Feldspar occurs partly or completely altered to sericite and fine muscovite, or to sericite and epidote, or entirely altered to a fine mat of tremolite, 

also to broad plates of tremolitc. 

Tremolite occurs in many large crystals. In one section it was noted as containing large 

grains of calcite, epidote, titanite, and green hornblende. The green hornblende in one section 

seems to fade into the tremolite, with an intermediate rim of glaucophane at many points; in 

some portions of the same section subordinate fresh feldspar is present. Tremolite also occurs 

in blades penetrating the rock, filling cracks in feldspar, etc.; also replacing feldspar, as noted 

in the preceding paragraph. 

Ilmenite is characteristically surrounded by relatively more abundant titanite, which 

probably represents a later crystallization. Locally the ilmenite is without this border. The 

ilmenite is in general closely associated with and is included in hornblende. In one rock noted, 

ilmenite and calcite are associated and seem contemporaneous. 

The titanite generally forms borders a~ound the il~enitc, and also occurs in separate grains. 

It is generally included in the hornblende. 

Epidote is rather common in the form of grains, and in some places is clearly one of the 

alteration products of the feldspar, occurring with sericite. 
.ROCKS OF THE SILVER PLUMB DISTRICT. 181 

ORDER OF CRYSTALLIZATION OF ROCK-MAKING ~IINERALS. 

The general order of crystallization of the. rock-making minerals may be stated as 

follows, with more or less reservation: (1) Green augite; (2) hornblende (largely due to the 

alteration of augite) with diopside, titanite, and probable rutile; (3) biotite, feldspar, and 

quartz; (4) in the last stage of consolidation or after practical consolidation muscovite (sericite), calcite, epidote, tremolite (and glaucophane). 

PEGMATITES. 

Pegmatites arc very abundant in the region shown on the Silver Plume special map. They 

belong to three different groups, which are distinct in age. 

EARLY PEGMATITE IN.JECTION. 

The pegmatite of the oldest group is by far the most abundant and forins intricate intrusions and injections in the gneiss and schist series throughout the area; it also occurs at many 

places in considerable masses. This pegmatite has muscovite granitic phases and alaskitic 

phases, but the different varieties change into one another within short distances. This peglhatite is distinctly older than the ordinary granite of the region, and, as before stated, is probably about the same age as the sheared diorite or hornblende gneiss. 

Th~ constituent minerals of the pegmatite are quartz, microeline, orthoclase, and in many 

places coarse muscovite. Accessory biotite occurs locally. Small scattered needles of sillimanite occur in much of the quartz and also in the feldspar. . A specimen of the alaskitie 

pegmatite, taken from the contact of the gneiss into which it is intrusive, showed considerable 

fibrolite in large and small bunches, practically confined to the muscovite crystals in the rock. 

This fibrolite is characteristic of all the muscovite in the rock, and the mode of occurrence suggests that it .has arisen from the alteration of the muscovite. 

PEGMATITIC PHASES OF MASSIVE DIORITIU ROCKS. 

Pegmatites of the second class are those derived from the coarse diorite already described 

·as a highly siliceous differentiation product. Th.e dioritic masses in the vicinity of the. Wisconsin and upper Maine workings contain seams of pegmatite, in part hornblendic, which are 

segregated from and in general are contemporaneous with the black hornblende-bearing part 

of the rock, though many of them are slightly later than this part, showing a relatively later 

crystallization. This pegmatite, together with the other phases of the coarse diorite here, is 

distinctly older than the Silver Plume granite, which cuts through it. Further characteristics 

of this pegmatite are mentionel) under the heading "Massive dioritic rocks" (p. 179). It is 

doubtful if it occurs except in close connection with the diorite as above described·. 

PEGMATITTC PHASE OF SILVER PLUME GRANITE. 

Connected with the eruption and consolid.ation of the typical granite o~ the region is a 

pegmatitic phase. Pegmatite of this class is much less abundant than that of the older period 

first described, but probably considerably more abundant than the pegmatitic phase of the diorite. 

The pegmatite connected with the granite represents the closing stages of consolidation of 

that rock. 

GENERAL COMPOSITION. 

The principal minerals of this pegmatite arc quartz and feldspar. · In much of the pegmatite that consists chiefly of these two minerals the feldspar is mostly on the edges and the 

quartz in the center, a distribution which has been previously noted in pegmatites in other 

regions. A large part of the pegmatite consists of quartz alone. In addition to quartz and 

feldspar, muscovite is very abundant and characteristic. 
182 GEOLOGY 01<' THE G.IWRGETOWN QUADHANGLE, COLOHADO. 

OCCURRENCE AND RELATIVE AGE 0~' MAGNE'l'l'l'J<J, MUSCOVITE, AND ZIRCON. 

Some specimens of pegmatite show nearly as much magnetite as quartz and feldspar. 

The granite itself contains considerable magnetite, and where it becomes coarse granite and 

approaches a pegmatitic structure, even though it is not pegmatite properly speaking, much 

of it contains considerable lumps of this mineral. 

A specimen of coarse pegmatitic granite from Leavenworth Mountain .contains a lump of 

magnetite 1 ~ inches long and half an inch .thick. A thin section of this rock shows an allotriomorphic granular structure, with a texture varying from medium fine to medium coarse. 

Feldspar (microcline and orthoclase) is predominant, with subordinate quartz and here and 

there small muscovite. Magnetite is present in crystals not larger than usual; but more numerous, and in many places bunched together. 

Near the same locality a pegmatitic vein in granite is made up entirely of quartz and magnetite. This vein is very regular and straight and from one-half to two-thirds of an inch in 

diameter. In part it consists mainly of magnetite and in part mainly of quartz, the magnetite 

on the whole being predominant. The vein contains no feldspar. One tiriy crystal of biotite 

was observed. While this vein is undoubtedly of pegmatitic origin, tllis fact would hardly be 

recognized without a general study of the pegmatites of the region. Near by tllis vein magnetite of pegmatitic origin occurs without quartz as a facing on a bowlder, representing a vein 

whC!se other wall has been broken off. 

Microscopic examination of the specimen of granite containing the quartz-magnetite vein 

above referred to shows that the structure of the granite is pegmatoid. The chief mineral is 

fresh microcline with large crystals of oligoclase-andesine, which include small areas of muscovite. There is a little zircon. Magnetite is present in small crystals. Near the quartz-magnetite veinlet the scattered crystals unite to form a solid vein, intergrown with coarse crystalline 

quartz. There is no sharp dividing line between the granite and the pegmatitic vein, but· the 

outline of the magnetite in the vein against the granite wall consists of an aggregate of idiomorphic magnetite crystals, in contact with the feldspar, muscovite, etc., of the granite. The 

magnetite and quartz of this pegmatitic vein have been the last minerals of the granite to 

crystallize. Some, at least, if not all of the isolated idiomorphic magnetite crystals in the rock 

away from the pegmatitic vein are of similar origin to the magnetite in the vein, as is shown 

by their clustering together in the proxtmity of the vein. 

This observation shows the futility of the ordinary criterion, by which magnetite is judged 

to be one of the first minerals to crystallize from a magma. The fact that magnetite typically 

occurs in idiomorphic crystals included in: the quartz, feldspar, biotite, etc., of granites, has 

led to this conclusion; but the present observation shows that magnetite may have this habit 

and nevertheless be one of the last minerals to crystallize, as is shown by its occurrence in 

large amounts in the pegmatite. This occurrence indicates that the magnetite has the property 

of being diffused through the already consolidated minerals and crystallizing as isolated individuals in them, without necessarily obtaining access through any defuiite channels. 

Microscopic examination was also made of the wall rock of the magnetite vein on the granite 

bowlder above :rpentionecl. The granite here is semipcgmatitic and contains quartz and feldspar with sparse biotite and more muscovite. The feldspar is about one-third microcline and 

about two-thirds striated feldspar, probably in part oligoclase-andesine. The striated feldspar contains numerous inclusions of calcite and muscovite. The muscovite varies from grains 

of a very tiny size, such as would usually be called sericite, to coarse ordinary granitic muscovite. Tht:l calcite grains show the same variation in size. These numerous fine inclusions 

give the feldspar a typically altered appearance. The coarser muscovite blades in the rock 

are undoubtedly of the same origin as the fine ones, and many of them folio";' cleavages in the 

feldspar. The microcline is fresh; some muscovite of the coarser type is here and there intergrown with it. Muscovite occurs also intergrown with quartz. Small rounded zircon grains 

are present. Ordinary magnetite in single crystals or bunches of crystals, not different in appearance from the magnetite of ordinary granites, is scattered here and there. Yet magnetite occurs 
ROCKS OF THE SILVER PLUME DISTRICT. 183 

on the face of this specimen as a mineral of the pegmatitic phase and the last mineral to crystallize. All of the magnetite, moreover, is probably of the same age here. 

The intergrowth of microcline and muscovite suggests that the older feldspar has been 

altered to muscovite before or during the crystallization of the microcline. The fact of this 

intergrowth is against the explanation that there has been selective alteration of the feldspars 

to muscovite, affecting the older striated feldspar and not affecting the clear unaltered microcline. The order of crystallization in this rock is as follows: ( 1) Feldspar (andesine-oligoclase~); 

(2) muscovite and calcite; (3) quartz and microcline; (4) magnetite (and zircon?). 

The relative age of zircon is not plainly shown in this specimen. In a specimen of magnetitebearing pegmatite obtained near Freeland, however, <lescribed on page 62, large crystals of 

zircon occur as one of the pegmatitic minerals, constituting 

with magnetite and quartz the last of the granitic minerals 

to crystallize. Zircon has usually been supposed to be one 

of the :first minerals to consolidate from a granitic magma,a 

the conclusion being based on the fact that,like magnetite, 

it occurs in idiomorphic crystals inclosed in all the more 

essential rock constituents, like quartz, feldspar, and biotite. 

The observation previously made concerning the magnetite, 

however, shows that this criterion is not reliable, and suggests that later-formed minerals may crystallize in the 

midst of earlier-formed ones by some process of molecular 

diffusion. 

A pegmatite vein containing quartz and muscovite 

observed near the magnetite-bearing pegmatites on Leavenworth Mountain shows pure quar'tz op. both sides and 

muscovite :filling.up the center in a regular band (fig. 32). 

This occurrence seems to indicate the filling of an opening 

in the solidified granite by the successive deposition of pegmatitic materials. . 

SILVER PLUME GRANITE. 

Granite of nearlyuniform structure and texture occupies 

about one-half of the area shown on the Silver Plume special 

map. This granite is younger than all the other rock 

formation previously described except the latest pegmatite, 

which represents the final stage of its consolidation. · The 

other rock formations are intruded by the granite. The 

older gneiss, which except the granite is the most widely 

distributed rock formation, is intricately cut by the granite, 

as is well shown on the geologic map, and where the contact is observed on a smaller scale the intrusion is still very 

FIG. 32.-Pegmatite vein (showing banding) in 

granite. a, Granite wall rock; b, pegmatite vein; 

c, pure quartz with occasionally a fleck of uncrystn1lized muscot"ite; d, solid tnuscovite, very 

regular. · 

intricate. ·On the extreme west edge of the mapped area, north of Clear- Creek, immense 

bowlders of gneiss occur in the granite, forming a gigantic breccia. 

As a rule the granite, though massive, has a porphyritic habit, marked by the more or less 

completely idiomorphic feldspars. The increase in size and number of these feldspars produced 

a highly porphyritic type known to the niiners as "corn rock." 

. · a See, for example, Rosen busch, ll., and Iddings, J. P ., Microscopical Physiography of the Rock-Making Minerals, 4th ed., p. 154. w. c. 

Brogger, however (Mineralien der Syenitpegmatitgiinge: Zeitschr. fiir Kyrst., vol. 16, 1890, p. 164), regards it as probable that zircon is formed 

by the action of mineralizers, probably fluorine, and notes that while it is formed during the ordinary magmatic consolidation, it is also formed 

during the pegmatite period. A. La Croix (Nouvelles archives du Museum, Paris, 1902, voJ. 4, p. 86) has described an alkaline granite from 

Madagascar, in which zircon is later than all the minerals except quartz. The present writer, in a study of the alaskites of Silver Peak. 

Nev>Lda (Prof. Paper U. S. Geol. Survey No. 55, 1906, p. 105; also Trans. Am. Inst. Min. Eng., vol. 36, 1906, p. 391), found that zircon was on~ 

of the last minerals to crystallize, and was especially associated with the quartz.-J. E. S. 
184 GEOLOGY OF THE GEORGETOWN QUADRANGLE,. COLORADO. 

The constituent minerals of the granite are feldspar, quartz, biotite, muscovite, and magnetite. The feldspar consists of microcline, orthoclase, and oligoclase. Ordinarily the feldspars 

are more or less altered, chiefly to sericite. In the process of alteration the oligoclase seems 

.to be most easily attacked, and the orthoclase next, while the microeline is invariably fresh. 

Original pyrite is present here and there. 

PORPHYRIES. 

The youngest of the rock formations in the Silver Plume area consists of dikes of porphyry. 

These are not very abundant and usually are of relatively small size. In the northwestern part 

of the area the porphyry is more abundant than elsewhere. The longest dike shown on the map 

is traceable from the northwest corner southeastward to Clear Creek near the Burleigh tunnel. 

Most of the dikes trend northwest and southeast; a number of others trend northeast and southwest, nearly at right angle,s to the first set. These dikes were not all intruded at the same time, 

but nevertheless seem to represent a single general period of igneous intrusion. They are all 

younger than all the other rock formations described, and are all, so far as known, older than the 

veins of the district. As a rule no determinations as to relative age have been possible; the only 

exception is in the case of the porphyry encountered in the Baltimore tunnel. This tunnel follows 

a dike of white porphyry with glassy borders, ,v-hich is intrusive into the gneiss. This lightcolored porphyry is found to be intrusive into a brownish porphyry which is exposed for a short 

distance only in the walls of the tunnel. The two porphyries are mainly fresh. As a rule the 

dikes of the district are thoroughly altered, so that no fme distinctions as to original composition 

can be made. All, however,· seem to be either alaskite pm·phyries, granite porphyries, or quartz 

monzonite porphyries, and in most cases it is not possible to tell, on account of their thoroughly 

altered condition, into which class a given dike falls. 

The white porphyry of the Baltimore tunnel is an alaskite porphyry containing scattered 

phenocrysts of quartz and orthoclase in a glassy to microgranular groundmass. The brown 

porphyry has a fine microlitic groundmass and contains numerous phenocrysts of biotite, andesine, some orthoclase, and quartz. In the hand specimen certain altered clark phenocrysts 

suggest pyroxene. 

The other dikes of the region, such as the dike through which the Seven-Thirty and other 

veins cut transversely, and the dikes along which the Bismarck, Pelican, and Corry City lodes 

have formed, are in a high state of alteration. Microscopic examination shows scattered quartz 

phenocrysts and abundant alteration pseudomorphs after biotite. The presence, of calcite among 

the alteration products indicates as a rule a certain amount of lime-bearing feldspars. Apatite 

is a constant accessory in all these dikes and is unaltered. 

DISPLACEMENT OF ROCKS. 

PREPORPHYUY MOVEMENTS. 

This region sem'ns to have suffered ·during a considerable period under the effects of dynamic 

stress. The porphyry dikes described above are part of a more extensive zone of similar dikes 

which extends in a northeast-southwest direction. The dikes themselves have either a northeast-southwest or 11 northwest-southeast extension. The general trend of these dikes, and the 

fact o'f their location in a definite belt extending through the ancient rocks of the region, suggest 

that the intrusion took place along this belt on account of its being a zone of weakness, and that 

the porphyry rose through the planes of weakness produced by the stress. 

POSTPOUPHYRY MOVEMENTR. 

It is at least certain that the porphyry dikes subsequent to consolidation were themselves 

zones of weakness, and that general dynamic stresses continued to be felt in the rocks of the 

region. In many places these stresses found relief most easily along the dikes, producing faulting parallel to their. strike and dip. The actual amount of displacement produced by this faulting was generally small. Numerous examples have been noted where it amounted to a move-
U. S. GEOLOGICAL SU RVEY PROFESSIONAL PAPER NO. 63 PL. XXIII 

A. SHERMAN AND BROWN MOUNTAIN S, FROM CEMETERY HILL. 

1 , Di amond tunne l ; 2, Pelican tunnel ; 3· Maine mine shaft; 4· Eagle Bird tunnel; 5, Dives vein, old 

work ings; 6, I llinois tunne l ; 7• Wisconsin mine, upper tunnel ; 8, Bismarck vein, upper workings; 

g, Backbone tunne l ; 10, Frostberg tunnel; ''· Montreal mine; 12, Burleigh t unnel and DivesPel ican-Seven-T hirty mill; '3· Victoria tunne l, Mendota mine; '4• Mendot a mi ll ; '5· Brown 

Mountain ; 16, Sherman Mountain . 

B. MINES NORTH OF SILVER PLUME, FROM SLOPE OF LEAVENWORTH MOUNTAIN. 

1, D1amond tunnel , 2, old workings on D1ves vein, 3· lll1nois mine, 4• Dunkirk shaft; 5• Silver Bank 

tunnel, Pay Rock mine; 6, Si lver Pl ume m1ne, 7, Vulcan mine . 

• 
U . S . GEOLOGICAL SURVEY 

GEORGE OTIS SMITH . DIRECTOR 

E. M. Douglas. Geographer. 

frank Tweedy. in charge of section. 

Topography by Pearson Chapman. 

Triangulation by frank Tweedy. 

Surveyed in 1904. 

~gp OF 'l"HE SILVER PLUME MINING DISTRICT, COLORADO 

SHOWING SKELETON OF PlUNCIP.AL UNDERGROUND ·woRKINGS 

0 .J, 

6 i =-r1DOO ~00 0 es===r=e=-< "* 200 100 0 

Contour interval 50 feet. 

Do1urnis7n~YA.n-sea level .. 

1906 

PROFESSIONAL PAPER NO. 63 PL. XXIV 

Mine workings compiled 

from mine maps by H.F.Ciarke. 
MINES OF THE SILVER PLUME DISTRICT. 185 

ment tanging from only a few inches to a few feet, while in many other places it probably did 

not exceed a score of feet. Here and there, however, the displacement was probably greater, 

though no well-proved cases can be cited. 

The direction of movement in these postporphyry displacements was in general horizontal, 

as is proved by numerous observations on the stri~D which mark the fault planes .. These show 

that the movement was slightly variable in different places, but that practically the maximum 

deviation from the horizontal direction was 30°, while the greater majority of strire show 

angles between this amount and the absolute horizontal, a set of fault slips of a given strike generally pitching in both directions from the horizontal, which therefore represents a fair average. 

Here and there some of the faults are not parallel to the preexisting dikes. The best known 

of these transverse faults are at right angles to the dikes and are detected by the fact that they 

have displaced the dikes. The northeast faults which displace the largest dike shown on the 

map, on the south side of Sherman Mountain; north of Clear Creek, are the best examples of these. 

The openings produced by these postporphyry fault movements became the sites of ore 

deposition, so that many of the veins run along porphyry dikes, at least for part of their course. 

POSTMINERAL MOVEMENTS. 

Subsequent to the· main period of ore deposition there has been some slight renewal of the 

movement, indicating that the inciting stress has not yet been satisfied. This disturbance is 

evidenced by postmineral movement parallel to the veins and in a few places across them. Like 

the premineral faulting, this relatively slight postmineral faulting has been in an approximately 

horizontal direction. It has not, however, been of any great amount. 

MINES. 

PELICAN-BISMARCK VEIN SYSTEM. 

HISTORY. 

The Bismarck lode was discovered in June, 1866, by a party of prospectors from Central 

City.a The Pelican (see Pl. XXIII, A) was discovered in 1868 and located December 19, 1869, 

by E. S. Streeter.b The claim was 1,400 feet in length and 50 feet in width, being located under 

the law of 1866. The narrow claims of both the Pelican and the Dives companies failed to cover 

the lodes and led to almost interminable litigation. It was not until early in 1871 that the mine 

began producing ore. In 1872 began the fierc.e quarrels between the owners of the Pelican and 

the Dives claims which culminated in May, 1875, in the death of Jacob Snyder, one of the heavy 

stockholders of the Pelican mine, at ~he hands of Jackson C. Bishop, a lessee on the Dives. The 

litigation, which also started in 1872 and included suits and counter suits, injunctions and cou.nter injunctions, :finally came to an end with the consolidation of the two properties in March, 

· 1880, when both came into the possession of the Pelican and Dives Mining Company.c This 

company then began buying up neighboring properties, among which were the Bismarck and 

Seve~-Thirty, the latter being purchased in 1896. 

DESCRIPTION. 

The Pelican-Bismarck vein system, consisting of a main trunk vein giving off important 

branches at both ends, is perhaps the strongest vein system in the Silver Plume region. (See 

Pl. L"CIV.) On account of the complexity of the trunk vein, which consists in large part of 

overlapping, splitting, and reuniting, or parallel veins occupying a relatively narrow zone 

rather than a single well-defined vein, and on account of the fact that the relations of the 

branches to the trunk vein are not everywhere clear, there has been a great deal of confusion 

in the nomenclature. The trunk vein has been variously called the Dives-Pelican, the Eagle 

Bird, and the Bismarck. From the northwestern portion of this vein splits off the Sevena Burchard, H. C., Rept. Director of Mint for 1883, p. 270. b Georgetown Courier, February 6, 1904. 

c Fossett, Frank, Colorado, 1880, p. 392. 
' 186 GEOLOGY 01!' THE GEORGETOWN QUADRANGLE, COLORADO. 

Thirty vein. The southeastern portion of the trunk vein gives off as a branch the WisconsinCorry City vein, and farther southeast splits into the Dives and Baxter veins, of which the 

former is stronger, its eastern extension going under the name of the Dunkirk vein. For the 

sake of simplicity the trunk vein between the ... branches will be called the Pelican; the same 

vein northwest of the intersection with the Seven-Thirty will be called the Bismarck. The 

trunk vein as here defined and its northwesterly extension past the intersection with the SevenThirty have a northwesterly strik.e and a dip to the. northeast at a variable angle, averaging 

about 65°. The principal mine which has exploited this trunk vein has long been well known 

by the name of the Dives-Pelican. This mine is said to comprise about 13 miles of underground workings. a 

PELICAN-BISMARCK LODE. 

NATURE OF VEIN. 

The Pelican-Bismarck vein has formed along a strong fault zone, whose course was determined by a preexisting dike of rhyolite porphyry (fig. 33). Evidence of this movement is 

afforded here and there by discordance 

N;:=====o=;:::;;:::;:==::;i:==::=:~~==.;.a::::;:=.===::;:~d~;:=;b;:::::;;=;s of the walls on the two sides· of the 

vein, and throughout the underground 

or__ ___ ___:cz ____ _;4 feet 

FIG. 33.-Sketch of Pelican tunnel, showing strong portion of Pelican lead. a, Zone 

of very much crushed and jointed porphyry, bounded by two definite walls, c 

and d; b, porphyry sheeted parallel to lode; e argentiferons galena and blende 

(100 ounces silver to the ton) along foot wall. When the sketch was made this 

ore was being worked, sorted, and shipped. 

workings (Pl. XXV) by zones of brecciation and trituration, by slickensided walls, and by surfaces striated 

by differential movement. The direc-

" tion of these strire indicates that the 

movement was nearly horizontal. 

The fault along which the vein 

subsequently formed has followed the 

porphyry dike for a maximum distance of half a mile, as measured in 

the Pelican tunnel. Along this portion. 

the vein has been most productive. 

This section of the vein underlies the 

summit of Sherman Mountain and the 

slope running down to Cherokee Gulch. 

Northwest of this portion, as seen in 

all the underground workings, the vein 

passes out of porphyry into granite with some gneiss in the region lying under the western slope -

of Sherman Mountain going down to Brown Gulch. The same lack of porphyry is characteristic also of the surface at this point, but farther northwest the porphyry reappears at the surface along the fault zone and passes beyond the limits of the area mapped. Along the southeastern portion of the vein also the porphyry virtually comes to an end in the ground underlying 

the southeastern slope of Sherman Mountain. Before Cherokee Gulch is reached the vein 

passes southeastward into gneiss and granite, containing only nonpersistent and untraceable 

streaks of porphyry. 

workings have not been pushed very far beyond the porphyry to the northwest, since 

beyond this point, so far as investigated, the vein ·has grown poorer and, finally, has contained 

little ore. This portion of the vein has been opened chiefly in the Hercules, Zero, and Pelican 

levels (Pl. XXVI). The Pelican level goes nearly 600 feet beyond the limits of the 'area 

shown on the map. The portion farthest in is now inaccessible, but it is reported that near the 

end of the level a drift following the Bismarck lodeb encountered a porphyry dike at right angles 

to the vein, or nearly so, and that at this point the vein was practically lost. This dike is probably that shown on the map as running northeastward across the saddle of Brown Mountain 

and is the second northeastward-trending dike counting from the northwest corner. 

a Proc. Colorado Sci. Soc., vol. 6, 1897, p. 43. b'l'o be distinguishcdfrom Bismarck vein, p. 307. 
MINES OF THE SILVER PLUME DISTRICT. 187 

Southeast of the portion of the Pelican-Bismarck lode which lies in or alongside the porphyry 

the vein, on passing into granite and gneiss, divides up into different branches, many of which 

are traceable for considerable distances and have yielded a considerable mass of ore. The junction of the different branches which belong to this vein system is characteristically inconspicuous. 

It is certain, for example, that the Seven-Thirty uni~es with the Pelican-Bismarck, as is shown 

11000 

10900 

10800 

10700 

10800 

10500 

10400 o. 

10300 

0 

10200 

10100 

10000 

9900 

9800 

9700 

960o 

9500 

9400 

0 200 400 600 feet 

~------~--------~------~ 

NE 

El Mahdi 

3d 

2d 

J st 

Hercules 

80-foot 

140-foot 

210-foot 

275-foot 

Caterello 

Grosso 

Zero 

Eagle Bird 

Bonomini 

Pelican 

9300 o Diamond tunnel 

~~~~~~~=-~~~--~~~~~~ 

Gneiss walJ 

rock 

Granite 

wall rock 

Alaskite 

porphyry 

Veins 

FIG. 34.-Vertical cross section of Bismarck lode, Sherman Mountain. 

on several of the levels of the Seven-Thirty mine, but the actual junction is at many points 

difficult to locate. 

On approaching the trunk vein the Seven-Thirty diverges from its normal trend and tends 

to become parallel with the stronger lode, and finally it becomes lost in the several parallel 

streaks which make up .the zone of the Pelican-Bismarck. Similar characteristics attend the 
188 GEOLOGY OF THE GEO~GETOWN QUADRANGLE, COLORADO. 

branching of the vein on the southeast to form the Dives q,nd Baxter veins, and, so far as can 

be learned, the same is true of the junction of the Wisconsin and Pelican lodes, which is now 

inaccessible . 

. The distribution of the porphyry along the vein is very irregular. At some places the 

porphyry is thick and at others thin; at still others it is wanting entirely. It may occur in 

irregular bunches or in streaks or slices iliclosed between movement planes. The relations of 

these different masses of porphyry to each other and to the vein shows that the horizontal fault 

movement along the porphyry dike has resulted in shredding the dike, so that in some places 

along the fault there is no porphyry, while in others the faulted portions overlap and increase 

largely the original thickness. 

Inasmuch as the whole of the porphyry dike, being a weaker zone included between stronger 

gneiss and granite walls, has been crushed by the faulting, there are numerous fault planes running through it or along it, and along any or all of these ore has formed (fig. 34).· Thus veins 

may occur on either contact of the porphyry or on neither, or it may occur in parallel streaks 

FIG. 35.-Horizontal sketch of brunch of Pelican vein, Pelican tunnel 

level, showing fault nature of vein channel. a, Quartz (gangue); 

b, granite; c, blende; d, gneiss; e, pyrite. In the center of the vein 

are vugs. 

with porphyry cin both walls. These veins 

may overlap without connection, they may 

split and reunite, or they may run parallel 

to one another. Inspection of the various 

levels shows that the relation of these veins 

to one another and to the porphyry is con~ 

stantly changing and that a vein may be at 

one point entirely· in porphyry, but may 

change 11long the strike or dip, or both, to 

the porphyry contact with the granite or 

gneiss. 

On account of the nonpersistence of these 

separate veins the whole dike has in the past 

been frequently considered as forming a single vein. While this is hardly true, yet it 

essentially forms or coincides with the relatively narrow vein zone, ~he different veins 

in which are generally too intimately related 

to be separat~d. These peculiarities are seen 

in the geologic maps of the separate mine 

levels. In places only the different veins 

within the porphyry dike remain separate 

and· distinct for a considerable distance. 

Thus on the Zero level two parallel veins, both in porphyry on both walls·, and lying about 70 

feet apart, run alongside each other for a distance of approximately 1,200 feet. In this case 

the northeast vein has been called the Bismarck, the southwest one the Pelican. On this 

level, where these two veins leave the porphyry dike and pass into granite and gneiss on the 

northwest and on the southeast, they seem to have resolved themselves into a single lode. 

STRUCTURE OF VEIN. 

Zones or planes of fault movement constitute the "lead" in these mines, and these fault 

zones may or may not be mineralized, the occurrence of ore being irregular (fig. 35). Where 

ore does occur the general structure is similar to that of the barren portions of the lode. In 

general the veins have a single good wall representing the main slip. On one side of this, 

generally above it, there is a rather indefinite zone several feet wide, consisting of fractured, 

broken, or completely crushed rock, usually decomposed and silicified. The best and the 

most continuous ore is to be looked for along the main wall or slip, and the adjacent zone of 

broken rock may be barren or it may be seamed or reticulated with ore. The opposite side 

of this zone is usually indistinct. 
Dip of vein 

OL-_________ 2_0L0 __________ 4J0_0 __________ 6~0-0 _________ ~00 foot 

Granite wall rock .Pitch of movement striae 

(from horizontal ) 

Alaskito porphyry Mineralized lead Unmineralize d lead 

PELICAN TUNNEL LEVEL 

DIAMOND TUNNEL 

ZERO TUNNEL LEVEL 

PELICAN TUNNE
U. S. GEOLOGICAL SURVEY PROFESSIONAl. PAPER NO. 63 PL. XXVII 

Stopes shown by shaded arc3$ 

VERTICAL LONGITUDINAL PROJECTION OF PELICAN-BISMARCK VEIN. 

Adapted from a projection by 0. 0. McReynolds, J une, 1903. 
MINES OF THE SILVER PLUME DISTRICT. 189 

AB a rule the ore appears to have formed mostly by replacement near cracks, as is shown 

both by field and by microscopic study (fig. 36). There are, however, numerous places where 

irregular cavities or druses have been filled with sulphides and gangue. . 

COMPOSl'l'ION OP. VEIN. 

The principal metallic minerals of the vein are blende and galena, with locally considerable 

pyrite. The chief gangue mineral is quartz, with some barite, calcite, and, especially near 

the surface, siderite. The galena and blende are both argentiferous, and the ore contains a 

little gold. In 1896 the average value of the ore shipped from the mine was $75 per ton. This 

ore contained 150 ounces of silver, 0.1 ounce of gold, 14 per cent of lead, and 20 per cent of 

zinc, the remainder being mostly silica and iron.a Sulphides of silver, including argentite, 

po'lybasite, and pyrargyrite, also occur, especially in the upper levels. Native silver was common near the surface. A specimen of this ore from 100 feet above the Pelican level, about 

e d c e r 

o ._ ----'---~--_.3:.._ _ __.4 feet 

FIG. 36.-Sketch of Pelican vein, Pelican tunnel level. n, Main lead; b, ore, chiefly blende; c, ore zone of brecciate<l, 

highly silicified alaskite porphyry; d, highly fractured silicified alaskite porphyry; e, sheeted silicified alaskite 

porphyry; j, altered granitic gneiss. 

4,000 feet in, and at a depth of 1,550 feet below the surface, shows predominating blende, 

with coarse galena, some polybasite or tetrahedrite, and a little dark-red ruby silver. The 

polybasite and pyrargyrite do not show any different origin from the galena or blenclc, but 

seem to be contemporaneous with them, so far· as one canjudge. 

LOCA'l'ION OF ORE llODIES. 

It is plain from a study of the stope projection (Pl. XXVII), combined with inspection 

of the available underground workings, that the ores lie chiefly in a definite shoot. The whole 

length of the vein, inclilding branches,·which has been developed underground is about 7,200 

feet from the northwest end of the Pelican tunnel to the ABhby tunnel. Most of the ore, however, came from a shoot that extended from 500 to 800 feet along the vein and was continuous 

from the surface vertically down to the Bonomini level, and from that level with an offset to 

the southeast developed clown to the lowest workings below the Diamond tunnel. The upper 

part of this shoot coincides with the junction of the main vein with the Seven-Thirty vcm. 

"Lawrence, 13. B., Proc .. Colorado Sci. Soc., vol. 6, 1897, p. 44. 
190 GEOLOGY OF THE GEOHGETOWN QUADHANGLE, COLORADO. 

At the surface this junction comes approximately at the outcrop of the shoot. On the Hercules 

tunnel level of the Seven-Thirty mine the shoot extends from the branching ,point for over 

SOO feet northwestward along the Bismarck lode. On the SO-foot level of the Seven-Thirty 

mine a shoot also begins at the junction and extends toward the northwest, and the same thing 

is seen in the 140-foot level. On the 210-foot and 275.:.foot levels of the Seven-Thirty the 

general relation of the junction to the ore shoot is to all appearances practically the same, 

although the actual il).tersection has not been proved. Below the 275-foot level the junction 

of the Seven-Thirty with the Bismarck has not been detected, and the Seven-Thirty levels 

have not been driven so as to approach anywhere near the main vein. However, tho SevenThirty is known to extend in depth probably as far down at least as the lowest workings on 

the Pelican-Bismarck lode, and its almost perpendicular attitude renders it easy to project 

the approximate point of its intersection with the main lode. On account of the steep southerly 

dip of the Seven-Thirty this intersection would pitch southeastward on the Bismarck, corresponding in general to tho pitch of the main ore shoot. (See Pl. XXVII.) 

It seems from this coincidence that the junction of the Seven-Thirty with the main PelicanBismarck lode bears a causal relation to the principal ore shoot on this lode, since tho shoot 

on· tho Bismarck reaches from the intersection 500 to 800 feet farther northwest. Yet the 

Seven-Thirty vein is not enriched near the junction; on the contrary, this junction is very 

weak, and the exact point is hardly recognizable even on those levels where it is best developed. 

It is b~st shown on the SO-foot level of the Seven-Thirty. On all the Seven-Thirty levels the vein 

appears near the intersection with the Pelican-Bismarck as a weak fracture zone o,r succession 

of fractures more or less mineralized but containing as a rule no notable quantity of ore. Therefore it is quite natural that the junction has not been observed on the lower levels. 

According to report the intersection of the Wisconsin lode with the Pelican-Bismarck 

had a similar effect, producing there an important and rich ore body. Neither the intersection 

nor the extension of the ore body, however, seems to have been identified for any considerable 

distance downward. · 

A similar relation may be detected in many places at the junction of branches along other 

portions of the vein system. _On the Diamo11.d tunnel level, for example, where the main vein 

splits up into the Dives and Baxter veins, the exact junction appears not to have been detected, 

yet the Dives vein is enriched for about 500 feet southeast of the probable branching point. 

RELATION 0~' ORES TO DEPTH FROM SURFACE. 

At the surface, as reported, the ore of the principal Bismarck shoot consisted largely of rich 

soft black silver sulphides with much wire silver. The old workings also show considerable 

blonde, galena, pyrite, and siderite. At the very surface and extending down about 40 feet 

oxidized ores having the appearance of brown clay are reported. These ores arc said to have 

been the richest in the vein, containing several hundred ounces of silver. Mixed with the oxidized ores and extending downward for 200 to 300 feet, especially along water courses, are the 

soft black sulphide ores, plainly of secondary origin. These ores consist chiefly of galena and 

blonde and contain several hundred ounces of silver. The ordinary ore of the vein consists of 

intercrystalllzed coarse blende and galena, the blonde being especially predominant. There is 

also some pyrite. In many localities, as, for example, in tho Zero and Eagle Bird levels (at a 

depth of, say, 500 to SOO feet), pyrite is scattered through the entirely altered porphyry near the 

vein and seems to be secondary from original iron in the rock. Coatings of pyrite on joint faces 

in the same rock have a very recent aspect. The suggestion arises that the scanty (except near 

the surface) pyrtie in the vein is derived from the wall rock. Tho best ore bodies are said to have 

been found at a depth of 300 feet, but good ore is also reported to have been encountered to a 

depth of l,SOO feet.a 

The general appearance of the coarse blonde and galena ore is much the same from the top to 

the bottom of the mine, except that in the lowest levels the bl~nde seems to become relatively 

a Lawrence, B. B., Proc. Colorado Sci. Soc., vol. 6, p. 44, 1897. 
U.S. GEOLOGiCAL SURVEY PROFESSIONAL PAPER NO. 63 PL. XXVIII 

N 

! 

EJ Gneiss Granite Alaskite porphyry Mineralized lead Unmineralized lead Dip of vein P1tch of striae 

on walls 

o.__ ___ 2oLo ____ 4oLo ___ -'6oLo ___ s-'oo feet 

GEOLOGICAL PLAN OF DIAMOND AND ASHBY TUNNELS. 
MINES OF THE SILVER PLUME DISTRICT. 191 

more abundant. It also appears probable that the amount of silver decreases gradually with 

depth. In the upper levels much of the blende is argentiferous, but in the lower levels this 

appears to be not the ease. A large part of the argentiferous blende appears to owe its silver 

content to intergrown part.ieles of polybasite or t.etrahedrite. The lowest workings are in the 

Diamond tunnel level, at a depth of about 1,800 feet. 

NATURE OF WALL ROCK. 

The granite and gneiss which constitute the wall roek of a considerable portion of the vein 

are similar to the roeks displayed a.t the surface and in the neighboring mines of the region. The 

porphyry, though almost invariably deeomposed, was evidently originally a rhyolitic or alaskitic 

variety containing phenocrysts of quartz, feldspar, some biotite, and probably hornblende in a 

glassy or microlitic ground mass. Apatite is N w 

almost always present. "'"'~--;:-;;""""'~~""'"""'"~~~--:-7-;:---;-._--rn 

ALTJ<~KATION OP WALL ROCK. 

By alteration the porphyry has become 

white and soft. Under the microscope it is 

shown to be invariably almost entirely decomposed. The feldspar is altered to serieite and 

fine quartz, the biotite to semistite and iron 

carbonate. Near veins the wall rock is highly 

silicified. Small vein lets and oval eavities are 

common, lined with quartz and filled with a 

clear, faintly spherulitic or apparently isotropic 

material which appears to be kaolin. (See 

p. 142.) Small grains of probable fluorite have 

also been detected in these veinlcts. The 

kaolinic alteration product has been deteeted 

at various depths, at least as far down as the 

Pelican level. 

POSTMINERAL FAULTING. 

Us{Jally, as is the case with all veins of this 

region, the amount of postmineral movement, 

except in some places along the plane of the 

vein itself, has been slight. Here and there, 

however, small transverse faults, in general 

FIG. 37.-Sketch of wall of Pelican vein, Pelican tunnel level. Shows 

in gneiss, (b) a zone of fragmental material (a) consisting of a compact hard gray matrix containing angular or sub rounded fragments 

of alaskite and granite. This zone has been displaced by a fault. 

involving an offset of only a few feet, have been noted. On the Zero tunnel level such faults were 

noted in ~wo places, in one the fault being at right angle~ to the vein and in the other diagonal. 

RUBBLE ZONE IN GNEISS. 

In a single locality in the mine an interesting occurrimce was noted. This is represented by 

fig. 37, a sketch of the wall of the stoped-out Pelican vein, on the Pelican tunnel level. A straight 

zone in the gneiss, 5 inches wide, and bounded by smooth parallel walls, consists of a compact, 

hard gray fragmental matrix, containing angular or subrouncled fragments of granitic rocks. 

The nearest granite noted on this level is nearly a thousand feet northwest of this locality. 

This zone has been slightly faulted, as shown in the sketch. Its relation to the lode faulting 

is obscure, though its occurrence in the wall of the lode would suggest that it represents a fissuring 

of greater age. 
192 GEOLOGY OF THE GEORGETOWN QUADRANGLE, COLORADO. 

DIVES-DUNKIRK LODE. 

D ~;scRIPTION. 

In the neighborhood of Cherokee Gulch the trunk vein known as the Pelican splits into the 

Dives and Baxter veins. Of these the Baxt.er is more nearly in a direct line, but is the weaker of 

s w N £ the two. The two branches are exposed 

//_,.//"'/ I - I I/ - / / - d d th p ,. t l I I /-'///;:: '/ // .//" "'..._/1 \---,:, -

1 -\> 1 --.1 _.. '::,___ ,--\;-1 \ .._ un ergroun on e e wan unne eve 

'///.//.-;<---/-- \ -;~--..:-- I/ '/ d I I tl D' d l '/:::-/;://:}!/. 1>!-v<·--1----"-)1 ;--/1

\'1' 1 an moreeearyon 1e 1amon tunnez 

~~$;f~. ~\f,i.f.-':.,{~(2?J>:J:-{; level, though the exact junction is not 

--//~/"";::/./::-- '/':__/' \-.... \J //..._ \f/ seen in either place. Farther southeast :;;--/0/~::; . .--.~ '-\"'\/ t-,1 -\ \ I<,; 'I both of these principal branches split //~//~~. 1\--/,/,/ 7> ...... 1\---- \ 

'/Yf'~_$zz · 71 \.-!-..../ 1/> l.-1 ~~/ repeatedly, no porphyry dike being pres-

~/~/~/· \ \2j (_!_\l_(\<~/\ ent to locali:w and control the zone of 

f,f~~~:::- -I<_~-~).:::':;~1 faulting and fracturing. / ~;: ~}:;:-/' 0 . / ~..! J.. d. 1\ ':..-- I y On the Diamond tunnel (Pl. XXVIII) 

'!//!/'/:(~~~ . · .-~)~\tf\ J.(-::_ the Baxter vein is seen to dip almostperJij/j{;/1f/:///{-;:;;~ ·c. L;1 ~~ ~ j 1 pendicularly, while the Dives has a dip of 

f./j//'101('//(;1/(:/; lJ?fp,~ · .... ~:?;!-!), ~~~ about60°NE. Thereforetl)etwo branches 

1 I/, 1, I; 1, 1;/lj 1; 1///, ;//:: / \ 1 tend to approach each other as distance 

!, 'lift I If jf rl;l!:;;ir ,..(;?,@' \:.-.I z (-I I from the surface i:S decreased. The Dives 

/;~'lj//j / !j flf;jf //!//{if!;/· . . ··~\!Y \ (I vein has been followed continuously from 

ft~ 11 1// /; ;// / li(/(,11///;//{[~; . . ·:. ~~i~ I its junction with the Pelican to the worko ings of the Dun,kirk mine, where it is 

FIG. 38.-Vertical cross section of Dives vein and Diamond tunnel level near 

tunnel. a, Gneiss; b, scams of crystalline bien de with smne pyrite; c, zone 

of crushed granite; d, granite. Shows fault nature of vein channel and the 

formation of a vein in the fault zone. Depth below surface 650 feet. 

known as the Dunkirk vein. The same 

Dunkirk vein has been drifted on at a 

lower level, in theAshbytunnel. On this 

level, at a point west of the main tunnel, 

the vein is overlapped by a parallel vein 100 feet farther north,east. This vein also goes by the 

name of the Dunkirk vein, and has been 

followed to a point about 450 feet southeast of the tunnel. It may be a branch of 

the main Dunkirk vein, the probable point 

of junction being just west of the Dunkirk 

shaft on the Ashby tunnel level. If so, it 

is considerably the stronger of the t>vo 

branches, for the southern branch, where 

the tunnel crosscuts it, is marked only 

by a slip in the gneiss with no mineralization. 

For about a thousand feet southeast 

of the southeast end of the Ashby tunnel 

workings on the Dunkirk vein this vein 

has not been traced underground or at 

the surface. Beyond this point comes 

the Lebanon group of veins, of which 

the Morning Star and the Alhambra 

are the principal branches. 'fhe MornQri f't. on ·vein 

ing Star lode, to judge from its trend sw '-'---'---'-'-=-~--£..-'--':....l4'-:-------:-:-z::::r---->~-"-' 

and position, may perhaps be the con- 3 

tinuation of the Dunkirk lode, though 

this surmise lacks confirmation. At the 

east end of the Dunkirk workings, howFIG. 39.-Vertical section of Dives vein in Diamond tunnel level near the 

tunnel, showing vein as two parallel seams (a) of lllende and pyrite in 

granite (b). The granite between the eemns is like that on either side. 

ever, the lode is still fairly strong. The Alhambra lode, as shown on the map, is a branch 

of the Morning Star. 
MINES OF THE SILVER PLUME DISTRICT. 193 

NATURE OF VEIN. 

The Dives-Dunkirk vein has formed along a strong fault slip which has a general northwest or west-northwest trend and a northeast dip varying from 50° to 80°. That the fracture 

zone along which the vein has formed is 

actually a zone of faulting is shown by 

polished and striated slip walls, the strire 

on which have usually a very moderate 

dip :from the horizonhal, not exceeding 

30° in any case noted. This indicates 

an approximately horizontal movement . 

. Faulting is -also shown locally by discordance of the walls (fig.38). In manyplaces 

there is only one strong slip along the zone, 

but in other places there is a well-defined 

zone bounded by two good walls (fig. 39). 

These peculiarities of course apply also to 

the vein, which has formed along the circulation channels thus offered. The mineralization is somewhat irregular, having 

little to do with the importance of the fault 

movement or with the presence of one or 

two movement walls. Some portions of 

the fault zone are quite. unmineralized, 

containing neither metallic minerals nor FIG. 40.-Vertical cross section of veinlet in gneiss, Diamond tunnel near 

Dives vein. Depth about 650 feet. Shows successive deposition of quartz 

gangue. At intervals, however, all along and pyri!erous carbonate. a, Empty v:ug; b, pyriferous .carbonate; 

the explored portion of the vein, ore is c, quartz; d, silicified gneiss; e, gneiss. 

found. The ore consists of blende, galena, and p.Jrite in various proportions; and the silver 

content range~ up to about 300 ounces. As a rule the mineralization evidently grows gradually 

less with the increasing distance from the Pelican vein. The upper portion of the vein nearer 

the surface seems to have produced more ore in proportion than the lower portions. ~he Diamond 

tunnel level is approximately 500feet below the outcrop 

On the Diamond tunnel level near the Dives vein a 

few small veinlets were noted, which are interesting 

from the point of view of the pars,genesis of the veinforming minerals (figs. 40, 41). One shows a small 

gash vein which has formed ·along one portion of a 

slight fault slip in gneiss. A cavity at this point has 

been partly filled by- a succession of minerals, comprising quartz, siderite, pyrite, and galena. The filling has been accomplished by successive crystallization from the walls inward, though part of the original 

opening still remains in the form of central druses! The 

general succession of the deposition of minerals is (1) 

quartz, (2) pyrite, (3) blende, and (4) siderite; but the 

periods of deposition of all the minerals overlap. Pyrite 

occurs here and there embedded in siderite, both being 

fresh and original, although as a rule siderite is a later 

formation. A thin section of another veinlet from the 

Fio, 41.-Detail of portion of vein shown in fig. 40, showing the relations of different vein materials. a, Vug 

lined with pyriferous carbonate crystals; b, pyriferous 

carbonate; c, comb quartz; d, zinc blende; e, pyrite; 

f, gneiss ('"all rock). same vicinity as the one above described shows the vein 

to be chiefly a replaced and silicified rock, possibly porphyry, although the walls show only gneiss. 

At present the gangue is a mixture of predominant sericite with fine granular quartz and numerous small crystals of apatite. There is some kaolin._ In this material a veinlet made up of carbonates is present. The larger part of the carbonates have the--optical properties of calcite; a 

31401-No. 63-08-' -13 
194 GEOLOGY OF THE GEORGETOWN QUADRANGLE, COLORADO. 

minor part, especially near the edges of the veinlet, consists of single crystals having an elongated rhombic form and curving extinction, probably siderite. Another veinlet in the same 

slide consists of quartz, blende, and galena. 

NATURE OF WALL ROCKS. 

No porphyry was actually detected in the wall material of the Dives-Dunkirk vein. The 

wall rock is characteristically gneiss, with some granite. As a rule, the strike of the schistosity 

is oblique to the lode, the angle between the two approximating 70°. Evidently, however, 

the attitude of the gneiss did not control the direction of the lode, but the lode fracture is a 

continuation of the main fracture controlled by the Pelican porphyry dike. 

A specimen of the gneiss in the Diamond tunnel, about 100 feet north of the Dives vein, 

shows on microscopic examination a moderately fine granular structure. This gneiss contains 

predominant quartz, with. areas of sericite nearly as abundant. The sericite is secondary 

after feldspar. Subordinate primary muscovite is present, and in many places altered biotite. 

Parallel thick wavy zones of fibrolite (sillimanite) seem to determine the parting planes of 

the schist. Sillimanite in isolated crystals is also included in the quartz, as are a few small 

·unaltered biotite flakes. Small zircon crystals are present in the quartz and in the sericite areas. 

HISTORY AND PRODUCTION. 

The Dives property was located May 27, 1869, by Thomas Burr. It produced but little 

until 1871, when a large body of ore was encountered. From this time until 1880 the Dives 

was involved in tho litigation with the Pelican mine, which was terminated only by the consolidation of the two properties. 

According to C. H. Morris "the yield or production from the Dunkirk group since the 

discovery and location of the Dunkirk claim in 1868 has been some 200,000 ounces fine silver." 

This, however, does not include the returns from large amounts of ore claimed to have been 

extracted by other parties from the west end of the vein between 1870 and 1880. 

BAXTER LODE. 

The Baxter lode, which is the direct southeast continuation of the Pelican lode and has 

the same trend, is best shown in the Diamond tunnel, which it crosses at something more than 

600 feet froin the mouth. It trends northwestward and either dips very steeply to the northeast 

or is perpendicular. Just west of the Diamond tunnel a branch diverges, opening out to the 

west. The drift on the main vein has caved about 100 feet northwest of this branch, but is 

said to have been opened for several feet farther. The diverging branch is exposed by a drift 

500 or 600 feet long. Both branches are along strong fault slips. The main vein is a strong 

mineralized lead in granite, marked by seams of galena and blendo. Ore has been taken out 

also from tho branch vein. A very considerable stope shows that ore has been taken out 

on the main vein for about 50 feet east of the junction. The location of this ore body seems 

to have been dependent on intersection. East of the Diamond tunnel workings the lode becomes 

practically barren. It has been traced for a considerable distance by drift tunnels. One 

tunnel, starting in just above the road which runs above the Diamond tunnel, follows this 

lode for 500 feet, overlapping and continuing tho drift on the Diamond tunnel level lower down. 

The smaller tunnel shows the lead as a strong single-wall fault, accompanied by a crushed 

zone from 3 inches to a foot thick. On the wall, strire pitch 15° NW., indicating a nearly horizontal movement. In this tunnel a branch .leaves the vein, opening out from it toward the 

east. This branch is also a strong fault lead, with very slight indications of mineral. 

Still farther southeast the Baxter lode is probably represented by a strong barren fault zone 

in the Ashby tunnel, about 130 foot in from the mouth. This fault shows strire pitching 5° E. 

Th~re are no signs of mineralization-only a little pyrite, which may or may not be connected 

with the ore deposition. Beyond this point the lode has not been traced. (Sec Pl. XXIX.) 

According to Fossett" the total yield of this mine to 1880 was about $360,000. During 

the latter year the lessees are reported to have employed forty or fifty men. 

a l<'ossctt, Frank, Colorado, 18~0, p. 394. 
U.S. GEOLOGICAL SURVEY 

·GEORGE OTIS SMITH, DIRECTOR PROFESSIONAL PAPER NO. 63 PL. XXIX 

J05' 45 ' 

~J~ ~-,~~ ~~~-r~~~~~~ "~~TT7J-r;~~ ~~~IT~~~77~~~~ ~~~VC~~ ~~ ,-,-y--,~~-,--i7--r---77~~~~~~~~ 

26' 

·E. M. Dou g la s, Ge og ra p he r. 

Fr a nk Tweedy, in ch arge ot s ection. 

Topog r a phy by. Pea rs on Chapman. 

Tri a n g ul atio n by Fr a nk Tweedy: 

Surveyed in \904. 

:Mineral v e:in.s 

43'' 

MAP SHOWING VEINS AND PORP.liYRY DIKES IN TillE SILVER PLUME REGION, COLORADO 

Scale~ 

1000 r,ooo :feet 

Geo loRY byJ. E.S p urr and. G1-i.Ga rrey. 
MINES OF THE SILVER PLUME DISTRICT. 195 

WISCONSIN-CORHY CITY LODE. 

RELATIONS TO PELICAN-BISMARCK TRUNK VEIN. 

The ores developed in the workings of the Wisconsin and Corry City mines appear to have 

been obtained from the same lode, which is apparently a branch of the main Pelican lode. 

The point of separation lies at the surface approximately in the gulch east of Sherman Mountain. The main lode, the Pelican, here has a northwest strike, while the diverging Wisconsin 

lode has a west-northwest strike, so that farther east the distance between the outcrops of the 

two branches continually increases. The difference in strike amounts to about 25 °. There is 

also a difference in dip, the Wisconsin-Corry City lode, with an inclination of 50° to 60°, being 

considerably flatter than the Bismarck, which dips at about 75°. Owing to this difference of 

strike and dip of the two branches the junction pitches very flatly, and with increasing depth 

travels very rapidly to the northwest. On the upper levels the junction of the two veins was 

encountered. The Illinois _level, running in on the Bismarck lode, and the .Wisconsin level, 

running in on the Wisconsin lode, came together in a large body of ore which had formed near 

FIG. 42.-Verticallongitudinal projection of the Wisconsin-Corry City mine workings. 

their junction, a fact which caused prolonged conflict and litigation. In the relatively lower 

levels, however, the junction either has not been encountered or has not been recognized. Even 

on the Zero level, which runs in on the Bismarck lode about 250 feet below the Illinois level, 

the projected- junction with the Wisconsin lode would come somewhere under Sherman Mountain, at a depth of about 1,200 feet, a distance from the surface which in most of the mines of 

this district has proved to be unfavorable for extensive ore deposition. In the still lower levels 

the junction of the two lodes would extend northwestward beyond the ore-bearing zone of the 

district and would be at still greater depths. 

NA'rURE OF VEIN. 

In the upper levels of the workings which run along the Wisconsin-Corry City lode (fig. 

42), especially in the Wisconsin mine, which exposes the portion relatively near the junction 

with the Bismarck, .the vein follows a very strong fault zone, in many pl~tces several feet wide 

and marked by brecciation and slickensided surfaces. On these surfaces strire caused by movement are abm1dant and are nearly horizontal or flatly pitching, the variation measured from 

the horizontal being ordinarily not more than 20°. Thus the movement is shown to have been 
196 GEOLOGY OF THE GEORGETOWN QUADRANGLE, COLORADO. 

approximately horizontal. The fault zone is associated with a dike of light-colored porphyry, 

which may lie on one side of the fault or locally on both sides. The porphyry dike seems to 

be continuous to the Bismarck vein and to unite with the dike along which the Bismarck has 

formed. It terminates on the east, however, within a relatively short distance, near the line 

of the gulch which separates the Wisconsin and Corry City mines and undergrouiid in a corresponding position, as may be seen by the maps of the different mine levels. Plainly the location of the fault movement has been determined by the preexisting porphyry dike, which has 

be~n weaker than the inclosing rocks and has offered a zone of weakness along which slipping 

took place. The lode extends eastward beyond the termination of the dike, with gneiss and 

granite as its walls, but after leaving the dike becomes weaker, branches repeatedly, and is at 

length virtually lost. The slips or leads followed in the gneiss or granite by this lode are also 

faults whose movements have had a pronounced horizontal direction, but these faults appear 

to 'have split up and scattered, distributing throughout the rock the movement which farther 

west was taken up along the relatively narrow zone of the porphyry dike. 

STRUCTURE OF VEIN. 

The ore has formed along thin seams in the fault zone, here on one wall, there on the other, 

and elsewhere along two or more parallel slips. It is in many places irregular, shooting across 

and reticulating the fault zone, and much of it occurs in blotches. As a rule the dimensions 

of the vcinlets are small, not more than a few inches. There arc no traces of mineral along 

many parts of the lead, but again in favorable localities mineralization has occurred. The vertical longitudinal projection of the vein (fig. 42) shows two distinct ore shoots, one on the Corry 

City, beneath Cherokee Gl!lch, and one on the Wisconsin, near or at the junction with the 

Bismarck vein. 

COMPOSITION OF VEIN. 

The ores contain lead and zinc in the form of intercrystallized galena, blende, and pyrite, 

with some tetrahedrite or polybasite. They run well in silver, the best ores containing several 

hundred ounces. 

NATURE OF WALL ROCK. 

The wall rock consists of porphyry, gneiss, and granite. The porphyry was originally 

siliceous and rhyolitic or alaskitic, containing phenocrysts of feldspar, biotite, and in places 

quartz, in a fine-grained groundmass. Accessory apatite is present. The rock was essentially 

like the porphyry accompanying the Bismarck vein, from which it is probably a branch. It 

is at present entirely altered. AL'l'ERA'l'ION 0~' WALL ROCK. 

The wall rocks are everywhere thoroughly ~ltered. The porphyry in the hand specimen 

is white and soft, or where more thoroughly mineralized is gray and siliceous. Under the 

microscope the feldspar phenocrysts are found to have been entirely decomposed to fine quartz 

and sericite; the biotite phenocrysts to quartz, sericite, cloudy carbonates, and· pyrite. The 

groundmass of the porphyry has been altered to quartz, sericite, and fine crystalline carbonates. These carbonates do not effervesce with dilute hydrochloric acid, and are therefore 

not calcite. 

INFLUENCE OF WALL ROCK. 

As stated above, the best part of the vein is that which accompanies the porphyry dike, 

or is not far removed from it, the reason appearing to be that the whole fault movement has 

been united where it runs along this dike, but has been separated into different parts where 

it passes into a more homogeneous mass of granite or of gneiss. In detail the different wall 

rocks seem to exercise some influence on the character of the lode here, ·as in other lodes of 

the district, the influence ·depending apparently on the relative rigidity of the different rocks .. 

Along this lode there is no definite rule. In the upper ·wisconsin tunnel the lode contains 

ore at the contact of porphyry and gneiss, but becomes barren on passing into porphyry 
U. S. GEOLOGICAL SURVEY 

A. SEVEN-THIRTY MINE, IN BROWN GULCH, LOOKING NORTH-NORTHEAST FROM 

GRIFFIN MONUMENT. 

B. BURLEIGH TUNNEL AND DIVES-PELICAN MILL. 

, , Burleigh tunnel; 2, Pelican tunnel. 
MINES OF THE SILVER PLUME DISTRICT. 197 

entirely. In the lower Wisconsin tunnel, however, tlie reverse is the case, as the maps show. 

It is probable th:;tt in the upper tunnel the porphyry was locally decomposed, and thus relatively plastic and impervious at the time of. the ore deposition; and that in the lower tunnel 

the rock was locally more .rigid and so capable of maintaining open fractures for the mineralizing solutions. The general order of events in this and the neighboring mines seems to have 

been (1) fracturing and slight faulting, producing circulating channels; (2) alteration of wall 

rock and partial softening of the porphyry; (3) ore deposition. 

CHANGES OF ORE IN DEP'rii. 

The greater portion of the ore taken from this lode has been obtained from the surface 

clown to a depth of 300 to 500 feet. Below this depth there was very little. At the very su~­

face dumps show probable rhodochrosite and other carbonates with very little sulphides. In 

the lower Wisconsin tunnel, 300 feet below the surface, the ore for the most part appears 

. primary, consisting of coarse intercr:vstallized galena and blonde with some pyrite. This ore 

contains a fair amount of silver. Some of the ore, however, shows along abundant crevices 

dull-black soft secondary su1phides, characteristic of the recent work of descending waters. 

This constitutes some of the best ore, and is very apt to occur in wet places in the mine. In 

the upper Wisconsin tunnel the deposition of ore which apparently constitutes the main vein 

·on top of the flatly dipping dike of impervious altered porphyry, suggests that the original 

deposition may also have been due to descending waters. In connection with these observations it is interesting to note· the position of the Corry City ore body beneath Cherokee Gulch 

(fig. 42). The belief that the best ores occur where ther~ is water and that the position of a 

vein directly beneath a gulch is especially favorable to good ore is common among Georgetown miners. The lowest level of the vein is in the Diamond tunnel, which is about 750 feet 

below the lower Wisconsin tunnel. Here the lode is very poorly mineralized, containing practically no filling whatever for considerable distances. In places, however, it contains a relatively large quantity of pyrite with some blonde. This pyrite carries no values. The fact 

.that the porphyry has been thoroughly altered throughout and the original iron which it contained leached out of it, together. with the presence of this barren pyrite, suggests that the 

pyrite may have been derived from the porphyry. 

SEVEN-THIRTY LODE.· 

LOCATION AND HISTORY. 

The Seven-Thirty mine, which is one of the group of mines owned by the Dives-Pelican 

a:r;J.d Seven-Thirty Mining Company, is on. an approximately east-west vein which crosses 

Brown Gulch at a point about 10,500 feet above sea level (Pl. XXX, A). The vein makes a 

junction ,~ith the Bismarck vein just east of the summit of Sherman Mountain; it then runs 

down the southwest slope of Sherman Mountain, crosses Brown Gulch,. and runs westward 

along Brown Mountain. 

The Seven-Thirty mine was first located about 1870 as a single claim. Numerous other 

claims were acquired by the owners, who, in 1896, transferred the entire group. to the owners 

of the Dives-Pelican mine. The purchasers immediately pulled up the pumps and allowed 

. the property to fill with water to the 275-foot level, from which level the water runs through 

the Dives-Pelican workings. This property, which has always been worked on the leasing 

system, has at various times in its history given employment to a force ranging from ·so to 

80 men. 

DEVELOPMENT. 

At the present time the Seven-Thirty vein is worked entirely through the Hercules tunnel, 

which runs about 150 feet due north from Brown Gulch before it intersects the vein. Above 

the Hercules tunnel level :five tunnels or drifts are driven east on the vein and three west. 

Work in these upper levels, however, was long ago discontinued. Below the Hercules level 
198 GEOLOGY OF THE GEORGETOWN QUADRANGLE, COLORADO. 

drifts have been driven on eleven levels both east and west of the Seven-Thirty shaft, which 

is sunk from the Hercules tunnel level to a depth of 975 feet: This shaft, which is on the vein, 

has a general dip of about 80° S. Near the end of the Burleigh tunnel vvhat is probably the 

Seven-Thirty vein has been cut, at a depth of 290 feet below the bottom of the shaft. 

The development in the lowest levels of the Seven-Thirty vein is not nearly as great as 

above, owing probably as much to the greater leanness of the ores at that depth as to the 

greater cost of production and the later period at which the development was undertaken. 

The aggregate length of the workings on this vein,· not including numerous raises and 

winzes, is between 5 and 6 miles. These figures represent the total amount of the original 

workings. Many hundreds of feet of workings, however, have subsequently been closed by 

cavmg. The owners are at present driving a crosscut from the Burleigh tunnel (Pl. XXX, B) 

to come under the Seven-Thirty shaft and join a continuation of that· shaft. The Burleigh 

tunnel is located on·a level only a little above Clear Creek, and by connecting with the SevenThirty shaft the workings below the 275-foot level will be drained of water, and the cost of 

ore transportation will possibly be diminished to such an extent that the low-grade ore bodies 

can be worked with profit. PRODUCTION. 

The yield of the vein from January 1, 1887, to January 1, 1896, was as follows: 

Year. 

Production of Se·ven-Thirty mine, 1887-1895.a 

Ore 

mined. 

Silver pro- Lead 

duced. produced. 

Average assay. 

Silver. Lead. 

Value. 

----------------·---.: ~----l-----·-------------

Tens. Ounces. Tens. Ounces. Per cent. 

1887 .............................................. ··················· 7G~. 4G 105, il7. 00 72.44 1~8 9.6 $83, G42. 91 

1888 ................................................................ . 98!. 39 134, il8. 47 85.43 151 9. 5 103,277.56 

1&~9 ................................................................ . 71!. 87 106,197.08 98.00 119 13.0 81.614. 39 1890 ................................................................ . 1, 303.81 185,047.89 180.40 J.ll 13.8 165,554.66 1891 .............................................................. . 1, 849.76 304,930. 40 183.19 164 10.0 248,659.42 1892 ................................................................ . 1893 ................................................................ . 1894 ................................................................ . 

1, 168. 45 111,806.63 31:25 g5 11.0 74,218.25 341.70 72,962.84 49.07 214 14.0 46,788. 67. 

485.65 92,337.95 71.37 190 14.0 41,902.75 1805 ................................................................ . 

1-----1---~--!-----1----

5!i0. 48 120,654.26 71.27 215 1:l.o GB, 420.82 

8, 076. 57 1, 234,372.521 942.42 154 11.5 914,079.43 

a From report on the mine by 0. 0. McReynolds. 

The following approximation was made by 0. 0. McReynolds in a report on the mme 

drawn up in 1903: 

Approximate total production of Seven-'l'hirty mine. 

Prior to 1879 ...................................... · ............................... . 

·From 1879 to 1887 ................................................................ . 

From 1887 to 1896 ................................................................ . 

During 1896 ...................................................................... . 

$151,617.00 

743,024.54 

914,079.43 

60,000.00 

1,868,720.97 

From 1896 until recently practically no active work was done on the Seven-Thirty vein. 

NATURE OF WAI,L ROCK. 

Granite, gnmss, pegmatite, and porphyry form the wall rocks. Of these granite is predominant. 

Granite.-The western part of the developed vein lies almost entirely in granite, which 

also occurs irregularly along the eastern portion. This granite (Silver Plume granite, pp. 58-60) 

is a medium-grained rock containing considerable biotite and usually showing small feldspar 

phenocrysts. In some places this porphyritic structure is more strongly developed by an 

increase in size of the phenocrysts and a diminution in size of the individual crystals of the 

groundmass, forming the "corn rock" of the miners. 

V'\Therever seen in the mine workings this granite is more or less decomposed. The freshest 

specimen examined microscopically contained original feldspar, biotite, Iimscovite, magnetite, 
MINRS OF THE SILVER PLUME DISTRICT. 199 

apatite, and pyrite. Microcline is abundant and very fresh. Another variety of feldspar is 

mainly altered to sericite; undecomposed portions show twinning striations and possess optical 

properties indicating andesine-oligoclase or oligoclase. Small idiomorphic crystals of largely 

sericitized feldspar, inclosed in microcline, probably belong to the same species. The biotite is 

nearly fresh, with some developments of sideritic material. The magnetite and pyrite are 

associated together. 

Gneiss.-The eastern part of the vein passes through gneiss, which varies considerably in 

structure and composition. Some portions are firm and siliceous; others are fissile and biotitic. 

Many of the schist laminre are crumpled, but the average strike is N. 40° W. and the average 

dip 65° NW. The gneisses inclose seams of pegmatite, porphyritic granite, and (in one place) 

graphlc granite. . 

A specimen of firm, biotitic gneiss, examined microscopically, proved to consist of primary 

quartz, biotite, sillimanite, feldspar, pyrite, and muscovite. The feldspar is partly fresh microcline and partly another feldspar which has been almost entirely altered to sericite. 

Porphyry.-The vein cuts diagonally across a porphyry dike in the vicinity of the SevenThirty shaft. This dike is 50 to 80 feet wide and dips flatly east-northeast, so that it lies between 

200 and 300 feet west of the shaft on the SO-foot level 

below the Hercules tunnel and between 225 and 375 

feet east of the shaft on the 275-foot level below 

the tunnel. The unaltered rock of this dike 1s a 

grayish quartz monzonite porphyry, containing 

phenocrysts of feldspar,· quartz, and biotite. 

NATUR!C OP V~~IN. 

The Seven-Thirty vein is a branch of the Bismarck, and although its junction with the Bismarck 

is inconspicuous, in many places obscure and attended by no well-marked effect on ore deposition, 

yet the productive portion of the Bismarck does not 

extend far to the northwest beyond the point where 

the Seven-Thirty leaves it. The last ore body of 

importance on the Bismarck is about 800 feet 

northwest of the split, but ore was found in the 

Seven-Thirty for 2,500 to 3,000 feet west of the 

branching point. Farther west the mineralization 

becomes obscure. It would therefore seem that the 

a b 

"-Q ----'~---"4 feet 

FIG. 43.-Sketch of barren lead (slip) on ITcrcules tunnel 

level. Seven-Thirty mine. a, Pegmntite; b, diorite. The 

discor<ln.nce of the roch-s on the two sides of the lead shows 

faulting. 

mineralizing solutions which circulated along the Bismarck lode openings were principally 

deflected into the Seven-Thirty lode openings at the branching. 

The veiJ;J. follows a line of movement in the rocks. The amount of movement is small, 

even though the openings created were sufficient to form a channel for mineralizing solutions 

for a horizontal distance of over half a mile. It was not sufficient to form a definite and continuous single fault or fracture zone, however, but resulted in a series of minor east-west fractures which run into and tie· on to one another to form the general vein channel. Similar 

fractures .branch off from the vein channel, but as they were not in the main string ·or line of 

openings these were but slightly mineralized or not at all (fig. 43). 

· Inasmuch as the veins lie along a composite fracture zone the strike and dip vary much 

locally. The strike ranges from east to northeast and th~ dip, while everywhere steep, changes 

repeatedly from south to north, the angle varying.from 80° N. to 55° S., although the average 

is about 80° S. The weakness of the movements which produced the fractures is also shown 

by the constant deflection .of strike and dip, due to the nature of the rocks which the' fractures 

traverse and to the changing of the vein from one fractu~e to another (fig. 44). There is, 

however, some slight faulting along the fractures, shown by discordances of the wall rock on 
200 GEOLOGY OF THE GEORGETOWN QUADRANGLE, COLORADO. 

either side. Slickensides and movement strire are also numerous and their trend varies from 

horizontal to a maximum pitch of 25° E. 

On account of the nature of the vein channels above described and the lack of definite 

and regular movement zones the vein which has formed does not possess. definite walls. The 

circulation openings were, wherever observed, mere cracks, and from these cracks .the mmB Mineralized lead Unmineralized lead 

FIG. 44.-Pian of portion of Seven-Thirty vein, 80-foot level, showing deflection of mineralizing solutions by different 

fracture channels. 

eralizing solutions penetrated the wall rock for a certain distance on each side, tllis distance 

depending on the amount of circulation at each point. The first result was a bleaching and 

alteration of the wall rock on both sides of the crack, and this effect is found along practically 

all fracture planes in the mine (fig. 45). The next step was the impregnation of the altered 

country rock with metallic sulphides, and finally the more or less complete replacement of the 

FIG. 45.-Vertical ~etch on wall of stope, Hercules level, 

Seven-Thirty mine, showing joints (b), in dark biotite 

granite (a), bordered on each side by a regular bnnd of 

bleached :1nd silicified granite (c). These bands form 

definite veins (single center wall veins). The increase 

in thickness of these veins causes them to unite to form 

a larger mass. 

rock by vein materials. As a rule the effects of this 

alteration extend to a regular distance on each side of 

the crack which has supplied the solutions, and so a 

definite band of altered granite, or (in the first stage) of 

ore, results, with, however, no definite division plane 

between it and the country rock. Veins of this type 

are numerous and important in the Georgetown region. 

In the Seven-Thirty mine the feeder crack is usually in 

the center of the resulting vein, but in other mines, 

described elsewhere, the alteration has been mainly on 

one side of the feeder only, producing a vein with a single 

side wall. Veins of this type may be called single-wall 

veins, to distinguish them from the more famiUar doublewall veins, which consist of a definite vein zone bounded 

by hanging and foot walls; and these single-wall veins 

may be defmed as single center-wall veins or single sidewall veins, according to the position of the feeder crack. 

STRUCTURE 0~' VEIN. 

The detailed structure of the vein depends on· the 

method of its formation, just described. In considerable portions of the underground workings the 

miners have followed rather poor "leads" consisting 

of strong fractures with or without considerable mineralization (fig. 46). These fractures 

may become mineralized, and the mineralized belt on each side may increase in thickness 

sufficiently to form an ore body. As a rule, however, an ore body is due to a bunching of mineralized fractures. Such a bunching is frequently seen at the intersection of two or more 

principal fractures, or it may take place along a single strong fracture by the local thickening 
U.S. GEOLOGICAL SURVEY 

w 

Tunnel level 

: ........... 80- fooT level 

·----·--·· 210-foot level 

L ···---- 275-foot level :(f) 

: ~ i·· ·';·---- 37C>-foot level 

: \ 

~----- ·:..:.·· 47C>-fooT level 

: :r 

t····--· ';i .. 5 75-foot I eve I 

:---·-·--- "i.-- 675-foot level 

~-- --- ~----· 775-foot level 

t··-------------- 875-foot level 

··· ··-··--·· ·- ........... 975-fool level 

Cross section of main Seven -Thirty shaft 

Dip so uth 

930' 

Sloped area 

N'!5L£V£L EAST 

210-F r LEIIEl. 

725 ' 

0 ?00 

shown thu s~ 

VERTICAL PROJECTION SHOWING WORKINGS ON SEVEN-THIRTY VEIN. 

Adapted from a section by 0 . 0 . McRevnolds, Jun e, 1903. 

PROFESSIONAL PAPER NO. 63 PL. XXXI 

E 

5~((~ 

275 · 

1441' 

<00 600 800 1000 feet 
MINES OF THE SILVER PLUME DIST.RICT. 201 

up of the fracture field. The latter condition produces a lenticular ore body. In such a field 

of close-set fractures th~ altered and mineralized zones bordering the individual cracks may 

broaden till they unite. 

The ore is thus almost entirely due to replace- N w s e: 

ment of the granitA (fig. 47). In such ore, seen 

under the microscope, the quartz has the retiform 

structure characteristic of replacement quartz. a 

COMPOSITION OP VEIN. 

The gangue material is chiefly quartz with 

local barite. Calcite veinings occur, but it is 

doubtful if they form part of the original gangue. 

The metallic minerals consist usually of galena, 

blende, polybasite, and tetrahedrite, with a little 

pyrite, some of it cupriferous. These ores contain 

a relatively large amount of silver and a small 

amount of gold (0.1 ounce .to the ton). As shown 

by the table on page 198, the average value of all 

the ore shipped during the nine years between 

1887 and 1895 was $113.17 per ton, and each ton 

carried an average of 154 ounces of silver and 11.5 

per cent of lead. Several mill runs have also 

yielded between 1,400 and 1,500 ounces of silver to 

the ton. The blendeismainly dark brown, but some 

of it is yellowish or ·light-brown "resin blende." 

It is held by the miners that where the blende is light 

colored, the silver content of the ore is greater. 

FIG. 46.-Sketch of breast of drift, Hercules level, Seven-Thirty 

mine, showing joints in granite, of which some have broadened 

into thin zones inclosing a few inches ofunrn!ncralized crushed 

granite. The walls of the zones usually show evidences of 

horizontal movement. These zones constitute the barren 

lea,ds followed by miners. 

N s Native silver in flakes is occasionally found. 

FIG. 47.-Vertical section of wall of stope, Hercules level, 

Seven-Thirty mine, showing formation of ore body by 

replacement of fractured granite at junction of main lead 

with minor.lcads. 

Argentite and pyrargyrite have also been reported. 

INFLUENCE OF DIFFEREN'r WALL ROCKS. 

The different wall rocks through which the vein 

passes influence the character of the vein, but, so far · 

as noted, on account of their physical and not their 

chemical peculiarities. 

Gmnite.-'Ihe comparatively fresh and rigid 

granite lias evidently afforded the plainest and 

most persistent fractures, as a result of the applied 

pressure. Iri. the granite (fig. 47) the vein runs due 

east and west, is usually well marked, and contains 

considerable ore. 

Gneiss.-vVhere the vein o'penings cross gneiss, 

they have been deflected from an easterly to a 

northeasterly trend. That this deflection is due to 

the gneiss is evident, for if the vein reaches granite 

again, after passing through gneiss, it resumes its 

normal east-west course. As the prevalent ·strike 

@f the gneiss is northwest, the result of the deflection 

seems to have been to make the trend of the vein 

fractures more nearly normal to this strike. 

The fissile character of the gneisses seems to have been unfavorable to the strength and persistence of the movement fractures, the movement having a continual tendency to be deflected 

a Spurr, J. E., Mon. U.S. Geol. Survey, vol. 31,1898, p. 218. 
202 GEOLOGY OF THE GEORGETOWN QUADRANGLE, COLORADO. 

and to be taken up in part along the schistosity planes. This action, of course, depende~ in 

brge part on the relative rigidity of the gneiss, which varies very widely in different portions. 

In the softer schistose gneisses the fractures were pour and veins are apt to be small and to split 

up continually; while in the hard gneisses the conditions were as fi!:vorable as in the granites. 

The miners of this mine as a rule regard the schistose gneiss with disfavor. 

Porphyry.-The porphyry dike which the vein cuts across, near the Seven-Thirty shaft, 

has no connection with the origin of the vein. The vein, as before stated, is a branch of the 

northwestward-:trending.Bismarck vein, whose direction is controlled by another porphyry dike. 

The dike which the Seven-Thirty crosses has also a northwest strike, so that the e;tst-west vein 

crosses it obliquely. The effect of the dike has been to tend to deflect the vein fracture in a 

direction normal to the dike walls-a tendency especially shown on the 275-foot leve_l (fig. 49). 

As a rule the fractures in the porphyry have not been so strong as in the granite and hard 

gneiss, and the vein is poorer (fig. 48). · 

ALTERATION OF WALL ROCK. 

The bleaching and alteration of the wall rock, noticeable along the fractures in the granite 

gneiss, or porphyry, is especially marked to the naked eye by the bleaching of the biotites and 

the alteration of the feldspars. This alteration was especially studied microscopically in the 

m : b 

FIG. 48.-Horizontal plan of portion of 80-foot level, Seven-Thirty mine, showing diminution of vein on passing through 

porphyry. a, Quartz monzonite porphyry; b, granite. 

case of the granite. Of the· original minerals of the granite, quartz, microcline, and muscovite 

are invariably unaltered. Other feldspars and biotite have been more or less completely altered. 

Andesine-oligoclase and oligoclase are often found largely altered, and in much of the rock are 

undoubtedly entirely altered; and orthoclase appears to have been entirely altered in all cases. 

The decomposition products are chiefly sericite, with some fine quartz, kaolin, ferriferous carbonates, and epidote. In these masses of alteration products fluorite crystals were detected 

and in one specimen a speck of native gold. Fluorite was also detected in other parts of the 

rock, among other places filling cracks in fresh microcline, and extending from the veinlet into 

the microcline as single isolated crystals; also filling cracks in quartz. Although some of this 

fluorite simulates the habit of an original mineral, its general relations leave no doubt as to its 

secondary nature. Marcasite and probable pyrite also occur as secondary minerals. The biotite is in general almost completely altered to sericite and a brown carbonate, in part, at least, 

siderite. 

A specimen of the porphyry near the vein also proved to be entirely decomposed. Of the 

phenocrysts, quartz, feldspar, and biotite, the last two were entirely altered. The feldspar was 

altered to sericite, kaolin, carbonates, kaolinic matter; quartz, etc. 

CHANGES OF ORE IN DEPTH. 

The oxidation of the ores extends down about 40 feet below the surface. (See Pl. XXXI.) 

The oxidized material looks like brown clay, much of it strained reddish, yellowish, or greenish. 

It occurs along cracks and watercourses, and is by far the richest ore of the mine, containing'several 
N 

200 400 

------. Gneiss 

HERCULES TUNNEL LEVEL 

140- FOOT L EVEL 

275- FOOT LEVEL 

1000 feet 

Porphyry 

( a laskite) 

GEOLOGICAL PLANS 

Granite 

(Silver Plume) 

PROFESSIONAL PA PER NO 

Mineral ized 

lead 

LEVELS, S EVEN-THIRTY 

EJ 

Unmineralized 

lead 

MINE. 

. 63 PL. XXXII 
MINES OF THE SILVER PLUME DISTRICT. 203 

hundred ounces of silver. Below this oxidized material, and in many places mixed with it, come 

sulphides, typically easily broken, powdery, and black. This material is also rich, and contains 

several hundred ounces of silver. A sample taken frmn the Bismarck vein on the Hercules tunnel level (see Pl. JG'C..:\:Il) of the Seven-Thirty, at a point not far below the surface, was examined 

by Doctor Hillebrand. It was found that the metallic minerals were almost wholly if not 

altogether in the sulphide condition. "The chief mineral is sphalerite, the next may he galena, 

though its disti~ctive features are not recognizable. There is a silver-antimony mineral which 

may be tetrahedrite, though possibly these elernents belong with the lead in a complex sulphantimony mineral. The only detectable oxidized mineral is a little lead sulphate. There is 

no silver chloride and no phosphate." 

Below these soft crumbling sulphides (called "sulphurets" by the miners), the solid sulphides continue to the bottom of the mine. From the surface down the amount of lead and 

silver in the ore is said to decrease and the zinc to increase. 

~ a 

Mineralized lead Unmineralized lead 

FIG. 49.-Horizontal plan oCportion of 275-foot level, Seven-Thirty mine, showing tho effect of softened porphyry dike on 

vein. Vein is deflected by dike and is impoverished, the dike having been a Jess rigid medium for fracturing than either the 

hard gneiss on the northeast or the granite on the southwest. · 

The ore was said to have been good clown to the sixth or seventh level. Below those levels 

it consisted of zinc-blende and pyrite ores, poor in lead and silver. The lowest level in the 

Seven-Thirty workings proper is 075 feet below the Hercules tunnel, and this is also practically 

the bottom of the shaft. About 920 feet lower, or 1,800 feet belm\r the Hercules tunnel, the 

vein, where cut in the Burleigh tunnel, shows only a seam of blende three-fourths of an inch 

thick. The lead here is in hard gneiss, with patches of pegmatite and granite. An unsuccessful 

attempt was made to follow it along the strike, but it becomes umnineralized and splits repeat 

edly into fractures passing into the gneiss. (See Burleigh tunnel plan, Pl. XL\:VII, p. 220.) 

DENVER LODE. 

The principal outcrop of the Denver lode lies 500 or 600 feet north of the Wisconsin-Corry 

City lode, with which it has a general parallelism. It is developed chiefly by a tunnel which 

follows the lode for a distance of 800 feet. The lode is traceable on the surface for the same 

distance that it is followed in the tunnel, and also is recognizable on the opposite or east side of 

Cherokee Gulch, making a total cli-;tance of nearly 2,000 feet. On the west the outcrop of the 
204 GEOLOGY OF THE GEORGETOWN QUADRANGLE,· COLORADO. 

lode passes into the broad depression filled with glacial drift which lies east of Shern1an Mountain. On the 'opposite side of this depression, on the side of the mountain, appears a lead or a 

series of leads going up the slope in the direction of the Bismarck lode. While no connection of 

these leads with the Denver lode exists, nothing corresponding to an extension of the Denver is 

found elsewhere, and it is possible that the leads in the two localities belong in one and the same 

zone. 

In the Denver tunnel drift the lode strikes west-northwest and dips to the north at an angle 

ranging from 55° to 75°. The vein is along a fault lead on whose walls strire dip 5° W. The 

rock is mostly gneiss with a little porphyritic granite. The lode for the most part seems to cut 

across the laminre of the gneiss at right angles. In general the lode is a strong one and still 

appears strong at both ends of the tunnel (fig. 50). The filling is largely quartz, which coats the 

walls of fissures in the lead, but is principally massive-the result of replacement along the fault 

·zone. The metallic minerals are chiefly blende, pyrite, and galena, which are in general later 

than the quartz and occur in many places as crystals lining druses and coating the earlier quartz 

crystals. In other places seams ofmassive ore traverse granite with no signs of crustification. 

Locally the ore has been raised qn somewhat, hut probably no great amount has been extra~ted. 

~ ' ' B D Gneiss Granite Pegmatite Mineralized lead Unmineralized lead 

.FIG. 50.-Plan of tunnel on Denver lode, showin~; geology. 

On the Hercules tunnel level of the Seven-Thirty and Bismarck mines the Kinney crosscut 

runs northeastward from the Hercules drift on the Bismarck lode and cuts what is known as the 

New vein, or New North vein, at a distance of 344 feet. This point is about 600 feet below the 

stirface. The New vein strikes a little north of east and dips 45° N .. It is entirely in granite 

It was drifted on along this level for about 300 feet. It resembles the Seven-Thirty vein in consisting of a connected- string of branching veins, many of them running into one another at an 

angle. These slips show movement strire, which were noted in one place dipping 15° to 25° E. 

on the fault plane. Scattered sulphides and some stringers of blende and galena occur along 

these slips. A shipment of 4,260 pounds of ore from a raise on this drift is recorded (May, 1903) 

as yielding 261 ounces of silver, 14 per cent of lead, and 16 per cent of zinc. The position of this 

vein renders it possible that it may be the same as the Denver lode, on the one hand, and as the 

series of leads above noted which run up the east side of Sherman Mountain toward the 

Bismarck vein, on the other. This vein has not been encountered in any of the other workings 

of the Dives-Pelican mine, and indeed on the lower levels its relatively flat dip would carry 

it rapidly farther and farther away from the Bism~rck-Pelican vein. 

PAY ROCK LODE. 

HISTORY AND PRODUCTION. 

The Pay Rock lode (Pl. XXXIII), which was discovered in 1872, was one of the active 

producers for years and all through the early eighties gave work to sixty or seventy men. In 

1884 a bucket tramway was used to convey many tons of low-grade ·material from the dump to 
GEOLOGICAL SURVEY 

f. \ 

0 

N 

1 

0~ 

0 200 400 

PROFESSIONAL PAPER NO. 63 PL. X!\XJII 

SNOWDRIFT 

L£YE.L 

VE/;.; 

LEVEL 

6o6 abo feet 

HORT'l.ON'T'AI . Pl. AN OF WORKINGS TN THF. PAY ROCK AND SNOWDRIFT MINES, INCLUDING ASHBY TUNNEL WORKINGS 
MINES OF THE SILVER PLUMB DISTRICT. 205 

the mill which had been c~nstructecl at the edge of Clear Creek. But little work has been done 

in this mine for years. . 

Fossetta states that the Pay Rock mine yielded about $60.000 in 1878 and "produced 

altogether some $450,000 up to 1880, or more than any other excepting the Dives and Pelican." 

E. 0. Leech, Director of the Mint, reported the production during three years as follows: 

Production of Pay Rock mine, 1889, 1890, and 1892. 

-----------------------------------------------------------~ 

Si!Yer. 

--------------------1----·----· 

1889 ................................................................................................ . 1890 ...................................... . 1892 .. . S135, 7:17. 50 

35. 20:l. 00 

217,470.00 

Lead. 

S3, 9.12. 00 

517.00 

6; 107.00 

Total. 

S139, 709. 56 

a.1, no.oo 223,586.00 

The total production of the Pay Rock vein is estimated by, E. F. Kendall, the superintendent, at about $2,000,000. 

"------'3~oo::___-----"6o-:::_o ____ .....:";900 feet 

Stope:!:> !>ho···<n by ruled areas 

FIG. 51.-Verticallongitudmal projection of Pay Rock mine. 

DESCRIPTION. 

The Pay Rock (or Silver Plume) lode may possibly have some connection with the PelicanBismarck vein system, as a branch of the Corry City lode. This relation, however, is not well 

established, and the productive portions of the two lodes are widely separated. The Pay Rock is 

a nearly perpendicular vein striking due northeast. At the southwest e"nd of the developed portion the dip is northwest, at an angle of about 80°; but farther northeast it becomes perpendicular, or even steeply inclined to the southeast. The length of the vein as developed by underground workings is about 2,700 feet, and the aggregate amount of developed \Vork done in the 

Pay Rock mine is approximately 5 miles. The lowest tunnel, that from the Ashby tunnel, ranges 

in depth from 1,000 feet below the surface at its southwest end to 1,500 feet at its northeast end 

(fig. 51). 

NATUI~E OF THE INCLOSING ROCK. 

The country roc;k is mostly hard gneiss, with some pegmatite and some porphyritic granite. 

The general trend of the schistosity and the angle of the vein to it are fairly constant, as seen in 

the Ashby and Silver Bank tunnel levels. The gneiss strikes north and so:uth, so that the vein 

cuts across it at an angle of 45°. 

a Fossett, Frank, Colorado, lbSO, p. 394. 
206 GEOLOGY OF THE GEORGETOWN QUADRANGLE, COLORADO. 

The productive portion of the vein ends on the west at the contact with a transverse dike 

of porphyry, "'hich has a northwest strike and a dip of about 50° NE. This dike was seen in 

the Ashby tunnel, where it has a thickness of 20 feet or more, and in the Silver Bank tunnel, 

where it has been disturbed by movement parallel with its course, resulting in considerable 

shredding of the porphyry, and the diminishing of the porphyry zone to about 6 feet. The 

same dike is reported as having been encountered at the southwest end of other levels of the 

mine, between the two above mentioned. 

In the field this porphyry was noted to be of the Corry City type. The microscope shows 

it to be entirely altered. Large phenocrysts of feldspar arc altered to calcite, sericite, and 

kaolinic products (the latter in the Silver Bank tunnel); phenocrysts of biotite are completely 

changed to silica, sericite, and cloudy carbonates. Apatite is frequently seen. Characteristic 

areas of cloudy siderite, rhombic or square in outline, probably represent altered pyrite. Many 

of these areas are fringed by hematite. The groundmass is a devitrified glass, now containing 

secondary carbonates, etc. 

This porphyry dike is locally known as the Silver Point lode, and has been somewhat 

explored. It is reported to be unmineralized, however. It is said to cut off and form the 

western termination not only of the Pay Rock vein, but also of the Vulcan, which lies not far 

northwest of the Pay Rock, and nearly parallel to it. Near the surface the dike is said to have 

been traced northwestward by shallow workings to Cherokee Gulch, as well as a considerable 

distance to the southeast; but at present many of these small tunnels are inaccessible, and 

the debris strewing the surface hinders the following out of the dike. The resemblance of the 

porphyry to that found in the Corry City workings is difficult to confirm, on account of the 

profound alteration which the rocks in both places have undergone. The position and trend 

of the Silver Point dike, however, suggest the possibility that it may be practically the same 

as that in the Corry City; its dip is also similar, and its relation to the probable extension of 

the Corry City lode on the surface, near the Silver Bank tunnel (of the Pay ~ock mine), is 

similar to that shown in the Diamond tunnel workings on the Corry City. In the Diamond 

tunnel the porphyry splits off from the Corry City lode at a point west of the tunnel, so. that 

in the crosscut tunnel only a little was detected, and that 200 feet northwest of the lode. On 

the surface, near the Silver Bank tunnel, the relation between tlie Silve.r Point porphyry dike 

and the extension of the Corry City lode is similar, the former lying about 200 feet northeast 

of the latter. 

NATURE OF VEIN. 

The Pay Rock vein has formed along a strong fault slip or fault zone. Movement along 

the slipping planes is shown by discordance of t~w opposing walls, by crushing, and by slickensiding. In the Silver Bank tunnel, in one place, the strire dip 25° SW.; in another locality 

on the sa~e level a polished surface shows an earlier and stronger set of grooves dipping 3° 

NE., and a weak later set dipping 20° SW. The. movement then was more nearly in a horizontal 

than a vertical direction, as was true in a general way of all the contemporary faults of this 

region. 

On the Silver Bank level (Pl. XXXIV) the fault lead is very strong and has been followed 

for 1,100 or 1,200 feet northeast of the main Pay Rock shaft. On the lowest level, that of the 

Ashby tunnel, the drifts west of the shaft also show a fairly strong fault lead; but a short distance northeast of the shaft the lead dies out, passing into mere fractures. In this place crosscutting was resorted to, and other fractures, having a similar trend, were found and followed. A 

similar condition was found 'in the first level, 140 feet above the tunnel level. The main lead 

divides, toward the northeast, into numerous little. slips in the gneiss. Many of these leads 

die out and are continued by parallel overlapping leads, and about 300 feet northeast of the 

shaft the. leads are practically lost. · 

In the Silver Bank tunnel the vein splits into a network of branches at its southwest end, 

just befure intersecting the Silver Point porphyry clike, and these branches, though representing in general weak slips; are highly mineralized. The same condition is not found at the 
U. S. GEOLOGICAL SURVEY PROFESSIONAL PAPER NO. 63 PL. XXXIV 

Mineralized lead Unm ine ralized lead 

200 400 600 feet 

N 

SILVER BANK TUNNEL 

GEOLOGICAL PLANS OF SILVER BANK AND ENNIS TUNNELS. 
MINES OF THE SILVE.R PLUME DISTRICT. 207 

Ashby tunnel level, where the lead is weak and unmineralized near the porphyry. The vein 

is not present in the porphyry, but on the southwest side was encountered in the upper workings (according to Mr. Kendall, the superintendent) as an entirely barren lead, which in one 

place was followed for several hundred feet. There is little evidence of fissure filling in any 

part of the mine; the ore seems to be rather the result of replacement and impregnation along 

fault zones. 

LOCATION OF ORE BODIES. 

It is reported that as a rule the best ore in the Pay Rock was at the junction of veins, the 

shoot beginning at the junction and extending -some distance along the united vein. At one 

such intersection the richest pocket in the mine was found. This principle is seen in the network of branches at the southwest 'end of the lode, in the Silver Bank tunnel, where chimneys, 

or pockets, of ore have been characteristically made at the intersection of branches. 

RELATION OF ORES TO DEPTH. 

The contrast is striking between the Pay Rock lode, as shown in the Silver Bank tunnel, 

the principal accessible upper level, and in the lowest level, that of the Ashby tunnel. The 

vertical distance between these two levels is about 640 feet. On the Silver Bank level are 

very strong, straight veins, consisting of slip zones cemented. chiefly by silica. On the lower 

level the slips are present constituting the same channels as those above, as is proved by their 

having been followed continuously downward; but they appear much weaker and less continuous than the same slips on the upper levels. The most striking difference, however, is 

in the quantity of mineralization. On the Ashby level (as also in the next level, 140 feet above) 

the slips are usually barren. On the Silver Bank level a slight fracture zone, weak and splitting, may serve as a locus for a deposit of quartz and rich ore, while along the main vein the 

mineralization is heavy and continuous; but directly below, in the Ashby level, the presence 

of an important vein would in many places not be suspected .. 

The ore contains galena, blende, pyrite, quartz, caleite, and siderite, all intercrystallized 

and contemporaneous. Polybasite was probably present in the richer ores. The carbonates, 

ealcite, and siderite, though intercrystallized with and eontemporaneous w:ith the sulphides, 

seem much more abundant near the surface than below. Siderite was not noted in the Ashby 

tunnel. A similar abundance of siderite and calcite was noted close to the surfaee of "the Bismarck and the Wisconsin lodes, and the relations to the sulphides were also similar. The 

surface at these different points is at different altitudes, ranging from 10,000 to 11,000 feet, 

so that these siderite ores can not be accounted for as a zone due to ascending waters, but 

must be clue to atmospheric agencies. 

In the Pay Rock mine the riehest and largest bodies of ore were elose to the surfaee. Pockets were opened near their outcrops and worked downward till barren rock -was reached. In 

many plaees these surface ores were very rich. Soft, black, rich secondary sulphides (locally 

called "sulphurets ") are reported to have been found up to about 50 feet below the surface. 

According to Mr. Kendall, large bodies of gold-bearing pyrite occurred near the surface, some 

of them containing as much as 2 ounces to the ton, while farther down, galena-bearing smelting 

ores came in, containing much less pyrite and only traces of gold. Similarly an unusual quantity of pyrite was noted by the writers in the sideritic superficial portions of the Bismarck and 

Wisconsin lodes, though whether or not these ores were auriferous has not been ascertained. 

The good ore on this vein appears to have reached locally a maximum depth of about 

1 ,200 feet below the surface. Near the bottom of the ore the largest amount of "glance" 

(polybasite n in the mine, including masses several inches in diameter, is reported to have been 

found. 

Besides the practical disappearance of the lode material, both gangue and metallic minerals, in depth, the greatest geologic interest in this mine centers around the fact that the vein 

is completely cut off by the transverse Silver Point porphyry dike in the upper levels. According to information obtained, the fault fracture passes through the porphyry, and has been 
208 GEOLOGY. OF THE GEORGETOWN QUADRANGLE, COLORADO. 

followed several hundred feet farther southwest, but is absolutely barren. On the northeast 

side the same fracture is highly silicified and mineralized. Moreover, close to the dike, on this 

side, there is an extraordinary amount of mineralization. The vein splits into many branches 

here, a condition which in general is elsewhere attended by weakening or lack of mineralization, but in this case good ore in considerable quantity has made along many -of these minor 

slips, so that a network of ore-bearing veins about 200 feet wide is present. According to 

Mr. Kendall, when the lode encountered the porphyry dike it spread out and followed the 

east side of the porphyry dike for some distance, forming considerable bodies of mostly 

low-grade ore. , 

The above.:.stated facts indicate very strongly that the mineralizing solutions were intercepted by the decomposed porphyry dike, and barred from the channels southwest of the dike; 

and that the solutions circulated most vigorously along the eastern contact of the dike. This 

dike dips about 50° NE., so that the solutions circulated along its upper surface; therefore the 

work of descending waters is suggested. 

All the above considerations indicate that the ore deposits in their entirety were deposited 

bj descending waters. That the secondary sulphides and other rich ores in the surface zone 

were the result of double or, let us say, multiple concentration and the partial separation of 

the different metals according to their relative solubilities is undoubtedly true, but there seems 

good reason to assign the low-grade ores also to ·deposition from similar waters. According 

to Mr. Kendall, before the Ashby tunnel was run (which changed the course of underground 

drainage) the presence of water was considered a good sign of ore, and where water was encol,lntered in any amount ore was looked for. 

PRESENT ACTION OF DESCENDING WATERS. 

In the Ashby tunnel special attention was paid to a remarkably active and impor~ant 

process observed everywhere in the underground workings throughout this district-the action 

of the present underground waters. The water in this country is close to the surface. Even 

on hilltops, as, for example, on Leavenworth Mountain, water was standing at 50 or 60 feet 

below the surface in shafts at the time of 'this examination. Underground, as in the Ashby 

tunnel, water circulates along slips and fractures in the rock. Most watercourses are found 

in granite and hard gneiss, rocks whose rigidity makes it possible for continuous open cracks 

to exist. The softer schist is without good fra~tures, and is free from any co:p.siderable amount 

of underground water. 

Every stream of water entering the Ashby tunnel along a water course has coated "the 

rocks, as well as the tunnel floor, with copious deposits of calcite, hydrated iron oxide, and 

manganese dioxide. The metallic oxides are in many places distinct from one another and 

from the calcite, though in other places the deposits are mixed. Generally the calcite lias been 

deposited first on the walls of a crevice, with the oxides above, indicating successive deposition. 

The invariable occurrence of these deposits wherever water courses enter the tunnel and their 

abundance in rocks remote from veins show that they have no connection with lodes, but 

represent the universal action of descending waters. 

This fact indicates that surface waters leach lime, iron, manganese, etc., from the granites 

and oth~r rocks, and carry these materials far down. The breast of the Ashby tunnel, where, 

as elsewhere in the tunnel this process has been active, is 1,200 feet below the surface. 

The deposition has been produced only by artificial means, by the introduction of air 

along tunnels. Where the waters emerge into the tunnel and .come into contact with the 

oxygen of the air they drop the material they hold in solution. These solutions are evidently 

carbonates, and the .process of deposition of the materials is similar to the precipitation of 

lime, carbonate, manganese dioxide, and limonite in bogs at the surface. A similar deposit of 

manganese and iron oxides on the floor of a cave has been noted at Aspen, Colo.a 

a Spurr, J. E., Mon. U. S. Geol. Survey, vol. 31, 1898, p. 239. 
MINES O.F THE SJLVER PLUME DISTRICT. 209 

At the breast of the Ashby tunnel the granite was noted to be darker and fresher than 

any of that seen above or at the surface, 'yhile along joints and other .cracks it has been bleached, 

plainly by percolating waters, to a light color, more like that of the surface rock. This locality 

is 150 feet north of the Pay Rock vein, which, moreover, is in this portion entirely unmineralized. 

A thin section of the fresh granite contains feldspar, quartz, biotite, and muscovite, with 

accessory apatite and zircon and a little pyrite. The muscovite is not abundant. The biotite 

·is fairly fresh. The feldspar may be divided into two classes, the first clear and unaltered, the 

second brown and clouded, partly altered to sericite, which in pa1;t becomes coarser, changing. 

to cle.ar, fine muscovite crystals. In the feldspars of the first class microcline is predominant. 

A second variety is an unstriated feldspar, transitional to microcline. Three determinations 

by the Fouque method indicate microcline-orthoclase. A third variety shows a single set of 

twimiing striations. A determination of this by the Fouque method also indicates probable 

microcline-anorthoclase. The partly sericitized feldspars constituting the second class are 

striated or unstriated. Nine determinations by the Fouque method show albite and oligoclasealbite. · 

The bleached granite near the joints is much inore altered than the rock just described. 

The feldspars, as in the fresh granite, may be divided into an unaltered and an altered class. In 

the unaltered elass the perfectly clear feldspars are, first, predominantly microcline; second, 

an unstriated feldspar transitional to microcline; third, a singly striated feldspar whose optical 

properties indicate probable microcline-anorthoclase. The altered feldspars are almost entirely 

decomposed-much more than in the fresher granite. There is a little residual feldspar, but 

. the species is indeterminable. The alteration products consist of sericite, fine crystalline muscovite, and carbonates, in good crystals. The biotite is entirely altered and bleached with the 

development of carbonates. 

The results of the alteration of this alkaline granite by carbonated waters descending from 

the surface are. the complete alteration of biotite and albite, while microcline, microcline-orthoclase, microcline-anorthoclase, and muscovite were free from attack. The biotite yielded to 

the waters abundant iron, which went into solution in the form of bicarbonate, and a little 

manganese. The'albite and albite-oligoclase yielded soda and some lime. The carbonate surface waters took these materials into solution as carbonates. 

The widespread importance of this process is shown by the surprising abundance of calcite, 

limonite, and manganese dioxide precipitated from underground waters in tunnels and other 

mine workings throughout the district, and by the universal alteration of the granites near the 

surface. 

The solutions which deposit the calcite ocher and wad in the tunnels would normally deposit 

their iron and mapganese along their channels as carbonates in the absence of excessive oxygen. 

The abundant calcite and siderite, much of the latter more or less manganiferous, in the superficial portion of the veins seem naturally referable .to this origin. Siderite-filled veinlets, such 

as that in the Diamond tunnel (p. 193), about 650 feet below the surface, moreover, seem the 

natural results of such waters at a greater depth. In the case just mentioned a gash fissure 

which had first been lined with quartz was again partly filled by a brown ferriferous carbonate, 

though spaces were still left empty. This carbonate, however, incloses some contemporaneous 

pyrite, galena, and blende, and the same is true of the carbonates that are characteristic of the 

surface portion of the principal veins. Moreover, the deposition of the carbonate layer on top 

of the quartz layer, and the fact that the sulphides occur here entirely within the quartz, there 

partly in the quartz and partly in the carbonate, and elsewhere entirely in the carbonate, suggests a continuous process of deposition for the whole vein. 

A specimen of "gouge," or soft triturated material, obtained along the Bismarck-Pelican 

fault lead, on the Pelican tunnel level, at a depth of 1,600 feet, examined by Doctor Sullivan, 

contained large amounts of ferrous carbonate (siderite). This gouge is being percolated 

through continually by descending waters, which have no doubt deposited the siderite. The 

fault lead at this point is on the contact of porphyry and gneiss. 

31401-No. 63-08-14 
~10 GEOLOGY OF THE GEORGETOWN QUADRANGLE, COLORADO. 

VULCAN LODE. 

The Vulcan vein lies about 300 feet northwest of the Pay Rock vein and has practically 

the same strike and dip. It is neither so strong nor so well mineralized as the Pay Rock, but 

has been followed underground for a distance of about 1,000 feet. The wall rock, so far as 

examined,· is gneiss, in some places thinly laminated, in others hard and granitic. As in the 

Pay Rock tunnel, the strike of the gneiss appears to be everywhere north and south, so that the 

vein traverses it at an angle of 45°. 

The pay ore taken from the Vulcan vein was near the surface, practically all being at a 

depth of 300 feet or less. In fact, there has been no encouragement for exploration of the vein 

much below this depth. The Ennis tunnel (Pl. XXXIV), which is the lowest working on the 

vein, exposes the vein at depths ranging from 100 to 400 feet, but on this level very little ore 

was encountered. The total production of the mine is very roughly estimated to have· been 

in the neighborhood of $100,000. BurcharcJ,a at the end of 1882, stated that the value of its 

production was over $41,976 from 132 tons of ore. 

In the Ennis tunnel workings the southwestern portion of the vein for about 500 feet consists of a very poor branch lead in gneiss with no mineralization. Farther northeast the lead 

., 

~ 

"" 

~ 

.~ 

l'! 

':! 

l!! 

:a "'- "' . 0 -:; ~ .. .:;, 

. 

-:; 

"' 

., 

.. 

-:; 

~--=:---'1~00--:---;-~200:___~300 feet 

· Stopes shown by shaded areas 

FIG. 52.-Vertical projection of Snowdrift mine. 

becomes a strong crushed zone with splits, probably indicating relatively slight faulting, and 

in this portion of the workings there is some mineralization. The metallic minerals are probably blende, galena, and pyrite, the blende being everywhere largely in excess. Some tetrahedrite is present here and there. This ore runs 20 to 30 ounces in silver, with a trace of gold. 

The gangue is in many places a dense cherty quartz. Examination of a thin section of 

this quartz under the microscope shows that it is probably a crushed and silicified gneiss, containing crystals of sulphides. Some of the sulphides in the ore represent the filling of small 

fissures which have first been lined with quartz .. 

SNOWDRIFT LODE. 

DESOIUPTION. 

The Snowdrift mine,b which was being worked by a single lessee at the time of the writers' 

examinations, lies about 1,200 feet east of_the main opening on the Pay Rock. The vein has 

an east-west trend, and, like the Pay Rock, a practically vertical dip. The ore has formed 

along slip planes, upon which striations showing movement are abundant. In. different places -----------------·-----------

a Burchard, H. C., Precious Metals in the United States, 1882, p. 427. ''Comprising, apparently, also the old Hopewell. 
U. S. GEOLOGICAL SURVEY PROFESSIONAL PAPER NO. 63 PL. XXXV 

I \ N 

0 200 400 600 soo feet 

LEBANON-EVERETT GROUP THE NOll/litiS ,.£Til/ItS eO . .'WASHINGTOI•I, 0. c . 
MINES OF THE SILVER PLUME DISTRICT. 211 

the angle of pitch of these striations on the faultplane, measured from the horizontal, was 5° E., 

7° E., and 15° E. This shows that the movement was nearly horizontal. The vein has been 

explored for a horizontal distance of nearly 1,000 feet and a vertical distance of about 400 feet, 

and a moderate quantity of ore has been stoped out. The ore extends down about 300 feet 

from t?e surface, but occurs in larger bodies farther up. In all about 4,500 feet of drifting and 

tunneling has been done in this mine, and two shafts aggregate over 300 feet in depth. (See 

figs. 52, 53.) At present, however, only a portion of the Snowdrift tunnel level is open. This 

. cuts the vein at a point about 250 feet below the surface. 

N 

I 

o~------~•oo~------~zo~o ______ ~3oo~ci 

EJ Mineralized lead. Unmincralized lead 

·~ 

Arrow indicates 

dip of vein 

FIG. 53.-Geological plan of Snowdrift tunnel. 

NATUltE OF VEIN. 

Pitch ~ of striae on walls 

The wall rocks in the tunnel level consist of soft gneiss alternating with patches of granite, 

·generally porphyritic. In the gneiss the vein lead repeatedly branches, in many places to such 

an extent that it is entirely 'lost among the gneiss laminre. Portions of these leads are wholly 

unmineralizecl; other portions have been somewhat stoped. The junctions of branches, so 

far as noted in the small areas exposed, are not particularly enriched. 

The whole vein, to judge from its location and attitude, is apparently a branch of the Pay 

Rock, but its junction with the Pay Rock does not seem to have beenfound,if present,nor does the 

vein seem to be accompanied at any point by an increase of mineralization suggesting such a 

junction, so far as can be made out. This is one of the most characteristic features of veins 
212 GEOLOGY OF THE GEORGETOWN QUADRANGLE, COLORADO. 

in this district. It has been repeatedly noted that strong veins, evidently branches of larger 

veins, grow weaker as they approach the junction. In many of them, as in the Snowdrift 

and Pay Rock, the junction is not seen; in others where a junction has been detected, it is 

very obscure, as, for example, that of the Seven-Thirty and Bismarck-Pelican lodes. 

Where the vein is most str~ngly developed in the Snowdrift tunnel (fig. 53) it shows 3 or 

4 inches of jaspery quartz containing abundant small vugs. These vugs are evidently due to the 

removal by solution of certain portions of the original veins, and on the walls of some of the 

cavities the molded imprints of crystal outlines suggest that the material removed was crystalline sulphides. Here and there residual fragments of crystalline galena and blende occur in 

other portions of the ore. The brown jaspery gangue contains much iron carbonate. When 

broken open the cavities are found to he full of water, though the rock itself is dry. On the walls 

of some of these cavities arc deposited layers of dark-brown botryoidal limonite, in general 

relatively thin. Other cavities have been lined with chalcedonic silica, of the same kind as 

that universally found in the mines and forming the last deposit in the veins. Plainly both 

the silica and the limonite are due to the action of surface waters. 

According to early reports, a much of the profitable ore in the upper workings was very rich 

in silver. This ore consisted of black decomposed sulphides, galena and hlende, and some 

gray copper. 

PRODUCTION. 

The Snowdrift mine was one of the early producers of the camp, for, according to Raymond, b it yielded largely in 1869~ 1870, and 1871. In 1870 the production was $96,316.80, 

currency value. 

In 1883 the mine gave regular employment to 23 men.c 

VICE-PRESIDENT LODE. 

LOCATION AND DEVELOPMENT. 

The Vice-President mine is located on the west side of Brown Gulch, at an elevation of 

about 10,400 feet above the sea. .The vein, which is parallel to the Seven-Thirty and only a 

couple of hundred feet south of it, is opened by a tunnel which runs westward from the gulch. 

o 100 2oofeet ~--~---' 

~~~\\~~ [IT] Gneiss Granite: Mineralized lead Unmineralized lead 

FIG. 54.-Plan of tunnel of Vice-President wi;,e, showing geology. 

At the time of visit this tunnel was caved shut 

at a distance of about 600 feet from its mouth. 

COUNTRY ROCK. 

The wall rock consists of biotite gneiss 

with patches of porphyritic biotite granite 

and seams of coarse pegmatite (fig. 54). 

DESCRIPTION 0~' .VEIN. 

The vein, which has a general S. 85° W. 

trend, in most places has a very steep dip to 

the south, hut the dip ranges from 65° S. to 

vertical. The lead in places is a mere clay slip 

which is hard to follow; in other places it is a quartz (or quartz and clay) lead 1 to 4 feet wide. 

It has a tendency to split into foot and hanging leads, and loop about" horses" of rock. The ore 

where present occurs in fractured zones from 1 to 3 feet in width, and forms a cement for angular fragments of silicified wall rock. These angular fragments, as well as the wall rocks themselves, are as a rule sep,aratecl from the ore by a thin coating of comb quartz. 

a Old, H. 0., Colorado, London, 1872. b Fossett, Fmnk, Colorado, 1880, p. 304. 

c Burchard, H. C., Precious Jlfctals in the United States, 1883, p. 268. 
MINES OF THE SILVER PLUME DISTRICT. 213 

ORE AND GANGUE. 

The ore consists of argentiferous galena and black sphalerite, with small amounts of 

chalcopyrite and pyrite. The gangue is chiefly quartz. 

OCCURRENCE OF ORE. 

The ore bodies pitch toward the west and appear to be due to the union of small branch 

veins with the main vein. According to B. J. O'Connell, of Georgetown, the best shoot in the 

mine is said to have occurred at the intersection of the comparatively fiat John J. Roe vein (a 

minor transverse vein) with the Vice-President. By the union of these two there was formed 

a V-shaped trough that pitched westward. Along the vein itself the best ore seems to occur 

close to the south or hanging-wall side of the fissure. The south wan· rock is usually more 

silicified than t.hat of the north wall, which is rather soft and kaolinized. 

The mine produced considerable ore, but has been idle for many years .. 

MONTREAL LODE. 

LOCATION AND DEVELOPMENT. 

The :Montreal lode is located about 300 feet south of the Vice-President lode, and like it 

strikes east and west. The chief development is a tunnel which runs westward on the vein 

from Brown Gulch for a distance of 600 feet. From this vein an irregular· crosscut runs 

in a general northeasterly direction to a 

stronger vein, which is probably the VicePresident. 

COUNTRY ROCK. 

The mine workings are entirely in por-:-

phyritic biotite granite, with the exception of a few feet of gneiss near the mouth 

of the tunnel (fig. 55). 

DESCRIPTION OF VEIN. 

Clay and a small amount of crushed 

granite mark the main lead, which is a 

fractured zone in granite. The Montreal 

100 2.0ofeet 

~ [3 Mineralized lead Unmineralized lead 

FIG. 55.-Plan of tunnel of Montreal mine, showing geology. 

vein proper dips 75° to 80° S. and is not very heavily mineralized. The vein intersected by 

the crosscut (probably the Vice-President) showed evidence of having contained some small 

and narrow but rieh streaks of silver ore and some stoping had heen done. The mine has long 

been unworked. 

SHIVELY LODE. 

The Shively lode, which was discovered and opened during the summer of 1873, a at one 

time had nearly 2,000 feet of tunnels, drifts; and crosscuts and a shaft about 500 feet deep. At 

the time of visit, however, these workings were entirely inaccessible, owing to the caving which 

had taken place during the years the mine had lain idle. · 

In the early eighties this mine gave employment to twelve or fifteen men and produced 

considerable ore. Fossettb estimates the production previous to 1880 at about $60,000. The 

vein is said to be a vein with 2 to 6 inches of very high-grade ore, some of which ran as high 

as 425 to 450 ounces in silver. 

The vein is an approximately east-west vein dipping about 80° S. Frank A. Maxwell c 

states that the workings of the Shivefy show three porphyry dikes running nearly at right 

angles to the vein, and also that the ore was all taken from the portion of the vein included 

a Raymond, R. W ., ~Iineral Resources West of the Rocky Mountains, 1873. p. 275. 

cOral communication. b Fossett, Frank, Colorado, 1880, p. 398. 
214 GEOLOGY OF '!'HE GEORGETOWN QUADRANGLE, COLORADO. 

between two of these porphyry dikes. The probable explanation for this occurrence of the ore 

is that the porphy~y became relatively impervious to waters on account of decomposition, 

and thus the minerali11ing solutions, circulating along the vein fissures, became confined 

mainly to the portion between the dikes. 

LEBANON GROUP OF LODES. 

DESCRIPTION. 

The name Lebanon is applied to a group of lodes of medium importance lying on the slopes 

-of Republican Mountain, in the eastern part of the area shown on the Silver Plume special 

map (Pl. XXI). This group consists of a network of branching and probably to some extent 

crossing lodes, which run in a general northeasterly or easterly direction. The two principal 

lodes of this group arc the Morning Star and the Alhambra, of which the former, as already 

noted, may be the extension of the Dunkirk, although there is an undevel_opcd section lying 

between the Dunkirk workings and the Lebanon mines (Pl. XXXV). The Alhambra is a 

branch of the Morning Star. These lodes subdivide and branch, most of the branches opening out 

to the east, but some to the west. Some of these branches, particularly those which open out to 

the west, are strong and persistent, and indicate a distinct set of northwestward-trending fracture 

zones traversing the main northeast minor faults and being deflected by them in many places 

at the junction, rather than actual branches of the northeast fissures. Undoubted branches, 

however, are common. A distinct feature of this group of lodes, differing from the group comprising the Pelican, Seven-Thirty, Pay Rock, etc., is that the intersections are as a rule well 

marked and notably enriched. In some places, as at the junction of the Alhambra and lode 

No. 9 of the Lebanon tunnel, both of the uniting veins and also the trunk vein arc mineralized 

at the junction. In other places, as at the junction of the Morning Star lode and the No. 4 

lode in the Lebanon tunnel, only the stronger vein is mineralized, the weaker branch being 

barren. · 

CHARACTERISTICS OF GROUP. 

Mineralogically the lodes of this group are characterized by the presence of a certain amount 

of chalcopyrite, which distinguishes them from the lodes of the Pelican-Seven-Thirty-Pay Rock 

group, to which they arc otherwise mineralogically similar. The outcrops therefore show in 

many places azurite with some malachite. Structurally the veins resemble those of the Pelican 

group, being replacements along fault zones of slight throw, but there is somewhat more fissure 

filling in the Republican Mountain group. 

It is reported that surface work on this group of lodes was begun in 1868-69. The most 

extensive and the lowest workings are those in the Everett and Lebanon tunnels. The Lebanon 

tunnel is 1,082 feet long from mouth to breast and was begun in 1870. The production of this 

tunnel is estimated by miners at $500,000. 

MORNING STAR LODE. 

DESCRiPTION. 

The Morning Star lode is well marked on the surface, but, like all the veins of this group, 

continually splits and branches in various directions, making the identification of a single main 

vein difficult. This lode is traceable on the surface for over 2,000 feet. At the east edge of 

the Silver Plume special area it is called the East Peru lode; on the west it unites with the 

Alhambra lode. The intersection of these two lodes dips to the east with increasing depth, 

so that where encountered in the Everett tunnel it is several hundred feet 'farther east than 

at the surface. 

The Morning Star lode is shown in both the Everett and Lebanon tunnels (fig. 56). In 

the Lebanon the veins are currently designated by numbers running from 1 to 10, No. 5 being 
MINES OJ!' THE SILVER PLUME DISTRICT. 215 

undoubtedly the Morning Star and No. 4 a branch. In this tunnel it is plain that the lode 

channel was due to faulting, as is shown by the discordant walls. The vein here forks repeatedly, the branches diverging both to the east and to the west from the trunk. · The country 

rock is gneiss, having a northeasterly trend, so that the vein crosses it. at an angle of 45°. The 

dip of the vein is to the north at angles ranging from 50° to 65°. At the intersection of branch 

No. 4 (which itself is an unmineralized slip) with the main lead an ore body has been found, 

extending both ways on the main vein, but farther toward the east, considerable ore having 

been stoped for nearly 200 feet in that direction. Beyond this poi1l.t the vein splits into several 

branches or slips in the gneiss. · 

'k3j- I .•• ·::· ... --:;:1 .··'·"·· ... . Mineralized lead Unminerallzed •lead 

Fw. 56.-Geological plan of Lebanon and Everett tunnel workings. 

RELATION TO PORPHYRITIC GRANITE DIKES. 

The strongest portion of the Morning Star vein in this tunnel, between the points where 

branching ·is common, runs along a dike of highly porphyritic granite (the corn rock of the 

miners). This dike is in gneiss and is only a few feet wide, being too small to be shown in the 

geologic plan of the tunnel. The lead lies in some places on top of this dike, in others on 

the bottom, and in still others it cuts through the dike. This rock is considered by the miners 

a favorable formation for a strong vein, Ev.ide.p.tly it plays the same r6le in deflecting and 

localizing the fault slips as do the porphyry dikes. Therefore along the contact of the porphyritic granite dikes, which are definite lines pf thickness, an unbroken and relatively strong 

lode is apt to run. 
216 GEOLOGY OF THE GEORGETOWN QUADRANGLE, COLORADO. 

EVERETT LODE. 

The Everett lode appears to be a branch of the Alhambra. The junction of the two, as 

nearly as can be found out, lies at the surface at the Alhambra discovery shaft. Here a body 

of ore was worked down for some distance. The location of the junction of these lodes underground is not certainly known, but from information obtained it is probable that the Everett, 

dipping more flatly than the Alhambra, joins the latter at a point on the line of the Everett 

tunnel about halfway from the surface to the tunnel level. Therefore the Everett lode is not 

encountered in the Lebanon and Everett tunnels. 

ALHAMBRA LODE. 

DESCRIPTION. 

The Alhambra lode and its branches are traceable on the surface for about 2,200 feet. It 

runs in a northeasterly direction and the dip as seen in the Lebanon tunnel is about 60° N. 

In this tunnel the slickensided fault faces of the lead exhibit strong strim dipping 10° to 12° 

E., indicating a nearly horizontal differential movement. 

LOCUS AND CHARACTER OF ORE. 

The most ore along the vein is found at its junction, in the Lebanon tunnel and above, with 

the No. 9 vein of the Lebanon tunnel, which is perhaps the Peru lode. Her,e there is a big stope 

going up 100 feet above the tunnel and representing the largest and richest ore body in the 

mine. This ore was richest at the junction of the two veins, but extended a short distance 

along both and a short distance also on the trunk vein beyond the intersection. J'his ore 

contained argentiferous tetrahedrite and ran 100 to 125 ounces of silver. At the time of the 

examination of the mine in 1904 the last unsorted shipment of the lessees was reported as 

yielding 26 per cent zinc and 19 per cent lead. The last sorted shipment ran 26 per cent lead, 

14 per cent zinc, 42 ounces silver, and 0.1 ounce of gold. A large part of the vein material is 

· coarsely crystallized brown blonde, which is never shipped, as it carries less than 30 ounces 

of silver to the ton and some of it runs as low as 6 ounces. This blende makes up the greater 

part of the vein material, so far as can be judged from what still remains in ~he stopes. Galena 

is abundant and chalcopyrite occurs here and there. The low-grade blonde orcs alternate 

with parallel streaks of relatively high-grade lead and silver ores. 

PARAGENESIS OF ORES. 

Examination of the ores indicates that the vein has been successively reopened and that 

later deposition has occurred in the reopened fissures. The galena and chalcopyrite arc closely 

associated; the blende does not seem to be associated with chalcopyrite, though locally it is 

with galena. A study of specimens indicates that at least some of the galena was precipitated 

later than some of the blende. The following succession of events is shown in places: (I) Opening of a vem channel; (2) deposition of blendc; (3) reopening; (4) deposition of galena; (5) 

reopening; (6) deposition of native silver on the walls of vugs. Another specimen shows the 

following order of deposition of minerals: (1) Quartz; (2) blende; (3) quartz; (4) siderite. 

The minerals of the last two periods of deposition are in free crystals, coating cavities in the 

blende. Tetrahedrite, as well as native silver, occurs coating cavities, and these minerals were 

found in the Lebanon tunnel 400 or 500 feet below the surface. 

On the surface the Alhambra is a strong vein, containing considerable galena, lead, etc., 

for 500 feet or so east of the Alhambra discovery sliaft. 

NORTHEASTERN EXTENSION OF ALHAMBHA LODE. 

The northeastern extension of the Alhambra is known as the Scott lode. From a mining standpoint this has been of no great importance, but it is a strong lode showing solid sulphides in places several inches thick, consisting of galena, blende, pyrite, and chalcopyrite. A 

thin section of a portion of the vein shows quartz, pyrite, barite, and siderite, all intercrystallized and contemporaneous. ~he lode lies in granitic gneiss and pegmatite, and is opened up, 
MINES OF THE SILVER PLUME DISTRICT. 217 

by a tunnel drift for nearly 300 feet along the vein. Near the mouth of the tunnel it makes a 

junction with an east-west lode which is very likely the Edinburgh lode. 

The Scott lode as seen in this tunnel is somewhat branching and splitting. A little ore 

occurs here and there. 

About 600 feet northeast of the mouth of the Scott tunnel and 300 feet northeast of the 

breast of the tunnel drift what is probably the same lode is again exposed in a erosscut tunnel 

running 100 feet into the hill. This shows an iron-stained lead with no ore, although small 

quantities of ore are found in the dump of a caved tunnel 100 feet or so farther southwest. 

PERU LODE. 

The Peru lode lies on the extreme eastern edge of the area covered by the Silver Plume 

special map. It has an east-west trend. As shown in the Iris tunnel, it is vertical and lies in 

porphyritic granite. It is here a strong, soft lead with much quartz, containing abundant 

pyrite and some galena, blende, and chalcopyrite. One specimen shows a veinlet in 

granite 1 inch in thickness, three-fourths of 

which is filled with quartz containing very 

little pyrite, while the central part is blende 

with a little pyrite (fig. 57). Barite is also 

present as a gangue mineral. The metallic 

minerals occur embedded in solid vein quartz 

and also as fissure fillings. There is no sign 

here of any considerable quantity of ore, 

though the lead is well defined. The Peru 

tunnel, which lies just above the wagon road, 

is caved at its mouth, but the maps show 

that it runs 260 feet into the mountain, crossing the East Peru (which is the extension of 

the Morning Star) and the Peru lodes. The 

tunnel on the hill 50 feet above the Bell 

(Pl. XXXV) runs in a drift on the Peru lode 

also. The latter tunnel showed a fairly strong 

lead in gneiss and porphyritic granite. 

FIG. 57.-Sketch showing successive deposits in fissure, Peru lode, 

Iris tunnel. a, Comb quartz, with occasionally a grain of pyrite; 

b, zinc hlende, with a little pyrite and galena; c, open cavity or 

vug; d, Silver Plume granite, country rock. Three-fourths natural 

size. 

Farther west the Peru lode at its junction with the Alhambra is very likely represented by 

No. 9 lode in Lebanon tunnel. At this junction, as already noted, a large body of ore has 

formed. 

ELIJAH RISE LODE. 

The location of the Elijah Hise lode. seems to have been considerably in doubt in past 

times, but as traced by the writers the lode runs on the surface from the Elijah Ilise discovery 

shaft in a northwesterly direction. The lode is strong near its junction with the Alhambra, 

and shows some ore containing galena. Just northeast of th13 junction the united vein is stronger 

than either of the uniting veins. The Elijah I!ise lode is traceable for 400 feet northwes~, and 

with a break in the outcrops considerably farther. It is in general slightly or not at all mineralized, except as before stated, and appears of very minor importance. Both the Elijah 

I-Iise lode and the Peru lode, though apparently branches of the Alhambra, belong to a set of 

eastward or southeastward striking lodes which are so persistent that they appear to be rather 

a set of crosscutting veins than true branches. 

EDINBURGH LODE. 

The Edinburgh lode is parallel with the Elijah Rise, and lies 400 o.r 500 feet farther northBast. It is traceable for 1,000 feet, and probably is continuous for 1 ,500 feet at least. It is 

·very likely the same lode as that which joins the Scott lode in the Scott tunnel. At the junction with the Scott it is a strong lode showing 6 to 8 inches of quartz, with iron oxide and galena. 
218 GEOLOGY OF THE GEORGETOWN QUADHANGLE, COLOHADO. 

Farther northwest, up the hill, the lode is opened up by a tunnel drift 2,500 feet long on the 

Edinburgh vein and by a shorter tunnel 100 feet higher up the hill. These tunnels show a 

strong fracture zone, dipping 80° S. or nearly vertical, in porphyritic granite. At the breast 

of the tunnel the lode splits into two branches which open up to the west. A little ore was 

taken out of this tunnel, but from what can be seen the streak was probabl:r small. One specinien taken shows a l-inch fissure vein with bands of clear comb quartz and a filling of blende, 

fine-grained galena, anJ pyrite. 

BARITE-BEARING LODE.' 

A lode near United States mineral monument No. 10 is shown on the map as running 

northeastward and crossing the northwesterly extension of the Elijah Hise. This lode affords 

an interesting study in vein formation. It follows a fractured zone in granite. At the crossing of this lode and the extension of the Elijah Hise there is a considerably silicifie(l altered 

zone in the granite, and veins have formed containing abundant barite and calcite, with jasper 

and a little comb quartz. The calcite and pyrite are intercrystallizecl and contemporaneous, 

and in many places line cavities. There are no signs of any metallic minerals in the vein, so 

that the color of the vein is similar to that of the granitic country rock. Therefore the contact 

between the two is not plainly marked, and the coarseness of the barite and calcite crystals 

give the vein at first glance the appearance of pegmatite. In places the barite and calcite 

form only a veneer on the walls of the fissure. The barite here is rather more abundant than 

has been observed anywhere else in the region, and its association with calcite and a little chalcedonic quartz suggests derivation from the wall rock. 

CHANGES IN VEIN-DEPOSITING WATERS. 

In general in this Lebanon group of lodes it is plain that a given fissure was occupied at 

different times by waters of different composition. The first deposit on the walls of a fissure is 

very commonly a layer of pure quartz. The quartz may fill the whole vein, or the deposition 

may be incomplete, leaving a cavity in the center; or, finally, the central part may be filled up 

with other materials, including metallic minerals. These facts show that the waters which 

formed the veins differed in separate fissures and at different times in the same fissure. 

DEPOSITION FROM PRESENT UNDERGROUND WATERS. 

In the Lebanon tunnel calcite is formed wherever water drips down, whether along a 

lode or not. The heaviest deposit of calcite, however, has "formed where the most abundant 

surface water enters the rhine-on the drift on the Alhambra vein in the Lebanon tunnel southwest of the crosscut tunne( Here the vein is marked by a strong mineralized lead in gneiss 

and pegmatite. The waters that enter the drift have formed on the floor a crust of calcite 

from 3 to 4 inches thick and on the· walls a beautiful deposit resembling a frozen waterfall. 

Much of the calcite of. this wall deposit is 1 to 3 inches thick. From the roof fine stalactites, 

upward of 6 inches long, have formed, and in the floor are pisolitic aggregates of calcite formed 

by the deposition of the mineral around small rock fragments. Fine calcite crystals have also 

formed on the walls. On the surface of pools of standing water in this drift a crust of calcite 

gradually forms by evaporation of the water, attaining a thickness of a small fraction of an 

inch. It then sinks to the bottom of the pool by its own weight, when a new crust begins to 

form. Thus at the bottom of some pools a pile of these crusts several inches thick has accumulated, and the different crusts are cemented together by calcite crystals. Much of the calcite 

is stained with iron and manganese, and some of it with a little copper. 

This drift in the Lebanon tunnel was begun in 1880 and closed in 1888. The portion 

where the calcite is most abundant was probably opened up in 1881 or 1882. These deposits 

have been accumulating, therefore, for not more than twenty-three years. According to this 

, the whole drift, 8 feet high, will be ce~ented solid with calcite in five or six hundred years. 
. GEOLOGICAL SURVEY 

PRINCIPAL UNDERGROUND WORKINGS 

OF THE 

TERRIBLE GROUP OF LODES 

0 200 ~00 600 :woo £eet ==~==~======~~~========~~~~ 

800 

1.906 

N 

I 

PROFESSIO 

BROWN 
MINES OF THE SILVER PLUME DISTRICT. 219 

TERRIBLE VEIN GROUP. 

DEFINITION A~TD GENERAL DESCRIPTION. 

The name Terrible vein group may be applied to an extensive and important group of 

veins of which the Terrible lode is one of the largest (Pl. XXXVI). Taken all together this 

vein group represents a northeastward-trending zone of minor faulting and heavy fracturing, 

about 1,000 feet in width and upward of 5,000 feet in length. This zone is practically vertical. 

The principal individual veins are also vertical or have a northerly dip, varying slightly from 

the perpendicular. Exceptions to these general statements will be mentioned later on. 

Considering this vein group in the largest sense, we may regard it as composed principally 

of three master veins parallel to one another and about 500 feet apart-the Mendota, the Terrible, and the l!~enton-.Mammoth. The Terrible and Fenton-Mammoth come together both on 

the northeast and on the southwest, the distance between the two fields of junction being about 

4,500 feet. These two master lodes thus constitute in a sense a single vein fascicle. The 

Mendota vein, however, is not known to be connected with the others. 

These veins follow slight branching fault slips in granite and gneiss. As a rule they are 

not accompanied by porphyry. In the northwestern portion, near the Seven-Thirty wagon 

road, they all pass at nearly right angles into the large porphyry dike which has been described 

(p. 199) as intersected also by the Seven-Thirty vein. Just northeast of this crossing, on the 

upper side of the wagon road, are two small porphyry dikes, resembling the porphyry of the 

Bismarck rather than that of the Seven-Thirty. In this district the Fenton-Mammoth and 

Terrible veins split repeatedly, and several of the branches are deflected by the east-west porphyry dikes, so that their main trend changes from southeast and northwest to east and west; 

farther east it becomes south of east, and the different branches apparently tend to unite into 

a single continuous vein-the Maine. The Maine vein swerves around until it strikes nearly 

parallel to the Bismarck-Pelican lode; it has been traced and worked southeastward nearly as 

far as the Pelican tunnel. 

FENTON-MAMMOTH-TERRIBLE VEIN SYSTEM. 

VEINS INCLUDED. 

The above term may be applied in a general sense to the branching and interlocking veins 

comprising the Terrible lode and branches, the Fenton-Mammoth lode and branches, the zone 

of branching and reuniting veins at the intersection of the Fenton-Mammoth and Terrible 

lodes, and the easterly or southeasterly continuation of this zone, the Maine lode. 

RELATIONS OF DIFFERENT LODES. 

FENTON-MAMMOTH LODE. 

Fenton lode.-The southwest end of the Fenton-Mammoth lode is the Fenton lode. In 

the area of the Silver Ph~me special map this lode is first recognized near the mouth of the 

Baltimore tunnel (fig. 58), whence it is drifted on for 500 or 600 feet to the northeast. Very 

near the point where this lode crosses the Baltimore tuimel an important branch leaves it on 

the north side and runs toward the west. This branch is called the Baltimore lode; it has an 

east-west strike and a northerly dip of about 60°. It is explored by a shaft more than 300 

feet deep and six levels which develop the lode laterally to a point 700 feet west of its junction 

with the Fenton. Northeast of the junction of the Baltimore and Fenton lodes the Fenton 

gives off another, branch, also on the north side. This branch is the Queen lode. It is practically parallel in strike and dip to the Baltimore lode, and west of the junction with the Fenton 

has been drifted on for about 700 feet on the Baltimore tunnel level. 

The exploration of the Fenton lode accomplished by the northeast drift on the Baltimore 

tunnel level is continued with very little break and at nearly the same level by a drift from the 

West Terrible tunnel, which follows the lode continuously for about 700 feet farther. From 
220 GEOLOGY OF THE GEORGETOWN QUADRANGLE, COLORADO. 

the northeast end of this drift there is a gap in the underground development where the vein 

has not been continuously traced on the surface, but about 400 feet farther northeast and 

approximately 250 feet higher up in the Silver Ore tunnel is a drift which is probably on pracFault leads 

Gneiss 

o~----~2xoo~-----~4~ooreet 

N 

FIG. 58.-Geological plan of Baltimore tunnel workings. 

tically the same lode, though 

this is locally known as the 

Brown lode. This drift extends northeastward about 

900 feet to a point beneath 

Brown Gulch. 

Relations of Fenton, 

Brown, and Glasgow lodes.-

At and near the surface in 

this region, especially just 

west of Brown Gulch, conditions are complicated by the 

intersection of minor east-west 

veins with the main lode. 

The chief east-west vein 

is theBrown, which crosses the 

main northeast lode (with 

probably some slight deflection resulting in offsetting by 

it) anq is mineralized and developed byminingexploration 

principally west of the intersection. Here in the Brown 

mine both the east-westBrown 

lode and the northeast Fenton lode have been followed. 

This mine is not worked at 

present and most of the workings are inaccessible, but, to 

judge from the mine maps, it 

appears probable that what 

was known in this mine as the 

south vein represents the main 

Fenton lode, and it is certain 

that the north vein represents 

the Brown lode proper. The 

relation of these two lodes at 

greater depth is obscure, but 

the maps of the different levels 

indicate that the Brown lode 

dips steeply into the Fenton 

lode and joins it with increasing depth. A branch of the 

north vein, known as the 

Coin lode, diverges from the 

Brown on the north side, 

opening out to the west. Workings on this lode indicate that it has a northerly dip. Probably 

as a result of the dipping together of the Brown and Fenton lodes between the surface and the 

level of the Silver Ore tunnel, this tunnel cuts only a single strong lode, which, as before stated, 

has the general trend of the main Fenton. 
MINES OF THE SILVER PLUME DISTRICT. 221 

The intersecting east-west veins continue also on the east side of the Fenton lode. One 

of these lodes was recognized at the surface and is practically· a continuation of the Brown. A 

little farther south and parallel to it is another, not very well recognized on the surface, but 

shown on mine maps as the Glasgow lode near the surface; The map of the workings of the 

Mammoth-Brown tunnel, which runs in to tap this group of lodes, shows near their intersection 

and beneath Brown Gulch a northeast drift on the line of the main Fenton-Mammoth and an 

east-west drift on the Glasgow. The Union tunnel (fig. 59), running in under these lodes at a 

depth 400 feet greater, shows an cast-west drift on a vertical lode directly under the Glasgow 

drift on the Mammoth-Brown tunnel, and this drift in the Union 

tunnel appears therefore to be probably also on the Glasgow lode. 

According to this the main FentonMammoth vein has not yet been 

cut in the Union tunnel. TheBurleigh tunnel, however, is on nearly 

the same level as the Union tunnel, 

and has a long drift on the Mammoth vein, locally called in this tunnel the Phillips vein. By projecting this vein in the Burleigh tunnel 

(Pl. XXXVII) southwestward past 

the last point of development to a 

point opposite the breast of the 

Union tunnel, it is seen to be probable that the Fenton-Mammoth or 

Mammoth lode lies 100 feet or so 

beyond the present breast of the 

Union tunnel. 

All the evidence available indicates that the intersection of the 

east-west lodes, of which the Brown 

is the principal representative, with 

the northeast or Fenton-Mammoth 

lode pitches steeply on the west. 

Mammoth Zode.-ThemainFenton-Mammoth northeast of its intersection with the Brown is called 

the Mammoth. It is open~cl up by 

tunnel drifts on the lode on the 

northeast side of Brown Gulch. 

Farther northeast, near the SevenThirty wagon road, the lode is cut 

by a small crosscut tunnel. Still 

Gneiss Mineralized lead 

EJ Unmineralized lead Dip of vein Pitch of striae 

on waHs 

FIG. 59.-Geological pl~n of Union tunnel, Terrible mine. 

farther northeast a tunnel drift beginning on the Seven-Thirty road and running northeastward 

exposes the lode for a distance of 200 feet. Near this point the lode branches; one branch 

is traceable on the surface eastward to a shaft on top of the ridge overlooking the DivesPelican workings. The clump from this shaft indicates considerable workings, but they have 

not been investigated. Another branch turns eastward farther up the ridge, and is well 

exposed in a tunnel drift on the Backbone claim. This tunnel starts from a point 100 feet or 

less above the Seven-Thirty road on the west side of the depression lying east of Sherman 

Mountain and runs westward into the mountain. So far as known this branch forks repeatedly to the east and practically ends in this vicinity, but the. branch first described connects by 

minor oblique leads with the northeast branches of the Terrible lode. 
222 GEOLOGY OF THE GEORGETOWN QUADRANGLE, COLORADO. 

The Mammoth section of the Fenton-Mammoth lode is most continuously shown in the 

Burleigh tunnel, wh.ere it is called the Phillips vein. In this tunnel the lode is strong, with a 

steep northerly dip. The southwest end of the drift on this lode is within 200 feet of a point 

directly ben~ath some of the upper tunnels of the Mammoth. The drift runs northeastward 

on a strong mineral-bearing lode, extending several hundred feet past the most northeasterly 

point where the Fenton-Mammoth has been traced on the surface and, as a strong though 

unmineralized slip or lead, for a few hundred feet stiU farther to a point under the southeastern 

slope of Sherman Mountain. At this last-named point the dip is vertical or at a high angle to 

the south. The relative position of the Fenton-Mammoth lode between the surface and the· 

Burleigh tunnel, involving a vertical difference of altitude of 1,200 to 1,400 feet, indicates that 

the average dip of the vein is to the north at an angle of about 83° or 84°. 

In the principal accessible tunnel of the Mammoth near the surface the dip of the lode is 

65° to ·75° SE.; in the small crosscut tunnel noted, farther northeast, it is goo, as it is also in the 

tunnel on the upper side of the Seven-Thirty road. The whole Fenton-Mammoth lode along 

its entire extent has a wavering dip that is near goo but averages slightly off the vertical, to 

the north. 

TERRIBLE LODE. 

Branches of main lode in and west of the Terrible mine workings.-The Terrible vein is best 

shown in the various workings of the Terrible and Dunderberg mines, now consolidated (fig. 60) . 

. I 

~etvnnel 

- level 

~] -c 

~~ 

.§~ 

"-.. 

"' 

Fra. 60.-Vertical section showinr;Icvels of Terrible mine. 

In the Terrible mine this vein is shown as a strong lode striking east-northeast and dipping 60° 

to 65° NW. On the west this lode splits into three principal branches, shown in some of the 

mine workings. The northern is called the Gunboat lode, the central the Elephant, and the 

southern the Silver Ore. All three near the point of their separation are seen near the mouth 

of the Silver Ore tunnel (fig. 61). All the branches turn more to the west than the strike of 

the main lode and approach the Fenton lode, "\vhich they probably join. A very plain branch 

of the Fenton lode, diverging from it on the southeast side and having an east-west strike, is 

very likely the continuation of the Elephant; and the western continuation of the Silver Ore 

lode is not improbably that known as the Blaine lode, which is developed at the surface and 

has been drifted on in the West Terrible lode. The Blaine lode is undoubtedly a branch of the 

Fenton. In the West Terrible workings the Blaine lode clips 70° to 75° N., and its probable 
U. S. GEOLOGIC AL SURVEY 

1 

. Gneiss 

~ Mineralized lead 

, ....................... , Unmineralized lead 

0 200 

GEOLOGICAL PLAN OF BURLEIGH T UNNEL 

PI'!OFESSIONAL ---PAP ER NO. 63 

_ PL. XXXVII 

800 feet 

WORKINGS. 
U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER NO. 63 PL. XXXVIII 

0 100 200 300 400 feet 

EJ Gneiss wall rock Granite wall rock Mineralized lead Unmineralized lead 

B LEVEL DUNDERBERG 

GEOLOGICAL PLANS OF B, NO. 4, AND EAST DUNDERBERG WORKINGS. 
MINES OF THE SILVER PLUME DISTRICT. ,, 223 

continuation, the Silver Ore lode, dips 60° N. in the Sil~er Ore tuimel. The Baltimore lode, 

described above as a branch of the Fenton lode separating from it on the northwest side and 

striking east and west, probably represents a continuation of the Terrible zone of fractures on 

the opposite side of the Fenton lode. 

From the position of the branches of the main Terrible lode at the surface and at various 

points underground it is evident that their intersection with one another to form the Terrible 

lode pitches flatly to the west, like tlie intersection of the Brown lode with the Fenton-Mammoth. 

This last-named field of intersection lies only about 500 feet north of that first mentioned. 

Dunderberg and Frostberg Zodes.-Northeast of the point where the Terrible lode separates 

into branches diverging to the west, and very near Brown Gulch, an important branch leaves 

N 

0 zoo o~-oofeet 

~--------~--------~ 

~~ Gneiss 

a 

Unmineralized lead 

Granite 

(Silver Plume) 

Dip 

~ of veins 

FIG. 61.-Geological plan of Silver Ore tunnel. 

B 

Mineralized lead 

1--~~·=J Pitch of striae 

on walls 

the main lode on the south side, opening out to the east. From this point of junction the main 

vein, continuing on with a rather more pronounced northeasterly trend, is called the Dunderberg 

(Pl. XXXVIII), while the minor branch, which may be called the Cascade, at first has a 

southeasterly trend, then swings around to the northeast parallel to the Dunderberg. 

The main Dunderberg is continuously developed undergroupd by different levels that make, 

together with the workings of the Terrible mine, an uninterrupted stretch of underground 

development extending for a distance of about 2,500 feet. At the northeast end of this region 

of development the Dunderberg veiQ branches again. A strong lode leaving the main vein on 

the southeast side and striking off to the east is shown in the Dunderberg B level and in the 

East Dunderberg; ~nd· another branch keeps on northeastward on the regular Dunderberg 

trend, but within n, short distance. 
224 GEOLOGY OF THE GEORGETOWN QUADRANGLE, COLORADO. 

The east branch, which is the Frostberg lode, is the stronger of the two. The workings 

on this branch connected with the workings of the Frostberg a short distance bBlow, causing. 

considerable litigation. In the workings of the Frostberg this lode was followed eastward 

along its strike for 1,100 or 1,200 feet· (fig. 62). On this level the Frostberg has a northerly 

dip of 50° to 72°. The position of the outcrop of this lode relative to its position in the Frostberg tunnel gives it .an average dip between these two points of 75° N. The lode as shown 

in tbis tunnel has a curving strike, which becomes due east in its eastern portion and south of 

east still farther east. 

This trend is similar to that of the Maine lode at this point, and this portion of the Frostberg is either actually a part of the :Maine or a parallel lode not far distant. On the surface the 

corresponding northwestern portion of the Maine lode is exposed by two tunnel drifts running 

northwestward into Sherman Mountain from the lode outcrop-one situated just below the 

Seven-Thirty wagon road on the southeast side of Sherman Mountain, and one about 250 feet 

farther southeast and down the hill. The Maine lode in the low:er of these tunnel drifts dips 

50° to 75° NE. In the first-named tunnel the dip ranges from 75° to 35° NE. This flat dip is due 

03 . 

• 

Quartz monzonite 

porphyry 

Mineralized lead 

a 

Unmineralized lead 

FIG. 62.-Gcological plan of upper Frostberg tunnel. 

to a small dike of porphyry striking east and west and dipping flatly to the north, which has 

deflected the vein so that it departs from its normal steep dip and follows the fiat dip of the dike. 

The normal-dip of the Frostberg-with wbich, as above stated, the Maine is actually or 

almost continuous-as measured from its outcrop at the surface to its position in the Mendota 

tunnel 1,000 feet in depth is 83° N., or the same average angle as that calculated for the Mammoth vein, which lies a few hundred feet north of the Frostberg at this point. There are 

numerous variations and irregularities in the dip, however. In the l\[endota and Burleigh 

tunnels, 1,000 feet above the outcrop, the Frostberg is a well-marked, plain lode, which has been 

followed for about 600 feet along the strike. The dip in these tunnels is variable, but is as a 

rule nearly vertical. 

Maine lo(le.-The Maine lode is traceable in an east-southeast direction from the tunnels 

above mentioned by a line of now mostly inaccessible workings. It is well shown, however, in 

a tunnel drift lying 300 feet northeast of the mouth of the Pelican tunnel. This tunnel runs 

northwestward into the hill, following the lode. Near the mouth of the tunnel the strike of 

the Maine lode has swerved so that it has become parallel with the Pelican, Which lies 400 feet 

to the northeast. The tunnel has a drift on the Maine lode for a distance of approximately 

350 feet. The lode has a dip of about 70° N. . 

Southeast of this tunnel the Maine lode has not been traced, nor have other lodes belonging 

to this general group been identified. 
MINES OF THE SILVER PLUME DISTRICT. 225 

Cascade lode and branches.-The 'Cascade lode, which has been mentioned above as branching 

off from the main Terrible-Dunderberg lode at the point where the latter crosses Brown Gulch, 

has at first a southeasterly strike and is well exposed by surface cuts, by shafts, and by a tunnel 

drift running 350 feet on the lode. This tunnel drift shows the lode with a northeast strike, 

having curved around from its initial southeast direction, and a nearly vertical dip. The vein 

in this vicinity shows considerable mineralization and some stoping has been done. 

Farther northeast this lode probably splits again, although the exact point of branching 

has not been identified. One branch of the lode is apparently represented by a tunnel drift 

about 250 feet long on the Cashier claim, which shows a strong mineralized lead striking to 

the east-northeast. The other branch appears to be represented by a tunnel drift on a lode . 

150 feet northwest of the Cashier and parallel to it. This tunnel is probably on the Groundhog 

claim; the lode clips to the northwest at angles ranging from 40° to 60°. Both of these branches 

seem from the surface showing to run into and unite with the eastward-trending portion of the 

main Terr~ble-Dunderberg-the Groundhog branch into the Frostberg portion, the Cashier 

into the Maine, both junctions occurring near the Seven-Thirty wagon road. 

In the Frostberg tunnel the Cashier lode is probably represented by a fairly strong mineralized lode crossing the tunnel halfway between the mouth and the drift on the Frostberg lode. 

In the Mendota tunnel an east-northeast lode diverging from the Frostberg, which here trends 

east and west, has been drifted on for upward of 400 feet. This is a definitely mineralized but 

FIG. 63.-Geological plan of Bush tunnel. Note local deflection of lode on passing through gneiss. 

' 

not very strong lode with a practically vertical dip. East of the drift on this lode in the 

Mendota tunnel the lode is represented in the Burleigh tunnel by an unmineralized clay lead. 

This may be the Cashier lode of the surface, as the flatter dip of the Groundhog branch would 

make it unite with the Frostherg above this point. If this lode is actually the downward 

extension of the Cashier, the average dip of the lode fr?m the surface would be about 73° NW. 

MENDOTA VEIN SYSTEM. 

The southwest end of the Mendota vein is known as the Smuggler vein. This portion is 

well opened up by tunnels and drifts, and is traceable from a point near the mouth of the Union 

· tunnel northeastward across Brown Gulch to the Petersen shaft, from which workings go down 

to the Victoria tunnel level. The workings near the surface are continued northeastward from 

the Petersen shaft, with very little break, by the drifts of the upper -Mendota tunnel which run 

along the road for more than 500 feet. About 200 feet northeast of the crosscut portion of 

the tunnel, along the drift on the lode, the Mendota vein divides into two branches. On the 

surface not far. above this point these branches can be identified and followed. One branch 

runs nearly due east to a shaft on top of the ridge on the Tishomingo claim, and is traceable 

eastward along the surface for about 600 feet, beyond which it is not recognizable. This 

Tishomingo lode has had some little production. 

About 250 feet northeast of the upper Mendota tunnel, and farther up the hill, is another 

tunnel drift whose mouth is situated ncar the branching. of the Tishomingo lode from the 

Mendota lode proper. This tunnel runs in on the main north branch. At a point less than 

200 feet in this branch divides again into two forks (both mineralized leads), dipping 70° N. 

31401-No. 63-08-15 
226 GEOLOGY OF THE GEORGETOWN QUADRANGLE, COLORADO. 

to 90°. Both of these branches can be followed on the surface. The southern of the two seems 

to curve around and rejoin the Tishomingo. The continuation of the other branch is shown 

chiefly in the underground workings of the last-named tunnel. Then, with a break of 250 feet, 

what is probably the same lode is shown in the workings of the Bush tunnel. In these wor!fings 

the lode was drifted on for about 400 feet. It runs in a generally east-west direction, with a 

variable dip which approximates the vertical. In the east end of the workings the lode passes 

Granite (Siher Plume) 

Pegmati~te 

El Mineralized lead 

N 

El Unmineralized lead 

f7Tl .. · l==:::=J Drp of vem 

~ ~ Pitch of striae on walls 

690 feet 

FIG. 64.-Geological plan of Mendota tnnnel·workings. 

transversely through the same porphyry dike that is crossed by the Seven-Thirty vein (fig. 63). 

The lode here near the Bush tunnel workings gives off a weak branch which strikes to the 

'southeast. · · 

Farther east the main lode splits into two parallel branches that can be traced to a point 

just beyond the summit overlooking the Pelican tunnel, qeyond which neither branch can be 

followed. 

Underground, the Mendota lode is followed continuously by workings on a number of 

levels in the Smuggler mine from a point near the mouth of the Union tunnel northeastward 
U. S. GEOLOGICAL SURVEY PROFESSIONAL PAPER NO. 63 PL. XXXIX 

E w 

~- '-' __ zo_o -~Joo_~4oofeet 

-~=============--~ ~=~-~=~~ 

VERTICAL LONGITUDINAL PROJECTION OF CHIEF WORKINGS ON MENDOTA VEIN. 
MINES· OF THE SILVER PLUME DISTRICT. 227 

to the Mendota workings. In the Mendota mine (Pls. XXXIX, XL, A) the lode is also exposed 

by many levels. The longest drift on the lode is that on the Victoria tunnel level of the Mendota, which. runs for more than 1,200 feet west of the tunnel (fig. 64). ·The position of the 

vein in the Mendota tunnel as compared with that at the surface shows an average southerly 

dip of about 83°. The Burleigh tunnel cuts the strongest branch of the Mendota, the Bush, at 

700 feet below the outcrop and vertically beneath it. In this tunnel the lode has been drifted 

on both east and west, but· the drifts are now caved. The western drift on the lode is reported 

to have connected with the workings of the Mendota tunnel. The Tishomingo branch of the . 

Mendota is not recognized in the Burleigh tunnel. Immediately above the tunnel this branch 

is also weak at this point. 

COMPOSITION QI<' VI<~ INS. 

All the lodes of the Terrible group have similar characteristics, which arc distinct from 

those of tlie veins of the Bismarck-Pelican-Corry City-Pay Rock group already described. 

In the lodes of the Bismarck group vein quartz is usually present, though ordinarily subordinate.in amount to the metallic minenils. Large quantities of coarsely crystalline, dark-colored 

blende are characteristic of the Terrible lodes. The other common metallic minerals are 

galena and pyrite, with some chalcopyrite. The galena is not invariably argentiferous to any 

important extent, though as a rule it contains a small amount of silver. From some of the 

mines considerable high-grade silver ore has been extracted, notably from the Terrible (Pl. XL). 

It is also reported that considerable high-grade ore, running from 200 to .500 ounces of silver, 

has been produced from the Baltimore. The high-grade ore from the Terrible ran from 100 to 

800 ounces or even higher in silver. Some of the lodes of the group, however, have produced 

very little high-grade ore. The Mendota and Dunderberg as a rule show low-grade ore almost 

to the surface. The great bulk of the Mendota ore assays frorri 25 to 30 ounces of silver, contained in coarse blende and galena within 50 feet of the surface down to a depth of 700 feet or 

more. 'fhe ores contain generally a very small quantity of gold, but sometjmes the amount 

is sufficient to be paid for by the smelters. The reports of the production of the Tishoming(} 

fodc in 1890 show $19,368 in silver and $1,950 in gold. In this case the value of the gold was 

10 per cent of that of the silver, but usually the proportion is much less. 

The ores near the surface show in many places considerable siderite as gangue. Barite 

was observed at one point in the Dunderberg workings on the third level, 1.50 feet from the, 

surface. The barite occurs as clear crystals lining the walls. of a cavity and associated witl1 

quartz. Where it was found· the Dunderberg lode was represented only hy a faint unrnineralizcd slip. 

In some of the lodes gray copper has been found, especially near the surface. Ruby silver 

and polybasitc also occur, most abundantly in the upper levels. Fine spcci~ens of light-red 

ruby silver (proustite) associated with polybasite have been described." The following is an 

analysis by Genth" of polybasite from the Terrible mine: · 

Analysis of poly basile from 'Terrible mine. 

Silver ... 0 0 0 0 0 0 0 0 0 °. 0 0 0 ° 0 0 0 0 ° 0 0 0 0 0 0 0 0 0 0 0 0 0. 0 0 • 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0. 0 0 0 0 0 0 0 0 0 0. 62. 70 

Copper 0 0 0 0 0 _ • 0 ••• 0 •• 0 ••••••• 0 • _ • 0 _ 0 • 0 • _ 0 _ 0 _ 0 •••• 0 0 0 •• 0 0 _ 0 0 __ 0 __ 0 __ 0 0 __ 0 0 0 _ 0 , 0 _ 0 _ 0 _ 0 _ _ _ 9. 57 

Iron._ 0 _ 0 _ 0 0 0. 0 0 ••• 0 0 0 0 0. 0 0 •• 0. 0 •• 0 •••••• 0 0. 0. 0 0 0 •• 0 0 0. 0. 0 _ 0. 0 0 0 0 ___ 0 _ 0 __ 0 0 0 0 0 0 _ 0 _ 0 0 _ _ _ • 07 

Arsenic __ 0 ___ 0 0 •• 0 0 ° 0 0 0 0 • 0 0 __ 0 _ 0 _ 0 ° ___ 0 _ 0 _ 0 0 0 0 0 •••• 0 ••• 0 • 0 • _ 0 _ 0 0 0 ___ 0 __ 0 0 0 _ 0 _ 0 0 0 _ 0 _ 0 _ _ _ • 78 

Antimony ........ 0 • 0 0 0 ° 0 0 0 0 0 •• 0 .. 0 ...... 0. 0. 0 •• 0 • 0 • 0 • 0 ° 0. __ 0 •• 0 0 0 ___ .. __ 0 ___ 0 0 _ 0 0 0 _ 0 0 0 0 10. 18 

· Sulphur (by difference) .... 0 0. 0 0 00 0. 0 •• 0 0 0 _·_ 0. 0. 0. 0. 0 ....... _____ 0. __ 00 __ • ____ 0 _ 0 _ __ __ __ 16.70 

100.00 

Although· the general composition of all the lodes is similar, there are slight differences 

throughout. It has even been noted that where a vein splits into two branches the ores in each 

have a slightly different character. 

a Genth, F 0 A., Proc. Am. Phil. Soc., YOL 23, 1886, p. 39. 
228 GEOLOGY 0.1!' THE GEORGETOWN QUADRANGLE, COLORADO. 

NATURE 01<' VEINS. 

The Terrible group of veins embraces the strongest lodes in the Silver Plume district. 

They are in the main well-defined fissure veins following strong fracture zones or' filling open 

fissures, many of which have been partly filled with angular rubble before cementation by the 

or~ minerals (fig. 65). The vein filling is characterized by large masses of hlendc and by 

relatively small amounts of silver. The proportion of blonde to galena is greater than in the 

· veins of the Bismarck group. 

While the more important lodes are filled fissures or heavy fracture zones the smaller ones 

are simply clay leads composed of altered rock material following along slip planes. Such 

decomposed zones may be several inches thick. Two strong fractures may form the walls of 

such a lead. In some places a third fracture, at first slightly divergent, may join the main .zone, 

forming a new wall, when one of the former wall fractures is forced to become a center fracture. 

This is often noted. In 

many places two nearly parallel small veins or leads approaching closely together 

form the walls of a larger 

vein, the intervening rock 

being replaced by ore. A 

good example of this is seen 

in a tunnel of the Brown 

mine (fig. 81). A vein thus 

formed has usually two good 

walls. 

AU the leads and veins 

have formed along definite 

hut relatively slight faults 

having an approximately 

horizontal movement. EviFm. 65.-Tioriwntal sketch showing Mendota vein as exposed in roof on second level east beloiY deuce of the faulting is furVictoria tunnel level, Mendota mine. Shows blcnde cementing a rubble-blockec! fissure. • l d · · b n1s 1e In many vems y 

discordance of the opposing .walls, but as a rule this discordance is only slight. Evidence of 

the movement and of its direction is also furnished by abundant strire. The following is a list 

of some of the strire observed along lode W!lllS: . . 

Fenton lode, West Terrible tunnel, strioo are horizontal and also pitch 25° \V. on lode. 

Fenton lode, Silver Ore tunnel, strioo pitch 5° E. and also 5° W. on lode. 

Mammoth lode, above Seven-Thirty road, strioo pitch 25° W. on lode. 

Glasgow vein, Union tunnel, sLrioo are horizontal on lode. 

Mendota vein, Smuggler mine, fifth level, strioo pitch 20° E. on lode. 

Frost berg vein, East Dunderberg tunnel, sLrioo are horizontal on lode. 

Terrible vein, level No. 3, Dunderberg mine, sLrim pitch 12° W. on lode. 

Mendota vein, Petersen shaft, 30 feet below surface, strim pitch 15° E. on lode. 

Mendota veii1, upper Mendota tunnel, stri:.e are horizontal on lode. 

Mendota IVein, Bush tunnel, stri:ce pitch 8° W. on lode. 

Mendota vein, second level below tunnel, stri:ce pitch 5° W. on lode. 

Mendota vein, third level below tunnel, stri:.e are horizontal on lode. 

Mendota vein, fourth level below tunnel, stri:ce pitch 30° IV. on lode. 

Maine vein, upper main tunnel, st.rioo pitch 15° IV. on lode. 

Maine vein, upper main tunnel, sLri:ce pitch 7° W. on lode. 

Branch of Mammoth lode, Backbone tunnel, sLri:ce pitch 10~ on lode. 

The lodes are prone to weakening by repeated branching. As a rule the branches open 

out in the direction of the weakening vein so that a main lode splits and feathers on both ends; 

in many lodes, however, minor branches diverge in the opposite di~·ection from that of the 

main branches. A combination of branching in the norma.I and abnormal directions constitutes 
U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER NO. 63 PL. XL 

A. MENDOTA MINE AND MILL. 

1, Mendota mill; 2, Victoria tunnel , Mendota min e; 3• Upper Mendota tunnel. 

B. TERRIBLE AND SMUGGLER MINES. 

Showinf?: also the great amount of wash from Seven-Thirty and other mine dumps brought down Brown 

Gulch by spring freshet. 1, T errible mill ; 2, Uni on tu nne l, T erri b le mine; 3, Smuggler mine; 

4• Si lver Ore tunn el, T errible mine; 5• wash from Seven-Thirty and other mine dumps. 
MINES OF THE SILVEH PLUME DISTRICT. 229 

a splitting and reuniting of the vein, leaving a horse of rock bet\Veen. This condition is not 

uncommon, and is well shown in the workings of the Brown tunnel. In extremely rare and 

unimportant cases leads cross. Such a crossing was noted in the East Dunder1,:>erg tunnel, where each of the leads contained a little 

mineral. 

STRUCTURE 01<, VEINS. 

CHARACTERIS'riC FISSURE FILLING. 

The most striking thing about the veins 

of this group, which are typically fissure veins 

formed in preexisting cavities, is that all these 

cavities are lined by comb quartz representing a first depositipn invariablyprecedi~1g that 

of the sulphides, which usually fill up the cen-

. tral and greater portion of the vein and are 

characteristically and practically free from 

admixture with intercrystallizecl quartz or 

other gang'ue (figs. 66, 67). These quartz 

linings show that in nearly all the veins the 

deposition of metallic minerals did not take 

place for a very definite period after the opening of the fissures. 

The Mendota lode is highly characteristic 

of this whole group, and its description may 

serve for all. In places this lode shows only 

slip walls with no mineralization. This tight 

slip, however, opens up rather abruptly into 

:Fra. GG.-Sketch of zone of Mendota vein, third level below tunnel 

level, showing (1) earlier deposition of quartz and subsequent 

deposition of metallic sulphides in open fissures; (2) displacement of 

vcinbypostmineralfaults. a, Granitecountry rock; b,combquartz 

without sulphides; c, sulphides without gangue (chiefly coarse blonde 

with a little galena, pyrite, and chalcopyrite). 

a strong breccia vein from 1 to 4 feet wide in which occur large bodies of blende with 

pyrite, inclosing numerous angular fragments of the country rock. This breccia vein is 

without doubt the cementing of a large rubble-filled fissure by ore. On the ·victoria tunnel 

Fra. 67.-Sketch of portion of a veinlet shown in fig. 

66, showing sheathing of quartz between sulphides 

and vein walls. a, Granite wall rock; b, comb 

quartz without sulphides; c, solid sulphides without gangue (principally blende with some galena 

and pyrite). 

level such a fissure vein continues for several hundred 

feet, then it narrows again to the east, and the lead 

changes into a zone of fractures marked by small seams 

of blende. Farther on the seams become very poor and 

are not minerali;r.ed. Still farther east the slip leads on 

the vein become stronger and, although not generally 

mineralized, contain bunches of galena, pyrite, blende, 

etc., up to a few inches in diameter (fig. 68). Beyond 

this part the lead opens out 11gain by the thickening of 

the sulphide streaks and within a short distance becomes 

a breccia vein 2!- feet thick filled with blende and including angular fragments of the country rock (figs: 60, 70). 

At one point here the structure is reversed and angular fragments of blende are included in. a hard rock which 

at frrst sight appears to be a uniform granite or porphyry. :Microscopic examination, however, shows that 

this rock is a finely ground granite cemented by quartz. 

The cement consists of fine crystalline to cryptocrystalline silica. This unusual occurrence indicates a rending apart of the vein subsequent to 

its formation and the deposition of the blende, and the filling of the new fissure thus created 

with a paste of ground granite probably brought in by waters, the granite catching up and 

cementing angular fragments of blende broken up by the rending of the original vein. 
230 GEOLOGY OF THJ'J G.EORGBTOWN QUADHANGLE, COLOHADO. 

Although this was subsequen,t to the ore deposition, the interspaces were afterward cemented 

hard by silica (fig. 71). The effect of this postmineral brecciation, however, was not extensive, for 10 feet farther west along the vein the normal 

structure reappears, where coarse blende cements a breccia of 

the granite country rock. A little farther west the vein again 

closes down to a slip and then to a bare fracture. 

Typical fissure veins are illustrated at many points in the 

other mines (figs. 72, 73). All gradations are illustrated· at 

various points, from ore reticulating a crushed zone of rock 

to that cementing a rubble-filled fissure, the gradations 

depending on the relative size of the openings in the fissured 

zone and of the included granite fragments. Fig. 74, a sketch 

of the Frost berg vein on the Frost berg tunnel level, shows this 

transition very well. 

FII,LING 0~' CAVITIES OF DISSOLUTION. 

While a large number of the cavities which have been filled 

with minerals are fissures clue to rending, another large number are irregular in shape and are plainly cavities of dissolution formed by the solvent action of waters circulating along 

a crushed fault · t: 

FIG, 68.-llorizont.1l section of roof of drift, 

Mendota mine, second loveleu.st, showing 

·deposition of !Jlende by impregnation of 

gouge zone. a, E.anging-wnll slip; b, footwall slip; c, zone of gouge (blackmnrkings 

are ore, chiefly blende); d, hard black 

gndss (walrrock), 

zone. Su bseq u en t to the 

f au l t i n g and 

during hardening and cementation of the 

faultgougethese 

irregular openings were in places extensively formed and were 

subsequently filled with. quartz and sulphides. 

The structure of these vein fillings is entirely 

similar to that of the fillings of cavities clue to 

rending, the quartz invariably forming a lining to 

the opening and the metallic mineral occupying 

the center. 

Similar cavities have formed at a later date, 

subsequent to the first filling of the vein, and 

indeed are probably being locally formed at the · 

present day by the dissolving action of descending 

surface waters ·along watercourses. Extensive 

elongated dissolution cavities of this sort were 

noted along the Mendota vein. These cavities 

are typically lined with quartz crystals which 

represent the lates,t deposition, probably the work 

of present descending waters. Some of these 

open vugs show incrustations of other minerals 

besides quartz. A specimen from the Brown 

mine shows a vug in a mixed sulphide ore conw "------"-2 -~-----..1 feet 

FIG. 69.-llorizontal section of roof of drift, :Mendota mine, 

second level east below Victoria tunnel Jeyel, showing fissure filling characteristic of Mendota vein. a, Hanging-wall 

slip; b, foot-wall slip; c, zone of coarse b1ende, pyrite, and 

galena containing angular fragments of gneiss; d, hard black 

gneiss (country rock). 

sisting of blende, pyrite, and, galenn. This vug is lined with free quartz 

representing a later deposition, are crystals of calcite and a brownish 

crystals upon which, 

cal"bonate, probably 

siderite. 
MINES OF THE SILVER PLUME DISTRICT. 231 

RENEWED MOVEMENTS SINCE BEGINNING OF ORE DEPOSITION. .. There is also evidence that the vein has been disturbed by renewal of stress subsequent to· · 

its deposition, resulting in reopening of the fissures. In many veins it is plain that this has 

happened repeatedly at intervals. 

A specimen from the Dunderberg vein shows (1) galena brecciated and cemented by· 

pyrite, (2) pyrite brecciated and cemented by siderite, (3) jaspery silica coating cavities, as the 

latest deposition. A specimen from the :Mendota at a depth of 650 feet shows the following: 

(1) Opening of the original fissure, (2) deposition of comb quartz on the walls, (3) filling of the 

remainder of the fissure by blende, (4) reopening of the fissure near its walls, (5) cementation 

of the new opening by galena, (6) reopening of the fissure, (7) cementation of the opening by 

pyrite. Similarly the Frostberg vein at the Burleigh tunnel, at a depth of 1,150 feet, shows a 

banding of different minerals. In ohe place, where the vein is 1 foot wide and consists of solid 

sulphides, there is a band of blende on both sides and the center consists of galena (fig. 75); 

in another place there is blende on both 

sides and the center consists of pyrite 

(fig. 76). In thelast-namedplacethereis 

not sufficient evidence to show whether 

this banding represents successive deposition or crustification in a single fissure 

or whether it represents the reopening 

of an original blencle-filled fissure and 

the cementation of the reopened fissure 

by galena and pyrite. The mass of 

evidence elsewhere, showing that this 

rending is extremely common, is in 

favor of the latter explanation. Any 

stresses applied to the rock after the 

formation of the vein would result in 

the splitting apart of the brittle sulphides rather than a reopening of the 

wall rock. This is not only a natural result, but one which is shown very convincingly to have taken piace elsewhere, 

as, for example, in the Griffith mine. 

A specimen from the e.ast end of 

the drift on the Frostberg vein, on the 

Victoria tunnel level, shows the folhwing 

history: (1) Opening of the original fisE 

w 

FIG. 70.---,Horizontal section of roof of drift, Mendota mine, second level cast 

below Victoria tunnel level, showing jog in Mendota vein. This jog is in· 

tcrpreted as probably due to prem!neralfaulting of the vein fissure, the fault 

having the strike a-a and the direction of movement wHhin a nearly vertical 

zone of the vein fissure. The effect of this movement was to bring a 

broader portion of the vein fissure opposite a narrow portion. b, Solid 

coarse blende with some pyrite and galena, containing angtt!ar fragments 

of granite; c, granite (countr~· rock). 

sure, (2) filling of the fracture with a soft, pasty mass of finely crushed and pulverized rock, 

inclosing a few angular fragments of porphyritic granite or "corn rock," (3) cementation of the 

crevice filling by silica, (4) fracturing of the cemented mass, (5) filling of these small cracks by 

blencle and quartz, (6) reopening of the main fissure, (7) deposition of comb quartz on the walls, 

(8) partial filling of the fissure by blende associated with a small amount of contemporaneous 

cupriferous pyrite, (9) deposition of copper-bearing pyrite and galena on large crystals of blende 

in vugs (the galena b.oth contemporaneous and later than the pyrite), (10) coating of the Qubooctahedral crystals or galena in vugs with minute crystals of pyrite or marcasite and coating of 

faces of the sphalerite crystals with a thin film of drusy white quartz. 

The fact that in this last stage the minute pyrite crystals are confined 

surfaces of the galena crystals, whereas the crusts of quartz occur only on 

indicates 'that the different minerals had caused selective deposition. 

entirely to the 

the sphalerite, 

It appears, therefore, that the solutions circulating along a given fissure were constantly 

changing in character, just as the slightly but constantly changing character of mineralization 

in different lodes testifies to different solutions or different blends of solutions circulating 
232 GEOLOGY OF THE GEORGETOWN QUADRANGLE, COLORADO. 

along different channels. The whole process of ore deposition was plainly very slow indeed, 

lasting through intervals of quiescence and the reopening of fissures which probably represent 

a long period of time. The evidence afforded by the veins, indeed, does not indicate clearly 

that the cementation by ore of the openings that still exist has ceased. Inasmuch as the pri-

. mary ore deposition was preceded by the lining of the cavities by quartz crystals, and as the 

abundant cavities, many of them large, that are now found along the veins are usually lined with 

similar quartz crystals of a later generation, the question arises whether the later deposition 

of quartz lining the cavities might not be succeeded by a new deposit of metallic sulphides. In 

beautiful specimens from the Argentine district, which is a few miles from the Silver Plume 

district, we have indeed examples of this occurrence. Here large cavities or druscs in the older 

ore are lined with comb quartz, and sitting free on the quartz crystals, as the last deposition 

from circulating waters, are beautiful and perfect crystals of metallic sulphides-pyrite, galena 

FIG. 71.-Horizontal section of roof of drift, Mendota mine, second level 

'vest belo'v Victoria tunnel level, showing exccpti011al and renuukable structure of Mendota vein due to postmlneral movement. 

a, Gra.nite waH rocksofYein; b,coinbquartz bordering vein; c, pyrite; 

d, coarse blende; e, uniformly ground-up granite, cemented hard by 

silica and having a close resemblance to the unbroken granite of the 

walls. 

and blende, copper pyrite, and polybasite, 

comprising all the species of the original 

deposition. These cavities are now the 

channels of descending atmospheric waters. 

It is plain that these waters are not hostile 

to the formation of the sulphides and are 

not dissolving them. Indeed, it is probably 

from these waters that they are being 

deposited. 

NATURE Ol!' WALL ROCKS. 

The wall rocks of this group of lodes 

consist of gneiss and granite. Granite unmixed with gneiss occurs over considerable 

areas here, anJ somg of the strongest lodes 

lie entirely in it. Other considerable portions of the wall rock consist of granite intimately mixed with gneiss, and pure gneiss, 

without any great admixture of granite, 

forms the walls in a smaller number of 

veins. Of the larger openings, the Baltimore tunnel is chiefly in hard gneiss; the 

Silver Ore tunnel is in massive granite; the 

Union tunnel workings are principally in 

granite with a minor quantity of gneiss; 

the Dunclerberg workings arc all in granite 

except at the north end, where they run 

into gneiss; the Mendota vein cuts through intimately mingled gneiss, pegmatite, and granite; 

the Burleigh tunnel is mainly in gneiss, with some granite; the West Terrible tunnel is all in 

gneiss, so far as it is now accessible. More detailed evidence of the relation of granite and 

gneiss as wall rock to the lodes is given in the general map (Pl. XXI), and in the detailed 

plans of different mine levels. 

The granite is identical with that found all over the area shown on the special map and 

alre&dy described. The gneiss is of different varieties, ranging from rather soft and schistose 

to hard and granitic. 

Porphyry is of very minor importance as a wall rock to this group of veins. The large 

northwestward-striking dike of porphyry, traceable on the surface from the northwest corner 

of the area mapped down to Clear Creek valley, east of the Burleigh tunnel, is cut through 

almost transversely by a number of lodes belonging to this group, such as the .Mammoth, 

Dunclerberg, and Frostberg. Northeast of the intersection of the porphyry dike with these 

lodes, in the area just north of the Seven-Thirty wagon road, where the principal lodes split 
MINES OF THE SILVER PLUME DISTRICT. 233 

into a number of branches with a general east-west trend, two small porphyry dikes have 

deflected some of these branches and act as walls to the lodes. These dikes, however, have 

been traced for only about 500 feet along the strike, and it is not probable that they extend 

very much farther. The veins are more persistent, and, passing on southeastward, come again 

into granit,e and gneiss as wall rocks. 

In the Baltimore tunnel, north of the vein zone, porphyry has been encountered and drifted on for some distance, but is entirely unconnected with any mineralization. 

Moreover, two porphyry dikes were encountered in the 

Burleigh tunnel in driving a cross-cut .from the east drift 

·on the Phillips vein to the Bismarck-Pelican lode. The N 

first of these dikes encountered was only 5 feet wide; 

the second, however, was a broad dike corresponding 

probably to the large dike of quartz monzonite porphyry, which runs northwestward on the surface and is 

cut by the Seven-Thirty, Frostberg, and other veins. 

ALTERATION OF WALL ROCKS. 

The wall rocks of the lodes are invariably altered. It 

is difficult to distingui~h the alteration which has been 

FIG. 72.-Cross section of Phillips vein, Burleigh tunnel level, showing angular fragments of wall rock 

in the vein filling. Gn, gneiss; Gr, granite; Kg, 

kaolinized gneiss and gouge. 

accomplished by the vein-forming waters from that which has been accomplished by the 

ordinary surface waters penetrating downward .. The effeets of these waters in portions of 

the rock remote from the veins will be described later. The following is a description of some 

of the alteration in the very walls of some of the .important veins: · 

Fro. 73.-Cross section of Phillips vein, west end of Burleigh tunnel lcvel,.showing horse of gneiss in Yein. 

a., }.ficaceous gneiss; b, gouge selvage; c, rniXtnre of 

quartz and crushed silicified gneiss; d, hornblende 

gneiss; e, fractured gneiss. 

A specimen of granite from the wall of the Dunderberg vein, on the D level, near the surface, proves under 

the microscope· to be slightly stained throughout. The 

microeline is fresh as a rule, but along cracks is altered 

to sericite and is somewhat impregnated by crystalline 

siderite. Muscovite and zircon are also unaltered. 

The other minerals are completely altered. Biotite 

is entirely decomposed and is recognized by cloudy, 

partly crystalline siderite, which obscures the bleached 

mica, and by webs of unaltered sagenite (rutile). The 

feldspar, other than mierocline, is entirely decomposed 

to coarse sericite. Along cracks in the rock siderite has 

crystallized, and one veinlet is lined with crystalline 

·siderite and the interior is filled with quartz. The 

relation of the siderite derived from the alteration 

of the biotite to that which has formed along cracks 

as veinlets indicates that the vein siderite also has been 

derived from the rock. 

It is only on extreme alteration that the microcline 

of the granite is decmnposed. A specimen of the granite 

wall rock of the Mendota vein, more highly altered 

than that just described, shows, of the original more 

common granitic minerals, only muscovite surviving. 

The feldspar is represented only by mats of sericite. Thls rock eontains isolated grains of 

·crystalline, idiomorphic quartz showing zones of growth on the outside. These grains probably 

represent the granitic quartz grains enlarged by deposition from mineralizing waters. Assoeiated espeeially with the quartz are large grains of intergrown blende and galena. 
234 GEOLOGY OF THE GEORGETOWN QUADRANGLE, COLORADO. 

Several specimens of extremely altered gneiss from the wall rock: of the northwest extension 

of the Mammoth lode in the Bac.kbone tunnel, above the Seven-Thirty road, show of the original 

0 2 .3 feet ~------~------~------~ 

Mixture ~ of hani Clay ~ gouge selvage 

~~cl Vuglined 

granite and gneiss with quartz crystals 

FIG. 74.-Sketch of Frost berg vein near shaft on upper tunnel JeyeJ, showing a condition of the vein transitional between a 

zonC' of rcticulnting veins in granite and a zone of ore containing nngular granite fragments. 

minerals only quartz and zoisite unaltered. The feldspars are represented by areas of fine 

sericite; the biotite is bleached, with the separation of abundant siderite, which is replaced 

in part by limonite. In. the last 

stage of alteration the rock becomes 

simply an aggregation of quart7., 

sericite, and crystalline siderite, with 

some pyrite and numerous zeolites. 

Siderite forms microscopic veinlets, 

and kaolin occupies portions of the 

rock that possibly represent shrinkage cracks. This localityisfromlOO 

to 300 feet below the surface. Originally this rock seems to have been 

a fine quartz-feldspar-biotite schist 

containing some muscovite. 

INFLUENCE OF WALL HOCK ON VEINS. 

RELATIVE INFLUENCE OF GRANI'l'E AND 

GNEISS 

It is proved everywhere in this 

group of veins, as indeed throughf'T-J 

~ Granite Galena ore Brown sphalerite ore Quartz 

carryjng a little galena 

0 

Clay selvage 

FIG. 75.-Cross section of Frostberg vein, showing probable reopening of blende vein · 

and deposition of galena. 

out all this general mining district, that the physical nature of the wall rock exerts a very 

delicate and important influence on the veins (figs. 77, 78). · 
MINES OF THE SILVER PLUME DISTRICT. 235 

The principal wall rocks of the Terrible group of veins, as before stated, are granite and 

gneiss. Of these rocks the granite has proved the most favorable for strong, persistent :fissures 

by reason of its homogeneous structure. These veins are strongest, straightest, and most 

productive when passing through granite. Moreover, in granite fissure fillings are very common 

FIG. 76.-Cross section of Frostherg vein, showing probable reopening of hlende ,·ein and cementation by pyrite. 

a, Granite; b, qunrtz~ c. .. sphalerite; d, [pyrite; e, mixcll pyrite-sphalerite ore and country-rock breccia fragments; 

/, pyrite-sphalerite ore. 

or characteristic, but when the same vein passes into gneiss the fissure vein becomes a decomposed rock lead with less mineralization. In granite the walls are generally well defined, 

whereas in gneiss, especially the softer varieties, the lead is mineralized much more irregularly. 

The effect of a vein on passing from granite into gneiss is well shown at the northeast end of 

the Dundcrberg workings. This strong vein here passes from granite to soft gneiss on the 

"FIG. 77.-Sketch of outcrop on hill north oi Brown vein, showing path chosen in homogeneous rock (porphyritic granite). a, Joints; b, joints with . silicification along 

margins. 

northeast; an immediate weakening and splitting of the lode is very 

plain and is noted on all levels, 

being especially oh;mrved on the 

Dunderberg B level (fig. 79). At 

the southeast end of this level the 

vein also passes into gneiss and the 

weakening and impoverishment of 

the vein is again noted.· The same 

thing is well shown on the Dunderberg A level. A strong shoot of ore 

in gtanite goes as far northeast as 

the granite contact with the gneiss. 

This shoot follows the granite contact from level to level, pitching at 

a flat angle with the contact, but 

does not enter the gneiss. ·Thelead continues on and enters the gneiss, but is poor and practically unmineralized. On the B level by far the greatest production of ore was from that portion 

which lies in granite near the gneiss, though some extended into the gneiss. It is probable 

that at this point the fractures may have scattered and forked in the granite by reason of the 

'resistance of the tough gneiss to the stress. Thus a number of forks were produced, furnishing 

the intersecting channels which have proved so favora;blc for ore bodies in this region. 
236 GEOLOGY OF THE GEORGETOWN QUADRANGLE, COLORADO. 

EFFEC'l' OF CHARAC1'~JR ANn ATTITUDE OF GNEISS. 

The character of the gneiss, whether soft and tough or hard and brittle, is of great importance to the existence _of a vein, as shown in this vein group. In the soft gneiss the vein splits 

repeatedly and is not far ~~'. J_.-\1

"";'1<. ,:.f,/,~';..-,\-; /'9\/-,'_,t .... l~l!..)t-/ traceable, but in the hard .'::--,. I ';. '/ I- I ) '-I I -/-1 / '/ !'_:-/,"'\~ ;:{-1

· >-. 1 '/

1

')_ 1 :~ ---r ->._1.----1';_ 1

1

'-< I ,--t-...1 }_.1\ I 1-/

1

"'-/1 "'/I ]-',L \ \ and rigid gneiss the condi- ' '-..·.\ - "'. -- ,...:_;,..- ....... './ \ " - \-.. -/ ......... - \ ..- / 

\

;\ '1/t- "'"J l '.( \ \/~"r,;..~_ '-1

. J \' 1-/ .._:: !-''-,'' -... 1 \-1 1-1

) 1 1 " ; ~I tionsarefavorablefor strong - -!-''''·' ....._..._,.; \- ·· .. ·. "- 1-'1- / / -\./-I l .l A . f . ,-1 (-1 > ;'>t:: . .-/

1 I:) ;:'1 I'/\>~'/ f /-..:-'/\'\ .._ 1.;\ 1/--'1-) 1

, \ -, 1 1 '-, oues. nuxturc o granite, 1 _ \. '"" '- "-' ··c- _:::: ,_'- ';--...,..._//l';'-\::",: 1 -I -I -I-,-!... I - \ --!.-~ --1 pegmatite, and gneiss has ;-; I ~ -._!,\,.·\_.! !_; 1\ ~'\," , ... J-\'; _!1 ""'--'- - I ,:: ,...-!_!, /~I '- I '.._!'-I "__1-J.:i>y;;(\~~~1'!~{:';:·\, 1 ,;.-!-"'....!; ;_ :_l 1 / .._: ~~ ::z--:>\1 proved in this vein group, \'~''...._rj_ \/ 1/, ~ \,,>-,-/.l>y_";~\f':::!~·.;;t,Tf/\ f-.. I- "'~I ,1,1 I/ 1 1 /-... as in other parts of the dis- /!-'\ I\.-\; ~;}'/'y\-\ ~~'~\/..:."":::;-,;_!__~ 1\ ;..::::;\'\?-/-/ r-I trict,afavorableveinfonna-

,.-.... \...( _ ~--yl ,,, 1:->-,/---;....;11 , _ 1

; -0 ?.:)/ 1

__..,/ \"" 1

_, -"'~ ;'::-/>1

\ /1- 1 1

c , \ ;.-r---:-1..:.-..., \ ~;..._'\-; ;..: 1 /"' \-....' 1 \-. tion, permitting the exist- _ ...... /\/~~- ~~~;.=;,.;/,i,\_:::-J-;:-~_-\/'\'j'...:::~·::::)- 'I--'\\ 1 --.......: /~, ' ence of strong and persistent 

__.I I \.-1/1~\ j'-V/c:b'I-:--,_:-\.....-J 1'--;_.'):.J "-/...._If-/ 1- f 

!

;-.,'-\/'- ,- 1 F' ~\-.~lJ...:: ~~-\ ;;-...... ~;v).y_<) .. -; \'I\""\/.\ 1-;' ractures. _-../\-'-\r_;;-~;>--i/..._J/'r,;''._-;~~,-1.).···(-/--'' rn1 t'k fth · ,_-........:....._ -1...-- ...--'-1 \\1-\;.!_1

...-- i;' .. J.;:,~v>;;.:.:o,: .. ...., 11 /, 1/!._\./ .11esn eo egnmss ...._ \r IJ-'f-/--,'~/ ·:, '-_.:.<~v<t"o'!--'·•.\_.\- '/ I;~ l . l \/;_1

'-yi-/ 1-H:',\\\ :-.;(;:'"""'.,>"~>,~"'-\/\)-/"'I-\/-..\ 1as m many paces un-

)' ;,~ (i~l-t-, ·:;.. _:-1 -..(l/_ ,:':_.;..;~-z;~;~'r..~/'/- ·,- ""\ 1/--= \!..-, doubtedly some influence on 

\ 1- ;- ;-, Y-···->''~">"~~",-...1-"' -\ I \1-:::..-/ I n-1/ 1-.... 1'-\•c-,~.;~d~~i~"t!~\--//~}l\-;-::-1 I /-'I the lode. In this group of 

"Z...j''rl-1 1 . V.....:/ /....._ \f "• "',..>~<~ .... --;._~-; .... ' ,(-.f/.!..:.0~ \f- veins the relations of the t5.'-;"::::::\::: 1

& -n r\!.. ---/~"~~"---.::::;,r;J...' (' --t ~' "'::./ "-> '/ /.; \ I -:-( ~..} J__-/ \ ':::-- ; '\, \ I ""!.:> v <\v<"' 1 \/ /_ !-' /.' 1 :::_1 ;h•.'3., {-1 strike oft he gneissic lamellre 

l-1 \ --::J\1 Y., /_ ~ "\.!- · / I -;;;;,.,v,~,~t<'..ll\! /J f.}_''/ 1-/, -:i}.!\- to the trend of the lode are ~,~';'')/J\1---,;- :;1;-\\,x~>v.f'l\"v~::"i',"...:l/-11~;,'_!_\'-, 'J/,"::."'·Y\' . 'fi 1 \ -"' .._ / /.. / _ \1 , ,v>v<~ ,a1_ 1 _ 1_ 1 _ / ; ~-"''! ..._/-...:· .. \-'1 not so sigm cant as e se- ~'/,~....l!~!\.-i- !\I,~ ~(,;,.,)/\;,'1 /\1;--J, .... I;-i-:}_t~<:\..!.\ :\ where, as the trend of the 

o 2 4 f'eet principal lodes was unFw. 78.-Sketch of ontcrops, same locality as fig, 77, showing deflection and displacement of d bt dl d t · d b 

lode following a joint in granite on encountering a bunch of pegmatite. a, Unmineralized OU e Y e ermine Y 

joints; b, joints with silicified borders forming quartz vein; c, porphyritic Silver Plume regional stresses operating 

granite (homogeneous); d, pegmatite. on the homogeneous granite 

which constitutes the principal rock mass here. It is therefore possibly accidental to a large 

degree in this group of veins that the strike of the lode generally quarters fWross the strike of. 

the gneiss. Where, however, a lode is deflected so 

that it is parallel with the 

strike of the gneiss, this 

relation seems as a rule 

rather unfavorable to mineralization. It is to be 

seen, for example, in the 

cast end of the drift on 

the Frostherg lode in the 

Frostberg tunnel. Here 

the Frost berg is parallel in 

strike and dip to the inclosing gneiss, and is weak 

and unmineralized. 

EFFECT OF PORPHYRY. 

N 

oL-~~~~s~o _________ ,o~o~------~'sofeffi 

FIG. 79.-Pian of northeaste=ost portion of No.4 Dunderbcrg level on Terrible lode, showing influence of relative rigidity of different rocks (controlling the formation of fractures) on the strength 

As stated above, por- of the veins which luwe formed along the fractures. A strong vein in passing from rigid granite 

Phyry pla vs a very lninor to yielding gneiss immediately branches and disappears, bnt farther on another dike of granite is 

.J encountered in which the vein is once more well marked, though on encountering gneiss beyond 

part as a wall rock to this dike it again disappears, showing that the strain passed through the gneiss without formthis group of veins. The ing a fracture, but was unable to do so in the rigid granite. 

large northwestward striking dike which is cut by the Seven-Thirty lode is also cut by a 

number of the lodes belonging to the Terrible group. This porphyry dike has a flat east-
MINES OF THE SILVER. PLUME DISTRICT. 237 

northeast dip. In the Frostberg tunnel, where the Frostberg vein traverses this lode diagonally, 

the vein has been largely productive on the west or under side of the dike. The productive 

portion continues eastward as far as the dike, and there are some gooa stringers of ore in the 

dike itself near its western contact; farther east, however, though the lode passes through 

the porphyry and into gneiss, it is unmineralized. The lode was followed in the 

gneiss for 600 feet west of the dike, but 

showed no minerali11ation. 

The Mendota or Bush lode cuts through 

the same dike at a point about 1,000 feet 

to the southeast. AB seen in the Bush tunnel, this porphyry has the same effect on 

the Mendota vein as it has on the Frostberg vein in the Frostberg tunnel. The 

vein on the west or under side of the porphyry is in granite with some gneiss, and 

is a very strong fissure vein containing 

blende, galena, etc. On entering the porphyry the vein becomes very little mineralFIG. SQ.-Horizontal plan of vein in one of the tunnels of the Maine mine, 

showing thickening of vein on passing from gneiss into harder and more 

rigid porphyry. Dotted line shows tnnne!. 

ized, and on passing through the porphyry to the granite, which lies on the east or upper side 

of the dike, the lead is still faint and contains little mineral. . 

The extension of the Dunderberg lode crosses the same porphyry dike near the surface in a 

tunnel on the upper side of the Seven-Thirty road. This tunnel starts in the porphyry and 

passes northeast of it into granite. In the porphyry the lead is well defined and there is a little 

ore but no stopes. In passing northeast of the porphyry into the granite the vein widens and 

becomes strong, showing considerable ore which has been stoped out. About 350 feet west of 

the above-mentioned tunnel another tunnel starts on the upper side of the same road on the 

continuation of the Mammoth lode. This tunnel starts in the same porphyry dike and runs 

~~2\j lz 4 Granite wall rock Mineralized lead 

FIG. 81.-Geological plan of tnnncl on Brown vein. 

f .... ::~:.-····J ::=~···· 

Unmineralized lead 

through it into gneiss on the northeast or upper side of the dike. In the porphyry the lead is 

"well defined and shows a little mineral, as was the case in the tunnel on the extension of the 

Dunderberg just mentioned. On passing into the gneiss the lead becomes very poor and splits 

into a number of branches, and there is no mineralization. 

The conditions in th,e important Frostberg and Mendota lodes, where the mineralization 

is cut off by the porphyry dike, suggests that the porphyryhas opposed a barrier to solutions and 

limited their mineralizing action to the west or underside of the dike. In the two tunnels 

described in the preceding paragraph the veins are not so strong as the Frostberg and Mendota. 

The porphyry is fairly hard and the lode fracture has extended through it to the other side. 
238 GEOLOGY OF THE GEORGETOWN QUADRANGLE, COLORADO. 

In the first mentioned of these two tunnel drifts the granite on the northeast side of the porphyry 

is a more rigid rock than the porphyry, and better fitted for the formation of lode fractures, so 

that the vein is stronger in it than in the porphyry. In the other tunnel, however, the porphyry 

is a more rigid rock than th.e 

1

r1\l 1

1

ijli\1

1

i

1

\\i:;'i ~~~~~~ '~~~:;Yf~~~>N-;,~~~7~~ ~(-' /~~J~~i~~/~~·~\~~~\\\\\\\~\~ \\\ soft e-neiss which ad]. oins it on IIIII ,-1/'llf "/11-/,, /-1,'/1-'/1'-l-~/,{\_1\\\\\1\ u 

\1~~~ lsi\1:~ 11~;::'[;<~ (;~/i;\!;'~~~~~{~:~> ~7;:~~~f;1~~ ;<,_::/S~.o\o~\( 101~\\ the northeast; therefore the 

I ', I \>- /- 1 o ~ -;/'I/ I I, I ' 1 - 1

'' I 90°/ '- · I~::;_/ A, ·c IWINZ<\\' vein, while Well defined in the llfi/l:dii l1i\':- 1 

~·1;,'~~~f !z~)J=~:. / '!"-?.e'L~~'-:=:-. :~-J;::~~:~~;~n;:.:~~-;\1'\1,· :'~~ )\ \I '--------- .l ~- -- - -- ----rF:--'7~-r·"~,r / c:,'lJnm•neralozed read~~\\\\ 1 \' \'\1\'\ porphyry loses itself in the ,~-~-lrljTI~'il_)_"""'';vi.;_-~;:,":lY:.c:c:.·q·-~--"-,,J_;"-:.MN'</· :--~"~./-:--~-.; ·:<:-::;::;'.:-::..!,~_\,iS\\\ ,\\ I\'~ . ' 1

1 I I 1"-::M•neralozeil leacJ'9uiC~-~-~- -_. ··T;:;·,'{5ush'a'n"a---- ·-'--:!-1-.:"\~\\Gne•ss\ gneiss. In these two tunnels 

Il l/ I 1'--•.: 1-::_ 1-;,'''/':.' 1 - CM 1 conpec,'_, 1-, 1 / 'list) •"'---.~"~ .. -,~\'\11\\\'\ 

(

Ill\ lii[:,:>,I,(~~~.-~::J::---~'~'---;-;;·,.:-;,'.o' :;::,'.:-/'._\-'.:' '-.-,-_:'~Jng-;,,,-:--..;\ ~~\\~\\ there was no opportunity for 1\) \,-'""/......._,...__,/,' /--\r r....:_,--, '_,_/--..;..._! ...... ,;/'/}-',;, ....... _,.;_Odes ~\! \\ 

II II j I 1 : 1 1 ' 1 ._~, • -~~; __ ,.;' 1 1/, 1; 1 --:_'r' '.___' 1 ~~ -~' '/' ,'.:-; ,_ /~ -",/,//'·\11:\\ .I \ study1'ng the relat1've strene-th I I I I t·-/- '-. I/ I I / '- ...... /. '...._ _...... - I" I / /- _\I\// I- \1 \\ \ \ ~ 1 .,, l 1•'/_, 1 _ 1

-, 1 j

0

\;

1

.:-/.'!. ~- ~''.'_.:...:-;.'-;oo',,_,J~~/.,~:;~~e~'111 :\ \ ,\ of the lode on the two sides 

FIG. 82.-Plan of ttmnel on branches of Mendota vein, showing forking n.nd ore deposition on tho of the porphyry· 

south branch and n.long the united vein at the intersection with tho north branch; also the On the northwestern eximbricn.tion or overlapping of the vein fractures of the north bra.nch. tension of the Niaine lode and 

the northeastern extension of the Mammoth and Terrible.lodes, where these lodes break into a 

number of branches, two porphyry dikes from 4 to 10 feet in thickness, striking east and west 

and clipping from 35° to 65° N., have governed the course of two of these branches and made 

the branches conform to themselves in strike 

·and clip. 

In a tunnel drift on the northwestern extension of the .Maine, star'ting about 10 feet 

above the Seven-Thirty road on the southeast side of Sherman Mountain, there is exposed a lode having a west-northwest strike 

and a dip of 45° N., which for a distance of 

150 feet cuts lengthwise through a· porphyry 

dike 6 or 8 feet thiek and passes diagonally 

out of this porphyry into gneiss at each end. 

Where the lode ha.s porphyry for both walls 

it is very strong and has been stopecl continuously, the width of the mineralization 

being from 1 to several feet (fig. 80). In the 

gneiss at both ends of the stoped region 

the lead is small and the mineralization 

_ is mueh less, and there has been no stoping. In this locality the porphyry is hard 

and has fissured better than the gneiss, 

so that the ore bodies have had better 

channels and better opportunities to form 

within the porphyry walls. The same vein 

is shown in another tunnel drift at an elevation 100 feet greater. Here the lode 

~o ____ ~5o~--~~~oo~--~~s~o~--~2oofuet 

still runs along the porphyry dike, for the FIG. 83.-Plan of a portion of No.4 Dunderberg tunnel level on Terrible 

lode, showing the effect of parallel fractures, which finally join, within 

most part entirely in the dike, though the main fracture zone. This junction produces enbrgement of the 

locally on one wall or the other. The Inin- united fracture and increases the strength of the vein channel. Mingling of solutions has also evidently taken place (just as it docs at the 

eralization has taken plaee in various par- intersection of fractures having different strikes) and resulted in the 

allel or overlapping slips throughout the formation of an ore shoot. 

dike. This occurrence of ore is similar to that in the porphyry dike along the BismarckPelican lode. A.s the dike represents a zone of relative weakness in the rock mass, the formation of later fissures has been induced along it. This dike is the northern of the two parallel 

east-west dikes in this locality. 
J\'I:TNES OF THE SILVER PLUME DISTRICT. 239 

The southern of the two dikes is exposed in the lower tunnel of the Maine, starting about 

150 feet below the Seven-Thirty road, and also 

Seven-Thirty road. The lower tunnel has a 

length of about 500 feet; it encounters the 

dike at a point 150 or 200 feet in and runs 

along it to the breast. The lode in the main 

f9llows the wall of the porphyry, which is 

highly silicified. The vein is stronger along 

the porphyry contact than elsewhere. 

m another tunnel on the same lode above the 

N 

LOCATION OF ORE BODIES. 

o;:__~25:___...:s::.:o:..._ __ •czcoo=----~Aso feet 

Fro. 84.-Horizontal plan of No.1 Dunderberg level, showing mineralization on two branching and uniting leads. 'rhe process carried further, as seen on another level, makes a solid ore lens along the lead. 

In the lodes of this group the ju:rction of 

branches has been a very favorable place for 

ore deposition (fig. 82). In the Silver Ore 

tunnel on the drift on the ·Fenton lode east of 

the tunnel an ore body was noted at the 

junction of several leads which, after running 

nearly parallel for some distance, came 

together. Similar conditions are shown on 

the Dunderberg No. 4 level (fig. 83). The 

effect or the junction of leads in. producing 

an ore body is well shown in the workings of 

the Brown mine (fig. 81), and also in tl~e Dunderberg. On the B level of the latter mine a large 

ore body is located at the branching off of the Frostberg lode. The splitting and reuniting of 

a lode inclosing a horse of country rock appears to have in many places a similar influence. 

Fro. 85.-Cross section of Mendota vein, Petersen 

shaft, showing zone of superficial enrichment. 

a, Surface light soil and vegetation roots; b, oxidized vein containing bunches of galena, some 

pyrite, and no blende, and 70 ,ounces of silver to 

the ton;· c, unoxidizcd ore, chiefly blende with 

galena and pyrite, containing 20 to 25 ounces of 

silver to the ton; d, gneiss country rock of vein. 

At both j1mctions there l.s enrichment, and locally the 

mineralization may be so great that the entire horse of 

country rock may be changed to ore, forming a lens-shaped 

shoot on the lode. The different stages in the production 

of such a lens-shaped shoot are shown in the Dundefberg 

workings (fig. 84). 

INl<'LUENCE OF DEPTH ON VEINS. 

In this group of veins the formation of soft secondary 

sulphides (locally called sulphurets) has taken place down 

to a limited depth in the same way that it has in the 

Bismarck group. The Mendota vein at the Petersen shaft 

(fig. 85) may serve for a general example for all the 

lodes of this group. At this shaft the roots of vegetation, 

slide rock, etc., cover the vein for a depth of 15 feet. 

Immediately below is an enriched zone 5 to 10 feet thick, 

consisting of oxidized clayey material containing bunches 

or nuggets of secondary galena and some pyrite, but no 

blende. These ores are said to carry 50 per cent of lead, 

$3 in gold, and 70 ounces· in silver. Below this altered 

zone, with almost a sharp line between the two, are 

ores consisting of coarse blende with some galena, pyrite, 

and chalcopyrite. These ores are of low grade in respect 

to their silve1: content, the maximum '!Jeing reported 

as about 20 ounces. The existence of this enriched 

zone near the 

they make a 

surface is well known to some of the miners who work on a small scale, and 

business of "gophering" out the enriched zone beneath the grass roots neal: 
240 GEOLOGY OF THE GEORGETOWN QUADRANGLE, COLORADO. 

the outcrops of the various lodes. The enriched ore is said to extend as much as 150 feet clown 

in some places, and to have run on the Mendota as high as 200 ounces in silver. 

The Mendota vein .extends downward from a point immediately beneath the enriched 

zone, or from a point about 30 feet below the surface, to the lowest depths examined, about 

800 feet below the surface. From top to bottom this vein is composed chiefly of blende, with 

galena, pyrite, and a little chalcopyrite. The silver content as a rule does not run over 25 or 

30 ounces; nevertheless, various reports, including those of miners familiar with the mine, 

indicate that somewhat better ore was obtained from the upper than from the lower portions: 

It is reported that rich ore, containing a little polybasite and ruby silver, was occasionally 

found near the surface. It is also reported by the miners that as the <lepth increases there is . 

less lead and more zinc in the ore. In general, also,it is said that the amount of silver decreases 

in depth. If this is true the rich ores have been locally concentrated, for it is certain that 

some of the uppermost workings of the Mendota contain heavy blende orcs of as low grade as 

' · any found in the lower workings. ~~-.-.OT~~~~~7r7T~><~nno I ( I ( (II ;~<\ \\\ \\ \ \ 1· \ II The workings on the group of branching veins \) ( ! \ \ \ \ ,~_>:;,, ~ \ \\\\ \ \ \ \ at the junction of the Maine, Man;!n?th, and 

I I / \ _o_ ~ · ... \\\\\ \ \ ~ j \ Dunderberg lodes above the Sevcn-llurty road 

I \ \ \ \\: .. )(~-,:~: .. \\\\ \~\\ \\I \ show a larger a~ount of p::rrite associated with / ll ·j \ \ ' 1 

- .,. \ ! the blende than Is the case m lodes encountered 

\(I I I \ \ ···:. -~/ :.\ \\ \ \\\ \ \ \ I farther down the hill or cut at greater depth, 

1 \ \ \ \ \ \~/ ' \ \\\ \ \ \ \ ~ho:rg~ othcrw~se the composition of the ve~ns f I \ \\ \ \ ) !. // :S:~ ·;:: \\ \\\\ \ \ \ j IS similar. The fact that the ab.m~dance of pynte 

I I J_ \ ) ) ~{7 --:7; '.': \ \ \ a ) \ I \ w~s also f?1md to be ch~ractenstiC ?f th~ super-

\lj / I \ ~~/'~ :;, \ \ \ \ I ! ~~ fi.Cial_ portiOns of the Bismarck, W~sco~su;, and 

\1 \ I \\ \ \ //,0~ /. ~ 1 \ \ \ I I\ \ ) Pay Hock lodes sugg~st~ that the pynte IS dire:~ly I( \} \ I -=/ c~ :: ) ) \ ~~~ \ I dependent on proximity to the surface. I he 

\ \ \ \ \ \ \ I -k'\ <:: I \ I l I \ abundance of pyrite can not be regarded as char- ! \ \ \\ \ ._., :::_Q~; ;: /)jl (I j ) 

1

) acterizin~ a zone h.aving .no relation to the sur- ' l \\ \ 1 \ ~~<~ " . :;. // I j ! I j face, f?r m the Mame vems these ores arc .at. an . \ \ ) ) [\II \ ) ~: '-'5/!· / :i I ! / II ( //· clevatwn 1,000 feet less than that of the simr.lar 

ljl ! ) . ;; 8i!: .i J / / 1

1 

j I orcs of the Bismarck. The effect of descendmg 

· / 1111 / \ II i: -~ ~ ::' / ·;////1 / l wa~ers therefore see~s to be the pro~ablc cxplajl \ 1/ -~ ' \ ;;:\ .-;' f / / ) / / j natwn .. The collectwn ~f mar!y speCime.ns fr?m 

I l1 :·:· -\~ : . I the regwn of the branclung vem:-; at the JUnctwn 

Fw.86.-Sketch of portion of veinlet near Mendota vein, Men- of the mairi Dunder:berg and JYlanrmoth zones 

dota mine, second !eye! belowVictorin, tunnel level. Shows shows that the pyrite is typically later than the 

paragenesis of siderite, calcite, and quartz ln a filled fissure. 

a, Gneiss wall rock; b, dark-brown siderite, comb structure; blonde, cementing it where it has been crushed c, white calcite, slightly ferriferous; d, crystals of quartz. and also succeeding the blendc as a regular subNatural size. 

sequent crust in cavities. . 

This same relation has been noted in manyother portions of this group of lodes. Specimens from the Dunclerberg D level show repeated brecciation and recementation by sulphides 

in which the pyrite is subsequent to the galena, the siderite is subsequent to the pyrite, and 

jaspery silica is the latest deposition. Of these minerals the jaspery silica has everywhere been 

noted as the last deposit in the veins, being probably due to descending waters. It has been 

concluded that in the Bismarck, Pelican, Corry City, and Pay Rock group of veins the siderite 

also has an atmospheric origin, and microscopic study of the altered granites indicates that 

the iron carbonate originates in part by the alteration of the iron in the biotite by carbonated 

surface waters. Much of the pyrite which is closely associated with the siderite and in many 

places contemporaneous with it must be considered to have a similar origin. A specimen from 

the Mendota lode at the junction of the Tishomingo at the surface shows a heavy pyritic ore. 

This specimen shows the rending of earlier crystallized galena and cementation by ·pyrite. 

There is no blende. A specimen from the Mendota, from the second level below the Victoria 

tunnel level, at a depth of 650 feet below the surface, shows also repeated opening and rece-
MINES OF THE SILVER PLUME DISTRICT. 241 

mentation by sulphides. In this specimen the galena is subsequent to blende, which was the 

first metallic mineral formed in the vein, and pyrite is subsequent to galena. Specimens from 

the East Dunderberg tunnel indicate the same rending and recementation. Here again pyrite 

is later than galena and siderite later than pyrite. • 

A barren veinlet 1 inch thick, on the second level of the Mendota below the tunnel level 

(fig. 86), shows thB following order of crystallization: ( 1) Siderite, ( 2) calcite. Quartz is contemporaneO\lS with both carbonates, a single quartz crystal reaching through both layers. 

It seems certain that a great deal of high-grade ore was taken from the upper workings of 

the Terrible mine, and the reports of polybasite and ruby silver in this ore suggest an enrichment 

by descending waters. The lower levels of this mine were under water at the time of examination and conditions were not favorable for comparing them with the upper workings.a 

~ 

~ Gneiss 

~{~. 

q.feet 

~·~ .• ~-~~---~ EJ Kaolin Iron-stained gneiss Mineralized lead Clay lead.Unmineralized (Pyrite, galena, and blende ore) 

FIG. 87.-Sketch showing postmincral fault affecting upper Frostberg vein. 

Some of the longer tunnels afford the best opportunities for comparing the nature of the 

veins near the surface and at. considerable depths. The Burleigh and Mendota tunnels show 

the Frostberg vein at a depth of 1,250 feet. In the Burleigh tunnel there is ore along the vein, 

but it is not of·good quality. The vein here consists of quartz and ore in places over 1 foot 

wide, and the lead is large and strongly mineralized. In the Mendota tunnel at the same depth 

there was some good galena ore containing 50 to 55 per cent of lead and from 50 to 60 ounces 

of silver. The ore consisted in places of l to 1~ feet of solid sulphides. The character of this 

vein seems to compare not unfavorably with that shown where the vein is drifted on in the 

·Frostberg tunnel and with its outcrop at the surface. 

a Mr. Waldemar Lindgren Yisitcd the Terrible mine in 1906, after it had been' pumped out, and communicated his observations to the writer. 

He found the vein to be fairly strong and well defined to the lowest or fourteenth level, \\;hich is 500 feet below the main (Union) tunnel level. 

The ore at this level was mostly blende, with a good deal of galena. A specimen sent by Mr. Lindgren shows as the oldest minerals blende, 

galena, and a little chalcopyrite. Later than these is a small vein of dense brownish carbonates that has formed along a fissure splitting and 

bisecting the galena and contains a little pyrite and considerable tetrahedrite, which ramifies out from the vein, cementing the broken fragments of galena. A similar veinlet, cutting the galena and blende, consists of pyrite, slightly cupriferous, and this is also associated with a 

little tetrahedrite. On fracture surfnces, cutting all the foregoing, is a coating of chalcopyrite, which is the last mineral deposited. This is 

similar to the greenish pyrite ernst noted as one of the most recently formed minerals in the Colorado Central mine. Mr. Lindgren observes 

that rich silver sulphides apparently do not occur below the tunnel level. The ore described is about 850 feet vertically below the surface.-

J. E. S. 

314(h-No. 63-08--16 
242 GEOLOGY OF THE GEORGETOWN QUADRANGLE, COLORADO. 

The Mammoth vein is drifted on at a depth of about 1,250 feet in the Burleigh tunnel, 

where it is called the Phillips. At one place in this tunnel the vein shows two parallel ore 

bodies, each consisting oj a fo9t of solid ore.' The vein here is of low grade, containing much 

blende, some galena, and a good deal of iron pyrite. Immediately above this level, ho,vever, 

and clear to the surface, the Mammoth has produced very little ore. 

It therefore appears that as a rule lodes of this group, to leave out of ·consideration the 

evident effects of enrichment close to the surface, have shown themselves fairly constant in 

character to a depth at least as great as 1,2.50 feet. They probably extend downward much 

farther. 

POSTMINERAL FAULTING. 

Faulting subsequent to the formation of the viens has not been important. Most of the 

movements have prdbably been parallel to the earlier movements and so parallel to the lodes, 

0 

FIG. 88.-Sketch of lower tunnel on Maine lode (ncar Pelicnn tunnel), 

showing Maine lode ofT set by a postmineral fault. This fault is per' 

pendicular, but with a horizontnl movement. The wall rocks are 

gneiss (b), with some gmnite (a). 

resulting in the repeated reopenings in the 

veins and recementations by new vein material 

which have been described. There is a slight 

fault transverse to the Mendota lode on the 

first level below the Victoria tunnel level. 

These small postmineral faults show nearly 

horizont&l striro, indicating that the postmineral faulting was a renewal of the same stress 

as that which produced the lode channels .. 

On the lower Maine tunnel near the Pelican 

tunnel there is a postmineral fault of considerable importance (fig. 88). Here the Maine 

lode is offset horizontally about 25 feet by an 

east-northeast fault. This fault shows as a 

strong gouge zone containing frag~nents of ore 

and accompanied by strong stri~ dipping 2° E. 

A very small postmineral fault in the Frostberg tunnel is shown in fig. 87. 

Those postmineral movements which were parallel to. the premineral faulting appear also 

to have taken place· in the same general horizontal or nearly horizontal direction. In the 

upper Mendota strong postinineral strire dipping 30° E. were noted along the lode. 

PRESENT EFFECT OF SURFACE WATERS. 

GENERAL DESCRIPTION. 

Ordinary surface waters descending through the rocks at the present day are accomplishing 

an immense amount of alteration in the way of abstraction and deposition of material. The 

effects of this work are visible in underground tunnels. . 

The maximum alteration takes place along watercourses that follow clown along fracture 

zones, and is not .directly dependent on distance from the surface. For example, in the Silver 

Ore tunnel, which is entirely in granit~, a specimen taken within 40 or 50 feet of the mouth 

and less than that distance vertically below the surface is an unusually fresh and unaltered 

type of rock, more so than any other specimen collected in the underground workings; whereas 

near the erid of this same tunnel, at a vertical depth of 800 or 900 feet, the granite is decomposed and disintegrated along a zone where abundant water seeps through the rocks and 

accumulates in the bottom of the drift. The fresh granite first mentioned contains quartz, 

. muscovite, magnetite, fresh biotite, and probably original pyrite, one large crystal of which 

is inclosed in fresh feldspar. Of the feldspars, microcline and orthoclase are entirely fresh, 

but another feldspar that is finely striated tmd is probably oligoclase is slightly altered to 

sericite. At the end of the tunnel the water which seeps through the decomposed granite 
U. S. GEOLOGICAL SURVEY PROFESSIONAL PAPER NO. 63 PL. XLI 

.11. SPECIMEN SHOWING STALACTITIC CRUST OF ZINC 

CARBONATE CONTAINING SOME BASIC COPPER 

SULPHATE WHICH HAS FORMED ON THE WALLS 

OF A DRIFT (VICTORIA LEVEL) ON THE MENDOTA 

VEIN WITHIN TWENTY. YEARS. 

Enlarged one-eighth. 

B. SPECIMEN FROM MENDOTA VEIN, VICTORIA TUNNEL LEVEL. 

a, Wall of vein (granite); b, comb quartz, first deposit in vein; c, finely ground 

granite (ce mented hard by quartz) in c losing blende fra gmen ts; d , fragment 

of coarse blende representing main vein fill ing subsequen t to quartz. Specimen illustrates rending apart of vein subsequent to main mineralization and 

filling of the new fissure with a paste of ground gran ite which aft e rward was 

cement ed by silica. Natural size. 
MINES OF THE SILVER PLUME DISTRICT. .243 

and forms pools in the drift accumulates on its surface a plentiful scum determined by W. F. 

Hillebrand to be calcite. On the walls of this drift· the waters have left soft stalactites, which 

Doctor Hillebrand determined to be manganese peroxide, with scales of calcite. 

The alteration of gneiss by atmospheric waters is shown by a specimen taken in the Baltimore tunnel 600 feet below the surface and 300 feet north of the Queen lode. It is not in the 

neighborhood of any vein. This ·is a granitic quartz gneiss showing quartz, fresh biotite, 

microcline, and orthoclase, and another feldspar which has been entirely altered to fme semispherulitic kaolin with some carbonates. It is probable, to judge from the result of the investigation of the altered feldspars in the granite of the Ashby tunnel, that the altered feldspar 

in this specimen also was a sod!!: feldspar. . 

The porphyry encountered near the end of the Baltimore tunnel at a depth of more than , 

1,100 feet, and nowhere near any known vein, shows a certain amount of alteration throughout, 

representing the general effect of percolating waters. The princip~l dike is of rhyolitic porphyry, which under the microscope shows a glassy microgranular groundmass somewhat kaolinizecl, with the development of a little calcite. Most of the sections show the feldspar mostly 

altered to kaolin. In one section the feldspars ar.e variously altered-one is partly altered 

to calcite; a second is entirely altered to sericite; a third is altered to quartz, kaolin, and a 

little pyrite; and a fourth is entirely altered to kaolin. · 

A monzonitic porphyry which is cut by alaskitic porphyry near the breast of the tuiLlcl 

shows under the microscope biotite partly alterecl to secondary quartz, calcite, and siderite. 

The feldspar, which is andesine, is largely or entirely altered to abundant carbonates and a 

fine kaolinic aggregate. This alteration product was separated and analyzed by Doctor Hillebrand. Under tl~e microscope the aggregate was fairly bright colored, the interference colors 

·reaching yellow of the first order, as ·seen under the high power. The material was semispherulitic in structure. From this material the carbonates were leached out and found to consist 

of carbonates of iron, calcium, and magnesium, the residual kaolinic matter having the following 

composition: Si02 , 50.0; Al20 3 , 31.3; igi1ition, 10.3. 

The drift on the Frost berg lode .in the Burleigh tunnel at 7 50 feet shows abundant precipitation of yellow iron oxide and also stains of white material that may be zinc carbonate. Yellow 

clay in the drift contains needle-lilm crystals which may be sodium sulphate. 

In the upper tunnel on the Maine vein, whose mouth is located about 150 feet below the 

Seven-Thirty r.oad, hairlike transpa.rent crystals were found covering fine debris left in the drift. 

These were examined by Doctor Sullivan, who found them to be chiefly sodium sulphate, containing also traces of calcium, magnesium, and chloride. The depth of this locality is about 300 

feet. 

In the same tunnel at one point an incrustation has been formed on the roof by dripping 

water, and similar material has been formed on the floor, covering the broken stones. Analysis 

of this material by Doctor Sullivan shows that it contains basic sulphate of copper and calcium. 

carbonate. Carbonate of copper may also be present. 

On the Victoria tunnel level of the Mendota vein, at a depth of 450 feet from the surface, 

stalactites and incrustations of a white material occur on the roof and walls'. These were 

examined by Doctor Sullivan, who found them to consist of carbonate of zinc. Some of the white 

carbonate was covered by a turquoise-blue incrustf!otion as a last deposition. Doctor Sullivan 

found that this incrustation is chiefly zinc carbonate containing some copper and sulphate, 

probably in the form of basic copper. sulphate, and traces of calcium and magnesium. This 

drift has been opened probably about twenty years. 

SU)IMARY. 

The universal effect of the underground waters which traverse the rocks seems therefore 

to be chiefly decomposition of the biotite and the soda feldspars, by means of which the waters 

obtain soda, iron, manganese, etc., leaving the feldspars altered to kaolin or to sericite. Precipitations from these waters include the limonite, manganese peroxide, and calcium formed 
244 GEOLOGY OF THE GEORGETOWN QUADHANGLE, COLORADO. 

when the air reaches the wate.r in deep tunnels, and result in the formation of sodium sulphate 

along veins. Along veins also it has been seen that surface waters have a very considerable 

power in dissolving the sulphides and taking the metals into solution. Some of the coatings of 

zinc carbonate noted above are as much as half an inch thick (Pl. XLI, .A), so that the aggregate amount of zinc removed from these lodes by the action of descending water since their 

formation must have been enormous. The numerous irregular cavities due to dissolution which 

occur in the veins, and many of which have a great extent, also testify to the mass of material 

that·has been removed by the underground waters recently-since the last cementation. 

. The zinc stalactites in the tunnel above described were precipitated only because the waters 

came in contact with the air in the tunnel. Under ordinary conditions in the rocks they would 

not have been so precipitated. At thit depth in the rocks no zinc or copper carbonates have 

been found, and a question arises as to the ultimate destination of the large amount of zinc 

that has been leached from the dissolution cavities which were found. It seems very likely 

that this zinc is normally deposited in the sulphide form, for we frequently find blende as well 

as other sulphides contemporary with siderite or forming the last deposit on the walls of geodes. 

Active solution and redeposition of this sort going on since the formation of the lodes must 

have brough~ about a very great amount of rearrangement of the metals. 
CHAPTER IV. 

MINES. OF THE GEORGETOvVN QUADRANGLE OU'l'SIDE 0]' THE SILVER 

PLUME DISTRICT. 

1\HNES NEAR GEORGETOWN. 

MINES OF LEAVENWORTH MOUNTAIN. 

COLORADO CENTRAL VEINS. 

LOCATION AND DEVELOPMENT. 

The Colorado Central mine, which lies in Leavenworth Gulch southwest of Georgetown, is 

one of 5he most famous of the region. It was discovered in 1872, and has been \Yorked more 

or less continuously up to the present clay, producing an estimated total of over $8,000,000.a 

The Colorado Central vein or veins strike to the northeast, and are nearly vertical; they have 

been continuously opened UI), both at the surface and underground, for a distance of threefourths of a mile. The workings reach a depth of about l ,050 feet, the lower levels (beneath 

the 600-foot or Ocean Wave. tunnel level) being worked only by shafts. A long tunnel-the 

Hall tunnel-was started from Clear Creek below Silver Plume, to pass under Leavenworth 

Mountain (Pl. XLII) and cut the Colorado Central veins at a depth approximately that of the 

present lowest workings, but was never carried in that far (fig. 89). 

The workings of the Colorado Central mine (Pl. XLIII) show a zone ofparallel, branching, 

or overlapping veins. ·whether this zone is a single lode, or whether the individual veins are 

separate lodes in the legal sense, has been the subject of much litigation in the past. b 

DESCRIPTION OF VEINS. 

The main Colorado Central vein has a northeasterly strike and a nearly vertical clip. In 

its most productive portion it skirts along the south(~ast contact of a porphyry dike which has 

a thickness of 8 to 40 feet. This may be called the Central vein, to choose the most applicabla 

from several names which it has received in the mine. 

On some of the mine levels-for example, on the 400-foot or Marshall tunnel level (sea 

Pl. XLIV)-there is an important forking of this vein. As seen on the 400·foot level, the 

southwesterly trend of the Central vein causes it to diverge gradually from the porphyry dike 

which it skirts. At a point where by reason of this divergence the porphyry and the vein have 

become separated by about 40 feet the vein splits, one branch running back diagonally to the 

porphyry contact, which it continues to skirt, farther southwest, and another keeping on in the 

same general direction as the main veins, but in gneiss on both sides. The last-named or 

southeastern branch may be held to represent the main vein, as it continues on the same strike, 

while the northwestern branch resembles the main vein in that, like it, it skirts the porphyry 

contact. For purposes of description, however, the separate names currently in use will be 

adopted, the northwestern branch being called the North vein and the southwestern branch 

the South vein. 

Two hundred feet below, on the 600-foot or Ocean vVave tunnel level, the branching of the 

Central vein into the North and the South vein is also seen, but it is 300 feet or more farther 

northeast, showing that the junction pitchef? northeastward. 

a Foster, E. Le Neve, Proc. Colorado Sci. Soc., vol. 7,1902, p. 43. b Foster, E. Le Neve, op. cit. 

245 
246 GEOLOGY OF THE GEORGETOWN QUADRANGLE, COLORADO. 

On the 7 50-foot level, 150 feet below the level of the Ocean Wave tunnel, the North vein, 

following the porphyry contact, is shown, and inaccessible workings on the South vein-converge 

toward the North vein to the northeast, as on the Ocean Wave level. The veins here, however, 

seem to become ill defined, overlapping, and unmineralizcd before the point of junction is 

. 

' Granite 

E Mineralized lead 

Unmineralizcd lead 

/+++++++] 

Porphyry dike 

(highly altered) 

100 

N 

200feet 

FIG. 89.-Geological plan of Hall tunnel. 

reached. The junction, if present, would be 

about 500 feet northeast of the corresponding 

point on the 600-foot level, showing that the 

northeasterly pitch is much flatter than on the 

levels above, making a horizontal difference 

of 500 feet with an increase in depth of 150 

feet. 

On the 850-foot level the North and South 

veins are well represented, and run parallel to 

each other and 80 to 100 feet apart, with no 

tendency to converge. This shows that the junction of the two veins has on this level, owing to 

the flatness of the pitch (probably still flatter 

than above shown), passed so far northeast as 

to be beyond the field of the workings. . 

On the 400-foot· or Marshall tunnel level 

(practically level No. 3 of the Aliunde workings) 

the South vein continues southwestward to a 

point opposite and about 30 feet northwest of 

the Aliunde shaft (J;>l. XLIV).· Here the normal 

nearly vertical dip changes to a northwesterly 

dip of 64°. Above this point, in the Aliunde 

workings, the vein does not appear to be present. 

Its place is taken by weaker veins, striking in 

general northeast, parallel to the stronger veins 

described, but dipping more flatly northwest, at 

angles averaging about 60°. These weaker veins 

are very apt to be discontinuous, to die out by 

repeated and irregular branching, and to overlap 

one another, as seen both in strike and in dip . 

(Sec Pl. XLIV.) On the Marshall tunnel and 

Aliunde No.· 3 levels (25 feet apart and represented together .on Pl. XLIV) the downward 

continuation of this zone of itnbricating veins 

is shown not only in the South vein, but in 

at least two veins lying between the North and 

the South veins, striking parallel to them and 

dipping 60° to 706 NW. (fig. 90). Of these two 

veins the more northwestern (see Pl. XLIV) is 

a branch of the North vein in the same manner that, on the sam~ level, the South vein is a 

branch of the Central vein. Coincident with 

the appearance of ·these intermediate flatter 

veins on this level, both North and South 

veins become weak, and have not been followed far to the southwest. 

The imbricating flat veins, intermediate between the North and the South veins, are also 

seen in. the Aliunde No. 4 and Colorado Central 500-foot levels, which are on nearly the same 

plane: (See Pl. XLIV.) As seen on these levels (the same f~ature is shown less markedly 

on the Aliunde Nos. 1 and 2 levels, Pl. XLIV), the flat veins have a tendency to twist around, 
U. S. GEOLOGICAL SU RVEY PROFESSIONAL PAPER NO. 63 PL. XLII 

.Jl. LEAVENWORTH AND ALPINE MOUNTAINS, FROM DUMP OF VICE-PRESIDENT MINE 

IN BROWN GULCH . 

T own of Si lver Plume i n foreground. 1 , We lch mine . 

• 

B. VIEW SHOWING RELATION OF RIDGE SEPARATING CLEAR AND LEAVENWORTH 

CREEKS TO THE CONTINENTAL DIVIDE, OF WHICH GRAYS AND TORREYS 

PEAKS FORM A PART. 

1, Grays Peak; 2 , Torreys Peak; 3· Colo rado Centra l mine; 4, W ide W est mi ne; 5· Geo rget own, 

6, H all t unnel; 7• Lebanon t unne l ; 8, Sil ver Pl um e; g. Leave nw orth Mountain. 
U . S . GEOLOGICAL SURVEY 

PROFESSIO N A L PAPER NO. 63 PL. XLI II 

".,;I 

MAP OF THE 

COLORADO CENTRAL, KIRTLEY, AKD ARGENTINE MINES 

0 100 zoo 300 soofeet 

J906 
MINES NEAR GEORGETOWN. 247 

so that the general strike becomes somewhat oblique to the regular trend of the main North and 

South veins, though the overlapping or imbrication of the separate veins preserves the normal 

alignment. Similar intermediate veins, many of them pitching more flatly than normal and 

·with a somewhat oblique trend, and typically discontinuous, are found at greater depth between 

the North and South veins, especially as the junction of these two veins is approached. Examples are shown on the plans of the 600-foot and 750-foot levels. On the 850-foot level, where 

the North and South veins run parallel, with the junction beyond the field of the workings, 

these intermediate veins are still represented, as, for example, in the one marked A ~·K 

on the plan (Pl. XLIV), which dips 30° .~lf-======~==;;f='!..-o 

NW. in general, though very irregular in 

detail. 

These discontinuous imbricating and 

branching veins, many of them oblique in 

strike or dip, or both, to the North and 

South veins, but lying between them and 

in places connecting them, have produced considerable ore, and have played 

a very conspicuous part in the mining 

litigation. . 

The New North vein, a separate 

vein frQm those already described, skirts· 

the northwest contact of the main por~ 

phyry dike. This vein is first shown 

on the Marshall' tunnel level, where it 

runs paraJlel to the Central-South vein. 

Like th'e Central-South vein, the New 

North diverges from the porphyry contact, but on the northeast instead of on 

the southwest, thereby maintaining its 

parallelism. On diverging, like the Central-South vein. it splits, and has not 

been fo.llowecl far northeast, though in 

the M1trshall tunnel crosscut there is a 

''lead" on the north side of the porphyry 

dike which may represent the whole or 

a part of the continuation of the New 

North vein. On the 750-foot level, how-

. ever, the New North approaches close to 

the North vein, with the thinning of the 

intervening porphyry dike, though no 

junction has actually been observed, 

either on this level or the 850-foot and 

l,ijOO-foot levels below. 

The above-described veins are the 

important ones of the Colorado Central 

No 2 level 

-·Colorado Central 1001}-foot level 

SE NW 

FIG. 90.-Vertical cross section along line A'-B', Pl. XLITI,.showing developed portion of Colorado Central veins and their relation to the porphyry 

dikes. · 

group, occupying the principal zone,'' which, as \vill be shown later, is a z~ne of fracture and 

nearly horizontal premineral faulting. This zone, however, is but the core of a wider zone, 

where the ro~ks have been sheeted and subsequently mineralir.ed. Some of the veins thus 

formed have had a considerable production. The principal is the Kirtley; lying a fe"' hundred 

feet northwest of the Colorado Central veins; another is the Munsill vein, lying to the 

southeast. 

A marked feature of all these veins is the tendency for ·an individual vein to discontinue, 

its place being taken by a parallel vein, offset more or less, and overlapping the first. This 
248 GEOLOGY OF THE GEORGETOWN QUADUA~GLE, COLORADO. 

characteristic is prominent in the flat intermediate veins of the Aliunde Nos. 1 and 2 levels 

and the 500-foot level of the Colorado Central. It is also well shown in the North vein on the 

750-, 850.:, an ell ,000-foot levels. The imbrication of the flat veins may be said to be left-handed; 

that is, on looking northward over a plan of the veins, the veins overlapping to the north are 

seen to be offset to the west from the veins to the south, in a direction opposite to the movement of the hands of a clock: On the 750-foot level the imbrication of the North vein is lefthanded to a point near the field of junction of the North and South veins, where it becomes 

right-handed, the offset being to the east instead of the west. On the 1 ,000-foot level also the 

imbrication is strong and left-handed in the southwestern part of the workings, but it appears 

to become distinctly right-handed in the northeastern part. 

What seems to be an example of this right-handed imbrication in the northwestern part 

of the productive region, and the best example in the mine, is afforded by the northeastern 

extension of the North vein, as seen on the plans of the Marshall tunnel and Ocean Wave 

tunnel workings (400-foot and 600-foot levels). On the 400-foot level the North vein becomes 

umnineralized before reaching the Marshall tunnel, but there is a fairly strong and nearly 

parallel vein overlapping it on the southeast-the Eqmttor vein. From this vein a good deal 

of ore was taken out. On the Ocean Wave tunnel and Equator tunnel workings also (these 

two have a diiierence of elevation of 10 feet, and are shown on the same horizontal plan) the 

North vein becomes barren almost exactly beneath the similar point on the Marshall tunnel 

level, but has been followed as an unmineralizecl slip 700 feet farther. In the Equator tunnel 

what is probably the Equator vein is found, overlapping the North vein on the east and contabling a little ore, though it has not been followed far. The Equator vein is in its turn overlapped on the east by the parallel Ocean·Wave lode, the total offset between the Ocean Wave 

and the North vein being about 100 feet. The Ocean Wave vein, continuously mineralized, 

though the ore is of low grade, has been followed for more than ·1,000 feet fa~ther northeast. 

· RELATION OF ORES TO DEPTH. 

The following statement is made by an engineer who had many years' experience with 

the mine, a and with it the more limited observation of the writers agrees: 

Concerning the effect of depth upon the value of the orcs, although the mine is worked to a depth of 1,000 feet, 

no particular difference is to be noted. Rich and poor ore channels are found at all depths. Within 100 feet of the 

surface some of the richest and some of the poorest were encountered, and the same might be stated of almost any 

depth. Upon the same horizon were found bodies of ore which were rich in silver and others which were poor in 

that metal. The poor orcs were apparently the same galena and zinc blende which in another place were enriched 

by ,the true silver minerals and were therefore rich in silver. 

While this is true, there are certain other features worthy of note. The rich and the 

. poor ore both occur practically at the surface. A portion of the vein outcrop examined by . 

the writers was 4 feet wide, and the vein filling consisted chiefly of zinc with some galena. The 

·nort.h half of the vein is reported to have run 500 to 600 ounces of silver, and the south half. 

only 40 ounces, the difference between the two being due to scattered polybasite. Lean blende 

ores, containing very little silver (8 to lO ounces), are common in the upper workings of the 

mines. Yet from all that can be gathered it appears probable that the amount of polybasite 

(and ruby silver) which makes the rich ore was distinctly greater in the ores nearer the surface 

than in those more remote. It is true that rich ore has persisted in this mine to a depth 

unequaled in the Silver Plume mines, but it is to be doubted whether some indications of falling 

off .In depth are not present. It is stated by miners that ruby silver was more abundant in 

the upper than the lower workings.· According to infortnation received, the richest ore shoot 

in the mine was worked downward more or less continuously from the surface to the 500-foot 

level. This shoot, between the 400-foot and 500-foot levels, contained the Carnahan stapes, 

the richest in the mine. It is reported not to have extended below the 500-foot level, though 

it split into rich spurs which were followed downward. 

The accessible portions of the different levels have been mapped, and barren leads distinguished from mineralized portions of the veins. On the 1 ,000-foot level the principal vein 

a Foster, E. Le Neve, !'roc. Colorado Sci. Soc., val. 7, 1902, p. 48. 
MINES NEAR GEORGETOWN. 249 

(North vein) is exposed for 1,700 feet, but the proportionate extent of mineralized vein is 

only 17 per cent of the whole development along this vein, the rest being unmineralized, whereas 

on the 400-, 600-, 750-, and 850-foot levels, which also have long stretches of development on 

this vein, the percentage is from 55 to 71 per cent. The suggestion derived from this observation, therefore, is not so much that the ore is passing from high grade to low grade in depth (for 

high-grade polybasite and ruby-silver ore was found on the 1,000-foot level) as that the entire 

vein is giving way to an unmineralized fault lead in depth, as has been noted for the Pay 

Rock and other mines of the Silver Plume and other adjacent areas . 

.The surface oxidation generally extended only a few feet downward on the vein, the groundwater level lying close to the surface. The oxidized ore resembles a soft iron-stained clay, and 

-was found forming the surface of the veins, or the harder portions occurred as loose bowlders 

in the overburden. It is reported that this oxidized stuff was exceptionally rich, nearly all 

running 1,000 to 2,000 ounces of silver. This corresponds with the condition at the Mm;dota 

vein, where the oxidized outcrop was especially rich in silver. 

No zonal distribution of the galena and blende has been noted in this mine. 

NATURE OF WALL ROCKS. 

The principal wall rocks of the Colorado Central veins are granite, gneiss, and pegmatite. 

The gneiss is mther the most abundant., but is characteristically intruded by many seams 

and dikes of granite and pegmatite, which give it greater rigidity. Solid pegmatite occurs in 

this mine as a wall rock to a greater extent than in most of the other mines of the region. 

Porphyry also in inany places forms one of the walls of the main lode, but with unimportant 

exeeptions does not form both walls. 

PORPHYRY. 

There are two parallel dikes of porphyry about 150 to 200 feet apart. The more northwestern of the two appears to be the thicker. · This dike shows a decided difference in texture· 

between the center and the edges, the rock at the edges being finer grained. A quick crystallization of the margins by chilling is indieated. In many places the border phase of the dike 

is glassy, forming the green or red obsidian which always excites the attention of the miner or 

visitor, and whose origin has been much discussed by them. This obsidian was observed at 

various levels, including the 400-foot, the 750-foot, and the 1,000-foot, and was seen in places 

· on· both contacts of the Central porphyry dike. It does not, however, everywhere form the 

margin of the porphyry, the principal reason being probably that the porphyry in some places 

has a normal intrusive contact, where obsidian may or· may not form the marginal phase, and 

in others is bounded by the parallel subsequent faults along which the vein has formed, in 

which case the present edge of the porphyry is not nceessarily the original boundary. Where 

the porphyry has a natural intrusive contact, as it does in many places, this is readily discernible from the wavy course of the contact and the little tongues of porphyry which run into 

the gneiss. A very characteristic feature of such intrusive contacts is the beautifully corrugated surface of the porphyry where it abuts against the gneiss. This corrugation conforms 

with the lamination of the gneiss and has evidently been formed by the molding of the plastic 

porphyry into the softer and harder gneiss laminm. 

A specimen from the southeastern or .tUiunde porphyry dike, obtaineq on the Aliunde 

No. 3 level, shows when examined microscopically a semiglassy, semimicrofelsitic groundmass 

with one small phenocryst of quartz and abundant perfectly fresh small crystals of sanidin'e; 

larger and more altered phenocrysts are of original biotite and probable soda-lime feldspar; 

accessory apatite and zircon are present. 

A specimen from the more northwestern or Central dike on the 850-foot level has a groundmass varying from glassy to crystalline, and contains phenocrysts of quartz and sanidine with 

pseudomorphs of kaolin and calcite after feldspar. A specimen from the same dike, obtained 

on the 1,000-foot level, has the same charaeteristics, containing fresh sanidine and quartz 

phenocrysts in a glassy groundmass. The Colorado Central dikes have been mapped by Ball as 

granite porphyry. · 
250 GEOLOGY OF THE GEORGETOWN· QUADRANGLE, COLORA.IiO, 

GRANITE. 

The g;anite is of the same nature and age as the principal intrusive granite in the area of 

the Silver Plume special map. In texture it ranges from massive to semiporphyritic, and 

even to the highly porphyritic type known by the miners as "cor11 rock." 

PEGMATITE. 

There are very large masses of pure pegmatite in the Colorado Central mine. The largest 

body exposed is seen in the. Ocean Wave tunnel, where it forms the wall rock 011 both sides 

of the Ocean W a.ve lode. Reddish feldspars are abundant. 

GNEISS. 

The gneiss in this mine is usually black and rather soft, but typically contains many lenses 

of pegmatite. In many places the pegmatite and gneiss are intimately intermingled. 

The relative ages of the different wall rocks arc shown in fig. 91. 

EXTENT OF POHPHYRY DIKES. 

There are two distinct dikes of porphyry associated with the Colorado Central veins and 

shown in the mine workings. They are parallel and are 100 to 150 feet apart. The more 

' FIG. 91.-Vertical section of face of drift, 350-foot level, 

Colorado Central mine, showing relative ages of rocks. 

Black gneiss with injected pegmatite scams (c) has 

been intruded by granite (b), and later both have been 

cut by porphyry (a). 

northwestern of the two is that along which the Central 

and North veins run, and it is the better developed 

underground. This may be called the Central dike. 

It is not shown in the accessible mine workings above 

the Marshall tunnel or 400-foot level. On this level it 

is shown in the mine workings for a distance of 2,000 

feet along the strike, and is still strong at both ends of 

the development work, its thickness at the northeast 

end in the Marshall tunnel being 40 feet and at the 

southwest end 25 feet, while its minimum thickness 

appears to be about 20 feet. 

On the 500-foot level the workings are mostly caved 

and the thickness and strike extent of the porphyry 

are not shown. On the 600-foot or Ocean Wave t11nnellevel the Central dike has an exposed strike extent 

of about 1,400 feet. Its thickness at different points 

is from 4 to 12 feet. Beyond the porphyry, hoth on 

the northeast and the southwest, the North vein runs 

for long distances in the gneiss and granite. The 

Equator tunnel, which crosscuts the whole formation, shows that the Central dike is not 

present, though, as above noted, it is present in the Marshall tunnel crosscut, 200 feet above 

and a little more than 200 feet to the southwest. This indicates that the Central dike 

pinches out on the northeast. On the 750-foot level the Central dike has a developed strike 

extent of about 1,200 feet. The probable wedging out of the dike on the northeast is shown 

on the plan (Pl. XLIV), about 250 feet southwest of the point where it probably wedges out 

on. the Ocean Wave level, 150 feet above. The thickness of the dike on this level is from 

8 to 15 feet. On the 850-foot level, ag~in, where the dike h~s- a strike extent of 950 feet, it 

probably pinches out on the northeast at a point about 70 feet west of the corresponding 

point on the 7 50-foot level above. The thickness of the dike on this level ranges from 10 to 25 

feet. The 1,000-foot level has a strike extent on this dike of 11050 feet, and shows a probable 

wedging out on the northeast almost directly below the corresponding locality on the 850-foot 

level 200 feet above. 

These observations indicate that the Central dike wedges out to the northeast, the line· 

of disappearance pitching southwestward at a flat angle on the levels first described, but becoming vertical at greater depth. On all these levels the vein continues northeastward past the 

porphyry, in gneiss, granite, and pegmatite wall rocks. 
U. S. GEOLOGICAL SU RV EY 

A . SPECIMEN FROM COLORADO CEN T.RAL MINE, SHOWING 

PRE-CAMBRIAN PEGMATITIC QUARTZ AND PEGMATITE, 

CRU SHED AND CEMENTED BY TERTIARY GREENISH 

JASPERY QUARTZ CONTAINING METALLIC SULPHIDES. 

a, Pegmatitic quartz cont a ining a litt le feldspar; h, met allic sulphides; 

r, jaspery quart z, t rue gangue of t he meta ll ic su lphides; d, pegmat itic fe ldspar. Nat ura l size. 

PROFESSIONAL PAPER NO. 63 PL. XLV 

B. POLISHED SPECIMEN FROM COLORADO CENTRAL MINE, SHOWIN G 

QUARTZ OF PRE-CAMBRIAN PEGMATITIC QUARTZ VEIN WHICH 

HAS BEEN FRACTURED AND PARTLY INVADED BY TERTIARY 

POLYBASITE, GALENA, AND BLENDE. 

T his vein has what may be termed a fa lse gangue. a, P re-Cambrian pegmatitic 

quart z, false gangue; b, Tertiary met allic su lphides ; c, T ert iary dark-gray 

jaspe ry quart z, t rue gangue. Nat ura l size. 
MINES NEAR GEORGETOWN. 251 

On all the levels above mentioned the Central dike disappears from the more southwestern 

workings also, but crosscuts here and there indicate that the vein has simply diverged from the 

dike, and do not show that the dike itself has pinched out. 

The more southeastern of the two dikes may be called the Aliunde dike. It is seen in the 

first, second, third, and fourth levels of the Aliunde and in the Ocean Wave level of the Colorado Central. On the last-named level its thickness is shown to be only a few feet, while higher 

up no point was seen where its thickness was displayed. This dike was not seen by the writers 

below the 600-foot Jevel. 

EFFECT OJ;' WALL HOCKS ON VEINS. 

In the Colorado Central mine the wall rock which is most favorable for strong and persistent veins is granite or pegv1atite. Hard gray granitic gneiss mixed with pegmatite and 

granite is abundant as a wall rock in the central portion of the mine, and in this rock the veins 

are strong and productive. In the western part of the mine workings soft black gneiss. is 

prevalent with only small seams of pegmatite. This rock is dry, containing very little or no 

seepage water, and the veins in it are characteristically weak and unmineralized. 

In general the veins cut diagonally across the lamin::e of the gneiss, as seen on the plans of 

the separate levels. The strike of the gneiss is fairly regular, being to the north or the west 

of north; the veins strike northeast, so that as a rule they quarter across the gneiss at an angle 

of 45° or more, up to 909, as seen at a few places in the Ocean Wave tunnel. The relative strike 

of the gneiss and of the veins is pla.inly accidental, as the veins depend on strong northeasterly 

differential movements regardless of the rocks traversed; but in detail the strike of the gneiss 

is apt to influence the direction and strength of the veins. Where the veins become parallel 

to the lamin::e of the gneiss they are usually weak. In soft gneiss the vein is apt to resolve 

itself into overlapping parallel lodes instead of continuing as a single strong lode. This is due 

to the planes of easy fissility afforded by the gneiss, which constantly tend to deflect the vein 

slip. This is especially well shown where the strike of the gneiss is not greatly different from 

that of the vein, as in the southwestern part of the 1 ,000-foot level wprkings. Here the vein 

fault is repeatedly deflected in the direction of the gneissic structure, but none of these deflected 

slips persists far out of the line of general stress shown by the strong unbroken fault vein, its 

position being taken by a parallel overlapping vein, and the different overlapping veins being 

stationed at intervals along a general zone parallel to that of the main vein. 

Where the vein cuts pegmatitic quartz veins which are phases of the pegmatite and are 

of pre-Cambrian age this older quartz is crushed and brecciated, and fractures and fissures in 

it are cemented with crystalline quartz belonging to the vein-forming period and with the 

metallic sulphides. The typical quart7, of the veins is dense, greenish, and impure, and is 

due chiefly to replacement, whereas the pegmatitic quartz is white and crystalline. Yet this 

pegmatitic quartz is similar in appearance to the comb quartz of the veins, which characteristically lines open cavities. So where the peg.matitic quartz has been crushed and rccemented 

by vein material, the difference between it and the greenish replacement quartz of a vein is 

easy to discern, but not that between the pegmatitic quartz and the comb quartz of the vein. 

Therefore it happens that many specimens of the pegmatitic quartz fractured and cemented 

with comb quartz and metallic sulphides appear to be really an integral part of the gangue; 

moreover, where pegmatite has been thus crushed and recemented the resemblance to an original mineral-bearing pegmatite is very close (Pl. XLV). 

As in the Bismarck-.:Pelican lode, the most productive portions of the Colorado veins are 

near the Central porphyry dike. The only discernible reason is a physical one· and is the same 

as that arrived at in the case of the Bismarck, namely, that the contact of the porphyry afforded 

a preexisting plane of weakness which governed the subsequent stress and made a single strong· 

and persistent circulation channel; whereas elsewhere, where there ·was no preexisting govern- , 

ing structure, the fault channel was more apt to branch and so to weaken. · 
252 GEOLOGY OF THE GEORGETOWN QUADHANGLE, COLORADO. 

ALTERATION OF WALL ROCK. 

ALTEHA'fiON OF PORPHYRY. 

A specimen of porphyry from the Central dike, Marshall tunnel crosscut, appears hard and 

fresh in the hand specimen, but under the microscope proves to be altered. It has an altered 

·microfelsitic groundmass. Phenocrysts are numerous and of fair size. Rather large and abundant feldspars are entirely altered to sericite, with some calcite. The plentiful biotite is entirely 

altered to muscovite, with the separation of siderite. Unaltered minerals comprise rare and 

small quartz phenocrysts, with accessory apatite and zircon crystals. Small areas of what 

appear to be specular iron and siderite occur throughout the slide. In many of these areas the 

siderite is rimmed by the iron. These minerals appear to be pseudomorphous after some 

original mineral. A thin section of a specimen from the Central dike, obtained on the 850-

foot level, shows· smile fresh sanidine, associated with larger crystals of another feldspar that 

is completely and uniformly altered to kaolin, with a little calcite. 

On the 600-foot level the porphyry iR usually much silicified near the vein. 

A specimen from the South or Aliunde dike, forming the wall of the South vein on the 

600-foot level, shows under the microscope the microgranular groundmass altered to sericite, calcite, quartz, etc. Biotite phenocrysts are entirely altered as above stated. Another 

specimen shows the feldspar phenocrysts entirely altered to aggregates of sericite, calcite, and 

kaolinic material. This specimen contains a veinlet of kaolin, apparently deposited in a 

crack. Specimens from this dike show scattered quartz phenocrysts and the same small areas 

of probable specular iron and siderite that arc characteristic of the other dike. 

AI,TERA'J'ION OF GRANITE. 

A specimen of granite which is practically a part of the North vein on the 400-foot level 

shows under the microscope fresh microcline and orthoclase. Other feldspars are entirely 

altered to what seems to be a fine sericitic aggregate. This altered feldspar shows fine twinning 

strim, but was indeterminable. The biotite is entirely bleached and altered to muscovite, 

with the development of siderite, much of which is oxidized to reddish ferritic stuff. A few 

small solid areas are made up of siderite, some of them with a smaller amount of inclosed 

pyrite. These minerals are perhaps pseudomorphs after original magnetite. Other specimens from the same vein on the same level show broken grains of perfectly fresh microcline, 

and fragments of granitic muscovite and of zircon with sericitic areas which evidently represent decomposed fragments of feldspar other than microcline, all cemented by vein quartz. 

In one specimen calcite seams traverse fresh microclinc and are probably derived from the 

alteration of the adjacent soda-lime feldspar. The granitic quartz and. muscovite of the 

granites are unattacked. The alteration products of the feldspar include, besides sericite, 

abundant carbonates. The fresh microcline wherever studied incloses small idiomorphic crystals of th~ completc~y altered feldspar. 

AJ,TERA'f!ON OF PEGMATITE, 

A specimen of pegmatite from the vein shows under the microscope fresh quartz and 

microcline crushed to a mosaic and shredded bits of muscovite. Areas of sericite represent 

original feldspar. The most crushed zone in the section consists of a fine aggregate containing 

in part fine secondary quartz, abundant siderite or ankerite, and some probable galena contemporary with the carbonate. Even in this zone there are fragments of fresh microcline. 

The microcline in these rocks appears never to have been decomposed, though in cases 

. of extreme ~ncroachment by mineralization it seems to be replaced. In one specimen studied 

microscopically microeline was seen in process of replacement by polybasite with a little pyrite, 

without decomposition of the feldspar. 
MINES NEAR GEORGETOWN. 253 

COMPOSITION OF VEIN. 

The principal metallic minerals· are gal~na and blende, with iron pyrite in small quantities; 

The blende of this mine has usually a light-yellow color. The galena contains a relatively small 

amount of silver. The silver values come from richer disseminated argentiferous ·minerals, 

of wl{ich E. Le Neve Fostera enumerates argentite, pyrargyrite, proustite, tetraheclrite, 

stephanite, and polybasite. The most abundant of these noted by tp.e writers is polybasite, 

which occurs massive and in cyrstals on the faces of vugs. Next, but much less abundant, 

appears to be ruby silver (chiefly pyrargyrite). A specimen of massive rich ore examined 

by ,V. F. Hillebrand for the writers proved to be mercurial tetrahedrite, and tetrahedrite was 

determined in two other specimens by W. T. Schaller. The polybasite, as well as the tetraheclrite, is locally known as "gray copper." Native silver is occasionally observed. · Only 

the richer ores, characterized by these silver-bearing minerals, have been worked. According to Fosterb the average silver content of the ore produced averaged 200 ounces to the ton 

for sixteen years. V cry little gold is present. During the period mentioned above only 

once was there enough gold to be paid for by the smelter, and that was in a lot of concentrates 

containing considerable pyrites, which yielded about 0.25 ounce of gold to the ton.c An 

experienced mii1er states that the only pocket of ore in the mine which carried gold in any 

amount was the .MacCarey stope, on the 850-foot level, much of which carried 0.1 ounce of gold 

to the ton. 

Hematite (specularite)d in aggregates of small crystals was found by the writers in the 

lowest part of the mine (1 ,050-foot level). This is barren of values. . 

Siderite or ankerite is rather common, especially in later veinlets. It is probable that 

various carbonate mixtures are present in the mine. According to J. S. Randall, of Georgetown, W. 0. Crosby determined the presence of small quantities of mesitite, an iron-magnesia 

carbonate, in the Colorado Central. 

The principal nonmetallic gangue mineral is quartz, which is locally abundant. There 

are two distinct varieties-the coarse crystalline, much of it with comb structure, and the fine 

chalce<;lonic, much of which under the microscope has a semispherulitic structure. Calcite 

occurs in and near the veins, very commonly in barren veinlets and in crystals lining the walls 

of small empty fissures and vugs. 

Where the vein is not miner ali mel the clay "lead" as a rule contains neither metallic 

sulphides nor gangue minerals. 

DEVELOPMENT AND CEMENTATION OF VEIN CHANNELS. 

The openings which gave access to the mineralizing waters were form.ed by sheeting or 

.. faulting of the rock along a northeastward-trending, nearly vertical zone. This zone was 

several hundred feet wide-say, roughly, a thousand feet-and several thousand feet long. The 

usually parallel fractures and slips which lie in it had variable strength and persistency, and 

along them veins of strength and size corresponding with the size of the channels were subsequently formed; but the strongest of these channels was probably that now occupied by the 

main Colorado Central vein, which lies near the center of the zone. This vein follows a strong 

fault plane or set of fault planes. The principal fault lies along the southeast contact of a 

·northwestward-striking porphyry dike-the Central dike of the above descriptions. The 

relation is similar to that of the Bismarck-Pelican fault vein to the porphyry dike which it 

follows, but differs in that the porphyry of the latter dike is soft and decomposed and the 

·slips have been mainly in it or along its contact, whereas in the Colorado Central the dike, 

though shown by the microscope to be in the main highly decomposed, is usually hard. The 

contact of this hard porphyry with the gneiss, granite, or pegmatite wall rock presented a 

plane of weakness which developed into a fault when stress was applied· at some time distinctly sub~equent to the consolidation of the dike, and where the fault slips do not follow 

a Proc. Colorado Sci. Soc .• vol. 7, 1902, p. 48. 

bldem. 

c Foster, E. Lc Neve, op. cit., p. 49. 

d Detennined by W. T. Schaller, of the United States Geological Survey. 
254 GEOLOGY Ol!' THE GEORGETOWN QUADRANGLE, COLORADO • 

.the contact they pass away from it into the gneiss or granite, and not into the hard porphyry. 

Exceptions to this rule are noted at a few places where fault slips and ore pass into porphyry. 

At one locality on the Marshall tunnel level the ore of the North vein occurs on both sides of 

the obsidian which forms the quickly chilled border phase of the dike, and a similar occura ,; a SE renee can be seen on the 

( 1(1 ~ r ~~~~~~~~~ !'( l ~~.>,~,._v.,,"/'V4AA V I\<CV4 v>yv_\v 750-foot and 1,000-foot 

1/ /) (/II J / f ~~~~ ~ 1) ql/ I(/ { ( ) \ l \ (j \ > .. ';;" ~ " : .t,.. : > : ~" • ,." . .'' \ levels. Moreover, the. vein 

) \/ (/I/) I ( /l/ ( 1

1/t/1/ / \ d I~ l1 \\ ", "v ".., ~": ~~ .. ,/" t; , ~ : : ~ " L.''~ fault, having naturally a 

lilll/((~)11 l/// I(! )~n~el_ /stls) 1)/ \ J

1

\ \\ /())\{ : ~:..,:.~~: ~~ig~itiie ~ ::: :,~": :"::> :;~:~I~::~~=~~l~e ~~a;:~= ( ( / ( 1 I ( / ( \ jl I 1) L ". ~" ".., ,.. " " , " ",. < tact, in places cuts through 

l~~(~~~~~\/l/(\\ll\l}'(\~~((\). v ~" ""~":' .. ,";"",•:""~.,~",." protuberant curves of the '1/ \ ! ) I ! " "

11 " ,.. > " " ., 7 < porphyry (figs. 92, 93). 

/11 ) /) ( 11 ~ /I! ( \ \( (I ~I ! / "L " : " " .. """ " : The other veins con-

~~~~~~/~1/./1((1/1 :~::: ... >~:;~ nectedwiththismainvein, 

I 1 1 I 1 \ 1 J •• " ·- • .. • • being either branches (South 

:~~~~~d(,_I ___ )HI ~;"~ .. ::"::: vein), parallel or oblique 

-; ~ ~~- :,: *' ~:--· .. " " v ~ ,.. .. " auxiliary veins (New North % ~ ~ * Drift " " ., .. vein), fiat intermediate jl/. ~~ * ,._ -.'-,."' .. " .. ~ »s ,.. >« "' • v ,.. .. ., veins, or imbricating veins, )\(. ~ * ~ % *" ~ ~ • ~ ~ v " ,.. " >- " , I II . 'I . . '~- * "' >~< ,.. "' ~ "' _, ".., .. " ~ 1ayc a a s1m1 ar ongin- 3\< >l< >~< " "' "' " • .. "' v .. they follow fault slips, and ~ ~ ~- ~ ~ >\t * * • .J\ " 1\ a. '7 r 

"' "'Porphyry* "' "' w • _., " ",.. "".., a study of their variations ill .. " "' * "' "' >/< ~ ..> ,.. .. 1\ 

a. * "' "' .. . "' >~< "" ~ 1!11 ~ ~ "' ., L resolves itself chiefly into a * '\1 * ~ ,_" ,.. \f ., "' * * "' * * " " "' £., study of the behavior of * * >l<>!<>l<\1: 'I<.. """"" * * "" "" .c "' "' * ., " " "'" < fault fractures (fig. 94). >l< * * ~ .... .. -e .. ,.. ,.. £ "**"'"' >~<,.~ "'*"'** * t."v, .. ",..~ ·Thefaultsareindicated 

o'------"'z ____ _.4feet 

FIG. 92.-'-Vertical cross section of Colorado Central vein on 1,000-foot level, showing relation 

of the vein (which has formed along the fault plane) to the und,ulating intrusive porphyry 

contact; a, Qougc, or kaolinized crushed rock along contacts; b, ore in main vein. 

by ground-up material, by 

discordance of opposing 

walls (which rarely can be 

matched so as to show the 

exact amount of displacement), and especially by numerous strong strim which show satisfactorily the direction of movement of one wall on another. The best of these records are tabulated below. 

Pitch of strix in t·eins of Colorado Central mine. 

[Degrees southwest from horizontal on lode.] 

North vein: 

600-foot leveL _____ ...... _________ .. __ , __ ._-{ 

750-foot leveL _________ .. _ .. _ .. _ ... _________ . 

851>-Ioot level.. ............................. ·l 

1,000-foot leveL __ .. ___________ ..... ___ .. __ .. 

Average of observations. ______ . __ ... ___ _ 

South vein: 

~ 600-foot level.. ____ .. _____________ . _________ -eo-!~ 

0 850-foot level.. ______________________________ 15-25 

35 Flat intermediate veins in Aliunde workings: 

30 Aliunde No. 2 leveL _______________ , __ .. ____ _ 

15 

15 

10 

45 

20 

20 

22 

Aliunde No. 4leveL. _' _____________________ .. { 

20 

12 

45 

All these observations show unequivocally that the movement of one of the walls on the 

other was to the southwest at a relatively small angle from the horizontal, varying to some 

extent locally, but averaging around 20°. These faults were strong and persistent, though the 

amount of displacement was not great. A maximum of somewhat moi·e than 100 feet may be 

roughly assigned, this figure being taken from the displacements indicated on the horizontal 
MINES NEAR GEORGETOWN. 255 

plan of the North vein of the Ocean Wave level. The displacement as seen on this plan is not 

far from the total displacement, since the line of movement on the fault is at a comparatively 

small angle from the horizontal. 

These fault openings have become the channels for 

circulating waters, which have altered the crushed fault 

material and to a lesser extent the strained wall rocks. 

Such waters have deposited vein materials, consisting of 

gangue and ore in some parts of these channels, but by rio. 

means at all points. The unmineralized portions of the 

chapnels are simply zones of crushing and decomposition in 

granite, gneiss, pegmatite, or rarely in porphyry, and are 

marked by clay ("gouge"). Such zones are called "leads," 

as by following them a mineralized portion of the fault 

channel can often be reached. As a rule they show no 

cementation whatever, but only the results of decomposition by the circulating waters. This condition is characteristic of parts of even the strongest fault channels, as that 

of the North vein, illustrated on the Ocean Wave or 600-foot 

level (Pl. XLIV). 

In many places the mineralized portions of the fault 

channels, constituting. the mineral veins, alternate irregularly with the barren "lead" portions, and the dying out 

of many of the veins is accomplished by the passage from 

a chiefly mineralized portion to a soft and up.mineralized 

portion, which may lose itself in the rock. Locally, however, the unmineralized "lead" may be very strong, so that 

it can be followed for many hundreds of feet. 

· FIG. 93.-Cross section of drift, 600-foot level,· 

Colorado Central mine, showing relation of 

South vein fa11lt lead (l-l) to the contact of 

the South or Aliunde porphyry dike. 

E w The mineralized portions of the veins have, as a r~ile, 

~--'-~3fect 

FIG. 94.- Vertical section at shaft, 850-foot level, Colorado Central mine, showing splitting by faults (c) 

in black gneiss (a) containing intrusive pegmatite 

bands (b). 

been cemented largely by the deposition of silica, forming 

a dense, greenish replacement quartz, or, in cavities, a 

white crystalline quartz, usually with comb structure. 

With the quartz are associated various other nonmetallic 

and metallic minerals. 

The porphyry dikes, which existed before the faulting that gave rise to the veins, were probably intruded 

along earlier planes of weakness, produced by stress that 

very likely terminated in faulting. These zones of weakness were cemented by the porphyry intrusion. Renewal 

of the stress brought about the postporphyry faulting, 

and these openings were cemented by the vein materials 

deposite'd from circulating waters. The movement was 

not entirely finished, however, but has been renewed, 

probably several times, since the initial opening of the 

vein channels. Thus the materials of the earlier mineralization have been brecciated and then cemented 

with new quartz and metallic minerals. New openings 

were thus repeatedly formed by movement, and in places 

they \Vere enlarged by waters through dissolution instead 

of cementation; and later these cavities became the seats 

of mineral deposition. The indications are that this 

process has gone on from the beginning of the faulting and mineralization down to the present 

day. The number and area of such unfilled openings at present existing in the vein are very 

large, and it is plain that some of them have been formed at a comparatively recent time by 

movement, while little vugs have been created along them by dissolution, 
256 GEOLOGY OF THE GEORGETOWN QUADRANGLE, COLORADO. 

A considera~le part 9f the gangue and metallic minerals in the vein has thus been deposited 

in open cavities, while probably a larger part is due to impregnation and replacement of crushed 

material. 

DISTRIBUTION OF ORE IN VEINS. 

All the pay ore contains polybasite and ruby silver. Where these minerals are absent the 

silver content rarely exceeds 25 ounces per ton. Pay ore has been found down to the bottom 

of the present workings, but apparently growing less in amount. .Much of the first-class ore, 

sorted out, has averaged 700 to 800 ounces of silver; the second-class ore has frequently averaged 200 to 300 ounces. 

The ores are found in irregular shoots which, as a rule, pitch westward on the vein. On 

the 400-foot level the Central vein splits up into a North and a South branch. It is said that on 

the North vein, for 200 feet southwest of the junction (Pl. XLIV) extending about 100 feet up 

and 100 feet clown from the level, occurred the richest body of ore in the mine, producing an 

estimated amount of $1,000,000. The part of it lying below the level included the Carnahan 

stapes, where from a portion of the vein about 100 feet long and 50 feet high $500,000 is said 

to have been extracterl. There is no particular enrichment at the junction of the two branches 

here. A similar deposit occurs on the 600-foot level, 200 feet below, at the same or a similar 

branching (here a few hundred feet farther northeast). On this level many thousand 

dollars' worth of ore was extracted from the North vein for about 100 feet southwest of the 

junction: From the next 300 feet of the vein beyond this 100-foot stretch about $300,000 

worth of ore is said to have been extracted. The location of these bonanzas indicates that 

the junction of the veins may have been important in determining ore deposition. A minor 

but fairly evident instance of the operation of this factor was observed on the Aliunde No. 2 

level. Here at the junction of two branching leads a ton or more of rich ruby-silver ore was 

taken out, but there was no other mineralization in any degree, either of the branches or of 

the united lead. 

PARAGENESIS OF VEIN MINERALS. 

DESCRIP'l'ION OF SPECIMENS. 

The Colorado Central mine affords an unusual amount of material showing the succession 

of minerals. Some of the most interesting specimens are described in the following notes: 

Specimens from the 500:foot level.-A specimen from the 500-foot level shows ( 1) medium 

coarse intercrystallized galena, blende, andjaspery replacement quartz; (2) in irregular geodes, 

which are probably the result of the eating out of this older ore by percolating waters, are 

subsequent kaolin, barite, and polybasite, contemporaneous with one another, the last two 

forming good free crystals. The kaolin, according to the chemical tests of W. T. Schaller, 

is mixed with zinc and cadmium minerals. There is no zinc carbonate present, and the zinc 

is probably in the form of sulphide. The relation of this kaolin to the other minerals shows 

that, like them, it is a deposit from solution, and is not residual. 

Another specimen from the same level shows coarse blende, bordering, with large free 

crystals, on a vug. On these blende crystals have been deposited numerous crystals of polybasite and barite, with less abundant siderite or ankerite and proustite. 

Still another specimen from this level shows massive blende, with massive galena cutting 

across the blende and characteristically later. In vugs in the blende and galena are fine 

small free crystals of quartz, polybasite, and blende, all abundant and contemporaneous. A 

fine skin of tiny calcite crystals covers all t}1ese minerals, as the latest crystallization .. This 

specimen, it will be noted, shows two distinct generations of blende. 

A fourth specimen from the same level shows a first formation of medium-coarse galena, 

blende, a little pyrite, and jaspery quartz. In geodes which have been formed in this mixture 

are free contemporaneous crystals of quartz, polybasite, proustite, and ,argentite. 

A specimen from the same locality as the last shows as the first formation medium-coarse 

galena and blende, with jaspery quartz. On the wall of a cavity in this material are beautiful 

small free crystals of contemporaneous polybasite and barite. 
MINES NEAR GEORGETOWN. 257 

Specimens from the 600-foot level.-A specimen from the 600-foot level contains massive 

galena, blende, and polybasite or tetrahedrite, the age relations of the three being· uncertain. 

On small crevices are contemporaneous tiny crystals of barite and pyrite or chalcopyrite. 

The latter is the thin skim of greenish cry!'tals called "green silver" by t~e miners and supposed 

to be rich in silver. As determined by W. F. Hillebrand, however, it probably does not contain 

any notable amount of silver. 

Another specimen from the same level shows massive galena and blende. Subsequent 

polybasite cuts these minerals and also appears in free crystals lining the walls of geodes, 

where there also occurs contemporaneous barite. 

Another specimen from this level shows veinlc~s of siderite and calcite, with a little galena. 

The calcite is locally intcrcrystallized with the siderite, but is on the whole a later deposit. 

Microscopic examination of this specimen shows that a siderite-calcite veinlet has been crushed, 

and the fragments cemented by barite. 

On the same level, on the Ocean Wave tunnel, about 700 feet in and 300 feet vertically 

below the surface, fissures have been lined with siderite, and upon this has been deposited· 

chiefly blende, which in some places has entirely 

cemented the fissure and in others has only partially 

filled it, the free blende crystals projecting into the 

cavity. Associated with these minerals arc also free 

crystals of galena, barite, and siderite or ankerite. 

Two generations of siderite deposition are shown in 

this specimen. · 

. Specimens from the 750-foot level.-A specimen 

from the 750-foot level shows fine galena with some 

blendc. These minerals arc cut across veinwise by 

pyrargyrite. Another specimen from the same level, 

and 100 feet distant from the first, shows massive 

blende, cut by seams of later pyrargyrite and some 

proustitc. 

Another specimen from this level shows free 

crystals of various minerals deposited successively on 

the walls of a fissure, in the following order: (1) 

Quartz, ( 2) blende, ( 3) pyrite, ( 4) a thin film of. 

calcite. There is here and there a slight intercrystallization of the quartz and the blende, though as a rule 

the blende is distinctly subsequent. This specimen was 

found close by others, showing the order of deposition of 

different minerals in cavities. In one specimen the or·der· 

was (1) quartz, (2) siderite; in another, (1) siderite, (2) 

FIG. 95.-Sketch of veinlet on 850-foot level, Colorado 

Central mine, showing a fissure which has been lined 

with brown carbonates and filled with fine galena, a 

single quartz crystal being contemporaneous with 

both deposits. a, Gneiss; b, quartz; c, galena; 

d. siderite. 

galena. The latter relation is very common, and was observed repeatedly in other localities, 

both here and on other levels-for example, the 600-foot level. In still another specimen 

the order is ( 1) quartz, with intercrystallizcd pyrite, galena, and blende; this material is 

traversed by a later fissure, which is lined by (2) chalcedony; and the last deposition consists 

of small crystals of (3) barite, sitting upon the chalcedony. Another specimen from the 

same level shows numerous small fissures in pegmatite, lined with quartz which is coated with 

subsequent siderite or ankerite. 

Specimens from the 850-foot level.-A specimen from the 850-foot level shows veinlcts 

filling fissures in gneiss. On the walls of the fissures siderite has crystallized. In many of 

the fissures the center is empty; ordinarily, however, it is filled with fine galena, which is 

always distinctly later than most of the siderite, though here and there a crystal of siderite 

may be found embedded in it. There is a very little quartz. In some places a single quartz 

crystal extends outward from the wall of the vein through both the siderite and the galena 

deposits, showing that its formation was contemporaneous with both of these (fig. 95). 

31401-No. 63-08-17 
258 GEOLOGY OF THE GEOHGETOWN QUADRANGLE, COLORADO. 

Another specimen from the same level shows veinlets containing chiefly galena, with some 

blende, polybasite, pyrite, and chalcopyrite. Druses in these veins are lined with free crystals 

of blende, polybasite, quartz, chalcopyrite, and siderite or ankerite. 

Specimens from the 1,000-joot level.-A specimen .from the 1,000-foot level shows the following succession of minerals in a fissure filling in gneiss: (1) Comb quartz, with a little pyrite, lining 

the fissure; (2) coarse galena and blende; (3) a thin coating of pyrite or chalcopyrite ("green 

silver" of miners); (4) fine crystalline caleite. Another specimen from the same level shows the 

following order of deposition in a fissure: (1) Comb quartz lining, with a little pyrite; (2) galena 

with a little quartz and pyrite; (3) comb quartz, of about the same thickness as 1; (4) crust of 

white calcite. Another speeimen shows, in gneiss, vugs filled with the following minerals, named 

in their order of deposition: (1) A lining of coarse comb quartz, with some intercrystallized pyrite; 

(2) galena; (3) polybasite, pyrargyrite, carbonate (probably ankerite), and quartz, in contemporaneous free crystals. Another specimen shows large crystals of polybasite (determined by 

E. C. Sullivan to contain much arsenic, and therefore to be grading toward pearccite), contem-

. poraneous, in druses, with fine carbonate (probably ankerite) and with chalcedonic quartz. 

Later than these minerals is a coating of fine pyrite. Another specimen shows quartz, kaolin, 

polybasite, and galena deposited separately but apparently contemporaneously in vugs. 

Specimens from the 1 ,050-Joot level.-A specimen from the 1,050-foot level (the deepest in the 

mine) shows a gangue of massive quartz and barite. In druses in this material are polybasite, 

pyrargyrite, galena, and blende. Another specimen shows a vein of polybasite cutting sharply 

coarse blonde and galena. In geodes in a third specimen from this level the following minerals were successively deposited in the order named: (1) Comb quartz, (2) polybasite, (3) a 

fine coating of carbonates (probably ankerite). A specimen near by shows the following order 

of deposition in a fissure: (1) Comb-quartz lining, (2) galena and blende, (3) pyrite film. Another 

shows (1) comb quartz; (2) blende, galena, and pyrite; (3) siderite or ankerite. 

On this level specimens were found showing. a considerable amount of hematite (specular 

iron), intercrystallized with quartz, blende, and pyrite (slightly cupriferous ?). These minerals 

occur cementing the fragments of a breccia of altered granite which contains pyrite, probably the 

result of alteration of the iron in the rock. The following order of deposition of minerals in the 

rock openings was noted: (1) Hematite and pyrite, intercrystallized; (2) clean comb quartz; 

(3) tetrahedrite, pyrite, and galena, intercrystallizcd, filling the central spaces between the combs 

of quartz. In other places a carbonate (siderite or ankerite) has been deposited on the quartz. 

Specimens from the mine dump.-Specimens taken from the mine dump, their original exact 

locality being unknown, show similar characteristics to those above described. One specimen 

shows the following order of events, which is similar to that repeatedly noted previously. The 

rock is pegmatite-mainly quartz, with a little feldspar. This rock has been crushed and cavities have been formed on whose walls the following minerals were successively deposited: (1) A 

lining of comb quartz, with some pyrite; (2) coarse galena and blende; (3) barite; (4) pyrite 

film; (5) carbonate (ankerite or siderite). Another specimen shows the same order of deposition, 

somewhat simplified, in large open cavities in gneiss: (1) Quartz with some pyrite; (2) blemle, 

galena, and some pyrite; (3) siderite or ankerite. In another specimen tetrahedrite occurs in 

small fissure veins in gneiss, associated ,i,rith and in general contemporaneous with pyrite, blende, 

and galena. In another specimen tlie following order of deposition was noted, on the walls of a 

geode: (1) Quartz, (2) siderite, (3) hematite and pyrite, intercrystallized. In another portion 

of the same specimen comb quartz is younger than massive hematite. Another specimen show!;J 

white kaolin occupying the spaces between interlocking crystals of galena and blendc, and apparently contemporaneously with them. Another specimen from the dump shows under the microscope a fragment of altered granitic rock surrounded by a fringe of crystalline quartz containing 

pyrite and a little galena. These materials have been subsequently fissured anlil then rccemented 

by veins of homogeneous, microcrystalline clear chert (of a grepnish color in the hand specimen). 
MINES NEAR GEORGETOWN. 259 

SUMMARY STAT~~MENTS CONCERNING PARAGENESIS OF MINERALS. 

Quartz.-Quartz is characteristic of the whole period of deposition and has been deposited 

from the beginning of the process to the end. As in the fissure veins of the Terrible group at 

Silver Plume, the first lining of the fissures in the Colorado Central mine has usually been quartz. 

Subsequently it is more or less plainly contemporaneous with other materials, and in many places· 

is among the very latest minerals, crystallizing on the walls of geodes, where it has been noted as 

contemporaneous with siderite or ankerite, blende, polybasite, pyrargyrite, proustite, and argentite. Chalcedony or chalcedonic quartz is not very common, though not rare. As a rule it 

belongs arriong the later formations, and has been noted cutting and distinctly subsequent 

to crystalline quartz. In places it is contemporaneous with galena and blonde, while barite, 

kaolinite, and polyhasite have been observed as minerals of later formation. 

Barite.-Barite was noted on the 400-foot level, forrning free crystals on the walls of a small 

fissure in crushed and silicifie<l vein material. Both the fissure and the barit~ crystals arc 

subsequent to the silicificatimi. On the 750-foot level barite was noted in separate crystals 

embedded in a soft 6-inch seam of clay. In neither of these localities was there any metallic 

mineral present, and as the crystals lay in the line of circulation of present waters their occur-' 

renee suggests that they have been d~posited from these waters. In the various specimens 

studied barite is characteristically among the last-formed minerals, and in many is thc.latest of 

all. In one specimen it was noted as younger than calcite, which in its turn was younger than 

siderite. In various specimens barite vms seen to be contemporaneous with siderite or ankerite, 

kaolin, galena, blende, pyrite, polybasite, and proustite. · Its favorite mode of occurrence is in 

fine crystals coating the walls of geodes, either alone or contemporaneous with free crystals of 

the above-named minerals, which in such eases usually belong among the last deposits. 

Siderite or ankerite.-Iron carbonates, probably comprising both the species siderite and 

ankerite, are more abundant throughout the Colorado Central mine than in the mines of Silver 

Plume here describe<!, and are more dosely associated with the ore. In many vei~lets in tllis 

mine siderite takes the place of the lining quartz and forms a barren first deposit on the walls of 

fissures which may subsequently be filled up by metallic Ininerals. Other veinlets of siderite are 

in barren rock, and are not accompanied by sulphides. In this respect the siderite (and ankerite) 

is like the pyrite and quartz, which have a similar habit, and its origin seems to be probably the 

same-that is, it represents the crystallir.ation along the walls of cavities of the carbonates 

abundantly formed by the alteration of the biotite and feldspar of the wall rock. Study of 

microscopic veinlets in altered rocks from other portions of the district makes it practiGallycertain that siderite veinlets may originate in this way. Much of this earlier siderite is intercrystallized with quartz. 

Siderite or ankerite, or more probably both, also forms one of the last and typically the very 

last deposit on the walls of cavities, coating all the other crystals.1 In other places siderite is 

contemporaneous with other of the latest-formed minerals, among whlch are quartz, barite, 

galena, blende, pyrite, hematite, chalcopyrite, polybasite, and pyrargyrite. 

Accordingly siderite belonging to two distinct periods of deposition may be seen in a single 

specimen. The fine coating of siderite which forms the last deposit in geodes is most intimately 

associated with the film of greenish cupriferous pyrite::, which usually occupies a similar position. 

The siderite is as a rule younger than the pyrite, though here and there the two are intercrystallized. 

Both seem to be cleai-Iy recent ueposits and due to ordinary ground waters. The change 

from an-iron sulphide to an iron carbonate may have been brought about by the opening of the 

mine workings, which artificially introduced oxidizing or semioxidizing conditions to influence 

the solutions in many places. 

Oalcite.-Calcite in the form of tiny crystals occurs_ in numerous places as the very last 

mineral to crystallize, its habit and probable origin being altogether similar to those of the 

siderite and ankerite with which it is associated. Calcite forms many barren veinS or veinlets 
260 GEOLOGY OF THE GEORGETOWN QUADRANGLE, COLORADO. 

in barren rock, where it has plainly collected from the calcite, which occurs abundantly within 

the rock as an alteration product. Both calcite and siderite form veinlets of this type in 

porphyry. 

The calcite was the very last mineral to form in all the specimens noted but one, which 

shows, later than the calcite, tiny free crystals of barite. Calcite has not been noted .~mung 

· the constituents of the earlier portions of the vein deposits. 

Kaolinite.-Kaolinite occurs, especially in small amounts, on the walls of geodes, where it is 

among the latest minerals to be deposited. In this mode of occurrence it forms a definite 

small mass on the geode walls, not interfering with the crystals of other minerals, which are 

contemporaneous. It is therefore plainly deposited from solution. Under these conditions 

it has been noted as contemporaneous with quartz, barite, galena, blende, a.nd polybasite. 

Galena and blende.-Galeiia and blende are closely aRsoeiated, and are usually eontemporaneous. In only one specimen was a differenee of age noted, the galena being there younger 

than the blende. These mincmlR do not belong to a single period of deposition, for although 

they are characteristic of the older massive vein material, yet they also occur in free crystals iR 

, druses of distinctly later age, where they are among the last minerals to crystallize. Frequently 

both generations can be seen in the same specimen (p. 136). In the latter mode of occurrence 

galena or blende, or both, have been noted as contemporaneous with quartz, barite, siderite or 

ankerite, kaolin, pyrite, hematite, chalcopyrite, polybasite, and pyrargyritc. 

Pyrite.-Pyritc occurs in fragments of the altered wall rocks inclosed in the veins in disseminated crystals so located that they appear to have been formed from the original iron in 

the rock. The presence of secondary pyrite, probably due to the alteration of magnetite and 

biotite, has indeed been determined by microscopic work, though siderite is a commoner form 

of alteration. Pyrite also occurs intercrystallized, generally in small amount, in the comb 

quartz which, as a rule (though not invariably, as is practically the case in the veins of the 

Mendota group at Silver Plume), forms the first deposit on the walls of open cavities in this 

mine. Where such a fringe of comb quartz carrying some pyrite surrounds a fragment of 

granite containing probable secondary pyrite all the pyrite has the appearance of having had 

a common ongm. According to this idea this (barren) pyrite has been secreted from the wall 

rock, to help in forming the first crust to heal the fissure. The comb quartz that is associated 

with the pyrite, and is usually much more abundant, may then have had a similar origin, its 

source being the abundant quart7; which, together with sericite and carbonates, is among the 

alteration products of the feldspars. It has been shown that in the Mendota veins, where the 

same principle which prevails in the Colorado Central is shown more strikingly, the sulphide 

deposition 'almost invariably took place during a period distinctly subsequent to the opening 

of the fissures, and in the meantime comb quartz crystals grew slowly and continuously outward 

perpendicularly from the walls, covering them everywhere. This action finds its best and most 

probable explanation in the derivation of the quartz from the adjacent rock. 

Pyrite also occurs, though in subordinate amount, as one of the minerals in the massive 

portions of the veins which are clue to impregnation and replacement of the crushed rock along 

fault zones. Here it is associated and contemporaneous with the jaspery vein quartz, together 

with galena and blende. It is usually very subordinate, and occurs in many localities in larger 

amount in the altered wall rock than in the veins, so that tho portion which occurs in the veins 

may easily have been derived from the wall rock. 

A thin film of greenish cupriferous pyrite also occurs very abundantly in nearly all vugs 

as one of the last minerals to crystallize, and of a distinctly later period and generation than 

that described in the preceding paragraphs. Pyrite of both generations occurs in many single 

specimens, as described above. The only minerals which as a rule are later than this latest 

pyrite are carbonates, including calcite and siderite or ankerite, or both. These carbonates 

commonly form a last coating of tiny crystals on the walls of cavities. The' carbonates and 

the latest pyrite are invariably very nearly contemporaneous, and in places some are fully so, 

and both seem to be doubtless the work of the present or recent circulating mine waters. 
MINES NEAR GEORGETOWN. 261 

Rematite.-Crystallized hematite (specular iron), though not noted as widespread, occurs 

in some specimens in considerable quantity. It has been observed contemporaneous with quartz, 

blende, and pyrite; as a later formation with siderite or ankerite, galena, and pyrite. 

· · Polybasite, tetrahedrite, pyrargyrite, proustite, argentite, ctnd native siZver.-Polybasite, 

pyrargyrite, proustite, and argentite, abundant in the relative order named, may be clisc~ssed 

together. The last two are relatively rare; the first two (especially polybasite) are very 

common, and their presence substantially determine_s the value of the ores. In most cases the 

age relations of these minerals are clear, and in every observed specimen they are among the 

.later minerals to crystallize. Where observed in connection with the galena-blende ores, 

which form the lean (unworkable) silver ores, these minerals are as a rule distinctly later, cutting 

veinwise across the older deposits. They are also typically formed in free crystals on the walls 

of druses. In this mode of occurrence polybasite has been noted contemporaneous with quartz, 

chalcedony, siderite or ankerite, kaolin! barite, blencle, chalcopyrite, pyrargyrite, proustite, 

and argentite. The only minerals observed as. of later origin are the carbonates which have 

been mentioned as forming the last fine crust in geode~. These include calcite and probable 

ankerite. Yet in other places polybasite, pyrargyrite, and proustite have been observed 

apparently contemporaneous with ankerite or siderite of this character, all sitting as free 

crystals on the walls of geodes. 

Tetrahedrite was observed in several localities. · Its genesis appears to be much like that 

of polybasite. Native silver is occasionally f,ound forming films on crevices. It is the last of 

the silver minerals to form, being later even than the rich secondary sulphides, and its formation seems relatively recent. 

GENERAl, CONCLUSIONS. 

The stu-dy of the paragenesis of the vein materials as a whole shows plainly that the deposition of the present vein materials has extended over a long period, which evidently lasted, and 

during which deposition was constantly though probably not uniformly active, from the formation of the first openings to the present day. The earliest deposits consist of comb quartz with 

occasional pyrite, which lined many of the fissures before the ,main deposits of metallic minerals; 

the latest are· the skins of more or less cupriferous pyrite and carbonates which crust the minerals 

and probably represent extremely recent deposition. 

It appears that at no time was the cementation of the opCJ~ings very thoro]lgh and complete, 

for portions of the fault zones are still soft and decomposed and appear never to have been 

cemented. Within the cemented portions of the veins, also, new openings have been repeatedly 

formed by movements in the veins and by the solvent action of dissolving waters that have 

followed depositing waters. The circulation of waters has always been active along the original 

openings, and so continues to the present clay. 

Of the gangue minerals, quartz has been continually deposite<l from the beginning to the end 

of the process. The deposition of siderite or ankerite has also been long continued. Barite and 

kaolinite, however, appear to be characteristic of the later periods, and calcite seems to be only a 

recent and relatively unimportant formation. 

Of the metallic minerals, pyrite, though always in small quantity, is the most persistent, and 

has been deposited as widely as has quartz.. The deposition of galena and blende began as a rule 

slightly after that of the first quartz and pyrite, but was repeated subsequently. Polybasite, 1 

tetraheclrite, pyrargyrite, proustite, and argentite seem to have been deposited only in the later 

stages and subsequently to the bulk of the older veins containing quartz, galena, blende, and 

pyrite. 

DERIVATION OF CERTAIN VEIN MATERIALS. 

DERIVATION OF RICH SILVER SULPHIDES AND ASSOCIATED LEAD AND ZINC SULPHIDES. 

Observations show that the highly argentiferous polybasite and ruby silver, on whose presence the :workable ore depends, are in general subsequent to massive galena-blencle ores carrying 

a few ounces (say 8 to 30) of silver. As formed during the later epoch of crystallization, markM 
262 GEOLOGY OF THE GEORGETOWN QUADRANGLE, COLOHADO. 

by these highly argentiferous minerals, are also to be reckoned some well-crystallized blende, 

pyrite, and galena. Similar last formations of galena and blende in cavities in the Waldorf 

mines, at Argentine fass, some miles southwest of the Colorado Central, and belonging in the 

same geologic district, are reported to be exceptionally low in silver. It seems probable that the 

polybasite, ruby silver, and other rich silver minerals have been concentrated from the leaner ores 

by circulating waters, subsequent to the first deposition. Certainly, however, the silver was not 

concentrated out of the surface zone and b~ought together in lower levels. The sup(lrficial portions of the sulphide vein are as rich as any portion, or richer, and the very thin oxidized zone is 

the richest of all. 

The most probable explanation is that these minerals were concentrated from the older lowgrade argentiferous sulphide orcs at all depths. Waters circulating through these ores have 

probably taken them into solution and effected a better separation of their constituents. The 

galena and blcnde of the second generation, lining geodes and in many places contemporaneous 

with the polybasite and ruby silver, may well represent the lead and zinc taken into solution 

and rcprccipitated with smaller silver values than originally. The deposition of the rich silverbearing sulphides in places more or less separate from that of the secondary lead and zinc 

sulphides-a separation whieli has brought about the formation of definite rich ore bodies-is 

probably clue to the differences in solubility of the materials. 

To follow this reasoning, considering the amoui1t of silver concentrated in the· rich shoots and 

the quantity of original blencle and galena which must have been taken into solution to furnish 

this silver, it seems probable that a large part of the original ore has been worked over by this 

process. Indeed, so abundant is the evidence of the continued work of solutions since the formation of the original vein opening that we can hardly be sure as to what is really primary and not 

rearranged ore. 

There appears to be in this mine, moreover, no clue as to the original source of the lead, zmc, 

and silver. Nevertheless, the presence of ore shoots in certain portions of the veins and their 

absence in other portions which contain only lead and zinc ores, of low grade in respect to silver, 

suggests an original difference in these separate portions. On the 600-foot level, for example, the 

portion of the Colorado Central lode known as the Ocean Wave lode carries zinc 'ores containing 

very little silver. Yet these ores also are traversed by later openings; affording the necessary 

conditions for concentration. It seems probable that such portions of the vein were originally 

different from the richer portions·ancl had originally a smaller amount of silver. This characteristic difference between veins of predominating blende, with little silver, and veins which carry 

rich silver ores, characterized by abundant, even though secondary silver sulphides, is striking 

enough in the Silver Plume area, as between the Mendota and the Terrible veins, for example, 

which lie side by side and have been exposed to similar conditions throughout; hut .here in the 

Colorado Central the two distinct ore .types are in different portions of the same lode. , 

The occurrence of the fresh, clean crystals of silver sulphides, ga}fma, blende, and other minerals, belonging to the later, probably concentrated generation, growing out from the walls of 

cavities which are full of circulating ground waters, suggests that such waters were the agents of 

concentration. Plainly at least these descending solutions have no dissolving effect on the Ciystals, which are as fresh in them as in a mother liquor.· On the Aliunde No. 2 level in the zone of 

the "flat" veins the writers noted a small unminenilized lead, a third of an inch thick. On 

breaking into this lead a streak of ruby silver was found, with water trickling clown over it. 

There was no more of this above or below. On the level above (Aliunde No. 1) ruby silver 

was found coating slickensided surfaces in galena ore. 

DERIVATION OF QUARTZ, PYRI'I'E, AND SIDERITE FROM 'l'HE WALL ROCK. 

The wall rocks of the veins are always altered. The chief alteration products are finely 

divided quartz, kaolin, muscovite (sericite), calcite, and siderite. On the walls of the vein fissures the first deposit has generally been comb quartz, with no evident relation to the deposition 

of galena and blende, and this has suggested the derivation of the quartz from that in the altered 
MINES NEAR GEORGETOWN. 263 

wall rock. The later quart11 may well have a similar origin. The amount of silica which has 

' been extracted from those portions of the lead which have not been cemented and are marked by 

clay zones is probably in itself sufficient to account for the quartz in the cemented porti'>ns of the 

lead. · 

A similar consideration applies to the rather scanty pyrite, which is in many places more 

characteristic of the altered rock than of the vein, and the iron for which (though not the sulphur) seems to have probably been original in the rock. 

The usual form of iron in the altered rock is siderite, showing that the altering waters were 

carbonated. Many fissures in such siderite-rich rock are lined with crystalline siderite, as the 

first deposit, in which case, as in that of the quartz, the derivation of the mineral from the wall 

rock seems probable, and in such fissures no other origin can be assigned to the subsequently 

deposited siderite. 

DERIVATION OF CALCITE. '· 

The sparse calcite, which is among the last minerals to form, needs no argument as to its 

formation, in view of the deposition of calcite from waters now traversing the rocks in workings, 

even crosscut tunnels, in this mine. This calcite has been shown elsewhere to be derived from 

altered feldspars by ordinary ground waters. 

DERIVATION OF KAOLINITE. 

Associated with these later carbonates, as well· as with the rich silver sulphides and other 

sulphides of the later generation, is kaolinite, a precipitated mineral whose derivation from· 

the wall rocks seems hardly questionable. This n1.ineral was observed in .this mine to· be an 

alteration product of the feldspar in the porphyry, together with sericite and calcite. Kaolinite 

is generally held to be insoluble,a yet in the porphyry it has been found separated from its 

associated decomposition products and in a pure, transparent crystalline form filling a fissure, 

being plainly the result of precipitation from solution, as it is where it occurs in vugs in the veins. 

Its relative insolubility, however, explains the small amount present in the veins as a chemical 

precipitate. 

DERIVATION OF BARITE. 

Barite, which is characteristically one of the later minerals and is in many places the very 

last to form, has been noted as contemporaneous with kaolinite. Barium is undoubtedly 

present in the feldspars of the wall rock, and some of it may pass into solution by circulating 

waters when these feldspars are decomposed. The common associate of barite, kaolinite, is 

frequently formed by the action of carbonic acid in waters, but it has been remarked by Lindgren b that it is formed abundantly in the upper zones of many ore deposits, where the action 

of stronger reagents, such as sulphuric acid, seems probable; and the senior writer has studied 

occurrences where this observation strikingly applies. Such sulphuric acid in the waters, 

derived from the oxidation of the sulphides and carried downwar<;l, would precipitate barite 

from solution in some other form, such as chloride, in other downward-moving currents. 

ThE' occurrence in this mine of separate barite crystals embedded in a seam of clay (see p. 259) 

calls for some such explanation. At the coal mines of Newcastle-on-Tyne barite is deposited 

from mine waters in boxes and pipes; these mine waters contain some barium chloride.c 

SUMMARY. 

It has been shown as probable that the gangue minerals are derived from the wall rocks. 

It has also been shown that a large part of the,.materials in the veins, both 1netallic mineral 

and gangue, are probably immediately due to precipitatiop from ordinary circulating ground 

watE<rs. The. original derivation of the metals, zinc, lead, silver, copper, arsenic, antimony, 

mercury, cadmium, gold, etc .. , and the source of the sulphur remain undetermined. 

a For solution of alumina, however, seep. lGl. bTrans. A-m. lnst. Min. Eng., vol. 30,1901, p. 614. 

c Dana, E. S., System of mineralogy, 6th ed., p. 903. 
264 GEOLOGY OF THE GEORGETOWN QUADHANGLE, COLORADO. 

DEPOSITION FROM MINE WATERS. 

At many places where surface waters enter crosscut tunnels in the rock, such as the Equator 

and the Marshall tunnels, they deposit iron oxide and calcite. On the walls of the Marshall 

tunnel a finely banded crust of pure calcite, ·one-third of an inch thick, was observed. This 

deposit resembled Mexican onyx in structure and was of a beautiful bluish-green color, owing 

s N 

b 

o~------'~0:___ ___ ____30 inches 

probably to the presence of 

some metallic mineral. A 

similar bu~ brown-banded deposit, over 0.4 inch thick, incrusts the pipe of a mine pump 

that has been used almost continuously for twelve or thirteen 

years. this deposit was determined by W. F. Hillebrand to 

be calcite with a little iron and 

a trace of manganese. 

TRANSVERSE POSTMINERAL 

FAULTS. 

FIG. 9G.-Vertical section showing minor postmineral faulting of sulphide veinlets in pegmatite, Intonnediate vein, 850-foot level, Colorado Central mine. a, Pegmatite; b, ore; 

c, faults. 

Most of the movement 

since the initial opening of the 

vein channels has been along the 

old slips, but occasional ruptures have taken place in a transverse direction, offsetting the veins. 

These movements are invariably small, nor are they in general abundant. On the .750-

foot level, in one locality, numerous slight faults, striking approximately north, were observed 

displacing a horizontal streak of galena (figs. 96, 97). These faults have, as a rule, only a few 

inches displacement. On the 850-foot level (Pl. XLIV) a slight transverse fault, striking westo~_..x_ _ __,w"-----"'30'-------'4"'-0 _ __,so inches 

a [TID 

Galena Faults 

FIG. 97.-Vertical section showing minor faulting of galena veinlet, 850-foot level, Colorado Central mine. Fault marked 

a is somewhat cemented by quart?-. · 

northwest, has a curious effect, shifting the porphyry and the veins about 12 feet to the northwest on the northeast side. This being the exact width of the porphyry, the North vein is 

here brought against the New North and in apparent continuation with it. At this point, 

however, the North vein is well mineralized and the New North practically barren, c,n~l the 

true continuation of the North vein was found by following up the fault, which was marked 

by a heavy sheeted zone. 
U. S. GEOLOGICAL SURVEY PROFESSIONAL PAPER NO. 63 PL. XLVI 

A . COLORADO CENTRAL AND KIRTLEY MINES, LOOKING SOUTH-SOUTHWEST UP THE 

VALLEY OF LEAVENWORTH CREEK FROM THE SLOPE OF ALPINE PEAK. 

1, Marshall tunn el, Col orado Central mine ; 2, Equ ator tu nnel, Colorado Central mine; 3• Kirtl ey mine; 

4• Aliund e tunnel and shaft, Colorado Central mine; 5• Colorado Central surface workin gs. 

B. WIDE WEST MINE, LOOKING NORTHWEST ACROSS THE VALLEY OF SOUTH CLEAR 

CREEK FROM THE SLOPE OF ALPINE MOUNTAIN. 
MINES NEAR GEORGETOWN. 265 

KIRTLEY GROUP OF VEINS. 

GENERAL DESCRIPTION. 

The Kirtley group of veins is located on the southeastern slope of Leavenworth Mountain, 

500 to 700 feet above the level of South Clear Creek (Pl. XLVI). The group consists of the 

Kirtley, Stranger, 0. K., Tilden, Argentine, Creole, and Gates veins. Of these the Kirtley is 

the main or master vein from which the other smaller veins branch in both a lateral and a vertical direction. Many of the branch veins successively subdivide until they finally die out. 

KIRTLEY VEIN. 

, The Kirtley vein was intersected by the Kirtley tunnel (fig. 98) about 650 feet from its 

mouth. It is opened up for a distance of over 1,200 feet on the tunnel level. The eastern 

Gneiss Pegmatite 

0 100 

Granite 

200 

Mineralized 

lead 

Unmineralized 

lead 

300 400 feet 

FIG. 98.-Geological plan of Kirtley tunnel workings. 

N 

part of this level, moreover, is connected with the workings on the Kirtley vein in the Argentine tunnel. Besides the development on the tunnel level, there are also about 250 feet of 

workings on the Kirtley vein, on the Marshall tunnel level of the Colorado Central mine, and 

100 feet of drifting on a poor lead in gneiss said to be the Kirtley vein near the breast of the 

Equator tunnel of the same mine. There are practically no workings on this vein at the surface. 

The general.strike of the vein underground is N. 45° E., but at the northeast end it swerves 

a little more eastward. The dip is practically vertical on the Kirtley tunnel level, but the 

vein rolls slightly so as to cause a gentle dip to the northwest in the upper workings and a dip 

in'the opposite direction in the lowest levels. 
266 GEOLOGY OF THE GEORGETOWN QUADHANGLE, COLORADO. 

The Kirtley has formed along a well-developed fault line, as shown by slickensides and by 

the strong gouge and crushed rock lead forming the portions of the vein where mineralization 

is wanting or is only slightly developed. Altho~gh the vein is strong to the southwest of the 

point where it is joined by the Stranger vein, it consists entirely of soft gouge and crushed rock, 

so far as shown by the open workings. 

The ore started abruptly at the junction of the Stranger vein and extended rather continuously along the Kirtley for more than 500 feet to the northeast, beyond which the· vein 

again changed to an unmineralized fault lead and gradually .became less pronounced. The 

richest a.p.d broadest part of the ore body was immediately northeast of this junction. 

In the Dunaway tunnel of the Argentine mine, located' about 800 feet' N. 28° E. of the 

Kirtley tunnel; the chief development has taken place on the Kirtley vein. Where encountered 

by the tunnel and for about 250 feet to the southwest the vein is chiefly a clay or gouge fault 

lead dipping 65° to 75° NW. and carrying no mineralization except a couple of small bodies of 

ore immediately at the junction of small branches with the main lead. Farther west the vein 

is irregular in trend owing to the fact that it follows a series of connecting but differently 

trending fractures. However, along this part of the vein zone considerable mineralization 

has taken place and the workings on the' Dunaway tunnel level are connected with the stopes 

of the Kirtley mine, which develop the east end of the long ore shoot resulting from the union 

of the Stranger and 0. K. veins with the Kirtley vem. 

STRANGER VEIN. 

The Stranger vein, the main branch of the Kirtley, also has a nearly vertical dip and cuts 

the schistosity of the gneiss wall rocks at approximately a right angle. The general trend of 

this vein isS. 85° W., but as the vein approaches the main vein it curves around to the northeast 

and runs almost parallel to the main vein just. before reaching the ju~ction. Ore extended 

along .the Stranger vein for more than 200 feet from the crossing, but the best pay ore started 

20 or 30 feet from the junction and extended westward for a short distance before it began to 

grow less in amount as the vein receded from the main vein. 

Another but minor branch of the main vein ran somewhat parallel to the Stranger vein 

and only 15 or 20 .feet to the southwest. This minor vein, although it was mineralized for a 

length of 200 or 300 feet, was not a strong lead. Its dip was about 76° N. 

0'. K. VEIN. 

The 0. K. vein was worked from the surface and produced considerable ore long before 

the Kirtley vein was discovered by the driving of the Kirtley tunnel. In 1872 about 20 men 

were employed on the 0. K. This vein, which from the mine maps is. seen to strike about 

N. 78° E., is a branch of the Kirtley vein and ranges in dip from 55° to 30° N. Where observed 

in the shaft just above the Kirtley tunnel, however, the vein had a· strike of N. 55° E. and a 

clip of only 30° to 35° from the horizontal. Whether the 0. K. vein makes simply a junction 

with the Kirtley vein, or whether it actually crosses this vein, is not well shown at present 

because the 0. K. becomes much more faint near the point of union, and also because all the 

. workings are not now open. Near the brea~t of the Kirtley tunnel is a strong lead in granite 

, and gneiss carrying no ore which strikes N. 60° W. and clips 35° NE. The vein where it nears 

the main vein curves around to the northeast and runs parallel to the Kirtley vein just before 

it joins it .. This vein was decided by a lawsuit to be the "upfaulted continuation of the 

0. K. vein," which is here supposed to cross the Kirtley. To judge from the dip of the two 

veins the junction should result in a northeastward-pitching trough. The ore on the 0. K. 

vei.n near the junction just above the Kirtley tunnel was not only 'less in quantity but also 

much leaner in values than the narrow but rich streak found near the surface. Possibly the 

underground waters circulating in the 0. K. vein, however, had a stimulating effect on ore 

deposition in the Kirtley and helped to cause the formation of the long ore· shoot on that vein. 
MINES NEAR GEORGETOWN. 267 

TILDEN VEIN. 

The Tilden vein, as shown by the mine maps, trends N. 87° W. and dips about 70° N. Jt 

lies intermediate between the 0. K. and Kirtley veins and, being somewhat parallel to and 

steeper than the 0. K., makes a junction with that vein. This junction is about 150 feet below 

the surface. The Tilden vein also joins the Kirtley vein to the west, but what effect the union 

has on ore "deposition is not shown in the present openings. 

ARGENTINE VEIN. 

The workings on the Argentine vein (fig. 99) were open only for a few feet at the time of 

visit, and so little could be determined about the relation of this vein to the other veins of the 

0 

Pegmati.te 

DUNAWAY TUNNEL 

100 

G!leiss 

200 feet 

Mineralized 

lead 

FIG. 99.-Geological plans of levels in Argentine mine. 

LOWER TUNNEL 

Unmineralized 

lead 

group. Where exposed near the mouth of the Argentine tunnel it had a dip of 75° SE. and a 

strike of S. 75° W. On the surface, however, from the point where a branch vein joins it from 

a S. 60° E. direction, about 100 feet north of the Creole tunnel, the Argentine vein runs S. 

52° W. · At the junction of this branch there was nearly 2 feet of coarse lead ore, and a inixture 

of galena and "honeycomb" . quartz occurs right at the surface. The Argentine vein, which 

is considered by some to be an extension of the Kirtley, may be an overlapping or imbricating 

vein,• although it is connected with the Kirtley by minor transverse fractures. 

CREOLE VEINS .. 

The two veins in the Creole tunnel, called the 11 foot-wall and hanging-wall leads of the 

Creole vein," are approximately parallel and are probably branches of the Argentine vein. 

The Creole tunnel, 150 feet in length, extends S. 78° W. along the south vein, which is a 

soft crushed granite, pegmatite, ·and gneiss lead except where a small body of ore was found 
26R GEOLOGY OJ!' THE GEORGETOWN QUADRANGLE, COLORADO. 

at the junction of a S. 80° E. branch. A 15-foot crosscut then connects the breast of the 

tunnel with a drift 35 feet in length along the north vein. Near the breast of this drift there 

is a junction with a slightly mineralized S. 43° W. vein which dips 70° NW. and may possibly 

be the Argentine vein. Flat transverse mineralized fractures were noted connecting the north 

and south Creole veins. 

GATES VEIN.· 

The Gates vein, which was actively worked in the early seventies and in 1874 employed 

about twenty-five men, has yielded considerable ore. It strikes N. 32° E. and dips about 55° 

NW., and is a strong fracture zone in gneiss and granite. In places it consists of foot and 

hanging leads 5 or 6 feet apart; with silicified granite and gn~iss or brown-stained quartz 

between and ore stringers interlacing all through the fractured zone. Both the walls and the 

rock fragments composing the fractured belt are coated with comb quartz from one-sixteenth 

to one-fourth of an inch thick, with the quartz crystals radiating into the spaces now occupied 

by a heavy zinc blende and galena ore carrying some "gray copper" (mainly polyb:asite and 

tetrahedrite). The galena is rather coarse textured. and much of it shows curved faces, probably due to some mechanical cause. The blonde is chiefly of a resinous nature and usually 

dark brown, though a little of it is greenish yellow. Small vugs lined with drusy quartz are 

present here and there and many of these are specked with minute tufts of a white mineral, 

probably calcite or some other carbonate. 

About 20 or 25 feet southeast of and parallel to the Gates vein is a strong lead made up 

of clay and decomposed gneiss and pegmatite, 1! feet wide, which has a nearly vertical dip 

and shows some mineralization, especially near its western workings. A cross flat vein dipping 

about 35° NW. runs parallel to this lead and connects it with the more steeply inclined Gates 

vein. The cross vein consists of a series of ore stringers in an inclined sheeted zone about 

4 feet wide. · 

OTHER VEINS OF THE GROUP. 

In the Kirtley tunnel, about 25 feet southeast of and parallel to the Gates lode, is the 

Saco vein, which on this level is small, but which, according to Raymond," produced about 

$55,000 of very rich ore in 1873, during the first five months after its discovery. A short 

~listance to the west of the tunnel the Saco vein makes a crossing with the Hidden Treasure 

vein, which trends S. 70° vV. until it joins the vein south of the Gates vein. The Hidden Treasure 

also was actively developed in 1873 and produced some ore. At the point of intersection with 

the Saco, both of the veins carry some qua:-tz and ore on the Kirtley tunnel level. 

Another strong vein carrying some rich ore near junctions with branch veins was encountered on the Equator tunnel level of the Colorado Central mine, about 150 feet from the breast. 

It trends N. 45° E. and dips 70° to 80° N. At the northeast end of the drift the vein is only 

75 to 100 feet from the Kirtley vein and trends slightly toward it as if to make a junction with it. 

However, this vein may not unite with the Kirtley but may be one of a set of imbricating or 

overlapping veins somewhat similar to the Colorado Central series of veins. 

The Branagan vein, which crosses the Dunaway tunnel about 215 feet from its mouth, is 

chiefly a gouge and crushed gneiss lead which is mineralized in places and has produced a little 

ore. It is a cross vein which strikes N. 10° W. and dips 70° E. 

COUNTRY ROCK. 

The country rock, for the most part, is gneiss associated with considerable pegmatite and 

granite. This gneiss in the southwestern portion of the workings on the Kirtley and Stranger 

lodes ranges from,a hard black rock resembling sheared diorite to a soft biotite gneiss with a 

strike of N. 15° to 25° E. and a dip of 70° to 80° vV. In places this gneiss forms an intimate 

mixture with pegmatite and granite. The eastern part of the vein on the Kirtley and the 

western part in the Dunaway tunnel levels consist of a hard granitic gneiss saturated with in-

? Raymond, R. W., Mineral resources west of the Rocky Mountains, 1873, p. 275. 
\ 

MINES NEAR GEORGETOWN. 269 

jccted pegmatite. Pegmatite, besides occurring intimately mixed with gneiss, is found in larger 

bodies just west of the junction of the Stranger and Kirtley veins, also about 150 feet to the 

northeast of this point and again on the 1iarshall tunnel level. Both biotite and muscovite 

pegmatite are present. The Kirtley crosscut tunnel is mainly in soft biotite gneiss with patches 

of pegmatite, and these same rocks occur along the eastern part of the Kirtley vein in the Dunaway tunnel of the Argentine mine . 

. Although the Kirtley and Argentine workings lie near the Colorado Central mine, in which 

porphyry is commonly associated with the veins, no porphyry whatever has been encountered 

in the Kirtley workings and only an isolated patch 2 or 3 feet in diameter of intensely altered 

rock which might be porphyry in the workings of the lower tmmel on the Argentine. 

STRUCTURE OF VEINS. 

The veins where unmineralized consist of fault fractures filled with crushed rock and clay 

gouge associated with some quartz and calcite. Although discordances in the wall rocks occur, 

the movements have caused only slight displacements and few well-marked slickensides were 

noted. Where ore occurs on the Kirtley vein the streak may consist of a single seam ranging 

·from 1 or 2 inches to several inches of high-grade ore, or to 2 feet or more where low-grade zinc 

ore comes in abundantly. However, in places the ore streak may widen and consist of several 

feet of fractured rock with stringers of ore running all through the mass. For example, the 

Kirtley vein near the intersection of the 0. K. contained nearly 3 feet of zinc ore which was of 

too low grade to pay foi.· extraction, but alongside of this low-grade belt on the south was a zone 

of fractured rock 6 or 8 feet wide which had seams of high-grade ore scattered throughout and 

paid well. 'In places the vein filling consists of a breccia of ore and rock recemented by quartz. 

On the Kirtley tunnel dump was found a specimen about 6 inches in width which represents 

part of the vein filling and shows a barite seam 1 ~ inches wide, traversing a hard, well-indurated 

mass which at first appearance seemed to be fine-grained granite or granitic gneiss, but on close 

·inspection proved to be finely crushed country rock containing small fragments of ore and 

crystals of barite cemented together by silica deposited from circulating waters. (Cf. p. 153.) 

Narrow tubular crystals Qf barite radiate out from the seam or barite into the crushed mass and 

appear to have been formed at a time when the crushed country rock was in a soft, pasty condition with the consistency of mud. On many of these barite crystals,is either a layer of comb 

quartz with the quartz crystals growing out from the barite or a thin coating of colorless quartz 

which gradually shades into the quartz matrix cementing the ·crushed rock. 

Possibly later barite is associated with this quartz in forming the cementing material to 

a less extent. Well-formed crystals of barite also occur in long, narrow vugs in the vein filling. 

The deposition of the barite as a whole seems to be subsequent to the ore deposition ( ef. p. 165), 

for angular fragments of galena and sphalerite are found embedded in the barite or the barite 

crystals loop about portions of the ore fragments. Particles of ore also occur in the crushed 

rock mass, which is cut by the seam of barite. Siderite and specularite, which are subsequent 

to the comb quartz coating the barite, are also sparingly present. 

The. veins in places show banding to some extent, clue to parallel sheeting and subsequent 

filling of the cracks with ore, both chalcedonic and comb quartz, calcite, and siderite. Much of 

the ore is probably due to a replacement of the gouge and crushed rock vein filling, though a 

considerable part is the result of impregnation of the wall rocks immediately adjacent to the 

fracture, especially in the vicinity of the vein junctions. 

COMPOSITION OF VRINS. 

Much of the ore from the main vein of the Kirtley group differs considerably in grade from 

the ore on the branches. Most of the best ore, however, consists mainly of galeha with some 

brownish to yellowish-green resin zinc blende and comparatively large amounts of polybasite 

and tetraheclrite. Copper-bearing pyrite is also present in very small quantities, and is usually 

associated with the polybasite and tetrahedrite ("gray copper") or forms a thin coating apparently 
270 GEOLOGY OF THE GEORGETOWN QUADRANGLE, COLORADO. 

of slightly earlier origin than these two minerals and may occur as thin films along conspicuous 

cleavage planes of the galena. Pyrite crystals are also associated with the chalcedonic quartz 

matrix cementing the crushed and altered rock fragments in the vein. Ruby silver, pyrargyrite, 

and some proustite are present here and there, but are rare in comparison with the amounts in 

the ores of the Colorado Central mine. Gold occurs as a mere trace. In one place a thin coating 

of a sectilc metallic dark-gray mineral resembling argentite (silver glance) was found along a 

crack in silicified cru'shed rock. 

Quartz is the chief gangue mineral, but barite is abundant and some calcite is also present. 

The quartz is either of the nature of comb quartz coating the walls, rock fragments, and the 

inside of vugs, or is a chalcedonic quartz cementing the crushed vein materials. Calcite., besides 

being intermixed with the other gangue minerals, also occurs with other carbonates as clusters 

of little white specks coating the ores in vugs. 

The silver values lie chiefly in the argentiferous sulphides (polybasite, tetrahedrite, pyrargyrite, and proustite) which occur both massive and as well-formed crystals lining vugs and which 

usually appear to be of later origin than the galena and sphalerite. The richness of the ore 

depends mainly on the quantity of these silver minerals disseminated through it, for the galena 

and sphalerite are as a rule lean in silver valm.;s. The low-grade orcs usually have a high content 

of zinc blimde, although they may consist of a mixture of coarse galen.a and blendc carrying 

little silver. Fine-grained galena or galena having a fibrous texture carries higher values. 

The ores of this mine when sorted usually fall into three classes, as follows: First class, 500 

to 600 ounces of silver; second class, 300 to 400 ounces of silver; third class, 100 to 200 ounces 

or less of silver. 

The Kirtley vein itself had the largest bodies of ore and ore of all grades. The Stranger 

vein produced relatively high-grade ore, and although it was a coarse galena, sphalerite, and 

barite mixture it belonged almost entirely to the first class. The parallel vein about 20 feet 

southwest of the Stranger yielded ore belonging to the lower grades. 

In the 0. K. and Tilden veins the ore near the surface, although it occurred in a narrow. 

streak, contained large amounts of the soft brownish oxidized ore and also of the rich powdery 

black sulphides that are associated in many places with the oxidized ores, termed locally 

"sulphurets of silver." On account of these highly argentiferous compounds, the ores at and 

just below the surface were of much higher grade than those taken from lower depths and 

are said to have carried a high percentage of lead and from 100 to 1,200 ounces of silver. According to Fossett, the Tilden in 1877 produced richer average ore than any other mine in the 

district. Just above the Kirtley tunnel level, however, the ore streak on the 0. K. vein consisted of 4 inches of galena and blende carrying but small amounts of silver. 

On the Argentine and Creole veins the ore, although averaging less than 100 ounces of 

silver, carries a high percentage of lead. Considerable zinc is associated with the lead and 

about 0.1 ounce of gold is also present. Cupriferous pyrite is abundant along cracks. Calcite 

veinlets and also incrustations on faces of fractures, both in the vein filling and in the country 

rock, are also conspicuous. These calcite deposits, which are normally white, have in places a 

greenish tinge due to the presence of copper. (Of. pp. 259-263.) 

LOCATION OF ORE. 

In the Kirtley group of veins practically every vein junction seems to have had a beneficial 

influence on ore deposition. As a rule the ore is located immediately at the junction and· 

along the united vein beyond the junction, but may extend along one or both of the uniting 

veins. For example, where .the Stranger vein un.ites with the Kirtley, the ore started immediately at the junction and extended northeastward for about 500 to 600 feet, and was stoped 

almost continuously for nearly the entire distance. However, the uniop. of the 0. K. and several 

small veinlets probably also assisted materially in causing the long stope. This ore shoot 

rrea.r the junction of the Stranger vein went up about 150 feet above the Kirtley tunnel but did 

not reach the surface. To the east the ore did not go up so high. The ore was also followed 

downward to a distance of about 90 feet below 'the Kirtley tunnel level. No shoot ort the 
MINES NEAR GEORGETOWN. 271 

Kirtley vein itself was .ever worked to the surface. On the Stranger vein ore extended from 

the junction with the main vein for a distance of more than 200 feet (fig. 100). However, 

the most and best ore was found 20 or 30 feet west of the intersection of a branch veinlet which 

crossed the Stranger vein about 25 feet west of the Kirtley vein. This branch veinlet connects 

the Stranger vein with the vein located about 20 feet to the south which runs parallel to the 

Stranger vein, and, although not an extra strong vein, is mineralized for a distance between 

100 and 200 feet. 

Both the 0. K. and Tilden veins were mineralized from the surface to a point so.me distance 

·below their junction. The amount of mineralization was greater near and below the line of 

union, but owing to oxidation and enrichment due to the action of surface waters the ore near 

the surface was of higher grade and paid better for 

mining. 

On the Dunaway tunnel level a couple of small 

shoots occurred on the Kirtley vein to the east 

of the long ore body, and each of these illustrated 

well the effect of uniting veins, for these isolated 

shoots extended for only 10 or 15 feet along the 

main vein beyond the union of branches, but followed the line of junction up and clown for considerable distances. In the Creole and Argentine 

tUiinels where ore was present it was also in the 

vicinity of vein junctions. 

PRODUCTION. 

Although the Kirtley vein was not intersected 

until October, 1877, it began to produce immediately, and up to April, 1880, the Kirtley mine had 

produced about $250,000.a Of this amount, about 

$200,000, according to :Mr. Weaver,b of Georgetown, 

the present owner, was taken from the Kirtley tunnel level or just below this level. It is also said 

that a large amount of ore was taken from the stope 

running east~ard from the Marshall tunnel of the 

Colorado Central mine. 

A good many thousand dollars were obtained 

from the 0. K. and Tilden veins, especially near 

the surface, for the ore in depth was not as rich as 

I 

FIG. 100.-Plan of Kirtley vein as shown in Kirtley tunnel. 

The formation of ores at the junction of branches is here 

illustrated. 

that higher up. The 0. K. vein yielded several small lots of very rich ore as early as 1871, and 

in 1877 the small amounts of ore taken from the Tilden vein were the richest in the camp. 

The Kirtley vein in the Dunaway and Argentine tunnels also produced considerable rich 

ore. The Argentine vein was fairly productive, but the ore. did not carry such high values. 

The Creole vein has produced a little low-grade ore. 

WIDE WEST LODE. 

LOCATION AND HISTORY. 

The Wide West lode, which is located mainly on the cast slope of Leavenworth Mountain, 

has been tracedfrom a point just above the large reservoir in the valley of South Clear Creek 

westward up the slope and over the crest of the mountain ridge separating the two forks of 

Clear Creek. 

This mine (Pl. XLVII), which had previously been worked to some extent, attracted very 

little attention until the spring of 1881, wheri a large body of very rich ore was encountered. 

a Fossett, Frank, Colorado, 1880, p. 402. b Verbal statemen~. 
272 GEOLOGY OF THE GEORGETOWN QUADRANGLE, COLORADO. 

Since that time the production, which has varied greatly from year to year, has aggregated an 

amount that is in the hundreds of thousands. Active work has recently been resumed owing 

to the discovery of another body of ore. 

DEVELOPMENT. 

More than 8,500 feet of drifts and crosscuts have been driven on the Wide West property, 

but many hundreds of feet of drifts run during the early history of the mine have since been 

allowed to cave shut. Most of the development has been carried on through tunnels on the 

vein and on shafts or raises connecting these tunnels. 

NATURE OF WALL ROCKS. 

The wall rocks for the most part consist of alternating patches of pegmatite and granitic 

gneiss (Pl. XLVIII). Granite is also common, especially near the east end of the lowest tunnel 

level, and a small amount of soft biotite gneiss is present in places. No porphyry is known to 

occur on the property. Owing to the small extent of the areas of the different kinds of rock 

the effect of each kind on the vein was not pronounced. The veins usually cut the gneiss nearly 

at right angles to the schistosity, which was in general slightly east of north. 

NATURE OF LODE AND LOCATION OF ORES. 

This lode has a general strike of S. 82° W. and a dip of 60° to 75° N. The lowest level 

of the.mine shows the Wide West lode made up of two veins only 10 to 20 feet apart, running 

parallel to each other for more than 1,000 feet. Of these two the south vein has a dip slightly 

flatter than the north vein, and is also a much narrower and less pronounced lead, owing to 

the fact that the movement was insufficient to form a continuous single fault, but resulted in 

a string of connecting minor fractures. The small ore streaks found in places along this southern 

string of fractures, though only 1 to 5 inches wide, usually contained much richer ore than the 

broader streaks along the north vein. Extensive movements along this north vein are indicated 

by slickensides as well as by the 6 inqhes to 3 feet of soft gouge and pulverized rock which occur 

continuously along it except where small bodies of quartz ore have occupied their place. However, to what extent the movements caused displacement is unknown, although that there is 

some dislocation is shown by the discordance of the wall rocks on either side. 

The main lode on level No. 4 is followed westward by the tun~el for about 600 feet to 

the point where it splits up into three veins that diverge from one another going westward. 

The "south" and "middle" branches are evidently formed by the splitting of the south 

vein of the Wiele West lode." Each of these three veins carried valuable deposits of ore 

not only on this level but also on the level above and on level No. 5 below. The largest 

and richest body of ore in these veins occurred along the "south branch," and according 

to Mr. Charles Pilz the stope resulting from the extraction of this ore was 200 feet long and had 

a vertical extent of about 300 feet, pitching slightly to the east. The ore body on the "north 

branch" was, moreover, "larger and richer than that on the "middle branch." 

The "south" and "middle" branches near their junction were soft gouge and crushed rock 

leads 2 to 5 feet wide, showing only the slightest evidence of mineralization. However, both 

the main vein and the two uniting branches carried ore bodies, but they were formed at a 

distance of about 75 feet from the junction in each vein. Those in the branches were the 

more extensive deposits. The point of junction of the "north branch" with the ''middle 

branch" or with the main vein is ill defined, but must exist if the north vein continues its general 

trend. The obscureness of the junction is possibly due either to the fact that the point where 

these two _veins unite was not mineralized and is therefore not easily detected, or to the fact 

that the north vein of the main lode may continue as the "north branch." 

The ore shoots on the three veins aresaid to have been cut off on the west _by a cross fault 

or soft lode composed of crushed and badly stained and altered rock mixed with soft clay and 

scattered angular pieces of ore. This part of the mine, however, was not accessible owing to 

caving. 
U S. GEOLOGICAL SURVEY PROFESSIONAL PAPER NO. 63 PL. XLVII 

N 

0 100 200 300 feet 

HORIZONTAL PLAN OF WORKINGS OF WIDE WEST MINE. 
U. S. GEOLOGICAL SURVEY 

0'-' -----1:..J9c.: 0 ____ _;_2)'9:':_0 ____ _.:'3?0 feet 

c::J ~ 

Mineralized lead 

I·· :t:-:: ...... 1 

Unmineralized lead 

GEOLOGICAL PLANS OF SEPARATE LEVELS, WIDE WEST MINE. 

PROFESSIONAL PAPER NO. 63 PL. XLVIII 
lVIINES NEAR GEORGETO\VN. 273 

A noticeable feature of the deposits on the three branch veins was the location of the 

larger ore masses on "flats" or places in the vein where the dip was much less .. The regular 

dip of the vein-ranges from 65° to 78°, but in places where the ore bodies were broadest and 

richest clips as low as 45° were obtained. This flattening .of the clip is especially noticeable on 

the "south" and "middle" branches on level No. 5. 

The Bambrick vein is parallel to the Wiele West main vein and about 200 feet farther west. 

STRUCTURE OF LODE. 

The north vein in the lower tunnel levels' consists of a single broad fracture filled as a rule 

with soft clay or gouge and crushed gneiss, pegmatite, and granite, which gives way in places 

to a gray quartz that in turn becomes ore bearing in spots. 'rhe string of minor fractures 

parallel to this vein, representing the south vein, in general run through comparatively hard 

rocks, and the orE;) where it occurs consists of narrow but rich stringers or seams, most of them 

tightly "frozen" to the "~all rocks. 

The three branches of the main lode at the west where mineralized consist either of parallel 

foot and hanging walls with a network of quartz and ore stringers ramifying between two 

fracture planes 2 to 12 feet apart, or else o~ a single well-defined wall from the crack along 

which the mineralizing solutions penetrated the adjacent wall rocks for varying distances and 

impregnated them with a greater or less quantity of metallic sulphides. 

Specimens of vein filling from the openings which are located southwest of the crest of 

the ridge of Leavenworth Mountain at first sight appear to be angular fragments of galena 

and blende ore included in porphyry. (Cf. p. 136.) On close examination, however, the 

matrix inclosing the ore fragments proves to be finely crushed granitic gneiss, granite, or pegmatite, the particles of which are compactly cemented by secondary silica. The na~;_;:~ cf 

the vein filling indicates that movement subsequent to the mineralization has resulted in a 

fissure filled with rubble comprised mainly of ore, that circulating waters have brought in the 

finely crushed rock matrix in a pasty condition and filled the interspaces, and finally that the 

underground waters have brought in the quartz which formed the hard siliceous cement. 

COMPOSITION 01•' LODE. 

Quartz, usually gray in color, is" the chief gangue mineral. Barite is found sparingly 

and extends to depths as great as 500 feet below the surface, in general as well-formed crystals 

lining vugs. Siderite and possibly also fer11uginous calcite arc common near the surface outcrops, bttt were not encountered in the lower levels. 

The metallic minerals are galena, blende, polybasite, tctrahcdrite, and pyrit;e, the last 

named generally copper bearing. The blendc is usually the yellow, light-brown, or green 

"resin blende." 

The ores are as a rule very rich in silver and there are several records of mill runs which 

ranged all the way from 1,000 to 2,000 "ounces of silver to the ton. A recent mill run on ore 

from a minor stope showed 236 ounces of silver, 8 per cent of lead, and 4 per cent of iinc per 

ton. Ore containing fine-grained massive galena commonly carries high values in silver. 

CHANGES OF ORE IX DEPTH. 

In some of the surface cuts the walls of 'the vein show numerous blotches or specks of the 

copper carbonates, a.zurite and malachite, chiefly the former. These carbonates, which have 

the nature of stains or thin films deposited on the hard rock walls from solution, were probably 

derived from the alteration of the cupriferous pyrite, and had apparently been transported 

for some distance in solution, as casts of dissolved sulphides were entirely lacking in the quartzose vein ftlling. Blue copper-carbonate stains were also noted in one stope that was more 

than 300 feet below the surface. A honeycombed porous gossan was found at several points 

where the vein outcropped. 

31401-No. ~3-08-18 
274 G.EOLOGY OF THE GEORGETOWN QUADRANGLE, COLORADO. 

For short distances below the surface siderite aud ferruginous calcite are common, both as 

massive stringers and as well-formed rhombic crystals lining small irregular vugs. The ores, 

which are locally called "sulphurets '' and which consist usually of soft brown and black oxidized 

ores and somewhat similar appearing mixtures of these oxides with sulphide ores of secondary 

origin, are said to have been common near the surface. 

The richest and largest bodies of ore were found between the third and fifth levels. It 

was from this locality that assays running several thousand ounces of silver to the ton were 

obtained from picked specimens. Good ore, however, running 200 to 400 ounces to the ton, 

has been taken from depths of more than 600 feet below the surface. 

CENTRAL COLORADO LODE. 

LOCATION AND DEVELOPMENT. 

The Central Colorado vein, which _is located on the northwest slope of Leavenworth 

Mountain, is developed by a series of tunnels. The main development being carried on at 

N 

1 

~ 

~ 

Gneiss 

wall rock 

o 100 200 feet L-~----~------~ 

·Granite 

~ 

~ 

wall rock 

Pegmatite 

~ 

~ 

wall rock 

~ EJ 

Mineralized Unmineralized 

lead lead 

FIG. 101.-Geological plan of tunnel workings, Central Colorado vein. 

present is through a tunnel about 450 feet in 

length, which enters the hillside at an elevation 

of. approximately 9,000 feet and intersects the 

vein about 400 ·feet from its mouth. About 225 

feet of drifting has been done along the vein 

and some stoping has been begun. 

NATURE OF WALLS. 

The country rock through which the tunnel 

(fig. 101) is driven is chiefly pegmatite with small 

patches of gneiss, usually of a hard granitic 

variety,·inclosed within it. The walls of the 

vein, however, are mainly hard and comparatively fresh biotite granite. No porphyry was 

observed in the workings. 

NATURE OF VEIN. 

The vein usually consists either of a single 

narrow fracture or of a series of small or tight 

fractures traversing hard granite in aN. 45° E. 

direction. Very little movement has occurred 

along the vein, as gouge, slickensides, and other 

evidences of faulting and diffm;ential movement are almost entirely lacking. Gouge as 

thin selvage one-eighth to one-fourth inch thick 

occurs locally between the ore and the wall rocks, but as a rule the ore is "frozen" to the walls. 

In the vicinity of the small stope from which ore was being extracted the vein had the nature 

of a stringer lo'de, and consisted of a series of.small mineralized veinlets running parallel to the 

strike of the vein and in places connected by a ~etwork of still finer veinlets. 

ORE. 

I 

The ore is mainly gold bearing and consists chiefly of pyrite, which is as a rule light in 

color, but in places carries so much copper that it verges on chalcopyrit~. Thin scams or coats 

of zinc blende occur here and there between the copper-bearing pyrite and the wall rock, as if 

the zinc sulphide were of earlier origin than the cupriferous pyrite. The ore, which carries from 

a fraction of an ounce to 2 or 3 ounces of gold and practically no silver, occurs either as fairly 

well formed crystals of pyrite lining vugs or as massive cupriferous pyrite which adheres 

., 
U. S. GEOLOGICAL SURVEY PROFESSIONAL PAPER NO. 63 PL. XLIX 

~------~'~oo ________ ~ooo~-------=~L--------·~oor~t 

Stopes shown by shaded areas 

soo -foot Jevel 

Tunnel le vel 

VERTICAL AND HORIZONTAL PLANS OF CENTENNIAL MINE WORKINGS. 
MTNBS NEAR GEORGETOWN. 275 

tightly to the comparatively fresh wall rocks or is separated fron1 these rocks b;v a thin selvage. 

In places where the ore adheres tightly to the walls the boundaries are rather irregular, owing 

to the replacement of more or less of the country rock adjoining the fracture. The chief gangue 

~neral is quartz, which in places shows comb structure. Small amounts of siderite and 

ferruginous rhodochrosite, as well as of barite, arc also present. 

WELCH LODE. 

·The Welch vein was actively developed in the early seventies and produced several shipments of very rich ore, hut serious difficulty was encountered in exploration work owing to 

the discontinuous nature of the vein, due either to faulting or to irregularities in the vein fracture 

itself. At the time of visit no work had been done for y~ars and practically all the workings 

were caved shut .except a short tunnel and shaft located on the upper part of the west slope 

of the north spur of Leavenworth Mountain. 

The country rock consists 

chiefly of soft .micaceous gneiss 

with patches of pegmatite and 

granite. The strike of the 

gneiss is slightly to the west of 

north. In the tunnel (:fig.l02) 

above mentioned the vein, 

which on the surface crosses 

the crest of Leavenworth 

Mountain ridge with a strike 

of S. 85° E., has a curved out-

~ 

~ Granite t3J ~~~ EJ B 

Pegmatite Gneiss Mineralized lead Unminerabzed lead 

·FIG. 102.-Geological plan of tunnel on \Velch lode. 

line with the concave part of the curve facing north and the strike ranging from S. 62° E. to 

N. 70° E. It dips northward from 50° to 55° and consists of a clay and crushed rock lead 

which at certain points is replaced to some extent by quartz or by quartz and ore. Small stopes 

in the parts of the workings still open indicate that considerable ore was probably extracted 

from the mine. 

CENTENNIAL CROUP OF VEINS. 

GENERAL DESCRIPTION. 

The Centennial lode and its branches outcrop on the southern edge of the town of Georgetown and· pass up over the northwestern shoU'lder of Leavenworth Mountain. The main 

or trunk vein, the Centennial, with a general strike of N. 40° E., passes on the northwest under 

the drift which here floors Clear Creek valley. Near the southwest end of the developed portion 

it gives off a number of branches, which strike S. 60° to 70° ·vv. These branches are. parallel 

and relatively close together, forming a zone a. few hundred yards wide. The best known 

is the Burrel, on which, not far from its point of junction with the main Centennial vPin, there 

is a.shaft. Lying south of and parallel to the Burrel vein is the Big Indian vein, which appears 

to be a branch of the Burrel and to unite with it at the Burrel shaft. Another lode, lying not 

far north of the Burrel and parallel to it, joins the Centennial independently of the Burrel. 

The junctions either have not been encountered or are obscure in the aceessible underground 

workings of the Centennial mine. All these veins are vertical or nearly so. 

Taken all together, these lodes have been developed for a linear distanee of more than half 

a mile. The main Centennial lode has been developed underground along a strike extent 

of upward of 1,400 feet. 

The Centennial mine (Pl. XLIX) is reported to have produced ore to the value of $300,000. 

The Burrel is reported to have produced about $17,000, takmi out mainly around the shaft, 

which is probably at the junction with the Big Indian vein and is not far from the junetion 

of the Burrel with the Centennial. 
276 GEOLOGY OF THE GEORGETOWN QUADHANGLE, COLORADO. 

The Centennial vein is opened up by a shaft more than 600 feet deep, from which run' 

various levels, as well as by some tunnels above the shaft. The Burrel vein was worked 

principally through a shaft. The Big Indian has a tunnel drift about 300 feet long on the vein. 

ORIGIN Ole VEIN. 

Like the other veins of this region, the veins of the Centennial group have formed along a 

zone of fracturing and minor faulting. On the northeast end of the sixth level drift, strire dip 

to the southwest at an angle of 32° from the horizontal, indicating the direction of differential 

movement. This fracture zone has been cemented by the vein materials. Little evidenee 

of fissure filling appears. There is, however, much reason to suppose that the fracture zone 

may have been first decomposed and leached by waters, thus causing it to shrink and become 

porous, before mineralization. Where not mineralized the vein zone is still soft but where 

it is mineralized it is cemented hard with quartz and sulphides. 

STRUCTURE OF VEIN. 

As seen on the 500-foot and 600-foot levels (Pl. L), the Centennial is a strong vein that 

branches and weakens at both ends. In the northeastern developed portion the branches 

open out to the riortheast; in the southeastern portion to the southeast. The Centennial is 

opened up on these levels for about 1,300 feet. On the northeast end the branching and weakening of the vein takes place under the drift of Clear Creek, but on the southwest several of 

the branches, as above described, are visible on the f?Urface and have been separately located 

and worked. 

The plan of the fifth level (Pl. L) shows, near the southwest end of this level, a branching 

arid reuniting of the vein, the two branches inclosing a horse of country rock. Similar features 

have been observed elsewhere in the district, as in the Dunderberg and Brown mines (figs. 84 

and 81). 

NATURE OF WAl~L ROOKS. 

The country rock of the Centennial vein is chiefly black biotitie gneiss, generally massive, 

with some pegmatite. There is also a little porphyritic granite. The same gneiss also forms 

the principal \vall roek of the Burrel vein and the other branches of the system which are parallel 

to it. The southwestern extensions of these branehes, however, run into a mass of porphyritic 

granite. The wall rock of the Big Chief tunnel is entirely granite. 

COMPOSITION OE VEINS. 

The chief gangue of the Centennial vein is quartz, which is relatively abundant. Here 

and there the vein is made up mainly of quartz and carries very little sulphides. The principal 

metallic minerals are pyrite, cupriferous pyrite, chaleopyrite, and tetrahedritc. A little galena 

and blende occur, but not in sufficient quantities to yield more than traces of lead and zinc 

in large shipments of ore. The values in the ore consist of gold, silver, and copper, the silver 

· content in the ore produced ranging from 20 to 30 ounces, the gold from 1 to 1! ounces, and 

the copper from 4 to 8 per cent. The average value of the ore taken out between the 300-foot 

and 500-foot levels is stated at· $18.60, of whieh about three-fourths was derived from the 

gold. The presence of gold in the ore seems to be dependent on that of eopper. It is reported 

that where copper pyrite is present in the ore gold occurs in it and in the iron pyrite, but that 

where there is no copper in the pyrites they are poor in gold or barren. Some small portions 

of the ore run very high in gold, one assay of 522 ounces being reported from the 500-foot 

level. On the 600-foot level a small patch of tetrahcdrite is said to carry $3 to the ton in platinum 

and iridium. 

Occasionally druses lined with siderite arc found. The Big Chief lode, which is probably 

a branch of the Centennial, shows different ore from that of the main vein. Here blendc and 

galena arc prominent, and chalcopyrite and pyrite arc also present. Silver, gold, ancl copper are 
PROFESSIONAL PAPER NO. 63 PL. L 

Gn eiss wall rock 

Mine ralized lead 

500-FOOT LEVEL 

Unmineralized lead 

Dip of "Vein 

Pitch of s t. nae on walls 

0 100 

GEOLOGICAL PLANS 0 F SEPARATE LEVELS, CENTENNIAL MINE. 
MINES NEAR GEORGETOWN. 277 

present in the ore, but in such small quantity that it can not be mined at a profit. The highest 

grade ore is reported as worth only about $10 to the ton. Fluorite occurs in this ore, being later 

than the mineralization and coating cavities in the old sulphides. 

DISTRIBUTION OF ORE. 

On the Centennial vein a definite ore shoot extends from the surfaee down below the fifth 

level. The stope map indieates that the e{'tent of tlus ore shoot along the vein inereases continuously to the bottom of the shoot. This downward termination of the ore comes between 

the fifth and sixth levels. Below this, on the 600-foot level, the vein is strong, but with nmch 

less values in silver, copper, and gold, the average value of the ore being from $6 to $10. On 

the 500-foot level one of the prineipal portions of the ore shoot seems to be due to the junction 

of two strong branches, converging to the southwest. At the junction the ore in plaees was 

from 10 to 14 feet wide. The branch vein is only slightly mineralized, but the other vein is 

4 to 5 feet thick, increasing in thickness toward the intersection. It is reported that ore forms 

at the same intersection in all the levels from the fifth to the surface. On the sixth level, at 

the shaft, the san1e junction of branches is seen, but it carries no ore. 

As a rule the ore produced does not contain any appreciable quantity· of lead. In one 

locality, however, on the 500-foot level, where the main vein branches, about 4.50 feet northeast 

of the sl~aft, the southeast branch carries galena in considerable quantityJ one mill run yielding 

n per cent of lead. The other branch shows the usual character. This variation of the mineralogical composition of the vein in different parts and also in different branches emphasizes 

the fact that rock openings of the same age and adjacent to one another have been penetrated 

by differing solutions.· The occurrence of ore at the intersection of branches noted in this 

group of veins and elsewhere in the district is probably due.to the precipitation resulting from 

the mingling of such solutions, flowing along distinct channels. . 

Although the main ore sho~t has a definite and somewhat abrupt downward termination 

midway between the 500-foot and 600-foot levels, at a vertical depth of about 550 feet, yet in 

the extreme southwest end of the .500-foot level, at the time of the wri\ers' visit, another separate body of ore was being opened up which was reported to be as good as any other that ever 

came out of the mine. This ore contained copper pyrite and tetrahedrite, like the ore of the 

main shoot. On account of the depth gained in this southwest drift by passing under the 

North vein of Leavenworth Mountain, this ore lies at a greater vertical distance from the surface than certain portions of the barren 600-foot level. · 

PAUAGENESIS OF ORE~ 

The gold content of the ores, as above stated, appears to depend on the presence of copper. 

The silver also appears to be chiefly associated with copper, probably ·in the form of tetrahedrite. The very rich assays of gold ore above mentioned are described as occurring in ore 

resembling tetrahedrite. Silver does 'not seem to be associated usually with the lead, for the 

galena ore described above as occurring in a branch vein on the 500-foot level does not contain 

more silver than the ordinary amount. 

The tetrahedrite as a rule i.s later than the chalcopyrite and is secondary to it, as is shown 

. by its occurrence as definite streaks along fractures in the chalcopyrite. It is also later than 

the pyrite. No evidence of the relative age of the pyrite and chalcopyrite was noted. The 

abrupt diminution in the qt<antity of both chalcopyrite and tetrahedrite between the fifth and 

sixth levels, however, with the incoming of large bodies of low-grade granitic ore on the sixth 

level, suggests that both chalcopyrite and tetrahedrite are later than the barren or relatively 

low-grade cupriferous original pyrite. 

CHANGES 0~' ORE IN DEPTH. 

Apart from the abrupt falling off in the principal ore shoot of the values in copper, silver, 

and gold, between the fifth and sixth levels, no marked variations in depth were noted. It is 

reported that in the main shoot tho gracle of orP was nearly the same at the surface as below, 
278 GEOLOGY OF THE GEORGETOWN QUADRANGLE, COLORADO. 

although probably the best and most abundant ore occurred near the bottom of the shoot, at 

the 500-foot level. 

Partial oxidation of the ore is reported to extend down 10 to 20 feet or more, but sulphides, 

chiefly pyrite, extend loeally quite up to the surface. Typically there is about 10 feet or more 

of glacial drift covering the outcrop. 

PRESE~T WOUK OF UNDERGlWUND WATERS: 

At present water is found dripping in all parts of the workings. This water contains sulphates of copper, as is shown by various occurrences. According to J. S. Randall, of Georgetown, crystallized hydrous cupric sulphate, chalcanthite, is found in the old workings of the 

Centennial mine. David Kennedy, owner and superintendent of the mine, reports that car 

rails in the mine get coated with metallic copper precipitated from the waters. The rust scale 

on the inside of a water pipe in the mine, assayed by .Mr. Kennedy, yielded 11 per cent of copper. 

These facts sh<iw that the copper sulphides have been transformed into soluble sulphate by 

oxidation. Recognition of this fact indieates that a considerable enrichment in copper of 

some of the sulphides has been brought about by such waters as these, and the downward 

termination of the richer copper orcs between the 500-foot and 600-foot levels suggests that this 

is the downward limit of the enriched ores, below which are the unaltered or only slightly altered 

pyritiferous ores. This suggestion, however, should be taken with some reservation. Although 

the influence of descending waters has probably been considerable in enriching the ore in the 

ore shoot, yet the irregularity of these ore bodies as shown in all the veins of the district is such 

as not to exclude the possibility that the ore shoot may have been in part original and formed 

previous to the enrichment. 

MINES OF GRIFFITH AND SAXON MOUNTAINS. 

COMET-JETNA LODE. 

The workings of the Comet and ABtna mines, which lie high up on the northwest slope of 

Griffith Mountain (Pl. LI, B), due east of the town of Georgetown, were undoubtedly driven in 

order to develop the same lode. At the time of visit, however, none of the ~vorkings of the 

ABtna were aceessible owing to caving. 

COMET MINE. 

LOCATION, DEVELOPMENT, AND PRODUCTION. 

The Comet vein, which was worked extensively even previous to 1871, is developed by 

three shafts located at an elevation of nearly 11,000 feet above sea level, at the point where the 

trail from Georgetown to Highland Park reaches the crest of the slope, and also by a tunnel on 

the west side of Griffith Mountain about 450 feet lower down. This lower tunnel follows the 

J. A. Hawkes vein (a crushed rock and clay lead dipping .55° to 65° NW. and trending N. 52° E.) 

for about 500 feet to the point where it connects with a crosscut drift driven S. 65° E. to 

its intersection with the Comet lode, which lies parallel to the J. A. Hawkes vei.a and about 600 

feet distant. 

Not even a rough estimate of the production from the Comet vein was obtainable, but 

undoubtedly a great deal of ore was taken from the old shafts near the crest of the slope in the 

early clays of mining about Georgetown. It is said that $60,000 was taken from a shoot of ore 

that was found a short distance above a level run from one of the shafts at a point 275 feet 

above the tunnel level. NA'l'URE OF WAT,L ROCKS. 

The wall rocks consist of much altered pegmatite, granite,· and gneiss, the last both granitic 

and micaceous. In the tunnel level the walls of the J .. A. Hawkes vein consist chiefly of fairly 

hard altered granitic gneiss, with irregular patches of pegmatite interspersed throughout. 

Where distinguishable the strike of the gneiss is about N. 80° W. and the clip 45° N. In the crosscut to the Comet lode (Pl. LII) the first third of the distance was also through gneiRs and pegmatite, 
U. S. GEOLOGICAL SURVEY PROFESSIONAL PAPER NO. 63 PL. Ll 

A. VIEW OF GEORGETOWN, LOOKING NORTHEAST FROM CEMETERY HILL, NEAR 

SILVER PLUME. 

Lebanon group of mines in left foreground; Griffith Mountain and Saxon Mountain mines in backg round . 1, Leba non tunn el; 2, H all tunnel; 3· Georgetown loop; 4• Georgetown; 5· Ann etteGriffith tunn el; 6, Capitol mine; 7, ktna mine; 8, Comet tunnel; 9, Com et shafts; 10, Ruler 

tunn el; ''• Magnet mine; 1 2, Woodley mine; ' 3· Summit mine; '4• Iri s tunn el ; '5• Lebano n group of mines. 

B. GRIFFITH AND SAXON MOUNTAINS AND MINES EAST OF GEORGETOWN, FROM 

MOUTH OF CLEAR CREEK CANYON. 

, , Annette-Griffith tunnel; 2 , Capitol mine; 3, Comet tunnel; 4• Saxon Mountain. 
MINES NEAR GEORGETOWN. 279 

but in places these rocks a·re soft, from alteration, and are cut by small dikes of the variety of 

porphyritic granite locally termed "corn rock." · The southeast portion of the crosscut is composed chiefly of granite cut by narrow inasses of pegmatite and "corn rock." In the vicinity of 

the Comet lode itself arid for a distance of 100 to 200 feet to the northwest of it the country 

rock is mostly a mixture of pegmatite and gneiss with small amounts of granite and "corn rock." 

In this belt, however, the alteration has been so intense that in many places it is difficult to 

distinguish one rock from the other. 

Two veinlets of gray jasper-like or ehalcedonic quartz intersect the crosscut from the J. A. 

Hawkes vein and run parallel to the Comet vein. This jasper-like quartz resembles porphyry 

in both texture and color, and also because of the presence of scattered small whitish rectangular 

specks that resemble altered feldspar phenocrysts embedded in a fine-grained groundmass. 

Under the microscope, however, a specimen of this rock exhibits a microgranular texture and 

appears to be composed chiefly of chalcedony, although a few small streaks cutting through the 

finer-grained matrix are composed essentially of minute angular fragments of quartz crystals, 

sericite and talc, and rarely shreds of nmscovite and leached biotite. A couple of minute grains 

. of zircon were also noted in one slide. These quartz veinlets contained no ore, although the 

microscope showed a little pyrite and magnetite. Tllis rock is probably a fmely crushed and 

highly silicified gneiss. No undoubted porphyry was found in the workings. 

Small stringers of soii1_ewhat similar hard clark-gray jasper-like quartz occur reticulated 

through the whole belt of softened country rock for a distance of 100 to 200 feet northwest of 

the main vein, and led to the prevalent idea that the vein here was nearly 200 feet wide. However, the most of this quartz contained no values whatever, although in the vicinity of the Comet 

vein proper a little mirteral occurred in stringers or in small isolated bunches. of a quartz of 

similar appearanee, but this probably represented simply a replacement of kaolin or gouge by 

silica along small slip zones. · 

NATURE OF VEIN. 

The Comet-JEtna vein, which is a strong fracture with 5 to 8 feet of quartz and clayey 

materials, is isolated from the rest of the veins of the district. The nearest· ore-bearing veins 

are separated from it by 3,000 to 5,000 feet of unexplored or unproved ground. Many people 

hold that the Comet-JEtna lode is an extension of the Colorado Central vein and this correlation 

is not without some foundation, for both of the lodes are very strong fractures which have the 

same general dip and strike. The dip of about 70° NW: could easily account for the northwestward swerving of the surface outcrops of the Comet and Colorado Central lodes as they cross 

the bottom of the valley of Leavenworth Creek. However, as there is fully a mile of territory 

betweenthe two recognized ends of the veins, no definite statement to the effect that the two 

veins are the same can be made. It is possible that the Comet vein represents only one of several 

branches of the Colorado Central main trunk: vein or that the two may be distinct veins which 

· die out by branching ~s they approach one anotl-ier. 

The Comet lode is very soft, and it is difficult to keep drifts open along it for any length of 

time. Consequently at the time of visit but little of the vein was exposed for study, as a result 

of the caving that had taken place during many years of comparative inactivity. From all 

appearances, however, the lode appears to be a single nonbranching one following two or more 

parallel planes of movement situated within a few feet of one another. It was not determined 

to what extent displacement had occurred along the fault planes, but discordances in the wall 

rocks on the two sides of the fractures were noted. 'l'hat extensive movements have occurred 

is well shown not only by polished and slickensided surfaces, but by zones of crushed and 

pulverized wall rock, by banded friction clays, and by qu~rtzose streaks which consist of wellrounded sandlike grains cemented together by a quartz matrix. 

Considerable fracturing of the country rock probably accompanied the faulting which 

caused the vein fracture, as shown by the zone 100 to 200 feet wide which contains numerous 

quartz stringers. More or less sheeting of the wall rocks parallel to the main vein has taken 

place, as can be seen from the number of minor gouge leads occurring between and parallel 
280 . GEOLOGY OF THE GEORGETOWN QUADR,c\NGLE, COLORADO. 

to both the Comet and J. A. Hawkes veins. Two well-defined open fractures or "water 

breaks" also occur within this same space. One of these is an .open fissure, in places nearly 

a foot wide, which furnishes a considerable flow of water and whose walls are heavily coated 

with the soft red and brown oxides of iron. 

About 50 to 100 feet northwest of the Comet lode as exposed on the tunnel level is a con~ 

nected series of leads with different strikes, forming a fairly persistent but irregular lead composed of 2 to 8 inches of crushed rock and gouge which in places grades into quartz or into quartz 

and ore sufficient in amount to have caused the· miners to gouge up on the streak here and there 

and to put up a raise about 60 feet high in one place near the point where a small branch joins 

the Stranger lead. · STRUC'l'URE OF VEIN. 

The vein where exposed on a level, connecting with one of the shafts at the crest of the 

mountain slope and located about 275 feet above the tunnel level, is extremely variable in width. 

One section of the vein (fig. 103) shows the foot wall, dipping 70° NW., to be somewhat altered 

granite. Resting upon this foot wall and separated from it by a narrow selvage is from 5 to 6 

feet of very highly altered micaceous gneiss; above this is about 1 foot of yellow, brown, green, 

and blue fine-grained clay in alternating hands. This clay, which is very tough and putty-like, 

probably represents gneiss, pegmatite, and granite finely pulverized by extensive movements 

FIG. 103.-Cross section of Cornet vein on level 270 feet above Comet tum1el level, showing the structure of the vein zone. 

lengthwise Qf the vein or fault. Embedded in this clay are scattered fragments of pegmatite 

or granite with their long axes oriented parallel to the clay seams in such a manner as to give 

the clay belt the appearance of the handed flow structure commonly seen in contact phases of 

porphyry dikes. , 

Above and northwest of the alternating ·variegated clay bands are from 2 to 3 feet of soft, 

crushed, altered gneiss and pegmatite containing a considerable amount of kaolin or "gouge." 

Still farther northwest 2 to 4 feet of low-grade ore was noted. This ore zone consists mainly of 

quartz, which in places either partially or entirely fills cracks in highly fractured gneiss and 

pegmatite or else coats the fragments of gneiss and pegmatite with a layer of dark-gray jaspery 

quartz one-eighth to one-fourth inch thick, closely resembling the widc1y separated veinlets of 

jaspery or chalc'edonic quartz that traverse the belt of altered country rock for 100 to 200 feet 

to the northwest of the Comet vein. 

Tliis dark-gray vein quartz is practically massive, with here and there a small pegmatite 

or gneiss inclusion that is not so much altered and replaced as to mak~ it impossible to recognize 

the original rock, although it is highly silicified and its irregular hGmndaries grade into the mass 

of jaspery quartz gangue. The ore in this silicified zone occurs mainly in narrow vuglike openings in the partially filled cracks in the fractured gneiss tmcl pegmatite, but also lining irregularshaped cavities, many of which show crustification but no comb quartz, and which are probably 

cavities of dissolution resulting from the action of the mineralizing waters. Small bunches 

of ore are also found sparsely disseminated through the quart"' mass. 
MINES NEAR GEORGETOWN. 281 

The northwest or hanging wall, in contact with the quartz and ore bolt, consists of a zone 

of kaolinized and somewhat silicified gneiss and pegmatite 100 to 200 feet wide. The structure 

of the vein, however, is not constant; for near the shaft, on tl~e level where the above-nientioned 

section was observed, the ore streak changes to a belt over 10 feet wide of fractured and in 

places brecciated pegmatite and gneiss, recemented by dark-gray jasper-like quartz which 

carries practically no ore. Between this broad zone and the putty-like banded clay strip is 6 

to 8 inches of rock which appears to. be a greenish-gray porphyry, but which on microscopic 

examination proves to have a groundmass ranging from cryptocrystalline to a fine-grained 

aggregate of crushed quart11 grains with irregular outlines and wavy or undulose extinction. 

In this groundmass are embedded larger rounded grains of quartz that in the hand specimen 

look like phenocrysts. This same rock section, moreover, shows that· small cracks and rectangular open cavities formerly existed, and that subsequently these were first lined with a 

coating of carbonates and then the remaining space was filled with kaolin. In some of the 

stringers filling the cracks, the carbonates, especially where they completely filled the minute 

veinlets, were associated with small amounts of the contemporaneous ore-bearing minerals, 

pyrite, zinc blende, and galena, the last appearing to be of slightly more recent origin than 

the pyrite. ORE. 

The ore noted consisted mainly of well-formed crystals of galena, 11inc blende, and a little 

cupriferous pyrite associated with some siderite or ankerite. This ore occurred generally lining 

narrow slitlike vugs along fractures or in irregularly shaped dissolution cavities lined with 

drusy, jasper-like, dark-gray quartz. Some of the blende was of a resinous nature and ranged 

in color from a light yellow or greenish yellow to a dark brown; other varieties were black and 

had a clark metallic luster. The ·ore exposed at the time of visit was of low grade owing to the 

predominance of the quartz gangue, but a large northeastward-pitching shoot of high-grade 

ore is said to have been stoped out at a point about 60 feet higher. The ore near the surface 

was also much richer. 

The siderite and ankerite where present have the nature of porous ~oats on drusy quartz. 

In places a white earthy substance is found incrusting the crystals of ga!ena and blonde in vugs. 

A chemical analysis by vV. T. Schaller shows this to be chiefly an alumina-bearing mineral 

resembling kaolinite, which contains some zinc but n0 carbonates. The zinc is probably in the 

form of a sulphide and the white subst:tnce may be either a physical mixture of finely powdered 

blende with the kaolinite, or kaolinite containing a small amount of the soft white amorphous 

form of zinc sulphide described by Dana.a 

MTNA MIN.E. 

The 1Etna mine, which is situated high up on the west slope of Griffith Mountain, was first 

located in 1867. The vein is undoubtedly the southwestern extension of the Comet lode, for it 

has been opened up by three short tunnels, one above another, run into the h.illside and extending almost to the limits of the Comet property. At the time of visit all these tunnels were caved 

shut. At present, however, a tunnel, now 1,900 feet in length, is being run by the Capital 

Mining and Tunnel Company from a point in the valley just east of Georgetown and south of 

the Griffith mine, with the purpose of intersecting the Comet-1Etna vein at a depth of about 

1,500 feet below the proved surface outcrops. It has been estimated by the owners that this 

tunnel will cut the vein at a point about 2,600 feet from the mouth. The vein is broad and 

soft and is said to resemble the Comet lode in every respect. Small shipments of ore that have 

been made from the mine have carried between 35 and 300 ounces of silver to the ton, but it is 

said that assays as high as 1,600 ounces in silver have been obtained from picked specimens. 

The ore consists chiefly of galena and zinc blende, with a small quantity of iron sulphide carrying 

considerable copper. A specimen of vein material picked up on one of the mine dumps showed 

specks of galena, greenish-yellow resin blende, and pyrite sparsely scattered through a grounda Text-book of mineralogy, p. 291. 
282 GEOLOGY OF THE GEORGETOWN QUADRANGLE, COLORADO. 

mass of finely crushed granite and gneiss containing a few larger fragments of pegrnatitic.milky 

quartz and vein quartz, the· whole recernented by silica. This well-indurated vein material, 

which closely resembles a solid igneous rock, is very similar in nature to the portion of the 

Comet lode which resembles porphyry. 

SPORTING TIMES MINE. 

The Sporting Times mine is located at an elevation of about 9,900 feet on Alpine Mountain, 

three-fourths Of a mile south-southwest of Georgetown. It is developed by two tunnels. The 

lower one, which· is· the only one accessible at present, is driven on the vein and is about 850 

feet in length. The wall rocks consist almost entirely of micaceous gneiss cut by intrusive 

pegmatite and granite·. At a point about 600 feet from the mouth of the lower tunnel, however,, 

porphyry forms the south wall of tho vein for a distance of 50 feet, and a patch of porphyry 

about 15 feet in length also occurs on the north side. This porphyry mass was too much altered 

to allow its variety to be determined. The -vein (fig. 104) consists of a soft clay, quartz, and 

pyrite lead, which here, and there gives place to quartz, and of sphalerite and galena ore. The 

I-I 1········:::.:·····1 

Mineralized lead Unmineralized lead 

FIG. 104.-Geological plan of Sporting Times lower tunnel. 

ore in places is comprised of breccia fragments of galena and sphalerite embedded in a matrix 

of pyritiferous quartz or silicified crushed rock. No stopes occur on the lower level, but small 

quantities of ore are said to have been produced by the upper workings. 

MAGNET LODE. 

LOCATION AND DEVELOPMENT. 

The Magnet mine (Pls. LUI, A; LIV), which produced small lots of rich ore prior to 1871, 

is situated near the head of the steep gulch extending from the valley of Clear Creek to the draw 

between Saxon Mountain and Woodchuck Peak. The development consists of a comparatively 

shallow shaft located at an elevation of over 11,000 feet and a series of five tunnels in the gulch 

to the west, the lowest one of which is more than 500 feet below the collar of the shaft. The 

tunnels and drifts aggregate over 6,000 feet in length, and although the mine has not been very 

actively worked for a number of years, yet most of these workings are still open. 

NATURE OF WALL ROCKS. 

In the vicinity of the principal ore bodies the walls of the vein are comprised chiefly of porphyritic biotite granite. On following the main vein to the· northeast, however, a mixture of 

pegmatite and granitic gneiss is encountered, and still farther northeast soft biotite gneiss with 

a few small patches of pegmatite. Gneiss also forms the walls of a clay and crushed rock lead, 
U. S. GEOLOGICAL SURVEY PROFESSIONAL PAPER NO. 63 PL. Lll 

0 100 200 .300 4oofeet ~------~------~------~------~ 

B EJ 

Mineralized lead Unmineralized lead 

GEOLOGICAL PLANS OF COMET TUNNEL AND ANGLO-SAXON EXTENSION LOWEST TUNNEL. 
U. S. GEOLOGICAL SURVEY PROFESSIONAL PAPER NO. 63 PL. Ll ll 

A . MAGNET MINE, ON SAXON MOUNTAIN, LOOKING SOUTHEAST. 

B . ANNETTE-GRIFFITH VEIN AND MINE OPENINGS, LOOKING EAST FROM SLOPE OF 

REPUBLICAN MOUNTAIN. 

1, Annette.Griffit h t unne l; 2, Capitol mine. 
U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER NO. 63 PL. LIV 

0 ;:. ____ ::.;100~-----=20U=---_.:_300~----4~00----__:500 feet 

HORIZONTAL PLAN OF MAGNET MINE WORKINGS. 
U. S. GEOLOGICAL SURVEY 

0 

Gran ite 

100 

Minera lized 

lead 

200 feet 

Unmineralized 

lead 

GEOLOGICAL PLANS OF SEPARATE LEVELS, MAGNET MINE. 

PROFESSIONAL PAPER NO. 63 PL. LV 
MINES N BAR GEORGETOWN. 283 

followed by tunnel No.4 for. the first 300 feet. The schistosity of the gneiss is variable but as a 

rule has a strike ranging from northeast to nearly due east. Granite forms the southern half 

of the long crosscut to the south near the breast of tunnel No. 4. 

The wall rocks near the vein are in many places much altered, the gneiss becominglocally 

soft and kaolinized and the granite changing to a decomposed or friable mass. No porphyry 

dikes are found in the present workings. 

NATU;RE OF VEINS. 

The main v,ein follows a strong line of movement, marked usually by a single fault line 

or narrow fractured zone from which a few minor fractures branch. The vein, where unmineralized, is a strong clay, crushed rock, and quartz lead where it passes through granite, pegmatite, or granitic gneiss; but on entering soft biotitic gneiss, as it does toward the northeast, 

it changes to a much narrower clay and crushed gneiss lead which tends not only to be deflected 

from its normal trend but to grow less marked and to .branch into a series of slips, owing to 

the fact that the movement was probably taken up in part along the schistosity planes. Besides 

the evidence of movement furnished by gouge and crushed rock, there are also well-developed 

movement strim, which range in dip from 30° to 63° SW. The Magnet vein varies greatly 

from point to point both in strike and dip, and this characteristic has caused considerable 

annoyance in exploration, especially on the lower levels (Pl. LV). In the southwest or principal ore-bearing portion of the workings, the vein ranges in strike from N. 45°E. on tunnel 

No. 1 level to N. 72° E. on tunnel No.3 level. The strike in the soft gneiss on'these levels was 

N. 77° E. and N. 55° E., respectively. On the main vein the dip was also flatter as a rule in 

the southwestern ore-bearing portion than at the northeast end. In the southwestern section 

the dip was about 45° to 55° NW.; at the northeast end it ranged from 60° to 7 5° in ·the same 

direction. 

Where mineralized, the vein may consist of a single seam of solid ore, but more commonly 

it is a stringer lode in granite or other hard rock, or consists of a fractured zone in which most 

of the rock fragments have been extremely altered and many of them wholly or partly 

dissolved away and their place taken by quartz or by gangue minerals and ore. However, 

evidences of the extensive solvent action of the underground waters is still seen in the porous 

nature of the vein filling and in the numerous irregular openings, which have every appearance 

of being cavities of dissolution. 

In a number of places where the vein has only one wall the mineral-bearing solutions have 

penetrated for some distance into the country rock on either side of the fracture, and impregnated it with ore minerals in sufficient quantities to make the extraction of the whole mass 

for a width of several feet pay well. Locally, however, where the vein is brqad, the proportion of quartz in the vein filling is so g>reat that, even though polybasite and tetrahedrite are 

found associated with the gale])Jl. and blendc present, the latter are in such minor quantities 

th11t the whole mass yields small values on mining. 

About 250 feet from the mouth of tunnel No. 1 two well-mineralized veins converge 

toward the cast and join the Magnet vein, the one northwest of the main vein coming from the 

S. 57° W. and the· other branch from the S. 28° W. These branches, which are rather obscure 

right at the junction with the main vein, are heavily mineralized a few feet away, but within a 

short distance diminish in size again as they recede from the main vein. 

The Sequel vein is a somewhat parallel vein, also dipping to the northwest, which trends 

slightly toward the Magnet vein and is thought by some of the miners to make a union with it; 

but no evidence of such a junction was noted. Several minor, slightly mineralized veins were 

also observed trending at high angles to the course of the main vein. However, these were 

rather isolated fractures whose relations to the main vein were indeterminable and which were 

not recognized on mo~e than a si.ngle level. 

An unmineralized fault line called the De Meli lode cuts across the Magnet vein on tunnels Nos. 1, 2, and 3. It is a strong barren clay and gouge lead 6 inches to 1-! feet wide which 
284 GEOLOGY OF THE GEORGETOWN QUADRANGLE, COLORADO. 

trends N. 58° W. It intersects the Magnet vein on tunnel No. 1 about 300 feet north from 

the mouth, but is found 50 feet or so farther northeast on No. 2 level, owing to its dip of 

55° NE. On tunnel level No. 3 its dip changes to 70° NE. 

COMPOSITION OF VEINS. 

The gangue material is chiefly quartz, ·but barite is also common. Carbonates are sparingly 

present. The quart7; is •mainly of the dark-gray chalcedonic variety. The barite is most 

prevalent as well-formed crystals in vugs, but also occurs massive, interspersed through the 

quartz and metallic minerals. · 

The metallic minerals consist as a rule of galena, blendc, polybasite, tetrahedrite, and 

some cupriferous pyrite or chalcopyrite. 

The ore is relatively high in both silver values and in the percentage of zinc. The amount 

of zinc present is commonly about twice as great as that of the lead. The zinc, which occurs 

chiefly as greenish-yellow blende but in part as the brown variety, is in places so abundant as 

to detract from the value of the ore. The amount of silver as a rule is greater the less the 

quantity of zinc present in the ore. However, small shipments have been made of ore averaging 

400 to 500 ounces of silver per ton in which the zinc content ranged from 15 to 20 per cent. 

The reason for this is that the main silver values probably lie in the polybasite and tetrahedrite, 

which arc very common in all the best orcs of the mine. These two argentiferous minerals 

occur in the massive form, associated with irregular masses of galena, sphalerite, and chalcedonic 

quartz, or as rough crusts and poorly formed crystals lining irregular-shaped openings which are 

probably mainly cavities of dissolution. 

The pyrite and chalcopyrite where present are generally associated with the tetrahcdrite, 

polybasite, and chalccdonic quartz. 

About 0.05 ounce of gold is usually present. Near the surface considerable soft oxidized 

ore carrying some copper carbonates was found, and this soft ore was also in many places associated with the soft pulverulent black sulphides rich in silver which arc common near the surface 

in many other parts of the district. Assays from different parts of the vein range from 35 to 

600 or 700 ounces of silver to the ton. Some of the small stringers on the Sequel vein gave 

assays as high as 1,400 ounces in silver. The average ore shipped, however, carries from 150 

to 300 ounces of silver to the ton. 

OCCURRENCE OF ORE. 

On tunnel No. 1 level ore was encountered about 50 feet from the mouth of the tunnel 

and extended eontinuously for nearly 250 feet, to a point within 10 or 15 feet of the barren 

De Meli cross lode. Ncar its east end the vein is joined on both sides by short but strong 

branch veins, which diverge to the southwest and contain small but well-developed ore shoots. 

The southern of these branches is probably represented on the level below by a similarly located 

vein carrying small streaks of ore. The main ore shoot on No.1 level pitches about 60° E., and 

was stoped from the surface eontinuously to tunnel level No. 3, on which ore also commenced 

10 or 15 feet southwest of the De Meli lode, but extended for only about 125 feet. The values 

in this ore shoot, which was the larges~ in the mine, however, do not seem to go much below 

level No. 3, a depth of 235 feet below the surface. An ore shoot due to the junction of minor 

leads also occurs to the northeast of the De l\1eli lode, and starts in about 10 feet from the cross 

lode. This ore body, which was only about 40 feet in length, also pitched eastward and extended 

between levels No. 2 and No. 3 and for 12 to 15 feet below No. 3. At the time of the visit a 

crosscut drift to the south was being run toward the vein at this point from a winze sunk to 

a depth of 65 feet below No. 3 level. A small ore body was also found on No. 3 level on the 

N. 55° W. vein in the drift located to the southwest of the large ore body west of the De Meli lode. 

The Sequel vein carries but few small stringers, and these are composed of very rich ore. 

All the main ore bodies seem to be confined to the areas of comparatively hard rocks such 

as granite, pegmatite, and granite gneiss, rather than to the areas of soft biotite gneiss.· The 

reason for this probably lies in the fact that the rocks first named, being harder and more 
MINES NEAR GEORGETOWN. 285 

brittle, under movement would result in more open fractures than the soft biotite gneiss, 

and so the mineralizing solutions would have a better chance to operate. Vein junctions, 

although in some cases obscure at the immediate point of union, also probably have a beneficial influence on the ore deposition; and a flattening of the dip of the veins appears to have 

further influenced the precipitation. 

PRODUCTION. 

The Magnet vem produced small quantities of ore as early as 1871, and according to 

Fossetta had yielded $50,000 up to 1875. According to some of the old lessees on the property, $70,000 worth of ore was taken from the large stope to the southwest of the De Meli 

lode, and about $50,000 was obtained from the ore body to the northeast of the same lode. 

A few sacks of very rich ore were also yielded by the Seq1,1el vein. 

GRIFFITH LODE. 

GENEl~AL DESCRIPTION .. 

The Griffith is a straight and strong lode, at present operated principally by two mines, 

the Griffith and the Annette, the former exploiting the southwestern portion, near the town 

of Georgetown, and the latter the northeastern portion, farther into Griffith Mountain 

(Pls. LIII, B; LVI).. The vein has in general a strike of N. 50° E. and a vertical dip. In the 

longest level, the Annette No. 1, the lode is developed for 1,900 feet in a straight line. The 

older levels, which are higher up on. Griffith Mountain, are mostly worked out and are now 

usually inaccessible. The main levels run into the mountain but little above the level of Clear 

Creek. The Griffith lode has the same general strike and dip as the Centennial, and the two 

lodes have certain features in common, together with very sharp differences. 

NATURE OF VEIN. 

The Qriffith lode has formed along a fracture zone, of which some parts are unmineralized, some are slightly altered, with vein and gangue material in thin seams, and others 

have been mineralized so as to form wide and important ore deposits. The vein shows the 

usual phenomena of branching, and on the Annette No. 1 level it forks and reunites-a conditiqn which was also observed in the Centennial (Pl. L). The east end of the Griffith lode, as 

developed underground in the Annette No. 1 level, shows only a weak mineralized slip which 

at the breast of the tunnel lies in soft gneiss and is barely perceptible. This suggests that 

the actual end of the lode is very near. 

Some of the phenomena of branching at least are probably due, as will be explained, to 

two distinct periods of vein formation, the veins of the second period being superimposed 

upon those of the first. The crossing of the main lode by a minor vein zone near the northeast end of the Annette No. 1 level is an unusual occurrence in lodes of this district, and is 

due in this case to the two periods of vein opening. 

Near the southwest end of the developed lode a branch diverges from the south side, 

opening out to the east. This is called the Sonora vein, and has a southeast strike and a dip 

of 70° NE. On the surface some high-grade ore carrying from 700 to 800 ounces in silver 

is said to have been taken from this vein; but where it joins the main Griffith lode, ·as seen in 

the Griffith No. 1 level, the vein contains only a small streak of pyrite and has not produced 

any pay ore. 

NATURE Cll<' WALL ROCK. 

The wall rocks include gneiss of several varieties, pegmatite, alaskite, and some hornblendite. Usually these rocks are all well mingled, though in some places gneiss and in others 

pegmatite and alaskite are 1predominant. 

a Fossett, Frank, Colorado, 1880, p. 316. 
286 GEOLOGY 01<' THE GEORGETOWN QUADHANGLE, COL011ADO. 

INFLUENCE OF WALL ROCK. 

A mixture of gneiss with alaskite and pegmatite has resulted in a rigid rock that has 

fractured well and formed a good medium for the slips along which the vein has formed. 

Locally, where soft gneiss predominates, as in the outer or southwestern part of the Griffith 

No. 1 level, the vein becomes weak or pinches out almost entirely and is unmineralized. 

Where the vein fracture passes into granite, on both sides of the g~eiss, the vein immediately 

becomes strong again. 

COMPOSITiON OF VEIN. 

The character of the ores in different parts of tl~e vein is very different, largely on account 

of the · two distinct periods of ore ·deposition. To take all parts of the vein together, 

it may be stated that the vein materials include considerable pure galena, some blencle, 

pyrite, and . chalcopyrite, with brown carbonates, including siderite, rhodochrosite, and 

magnesite, as the chief gangue minerals. Pure kaolin occurs in the vein at many places 

and is locally abundant. It is evidently a ·chemical precipitation, as was determined to be 

the case in the Colorado Central mine. · A peculiar feature of this mine, not determined in 

other mines of the district, is the presence of gold and silver telluride. This mineral has been 

described by Richard Pearce," who notes that C. A. Martine, of Georgetown, discovered the 

presence of tellurium in the mineral and sent it to him for in.vestigation. Doctor Pearce found 

that the mineral occurred in small quantities, associated with pyrite, galenite, and chalcopyrite, in a matrix containing a little magnesite. ' 

The color is dark gray, with a bright metallic luster; hardness about 3. An analysis was made by Mr. F. C. 

Knight on carefully selected material, but, owing to its intimate association with the minerals named above, it was 

impossible to obtain .the mineral in a pure form. The result of the analysis is as follows: · 

Ag ...................................................................................... 50.65 

Te ........................................................................................ 18. 80 

Pb ................................ : ..................................................... 9.34 

Cu...................................................................................... 4. 65 

Fe .......................................................................... _. ......... ·... 4. 00 

Bi. .............................................................................. ·_....... 1.16 

S .......................................................................................... 8.06 

~~~~~r·A·t~--~;lci-~\~_-_-_:: :_·_·_·_·_·_:::::::::::::::::::::::::: ::::::::::::::::::::::::::::::::: :!~ Insoluble residue ....................................................... -...... . . . . . . . . . . . 1. 07 

100.16 

. It was supposed by Mr. Martine that the gold present in the mineral was combined with t~llurium; the results 

of my investigations show, however, that the gold exists in the form of a rich alloy of Au and Ag. One small piece 

of the mineral showed distinctly a coating of gold and silver alloy on its deavage plane. On treating the powdered 

mineral with nitric acid the residue showed no indication whatever of brown sponge gold, which would have been 

the case if this metal Wl).S combined with tellurium. On the other hand, the residue contained the gold only in the 

form of a bright pale-yellow alloy which proved to he 0. 725 fine, the rest being silver. 

The mineral, judging from the above analysis, inay be said to consist of a-mixture of hessite and argentite, Ag2Te 

and Ag2S, associated with other min~rals in the following proportions: 

Hessite ............................ · ....................................... : ............. 51.22 

Argentite .............................................................................. -... 20.93 

~:ss}(ehalcopyrite probably) ..... __ ., _____ . ____ ... _ ... _ .... _ .. ___ . __ ._. _________ ... __ . _ _.{ ~: ~~ 

PbS (galenite) .. _ ...................... _ ................. ~ .......... _ .... _ . . . . . . . . . . . . . . . . 10. 78 

Bi2S3 .................... : •.•..•.•.•...•.••.•••.•.•••• ~ ••••••••••••. _ •••. _ .. _ . • • . . . . • • • • • 1. 42 

Au and Ag alloy .............................................. :........................... .48 

MgC03 ••••.••••••.•••.••...•.•••.......•••.••••.•••••••.•••••••••••••••.••....•.. _ ...•.• 1.95 

Insoluble residue ................ _ ....... _................................................ 1. 07 

99.95 

It is interesting to note that the relation between the hessite and the argentite corresponds closely to the formula. 

2(Ag2Te)+Ag2S. • 

a l'roc. Colorado Sci. Soc., voL 5, 1894-1896, pp. 242-243. 
U. S. GEOLOGICAL SURVEY PROFESSIONAL PAPER NO. 63 PL. LVI 

NE s w 

o soo 1ooo f eet ~~--~--~--~~--~--~~~ ~--~ 

ANNETTE MINE 

GRIFFITH MIN£ 

VERTICAL AND HORIZONTAL PLANS OF WORKINGS ON GRIFFITH VEIN COVERED BY GRIFFITH AND ANNETTE MINES. 

Ann ette workings incomp let e; other workings less so. 
MINES NEAR GEORGETOWN. 287 

A specimen of this telluride was assayed by H. K. Miller, of Georgetown, who found .t7 .478 

ounces of silver and 1.970 ounces of gold to the ton, with 13.75 per cent of lead and 5.80 per 

cent of copper. Another specimen given to the senior writer by the owners of the Griffith 

mine was submitted toW. F. Hillebrand, of the United States Geological Survey, who found in 

it qualitatively the elements reported by Doctor Pearce, and remarked that the evidence 

afforded by a partial analysis supported his conclusion as to the presence of a mixture of hessite, 

argentite, and other minerals. This telluride occurred in the Griffith vein, in a shoot of ore 

which extends from the surface downward" almost vertically. 

In general the ores of the mine may be divided into two groups-the galena ores associated with blende, chalcopyrite, and some pyrite, and the carbonate ores associated with 

pyrite and some barille. These two classes of ore were deposited at distinctly different 

periods. In general the carbonate ore is not profitable for mining, the average content 

being stated at $2 to $3 in gold and 10 to 12 ounces in silver, with little or no lead. It is, 

however, reported that one stope of carbonate ore containing pyrite yielded 20 to 25 ounces in 

silver and $6 to $10 in gold. A specimen of typical carbonate-pyrite ore was taken by the 

senior writer from· the more northern of the two branches near the northeast end of the Annette 

No. 1 level, 60 to 70 feet from the breast of "the branch. A seam of ore here was a few inches 

thick, with no traces of galena, but the ore was rather soft and crumbling, as if decomposed by 

circulating waters. An assay made by R. H. Officer & Co., of Salt Lake City, yielded 0.16 

ounce of gold and 63 ounces of silver to the ton. 

In the orcs of the galena class in the Griffith mine the values are chiefly in lead, next in 

silver, and $4 to $5 in gold. It is reported that farther into the mountain on the same vein, 

in the Annette mine, the values run more in gold, silver, and copper and less in lead. In both 

mines it is these masses of galena ore which are sought for and mined. These lead ores are 

subordinate in quantity to the carbonate ores above discussed. They are reported to contain 

on an average about 40 ounces of silver and $8 to $9 in gold. As a rule the copper is not 

present in large enough quantity to be paid for by the smelter, although one shoot yielded the 

exceptional amount of 1 to 1?! per cent of copper. 

Owing to the origin of the two classes of ore at different periods, mixtures of the two generations of vein materials in all proportions are found at many places. Such mixed ore was 

seen, for example, in the Annette No. 1 level a few hundred feet from the entrance and was 

reported to yield 20 ounces of silver and 0.05 to 0.10 ounces of gold. Similarly, about 300 

feet in on the Griffith No. 1 level is a small body of rich ore in a low-grade vein. This ore is 

reported to yield 300 ounces in silver, $2 to $3 in gold, and some copper. A specimen of this 

ore taken by the senior writer and examined by E. C. Sullivan, of the Survey, was found to ' 

contain a sulphantimony compound of silver." Sphalerite was also present and smaller quantities of copper, cadmium, lead, arsenic, etc. 

STRUCTURE OF VEIN. 

The vein shows plainly two distinct periods of ·fissuring and cementation by vein materials. 

The result of the first dislocation of the rocks along this line was a strong fissure, in part 

open for a considerable width, as is shown by the angular fragments of wall' rock that occur 

in and are cemented by the vein materials which were deposited subsequent to this dislocation. 

These angular fragments of wall rock indicate an original rubble-filled fissure, like that now 

occupi~d by portions of the Mendota and similar :veins. The filling of this fissur~ was also 

similar to veins of the class mentioned. The first deposition was a line of comb quartz, and 

the interior of the fissure was filled with solid"sulphides, practically without gangue, consisting o.f galena, blencle, pyrite, and chalcopyrite. The abundance of chalcopyrite is distinctive 

of this vein, in contrast with the Mendota and other similar veins referred to. 

Subsequent to the formation of this sulphide vein there was another dislocation, resulting 

in much larger openmgs than t~1e first disturbance produced. The new fissure in general 

a Probably tetrahedrite or polybasite.-J. E. S. 
288 GEOLOGY OF THE GEORGETOWN QUADRANGLE, COLORADO. 

followed along the line of the old one, splitting the older vein and breaking it up in various 

ways. On account of the greater brittleness of the original sulphide filling, as compared with 

'(__ __ _...,.. __ ___. zinches. 

that of the wall rocks, the new fissure split the vein in the 

middle at many points, leaving a layer of sulphides clinging 

to each wall, a result whieh, after the new fissure was 

cemented with the carbonate ore of the seeond period, ·gave a 

false appearance of erustification. In numerous other places 

the older vein was broken into angular fragments, which 

rested in the new-fonned fissure in the shape of rubble (fig. 

105), precisely as the fragments of country rock had filled the 

original fissures. These angular fragments of sulphide ore 

now appear as inclusions in the later carbonate ore in varying 

amounts and degrees of coarseness. 

Elsewhere the new fissure departed locally or completely 

from the course of the original one (fig. 106). Thus on the 

Annette No. 1 level, where the composite Griffith vein splits 

and reunites, the· two branches inclosing a horse of country 

rock, it is probable that one of the branches was formed 

during the first period of vein deposition and the other after 

the second period. It seems highly probable also that certain of the branches diverging from the trunk vein were formed 

entirely during the second period of vein deposition. Near 

the northeast end of the Annette No. 1 level are two branches 

separating from the trunk vein on the north side and striking 

almost due north. The ore in these branches is entirely the 

carbonate-pyrite ore characteristic of the second period. The 

more northeastern of the two branches crosses the trunk 

vein and has been explored for a considerable distance in 

both directions. The probability that this branch is due to 

the sul->sequent period of fissuring explains the otherwise 

unusual phenomenon of crossing. 

Where, as is usual, the path of the earlier and the later 

FIG. 105.-Sketch of specimen from Griffith 

vein, showing cementation of fragrnents of 

galena (a) belonging to the first period of 

deposition by a crustified material belonging 

to the second period and deposited in the 

following order: 1, Finely banded (crustified) brown carbonates (b); 2,a thin crust of 

quartz with sulphides (c); 3, mingled bwwn 

carbonates and fine quartz (d) containing 

some pyrite (e) and a little galena (a). fissures practically coincided, the proportion of the vein filling belonging to the two periods varies greatly (fig. 107). 

In general, however, the spaces afiorded by 

the reopening of the fissure, and consequently 

the amount of ore now present in the vein as 

a result of cementation of this reopened fissure, are estimated to be many times as great 

as in the first period. In places the earlier 

sulphide ore may form the bulk of the present 

vein, constituting what is locally (though perhaps not correctly from a technical standpoint) 

known as an ore shoot. 

PARAGENESIS OF MINERALS. 

The principal minemls of the orcs belonging to the first period are galena, blencle, chalcoFIG. 106.-Vertical cross section of Griffith vein, Griffith No. 1 level, 

showing veins o[ two distinct periods running side by side and parallel 

in the same drift. 1, Vein of earlier period consisting o[ blende, galena, 

and a little pyrite; 2, vein o[ lateJ; period consisting of brown carbo· 

nates with pyrite; 3, sheetingingneisscountryrock, parallel with vein. 

pyrite or cupriferous pyrite, and some iron pyrite. Copper-bearing pyrites seem to be invariably associated with this earlier vein material, but nowhere in any quantity with the material 

of the second period. Examinations of hand specimens and thin sections of this earlier sui-
MINES NEAR GEORGETOWN. 289 

phide ore show that the various sulphides are almost contemporaneous, but in detail they have 

crystallized out in the following order: (1) Pyrite, (2) chalcopyrite, (3) galena. The reiative 

age of the blende has not been observed, though it seems to be very nearly contemporaneous 

with the galena. In various places galena was noted filling cracks in chalcopyrite. The earliest 

formation in the veins of this first period was comb quartz, which lined the open spaces. 

Tetrahcdritc was noted along cracks in chalcopyrite and distinctly subsequent to it. 

In the vein material belonging to the second period of deposition the pyrite, which is 

abundant, is contemporaneous with the more abundant carbonates, whereas the sulphides of 

the first formation, where they are included in the carbonate gangue, occur in broken and 

angular fragments. Microscopic examination, however, indicates that a small amount of 

chalcopyrite, galena, and blende was deposited contemporaneously with the carbonate gangue. 

A specimen of the carbonate in the later ore, submitted to E. C. Sullivan, of the United 

States Geological Survey, proved to be mainly ferriferous rhodochrosite, with some magnesite 

and traces of calcite. Doctor Pearce, quoted above, mentions magnesite as a gangue material. 

According to J. S. Randall, of Georgetown, the carbonate gangue of the Griffith mine is largely 

siderite, with some magnesite. It is probable that the 

brown carbonate is a mixed carbonate of iron, magnesite, manganese, and lime, as in other mines of the 

region. 

Typically small contemporaneous crystals of quartz 

arc scattered in the massive carbonate. As a rule also 

the deposition of quartz has persisted longer than that 

of carbonate, for abundant geodes and Jruses, which a:re 

found in the carbonate ore, are lined with crystals of· 

quartz. With this predominant quartz there is also here 

and there some lesser amount of brown carbonates, and 

even small and scanty crystals of sulphides, such as chalcopyrite, galena, and blende. Barite also is very commonly 

found lining the druses as the last mineral to crystallize, 

and kaolin occurs under the same conditions. 

In some places the gangue may be mainly quartz, 

with a subordinate amount of intercrystallized carbonates. In other mines of this region, such as the Mendota, Dunderberg, Bismarck, Wisconsin, Pay Rock, and 

Colorado Central, brown carbonates of iron, manganese, 

etc., with contemporaneous pyrite, have been found to 

be in general later than galena and blende, but nowhere 

on such a scale as is found in the Griffith mine. In 

FIG. 107.-Vertical sketch section of Griffith vein, Annette No_ 1 hwel, ahou~l,600 feet from 1nouth, showing 

ore of two periods of vein fOrmation. 1, Coarse galena 

ore of first period; 2, carbonate-pyrite ore of second 

period inclosing broken angular fragments of No.1. 

The wall rock is gneiss. 

the other mines mentioned also this carbonate-pyrite ore pas usuaJly a very evident coanection with the present topographic surface, but the somewhat similar ore of the Griffith has no 

su,ch connection. · 

INFLUENCE 0~' DEPTH. 

On considering the·influence of depth on the ores of the first period, it appears probable 

that the action of surface waters has brought about some local concentration and impoverishment. As is usual in mines of this district, the surface ores were very rich, and according to 

Raymond a contained sufficient gold to repay sluicing. It is reported by those familiar with 

the mine that there was more copper on the upper levels than in those more remote from the 

surface. The occurrence of tetrahedrite along cracks in chalcopyrite, like that noted in the 

Centennial mine, suggests the secondary nature of the tetrahedrite and descending ground 

waters as the depositing agency. It is also reported that these upper ores contained more 

silver and gold and less galena. 

a Mines and mining west of the Rocky Mountains: Ex. Doc. No. 207, 41st Cong., 2d sess., p. 3G8. 

31401--~o. 63--08----19 
290 GEOLOGY OF THE GEORGETOWN QUADRANGLE, COLORADO. 

It appears probable that this action of shallow descending waters had some enriching influence on the subsequent carbonate-pyrite ore, for it is reported that in the upper levels much of 

the ore of this class was rich enough to pay. · 

POSTMINEl~AL FAULTING. 

On the Griffith No. 1 level, 100 feet from the mouth, the Griffith vein is cut by a transverse 

fault striking N. 43° W. and dipping 76° NE. At the point where the lode is cut it is a composite lode, the result of the two periods of vein cementation. It is offset about 10 feet to the 

northwest on the northeast side by the fault. It is reported that this same fault is found all 

the way down from the surface, hut that the offset diminishes ih depth. For example, 60 feet 

above this level the offset is 15 feet, but 25 feet below it is only 5 or 6 feet. Near the surface 

the fault is marked by a water course filled with bowlders. 

PRESENT UNDERGROUND WATERS. 

In the southwestern, or outer, portion of the Annette No. 1 level coatings were observed 

on the drift walls, in some places stained with copper. Water drips from the rocks a:nd vein at 

intervals all along these drifts. On the Annette No. 1level water was observed from the mouth 

for 1,200 feet in, to a point having an estimated vertical distance from the surface of 650 feet. 

From this point to the end of the level, which is about 1,900 feet from the surface, at an actual 

depth of about 1,050 feet, the rocks were quite dry. 

ANGLO-SAXON LODE. 

ANGLO-SAXON MINE. 

HISTORY, DEYEL6PMENT, AND PRODUCTION. 

The Anglo-Saxon lode is located on the northwest slope of Saxon Mountain and runs northnortheastward up the slope to an elevation of 10,500 feet. In 1867 it caused much excitement 

on account of the extremely rich ore found along it. The developments consist of four levels 

with numerous crosscuts and shafts, but all these workings are at present caved shut. 

Accordip.g to Burchard a the Anglo-Saxon mine had yielded about $700,000 previous to 

1883. But little ore has been extracted since that time, although small quantities of rich "float 

ore" are at present occasionally found scattered through the loose surface material along the 

trend of the vein. NATURE OF ROCKS. 

As all the workings were closed by caving, the exact nature of the rocks occurring along 

them is unknown, but the surface rocks in the vicinity of the vein consist chiefly of much-altered 

friable granite and pegmatite. Granitic and biotitic gneiss also occur. The gneiss has.a general 

strike of N. 40° W. and a dip of 20° NE. 

VEIN AND ORE. 

· The Anglo-Saxon vein on t~e surface has a general strike of N. 70° E. and a dip ranging 

from vertical to a slight inclination toward the north. Where the vein crosses a spur of Saxon 

Mountain, at an elevation of about 10,300 feet, the wall rocks for some distance on either side 

of the vein consist of yellowish J,ltered and friable granite. Running through this soft granite, 

much of which is probably not in place, are irregular branching seams of loose material in which 

are embedded here and there nodular masses or nuggets of very rich ore. These masses consist either of yellowish and greenish light-colored oxidized ore, or of rounded lumps of a black 

sectile waxy substance, high in silver values, which vV. T. Schaller found by chemical analysis 

to be argentite (Ag2S), or silver glance. Argentite occurs also as small stringer-like masses in 

the sandy matrix, which resembles loose surface material. These stringers may represent the 

portions of displaced surface croppings of the vein, for the seams have been successfully 

followed for only a few feet in depth. 

u. Burchard, H. C., Precious metals in the United States, 1883, p. 277. 
U. S. GEOLOGICAL SURVEY PROFESSIONAL PAPER NO. 63 PL. LVII 

.!!.. GEORGETOWN AND REPUBLICAN MOUNTAIN , LOOKING NORTH IN SILVERDALE 

CANYON TO SOUTH CLEAR CREEK. 

1, Kelly-tunnel, 2, New Boston mine; 3, Beecher mine; 4· Muscovite mine. 

B. REPUBLICAN, DEMOCRAT, AND COLUMBIA MOUNTAIN MINES, LOOKING WEST 

FROM DUMP OF MAGNET MINE. 

1, Kelly tunnel; 2, Moline tunnel; 3• New Boston mine ; 4• Spart an tunn el ; 5• Beecher mine; 6, Mi neral 

Chief mine; 7, Muscovite mine; 8, Sceptre mine; g, Qu een mine ; 10, Ast or tunnel; 11, Cliff l owest 

level , 12, Junction lower level; 13, Edgar tunnel; 14, Buckeye tunnel. 
MINES NEAR GEORGETOWN. 291 

In all probability the rich sulphide ore is the result of the secondary action of surface 

waters and has originated since the formation of approximately the present topography. These 

sulphides which are residual in the oxidized ores are much the richest of the ores in the mine 

and in respect to their high content of the precious metals are very similar to ores noted at 

Monte Cristo, Wash.,a by Spurr, and also at LeadviJleb by Emmons, who stated that it was 

"evident that the action of the surface waters has been to concentrate the silver in the sulphide 

ore, not in the oxidized product." 

Small shipments of high-grade ore obtained near the surface have brought returns ranging 

from 1,000 to several thousand ounces in silver to the ton. 

No active work has been done on this mine for years, although at present a few sacks of 

ore are being obtained from surface pits by picking out the rich pieces of ore which are sparsely 

scattered as iwdules or irregular seams in the friable granite and wash. 

ANGI.O-SAXON EXTENSION VEIN. 

LOCATION AND DEVELOPMENT. 

The Anglo-Saxon Extension vein, which is probably either the continuation of the AngloSaxon vein or one of its branches, is located to the west and south of the old Anglo-Saxon 

workings and is opened by a series of tunnels which aggregate bver 2,500 feet of drifting. Most 

of the upper workings are inaccessible. The lowest, or seventh, level, the longest one in the mine, 

was about 825 feet in length at the time of visit, but was being extended. The first part of the 

lowest tunnel, the mouth of wnich is located 600 or 700 feet above Clear· Creek, is a crosscut 

run through fractured biotite granite and pegmatite. The rest of the level and the vein itself 

are entirely in gneiss, which as a tule strikes to the northwest. 

VEIN AND ORt~. 

The vein on the lowest level has a strike of N. 73° E. and a slight dip to the northwest. It 

is for the most part a crushed gneiss and clay lead, from a few inches to 2 feet wide, which is 

in many places highly colored by irc)n and copper salts. That the vein is along a fault line is 

well shown by the gouge and the putty-like clay, which represent country rock finely pulverized as a result of movement, and also by slickensides and movement strim dipping 65° NE. 

In a few places the clay and crushed rock lead changes to quartz or to a low-grade ore, or "mill 

dirt," consisting of a mixture of quartz and decomposed pegmatite and gneiss over a foot wide 

with specks of galena all through the mass, which also carries a little lead carbonate and some 

copper compounds, as shown by the presence of copper sulphate, and the carbonates, azurite 

and malachite. 

This lean ore is said to carry values ranging from $10 to $18 per ton. Some portions of 

the vein nearest the surface carry a porous or honeycombed mass of quartz which shows leaching by descending surface waters. The presence of iron oxides in abundance shows that the 

lower workings are still in the oxidized zone. 

Most of the ore produced by this mine was found betw~en the Blacksmith Shop level and 

the seventh level, which are about 225 feet apart. The greater part of the ore was from shoots 

which are said to have clipped to the northeast. 

PRODUCTION~ 

Little information was obtainable about the production of the Anglo-Saxon Extension 

mine, but the Director of the Mint reports the production for 1889c as $2,017.60, divided as 

follows: Gold, $880; silver, $768; lead, $250.80; copper, $118.80. In the report for 1890 d 

the yield was given as $3,955, divided thus: Gold, $1,100; silver, $2,246; lead, $609. 

a Spurr. J. E., Twenty-second Ann. Rept. U.S. Geol. Survey, pt. 2,1901, p. 777-865. 

b Emmons, S. F., Secondary enrichment of ore deposits: Trans. Am. Inst. Min. Eng., February, 1900, p. 7. 

c Leech, E. 0., l'roduction of gold and silver in United States, 1889, p. 144. 

rl Idem, 1890, p. 130. 
292 GEOLOGY OF THE GEORGETOWN QUADRANGLE, COLORADO. 

MINES OF REPUBLICAN. DEMOCRAT. AND COLUMBIA ~OUNTAINS. 

REPUBLICAN MOUNTAIN GROUP. 

EXISTENCE OF A ZONE OF MINERALIZATION AND INTRUSION. 

The so-called Republican Mountain group of veins occupies a definite east-west zone 

which extends from Clear Creek on the east westward up the valley of Silver Creek nearly to 

the summit of the mountain ridge. (See Pl. LVII.) The difference of elevation of the surface 

along various parts of this zone amounts to about 3,000 feet. The separate veins in this zone 

have a generally eastward trend corresponding with that of the zone as a whole, but also vary 

· from that direction to the southeast and the northeast. 

This vein zone is also a zone of porphyry intrusion. 'The principal dike runs east and west 

.along, the Sunburst-Sceptre vein. Farther east, what may he the same dike is encountered 

near the Boston vein, in some places forming one or both of the walls of this vein. Smaller 

-dikes have also been noted in this zone. The porphyry is definitely earlier than the formation 

-of the vein zones. · 

It is evident that an east-west zone of weakness existed here previous to the intrusion of 

the porphyry. The first dislocations were probably followed by the igneous intrusion. Subsequently, the stress continuing, renewed movement took place, producing fault zones which 

.in part followed along or near the older dike. These later openings were in part cemented by 

vein material. Later still movement was again resumed, also on an important scale. The 

result \vas that the principal vein of the zone, the Sunburst-Sceptre, was reopened and fissured 

in the same way as the Griffith vein on Saxon Mountain. The fissures in the reopened veins 

-of Republican Mountain were subsequently cemented by the same materials as in the Griffith 

-vein, namely, brown carbonates with pyrite. Subsequent to this cementation the SunburstSceptr·e vein was again reopened and the fissures were cemented with chalcedonic quartz. 

Thus fissuring and subsequent cementation have taken plaue repeatedly during a period of time 

·probably beginning in the late Cretaceous a and lasting to a recent date, or very likely to the 

present day. 

PRINCIPAL VEINS OF THE GUOUP. 

The principal vein in the western portion of this zone is the Sunburst-Sceptre. The Queen 

]ode is a smaller vein near and parallel to the Sunburst-Sceptre, and the Astor is a similar nearly 

parallel vein. At the east end of the zone the chief veins are the Boston and the Mineral Chief, 

·with a number of smaller ones, such as the Muscovite, Spartan, and Beecher. The connection 

between these veins has not been well established, but it is possible that some of those which 

:go by different names may be in reality parts of the same lode.. For example, the Spartan is 

probably the northeastern continuation of the Mineral Chief, and the western continuation of 

the Mineral Chief is commonly held to be the Smith & Wesson. Similarly it is possible, as is 

lleld by many, that the Spartan-Mineral Chief vein may be the continuation of the Boston·vein. 

'The Muscovite vein may be the cop.tinuation of the Beecher, which is a branch of the Boston, 

;although no connection has been traced either on the surface or underground. 

CHARACTERISTICS OF VEIN FILUNG. 

The lodes of this zone contain ores which possess characteristic distinctions from those of 

·other groups of veins. Those near the foot of the mountain or at the east end of the zone, such 

:as the Boston, Mineral Chief, Beecher, Spartan, and Muscovite, all contain ore of the same kind, 

:marked in many plaues by large amounts of pure galena, a variable amount of very dark blende, 

.and considerable pyrites, with a low content of silver and very little gold. Siderite and a little 

:kaolin also occur. When sorted this material forms a good grade of commercial lead ore. It 

::resembles in many ways the Griffith ore of the first period of deposition, even containing the 

:same peculiar dark-colored zinc blende. The lodes farther up the mountain, at the west end of 

the zone, are similar to the others, being usually of low grade in respect to silver. However, 

a Seep. 109. 
MINES NEAR GEORGETOWN. 293 

they contain relatively more silver and less lead than the group of 'veins first mentioned. As a. 

whole, the Republican Mountain ores are distinguished from those of the so-called Democrat 

Mountain group of veins in that the latter are high-grade silver ores. 

KELLY TUNNEL. 

Like nearly every other group of lodes in the region, this group, chiefly worked out near 

the surface, has been the object of one of those tunnel projects for which the local topography 

is so favorable. The Kelly tunnel (Pl. LVIII) was started on the west side of Clear Creek, 

near the ·north end of the town of Georgetown, to run under both the Republican and the 

Democrat Mountain groups .of lodes and tap them at great depth. It was driven for 2,680 feet 

in a northwest direction, at a reputed cost of $160,000. A number of minor veins were first 

encountered, and, farther in, the Boston vein, at the point where it is joined by its principal 

branch; the Beecher vein. This junction at the surface and in the upper workings had been 

responsible for an important ore shoot. Where the same junction was encountered in the 

tunnel, however, at a depth of about 1,000 feet, it was absolutely barren, and the vein zones, 

though well marked as faults, showed practically no mineralization. The tunnel undertaking 

was therefore given up,. at least temporarily. 

In the Kelly tunn'el, '"for the :first 750 feet or thereabouts, abundant water drips from the 

roof and deposits iron ocher in the same way that has been described for other underground 

workings in the region. Farther in, however, as far as the breast of the tunnel, the rocks are 

practically dry and there has been no deposition of iron or lime. A small pool of water backed 

up near the breast by the fall of material ftom the roof of the tunnel shows that these deeper 

rocks are not entirely dry, although relatively much drier than those of the superficial zone. 

One of the lodes cut in the outer part of the tunnel, the Deming lode, at a vertical distance 

of 300 feet from the surface, shows 3 or 4 feet of silicified material cementing the fault zone, 

with local sulphides. Farther in the tunnel the lodes cut at.depths ranging from 500 to 1,200 

feet are nearly all soft fault zones with only traces of cementation, even silicification being almost 

absent. In some of these lodes the superficial portion for a few hundred feet below the surface 

shows strong silicification, resulting in the formation of quartz veins, some of them several feet 

thick, with accompanying sulphides. · Such, for example, were the conditions in the upper 

portion of the Boston and Beecher lodes. Moreover, where these soft lodes, which are cut at 

greater depths underground, are at all silicified, there is a relatively thin quartz streak, in some 

places containing pyrite, which invariably, so far as noted, lies upon the foot wall of the soft, 

decomposed zone.. These two circumstances indicate that the silicification is probably the work 

of descending surface waters and that the sulphides which accompany the quartz have probably 

a similar origin. 

BOSTON AND BEECHER VEINS~ 

DESCRIPTION. 

The Boston vein has a northeasterly strike and a steep southerly dip. It is opened underground by three tunnels, of which the uppermost, the New Boston, also called the Kelly No. 2 

level, is about 170 feet below the outcrop. Below this is the Moline tunnel, also known as the 

Kelly No. 1 level, which is 400 feet below the outcrop. Still below this the Kelly tunnel cuts 

the vein at a depth of somewhat more than 1,000 feet. The chief branch of the Boston vein is 

called the Beecher. It diverges from the main vein on the north side, opening up to the west 

and striking approximately east and west. Like the Boston, it has a steep southeast pitch. 

In the Moline tunnel a zone of slight mineralization occurs on the south side of the porphyry 

dike, which is here 200 feet or more south of the Boston vein and has an east-west trend. This 

mineralized zone is called the Galena vein. It is marked by tight seams of galena, pyrite, and 

blendc carrying a little silver.· The same slightly mineralized lead is said to have been recognized at the surface. On the Kelly tunnel level, at the same porphyry contact, a similar lead 

was noted, also slightly mineralized. As the trend of the porphyry contact is diagonal to that 

of the Boston vein, this so-called Galena lode runs into_ that vein and is to be considered as a· 

branch of it. The junction was actually observed on the Kelly tunnel level (Pl. LVIII)' 
294 GEOLOGY OF THE GEORGETOWN QUADRANGLE, COLORADO. 

ORIGIN AND STRUCTURE OF VEINS. 

The main Boston vein and its branches follow fault zones marked by bands of crushed and 

decomposed country rock, in many places by contortions of the gneiss against the fault plan<?s, 

and also by the strire, which in this region afford a reliable indication of the direction of movement. Such striations along the Beecher lode in the Moline tunnel have a pitch of 30° E. 

Along the Galena lode in the Kelly tunnel they pitch 20° E. These zones of crushing have 

locally been cemented by cherty quartz and sulphides. A study of the veins and the vein 

material leads to the conclusion that the process of mineralization has been largely replacement 

and interstitial filling rather than fissure filling. As a rule different hard slices of the lode are 

separated from one another by bands of soft gougewhich are apparently the result of renewed 

movement subsequent to the first cementation. 

CHARACTER OF WALL ROCKS. 

The principal wall rock of the vein is a black biotite gneiss. Here and there, however, 

there is considerable granite. In the Moline tunnel it was noted that the rock on the north 

side of the vein was chiefly granite, whereas that on the south side was mainly gneiss, as if 

the vein had been localized near the contact. 

A dike of porphyry 30 to 35 feet thick goes down perpendicularly from the surface, and 

has been cut in both the Moline and the Kelly tunnels. ~ocally, as in one place in the Keliy 

tunnel, the thickness of this dike increases to 150 feet. In the vertical section taken along 

the Kelly tunnel the dike is at some little distance from the vein, but as the trend of the two 

is diagonal they come together as shown in the Kelly tunnel, where the junction occurs a short 

distance southwest of the main tunnel. On this level the main Boston vein pursues its course, 

passing diagonally through the porphyry dike to the opposite or southern contact, but the 

Beecher vein on encountering the north contact of the porphyry appears to follow it westward. 

ALTERATION OF WALL ROCKS. 

It is characteristic of the altered gneiss in the vicinity of some of these veins that it has 

assumed a peculiar bright-green color. Some of this gneiss examined microscopically was 

found to consist chiefly of quar.tz and a pale-green mineral, evidently a mica. Analysis by 

W. T. Schaller showed the coloring to be due to chromium, so that the mineral is a chromebearing muscovite (fuchsite). Other specimens of the altered gneiss showed, besides these 

alteration products, abundant cloudy carbonates with original sphene and ilmenite. Through 

the Kelly tunnel it was shown that the black gneiss contains veinlets of brownish carbonate 

which have the appearance of being derived from the neighboring wall rock. 

In some hand specimens the porphyry appears decomposed and in others hard and fresh. 

Microscopic examination, however, indicates that it is all decomposed. A specimen from 

the Kelly tunnel showed phenocrysts of quartz and others of feldspar entirely altered to sericite and brown carbonates. The groundmass was a fine body of secondary quartz, sericite, 

and a little brown carbonate, the section containing a small veinlet of brown carbonate and 

another of fine crystalline quartz. In the other veinlets the two minerals were crystallized 

together. Another specimen of altered porphyry from the wall rock of the Beecher vein in 

the Kelly tunnel was so thoroughly decomposed as to show under the microscope only a confused mass of brownish carbonates with sericite and quartz. In general, the porphyry as seen 

in the Kelly tunnel contains numerous small calcite veinlets apparently derived from the altered 

rock. RELATION TO WALL ROCK. 

The Boston vein where it enters the porphyry dike on the Kelly tunnel level becomes 

wider and stronger and shows more mineralization than where it lies iri the gneiss. 

COMPOSI'l'ION OF vgiN. 

The vein contains considerable quartz and pyrite, and these two materials are the chief 

constituents of the poor ore. There is also some siderite, which is probably contemporaneous 
U. S. GEOLOGICAL SURVEY 

Elevation 1500 feet 

Elevation 1000 feet 

PROFE'.SSIONAL PAPER NO. 63 PL. LVIII 

NEW BOSTON TUNNEL 

0 100 200 feet '-----'------' 

VERTICAL PLAN 

KELLY TUNNEL 

VERTICAL AND HORIZONTAL PLANS OF KELLY TUNNEL 'AND ASSOCIATED MINE WORKINGS. 

0 100 200 300 feet 

Gneiss1 usually biotitic, Pegmatite with some Granite, usually A\askite porphyry 

in places hornblendic hornblendite porphyritic 

Glacial ddft 

EJ [ZJ Mineralized lead 

(in fault .wne) 

Unmlneralize-d Jead 
MINES NEAR GEORGETOWN. 295 

with the pyrite. The ore of the better quality carries large quantities of massive pure galena, 

with abundant pyrite, considerable blende, ·and a little chalcopyrite. This ore contains a 

small amount of gold and silver. Picked ore fTom the Moline tunnel is reported to yield 50 

to 60 per cent of lead, 14 ounces of silver, and 0.1 of gold. Tetrahedrite is practically absent, 

only one or two small fTagments of it having been reported. 

J,OCATION OF ORE BODIES. 

Only a single ore shoot has been found on the Boston vein, and this is located at the junction of the Boston with the branch called the Beecher. The Boston vein has been traced on 

the surface eastward nearly to' the foot of the mountain withoutany other discoveries. Except 

for this shoot, the vein is practically unmineralized, as is shown not only at the surface but 

in the underground workings. In the Moline tunnel a drift which is now caved was run along 

the Boston vein for 200 feet east of the ore shoot. The lead as developed in this drift showed 

neither ore nor quartz, but only a zone of crushed and decomposed rock. Near the junction 

on this level the quartz and ore are several feet wide, but along this drift the quartz dwindled 

in thickness to 3 or 4 inches, lying upon the foot wall of the fault zone and containing nothing 

but a little barren pyrites. 

The ore shoot was worked downward from the surface. At the surface it was ·worked 

for 50 feet east of the junction along the united veins, and for 100 feet west of the junction along 

the Boston. On the New Boston tunnel, which cuts the shoot about 170 feet 

below the outcrop, the body of quartz 

and sulphides is situated at the exact 

junction of the main vein with the 

branch vein and is 8 or 9 feet wide. 

From this junction the thickness and 

strength of the vein diminish rapidly 

on both of the uniting veins and on the 

trunk vein. On the .Moline tunnel level, 

400 feet below the outcrop, the principal ore has formed fTom the junction of 

the Beecher with the Boston on the west, 

eastward along ·the united vein for 75 

to 80 feet. Here there is a large body 

of ore, consisting of a gangue of jaspery 

white and dark quartz carrying large 

quantities of sulphides. 

INFLUENCE OF DEPTH .. 

The principal ore shoot of the Boston vein was followed from the surface 

downward below the Moline tunnel, to 

a depth of more than 400 feet, but at 

the point where the same junction was 

encountered in the Kelly tunnel at a 

depth of 1,000 feet it was almost entirely unmineralized, being marked only 

by a thin silicified streak which locally 

lies upon the foot wall of the fault zone, 

Nw 

FIG. 108.-Vertical cross section of New Boston vein. Sketched from breast 

of drift in Kelly tunnel. Shows quartz streak on foot wall of cl>Ly lead 

(watercourse). a, Gneiss with pegmatite streaks; b, soft clayey gouge; 

c, quartz with specks of pyrite. 

and by local bunches of iron pyrite and cupriferous pyrite carrying a little silver and a trace 

of gold. Usually, however, there is no mineralization at all. This soft fault zone in the 

Kelly tunnel has no resemblance to the. strong, wide vein formed at the same intersection in 

the upper levels. 
296 GEOLOGY OJ!' THE GEORGETOWN QUADRANGLE, COLORADO. 

The Beecher vein, which unites with the Boston to form the ore shoot in the upper levels, 

appears stronger in the New Boston tunnel, where it extends as a hard definite vein for some 

distance away from the junction, than it does ·more than 200 feet deeper in the Moline tunnel, 

where, at a distance of not many,feet away from the junction, it turns into a soft uncementeci 

fault zone·. Still deeper, in the Kelly tunnel, the Beecher bra~ch is represented only by a 

mere slip. 

This noted diminution in depth of strength of the ore shoot and of the veins which unite 

to form the ore shoot suggests the conclusion that the increasing barrenness is necessarily 

due to increasing distance from surface influences and that the veins have been formed by 

descending surface waters. Although this conclusion is probably in the main true, it is not 

unequivocally demonstrated by the above-mentioned occurrences, for when fellowed laterally 

. the veins which in the region of the principal shoot in the upper levels are strong and well mineralized, become soft and unmineralized, just as they do when followed downward. Certainly 

the mineralized region here has been limited laterally as well as in depth, and the ore deposition 

has been due to waters circulating or uniting along the junction named and not mineralizing 

other porLions of the vein channel. 

Another circumstance, however, which bears on this problem is that where the soft decomposed fault zones carry a small streak of gua.rtz containing specks of sulphides, this streak 

almost invariably runs on the foot wall of the zone, as if deposited by waters which sink through 

the porOUt? material and flowed along the foot (fig. 108). 

MUSCOVITE VEIN. 

The Muscovite vein has not been traced very far on the surface. Underground it has been 

qpened up in different places by tunnel workings, of which the most extensive has a drift following 

'-----~'o--~~oo feet 

0 Gneiss Pegmatite Mineralized lead 

the lode for 300 feet (fig. 109). The wall 

rocks of the lode are mixed gneiss and 

porphyritic granite. The marketable 

ores consist of pure galena containing 

some pyrite and approximately 12 ounces 

per ton of silver. There is some chalcopyrite but no tetrahedrite. Blende occurs very rarely; as a rule the ores do 

not contain any appreciable quantity of 

ZinC. 

MINERAL CHIEF VEIN. 

The Mineral Chief vein, of which the 

FIG. 109.~Geological plan of Muscovite tunnel. Smith & wesson is generally held to be 

a continuation, is opened up by tunnel drifts of considerable length. A plan of the most 

extensive of these tunnels now open to observation is shown in fig. 110. 

The vein is of irregular thickness, being strong in many pYaces but not very continuous. 

The wall rocks consist of gneiss and porphyTitic granite in about equal proportions. The 

gneiss is typically silicified near the veins; much of it has been altered so that it has the same 

green color as that noted in the walls of the Boston vein. The ores arc of exactly the same 

kind as those of the Boston and Spartan workings, the veins in which, as previously mentioned, may possibly have some connection with the .Mineral Chief. The picked ore contains 

large blocks of pure galena with some pyrite and a little chalcopyrite. It is low grade in 

silver. The characteristic feature of this vein, as well as of others in the same general district, 

is the soft gouge which occurs in bands along it. I ' 

SP.ARTAN VEIN. 

The Spartan tunnel runs in from the bottom of Silver Creek from a point directly northeast 

of the mouth of the opening on the Mineral Chief vein, of which it is very likely a continuation. 
MINES NEAR GEORGETOWN. 297 

This tunnel shows a veili. 2 or 3 feet thick, consisting of mixed silicified country rock and quartz 

with considerable clean galena containing pyrite. Blende occurs also in streaks and bunches 

and is of the same dark color as in other lodes of this group. A 6-inch streak of soft gouge 

follows the vein. The wall rock is gneiss. 

MINOR VEINS OF THE NEW BOSTON-MINERAL CHIEF REGION. 

The other lodes of this group have not produced any ore of importance. Many of them 

are strong fault zones which may be traced for some distance on the surface and are well marked 

when tapped by tunnels underground, but they are slightly or not at all mineralized. The 

Deming lode is recognizable on the surface. A little ore was taken out here to a depth of 25 

to 30 feet. This ore consisted of galena and pyrite, containing approximately 35 ounces of 

silver .and $4 to $5 in gold to the ton, although some of it went higher. This lode is cut in 

the Kelly tunnel at a distance of 700 feet from the mouth. It strikes east and west, and 

like most of the other lodes dips steeply to the south.. At the point where the tunnel intersects 

the lode a cubic yard or so of ore showed wire gold associated with galena. This was at a 

vertical depth of about 300 feet. This vein forks just west of the tunnel on this level. The 

south branch is called the Egyptian lode and is silicified for a width of about 1 ·foot. The 

lode from its forks back to the tunnel is strong and is marked by quartz, galena, blendc, and 

pyrite containing silver and a little gold. The wall rock is of different varieties, including 

hornblendite, black biotite gneiss, pegmatite, and granite. This lode, wherever developed 

on this level, is not soft, but shows 3 or 4 feet of hard silicified material and quartz. 

; The Washington lode, shown on the plan of the Kelly tunnel, is also said to be recogniza.ble 

on the surface, where, however, it yielded no ore of importance. Very small quantities of ore 

found on this lode in the Kelly tunnel assayed from 10 to 30 ounces of silver, 4 to 5 per cent of 

lead, a little zinc, some 

pyrite, and ·a very little 

gold. The wall rock is 

black biotite gneiss and 

some pegmatite. 

The Great Western 

lode, shown in the Kelly 

tunnel plan 60 to 70 feet 

northwest of theW ashingtuu lode, ·is reported 

not to have been recognized on the surface. It 

follows a fault zone 

marked by a layer of 

gouge materials and 

smooth slip surfaces 

showing strim dipping 

eastward at an angle of 

I~Z2@ Gneiss wall rock 

o~------~~oo~----~z~oo~----~~ofuet 

B Mineralized lead Unmineralized fissure 

FIG. 110.-Geological plan of Mineral Chief tunnel. 

20° to 32° from the horizontal. For a distance of 20 to 30 feet along the drift on this lode 

there is a little quartz and pyrite, from 1 to 2 inches thick and resting upon the foot wall of 

the fault zone. The remainder of the lode, as exposed by the drift on this level, shows no 

quartz or mineralization save for a few specks of pyrite. 

The Tennessee lode is reported to have been recognized at the surface, but it shows there 

only soft decomposed material without mineralization. It is opened up only by a prospect 

hole. Where cut in the Kelly tunnel; at a depth of 1,200 feet or more, it exhibits the same 

characteristics, being entirely unmineralized. 
\ 

298 GEOLOGY OF THE GEOUGETOWN QUADUANGLE, COLORADO. 

SUNBURST-SCEPTRE VEIN. 

SCEPTRE JJflNE. 

LOCATION AND DEVELOPMENT. 

The buildings and tunnel mouth of the Sceptre mine are located in Silver Creek gulch on 

the east side of Republican Mountain, at an elevation of about 10,700 feet. The vein is opened 

by a single tunnel, approximately 2,300 feet in length, all on the vein. On the western portion 

of the same vein are the Sunburst mine workings. (See Pl. LIX.) 

COUNTRY HOCK. 

Gneiss and pegmatite with local patches of "corn rock" are found on both walls, except 

where the porphyry dike that extends the whole length of the vein on the southwest side comes 

into direct contact with the vein materiaJ. The gneiss, especially that toward the northwest end, 

is a highly micaceous biotitic variety. 

The porphyry dike, which has a width of 15 to 18 feet, is undoubtedly a continuation of 

the dike which holds a similar relation to the vein in the Sunburst mine. It is made up chiefly 

2_ 3 f'eet 

FIG. 111.-Cross section of S~eptre vein, showing the vein structure. a, Alaskitc porphyry; b, silicified 

alaskite porphyry; c, silicified gneiss 11ndpegmatitc; d, ore and rock breccia; e, gouge and crushed rock 

lead; j, granite. 

of dark-gray porphyry with phenocrysts of feldspar up to 0.1 inch in length, especially well 

developed toward the nwthwest end; but nearer the mouth of the tunnel it changes in places 

to an almost white rock with biotite phenocrysts up to 0.2 inch in length. The vein, which is 

considerably altered and silicified in the vicinity-of the vein, is probably an alaskite porphyry 

dike corresponding to one recognized on the surface. 

'\)ESCRIPTION OF VEIN. 

This vein has a general trend of N. 73° W. and a clip of 70° SW. 'l'hrough the entire length 

of the tunnel, about 2,300 feet at the time of visit, the porphyry dike, with the same dip as the 

vein, is on the southwest (hanging wall) side of the lode, either in direct contact with the vein 

or separated from it by masses of granite or gneiss up to 18 feet in width. 

The vein (fig. 111) is usually accompanied by a zone of dense clay or crushed rock, from 

6 inches to 4 feet wide, located just above the foot wall. Immediately above this rather impervious layer lies the ore or mixture of ore and quartz of brecciated country rock, ore, and 

quartz. In some places the quartz and ore streak or the ore breccia is entirely absent; in others 

it reaches a width of several feet. Above the ore-bearing band is usually a gray jaspery quartz 

seam from 1 inch to 4 feet wide, or a more or less silicified belt of crushed gneiss, pegmatite, 
Gneiss 

MAP OF MINE 

Astor tunnel level 

0 100 

Pegmatite 

WORKINGS AND GEOLOGICA L PLANS 

0 -~~--_:2~00:_ 400 feet 100 ___ 30~0----

200 

Alas kite porphyry 

~ 

--==~_:_----:==:~E GROUP OF MINES. OF SEPARATE LEVELS, SUN BURST-SCEPTR 

Mineralized lead 

PROFESSIONAL PA PER NO. 63 

[;3 

Unmineralized lead_ ------------
MINES NEAR GEORGETOWN: 299 

or granite, of varying width, which separates the ore from the porphyry dike. Nearly midway on the tunnel about 1 foot of crushed quart.z and white clay occur between the porphyry 

dike and the 4 to 5 feet of quartz which lie just above the clay lead. 

JIIINOR VEINS. 

About 1,300 feet from the mouth of the tunnel a crosscut runs for70 feet or so to the west 

of south, where it intersects a drift on a lead trending S. 69° W. This lead, which is of clay 

and crushed rock, is 6 inches to 2 feet wide. It clips 

75° N. and shows traces of vein quartz and in some 

places specks of ore. The wall rocks consist of gneiss 

with patches of pegmatite and "corn rock." · 

About 550 feet farther in on the tunnel another 

crosscut extends 325 feet to th~ west and south, but 

does not cut any important .leads and is not extended 

far enough to cut the vein mentioned immediately 

above. 

At 300 feet from the mouth a crosscut of 25 feet 

to the south encounters a mineralized lead trending 

N. 75° W. and dipping 70° S. This leacl~shows a 

streak of brecciated quartz and sphalerite ore from 

one-half inch to 5 inches wide. 

Some sma1l stringers of ore consisting of galena, 

sphalerite, and pyrite were found at_ the contact of 

the porphyry and the count.I'y rocks on the southwest 

side of the porphyry dike, on the contact opposite 

from that where the Sceptre vein is situated; however, 

no "pay ore" has ever been discovered, though a few 

stringers of ore were found running from the main 

vein and entering the porphyry. Fig. 112 is a vertical 

section across one of these stringers in the porphyry. 

THE ORE. 

The ore of this vein, like that of the rest of the 

group of veins in the same locality, is of low grade. 

That is, although the veins are strong and the quartz 

and ore streak where p'r·esent is comparatively broad, 

yet the best assays rarely show more than 200 or 300 

ounces of silver and the average is far below this. 

Besides the silver content, the ore carries from 3 to 5 

per cent of galena, 5 to 12 per cent of sphalerite, and 

0.08 to 0.12 ounce per ton of gold. Traces of polybasite .and gray copper occur here and there, but as 

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Siderite Lead 

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and zinc ore 

FIG. 112.-Crosssection of veinlet running into Sceptre 

vein, showing· reopening of a fissure filled with 

galena and blcnde ore and subsequent deposition of 

siderite. 

a rule the silver is carried mainly by the galena and to a less extent by the sphalerite, which is 

usually of the black variety. The ores, especially toward the southeast end, carry more or 

less pyrite, which is locally copper bearing. · 

The notable features of the ore of this mine are the characteristic siuerite gangue, the 

small percentage of galena, and the fairly constant content of 0.0~ to 0.12 ounce of gold. For 

the most part t.he ore is a breccia, in which fragments of galena, sphalerite, siderite, and country 

rock have bee11 recemented by gray jasper-like quartz. 
300 GEOLOGY OF THE GEORGETOWN QUADRANGLE, COLORADO. 

SEQUENCE OF ORE DEPOSITION. 

The sequence of events and the order of deposition of the minerals composing the ore 

breccia is probably as follows: 

1. Formation of fracture and brecciation of wall rocks . 

. 2. Cementation of angular fragments of wall rocks by quartz or by pyrite and sphalerite. 

3. Deposition of galena. 

4. Brecciation of above. 

5. Deposition of siderite: 

6. Deposition of small stringers of sphalerite-galena ore. 

7. Deposition of pyrite. 

8. Brecciation. 

9. Deposition of finely granular crystalline to coarsely chalceuonic vein quartz. 

10. Deposition of pyrite. 

11. Deposition of very fine chalcedonic quartz. 

The evidence for the foregoing conclusions is based on the following observations from 

thin microscopic sections and hand specimens and the study of ore in the vein itself: 

(1) The vein contains angular fragments of gneiss and pegmatite. 

· (2) The vein shows broad ore belts, in which black sphalerite is the matrix inclosing the 

angular fragments of wall rock and quartz. A small amount of contemporaneous pyrite accompanied the sphalerite. · 

(3) In microscopic sections galena is seen to fill interstices between the sphalerite crystals, 

to line cracks in the sphalerite, and to form a fringe about the sphalerite crystals. 

( 4 and 5) In places along the main vein, as well as in some of the branch stringers which 

here and there leave the main ore streak, siderite forms a cementing material for angular fragments of the sphalerite and galena qre of Nos. 2 and 3, and also fills reopened fissures in this 

older ore. 

Fig. 112 represents a small branch veinlet in porphyry which shows predominant blende, 

with galena and pyrite bordering the sides of the old fissures, and siderite filling the later opening in the center of this ore; or, where the galena-sphalerite ore has broken away from the 

sides, the siderite forms the matrix cementing the angular fragments of porphyry and sulphide 

ore. The pyrite in this veinlet occurs more abundantly near the borders of the other sulphides. 

In other parts of the vein angular fragments of country rocks or quartz, with a border of siderite 

surrounding them, are found inclosed in later chalcedonic quartz. 

(6) Microscopic examination revealed also some small stringers which showed contemporaneous comb quartz and toothlike crystals of siderite lining a small opening, with later galena and 

sphalerite filling the center. The second period of sulphide deposition, however, was probably 

only locally developed, and was not of much economic importance. 

(7) Pyrite crystals were noted on top of siderite crystals which lined vugs. Thin sections 

of the ore also showed pyrite along the cleavage planes of siderite and forming a coating a.bout 

siderite crystals. .This pyrite was contemporaneous with some quartz. 

(8 and 9) The brecciated fragments of the ore formed by the foregoing processes were 

inclosed in the finely granular crystalline quartz of No. 9, as seen in rock sections. 

(10) In thin sections forking stringers of pyrite were noted which cut through the quartz 

·crystal aggregates of No. 8 and the rock fragments. , 

(11) Very fine chalcedonic quartz stringers were found cutting all pf the above-described 

ores as well as including angular fragments of them. This observation was noted both in hand 

specimens and in thin microscopic sections. 

LOCATION OF ORE BODn;s. 

, Two important ore bodies were noted. The main body, about 320 feet long, was about 

1,250 feet northwest of the mouth of the tunnel and had a pitch of about 16° SE. The second 

body of ore was about 150 feet in length and carried a larger percentage of low-grade ore 

(30 ounces silver), containing considerable sphalerite. This mass of ore was encountered about 

1,700 feet from the tunnel mouth~ 
MINES NEAR GEORGETOWN. 301 

SUNBURST MINE. 

LOCATION AND DgVELOPMENT. 

The Sunburst mine (see Pl. LIX) is located at an elevation of about 11,300 feet on the 

northwest slope of Republican lVlountain, a short distance below the ridge connecting Republican and Democrat mountains. 

The vein is opened by two tunnels. The upper tunnel, which is 132 feet above the lower 

one, is about 1,000 feet in length and follows the vein all the way. The lower tunnel consists 

of a crosscut of 225 feet and a drift of about 000 feet on the vein. 

Besides the two drifts on the vein, a single exploratory crosscut about 200 feet in length 

runs a little east of south from the lower tunnel level, at a point about 450 feet from the mouth 

of the tunnel. At the time of visit two faint leads had been intersected by this crosscut, one 

with a strike of S. 72° E. and the other of S. 81 o E. Both of these leads .had a dip of 80° S . ... COUNTRY ROCK. 

The Sunburst vein runs parallel to and is found immediately in contact with or within a 

short distance of the porphyry dike that holds a similar relation to the Sceptre vein, of which the 

Sunburst is the westward . 

continuation. The rock 

forming this dike is in the 

main dark gray, and has 

a compact ground mass and 

conspicuous phenocrysts of 

feldspar ·up to 0.1 inch in 

diameter. In some places, 

however, the porphyry is 

white and has hexagonal 

plates of biotite, and elsewhere it forms a lightcolored, finely granular, 

altered mass. The dike, 

which dips 70° SW. and is 

approximately 18 feet wide, 

lies in general on the southwest side of the vein, where 

FIG. 113.-Cross section of Sunburst-Sceptre vein in stope SO feet above lower Sunburst 

tunnel showing lens or horse of porphyry within tho vein zone. · 

it is either in direct contact with the lode material or is separated from it by lenses of gneiss 

and pegmatite or altered porphyritic granite. 

The northeast wall consists principally of micaceous biotite, gneiss, and pegmatite, with 

small patches of porphyritic granite or "corn rock." Southwest of the porphyry dike the 

country rock, where exposed, was coarse pegmatite with a small amount of gneiss. 

DESCRIPTION OF VEIN. 

The vein has a general strike of N. 70° W. and a uniform dip ()f 70° to the west of south. 

A band of white clay gouge or a mixture of clay and crushed and kaolinized gneiss and 

pegmatite separates the ore or quartz streak from the northeast or foot wall of the lode, and 

silicified porphyry occurs on the hanging-wall side with a belt (horse) of silicified gneiss and 

pegmatite between it and the ore in many places. Fig. 113 illustrates a vertical section across 

the inner stope above the lower level, where a lens of porphyry about 1 foot wide comes in 

between the foot-wall "gouge" lead and a belt of crushed and kaolinized pegmatite and gneiss 

farther southwest. In places, especially near the breast of the lower tunnel, the soft white 

"gouge" lead changes to a very tough grayish putty-like clay belt. 

That the relation between the vein fracture and the porphyry is purely physical, depending on the fact that a zone of weakness exists either in the porphyry dike itself or in the rocks 

adjacent to the dike, ~s shown not only by the belt of gneiss, pegmatite, or granite, from a few 
302 'GEOLOGY OF THE GEORGETOWN QUADRANGLE, COLORADO. 

inches to 17 or 18 feet wide, which generally occurs between the vein proper and the porphyry 

contact, but also by the fact that toward the west end of the upper tunnel the vein crosses to 

the north side of the dike, whereas in the Jower tunnel the vein fracture runs along in the 

porphyry dike itself and aln1cist entirely disappears within the dike by fracturing. 

Except where the vein runs through porphyry, the clay and crushed rock lead next to 

the foot wall is continuous throughout the length of the vein, but ranges from a few inches to 

4 or 5 feet in width. Between this clay streak and the hanging wall, in the barren sections, 

.occur small irregular lenses or-,discontinuous strips of dark-gray quartz. As ore-bearing areas 

are-approached this quartz strip becomes continuous andmore and more speckled with metallic 

minerals, grading into an ore shoot in which the quartz and ore streak may reach a width as 

great as 4 feet. · · 

NATURE OF ORE. 

The ore streak, although broad, rarely yields what would be called high-grade ore in the 

Georgetown district. Assays have shown the values in silver to run as high as 300 ounces, 

but these are exceptional. . ?'he mill runs probably average about 100 ounces of silver, 4 to 5 

per cent of lead, and 8 to HJper cent of zinc, with a small but rather constant content of gold 

(0.08 to 0.1 ounce). The' ~re for the inost part is a breccia, in which angular fragments of 

galena, sphalerite, siderit~, and'country rock have been recemented by a later gray jasper-like 

quartz. , 

The sequence of the depo~ition of the various constituents ·of the ore is similar to that of 

the ores in the Sceptre mine~ (See p. 300.) The Sunburst ores differ from the Sceptre orcs in 

having less pyrite ·and cupriferous pyrite. · 

Small masses of semioxidized ores are found in places down to a depth of 150 feet from 

·the surface. They usually consist of rather porous masses of poorly formed crystals of black 

dphalerite and galena, covered by a reddish or brownish coating of oxidized minerals, probably for the most part iron and manganese oxides derived from the oxidation of siderite, 

rhodochrosite, and pyrite. The oxidized and semioxidized ores (locally called ''sulphurets ") 

are said to carry values but slightly higher than the unoxidized ores. 

Native silver in the form of "wire silver" was found intimately mixed with sphalerite in 

the innermost stope abov:e the lower level, at a distance of 200 feet .from the surface, along a 

postmineral plane of movement. Small quantities of tetrahedrite (gray copper) and poly-

. basite occur here and there iri the quartz and ore streak. 

The characteristic feature of the Sunburst-Sceptre ores is the abundance of ferruginous 

rhodochrosite occurring as a ga;ngue mineral. It is found in considerable quantity even in the 

highest grades of ore. The ore in the upper parts of the shoots was usually of a little higher 

grade than that at greater depths. LOCATION OF ORE DODIES. 

The ore appears to occur in two distinct ore bodies. The first one, which is encountered 

about 125 feet from the mouth of the upper tunnel, has a width of about 200 feet and extends 

from the surface to the lower level, with a vertical to steeply inclined westerly pitch. The 

second ore body, which lies about 490 feet southeast of the mouth of the upper tunnel, is 275 

feet wide and extends from a point 115 feet above the upper level to the lowest level also. 

This ore body pitches toward the northwest. 

QUEEN CITY LODE. 

LOCATION AND DEVF>LOPMENT. 

The Queen City mine is opened by a single tunnel, the mouth of which is located in the 

valley of Silver Creek, at an elevation of about 10,700 feet and at a distance of 250 to 300 feet 

soutp. of the mouth of Sceptre tunnel.' The tunnel has followed the vein for 525 feet. 

COUNTRY ROCK. 

The country rock consists almost entirely of micaceous biotite gneiss, with seams or patches 

of pegmatite. 
MINES NEAR GEORGETOWN. 303 

DESCRIPTION. OF VEIN. 

The vein strikes N. 65° W., being thus nearly parallel to the Sceptre, and dips about 75° NW. 

1t consists of a single crushed rock and clay lead from the mouth to a point about 380 feet in, 

where this lead is replaced by a narrow quartz streak that gradually increases in width up to 2 

feet at a point 480 feet from the mouth, where a branch vein enters from the northwest (striking 

N. 75° W.). This quartz streak carries some specks of galena and sphalerite, .although up to 

the time of visit it had yielded no pay ore. Its existence is undoubtedly due to the union of 

the branch vein with the main vein, for it narrows up again beyond the union. 

ASTOH MINE. 

The Astor mine is located at the head of Silver Creek Valley, near the summit of the ridge 

extending between Republican and Democrat'mountains, at an elevation of nearly 11,500 

feet above sea level. This mine, which was worked extensively previous to 1868, formerly had 

more than 2,500 feet of underground workings,/ but at present access can be had to practically 

none of these on account of excessive caving. The total yield of this mine up to 1880, according to Fossett," was between $60,000 and $100,000, and was due to numerous small shipments 

of comparatively rich ore. Most of tllis ore was obtained near the surface and from a stope 

about 150 feet in length, whlch was reached by a crosscut tunnel 450 feet long and a drift 

extending 75 feet along the vein. . · 

The W. B. Astor vein, whlch runs approximately parallel to the Sunburst-Sceptre vein, 

is a rather broad, soft vein, striking N. 75° vV. The mine workings on the west side of the 

ridge arc partly on the Astor vein and partly on the Wolverine vein, both of which are supposed to have been encountered also in the workings of the Sunburst mine. The Astor vein 

is considered to have been cut in the Sceptre workings, too, but its identity could not be 

established. · 

The ore, which consisted chlefly of galena, zinc blencle, . and traces of pyrite associated 

with a quartz-siderite gangue, is said to have averaged between 75 and 300 ounces of silver to 

the ton, but assays as high as 1,200 to 1,600 ounces of silver per ton have been obtained from 

picked specimens. -

DEMOCRAT MOUNTAIN GROUP OF VEINS. 

GENERAL -DESCRIPTION. 

The so-called Democrat Mountain group of veins (Pl. LX) occupies an are·a of irregular 

extent, a few thousand feet in diameter, situated on the very summit of Democrat Mountain, 

and extends from the ridge of the mountain a short distance clown the slope both to the east 

and to the west. When the veins of the group are plotted together, there is a noticeable general 

east-west tre-nd, more or less parallel with that of the Republican Mountain zone of veins. To 

speak more accurately, however, the veins represent two distinct systems-a northeast system 

and a northwest system. 

There was considerable activity in this district about thirty years ago, and it is estimated _ 

- that approximately $2,000,000 worth of ore was produced, but for many years past the mining 

operations have been of an unimportant character. 

NATURE OF VEDfS. 

The relations of different veins to this group are unusual as compared with those of neigh.,-

boring districts in that the veins can be referred to two distinct systems striking in different 

directions and running into or crossing one another. 

One set of veins has a northwest strike and is represented chiefly by the Emllla-Galie, Polar 

Star-Junction, and Fletcher-Silver Cloud veins. Another set has a northeast strike and is 

represented chiefly by the Rogers and Cliff veins. There are also minor veins belonging to these 

systems. Veins having· strikes transverse to one another may run into each other without 

a Fossett, Frank, Colorado, 1880, p. 387. 
304 GEOLOGY OF THE GEORGETOWN QUADRANGLE, COLORADO. 

crossing or they may cross, but where they cross, so far as noted, there has been no appreciable 

displacement of one vein by another. Some of the relatively weaker lodes have an irregular 

and curving trend, as if wavering between the two principal strikes. This is characteristicto take the best examples-of the Nyanza and Silver Glance lodes, whose course is curving and 

in part oblique between the trends of the two systems, but which on their east ends conform 

entirely in strike to the northwest system and on their west ends curve around so as to be parallel with the northeast system. (See :fig. 114.) 

From these different relations it is probable that one set of vein fractures is not later than 

the other, but that both were formed contemporaneously and were the results of a single stress. 

The character of the vein filling is similar in both systems, indicating a contemporaneous mineralization of the vein zones. 

The main lodes are usually well-defined zones of crushed and altered rock of varying 

thickness. These zones are locally cemented with vein material, including quartz and sulphides. Except where these ore bodies occur, the lodes are in general soft and barren looking, 

N 

t 

T 

FIG.ll4.-Horizontal sketch plan of developed veins (so far as known) iri Democrat Mountain group. At the west end of 

the developed portion of the Queen vein is the only porphyry dike noted in the district. The wall rocks are principally 

mixed gneiss and granite. 

hardly resembling mineral veins at all-a particular in which they resemble the veins of the 

Republican Mountain group. 

In the Buckeye mine evidence of movement subsequent to mineralization was noted. In . 

one place the vein :filling consists of angular fragments of ore embedded in a matrix of crushed 

country rock, which has been indurated by a silica cement filling the interstices. This recalls 

a similar phenomenon in the Mendota lode, ncar Silver Plume. (See p. 229.) 

NATURE OF WALL ROCKS. 

The principal wall rock of the Democrat Mountain group of veins is biotite gneiss, with 

considerable granite, much of which is porphyritic, and some pegmatite and diorite. & single 

porphyry dike was noted in the Queen tunnel (fig. 114). 'fhis dike is about 30 feet wide and 

entirely decomposed. It has a strike at right angles to that of the Queen of the vVest vein, 

which runs into it in the tunnel workings, and parallel with that of the veins of the northeast 

system, such as the Rogers and Cliff. However, so far as noted, it has no connection with any 

vein of importance. 
U. S. GEOLOGICAL SURVEY 

MArrHEWS 

LITTL£ EMUA V£/N) 

BONANZA 

POLAR .Sr~lf 

ELDRIDGE 

LEYEL' 

MAP (INCOMPLETE) SHOWING MINE WORKINGS OF DEMOCRAT MOUNTAIN DISTRICT. 

IOROFESSIONAL PAPER NO. 63 PL. LX 

• = DISCOVERY 
MINES NEAR GEORGETOWN. ' 305 

ALTERATION OF WALL ROCKS. 

The rocks adjacent to the veins of this group are commonly altered in the usual way characteristic of wall rocks in this general region. A specimen of the porphyry dike in the Queen 

tunnel above mentioned, when examined microscopically, proved to be entirely altered to :fine 

sericite and quartz, with a little iron oxide. 

• 

INFLUENCE OF WALL ROCK ON VEINS. 

The favorite wall rock of the miners in this district is the porphyritic granite, which is frequently called "corn rock." The reasori appears to be, here as elsewhere, that this rock has 

much greater rigidity than the biotite gneiss and has lent itself better to the formation of strong 

and persistent fractures. It \vas noted in several places that lodes on passing from granite into 

gneiss have a tendency to split. and scatter, passing between the laminre. 

COMPOSiTION OF VEINS. 

The chief gangue material is quartz, which is found in two varieties-one having a typical 

crystalline texture and the other having the dense, :fine texture of chert. Specimens from the 

Rogers vein in the Edgar tunnel and from the Jordan vein in the Bonanza tunnel show fragments of strained crystalline quartz, cemented by chert containing fine crystalline siderite and 

areas of sericite. The chief sulphide is galena. More or less blende is present, especially on the 

eastern slope of the mountain-as, for example, in the Cliff workings. · This blende is typically 

dark colored, like that in the mines of Republican Mountain and in the Griffith mine. Scarcely 

any iron pyrites is present, but there is locally some chalcopyrite. In the Cliff mine brown 

carbonates, probably of iron or maganese or both, were noted. 

The most characteristic ores of this district have been high-grade silver ores. They are. 

distinct from those of the Republican Mountain mines in that they are high in silver and contain no gold and less lead, iron, and zinc. The best ore has run up to several hundred ounces 

in silver per ton, and the amount of low-grade ore is not very great. Native silver has been 

frequently found in these mines .. 

PARAGENESIS OF VEIN MATERIALS. 

It was noted that, in a veinlet containing quartz and galena which was observed in the 

Rogers vein zone in. the Edgar tunnel workings the two minerals were of distinctly different 

ages, the quarb:: being first, coating the walls of the fissure, and the galena being later, filling 

the remaining space. In the Cliff mine it was noted that in some places brown carbonates 

(probably carbonates of iron or manganese or both) had the same age relation to the sulphides 

as the quartz just mentioned, constituting the first crust which formed upon the walls of the 

vein cavity. · 

As already noted, specimens from the Rogers and Jordan veins indicate a crushing of the 

original crystalline quartz and a cementation of the breccia thus formed by :fine chert. Similarly it was noted in the Buckeye mine that the sulphides of the original deposition had been 

brecciated and the interstices between the fragments of ore filled with crushed country rock, 

which in turn was cemented by fme silica. 

LOCATION OF ORE BODIES. 

The ore bodies encountered in this group of veins have been relatively small in proportion 

to the amount of exploration that has been done, but as a rule have been of high grade. Apart 

from these ore bodies the veins are in many places very little or not at all mineralized, or even 

not cemented by silica. 

Most of the old workings are at present inaccessible. According to reliable reports, however, the chief ore occurred in the vicinity of the principal crossing or intersection of the main 

lodes in the region, where the powerful Rogers vein runs into the Polar Star and Emma veins, 

31401-No. 63-08 -20 

• 
306 GEOLOGY OF THE GEORGETOWN QUADHANGLE, COLORADO. 

the Rogers belonging to the northeast system, and the other two to the northwest system. It 

is reported that around this junction, within a horizontal area having a diameter of several 

hundred feet, a million dollars' worth of ore was extracted. In this region nearly all the intersecting lodes had ore shoots, of which that of the Rogers seems to have persisted farther away 

(southeast) from the junction than any of the others. . It is reported, however, that at the 

exact crossing of the lodes as a rule there was little ore, only leads extending to ore shoots at a 

little distance being present. This productive area is situated on the very top of the Democrat 

Mountain ridge, in a little saddle. It is reported that the ore shoots here dip both ways, according to the slope, the pitch on the eastern or Clear Creflk slope being to the east, and that on the 

western or Bard Creek slope being to the west, thus saddling the ridge. 

Outside of this principal field of mineralization, lesser pockets of ore were found on the 

Jordan, Silver Glance, Silver Cloud, Queen of the West, Cliff, Buckeye, and other veins. In 

some of these places the intersection of two veins has evidently had a stimulating effect on ore 

deposition. For example, in ~he Buckeye the main vein trends N. 85° W. and is intersected by 

a branch striking N. 55° W. From the junction of this branch, which is practically unmineralized for 40 feet along the united vein, high-grade ore has been taken out and low-grade ore 

extends for 80 feet farther. At another junction, however, there is no increased mineralization whatever. 

The ore bodies which have been worked out in this region are characteristically shallow, 

most of the ore having been taken out within 200 feet of the surface. Locally the ore has been 

found to a depth of 400 feet or more, but as a rule the lower workings, including the long 

tunnels, have encountered mainly barren and soft unmineralized lodes beneath the wellmineralized superficial portions. 

ORIGIN OF ORES. 

The proximity of the known shoots to the surface and especially the conformation of their 

pitch with the surface slopes, together with the characteristically almost unmineralized nature 

of the same lodes at the depth of a few hundred feet, suggest that the ores were deposited by 

waters which descended from the surface. 

DESCRIPTIONS OJ<'. SEPARATE VEINS. 

The principal lodes of the northwest system are the Emma-Galie and Polar Star-Junction 

on the north side of the district, and the Silver Gh1nce and Silver Cloud-Fletcher on the south 

side. Minor veins, which, from the best information known to the writers, seem to form with 

the Silver Glance lode a northwestward-trending zone of imbricating veins, include the Queen 

of the West, Konigsburg, Edgar, Nyanza, and Buckeye. 

The Emma-Galie lode probably crosses .the Rogers lode, the portion on the east side of 

the Rogers being known as the Galie and that on the west side as the Emma. The workings 

are not now accessible, so that this conclusion could not be actually proved. It is reported 

that the chief ore shoot on the Emma occurred where a minor diagonal lode styled the Little 

Emma intersected the main vein and ran with it for some little distance before crossing and 

leaving it on the opposite side. The ore in this shoot was of high grade. The Galie vein, on 

the east side of the intersection, is reported to have contained a body of relatively low-grade 

ore near the field of intersection. 

The Polar Star lode is probably the same as the Junction, the former lying on the west 

side of .the intersection with the Rogers and the latter on the east. All these workings, however, are now inaccessible, so that only the probability can be stated. The Polar Star-Junction, 

when considered as one, has been developed by underground workings along its strike extent 

for about 2,200 feet. The ore on this lode was taken chiefly from the superficial portions. 

The Bonanza tunnel, which has a long drift for about 1,400 feet on the vein, shows a little ore 

in it, the lode being soft, semisilicified barren material 3 or 4 feet wide and underlying the 

zone where the rich pockets were present. In the workings of this tunnel the Polar Star divides 
MINES NEAR GEORGETOWN. 307 

into two branches opening out to the west, the more northern retaining the name Polar Star 

and the southern, which appears to be the stronger, being called the Jordan. A small body 

of ore was found on the Jordan branch not far from the junction. A small lode cut by the 

Bonanza tunnel about 80 feet south of the Jordan is parallel to it, and is probably either a 

branch or an auxiliary vein. 

· The Edgar lode as opened up in the Edgar tunnel runs transversely across from the Rogers 

lode on the northeast to a minor lode parallel with the Rogers on the southwest. It shows no 

ore of importance. 

The Nyanza tunnel is on the same level with the Edgar tunnel and connects with it, the 

former starting from the west slope of the hill and the latter from the east. The Nyanza tunnel 

runs along the Nyanza lode, which is of irregular strike and is in most places of no importance. 

The Buckeye vein has a general strike of N. 85° W. and a dip of 60° to 80° S. It is opened 

up by two tunnels, of which only the upper was accessible at the time of examination. (See 

fig. 115.) The country rock is gneiss with a little pegmatite. The vein follows a fairly strong 

but rather irregular fracture which tends to break up toward the east into a number of branches. 

Tlie most important branch strikes N. 55° W. and joins the main vein 'about 275 feet from the 

EJ }····· .. ·! 

Pegmatite Mineralized lead Unmineralized lead 

FIG. 115.-Geological plan of tunnel on Buckeye vein. 

mouth of the upper tunnel. This vein is very slightly mineralized, but just beyond the junction with the main vein the. chief body of ore was formed, extending from the junction for a 

distance of about 40 feet to the east, along the main vein. 

The Queen of the West lode strikes northwestward and dips steeply to the southwest or 

is vertical. As opened up in the Queen tunnel it is fairly strong, but weakens at both ends of 

the development work. It contains a little ore. 

The Silver Glance is a somewhat curving and oblique vein, belonging on the whole distinctly to the northwest system. Its dip is flatter than that of :most of these veins and in the 

lowest level, the Queen tunnel or seventh level, was noted to be about 55° SW. This lode is 

developed by underground workings along a strike extent of more than 1,000 feet. It was 

principally exploited through the Silver Glance shaft. Six levels were run on the vein, the 

seventh level being run in from the Queen tunnel. Considerable ore in bunches has been taken 

out from this lode. 

The workings on the Fletcher vein were all inaccessible at the time of examination, on 

account of caving. On taking the general course of the dumps which were reported to be up 

the line of the Fletcher, it appears that this vein is one of the northwest system. It is stated 

that this lode has yielded three distinct pockets of ore, and has had a fairly good production. 

The dip is said to be nearly vertical. The Fletcher vein is reported to run on the northwest 

into the Silver Cloud, which has approximately the same strike and dip and is currently supposed to be practically the same vein. 

/ 
308 GEOLOGY OF THE GEORGETOWN QUADRANGLE, COLORADO. 

But little is known of the Konigsburg vein, whose workings are mostly caved shut and 

of which no maps were obtainable. It is reported to have been traced for a long distance on 

the surface, and to have yielded a little rich ore. A tunnel examined by the writers and reported 

Z+-------

-

0 

to be on the Konigsburg lode shows a 

fairly strong vertical vein with a northwesterly trend, somewhat oblique to the 

trend of the more important veins of the 

northwest system. 

The northeast system of lodes is 

repres~mted principally by the Rogers 

and Cliff veins, which are parallel and 

nearly 1,000 feet apart. Both are strong 

lodes, the Rogers having been traced by 

underground workings for about 2,500 

feet and the Cliff fo:r about 1,500 feet. 

Minor veins of the same system include 

the White Metal, Premium, La Plata, 

and the northeastward-striking vein zone 

in the Edgar tunnel workings. 

The Rogers vein, perhaps the strongest and most mineralized lode of the. 

district, probably crosses the Polar StarJunction and Emma-Galie veins, its continuation beyond the intersection being 

known as the Polar Star Extension. At 

the time of examination the Rogers vein 

was visible in only three places-in the 

Edgar, Bonanza, and White Pine tunnels. In these localities it appears as 

a broad crushed zone in granite and 

gneiss, more or less silicified and containing sulphides. 

The White Pine lode is intersected 

in the Bonanza tunnel and also in the 

White Pine tunnel. Like the Rogers, it 

it> nearly vertical and has a northeast 

trend. Its relations to the Rogers indicate that it is very likely a branch of 

this lode, although the connection is not 

known to the writers. This vein contains, in a silicified gangue, galena and 

blende; the best orcs are reported to 

carry chalcopyrite and gray copper. It 

has not been well developed. 

The Cliff mine (fig. 116) was actively 

worked as early as 1857. Up to the 

end of 1883 the production, as estimated 

by Burchard,a was about $60,000. Kimball b gives the yield of the mine for 1887 as 

$1,939.13 and for 1888 as $2,762.80, of which $2,586 represented the silver and $176.80 

the gold content. The vein is developed by four tunnels, aggregating about 2,500 feet in 

a Burchard, H. C., Precious metals in the United States in 1883. . 

b Kimball, J. P., Production of gold and silver in the United States, 1887, p. 159; idem, 1888, p. 100. · 
MINES NEAR GEORGETOWN. ·309 

length, of which all except the lowest, or fourth level, were caved shut at the time of visit. 

The vein is a strong, soft gouge in gneiss and pegmatite, with some granite. A number of 

small branches which leave the main vein appear to have influenced ore deposition favorably, as the vein is somewhat better mineralized' immediately beyond the junctions of the more 

prominent branches than elsewhere (fig. 117). The ore is made up of galena and dark-colored 

blende, the gangue of quartz and brown carbonates; pyrite and chalcopyrite are lacking. 

The Premium lode, which is developed in the White Pine tunnel, is a minor vein parallel 

to the Rogers and lying in an intermediate position between the White Metal and the La Plata 

veins, which are also parallel. Of these minor northeast veins some are probably continuous 

with the Nyanza or with the minor northeast leads that are ·encountered in the outer portion 

of the Nyanza tunnel, lying .between the Nyanza and the Rogers lodes. As a rule they are soft 

lodes, containing only a little ore. 

1-----:1 ! ......... ,-::>·} Mineralized lead Unmir:eralized lead 

FIG. 117.-Geological plan of fourth level, Cliff mine. 

MINES OF LINCOLN MOUNTAIN WITHIN THE GEORGETOWN QUADRANGLE.a 

RAMSDALE TUNNEL. 

LOCATION. 

The Ramsdale tunnel is located at an elevation of a little over 9,400 feet on the south slope 

of Lincoln Mountain, about a mile west and a little north of the summit of the pass on the road 

between Georgetown and Empire., The tunnel, which runs a little west of north for about 400 

feet, cuts the Ramsdale vein at a point about 340 feet from its mouth. 

NATURE OF WALL ROCKS. 

The wall rocks consist of granite and porphyries. The porphyries are ·probably of different 

kinds, which, though highly decomposed, may be described as alaskite porphyry and quartz 

monzonite porphyry. The alaskite porphyry was found in a 15-foot dike which crosses the 

tunnel about 65 feet from its mouth, with a strike of N. 60° E. and a dip of 65° NW. At a 

distance of about 230 feet from the mouth of the tunnel a dike of very much altered porphyry 

30 feet wide, probably belonging to the quartz monzonite class, crosses the tunnel in aN. 88° E. 

direCtion. This dike had a quartz streak carrying some pyrite and traces of sphalerite along 

the north side, at the contact with the granite. The clip of the vein and the porphyry dike is 

70° N. There had not been sufficient development work at the time of visit to show the relations between the two dikes, although they appeared to be converging toward the east. 

a For description of the mines of Lincoln Mountain north of the Georgetown quadrangle see pp. 408-410 . 

• 
310 GEOLOGY OF THE GEORGETOWN QUADRANGLE, COLORADO. 

NATURE OF VEIN. 

The Ramsclale vein was encountered in the tunnel about 40 feet north of the 30-foot porphyry dike, at which point both walls consisted of granite. (See fig. 118.) The vein, which 

has a general strike of S. 80° E. and a clip of about 70° N., varies considerably in width. Where 

cut by the tunnel it consists of a quartz streak 2 feet wide, carrying some pyrite and a little 

zinc blencle, but a short distance farther east it is 5 to 10 feet wide and is composed (a) of a 

o.__ ___ _:..so~ __ _:..;•oo feet 

l~i B 

Mineralized Unmineralized lead , lead 

~~\~(~~j -

-

breccia of light-gray quartz and granite, cemented by 

a clark-gray to black quartz carrying specks of pyrite 

and small masses of galena and cupriferous pyrite ore, 

or (b) of a fractured zone of granite almost entirely replaced by quartz and containing disseminated specks of 

pyrite. Some of the vein filling shows movement subsequent to the ore deposition, by which the ore was 

brecciated and mixed with crushed and pulverized 

country rock. This crushed mass, however, was subsequently recementecl by silica carrying pyrite. Slickensides were noticed along the north wall of the 1-cacl, 

which showed movement strire dipping 40° E. · 

The best ore consists of a banded mixture of galena 

and copper-bearing pyrite associated with a little zinc 

blende. The blende is usually black in color, but some 

resin blende is found. Quartz is practically the only 

gangue mineral, although a little siderite is locally present. 

Black, sooty sulphides are here and there associated 

with the galena. The miners hold that the higher the 

copper content in the ore the greaterthe value. Assays 

as high as 15 to 25 ounces in gold to the ton are reported by some of the leasers, and some of the ore is 

said to carry 8 to 11 per cent of lead and 10 per cent of 

copper, with a maximum of 10 to 15 per cent of zinc. 

An assay of a sample of ore from this mine made for the 

writers by R. H. Off1cer & Co., of Salt Lake City, Utah, 

showed 3.53 ounces of gold and 12 ounces of silver to 

the ton. 

From ,25 to 30 feet north of the Ramsdale vein and 

running parallel to it is a vein of crushed granite, clay, 

and quartz carrying a little pyrite and dipping 45° N. 

VIRGINIA CITY MfNE. 

LOCATION AND DEVELOPMENT. 

The shaft on the Virginia City lode is located just 

south of the crest of the ridge running east from Lincoln 

Mountain, at an elevation of about 10,300 feet. No 

work had been done on the vein for some time previous to the visit and the shaft was filled 

with water up to a point about 80 feet below the collar. 

rn 

Granite /daskite porphyry Quartz monzonite 

porphyry ----

FIG. 118.-Geological plan of tunnel of Ramsdale rnineA short tunnel about 50 feet below the collar connects with a drift which extends for a 

short distance on either side of the shaft. At the time of visit a long tunnel had just been 

started for the purpose of connecting with the lode at a point in the shaft below the lowest 

level. 
MINES NEAR IDAHO SPRINGS. 311 

PRODUCTION. 

The exact production of this mine could not be learned, but according· to Mr. Frank A. 

Maxwell, of Georgctown,a there are still in existence mill-run sheets which aggregate $20,000. 

COUNTRY ROCK. 

The wall rocks are chiefly granitic gneiss and pegmatite. The gneiss is somewhat decomposed, and some of the biotite gneiss at a considerable distance from the vein shows small, 

irregular, isolated bunches of pyrite, carrying no values so far as known. 

DESCLUPTION OF VEIN. 

Above the sulphide zone, which on this hill came in at a depth of about 25 feet below the surface, the vein filling was chiefly a gold-bearing honeycombed quartz which had evidently resulted 

from the dissolving of the sulphide -«onstituents from the quartzose vein filling. On the 50-foot 

level the vein mttterial consists of a breccia of quartz and ore (galena, sphalerite, and pyrite) 

embedded in a matrix of crushed pegmatite, gneiss,· and granite; this matrix, with the inclosed 

fragments, has also been crushed and fractured and again recemented by silica. A few very 

small stringers of still younger galena and sphalerite ore were observed running through the whole 

vein mass. Small stringers of pyrite, up to three-eighths of an inch wide, enter the gneiss of the 

hanging wall here and there. These carry very low values. 

The vein and stope on the 50-foot level strike about S. 45° W. and dip from 70° to 80° SE. 

On the surface, however, the trend of the vein outcrop appears to be approximately S. 31° W. 

CHARACTEH OF ORE. 

On the surface and in many places to a depth of 25 feet the values were mainly in free gold, 

which occurred either in fractures in the honeycombed quartz of the oxidized zone or within the 

quartz itself. The sulphide ores were chiefly silver bearing, some of the ores running as high as 

300 ounces in silver and 0.2 to 0.5 ounce of gold. Some mill runs contained as high as 66 per 

cent of lead, but others showed practically no lead whatever, probably as a result of sorting 

the ores into a lead class and a siliceous class. Zinc and copper were not present in sufficient 

quantity to receive mention in the mill-run sheets. 

MINES NEAR IDAHO SPRINGS. 

PLACER DEPOSITS. 

HISTORY. 

The first discqvery of pay gold in Qlear Creek County, and one of the first in Colorado, was 

made by George Jackson, about April 1, 1859,b on Chicago Creek, just above its junction with 

Clear Creek. This is considered by many to have also been the first discovery of gold in Colorado, 

but, according to Hollister,6 B. F. Langley had about the end of the preceding January found 

placer deposits in a gulch on South Boulder Creek. These deposits, which were known as 

"Deadwood diggings," had by the end of March produced considerable gold and were affording 

employment to a number of men. Small amounts of gold had also previously been encountered 

near Golden and on Ralston Creek, a small tributary of Clear Creek .. On South Clear Creek 

placer mining was prosecuted from Grass Valley Bar, 3 miles below the mouth of Chicago Creek, 

to the forks of the south branch, about 9 miles above this same point. By the end of May, 

1859, there were nearly 300 men employed at" Jackson's diggings" and on the various claims for 

several miles up and down Clear Creek. (See Pl. LXI.) ----------------------------------------------- a Oral coiDinunication. b The mines of Colorado, 18G7, p. 60. 
312 GEOLOGY OF THE GEORGETOWN QUADRANGLE, COLORADO. 

Shortly after Jackson's discovery complications began to arise over the question of ownership, owing to the nonexistence of mining laws. Finally, on May 9, 1859, the following laws 

were adopted at a miners' meeting at Jackson's diggings: 

l. Each claim shall be 50 fc:ct front by 200 feet dccp.a 

2. Every claim shall be marked and staked with at least two stakes, and shall be improved within ten days after 

taking. 

3. The discoverers of new diggings shall be entitled to one extra claim each. 

Owing to the small extent of the claims located under this law, extensive mining operations 

were impossible and as a result many of the deposits which if worked on a large scale would have 

paid proved unprofitable. Some of' the deposits, moreover, such as those. on Spanish Bar, 

offered serious difliculties, owing to the large size of the bowlders and the depth and roughness of 

the beds of the narrow streams. Accordingly, many of the unsuccessful miners were diverted to 

the newfields resulting from the discovery of the gold-bearing "Gregory lode" on May 6, 1859, by 

John H. Gregory. Here, in the vicinity of what is now Central City, the rich surface ores of the 

veins were sluiced for a year or two, with comparative ease, as contrasted with the obstacles 

encountered at Spanish Bar and some of thc.other placer deposits along Clear Creek. 

The Empire mines, which were located in 1860 and were the scenes of great excitement for 

three or four years, while the· surface ores were being sluiced, also attracted many of the Clear 

Creek miners. 

In spite of the fact that some forsook the Clear Creek placer diggings, these deposits yielded 

good returns to a number of the workers for a great many years. In fact, the yield of the precious metals in Colorado from 1859 to 1864 was almost entirely from gold derived from these 

placers and from the sluicing of the decomposed surface ores of the veins, for the silver mines 

were not operated until 1865 and the unaltered gold-bearing sulphides could not be treated with 

profit. The placers were most profitably worked between the years 1859 and 1863. In 1883 the 

most extensive placer-mining operations in the county were being carried on in the vicinity of 

Alice, which is located on Silver Creek, a branch of Fall River. · 

Placer mining remained an important industry in Clear Creek County and added materially 

to the gold production up to the early eighties. Since then, however, although considerable 

work has been carried on at various times, it has met with variable success and as a rule has not 

been highly profitable, though it has yielded good wages to a few miners and has added slightly 

to the gold output of the county. . 

OCCURRENCE AND NATURE OF THE DEPOSITS. 

The placer-gold deposits of Clear Creek County may be classed as ( 1) river-bar placers, (2) 

stream placers, and (3) bench placers. The river-bar deposits are those on gravel flats in or 

adjacent to the beds of the streams. The stream deposits consist of b'owlders, gravel, and sand 

filling the channels and forming the beds of the streams. The bench placers are ancient stream 

deposits, which arc now represented by remnants of old stream terraces located at a considerable 

elevation above the level of the present watercourses (Pl. LXII, B). There are three conspicuous lines of terraces. The lowest one of these lines occurs about 25 feet above the present stream 

beds. Though not of glacial origin, the deposit consists largely of glacial material which has been 

worked over and transported to its present position by streams which were developed subsequent 

to the period of glaciation. The second gravel terrace, which according to Mr. Ball is possibly 

preglacial in origin, lies 55 feet above the creek beds. The terraces of the third set are 180 feet 

above the channels of the present streams and are covered with indurated riv{)r gravels of late 

Tertiary or early Pleistocene age. 

'Ihe principal deposits along Clear Creek occur in the vicinity of Idaho Springs and extend 

from the junction of Fall River and Clear Creek to the Forks, a point several miles below the town, 

where North Clear Creek, which flows through Blackhawk, joins the main.stream. 

The exact site where Jackson first found gold is said to be beneath one of the large willow 

trees along the road on the north side of Chicago Creek, about halfway between the Jackson 

a The size of a plac\'r claim at present is limited to 20 acres lor each individual. 
U. S. GEOLOGICAL SURVEY PROFESSIONAL PAPER NO. 63 PL. LXI 

.. 

A. OLD PLACER DIGGINGS ON CHICAGO CREEK NEAR JUNCTION WITH CLEAR CREEK; 

ALSO WALTHAM MINE AND MILL. 

Looking northwest from s lope of Fl irtation Peak. 

B. OLD PLACER DIGGINGS NEAR POINT WHERE JACKSON DISCOVERED GOLD IN 1859 

(IN FOREGROUND), AND MILLS NEAR JUNCTION OF CHICAGO AND CLEAR CREEKS. 

Looking down va ll ey of Chicago Creek. 
U. S. GEOLOGICAL SURVEY PROFESSIONAL PAPER NO. 63 PL. LXII 

.11. IDAHO SPRINGS AND SOME OF THE MINES AND MILLS IN THE IMMEDIATE 

VICINITY. 

Looking west toward junction of Clear and Chicago creeks. 1, Blue Ribbon Spring tunnel; 2, Waltham 

mine and mill; 3• Jackson mill; 4• Hud son mill; 5· Newton mills; 7• Bertha mill; 8, Alpine mill; 

g, Big Fi ve power house; 10, Golden L ink shaft, Stanley mine; 11, Cardigan mine; 12, En g land 

mine; 13, Bullion King mine; 14, 15, bench gravels. 

B . OLD PLACER DIGGINGS IN BENCH GRAVELS SOUTHEAST OF THE JUNCTION OF 

SODA AND CLEAR CREEKS, IDAHO SPRINGS. 

Looking south·southwest from mountain slope above Newhouse tunnel. 
U. S. GEOLOGICAL SURVEY PROFESSIONAL PAPER NO. 63 PL. LXII I 

.11.. LAMARTINE MINE SHAFT, LOOKING NORTH-NORTHEAST. 

B. V ILLAG E OF LAMARTINE AND LAMARTINE MINE WORKINGS, LOOKING EAST. 
MINES NEAR IDAHO SPRINGS. 313 

and Waltham concentrating mills. This site is now included in what is known as the Edwards 

placer. (See Pl. LXI, A.) The "Jackson diggings" consisted of coarse gravel and sand, 

occurring· as bars or flats in or adjacent to the streams, and of deposits forming the stream bed 

itself. The low benches which occur along the sides and in places on the top of the low ridge 

or spur of land that separates Chicago Creek from Clear Creek just above their union were 

also worked extensively. Much of this material was sluiced at a very high profit in the early 

days. The deposits of Spanish Bar, which was the name given to the portion of Clear Creek 

extending from the junction of Fall River and Clear Creek to a point below the Stanley mine, 

consisted mainly of a mass of bowlders of all sizes, gravel, and sand filling the channel of the 

stream which here runs in a comparatively narrow valley. According to Hollister,a the creek 

at this point' "was very rapid, the valley narrow and rough, the bars were masses of bowlders 

of all sizes, the gold very deep and below the bed of the creek, and the pits were constantly 

flooded with water." On account of the difficulty in handling the large bowlders, the depth, 

and the troublesome waters, but little of the bed rock was ever uncovered along Spanish Bar, 

and for this reason also these deposits were less remunerative. 

The uppermost old river terrace is to be seen on the south side of Clear Creek and extends 

from the eastern edge of the area included in the Georgetown quadrangle along the creek to 

the western edge of the town of Idaho Springs. This terrace is cut through by both Soda and 

Chicago creeks and is beveled by Clear ·creek. The upper part of the scarp, due to the action 

:of the present stream, is about 100 feet above the present valley, and the upper part of the 

terrace, which has a slope toward the present stream, has an elevation of about 180 feet above 

Clear Creek. The gravels on this terrace, which are now covered by the Arthur and Doherty 

placer claims, were formerly extensively worked with very profitable results. In the southeast angle formed by the union of Soda Creek with Clear Creek this terrace is underlain by a 

bench of bed rock which in few places reaches the surface, being usually covered with a river 

deposit averaging 40 feet in thickness. The lower 20 feet of this deposit consists of large 

rounded bowlders, gravel, and sand; the upper portion is composed of finer gravels and silts 

capped by aerial talus or silt having an average thickness of 6 feet. 

Good gold values were found in these deposits, the best values usually occurring either in 

the deeper gravel-filled channels or immediately on bed rock. Where the gravel above bed 

rock was very thick, tunnels were run, shafts were sunk, and the deposit was worked by drift 

mmmg. Many hundreds of feet of tunneling and drifting on bed rock were run during the 

:Reriod of active development, and according to Fo~sett b there was one tunnel over 900 feet 

long. That the deposits are fa.irly well consolidated is shown by the fact that untimbered 

drifts and tunnels run forty or fifty years ago can still be explored to a considerable extent. 

The high bench just described as lying at the junction of Soda and Clear creeks is not 

visible below this point on the south side of Clear Creek, but.reappears on the north side about 

one-fourth of a mile downstream. At this place there is a horizontal· or slightly sloping shelf 

or ledge of coarse conglomerate and soft sandstone or granitic arkose, similar in character to 

the placer deposits on the other side of the valley., This fairly well indurated conglomerate, 

which is composed chiefly of rounded masses of gneiss and granite but also contains some 

rounded pebbles of porphyry, is of moderate extent and is plastered unconformably upon the 

rather steep gneiss walls of the valley at this point. Through this conglomerate and to a less 

extent in the underlying gneiss are veins (fissure fillings) of calcite .. 

The placer deposits in the Alice district, which were extensively worked in the eighties, 

wer.e located mainly on Silver Creek or at the junction of Silver Creek with Fall River, 7 miles 

above the point where the latter stream empties into Clear Creek. The principal deposits 

occurred on the south slope of the high spur lying between Silver Creek and Fall River and 

between 1 and 2 miles north of their junction. These deposits were not examined by the 

writers, but according to Burchard c "the debris removed consisted of from 3 to 6 feet of coarse 

gravel and soil, 2 feet of fine yellow dirt, and about 3 feet of shattered bed rock." Burchard 

a The mines of Colorado, 1867, p. 70. b Fossett. Frank, Color!J.do, 1880, p. 381. 

c Burchard, ll. C., Precious metals in the United States, 1883, p. 280. 
U. S. GEOLOGICAL SURVEY 

0 100 200 400 600 800 1000 feet 

u' "ili'·~ .. ~~~--~·--~----~· ---L--~·--_J--~·~--~~· 

vein ! 4-«J /eel I 

7rh i•vel 

·!'! " 

I MPER IAL 

.:::_ 

~ 

~ 

I M O N EY MUS K ~ 

I ~ ~ 

~ 

HORIZONTAL AND VERTICAL PLNS OF LAMARTINE MINE WORKINGS 

PROFESSIONAL PAPER NO. 6 3 PL. LXIV 

BEN HARRISON LAMA RTINE 

EAST 
314 GEOLOGY OF THE GEORGETOWN QUADRANGLE, COLORADO. 

also states that of these materials the yellow dirt seemed to carry the highest values and also 

that considerable native silver was found in the sluice boxes. The scarcity of large bowlders 

and the favorable location on the hill slope allowed the deposits to be easily rem~ved by the 

processes of hydraulicking then employed. 

PRODUCTION. 

It is difficult to make even a rough estimate of the yield of the placers, owing to the fact 

that in compiling early statistics the production of gold from this source was not kept separate 

from that derived from the hydraulicking of the decomposed surface outcroppings of the goldbearing veins. However, although the separate production of the placers and surface lode 

materials is unknown, the yield of the precious metals in Clear Creek County between 1859 

and 1864 was almost wholly from these two sources. 

According to Fossett a South Clear Creek and its branches, including Chicago Creek, from 

1859 to 1880 probably yielded $750,000 in gold dust. 

The following miscellaneous items on the production of the placers of Clear Creek County 

have been c'ollected from ~arious reports of the Director of the Mint: 

Year. 

1872 ................................... . 1875 ............. -.. -.......... -- ...... . 1876 .. -............... -... -... -- ....... . 1880 ... ···- ............................ . 1881 ............. : ..................... . 1882 ..................... -- ....... -.-. 1884 ... -.. -......... -................ -.. 1889 .... -- ....... --.- ....... -.......... . 1890 ...... -.. -- ..... -. ··- .. --··- ....... . 1890 ..•. -.- ........ -..... --- --- .. --·· .. - 1890 ... ------ .. -- .. --- --.- -· ....... ·---- 1890 ..... --·-.- -- .. ----.----- .. --- ..... . 1891. ... -.... ------ .. -·.- -· -- ·- .. -.. -.-. 1891. .. --- .. -.- .. ---- .. -----.--.--- ·---- 189'2 ... ·- .. ------ .. ---- -·.- ... --· ---- ... 1897. ·- -·. --- -· ·-· --· ·- -·.-. ·-- ---· --·--

Pror{uction of gold placers in Clear Creek County . 

.An10unt. Source. 

$24,000 Bars and placers ........................................... __ . 42, 500 Placer and stamp gold ..... ____ .. _ .. _ ...... _ ... _ ..... _ ... _ ••.. 80,000 Gulch gold on and below Spanish Bar .. _._ ...... _ .. _._ ...... _ 5,000 Placers .................... _ ......................... _ ..... _ .. . 5,000 _ .... do ............................ _ ............. _ ...... _ ...... . 6,000 ..... do ....................................................... . 12,000 Alice placers ...... __ .. __ ..... _ ................ _ .... _ ..... _ .... . 25,-000 Pauline placer.: ..... -.. --_ .... -._ ...... __ ............... _ .. . 20,000 ..... do .............. _ .. _ ... _._ ..... _ .. __ .... _ ............ __ ... . 

~:g!g ~~m~~~r~r~a~r!~~~:::::::::::::::::::::::::::::::::::::::::: 1,875 Trout Park placer ...................................... _ .... . 1, 000 Gold Dirt (surface lode)_ .... _ ..... _ ......................... . 2,220 ..... do ........................................................ . 50,000 Placers (approximately) .... __ ., ............................. . 

MINES. 

LAMARTINI!-TRAIL CREEK GROUP. 

JtA~ARTINE AND CONNECTING VEINS 

HISTORY AND PRODUCTION. 

Reference. 

Raymond, 1872, p. 276. Haymond, 1875, p. 305. Raymond,1876, p. 297. 

Burchard, 1880, p. 145. Burchard, 1881, p. 374. 13urchard, 1882, p. 425. Burchard, 1884, p. 202. Leech, 1889, p. 146. 

Leech, 1890, p. 13!. Leech, 1890, p. 130. Leech, 1890, p. 132. Leech, 1890, p. 132. Leech, 1891, p. 177. Leech, 1891, p. 177. Leech, 1892, p. 122. Hoberts, 1897, p. 118. 

The Lamartine mine (Pl. LXIII) was located in 1867. The original discovery was made 

by Peter Cooper, John J: Bougher, and Peter Chavanne, who were searching for a gold mine. 

As the surface indications of this lode were not favorable, one of these men sold a fourth interest 

for $25, another for $250, and Chavanne sold out for $5. The property was acquired by Peter 

Himrod, of New York City, who in 1887 let a contract for sinking a shaft in the vein. This 

shaft encountered the Lamartine ore body at a depth of about 100 feet, in May, 1888, one 

month after the death of Mr. Himrod. Freel E. Himrod, his son, gave a lease on a small block 

of ground to Messrs. Armstrong, Burns, Williams, and Hanchett, who produced $616,000 from 

it in sixteen months. At the expi_ration of this lease, vigorous development work was carried 

on by the owner, under the direction of Silas Hanchett. 

The following- statement of.thc production of the mine was taken from the record books _by 

the courtesy of the present owners: Total production to August, 1905, ore, net weight, dried, 

67,946,019 pounds, yielding 39,291.81 ounces of gold, 2,677,470. 79 ounces of silver, and 3,232,020 

pounds of lead, having a total actual value of $2,361,039.15. No copper appears in the returns. 

a Fossett, Frank, Colorado, 1880, p. 380. 
U. S. GEOLOGICAL SURVEY 

SEVENTH LEVEL 

N 

TENTH LEVEL 

GEOLOGICAL PLANS OF SEPARATE LEVELS ' LAMARTINE MINE. 

PROFESSIONAL PAPER NO. 63 PL. LXV 

0 

------~2~o~o------~4yoo~----~6~o~o ______ ~eo~o~----~ I 000 feet 

Biotite gneiss 

wall rack 

' , 

Gneissoid granite 

wall rock 

Bostonite porphyry Mineralized lead 

Massive granite 

B 

Unmineralized lead 
MINES NEAU IDAHO SPRINGS. 315 

The underground workings of the mine have a total extent of 12 miles, included in which is 

a drainage tunnel 8,000 feet long (Pl. LXIV). From 1894 to 1904 the mine was· under the 

management of Lafayette Hanchett. · 

At present not much \Vork is going on at the mine, and many of the. old workings are 

inaccessible. 

COMPOSITION O>' VEIN. 

The vein material contains abundant galena and blonde, with some pyrite. The product 

has been chiefly galena ore which has been selected from ore containing blonde. The zinc-bearing ore as a rule is poor in gold and silver. The best ore appears to be a fine galena, much of the 

coarse galena being of lower grade as regards gold and silver, as in many other mines of the region. 

The gold content ranges from little more than nothing up to several ounces per ton; the silver 

in a good grade of ore amounts to about 150 ounces. A few years ago the total value of the ore 

per ton was about $100, but it has grown less in recent years. 

NATUHE OF WALL ROCK. 

The vein walls comprise different rock formations, which are usually present in relatively 

large masses. They include porphyritic granite like that around Georgetown (Silver Plume 

granite), gneissoid granite, biotite gneiss, and pegmatite. · The principal wall rock is the 

gneissoid granite, which in some places contains biotite and in others is free from it. This 

granite is locally porphyritic, but becomes also pegmatitic and aplitic. 

A.dike of porphyry lies on t)le,south side of the vein where it is most productive. As the 

mine workings arc now partly caved this porphyry was not noted, at the time of the writers' 

examination, above the sixth level. On this level (Pl. LXV) the porphyry was found in two 

places, about 200 feet apart, forming the south wall of the vein. Apparently the dike is not 

continuous between these two points, as a drift on a branch vein which runs into the south 

wall of the main lode between these points shows n'o porphyry. 

On the seventh level west of the ~amartine shaft the same dike Was found in one place only, 

along the south wall of the vein .. On the tenth level, however, the south wall of the vein for a 

distance of 1,600 feet is chiefly formed by this porphyry dike. Where it does not form the exact 

wall, it usually lies only a few feet south of the wall, as is proved by several crosscuts. On the 

surface this dike has been recognized by Mr. Ball, where it has been found to outcrop at intervals for nearly half a mile. The relation of these outcrops to one 'another indicates that the dike 

is not continuous, but intermittent, a conclusion which probably applies underground as well. 

As determined by Mr. Ball this porphyry is to be classed as bostonite. 

NATURE OF VEINS. 

A general view of the veins connected with the Lamartine mine is shown in fig. 119. There 

are two parallel northeastward striking and nearly or quite vertical veins-the Oneida and the 

R. E. Lee. The Oneida vein is opened up in the Lamartine workings chiefly by the long Laniar-

. tine tunnel, which serves principally as a drainage tunnel, but follows this vein nearly all the way 

from the Lamartine shaft to the surface ancl shows that it is continuous with the Lamartine vein. 

The Oneida vein is also opened up by the Oneida tuimel aoout 100 feet below the Lamartine 

tunnel. A shaft about 200 feet from the mouth of the Lamartine tunnel corinects the two 

tunnels. The Oneida vein contained considerable ore at the surface, but is for the most part, 

as shown in the Lamartine tunnel, very little mineralized, though some low-grade ore was , 

encountered about 2,500 feet in. _ 

This vein has been cut at a depth of (roughly) 150 feet below the Lamartine tunn(}l, at a 

point approximately below the mouth of this tunnel (following down on the dip of the vein), 

by the Old Stag tunnel (fig. 120), which runs in from Trail Creek valley due south, the distance 

from the mouth to Lhe vein being 820 feet. The vein on this tunnel has produced some ore. 

A shipment of 120 tons without sorting yielded $15 per ton, the contents being 0.6 ounce of gold, 

5 ounces of silver, and 0.5 per coot of copper. It is reported that from one of the two shoots 
316 GEOLOGY OF THE GEORGETOWN QUADRANGLE, COLORADO, 

worked out in the Oneida workings above $600,000 worth of ore was produced. The following 

returns fro~ 1,000 tons of ore are reported: Gold, 1.26 ounces; silver, 16 ounces; copper, 

2+per cent. 

The R. E. Lee lode has been drifted on for a distance of 2,500 feet on the sixth level of the 

Lamartine. It is a smooth, straight fault zone marked· by soft crushed and decomposed material jn granite walls. Its dip ranges from steeply northwest to nearly vertical. On this level it 

contains no ore and no quartz. Locally there is a little iron pyrite. 

The Oneida vein encountered the zone of an earlier porphyry dike which strikes east and 

west. The vein zone, which was a zone of fracture and probably slight faulting, was largely 

deflected from its normal course along this dike. Thus this portion of the vein now possesses an 

east-west strike and the dike is in most places its south wall. This is the Lamartine vein proper. 

At the place where the fault has been deflected from its northeast course by the porphyry the 

angle is occupied by a number of minor intersecting veins representing the directions in which the 

main vein trends on either side of the point of deflection, some striking east and west and others 

0 1000 2000 3000 feet 

N 

Lamartine vein 

FIG. 119.-Gencralized horizontal plan of VPins exposed in Lamartine workings, compiled chiefly from the plans of the 

sixth and tenth levels. 

northeast and southwest. The Oneida vein as developed in the Lamartine tunnel has an 

horizontal extent of about 4,000 feet from the Lamartine shaft to the mouth of the tunnel. 

The Lamartine vein has a horizontal length of about 4,000 feet measured from the Lamartine shaft westward to the last point where it has been identified. On the si:x:th level west the 

vein weakens at a point 3,000 feet west of the shaft; also at about this place it passes westward 

from hard gneiss into granite, in which it assumes the character of a chain of branching arid 

splintering minor fractures, continually weakening. As the R. E. Lee lode is approached this 

character becomes more pronounced, so that the junction of the two has not been observed, and 

it is doubtful if it exists. This weakening of a strong fault lode when entering the field of immediate proximity of another transverse or oblique locie is a common occurrence in this general 

region. It seems to indicate that a general internal stress experienced by the rock was taken 

up along a zone of movement and crushing, which was strong and well marked in proportion 

as it was obliged to assume the entire work of readjustment while the process of discharging the 

strains was going on. Where, however, such a zone entered the field of a stronger zone 

the internal rock stress was taken up by the latter, and the weaker zone, on nearing it, found 

progressively less and less cause for existence. 

It is probable that the northeastward striking vertical line of favlting represented by the 

Oneida vein is continued to some degree also southwest of the Lamartine shaft, past the point 

where a large part of the movement has been deflected along the preexisting porphyry dike, to 

j 
MINES NEAR IDAHO SPRINGS. 317 

form the Lamartine vein. The plan of the si..xth and seventh levels (Pl. LXV) shows branches 

which diverge to the southwest from. the south side of the main Lamartine vein, leaving the 

region of minor intersecting veins (which occupies the bend between the Oneida and the Lamartine veins) on the southwest, much as the main Oneida vein does on the northeast side. These 

branches have not been followed far in the underground workings, so their extent is unknown. 

From the surface, however, a vertical shaft 

goes down on the Financier lode, which appears to strike northeast, approximately parallel to the Oneida vein, and an extension of 

which northeastward from its observed position at the surface would bring it into the 

Lamartine lode near the west end· of the field 

of intersecting minor veins above mentioned. 

From some of the underground workings 

shown on the map (Pl. LXIV), but now inaccessible, especially those on the second level, 

it is probable that the east-west Lamartine 

fracture zone continues for a short distance at 

least beyond and to the cast of the incoming of 

the Oneida. 

The character of the Lamartine vein 

proper as a zone of sheeting and faulting is· 

shown in many places. Strire indicating the 

direction of movement were observed here 

and there. At one place, near the breast of 

the third level west, they dip 75°W. Although 

the vein, especially on the tenth level, borders 

the north wall of a porphyry dike, that it follows a zone of faulting is clearly shown. by 

the fact that it does not keep close to the 

porphyry, but at many points deviates more 

or less from the contact. Moreover, it was 

rioted that in one place the fault lead cuts 

through a projecting curve of the porphyry 

dike. The porphyry contact, then, simply 

afforded a zone of weakness or of minimum 

resistance, which was followed by the later 

movement. In some places the vein is accompanied by a band of soft clay or gouge; in 

others, the vein zone, which has normally 

been simply sheeted by the movement, has 

been partly brecciated. The vein cementation 

was subsequent to this brecciation. 

J,OCATION OF ORES. 

The Lamartine -ore shoot is rather the 

exception among the ore shoots of the region 

in that it does not come to the surface. Mr. 

N 

1 

Mineralized lead 

FIG. 120.-Geological plan of Old Stag tunnel. 

W. H. Wiley, of Idaho Springs, informs the writers that he sampled the outcrop of the vein 

thoroughly and that it did not go over $4 anywhere. The relation of the ore body to 

the surface is best shown in the stope plan (Pl.· LXIV), where it is seen that in the 

neighborhood of the Lamurtine shaft . the ore approaches within 100 feet of the surface, 

but that at most other points the stopes have kept at a vertical distance of 400 feet or 
318 GEOLOGY OF THE GEORGETOWN QUADRANGLE, COLOUADO. 

more on the vein from the outcrop. The general form of the ore body as seen in this stope 

plan is roughly horizontal, or, more correctly, the form of the highly mineralized portion is like 

the cross section of a thick saucer, having a length of nearly 3,000 feet and a vertical thickness 

of 400 feet, more or less. Below as well as above this great ore body, the vein channel is relatively little mineralized. 

The richest portion of this ore body occupies the field of intersecting minor veins which 

have formed in the angle between the Oneida vein and the Lamartine, near the point where the 

fracture has been deflected on encountering the porphyry dike. These minor veins are well 

exposed on all the accessible levels in the region just west of the Lamartine shaft, and arc shown 

on the plans of the different mine levels. Some of the veins are parallel to the Oneida, striking 

northeast; others arc parallel to the Lamartine, striking east and west. The intersection of 

these veins is characteristically mineralized to a high degree and large stopes have been made 

at these points. The ore, which at many places extends a long distance away from the intersections, becomes richer and more abundant as the junctions are approached. Inasmuch as 

these minor veins are all nearly or quite perpendicular, conforming with the attitude of the 

master veins to which they are auxiliary, a given vein can be recognized on different levels, 

and the relation of these veins and their intersections as observed on these levels shows 

that the whole field of cross faulting extends downward almost vertically. Therefore this 

principal portion of an ore shoot, which appears when entirely mapped out to have a greater 

horizontal than vertical extent, was actually formed along a vertical chimney. Plainly the field 

of intersecting veins has furnished a chimney of maximum water circulation. On the tenth level, 

however, the intersecting veins characterizing this chimney, if present, have not been opened 

up or were not observed, the Lamartine on this level having the form of a plain straight vein, 

very slightly ·mineralized as compared with the portion of the vein immediately above, on the 

upper levels. . 

The best portion of the nearly horizontal ore shoot, according to this conclusion, has been 

formed by the intersection of perpendicular veins forming a perpendicular channel of water 

circulation. The mineralization extends for a considerable distance west of the principal field 

of intersection, to a point about 3,000 feet west of the Lamartine shaft, which is practically at 

the east end of the ore shoot and also of the field of vein intersection. It is probable that 

much of the western portion of the shoot is also marked by minor fractures intersecting the 

main lode. For example, on the sixth level there is a northeastward striking mineralized· 

branch which runs into th._e main east-west lode, in the center of a vertical ore shoot. This 

locality is about 1,500 feet west of the shaft. 

· The most important feature about this ore body from a theoretical standpoint is that 

nowhere does it reach the surface. In fact, if we may credit the commonly accepted account, 

its discovery was accidental and was the consequence of a hoax. Under ordinary conditions 

it may very well have entirely escaped discovery down to the present clay. The significance 

of this fact becomes immediately apparent. An explanation which naturally arises to fit the 

case is that the upper portion of the vein zone has been leached out and the minerals carried 

to the lower portion-an explanation which is apparently favored by the general parallelism 

of the ore shoot to the surface. In this general region, however, most of the known ore bodies 

outcrop at the surface, are similar in composition to this vein, and arc exposed to the same 

climatic conditions as affect it. 

The definite rule observed in these bodies is that the ores are not leached out from the 

superficial portions to .be concentrated below, but on the contrary the ricpest zone lies 

immediately at the surface, and the ores, if any change is noticeable, decrease in value and 

abundance with increasing depth. ·It seems probable, therefore, that the Lamartine ore body 

was formed in it;; present position and that the plane of the surface, which is being continually 

carried lower by the process of erosion, has not yet reached it. In this connection attention 

should be called to the fact that the situation of the mine is high, the collar of the Lamartine 

shaft being 10,500 feet or more ahovc sea level. If the location were somewhat different topographically-for example, if it were 500 feet farther down the side of the mountain instead of 
I 

MINES NEAR IDAHO SPIUNGS. 319 

being at the actual top-the plane of erosion would have reached the ore body and we should 

have precisely the same phenomenon as is observed in most of the other veins-an outcropping 

ore body e~riched near the outcrop by atmospheric waters. 

BRAZIL MINE. 

The Brazil mine is located near the head of Trail Creek and develops a vein which has 

been opened at an elevation of 9,800 feet in Trail Gulch and is traced on the surface beyond 

the point where it crosses a ridge half a mile farther N. 80° W., at an elevation of 10,500 feet. 

The wall rocks consist entirely of micaceous gneiss and pegmatite. 

The Brazil vein is said to be joined just west of the main shaft near the crest of the ridge 

by the Poor Boy vein, which dips 75° N. and runs a little south of west. 

The vein is developed by three tunnels and two shafts. The upper two tunnels, however, 

are caved shut. The lower tunnel (fig. 121) is open for a distance of more than 800 feet and 

follows a lead all the way. For the first 450 feet the tunnel follows a N. 70° W. vein which 

Micaceous gneiss intruded 

by small masses or pegmatite 

Mineralized lead 

EJ 

U nmineralized lead 

FIG. 121.-Geological plan of lower Brazil tunnel. 

dips 60° NE. At that point, however, there is a split in the lead; the south branch enters the 

wall and the north branch trends N. 55° W. for 125 feet and then bends to N. 85° W., with a 

dip of 50° N. The vein is a crushed gneiss and clay lead 1 to 6 inches in width, showing little 

if any mineralization for the first 600 feet. Where the vein assumes a N. 85° W., direction, 

however, a stope 150 feet or more in length and 20 to 40 feet high has been'made. This stope 

yielded a yellowish-brown oxidized ore, containing abundant iron oxides. Black sooty copper 

sulphides were also observed associated with some of the oxidized ores, as were also fibrous 

white crystals of cerusite (PbC03). Solid sulphide ores, howe~er, occur along the vein at greater 

depth and consist of black sphalerite, pyrite, and some galena. The ores obtained from this 

mine have been chiefly silver bearing. No ore has been extracted in the last few years. 

POOH MAN MINE. 

The Poor Man mine is located near the head of Trail Creek, several hundred feet northnorthwest of the lower Brazil tunnel. The development consists of a crosscut tunnel about 

250 feet in length trending N. 20° W., and a drift on a vein which runs a l~ttle south of west. 

The rocks in the vicinity consist of gneiss, pegmatite, and granite. The pegmatite and granite 
320 GEOLOGY OF THE GEORGETOWN QUADRANGLE, COLORADO. 

were observed to contain small scattered masses of magnetite. The vein dips 75° N. and is 

said to intersect the Brazil vein just west of the Brazil shaft, near the crest of the ridge. It 

consists of a hard quartz streak adhering rather tightly to the walls and containing a little 

mineral. 

NEW EltA MINE. 

GENERAL DESCIUP'riON 0~' VEINS. 

The New Era mine is located in the valley of Trail Creek, at an elevation of about 9,150 

feet, a short distance west of the town of Freeland. The mine workings for the main part were 

driven for the purpose of developing the New Era and Great Western veins, which run approximately parallel to one another and only 40 to 90 feet apart. 

The New Era vein on the surface runs about S. 27° W. and is probably a continuation of 

the Lone Tree vein, which runs up the hill slope to the sout~-southwest and has been opened 

up to a point 100 feet or so west of the Teller shaft. Formerly the vein was extensively developed by the workin~s of the Lone Tree mine, most of which have long since been inaccessible, 

due to caving resulting from the mines having lain idle for a considerable number of years. 

Northeast of the New Era mine, on the surface, the New Era vein runs up the hill with a trend 

of about N. 45° E., but the Great Western vein straightens somewhat, runs N. 30° E., and is 

probably continued as the Rough and Ready vein. 

UNDERGROUND DEVELOPM~JNT. 

Underground the New Era and Great Western veins are developed by workings connecting 

with two tunnels and a shaft (Pl. LXVI). On the south side of the creek is the main tunnel, 

driven on the New Era vein for 700 or 800 feet. This tunnel is connected by two crosscuts 

with a drift 800 or 900 feet in length ori. the Great Western vein, and by raises and winzes with 

other minor drifts. On the north side of the creek is the shaft, which.is connected with drifts on 

both the New Era and Great Western veins, and also a tunnel level with about 350 feet of drifting, 

which shows the New Era and Great Western veins approximately 75 feet apart. The main 

level connecting with the shaft shows 1,500 feet of drifting, all but about 500 feet of this being 

on the Great Western vein. 

NATURE OF WALL ROCK. 

All the workings on both veins are chiefly in gneiss, which is both of the micaceous and 

granitic varieties. Some pegmatite is also present in places,· especially near the shaft, where 

the mixture of gneiss and pegmatite has locally a crushed or gnarled appearance. 

NEW ERA VEIN. 

The New Era vein, which has a general trend of S. 40° W. and a dip of 60° to 75° NW., 

consists for the most part of foot and hanging wall leads separated by 1 foot to several feet of 

altered pegmatite or granite heavily impregnated with pyrite. On the main shaft level'the two 

leads, which run about 15 feet apart for so~e distance, consist mainly of small clay or gouge 

selvage streaks, many of them stained black, which are here and there associated with narrow 

seams of low-grade galena and pyrite ore. However, little "pay ore" has ever been taken from 

the New Era vein itself. The only good body of ore so far opened up on this vein was located 

on the south tunnel level, near the first crosscut to the Great Western vein. This ore body, 

which was stoped out years ago, had a length of 100 feet along the drift and extended up to a 

blind level about 75 feet above the tunnel. Near this ore body the walls of the vein approached 

within about 2 to 3 feet of each other, and the intervening gneiss and pegmatite was almost completely replaced by quartz and pyrite or by quartz and a galena-pyrite ore containing traces of 

sphalerite. In places the vein filling was heavily stained by iron oxides. 
U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER NO. 63 PL. LXVI 

LOWER LEVEL FROM SHAFT 

100 200 300 400 feet 

EB 

Mineralized lead Unminerallzed lea.d 

GEOLOGICAL PLANS OF SEPARATE LEVELS, NEW ERA MINE. 
MINES NEAR IDAHO SPRINGS. 321 

GREAT. WESTERN VEIN. 

The Great Western vein, which lies 40 to 90 feet west of the New Era vein, is flatter, 

·ranging in dip from 45°. to 60° and striking S. 25° to 40° W. Where lacking in ore this vein 

consists of a 'crushed and silicified rock lead, containing some quartz and pyrite and ranging from 

a few inches to 2 feet in width. In the main shaft level the vein is mineralized to some extent, 

for near the southwest breast there was a drift stope 150 to 200 feet in length on a 6-inch streak 

of quartz and ore. At the time of examination some good-looking galena-chalcopyrite ore was 

also being obtained from a drift stope near the northeast breast of the long drift on the Great 

Western on this same level, at a point about 400 feet southwest of the shaft. On the tunnel level 

the Great Western vein was comparatively heavily mineralized for most of its length. Near the 

northeast end of the drift on it the vein zone was about 2 feet wide and consisted of small galena 

and pyrite seams running parallel to the schistosity of the gneiss, the strike of which at this point 

corresponded with the trend of the vein. The low stope at this northeast end was between 150 and 

200 feet long. The main ore body, however, was one extending continuously from a point near 

the southwest breast of the drift on the Great Western vein to a point more than 400 feet to the 

northeast. For much of this distance stopes had been made on the vein for only 20 to 40 feet 

above the tunnel level. Most of the best and largest bodies of ore along the vein appeared to be 

located at or near the junctions of minor, slightly mineralized leads with the main vein. At one 

point where several small veinlets joined the main vein there was an ore body about 10 feet in 

width, on which stopes were made for a vertical cli.stance of about 60 feet. 

CROSS J,EADS. 

A little more than 500 feet southwest of the mouth of the main tunnel, on the south side of 

the creek, is an east-west vein consisting of 1 inch of clay or gouge and about 5 inches of crushed 

gneiss. This vein, which dips 52° N. and is practically unmineralizecl, crosses both the New Era 

and Great Western veins. Its course is altered very little where it crosses theN ew Era vein, but 

it is faulted about 31 feet by the Great Western vein, the west portion of the cross vein being 

offset to the south. At the point of crossing the Great Western vein itself also seems to have been 

somewhat disturbed and the segments displaced about 1 foot. The junction of the two veins 

does not appear to have had any effect on the ore deposition. 

On the main shaft level, about 800 feet south-southwest of the shaft, the Great Western vein 

splits in going southwest, the east branch entering the wall as a lead of quartz, pyrite, and 

silicified gneiss, and the west branch swerving slightly more to the southwest and continuing 

about 90 feet farther, to a point where it appears to be intersected and faulted by aN. 75° W. 

~gouge vein dipping 65°. At this point the southwest portion of the Great Western vein, 

which here consists of a 6-inch streak of quartz and ore trending N. 32° E., is offset 5 or 6 feet to 

the west of the northeast portion, which a few feet from the fault contains a streak of low-grade 

galena and pyrite ore over 1 foot wide trending N. 60° E. This fault lead on the lower level is 

. possibly the same one that crosses both the New Era and Great Western veins on the south 

tunnel level, although the effect on the crossing veins is somewhat different. However, the 

definite correlation of the two was made impossible by the lack of accurate mine maps showing 

the relation of thetunnellevel to the shaft workings. 

NATURE OF ORE. 

Most of the ore from theN ew Era mine consists of a mixture of coarse galena, chalcopyrit~, 

and pyrite, associated with a gangue composed mainly of quartz but carrying locally a little siderite or ferruginous rhodochrosite. A dark-gray, shiny, metallic mineral resembling tetrahedrite 

{gray copper) or tennantite is also rather abundant, but does not seem to carry any larger 

values of gold and silver than the other ore minerals. 

The ore is notable for the abundance of its chalcopyrite and for the lack of zinc blende, 

although a little of the latter mineral is occasionally found. The' vein filling in places consists of 

31401--~o. 63--08----21 
322 GEOLOGY OF THE GEORGETOWN QUADRANGLE, COLORADO. 

a thin layer of comb quartz and slightly cupriferous pyrite next the two walls, with a mixture of 

approximately contemporaneous galena and chalcoyprite in the center. Some of this galena is 

:fibrous in structure; but where it has been crushed by movement subsequent to deposition it is 

finely granular. In somewhat porous vuglike portions of the massive galena and chalcopyrite 

ore, a mixture of crystalline pyrite and the dark-gray shiny mineral resembling a gray copper" 

occurs in association with a little siderite. That the gray copper and pyrite. are of later origin 

than the galena and chalcoyprite is shown by the fact that seams of the :first-named minerals 

up to 2 inches in width cut the latter or include angular fragments of t}1em. 

In many places the vein consists, instead of a single fracture, of a number of parallel seams 

somewhat similar in structure to the vein described above. In places, however, much of the ore 

seems to have resulted from a replacement of the altered wall rocks included between a series of 

fractures. 

The :first-class ore of the mine averages about $60 to the ton and carries from 1 i to 2 ounces 

of gold, 5 to 20 ounces of silver, and 45 to 50 per cent of lead. Copper up to 5 per cent and zinc 

up to 3 per cent have at times also been obtained, although in quantities so small that even the 

copper has not been paid for by the smelters. The second-class orcs give mill concentrates that 

consist of ore carrying about 1.4 ounces of gold, 5 ounces of silver; 17 or 18 per cent of lead, and 

a little copper" The richest ore found in the mine came from a small seam of galena and gray 

copper only one-half inch to 3 inches in width, said by the superintendent to have given assays 

of 1,100 ounces in silver, 33 ounces in gold, and 51 per cent of lead. However, this small seam 

was only a few feet in length, and so yielded but a small quantity of ore. This rich ore was not 

located along the main vein, but was 30 or 40 feet west of the Great Western vein, at the junction· 

of 'two small veins, one of which joined the Great Western vein and caused another body of 

good ore there. 

The total production of the New Era mine is not known, but besides the :first-class ore 

being shipped the mine was supplying 18 to 20 tons of lo\v-grade ore to the 10-stamp concentrating mill of the New Era Mining Company. The output of concentrates from this mill at the 

time of visit was about one carload a week. 

FALU MINE. 

GENERAL DESCRIPTION. 

The Falu mine is located at an elevation of 9,250 feet a little north of the toWn. of Freeland. 

The vein is developed through a shaft and a series of drifts connecting with it. The mine had 

been idle for some time when visited, and as the shaft was caving no attempt was made to 

exan~ine the underground workings. ~ NATURE OF VEIN, 

The vein near the shaft has a general strike of S. 70° W. and a dip of about 50° NW. A 

little southwest of the shaft, however, the vein swerves a little more to the west and runs toward 

the New Era vein. The material in the ore bins consisted of gray jaspery quartz and pyrite, no 

galena or zinc blende being observed. 

The vein developed in this mine, or at least a branch of it, is pqssibly a continuation of 

one observed in the open cuts and caved shafts of the Turner mine, which is located about 1,000 

feet distant in a N. 78° E. direction. 

TELLER MINE. 

The Teller mine consists of workings connected with a shaft located approximately 3,000 

feet southwest of Freeland. at an elevation of about 9,750 feet, and of a tunnel about 1,800 feet 

N. 42° E. of the shaft at an elevation 400 feet lower. Tlie shaft develops a northeast-southwest 

vein, which has produced some coarse galena, chalcopyrite, and sphalerite ore carrying from onehalf to 1 ounce of gold and occasionally as high as 25 to 50 ounces in silver. 

The tunnel at the time of visit was being run with the purpose of intersecting the vein at 

u.lower depth. (See :fig. 122.) It follows a minor lead which clips 60° to 65° NW. and is marked 
, 

MINES NEAR IDAHO SPRTNGS. 323 

by poorly mineralized crushed gneiss and pegmatite, carrying a little pyrite and traces of galena 

and sphalerite. The tunnel is driven on this lead for 300 feet to the southwest and then for 

200 feet S. 80° W., to a point where the lead intersects a slightly mineralized vein striking 

N. 38° E. and dipping 65° to 80° NvV. The last-mentioned lead is marked by a seam of clay, 

quartz, arid crushed rock 2 to 6 inches wide, which in places carries a little pyrite and.siderite 

or ferruginous rhodochrosite ore, with local galena and sphalerite. High assays in gold are said 

to have been obtained from this vein, from small streaks of light-colored pyrite and milky 

quartz. 

TheN. 80° E. tunnel vein faults the northeast-southwest vein about 1 foot, but at the same 

time has its own course changed slightly. · 

No pay ore had been mined f{om the tunnel level at the time of visit. The relation of the 

veins on the tunnel level to the vf'in developed by the shaft workings was not determined. 

Gneiss Mineralized 

lead 

FIG. 122.-Geological plan of lo\l·er Teller tunnel. 

Unmineralized 

lead 

FREELAND VEIN AND CONNECTED VEINS. 

GENERAL DESCRIPTION OF VEINS. 

Freeland vein.-The Freeland vein is a strong trunk vein having a northeast strike and a 

comparatively flat northwest dip which is said to average about 36°. This trunk vein is about 

2,200 feet long. On the southwest end there are two s.~rong branches, each of which is ore 

bearing; the general direction of the trunk vein bisects the _angle between the two. In the 

Freeland mine (Pl. LXVII) these two branches are known as the Freeland Extension vein and 

the Split vein. The Freeland Extension vein, which is the more southern of the two, has a 

southwest strike, more southerly than that of the main vein, and a northwest dip. On the 

Freeland tunnel level this dip was noted in one place to be 60°. The average angle, however, 

in the interval between this level and the surface at the top of the Freeland Extension shaft is 

41° or 42°. Nearer the surface the dip of this branch flattens markedly, and for the first 200 or 

300 feet of vertical depth it is only about 30° or a little more. The more northern branch, the 

Split vein, has a northwest strike, which approaches very nearly due cast and west, and a northerly dip, the angle of which on the Freeland tunnel leYel was observed in one place to be 42°. 

Each of these branches is drifted on in the Freeland tunnel level for about 600 feet from the 
324 GEOLOGY OF THE GEORGETOWN QUADRAN?LE, COLORADO. 

junction. The end of the drift on the Split vein was inaccessible at the time of visit, but the 

maps indicate that another branch diverges from the Split vein at this point and runs off to the 

southwest, parallel to the general direction of the Freeland vein and the Freeland Extension. 

However, there appears to have been very little drifting on either of the veins past this junction. 

Toledo, Gum Tree, and Anchor veins.-At its northwest end the Freeland trunk vein has 

been followed nearly to Trail Creek, beneath which there are no workings. On the opposite 

(north) side of the creek there are two ore-bearing veins whose trends converge to the southeast. 

The junction of these two veins is not visible at the surface on account of the overburden and has 

not yet been developed underground, but their trends make it probable that the junction or 

intersection takes place not far north of the mouth of the Freeland tunnel, near the bottom of 

Trail Creek. (See fig. 123.) 

The main northeasterly trend of the Freeland vein is continued by the Toledo vein, with a 

similar relatively fiat northwesterly dip. The angle of dip is about 45° in the western part of 

the Toledo tunnel, but becomes flatter near the east end of the tunnel, being noted in different 

places as 22° and 24°. At the east end of the tunnel, also, the Toledo vein becomes very weak. 

Vein 

Friction breccia caused by renewal of mO\'ement along vein 

500 1000 1500feet 

~~~~~------~--------~ 

FIG. 123.-Generai plan of Freeland vein and some of its branches. 

The Gum Tree vein diverges from the Freela:nd trunk vein with a general east-west strike. 

About 600 feet away from the probable junction or intersection with the Freeland, the Gum 

Tree splits, the weaker brandt continuing the normal· east-west strike of the vein from its 

junction with the Freeland, and the stronger branch having a northeast trend, divergent from 

that of the Gum Tree and parallel in general with that of the Toledo and Freeland. Both of 

these branches are mineral bearing to a greater or less extent. They dip 45° N. Farther 

east on the Gum Tree workings the stronger branch again divides and becomes weak. 

The Toledo vein has been opened up underground to a point about 1,000 feet from the 

junction (see fig. 124); the Gum Tree to about 1,300 feet. 

A few hundred feet northeast of the mouth of the Freeland tunnel is the Shakespeare 

tunnel, which runs 200 or 300 feet southwest until it cuts a fairly strong vein called the Anchpr, 

which has been drifted on for more than 400 feet. This vein (Pl. LXVIII) has an east-west 

strike similar to the normal strike of the Gum Tree as stated above and a flat northerly dip of 

25° to 45°, typical of the Freeland group of veins. Its strike is such that it probably joins or 

intersects the Freeland beneath Trail Creek. 

It will be seen that the somewhat obscure relations of the Freeland trunk vein to the 

Toledo, Gum Tree, and Anchor veins are concealed beneath the drift of Trail Creek. What 

appears to be the most likely solution i~ shown in fig. 123. According to this hypothesis the 
U.S. GEOLOGICAL SURVEY 

-... ___ ----------

tu rret level 

Minnie 

HORIZONTAL PLAN OF FREELAND MINE WORKINGS. 

PROFESSIONAL PAPER NO. 63 PL. LXVII 

0 •OO 200 300 <00 soo feet ~--~---4----~--~--~ 
MINES NEAR IDAHO SPRINGS. 325 

Toledo is the extension of the Freeland, and the Anchor and the Gum Tree are the same vein, 

which crosses the Freeland-Toledo obliquely and without great displacement. 

Brighton and Harrisburg veins.-Veins representing an extension of some of the veins 

of the Freeland group are seen in the Brighton and Harrisburg workings, which lie west of the 

Freeland Extension workings. The workings of these mines, however, are separated from 

those of the Freeland and the Freeland Extension. The Brighton shaft is approximately 

1,000 feet S. 60° W. of the Freeland Extension shaft, on the outcrop of a vein which· strikes 

northeast, parallel to the Freeland, and has also the flat northwest dip of the Freeland vein and 

its different branches. The average angle of dip, calculat~d from the collar of the Brighton 

Toledo shaft 

FIG. 124.-Vertical and horizontal plans of Toledo mine. Figures represent elevations in feet with reference to the 

arbitrary datum plane (1,000 feet) shown west of Trail Creek. 

shaft to the fourth level, an actual difference of elevation of 215 feet, is 31 • This dip corresponds with the dip of the Freeland Extension vein as measured in the Freeland Extension 

inclined shaft. The average angle of clip from the surface of the Freeland Extension vein to 

the Platt level (nearly 300 feet difference of elevation) is 30°. 

The Brighton vein is opened up underground by five levels, of which the most extensive 

is the fourth (fig. 125). The Brighton outcrop is in the same rough general line as the outcrops 

of the Freeland, as traced from the mouth of the Freeland tunnel to the top of the Freeland 

Extension shaft. It is probable that the Brighton is a vein of the Freeland system, distinct 

from the Freeland Extension vein, which it probably joins on the northeast. 
326 GEOLOGY OF THE GEORGETOWN QUADRANGLE, COLORADO. 

The Harrisburg shaft is situated N. 20° W. of the Brighton shaft, at a distance of 300 or 

400 feet. It is located on the Harrisburg vein, which has a general northwest or nearly eastwest strike, about the same as that of the Split vein of the .Freeland workings. Like all the 

,veins of this group, the Harrisburg has a flat northerly dip, noted as approximately 40° in 

. the upper parts of the workings. The exact position of the Harrisburg on the level of the 

Freeland tunnel is conjectural. 

It-is reporte<;l that the junction of the Harrisburg and Brighton veins has been encountered 

in the workings from both mines, and that the i1ature of this junction shows that the Brighton 

:is the main vein, for it continues northeastward past the junction,. whereas the Harrisburg 

docs not cross the Brighton. 

FrG.l25.-Map o! Brighton mine workings. 

St. Patrick lode.-On the Freeland tunnel level of the Freeland mine, abotit 600 feet from 

the mouth, a crosscut runs northwestward in the hanging wall, exposing first a porphyry <:like and 

beyond that a minor vein parallel to the Freeland. This vein lies on the north side of the dike, 

about 20 feet from the contact, with gneiss for both hanging and foot walls. It has a northeast strike and a dip of ·35° NW. 'The portion opened up in a short drift at this point shows a 

streak of pyri~ic ore 3 or 4 inches thick. This is called the St. Patrick lode. 

PRODUCTION. 

The Freeland vein was discovered in 1861. In 1879 the inine was bought by California 

mining men. From 1880 to 1888 it was owned by the California millionaire John W. Mackey. 
MINES NEAR IDAHO SPRINGS. 327 

The mine is claimed to have produced during Mackey's ownership over $3,000,000 worth of 

ore. The following are the estimated figures of the total production of the Freeland, furnished 

by the present management of the mine: 

Production of Freeland mine. 

Estimated production from 1861 to 1879, of which no records have been kept ............ $1,000,000 

Production from 1879 to 1888 .................................... ·. . . . . . . . . . . . . . . . . . . . . 3, 299, 000 

Estimated increase of this amount to account for four months' production when the records were improperly kept ....................................................... . 

Record of production since 1888 by lessees ....................................... . 

200,000 

156,000 

. Total ..... ".. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4, 655, 000 

The production of the Gum Tree is stated by the owner at $300,000; that of the T0ledo 

at $130,000. . 

The Harrisburg mine was worked some time ago, but no figures as to the production have 

been obtained. The Brighton mine was only two years old at the time of the writers' examination in 1905. At that time no ore had been produced, although considerable development 

work had been done and ,ore had been accumulated on the clump awaiting the completion of a 

mill on Fall River. EXTENT OF ~liNE WORKINGS. 

The Freeland vein is opened up by several tunnel drifts, of whicli the lowest starts in on 

the outcrop of the vein near the bottom of Trail Creek. It is also opened up by an inclined 

shaft running from the surface clown through the various tunnel drifts and thence clown to the 

lowest workings. There are six levels beneath the lowest tunnel drift (the Freeland tunnel), the 

actual difference in elevation between the Freeland tunnel and the bottom of the Freeland 

shaft being about 630 feet. A few years ago the McClelland tunnel was begun, to cut the 

Freeland and other veins of this region at a considerable depth and to drain this· portion of 

the country. This tunnel starts in from Clear Creek, below Dumont, and will cut the Freeland 

vein at a point very roughly estimated as about 400 feet beneath the bottom of the Freeland shaft. 

In July, 1906, ·this tunnel was in more than 5,100 feet. The map of the Freeland mine 

(Pl. LXXX) shows about 4 miles of underground workings. 

The Gum Tree mine, which is under a separate ownership from the Freeland and Toledo, is 

opened up by an inclined shaft running from the outcrop of the vein down on its dip and by four 

drift levels on the vein, aggregating in all several thousand feet of workings. Only the upper 

two'levels are shown on the accompanying map (fig. 126). 

The Toledo vein also is chiefly opened up by an inclined shaft, connecting with the tunnel 

drift running northeastward for about 700 feet from the outcrop of a vein at the surface near 

Trail Creek. . · 

The Anchor vein is opened up by the crosscut Shakespeare tunnel and by a drift several 

hun~lred feet in length. 

The Brighton vein is opened up by an inclined shaft from the outcrop. Drifts extend 

both ways from this shaft at five levels, of which the fourth is the most extensive. The total 

difference of elevation between the collar of the shaft and the drift level is 264 feet. 

The Harrisburg vein is developed by an inclined shaft and various levels. Not much 

information is at hand concerning it. 

NATURE OF VEIXS. 

These veins lie along lines or zones of fr~cturing and minor faulting. It is probable that 

the displacement has been relatively slight. On the second level of the Gum Tree mine 

(:fig. 127) the main branch of the vein shows on its walls slickensides dipping 70° E. On the 

third level strire dipping 45° W. were found. 

Some indication of faulting along the Harrisburg vein is seen in the Harrisburg mine, 

where on the first level a transverse porphyry dike was observed in the foot wall and not in 

the hanging wall. Inasmuch as the line of surface exposures of this dike is practically straight, 

showing that it trends in generai across the strike of the vein, it is probable that it has been 

'offset at this point by movement along the vein, but that the displacement has not bee11 great. 
328 GEOLOGY OF THE GEORGETOWN QUADRANGLE, COLORADO. 

NATURE OF WALL ROCKS. 

The wall rocks of the Freeland trunk vein and of all its different branches or connected 

veins above mentioned consist chiefly of black gneiss with some pegmatite. (See Pl. LXVIII.) 

The schjstosity in the gneiss has in general an oblique trend as compared to the strike of the 

vems. 

Porphyry dikes are present here and there in the walls of the veins or close by. On the 

Freeland tunnel level of the Freeland mine, about 600 feet in from the mouth, a crosscut to 

the northwest in the hanging wall of the vein, which at this point is as usual of gneiss, cuts 

at a distance of 10 feet a porphyry dike which is about 60 feet thick, as measured by a horizontal section. This dike has a strike oblique to that of the vein, or nearly due east and west, 

and a northerly dip of 55°. It consists of an entirely altered rock showing numerous large 

o.__ __ •o~o--~~~o_o _ __c:3~oofeet 

FIG. 126.-Vertical section and plan of two upper levels of the Gum Tree mine. 

altered feldspar phenocrysts and, under the microscope, pseudomorphs of alteration products 

after biotite ph()nocrysts. Near the mouth of the Freeland tunnel level an outcrop of the 

same porphyry was noted by Mr. Ball near the hanging wall of the vein. This rock was determined by him to be probably alaskitic quartz monzonite porphyry. It was reported by the 

foreman that porphyry is encountered in a number of other places in the Freeland mine in the 

hanging wall of the vein. It does not occur on the Freeland tunnel level, however, so far as 

known, at any other place than that described: It is reported that porphyry occurs on the 

Minnie level, forming both walls of the vein. · 

No porphyry was observed in the Gum Tree or Toledo workings. 

In the Brighton mine, on the fourth level (see Pl. LXIX), porphyry comes into the hanging 

wall of the vein at the southeast end of the drift, in the form of a dike having a width as measured in horizontal section of about 40 feet, which represents an actual thickness of about 20 

feet, as the dike has a northwest dip of 45°. The strike of this dike diverges from that of the 

Brighton vein, opening out from it to the north when followed in a northeasterly direction. 

( 
U. S. GEOLOGICAL SURVEY 

----d ------- -----rtee\aX' -----........._ ________ __ Qu~(O? 

0 

Surface slide Gneiss 

----~e\'("\ -----

200 400 

Alaskitic quartz 

monzonite porphyry 

--

600 feet 

Mineralized 

lead 

SHAKESPEARE TUNNEl 

Unmineralized 

lead 

PROFESSIONAL PAPER NO. 63 PL. LXVIII 

GEOLOGICAL PLAN OF FREELAND, TOLEDO, GUM TREE, AND SHAKESPEARE TUNNELS, ALL ON NEARLY THE SAME LEVEL. 
U. S. GEOLOGICAL SURVEY PI'IOI'ESSIONAL PAPER NO. 63 PL. LXIX 

a._ ___ _:.:, 0:;,:0 ____ .:..:;200 feet 

~ Bostonite porphyry . 

B Mineralized lead 

/ .......... ··/ Unmineralized lead 

N 

t 

GEOLOGICAL PLANS OF SEPARATE LEVELS, BRIGH10N MINE. 
MINES NEAR IDAHO SPRINGS. 329 

It has not been observed elsewhere in the mine. In the Harrisburg mine, on the first level 

near the shaft, porphyry forms the foot wall, the hanging wall consisting of pegmatite and 

gneiss. Farther east on the first level porphyry also occ:urs as a layer in the hanging wall. 

Specimens of the porphyry from the Harrisburg and the Brighton mines prove to be of 

the same kind of rock. Both of these occurrences are evidently a part of the strong dike of 

bostonite which has been followed on the surface from Trail Creek, where its outcrop occurs 

about 1,000 feet northwest of the outcrop of the Freeland vein at the opening of the Freeland 

tunnel, southwestward up the hill. This outcrop is nearly parallel to that of the main Freeland 

vein, but the two outcrops converge to the southwest. This dike evidently crosses the Harrisburg vein near the shaft and runs transver~ely across to the hanging wall of the Brighton vein. 

COMPOSITION OF VEINS. 

The nature of the vein materials is similar in the main Freeland vein and its different 

branches. The deeper workings of the Freeland mine show ore composed mainly of pyrite and 

I 

I 4f' I 

Dip of vein 

EJ 

Mineralized lead 

~ 

~ 

EJ Umnineralized lead 

Pitch of striae on vein walls 

0 100 

FIG. 127.-Geological plan of second level, Gum Tree mine. 

200 

chalcopyrite, with a little tetrahedrite. Only a small amount of blende is present-not enough 

to make the zinc enter into the smelter returns. Galena occurs chiefly in bunches near the 

surface; in the deeper workings there is hardly any. The average ore is roughly stated to be 

worth about $30, containing 1 ounce or more of gold, 4 to 20 ounces of silver, and 3 to 4 per 

cent of copper. St~tements of returns from different lots of smelting ore taken from the Minnie 

level of the mine showed total values ranging from $15 to $40. A typical return sheet, picked 

out by the writers, is as follows: 

Total value .................... __ ...... _ . _ ..... _ . _ .... _ .. _ .... __ . _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ $30. 87 

Gold ..... -.. -.----- .. -.. -....................................... _____ . ____ . _____ .ounces.. l. 35 

Silver ___________ ......... ____ -____ . _______ ._._ .. _. ____ . __ . ______________________ .. do.... 9 

~~~~~~~---_-_-_- ~::: -_-_-_-_-_-_- ~ ~ ~ ~ ~:: ~: ~: ~:::: _-: _- ~ _- ::: _·:: _- _-:: _-_- _- _- _- _- _- _- _- _- _- _- _- _- _- _- _-_- _-: _- __ ---~e_r_ c~~~:: . ~.8 Zinc ........... -............. _____ ... _ ......................... _____ ... _. __________ .____ 0 

It is reported that the gold and silver in the ore are very closely associated, the relative 

amount. of one rising with that of the other, but that there is no close connection between the 

gold and the copper. Some of the iron pyrite carries very high values in gold. 
330 GEOLOGY OF THE GEORGETOWN QUADRANGLE, COLORADO. 

The ore of the Toledo mine is of similar character, consisting of pyrite, much of it cupriferous, and some tctrahedrite. The ore showed very little galena except some near the surface 

associated with quartz. 

The ore of the Gum Tree mine consists largely also of pyrite and chalcopyrite. Tetrahedrite occurs in small quantities, in many places following cracks in the chalcopyrite and 

evidently secondary to this sulphide. The ore of this mine, however, contains considerable 

galena down to the fourth level, which was the deepest level ·at the time of examination; and 

at a depth vertically beneath the surface of, roughly, 400 feet .the lead still forms an important 

part of the ore from a commercial standpoint. The massive pyritic ore is said to contain 2 to 3 

per cent of lead and 3 to 4 per cent of copper. A s?-ipment of galena ore carefully sorted out· 

yielded the following returns: Gold, 0.42 ounce; silver, 13 ounces; lead', 56.5 per cent; copper, 

2.2 per cent; value, $39. · · 

The material of the Anchor vein is mostly quartz and pyrite, but chalcopyrite and a little 

blende also occur. 

The Brighton vein shows, like the Gum Tree vein, ·considerable galena associated with 

pyrite. There is also some chalcopyrite, though much of this mineral is considerably altered 

to tetrahedrite. . Sphalerite occurs in much of the ore. The gangue consists of quartz (some 

of it being comb quartz), rhodochrosite, siderite,a and barite. The walls of many open druses 

have a coating of soft black or dark-gray pulverulent secondary sulphides. 

Ore from the Harrisburg vein shows galena and pyrite, with barite, quartz, and siderite. 

LOCATION OF ORES. 

The main Freeland trunk vein splits up ,on the southwest into two branches. The ore 

continues along the more southern of the branches, the Freeland Extension, although not in 

such large amounts as on the trunk vein. On the Split vein, the more northern of the two 

branches, no stopes were observed on the Freeland tunnel level for the first 225 feet from the 

junction, but on the remainder of the vein ore has been stoped. The largest and most persistent 

body of ore on the Freeland vein appears to extend from the junction back along the trunk 

vein for a distance of approximately 1,800 feet. This ore body appears to be thickest and 

strongest at .the junction, and on the whole to decrease gradually toward the northeast. It 

has been followed from the surface down to the lowest workings of the mine. (See fig. 128.) 

These lowest workings are vertically 1,040 feet below the collar of the Freeland shaft and about 

1,340 feet below the collar of the Freeland Extension shaft. Thus the actual length of the 

shoot, measured downward along the dip, is about 2,000 feet. 

The Toledo vein shows a similar ore body about 600 feet in length along the vein. The 

southeast end of this body is located about 80 feet east of the mouth ·of the Toledo tunnel. 

,Here a small mineralized branch vein, striking N. 75° W., join~ the main vein in the mmmer 

of a reversed branch, opening out on it to the southwest, instead of to the northeast, as is 

normal in this portion of the Freeland vein group. The ore begins at the junction of the br:anch 

with the main Toledo vein and extends eastward. It has been followed down along the dip 

of the vein by a shaft, the bottom of which is 28 feet below the tunnel level. 

In the Anchor vein ore has formed at the junction of a branch striking northeast and 

dipping flatly northwest. RENEWAL OF MOV~;MENT ALONG VEINS. 

On the Freeland tunnel level of the Freeland mine, oii the south side of the horse of country 

rock which occurs in the trunk vein near the point of branching, hard consolidated breccia 

was observed. This material contained broken and rounder fragments of gneiss, pegmatite, 

and ore in a hard fragmental matrix. The included fragments in general have the appearance 

of pebbles, being subangular and rounded like pebbles that have been rolled in a stream bed. 

The general relations of this material, however, indicate that it is probably a breccia due to 

friction. The ve_in has been reopened subsequent to the chief period of mineralization, and 

the new differential movement along the vein walls has produced a zone of crushed Ii1aterial, ----·---~ ·--~---------

a Yellow-brown carbonate from this mine, ~nalyzed by W. T. Schaller, was found to be mixed carbonate of iron and manganese. 
MINES NEAR IDAHO SPRINGS. R31 

·the harder and larger fragments of which have been rounded by rubbing against one another. 

Evidently the pressure on this zone of fragmental material has not been great, otherwise it would 

have been ground to a soft paste rather more typical of a fault zone than the material in question. 

Given, however, differential movement along the walls of this crushed zone and rel~J;tively slight 

pressure, the origip. of the rounded forms, which are characteristic ofthis friction breccia and of 

other similar breccias, which are conunonly 

found along other veins in this region, is easily 

. understood, and the resemblance to stream 

pebbles is due to the fact that both have a 

somewhat similar origin. Stream pebbles are 

not, strictly speaking, waterworn, but are worn 

by rubbing against one another when they 

. are set in motion by the current of stream _ 

til water. Similarly, these pebbles have been ·

1 

rounded by friction, though the movement ;;: 

;::. which has agitated them has originated in ~ 

stresses in the rocks of the crust, not in the o 

action of atmospheric waters clue to gravity. ~ 

At "' the point above described this fric- ~-

tion breccia or conglomerate is· about 3 feet "" 

thick. It was reported that this formation 8. 

was II ~ seen at intervals a along the Freeland ::;· 

vein, generally on the hanging wall. It also t:l 

continues along the Split vein. The Freeland ~ 

.Extension branch, however, has no gouge or i5: 

breccia, and therefore wa.s not disturbed by ~ reopening of the fissure subsequen~ to the ~ 

principal period of cementation. ~ §" Where observed in the Freeland level, "' 

'· most of the ore in this breccia lies in broken 6' 

[ fragments and is evidently older than the o 

period of. brecciation. Some of "" it, however, ~-

lies in regular seam.s in the breccia, suggesting f!l 

that it was formed later than the breccia, at .g 

~ a period altogether distinct from that of the 

maii1 vein filling. 

Gouge, probably clue to movement along 

the ·vein since the main period of mineralization, has been noted along the Gum Tree vein. 

The-Harrisburg vein also shows signs of postmineral reopening. In places near the Harrisburg shaft open fissures were observed 

along the vein. 

CHANGES OF ORE IN DEPTH. 

It is reported that the ore of these veins 

shows no marked changes in depth, but the 

z 

,., 

examination has not been complete enough to give many data on this point. The only feature 

noted was that the galena appears, so far as reported, to be more characteristic of the uppermost 200 or 300 feet of the veins rather than of those portions more remote from the surfacfl. 

In the Freeland trunk vein galena is practically absent from the. lower workings, although it 

occurred near the surface in bunches. The same observation applies to the Toledo. In the 
332 GEOLOGY OF THE GEORGETOWN QUADRANGLE, COLORADO. 

Gum Tree vein galena is present, but the lower stopes at the time of examination were only 

about 400 feet vertically below the surface. In the Brighton mine, where ore has been developed 

to a vertical depth of only 215 feet below the surface, there is considerable galena associated 

with pyrite, rhodochrosite, siderite, an<j barite. Some of the pyrite is distinctly contemporaneous with the siderite. In several places it was shown that siderite, barite2 and clear cerusite 

crystals a were younger than the galena. 

It appears possible that the carbonates of iron and manganese, with the associated pyrite 

and barite, found in these upper workings may be characteristic of the more superficial portions of the ore ·and may be due to atmospheric waters. It is also possible that the greater 

abundance of galena near the surface is due to the same cause. 

PARAGENESIS OF MINERALS. 

The principal ore which occurs in the lower portions of the veins consists of pyrite and 

chalcopyrite or cupriferous pyrite, with a very little galena and blende. The relatively more 

abundant galena observed in many places near the surface is, as above noted, very probably 

of later origin than the scanty galena found at greater depths. 

Tetrahedrite, observed in a ·number of places, has characteristically the appearance of 

being due to the alteration of chalcopyrite along the cracks in which it is usually formed. This 

is probably the result of descending surface waters. 

Ore from the Brighton mine shows galena brecciated and cemented by brown carbonates, 

probably siderite. Vugs in this material. are lined with crystals of siderite and cerusite. In 

many other specimens this siderite is uncrystallized and contemporaneous with pyrite, which 

' is therefore in general later than the galena above mentioned. Some quartz and galena, however, occur as crystals contemporaneous with this ·latest iron mineralization. A specimen 

from the Harrisburg mine shows the reopening of a fissure along an original vein of galena. 

Although the bulk of the new fissure was filled by crystalline barite, the side of the fissure 

which joined the wall rock of the vein was coated with crystalline siderite contemporaneous 

with the barite. The gneiss wall rock of this vein was, at the time of the earlier mineralization 

and before the opening of the new fissure, highly impregnated with pyrite, and the occurrence 

of the siderite fringing a fissure in this pyritic rock suggests that the iron for the carbonate 

was derived from the older sulphide, while the carbonic acid was supplied by the waters which 

circulated through the subsequent opening. 

Some of the specimens of ore from the Brighton and Harrisburg workings show on the walls 

of geodes a sooty black or gray coating, determined .by W. T. Schaller to contain sulphide of 

lead, in many places associated with and contemporaneous with small crystals of quartz and 

beautiful crystals of cerusitc. All these minerals have formed in druses which exist in ore made 

up of quartz, pyrite, and galena and which have the appearance of having been dissolved out 

subsequent to the entire cementation of the original vein opening. Both the sooty black 

material and the cerusite are formed on the surface of galena, which is part of the walls of the 

geodes and is apparently older than the opening, in such a way as to suggest that both have 

originated from the galena. In another specimen similar sooty black material (secondary 

sulphide material) is associated with probable tetrahedrite, and both are plainly derived from 

the alteration of older chalcopyrite along cracks and in geodes. 

PRESENT ACTION OF UNDERGROUND WATERS. 

The mine waters of the Freeland mine contain the sulphates of copper and iron and car-

. bonate of lime. This is shown by the deposition, on the walls, of copper and iron sulphates 

and calcite. It is said that nails have been changed to native copper by the mine waters, 

through the well-known action of metallic iron upon copper in solution. 

a Determined by W. T. Schaller. 
MINES NEAR IDAHO SPRINGS. 333 

INVINCIBLE MINE. 

LOCATION. 

The Invincible mine is located on the Lamartine road about 3,000 feet N. 60° W. of Alps 

Mountain. The mine has long been unworked and most of the openings have caved so that 

they arc inaccessible. One tunnel and the shaft, however, show a S. 53° W. vein which dips 

about 70° NW. As the Invincible mine is located in a line between the Brighton and Cecil 

mines and develops a vein which heads in the direction of each, it is probable that the vein 

belongs at least to the same general vein system as these mines.· 

CHARACTER OF ROCKS AND VEIN. 

The rocks in the vicinity consist chiefly of micaceous gneiss associated with a little pegmatite. Fragments of porphyry were found on the mine dump, showing that a dike was probably encountered in the mine workings. 

The dump contained vein material which. 

consisted of galena and quartz, finely 

crushed and recemented by silica. 

ALPINE .MINE. 

DESCRIPTION. 

The Alpine mine lies about 1,000 

feet south-southwest of Alps Mountain, 

at an elevation of 10,300 feet. A fire 

has destroyed the shaft house and the 

mine workings were inaccessible. The 

vein, which has a general northeastsouthwest trend, dips slightly to the 

northwest and cuts through an area of 

micaceous gneiss associated with a little 

intrusive quartz monzonite, pegmatite, 

and granite. 

CHARACTER OF ORE. 

The ore from the ore bins showed 

narrow seams· of galena and grayishbrown sphalerite, associated with a little FIG. 129.-Geological plan of Lanagan tunnel. 

tetrahedrite and argentite. Much of the tetrahedrite occurs in minute but well-formed crystals. Pyrite, although rare, is present in some of the ore. In a few specimens thi~ seams of 

ore were noted between the laminre of the gneiss and conforming with its schistosity. The 

Alpine mine has produced considerable ore and was worked up to the time when the fire 

destroyed the mine buildings. 

LANAGAN MINE. 

LOCATION AND DEVELOPMENT. 

The Lanagan mine is located at an elevation of about 10,200 feet on the slope of Alps 

. Mountain, 700 feet north of the summit. The developments consist of a tunnel about 350 feet 

in length, which runs through pegmatite, gneiss, and granite and follows a slightly mineralized 

clay lead (fig. 129). 
334 GEOLOGY 01<' THE GEORGETOWN QUADRANGLE, COLORADO. 

DJ<;SCRIPT!ON OF VEINH. 

This tunnel vein conneets with a short drift and stope on a N. 42° E. vein, which dips from 

55° to 70° NW. and runs somewhat parallel to and only a short distance from the tunnel vein 

all the way from the mouth of the tunneL In places this second vein consists of a broad clay, 

quartz, and crushed rock lead several feet wide; in others it comprises a network of small 

stringers of quartz or ore cementing fractured wall rock. It is approximately parallel to a 

10-foot dike of bostonite porphyry. Locally it runs along the northwest or hanging-wall contact 

of the porphyry dike, but elsewhere it is separated froni. the dike by several feet of pegmatite or 

other country rock. In some places the vein evidently cuts through portions of the dike, for a 

few small patches of porphyry occur also on the hanging-wall side of the vein. Specimens 

found on the mine dump showed small stringers of ore runmng all through and cementing 

fractured porphyry. 

CHARACTER OF ORE. 

The ore which has been mined, to judge from specimens found on the dump, consisted of 

galena, zinc blende, both resin blende and "black jack," and some pyrite in a quartz gangue. 

Minute blotches and fibrils of probable tetrahedrite were also noted in some of the specimens. 

According to the owner, the average of all the ore shipped from the mine gave values of 2.21 

ounces in gold, 150 ounces in silver, and 5 per cent of lead. 

LOCATION AND PRODUCrl:ON OF ORE. 

The only stope in the mine is near the junction of the S. 42° E. vein and the tunnel vein, 

and is a good illustration of ore deposition at the point of union of two veins. 

The production of the mine has consisted of four shipments of ore, aggregating 14 tons in ali. 

MILLER MINE. 

LOCATION AND DEVELOPMEN1'. 

The Miller mine is located at an elevation of about 9,950 feet, just below· the Lamartine 

road, about 1,500 feet north-northeast of the summit of Alps Mountain. The development consists of a crosscut tunnel about 80 feet in length to a vein which is drifted on to the southwest 

for about 850 feet (fig. 130). 

WALL ROCKS. 

Except for small amounts of gneiss, pegmatite, and bostonitc porphyry near the point 

where the tunnel cuts the vein, the mine workings are all in dark-gray biotite granite. This 

granite immediately in contact with the vein shows striking examples of the development of 

secondary adularia. The rhombie colorless crysta]t; of t~is mineral occur as aggregat(.ls of grains 

associated with secondary quartz and also as border fringes surrounding altered orthoclase 

crystals which have been clouded by kaolin. 

DESCRIPTION OF V"lN, 

The vein, which dips 75° NW. and strikes about N. 60° E., consists of a clay, quartz, and 

friable granite lead several inches wide. Here and there, however, quartz and low-grade ore 

appear along the vein in small streaks. Near the inner or southwest breast of the drift, which 

was being driven ahead at the time of visit; the vein split into three small branches, striking 

S. 50° W., S. 60° 'lf., and S. 70° W. and dipping from 78° to 82° NW. The southern two of 

these leads were mineralized to a small extent, but the most ore occurred at the very junction 

of these two leads a short distance to the. northeast, where the vein formed a small trough 

pitching to the southwest and containing from 1 to 4 inches of galena-sphalerite ore. 'T'he granite 

between stringers of ore and the wall rocks near the vein were pretty well impregnated with 

pyrite. CHARACTER OF ORE. 

The little ore produced by the mine has consisted of galena, dull-black sphalerite, and some 

pyrite in the quartz gangue. Specks of probable tetrahedrite were observed, associated with 

• 
MINES NEAR IDAHO SPRINGS. 335 

the galena and sphalerite, but possibly of slightly later orig\n. The first-class ore averages 

about $30 to $40 to the ton, the·main values being in silver, although the owners are paid about 

$2 a ton for the gold. The sphalerite, which is of a peculiar dull-black variety, occurs at many 

places in considerable abundance, to the detriment of the ore. Black, sooty sulphides, resembling the "black sulphurets" of the miners of the district, arc also here and there associated 

with the lead and zinc ore, but as they carry no higher values than the rest of the ore they probably represent simply pulverized zinc blende and galena. ·Recognizable dull-gray crushed galena 

is found at some places. 

Besides the galena-sphalerite ore, which occurred with quartz as a fissure-vein filling and as 

small stringers in fractured granite, stringers of solid pyrite also cut the wall rocks. Good exam-

. pies of pyrite gradually replacing the granite wall rock were also seen. So far as known, the 

pyrite carries but slight values inthis mine. 

RELATION OF VEIN TO PORPHYRY DTK~;. 

A noteworthy feature. in this mine is the fact that .the vein cuts through and faults for a 

distance of several feet a dike of bostonite porphyry. 

N 

100 200 feet 

EJ B Unrnineralized lead Mineralized lead 

~ Bostonite 

porphyry 

Mixed gneiss 

and pegmatite 

FIG. 130.-Geological plan of Miller tunnel. 

CRAZY GIRL MINE. 

LOCATION. 

The Crazy Girl mine is located about 3,000 feet north-northeast of Alps Mountain. As the 

workings were not accessible at the time of visit, the underground development was not observed. 

CHARACTER OF VEIN MATERIAL. 

The mine is situated in a granite area, and, to judge from material found 011 the dump, the 

vein consists of a series of quartz stringers running through fractured granite. The mine· is 

reported to have produced a small amount of ore, and some ore that was found on the clump 

consisted of galena, cupriferous pyrite, zinc blencle, and quartz. A little speeularite ·and tetrahedrite was also observed associated with the sulphides, and clusters of white to brownish 

fibrous crystals of ccrusite or pyromorphite were noted in vugs in the oxidized ore. 

· In some places the ore appears to have been brecciated and recementecl by quartz crystals; 

in others it fills the space between the comb-quartz coatings on the granite walls. 
336 GEOLOGY OF THE GEORGETOWN QUADRANGLE, COLORADO. 

DONALDSON-CHAMPION DIRT GIWUP. 

DESCRIPTION OF VEINS. 

The Donaldson-Champion Dirt group consists mainly of a single well-defined vein with 

several minor branches, which outcrops first on one side of Trail Creek and then on the other 

from a point near the junction of that stream with Clear Creek to a point within·half a mile of the 

village of Freeland, the circuitous outcrop being 10,000 feet in length. The vein has been 

extensively developed by the Wheatland, Donaldson, Kelly, Little Albert, Champion Dirt, 

Harrison (and Union), and Morgan mines. (See Pl. LXX.) 

The vein, although strong and well defined, has a flat northward dip, usually not over 35°. 

In consequence of this dip, the rugged topography of the surrounding region, and the slight 

variations in strike, the surface outcrops have an exceptionally tortuous course. 

On the surface the vein was recognized trending about east and west along the south slope 

of the mountain spur separating Trail and Clear creeks, a short distance above their junction; 

it then crosses Trail Creek gulch at an elevation of 7,900 feet near the ore house of the Wheatland 

mine, turns abruptly to the south, and runs up the steep mountain slope on the Golden Link 

claim to the point where it is developed by the workings of the Donaldson mine. At an elevation of about 8,850 feet there is apparently a split in the vein, so that on the crest of the ridge, 

at an elevation of about 8,925 feet, two veins appear. The southern of these two, where last 

exposed by workings, had a trend of S. 73° W. The northern vein, which may possibly coincide 

with the so-called "Mount Vesuvius lode," connects with the main workings of the Kelly mihe, 

and after running at the very crest of the ridge somewhat parallel to the southern vein, swerves to a 

N. 60° W. course as it proceeds down the steep northwest hill slope, although the actual strike · 

is to the southwest. At an elevation of about 8,700 feet, where the vein crosses a small gulch 

150 or 200 feet south of the mouth of the Little Albert tunnel No.4, the effect of topography is 

again shown and the surface outcrop of the vein makes an abrupt turn to aS. 42° W. direction 

u'ltil it crosses the crest of another small mountain spur. Beyond this point, where it is developed by the Champion Dirt workings, it trends irregularly N. 75° W. The vein th~n crosses the 

valley of Trail Creek and is probably the one which strikes about S. 72° W. and is developed 

in the Harrison and Union and the Morgan mines. 

MINE DEVELOPMENT. 

All the way from the lowest Wheatland tunnel to the top of the hill above the Donaldson 

mine the vein is almost continuously opened by tunnels at short intervals, from which there 

are numerous stopes and raises. On the Donaldson there are four tunnels, all driven on the 

vein and aggregating more than 2,000 feet in length. Both the Donaldson No. 1 and No. 2 

tunilels connect with drifts running into the Kelly tunnels Nos. 3 and 4, respectively, which 

open to the surface on the opposite or west side of the mountain~spur. The Kelly mine com:.. 

prises about 1,700 feet of crosscuts, drifts, and tunnels. The Little Albert mine has about 1,400 

feet of crosscut tunnels, which connect with 2,100 or 2,200 feet of drifting. The Champion 

Dirt is developed by the Centurion tunnel, which is about 1,100 feet in length and connects with 

a shaft and drifts on four levels aggregating about 2,100 feet in length; and also by six tunnels 

which connect with a total of nearly 1,500 feet of drifts. The Harrison and Union mine develops the vein by means of a shaft from which drifts run, and by a tun~el 60 feet in length connecting with a drift on the vein. In the Morgan mine the workings on the vein all connect 

with the shaft. 

NATURE OF WALL ROCKS. 

The veins lie in an area of gneiss intruded by dikes of pegmatite. As a rule the gneiss 

strikes northeast and southwest and dips toward the west. In the Donaldson mine workings 

the gneiss is chiefly of the compact granitic variety, but farther west, in the Kelly mine, considerable micaceous gneiss is present. Near the Morgan mine both walls are composed of granite, 

and a small amount of granite also appears in the southwest end of the Centurion tunnel level 

on the Champion Dirt vein. 
U.S. GE.OLOGICAL SURVE.Y PROFE.SSONAL PAPE.R NO. 63 PL. LXX 

NW 

N 

---

HORIZONTAL PLAN OF DONALDSON-CHAMPION DIRT MINE WORKINGS. 
U. S. GEOLOGICAL SURVEY 

Alaskite porphyry 

Bostonite porphyry 

Gneiss with small intrusive 

bodies of pegmatite 

~(Vein 

~{ nminera lized lead 

{Vein r==.J M1neral1zed lead 

Pegmatite 

PROFESSIONAL PAPER NO 63 PL. LX XI 

0 100 200 300 400 feet 

~----~------~------~----~ 

GEOLOGICAL PLAN OF DONALDSON-KELLY MINE WORKINGS. 
I ' 

MINES' NEAR IDAHO SPIUNGS. 337 

Porphyry dikes are encountered in some of the workings of .the different mines. Most of 

the porphyry, however, is altered almost beyond recognition. Bostonite porphyry was found 

in the west end of the second level below the Centurion tunnel. In the Kelly mine a much 

altered white, shelly, banded porphyry, possibly of the alaskitic variety, forms a dike from 2 to 8 

feet wide. This same porphyry body was also encountered near the breast of No. 2 tunnel on 

the Donaldson. A 40-foot dike of altered bostonite porphyry which occurs on the surface just 

southeast of No. 0 tunnel on the Donaldson vein forms the foot wall of the vein for a distance 

of more than 100 feet from the mouth of the tunnel, and then disappears to the southeast, behind 

the pegmatite and gneiss which come in on the fo~t wall. 

CO)lPOSITION OF VEINS. 

The ore found throughout this whole group of veins consists chiefly of pyrite and quartz, 

which, according to the miners, carry gold values almost exclusively. According to the reports 

of the Director of the Mint for the years 1890 to 1892, however, which give figures representing 

the production of the different metals, the silver output seems to have averaged about one-fourth 

the value of the gold output. A little siderite or ferruginous rhodochrosite is also associated 

with the quartz. The pyrite, which is as a rule only slightly cupriferous, occurs both in the 

massive and in the crystalline forms, associated with either massive or crystalline vein quartz. 

Cubes of pyrite fully 2 inches in diameter were seen in ore from the Centurion tunnel, but the 

crystals had evidently been severely strained subsequent to deposition, and crumbled readily. 

Some of the ore obtained along different parts of the vein is pegmatite or gneiss hea·vily 

impregnated with pyrite; elsewhere these rocks have been completely replaced by the pyrite. 

On the 50-foot level below the Centurion tunnel a small pocket yielded about 700 pounds 

of ore, which consisted of massive galena and chalcopyrite, associated with smaller amounts of 

pyrite, 11 gray copper," and siderite or ferruginous rhodochrosite. 

The best ore taken from the Donaldson mine is reported to have been oxidized ore averaging 

6 ounces to the ton in mill-run lots. At the time of the visit to this mine a little desultory work 

was being done by lessees, who were obtaining a small quantity of oxidized ore, consisting of quartz, 

limonite, and yellow clay, which is said to have averaged from 2.25 to 2.36 ounces of gold to the 

ton. According to some of the miners, the last shipment of ore taken near the west breast of the 

Centurion tunnel level yielded 4. 7 4 ounces in gold and 4 ounces in silver to the ton. 

The Mount Vesuvius vein, although it has produced little ore, is said to have yielded some 

fine specimens of free gold. 

PRODUCTION. 

The output from the group at the time of the examination was not large, as the portion of 

the main vein developed through the Centurion tunnel was the only section of the vein which 

was being actively developed, and was yielding much ore. However, in times past the production 

from this group has been large, and the total has been estimated to be nearly $500,000. 

DONALDSON llliNE. 

The main vein, after being extensively developed in the Wheatland mine, some of the workings of wJ;llch extend along the part of the vein crossing the northern boundary of the Georgetown quadrangle, is opened up by the. different levels of the Donaldson mine. (See Pl. LXXI.) 

In the Donaldson workings the main vein strikes N. 60° to 65° E. and clips 25° to 30° NW. 

To the west this vein spiits into two parts; the southern branch continues with approximately the 

trend of the majn vein and the northern branch swerves slightly northward and then continues 

S. 75° to 85° W. to the Kelly mine workings. 

Toward the west end of the No. 2 Donaldson tunnel there is a dike of white highly altered 

porphyry (probably alaskite porphyry), which shows in places banded and glassy contact 

phases. This porphyry extends along the drift for only about 40 feet; on the east its contact 

turns into the vein wall and on the west it narrows and wedges out. It is probably the same 

dike that runs along the inner drift on the Kelly No. 4 mine and forms the hanging wall of the 

31401-No. 63-08-22 
/. 

338 GEOLOGY OF THE GEORGETOWN QUADRANGLE, COLOHADO. 

small stope near the breast of that drift. On the surface what is probably the same dike was 

found about 250 feet north of the mouth of the No. 2 tunnel. . 

The line of the junction of the two branch veins with the main vein pitches slightly in a 

west-of-north direction, owing to the fact that the south branch has in general a slightly flatter 

dip than the north one~ The north branch is inclined from 30° to 35° N., whereas the dip of the 

south branch ranges from 20° to 35° at various points. Although not yet located, the junction 

of the two branches probably reaches the surface somewhere between the No. 0 and No. 1 

tunnels, as the fork in the vein, which occurs on all the levels below No. l Donaldson, is not 

found on No. 0 level. 

Most of the ore in the Donaldson mine occurred on the main vein at and immediately east 

of the junction of the two branch veins, and the part of the vein to the east of this union is 

almost entirely stoped out between the different levels up to the surface. On No. 3 level the 

length of this ore body was 275 feet, on No. 2level it was 175 feet, and on No. 1lev~l the junction 

occurs at a distance of only about 60 feet from the mouth of the tunnel. 

Both of the branch veins also carry considerable ore near the junction, but the south vein, 

although heavily mineralized near the point of union, diminishes in strength rapidly toward 

the west and gradually becomes inconspicuous through branching. The north vein, however, 

carries considerable low-grade ore to the point where it leaves the Donaldson workings, and 

stopes from 20 to 40 feet in height are common on most of the levels. A little more than 200 

feet west of the main fork in the vein a minor branch vein enters the north vein, and at this 

junction also there was a rich ore shoot, about 20 feet wide, which was followed from a point 

below No.2 level to a point halfway between No. 1 and No. 0 levels. On No. 0 level a 40-foot 

dike of porphyry with conspicuous large feldspar phenocrysts (probably altered bostonite) 

forms the foot wall of the vein for a distance of more than 100 feet from the mouth of the tunnel. 

For most of this distance there was no stoping, as little ore was encountered. From the point 

where the porphyry dike disappears behind pegmatite and gneiss, however, the vein is stoped 

for a distance of nearly 100 feet along the drift. 

An interesting feature well shown by the Donaldson mine workings is the relation of the 

sulphide-ore zone to the oxidized portion of the vein. The original vein filling consists of streaks 

of slightly cupriferous pyrite, tightly adhering to the wall rocks. The oxidized zone is 40 to 70 

ieet deep and follows approximately the configuration of the present topography fQr the whole 

length of the northeast hill slope. The oxidized material is a porous, skeletal mass of quartz 

from which the sulphides have been disolved, or of quartz and limonite. Soft light-yellow to 

brown clay is also present in many places. 

KJlf,r,y MINE. 

In the Kelly mine the workings are very tortuous. The vein developed on the No. 1 level 

of the Donaldson mine was followed up an inclined passageway until it connected with a drift 

on No. 3 level of the Kelly mine, and along this drift for 40 or 50 feet to the point where the 

vein, which dips 35° N. and trends a little south of west, enters the south wall of the drift. The 

drift just mentioned continues N. 78° W. along another and steeper lead dipping 65° N., which 

emerges from the north wall of the drift near the same point. This last vein, where developed, 

shows but little mineralization. It is very likely that exploratory wo~k will show another vein 

(the continuation of the lead which entered the wall) south of the west end of the drift, though 

whether it will be well mineralized can only be determined by actual development work. 

Although a connection ·also exists between the Donaldson No. 2 and the Kelly No. 4 levels, 

this connection was not open at the time of visit. However, in the portions of the two levels 

adjacent to one another the vein in each level has white shelly porphyry either immediately at 

or a short distance from the north wall, showing the probable identity of the veins. Some ore 

was obtained from a narrow stope extending a little way above and below the drift at the cast 

end of the fourth level on the Kelly, but the vein followed westward from this point shows but 

little evidence of mineralization. 
U. S. GEOLOGICAL SURVEY 

Bostonite 

~ 

~ porphyry Mineralized lead 

PROFESSIONAL PAPER NO. 63 PL. LXXII 

No.2 LEVEL BELOW CENTURION TUNNEL LEVU 

N 

No.1 TUNNEL ON CHAMPION DIRT VEIN 

~----1~0_0 ____ _,200 feet 

Unminera/ized lead 

GEOLOGICAL PLANS OF SEPARATE LEVELS, CHAMPION DIRT VEIN . 
MINES NEAR IDAHO SPRINGS. 339 

For the whole distance exposed this vein has a 2- to 8-foot dike of white porphyry of very 

irregular trend as its northwest wall or is separated from the porphyry either by a breccia zone 

2 feet wide or (where the porphyry loops away from the contact to the north) by from 1 to 25 

feet of pegmatite and gneiss. In places the porphyry dike clips to the north only 10° to 30° 

from the horizontal. The same porphyry dike, with a width of 3 to 6 feet, is encountered 

on No, 3 level, where, although close to the vein near its south westernmost exposure, it is about 

JOO feet distant where exposed farthest northeast. Near the latter portion of the dike a minor 

clay and crushed-rock lead follows the northwest contact of the flike, lopping off protuberances of 

porphyry here and there, and as it approaches the breast of the drift this lead enters the porphyry 

and crosses nearly to the foot-wall side of the dike, which is here only 3 feet wide. It then 

splits into two light-colored clay leads, each one-half inch in thickness, one of which swerves 

back to the hanging-wall side of the dike and the other one follows the foot-wall side. In the · / 

breast of the drift this porphyry is marked by a series of curved concentric varicolored lamellre, 

which have probably resulted from contraction attendant on the loss of a part of the bulk by 

alteration. The banding is probably due to the concentration of iron and other colored oxides 

along the planes of weakness between the lamellre thus formed. Similiar occurrences of ba~ded 

porphyry have been described from the Mercur mining distr.ict in Utah.a 

LITTL~~ ALBBR'l' ~liNE. 

Practically all of the ore obtained in the Little Albert mine is reported to have come from 

the upper workings located near the surface. However, all these upper four levels were 

caved shut, and the lower and more recently run tunnels Nos. 5 and 6. were not visited. The 

general trend of the vein where still exposed on No. 4level is N. 70° E., and the dip at the point 

where a winze to No. 5 level is situated is 55° NW. · 

CHAMPION DIRT O.R ·CENTURION MINE. 

Champion Dirt vein.-In the Champion Pirt mine the vein has a general strike of N. 80° 

to 85° E. On the levels connecting with the shaft leading down from the Centurion tunnel 

level it clips from 35° to 48° N., and on the upper tunnel levels the vein" ranges in inclination 

from 15° to 60° from the horizontal. The flattest dip was noted on the Champion Dirt tuimel 

No. 1, at a point where the vein splits and loops around a large horse of rock. (See Pl. LXXII.) 

In the upper levels, near the surface, the vein zone is 2 inches to 1! feet wide, and usually comprises a series of quartz stringers (many of which show comb structure), in gneiss and pegmatite. 

Alth~ugh some of this qua.rtz carrifls a little sulphide ore, the ore is usually a soft brown oxidized 

material which either runs in a streak beside the quartz or occurs in cavities in it. On the 

lower levels connected with the Centurion tunnel, however, the vein consists either of solid 

streaks of massive pyrite and quartz ore, separated from the wall rocks on each side by gouge 

selvage, or of pyrite and quartz stringers in gneiss and pegmatite. In places, however, pyrite 

ore seems to have been formed by a replacement of the gneiss or pegmatite adjacent to the 

fractures. 

Near the west end of the drift on the second level below the tuimel level the Champion 

Dirt vein cuts through a dike of bostonite porphyry and appears to have offset the north portion 

of the. dike 25 to 35 feet to the west of the south portion. 

Postmineral movement.-An interesting fault zone having its origin subsequent to the period 

of deposition of the ore was noted crossing the Centurion tunnel about 220 feet from its mouth. 

At this point there is a band of soft breccia from 2! to 4 feet wide, containing crushed and angular 

fragments of gneiss, pegmatite, quartz, and pyrite ore, embedded in a gouge matrix. A few 

fragments of porphyry were also noted in the vein at this point, though no porphyry was 

observed in place within several hundred feet. 

Although low assay values have been obtained from this material, no pay ore has been 

struck and all the values are probably fro~ ore "drag" from the older mineralized vein. This 

a Spurr, J. E., Sixteenth Ann. Rept. U.S .. Geol. Survey, pt. 2, 1895, p. 413. 
340 GEOLOGY OF THE GEORGETOWN QUADRANGLE, COLORADO. 

fault zone enters the wall, trending S. 47° W., and was encountered again on this same level, 

at the point where it emerges from the north wall of the drift on the Champion Dirt vein, 200 

feet west of the Centurion tunnel. Th,e breccia zone, after joining the main vein, runs along 

in contact with it to. the west breast of the tunnel-level drift, a distance of 225 feet or more. 

On the second level below the tunnel level, however, the same soft zone joins the Champion 

Dirt vein at a point about 125 feet west of the shaft, and then, after following the hanging-wall 

side of the main vein for a distance of 150 feet, cuts through the vein and disappears in the 

rocks on the foot-wall side. (See Pl. LXXII.) At the point where it cuts through it faults 

the main vein, causing an offset and gap between the two faulted ends equal to its own widthabout 3 feet. Where the vein and the breccia zone were in contact, the continuous streak 

of hard pyrite and quartz ore which usually exists in other portions of the Champion Dirt vein 

was badly broken up by movement and in places mixed with so much crushed and triturated 

wall rock that what would normally have been good ore was rendered valueless. 

Occurrence of ore bodies.-Considerable stoping was done years ago in the upper tunnel 

level<: on the Champion Dirt vein, and a good deal of ore was taken out. A series of small 

stopes. separated by low-grade pyrite and quartz streaks several inches wide were also made in 

tunnel level No. 1. There are large stapes in the workings connecting with the Centurion 

tunnel, the largest of these being immediately east of the point where the broad, soft breccia 

zone joins the main vein, but they do not extend far east of the shaft on any of the levels. In 

the upper levels minor branches enter the main vein and here and there the points of junction 

seem to have been the seats of somewhat more extensive ore deposition, but on levels connecting with the Centurion tunnel branches to the main vein are rare. 

HAHRISON AND UNION MINE AND MORGAN MINE. 

The Harrison and Union mine ·and the Morgan mine were both inactive at the time of visit, 

and as no one in authority could be found the underground workings were not examined. Each 

of these mines is said to have yielded a little ore, although the development in them is slight in 

comparison with that done at other points along the main vein. The vein, w'l'iere intersected 

by a 60-foot crosscut tunnel 100 feet west of the Harrison and Union shaft, has a trend of 

N. 85° Vv. and a dip of 45° to 50° N. Farther west on the surface, however, the vein ranges 

in trend from N. 85° E. toN. 70° E. 

In the vicinity of the Morgan inine a vein which is supposed by the miners to be a continuation of the Harrison-Champion Dirt vein is entirely in granite and has a clip of only about 

20° NW. 

OLD SETTLER MINE. 

HISTORY. 

The Old Settler mine, which is situated at the summit .of the divide separating Turkey 

Gulch from the. valley of Trail Creek, was discovered in 1860,· and in the following few years 

yielded large quantities of rich oxidized quartz ore from points near the surface. With depth, 

however, sulphide ore carrying less values came in and the mine became less remunerative, 

and consequently little work was clone for years. In 1884 work was resumed for a short period, 

but at the time of visit the mine had again long been idle, although some work was contemplated. 

DESCRIPTION OF VEIN. 

The vein consists of a very irregular lead, striking approximately east and west and dipping to the north. The vein is developed by a series of levels running from an inclined shaft 

following the dip of the vein, which near the surface is .50° NW. but flattens below to 30°, so 

that the dip nearly corresponds with the north slope of the hill. The workings are mainly in 

micaceous gneiss, the schistosity of which has a strike parallel to the vein but a somewhat 

flatter ·clip. 

The vein on the first and seconcllevel,s consists •of a soft crushed-rock lead heavily stained 

by iron o~ides, of a granulated mixture of crystalline and massive pyrite with quartz, or of a 
MINES NEAR IDAHO SPRINGS. 341 

series of small pyrite stringers in gneiss. The pyrite, especially on the lower level examined, 

carries considerable copper and the walls in many places show traces of copper sulphate and 

also of the carbonates azurite and malachite. Near the surface the soft oxidized ore has practically all been stoped out for some distance on either side of the shaft. 

CHARACTER OF OR]<;. 

It is reported that the best ore recorded from this mine, as shm\'n by the old Freeland 

smelter-return sheets, gave values of 3 to 4 ounces in gold to the ton and 22 per cent of copper. 

Assays of 3 ounces in gold and 5 per cent of copper have also been obtained, but most of the 

cupriferous pyrite ore ran less than 1 ounce in gold to the ton. Some pyrite ore associated 

with a black sooty copper sulphide, probably chalcocite (Cu2S), is said to have averaged 0.85 

ounce in gold per ton. The production frohl this mine has been considerable, but as most of 

the records have been lost its total production is unknown; probably, however, it is at least 

to be valued in thetens of thousands of dollars. 

STANLEY-LITTLE ]}!ATTIE GROUP. 

STANLEY MINE. 

HISTORY AND PRODUCTION. 

According to Hollister,a the Whale Mining Company 0was organized in 1864 to develop 

the Whale lode and other lodes of the vicinity. This is the lode which is at present known as 

the Stanley. In 1871 b development of the Hukill lode was begun by John M. Dumont. The 

lode known by this name is the portion of the Stanley vein which lies on the northern side of 

Clear Creek. The identity of the Hukill and Whale lodes was afterward established by drifts 

along the vein beneath the creek. In June, 1878, the Hukill and 'Vhale mines were consolidated. At present the property is known as the Stanley and is owned by the Stanley Mines 

Company, of which A. G. Brownlee is president and general manager. 

The Hukill lode, or that portion of the Stanley vein which lies on the northeast side of 

Clear Creek, is reported to have paid well from the start. Development was begun in 1871, 

and at the end of five years the mine was sold for $200 ,ooo·. The excavation of 1,600 feet of 

adits, drifts, and shafts had returned, besides a large amount of low-grade ore, 1,700 tons which 

sold for $195,200. The cost was $167,140 and the profit $20,060. The average yield was 

$114.82 per ton. From 1871 to the summer of 1879 the Hukill mine yielded $55,000, of which 

$25,000 came out in the last two years. In 1~79 the mine was bought by the same Californians 

who had bought the Freeland, and passed under the management of F. F. Osbiston, an experienced Comstock miner. 

The total production of the Stanley mine to elate is rather uncertain, but is estimated to 

be $3,600,000 by Colonel Brownlee, the present manager. 

From $25,000 to $75,000 worth of.ore is roughly estimated to have been shipped from the 

Crockett vein, which is a branch of the Stanley and is situated on the southwest side of Clear 

Creek. 

GENERAL DESCRIPTION OF VEIN. 

The Stanley vein extends from a point beyond the northern boundary of the Georgetown 

quadrangle southwestward across Clear Creek and up the slope. of the ridge between Clear 

Creek and Spring Gulch. Midway of this slope is the Golden Link shaft, which has been sunl>: 

on the vein. Farther southwest the vein splits and the branches as they cross the summit of 

the ridge and descend into Spring Gulch diverge to the southwest, the western branch probably 

uniting farther southwest with the Niagara-Lord Byron vein. The mines situated along Spring 

Gulch, including the Niagara, Lord Byron, Fraction, Kitty Clyde, and Gomer, are situated 

on veins that either run together in such a way as to prove their identity or overlap one upon 

a Mines of Colorado, 1867, p. 234. b Fossett, Frank, Colorado, 1880, p. 368. 
342 GEOLOGY OF THE GEORGETOWN QUADRANGLE, COLORADO. 

the other with relatively little distance between and form a zone of veins which taken together 

may be considered as a general trunk lode, of which the· Stanley, Phoenix, and Cardigan veins 

are probably extensions. 

On leaving Spring Gulch the trend of the Stanley deviates to the east of the general northeast strike of the trunk vein, but recovers this typical northeast direction near the Golden 

Link shaft and preserves it frorA this point on. Near Clear Creek the Stanley splits into two 

branches, slightly divergent from one another to the northeast. The exact point at which this 

bifurcation takes place is reported to be about 200 feet west of the Stanley shaft house, which 

would be near the mouth of the Road level. Of these two branches the more northern is the 

principal one and retains the name of the Stanley vein; the other is called the Crockett. At the 

Crockett shaft, situated on the northeast side of Clear Creek, the south branch, which.strikes 

N. 57° E. and dips 80° NW., is located a few hundred feet southeast of the Stanley vein. 

The underground workings of the Stanley show the same change in the strike of. the lode 

as was observed at the surface. Near the Golden Link shaft there is a sharp curve to the 

northeast, beyond which the vein has almost a due northeast strike, whereas to the s'outhwest 

the strike is more nearly east and west. 

MINH WORKINGS. 

The Stanley mine is developed by tunnel drifts which follow the vein from the surface into 

the mountain which lies on the southwest side of Clear Creek. Of these tunnels the chief is 

the Road level, which starts in near the bottom of the creek. Above this one to the southwest 

come the Whale, York, Golden Link, and smaller tunnel drifts. On the opposite or northeast 

side of the creek is the Hukill tunnel. The vein has also been developed by inclined shafts 

following down the dip of the vein. Of these the most important is the main or Gehrman shaft, 

which starts in Clear Creek valley on the level of the Roa(~ tunnel and follows the vein downward. 

Eight levels start from this shaft below the Road level. Another important shaft is the Golden 

Link, which starts at the outcrop of the vein about halfway up the mountain ridge to the 

southwest and follows it down to the Road level. The general dip of the vein as shown in these 

workings is about 60° NW. 

The underground workings of the Stanley (Pl. LXXIII) have developed the vein along the 

strike for a distance of about 4,000 feet, and the difference in elevation from the highest outcrops 

of the vein to the lowest workings is not far from 1,500 feet. In all, the mine maps show over 

4 miles of drifts and inclined shafts. 

PHYSICAL NATURE 0~' VEIN ZONE. 

The Stanley vein follows a zone of crushing, sheeting, and faulting. This zone, as explained 

on page 130, is of compound origin and records repeated movements. The vein is formed 

mainly along a single zone, which in accordance with most of the veins of this region tlhows 

local branching. The most important branch noted underground is that which on the Road 

level is called the Joker vein. The junction of this vein with the main lode takes place about 7 50 

feet southwest of thc.Golden Link shaft. The branch is nearly parallel with the trunk vein, but 

diverges from it in general in a northeasterly direction. It is a strong vein, which has been 

followed for more than 200 feet from the junction. The dip of the Joker vein on this level 

is 80° S., but that of the Stanley near the junction is 60° N., which is the normal dip for the 

trunk vein. The Joker vein therefore probably runs into and unites with the Stanley above 

the Road level. What is probably the same branch was noted on the Whale or Intermediate 

level, 83 feet above the Road level, but was not drifted on. 

About 800 feet from the mouth of the Road level another branch diverges from the main 

lode, as shovm in the plan (Pl. LXXIV). This branch is a fairly strong crushed zone containing 

several streaks of pyritic ore. 

At the surface on the north side of Clear Creek the local splitting and reuniting of the 

Stanley vein is well illustrated. 
U. S. GEOLOGICAL SURVEY 

Old Stanley shaft 

200 400 600 800 \jr------, '------' 

Golden Link shaft Stapes shown by shaded areas 

Golden Link level 

York level 

Gehrmann shaft 

Sixth level. 

= 

'' 

HORIZONTAL AND VERTICAL PLANS OF STANLEY MINE WORKINGS. 

Hukill tunnel 

1000 feet 

PROFESSIONAL PAPER NO. 63 PL. LXXIII 

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ROAD LEVEL 

LEGE:ND 

1\~\t~t&ll~J Surface slide 

Pegma \ito wall rock 

6 Mineralized lead 

17\ Unmineralized lead 

c.::::::::1 

GEOLOGICAL 

INTER MEDIA TE LEVEL 

PLANS OF SEPARA TE LEVELS, STA NLEY MINE. 

PROFESSIONAL PA PER NO. 63 PL. LXXIV 

N 
MINES NEAB. IDAHO SPRINGS. 343 

NATURE OF WALL ROCKS. 

The principal wall rock of the Stanley vein is gneiss of different varieties, some siliceous and 

quartzose, some dark and biotitic, and some hornblendic. Considerable pegmatite is present, 

generally in small bodies intrusive into the gneiss. Two distinct varieties of porphyry occur also 

in places forming the walls. One of these :~:ocks is a bostonite and the other a biotite latite. 

COMPOSITION 0~' VEIN. 

The chief gangue material is quartz; the metallic ores consist mainly of pyrite and also considerable galena and chalcopyrite. Tetrahedrite is reported to have been encountered in some 

places in important ainount. The ore where it is now being worked is stated to have, after being 

sorted, an average value of $40 to the ton. It contains 1 to 2 ounces of gold, 20 to 40 ounces 

of silver, 25 to 30 per cent of lead, and 2 to 3 per cent of copper. The zinc in the mine is insufficient 

to figure in the smelter returns. 

A little siderite was noted in the mine, some crystals of this mineral forming free in vugs. 

Cherty silica was also noted as one of the last minerals to be deposited, forming a coating in the 

walls and geodes. 

STUDY OF THE DEVELOPMEN'l' OF THE LODE. 

Older wall rocks.-The oldest rocks of this region are gneisses, into which pegmatitcs have 

been injected. All these rocks are of pre-Cambrian age. In general the gneiss belongs to the 

Idaho Springs formation and consists of quartz, biotite, and feldspar in varying proportions, so 

that in some places the rock is hard and quartzose and in others soft and biotitic. In general the 

rock is fairly rigid and is of a character to yield strong and persistent fractures when stress is 

applied to it. 

First porphyry intrusion.-The line of weakness occupied by the Stanley vein was, so far 

as we know, initiated at the time of the intrusion of the porphyry dikes, which occupy a belt in 

this country roughly coincident with the belt of mineral veins. The formation of the primary 

northeast fracture was followed by the intrusion of the porphyry dike. Only shreds of this 

dike are recognizable at present, as it has been torn apart and displaced by repeated reopening 

and differential movement along the original fissure zone. This older porphyry is a bostonite 

and has been recognized by Mr. Ball as similar to the other bostonite dikes which he has traced 

on the surface in this region. In hand specimens the rock shows large phenocrysts, many of 

which have a reddish tinge. A specimen from the wall of the Joker vein in the Road level 

shows under the microscope large feldspar phenocrysts, partly of orthoclase and partly of a 

finely striated feldspar, possibly anorthoclase. There are also pseudomorphs of decomposition 

products after entirely altered pyroxene. · The groundmass has a trachytic structure and. 

consists of moderately small cryst11,ls, main!y of feldspar. The edges of this dike are hard an<l 

dense on account of the more rapid cooling. 

Two unconnected fragments of this dike have been recognized by Mr. Ball near the outcrop 

of the vein on the surface. These fragments, which occur on the ridge on the south side of Clear 

Creek, have a northeast strike and are aligned in a northeasterly direction. 

Underground the bostonite porphyry is recognized on the Road level in two localities, one 

on the hanging or north wall of the Stanley and another on the south wall of the nearly parallel 

branch vein called the Joker. These two streaks are not aligned but are nearly opposite one 

another. They are shown by drifts and prospects to be also of limited extent. Their separation and lack of continuity is to be explained by the repeated faulting (essentially strike faulting but with minor concomitant transverse and oblique displacement) which is proved to have 

taken place since the bostonite intrusion. On the Intermediate level, 3 feet above the Road 

level, the bostonite porphyry is found on the north side of the Stanley vein from 850 to 1,050 

feet southwest of the Golden Link shaft and not elsewhere~ The position of the strips above 

described on the Road level is froin 500 to 700 feet southwest of the same shaft. Thus a combination of the observations on the two levels shows that strips of the dike occur along the 
344 GEOLOGY OF THE GEORGETOWN QUADRANGLE, COLORADO. 

vein for a distance of 550 feet at least, and proves that the extension of this older dike is the 

same as that of the later dike, mineral vein, and friction conglomerate which will presently be 

described. 

Later por·phyry intrusion.-At a distinctly later time than the intrusion of the bostonite 

porphyry, the line of weakness occupied and cemented by the bostonite dike was reopened 

and again filled and cemented by a later dike of quite different composition and appearance. 

This later dike is a biotite latite. It incloses angular fragments of the earlier bostonitc porphyry, as well as of pegmatite, gneiss, and alaskite. This later porphyry under the microscope 

shows a fine trachytic groundmass containing abundant phenocrysts of quartz, feldspar, and 

biotite. There is considerable magnetite and apatite; zircon has also been noted. This dike 

belongs to a series determined and mapped by Mr. Ball. The dikes of this series have a northeast strike and are aligned in a northeast direction along Chicago Creek, where they occur at 

intervals for a distance of more than 4 miles. The dike which occurs in the Stanley is the only 

one known of its kind outside of the group mentioned. It lies about 2! 'miles northeast of the 

northeasternmost of the dikes in the group, 

at a point which is only a little out of line 

with the general trend of this string of 

dikes. 

The Stanley latitc dike was not recognized by Mr. Ball on the surface, although 

the exposures here are good. It is probable, therefore, that it does not outcrop, 

the upper portion of the dike having been 

faulted, brecciated, and effaced by the later 

differential movement along this line of 

weakness. 

Periods of mineralization.-The data 

accumulated by a study of the Stanley 

mine point rather clearly to the conclusion 

that the formation of the Stanley vein took 

place mainly between the bostonite intrusion and that of the latite. Subsequent 

to the injection of the bostonite dike 

renewed stress in the rock reopened the 

old fissure and afforded a channel that 

was occupied and traversed by mineralizing waters, which finally cemented this 

new fissure with sulphides and quartz. FIG.131.-Biotite latite porphyry (c) from the Stanley mine, containing 

angular blocks of pegmatite (b) with galena-pyrite ore seams (a). 

Shows intrusion subsequent to ore deposition. Observations show that the period of 

this mineralization was essentially subsequent to the bostonite intrusion and older than that of 

the latite. The breast of the drift on the Joker vein on the Road level shows the vein running 

through the middle of the bostonite dike, which is here only 3 feet thick and is hard and dense. 

The vein is a strong, unbroken one at this point, consisting of quartz with considerable pyrite. 

On the other hand, angular inclusions of vein material corresponding exactly with the 

ore which has been mined have repeatedly been found in the younger porphyry. This fact has 

been proved both by observations on a large scale and by microscopic work. Fig. 131 shows 

large angular fragments of the vein embedded in the dike. Observations have also been made 

indicating that the flowage in the porphyry dike continued in some places after the catching up 

of old fragments into its mass. The result of this flowage of the viscous magma is especially 

noticeable near the contacts, where streaks of galena, pyrite, quartz, etc., have been drawn out 

together with the igneous rock into fine wavy metallic threads. 

It is thus shown that a considerable amount of the ore deposition took place between the 

two periods of porphyry intrusion. Other considerations indicate that practically the whole 
MINES NEAR IDAHO SPRINGS. 345 

formation of the vein was included in this interporphyry period. The strongest, wide~t, and 

best portion of the vein as a rule lies entirely within gneiss walls. The great stope seen on the 

Road level extends for 500 feet within gneiss walls, but becomes discontinuous and weak where 

the latite dike enters the vein zone, as seen on the plan. The general relation of dike to veins, 

as shown on this plan, suggests that the former has been intruded into the latter. Some ore 

is fo:und at intervals on both sides of this intruded dike but without the same continuity as is 

characteristic farther northeast, where the dike is not present. The situation suggests that 

the mineral vein has been split open subs~quent to the mineralization; that in places the chief 

fissure ran along the center .of the vein, as the brittle sulphides were more easily fissured than 

the wall rocks; and that the igneous mass was injected into this fissure, to both walls of which 

fragments of the vein still adhered. This fissuring of the vein, leaving fragments of ore clinging to both sides of the opening, is exactly like what took place in the Griffith vein, as already 

described. The difference between the Griff1th and the Stanley veins is that in the latter the 

fissure was filled by an igneous injection of porphyry and in the former it was cemented by a 

deposition of minerals from solution, which, however, were of a character so strikingly different 

from those of the original mineralization that the history of the vein is quite as clearly recorded 

for one period as for the other. 

A considerable proportion of the drifts in the Stanley mine follow the contact of the latite 

dike. The dike shows the typical fluted contact due to a molding of the plastic magma into 

the laminffl of the gneiss, as was described in the discussion of the Colorado Central dikes. Considerable reaches of this contact are marked by no trace of mineralization, and where ore does 

occur between the dike and the gneiss it stops abruptly in many places as if cut off by the 

intrusion. 

As further evidence that the main mineralization occurred previous to the latite intrusion 

is to be cited the fact that the latite shows very little subsequent mineralization and moreover 

. is relatively fresh, as it could hardly be if it had been subjected to attack by strong mineralizing solutions. This is the freshest porphyry noted in the district in association with an ore 

deposit. The almost unfailing rule for the porphyry associated with the vein is that the igneous 

rock is highly altered, so that this is a striking exception. 

In a few places the later porphyry was observed to be traversed by pyritiferous quartz 

veins representing the mineralization subsequent to its intrusion. One such instance was · 

noted in a specimen taken fron1. the foot wall of a dike, at the contact with the gneiss, in the 

crosscut on the Road level, about 60 feet northeast of the mule winze. The porphyry near 

these quartz veinlets is more altered than usual. However, there is no ore streak at this point, 

which is separated from the ore vein on the southeast by. 10 or 15 feet of gneiss .. This vein 

has been stoped and lies entirely in gneiss. It probably represents the main mineralization, 

older than the latite. ' 

The watercourse along which the Stanley vein formed, in the interval between the intrusion of the bostonite and that of the latite, must have been in part an open rubble-filled fissure 

like that in which many of the veins of the Silver Plume region were, in part at least, formed, 

as, for example, the Mendota. This fact is indicated in the Stanley mine by the same phenomena as observed in the Mendota and other veins of this type. In the Hukill tunnel, and also in 

.the Intermediate level, places were observed where the original vein consists of a cement of 

quartz, pyrite, chalcopyrite, and galena around angular fragments of gneiss, pegmatite, and 

bostonite porphyry. 

Formation of friction conglomerate.-Subsequent to the injection of the latite dike the old 

fissure was again reopened by a renewal of rock stress similar to that which had caused the 

successive reopenings marked by the successive cementations by the bostonite porphyry, the 

mineral vein, and the latite porphyry. This postlatite reopening was marked, as were the 

earlier ones, by differential movement along the walls of the fissure. zone, bringing about brecciation and faulting of the earlier cementing materials, including both the porphyries and the 

vein minerals. One of the most striking results of this last movement was a breccia, many of 

whose component fragments are subangular. or even perfectly rounded, and which may there-
346 GEOLOGY OF THE GEORGETOWN QUADRANGLE, COLORADO. 

fore be called a conglomerate. The evidence is plain that these rounded forms are due to the 

rubbing of one fragment against another in the course of the differential movement of the walls, 

so that the deposit may be styled a friction conglomerate. This conglomerate zone runs along 

the vein· or along the porphyry and ranges up to several feet in thickness. The fragments in 

this breccia consist of both the bostonite and latite porphyries, the various older wall rocks, 

and the vein materials. The crushing and subsequent dragging out by faulting have been 

locally at least very extensive, and large sections of the dikes and veins have been involved 

and mingled with fragments of gneiss in a heterogeneous mixture. As the exact plane of 

movement represented by this friction conglomerate did not always coincide exactly with the 

earlier movements along this mne, the breccia in some places follows the contact of the core of 

the porphyry and in others cuts obliquely across these older formations, shearing them off. 

For example, on the Road level near the mule winze the latite dike is cut off by the friction 

conglomerate. Farther on, along the same level near the principal crosscut to the south, the 

same breccia zone also faults the latite dike, separating it into two portions which are 15 to 20 

feet from one another measured along the strike. Still farther southwest, beyond the point 

where the latite porphyry leaves the drift, the lead which has been followed consists only of a 

slip and breccia representing this last period of differential movement and containing practically 

no ore. 

This portion of the drift is probably off from the original lode, which may lie to the south 

and from which the line of the last movement seems to have here, as elsewhere, locally deviated. 

An instance of deviation of the line of the friction conglomerate from that of the vein is shown 

in the Hukill tunnel, where there are two veins 15 feet apart but uniting at both ends so as to 

leave a horse of gneiss between. One of these veins is a solid lode of gneiss consisting of pyrite, 

chalcopyrite, and galena, and representing the main period of mineralization. The other vein 

is a breccia lode containing fragments of gneiss, pegmatite, and ore similar to that in the firstdescribed vein, all inclosed in a gray clay matrix. Some of the angular fragments of ore in this 

breccia show, included within the ore, angular fragments of pegmatite and gneiss, to which 

the ore acts as. a cement. These fragments, which are from the older solid vein, record the 

earlier period of opening, brecciation, and cementation; the soft breccia represents the postlatite movement.· 

The minor oblique and transverse faulting which accompanied the more important strike 

faulting accomplished by the postlatite movement is illustrated also on the Whale level about 

180 feet northeast of the Golden Link shaft. Here, as seen on the map, the dike of latite porphyry is offset in such a way as to indicate faulting oblique to the vein. A specimen of the 

porphyry taken at this point and examined microscopically shows internal crushing and straining 

since cooling. The phenocrysts have been step faulted along minute close-setting fractures. 

The general direction of movement along the zone of friction conglomerate is indicated by 

striations of the walls observed in two places on the Road level. Near the Mule winze strire 

dip 60° SW., and at the end of the level they dip 34° SW. 

At the time of this latest dislocatioh the rocks were frequently split open so as to leave 

open fissures. The records of these openings arc especially plain in the prophyry, which was 

hard and brittle because it had not been subjected to the softening influences of alteration by 

earlier mineralizing waters. Many such open fissures in the porphyry were filled by deposits 

which can not be ascribed directly to crushing, but rather to infilling by water-borne detritus. 

The shape of these fissures indicates rending rather than shearing stress; yet the filling material 

is substantially of the same nature as other portions of the breccia zone where the materials 

have originated largely by grinding, and it is probable that the filling of the fissures was simply 

transferred by water from the supply of material due to attrition (fig. 132). 

Under the microscope the detrital veins filling the fissures in the porphyry show the general 

structure of sandstone or arkose, containing rounded, angular, or subangular grains of kaolinized feldspar, quartz, microcline, etc. 

Mineralization subsequent to formation of fricti~n conglomerate.-In many places along the 

sides of the zones of friction conglomerate are solid residual fragments of vein material clinging 
MINES NEAR IDAHO SPRINGS. 347 

to the original walls. Open fragments of this ore locally form a very large part of the breccia or 

conglomerate, and where present in sufficient abundance may be mined, although in many places 

they have been so scattered about in the breccia as to spoil the value of the vein. Aside from 

these fragments of included older ores, there is usually very little or no evidence of mineralization in the conglomerate. Nevertheless, there are indications that a certain subordinate 

amount of ore deposition has taken place, somewhat similar in quantity to that which has been 

inferred from observed data to be subsequent to the introduction of the latite dike. Here and 

there the arkose or conglomerate veins contain scattered or locally rather abundant sulphides, 

including galena and copper pyrite, which are not fragmental but have acted as interstitial 

cement to the rock fragments. This was noted both in the hand specimen and under the 

microscope. In the microscopic study it was observed that sulphides had thus formed in an 

arkose veinlet which had been injected along the fracture in the later porphyry. 

In a few places this postconglomerate mineralization may have formed seams of sufficient 

size to be of economic value, or at least to encourage the 

miner to further search. On the whole, however, it bas 

• been of little consequence from an economic standpoint. 

More important than the deposition of sulphides has been 

that of silica, which has thoroughly cemented the fragmental material so as to form a rock that is in many 

places quite .as hard and firm as the later porphyry or any 

of the wall rocks. Indeed, much of the hard, fine arkose 

is not distinguishable with· grGat facility from some of the 

dense varieties of latite. 

Alteration of rocks.-A specimen of gneiss from the wall 

rock of the lode in the Road level some distance from any 

porphyry when examined microscopically shows abundant 

ealeite and chlorite as decomposition products. The original nature of the rock is not clear, but it may be a sheared 

and altered hornblendic gneiss. 'l'he extremely abundant 

calcite is probably derived from original feldspar, and the 

chlorite may be due to the alteration of hornblende. Orthoelase, which is present in subordinate amount, is fairly fresh,. 

and biotite, which is more abundant, has been changed to 

a green color and eloude<l but otherwise not greatly altered. 

Fragments of the old gneiss forming \valls to ore yeinlets, 

the whole being included in the later porphyry (latite) dike, 

show under the microscope feldspars which have been entirely altered to sericite. 

The bostonite porphyry is highly altered, as is rock of 

this type wherever it has been found elsewhere. The phenoFw. 132.-Sketch (aetna! size) of specimen from 

· Stanley mine, showing biotite latite (a) cut by 

firmly consolidated small vein or dike (b) of 

fragmental material derived from granite, gnefss, 

pegmatite, ore, and porphyries. . The biotite 

latite is bleached (c) near the walls of the dike 

by the waters accompanying the plastic material, 

which has been injected or squeezed into a fissure. 

crysts consisted chiefly of feldspar and pyroxene. The feldspar was apparently orthoclase with 

some finely striated probable anortJ:loc.lase. These minerals have been altered to kaolin, the· 

alteration extending from the outside inward, so that many residual cores of comparatively fresh 

feldspar remain. One specimen shows the feldspar altered to kaolin and sericite. The pyroxenes in all specimens have been completely destroyed, and are represented by decomposition 

products, ehiefly calcite, magnetite or hematite, and abundant deep-purple fluorite, which also 

occurs along cracks in the rock and is abundant in some particles in the groundmass. Fluorite 

has been frequently found by Mr. Ball in the dikes of this composition. The mineral in this 

case is closely associated with calcite and is of secondary origin. The pre~ence of fluorine and 

carbonic acid in the waters which accomplished the alteration of the rock subsequent to its 

intrusion is suggested, and it is highly probable that these waters deposited the vein. 

As before noted, the later porphyry is unusually hard and fresh, and the partial alteration 

which it has undergone is of a constant and characteristic nature. The biotites are almost invari-
048 GEOLOGY OF THE GEORGETOWN QUADRANGLE, COLORADO. 

ably quite fresh. The feldspar includes both orthoclase and caleic feldspars in varying proportions 

in different sections, but the former on the whole predominating. Some of these feldspars are 

almost entirely clear and fresh. Others, though retaining their general freshness, are altered to 

a greater or less extent along the cleavage cracks, and locally the alteration has involved most 

of the feldspar. The calcic feldspars have yielded calcite on alteration; the orthoclase feldspars 

kaolin. In one specimen examined considerable brown carbonate was noted, taking the place of 

the calcite and especially abundant in the altered groundmass. In one exceptional specimen, 

where the porphyry contained subsequent pyritiferous quartz veinlets, the microscope showed 

that the feldspar phenocrysts were more altered than usual and contained secondary quartz 

with some sericite and kaolin. 

Two interesting examples of the effects of different periods of alteration were found by 

microscopic study. A specimen of latite porphyry from the Intermediate level contains intrusive 

veinlets of arkose, whose grains are in part cemented by subsequent galena. The alteration of 

the porphyry is of the usual type, with fresh biotite and orthoclase almost entirely altered 

to kaolin, but among the fragments which occur in the arkose vcinlets orthoclase of granitic or 

pegmatitic origin is also present and is only very slightly kaolinized. The difference of freshness 

presented by the veinlet and the intruded rock is striking-and shows that the main alteration of 

the porphyry preceded the injection of the arkose. 

Illustrating a different point is a specimen of latite porphyry from the Intermediate level, 

about 75 feet northeast of the Golden Link shaft. This porphyry incloses fragments of older 

ore and of older gneiss wall rock. The porphyry has fresh biotite phenocrysts and feldspar 

phenocrysts that are mainly fresh though partly al~ered.to calcite. In the fragments included 

in the porphyry the gneiss wall rock of the veinlets contains relatively fresh orthoclase, singly 

striated feldspar whi~h· is partly sericitized, and blocks of entirely sericitized feldspar. This 

alteration is that typical of the wall rock of lodes in this region and it has been repeatedly described 

elsewhere in this volume. The striking difference in the alteration of the included gneiss and the 

porphyry shows that the gneiss WI1S altered previous to its being taken up into the dike, and it 

is probable that its alteration was the effect of the solutions which deposited the vein. 

LOCATION 0~' ORE. 

The stope projection of the Stanley mine shows an unusually irregular shaping of the 

principal ore bodies. The stopes do not go much above the Whale level. Above this on the 

Golden Link and York levels the vein is represented by a zone of breccia or friction conglomerate 

containing only broken fragments of ore. In this vein successive reopenings and brecciation 

subsequent to the main period of mineralization have worked the same result as in the Griffith. 

Large bodies of the original vein have been left intact or not much broken and mixed up. These 

constitute the greater part of the present stopes. In many places, however, the original vein has 

been entirely broken up for long stretches. The ore has been dragged out and mixed with so 

much rock breccia as to be unavailable for economic purposes. 

Originally some of the best ore seems to have formed at the intersection of branches, as is 

common in mines in this region. On the Road level, at the junction of the main Stanley vein 

·with the Joker vein, a large quantity of high-grade ore has formed. The first big stope in the 

Stanley vein coming in from Clear Creek is said to have formed by the junction of this vein with 

the Crockett. There was no ore on the Crockett near the junction. 

SUllfM.A.RY 0~' .GEOLOGIC HISTORY. 

The hist01y of this vein is one of repeated openings along a single zone of weakness, resulting 

in fissuring and accompanied by faulting. This process has been going on since a time before or 

at the beginning of the period of dike intrusion, and has probably not yet come to an end. The 

original formation of a zone of fracturing and faulting in the pre-Cambrian gneiss was followed by 

the intrusion of a dike of bostonite porphyry. Subsequent to the cooling of this dike the movement was renewed, the dike filling was ruptured, and oblique and strike faulting along the dike 

separated one portion from another, resulting in an intermittent dike marked by a string of 
U. S. GEOLOGICAL SURVEY PROFESSIONAL PAPER NO. 63 PL. LXXV 

"' 

I 

u 

Kitty Clyde sha/1 

100 200 300 

· Stopes shown by shaded areas 

HORIZONTAL AND VERTICAL PLANS OF CONSOLIDATED FRACTION MINE WORKINGS. 
MINES NEAR IDAHO SPRINGS: 349 

discomi.ected fragments. This is the general character of dikes in this whole region, and is 

probably due to the same cause throughout. They were probably not originally intermittent, 

but have been made so by strike faulting. One of the consequences of the renewed movement 

was a series of porous zones of crushed material and open rubble-filled fissures. These became 

the channels for powerful mineralizing solutions which cemented the openings with quartz and 

metallic minerals. Again the renewal of strain brought about a reopening of the old break, and a 

dike of laLite was intruded, splitting and brecciating the vein. Subsequent to the consolidation 

of the latite porphyry the same fissuring process was repeatc<l, with, as before, attendant differential movement or faulting. The result was the formation of the friction conglomerate and of 

open fissures which were filled by detritus c11rried by waters from the ,crushed material or 

squeezed in, in the form of mud. Certain features in the mine, such as the presence of open 

fissures and watercourses, indicate that this movement has not yet entirely ceased and that the 

rock strains have not been fully relieved. 

The main features of this history arc the repeated, almost continual, movements along the 

same zones of weakness, and the openness and lack of great crushing along the fissures and 

faults. The latter feature is indicated by the evidence of the presence of open rubble-filled 

fissures following each violent dislocation. The friction conglomerates characteristic of many 

of the mines of this region have excited much comment on account of their rather unusual 

nature. Ordinarily the material along faults lias been crushed to a paste bi the tremendous 

shearing stresses. The fact that the fragmental material along the veins in this region has been 

affected almost exclusively by rubbing of one fragment against another in the course .of the mass 

movement shows that there has been practically no grinding stress, and that the movement, 

even if attended by faulting, was one of rending rather than of shearing. A corollary of these 

deductions is that the friction conglomerate was formed relatively near the surface, and the same 

observation applies to a less extent to the older openings. 

NIAGARA MINE. 

The Niagara mine is located at an elevation of 8,440 feet in Spring G'ulch, a little over 

2! miles N. 77° W. of the junction of. the gulch with Clear Creek va.lley. The vein, which was 

formerly developed through a shaft, strikes N. 60° to 65° E. and dips 55° NW. The Niagara 

vein continues to the southwest as the Lord Byron vein; to the northeast it probably joins the 

vein exposed in the shaft on the crest of the hill to the northeast, which is termed the "Old 

Stanley shaft." The Niagara mine at one time produced considerable ore, but has long laid idle. 

JAMES MINE. 

The James mine, which is only in the prospect stage, is developed by two shallow shafts 

located in Spring Gulch, 700 or 800 feet cast of•the Niagara mine. The main vein, which, where 

exposed in the workings, has a trend orN. 62° E. and a dip of 55° to 60° NW., probably joins 

the main Stanley vein, of which it is either a branch or a continuation. The vein consists of a 

quartz and crushed rock lead 4 to 10 inche~ wide. Near the shaft on the James vein a branch 

vein enters the main vein from the N. 78° E. 

LORD BYRON MINE. 

The Lord Byron mine is located in Spring Gulch, at an elevation of about 8,600 feet, about 

1,700 feet northeast of the Fraction engine shaft. The development consists of a shaft from 

which rather long drifts run on several levels. However, although extensive development work 

has been done, the mine is reported to have produced little ore. At the time of visit the workings were all filled with water within about 50 feet of the collar of the shaft. The vein on the 

surface appears to trend about N. 60° E. and to dip 75° to 80° NW. · 

The wall rocks in the vicinity and on the clump are chiefly gneiss, but a dike of bostonite 

porphyry outcrops beside the shaft house in a position corresponding to the hanging wall of the 

vem. The Lord Byron vein follows this porphyry more or less closely for 800 or 900 feet to the 
350 GEOLOGY OF THE GEORGETOWN QUADRANGLE, COLORADO. 

northeast, to a point where the vein splits. The southeast branch has been traced in aN. 62° E. 

direction continuously until it joins the vein in the Niagara mine; the northwest branch fqllows 

the southeast contact of the porphyry dike, which swerves to the N. 35° E. and was traced to 

the gulch several hundred feet distant. 

This vein possibly continues across to the northeast side of the gulch, for a simila~ porphyry 

dike with a like trend is found on that side, and continues up the hill slope to a point between 

the Phoenix and England mines. A short distance northwest of the Lord Byron shaft house is a 

vein which, on the surface, trends N. 45° E. and dips f).5° to 60° NW. This vein shows some signs 

of mineralization. 

}<'!~ACTION AND KITTY CLYDE MINES. 

flENERAL DESCRIPTION. 

The Fraction and Kitty Clyde mines are located on the northwest side of Spring Gulch, a 

little over 2~ miles N. 80° to 85° W. of the town of Idaho Springs. These two mines develop a 

series of veins which formerly composed one of the most productive portions of the StanleyKitty Clyde vein system. 

The main vein may continue to the northeast as the Lord Byron vein, but surface outcrops 

widely separated by the gulch to the southwest of the Lord Byron seem to indicate rather that 

the Fraction vein passes a sho1;t distance to the northwest of the Lord Byron shaft and thus 

either overlaps the Lord Byron vein or continues as a vein into which the Lord Byron runs. 

Between the main Fraction shaft and the Pierce shaft, about 400 feet N. 72° E., there seems to be 

a loop formed by the veins splitting and nmning around a large horse of rock. 

Between the Fraction and Kitty Clyde shafts the vein swerves slightly 'to the south, and 

there is another branching, the forks diverging toward the west. The west or Kitty Clyde shaft 

is on the chief of these branches, which probably continues with about a S. 55° W. trend and a 

48° NW. dip to the point where it is joined' by the Gomer vein. Quartz leads showing some 

mineralization were also noted about 150 feet south of both the Kitty Clyde and Fraction shafts. 

These leads are approximately parallel to the main vein at the points observed, but possibly 

join the main vein or some of its branches farther northeast. 

As little work has been done in either the Kitty Clyde or the Fraction mine for nearly 

fifteen years, they are practically inaccessible through caving and the filling of the lower levels 

with water. (See Pl. LXXV.) 

NATURE OF WAT,L ROCKS. 

No porphyry was observed either on the portion qf the second level examined or on the 

mine dump, although about 250 feet east of the Kitty Clyde shaft a body of porphyry was noted 

on the foot-wall side of the vein. This porphyry, which probably forms a· portion of the dike 

that runs fairly parallel to the vein to the vicinity of the Lord Byron shaft, was probably 

encountered in the underground workings of the Kitty Clyde mine, for a few pieces of porphyry 

were found on the dump of this mine. The gneiss from the Fraction mine was of both the 

micaceous and the granitic varieties, and some of it was highly garnetiferous. Garnets up to 

one-fourth inch in size were found. · 

CHARACTER OF VEIN FIJ,LING. 

On the second level of the Fraction mine, except where stoping had been done, the vein 

was a narrow quartz and pyrite lead, adhering tightly to the gneiss and pegmatite walls. Specimens of ore found in the ore bin were composed of galena and cupriierous pyrite associated with 

quartz and ferruginous rhodochrosite. Some of the ore also showed specks of tetrahedrite or 

polybasite. Quartz and pyrite stringers were moreover noted cutting micaceous gneiss. In 

every specimen examined the ore stringers were tightly "frozen" to the wall rocks. 

Ore found on the dump of the Kitty Clyde mine consisted of galena, blende, and pyrite, in a 

gangue of quartz, ferruginous rhodochrosite, and some barite. 
MINES NEAR IDAHO SPRINGS. 351 

DEVEI,OPMENT AND PRODUCTION . 

. An idea of the amount of development work and stoping done in ~he Fraction and Kitty 

Clyde mines can best be obtained by examining the vertical longitudidnal section of the veins. 

(See Pl. LXXV.) 

The estimated production of the Fraction mine is between $7 5,000 and $100,00.0; the Kitty 

Clyde mine is also said to have yielded about $75,000. 

According to Dr. James Underhill,a both the Kitty Clyde and the Fraction mines show 

excellent examples of ore occurring at and beyond the junctions of veins, for in the Kitty Clyde 

two veins unite going northeastward to form a flat vein and ore body, and in the Fraction three 

veins unite going westward to form an ore body which is continuous with the ore shoot just 

mentioned as occurring in the Kitty Clyde mine. 

G!leis.s 

rRlJ77!!7l!i 

~ and pegmatite Mineralized lead 

FIG. 133.-Sketch of fir'st level of Gomer mine, showing deflecting influence of porphyry dike on vein where 

vein cuts through dike. 

GOMER OR HEDDENSBURG MINE. 

GENERAl, DESCRIPTION. 

·The Gomer or Heclclensburg mine is located in Spring Gulch at an elevation of 9,900 feet, 

about 3 miles west-southwest of Idaho Springs. The mine is developed by a shaft and drifts 

situated on a vein which is probably a branch of the Kitty Clyde-Fraction vein farther northeast. 

The shaft follows the vein, which clips 45° NW. for most of the way, but near the bottom st~epens 

to 60° in the same direction. Water filled the shaft within about 180 feet of the collar. A drift 

between 240 and 250 feet in length leaves the shaft at the 180-foot level and runs eastward. 

(See sketch map, fig. 133.) NATURE OF VEIN. 

On this level the vein has a very irregular strike, owing to its deflection where it cuts across 

a dike of bostonite porphyry about 20 feet in width. As seen from fig. 133, the mineralized 

vein trends about N. 53° E. between walls of fractured gneiss and pegmatite, until it encounters 

the nearly east-west porphyry dike, whereupon it follows the south contact of the dike in a· 

N. 64° E. direction for a short distance before cutting through the porphyry. On emerging on 

the north side of the dike the vein follows the north contact ,for 60 or 70 feet be for~ it leaves the 

porphyry dike, and continues to theN. 83° E. with both walls pegmatite and granitic gneiss. The ----------------- a Oral communication. 
352 GEOLOGY OF THE GEORGETOWN QUADRANGLE, COLORADO. 

vein ncar the center of the dike IS narrow and contains only a small seam of quartz, crushed 

porphyry, and mineral, but as it approaches the margins of the porphyry mass it becomes stronger 

and in places carries considerable galena ore. What little mineral occurred along the vein near 

the center of the dike consisted of sphalerite mixed with some quartz and a little pyrite. 

The ore, where observed along the vein, consists of small bunches irregularly distributed 

and a series of seams in gneiss and pegmatite. 

CHARACTER OF ORE. 

Specimens of massive, low-grade ore from the bins showed an intimate mixture oi galena 

and green to bl&ek zinc blende, associated with considerable contemporaneous pyrite and 

pinkish to yellowish- brown ferruginous rhodochrosite. Some small specks of tetrahedritc 

and specularitc were also observed. 

In places the ore gives evidence of having been deposited in an open fissure partly filled by 

rubble, for comb quartz, though not abundant, is present and forms a layer about angular rock 

fragments as well as a coating on the walls of the fracture. Vugs are also common and may be 

lined with well-formed crystals of sphalerite and galena, but more generally with crystals of 

slightly later quartz, ferruginous rhodochrosite, or barite, which occur upon the faces of the· 

galena and sphalerite crystals. 

BANTY MINE. 

The Banty mine is located in Spring Gulch between the Lord Byron and Metropolitan mines. 

The vein, which on the surface runs N. 60° E. and clips 65° NW., is developed by a shaft from 

which a series of short drifts have been driven. The mine wa~ closed at the time of visit. The 

country rock in the vicinity and on the clump consists chiefly of granitic gneiss. 

Some specimens of ore found in the ore bin showed quartz and pyrite occurring as stringers 

in gneiss; others showed angular fragments of quartz and pyrite ore embedded in a cementing 

matrix of dull-yellow jasper-like quartz; a few showed galena, zinc blende, cupriferous pyrite, 

and specks of tetraheclrite. Considerable fenuginous rhodochrosite was also observed. 

METROPOLITAN MINE. 

The Metropolitan mine is located on the northwest side of Spring Gulch, about 800 feet 

northwest of the Lord Byron shaft house. The walls of the vein, which in this mine runs about 

N. 75° E. and dips about 70° NW., consist mainly of granitic gneiss. The vein is developed 

by a shaft on the vein and by short drifts from this shaft. 

A small quantity of granulated-appearing quartz, heavily impregnated with pyrite, was 

observed in the mine clump. The mine, which was not working at the time of visit, is reported 

to have produced a little ore. 

PHOENIX MINE. 

The Phoenix mine and vein are located in Spring Gulch about 300 to 400 feet northwest 

of the'Niagara vein. The vein, which is worked through a shallow shaft, strikes about N. 70° E., 

and if it continues in this direction w )Uld join the northwest branch of the Lord Byron vein. 

ENGLAND MINE. 

LOCATION. 

The England mine consists simply of a tunnel level, the mouth of the tunnel being located 

at an elevation of 8,550 feet on the northeast side of the mountain spur a little more than a mile 

due west of Idaho Springs. 

WALL ROCKS. 

The wall .rocks adjacent to the vein consist entirely of gneiss and pegmatite, although on the 

surface a short distance east of the tunnel there arc two parallel porphyry dikes about 150 feet 

apart which appeared to trend S. 20° W. The western one of these dikes is about 10 feet wide 
MINES NEAR IDAHO SPRINGS. 353 

and consists of a grayish porphyry the variety of which was not determined; the eastern dike 

is about 30 feet in width and is composed of reddish bostonite porphyry. In the mine the gneissic 

structure is in general parallel to the vein. 

VEIN AND VEIN FILLING. 

The strike of the vein, which is developed by a drift approximately 400 feet in length 

(fig. 134), is about S. 60° W. and its clip is about 45° to 50° NW. The vein is not an especially 

strong one, being formed of a series ofconnectecl minor fractures. Locally the vein consists of a 

series of stringers of quartz, here and there showing comb structure, in gneiss; elsewhere it consists of gray quartz containing a few angular fragments of wall rock and showing some little 

replacement by mineral. The vein is only slightly mineralized, but in places carries a little 

ore consisting of galena, 

pyrite, quartz, and both 

brown and resin zmc 

blende. 

CARDIGAN MINE. 

The Cardigan mine is 

located in a ravine termed 

Cardigan Gulch, on the 

south side of Clear Creek, 

1 mile west of Idaho 

Springs. It is developed 

by three tunnel levels, 

aggregating about 2,000 

feet of workings. However, the mine has long 

laid idle and nearly all 

the workings are inaccessible through caving. 

11ost of the rock in 

the vicinity appears to be 

gneiss, chiefly of the micaceous variety, the attiGneiss 

Mineralized lead 

~ 

~ 

Pegmatite 

B 

o;:__ __ ..;;.•oo-=-----=2~oo:__ _ _::.;3oo teet 

FIG. 134.-Geological plan of tunnel on England vein. 

tude of which corresponds in general to that of the vein. In pla.ces a. mixture of gneiss and 

pegmatite having a gnarled or crumpled appearance was observed. Specimens of pinkish bostonite porphyry were found on the dump of the lower tunnel, so it is possible that a dike of this 

porphyry was encountered in the mine, especially as such a dike is present on the surface southeast of the shaft. Near the shaft on the lower leve', where two nearly parallel leads were 

noted, the vein consists of leads of crushed' gneiss and clay 2 inches to 1 t feet in width. In places 

these leads contained vein quartz and pyrite. The general trend of the vein, as seen from the 

mine maps, was about N. 60° E. and the dip about 40° NW. 

GLADSTONE TUNNEL. 

The Gladstone tunnel is situated on the south bank of 'Clear Creek, near the point where 

Cardigan Gulch joins the valley of this stream. The wall rocks consist entirely of gneiss and 

pegmatite. 

The main vein strikes about S. 65° vV. and clips from 60° to 70° NW. The vein consists 

of a quartz lead, ranging from 2 inches to over a foot in width, which locally is made up solely 

of quartz with a little pyrite, though elsewhere it is composed of quartz, pyrite, galena, and 

brown sphalerite. 

About 325 feet from the mouth of the tunnel a crosscut spur to the northwest cuts a clay 

and crushed gneiss lead several inches wide, which carries some pyritiferous quartz. Near the 

31401-No. 63-08-23 
354 GEOLOGY 0!< THE GEORGETOWN QUADRANGLE, COLORADO. 

point where this spur leaves the tunnel the main vein is joined from the west by a clay lead 

several inches wide, which runs in contact with the vein for about 50 feet and then crosses it and 

disappears in the foot-wall gneiss. A considerable number of small stopes oceur scattered 

along the vein and indicate that the mine produced some ore in times past. 

About 300 feet N. 35° W. of the Gladstone tunnel is a small tunnel called the Donna Juanita, 

which was driven for about 250 feet S. 6"5° vV. on a small vein dipping 40° NW. Two minor veins 

striking N. 80° E. and N. 25° E. were encountered in a short tunnel located between the Gladstone 

and Donna Juanita tunnels. 

SALISBURY l\IINE. 

The Salisbury mine is located on the north side of Clear Creek, about 600 feet north-northwest of the main Stanley shaft. The developments consist of a crosscut tunnel about 17 5 feet in 

length, and a drift on the vein about 400 feet in length which connects with winzes and raises 

(fig. 135). The country rock consists chiefly of gneiss, principally of the mieacoous variety. 

The .schistosity of the gneiss ranges in strikes from N. 65° E. to east and west, but as a rule is 

nearly parallel to the vein; the dip is between 45° and 55° N. 

The vein is a strong one. It dips 30° to 45° N. and has a very irregular strike, which in 

general is a little south of east. Tho lode consists of a quartz and crushed rock vein, which at 

many places splits into foot-wall and hanging-wall leads. About 250 feet east of the tunnel 

[ 

B Micaceous gneiss Mineralized lead Unmineralizcd lead 

FIG. 135.-Geological plan of Salisbury tunnel. 

there are two heavily mineralized leads 8 to 10 feet 

apart. Considerable stoping has boon done along 

the vein, both above and 

below the tunnel level, at 

various points along a distance of 350 feet east of 

tho tunnel. Most of the 

mineral observed along 

the vein consisted of both 

massive and crystalline 

copper-bearing pyri to, and 

possibly some chalcopyrite, a.'lsoeiated with quartz, and ,occurred as scams running through 

fractured gneiss. A little galena is present. In places the chalcopyrite in the orcs has been 

altered to a grayish-black or bluish-black mineral, probably chalcocite. 

The Salisbury mine formerly produced considerable ore and for a long time supplied most of 

the ore used in running tho 10-stamp mill located on Clear Creek a short distance above the mine. 

LINCOLN GROUP. 

GENERAL DESCRIPTION. 

The Lincoln group of veins (Pl. LXXVI, A) is located on the southwest side of Clear Creek, 

between 1,000 and 2,000 feet below the mouth of Trail Creek. The group consists of the Lincoln, 

South Lincoln, Josephine, and Barber and Elliott veins. These veins, which have a general 

northeast-southwest trend and a comparatively flat dip to the northwest, arc located in an area of 

gneiss, usually of the micaceous variety, which has been intruded by small bodies of pegmatite. 

Porphyry of the biotite latite variety occurs along both the Josephine and South Lincoln veins, 

but is absent from the workings on the other veins. 

LINCOLN VEIN. 

The main Lincoln vein, which is the westernmost of the group, has on the lower tunnel level 

a trend of about N. 35° to 40° E. and a dip of only 25° to 35° NW. for the whole length of the tunnel, which was followed to a point about 950 feet southwest of the mouth, where a cave dams 

back the water and prevents access to the workings beyond. The levels connecting with the 
U. S. GEOLOGICAL SURVEY PROFESSIONAL PAPER NO. 63 PL. LXXVI 

A . MINES OF LINCOLN GROUP OF VEINS, LOOKING WEST FROM CLEAR CREEK 

1 MILE ABOVE IDAHO SPRINGS. 

1, Elliott & Barber mine; 2, J osephine mine ; 3· Sout h L inco l n ve i n; 4• L inco l n mine and mil l. 

B. WALTHAM MINE 'TUNNELS. 

Showing two northeast ward .dipp i ng vein leads i n much fract ured pegmatite and gneiss. 
( 

I 

I 

\ 

MINES NEAR IDAHO SPHINGS. 355 

shaft sunk from the lower tunnel were also inaccessible, as was the intermediate tunnel on the 

Lincoln vein. The upper level on the Lincoln vein consists of a single drift and stope about 250 

feet in length, connected by an 80-foot crosscut with the upper tunnel which runs on what is 

termed the South Lincoln vein. In these upper workings the Lincoln vein trends N. 52° E. and 

dips 45° to 52° NW. On the surface, however, owing to the flat dip and the steepness of the hill 

slope up which the vein runs, the outcrop trends about N. 20° E. The walls of the vein, so far as 

observed, consist entirely of siliceous or granitic phases of gneiss, to which the quartz and mineral 

streak appeared in general to adhere rather tightly. 

Extensive stopes occur along the vein on t~e lower tunnel level and the yield from this part 

of the mine is said to have been large, although no definite figures concerning the production were 

1 

_ ;; ,t-<" LINCOLN LOWER TUNNEL 

.::::::::~-l{:_==._ 

__ ---=:::~2== _ ,,,-_ 

;;_ jL--:_~ 

-~~;..~,---

~ -.d:::- - -=--:h:'-=-===-

_cfjfl~- 4~;~~w'i:\'~~'" -- -~f{_S~-=- ;-:{t~:;;~~:{;-

=-:: A&~~;_~~1ifA~ti!f!:J-;c:- ~35Q_ ~RAiSE-& CHUTE .,·rr--~~~~~~-===-- --- --- -..JL..'\"\:~1.·~--- -:::.._---=:::......=t: ,._~ -=]._J, -~~~·r-:---

~~ ~~~ _- -;::11[~ -=:::::-- - ::-.: ;.'/- - 1\"' v-;,-"1\ -r:=. 

~~1k~ :3~tt!!Y= ~;. SHAFT,:-- ,.,-J-,..~.r,~~C··<-,--:-

0 100 

Mineralized lead 

FIG. 136.-Geological plans of sepnrate levels, Lincoln veins. 

200 feet 

Pegmatite 

~ 

~ 

Unminer.:lized 

lead 

obtainable. On the lower level in leaner portions of the vein where only drifting had been done, 

so that the vein material was still observable in the roof and walls, it was found to consist entirely 

of pyrite and quartz. Patches of pyrite and quartz were found adhering to the walls of some of 

the stopes. No galena was observed on this level. 

On the upper level (fig. 136) at the time of exainination two lessees were stoping on an 8-inch 

streak of cupriferous pyrite, which carried a little galena and contained scattered vugs or dissolution cavities filled with a black pulverulent sulphide, possibly chalcocite (Cu2

S). The copperbearing pyrite was tarnished in places by a varicolored film probably resulting from the 

oxidizing influence of underground waters. The ore being removed was said to carry 6 or 7 ounces 

of silver to the ton and enough gold to bring the values up to an average of about $14 to the ton. 

The most gold appeared to be associated with highly cupriferous pyrite, while the black sooty 
356 GEOLOGY OF THE GEORGETOWN QUADRANGLE, COLORADO. 

sulphides were reported to carry little if any more values than the plain pyrite ore, as some ore 

especially rich in them returned values of only $19 to $27 to the ton. Near the surface the vein 

filling for a shallow belt consists of a porous gossan or quartz heavily impregnated and stained 

with iron oxides. 

Years ago a 15-stamp mill, located just iwrth of the mouth of the lower Lincoln tunnel, was 

run on ore from the Lincoln mine, but this mill, like the lower workings of the mine, has long been 

idle. The present output from the upper level is small. 

SOUTH LINCOJ,N VEIN (POSTMINERAI, BRECCIA). 

The mouth of the lower tunnel on the South Lincoln vein is located on about the same level 

as the lower Lincoln tunnel and only about 225 feet S. 35° E. of the entrance to the latter. The 

vein, which strikes N. 35° E. and dips about 60° NvV., runs parallel to the Lincoln vein, which is 

about 200 feet farther southeast, and is followed by the two tunnels for their whole lengths of 700 

and 225 feet. The vein is a stroJ?.g breccia lead ranging from 6 inches to several feet in width, and 

consists chiefly of fragments of pegmatite and gneiss embedded in a matrix of clay or soft gouge, 

although here and there bowlders of pyritiferous quartz occur in the soft matrix. For the most 

o~------~•o.._o _____ ---'..Jzoo feet 

L:'>-'>1 ~~ ~~~-~~.~:~·1 Biotite latite porphyry Gneiss Unmineralized lead Pegmatite 

FIG. 137.-Gcological plans of separate levels, Josephine vein. 

part the walls are of siliceous 

gneiss, micaceous gneiss, and 

later pegmatite. The schistosity of the gneiss strikes 

northeast. On the lower level 

the foot wall of the vein for 

a distance of 250 to 300 feet 

from the breast of the tunnel is formed by a porphyry 

dike about 17 feet wide. The 

dike is of biotite latite, in 

which both the phenocrysts 

and the groundmass have 

been highly altered. Although small pyrite and 

quartz stringers cut the 

gneiss, running somewhat 

parallel to the dike, they are 

invariably separated from the porphyry by a clay seam which runs along the contact. Near the 

point where the dike at the northeast swerves abruptly to the east behind a considerable thickness of gneiss and pegmatite which comes in as the foot wall the clay and breccia lead begins 

to widen, and it reaches a maximum width of 10 feet at a point, 225 feet from the mouth of the 

tunnel, where the main vein is joined by a branch lead from the north. On the southeast side of 

the 17-foot dike a small clay lead runs parallel to the dike, but though at some places this lead 

is in contact with the porphyry, at others it cuts through the gneiss and leaves lobes of gneiss 

in undulations along the surface of the dike. No fragments of the biotite latite porphyry 

were noted in the breccia zone. No stoping has been done on this breccia zone, which is called 

the South Lincoln vein, and no ore was noticed except the fragments of pyrite and quartz in the 

breccia lead and the small pyrite stringers cutting the gneiss. The zone probably represents a 

faulting subsequent to the main period of ore deposition. 

JOSEP!IINE MINE. 

'fhe Josephine vein, which lies about 200 feet southeast of the South Lincoln, is developed by 

three short tunnels, the lowest of which, however, was caved shut a short distance from the mouth 

at the time of visit. The middle and upper tunnels are only ~00 and 150 feet in length, respectively. A shaft is sunk from the intermediate level at a point 50 feet from the mouth of the 

tunnel. 
MINES NEAlt IDAHO SPRINGS. 357 

Except where porphyry is present the wall rocks are gneiss and pegmatite, the attitude of 

the gneiss in general being approximately parallel to that of the vein. On the middle and lower 

levels biotite latite porphyry forms the hanging wall of the vein in places, but is separated from 

the vein at other points by horses or lenses of gneiss and pegmatite. In the upper tunnel 

(fig. 137) the vein lies entirely in gneiss and pegmatite. The porphyry on the hanging wall of the 

Josephine is similar in character to the porphyry forming the foot wall of the South Lincoln vein 

and may be a portion of the same porphyry mass, as the trend of the dike in both mines is very 

irregular. The vein, which varies in dip between 40° and 50° from the horizontal in both the 

upper tunnels, trends S. 35° W. for most of the way and then swerves slightly more to the west. 

On the middle level this bend corresponds somewhat closely to a bend in the porphyry dike 

forming the wall. The vein is a breccia lead ranging from a few inches to 2 feet in width at various points and consisting of angular fragments of gneiss, pegmatite, granite, pyrite-bearing 

quartz, and highly altered porphyry embedded in a white to gray clay or gouge. In places the 

vein filling is well indurated through silicification subsequent to the movements which caused the 

breccia, but as a rule it is comparatively. soft. The fact that angular fragments of porphyry are 

included in the vein would indicate that movements have taken plrtce since the intrusion of the 

porphyry. Aside from the few angular fragments of pyritiferous quartz found in the breccia 

lead and a few small pyrite 

stringers running alongside of and 

parallel to the breccia lead, no 

mineral was observed on the two 

upper levels of the mine. · "N othing could be learned concerning 

the lower inaccessible workings. 

This so-called vein is probably a 

breccia zone which has developed 

subsequent to the main period of 

mineralization.a 

HARBER AND ELLIOTT JIIINE. 

The veins developed in the 

Barber and Elliott mine are located 

r-=:::::1 r--:::::1 

Mineralize(! 

t=_j Unmineralized ~ 

lead lead· 

FIG. 138.-Geological plan of intermediate tunnel, Barber and Elliott lode. 

150 to 200 feet southeast of the Josephine mine workings. The development consists of three 

tunnels. Neither of these tunnels, however, was accessible for its whole length, owing to caving 

at points between 300 and 400 feet from the mouth. 

The wa.ll rocks consist entirely of gneiss, with small intrusive masses of pegmatite. The 

schistosity of the gneiss is approximately parallel to the vein. The veins in general strike about 

N. 60° to 65° E. and dip from 50° to 75° NW. 0~ the lowest level, and also for a distance of 

17 5 feet from the mouth of the intermediate level (fig. 138), the vein consists of a single strong 

vein, but at the latter point on the intermediate level it splits into foot and hanging leads, which 

diverge as they proceed southwestward until, at a point about 100 feet distant, the foot-wall 

lead, which is the less mineralized, enters the foot-wall side of the drift. On the upper tunnel 

level two veins run parallel and only about 20 or 25 feet apart, and these may represent the two 

leads formed by the splitting of the vein on the intermediate level. 

The vein consists usually of a narrow quartz and pyrite lead adhering tightly to the wall rocks, 

but in places is composed of a quartz and clay streak several inches wide. Near some of the 

stopes the quartz and pyrite vein filling contains specks of galena scattered through it, so the 

best ore probably also carried galena. Considerable stoping has been done along the vein on all 

· the levels. On the intermediate level the main stope is just east of the point where the two leads 

make a junction going east; on the upper level the stope on the northwest vein is just southwest 

a See, in the description of the Stanley mine (pp. 345-346), the account of the ''friction conglomerate" which corresponds to the Josephine 

vein and which nlso was lonned subsequent to the intrusion of a dike of biotite latite, itself later than the main mineralization period. 
358 GEOLOGY OF THE GEORGETOWN QUADRANGLE) COLORADO. 

of the point where the vein is joined by two minor branches from the northeast. Vein junctions 

have therefore probably had an important influence on ore deposition in tllis vicinity. 

The fact that in the Barber and Elliott and the Lincoln mines, where no porphyry was found, 

the veins consist of continuous quartz and ore streaks tightly adhering to the wall rocks, whereas· 

in the Josephine and South Lincoln mines, where porphyry occurs, the veins consist solely of a 

breccia lead containing scattered angular fragments of porphyry and of pyrite and quartz ore, 

would indicate that at least a part of the movement along the veins in the Josephine and South 

Hncoln mines is subsequent to both the porphyry intrusion and to the period of formation of the 

solid quartz and ore veins. Possibly the intrusion of the porphyry was also subsequent to the 

formation of the earlier pyrite and galena veins, as was found to be the case with the same variety 

of porphyry in .the Stanley mine. 

LITTLE MATTIE-NEWTON VEIN SYSTEM. 

GENERAL D~>SCRIPTION OF. VEINS. 

The Little Mattie-Newton group is so named because the principal vein has been most 

extensively developed in the Little Mattie and Newton mines, which are located from 2 to 2~ 

miles west-southwest of Idaho Springs, in steep gulches running southeastward to the valley of 

Chicago Creek. 

The group consists essentially of a single exceptionally strong and well-mineralized northeastsouthwest vein and the branches into whlch this main vein breaks up at the two ends. Throughout its course the vein is intimately associated with one or more porphyry dikes. Northeast 

of the Little Mattie mine the main vein follows in a general way the line represented by the 

Great Hepublican, Decator, General Thomas, Newton, and Wild Rose claims. In the Wild Rose 

claim the vein becomes hard to follow, owing to the lack of surface outcrops and to the fact that 

the vein, after crossing the ridge extending northeastward from Alpine Mountain, probably 

splits into at least two branch veins which eontinue northeastward and cross Spring Gulch. 

Two probable branches are distinguished in the Star and Red Lyon tunnels. These branch 

veins probably follow approximately the course of two porphyry dikes which run some distance 

apart and then join near the summit of the ridge jtist southwest of the Bullion King shaft. 

It is very likely one of the branches of the same vein system which is developed by the 

workings of the Bullion King and Mayflower mines. Another minor vein follows the porphyry 

dike, which swerves northward and may possibly be the same as one of the porphyry dikes 

found in the Stanley mine. 

Southwest of the I.1ittle Mattie mine the main vein is strongly developed in the workings 

of the Silver Glance mine, but a short distance farther southwest it breaks up into several 

well-marked but less highly mineralized fractures, of which one is represented by the vein in 

the Arthur mine and another by the vein in the King Solomon and Dorritt workings. The 

junctions of these branches with the main vein .were not seen by the writers, but it is claimed 

by some of the old miners familiar with the mines in the vicinity that the splitting of the main 

vein takes place in the now inaccessible levels in the southwestern part of the Silver Glance mine. 

The Little Mattie-Newton vein, together with the lodes into which it splits, is opened at 

short intervals for a distance of about 3l miles along the strike. In the Little Mattie mine 

the lode h1ts been developed mainly by a shaft which extends to a vertical distance of about 

650 feet below the surface in the lowest part of the gulch and by a series of eight levels, which 

are represented by drifts that extend along the vein in both directions from the shaft. Besides 

these workings a tunnel between 2,300 and 2,400 feet in length follows the vein northeastward 

into the hill to a point 500 or 600 feet beyond the Newton shaft, with which it connects. The 

drifts and tunnels in the Little Mattie mine (Pl. LXXVII) alone aggregate about 10,000 feet 

in length, and most of them are still open. Besides these and the several hundred feet of · 

development in the Silver Glance and Decator mines (which were inaccessible), the vein has 

been opened up on the Newton and General Thomas claims by inore than 4,000 feet of drifts 

run from shafts. The Newton and General Thomas shafts, which reach depths below the 
U. $.GEOLOGICAL SURVEY 

Discovery tunnel 

0 200 

VERTICAL AND HORIZONTAL PLANS OF UTTLE MATTIE WORKINGS. 

PROFESSIONAL PAPER NO. 63 PL. LXXVII 

400 800 

Stapes are shown by shaded areas 

1000 feet 

" 

,, 

,, 

:~-03 

" 

,, 
• S. GEOLOGICAL SURVEY 

Bostonite porphyry 

,, REPUBLICAN SHAFT I~ 

0 

Gneiss 

100 200 

Unidentified porphyry 

N 

! 

300 400 

Pegmatite 

Mineralized lead 

TUNNEL LEVEL 

sao feet 

Mixed gneiss and 

pegmatite 

SIXTH LEVEL 

EIGHTH LEVEL 

PROFESSIONAL PAPER NO. 63 PL. LXXVIII 
MINES NEAR IDAHO SPRINGS. 359 

surface of 275 and 250 feet, respectively, have not been used for a good many years, the development on these properties having been carried on through the Little Mattie tunnel, which 

intersects the Newton shaft about 230 feet below its collar. 

NATURE OF WALL ROCKS. 

The country rock along the Little Mattie-Newton vein is mainly pegmatite, gneiss, and 

porphyry. (See Pl. LXXVIII.) In places the pegmatite occurs by itself for long distances, 

but at other points it is intimately mixed with the gneiss. The gneiss is chiefly of the dark 

micaceous variety. Its strike varies considerably, but in general corresponds somewhat to the 

trend of the vein, or runs a little more toward the east and is intersected by the vein fracture 

at a very low angle. The clip as a rule is between 60° and 75° NW. However, just southwest 

of the cross vein on the seventh level the gneiss strikes S. 50° E. and clips northeast. Altered 

porphyritic granite or "corn rock" is common in the vicinity of the "cross vein." 

At many places porphyry forms one or both walls of the vein. There are either two dikes 

which in places come into coritact with each other, though usually parallel and separated by 

. from 2 to 25 or 30 feet of country rock, or else one large porphyry dike which splits into two 

or more parts that loop around large masses of pegmatite and gneiss and then reunite. Where 

there are two distinct belts the one to the northwest is usually the broader. On the surf~ce the 

southeast dike appears to be about 10 feet wide, and the northwest dike ranges from 10 to 20 

feet or more. In places the surface of the dikes is in uneven rolls, and at certain points, 

as on the seventh level of the Little Mattie, the margins show a banded structure, probably 

due to flow. 

Apophyses of porphyry are in a very few places given off from the main porphyry mass. 

One of these narrow spurs of porphyry was observed intruded between the laminre of the gneiss 

a short distance northeast of the cross vein on the tunnel level. In the breast of the fourth 

level on the Little Mattie is a 4-foot porphyry belt separated from the main hanging-wall porphyry dike to the northwest by 1 foot of crushed gneiss. Traced westward, this 4-foot belt 

of porphyry narrows and disappears. In the Little Mattie tunnel porphyry forms the hanging 

wall of the vein, or is only a short distance northwest of the vein for the whole length of the 

level-about 2,400 feet. On this same level porphyry is also found on the foot wall for several 

· hundred feet both to the east and to the west of the Newton shaft. On the different levels 

run from the Little Mattie shaft porphyry forms portions of both walls. It is more common on 

the foot wall near the shaft and along the vein to the southwest, but more prevalent as a hangingwall rock to the northeast of the shaft. 

The porphyry, which falls in the class of bostonites, is, when comparatively fresh, pinkish 

gray to reddish brown, with usually conspicuous phenocrysts of feldspar. The feldspar crystals, 

however, are generally altered to a pale-greenish color. A few altered barlike crystals of darker 

silicates are present.· In much altered forms the rock mass bleaches to a white or light.:gray 

color, except where stained brown by iron oxide or black by manganese oxide. Under the 

microscope the groundmass in most of the specimens of the porphyries ~hows the trachytic 

texture characteristic of this class of rocks. 

NATURE OF VEIN. 

The Little Mattie-Newton vein has formed along a strong fault line, the course of which 

was determined by one or more preexisting dikes of bostonite porphyry. The strike of the vein 

is approximately N. 40° E., but on the surface the direction of the outcrop is about N. 47° E., 

with divergences to the northwest here and there when crossing gulches, owing to its north-· 

west clip. ·The clip usually ranges between 65° and 70°, but in places the vein rolls slightly, as 

shown by the fact that the shaft, which follows the average clip, is about 15 feet southeast of the 

veiri on the eighth level. Movement along this fault plane is shown by discordances in the 

wall rocks on the two sides of the vein and by the zones of brecciation and trituration which are 

general throughout the underground workings. Slickensides and strire on these smooth polished 

surfaces also indicate movement. The attitude of the movement strire ranges from horizontal 

to a clip 15° SW. 
360 GEOLOGY OF THE GEORGETOWN QUADRANGLE, COLORADO. 

The fault along which the vein subsequently formed .followed alo11g, close to, or entirely 

within one or the other of the porphyry dikes for the whole distance now covered by the main 

vein, which disappears by branching at the two ends. 

The vein, while consisting locally of a single mineralized fracture, as a rule belongs to the 

double-wall type of veins (see p. 99), which consist of a vein zone from 1 foot to several feet 

wide, bounded by definite hanging and foot walls. The leads along the two walls are about 

2 to 4 feet apart on an average, but the distance between them ranges from a few inches to 

14 feet in some places. The vein filling, which in the mineralized areas consists chiefly of gouge 

and crushed wall rocks, gives way to quartz and then to quartz and ore as the ore bodies are 

approached. The crushed vein filling as a rule consists chiefly of gneiss and pegmatite, but 

in some places contains fragments or bowlders of porphyry. 

The vein follows in the main the contacts of the two porphyry dikes with pegmatite and 

gneiss on either side. Many portions of it, however, lie entirely within the horses or longer 

belts of country rock included between the two parallel dikes. . A much smaller part of the 

Little Mattie-Newton vein is entirely in porphyry and the vein may consist of a single frac- . 

ture plane or may be represented by two distinct fractures bounded on the outside by two 

definite walls of porphyry and with a belt of porphyry 4 or 5 feet wide between them, as in the 

second tunnel northeast of the Newton shaft. Here also was noted a lens of pegmatite 2 feet 

wide about 2 feet northwest of the hanging-wall lead and apparently embedded entirely in 

porphyry. 

The irregular distribution of the porphyry along the vein, as well as the great variation 

in the apparent thickness of the individual dikes, may be due to a series of strike or oblique 

fault planes following the weak zone represented by one of the porphyry dikes, and to horizontal 

displacement along these planes, causing the irregular slices or slabs of porphyry to be entirely 

lacking in one place while they overlap in others and produce an apparent thickening of the 

dike, in a manner somewhat similar to the occurrence in the Pelican-Bismarck vein system at 

Silver Plume. This would account for the occurrence of the porphyry in tlie tunnel east of 

the Newton shaft. The irregular distribution of the porphyry is also shown on the fourth 

level of the Little Mattie, where a 5-foot dike in the northeast breast of the drift narrows toward 

the west and disappears. In certain places where the surface of the. porphyry dike is in a 

series of, rolls the vein is in contact with the crests of the rolls or undulations, and disconnected 

masses of gneiss and pegmatite occupy the hollows between the vein and the porphyry. Except 

where the vein follows a porphyry dike, the contact of the porphyry with the adjacent pegmatite 

or gneiss is very tight and practically no gouge or other indication of movement is present. 

Between the Little Mattie and Newton shafts on the surface two porphyry dikes run along 

for some distance, separated only by 2 to 4 feet of pegmatite and gneiss. This same condition 

was locally observed underground, where the contacts with the two dikes represented the footwall 'and hanging-wall sides of the vein. 

STRUCTURE OF VEIN. 

Where the vein belongs to the double-wall type, the material between the two walls may 

originally have been comparatively solid rock around which the fractures looped, or a fractured 

or crushed and pulverized rock mass filling the space between the vein fractures. 

From the hanging and foot fractures the mineralizing solutions penetrated the rock mass 

between and] probably first caused the alteration and replacement with ore minerals of the 

intensely crushed and fractured portions of vein zone, at the same time that the harder and 

less fractured horses of rock were being bleached and altered so that later they offer~d little 

resistance to impregnation by metallic ~ulphide or to complete replacement by vein materials. " 

The walls bordering the vein zone were also altered for short distances on either side, and in a 

few places were impregnated with sulphides to such an extent that portions of the walls immediately adjacent to the vein paid well for extraction. In places where the vein zone between 

the two walls is rather wide, the mineralized portions consist of foot- and hanging-wall streaks 

of ore connected by a network of minute stringers or seams of ore which intersect the inter-
MINES NEAR IDAHO SP!UNGS. 361 

vening D"tass of fractured rock and mineralize it to such an extent that practically the whole 

zone can be shipped as ore. Such an ore body occurred about 140 feet east of the shaft on the 

sixth level, where a continuous streak of galena-tetrahedrite ore was found on each wall and 

where about 65 per cent of the 10-foot zone, with ore stringers all through it, was shipped 

directly as ore. On the same level west of the shaft the ore zone was 3 to 11 feet wide and 

75 per cent of the material was sent directly to the sampler. The ore streaks along the foot-wall 

side are said to be usually a little better developed, but may play out, and a seam of ore may 

cross over to the hanging-wall side and continue along in the same direction. Quartz usually 

coats the wall rocks next to the vein fracture and also forms a thin layer between the ore and 

the angular fragments of country rock in the vein zone. · . 

Just southwest of the Little Mattie shaft on the fourth level a 5-foot belt of crushed pegmatite and gneiss occurs between a dike of porphyry which forms most of the foot wall of the lode 

and a 2-foot porphyry dike which lies to the northwest. Still farther northwest, beyond the 

2-foot porphyry dike, is more pegmatite and gneiss. The main lead here is between the 5-foot 

pegmatite and gneiss belt and the 2-foot porphyry dike to the northwest. Followed southwestward this rock belt between the two porphyry dikes narrows to 2 feet and first becomes 

highly silicified and then changes to quartz and ore. To the east of the shaft also the same 

crushed pegmatite and gneiss seam narrows, becomes silicified, and changes to quartz and ore 

as the two bordering dikes approach within 6 inches to 1 foot of each other. In tlus group of 

mines narrow belts of crushed rock bounded on two sides by porphyry dikes seem to have been 

locations favorable to the deposition of ore. 

Where the foot and hanging leads of the vein occur along the two sides of a porph:yTy belt 

only 2 or 3 feet wide, the fractured porphyry has been highly altered, silicified, and in some 

localities either replaced more or less coinpletely by metallic sulphides and gangue minerals or 

mineralized to such an extent along the infinite number of cracks in .the fractured porphyry 

that the whole mass can be profitably mined. 

COMPOSITION OF VEIN. 

The ores from the main vein of the Little Mattie-Newton group are predominantly gold 

bearing along certain portions, but silver is of greater value in others. It is claimed that in 

the Little Mattie mine the ores to the west of the so-called" cross vein" generally yield a greater 

percentage of silver than gold, whereas to the east of the same fault gold is the more va)uable 

constituent of the ore, although there arc a few exceptions to these general statements. 

Quartz is the chief and practically the only gangue mineral. The metallic constituents 

comprise galena, zinc blende, tetrahedrite (gray copper), polybasite, and possibly small quantities of argentite (silver glance); pyrargyrite (ruby silver) has also been found. Both native 

silver and native gold occur in a few places. About 100 feet east of the shaft on the seventh 

level of the Little Mattie mine was found a small pocket, 4 or 5 feet wide, of finely fractured 

quartzose rock associated with a brownish to yellowish clay which carried $32 in values and 

on panning showed free gold. On the same level ore stoped west of the "cross vein" is said by 

Mr. Sutherland, the superintendent of the mine, to have averaged between 100 and 300 ounces 

of silver and 2 or 3 ounces of gold, whereas to the east of the "cross vein" the ore generally carried 

only 7 to 30 ounces of silver with a gold content of about 2~ ounces. 

In the double-wall veins, where mineralized, there is usually a solid streak from 1 inch to 

several inches wide of high-grade smelting ore on one or both walls, with values averaging 

between $200 and $400, and the fractured zone between the two walls has stringers of ore 

throughout and is in many places sufficiently well mineralized to make good milling ore. Small 

pockets of ore, however, have at various times within the history of the mine yielded several 

hundred pounds of ore running more than $1,000 to the ton. Some of tllis ore carried from 

30 to 42 ounces of gold, 400 to 600 ounces of silver, 5 to 10 per cent of lead, and 5 to 10 per cent 

of copper. Near the surface some of the ores were of the soft oxidized kind; lower down were 

found some strea.ks of the soft black sulphides locally called "black sulphurets," wllich were 

associated with the galena, blende, and other heavy sulphides. Lobes or patches of ore con-
362 GEOLOGY OF THE GEORGETOWN QUADRANGLE, COLORADO. 

sisting largely of these soft black sulphides were obtained as far down as the sixth level, where 

in the Pike & Rowe lease, nearly 750 feet below the surface of the ground, the vein filling consisted of 6 inches to 3 feet of black sooty sulphides and galena ore mixed with crushed quartz 

and angular fragments of silicified gneiss, pegmatite, and porphyry, which had been cemented 

to some extent by quartz. Ore from the Little Mattie mine that is of too low grade to ship 

directly to the smelter is treated in the 20-stamp concentrating and amalgamating mill belonging to the same company. RELATION OF ORE DEPOSITION TO DEPTH. 

The highest grade of ore and the largest ore bodies in the Little Mattie mine, according to 

Mr. Sutherland, the superintendent, were found between the fourth and seventh levels, the 

portions of the ore shoots in the vicinity of the sixth and seventh levels being especially good. 

The ore bodies in places extended continuously from these lower levels to the sur-face, but above 

the fourth level were of no better grade and were not as wide as between that point and the 

seventh level. Moreover, some of the ore from the second level ran only $40 to $60 to the ton, 

whereas immediately beneath on the sixth level ore running over $200 to the ton was obtained. 

At the time of the examination considerable 6re, said to average about $200 to the ton, was being 

obtained from the Pike & Rowe lease near the east breast of the sixth level of the Little Mattie 

mine. 

Below the seventh level the ore, so far as development has shown, has been decidedly leaner 

in values, although some milling ore has been obtained just above the lowest or eighth level. 

FAULTS. 

In the Little Mattie mine workings the main vein is faulted from 18 to 20 feet on the fourth 

level, and from 6 to 12 feet on the other levels of the mine, by a strong but only slightly mineralized vein, termed in this mine the "cross vein." This is supposed to be the same vein which 

is being followed in the Star tunnel, driven in the direction of the Little Mattie from Chicago 

Creek. This vein, where intersected on the Little Mattie tunnel level, about 280 feet from the 

mouth, has a strike of N. 28° W. and a dip of 65° NE. On the other levels the trend varies 

somewhat from that on the tunnel level, and the dip, while always to the northeast, ranges from 

40° to 70°, the fiQ.ttest inclination recorded being on the fourth level. 

The "cross vein" carries no pay ore, although assays of $2 to $6 in gold have been obtained 

from some of the material of the fault zone. The material :filling the fracture consists of crushed 

and pulverized country rock or gouge. The wall rocks in the vicinity of this fault are highly 

altered for 5 to 30 feet from the fault plane, especially on the foot-wall side. Altered pegmatite or coarse porphyritic granite ("corn rock") is a characteristic wall rock in the vicinity of 

this "cross vein." Where porphyry is cut by this fault the dike is displaced and is badly fractured for some distance from the crossing. · 

Another fault in the niain vein was noticed in the Little Mattie tunnel about 25 feet southwest of the raise to the Decator shaft. The strike of the fitult mine was about N. 12° E. and 

the clip 45° W. Here also the northeast portion of the main vein appears to have been offset 

a short distance to the south. 

In a stope worked to the surface, near the Thomas shaft, the main vein is intersected by a 

cross fracture dipping northeastward only 35° from the horizontal and trending S. 70° E. This 

flat fault offsets the northeast portion of the Little Mattie-Newton vein and the porphyry dike 

forming its foot wall about 5 feet to the S. 70° E. Movement strire on the slickenside along 

this fault plane dipped to the southeast 10° from the horizontal. About 40 feet farther northeast a parallel cross vein caused a step fault by offsetting the northeastern portion still farther 

to the southeast.a · 

Another strong fault line is represented by the Old Chicago-Bismarck "vein," a wide soft 

zone which crosses the Little Mattie-Newton vein several hundred feet east of the Newton shaft. 

As no work had been done in the Newton, General Thomas, and Decator shafts for many years, 

a Possibly one of the above-described faults constitutes the so-<lailed Guy Irving vein, which is supposed to be a tlat vein said to cross the 

Little Mattie-Newton ve;n a few hundred feet west of the Newton shaft. 
MINES NEAR IDAHO SPRINGS. 363 

the levels extending from them were inaccessible and the relations of these faults to the main 

vein were here undetermineu. The mine maps show, on the third level of the Newton, about 

660 feet east of the shaft a cross" vein" that probably corresponds to the Old Chicago-Bismarck 

"vein," which on the third level of the Bismarck mine had a strike of N. 70° W. and a dip .of 

60° NE. 

It is reported that no difference was noticed in the character of the ore in the N cwton, on 

the two sides of the Old Chicago-Bismarck zone. This fault zone is practically unmineralized 

where it crosses the Little M:attie-N ewton vein. 

ARTHUR MINE. 

The Arthur mine is situated nearly 3,000 feet, S. 45° W. of the Little Mattie shaft house, at 

an elevation of 8,400 feet. The. mine is located on a branch of the Little M:attie-Newton vein 

that is almost directly in line with. the main vein, which splits several hundred feet west of the 

Silver Glance shaft house. The Arthur vein is developed by a shaft from which several levels 

run, and also by a tunnel about 450 feet in length that starts near the shaft house and runs 

northeastward on the vein. The levels from the shaft were not examined: 

On the tunnel level the wall rocks consist entirely of gneiss and pegmatite, which in many 

places form an intimate mixture having a gnarled appearance. No porphyry was observed 

·underground on this level, but Mr. Ball found a dike of bostonite porphyry on the surface a short 

distance north of the mine and traced it to its junction with the Little Mattie porphyry dike. 

The general strike of the vein is N. 50° to 55° E. and the dip about 75° NW. The vein is a lead 

of crushed gneiss, pegmatite, and clay, 6 inches to 2 feet wide, which gives way in places to a 

silicified gneiss or quartz streak of the same width carrying some galena and zinc blende, the 

la'tter usually of the black or dark-brown variety, termed "black jack." In places the vein 

consists of mineralized stringers of quartz showing comb structure, scattered through a belt of 

gneiss and pegmatite. 

A small stope has been made about 100 feet from the breast of the tunnel level and a little 

drift stoping has also been done at places along this same level. Somewhat more ore was shipped 

from the shaft workings. 

DORRITT AND KING SOLOMON MINES. 

The Dorritt and King· Solomon mines arc located on one of the branch veins southwest of the 

Silver Glance minewhichresultfrom thesplittingof the Little Mattic-N ewton vein at its southwest 

encl. A crosscut tunnel run· on the boundary line between the King Solomon claim on the east 

and the Dorritt. claim on the west enters the hill from a point about 3,000 feet S. 30° W. of the 

Little Mattie shaft and intersects the vein at about 380 feet from its mouth. A drift on the 

vein to the northeast, on the King Solomon claim, shows the vein trending N. 40° to 45° E. 

and clipping 75° NW., while a drift in the opposite direction shows the vein with the same strike 

but a clip of only about 58° NW. · 

The country rock is gneiss and pegmatite, with a porphyry dike running along or a short 

distance northwest of the hanging wall of the vein. This dike, which consists of bostonite 

porphyry containing pseudomorphs of serpentine after pyroxene, is 4 or 5 feet wide on the 

King Solomon claim, but in the Dorritt workings· narrows in places to only 6 inches to 1 foot. 

Where the vein is not adjacent to the dike the contact of the dike with the country rock is a 

very tight flow contact with no gouge or mineral lead intervening. 

The vein, which is usually a crushed rock and gouge lead, changes in places to a quartz or 

silicified gneiss streak 2 inches to 1 foot wide, containing specks and streaks of dull-brown 

zinc blcncle and galena scattered through it. In certain parts of the mine the vein consists of a 

fairly well indurated belt or zone of material composed of numerous slightly rounded to angular 

fragments of pegmatite and gneiss up to It inches in diameter, in a gray matrix of pulverized 

rock consolidated by silicification. · 

Small ore bodies have been found and worked out nearer the surface along this vein on 

both the King Solomon and the Dorritt claims, but at the time of visit there was only one 

man working. 
364 GEOLOGY OF THE GEORGETOWN QUADRANGLE, COLORADO. 

WILD ROSE MINE.-

The Wild Rose mine is located on the north slope of the ridge between Spring Gulch and 

Chicago Creek, about 1,500 feet N. 50° E. of the Newton shaft. The workings, which are now 

inaccessible on account of caving, follow, according to Mr. Tarr, owner of the Red Lyon mine, 

two veins 20 feet apart that are branches of the Little Mattie-Newton vein, which splits near the 

crest of the hill between the Newton and Wild Rose shafts. 

RED LYON MINE. 

'fhe Reel Lyon mine is located in Spring Gulch on the west side of the creek, at an elevation 

of about 8,300 feet. It is about 300 feet northeast of the Star mine. The developments consist 

of a short tunnel and a winze from which short drifts run. 

The workings develop two veins which follow in a general way the contacts on the two 

sides of a dike of red mottled bostonite porphyry very similar to that in the Newton and Little 

Mattie mines. These veins are probably branches of the Little Mattie-Newton vein, which 

forks to the northeast near the top of the ridge between the Newton and Wild Rose mines. 

The dike of porphyry in the Red Lyon workings splits just southwest of·the winze. The east 

branch of the dike dips 75° NW. and trends on the surface N. 55° E. It is probably the dike 

encountered in the Star tunnel, where from a point 150 feet from the mouth of the tunnel to 

the breast it forms the hanging wall of the vein. The other or west branch of the dike runs 

N. 40° E. up the hill slope, where in places it is separated from the east dike by about 200 feet 

of pegmatite and gneiss. Near the crest of the ridge northwest of the Bullion King shaft 

house, however, the two dikes appear to reunite, forming a single dike nearly 30 feet wide. 

The Red Lyon mine has produced no ·ore. Some mineralization has taken place, however, on 

both sides of the porphyry dike, but mainly along the southeast contact. 

STAR MINE.a 

The Star mine is located in Spring Gulch at an elevation of about 8,300 feet, approximately 

300 feet northeast of the Red Lyon tunnel. The workings consist of a shallow shaft and a 

tunncl.about 250 feet in length, which runs N. 40° E. on a vein that clips 70° to 80° NW. In 

the first 150 feet of the tunnel the wall rocks of the vein consist entirely of pegmatite and gneiss, 

but along the last 100 feet altered bostonite porphyry forms the northwest or hanging wall 

of the vein and pegmatite and gneiss the foot wall. 

The vein is a strong lead marked by quartz, pyrite, and clay, which in places gives way 

to fractured gneiss and pegmatite cemented by ore. There is also usually prasent an unconsolidated postmineral breccia zone 6 inches to 1 foot wide, consisting of clay, crushed rock, and 

angular fragments of quartz and ore. Remnants of the ore streak left adhering to the wall 

show seams of galena ore cementing fragments of highly silicified and pyritized gneiss. Little 

veins in the galena ore show specks of slightly later tetrahedrite (gray copper) or polybasite 

and also well-formed quartz, siderite, and pyrite crystals coating the galena. The mine has 

produced a little ore, but has not been worked for some time. 

The vein in the Star tunnel is probably another one of the branch veins resulting from 

the splitting of the Little Mattie-Newton vein northeast of the Newton mine. The porphyry 

dike forming a portion of the hanging wall ncar the breast of the tunnel is probably the east 

branch of the bostonite porphyry dike which splits just southwest of the winze in the Red 

Lyon mine workings. This dike was traced up the hill slope to the northeast and over the 

crest of the ridge to a point about 100 feet west of the Bullion King shaft. 

BULLION KING MINE. 

The Bullion King mine is located at an elevation of over 400 feet near the top of the north 

slope of the ridge separating Clear Creek from Spring Gulch, about 1,700 feet S. 20° W. of the 

ore house of the Stanley mine. An underground study of this mine was not made, as the openings 

a To be distinguished from the Star tunnel on Chicago Creek (p. 366). 
MINES NEAR IDAHO SPRINGS. . 365 

to the mine were closed and the owners were not to be found. The rock in the vicinity of· the 

vein on the surface is all pegmatite and granite, although a dike of bostonite about 18 feet 

wide. occurs about 100 feet west of the shaft house. . 

The vein seems to have a general N. 55° E. trend. It is probably a branch of the Little 

Mattie-Newton system of veins and joins the vein along the porphyry dike, which is located 

farther southwest. It is probably also the same vein as is developed in the Mayflower mine 

to the northeast. 

The low-grade ore found on the mine dump contains galena, pyrite, and black sphalerite, 

and occurs as small seams running through granite and pegmatite or as a. cementing material 

inclosing angular fragments of the same rocks. A few small specks of tetrahedrite or polybasite 

were also found associated with the galena. , 

Considerable ore is reported to have been obtained from the Bullion King workings. 

MAYFLOWER MINE. 

The Mayflower rrtine is located on the south bank of Clear Creek about 1! miles west of 

the central part of the town of Idaho Springs. Although on the slirface a dike of bostonite 

Gneiss with small 

intrusive masses 

of pegma.tlte 

FIG. 139.-G~ologica\ plan of Mayflower tunnel. 

Mineralized 

lead 

Unmineralized 

lead 

porphyry is found a short distance away from the point where the vein should outcrop, the 

wall rocks of the vein, as shown in the underground workings on the tunnel level; consist entirely 

of gneiss, cut by small intrusive masses of pegmatite. 

The development consists of about 1,800 feet of tunneling and drifting. A crosscut tunnel about 90 feet long connects with a drift following a vein which strikes N. 86° E. and ranges 

in dip from 75° NW. to 84° SE. Near the breast of this drift a crosscut to the northwest 

about 60 feet in length, connects with a drift extending 400 feet S. 70° W. along another welldeveloped vein which dips 50° to 55° NW. (See fig. 139.) This second vein possibly connects 

.with the southeast vein followed by the tunnel farther to the northeast, either through cross 

fractures between the two or by swerving slightly to the south until it joins the southeast vein. 

Several minor branch veins were noted entering the southeast vein from the west, and one of 

these may represent the point of junction of the two main veins. · 

Both of the principal veins consist of clay and crushed-rock leads only a few inches in 

width, which in places give way to quartz or to quartz and ore. The ore found along the southeast vein consisted of galena, resin, zinc. blende, and cupriferous pyrite, associated with quartz 

and siderite or ferruginous rhodochrosite gangue. Of these minerals the galena and zinc blende 

appear to have been the earlier to be deposited, for although some cupriferous pyrite is associated with these two minerals most of the pyrite occurs in small veinlets or seams cutting them. 
GEOLOGY OF THE GEORGETOWN QUADRANGLE, COLORADO. 

The carbonates, siderite, and rhodochrosite also occur in veinlets of later origin than the galena 

and sphalerite.a Quartz and pyrite crystals, associated with specks of galena and sphalerite, 

were found in vugs along the northwest vein. 

About 100 feet from the mouth of the tunnel an ore body was encountered and stoped for 

a distance of nearly 100 feet along the vein to the southwest. Aside from this body of ore, 

however, little mineralization was encountered on either of the two main veins, although small 

streaks of ore were found here and there near the junction of minor branches with the main leads. 

The mine is reported to have produced considerable ore, but most of it evidently came 

from the lower workings connecting with the shaft, which is now filled with water. 

STAR TUNNEL.b 

The Star tunnel enters the north hill slope bordering Chicago Creek about 1! miles above the 

entrance to Spring Gulch. The tunnel for most of its length follows a large soft vein considered to be the continuation of the so-called "cross lode," or postmineral fault zone, which intersects and faults the Little Mattie vein from 10 to 20 feet. Toward its inner end, however, the 

tunnel leaves the "cross vein" and becomes a crosscut run for the purpose of cutting the vein 

developed in the Columbia mine, located on the surface a little more than 2,500 feet N. 53° W. 

of the mouth of the tunnel. Owing to the northward dip of the Columbia vein the distance 

to be driven at the depth of the tunnel, which is between 700 and 800 feet lower, would be 

somewhat greater. Although the tunnel is reported to be in the supposed neighborhood of 

the Columbia vein, no vein recognized as the lode sought had been encountered. 

Some mineralization has been found along the main tunnel lode in places, but this vein 

has yielded but little pay ore. Work was suspended on the Star tunnel during the fall of 1905. 

COLUMBIA 1\flNE. 

The Columbia mine is situated at an elevation of 8,550 feet on the south slope of the ridge 

separating Chicago Creek from Spring Gulch, about 800 feet N. 75° E. of the Little Mattie 

shaft house. The vein, which is developed by a shaft and three short levels aggregating 600 

feet in length, trends from east and west to N. 75° E. on the different levels and dips slightly 

to ihe nprth. It splits on the upper level about 40 feet west of the shaft, the branches diverging to the west. A similar split is noticed oii the 180-foot level, at a point about 100 feet west 

of the shaft. . , 

The vein as a rule is very narrow and consists of a seam of comb quartz, clay, and iron 

oxides one-half to 1 inch wide, but in places where the best ore occurs it widens to 2 to 4 inches 

of dark-gray quartz which carri.es considerable pyrite. Locally, however, the vein changes to 

a silicified streak 6 or 8 inches wide, composed of crushed, fractured, and altered pegmatite 

and gneiss, heavily stained with iron oxides and carrying low values. · The best ore is the clarkgray quartz and pyrite ore which occurs in narrow seams. Most of its values are in gold, and 

fine specimens which show thin leaves of gold along small cracks in the quartz have been obtained. 

Although the pay streak is very narrow, considerable stoping has been done, but most of 

it is confined to the immediate vicinity of the split ip the vein, where the junction of the two 

branch veins has evidently had a favorable influence on ore deposition. The ore occurs both 

on the main vein and on the branches for a short distance from the junction. 

In the shaft about 40 feet below the third or lowest level a veinlet or seam of chloropal 

1 to 1 i inches in width cuts across the ore-bearing vein nearly at right angles to the strike and 

enters the gneiss and pegmatite walls on either side. The material composing the mass (which 

is of later origin than the ore of the veins) is a bright yellowish-green granular substance. Under 

the microscope this is shown to be composed of aggregates of little rounded green grains showing radiating or spherulitic structure. Many of the spaces between these aggr:egates are occupied by opal. Associated with the chloropal, which is a hydrated iron silicate, is some alumina. ----·--·----·-·---------· ·-----

a This is possibly a' 'compound" vein. Seep. 3G4. 

b To be distinguished from the Star mine in Spring Gulch (p. J64). 
MINES NEAR IDAHO SPRINGS. M7 

BLACK EAGLE AND BISl\fARCK VEINS. 

DEVEJ,OPMENT. 

The Black Eagle and Bismarck veins are developed by workings connecting with shafts 

located at an elevation of about 8,550 feet on the south side of the ridge between Spring Gulch 

and Chicago Creek, 1,000 to 1,200 feet east of the Newton shaft. As.thc Bismarck minea had 

been closed for some time, only the· portions of the mine directly connected with the Black 

Eagle mine workings W(:)re observed. 

0 

! ... ·········! Micaceous gneiss intruded Mineralized lead Unmineralized lead 

by small masses of pegmatite 

FIG. 140.-Geological plans of separate levels, Black Eagle-Bismarck veins. 

The Black Eagle mine consists of five levels connected with a shaft sunk on the Black 

Eagle vein. , A mill was constructed by the company owning the mine to treat the ore. This 

mill, which contains five stamps, amalgamating tables, and cyanide tanks, however, has proved 

to be not well adapted to trea.t the ore from the mine. 

DESCRIPTION OF VEINS . 

. The. Black Eagle vein dips 60° to 80° NW., strikes N. 60° to 75° E., and intersects and 

crosses the Bismarck vein, which strikes northwest, so that the junction of the two is, on some 

a To be distinguished from the Bismarck lode at Silver Plume, near Georgetown (p. 186). 
368 GEOLOGY OF THE GEORGETOWN QUADRANGLE, COLORADO. 

levels at least, nearly at right angles. Where the Black Eagle vein crosses the Bismarck th£ 

latter, as is well shown on the third and fourth levels (fig. 140), is faulted about 6 feet, the 

southeast portion being offset to the southwest. Ncar the point of intersection the Bismarck 

vein, which dips 5.5° to 65° NE., strikes about N. 60° W., but about 100 feet northwest of the 

junction it splits into two veins which loop about a horse of rock and then unite and continue 

almost due west to one of the shafts on the Bismarck vei:o.. This shaft, which is about 275 feet 

along the Bismarck vein from the junction, or 200 feet N. 60° W. of the Black Eagle shaft, 

has a dip of 55° to 60° N. and is connected with the workings of the Black Eagle mine on the 

third and fourth levels. On the third level a drift on the vein extends N. 70° W. for 150 feet 

from the shaft and probably represents the general direction of this vein to the point where it 

crosses the Little Mattie-Newton vein a short distance northeast of the Newton shaft. 

WALL ROCKS. 

The wall rocks of both veins consist entirely of micaceous gneiss and pegmatite, the former 

being in many places intruded by the latter in such a way as to give the wall rock a gnarled 

or crumpled appearance. LOCATION OF ORE. 

Both veins, where not especially mineralized, consist of a crushed-rock and clay lead several 

inches wide containing a little quartz and scattered specks of mineral. The ore on the levels 

examined was confined to the immediate vicinity of the junction. of the two veins. The 

largest body of ore was encountered on the third level immediately at the junction, along 

the Bismarck vein for a distance of 125 feet northwest, and along the Black Eagle for 50 feet 

southwest. On this level little ore occurred on the veins to the southeast and northeast of the 

crossing. The stope immediately at the junction on the third level was about 20 feet in width, 

having followed a series of ore stringers into the fractured wall rock adjoining the main veins. 

On the fourth level ore occurred on all four arms of the crossing veins, but most of the ore 

was a short distance from the junction, and the Bismarck vein was the more heavily mineralized 

of the two. On the fifth level the intersection of the two veins is not well shown, but stoping has 

been done on portions of the veins corresponding to the Black Eagle southwest of the junction 

and to the Bismarck southeast of the junction. Both of these intersecting fault leads are therefore older than the main minemlization. Several minor. branches which join the main veins near 

their crossing may also have had some influence on the deposition of ore. 

CHARAC'J'ER 0~' ORE. 

The ore which is being mined at present consists of galena, black sphalerite, and a little P,.YTite, 

and is said to average about $20 to the ton. The values are chiefly in silver, as the ore usually 

carries only about 0.01 ounce of gold for every 35.ounces of silver. Howev.er, assays which gave 

0.35 ounce of gold and 100 ounces in silver to the ton have been obtained. Considerable native 

silver is found at times; it occurs either as wire silver in vuglikc openings associated with black 

sooty sulphides or as flakes along small cracks in the fractured wall rock. 

UT£ CREEK-CHICAGO CREEK GI~OUP. 

CECIL-ARGO GROUP. 

DESCRIPTION OF GROUP. 

The Cecil-Argo group of veins is located in Ute Canyon and on the hill slopes due south of the 

mining village of Lamartine. It consists of the veins developed in the Cecil, Wallace, Argonaut, 

Argo, Ouida, Sappho, and Gold Coin mines. The main vein, which is developed extensively by 

the Cecil and Argo mine workings, is a very broad, strong vein with a rather irregular strike which 

ranges from N. 45° E. to N. 65° E. The clip, as nearly as could be determined from surface 

outcrops and shafts, ranges from nearly vertical to northwest. 

To the southwest the main vein evidently breaks up into the Ouida, Sappho, and several 

other veins, which diverge slightly in going southwestward. The Gold Coin is a transverse lode 
MINES NEAR IDAHO SPRINGS. 369 

which either joins or crosses the Sappho lode. Exctfpt in the Sappho and Gold Coin mines, no 

work has been done on these veins for years, and practically no11e of the extensive workings 

on them are accessible. 

Northeast of the main Cecil shaft is a line of prospect pits on the vein which trend N. 45° E. 

If this direction is maintained by the vein underground, it is possible that its continuation would 

join the vein in the Invincible mine, which is located on the road between Lamartine and Idaho 

Springs about 2,500 feet east of the Lamartine shaft. The Invincible vein in turn may be a continuation of the Brighton or Freeland Extension vein. However, although these vein segments 

when plotted on the map seem to have the same general alignment, the correlation of portions of 

veins so widely separated by unexplored ground can only be guess work, for a slight variation in 

the strike or dip might within half a mile throw the vein several hundred feet to one side or the 

other of the projected path. Therefore these vein outcrops, which resemble one another somewhat both in character of the lodes as well as in strike and dip, may be proved by future development to belong to portions of somewhat parallel veins, rather than to connecting veins. 

CECIL MINE. 

The Cecil mine was opened by extensive drifting from a shaft and by tunnels, but all of these 

are inaccessible either through caving or through being filled with water. The Cecil shaft is 

about 2,400 feet S. 7° E. of the main Lamartine shaft. The general trend of the vein at the 

Cecil shaft is N. 45° E. and corresponds to the line of prospect pits to the northeast. 

The country rock in the immediate vicinity is composed of micaceous and granitic gneiss cut 

by dikes of quartz monzonite, granite, and pegmatite. 

To judge from the material on the clump, the vein consists of a broad zone of crushed, 

highly altered and silicified rock, or of quartz and ore stringers running through the fractured 

and altered rock mass. Specimens of ore found show galena and zinc blende associated 

with pyrite. The blende is chiefly of the black variety, although some resin blende is present. 

According to a miner who had worked in the Cecil, most of the ore produced was of a comparatively low grade and carried only a small percentage of lead, although small amounts of ore 

averaging 200 ounces of silver to the ton were at times obtained. Small bunches of native 

silver in flakes and wires were also occasionally found. 

Little is known of the production of the Cecil, but E. 0. Leech, Director of the Mint, in his 

report for 1892, represented the yield as follows: Gold, $7 40; silver, $9,578; lead, $2,295. 

ARGO MINE. 

The main Argo shaft is about 1,000 feet S. 65° W. of the Cecil shaft. The vein to the 

northeast has a trend of N. 65° E. and a dip ranging from vertical to a few degrees northwest, 

as is shown in the Wallace shaft located about halfway between the Cecil and Argo shafts. 

Southwest of the main Argo shaft the vein, from surface indications, appears to curve around 

to almost due southwest for a short distance before splitting up into several branches. The 

large dumps are e~idence that extensive development work was done in the mine also, but 

as in the Cecil the underground workings are now inaccessible. The ore streak is reported 

to have ranged in width from 3 or 4 inches to 2 feet and to have been composed of black 

zinc ore, usually carrying less than 5 per cent of lead. Where lead ore itself was obtained it 

carried only 50 to 60 ounces of silver. Here and there, however, a small streak of soft, black, 

sooty sulphide ore came in between the hanging wall and the zinc ore, and some of this carried 

several ounces in gold per ton. The production of the Argo mine has been fairly large, but 

as for-many of the other mines of the district the records are either not obtainable or are incom 

plete. The following estimates of production are from the reports of the Director of the Mint: 

Prod!tetion of Argo minr. 

1887: Gold, $4,040; silver, $27,357 ................................................... $31,397.00 

1888: Gold, $414.40; silver, $13,549.88; lead, $582.56.................................. 14,546.84 

1889: Gold, $4,500; silver, $32,322.50; lead, $1,900.................................... 38,722.50 

1890: Gold, $30; silver, $291........................... ............ ..... .... ... ...... 321.00 

1892: Gold, $640; silver, $3,044 ................... : .... _............................... 3, 684.00 

88, 671.34 

31401-No. tm-OS-24 
370 GEOLOGY OF THE GEORGETOWN QUADRANGLE, COLORADO. 

ARGONAUT AND WALLACE CLAIMS. 

The workings on the Argonaut and Wallace claims are inaccessible, but are known to be 

rather extensive. Moreover, the vein at these points also has produced considerable ore. 

According to Leech, the Argonaut yielded in 1890 gold, $1,400; silver, $i0,443; total, $11,843; 

and in 1892 gold, $1,954; silver, $16,381; lead, $157; total, $18,492; and the Wallace, which 

in 1887 was reported to have yielded only $43 in silver, was estimated to have produced during 1892 $600 in gold, $10,965 in silver, and $1,275 in lead, or a total of $12,890 for the year. 

OUIDA VEIN. 

The Ouida vein, which is probably a southwest branch of the Argo vein, is a short distance 

northwest of and runs somewhat parallel to the Sappho vein, trending up the hill to the south 

of Ute Canyon in a S. 41° W. direction. About 150 feet southwest of the mouth of the lower 

tunnel a small ore shoot, which is said to have yielded in the neighborhood of $63,000, was 

encountered near the surface, and another body of ore still farther southwest produced about 

$3,000 worth of ore. The ore, the chief values of which were in silver, consisted mainly of 

small stringers of galena-zinc ore in quartz monzonite. 

8.\PPHO VEIN. 

The Sappho mine is located about 4,500 feet S. 20° W. of the Lamartine shaft on the southwest side of Ute Creek. The vein is developed by a tunnel about 550 feet in length that intersects the vein at a distance of about 150 feet from the mouth and then continues along the 

vein to the breast. The wall rocks are chiefly quartz monzonite, which here is a dark granitelike rock containing abundant crystals of black mica. The vein, which has a strike of S. 32° W., 

consists mainly of a crushed-rock lead that in places gives way, to quartz or to quartz and ore. 

One small stope on the tunnel level yielded $3,500 worth of ore. This ore body seems to have 

been due largely to the entrance of a small branch vein into the main vein from the north, for 

the ore occurred on the main vein just beyond the junction. In two other places a slightly 

increased amount of quartz and mineral formed at the junction of minor veins with the main 

vein. At the time of visit a few sacks of ore were being obtained from a level about 40 feet 

above the tunnel. This ore, which consisted of quartz specked with resin zinc blonde and a 

mineral resembling tetrahedrite, was Sfl,id to carry values amounting to $118 in silver and $4 

in gold per ton. Some barite is also associated with the quartz gangue in the ,ores of this mine. 

GOLD COIN VEIN. 

The Gold Coin lode is developed by a shaft from which a number of levels run and also 

by a tunnel level. The wall rocks are mainly quartz monzonite and pegmatite. The vein, 

which appears to strike S. 70° W. and to dip 60° NW., probably either joins the Sappho vein 

or crosses the Sappho, Ouida, and other branches of the Argo-Cecil vein. The ore bins show 

a considerable quantity of low-grade ore high in its content of resin zinc blende, which ranges 

in color from yellow to brown. Considerable galena and traces of pyrite are also present. 

The ore appears to be chiefly a breccia containing fragments of ore and wall rock cemented by 

quartz, much of which shows comb structure. The Gold Coin has produced considerable ore, 

some of which is said to have carried several hundred ounces of silver to the ton. 

HUMBOLDT-MARY FOSTER VEIN. 

DESCRIPTION OF VEI};. 

The Humboldt-Mary Foster vein is developed rather extensively from a point several 

hundred feet west of the Mary Foster shaft, which is located at an elevation of about 9,750 

feet on the crest of the ridge separating Cascade and Ute creeks, northeastward through the 

workings of the Humboldt mine to a point where it crosses Ute creek. Many of the workings 

of both mines are caved shut, as, except for desultory work by one man, the mines have been 

idle for years. The lower or mill adit on the Humboldt mine, which is open for about 900 feet, 

is entirely in dark mottled quartz monzonite, with a few masses of intrusive pegmatite. 
MINES NEAR IDAHO SPRINGS. 371 

The average strike of the vein isS. 51° W. and the dip is 80° NW. The vein on the lower 

level consists of a clay and crushed quartz monzonite lead, giving way in places to quartz and 

silicified crushed rock or to a series of quartz stringers in fractured quartz monzonite. A 

little lean ore occurs near the breast of the lower tunnel, but no stoping of importance has 

been done on this level. The best and largest bodi.es of ore were found near the boundary 

line between the Humboldt and Mary Foster claims. According to Mr. Julius Hell, ore extended 

continuously along the vein on the Mary Foster claim for a height of 60 feet and a length of 

100 feet immediately southwest of the 150-foot shaft sunk on the boundary between the Humboldt and Mary Foster claims. Ore was also stoped for a distance of 150 feet northeast of 

the same shaft along the upper Humboldt tunnel level, which is about 150 feet above the lower 

tunnel. · CHARACTER OF ORE. 

The ore is a galena and zinc blende ore carrying some pyrite associated with quartz. Tetrahedrite and polybasite are also present in the best ores. Mr. Hell stated that the ore northeast 

of the line between the two claims carried only from 10 to 30 ounces of silver, but that toward 

the southwest the amount of "gray copper" (tetrahedrite) and the silver content in the ore 

increased, so that many ore samples from the Mary Foster carried between 200 ancl400 ounces of 

silver. The regular mill runs ranged from 16 to 111 ounces of silver as a rule. One smallmass 

of ore from a depth of 50 feet below the surface near the crest of the ridge is said to have carried 

10 ounces in gold and 1,300 ounces in silver. Specimens of native gold in fme grains associated 

with resin blencle are also reported to have been obtained from the Humboldt mine. 

ALEXANDEH. LODE. 

The Alexander lode runs somewhat parallel to the Humboldt vein and about 500 feet farther 

to the northwest. Its strike is in general S. 65° W. and its clip 80° NW. The country rock in 

the vicinity is quartz monzonite. The ore consists of small streaks of quartz containing specks 

of galena and some tetraheclrite in fractured quartz monzonite. A little ore running between 

200 and 300 ounces in silver to the ton has been obtained from this vein. 

SILVER HORN OR GOLDEN HECLA MINE. 

LOCATION AND DEVELOPMENT. 

The Silver Horn or Golden Hecla mine is located in the valley of Ute Creek at an elevation 

of about 9,000 feet. The workings, which are mainly in quartz monzonite, cut by a few dikes of 

pegmatite, consist of a shaft from which drifts between 100 and 200 feet long are run on two 

levels, and of a tunnel level (see sketch map, fig. 141) which consists of a short crosscut 

tunnel and about 700 feet of drifting on the two veins encountered. 

SILVER HORN VEIN. 

The main vein developed by the mine workings is the Silver Horn vein, which immediately 

northeast of the shaft trends N. 55° E. and dips 75° NW., but which between 200 and 400 feet 

east of the shaft swerves slightly more to the north and then turns again and continues in a 

N. 68° E. direction until it disappears in the breast of the drift. About 400 feet northeast of the 

shaft the Silver Horn vein comes into contact with a broad vein termed by the miners the" soft 

lode," which is a large, unconsolidated lead from 1 foot to several feet wide, composed of clay, 

quartz, and crushed country rock, bearing only faint evidence of mineralization. Its general 

strike, which is S. 60° W., would carry it a short distance northwest of the shaft. Slickensides 

and movement strire dipping northeast at 2° from the horizontal, together with the crushed-rock 

vein filling, indicate that the "soft lode" is a fault line along which movement has taken place 

since the deposition of the ore in the Silver Horn vein. Although a little ore is found in places 

along this "soft lode," it usually occurs as isolati3d fragments or as lens-shaped slices of quartz 

and crushed galena-blende ore resembling in nature the ore from the Silver Horn vein. As the 
372 GEOLOGY OF THE GEORGETOWN QUADRANGLE, COLORADO. 

latter vein loses its identity for a distance of about 25 feet, where it comes into contact-with the 

"soft lode," it is probable that the "soft lode" cuts off a portion of the Silver Horn vein at the 

point where it curves farthest to the west (see fig. i41), and that the ore occurring along the 

"soft lode" is simply "drag" from the crushed vein filling of the Silver Horn lode. 

The Silver Horn vein itself is comparatively narrow, consisting either of a single crushedrock and quartz lead which in places gives way to quartz and ore, or of foot- and hanging-wall 

leads separated by 1' to 3 feet of country rock which is intersected by a series of small quartz 

or quartz and ore stringers. 

On the first level below the tunnel level, about 60 feet northeast of the shaft, a small pegmatite 

dike intrusive into the quartz monzonite forming the wall roc.ks has been step faulted and irregularly dissected by a series of fractures which include the foot- and hanging-wall leads (fig. 142). 

Some of these fractures are filled with narrow seams of clay; others are occupied by slightly 

mineralized quartz. 

N 

l 

Quartz monzonite cut by Dip and direction of 

occasional small dikes Or movement striae on 

pegmatite . the walls 

_100 

Mineralized lead 

200 feet 

r==::J 

t::::::::_] 

U nmineraliZed lead 

FIG. 141.-Geological plan of Silver Horn (Golden Hecla) tunnel. 

The ore consists of galena and zinc blende, carrying a little pyrite and some tetrahedrite. 

Quartz is the chief gangue mineral. The zinc in some of the ore is high and ranges from a yellow 

or a brown to a peculiar reddish variety of sphalerite. The red sphalerite was especially prevalent in a streak of zinc ore encountered in sinking the shaft. The ore, while usually occurring 

in narrow seams which must be mined along with considerable country rock, is of comparatively 

high grade. Exceptionally high values in both gold and silver are reported as having been 

obtained from picked samples, and the mill run showing the lowest values consisted of ore 

averaging 0.5 ounce in gold and 70 ounces in silver to the ton. Small shipments, however, are 

said to have showed as much as 6 or 7 ounces of gold to the ton. But little stoping has been 

done in the mine, and most of it was confined to the tunnel level. The largest and best ore boCly 

was located about 240 feet east of the shaft and was stoped out for a length of about 35 feet and 
MINES NEAR IDAHO SPRINGS. 373 

a height of 50 to 75 feet. At several other places, however, on the tunnel level the vein is· 

stoped up from a few feet to 25 feet. · 

About 25 feet east of the breast of the drift on the Silver Horn vein, in the face of a short 

spur, was found a veinlet of hematite and calcite about 1 to 1! inches wide. On the foot-wall 

side of this stringer was a 2-foot dike of pegmatite carrying some included magnetite which 

appeared. to be a primary constituent of the pegmatite. The hematite was probably of secondary 

origin, due to the action of underground waters on the primary magnetite in the pegmatite or on 

the pyrite in the vein. ARGUS VEIN. 

A short distance southeast of the Silver Hon~ shaft house it: a broad stained, crushed, and 

friable rock lead called the Argus vein. This vein, which has a nearly vertical dip, crosses the 

· valley of Ute Creek in aS. 53° W. direction. It shows only faint signs of mineralization. 

NORMAN VEIN. 

Several hundred feet east-northeast of the Silver Horn mine is the Norman lode, which 

has a trend on the surface ranging from N. 60° E. toN. 68° E. and a dip of about 80°NW. This 

lode is developed by a shaft 95 feet deep with short drifts at the bottom. The lead consists 

. 

.. .. " . . '-~~-====----"·10!"' 

~ B B Pegmatite Unminemlized lead Quartz lead slightly mineralized B 

WallofdrifL 

FIG. 142.~Horizontal plan of portion of 50-foot level, Silver Horn mine, showing horizontal offsetting of-pegmatite dike by 

the vein faults of the ore zone. 

of crushed rock recemented by silica. The little ore taken from the vein consisted of galena 

and zinc blende carrying small amounts of a gray mineral which is probably tetrahedrite or 

specularite. 

"M AND M" TUNNEL. 

The "M and M" tunnel is located in the valley of Ute Creek about 400 feet above the Silver 

Horn (Golden Hecla) mine. The development consists of a very tortuous crosscut tunnel 

between 700 and 750 feet in length, which in places follows slips in the fractured country rock, 

and short drifts along three somewhat parallel veins. The country rock consists chiefly of quartz 

monzonite cut by pegmatite dikes. · 

What is called the Little Helen vein is intersected at a point between 400 and 450 feet from 

the mouth of the tunnel, and has been drifted on for about 125 feet. This vein has a strike of 

N. 70° E. and a dip of 80° to 85° NW. It consists of a 1- to 2-inch quartz streak in a cmshed 

rock lead 1 foot wide. 

Near the breast of the tunnel what is termed the "M and M" vein was encountered. It 

strikes N. 66° E., dips 80° NVV., and so far shows only faint signs of mineralization. A small, 

slightly mineralized veir!. runs parallel to the "M and M" vein about 60 feet to the southeast. 

The mine has produced no ore, although narrow seams of black zinc blende, associated with 

comb quartz and a little galena, have been found cutting fractured quartz monzonite and pegmatite along the vein zone. 

A shaft on the road above and to the north of the "M and M" tunnel isprobably sunk on 

one of the veins exposed on the tunnel level. . 
374 GEOLOGY OF THE GEORGETOWN QUADRANGLE, COLORADO. 

ELLA M 1KINNEY MINE. 

The Ella McKinney mine is located in the valley of Ute Creek between 500 and 600 feet 

southeast of·the Silver Horn (Golden Hecla) mine. The development consists of about 700 

feet of tunneling and drifting. The first 500 feet of workings consist of very tortuous crosscut 

drifts in hard, fresh quartz monzonite. The inner 200 feet follow a weak crushed-rock lead 

2 to 4 inches wide, which in places carries a little quartz and pyrite. The quartz in spots was 

stained black, but showed no recognizable galena or zinc blende. This mine has evidently 

produced no ore. · 

CHARTER OAK MINE. 

The Charter Oak vein is developed through a shaft los:~ated at an elevation of about 9,550 

feet on the summit of the ridge separating Cascade and Ute creeks. This mine was closed down 

at the time of visit, but had been in operation only a year or so previously. The underground 

workings, though probably in good condition, were not visited. 

On the surface in the vicinity of the shaft house two veins a short distance apart were 

found running parallel to each other with a strike of N. 65° E. and a dip of 80° NW. 

The ore found on the dump consisted of galena, resin blende, and pyrite, associated with 

quartz, barite, and a considerable amount of oxides of iron (both hematite and limonite) and 

manganese. The ore occurs both as stringers traversing quartz monzonite and pegmatite and 

as a cementing material about angular fragments of these rocks. Some of the specimens show 

crustification in which galena associated with a little blende is of earlier deposition than the 

pyrite. Magnetite, probably of primary origin, was noted in some of the pegmatite masses 

from the mine. Hematite seams of secondary origin which cut.across stringers of ore were also 

observed. The mine has yielded considerable ore. 

KITTY EMMET MINE. 

The Kitty Emmet mine is located at an elevation of about 9,_250 feet on the south slope of 

the bridge bordering Cascade Creek, 1 mile N. 70° W. of the junction of Cascade and Chicago 

creeks. The vein, where opened up on the surface, trends N. 50° E. and dips 75° NW., and is 

composed of a quartz and crushed and stained rock zone. The country rock was mainly quartz 

monzonite. The mine has been idle for some time. 

MARY MATILDA AND LITTLE FLAT MINES. 

The veins on the Mary Matilda and Little Flat claims are developed by a tunnel about 325 

feet in length and by a very flat inclined shaft from which short levels run. The country rock 

is quartz monzonite. The vein in the tunnel consists of a series of connecting fractures which 

trend in general N. 55° to 60° E. These irregular fractures show but little mineral, except at a 

point about 200 feet from the mouth of the tunnel, where a streak of good-looking quartz and 

galena ore 2 to 5 inches wide extends for a distance of about 30 feet along the vein, which here 

dips flatly to the northwest. The ore from this streak consists of a fine breccia of country rock 

cemented by galena, which is associated with small amounts of sooty black sulphides and iron 

oxides. Crusts and small crystals of ferruginous rhodochrosite lining small vugs are also occasionally found in. this ore. 

The shaft, which follows the flatly dipping vein for much of the way, dips only 15° NW. 

near the top, but lower down has an inclination of 23° from the horizontal. Short drifts are run 

on four levels. The longest two of these are on the bottom or fourth level. One drift on this 

lowest level follows the vein to the N. 55° E. for about 70 feet, .while the other extends irregularly to a point about 100 feet S. 45° W. of the shaft. The vein on the bottom level consists of 

two black selvage leads 2 to 6 feet apart, with crushed and very highly altered quartz monzonite 

between. About 1 inch of dark-gray quartz showing faint indications of mineral is associated 

with the black selvage streaks, but no ore has been obtained from any of the shaft workings. 
MINES NEAR IDAHO SPRINGS. 375 

BURNS-MOORE TUNNEL. 

The mouth of the Burns-Moore tunnel is located on the north side of Chicago Creek between 

Ute and Cascade creeks. At the time of visit the tunnel, which was being actively driven, was 

about 2,200 feet in length. The whole tunnel was driven in comparatively fresh, massive _quartz 

monzonite. No veins of importance had been encountered in the whole distance, although 

between 1,100 and 1,200 feet from the mouth of the tunnel some drifting had been done on the 

so-called Modoc vein, which here strikes N. 66° E. and consists of a series of 'small white clay 

slips in fractured quartz monzonite. 

About 100 feet south of the Burns-Moore tunnel is a shaft from which a drift runs 60 feet to 

the southwest on a vein striking S. 54° W. and dipping 50° NW. Near the breast of the drift, 

on the northwest side, was a small streak of galena-sphalerite ore carrying a little tetrahedrite 

(gray copper), and to the southeast of this was a 2-foot belt of fractured quartz monzonite 

with minute pyrite stringers running throughout, which formed the rest of the vein zone. Quartz 

is the chief gangue mineral of the ore. 

DUMBARDIN MINE. 

The Dumbardin mine is located at an elevation of about 8,900 feet on the crest of the ridge 

separating Maximilian Gulch from the valley of Chicago Creek. It develops a vein which on 

the surface runs in a general northeast-southwest direction and to the southwest probably joins 

theN. 54° E. vein found in the shaft located about 100 feet south of the mouth of the BurnsMoore tunnel. The Dumbardin n1.ine is said to have produced about $8,000. 

G~teiss intruded by sm~ll 

.masses of pegmatite 

k?l 

Mineralized lead 

.F'IG. 143.-Geological pla.n of lower ~!uscovite tunnel. R, Bais~. 

MUSCOVITE-SIL VERINE LODE. 

GENERAL DESCRIPTION. 

I· ......... J Unrnineralized lead 

The .Muscovite-Silverine lode crosses the head of Eclipse Gulch at an elevation of about 

9,050 feet. From the upper workings on the Muscovite, on the ridge to the west of the gulch, to 

the openings on the Silverine, on the· ridge to the east, the vein on the surface strikes N. 60° E. 

except where it swerves slightly to the northwest in the gulch, owing to its average dip of about 

72° NW. 

The }/Iuscovite mine consists of tunnels on the part of the vein lying southwest of the gulch; 

the Silverine mine is composed of tunnels located. on the northeast part. 

CHARACTER OF VEIN. 

The lowest tunnel on the Muscovite (fig. 143), which is more than 800 feet in length, runs 

S. 80° W. and follows a strong lead of crushed quartz, gneiss, and pegmatite. This vein, though 

containing considerable quartz, is for the most part but slightly mineralized. For most of the 

way it follows immediately along the northwest contact of a 10- to 14-foot porphyry dike, 

though in a few places lenses or horses of pegmatite and gneiss come in between the vein and the 
376 GEOLOGY OF THE GEORGETOWN QUADRANGLE, COLORADO. 

porphyry. In the upper tunnel of the Muscovite the vein has the same general relation to the 

porphyry dike. Across the gulch to the east, in the workings of the Silverine mine, the vein 

consists of 1 to 2~ feet of clay, crushed granite, and porphyry, which in places gives way to 

quartz. Slightly mineralized quartz stringers were also observed forming a network in fractured 

granite. The porphyry in general forms the foot wall of tht; Silverine vein also, but evidently has 

been cut in places by the vein, which is of later origin, as shown by angular breccia fragments of 

porphyry in the lead and also by small quartz, siderite, and galena stringers cutting the porphyry. 

The porphyry along the vein in both the Muscovite and the Silverine mines corresponds to the 

rock in a dike that Mr. Ball found. on the surface and classified as alaskitic quartz monzonite 

porphyry. CHARACTF:R OF ORE AND PRODUCTION. 

According to miners working in the vicinity, lessees took out something like $120,000 worth 

of ore from the lower level on the Silverine. The values in this ore were chiefly in gold, although 

considerable silver was present. Some of the ore is said to have carried between 7 and 8 ounces 

in gold. No work has been done in the Silverine mine in recent years and nearly all the workings 

are caved shut. 

But little ore has been produced by the portion of the vein southwest of Eclipse Gulch, but 

at the time of visit a small amount was being obtained from the upper tunnel of the Muscovite 

mine. This ore, which occurred in small bunches irregularly distributed along the vein and was 

composed of dark-gray pyritiferous quartz and silicified fragments of wall rock carrying galena 

and dark-brown sphalerite, was said to carry values between $100 and $120 to the ton, of which 

$70 to $80 was in silver. A characteristic feature of the Muscovite-Silverine vein, according to 

the miners, was that the ore to the west of Eclipse Gulch carried more values in silver than ingold, 

whereas on the east or.Silverine side of the gulch the reverse was'true. 

ROCKY CLIFF LODE. 

About 100 feet northwest of the Silverine lode is situated a minor vein termed the Rocky 

Cliff lode, which runs in granite parallel to the Silverine vein. No production is reported. 

BUCKEYE TUNNEL (ON ALPS MOUNTAIN). 

The Buckeye tunnel is located about 250 feet north of the lowest workings on the Silverine 

lode and consists simply of a crosscut tunnel 200 feet in length, which was being driven for a vein 

found outcropping on the surface several hundred feet farther north. 

ECLIPSE MINE. 

DESCRIPTION. 

A mine located about 250 feet south of the lower tunnel on the Muscovite v~in, near the 

head of Eclipse Gulch, was termed the Eclipse mine by miners in the vicinity, who also stated 

that the production of the mine had been considerable. The development appeared to be 

extensive and to consist of a tunnel level and of a shaft connecting with lower levels, but the 

mine had not been worked for a considerable length of time and the workings were not accessible. 

The vein runs approximately parallel to the Muscovite vein and strikes about S. 67° W. and dips 

70° NW. CHARACTER 0~' THE ORE. 

The specimens of ore found in the shaft house and on the dump consist of galena, black zinc 

blende, and pyrite, associated with a.little tetrahedrite and argentite. In some places the ore 

evidently occurred as small stringers cutting granite and gneiss; in others it was a partial replacement of these same rocks. 

BIG FORTY MINE. 

The Big Forty mine is located in Maximilian Gulch, near its junction with the valley of 

Chicago Creek. Its development consists of a crosscut tunnel about 550 feet in length, which 

runs about N. 40° W., and short drifts along the three small veins intersected by the tunnel. 
MINES NEAR IDAHO SPRINGS. 377 

The workings are all in gneiss, consisting both of the soft biotitic and hard granitic v~rieties. 

The schistosity of the gneiss ranges in trend from N. 80° E. toN. 55° E., and in places is parallel· 

to the veins. The three veins or leads, which, in the order encountered in driving the tunnel, 

have strikes of N. 75° E., N. 45° E., and N. 55° E., respectively, are narrow quartz and crushedrock leads only 1 to 3 inches wide, which show but slight mineralization. About 150 feet due 

east of the mouth of the Big Forty tunnel is a tunnel 40 feet or so in length, which runs northwestward to strike a clay and crushed-rock lead striking from N. 65° E. toN. 45° E. and dipping 

85° NW. Near the breast of the 150-foot drift on the vein a dike of~dark-gray brittle porphyry 

comes in on the northwest side and forms the hanging wall to the vein. The porphyry has 

conspicuous black bi0tite phenocrysts and belongs to the biotite latite porphyry class. . . 

GOLDEN GLEN MINJ<:. 

The Golden Glen mine is located in Golden Glen Gulch at an elevation of about 8,600 feet, on 

a northeast-southwest vein. The production of the mine has been about 10 tons of ore, averaging $95 to the ton. The ore, in which the values are about equally divided between gold and 

silver, is a quartz and pyrite ore containing specks of galena and here and there a little tetrahedrite (groy copper). 

BEAVER MINE. 

LOCATION AND DEVELOPMEN'r. 

The Beaver mine is located at an elevation of a trifle over 8,000 feet on the north side of 

Clear Creek valley, a little more than a mile above the entrance to Spring Gulch. The development consists of an inclined shaft about 250 feet deep and levels aggregating 800 feet in length 

connecting with the shaft. 

WALL ROCKS. 

The workings are run in a complex of gneiss, granite, and pegmatite. The gneiss is. both 

granitic and micaceous; where schistosity is developed it strikes about N. 60° E. and is in places 

parallel to the vein but usually dips 60° to 85° NW., the vein being much flatter. 

ROLLING CHARACTER OF MAIN VEIN. 

The vein, which is very flat and strikingly irregular, is developed by a shaft which follows 

it for most of its length, and by several very tortuous drifts which leave the shaft at irregular 

intervals. The great irregularities in the workings are due to the nature of the flat vein, which 

at many places changes both its strike and dip within exceedingly short distances. The vein 

not only forms a series of wavelike rolls along the length of the shaft to the north, but also undulates where drifted on to the east and west. At a point 190 feet from the collar of the shaft the 

vein, just beyond the lowest drift, after clipping 15° to 30° N. for nearly its whole length, flattens 

to horizontal and then begins to rise, changing its dip to 15° S., so that the shaft in following the 

vein starts up hill and rejoins a drift that follows the vein first to the west and then to the north 

and northwest from the point where the lowest level leaves the shaft. At this point the vein is 

in the back of the drift and shaft and dips to the south, so that the change in dip has been very 

decided. 

The dips in general range up to 35° from the horizontal in a series of step like rolls. Near the 

northeast breast of the lowest level an inclination as great as 60° from the horizontal was observed, 

but this is an exception to the general dip. An idea of the abrupt changes in dip, however, can 

best be obtained from the dip readings as plotted in the sketch of the underground workings 

shown in fig. 144. DESCRIPTION OF OTHER VEINS. 

The shaft was sunk for a distance of about 100 feet on the incline before the main vein was 

encountered.. at a point where it appeared to be cut off to the south and southeast by a clay and 

crushed-rock cross lead striking N. 85° W. and dipping 50° S. About 75 feet west of the shaft on 

the first level a clay and crushed-rock lead emerges from the north wall and simply comes into contact with or else barely cuts through. the ore filling of the Beaver vein, and then swerves awl leaves 
378 GEOLOGY OF THE GEORGETOWN QUADRANGLE, COLORADO. 

the Beaver vein again on the same side as it entered, trending S. 75° W. for 150 feet to the point 

where it enters the breast of the drift on it. This so-called cross vein, which is termed the Washington vein, carries some ore near the west end of the level, where small stope anq raise 27 feet in 

height connect with a shaft to the surface. A vein near the west breast of the lowest level of the 

mine, with a strike of S. 85° W. and a dip of 32° N., has an attitude corresponding somewhat to 

that of the Washington vein on the level above, but seems to cross the Beaver vein and to fault 

it slightly, so that the portion of the Beaver vein to the south of the cross vein is about 6 inches 

lower than the north portion. This cross vein consists of a gray to black gouge streak one-half 

inch wide, associated with several inches of crushed pegmatite and granite stained by iron oxides, 

or of a series of minute reddish blende stringers in hard granite. 

[0?1 [:&}i 0 Cne1:;s Granite Pegmatite Mineralized lead 

(Sih·er Plwnc:) 

l77l L:L__j 

Drifts 

M I / 

Outline of stopes Dip 

~ of gneiss 

FIG. 144.-Geological plan of Beaver mine. The undulations of the vein are shown by arrows (showing 

local dips). 

CHARACTER OF ORE. 

The ore obtained from the mine consists of coarse to fine grained galena, black sphalerite, 

and pyrite, associated with some black sooty sulphides. The ore of the first class (sorted), which. 

is comparatively free from zinc blend~ and contains considerable fine-grained galena, is said to 

run 65 per cent of lead, 90 ounces in silver, and 0.35 ounce in gold. However, a small shipment of 

sorted coarse galena ore showed 80 per cent of lead, 65 ounces of silver, and 0.45 ounce of gold. 

The ore streak, as a rule, is narrow, rarely attaining a thickness of 4 or 5 inches. 

LOCATION 0~' ORES. 

A striking peculiarity in connection with the distribution of the ore was mentioned by Mr. 

F. F. Hoban, the manager of the mine, who stated that where the vein descends in a series of steplike rolls or waves and comparatively level stretches of the vein connect places of steeper dip, the 

most and best ore is located at the crest of the roll. (See fig. 144.) A possible explanation for 

this is that movement took place subsequent to some mineralization along the vein, and that this 
MINES NEAR IDAHO SPRINGS. 379 

movement~ instead of causing a reopening of the vein in a manner similar to that occurring in 

other veins of the district, resulted in a buckling or folding of the vein and inclosing rocks due to 

some compressive force which had developed. The effect of the ·buckling or folding being most 

strongly felt on the crests of the folds, fractures or openings formed t.!:lere, in which the main ore 

deposition took place. 

BLACK ~WAN MINE. 

The Black Swan mine is located on the north side of Chicago Creek 4,000 feet west of the 

point where the valley is joined by Spring Gulch. The mine consists of a crosscut tunnel about 

125 feet in length and a very tortuous drift on a vein-running N. 50°W. for about 535 feet. The 

vein usually consists of a quartz and pyrite lead one-half inch to 2! inches wide in gneis's. At a 

distance of about 275 feet from the mouth, however, there is a small stope about 50 feet in length 

which yielded some ore. Owing to its flat dip its course on the surface is N. 70° to 75° W. 

P. T. MINE. 

GENERAL DESCRIP'riON. 

The P. T. mine lies at an elevation of about 8,000 feet on the spur separating Spring Gulch 

and the valley of Clear Creek, and is distant about three-fourths ota mileS. 75° W. from the junction of these two valleys. The mine develops an irregular east-west vein, by means of a shaft and 

three short levels aggregating about 500 feet in length. The vein, which dips from 50° to 55° N., 

on both the 50-foot and 150-foot levels has a dike of porphyry as a. foot wall for most of the way. 

ALTERATION OF PORPHYRY. 

This porphyry on microscopic examination proves to be a bostonite which shows a trachytic 

groundmass of lath-shaped feldspars, partly altered to kaolin and sericite and containing secondary adularia, pyrite, and quartz, both in vein!ets cutting the groundmass and as small, isolated 

patches in it. Possibly some of the pyrite in the rock is an original constituent, for well-formed 

crystals of the mineral contain inclusions of feldspar; moreover, some of the crystals have been 

fractured and the cracks cemented by the later quartz and adularia veinlets. 

RELATIVE AGE OF DIKE AND VEIN. 

The bostonite porphyry was evidently intruded prior to the vein formation, as much of the 

vein filling consists of a clay and crushed porphyry, pegmatite, or gneiss lead. Furthermore, on 

the 150-foot level west of the shaft narrow lenses of porphyry 6 inches to It feet wide occur in the 

vein zone, and the mineralized leads loop about them. 

CROSS LEAD. 

On the 50-foot level just west of the shaft the main vein and the porphyry foot wall both 

seem to end abruptly against aN. 38° E. lead which cuts across and possibly faults the vein a 

short distance. The mine workings on this level were not sufficient to show the extent of the 

faulting. The cross vein, however, probably corresponds to the N. 45° E. vein found about 

75 feet east of the shaft on the 100-foot level, where it comprises foot- and hanging-wall leads 

dipping 50° to 65° SE. and connects the N. 83° W. portion of the main vein with the east 

portion, which strikes S. 75° E. and is located about 15 feet farther north. 

CHARACTEH OF OHE. 

The ore obtained from the mine consists of copper-bearing pyrite and quartz, associated 

with considerable tetrahedrite and siderite. The minerals are in many places well crystallized. 

The tetrahedrite and siderite are of a later period of deposition than the pyrite and quartz, and 

occur only in vugs coating crystals of these minerals.. Much of the pyrite ore, especially where 

cupriferous, is coated by a dark-blue to iridescent film which is probably chiefly chalcocite, a 

sulphide of copper. The last shipment of first-class sorted ore ran 4.41 ounces in gold and 

3.71 ounces in silver to the ton. 

-, 
380 GEOLOGY OF THE GEORGETOWN QUADRANGLE, COLORADO. 

The wiin is stoped almost cOiitinuously from the 50-foot level to the surface, 

length of 75 feet, and smaller stopes have been made on both of the lower levels. 

has produced about $70,000 and is still shipping ore. 

QUITO MINE. 

and for a 

The mine 

The Quito mine, which is located on the north side of Chicago Creek about 3,000 feet west 

of Spring Gulch, is one of the first mines to produce ore in this part of the area. The vein, 

which runs approximately east and west through an area of gneiss and pegmatite and dips 

very slightly to the north, probably joins on the west the extension of the vein developed in 

the P. T. mine. The mine workings below the tunnel level are filled with water. The Quito 

has produced a good many thousand dollars' worth of ore, but the exact amount could not be 

ascertained. 

TORPEDO MINE. 

The Torpedo mine is located at an elevation of. 8,I50 feet in Spring Gulch, about I mile 

N. 85° W. of the junction of Spring Gulch and Chicago Creek. The wall rocks consist of fresh, 

hard, black gneiss and pegmatite. The strike of the vein where it outcrops on the surface is 

S. 52° W. and the dip is 65° NW. The lead, where mineralized, consists in the main of darkgray quartz, inclosing small masses of galena, sphalerite, and pyrite. However, small veinlets 

of telluride ore _have also been found. Specimens on the mine dump showed a series of small 

quartz and mineral veinlets cutting fractured gneiss and frozen or adhering tightly to the wall 

rocks. Although extensive development work has been done on this mine, through a tunnel 

and also through drifts connecting with a shaft, it has produced little ore. 

WALTHAM MINE. 

The workings of the Waltham mine are located on th~ southeast slope of the low spur 

separating Chicago and Clear creeks, near the junction of the two streams. The development 

consists of three short tunnels and a shaft 200 feet in depth from which short drifts run. (See 

Pl. LXXVI, B.) 

On the ~pper tunnel level two short tunnels, about 20 feet apart, expose two veins striking 

N. 43° W. and N. 58° W., both of which dip 65° NE. The I40-foot level connecting with the 

shaft shows two somewhat parallel veins running N. 38° W. and N. 44° W. and dipping 60° to 

65° NE. These two veins are connected by a cross vein running N. 80° W. and dipping 60° N. 

The southwest vein on this level consists of a clay and crushed-rock lead 6 inches to I! feet 

wide, containing scattered nodules of pyritiferous quartz. The northeast vein consists of a 

crushed-rock zone 4 to 6 feet wide, which in places carries stringers of mineral, consisting chiefly 

of pyrite with some galena. The wall rocks arc gneiss, pegmatite, and sheared diorite. They 

arc much fractured and altered between and around the different veins, and are in many places 

impregnated with pyrite to such an extent that the whole mass carries slight values in gold. 

The pay streaks, however, are so far confined to the immediate vicinity of the veins, especially 

near places where the wall rocks are firmer and less fractured. 

The ore, which has been of comparatively low grade, was first obtained from an open cut 

at the surface where the vein zone was worked for a width of 25 feet, but as this ore contained 

too much waste for profitable treatment the work is now limited to ore derived from the immediate vicinity of the vein fractures. The output, which so far has been small, has been run 

through an amalgamating and concentrating mill just below the mine, and the tailings derived 

from the mill have been treated in the large cyanide plant adjacent to it. 

The vein zone developed by the workings of the Waltham mine is considered to be a. part 

of the broad cross lode which has been traced on the surface for a long distance, and runs about 

N. 57° W., cutting the main northeast-southwest belt of veins. The vein is probably the same 

as the one developed on the Ward, Michigan, Little Mack, and other claims. 
MINES NEAR IDAHO SPRINGS. 381 

SODA. CRE_EK GROUP. 

To the southeast of Idaho Springs along the valleys of Soda Creek and its tributaries are a 

number of small mines. Little ore, however, has ever been produced by this section of the 

mineralized area, and therefore, although considerable work has been done in some of the mines, 

practically all of them can be considered only as prospects. 

SODA CREEK l\IINES. 

In the main valley of Soda Creek several mine openings are to be found, 'and among the 

most prominent of these are the Columbia and the Coming Nat ion. Both of these are said to 

have produced some ore, the Columbia having yielded some free gold. The Coming Nation 

mine is interesting because a little nagyagite, a sulphotelluride of gold and lead, is said to have 

been found in it. The veins in both of these mines have a general northwest-southeast trend. 

BARBOUR FORK VEIN"S. 

A few veins were also encountered in Barbour Fork valley about a mile above the pomt 

where it joins Soda Creek. Among the properties located here are the Gold Leaf, Gold King, 

and Maple Leaf. 

Some of these are reported to have carried a little- native gold in the oxidized portions of 

the veins near the surface. All the workings have been abandoned for years, however. The 

main veins observed were very irregular in trend, o\ving to the flat dips of ·wo to 35°. The veins 

near the surface consist of a soft yellowish to purplish streak of crushed rock and quartz 

heavily stained by oxides and carbonates. 

A few prospects of minor Importance have also been developed to some extent in both 

Soda Creek Valley and Squirrel Gulch, above the junction of the two. 

LEXINGTON MINE. 

The Lexington mine, situated at an elevation of about 9,000 feet on the divide between 

Squirrel and Warren gulches, develops the most important vein opened up so far to the southeast of the town of Idaho Springs. The vein is in an area composed chiefly of biotite granite, 

but containing considerable pegmatite of both the biotitic and muscovitic varieties, and also 

a little gneiss. The main vein is extensively developed by a shaft from which several levels 

run. The workings, however, arc inaccessible owing to filling hy water and to excessive caving 

during a long period of idleness. The main vein at the shaft has a strike of N. 75° W. and a 

dip of 48° N. About 350 feet northwest of the shaft, however, the vein where it outcrops on 

the surface splits, one branch trending S. 85° W. and dipping 50° N., while the other branch 

runs N. 45° W. with a like dip for a short distance and then in turn splits up into still other 

branches. 

The mineral found on the clump consisted entirely of pyrite, associated with massive gray 

quartz or with comb quartz in the form of stringers in the country rock. 

LITTLE RICHARD MINE. 

The Little Richard mine is located about 2,000 feet northeast of the Lexington shaft in 

Warren Gulch, at an elevation of about 8,250 feet. It consists of a shaft 600 feet deep and of a 

long tunnel level. The tunnel follows a N. 70° W., slightly mineralized lode for a distance of 

500 or 600 feet, to the point where a split in the vein takes place. Beyond that point the tunnel 

becomes very tortuous and follows several mineraliz'ld slips in biotite gneiss but in general 

becomes a crosscut drift heading in an irregular way for the Lexington and Preacher lodes, which 

lie a considerable distance to the southwest. A little ore, averaging about $15 to the ton, was 

found along the tunnel vein for a distance of 60 feet southeast of the point where the vein splits. 

The production, however, has been slight. 
382 GEOLOGY m' THE GEORGETOWN QUADUANGLE, COLORADO. 

BLIZZARD J,ODE. 

Directly across Warren Gulch from the Little Richard mine is the Blizzard lode, which 

has a strike of about N. 40° W. and a dip of 70° NE. This vein is reported to have yielded several thousand dollars' worth of ore, the chief values being in gold and copper, although some of 

the ore carried up to $2 per ton in silver. 

INTERNATIONAL LODE. 

On the southwest slope of Santa Fe Mountain, in the valley of Little Bear Creek, is the International lode, a broad soft lode of clay gouge and crushed and altered country rock that dips 

50° to 60° NE., and where observed on surface was headed due northwest. This vein is regarded as a continuation of the Columbia and Sterling lodes in Soda Creek Valley. The vein 

where developed in the International mine has produced considerable ore, but the ore has been 

of very low grade. From material found on the dump and in the ore bins, the ore appears to 

consist entirely of pyrite and quartz or of altered gneiss impregnated with pyrite. The 

International mine was not being worked at the time of visit, although it had been closed but a 

comparatively short time. 
CHAPTER V. 

THE EMPIRE MINING DISTRIO'l'. 

GENERAL GEOLOGY. 

By s. H. BALL. 

LOCATION. 

The Empire mining district lies in the vicinity of Empire, Colo., in the southwestern portion 

of the Central City quadrangle, which adjoins the Georgetown quadrangle on the north. (See 

Pls. LXXIX, LXXX, LXXXI.) 

TOPOGRAPHY. 

Topographically the district is similar to the Georgetown quadrangle and has had a similar 

history. (Seep. 30.) The most striking land forms are the broad valley of the West Fork of 

Clear Creek, which is filled in with glacia.l H.nd postglacial deposits, and the level crest of the ridge 

east of North Empire Creek, a remnant of the old mountainous upland. 

DESCRIPTION OF FORMATIONS. 

PRE-CAMBRIAN ROCKS. 

The pre-Cambrian rocks of the Empire mining district include the Idaho Springs formation, 

the hornblende gneiss, the gneissoid ·granite, the Silver Plume granite, and the granite-pegmatite. 

These are identical with the formations of the Georgetown quadrangle bearing the same names. 

(See pp. 37-96.) All the members of the Idaho Springs formation are represented, the limesilicate area on Douglas Mountain being a granular intergrowth of epidote, quartz, and garnet. 

The conglomeratic facies outcrops east of the Gold Dirt tunnel. In the northeast portion of the 

district dikes· of the Silver Plume granite cut the gneissoid granite. On the ridge east of Miller 

Creek the feldspar phenocrysts in the Silver Plume granite are 1 inch long. 

INTRUSIVE PORPHYRIES. 

The Empire mining district is included in the belt of intrusive porphyries pJready delimited 

(see pp. 67-70), and within it are many dikes and stocks of rocks which are similar lithologically 

to those of the Georgetown quadrangle and are probably contemporaneous with them. 'rhe 

generalizations concerning the form and structure of dikes already made apply with equal force 

to those of the Empire district. 

QUARTZ MONZONITE AND ITS DIFFERENTIATION FACIES. 

Distribution and structure.-A stock of monzonite 1! miles in diameter extends into the 

western portion of the Empire mining district; a smaller stock lies between Lion and North 

Empire creeks, and a small mass is situated east of Miller Creek, near its mouth. The largest 

stock is approximately circular, although it sends a few lobes into the pre-Cambrian rocks. In 

various parts of .the area surrounding Empire are quartz diorites and andesitic rocks that are in 

all probability simply differentiation phases of a magma corresponding in character to the 

quartz monzonite. As these dark-colored basic facies are rather .distinct in character, they 

have been mapped as dikes of andesite porphyry. Messrs. Spurr and Garrey found some of 

these quartz diorites of closely related character in the Aorta tunnel of the Gold Dirt mine and 

383 
384 GEOLOGY OF THE GEORGETOWN QUADRANGLE, COLORADO. 

in the Silver Mountain mine. The mass in the Aorta tunnel is probably an independent intrusive body which does not reach the surface; that in the Silver Mountain mine may be an offshoot 

from the mass between Lion and North Empire creeks. Syenitic rocks associated with the 

quartz monzonite and aplitic dikes cutting the same ·rock probably represent the more feldspathic phase formed by the differentiation of the quartz monzonite magma. 

Petrography.-The monzonite is typically a gray rock of medium grain, composed essentially of feldspar and hornblende, much of the latter mineral occurring in columns. The feldspar in some hand specimens is white plagioclase, and in others pink orthoclase. In consequence, 

though the rock is usually monzonite, some of it has dioritic and some of it syenitic affinities. 

Hornblende varies in amount considerably from place to place, but rarely exceeds 25 per cent, 

and is locally almost absent. Quartz is usually and biotite in some places macroscopically visible, and titanite and magnetite are common accessories. Facies of the large stock contain darkgray feldspar phenocrysts, and monzonite from the area between Lion and North Empire creeks 

contains tabular crystals of pink orthoclase 1 t inches long. These inclose, zonally, small hornblende rods. The monzonite is usually well jointed and breaks down into cube~.> with rounded 

angles. Granular aggregates of epidote in some places blotch the rock and epidote films develop 

upon joint faces. The monzonite is, cut by aplitic dikes, some of which are sinuous. This aplite 

resembles the monzonite in a general way, although fmer grained and richer in orthoclase, and 

in consequence commonly pink in color. 

Under the microscope the rock is seen to vary from syenite to quartz diorite, although quartz 

monzonite is most widely distributed. Notwithstanding the range in composition, indicating 

considerable differentiation of the original magma, the various types occur at haphazard throughout the mass and appear to bear no definite relation to its form. Though locally allotriomorphic, 

the rocks are usually hypidiomorphic through the partial development of crystals of plagioclase 

or hornblende and augite. The order of solidification was ordinarily zircon, apatite, ilmenite 

and titanite, pyroxene, hornblende and plagioclase, and orthoclase and quartz. Some titanite 

occurs in large irregular granules, wrapping around plagioclase and hornblende, and in such 

places was one of the last minerals to cr_ystallize. Plagi~clase, which ranges in composition 

from oligoclase to andesine, is in some specimens zonally built. Orthoclase, in optical orientation with one set of the albite twins, not unusually rims the plagioclase. Orthoclase, much of it 

microperthitic, occurs also in irregular grains or areas which micropoikilitically inclose plagioclase and pyroxene. Carlsbad twins are common. Some of the feldspars are slightly cracked and 

more rarely they show faint undulose extinction. Of their alteration products, kaolin predominates over sericite, and calcite is unusual. Large irregular plates of n:uscovite may completely 

or partly replace orthoclase. Both light and deep green augite arc present, the darker variety 

probably containing a little of the regirite molecule. Pyroxene alters to hornblende, with or 

without quartz, and calcite, to biotite, and to a serpentine-like mineral. Much of the hornblende 

is original and alters to biotite or to epidote, zoisitc, and calcite. Biotite, besides being a secondary mineral, probably produced prior to the final consolidation of the magma, is also in places 

original, occurring in hexagonal plates or in irregular areas which crystallized after orthoclase. 

Quartz, present in most thin sections, is nowhere abundant. Titanite is constantly present, and 

in some areas is as abundant as augite or hornblende. Apatite, ilmenite, and titaniferousmagnetite are constant accessories, and allanite and zircon are present·in many specimens. Crystals 

of allanite are locally large and abundant. Fluorite occurs here and there in wedges between tlie 

other constituents; and then has the habit of quartz in granite. This fluorite appears to be 

original, but other fluorite in tiny areas replacing feldspar was probably introduced by gases 

after the rock solidified. 

The aplite under the microscope proves to be. a quartz monzonite, more acidic and probably 

richer in soda than the normal rock of the stocks. Its texture is between the hypidiomorphic 

granular and the porphyritic. 

·Age.-The monzonite cuts the supposed pre-Cambrian rocks, and from its massive character 

is evidently younger than any of them. In the Silver Mountain mine it has been found grading 

into quartz diorite porphyry, a eonclition similar to that observed by the writer in the late Creta-
U. S. GEOLOGICAL SURVEY PROFESSIONAL PAPER NO. 63 PL. L XXIX 

A. TOWN OF EMPIRE AND MINES IMMEDIATELY EAST. 

B. TOWN OF EMPIRE AND GOLD BUG GROUP OF VEINS. 

1, Empire City tunne l ; 2, Golden Chariot ; 3• Jackpot ; 4· Oro Cash; 5· Go ld Bu g. 
THE E.lVIPIRE .lVIINING DISTRICT. 385 

ceous quartz monzonite and the quartz monzonite porphyry of Lincoln Mountain in the Georgetown quadrangle. (See p. 73.) The two granular rocks are practically identical. Furthermore, the rock is closely allied to late' Cretaceous diorites of Leadville, Colo., described by Cross, a 

and like them is characterized by the presence of allanite. In consequence the monzonite is considered of late Cretaceous age-in short, a granular phase of the intrusive porphyries, although 

it may possibly be the youngest of the pre-Cambrian rocks. 

QUARTZ MONZONITE PORPHYRY. 

Distribution.-Dikes of quartz monzonite porphyry are confined to Douglas and Lincoln 

mountains, the valley of Miller Creek, and the ridge between Lion and North Empire creeks. 

Similar rocks, which, however, are perhaps best described as andesite, occur in dikes 6 miles 

east of north of Empire, and on the . ridge east of Miller Creek. In the Empire tunnel arc 

dikes of quartz diorite porphyry, evidently differentiation products of the same magma. (See 

p. 387.) 

Petrography.-The porphyry of the dikes on Douglas and Lincoln mountains is similar in 

every way to that already described from the Georgetown quadrangle. (See pp. 73-83.) Phenocrysts are so abundant in the quartz monzonite porphyry from Miller Creek that the rock has a 

granitic aspect. Under the microscope the groundmass appears as a mosaic of orthoclase, quartz, 

and plagioclase., and the phenocrysts are plagioclase (oligoclase-andesine), orthoclase, quartz, and 

biotite. The dike west of the monzonite stock between Lion and North Empire creeks is a porphyritic rock containing phenocrysts which equal in volume the finely granular greenish-gray 

groundmass. The most conspicuous phenocrysts are rounded tablets of pink feldspar threefourths of an inch in length. The·se feldspars, which are Carslbad twins containing tiny plates 

of biotite, are exceeded in bulk by smaller white plagioclase phenocrysts and aggregates of biotite which have the form of hornblende and reach a maximum length of one-fourth inch. Under 

the microscope the groundmass is seen to be finely microgranitic, consisting of orthoclase, quartz, 

plagioclase, and biotite. A few rounded quartz and ragged biotite phenocrysts are associated 

with those already mentioned. The phenocrysts with the form of hornblende arc aggregates of 

green hornblende and biotite, or of biotite and epidote. The biotite occurs as diversely oriented 

plates, which arc in such sharp contact with epidote granules that it is probable that the two 

minerals were formed contemporaneously from hornblende prior to the final solidification 9f the 

rock. · 

The andesite dikes on the ridge east of Lion Creek south of the 9,950-foot hill are formed of 

a medium-gray rock with numerous phenocrysts 0f feldspar. and hornblende, the latter being in 

flow orientation. Under the microscope this andesite shows a pilotaxitic groundmass composed 

of plagio~lase and orthoclase. The feldspar phenocrysts (plagioclase) are much altered to sericite 

and epidote. The brown hornblende columns arc almost wholly altered either to actinolite or 

to chlorite (pennine in part) and epidote. Apatite and magnetite arc accessory minerals. 

Correlation.-The quartz monzonite porphyry resembles in mineralogical composition the 

monzonite of the near-by stock, and like it is characterized by large pink orthoclase phenocrysts. 

The other quartz monzonite porphyries are similar to rocks which on Lincoln Mountain in the 

Georgetown quadrangle appear to grade into quartz monzonite, probably to be correlated with 

the monzonite of the Empire mining district. The quartz diorite porphyry in the Empire tunnel 

grades into quartz diorite, a member of the monzonitic series. Therefore one and the same 

magma solidified in stocks as a quartz-bearing monzonite or diorite and in dikes as quartz monzonite porphyry, quartz diorite porphyry, or andesite. 

l30STONITE AND BOSTOXITE PORPHYRY. 

Bostonite and bost<mite porphyry, differing .in no essential particular from those of the 

Georgetown quadrQ.ngle, form a number of dikes iri the northern portion of the Empire mining 

district and occur also near the south end of the ridge east of Lion Creek. One of these dikes 

was encountered underground in the Golden Chariot mine. 

aCross, Whitman, Mon. U.S. Oeol. Survey, vol. 12, 1886, p. 334. 

31401--~o. 63--08----25 
386 GEOLOGY OF THE GEORGETOWN QUADRANGLE, COLORADO. 

GRANITE PORPHYRY. 

A few dikes ·of granite porphyry have been found underground. 

AT,ASKITIC QUARTZ MONZONITE PORPHYRY. 

A few of the dikes of the Empire ~ining district consist of much altered alaskitic quartz 

monzonite porphyry. Underground rocks probably o,f this class occur in the Empire City, 

Golden Chariot, Gold Bug, Oro Cash, and Conqueror mines. 

ALASKITE PORPHYRY. 

Alaskite porphyry similar to that of the Georgetown quadrangle (see p. 77) occurs as· a 

stock 0.6 mile.long 1 mile west of Empire, and as dikes on the ridge east of North Empire Creek 

and on the divide between North Empire and Mill creeks. The stock, from its form, probably 

intrudes the monzonite, an inference in harmony with the age relations of the quartz monzonite porphyry and the alaskite porphyry at Silver Plume. (Seep. 131) ·Alaskite porphyry was 

found underground in the Empire City tunnel, the Golden Chariot tunnel, and the Oro Cash 

mine. Specimens from the Empire City tunnel show beautiful flow banding and, under the 

microscope, a devitrified glassy groundmass. 

BIOTITE LATITE. 

A dike of biotite latite 2! feet wide showing. considerable flow banding was encountered in 

the Elizabeth tunnel. It is a brown flinty rock. Tiny phenocrysts of feldspar and biotite 

and probably of quartz are visible. Geodes of quartz fill large cavities which are probably 

vesicules. Under the microscope the groundmass appears as a fresh glass containing tiny phenocrysts of orthoclase and andesine. These grade into larger phenocrysts, with which are 

associated biotite crystals. Microscopically the rock is strikingly similar to the biotite latite of 

thE\ Georgetown quadrangle. It appears, however, to be more siliceous than that of Chicago 

Creek. (See p. 81.) 

PLEISTOCENE DEPOSITS. 

The Pleistocene deposits of the Empire district (see Pl. LL\:X) include drift of the earlier 

glacial epoch; drift, kames, and overwash gravels of the later glacial epoch; and alluvium, 

filled in behind moraines of the later glacial epoch. Lateral morainal drift (see pp. 85-86) deposited by the earlier Clear Creek and West Fork glaciers occurs in patches along Clear Creek below 

an elevation of 10,000 feet-approximately 9,500 feet at the east border of the district. 

Of the later glaciers the Mad Creek, West Fork, Bard Creek, and Clear Creek glaciers terminated within the Empire mining district, but the Mill Creek Glacier passed through its northeast corner. A considerable area of later glacial overwash gravels is located south of Empire, 

and three small areas lie south of the mouth of Miller Creek. On vVest Fork of Clear Creek, 

5 miles below Empire, is a gently undulating kame tract. The material is of glacial origin, and 

though imperfectly assorted and stratified was clearly deposited by water at the front o£ the 

West Fork Glacier. Similar material on the east border of the district was deposited by the 

retreating glacier of Clear Creek · 

ECONOMIC GEOLOGY. 

By J. E. SPURR and G. H. GARREY. 

EMPIRE TUNNEL. 

The Empire tunnel starts in below Empire near the level of West Fork ot Clear Creek, and 

runs in a northwesterly direction, with the design of cutting at considerable depth the principal 

lodes of the Empire district. At the time of examination this tunnel was in about 3,000 feet 

and had not yet reach,ed the region of the more productive veins. 
MINES AND TUNNELS 

1. Empire tunnel. 

2. Harrison mine . 

3. Hecla mine. 

4. Mint and Cashier mine. 

5. Empire City tunnel. 

6. Golden Chariot tunn&l. 

7. Jac~pot tunnel. 

8. Gold Bug tunnel. 

9. Oro Cash tunnol. 

10. Elizabeth tunnel. 

11. Aorta tunnel of Gold Dirt mine. 

12. B. B. tunnel. 

13. MacGregor mine sh aft. 

14. Benton mine shaft. 

15. Silver Mountain mine shaft. 

16. Gold Fissure mine. 

17. Badger tunnel. 

18. Conqueror mine. 

19. Union tunnel. 

20. Rosecrans tunnel. 

21. Atlantic mine. 

22. Con1et Extension mine. 

23. Crown Prince mine. 

24. Cleopatra mine. 

2 5. Apex-Surprise mine. 

26. Three Friends mine. 

27 . Hidden Treasure mine . 

28. Allen mine. 

29. Headli&ht (Mickmack) mine 

U . S . GEOLOGICAL SURVEY 

GEORGE OTIS SMITH DIRECTOR PRO FESSIONAL PAPER NO. 63 PL. 

Topography from 

Centra I City sheet 

U.S.Geological Survey 

.TOPOGRAPHIC AND GEOLOGIC MAP OF THE EMPIRE MINING DISTRICT, COLORADO Geology by S.H.Bal l 

assisted by O.H.Hershey 

Surveyed rn 1904- 0 500 lOOO 2000 3000 <ooOO CHAHAH 

Contour in.tervallOO £eet 

l006 

5000 

Veins by J. E.Spu rr and 

G.H.Garrey 

LEGEND 

SEDIMENTARY ROCKS 

Overwa.sh gravels 

(•"%' trninR of' latEr g laaia:L pe7<io&) 

'Kmnes 

@a.cial period 

g 

Limit of gL'l.ciers o£ latex gJa.Cia1 period 

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Ill 

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U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER NO. 63 PL. LXXXI 

MAP OF MINING CLAIMS IN EMPIRE DISTRICT. 
THE EMPIRE MINING DISTRICT. 387 

NATURE 0~' ROCKS. 

General description.-Probably the oldest rock encountered in the Empire tunnel is a coarse 

black gneiss containing a large proportion of biotite. This rock occurs abundantJy. It is 

usually intermixed with seams of pegmatite which are of later age and are a phase of the intrusion of porphyritic granite described below. 

Gneissoid granite forms part of the material through which the tunnel has passed. This 

·rock in the Empire district, as elsewhere, is younger than the biotite gneiss and older than the 

rocks described in the following paragraphs. · 

One of the most important rocks in the tunnel is a biotite granite, typically more or less 

P<:>rphyritic. This rock is intrusive into the biotite gneiss and into the gneissoid granite. The 

essential minerals are feldspar, quartz, biotite, and muscovite, with more or less magnetite, 

pyrite, sphene, apatite, and zircon. The feldspar consists in part of orthoclase and in part of 

oligoclase. Portions of this granite approach alaskite in composition, the biotite and muscovite of the typical granite becoming subordinate, and the rock consisting mainly of quartz and 

feldspar.. Very commonly also the granite becomes semipegmatitic and passes over into true 

pegmatite, which forms seams and blotches in the granite itself and intrudes the oldest gneissic 

rocks on a very large scale, being represented in them much more abundantly than is the 

granite. Much of the pegmatite passes into phases consisting only of quartz. On the whole 

the constituent minerals of the pegmatite are similar to those of the parent granite, but irregular anCI coarse crystallization is accompanied by irregular massing of the different constituents, 

one extreme of which is shown in the quartz veins. Other minerals are in places relatively 

more abundant in the pegmatite than in the granite. Magnetite and pyrite, for example, are 

in many phtces segregated into bunches of considerable size, both in the pegmatites and in the 

pegmatitic quartz veins. 

Dikes of basic rock. cut all the other rocks above mentioned. Mineralogically these basic 

rocks arc identical, but texturally and to a certain extent structurally they arc highly variable. 

The narrow dikes are distinctly porphyritic and fine grained, but the larger masses, though 

they 1i1ay have a porphyritic contact phase, become relatively coarse grained away from the 

margin. The numerous fine-grained porphyritic dikes contain phenocrysts of feldspar, green 

hornblende, and biotite. Some also carry phenocrysts of quartz. Magnetite, pyrite, and apatite occur in crystals of smaller size. The feldspar has characteristically a zonal structure. In 

one specimen that was studied the inner portion of such a phenocryst consisted of labradoritebytownite, another of labradorite-andesine. The smaller phenocrysts occurring in the rock 

were apparently all andesine and andesine-oligoclase. It is evident that the cores of the large 

zonal phenocrysts represent the first feldspar crystallized from the magma; this was more basic 

than the later-formed feldspar, which includes the smaller phenocrysts and the outer zone of 

the larger ones. The groundmass of these rocks is as a rule finely microgranular. 

Larger masses of basic rock encountered in the tunnel have the same mineralogical composition as the basic porphyritic rocks, but a granular texture and irregular and variable massing of the different miJ!erals. One specimen of such rock consists of about two-thirds green 

hornblende and brown biotite, most of the rest of the rock consisting of feldspar with some 

quartz. Magnetite was very abundant, with original pyrite .. Another specimen from the 

tunnel is practically hornblendite, containing 80 to 90 per cent of green hornblende. Magnetite is very abundant in large grains with a little associated original pyrite. Feldspar and 

quartz are also present. This specimen was in contact with asmall mass of pegmatitic quartz 

containing pyrite. This pegmatitic quartz is probably an integral part of the same rock mass, 

representing one extreme of the segregation (probably magmatic differ()ntiation) of the diorite, 

as the hornblendite does the other extreme. 

The general rela.tions make it appear probable that all these dioritic rocks, whether fine 

grained and porphyritic, granitic, or pegmatitic, are phases of a single magma and a single 

period of intrusion. 
3::38 GEOLOGY OF THE GEORGETOWN QUADRANGLE, COLORADO. 

Dikes of porphyry belonging to the types which are associated with the mineral veins of 

this district are also encountered in the tunnel. A: decomposed dike near the breast is probably one of the group of bostonite dikes which occur in this region. The original phenocrysts 

consisted of feldspar and either pyroxene or hornblende in a feldspathic groundmass. Magnetite and apatite were plentiful. Another dike, the first encountered in the tunnel, is of granite porphyry containing abundant medium-sized phenocrysts of quartz, feldspar, and biotite. 

Pyrite in the rocks.-'The presence _of original pyrite in the rocks traversed by the Empire 

tunnel is one of the most striking features in its geology. The older gneissic rocks, including 

the biotite gneiss arid the gneissoid granite, contain very little pyrite and would not be worthy 

of particular mention on this score. The basic porphyries contain as a rule original pyrite, 

Quartz veins ~ formed by replacement 

of granite along fractures 

O,__ __ _.c>.:Z:.._ __________ _jz feet 

Fro. 145.-Sketch of portion of vertical wall of Empire tunnel, showing barren veins 

of replacement quartz which have formed along fractures belonging to an intricate 

system. 

together with magnetite, and in 

some of them the amount of the 

former as well as of the latter 

becomes considerable. The porphyritic granite, however, contains pyrite in unusual abundance. 

This mineral can be detected in 

nearly every specimen of the 

granite,- wherever collected. 

The abundance of the pyrite 

does not depend on the alteration 

of the rocks, for the mineral occurs in quite as large quantity in 

the fresh as in the altered rock. 

In the pegmatitic portions of the 

granite and in the pegmatite the 

pyrite is more abundant than in 

the ordinary granular-porphyritic 

phase, and has segregated into 

larger bunches. The pegmatite 

is therefure the chief pyritiferous 

rock encountered in the tunnel. 

In most places where pyrite occurs in fresh granite it follows 

tiny seams or gashes in the rock, 

indicating that its origin is subsequent to that of the other minerals. Very commonly, however, 

it occurs in such a way as to be 

positively a primary constituent, 

appearing as crystals in fresh 

feldspar, quartz, and biotite. In 

the pegmatites also it occurs as 

isolated particles and crystals in unbroken masses of quartz and feldspar. Magnetite and 

pyrite are in many places associated and contemporaneous, both in the granites and pegmatites; the two minerals are locally intergrown, and in the pcgmatites each may occur in 

con,siderable· lumps. The occurrence of large quantities of magnetite, much of it in considerable lumps, in pegmatite is characteristic of this whole region, but the Empire district 

shows more pegmatitic pyrite than is present elsewhere. Practically all of the pyrite in the 

granite and pegmatite here appears indigenous, and its occurrence in tiny seams and gashes 

is probably due largely to the fact that the pyrite was in great part a mineral of the pegmatitic period and was introduced along with the other pcgmatitic minerals subsequent to 

the consolidation of the main rock mass. This pyrite not only crystallized, at many places in 

consolidated form, in the spaces occupied by the pegmatite, but perm.eated the already con-
THE EMPIRE MINING DISTRICT. 389 

solidated granite. A -similar conclusion has already been reached in regard to the introduction 

of the magnetite in the magnetite-bearing 'pegmatites and pegmatitic quartz veins near Georgetown, and to a certain as yet undefined extent also in the previously consolidated granite. 

(Seep. 182.) The original pyrite in the Empire granites~ being more easily attacked by percolating waters than the magnetite, has migrated from its original position to a greater extent, 

so that it is now found coating fractures which are probably subsequent to all granitic consolidation, and also has been transferred into fracture zones which have formed the channels for 

circulating waters. 

It was stated to the writer by an experienced mine foreman at Empire that free gold was 

occasionally found in the pegmatitic quartz. 

NA'l'URE OF VEINS. 

A number of small lodes were cut in the 

tunnel. Most of them are pyritic, like the typical lodes of. the Empire district, and contain 

only pyrite and. chalcopyrite with a quartz. 

gangue. Some, however, contrary to the general rule, contain argentiferous galena and 

blende. This difference is found in these veins 

at the surface as well as underground. . The 

presence of these two chemically distinct types 

of veins in the same neighborhood makes the 

district a·sort of link between the other mining 

districts in which only one or the other vein 

type occurs. 

A specimen froll). a pyritiferous quartz vein 

encountered in the tunnel showed a deposit of 

ankerite in cracks and lining cavities, having 

formed later than the quartz and pyrite. Another specimen shows crystalline calcite with a 

similar relation. 

V einlets of pyritiferous quartz, although 

usually of no economic importance, are very 

·abundant, especially in the granite. These 

granites and associated pegmatites show on 

fracture faces secondary pyrite, which thus has 

the aspect of having been derived from a solution and reprecipitation of that originally contained in the rock. What appears to be a further step in the segregation is the presence of 

small veinlets, which are locally abundant in 

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FIG. 146.-Sketch of portion of vertical wall of Empire tunnel, show· 

ing pyritiferous pegmatite vein (a), mainly quartz with some feld· 

spar, faulted by later fracture, on both sides of which the silicifica· 

tion and replacement of the granite (b) has formed a vein of gray 

quartz (c). This later vein contains pyrite below the intersection 

with the pyritiferous pegmatite vein, but not above, showing that 

the later pyrite was probably derived from the older pyrite, and 

that the transferring agent has been descending water. ' 

the porphyritic granite and consist chiefly of jaspery gray quartz and pyrite (fig. 145). These 

have altered and bleached the granite along 11 narrow zone on either side of a central fracture. 

An interesting occurrence· is shown in fig. 146. An original vein of pegmatitic quartz containing 

local feldspar and some pyrite crystals had been traversed by a slight fault. Along this fault 

fracture, wi1ich was marked by gouge as a result of the movement, had been deposited a very 

moderate amount of gray jaspery quartz with considerable pyrite. This pli.ase contained relatively more pyrite than the original pegmatitic vein, especially'helow the junction of this vein. 

The derivation of the pyrite in the fault lode from that in the pegmatitic vein is strongly suggestive, and the location of the largest amount of pyrite in the former directly below its interse~tion with the latter indicates that the agency may have been descending water. On 

the whole, the inspeCtion of many of these fracture zones in fresh granite that contain quartz 

and pyrite makes it. appear very clear that the vein filling 'is derived from the adjacent 

\ 
390 GEOLOGY 01•' THE GEORGETOV/N QUADRANGLE, COLORADO. 

. . 

granite, which is altered for a short distance in their vicinity. This later-formed vein quartz 

is dense and grayish in color and is not to be confounded for a moment with the quartz of 

pegmatitic origin. · 

ALTERATION OF ROCKS. 

The alteration of the rocks older than the diorites in this tunnel has not been investigat~d. ' 

The dioritic rocks as a rule are relatively fresh. Feldspars are typically largely or entirely 

clear, but a few are more or less altered. One zonal phenocryst which consisted inside of 

labradorite-bytownite and outside of labradorite-andesine had its core partly altered to sericite,' 

while the outside was entirely cle:tr. In other places feldspar has been altered to sericite and 

quartz. The hornblende is usually fresh, but some of it is more or less completely altered to 

chlorite, quartz, calcite, and epidote. In the neighborhood of fracture zones in the rock, both 

the feldspar and hornblende are altered and the biotite is bleached, with the separation of 

cloudy carbonates. 

In the porphyritic granite the orthoclase is usually very clear, being only a little turbid 

from kaolinization, and the· oligoclase is partly altered to 'sericite and calcite. The biotite is 

as a rule mainly fresh, but is in some places partly altered and bleached to a green color. 

The bostonite dike described above is highly altered, as is usual with all rocks of this class. 

The abundant feldspar phenocrysts are altered to coarse sericite and calcite, and the feldspathic 

groundmass is also decomposed. A hand specimen of this roek shows deep purple fluorite 

associated with calcite, both being secondary. Phenocrysts of pyroxene and hornblende are 

also entirely altered. 

A specimen of the granite porphyry, which is the first dike encountered in the tunnel, shows 

an altered groundmass consisting mainly of sericite. The feldspar is entirely altered to sericite 

and calcite, and the biotite is bleached, with the separation of carbonates. 

HARRISON MINE. 

LOCATION AND D~WELOPMENT. 

The Harrison mine is located at an elevation of about 8,800 feet, on a southward-facing 

mountain slope about 1, 700 feet N. 87° E. of the central portion of the town of Empire. The 

main tunnel, which is 400 feet or so in length, follows a vein that strikes in general N. 10° E. 

and clips 70° E. 

RELA'l'ION o~· DIFFERENT VEINS. 

This north-south vein consists of a clay an-d crushed-rock lea?- from 4 to 10 inches in width, 

which is replaeed along certain parts by clark jasper-like quartz heavily impregnated with 

pyrite. About 200 feet from the mouth of the tunnel is a vein trending N. 52° E. which 

crosses the north-south tunnel lead, though little if any displacement has occurred at the crossing, where both leads consist of quartz, crushed gneiss, and pyrite. TheN. 52° E. vein, which 

breaks up in micaceous gneiss at the east breast of the 125-foot drift, carries specks of sphalerite 

mixed with the pyrite and quartz, but these occur only near places where stronger seams of 

sphalerite ore join the vein from the west. 

The pyrite-bearing tunnd vein at distances of 250 and 335 feet from the mouth of the tunnel also intersects and faults two approximately east-west veins which carry sphalerite ore. 

The first of these sphalerite veins, which strikes S. 80° W. and clips 73° N., is faulted about 

10 inches, the east-portion of the vein being offset to the south. The vein at 335 feet from the 

mouth of the tunnel is relatively stronger, and consists of 3 to 8 inches of sphalerite, quartz, and 

pyrite, separated from the granite walls by only a thin selvage of gouge. This vein, where 

crossed by the north-south pyrite and quartz vein, is faulted for a distance of about 3 feet (see 

fig. 147); the east portion of the vein, which has a trend of N. 65° E., is offset to the south of 

the west portion, which trends K. 58° E. The clip of the eastern portion of the faulted vein, 
THE EMPIRE MINING DISTRICT. 391 

which is about 67° N., is somewhat flatter than that of the western portion. Between the 

faulted ends of the sphalerite-bearing vein on the west side of the pyrite and quart7. vein is a 

small area of crushed rock inclosing angular fragments of ·granite and of sphalerite, pyrite, and 

quartz ore, the latter probably representing the drag from the east-west vein along the line of 

the displacement. · 

The east-west vein in places carries black zinc blende next to the two quartz layers which coat 

the granite walls, a mixture of quartz and pyrite occupying the center of the vein. In other 

places along the same vein pyrite streaks occur along the two walls, the center of the veil). containing a breccia made up of angular fragments of sphalerite ore in a cementing matrix of pyrite 

and quartz. These occurrences would indicate that sphalerite ore had at one time filled a fracture which was later reopened, the new fracture being subsequently filled by pyrite and quartz. 

FIG. 147.-Horizontal plan of portion of Harrison tunnel, showing faulting of earlier sphalerite-hearing vein (Harrison 

lode) by later pyrite-bearing vein: a, Granite; b, breccia of sphalerite ore and crushed rock, representing drag along fault 

line; c, pyrite vein; d, sphalerite vein. 

Midway between the two east-west sphalerite veins the north-south vein of pyrite and 

quartz which faults them is itself faulted by a vertical and apparently barren clay lead which 

runs N. 84° E. and offsets the south portion of the pyrite vein 3! to 4 feet to the east of the 

north portion. HISTORY OF VEIN FORMATION. 

From these different veins and their relations to one another the following outline of the 

geologic history has been worked out: (1) Fracturi?g of the granite and gneiss country rock by 

a series of approximately east-west fissures; (2) filling of east-west fissures by black sphalerite 

ore containing some contemporaneous pyrite and rarely specks of galena; (3) reopening of 

fissures and brecciation of sphalerite ore; (4) filling of new fractures by quartz and pyrite; 

(5) formation of north-south fractures which fault the earlier east-west sphalerite-bearing veins; 

(6) filling of north-south veins by pyrite and quartz; (7) formation of a new set of east-west 

fractures, at present probably unmineralized, which fault the J1orth-south pyrite and quartz 

vems. 
392 GEOLOGY OF TUE GEORGETOWN QUADRANGLE, COLORADO. 

UPPER HARHISON TUNNEL. 

About 200 feet up the mountain slope to th~ north from the tunnel just described is a second 

tunnel 100 feet in length which connects with a 190-foot drift on a vein that strikes N. 60° E. and 

dips 70° to 75° NW. This vein is approximately parallel to and may possibly be a part of the 

east-west vein described as occurring 335 feet from the mouth of the lower tunnel. The vein 

in the upper tunnel is sphalerite bearing to a slight degree but consists mainly of a strong crushedgranite, quartz, and pyrite lead 1 to 2 feet wide. The sphalerite where present occurs as small 

discontinuous streaks. This east-west vein is also faulted by a N. 10° E. quartz- and pyrite-

~---''"'-o __ _....:;IOO feet 

~ 

~a Pegmatite l2J 

Mineralized lead C3 Unmineralized lead 

FIG. 148.-Gcological plan of Hecla tunnel. 

many years. In view of the small amount of 

tioned, it has probably not yielded much ore. 

bearing vein, very similar in attitude and structure to the lower tunnel lead. 

FEATURES OF THE ORES. 

T3e ores from the east-west veins of both 

tunnels are remarkable for the scarcity of the 

galena associated with the black sphalerite. A 

little pyrite is found associated with the sphalerite in both places, but in the ores from the upper 

tunnel the pyrite seems to be much more cupriferous. The ores from the east-west vein on the 

upper tunnel level also show angular fragments 

of sphalerite ore wit)l specks of cupriferous 

pyrite embedded in the matrix of later pyrite 

and gray quartz. 

HECLA TUNNEL. 

The Hecla tunnel is located about 1,700 feet 

due east of the central portion of the town of 

Empire. The workings are entirely in a mixture of hard micaceous gneiss, pegmatite, and 

granite. The crosscut tunnel (fig. 148) is more 

than 400 feet in length. At a distance of about 

225 feet from its mouth it intersects a pyrite 

and quartz vein striking N. 77° E. ' On this vein 

there is a drift stope 100 feet long. The east 

end of this drift is joined by a north-south drift 

about 250 feet long, which follows for most of 

its length a small clay and pyrite lead dipping 

50° to 70° E. 

This mine has not been worked for a good 

material taken from the drift stope above menMINT AND CASHIER MINES. 

LOCATION AND DEVELOPMENT. 

The Mint mine is located at an elevation of about 8,650 feet just east of the town of Empire. 

The mouth of the Cashier tunnel is situated about 50 feet farther east. The development on 

the veins in the Mint mine consists of about 500 feet of tunneling and of a drift on a level 150 

feet below the tunnel level, connected with a crosscut from the Cashier workings that starts at a 

point 160 feet in from the Cashier shaft. Besides raises and stopes, there are also two shafts 

sunk from the Mint tunnel level to depths of 80 and 50 feet at points distant about 50 and 250 

.. 
THE EMPIRE MINING DISTRICT. 

feet, respectively, from the mouth. The Cashier mine consists of a tunnel level with drifts aggregating 450 feet and a shaft from which several drifts run. How·ever, all the workings of both 

the Mint and Cashier mines below the tunnel levels are at present filled with water. 

NATURE OF WAI,L ROCKS. 

The wall rocks consist entirely of granite, cut in places by seams of pegmatite and of what 

appears to be white pegmatitic quartz, which contain small inclusions of probably original 

pyrite. Near the northeast breast of the tunnel some of these pegmatite dikes run parallel to 

and are closely associated with the vein. The granite forming the walls is here and there highly 

altered. NA'rURE OF VEINS. 

The two mines have developed a series of branching veins which diverge going to the northeast. The general trend of the vein followed by the Mint tunnel is N. 20° E.; the Cashier vein 

runs N. 50° E. for most of the way, 

but as it nears the Mint vein it 

turns slightly and runs somewhat 

parallel to that vein, although it 

probably joins it a little farther to 

the south. 

The Mint tunnel (fig. 149) for 

150 feet follows a slightly mineralized crushed-granite vein, which 

consists either of a single lead several inches wide or of foot- and 

hanging-wall leads that loop apart 

several feet and inclose horses of 

granite. At 150 feet from the 

mouth a split in the vein takes 

place, the west branch consisting 

of a vertical lead 6 inches to 1 

foot wide composed of dark-gray 

quartz and crushed granite impregnated with pyrite, and the 

east branch forming a small lead 

which connects the tunnel lead 

just mentioned with a somewhat 

parallel and overlapping vein 

trending N. 32° E. and dipping 

80° to 85° SE. This vein, near 

the point where it was first encountered, contained considerable 

ore for a distance of more than 

o~---~so_,____!.,oo feet 

Mineralized 

lead 

FIG. 149.-Geological plans of Mint and Cashier tunnels. 

EJ 

U m:nineralized I 

lead 

50 feet. It was stoped for a height of 30 feet or more above the tunnel level and was worked 

out below the tunnel to either side of the 50-foot shaft sunk at this place. 

The tunnel of the Cashier mine (fig. 149) follows a faint quartz lead for about 100 feet from 

the mouth to the point where it encounters the main vein, which strikes N. 50° E., dips 70° SE., 

and is composed of clay or of clay and crushed_granite that in places gives way to quartz anci 

pyrite. Just northeast of the point where the main vein was first encountered by the tunnel 

there is a small stope worked up nearly to the surface, and about 50 feet farther northeast is 

another small stope. Both the Mint and the Cashier veins break up toward the northeast end 

of the tunnel levels into several smaller veins. The lower workings are filled with water. 
394 GEOLOGY OF THE GEORGETOWN QUADRANGLE, COLORADO. 

COMPOSITION OF VEINS. 

All of the vein material observed in the two tunnels consisted of pyrite and chalcopyrite 

associated with quartz, and, according to Mr. Swanson, one of the owners of the mines, all the 

ore obtained along the Cashier vein, both on the tunnel level and on the workings reached by · 

the shaft, consisted of a mixture of these same minerals, which carried values a.s high as 3! to 

6 ounces in gold to the ton. On the Mint vein, according to the same authority, the noticeable 

feature was the change in character of the ore with depth. Near the surface and for some 

distance below the <tunnel level the ores consisted of cupriferous pyrite or of a mixture of pyrite 

and chalcopyrite, carrying from 3! to 4! ounces in gold and 14 to 18 ounces in silver to the ton. 

However, in the bott9m of the 50-foot shaft, located 225 feet from the mouth of the tunnel, 

and also on the drift on the Mint vein connecting with the Cashier shaft, about 150 feet below 

the tunnel level, the ore consisted of a mixture of galena, chalcopyrite, pyrite, and a little zinc 

blende, which carried about 2! ounces in gold and 23 ounces in. silver to the ton. 

EMPIRE CITY-GOLD BUG GROUP. 

LOCATION AND DEVELOPMENT. 

The Empire City-Gold Bug group of mines (see Pl. LXXIX, B) is situated in the small 

gulch which starts near the northern edge of the town of Empire and extends about N. 35° E. 

for a little more than half a mile up the steep mountain slope. The group consiEl,ts of the veins 

developed in the Empire City, Golden Chariot, Jackpot, Oro Cash, and Gold Bug mines. Belonging to it is a whole series of veins running more or less parallel to and at or near the contacts 

of a series of porphyry dikes, which in general trend northeastward. · 

NATURE OF WALL ROCKS. 

The country rock forming the walls of the veins consists of granite, biotite gneiss, and 

pegmatite. This series of rocks is intersected by a number of porphyry dikes, which vary 

widely from one another in composition. The dikes fall into four different classes, which 

include the following porphyries: Alaskite, bostonite, alaskitic quartz monzonite, and andesite. 

In •the Empire City tunnel, which is located just east of the road at the north end of the town 

of Empire, is an alaskite porphyry dike, occurring between two dikes of alaskitic quartz monzonite. The alaskite dike in going northeastward splits up into as many as three branch dikes, 

separated from one another by about 5 feet of black crushed biotite gneiss. In the Golden 

Chariot mine, which is about 1,500 feet N. 32° E. of the Empire City tunnel and 485 feet higher 

in elevation, there are two alaskite dikes which diverge going northward and one alaskitic 

quartz monzonite dike which runs along the southeast side of the eastern alaskite dike. 

(See Pl. I~XXXII.) On the northwest wall, near the breast of the Golden Chariot tunnel, was 

found a small patch of bostonite porphyry, separated from the northeast alaskite porphyry dike 

by a· slightly mineralized vein. In .the Gold Bug mine, still farther northeast, nt an clevn#on 

of 9,500 feet, is an alaskitic quartz monzonite dike which runs the whole length of the northwest wall of the vein. A small isolated patch of alaskite porphyry was also noticed on the 

southeast wall of the snme mine. In the Jackpot mine, several hundred feet south of the 

Gold Bug, are two dikes 30 and 45 feet wide, of dark-colored andesite porphyry. Near the 

brenst of the Oro Cash tunnel, which is about 400 feet east of the Gold Bug, nre found three 

different kinds of porphyry. A dnrk-colorcd rock, probably andesite, forms the northwest 

wall of the vein near the breast of the drift, and 11 20-foot dike of alaskite porphyry occurs 

near the breast of the crosscut running southeastward from the end of the main tunnel. Between 

these two dikes, and separated from them by several feet of crushed granite and micaceous 

gneiss, is a 12-foot dike of granitic appearing alnskitic quartz monzonite porphyry. 

NATURE OF VEINS. 

Veins in Empire City mine.-In the Empire City mine the main tunnel follows what is 

termed the Empire City vein for over 650 feet. A crosscut drift, about 60 feet in length, connects with a second somewhat parallel vein, which is drifted on for about 350 feet farther to 
U. S. GEOLOGICAL SURVEY 

TUNNEL 

Gneiss Bostonite 

porphyry 

Alaskitic quartz monzonite porphyry 

GOLDEN 

/9r.* >«,.i /iV~~···.,_ + + + \ fo -*-t ':i _,.%,_.·;.•' + .,. ........ 

''~\-0\\, **'~;' ~~/··· + + + + 

........ ~':...'S.~·:::\~~/'f. "*,:::)\/\_\(;I /i.i~l'"' .. / of // CHARIOT TUNNEL r"~.:-":;{t *:<~-',?}~~~·/'" .,_ + 75+ + 

,,~~;r-i"~t.:~<--'-_,1~//,+ t + + + 

'~11' %~ .. '..:'1':::--\...;'/.';' + + + 

100 200 

Gran ite 

* -t\ * r~-~~.{i;->' + + ~ + + 

-:?t/" + + + + 

'.-It + + ~ 

+ + 

300 feet 

Pegmatite Andesite 

PROFESSIONAL PAPER NO. 63 PL. LXXXII 

Mineralized 

lead 

\\~\~~ i~~:>;~: 

L3 

Unmineralized 

lead 

GEOLOGICAL PLANS OF TUNNELS OF EMPIRE CITY, GOLDEN CHARIOT, JACKPOT, GOLD BUG, AND ORO CASH MINES. 
THE EMPIRE MINING DISTRICT. 395 

the northeast. The Empire City vein itself consists of a clay and crushed-gneiss lead, in many 

places containing considerable quartz, which gives way, locally, to the pyrite, chalcopyrite, and 

quartz ~re. The strike of the vein is about N. 43° E. and the dip ranges from 70° or 80° NW. 

to 80° SE. For the entire length of the drift the walls' of the vein consist of soft biotite gneiss 

containing numerous patches of pegamite." Toward the northeast end, however, the vein 

splits into two branches that enter soft biotite gneiss, the schistosity of which strikes a little 

east of north. The somewhat parallel vein, about 60 feet southeast of the Empire City vein, 

has a strike of N. 36° E., and consists of 1 to 1! feet of crushed pegmatite, gneiss, and quartz 

which in places changes to small streaks of pyrite and quartz. This crushed-gneiss and clay 

lead runs for its whole length between two bodies of porphyry, but is at many places separated 

from the northwest porphyry mass by about 1Z feet of a soft friction breccia, which is gray· 

in color and has a few white specks resembling altered feldspar crystals. In the northeast end 

of the mine workings this vein separates two tongues of the alaskite porphyry, the one on the 

southeast being about 22 feet in width and the one on the northwest only about 4 to 6 feet. 

Where first encountered in the crosscut from the Empire City vein two somewhat similar 

alaskite porphyry dikes form its walls, but on the northwest of the 6-foot porphyry dike 

forming the northwest wall is a 15-foot dike of alaskitic quartz monzonite which, according to 

Mr. W. P. Clough,, one of the owners of the mine, swerves to the northeast and cuts through the 

4- to 6-foot porphyry dike, and then, without entering the alaskite porphyry body forming the 

southeast wall of. the vein, turns northward again and disappears from the workings. However, 

this alaskitic quartz monzonite dike is probably encountered again in the Golden Chariot mine, 

farther up the hill. . , 

What could possibly be termed a vein was also found between the southeast dikes of alaskite porphyry near the breast of the mine workings. This vein consisted of 5 feet of crushed 

micaceous gneiss and clay containing stringers of quartz, chalcopyrite, and pyrite, which were 

reported to carry about $8 in values. Near the southwest breast of the crosscut connecting· 

the two main veins there was also a small unmineralized lead of white clay, which had alaskite 

porphyry for both of its walls near the back of the drift, while near the bottom of the drift 

the southeast wall was alaskitic monzonite and the northwest wall the 15-foot alaskite porphyry 

dike which forms the southeast wall of the main southeast vein. 

Veins in Golden Chariot mine.-In the Golden Chariot mine the main tunnel for 150 feet 

follows a white unmineralized clay lead only one-half to 1 inch wide, which dips 75° SW. 

and separates a 4-foot dike of alaskite porphyry on the northwest from a 50-foot dike of alaskitic quartz monzonite on the southeast. Where the two dikes separate and loop about a 

lens or horse of granite several hundred feet long and25 feet wide in the widest part, the tunnel 

follows a faint lead along the southeast side of the alaskite porphyry dike to a point a little 

more than 500 feet from the mou.th, where it crosses to the northwest side of the dike and 

continues to the breast of the tunnel, 200 feet distant, along a clay, quartz, and crushed-rock 

lead 1 to 3 feet wide which in places carries pyrite and chalcopyrite. 

The strike of this vein, which ranges in dip from vertical to 75° NW., is N. 50° E. In 

the breast of the main tunnel both walls of the vein are gneiss, but a short distance southwest 

of the breast the northwest or hanging wall of the vein is a dense fine-grained porphyry, which 

on microscopic examination· proves to be bostonite. The southeast wall at the same point 

is pegmatite for a portion of the way and alaskite porphyry for the rest. In a crosscut spur 

to the southeast near the breast this alaskite porphyry dike is 11 feet wide and is separated 

from the 50-foot dike of alaskitie quartz monzonite to the southeast by an 8- to 10-inch clay 

and crushed-rock lead which in places carries pyritized quartz. Along the southeast side of 

the 50-foot dike and between it and granite is a clay and crushed-gneiss contact lead 8 inches 

to 2 feet in width, which strikes N. 35° E. and dips 80° SE. About midway of the crosscut 

tunnel, whose mouth is about 125 feet N. 40° vV. of the mouth of the main tunnel and which 

runs southeastward to meet the latter at a point about 75 feet in, is a second alaskite porphyry 

dike, approximately 30 feet wide, that trends N. 20° E. This porphyry body, which is. badly 

altered and heavily stained with iron and manganese oxides near the margins, is separated 
396 GEOLOGY OF THE GEORGETOWN QUADRANGLE, COLORADO, 

by a soft unmineralized gouge lead from tho granite on the southeast side, whereas on the northwest side the contact with the micaceous gneiss is rather tight. 

Vein in Gold Bug mine.-In the Gold Bug mine, located 700 or 800 feet N. 55° E. 

of the Golden Chariot mine, the vein, which is followed by a tunnel about 550, feet in length, 

strikes N. 40° E. and dips 8° SVT. For this whoie distance the southeast or hanging wall of 

the vein, aside from a small patch of alaskite porphyry about 350 feet from the mouth, is 

granite containing a small amount of pegmatite. The foot wall of the vein is usually alaskitic quartz monzonite porphyry, which is probably the continuation of the dike in th.e Golden 

Chariot mine. However, about 225 feet from the mouth of the tunnel there is a split in the 

. vein, and though a small clay selvage streak continues along the contact' with the porphyry, 

which here swerves slightly to the northwest, the main vein turns slightly eastward and loops 

around a horse of granite about 125 feet in length before again continuing along the porphyry 

contact farther to the northeast. 

Vein in Jackpot mine.-The vein in the Jackpot mine, situated several hundred feet 

S. 25° W. of the Gold Bug, is in granite for the whole length of tho tunnel, which is more than 

300 feet. A crosscut drift to the north, however, about 75 feet from the breast of the tunnel, 

intersects two dark-colored andesite porphyry dikes 20 and 35 feet in width. The vein in 

the Jackpot mine in places consists of a broad, soft lode impregnated with pyrite; elsewhere 

it is made up of hard quartz and cupriferous pyrite stringers intersecting granite. 

Vein in Oro Cash mine.-The Oro Cash mine, located on the road about 400 feet west of 

the Gold Bug mine, is developed by a tunnel on a vein which runs N. 22° E. for about 300 

feet to a point 'vhere the drift was caved shut. For most of this distance the walls of the vein 

consist of gray granite, but porphyry is probably but a few feet distant from the vein on both 

sides, as near the breast of tho tunnel three dikes of different kinds of porphyry were encountered. At this point dark andesite porphyry formed the northwest wall of the main vein, 

and a gray speckled porphyry, probably alaskitic quartz monzonite, formed the southeast wall. 

This dike, which is about 12 feet in width, was separated from a 20-foot alaskite dike farther 

to the southeast by about 7 feet of granite and micaceous gneiss. Although some pyrite and 

quartz occurred along a clay and crushed-rock vein, but little signs of ore were noticed along 

the vein up to the point where the cave in the drift occurred. 

NATURE OF ORE. 

In the Empire City mine the ore occurring along the Empire City vein consisted of quartz 

and copper-bearing pyrite, with possibly some chalcopyrite. According to Mr. Clough, this 

ore in spots ran as high as 8 ounces in gold. Although considerable stoping has been done 

along this vein, however, the mineralized streaks have been so narrow and the quantity of 

ore so small in comparison with, the amount of work necessary for mining it that the mine 

has not been highly productive. 'The middle tongue of white alaskite porphyry near the 

breast of the Empire City worki~gs, which is highly altered and impregna~ed with some pyrite, 

is said to have given assays running as high as 8 ounces in silver and $3 or $4 in gold. None 

of this porphyry, however, has yet been shipped as ore. In the breast of the drift running 

northwestward from the end of the tunnel on the Empire City vein is a seam of pyrite ore 1 to 1} 

inches wide, which is said to have assayed 18 ounces in silver to the ton, an extraordinarily 

large amount of silver for straight pyrite ore. Several of the minor leads associated with the 

porphyry dikes in the Empire City tunnel also showed some values in gold and silver, but 

these were not of sufficient richness or size to warrant the mining of the material as ore. 

In the Golden Chariot mine the only vein that showed evidence of much mineralization was 

the one along the inner 175 feet of the main tunnel, on the northwest side of the southeast alaskite dike. No stoping had been done on this level, however, and therefore there was little if any 

production. · . 

The Gold Bug vein shows evidences of mineralization for much of its length. This mineralization consists of solid pyrite and quartz streaks a few inches in width, or of small stringers of 

pyrite forming a network several feet wide in granite. Stoping, however, has been confined to 
THE EMPIRE MINING DISTRICT. 397 

two places, about 50 feet in length, at distances of 100 and 275 feet from the mouth of the tunnel. 

The stope at 275 feet, which was much the larger one, was entirely in granite on the southeast 

branch of the vein, where the vein splits and loops about a horse. of granite about 125 feet in 

length. 

The vein in the workings of the Jackpot mine shows considerable pyrite ore for its whole 

lengt.h. Much of this pyrite, however, carries but little copper and is probably of rather low 

grade, for aside from some drift stoping little effort has been made to mine the ore. 

SILVER MOUNTAIN ORE ZONE. 

NATURR CH' ORE ZONE. 

Some of the principal mines in the Empire district occur in the zone of crushing and decomposition which has a general northeast strike. The zone has been traced along the strike, as 

shown on the accompanying sketch (Pl. LXXXIII, A), for about 3,500 feet, and is several hundred feet wide. Within this zone the rock is in a general fractured and altered condition, but the 

fracturing and alteration are by no means regular, being more intense .along certain zones. The 

principal zones of maximum alteration and mineralization have a northeast strike parallel with 

the general trend of the main zone of crushing, but there arc many other slips and fractures intricately branching and crossing one another. In many places one slip passes into another in 

zigzag fashion. CHARACTER OF LODES WITHIN TIH~ ORE ZONE. 

The minor ore zone that is known as the Silver Mountain lode in the Silver Mountain mine 

has a northeast trend and is a thoroughly crushed and altered zone in gneiss. It is in places 

60 or 70 feet. wide and is exposed both at the surface and in the Aorta tunnel, which is the lowest 

level of the mines. The line drawn on the plan (Pl. LXXXIII, A) to represent the Silver Mountain lode delineates the· course of a small gouge slip, which is the most persistent feature of this 

crushed zone, for the altt?red rock adjoining it is without definite walls and is irregular in its outline. At the surface this soft' and decomposed rock is iron stained, but in the lower levels the 

,origin of this stain is seen in the pyrite which is everywhere disseminated in it. On the first 

level of the Silver Mountain mine this lode shows 30 feet of irregularly decomposed and mineralized porphyritic granite with bunches of pay ore scattered throughout. 

The Silver Mountain lode is typical of the other lodes within this general crushed zone. All 

have formed along slips or zones of especially intense crushing within the disturbed belt and are 

bordered on each side by highly altered and mineralized rock. As a rule some of the best ore 

occurs along the main slips or narrow fracture zones, so that relatively definite veins exist; but 

the wall material is also in many places sufficiently mineralized to become an ore, and the resulting stopes are of very irregular form and extent. Here and there good walls are present along 

the veins, there being in som~ places a single wall, in others two, and in still others three or more. 

At many points there are none, and some of the largest and richest ore bodies consist only of an 

irregular impregnation of the country rock by sulphides on either side of a central small crack. 

As compared with most of the other veins in this region, these separate lodes are weak and 

unreliable. Since the definite slips or narrow fracture zones may run out into broad zones of 

decomp6sition, and since other irregularities, such as the diverging of the two walls of a lode, 

the branching, and the intricate crossing of one mineralized seam by another, are eonstantly 

encountered, the lodes are in general not easy to follow for any long distance, and baffling complications may be met at any point. 

PROCESS OF MINERAUZATION. 

The mineralization which has. resulted from waters circulating through the crushed zone, 

especially along the main slips and narrow fracture zone which form the present lodes, has been 

accomplished chiefly by the filling of microscopic spaces in the altered and shru~ken rock (the 

process of impregnation), with undoubtedly some replacement. There is no evidence anywhere 

of fissure filling. · 
398 GEOLOGY OF THE GEORGETOWN QUADRANGLE, COLORADO. 

POSTMINERAL MOVEl\IENTS. 

The movements along tlus general zone of crushing and fracturing are not all of ancient date. 

Zones of crushed and broken rock constituting masses of uncemented rubble are not infrequently encountered underground, even at depths of several hundred feet from the surface. For 

example, in the Aorta tunnel, about 700 feet in and 200 feet below the surface, a belt of quartz 

diorite has been thus broken into fragments by comparatively recent movements, as there is no 

indication of any cementation, such as would have set in had the date of the crushing been even 

moderately remote. 

On the third level of the Silver Mountain mine also, in the northeast workings, the rocks have 

been scattered so as to form a loose, unconsolidated breccia. This is about 270 feet from the 

surface. The same breccia, apparently representing a relatively recent zone of crushing transverse to the main northeast trend of the veins at this point, is encountered also on the fifth level, 

100 feet below. Near this breccia on the fifth level the Silver Mountain vein is displaced by an 

open fault fissure 18 inches to the southeast on the northeast side. 

Such postmineral transverse faults have been obseroved elsewhere; for example, in the B. B. 

tunnel the Lafayette vein is faulted at one point by a movement involving a horizontal offset of 

about 3 feet. 

It is probable that other postnlineral faults have a larger displacement, for in several places 

the nlineralized vein slip was followed up to a poorly mineralized or barren transverse slip, which 

ternlinated the ore. 

ORIGIN AND DBVELOPMENT OF ORE ZONg. 

This broad general zone of crushing movement and faulting probably originated as early as 

the period of porphyry intrusion. This is indicated by the porphyry dike which .is shown on 

the plan (Pl. LXXXIII, A) as having been encountered within this zone at several points, and is 

approximately parallel to its main trend, as if at the time of the intr,usion the zone had already 

offered an inviting zone of weakness. This dike is shown in the B. B. tunnel, and what is probably the same dike, having a similar composition, thickness, and attitude, is found at several 

points in the workings of the Gold Dirt mine. 

Movement took place along the zone subsequent to the intrusion of the dike, for on the 

different levels of the Gold Dirt mine where this porphyry is encountered it is found only in 

fragments of small extent which are separated from one another. These fragments have evidently been torn apart by strike faulting complicated with oblique movement. 

The principal mineralization was subsequent to the faulting disturbance of the dike, and 

occurred principally along the same fault planes, where, as above stated, there is evidence of a 

not inconsiderable amount of faulting subsequent to the main mineralization. Other than this, 

however, there is no evidence fixing definitely the period of mineralization, and the complicated 

crossing of the mineralized and unmineralized slips suggests that different sets of fractures, 

which were subsequently mineralized, were formed at distinct periods. 

PRINCIPAl, VEINS WITHIN TIIB ORE ZONE. 

Though some of the mineralized slips or veins within the ore zone can not be followed for 

any considerable distance, as at many places they branch and zigzag and pass from veins to 

unmineralized and splintering slips, there are others which are so strong and persistent as to be 

reliable for long distances, and some of these have been traced for hundreds of feet. 

The chief veins have a northeast direction and are represented principally by the Silver 

Mountain lode and the Gold Fissure lode. From these veins branch out others which are also 

more or less persistent. Of these branches the principal ones arc perhaps the Gold Dirt and the 

Tenth Legion veins, both of which have been followed fro1n the surface down to the Aorta 

tunnel level and probably extend downward considerably farther, although they arc locally 

confused and complicated by intersecting slips. In the Silver Mountain mine there is a branching of the main lode opening out to the northeast. The direct extension of the main'lode to the 

northeast has been locally called the Pittsburg vein, and the southwestern branch retains the 
U.S. GEOLOGICAL SURVEY 

1 . 

. 

' ' 

(ff 

~ 

(Ej 

Vein 

N 

I 

( 

(A) 

0 L----~~~o _____ z~oo ______ 3o~o _____ '"'~o----~~ofeet 

[1] ' 

' Alaskitic quartz monzonite 

porphyry dike 

--

Granite porphyry dike 

-- __ ___ __ , ___ ~- ••• n .. ~ ...... ~ ~.- ..... ,,_.,., "1:0 1"\1:0\/1:01 f'\01:01"\ \/!:IN~ IN TI-n= ~II VFR MOUNTAIN ORE ZONE. 

N 

PROFESSIONAL PAPER NO. 63 PL LXXXIII 

(B) 

100 200 300 '100 soofuet ~--~--~--~--~~~ 

[];[] 

Surface slide 

' ' 

' ' Monzonite porphyry 

G77l 

L2J 

Altered and mineralized 

rock adjacent to vein 

Mineralized fissure 

B. GEOLOGICAL PLAN OF AORTA TUNNEL. 
U. S. GEOLOGICAL SURVEY 

1

/JCNTOH ~NAI'T 

v 

• 

v 

< r 

BE.NTON MINE 

PROFESSIONAL PAPER NO. 63 PL LXXXIV 

N 

MINE 

SILVER MOUNTAIN MINE 

~~1~00~----~·~0~0------~~------4-®~----~ 
U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER NO. 63 PL. LXXXV 

A . VALLEY OF CLEAR CREEK FROM EMPIRE PASS ROAD. 

Showing meand ering of stream. 

B. AORTA TUNNEL, EMPIRE, AND SURFACE WORKINGS ON SILVER MOUNTAIN AND 

BENTON LODES. 
THE EMPIRE MINING DISTRICT. 399 

name Silver Mountain. The northeastern branch is persistent from the surface down to the 

lowest (Aorta) tunnel level; the other branch, which is strong on the first level, is very weak on 

the third and not recognizable on the filth. 

The accompanying plans (Pl. LXXXIII) show only the veins which have been noted in 

the writers' study of the mine workings, with a little conservative projection in some places . 

. They undoubtedly do not represent all the veins or principal mineralized slips within the ore 

zones, a full list of which would be much larger. Moreover; some of the veins which are shown 

may unite and be practically the same lode; for example, the Silver Mountain and the Lafayette veins are in line, and also the Whangdoodle and the Benton. In general, however, in this 

district the veins are so irregular that it_ is unsafe to correlate them across any considerable 

intervening space. 

MINt; WORKINGS. 

The principal mine workings in this zone are those of the Gold Dirt, Gold Fissure, and 

Silver Mountain mines, which are close to one another. (See Pl. LXXXIV.) These were all originally worked by shallow tunnels and shafts, from which deeper levels were run. Later the 

Aorta crosscut tunnel was run in from the gulch to the south of the mines to tap the veins at a 

lower level. The Aorta tunnel level (Pis. r_.XXXIII, B>· LXXXV, B) passes under the Silver 

Mountain and Gold Dirt mines and forms the sixth level of the latter. Along the zone southwest of the mines, mentioned above, is the Bay State tunnel, which starts in on nearly the same 

level as the Aorta and opens up some of·the veins of the zone. Northwest of this tunnel and 

southwest of the other min:es mentioned is the Benton shaft, from which a number of different 

levels have been run. Several hundred feet north of the Gold Fissure shaft there is a tunnel on 

the Badger lode. COMPOSITION OF VEINS. 

The metallic minerals consist chiefly of pyrite and cl_talcopyrite with a gangue of quartz. 

Galena and blende were not observed in these mines. Subsequent black sulphides of copper, 

which have formed along fractures in the chalcopyrite, occur in the upper workings, especially 

in the Gold Fissure mine. Native copper in small amounts is reported in different places, 

especially in the upper portions of the veins. A small quantity of native bismuth is reported 

as having occurred on the third level of the Gold Fissure mine: (See Pl. LXXXVI.) In the 

Silver Mountain mine along a barren slip tabular crystals of barite were found. A specimen of 

the ore from a vein in the Aorta tunnel level of the Gold Dirt mine examined microscopically 

was found to consist of abundant pyrite, finely crystalline quartz, disseminated brown crystals 

of siderite, and some :fine sericitic material. 

The valuable materials in this ore are chiefly gold and secondarily copper. Silver is 

present in small quantity. The gold is apparently confined mainly to the chalcopyrite; the 

pyrite is usually of low grade or barren, although exceptionally it may contain enough gold to 

be a profitable ore. Up to the present time the ore has been treated by hand sorting, to separate the chalcopyrite as well as possible from the pyrite, and the material which is of sufficiently 

high grade is smelted. The ore that has a value of less than $10 is characterized as ''mill 

dirt," and no satisfactory process for treating it has yet been found. Mechanical concentration 

has failed to separate the chalcopyrite from the pyrite, and on account of the presence of so 

much copper the ore is not well adapted for cyaniding even after roasting. According to 

Mr. F. A. Maxwell, of Georgetown, the sulphides yield on assay only about 80 per cent of the 

values indicated by assay of the whple ore; the rest must occur as free gold, for this can he 

obtained by panning the ore. 

The hi~hest grade ore which has been extracted contained several ounces of gold to the 

ton. One carload of ore from the Sprangle vein on the fifth level of the Gold Dirt is reported 

to .have averaged several ounces to the ton. This was the richest ore in the mine. Picked 

smelting ore from the third level of the same mine is reported as containing 3 ounces. On the 

Tenth Legion vein some ore rich in copper was encountered. This ore carried as high as 

12t per cent of copper, with 7 ounces of gold and 20 ounces of silver to the ton. These values 
400 GEOLOGY OF THE GEORGETOWN QUADRANGLE, COLORADO. 

apparently represent exceptional spots, for 2 feet of high-grade ore from the same vein on the 

125-foot level is reported as having yielded from $45 to $50 per ton in gold. There are, however, apparently large masses of low-grade ores. 

LOCATION OF ORgs. 

In this zone the ore bodies seem to have no relation to the junction of slips or leads, thus 

forming an exception to the general rule for this region. It is quite likely that this fact is due 

to the successive develop:nent of the intersecting fractures, which obscured the original junctions, if there were any, and created a number of junctions subsequent to the earlier stages of 

mineralization. · 

NATURE OF WALL ROCKS. 

The oldest rock within this ore zone is a dark, usually biotitic gneiss, which is also the oldest 

formation known in this whole region-the Idaho Springs formation described by Mr. Ball. A 

large amount of gneissoid granite was encountered in the vnderground workings, especially in 

the Aorta tunnel. This rock is cut by the other igneous rocks described below. 

Dark-colored dioritic rocks occur at numerous places unJerground in dikes and larger intru- . 

sivc masses. These rocks are similar to those in the Empire tunnel already described. They 

range in texture from relatively fine grained porphyries to coarse porphyritic granular rocks and 

to both fine and coarse nonporphyritic granular rocks. ·The mineral composition, however, is 

similar throughout, although the relative proportions differ somewhat, especially in the granular parts, where there has evidently been some segregation and differentiation, further indicated 

by pegmatitic streaks and bands of fine siliceous aplitic rock, apparently contemporaneous in 

origin with the inclosing diorite. The principal minerals are feldspar, hornblende, and biotite. 

Quartz is usually present in varying quantity. As accessory minerals occur sphene, zircon, 

magnetite, pyrite, and apatite. The feldspar is mainly andesine-oligoclase and andesine. In 

the porphyritic rocks some of the phenocrysts, which in the coarse-grained phases are an inch or 

more in length, show a zonal structure .. The core of one of these crystals was determined to be 

labradorite and the outside layer andesine. Small fragments of augite were noted in one or two 

places. . 

The porphyry dike that follows the W11angdoodle vein in the B. B. tunnel and the Tenth 

Legion vein in the Aorta tunnel and the upper levels of the Gold Dirt mine is for the most part 

entirely altered. The original phenocrysts appear to have consisted chiefly of feldspar and 

biotite; apatite and magnetite still remain unaltered. The rock appears to belong to the class 

of alaskitic quartz monzonite porphyries, as determined by Mr: Ball. 

The porphyry dike encountered in the e)\treme end of the Gold Fissi1re workings belongs 

to the class of granite porphyries. 

ALTERATION OF WALL ROCKS. 

The gra:n:itic gneiss, which wheh fresh has the composition of an alkaline or alaskitic granite, 

is as a rule entirely cj_ecomposed within the ore zone, and is changed to secondary quartz and 

sericite, the original quartz and zircon of the rock remaining unattackcd. A specimen of this 

gneiss from the Aorta tunnel appeared at first sight to be altered to quartz and sericite, but an 

investigation of the latter mineral showed it to be a pale-brown pleochroic mica of sericitic habit. 

Pyrite is always abundant in these altered rocks. A specimen of the gneissoid granite from the 

dump of the Aorta tunnel contains a small seam of brilliant specular iron, apparently secondary 

n1 ong1n. 

The porphyry dike encountered along the Tenth Legion vein is probably identical with that 

encountered along the Whangdoodle vein, and both are everywhere highly altered. The feldspar is characteristically changed to clear, translucent ka.olin. The biotite is bleached and is 

filled with reddish cloudy carbonates, chiefly siderite. Apatite and probably magnetite remain 

unaltered. The groundmass is usually altered to quartz with some siderite. Specimens of the 
/ 

THE EMPIRE MINING DISTRICT. 401 

dike from the B. B. tunnel show a slightly different and rather more intense alteration than 

those from the Tenth Legion vein, above referred to. They are entirely altered to quartz, 

sericite, and siderite, the original structure as well as the composition of the porphyry being 

completely obscured. : 

A specimen of the porphyritic granite taken from the north breast of the Sprangle vein on 

the fifth level of the Goid Dirt mine shows some of its feldspars entirely altered to sericite, while 

some show no alteration except that they are slightly turbid as if from kaolinization. There is 

considerable crystalline pyrite, associated with a fine crystalline aggregate of brownish siderite, 

the iron minerals being probably qerived from the alteration of original biotite. The original 

quartz and :ilircon of the granite remain in the unaltered mass. The rock also contains microscopic veinlets, the borders of which, representing the first deposition in the tiny fissure, are of 

fine crystalline quartz, while the interiors are filled with clear; translucent kaolin. 

The quartz diorites examined from this zone underground arc as a rule relatively fresh; 

~ome of the feldspars are altered more or less to sericite or to kaolin. The hornblende is in 

places slightly altered to epidote. A specimen of the diorite from the crushed zone which has 

been described as appearing along the Aorta tunnel about· 700 feet from the mouth is, however, 

unusually altered, the feldspars being partly clouded by alteration products which appear to be 

mainly kaolin with some sericite. The hornblende also in this rock is entirely decomposed. 

The unusual alteration in this specimen i$ evidently clue to ordinary waters circulating through 

the breccia zone. 

RELATION OF ORES TO DEPTH. 

No accurate data as to the relation. of the strength of the veins to depth from the surface 

were obtained. The general impression received, however, was that the several lodes were 

decidedly stronger in the upper portions of the mines than, for example, in the Aorta tunnel 

level. On this level the different lodes, which in the overlying workings were represented by 

numerous definite and stronger deposits of quartz and sulphides,. are characteristically only 

mere slips in the zones of altered and pyritized rock, and many of them are marked by very little 

or no especial mineralization. On the upper levels of the mines the veins are stoped at many 

places for a width of severalfeet and may be relatively regular. It also seems probable that 

the proportion of chalcopyrite to pyrite is much larger on the upper than on the lower levels 

and that the Aorta tunnel level is mostly below the rich chalcopyrite zone. The superficial 

portion' of the ores, extending from the surface down to a depth of 40. feet or more, is entirely 

oxidized, and the gold below, associated with and mostly contained in sulphides, is 'in a free 

state. . 

The discovery of gold in this oxidized and decomposed superficial rock was the cause of the 

first mining activity in the Empire in 1862 and 1863. The softened material was washed into 

sluices and treated in the same way as placer gravels, yielding a good profit. Water was brought 

in a ditch from Mill Creek, 3! miles distant, and most of the surface material was sluiced off. 

The diggings were scenes of great excitement for three or four years. 0 Later, when the surface 

material had been mostly removed, sulphides were encountered in depth, and as these ores were 

not amenable to the same simple treatment the mining industry suddenly languished. 

The writers were informed by the miners at Empire that the oxidized and decomposed surface ores were much poorer than the sulphides below, and this information is published in an earlier 

report.b ,Further information renders it doubtful whether this is strictly true when applied to 

the material which has been removed, although it may be true with reference to the poorer portions that have not been washed away. These :residual portions are said to contain from $2.50 

to $7 in gold to the ton. It is stated by earlier writers, however, that the material sluiced away 

and passed through a stamp mill yielded $70 to $85 per ton. The surface of the Tenth Legion 

lode is said to have yielded, locally at least, $35 per ton.c According to this, it seems pr~bable 

that the surface material was as high in gohl or higher than the sulphide material below. 

a Fossett, Frank, Colorado, 1883, p. fl. I> Bull. U. S. Geol. Survey No. 260, 1905, p. 120. c Hollister, Mines of Colorado, 1867, p. 245. 

31401-Ko. ()3-08--26 
402 GEOLOGY OF THE GEORGETOWN QUADRANGLE, COLORADO • 

.Not only do the sulphides immediately beneath the oxidized zone contain a large amount of 

chalcopyrite, but along the fractures in this ore black sulphides of copper a have formed which 

are undoubtedly of secondary origin. In the Gold Fissure mine the best ore was of this character, 

and extended from 25 to 150 feet beneath the surface. Much of this ore was of high grade, some 

of it assaying 10 ounces of gold to the ton. 

The actual difference of elevation between the surface at the outcrop of the Gold Fissure 

vein and the Aorta tunnel level, where the quantity of chalcopyrite seems very much less, is 

about 500 feet. This recalls the Centennial vein, where similar gold ores whose values are associated closely with chalcopyrite and black or gray copper sulphides (tetrahedrite, in part at least) 

give way to lean orcs at about 550 feet below the surface. In the Centennial min~ and perhaps 

also in the Empire mines the effect of descending surface waters is suggested. In veins carrying 

considerable copper the amount of rearrangement produced by a given quantity of descending 

surface waters within a given period is usually much greater than in veins of different composition, on account of the easy solubllity of copper. 

PRESENT WORK OF SURFACE WATERS. 

The water in the mine is very acid, evidently as a result of the superficial oxidation of sulphides. It also contains copper. Iron rails in the mine are rapidly eaten up and in places shells 

of copper arc deposited on them. Deposits of amorphous copper minerals, probably both carbonate and sulphate, were noted at several places as having formed since the opening of drifts . 

.Fine greenish stalactites were taken from the roof of the Bay State tunnel on the first level of the 

Silver Mountain mine. These were examined by E. C. Sullivan and determined to be hydrous 

ferrous sulphate of iron containing a trace of calcium, a little magnesium, and a faint trace of 

copper. The underground waters which circulate along the zone .of brecciated diorite described 

above as occurring in the Aorta tunnel about 700 feet from the mouth deposit considerable iron 

oxide (limonite). Pools of water standing in the bottom of the drift at this point have a film 

of blood-red iron oxide on top. 

CONQUEROR 1\fiNE. 

LOCATION AND DEVELOPMENT. 

The Conqueror mine is located on the west side of North Empire Creek, 1. 7 miles clue north 

of the town of Empire. On the tunnel level the development consists of a crosscut ·tunnel 

about 1,100 feet in length and 2,000 feet or more of drifting. A shaft also extends up and 

down from the tunnel level on the Conqueror vein, but the extent of the workings connecting 

with the shaft was not learned. NA'l'URE OF WALL ROCKS. 

The wall rocks throughout the mine consist almost entirely of biotite granite of the Silver 

Plume variety, hut a few small patches of black gneiss and a little pegmatite were also noted, 

and at two points along the northeast workings on the Conqueror vein portions of a fine~grained 

alaskitic quartz monzonite porphyry dike 3! feet wide appeared as the northwest wall of the 

vein and ran parallel to the vein for a short distance. A mass of dark-colored, highly silicified, 

and badly altered porphyry about 40 feet in width was cut by the tunnel about 50 feet south of 

the Conqueror vein. (See Pl. LXXXVII.) This porphyry was so 'intensely altered that its 

variety was indeterminable. On the surface an outcrop of bostonite porphyry was observed 

just west of the boiler house. The grourtdmass of this rock was very fine grained and contained 

only a very few minute phenocrysts embedded in it. Under the microscope this groundmass 

showed the trachytic structure formed by intergrown lath-shaped feldspar crystals, 

NATURE OF VEINS. 

The main vein developed in this mine is the Conqueror vein, which was encountered in 

the tunnel at a point between 1,000 and 1,050 feet from the mouth. The general strike of the 

' a A specimen taken proved to be too thin a crust for chemical determination, but is probably chalcocite.,. 
U. S. GEOLOGICAL SURVEY 

Region of genera l brecciation 

GOLD DIRT TH IRD LEVEL 

GOLD DIRT FIFTH LEVEL 

0 100 200 300 feet ~----~------~----~ 

Granite Gneiss Alaskitic Mine ralized lead 

quartz monzonite porphyry 

PROFESSIONAL PAPER NO. 63 PL. LXXXVI 

N 

SILVER MOUNTAIN FIRST LEVEL 

SILVER MOUNTAIN THIRD LEVEL 

SILVER MOUNTAIN FIFTH LEVEL 

Unmineralize d lead Altered and mineralized 

country rock adjoining 

central vein fissure 

GEOLOGICAL PLANS OF SEPARATE LEVELS, SILVER MOUNTAIN ORE ZONE. 
THE EMPIRE MINING DISTRICT. 403 

Conqueror vein is about N. 70° E., and its average dip is 75° NW. The vein is a strong one, 

and consists of a zone of clay and crushed wall rock several feet in width, which represents a 

line of faulting or movement, as indicated by slickensided surfaces darkened by pulverized 

pyrite and showing movement strire.dipping 45° NE. 

The Conqueror shaft, located about 80 feet northwest of the tunnel, is at a point where 

the Conqueror vein splits going northeast and loops about a horse of granite 100 feet or more 

in length. The point where these branches come together again is not shown in the present 

workings, but the continuation of the vein as exposed about 100 feet farther northeast, in a 

drift connecting with what is called the drift on the "Patsy," consists of a single lead, showing that the union has probably taken place. To the west of the shaft the vein has been opened 

for 600 feet or more. In the drift on the Conqueror vein in the northeast part of the mine the 

vein consists of 2 feet of clay or gougelike triturated material, resting to the southeast upon 

an altered granite foot wall highly impregnated with pyrite, and to the northwest of the clay 

streak is a belt of soft altered granite and gneiss permeated with pyrite. In spots this hangingwall belt gave good values and was being stoped at the time the mine was visited. 

Most of the stoping on the Conqueror vein, however, has been in the immediate vicinity of 

and to the west of the shaft. Although the two branches of the vein to the east of the shaft 

yielded some ore, th_e highest grade and largest body of ore was obtained near the shaft, where 

the two branches come together. At this point the ore was stoped for a height of 125 feet 

above the level, and for a width of 5 feet is said to have averaged between 3 and 4 ounces in 

gold to the ton. The stope continued for a distance of nearly 250 feet southwest of the shaft, 

but to the west the ore streak was neither so wide nor so rich as near the shaft. Some ore 

was also found along the vein for 300 or 400 feet t? the southwest of the large stop·e, and a 

little drift stoping was clone at various points along this stretch, although little pay ore was 

obtained. The best ore procured, which carried about 5 ounces in gold to the ton, came from 

the tunnel level near the shaft; the ore both above and below this level decreased in value. 

Ncar the breast of the northeast workings of the mine the Conqueror vein intersects and 

faults, by a distance of about 3 feet, a vein termed the Patsy vein. The Patsy vein, which has 

a general N. 23° E. trend and a dip of about 80° SE., consists of a fairly strong crushed jaspery 

quartz and gouge lead, which was drifted on for a distance of more than 300 feet southwest 

of the Conqueror vein, to a point where it either terminates or changes rather abruptly to a 

series of faint leads in friable granite. It is very possible that the Patsy vein is cut off on the 

southwest by a cross lead somewhat parallel to the Conqueror vein. Although showing some 

pyrite in places, the Patsy vein usually carries little mineral of value and has produced only a 

small quantity of ore, if any. 

About 250 feet S. 33° ·vv. of the point where the Patsy vein was intersected by the tunnel, 

another vein, termed the General Harrison vein, a was intersected by a drift that followed 

the Patsy vein for about 50 feet and then continued to the southwest as a crosscut drift in 

friable granite. The General Harrison vein, which is considered to be the same vein as the 

one developed in the Rosecrans mine, is a soft lode from 3 to 7 feet wide, consisting of quartz, 

pyrite, and soft kaolinized and otherwise altered granite, containing disseminated pyrite. 

Some stoping has been done on this portion of the vein, which has yielded some ore. Although 

the General Harrison is a much broader and softer vein than the Patsy and differs from the 

latter somewhat in structure, it possibly represents a continuation of the Patsy vein, which 

has been faulted some distance to the southeast. 

About 70 feet southeast of and parallel to the General Harrison vein is another vein called 

the Rosecrans, which is thought to be a continua,tion of the vein of that name developed in 

the _Rosecrans mine. Where encountered this vein, which was trending N. 32° E., showed 

nearly 4 feet of quartz and altered granite impregnated with pyrite, much of which was in 

well-formed cubical crystals. Practically no development work had been done on this vein, 

which at this point probably carried very low values. -------------~ ----

a '!'his is to be distinguished from the Harrison vein in the 1Iarrison tnnnel (pp. 390-391.). 
•I 

404 GEOLOGY OF THE. GEORGETOWN QUADRANGLE, COLORADO. 

Still another vein is found in the mine. It is located about 60 feet northwest of the Conqueror vein, near the shaft, and has been drifted on for about 100 feet. The vein, which strikes 

S. 63° W. and is generally vertical or dips slightly to the northwest, is a slight lead of quartz 

and pyrite 1 to 3 inches wide, which has yielded no ore. 

NATURE OF ORE. 

The ore from the different veins encountered in the workings of the Conqueror mine consists in the main of pyrite and quartz from the main lead, or of a pyrite ore formed by the 

impregnation and partial or entire replacement of the wall rocks on the two sides of the main 

fracture. 

The pyrite is slightly cupriferous, and in places along the Conqueror vein carries as much as 

5 ounces in gold to the ton and a small amount of silver. In the northeast drift on the Conqueror 

vein patches of ore gave assays of 3 ounces in gold and 3 or 4 ounces in silver to the ton, but 

in general the ore throughout the mine averaged much lower in value. The exact production 

of the mine could not be ascertained, but is known to have been considerable; practically all 

having been from the Conqueror vein itself. · 

FAULTING. 

The only faulting recognizable in the mine is that which resulted in . the dislocation of 

. portions of the Patsy vein. As already stated, the Conqueror vein cuts through the Patsy 

vein and causes a displacement of the south portion of that vein 3 feet to the northeast of the 

'north portion.· Near the fault the Cqnqueror vein dips 72° NW. and the Patsy dips 8° E. 

About 100 feet farther south the Patsy vein is again faulted by a vein running approximately. parallel to the Conqueror vein. Here also the south portion of the Patsy ·vein is offset 

about 3 feet to the east of the north portion. This cross vein, which dips about 75° SE., consists of 

a narrow clay and crushed-rock lead, apparently mineralized but slightly, if at all. It is possible 

that where the Patsy vein appears to end abruptly at the southwest end it is cut off by a cross 

lead, by a displacement similar to that of the faults which intersect it in the other two places. 

A small east-west pyrite and quartz lead observed 40 or 50 feet southeast of the Patsy vein 

in the tunnel may be the lead which is responsible for the faulting. The General Harrison 

vein, developed farther to the southwest, has a strike like that of the Patsy, and if projected 

to the northeast would run only a short distance southeast of it. As the General Harrison 

vein is not recognized in the tunnel, which it should cross if its course is undisturbed, it is very 

likely that it represents the south portion of the Patsy vein, which has been offset to the southeast by a displacement similar to that of the faulting which has affected it in the other two 

places. 

ROSECRANS MINE. 

LOCATION AND DEVELOPMENT. 

The Rosecrans tunnel mouth is located on the west side of North Empire Gulch, 500 or 

600 feet due west of the Conqueror mine. 'fhe development consists of a tunnel level with 

about 600 feet of drifting and a shaft extending for a short distance below the tunnel level 

and connecting it with the surface. 

NATURE OF WALL ROCKS. 

The workings are all in granite, which is in general highly altered. The friable altered granite 

is as a rule highly impregnated with pyrite for a distance of several feet on either side of the 

main leads, or has small pyrite stringers scattered all through it. 

DESCRIPTION 0~' V~~INS. 

Th~t Rosecrans vein, which was encountered about 150 feet· west of the mouth of the tunnel, 

dips 65° to 70° SE. and trends S. 45° W. for 100 feet and then bends to S. 30° W., continuing 

in the latter direction to the breast of the drift, about 200 feet distant. That the vein represents 

a prominent line of faulting or movement is shown by the smooth wall along the southwest 200 
U. S. GEOLOGICAL SURVEY 

0 I 00 

J ............ J 

Mineralized lead Unmineralized lead 

200 300 

Arrow indicates direction 

and dip of movement striae 

400 feet 

Arrow indicates dip 

of vein 

PROFESSIONAL PAPER NO. 63 PL. LXXXVII 

J~! +!+ 

Alaskitic quartz monzonite 

p orphyry 

GEOLOGICAL PLANS OF TUNNELS IN CONQUEROR AND ROSECRANS MINES, EMPIRE. 
THE EMPIRE MINING DISTRICT. 405 

·feet of the vein. Movement strire dipping 70° SW. were observed on this smooth wall, which 

probably represents the southeast or hanging-wall lead of the vein, which possibly occurs 

slightly northwest of the present drift. Along the northeast 100 feet of the drift the Rosecrans · 

vein, which consists mainly of clay and quartz, is rather heavily mineralized in places anc). 

has been worked upon nearly to the surface along the line of the shaft. The granite south 

and southeast of the shaft is altered to a white gougelike mass and heavily impregnated with 

pyrite for a distance of 30 feet or more from the main lead. According to lessees at work near 

this point, assays made from material taken from a groove 10 feet in length along the wall 

of a crosscut spur in this impregnated granite gave an average value of $4.50 per ton for the 

whole mass. 

A plausible reason for the large amount of mineralization 

in this place is that the branch mineralized lead which was 

encountered about 50 feet from the mouth of the tunnel and 

drifted on to the S. 68° W. probably joins the H,osecrans vein at 

this point. 

Another vein, termed the General Harrison vein, which 

runs S. 509 W., approximately parallel ,to the Rosecrans vein, 

is located about 40 feet west of the latter vein where out 

by the tunnel. The General Harrison vein consists of a 5-foot 

belt of altered granite and pyrite in equal proportions, the 

mass being cut by local streaks of solid pyrite up to 1 foot in 

width. This vein was opened only for a length of about 50 

feet, but at the time of visit a small amount of stoping was 

being done on it. 

Both the General Harrison and Rosecrans veins are supposed to have been encountered at a lower depth in the workings of the Conqueror mine. 

CHARACTER 0~' ORES. 

The ores from this mine are chiefly low-grade gold ores 

consisting of pyrite associated with consicierable quartz or silicified and altered granite. The pyrite as a rule is not highly 

cupriferous, but locally contains some copper, as is shown by 

the fact that tlie ores· obtained from the present workings, 

which are comparatively near the surface, are here and there 

·associated with a dull-black sooty mineral of secondary origin 

which W. T. Schaller, of the Geological Survey, found to contain both copper and sulphur and considered to be chalcocite. 

[3EJ 

Mineralized Unmineralized lead lead 

FIG. 150.-Geological plan of Union tunnel. 

Near the shaft on the Rosecrans vein some porous or spongy quartz ore which on panning 

showed a little free gold was also obtained. 

UNION TUNNEL. 

The Union tunnel is located in North Empire Gulch, between 200 and 300feet S. 57° W. of 

the mouth of the tunnel of the Conqueror mine. The tunnel (fig. 150),.which follows the vein 

for 400 feet in a S. 25° W. direction, at about 125 feet from its mouth intersects a shaft which 

extends to the surface, a distance of about 86 feet, and has been s1.mk about 20 feet below the 

tunnel level. 

The wall rocks in the northern part of the drift consist chiefly of gneiss, patches of which 

are so altered and silicified that they form a fine-grained mass resembling porphyry. Near the 

south end of the tunnel the wall rocks are chiefly granite, with some gneiss; 

The vein is an irregular or wavy fault plane trending S. 25° W. and either vertical or dipping slightly to the southeast. In places the single vein splits and loops about horses of rock. 

In the main the vein consists of a gouge lead 4 inches to lt feet wide, one-fourth inch to 4 inches. 
406 GEOLOGY OF THE GEORGETOWN QUADRANGLE, COLORADO. 

of which is in many places heavily specked with pyrite. The vein in general contains but little 

pay ore, although some values were obtained near the shaft, where the vein loops about a horse 

of rock. Here the ore was a pyrite ore slightly stained by blue copper carbonate and containing 

some sooty black sulphides, probably chalcocite for the most part, in small vugs in the porous 

quartz. Some well-formed crystals of barite were found in vugs in the quartz gangue of the ore. 

CROWN PRINCE-ATLANTIC GROUP. 

GENERAL DESCRIPTION. 

J.'he Crown Prince-Atlantic group of veins includes the veins developed in the Crown Prince, 

Comet Extension, Atlantic, and Cleopatra mines. The first three of these mines, although separated by considerable unexplored ground, are probably either located on the same strong vein 

or belong to a set of nearly parallel veins a short distance apart which occupy the same fault 

zone. On the surface this fault zone runs N. 40° E. from the point >vhere it crosses Lion Creek, 

,!~,..,I I .. 

~~-J)It;t"'fj 7 

1;!'// IJ/!,'i":>r1'1 ''·J/~ /[../6'!, ')lit~ "~~t..es /Jf .. ~ v;t/ ~ftl/1 

1;:, 'I'JJJ;((!~"/, ''.y"~ .. ,~·;t,i'; I; J ., -.J,-<i .-1Y /, "} ~~~" 1-1 

/1;1./(1><; (/If'' 1v,l'l'/! lf.;?J;; /,'' l!.t"/'" 

f;/;,~p;~~1}/f;~l I 

I 1/!JI/ {1~17 I 

,'/rljj/,:v,v';' ";!!; /.'f·,f/<:1"' 

at an elevation of about 0,700 feet, to a point some distance 

northeast of the Atlantic mine, which is located on the crest 

of the same mountain spur at an elevation of 10,280 feet. The 

Cleopatra mine is located on a vein which evidently is a branch 

of the main Crown Prince-Atlantic vein series. The veins van 

considerably in width and structure throughout the differe~t 

mines, owing mainly to the nature of the country rock. The 

mines will be described separately. 

I..;J""iJA/!Ji~t~~r CROWN PRINCE MINE. /;!/'I.'/) J /y :;; 

/'"~iJrJ.k~~·/ft/1-;. The workings of the Crown Prince mine consist of three 

1

, ~'"I"J:L~r;;, /1 I; .. ._~f/;/, .. 'X{;;qL-_ff' tunnels running into the hillside and following the vein to the 

... --;!'J.(I;j)!.:ft'L;l::;-'- north-northeast., The lower two of these tunnels were caved -"k\'···"/(" ...,< (J/ 

'lJ.i~'!.3jJI'o" :t 50 1oofeet shut at the time of visit, no work ha,ving been done on the 

;_Jf>/1'1t;'','----'-----' mine for a year. 

{/;;//!;1 The upper tunnel, which is located a little over 7,000 feet 

~ 

~ t::::::..J 

~ 

Gneiss intruded by Mineralized 

small masses of lead 

pegmatite 

B 

U nmineralized 

lead 

N. 15° W. of the central portion of the town of Empire, follows 

in a N. 45° E. direction along a branch quartz and pyrite lead 

6 inches to 1} feet wide for about 125 feet, to a point where it 

FIG.l5I.-Geological plan of upper Crown joins the main vein, along which it runs in a N. 32° E. direction . Prince tun;wl. for 125 feet more (fig. 151). The main vein consists of a zone 

of gray jaspery quartz and highly silicified crushed country rock from 3 to 8 feet wide, which is 

in many places highly impregnated with pyrite for the whole width and contains numerous angular embedded fragments pf pyrite. This massive pyrite and quartz ore, which contains a few 

small specks of crushed galena, evidently is of too low grade to warrant extraction, for no stoping 

has been done on the main vein on this level. Near the breast of the upper tunnel a crosscut to 

the northwest cuts two veins at distances of 30 and 45 feet from the main vein~ both of these 

veins are evidently branches of the main vein, and if followed would be found to unite with it 

farther to the northeast, for they have strikes of N. 50° E: and N. 55° E. and dips of 65° and 

85° NW., respectively. Both of these branch veins show some quartz and pyrite associated 

with a clay lead, and the branch nearer the main vein has been stoped up to a height of 30 feet, 

although the vein weakens in fractured gneiss and.pegmatite. 

All the upper tunnel workings on the Crown Prince are in granitic gneiss, with patches of 

granite and pegmatite scattered throughout. In places some of the granitic gneiss is so fine 

grained and so highly silicified that it resembles porphyry. 
THE EMPIRE MINING DISTRICT. 407 

COMET EXTENSION MINE. 

The Comet Extension mine, which is located a little more than 500 feet northeast of the 

upper tunnel on the Crown Prince, is developed by a single tunnel about 400 feet in len~th which 

follows the vein all the way (fig. 152). The wall rock consists entirely of granite. 

The general strike of this vein underground is about N. 34° E. and the dip 65° SE. For the 

first 100 feet the tunnel follows a small quartz and crushed~granite lead which beyond this stretch 

splits into two forks, each of which is mineralized beyond the junction. These two branches, 

· after looping about a horse of fractured granite, approach each other again and form the foot- and 

hanging-wall leads of the vein, which for the rest of the 250 feet to the breast consists of two 

streaks of solid low-grade pyrite ore 1 inch to 1 foot wide along each of the walls. These ore 

streaks are separated by 5 to 8 feet of fractured and silicified granite, with irregular stringers 

of pyrit·e up to several inches in thickness running throughout. Moreover, in some places the 

granite is highly impregnated, and in others it is partly 

or completely replaced by pyrite. The ore consists of 

pyrite containing only a small percentage of copper and 

quartz. Considerable stoping has been done along the 

vein, northeast of the point where it splits: 

ATLANTIC MINE. 

,_,~ .. x ';- I' 1 

{\ \'f1~\ 

;-!"'/, rt>- \ 

1 1~ :;-I< 

/-;'>•_\ /I 

\ \ I '-'I, I\ /-/,\'I I,, .:_ \ ~:// ;-/--- ,, /J/'/..:.i;'~'\1 I /- \~_~~'/'- -/1 TheAtlanticmine is located at an elevation of 10,280 , 1 ....-/;'\ ---1 /~-

feet, on the crest of a ridge 1! miles N. 6° W. of the cen- :=;70;_i):;}: 1 

tral part of the town of Empire. The mine is developed .... _!_::~ ·:,(,_soj!..' 

by a shaft several hundred feet d()ep and a considerable :_ ~·.';';'-!;' 

number of drifts, but at the time of visit it had long been _ ~'__:;_ \:,(~(::: ..... 

idle and the workings were inaccessible owing to water, {7_>~·;-,;s~:~ 1 

which partly filled even the first level connecting with 7~; );:,{!_i'~ 

h h h _,,,.· .. ·,I' t e s aft. Granite is the prevailing country rock in t e _ 1 ~ .. ..:'! 7 1 

immediate vicinity. The general strike of the vein on '-..... ·.·-';-, 1- ,~ I 1<,•/ I I I/ 

this first level is N. 38° E., and the shaft and vein dip -~~}~>-.:::;~~-

about 80° to 85° SE. The vein at this point consisted -~}. ;,;)1/'-r"-

f f f b ~ 1 , \,,.,I- I , o 5 to 7 feet o gouge and decomposed ria le and \ /-1-:.;- 1 1 , 1 

·crushed granite impregnated with pyrite, some of which -·1 {/):/_!;. .:-: 1 

is copper bearing. 1-\~~~~;. 

Specimens obtained from the dump showed the vein ~j;:Sr~, r=::1 ~ 

filling to consist of a mixture of angular fragments of ~ L:___j Granite Mineralized I~ Unmineratized 'ead 

pyrite and finely crushed granite, cemented by quartz FIG. 1.52.-Geological plan of Comet Extension tunnel. 

and cryst;-tlline pyrite, showing' two periods of ore formation or a movement midway in the process of mineralization. The chief values in the ore 

of this vein were in gold, with less copper. Considerable stoping has been done in this mine. 

CLEOPATRA MH.-E. 

The vein in the Cleopatra mine is developed by two tunnels, the lower one of which is a little 

more than 250 feet and the upper one about 400 feet in length (fig. 153). The upper tunnel 

is located at the end of the same branch road as the Comet Extension, but between 300 and 

400 feet farther northwest, at a distance of about 1! miles N. 12° W. of Empire. 

Aside from a crosscut about 100 feet in length through soft biotite gneiss near the mouth, 

the lower tunnel is entirely in granite. The vein in the lower tunnel consists of a series of 

imbricating or overlapping fractures connected by transvers~ veinlets, .and is composed of 

small quartz and pyrite stringers in friable biotite granite. On the upper level, however, the 

vein is stronger, more highly mineralized, and more regular in trend, the strike being about 

N. 35° E. and the dip from 60° to 70° NW. In contrast to the soft, friable granite of the lower 
408 GEOLOGY OJt' THE GEORGETOWN QUADRANGLE, COLORADO. 

level, the granite walls on the upper level are comparatively hard. The vein on the upper 

level consists as a rule of a fractured zone in granite with stringers of pyrite and chalcopyrite 

ore cementing the granite ftagments. Vugs lined with drusy quartz and pyrite are common. 

· The pyrite in places is very light 

Granite Mineralized lead Unmineralized lead 

FIG. 153.-Geological plans of upper and lower Cleopatra tunnels. 

'' colored and shows little evidence 

of copper, but elsewhere the copper content is much larger and 

the pyrite becomes coated with a 

bluish-black film of copper sulphide, or with a brightly colored 

iridescent film. Considerable stoping has been done along the vein 

on the upper level, but the yield 

so far has not been very large. 

MINF~S OF LINCOLN MOUNTAIN 

WITHIN THE AREA SHOWN ON 

TilE EMPIRE MAP. a 

HEADLIGHT MINE. 

The Headlight or Mickmac 

mine, which is located on the 

Swanson lode about 3,600 feet 

south-southwest from the central 

portion of the town of Empire, is 

developed by a shaft 215 feet 

deep, from which short drifts are 

run on three levels, and also by a 

tunnel 125 feet in length which 

connects with a drift 150 feet in 

length on the Swanson lode on the :first level (fig. 1M). The latter drift connects with the 

shaft about 75 feet below its collar. The largest drift in the mine starts from the shaft at a 

point 60 feet below the tunnel level and runs for 260 feet tu the south. The drifts on the 

mine aggregate about 700 feet in length. 

The wall rocks immediately in contact with the 

vein consist of biotite granite, but ~orne granitic 

gneiss was encountered in the crosscut tunnel. 

The vein, which has a dip of 50° NVr., strikes 

N. 25° E. and consists in some places of a mere 

fracture or a series of small quartz stringers in fractured granite, while in others it is a pyrite-specked 

quartz lead from 1 to 8 inches wide. This quartz 

was "frozen" to the walls and graded directly into 

silicified granite, and this in turn into comparatively fresh granite. Most of the ore taken out 

came from between the first or tunnel level and the 

surface, where several small pockets of very rich 

ore were stoped out. The best ore was a chalcopyrite~or cupriferous pyrite and quartz ore, carryGranite Mineralized lead 

Unmineralized lead 

FIG. 154.-Geological plan of Headlight tunnel. 

ing, it is stated, from 3 to 15 ounces of gold per ton. Some milky quartz ore, containing 

only a few specks of mineral, was also s·aid to carry several ounces of gold to the ton. 

a For descriptions of the mines of Lincoln Mountain within the area of the Georgetown quadrangle, see pp. 309-311. 
THE EMPIRE MINING DISTRTCT, 40\) 

The Halifax or Anderson lode, which lies a short . distance northwest of the Headlight 

Jf.ine, on the surface shows a silicified streak with a few specks of galena and pyrite scattered 

throughout. The Halifax is said to have a porphyry dike running along its cast side. 

HIDDEN TREASURE MINE. 

The lower level of the Hidden Treasure mine is located at an elevation of 9,000 feet about 

1} miles S. 75° vV. of Rmpire, on the north slope of Lincoln Mountain, just beyond the north 

boundary of the Georgetown quadrangle. The mine is developed by two tunnels, but the 

upper one, which is about 700 feet above the lower one just mentioned, has been caved shut 

for some years, though formerly it produced considerable ore of an exceptionally rich nature, 

It is claimed that this mine produced the largest and finest specimens of native gold obtained 

in the district. This free gold consisted both of leaf gold and of small irregular masses resembling nuggets. 

On the dump of the upper tunnel were found some chunks of massive cupriferous pyrite 

ore, some of which contained angular frag"ments of granite, quartz, and granitic gneiss entirely 

surrounded by the pyrite, although a thin layer or film of quartz usually intervened. One of 

these specimens showed crustification, for small specks of galena and sphalerite· were asso·-

k?1 c=J 

Mineralized lead Unmineralized lead 

FIG. 155.-Geological plan of lower Hidden Treasure tunnel. 

ciatecl with the narrow quartz layer between the massive pyrite ore and the included fragment, 

thus indicating that the galena and sphalerite had been deposited at an earlier stage in t~e period 

of ~re deposition. 

The lower tunnel (fig. 155), which was run to cut the same vein that produced the rieh 

ore on the level above, extends for about 225 feet S. 32° W. to the breast, where a clay lead 

striking S. 78° W. was encountered. At 100 feet from the mouth of the tunnel a strong elay 

and crushed-rock lead slightly mineralized in places was also encountered and drifted on about 

600 feet to the S. 82° W., to a point where it joined a clay lead striking N. 75° W. All the 

drifts of the lower tunnel were in granite and granitic gneiss. 

ALLEN MINE. 

General description.-The Allen mine, which is located between the Virginia City and Hidden Treasure mines on the north slope of Lincoln Mountain, at an elevation of about 10,000 feet, 

is about 9,000 feet south-southwest from the town of Empire. The workings consist of a 

crosscut tunnel 130 feet in length which runs S. 20° W. and connects with a drift about 200 

feet long to the southwest on the lode. 

The workings are all in gray granite. This rock is slightly fractured and mineralized for 

some dist~nce from the vein, at least on the foot-wall side, as is shown by the walls of the crosscut tunnel, where small irregular stringers of pyrite run through the granite mass in lines somewhat parallel to the mam vem. The vein represents a strong fault line trending S: 48° W. 

and dipping 50° SE. 
410 GEOLOGY OF THE GEORGETOW~ QUADRANGLE, COLOHADO. 

Structure ofvein.-This vein, although it is mineralized to only a slight extent, ha& an 

interesting structure. Next to the foot wall, which consists of gray granite, is present in places 

a thin seam of sphalerite and galena only three-eighths to 1 inch in thickness, separated from 

a somewhat similar but discontinuous seam of sphalerite and galena farther southeast and within 

the vein by 3 inches to 1 foot of pyrite-bearing gray jasper-like quart?, which locally changes to 

massive pyrite. Next to the southeast sphalerite-galena streak, or separated from it by a nonmineralized zone of crushed and friable granite 1 to 2 fe(lt wide, is a belt of soft friction conglomerate 4 inches to 1 foot in width having a stiff, putty-like blue-clay or pulverized-rock matrix 

in which are embedded rounded and subangular pebbles of granite and quartz as well as of quartz 

and ore. The ore consists of cupriferous pyrite and massive galena and sphalerite. This 

friction-conglomerate belt is for the most part immediately in contact with the gray granite 

of the hanging wall. 

Evidences of two periods ·of mineralization.-The fact that the sphalerite-galena streaks near 

the two walls are separated by a central seam of pyrite-bearing quartz suggests the possibility 

that there were at least two periods of mineralization and that the zinc-lead ore represents an 

earlier deposit filling a vein that was reopened by a fissure in which pyritiferous quartz was· 

subsequently deposited. 

The finding of rounded pebbles of both galena-sphalerite and pyrite ores in the soft friction 

conglomerate would indicate that this belt of soft crushed rock was the result of movements 

of a still later elate 'than that at which the pyrite ores were deposited. Specimens of massive 

pyrite ore on the dump showed vuglike openings lined by a layer of marcasite one-eighth inch 

thick. 
INDEX. 

A. Page. 

Ad 'trellitc. Sec Quartz monzonite. 

Ad"ms, F. D., on micropegmatite........ .. . . .. .. . . . .. .. . . . . . 95 

.Et.na mine, account of............................... 281-282 

Alaskite porphyry, age of. ___ .......... _. __ . 133 

correlation oL. ___ .................. ______ .. ___ . _ ....... _ .. . 132-133 

definition oL .............. _ .... __ ...... _. _ .... __ . _ 77 

distribution of.. ............................................. 77,386 

petrography of........................ . ..... 77,132 

section of, figure showing .... _ .. _ .. _ .... _ .... _ ....... _ .. _..... 77 

Alaskitic quartz monzonite porphyry, age of ........ ·'·.. 135 

correlation of.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 134 

dcfmition of.................................................. 80 

distribution of.. ............................................. 80,386 

petrography of. ......................................... 80, 133-134 

Alexander lode, description of............................ 371 

Alhambra lode. account of....................................... 216 

extension of. .... _ .............................. : ........... 216-217 

ores of.. ..................................................... . 216 

Alice, placer mines at ..... _ ................................ 312,313-314 

Alkali-lava eruptions, centers of................... 123-129 

Alkaline magma. See Eruptions. 

Alkali-soda syenite, classification of ...... . 

Alkali syenite porphyry, age of.. ........ . 

analysis of. ................ _ ................................ . 

correlation of.. ................ : . .... . 

definition of.. ..................................... . 

distribution of. .............. _ .................... . 

83 

135 

83 

134 

81 

82 

petrography of. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82-83,133 

Allen mine, description of. ..................................... 409-410 

veins of .................................................... 101, <aO 

Alluvium, character and distribution of..................... 88 

Alpine district, ores of.................................. lOG 

Alpine mine, description of.... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 333 

Alps Mountain, rockson..................................... 58,80 

view of....................................................... 244 

A Iteration. See Metamorphism. 

Ancbor vein, description of ................... _ ......... 324-325,327,330 

Anglo-Saxon extension tunnel, plan of, plate showing............ 280 

Anglo-Saxon extension vein, description of....................... 291 

Anglo-Saxon lode, account of.................... . .......... 290-292 

description of. .......... ·..................... 290 

veins of. ............... , . . . . . . . . . . . . . . . . . . . . . ..... 98, 290-291 

wall rock of ............ :...................... 290 

Ankerite. See Siderite. 

Annette mine, location of. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 285 

view of. .. _.................................................. 282 

workings of, plate showi)lg............................... 286 

Anthracite-Crested Butte region, ores of................... 106 

Aorta tunnel, plan of, plate showing.. . . . . . . . . . . . . . . . . . . . . . . . 398 

rocks in................................................... 152; 154 

view of. ............................................... _. . . . . . 402 

Argentifcrous sulphides, occurrence and character of. ... _ ...... 143-144 

A rgentiferous veins. See Silver-bearin~ veins. 

Argentine district, galena blende at, platesllowing.............. 136 

ores of...................................................... 104 

rocks of...................................................... 69 

Argentine mine, description of. ............................ 267,270, 27i 

map of. .................... -... . . . . . . . . . . . . . . . . . 246 

working of, plan ol. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 267 

Argentite, genesis of. ........... -................................ 261 

Argo mine, description of........................................ 369 

Argonaut mlne, description of............................... 370 

Arthur mine, description of...................................... 363 

Page. 

Ashby tunnel, ores in ...... . . . . . . ... . . . . .. . . . .. . . 144-1'5,207-208 

plans of, plates showing. . ............ 192,204 

water in ... __ .. _._ .... _ .. _ 

Ashcroft. rocks at ............ . 

Aspen, rocks at ... _ .......... . 

Aspen district, faulting and folding in ..... 

intrusion in .. _._ .... _ .... __ 

ores of. ............................... . 

Astor mine, description of. ..................... . 

...... 139,208-209 

70 

. ............. 70, il 

. ....... 109,121-122 

.......... 119-121 

. . 105-106,121,132 

303 

Atlantic mine, description of.. ............................ . 407 

Auriferous veins. See Gold-bearing veins. 

B. 

Ball, S. H., on general geology_ ............ . 

on intn1sive porphyries . ..... _ .. _ .... _ ..... _ .... _ .. . 

Baltimore lode, description of. .............. . 

·Baltimore tunnel, plan of, figure showing ....... . 

29-96, 383-386 

. . 107,131,134 

219• 

. \Vater in . ........................... -......... -... -.. -

220 

... 138,243 

Banty mine, description of. ............................... . 352 

Barber & Elliott mine, description of. .. _ ......... . . . -. 357-358 

tunnel of, plan of.. ............................. . 

Barbour Creek, mines on .... _ .. _ .... _ .... __ ... _ ... . 

sheets on ........................... . 

Bard Creek, glacial till on.......................... . ......... . 

357 

381 

72 

84 

glacier on .......... _ .................. . 85 

Barite, derivation of. .................... . 

genesis of. ............................. . 

lode bearing .......................... . 

occurr.encc and character of. .... .'. . . . . . ......... . 

112, 153, 165,263 

259 

218 

137 

Bassick agglomerate, description of. .................... 124-125 

orcs in ......................... _ .. 

Baxter lode, description of. ....... . 

production of. .................... . 

Bayley, W. S., on microcline ........... . 

Bear Creek, description of. ............ . 

glacier on ..................... . 

rocks on ................... _ .. _ ....... _ .. 

Beartrack Creek, glacier OIL ..•...... 

Beaver IUine, description of ... _ ... _. 

ores of. ........................ . 

'reins of . ...... _ ......... _ ...... _ 

wall rock of. .................... . 

workings of, plan of. ........ . 

Becke, F., definitions by ............... . 

Beecher veins. See Boston and Beecher veins. 

Belmont lode, discovery of. ............ . 

Belts, mineral. See Mineral belts. 

Benton lode, surface workings of, view or. .... _ 

Beresite, correlation of .......... . 

Beryl, occurrence of. ............. . 

Big Chief mine, minerals in ........ . 

ores of. .......... ~ ....... . 

Big Forty mine, description of. ... 

Big Indian vein, description of.. 

Biotite granite, definition of. ..... . 

Biotite latite, age of.. ........... . 

correlation of.. ............. . 

defmition of.. ....•............. 

distribution of. ................ . 

occurrence of.. ........... _ ....... . 

petrography of. _ ....... _ .............. . 

sections of, figures showing .. 

Biotite schist, distribution of. ......... . 

126 

194 

194 

94 

30,32 

85 

57,70 

85 

377 

. ... 378-379 

.... 377-378 

377 

378 

.1G 

173 

402 

61 

63 

153 

276 

. . 376-377 

275 

49 

135 

134 

81 

. .... 81,386 

344 

. .... 81,133 

...... 344,347 

41 
412 INDEX. 

Page. 

Biotite schist, petrography of ..................................• 40-41 

See also Schists. 

Biotite-sillimanite schist, distribution of... . . . . . . . . . . . . . . . . . . . . . . 40 

metamorphosis oL ...................................... _. _ _ _ 40 

pegmatite injections in ... _ ............................... __ ._ 38-39 

figure showing._ .. _._.......... . ......... -. 39 

petrography of. ... _ ... _. . . . . . . . . . . . ................... 38-39 

See also Schists. 

Bismarck lode, workings of, plan of. __ .............•.......... Pocket. 

See also Pelican-Bismarck lode. 

Bismarck vein. See Black Eagle-Bismarck veins. 

Black Eagle-Bismarck veins, description of...... . ........ 367-368 

ores of. . _____ ........... _ ............... _ .................. _. 368 

veins of.................................... .. ......... 368 

Blackhawk, silver veins near ................ _. . ............. -. 98 

Black Swan mine, description of.. ....... _ .. _.............. 379 

Blain lode, description oL ..... _ ... _........ . . .... 222-223 

"Blende, genesis of.......................... . . . . . . . . . . . . . . . . . . . . 260 

occurrence and character of ...... _. . . 136 

Blizzard lode, description of..._ ..... _... 382 

Blow, A. A., on Leadville ores....................... 105 

Blue Ribbon Springs. See Idaho Springs. 

Blue River, rocks on.................. 69 

Boston and Beecher veins, account of........ _ ............. 293--29G 

description of._ .. _._............. . . . . . . . . . . . . 293 

ores oL ............. _ ... : _... . . . . . ......... 295-296 

veins of._ ...... _................ . .... 294--295 

wall rocks of. . _ . _. . . . . . . . . . . . . . . . . . . . . . . . . . 294 

Bostonite, correlation of._........ . . . . . . . . . . . 134 

definition of. . . . . . _. . . . . . . . . . 78 

description of.._............ . .... 150-151 

distribution of. .. ___ ........ . . . . . . . . . . . 78 

occurrence of.................. 343-344,38.5 

petrography of......... . ...... 78-79, 133 

Bostonitc porphyry, age of....... . . . . . . . . . . . . 135 

Boulder, rocks near._............... . ..................•..... 68,71 

Boulder County, dynamic movements in ... _ .................. 108-109 

orcs of. _ ..... _.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103 

Bowlders, occurrence and character of. ....................... 24,66-67 

Branagan vein, description of................. 268 

Brazil mine, description of. .............. -.. . . . . . . . . . . . . . . . . . . 319 

tunnel of, plan of. . ___ . . . . . . . . . . . . . . . . . . . . 319 

Breccia lodes, occurreiJCe and character of. ............. 149-150,345-346 

Breckenridge, ores near ....... _ ..... -................ _ . . . . . . . . . . . 104 

rocks ncar .... _ . _ ........ _. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69 

Brighton mine, description of_ ....... _ ............. _ ... 32.'>,327,328,332 

levels of, plans of. .......... -.-. . . . . . . . . . . . . . . . . . . . . 328 

workings of, map of._ ... ___ .-._............. 326 

BrOgger, W. C., on rnonzonite .... _ .... _ ......... . 46 

on zircon.................................. 183 

Brown Gulch, view in..................... 196 

Brown lode, outcrop near, sketch showing ... _ . . . . . . . . . . 235 

relation of, to other lodes .. · ................................. 220-221 

tunnel on, figure showing .. _. _................. 237 

Brown Mountain, view of. ... _............ . .. _.......... 184 

Brush, G. J., and Dana, E. S., on microcline...... 93 

Brush tunnel, description of. ___ ._...... . ........... _. 226 

plan of, figure showing..................... .............. 225 

Buckeye lode, description of.- .... -............ . ....... 304,306,307 

tunnel on, plan of.. . . . . . . . . . . • . . . . . . . . . ............. -. . . . 307 

Buckeye tunnel, description of......... 376 

Building stone, occurrence and character of.. . . . . . . . . ..... _..... 91 

Bull-Domingo mine, ores of............ . .............. 125,126 

Bullion King mine, description of.. . . . . . . . . . . .. 364--36.5 

Burchard, II. C., on Colorado mines........ 185,210,212,290,308,313 

Burleigh timnel, description of......... . ............. 221,222 

plans of, plates showing............ . .. 220,pocket. 

view of.._. ___ ... _ ... _.............. . . . . . . . . . . . . . . 196 

water in._ .... _ .. _~ .. _ ...... _._ .. _ 

Bums-Moore tunnel, description of.. ..... 

Burrell vein, description of.. ..... . 

C. 

139,243 

37.5 

275,276 

Calcite, derivation of. ...... _.. . ................ _ ....... 141,263 

genesis of. . _ . _ . . . . . . . . . . . . . . . . 259-260 

occurrence and character of. . . . . . . . . . . . . . . . . . . . . . . . . 137, 138 

Carboniferous sandstone, bowlders of, character and distribution 

· of. ......... _ ..... _ ..............•.................... 66-67 

Pag•l. 

Cardigan mine, description of._ .............. _.. 353 

Cascade lode, description of._ .. _ ....... _............ 225 

Cascade Creek, rocks on and near .. _................ 66 

Cashier mine, description of ........................... ,_._ ..... 392-393 

tunnel of, plan of. . __ . _ . . . . . . . • . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 393 

veins 9f. - -- .... - .. -... . . . . . . . . . . . . . ......... 393--394 

wall rock of._-------··............................... 393 

Cecil-Argo group, description of....... . ................ S68-369 

mines'of. __ . _ .. -...... _.--.... . . . . . . . . . . . .. 369-370 

veins of..------·········-····.......... . ........ 98,368-369 

Cecil mine, description of. __ .. _. . . . . . . . . . . . . . . . . . . . . . . . . . . . 369 

Centennial group, account of.......... . ....... _ ... 275-278 

description of. .... _................. . . . . . . . . . . . . . 275 

ores of. .... -- .. -.. -.- ..... -._.. 147,148,277-278,402 

production of..--- ... -...... _..... . ..... 97-98,275 

veins of. .. -.... -.- ........... -... 276-277 

wall rocks of..- ..•..•........ -._..... 276 

waters of. .. _ .. _ ......... _ ... _ .. _ .... _ .. . .... 154,278 

Centennial mine, plans of, plates showing ...................... 274,276 

Central City. dikes at._ ......... _ .. _..... 69 

Central City quadrangle, maps of, showing former topography .. _ 30 

maps of, :sliowing glaciers ... _ .. _. 1 

showing present topography ......... . 

Central Colorado lode, account of.. ............. . 

plan of, figure showing ____ ........... . 

veins of . .. _. ~ ... __ ............ __ ........ . 

Centurion mine. See Champion Dirt mine. 

32,34 

36 

274-27:; 

274 

98,100,274 

Champion Dirt mine, description of_........ 339 

faulting in ....... _...................... 339-340 

ores of. .... ----------···················· 150,340 

levels of, plans of.. . . . . . . . . . . . . . . . . . . . . . . . . . . 340 

Charter Oak mine, description of.. ... _.... . . . . . . . . . 37 4 

Chicago Creek, description of.................................. 30 

goldon ...................................................... 311 

placers on _____ ·--· ...• -- ...... _ .. _.- ........................ 3!1-312 

view of... __ .... _. ____ ........... __ .. _..................... 312 

rocl{s on ....................................... 40, m, nr.. 71, 72, 78,81 

terrace gravels on ....•.. _ ... _ ............ _ .. _.............. 83-84 

Chicago Lake, rocks near •. _ ... _ .. _· ............ _........... 47 

rocks near, section of, figure showing .. _...... . . . . . . . . . . . . . 61 

Chief Mountain, altitude oL _ .... _ .... _. _ ... _ .. _............ 30 

rocks of... ~ . -.... ___ .- .......... _ ....... _. . . . . . . . . . . . . . . . . 41, 61, 71 

Cirques, descriptionoL--------------------,--···-··············· 33 

Claims, law of. ........ -------- .•.. -.- .................. ·. 312 

Clea~e~~~~~ti~~~~~~-~~:::::::: ::::::::::::::::::::::::: ::::':: .. 30,~~ glttciation on---------- •. ·---.---.-.......................... 84-87 

placers on .. _-- .• __ ••.•.. --- ... -- ...... _ .................... 311-314 

views of.. .•.••••••...•............... :................ 312 

rockS from·------- ..•.•. -----·................ . . . . . . . . . . . . 77,82 

analysis of.. ..••.. _-.--... . . . . . . . . . . . ............. _ 83 

section of. tignre showing................................. 63 

terrace gravels on •..... -- __ -- ....... -..................... : ... 83-84 

valley of, view of..._ .. ___ .-- .... -..................... -..... 96-402 

Clear Creek County, gold of ....•... _ .. _ ...................... , .. 312-314 

gold in, discovery oL .. :.................... .. ..... ... .. ... 172 

ores of.. .. -....•... ----.--.-- ... -.... -- ..................... 103-104 

Cleopatra mine, description of •.... _ .. _ ................... _ ..... 407-408 

tunnel of, plan of .............. ,---···················-····-·- 408 

Cliff mine, description of.. ..•.•.......... -................ -.. 305,308--309 

workings of, plansof.. ...................................... 308,309 

Climate, character of.. ..••••. -- __ .. -.. -............. . ........ -. 30 

Coin lode, description of.................................. 220 

Cold Snlphm Springs. See Idaho Springs. 

Colorado, distribution of gold, silver, and lead in, mop showing... 128 

early history oL ... ___ ---- _ .. --....... . . 172-174 

index map of.. ...... -------··--·........ 29 

mineral belts in....................... .. .. 98-99 

mountaiuous uplands ill---~----·······.... 32 

porphyries (intrusive) In, belt of. map showing...... 70 

Colorado Central mine, account of................ . ... 245-264 

calcite in, plate showing ..... -...................... 138 

development of .. _ •. __ ... -. . . . . . . . . . . . . . . . . . . . . . . . . 245 

faults in........................................... 264 

.figuresshowing............. ................... 264 

levels of, plans of.. ...• _ •.... _ ..... --. . . . . . . . . . . . . . . . . . . . . . . Po.Qket. 

location oL .. _. _ .... _..................... . . . . . . . . . . . . . 245 

map of.. ••............ ···---------· ................... -· 246 
INDEX. 413 

Page. 

Colorado Central mine, minerals of ____________________________ 25fi--263 

orcs oL .. _ ......... ______ . __ . __________ . _ .. _____ .. _ .. _ _ 248--249, 256 

porphyry dikes in .. _____ .... _______ .____ _ ____ ... _____ 249-250 

sections of, figures showing. ________ .. _._. ___ ·_. 247,250,254,255,257 

wall rock of, grooving of, plate showing .... ___ .. __ ..... _._._.. 138 

vein ·ehannels in •.••. ____ ..... _._. __ .. __ .. _. __ .. __ .. _ 253-256 

veins of.. ....... __ .--- .. ____ .. ------._--- .. -

specilliens from, views of ............. __ 

wall roeks oL ............. --·- __ .... ____ . _ ... 

water in ..... ---------- ....... __ ... ___ ... ____ . 

Columbia mine, description of.._ .. ___ .... _._ .. _ 

Columbia Mountain, view of.. ....... ____ .. ~ __ _ 

Comet-JEtna lode, account of.. ....... _ .. _ ... __ 

See al80 Comet mine; 1Etna mine. 

97,245-248,251,256 

----------- 250 

249-250,251-252 

---- .. 139, 264 

366 

292 

--- 278-282 

Co :net Extension mine, description of ..... __ .. _ 407 

cunnel of, plan of.._ ... _ .. _ . _ .. _. __ .. ____ .. . .. 407 

Comet mine, account of.. ......... _ ..... ___ ._ .. _ .. __ 278--281 

description of.. ... __ ... __ . _ ...... .- _ .. __ .... ___ . _ _ _ 278 

orcs of ..... _ ..................... · .... ____ .... ______ 281 

section of, figure showing .... __ .. · ... _ .. __ ...... _ . _ _ _ 280 

tunnel of, plan of.. .... _ ......... __ . __ ._, .. ________ ... _ 280 

veins of.. .. _ .. _ .... __ .... -- ..... ____ .. __ .... _ ....... _ ... _ 279-281 

wall rock of.. ..... _._ ................... _ ..... ; ____ ..... ____ . 278-279 

Coming Notion mine, ores of.. ..... __ .... ·. ___ .. _ .. _______________ . 147 

Compound vcin.s, occurrence and character oL . ..... __ .. __ . _ 135 

Conglomerate. See Idaho Springs formation; Stanley mine; 

Breccia lodes. 

Conqueror mine, description of.._____ 402 

faulting in .... __ .. _ ... _. ____ .___ 104 

orcs of.. .. ___ ...... _ ..... __ . : .. _ _ 404 

plan of, plate showing ........... __ .. 404 

veins of.. ..... _ ...................... _ .................. _ .. _ 402-404 

wall rock in .............. _ ................... ___ ........... _.. 402 

Continental divide, relation of, to subordinate ridges, view 

showing ...... _ ... ___ .......... _ ... __ .... _.. 244 

Copper minerals, derivation of................. 153 

Corn roCl<, identity of...' ......... __ ........ _........... 183 

Corry City lode, workings of, plan of ...•.......... _ Pocket. 

See also Wisconsin-Corry City mine. 

Corundum, occurrence of... 40 

Crazy Girl mine, description of........ 335 

Creede, ores near .. _ ..... _ . .. . . . . .. .. . 116 

Creole veins, description of............ 267-268,270 

Crested nutte quadrangle, rocks of. .. _ _ _ .. _ ...... _ .. _ _ 44 

Cripple Creek district, comparison of Rosita Hills and ....... _ .. 128-129 

geology of.. .... _ .. _ ..... __ .. -_ ...... __ ... _ .... _ . __ ....... __ . 127 

ore deposits of.. ............ __ ......... ___ .. 127-128,133,159 

origin of.. .... _ ....... __ ..... _ .... _ .. . . . _'. . . . . 168, 170 

Cripple Creek quadrangle, rocks of. . ______ .. ___ .... __ . ____ .. __ . 44 

Cross, Whitman, on Cripple Creek district.... .. __ 128-129 

on geology of region .............. _ .... 32, 44, 69, 70, 71, 73, 83,112, 14.) 

on intrusion .............................. __ ...... 92,110,119-120 

on minerals .. _ ......... _ ................ _ .. .. .. . . . . 132 

on Silver Cliff and Rosita Hills ..... ____ .. __ ... ___ .. __ . 123-124,128 

Cross, W ., and others, on Needle Mountains quadrangle.. 115 

Cross, W., and Spencer, A. C., on Rico quadrangle... 114 

Crown Prince-Atlantic group, description of ........ _ 406 

mines of, descriptions of ... _ ............ _..... 406~408 

Crown Prince mine, description of ..•.....• _ 406 

tunnel of, plan of.. ...................... _ _ __ __ 406 

Culture, description of ..................... ___ . __ . __ ... _ 31 

D. 

Dacite, correlation or. .. _ ............. __ .. _ 

definition of.. ................... _ ... . 

132-133 

75 

distribution of. .............. ___ ........ __ __ 75 

petrography of.. ..... ____ ...... _.. _ ... 75, 132 

section of, figure showing ................... _ . _ ..... __ .. 75 

Daly, R. A., on overhead stoping...... 51 

Deadwood diggings, gold of............. 311 

Debris sheets, location and character of... 87-88 

Deer Creek, description of ...... " ...... _. 30 

glacier on .... __ . _ ..... _ .... __ .... _ . . . 85 

De Mel! lode, description of ........... _... 284 

Democrat Mountain, mines on ... __ .... _ .............. _ 303-309 

mines on, ores of. . _ .. _____ .. _____ .. __ ...... _ .. __ ..... _ _ . .. 305-306 

workings of, map of._ ...... __ ....... __ . __ . __ .... __ ._ 304 

plans or. .............................. __ ........... 307,308 

Page. 

Democrat Mountain, veins of. .. _......... 97,145,303-305 

veins of, description of ................. · ..... ___ .. 306-309 

plan of, figure showing .... _ ..... _ . . 304 

view of. .......... _______ .... __ ..... _ __ 292 

Denver-formation, character of. ......................... __ ... __ 7l 

Denver lode, description of..._ ....... ; ...... __ ... _ ..... ______ .. 203-204 

·plan of, figure showing....................................... 204 

Deptb, relation of, to ores ................. 143-146,148-149, 190~191,197, 

202-203,239-242,248-249,273-274,277-278,289-290,331-332,362 

to spring waters ..................... __ ........... ____ ........ - 167 

Diamond tunnel, ores in . . .. . . .. . .. . .. . .. . _ . _ . ____ .. 193-194 

plans of, plates showing . _ ........... _ . _ ... _. 192, pocket. 

Dikes, age of. ......... _ ............. · ..... __ . __ ........ 131 

character and distribution of. ....... __ 23-25,67-71,132 

classes of. ........ _ . -.. .. .. .. . .. . . . . . . . . 131 

intrusion of, forms of._._ .... _. . . _ .. 71-73 

forms of, figure showing.. . .. . . _ ........ _ _ 72 

plate showing .. _ . _ ... __ ...... _ _ . __ ....... _ . 178 

location of, maps showing........ . . _. _ ...... __ . 102, 194 

relation of ores and_. _ . _. _ .. __ ... _ 25. 

relatioss of veins and ...... · ...... ___ .. _ .... __ ...... . ... 130--132 

weathering of. ...... _ ...... __ ... __ ... _ ..... __ _ 36 

See also Porphyries, intrusive; Fe1dspathic dikes; Porphyry 

dikes. 

Diorite, character of ............ _____ ... __ ... _ ..... _ ...... __ ... __ 179 

definition of.. . . . . . .. , .... , .. __ .. _ ..... __ . _ .. _ ........... _ . 

pcgmatitic phase of .... _ .... __ ............... __ ... __ ....... __ 

See also Quartz-bearing diorite .• 

Dioritic rocks, descriptions of. ..... _ .. . ------ .. - 179--181 

minerals in ... _ _ ......... __ _ --- -- .. -- -- --- --- .. -- - 180 

relations of, plate showing 178 .. __ .. __ .. _ 

Dives-Dunkirk lode, description o!. .. · .. __ .. -------- 192-194 

history and production of. ............ . -- -. --- -- .. --- --- .. - 194 

veins of. .... _ ........... __ .. ____ .... .. ----------------_--- 192-194 

_figures showing ............ __ .. _ 

wall rock of. ..... ___ ....... __ . __ .... _ ---- .... ------------ 192, 193 

194 

working of, plan of ............. _ . . . . . _ ..... __ .. . . .. . Pocket. 

Dives-Pelican lode, description of._ 186 

levels of, plan of. ..... __ ....... Pocket. 

mill of, view of. ___ .. __ ........ _ __ .. ____ .. _, __ 196 

workings of, plan of ... __ . . . . Pocket. 

Do mal weathering, description of .. . . . __ ..... __ ...... _ .. .. .. 36 

figure showing .......... ____ . __ ... __ 36 

Donaldson-Champion Dirt group, description of ..... _........... 336 

mines of, descriptions of.. ....... _ .... ___ .. _._ .... __ ... __ ._. 337-340 

ores of. ........... _ ...... _ ...... _ .. __ .. __ .... __ .. __ .. _ 148, 337 

veins of. ... : ....... __ ...... _ _ 336, 337 

wall rock of. . _ ............ __ .. __ ... __ ... _ .. __ 336-337 

workings of, plan of.._ ....... _ .. _ .. __ .... __ .. . 

Donaldson mine, account of. . ____ .... __ .. 

workings of, plan of. .. _ .. _____ ........ __ _ 

Dorrit mine, description of. ... __ ..... __ .. _ 

Douglas Mountain, glaciation at ........... _ ..... __ _ 

Drainage, description of. ....... _ .... __ .. . 

See also Topography. 

Duck Creek, glacier on ........... __ .. _ ... ___ . 

~uck Lake, glaciation at. ....... __ .. __ ..... _ .. 

glaCier at. .... _ .. _ ......... __ ..... _ .... __ .. 

rocks near . . _ ........ _ ................ ___ .. _ 

Dnmbardin mine, description of. .... ___ ._...... __ .. __ .. __ _ 

Dumont, moraine at .. __ ..... __ .... _ ..... _ ..... _____ . ___ ..... _ 

Dunderbetg lode, description of. ... _ ............. _ .... _ 

levels of, plan of._ ..... __ . __ .......... __ ... _ ........ _ 

336 

337-338 

338 

363 

86 

30,32 

85 

86 

85 

60 

375 

84 

223-224 

222 

239 section in. figure showing ... ___ ......... __ . ___ ..... _. _ .... __ . 

_Dunkirk mine. See Dives-Dunkirk lode. 

E. 

Eagle River, ore deposits on .. _ ........ _ ...... ______ ........ ____ . 68 

East Chicago Creek, glacier on ............. __ .... _ _ __ .... _.. . 85 

East Geneva Creek, rocks on and near ....... _ .... _ ..... ______ .. . 57 

Eclipse mine, description of. .......... ______ .. _____ . ____ .. ____ . _ _ 376 

ores of. ......... __ . _ . __ : . . ____ ...... __ .. _ ... __ ...... __ .. __ .. . 376 

Economic geology. See Geology, economic. 

Edgar lode, description of. ............ __ ..... _ ... __ ._ .... ,_ .... __ 307 

Edinburgh lode, description of. .... _ .. __ ....... _,._.,. __ ....... 217-218 

Egyptian lode, description of. ..... ___ _ _ .... __ . ___ . _. _ ..... _... 297 

Elephant lode, description of.. .. _ ... _ .. ____ .... __ ... _ .... __ ..... 222 

Elevations, data on .. ~ ............ ___ . _____ ........ ___ . __ ... _ .. _ . 30 
414 INDEX. 

Page. 

Elijah His~ lode, description oL _. _ ...... _ _ __ .............. _ 217 

Elizabethtown, gold at ............ __ .. .. .. . .. .. .. .. . 172 

Elk Creek, description of. ........ _._ ... __ . . ...... __ .. _ an 

Ella McKinney mine, description oL.... . .. _......... 374 

El Oro, Mexico, ores from...... .. _ ........... _ 170 

Emma-Galie lode, description of._ ...... _. . ......... _..... 306 

... Emmons, S. F"~ on Colorado intrusive porphyries ................ 67-71 

on Eagle River ores .......... __ __................ 68 

on Leadville district .... __ ....... _ __ .. _ 105, 109-110,119-120,169 

on Needle Mountains quadran~le ... _ ... ___ ...... ___ ....... _. 115 

on Silver ClifT and Rosita Hills_._._ .. __ ._ ... _._._ .... _ 123;12.5,126 

on Ten,nile district. .. · ........... _ .. _ ............ __ . 104, 110, 119 

Empire, views of.. .............. __ ... __ .. _..................... 3R4 

Empire City-Gold Bug group, des~ription of..................... 394 

ores of. .......... _ ............... _ .................... _... . 396-397 

tunnels of, plans of.. ... _ ....... _._ ... _....................... 394 

veins of. ...................... _ ............................ 394-396 

view of.. ..................... ___ ...... _ .. _ ....... _ . _ .. .. . . . . . 384 

wall rocks of. ................. ____ .... ___ .................... 394 

Empire City mine. See Empire City-Gold Bug group. 

Empire district, alteration in ... _ ....... _ .......... _ .. __ ......... 150 

dikes of. ... _ .............. _ ... __ .. _._ .... __ .... _ ...... __ 131 

geology of. ..... _ ... _ ... __ . __ ... _ _ .. _ ......... _ . _ ... 383-386 

gold in, discovery of. .. .. . .. .. .. _ .. _ ............ 172,312, 401 

location of .............. _ .. . .. . .. . .. .. .. .. .. .. . 383 

map (geologic and topographic) of.. .............. _ ..... __ .... 38G 

tniningclaims in, map of............ ---·-···------- 388 

Inovement in ........ _._ ... _._-~.... . . . . .. .. . . . ... . 150 

placers of. , ......... _ .. . .. .. .. .. .. .. .. .. .. .. . 312 

topography of. ....... _ ..... _._ ..... _ _ ... _ .. _....... 383 

v·eins of ....... __ ._ .......... __ ... _ 24, 98, 100,131, 148-149 

Empire tunnel, description of ........ _. ................... 386 

'Teitls of .... _. __ .. _ ... _._._ .... _ .. _ .................. 389-390 

figures showing. . . . .. .. .. . .. .. .. . . .. .. ......... 388, 389 

wall rock of.._._ .... ____ ......... _ .. ,_ ........ _ 152,387-389,390 

England mine, description of. .. _._ ........................... _.. 352 

tunnel of. plan of. _ .. .. .. . .. .. ................ _ .. . 353 

veins of._ .. _._ ... _._ .................... _ .. _ ... _.... 353 

wall rock of. ....... _._ ..... _._ ............................. 352-3.53 

Enlargement, secondary, process of. ...... _ ............ _._....... 94 

Ennis tunnel, description oL..................................... 210 

plan of, plate showing ...... __ ...................... _ ..... _._. 206 

Enrichment, secondary, description of. ...... _ ....... _ ..... 143-144,146 

Epidote quartz, plate showing ....... _........................... 42 

Erosion, progress of. .............. _ ....................... 32, 37,92,145 

sections showing............................................. 92 

J<:ruptions. See Volcanism. 

Everett lode, description of. ................... __ ..... _......... 216 

plan of, figitre showing ... _ .. _................................ 215 

workings of, plate showing................................... 214 

Export of ores, beginning of...................................... 173 

F. 

Fairplay, rocks at.................... .. .. . . .. .. .. . . .. .. . .. 70 

Falu mine, description of. ................. --.................... 322 

Farish, J. B., on Boulder County ores............................ 103 

Faulting, displacement and...................................... 108 

occurrence and character of.. ......... _ ....... _ .......... 90,107-108 

relation of, to replacement deposits ......................... 160-161 

See also Gouge; particular mine.~. 

Faults, movements along ............................. 26-27,108,161-163 

movements along, diagrams illustrating...................... 162 

Fe!dspar-magnetito pegmatite, plate showing..................... GO 

Feldspathic dikes, Intrusion and age of. ..................... _ .. 1.54-155 

mineralization and relations of. .... _ . _ ............. _ ...... _ 155-157 

~'enneman, N. M., on Colorado intrusives ...... _ ......... _ ....... 68,71 

Fenton lode, description of._._ ..... _ .......... _ ..... _ .. _ ....... 219-220 

relation of, to other lodes._ ...... _ ................ __ ._ ...... 22(}-221 

Fenton-Mammoth lode, description of.. ...... _ ........ _ ....... 219-222 

Fenton-Mammoth-Terrible vein system, lodes of, descriptions of. 219-225 

Ferromagnesian-poor granite porphyry. See Alaskite porphyry. 

Fissure filling, character of. ... _ ... _ ..... _ ......... _ ............ 229-230 

Fluorite, occurrence and character of .................. 136, 1~7, 153-154 

· origin of..~ ... __ .. _. ___ .. _._ ..... _ ....................... 96,142,154 

relation oL to mineralization ..... _ ......... _................. 15S 

section of, figure showing ..................... _............... 96 

Folding, processes of ........ ___ ... _ ... _ ... _...................... 89 

Fossett. Frank, on Colorado mines .. __ ......................... 185,401 

Pnge. 

Fossett, Frank, on Colorado ores ........... _ ... ___ . 98, 103, 104,106,116 

on Colorado output ..... 194,205,212,213,270,271, 28,?, 303,313,314,341 

Foster, E. LeN., on Colorado orcs ............... _ ...... 68,24.?, 248,253 

on Colorado rocks .......... _ .............. __ 70 

Fourmile Creek, rocks on .................... _ 70 

Fraction mine, description of. .. _............. . .... 350,351 

veins of. .............................. _... 350 

wall rock of. ... _ ....... _ . _ ........ _ .. __ .. _ 3.50 

workings of. plate showing ....... _ ... _ . _ .... _ . _ . .. . . . 350 

Fractures, effect of, on ore deposition ...... _.............. 159 

origh1 of .................... _._ ....... _ .. _ ... _. __ ..... . 92 

Freeland, rocks ncar ....................................... 40,62,79,80 

Freeland extension vein, description of.. .......... _ ...... __ .. __ 323-324 

Freeland group, accounL of. .... _._ ... _._._.. 323-332 

description of. .......... _ ............... _._ .... 323-326,327,329-330 

faulting on._ .......................... _.. . _ ..... _ ... _ 330-331 

minerals of ........... _ ....... _ ... _ ..... __ ....... _ ... _._._... 332 

oresof.. ............................................ 149,330,331-332 

plan of, figure showing ...... _ ............... _ . 324 

production of. .... _ ............ _ ....... _ ...... ___ .. _ ......... 326-327 

wall rocks of. .......................................... _ ... 328-329 

waters of._._ ..................... _._......................... 332 

workings on ............... _: ............ :.................... 327 

Freeland mine, description of. ...................... _ ......... _ 323-324 

plans of, plates showing ............ _ ......... _._ ... _ ....... 324,326 

Frost berg lode, description of.. ........ _ .... _ .. _ ... _ .... __ .... _ 224 

faulting on, figure showing .... _ ... _. __ ... _ ................ __ 241 

sections of, figures showing .. _ ..... _._ ... _ ....... __ ..... _ .. _ 234,235 

Fum:lroles\ gases of, sequence of.................................. 156 

gases of, sequence of, relation of, to mineralization .... _._..... 15g 

Futtcrer, Karl, on igneous rocks .............. _ .. _ ....... _........ 63 

G. 

Galena, genesis of................................................ 260 

occurrence and character of. .............................. _.. 136 

plate showing ..... _ .. : ..... _ .... _ .. _ ...... _............. 136 

Galena bien de. See Silver-bearing veins. 

Gamet quartz, occurrence of ... :._ ... _._._ ... _ .... _ .. _ .... _...... 43 

plate showing ....... _ .......... _._ ...... _. __ ..... _ ........ _.. 42. 

Garrey, G. H., and Spurr,J. E., oneconomlcgeology..... 97-382,384-410 

Gates vein, description of. .... _ .. _ .... _ .... _ .... _............. 268 

General Harrison vein, description of.. ........ _.................. 403 

Geneva Creek, description of. ..... _ ........ _ ..... _ ......... _..... 30 

rocks ncar .......... _ .................. _ ........ _............. 37 

Geneva Park, elevation of.. ................. _ ............ _....... 45 

rocks of. .............................. ,_ ..................... 45,47 

Geblogic history, account of. ..... _ .... _ ........ _. __ .... _ 23,24-25,92-93 

Geologic relations, plate showing. _ ....... _ ... _ .. . . .. . . . . . 178 

Geology, description of. ..................... 23,37-96,176-185,383-385 

Geology, economic, description of. ............... _ ..... _ 97-171,386-390 

See also Mines. 

George, R. D., work of. ................... _ .......... _._ ......... 68,71 

Georgetown, description of....................................... 31 

gold at, discovery of.. ... _ ............... __ ................... 172 

mines near, descriptions of. ......... _ ...................... 24!",-311 

view of................................................... 278 

rocks near.- ................................... 48, 40,54,.18, 63, 65,77 

views of. ........................................ -...... - 90,278,292 

Georgetown district, dikes of. ............................... 25,132, 157 

dikes of, description and correlation of._ .............. _. _. 132-133 

See also Alaskite porphyry; Granite porphyry; Quartz 

monzonite porphyry; Dacite. 

metamorphism in............................................ 158 

See also Metamorphism. 

ore, zones of, elevation of..................................... 158 

veins of. ..... _ ... _ ..................................... 24-25,97-98 

See also Silver Plume district; Silver-bearing veins. 

Georgetown quadrangle, maps of, geologic and topographic .... Pocket. 

northern part of, map of, showing veins, etc.................. 102 

topogrnphy of. See Topography. 

Geyser mine, oresot .................... '!....... ................. 125 

waters in ..... _.......................... -.- ....... 125-126 

Gilpin County, dynamic movements in ... __ ................... 108-109 

ores of. ........ _.. .. .. .. .. .. .. .. .. .. .. -.. -.. -.... .. .. 103 

Glacial lakes, description of. ..................................... 33-34 

Glaciation, debris from ............................... -........... 87-88 

forms produeed by ........ __ .............. . ......... _. 33-34,85 

occurrence and character of. ....... _ .. .. .. . .. .. .. .. .. .. .. . 23, 84-87 
INDEX. 415 

Page. 

Glaciers, location and character ot ............................... 84-85 

maps showing ................................................ 32,34 

Gladstone tunnel, description of.. .............................. 353-354 

Glasgow lode, relation of, to other lodes ........................ 220-221 

Glenwood, hot springs at ....................................... 167-168 

Gneiss, alteration ot............................................. 2.52 

effect of, on veins...................... . . . . . . . . . . . . . . . . . .. . . 236 

figures showing .................................... , . . . 236-237 

injection of, by pegmatite............... . ............ 178-179 

figure showing. . . . . . . . . . . . . . . . . . . . . . 178 

occurrence of, as wall rock ....... : ..... 199,201-202,232,234-236,250 

origin of... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 176 

rubble zone in............................................... 191 

varieties of, descriptions of.......................... 41-43,170-179 

weathering of................................................ 35 

See also Quartz gneiss; Pebble-bearing gneiss. 

Gneissic structure, discussion of. ................................. 89-90 

plate showing... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60 

Gneissoid granite, age of......................................... 51 

definition of.................................................. 49 

distribution and structure of. ................................ 49-50 

petrography of. .............................................. 49-51 

sections of, figures showing ................................... 49-50 

Gold, discovery of.. ......................................... 23,172-173 

production of. .............................................. 23,175 

See also Mines. 

Gold-hearing veins, changes in, with depth .................... 148-149 

character of. ........ ,. ......................... 25, 26, 99,103--106,130 

composition of. ............................................ 147-148 

dikes associated with. See Bostonite; Alaskitic quartz monzonite; Biotite latite; Alkali syenite porphyry. 

distribution of. ............................................. 24,130 

map showing. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . 128 

fonnation of.............................. . ................ 154-157 

metamorphism of............................................ 154 

minerals of. See Minerals. 

movements along ...................................... 149-150,163 

origin of.... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . 2& 

relation of, to character of dikes ............................. 25,131 

to silver-bearing veins ................................... 97-101 

wall rocks of: See Wall rocks. 

Sec also Veins; Idaho Springs district. 

Gold Bug mine, description of. ............................ 394,390-397 

' See also Empire City-Gold Bug group. · 

Gold Coin vein, description of.................................... 370 

Gold Dirt mine, alteration in..................................... 150 

description of. . . .. .. .. . .. . .. .. .. .. .. .. .. .. . .. . .. .. .. .. . .. .. .. 398 

Golden Chariot mine, description of. ....................... 394,395,396 

tunnel of, plan of. ................. ,.......................... 394 

Golden Glen Gulch, fault at............... .. .. .. .. . .. . .. .. .. 90 

<Jolden Glen mine, description of................................. 377 

Golden Hecla mine. See Silver Rom mine. 

Gold Fissure lode, description of. .............................. 398,399 

Gold IT ill, geology at............................................. 68 

Gold mining, development of. .............................. 23,172-174 

Gomer mine, description of...................................... 351 

dike in, figure showing....................................... 351 

oresof....................................................... 352 

vein of....................................................... 351 

Good Rope mine, description of.. .............................. 110-111 

Gouge, replacement deposits in ................................. ·160-161 

Grand Encampment district, Wyo., gold in...................... 152 

Granite, alteration of............................................. 252 

character and distribution of ........................ 23,60-61,64-66 

definition of.................................................. 49 

effect of, on veins.. . .. . .. . .. . .. .. .. .. .. .. .. .. .. .. .. .. .. . .. .. . 236 

figure showing........................................... 236 

occurrence of, as wall rock ............ 198-199,201,232,234-235,250 

petrography of. . . .. . .. .. . .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. 64-66 

replacement deposits in, figure showing...................... 160 

rocksat .......... c .................. ·......................... 70 

weathering of. ............................................... 35-36 

See also Oneissoid granite: Silver Plume gra,nite. 

Granite-pegmatite, definition of.................................. 60 

distribution and structure of. ................................ 60-61 

injection by, figures showing ........................ _ ....... .. 

magma of, character of. .................................... .. 

pet;rography of ............................................. .. 

60 

63--64 

61 

Pa!!"e. 

Granite porphyry, correlation of.. ............................... 132-133 

definition of.. ...... :......................................... GO 

distribution and structure of. ...................... 60-61,75--77 386 

petrography of. ........................... : .... , ... 65-6G, 75--77, 132 

Graton, L. C., on Cripple Creek district.......................... 128 

Great Western vein, description of.................... 297,320-321 

Gregory, J. H., discovery by............................. . .... 172,312 

Griffith mine, description of............................... 285 

faulting in........................... . . .. . . . . .. . . 290 

levels in, plan of...................... . . . . . . . . .. .. . 98 

minerals of........................... . ....... 288-289 

section of, figure showing......... 288 

ores oL.............................. 289-290 

sections of, figures showing............. 288,289 

veins of............................ 98, 99,147,285,286-290 

viewof.............................. ................. 282 

wall rock of. .. . . .. .. . .. . .. .. .. .. .. .. .. ......... 285-286 

waters in......... . .. . .. . .. .. .. .. . .. .. .. .. .. .. .. . .. . 290 

workings in, plate showing............................ 286 

Griffith Mountain, mines of. ........................... 278-282.285-290 

rocks of. .................................................... ·.. 37 

view of................. . .. . .. . . . . . .. .. . . .. . . . .. . .. ..... .. .. 278. 

water in ................... :.................................. 154 

Grits, occnrrence of. ........................................... 128-129 

Gnanajuato, Mexico, ores at ................................ 158-159,170 

Gum Tree mine, plan of, plate showing. . . . .. .. .. .. .. .. .. .. .. .. .. 326 

vein of. ................................. ,, ..... 324-325,327,330-332 

workings of, figure showing.................................. 328 

Gunboat lode, description of................................. 222 

Gunnison region, intrusion in ..... ___________ . _ ...... __ .. _ ___ ___ 119 

H. 

Hall, C. W., on microcline . .. . . .. .. .. .. .. .. .. .. .. . .. .. .. • .. .. .. . 94 

Hall tunnel, description of...................................... 245 

geologic plan o!, figure showing.. .. .. . . .. . .. . .. .. . .. . . .. . .. . . . 246 

Hanging valleys, occurrence and c:haracter of..................... 34 

Harrisburg vein, description of..... . ......... 325,327, :l31. 332 

Harrison and Union mine, description of................. 340 

Harrison mine, description of.................................... 390 

oresof................................................. 392 

plan of, map showing...................................... 100 

tunnel of, plan of............................................. 391 

veins of. ............................................... 101,3\J0-391 

fonnation of... . .. . .. . . .. . .. . .. .. . . . . .. . .. . .. . . . . . . .. 391 

Hayden, F. V., onColoradorocks.: ......... : ................... 70,110 

Haworth, Erasmus, on mineral controL......................... 94 

Headlight mine, description of. ................................ 408-409 

tunnel of, plan of.. .................... •. ... . . . . .. . . .. . . . .. .. 408 

Hecla tum1el, description of................................... 392 

plan of, figure showing_ . .... __ ..... _ _ _ 392 

Heddensburg mine. See Gomer mine. 

Hematite, genesis of._ .. ____ .. _._. __ ...... : ______ ... ___ ._ 2ti0 

occurrence and character of........................... 136 

Hershey, 0. H., aid of.................................... 29 

Hidden Treasure mine, description of. . . . . . . . . . . . . . . . . . . . . . . . . . 2!\8, 409 

tunnel of, plan of.................. . .. .. .. .. .. .. .. . .. .. .. .. .. . 409 

Ilillebrand, W. F., on calcite deposition............ 243 

Hills, R. C., on Spanish Peaks district........................... 123 

History, geologic, account of. ........................... n, 24-25,92-93 

History, mining, of quadrangle ......................... 172-174,311-312 

Hollister, 0. J., on Colorado mines ...................... ,........ 341 

on placer mining............................................. 313 

TI ornblende-diopside gneiss, description of. . . . . . . . . . . . . . . . . . . . . . . . 42 

section pi, figure showing..................................... 43 

Hornblende gneiss, age of........................................ 46 

distribution and structure of. ............................... 45.179 

injection in, figure showing................................... · 60 

petrography of. . .. .. . .. . .. .. .. .. . .. . . . .. .. . . .. . .. .. .. . .. .. . . . 45-46 

relations of, plate showing.................................... 178 

section of, figure showing............ . .. .. .. .. .. .. .. .. .. .. .. . 46 

Hornblendite. Sec Quartz diorite. 

Hot springs, discussion of. .................................. 163-169,171 

Hot Springs. See Idaho Springs. 

Howe, Ernest, and others, on Needle Mountains quadrangle. 

Hukill lode, description of.................. . .. . .. .. . . . . .. . .. . . . 341 

Humboldt-Mary Foster vein, description of. ................... 370-371 

ores of. ............. _........ . .. .. .. .. .. .. .. .. .. .. .. .. .. .. 371 

Hydrometamorphism. See Metamorphism. 
416 INDEX. 

I. Page. 

Idaho Springs, description of. ................... ·.. . . . . . . . . . . . . . 31 

gold at, discovery of.......................................... 172 

gravels near ................... ." .............................. 83-84 

hot springs at. .................................... ·. . . . . . . . . . . 27 

mines near ................................................. 314-382 

view of................................................... 312 

placers near ................................................ 311-314 

history of. ............................................. 311-312 

production of............................................ 314 

view of........................ . . .. . . .. . ..... .. ....... 312 

rocks near ........................................... 40, 49, 50, 54, 73 

springs at ............................................... 82, 163-167 

origin of.. .............................................. 166-167 

views of ................................................. 96,312 

waters of. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 164-166, 167 

comparison of, with other springs .................. 167-168 

Idaho Springs district, dikes of. ......................... 25,131,132,157 

dikes of, description and correlation of..................... 133-135 

See also Alkali syenite porphyry; Bostonite; Biotite latite; 

Alaskitic quartz monzonite porphyry. 

metamorphism in............................................ 158 

See also Metamorphism. 

ores of .... _ .. _ .. _ ... _ .......... _ .... _ .... _ .... _ 

ore zones of, elevation of .............. --------- .............. . 

132 

158 

veins of. . _..... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .......... 25, 98 

deposition of.............................. . . . . . . . . . . . . . 26 

See also Gold-bearing veins. 

Idaho Springs formation, age of................ . . . . . . . . . . . . . . 44 

character and distribution of. .................... ~7-44,176-177,383 

conglomeratic type of................. . ....... 176-177 

plates showing ........... ·~ ............................. 38,40 

dikes in..................................... 71 

sections of, figures showing ................... , . . . . 72 

epidote-quartz rock of............................... 42 

intrusion on, plate showing.................................. 178 

joints in ...................................................... 90-91 

metamorphic members of. .................................. 38,176 

See also Biotite-sillimanite schist; .Biotite schist; Limesilicate rocks; Pebble-hearing gneiss. 

origin of.. .......... , ........................................ 44,176 

quartz-garnet rock of, plate showing.......................... 42 

Iddings, J.P., on rhyolite........................................ 77 

Iddings, J. P., and Rosenbusch, H., on zircon................... . 183 

Igneous rocks. See Intrusive rocks. 

Indus tries, description of.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31 

International lode, description of................................. 382 

Intrusion, mineral deposits connected with. See Mineralization. 

phenomena of, plate showing.................................. 92 

Intrusive rocks, age of............................................ 23 

character of.................................................. 23 

descriptions of. .................. : ........................... 45-83 

intrusion of. ........... · ...................................... 91,92 

minerals of, descriptions of. .................................. 93-96 

See also Dikes; .Porphyries, intrusive. 

Invincible mille, description of................................... 333 

Irving, J.D., on Needle Mountains quadrangle.................. 115 

J. 

Jackpot mine, description of................. . ..... 394,396,397 

tunnel of, plan of.... .. . . .. . . . . .. .. .. .. . . . ......... :.. 394 

Jackson, G. A., discoveries by .............................. 172,311,312 

Jackson diggings, description of. ............................... 312-313 

view near. . . . . . . . . . . . .. . . . . . . . . . . . . .. . . . . . . . . . .. . . .. . . .. . . . . . 312 

J. A. Hawkes vein, description of. ............................. 278-280 

James mine, description or................................ 349 

Joints, occurrence and character of. .............................. 90-91 

Joker vein, description of....................................... 342 

Jordan vein, description of....................................... 305 

Josephine mine, description of. ................................ 356-357 

levels of, plans of.. .. .. .. .. .. .. .. .. .. . .. .. . .. .. . . .. . . 356 

K. 

Kaolinite, derivation of. .......... · ............................. 142,263 

genesis of. .. .. .. .. .. . .. . .. .. . .. . .. .. . . . .. . .. .. .. .. . 260 

occurrence and character of. . . . . . . . . . . . .. . . . .. .. . .. . . .. 137 

Kelly mine, account of. . .. .. . .. . .. .. . . . . . . 338-339 

workings of, plan of ................. ·................. 338 

. Page. 

Kelly tunnel, description of................................... 293 

ores in ..................................................... 294-295 

workings of, plate showing ................. , . .. . . .. . . . .. . . .. . 294 

Kemp, J. F., on Leadville ores................................... 106 

Kemp, J. F., and Marsters, V. F., on igneous rocks.............. 83 

King Solomon mine, description of....... .. .. . . . . .. . . . . . . .. .. 363 

Kirtley group, account of. .............................. :: :: ... 265-27! 

mapof....................................................... 246 

ores of. .................................................... 270-271 

production of..,............................................. 271 

veillS of. .................... , ...................... 2ti5-268, 269-270 

wall rock of. ............................................... 268-269 

Kirtley mine, map of. . . . .. . .. . .. . . . .. . . . . . . . . . . .. . . . . . . . 246 

viewof...................................................... 264 

See also Kirtley vein. 

Kirtley tunnel, plan of, figure showing........................... 265 

Kirtley vein, description of ............................ 265-266,269-270 

ore of. ..................................................... 270-271 

plan of, figure showing. .. .. .. .. .. . . . . . . . . . . . . . .. . . .. . 271 

production of................................................ 271 

wall rock of. . .. .. .. . .. .. .. .. . .. . . . . . . . . . . . . . . . . . . . .. . . . . . . 268-269 

Kitty Clyde mine, description of. .............................. 350,351 

veillS of. . .. . .. .. .. .. . . . . .. .. .. .. .. .. . . . . . . . . . .. . . . .. . .. . .. .. . 350 

wall rock of. . .. .. .. .. .. .. . .. .. .. .. .. .. .. . . . . .. . . .. .. .. .. .. .. . 350 

Kitty Emmet mine, description of.......... 374 

Konigsburg lode, description of.................................. 308 

L. 

Laccoliths, character of. ...................... :. .. .. . . .. .. .. .. . • . 112 

intrusion of. . . .. .. .. . .. . .. .. . . . . . . . . . . . . . 110, 120 

LaCroix, A., onzircon.......................................... 183 

Lake City district, ores of. ...................................... .' 116 

Lake Creek, rocks on............................................ 70 

Lake Fork of Geneva Creek, glacier on........................... 85 

Lakes, Arthur, on Colorado ores, .......................... 104,108,111 

Lamartine, view of.............................................. 314 

Lamartine mine, description of. .......................... , .... 314-315 

ores of. ............................................ 149,158,317-319 

plallS of, piate showing....................................... 316 

production of................................................ 314 

::e~:~: ~1-a~ -~~·-·. ·.:::::::::::::::::::::::::::::::::::::::. a1J~~ plan of, figure showing................................... 316 

viowof...................................................... 314 

wall rock of. ........ :........................................ 315 

workings of, plate showing ................................. .'. 314 

Lamartine-Trail Creek group, account of.............. 314-341 

L!!nagan mine, description of... . . . . . . .. .. . . . . .. . . . . 333 

ore of........................................... 334 

tunnel of, plan of............................... 333 

veins of. .. . . . . .. .. . . .. .. .. .. .. .. .. .. . . .. .. .. . . . . . . . . .. . . . . .. . 334 

Landslides, character and distribution of......................... 88 

Langley, B. F., discoveries by ................................. 1:2,311 

La Plata quadrangle, ores of. .................................. 114-115 

Latite porphyry, occurrence and character of.... . . .. . . .. . . . . . . . 151 

Lawrence, B. B., on Colorado ores...................... 189,190 

Lawson, A. C., on geornorphogeny............................ 32 

Lead-silver deposits,. features of. ............................... 136-147 

Lead-silver veins, composition of.............................. 136-138 

distribution of, map showing. . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . 128 

See also Silver-bearing veillS. 

Leadville, mineral belt ncar...................................... 24 

rocks at ...................................................... 70,71 

Leadville district, faulting in........... . .. . .. .. . . .. . . 109 

intrusion in......................... . ........ 109-110, 119-120 

ores of ..................................................... 104-105 

origin of...................................... 169 

Leavenworth Creek, dikes on................................ 48, 71,75 

glacier on .... :............................................... 85 

Leavenworth Mountain, gold veins of ............................ 97-98 

minerals at............................. 154 

mines of, descriptions of................. . ........ 245-278 

viewof...................................................... 244 

Lebanon group, character of................ 214 

description of. . . . . . .. . . . .. .. . . . . . . . . . . . . . . . .. . . . . . . . . . . . . .. . . 214 

lodes of, descriptions of. ... : ................................ 214-218 

workings of, plate showing......................... . . . 214 
INDEX. 417 

Page. 

Lebanon. tunnels, plan of, figure showing............... . 215 

water in ....................................... _ ... _._. 138,218-219 

Lenado district, ores of. ............ __ ................. _ ....... 105-106 

Lexington mine, description of.. .. _ .. _ ..... _ . . . . . . . . . . . . . . . . . . . . . 381 

Lime-silicate rocks, description of. ... _ ... _. __ .................... 41-43 

Lincoln group, description of. ........... _._ ......... _........... 354 

mines of, descriptions of. ...................... __ ...... :. . . 354-358 

view of...................................................... 354 

Lincoln Lake, glacier at.......................................... 85 

Lincoln Mountain, mines on ..... _ ..... _ ............... 309-311,408-410 

rocks on. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71, 73, 75,309, 311 

veins of. .......................... _ .......... _.. . . . . 98, 100,310, 311 

Lincoln vein, description of. ........................... _ .... ' ... 354-356 

levels on;plans of. ................................ _ .. _....... 35S 

Lindgren, Waldemar, on hot-springs deposits .... _............... 164 

on magmatic theory ...•................................... 169-170 

on Terrible mine............ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 241 

Lindgren, W ., and Ransome, F. L., on Cripple Creek •. 128-129,168,170 

Little Albert mine, description of................................ 339 

Little Bear Creek, rocks on ................................... 42, 78,79 

Little Flat mine, description of. ................ _-_............... 374 

. Little Mattie--Newton system, description of. ................... l58-359 

faults of. ............. _ .... _ .......... _: .................... 362-363 

ores of. ....................... _ ............. , .. . . .. . .. .. . . . 361-362 

vein of. ........................................... 00 ....... 359-362 

wall rocks of. ........ 00 ........ 00 .... 00 00 ........ 00.......... 3S9 

Little Mattie mine, description of. ........ 00..................... 358 

faults in ................... 0000 oo· •• _ ••••••• oo .... 00 ...... 00. 362 

levels of, plans of. ..... 00 00 ......................... 00. 00 •• 00 • 360 

veins in ............ oo ................. 00 ..... 00 ............ 150,361 

workings of, plate showing ............ 00.... .... .. .. .. .. • • ... 358 

See al.,o Little Mattie-Newton system. 

Little Richard mine, description of.............................. 381 

Lord Byron mine, descriptioJl of. ............................. 349-350 

M. 

McClellan Mountain, lodes of.. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. . 104 

Magma, waters from ................................ , ..... 00 ... 16!\-167 

waters from, importance of................................... 171 

See also Quartz monzonite; Volcanism. 

Magmatic theory, progress o!.. ................................. 169-170 

statement of. ............. 00 .. 00 ........ 00 .......... _ ..... 00 • 168 

Magnetite, crystal formation of, plate showing................... 60 

occurrence and character of. .. . . .. .. .. .. .. .. .. .. .. .. .. 182 

Magnet lode, account of. ... _ .... 00 .. • • .. .. • • • • .. .. .. .. • .. • •••• 282-285 

ores of. .............. 00 ..... 00 • _ ........... 00 •• _ • .. .. • • • .. • 284-285 

plans of,,plate showing ................. 00............. 284 

production of. .............. 00. 00 ......... 00.: ........... _... 28/i 

veins of. .............................. 00 • • • .. .. .. • .. • • .. .. • 283-284 

wn.ll rocks of ............. 00 .............. oo .. 00 ...... 00 .... 282-283 

Magnet mine, view of ........... _ .... _ ... 00 ......... _ ........ _ 00 282 

workings of, plan of. .... : ....... 00 ................ _ • .. • • .. .. • 284 

Maine lode, description of. ..................... _ ...... 00 224 

faulting on, figure showing .......... _ .. _ .. _ ....... 00 242 

section in, figure showing....... .. ........... 00 237 

Maine tunnel, water in .. 00 00 .... 00. _. _ ..... __ ........... 139 

Mammoth lode, description of.._.... _ ................. 00. 221-222 

M. and M. tunnel, description of. ....................... 00 .... 00. 373 

Map, index, of Colorado ........ _._ 00 .... 00 ...... 00 ...... 00 •• 00... 29 

Map of Central City quadrangle, showing former topography..... 30 

of Central City quadrangle, showing glaciers ................. 32,34 

showing present topography .......................... 00 • 36 

of Colorado, showing belt of intrusive porphyries ... _......... 70 

showing distribution of gold, silver, and lead............. 128 

of l<~mpire district. .. 00 •• 00 ... 00 ............. 00 00 .. _.......... 386 

showing mining claims .... _ ... _ ... __ . __ ...... _ 388 

of Georgetown quarlrangle. 00 .............................. Pocket. 

northern part of, showing veins, etc .......... _. _ . . 102 

showing former topography .... _......... 30 

showing glaci_ers.................................. 32,'34 

showing present topography ..... __ ..... _......... 36 

of Silver Plume district............................... 138 

showing dikes and veins ...... 00 ....... • ...... 00... 194 

showing workings .. 00 ........ _ ......... _.......... 184 

Ma]), geologic, of Empire district. 00 • 00 • 00 .... _ • • • • • • .. .. • • 386 

of Georgetown quadrangle ........ _ .... _ ............. _ ... -... Pocket. 

of Silver Plume district. .... _. . ... 00........... 138 

31401-No. 63-08--27 

Page. 

·Martine, C. A., on Colorado ores ................ 00............. 286 

Marvine, A. R., on Colorado intrusives .............. -... _ 68 

Mary Matilda mine, descriptiou of. . . . . .. ........ 00 ........ _ •• _ 37 4 

Matehuala, Mexico, ores at ................. 00 ............ 00...... 170 

Mathews, G. D., on Colorado granites .... 00 ................... 58,59,66 

Maximilian Gulch, rocks in ................... 00 .............. 000 62 

Mayflower mine, description of..., ... _ ......................... 364-365 

tunnel of, plan of. .............. 00 .... 00 .......... 00.... .. .. • 365 

Mendota mine, description of. .... 00 ...... : ...... , ....... 00 ••• 00. 227 

mill of, view of.. 00 ...... 00 .... 00 .................... 00....... 228 

Mendota tunnel, deposition in, plate showing . . . . .. . .. .. . .. .. .. . 242 

plan of, figure showing ........ 00 .................... 00 .·...... 225 

Mendota vein system, account of. .......................... 144,225-227 

sections of, sketches showing ........... 228,229,230,231,232,238,239 

workings on, plate showing.................................. 226 

M~rcur district, ores of. ........... 00 ......................... 00.. 169 

Meridian Hill, rocks near. .. . . . • .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. 47, 58, 65 

Metadolerite, intrusion of, figure showing ............. 00 00. 00 .. 00 46 

See alilo Hornblende gneiss. 

Metals, magmatic origin of ......................... : .. : ..... _ .... _ 157 

Metamorphic rocks, minerals of, descriptions of. .......... 93-96, 17&-179 

Metamorphism, occurrence and character of. ....... 143,145-146,150--152 

See alilo Wall rock; Waters. 

Metropolitan mine, description of. ............... 00 ........... 00 • 352 

Michigan Creek, rocks on .......... 00 ... 00..... .. .. .. .. .. .. .. • .. .. 69 

Microcline, occurrence and character of. .................. _ ... 93-94, 143 

Micropegmatite, character of. ................. 00 ................. 95-96 

Miller mine, description of. .............. 00 ... 00 .............. 00 • 334 

ore of. ........................ 00 ..... ·... .. .. .. .. • .. • • .. • • .. • • 335 

tunnel of, plan of ............ 00 .... 00 .............. 00.. ...... 335 

vein of. ....... 00 .............. 00000 oooo• .... 00 ... .. ..... • ... • 334 

wall rock of. ..................... : ............... 00 .. 00.. ... • 334 

Mineral helts, character and distribution of. ............... 24-25,97-99 

origin oi. ............. : ............... 00 .. .. • .. .. • .. .. • • .. .. • 25 

See a!so Gold-bearing veins; Silver-bearing veins; Northeastsouthwest mineral belt. 

Mineral Chief vein, description of. ...... 00 • : ..... _ ........... 00 29!\-297 

tunnel on, plan of. .......... 00. _ 00 00... .. .. .. • .. .. .. .. • .. .. • • 297 

Mineralization, causes of. ............. 00 .............. 00 ..... _ 00. 159 

duration of. ............ -- .. 00 ............. 00 .............. 119-122 

methods and forms of.. ..... 00 ...... 00 ..................... 159-161 

periods of. ...................................... 00. 120-122, 128-129 

progress of. .. .- ................ 00 .................. 00. 27-28, 155--157 

relation of, to feldspathic dikes ...... 00 .. 00 ................. 155-157 

to porphyry dikes ... _ ........ 00 ....................... 101, 107 

to sequence of fumarolic gases .......... 00.......... .... • • 156 

Mineralizing solutions, mingling of ....... _ ............ 00 ••• _..... 159 

nature and effects of. ............................. 00 ....... 155-156 

origin of. .................................................. 15&-157 

See also Metals. 

Minerals, derivation of. ................................ 141-142,261-264 

descriptions of. ............ 00 ... 00 ................. 00 ... 93,256-258 

occurrence and character of. .......... 00 00 00 ... 00 00. 24, 13!\-138, ISO 

paragenesis of. ....... 00 .... 00 • • 136-148, 181, 259-261, 277, 288-289, 300 

plate showing .... : .. .. · ................ 00 ... 00 00 ..... 00.. • 98 

See al8o particular mines; Ores. 

Mines, detailed descriptions of. .................... :. . . 185-244, 390-411 

history of. .... 00 ...... 00 ...... : ................ _ .... : .. 00 .. 172-174 

location. of, map showing .. 00 ............ 00 .............. 00 ... • 102 

production of. ..... _ ............ 00........................... _175 

Mine waters, character of. ..... · ... 00 .... 00 ..... _ ...... _ .. 00 _ ••• 125-126 

Mining districts, miscellaneous descriptions of. ................. 122-129 

Mint mine, description of. ............... _ ....... _ . _ 00 ......... 392-393 

tunnel of, plan of. ............. _ .. 00 .. .. .. .. .. .. .. .. .. .. .. .. • 393 

veins of. ............ : . ............... 00 ............ 00 _ ..... 393-394 

wall rock of. ......... 00 ...... 00 ..... 00 ................... 00 • • 393 

Moline tunnel, orcs in .... _ ..... _ . _ ......... 00 ................. ." 293-295 

Monarch, rocks at ... · ..... _._ ............. 00 ....... 00............. 70 

Monarch district, orcs of. .. _._._._._............................. 106 

Montezuma, rocks at ............... : . ..................... 00 .... _ 69 

Montreal lode, 8,ccount of. ...... 00 ................ 00 ............ _ 213 

tunnel of, figure showing .......... _ .. .. .. .. .. .. .. .. .. .. .. . 213 

Monzonite, definition of. ....... _................................. 46 

· See a!siJ Quartz monzonite gneiss; Quartz monzonite porphyry. 

Mon~onitlc eru~tions, centers of. . . . . . . . . . . • . . . .. . . . • . . . . . . . . . . . . . . 123 

Morames, gold 111. • • • .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. . 86 

occurrence and character of. .................... 00 ...... : • •••• 84-86 
418 INDEX. 

Page. 

Morgan mine, description of..................................... 340 

Morning Star lode, account of .............. , ................... 214-215 

dikes of. . .. . .. .. . . .. . .. .. .. .. . .. .. . .. . . .. .. .. .. .. .. . . .. . .. .. . 215 

plan of, figure showing. .. . .. .. .. .. .. .. .. . . . .. . . . .. . .. . . . . .. . . 215 

Mosquito Gulch, ore deposits on................................. 68 

rocks near ................................ __ . . . . .. .. .. . . . .. . . 69 

Mountains, description of. ................... __ .................. 31-32 

Mount Evans, altitude of......................................... 30 

glaciation on. .. . .. . .. .. .. .. .. .. .. .. . . .. . .. .. .. . .. . .. . . . . .. . . . 33 

rocks near ............................................... 47, 57,62 

Mount Sugarloaf, rocks at ............... __ .. _ .................... 41,68 

Muscovite, introduction of, figure showing.. . . . . .. . . .. . . . . . . . .. . . 64 

occurrence aud character of. ........................... 132,182-183 

Muscovite-Silvcrine lode, description of_ ....... _................. 375 

ores of....................................................... 376 

vein of. .................................................... 375-370 

Muscovite tunnel, plan of, figure showing........................ 375 

Muscovite vein, description of................................... 296 

tunnel in, plan !)f ........... __ . . .. .. . . . .. .. . . .. .. . .. .. .. .. .. . 296 

N. 

Naylor Lake, glaciation near..................................... 86 

glacier at ....................... ____ .... .. . . . .. . .. .. . .. .. .. .. 85 

rocks near................................................... 63 

Needle Mountains quadrangle, ores of .......................... 115-116 

Negro Hill, rocks on............................................. 69 

New Boston vein, ores of................................. 295 

section of, figure showing ........ _ .. : ............ :. . .. .. . . . . . 295 

New Em mine, description of.................................... 320 

orcs of ..................................................... 321--'122 

plans of, plate showing....................................... 320 

veins of. ................................................... 320-'121 

wall rock of. .................. ___ ............................ 320 

Newton mine. See Little Mattie-Newton system. 

New vein, description of......................................... 204 

Niagara mine, description of. ........... __ .... ____ ............... 349 

Northeast-southwest mineral l>elt, dynamic history of............ 109 

discussion of ............................................... 101-122 

eruption in ................................................ 119--122 

existence of. . . . .. .. .. . .. .. .. .. . . .. . . .. .. . . .. .. . .. .. . .. .. .. . . . 101 

extent of. .................................................. 101-102 

faulting along .............................................. 101-109 

intrusive rocks and, coincidence of ......................... 101,107 

intrusive rocks in, age of. .................................. 109-110 

mineralization of. .......................................... 119-122 

nature of. .................................................. 107-109 

ores of ................... __ ... : ............................ 102-107 

detailed descriptions of.--- ... -.--- .... ---.--.-.- .. :----- no origin of. . . . . . . .. .. . . .. .. .. .. . .. .. .. . . . .. . .. .. .. .. . .. . .. .. . . . 118 

zone of weakness and .................................. 107-108,118 

Nyanza lode, description of...................................... 307 

0. 

0. K. vein, description of .................................. 266, 270, 271 

Old Settler mine, description of. ............................ '.... 340 

ores of. ...................................................... · 341 

vein of. ........................ · ............................ 340-341 

Old Stag tunnel, plan of . .. . .. .. .. . . .. . . .. . . .. .. . .. . .. .. . . . . .. . . . 317 

Oneida vein, description of. ............... __ .................. 315-316 

Ore deposits, classes of. ............... , ........................ 129--130 

deposition of, history of, plate showing...................... 98 

detailed descriptions of..·_ ................... _ ..... 103-106,136--157 

form of. ...................... · ............................. 102-103 

zones of. ................................................... 158-159 

See also particular districts, mines, etc.; Lead-silver deposits; 

Gold-bearing deposits. 

Ores, changes in, with depth. See Depth. 

deposition of. ........................ _ ...... __ ............... 147 

cljaracter and distribution of. ............... __ ............ __ . 24,26 

classes of. .................................................. 129--130 

deposition of. .................................. _ ..... :. 26, 120--122 

plate showing....... . .. .. . .. .. .. .. . .. . . .. .. . .. .. .. .. .. .. . 98 

effects of waters on ...................................... __ . 138-140 

exportation of.............................................. 173 

localization of. ........... : . . : . ..... :.. .. . .. . . ... ... .. .. . .. .. 159-161 

paragenesis of. See Minerals. 

See also particular mines; Ores. 

Page. 

Oro Cash mine, description of. ................................. 394, 39U 

tunnel of, map of. .............................. ____ ..... 394 

Onida vein, description of.......................... 370 

Ouray district, ores of. .. .. . . . .. .. . . . .. . . .. . .. .. . .. .. .. .. .. .. .. .. . li6 

Oxidation, zone of. ............................................ 143,148 

P. 

Paines Mountain, rocks on....... .. .................... 46--47,75 

Palmer, C. S., on Colorado dikes................................. 68 

Pandora vein, ores of. . . . . . .. . . . .. . . . . . .. . . . .. . . .. .. . .. .. . .. .. . . 113 

Paragenesis. See Minerals. 

Park County, ores of................. . . . . .. .. . . . .. .. .. .. . ..... 104 

Patsy vein, description oL........... . ... __ ................. 403,404 

Pay Rock lode, account of........... . ...................... 204-209 

history of.. ................................................. 204-205 

ores of............................ . ........... 144,207-208 

production of....... .. . . . . . . .. . . . . . . . . .. . . .. .. .. .. .. . . 205 

vein of......................... .. .................. 206--207 

wall rock oL ............................................... 205-206 

waters in ................................................... 20&-209 

workings of, figure showing .................... -.-............. 205 

plate showing............................................. 204 

Peale, A. C., on Colorado rocks........... .. ........... 37,69-70 

Pearce, Richard, on Colorado ores. . .. . .. . . .. . .. 103, 104, 105,286, 288,289 

Pcbhle-iJearing gneiss, description of.. ....................... 41,176--177 

figure showing .. -----.-----.-- .. --- .. --.- .. -------- .. --------- 176 

plates showing ................................................ 38,40 

Pegmatite, character and distribution of. .......... 23-24,66--61, 181-183 

definition of. ...................................... __ ... .. .. .. 60 

dike of, section of, figure showing .......... __ ...... __ .... __ ... 63 

effect of, on vein.............................................. 236 

figure showing .......... : .. .. . .. .. . . .. . .. .. .. .. .. .. .. 236 

injections of. ..................................... 38--39,178--179,181 

figures showing..... .. . .. .. . . . . . ....... 39, 60, 64, 178 

magma of, character of........ .. . .. .. .. . .. .. .. . . . .. .. .. .. .. .. . 63-64 

petrography of........ .. .. .. . . . . . . .. __ .. Gl-63, 181-183 

occurrence of, as wall rock .. .. . . . . .. . . . .. .. .. .. 250 

plates showing................... .. ..... c ...... 60,250 

relations of, plate showing.............. . . . . . .. . . . . . .. . 178 

vein of, figure showing........................................ 183 

Pclican-llismarck lode, description of.............. . ........ 186--191 

ores of................................. .. ....... 189--191 

figures showing................... . . .. .. .. .. . 186 

section of. figure showing ..... ___ ............................. 187 

vein of.......... .. .. .. .. .. .. .. .. .. .. . . .. .. .. .. 186--189, Hll 

figures showing ................................. 180,188,189,191 

wall rock of................................................... 191 

Pellca'n-Bismarck vein system, description of. ........ _ .. ~ ..... 185-186 

history of.................. .. .. .. .. .. .. . .. . .. .. .. .. .. .. 185 

lodes of, descriptions of.. ............ _..... . .......... 186--203 

plate showing................ .. .. .. . . .. . . . .. .. . . .. .. 188 

Pendleton Mountain, gold veins on ............ _. 98 

rocks on.... . .. .. . .. .. .. .. .. .. .. . .. .. . . 41,75 

Peru lode, account of.................................. 217 

ores of, section of, figure showing............. 217 

Phenocrysts, influ_ence of, on groundmass....... 94 

ongm of... . .. .. . . .. . . . .. .. . . .. . .. . . .. . . . . . . ........... 94-95 

Phillips vein, sections of, figures showing ...... _.................. 233 

Phoenix mine, description of..' ................ _ .. _............... 352 

Placer deposits, diggings on, view of.. ............ _ . . . . . . . . . .. .. . . 312 

history of.. ................................................. 311-312 

occurrence and character of............... . ............ 312--314 

production from .. : ....... ."................................... 314 

Pleistocene deposits, description of. ......... -~ .. _ ..... _ .. _ ... 83-88,386 

Polar Star-Junction lode, description of ........................ 306--307 

Polybaslte, analysis of.. ........................... _.............. 227 

deposition of, section showing................................. 140 

genesis oL................................................... 251 

Poor Man mine, description of.. ................................ 319-320 

Porphyries, intrusive, age of. ....................... 71,109-110,129-130 

belt of, map showing ............... _......................... 70 

description of. ............................................. 67,73-83 

distribution of, in Colorado ............................... 67-70,383 

in Georgetown quadrangle...... . . . . . .. . . . . . . . . . . .. .. .. .. . 71 

Intrusion of, centers of ..................................... 123, 129 

forms of ................................................... 71-73 

fonns of, figures showing........ .. .. .. .. .. .. .. .. .. .. .. .. .. 72 

mineralization along ................................. 68, 101, 107, 129 
INDEX. 419 

Page. 

Porphyries, replacements deposits in, figure showing............. . 161 

See also Alaskite porphyry; Granite porphyry; Quartz monzonite porphyry; Dacite; Alkali syenite porphyry; 

Bostonite; Biotite latite; Alaskitic quartz monzonite 

porphyry. 

Porphyry dikes, alteration of..................................... 252 

character and distribution of.................................. 184 

displacement in ............................................. 184-185 

effect of, on veins ............................................ 236-237 

figure showing............................................ 233 

intrusion of.. ............................. _ ................ 24,71-79 

sections· of, figures showing ..... _ ...... _ ...... _ ..... _..... 72 

mineralization and, connection of. ...... _ .......... _ ......... 25,107 

See also particular mines. 

occurrence as wall rock ........... _. 199,202,206,232-233,236-239,240 

weathering of.. ................... _ ...... _ ... _ . __ ..... _ . . . . . . . 36 

Potosi volcanic series, deposition of.. ......... _ ... _ .. __ ..... _ .. - 111-112 

Premium lode, description of.. .... ___ . __ . __ ...... _ .. _ .. _......... 309 

Pressure, relief from, deposition due to ............ _ ... _ .. ____ ... _. 158 

Production, statistics of.. ........... _ ... __ ._ ........ _ ... _ ... _..... 175 

Proustite, genesis of! ................. _ ... -.. ___ ..... -·- .. __ ...... 261 

Prudential tunnel, minerals in ......... __ .. _._. ___ ..... ____ .. _.... 153 

P. T. mines, description of.. .................. __ .. __ ........ 151-152,379 

. ores of.. ................................. _ .. _ .. _ .. · ...... __ .. 371}-380 

veins of. ....................... __ .. _ ..... __ . . . . . . . . 371}-380 

wall rock of.. .......... _ ............. -- _ ..... -... - -... -. . . . . . . 379 

Pulaskite porphyry. See Alkali syenite porphyry. 

Purington, C. W ., on Telluride quadrangle._ ........ ___ ... _ .... 113,169 

Pyrargyrite, genesis of.. ...................... _ ........... __ ....... 261 

Pyrite, derivation of.. .............................. _ ....... 152,262-263 

genesis of.. .................................. - __ . . . . . . . . . . . . . . 260 

occurrence and character of ................ _ ........ -_ .. 136,388-389 

Pyrite, secondary, occurrence of.................................. 144 

Q. 

Quartz, derivation of ....... _ ................ _ .... __ ........ 141,262-263 

genesis of.. ............. -........ --.. . . . . . . . . . . . . . . . . . . . . . . . . . 259 

Quartz-bearing diorite, age of.. ......... ___ ._._ .... _..... . ... 56-57 

•listrihutior\ l!lld structure of. .. -............ _ 54 

petrography of.. ....... __ ....... __ ............ ____ 54-S6 

section of, figure showing ......... __ . _ ........... _ 56 

Quartz gneiss, distribution of. ............. ___ ......... __ ._... 41 

petrography ..................... _ .... _ ... _ .. ___ .............. 41 

Quartz-magnetite gneiss, description of.. .... ___ ..... __ ........... 42 

Quartz-magnetite pegmatite, plate-showing ........ _ ..... _ ........ 40,60 

Quartz monzonite, age of ... .' ............. ____ ._ .... _ ........ .14,384-38.> 

definition of.. ............................. _. ____ ............. 4R 

distribution and structure of ......... ___ .............. 51-52,383-384 

injection in, figure showing .... _ ......... __ . . . . . . . . . . . . . . . . . . . 60 

intrusion of. ..... _..................... _ ............. 24-2~ 

petrography of. ......•......... : ......... _____ ............ .'>2-54,384 

section of, figure showing •............... _ ...... __ . . . . . . . . . . . . 52 

weathering of.. ................................ _ ......... _. . . . 3.5-36 

figure showing .................................... _ 36 

Quartz monzonite gneiss, age of.. ............. _ ............... _... 49 

definition of.. .............. __ ........... ______ . __ ............ 4fi 

distribution and structure of. ......... _ .. _ ........ _ ... __ ...... 46-47. 

petrography of.. ........ _ ..................... _ ........... _ ... 47-48 

Quartz monzonite porphyry, age of .......... ____ ... _ .... _. 133 

correlation of .................... _ .. _ ....... __ ._ ....... _ 132-133,385 

definition of ..........•.............. _ .. __ ...... _ . ___ .. _ .... _ . 73 

distribution of .............................. :,._ ............. 73,385 

petrography of.. ............................... __ ..... 73-74, 132,385 

Quaternary deposits, description of. .... _ ............ __ ....... 83'-88,386 

Queen City lode, description of .......... _. _ ............ _........ 302 

veins of. ......................... _ .......... ___ ........... _ . . . 303 

wall rock of. ............................................ __ . . 302-303 

Queen of the West lode, description of .... _ ....... _ ............ _. 307 

Quito mine, description of.. ............... · .. _ ......... ,.......... "380 

R. 

Railroads, construction of ........ -................... __ ... _...... 173 

Ramsdale mine, description of. ................... _ .... _ ........ 309-310 

tunnel of, plan of.. ............ __ .. _ ........... __ .... __ .. _. . . . 310 

veins of.. ..................... -_ ....... __ ........ ____ .. __ ... 100,310 

Randall, J. S., on Colorado ores ....... _ ..... _ ... ___ ....... __ . 289 

Ransome, F. L., on S•lverton quadrangle ..... _._ .. _ ....... _...... 113 

Ransome, F. L.;and Lindgren, W., on Cripple Creek .. 128-129,168,170 

Page. 

Raymond, R. W., on Colorado output .....•................ 173,213,268 

Red Cliff, rocks at ................ __ .............................. 70 

Redcliff district, ores of. ................ _. . . . . . . . . . . . . . . . . . . . . . . . . . 105 

Red Elephant Hill, veins on.-·-_ .. _._ ... __ ...... _................ 98 

Red Lyon mine, description of.._ ... _ ...... _ ........ __ ._.......... 364 

R. E. Lee vein, description of. ........... _ .. _._ ................. 315-316 

Relief, description of. ............... __ ..... _____ .............. _ 30 

See also Topography. 

Replacement, ore deposition by... . .................. 160-161 

ore deposition by, figure showing ........ ____ . __ ..... _........ 160 

Republican Mountain, mines of.. ..... __ ............. _ .....•.... 292-303 

veins of.. ............................................ 97,145,292-293 

detailed descriptions of.. ...... -- ....................... 293-303 

views of.. .......................... _· __ ......... _-·-· __ ....... 292 

Rhyolite, occurrence of.. ... _ ............. _....................... 76 

Rickard, Forbes, on Colorado intrusives.......................... 69 

on Colorado ores ...................... -- ____ ._ 68, 98,103,106,108,111 

Rico quadrangle, ores of..._...................................... 114 

Rock formations, character of, relation of, to topography _____ ..... 35-36 

descriptions of. See Geology. 

See also particular formations, rocks, etc. 

Rocky Cliff lode, description of.. .......... _ ...... _ ...... , . . . . . . . . 37C 

Rogers vein, description of. .... _ ............................... 305,308 

Rollinsville, dikes near ....... _ ...... _ .............. _ ... :. . . . . . . . . 69 

Rosalie granite, age of .... __ ......... _ .. ·.......................... 58 

character and distribution of. ............ , ........ _ ......... _ 23,57 

petrography of. .......................... _ . . . . . . . . . . . . . . . . . . . . 57 

use of, for bpi! ding........................................... 91 

weathering of. ......................... ___ . . . . . . . . . . . . . . . . . . . 36 

figure showing. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36 

Rosalie Peak, rocks near ......................................... 47,57 

Rosecrans mine, description of. ............. _ ........ , . . . . . . . . . . . 404 

ores of.. ................... , ..... _ ............. __ .. _. ___ ... _.. 405 

plan of, plate showing ........................ _. _____ ._....... 404 

veins of. ... -........................... _ . . . . . . . . . . . . . . . . . . . 404-405 

wall rock of. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 404 

Rosen busch. H., on microcline................................ .. . 93 

Rosen busch, H., and Iddings, J. P ., on zircon ..... _'_. ______ . __ ... 183 

Rosita Ilills. See Silver Cliff and Rosita Hills. 

Rubble zone, occurrrence and character of.. ............ ,......... 191 

Ruby district, ores of ....................................... ·...... 106 

Russell, W. J., prospectingby................................... 172 

s. 

Saco vein, description of. ................ _ ... _.. . . . . . . . . . . . . 268 

St. Patrick lode, description of................................... 326 

Salida, rocks near .. __ ......................................... _. . 70 

Salisbury mine, description of.. ...... _ .... _ ... _.................. 354 

tunnel of, plan of.. .......••................. ·····-·---······· 354 

Sandstone. See Carboniferous sandstone. 

Sangre de Cristo Mountains, rocks in .. _ ............. _ .... _ 

San Juan region, districts of, ores of. ....... __ .. __ ........ . 

domingin .......................................... _ 

geology of. ................... __ ...................... _ .... . 

70 

113-117 

121-122 

118-113 

intrusion in .............. : ................................. 111}-120 · 

location of. .............. _ .......... _ . . . . . . . . . . . . . . . . . . . . . . . . 118 

ore deposits of.. .. _ ........................... 24-25,113-118,120-121 

similarity of Gunnison-Boulder district and ..••.......... _._. 118 

volcanism in ...... _ ...................•........•.. : ........ 111-112 

San Juan tuff, deposition of. ............ , ............... _........ Ill 

Sappho vein, description of. .................................. __ . 370 

Sa watch Range, ores of. ..................................... _... 106 

Saxon Mountain, beryl on........................................ 63 

gold veins of. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98 

mines on ....... -....................... _. . . . . . . . . . . 282-285, 290-291 

rocks on and near .............................. __ . . . . . . . . . . . . 78 

vie'v of .... ..... _ ........ ~ ...... _ ........ _ .... __ . _. _. _ ...... _. 278 

Scapolite, occurrence of. ......................................... 62-63 

Sceptre mine, arcount of. ........................ _ . . . . . . . . . . . . . 298-300 

description of. .................................... :.......... 298 

ores of. ...... _ ...................................... _ ....... 291}-300 

sections of, figures showing ................................. 298,299 

veins of. ................................. ___ . . . . . . . . . . . . . . . 298-299 

wall rock in ................................ _................. 298 

See al"o Sunburst-Sceptre vein. 

Schaller, W. 'l'., on biotite schist ............................. _.. 52 

on garnet. . . . . . . . • . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43 

Schistosity, discussion of. ..................................... _ .. 89-90 
420 INDEX. 

Page. 

Schists, origin of................................................. 176 

See also Biotite-sillimanite schist; lliotite schist. • 

Scott lode, description of. ...................................... 21(;..217 

Seaton Mountain, veins on ............................... .-....... 98 

Secondary enlargement, process of..... . . . . . . . . . . . . . . . . . . . . . . . . . . . 94 

Secondary enrichment, descript.ion of ...................... 143-144,146 

Sedimen:ary rocks, age of. ....................................... 23,37 

character of. .................................... 23,37-44, 6tl-67, 383 

Sequel vein, descript.ion of. .................. _ ...... _ .......... 283-2Sii 

Seven-Thirty lode, account of ........ _ .. _ .................. ___ . 197-203 

barren lead in, figure showing ....... __ ...... __ .... _ .......... , 199 

history of.. ··················-~·-·-······-······--········ 197-198 

ores of, depth and .... _ ................. _ . . . . __ ......... - 202-203 

production of. .............. _................................ 1113 

vein of ............................. -..... __ ............... 199-201 

figures showing ....... __ .......... 200,201,202,203 

view of···-·············-·····-········-······-········-----· 196 

wall rock in ......................... _............. 198-199,201-202 

workings of, plates showing ...... _ ........ _ .... _ .. 200,202, pocket. 

Shakespeare tunnel, description of. ................ _ ........ --... 324 

plan of, plate showing _ ...•... ___ ............ _ ....... _....... 321\ 

Sherman Mountain, view of. .... _ ......... _ ... _ ............... _. . 184 

Shively lode, account of.. ...... _ ............................ _ .. 213-214 

Siderite, derivation of .••••... _ ........ _ ................... 153,262-26.1 

genesis or .... -..... ---.-- ...... -.-.- .. -..... -... -- ........ -.-- 259 

Silica, derivation of .................... _ .............. _........... 15:1 

Sillimanite. See Biotite-sillimanite schist. 

Silver, discovery of. ....................... _ ..... _ ....... __ ...... 173 

'Productionof .............. : ............................ ,.... 175 

Silver, native, genesis oL ....... _ ..... _ ...... _ ...... _ ... _........ .261 

Silver Bank tunnel, ores in .•... _ ...................... :. _ ..... 206-207 

plan of, plate showing .......................... _ . . .. .. .. .. .. 206 

Silver-bearing veins, changes in, with depth ................... 143-146 

character of. ........................ _ .. _ ....... 25-26, 99, 103-106, 130 

composition of. .... _ ..................... _._ ..... _ ......... 136-138 

distribution of. ........... _ ... _._ .............. _ ...... 24-25,99, 130 

map showing............................................ 128 

dikes associated' with. See Alkali syenite porphyry; BostonHe; Biotite latite; AlfiSkitic quartz monzonite porphyry. 

formation of. ................... __ ... _ .. _ ..... . 

metamorphism of. ................. _ .· .. _ ...... . 

minerals of. See Minerals. 

. ....... 26,157 

143,145-146 

movements along .................... _ ...... _ .............. 162-163 

relation of, to character of dikes ..... __ ...................... 25, 131 

to gold-bearing veins ......................... _ .......... 97-101 

wall rocks of. See Wall rocks. 

See also Veins; Georgetown district; particular veins. 

Silver Cliff and Rosita Hills, comparison of Cripple Creek and.... 126, 

128-129 

geology of .. _ ................. _ .... _ .. _ 

rnine waters of.-- .......... _ ...... . 

ores of. ............................................ . 

123-124 

125-126 

124-125 

origin of. ........................ _ ....... _ ......... 126-127,168 

Silver Cloud-Fletcher lode, description of. ....... _ ............. 306,307 

Silver Creek (of Clear Creek), dikes on ......... _._......... 71 

dikes on, section of, figure showing ............. _........ 72 

glacial debris on .............. _ .............................. 86,88 

gold on .................................................... 312-313 

section of, figure showing ........... _ ...... _................. 92 

Silver Creek (of Clear Creek), glaciation on ................... ·.... 86 

section of, figure showing .................... _............... 92 

Silver Glance lode, description of. ............................. 306,307 

Silver Horn mine, description of._ ............ _ .............. _... 371 

plans of, fignres showing .. -.... -........................... 372, 373 

veins of. ........................ -................. -........ 371-373 

Silver minerals, derivation of. .............. _,................... 140 

See also Minerals. 

Silver mining, history of. ..... _ ....................... _ ........ 173-175. 

Silver Mountain, brecciation at.................................. 150 

orcs at ........................... _ ................. 148-149,397,402 

veinsat...................................................... 150 

relations of, plan showing .......................... __ .... 396 

waters in . ..... _ ........ : . _ .... _ .. ____ . ____ . __ . __ .... __ ..... . 

See also Silver Mountain ore zone. 

lver Mountain lode, description of.·_ .................. _ ...... _ .. 

surface workings on, view of._ .... _ ........... _ ............. . 

1541 

397 

402 

Silver Mountain ore zone, character of. ..... _ .. _ ..... _. . .. _ 

faulting in ........................ _ ......... _._ ::: ..... .. 

levels of, plans of.................. . ....... _ .... , .......... . 

location of, plate showing ............. _ ... _ ... __ ...... __ .... _ 

lodes of. ............................................. _ ..... .. 

Page. 

397 

398 

402 

396 

397 

mineralization of. ........ _ ...... _ .... .... ...... . 397 

mines 011. ............................. _ •• _ ..... _. • . .. • • • .. .. • 399 

~~~;i~f~r.·.· .·.·_-:::::::::::::::::::::::::::::::::: ::::::::-~~o,_ ~1-~~ veins of. ....................... __ ... _ ......... , __ .... _ .. _ .. 398-400 

wall rock of ................... _ ....... _._ ....... _._._.... 400-401 

waters of. ................ _._ ......................... _....... 402 

workings along, map of. ....... _ ..... _ ......... _ ... _ .. . . . . . . . 398 

Silver Ore lode, description of ..... _ ... __ .... _ .... __ .... _ ... __ .. _. 222 

Silver Ore tunnel. waters in ...... _ ..... _ ....... _ ...... _ .... _ ... 138,242 

Silver Plume, mines north of, view of. .......... __ ... _........... 184 

rocks at and near ................ _ ......... __ .... 54, 58, 61, 71, 73, 77 

Silver Plume district, dikes of ............. _ .. _ ...... _ .. _ ...... 131, 184 

dikes or, location of, map showing .......... _ .......... _ .. _ 194 

faulting in .................. _ ........ _.. . .· ...... 108, 184-185 

geology of. ............................ _._ .... __ ............ 17tl-185 

map, geologic and topographic, of. .... __ ... _ . 176 

mines of, descriptions of. ......... _ .. _ ......... __ ... _ .. _ ... 185-244 

workings of, map showing ...... _ ..... _... 184 

veins of, location of, map showing._ ..... _._ .. _ .... _ .... _..... 194 

Silver Plume granite, age of. .... _ .... _ .... _ .... __ ................ 59-60 

character and distribution of ............. __ ... _ .. 23, 58, 183-184, 383 

pegmatitic phase of ....... _ ....... _ .. _._ ......... _ ......... 181-183 

petrography of. ............ _ ....... _ .. _ .... _ ......... 58-59, 183-184 

sections of, figures showing ........ _._ .................... 59, 63, 183 

use of, for building ................. ___ .... __ ................. 91 

Silver l'oin t lode, description of. ............... _ ...... _ ......... _ · 206 

Silvertori quadrangle, ores of ............................ _ .. _..... 113 

Smelting, beginning of. ..... _ .. _ .. _ .............. _............... 173 

Smuggler mine, description of.. .. _ .... __ ......... _ .. _._.......... 22S 

view of. .......................... _ .. _....................... 228 

Snowdrift lode, accoun.t of. .............. _ . _ ......... _ ... _. . . .. 210-212 

production of. ................. _ .... _ ..................... _. . 212 

vein of. ............................................. · ........ 2•.~-221, 

waterin................................................ 139 

workings of, figures showing .. _ .... __ ._ .............. _._._. 210,211 

planof.......................................... 204 

Soda Creek, description of.._ .......... _ ..... __ ........... _. _ .... _ 30 

dikeson .................................................. 4.1,71,80 

goldon........................................... 313 

mineson ............................. ."........ 381 

placers on, view of. .............. _ ....... _ . _ .... _ ........ _ .. _ 312 

travertine on ................... _ .......... _ .... _.. 88 

Soda Creek group, mines of, description of. _ ....... _ .. _ .. _.. . . . 381-382 

Solutions. See Mineralizing solutions. 

Sonora vein, description of. ...... :.' ........ __ .. _ ..... _.......... 285 

South Douidcr Creek, gold on ........ _ ...... _ ........ _ ........ _.. 311 

rocks on .............................. __ ................. _... 44 

South Clear Creek, glacial till on .......... _.: .. _ .. _ .... _ .... _.... 84 

glacieron.......................................... 85 

gold on............................................. 311 

rocks on_.......................................... 37,75 

South Clear Creek canyon, dacite in ...................... _....... 75 

dacite in, section of, figure showing.......................... 75 

South Lincoln vein, description of. .................. _ ...... _ 356 

Span ish Peaks district, description of. .. · .......... _ . . . . . . . . . . . . . . 123 

Spartan vein, description of. ....... _ ............... _....... 296 

Spencer, A. C., on Wyoming rocks ............................ 32,46,56 

Spencer, A. C., and Cross, W .. on Rico quadrangle ....... _.:..... 114 

Split vein, description of. .... : ................................. 323-324 

Sporting Times mine, description of.............................. 282 

tunnel of, plan of. .................................. _.... .. .. 282 

Sprangle vein, ores from ...................................... 399,401 

Spring Gulch, dikes near.......................................... 56 

section of,-figure showing. . . . . . . .. .. . . .. .. .. .. . .. .. .. . .. . .. . • 72 

Springs. See Idaho Springs. 

Spurr, J. E., on Colorado intrusives ................. 49, 62, 69, 70, 71,119 

on glaciation !n Aspen district............. . ..... _......... 87 

on magmatic theory ..................................... 169-170 

on Nevada 'rocks ....................... _ ........ _ .... _ ..... _. 61 

outlineofrcsultsby .......................................... 23-27 

Spurr, J. E., and Garrey, G. H., on economic geology ... 97-382, 38tl-410 
INDEX. 421 

Page. 

Stanley Mountain, alteration in ................................ 150-151 

Stanley-Little Mattie group, mines of, descriptions of .......... 341-368 

Stan ley mine, description of. . . . . . .. .. .. .. .. . . .. .. .. .. .. .. .. .. . 341, 342 

dikes of............. .. . .. .. . . . .. .. .. .. . . .. . 135,157,343-344 

sections of, figures showing ............................. 344,347 

friction conglomerate in...... .. .. .. . . .. .. .. . . ........ 345-346 

levels of, plans of.. . .. . .. . . . . .. . . . . .. .. .. .. . . . . . . . . . . .. . .. .. . . 344 

mineralization of. ............... : .................. 344-345,346--347 

ores of.......................... .. .. .. .. .. .. .. .. . .. .. .. .. 348 

production of. .............................................. . 341 

veins of. ....................................... 130-131,150,341-343 

development of. ....................................... 343-349 

wallrockof. ........................................... 343,347-348 

workings of, plate showing. . .. .. . . .. . . .. . . . . . . .. .. . . .. . . . . . . . 342 

Star mine, description of......................................... 364 

Star tunnel, description of... . . . .. . .. . . . .... . .. . . . . . . .. . .. . . . . .. .. 366 

Steamboat Springs, waters of. .............................. 27,167-168 

Stocks, occurrence of. ............................................ 72-73 

Stranger vein, description of. .............................. 266,270-271 

Streams, adjustment of. ....................... ~................. 33 

Strncture, discussion of. ......................................... 89-91 

plate showing..................................... 138 

Sugarloaf, rocks at...................... . .. . . . . .. . . . . . . .. . . .. .. . 69 

Sullivan, E. C., on renJacement ................................ !60-161 

on rock alteration............................................ 243 

Sulphide ores, difficulty with.................................... 173 

Summit, rocks at. .............................. ·.·............... 70 

Summit County, ores of................................ 104 

Summitville, ores near.................... 116 · 

Hun burst mine, account of. .................................... 301-302 

ores of....................................................... 302 

veins of............................................ . .. 301-302 

wall rocks of. . . . . . . .. .. . . . . .. . . . . . . . .. . . . . .. . . . . .. . 301 

Sunbnrst-Sceptre vein, account of.·:........ . .............. 298-302 

workings on, plate showing............................... 298 

'T. 

Talus, character ami distribution of.............................. 88 

Teller mine, description of.......... . . . . . . . .. . . .. . .. . 322--:l23 

tunnel of, plan of. .. . .. .. .. .. .. .. .. .. .. .. . .. .. .. .. .. . :l23 

Telluride conglomerate, description of.... 111 

Telluride qnadrangle, ores of................ . 113-ll4,J69 

'rem perature, records ot ... __ ........... _ ..................... _.. 30 

Tenmile district, faulting in . . . . .. .. . . . . .. . . . . .. . . .. . . . . .. .. . . .. . 109 

intrusions in ................................................ . 110 

ores of.................................................... 104 

'l'enmile quadrangle, rocks of...... . .. . . .. .. .. .. .. . .. . . . .. . . .. .. . 70 

Tennessee Pass, rocks in ...................... _ ........ _......... 70 

Tenth Legion vein, <lescription of. ..................... 398,399,400,401 

Terrace gravels, description of ..... ............ __ . _ . ·_. 83-84 

Terrible lode, description of. ............................... 144,222--225 

section on, figure showing................................. 238 

view of...................................................... 228 

workings of, figure showing. . . . . .. . . .. .. .. . . .. .. . . .. .. .. . .. .. 222 

Terrible vein group, description of............................... 219 

movements in ......................................... 242,231-232 

ores of....................................................... 239 

veins of, composition of...................................... 227 

depth and. . . . . . .. .. .. . . . . .. .. .. . . .. .. . . .. .. .. . .. . . .. .. 239-241 

descriptions of........................................ 219-232 

nature of.. . . . . .. .. . .. . . .. . . . .. . . . . . . .. . . .. . . . . .. . . . . .. . 22&-229 

structure of. . . . . .. . .. . .. . . . . . . . . .. . . . . . .. .. . . . . .. . .. .. . 229-232 

wall rock of. . . .. . . .. .. .. . . .. . . .. .. .. . .. . .. .. .. .. . . .. .. . . .. . 232-239 

waters of.. . . .. .. . . .. .. . . . . .. .. . . . . . . . . . . . . . . . . . . . . .. . . .. .. . 242-244 

workings of, map showing................................... 218 

Tetrahedrite, genesis of.......................................... 261 

Tilden vein, description of. ................................ 267,270,271 

Tincup district, ores of....................................... 106 

Tishomingo-lode, description of. ....... · ....................... 225-226 

Titanite, age of............................................... 96 

sections of, figures showing.. . . . . .. .. .. .. .. .. .. .. . . . . . . . .. . 96 

Toledo mine, plan of, plate showing........................... 326 

vein of.... .. .. .. .. .. .. .. .. .. .. ............. 324-325,327,330,331 

workings of, plan showing................................... 325 

Tonopah district, Nevada, ores of................................ 170 

Topography, description of. .................................... 30,383 

evolution of. ...................................... · .......... 31-34 

Page. 

Topography, maps showing ..................................... 30,36 

relation of, to rock fonnations .. ______ . __ . _ .. _ .......... __ . _ .. 35-36 

Torpedo mine, description of.. ................ :.. . .. . . . . .. .. . . .. . 380 

·Trail Creek, rocks on ... _ .. .. . . . . .. . . .. .. . .. . . .. .. . . . . . . . . 42 

TtaYertine, occurrence and character of ___ ... _ .............. _.... 88 

Trout Creek, rocks on.......... .. ..... . .......... ........ 70 

Truesdale Creek, rocks near .... : ............. :.. .. .. .. .. .. .. .. . . . 57 

u. 

Underhill, James, on Colorado dikes ................... •.......... 69 

Union tunnel, description oL .................................. 405-406 

plan of, fignre showing ..................................... 221,405 

Upper East Geneva Creek, glacier on............................ 8.'i 

Upper Twin Lake. rocks near.................................... 70 

Ute Creek. rocks on....... .. .. .. .. .. .. .. .. . 64-65 

Ute Creek-Chicago Creek group, mines of, descriptions of. ...... ~68-380 

v. 

Valleys, incised, description of. ................................. :JZ-33 

Yan Diest, r. TT., on Colorado intrusives ................. :.. 68 

Van IIise, C. R., definitions by. . . . . . . . . . . .. . . .. . .. .. . . . . . . . . .. .. ~8, 56 

on geology of region. .. .. . . .. . .. .. . . . . . . . .. . .. . . . .. .. .. .. .. . . . 44 

Vegetation, character of. .......... :..... . ................... 30-31 

Vein channels, developrrmnt and cementation of. .............. 25~256 

Veins, cementation in ... __ ._ ... __ ... _ ...... __ ... _ .............. _. 107 

classification of.............................................. 99 

compound character of...... .. .. . .. . . . . .. . .. . .. .. .. .. . . .. .. . 135 

dikes and, relations of.. . .. , ..................... 1:J0-132, 146--147 

faulting and intersection of.. ........................... 101,108, 159 

formation of. ......... ·.. .. ............. 24,107-108,146-147,159 

mineral characteristics of. ................................... 99-101 

limestone replacements and.................................. 108 

location of, map showing ................ _............ 102 

materials of, derivation of. ................................. 140-142 

minerals in. See Minerals. 

movement in ..................................... 101,108, 149-1SO 

occurrence of.. ......................................... 102-106,107 

relati vc age of. ................. , ......................... 24, 99-101 

types of...................................................... 99 

See also particulnr mines. 

Veins, gold and silver bearing, distribution and interrelations of. 97-100 

See also Gold-bearing veins; Silver-bearing veins. 

Vice-President lode, account of..: ............................. 212--213 

orcs of. . . . . .. . .. .. . . .. . . . . . . . . .. . . . . . . . . .. .. .. . .. .. . . . . . .. .. . 213 

tunnel or, figure sho\1~ng..................................... 212 

vein of. ....... _,........................................ 212 

Victoria tunnel, waters in ...................................... 139,243 

Virginia Canyon, view of.. __ . ........... · ... _.................... 96 

Virgini£1 City mine, dBSCription of ....... _ ........... _ .......... 31o-311 

veins of. ................................................... 100,311 

Volcanic eruptions, mineral deposits connected with............. 25 

Volcanism, age of. ........................................... :... 120 

centers of. ............................................ 123-129,132 

duration of........... . ........................ 119-122, 12&-129 

Vulcan district, description of. ................................. 110-111 

Vulcan lode, dcscri ptioi:l of... .. . .. .. .. .. . . .. .. . .. .. . .. .. .. .. .. .. . · 210 

w. 

Waldorf mine, galena ])len de from, plate showing. . . . . . . .. .. . . .. . 136 

Wallace claim, description of..................................... 370 

Wall rocks, alteration of. .......................... 143,150-152,165,168 

nature of................................................... 99 

See also particular mines. 

Waltham mine, description of. .................... :............. 380 

tunnels of, view of. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 354 

Warren Gulch, rocks in.......................................... 61 

Washington, lL F., on igneous rocks............................. 33 

Washington lode, dcscri ption of... . .. .. .. .. .. .. .. .. . .. .. .. . . .. .. . 297 

Waters, spring, composition of. ................... : ............ 164-166 

Waters, underground, channels for............................... 159 

effec(,s of. .................................. 138-140,154,158,165-171 

See also Metamorphism; particular mines. 

Weathering, work of............. ............................... 35 

work o[, fignre showing...................................... 36 

Weed, W. H., and Pirsson, L. V ., on igneous rocks.............. 83 
422 INDEX. 

Page. Page. 

Welsh mine, description of....................................... 275 Wisconsin-Corry City lode, description of ..................... 195-197 

·oresof....................................................... 1M ores of, depth and.......................... .. .. .. .. .. .. .. .. 197 

West Chicago Creek, glacier on................................... 85 vein of .................................................... 195-196 

rocks near ................................................ 66, 71,81 wall rock in . .' .............................................. 196-197 

West Geneva Creek, glacier on. . . .. . .. .. . .. . . .. . . . .. . .. . .. .. . . .. . 85 workings of, figure showing. . . . . .. .. .. .. .. .. .. .. .. . . .. . . .. . .. , 195 

White Pine lode, description of... . . .. .. .. .. .. . . . .. .. . .. . . . . . . 308 

Wide West lode, account of. ................................... 271-274 y_ 

description of .............................................. 271-272 

ores of. .................................................... 272-274 Yukon district, mineralization in_ .. _ ...... _ .................... _ 169 

plans of, plates showing ..................................... . 270 

veins of._ ... __ ...... ______ .... ________ .. _____ ............. __ . 273 z. 

viewof........................................ ............. 264 Zinc, precipitation or............................................ 244 

wall rock of. ................. :............................... 272 Zinc mining, development of..................................... 23 

Wild Rose mine, description of.................................. 364 Zircon, age of................................... . 183 

Williarns1 G. H., on micropegmutite ........ . 95 occurrence and character of, ... _ ........................... 182-1&1 

Wisconsin lode, workings of, plan of .......................... Pocket. .Zone of weakness, mineral!zation and .......................... 107-108 

See also Wiscopsin-Corry City lode. origin of. . . . . . .. .. .. .. .. .. .. .. .. . .. .. .. .. .. .. .. . .. .. .. . .. .. .. i18 

0 
SEP 2 5 1963 

LIBRARY 

8i1Pftlil Of VIN£1 

~B~ARY 

SPIJI\AIH. hASH. 

JUN - 11971 

IH l A s f It fT iJ R N 

&o ue·a AI~ 

j