UNITED STATES DEPARTMENT OF THE INTERIOR 

Harold L. Ickes, Secretary

GEOLOGICAL SURVEY 

W. C. Mendenhall, Director

Professional Paper 176

GEOLOGY AND ORE DEPOSITS

OF THE

BRECKENRIDGE MINING DISTRICT, COLORADO

BY

T. S. LOVERING

UNITED STATES

GOVERNMENT PRINTING OFFICE 

WASHINGTON : 1934

For sale by the Superintendent of Documents, Washington, D.C. ........... Price $1.00

CONTENTS

Abstract.._....____.._.___.____._________.

Introduction. ______-_____--____--.________

Scope of the report.____ _______________

Field work and acknowledgments..------

Location and topography _______________

History of mining..__-._..-_-..----..__

General geology___...__-_--_--_.___._--... 

Maroon formation_--_---_-_--__-_----.

Morrison formation____________________

Dakota quartzite______________________

Benton shale._-_______.--____.__---...

Niobrara formation ____________________

Pierre shale_---_--__--------------_---

Quaternary gravel-__------__------__--

Early glacial deposits..-__---__-----

Late glacia) deposits___"___._________

Recent deposits____--____---_--___.

Igneous rocks.-_______-__-_---__--._--

General features._--_--_-_-_-_.____

Monzonite porphyry____-__..-__.__

Intermediate type.._-___--_-_---_-- 

Quartz monzonite porphyry.........

Rock alteration..____-_________._--.._.

''Contact metamorphism"_.......__

Hydrothermal alteration..____-_.___

Structure ______--__-_-----_--__------.

Regional features._________________

Local features._-_-__--_--__-_.--__

Relation to Front Range mineral belt.

Geologic history___________________________

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Mineralogy ___-___________--__-_-_____--_____---._ 23

Ore deposits._--______.___---__--__^_-__-._.__---._ 24

Classification-----_____--__------__-_----_----- 24

High-temperature ("contact-rnetamorphic") deposits ______________________________________ 25

Moderate-temperature (mesothermal) deposits _.___ 25

Stockworks.___---_-_-_--_-__---_____---__ 25

Veins...__________________________________ 26

Relation of veins to country rock._______--__ 26

Ore channels._____-_______-_.___-_-_--_-__ 26

Character of the primary ores-_.____..______ 27

Relation of veins and ore to topography. _____ 28

Supergene enrichment--_-----__-_____________!_ 28

Criteria of enrichment in the Breckenriclge district......__._.._-._-..._.__.___-_._ 28

Enrichment of zinc_____-______._______--_-__ 29

Enrichment of lead.___.__--____-_________._ 29

Solvents of lead minerals.....__.-____-___.-_ 30

Leaching of the veins.._____________________ 30

Precipitation of lead.._-___-._____-___----.__ 31

Probable reactions in the vadose zone. _______ 31

Enrichment of gold________________________ 32

Blanket deposits..__________._-._-___________-- 32

Relation to country rock-_._________-__-_-._ 32

Relation to structure.______________________ 33

Character of ore____--------------_-------- 33

Mine descriptions.________-_-___-_______-_____---__ 33

Guides to ore and future of the district_______________ 59

Production of mines in the district________-____.-__-_ 59

Index._____...______._._.--...._.._._....___.__.. 63

ILLUSTRATIONS

Page 

PLATE 1. Map of part of the Breckenridge district, Colorado, showing some of the principal mine workings and known faults

and veins-_--_-------__-_---------_--_---_--_---_--_----------__---------------------_------- In pocket

2. Geologic map of part of the Breckenridge district._____-__--__--_---_____________--_--------_____-__ In pocket

3. Correlation of formations in the Breckenridge district.-,.---__-_-__-------__-____-------__----___------- 10

4. A, Upper member of Dakota quartzite; J5, View looking east up French Gulch from north slope of Nigger Hill.. 10

5. Structure sections of part of the Breckenridge district..-_____---_--_--_____._____---_-__--_---..--___-- In pocket

6. A, Ore from Spur vein, Wellington mine; B, Photomicrograph of ore shown in A.------------------------- 26

7. A, Late sphalerite cementing and replacing brecciated galena, Spur vein, Wellington mine; B, Late galena following crystal boundaries of early sphalerite, Wellington Main vein, at winze, level 8; C, Replacement ore 

from level 8, Wellington mine.____--_----__-_-_-_____------_____-_-___-__.-------_---___--_._-----. 27

8. A, "Roll" in thin-bedded Dakota quartzite, Detroit mine; B, Galena altering to cerusite, Detroit mine.----. 34

9. Plan and section of Juniata mine.___--___-______-_____-_-_-____-_-___--_____---------_--_---___.---_- 42

10. Geologic and assay map of Puzzle, Golddust, and Pacific veins.__-_--____--_____---_--------__-_______--- 42

11. A, Open stope on Puzzle vein; B, Wellington mills and mine in 1928.._____________-----__--_--______---_- 42

12. Stope maps of Puzzle and Golddust veins.._-________-_______________--_-______--___---_--__-_-___-___- 42

13. Mine map of Oro and Wellington workings.__-______--.__.---__-_--_-____-__-_-_-----_._---_-_._-_- In pocket

14. A, Main vein, Wellington mine; B, Partial replacement of shale wall of Main vein__--_--_-----_---_-.---__ 42

15. Geology and sections of parts of Oro and Wellington mines.__________________________________________ In pocket

FIGCJHK 1. Index map of Breckenridge district._-_-_______.________----___--____-.____-__--___---__--_-__._-----.

2. Columnar section for Breckenridge district------..-----------------------.___---------------_----.----.

3. Correlation of sections measured from Boreas Pass to Keystone.___-_-___-_______--__-----__--__--__-_--.

4. Correlation of sections measured at Palmer ranch and Keystone._---__-__--__-__--__-----_------_-_---_-.

5. Generalized section of Farncomb Hill gold vein.._______________________________________________________

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IV ILLUSTRATIONS

Page

FIGURE 6. Map of Detroit mine._________________-_-_--______-_-_____-_-___-----_----------_-___--__--__--_-_-. 34

7. Geology of Detroit mine..______________---_-___--___-_---__--_____-_------_---_---__--__-___-_-_-..__ 35

8. Development of "roll" in Detroit mine....__________________________________ _ ________________________ 36

9. Detail of "roll" in Detroit mine-______-_----_-_-_.________--_____________-______-___-____-___-----.__ 37

10. Galena ore in shale, Detroit mine.__._________-_-____________--__--------_--------_-_-___-_-__-----__. 37

11. Map of Dunkin mine..______________..___---..__________-_____-_______-----_-_-___-_-___________--..___ 38

12. Occurrence of rich gold ore in Dunkin mine.___________-_______-_____-------------_-___-_-___._-__.-__. 39

13. Plan of the Germania and South Elkhorn tunnels in Little Mountain.__-_-_-____------___-_-__-_-_-----_-_ 41

14. Cross section showing mode of occurrence of a rich pocket of oxidized gold-silver ore in the quartzite of Little

Mountain _____________-__-_____--___..-_--_______________-____-_---_-_-_-----_-___-___-_____--,_-_ 42

15. Sublevels of Golddust mine____________.______________.-__-___-_______-__-_____---._.-________-___-___ 43

16. Occurrence of ore in Golddust vein....___-_---__-____________---___-__--_---_---___.__._.__-.__-____-_ 44

17. Map and sections of Washington and Mayo mines_----_-_-_______-__------------_-_-__-__---___-----___ 45

18. Main vein, Wellington mine, paralleling intrusive contact of cross-breaking monzonite porphyry.____.--_-.__ 47

19. Fault later than Puzzle vein fissure but in part premineral.____-_-_--__--__-__-_-_--_____-_-__-_-.-_-_-__ 50

20. Generalized surface map of chief veins and faults of Wellington and Oro mines_____-_-____-_--_____-__--.__- 51

21. Stope map of Great Northern vein..___________________________________________________________________ 52

22. Stope map of main ore shoot and shale shoot on Main vein___-________---_--_-. ___-._._________._--_____ 54

23. Stope map of Puzzle vein____________________________-_-__--_-._..-_--_-_-___________._..._.__I-_____ 55

24. Ore enlargement at premineral fault, level 7, Oro mine.___________.____.___-__-_---_._________.____.____ 57

25. Enlargement of ore as vein approaches premineral fault, level 3, Oro mine_----.---_-___--_-.-_-_..___-__._ 57

26. Local enlargement of ore at intersection of branch vein, level 6, Oro mine..._-.______._.__._____.___.____.._ 58

27. Fracturing of interbedded shale and quartzite, level 6, Wellington mine.. _--__--_--__.____.___..__-----___ 58

28. Brecciated marmatite cemented by ankerite, level 7, Oro mine._____.._-_---_---_--..____.________-__-___ 58
ABSTRACT OF REPORT

A small portion of the Breckenridge mining district, originally 

mapped by Ransome, was remapped by the writer on a larger 

scale. In addition to the geologic data supplied by deeper 

mining since the earlier report was written, information gained 

in a study of the surrounding region has helped interpret the 

stratigraphy and complicated structure of the district itself.

Pre-Cambrian schist, granite, and granite gneiss underlie the 

sedimentary rocks of the district and come to the surface in the 

northern and southern parts of the area mapped. The lower 

Paleozoic beds are not present. The Maroon formation (of 

Permian and Pennsylvania!!? age) lies directly upon the preGambrian rocks in the Breckenridge district. It is missing a 

short, distance farther east, where it is overlapped by the Morrison and Dakota formations, but it thickens greatly to the 

south and southwest. The underlying Weber (?), if present, 

occurs only under a thick cover in the southern part of the 

district. The Maroon formation consists of 600 to 900 feet of 

rod, gray, and black micaceous sandy shale with some limestones and some red and gray micaceous grit and conglomerate. 

The Maroon is overlain by about 200 feet of sandstone and 

variegated shale belonging to the Morrison formation, of Upper 

Jurassic age.

The Upper Cretaceous Dakota quartzite, which rests upon 

the Morrison formation, ranges from 125 to 165 feet in thickness in the special area mapped by the writer. In the surrounding region it ranges from 20 to 225 feet. Probably 

because of its rigidity and its ability to contain open fractures, 

ore bodies are commonly localized in the quartzite where veins 

pass through it.

The dark Benton shale overlies the Dakota quartzite and is 

about 360 feet thick. At the top of the Benton is a bed of 

shaly limestone about 25 feet thick, which is probably the 

equivalent of the fNio-Benton sand 1 of North Park. The 

Niobrara formation is about 350 feet thick and consists of black 

or gray limy shale interbedded with thin-bedded dark limestone.

Overlying the Niobrara formation is the Pierre shale, which 

has a thickness of about 4,000 feet near Keystone, 10 miles 

north of Breckenridge, but only the lower 500 feet is present 

in the special area. It consists chiefly of dark olive-colored 

and dark-brown clay shale.

Quaternary glacial and stream deposits rest upon the uneven 

surface of the older rocks in the Breckenridge district. The 

outwash from early Pleistocene glaciers forms high-terrace 

gravel. Moraines and outwash of the late glacial (Wisconsin) 

stage are also present. Valuable gold placer deposits have been 

found in the early glacial and late glacial outwash gravel and 

in the late glacial ground moraine.

Intrusive monzonitic porphyry of early Eocene age is common 

in the Breckenridge district. Most of the porphyry is a true 

monzonite in composition, but both diorite porphyry and 

quartz monzonite porphyry are also present. The monzonite 

porphyry generally occurs as sills, some of which are very thick, 

and only in a few places does it break across the sediments. 

It is cut by dikes and stocks of coarsely porphyritic quartz 

monzonite. A porphyry intermediate in composition and appearance as well as in age between the typical coarse-grained

1 A dagger (t) preceding a geologic name indicates that the name has been abandoned or rejected for use in classification In publications of the U.S. Geological 

Survey.

quartz monzonite porphyry and the monzonite porphyry occurs 

in sills, dikes, and small stocklike masses. In the remapped 

area a small stock near French Gulch has caused contact rhetamorphism in the limy shale of the Morrison formation nearby, 

and similar contact metamorphism has taken place elsewhere 

in the district.

The hydrotherrnal alteration of the wall rocks of the veins 

was thoroughly studied by Ransome, and little has been added 

to his conclusions. The solutions were of reducing character 

in most places, and the bright red sediments of the Maroon 

formation are changed to green or gray in contact-metamorphic 

zones and near ore channels. Where alteration was intense, 

silicification occurred on a large scale, and much of the dark 

Cretaceous shale near the Wellington mine has been completely 

converted into jaspery silica, similar to the flint or jasperoid of 

the Leadville district. Near the veins the porphyritic rocks 

have in places been completely altered to sericite, quartz, and 

ankerite with minor amounts of pyrite. A more widespread 

type of alteration is that in which the ferromagnesian minerals 

of the porphyry are changed to calcite and epidote and the 

feldspars are incompletely converted to calcite, sericite, and 

kaolinite.

The sediments of the Breckenridge district lie in an asymmetric syncline in which the general dip is toward the east. 

At the edge of this trough, 5 miles east of Breckenridge, the 

sediments are limited by an overturned fold that passes northward, through an east-northeast shear zone, into the large 

Williams Range thrust fault. At Keystone, 10 miles north 

of Breckenridge, this thrust fault has a known horizontal displacement of 4- miles. The trough of the syncline has been the 

site of much intrusive activity from South Park to the region a 

few miles south of Keystone. Much of the igneous rock occurs 

in large sills of diorite and monzonite porphyry, but stocks of 

quartz monzonite porphyry are also present.

The remapped area of the Breckenridge district is on the 

western limb of the regional syncline about 2 miles west of its 

axis. In the southwestern part of this area the prevailing 

northeasterly dip is broken by an open anticline and a com > 

pressed syncline, both of which trend northwest. Two broken 

down-faulted belts cross these folds and intersect near the 

central part of the area. One of these belts trends northnortheast and the other east-northeast. Both normal and 

reverse faults are common. The most productive mines of the 

district, the Wellington, Washington, Golddust, Dunkin, and 

Puzzle-Ouray, are situated in the east-northeast down-faulted 

belt. The veins in these mines follow premineral faults of small 

throw. Nearly all the productive veins strike between N. 40° 

E. and N. 80° E., and the fissures that they follow are generally 

older than the faults that strike N. 10° W. to N. 20° E. The 

bulk of the porphyry of the district was intruded after folding 

was completed, and part of it is later than the initial eastnortheast faults, which were later mineralized. The large 

thrust fault at the east side of the district is earlier than the 

coarsely porphyritic quartz monzonite, but much of the faulting 

is later than the latest porphyry masses.

A study of the regional geology shows that the Breckenridge 

district lies in a northeastward-trending zone of weakness which 

first localized the intrusive activity and, slightly later, the 

mineralization of the region. This zone is known as the por-
VI ABSTRACT OF .REPORT

phyry belt or the mineral belt and extends northeast across the 

Front Range to Boulder, Colo. This structurally incompetent 

belt comes to the western edge of the Front Range, where the 

Williams Range thrust fault breaks from the overturned fold. 

In the Breckenridge district the mineral belt is approximately 

6 miles wide, and the complex fault mosaic represents largely 

the response of interbedded incompetent rocks to an eastnortheast compression localized along a 6-mile front. The ores 

are largely confined to open premineral faults, but many of the 

veins are related to channels of mineralization that trend more 

nearly north than the veins. This relation is well illustrated 

by the occurrence of ore in the Wellington mine.

A wide variety of ore and gangue minerals has been found in 

the Breckenridge district, but the only ore minerals sufficiently 

abundant to be of commercial importance are gold, silver, 

sphalerite, galena, and pyrite. The primary mineralization is 

believed to be related to the solidification of the deeper parts of 

the quartz monzonite porphyry and is later than any of the 

porphyry intrusions. Enrichment has greatly modified many 

of the deposits. Contact-metamorphic ores, stockworks, veins, 

blanket ores, and placers occur.

Contact-metamorphic deposits have little economic significance in the Breckenridge district, but dense, irregular masses of 

silicates, intergrown with metallic oxides and sulphides, occur 

in several places and are best developed in the limy beds of the 

Niobrara and Morrison formations. These deposits are resistant 

to weathering and show little enrichment. Some of them contain 

appreciable amounts of gold and copper, but the ore bodies are 

usually small, spotty, and irregular in plan. They cluster 

around the stock of quartz monzonite porphyry to which ore 

deposition is closely related.

Stockworks are found chiefly in the northeast quarter of the 

Breckenridge district in a large mass of quartz monzonite 

porphyry outside of the remapped area. They are probably 

mineralized shattered zones formed in response to residual 

stresses in the shear zone between the major thrust fault and the 

overturned fold to the south. The country rock of the stockworks has been fissured in many directions, but the strongest 

and most persistent fractures are those that strike east to 

northeast. The primary ores are generally low-grade pyritic 

gold ores, containing small amounts of galena and sphalerite. 

Most of the ore occurs in small open fissures, and there has been 

little replacement of the country rock. Enrichment has been a 

notable factor in the upper part of the stockworks.

Most of the productive veins lie in a short, narrow eastnortheastward-trending belt that extends from Little Mountain 

to Mineral Hill. With the exception of the rich narrow gold 

veins of Farncomb Hill, very few veins outside this belt have 

contributed materially to the output of the camp. Most of the 

veins occupy normal faults striking N. 40°-80° E. and dipping 

60°-80° SE. or NW. They show pronounced irregularities in 

both strike and dip.

All the bedrock formations in the district are cut by veins, 

but most of the ore has been found where the vein walls are 

monzonite porphyry or Dakota quartzite. Minor quantities 

have also been produced from veins in each of the other formations except the Morrison, which has yielded little or none. 

The influence of the wall rock is largely physical. Faults passing through porphyry, quartzite, and silicified shale commonly 

have open fault breccias but on passing into unaltered shaly 

formations are marked by soft impervious clay-filled fissures. 

The chemical character of the wall rock has been less influential 

than its physical properties in determining the presence of ore, 

but certain limy layers in the Cretaceous beds are replaced 

by ore next to some of the larger veins. The ore shoots 

were localized in the more open parts of faults of moderate 

movement.

In the remapped area the primary ores consist largely of lead, 

zinc, and iron sulphides with some native gold and some silver 

whose form is uncertain. Ankerite, calcite, quartz, and sericite 

are the common gangue minerals. Some of the veins consist 

chiefly of pyrite and sphalerite and contain very little lead.

The bulk of the ore produced from these veins is pyritic zinc 

blende, which typically changes abruptly downward into nearly 

pure pyrite. More abundant than the ore of this class are the 

primary zinc-lead ores, which have a much larger proportion 

of galena throughout the vein and which commonly pass 

upward rather abruptly into a zone of sphalerite-free galena 

near the surface. The sulphides are generally massive and 

show no evidence of depositional banding or crustification.

Ore shoots made up chiefly of the more readily removed 

minerals, pyrite or sphalerite, show a negative relation to 

topography and occur below the zone of Tertiary oxidation. 

Gold ore and high-grade galena ores, however, appear limited 

to a zone that ranges from 200 to 300 feet in depth, corresponding roughly to the present topography. The mixed zinc-lead 

ores have a much greater range than the high-grade lead ores, 

and in the Wellington mine they have a vertical range of more 

than 800 feet. Practically no high-grade zinc ores have been 

found in the upper parts of veins that come to the surface in 

areas where preglacial topography is well preserved. The 

largest zinc shoots have all been found several hundred feet 

below the level of the Tertiary erosion surface, and their position probably reflects the read}7 removal of zinc sulphide from 

the zone above the Tertiary ground-water level.

A study of the topographic, mineralogic, and textural relations of the ores indicates that the presence of high-grade lead 

ore shoots near the surface is due to the leaching of the more 

soluble sphalerite and pyrite from a primary lead-zinc ore and 

that there has been little or no secondary deposition of the 

galena itself. The enrichment of the gold ores is easily demonstrated, and it is unquestionably concentrated by the agency 

of surface waters moving down toward the water table.

The mineral deposits that follow bedding planes, locally 

known as "contacts", are practically confined to Gibson Hill, 

Shock Hill, and Little Mountain and occur in replaceable beds 

of the Dakota quartzite and Maroon formation. Both primary 

and secondary ores of this type have been mined, but the most 

valuable concentrations have been apparently caused by the 

action of surface water. The most favorable situation for these 

replacement deposits seems to have been at the upper termination of minor veins.
GEOLOGY AND ORE DEPOSITS OF THE BRECKENRIDGE MINING

DISTRICT, COLORADO

By T. S. LOVERING

INTRODUCTION

SCOPE OF THE REPORT

In 1909 the Breckenridge district was studied by 

F. L. Ransome, assisted by E. S. Bastin. The results 

of this work appear in Professional Paper 75 of the 

United States Geological Survey. Ransome concluded 

that most of the essentially lead ore was confined to 

shallow depths, and, as the metallurgy of mixed leadzinc ores had not at that time been satisfactorily 

solved, he was not optimistic over the future of the 

district. At the time of his study there were no accessible underground workings that were more than 400 

feet below the surface.

The structure of the district is very complex, and 

much of the geology is hidden beneath a deep mantle 

of slope wash on the hills and a thick cover of alluvium 

in the broad valleys of the Blue River, French Creek, 

and the branches of the Swan River. It is obvious 

that a geologic map of 45 square miles in such a region, 

completed in 3 months of field work, must be more or 

less generalized. The extensive development of the 

Wellington mine, the additional work in the Detroit, 

Golddust, and Puzzle mines, and the large amount of 

dredging that has taken place since 1909 have supplied 

many significant geologic facts not available at the 

time the earlier report was written. The present study 

records new data and reviews the geology in the light 

of these new data as well as additional information 

gained in the surrounding region. A small portion of 

the district, which is the subject of this paper, was 

remapped on a scale of 1,000 feet to the inch, but only 

the geology of the bedrock surface, which lies from 1 

to 80 feet below the actual surface, is shown on plate 2. 

In constructing the map the writer used underground 

data wherever they were available and then plotted 

the bedrock geology from the facts revealed by subsurface work. Little reliance can be placed on the 

materials found in the hillside wash as indicators of 

underlying bedrock, and only the large, number of 

prospect holes sunk through the wash enable one to 

map the geology in much of the special area studied. 

The extensive dredging on French Creek has exposed 

bedrock fragments on most of the tailing piles. With 

a little study the weathered, angular fragments of bedrock can readily be recognized, and with allowance for 

the length and swing of the dredge a very satisfactory 

map of the underlying formations can be made. Such 

work outlined the quartz monzonite stock half a mile 

east of Breckenridge that is believed to be intimately 

related to the source of the adjacent ore deposits and 

was of great value in correlating the geology and structure on the north and south sides of French Gulch.

The stratigraphy has been revised, and 'the new subdivisions of the formations, shown on the special map, 

make it possible to give a more detailed picture of the 

structure than was possible on Ransome's smaller-scale 

map. The folio wing are the outstanding changes: The 

fWyoming formation is correlated with the Maroon 

formation of the areas to the west; the Morrison formation is separated from the upper part of the f Wyoming and the lower part of the Dakota as previously 

mapped in this region; the true Dakota (here chiefly 

quartzite) was studied in detail, its variations in tliickness are considered with respect to its general distribution, and its overlap of the older formations is noted; 

the Upper Cretaceous shale of Ransome's report is 

subdivided into the Benton shale, the Niobrara formation, and the Pierre shale; and detailed sections of all 

the formations except the Maroon are given.

The detailed structure of -'the small special area is 

discussed and its relation to the regional structure is 

brought out. The problem of the enrichment of lead 

is considered, and experimental work on the solution 

and precipitation of lead compounds.is presented; as a 

result it is concluded that moderate amounts of lead 

have been dissolved and reprecipitated in the form of 

cerusite but that very little supergene galena has been 

formed. Mine maps and stope maps of most of the 

mines in the special area are shown, and their geology 

is discussed briefly. The' study of the Wellington 

niine, the largest producer in the Breckenridge district, 

is given in more detail, as the occurrence of ore in this 

mine suggests many factors that bear on the localization of ore shoots, and the conclusions reached find 

wide application in the district.

FIELD WORK AND ACKNOWLEDGMENTS

The surface geology of the special area of the Breckenridge district was mapped by the writer in the inter-

 . , - . 1
BRECKENRIDGE MINING DISTRICT, COLORADO

val from May 27 to June 28, 1928, and the underground 

geology of the mines was studied at different times 

during 1927 and 1928, probably consuming about five 

additional weeks. Mr. L. B. Graff and Mr. H. W. Putnam, as field assistants, contributed materially to the 

speed of the surface work. It is impossible to name 

everyone to whom the writer should make acknowledgments for help and information on the district, but 

special mention should be made of the assistance and

of Denver and 20 miles northeast of Leadville. The 

district mapped by Ransome lies between latitude 

39°27' and 39°33' and longitude 105°57' and 106°4' 

and covers 45 square miles. The small part of this 

region restudied by the writer is near Breckenridge, 

where most of the chief productive mines of the district are situated. In this report it is distinguished as 

the special area; its relation to the larger district 

mapped by Ransome is shown in figure 1.

FIGURE 1. Index map of Breckenridge district showing area restudied for this report (dark shading).

courtesies extended by Mr. R. M. Henderson, Mr. H. 

L. Tedrow, Mr. George Robinson, Mr. Neucombe, 

and Mr. John Nelson. The writer was freely admitted 

to all the mines in the Breckenridge district that were 

open at the time of his visit except the Royal Tiger.

LOCATION AND TOPOGRAPHY

The Breckenridge district is only a few miles northwest of the geographic center of Colorado and lies on 

the headwaters of the Blue River, a tributary of the 

Colorado River. Its relation to nearby cities and 

towns is shown in figure 1; it is about 60 miles west

The district mapped by Ransome ranges in altitude 

from 9,100 to 13,100 feet, and to the east, west, and 

south the mountains attain even greater heights. In 

general the district is decidedly hilly, although little 

of it would be described as rugged. The area covered 

by this supplementary report includes only a few hills, 

of minor prominence. The lowest point is on the Blue 

River at an altitude of 9,500 feet, and the highest is 

near the top of the hill a mile east of Bacon, at 11,500 

feet, a difference in altitude of 2,000 feet.

French Gulch follows a westerly course across the 

northern part of the special area and separates Gibson
GENERAL GEOLOGY

Hill and Prospect Hill on the north from Nigger Hill 

and the northwestern spur of Bald Mountain on the 

south. (See pi. 4, B.) South of this spur and Nigger 

Hill the special area is crossed in a northwest course 

by Illinois Gulch. Little Mountain and the unnamed 

ridge southeast of Rocky Point lie between Illinois 

Gulch and the Blue River. The relation of these 

streams to one another is shown in figure 1, and the 

details of the topography can best be seen on plate 1.

HISTORY OF MINING

Ransome 3 gave a summary of the history of the 

Breckenridge district from 1859 to 1909, and Henderson 4 has recently given a detailed account of the mining activities in the district from 1859 to 1924. With 

these two excellent histories of the district easily available, it would be useless repetition for the writer to 

give a detailed account of the development of the district. However, a summary of the most significant 

events is given here before the geology and the individual mines of the special area are considered.

A group of prospectors discovered rich placer ground 

on the north side of Farncomb Hill in the summer of 

1859, and within the next 3 years $3,000,000 in gold 

was Avashed from the placers in Georgia Gulch, the 

Swan River, Gold Run Gulch, Galena Gulch, American Gulch, Humbug Gulch, Delaware Flats, the Blue 

River, and French Gulch and its tributaries Gibson, 

Nigger, Corkscrew, Illinois, and Hoosier Gulches. In 

1870 there were 100 miles of ditches and flumes in the 

district and $0,000,000 had been recovered from the 

placers. During the next 10 years the output of gold 

dwindled to a little less than $1,000,000, all of which 

came from placers. In 1880 gold was found in place 

on Farncomb Hill, and for the next 10 years the rich 

narrow veins of crystallized gold, for which the district is justly famous, yielded most of the gold output. Gold-bearing lodes were discovered in other 

parts of the district soon after those on Farncomb 

Hill were uncovered, and veins were the chief source 

of gold until dredging began on a large scale. Gold 

dredges were introduced in 1898 and have been working the deep gravel of the large gulches ever since. In 

the first few years of their operation they were largely 

experimental and did not contribute materially to the 

production of the district, but the construction of 

larger and stronger boats enabled the later dredging 

enterprises to work large tonnages of the coarse gravel 

successfully. It is probable that $7,000,000 in gold 

has been produced by dredging low-grade placers in 

the district.

The earliest attempt to mine lead-silver ores in the 

Breckenridge district occurred in 1869, when some 

argentiferous lead ore was taken from the. Old Reliable

3 Kansomo, F. L., Geology and ore deposits of the Breckenridgo district, Colorado: U.S. Gcol. Survey Prof. Paper 75, pp. 17-20,1911.

'Hendersou, C. W., M'iuing in Colorado: U.S. Qeol. Survey Prof. Paper 138, pp. 

32-30, 227-2245, 1020.

vein, in French Gulch near Lincoln, about 3 miles east 

of Breckenridge. A small amount of lead was smelted 

here in 1873-75, but little interest was shown in lead 

veins until the completion of the railroad from Denver 

to Breckenridge in 1880. The advent of cheap transportation greatly stimulated lode mining, and within 

a few years most of the mines that have produced lead 

ore were developed. The first flush of production 

came in the late eighties and early nineties, and after 

this period the production of lead showed a decided 

downward trend until 1910. At this time the Wellington mine started active development, and it was a large 

and moderately steady producer of lead and zinc up 

to 1929. The output of this mine has made up the 

bulk of the lead and zinc ore shipped from the Breckenridge district in the period 1910 to 1929 and compares 

favorably with the large production of the district in 

the early nineties.

Tables showing the production of individual mines 

are given on pages 60-62.

GENERAL GEOLOGY

The general geology of the Breckenridge district 

has been discussed by Ransome,5 and in the following 

pages the formations will be described only briefly 

unless information can be given that is not found in 

the earlier report.

Sedimentary rocks in the Breckenridge district include formations from the Pennsylvania!! (?) to the 

Upper Cretaceous. Their character, approximate 

thickness, and relations are shown in figure 2.

As a result of detailed studies in the district and at 

places nearby,'the writer has been able to recognize 

several subdivisions that have not been made in this 

region before. The Dakota sandstone of Ransome's 

report on the district and the uppermost beds of his 

fWyoining formation have been divided into the 

Morrison formation (Upper Jurassic) and the Dakota 

quartzite (Upper Cretaceous). The Upper Cretaceous 

shale has been subdivided into the Benton shale, the 

Niobrara formation, and the Pierre shale. The 

tWyoming formation, as previously recognized in this 

region, is considered equivalent to the Maroon formation of present terminology. These changes in correlation are discussed in detail under their respective 

headings.

Pre-Cambrian rocks are exposed in two places in the 

area studied, and they underlie the sedimentary rocks 

throughout the district. A quartz-biotite-muscovite 

schist is exposed at the Owl tunnel, on French Gulch, 

near the northwest corner of the area mapped. The 

foliation of this schist strikes about N. 20° W. and dips 

about 60° E. Schist, granite, and granite gneiss come 

to the surface in the upper part of Illinois Gulch, but 

the trend of the schistosity could not be seen. The 

regional strike of the schistosity, however, over a large

5 Ranioma, F. L., op. cit., pp. 25-40.
BRECKENEIDGE MINING DISTRICT, COLORADO

area to the north, east, and south is known to be north

or northwest, and the dips are consistently east or

heast; thus it is probable that the foliation of the

and gneiss throughout the special area of the

He called the formation Maroon conglomerate and 

included in it all beds between his Weber limestone 

below and the unconformably overlying fGunnison 

formation, and he correctly assigned to it a Carboniferous age. As defined by Eldridge, 

the Maroon included at the base the 

so-calledf Weber grits, which Emmons 7 

excluded from the formation in 1898 

and which were thenceforth regarded 

as separate from the Maroon. The 

name fGunnison formation is no longer 

used, as the rocks are equivalent to 

the Morrison formation, and Morrison 

is the older name.

In the Denver Basin the f Wyoming 

formation was defined by Emmons 

and Eldridge 8 as the beds between 

the pre-Cambrian rocks and the Morrison formation. It was at first assigned to the Triassic system, but 

later work showed that most, if not 

all, of the beds below the Morrison 

belong to the Carboniferous system. 

As shown in the correlation chart in 

plate 3, the Maroon formation, as 

first defined, is practically equivalent to the original f Wyoming group 

of the Denver Basin, on the east side 

of the Front Range, which has since 

been subdivided into the Fountain,

Lyons, and Lykins formations, and 

the term f Wyoming is no longer 

used.

In the Tenmile folio Emmons 9 

subdivided the Carboniferous rocks 

overlying the fWeber grits into the 

Maroon and Wyoming formations. 

He considered his Jacque Mountain 

limestone the uppermost bed of the 

Maroon formation and the overlying 

sandstone the base of the fWyommg 

formation. Limestone is much less 

common in the fWyoming formation, 

as he defined it, than in his Maroon 

formation, and the red color of the 

sandstone is more pronounced in the 

so-called f Wyoming than in the underlying beds. No other changes in 

the lithology are mentioned, and no

FIGURE 2. Columnar section for Breckenridge district. Tertiary intrusives: Tqmp, Eocene quartz evidence of an UnCOnformitV was

found between the two subdivisions. 

Emmons I0 correlated the upper red sandstones in the 

Tenmile district with the fWyommg of the Denver

Vertical scale

80O FEET

monzonite porphyry; Tmp, Eocene monzonite porphyry.

Breckenridge district does not deviate greatly from 

the strike and dip observed in the Owl tunnel.

MAROON FORMATION

Correlation. The Maroon formation was named by 

Eldridge 6 in the Anthracite-Crested Butte folio.

« Cross, Whitman, and Eldridge, G. H., U.S. Geol. Survey Geol. Atlas, Anthracite-Crested Butte folio (no. 9), p. 6,1894. '

' Emmons, S. F., U.S. Geol. Survey Geol. Atlas, Tenmile district folio (no. 48) 

1898.

8 Emmons, S. F., Cross, Whitman, and Eldridge, G. H., Geology of the Denver 

Basin: U.S. Geol. Survey Mon. 27, pp. 51-60, 1896.

 Emmons, S. F., U.S. Geol. Survey Geol. Atlas, Tenmile district folio (no. 48), 

1898. 

10 Idem, p. 2.
GENERAL GEOLOGY

Basin, "not because of any fossil evidence of their age 

that could be found, but because by their position and 

petrological character they most nearly correspond to 

the beds of this -formation which elsewhere in the 

Rocky Mountain region have, on fossil evidence, been 

determined to be Triassic."

Ransome n says that the fWyoming of the Breckenridge and Tenmile districts is probably the stratigraphic equivalent of the Lykins of the Boulder district (called fUpper Wyoming by Emmons, Cross, and 

Eldridge). The f Weber grits and Maroon of the Tenmile folio are probably equivalent in a general way to 

the Fountain and Lyons formations or fLower Wyoming of the Denver Basin.

It is evident that all the beds overlying the fWeber 

grits in the Tenmile district are to be correlated with 

the Maroon formation, as now recognized in the 

Anthracite-Crested Butte area, and with the central 

and upper parts of the original fWyoming of the 

Denver Basin, and that the underlying so-called Weber 

strata are to be correlated with the lower part of the 

original fWyoming formation. The name fWyoming 

is obsolete and has long been abandoned by the United 

States Geological Survey. The beds between the 

Morrison and the fWeber grits in the region south and 

west of Breckenridge are therefore designated Maroon 

formation in this report, and the underlying grits and 

conglomerates are called Weber (?) formation, because 

the equivalency of these beds to the typical Weber of 

eastern Utah is now seriously questioned. Because 

of the Permian overlap the Weber (?) formation does 

not show in the area mapped, but it may be present at 

depth in the southwestern part of the region.

Lithology. It will be helpful to summarize the chief 

features of the Weber (?) and Maroon formations in 

central Colorado before considering their lithology in 

the Breckenridge district. The contact of the fWeber 

grits with the underlying fWeber shales is gradational 

in some places and sharp in others. The so-called 

"grits" consist of micaceous sandstone, shale, grit, and 

conglomerate, strongly cross-bedded and locally containing beds of limestone. The color is generally gray. 

The lower part of the overlying Maroon formation is 

commonly more calcareous than the upper part, and 

the predominant color is dark red. In the upper part 

of the Maroon formation limestone becomes rare and 

the dark-red color gradually gives place to a brilliant 

brick-red, although some beds of gray shale and sandstone are present.

Throughout the formation the grit and sandstone 

are strongly cross-bedded and may grade into shale 

or conglomerate within very short distances, both along 

and across the bedding. The lenticular character of 

the beds is a striking feature of the entire formation and 

is well shown in the structure-section sheets of the 

Tenmile folio. Most of the material in the grit and

11 Rausouie, F. L., op. clt., p. 31.

conglomerate was derived from pre-Cambrian rocks, 

but pebbles of limestone are common in the conglomerate near the base of the Weber (?) formation. The 

combined thickness of the Weber (?) and Maroon formations ranges from a few feet to nearly 10,000 feet 

in central Colorado and is commonly about 5,000 feet. 

A short distance east of Breckenridge the Morrison 

and Dakota formations successively overlap the preCambrian rocks, and in the district only the thin 

edge of the Maroon formation, from 600 to 900 feet 

thick, is exposed; but the beds show all the variations 

of lithology that the thickest sections of the Weber (?) 

and Maroon formations show elsewhere. The varying 

character is well illustrated near Gibson Hill. At the 

Owl tunnel the base of the formation is a thick coarse 

conglomerate, containing boulders as much as 2 feet 

in diameter, overlain by a black carbonaceous shale, 

which in turn is succeeded by another coarse conglomerate. This succession is typical of the lower part of 

the fWeber grits, near a gradational contact with the 

underlying fWeber shales, although the conglomerate 

is not generally so coarse. The lithology suggests 

reworked fWeber at the base of the overlapping 

Maroon formation. Near the Sultana mine, 1 mile 

to the northwest, the base of the Maroon formation is a 

bed of grit 2 feet thick and is overlain by a thick bed 

of bright-red micaceous sandy shale, such as typifies 

the uppermost beds of the Maroon formation. The 

sedimentary contact of the beds with the pre-Cambrian 

rocks is exposed in both places and leaves no doubt as 

to the rapid lateral variation that has taken place. 

It is impossible to give a helpful detailed section of the 

Maroon formation in a district where such changes 

have occurred. In general, the coarse conglomerate 

beds are more common near the base of the formation 

than they are near the top. Black shale, so far as 

the writer knows, is confined to the base of the formation. Micaceous red shale is most common in the 

upper part of the section. Beds of grit and sandstone 

are well represented throughout the formation but 

are not as abundant as the shale and conglomerate. 

A moderately persistent sandstone, varying from 

yellowish brown to light orange-yellow, occurs near 

the top of the section. Limestone beds from 2 to 5 

feet thick are interbedded with shale and grit. These 

beds are lenticular and although confined to no definite 

horizon seem to be more common near the base of the 

formation than in the upper part. For further information on the character of the material in the grit and 

conglomerate and a discussion of the cause of the red 

color, the reader is referred to Ransome's description 

of the fWyoming. 12 In the special area studied, the 

thickness of the Maroon formation ranges from about 

600 feet in the northwestern part to about 900 feet in 

the southeastern part. The average thickness is 

probably close to 700 feet.

ia Ransome, F. L., op. cit., pp. 31-33.
6 BRECKENRIDGE MINING DISTRICT, COLORADO

The Maroon formation is well exposed along the 

north bank of Indiana Gulch and in the upper part of 

Illinois Gulch. It passes from sight beneath the 

younger formations in the central part of the special 

area and reappears in the northwestern part at the 

base of Gibson Hill.

Near intrusive masses and in the vicinity of metalliferous veins, the normal color and character of the 

formation is locally changed. The bright-red color of 

the Maroon sandstone and shale may be reduced to 

greenish gray near a vein, and garnet, epidote, and

can be seen at Rocky Point, but its lower part is 

poorly exposed and is separated from the overlying 

part by a thick porphyry sill. No other place in the 

district, however, shows so much-of the formation. 

Its shaly character makes it weather into smooth 

slopes, and only where man or some accident of 

erosion has cut into it, are exposures found. It is 

well represented in the fault mosaic of Gibson Hill 

and forms the lower slopes of Little Mountain and 

the southern slope of Nigger Hill, but very few outcrops can be found in these localities.

Boreas 

(according to 

Ransome)

X Rocky Point

Detroit 

mine

Covered

Kb - Benton shale 

Kd - Dakota quartzite 

Jm - Morrison formation 

Cm - Maroon formation

FIGURE 3. Correlation of sections measured from Boreas Pass to Keystone.

other contact-metamorphic minerals are common in 

the calcareous beds- near certain bodies of quartz 

monzonite porphyry.

MORRISON FORMATION

Correlation. The Morrison formation (now classified as Upper Jurassic) is widely distributed in the 

region surrounding Breckenridge. It has not hitherto 

been separated from the overlying or underlying beds, 

but in the detailed study of a small area it was found 

possible and helpful to make the Morrison a distinct 

cartographic unit. Although Ransome 13 recognized 

the probability that the Morrison was present in the 

district, he did not find sufficient evidence to map it 

as an individual formation and included most of it 

with the Dakota quartzite. The uppermost beds of 

his fWyoming and the lower part of his Dakota, specifically those beds below bed 5 in his Rocky Point 

section, are included by the present writer in the 

Morrison. This formation underlies the Dakota 

quartzite everywhere in the Breckenridge district. 

Nearly the entire thickness of the Morrison formation

13 Kansoine, F. L., op. cit., pp. 34, 36.

The Morrison can be traced from the Breckenridge 

district through the knob south of Boreas Pass into 

the Tarryall district, where Muilenberg 14 found fossils of Morrison age.

Lithology. Its lithology is extremely varied in detail, 

but the following sections are typical in the Breckenridge 

district and are graphically correlated in figure 3.

Section of Morrison formation near Rocky Point

Dakota quart/ite. Base of the Dakota concealed; lightgray pebbly quartzite carrying fragments of shells in 

float between outcrops of unquestionable Dakota and 

the red shale; base assumed to be 5 feet below outcrop.

Feet

Covered _____-____--___-___._..____-_---__-_------

Morrison formation:

Red shale, thin-bedded, not micaceous.____._---__

Blue-green and olive-green shale, thin-bedded, 

locally with pinkish layers._____________--_---

Thin-bedded and blocky shales, brownish purple 

and maroon; some layers weather gray. __--___ _

Hard limy and sandy dark-gray medium-bedded 

shale, alternating with thin-bedded purplishbrown shale._______-_____-______-_-_-_----_-

20

12

35

43

26

14 Muilenberg, G. A., Geology of the Tarryall district, Park County, Colo.: 

Colorado Geol. Survey Bull. 31, p. 21,1925.
GENEEAL GEOLOGY

Section of Morrison formation near Rocky Point Continued

Morrison formation Continued. . Massive grayish-white cross-bedded gritty sandstone; numerous brown spots and discontinuous feet 

brown bands a quarter of an inch wide._.________ 15

Thin-bedded shale, brownish, maroon, and black;

mostly covered-______-_--___--_-_--__-____-- 25

Very thin-bedded gray and brown shale.----------- 18

Massive and medium-bedded gray shale; weathers 

buff_--_--_-_--_-_-__---____-___---__-__-_-_ 45

Thin-bedded limy shale and shaly limestone, grayishgreen; weathers buff--__---_-_-________-____-_ 6

Massive grayish-green limestone; color probably 

caused by metamorphism of underlying porphyry 

sill; both epidote and hematite are developed on 

joint surfaces..---------------------.-------- 12

Porphyry sill about 450 feet thick separates the limestone above from the remainder of the section, 

which is very poorly exposed.

Light-red nonmicaceous thin-bedded shale, about. - _ 15

Medium-bedded medium-grained white sandstone 

containing many layers of grit and a few layers of 

conglomerate. The sandstone contains conspicuous rusty blotches a quarter to half an inch in 

diameter. The conglomerate contains pebbles of 

micaceous red shale and pre-Cambrian pegmatite 

from half to three quarters of an inch in diameter 

in a matrix of light-colored quartz grit. About... 15

Conglomerate, largely made up of pebbles of micaceous red shale about half an inch in diameter in 

a matrix of pink sand-_.__.___.__.____..._____ 2

Red micaceous shale (Maroon formation). ~~

284

Section half a mile west of Boreas Pass

Dakota quartzite. Hard medium-grained light-gray

cross-bedded quartzitic sandstone and grit. 

Morrison formation: Feet 

Thin-bedded red shale-------------------------- 22

Light purple limestone----____-____________--_-_ 1

Thin-bedded sandy red shale____________________ 80 }i

Porphyry sill, 14H feet thick.

Thin-bedded maroon shale.___--________--___-__ 5

Coarse-grained brownish grit..._________________ 3

Coarse-grained white gritty sandstone.___._-_____ 3

Conglomeratic quartz grit-_--____--_-__.________ 10

Thin-bedded nonmicaceous red sandy shale, with a 

few thin beds of grit._________________________ 75

Arkosic sandstone and conglomerate; contain pebbles 

of micaceous red sandstone and micaceous red 

shale.___-_--_--_-----_---_______-_-______-_ 2

Total thickness of Morrison_______________ 201/4

Maroon formation:

Thin-bedded fine-grained arkosic pink sandstone.__ 20 

Thin-bedded coarse-grained light-gray sandstone.._ 35 

Thin-bedded micaceous reddish-brown sandy shale. 200

In the Breckenridge district the Morrison formation 

is commonly made up of a gritty sandstone overlain 

by a series of beds of dark-gray and maroon shale 

containing local limy beds, which in turn are overlain 

by a very persistent cross-bedded sandstone (no. 8 in 

Ransome's Rocky Point section; nos. 3-5 in his 

Boreas section); the remainder of the formation consists chiefly of gray and maroon shale, some of which

is decidedly calcareous. The character of the formation in the northeastern part of the special area differs 

somewhat from that of the sections given above. In 

that locality the upper part of the formation contains 

thin lenticular beds of grit and sandstone and a bed of 

limestone interbedded with shale that is generally light 

gray and very thin-bedded. At the base of the formation there is generally a bed of gray gritty quartzite 

about 20 feet thick.

The micaceous appearance of the shale and sandstone of the Maroon formation is one of the most 

outstanding characteristics, and the shale of the Morrison may be easily distinguished from that of the 

Maroon by its complete lack of visible mica. Shale of 

the Morrison having a reddish tinge will not be confused with that of the Dakota, which is gray or black, 

but the gray shale of the two formations is indistinguishable. The sandstone of the Morrison is generally 

softer, coarser-grained, and more cross-bedded than 

similar beds in the Dakota quartzite.

The limy beds of the Morrison are completely 

converted into garnet, epidote, magnetite, hematite, 

and other contact-metamorphic minerals on Prospect 

Hill, just west of the Wellington mine. On the south 

slope of Gibson Hill contact-metamorphic minerals 

have been developed at the same horizon. More detailed consideration of the contact metamorphism of 

these beds is given on pages 16-17.

DAKOTA QUARTZITE

The Dakota quartzite is well represented in the 

Breckenridge district and is one of the most persistent 

and easily recognized formations present. The best 

section of the Dakota was found at Rocky Point, 

which is the only place in the district where there is a 

continuous exposure of the rocks from the Morrison to 

the Ben ton. At Nigger Hill and Little Mountain 

there are good sections of the massive lower quartzite 

member, and the upper quartzite is well exposed in the 

open pits on the Puzzle-Ouray vein, nearby. The 

shaly middle member of the formation is not well 

exposed at any place other than Rocky Point, although 

it can be recognized on Little Mountain and in the 

underground workings on Gibson Hill.

Lithology. The three members recognized by Ransome 15 at Boreas Pass, 5 miles southeast of Breckenridge, can also be recognized in the district. The most 

significant changes noted in following the beds to the 

northwest are the loss of the light-red color of the middle shaly member and the transition from a sandy and 

shaly limestone to carbonaceous shale and quartzite. 

Evidently a similar change occurs east and southeast 

of Boreas Pass, for, according to Muilenberg: 16

15 Ransome, F. L., op. cit., p. 35.

16 Muilenberg, Q. A., Geology of the Tarryall district, Park County, Colo.: Colorado Geol. Survey Bull. 31, p. 26, 1925.
8 BRECKENRIDGE MINING DISTRICT, COLORADO

On the east side of the pass this section cannot be recognized. * * * No evidence was found of the sandy limestones which are there encountered, although the dark shale is 

probably present, as are the conglomeratic beds.

Between Dillon and Keystone, 11 miles north of 

Breckenridge, the three members of the formation can 

be recognized and show the same characteristics as at 

Breckenridge. It therefore seems probable that the 

Boreas Pass section represents an unusual facies of the 

formation and that its general character is well illustrated in the Rocky Point section, which is given below. 

However, the middle member differs greatly in thickness and in the details of its lithology within short distances, and the formation as a whole thins rapidly 

eastward. At Georgia Pass, 4 miles northeast of 

Boreas Pass, the overturned sedimentary contact between the Dakota and the pre-Cambrian rocks is well 

exposed. At this locality there is no faulting between 

the quartzite and the underlying schist or the overlying black shale beds, which are more than 200 feet 

thick. The shale is probably Benton, yet the quartzite here is only 20 feet thick. The regional structure 

indicates that the beds now exposed at Georgia Pass 

were probably deposited at least 7 or 8 miles northeast 

of the exposures at Boreas Pass and that folding has 

brought these distant parts of the formation within 4 

miles of each other. The distinct northeastward thinning of the Dakota probably marks the overlap of the 

formation on the slowly submerging pre-Cambrian 

highland, which the writer believes had been a source 

of sediments for the Weber (?), Maroon, and Morrison 

formations. 17

The thickness of the Dakota in the region discussed 

above ranges from 225 to 20 feet, but in the special 

area of the Breckenridge district the thickness ranges 

from about 125 feet on Gibson Hill to 175 feet at 

Rocky Point. Detailed sections at various localities are 

given below and are graphically correlated in figure 3.

Section of the Dakota quartzite near Boreas Pass

[After Ransome]

Feel 

3. Massive, buff, fine-grained sandstone. _._._______.__ 50

2. Thin-bedded reddish and gray sandy limestone with 

some pebbly grit and a little dark shale. Weathers 

pink or light red as a whole______________.______ 100

1. Light or buff sandstone with pebbly streaks. Beds 

thinner than no. 3. Bottom bed, about 6 feet thick, 

contains abundant pebbles of white quartz, mostly 

less than half an inch in diameter. This bed ranges 

from coarse sandstone to fine conglomerate..___.. 50

200

17 Levering, T. S., Geologic history of the Front Range, Colorado: Colorado Sci. 

Soc. Proc., vol. 12, pp. 81-86, 1929.

Section of the Dakota quartzite at Rocky Point 

[By T. S. Levering]

Benton shale: Black shale. 

Dakota quartzite: 

Upper member:

Medium-bedded light-gray quartzite with a few Feet 

black-shale partings._____________________ 12. 5

Massive, moderately fine grained, slightly 

banded light-gray quartzite, heavily stained 

with limonite...-----_--___-----___--__-- 5. 5

Thickness of upper member. ___________ 18

Middle member:

Gray shale-_------_-___-__-_---___-------- 1. 5

Medium-grained gray quartzite-------------- . 8

Dark-gray to black shale. __________________ 2. 5

Thin bedded light-gray quartzite and gray shale. 3. 5 

Black shale__--__-__-__-__-_-____-__---_-_ 2. 5

Thin-bedded b] ack shale and gray quartzite--- 4. 5 

Black shale___-_--_____--__________--___-- 4. 5

Thin-bedded dark-gray shale and quartzite---- 5

Grayish-white quartzite-___________________ 2. 5

Grayish-black shale._______________________ .5

Grayish-white quartzite_ ___________________ .5

Brownish-black coaly shale, containing marcasite and carbonized plant remains..----.- 2. 5 

Massive, medium-grained gray quartzite, gritty

at bottom with pitted surface at base. _____ 13

Black shale.___-____-_-_-_______-__.-_-_-- .5

Massive, moderately fine grained light-gray

quartzite---_------------__-_------___-- 5

Black coal-bearing shale.-_____-_____-____-_ 2

Dark-gray shaly quartzite. _________________ .5

Thin-bedded gray shale and quartzite-------- 1

Medium-bedded dark-gray banded quartzite- _ 2. 5 

Sandy gray shaJe__________________________ 2

Thickness of middle member.__._._-.___ 59. 3

Lower member:

Massive, moderately fine grained grayishwhite quartzite, slightly banded at the base. 26 

Medium-bedded black quartzite and black 

shale_-_-____.______________-_____--__-. 5

Medium and fine grained dark-gray quartzite. 2 

Black shale.__-________-___________---__-- 1

Thin-bedded grayish-white fine and medium 

grained quartzite, containing white chalky 

lines suggesting shell fragments____________ 4

Alternations of 8-inch beds of black shale and 

quartzite ___-_------_----__----_------_- 5

Black shale.______________________________ 5

Buff quartzite.---___________________________ .5

Sandy gray shale.__________________________ 3

Massive medium-grained gray quartzite- _____ 10

Grit, containing chalky marks suggesting shell

fragments_--_------._-___._______.^_____ . 5

Thin-bedded, finely banded buff and grayishwhite sandy quartzite__--_---____-_-___-_ 20

Massive coarse-grained grit and quartzite, conx taining pebbles half an inch in diameter. ___ 5
GENERAL GEOLOGY

Section of the Dakota quartzite at Rocky Point Continued

Dakota quartzite Continued. 

Lower member Continued.

Massive fine-grained light-gray sandy quartzite; carries many plant remains preserved in Feet 

limonite. _____________ __________________ 5

Covered _____----_-_.._____________________ 20

Thickness of lower member of the Dakota, 

base assumed 5 feet below covered portion between sandy quartzite and the 

red shale..._._.__________..____.___ 97

Total thickness of the Dakota.-----.--.. 174. 3

Morrison formation (red fissile shales, not micaceous)... 12

Partial section of Dakota quartzite in the Detroit mine, 

Gibson Hill

Benton shale: Black shale.

Dakota quartzite:

Upper member: Feet 

Massive dark-gray quartzite- ____.._____.___ 3

Massive gray quartzite-_____________-__--_.. 7

Thin-bedded gray quartzite_--_--__-_--_-.-- 8. 5

Thickness of upper member.------------ 18. 5

Middle member:

Black shale,_---.---_-____-..-..-_---_-.-_ 4

Gray quartzite._-_-_--_--_-.-.._______---_ 2

Sandy gray shale.___-----_-_--__-_-----.-- 4

Light-gray quartzite---_____..--_-----__---_ 4.5 

Banded gray shale.----.-------.----------- 4. 5

Thickness of middle member.___________ 19

Fault.

Lower member: Gray quartzite to bottom of shaft.

Section of the Dakota 1 mile west of Keystone

Benton shale: Gray shale.

Dakota quartzite: 

Upper member:

Medium-bedded gray quartzite-_____________ 

Thin-bedded gray quartzite__________________

Massive light-gray quartzite_-___----------_

60

Feet

10

5

32

Thickness of upper member. 47

Middle member:

Black shale.______________________________ 13

Massive gray quartzite-_-____-.________-_-_ 4

Black shale.______-_-_--_-_____-__-_______ 1

Massive quartzite-.-_______________________ 25

Interbedded black shale and gray quartzite-__ 50

Thickness of middle member.___________ 93

Lower member:

Massive gray sandy quartzite. 

Hard massive gray quartzite-.

16

69

Thickness of lower member.

Thickness of the Dakota. 

Green shale of the Morrison.

85

225

Section of Dakota quartzite on Little Mountain

Beuton shale: Black shale. 

Dakota quartzite:

Upper member: peet 

Massive light-gray quartzite.----__-_-__-____ 10

Thin-bedded banded light and dark gray

quartzite.______________________________ 10

Massive banded light and dark gray quartzite- 12 

Massive black quartzite. ___________________ 5

Thickness of upper member. 37

Middle member:

Thin-bedded black shale and quartzite. 

Massive white sandy quartzite-------.

Limy gray shale___________-__..____

6

7

10

Thickness of middle member. 23

Lower member:

Massive grayish-white quartzite. __-.-__.____ 45 

Gray shale._______________________________ 3

Massive grayish-white quartzite with a few 

gray-shale partings. ______________________ 40

Thickness of lower member. 

Covered.

Total thickness exposed-... 148

Partial section of the lower member of Dakota quartzile exposed 

on Nigger Hill

Feet 

Gray quartzite.--.---..--------------._-_._---_--_ 2

Gray shale._-_-________.-_---_-__--_--__-_-__-.-__ 3

Massive medium and coarse grained grayish-white

quartzite _.._________--_-__----_---______-_______ 20

Gray shale.--_-----_-__------_---------_-_----_-.. 1

Massive gray quartzite. __------_------_____-___-___ 3

Gray shale._________-__---_--_-_-___-_____-_-_-___ 2

Medium-bedded light-gray quartzite with shaly partings,

some of which contain plant fragments.______.___-. 6

Light-gray sandstone.._____________________________ 1

Massive grayish-white quartzite, darker at base.----.-. 19

Massive light-brown sandy and gritty quartzite__._._ 3

60

Morrison formation: Greenish-white, brownish-green, and 

reddish-brown shale___------_------__-.-__------- 13

A consideration of the detailed sections given above 

and the graphic correlation chart shown in figure 3 

should give the reader an understanding of the general 

character of the formation. The three members may 

be clearly distinguished at Rocky Point, where differential weathering of the steeply dipping formation has 

served to accentuate the essential unity of the individual members. From the flats south of Little 

Mountain the Rocky Point exposure stands out prominently and appears to consist of three beds a thick 

hard light-colored basal bed, a softer black middle bed, 

and a thin light-colored top bed. As the Dakota is 

the most significant key formation of the district, its 

lithology is worth careful study, and the following
10 BRECKENRIDGE MINING DISTRICT, COLORADO

summary may be helpful to anyone working in this 

region.

The lower member is predominantly massive lightgray quartzite and ranges in thickness from 95 to 120 

feet. Gritty or conglomeratic beds are usually found 

near the base, and a few thin beds of gray or, less 

commonly, black shale are nearly everywhere present. 

The shale beds are not persistent and may thin and' 

disappear in short distances.

The middle member is the most variable of the three 

in thickness as well as in lithology. It contains more 

shale than any other part of the formation, but the 

individual shale beds are lenticular and can seldom be 

correlated with any confidence. Both black and gray 

shale beds are common, and the quartzite beds belonging to this member are more generally dark colored or 

black than those in other parts of the formation. A 

bed of shale may grade within a short distance through 

sandy shale into quartzite, or it may thin from a 5-foot 

bed to a shale parting within a few hundred feet. The 

variation in color, thickness, and composition of the 

individual beds in this member makes it difficult to 

estimate the stratigraphic position of isolated or small 

exposures of interbedded shale and quartzite. The 

thickness of the middle member ranges from 20 to 60 

feet and is greatest in the southern part of the district.

The upper member of the Dakota (pi. 4, A) is the 

least variable of the three. It consists of massive and 

thin-bedded light-colored quartzite, having a few shale 

partings, and is commonly from 20 to 30 feet thick in 

the special area. The lower part of the Benton shale 

contains thin beds of black quartzite at some places 

near Breckenridge, but the contact of the two formations is clearly marked.

No identifiable fossils were found in the Dakota 

quartzite by either Kansome or the writer, although 

plant fragments are abundant at certain horizons in 

several localities. Its lithologic character and its 

presence immediately below beds of definite Benton 

age leave no doubt as to the propriety of correlating 

it with the Dakota sandstone of central and eastern 

Colorado.

BENTON SHALE

Correlation. The Benton shale has not heretofore 

been mapped as a separate unit in the Breckenridge 

district or the surrounding region. Its presence was 

recognized by Ransome 1S and by Muilenberg, 19 

but no attempt was made to establish the thickness 

of the formation. Benton fossils have been found at 

many localities west of the Front Range, and it is evident that the Benton shale is persistent for long distances to the north, west, and south of Breckenridge, 

but comparatively little study has been accorded it

18 Ransome, F. L., op. cit., p. 38.

19 Muilenberg, Q. H., op. cit., p. 27.

in this region. Spurr 20 found 350 feet of black calcareous shale of Benton age underlying the Niobrara 

limestone at Aspen, 50 miles west of Breckenridge. 

Eldridge 21 found 150 to 300 feet of black shale and 

interbedded fetid limestone of Benton age in the 

Anthracite and Crested Butte quadrangles, 60 miles 

southwest of the Breckenridge district. The Benton 

fauna found in the Tarryall district by Muilenberg was 

in a black bituminous shale and associated beds of fetid 

limestone immediately over lying the Dakota quartzite..

Henderson 22 assigns a thickness of 300 to 400 feet 

to the Benton in the Rabbit Ear district and in the 

vicinity of Kremmling. According to drill records 

obtained near Walden, the Benton is 550 feet thick in 

that part of North Park, but Beekly 23 gives the thickness as only 165 feet.

Lithology. There are no localities in the Breckenridge district where a complete section of the Benton 

shale was found. The information regarding thickness 

and lithology was pieced together from many incomplete exposures, supplemented by study of the region 

north of the district. The average thickness of the 

shale is probably very close to 360 feet, but the complex faulting and intense intrusive activity that have 

occurred in the district, together with the lack of outcrops, makes it difficult to give a confident estimate of 

the thickness.

The best section found is in the railroad cut just 

northeast of Rocky Point. At this locality nearly 300 

feet of the Benton shale is exposed, by far the most 

continuous section in the district, and although the 

beds have been disturbed by quartz monzonite porphyry intrusions and locally overturned, the general 

character of the section can be easily determined. 

The uppermost part of the section can be seen at the 

railroad cut on Nigger Hill, at the north end of the 

trestle crossing Illinois Gulch.

Throughout the district the Dakota quartzite is 

overlain by black shale for 30 to 50 feet. Between the 

lower black shale and a point about 250 feet above the 

quartzite, the shale usually has a decided grayish cast, 

varying from dark gray to grayish black; the shale up 

to this place in the section is rather fissile. Thinbedded black shale and limestone alternate for about 

35 feet above the grayish-black shale and are overlain 

by about 50 feet of black fissile shale. Capping the 

black fissile shale is a bed of shaly limestone about 25 

feet thick. It is a fetid black fossiliferous limestone, 

probably the equivalent of the f Nio-Benton sand of the 

oil-well drillers of North Park, as it forms the top of the 

Benton shale and the base of the Niobrara formation.

20 Spurr, J. E., Geology of the Aspen mining district, Colo.: U.S. Qeol. Survey 

Mon. 31, p. 41,1898. 

si Eldridge, O. H., and Cross, Whitman, op. cit., p. 6.

22 Grout, F. F., Worcester, P. G., and Henderson, Junius, Geology of the Rabbit 

Ear region: Colorado Geol. Survey Bull. 5, p. 31, 1913.

23 Beekly, A. L., Geology and coal resources of North Park, Colorado: U.S. Geol. 

Survey Bull. 596, P. 35, 1915.
U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 176 PLATE 3

GUNNISON REGION 

(Campbell, I9Z2)

CRESTED BUTTE 

(Eldridge,l894)

ASPEN 

(Spurr.1898)

MONARCH 

(CrawforxJ,l9l3)

RED CLIFF- NEAR 

TURKEY CREEK LEADVILLE 

(Johnson and Behre. DISTRICT 

1931) IN

MOSQUITO RANGE 

(Behre, 1929)

ALMA TENMILE

ld (Emmons,t898) 

Butler 

1933)

BRECKEN RIDGE

, DENVER BASIN

(Emmons and Eld ridge;

COLORADO SPRINGS 

(Finlay

llsap limestone 30 

rernont Is. 100'

nd stone 100 

nitouBMOO' 

Quartzite

lt~Parting tfuartzite 

r-White limestone

Bentpn 

Dakota 100' 

Purgatoire 300' 

Morrison 225' 

Lykins 180'

Handing ss. 

and Manitou 

Is. 50- 2SKf 

 PikesPeak.Cripple

Creek,and Mt,Rosa

quartzites
D.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 176 PLATE 4

A. UPPER MEMBER OF DAKOTA QUARTZITE.

B. VIEW LOOKING EAST UP FRENCH GULCH FROM NORTH SLOPE OF NIGGER HILL.
GENERAL GEOLOGY 11

'There is no break evident between the Benton and 

Niobrara, and Niobrara fossils have been obtained 

from the shale at the top of the lunestonj^tkaj. yields 

t3£pi«al Benton forms." Near Keystone, 11 miles to 

the north, Niobrara fossils were found in a limestorie 

immediately overlying a black fetid imfossiliferous 

limes tone-370 feet above the Dakota quartzite.. The 

break between the Dakota and the Benton is usually 

abrupt, but in some localities there is a transition zone 

in which thin beds of quartzite alternate with black 

shale through a distance of about 10 feet. Near 

Dillon the base of the Benton is marked by a thin bed 

of conglomerate containing fragments of Dakota, 

Morrison, and pre-Cambrian formations. The sections of the Upper Cretaceous shale given in figures 

3 and 4 show the general relations of the Benton to the 

other Cretaceous formations.

The Benton shale, in common with the other sediments of the district, may be greatly altered by intrusive rocks, and this subject is considered under the 

heading "Rock alteration" (pp. 16-17).

In the special area of the Breckenridge district the 

largest exposure of Benton shale occurs in the syncline 

between Little Mountain and Rocky Point. The shale 

underlies the Niobrara formation in French Gulch and 

elsewhere and crops out on the top of Gibson Hill.

NIOBRARA FORMATION

The stratigraphic limits of the Niobrara formation 

cannot be worked out from the exposures in the Breckenridge district, nor can the lithology of the formation 

be satisfactorily established. A study of the sections 

between Dillon and Keystone, a reconnaissance of the 

region from Breckenridge to North Park, and detailed 

work on the exposures in the special area form the basis 

of the conclusions presented here.

In the Anthracite-Crested Butte area the Niobrara 

limestone consists- of a limestone bed about 30 feet 

thick overlain by approximately 150 feet of calcareous 

gray shale. At Aspen the Niobrara limestone is said 

to be about 100 feet thick and to grade upward into 

the Montana shales without any perceptible break. 

In North Park, according to Beekly,24 the average 

thickness of the Niobrara* formation is about 800 

feet. Well records in the park later confirmed this 

estimate. The formation in this region, as well as 

in the country to the south, is made up of a basal 

thin-bedded limestone overlain by interbedded calcareous and clay shale and locally capped by a thin 

limy sandstone.

In the Keystone section measured by the writer 

(fig. 4) the calcareous beds definitely assignable to the 

Nipbrara are 350 feet thick, but it is possible that a 

thin limy layer in the overlying blue-black clay shale 

carrying poorly preserved fragments of Hypsodon,

« Beekly, A. L., op. cit., p. 39. 

18293 34 3

Ostrea, and Baculites may 

tion, suggesting a possible 

No fossils were-found in 

the nearly uniform succession of somber-colored 

clay shale overlying this 

layer for nearly 1,500 feet. 

The present lack of fossil 

evidence that the clay 

shale belongs to the Niobrara and the decided 

change that occurs in lithology make it advisable to 

limit the Niobrara formation to the calcareous beds. 

In the Breckenridge district the upper limit of the 

formation is placed at the 

top of the calcareous beds 

350 feet above the Benton. 

This calcareous zone in 

the Cretaceous is widely 

distributed in the region 

around Breckenridge and 

is nearly uniform in thickness and in general lithology. It carries Niobrara 

fossils in many localities 

and is undoubtedly of 

Niobrara age from top to 

bottom and is overlain by 

thick somber-colored clay 

shale.

The Niobrara is well exposed in the Wellington 

mine and in some prospects 

in Illinois Gulch. The 

formation has its greatest 

areal extent in the syncline between Little Mountain and Rocky Point and 

is also well developed in 

French Gulch near the 

Wellington mine.

A section of the Niobrara formation near Keystone is shown in figure 4, 

and a composite section of 

the formation as it occurs 

in the special area is given 

below:

mark the top of the formathickness of about 475 feet.

Keystone

Kp

Kp

Kp-Pierre shale 

Kn-Niobrara formation Kb-Benton shale ~~ 

Kd-Dakpta quartzite

Kn

Kb

Vertical scale GOO o soo Feet

FIGURE 4. Correlation of sections measured at Palmer ranch and Keystone.

Composite section of Niobrara formation in special area

Pierre shale: Black noncalcareous clay shale.

Niobrara formation:

Black limy shale and thin-bedded limestone containing 

abundant lenses and veinlets of secondary white Feet 

calcite.--------_.__.___.._____.____._.__..__._ 50
12 BRECKENRIDGE MINING DISTRICT, COLORADO

Composite section of Niobrara formation in special area Contd.

Niobrara formation Continued.

Gray and black limy shales with some interbedded

noncalcareous clay shale._______________________ 250

Massive greenish-gray limy shale, locally grading into

gray limestone at the base. _____---____-_-__--__ 50

Benton shale: Black fetid limestone. 350

Ransome found imperfectly preserved specimens of 

Inoceramus deformis in the basal limy bed of the 

Niobrara 1,500 feet southeast of the summit of Little 

Mountain, close to a railroad cut. The following 

fauna was collected by the writer from the basal limestone of the Niobrara formation near Keystone and 

was identified by J. B. Reeside, Jr.:

Inoceramus aff. I. subquadratus Schluter 

Ostrea aff. 0. congesta Conrad 

Baculites sp. 

Globigerina sp.

PIERRE SHALE

The widespread blanket of Upper Cretaceous shale 

that covers most of Colorado is well developed on both 

sides of the Continental Divide near Breckenridge. 

Large portions of South Park, Middle Park, and North 

Park are underlain by shale of Montana age, and the 

shale overlying the Niobrara formation at Breckenridge can be traced down the Blue River to Middle 

Park and thence up Muddy Creek into North Park. 

No detailed work has been done on these shales in 

either South Park or in Middle Park, but according to 

Beekly 25 the Pierre shale in North Park was definitely 

identified and is about 4,500 feet thick. It is predominantly dark olive-gray shale but contains a few 

thin beds of limy sandstone; in general its lithology is 

similar to that of the shale of Montana age exposed 

along the Blue River from Breckenridge to Kremmling. 

In the Aspen district the shale of Montana age was 

estimated to be about 4,000 feet thick by Spurr, 26 but 

in the Anthracite and Crested Butte quadrangles, a 

short distance south of Aspen, Eldridge 27 found a 

maximum thickness of 2,800 feet.

Ransome did not subdivide the Cretaceous in the 

Breckenridge district and estimated that the shale 

present had a thickness of at least 3,500 feet and probably as much as 5,500 feet, but the presence of an unrecognized overturned isoclinal fold in the wash-covered 

region of the eastern part of the district caused the 

duplication of some shale and made the latter amount 

somewhat excessive for the district. As pointed out 

by Ransome,28 the thickness of the shale can be best 

estimated by a study of the little-disturbed exposures 

about 10 miles down the Blue River. In figure 4 are 

shown the sections made by the writer at Keystone

M Beekly, A. L., op. cit., pp. 43-49. 

!'" «Spurr, spuJ. jE., £>.,op. cit.,c p. 43. in.

27 Cross, Whitman, and Eldridge, Q. H., op. cit., p. 6.

28 Ransome, F. L., op. cit., p. 40.

and at the Palmer ranch, about 20 miles below Dillon, 

and they indicate that at least 4,750 feet of Pierre shale 

overlies the Niobrara formation. The greatest thickness of Pierre known in the Breckenridge district 

o'ccurs in the southeastern part, where there may be 

as much as 3,500 feet, with due allowance for the overturned fold and local faulting.

In the special area the Pierre shale does not exceed 

500 feet in thickness. It is uniformly thin-bedded 

black or dark olive-gray noncalcareous clay shale, containing no fossils and no beds distinguishable through 

differences in color or in lithology. The following sections show the character of the Pierre shale some distance north of Breckenridge and are presented here 

in the hope that they may be of use to anyone working 

in the district outside of the special area considered.

Section of Cretaceous on north side of Snake River at Keystone, 11 

miles north of Breckenridge

Pre-Cambrian schist thrust-faulted onto Pierre shale. 

Pierre shale: Feet 

Black fissile shale.__________________--_--____-- 20

Black sandy sha]e-_-------__--_------_----__--- 10

Greenish-gray limy sandstone____-___--__-__._-_ 25

Dark-gray sandstone.________-_______-__-_-._-- 15

Interbedded shaly gray sandstone and shale._____ 30

Black and dark olive-gray shale-.--___--__-..__-__ 1, 400 

Thin-bedded greenish-gray sandy shale ___________ 200

Thin-bedded fossiliferous limy brownish-gray sandstone, ridge-forming member. ______---_-______ 120

Fissile gray limy shale._____________________ ____ 150

Thin-bedded hard sandy and limy shale.__________ 30

Thin-bedded to fissile black, dark olive-gray, and

dark-brown clay shale.-_______-_--_----____-_ 1, 850

Thin-bedded slightly limy brown shale containing 

fossil fragments, interbedded with blue-black noncalcareous shale.---_-_------_----- ----------

Blue-black fissile clay shale..----------...._--___.

10

115

3,975

Niobrara formation:

Blue-gray thin-bedded limy shale containing abundant secondary calcite..__ ____________________ 30

Fissile blue-black limy shale, containing several

beds of coarse gray limestone 1 to 2 inches thick. 30

Fissile gray limy shale, separated from overlying

bed by 3-inch layer of white calcite____________ 40

Thin-bedded shale, largely covered.________-_..__ 240

Dark grayish-black thick-bedded, dense fossiliferous limestone..-- ___.____________-_---___-_ 10

350

Benton shale:

Thin-bedded bluish-gray limestone, grading downward into thin-bedded purplish limy shale.._ _ _ _ _ 30 

Covered but mostly black shale _________________ 340

370

Dakota quartzite: 

Massive and 

quartzite--.

medium-bedded fine-grained gray

47
GENERAL GEOLOGY 13

Section of Cretaceous shale exposed at and above Palmer ranch, on 

northeast side of Blue River about 20 miles northwest of Dillon

Pre-Cambrian gneiss faulted onto the Cretaceous rocks. 

Pierre shale: Feet 

Dark-gray limy sandstone.______________________ 100

Massive soft white sandstone.-_-____._-_____---- 25

Dark sandy shale._..__________________.--.-_-- 25

Black fissile shale._._._.._.--.--...---_..-----_ 400

Dark-gray limestone.__________--___-------_--_. 2

Black fissile shale........_._._..._.._...._.__.. 200

Gray fissile shale...____--_---_-_-_-_---___----- 118

Medium-bedded fossiliferous, cross-bedded lightbrown sandstone.___________-______-___--__-- 50

Dark shale containing a few thin limy layers near

base...__.._....-._.-..__-..-__.--.._....--- 1, 100

Thin-bedded brown sandy limestone with thin shale

partings-.--^--..--------------------------- 25

Dark-gray shale.-.___-_.___-___.___-______---- 225

Medium-bedded gray limy shale.__._-----_._---- 10

Thin-bedded dark-gray shale_.________-_-___---- 10

Thin-bedded, slightly oolitic limy sandstone.._..-- 10 

Thin-bedded grayish-brown limestone._____._.--- 10

Medium-bedded light-brown limestone, cliff-forming. 15 

Grayish-black limy thin-bedded shale....___-_---- 15

Medium-bedded brown shaly limestone; cliffforming member.___________-______..______--- 10

Thin-bedded black shale..._-.___-___--.___..--. 150 

Thin-bedded limestone and dark-gray limy shale... 125 

Medium-bedded light-brown sandy limestone....-- 10

Fissile grayish-black shale..-...---.-.----------- 180

Thin-bedded gray sandy limestone, with shale

partings; cliff-forming member ____-_.__--_---_ 20

Fissile grayish-black shale with a few ^-inch layers

of limestone at the base.....___'___._______.--- 1, 160

Covered by river gravel. 3, 995

The correlation of these two sections is indicated in 

the graphic sections shown on figure 4. The following 

table shows the fossils found in making the sections 

and their distance above the Niobrara. The fossils 

were identified by J. B. Reeside, Jr. '

Keystone: Feet 

Baculites ovatus Say var. haresi Reeside.._.______ 2, 600

Clionia sp......____....._........_.__.....:... 2, 300

'° Inoceramus sagensis Owen._______________ 2, 300, 2, 200

Mactra aft. M. nitidula Meek and Hayden.___._-_ 2, 200 

Inoceramus barabini Morton._____________ 3, 930, 2, 200

Baculites ovatus Say.________._____--..-..___-_ 2, 200

Palmer ranch:

Ostrea cf. 0. larva Lamarck.___._______________ 4,600

Inoceramus afF. I. barabini Morton.______._._.___ 3, 850

Pteria linguaeformis Evans and Shumard. ____..-. 3, 850

Protocardia subquadrata Evans and Shumard.____ 3, 850

Dentalium gracile Evans and Shumard___________ 3, 850

Baculites ovatus Say.____________________ 2, 150-2, 750

Baculites compressus Say?.__----_ 2, 150, 2, 750, 3, 850

Inoceramus sagensis Owen. 2,150, 2,750, 3,850, 4,600, 4,750 

Vanikoro sp._.___---._____--_--___-____--._--. 1, 650

Fish scale.-----__----.___------_-__-__--____-. 1, 650

QUATERNARY GRAVEL

The Quaternary deposits of the Breckenridge district 

were not mapped by the writer, as they were carefully 

studied and mapped by Ransome, 29 and the reader is 

referred to his report for a map and a comprehensive

«« Rausouie, F. L., op. cit., pp. 72-80.

discussion of them. The materials deposited by the 

ice and streams of the Quaternary period may be 

classified as early glacial, late glacial, and Recent 

deposits. It is very difficult, however, to make age 

distinctions in the various kinds of soil and slope wash 

covering the hills between the valley deposits.

Early glacial deposits. The high-terrace gravel 

deposited by the streams of the early glacial stage is 

well developed on both sides of the Blue River from a 

point about a mile south of Breckenridge north to 

Dickey and is well shown on the spur east of Breckenridge between French Gulch and the Blue River, where 

the deposits are over 250 feet above the present stream 

channels. Ransome reports that west and southwest 

of Breckenridge they probably attain altitudes of 650 

feet above the Blue River.

The terrace-gravel deposits are made up chiefly of 

well-rounded boulders and pebbles and contain very 

little fine sand or clay. The largest boulders are about 

4 feet in diameter, but most of them are less than 15 

inches. Quartzite, porphyry, granite, and schist are 

the most common materials but are intermingled with 

micaceous red grit, red shale, and black shale. Most of 

the fragments of these formations are deeply weathered, 

and generally the boulders of granite, gneiss, and red 

sandstone readily go to pieces under the hammer.

The older slope wash contains a much larger proportion of admixed soil than the high-terrace gravel, 

and the fragments of bedrock are much more angular 

and as a rule are less weathered.

Late glacial deposits. The moraines of the late 

glacial stage are well exposed on the Blue River about 

half a mile above Breckenridge. Moraines are also 

present in French Gulch a short distance below Lincoln 

and can be found in most of the other valleys that head 

in the high mountains. The extensive flats about 15 or 

20 feet above the level of the present streams are 

composed of the outwash gravel from the last glaciers. 

These low-level gravel deposits are about 90 feet thick 

at Breckenridge and gradually decrease to about 60 feet 

near the mouth of the Swan River. The terminal 

moraine rises about 80 feet above the low-level gravel 

at the north end of Little Mountain, and its thickness 

must be at least 170 feet in this region.

All the bedrock formations are represented in the 

pebbles and boulders of the late glacial deposits and 

are usually fresh and unweathered. The largest 

boulders in the low-level terraces near Breckenridge 

are about 6 feet in diameter, but the maximum size 

decreases steadily downstream. The material in the 

low-level terraces is very poorly sorted, and there is 

little evidence of stratification in any of the exposures.

The writer has elsewhere presented evidence for 

correlating the early glacial gravel of the Front Range 

with the Yarmouth deglaciation of the central United 

States,30 and the late glacial deposits have long been 

correlated with the Wisconsin glacial stage.

so Levering, T. S., op. cit., pp. 104-106.
14 BRECKENRIDGE MINING DISTRICT, COLORADO

Recent deposits. The postglacial deposits consist 

chiefly of small alluvial fans, lake beds, stream 

alluvium, and slope wash. In the steep-walled glacial 

valleys tributary streams have formed small alluvial 

fans at many places. Here and there in the terminal 

moraines and the ground moraines poorly drained 

hollows are the sites of swampy lake beds. The most 

extensive of these is the swampy meadowland known 

as the Goose Pasture, about 2 miles south of Breckenridge. The flood-plain deposits of the present streams 

are areally insignificant, but narrow belts of alluvium, 

generally covered with willow thickets, occur at many 

places along the watercourses of the district. Slope 

wash of recent age is almost confined to the lower 

slopes of the valley sides, where post-Yarmouth 

erosion has exposed new bedrock surfaces.

All the recent deposits are thin and discontinuous. 

The sorted material ranges from fine lake silts through 

the coarse sand of the stream deposits to the coarse 

gravel of the fans. The recent slope wash is indistinguishable from the older slope wash and consists of 

subangular rock fragments in a generous admixture of

soil.

IGNEOUS ROCKS

GENERAL. FEATURES

The igneous rocks in the Breckenridge district are 

monzonitic porphyries. In common with the intrusive rocks of the nearby mining districts, most of the 

porphyry is close to a true monzonite in composition, 

but both silicic and calcic facies are common. The 

silicic facies or quartz monzonite porphyry is more 

abundant than the calcic or dioritic facies. Ransome 31 

recognized three main types (1) the silicic type, 

exemplified by quartz monzonite porphyry with 

phenocrystic quartz and orthoclase; (2) the calcic 

type, exemplified by monzonite porphyry grading into 

diorite porphyry, with only incidental quartz in the 

groundmass; and (3) the intermediate type, which 

lacks orthoclase phenocrysts but in which quartz is 

commonly present as phenocrysts.

Ransome did not find it practicable to attempt the 

separate mapping of the intermediate type in the time 

available. Although the structural relations of these 

porphyries to one another were obscure, he concluded, 

on the basis of the scant evidence then available, that 

the porphyries on the whole were probably intruded 

close together in time, but that the monzonite porphyry was intruded before quartz monzonite.32 The 

present writer has confirmed this general relation. 

The monzonite porphyry usually occurs in sills and 

is cut by the quartz monzonite, which commonly 

occurs in stocks and dikes.

The intermediate type was not found cutting the 

monzonite porphyry, and the only basis for regard31 Ransome, F. L., op. cit., pp. 44, 50, 57. 

" Idem, p. 71.

ing it as somewhat younger is that in the normal 

sequence of differentiation calcic rocks precede silicic 

rocks. The coarsely porphyritic quartz monzonite 

cuts the intermediate type a short distance north of 

Bacon. The intermediate type has been mapped as 

a separate unit only in the southern part of the special 

area. It occurs elsewhere in the district, notably on 

Mount Guyot and at the mouth of Gibson Gulch in 

French Gulch, where its contact with the monzonite 

porphyry was so indistinct that it could not be mapped 

separately.

The association of monzonite porphyry with the 

"mineral belt" of Colorado has long been recognized.33 

From Leadville to Boulder the productive mines are 

confined to a narrow belt trending northeast, and the 

same belt includes nearly all the monzonitic porphyry 

in the region. There has been a growing tendency 

to recognize a genetic relationship between the quartz 

monzonite porphyry and the ore deposits of central 

Colorado.34 The work of the writer in the Breckenridge and Montezuma districts has only served to 

confirm the theory that the ore deposits are the result 

of emanations from the deeper portions of the quartz 

monzonite magma.

The age of the monzonite porphyry is commonly 

given as late Cretaceous or early Tertiary 3S because 

dikes have been found cutting the Pierre shale and 

other sediments at the northeast end of the mineralized 

belt, near Boulder, in such association with folds as 

to suggest that the intrusion was contemporaneous 

with the major folding that formed the great foothill 

monocline 36 during the Laramide revolution. A large 

stock of quartz monzonite prophyry in the Montesuma district cuts a thrust fault that has brought preCambria,n gneiss above Pierre shale. As this fault 

is one of the major structural features of the west side 

of the Front Range and can be traced for 50 miles, it 

also is probably related to the Laramide revolution, 

which is commonly regarded as marking the transition 

from the Cretaceous to the Tertiary. The intense 

folding that occurred at this time involved the Denver 

and Middle Park formations on both sides of the 

Front Range but did not affect Oligocene sediments 

nor any unquestionable Eocene sediments. The 

Flattop peneplain, which was well developed before 

the end of the Eocene epoch, is distinctly later than 

the quartz monzonite prophyry stock referred to above. 

These facts indicate that the age of the quartz monzonite porphyry is Eocene.

33 Spurr, J. E., Garrey, G. H., and Ball, S. H., Economic geology of the Georgetown quadrangle, Colo.: U.S. Geol. Survey Prof. Paper 63, pp. 67-71,1908.

« Crawford, B. D., A contribution to the igneous geology of central Colorado: 

Am. Jour. Sci., 5th ser., vol. 7, pp. 381-388, 1924. Emmons, S. F., Irving, J. D., 

and Loughlin, G. F., Geology and ore deposits of the Leadville mining district, 

Colo.: U.S. Geol. Survey Prof. Paper 148, p. 209, 1927.

» Spurr, J. E., Garrey, G. H., and Ball, S. H., op. cit., p. 71.

8« Fonneman, N. M., Geology of the Boulder district, Colo.: U.S. Geol. Survey 

Bull. 265, p. 40, 1905.
GENERAL GEOLOGY 15

MONZONITE PORPHYRY,

The petrography of the igneous rocks of the Breckenridge district has been exhaustively studied by 

Ransome,37 and the reader is referred to his report for 

detailed descriptions of the different varieties of 

porphyry. Only a summary of their general features 

will be given here.

Unaltered specimens of the monzonite porphyry 

arc dark gray on fresh fracture. Nearly everywhere 

in the Breckenridge district where the rock is exposed 

at the surface it presents some shade of green owing 

to the development of secondary chlorite and epidote. 

The porpbyritic texture is inconspicuous in the hand 

specimen, as few of the phenocrysts exceed 5 millimeters in length. Hornblende is usually the most 

conspicuous of the phenocrysts, but on some facies 

biotito is more abundant. The feldspar crystals are 

uniformly small and generally dull white.

Under the microscope biotite appears to be as 

common as hornblende, and both minerals are altered 

to chlorite, epidote, quartz, and calcite in varying 

degrees of completeness. Some augite is present and 

magnetite is common. The foregoing minerals are 

usually more or less intergrown with one another, 

whereas the plagioclase shows little tendency to form 

aggregates. The plagioclase ranges from a calcic 

andesine (AbiAn,) to a labradorite (Ab2An3 ). In the 

more calcic varieties of monzonite prophyry the 

orthoclase is confined to the groundmass, and in the 

less calcic varieties the orthoclase phenocrysts have 

no unusual features. In addition to orthoclase the 

groundmass contains abundant laths of plagioclase 

and minor amounts of quartz, biotite, augite, hypersthene, hornblende, magnetite, apatite, allanite, and 

zircon. The texture of the groundmass is usually 

holocrystalline.

The common alteration products of the feldspar are 

calcite, epidote, and sericite, and the ferromagnesian 

minerals are more or less changed to chlorite, epidote, 

quartz, and calcite.

Most of the porphyry in the special area is monzonite. It generally occurs as sills, some of which are 

very thick, and only in a few places was it observed to 

break across the bedding of the sediments which it 

invades. It is conspicuous on Nigger Hill, Gibson 

Hill, and Prospect Hill and in the valley of French 

Gulch, and forms large masses in Mount Guyot and 

Bald Mountain, south of the special area. A dike of 

quartz monzonite porphyry cuts the older monzonite 

porphyry above the Wellington mine on Prospect Hill 

and establishes the age relations already indicated.

INTERMEDIATE TYPE

The intermediate type of porphyry is gray where 

fresh and grayish green where altered. Fresh material

8' Ransomo, F. L., op. cit., pp. 43-62.

is more frequently observed in this type than in the 

monzonite porphyry. It usually has many phenocrysts of biotite, or locally of hornblende, from 3 to 

5 millimeters in diameter. The gray plagioclase crystals are easily visible but are not conspicuous. The 

size and abundance of the quartz phenocrysts vary 

greatly; in many localities they can scarcely be detected in the hand specimen, but in others quartz is 

one of the most conspicuous phenocrystic minerals. 

Orthoclase is practically confined to the groundmass 

and was not observed to form phenocrysts in any locality. The presence of quartz phenocrysts distinguishes 

'this porphyry from the monzonite porphyry, and the 

lack of conspicuous crystals of orthoclase makes it 

easy to separate from the quartz monzonite porphyry 

discussed in the next section.

Under the microscope the rock has the general appearance of the monzonite porphyry except in the 

character of the groundmass and the presence of corroded phenocrysts of quartz. The phenocrysts are 

andesine, biotite, hornblende, magnetite, and quartz. 

The groundmass is strikingly different from that 

typical of the monzonite porphyry; the network of 

minute plagioclase laths is absent, and in its place 

is a mosaic of fine-grained orthoclase and quartz, very 

similar to the groundmass of the coarsely porphyritic 

quartz monzonite described below.

QUARTZ MONZONITE PORPHYRY

The fresh quartz monzonite porphyry is generally 

light gray and weathers to buff or light-brown tints. 

The altered facies are usually of a much lighter shade 

of greenish gray than the monzonite porphyry altered 

under similar conditions. An outstanding feature of 

the quartz monzonite porphyry, and one of the best 

guides to its recognition in the field, is its conspicuously porphyritic appearance. Phenocrysts of orthoclase as much as 8 centimeters in length are common; 

quartz crystals as large as 1 centimeter in diameter and 

plagioclase laths as much as 1 centimeter in length are 

also abundant. Phenocrysts of hornblende are uncommon, and biotite is found only in moderate amounts 

as a megascopic constituent. The groundmass is generally very fine grained and presents a marked contrast to the large phenocrysts a contrast that is lacking in the monzonite porphyry and enables the two 

rocks to be easily distinguished in the field.

Under the microscope the difference between the 

groundmass and the phenocrysts is accentuated and 

is much more striking than in the monzonite porphyry. 

The scattered phenocrysts of orthoclase are so large 

that they dwarf the other porphyritic constituents, 

but seldom does an orthoclase phenocryst appear in a 

thin section. The moderately large crystals of andesine-labradorite (AbiAnO and corroded quartz are invariably conspicuous in the fine-grained mosaic of 

quartz and orthoclase that makes up the groundmass.
16 BRECKENRIDGE MINING DISTRICT, COLORADO

Some phenocrystic biotite is present in nearly all 

specimens, but hornblende is rare. The common accessory minerals include allanite, magnetite, titanite, 

zircon, and apatite. Alteration products are much 

less common than in the monzonite porphyry and include chiefly chlorite and calcite.

The quartz monzonite porphyry is abundant in 

French Gulch, near the northwestern boundary of the 

special area. It forms a small stock that has caused 

contact metamorphism in the limy shale of the Morrison formation nearby. Dikes of the coarsely porphyritic quartz monzonite cut monzonite porphyry on Prospect Hill and in Prospect Gulch, and the intermediate 

quartz monzonite porphyry is cut by the coarsely 

porphyritic quartz monzonite a short distance north 

of Bacon.

The late age of the porphyry is established by these 

relations, which are similar to those existing between 

these types of porphyry in other districts.38 Ransome 

correlated the quartz monzonite porphyry of the 

Breckenridge district with the Lincoln porphyry of the 

Tenmile and LeadviJle districts because of their petrographic similarity, and the age relations a well as the 

form of occurrence of the two porphyries confirms this 

correlation.

ROCK ALTERATION 

 'CONTACT METAMORPHISM"

The larger masses of monzonite porphyry generally 

occur as thick sills, and this form of intrusion, here as 

elsewhere, rarely caused much change in the rocks 

which it invaded. Shale was locally indurated; limy 

shale assumed a greenish cast, probably caused by the 

development of small amounts of epidote; and the 

rocks near, the contacts of the sill may have been shattered and show slight variations of color and hardness 

from that normal to them.

The atocklike masses of quartz monzonite porphyry, 

however, have caused marked alteration in many 

places. Contact-metamorphic minerals were noted at 

several localities by Ransome,39 but only one of these 

is in the special area. The small stock in French 

Gulch, due east of Breckenridge, has had a marked 

effect on the calcareous beds near by. On Prospect 

Hill and Gibson Hill the limy shale beds of the Morrison have provided favorable horizons for the development of garnet, epidote, dark-green amphibole, quartz, 

specularite, magnetite, and small amounts of pyrite 

and chalcopyrite. Close to the quartz monzonite porphyry the garnet and epidote are the most conspicuous 

minerals in the limy beds, but the garnetization was 

generally limited to the formations within 1,500 feet of 

the stock, whereas epidote and quartz replaced favorable beds at much greater distances. The other contact minerals mentioned are locally abundant but are 

sparingly developed in general.

»« Crawford, B. D., op. cit., pp. 377, 378. 

" RaDsome, F. L., op. cit., pp. 93-94.

Metamorphism is shown only on the north side of 

French Gulch, where garnetized shale crops out from 

the Wellington mine to the western slope of Gibson 

Gulch, a distance of about a mile. The apparent lack 

of metamorphism on the south side of French Gulch 

may simply reflect the absence of favorable horizons at 

the surface, and it is possible that the calcareous beds 

that underlie the Dakota quartzite in this region have 

undergone changes similar to those which they show 

on the north side of the valley.

The development of " contact-metamorphic" minerals is thought to result from the action of hot magmatic fluids given off from the deeper portions of the 

slowly cooling quartz monzonite mass. As Ransome 40 

pointed out, there is probably no sharp line of demarcation between these solutions and those that deposited 

the sulphide ores of the district. The rarity of hematite and the presence of sulphides in the metamorphic 

aggregates indicate that the solutions were probably 

reducing in character shortly after the early stage, 

when '' contact'' silicates were formed. This character 

is reflected in the alteration of the different formations.

HYDROTHERMAL ALTERATION

Ransome 41 made a thorough study of the alteration 

of the porphyry wall rocks of the veins, and the reader 

is referred to his report for detailed consideration of 

this subject. The following summary of the changes 

in the rocks near the veins gives the results of Ransome ;s work supplemented to a small degree by the 

writer's observations.

The bright-red sediments of the Maroon formation 

were changed to green or gray in zones of contactmetamorphic action or near ore channels, and a similar 

change was noted in the red shale of the Morrison formation. This change in color resulted from the reduction of the oxidized iron in the red beds to a ferrous 

state. The black shale of the various formations generally remains black even where the beds have been 

completely silicified. The addition of pyrite to the red 

sediments was invariably attended by a bleaching of 

the color, but weathering of pyritized rocks resulted in 

the oxidation of the iron, and under these conditions 

the rock became reddish brown at the surface, regardless of its original color. The warm reddish brown of 

the weathered Pierre shale on Mount Guyot, west of 

Georgia Pass, is due to the oxidation of finely disseminated pyrite.

In regions where hydrothermal alteration was intense the arkosic grit of the Maroon formation was 

altered to dense aggregates of quartz, chlorite, ankerite, pyrite, and sericite, but in the shale of this.formation the chief effects of the ore-bearing solutions were 

the bleaching of the color and the development of chlorite, pyrite, and chalcedonic quartz. Where calcareous

<° Ransome, F. L., op. cit., p. 93. 

<i Idem, pp. 94-102.
GENERAL GEOLOGY 17

bods formed the walls of veins they were usually 

silicified and replaced by pyrite and to a less extent 

by other sulphides.

The dark Cretaceous shales have been almost completely converted into jaspery silica near some of the 

larger veins of the district. This dense black silica 

is similar to the "flint" or jasperoid of the Leadville 

district in its occurrence. 42 It is in part associated 

with ore channels but occurs as abundantly in the 

barren parts of a vein as it does near ore shoots.

In many places near veins the monzonite porphyry 

and the quartz monzonite porphyry were almost completely altered to sericite, quartz, and ankerite with 

minor amounts of pyrjte. Commonly as a vein is 

approached ankerite increases, and sericite at first 

increases rapidly and then decreases, but quartz increases steadily, although owing to the replacement 

of the original silicates by ankerite and sericite the 

total silica content decreases, as is illustrated by a 

series of specimens collected by Ransome, grading 

from fresh, unaltered monzonite porphyry 26 feet 

from the Wellington vein to the completely altered 

wall rock 6 inches from the vein. During its alteration the monzonite porphyry lost approximately 19 

percent of its silica, 22 percent of its alumina, all of 

its ferric iron, 73 percent of its calcium oxide, 70 percent of its sodium oxide, and 50 percent of its potassium oxide, and gained 260 percent in ferrous iron, 

165 percent in magnesia, 485 percent in water, 2,500 

percent in carbon dioxide, whereas titanium and 

phosphorus showed little change. Small amounts of 

pyrite (2 percent), sphalerite (1 percent), and galena 

(0.5 percent) were introduced.

Near some of the gold veins in the quartz monzonite 

porphyry the rocks have been strongly silicified and 

sericitized, pyrite is abundant, and no ankerite or 

other carbonate is present. Near the Jessie mine the 

quartz monzonite had apparently lost all its ferrous 

and ferric oxide, carbon dioxide, and magnesia, almost 

all of its calcium oxide, and 87 percent of its sodium 

oxide and had gained about 770 percent of pyrite (from 

0.09 to 9.92 percent), 300 percent of water, 18 percent 

of potassium oxide, and 3 percent of silica; alumina, 

titanium, and phosphorus showed little change. Small 

amounts of lead, zinc, and copper sulphides were introduced. The analyses on which these figures are based 

show the effects of moving solutions whose composition was constantly changing. The different types of 

rock alteration may be explained by assuming either 

that they were typical of different distances from the 

magmatic source of the solutions or that they resulted 

from magmatic solutions whose initial compositions 

were different. In general, sericitization has been the 

dominant type of rock alteration in the Breckenridge 

district and was commonly accompanied by the formation of ankerite. Silicification has been extensive

Einmons, S. F., Irving, J. D., and Loughlin, O. F., op. cit., pp. 217-218.

in many places and commonly represented alteration 

at a higher temperature than the sericitization.

Another kind of rock alteration, and the one that 

has been most widespread in the district, is ascribed 

by Ransome to the action of solutions rich in carbon 

dioxide that were derived from the zone of weathering 

but were active well below the level of ground water. 

In all the specimens of monzonite porphyry collected 

from surface exposures the ferromagnesian minerals 

were altered chiefly to chlorite and to a lesser degree 

to calcite and epidote, and the feldspars generally 

contained some calcite, sericite, and kaolinite. Some 

pyrite and secondary quartz are also common.. The 

alteration of the ferromagnesian minerals to chlorite 

is typical of the rock alteration called propylitization 

and is usually associated with the milder phases of 

hydro thermal action accompanying ore deposition. 

However, Ransome states that the propylitized rocks 

are restricted to a zone within 300 feet of the surface 

and that this type of rock alteration shows no relation to the channels of mineralization. The writer is 

unable to add new data on this subject. The relation 

of the propylitized rock to the surface must be more 

than accidental, but the development of sericite and 

epidote indicates the action of hot solutions, and it is 

worthy of note that the later porphyry is less propylitized than the monzonite porphyry.

STRUCTURE

Regional features. The structure of the Breckenridge district is only a small detail of the structure of 

a much larger region that embraces all the mountainous part of Colorado and continues into Wyoming 

and New Mexico. The outstanding features of the 

region surrounding Breckenridge have been described 

elsewhere by the writer 43 and will only be briefly 

reviewed here.

The sediments of the Breckenridge district lie in a 

structural trough about 10 miles wide that extends 

north-northwestward from South Park to Middle Park. 

The belt of sediments is about 8 miles wide and is 

flanked by pre-Cambrian rocks on the east and west. 

The older formations in South Park, such as the 

Cambrian, Ordovician, Devonian, and Carboniferous, 

thin rapidly to the north and disappear successively, 

allowing younger and younger sediments to rest on 

the pre-Cambrian floor. This progressive overlap is of 

economic importance, as it has resulted in the absence 

from the Breckenridge district of the Ordovician and 

Leadville limestones, noteworthy ore-bearing beds in 

Leadville, Aspen, and other mining districts of central 

Colorado.

Owing to isoclinal folding, the regional dip is eastward throughout the structural trough. The oldest 

formations rest on the pre-Cambrian rocks of the Tenmile and Gore Mountains and pass eastward under an

"Levering, T. S., Geologic history of the Front Range, Colorado: Colorado Sci. 

Soc. Proc., vol. 12, no. 4, pp. 59-111,1929.
18

ever increasing thickness of sediments until folded and 

faulted back on themselves at the western edge of the 

Front Range. About 5 miles east of Breckenridge, the 

sediments are limited by an overturned fold, which 

passes northward into a thrust fault of large magnitude. 

The overturned fold is well shown a short distance 

southwest of Georgia Pass, where the contact of the 

Dakota quartzite and the overlying pre-Cambrian 

gneiss dips 65°-70° E. This fold passes northward, 

about a mile north of Tiger, into the Williams Range 

thrust fault,44 which the writer has interpreted as an 

under thrust. From this locality to the Lower Muddy 

Butte, a few miles north of Kremmling, the Cretaceous 

shales are thrust eastward under the pre-Cambrian 

rocks. Near Keystone there is a known horizontal 

movement of 4 miles, and the Cretaceous shales may 

continue their eastward dip under the pre-Cambrian 

rocks for some distance farther.

The Mosquito fault forms the western boundary of 

the tetonic block that is limited oh the east by the 

Williams Range thrust fault and the overturned fold 

and is regarded as a structural unit. The Mosquito 

fault extends from Leadville to the Tenmile district, 

and according to Emmons,45 it probably continued 25 

miles to the northwest of the west fork of Tenmile 

Creek. It is a normal fault near Leadville, but the 

fault plane steepens as it is followed to the north, and 

in the Tenmile district it overturns and becomes a 

reverse or thrust fault. Where it is a thrust fault, 

the pre-Cambrian rocks overlie the Maroon formation, 

just as they overlie the Cretaceous shales along the 

Williams Range thrust fault 10 miles to. the east.

The trough of the overturned synclinal fold must 

have been a weakened zone, especially favorable for 

intrusive activity. From South Park to the region 

north of Tiger, where the overturned fold gives way to 

the Williams Range thrust fault, the axis of the fold is 

marked by an almost continuous series of large 

porphyry masses. Much of this porphyry occurs as 

large sills of monzonite and diorite porphyry, but stocks 

of quartz monzonite porphyry are also present. The 

intrusion of the porphyry into the strongly folded sediments was preceded and accompanied by faulting, 

and most of the faults and nearly all the porphyry 

masses occur in the eastern half of the tectonic block 

outlined above. These faults and porphyry masses 

form the westernmost part of a transverse zone of 

fissuring and subsequent ore deposition that extends 

from Breckenridge to Boulder. This zone is more 

conveniently described and interpreted on pages 20-21.

Local features. In the region north of the Breckenridge district, where exposures are better, studies by 

the writer have resulted in a better understanding of

« Levering, T. S., Williams thrust fault: Geol. Soc. America Bull., vol. 39, p. 173, 

1928; Field evidence to distinguish overthru'sting from underthrusting: Jour. Geology, vol. 40, pp. 660-663,1932.

« Emmons, S. M., U.S. Geol. Survey Geol, Atlas, Tenmile district folio (no. 48), 

p. 3, 1898.

the details of the geologic section and the general 

structure than is possible from work confined to the 

district. Much helpful information, which was also 

unavailable to Ransome, has been gained in the lower 

levels of the Wellington and Golddust mines and in a, 

study of the bedrock fragments on the dredge piles. 

The interpretation of local structure which follows 

thus differs in many respects from that in Professional 

Paper 75. The overturned asymmetric regional syncline broken by the underthrust fault in the northeastern part of the district, the closely compressed 

syncline north of Rocky Point, and many details of 

the fault pattern and of the distribution of the sediments, shown by the writer, were not recognized 

during Ransome's work in the district.

The special area in the Breckenridge district is on 

the western limb of the regional syncline about 2 miles 

west of its axis. As shown on the geologic map (pi. 2) 

and the cross sections (pi. 5), both open and moderately 

close minor folds and crumplings, which are present at 

places on both limbs of the regional fold, are well 

defined in the special area. The larger elements of 

structure in the special area are moderately simple and 

should be pictured before their features are considered 

in detail. In the southwestern part the prevailing 

northeasterly dip of the region is interrupted by an 

anticline and syncline, both of which trend northwest. 

Two broken, down-faulted belts cross these folds and 

intersect near the central part of the special area. One 

of the belts trends north-northeast and the other eastnortheast.

A compressed syncline extends northwest from 

Bacon to a point about 500 feet south of Little Mountain. Both limbs dip toward the axis at about 60°, 

and formations from the Pierre to the Maroon are 

uniformly exposed on the southwestern ,limb. The 

northeast side of the fold passes into a gentle anticline, 

with northwest pitch, which extends northwestward 

from the head of Illinois Gulch through Nigger Hill to 

Breckenridge. This anticline is broken by cross faults 

and is the site of large porphyry intrusions. As a 

result the formations on each side of the anticlinal' 

axis are markedly discontinuous and contrast with 

the uniform linear belts of sediments to the southwest. 

North and northeast of this anticline the formations 

have a general dip of 20°-30° NE., conforming to the 

regional structure, as shown in section A (pi. 5). 

There is evidence of gentle doming on Gibson Hill, 

but even here the prevailing dips are toward the east 

or northeast.

All of the area northeast of the compressed syncline 

is so complexly faulted and so broken by monzonite 

porphyry sills and cross-cutting masses of quartz 

monzonite porphyry that the structure is none too 

clear on the map; neither does the detail of the intricate 

faulting lend itself to clear description, but, as noted 

above, the major features can easily be pictured.
GENERAL GEOLOGY 19

The southern part of the north-northeasterly belt 

of faults is bounded on the west by the strong fault 

that trends north-northeastward through the saddle 

on Little Mountain, across Illinois Gulch, and through 

the saddle 1,000 feet east of the United States locating 

monument on Nigger Hill. This fault marks the 

western limit of the large masses of monzonite porphyry 

that extend from Nigger Hill southeast to Bald Mountain. 40 This fault has a vertical displacement of about 

350 feet at Little Mountain and about 750 feet on the 

south side of Nigger Hill, but this decided increase in 

throw is probably due to the presence of a branch 

fault striking nearly east in Illinois Gulch. The east 

side of the major fault has dropped, so that porphyry 

and Cretaceous shale on the east are brought against 

Morrison on the west. On the north side of Nigger 

Hill the displacement is distributed on several faults 

striking northeast, and the zone of movement is 

stepped eastward as it is followed to the north.

The dropped belt east of this fault zone continues 

across French Gulch, where it is limited on the west by 

the Bullhide fault, which extends up the small gully 

about 200 feet west of the Wellington mill. In this 

locality the displacement is hard to calculate because 

of the many branching faults that are present. The 

Pierre shale is faulted against the Morrison formation 

near the Wellington mill, and a vertical movement of 

about 800 feet is indicated. At 800 feet north of the 

mill a quartz monzonite porphyry dike is offset 300 

feet along the Bullhide fault. Underground work 

shows that the Bullhide fault dips about 58° E. and is 

therefore a normal fault. If the dike is vertical the 

displacement indicates that the western wall of the 

fault moved 300 feet south, but if the porphyry dike 

dips to the south the apparent lateral movement may 

represent the normal offset caused by a fault with a 

large vertical displacement and a small horizontal 

component.

South of French Gulch the east side of the northnortheasterly belt of dropped ground is raised gradually 

along a series of north-northeast faults of comparatively 

small throw. As the Nigger Hill anticline plunges 

northwest, older and older rocks rise to the surface 

southeastward along the fold. This upward trend of 

the formations is accelerated by the step faulting along 

the eastern border of the downthrown belt. Most of 

the faults of this series whose dips are known are 

normal.

The other down-faulted grabenlike belt extends east 

and northeast from Breckenridge, crossing and accentuating the central part of the north-northeastward 

trending dropped belt described above. The northern 

limit of this second down-faulted belt runs nearly due 

east from Breckenridge across French Gulch and turns 

northeast into Prospect Hill at the Wellington mill. 

In Prospect Hill the Wellington vein is in the chief

« Ransonie, F. L., op. cit., pi. 1. 

18293 -34 4

fault on the north side of the depressed belt. This 

fault is normal, dips about 63° SE., and has a dip slip 

of about 110 feet.

The south limit of this belt extends eastward from 

the north side of Little Mountain across Illinois Gulch 

and enters Nigger Hill near the Golddust shaft. Beyond 

this place it turns gradually to the northeast and 

appears on the southeast side of French Gulch near the 

mouth of Australia Gulch. The Golddust vein and 

the Country Boy veins are the best-known veins on 

fault fissures along the southern border of this downthrown belt. The Country Boy vein is in a normal 

fault, but the Golddust vein is in a steep reverse fault. 

This complexly faulted belt is limited on the north and 

south by discontinuous faults. In some places the 

movement is taken up by a series of step faults, in 

other places a single fault will absorb all the movement, 

and in still other places a zone of shattered rock or a 

wide sheeted zone marks the edge of the belt.

Very few faults that trend northwest were found, and 

the relative ages of those within the special area could 

not be determined, but in the surrounding region most 

of the northwesterly faults are older than those that 

trend north and northeast. The faults that strike 

N. 40°-80° E. are by far the most common in the 

Breckenridge district. They are generally older than 

those that strike N. 10° W. to N. 20° E. The latter 

fault system includes most of the normal faults with 

large displacement, and much of the movement along 

them was later than ore deposition. Nearly all the 

veins in the district are found in fault fissures of the 

east-northeast system. On Gibson Hill, however, 

many of the "blanket veins" occur in replaceable beds 

of different formations and along gently dipping 

bedding-plane "slips."

Most of the monzonite in the special area is part of 

one large irregular laccolithic mass that formerly 

extended in a continuous sheet from Prospect Hill 

south to Nigger Hill and southeast to Bald Mountain. 

This mass was fed by many conduits and is probably 

composed of many distinct sills that have united to 

form the larger body. The base of the large intrusive 

sheet is irregular and follows different horizons in 

different places but most commonly occurs a short 

distance above or below the Dakota. The bulk of the 

porphyry was intruded after folding was completed, 

and part of it is later than the initial east-northeast 

faults (see description of Wellington mine, p. 48), but 

many of the faults in the district are later than the 

monzonite porphyry. It is probable that the large 

sheet was broken by the fault that bounds it on Nigger 

Hill and that its northwestern extension has been removed by erosion. Structure sections H, I, and J 

(pi. 5), show what is known of the base of the monzonite mass.

In general, the crosscutting bodies of monzonite 

and quartz monzonite porphyry followed the earlier
20 BKECKENRIDGE MINING DISTRICT, COLORADO

lines of weakness indicated by the east-northeast 

faults, but in a few places the porphyries broke across 

the sediments in a north-south line. This feature is 

illustrated on the eastern edge of the quartz monzonite 

porphyry stock south of Gibson Hill, and hi the crossbreaking mass of monzonite porphyry north of it. As 

most of the north-south fractures are distinctly later 

than the east-northeast fractures and in places offset 

quartz monzonite dikes, it is probable that lines of 

weakness were just beginning to develop in the northerly directions as the intrusive activity was drawing 

to an end.

Some local faulting accompanied and resulted from 

the intrusion of the porphyries. The faults along the 

north side of Nigger Hill may be related to the intrusion of the quartz monzonite stock north of them. In 

many places the intrusion of sill-like branches from 

dikes that followed preexisting fault planes has either 

accentuated or diminished the original throw of the 

fault above the sill; a fault may be normal below a sill, 

but above the sill its hanging wall may have been so 

much raised by the intruding porphyry that it now 

has a reverse appearance.

Relation to Front Range mineral belt. The detailed 

study of the special area of the Breckenridge district 

shows a complicated fault mosaic whose genesis is 

clouded with detail. Knowledge of the regional geology, however, makes it clear that this badly broken 

ground is the natural result of the forces involved in 

the diastrophic movements that formed the Front 

Range of the Rocky Mountains. The earliest record 

of these movements is contained in the overturned 

fold that passes northward into the Williams Range 

underthrust fault. The general trend of the fault, 

N. 30° W., indicates a compressive force operating in 

a direction approximately S. 60° W. or N. 60° E. 

Compression in this direction has been recognized in 

the Leadville district also, where Loughlin 47 concluded 

that a compressive force must have acted S. 60° W., 

because the folds that trend N. 30° W. have steep 

western limbs and several of these limbs are broken 

by reverse faults.

Little evidence has been found that regional compressive forces acted at any later time in directions other 

than west-southwest or east-northeast. The forces 

involved in the thrusting were of large magnitude and 

accumulated so rapidly during the Laramide revolution 

that rock flowage was unable to keep pace with the increasing stresses, and fracturing resulted. The first fractures near Breckenridge followed the line of the great 

overturned fold of the region and developed into the 

Williams Range thrust fault, north of Tiger. This 

fault passes into the overturned fold south of Tiger, 

where compression was evidently less severe than it 

was in the region farther north, near Keystone. A 

few miles east of Keystone, at the junction of the North

«' Emmons, S. F., Irving, J. D., and Loughlin. G. F., op. cit., p. 96.

Fork and South Fork of the Snake River, Pierre shale 

is exposed in a "window" which has been eroded 

through the overlying pre-Cambrian rocks. This shale 

is several miles east-northeast of a line passing through 

and paralleling the overturned sedimentary contact of 

the Dakota quartzite with the pre-Cambrian gneisses 

at Georgia Pass, 4}£ miles south-southeast of Tiger. 

This relative position shows that the shale north of 

Tiger was thrust much farther eastward under the 

rising pre-Cambrian rocks than the shale south of 

the town. Although the regional geology shows that 

compressive, mountain-building forces were acting in 

an east-northeast or west-southwest direction over a 

wide area, it is evident that locally the forces differed 

greatly in intensity. Thus the underthrust of the sediments near Tiger, several miles east of the corresponding beds south of the town, indicates a strong regional 

shearing force in an east-northeast or west-southwest 

direction.

The compressive and shearing forces that caused the 

unsymmetrical infolding and underthrusting of the 

Cretaceous sediments near Tiger had little effect on the 

large batholithic masses of granite in the Front Range, 

but they greatly affected a northeasterly zone of incompetent schist and gneiss between strong granite 

buttresses. In this belt, which follows a sinuous 

course from Tiger northeast to Boulder, shear faults 

are common, and hi nearly ah1 these faults the north 

wall has moved eastward past the south wall. Intrusive activity as well as fracturing was concentrated in 

this irregular, incompetent zone, and emanations from 

the deeper parts of the intrusive magma later formed 

many ore deposits in or near the intrusive masses now 

exposed. Compression was locally relieved by interlaminal movement in the soft schists, but the stronger 

ribs of granite and granite gneiss either transmitted the 

force without rupturing or absorbed some of the force 

by fracturing. These fractures later provided channels of circulation for ore solutions and thus helped to 

localize mineralization. The restriction of Tertiary 

intrusive masses and veins to the incompetent zone 

has caused this strip of country to become known as 

the "porphyry belt" or the "mineral belt." 48 This 

belt is about 6 miles wide where it comes to the western 

edge of the pre-Cambrian mass 5 miles east of Breckenridge but is narrower .a few miles to the northeast.

North of Tiger the compressive forces found relief 

through movement along the Williams Range thrust 

fault, and it is probable that little faulting transverse 

to the thrust fault occurred[in the sediments. Peripheral faults related to the subsequent intrusion of a 

northward-trending quartz monzonite stock are common, however. In the region south of Tiger, where 

the underthrust passes into an overturned fold, transverse shearing and compressive faulting were at a

« Levering, T. S., Localization of ore in the schists and gneisses of the mineral 

belt of the Front Eange, Colo.: Colorado Sci. Soc. Proc., vol. 12, pp. 237-242,1930.
GEOLOGIC HISTORY 21

maximum in both the sediments and the pre-Cambrian 

rocks. As a result the sediments are much less 

fractured north and south of the mineral belt. Southwest of the pre-Cambrian rocks, in line with the 

mineral belt, the stresses that existed were, in effect, 

(1) compression from the west-southwest, (2) shearing 

directed to the east-northeast, and (3) tension or only 

slight compression in a north-northwesterly direction. 

The fracturing that would result from such a combination of stresses would have the most open fissures 

parallel to the direction of major compression and at 

right angles to the direction of tension. The fractures 

caused largely by compression would make angles of 

less than 45° to the direction of compression; steeply 

dipping compression fractures in this region should 

therefore trend east or slightly east of north. 49

Although the fractures and faults in the district were 

largely caused by the same forces and in a broad way 

belong to one period of fracturing, they are not strictly 

contemporaneous. Experimental work with bodies 

compressed under differences in stress analogous to 

those described above show that the shear cracks and 

open tension cracks form parallel to the direction of 

compression and are commonly a little earlier than the 

compression cracks. The compression fissures form 

in two sets at angles of less than 45° to the direction of 

compression, and these two sets may develop simultaneously, but more commonly one set develops before 

the other. Each of these three stages compression, 

shearing, and tension increases to a maximum and 

then wanes, but there is an overlap of one stage on 

another. Further relief from compression is obtained 

by the shoving and rotation of the blocks into which 

the mass has thus been broken. The faults in the 

Breckenridge district are largely the response of interbedded competent and incompetent rocks to these 

compressive and shearing forces localized along a 

6-mile front, but the pressure of large irregular masses 

of porphyry caused many local variations in the courses 

of the fractures. There is no sharp distinction in age 

between the different fault systems of the district, and 

they are in part contemporaneous, the later faulting of 

one stage having overlapped the earlier faulting of 

another. Movement on many of the faults recurred 

at intervals over long periods of time, and it is probable 

that the stress that caused the last faults also caused 

some readjustment on the existing fault planes.

The veins of the district occupy those older faults 

that served as good channels of circulation for the 

metallizing fluids from the deeper parts of the quartz 

monzonite magma. As the solutions followed the 

easiest avenues of escape, the ores are largely confined 

to the more open of the older faults. As suggested 

above, such faults commonly trend northeast or eastnortheast. Some of the strong northerly fractures

< 9 Loverlng, T. S., The fracturing of incompetent beds: Jour. Geology, vol. 36, 

pp. 709-717.1928.

were also mineralized, but as they were comparatively 

tight no deposits of commercial size were formed in 

them. Their relation to important veins suggests 

that some*'of these slightly mineralized faults may 

have been deep-reaching channels of mineralization. 

Such relations are illustrated by the occurrence of ore 

at the Wellington mine. (See pp. 26, 27, 50, 51.)

GEOLOGIC HISTORY

The geologic history of the Breckenridge district is 

only partly recorded in the formations that are now 

exposed here. Many details that are of interest in 

presenting a general picture of the geologic past must 

be gathered in regions remote from this district. The 

summary given below brings together the features of 

the geologic history necessary to an understanding of 

the district. It is based largely on the writer's study 

of the Front Range from 1926 to 1929, and the evidence 

for the statements made has been presented elsewhere 50 

and will not be repeated here.

During pre-Cambrian time thick shaly sediments 

were deposited on an unknown basement, folded and 

metamorphosed into schists, intruded by large masses 

of granite, raised into mountains, and worn down to 

nearly featureless plains on both sides of an area of 

slightly higher land that existed along the site of the 

present Front Range. This .highland was separated 

by a broad basin from a similar high area stretchingnorthwest and southeast of the present town of Creede. 

The Cambrian sea spread over the pre-Cambrian 

lowlands and may have covered the Breckenridge district. At the end of the Cambrian period gentle but 

widespread uplift caused the sea to withdraw.

Oscillations of the earth's surface resulted in successive invasions and withdrawals of the sea in the 

Ordovician, Devonian, and Carboniferous periods. 

The areas occupied by the sea in each period were 

much the same, but in general each marine invasion 

was a little more widespread than the one before.

There is little in the character of the Cambrian, 

Ordovician, Devonian, and Mississippian sediments to 

indicate proximity to a rugged land surface, but there 

is clear evidence of the rejuvenation of erosive agents 

in the Pennsylvanian epoch. Thick beds of conglomerate, cross-bedded sandstone, micaceous shale, and 

grit all testify to the activity of erosion in nearby land 

areas, and the presence of coarse boulder conglomerate 

interbedded with fine grit and shale indicates that the 

earlier mild climates had given way to one in which 

violent fluctuations were common. This change was 

evidently in the direction of increasing aridity, as 

carbonaceous shale is absent in the upper part of the 

Pennsylvanian series, and the increasing prominence

M Lovering T. S., Geologic history of the Front Range, Colorado: Colorado Sci. 

Soc. Proc., vol. 12, pp. 59-111, 1929; Localization of ore in the schists and gneisses 

of the mineral belt of the Front Range, Colorado: Idem, pp. 234-268, 1930. Spurr, 

J. E., Garrey, G. H., and Ball, S. H., op. cit., pp. 92-93. Ransome, F. L., op. cit., 

pp. 182-183.
22 BRECKENEIDGE MINING DISTRICT, COLORADO

of red micaceous sediments in the upper part of the 

section also suggests semiarid conditions. The eastern 

shore line of the Pennsylvanian sea passed through the 

Breckenridge district, and it is probable that in the 

succeeding epoch Permian sediments lapped over the 

pre-Cambrian rocks for several miles eastward. Probably erosion was mildly active in this region from the 

end of the Permian epoch to the beginning of Cretaceous time. The geologic record of the Triassic 

period is difficult to interpret, but it is probable that 

arid conditions prevailed and that sedimentation was 

insignificant.

The Jurassic period saw a return to a more humid 

climate as shown by the fresh-water sediments of the 

Morrison (Upper Jurassic) and the lack of rugged 

country nearby is indicated by the character of the 

shale and sandstone in that formation. The lack of 

relief in the Jurassic land areas suggests the old-age 

topography of a sinking land mass.

The highlands of Paleozoic and early Mesozoic time 

slowly sank below the level of the Cretaceous sea until, 

at the end of Dakota time, only a few islands marked 

the high points of the ancestral Front Range. The 

humid climate of Dakota time is recorded in the many 

seams of coaly shaje near Breckenridge and in similar 

beds of carbonaceous shale interbedded with fire clay 

elsewhere. During Benton and Niobrara time and the 

first half of Pierre time subsidence continued and 

probably resulted in the complete submergence of the 

Front Range highland to a moderate depth. In the 

later half of Pierre time, however, this ancient highland slowly emerged and was again laid bare to the 

attack of erosion. The emerging highland gradually 

pushed the shore of the receding ocean farther and 

farther out, and this shoaling of the sea is recorded 

in the alternations of the marine and continental beds 

.in the equivalents of the upper part of the Pierre and 

of the Fox Hills formation. The emergence of the 

land was not accompanied by folding but seemed to be 

the result of a vertical uplift that raised most of North 

America above the ocean. Although pronounced 

 folding did not occur until the end of the Denver 

epoch, it is certain that the Front Range highland was 

elevated well above the level of the plains at the end 

of Laramie time. It is probable that this elevation 

was caused by a gentle doming of the area now occupied by the Front Range and was accompanied by the 

pronounced volcanic activity that is believed to have 

covered the Front Range with lavas during Denver 

time. The doming was a forerunner of the violent 

mountain-forming movement called the Laramide revolution that marked the transition from the Cretaceous 

period to the Tertiary period. This widespread diastrophism, which in large measure built the Rocky 

Mountain system, occurred at the end of this very 

early Eocene stage, when the Denver and all the older 

formations were strongly folded, in places overturned,

and faulted on a large scale where compression was 

greatest. The Williams Range thrust fault and the 

related shear and tension faults were formed during the 

Laramide revolution, and it is probable that the reverse 

faults on the east side of the Front Range, near Golden 

and Boulder, were also formed then. Shortly after the 

beginning of the stage of shear faulting, sills of diorite 

and monzonite were intruded in the Breckenridge district, and intermittent intrusion of increasingly silicic 

rock continued throughout the Laramide revolution, 

culminating after the thrust faulting in the emplacement of large stocks of coarsely porphyritic quartz 

monzonite. Minor faulting accompanied and followed 

their emplacement but waned rapidly during the ensuing stage of ore deposition.

The folding and faulting must have raised the tops 

of the resulting mountains to high.altitudes and early 

Tertiary time probably found the ancient highland 

elevated to a lofty mountain range containing many 

active volcanoes. The rejuvenation of the streams 

that accompanied the great mountain uplift caused 

rapid erosion. The volcanic rocks that covered the 

pre-Cambrian core of the Front Range were cut 

through and removed over large areas. Before the 

end of Eocene time erosion had reduced the rugged 

surface to a gently undulating upland plain, above 

which rose a few monadnocks of andesite and granite. 

It is probable that some of the conspicuously even 

surfaces of this plain, now called the Flattop peneplain, 

were remnants of the earlier erosion surface over which 

the Cretaceous sea advanced at the beginning of Dakota time and which were uncovered in late Cretaceous 

and early Eocene time.

Very early in the Eocene epoch, before the cover had 

been completely stripped from the pre-Cambrian core 

of the Front Range highland, a series of monzonitic 

sills, dikes, and stocks were intruded in the region now 

known as the mineral belt of Colorado. The emanations from these porphyry masses formed many metalliferous veins throughout this belt and were the source 

of the ores of the Breckenridge district. The ores 

were formed under a moderately thick cover, and it is 

probable that they did not come within reach of ground 

water until erosion had decreased greatly from its 

early Eocene activity. From this time on, however, 

the ore deposits were slowly reconcentrated and enriched through the action of meteoric waters. The 

climate and the activity of erosion are determinative 

factors in enrichment, and thus the climatic as well as 

the geologic history of the Tertiary and Quaternary 

periods is worthy of consideration.

During the Eocene epoch a mild and moderately 

humid climate prevailed in the lowlands bordering 

the newly formed mountains. A lush vegetation 

flourished in the plains and basins, and though there 

is no record of the fact, the mountains themselves 

were undoubtedly forest-clad throughout this epoch.
MINERALOGY 23

There is evidence of uplift at the end of the Eocene 

and the consequent rejuvenation of erosive agents. 

Faulting probably accompanied the uplift.

The Oligocene epoch was characterized by a continuance of the mild Eocene climate, but the rainfall 

was distinctly less than that of the Eocene. Although 

some uplift occurred in middle and late Oligocene time, 

the epoch saw no marked rejuvenation of the streams.

The Miocene climate was mild but drier than that 

of the earlier Tertiary epochs. Volcanic activity and 

pronounced uplift in lower and upper Miocene time 

are recorded in the Front Range, and it is probable 

that intermittent volcanic activity continued from 

the beginning to the end of this epoch in the Cripple 

Creek region and in Middle Park. The Miocene 

plains deposits contain hackberry fruits, indicative 

of a semiarid environment, but the Florissant lake 

beds contain a diversified flora and fauna, indicating 

that the Miocene mountains were covered with abundant vegetation. The activity of erosion that was 

begun by the Miocene uplift caused rapid degradation 

of the mountain fronts, as it was there that the change 

in the gradient of the streams was most effective. 

During Miocene and Pliocene time an uneven compound erosion surface, known as the Rocky Mountain 

peneplain, was carved on the eastern flank of the Front 

Range. This surface is not so well developed in the 

pre-Cambrian rocks on the west side of the Front 

Range, but remnants of the older Flattop peneplain 

are conspicuous there. The pronounced physical 

differences of the Cretaceous shale and the Tertiary 

monzonite porphyry in the Breckenridge district 

greatly influenced the development of topographic 

form after the Miocene uplift and resulted in a moderately accented surface instead of a piedmont plain.

The early and middle Pliocene were characterized 

by a return of mild and humid conditions throughout 

most of North America. There is some evidence of 

widespread and severely arid conditions in upper 

Pliocene time, but most of our information on the late 

Pliocene is obtained by extrapolation from the conditions known to have prevailed at the beginning of 

Pleistocene time. There can be no doubt, however, 

that the upper Pliocene was decidedly more arid than 

the earlier Tertiary epochs. This period of extreme

aridity was ideal for the enrichment of lead, silver, 

and gold, and there must have been a notable concentration of these metals in the upper parts of the veins 

at this time.

The Front Range was raised into a broad arch at 

the beginning of the Quaternary period, and it is probable that some of the postmineral faulting exhibited, 

in the Breckenridge ore deposits occurred at this time. 

This uplift or the coincident change to a humid climate 

brought about the period of vigorous erosion that has 

lasted to the present time. Two epochs of notable glaciation followed the uplift, separated by a long interval during which some streams cut channels as much 

as 160 feet deep in bedrock. It is unlikely that a great 

amount of enrichment occurred in the veins in glacial 

time, but the streams were vigorously and effectively 

concentrating gold in the placers that have supplied 

most of the gold output of Summit County.

MINERALOGY

Ransome 61 has described most of the minerals 

occurring in the Breckenridge district, and the reader 

is referred to his paper for such details as are not 

included in the tabular summary shown below. The 

writer has added a few minerals to the list given by 

Ransome, but it would be unprofitable to take space 

for a detailed description, of these additions. The 

composition and genesis of every mineral is indicated 

in a general way in the table, but the relations of the 

common ore minerals are considered more fully below, 

and further information regarding them can be found 

in the descriptions of the individual mines.

Pyrite is the earliest of the common sulphides. 

Marmatite (dark, ferruginous zinc blende) containing 

minute blebs and Wringers of chalcopyrite, followed 

the pyrite; it is usually earlier than galena, but in 

some ores it is contemporaneous with galena and 

locally it is later. (See pi. 7, A.) Most of the lightcolored zinc blende is later than galena. Gold and a 

small amount of the blende are contemporaneous 

with the last stage of galena formation. Quartz and 

ankerite followed the deposition of the sulphides in 

most places, but the gangue minerals form only a 

small portion of the ore in most veins in the district.

" Ransome, F. L., op. cit.. up. 81-91.
24 BRECKENRIDGE MINING DISTRICT, COLORADO

Minerals of Breckenridge district, Colorado

Name and composition

Allanite, Ca2(Al.OH)(Al,Fe,Ce,La,Di)2(Si04 )3 -------------- __ -

Amphibole, CaMg3 (SiO3) 4 ____-.-_.___-___-____ _-.______-.____

Ankerite, (Caf Mg,Fe)C08 -- --- ------ -

Apatite, (Ca,F)Ca4P3 O12 - _ ________ ._ -.___. _ - ___ _. _____

Augite, CaMgSi2O6, with (Mg.Fe) (Al,Fe)O2Si6 .. ..._.. _._._.._.-_-____

Azurite, 2CuC03 .Cu(OH) 2 ... ______________________________________

Barite, BaSO4 _____----_--------_---_-_-_-_---____--_-____-_--_-_-

Biotite, (H,K) 2 (Mg,Fe) 2Al2Si3O12 ..__. ______ _.__-_.__.___..__._.___._

Calcite, CaC08 ___ .__ --_-__ -_-_ -_ _-___ _-_-_ __

Cerusite, PbCO3 -..-_- __ ___._.___ ____________________________

Chalcedony, Si02 ______-_---_-_--_--_--____---___-___-_____-_-____

Chalcocite, Cu2S ---_--_-_--------_----_-_-_---___-___-___-_----___

Chalcopyrite, CuFeS2 -___-_-_-_-------_-___-_-_-_--______---_-_--_

Chlorite _________________________________________________________

ChrysocoUa, CuSi03 .2H2 0 --. ___________:_______________________

Clay___------_------------------------------- -------------------

Copper, Cu ______-_--_- _-_-----_--_--_-_-_-----_---_-__---_-__-__

Diopside, CaMg(SiO3) 2 .- - ---------- -__-____--_-.-__-_______-____.

Dolomite, CaMg(CO3) 2 __ ___-_--__---_-___---__________________._

Epidote, H2 0.4Ca0.3(Al,Fe) 2O3 .6SiO2 _ ___________________._>_...____

Ferric sulphate, basic, Fe2 03 .SO3 .2H2O ______________________________

Freibergite, 4(Cu2 S,Ag2S) 3 .Sb_S-_ ___________ _ ._ _______

Galena, PbS____ _ --_ ___---_ _ _-___~_~_____-_~-_-

Garnet, R"3R'"2 (SiO4) 3 or 3RO.R2O3 .3SiO2 , when R" equals Ca,Mg,Fe, 

Mn and R'" equals Al,Fe,(Mn),Cr,Ti.____________________________

Gold, Au__-_---_----__------_----------- ---------------_---__---

Gray copper, Cu8Sb2S7 or 4Cu2 S.Sb2S3 _.._____ _ _ _________________

Gypsum, CaS04 .2H2 0._ __ ____________________________ _ _ ______

Hematite, Fe2 03 - -_----_-------_-___---___-____________ _ _

Hornblende, Ca(Mg,Fe)_(SiO_)2- -----------_--__-_--_-----_----____

Hypersthene, (Mg,Fe)SiO3 ._ ___ --------_----_---___.._____-_-_--_

Jarosite, K2O.3Fe2O.4SO3 .6H2O-__----_- ____________________________

Kaolin, 2H2O.Al2O3 .2SiO2 _-_--------- _ ___________________________

"Limonite," 2Fe2 03 .3H2 0_ _ __________________________ __ ________

Malachite, CuCO3 .Cu(OH) 2 -..____-___. ______________ __ __________

Manganosiderite, (Fe, Mn, Mg) C03 __________________________________

Marmatite, (Zn3Fe) S_ . _________ __-_-_-_-_---___________________.

Melanterite, FeSO4 .7H2 0__-----_---------___--___---_____-_-._____

Muscovite, KH2 .Al3 (Si04 ) 3 -------------_---_____-_ ----______-___ _

Oligoclase, Ab6Ani__________-_--__-__-_ _ __ ___________ _________

Orthoclase, KAlSi3 O8 --__ ------- _ _________ _-___j__.-____________._

Plagioclase, z(NaAlSi3 08)i/(CaAl2Si2 08)_-__ __ ___ __________________

Psilomelane, H4 MnO5 _____________ ____ __ _____ ________________

Pyrite, FeS2 _ ----------------------------------------------------

Pyrolusite, Mn02 -------_-_---_ _ __._ ____ ______ ______________

Rutile, Ti02 ____ ---_-_--__-___-___-_______-_____.______________

Sericite, KH2Al3 (Si04) 3 _ -------------------------------------------

Siderite, FeCO8 - -----_----------_---_---------__-_- _ ___________

Silver, native, Ag_ _____________ _ _ _ _ ______ _____ ________________

Smithsonite, ZnCO3 --_ _----_--___ _______ _ _____________________

Sphalerite, ZnS__.__ _____________ - - __' _ _ _----__-__-__-_____

Sulphur, S-_______-_-___-___- -------- __-_ --_-_____-_-. ____

Tetrahedrite, Cu8Sb2 S7 _. . ___-____--_-__- .-----_--__-_______-______

Titanite, CaTiSi06 or CaO.TiO2 .SiO2 . ---___-_--_-__--- _ ___________

Vanadinite, (PbCl) Pb4V3 0]2 - _ __ _ ___________________________________

Wad, mixtures of oxides, chieflv of manganese, with water_____________

Wulfenite, PbMo04 ___. ___________ _______________________ _-_-_-___

Zircon. ZrSiO*--.--------------- __ ____ _ _ __-.__-___ __________

Hypogene

In igneous 

rocks

X 

X 

X

X 

X

X

X 

X

X

X

X 

X 

X 

X 

X

X 

X

X

X

In contactmetamorphic 

deposits

X

X

X

X

X 

X

X

X '

X

X

X

In hightemperature 

deposits

X

X

X

X

X

X

X

X

In moderate-temperature 

deposits

X

X 

X

X 

X 

X 

X

X 

X

X

X

X 

X

X 

X

X

X

X

X 

?

X

X

Supergene

In secondary 

sulphide 

zone

X

X 

X

X

?

X

X

X 

X

X

X

?

In oxidized 

zone

X

X 

X 

X

X 

X 

X

X

X

X

X 

X

X

X

X

X

X

X 

X

X

X 

X 

X

ORE DEPOSITS

CLASSIFICATION

The ore deposits of the Breckenridge district were 

formed in one period of hypogene mineralization, and 

some of them were subsequently reconcentrated and 

modified by the action of surface water. The primary 

mineralization was directly related to the solidification

of the quartz monzonite porphyry and was later than 

any of the porphyry intrusions in the region. Enrichment has greatly modified the original character of 

many deposits.

According to Ransome,52 the differences in the ore 

deposits of the district are largely matters of mere size

"Ransome, F. L., op. cit, pp. Ill, 112.
ORE DEPOSITS 25

and shape of relative proportions of constituents 

rather than of essentially distinct types or periods of 

mineralization, and the relation of deposits of apparently diverse types can be traced through those of intermediate character. For convenience of description 

he grouped the deposits under the following heads:

(1) Veins of the zinc-lead-silver-gold series.

(2) Stockworks and veins of the gold-silver-lead series.

(3) The Farncomb Hill gold veins.

(4) Veins in the pre-Cambrian rocks.

(5) Metasomatic replacement deposits. '

(6) Gold-silver deposits in the Dakota quartzite.

Deposits of the second and third classes are not 

present in the special area but will be briefly considered below. For this report it seems best to condense 

Ransome's classification and to summarize the general 

features of the ore deposits under the heads of (a) hightemperature deposits (" contact-metamorphic" ores) 

and (6) moderate-temperature deposits, including 

stockworks, veins, and blanket ores.

HIGH-TEMPERATURE (''CONTACT-METAMORPHIC'') 

DEPOSITS

High-temperature deposits are generally regarded as 

having been formed under high pressures between 

temperatures of 300° and 500° C. The only ores in 

the district that come under this heading are the contact-metasomatic or "contact-metamorphic" ores, 

which are best developed on Gibson Hill. They are 

of too low grade to encourage development.

The "contact-metamorphic" deposits are related 

to crosscutting bodies of quartz monzonite porphyry 

and are best developed in limy beds. Limy beds occur 

in the Maroon, Morrison, and Niobrara formations, 

but the most favorable beds occur in the Morrison. 

Structurally the best conditions are found where the 

limy beds dip gently toward the cross-breaking body 

of quartz monzonite porphyry and are slightly fractured in directions nearly at right angles to the contact of the porphyry and the sediments. Strong 

gouge-filled faults or more open fractures that dip 

away from the contact with the porphyry are not 

structurally favorable for the formation of "contactmetarnorphic " deposits.

These deposits are usually dense, irregular, tabular 

masses of silicates intergrown with metallic oxides and 

sulphides. The impervious character of the material 

as well as its chemical nature makes it resistant to 

weathering and enrichment. The garnetized limestone of the Morrison therefore stands out in the form 

of low cliffs on Gibson and Prospect Hills. (See pi. 

4,5.)

"Contact-metamorphic" deposits cluster around the 

stocks that seem genetically related to their formation 

and differ from the veins in showing no general trend. 

In the Breckenridge district the proportion of metallization is small in these dep.osits, and extensive garnetization has occurred without the development of any 

ore. Where ore occurs it is usually small, spotty, and 

irregular in plan. Its inclination is that of the replaceable limy bed that formed the host rock and is therefore fairly regular. The ore commonly occurs in 

bunches, and its terminations may be very abrupt; 

but some of these bodies contain profitable ore which 

is rather evenly disseminated through them and grades, 

through lean ore into the country rock.

In the zones of most intense metamorphism magnetite and hematite, although not abundant, are the 

chief metallic minerals present and were probably 

formed at high temperatures and high pressures, but 

chalcopyrite and pyrite were formed under pressures 

and temperatures only slightly less intense. The tonnage of copper ore in the district is small. The 

pyritic ores that are low in copper and of contact-metasomatic origin contain very little gold and cannot be 

worked below a shallow zone of enrichment, except 

where they grade into the pyritic galena blanket ores.

As distance from the mineralizing stock increases, 

the ore deposits lose their high-temperature characteristics and assume the appearance of deposits formed under 

moderate temperatures and pressures. Some deposits, 

such as the Sultana and Fox Lake, are evidently 

transitional between the blanket deposits considered 

on pages 32-33 and the "contact-metamorphic" 

deposits. It was impossible to study the ores of these 

mines during the visits of either Ransome or the writer, 

but it is known that some oxidized lead ore was shipped 

from them in the eighties and nineties. The sulphide 

ore exposed on the dumps at the present time is a 

heavy pyrite ore containing a little sphalerite.

MODERATE-TEMPERATURE (MESOTHERMAL) DEPOSITS

Stockworks. The stockworks are practically confined to the quartz monzonite porphyry mass in the 

northeast quarter of the Breckenridge district and are 

not found in the special area studied by the writer. 

All the stockworks occur close to the axis of the regional 

syncline, and most of them lie about S. 60° W. of the 

place where the Williams Range thrust fault breaks 

away from the overturned fold that borders the preCambrian terrane from Tiger to South Park. These 

shattered zones are younger than the quartz monzonite 

stocks, which in turn are younger than the thrust 

fault and most of the transverse faults. The stockworks therefore mark a late pre-ore adjustment along 

the dominant zone of weakness in the region. They 

probably mark the northern limit of the transverse 

belt effectively opened by northeast shear and tension 

fissures after the formation of the thrust fault, and 

thus they indicate the probable northern limit of 

profitable lode mining. (See "Structure", pp. 17-21.)

The following excerpts from Ransome's report 53 

give the general features of these deposits, and the

" Ranspine, F. L., op. cit., pp. 143-144, '
26 BRECKENRIDGE MINING DISTRICT, COLORADO

reader is referred to the original report for further 

details:

A distinctive feature common to these deposits is the occurrence of the ore in much fissured and minutely veined rock, 

ordinarily quartz monzonite porphyry, rather than in welldefined lodes. The fissuring may vary widely in character. 

In the Hamilton ore bodies it is concentrated along nearly 

parallel zones, so that the pay shoots have a certain regularity 

and might be classed as stringer lodes having widths up to 

about 15 feet and being separated from each other by 50 feet 

or less of relatively barren porphyry. The ore as a whole, 

however, has no definite walls. The adjacent porphyry is also 

more or less fissured and contains many small stringers of 

sulphides.

In the Jessie mine there is not the same general agreement in 

direction of the fissures. Certain groups of them are approximately parallel, but these are associated with others having 

decidedly different strikes. In many places one set joins or 

crosses another without any general displacement of either. 

* * * At the Jessie mine a mass of porphyry, oval in plan, 

fully 900 feet long, 600 feet wide, and 300 feet deep, has been 

fissured in many directions, but especially by fractures striking 

from east to northeast. * * * There is generally no great 

persistency to these lodelike zones or stringers. Some die 

:>ut within surprisingly short distances or merge with other 

zones. * * *

The deposits of this group yield generally a low-grade pyritic 

ore, concentrated principally for its gold and silver contents. 

There may be or may not be enough lead present to add to the 

value of the product. Mineralogically they consist of pyrite, 

sphalerite, and galena in a gangue of sericitized porphyry. 

Report indicates that the oxidized and partly oxidized upper 

portions of these deposits were considerably richer in the 

precious metals and in lead than the deeper sulphides, and it is 

known that bunches of rich ore containing galena and native 

gold were found in some of the upper workings. The greater 

part of the sulphides fills small open fissures, * * * but the 

veining is associated with some replacement. In the Jessie 

mine this metasomatism does not result in any general replacement of the porphyry by sulphides, so far as observed, but 

pyrite, sphalerite, and galena all develop sporadically at the 

expense of the orthoclase phenocrysts. In the Wire Patch ore 

bodies, however, the sulphides appear to have replaced the 

porphyry bodily to some extent.

Veins. There are in the Breckenridge district from 

25 to 30 veins that have each produced 1,000 tons of 

ore or more. Most of these veins lie in a short, narrow 

northeastward-trending belt, extending from Little 

Mountain to Mineral Hill, but a few, such as the 

Lauriam, lie slightly to one side. Smaller veins occur 

in this northeasterly belt and elsewhere in the district, 

but with the exception of the rich, narrow gold veins of 

Farncomb Hill none of them have contributed materially to the output of the district.

Nearly all the veins occupy premineral normal faults 

which strike N. 40°-80° E. and average about N. 65° 

E. They dip from 60° to 80°, and most of them show 

pronounced irregularities along their strike and dip. 

The undulations of the veins are illustrated in the plan 

and cross sections of the Wellington veins shown on 

plate 13.

Relation of veins to country rock. The veins in the 

Breckenridge district cut all the bedrock formations,

and the influence of the wall rock on the character of 

the ore is notable. Most of the ore in the district has 

come from veins in the monzonite porphyry and Dakota 

quartzite, but much ore has also been produced from 

veins in the quartz monzonite porphyry, Pierre shale, 

Niobrara formation, Benton shale, and Maroon formation. The writer does not know of any vein or blanket 

deposit that has produced ore from the Morrison formation, although, as stated on page 25, the limy beds 

are considerably replaced by contact-metamorphic 

minerals. Very little exploration has been done in 

the Morrison, however, and ore-bearing veins may 

occur in it.

The influence of the wall rock on the veins is largely 

physical. Comparatively strong, brittle rocks, such as 

porphyry and quartzite, will break and form open 

fault breccias, whereas shale bends or forms tight, impervious clay-filled fissures. For these reasons ore 

solutions more frequently followed open channels 

along faults in the quartzite and porphyry than in the 

other rocks. In some places, however, as along the 

western part of the Wellington vein, the shales were 

hardened and strengthened by silicification before 

mineralization. In such places valuable ore shoots 

may be found between shale walls, and though the 

width of the vein almost invariably decreases on passing from porphyry into shale, the tenor of the ore 

commonly rises. If a vein along a fault fissure of 

small displacement is followed into shale, it commonly 

pinches out as the fault fades into a sharp flexure in 

the shale. A fault of large displacement, on the other 

hand, may contain ore in the quartzite or porphyry but 

become barren on passing into shale, although the 

fault itself persists for long distances between the 

shale walls. Faults that have a throw of 50 to 150 

feet are more likely to contain ore in shale than those 

of greater or less displacement. Even in strong 

rocks, such as porphyry and quartzite, faults that show 

displacements of more than 300 feet produced large 

amounts of nearly impervious gouge. In premineral 

faults of this class the ore occurs in bunches and chimneys, is commonly brecciated by later movements, 

and consequently is often mistaken for "drag ore" 

from some faulted vein.

The chemical character of the wall rock is much less 

important in determining the presence of ore than its 

physical properties. Certain limy beds in the Cretaceous rocks are slightly replaced next to some veins. 

In general the replacement deposits are largely pyritic 

and of too low grade to be mined profitably. These 

replacement deposits are intermediate between the 

veins and the blanket deposits described in the next 

section but have not proved of commercial value.

Ore channels. The faults having large displacements 

persist to greater depths than the faults of small movement. Thus the strongest pre-mineral faults should 

tap the deep upward-moving ore solution first and act
U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 176 PLATE 6

A. ORE FROM SPUR VEIN, WELLINGTON MINE. 

About 200 feet from surface, near junction with Main vein.

B. 1'HOTOMICROGRAPH OF ORE SHOWN IN A. 

Showing solution cavities, g, Galena; sp, sphalerite; py, pyrite.
U.S. GEOLOGICAL SURVEY PRO

A. LATE SPHALERITE CEMENTING AND Rlil'LAI .1 Mi BKKGCIATKI) G\LKV\. SPUR VEIN, WELLINGTON MINK. If. LATE GALENA FOLLOWING C

SPHALERITE, WKLLINGTON MAIN VKIN AT WINZE, LEVELS. C. REPLACEMENT ORE FROM LEVEL 8, WELLINGTON M

g. Galena; sp, sphalerite; py, pyrite; a, ankerite; ch, chalcapyrite; q, quart/.
ORE DEPOSITS 27

as guides in their subsequent journey. There is a 

striking relation between the Great Northern-J fault 

and the ore bodies in the veins on the east side of the 

Wellington ground. (See fig. 20.) A similar relation 

exists between the Bullhide fault and the veins on the 

west side of the property. Ore solutions under high 

temperatures and pressures probably followed an intricate course through local channels in the wide zones of 

selvage that mark these large faults and passed into the 

more open channels of moderate-sized faults at the first 

opportunity. The more open character of these newly 

reached fissures and the large area of their walls, which j 

appreciably cooled the solutions, were evidently effective 

in causing the deposition of the primary sulphide ores. 

The localization of ore shoots was further influenced 

by the ease of movement in different directions in the 

more open fissures. It is 'evident that movement of 

the irregular surfaces forming the fault walls resulted 

in openings in some places and the squeezing and 

grinding of the walls in others. The ore solutions 

would follow the easiest road to the areas of low pressure and travel through the most open channels available, thus localizing ore shoots in the more open parts 

of the faults of moderate movement. In some places 

small gouge-filled cross faults displace the veins and 

apparently cause a notable change in the vein filling 

on each side. The ore is commonly wider than the 

average on one side of the cross fault and much narrower on the other. In many places it is clear that 

the cross fault is premineral and that either the cross 

faulting has dragged the vein walls together on one 

side while pulling them apart on the other or that the 

gouge seam has acted as a baffle and deflected the 

mineralizing solutions away from the barren zone 

found on one side of the fault.

Mineralized sheeted zones usually contain one major 

vein and one or more parallel minor veins. The minor 

veins are irregularly distributed and lack persistence. 

The major vein may thin out at the place where a 

parallel minor vein thickens, but more commonly the 

major vein is continuous and consistently follows the 

footwall or the hanging wall of the sheeted zone.

Character of the primary ores. 54 The primary ores of 

the Breckenridge district consist largely of lead, zinc, 

and iron sulphides, with some native gold and some 

silver whose form in the primary ores is uncertain. 

The most common gangue minerals are ankerite, calcite, quartz, and sericite. Copper is a minor constituent of most of the ores but is locally abundant in 

the narrow gold veins of Farncomb Hill.

Pyrite is the most abundant and widespread of the 

sulphides, occurring in the high-temperature deposits

14 Ransome (op. cit., p. Ill) applied the word "primary" to all lode deposits as 

distinguished from detrital or placer deposits. As the present writer is dealing only 

with the lode deposits, he is following the more common practice of calling original, 

hypogeno ores primary and distinguishing them from ores that have undergone 

secondary or supergene changes, either In the oxidized zone or in the zone of sulphide 

enrichment.

18293 34 5

close to the quartz monzonite stock north of Nigger 

Hill, in the zinc veins a short distance east of it, in 

the mixed lead-zinc ores nearby, and to some extent 

in the high-grade galena shoots found scattered throughout the district. Sphalerite is also very abundant and 

is a moderately persistent mineral, although it is uncommon in the high-temperature deposits and in the 

essentially lead ores. Galena is rare in the high-temperature deposits and is more abundant in the upper 

part of some mixed lead-zinc ores than in depth. In 

others it apparently has as great a vertical range as 

sphalerite.

The ores of the high-temperature or "contactmetamorphic" deposits are not economically important 

and have not been exploited enough to give any information on their changes with depth.

The pyritic sphalerite veins are illustrated by the 

Great Northern vein of the Wellington mine, which is 

the only one in the district that was essentially a zinc 

ore throughout. The lead content of this vein was 

less than 2% percent at the surface. Pockets of galena 

occurred here and there throughout the vein, but the 

average lead content of the ore decreased with depth 

from 2% percent at the surface to less than 0.5 percent 

near the bottom of the ore shoot. The bulk of the 

ore was a pyritic zinc blende, which changed abruptly 

into nearly pure pyrite at the bottom and at the east 

end of the shoot. The sphalerite contains more than 

10 percent of iron and should be classed as marmatite. 

The silver and gold content of the ore was very low. 

Comparatively little gangue was found in the vein, 

and it consisted chiefly of sericitized wall-rock inclusions and ankerite.

The primary zinc-lead ores differ from the ore just 

described in having a much larger proportion of galena 

throughout the vein and in passing upward rather 

abruptly into a zone of sphalerite-free galena near the 

surface. Below the upper galena zone the lead content of the primary ore generally decreases slowly 

with depth. Little gangue is found in the sulphides, 

but siderite veinlets are locally abundant where movement subsequent to the deposition of the sulphides 

has reopened or brecciated the ore. Siliceous gangue 

material is uncommon. The sulphides are generally 

massive and show no evidence of depositional banding 

or crustification, although light-colored veinlets of 

ankerite parallel to the walls in places give the ore a 

slightly banded appearance. (See pi. 14, B.)

The primary ore of the thin gold veins on Farncomb 

Hill is a mixture of pyrite, galena, sphalerite, and 

chalcopyrite in various proportions, accompanied by 

native gold in a calcite gangue. At other places in the 

district gold is found in veins made up dominantly of 

pyrite but containing some zinc blende and galena. 

In general the gold deposits are associated with calcite 

and quartz gangue, in contrast to the lead-zinc veins
28 BRECKENRIDGE MINING DISTRICT, COLORADO

low in gold, which generally have ankerite and siderite 

as gangue minerals.

Relation of veins and ore to topography. In a few 

places rusty, silicified vein outcrops can be traced for 

some distance on the surface, but generally the veins 

are hidden beneath a cover of slope wash or gravel. 

However, most of the veins were found by prospecting 

based on the distribution of rich oxidized "float" ore 

at the surface, although some highly productive veins, 

such as the Great Northern, remained unknown until 

found in the course of underground development.

The relation of ore shoots to topography is somewhat 

complicated by the original distribution of ore minerals 

.and by the work of downward-moving waters in postEocene time. Ore shoots that are made up chiefly of 

the comparatively soluble minerals pyrite or sphalerite 

show little or no relation to topography other than 

that they lie below the zone of Tertiary oxidation. 

Galena, however, shows a rather definite relation, 

though interpretation of this relation is not simple.

Ransome 55 noted that all the high-grade galena ore 

had been found within 300 feet of the surface and says:

Even the imperfect data now available establish without 

much question the limitation of the lead ores to a zone ranging 

from 200 to 300 feet deep and corresponding roughly to the 

present strongly accented topography. * * * All available 

evidence indicates that in this district 300 feet, or perhaps more 

safely 400 feet, is the limiting depth for the occurrence of considerable bodies of essentially galena ore as opposed to sphaleritic 

or pyritic ores.

No high-grade lead ore has been opened below this 

depth since Ransome's visit.

Most of the rich lead ores so clearly related to the 

surface probably resulted from the leaching of lowergrade mixed sulphide ores. Both sphalerite and pyrite 

are soluble in meteoric waters and tend to disappear 

from the upper parts of the veins, while galena, protected by a thin but comparatively insoluble coating 

of lead sulphate, remains.

Mixed lead-zinc ores have a much greater range 

than the high-grade lead ore. In the Wellington mine 

they show little relation to the topography and are 

known to have a vertical range of more than 800 feet. 

The upper parts of these veins, however, are richer in 

lead than the lower parts, and the shoots of lead ore 

conform to the topography in a general way.

The zinc ores show a negative relation to the topography. Practically no zinc has been found in the 

upper parts of veins that come 'to the surface in areas 

where the preglacial topography is well preserved. 

(See pi. 4, B.) The most productive zinc veins, such 

as the Great Northern, Country Boy, and Sallie 

Barber, are well down the slopes of French Gulch, 

several hundred feet below the level of the.Tertiary 

land surface. This relation may be accidental, but 

the writer believes that it reflects the leaching of the

»« Ransome, F. L., op. cit., p. 167.

readily soluble zinc sulphide above the Tertiary 

ground-water level.

The rich gold ores bear a positive relation to the 

surface compatible with their secondary origin. Rich 

gold veins are not found in the floors of the large 

valleys but are closely associated with the tops of the 

hills, which represent the remaining parts of the old 

Tertiary land surface. This is true not only of veins, 

such as those on Nigger Hill and Farncomb Hill, but 

also of the blanket deposits, such as those on Schock 

Hill, Little Mountain, and Gibson Hill. The rich 

gold ore is limited to the oxidized zone, and most of 

the gold produced in the district has come from depths 

of less than 350 feet.

SUPERGENE ENRICHMENT

After careful study of the district Ransome 50 concluded that most of the native gold and much of the 

galena were concentrated by the agency of surface 

water, but there has been a general reluctance on the 

part of miners and geologists to accept this part of 

his report. The present writer believes that much of 

the native gold and some of the galena are secondary 

but that most .of the galena and essentially all the 

sphalerite are primary. The evidence that has led 

the writer to these conclusions is presented below.

Criteria of enrichment in the Breckenridge district. 

Before discussing the Breckenridge deposits further 

it will be advisable to consider the criteria of enrichment. Lindgren " concurs with Ransome s8 that 

the best geological evidence of enrichment consists in the progressive, uniform impoverishment of all similar deposits in a 

given district, coupled with the condition that the change in 

ore. should be dependent upon postmineral topographic development.

Several additional criteria are suggested by Emmons: 59 A secondary ore is commonly porous and 

open-textured; it is later than the primary ore and 

may form in cracks cutting the earlier ore or replace 

it along such cracks; a leached zone near the surface, 

grading downward into .a zone of higher-grade sulphide 

and this into a leaner ore zone, suggests enrichment; 

metals that are reprecipitated are those that go into 

solution and consequently are those that are leached 

from the outcrop; the presence of groups of minerals 

known to be characteristic of enrichment in other 

deposits is suggestive; the vertical extent of a secondary sulphide zone should show a relation to the composition and permeability of the primary ore. A detailed study of the paragenesis of an ore may be helpful, 

although hypogene sulphide replacement is very common and may be difficult to distinguish from supergene 

replacement.

56 Ransome, F. L., op. cit., pp. 169-170.

» Lindgren, Waldemar, Mineral deposits, 3d ed., p. 934, 1928.

58 Kansome, F. L., Criteria of downward sulphide enrichment: Econ. Geology, 

vol. 5, pp. 205-220, 1910.

59 Emmons, W. H., The enrichment of ore deposits: U.S. Qeol. Survey Bull. 625, 

pp. 81-83, 1917.
ORE DEPOSITS 29

Enrichment of zinc. The most significant criterion 

of enrichment is the relation of the ore to the post 

mineral topography, which has been discussed briefly 

on page 28. The negative relation of the zinc ores 

to topography and the gradual instead of abrupt increase of sphalerite with depth clearly reflect leaching 

of zinc sulphide from the outcrops and its dissipation 

by ground water at depth. The porosity developed in 

mixed lead zinc ore close to the surface by the leaching 

of sphalerite is illustrated in plate 6, B. The lack of 

replaceable limy beds near the ore bodies prevented 

the fixation of zinc as the insoluble smithsonite.. 

Insignificant amounts of calamine and of late lightcolored sphalerite that may be of supergene origin have 

been noted, but no commercial quantities of zinc ore 

were formed by enrichment in the Breckenridge 

district.

Enrichment of lead. The relative insolubility of lead 

sulphide, lead carbonate, and lead sulphate during the 

weathering of mixed sulphide ores causes these compounds to linger in the oxidized zone, and residual 

galena is common in the outcrops of veins. As a 

result, it is commonly thought that lead deposits 

become enriched chiefly through the removal of soluble 

material, which leaves the lead compounds as residual 

concentrates in the upper part of a vein, and very little 

mobility is ascribed to lead in the weathering of a vein. 

Galena is tacitly assumed to be primary unless there is 

strong evidence to the contrary, and so a somewhat 

detailed consideration of its occurrence and alteration 

is given.

In all the productive lead mines in the district 

cerusite with more or less galena occurred at the surface. This oxidized ore commonly passed downward 

into a sulphide zone rich in galena, but probably 

nowhere in the district has this rich lead ore been found 

more than 350 feet below the surface. The longitudinal 

sections and stope maps of the mines described on pages 

41-44 are convincing illustrations of the fact that rich 

lead ores are restricted to a comparatively shallow 

zone.

The uniform impoverishment of the essentially lead 

veins at shallow depths is shown by the records of 

the Old Reliable, Cincinnati, Union, Lucky, Iron, 

Wellington, Prize Box, Iron Mask, Ohio, Minnie, Ella 

Kellog, Washington, Mayo, Dunkin, Puzzle-Ouray, 

Germania, and Golddust veins. The relation of the 

cerusite in these veins to the old land surface is accepted 

as a consequence of its secondary nature; the parallel 

relation of galena to the ancient topography cannot be 

regarded as accidental and is the reason for Ransome's 

conclusion that "a large proportion of the galena in 

the Breckenridge ores is the result of downward concentration by atmospheric water." co '

o« Ransome. F. L., op cit. (Prof. Paper 75), p. 109.

The criteria of enrichment listed from Emmons' 

work can be conveniently grouped under texture, 

paragenesis, and downward changes in minerals.

The shallow galena ores seen by the writer are conspicuously open textured, and although the primary 

ore exposed in the deeper workings of the Wellington 

mine is porous, it is much less open textured than the 

shallow galena ore. (See pi. 6.) Many of the openings 

in the shallow ore are cavities left by the removal, of 

zinc blende. Remnants of sphalerite can be seen in the 

openings in shallow ore of the Wellington vein, as 

illustrated in plate 6, B. According to experiment and 

observation,61 sphalerite is one of the most readily 

soluble of the common sulphide ores, and in the zone 

of oxidation the sulphides are dissolved in the order (1") 

sphalerite, (2) chalcopyrite, (3) pyrite, (4) galena 

It is difficult to estimate how much of the porosity of 

the shallow ores has resulted from leaching, but it is 

plain that the open texture is no proof of the secondary 

nature of the galena and pyrite present.

A study of the paragenesis of an ore may be very 

helpful in working out the problem of its origin. 

Some investigators believe that certain types of microscopic intergrowths and textures are found only in 

supergene deposits and that others are confined to 

primary ores, but there is disagreement on the meaning 

of many textural relations. A study of the problem 

with a reflecting microscope is not conclusive, but no 

evidence of secondary galena was found. The relation 

of galena to the other sulphides in the shallow ores is 

discussed on page 56 and illustrated in plate 7. Coarse 

cubical galena coated with films of late fine-grained 

pyrite is abundant and is earlier than some mediumgrained pyrite and dark sphalerite that contains many 

small blebs of chalcopyrite. (See pi. 7, A.)

The downward changes in an ore deposit are of great 

aid in determining the origin of the different mineral 

zones.

In the Breckenridge district the primary ores contain very little copper, and the small amount present 

commonly occurs in erratic bunches of chalcopyrite in 

the lead-zinc ores, in small masses in the contactmetasomatic ores, and as an evenly disseminated constituent in the thin gold veins of Farncomb Hill. The 

upper parts of the lead veins carried more silver than 

the lower parts of the same veins. In a few places some 

secondary copper sulphides have been found near the 

outcrop, associated with masses of galena, but this 

may pass downward into a mixed lead-zinc ore, a 

nearly pure sphalerite ore, or a heavy pyritic material 

containing very little zinc. In primary ores sphalerite 

would normally increase with depth, but a change 

from galena to pyrite low in zinc is less common in the

6" Emmons, W. H., The enrichment of ore deposits: U.S. Qeol. Survey Bull. 625, 

p. 131, 1917.
30 BRECKENRIDGE MINING DISTRICT, COLORADO

primary ores of a -lead-zinc district. It is probable, 

however, that zinc was originally present in all the ores 

and has been completely leached from the veins now 

showing galena ores bottoming in pyrite.

As galena is more abundant in the upper part of a 

primary galena-sphalerite-pyrite ore shoot, and as 

sphalerite and pyrite are leached from the oxidized 

zone much more rapidly than galena, it follows that 

galena should invariably be much more abundant 

than sphalerite and pyrite near the surface, unless the 

rate of erosion was too fast to permit the leaching of 

pyrite and sphalerite. Whether the veins suffered 

little or much erosion, the ore consisting essentially 

of lead would probably appear related to the surface 

unless primary galena ore shoots went to great depths. 

Thus in attempting to apply the criteria of sulphide 

enrichment to the galena ores of the Breckenridge 

district, only two relation to surface and open 

texture are of any significance, and even these may 

be rather satisfactorily explained by the leaching 

of more soluble minerals from a primary ore.

However, if it were not for the comparative insolubility of secondary lead minerals the downward changes 

in the ores of the Breckenridge district would be as 

compatible with the theory that much of the galena 

is secondary as with the opposite theory of primary 

origin. Therefore, a brief discussion of possible lead 

solvents is given below.

Solvents of lead minerals. The search for lead solvents must be restricted to the solutions that may be 

expected to occur in nature in the Breckenridge district. Carbonated waters, chloride waters, sulphate 

waters, alkaline-earth waters, and water containing 

organic acids are the only meteoric solutions that are 

likely to have affected the veins. The solubility of 

the carbonate, sulphate, and sulphide of lead in pure 

water is low, but the chloride is moderately soluble. 

Both the sulphate and the chloride are moderately 

soluble in concentrated brine solutions, but their solubility in weak chloride solutions is less than in pure 

water.62 Strong chloride waters could be present only 

under very arid conditions, but the Tertiary history 

of the region shows that aridity may have prevailed 

in the later part of the Pliocene epoch. Throughout 

nearly all post-Cretaceous time, however, the Breckenridge district had a moderately humid climate, and 

it is unlikely that much lead was moved in chloride 

solutions. Waters that contain the carbonate radicle, 

especially as bicarbonates of the alkaline earths, are 

effective precipitants of lead salts and need not be 

further considered. Oxidizing sulphate waters, which 

are abundant in many of the veins in the district, 

react with galena to form lead sulphate, which is 

only sparingly soluble (41 parts per million) but is 

nevertheless 21 times more soluble than the carbonate,

« Ralston, 0. C., Hydrometallurgy of lead: Am. Inst. Min. and Met. Eng. 

Trans., vol. 70, pp. 447-449,1924.

and this in turn is slightly more soluble than the 

sulphide. A solution of bicarbonate of lead is formed 

from lead carbonate in the presence of free carbon 

dioxide, and the solubility of the carbonate is directly 

related to the concentration of carbon dioxide in the 

water. The solubility of lead carbonate in a solution 

saturated with carbon dioxide 63 is slightly less than that 

of lead sulphate in pure water, and it is unlikely that 

the concentration of free carbon dioxide ever approached saturation in the ground water. The abundance of lead carbonate in the oxidized zone suggests 

that the carbon dioxide was combined with alkalineearth metals and that lead carbonate was precipitated 

instead of transported.

It has long been known that lead carbonate is 

soluble in certain organic acids. A cold-water extract 

of decaying pine needles was prepared by the writer 

and was tested as a solvent for lead sulphate, lead 

carbonate, and lead sulphide by R. C. Wells of the 

United States Geological Survey. The equilibrium 

solubilities of these compounds in grams per liter, as 

determined by Mr. Wells, together with their solubility in pure water, are given in the table below.

Solubility of lead compounds (grams per liter)

Lead sulphate. __ ___ __________

Lead sulphide __ ..- _-_ __ _ .

Pine 

extract

0. 2100

.0215

«. 0089

Water

0. 0019

. 0410

°. 0013

" Lead weighed and reported as lead sulphate.

The solubility of the carbonate in the pine extract 

is greater than that of lead compounds in any other 

probable natural solution. A pound of lead carbonate would require 540 gallons of the pine extract to 

dissolve it, whereas 2,900 gallons of pure water is 

needed to dissolve a pound of lead sulphate, the next 

most soluble compound that can be considered in the 

Breckenridge district. Lead sulphate and carbonate 

are also readily soluble in solutions of ammonium 

acetate, calcium acetate, or magnesium acetate, and 

some of these substances would be present in the soil 

solutions after they came in contact with the carbonate gangue of the vein. As it is probable that this 

region has been forest-clad ever since the Laramide 

revolution, organic acids from decaying vegetation 

have probably been present in the ground water 

throughout the time that the veins have been undergoing erosion.

Leaching of the veins. The open texture of oxidized 

ore favors the rapid passage of ground water from the 

surface to the level of the water table, and the upper 

part of a vein commonly forms a persistent, porous

" Siebenthal, C. E., Zinc and lead deposits of the Joplin region: U.S. Qeol. Survey 

Bull. 606, p. 53, 1915.
ORE DEPOSITS 31

crevice, which drains the surface for considerable distances on both sides of its outcrop. The ample flow 

of water from adits driven along veins indicates that 

the circulation of ground water is very active in these 

veins at present. There is little doubt that during 

the, Tertiary period there was time for a large quantity of ground water to sink through the veins to the 

water table. It is very improbable that the ground 

water carried as high a percentage of organic acids as 

the pine extract or that it would become saturated 

with lead salts before it reached the water table. If 

only a small amount of an ore shoot has been eroded 

and ground water has always been close to the surface, 

only a small amount of lead would be exposed to 

oxidation, and no matter how much water passed 

through the outcrop only a small amount of lead 

could be moved. Thus the actual amount of lead 

dissolved would be many times less than that theoretically possible. Nevertheless it is quite certain 

that very appreciable amounts of lead could have 

gone into solution since Eocene time.

Precipitation of lead. In spite of the comparative 

insolubility of lead minerals it is reported that secondary galena may be formed rapidly. In the Mission mine, near Lincolnville, Okla.,04 crystals ranging 

from one fiftieth to one half inch in diameter were 

deposited on iron spikes and tools that had been submerged during a 2-year period of idleness. Wheeler 

believes that the galena was precipitated from lead 

solutions through the reducing action of the iron. 

The iron spike shown in his paper was strongly altered 

to limonite; sand and angular pieces of chert, the 

largest half an inch in length, as well as galena, are 

cemented to the spike by this limonite. Most of the 

galena crystals were less than a quarter of an inch in 

diameter, but two crystals about half an inch long 

are present. One of these large crystals is beside a 

%-inch fragment of chert, which Wheeler suggests was 

lying on the bottom of the drift, and it therefore seems 

just as probable that the galena was also lying on the 

bottom of the drift. The other large piece of galena 

is separated from the metallic iron by about a quarter 

of an inch of chert-bearing limonite. As the chert 

and sand indicate transportation by moderately 

strong currents, it seems probable that much of the 

galena was also mechanically transported and was 

entrapped by the cementing limonite. Rapid precipitation of galena from mine waters does not seem 

clearly established.

Experimental work shows that hydrogen sulphide 

immediately precipitates lead sulphide from solutions 

of organic lead compounds, lead sulphate, or lead 

bicarbonate. Sulphate solutions immediately precipitate lead sulphate from a pine extract. Solutions containing carbon dioxide or alkaline-earth carbonate will

« Wheeler, H. A., The rapid formation of lead ore: Am. Inst. Min. Eng. Trans., 

vol. 03, pp. 311-323, 1920.

precipitate lead carbonate from a solution of lead 

sulphate if no free acid is present, but if a small amount 

of free organic acid is present carbon dioxide will not 

precipitate solutions of lead. Similarly, solutions of 

alkali carbonates, after neutralizing any free organic 

acid, precipitate lead carbonate.

Probable reactions in the vadose zone. In a galenasphalerite-pyrite vein cold oxygen-bearing waters first 

attack sphalerite, convert it to the sulphate, and leach 

it from the upper part of the vein. Galena has a lower 

oxidation potential than pyrite and therefore tends to 

change to the sulphate before pyrite is oxidized. However, as soon as a protective coating of the insoluble, 

poorly conducting sulphate has formed on the galena, 

it oxidizes more slowly, and the oxygen-bearing waters 

start their attack on pyrite. Some of the pyrite is 

oxidized to inert limonite, but much of it is converted 

into ferric sulphate and sulphuric acid, which are 

mobile and move down, toward the water table.

Lead sulphate is more soluble than the carbonate, 

and the sulphate shell surrounding the galena masses is 

converted to the carbonate by water containing carbon 

dioxide or alkaline-earth carbonates. As ankerite is 

present in most of the" lead veins and in their altered 

wall rocks, the ground water soon contains carbonates 

and bicarbonates of lime and magnesia, which can react 

with the slightly soluble lead sulphate. In many 

places galena is apparently converted directly to 

cerusite, but it is probable that a submicroscopic film 

of sulphate intervenes between the sulphide and 

carbonate.

At the surface there is commonly a layer of decaying 

pine needles which supplies organic acids to the water 

soaking through it. The results of Harrar's work 65 

suggest that organic acids dissolve limonite much more 

slowly than lead carbonate. Thus the lead carbonate 

near the surface would be attacked by the compounds 

extracted from the pine needles and go into solution 

while limonite remained behind. Unless this solution 

were moving very rapidly or escaped an appreciable 

contact with ankerite or some other neutralizing substance, the dissolved lead would soon be reprecipitated 

as the carbonate. In the places where lead solutions 

reached the lower part of the vadose zone they might 

be reprecipitated as sulphate, carbonate, or sulphide. 

If any hydrogen sulphide were present lead sulphide 

would be precipitated regardless of the presence of sulphates or carbonates. As dilute sulphuric acid attacks 

sphalerite and the other sulphides, with the evolution 

of hydrogen sulphide, it is probable that the lead would 

be precipitated as galena. The chemistry of lead in a 

galena-sphalerite-pyrite-ankerite deposit within the 

reach of meteoric water, as sketched above, suggests 

that in the Breckenridge district moderate amounts of 

lead were dissolved and reprecipitated near the surface

83 Harrar, N. T., Solvent effects of organic acids on oxides of iron: Econ. Geology, 

vol. 29, p. 55, 1929.
32 BRECKENRIDGE MINING DISTRICT, COLORADO

in the form of cerusite, but that little secondary galena 

was formed.

It must be recognized that enrichment processes are 

cyclic and depend largely on climate and topography. 

The general nature of the Tertiary period has been 

discussed on pages 22-23, but the placers bear witness 

that at times erosion overtook the tops of the veins and 

swept away much valuable material. At other times 

the downward concentration of the metals proceeded 

with greater rapidity, and some lead was carried down

and reprecipitated, 

chiefly as carbonate 

but to a less extent 

as sulphate and sulphide.

In conclusion, it 

seems that the evidence for the residual enrichment of 

the lead ores in the 

Breckenridge d i strict is established 

by their relation to 

topography and 

supported by their 

mineral changes in 

depth but that 

some solution and 

reprecipitation 

have also occurred. 

Carbonated waters 

are shown to be an 

inadequate agent 

for the transportation of an appreciable amount of lead, 

but a considerable 

quantity was probably carried as organic compounds, 

a small amount as 

sulphate, and only very minor amounts in the form of 

chloride. Most of the lead was reprecipitated as cerusite, but some secondary galena may have formed.

Enrichment of gold. The enrichment of gold is 

easily demonstrable. 'It is closely related to topography, and it is definitely concentrated in the Farncomb Hill veins at the intersection of postmineral 

bedding-plane slips, as shown in figure 5. Its occurrence in the Dunkin vein is clearly related to the 

presence of the impervious shale underlying the open 

fissure in the porphyry and indicates the reaction of 

downward-moving solutions with reducing material 

in the shale. (See fig. 12.)

Although the gold is unquestionably concentrated 

by the agency of surface water the solvent that was

FIGURE 5. Generalized section of Farncomb 

Hill gold vein. After Ransome.

effective in its solution has not been determined. 

Ransome 68 was unable to suggest an effective agent. 

Emmons 67 believes that chlorine is essential for the 

solution of gold and that the presence of manganese 

is almost necessary to release chlorine from chlorides. 

The primary gold ores contain manganese, but chlorides are not abundant enough in the present humid 

climate to furnish a source of the necessary chlorine. 

If the mechanism suggested by Emmons caused the 

gold enrichment, which is clearly related to the Tertiary land surface, it must have operated during a former 

arid climate, when chlorides would have been abundant. The only arid stage in the Tertiary history of 

the region is believed to be the late Pliocene, but it is 

very doubtful if even at that time the mountainous 

western slope of the Front Range, where the Breckenridge district lies, would have been so arid that the 

ground waters were briny. The work of Freise 68 

shows that gold is soluble in humic acids derived from 

decaying vegetation if no oxygen is present. It seems 

probable that solutions similar to the pine extracts of 

the writer were generated through much of the 

Tertiary and Quaternary periods and were probably 

the effective solvents of gold in the Breckenridge 

district.

BLANKET DEPOSITS

Mineral deposits that lie along bedding planes are 

locally known as "contacts" but will be described in 

this report as blanket deposits, or simply "blankets", 

because of the more widespread use of that term. 

Both primary and secondary mineralization are represented in the blanket deposits of the Breckenridge 

district, but the valuable concentrations were apparently caused by the action of surface water.

Relation to country rock. The blanket deposits are 

practically confined to Gibson Hill, Schock Hill, and 

Little Mountain and occur in replaceable beds of the 

Maroon formation and the Dakota quartzite. It is 

possible that the limy beds of the Morrison formation 

contain blanket deposits also, but none were observed 

by the writer. The discontinuous character of the 

beds in the' Maroon formation suggests that layers 

containing ore would not be persistent, and thus the 

value of finding their exact stratigraphic position is 

questionable. At the Sultana and Fox Lake mines 

the ore follows replaceable beds close to the bottom of 

the Maroon formation, but the stratigraphic position 

of the other blanket deposits in this formation is not 

known.

On Little Mountain the blankets occur in both the 

middle and lower members of the Dakota quartzite, 

whereas on Gibson Hill the valuable deposits are found 

chiefly in the middle shaly member. Blanket deposits

69 Ransome, F. L., op. cit., p. 170. 

6? Emmons, W. H., op. cit., p. 308.

68 Freise, F. W., Transportation of gold by organic underground solutions: Econ. 

Geology, vol. 26, pp. 421-431, 1931.
BRECKENRIDGE MINING DISTRICT, COLORADO 33

are not known in any formations younger than the 

Dakota.

Relation to structure. The blanket deposits are not 

related to strong faults or veins. The most favorable 

situation for large replacement deposits seems to be 

at the upward termination of minor veins. It is 

probable that a large open vein permitted such easy 

passage for ore solutions that they had little reason 

for following replaceable beds where these were intersected by the vein; on the other hand, if a fissure filled 

with ore solutions died out in a series of replaceable 

beds the solutions would be forced into an intimate 

contact with these beds that would be highly favorable for the formation of ore deposits of minable 

proportions.

Character of the ore. In Little Mountain the primary 

ore probably consisted of gold and silver accompanied 

by pyrite. Weathering has changed the primary ores 

here into spongy masses of limonite with a relatively 

high content of gold and silver. The ore occurs in 

small bodies, and the richest masses were found in 

pockets along the bedding planes close to the surface.

In Gibson Hill the blankets contain both primary 

and seconday ore and are much larger than the deposits 

on Little Mountain. The primary ore is a mixed 

pyritic lead-zinc ore containing both gold and silver. 

In some places native gold and native silver have been 

found coating galena in the oxidized zone. "Mud" 

seams, consisting of altered rock selvage along beddingplane slips, are mined for their gold content in some 

places, and the adjoining quartzite wall rock contains 

varying amounts of gold disseminated through' it. 

No evidence of mineralization can be seen in the goldbearing quartzite in these places, and the presence of the 

metal can be determined only by an assay. Little is 

known of the character of the ore at the west base of 

Gibson Hill in the blanket deposits in the Maroon formation. In general these were also pjrritic lead-zinc 

ores, having a shallow zone of galena and cerusite, 

and some of the material on the dumps suggests 

that the mineralization was intermediate between that 

of the deposits formed at moderate temperatures 

and that of the deposits formed at high temperatures. 

Specular hematite, epidote, and chlorite are associated 

with pyritic zinc ore in some of the latest material 

mined.

On Schock Hill shallow high-grade gold ores were 

found similar to those in Little Mountain, and some 

deeply weathered lead deposits resembling the deposits of Gibson Hill were also exploited. Some 

pyritic zinc ore was found in the Finding shaft and 

suggests the same downward changes observed in the 

blanket deposits in the Maroon formation at the west 

base of Gibson Hill.

MINE DESCRIPTIONS

COUNTRY BOY

The Country Boy mine is about 1% miles east of 

Breckenridge on the south side of French Gulch 

directly across from the Wellington mill. (See pi. 4, 5.) 

The mine has been operated intermittently since about 

1889, and the known production is given in the table 

on page 60. It is said to have produced, when first 

opened, a high-grade lead-silver ore carrying some gold, 

but it is best known for the high-grade zinc ore that 

has made up the bulk of the production.

The mine was not accessible in 1928, and the following summary is taken from Ransome's report.69

The mine is opened by two adits about 170 feet apart 

vertically and not connected by raises underground. 

The upper tunnel, at an altitude of about 10,000 feet, 

runs S. 18° E. for 700 feet and then turns slightly and 

continues in a S. 25° E. direction for about 1,000 feet 

more. This tunnel cuts the Country Boy vein between 

400 and 450 feet from the portal and is connected with 

drifts and stopes on the vein, most of which are inaccessible. At 350 feet beyond the Country Boy vein 

a drift turns off on the Hite vein, which was being 

prospected at the time that Ransome's report was 

made. The lower tunnel runs southeast for 1,050 feet 

to the Country Boy vein, where it connects with short 

drifts and stopes.

The thick monzonite porphyry sheet of Nigger Hill 

frays out near the Country Boy mine into a series of 

northeastward-dipping sills that intrude the Niobrara, 

Benton, Dakota, and Morrison formations. The 

whole mass has. been strongly faulted, but unfortunately the rocks are very poorly exposed, and it is 

difficult to make out the exact relations. The triangular patch of black shale near the mine is probably 

in large part Niobrara, as the thickness of limy shale 

underground suggests that the Benton does not 

appear at the surface here. The shale is intruded by 

a monzonite porphyry, which crops out at the portal 

of the upper tunnel. Both shale and porphyry are 

cut by a strong northerly fault that offsets the west 

side to the south. The throw of the fault is probably 

less than 300 feet. This fault may be a continuation 

of the Great Northern fault zone of the Wellington 

mine. The throw is apparently less at the Country 

Boy mine, but the relations are so obscure that it is 

possible to point out only the most probable of the 

many possible interpretations of the areal geology. 

The wedge-shaped area of black shale east of this 

fault is probably Benton and is limited on the southeast by the Country Boy fault vein, which brings 

Dakota quartzite against it. The Dakota quartzite 

in turn is limited on the southeast by another fault,

69 Ransome, F. L., op. cit., pp. 134-136.
BRECKENRIDGE MINING DISTRICT, COLORADO

which brings monzonite porphyry against the quartzite. In all these faults the down thrown side is on the 

northwest or west. Apparently the porphyry sill 

southeast of the Country Boy vein is underlain by the 

gray shale of the Morrison about 300 feet below the

5 feet. The ore consisted of dark sphalerite with very 

little gangue material and graded into pyritic ore at 

both ends of the 250-foot stope on the lower level. 

Galena is rare in this level and occurs only in small 

bunches less than 5 inches in diameter. Most of the

FIGURE 6. Map of Detroit mine.

surface. The general dip of the gray shale is about 

17° NNW., and the dips of the black Cretaceous shale 

range from 25° to 50° N. or NE.

The Country Boy vein strikes approximately northeast and dips on the average about 80° NW. Where 

Ransome examined the vein, it was entirely in monzonite porphyry and had a maximum width of about

ore cements and replaces crushed porphyry in a zone 

of fissuring and brecciation, but some of it fills cleancut fissures in the porphyry. The boundary between 

the vein and the country rock is usually very definite, 

but the wall rock shows marked metasomatic alteration and contains pyrite and sphalerite in veinlets 

and disseminated crystals. Where the ore pinched
U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 176 PLATE 8

A. "ROLL" IN THIN-REDDED DAKOTA QUARTZITE, STANDARD STOPE, DETROIT MINE.

B. GALENA ALTERING TO CERTJSITE, DETROIT MINE.
MINE DESCRIPTIONS 35

the course of the vein was marked by a tough seam 

of gouge, but it was by no means certain that the 

gouge is younger than the ore.

If the writer is correct in believing that the Great 

Northern fault zone intersects the Country Boy vein 

a few hundred feet to the southwest, the ore of the 

Country Boy prt)bably bears the sa.me relation to 

this ore channel as the ore shoots of the Wellington

bile road of moderate grade. The mine was first 

worked about 40 years ago, when rich gold ore was 

taken from a bedding-plane deposit through an inclined shaft. Accurate data on the early production 

are lacking. After a long period of idleness the mine 

was reopened by George Robinson, of Breckenridge, 

in 1923. A vertical shaft was sunk 151 feet, and 

three levels were turned. Lead ore carrying conx- Massive '.'''. '. . . 

:C'X' <ilJ ?':.tz'.t?'. '.' . ' 

EXPLANATION

Benton shale

Dakota quartzite 

(interbec/dect shafe and tpuortzi'te)

Dakota quartzite 

t'Massive yuartzite)

Fault with less than l"of gouge

Fault with from l"to I'of gouge

Fault with more than l' of gbu£e

Fault showing directfon .of downthrow

Strike and dip of rocks

Pitch of striae

Winze ' ' '

LEVEL Z

FIGURE 7. Geology of Detroit mine.

mine. Prospecting east of the present ore shoot 

would be unlikely to find an extension of the ore if it 

behaves like the veins of the Wellington mine.

DETROIT

General features. The Detroit mine lies 1)£ miles 

northeast of Breckenridge and about 1,000 feet east 

of the summit of Gibson Hill. It is at an altitude of 

10,325 feet and is accessible by means of an automosiderable gold and silver was found, and since 1924 

many shipments have been made. The known production of the mine is given in the table on page 60. 

The mine is opened by two vertical shafts the 

"old shaft", 151 feet deep, connecting with the productive stopes, and the "new shaft", 80 feet deep, 

which has been sunk to develop possible ore bodies 

to the southwest of the "old shaft." Levels have 

been turned at 40, 90, and 140 feet from the "old
36 BRECKENRIDGE MINING DISTRICT, COLORADO

shaft" and are shown in figure 6. The drifts from 

this shaft aggregate about 2,000 feet. In 1928 the 

underground work was done by hand, and mining 

was confined to the first and second levels. The ore 

was sorted and raised to the surface by a steam hoist 

and placed in appropriate bins. High-grade ore was 

hauled to Breckenridge for shipment, and low-grade 

ore was taken to the Extension mill, about a mile to 

the north. The mill treated about 25 tons of ore a 

day, producing a high-grade lead concentrate on tables 

and gold and silver bullion from cyanide treatment of 

the tails. In 1928 the mill superintendent reported 

that 80 percent of the gold and 70 percent of the 

silver were recovered.

Geology, The Detroit mine is exploiting some of 

the bedding-plane deposits or blanket veins that are 

characteristic of Gibson Hill. (See fig. 7.) These 

deposits occur in replaceable beds of the upper memMany small normal faults occur and either parallel 

the bedding planes of the shaly quartzite or cut across 

them at low angles. At several places the faults cut 

through the top of a steep monoclinal fold and produce open broken ground very favorable for the localization of ore. As shown in figures 8 and 9, the weak 

shaly beds are dragged and crumpleM near the intersection of the fault and the fold, resulting in a structure known to miners as a "roll."

The gently dipping faults are cut by a system of 

steeply dipping normal faults that trend from north 

to northeast and dip 55°-SO° E. or NE.

Ore deposits. On the first level the Standard stope 

extends southwest for a distance of about 200 feet and 

has an average width of 60 feet. The hanging wall of 

the Standard stope is thin-bedded shaly quartzite, and 

the footwall or floor is a layer of massive quartzite. 

The ore occurs in gouge seams of bedding-plane faults

FIGURE 8. Development of "roll" in Detroit mine through bedding-plane faults.

ber of the Dakota quartzite. The "old shaft" 

exposed the following section:

Feet 

3

Section in "old shaft" at Detroit mine 

Wash....__..____._.__._________._.__. -.______

Benton shale:

Light-gray shale---_--_-_--________-_-_-_____..___ 56

Jet-black shale._ -_______-________--_-___-_____-_ .15

Dakota quartzite:

Black shale interbedded with thin layers of quartzite; shale and quartzite beds ranging from 6 inches 

to 2 feet in thickness; quartzite becoming more and 

more prominent toward the base_______'_______.__ 18

Massive white quartzite; gray to white on fresh fracture but heavily stained with iron on joints. ______ 59

The general dip of the sediments near the Detroit 

mine is about 20° SE. Locally sharp monoclinal 

folds cause steep dips, and near porphyry masses the 

beds may be much disturbed. About 100 feet east 

of the "old shaft" a wide dike of altered monzonite 

porphyry cuts through the sediments. There are 

many small irregular offshoots from this mass, and 

two that were found underground dip about 65° E.

and as invisible impregnations in shale and shaly quartzite and can be delimited only by careful sampling. The 

best.and thickest ore coincides with the axis of a "shale 

roll", where the distance between the floor and back 

of the stope is 10 feet. (See pi. 8, A.} As the ore is 

followed downward to the northeast, it becomes confined to narrow zones near some bedding-plane faults, 

which gradually diverge from the gently dipping fault 

that forms the floor of the stope under the "shale roll." 

To the northwest the ore decreases in thickness as the 

faults forming the floor and back of the stope approach 

each other. Nearly all of the ore from this stope was 

oxidized, but some sulphides, chiefly galena, were 

found in its deeper parts. Partly oxidized ore from 

this stope is shown in plate 8, B. A strong northwest 

fault forms the northeastern limit of the stope and drops 

the ore-bearing beds to the second level.

The conspicuous bedding-plane faults are marked by 

seams of white, gray, or brown claylike material from 

2 to 6 inches thick. According to the miners, the color 

of the gouge is a guide to the tenor of the seam.
MINE DESCRIPTIONS 37

Brown "mud seams" will nearly always assay 2 ounces 

of gold to the ton; gray "mud seams" vary in value, 

but usually carry at least 1 ounce of gold to the ton;

Black shale Gouge slips-

"TTT^'/ ' .' ' !' '. - K'. ' '' '.' .' .Quartzite'. '> ' .'^V-.. ' '.' '. 

FIGURE 9. Detail of "roll" in Detroit mine.

the white "mud seams" are the most uncertain and 

range from half an ounce to 2 ounces of gold to the ton. 

The small masses of sulphide found in the Standard 

stope were commonly embedded in gray or brown 

gouge, and many masses of galena were discovered that 

were coated with wire silver on the upper surface and 

with fine-grained gold on the lower surface.

It had replaced black sandy shale between two layers 

of quartzite about 4 feet apart. In some places 

the shale was entirely replaced by galena, and 4}£ 

feet of solid ore was mined from the best parts of 

the stope. The stope is limited on the east by a 

northward-trending fault, beyond which no commercial ore has been found. A raise was put up from the 

third level to test the ground east of the fault, in the 

hope that a faulted continuation of the ore would be 

found below the second level, but nothing of value 

was found.

About 15 feet above the Sulphide stope a thin seam 

of high-grade ore has been stoped. The ore is chiefly 

galena but contains some sphalerite and pyrite. It lies 

beneath a thin bedding-plane slip in black shale and is 

from 3 inches to If oo tin thickness. (See fig. 10.) A few 

steeply dipping seams of lead-zinc ore have been found 

in the masses of porphyry cut 100 feet southwest of the 

shaft. The seams were narrow, and no attempt was 

made to follow them.

DUNKIN

The Dunkin mine lies on the southeast side of Nigger 

Hill about 1}£ miles northeast of Breckenridge. It has 

been worked intermittently since about 1895, has produced oxidized lead ore carrying considerable amounts 

of gold, and is best known for the shoots of crystallized 

gold found on the bottom level. The mine was orig3 FEET

FIGURE 10. Galena ore in shale, Detroit mine.

On the second level ore has been found at two horizons northeast of the fault that cuts off the Standard 

stope on the first level. The largest ore body was 

found in the Sulphide stope and is thought to be the 

unoxidized equivalent of the Standard stope ore body.

inally opened by the Dunkin Mining Co. with the expectation of developing large bodies of milling ore. 

After about 7 years of development work, in which 

little attempt was made to stope and ship ore, the mine 

was leased to a Mr. Gallager, who drove most of the
38 BRECKENRIDGE MINING DISTRICT, COLORADO

Gallager tunnel (see fig. 11) and stoped the ore above 

it in 1902 to 1906. T. H. Knorr leased the mine from 

1906 until 1916 and exploited the mixed sulphidecarbonate ore found below the Gallager tunnel. The 

high-grade gold ore found in the lowest adit was shipped 

in 1914 and 1915, and since that time little work has been 

done on the property. In 1928 George Robinson was 

reopening the intermediate, or Railroad tunnel, with 

the hope of reworking some of the old ore shoots and 

discovering new ore bodies.

The underground workings consist of three main 

adits the Gallager tunnel, the Railroad tunnel, and 

the Redwing tunnel driven northeastward into Nigger

sill that forms much of Nigger Hill, the vein becoming 

nearly barren soon after it passes into the underlying 

Ben ton shale on the Redwing tunnel level. The vein 

occupies a fault fissure of small vertical displacement 

that trends northeast in the mine but swings east a 

short distance beyond the northeast end lines of the 

Redwing claim. The dip of the vein shows many 

minor irregularities but does not depart much from an 

average of 57° SE.

The ore in the Dunkin mine occurred in several 

lenticular ore shoots separated by barren ground, in 

which the vein pinched to a tight, narrow seam of 

gouge. The ore shoots were 50 to 100 feet in breadth

600 FEET

FIGURE 11. Map of Dunkin mine.

Hill and a few short exploratory openings long since 

inaccessible. The portal of the Railroad tunnel is 

on the Colorado & Southern narrow-gage railroad at 

an altitude of about 10,135 feet. The Gallager tunnel 

is about 165 feet vertically above it, and the Redwing 

tunnel is about 135 feet below it. These adits are 

connected by raises, but the portals of the upper and 

lower adits were caved in 1928, and only the Railroad 

tunnel could be entered.

The productive portion of the Dunkin vein lies 

almost wholly within the thick monzonite porphyry

and 200 to 500 feet in pitch length, but only one of 

them is known to have extended continuously from 

the bottom level to the surface. The shoots are said 

to pitch at about 60°-70° NE! The width of the ore 

in the shoots ranged from 6 to 40 inches between the 

porphyry walls but narrowed to a few inches where the 

vein passed into shale, and the vein became a narrow 

barren seam of quartz and pyrite 15 or 20 feet below the 

porphyry sill.

The ore in the upper levels consisted almost entirely 

of cerusite carrying about 1 ounce of gold and 12
39

ounces of silver to the ton. Below the Railroad tunnel 

galena and some anglesite were found in addition to 

the cerusite, which was a notable constituent of all 

the lead ore mined. No record was made of the zinc 

in the ore, but it was invariably below 8 percent, as no 

shipment was ever penalized for its zinc content. 

Pyrite was found only on the Redwing tunnel level 

associated with the rich gold ore hi the fetid limy 

shales of the upper Benton beds. The gangue was 

chiefly sugary iron-stained quartz, according to Mr. 

Knorr, and the writer found some brown jasper and 

jarosite in the Railroad tunnel, but no ankerite was 

observed. Some gold was associated with the jarosite 

and galena and occurred as small flakes disseminated 

through the ore.

At the bottom of the long ore shoot that extended 

from the surface to the Redwing tunnel a rich seam of 

gold was found. This ore shoot, which is the one 

farthest from the portal, has a stope length of about 

100 feet on the bottom level. Throughout this distance gold was found in paying quantities, commonly 

in spectacular spongy masses of coarsely crystalline 

gold from 1 to 4 inches thick and as much as 40 inches 

in length and height. Some of these pockets netted 

$10,000 to $35,000 in a few hours, according to Mr. 

Knorr. A specimen of this high-grade ore shown to 

the writer was about 2}£ inches thick and consisted of 

a well-crystallized slightly vuggy mass of gold, free 

from other minerals. White gouge on one side of the 

specimen and black manganese-stained gouge on the 

other suggested that it represented the width of the 

vein at the place where it was obtained. Octahedral 

faces were prominently developed on the gold crystals, 

and stout columns of twinned crystals rose from the 

vuggy openings. Some of the gold is said to have 

been associated with cubes of pyrite and some of it 

with galena.

The small vertical displacement on the Dunkin 

fault vein did not displace the shale-porphyry contact 

more than 10 feet at any place where it was explored, 

according to the miners. The report that a shale 

footwall and a porphyry hanging wall would give way 

to a porphyry footwall and shale hanging wall suggests 

that the irregular contact of the sill and shale was 

moved farther horizontally than vertically. The rich 

gold ore was practically confined to the upper 10 feet 

of shale and was best developed where the shale and 

porphyry overlapped each other on different sides of 

the vein. (See fig. 12.) The oxidized nature of the 

ore and the structural relations of the rich gold ore 

shoot to the shale indicate that it was formed by enrichment through the agency of surface water. For this 

reason a downward extension of the high-grade gold 

ore would not be probable. The horizon at which the 

free gold occurs is nearly 300 feet above the Dakota

quartzite, and it is problematic whether the Dunkin 

vein persists through the thick layer of shale. If 

there was a large horizontal movement along the vein 

it is likely that the fissure continues into the quartzite, 

but if the horizontal component was small the vein 

probably dies out before reaching the quartzite.

According to Mr. Page, the Redwing tunnel was 

driven in large part on a fissure parallel to the Dunkin 

vein, and the ore shoot was discovered by driving a 

crosscut to it. Thus it is possible that other concentrations of gold ore exist in the unexplored vein between the first and last ore shoots. However, as most

^Sflix Monzonite porphyry J

< ' \\V v X. v r ' ' »

FIGURE 12. -Occurrence of rich gold ore iu Duukin mine.

of the information on the Dunkin mine was obtained 

from others, the writer does not feel qualified to offer 

definite opinions.

The western branch of the Redwing tunnel, according to Mr. Knorr, explored a vein containing from 1 to 

3 feet of "mill ore" that contained some lead, zinc, and 

iron sulphide. Its strike and dip suggest that it is a 

continuation of the Juniata vein.

JUNIATA

The Juniata vein is on Nigger Hill about 1% miles 

east of Breckenridge. Most of the workings are on 

the north slope of the hill a short distance west of the 

head of Nigger Gulch. In 1890 the Dunkin Mining 

Co. owned the Wolf tone claim and commenced development work on the Juniata vein within its end
40 BRECKENRIDGE MINING DISTRICT, COLORADO

lines. The Wolf tone shaft was sunk and two levels 

turned from it, as shown in plate 9. Unfortunately, 

the vein turns sharply as it enters the Wolf tone claim, 

and in a lawsuit between the Juniata and Dunkin 

mining companies it was shown that the extralateral 

rights of the Juniata claim gave its possessor the ownership of nearly all the vein under the Wolftone claim 

that had been opened by the Dunkin Mining Co. 

Since the settlement of this lawsuit in 1895 little work 

has been done.

As shown in plate 9, the vein is opened by several 

shafts and two adits, one on each side of Nigger Gulch. 

The Juniata main shaft, an incline said to be about 280 

feet deep, and the Wolftone shaft, said to be about 250 

feet deep, are the principal shafts on the vein. The 

4 levels on the Juniata and the 2 levels on the Wolftone were all inaccessible in 1928, and the available 

information on the mine is necessarily meager.

The Juniata vein is wholly within porphyry, so far 

as known, and strikes from N. 50 E. on the Wolftone 

claim to N. 80 E. on the Juniata claim and dips about 

59° SW. The ore was chiefly a high-grade galenacerusite ore having galena as the most abundant constituent. It is said to have carried more silver than 

gold in the Wolftone workings, but gold contributed 

greatly to the value of the ore shipped by the Juniata 

Co. The galena on the lower levels contained a small 

amount of sphalerite and pyrite, but there was still a 

decided preponderance of lead. The vein is said to 

have pinched to' a width of about 4 inches on the third 

level of the Juniata and is not certainly recognized on 

the fourth level.

MINES ON IITTIE MOUNTAIN 

GERMANIA

The Germania vein is about a mile south of Breckenridge, on the east side of Little Mountain, about 250 

3rards west of the portal of the Willard tunnel. No 

map of the underground workings on this vein could 

be found, but according to reliable information the 

Germania shaft was 300 feet deep and three productive 

levels were turned from it. These levels are said to 

have followed the vein closely, and very few crosscuts 

were driven to explore the country to the northwest 

or southeast of the vein. Most of the ore, which was a 

.high-grade lead-silver and gold ore, is said to have come 

from a large stope 160 feet long and 80 feet wide on the 

second level.

Geology. The bottom of the shaft is said to be in red 

shale, but most of the workings were in quartzite. 

The black shaly quartzite of the upper part of the 

Dakota formation crops out a short distance from the 

shaft, and most of the dump is made up of quartzite. 

The vein strikes N. 40° E. and dips 80° SE. It is 

nearly parallel to the large fault 350 feet northwest of 

the Germania shaft, which has dropped the region to

the east a distance of several hundred feet. Southeast 

of the Germania vein the rocks are influenced by the 

Rocky Point syncline and dip southeast. The intermediate quartz monzonite porphyry at the south end of 

Little Mountain sent several fingers into the Germania ground, but so far as could be learned little 

porphyry was encountered in mining.

There are several prospects and abandoned mines on 

Little Mountain, and oxidized gold-silver ore was found 

in small pockets in many of them. The description of 

the Germania tunnel given below is quoted at length 

from Ransome's report because it adequately describes 

a deposit typical of the occurrence of the gold-silver 

ores of Little Mountain, and the writer does not believe that anything will be gained by the enumeration 

and description of the other occurrences of ore.

Germania tunnel. The Germania tunnel was driven 

with the hope of finding the Germania vein on the 

west slope of Little Mountain. It enters the hill a 

short distance above the Blue River, about 20 feet 

below the level of the.Hoosier Pass highway. It was 

inaccessible at the time of the writer's visit, but, according to Ransome,70 it

runs northeast for about 300 feet through boulder till and then 

through about 140 feet of much disturbed black shale, quartzite, 

and monzonite porphyry. The structure of these rocks cannot 

be determined from the present exposures in the tunnel. From 

the main tunnel a drift turns north on a strong zone of fissuring 

which dips about 55° W. The rocks along this fissure are 

crushed and exposures in the drift give no hint of their general 

structure. At the winze (see fig. 13) a seam of ore was found in 

hard massive quartzite which was stoped up an incline, as 

shown. This ore is said to have been of good grade, but the 

body evidently was small. Its general dip was to the south at 

about 28°, whereas the dip of the quartzite is about 40°. The 

quartzite has been irregularly fractured along a plane lying at a 

little lower angle than the bedding, and the small fissures and 

interstitial spaces, some of them enlarged by solution, have been 

coated with quartz druses and are filled with a porous rusty 

material that is chiefly limonite. This material, which in 

places carries enough gold and silver to constitute rich ore, is 

probably an oxidation product of pyrite.

The richest bunch of ore ever found in the quartzite of Little 

Mountain lay within 10 feet of the surface in the footwall of the 

strong fissure which is cut in the Germania tunnel and on whose 

hanging-wall side was the pay shoot that has just been described. A little flat seam of limonite that was followed into 

the footwall of this fissure was found to expand into an ore 

body which turned down to the east with the bedding of the 

quartzite. (See fig. 14.) This mass was about 20 feet in 

diameter and up to 6 feet thick. Around its edge the ore 

graded into spongy limonite of no value. The ore itself differed 

little from this material in appearance, being a very porous 

mixture of limonite and silica which carried gold and much 

silver. The silver was for the most part probably native or in 

the form of cerargyrite, but Thomas West, who discovered the 

pocket, reports that there was also some "sulphurets of silver" . 

possibly argentite. A few feet southeast of the ore body the 

quartzite is cut by monzonite porphyry; the ore thus accumulated in an angle between the fissure and the porphyry.

M Ransome, F. L., op. cit., pp. 161-163.
MINE DESCRIPTIONS 41

PUZZLE AND GOLDDDST

Puzzle-Ouray vein. The Puzzle-Ouray vein is about 

a mile southeast of Breckenridge. It was opened about 

1885 and was productive from about 1888 to 1900. 

Since that time intermittent production has been made 

by lessees but has not been large. About 1890 the 

Puzzle and Ouray mining companies became involved 

in a lawsuit over the ownership of the vein. This suit

tested the Puzzle vein 65 feet below that level, and 

the ground to the northwest has been explored from 

the Willard tunnel through a crosscut that connects it 

to the Golddust shaft on the Golddust vein. Ransome 

estimates the total production of the Puzzle-Ouray 

vein prior to 1909 at $960,000.

The Willard tunnel level is the principal level of the 

Puzzle mine as well as of the Golddust mine. The

FIGURE 13. Plan of the Qennauia and South Elkhorn tunnels in Little Mountain. After Ransome.

was settled in favor of the Puzzle Mining Co. in 1897, 

and no further work was done on the Ouray claim after 

that year. Most of the work since 1900 has been of 

an exploratory nature. A crosscut to the southeast 

on the second level of the Puzzle Extension shaft was 

intended to test the Washington vein but was discontinued before it reached its objective; a winze (at A-A, 

pi. 10, A) sunk from the Willard tunnel level in 1927

portal is directly east of Little Mountain at an altitude 

of about 9,840 feet, and is a quarter of a mile north of 

the "horseshoe" the 350° curve in the railroad below 

Rocky Point. The tunnel runs 715 feet southeast to 

the Puzzle vein and turns northeast on the vein, which 

it follows to the Puzzle Extension shaft. At this point 

a crosscut connects it with the Golddust shaft and 

the Pacific vein, as shown in plate 10. The second
42 BRECKENRIDGE MINING DISTRICT, COLORADO

level, about 95 feet above the Willard level, is open 

near the Puzzle Extension shaft, and the Sauer's 

crosscut on this level extends 700 feet southeast of the 

Puzzle vein and was in good condition in 1928. The 

rest of the upper levels of the Puzzle, all the Ouray 

workings, and all the upper workings of the Golddust 

were inaccessible during the writer's visit. Most.of 

the workings are shallow, and very little production 

came from .the ground below the Willard tunnel, which 

is only 280 feet below the surface at its deepest point. 

The Ouray shaft, however, is said to extend 268 feet 

below the Willard tunnel level, but the section in plate 

12 indicates that most of the work iii this part of the 

vein was also confined to a zone less than 280 feet from 

the surface.

The surface geology is shown in plate 2 and the 

underground geology in plate 10. As shown in plate 

2, the predominant country rock of the Puzzle vein at 

the surface is Dakota quartzite. The vein cuts diagonally across a local anticline that has a steep southFIGURE 14. Cross section showing mode of occurrence of a rich pocket of oxidized 

gold-silver ore in the quartzite of Little Mountain. After Ransome.

east limb and a gently dipping northwest limb. The 

Benton shale is prominent near the mouth of the 

Willard tunnel but is not cut by the vein in the workings now accessible, although it is almost certainly 

present in the old Ouray workings at the southwest end 

of the vein. The black Benton shale foXind at the portal 

of the Willard tunnel is faulted against Dakota quartzite 

along the Eva vein 165 feet from the entrance. At 295 

feet the quartzite is overlain by a sill of monzonite 

porphyry, and this rock is faulted against the lower 

part of the Dakota formation 375 feet from the portal. 

The black shale found at 475 feet overlies a coarsegrained gritty quartzite or arkose. which- is probably 

the basal member of the Dakota formation. Below it 

a few thin beds of quartzite interstratified with gray 

calcareous shale suggest a gradual transition to the 

Morrison formation. Gray shale forms both walls of 

the Puzzle vein where it is cut by the Willard crosscut 

and continues on both sides of the drift for 225 feet, 

where a thick monzonite porphyry sill appears abruptly 

in the footwall on the northwest side of the drift. 

From this point to the end of the drift on the Puzzle 

vein, a distance of 700 feet, the gray shale of the Morrison forms the country rock southeast of the vein and 

the monzonite porphyry sill forms the northwest wall 

of the vein. A 20-foot winze at the junction of the vein 

. and the Willard crosscut showed no ore but revealed a 

bed of quartzite under gray shale and faulted against 

the gray shale that underlies black shale. The presence of interbedded quartzite, black shale, and gray 

shale is well shown near the Puzzle Extension shaft and 

is indicated in plate 10. At a point 375 feet southwest 

of the shaft a winze was sunk to a depth of 65 feet in 

1927. The vein contained a small amount of sphalerite, and gold was reported in a sample obtained at the 

collar of the winze. The vein, which was nearly 

vertical here, dipped out of the winze to the northwest 

about 30 feet below the collar but was found on the 

sublevel by a short crosscut to the north.

The general course of the Puzzle vein is N. 50°-70° 

E., and above the Willard tunnel level it dips about 80° 

N. The pronounced drag of the beds observed in the 

open cuts on the surface, shown in plate 11, A, clearly 

indicates that the vein occupies a normal fault. The 

original displacement of this fault was probably about 

120 feet, but the intrusion of the porphyry sill on the 

hanging-wall side has raised the overlying beds and 

decreased the initial displacement by varying amounts, 

dependent on the thickness of the sill. According to 

the miners thin porphyry sills were also found in the 

fobtwall of the vein. Locally the porphyry has cut 

through the fault, but in general it followed the fault 

plane very closely.

Profitable ore was found only where one or both walls 

of the vein were quartzite or porphyry. The gray shale 

of the Morrison underlying the Dakota formation 

apparently causes a marked decrease in the value of 

the ore. The relation of the ore to the surface has 

already been commented on and is well shown in the 

stope map of the Puzzle-Ouray vein (pi. 12). The 

relation to the surface suggests enrichment. In a 

.fissure cutting the Morrison and Dakota formations 

the movement of ground water would be largely confined to the more open fractures above the Morrison, 

and as enrichment is dependent on the movement of 

ground water, the double relation of the ore to the 

surface and the wall rocks can be satisfactorily 

accounted for by assuming the ore to be secondary. 

However, primary ore is also localized in open fissures 

and shows a marked relation to the character of the 

wall rock. The superior hardness of the quartzite 

makes it resistant to erosion and relates the surface to it 

through the very qualities that would localize a primary 

ore shoot between walls of this formation. The ore 

seen by the writer was a mixture of galena, zinc blende, 

and pyrite, which suggested primary and not secondary 

metallization.

The material shipped from the Puzzle-Ouray vein is 

reported to have been largely a lead ore carrying moderate amounts of silver, some iron, and less than 8 per-
O.S G&OLiOGICAI.. SURVEY PROFESSIONAL PAPER 176 PLATE 9

300 Feet 

'

WOLFTONE

SHAFT \ SHAFT

JUN/ATA

SHAFT

HOUSE

fl

SECTION

PLAN AND SECTION OF JUNIATA MINE.
U.S. GEOLOGICAL SURVEY PKOFESSIONAT PAPER J76 PLATE 10

PORTAL 

WILLARD TUNNEL

 ^XfCPUZZLE EXTENS/ON SHAFT 

'35 feet abore Wi/lard 

cut level

, PA, anc/ ?% Cu

ncf $4,OOO production

-' .' * "X \'. / * *«* ''^V'VS* £9° ' J^* 1 /.* ' ' 1* 

fs ven

on su'r>a'c4"*""'V-«...».

SAUERS 

SHAFT 

El. 140 feet 

above crosscut -_n

.'/ '. ' -\^\Coarse'-ffrained

' ''' " " ' ' ""

£5 ?/A'$i>-i< J J < i

" 

t&t:$0?$j$iii&^:.. < ,

|^/,',\\- , .^'-^ ^"^^^S*:^

^^£<^:^^^llllSfc^/^ '-Ss|£^fe:<v H-'-^^ -' v^'i^^v

Morrison formation,-": .;'-'^$^> .A, ^ < A//v , < * t- i J1 f. v \>^T /A /A/^SiA -' '' 

-^^^^^^^^V/y^7 ^ > < > ^ r ^v^^:^;^

>< > ^ ^^^//.^^Ui^ofA , J u 7 r A v A , « 4 . > 4 * AA^-;...

:.:/:

Ore-bearing part of vein

7"A/s ro-we /s 7S feet fron 

breast and breast is SO 

feet from shaft

C"' -^\^ sftGEOLOGIC AND ASSAY MAP OF PUZZLE, GOLDDUST, AND PACIFIC VEINS.

A, Willnrd tunnel level; B, Sauer's crosscut level.
D.S. GEOLOGICAL SUKVEY PROFESSIONAL PAPEE 176 PLATE 11

A. OPEN STOPE ON PUZZLE VEIN, LOOKING NORTHEAST.

B. WELLINGTON MILLS AND MINE IN 1928.
U.S. GEOLOGICAL SUKVEY PROFESSIONAL PAPER 176 PLATE 12

w.

Shale

Reported to be 

lean ore

Shale

May 6e stopecfout

Quartzite

w.

Shale

A

^- Ele va tion of portal of 

Wi/lard crosscut

E.

IOO 3 

i i i i i i

Known stoped area

500 Feet

GOLDOUST

B

STOPE MAPS OF PUZZLE (A) AND GOLDDUST (B) VEINS.
U.S. GEOLOGICAL STJEVEY PROFESSIONAL PAPER 176 PLATE 14

A. MAIN VEIN, WELLINGTON MINE. 

Parallel to intrusive contact of cross-breaking monzonite porphyry cut by gently dipping normal fault. (See fig. 18.)

B. PARTIAL REPLACEMENT OF SHALE \\ALL OF MAIN VEIN, WELLINGTON MINE.
MINE DESCRIPTIONS 43

cen t of zinc. On the upper levels anglesite and cerusite 

occurred, but, in general, galena made up the bulk of the 

material mined at a profit. The vein is said to have 

been from 1 to 15 feet wide, and although no ore 

remained for study in the productive parts of the vein, 

tho width of the old stopes and the exposures of the 

loan portions of the vein suggest that it was from 3 to 5 

feet wide in most places.

Golddust vein. The Golddust vein could be entered 

only from the Willard tunnel at the time of the writer's 

visit. The bottom of the Golddust shaft was originally at the level of the Willard tunnel, but it was 

later deepened by lessees, who turned a level at 28 

feet in 1915, another level at 52 feet in 1916, and a 

third at 87 feet in 1928. All the Golddust workings

the Puzzle Extension shaft can be followed along the 

crosscut to the Golddust shaft. It is underlain by 

gray shale on the 87-foot sublevel and in the eastward 

extension of the Puzzle drift. The monzonite porphyry sill forming the north wall of the vein at the 

Golddust shaft is underlain by quartzite on the 52- 

foot sublevel and is evidently a different sill from that 

forming the south wall. On the 87-foot sublevel an 

irregular dike of porphyry follows the sheeted zone 

and is clearly later than the first movement on the 

fault but earlier than mineralization. It probably 

represents the conduit through which the magma rose 

to form the sills on the higher levels. Subsequent 

movement along the sheeted zone resulted in local 

shattering and the displacement of the sills. The

Dakc

Drift cavea? 

here

Monzonite* < 

^orp^ycy- 1-

^/WW///M^/W^

''///// ' '^Morrfson format!'

 ayish-blactt 'shale 

'////

mation Pitch ,.'.' .' ' '.

. Dakota quartzite ' 

 / ~,r .. 

.'. .''Black tag ray ytzte.' '. ' ' ' ' ' ' '' ' 

'Mineralized (ZnS and fe5z) 

crushed and ottered 

monzonite porphyry

EXPLANATION

Vein more than iz"thick 

Vfein less than I2"thick

Fault or fissure

I Arrow shows dip)

Stops

FIOUBE 15. Sublevels of Oolddust mine. A, 52-foot sublevel; B, 87-foot sublevel.

were inaccessible in 1928 except the 87-foot sublevel 

and a small part of the 52-foot sublevel. (See fig. 15.) 

The shaft was open to the surface, but the development work was carried on from the Willard tunnel 

level, where a small underground hoist was installed. 

A stope map is shown in plate 12.

The Golddust vein is a sheeted zone 15 to 30 feet 

wide, striking about N. 75° E. and dipping about 

80° S. The sheeted zone has fairly definite walls, and 

the vein matter between them consists of much disturbed and fractured shale, porphyry, and quartzite, 

replaced in varying degree by galena, pyrite, and 

sphalerite. This zone was formed by reverse faults.

On the Willard tunnel level at the Golddust shaft 

monzonite porphyry forms both walls of the vein. 

The porphyry on the north wall of the Puzzle vein at

details of the geology of the 52-foot and 87-foot sublevels are shown in figure 15, and the geology of the 

Golddust vein on the Willard tunnel level, largely 

taken from Ransome's report but modified in the light 

of later developments, is shown in plate 10.

The ore consisted of irregular stringers of sulphides 

ramifying through the sheeted zone. Usually there 

was a persistent vein on the hanging wall and the 

footwall of the sheeted zone. These veins ranged 

from a few inches to 3 feet in width. In the disturbed 

rock between the two walls of the sheeted zone the 

stringers of ore branched and disappeared, or joined 

and formed a workable seam of ore. According to 

several miners, the vein was at one place 30 feet wide, 

and 4 wide seams of ore furnished material for cars 

on 4 parallel tracks. The sketch of the occurrence
44 BRECKENRIDGE MINING DISTRICT, COLORADO

of ore shown in figure 16, taken from Ransome's 

report, illustrates the irregular nature of the seams of 

ore on this level. On the 52-foot sublevel the ore was 

more compact and occurred chiefly in a vein on the 

footwall of the sheeted zone. In places on this sublevel the ore was nearly 4 feet wide and consisted 

chiefly of galena. On much of this level underhand 

stopes 10 to 15 feet deep were made. On the 87-foot 

sublevel no shipping ore was found. The porphyry 

vein filling was impregnated with thin seams of sphalerite, generally less than 30 inches in length, and 

galena was disseminated through it in the form of 

fine grains. On this sublevel a vein of galena and 

sphalerite from one eighth to 3 inches wide was found 

on the footwall between quartzite and porphyry Walls 

about 80 feet east of the shaft. The ore extended to 

the bottom of the drift in only a few places until a 

point about 165 feet from the shaft was reached. Here

FIGURE 16. Occurrence of ore in Qolddust vein. After Ransome. a, Disturbed 

black shale; 6, porphyry; c, gouge; d, ore; e, quartzite.

the vein thickened to about 6 inches, and at 200 feet 

from the shaft it became about 12 inches wide at the 

top of the drift. A-raise upon this ore broke into an 

underhand stope 28 feet above the level, where the 

ore had widened to about 2% feet. Evidently the 

southwestern part of the ore shoot bottomed very 

abruptly between the 52-foot and 87-foot sublevels. 

Although little ore was found on the bottom level, the 

sheeted zone was very well defined, and the shale, 

porphyry, and quartzite filling were strongly impregnated with pyrite and to a less extent with sphalerite.

The Golddust ore is reported to have been largely 

a galena ore, and the record of the shipments from 

this vein show that the value lay chiefly in lead and 

to a less extent in silver. Apparently zinc became 

more abundant in the lower levels, but the shipments 

from the 52-foot sublevel were not penalized for their 

zinc content, and it is safe to conclude that sphalerite 

was only locally an abundant constituent of the 

Golddust ore.

Pacific vein. The Pacific vein is parallel to the Golddust vein and lies about 150 feet north of it on the

Willard tunnel level. The adits that explored the 

upper part of the Pacific vein have long been inaccessible. A continuation of the crosscut that connects 

the Puzzle Extension shaft with the Golddust shaft 

has been the means of exploring the Pacific vein on 

the Willard tunnel level. During the summers of 

1915-25 lessees stoped and shipped 218% tons of ore, 

which averaged about 0.5 ounce of gold and 15 ounces 

of silver to the ton, 30 percent of lead, and about 13 

percent of zinc. A small amount of development work 

has been done since 1916, but no shipments have been 

made.

The Pacific vein occupies a small normal fault that 

dips 85° NW. The fault and the vein abruptly 

decrease in strength southwest of the Golddust 

crosscut, most of the movement having been taken up 

by folding and minor fissuring in the black Benton 

shale. The quartzite underlying the shale rises to the 

east, and in this formation the vein is much stronger. 

The shipping ore came from a small stope in.the 

quartzite immediately under the Benton shale. The 

monzonite porphyry southeast of the quartzite clearly 

underlies it and is sill-like in form where it is cut by 

the Golddust crosscut, but near the east end of the 

Pacific drift, where the porphyry forms the southeast 

wall of the Pacific vein, it is in part cross-breaking. 

The surface geology suggests that this porphyry is 

part of an irregular pipelike conduit connected with 

the Nigger Hill sill.

The ore of the Pacific vein taken from the surface 

workings is reported to have been an oxidized lead-gold 

ore. The ore from the stope on the Willard tunnel 

level was entirely a sulphide ore and contained pyrite 

and sphalerite in sufficient amounts to detract from 

its value as a lead ore. The vein and its walls were 

heavily pyritized as far as it had been explored, but 

galena was confined to the region stoped. The 

character of the ore and its occurrence below the 

Benton shale indicate that it is primary and has not 

been reconcentrated by surface waters.

Washington and Emmet veins. The Washington 

mine is \% miles southeast of Breckenridge, on the 

north side of Illinois Gulch. According to Ransome,71

This is one of the older mines of the district and as early as 

1883 was giving employment to over 30 men. A 20-stamp 

mill was built about 1885. At this time and for a few years 

later the mine was worked through the Watson shaft, situated 

at an elevation of 10,550 feet on the spur connecting Nigger 

Hill with Bald Mountain. The mine continued steadily 

productive until 1891, after which it appears to have been 

turned over to lessees, who made shipments up to 1897 and 

perhaps later. The total output of the Washington mine, as 

given in a prospectus issued by the Washington-Joliet Mining 

& Milling Co., capitalized at $1,500,000, is between $400,000 

and $500,000. There is no reason to suppose that this statement is exaggerated.

The underground workings consist of the old shaft on the hill, 

from which considerable drifting and stoping was done, and of

« Ransome, F. L., op. cit., p. 141.
MINE DESCRIPTIONS 45

NE.

SW.

DUMP

DUMPM,LL TUNNEL.

WASHINGTON SHAFT

Conditions here not known 

COKNISH TUNNEL________

WILLARO TUNNEL LEVEL

Profile section of Washington mine

WASHING TON SHAFT

SW.

NE.

]j CHKISTIENSEN TUNNEL 

WINZE

300 Feet

WILLARD TUNNEL LEVEL

Profile section of Mayo mine

FIGURE 17. Map and sections of Washington and Mayo mines.
46 BRECKENRIDGE MINING DISTRICT, COLORADO

six tunnels on the slope from the shaft down to Illinois Gulch. 

Of these the most important is the Cornish tunnel, which 

follows the Washington vein in a general northeast direction, for 

about 1,400 feet and connects with the shaft at a depth of 250 

feet. At 115 feet-below the Cornish tunnel is the Berlin tunnel, 

supposed to be on the Emmet, a vein parallel with and a short 

distance southeast of the Washington.

The Berlin tunnel is also known as the Shonty 

tunnel and the Mill tunnel. A map and section of the 

underground workings and the stopes, so far as known, 

are shown in figure 17. According to John Nelson, 

the breast of the Berlin tunnel is now about 1,150 feet 

from the portal. A raise 630 feet from the portal is 

said to connect the lower adit with the Cornish tunnel. 

Unfortunately the underground workings of the 

Washington mine were inaccessible at the time of 

Ransome's visit and also in 1928, when the writer 

studied the district.

The Washington vein strikes N. 50° E. and dips 

about 70° SE. The Emmet vein is a parallel vein 

from 15 to 20 feet southeast of the Washington vein 

and may be considered part of the same mineralized 

fault zone. The Berlin tunnel is said to pass through 

some red shale near the portal but to traverse porphyry 

for most of its length. The Cornish tunnel enters the 

hill in Dakota quartzite but is said to be in porphyry 

for most of its length, passing into black shale near the 

shaft. By far the bulk of the material on the dumps 

of the two adits consists of porphyry. The ore is said 

to be a lead-silver ore containing small amounts of 

gold and zinc similar to the ore found in the Puzzle 

and Golddust veins.

Mayo vein. The Mayo vein lies about 600 feet 

southeast of the Washington vein and was .opened 

through the Christensen tunnel, the Mayo incline, 

and the Weinland tunnel. All the workings were in 

bad condition at the time of the writer's visit and were 

not studied underground. A map and section of the 

Mayo workings are shown in figure 17. The Mayo 

vein has produced at least $50,000 in lead-silver ore 

but has not been worked for 30 years.

The Mayo vein trends northeast and dips about 

65° SE., but most of the ore was obtained from a 

"flat vein" striking about N. 75° E. and dipping 25° S. 

It is probable that the ore found in the "flat vein" was 

a bedding-plane deposit in the Dakota quartzite, 

which crops out near the Mayo incline and strikes 

N. 65° E. and dips 21° S.

Horn vein. About 300 feet southeast of the Mayo 

vein is another vein striking N. 25° E. and dipping 

75° NW. It was developed through the Horn tunnel, 

long since caved and inaccessible, and is known to have 

supplied some good oxidized lead ore containing about 

25 ounces of silver and half an ounce of gold to the ton 

when it was worked by the Washington Mining Co.

WELLINGTON

ACKNOWLEDGMENTS

The well-kept mine records of the Wellington Mines 

Co. and the intimate personal knowledge of the mine 

which Mr. R. M. Henderson, the general manager, and 

Mr. H. L. Tedrow, the mine superintendent, placed at 

the writer's disposal, made the study of the mine an 

unusually profitable and pleasurable task. Information furnished by them on inaccessible parts of the 

mine was indispensable in obtaining the picture of the 

geology given below. In addition maps and reports 

of the old workings, made by Mr. John Wellington 

Finch at various times prior to 1920, were placed in 

the writer's hands and were invaluable.

HISTORY

The Wellington Mines. Co. in 1929 owned the old 

Oro mine and the original Wellington mine. The 

Oro mine first became productive about 1887, and the 

Wellington was opened a short time later; during the 

period of their separate existence the Oro mine was 

more productive than the Wellington. Shipments at 

that time consisted of argentiferous lead ore, made up 

largely of cerusite and galena. As zinc was then regarded as waste and penalized by the lead smelters, no 

effort was made to save it. The Wellington and Oro 

mines were consolidated in 1902 by the Colorado & 

Wyoming Development Co., but little work was done 

until the present Wellington Mines Co. took over the 

property about 5 years later.

A 100-ton gravity concentration mill was built at 

the mine in 1908, and by the following year 25 tons of 

galena concentrates and about 10 tons of zinc middlings 

were being shipped daily. High-grade zinc concentrates and crude lead ore of smelting grade were also 

shipped from time to time. A 50-ton roaster and 

magnetic separating mill were built in 1912 to remove 

the iron from the zinc middlings. The mixed leadzinc ore of the mine was successfully treated by the two 

mills until 1927, when the gravity concentration plant 

was changed into a flotation mill because of the greater 

economy of its operation. The two mills and the Oro 

shaft are shown in plate 11,5. The roaster and magnetic separation plant have been entirely abandoned 

since August 1927, when the new mill started work.

Except for a 2-year period of idleness in 1920 and 

1921, the mine produced large amounts of both lead 

and zinc from 1909 to 1929. The production of the 

property is given in the table on page 62.

DEVELOPMENT

The relations of the levels and shafts are shown in 

plate 13. The original Wellington mine was opened 

by several adits, the largest of which was the X-10-U-8, 

driven north-northwest at an altitude of 10,040 feet.
MINE DESCRIPTIONS 47

This adit connects with the collar of an underground 

inclined shaft from which five levels have been turned. 

Close to the mill and 1,700 feet S. 35° W. of the inclined 

shaft, the Oro workings are opened by the vertical Oro 

shaft. The two shafts are joined by their fifth and 

sixth levels, but the other levels connect with only one 

shaft. The deepest level in 1928 the Oro eighth 

was not directly connected with the Oro shaft but was 

opened by a winze near the east end of the seventh 

level. The altitudes of the levels at the Wellington

sixth level, which handles all the underground water of 

the mine.

GENERAL GEOLOGY

The surface geology is shown on plate 2 and figure 

20. The Wellington Main vein (pi. 14) lies in the 

downthrown Oro fault block, which is bounded on the 

west by the Bullhide fault and on the east by the 

Great Northern-J fault zone. (See fig. 20.) Most of 

the Oro workings are in this fault block, but the Wellington levels develop ground in both this downthrown

FIQUKK 18. Main vein, Wellington mine, paralleling intrusive contact of cross-breaking monzonite porphyry.

and Oro shafts and their approximate extent are given 

below.

Wellington levels: 

First... ____.--..._--.._-.__.-..

Second. ._.______.__.__ _..-.-

Third.. ...............^ __........

Fourth...... ...._.._....._.....

Fifth. ___________________ _____..

Sixth.. _._.._._.._.__.._..._._.

Oro levels: 

First..... ......................

Second- _ ___.___._.__:__.._____.

Third..........................

Fourth.... .__........_.........

Fifth. ................_......_.

Sixth..... .....................

Seventh. _ ...I..................

Eighth _ ._.__..-......__.__.__

Si am tunnel _--__-.-_---_--__-__--_.

Prize Box tunnel .-.-_---.__--.-_---.

Other surface workings. -___----__---_

Drifts and crosscuts, total............

Shafts.............................

Extent (feet)

8,090

6,590

6,480

5,700

8,220

6,720

1,020

2,020

1,320

1,480

420

(8, 220)

(6, 720)

2,990

2,200

1, 150

1,610

1,410

9, 150

66, 570

1,346

Altitude at 

shaft (feet)

10, 048

9,962

9,863

9,730

9,601

9,470

9,844

9,774

9,715

9,637

9,612

9,488

9,361

9,260

Active mining has been confined to the Oro workings 

for several years, and the Wellington inclined shaft has 

been used only for the care of the pump station on the

block and in the upthrown Wellington block east of the 

Great Northern-J fault zone. The upper levels are 

almost wholly in monzonite porphyry, but the lower 

levels expose Pierre, Niobrara, Benton, Dakota, and 

Morrison beds. The monzonite porphyry is part of 

the northern border of the large irregular intrusive 

mass that forms much of the upper part of Bald 

Mountain and was formerly continuous with the porphyry mass of Nigger Hill.

Porphyry is exposed on the surface near the Oro 

shaft, but Dakota quartzite crops out in a low cliff a 

few hundred feet farther west, near the small gully 

where the Bullhide fault comes to the surface. Above 

the quartzite alternations of shale and porphyry indicate sills of monzonite porphyry in the Benton shale. 

On the east side of the fault monzonite porphyry is 

exposed over a wide area and is cu^ by a dike of coarsegrained quartz monzonite porphyry near the top of the 

hill. A study of the dredge tailings shows that black 

shale forms the bedrock east of the Bullhide fault for a 

thousand feet. Monzonite porphyry next appears, 

and no more shale is seen until the Great Northern-J 

fault zone is passed, beyond which shale again appears 

in the dredgings.

Most of the rock exposed underground is monzonite 

porphyry. It has been intruded in large irregular
48 BRECKENRIDGE MINING DISTRICT, COLORADO

bodies which are roughly parallel to the bedding of the 

sediments but which to some extent follow early faults 

and break across the bedding, as shown in the geologic 

sections in plate 15. In places these early planes of 

weakness were reopened many times, and the Main 

vein is in part along a fault fissure that follows the 

cross-breaking intrusive contact of a monzonite 

porphyry mass. At the place shown in figure 18 

monzonite porphyry was intruded along a fault fissure 

in the sediments, and later movement along the intrusive contact opened channels for ore-bearing solutions. A small gougy fault offset the porphyry and 

vein before mineralization was completed, and much 

ore was dragged into the gouge, but some pyrite and 

ankerite cut through the gouge. It is impossible to 

work out the exact amount of movement that has 

occurred on all the many faults found underground or 

to be sure of the amount of displacement at different 

points along a major fault. The history is complex, 

and the adjustments that occurred during faulting 

before and after the intrusion of the porphyry have 

been obscured by the disruptions that attended the 

intrusion. Only the major features of the structure 

are considered, and many interesting and puzzling 

details are left unmentioned, but it is hoped that the 

text, supplemented by the geologic map and sections, 

will give an adequate picture of- the geology.

Most of the intrusive rock is moderately fine grained 

light-colored monzonite porphyry and does not appear 

conspicuously porphyritic to the unaided eye. Much 

of it is bleached and altered, but irregular masses of 

moderately fresh rock occur in the lower levels. The 

most common facies of unaltered porphyry is a darkgray medium-grained rock containing many phenocrysts of biotite. It is locally known as diorite and 

seems to be richer in ferromagnesian minerals than 

the large mass of altered monzonite porphyry found 

in the upper levels. This diorite is abundant near the 

inclined shaft on the fifth and sixth levels and occurs 

in small bodies on the Oro seventh level; a diamonddrill core obtained 510 feet east of the Oro shaft shows 

that a thick mass lies 200 feet below the seventh level.

Gray siliceous shale belonging to the Morrison formation was recognized on the sixth level south of the 

Wellington shaft. On the same level pink shale, sandy 

quartzite, and gray shale of Morrison age were also 

found in the Bullhide fault zone west of the Oro shaft.

Massive light-gray quartzite, thin-bedded shaly 

quartzite, and gray sandy shale belonging to the 

Dakota formation occur on the Wellington sixth level, 

as shown on plate 15, and massive gray quartzite was 

found on the Oro fifth level west of the Bullhide fault.

Black shale belonging to the Benton is probably 

present at the west end of the Oro seventh and eighth 

levels and may occur on the Wellington sixth level, 

but the stratigraphic relations are not clear in the latter

place, and it is generally impossible to distinguish the 

Benton shale from the Pierre shale lithologically.

The limy Niobrara shales are abundant on the sixth, 

seventh, and eighth levels of the Oro shaft. The upper 

member, characterized by numerous veinlets of secondary calcite, is well exposed 100 feet east of the Oro 

shaft on the sixth level. This horizon is about 650 

feet above the Dakota quartzite. The limy shale beds 

of the Niobrara, which underlie this member for 300 

feet, are the only sediments exposed on the seventh 

level, and the lower part of the formation is well shown 

near the winze on the eighth level. Much of the limy 

shale has been strongly silicified near the veins and 

is converted into black jasperoid, locally called jasper. 

Ore has replaced favorable beds in the lower part of 

the formation at several places, but the replaceable 

beds are generally less than a foot thick, and ore bodies 

of .this type have not proved profitable. The Oro third 

level is almost entirely in the somber-colored Pierre 

shale west of the shaft, and the large masses of shale 

found on the fifth level belong to this formation. The 

Pierre shale is generally much less silicified near the 

veins than the Niobrara formation.

STRUCTURE .

The chief elements of the structure and the location 

of the veins and faults referred to below are shown in 

figure 20. The two strong northerly normal faults, 

the Bullhide and the Great Northern-J, are the dominant structural features of the mine and limit the 

downthrown Oro fault block to the west and east respectively. The Bullhide fault, which comes to the 

surface about 500 feet west of the Oro shaft, strikes 

N. 26° E. and dips 58° E. The contact of the Pierre 

shale and Niobrara formation is probably a very short 

distance below the Oro fifth level, where that level 

cuts the Bullhide fault. The lower member of the 

Dakota quartzite is on the west side of the fault on 

this level, but the exact position of the base of the 

quartzite is not known. If the base is assumed to be 

60 feet below this level, it is very unlikely that an 

error of more than 50 feet will be made. Under this 

assumption the top of the Niobrara formation is 

faulted down to a point about 70 feet below the top 

of the Dakota quartzite. If there are no sills between 

the quartzite and the Niobrara shale, the vertical 

displacement corresponding to such faulting is 790 

feet, with a possible error of 50 feet. Until more exact 

measurements can be made, however, it will be assumed 

that the throw on the Bullhide fault is approximately 

800 feet and that the dip slip is about 900 feet. The 

quartz monzonite porphyry dike on the upper part of 

Prospect Hill is offset by the Bullhide fault, the western 

segment cropping out about 300 feet to the south of 

the eastern segment. As this dike dips 65°-80° S. in 

the upper Wellington.workings, the offset may be due
MINE DESCRIPTIONS 49

to the displacement of a southward-dipping mass and 

cannot be taken as proof of a horizontal component of 

movement. The bedrock geology revealed by the 

drcdgings west of the Bullhide fault shows that the 

offset of the southward-dipping Main vein cannot 

exceed 350 feet and does not indicate a horizontal component of movement along the fault. The displacement given above cannot be taken as general and will 

be increased or diminished wherever earlier cross faults 

are cut. As the Main vein follows a fault that has a 

throw of about 110 feet, the displacement along the 

Bullhide fault between the offset segments of the vein 

should be about 900 feet.

The Bullhide fault zone ranges from 5 to 15 feet in 

width and is characterized by an abundance of gouge 

and broken quartzite, shale, and porphyry intimately 

mixed and having little relation to the adjacent walls. 

Sulphide bunches, ranging in size from those comprising 

a few cubic feet to bodies large enough to furnish 

several mine cars of ore, are common in this broken 

ground. Quartzite fragments are commonly mineralized and partly replaced by pyrite, galena, and sphalerite. Some of the sulphide bunches are massive and 

unbroken, but many of them are shattered and pulverized by postmineral movement. In places veins of 

sulphide cut across the gouge and clearly were formed 

after a large amount of movement had occurred and 

show that the fault is in part premineral. The postmineral movement was probably confined to one of 

the walls of the fault, where the veins that cut the 

gouge appear, and conversely, in the places where 

shattered bunches of ore are found, postmineral movement affected the entire width of the fault zone.

The Great Northern-J fault zone was traced on the 

surface by the writer and shows best in the dredge 

tailings and where it offsets the quartz monzonite 

porphyry dike, a short distance below the saddle separating Mineral Hill from Prospect Hill. It is regrettable that the condition of the Wellington workings 

made it impossible to study the fault underground, 

but, as already noted, this part of the mine was mapped 

by Mr. John Wellington Finch when conditions were 

more favorable, and these maps were courteously 

made available to the writer.

The strong fault zone that forms the western limit of 

the Great Northern vein is called the Great Northern 

fault. (See fig. 20 and pi. 15.) It strikes about N. 

15° E. and dips approximately 60° W. It is a strong, 

narrow fault in its southernmost exposures, but to the 

north it splits into many minor fractures. It is last 

recognized at a point about 475 feet southeast of the 

Wellington shaft on the second level, where it shows as 

a crushed zone in the porphyry about 15 feet wide, 

dipping 66° W. On the third level, 80 feet to the south, 

it is much stronger and makes a crushed zone 60 feet 

wide, limited on the east by a definite fissure that dips 

05° W. On the fourth level the movement was taken

up chiefly on one strong gouge-filled fissure that 

strikes N. 10° E. and dips 60° W. The walls of the 

Great Northern vein were shattered near the fault on 

this level, and a chamber of ore about 35 feet wide was 

found. Grooves on the walls of the fault are said to 

dip steeply to the south. On the sixth and seventh 

levels the fault is strong and definite and strikes about 

N. 20° E. and dips 50°-70° W. It is reported that 

quartzite was found in the footwall on the sixth level 

and that the hanging wall was made up of black limestone and porphyry. At the eastern end of the Oro 

seventh level, about 80 feet north of the point where the 

fault was cut on the sixth level, the drift follows a steeply 

dipping fault having porphyry on the west and Niobrara limestone on the east. The porphyry is part of 

a thick sill and is intruded slightly below the top of the 

Niobrara formation so that the top has been lifted far 

above its normal position. However, in order to arrive 

at the general order of magnitude of the Great Northern fault, it is inferred that the upper part of the Niobrara formation is west of the fault on the seventh 

level and that the lower part of the Dakota quartzite 

is east of the fault on the sixth level. These relations 

indicate a vertical displacement of 850 feet, which is 

comparable to that of the Bullhide fault, but this 

amount may be in error by 100 feet.

The movement taken up by the Great Northern fault 

is gradually transferred to a more steeply dipping fault 

in a wide zone of broken ground a few hundred feet 

south of the Wellington shaft. (See fig. 20.) North of 

the shaft the movement is confined to one or two strong 

faults that strike north to N. 10° E. and dip 65°-85° 

W. The strongest of the steeply dipping faults is 

named the J fault, and there is no sharp junction 

between it and the Great Northern fault. Strong 

gouge-filled fractures in the much broken ground 

where the two master faults merge lack continuity both 

vertically and horizontally. A fault marked by a 

gouge seam 3 feet wide may not persist 200 feet vertically in this transition zone.

As shown in figure 20, the quartz monzonite porphyry 

dike west of the J fault, in the upper part of the Wellington workings, is offset about 80 feet to the south of the 

eastern segment. As the dike dips steeply to the south 

this offset probably reflects the horizontal component 

of movement suggested by the grooves noted on the 

walls of the Great Northern fault. The amount of 

vertical displacement on the J fault is not known but is 

probably similar to that on the Great Northern fault.

Many small masses of ore have been found in the 

Great Northern-J fault zone, and there is a marked 

increase in the width of the ore in the veins near the 

faults. Most of the ore within the fault is crushed, 

showing that postmineral movement has taken place. 

The increase in strength shown by the veins as they 

approach the fault zone, however, can hardly be 

a coincidence and indicates that the vein fissures were
50 BRECKENRIDGE MINING DISTRICT, COLORADO

opened near the fault by movement along it, before or 

during mineralization.

In the western part of the Oro fault block an eastward-dipping fracture known as the 11-10 fault has 

been traced from the surface to the eighth level. (See 

pi. 15 and fig. 20.) It strikes nearly north and dips 

30°-40° E. The throw of the fault is not large and is 

probably about 50 feet, where the movement is not 

partly absorbed on minor faults nearby. There is a 

decided horizontal component of movement, however, 

and grooves in the walls on the eighth level indicate 

that here the direction of slip was about 30° south of 

the direction of dip. The calculated net slip is about 

llQ feet, and the strike slip is 45 to 50 feet. All the 

veins west of the fault are 45 to 90 feet farther south 

on the east side of the fault, the distance depending on 

their direction and amount of dip. The movement in 

the lower levels seems to be somewhat less than that

FIGURE 19. Fault later than Puzzle vein fissure but in part preraiaeral.

in the upper levels and probably foreshadows the disappearance of the fault in^ depth.

Crushed ore mixed with gouge and the pronounced 

drag of the ore at the 11-10 fault indicate that considerable postmineral movement has occurred along it. 

The localization of ore shoots at the junction of the 

veins and this fault, however, suggests that the fault 

has determined the position of the ore shoots and that 

it must be in part premineral. Sulphide veins an inch 

wide cut the gouge of a minor fault nearby which 

offsets the Puzzle vein in the same direction as the 

11-10 fault, clearly showing that much of the movement 

on the smaller related faults is premineral. (See 

%. 19.)

Where most of the movement has been taken up on 

a single plane the 11-10 fault is marked by a seam of 

gouge and breccia 2 or 3 feet wide. In many places, 

however, it splits and branches, and the movement is 

distributed on several parallel and branching faults in 

a zone from 50 to 75 feet wide. In addition to the 

11-10 fault and the fractures directly related to it, 

there are many smaller faults that dip gently to the 

east in the Oro fault block. They do not warrant detailed consideration but are shown in the cross sections 

and on the geologic maps of the levels on plate 15. 

Many of the gently dipping faults pass into beddingplane slips that soon disappear, and most of them lack 

persistence in any direction and have no influence on 

the localization of ore shoots.

VEINS IN THE WELLINGTON FAULT BLOCK

The general relations of the veins and the major 

faults are shown in figure 20. The veins in the Wellington fault block east of the Great Northern and J 

faults trend about N. 70° E., whereas the main veins 

in the Oro fault block to the west strike nearly northeast, except in the region close to the Bullhide fault, 

where they assume an east-northeast direction. Many 

small fissures carrying ore were found east of the 

Great Northern-J fault zone, but only four strong 

veins were recognized; from north to south they are 

the East Iron vein, the East vein, the Orthodox vein, 

and the Great Northern vein. The veins are spaced 

about 200 feet apart on the upper levels of the Wellington workings and dip 45°-85° S. The ore varied 

from mixed lead-zinc ore in the northern veins to highgrade zinc ore in the southern veins, and the northern 

veins bottomed at shallower depths than those to the 

south. The veins carry ore close to the Great Northern-J fault and become unproductive 400 to 600 feet 

to the east..

East Iron vein, The East Iron vein is the northernmost vein in the ^Wellington fault block and comes to 

the surface on the east side of the J fault in the quartz 

monzonite porphyry dike on Prospect Hill. It strikes 

about N. 60° E. and dips 55° SE. The vein was mined 

long ago and is reported to have yielded little but 

oxidized lead ore. According to R. M. Henderson, 

the general manager of the Wellington mines, the vein 

definitely bottomed about 200 feet above the Wellington first level and was stoped to the surface.. The ore 

shoot extended about 220 feet to the east of the J fault 

and was broken by a few minor cross faults.

East vein. The East vein is about 250 feet south of 

the East Iron vein and about 225 feet east of the inclined shaft on the Wellington no. 1 level. It strikes 

N. 70° E. and dips 65° S. on the first level but flattens 

to 40° and bends sharply to the south as it approaches 

the J fault on the second level. The ore bottoms from 

15 to 30 feet below the no. 2 level and has been stoped 

to a point about 75 feet' above the first level. The 

J fault sharply limits the vein on the west, and some 

very large masses of ore occurred at the junction. 

On the second level the ore shoot extended without 

interruption from the J fault to a small cross fault 

300 feet east, beyond which the ore was of too low 

grade to mine. On the first level the ore shoot is 

broken by several nonpersistent faults, but good ore 

was found for 380 feet beyond the J fault. The average width of the vein is about 4}£ feet, but it ranges
MINE DESCRIPTIONS 51

from 2 to 12 feet. The vein carried a mixed lead-zinc 

ore whose average tenor was about 3 percent of lead, 

20 percent of zinc, and 18 percent of iron.

Orthodox vein. The Orthodox vein was cut by the 

X-10-U-8 tunnel 390 feet south of the inclined shaft 

and is about 250 feet south of the East vein on this 

level. West of the J fault the vein strikes about N. 

75° E. and dips steeply to the north, but east of the 

fault it strikes about N. 50° E. Its dip, which is 

vertical or steeply north near the fault, changes to 

steeply south about 200 feet to the east. Coincident 

with the change to a southward dip, a large ore shoot 

occurred. The Orthodox vein is mineralized throughout

show no regularity in arrangement of direction and appear to 

have little individual persistency; but better exposures would 

probably show the vein to be cut by north-south faults much 

as is the East vein. Doubtless some of these fissures continue 

from one vein to the other. At about 200 feet from the Extenuate tunnel the vein is displaced by a strong gouge-filled fissure 

with a dip to the east of only 15°. The general disturbance in 

the neighborhood of this fault, however, is so great as to 

obscure its effect on the vein and the direction and character of 

the displacement were not ascertained.

The zone described by Ransome is near the south 

end of the J fault and within the influence of the 

northern part of the Great Northern fault, which 

branches and splits into many slips a short distance to

EXPLANATION

FIGURE 20. Generalized surface map of chief veins and faults of Wellington and Oro mines.

most of its length, but no commercial ore was found 

west of the J fault, although it was explored to the Main 

vein, which offsets its west continuation about 35 feet 

to the northeast. Exploration of the offset portion of 

the Orthodox vein for 260 feet west of the Main vein 

on the Wellington no. 2 level did not reveal commercial 

ore.

East of the X-10-U-S tunnel the Orthodox vein turns 

north into the J fault, through which it was followed 

with difficulty for 90 feet before it emerged and turned 

northeast. According to Ransome, 72

East of the tunnel, however, the entire drift supposed to be 

on the Orthodox vein is in soft, very much disturbed rock 

in which the vein can be followed only as a series of crushed 

fragments. The numerous gouge-filled slips for the most part

71 Rnnsomo, F. L., op. cit., p. 133.

the southeast. The J fault offsets the eastern side of 

the Orthodox vein about 75 feet to the north of its 

western continuation. Several smaller northerly 

faults offset the vein in the same direction as the J 

fault in the workings farther east.

There is a, series of old stopes a.bove the no. 1 level 

which starts just east of the J fault, and the ore taken 

from this part of the vein is reported to have carried 

considerable lead. The vein dips to the north throughout its length on the first level, and no commercial ore 

was found. The principal ore shoot on the Orthodox 

vein occurred 225 feet east of the J fault on the second 

level, where the vein dips to the south. The shoot was 

about 280 feet long on the second level and 320 feet 

long on the third level and bottoms 60 feet below. Near 

the western limit of the shoot some good ore was found
52 BRECKENRIDGE MINING DISTRICT, COLORADO

between the first and second levels along fissures 

dipping gently to the south. This "fiat ore" branched 

off into the footwall of the vein and was entirely in 

porphyry. Steeply dipping northerly faults limited 

the ore shoot at each end. The ore was almost wholly 

dark sphalerite and contained little lead, iron, or 

gangue. The productive part of the vein was about 3 

feet wide and was entirely in porphyry.

Great Northern vein. The Great Northern vein is cut 

by the no. 1 level 140 feet south of the Orthodox vein 

and about 550 feet east-southeast of the Wellington 

shaft. The general strike of the vein is N. 65° E., and 

its dip is 30°-55° S., the steeper dips occurring below 

the third level. It has been explored a maximum

W.

changes in wall rock, for though most of the vein is in 

porphyry, the fifth level is largely in shale, and here the 

vein showed no decrease in strength. At many places, 

notably on the fourth level, the ore widened as the 

Great Northern fault was approached, reaching a maximum width of 35 feet at this place. This width of ore 

probably is due to the mineralization of a crushed wedge 

of rock between the vein and a northwestern branch 

that splits off about 50 feet east of the fault.

The ore from the Great Northern vein was largely 

sphalerite and pyrite. Little lead occurred, and 

rarely did it exceed 2 percent of the ore as mined. 

Some large bunches of galena were found on the fourth 

level near the Great Northern fault, and for a time the

S^l:,yi'-'X'H_igh' gra'de';py'rite.v-v ' ' 

Vyv.V.Y ;' :: : ' ' ' ' '

FIGURE 2L^Stope map of Great Northern vein.

distance of 1,200 feet to the east of the Great Northern 

fault but has not been recognized west of the fault. As 

shown on the stope map '(fig. 21), the ore is limited on 

the west by the Great Northern fault and changes into 

low-grade pyritic ore to the east. The length of the 

shoot ranges from 800 feet on the fourth level to 500 

feet on the first level and is less than 200 feet at the 

surface. Apparently only the very top of the ore 

shoot has been removed by erosion. The ore bottoms 

abruptly about 40 feet below the fifth level, at an 

altitude of about 9,560 feet, but because of the gentle 

dip of the vein in the upper levels the shoot has a dip 

length of 900 feet.

The Great Northern vein is one of the most productive veins developed by the Wellington Mines Co. 

Its width ranged from 3 to 20 feet and averaged about 

6 feet. The changes in width were not related to

average lead content of the ore increased to 2.4 percent. In general the lead content was slightly higher 

in the upper part of the vein than elsewhere, changing 

from about 0.5 percent on the fifth level to about 

1.7 percent on the first level. At the bottom and 

east end of the shoot the zinc ore changes within a 

short distance into pyrite. The transition from 25 

percent zinc ore into nearly pure pyrite occurred in 

about 20 feet at many places on the eastern edge of the 

shoot. This change in character was not coincident 

with a decrease in the width of the vein, and on the 

fifth level some fair-sized stopes were made in the pyrite, which was shipped to the Western Chemical Co. 

at Denver and used in the manufacture of sulphuric 

acid. The vein becomes small east of the pyritic border, and no ore was found in an exploratory drift driven 

500 feet east of the ore shoot on the fifth level.
MINE DESCRIPTIONS 53

VJEINS IN THE ORO FAULT BLOCK

In the Oro fault block the most notable vein is the 

Siam vein, or Main vein, as it is now called. It strikes 

N. 45° E. and dips about 62° SE. It can be clearly 

traced from the Oro shaft to a point about 1,500 feet 

northeast, where it is cut by the eastward-trending 

Fault vein. The Iron vein, which strikes northeast 

and dips 65° SE., leaves the Fault vein on the north at 

a point about 50 feet west of the Main vein. According to Ransome,73 it is probably a segment of the Main 

vein that has been displaced by the Fault vein. The 

Spur vein branches west from the Main vein about 400 

feet southwest of the Fault vein. Near the Oro shaft 

the Main vein splits into two or three parts. West of 

the 11-10 fault the northern fracture is called the 

Puzzle vein and the strongest of the southern veins is 

called the Main vein. The Puzzle vein dips about 70° 

N. and trends about N. 70° E., being nearly parallel 

in strike to the southward-dipping fractures correlated with the Main vein.

Fault vein and Iron vein. The Fault vein was cut by 

the X-10-U-S tunnel 110 feet north of the collar of the 

inclined shaft. The vein strikes N. 70° E. and dips 

55°-75° S. It consists chiefly of broken rock and 

strong seams of gouge but contains a little ore and is 

probably in large part premineral. The movement 

along the Fault vein, however, is later than the fault 

fissure which became the Main vein and which, as 

noted above, offsets it about 50 feet, the northern part 

or Iron vein moving west. In the Iron vein the ore 

bottomed about 75 feet above the X-10-U-8 level and 

was stoped to the surface. The ore shoot was about 

3 feet wide and was entirely in porphyry. The ore 

was chiefly galena and cerusite close to the surface but 

abruptly changed into pyritic galena-sphalerite ore 

with depth.

Spur vein. The'Spur vein branches westward from 

the Main vein about halfway between the Orthodox 

and Fault veins. It strikes about N. 70° E. and dips 

60°-65° S. It contained high-grade galena ore close 

to the surface, in the Prize Box tunnel, and two tapering 

branches of this shoot extended down to the Wellington first level, and the western branch continued to the 

Wellington second level. The ore below this level was 

bunchy and erratic in occurrence but contained more 

galena, than the ore in the Main vein nearby. Ore 

from the Spur vein from the Price Box tunnel (altitude, 

10,250 feet) is shown in plates 6 and 7, A.

Main vein. As shown in figure 20, the Main vein 

extends from the J fault southwest to the Bullhide 

fault, a distance of nearly 3,000 feet at the surface. 

Its course is interrupted by several cross faults, the 

most notable of which a,re the 11-10 fault and the 

Fault vein. The segment northeast of the Fault vein 

has been described. Throughout most of its length 

the Main vein strikes N. 45° E. and dips about 60° SE.

" Ransome, F. L., op. cit., p. 132.

It is not a single mineralized fissure but a mineralized 

fault zone, and at several places it splits into two or 

more veins. The vein was not productive throughout 

its length, and some drifts followed barren zones as 

much as 1,000 feet long.

One of the best ore shoots was found a short distance 

east of the J fault where the Main vein is cut by the 

Fault vein. This shoot was roughly triangular in 

shape, coming to a point about 100 feet northeast of 

the inclined shaft between the fourth and fifth levels. 

A northerly fault, dipping 50° E., marked the western 

edge of the ore shoot; the J fault limited it on the east 

below the third level; and the Fault vein formed the 

east side of the shoot above the third level. The ore 

ranged from 2 to 18 feet in width and averaged 12 feet 

throughout the second level. It was chiefly sphalerite 

but contained moderate amounts of galena.

Southeastward along the vein from the inclined shaft 

several small ore shoots occur, but none of them extend 

below the Wellington fourth level until the Main ore 

shoot is entered, 500 feet southwest of the fifth station 

of the inclined shaft. (See pi. 15 and fig. 22.) This 

ore shoot has been stoped from the Oro eighth level 

nearly to the surface and was one of the largest bodies 

of ore in the mine. On the eighth level the shoot is 

only 150 feet long and is cut off on the east by northerly 

faults that dip steeply to the west. The intersection 

of the vein with the faults dips gently to the southwest 

and forms the bottom of the ore shoot between the 

eighth and seventh levels. The east end of the shoot 

is not marked by faults above the seventh level, and 

the west side of the ore shoot is not related to faults 

on any of the levels. In most places the limits of the 

shoot are marked only by a decrease in the width and 

tenor of the vein. On the seventh and eighth levels, 

however, the vein splits into several smaller veins soon 

after it passes into the shale at the west end of the 

shoot, and the branches contain little workable ore 

east of the 11-10 fault. Above the sixth level the shoot 

splits, the eastern part extending almost vertically upward to the Oro second level, whereas the western part 

trends upward to the west. The eastern branch probably extended to the surface, as it was apparently 

stoped above the Wellington second level before 1900. 

The western branch of the ore shoot is not known to 

extend more than 100 feet above the Wellington fourth 

level. On the fifth level the two branches of the shoot 

are very short, neither branch extending along the drift 

for more than 50 feet. Above the Wellington fourth 

level, however, both branches show a marked increase 

in their horizontal dimensions. Ore was found for 

nearly 700 feet in the western branch above the level 

and for 300 feet in the eastern branch. Both walls 

are monzonite porphyry above the sixth level, but between the sixth and seventh levels the limy Niobrara 

shale appears in the footwall in the southwestern part 

of the shoot. Niobrara shale forms both walls on the
54 BRECKENRIDGE MINING DISTRICT, COLORADO

eighth level, except in the northeastern part of the 

workings, where the hanging wall is monzonite porphyry. The shale dips 15°-20° E. and underlies the 

monzonite porphyry, which in some places follows the 

bedding and in other places breaks steeply across it, 

rising to the west much more steeply than the bedding 

planes of the shale. In general the strength of the 

ore shoot decreased as the vein passed from porphyry

sw.

the 11-10 fault about halfway between the sixth and 

seventh levels and probably extends to the surface. 

In longitudinal section, as shown in figure 22, the shoot 

has a chimneylike shape and pitches about 75° SW. 

In the upper levels both walls are monzonite porphyry, 

but below the Oro third level the shoot lies in part 

between shale and porphyry walls and in part between 

shale walls. Where both walls are shale the ore is

NE.

l°7oPb

25% Zn 

Little Fe

FIGURE 22. Stope map of Main ore shoot and shale shoot on Main vein.

into shale, but porphyry walls are no assurance that 

the vein contains ore.

Much of the ore above the Wellington fourth level 

averaged about 8 percent of lead, 15 percent of zinc, 

and about 0.05 ounce of gold and 5 ounces of silver 

to the ton. The ore from the lower levels carries decidedly less lead and more zinc. In the upper portions 

of this ore shoot good lead ore was in some places underlain by heavy pyritic material carrying very little lead 

or zinc. (See fig. 22.)

The ore shoot in shale is the first notable one southwest of the Main ore shoot. An almost barren stretch 

of vein about 800 feet in length separates the two ore 

shoots on the fifth and sixth levels. The stope length 

of the shale shoot is about 150 feet, and the pitch 

length is about 500 feet. The shale shoot bottoms on

said to be narrower but of higher grade than where 

one or both walls are porphyry. The ore contained 

more lead than the average of the Main shoot and 

showed little change with depth.

Several small ore bodies have been found on southward-dipping branches of the Main vein west of the 

shale ore shoot. The location and relation of these 

ore bodies is shown on the level maps in plate 15.

Puzzle vein. In the broken ground west of the 11-10 

fault the most conspicuous and easily followed vein is 

known as "The Puzzle." It strikes N. 65° E. near the 

Bullhide fault and swings to N. 85° E. a few hundred 

feet farther east, but unlike the veins previously described, it dips about 75° N. It is not certainly known 

to be a continuous fissure, and the West Puzzle ore 

found on the Oro third level may occur in a fissure
MINE DESCRIPTIONS 55

parallel to and north of the vein in which the ore is 

found on the fifth level. The relations shown on plate 

15 indicate that both shoots occur in the same fracture 

zone, but the disturbed character of the ground west 

of the 11-10 fault makes it difficult to correlate individual fissures.

On the eighth level several good bunches of ore were 

found in the faulted segments of the Puzzle vein that 

were being explored when the mine closed in 1929. 

The ore on this level was not as abundant as that on 

the fifth, sixth, and seventh levels, but two stopes in

sw.

much broken by cross faults, extends west from the 

11-10 fault for about 100 feet. The complexity of the 

fissuring on this level can only be suggested in maps 

whose scale is less than 50 feet to the inch, but the 

general relations are indicated on plate 15 and figure 

23. The Puzzle ore shoot lies entirely between porphyry walls and is distinctly higher in lead than the 

ore in the Main vein east of the 11-10 fault on the 

same level.

The West Puzzle ore shoot was found about 325 

feet west of the Oro shaft on the third level. It

NE.

SHAFT 3

Slope above 11-10 fault 

on offset'segment 

of vein

(IO-I2 % Pb 

\14-I6 % Zn 

\fittle iron

FIGURE 23. Stope map of Puzzle vein.

good lead ore were carried up to the seventh level, as 

shown in figure 23. On the seventh level good ore was 

found from the Bullhide fault to a point about 200 

feet to the east, where a small .premineral fault cuts 

across the vein, throwing the east side about 3 feet to 

the north. Beyond this fault the vein is much narrower, and as it is followed through successive faults 

it becomes less and less productive. The ore shoot is 

strongest near the Bullhide fault on the seventh level 

but diverges from the fault as it is followed upward. 

On the sixth level the ore shoot begins about 80 feet 

east of the fault and continues to the 11-10 fault, about 

260 feet farther east. On the fifth level the ore shoot,

occurs in a vein that strikes N. 60° E. aud dips 65° N.; 

the position, trend, and dip suggest that it is a continuation of the Puzzle vein opened in the lower workings. The ore has been stoped up to the 11-10 fault, 

about 110 feet above the level, and has been followed 

along the strike for about 100 feet. About 75 feet 

above the level, near the western edge of the ore shoot, 

the vein becomes vertical, and above this point it dips 

70° S. The widest part of the shoot was found where 

several strong fissures came into the vein at acute 

angles. (See pi. 15.) The ore in the West Puzzle 

shoot lies between a shale footwall and a monzonite 

porphyry hanging wall. It is very similar in character
56 BRECKENRIDGE MINING DISTRICT, COLORADO

to the ore found in the eastern stopes on the Puzzle 

vein but contains more gold.

GENERAL FEATURES OF THE ORE AND THE VEINS

The character of the primary ore has been well 

described by Ransome 74 as follows:

The vein material is composed essentially of pyrite, sphalerite, 

and galena in various proportions with relatively little gangue. 

Such waste as occurs within the ore bodies consists as a rule of 

horses or small fragments of metallized porphyry or of pyrite 

containing too small a proportion of galena and sphalerite to be 

classed as ore. The gangue, where present, is siderite or barite. 

These materials, however, are nowhere abundant and are 

younger than the sulphides, in which they occur as veinlets or 

as the lining of vugs.

Metasomatic replacement has played an important part in 

the formation of the veins, but the process * * * has been 

confined to fissured or crushed rock. Consequently the ores 

* * * lie as a rule between fairly well defined walls, which 

correspond closely to the original boundaries of the main 

fissures. Fragments of porphyry within the fissure zone, 

especially if of small size or traversed by many cracks, have 

been more or less changed to ore, partly by the filling of interstices and open spaces, partly by replacement. The ore is 

generally solid and firm and is accompanied by no persistent 

gouge along either wall. Locally gouge may be present between 

walls and ore, and the ore bodies in a few places are traversed by 

fissures filled with crushed ore or gouge. In the vicinity of 

faults, also, the ore is fissured and broken. * * *

The prevailing texture is that of a granular aggregate of 

galena, sphalerite, and pyrite,' the three being combined in 

different proportions in different places and showing much 

variation in coarseness of crystallization. Crystals of galena 

attain the largest size, some of them being 2 inches across. This, 

however, is exceptional.

A very characteristic feature of the Wellington ore is the 

manner in which it is traversed by white or pale-buff veinlets 

of carbonate approximating siderite in composition. In some 

parts of the veins these veinlets, most of which are less than an 

inch in width, and are inclined to be vuggy along their middle 

planes, are arranged parallel with the walls of the Main vein, 

giving the ore a banded appearance. In other places the carbonate veinlets branch irregularly through the sulphides in all 

directions [see pi. 14, B], and in a few places the sulphides have 

been brecciated and the fragments are now cemented by the 

sideritic carbonate. * * * The siderite is present in some 

form in nearly all of the ore, either as the veinlets just described 

or as the lining or filling of small interstices in the original porous 

bodies of sulphides.

The carbonate veinlets described by Ransome are 

common in the sulphide ore of the eighth level and are 

undoubtedly primary. The ore in the deepest workings does not differ appreciably in appearance from 

much of the ore found in the upper levels. Nevertheless, there are changes in composition that are related 

to depth and structure. The highest-grade lead ore 

was found close to the surface and usually became more 

zinciferous in depth. In some places high-grade lead 

ore changed downward into a heavy pyritic ore, which 

marked the bottom of that part of the ore shoot. Ores 

composed chiefly of sphalerite bottomed in places with

« Ransome, F. L., op. cit., pp. 133-134.

no appreciable change in composition and in other 

places gave way to massive pyrite both vertically and 

laterally. Sudden changes in composition commonly 

mark the top, bottom, or sides of an ore shoot, but 

most of the ore from a given shoot is remarkably similar 

and is commonly characteristic of the particular shoot 

from which it was taken.

On the seventh and eighth levels sphalerite is distinctly younger than pyrite, and galena cuts both 

minerals in veinlets. (See pi. 7, B, C.) In the upper 

levels this order is not everywhere the same. In the 

Main vein west of the 11-10 fault on the third level 

pyrite, sphalerite, and galena seem to be contemporaneous in all the specimens examined. Ore from the shale 

shoot on the Wellington fourth level showed galena 

and sphalerite that were apparently contemporaneous, 

galena cut by many veinlets of dark sphalerite, and 

sphalerite cut by veinlets of galena. Some fine-grained 

pyrite accompanied the late sphalerite, but most of 

the pyrite was early and moderately coarse grained. 

Chalcopyrite occurs in minute blebs and stringers in 

the dark sphalerite and is most abundant in the early 

variety of sphalerite. In ore from the third level 

chalcopyrite also occurs in small grains at the edges 

of moderately light colored late sphalerite that does 

not contain the minute inclusions of chalcopyrite that 

characterize the early variety. Porous, nodular ore 

from the ground above the first level is shown in plates 

6 and 7, A. Early galena was brecciated and recemented by dark sphalerite containing many inclusions of chalcopyrite. The sphalerite is surrounded by 

coarse pyrite and galena that are probably still later. 

Very fine grained pyrite, which may be supergene, encrusts some of the coarse-grained galena.

Replacement ore from the West Puzzle stope contained veinlets of galena, gold, and a small amount of 

sphalerite, pyrite, and quartz, cutting silicified limy 

shale in which fine-grained galena was abundantly 

disseminated.

The general order of deposition interpreted from the 

relations described above is as follows: (1) Pyrite, 

(2) quartz locally, (3) sphalerite (variety marmatite) 

and intergrown chalcopyrite, (4) sphalerite and galena, 

(5) sphalerite and chalcopyrite, (6) galena and pyrite, 

(7) galena, sphalerite, and gold, (8) quartz, (9) ankerite, 

siderite, calcite, and barite.

As the pyrite and sphalerite on the lower levels were 

deposited earlier than galena, and as galena is contemporaneous with much of the sphalerite on the upper 

levels, the sphalerite in the lower part of an ore shoot 

was probably formed at an earlier stage in the epoch 

of mineralization than the sphalerite in the upper parts 

of the same shoot. In other words, the deposition of 

the sulphides extended over an appreciable period of 

time, and ore formed in large amounts at the bottom 

of an ore shoot sooner than it did at the top.
GUIDES TO ORE AND FUTURE OF THE DISTRICT 57

OXIDATION OF THE ORE

It was impossible to study the upper parts of the 

veins in 1928, and the following paragraph on the 

oxidation of the ores is quoted from Ransome: 75

sulphide ore. The normal sequence, however, from the surface 

down, appears to be (1) a soft, heavy, yellowish claylike ore 

consisting largely of earthy cerusite and containing residual 

nodules of galena; (2) a lead-silver ore in which the galena is 

only in part oxidized, while the pyrite has been for the most part

 H it Monzonite \porphyry

10 10 20 FEET

FIGURE 24. Ore enlargement at premineral fault, level 7, Oro mine. Note stringers of ore following faults.

Even along the outcrops of the veins most of the shafts and 

tunnels show some galena in the claylike product resulting 

from thorough oxidation, and the change to essentially sulphide 

ore generally takes place at depths of less than 300 feet. The 

depth of the oxidized zone, however, varies, being greatest in 

general near the crest of the ridge in which the ore bodies occur

changed to limonite and the sphalerite altered to smithsonite 

and limonite, with removal of zinc in solution, and finally (3) 

a lead-silver-zinc ore in which galena predominates and in which 

the early stages of oxidation are indicated by the formation of a 

little spongy smithsonite, or "dry bone", as the miners call it, 

at the expense of the zinc blende.

45 FEET

FIGURE 25. Enlargement of ore as vein approaches premineral fault, level 3, Oro mine.

and least along the lower slopes. On the main levels of the 

Wellington mine the ore as a rule shows no oxidation, although 

in a few places there has been a slight development of smithsonite in interstices of sphaleritic ore. Owing to the inaccessibility of the old upper workings there is little opportunity at 

present for studying the transition from partly oxidized ore to

" Ransome, F. L., op. cit., p. 134.

OCCURRENCE OF ORE

Evidence has been given to show that the major 

faults originated before the period of mineralization. 

In figures 24 to 28 the influence of preexisting faults 

on ore deposition is illustrated. In some places premineral cross faults acted as guides to the mineralizing
58 BRECKENRIDGE MINING DISTRICT, COLORADO

solutions and determined the limits of an ore shoot; 

in other places the cross fault opened up the vein on 

one side and closed it on the other and thus caused a

steepens, whereas a reverse fault becomes open where 

the dip flattens. So far as known, all the veins of the 

Wellington mine are in normal faults, and most of the

" * * II . * >V « . // - V * " " .

FIGUKE 26. Local enlargement of 'ore at intersection of branch vein, level 6, Oro mine.

local enlargement and "pinch" in the ore. Some of 

the ore shoots seem to be related to the variations in 

the dip of the vein. The slipping of the irregular

2 FEET

FIGURE 27. Fracturing of interbedded shale and quartzite, level, 6 Wellington

mine.

surfaces of a fault will produce open spaces as well 

as tight contacts between the walls, and a normal 

fault will tend to develop openings wherever the dip

ore shoots occur where the vein becomes steeper or 

where it changes its course. Thus the decrease in ore 

at the top of the Great Northern vein coincides with 

a marked flattening of the vein, and the steepening 

of the Main vein in the upper levels is marked by an 

increase in the width and length of the Main ore shoot. 

The physical character of the wall rock influences the 

permeability of the material in a fault, and ore chanFIGUEE 28. Brecciated marmatite cemented by ankerite, level 7, Oro mine.

nels are stronger and more persistent in porphyry or 

jasper than in unsilicified shale. (See fig. 27.)

The presence of two ore shoots that have not suffered 

appreciable erosion the western branch of the Main 

ore shoot and the Great Northern ore shoot gives an 

unusual opportunity to study the character of the 

primary ore and to trace the relation of the ore to the
PKODUCTION 59

structure of the region. The zinciferous character of 

the Great Northern vein and its relation to the Great 

Northern fault have already been pointed out. As 

pyrite and sphalerite are early minerals, it is probable 

that this vein was filled before the shoots on the Main 

vein or those in the veins farther north, which were 

richer in lead. If the Great Northern fault acted as a 

guide for the metallizing solutions and they filled the 

Great Northern vein before the other veins, the solutions must have followed a northerly course along the 

master fault. Thus the open fissures communicating 

with the foot wall of the fault were filled as the northward-moving solutions reached them; in a general way 

the southern fractures were filled and healed before 

those to the north. Intermmcralization movement 

along the Great Northern-J fault probably opened ore 

channels communicating with the Bullhide fault and 

with the Main vein. The higher proportion of lead in 

the ore shoots near the Bullhide fault suggests that the 

solutions did not have access to them until late in the 

period of mineralization.

GUIDES TO ORE AND FUTURE OF THE DISTRICT

Only a, small part of the Breckenridge district has 

been studied by the writer. Most of the known ore 

has been exhausted in this special area, but the relation of ore to structure seems established, and there 

are some places where the writer believes that more 

work should be done. The occurrence of ore in the 

Wellington mine, summarized on pages 50-59, suggests 

that shoots of ore are most likely to be found in northeastward-trending veins close to strong northwardtrending faults, which acted as trunk channels for the 

metallizing solutions. Thus the ground close to the 

Bullhide fault north of the Puzzle vein may contain 

workable bodies of ore on some of the fissures that are 

known to branch from the Main vein farther east. 

The Wellington Main vein, as noted on page 49, is 

offset by the Bullhide fault, and, as shown on plate 2, 

the western continuation of the vein, mantled by wash 

and completely unexplored, is probably directly 

beneath the wagon road to the Wellington mine. 

This unexplored portion of the vein is about 800 feet 

long. Good galena ore was picked up by the dredge 

where it crossed a vein parallel to the Bullhide fault 

and about 150 .feet farther west. This vein should 

intersect the western segment of the Wellington Main 

vein about 100 feet west of the Bullhide fault. Although the wall rocks of the vein here are not as 

competent to form open fissures as porphyry or 

quartzite, the distance to quartzite on the hanging 

wall is probably not more than 50 feet. This is one 

of the most promising unexplored localities in the 

special area.

In Gibson Hill the blanket deposits have been found 

chiefly in the upper part of the Dakota formation, but

the occurrence of replacement deposits in the upper 

part of the Maroon formation and the presence of 

calcareous beds in the Morrison formation, which 

might carry ore, suggest the possibility of deeper ore 

shoots beneath the blanket ores of the Dakota. Most 

of these deposits have been small, however, and it is 

doubtful if the cost of exploration for such deep deposits 

could be justified. In the faulted area east of Gibson 

Hill, where the Benton shale crops out, it is possible 

that core drilling into the upper part of the Dakota 

formation might discover some ore shoots that could 

be profitably mined.

South of French Gulch there is a possibility of finding 

ore in cross fissures close to the southward continuation of the Great Northern-J fault zone, but the mineralization on this side of the gulch was apparently 

less intense than on the northern side. The fissures 

in the thick porphyry sill that makes up most of 

Nigger Hill are probably weak and poorly mineralized 

in the underlying shale, but there is a strong probability that gold enrichment occurs in some of the veins 

at the shale-porphyry contact, as it did in the Dunkin 

vein. It is possible that the veins are stronger and 

carry ore in the Dakota quartzite, which lies at different altitudes near Nigger Hill, as shown in plates 2 and 

5. South of Nigger Hill, in the mines between Dry 

Gulch and Illinois Gulch, ore has been found only in 

the rocks above the Morrison formation. If this relation is not fortuitous, the only area of much promise 

that has not been explored is under the Benton and 

Niobrara formations west of the Golddust vein. This 

unexplored ground is chiefly in the bottom of Illinois 

Gulch and undoubtedly contains much water, so that 

exploration may be too expensive unless it can be done 

by geophysical means.

In conclusion, the amount of known ore left in the 

special area is a very small part of that originally 

present, and the discovery of additional ore will probably depend on the correct interpretation of the 

structural geology. Geophysical work in areas that 

are structurally favorable for occurrence of ore, supplemented by core drilling, will probably be the most 

economical method of ore hunting in most places, 

although the western continuation of the Wellington 

Main vein can probably be cheaply prospected by 

means of test pits.

PRODUCTION TABLES

The available information on the production of most 

of the mines described in this report is summarized in 

the following tables. The production since 1906 is 

believed to be accurately shown, but as the figures for 

the preceding years are incomplete no totals are given. 

The writer expresses his appreciation of the fact that 

with the exception of the Royal Tiger Mining Co. none 

of the owners or lessees have withheld permission to 

publish the production figures for their properties.
60 BRECKENRIDGE MINING DISTRICT, COLORADO

Metals produced from crude ore shipped to smelters from Country 

Boy mine

1889-:-....

1902 «__._..

1906. .-.

1907 

1909 

1911 »______

1912 6 _..._.

1913 

1914... ._-_.

1915 ..-

1916 

Ore (short 

tons)

(?)

1,500

1,640

727

2,876

2

101

276

267

« 436

<*458

Gold (fine 

ounces)

23. 77

. 37

14. 37

2. 26

Silver 

(fine 

ounces)

477

20

352

72

Lead 

(pounds; 

wet assay)

28, 553

1,976

106

Zinc (pounds; 

gross)

1, 223, 623

727, 000

2, 588, 400

73, 478

250, 884

244, 127

166, 268

320, 576

No production in 1887-88, 1901, 1903-5, 1908,1910,1917-28; no record for 1890-1900.

« Probably Juniata.

» May include production from the Juniata.

«Includes 396 tons of crude ore that was treated in concentrating mills. About 

132 tons of zinc concentrates were produced, containing 77,173 pounds of zinc. The 

40 tons of smelting ore contained 36,848 pounds of zinc. No gold, silver, or lead 

recovered.

d Estimated tonnage and content of concentrates produced from 939 tons of crude 

ore.

Metals produced from crude ore shipped to smelters from DetroitHicks mine

[See also table for milling ore]

Dry gold and silver ore: 

1922_ -_.....

1923-----..-

1924__._ _.

1925.-------

1926__-. 

1927------.-

Lead ore: 

1923. -------

1924--.---..

1925.... _.

1927________

Lead-zinc ore: 

1923... ..

1924______ __

1926-------.

Ore 

(short 

tons)

68 

141 

136 

188 

52 (")

1,632 

1,044 

21 

16

102 

118

» 311

Gold 

(fine 

ounces)

38. 80 

106. 30 

33. 40 

96.70 

8.40 

17.00

252. 30 

180. 15 

6.30 

9. 41

13.80 

18. 29 

176. 79

Silver 

(fine 

ounces)

1,498 

1,237 

1,351 

2,758 

429 

816

49, 644 

32, 822 

514 

1,083

1,448 

1,641 

2,566

Lead 

(pounds; 

wet 

assay)

5,287 

2,514 

415 

7,764 

1,683

521, 139 

359, 316 

2,930

7,457

11, 984 

10, 397 

13, 103

Copper 

(pounds; 

wet 

assay)

105

2, 228 

201

2, 225

Zinc 

(pounds; 

gross)

51, 951 

54, 465 

78, 341

 Less than H ton.

6 Content of crude milling ore, only estimated figures available for concentrates 

produced.

Ore from Detroit-Hicks mine treated at mills, and gross content 

of concentrates produced from it

[See also table for smelting ore]

1927 

1928 « _-_..

Ore to mills 

(short 

tons)

200

1,835

Concentrates 

produced 

(short

tons)

r-H

{ 93

Gold 

(ine 

ounces)

« 12.71

0 1. 47

67.90

Silver 

(fine 

ounces)

a Q7Q

a 152

3,000

Lead 

(pounds; 

wet 

assay)

15, 615

Copper 

(pounds; 

wet 

assay)

96

Zinc 

(pounds; 

gross)

18, 937

' Bullion. Includes Extension mine.

Metals produced from crude ore shipped to smelters from Extension

1912. ___.....____.___.__

1913___-_. ........ ------

1914____-_-___-___._.__.

1915-_._-_--_-_. ________

1916 ___ 

1926--.------------.-. ._

Ore (short 

tons)

4

12

101

' (?)

(?)

13

Gold (fine 

ounces)

1.32

5.87

116. 69

3.92

5.26

6 30. 30

Silver (fine 

ounces)

68

158

134

1

2

e 1

No production in 1901-11, 1917-25, 1927; for 1928 see Detroit-Hicks.

" From milling ore.

b Includes 3.83 ounces gold bullion (amalgam).

c Bullion (amalgam).

Metals produced from crude ore shipped to smelters from Dunkin

mine

[See also table for milling ore]

1890 

1891 

1901 -

1902_____..__]

1903 

1904-_-_-.-___

1906 

1907 

1908 

1909 

1910 

1911 - 

1912____ .__.

1913 -------

1914 >____--__

1915 b .-. ------

1916 - ^

1917 

1918 

1920 

1926 --

1928 

 Ore 

(short 

tons)

(?)

(?)

230

125

150

(?)

80

250

675

110

205

94

688

259

100

127

125

186

73

20

8

14

Gold (fine 

ounces)

15.00

129. 00

101. 60

63.00

75.00

1. 11

59. 99

150. 01

445. 00

102. 56

73.24

94. 01

568. 55

165. 83

357. 72

534. 65

1, 414. 74

252. 57

62. 70

15.60

3.80

21. 10

Silver 

(fine 

ounces)

339

3,492

3,335

544

1,500

748

1,200

3,250

7,425

1,273

10, 130

1,034

9,603

2,497

812

1,249

1,706

658

611

112

78

101

Lead (pounds; 

wet assay)

29, 855

.402, 362

117, 647

105, 882

56, 471

176, 470

457, 227

72, 013

129, 519

59, 220

435, 491

145, 342

51, 581

58, 062

39, 989

98, 491

31, 777

3,272

3,610

3,270

Copper 

(pounds; 

wet 

assay)

1,840

49

51

608

144

320

146

No production in 1905, 1919, 1921-25, 1927; no records for 1892-1900.

0 Includes 3 tons of dry ore containing 170.44 ounces of gold and 136 ounces of 

silver.

6 Includes 23 tons of dry ore containing 469.22 ounces of gold and 288 ounces of 

silver.

Gold and silver produced from ore from Dunkin mine treated at

mills

[See also table for smelting ore]

1910.- 

1913. 

1914 

1915 -

1916. ------

1917..--- 

1924- . 

Crude metal 

(ounces)

44. 56 

2, 344. 79 

2, 562. 39 

2, 931. 85 

1, 908. 52 

23. 63 

' 85. 98

Gold

Fine ounces

31.31 

1, 383. 02 

1, 701. 63 

1, 992. 26 

1, 218. 81 

15. 66 

51.39

Fineness

0. 7026 

. 5898 

. 6641 

. 6795 

. 6386 

. 6625 

.5977

Silver

Fine 

ounces

11 

901 

834 

864 

556 

8 

24

Fineness

0. 2468 

.3842 

.3254 

. 2948 

. 2912 

. 3279 

.278
PRODUCTION 61

Metals produced from crude ore shipped to smelters from 

Germania group °

1889..---..-..-..-.

1890..............

1901..............

1902..............

1903. ..............

1904.. ........ ....

1905... .. ---------

1906-.-..------...

1907..............

1908.............

1909.....-..-.--..

1912..............

1913*.. ............

1915-.--.------...

1916. __-----.__--.

1920. . .------.-..-

1928..............

Ore 

(short 

tons)

(?)

(?)

150

87

240

94

133

164

11

20

75

5

21

73

307

156

3

Gold (fine 

ounces)

6.82

100. 00

675. 00

64.33

320. 00

40. 15

78. 37

38.31

1. 11

8.57

1.60

.64

2. 62

2. 40

26.43

6.50

. 20

Silver (fine 

ounces)

1,929

7,500

4,968

49, 800

4,404

8,079

4,482

835

1,741

531

146

584

2,978

9,213

2,306

121

Lead 

(pounds; 

wet assay)

19, 374

198, 924

28, 586

5,705

4,211

2,445

2,380

12, 755

297

Copper 

(pounds; 

wet 

assay)

1 

' 714

No production in 1887-88, 1910-11, 1914, 1917-19, 1921-27; no records for 1891-1900.

« Figures for some years may include production from Morning Star. 

» Includes 13 tons of dry ore containing 221 ounces of silver.

Metals produced from crude ore shipped to smelters by Little 

Mountain Milling & Mining Co. 0

1887-. ---.-.-. -.-..

1888 ...........

Ore (short 

tons)

(?)

(?)

Gold (fine 

ounces)

15. 75

19. 35

Silver (fine 

ounces)

6,011

9,393

Lead (pounds; 

wet assay)

31, 479

0 Probably belongs to Germania group.

Metals produced from crude ore shipped to smelters from Juniala

mine

.1.889........-.-..-.

1890-.-..-.--.-...

1892..............

1914

Ore (short 

tons)

(?)

(?)

(?)

11

Gold (fine 

ounces)

206. 68

500. 00

1, 162. 14

3. 43

Silver (fine 

ounces)

3,370

7,540

2,694

117

Lead (pounds; 

wet assay)

120, 507

1, 662, 430

185, 393

8,635

No production in 1901, 1903-10, 1913, 1915-28; no records for 1891, 1893-1900; 

for 1902, 1911-12, see Country Boy.

Metals produced from crude ore shipped to smeller from Ouray mine

1890..............

1891...-.-......-.

1892........._.__.

1915.. .........

Ore (short 

tons)

(?)

(?)

(?)

5

Gold (fine 

ounces)

334. 50

566. 41

1, 342. 33

4. 08

Silver (fine 

ounces)

12,611

58, 505

118, 022

128

Lead (pounds; 

wet assay)

633, 713

610, 728

2, 043, 515

768

No production in 1901-14,1918-28; no records for 1893-1900; according to the report 

of the Director of the Mint, the report of production for 1889 was confidential; production for 1910-17 may bo included in report for Qolddust and Puzzle.

Metals produced from crude ore shipped to smelter from Pacific

mine

1905...--.-...

1906..-..,....

1914... .......

1916..........

1928. .........

Ore (short 

tons)

300

175

8

180

9

Gold (fine 

ounces)

24. 19

24. 19

1.88

110. 48

4. 40

Silver (fine 

ounces)

331

330

86

2,728

119

Lead 

(pounds; 

wet assay)

7,509

7,513

5,649

105, 963

1.221

Zinc 

(pounds; 

gross)

452

No production in 1901-4, 1907-13,1917-27.

Metals produced from crude ore shipped to smelters from Puzzle 

mine and dump

1887. ......

1888---..--

Lead ore: 

1915.--

1916...

1917--.

1925--.

L e a d-z i n c 

ore: 1916.

Ore 

(short 

tons)

(?)

(?)

21

116

111

13

356

Gold (fine 

'ounces)

4.79

53.95

12. 12

67.03

70.64

4. 10

214. 40

Silver 

(fine 

ounces)

777

379

266

936

1, 142

119

4,623

Lead 

(pounds; 

wet assay)

18, 973

5,529

29, 772

28, 651

2,677

124, 750

Copper 

(pounds; 

wet assay) 

242

418

2,731

Zinc 

(pounds; 

gross)

207, 313

According to the report of the Director of the Mint, the report of production for 

1889 was confidential.

Ore from Puzzle mine and dump treated at mills and gross content 

of concentrates produced from it

1902... .......

1903-.------..

1916. ---------

Ore to concentrating 

mills (short 

tons)

400

500

350

Concentrates produced 

(short tons)

(?)

(?)

70

Gold (fine 

ounces)

50.00

58. 06

119. 44

Silver (fine 

ounces)

1,400

1, 400 ~

1. 102

Lead 

(pounds; 

wet assay)

11, 765

11, 800

66. 843

Metals produced from crude ore shipped to smelters from Golddust

mine °

10(19

1Q04

1 Q0 1!

1906-..-.-

1907-----.

1908----.-

1QOQ

1911----.-

1914---..-

1915.-----

1916......

1Q17

1928_-_--_

Ore 

(short 

tons)

1

6 1 461

fcQQtl

b 348

"795

236

85

17

65

174

199

20

7

Gold (fine 

ounces)

3. 53

1, 022. 00

194. 04

68.65

214. 01

1 "SQ 4Q

41.61

5. 97

48.00

55.03

60. 38

18.66

.80

Silver 

(fine 

ounces)

2

23, 376

5,589

2,921

6,665

3,245

1,023

267

1,016

2,702

3,044

302

45

Lead 

(pounds; 

wet assay)

687, 616

27, 909

160, 331

137, 825

122, 520

41, 085

16, 404

16, 601

164, 626

206, 171

10, 000

2.397

Copper 

(pounds; 

wet assay)

321

125

31

Zinc 

(pounds; 

gross)

61.3

No production in 1901,1910, 1912-13, 1918-27; production for 1903 may be included 

in table for Puzzle mine.

" For some years production for Puzzle mine is included.

* Probably includes crude ore shipped to smelters and concentrates prodnoed 

from milling ore.

Metals produced from crude ore shipped to smelters from Washington mine

1890. -----------------

1909__----------------

1911---..--------------

1912-.-_-_------ ------

1913.. ._--..-. --------

1915-.--. ... ----------

1916_-____.- ------

1917-------------.----

Ore (short 

tons)

(?)

34

9

18

22

29

46

36

Gold (fine 

ounces)

250. 00

8.31

5. 64

6.06

5. 58

17. 50

27. 53

17.04

Silver (fine 

ounces)

4,713

723

187

341

432

305

484

467

Lead 

(pounds: 

wet assay)

390, 759

47, 238

4,392

11,499

19, 452

7,480

15, 827

17, 564

Gold and silver produced from ore of Washington mine treated at

mills

1904_... --------------

1916.-.--.. -----------

1917-----------.------

Ore to mills

(short tons)

1, 500

Concentrates produced

(short tons)

(?)

Gold (fine

ounces)

264. 00

«6. 88

« 5. 95

Silver (fine

ounces)

7, 500

o 7

o 4

' Bullion.
62 BRECKENRIDGE MINING DISTRICT, COLORADO

Metals produced from crude ore shipped to smelters from mines of 

Wellington group

1887 (Oro 

claim) .... 

1888 (Oro 

claim) 

1889 (Oro 

claim) 

1890 (Oro 

claim) 

1891------

1889 (Prize 

Box Vein) - 

Lea d-z i n c 

sulphide 

smelting 

ore: 

1904 

1905 

1909 

Zinc sulphide 

smel ting 

ore: 

1910-.-. 

1911-- 

1912.... 

1916 

. 1917...-

1918-.-- 

1919.... 

1922.... 

Lead smelting ore: 

1908.... 

1915 

.1918.... 

1920--.- 

1922 

1923 

1924.... 

1925.... 

1926-... 

1927-.-- 

1928... . 

Dry silver 

smelting 

ore: 

1916..-- 

1920... .

Ore

(short 

tons)

(?) 

(?) 

(?)

(?) 

(?)

(?)

4,200 

583 

1,950

1, 342 

150 

1, 260 

2,025 

5, 873

1,898 

209 

750

 197 

35 

37 

31 

766 

364 

162 

251 

48 

118 

170

218 

612

Gold 

(fine 

ounces)

102. 60 

211.40 

103. 54

480. 00 

45. 23

118. 84

210. 00 

17. 46

4. 83 

4. 78 

2. 00 

7.08 

84. 50 

23. 30 

11.40 

10.40 

1.70 

15. 49 

20. 91

11. 23 

24. 48

Silver 

(fine 

ounces)

2,874 

3, 518 

7, 158

20, 359 

1,045

796

84, 000 

3, 001 

2, 120

247 

854 

500 

706 

7, 131 

3,942 

1,611 

3, 360 

449 

1,367 

2, 124

1,089 

2.448

Lead 

(pounds; 

wet assay)

325, 382

47, 275 

2, 557, 586

53, 301

4, 941, 176 

158, 344 

745, 883

228, 395 

52, 386 

18, 744 

33, 021 

287, 226 

117, 630 

50, 367 

154, 351 

17, 579 

64, 139 

123, 137

Copper 

(pounds; wet assay

3,480

42

Zinc 

(pounds; 

gross)

2, 016, 000 

291, 325 

928, 000

1, 017, 200 

108, 783 

819, 000 

1, 832, 810 

5, 006, 578 

1, 717, 584 

177, 232 

846, 006

Lead-zinc ore from mines of Wellington group treated at Wellington

mills 

[Short tons]

1905------.--.------ 2,770

1906-----...-----.- 862

1907--..----------. 5,200

1908--------------- 2,811

1909------------- 10,500

1910--------------- 23,496

1911--.--.---..--.- 30,930

1912--.--..--------- 33,322

1913- --------- 19,013

1914--_-__--.---- 17,403

1915------.------ 38,846-

1916------------- 39,230

1917--.----------- 41, 900

1918--.--- ------ 39, 117

1919---.. --..--_- 11, 103

1920--- --------- 30,401

1922.-. ---------- 2,114

1923------------- 42, 552

1924--.--. -------- 21, 586

1925.--_----.-..- 23, 743

1926- - --------- 31,740

1927--. - - 20, 179

1928-- - .-- 19,810

Metals produced from ore from mines of Wellington group treated 

at Wellington mills

Lead concentrates: 

1910 

1911..: 

1912 

1913... 

1914 

1915 

1916--. 

1918 

1919--. 

1920 

1922.-. 

1923-... 

1924 

1925 

1926 

1927 

1928 

Iron concentrates: 

1916 

1917 

1918 

1920 

1923 

Zinc concentrates: 

1905 

1906 

1907 

1908 

1910°..

1911 

1912 

1913... 

1914 

1915 

1916 

1917___ 

1918-.. 

1919 

1920 

1922 

1923 

1924 

1925.-. 

1926 

1927 

1928 

Lead - zinc 

concentrates: 

1909 

1925 

Concentrates 

(short 

tons)

3,452 

4,718 

3,222 

2,617 

1,307 

1,433 

298 

114 

122 

243 

1,70 

4,282 

2, 168 

2,913 

2,832 

843 

1, 190

3,388 

8,061 

2,353 

297

448

(?) 

(?) 

(?) 

790 

6,630

9,266 

11,804 

8,678 

5,798

10, 262 

15, 444 

20, 715 

17, 201 

4,782 

10, 394 

418 

9,454 

3,666 

1,848 

4,715

5,852

4, 114

5,250 

1,585

Gold 

(fine 

ounces)

156. 58 

50.25 

86.67 

143. 94 

117. 22 

86.84 

43.33 

15. 90 

7. 20 

37. 78 

7. 58 

775. 50 

353. 50 

333. 30 

418. 70 

126. 24 

316. 76

152. 60 

341. 34 

120. 20 

14. 11 

38. 40

69. 03 

16. 06 

117. 02

1. 47

49. 10

90. 50

883. 96 

30.00

Silver 

(fine 

ounces)

38, 990 

56, 591 

50, 073 

30, 491 

21, 518 

21,919 

9,671 

3,832 

2,694 

4,742 

2,040 

47, 117 

21, 303 

30, 531 

29, 281 

16, 682 

20, 856

14, 319 

38, 098 

14, 654 

1, 188 

1, 588

14, 188 

4,249 

23, 400

4,545

4,573

9,661

55, 988 

4,000

Lead 

(pounds; 

wet assay)

2, 999, 237 

4,035,711 

3, 053, 776 

2, 547, 240 

1, 158, 306 

1, 417, 142 

352, 537 

133, 662 

68, Oil 

168, 858 

117,920 

3, 461, 772 

1, 870, 110 

2, 734, 619 

2, 662, 886 

972, 832 

1, 453, 346

10, 823

7,277 

28, 745

7,362

778, 974 

251, 538 

1, 127, 200 

451, 988

19, 857

89, 456

91, 439

3, 161, 122 

504, 734

Copper 

(pounds 

wet 

assay)

1,845

4,815

2,613

70

82

318 

953

104

17, 897

41, 650

99, 380

Zinc 

(pounds; 

Rross)

(?) 

(?) 

(?) 

(?) 

(?) 

(?) 

(?) 

(?) 

(?) 

(?) 

(?) 

(?) 

(?) 

(?) 

' (?) 

(?) 

534, 820

1, 325, 021 

416, 826 

1, 996, 800 

1, 138, 852 

4, 325, 500 

7, 726, 403 

9, 862, 747 

7, 458, 723 

5, 356, 157 

9, 428, 807 

14, 047, 189 

19, 347, 039 

17, 304, 206 

4, 881, 634 

10, 322, 902 

401, 000 

9, 061, 002 

3, 792, 338 

1, 790, 175 

4, 666, 173 

6, 160, 015 

4, 412, 091

2, 492, 000 

1, 557, 897

> Includes 2,972 tons of zinc middlings containing 1,545,500 pounds of zinc.
INDEX

Abstract of report.........

Acknowledgments for aid..

Page 

. v-vi 

1-2,40

B

Benton slmlo, corrolatlou of................................................... 10

llthology of.............._....-.--.-...--.._.__.._..._..._-_....... .._.-. 10-11

Berlin tunnel, Washington mine........ .-_..-..-.._....-.._...__..---.---_---.. 45

Blanket deposits, cliaraetcr of ore of...----...........-_...-.....-....---.-..---- 33

relation of, to country rock.. ................... ...._................. 32-33

to structure............................... ............................. . 33

Boreas Pass, section of Dakota quartzite near_................ .............. 8

section of Morrlsou formation near......... . ........... . ........ ... 7

Bullhido fault, Wellington mine....... . .......... . . ....... .. .... . 48-49

Cambrian time, events of......................

Carboniferous time, events of................

r.hrlstonson tunnel, Mayo vein_.............

Contact-niotamorphlc deposits, features of. 

Contact motamorphisin, occurrence of.....

Copper, occurrence of, on Farncouib Hill....

Cornish tunnel, Washington mine...........

Correlation of formations In Breckeuridge district .............................. pi. 3

Country Boy mine, features of__................ . ............._......-....... 33-35

production of.. ......................... ................_..............._....... 60

Cretaceous time, events of.............. ...... . ................................. 22

D

Dakota ciuurtzite, general features of ........... ........................... 7

llthology of... .........................._......_._............................... 7-10

"roll" lu thin-bedded, at Detroit mine........................................30, pi. 8

upper member of....................................... ...........................10, pi. 4

Detroit mine, general features of............ .......................................... 35

geology of...................................... - .................__..___..,_-....--.-... 36

ore deposits of................................... ................................ 36-37

ore from.............................................. ......_................ .. pi. 8

section of Dakota quartzlte lu................................................. 9

Detroit-Hicks mine, production of......--.............."-...........-........, 60

Devonian time, events of._................._.-.-.......................-.-. 21

Dunkin mine, features of......._.... .-.................... ........... 37-39

production of............................................................. 60

E 

Kast Iron vein, Wellington fault block....................................... 50

East vein, Wellington fault block............................................ 50-51

Kmmet vein, workings on..................................................... 44-45

Eocene time, events of._........._...--........._........................... 22-23

Extension mine, production of................................. ........................ 60

Extenuate adit. See X-10-U-8 adit.

F 

Faults, features of.___............. ................_...........___ 18-20

map showing................................................. pi. l (in pocket)

Fault vein, Oro fault block......................................... . 53

Field work................- . ........-.--......-......-.-......-----.--...... 1

French Gulch, view looking east up........................................... pi. 4

O 

(lullager tunnel, Dunkin mine.......... ................................. 38

Geologic history.... ......................._...-... ................................. 21-23

Geologic map of Breckenridgo district-.-..-.-..................... pi. 2 (in pocket)

Uoology of the district, general features of................_.................... 3-20

Oermaula group,'production of.._.... ............_.............-.......-. 61

Ueruiunla shaft and tunnel, Little Mountain................................ 40

GIbson Hill, blanket deposits of...-....-..-..................-------..-.-..- 32-33

Glacial deposits, character of..................____..................... 13

Goldclust mine, production of...._......-.... ....-.. .................. 01

Golddust vein, geologic and assay map of.................................... pi. 10

slope map of............_............................................ pi. 12

workings on.........._................................................ 43-44

Gold, enrichment of. .... ............... .................. ... 32

occurrence of..._____.__.. ......__........................ 27-28

Great Northern fault, Wellington mine. _.............................. 49

Great Northern vein. Wellington fault block.............._..............-.. 52

H Page 

High-temperature deposits, features^ of....................................... 25

Horn vein, workings on........_.......................................... 45

Hydrothennal alteration of rocks, occurrence of.............................. 16-17

Igneous rocks, general features of. 

Iron vein, Oro fault block........

J

J fault, Wellington mine_.................................................. 49

Juniatarnine, features of............._....................... .......... 39-40

plan and section of..........--..-...-.-........-...--.-.-...-.------.... pi 9

production of.........................._._............................... 60,61

Jurassic time, events of........----------------------............................ 22

Keystone, section of Cretaceous at.............

section of Dakota quartzite 1 mile west of.

Lead, enrichment of......................................................... 29-30

precipitation of........................................................... 31

Lead minerals, solvents of......................................... .......1.. 30

Lead ores, occurrence and character of...._..............-............-........ 27

relation of, to topography...................._........._-.......-......... 28

Little Mountain, blanket deposits of......................................... 32-33

mines on......................_.......................-.-..-........-..--. 40-46

section of Dakota quartzite on...........__...................... ... 9

Little Mountain Mining & Milling Co., shipments of ore by................. 61

Location of the district........................................................ 2

Main vein, Oro fault block........................................... 53-54, pi. 14

Maps of Breckenridge district................................. pis. 1, 2 (in pocket)

Maroon formation, correlation of............................................ 4-5

lithology of...........-..------.----..--.....---.-.---.-.-..--.-.-.------ 5-6

Mayo vein, workings on_.................................. ............. 45

Mesothermal deposits, features of............................................ 25-28

Mill tunnel. See Berlin tunnel.

Mineralogy........_...................................................... 23-24

Mine workings, map showing principal ........................ pi. 1 (in pocket)

Mining, future of__...... ............................................. 59

history of...___.__....... ........ ....... ..... 3

Miocene time, events of.-.-.--------.-.--.-.-...-.....--------------.------- 23

Mississippian time, events of... .......................................... 21

Moderate-temperature deposits, features of.................................. 25-28

Monzouite porphyry, features of._.......................-.--.--..-..-._... 15

Morning Star mine, production of...__.................................... 61

Morrison formation, correlation of........................................... 6

lithology of...-........-.---------.---.-.----.-----..-.----------.---..-.. 6-7

N 

Nigger Hill, section of lower member of Dakota quartzite on................. 9

Niobrara formation, composite section of.................................... 11

features of...._..._.. ............... ......................... 11-12

Oligocene time, events of..-.....-.-..---......-.-...-.-.----.--.--.----.---. 23

Ordovician time, events of... ......................... ............... 21

Ore, guides to occurrence of............................. .................. 59

localization of........................................................ 26,68-59

paragenesis of........................................................... 33,66

Ore channels, features of.._................._............................ 26-27

Ore deposits, classification of... ............ .......... ........ 24-25

Ores, primary, character of....---------..-.......-.-...-..--.-..-...-..--... 27-28

Oro fault block, structure in. . ...............__... . ........ 48-60

veins in..__.____... ......................... ............. 53-56

Oro mine, geology and sections in.... .............. ................. pi. 15

Oro workings, extent of.. ................................................. 47

map of..______._._--.-.__._.................__......... pi. 13

Orthodox vein, Wellington fault block.._____...__......__.......... 51-52

Ouray mine, production of................ ....'.............................. 61

Ouray vein. See Puzzle-Ouray vein.

63
64 INDEX

P Page 

Pacific mine, production of......__....__.-__-.-----__--_..__.._._---._ 61

Pacific vein, geologic and assay map of--..--..-..------.----.-___.__.---.. pi. 10

workings on......____.______..-._....._....._......... 44

Palmer ranch, section of Cretaceous shale at and above.._---------_..-.. 12-13

Peunsylvanian time, events of.-_.-.-.-...___-.--..----......-.__- 21-22

Permian time, events of.......... --...I.........._..---.......____ 22

Pierre shale, features of____. ________________._______ 12-13

fossils of. . ___ -.-.-.-._____.__________ 13 

Pliocene time, events of..___-_________.........___..__. 23

Porphyry, intermediate type of......___________.______.__.. 15

Pre-Cambrian rocks, features of.._-..-------._._.-.-----._............ 3,4

Pre-Cambrian time, events of......_.................._...........__. 21

Production, tables showing____.........................._....-.----.-- 60-62

Puzzle Extension shaft, Puzzle mine....................__--------__.... 41

Puzzle vein, Oro fault block__...__:.__....__......___.....__. 54-56

geologic and assay map of...---------------.....-..----.-------.--.--.-- pi. 10

open stope on...__.................................................... pi. 11

Puzzle mine, production of..---.--..-----.-.-.-.---.-----.-------...-.------ 61

itopemap of...____-__-_..___...___.......__......_.... pi. 12

Puzzle-Ouray vein, workings on.......:......__._.....__...__._ 41-43

Q 

Quartz monzonite porphyry, features of..-..--.-.-.--.-.-.--------..-..-..-- 15-16

Quaternary gravel, deposits of............__._._--------_.......... 13-14

Quaternary time, events of....--------....-..----.-------.---.-.-----...-.-. 23

R 

Railroad tunnel, Dunkin mine.....-..-.-.---.-----------.------.-.-....---- 38

Recent deposits, character of....__...-.---.-..-.........----.-....-..--.- 14

Redwing tunnel, Dunkin mine, ore deposits in.............................. 38,39

Reeside, .J. B., Jr., fossils identified by....................................... 13

Rock alteration, character of--......___----.....___.__-.-.-....... 16-17

Rocky Point, section of Dakota quartzite at..__.__....___.__..... 8-9

section of Morrison formation near.... -.-..------.-..-..-........---. 6-7

Royal Tiger mine. ............................^............................. 2

Sauer's crosscut, Puzzle mine.--.-.-..______.._...._____...... 42

Schock Hill, blanket deposits of..._____.____...-.___......_ 32-33

Scope of report ____--.....--...-------.-..........---.....---..--.. 1

Shonty tunnel. See Berlin tunnel.

Spur vein, Oro fault block__ .._..... ___.........__.......... 53

ore from_ _________..____.____....___._...... pis. 6,7

Standard stope, Detroit mine, ore deposits in........._.................... 36-37

Stockworks, occurrence and features of.. -.-----.-....--.-.-.---------- 25-26

Page 

Structure, local features of-. .. ... .... .... . 18-20

regional features of...... ...-...------. ...---...---...----.---.-.. 17-18

relation of, to Front Range mineral belt.._---.-...---.---....-----.--.-. 20-21

sections showing.._____-__......__-..--.-..... ..---.----.. pi. 5

Sulphide stope, Detroit mine, ore deposits in....___.....-.---..-..--...- 37

Supergene enrichment, criteria of...----------- ...-..--..-...----.--..-.- 28

T

Tertiary time, events of--.--------..-------------..--.---------------.------ 22-23

Topography of the district...---------__..___....__.... 2-3

Triassic time, events of---------.-.---. ........---.. ..... ........... 22

V 

Vadose zone, probable reactions in....._.----......................-.-....- 31-32

Veins, leaching of_____..._--._-_-------.---.-_-.-----.--.-....-.-.-- 30-31

map showing_____... ...._...........___... pi. 1 (in pocket)

occurrence of...___..-..---.-.------.----.-.-.-.--...--.-.-. ...... 26

relation of, to country rock... ....... ..................... 26

W 

Washington mine, features of..----.___---_.-.---.__._....................... 44-45

production of.___. .............................................. 61

Watson shaft, Washington mine.__...--_--.----..--..----..-..-.-.---... 44

Weber (?) formation__ - ___ ......_.. ...... 4-5

Weinland tunnel, Mayo vein.......... ...--............-....-----.-... 45

Wellington fault block, veins in....____....___..__..._........ 50-56

Wellington mills and mine, view of...-.---.--.-...---.-.--------.------.---. pi. 11

Wellington mine, development of.._-...--......-.---------.......-.--..... 46-47

general geology of... .--..-------..-........----..------.--.--.... 47-48

geology and sections in... ............................................ pi. 15

history of.__.-......-.-----.---..----..-...-.---.---.-----...----.-.... 46

Main vein in....-____._..._.....____.__-----_...... 53-54, pi. 14

map of...... ...............-....-.....--...-.-....-.-.......- .. pi. 13

ore from.______.__ --------- _.___............ pis. 6,7

ore in, general features of--..-...---------- .--.---..-----.-.-....-..-. 56

occurrence of.... .---------__--- .---------.... ...------...-. 57-59

oxidation of.------------.-.------.---.--..-.-------..-......-------- 67

production of.....--.----------.-.--.-----------.-.-----...------------.. 62

structure in.----------_..-.-----.----.._._---.-........._....... 48-50

Willard tunnel, Puzzle mine........_....____.___......__...... 41-43

Wolftone shaft, Juniata mine.---------------------.-----....-..----------.-. 40

X 

X-10-U-8 adit, Wellington mine.-..--..--...-......-..-....-.-...-......-.- 46-47

Z

Zinc, enrichment of._______.._....j...._-.--.---.-...-_..-.....-- 29

Zinc ores, occurrence and character of....-.-.-.---..-.-.----..-..---.---.---- 27

relation of, to topography ........................................... 28

o