Petrography of Radioactive 

Tertiary Igneous Rocks 

Front Range Mineral Belt 

Colorado 

GEOLOGICAL SURVEY BULLETIN 1032-E 

Prepared on behalf of the United States 

Atontic Energy Commission and published with the permission of the 

Commission 
Petrography of Radioactive 

Tertiary Igneous Rocks 

Front Range Mineral Belt 

'·colorado 

By]. D. WELLS 

GEOLOGY AND ORE DEPOSITS OF CLEAR CREEK, 

GILPIN, AND LARIMER COUNTIES, COLORADO 

GEOLOGICAL SURVEY BULLETIN 1032-E 

Prepared on behalf of the United States 

Atomic Energy Commission and published with the permission of the 

Commission 

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON • 1960 
UNITED STATES DEPARTMENT OF THE INTERIOR 

FRED A. SEATON, Secretary 

GEOLOGICAL SURVEY 

Thomas B. Nolan, Director 

For sale by the Superintendent of Documents, U S. Government Prlntln~ Office 

Washln~ton 25, DC. 
CONTENTS 

Page 

Abstract------------------------------------------------------~--- 223 

Introduchon------------------------------------------------------ 224 

GeologiC settmg __ --------------------- _ --- ________ -------- _ ----- _ _ 227 

Form of mtrusive rocks _______ --------- __ -- _________ --- __ ----______ 228 

Age and sequence of mtruswn_______________________________________ 228 

Texture and mternal structures-------------------------------------- 230 

PetrographY------------------------------------------------------ 231 

ClassificatiOn _______ -----_-----_--- _____ ---_----------___________ 231 

DescriptiOn of rocks_____________________________________________ 232 

Light-coloicd granod10nte group_________________________________ 232 

Light-colored granodwnte porphyry___________________________ 233 

Albite granodwnte porphyry _______________ ---_---____________ 237 

Alkahc syemte porphyry-- ____ -- ____ -- __ -- __ ---______________ 238 

Quartz monzomte group _______________________ ----_____________ 240 

Quartz monzomte porphyry---- ______________ ------___________ 240 

Granrte porphyrY-------------------------------------------- 243 

A Iaski te porphyry___________________________________________ 244 

Bostomte group ____ -------- __ ----- ___ -_-_-----_-______________ 245 

Bostomte and garnetiferous bostomte porphyry_________________ 246 

Bostomte porphyry ___ -____________________________________ 246 

Garnctiferous bostomte porphyry ___________ ---______________ 247 

Trachytic gramte porphyry ________________ -- __ ---____________ 248 

Quart.6 bostomte porphyry __________ -----------------_________ 249 

Hornblende granodwnte group ____________________________ ------ 251 

Hornblende granodwnte porphyry_____________________________ 251 

BIOtite granodwnte porphyry __ --- _________ --_________________ 252 

Biotite-quartz monzomte porphyry _____ -- __ ---_--_____________ 253 

Bwtr te-q uartz la tr te porphyry___ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 254 

Alteratton-------------------------------------------------------- 255 

RadwactrVIty and uramum m the Igneous sequence____________________ 257 

Distribution _______ -------- ___________________ --- __ ---____________ 261 

Mode of mtruswn ______________________________ - ___ - _ _ _ _ _ _ _ _ _ _ _ _ _ _ 262 

Ongm of the Igneous rocks------------------------------------------ 265 

D1fferentmt10n of uramum and thonum ________ ---------- ________ --- _ 266 

Literature ctted _______________________________ -- ___ - _ _ _ _ _ _ _ _ _ _ _ _ _ _ 269 

IndeX------------------------------------------------------------ 271 

III 
IV CONTF.NTS 

ILLUSTRATIONS 

PLATE 19 GeologiC map showmg the Tertiary Igneous rocks __________ Jn pocket 

Page 

FIGURE 57 Index map of the Front Range, Colorado__________________ 225 

58 Diagram shoWing the probable sequence of mtruswn of the 

Tertmry Igneous rocks_________________________________ 229 

59 Histograms showmg distributiOn of radiOactivity (eqmvalent 

uramum) and uramum mall Tertiary rocks______________ 256 

60 Histograms showmg distnbutiOn of radioactivity (eqmvalent 

uramum) uramum m the hornblende granodwnte group____ 258 

61 Histograms showmg distributiOn of radiOactivity (eqmvalent 

uramum) and uramum m the light-colored granodwnte 

grouP------------------------------------------------ 258 

62 Histograms showmg d1stnbutwn of radiOactivity (eqmvalent 

uramum) and uramum m the quartz monzomte group______ 259 

63 Histograms showmg distnbutiOn of radioactivity (eqmvalent 

uramum) and uramum m the bostomte group_____________ 260 

64. Diagram showmg vanatwns of eqmvalent uramum (radiOactivIty) and uramum with rock type arranged m order of decreasmg age w1thm groups_____________________________ 261 

65. Dtagram shoWing vanation of equivalent uramum (radiOactivIty) and uramum with rock type and ttme_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 268 

TABLE 

TABLE 1 Petrographic charactenstics of the Tertiary Igneous rocks____ 234 
GEOLOGY AND ORE DEPOSITS OF CLEAR CREEK, GILPIN, 

AND LARIMER COUNTIES, COLORADO 

PETROGRAPHY OF RADIOACTIVE TERTIARY IGNEOUS 

ROCKS, FRONT RANGE MINERAL BELT, COLORADO 

By JoHN D. WELLS 

ABSTRACT 

The centlal pa1t of the F10nt Range mme1al belt, an a1ea of P1ecambuan 

metamo1pluc and igneous rocks, was mtruded durmg early Te1 tiary time by 

small 1uegular plutons and d1kes of 1adwact1Ve po1phyutic rocks Two of the 

1ock types 1ep1esented, bostomte porphyry and qua1tz bostomte porphy1y, are 

highly 1adwactive 

These Te1 t1a1y 1gneous rocks have several characteristics in common The 

g1oundmass of the porphyues IS genetally ve1y fine gtamed feldspar and 

quattz The phenoctysts tange m gram f1om coatse to medmm and range m 

quantity f10m abundant to sparse The most common phenocrysts ate one or 

more of the plaglioclase or potassic feldspars, phenoc1ysts of ferromagnesmn 

mme1als ate common though not always present; qua1 tz and garnet phenoClysts occm The rocks ate gene1ally alte1ed 

The potphyutlc rocks, which constst of 13 mappable umts, can be classified 

acc01 dmg to petrographic Similarities mto 4 g10ups These g10ups are the lightcolored gianodwnte g10up which mcludes the light-colored granodwute polphyry, albite g1anod10r1te porphyry, and alkahc syemte porphyry, the qua1 tz 

monzomte g10up which mcludes the quartz monzomte porphyry, gramte porphyry, and alask1te p01phyry, the bostomte g10up wh1ch mcludes the bostomte 

p01phyry, trachytlc gramte porphyry, and quartz bostomte porphyry, and the 

hornblende gianodlorite group which includes the homblende granodwute polphyry, biotite granodwute porphyry, b10tite-qua1 tz monzomte porphyty, and 

biotite-qual tz latite porphyry. 

The bostomte g10up has a distinctive trachytlc or bostomtic textme, contams 

abundant alkalic mmerals, IS characteristically reddish brown to "lilac", and 

may contam aegirine-augite The other groups have a granular textme and 

are gene1ally gray The light-colored granodiOrite g10up contams abundant 

albite and sparse aeglrme-augite (?) The qua1 tz monzomte g10up con tams 

abundant quartz in the groundmass, abundant oligoclase and potasstc feldspar, 

and spa1 se to abundant aegirine-augite (?) The hornblende granodloute g10up 

IS chm acteuzed by calcic oligoclase, and one or mote of hornblende, py10xene, 

or biOtite The hornblende g1anod10r1te group 1s hmtted geographtcally to the 

lower Fall River area, and the light-colored granodiOrite group IS found mostly 

in the easte1 n part of the area. The members wtthm each of the light-colored 

223 
224 GEOLOGY AND ORE DEPOSITS, COLORADO 

granodwnte, quat tz monzomte, and bostomte groups form an intrustve sequence of nearly the same age 

The porphyrtes wete mti uded along preexistmg planes of weakness in the 

Precambrian 1ocks, particularly along JOmts, foliatiOn surfaces, contacts, faults, 

and axtal planes of folds The form of mdiVldual bodies changed progtesstvely 

With time f10m Irregularly shaped steep-walled plutons to long, thm dikes 

Inti uswn of the rocks was la1gely passive, the plugs formed dommantly by 

stopmg, and the dikes entered fractm es opened by tectomc movements duung 

the Laramide orogeny Local explosive activity that resulted m the formatwn 

of breccias appears to have 1mtiated some of the pertods of magmatic activity 

The 1ocks of the Igneous sequence are tentatively considered to have been 

deuved from two separate magma chambers The hornblende granodwnte 

g10up, which constitutes a mmor magma senes, appeals to have stemmed f10m 

a relatively small magma chamber The light-colored granodiOrite, quartz 

monzomte, and bostomte groups, which constitute a maJOr magma series, on 

the other hand appear to have been denved from a much larger magma chambel Success1 vely younge1 members w1thm each group are more felsic than 

the ptevwusly mtruded one 

The members of each group and each senes of the rock sequence show a 

progressive ennchment m uramum and thormm from the more mafic to the 

more felsic phases A late phase of the quartz bostomte porphyry, the youngest member of the maJor magma senes, was particularly enriched m the radiO· 

active elements, and 1t probably was the source for the solutiOns that p1oduced 

the manmm m the hydrothetmal vem deposits at Central C1ty and adJacent 

distncts The biOtite-quai tz labte porphyry may also have been a local mmor 

source of uramum 

INTRODUCTION 

The area covered by this report occupies 48 square miles In the 

central part of the Front Range nnneral belt (fig. 57), within Gilpin 

and Clear Creek Counties, Colo. It Includes the mining districts of 

Central City, Idaho Springs, Freeland-Lamartine, Chicago Creek, 

and Lawson-Dumont-Fall River. 

Abundant small bodies of radiOactive porphyritic rocks of early 

Tertiary age occur in Precambnan crystalline rocks, which constitute 

the core of the Front Range. For the most part the Intrusive rocks 

are in well-foliated metasedimentary gneisses and schists, but to a 

lesser extent they are in the more massive igneous rocks. The 

porphyries range in compositiOn from granodiOrite to alkahc granite 

and were intruded in a time sequence as small Irregular plutons and 

dikes. 

The first intensive study of the Tertiary intrusive rocks in the area 

was made by Spurr, Garrey, and Ball (1908) In the Georgetown 

quadrangle, whiCh includes the southwest part of the area of this 

report. Later, Bastin and Hill (1917) studied the Central City quadrangle, part of which is included in the northern part of the area. 

Data from these reports were used as the basis for further studies in 

the mineral belt by Lovering and Goddard (1938) who presented 

an excellent summary of some of the broader regional aspects of the 
PETROGRAPHY OF TERTIARY ROCKS, FRONT ~ANGE, COLO. 225 

LARIMER 

) 

w 

(!J 

z 

<:( 

')' t BOULDER~ 

I ---~--1 I 

I I I I 

I I 

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. I --------

INDEX MAP OF COLORADO 

~ I Fort Colhns 1 

WELD 

GRAND \.. ,-.l 

r;t \ r" ._/ " \ I ' )I §:7 I I \, ~) tt <o H ~Boulder : - ---

\~, -~~< \ -1 ~) ~,~_:· ~~ r 1 \ -- 1 ADAMs 

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' --1 / ~~ rmED AREA ~ :r 

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( PARK lL i / '\ I 

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\ I I \\ I TELLER I ~) I 

10 20 MILES ( 

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Colorado 

~Sprmgs 

After Lovenng and Goddard 1950 

FIGURE 57 -Index map of the Front Range, Colo, showing location of the mapped area 

with reference to the mineral belt. 
226 GEOLOGY AND ORE DEPOSITS, COLORADO 

intrusive rocks and related them to the Laramide deformation and 

to the accumulation of sediment in basins adjoining the Front Range 

highland. Later they summarized the data concerning these rocks 

throughout the Front Range (Lovering and Goddard, 1950). Beginning in 1949 and continuing for several years afterwards, George 

Phair, of the U.S. Geological Survey, studied the petrology and 

radioactivity of the Tertiary porphyries in the middle and northern 

part of the Front Range mineral belt. A preliminary report concerning the Central City district has been completed (Phair, 1952) ; 

a more comprehensive report is in preparation. 

This report is based on investigatiOns by the U.S. Geological Survey on behalf of the Division of Raw Materials of the U.S. Atomic 

Energy CommissiOn. The investigations were started in 1952 and 

continued through the next two field seasons. The mining districts 

of Central City, Idaho Springs, Lawson-Dumont-Fall River, Freeland-Lamartine, and Chicago Creek were mapped in detail (1: 6,000) 

as part of an Investigation of the geology of the uranium deposits in 

the central part of the Front Range. A special study of the Tertiary 

igneous rocks was undertaken to classify the rocks and to determine 

the genetic relationships, if any, between the Intrusive rocks and 

uranium mineralization. Although the geologic mapping was done 

largely by the field members working In the respective districts, the 

author assisted in identification of the Tertiary rocks and developed 

the rock classification used in this report. Also, at places he studied 

critical areas of outcrop. 

The map of the Tertiary igneous sequence (pl. 19) presented with 

this report was compiled by the author from the geologic maps of 

the Central City (P. K. Sims and others, written communicatiOn, 

1956), Idaho Springs (R. H. Moench and A. A. Drake, written 

communication, 1955), Freeland-Lamartine (Harrison and Wells, 

1956), Chicago Creek (Harrison and Wells, 1959), and LawsonDumont-Fall River districts (C. C. Hawley, and F. B. Moore, written 

communication, 1955). As the mapping was done in much greater 

detail than previously, much new information was accumulated regarding the distribution of the rock types and the structural relations of the rocks. Because the author believes that the rock units 

should be mappable units, the rocks are classified by features that 

can be observed megascopically and microscopiCally. 

The laboratory investigatiOn of the Tertiary igneous rocks consisted principally of petrographic studies. About 430 thin sections 

were studied. In addition, many analyses for chemical and equivalent. uranium were made. George Phair, of the Geological Survey, 

kindly made available to the author thin sections and uranium determinations from rocks collected by him in the Central City dis-
PETROGRAPHY OF TERTIARY ROCKS, FRJONT RANGE, COLO. 227 

trict. Chemical studies of the rocks were beyond the scope of the 

present work. 

The investigation has shown that the Tertiary porphyritic igneous 

rocks can be separated into mappable units that have consistent age 

relatiOns; that during emplacement of the sequence there was a 

progressive change In mode of intrusiOn and form of the bodies from 

irregular plutons to well-defined dikes; that most bodies were intruded along preexisting structural weaknesses In the Precambrian 

crystalline rocks, principally along joints, faults, and fohatwn surfaces; that the rocks possibly were derived from two different magmatic sources; and that the members of each of the two differentiation series show a progressive enrichment In thoriun1 and uranium 

from the more mafic to the more felsic members. 

GEOLOGIC SETTING 

The Tertiary igneous rocks of the area are part of a narrow belt 

containing these rocks that extends from near Boulder to Breck enndge, Colo. The rocks vary widely ill composition and form 

(Lovering and Goddard, 1938, p. 36). Other Similar Tertiary porphyritic igneous rocks are present In the Mosquito and Sawatch 

Ranges, the Cameron Pass area northwest of Estes Park, and the 

Cripple Creek area west of Colorado Springs; basaltic flows, Interbedded with the Denver formation, occur on the eastern flank of the 

Front Range of Colorado. 

The Tertiary intrusive rocks in the Front Range mineral belt 

occur in Precambnan crystalline rocks. In the area of this report, 

as throughout the mineral belt (Lovering and Goddard, 1950, pl. 2) , 

the intrusive rocks are found mostly in terranes containing wellfoliated, folded gneiss and schist. They are much less common in 

the large bodies of relatively massive Precambrian Igneous rocks, 

and where present, they are confined to the margins of these bod1es. 

With the exception of biotite-quartz latite, the porphyries in the 

mapped area (pl. 19) were emplaced before the mineralization that 

yielded the ore depositS of the regwn. Genenilly the dikes and 

metalliferous veins occupy different fractures. 

Thirteen types of porphyptiC intrusive rocks, which can be combined into four groups whose members have similarities in age, 

petrography, and geographic distributiOn, are described ill this report. The four groups of porphyries and the types they include are 

hornblende granodiorite group, which Includes hornblende granodiorite, biotite granodiorite, biotite-quartz monzonite, and biOtitequartz latite porphyries; light-colored granodiorite group, which includes light-colored granodiorite, albite granodiorite, and alkahc 
228 GEOLOGY AND ORE DEPOSITS, COLORADO 

syenite porphyries; quartz monzonite group, which includes quartz 

monzonite, granite, and alaskite porphyries; and bostonite group, 

which includes bostonite, trachytic granite, and quartz bostonite 

porphyries. Within each of the rock types, phases are recognized 

that differ somewhat from the normal phases. Intrusion breccias 

occur in or are spatially associated with light-colored granodiorite 

porphyry and quartz monzonite porphyry bodies in the eastern and 

northern parts of the area. They consist of variable quantities of 

wallrock fragments embedded in an igneous matrix. 

FORM OF INTRUSIVE ROCKS 

The Tertiary intrusive rocks occur as plutons with short radiating 

dikes, as plutons with long dikes extending in a preferred direction, 

as long continuous dikes, and as short dikes whiCh have a common 

trend. The plutons are step-walled irregular oblong igneous bodies 

that range from maximum horizontal dimensions of a few hundred 

feet to a few thousand feet. The dikes range in length from a few 

hundred feet to several miles, and in width from a few Inches to a 

few hundred feet; the average width is about 15 feet. Most of the 

dikes dip more than 60°: generally they dip to the north, but some 

dip south. 

The form of the intrusive bodies is closely related to the rock 

type. With few exceptions the older intrusive rocks occur as plutons, 

whereas the younger intrusive rocks form dikes. The members of 

the granodiOrites groups tend to occur mostly as plutons, the members of the quartz monzonite group occur mostly as small plutons 

and dikes, and the members of the bostonite group mostly form long 

continuous dikes. 

AGE AND SEQUENCE OF INTRUSION 

The porphyritic igneous rocks in the eastern part of the Front 

Range have been shown by Lovering and Goddard (1950, p. 47) to 

be early Tertiary in age. They are slightly older than the metalliferous veins of the regiOn (Sims, 1956) which have been shown by 

lead-uranium ratios in pitchblende to be about 60 million years old 

(Holmes, 1946; Faul, 1954, p. 263). 

The sequence of intrusion of the porphyritic rocks is determined 

mainly by crosscutting relations and faulting relations, but in part 

by structural, textural, and compositional variations. The rocks in 

order of their probable age are given in figure 58. 

The hornblende granodiorite and biotite granodiorite porphyries 

are inferred to be the oldest Tertiary intrusive rocks in the region 

because they occur as small plutons with few associated dikes. This 
PETROGRAPHY OF TERTIARY ROCKS, F~ONT RANGE, COLO. 229 

rlornblende granod1onte 

31ot1te granod1onte 

Light colored granod1onte 

Alb1te granod1onte 

Alkalic syemte 

Quartz monzon1te 

810t1te quartz monzomte 

Gran1te 

Alask1te 

Boston1te 

Trachyt1c gran1te 

Quartz boston1te 

810t1te quartz lat1te 

----------------~T~IM~E--------------------~~ 

---' 

-----' __ K 

__ K 

__ K 

__ K 

___ K 

{ -----' 

K -----' 

{ 

---K ~ 

~ 

K ---1 o ~ __1K - __ ~ K 

Age known from f1eld relat1ons K. age mferred 1n th1s report I 

FIGURE 58 -Diagram showing the probable sequence of intrusion of the Tertiary igneous 

rocks 

inference IS based on the tendency of the earlier intrusive rocks to 

occur as plutons. The hornblende granodiorite porphyry IS thought 

to be older than the biOtite granodiorite porphyry because It has a 

more basic mineral suite. A body of light-colored granodiOrite 

porphyry in the western part of the Gilson Gulch area IS cut by an 

albite granodwnte porphyry dike, and a pluton of albite granodiOrite porphyry on the east side of the Elkhorn pluton IS cut by a 

dike of alkalic syenite porphyry. Quartz monzonite porphyry cuts 

albite granodiorite porphyry in the western part of the Gilson Gulch 

area and cuts alkalic syenite porphyry on Banta Hill. The age relation between granite porphyry and alaskite porphyry is not definitely 

known; but they are thought to be essentially contemporaneous because of similarities in form, mineralogy, texture, and radioactivity. 

The alaskite porphyry is indicated by Lovering and Goddard ( 1950, 

p. 44-47) to be younger than the quartz monzonite porphyry and 

older than the members of the bostonite group. The granite porphyry is cut by biotite-quartz latite near Cottonwood Gulch on 

Chicago Creek. Quartz monzonite porphyry IS cut by biotite-quartz 

monzonite porphyry on the ridge west of Dumont, and both nlonzonites are cut and offset by bostonite porphyry east of Dumont. 

The trachytic granite porphyry IS inferred on field relatiOns and 

petrographic character to be intermedmte in age between the older 

bostonite porphyry and younger quartz bostonite porphyry. South 

of Dumont and in the lower Spring Gulch area of Chicago Creek, 

trachytic granite porphyry both cuts and is cut by quartz bostonite 

porphyry. This suggests that it is in part younger and in part older 
230 GEOLOGY AND ORE DEPOSITS, COLORADO 

than the quartz bostonite porphyry. The quartz bostonite porphyry, 

where It was observed on Quartz Hill, cuts bostonite porphyry. A 

relatively nonporphyritic phase of quartz bostonite porphyry cuts 

and offsets a porphyritic phase of the rock west of Central City, but 

the reverse age relatiOn is shown on the ridge between Chicago Creek 

and Spring Gulch. 

The metalliferous veins throughout the area cut all the porphyritic 

rocks except biotite-quartz latite porphyry, whiCh cuts maJor veins 

in the Stanley, Lincoln, and Donaldson mines west of Idaho Springs. 

Biotite-quartz latite porphyry cuts quartz bostonite porphyry between Cottonwood Gulch and Spring Gulch along Chicago Creek. 

Local minor sulfide veins cut bwtite-quartz latite porphyry in the 

Stanley mine and Dorit mine on Chicago Creek. This IndiCates that 

some mineralizatiOn, probably unimportant economically, occurred 

after the emplacement of the igneous sequence. The intrusive breccias are essentially contemporaneous with the rocks with which they 

are associated; therefore, periods of intrusive brecciation occurred 

during hght-colored granodiOrite porphyry and quartz monzonite 

porphyry Intrusion. 

TEXTURE AND INTERNAL STRUCTURES 

The Tertiary igneous rocks are characteristically porphyritic; but 

a few dikes of bostonite porphyry, bwtite-quartz latite porphyry, 

and alaskite porphyry are nearly equigranular and very fine grained. 

The phenocrysts In the Tertiary rocks are generally less than a 

quarter of an inch long, and they are embedded In a fine-grained 

groundmass. An exceptiOn is the hornblende granodwrite porphyry, 

whiCh has phenocrysts as much as an inch long set in a mediumgrained groundmass The size and number of phenocrysts and the 

groundmass texture vary from dike to dike, but individual dikes are 

similar throughout. Within each group, the younger rocks are finer 

grained and contain fewer phenocrysts than the older rocks of the 

group. 

Various internal structures, flow structures, joints, and chilled 

margins occur in the rocks. Flow structures, shown by the alinement 

of feldspar crystals, can be seen both megascopically and microscopiCally in the younger intrusive rocks and are particularly evident 

in the bostonite group and the biotite-quartz latite porphyry. Closespaced joints are present near the margins of some of the plutons. 

The dikes of the younger intrusive rocks, particularly the biotitequartz latite porphyry and the bostonite group, commonly have 

chilled borders that are as much as an inch thick; but chilled borders 

were not observed in the older rocks. The presence of chilled border 
PETROGRAPHY OF TERTIARY ROCKS, FRONT RANGE, COLO. 231 

phases along the younger dikes indicates that these dikes probably 

were Inti uded into relatively cool country rocks. 

PETROGRAPHY 

The porphyritic igneous rocks of the region can be separated into 

mappable rock units on the basis of color, texture of the groundmass, 

size, shape, and abundance of the phenocrysts, qualitative mineralogy, 

approximate quantitative mineralogy, and character of fractured 

surfaces. 

Although most of the 13 kinds of porphyries are distinct in their 

petrographic characteristics, each has varmtwns from the normal 

or typical rock. These vanations tend to cause gradations from one 

distinctive kind to another, as might be expected from a series of 

closely related intrusive bodies. The alteration partly or completely 

destroys the mafic minerals and feldspars, which are critical for the 

identification of members of the sequence As a consequence of the 

slight gradatiOn between kinds of rock and the Intense alteration of 

many of the rocks, identificatiOn of the atypical varieties may not 

always be correct. The age relations and estimated modes given m 

this report are those of the least altered typical rock and its varieties 

that can be distinguished with a reasonable degree of certainty. 

Because of the fine-grained groundmass and the widespread alteration of the rocks, staining procedures were used to supplement thinsectiOn analysis. Only in the coarsest of the rocks Is an accurate 

modal analysis possible; in the others it is necessary to estimate the 

percentages of the mineral constituents. Staining procedures were 

used to assist in the identificatiOn of the extremely fine grained 

minerals and to distinguish the alkalic feldspars, especially when 

they were altered. With some modification the sodium cobaltinitrite 

stain for potassium (Gabriel and Cox, 1929) and the malachite green 

stain for feldspar (Russell, 1935) were used. Most of the staining 

was by the sodium cobaltinitrite method on rock slabs and thin sections, but commonly both methods were used together. The color of 

hand spemmens was determined by comparison with the Rock color 

Chart of the National Research Council distributed by the Geological 

Society of America. In the section that follows, the very fine 

grained, low-birefringence minerals are referred to as clay minerals; 

fine-grained highly birefringent miCaceous material is called sericite. 

CLASSIFICATION 

The classification of the Tertiary intrusive rocks used in this report 

is essentially the same as that used by Spurr, Garrey, and Ball 

(1908) in the Georgetown quadrangle and later adopted by Bastin 
232 GEOLOGY AND ORE DEPOSITS, COLORADO 

and Hill (1917) in the Central City quadrangle and by Lovering 

and Goddard ( 1950) throughout the Front Range. Some of the 

modifications In termmology proposed by Phair ( 1952) are used by 

the author; in addition, new names are introduced by the author for 

previOusly unrecognized rock types. Although the classification IS 

based on the kind and quantity of minerals found In the rocks, It IS 

often necessary to distmguish rocks by qualitative mineralogy and 

texture rather than by quantitative mineralogy. 

The rock types of the regiOn can be combined into four groups, 

the light-colored granodiorite group, the quartz monzonite group, 

the bostonite group, and the hornblende granodiOrite group and 

according to similarities into groundmass texture, mineral suite, and 

color. Some of these groups have similar ages and geographic distributiOns. The bostonite group has a distinctive trachytic or bostonitic texture, contains abundant alkalic minerals, and has a characteristic reddish-brown to lilac color; the other groups have a 

granular texture and are various shades of gray. The hornblende 

granodiOrite group characteristically contains a greater abundance 

of more calcic plagioclase than the other groups and a suite of 

ferromagnesian minerals, hornblende, and pyroxene or bwtite. ~{embers of this group are concentrated In the lower Fall River area. 

The members of the light-colored granodwnte group contain abundant sodiC minerals and sparse ferromagnesmn minerals, they are 

nearly contemporaneous in age (fig 58), and are restricted mostly 

to the eastern part of the area. The members of the quartz monzmnte group are nearly of equivalent age and contain an abundance 

of potassiC minerals and quartz In the groundmass. 

DESCRIPTION OF ROCKS 

LIGHT-COLORED GRANODIORITE GROUP 

The hght-colored granodiOrite porphyry, albite granodiOrite porphyry, and alkalic syenite porphyry are placed In the light-colored 

granodiOrite group because of Similarity In texture of the granular 

groundmass, the gray color, sparsity of ferromagnesian minerals, 

and abundance of sodiC minerals. The rocks also show, from the 

oldest to the youngest, a gradual increase In their content of sodiC 

minerals. 

The rock types in the group are not closely similar in megascopic 

appearance. The distinguishing feature of the light-colored granodiorite porphyry that is seen in hand speCimen is a general 

"dotted" appearance, produced by small, rounded, uniform-sized 

phenocrysts that are embedded In aphanitic gray groundmass. The 

rock is rather similar in appearance to the quartz monzonite por-
PETROGRAPHY OF TERTIARY ROCKS, FRONT RANGE, COLO. 233 

phyry, but the small phenocrysts in the latter are vanable in size 

and form euhedral laths. Albite granodiOrite porphyry IS distinguished by inconspicuous pinkish-gray phenocrysts, about one-fourth 

of an inch long, In a pinkish-gray groundmass flecked with small 

grains of ferromagnesian minerals. Large gray conspicuous commonly diamond-shaped phenocrysts that are bent and show wavy 

reflectiOns are characteristic of the alkahc syenite porphyry. 

The microscopic features that distinguish the hght-colored granodiorite porphyry from the hornblende granodiOrite, biOtite granodiOrite, and quartz monzonite porphyries are an abundance of oligoclase and a general absence of ferromagnesran minerals. An abundance of antlperthitic albite Is a characteristic feature of the albite 

granodiOrite porphyry. Abundant albite and very minor quartz 

distinguish the alkahc syenite from other similarly appearing rocks. 

The petrographic characteristics of this group are summarized 1n 

table 1. 

LIGHT-COLORED GRANODIORITE PORPHYRY 

The light-colored granodwnte porphyry occurs as plutons as much 

as 3,000 feet across wrth assocrated dikes In the east-central part of 

the area and as dikes in the southeast and south-central part (pl. 19). 

The Gregory Hill pluton, about half a mile southeast of Central 

City, has several radiating d1kes; the southern part of the pluton 

contains some brecciated wallrock. An Irregular crescent-shaped 

body crops out southwest of the Gregory Hill pluton near the intersection of Russell and Leavenworth Gulches; a dike extends eastward from the pluton. Several small bodres and dikes of the rock 

are present on the south side of Russell Gulch. Farther south on 

Banta Hill, another pluton, wluch has several radiating drkes, is cut 

by branching bostonrte drkes. The northeast proJectiOn of the pluton 

is free from wallrock contamrnatwn, but the remainder contains two 

large mappable blocks and many breccia fragments of the country 

rock. Short dikes trendrng east and northwest occur west of the 

Banta Hill pluton. Thin branclung dikes of hght-colored granodiorite porphyry are present about three-fourths of a mile south of 

the Banta Hill pluton In the headwaters area of Gilson Gulch. A 

nearly eastward trending dike that is cut by a quartz monzonite 

porphyry dike is on the east slope of the headwaters of Gilson Gulch. 

Short, nearly eastward trending dikes occur about 1.5 miles northwest of Idaho Springs near the junction of Trail Creek and Clear 

Creek and about 2 miles southwest of Idaho Springs near the head 

of Cottonwood Gulch. 
TABLE I.-Petrographic charactenst1cs of the Tertiary Igneous rocks 

[X indicates mmeral is present but vanety not determmedJ 

Phenocrysts Ground mass Rock name 

I I I 

minerals Remarks 

Potassic feldspar Plagioclase Quartz Ferromagnesian mmeral 

L~ght-rolored granodJonte group (granular groundmass, sparse ferromagnes.an mmeraJs, generally brownish gray, ant&perthJt&c albite c:ommon) 

Light-colored granodiorite Mmor anorthoclase. Oligoclase. __________ -------------------- Sparse bladed mmeraL Same as phenocrysts plus Small rounded phenocrysts porphyry quartz give "dotted" appear· ance Albite granodiorite Antiperthite •• ------ Albite and ohgo- -------------------- Aegrrme-augite ________ Same as phenocrysts plus One body is garnetiferous porphyry clase quartz Alkalic syenite porphyry ____ Antiperthite.------- Albite and oligo- -------------------- Sparseoladed mmeraL Same as phenocrysts plus Less than 5 percent quartz clase very minor quartz Large albite phenocrysts are diamond shaped 

------------ - - -- ·- ----- - - ----~--------- ------~------~ 

Quartz monzomte group (granular groundmass, generally sparse but locally mmor ferromagnes1an content, gray, lugh quartz content m groundmass) 

Quartz monzonite porphyry_ Sparse sanidme •• ___ Oligoclase, sparse ... ------------------- Sparse to abundant Quartz, potassic feldspar, SiZe and quantity of pheno- albite bladed •mmeral and plagioclase cry~ts variable 

Granite porphyry----------- X Albite-oligoclase _____ ---.. ---------------... Sparse bladed mmeraL Same as phenocrysts plus Usually conchoidal por· 

quartz. celaneous or funty frac· ture Alaskite porphyry __ .------- X --- __________ ... -------- In one variety _____ Essentially absent _____ Same as phenocrysts plus Conchoidal porcelaneous or quartz flinty fracture, pitted weathered surface 

-- - - - T - -

t-:) 

Cl-j 

~ 

0 

t:;1 

0 

t:""l 

0 

0 

~ 

~ 

0 "' 

::0 

t:<.1 

t:l 

t_:l:j 

t,;j 

0 

m 

H 

~ 

... r:tJ 

c 

0 

t:""l 

0 

::0 

£; 

0 
01 

01 

~ 

(0 

00 

i 

f 

~ 

Bostonlte porphyry---------

Oametiferous bostonlte por- phyry 

Tr .lChytic granite porphyry_ 

Quartz bostonlte porphyry __ 

Hornblende granodiorite porphyry 

B10t1te granodiorite porphyry 

Bwtite-quartz monzonite 

porphyry 

Biot1te-quartz latite 

porphyry 

Bostomte group (bostombc groundmass, sparse- ferromagnes1an mmerals, general reddiSh brown to hlac) 

X Albite-oligoclase _____ ----------... --------- Bladed mmeraL ______ Same as phenocrysts plus mmor quartz 

X Albite-oligoclase _____ -------------------- Aegirine-augite ________ Same as phenocrysts plus minor quartz 

X Albite-oligoclase _____ -------------------... Bladed mmeraL ______ Same as phenocrysts plus anhedral grams between 

feldspar laths Anorthoclase ________ ---------------------. -------------------- ------------------------ Same as phenocrysts plus anhedral quartz grains between feldspar laths 

Hornblende gr:modJOnte (granular grounclmaEs, ferromagnesmn mmerals common, generally gray to black) 

Mmor --------------- Oligoclase and some X Hornblende and mi- Same minerals as pbeno- albite nor aug1te crysts Mmor _______________ Oligoclase and some X B10t1te m flakes _______ Same as phenocrysts ______ alb1te 

X Oligoclase ___________ Many b1pyra- Biot1te books and Mostly quartz and potas- mids flakes sic feldspar 

---------------------- Ollgoclase-andesine __ -------------------- B10tite 

fl.akes 

books and Same as phenocrysts with 

potassic feldspar 

Less than 10 percent quartz 

Contains andraditie gar- net Less than 10 per· cent quartz 

Very fine gramed phase bas 

conchoidal fracture 

"d 

~ l:tl 

0 

0 

l:tl 

> "d 

~ 

0 

l%j 

8 

t;l 

l:tl 

:j 

> l:tl 

~ 

l:tl 

0 

c 

>1 

.... m 

l%j 

!:tl 

0 

z 

8 

0 

~ .... trj 

c 

0 

t'l 

9 

'-"=> Clj 

CJl 
236 GEOLOGY AND ORE DEPOSITS, COLORADO 

The light-colored granodiOrite porphyry contains very light-gray 

feldspar phenocrysts in a hght-gray aphanitic groundmass. In the 

observed hand specimen, the phenocrysts are of two different sizesthe larger euhedral crystals are about half an inch long and the 

smaller ones range from one-sixteenth to one-eighth of an inch in 

length. The smaller phenocrysts are generally equidimensional, 

appear rounded, and are uniformly scattered through the rock givmg 

it a characteristic "dotted" appearance. In the dikes large phenocrysts are rare, and the smaller Sized phenocrysts, being about oneSixteenth of an Inch in diameter are barely VISible. Brown pseudomorphs of an unknown mmeral replace sparse ferromagnesmn minerals. Weathered surfaces are essentially the same color as the freshly 

broken rock. The wallrock fragments, which are locally abundant m 

parts of the plutons, vary widely In Size and angulanty; some 

inclusiOns are mappable, whereas others can be seen only with the 

aid of the microscope. Some of the fragments are sharply angular 

while others are well rounded. The fragments are only slightly 

altered. In the highly contaminated parts of the plutons, the color 

of the rock remains light gray, but the contaminated branching dike 

near the head of Gilson Gulch is brownish gray. The most highly 

contaminated parts of the hght-gray, hght-colored granodiOrite 

porphyry resembles typiCal albite granodiOrite porphyry. 

The texture of the groundmass of the hght-colored granodiorite 

porphyry IS hypidwmorphic granular to pmkihtic granular. Pmkihtic granular texture IS used in this report to describe a texture in 

which Irregular to circular mutually sutured quartz grains contain 

minute laths and irregular grains of feldspar. Quartz occurs as 

uregularly shaped clear anhedral grains and as equidimensional 

grains with minute, generally less than 0 02 mm In diameter, incluSions of anhedral grains and euhedral laths of potassic feldspar and 

plagiOclase. Potassic feldspar and plagioclase also occur as larger 

grains, as much as 0.2 mm In diameter, between the quartz grains. 

The large phenocrysts observable in hand speCimens are euhedral to 

subhedral anorthoclase crystals that are generally unaltered In contrast to the highly altered small phenocrysts and groundmass. The 

name "anorthoclase" IS apphed to the feldspar that IS biaxial (-) 

with a moderate 2V and an index less than balsam; fine twinning 

lamellae are observed in some speCimens and staining shows the 

presence of potassium. The oligoclase, which is highly altered to 

clay, occurs as euhedral to subhedral, mostly equidimensional crystals 

ranging from 0.1 to 4 mm across. The crystals are zoned ond show 

pericline-albite twinning. In some rocks and twinned oligoclase 

crystals have borders of untwinned albite. Small phenocrysts of 

untwinned albite also occur in some samples. A bladed ferromag-
PETROGRAPHY OF TERTIARY ROCKS, FR·ONT RANGE, COLO. 237 

nesian mineral is completely replaced by sericite and magnetite. Accessory minerals of the rock are magnetite-Ilmenite, sphene, apatite, 

and zircon. 

Thin sections of the rock from contaminated parts of the plutons 

show that the phenocrysts have been broken and that the textural 

features characteristic of the uncontaminated rock are partly obscured by the addition of foreign material. In the contaminated, 

branching dikes at the head of Gilson Gulch the overall texture of 

the rock is Similar to the normal texture, but certain features of the 

rock are notably different. The groundmass has an allotrwmorplucgranular instead of a hypidwmorphiC-granular texture and consists 

of anhedral grains of quartz, orthoclase, and plagwclase that range 

In size from 0.05 to 0.2 mm. The untwinned albite grains and albite 

margins of oligoclase grains differ from the mineral In the normal 

rock in containing antiperthite. 

The light-colored granodiOrite porphyry is variable in compositiOn 

mostly because the groundmass of the finer gramed dike rocks contains unusually large quantities of potassic feldspar. The approximate quantities of minerals present are quartz, 20 to 35 percent; 

anorthoclase, 0 to 5 percent; potassic feldspar, 15 to 25 percent; 

oligoclase, 40 to 60 percent; albite, 0 to 10 percent; and a ferromagnesian mineral, 1 to 5 percent; accessory minerals occur In trace 

quantities. 

ALBITE GRANODIORITE PORPHYRY 

Albite granodiorite porphyry is exposed only In the east-central 

part of the mapped area (pl. 19) between the headwaters area of 

Gilson Gulch and Pleasant Valley. Three maJor plutons are exposed 

in this area; they are the Elkhorn pluton to the northeast, the Sun 

and Moon pluton immediately west of the southern part of the Elkhorn pluton, and the Gilson pluton to the southeast. A small body 

crops out southwest of the Sun and Moon pluton. One maJor northeastward-trending dike, as well as several small dikes, is exposed; 

some dikes extend outward from the plutons, whereas others are 

isolated. The Elkhorn pluton IS a compound Igneous Intrusive body 

for It contams Intrusive rocks other than albite granodiOrite 

porphyry. 

The albite granodiorite porphyry is a pinkish-gray seriate porphyritic rock with generally inconspicuous phenocrysts. The feldspar 

phenocrysts, constituting about 40 percent of the rock, are In a 

groundmass that is flecked with dark, mafic minerals In most specimens they are about one-fourth of an inch long, but In some they are 

as much as half an inch long. The granodiorite that constitutes the 

Sun and Moon pluton is unusual because it is brownish gray rather 

than pinkish gray and contains crystals of garnet that appear black 
238 GEOLOGY AND ORE DEPOSITS, COLORADO 

In hand specimen. In typiCal weathered specimens the phenocrysts 

are conspiCuous In the brown groundmass that is colored by limonitiC 

alteration of the ferromagnesian minerals. 

The rock has a hypidwmorphic granular texture. Three feldspars 

are present; they are oligoclase, albite, and potassic feldspar. The 

oligoclase occurs as euhedral to subhedral laths, rangmg in length 

from 0.1 to 6.5 mm, that are zoned and show albite twinning. The 

composition of these crystals range from intermediate oligoclase in 

the center to albite at the margins. The larger crystals contam 

Inclusions of small plagioclase crystals. The albite is untwinned~ 

some occurs as margins of varying width on oligoclase crystals, and 

the remainder occurs as individual zoned crystals ranging in size 

from 0.1 to 3 mm. Although most of the albite forms subhedral to 

anhedral grains, some of it forms euhedral equidimensional crystals. 

The albite contains most of the potassic feldspar in the rock In 

antiperthitic intergrowths. The potassic feldspar forms stringers 

and patches intergrown with albite and rarely with oligoclase; 

locally it occurs as small subhedral to anhedral grains in the groundmass. Anhedral quartz grains, ranging in length from 0.02 to 0.2 

mm, occur In the groundmass and in crosscutting veinlets. Aegirineaugite occurs as subhedral to anhedral laths and grains that range In 

length from 0.1 to 1 mm. Some of the crystals of aegirine-augite 

show color zoning with green on the margins and hght green In the 

center. Some specimens of the rock have scattered biotite flakes. 

The garnet in the Sun and Moon pluton, which is brown and similar 

to the andradite garnet identified in the garnetiferous bostonite, 

forms rounded, subhedral to anhedral grains that range from 0 3 

to 1.5 mm in diameter. Accessory minerals in the albite granodorite 

porphyry are sphene, magnetite-ilmenite, apatite, allanite, and zircon. 

The typical albite granodwrite porphyry is remarkably uniforn~ 

in composition. The approximate range m mineral composition is 

quartz, 5 to 20 percent; albite, 35 to 40 percent; potassic feldspar~ 

10 to 20 percent; oligoclase, 30 to 35 percent; and aegirine-augite,. 

3 to 5 percent; the accessory minerals constitute less than 1 percent 

of the rock. A sample from the Sun and Moon pluton is unusual 

because it contains approximately 48 percent albite, 18 percent 

potassic feldspar, 9 percent oligoclase, 20 percent aegirine-augite, 

and 2 percent garnet. 

ALKALIC SYENITE PORPHYRY 

Alkalic syenite porphyry is exposed in the east-central part of the 

area. It forms a small pluton on Banta Hill, adjacent to the north 

end of the Elkhorn albite granodiorite porphyry plutons, a small 

body about 1,500 feet east of the junction of Russell and Leaven-
PETROGRAPHY OF TERTIARY ROCKS, FRONT RANGE, COLO. 239 

worth Gulches, and small dikes about 1,000 feet southeast of the plug 

on Banta Hill (pl. 19). 

The alkahc syenite porphyry is a hght-brownish-gray rock with 

conspicuous dark-gray phenocrysts. The phenocrysts are entirely 

feldspar, and they constitute from 25 to 50 percent of the rock. They 

range In length from one-eighth to one-half inch, are commonly 

diamond shaped, are usually bent and give a wavy reflection from 

cleavage surfaces, and locally have rims or centers that are altered 

white. Weathered surfaces are light gray, and exposed areas of the 

rock appear pohshed. 

The groundmass of the alkalic syenite porphyry is hypidiOmorphiC 

granular and variable In grain size and 1nineralogy. In the Banta 

Hill body the groundmass grains range from 0.1 to 0.5 mm in 

diameter, whereas In the Russell Gulch body they range from 0.02 

to 0.1 mm. The feldspars In the two bodies vary In compositiOn 

and Size. The porphyry of Banta Hill contains albite and oligoclase, 

"hereas the rock near Russell Gulch contains sodiC oligoclase. The 

albite occurs in euhedral to subhedral crystals that range in length 

from 0.2 to 10 mm and show zoning but no twinning. Ohgoclase 

from the Banta Hill rocks occurs as euhedral to subhedral laths, 0.2 

to 1 mm 1n length, that show periChne-albite twinning. The oligoclase In the Russell Gulch body occurs in euhedral to subhedral 

crystals that range In size from 0.2 to 1.3 mm. Generally the crystals 

are zoned and untwinned, but in a few crystals carlsbad twinning IS 

present, and some of the smaller grains show fine albite twinning. 

All the plagioclase feldspars contain anbperthitic intergrowths of 

potassic feldspar, principally as patches and stringers. A euhedral 

to subhedral ferromagnesian mineral, which forms laths 0.2 to 1 0 

mm long, IS replaced by clay, limonite, magnetite, and quartz. Accessory minerals of the rock are apatite, magnetite, ilmenite, and 

sphene. 

Although the mineralogy of the varwus bodies of alkahc syenite 

porphyry differ in detail, it 1s grossly Similar, as can be seen by 

comparison of modal analyses from the Banta Hill and Russell 

Gulch bodies. The Banta Hill body contains approximately 3 percent quartz, 44 percent albite, 15 percent oligoclase, 32 percent potassic feldspar, 4 percent a ferro magnesian mineral, and trace amounts 

of the accessory minerals. The Russell Gulch body contains about 

2 percent quartz, 68 percent oligoclase, 26 percent potassic feldspar, 

4 percent a ferromagnesian mineral, and trace amounts of the accessory minerals. The total plagioclase and the ratios of sodium to 

calcium of the plagioclase in the two samples are about the same, 

for the mixture of the oligoclase and albite of the Banta Hill sample 

is essentially equivalent to the sodic oligoclase of the Russell Gulch 

sample. 
240 GEOLOGY AND ORE DEPOSITS, COLORADO 

QUARTZ MONZONITE GROUP 

The members of the quartz monzonite group-quartz monzonite 

porphyry, granite porphyry, and alaskite porphyry-are grouped 

together because the feldspars are mostly oligoclase and orthoclase, 

the quartz content IS generally high, the ferromagnesian nuneral 

content is low in most of the speCimens, and the appearance of the 

groundmass is similar. (See table 1 for a summary of the petrographic characteristics of the group ) The groundmass of the 

granite, alaskite, and part o£ the quartz monzonite porphyries has a 

characteristic conchmdal, fhnty to porcelaneous fracture. 

The quartz monzonite porphyry IS characterized by medium to 

large feldspar phenocrysts, from one-mghth to one-hal£ Inch In 

diameter, which are generally euhedrallaths that vary widely within 

individual speCimens. Some of the rock has a reddish-brown groundmass and is difficult to distinguish from bostonite porphyry, except 

under a miCroscope, where the granular groundmass can be seen. 

The granite and alaskite porphyries are nearly Identical In n1egascopic appearance, but they can be separated microscopiCally, for the 

feldspar of the granite IS partly plagioclase. Both rocks fracture 

similarly and contain sparse phenocrysts. 

QUARTZ MONZONITE PORPHYRY 

The quartz monzonite porphyry Is the most abundant, widespread, 

and variable rock In the area. It occurs throughout the area as dikes 

of diverse trends and rarely as small plutons and IS particularly 

abundant In the eastern part o£ the regwn (pl. 19). Although the 

rock on the average has the compositiOn o£ quartz monzonite, It 

ranges in composition from granodiOrite to granite. Two dominant 

phases are recognized: a phase that contains small phenocrysts and 

sparse quantities o£ ferromagnesian minerals and a phase that contains relatively large phenocrysts and several percent of ferromagnesian minerals. Other minor phases that differ slightly in composition, texture, or both are present locally but are not quantitatively 

important. 

Dikes of quartz monzonite porphyry, which trend In several directions, are present throughout the area (pl. 19). The dikes in the 

Lawson-Dumont area vary in length and generally trend eastward; 

those in the northwest part of the area trend northeastward. A 

remarkably long zone o£ dikes, which follow a northwestward-trending fault zone, extends from Fall River southeastward to the south 

slope of Bellevue Mountain, at whiCh point the dikes curve northeast. A large proportion o£ the dikes occur in a belt about a mile 

wide that extends from the viCinity of Alps Mountain, west o£ Idaho 

Springs, to Black Hawk, a distance o£ about 8 miles; Individual 
PETROGRAPHY OF TERTIARY ROCKS, FRONT RANGE, COLO 241 

dikes trend northeast, essentially parallel to the trend of the belt; 

but in detail they vary In trend from north to east, commonly striking either north-northeast or east-northeast. 

Two small plutons of quartz monzonite porphyry occur in the area 

-a crescent-shaped body on the south slope of Bellevue Mountain 

and a body In upper Chase Gulch. The latter consists of two separate plugs that stand out as topographic knobs above a flat surface 

eroded on a breccia that is Interpreted by Sims, Drake, and Tooker 

(written communicatiOn, 1957) to be Intrusive brecCia. The breccia 

consists of subrounded fragments of wallrock, generally ony a few 

Inches in maximum diameter, embedded In a matrix of Igneous 

material. 

Although the rocks included in quartz monzonite porphyry differ 

widely in compositiOn, texture, or both, most can be grouped In t"o 

distinctive phases, each characteristic of a specific geographic area. 

The quartz monzonite porphyry In the long belt extend1ng along the 

eastern part of the area is characterized by small phenocrysts (less 

than one-fourth of an inch long) and sparse ferromagnesmn minerals. The rock In the area north of Clear Creek and 'Yest of Fall 

River and In part of the southeast corner of the area IS characterized 

by large phenocrysts (one-fourth to one-half Inch long) and several 

percent of ferromagnesian minerals. 

The quartz monzonite porphyry typically contains veiy hght gray 

feldspar phenocrysts in a hght-gray aphanitic groundmass. The 

northeastward-trending quartz monzonite porphyry dike that crosses 

the north slope of Maryland 1\1ountain has an unusual pale-red 

groundmass and because of the color resembles the bostonites, wluch 

are so abundant In this area. Certain dikes, hke those near the 

n1outh of Chicago Creek and in the Gilson Gulch area, contain 

inconspicuous phenocrysts. Some of the more mafic dikes are altered 

to chlorite, epidote, and calcite, as well as the more common clay 

minerals and sericite; and these rocks have a dark gray groundmass 

and green phenocrysts. 

The phase of quartz monzonite porphyry that contains large 

phenocrysts is characterized by thin, tabular crystals one-fourth to 

one-half inch long that constitute from about 25 to 50 percent of the 

rock. These rocks generally contain from 5 to 20 percent ferromagnesian mineral which occurs as thin blades generally less than 

one-eighth of an inch long but at places as much as three-eighths 

of an inch long. The phase that is characterized by small phenocrysts is seriate porphyritic with equidimensional crystals, generally 

less than one-fourth of an inch long, that constitute as much as 50 

percent of the rock and a ferromagnesian mineral that constitutes 

generally less than 5 percent. Non porphyritic dikes of the rock occur 
242 GEOLOGY AND ORE DEPOSITS, COLORADO 

near the mouth of Trail Creek and on the east slope of Bellevue 

Mountain. 

The groundmass of the quartz monzonite porphyry as seen under 

the miCroscope is allotriomorphic granular and consists essentially 

of quartz, potassic feldspar, and plagiOclase. The texture varies 

from an intimate mixture of these minerals in anhedral grains less 

than 0.1 mm In size to poikihtiC granular in which the quartz grains 

are from 0.1 to 0.4 mm in length and the feldspar InclusiOns generally less than 0.05 mm. In a group of dikes along the upper part 

of Mill Creek, the feldspar laths In the groundmass are usually large, 

and they are intergrown in a subparallel arrangement to produce a 

trachytic texture. 

The feldspar phenocrysts in the coarse phenocrystic phase are 

oligoclase, whiCh ranges in composition from sodic oligoclase in the 

smaller grains to calcic oligoclase In the larger grains; albite IS also 

present in a few speCimens. The oligoclase crystals are euhedral to 

subhedral plates that range from 0.5 to 10 millimeters in length, are 

zoned, and have carlsbad-perichne-albite twinning. The larger crystals generally show better twinning than the smaller. The albite 

crystals are zoned, untwinned, and about the same size as the oligoclase grains. Pseudomorphs of chlorite, epidote, and magnetite 

replace laths of a ferromagnesmn mineral that ranges in length from 

0 2 to 2 mm. Epidote and clay have replaced much of the feldspar 

In some of the specimens, and calcite is present as a secondary mineral In the groundmass of some of the samples. 

The phenocrysts in the phase characterized by small crystals consist of four kinds of feldspar: oligoclase, orthoclase, albite, and 

sanidine. The name "sanidine" is applied to the feldspar that is 

biaxial with a small 2V and an index much lower than balsam; 

crystals are generally clear in contrast to the clouded orthoclase. 

Euhedral to subhedral laths and aggregates of laths of plagioclase 

0 5 to 5 mm long and ranging in composition from sodiC oligoclase 

to calcic albite are always present in this group of rocks. Twinning 

according to the albite and combined albite-carlsbad laws is commonly present, but perichne twinning is rare. Some of the crystals 

have centers of untwinned oligoclase. The orthoclase forms equidimenswnal, euhedral to subhedral crystals that range in length from 

0 5 to 2 mm and are smaller than the oligoclase. Untwinned albite 

in crystals similar in size and shape to the orthoclase IS present in 

some of the samples. Sanidine is rarely present in this rock. A 

ferromagnesian mineral IS generally present in small quantities. 

Primary accessory minerals are sphene, apatite, topaz, magnetiteilmenite, and zircon. 
PETROGRAPHY OF TERTIARY ROCKS, FRONT RANGE, COLO. 243 

Some specimens of both phases of the rock contain quartz grains 

and aggregates of grains in veinlets. Most of this quartz is probably the result of wallrock contaminatiOn or secondary filling of 

openings. Other samples show obvious InclusiOns of wallrock. 

The quartz monzonite porphyry ranges m composition from 

granite to granodiOrite. The phase characterized by small phenocrysts contains approximately 40 to 50 percent quartz, 25 to 30 percent orthoclase, 20 to 30 percent plagioclase, and less than 5 percent 

a ferromagnesian mineral. The dike with the pale-red groundmass 

contains approximately 18 percent quartz, 52 percent orthoclase, 27 

percent plagwclase, and 2 percent ferromagnesmn mineral. The 

large-phenocryst phase contains approximately 25 percent quartz, 

20 to 30 percent orthoclase, 0 to 10 percent albite, 20 to 40 percent 

plagioclase, and 5 to 20 percent ferromagnesian mineral. 

GRANITE PORPHYRY 

The granite porphyry IS exposed as short, narrow, generally northeastward-trending dikes at road level along Chicago Creek, on the 

slope north of Chicago Creek in the VICinity of Cottonwood Gulch, 

west of Virginia Canyon and half a mile north of Idaho Springs, 

and In the VICinity of Banta Hill (pl. 19). 

Two types of granite porphyry are present in the area. The more 

abundant type has a gray to pinkish-gray aphanitic groundmass and 

sparse pinkish-gray feldspar phenocrysts. The groundmass usually 

has a characteristic conchOidal porcelaneous or fhnty fracture, but 

an irregular granular-appearing fracture also occurs. The phenocrysts are generally less than one-fourth of an inch long and constitute less than 5 percent of the rock. Some specimens have small 

reddish-brown pseudomorphs, which probably formed after ferromagnesian mineral phenocrysts. Weathered surfaces are tan and 

characteristically pitted by the removal of the phenocrysts. Although granite porphyry closely resembles alaskite porphyry In hand 

specimen, the granite porphyry contains fewer feldspar phenocrysts, 

is devoid of quartz phenocrysts, and contains ferromagnesian 

minerals. 

The less abundant type of granite porphyry is a fine-grained lightbrownish-gray rock that has the appearance of aplite. Phenocrysts 

are few and inconspicuous. This type of granite porphyry is exposed 

as a dike on the south side of the group of granite porphyry dikes 

north of Idaho Springs. 

The groundmass texture of the more abundant type is hypidiomorphic granular, and the rock consists essentially of irregular 

g-rains of quartz and subhedral to anhedral grains of alkalic feldspar. 

The phenocrysts are albite-oligoclase, orthoclase, and in some speci-
244 GEOLOGY AND ORE, DEPOSITS, COLORADO 

mens, a ferromagnesian mineral. The albite-oligoclase is euhedral 

albite-pericline-twinned laths that range from 0.4 to 5 mm In length. 

Orthoclase phenocrysts are subhedral to anhedral laths and plates 

that range from 0.5 to 3 rom In length. The orthoclase and plagiOclase commonly have sutured mutual boundaries. Aggregates of 

clay, magnetite, and limonite fill euhedral forms 0.1 to 1 rom long 

that were probably ferromagnesian minerals. Accessory minerals are 

sphene and magnetite-Ilmenite. 

The less abundant fine-grained granite porphyry has a hypidiomorphiC-granular texture and consists of albite-oligoclase, orthoclase, 

quartz, and bwtite. The albite-oligoclase occurs In subhedral to anhedral, partly twinned to untwinned, antiperthitic crystals that range 

In length from 0 2 to 1 5 rom. The twinning present In the plagioclase IS albite or carlsbad. The orthoclase is In subhedral to anhedral 

grains that range from 01 to 1 rom in length. Quartz occurs as 

scattered, bent anhedral grains and aggregates that range from 0.04 

to 0 2 rom in length; most of the crystals in the rock show pronounced cleavage or parting. Accessory minerals are zircon and 

magnetite-Ilmenite. 

Aside from quartz, which is present in greater quantity in the 

aplitiC type, the minerals In the two types of granite porphyry occur 

In about the same proportions. The approximate average composition of the granite porphyry is 20 percent quartz, 45 percent potassic 

feldspar, 34 percent plagioclase, and 1 percent a ferromagnesian 

mineral. A modal analysis of a sample from the aplitic dike shows 

quartz, 43 percent; orthoclase, 34 percent; plagioclase, 22 percent; 

and bwtite, 1 percent. Trace amounts of the accessory minerals are 

present. 

ALASKITE PORPHYRY 

Alaskite porphyry is exposed only in the vicinity of Alps Mountain west of Idaho Springs (pl. 19). A small pluton with radiating 

dikes crops out about half a mile north of Alps Mountain; and a 

short, nearly east-striking dike is present about 1 mile to the northwest. A long, east-northeast-striking dike that has several branches 

is exposed about half a mile south of Alps Mountain. 

Two varieties of alaskite porphyry are recognized. One contains 

numerous' feldspar phenocrysts, and the other contains quartz phenocrysts and sparse feldspar phenocrysts. The variety with the abundant phenocrysts is restricted to the area north of Alps Mountain. 

It has a light-gray, yellowish-gray to pinkish-gray groundmass 

and very hght-gray eqtudimenswnal phenocrysts, generally less than 

one-fourth of an Inch long, which constitute :from 15 to 20 percent 

of the rock. The rock has a characteristic porcelaneous to flinty 

conchmdal :fracture. Weathered surfaces are very light gray and 
PETROGRAPHY OF TERTIARY ROCKS, FRONT RANGE, COLO 245 

are generally pitted by the removal of the feldspar phenocrysts. 

The alaslnte porphyry south of Alps Mountain is similar in general 

appearance to that to the north. The groundmass Is Similar In 

'Composition and texture, and the feldspar phenocrysts have similar 

s1zes and shapes, but they are generally pink and fewer m number, 

'COnstituting fron1 5 to 10 percent of the rock. Clear quartz pheno-

<:-rysts generally less than one-mghth of an Inch long constitute a 

few percent of tlus rock. 

The groundmass IS generally allotriOmorphic granular, but most 

of the specimens from the bodies on the north side of Alps Mountain 

are pOilnhtic granular. The groundmass consists of quartz and 

potassic feldspar In anhedral grains, 0.02 to 0.3 mm In drameter In 

about equal proportiOns. The feldspar IS orthoclase, which occurs 

in euhedral equidimensional crystals 0 5 to 3 mm long, some of 

which show carlsbad twinning. The quartz phenocrysts are anhedral and 0 5 to 2 mm in diameter. Deep embayments in the 

quartz caused by corrosiOn are filled by the groundmass constituents. 

Small quantities of euhedral to subhedral ferromagnesian mineral 

laths, 0.2 to 1 mm long, whiCh have been replaced by quartz and 

magnetite, are present In some of the speCimens. Accessory minerals 

are sphene, apatite, topaz, magnetite-Ilmenite, and zircon. 

The compositiOn of both varieties of the rock appears to be 

smular. The alaskite porphyry contains approximately 40 to 45 

percent quartz, 50 to 60 percent potassic feldspar, as much as 2 

percent ferromagnesian mineral In a few specm1ens, and trace 

amounts of accessory minerals. 

BOSTONITE GROUP 

The bostonite group, consisting of bostmute porphyry with a 

garnehferous variety, trachytiC granite porphyry, and quartz bostonite porphyry, can be distinguished from all other Tertiary Intrusive 

rocks 1n the area by its trachybc or bostonitlc groundmass texture, 

wh1ch IS observable under the microscope. 

The members of tlns group cannot easily be recognized in hand 

specimen, but certain dmgnosbc megascopic features are readily apparent The donunant variety of trachytiC granite porphyry is characterized by distinctive large white subrounded phenocrysts; the 

garnebferous bostonite porphyry can be distlngtnshed by VIsible 

garnets; and the nearly nonporphyntic phase of quartz bostonite 

porphyry is characterized by Its uniformly dark red color. The 

quartz bostonite porphyry Is distinguished microscopiCally from 

the other two members of the group by Its lack of ferromagnesian 

minerals and plagioclase. Bostonite Is distinguished from trachytic 

gran1te by containing generally less than 5 percent quartz. (See 
246 GEOLOGY AND ORE DEPOSITS, COLORADO 

table 1 :for a summary o:f the petrographiC characteristics o:f the 

group.) 

BOSTONITE AND GARNETIFEROUS BOSTONITE PORPHYRY 

Bostonite and garnetlferous bostonite porphyry are distributed 

over most o:f the eastern part o:f the mapped area (pl. 19). Bos· 

tonite porphyry is evenly distributed In the area between Central 

City and Idaho Springs, and d1kes o:f the rock extend southwestward to the south slope o:f Alps Mountain. Other dikes occur 

west and south o:f Dumont. Garneti:ferous bostonite porphyry IS 

present :fron1 west o:f Alps ]\fountain northeastward and northward 

to Spring Gulch, east o:f Dumont. Both varieties o:f bostonites 

generally occur as long, narrow branching dikes, although small 

plutons and short lenticular dikes are common. South o:f Central 

City the dominant trend o:f the dikes Is east and northeast; :from 

north o:f Idaho Springs to Alps Mountain, the trend IS dominantly 

northeast. Northwest o:f Alps Mountain and near Dumont, the dikes 

trend east, northeast, and north. Northwestward-trending dikes are 

rare In the area Most o:f the dikes dip steeply, but those south 

and west o:f Alps Mountan1 dip at angles as low as 33°. 

The two varieties o:f bostonite porphyry, although similar in 

essential characteristics, differ significantly In mineralogy, color, 

and grain size. 

BOSTONITE PORPHYRY 

The bostomte porphyry is a seriate porphyritic rock that has a 

pink aphanitic groundmass, white :feldspar phenocrysts, and generally small phenocrysts o:f a bladed :ferromagnesian mineral. In 

typical specimens the :feldspar phenocrysts constitute 15 to 30 per· 

cent o:f the rock, but in some dikes in the vicinity o:f Alps Mountain they make up less than 5 percent o:f the rock. The phenocrysts 

are generally Individual laths or aggregates less than one-fourth 

o:f an inch long, which are commonly ahned and give flow structure 

to the rock. The :feldspar phenocrysts in some o:f the dikes along 

Chicago Creek occur as X-shaped aggregates, as much as half an 

Inch long, that have been altered to a gray-green clay bordered by 

white clay. The gray-green clay has been Identified (Harrison and 

Wells, 1959) as illite and the white clay as sericite. Most o:f the 

:ferromagnesian mineral blades have been altered to brown pseudo· 

morphs o:f an unknown mn1eral; these are generally less than onefourth o:f an inch long and make up less than 5 percent o:f the rock. 

Parallel alinement o:f the :ferromagnesian mineral Is common. Weath· 

ered surfaces o:f the rock are tan and pitted by the selective removal o:f the :ferromagnesian mineral. 

The texture o:f the groundmass, as seen under the microscope, is 

bostonitic-the :feldspar laths are interlocking and have an imper-
PETROGRAPHY OF TERTIARY ROCKS, FR<QNT RANGE, COLO. 24 7 

feet parallel orientation. The groundmass consists of subhedral 

to anhedral albite laths, anhedral grains of potassic feldspar, and 

anhedral grains of quartz. The grains of the groundmass generally 

range In size from 0.02 to 0.01 mm but In some specimens they may 

be as much as 0.3 mm long. The quartz and most of the potash 

feldspar in the rock are restricted to the groundmass. The phenocrysts are plagioclase, orthoclase, and a ferromagnesian mineral. 

The plaglioclase phenocrysts occur as euhedral to subhedral laths 

and aggregates of laths that range from 0.5 to 3 mm in length and 

whiCh are compositiOnally zoned. They contain oligoclase in the 

center and calcic albite on the margins; typically the plagiOclase 

shows combined periChne-albite twinning. The orthoclase phenocrysts are similar in size but more nearly equidimensional than the 

plagiOclase, and generally they are untwinned. The ferromagne5Jan phenocrysts, which form single blades and aggregates, ranging 

from 0 4 to 2 mm in length, are entirely altered and replaced by 

quartz, magnetite, and serpentine in most specimens. The ferromagnesian mineral is probably aegirine-augite, for this mineral 

has been identified from a nearly unaltered specimen of garnetiferous bostonite porphyry. The accessory minerals are sphene, apatlte, and zircon 

The mineralogiC composition of the bostonite is variable; the 

approximate estimated range In mineral compositiOn is 1 to 10 

percent quartz, 50 to 70 percent potassic feldspar, 20 to 45 percent 

plagioclase, 1 to 10 percent aegirine-augite, and a trace of accessory 

minerals. Although bostonite porphyry, as defined by Phair ( 1952, 

p. 10), should not contain more than 5 percent quartz by volume, a few 

speCimens contam excess quartz in veinlets In small vugs. Other 

f:,peCimens, some of which contain wallrock inclusions, also contain 

excess quartz In the groundmass. Although It IS not definitely known, 

It IS probable that the excess quartz IS of secondary ongm or IS related 

to contaminatiOn from quartz-bearing n1cluswns of wallrock. 

GARNETIFEROUS BOSTONITE PORPHYRY 

The garnetiferous bostonite porphyry is a seriate porphyritic 

rock that has a reddish-brown aphanitic groundmass and contains 

white to pink zoned feldspar phenocrysts, black garnet, and aegirineaugite. The feldspar phenocrysts may be larger than in the nongarnetiferous variety, generally about one-eighth of an inch but as 

much as half an Inch long, and are more abundant and constitute 

about 50 percent of the rock. The aeginne-augite commonly occurs 

as small blades that are altered to hydrous Iron oxides, and the 

garnet occurs as round grains, approximately one-sixteenth of an 

inch In dmmeter, that also are altered to the same oxides. Some 

specimens are altered to shades of dark gray; others are altered to 
248 GEOLOGY AND ORE DEPOSITS, COLORADO 

pale pinkish gray. Weathered specimens of the rock are a darker 

reddish brown than the fresh rock. 

The groundmass of the garnetiferous bostonite porphyry is similar 

to that of the bostonite porphyries. Euhedral to subhedral phenocrysts of compositiOnally zoned oligoclase-albite in the form of 

laths and plates constitute most of the feldspar phenocrysts. Characteristically, the feldspar Is twinned according to the perichnealbite twin laws. The smaller oligoclase-albite phenocrysts-those 

less than 3 mm long-have sharp, straight crystal faces, whereas, 

those that are about 2 em long and whiCh show euhedral growth 

zones have sutured boundaries because each of these larger crystals 

has an overgrowth of potassic feldspar that Is intergrown with the 

feldspar laths of the groundmass. The plagwclase near the margins 

of the larger crystals Is more albitic than the smaller crystals. Tintwinned albite crystals are small equidimenswn subhedral to anhedral phenocrysts. Potassic feldspar usually does not occur as 

individual grains but Instead is an overgrowth of the plagioclase,. 

as descrtbed above. The garnet forms euhedral crystals that range 

from 0 5 to 1.5 mm In diameter. X-ray and index of refraction 

determinatiOns show the garnet to be andradibc (Harrison and 

Wells, 1956). Sphene and aegirine-augite crystals commonly are 

InclusiOns In the garnet. The aeguine-augite also occurs as small 

anhedral grains In the groundmass and as euhedral bladed phenocrysts as much as 1.5 mm long. The accessory minerals are sphene,. 

apatite, and zircon. Specimens from half a mile east of Dumont 

contain carbonate, epidote, and chlorite as alteratiOn products. 

The mineral compositiOn of the garnetiferous bostonite porphyry 

Is similar to that of the bostonite porphyry, but plagwclase is more 

abundant and generally constitutes nearly 45 percent of the rock, 

and quartz Is less abundant. Garnet characteristically makes up 

about 5 percent of the rock by volume. 

TRACHYTIC GRANITE PORPHYRY 

Trachytic granite porphyry IS exposed as a series of dikes in the 

vicinity of Lawson (pl. 19) and along Chicago Creek and lowerSpring Gulch; a single northwestward-striking dike is exposed on 

the north side of Pewabic Mountain. The dike series near Lawson 

consists of a wide northward-trending dike and narrow eastwardand northward-trending dikes. Along Chicago Creek and lower 

Trail Creek, the dikes are of varying length, but they are generally 

long, narrow, northeastward-trending dikes. 

The trachytic granite porphyry is seriate porphyritic and has an 

aphanitic pale-red to reddish-brown groundmass. It contains pink 

to white feldspar phenocrysts and at places small phenocrysts of a 

ferro magnesian mineral. Two phases of this rock can be distin-
PETROGRAPHY OF TERTIARY ROCKS, FRONT RANGE, COLO 249 

guished on the basis of the feldspar phenocrysts whiCh vary in size, 

quantity, and crystal habit. A coarse-grained phase contains rounded 

phenocrysts which are as much as three-fourths of an Inch long 

and which constitute about 25 percent of the rock. A relatively 

fine-grained phase contains both lathhke and subrounded phenocrysts 

that are as much as one-fourth of an Inch long and constitute about 

10 percent of the rock. The rock has an Irregular granular-appearIng fractuie. Upon weathering, the rock becomes either a darker 

or hghter red. 

The trachytic granite porphyry closely resembles the bostonite 

porphyry because of the color of the groundmass and Its general 

appearance, especially the finer grained phase; and the two can 

be distinguished only with the aid of a microscope. The phenocrysts 

of the trachytic granite porphyry resemble those irl bostonite, and 

the groundmass Is characteristic of that In quartz bostonite. 

MICroscopiCally, the groundmass IS hypidiOmorphiC granular, and 

It consists of stubby, euhedral to subhedral plagiOclase laths, subhedral potassic feldspar grains, and anhedral quartz grains that 

fill the Interstices between the feldspar grains. Most speCimens tend 

to have subhedral plagiOclase laths, but speCimens from dikes along 

Chicago Creek show conspicuous euhedral laths. 

The size of the groundmass grmns range from 0 02 to 0.1 mm in 

length; the samples with euhedral plagioclase contain grains as 

large as 0 3 mm Most of the plagiOclase phenocrysts are euhedral 

plates and crystal aggregates, as much as 1.5 mm In length, which 

are compositionally zoned from middle oligoclase in the center to 

albite on the margins. Perichne and albite twinning are generally 

present but poorly formed. Some of the speCimens of the finegrained phase have phenocrysts of zoned, untwinned albite. Phenocrysts of orthoclase occur as plates and crystal aggregates as large 

as 3 mm. Euhedral pseudomorphs, 0 8 to 15 mm long, of quartz, 

magnetite, and clay minerals replace a ferromagnesmn mineral. 

Euhedral gra1ns of sphene are altered to magnetite-Ilmenite and 

leucoxene. Accessory minerals are sphene, magnetite-ilmenite, apatite, and zircon. 

The approximate estimated range of mineral composition of the 

trachytic granite porphyry Is 5 to 15 percent quartz, 20 to 40 percent plagwclase, 45 to 65 percent potassic feldspar, as much as 5 

percent ferromagnesmn mineral, and trace amounts of the accessory 

minerals. The samples that contain the large phenocrysts also contaln the most plagiOclase. 

QUARTZ BOSTONITE PORPHYRY 

The quartz bostonite porphyry occurs as long, thin steeply dippmg, branclung d1kes that locally extend from small plutons or 
250 GEOLOGY AND ORE DEPOSITS, COLORADO 

plutonhke centers. Some of the dikes are several miles in length. 

Near Central City (pl. 19} the dikes strike northwest and northeast 

to form a prominent grid pattern; shorter dikes, which have 

essentially the same strike but do not form a grid pattern, occur 

in the lower Spring Gulch area along Chicago Creek. From lower 

Trail Creek west to the map boundary, the dike direction is predominantly eastward; but some dikes trend north, northeast, and 

northwest. 

The quartz bostonite porphyry has a aphanitic groundmass of 

varwus shades of hlac, and it contains variable quantities o.f phenocrysts, ranging from 0 to about 25 percent. The less porphyritic 

phases have a groundmass that is pale red purple to grayish purple, 

whereas the more porphyritic phases are pale red; with bleaching or 

alteratwn, which are common in these rocks, the color grades to 

vmy hght gray and locally to chalky white. The phenocrysts are 

mostly pink to gray feldspar crystals, generally less than one-fourth 

of an Inch long, whiCh occur as Individuals and irregular aggregates. 

The phenocrysts are commonly altered to gray green or white clay 

minerals Some specimens show small rounded areas of hydrous 

Iron oxides, probably formed by the alteratiOn of magnetite. In 

some specimens the grains in the groundmass are large enough that 

the trachytiC texture can be seen with the aid of a hand lens. 

Microscopically, the groundmass has a bostonitlc texture and consists of quartz and long, slender feldspar laths, probably anorthoclase. The laths are subhedral to anhedral, commonly show carlsbad twinning, and range In length from 0.1 to 0.3 mm; chilled 

margins have smaller grains. The quartz forms anhedral grains 

between and around feldspar laths. In some speCimens, which contanl abundant quartz, the characterjstic texture formed by the interlocking feldspar laths is partly destroyed because of the separation by quartz grains. In smne speCimens minute grains of a black 

opague mineral are scattered through the groundmass. The feldspar 

phenocrysts are euhedral to subhedral crystals and aggregates of 

carlsbad -twinned anorthoclase ( e). The crystals range from 1 to 3 

mm In ·length. Magnetite grains as much as 1 mm in diameter are 

present in some specimens, particularly from the lower Spring 

Gulch area of Chicago Creek. These grains are commonly replaced 

by hydrous iron oxides and quartz. Accessory minerals are apatite, 

zircon, and rarely fluorite and carbonate. 

The quartz bostonite porphyry has an estimated mineralogic composition ranging from 10 to 25 percent quartz, 70 to 80 percent 

anorthoclase ( ~), and 0 to 10 percent magnetite. Trace amounts 

of the accessory minerals are present. 
PETROGRAPHY OF TERTIARY ROCKS, FRONT RANGE, COLO. 251 

HORNBLENDE GRANODIORI~ GROUP 

The members of the hornblende granodiOrite group-hornblende 

granodiorite porphyry, biotite granodiOrite porphyry, biotite-quartz 

monzonite porphyry, and bwtite-quartz latlte porphyry-are classed 

together because they contain biotite or other ferromagnesian minerals, generally contain more calcic plagioclase than any of the other 

intrusive rocks, and are closely associated geographically. 

The hornblende granodiorite and the biotite granodiorite porphyries can be distinguished In hand speCimen by the presence of their 

respective ferromagnesian minerals. The bwtlte-quartz latite and 

bwtite-quartz monzonite porphyries are distinguished from the 

granodiOrites miCroscopically by their lower ratiO of plagiOclase to 

potassic feldspar and megascopically by the presence of quartz 

phenocrysts In the form of bipyramids In the monzonite and the 

presence of small sparse quartz phenocrysts in a very fine-grained 

groundmass in the lahte porphyry. (See table 1 for a summary of 

the petrographic characteristics of the group.) 

HORNBLENDE GRANODIORITE PORPHYRY 

Hornblende granodiOrite porphyry is exposed as an irregular 

pluton about 3,500 feet long and 2,500 feet wide along Fall River 

In the central part of the area (pl. 19). Large bodies of country 

rock are included jn the pluton, but small fragments are not known 

to be present In the body. Most of the pluton is a seriate porphyritic rock that IS gray with black flecks; the rock consists essentially of plagioclase, orthoclase, quartz, and ferromagnesmn minerals. 

Crystals of sphene can be recognized megascopically in some specimens. A few large feldspar phenocrysts about three-fourths of an 

inch long are conspiCuous, but the smaller plagioclase phenocrysts 

are inconspicuous; thus those parts free from the large phenocrysts 

appear to be medium grained. The ferromagnesian minerals are 

scattered through the groundmass with feldspar and quartz. Some 

of the pluton consists of a seriate porphyritic rock that has a 

medium-gray aphanitic groundmass and conspicuous light-gray 

rounded feldspar phenocrysts as much as three-eighths of an inch 

long, which constitute about 40 percent of the rock. The weathered 

surface is light gray. 

Microscopically, the texture of the hornblende granodiorite porphyry is hypidiomorphic granular; the phase that contains smaller, 

more abundant phenocrysts has a fine-grained allotriomorphic granular groundmass. The large, 5 to 15 mm, phenocrysts are untwinned 

unzoned albite. Oligoclase occurs as euhedral to subhedral crystals 

that have sutured boundaries with the fine-grained groundmass and 

range from 0.01 mm to 5 mm in length. Compositional zoning in the 

552984~61-3 
252 GEOLOGY AND ORE DEPOSITS, COLORADO 

plagwclase ranges from calcic ohgoclase in the center to sodic oligoclase at the margins. The crystals generally consists of several 

parts that have a shghtly different orientatiOn. PeriChne and albite 

twinning either alone or In combination is common, whereas combinatiOns with carlsbad twinning are uncommon. Orthoclase occurs 

as subhedral to anhedral grains ranging from 0 01 mm to 2 5 mm 

in length Situated between the larger plagioclase grains. The orthoclase Is perthitic, and some grains show carlsbad twinning. Irregular patches of orthoclase are enclosed by the large albite grains. 

Two ferromagnesian minerals-hornblende and a pyroxene that IS 

probably augite-are present In this rock. The hornblende occurs 

in euhedral to subhedral laths that range from 0.1 to 2 mm In 

length. Some grains have been replaced by fine-grained aggregates 

o£ biotite and magnetite, possibly during the hydrothermal stage, 

for ve1nlets of quartz and grains of pyrite and calcite are assocmted 

with the biotite. The augite occurs as euhedral to subhedral crystals, 

ranging from 0.5 to 2.0 mm in length, associated with the hornblende. Quartz Is in anhedral grains as much as 1 mm across that 

fill the spaces between other grains. Some of the quartz probably 

was Introduced during the hydrothermal stage. 

Accessory minerals are sphene, apatite, topaz, nlagnetlte-Ilmenite, 

allanite, calcite, pyrite, zircon, epidote, and serpentine. The sphene 

()ccurs In unusually large euhedral twinned crystals as much as 15 

mm In length. Calcite and pyrite are not present In all samples 

but occur together with quartz vmnlets. Epidote and serpentine are 

uncommon alteration products of the ferromagnesmn minerals. 

Approximate modal analyses show that the hornblende granodiorite contains approximately 5 to 30 percent quartz, 40 to 75 

percent plagiOclase, 15 to 20 percent orthoclase, 0 to 5 percent 

albite, 3 to 8 percent hornblende, 0 to 3 percent augite, and trace 

amounts of the accessory minerals. 

BIOTITE GRANODIORITE PORPHYRY 

Biotite granodiorite porphyry Is exposed as a pluton about 1,500 

feet long and 600 feet wide and as short dikes between Spr1ng 

Gulch and Fall River In the central part of the area, and a5 a small 

body near the western boundary of the area ( pl 19). 

The biotite granodiorite porphyry Is a hght-greenish-gray seriate 

porphyritic rock with conspicuous black flakes of bwtite. The 

quartz Is in rounded grains as large as one-fourth of an inch in 

diameter. The feldspar occurs as pinkish laths as much as an Inch 

long and as smaller, inconspicuous gray grains. Bwtite is present 

as books and flakes among the other minerals. The weathered 

surface-Is light gray. 
PETROGRAPHY OF TERTIARY ROCKS, FRONT RANGE, COLO. 253 

Microscopically, the texture of the biotite granodiorite porphyry 

is hypidiomorphic granular. The three feldspars present in the 

rock are albite, oligoclase, and orthoclase. The large pinkish phenocrysts seen in hand specimen are untwinned subhedral albite crystals 

that have inclusions of small quartz grains around the margins. 

Anhedral grains of albite, 0.02 mm or less in diameter, occur in 

the groundmass. Oligoclase to sodiC oligoclase occurs as anhedral to 

subhedral crystals and aggregates that show zoning and combined 

carlsbad-albite-pericline twinning. Single crystals range in diameter 

from 0.1 to 5 mm and aggregates range up to 1 em in diameter. 

Orthoclase forms anhedral grains 0.1 to 1 mm across which show 

carlsbad twinning and generally are more altered to clay than the 

other feldspars. Quartz occurs as anhedral rounded grains with 

sutured boundaries and ranges from 0.01 to 3 n1m in diameter. 

The. biotite occurs as subhedral to anhedral crystals with irregular 

boundaries caused by embayment by quartz grains. The biotite 

crystals range in length from 0.5 to 1.5 mm. 

Accessory minerals are sphene, apatite, magnetite-ilmenite, and 

zircon. Much of the sphene has been altered to leucoxene. 

Two modal analyses of samples of biotite granodiorite porphyry 

show the composition to be quartz, 31 and 34 percent; orthoclase, 

8 and 17 percent; albite, 11 and 12 percent; oligoclase, 48 and 34-

percent; biOtite, 3 and 3 percent. 

BIOTITE-QUARTZ MONZONITE PORPHYRY 

Biotite-quartz monzonite porphyry occurs as a single eastwardtrending dike, about 2 miles long, north of Dumont (pl. 19). The 

porphyry has a light-gray aphanitic groundmass; the phenocrysts 

are yellowish-gray feldspar, clear quartz, some of which is bipyramidal, and biotite in books and flakes. The feldspar and quartz phenocrysts are generally less than one-fourth of an inch long and the biotite 

less than one-sixteenth of an inch long. The phenocrysts constitute 

about 30 percent of the rock. The weathered surfaces are very 

hght gray, and the quartz phenocrysts stand out prominently. 

lVhcroscopically, the groundmass is allotriomorphic granular, and 

it consists essentially of quartz and potassic feldspar. Both Ininerals are anhedral and range in diameter from 0.01 to 0.1 mm. 

Carbonate is present between the larger grains as small irregular 

areas in the groundmass The feldspar phenocrysts are euhedral 

laths and aggregates that range from 0 2 to 3 mm in length. Most 

of the crystals have periCline-albite twinning. A considerable 

amount of clay has replaced the oligoclase. The quartz crystals are 

euhedral with some showing embayment of groundmass into the 

quartz; the length ranges from 0.2 to 2 mm. The biotite crystals 
254 GEOLOGY AND ORE DEPOSITS, COLORADO 

are euhedral and range from 0.1 to 1 mm in length; chlorite and 

magnetite have completely replaced most of the grains. Accessory 

minerals are apatite, sphene, magnetite-ilmenite, zircon, and allanite. 

The composition of the biotite-quartz monzonite porphyry is 

appro:x;imately 25 percent quartz, 45 percent potassic feldspar, 25 

percent oligoclase, and 5 percent biotite. Trace amounts of the 

accessory minerals are present. 

BIOTITE-QUARTZ LATITE PORPHYRY 

Exposures of bwtite-quartz latite porphyry are limited to the 

area Immediately west of Idaho Springs along Chicago Creek and 

to the slope a mile southwest of Lawson (pl. 19). All the dikes 

strike northeast, except near Lawson where some trend northwest. 

A series of short dikes occur south of the junctiOn of Trail and 

Clear Creeks, and two longer dikes extend westward nearly parallel 

to Chicago Creek. Biotite-quartz-latite dikes are exposed In underground workings of the Dorit, l{Ing Solomon, Stanley, Lincoln and 

Donaldson mines, west of Idaho Springs; some of these dikes have 

no known surface exposure. 

Two textural phases of biotite-quartz latite porphyry, both of 

whiCh locally show well-developed flow layering espeiCally along the 

Inargms, are recogrnzed. One phase Is very fine grained and contains sparse phenocrysts, and the other is coarser grained and contains relatively abundant phenocrysts. The very fine grained phase 

has a greenish-gray to dark-gray aphanitic groundmass; phenocrysts 

consist of biotite flakes and books and Irregular feldspar crystals 

as much as one-sixteenth of an inch across. The phenocrysts constitute about 5 percent of the rock. An irregular to smooth conchoidal flinty fracture is characteristic of the fine-grained phase. 

This phase weathers to a reddish tan. The fresh coarser grained 

phase has a brown-gray aphanitic groundmass and contains white 

feldspar phenocrysts that constitute from about 20 to 50 percent 

of the rock. The feldspar laths are generally less than one-sixteenth 

of an inch long but may be as much as half an inch long. Biotite 

books and flakes are scattered through the rock. The coarser phase 

weathers to a light gray. Two of the coarser grained dikes, one 

of which shows conspicuous flow layering, have been profoundly 

altered; the color has been changed to yellowish gray; and the bio- , 

tite has been replaced by limonitic material. The dike having a 

conspicuous flow layering crops out near Trail Creek; the other is 

the longest dike along Chicago Creek. 

MICroscopically, the groundmass ranges In texture from allotriomorphic granular in the very fine grained or cryptocrystalline phase 
PETROGRAPHY OF TERTIARY ROCKS, FRONT RANGE, COLO. 255 

to hypidwmorphlC granular in the coarser grained or miCrocrystalline phase. The groundmass consists essentially of oligoclase-andesine, potassic feldspar, and quartz. The cryptocrystalline phase has 

a felty appearance caused by conspicuous crudely alined oligoclaseandesine laths. The grain size ranges from cryptocrystalline to 

0.1 mm. In the groundmass the plagioclase is in euhedral to subhedral laths; the potassic feldspar In subhedral to anhedral grains; 

and the quartz in anhedral grains. The groundmass of the microcrystalline phase ranges in size from 0.02 to 0.1 mm. The plagiOclase grains are equidimensional and not conspicuous ; otherwise 

the groundmass of the two phases Is essentially the same. The 

oligoclase-andesine phenocrysts, ranging In size from 0.3 to 2 mm, 

are euhedral to subhedral laths and plates which have albite twinning occurring alonE:' or in combination With perichne or carlsbad 

twining. The plagiOclase in the coarser or microcrystalline phase 

tends to be more sodic than In the finer grain phase. Subhedral 

biotite laths and books range In Size from 0 3 to 1 mm. In the 

altered specimens the biotite is replaced by hydrous iron oxides, 

magnetite, quartz, and chlorite. In some speCimens a few rounded 

quartz phenocrysts, about 0 5 mm In dmmeter, are present. 

The approximate estimated mineral composition of the biOtitequartz latite porphyry IS 35 percent quartz, 40 percent plagioclase, 

20 percent potassic feldspar, and 5 percent bwtite. Accessory minerals are apatite, magnetite-Ilmenite, sphene, and zircon. 

ALTERATION 

The Tertiary intrusive rocks are altered to varying degrees. Generally the ferromagnesian minerals are completely changed, and the 

feldspars partly changed to secondary minerals, but at places these 

minerals are only partly altered. 

The alteration IS pervasive; and, although the rocks that constitute the walls of vein-fissures generally are more Intensely altered 

than those more distant from veins, all rocks are altered to some 

degree. Generally the feldspars and ferromagnesian minerals are 

changed to clay minerals and sericite, but at places they are 

changed to epidote, calcite, and chlorite. The development of clay 

minerals changes the color of the rocks to gray, whereas the 

saussuritic alteration changes it to a dark greenish gray. 

As the Tertiary igneous rocks In all environments are always 

more altered than the adJacent crystalline rocks, the writer believes 

that the alteration largely resulted from late magmatic or deuteric 

solutions. At places, later hydrothermal solutions intensified the 

alteration in the vein enviromnent. 
256 GEOLOGY AND ORE DEPOSITS, COLORADO 

80~ 

-

70 -

-

60 -

-

50 

40 

-

30 

20 

10 

r nnn.~n-n l 0 I 

0 0.006 1 0.010 0.020 

PERCENT eU PERCENT U 

li'IGURII 59.-Hlstograms showing dlstrlbutlon of radioactivity (equivalent uranium) 

and uranium in all Tertiary Igneous rocks, central part of Front Range mineral 

belt, Colorado. 
PETROGRAPHY OF TERTIARY ROCKS, FRONT RANGE, COLO. 257 

RADIOACTIVITY AND URANIUM IN THE IGNEOUS 

SEQUENCE 

The Tertiary Igneous sequence of the region IS one of the most 

radiOactive groups of Intrusive rocks In the world, according to the 

present literature. All the rocks are more radioactive than the 

average granitic rock of Keevil (Phair, 1952, fig. 2) ; and quartz 

bostonite porphyry, the most radiOactive rock of the sequence, is 

15 to 25 times as radiOactive as the average granitic rock (Phair, 

1952, p. 25). 

The work of Phair (1952) has shown that the radiOactivity of 

the rock results from both thorium and uranium, which are essentially In equilibrium with thmr daughter products, and that these 

metals are pnmary constituents of the rock, occurring In part In the 

mineral zircon. He further has shown (p. 26) that the ratios of 

thorium to uranium In the least altered quartz bostonite porphyry 

averages about 6.4 and seems to be fairly constant from dike to dike. 

The radiOactivity and the uranium content of all Tertiary Intrusive rocks of the regiOn are shown In the histograms on figure 

59. The histograms show the frequency of occurrence and distribu .. 

twn of values of radiOactivity and uranium content as well as the 

arithmetic average of these values for all the Tertiary mtrusive 

rocks, each of the groups, and the individual rock types that constitute the groups. Although a large proportiOn of the samples were 

reported to contain no uranium because the sensitivity limit for 

standard chemical uranium determinations Is 10 parts per milhon, 

many of this group probably contain a few parts per million 

uranium. 

The rocks constituting the igneous sequence contain an average 

of 0.0061 percent equivalent uranium and 0.0018 percent uranium, 

but vary widely from these figures (fig. 59). The values of both 

equivalent uranium and uranium with respect to the averages are 

low In n-Il except the youngest rocks. The average equivalent 

uranium and uran1um content of the hornblende granodiorite (fig. 

60), light-colored granodiorite (fig. 61), and quartz monzonite 

(fig. 62) groups is below the average of all the rocks of the sequence, whereas the averages of the bostonite group (fig. 63) are 

above the average for all rocks. The variatiOns In average values 

for the equivalent uranium and uranium content of each rock type 

IS graphically shown In figure 64. These, arranged according to 

age within each group, show an increase In uranium and equivalent uranium content from the oldest to the youngest members of 

each group. The values for uranium and equivalent uranium are 

generally lower for the hornblende granodiOrite group than for the 

others. 
258 GEOLOGY AND ORE DEPOSITS, COLORADO 

10 

-tL-JLLJ'+r'...L.l.-,..---,...--o-.y.ow ]~_00 --o-.-ro-1o---o-".o...,w ]~~,; oci10 ]....,~<-o.-oo-o-.--o ... ow 

PERCENT eU PERCENT U PERCENT eU PERCENT U 

HORNBLENDE GRANODIORITE GROUP HORNBLENDE GRANODIORITE 

10] 10] lOL lOl 

o~ o Q o o n,."',____,,----., o 0_

0023 o.o1o o0_

000 0.010 o 0_

020 o.o1o o 0_

000 o.o1o 

PERCENT eU PERCENT U PERCENT eU PERCENT U 

BIOTITE GRANODIORITE BIOTITE-QUARTZ MONZONITE 

10 

-~LL~W--r-~-~p I 

0.010 

,, 0 0.0190°·020 

PERCENT eU PERCENT U 

BIOTITE QUARTZ LATITE 

FIGURE 60.-Histograms showing distribution of radioactivity (equivalent uranium) and 

uranium in the hornblende granodiorite group. Blackened areas indicate unusual 

samples discussed in text. 

"] rfl 10 l 10] en . 101 

~10 0.0044 ° 0 0.0017 0010 °~10 0.0042 ° kiO 0.0016 

PERCENT eU PERCENT U . PERCENT eU PERCENT U 

LIGHT-COLORED GRANODIORITE GROUP .LIGHT-COLORED GRANODIORITE 

:~10 1,-'--l-L---,~ ----,0.010 

0.0040 0.0017 

PERCENT eU PERCENT U 

ALBITE GRANODIORITE 

1 o

0

]____LL__Lj__ 

J -9.-n-, 

0 t 0.010 0.0065 

PERCENT eU 

ALKALIC 

101 

0 o \ ' o.o'w 0.0025 

PERCENT U 

SYENITE 

FIGURE 61.-Histograms showing distribution of radioactivity (equivalent uranium) and 

uranium in the light--colored granodiorite group. 
PETROGRAPHY OF TERTIARY ROCKS, FRONT RANGE, COLO. 259 

30 

20 20 20 

10 10 10 10 

0 lr .lk. 0 0 t -'-'--'-T"-'---, 

0 0.010 0 0.0013 0.010 0 0.0037 0.010 0 0.0010 °·010 

PERCENT eU PERCENT U PERCENT eU PERCENT U 

QUARTZ MONZONITE GROUP QUARTZ MONZOf~ITE 

1~10 ]~10 1-~- 1

: &kw 0.0056 0.0016 0.0073 0.0110 0.0025 0.0040 

PERCENT eU PERCENT U PERCENT eU PERCENT l:l 

GRANITE ALASKlTE 

FIGURE 62.-Histograms showing distribution of radioactivity (equivalent uranium) and 

uranium in the quartz monzonite group. Blackened areas indicate unusual samples 

discussed in text. 

The quartz bostonite dikes of Prosser Gulch and Nigger Hill, the 

youngest quartz bostonites in the Central City district, contain unusually large amounts of equivalent uranium and uranium; samples 

from these dikes are shown by blackened parts of the histograms 

in figure 63. Some samples from the youngest members of the 

quartz monzonite and hornblende granodiorite groups have unusually high radioactivity for their respective rock types. Two such 

samples of alaskite porphyry (blackened part of histograms fig. 62), 

one from the small pluton north of and one from the dike south 

o:f Alps Mountain, present respectively one of several taken from 

each of the two bodies. A highly radioactive sample of biotitequartz latite porphyry was taken from the Donaldson mine in the 

lower Trail Creek area (blackened part of histogram fig. 60). 

Although the alaskite porphyry samples may have been contaminated 

by hydrothermal solutions which deposited uranium nearby, the 

biotite-quartz latite sample was taken several thousand :feet from 

known uranium deposits. 

Postconsolidation alteration did not appreciably change the amount 

of equivalent uranium (measured radioactivity) or uranium in the 

rocks nor did it materially affect the ratio of equivalent uranium 
260 GEOLOGY AND ORE DEPOSITS, COLORADO 

40 

30 

20 

10 

0.010 0.0072 

50 

40 

30 

20 

10 

0.020 

30 

20 

10 

0.0021 

0.010 0.0056 

20 

10 

oi o.o1o 0.0011 

PERCENT eU PERCENT U PERCENT eU PERCENT U 

BOSTONITE GROUP BOSTONlTE 

0 0.010 0.0039 0.0062 

10Ul10~10 

O 0 O~LU~WU~~~~u.-, 

0 0.010 0 0.010 0 0.0047 0.0013 

PERCENT eU PERCENT U PERCENT eU PERCENT U 

TRACHYTIC GRANITE QUARTZ BOSTONITE 

FIGURE! 63.-Histograms showing distribution of radioactivity (equivalent uranium) and 

uranium in the bostonite group. Blackened areas indicate unusual samples discussed 

in text. 

to uranium. Except locally in the Central City district (Phair, 

1952, p. 44), there is no apparent correlation between the degree of 

alteration and the equivalent uranium and uranium content of a 

rock. If alteration had materially affected the quantities of the 

radioactive elements in the rocks, the relatively leachable uranium 

should have been preferentially leached; this would change the ratio 

of uranium to equivalent uranium. Such is not the case, for although both the altered and the relatively unaltered phases of the 

rocks have a wide range of radioactivity and of uranium content, 

most of the samples of each rock type have within the limits of 

analytical error approximately the same ratio of equivalent uranium 

to uranium. Those ratios that are erratic show no relation to the 

degree of alteration. 
PETROGRAPHY OF TERTIARY ROCKS, FRONT RANGE, COLO. 261 

LIGHT·COLORED GRANODIORITE QUARTZ MONZONITE 

GROUP GROUP 

0.015 0015 

0.010 0010 

~ 

/ 0.0051 / ..__. 

_....... 

' ' !' 0.005 

' 

-

/ 

Average equivalent uramum 

BOSTONITE 

GROUP 

0015 

0010 

.... 

z 

UJ 

u 

~ 

0.005 ... 

, 

I 

I 

, 

I 

I 

I 

I 

I 

...... 

J 

EXPLANATION 

HORNBLENDE GRANODIORITE 

GROUP 

0015 

0010 

0.005 

~ 

l .. t '0 

~ 

~ .. 2 0 .g J: Iii ii5 

Average uranium 

.. 

., 

~ 

z 

.g 

Iii 

FIGURE 64.-Diagram showing variations of equivalent uranium (radioactivity) and 

uranium with rock type arranged in order of decreasing age within groups. 

DISTRIBUTION 

Although some rock units are present throughout the mapped 

region (pl. 19), others are limited to relatively small, local areas. 

The quartz n1onzonite, granite, bostonite, and quartz bostonite 

porphyries occur throughout the region but are characteristically 

grouped around centers of radiating dikes, intersecting dikes, or 

plutons. Quartz bostonite porphyry and bostonite porphyry form 

conspicuous centers on Nigger Hill, IGngs Flat, and Quartz Ilill 

that result from two or more intersecting dikes. The quartz bostonite porphyry dikes on Pewabic Mountain extend outward from a 

small pluton on the north slope of the mountain; another prominent, 

branching dike of the same rock type extends from a small plutonic 

center south ()f Dumont. Quartz monzonite dikes are locally abundant, as in the area north of Idaho Springs, and generally are 

particularly common near a large pluton of the rock, as on the 

south slope of Bellevue Mountain. Irregularly branching dikes of 

garnetiferous bostonite porphyry, a distinctive phase of the bostonite porphyry, are abundant in the upper Trail Creek area. 

The light-colored granodiorite group, hornblende granodiorite 

group, trachytic granite porphyry, and alaskite porphyry have a 

limited geographic occurrence. The light-colored granodiorite group 

is found only in the eastern part of the area; and the hornblende 

granodiorite porphyry occurs only in the north western part, mainly 

in the lower Fall River area. The trachytie granite porphyry and 
262 GEOLOGY AND ORE DEPOSITS, COLORADO 

alaslnte porphyry are limited to the southwestern part of the region. 

The grouping of the various rock types around centers of intrusiOn probably reflect apices of magma chambers at depth. 

MODE OF INTRUSION 

The Tertiary igneous rocks were intruded largely into preexisting fractures In the Precambrian country rock. Joints, foliation 

surfaces, contacts, faults, and axml planes of folds were particularly favorable sites for emplacement of the bodies. The fractures 

not only controlled the localizatiOn of the Intrusive rocks, but also 

to a marked degree controlled the shape and trend of the bodies. 

'the older intrusive rocks formed plutons, which only locally appear to be related to the structural fabric of the Precambrian rocks, 

whereas the younger intrusive rocks formed dikes, whiCh are dominantly controlled by linear fractures in the Precambrian rocks. Intrusion of all types of bodies was probably dominantly passive, 

with the magma stoping Its way in or rarely being forced into the 

fractures that offered the least resistance to its movemen1. Intrusion breccias are associated with some of the plutons and probably 

resulted from local explosive force. 

At many places throughout the region, it can be demonstrated 

that the igneous bodies are localized by joints, foliation surfaces, 

axial planes of tight folds, faults, and contacts Many dikes trend 

parallel to well-defined joint sets and in detail are seen to occur in 

particular joints A close correlatiOn between dikes and joint sets 

is particularly striking in the Central City district. The Intersecting northwestward-trending and northeastward-trending quartz bostonite porphyry dikes, which form a prominent grid pattern in the 

Central City district (pl. 19), respectively occupy northwestwardstriking and northeastward-striking jmnts. In detail, however, the 

dikes wiilnn each of the dike groups, occur in two or more specific 

j<>int sets (Sims, Osterwald, and Tooker, 1955, p 11; Sims, Drake, 

and Tooker, written communication, 1957). Foliation. surfaces and 

axial planes of tight folds are the dominant structural features 

controlling the localization of dikes in the Chicago Creek area 

(Harrison and Wells, 1959), and probably also the southern part 

of the Idaho Springs district (Moench and Drake, written communication, 1957). In these areas the dikes trend mainly northeastward, parallel to the prevailing structural fabric of the Precambrian country rock On Bellevue Mountain a large body of 

quartz mon?:onite porphyry occurs at the contact between two Precambrian rock units and possibly is a laccolith (pl. 19). Along the 

Chicago Creek dikes follow contacts for short distances. Dikes of 

quartz monzonite porphyry and bostonite porphyry locally occur 
PETROGRAPHY OF TERTIARY ROCKS, FRONT RANGE, COLO. 263 

along the Idaho Spring fault, west of Idaho Springs. Dikes ofbiotite-quartz latite porphyry follow vein fissures along part of 

their length. 

The localization of the igneous plutons by earlier structural features IS not conspicuous, but most of these bodies are elongate parallel to a conspicuous jmnt duectwn. Most plutons In the eastern part 

of the regiOn (pl. 19) are elongated in a northeast directiOn, subparallel to a prominent joint set. Commonly, dikes assoCiated with 

these bodies extend outward from the margins of these bodies In 

the same directiOn. 

Although all the intrusive bodies were largely localized by preexisting planar structural features In the Precambrian bedrock, the 

Influence of these features on the shape of the bodies was more 

pronounced in the younger members of the sequence. The earlier 

intrusive rocks, as the hornblende granodiOrite group, formed moderate-sized irregular-shaped bodies with only rare associated short 

dikes; the slightly younger Intrusive rocks of the quartz monzonite 

group and the biotite-quartz monzonite porphyry formed small Irregular bodies with associated long branching dikes; and the youngest intrusive rocks, the bostonite group and the biotite-quartz latite 

porphyry, almost exclusively formed long, thin continuous dikes. 

Rarely, small bulbous bodies developed along the younger dikes. 

Intrusion breccias consisting of wallrock fragments In a matrix 

of igneous rock are assoCiated with plutons of light-colored granodiorite porphyry and quartz monzonite porphyry in the Central 

City district. The wallrock fragments differ in size, angularity, 

and abundance. Generally, they range from mappable blocks to 

microscopic fragments; they are rounded to angular; and they are 

sparse to abundant in the plutons of partly brecciated light-colored 

granodiorite porphyry and are abundant in the pluton of largely 

brecciated quartz monzonite porphyry in Chase Gulch. Most of 

the larger blocks probably were the result of stoping, but the fragmentation necessary to produce the smaller fragments was most 

likely the result of explosiOn. 

The Patch, on Quartz Hill, is an oval-shaped breccia pipe consisting of intricately fractured, altered, and mineralized wallrock 

without an igneous matrix. Possibly this breccia formed by explosive activity above a "plug" of magma (Sims, Drake, and Tooker, 

written communication, 1957). 

It is inferred from the nature of the breccias, the relation of the 

fragments to the matrix, and the form of the intrusive igneous 

bodies that the breccias were formed at different levels in the 

earth's crust; and by analogy with classical explosion pipes 

(Walker, 1928), the present erosional surfaces of the plutons rep-
264 GEOLOGY AND ORE, DEPOSITS, COLORADO 

resent different levels in an explosion pipe. If it IS assumed that 

all the plutons were formed in a similar manner, the plutons of 

hght-colored granodiorite porphyry with sparse breccia fragments 

represent a level low in the pipe, where most of the space was occupied by magma; the quartz monzonite porphyry pluton would be 

Intermediate, at a position where brecciated wallrock would be 

abundant with only a small quantity of magma; and the Patch 

would represent a level high In the pipe where brecciation was intense above the magma level. As the granodiOrite porphyry plutons, quartz monzonite porphyry plutons, and the Patch have been 

eroded to progressively lesser depths, the Patch is presumed to be 

the youngest of the breccias. 

It is probable that local explosive activity initiated the intrusion 

of the magmas that consolidated to form light-colored granodiorite, 

quartz monzonite, and bostonite groups. The known intrusion breccias are restricted to bodies of hght-colored granodorite porphyry 

and quartz monzonite porphyry; these porphyries were the first 

igneous rocks of their respective groups to be emplaced. By extrapolation the next IntrusiOn breccia would occur with the bostonite 

porphyry, the first intrusive of the bostonite group The Patch may 

be inferred to be assocmted with bostonite porphyry, because the 

Patch is probably the youngest breccia. If this is true, then the 

breccms are associated only with rock types that constituted the 

earhest intrusive body of their respective groups; and the Inference may be made that local exploswn, resulting in intrusiOn breccias, "triggered" the intrusion of each group. 

As the walls of the igneous bodies are nmther displaced nor brecciated, except locally, it appears that the igneous rocks were largely 

intruded passively Into relatively open fractures In the country rock. 

Presumably the walls of the fractures were forced apart by tensional forces of unknown origin that preceded or accompanied the 

magma emplacement. The local exploswns that are inferred to 

have caused the volcanic breccias were probably the result of sudden release of gas pressure concentrated In the apex of a magmatic 

chamber; this is a process s1milar to that suggested by Walker 

(1928, p. 895). The pressure may have been released when the 

magmatic chamber approached so near the surface that the rock 

load was no longer sufficient to confine the pressure or when an In .. 

tersection occurred between the magmatic chamber and some of the 

early Laramide faults described by Sims ( 1956, p 7 43). 

The earher formed bodies were more equidimensional and contained considerable quantities of Inclusions, whereas the later Intrusive bodies were long dikes essentially free from inclusions This 

is interpreted by the author to indicate that the early intrusions 
PETROGRAPHY OF TERTIARY ROCKS, FRONT RANGE, COLO. 265 

were accomplished mainly by stoping with local explosive activity 

along the fractures which controlled the position and shape of the 

pluton. Later, tensiOnal forces opened fractures or tended to open 

incipient fractures. This tensional stress would have allowed the 

magma, under slight pressure, to open and occupy or create and 

occupy fractures. Similar mechanics of dike IntrusiOn have been 

described by Anderson ( 1942, p. 22-28). 

ORIGIN OF THE IGNEOUS ROCKS 

Although the principal purpose of this report is the descriptiOn 

of the field occurrence and petrography of the intrusive rocks, the 

data acquired during field and laboratory studies suggest an hypothesis of differentiation of these rocks that parallels in some 

aspects and differs in other aspects from that proposed by Lovering 

and Goddard (1938). Lovering and Goddard concluded that three 

rock series occur in this area. These are the dwrite-quartz monzonite-alaskite series, an alkalic series of dwrite-syenite-bostonite, 

and a biotite-monzonite-latite series. The first "* * * may be explained as the normal differentiation from mafic to silicic faCies 

* * *"; the second was explained by subtractive differentiation in 

the parent magma chamber to form the syenitic alkalic series; and 

the third was regarded as the "* * * last material to consolidate 

from a deep gabbroic magma in which equilibrium conditions existed essentially throughout consolidatiOn." 

The author infers that the rocks constituting the Tertiary igneous 

sequence in this area were derived from two magmatic sources that 

existed simultaneously. One yielded a "major series" including the 

light-colored granodiorite, quartz monzonite, and bostonite groups 

of rocks, the other yielded a "minor series" includrng the hornblende 

granodiorite group of rocks. The rocks of the major series can 

reasonably be inferred to have been derived from a relatively large, 

broad magmatic chamber, whereas the rocks of the minor series are 

inferred to have been derived from a smaller, more restricted magmatic chamber. · 

These inferences are based on field and petrographic evidence. 

The rocks of the minor series are limited geographically to the 

lower Fall River area, contain relatively large quantities of calcic 

oligoclase, and contain the ferromagnesian minerals hornblende, 

pyroxene, and biotite. The rocks of the major series contain generally abundant albite and potassic feldspars; the only characteristic ferromagnesian mineral identified was aegirine-augite. Natwithstanding the fact that the major magmatic chamber possibly 

had three subchambers or two periods of rejuvenation to yield the 

three groups, the evolution of the grain s1ze of the rocks, shapes of' 
266 GEOLOGY AND ORE' DEPOSITS, -COLORADO 

the bodies, and radioactivity of the two series is parallel. The oldest members of each series occur in medium- to coarse-grained equidimensional plutons while the youngest members occur m very finegrained, long, continuous thin dikes. The radioactivity of thQ 

minor series is generally lower than that of the major series; both 

show systematic variations with age. (See following sectiOn for 

details.) 

Aside from the physical features that prompted the classification of the rocks Into a major and a minor series, the general scheme 

of magmatic differentiation from two separate sources simplifies the 

overall differentiation sequence. If only one source had been present, it would be necessary to account for the culminatiOn of a long 

sequence of intrusion with biotite-quartz latite porphyry (of quartz. 

monzonite composition) immediately following the potassium-rich 

quartz bostonite. Simultaneous differentiation from two separate 

sources of somewhat different compositiOn accounts for the Intrusion of magmas of different composition at nearly the same time~ 

as is exhibited in this area. 

The rocks of the Tertiary igneous sequence were intruded during 

the Laramide uplift; and, as erosion stripped off some of the rock 

load during uplift, confining pressures were reduced and fractures 

were allowed to open more easily. This change in lithostatic pressure is indicated by the gradual change in form of the intrusive 

bodies from the nearly equidimensional granodiorite porphyry plutons to the long narrow dikes of the bostonite group and biOtitequartz latite porphyry. The coarse crystalline texture of the earliest intrusive rock, the hornblende granodiorite, as contrasted to 

that of the later intrusive rocks, particularly the cryptocrystalline 

phase of the biotite-quartz latite, indicates a slow rate of cooling of 

the early magma at a position of considerable depth. These changes 

in texture and shape of the bodies show that the magmas were 

probably intruded into progressively shallower depths of the earth's 

crust. 

DIFFERENTIATION OF URANIUM AND THORIUM 

In consideration of the radioactive properties of the Tertiary 

igneous rocks and the geologic distribution of the uranium depoSition relative to the location of centers of quartz bostonite intrusive bodies, the author believes that uranium and thorium were 

concentrated by magmatic differentiation into the late phases of the 

igneous activity and that uranium was transported to the sites of 

deposition by hydrothermal solutions mostly derived from the quartz 

bostonite porphyry. 
PETROGRAPHY OF TERTIARY ROCKS, FRONT RANGE, COLO. 267 

To Illustrate the reasons for this belief the data shown in figure 

64 is shown in figure 65 with the rock types arranged in their intrusive o1der shown 1n figure 58. The quartz boston1te d1kes of the 

Prosser Gulch and Nigger H1ll (designated PN In figure 65) are 

shown separately because of their unusually high radioactivity and 

uranium content. Individual samples from the Prosser Gulch and 

Nigger Hill dikes are shown as blackened areas in the histograms 

In figures 62 and 63. 

In the 3 groups-hght-colored granodiorite, quartz monzonite, 

and bostonite-that comprise the major magmatic series (fig. 65), 

the radiOactivity and uranium content IS progressively greater in 

each succeeding group; similarly the maximum values are progressively greater in each _succeeding group. The hornblende granodiorite group, which comprises the minor magmatic series, shows a 

similar but smaller rate of Increase in equivalent uranium and uranium. The quartz bostonite dikes of Prosser Gulch and NIgger Hill, 

designated (PN) in figure 65, are the youngest quartz bostonites In 

the Central City district and contain unusually large amounts of 

equivalent uranium and uranium. The high average radioactivity 

and uranium content of these dikes IndiCate further enrichment In 

uranium in the latest rocks of the major magmatic series. 

The consistent progression in radiOactivity and uranium content 

are readily apparent accordrng to the rock classification presented 

here. If these were plotted as one magmatic series, the variations 

would be random; therefore, the plausibility of the existence of the 

two magmatic sources is enhanced. 

The progressive Increase in the radioactive elements in successively 

younger members of each rock group plus the concentration of most 

of these ele1nents In the last members of each group (fig. 65) indicate that uranium and thorium were concentrated during differentiation of the magmatic series within the magmatic chambers. If 

differentiatiOn occurred in the magmatic chambers and the enrichment continued with the uranium entering a residual aqueous phase, 

the magmas forming any of the four groups could have provided a 

source for the uranium-bearing solutions that contributed to the 

uranium deposits of the region. However, suitable sites for deposition of uranium were not available until the vein fissures were 

formed after the consolidation of the quartz bostonites (figs. 58 

and 65) . Therefore, only the magmas of the quartz bostonite porphyry and biotite-quartz latite porphyry may be considered as possible sources of uranium mineralization. 

The bwtite-quartz latite magma could have been a minor contributor of uranium-bearing fluids to the vein fissures that were open 

prior to its intrusion or to those minor fissures that formed after 

55.2984-61-4 
268 

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GEOLOGY AND ORE DEPOSITS, COLORADO 

EXPLANATION 

Average equivalent uranium of the major series including the Light 

colored granodiorite, quartz monzonite, and bostonite groups 

Average uranium of the major series 

Average equivalent uranium of the minor series including the hornblende granodiorite group 

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FIGURE 65.-Diagram showing variation of equivalent uranium (radioactivity) and 

uranium with rock type and time. Rock types are arranged in order of their intrusion, 

and the average value of each type is plotted on the vertical axis. Different members 

of each group are connected by straight lines; members of the major magmatic series 

are connected by dotted lines ; members of the minor magmatic series are connected by 

solid lines. 

.J 
PETROGRAPHY OF TERTIARY ROCKS, FRONT RANGE, COLO. 269 

consolidation. If this magma is a source of uranium, then the uranium deposits In the south-central part of the area may have originated fiom the bwtite-quartz latite porphyry that crops out there. 

The close geographic assoCiation of quartz bostonite with uraniumbearing veins In the NIgger Hill, Quartz Hill, and Lawson areas 

(Phair, 1952; Sims, 1956, p. 750; Sims and Tooker, 1956, p. 109; 

and Sims, Osterwald and Tooker, 1955, p. 18 and pl. 1) suggest 

that the magma that formed the quartz bostonite porphyry was by 

far the most important source of uranium mineralizatiOn. In the 

Central City district (Sims, 1956, p. 745 to 749), uranium deposits 

are closely related to the distributiOn of the quartz bostonite porphyry and are not related to the mineral zoning of the sulfide deposits That uranium is highly concentrated In the quartz bostonite 

porphyry of the Central City district is shown in figure 65 and by 

Phair (1952). A process by which the uranium IS taken into solution by the aqueous phase is suggested by Phair (1952). These 

uranium-bearing solutions would move along fractures and contacts 

to the vein fissures and then to the sites of the uranium deposits. 

LITERATURE CITED 

Andetson, E M, 1942, The dynamics of faultmg London, Oliver & Boyd 

Bastm, E S , and Hill, J M , 1917, Economic geology of Gilpm County and 

adJacent pat ts of Clear Creek and Boulde1 Counties, Colo : U S Geol 

Survey Ptof Paper 94, 379 p 

FauJ, Henry, ed, 1954, Nucleai geology New Yotk, John Wiley & Sons, 414 p 

Gabriel, Alton, and Cox, E P, 1929, A stammg method for the quantitative 

deteimmatwn of ce1 tam 1ock mmeials Am Mmetalogist, v 14, p 29o292 

Harrison, J E, and Wells, J D, 1956, Geology and 01e deposits of the Freeland-Lamat tme distuct, Clear Cteek County, Colo U S Geol Survey Bull 

1032-B, p 33-127 

Harrison, J E, and Wells, J D, 1959, Geology and 01e deposits of the Chicago 

Cteek a1ea, Clear Creek County, Colo US Geol Survey Ptof Paper 319 

Holmes, A1 thur, 1946, The constl uctwn of a geological time-scale Geol Soc 

Glasgow Ttans, v 21, pt 1, p 117-152 

Loveung, T S, and Goddaid, E N, 1938, Latamide 1gneous sequence and diffetentmtwn m the Ftont Range, Colo Geol Soc AmeriCa Bull, v 49, 

p 35-68 

Love1mg, T S , and Godda1 d, E N , 1950, Geology and ore deposits of the 

Ftont Range, Colo US Geol Survey Prof Paper 223, 319 p 

Phan, George, 1952, Radwactl ve Te1 hary porphyries m the Central City dtstuct, Colotado, and thetr beaung upon pttchblende depos1t10n, US Geol 

Stu vey TEI-247, Issued by US AtomiC Energy Comm Tech Inf Set vice, 

Oak Rtdge, Tenn 

Ru::.sell, R D, 1935, Ftequency percentage determmatwns of detrital qua1tz 

and feldspar Jour Sed Petrology, v 5, p 109-114 

Stms, P K , Oste1 wald, F W , and Tookei, E W , 1955, Uramum deposits m 

the Euieka Gulch atea, Cenbal City d1stuct, Gtlpm County, Colo US 

Geol Survey Bull 1032-A, p 1-31 
270 GEOLOGY AND ORE, DEPOSITS, COLORADO 

Sims, P K, and Tooker, E W, 1956, Pitchblende deposits m the Central City 

district and adjoinmg areas, Gilpin and Clear Creek Counties, Colo ~n 

Page, L R , Stocking, H E , and Smith, H B , Contnbutwns to the geology 

of uranium and thormm by the Umted States GeologiCal Survey and 

Atomic Energy Commission for the Umted Natwns InternatiOnal Conference on Peaceful Uses of Atomic Energy, Geneva, Switzerland, 1955 U S 

Geol Survey Prof Paper 300, p 105-111 

Sims, P K , 1956, The paragenesis and structure of pitchblende-bearmg vems 

in the Central City distnct, Colorado Econ Geology, v 51, p 739-756 

Spurr, J E , Garrey, G H , and Ball, S H , 1908, Economic geology of the 

Georgetown quadrangle, Colorado US Geol Survey Prof Paper 63, 422 p 

Walker, R T, 1928, Mmeralized volcamc explosiOn pipes Eng and Mm Jour, 

v. 126, p. 895-898, 939-942, and 976-984 
INDEX 

Page 

Abstract- ___ ------------------------------- 223-224 

Age or intrusive rocks---------------------- 227,228 

Alasklte porphyry, composition______________ 245 

occurrence-------------------------- 244, 261-262 

Petrographic characteristics_______________ 234 

texture. __ -------------------------------- 245 uranium content_ ______________________ 259,268 

varieties ••• ----------------------- ______ 244-245 

Albite granodiorite porphyry. See Granodiorite porphyry, albite 

Alkalic syenite porphyry See Syenite 

porphyry 

Alps Mountain, rocks ______ 24G-241, 244-245,246,259 

Page 

Dikes, alaskite porphyry--------------------- 244 

biotite-quartZ' latite porphyry------------ 254 

biotite-quartz monwnlte porphyry------- 253 

bostonite _- ___ -- __ ------------------------ 246 

chilled borders oL---------------------- 23G-231 

granite porphyry------------------------- 243 

in Gilson Gulch •• ------------------------ 237 

in Prosser Gulch.---------------------- 259,267 

mode or intrusion----------------------- 262-263 

quartz bostonite •• ------------------ 249-250,259 

quartz monwnite.-- _ --------------- _ -- 24G-241 

Dumont, rocks near-------- 228-230,246,248, 253,261 

Elkhorn pluton ____________________________ 229,237 

Banta Hill, rocks--------------- 228-230,243,238-239 Eqmvalent uranium.------------------ 257-261,268 Banta Hill pluton ______________________ 233,238-239 

Bellevue Mountain, rocks ______ 24G-242, 261,262-263 Fall River, rocks _______________ 232,240,241,252,261 

Biotite granodiorite porphyry, composition Fieldwork •• -------------------------------- 226 

texture.-------------------------- 253 Flow structure __ --------------------------- 23o-231 

description and exposures_________________ 252 Front Range mineral belL----------------- 225,227 

petrographic characteristics_______________ 235 

uranium content_ ______________________ 258,268 Gilson Gulch, dikes ____________ 233-234,236,237,241 

Biotite-quartz latite porphyry, composition__ 255 rocks or _________________________ 228-230,233,237 

exposures ••• ------------------------------ 254 Gilson pluton.------------------------------- 237 

petrographic characteristics_______________ 235 Granite porphyry, composition_______________ 244 

texture.-------------------------------- 254-255 exposures------------------------------- 243, 261 

uranium in--------------------- 258,259,267-269 petrographic characteristics--------------- 234 

Biotite-quartz monwnitc, uranium _________ 258,268 texture--------------------------------- 243-244 

Biotite-quartz monwnite porphyry, compos!- trachytlc, composition____________________ 249 

tion. _ ---------------------------- 254 exposures----------------------- 248, 261-262 

exposure and description ________________ 253-254 petrographic characteristics___________ 235 

petrographic characteristics._.------_____ 253 texture.------------------------------ 249 

Bostonite group, description ________________ 245-250 uranium ____________________________ 260,268 

histograms showing radioactivity------- 260,261 uranium ________________________________ 259,268 

Bostonite porphyry, composition------------ 247 Granodiorite group, llght-colored,descnption_ 232-

descrlptlon. ---------------------------- 246-247 239 

gametlferous, composition________________ 248 histograms showing radioactivity _______ 258,261 

description _________________________ 247-248 Granod10nte porphyry, albite, composition___ 238 

occurrences----------------------------- 246,261 albite, exposures.------------------------- 237 

petrographic characteristics_______________ 235 phenocrysts •. ------------------ 234,237-238 

quartz See Quartz, bostonite porphyry texture .• ----------------------------- 238 

texture.-------------------------------- 246-247 uranium content._----------------- 258, 268 

uranium ________________________________ 260,268 biotite, description. ________ 235,252-253,258,268 

Breccias, intrusion 0 r_ ______________________ 263_264 hornblende, description •• __ -------- 235,251-252 

light-colored, composition ••• ------------- 237 

Central City, dikes near ____ 233,246,249-250,259,262 

rocks ___________________________ 228-230,262-263 

Chase Gulch--------------------------------- 241 

Chicago Creek area, dikes.------------------- 241, 

243, 246, 248, 249-250, 253, 262 

rocks.-------------------------- 228-230,249,250 

Clear Creek, dikes near _________________ 233, 241, 254 

Cottonwood Gulch, rocks __________ 228-230,233,243 

phenocrysts.----------------------- 234,236 occurrence ______________________ 233,261-262 

texture •• _------------ __ -------------- 236 

petrographic characteristics--------------- 234 

Gregory Hill pluton •. ------------------------ 233 

Hornblende granodiorite group, description. 235, 

251-255 

histograms showing radioactivity------- 258,261 

271 
272 INDEX 

Page Page 

Hornblende granodionte porphyry, accessory Petrograplue studies, basiS for classification____ 2J1 

minerals-------------------------- 252 characteristics of Igneous rocks _________ 234-235 

compositwn. ____ --------- ______ __ ___ _ ____ 252 scope_---------------------------------- 226-227 

exposures ... __ -------------------- __ 251, 261-262 procedures. ____ -------------------------- 231 

texture .... ----------------------------- 251-252 Pewabic Mountam, dikes near _____________ 248,261 

uranium ... ------------------------- 257,258,268 Plutons ___ 228,233,237,241,244,246,251,252,262-263, 

Idaho Springs, rocks _______________________ 230,233, 

24Q-241, 243, 246, 253, 259, 261, 262-263 

Igneous rocks See Intrusive rocks 

Introduction. _________ --------------------- 224-227 

Intrusion, mode oL ________________________ 262-265 

sequence of.-------------------------_-- 228-230 

Intrusive rocks, alteratwn____________________ 255 

classification .... ------------------------ 231-232 

descnption. ---------------------------- 232-255 

di1Icrentmtlon of uranmm and thonum. 266-269 

distributwn. _ -------------------------- 261-262 

form oL ---------------- ___ --------------- 228 

ongin ______ ------------------------ ____ 265-266 

petrographic charactenstws, table. _____ 234-235 

radioactivity ___ -------- ____ ------------ 257-260 

texture. __ -------------------------------- 230 

Joints._------------------------------------ 23Q-231 

I 

Kings Flat. __ -------------------------------- 261 

J.1awson, dikes near_ ________________________ 248, 254 

Leavenworth Gulch, rocks _____________ 233,238-239 

Literature cited. ___ --- __ ------------------- 269-270 

Prosser Gulch, uranmm deposits ___________ 259,267 

Quartz bostonite porphyiy, composition______ 250 

occurrence.------------------------- 249-250, 261 

phenocrysts. ___ -------------------------- 250 

petrographic charactenstics_______________ 235 

radioactivity oL------- 257, 260, 261, 267,268, 269 

texture. ____ ------------------------------ 250 

Quartz Hill, rocks ______________ 228-230, 261,263,269 

Quartz monwmte group, descnptlon _______ 24Q-245 

histograms showmg radioactivity------- 259,260 

Quartz monwnite porphyry, composition.___ 240 

occurrences----------------------~--- 24G-241, 261 

phases.----------------------------- 240,241-242 

phenocrysts. ___ ------------------------ 241-242 uranium ____________________________ 259, 267,268 

Russell Gulch, rocks __ ---------------------- 233,239 

Spring Gulch, rocks m. 228-230,246,248, 249-250,252 

Sun and Moon pluton. _____________________ 237,238 

Syemte porphyry, alkalic, composition_______ 239 

exposures _______________________________ 238-239 

petrographic characteriStics_______________ 234 

phenocrysts. __ ------------------------- 234, 239 

texture._--------------------------------- 239 uranium ________________________________ 258,268 

Location of the area.----------------------- 224• 225 Tertiary Igneous rocks ______________________ 227-228 

The Patch _________________________________ 263-264 

Mapping_--- ___ -- ___________________________ _ 226 Trachytw gramte porphyry See Granite porphyry, Maryland Mountam ________________________ _ 241 trachytw 

Mill Creek.---------------------------------- 242 Trail Creek. ________ 233,241-242,248,249-250,259,261 

Nigger Hill _________________________ 259, 

261, 

267,269 Uranium, differentiation of _________________ 266-269 

content m rock of the area.-----·--·-··- 257-260 

Origm of Igneous rocks--------------------- 265-266 Virginia Canyon_____________________________ 243 

0