Uranium Deposits in the 

Eureka Gulch Area 

Central City District 

Gilpin County 

Colorado

GEOLOGICAL SURVEY BULLETIN 1032-A

This report concerns work done on behalf 

of the U. S. Atomic Energy Commission 

and is published with the permission of 

the Commission

Uranium Deposits in the 

Eureka Gulch Area 

Central City District 

Gilpin County 

Colorado

By P. K. SIMS, F. W. OSTERWALD, and E. W. TOOKER

GEOLOGY AND ORE DEPOSITS OF CLEAR CREEK, 

GILPIN, AND LARIMER COUNTIES, COLORADO

GEOLOGICAL SURVEY BULLETIN 1032-A

This report concerns work done on behalf 

of the U. S. Atomic Energy Commission 

and is published with the permission of 

the Commission

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON s 1955
UNITED STATES DEPARTMENT OF THE INTERIOR 

Douglas McKay, Secretary

GEOLOGICAL SURVEY 

W. E. Wrather, Director

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

Washington 25, D. G. - Price 50 cents (paper cover)
CONTENTS

Pag«

Purpose and scope of report. __ ________________________________ 1

Previous geologic studies_______________________________________ 2

Field work and acknowledgments _ _____________________________ 4

General geology. ___________ __ ______ __ ____________________ 4

Pre-Cambrian rocks __ __ _ __ _ ________________________ _ _____ 5

Biotite-quartz-plagioclase gneiss. ____________________________ 6

Sillimanitic biotite- quartz gneiss _ __________________________ 6

Lime silicate gneiss___________________________ _________ 6

Cordierite-anthophyllite-garnet gneiss. _______________________ 7

Amphibolite. _______ __ _ ________________ ________ 7

Migmatite____ ___________ _ ____________ _ ____ ___________ 7

Granite gneiss __ _________________________________________ 8

Granite pegmatite____ ______________________ ___________ 8

Tertiary rocks __ __________________________________________ _ 8

Monzonite porphyry________________________ ________ ___ 9

Quartz bostonite______________________.____ ______ 9

Structure of pre-Cambrian rocks. __ 10

Foliation and lineation__________ _ ________ _ __ _ ______ _ ___ 10

Folds. ____________ ___ __________________ __ ___ _.___. 10

Structure of Tertiary dike rocks __ ______ 11

Uranium deposits __ _______ ____ _ _ 12

Mineralogy and grade _ __________ _______ 12

Localization. ______________ ________________ _________ 13

Wall-rock alteration _ ._._ _____ ______ 15

Supergene zone __ _________ _ ______________ ____ _____ 16

Hydrothermal zone__ _______________ _ ______ ______ ___ 17

Relation of metatorbernite to altered host rocks ______ ___________ 17

Origin of pitchblende and metatorbernite- ___ ___ _ _____ 18

Economic evaluation_____ __ _ ______ ____ _____ 19

Descriptions of individual mines ___ ____________ 19

Buckley mine__________ _ ________________ _ _ _ _ _ ___ _ _ ___ 20

Bullion mine«,_____________ _ _ _ ______ _ __ _ __ __ 20

Carroll mine____________ _ _______________ _ ____ __ _ 20

Claire Marie mine_________________ _ _ _ ________ _ ___ ___ 22

Rara Avis mine... _____________ _ ______________ _____ _ 23

R. H. D. claim ________________________________ 24

Two Sisters mine_______ __ 27

Literature cited. ______ 29

Index _ _ ___ ... ________________________________ 31

in
IV GEOLOGY AND ORE DEPOSITS, COLORADO

ILLUSTKATJONS

PLATE 1. Geologic maps of the Eureka Gulch area and the north slope of

Nigger Hill______ _ _______ _ _____________________ In pocket

FIQUEE 1. Index map showing location of Eureka Gulch area, Gilpin

County, Colo_________________-.._____--___---__----__- 3

2. Geologic map and section of McKay shaft, R. H. D. claim _ . 25

TABLES

Page 

TABLE 1. Uranium analyses of selected dump material, Eureka Gulch

area_________________________________________ 14

2, Uranium analyses from McKay shaft workings, R. H. D.

claim______________________________ 26
GEOLOGY AND ORE DEPOSITS OF CLEAR CREEK, 

GILPIN, AND LARIMER COUNTIES, COLORADO

URANIUM DEPOSITS IN THE EUREKA GULCH AREA,

CENTRAL CITY DISTRICT, GILPIN COUNTY,

COLORADO

By P. K. SIMS, F. W. OSTERWALD, and E. W. TOOKER

ABSTRACT

The Eureka, Gulch area of the Central City district, Gilpin County, Colo., was 

mined for ores of gold, silver, copper, lead, and zinc, but there has been little 

mining activity in the area since World War I. Between 1951 and 1953 nine 

radioactive mine dumps were discovered in the area by the U. S. Geological Survey and by prospectors. The importance of the discoveries has not been determined as most of the mines are inaccessible, but the distribution, quantity, and 

grade of the radioactive materials found on the mine dumps indicate that the 

area is worthy of additional exploration as a possible source of uranium ore.

The uranium and other metals are in or adjacent to steeply dipping mesothermal veins of Laramide age that cut pre-Cambrian metasedimentary rocks, granite gneiss, pegmatite, and Laramide intrusive rocks. Pitchblende is present in 

at least four veins, and metatorbernite, which is associated at places with kasolite, is found along two veins for a linear distance of about 700 feet. The pitchblende- and metatorbernite-bearing ores appear to be mutually exclusive and 

seem to occur in different veins. Colloform grains of pitchblende were deposited 

in the veins essentially contemporaneously with pyrite. The pitchblende is 

earlier in the sequence of deposition than galena and sphalerite. The metatorbernite replaces altered biotite-quartz-plagioclase gneiss and altered amphibolite, 

and to a lesser extent forms coatings on fractures in these rocks adjacent to the 

veins; the kasolite fills vugs in highly altered vein material and in altered wall 

rocks. Much of the pitchblende found on the dumps has been partly leached and 

is out of equilibrium. Selected samples of metatorbernite-bearing rock from one 

mine dump contain as much as 6.11 percent uranium.

The pitchblende is a primary vein mineral deposited from uranium-bearing 

hydrothermal solutions. The-metatorbernite probably formed by oxidation, solution, and transportation of uranium from primary pitchblende, but it may be a 

primary mineral deposited directly from fluids of different composition from 

those that deposited pitchblende.

INTRODUCTION

PURPOSE AND SCOPE OF REPORT

Prior to World War I, the Central City district, Gilpin County, 

Colo., was an important domestic source of pitchblende, the black
2 GEOLOGY AND ORE DEPOSITS, COLORADO

oxide of uranium, but since that time no uranium has been produced 

in the district.1 As early as 1871, pitchblende was noticed on the Wood 

mine dump in the Quartz Hill area by Pearce (1898); since 1872, when 

uranium production started, the Quartz Hill area has yielded 324 tons 

of high-grade ore containing 110,575 pounds of IT3O8 . 2

During and after World War II, because of the strategic importance 

of uranium, interest was revived in the Central City and adjacent mining districts, and as a result several new discoveries have been made by 

prospectors and Government agencies. Many of the finds are outside 

of the previously known productive area on Quartz Hill, and some of 

them appear to be of possible commercial importance.

Uranium was first discoverd during 1951 in the Eureka Gulch area, 

which is in the northern part of the Central City district (fig. 1). A 

prospector found the dump of the adit on the Kara Avis millsite to be 

radioactive and submitted a sample of the ore to the Atomic Energy 

Commission. Subsequently, several additional uranium occurrences 

have been found in this area by the U. S. Geological Survey. Although 

pitchblende is the uranium mineral in most of the deposits, several 

veins on the north slope of Nigger Hill (pi. IB) contain green metatorbernite; this mineral was formerly believed to be sparse in the Central City district. Aside from the McKay shaft, on the K. H. D. 

claim, the discoveries of radioactive minerals have been made on the 

dumps of abandoned mines. Subsequent to the discovery of uranium 

minerals on the dumps, private mining interests have started rehabilitation of the Carroll, Two Sisters, K. H. D., and Claire Marie mines, to 

determine the economic potential of the uranium occurrences; in September 1954, only the Carroll mine was accessible for examination. 

The Eureka Gulch area formerly yielded gold, silver, lead, zinc, and 

copper, but mining has been sporadic since World War I.

This report, prepared because of the potential economic importance 

of the new discoveries in the Eureka Gulch area, presents a preliminary account of the geology and economic aspects of the uranium deposits in this part of the district. A more comprehensive report will 

be included in a later publication.

PREVIOUS GEOLOGIC STUDIES

Several reports describing the uranium deposits in the Central City 

district were published before World War I. The most important 

are Pearce (1898), Kickard (1913), Moore and Kithil (1913), Alsdorf 

(1916), Bastin (1916), and Bastin and Hill (1917). The report by 

Bastin and Hill gives a comprehensive summary of the information

1 Since the preparation of this manuscript (1954), a few tons of uranium ore has been 

shipped from the district.

2 Armstrong, F. C., Unpublished report.
URANIUM DEPOSITS OF EUREKA GULCH AREA, GILPIN COUNTY

105" 32' 30" 105"

Maryland 

Mountain

A

Elev. 9201*

FIGURE 1. Index map showing location of Eureka Gulch area, Gilpin County, Colo.

known to that time. Lovering and Goddard (1950) described the 

geology of the entire Colorado Front Range and their report is the 

most authoritative source of information on the ore deposits of this 

vast region.

The results of the work of the various government agencies investigating the uranium resources of the Central City region have 

not for the most part been published. A report on the Wood and 

Calhoun mines (Moore and Butler, 1952) was released, and a report 

on the Quartz Hill area is being prepared for publication by Armstrong (see footnote, p. 2).
4 GEOLOGY AND ORE DEPOSITS, COLORADO

FIELD WORK AND ACKNOWLEDGMENTS

The U. S. Geological Survey's uranium investigations in the Colorado Front Range began in 1944. The work at first was on behalf of 

the Manhattan Engineer District, and later on behalf of the U. S. 

Atomic Energy Commission. In 1952 the Geological Survey began 

a comprehensive geologic study of a 60-square mile area in the Central 

City and Idaho Springs region, largely on behalf of the Division of 

Raw Materials of the U. S. Atomic Energy Commission. As a part 

of this survey, a systematic reconnaissance for radioactivity was made 

in the Eureka Gulch area by A. E. Dearth and A. R. Krueger during 

the 1953 field season; both a ground scintillation counter and a rate 

meter with a 6-inch probe were used. A geologic map of the area 

was prepared by P. K. Sims and E. W. Tooker (pi. 1A).

Most of the mine dumps in the area were checked during the radioactivity survey, and ten were found to contain radioactive materials. 

These were the Buckley shaft, Bullion No. 2 shaft, Carroll shaft, 

Claire Marie shaft, J. P. Whitney shaft, adit on Rara Avis millsite, 

Two Sisters shaft, R. H. D. shaft, St. Anthony shaft, and McKay 

shaft (pi. IB). Aside from the dump on the Rara Avis millsite, none 

of the localities had previously been reported as radioactive. Because of the possible economic interest in the uranium deposits on 

Nigger Hill, a detailed map of the north slope of the hill was made 

during August and September 1953 by F. W. Osterwald, using a 

plane table and alidade (pi. IB). The McKay shaft on the R. H. D. 

claim was mapped and samples were collected by Sims and A. A. 

Drake in September 1953 (fig. 2); the Carroll mine is being studied 

by Sims. E. W. Tooker collected samples of the altered rocks in 

the McKay shaft workings and on the Two Sisters shaft dump; these 

samples have been partially studied by X-ray diffraction and microscopic methods. A. E. Dearth and R. H. Campbell of the Geological 

Survey assisted in the preparation of the plane-table map.

The writers are indebted to several local people, and particularly 

to Henry DeLinde, Claude McKay, Van McKay, Joseph Pughes, W. C. 

Russell, Jr., and Noah Williams for information on inaccessible mines 

in the Eureka Gulch area. Many discussions of problems pertaining 

to the work were held with personnel in the Denver Exploration 

Branch of the U. S. Atomic Energy Commission.

GENERAL GEOLOGY

Most of the region described in this report is on the northwest 

flank of a northeast-trending, broad, upright anticline, which is the 

dominant structural feature in this part of the Central City district 

(pi. 1A). Granite gneiss of pre-Cambrian age, described by Bastin 

and Hill (1917, p. 30-32), constitutes the core of the anticline. It con-
URANIUM DEPOSITS OF EUREKA GULCH AREA, GILPIN COUNTT 5

tains abundant, small, discontinuous, folded layers and lenses of 

metasedimentary rocks and lenticular bodies of granite pegmatite. 

Pre-Cambrian metasedimentary rocks, principally interlayered sillimanitic biotite-quartz gneiss and biotite-quartz-plagioclase gneiss, 

that contain conformable layers of granite pegmatite, flank the 

granitic core on the northwest. The metasediments and granite gneiss 

clip gently away from the crest of the anticline and locally are deformed into broad warps or drag folds.

Intrusive porphyritie monzonite and quartz bostonite dikes, Tertiary in age, cut the pre-Cambrian rocks. Most of the dikes occupy 

northwest- and northeast-trending fractures.

Many metalliferous veins cut all the rock types in the Central City 

district. Most of the veins contain quartz gangue and have been the 

source for the gold, silver, lead, zinc, copper, and uranium that have 

been produced in the district.

PRE-CAMBRIAN BOCKS

The pre-Cambrian rocks in the Eureka Gulch area consist predominantly of biotite-quartz-plagioclase gneiss, sillimanitic biotitequartz gneiss, amphibolite, lime silicate gneiss, cordierite-anthophyllite-garnet gneiss, granite gneiss, and granite pegmatite. All the rocks 

except some of the granite pegmatite have been deformed and recrystallized and have a metamorphic texture. Altered biotite-quartzplagioclase gneiss and amphibolite constitute the host rocks for metatorbernite-bearing uranium deposits on Nigger Hill.

The biotite-quartz-plagioclase gneiss, sillimanitic biotite-quartz 

gneiss, amphibolite, and lime silicate gneiss are believed to be of metasedimentary origin, and are widespread in the Central City district 

and the adjacent region. Bastin and Hill (1917) and Lovering and 

Goddard (1950) included these rocks in the pre-Cambrian Idaho 

Springs formation. In the northwest part of the region (pi. 1A) 

biotite-quartz-plagioclase gneiss is interlayered with sillimanitic biotite-quartz gneiss and accordingly the rocks are mapped as interlayered 

biotite gneisses. The cordierite-anthophyllite-garnet gneiss is probably also of metasedimentary origin, and had not been described previously from this part of the Front Range; so far as known it occurs 

sparsely in the Central City district. The granite gneiss, as mapped 

by Bastin and Hill, is a metamorphic rock of uncertain origin. Studies of this rock indicate that it has an average composition of quartz 

monzonite; accordingly in a later report it will be defined as quartzmonzonite gneiss. In this report, however, the writers follow the 

usage of Bastin and Hill and refer to it as granite gneiss. The granite 

pegmatite has a simple composition and is clearly younger than the 

granite gneiss. It is similar and probably correlative to the granite

344986 55 2
6 GEOLOGY AND ORE DEPOSITS, COLORADO

gneiss and pegmatite that is widely distributed south of the Central 

City district in north-central Clear Creek County (Harrison and 

Wells, 1955).

BIOT1TE-QTJARTZ-PLAGIOCLASE GNEISS

Biotite-quartz-plagioclase gneiss is intimately interlayered with sillimanitic biotite-quartz gneiss in the northwestern part of the Eureka 

Gulch area (pi. 1A), and it forms generally thin, discontinuous layers 

and lenses within the granite gneiss. Nearly all of the rock contains 

conformable, but locally crosscutting, bodies of pegmatite ranging 

from a few inches to several feet thick.

The gneiss is a variable, generally medium-grained rock that consists principally of quartz, oligoclase, and biotite. Within the McKay 

shaft workings the gneiss consists of two principal facies a lightgray uniform gneiss containing a maximum of 15 to 20 percent biotite 

and a dark-gray well-layered gneiss containing as much as 30 percent 

biotite. Individual layers in the mine generally range from 6 inches 

to 3 feet in width. Soft biotite-quartz schist layers, commonly a foot 

or less thick, are present at places in the rock. Some parts of the gneiss 

contain scattered crystals of red garnet.

Altered facies of biotite-quartz-plagioclase gneiss on the dump at 

the Two Sisters shaft contain discrete crystals of metatorbernite, disseminated through the rock or on fracture surfaces.

SILLIMANITIC BIOTITE-QUARTZ GNEISS

Sillimanitic biotite-quartz gneiss is the predominant rock type in 

the northwestern part of the mapped area (pi. 1A), but it contains 

abundant interlayers of biotite-quartz-plagioclase gneiss. It is distinguished from biotite-quartz-plagioclase gneiss by the presence of 

visible white sillimanite and by smaller quantities of plagioclase feldspar. Pegmatite bodies, generally conformable to the structure of the 

gneiss, occur throughout the rock.

Typically, sillimanitic biotite-quartz gneiss is a light-gray to darkgray, medium-grained, granoblastic rock that has a conspicuous gneissic structure because of an excellent preferred orientation of biotite 

and sillimanite. Weathered surfaces commonly are light gray or 

silvery gray to grayish brown. The sillimanite occurs as delicate fibers 

and bunches or in elliptical aggregates intergrown with quartz.

LIME SILICATE GNEISS

Lime silicate gneiss consists of massive to layered rocks that principally contain variable quantities of quartz, garnet, and pyroxene, 

and smaller amounts of hornblende, plagioclase, epidote, apatite, and 

sphene.
URANIUM DEPOSITS OF EUREKA GULCH AREA, GILPIN COUNTY 7

Two irregular zones of lime silicate gneiss crop out on opposite sides 

of the major anticline, close to the trace of the axial plane (pi. 1A). 

Individual layers within each zone are discontinuous and contorted 

and are intimately associated with layers of amphibolite and epidoterich hornblendic rocks.

CORDIERITE-ANTHOPHYLLITE-GARNET GNEISS

Rocks composed principally of cordierite, anthophyllite, garnet, 

cummingtonite, and quartz are present at places within the Central 

City district. A few small bodies that are generally less than 500 feet 

in length have been mapped in the Quartz Hill area; 3 one body, near 

the crest of Nigger Hill, is known in the Eureka Gulch area 

(pi. 1). The gneiss is a medium-gray, generally medium-grained, but 

locally coarse-grained, layered rock of variable composition. Fresh 

surfaces commonly have a greasy, lustrous appearance. Weathered 

surfaces are grayish brown to reddish brown and at places are ribbed. 

One layer is composed almost entirely of coarse, matted or felted 

anthophyllite and cummingtonite crystals. Cordierite layers that are 

an inch or less thick and are parallel to the gneissic structure of the 

rock are common.

AMPHIBOLITE

Small, discrete, lenticular or podlike bodies of dark-gray to nearly 

black, medium-grained, equigranular amphibolite are scattered 

through the granite gneiss and biotite-quartz-plagioclase gneiss. The 

amphibolite is composed of nearly equal parts of oligoclase-andesine 

and hornblende and has as much as 10 percent clinopyroxene; accessory minerals include microcline, apatite, sphene, and epidote.

Several long discontinuous layers of amphibolite are associated with 

lime silicate gneiss and make up a conspicuous stratigraphic marker 

bed near the axis of the major anticline. In the McKay shaft workings (fig. 2), contorted pods of amphibolite that range from about 

1 foot to 7 feet in diameter occur within biotite-quartz-plagioclase 

gneiss. The amphibolite in the mine is largely altered to clay minerals, 

and it is the principal host rock for metatorbernite.

MIGMATITE

Interlayered rocks, which are termed migmatite in this report, consisting of roughly 50 percent biotite-quartz-plagioclase gneiss and 50 

percent granite pegmatite were mapped in the McKay shaft workings 

(fig. 2). In the gneiss the pegmatite forms closely spaced, generally 

conformable layers and lenses that range in thickness from less than 

a foot to about 3 feet. At places the pegmatite cuts across the foliation of the gneiss.

3 Sims, P. K., Drake, A. A., and Moencli, K. H., Unpublished report.
8 GEOLOGY AND ORE DEPOSITS, COLORADO

GRANITE GNEISS

The granite gneiss consists essentially of quartz, plagioclase, microcline, and biotite. The composition varies from granodiorite to quartz 

monzonite, according to Johannsen's classification (Johannsen, 1931, 

p. 143-147). The rock is generally medium grained, and on freshly 

broken surfaces is light gray to dark gray, but generally changes to 

buff when weathered. Characteristically, granite gneiss has a welldefined foliation marked by alternating biotite-rich layers and layers 

rich in quartz and feldspar which are a fraction of an inch thick, and 

by a subparallel arrangement of the tabular and platy minerals. 

Some outcrops of the gneiss, which appear to be massive, show a fair 

to good preferred orientation of biotite on close inspection; the biotife 

is more or less evenly distributed through this rock. Some of the 

granite gneiss contains abundant, closely spaced, relatively continuous 

mafic layers, but at many outcrops the mafic minerals consist only of 

discontinuous, parallel streaks. Discrete lenses, layers, and irregularshaped bodies of metasedimentary rocks, usually only a few tens of 

feet wide and a few hundreds of feet long, are scattered through the 

gneiss at places. Younger granite pegmatite intrudes the gneiss.

Granite gneiss is the predominant rock in the area and crops out in 

the core of the major anticline (pi. 1A). Contacts of the gneiss are 

sharp and conformable to the layering of adjacent rock types.

GRANITE PEGMATITE

The granite pegmatite is generally white and coarse grained; it 

consists predominantly of quartz and feldspar. Magnetite and biotite are conspicuous accessory minerals; magnetite is more abundant 

and widespread.

Pegmatite bodies ranging from a few feet to a few tens of feet in 

width form generally conformable bodies in granite gneiss and the 

biotite gneisses. The pegmatite bodies within granite gneiss are discrete, discontinuous layers along the limbs of folds and small, irregular-shaped masses in the crests of minor folds (pi. 1A). Pegmatites 

in the biotite gneisses are partly discrete bodies, some large enough 

to be mapped (pi. 1A), and partly thin, conformable, discontinuous 

layers along the foliation.

TERTIARY BOOKS

The intrusive dike rocks in the Eureka Gulch area monzonite porphyry and quartz bostonite belong to the Tertiary intrusive sequence 

in the Front Range, which is among the most radioactive igneous 

series in the world.4 The quartz -bostonites of Eureka Gulch contain

'Phair, George, Unpublished report.
URANIUM DEPOSITS OF EUREKA GULCH AREA, GILPIN COUNTY 9

from 0.010 to 0.024 percent equivalent uranium and are 10 to 20 times 

more radioactive than the surrounding pre-Cambrian rocks.

MONZONITE PORPHYRY

Fresh monzonite porphyry is gray, medium grained to fine grained 

and contains abundant feldspar phenocrysts. The rock is altered and 

bleached to gray or white at most places; weathered surfaces where 

feldspar phenocrysts have weathered out are characteristically deeply 

pitted.

A monzonite porphyry dike, generally less than 5 feet thick, crops 

out at places on the south slope of Nigger Hill (pi. 1A). It can be 

traced from the vicinity of the Tom Martin mine (13) southwestward 

to the road in Eureka Gulch. A branching dike, which is exposed 

at places on the south side of Eureka Gulch, probably is correlative. 

Two analyses from the dike gave 0.001 and 0.002 percent chemical 

uranium (Phair, oral communication).

QTJARTZ BOSTONITE

Bostonite, a fine-grained rock resembling trachyte, is widespread 

and conspicuous in the Central City district (Bastin and Hill, 1917, 

p. 52). Fresh bostonite is lilac colored to reddish brown and contains 

abundant to sparse salmon-pink feldspar phenocrysts. Light-gray 

altered and bleached bostonite is common in the district; feldspar 

phenocrysts are changed to soft, green aggregates of sericite, or to a 

chalky, white clay mineral, and the groundmass is bleached to gray 

or white.

Three types of bostonite have been distinguished in the Central 

City district by Phair. These are quartz bostonite porphyry, nonporphyritic quartz bostonite, and syenitic bostonite porphyry; only 

nonporphyritic quartz bostonite is found in the Eureka Gulch area. 

The nonporphyritic quartz bostonite is shown on plate 1 as quartz 

bostonite.

Two long, narrow, branching quartz bostonite dikes traverse the 

Eureka Gulch area (pi. 1A). One dike trends northwestward from 

the reservoir in Central City, about 1 mile south of the area shown on 

plate 1A, to Mammoth Gulch, west of Apex, which is about 8 miles 

north of Central City (Bastin and Hill, 1917, pi. 1). Phair called 

this dike the Nigger Hill dike. Another dike trends northeastward 

from Bald Mountain, across the center of the mapped area; this has 

been called the Prosser Gulch dike.

Most nonporphyritic quartz bostonites are uniformly fine grained, 

but at a few outcrops they contain visible feldspar phenocrysts. A 

faint to pronounced planar flow structure parallels the walls of the 

dikes at most places. Outcrops of the dikes are usually low, rounded
10 GEOLOGY AND ORE DEPOSITS, COLORADO

ridges mantled with angular fragments of bostonite; even where exposures are poor the dikes can be traced by float.

The Nigger Hill dike is younger than the monzonite because it cuts 

and offsets the monzonite porphyry on the southeast side of Nigger 

Hill, about 250 feet northwest of the Iowa mine (18). The apparent 

horizontal displacement is 75 feet, the northeast side having moved 

northwestward relative to the southwest side.

The unbleached parts of the Nigger Hill dike contain an average of 

about 0.022 percent equivalent uranium, generally less than 0.010 percent uranium, and about 0.020 percent thorium (see footnote p. 8); 

one analysis from the Prosser Gulch dike gave 0.018 percent equivalent 

uranium, 0.006 percent uranium, and 0.049 percent thorium. A sample of bleached bostonite from the Nigger Hill dike contained 0.010 

percent equivalent uranium, 0.003 percent uranium, and 0.020 percent 

thorium.

STRUCTURE OF PRE-CAMBRIAN ROCKS

FOLIATION AND I/ENEATION

The pre-Cambrian rocks have a fair to excellent lithologic layering 

and subparallel alinement of tabular and platy feldspar, biotite, and 

hornblende. The foliation in the metasedimentary rocks everywhere 

parallels the layering produced by differences in mineral composition; 

hence, it is assumed that the foliation parallels the original bedding in 

the rocks. The contacts and megascopic gneissic structure of the 

granite pegmatite generally are conformable but at places they cut 

across the gneissic structure of the older rocks.

Subparallel alinement of elongate minerals, streaming, crinkling, 

warps, and minor fold axes is conspicuous in all rocks except pegmatite. 

The most prominent lineation trends about N. 40° E. and plunges 

northeast or southwest at angles of less than 20°. The angle of plunge 

varies considerably, even within individual outcrops, because of cross 

warps described below. This lineation is marked chiefly by mineral 

parallelism, by streaking, and by minor fold axes; it essentially parallels the axis of the major fold in the region.

A second locally conspicuous lineation plunges northwest and is 

marked by warps in the foliation planes. Less commonly the warps 

plunge southeast. Most warps in the mapped area plunge 20°-40° N. 

50°-70° W. The warps vary greatly in size, and the ratio of amplitude 

to wave length is usually greater than 1: 3.

FOLDS

The attitude of the foliation and the outcrop patterns of the various 

rock units indicate that the pre-Cambrian rocks are folded along 

northeast-trending axes (pi. 1A). The predominant fold in this part 

of the Central City district is a large, broad, symmetrical anticline
URANIUM DEPOSITS OF EUREKA GULCH AREA, GILPIN COUNTY 11

that trends about N. 40° E. Locally the fold axis plunges gently either 

northeastward or southwestward because of minor variations in the 

plunge angle. On the flanks of the anticline the rocks generally dip 

less than 45° away from the crest, but at some places the dips are 

steeper. The structures on the flanks of the anticline are principally 

minor open folds with gently dipping limbs. Some minor folds are 

asymmetrical; the axial planes converge upward toward the axes of 

the major anticline. Cross warps plunging northwest on the northwest limb of the anticline and southeast on the southeast limb are 

common and locally abundant. The folds on the flanks of the major 

anticline are essentially parallel to the major fold axis and represent 

drag folds related to the major fold.

STRUCTURE OF TERTIARY DIKE ROCKS

The Tertiary dikes are long and continuous and rarely are over 25 

feet thick. They fill steeply dipping fractures in the older pre-Cambrian rocks and are in different fracture systems than the metalliferous 

veins. Because the dikes are fractured (pi. IB) and offset along 

planes roughly parallel to some of the veins, it is inferred that the 

dikes fill older fracture systems than the metalliferous veins.

Although the general trend of each dike is consistent over long 

distances, local trends deviate considerably from the average. The 

Nigger Hill dike trends about N. 45°-50° W.; it occupies two sets of 

fractures, one of which strikes between IS". 40°-60° W. and the other 

about N. 10° W. Near the I. X. L. mine (25) a branch of the Nigger 

Hill dike trends about N. 50° E. The apparent displacement along 

the dike-filled fractures is small. On Nigger Hill, the Nigger Hill 

dike cuts and offsets a small lens of cordierite-anthophyllite-garnet 

gneiss; the apparent horizontal displacement is about 12 feet, with 

the northeast side relatively displaced to the southeast. The same 

dike cuts the Tertiary monzonite porphyry dike about 500 feet southeast of the cordierite-anthophyllite-garnet gneiss lens; the monzonite 

is displaced about 75 feet with relative northwestward motion of the 

northeast block. The difference in apparent horizontal displacement 

of the lens and monzonite dike probably is due to the difference in 

dip of the two intersected rock bodies.

The average trend of the Prosser Gulch dike is about N. 50° E., but 

at places it occupies fractures that strike N. 80°-90° E.; these are 

probably joints, as there has been little or no offset along the fractures.

The monzonite porphyry dike has many short branches and is more 

irregular in trend than the bostonite dikes. Throughout most of its 

length the monzonite fills fractures trending northeastward, which 

are probably the same as the N. 50° E. set occupied by the Prosser 

Gulch dike, but near Eureka Gulch the monzonite fills eastwardtrending fractures (pi. 1A).
12 GEOLOGY AND ORE DEPOSITS, COLORADO

URANIUM DEPOSITS

The gold, silver, copper, lead, zinc, and uranium deposits of the 

Central City district are found in veins, or less commonly in stockworks, that are believed to have formed at intermediate temperatures. 

These deposits are early Tertiary in age and are genetically related 

to the intrusion of porphyritic rocks (Levering and Goddard, 1950, 

p. 170-191; see also footnote p. 8). Gold and silver accounted for 

most of the dollar value of the ore produced.

Two main types of deposits give the district a zonal arrangement. 

A core of pyrite-quartz veins, about 2 miles in diameter, is surrounded 

by a rather wide outer zone of silver-lead-zinc veins (King and others, 

1953, p. 6). In the zone of overlap between the two zones, transitional 

veins contain gold, silver, copper, zinc, and lead. All the known 

uranium deposits in the Central City district except those in the 

Eureka Gulch area are in the transition zones as outlined by Leonard 

(1952).

The uranium deposits in the Eureka Gulch area are of two types, 

one carrying pitchblende and the other metatorbernite. So far as 

known the two types of ores are mutually exclusive. Pitchblende 

occurs at places along metalliferous veins; it was formed earlier than 

sphalerite, galena, chalcopyrite, and most of the pyrite. The metatorbernite primarily is in altered biotite-quartz-plagioclase gneiss and 

amphibolite wall rocks adjacent to veins in the oxidized zone.

MINERALOGY AND GRADE

Pitchblende, metatorbernite [Cu(UO2) 2 (PO4) 2 *8H2O], and kasolite [Pb(UO2 )SiO4 -H2O] have been identified in the Eureka Gulch 

area. Some of the metatorbernite may contain torbernite [Cu (UO2 ) 2 

(PO4 ) 2 *8-12H2O]. The pitchblende and metatorbernite are sufficiently concentrated locally to be of possible commercial value; 

kasolite has been found only on the Two Sisters mine dump in small 

quantities.

Black, heavy pitchblende with a dull metallic luster was found on 

the dumps of the J. P. Whitney shaft (33), Bullion No. 2 shaft (32), 

Carroll shaft (6), and the adit on the Eara Avis millsite (34). Under 

a reflecting microscope, the pitchblende from the J. P. Whitney 

shaft shows a colloform structure with radiating syneresis cracks. 

The pitchblende is brecciated at places, and is veined by sphalerite, 

galena, pyrite, and chalcopyrite of a later age.

Metatorbernite (identified by W. F. Outerbridge, Denver X-ray 

laboratory, U. S. Geological Survey) was found on the dumps of the 

Two Sisters, E. H. D., McKay, and Claire Marie shafts (pi. IB) and 

underground at the McKay shaft (fig. 2); it is also present at places 

in the upper part of the Carroll mine. Except at the Carroll mine,
URANIUM DEPOSITS OF EUREKA GULCH AREA, GILPIN COUNTY 13

the metatorbernite is disseminated as tiny, apple-green crystals or 

forms fracture coatings in altered biotite-quartz-plagioclase gneiss 

and amphibolite. At the Carroll mine metatorbernite locally coats 

partly leached pitchblende.

Yellowish-brown kasolite occurs in thin mats of closely spaced 

fibers, rosettes, and tabular crystals that coat fractures and partly 

fill vuggy openings in highly altered, porous, hydrous-iron vein 

material and in altered wall rocks.

The grade of selected material from radioactive dumps in the 

Eureka Gulch area is given in table 1; the grade of samples taken 

from the mine workings of the McKay shaft (fig. 2) is given in table 2.

Dump samples of material containing pitchblende differ widely in 

grade (table 1), and in many of them the equivalent-uranium content, 

which is determined by radioactivity analysis, far exceeds the chemical uranium. Sample N16 from the Carroll dump contains 6.4 percent equivalent uranium, but only 0.62 percent chemical uranium; 

and sample N16-C from the same dump contains 10.5 percent equivalent uranium but only 0.095 percent chemical uranium. Eadium and 

other daughter products of uranium, principally radon and radiogenic lead, account for the disequilibrium. Phair and Levine (1953) 

demonstrated that pitchblende from pyritic dumps in the Central City 

district oxidizes rapidly. The uranium is leached by sulfuric 

acid derived from pyrite, and both radium and lead remain in the 

original material in approximately proportional amounts. Accordingly, many of the highly radioactive "hot spots" on pyritic dumps 

in the Central City district are caused by residual concentrations of 

radium and other decay products of uranium. For this reason the 

equivalent-uranium analyses indicate the approximate uranium content of the original material when removed from the vein.

Sample analyses of selected metatorbernite-bearing material from 

the Two Sisters mine dump show as much as 6.11 percent chemical 

uranium (table 1). A grab sample of metatorbernite-bearing rock 

from the 40-foot level of the McKay shaft workings assayed 0.24 

percent chemical uranium, and two other samples assayed more than 

0.1 percent chemical uranium (table 2). Two analyses of radioactive 

material from the Claire Marie dump (table 1) indicate a low uranium 

content.

LOCALIZATION

Pitchblende, associated with gangue quartz and sulfide minerals, 

is a primary vein mineral. Metatorbernite is principally confined to 

altered rocks adjacent to oxidized veins or is sporadically distributed 

within the upper parts of some veins. So far as known, the metatorbernite is associated with gold- and silver-bearing veins of the 

galena-sphalerite type (Bastin and Hill, 1917, p. 110). Veins cori344986 55 3
14 GEOLOGY AND ORE DEPOSITS, COLORADO

TABLE 1. Uranium analyses of selected dump material, Eureka Gulch area 

[Analyses by Denver laboratory of the IT. S. Geological Survey. All are grab samples]

Field no.

Bl ._.

N109 ...

N20 __.

N20 

N16_ __

N16-AN16-B _

N16-C _

N23- ....

N23-A 

N45 

N18 ._

SI-53... . .

El. _______

E2_. ......

N44__.-__.

N44-C .

N44-A -

N44-B__._

Locality

Buckley mine. ____ __

.-. do .-_..............

Carroll mine - __

.....do...... .........

____.do..___._ _._-._______

.-..do ................

-_._ do.. ................

J. P. Whitney ...... -

Adit, Rara Avis millsite. 

... ..do_ ........... ....

.... .do..... ............ .

 ..do - ..............

Two Sisters shaft . _ _

 ..do.. - _-___

 do . ..............

 ..do..... _ .- 

Equivalent uranium (percent)

0.013

.31

.021 

6.7 

6.4 

1.7 

.93 

10.5 

.027

.054 

8.9 

.80 

.17 

.22 

1.7 

.92

5.6 

4.4 

2.6

Chemical uranium (percent)

0.003

.33

.007 

6.87 

.62 

.025 

.68 

.095 

.017

.034 

5.92 

.59 

.15 

.03 

.94 

1.42

6.11 

4.70 

3.05

Rn Pb 2I°

CuriesXHH

45

64

45

68

Remarks

Probably middlings from radioactive ore milled from Wood 

mine. 

Black uranium oxide, probably 

pitchblende.

JFitchblende. 

[pitchblende has been leached. 

\ Green secondary mineral.

Pitchblende.

Uranium principally contained 

in metatorbernite. 

Do. 

Do. 

Uranium principally contained 

in kasolite.

taining pitchblende are not known to bear appreciable quantities of 

metatorbernite, and no pitchblende has been observed in the metatorbernite-bearing rock. Pitchblende and metatorbernite ores, therefore, seem to be mutually exclusive in the Eureka Gulch area and 

appear to occur in different veins (pi. 1A).

Pitchblende has been observed in place only at the Carroll mine and 

is confined to several small shoots along the Carroll vein. On the 

177-foot level, which was partly mined during earlier operations, and 

in the stopes above this level, the pitchblende-bearing ore appears to 

occur in steeply-plunging shoots that have a maximum height of BO 

feet, a stope length from 10 to 15 feet, and a width from an inch or less 

to 12 inches. To judge from the limited exposures of ore left in the 

stopes, individual shoots are alined en echelon. Within each shoot 

the pitchblende ore forms small pods and lenses, generally only a few 

feet in maximum dimension. A small pod of ore extracted from the 

underhand stope below the 177-foot level occurred at the junction of 

two pitchblende-bearing seams and plunged 20° NW.; it averaged 

about 12 inches in width and was 10 feet long.

The pitchblende shoot on the 228-foot level has a known horizontal 

length of nearly 40 feet and possibly is as much as 65 feet long; its 

height has not been determined. The shoot consists of three principal 

subparallel pitchblende seams that are from an inch to about 4 inches 

thick and at places are separated by radioactive vein and altered wall 

rock. At junctions of the seams the intervening rock is veined by
URANIUM DEPOSITS OF EUREKA GULCH AREA, GILPIN COUNTY 15

abundant pitchblende seams a fraction of an inch thick; the whole 

constitutes minable ore. The distribution and quantity of pitchblende 

and the concentration of radioactive material in two or more distinct 

places on the J. P. Whitney shaft (33) and the Rara Avis adit (34) 

dumps indicate that uranium-bearing material probably was mined 

from at least two places in each mine. At the Bullion mine dump 

(32), radioactive rock was localized in one spot; hence, the pitchblende 

probably came from a single body.

Metatorbernite-bearing rock on the dumps at the McKay, B. H. D., 

and St. Anthony shafts (pi. IB) probably came entirely from the 

R. H. D. vein; because most mines are inaccessible the writers do not 

know definitely which veins are radioactive. The uranium-bearing 

material on the Two Sisters dump (pi. IB) could have been mined 

from either or both the Two Sisters or the Claire Marie veins, because 

both veins are reported to have been worked underground in the Two 

Sisters mine.

WALL-ROCK ALTERATION

The wall-rock alteration of vein deposits in the Eureka Gulch area 

is not fully known; however, some preliminary observations can be 

presented. The alteration was studied in the McKay shaft workings 

(E. H. D. vein), on three levels of the Carroll mine (Carroll vein), 

and in dump samples from the Two Sisters mine (Two Sisters and 

Claire Marie veins).

The wall rocks were altered by two types of solutions: hydrothermal, related to base metal and uranium ore deposition, and supergene, related to circulating ground water of a later time. The supergene alteration was superimposed upon the hydrothermally altered 

rock. A transition from the supergene zone to the primary zone has 

been observed only in the Carroll mine.

The mineralogy and width of wall-rock alteration zones are complicated by the occurrence of diverse rock types; however, the rocks 

studied have related trends of alteration. Plagioclase feldspar was 

the first mineral to completely change to montmorillonite and minor 

amounts of kaolinite, chlorite, and illite. Hornblende, where present, 

was partly replaced by biotite. Biotite in turn was converted into 

chlorite, kaolinite, and possibly some montmorillonite. Illite is present in small amounts in the supergene zone, but is nearly absent or 

masked in hydrothermally altered rock. Sericite occurs as an alteration product in the immediate vein zone in the hydrothermal environ 

ment. Silicified wall rock is apparent megascopically only in the 

hydrothermal zone. Supergene action seems to have intensified 

argillic alteration at the expense of sericite and outlying host rock; 

however, no well-defined argillic subzones are evident. Montmorillonite is concentrated in the most intense phase, but occurs throughout
16 GEOLOGY AND ORE DEPOSITS, COLORADO

the clay-mineral alteration zone. Chlorite and illite are concentrated 

in the moderately altered rock, but in smaller amounts than montmorillonite; kaolinite is present in small amounts throughout the 

altered zone. These relations suggest a moderate temperature for 

hydrothermal alterating solutions.

SUPERGENE ZONE

A study of the metatorbernite-bearing rocks biotite-quartzplagioclase gneiss and amphibolite indicates that clay-mineral 

alteration predominates in the supergene zone. The principal claymineral constituent is montmorillonite.

The alteration of biotite-quartz-plagioclase gneiss produced the 

following mineralogical changes: Plagioclase feldspar was first converted to a mixture of montmorillonite and kaolinite (mont.>kaol.) ; 

biotite later was altered to chlorite, illite, and, with more intense 

alteration, in part to kaolinite and montmorillonite.

The fresh rock is dense, hard, and has a prominent foliation resulting from a subparallel arrangement of biotite crystals. A study of 

thin sections indicates that the plagioclase (An25-3o) has been slightly 

altered to montmorillonite along crystal margins, cleavages, twin 

planes, and less frequently irregularly over the crystals. With more 

intense alteration either closer to the vein or along open foliation 

planes away from the vein the rock was softened, and white and 

light-green clay-mineral mixtures of kaolinite and illite-montmorillonite replaced the plagioclase feldspar. Biotite initially was not 

visibly altered.

In the most intense stage of alteration the gneiss was changed to a 

soft mass of plastic green clay, primarily 15A type montmorillonite. 

At places along subsidiary shear surfaces, foliation planes, and other 

irregular surfaces within the green clay, fragile sericite-like aggregates, and less commonly, aggregates of biotite were developed. Both 

new minerals are oriented irregularly with respect to the original rock 

structure. Within the supergene zone there is no visible silicification; 

sericite, if present as a true phase of alteration, is poorly developed.

The alteration of amphibolite is predominantly argillic; the mineralogy is similar in many respects to that for biotite-quartz-plagioclase gneiss. Plagioclase feldspar was altered first, or more rapidly 

than hornblende, to montmorillonite; hornblende altered primarily 

to biotite; biotite altered to chlorite, illite, and probably later to 

kaolinite and montmorillonite.

Fresh amphibolite is a dark-gray to black, dense, poorly foliated 

rock that is composed of hornblende, plagioclase feldspar, and minor 

quartz. The rock occurs as small podlike bodies in biotite-quartzplagioclase gneiss. Where altered, the amphibolite grades into a mas-
URANIUM DEPOSITS OF EUREKA GULCH AREA, GILPIN COUNTY 17

sive, soft, white and green clay-biotite-hornblende rock. Where intensely altered a soft, plastic, green clay was formed at the expense of 

biotite; however, the green clay often contains irregularly distributed 

flakes of mixed green-brown and red-brown biotite crystals. The 

former are partly altered to chlorite, whereas the latter are in part 

changed to kaolinite. The green color of the rock results from th* 

large quantity of montmorillonite, 15A type; a small amount of white 

clay aggregate, separated during sedimentation, is kaolinite and illite 

(kaolinite > > illite).

Some amphibolite pods, not observed to be cut by the vein, are 

similarly altered but alteration was most intense at their centers and 

decreased outward. Metatorbernite is localized at the junction of 

green clay and moderately altered zones, or in some cases where the pod 

is completely converted to green clay, at the edge of the pod.

No silicification or sericitization of amphibolite wall rocks was observed at the McKay shaft workings.

At places granite pegmatite is cut by the vein, and, as noted elsewhere in the district, the potash feldspar is not as highly altered as 

plagioclase (sodium-calcium) feldspar.

The alteration of biotite-quartz-plagioclase gneiss from the Two 

Sisters mine dump and from the upper levels of the Carroll mine is 

generally similar to the foregoing description.

HYDROTHERMAL. ZONE

The hydrothermal alteration, as determined from megascopic observations on the 228-foot level of the Carroll mine, differs from the 

supergene zone alteration in the presence of a conspicuous but narrow 

silicified and sericitized wall-rock zone. Away from the vein outside 

of the silicified and sericitized zone and in host-rock fragments in 

cockade ore, white and greenish clay and bleached biotite grains grade 

into soft white clay and biotite, and finally into fresh rock. Pods of 

amphibolite in the granite gneiss are partly altered to soft green clay 

and biotite.

RELATION OE METATORBERNITE TO ALTERED HOST BOCKS

The metatorbernite at the Two Sisters mine and the McKay shaft 

workings is disseminated through altered biotite-quartz-plagioclase 

gneiss and amphibolite pods in the gneiss; it replaces constituent minerals of the host rocks. At places it forms coatings on fractures in the 

rocks.

In the McKay shaft workings (fig. 2) the metatorbernite is concentrated in the margins of the altered amphibolite pods, but it is also 

abundantly disseminated through the green clay. Samples of altered 

biotite-quartz-plagioclase gneiss from the Two Sisters dump contain
18 GEOLOGY AND ORE DEPOSITS, COLORADO

disseminated metatorbernite granules or aggregates of granules in the 

moderately altered rock. The granules are closely associated with 

biotite and commonly embay the biotite flakes as seen in two dimensions 

in thin sections. Evidence for replacement of the biotite by metatorbernite is inconclusive; however, many granules occur where the 

biotite has been bent or flexed. The grain boundaries are generally 

smooth and regular. A marked concentration of metatorbernite occurs in the more intensely altered moderate or bleached zone at its 

boundary with the moderately altered zone. In this environment the 

metatorbernite appears to replace the biotite along cleavage planes and 

crystal edges, and it assumes tabular or flake-shaped aggregations, 

rather than a granular form. Under the microscope it is seen to be 

intimately mixed with sericite and illite. Granules persist into the 

bleached zone, and, away from the zone of concentration of flake-type 

metatorbernite in the boundary zone, granules are present in about 

equal proportions with the flake type. Along fractures, and at places 

along folia in the rock, the flake type predominates over the granular 

type of metatorbernite in the bleached zone.

The two different morphological types of metatorbernite granules 

and flakes from the Two Sisters mine dump samples are difficult to 

explain from the present inadequate data. Two alternative explanations may be suggested for their development. Either they were 

formed at different times, perhaps under slightly different environmental conditions directly related to the degree of completion of the 

wall-rock alteration, meaning that flakes relate to very late supergene 

action; or they were more nearly contemporaneous, where the physical 

and structural conditions were such that the flake type was favored in 

the more accessible parts of the system. The granular phase may be 

slightly older than the flake phase, but there is no other relation of the 

granules to a particular texture or structure; the formation and concentration of the flake-type metatorbernite, however, appears to have 

a direct relation with the early stages of breakdown of biotite.

ORIGIN OF PITCHBLENDE AND METATORBERNITE

Pitchblende deposits and quartz bostonite dikes have a close spatial 

relationship within the Central City district (Alsdorf, 1916; Bastin, 

1916; see also footnote p. 8). Phair concluded that the uranium was 

derived from a cooling quartz bostonite mass at depth; additional 

uranium could have been obtained by leaching of uranium from quartz 

bostonite during the rise of the uranium-rich solutions. The dikes also 

contain a relatively high proportion of thorium; the veins contain almost none. This suggests that either the uranium in the dikes was 

more readily available than thorium, or that the uranium in veins (and 

dikes) was derived from some other common source. The pitchblende 

was deposited in fractures which were open at the time of its formation.
URANIUM DEPOSITS OF EUREKA GULCH AREA, GILPIN COUNTY 19

The pitchblende in the Central City district was deposited early in 

the sequence of Tertiary vein formation; in some veins it crystallized 

essentially contemporaneously with pyrite; in others it formed before 

the pyrite. So far as known, it always formed earlier than sphalerite, 

galena, and chalcopyrite, and commonly was brecciated before the 

deposition of these metals.

Uranium minerals other than pitchblende are sparse in the Central 

City district. The occurrences of metatorbernite for a distance of 

about 700 feet along the Two Sisters and K. H. D. veins (pi. IB) are 

unique in the district. The metatorbernite was probably formed by 

oxidation, solution, and transportation of uranium from primary 

pitchblende, but it may be a primary mineral deposited directly from 

mineralizing fluids of different composition from those which deposited pitchblende. The PO4 ion, required for crystallization of metatorbernite, possibly was derived from apatite in the host rock or from 

adjacent rocks. The SiO4 ion, required for the precipitation of kasolite, could have been derived from the silica within the quartz vein or 

from alteration of feldspar in wall rocks. The necessary copper and 

lead were primary vein constituents, because the veins are known to 

contain copper and lead sulfides.

ECONOMIC EVALUATION

If the metatorbernite and kasolite are secondary minerals deposited 

by circulating waters which dissolved uranium from pitchblende, then 

primary pitchblende deposits may be encountered at depth. The 

quantity and location of the pitchblende cannot be determined with the 

existing information, but it would be below the zone of oxidation. 

However, if the metatorbernite is a primary mineral deposited directly 

from a deep-seated source, it is unlikely that any pitchblende deposits 

will be found at depth. The grade, tonnage, and depth extension of 

the metatorbernite deposits cannot be. estimated on the basis of existing information. The abundant disseminated metatorbernite in 

altered wall rocks in the Nigger Hill area so far as is known is a unique 

deposit in the United States.

The pitchblende deposits of the area, to judge from the Carroll 

mine, are similar to those that have been mined in the Quartz Hill 

area. The ore forms small pods, lenses, and seams within pitchblende 

shoots along the veins. These deposits can best be mined together 

with other metals contained in the veins. Some of the veins, however, might contain bodies of sufficient size and grade to be mined 

profitably for uranium alone.

DESCRIPTION OF INDIVIDUAL MINES

Within the Eureka Gulch area there are 68 principal shafts and 

adits, locally referred to as tunnels. A list of mines with their cor-
20 GEOLOGY AND ORE DEPOSITS, COLORADO

responding numbers is given on plate 1 A. Because most of the mines 

were worked several decades ago and subsequently have partly caved, 

few mine openings were accessible in 1953 and 1954.

In the following pages only those mines that are known to contain radioactive materials are described, and of these only two, the 

McKay shaft, K. H. D. claim (10), and the Carroll mine (8), were 

accessible for underground examination in 1953 and 1954.

BUCKLEY MINE (56)

The Buckley mine (pi. 1A), now inaccessible, was opened by a twocompartment shaft inclined 80° S. 15° W. The shaft is open at the 

collar but appears to be jammed at a depth of about 40 feet. To 

judge from the size of the dump, the mine contains several hundred 

feet of underground workings. Between 1902 and 1922, 57.42 ounces 

gold and 93 ounces silver were produced (A. J. Martin, Metal Economics Branch, U. S. Bureau of Mines). Considerable ore probably 

was extracted prior to that time.

The shaft was sunk on a vein trending N. 65°-70° W. The Wood 

shaft (55), 650 feet to the northwest across Prosser Gulch, probably 

was sunk on the same vein. According to Bastin and Hill (1917, 

pi. 4) the vein belongs to the pyritic gold type. Pyrite and sparse 

galena and sphalerite can be recognized on the dump.

A small radioactive area on the surface of the dump, about 2 feet 

in diameter, was sampled, and it assayed 0.31 percent equivalent 

uranium and 0.33 percent chemical uranium (table 1). The radioactive material is black and presumably contains pitchblende.

BULLION MINE (32)

The Bullion mine, on the Bullion No. 2 claim, is about 1,000 feet 

west of the portal of the Kara Avis tunnel, on the south side of Eureka Gulch (pi. 1A). The mine is inaccessible and little is known 

of the extent of the mine workings. According to W. C. Russell, Jr. 

(oral communication), the mine consists of two levels about 100 feet 

apart vertically, and a vertical shaft, now caved at the collar. Some 

drifting was done on both levels west of the shaft; probably little 

ore was extracted. The mine was last worked during World War II.

CARROLL MINE (6)

The Carroll mine is on the west slope of Nigger Hill near the southeast corner of the cemetery (pi. IB). It was closed at the time of 

the discovery of radioactive materials on the dump in 1953, but subsequently it has been rehabilitated by United Mining and Leasing 

Corporation. It was last worked about 1903.
URANIUM DEPOSITS OP EUREKA GULCH AREA, GILPIN COUNTY 21

The mine workings consist principally of a two-compartment shaft, 

inclined about 65° NE., which is 230 feet deep; short levels at vertical 

depths of 102, 177, and 228 feet; and stopes above the 177-foot and 

102-foot levels, southeast of the shaft.

The shaft is located at the approximate junction of three veins, as 

shown on plate IB. The Carroll vein where opened underground 

strikes N. 70°-80° W. and dips on the average about 65° NE., but 

there are local minor deflections in dip and strike. The Claire Marie 

vein (pi. IB) is inferred to be present in the shallow prospects pits 

between the Carroll and Claire Marie shafts; on this assumption it 

should junction with the Carroll vein near the Carroll shaft. The 

northwest-trending vein to the southwest of the Carroll vein (pi. IB) 

probably was explored by the short drift on the southeast side of the 

shaft at a vertical depth of 52 feet.

The radioactive material discovered on the dump occurred within 

an area several feet in diameter. Although pitchblende was recognized in certain samples, the radioactivity largely resulted from 

residual concentrations of radium and other daughter products of 

uranium. (See p. 14). Four analyses of selected radioactive material 

are given in table 1. Subsequent rehabilitation has disclosed the 

presence of pitchblende and sparse colored secondary uranium 

minerals in the mine workings.

The Carroll vein cuts granite gneiss, biotite-quartz-plagioclase 

gneiss, and pegmatite that strike northeasterly and except locally dip 

moderately to gently northwestward. Granite gneiss is the principal 

wall rock except on the 228-foot level, where the wall rock is largely 

biotite-guartz-plagioclase gneiss at places containing conformable 

bodies of granite pegmatite.

The Carroll vein strikes N. 70°-80° W., and dips on the average 

65°-70° NE. It is a strong structure consisting of two or more subparallel, but locally diverging, fractures that are separated by brecciated, altered, and mineralized wall rock. The width of the vein 

ranges from less than a foot to about 9 feet. The vein contains seams 

of massive sulfides as much as a foot thick, but generally less than 

6 inches thick, and cockade ore formed by sulfides and quartz that 

coat breccia fragments of wall rocks. Vugs coated by terminated 

quartz crystals are common. Galena and sphalerite are the most 

abundant ore minerals; chalcopyrite and pyrite are sparse. Pitchblende and local colored secondary minerals are confined to shoots 

within the vein, as described below.

Pitchblende is the principal uranium mineral found in the mine; 

sparse colored secondary minerals are present at places in the upper 

parts of the mine. Colored uranium minerals torbernite and a yel-
22 GEOLOGY AND ORE DEPOSITS, COLORADO

low unknown mineral are present locally in the hanging wall of the 

vein exposed in the sublevel above the 102-foot level. Pitchblendebearing ore, considerably out of equilibrium, is exposed in the f ootwall 

of the vein in the raise, about 10 feet below the 102-foot level, and in 

the stope at an altitude of about 8,875 feet. A sample, 12 inches thick, 

of the radioactive vein, from the raise gave 5.0 percent equivalent 

uranium and 0.88 percent uranium. In this specimen, sparse torbernite coats the black radioactive mineral, presumably leached pitchblende. Samples cut from the pillar in the raise, subsequently mined, 

showed that in part the equivalent-uranium content of this ore exceeds 

the chemical uranium. The disequilibrium is thought to result from 

leaching of uranium in place. Analyses from the body of pitchblende 

ore found on the 177-foot level, which subsequently was mined, indicate this ore is essentially in equilibrium. On the 228-foot level, 

pitchblende is present along the vein for a distance of at least 40 feet. 

Without exception this ore is in equilibrium.

The uranium minerals are not present everywhere along the Carroll 

vein; instead they are confined to shoots. Between the shoots the 

vein is essentially barren of uranium. The uranium ore shoots defined 

by mining above the 177-foot level are small; the shoot exposed on 

this level has a height of 30 feet, a stope length of 12 feet or less, and 

an average width from 4 to 6 inches. It is inferred that the other 

uranium occurrences noted in the stoped ground above the 177-foot 

level also form steeply plunging shoots, which are arranged en 

echelon. The shoot found on the 228-foot level has not been defined; 

where exposed in the drift, it has a length of 40 feet and possibly as 

much as 65 feet.

Within the shoots, the pitchblende occurs as small pods, lenses, and 

seams that are separated by inch-thin pitchblende-bearing seams generally too small to be extracted profitably. A podlike body of highgrade pitchblende ore, extracted from the underhand stope on the 177- 

foot level, plunged 20° NW. and measured 10 feet in length and an 

average of 1 foot in height and width. This body occurred at the junction of two closely spaced pitchblende-bearing seams. A similar pod of 

high-grade ore, likewise formed at the junction of two seams, was a 

foot thick, 3 feet high, and about 5 feet long, and was mined from the 

228-foot level, 75 feet northwest of the shaft. Immediately southeast 

of this body, the two seams are from 12 to 15 inches apart; the rock 

between the seams contains irregular, discontinuous pitchblende-bearing seams, generally less than half an inch thick, the whole constituting 

minable ore.

CLAIRE MARIE MINE (7)

The Claire Marie mine is on the north slope of Nigger Hill about 280 

feet southwesterly from the Two Sisters shaft (pi. IB). The mine
URANIUM DEPOSITS OF EUREKA GULCH AREA, GILPIN COUNTY 23

workings consist of a 255-foot shaft, which is inclined on the average 

75° SE., and levels at depths of 125 and 206 feet. The workings connect to those from the Two Sister mine through a filled stope on the 

Claire Marie vein. So far as known, little mining was done from the 

Claire Marie shaft, and the value of the shipments probably was small. 

A shipment of concentrates (about 3 tons) contained 5.125 ounces gold, 

10.9 ounces silver, 25.55 percent lead, and 10.7 percent zinc. The gross 

value of this shipment was $114.40 per ton, based on a gold price of 

$19. The mine was worked principally during the 1890's. It was 

reopened in 1928 by the Claire Marie Mining Company for examination and sampling, but no ore was shipped at this time. The shaft 

was sunk on the Claire Marie vein, which trends about N. 70° E. and 

dips an average of 75° S^p. According to a private company report 

by Horace F. Lunt, prepared in 1928, the vein is weak where exposed 

on both the 125- and 206-foot levels and nowhere is sufficiently thick 

to mine profitably. On the 206-foot level, the vein contains a small 

body of galena, pyrite, and sphalerite that is generally less than 6 

inches thick and carries varying, but generally small, quantities of gold 

and silver. Minable ore might be found below the 206-foot level, 

because the ore mined in the stope northeast of the shaft appears to 

rake to the southwest.

A small quantity of radioactive material, probably metatorbernite, 

was found on the dump. Some of the uranium mineral forms coatings 

on wood fragments.

RARA AVIS MINE

The Rara Avis mine, as used in this report, includes the workings 

from the shaft on the J. P. Whitney claim (33), situated on the north 

slope near the top of the hill between Eureka and Prosser Gulches, 

and the adit on the Rara Avis millsite (34), on the south side of 

Eureka Gulch (fig. 2). Both the dump at the shaft and the dump at 

the portal of the adit contain appreciable quantities of pitchblendebearing rock.

The J. P. Whitney shaft, now inaccessible, is nearly vertical and is 

reported to be 589 feet deep. Six levels about 100 feet apart vertically 

connect with the shaft and consist of a total of 1,190 feet of drifts. 

The shaft collar is at an altitude of about 9,102 feet. At an altitude 

of about 8,800 feet the adit bears S. 25° W. and connects with the 

workings from the shaft on the 300-foot level. The portal is caved 

and accordingly the adit is partly filled with water. There are no 

known records of production.

At the surface the vein intersected by the J. P. Whitney shaft, where 

observed, trends N. 75°-85° E. and dips 84° SE. At the shaft collar 

the vein is about a foot wide. Presumably this was the principal
24 GEOLOGY AND ORE DEPOSITS, COLORADO

vein worked underground. The ore consists predominantly of galena 

and sphalerite. The pitchblende has a colloform structure and is 

earlier in age than sphalerate, galena, chalcopyrite, and at least part 

of the pyrite.

Pitchblende-bearing material is present at three places on the shaft 

dump and at two places on the adit portal dump. According to Henry 

DeLinde (oral communication) pitchblende was reported from the 

third level below the adit, presumably the 600-foot level. It is possible, however, that more than one pitchblende body was found in the 

underground workings. Selected grab samples from the dumps assayed as much as 8.9 percent equivalent uranium and as much as 5.92 

percent chemical uranium (table 1). The analyses indicate that there 

has been some leaching of uranium since the material was placed on 

the dumps; the equivalent-uranium content is thought to be indicative 

of the grade of ore mined underground.

B. H. D. CLAIM

The R. H. D. claim adjoins the Two Sisters claim on the east; the 

west end line is just west of the R. H. D. shaft (9), and east of the St. 

Anthony shaft (pi. IB). The principal mine openings on the claim 

are the R. H. D. shaft and McKay shaft (10), 70 feet to the east. The 

R. H. D. shaft, inaccessible in September 1953, is a nearly vertical 

shaft that extends downward into an open stope (Van McKay, oral 

communication). At a vertical depth of about 100 feet a drift extends 

eastward beneath the workings from the McKay shaft.

The McKay shaft, vertical to a depth of 23 feet, then inclined 80° 

N., is 42 feet deep; a 117-foot drift extends eastward from the bottom 

of the shaft a distance of 105 feet and connects to the west with a 

stope from the R. H. D. shaft workings (fig. 2). In September 1953 

the connection was closed by a cave in the drift. The bottom of an old 

shaft was entered 100 feet east of the McKay shaft; this opening has 

been timbered and its exact location could not be determined.

West of the McKay shaft the R. H. D. vein trends northwestward, 

and east of the shaft it trends slightly north of east (pi. IB); the vein 

dips 60°-80° N. In the McKay shaft workings the vein is curved and 

has many irregular branches and splits, as shown on figure 2. Whereas the main vein trends roughly eastward, the subsidiary veins branch 

to the northeast or to the southwest; many branches roll into the 

foliation of the wall rocks and die out within a few feet from the 

main vein. Horses between splits in the vein typically are sheared 

and altered and at places are brecciated.

The main vein consists largely of white clay gouge, at places associated with some breccia, and sparse white quartz; near the shaft 

the vein contains as much as 2 inches of gray, fine-grained quartz.
URANIUM DEPOSITS OF EUREKA GULCH AREA, GILPIN COUNTY 25

Pods of altered amphibolite 

containing scattered metatorbernite40-FOOT LEVEL 

12 in. gouge

Vein, showing plunge of grooves 

and sl

SECTION THROUGH 

MCKAY SHAFT

Generalized strike of warped 

foliation, showing plunge of fold axes

Strike and dip of foliation

Gob in drift

TS4

Sample numbers, referred to 

in table 2

20

Geology by P. K. Sims and A. A. Drake, September 1953 

0 40 Feet

Datum is approximate mean sea level 

FIGURE 2. Geologic map and section of McKay sbaft, R. H. D. claim, Qilpin County, Colo.

The vein ranges in thickness from less than an inch to a maximum 

of about 12 inches. The subsidiary veins generally are less than 6 

inches thick, and many are less than an inch thick. The vein is reported to contain local pockets of galena surrounded by cerussite, but 

sulfides were not observed by the writers. Van McKay (oral communication) reports that a small shoot containing free gold is present 

east of the McKay shaft.

The wall rocks consist predominantly of biotite-quartz-plagioclase 

gneiss, locally garnetiferous, which at places contains interlayers of 

granite pegmatite and pods of amphibolite. Most of the biotitequartz-plagioclase gneiss and nearly all of the amphibolite have been 

intensely altered. The amphibolite has been altered to biotite and 

subsequently to a soft, green clay that when wet resembles olive-green
26 GEOLOGY AND ORE DEPOSITS, COLORADO

soap. (See p. 17.) The biotite-quartz-plagioclase gneiss and associated pegmatite layers trend northeastward and on the average dip 

moderately northwestward. The amphibolite occurs within the gneiss 

in podlike bodies that are as much as 7 feet in diameter and which 

plunge gently northeastward. Most of the amphibolite bodies are 

too small to be shown on figure 2.

Abnormal radioactivity is widespread on the 40-foot level and in 

the lower part of the McKay shaft; uranium analyses are given in 

table 2. The radioactivity is caused by metatorbernite, except for 

sample TS7, which possibly contains kasolite.

Metatorbernite occurs sporadically along the R. H. D. vein and 

subsidiary branching veins but generally is not abundant; an exception is the hanging-wall vein in the lower part of the McKay shaft 

(fig. 2), where a vein, one inch wide, contains 0.19 percent chemical 

uranium. Substantial quantities of metatorbernite occur in altered 

amphibolite adjacent to the main vein or subsidiary veins. The metatorbernite primarily is concentrated in the margins of the altered 

biotitic amphibolite pods, but it is also disseminated through the green 

clay. A selected sample (TS2, table 2) of metatorbernite-bearing 

clay from a pod 68 feet east of the McKay shaft contains 0.24 percent 

chemical uranium. Other adjacent pods contain comparable, but 

generally somewhat smaller quantities of uranium. Small bodies of 

altered amphibolite near the shaft (samples TS8 and TS9) from a 

horse between two veins contain respectively 0.004 and 0.018 percent 

equivalent uranium (table 2).

TABLE 2. Uranium analyses from McKay shaft workings, R. H. D. claim

[All locations are east of the shaft except those marked with *; these are west of the shaft. Analyses by 

Denver laboratory of the U. S. Geological Survey]

Field no.

TS2 ...

TS3 .

TS4 _

TS5 - ...

TS6 

TS7.......

TS8 ....

TS9-. ...

TSIO... ...

TSA3..-.

Type of 

sample

Grab--

Chip....

Grab. _

Chip....

 do.....

Grab .

... do ..

.-do 

Chip....

_ do.. _

Width 

(inches)

6

8

12

1

4

Material

 ..do....... __ .......... ____ ......

.. do ...__..-_ .. ......-. 

Location 

(feet)

68

62

52

20

7

*2

*10

*2

At shaft

69

Equivalent 

uranium 

(percent)

0.18

.12

.025

.006

.036

.031

.004

.018

.15

.053

Chemical 

uranium 

(percent)

0.24

.14

.025

.003

.033

.034

.011

.19

.062

Abnormal radioactivity extends to within 25 feet of the surface in 

the McKay shaft; above that altitude the vein is not radioactive and 

presumably the uranium, if ever present, has been leached. The 

radioactive material can be expected to extend to lower depths but 

possibly will not be found below the lower limit of the zone of 

oxidation.
URANIUM DEPOSITS OF EUREKA GULCH AREA, GILPIN COUNTY 27

TWO SISTERS MINE (8)

The Two Sisters shaft, on the north slope of Nigger Hill (pi. IB), 

is the largest mine in the Nigger Hill area but like most others in the 

region has been closed for many years. In 1954, the Two Sisters 

shaft was caved at the collar and the mine was inaccessible. The mine 

workings, according to a private company report by Daniel Munday, 

consist principally of a 700-foot shaft, which is inclined about 75° SE., 

and three levels at depths of 100,150, and 300 feet. Extensive stoping 

was done northeast of the shaft on the Two Sisters vein (?) from the 

100-foot level to the surface, and gold ore valued at nearly $100,000 

was extracted. West of the shaft, some stoping was done on the 

Claire Marie vein between the 100- and 150-foot levels, and Munday 

reports that ore valued at about $35,000 was removed. Apparently 

little, if any, stoping was done on the 300-foot level. West of the 

shaft on the 100-foot level, the workings on the eastward-trending 

Two Sisters vein connect to the workings on the northeast-trending 

Claire Marie vein through a crosscut about 60 feet long. According 

to the company report the drift to the west on the 100-foot level is 750 

feet long, but it is probable that this drift is much shorter. On the 

150-foot level the drift extends 70 feet to the east and 95 feet to the 

west; on the 350-foot level the drift extends 190 feet to the east and 75 

feet to the west. Munday reported good showings of lead ore in the 

east drift, 300-foot level.

The St. Anthony shaft on the Two Sisters claim is near the east 

end line (pi. IB) and apparently was sunk on the R. H. D. vein. 

According to the report by Munday, the St. Anthony shaft is 230 feet 

deep and drifts were driven from it at depths of 50 and 100 feet. At 

the 50-foot level the drift extends 60 feet southeastward and 35 feet 

northwestward; from the east drift the vein was stoped nearly to the 

surface. At the 100-foot level the drift extends 125 feet to the east 

and 40 feet to the west. According to Munday the westward drift on 

the 100-foot level crossed the Claire Marie vein 40 feet northwest of 

the shaft. If this is true, the Claire Marie vein must intersect the 

Two Sisters vein near the collar of the Two Sisters shaft; possibly the 

shallow pits that are alined northeasterly between the Two Sisters 

and St. Anthony dumps were dug on the Claire Marie vein (pi. IB) 

north of its intersection with the Two Sisters vein.

The Two Sisters shaft was sunk at the approximate junction of the 

northeastward-trending Claire Marie vein and the eastward-trending 

Two Sisters vein. The Two Sisters vein was prospected to shallow 

depths in the pits west of the Two Sisters shaft, south of the road 

(pi. 1A). East of the Two Sisters shaft the Two Sisters vein (?) 

was mined in "grass root" stopes. In these workings the vein makes
28 GEOLOGY AND ORE DEPOSITS, COLORADO

a very flat S-shaped bend, changing in strike from N. 86° E. to N. 

70° E. and back to N. 86° E. This dip ranges from 68° to 75° SE. 

Probably the vein joins with the R. H. D. vein in the vicinity of the 

McKay shaft, but this cannot be proved (pi. IB). In the open stope 

160 feet east of the Two Sisters shaft the vein trends N. 70° E. and 

dips 75° S. where the stope is from 3 to 4 feet wide, but is from 5 to 6 

feet wide where the trend is N. 86° E. and the dip is 68° S.

Bastin and Hill (1917, pi. 4) classified the Two Sisters vein as 

a gold-silver vein of the galena-sphalerite type. So far as known 

galena and sphalerite are the predominant metallic minerals, with 

smaller quantities of pyrite. Free gold is present, at least in the 

near-surface part of the vein.

Substantial quantities of metatorbernite and kasolite are present on 

the Two Sisters mine dump; no pitchblende was found. Most of the 

uranium-bearing material is near the toe of the dump, near the shaft 

on the west side, and on the east side of the lobe that extends east from 

the shaft (pi. IB). The dump contains a minimum of 3.5 tons of 

material of ore grade.

The metatorbernite occurs primarily in a distinctive, green altered 

biotite-quartz-plagioclase gneiss, but also is present in smaller quantities in other rock types. Although most of the metatorbernite is disseminated through the altered gneiss, it also is present along fractures 

in the rock. A description of the relation of the uranium mineral to 

the host rock has been given on pages 17-18. Analyses of selected 

pieces of metatorbernite-bearing altered biotite-quartz-plagioclase 

gneiss (N44, N44-A, N44-C, table 1) indicate a range from 1.42 to 

6.11 percent chemical uranium. The equivalent-uranium content of 

the samples is from 8 to 25 percent less than the chemical uranium.

Kasolite is much less abundant than metatorbernite; most of it is in 

the west side of the dump near the shaft. The kasolite occurs in vugs 

within hydrous iron-rich vein material that resembles gossan and to 

a lesser extent as coatings on fractures in altered vein material. An 

analysis of a selected kasolite-bearing specimen (N44-B, table 1) of 

the most radioactive material indicated 2.6 percent equivalent 

uranium and 3.05 percent chemical uranium.

Because the mine is inaccessible, an evaluation of the uranium 

deposit must be based entirely upon the known geology of the region. 

It is probable that the biotite-quartz-plagioclase gneiss that is the 

principal host rock for the metatorbernite constitutes less than 5 percent of the wall rocks in the mine. It also is likely that the uranium 

is mainly confined to the upper part of the mine, within the zone of 

oxidation; the depth of this zone is unknown. To judge from the 

high proportion of altered rocks on the dump, it is quite possible, 

however, that the oxidized zone extends to the 150-foot level.
URANIUM DEPOSITS OF EUREKA GULCH AREA, GILPIN COUNTY 29

LITERATURE CITED

Alsdorf, P. B., 1916, Occurrence, geology and economic value of the pitchblende

deposits of Gilpin County, Colo.: Econ. Geology, v. 11, p. 266-275. 

3astin, E. S., 1916, Discussion Occurrence, geology and economic value of the

pitchblende deposits of Gilpin County, Colo.: Econ. Geology, v. 11, p. 681-685. 

Bastin, E. S., and Hill, J. M., 1917, Economic geology of Gilpin County and

adjacent parts of Clear Creek and Boulder Counties, Colo.: U. S. Geol.

Survey Prof. Paper 94. 

Harrison, J. E., and Wells, J. D., 1955, Geology and ore deposits of the Freeland--

Lamartine district, Clear Creek County, Colo.: U. S. Geol. Survey Bulletin

1032- (in press). 

Johannsen, Albert, 1921, A descriptive petrography of the igneous rocks; V. 1,

Introduction, textures, classifications, and glossary: Univ. Chicago Press,

267 p. 

King, B. U., Leonard, B. F., Moore, F. B., and Pierson, C. T., 1953, Uranium in

the metal-mining districts of Colorado: U. S. Geol. Survey Circ. 215. 

Leonard, B. F., 1952, Belation of pitchblende deposits to hypogene zoning in the

Front Bange mineral belt, Colorado: Geol. Soc. America Bull., T. 63, no. 12,

pt 2, p. 1274-1275. 

Lovering, T. S., and Goddard, E. N., 1950, Geology and ore deposits of the Front

Bange, Colorado: U. S. Geol. Survey Prof. Paper 223. 

Moore, F. B., and Butler, C. B., 1952, Pitchblende deposits at the Wood and

Calhoun mines, Central City mining district, Gilpin County, Colo.: U. S.

Geol. Survey Circ. 186. 

Moore, B. B., and Kithil, K. L., 1913, A preliminary report on uranium, radium,

and vanadium: U. S. Bur. Mines Bull. 70, Min. Tech. 2, p. 43-47. 

Pearce, Bichard, 1898, Some notes on the occurrence of uraninite in Colorado:

Colo. Sci. Soc. Proc., v. 5, p. 156-158. 

Phair, George, and Levine, Harry, 1953, Notes on the differential leaching of

uranium, radium, and lead from pitchblende in H2SO4 solutions: Econ.

Geology, v. 48, p. 358-369. 

Bickard, Forbes, 1913, Pitchblende from Quartz Hill, Gilpin County, Colo.:

Min. Sci. Press, v. 106, no. 23, p. 851-856.

INDEX

Page

Abstract______-.-_._.-.._----- 1

Acknowledgments. __---__--_---_------------- 4

Amphibolite. ..- -------_---_.-- 5,7,16-17

Analyses of uranium samples_ _-_ _-._ 14,26

Bibliography.. -_____ _ __--_ -_-- 29

Boodle mine, analysis of uranium material _ _. 14 

Bostonite, quartz_______.....____ 8,9-10

Buckleymine..._____..........._.__ 14,20

Bullion mine-..-.. _. 14,20

Carrollmine..... _ ___ 14,20-22 

Claire Marie mine..---------_---_--------. 14,22-23

Dikes, composition and age-----------_.... 8-9,11

Economicevaluation...- _- - _--. 19 

Folds...-...--..-............................. 10-11

Foliation of pre-Cambrian rocks. 10

Gneiss, biotite-quartz-plagioclase... _ 5,6,16,21,22 

cordierite-anthophyllite-garnet ..... _. 7

granite....__..___-..._......._ 8

limesfflcate..___________...__ 6-7 

sillimaniticbiotite-quartz -- __ 6

Hydrothermalzone--.____.______ 15,17

J. P. Whitney shaft........ ....._...-...... 14,23

Kasolite................................ 12,13,19,28

Lineation of pre-Cambrian rocks.

Page

10

McKayshaft 24-25,26

Metatorbernite, localization .... 12,13,14,15,19,28

origin J8-19

relation to altered host rocks__..-. ___ 17-18 

Migmatite _____...____.____ 7 

Monzonite porphyry____.--.--.-..-------. 9,11

Nigger Hill dike ... .. ......... 9,10,11

Pegmatite, granite.__.................._.- 8

Pitchblende, localization - 12,13,14,15,19,22,24 

origin 18-19

Pre-Cambrian rocks---.------.--...------ 5-8,10-11

Pre vious reports..-..-_..._.- __ 2-3 

Prosser Gulch dike.......... ..-..-.- 9,10,11

Purpose of report ------------------------ 1-2

R. H. D. claim. 24-26

Kara Avis mine ---------------------- 14,23-24

St. Anthony shaft... ..------.-- -. - 27

Supergene zone ----- -. .,- --. 15,16-17

Tertiary rocks.... _ - 8-10,11 

Two Sisters mine... _____. _______ 14,27-28

Uranium, analyses__-------.-...---- . 13,14,26

leaching-_ . .. . . 13

Uranium deposits, types -------- ----- 12

Wall-rock alteration 15-16,17

31

O