Geology of the Copper 

King Uranium Mine 

Larimer County 

Colorado

GEOLOGICAL SURVEY BULLETIN 1032-D

This report concerns work done on behalf

\ of the Atomic Energy Commission and is

published with the permission of the 

Commission

Geology of the Copper 

King Uranium Mine 

Larimer County 

Colorado

By P. K. SIMS, GEORGE PHAIR, and R. H. MOENCH

GEOLOGY AND ORE DEPOSITS, CLEAR CREEK, GILPIN, AND 

LARIMER COUNTIES, COLORADO

GEOLOGICAL SURVEY BULLETIN 1032-D

This report concerns work done on behalf 

of the Atomic Energy Commission and is 

published with the permission of the 

Commission

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1958
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 Printing Office 

Washington 25, D. G. - Price 75 cents (paper cover)
CONTENTS

	Page

Abstract______._________________.______________ 171

Introduction________ _ _ _______.._______ . 172

Purpose and scope of report__________._____ 172

Acknowledgments______ ________._________-_-_ 173

Geography_____________________________ _ ___ 173

History and production____________ ._______________ 173

Mine workings___ ______________.__ _ _ 175

Rock units_______________________-__________ __ 175

Metasedimentary rocks________..___.._______________ 176

Biotite-quartz-plagioclase gneiss_____________ _ 177

Biotite schist________________._______________ 178

Amphibolite_________________-_______________ 179

Quartzite and related rocks________________________ 179

Amphibole skarn______________.._______________ 180

Origin of metasedimentary rocks_______-_______________ 184

Igneous rocks__________________________________ 185

Biotite granite_______________. _______________ 186

Granite pegmatite-_____________.._______________ 188

Origin and age of granites and pegmatite__.__________ __ 189

Structure________________________.._______________ 189

Foliation and lineation_____________._______________ 189

Joints________________________.._______________ 190

Faults_______________________._______________ 190

Copper King fault_____________.._______________ 191

B fault____________________.._______________ 191

A fault_____________________________________ 192

Mineral deposits_________ _________._______________ 192

Sulfide-magnetite deposits___________._______________ 192

General character and structure_____________________ 193

Mineralogy_________________.______________ 194

Magnetite________________._______________ 194

Pyrrhotite_______________.._______________ 194

Pyrite__________________________________ 194

Sphalerite________________________________ 197

Chalcopyrite______________.._______________ 197

Molybdenite_______________._______________ 197

Trace elements____________._______________ 199

Paragenesis_________________._______________ 199

Age and origin -__________._______________ 200

Pitchblende deposit_______________._______________ 200

Copper King vein______________________________ 201

Pyrite boxwork_______________._______________ 201

Pitchblende shoot__ __________._______________ 203

Mineralogy__________________._______________ 205

Pitchblende_______________.._______________ 206

Siderite_________________-_______________ 209

in
IV COKTENTS

Mineral deposits Continued

Pitchblende deposit Continued

Mineralogy Continued Page

Quartz__________________________ _______ ____________ 209

Pyrite___________________________ __________________ 210

Marcasite_ ___________________________________________ 211

Sphalerite.___________________________________________ 211

Chalcopyrite_ _________________________________________ 211

Goethite__________________________________________ 211

Fluorite___________________________.______ _____ ______ 211

Trace elements__-_______-_---------___-_______________ 211

Paragenesis___-___________-____-_-___________ _____________ 213

Age and origin.______________---_--__-_-__-_______________ 213

Radioactivity of A and B fault zones_ _______________________ 215

Alteration of wall rock. ________________________________________ 215

Results of exploration._________________________________________ 216

Reconnaissance for radioactivity_________________________________ 216

Suggestions for prospecting. _________-__-_----_____--________ ._. 218

References.-______________________________________________________ 218

Index.____________________________________________________________ 219

ILLUSTRATIONS

[Plates in pocket]

PLATE 16. Outcrop map of Copper King mine area showing sections 

through mine.

17. Geologic maps of mine.

18. Section through east wall of main shaft. 

FIGURE 41. Index map showing location of Copper King mine..... _______ 174

42. Vertical longitudinal projection of mine workings. ___________ 176

43. Section through drill hole BH-8_--_-._-_-__--.--_ _-----___ 181

44. Photograph of amphibole skarn_____-__-__________________ 183

45. Photomicrograph showing typical elliptical form of early

pyrite _______________________________________________ 195

46. Photomicrographs of early pyrite showing oriented inclusions

of amphibole______________-._______-__-_--_-_--__-___. 195

47. Photomicrographs of early pyrite_________________________ 196

48. Par agenesis of the massive sulfide-magnetite ore minerals _____ 199

49. Polished surface and autoradiograph of specimen from

pitchblende-bearing vein.._______________________________ 202

50. Photograph of boxwork pyrite ore______ ________________ __ 203

51. Photograph of lacy ore__________________________________ 204

52. Photomicrograph showing pitchblende veinlets in granite

host rock__-________------_--_-----_--_-----_-_-----_ 207

53. Photomicrograph of pitchblende showing colloform texture

and well-developed concentric cracks____________________ 207

54. Photomicrograph of colloform pitchblende with radial cracks

filled with siderite (dark gray)_____-___-_-__-___________ 208

55. Photomicrograph of UO3-rich pitchblende______. ___________ 208

56. Photomicrograph showing pyrite veinlets that cut sphalerite. _ 210
CONTENTS V 

TABLES

N Page 

TABLE 1. Modal analyses of metasedimentary rocks_.________________ 178

2. Modal analyses of granites__ ________-_____--_-----__---- 186

3. Semiquantitative spectrographic analyses of sulfide-magnetite

ores._________________________________________ 198

4. Semiquantitative spectrographic analyses of Copper King

vein and^l and Bfaults_________________-__-------__--- 212

5. Semiquantitative spectrographic analyses of granite wall

rocks.__________________-________-__-___---_---_-_--- 217

GEOLOGY AND ORE DEPOSITS, CLEAR CREEK, GILPIN, AND 

LARIMER COUNTIES, COLORADO

GEOLOGY OF THE COPPER KING URANIUM MINE 

LARIMER COUNTY, COLORADO

By P. K. SIMS, GEORGE PHAIR, and K. H. MOENCH

ABSTRACT

The Copper King mine, in the Prairie Divide region of Larimer County, Colo., 

in the northern part of the Front Range of Colorado, was opened during World 

War I in an unsuccessful attempt to mine copper and zinc ore. In 1949, following the discovery of pitchblende on the dump, the mine was reopened; it was 

worked until 1953 for uranium. A total of 652 tons of ore that contained an 

average of 0.28 percent U3O8 was shipped; 622 tons of the ore was mined from 

an ore shoot opened under a Defense Minerals Exploration Administration 

contract.

The bedrock consists predominantly of biotite granite, part of the Precambrian 

Log Cabin batholith, and minor metasedimentary rocks, biotite-quartz-plagioclase gneiss, amphibole skarn, biotite schist, quartzite, amphibolite, and biotite 

skols. The metasedimentary rocks occur as inclusions that trend northeast, 

almost parallel to the prevailing foliation in the granite. In places the metasedimentary rocks are crosscut sharply by the granite and form angular, steepwalled blocks in the granite. Faults, confined to a narrow eastward-trending 

zone through the mine, cut all the Precambrian rocks.

Mineral deposits of two types are present at the mine: pyrometasomatic sulfidemagnetite deposits and a vein pitchblende deposit that cuts the skarn.

The sulfide-magnetite deposits are small and consist of pyrite, sphalerite, 

chalcopyrite, pyrrhotite, and, at places, magnetite. Alpha-helium age determinations on ore magnetite indicate that the mineral assemblage is late Precambrian in age.

The uranium deposit consists of pitchblende and associated vein-forming 

minerals that occur in the Copper King fault and locally in pyrite boxwork 

adjacent to the fault. Three black, uranium-rich phases have been identified: 

uraninite, comnite, and UOs-rich pitchblende. Colored secondary minerals are 

absent. The pitchblende occurs in a steeply plunging, tabular shoot that is 

45 to 135 feet below the surface and that has a horizontal length of about 50 

feet. Within the shoot the pitchblende occurs in pods or layers that are generally a few feet in height and length and as much as a foot thick, and that 

are separated by nearly barren vein. The grade of the ore within the pods 

ranges from 0.2 percent uranium to as much as 20 percent but averages about 

1 to 2 percent. Age determinations by the Pb^/U238 and Pb807/!!235 methods on 

two samples of hard pitchblende from the vein, not from the pyrite boxwork,

171
172 GEOLOGY AND ORE, DEPOSITS, COLORADO

gave ages, after suitable common lead correction, ranging from 55 to 76 million 

years, corresponding to an early Tertiary age.

Diamond-core drilling and reconnaissance for radioactivity have not disclosed other uranium deposits in the Prairie Divide region; nevertheless it 

seems likely that other deposits are present.

INTRODUCTION 

PURPOSE AND SCOPE OF REPORT

In 1949 a black radioactive mineral, later identified by Phair as 

pitchblende, was found by two prospectors on the dump of the Copper 

King mine, at Prairie Divide in Larimer County, Colo. The pitchblende on the dump was sufficiently abundant and high in grade to 

indicate the possible occurrence of an economic deposit of uranium 

underground and accordingly the mine was unwatered. R. U. King 

and H. C. Granger (written communication, 1951) examined the 

workings and found that the ore came from the 70-foot level which 

was worked by the earlier operators. Subsequent exploration by 

Cherokee Mines during 1950 disclosed uranium ore of shipping grade, 

and in 1951 the company requested and obtained an exploration contract from the Defense Minerals Exploration Administration to explore the vein further. Because of the economic and scientific importance of the deposit, the writers, on behalf of the Division of Raw 

Materials of the U. S. Atomic Energy Commission, began a detailed 

geologic investigation of the mine. The study disclosed that the uranium occurred in a vein that cut the massive sulfide ore, which was 

worked by the early operators, and in ore of box work type adjacent 

to this vein.

During September 1951, Sims and Phair mapped the geology of 

the surface and the mine workings. The surface geology was mapped 

on a scale of 40 feet to 1 inch by plane table and telescopic alidade 

(pi. 16); the mine was mapped on a scale of 5 feet to 1 inch, using a 

base map made by a transit, tape, and brunton compass survey 

(pis. 17 and 18). Subsequently as the DMEA exploration progressed 

and mining continued, the new work was mapped by Sims. To support 

the field investigations, 65 thin sections and 47 polished sections were 

studied in the laboratory.

During the course of the mining, the U. S. Atomic Energy Commission sampled the pitchblende deposit to determine grade and tonnage. From September to November 1952 the Commission conducted 

a diamond-core drilling project in the search for an extension of the 

uranium ore body indicated by mining, and the results of this exploration are given by Derzay and Baker (1953).

This report presents the results of the field and laboratory investigations carried on by the Geological Survey at the Copper King mine. 

The Precambrian rocks and massive sulfide ores that are present in
GEOLOGY OF COPPER KING URANIUM MINE 173

the mine area are discussed together with the uranium ores. An 

interim report on the general geology of the mine (Sims and Phair, 

1952), intended mainly to assist the Atomic Energy Commission in 

the interpretation of the exploration, and a report on the paragenesis 

and age of the ores (Phair and Sims, 1954) were prepared previously.

ACKNOWLEDGMENTS

This investigation was carried out jointly by Sims and Phair. The 

field mapping and the laboratory studies of the Precambrian rocks 

and ores were the responsibility of Sims; the laboratory investigations 

of the uranium minerals were the responsibility of Phair. Moench 

assisted in the study of the petrography of the rocks and ores. Most 

of the report was written by Sims. J. W. Adams assisted in the field 

work in September 1951, and several other members of the U. S. 

Geological Survey assisted for short periods during the course of 

the project. This report is based on work done on behalf of the Division of Raw Materials of the U. S. Atomic Energy Commission.

The writers are indebted to Cherokee Mines, and particularly to 

T. H. Sackett, for full cooperation during the survey and for permission to publish the production data. During the investigation 

many of the economic problems were discussed with geologists and 

engineers of the Denver Exploration Branch of the U. S. Atomic 

Energy Commission.

GEOGRAPHY

The Copper King mine is on the Black Hawk No. 1 claim in sec. 8, 

T. 10 N., R. 72 W., 6th principal meridian, Larimer County, Colo., in 

the northern part of the Front Range (fig. 41). From Fort Collins, 

Colo., it can be reached by following the road log below:

Miles

From Fort Collins proceed north along U. S. Highway 287 for a distance of _________________________________________ 23. 3 

Turn left (west) on gravel road and proceed to road fork_________ 12.4 

At road fork take sharp left turn on road to Red Feather Lakes; proceed

along this road to Copper King mine____________________ 5. 3

The mine is near the southwestern edge of Prairie Divide, a broad, 

gently rolling mountain flat, at an altitude of about 8,000 feet. This 

flat is the result of a widespread pre-Wisconsin glaciation that has 

been named the Prairie Divide stage (Ray, 1940, p. 1856). The plain 

is mantled by weathered gravel that rests on deeply weathered till. 

At places, rounded knobs and hills composed largely of granite protrude above the plain.

HISTORY AND PRODUCTION

The Copper King mine and several nearby shafts were opened 

during World War I in the search for copper and zinc ores. No ore

448648 58 -2
174 GEOLOGY AND ORE: DEPOSITS, COLORADO

R. 74 W.

WYOMING

72 71

10 10 Miles

Front of Southern Rocky Mountain 

physiographic province 

Fort Collins

Denver

COLORADO

'Pueblo

FIGURE 41. Index map showing location of the Copper King mine, Larimer County, Colo.

is known to have been shipped, however, until 1920, when a carload 

of zinc ore was mined on the first level of the Copper King mine from 

a deposit of massive sulfides and oxides. This carload failed to pay 

the cost of milling and the mine and prospect shafts were abandoned. 

In 1936, Richard Kyle of Idaho Springs, Colo., operated the Copper 

King mine briefly for zinc but, so far as known, no ore was shipped.
GEOLOGY OF COPPER KING URANIUM MINE 175

In 1949, after finding the dump to be highly radioactive, A. H. 

Brown and H. G. Ismert filed claim on the Copper King mine on 

July 14, 1949, naming it the Black Hawk No. 1 claim. Shortly afterwards the mine workings, which consisted of a 64-foot vertical shaft, 

a level at a depth of 64 feet, and a 12-foot sump at the position of 

the present main shaft were unwatered. In 1950 and 1951 the operators extracted several tons of massive sulfide ore and some pitchblende-bearing rock from the level and sump. Fifty-five tons of 

hand-sorted sulfide ore that contained 18.2 percent zinc was shipped 

to Salt Lake City in 1951.

In 1951, Cherokee Mines, the mine operator, was granted a Defense Minerals Exploration Administration exploration contract, 

DMEA Docket 1702, contract Idm-E61, to sink a new 2-compartment shaft. This shaft, located 50 feet east of the old (west) shaft, 

was made by raising to the surface from the first (70-foot) level, 

and sinking below that level. During sinking, stations were cut at 

the 110-foot and 160-foot levels. The shaft was completed to a depth 

of 170 feet in May 1952. In the summer of 1952, an inclined raise 

was driven from the 160-foot level to the bottom of the pitchblende 

ore shoot, and a level at a depth of 140 feet was connected to the 

main shaft. Subsequently, during mining, a raise was completed 

connecting the 140-foot level and the 110-foot level.

As a result of Defense Minerals Exploration Administration exploration, a discovery was certified by the Government.

From December 1951 until the mine closed in August 1953 a total 

of 652.33 tons of ore that had an average grade of 0.28 percent 

U3O8 was shipped; 622 tons of the ore was mined from an ore shoot 

opened under the Defense Minerals Exploration Administration 

contract.

MINE WORKINGS

The mine workings consist of two vertical shafts 52 feet apart totaling 234 feet in length, 4 short levels, and 3 main raises (fig. 42). 

The principal manway and hoisting shaft, the main or east shaft, is 

a 2-compartment cribbed opening 170 feet deep. The west shaft, a 

single-compartment cribbed shaft, is 64 feet deep and bottoms at the 

first level. The drift on the 70-foot level is 118 feet long and connects 

the two shafts. The drifts on lower levels were driven to the east 

from the main shaft. All levels except the lowest were driven on ore. 

Raises were driven between the various levels. The main shaft and 

all workings below the 70-foot level were driven under DMEA 

contract.

ROCK UNITS

The Copper King mine is near the central part of a Precambrian 

batholith of Silver Plume granite known as the Log Cabin batholith.
176 GEOLOGY AND ORE DEPOSITS, COLORADO

WNW. ESE.

8000'-

7950'-

_JO-FOOT LEVEL

7925'-

79001-

7875'-

_ Dump 

Approximate surface

| Altitude, collar 

8012 feet

Approximate boundary of 

pitchblende deposit

J_W-FOOJ LEVEL

.UO-FOOT LEVEL

J.60-FOOT LEI/EL

Note: Pitchblende-bearing ore 

also mined from shaft 

and drifts

2 RAISE Boxwork pyrite ore

Geology and mine workings

by P. K. Sims 1951-53 

40 Feet

Datum assumed

FIGURE 42. Vertical longitudinal projection of mine workings, Copper King mine, 

showing approximate outline of pitchblende deposit.

Alpha/lead determinations upon zircon from the granite and isotopically corrected Pb208/Th232 age determinations upon two monazite 

crystals from a pegmatite within the body, believed to be genetically 

related to the granite, indicate a late Precambrian age. At the mine 

the granite contains less mica than typical phases of the batholith 

and has the average composition of a leucocratic biotite granite. 

Within the granite are small inclusions of metasedimentary rocks, 

principally biotite-quartz-plagioclase gneiss and amphibole skarn.

METASEDIMENTARY ROCKS

Metasedimentary rocks constitute about 10 percent of the exposed 

bedrock in the Copper King mine area (pi. 16). Besides the biotite-
GEOLOGY OF COPPER KING URANIUM MINE 177

quartz-plagioclase gneiss and amphibole skarn, which are the most 

abundant types, the metasedimentary rocks include biotite schist, amphibolite, and quartzite and related rocks. Approximate modes in 

volume percent of typical facies of these rocks, determined by Rosiwal 

analyses of thin sections, are given in table 1. The metasedimentary 

rocks are believed to represent metamorphosed alumino-siliceous, 

quartzose, and carbonate sediments that have been recrystallized and 

reconstituted; all evidences of the original textures have been destroyed. Because these rocks have been folded and then intruded 

by granitic rocks, the age relations among the metasedimentary rocks 

are unknown.

BIOTITE-QUARTZ-PLAGIOCLASE GNEISS

Exposures of biotite-quartz-plagioclase gneiss occur sporadically 

in the south half of the mapped area (pi. 16), and rocks of this type 

were intersected in some drill cores. From the surface exposures and 

the drill cores it is inferred that individual bodies of the gneiss are 

small and that many are crosscut by the granite, to form steep-walled 

inclusions. At the surface west of the mine small exposures of biotitequartz-plagioclase gneiss, interpreted as parts of a larger body of 

gneiss which has a northeastward-striking foliation, abut against 

biotite granite; the contact between the two rock types strikes northwestward and dips nearly vertically. At the contact and for a distance of a few feet from it, the biotite-quartz-plagioclase gneiss has 

been altered to a streaked granite. (See p. 188.)

The biotite-quartz-plagioclase gneiss is a dark-gray fine-grained 

equigranular foliated rock that consists almost entirely of biotite, 

quartz, and plagioclase (table 1). Biotite constitutes 15 to 20 percent 

of the rock. It is pleochroic and varies from straw yellow to dark 

brownish green; nY=1.685±.003, but it varies within a single crystal 

to as low as 1.630±.003. Plagioclase (An3o ) constitutes 30 to 40 percent of the rock; single grains are uniform in composition. Quartz, 

which shows strain shadows, consistently comprises about 45 percent 

of the rock. The characteristic accessory minerals are muscovite, 

apatite, opaque iron oxides, and zircon.

The gneiss has a granoblastic texture. The biotite, concentrated 

into thin layers and wisps, has a strong dimensional and crystallographic orientation, which produces a conspicuous foliation. At 

places a later, weaker foliation, visible only under the microscope, is 

produced by biotite foliae that are partially oriented at an angle of 

about 30° to the dominant direction. The magnetite throughout the 

gneiss characteristically is surrounded by round or lenticular halos 

composed of quartz, plagioclase, and small quantities of muscovite 

and apatite. In one specimen (A-31, table 1), a magnetite crystal 

with a maximum diameter of 1.5 mm is surrounded by a halo 0.75-1.25
178 GEOLOGY AND ORE DEPOSITS, COLORADO

TABLE 1. Modal analyses of metasedimentary rocks, Copper King mine

Mineral 

Quartz. ___

Plagioclase _ _ _

Biotite

Mica

Zircon

Rutile(?)____________________

Sphene(?)____ ___ ____ _____

Enidote __ __ __

A-31

______ 46

______ 33

______ tr

______ 21

______ tr

______ tr

tr

tr

45

40

15

tr

tr

tr

tr

D-17-B

16

tr

82

tr

2

tr

UO-9

86

11

tr

3

tr

tr

tr

UO-1S

47

10

37

tr

6

tr

C-lr% 

tr

49

50

1

tr

tr

tr

tr

Average grain diameter, in mm_______ 0.2 0.24 ____ 0.75 ____ _____

A-31 Biotlte-quartz-plagioclase gneiss. Plagioclase is Anso.

A-32 Biotite-quartz-plagioclase gneiss.

D-17-B Biotite schist.

TJQ-9 Phlogopitic quartzite.

TJG-15 Quartz-mica-hornblende schist.

C-4-2 Amphibolite. Plagioclase is An<?; hornblende is hastingsite.

mm in diameter. In specimen A-32 (table 1) the halos are lenticular 

and flattened parallel to the foliation; one magnetite crystal, 1.25 by 

2 mm in size, is surrounded by a lenticular halo 12 mm long and 2 mm 

wide. The quartz and feldspar within the halos have a granoblastic 

texture, similar to that throughout the gneiss.

BIOTITE SCHIST

A layer of biotite schist, 2 feet thick, crops out between granite 

and biotite-quartz-plagioclase gneiss, 100 feet south-southeast of the 

main shaft of Copper King mine, but schist is not exposed elsewhere 

in the area. Sparse thin layers of schist, commonly a foot or less 

thick, were cut in the drill cores. In drill hole BH-8, 3 feet of schist 

in biotite-quartz-plagioclase gneiss was intersected at a depth of 67 

feet (fig. 43).

The biotite schist is a black uniform rock that contains about 80 

percent biotite, 15 percent quartz, 2 percent apatite, and sparse 

phlogopite, zircon, and muscovite (D-17-B, table 1). The zircon 

occurs as inclusions in the biotite and is surrounded by conspicuous 

pleochroic halos. The quartz forms subrounded inclusions in the 

biotite. The biotite is dark brown; riY= 1.630. The biotite is of two 

generations. The first generation consists of compact aggregates of 

oriented laths that produce a definite foliation. The laths average 

about 1 mm in diameter and .1 mm in thickness. The second gen-
GEOLOGY OF COPPER KING URANIUM MINE 179

eration consists of much larger single crystals with a decussated texture 

that clearly cuts the foliation at all angles. The color and refractive 

index of both generations of biotite are identical.

AMPHIBOLITE

A black fine-grained foliated gneiss consisting mainly of green 

hornblende (hastingsite) and plagioclase of the composition An47 is 

found on the dump of shaft 8 and locally in the Copper King mine, 

but does not crop out at the surface. The foliation is produced 

by sparse streaks and lenses of plagioclase, generally less than onefourth inch thick, and by the dimensional orientation of the hornblende and plagioclase. The rock has a crystalloblastic texture. Hornblende constitutes 50 percent of the amphibolite (C-4-2, table 1); 

locally it is slightly altered(?) to biotite, which occurs as small 

anhedral brown flakes both parallel and transverse to the hornblende cleavage traces. The hornblende is strongly pleochroic: 

X=pale yellow-green, Y=dark green, and Z=dark bluish green. 

Measurements of the refractive index indicate that nY=1.683±.003; 

dispersion of the optic axes is strong, with r>v. The rock contains a small amount of quartz and traces of magnetite, apatite, 

muscovite, and epidote.

QUAKTZITE AND RELATED ROCKS

Small bodies of dark-gray to black fine- to medium-grained quartz 

ite are interlayered with amphibole skarn on the south wall of the 

70-foot level of the Copper King mine, immediately east of the main 

shaft (pi. 17); and two bodies, each less than 2 feet thick, associated 

with skarn, mica skols, and biotite-quartz-plagioclase gneiss, were 

intersected in drill hole BH-8. The quartzite is principally a massive, 

nearly equigranular, heterogeneous rock that contains about 11 percent phlogopite and sparse to abundant magnetite (UG-9, table 1). 

The quartz occurs as clear polygonal grains; it includes abundant 

euhedral apatite and subhedral phlogopite and magnetite. The 

phlogopite varies from colorless to pale brown; nY= 1.610. The mica 

crystals are for the most part rounded at their terminations.

Intercalated with the quartzite on the 70-foot level are gray to 

black, foliated to schistose rocks that are composed of variable quantities of quartz, mica, and hornblende. Magnetite is present in almost 

the same quantities as in the quartzite. The typical composition of 

this rock is given in table 1 (UG-15). The foliation is produced by 

alternating layers, less than an inch thick, of light and dark minerals. 

The dark layers consist predominantly of hornblende and quartz; 

the light layers consist mostly of quartz and mica. The hornblende 

(hastingsite) occurs as euhedral to anhedral blades, some of which
180 GEOLOGY AND ORE DEPOSITS, COLORADO

are poikilitic with abundant inclusions of subrounded quartz. The 

hornblende is pleochroic: X=very pale yellow-green, Y=green, 

Z=blue-green; ZAC=28°; nZ= 1.667±.003. The mica varies in individual flakes from a deep grass green near the margins and along 

some cleavage cracks to colorless in the center; the index of refraction of the green mica indicates that it is biotite with an appreciable 

content of iron, whereas the colorless mica is phlogopite. Mica of 

similar type occurs in the cummingtonite skarn. The quartz is anhedral and it contains minute inclusions of mica, hornblende, and 

magnetite. The texture of the rock is granoblastic; the average grain 

diameter is about 1 mm.

AMPHIBOLJE SKARN

The rocks that consist almost entirely of anthophyllite, cummingtonite, or actinolite, and which are related to the massive sulfide deposits, are mapped as amphibole skarn (pi. 17). Although lime-rich 

layers or unaltered carbonate "beds" are not present, the rocks, in 

mineral composition, texture, and structure, resemble demonstrable 

skarn deposits from Fennoscandia. The writers believe, therefore, 

that the rock probably represents metasomatized limy layers. An 

alternative interpretation is that the anthophyllite skarn represents 

a layer originally rich in magnesia that was regionally metamorphosed 

to the anthophyllite-phlogopite assemblage and subsequently replaced 

locally by cummingtonite, green biotite, and sulfide minerals.

Thin selvages and partings of biotite occur in the skarn and sulfide 

ores. These selvages and partings, because they resemble one type 

of skol, or shell, that has been described from Fennoscandia (Eskola, 

1914, p. 226, 259) and from the Northeastern United States 1 (Sims, 

1953, p. 271), are referred to as biotite skols.

Skarn does not crop out at the surface but is present in the Copper 

King mine and on the dump of shaft 1. The skarn exposed within 

granite in the mine (pi. 16) forms a roughly tabular complexly folded 

steeply plunging body that has a probable stope length of more than 

100 feet, a width of as much as 20 feet, and a maximum breadth of 

about 50 feet. The skarn body extends east of the mine workings, 

for it was intersected in drill holes BH-8 (fig. 43) and BH-3; where 

cut by hole BH-3, 80 feet east of the Main shaft, however, the skarn 

is less than six feet thick. The body becomes thinner as depth increases; probably it pinches out at a depth of about 170 feet below 

the surface. The skarn mass is generally conformable to the enclosing granite, except at its southwest end, where it is crosscut sharply 

by the granite (pi. 18, 70-ft. level). At places the skarn contains

1 Leonard, B. F., 1951, Magnetite deposits of the St. Lawrence County district, New 

York : unpublished Ph. D. thesis, Princeton Univ.
7960'-

7920'-

FAULT A (?)-J;l + + +

l;';: :':-| 'j ' ~ ** / \ 

|:;;i§j 4 + + + '^v8-in. pitchblen

7880'

40 80 Feet

Datum assumed 

FIGURE 43. Section through drill hole 
182 GEOLOGY AND ORE DEPOSITS, COLORADO

small bodies of granite and pegmatite that were intruded prior to 

the sulfide mineralization; and it is associated closely with biotitequartz-plagioclase gneiss. Thin partings and selvages of biotite skols 

as much as a foot thick commonly occur in the skarii.

The internal structure of the skarii body at Copper King mine is 

complex. The skarn is contorted into irregular layers and pods of 

varying mineral composition that plunge about 45° NE. Some of 

the layers and pods have been largely replaced by sulfide minerals 

and magnetite, whereas others are nearly barren.

The skarn in the mapped area can be divided according to mineralogy into three principal types: anthophyllite skarn, cuiimiingtonite skarn, and actinolite skarn. The anthophyllite skarn nearly 

always contains some cummingtonite and it grades into cimimingtonite skarn, a rock that always contains moderate or abundant sulfides. The actinolite skarn is essentially monommeralic; so far as 

known it does not grade into skarn composed of other amphiboles. 

The skarns contain variable quantities of sulfides and magnetite and 

at places these minerals are sufficiently abundant to constitute minable 

ore.

Anthophyllite skarn and cummingtonite skarii comprise nearly all 

of the skarii body that has been cut in the workings of the Copper 

King mine. Actinolite skarn is only locally present. In general the 

barren skarii consists predominantly of anthophyllite, whereas the 

mineralized skarn consists principally of cummingtonite.

The anthophyllite skarii is a gray, tan, or pale-green inequigranular massive rock of variable grain size consisting of anthophyllite and 

minor quantities of cummingtonite, phlogopite, apatite, quartz, calcite, sulfides, and magnetite. The anthophyllite typically occurs as 

sheaves of curved fibers, oriented at random, as much as an inch long 

and averaging about half an inch in length. If mica and quartz 

are present in the rock, the anthophyllite fibers are imbedded in a finegrained groundmass of these minerals. The cummingtonite, where 

present, forms straight euhedral blades that distinctly cut the anthophyllite sheaves and appear to be later; less commonly the cummingtonite occurs in parallel growth with anthophyllite. A typical specimen containing both anthophyllite and cummingtonite is shown in 

figure ±±. The index of refraction of the anthophyllite is variable, 

but nZ averages about 1.652±.00o. The anthophyllite is colorless, 

r<v, and the birefringence is moderate. The mica associated in variable quantities with the anthophyllite is homogeneous phlogopite. In 

three specimens examined, riY= 1.595±.003; 2V=0°-5°.

The skarns that contain cummingtoiiite as their principal silicate 

mineral always contain appreciable quantities of sulfides or magnetite; in addition they contain mica and sparse or moderate quartz.
GEOLOGY OF COPPER KING URANIUM MINE 183

FIGURE 44. Photograph of amphibole skarn, principal host rock for the sulflde-magnetite 

assemblage. The anthophyllite blades have a random orientation, whereas the cummingtonite blades are elongate and oriented approximately parallel to the lower edge of 

the photograph.

Because of the abundance of sulfide and magnetite, the original composition of the skarn is not known. The cummingtonite forms straight 

euhedral to subhedral blades, as much as 6 inches long but more commonly half an inch or less in length. Most, but not all of the blades, 

have a preferred orientation that imparts a lamellar structure to the 

rock (fig. 44). Quartz, where present, contains abundant inclusions 

of euhedral cummingtonite, phlogopite, and magnetite. Some of the 

quartz fills fractures and vugs in sulfide minerals. The mica varies 

from colorless phlogopite in the center to green biotite along crystal 

edges and cleavages, The refractive index changes with the color; 

for the green biotite nL is 1.620±.003, but in one specimen nL is 

1.612±.003; for the associated phlogopite nZ is 1.605±.003. The 

accessory minerals commonly associated with this rock are zircon, 

calcite, and rare microcline.

The actinolite skarn is a dark-green equigranular massive rock consisting of 95 percent or more of actinolite and traces of quartz and 

brown mica. At places it contains sparse sulfides and magnetite. 

So far as known, in the main skarn body at the Copper King mine 

actinolite occurs only as local small bodies. It is abundant on the 

dump of shaft 2 (pi. 16).

The actinolite occurs as short, stubby, randomly orientated crystals 

as much as 4 mm long. The quartz forms small interstitial grains.
184 GEOLOGY AND ORE DEPOSITS, COLORADO

The biotite skols form selvages as much as a foot thick, but generally less than 6 inches thick, around folded podlike bodies of skarn; 

others occur as wisps and layers up to 6 inches thick, and still others 

occur along contorted partings within the skarn and sulfide ore; they 

are dark green. The biotite generally has a strong dimensional orientation parallel to the compositional layering of the skarn and skols; 

in one specimen it is dark greenish brown: nY = 1.611.

ORIGIN OF METASEDIMENTAKY KOCKS

The metamorphic rocks in the mapped area were formed by dynamothermal metamorphism and metasomatism. The biotite-quartzplagioclase gneiss, biotite schist, quartzite, and amphibolite are 

interpreted as regionally metamorphosed sedimentary rocks, whereas 

the skarn and biotite skols, although possibly of dynamothermal 

origin, probably formed by metasomatic alteration.

The mineralogic composition of the biotite-quartz-plagioclase 

gneiss and the biotite schist indicates that these rocks were derived 

from alumino-siliceous sediments, perhaps without the addition or 

subtraction of materials. There is no evidence in this area that these 

rocks were derived originally from amphibolites by granitizatioii 

processes, which Steven (1957) noted in the Xorthgate area, Colorado, about 25 miles to the west. The quartzite represents an impure quartz sand. The amphibolite, because it occurs only as local 

small lenticular bodies, is assumed to be of metasedimentarv origin, 

probably metamorphosed impure carbonate rock. Similar amphibolites have been interpreted as metamorphosed igneous rocks 

(Adams, 1909; Dodge, 1942, p. 561-83).

The skarn body in the Copper King mine probably consisted originally of anthophyllite and small amounts of phlogopite. Later a new 

amphibole mineral, cummingtonite, formed at the expense of anthophyllite, and green biotite partly replaced the phlogopite. At this 

time iron was introduced to form magnetite and slightly later the 

ore-forming solutions deposited the sulfide minerals and quartz; the 

quartz continued to crystallize after the cessation of deposition of the 

sulfides.

The skarn is thought to be metasomatized carbonate rocks because 

of its mineralogic assemblage, massive to lamellar texture, spatial 

relation to the granite, and resemblance to known skarn rocks. Nearly 

all the original carbonate was altered to magnesium-rich skarn minerals; only scattered interstitial carbonate grains remain. Metasomatism probably was effected by solutions from the granite magma.

The alternative hypothesis for the origin of the skarn is that the 

amphibole rock represents an original magnesia-rich layer that was
GEOLOGY OF COPPER KING URANIUM MINE 185

metamorphosed during regional metamorphism and subsequently 

modified by hydrothermal solutions that formed the ores. The principal argument against this origin, however, is the absence of marked 

lineation of the amphiboles, for amphibole rocks of this origin in 

Fennoscandia typically have a conspicuous lineation, whereas the 

true skarns, although commonly layered, generally lack lineation.

The amphibole rocks at the Copper King mine are similar to magnesian skarns that have been described from many localities in Fennoscandia. For an excellent summary of these rocks and ideas concerning their origin the reader is referred to Sundius (1935). Anthophyllite skarns have also been described from Jayville, N. Y., in 

the northwest Adirondacks, but these rocks do not contain cummingtonite.2

The biotite skols, intimately associated with the skarn, could have 

been deposited directly from hydrothermal solutions; or they may 

be the result of the alteration of the amphiboles of the skarn by pegmatitic or hydrothermal fluids.

The metamorphic rocks in the mapped area are typical of many 

Precambrian terranes throughout the world and are characteristic 

of the amphibolite facies (Eskola, 1939, p. 351-355; Turner, 

1948, p. 76).

The dynamothermal metamorphism that resulted in the recrystallization and reconstitution of the original diverse sediments probably 

occurred during a major period of Precambrian orogeny. With rare 

exceptions all of the rocks have a texture indicative of a single period 

of metamorphism; a dominant northeast-trending gneissic structure 

was developed. (See p. 189.) After this major deformation, local or 

weak metamorphism, produced by different stresses and probably 

under more nearly hydrostatic conditions, resulted in the local recrystallization of biotite: large crystals formed at the expense of 

the smaller (earlier) crystals and were oriented at an angle to the 

minerals formed earlier.

IGNEOUS KOCKS

Granite and minor amounts of pegmatite constitute 90 percent of 

the exposed bedrock (pi. 16) and probably underlie most of the 

covered areas. The granite varies from a leucogranite (Johannsen, 

1939, p. 154) to a biotite granite. The inclusions of metasedimentary 

rocks within the granite are small, probably not more than a few 

tens of feet in maximum width. The pegmatite and minor amounts 

of associated aplite form small dikes in the granite and irregular 

masses in the skarn.

2 Leonard, op. cit.
186 GEOLOGY AND ORE DEPOSITS, COLORADO

BIOTITE GRANITE

The granite is megascopically homogeneous but the proportions of 

the essential minerals vary considerably, as shown in table 2.

An average of the modes of 7 thin sections indicates that the rock 

has the composition of a leucocratic biotite granite. Small bodies 

of streaked granite occur as transitional phases to biotite-quartzplagioclase gneiss.

Sparse, angular inclusions of metasediments are present in the 

granite. Generally the inclusions are conformable with the granite, 

but locally the granite cuts sharply across the foliation of the 

inclusions.

TABLE 2. Modal analyses of granites, Copper King mine

Mineral

Biotite -- - _ - --_

Anorthite content of plagioclase

Anorthite content of rims. ________

Anorthite content of rclaeioclase .-

UO-1

27

40

8

1

tr

tr

Ana

.4-47 C-4-1

44

32

20

3

1

tr

Am -_

40

30

4

1

tr

tr

tr

G-ll

63

12

tr

tr

tr

Ails

D-2

34

32

31

2

1

tr

tr

tr

Ang

G-K

30

40

26

3

1

tr

tr

tr

tr

D-5-B

31

48

20

tr

tr

1

tr

tr

tr

Am

G-8

31

28

31

9

1

tr

tr

tr

-__-

An 24

C-l

38

46

9

6

1

tr

tr

D-17-A

47

31

15

6

1

tr

tr

tr

-------

An?<

.5

UG-1 Biotite granite, 70-foot level, Copper King mine.

A-47 Leucocratic biotite granite, surface.

C-4-1 Leucocratic biotite granite, surface.

G-ll Leucogranite, surface.

D-2 Biotite granite.

G-23 Leucocratic biotite granite, surface.

D-5-2 Leucogranite, surface.

G-8 Streaked granite.

C-l Streaked granite.

D-17-A Streaked granite.

1 Mostly perthitic.

The granite has a poor or moderate foliation produced by the 

subparallel alinement of tabular microcline crystals. The biotite 

is dispersed uniformly through the rock and has a random orientation. The porphyritic microcline crystals, although tabular, do not 

show a preferred orientation. The granite throughout the area contains abundant closely spaced incipient fractures almost parallel to 

the foliation.

The granite is typically a buff or pink medium-grained gneissoid 

rock consisting essentially of perthitic microline, quartz, and sodic 

plagioclase. The rock generally is equigranular but locally is 

porphyritic.
GEOLOGY OF COPPER KING URANIUM MINE 187

Microcline constitutes 30 to 60 percent of the rock; it typically 

occurs as interstitial anhedral, but sometimes as subhedral, grains 

as much as 1 inch long; most of the microcline is perthitic. Albite, 

occurring as smooth-walled blebs rods and strings (Ailing, 1938, 

p. 142-147) or as irregular patches, constitutes from a trace to 

about 20 percent of the volume of the perthite grains. The (010) 

plane of the plagioclase in the patch perthite is oriented parallel 

either to (010) or to (001) of the microcline. The microperthite 

containing rods and strings is similar to the perthites that Ailing 

interprets as resulting from exsolution.

The plagioclase grains are zoned; cores of oligoclase, generally 

partly altered to muscovite and epidote, are bordered by rims of 

clear albite. The contacts between the cores and rims are sharp. 

The plagioclase in the cores varies from Ani2 to An22 and shows 

a normal continuous zonation. The plagioclase in the rims varies 

from Aii4 to An9 . Rims occur on plagioclase grains at the contacts 

of the grains with microcline; they rarely are present against quartz. 

The albite rims in some grains embay the plagioclase of the cores; 

in others the rims parallel the zonation of the core and the outer 

margins penetrate the microcline; and in still others the albite rims 

embay both the plagioclase cores and the microcline.

Quartz constitutes 25 to 47 percent of the rock. It forms anhedral 

grains that show strain shadows and some mortar structure. For 

the most part the quartz embays the plagioclase and microcline, but 

in part it occurs as subrounded grains that show mutual extinction 

in microcline.

Biotite makes up as much as 9 percent but generally is slightly less 

than 5 percent of the rock. The biotite is strongly pleochroic, ranges 

from straw yellow to dark brownish green, and is in part altered to 

chlorite. (001) forms are well developed, whereas (hkO) forms are 

either ragged or rounded in abutment with quartz. Zircon inclusions, surrounded by pleochroic halos, are common.

Muscovite is ubiquitous but constitutes only 1 percent or less of 

the rock. It is mostly fine grained and occurs as ragged crystals 

in plagioclase cores, along cleavage cracks, fractures, and crystal 

boundaries, but also occurs as sharply defined crystals in parallel 

and penetrative growth with biotite.

Epidote, in minor amounts, is present in most of the rocks examined as small grains in fractures, cleavage cracks, and crystal 

boundaries. Apatite occurs as minute euhedral grains in quartz and 

microcline or as larger anhedral grains along crystal boundaries. 

Zircon forms minute grains in biotite and larger crystals in quartz and 

albite. The opaque iron oxides are dispersed through the rock as 

minute subhedral grains.
188 GEOLOGY AND ORE DEPOSITS, COLORADO

Streaked granite was noted at three places on the surface and 

locally in the drill cores. It occurs in layers of biotite-quartz-plagioclase gneiss at and near the contact of the gneiss with biotite granite. 

It always is within 25 feet of a contact with biotite granite, and farther away from the contact it grades into biotite-quartz-plagioclase 

gneiss. The bodies never exceed a few feet in width and because of 

their small size they are included with biotite-quartz-plagioclase 

gneiss on plate 16. Modes of streaked granite are given in table 2.

The streaked granite is a pink fine- to medium-grained generally 

equigranular rock streaked with gray. The streaked appearance is 

caused by paper-thin films and wisps of oriented biotite. It differs 

from the biotite granite principally in having a conspicuous gneisslike 

structure and somewhat more biotite (table 2).

The plagioclase of the streaked granite is An24, similar to that in 

biotite granite but more sodic than the plagioclase in the biotitequartz-feldspar gneiss. Clear albite rims occur on most plagioclase 

grains but they are smaller than those in the biotite granite. Porphyritic perthitic microcline crystals are euhedral and as much as half 

an inch long; generally they poikilitically include small grains of 

plagioclase, subrounded quartz, and biotite. The accessory minerals, 

opaque iron oxides, apatite, zircon, muscovite, and epidote, are the 

same suite found in the biotite granite.

GRANITE PEGMATITE

Small bodies of granite pegmatite are present within the granite, 

at the contacts between the granite and the metasediments, and locally 

within the skani. Within the granite, the pegmatite occurs as dikes, 

generally not more than a foot or two wide, along joints and foliation planes. The contacts are sharp in some places and gradational 

elsewhere. The pegmatite at the contacts with, and within, the skarn 

forms small bodies, irregular in shape.

The pegmatites are for the most part homogeneous bodies composed 

essentially of quartz and feldspar; at places in and adjacent to skarn 

some pegmatite contains a few percent of biotite. The grain size of 

the pegmatite minerals is uniform and generally is less than 2 inches 

in maximum dimension.

The feldspars, microcline perthite and sodic plagioclase, occur in 

varying proportions, but perthite is dominant. The plagioclase apparently is homogeneous and is oligoclase (Ani7 _20 ). Most plagioclase 

shows some alteration to clay minerals. The biotite is dark greenish 

brown and occurs as books as much as an inch long and one-fourth inch 

thick. Refractive index measurements indicate that nY ranges from 

1.640±.003 to 1.690±.003. The biotite and the perthite in the pegmatites are similar to those in the granite. The accessory minerals, 

muscovite and opaque iron oxides, are sparse.
GEOLOGY OF COPPER KING URANIUM MINE 189 

ORIGIN AND AGE OF GRANITES AND PEGMATITE

The granite of the mapped area, part of the Log Cabin batholith, 

is considered by Levering and Goddard (1950, pi. 1) to be Silver 

Plume granite, which is the youngest Precambrian granite in the Front 

Range. Alpha/lead determinations upon zircon from the biotite 

granite in the vicinity of the main shaft indicate a late Precambrian 

age according to the presently accepted geologic time-scale. Providing this age gives the time of emplacement of the enclosing granite, 

it is in good agreement with the geologic age as inferred from the field 

evidence.

The biotite granite, because of its granitic texture and crosscutting 

relations to metasedimentary inclusions, is thought by the writers to 

have consolidated from a magma. Regardless of its initial origin, 

however, the granite was altered after consolidation. Evidence for 

postconsolidation alteration are: clear albite rims around altered oligoclase cores along the boundaries between plagioclase and microline; 

clear albite replacing both the plagioclase cores and microcline; quartz 

truncating rimmed plagioclase; ragged muscovite flakes in plagioclase 

cores, along cleavages, fractures, and crystal boundaries; and epidote 

grains along cleavages, fractures, and crystal boundaries.

The streaked granite, on the other hand, is thought to be of replacement origin because of its spatial relations to the biotite granite and 

biotite-quartz-plagioclase gneiss, its gneisslike structure, and its gradation into biotite-quartz plagioclase gneiss. The wisps and streaks 

of biotite are interpreted to be inherited from the original gneiss.

The pegmatite is thought to be genetically related to the biotite 

granite, for so far as known, the granite is the youngest Precambrian 

intrusive in the area. Age determinations by the Pb208/Th232 method 

on two monazite crystals from a pegmatite quarry about 5 miles east 

of the Copper King mine by Phair indicate a late Precambrian age 

for the pegmatite (Phair and Sims, 1954).

STRUCTURE

The Precambrian rocks at the Copper King mine have a definite 

gneissic structure that trends northeastward and is nearly vertical. 

The rocks are broken at places by eastward-trending, steeply dipping 

faults of small displacement and of probable Laramide age.

FOLIATION AND LINEATION

The foliation in the metasedimentary and igneous rocks (pi. 16) 

trends northeast and generally dips steeply to the northwest or southeast. The metasediments occur as inclusions of various sizes in the 

granite (pi. 16) and trend northeast parallel to the foliation, but they 

are crosscut sharply by the granite and for the most part form angular,

448648 58 4
190 GEOLOGY AND ORE DEPOSITS, COLORADO

discordant blocks. To determine the general structural setting, mapping of a larger area would be necessary.

Lineation, produced by elongated plates of biotite and hornblende 

and by mineral streaking, is conspicuous in the biotite-quartz-plagioclase gneiss, the amphibolite, and the biotite schist; it is poorly developed in the granite, however, except locally where elongated quartz 

and feldspar grains show a subparallel alinement. The bearing of 

the lineation in all the rocks is nearly normal to the strike of 

the foliation; the lineation in the metasediments in the mine workings 

is parallel to small folds and plunges on the average about 1ST. 45° E.

JOINTS

Joints are conspicuous in both the metasediments and granites, but 

they are most striking in the biotite granite. The joints are nearly 

vertical, and can be grouped roughly into two sets, northeastward and 

northwestward trending. Although most joints are barren, a few 

contain discontinuous dikes of pegmatite or, less commonly, aplite, 

generally less than a foot thick, suggesting that the joints formed 

before the emplacement of these dikes.

Joints that trend about N. 80° W. are present at the surface near 

the inferred position of the surface trace of A fault, which was intersected in the mine workings (pi. 17). Study of drill cores indicates 

that this joint set is nearly parallel to the fault; joints of this attitude 

were not observed elsewhere in the mapped area.

FAULTS

Steeply dipping faults of small displacement cut the sulfidemagnetite deposit, skarn, and biotite granite at the Copper King mine 

(pi. IT). The faults are probably Laramide in age. They are 

confined to an eastward-trending zone that is a few tens of feet wide 

and extends the length of the mine; they also were cut in some drill 

cores. None of the faults has been recognized at the surface. The 

principal faults, in order of probable age, are herein named: Copper 

King fault, B fault, and A fault. Other minor fractures are common, 

but these are not discussed individually in the pages that follow. The 

oldest fracture, the Copper King fault, and the complexly branching 

fractures probably related to it contain pitchblende and were the 

source of most of the uranium ore mined.

The structural relations of the faults and the mineralogic differences 

in the fault zones indicate that the faults were open at different times, 

and there is no evidence that the fractures are contemporaneous. 

The uranium-bearing Copper King fault and its subsidiary fractures 

are clearly cut by B fault; B fault likewise is cut by A fault.
GEOLOGY OF COPPER KING URANIUM MINE 191 

COPPER KING FAULT

The Copper King fault is a nearly vertical fracture zone that trends 

N. 60° W. It has been traced in the mine and in drill cores for a 

horizontal distance of more than 50 feet; it dies out on the lower 

level by passing into many thin, branching "horsetail" fractures. The 

fault has not been recognized at the surface.

Above the 110-foot level the fault is a well-defined, tabular fracture 

that is filled with angular fragments of the wall rocks and pitchblende 

and associated ore-forming minerals. At the 110-foot level (pi. 17) 

the Copper King fault is cut by B fault, and this intersection plunges 

southwestward. Below the intersection, instead of being a welldefined tabular structure, the Copper King fault is characterized by 

horsetail fractures. The fractures trend eastward, northeastward, 

and northwestward; fractures of all trends are locally ore-bearing. 

On the 110-foot level, ore-bearing fractures of one set strike eastward 

and dip nearly vertically; fractures of another set, apparently subsidiary to the easterly set, strike about N. 65° E. and dip 60°-90° in 

either direction. Similar fractures were encountered in the stoped 

ground above the 140-foot level. In the raise from the end of the 

crosscut on the 140-foot level, however, according to the miners, the 

principal fracture trended X. 50° W. and dipped about 80° SW.; 

thin fractures branched eastward from it.

The Copper King fault was reopened several times during mineralization, and grooves and striations on the vein-forming minerals 

indicate that some movement also took place after deposition of 

the ores.

B FAULT

B fault is exposed at and below the 110-foot level in the mine. It 

probably was cut in drill hole BH-8 at a depth of 90 feet (fig. 43), 

but its identification in the core is not certain.

In the mine, the fault trends K 80°-85° E. and dips 68°-85° SE. 

On the 110-foot level (pi. 17) the fault zone is 3 to 9 inches wide and 

is filled with hematite, goethite, a dark-brown hydrous iron oxide, 

light-brown hydrous iron oxide, and vuggy quartz. The hydrous 

iron oxides are colloform; the quartz exhibits comb structure or is 

platy and commonly fills fractures and vugs in the hydrous iron oxides. 

The sulfides adjacent to the fault on the 110-foot level are fractured 

for a distance of about 2 feet from the fracture.

In the stopes between the 110-foot level and the 140-foot sublevel 

the fault is obscure, but it probably continues through the stopes into 

the conspicuous breccia and gouge zone on the 140-foot level (pi. 17). 

The B fault zone is abnormally radioactive but nowhere contains sufficient uranium to be ore (pi. 17 and p. 215) ; no uranium minerals 

have been identified from the fault zone.
192 GEOLOGY AXD ORE DEPOSITS, COLORADO

A FAULT

A fault, the most conspicuous and continuous break in the mine, 

was intersected on all levels and was noted in many of the drill cores. 

The fault extends nearly the length of the 70-foot level, where it 

strikes X. 55°-70° W. and dips about 80° SW. It can be traced 

in the shaft to a depth of about 130 feet (pi. 18). At this depth it 

passes into the southwest wall, and probably was cut on the 160-foot 

level (pi. 17). Breccia and gouge zones as much as 5 feet thick, 

believed to correlate with A fault, were intersected in drill holes 

BH-1, BH-2, BH-11, BH-8 (fig. 43), BH-3, BH-5, BH-6, and 

BH-7. The fault apparently was not cut, however, in the holes to 

the west of the mine and may die out in that direction.

The fault consists principally of soft gouge. Commonly the adjacent wall rocks, particularly the granite, are broken and oxidized. 

On the 70-foot level the fault contains abundant hydrous iron oxides, 

discontinuous thin pyrite seams, and veinlets of dense white quartz 

as much as an inch thick; but these minerals are sparse or absent, 

so far as known, in the lower workings. Throughout its length the 

fault is abnormally radioactive. (See page 215.)

A fault cuts B fault. This relationship was seen most clearly in 

the stope pillar east of the main shaft about 10 feet below the 110-foot 

level.

MINERAL DEPOSITS

Mineral deposits of two types differing widely in origin, age, and 

character are present in the Copper King mine area: massive sulfide-magnetite deposits of Precambrian age, and a pitchblendebearing vein deposit of early Tertiary age. The sulfides and magnetite replace amphibole skarn and associated rocks and constitute a 

typical pyrometasomatic deposit. The age of the magnetite in these 

deposits, as determined by the helium method of Hurley on two 

samples, is late Precambrian. The pitchblende-bearing vein deposit 

clearly cuts the sulfide-magnetite ores and surrounding country rocks 

and was determined, by isotopically corrected lead-uranium ratios, 

to be of early Tertiary age (Phair and Sims, 1954). Boxwork pyrite, 

locally containing pitchblende, occurs in the wall rock of the pitchblende-bearing vein.

SULFIDE-MAGNETITE DEPOSITS

Deposits of pyrite, sphalerite, pyrrhotite, chalcopyrite, magnetite, 

and sparse molybdenite, in order of decreasing abundance, are present 

in the Copper King mine and at shafts 1 and 2 (pi. 16). The Copper 

King deposit has a gross layering. The layers range from less than 

a foot to several feet in thickness and reflect variations in mineralogy,
GEOLOGY OF COPPER KING URANIUM MINE 193

which probably in turn reflect original layering in the skarn host 

rock.

GENERAL CHARACTER AND STRUCTURE

The sulficle-magnetite deposits are mainly in amphibole skarn; to 

a lesser extent they occur in the quartzite and related rocks (see page 

179) that are closely associated with the skarn. The Copper King 

deposit is principally in a body of anthophyllite-cummingtonite 

skarn; the deposits at shafts 1 and 2, to judge from the dumps, are 

largely in actinolite skarn.

The ore minerals form massive layers, streaks, and knots or are 

dispersed through the skarn. The massive ore forms lens-shaped 

layers, some of which have a circular cross section, that conform 

to the layering in the skarn and are elongated in the direction of the 

lineation. The sizes and shapes of individual layers and pipes are 

extremely variable, but commonly they do not exceed two or three 

feet in thickness.

The deposit at the Copper King mine is an elongate, roughly tabular body that trends east-northeast and dips steeply (pi. 16). It has 

a maximum stope length of about 40 feet, a width of as much as 14 

feet, and a breadth of approximately 25 feet. The pitch length of 

the body (as defined by Lindgren, 1933, p. 192) is not known. The 

deposit is smaller than the skarn mass that constitutes the host, for 

the skarn is in part barren or contains only sparse ore minerals.

The principal shoots of magnetite and massive sulfides within the 

deposit are distinguished on plates 17 and 18. Pyrite, the most abundant sulfide, forms a steeply dipping layer, 2 to 3 feet thick, on the 

70-foot level along the north wall of the drift (pi. 17). Sparse sphalerite, chalcppyrite, and pyrrhotite occur with the pyrite. Magnetite without appreciable sulfides forms a small body on the south 

wall 10 feet east of the east shaft.

A shoot of high-grade sphalerite ore just below the 70-foot level 

yielded 55 tons of shipping ore during shaft sinking (pi. 18). The 

shoot is a lens-shaped layer about 5 feet thick that plunges 45° to 

the east. Beneath it a 12-foot layer consisting predominantly of 

pyrite with sparse sphalerite and chalcopyrite is underlain by almost 

barren skarn.

On the 110-foot level pods of massive pyrite, pyrrhotite, sphalerite, 

and chalcopyrite occur on the north wall opposite the shaft and plunge 

about N". 50° to 60° E. Pyrrhotite is more abundant here than elsewhere in the mine, and at places it occurs with other sulfides as lenses 

as much as 3 feet across.

On the 140-foot level pyrite is the most abundant sulfide, but a 

few layers of high-grade sphalerite ore, a foot or less thick, that
194 GEOLOGY AND ORE DEPOSITS, COLORADO

trend across the drift and dip N. 50° to 60° E. were cut about 25 feet 

northeast of the main shaft.

MINERALOGY

Pyrite is the most abundant and widespread component of the 

sulfide-magnetite deposits; pyrrhotite, sphalerite, chalcopyrite, and 

magnetite are locally abundant. Molybdenite is sparsely disseminated 

at places in the wall rocks at the margins of the deposits but does 

not occur with the other ore minerals. There is no gold and less 

than O.OOX percent silver in the ores that have been analyzed. (See 

table 4.)

At the Copper King mine the sulfide-magnetite body has been brecciated where it is cut by the pitchblende vein, and fragments of the 

sulfides and magnetite are locally incorporated in the vein.

MAGNETITE

Magnetite is abundant only on the 70-foot level where it forms a 

massive, irregular layer (pi. 17). Where intergrown with the sulfides, the magnetite occurs sparsely as anhedral or subhedral crystals 

1 mm or less in diameter, corroded slightly by the younger sulfides. 

The magnetite ore on the 70-foot level consists of aggregates of 

magnetite grains that embay and corrode cummingtonite, green 

biotite, and quartz. Pyrrhotite, pyrite, chalcopyrite, and sphalerite 

occur as sparse grains in the interstices of the magnetite grains.

PYRRHOTITE

Pyrrhotite is widespread but not abundant. It occurs as aggregates of anhedral grains, commonly more than 1 mm in diameter, 

which are intergrown with pyrite and other ore minerals, and also 

as blebs within sphalerite. The pyrrhotite aggregates embay magnetite and occur between magnetite and silicate grains; in turn, they 

are apparently embayed by sphalerite. The blebs of pyrrhotite in 

the sphalerite commonly are associated with blebs of chalcopyrite. 

The smaller blebs are arranged in straight or curved films. The 

straight films are along the traces of definite crystallographic planes 

in the sphalerite. The curved films, however, in part appear to disregard the crystal planes. At places pyrrhotite and chalcopyrite 

occur together in the same bleb in sphalerite, in which case the 

pyrrhotite appears to embay the chalcopyrite. Where fractured, the 

pyrrhotite is veined by pyrite or, less commonly, by marcasite.

PYRITE

The pyrite belonging to this period of ore formation is a distinctive 

and unusual morphologic type. Most of it is weakly anisotropic. 

It is characterized by many subrounded to elliptical forms (fig. 45)
GEOLOGY OF COPPER KING URANIUM MINE 195

FIGUHE 45. Photomicrograph showing typical elliptical form of early pyrite. 

Note that the pyrite with elliptical form grades into massive homogeneous 

pyrite. The pyrite is emhayed hy sphalerite (dark gray). Both minerals are 

veined hy late pyrite.

that commonly are surrounded by, and gradational into, massive anheclral or subhedral pyrite that appears homogeneous. As shown in 

figure 45, the massive, homogeneous pyrite at places cuts the elliptical 

forms. In figure 46 it can be seen that the concentric rings in the

FIGURE 40. Photomicrograph of early pyrite showing oriented inclusions of 

amphihole. Sphalerite (dark gray area in right of photomicrograph) emhays 

and veins the pyrite.
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FIGURE 47. Photomicrographs of early pyrite showing stages in the replacements of cummingtonite. A, Pyrite containing oriented inclusions of cummingtonite. The ends of the cummingtonite blades are rounded. B, Pyrite with elliptical forms that contain ragged 

remnants of cummingtonite blades. The elliptical form is interpreted as a relict texture produced by pseudomorphic replacement 

of elongate oriented slightly warped cummingtonite blades. C, Pyrite characterized by well-developed concentric nonopaque layers. 

Replacement of the host is nearly complete.

COO5
GEOLOGY OF COPPER KING URANIUM MINE 197

pyrite consist of oriented inclusions of the amphibole host. As shown 

in the large grain in figure 46, the host remnants within an individual 

concentric ring have a subparallel arrangement. In other pyrite 

grains the inclusions similarly have a common orientation but do not 

form a concentric ring.

Because the cummingtonite gangue remnants in the pyrite have a 

marked preferred orientation, pyritiferous layers have a prominent 

lamellar structure. Stages in the replacement of the gangue by pyrite 

are shown in figure 47. Figure 47J. shows oriented inclusions of 

cummingtonite in massive pyrite. The cummingtonite is little replaced, and only the ends of the crystals are slightly rounded. In 

figure 475, ragged remnants of cummingtonite blades occur in pyrite 

characterized by well-developed elliptical forms. The pyrite with the 

elliptical forms has concentric layers containing minute inclusions of 

the host. This structure is inherited by pseudomorphic replacement 

of the elongate oriented slightly warped cummingtonite blades. In 

figure 476", where replacement of cummingtonite is nearly complete, 

the concentric nonopaque layers are clearly defined.

SPHALERITE

Sphalerite is locally abundant in the sulfide-magnetite deposits. 

It is dark brown and iron rich (marmatite) and contains tiny exsolution blebs, films, and blades of chalcopyrite. At places it also contains 

exsolution blebs of pyrrhotite. The sphalerite commonly is intergrown with chalcopyrite, pyrite, and pyrrhotite. It embays the early 

pyrite (figs. 45 and 46) and in turn is embayed and corroded by chalcopyrite. At places it fills interstices between magnetite grains and 

at other places it corrodes magnetite. The sphalerite commonly 

forms large anhedral grains 1-3 mm in size. In most specimens the 

chalcopyrite blebs constitute about 5 percent of the volume; they are 

oriented principally along the (111) planes of the sphalerite, although 

some appear to be distributed at random.

CHALCOPYRITE

Chalcopyrite is widespread but it constitutes only a percent or two 

of the deposits. It occurs (1) as clots and layers of anhedral grains 

that alternate with layers of sphalerite, pyrite, and pyrrhotite, and 

(2) as minute blebs in sphalerite. The grains of chalcopyrite embay 

both pyrrhotite and sphalerite. The textures resulting from minute 

blebs and films of chalcopyrite in sphalerite are similar to those described by Edwards (1947, p. 79-82).

MOLYBDENITE

Scattered plates of molybdenite are present in the granite wall 

rocks near the contacts with the skarn ores. So far as known, the
198 GEOLOGY AND ORE DEPOSITS, COLORADO

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GEOLOGY OF COPPER KING URANIUM MINE 199

molybdenite is most abundant on the 140-foot level. Spectrographic 

analyses for molybdenum indicate that it constitutes O.OOX percent or 

less of the massive ores (table 3). Because molybdenite has not been 

observed in the ores, little is known concerning its relation to the ore 

minerals.

TRACE ELEMENTS

Semiquantitative Spectrographic analyses that show the minor element content of the ores are given in table 3. It can be seen that the 

sulfide ores contain sparse quantities of cobalt and nickel, which are 

characteristic of pyrrhotite-bearing ores. Indium and cadmium are 

present in some ores, and are particularly abundant in ores rich in 

sphalerite. The magnetite ore contains as much as O.OX titanium.

The theoretical ranges for the values reported in tables 3, 4, 

and 5 are:

Subgroup Theoretical range

0. X + 0. 464-1. 0

. X . 215- . 464

. X- . 10 - . 215

PABAGENESIS

The minerals of the sulfide-magnetite deposits are believed to have 

been formed during a single period of mineralization. The paragenesis of the minerals is shown in figure 48. The order of introducMagnetite Blebs in

sphalerite

Pyrrhotite ~^^^*

Pyrite

Sphalerite

Chalcopyrite "~\

s Exsolution 

Quartz 

FIGURE 48. Paragenetic sequence of the massive sulfide-magnetite ore minerals.

tion and replacement of the different minerals follows closely the 

usual sequence in pyrometasomatic deposits.

Magnetite was the first ore mineral to be deposited. It replaced 

cummingtonite skarn and quartzite; and oriented, corroded blades 

of cummingtonite and biotite are widespread in the iron ore. Subsequent to its formation, small quantities of quartz were introduced. 

The quartz replaced magnetite or filled the interstices between magnetite grains.
200 GEOLOGY AND ORE DEPOSITS, COLO'RADO

Pyrrhotite, the next mineral to be deposited, replaced magnetite 

and the host minerals. At places it occurs between magnetite and 

silicate grains. Some of it may have filled cavities in the host and 

spaces between ore mineral grains.

Pyrite replaced the gangue minerals and also pyrrhotite. It crystallized mostly as spheroidal forms (fig. 45) but in part as massive 

anhedral or subhedral crystals.

Sphalerite is later than the magnetite, pyrrhotite, and pyrite, for 

it corrodes and embays these minerals. At places it contains corroded fragments of oriented silicate grains. Some chalcopyrite in 

solid solution with sphalerite exsolved during cooling as minute blebs 

and blades; and this generation was closely followed by the deposition of homogeneous chalcopyrite, grains of which replaced the 

silicate grains of the host and sphalerite along crystal interfaces. 

Contemporaneously with the chalcopyrite, blebs and films of pyrrhotite exsolved from sphalerite.

AGE AND ORIGIN

The sulfide-magnetite deposits are replacements of amphibole skarn 

and related biotite skols and quartzite. The ore minerals were deposited after the skarn had formed and they embay and corrode the 

host minerals. At the Copper King mine, cummingtonite skarn was 

a more favorable host than anthophyllite skarn.

Because of the close spatial relationship of the deposits to the biotite granite, it is probable that the ore-forming fluids were derived 

from the magma that consolidated to form the granite. The correspondence in age of the magnetite of the deposit (see p. 192) and 

the biotite granite which surrounds the deposit, as reported previously (p. 189), lends strong support to the pyrometasomatic origin 

of the ore deposit as inferred originally from the field evidence.

The deposits are typical of the class of ore deposits that Lindgren 

(1933, p. 35-49) has called pyrometasomatic, and they resemble many 

other deposits of this class in both the western (Knopf, 1933, 

p. 537-539; Lovering and Goddard, 1950, p. 64-71) and eastern 

(Leonard 3 ; Sims and Leonard, 1952) parts of the United States. The 

deposits also are strikingly similar to some deposits of sulfides and 

magnetite in central Sweden (Geijer, 1917; Magnusson, 1940, 1950).

PITCHBLENDE DEPOSIT

Pitchblende and associated minerals occur in the Copper King 

fault and subsidiary fractures and locally in the interstices of pyrite 

boxwork in the wall rock adjacent to the fault. Both types of ore 

provided important quantities of uranium. In order of decreasing

s Op. cit.
GEOLOGY OF COPPER KING URANIUM MINE 201

abundance, siderite, pyrite, marcasite, quartz, sphalerite, and chalcopyrite were deposited with the pitchblende.

The pitchblende-bearing vein occurs in the Copper King fault. 

The vein minerals formed by deposition in open spaces and to a small 

extent by replacement. In the upper workings the Copper King vein 

is well-defined, tabular, and nearly vertical and it strikes N. 60° W. 

(pi. 17), but below the 110-foot level the vein is ill-defined and consists of branching horsetail veinlets of several trends, as described on 

page 191. The vein dies out laterally and vertically and passes into 

barren fractures. Above the 110-foot level the wall rock of the vein 

is mainly ore-bearing amphibole skarn, with subordinate biotite 

granite, but below this level the wall rock is biotite granite (pi. 18).

Where the vein cuts skarn and massive sulfides, it is hard and compact and consists of angular, brecciated fragments and shreds of the 

wall rocks that are veined and cemented by pitchblende and associated vein-forming minerals (fig. 49^4). As seen in the photograph, 

individual veinlets are extremely variable in trend and in mineralogy; 

the vein margins are irregular and jagged. Although most veinlets 

are subparallel to the vein walls many are at a marked angle. As 

shown in figure 49Z?, most of the radioactivity is limited to the dark 

veinlets; the variation in pitchblende content of the veinlets is shown 

by the differences in intensity of radioactivity. The maximum width 

of the vein in this part of the mine is about 12 inches but the average 

width is 3 to 6 inches. The walls are frozen; vugs are small and rare.

In the lower workings wThere the vein cuts biotite granite, it consists of many branching veinlets which are composed largely of pitchblende and siderite. Most veinlets are less than an inch wy ide, and 

profitable mining of this part of the deposit is dependent upon a closespacing of individual veinlets. (See pi. 17.) 

" ''^

PYRITE BOXWORK

Throughout the mine those parts of the sulfide-magnetite deposit 

that are mostly pyrite have been altered either to boxwork ore or to 

lacy ore. At places near the Copper King vein the pyrite boxwy ork 

contains minable quantities of pitchblende.

The pyrite boxwork is characterized by thin branching and intersecting veinlets of pyrite and marcasite separated by void spaces 

(fig. 50). At places siderite and pitchblende occur wy ith the pyrite 

and marcasite in the veinlets, and UO3 -rich pitchblende partly fills 

the vuggy openings. The half-inch layer parallel to the lower edge 

of the photo in the middle of figure 50 is sphalerite and pyrite that 

conforms to the relic layering of the skarn host rock. The remainder
202 GEOLOGY AXD ORE DEPOSITS, COLO'RADO

FIGURE 49. A (upper photograph), Specimen showing polished surface of pitchblende-bearing vein from upper part of Copper King mine. A, The pitchblende 

(black) forms irregular veinlets that cut the breccia. (WR, wall rock; Sp, 

sphalerite ; Py, pyrite ; Cp, chalcopyrite ; Bid, siderite ; P, pitchblende.) B, Autoradiograph of A.

of the specimen, aside from the veinlets, consists of gangue minerals, 

principally cummingtonite and biotite, that are corroded and veined 

by pyrite and oriented parallel to the middle sulfide layer. The pyrite 

veinlets that are oblique to the laminae contain thin films of siderite 

and at places pitchblende. They are most conspicuous and best developed where they cut the amphibole and associated biotitic layers. 

Where the veinlets cut the sulfide layer they are wy eak and composed 

mostly of siderite.
GEOLOGY OF COPPER KING URANIUM MINE 203

*u. »'

FIGURE 50. Photograph of pyrite boxwork. Pitchblende, siderite, pyrite, and local marcasite form intersecting veinlets that cut layered amphibole skarn. Pitchblende rich in 

UO3 is present in vugs between the veinlets. The half-inch layer in the middle of the 

specimen is massive sulflde.

Biotite from replaced skols is the principal gangue in the vugs between the veinlets. Marcasite rather than pyrite may be the principal 

iron sulfide in the veinlets.

At places the veinlets do not form a boxwork but are highly irregular and delicate, and form a lacy pattern (fig. 51). Material of 

this type occurs adjacent to massive sphalerite-pyrite-chalcopyrite 

ore and is characterized by irregular, small vugs that locally are coated 

by pitchblende.

PITCHBLENDE SHOOT

The pitchblende-bearing portion of the vein and associated boxwork 

ore form a nearly vertical shoot extending from 45 to 135 feet below 

the surface and about 50 feet horizontally (fig. 42). Because the pitchblende is irregularly distributed, not all of the shoot could be mined 

profitably. The outline of the ore that was extracted from the shoot 

is shown on figure 42.
204 GEOLOGY AND ORE DEPOSITS, COLORADO

FIGURE 51. Photograph of lacy ore. Pitchblende (black) is in vugs formed by the 

leaching of sphalerite from pyrite-sphalerite replacement ore.

The ore within the shoot occurs in three structural envoironments: 

Above the 110-foot level, the pitchblende primarily is in the tabular 

Copper King fault; below this level, the uranium occurs in branching 

fractures related to the Copper King fault, and profitable extraction 

is dependent upon close spacing and high uranium content of the 

separate mineralized fractures; and at places adjacent to the Copper 

King vein high-grade uranium ore occurs in pyrite boxwork.

Mining indicated that the shoot generally is sharply delineated. The 

western edge of the shoot occurs in the shaft. Except at the 110-foot 

and 140-foot levels the east wall of the shaft was in ore (pi. 18), but 

the west wall was barren. Above the 110-foot level, the change from 

ore to barren vein is abrupt, for at places the ore was as much as a foot 

thick on the east wall. The eastern limit of the shoot is less welldefined. On the 70-foot level, 35 feet east of the main shaft, the vein 

is an inch thick and high in grade. Apparently this vein extends into 

the wall, but it is too thin to be worked profitably. On the 110-foot 

level the vein is exposed throughout the length of the drift, but beyond 

a distance of 24 feet east of the shaft it is thin and nearly barren 

(pi. 18). Uranium-bearing fractures extend into the east face of the 

140-foot level and were cut in drill holes BH-3 and BH-8 (fig. 43), but 

these fractures are too small and too widely-spaced to constitute ore. 

Similarly, on the 140-foot level, the veinlets that continue into the floor 

of the drift are not sufficiently abundant or rich to be ore.
GEOLOGY OF COPPER KING URANIUM MINE 205

Within the shoot the pitchblende occurs in lenses and pods separated by slightly mineralized vein matter. The grade of the ore within 

the lenses is variable, but ranges from about .2 percent uranium to 

as much as 20 percent uranium. The largest known lenses are above 

the 110-foot level. On this level west of the shaft a pod of ore about 

12 feet long, 10 feet high, and as much as 12 inches thick was mined 

(fig. 42). Part of a somewhat larger lens, a maximum of 12 inches 

thick but averaging 2 to 3 inches thick, was mined east of the shaft. 

Several tons of ore from this lens was left in the pillar on the east 

side of the shaft.

The horsetail fractures that comprise most of the ore shoot below 

the 110-foot level contain veinlets of high-grade uranium generally 

one-eighth to one-half inch thick. Mining of this part of the vein 

was largely dependent upon the relative abundance of the veinlets 

over a mining width. At places along the east side of the shaft 

(fig. 42) the fractures were sufficiently closely spaced to be mined 

across a width of as much as 4 feet.

At two places within the deposit uranium-bearing pyrite boxwork 

ore was extracted (fig. 42). This type of ore occurs only within 5 feet 

of the Copper King vein or a uranium-bearing horsetail fracture. 

The pyrite boxwork more distant from a uranium-bearing vein, so 

far as known, is barren or only slightly uraniferous.

Several tons of high-grade boxwork pyrite ore and lacy ore was 

mined from the shaft 5 to 10 feet below the 70-foot level (pi. 18 and 

fig. 42). Plates 50 and 51 show typical ore from this place. The ore 

occurred in the lower part of the sphalerite-rich pod and in the upper 

part of the pyrite-rich pod shown on plate 18. The ore was high in 

grade and much of it contained more than 2 percent uranium. Another body of pyrite boxwork ore, comparable in size and grade, was 

mined from the 140-foot level, about 35 feet east of the main shaft 

(pi. 17). This ore occurred in a pyrite boxwork layer, 1 to 2 feet thick, 

that trended northwestward across the strike of the northeastwardtrendiiig veinlets; it decreased in tenor away from uranium-bearing 

veinlets. Boxwork pyrite containing low-grade uranium is exposed 

on the 110-foot level opposite the shaft (pi. 17); no attempt was made 

to mine it. An analysis of this rock (CK-83) gave 0.21 percent equivalent uranium and 0.08 percent uranium.

MINERALOGY

In addition to pitchblende, the deposit consists, in order of decreasing abundance, of siderite, pyrite, marcasite, quartz, sphalerite, and 

chalcopyrite. Perhaps some of the minerals that coat older minerals 

and fill vugs and probably all of the sphalerite are of supergene 

origin.
206 GEOLOGY AND ORE DEPOSITS, COLORADO

PITCHBLENDE

Pitchblende is used in this report as a mining term, analogous to 

limonite or wad, to include 3 distinct uranium-rich phases, which are 

black and commonly indistinguishable in the lield. Only where X-ray 

diffraction results have confirmed the identification are the specific 

names uraninite, coffinite, and UO3-rich pitchblende used. The uraninite is a primary mineral presumably deposited from hydrothermal 

solutions. The UO3 -rich pitchblende is a secondary mineral attributed to supergene solutions. The origin of the coffinite is not definitely known, but as the coffinite-rich mixtures from the mine gave 

early Tertiary lead/uranium ages similar to that of the uraninite, it 

is also probably hydrothermal. It occurs intergrown minutely with 

uraninite. Coffinite is a newly identified uranium mineral, a hydrous 

silicate having the zircon structure; hydroxyl groups substitute for 

a part of the silica (Stieff, Stern, and Sherwood, 1955).

Except in high-grade veinlets, the pitchblende is difficult to distinguish megascopically from nearly barren siderite-rich vein material 

without radioactivity measurements or chemical analyses. It is also 

difficult to distinguish under the microscope because of its varied 

morphologic forms and its generally fine texture and intimate intergrowth with siderite,

The pitchblende has four principal modes of occurrence, in part 

apparently related to differences in morphologic types. Most commonly, the pitchblende is minutely intergrown with dark-brown 

siderite. Individual grains are tiny; the exact form cannot be determined even with magnifications greater than 200 times. Possibly this 

pitchblende is microgranular and constitutes extremely fine-grained 

aggregates.

Pitchblende also is present as tiny veinlets as much as a few millimeters wide and several millimeters long. A typical occurrence of 

this type of pitchblende can be seen in figure 52, which shows irregular, anastomosing pitchblende veinlets in granite host rock. As can 

be seen in the photomicrograph, the pitchblende veinlets appear 

mottled; tiny subrounded to irregular forms with a bluish tinge are 

surrounded by irregular areas of less reflective pitchblende.

Colloform grains with prominent concentric cracks commonly filled 

with siderite are sometimes found in the vein (fig. 53) ; the largest 

of these grains measure about 3 mm across. Radial cracks, typical of 

many pitchblendes with spheroidal outlines, are not common; they are 

best developed in small rounded forms (fig. 54).

As a fourth mode of occurrence, particularly in the boxwork pyrite 

ores, the UO3 -rich variety of pitchblende is found in vugs as coatings 

on older minerals, principally sphalerite and chalcopyrite and less 

commonly pyrite (fig. 55). This pitchblende forms thin layers and 

has a generally conspicuous concentric layering.
GEOLOGY OF COFFEE KING URANIUM MINE 207

FIGURE 52. Photomicrograph showing pitchblende veinlets in granite host rock. 

The veinlets occupy irregular fractures. The white areas within the pitchblende 

are marcasite.

As the separate phases of the ore cannot be determined in the field, 

the distribution of each phase in the deposit is not fully known. 

Uraninite probably is the most abundant and widespread phase, occurring throughout the deposit, but commonly it is minutely intergrown with coffinite. UO3-rich pitchblende characteristically occurs

FIGUKE 53. Photomicrograph of pitchblende showing colloform texture and welldeveloped concentric cracks. Pyrite (white), associated with siderite (dark 

gray), at places forms a partial rim on pitchblende.
208 GEOLOGY AND ORE DEPOSITS, COLORADO

FIGURE 54. Photomicrograph of colloform pitchblende with radial cracks filled 

with siderite (dark gray). Siderite also veins the pitchblende. Pyrite (white) 

forms uniform, complete rims on the pitchblende.

in the boxwork and lacy ores, but it also is present locally within the 

vein.

Veinlets of high-grade uraninite with a colloform structure occur 

in fractures within massive sulfide ores. The uraninite has a pitchy

FIGURE 55. Photomicrograph of pitchblende rich in UO3. The pitchblende (P) forms 

colloform coatings on fragments of sphalerite (Sp) and chalcopyrite (Cp). Note that 

pitchblende is mottled light and dark gray.
GEOLOGY OF COPPER KING URANIUM MINE 209

or glossy luster and a specific gravity above 6.0. In thin section it 

is opaque and has a greenish color. It tends to be associated with 

pyrite and chalcopyrite. Commonly it is intimately intergrown with 

coffinite and UO3-rich pitchblende. It also coats wall rock and sulfide fragments in the breccia and forms veins in the crushed matrix.

Pure coffinite has not been obtained. In contrast to uraninite, it 

has a dark-gray color, a much lower specific gravity, and an earthy 

appearance. In thin sections it is a translucent brown.

UO3 -rich pitchblende occurs both in the boxwork and lacy ores 

and in the vein. In the boxwork ores it lines cavities and is dusted 

on amphibole blades that project into the open spaces; in the lacy 

ore it fills cavities formed by the selective leaching of sphalerite. The 

UOs-rich pitchblende in the veins is intergrown with uraninite and 

coffinite. The UO3 pitchblende in part is soft and sooty, but at places 

it has spheroidal outlines characteristic of colloform structures. It 

gives no X-ray diffraction pattern and according to the data of Brooker 

and Nuffield (1952) should contain 80 percent or more of its total 

uranium as UO3 .

In thin and polished sections, an isotropic mineral with a high 

index of refraction locally veins the pitchblende or forms rims around 

it. So far as known, this mineral occurs in the locality studied only 

in close association with pitchblende; it may be a secondary uranium 

mineral.

SIDERITE

Siderite, the most abundant and conspicuous gangue mineral in 

the vein, was deposited at several stages. In general, the earlier 

siderite is relatively coarse grained and colorless in thin section and 

forms comb structures in vein fillings, whereas the later siderite is 

fine grained, tan to dark brown, and forms veinlets that cut the earlier 

minerals, including pitchblende (fig. 54). Some coarse clear siderite 

is obviously later than the tan siderite. Regardless of color, all of 

the siderites have a ^ index of 1.86 or greater. Pitchblende apparently occurs generally with the dark-colored siderite.

QUARTZ

Quartz of several varieties was deposited in small lenses and layers 

a maximum of a few millimeters thick and is generally inconspicuous. 

Most of the quartz formed early or intermediately in the paragenetic 

sequence. The earliest quartz typically was clear and formed euhedral 

crystals. Subsequent to its deposition it generally was brecciated by 

movements along the vein. Other varieties, in approximate decreasing age, are clear, fine-grained mosaic quartz, euhedral smoky quartz, 

and yellow botryoidal quartz with concentric layering. In B fault 

clear quartz is concentrically interlayered with goethite in colloform 

structure.
210 GEOLiOGY AND ORE DEPOSITS, COLORADO

PYRITE

The pyrite of these ores is characterized by its occurrence in and 

along fractures and by its generally close spatial association with 

siderite. It is present in the Copper King vein and subsidiary veinlets and in the pyrite boxwork ores. All of this pyrite is believed to 

be distinctly later than the pyrite of the sulfide-magnetite ore assemblage, but there is no certainty that all is closely related in time to 

uranium deposition; possibly some of the pyrite is supergene in origin.

The pyrite is pale brass yellow and has a dull or moderate metallic 

luster. Typically it occurs in fractures that cut the older minerals 

(fig. 56). In figure 56, a photomicrograph of pyrite boxwork ore, the

FIGURE 56. Photomicrograph showing pyrite veinlets that cut sphalerite. The 

sphalerite formed during the early (Precambrian) period of mineralization. 

Subsequently it was fractured and veined by the pyrite.

pyrite is seen to fill fractures cutting sphalerite, which formed during 

the early pyrometasomatic period of mineralization. In figure 45, a 

thin veinlet of pyrite is seen to cut sphalerite and elliptical pyrite 

forms. In many specimens pyrite is intergrown with or rimmed by 

marcasite.

A variety of pyrite common in the vein but sparse in the pyrite 

boxwork consists of fine-grained aggregates of minute pelletlike grains 

with spheroidal outlines that are dispersed through tan to brown 

siderite. Individual pellets are less than 1 mm in diameter. Another 

variety, less abundant, occurs as tiny wirelike forms.
GEOLOGY OF COPPER KING URANIUM MINE 211 

MARCASITE

Marcasite is a widespread component of the pitchblende-bearing 

ores. It appears to be more abundant in the lower mine workings 

where it is associated with siderite and pitchblende. The marcasite 

occurs as layers, generally less than 2 mm across, that alternate with 

layers of quartz and siderite in the Copper King vein; as distinct 

veinlets that occupy fractures, particularly in the boxwork pyrite ores 

but also in the vein; and as rims on pyrite. The marcasite generally 

has a well-developed comb structure.

SPHALERITE

Resin sphalerite is common in the pitchblende-bearing ores. It is 

distinguished from the early generation of sphalerite by the absence 

of chalcopyrite blebs, strong internal reflections, and low reflectivity. 

Most commonly it coats vugs, but at places it occurs as anhedral grains 

and veinlets associated with siderite and pitchblende.

CHALCOPYRITE

A minor amount of chalcopyrite is present in the Copper King vein. 

It is distinguished from the earlier generation of chalcopyrite by its 

occurrences in veinlets that were formed later than some of the siderite.

GOETHITE

Goethite is abundant in B fault at the 110-foot level, but so far 

as known is not present in the Copper King vein. It is concentrically 

interlayered with clear quartz to constitute a colloform structure. 

Locally it is associated with sparse crystalline hematite and brown 

hydrous iron oxides.

FLUORITE

A small quantity of purple anhedral fluorite was observed in the 

altered granite adjacent to a pitchblende-siderite veinlet. The fluorite was dispersed through altered plagioclase and along crystal 

boundaries. Fluorite was also observed in a prospect pit a quarter 

of a mile from the Copper King mine.

TRACE ELEMENTS

A spectographic analysis of a uranium ore sample from the stope 

above the 110-foot level (CK-82) is given in table 4. For comparison, 

spectrographic analyses, together with the equivalent uranium and 

chemical uranium analyses, are given for gouge taken from A and B 

faults.

Aside from manganese and zinc, which exceed one percent in the 

vein, the trace element suite of the Copper King vein is similar to that
TABLE 4. Semiquantitative spectrographic analyses of material in Copper King ve

[Analyst: A. T. Myers]

Field 

No. Material in 

CK-80 B Fault 

CK-81 ....do -_-_

CK-82 Copper King 

vein. 

CK-85 A Fault......

CK-86 ..._do. ..

CK-87 _do .....

CK-88 - do 

Field 

No. Material 2 in 

CK-80 B Fault 

CK-82 Copper King 

vein. 

CK-85 A Fault ...

CK-86 ...-do ....

Width 

of 

sample 

(feet) Location 

1. 0 110-ft level

.2 .....do ..

1. 0 Crosscut, 

160-ft 

level. 

. 4 70-ft level.

.5 -_._do_ 

.9 ___ _do -

Width

sample 

(feet) Location

1.0 110-ft level

. 2 do

1. 0 Crosscut, 

160-ft 

level. 

. 4 70-ft level. 

.5 do ..

XX. 0. 

XX. 

X.

XX. X.

X. 

XX. X. 

XX. X.

Ga 

0. 000X4-

. 000x4-

. OOOX+ 

.000X4-

.oox- 

.ooxAl Fe 

X- XX. 0. 

0X4- XX.

00x4- xx. 

4- X.- .

X XX. 

- XX. 

XX.

Mo Nb 

O.X- ......

.OX

.OX- ______

________ O.OOX

.00X4- -----

.00x4- 

.ox .oox

Ti

00x4- o. 

oox

OX- X. 

Xoxoxox

Ni 

0. OOX-

.oox- 

.oox-

.ooox- 

.ooxMn Ca

0x4- o. ox o. 

ox .ox- 

- . 0X4-

00x4- x0X4- . OX 

OX+ . X- X. 

0x4- . 0x4-

Pb Sc

o.oox- .......

.oox- .......

.ox- _______

.00x4- o.ooox 

.oox- ..._._.

.ox- _______

.0X4- .

Mg Na 

X- O.X 

X- .X 

X4- .X 

X- .X

X4- 

.0X

X- . X

Sr 

._ __ _._ 

- --- 

0.00x4- 

 _ 

. OOOX- 

. OOX 

i Looked for but not found: P, As, Au, B, Bi, Cd, Ce, Cr, Dy, Er, Gd, Ge, Hf, B>, 

In, Ir, La, Li, Nd, Os, Pd, Pt, Re, Rh, Ru, Sb, Sm, Sn, Ta, Th, Tl, Te, and W. 

Some elements of this group possibly are present in trace amounts, but because of 

their low-standard sensitivities were not detected.

* All are chip sam

3 Determined by 

4 Determined by 
GEOLOGY OF COPPER KING URANIUM MINE 213

of the gouge in A and B faults. The zinc in the vein is contained 

in sphalerite. The source of the manganese, however, is not known; 

possibly it is contained in a manganiferous siderite.

The gouge in both A and B faults contains elements common to the 

granite and skarn ore wall rocks and elements probably introduced 

by the Tertiary ore-forming solutions. The elements found in the 

gouge (table 4) are similar in abundance to those occurring in the 

fresh granite. (See table 5.) The gouge, however, contains more 

iron, and less aluminum, calcium, sodium, and barium than fresh 

granite. In addition the gouge, like the vein material, contains the 

elements cobalt, silver, copper, niobium, nickel, and zinc which are 

not present in measurable quantities in the granite.

PARAGENESIS

The paragenesis of the vein-forming minerals is complex. During 

mineral deposition the vein was repeatedly reopened, as shown by 

several stages of fracturing and recementation by the vein-forming 

minerals. Subsequently, supergene solutions altered parts of the 

deposit.

During initial fracturing, fragments of the wall rocks were incorporated in the vein, particularly in the upper mine workings, and 

skarn ore minerals, mainly pyrite, sphalerite, and chalcopyrite, were 

broken into tiny fragments to form a microbreccia texture (Schwartz, 

1951, p.589).

Later the vein was invaded by barren siderite and by pyrite, marcasite, and minor quartz. Some sphalerite and chalcopyrite may also 

have formed at this time. The main period of pitchblende deposition 

followed a period of fracturing. " The pitchblende, together with finegrained tan to brown siderite, veined the earlier minerals and filled 

openings between them. The siderite began to form before the 

pitchblende and continued to crystallize after the cessation of deposition of the uranium. In part it is in concentric cracks in the pitchblende ; much of it also veins pitchblende. A small amount of pyrite 

was deposited after the pitchblende, largely as rims on pitchblende 

(fig. 54). Much later, resinous sphalerite, some siderite, and finegrained quartz were formed, probably by supergene solutions. Also 

late in the sequence, UO3-rich pitchblende was deposited as thin 

coatings and colloform layers in vugs in the vein (fig. 55) and in the 

boxwork pyrite ore.

AGE AND ORIGIN

Determinations by the methods Pb 206/U 238 and Pb 207/U 235 on two 

samples of hard pitchblende from the Copper King vein, not from the 

boxwork pyrite ore, gave ages ranging from 55 to 76 million years after 

suitable common lead corrections. This range is similar to that ob-
214 GEOLOGY AND ORE DEPOSITS, COLORADO

tained for four samples from the Central City, Colo., district when 

similarly corrected and shows that the Uranium deposition in both 

areas was contemporaneous within those limits.

The hard pitchblende and associated vein minerals in the Copper 

King vein are thought to have been deposited by hypogene solutions, 

although subsequently they were somewhat modified by supergene 

solutions. The primary ore minerals were deposited in the relatively 

open parts of the Copper King fault. The amphibole skarn and 

associated sulfide-magnetite deposits, because they were more competent than the biotite granite, remained relatively open when fractured, 

and hence more permeable to the ore-forming solutions than adjacent 

broken rocks.

Deposition of the uranium was possibly related to some extent to a 

chemical reaction between the ore-forming solutions and the wall 

rocks. It is known from field occurrences that some important deposits of pitchblende are localized in mafic wall rocks (Robinson, 

1951; Murphy, 1946, p. 433; Adams, Gude, and Beroni, 1953, p. 2; 

Adams and Stugard, 1956, p. 202-204). At the Copper King mine, the 

sulfide-magnetite ores and skarn, a mafic iron- and magnesium-rich 

host, are the principal wall rocks for the vein, but important quantities of uranium also occur at places in the adjacent granite. Consequently, if the mafic wall rocks did exert a chemical control on ore 

deposition, the ore was not deposited entirely within the limits of 

these rocks.

The hard pitchblende-siderite-pyrite-marcasite yeinlets within the 

pyrite boxwork are thought to be related to, and almost contemporaneous with, ore deposition in the Copper King vein. The development of these boxwork ores was complex. The sulfide-magnetite ore 

assemblage formed during the Precambrian period of pyrometasomatic mineralization. (See page 200.) During Laramide(?) time 

these ores were fractured contemporaneously with the development 

of the Copper King fault and subsequently were leached. Pitchblende, pyrite, and marcasite were deposited in the fractures 

(fig. 50). Subsequently, through the action of circulating supergene 

solutions, vugs were formed between the veinlets by the leaching of 

materials, chiefly sphalerite and silicate minerals; and pitchblende 

in the veinlets was partly oxidized and hydrated (?) and redeposited 

in the vugs. The lacy ores (fig. 51) formed essentially contemporaneously with the pyrite boxwork, largely by the leaching of sphalerite from sphalerite-pyrite-chalcopyrite ores of Precambrian age.

Supergene alteration extended to the lowest workings in the mine; 

pyrite boxwork and lacy ores containing UO3 -rich pitchblende are 

found on the 140-foot level. Because the supergene mine waters were 

highly acid, colored secondary uranium minerals were not formed. 

(Phair and Levine, 1953, p. 36Y).
GEOLOGY OF COPPER KING URANIUM MINE 215

The hypogene uranium-bearing solutions probably were derived 

from a magmatic source. The source is not known, however, as the 

nearest known exposures of igneous rocks of Tertiary age are 10 miles 

to the southwest in the Manhattan mining district.

RADIOACTIVITY OF A AND B FATJI/T ZONES

The gouge in A and B faults is abnormally radioactive throughout 

the Copper King mine, but no uranium minerals have been identified from them. Analyses giving the equivalent uranium and chemical uranium content of the gouge are presented in table 4.

The anomalous radioactivity of the fault gouge is low, except for 

sample CK-88 from A fault, near the west shaft on the 70-foot level 

(pi. 17). Without exception, the analyses indicate disequilibrium: 

There is insufficient uranium to account for the radioactivity. Analysis of a 4-foot chip sample collected from A fault in the southwest 

corner of the main shaft at the sill of the 110-foot level (pi. 17) indicates that the excess radioactivity can be accounted for largely by 

radium and other disintegration products of uranium, as shown in 

the following table:

[Analysts: S. P. Furman, J. N. Rosholt, and James Wahlberg] 

Field No. Percent of 10-* curies per gram of

eU U Ea En Pb^ 

CK-92 0. 050 0. 007 0. 32 0. 28 0. 28

The interpretation is that uranium has been leached recently from 

the fault zone, leaving the radium content fixed almost in place (Phair 

and Le vine, 1953).

ALTERATION OF WALL BOCK

Aside from the pyrite boxwork and lacy ores formed from the sulfide-magnetite deposit, the wall rocks adjacent to the Copper King 

vein were little altered.

The amphibole skarn, biotite skols, and sulfide-magnetite ores, where 

cut by the vein, are at places strongly sheared for as much as a foot 

from the vein but little changed mineralogically. Fragments of skarn 

and mica skols within and adjacent to the vein were locally chloritized. 

The chlorite probably was derived from the alteration of biotite.

The granite wall rocks were somewhat more altered than the amphibole skarn and associated ores. Alteration was most intense where 

many closely-spaced mineralized fractures cut the granite. In the 

stope east of the shaft, between the 110- and 140-foot levels (fig. 42) 

where the granite is cut by abundant pitchblende-bearing fractures, 

the rock is altered to a soft, crumbly rock. Possibly the breakdown 

largely resulted from saussuritic alteration. The cores of the plagioclase feldspar grains were changed to a fine-grained aggregate of a
216 GEOLOGY AND ORE DEPOSITS, COLORADO

greenish mineral and clear crystalline albite. Accompanying this 

change, a new mica formed along fractures, cleavages, and interfaces 

in quartz and microcline. The mica is slightly pleochroic, varying 

from colorless to pale olive green. It is biaxial with a 2V of approximately 15°; E>V, strong; nY is 1.59±.01; birefringence is strong. 

The mica may be vermiculite. The biotite, except w^here it occurs as 

inclusions in quartz, was altered to a pale-green mica with a high birefringence. Small quantities of purple fluorite and calcite were formed 

locally in the saussuritized plagioclase feldspars. In the most intensely 

altered granite, which is mottled red and green, red flakes of an unidentified mineral occur in the saussuritized feldspar and in fractures. 

The quartz and microcline in the granite remained unchanged.

Semiquantitative spectrographic analyses of the altered and fresh 

granite indicate that the principal change accompanying the alteration 

was the addition of uranium, cobalt, neodymium, samarium, ytterbium, 

molybdenum, and locally zinc to the rock (table 5). The other elements common to both altered and fresh granite quantitatively show 

little variation.

RESULTS OF EXPLORATION

To explore for an extension of the Copper King ore body, the 

Atomic Energy Commission conducted a diamond core drilling project 

in 1952 (Derzay and Baker, 1953), after the approximate outline of 

the ore body had been determined underground. Fourteen surface 

holes totalling 3001 feet were cored. The holes were drilled on a 

grid system designed to penetrate the ore-bearing zone at a 50-foot 

vertical and T5-foot horizontal spacing. The drilling explored the 

zone to a distance of 225 feet west of the main shaft, 350 feet east 

of the shaft, and to a maximum vertical depth of 350 feet. Although 

no uranium ore of commercial importance was found, pitchblendecarbonate veinlets were intersected in hole BH-8 (fig. 43), and traces 

of uranium were found in hole BH-3 at drill depths of 130 and 145 

feet and in hole BH-11 at a depth of 189 feet. The uranium in hole 

BH-8 assayed less than 0.05 percent U3O8 for a 2-foot width; the 

veinlets represent an extension of the uranium mineralization encountered near the face of the 140-foot level.

RECONNAISSANCE FOR RADIOACTIVITY

Radioactivity surveys in the mine area and the surrounding region 

have failed to locate radioactive anomalies other than pegmatites. 

In 1952 the Atomic Energy Commission surveyed an area of 700 

feet by 1,200 feet in the vicinity of the Copper King mine, using 

a portable scintillation counter. The data were recorded on an isorad map (Derzay and Baker, 1953) ; no anomalies other than the
TABLE 5. Semiquantitative spectrographic analyses of granite wa

[Analysts: P. J. Dunton and R. G. Havens]

Field 

No. 

D-2 

UG-40 

UG-66 

UG-67 

CK-50

CK-51 

CK-52

Field 

No. 

D-2 

UG-40 

UG-66 

UG-67 

CK-50

CK-51 

CK-52

Field 

No. 

D-2 

UG-40 

UG-66 

UG-67 

CK-50 

CK-51 

CK-52

Type of Material Location 

Fresh granite _ .. _ Surfane.

Altered granite....... _ -. -_--_ . .. 110-ft level. _.

Fresh(?) granite... -_.-. --_-_-. ._ .. ..... _______________ do . _

Si

... __ XX.

..... XX.

Altered green granite containing pitchblende Stope below 110-ft XX. 

seams. level.

seams.

blende seams. 

Type of Material Location

Altered granite.. __ _ _ ... __ _ ._ 110-ft level. _.

Altered green granite containing pitch- Stope below 

blende seams. 110-ft level,

blende seams.

ing pitchblende seams. 

Type of Material

Altered green granite containing pitchblende seams.

Altered greenish-gray granite containing pitchblende seams.

..do ......

Ce 

... O.OOX

... .ox-

--. .OOX

.0X4-

..... do ...

Stope below 

. do- ..

.....do ..

XX

Al 

X +

X.+ 

X.+

X.+

Cr Cu, 

O.OOOX O.OOOX- 

.OOOX- .OOOX+ 

. OOOX . OOOX 

.OOOX .OOX-

.ooox . oox- 

.ooox .oox- 

.ooox .ooxLocation

Fe 1 

X. - 0. 03 

X. - .05 

X. - .X 

X. - .X 

X.+ .X

X. .X

X.+ .X

Ga 

O.OOX- 

.OOX- 

.OOX- 

.OOX-

.ooox

.OOOX 

.OOOX

Y

0 00X4-

.oox- 

.oox

.OOX

.ox-

.oox

.OOX

:+ o.

^4- 

La 

0.003

.ox

.003

.ox

.003 

.003

7n

O.OX

 -

i Looked for but not found: P, Ag, As, Au, B, Bi, Cd, Ge, In, Ir, Hf, Hg, Li, Nb, 

Ni, Os, Pd, Pt, Re, Rh, Ru, Sb, Ta, Te, Th, Tl, W. Some elements of this group 

possibly are present in trace amounts, but because of their low standard sensitivities 

were not detected.

2 Determined by r

3 Determined by c
218 GEOLOGY AND ORE DEPOSITS, COLORADO

radioactive mine dump were disclosed. Subsequently a radioactivity 

reconnaissance was carried out in the immediate region and to the 

southwest in the Manhattan mining district, but the results of these 

surveys likewise were negative.

SUGGESTIONS FOB PROSPECTING

It seems unlikely that the Copper King deposit is the only uraniumbearing vein in the region. Accordingly the writers believe that 

further search for uranium is justified in this area.

The uranium-bearing vein at the Copper King mine is the only 

vein-type deposit known in the Prairie Divide region. Many small 

copper-bearing sulfide deposits of Precambrian age have, however, 

been prospected. The nearest known vein deposits are the gold-bearing pyritic deposits in the Manhattan district, 10 miles to the southwest (Levering and Goddard, 1950, p. 285-287). These deposits, 

together with those at the nearby Mayesville and Home districts, 

are related spatially to a northeast-trending belt of Tertiary (?) intrusive porphyries, ranging in composition from granodiorite to rhyolite, that extends from Eadial Mountain, on the divide between Middle 

Park and North Park, through Cameron Pass to the town of Manhattan, Colo. Exposures of porphyritic intrusive rocks are not known 

northeast of Manhattan.

The radioactive Copper King vein did not crop out at the surface, 

nor could it be detected by portable ground scintillation instruments. 

Other veins in the Prairie Divide area, if present, in all probability 

likewise would not be observable at the surface, for Prairie Divide is 

largely mantled by a veneer of glacial deposits of unknown thickness.

It is thought that the search for new radioactive deposits could 

most effectively be carried on by the use of airborne scintillation 

equipment. Anomalies, if found, could then be checked on the ground. 

As radioactive pegmatites are known in the region, it would be necessary to distinguish the anomalies resulting from these bodies. If 

anomalies of possible economic significance are found, regional geologic studies should be carried out to relate these anomalies to the 

geologic setting.

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of Canada: Jour. Geology, v. 17, p. 1-18.

Adams, J. "VV., Gude, A. J., 3rd, and Beroni, E. P., 1953, Uranium occurrences 

in the Golden Gate canyon and Ralston Creek areas, Jefferson County, 

Colo.: TJ. S. Geol. Survey Circ. 320.

Adams, J. W., and Stugard, Frederick, 1956, Wall-rock control of certain pitchblende deposits in Golden Gate Canyon, Jefferson County, Colo.: TJ. S. 

Geol. Survey Bull. 1030-G.
GEOLOGY OF COPPER KING URANIUM MINE 219

Ailing, H. L., 1938, Plutonic perthites: Jour. Geology, v. 46, p. 142-165. 

Brooker, E. J., and Nuffield, E. W., 1952, Studies of radioactive compounds, pt.

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Buerger, M. J., 1934, The pyrite-marcasite relation: Am. Mineralogist, v. 19,

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Eskola, Pentti, 1914, On the petrology of the Orijarvi region in southwestern

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 1939, in Earth, T. F. W., Correns, C. W., and Eskola P., Die Entstehung der

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Geijer, Per, 1917, Falutraktens berggrund och malmfyndigheter: Sveriges geol.

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Johannsen, Albert, 1939, A descriptive petrography of the igneous rocks, v. 1,

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Knopf, Adolph, 1933, Pyrometasomatic deposits, in Ore deposits of the Western

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Lindgren, Waldemar, 1933, Mineral deposits. McGraw-Hill Book Co., 4th ed. 

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

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Magnusson, N. H., 1940, Ljusnarsbergs malmtrakt, berggrund ach malmfyndigheter : Sveriges geol. undersokning ser. Ca no. 30, Nummary: Geology and

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 1950, Zinc and lead deposits of central Sweden, Pt. VII, The geology, 

paragenesis and reserves of the ores of zinc and lead, XVIII Internat. Geol. 

Congress, London, p. 371-379.

Murphy, Richard, 1946, Geology and mineralogy at Eldorado mine: Canadian 

Min. Metall. Bull. no. 413, p. 426-435, and Canadian Inst. Mining and Metallurgy Trans., v. 49, p. 426-435.

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

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220 GEOLOGY AND ORE DEPOSITS, COLORADO

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INDEX

Page

Acknowledgments ______ _ __ 173 

Alluvium ___________- 181 

AmpMbolite ______________ 179,184

Black Hawk No. 1 claim______ 173, 175

Chalcopyrite_____ 192, 193, 197, 199, 200, 

202, 205, 206, 208, 209, 211 

Cherokee Mines__________ 172, 173, 175 

Cobalt_____________ 213, 216 

Cofflnite __________________ 206, 209 

Copper ____________ 173 

Copper King fault. /See Faults. 

Copper King mine, workings _ 

Copper King vein, mineralogy 

paragenesis ___ _ .

pitchblende deposit_ 201

semiquantitative analyses

samples _ ____

wall-rock alteration-- 215

175,176

205-212

213

,203-205 

of

212 

.-216, 217

Defense Minerals Exploration Administration_________ 172,175 

Diamond-core drilling _ 172, 

181,190-192, 204, 216

Faults________________ 190, 191-192,

204, 212,213,214,215

Fluorite ____________________ 211

Geography _______ _ __ 173

Goethite __________________ 211

Gneiss, biotite-quartz-plagioclase 176,

177-178, 179, 184

Granite, biotite____________ 181, 185,

186-188, 189, 190, 200, 201

modal analyses___ _ 186

History, Copper King mine 173-175

Igneous rocks, age____ ____ 176, 189 

composition _____ _ 185-189 

foliation ________________ 189 

origin___________________ 189 

structure______________ 189-190

Joints 190

Lacy ore________________ 204,214

Log Cabin batholith__________ 175, 189

Magnetite__________ 192, 193, 194, 199 

Manganese __ 211 

Marcasite __ _ 211 

Metasedimentary rocks, composition __________ 176-185 

foliation ____________ __ 189 

modal analyses ______ 178 

origin _____________ 184-185, 189 

structure_____________- 189-190 

Mineral deposits, types_ 192 

Molybdenite ________..______ 197, 199 

Monazite_______________ __ 189

Neodymium 

Nickel _____ 

Niobium ___

Page 

216 

213 

213

Pegmatite, granite_________ 188, 189

Pitchblende, age_________ 192, 213-214

ore shoot___________ 176, 203-205

terminology used in report____ 206

texture________________ 206-209

Pitchblende deposit, location_____ 172,

176,200-205

origin ________________ 213-215

structure___________ 176, 200-205

Production, Copper King mine__ 173 175

Prospecting, suggestions_______ 218

Purpose of report_____________ 172

Pyrite___________________ 192, 193,

194-197, 199, 200, 201-203,

205, 206, 207, 209, 210, 213

Pyrite boxwork, description____ 201-203

origin __________________ 214

Pyrrhotite ______ 192, 193, 194,199, 200

Quartz _________ 178, 183, 187, 205, 209 

Quartzite ______________ 179, 184, 200

Radioactivity, A and B fault zones_ 

reconnaissance________________

215

216

Samarium________________ 216

Schist, biotite______________ 178, 184 

Scope of report-___________ 172

Siderite_________ 205, 207, 208, 209, 213

Silver _____________________ 213

Silver Plume granite_________ 175, 189

Skarn, amphibole ____________ 176,

179, 180-184, 193, 201, 203

Skols, biotite________ 180, 184, 185, 200

mica ________________ 179, 215

Sphalerite__________________ 192,

193, 195, 197, 199, 200, 202.

205, 206, 208, 210, 211, 213

Sulflde-magnetite deposits, age____ 200

location________________ 193

mineralogy____________ 194-199

origin _______________ 200

paragenesis-__________ 199 200

semiquantitative spectrographic

analyses _________ 198 

structure______________ 193-194

Titanium___________________ 199 

Trace elements_______ 198-199, 211-213

U. S. Atomic Energy Commission__ 172,

173,216

Uranium___________________ 190, 

204, 205, 206, 215, 216, 218

Ytterbium. 216

Zinc____________ 173, 174, 211, 213, 216

o

221