Contributions to the 

Geology of Uranium 

1953-54

GEOLOGICAL S U R*V E^Y BULLETIN 1009

4 6 b

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1956
. 1009 -

UNITED STATES DEPARTMENT OF THE INTERIOR 

Douglas McKay, Secretary

GEOLOGICAL SURVEY 

W. E. Wrather, Director
CONTENTS

The letters in parentheses preceding the titles are those used to designate the separate chapters

(A) Uranophane at Silver Cliff mine, Lusk, Wyo., by V. R. Wilmarth and

D. H. Johnson.-____-_____--______-___-__-______-____-___-__ 1

(B) Identification and occurrence of uranium and vanadium minerals

from the Colorado Plateaus, by A. D. Weeks and M. E. Thompson. 13

(C) Radioactive deposits of Nevada, by T. G. Lovering___.-__.___.____ 63

(D) Further studies of the distribution of uranium in rich phosphate beds

of the Phosphoria formation, by M. E. Thompson_______________ 107

(E) Stratigraphy of the Morrison and related formations, Colorado Plateau

region, a preliminary report, by L. C. Craig and others..-...-..-. 125

(F) Glossary of uranium- and thorium-bearing minerals, by J. W. Frondel

and M. Fleischer.---_________--____________-____-___--______ 169

(G) Uranium deposits in Fall River County, S. Dak., by H. Bell and W. E.

Bales.--.__.._...--....___.._..__-_..-_.._.---._..-..----__ 211

(H) Geology of the Happy Jack mine, White Canyon area, San Juan

County, Utah, by A. F. Trites, Jr., and R. T. Chew III._.._.._._ 235

(I) Carnotite-bearing sandstone in Cedar Canyon, Slim Buttes, Harding

County, S. Dak., by J. R. Gill and G. W. Moore.-.-...._.._..... 249

(J) Criteria for outlining areas favorable for uranium deposits in parts of

Colorado and Utah, by E. J. McKay.--_--_-....._.__..__.___._ 265

(K) Uranium-bearing nickel-cobalt-native silver deposits, Black Hawk 

district, Grant County, N. Mex., by E. Gillerman and D. Whitebread--- --_-__--___.__-__-___-____-----__---_--.--_----_--- 283

(L) Radioactive deposits in New Mexico, by T. G. Levering.___________ 315

(M) Botanical prospecting for uranium on La Ventana Mesa, Sandoval

County, N. Mex., by H. L. Cannon and W. H. Starrett...-.-_ 391

in
Uranophane at

Silver Cliff Mine

Lusk, Wyoming

GEOLOGICAL SURVEY BULLETIN 10 09 -A

This report concerns work done on 

behalf of the U. S. Atomic Energy 

Commission and is published with 

the permission of the Commission

A CONTRIBUTION TO THE GEOLOGY OF URANIUM

URANOPHANE AT SILVER CLIFF MINE, LUSK, WYOMING

By VERL R. WILMARTH and D. H. JOHNSON

ABSTRACT

The uranium deposit at the Silver Cliff mine near Lusk, Wyo., consists primarily 

of uranophane which occurs as fracture fillings and small replacement pockets in 

faulted and fractured calcareous sandstone of Cambrian (?) age. The country 

rock in the vicinity of the mine is schist of pre-Cambrian age intruded by pegmatite dikes and is unconformably overlain by almost horizontal sandstone of Cambrian (?) age.

The mine is on the southern end of the Lusk Dome, a local structure probably 

related to the Hartville uplift. In the immediate vicinity of the mine, the dome 

is cut by the Silver .Cliff fault, a north-trending high-angle reverse fault about 

1,200 feet in length with a stratigraphic throw of 70 feet. Uranophane, metatorbernite, pitchblende, calcite, native silver, native copper, chalcocite, azurite, 

malachite, chrysocolla, and cuprite have been deposited in fractured sandstone.

The fault was probably mineralized throughout its length, but because of 

erosion, the mineralized zone is discontinuous. The principal ore body is 

about 800 feet long. The width and depth of the mineralized zone. are not 

accurately known but are at least 20 feet and 60 feet respectively.

The uranium content of material sampled in the mine ranges from 0.001 to 

0.23 percent uranium, whereas dump samples, range from 0.076 to 3.39 percent 

uranium.

INTRODUCTION

The ore deposit of the Silver Cliff mine near Lusk (population 2,000), 

in south-central Niobrara County, Wyo., was studied by the U. S. 

Geological Survey on behalf of the Division of Raw Materials of the 

U. S. Atomic Energy Commission. The purpose of this study was to 

determine the mode of occurrence, extent, and reserves of uranium in 

the deposit.

The Silver Cliff mine is near the top of Silver Cliff Hill, a prominent 

topographic feature that rises nearly 300 feet above the surrounding 

plains. The mine is half a mile west of Lusk, in sec. 7, T. 32 N., 

R. 63 W.; 6th principal meridian (fig. 1).

GEOLOGICAL INVESTIGATIONS

The copper deposits at the Silver Cliff mine were first described by 

Ball (1906, p. 104). The uranium deposit at the Silver CM mine was 

first described by Lind and Davis (1919, p. 441-443). In 1926, 

Larsen, Hess, and Schaller (1926, p. 155-164) described the mineralogy of the ores at the Silver Cliff mine. They identified pitchblende 

in the calcite veins and found uranophane to be the predominant 

secondary uranium mineral. Dietz (1929, p. 17-20) briefly described

1
CONTRIBUTIONS TO THE GEOLOGY OF URANIUM

109' . 1W

FIOUKE 1. Index map of Wyoming showing location of Silver Cliff mine.

the deposit and gave the value of the production from the mine as 

$33,857.48 in uranium ore. In 1944, West 1 examined and sampled 

the underground workings of the mine for the Union Mines Development Corporation. In 1946, Slaughter and Nelson 2 briefly visited 

the mine and in 1949, it was examined by Wyant.3 In 1949, a reconnaissance geologic map of the area was made by Denson and Botinelly 

(1949). In August 1950, the writers mapped the surface and accessible 

mine workings on a scale of 1 inch to 40 feet. The accessible mine 

workings and dumps were examined with a Geiger counter and an 

ultraviolet light, and all roads hi the vicinity of the mine were traversed in a .car equipped with instruments to detect abnormal radioactivity.

HISTOBY AND PRODUCTION *

In the winter of 1879, a Mr. McHenry discovered the deposit and 

located the claim, which he -named the Rochelle silver mine. In 

February 1880, the Great Wyoming Mining and Milling Company 

was organized to operate the property. On the basis of samples that

' West, J. F., 1944, Examination of the Silver Cliff mine, Lusk district, Hartville uplift area, Wyoming: 

Union Mines Development Corporation Kept. 48-1, p. 1-11.

i Slaughter, A. L., and Nelson, J. T., 1946, Trace elements reconnaissance in South Dakota and Wyoming, 

Preliminary Keport: TJ. S. Geol. Survey Trace Elements Inv. Rept. 21, p. 20-22.

«Wyant, D. G., 1949, Uranophane deposit, Lusk, Wyo: U. S. Geol. Survey memorandum.

< The Lusk Herald, Lusk, Wyo., v. 50, no. 2, May 28,1936.
URANOPHANE AT SILVER CLIFF MINE, LUSK, WYO. 3

contained as much as $6,000 per ton in gold and silver, this company 

built a 400-ton mill which was operated only intermittently because 

the grade of the ore was lower than anticipated. Between 1880 and 

1884, a 285-foot shaft was sunk from the top of Silver Cliff Hill and a 

1,200-foot adit was driven to the bottom of the shaft from the north 

side of Silver Cliff Hill. A winze from the 1,200-foot adit was abandoned because of excess water. According to Mrs. Jamesetta Hale 

of Lusk, Wyo. (oral communication) the vertical shaft was caved in 

1884, and the mine abandoned. The mill was dismantled in 1886. 

In 1918, E. D. Lorimer set up a small mill and began milling material 

from the mine dumps for radium ore, and between 1918 and 1922, 

six carloads of ore that contained about 3 percent uranium oxide were 

shipped to the Radium Company of Denver, Colo. These operations 

at the mine ceased in 1922.

MINE WORKINGS

The workings of the Silver Cliff mine (pi. 1) consist of four large pits, 

a 285-foot vertical shaft, five levels having a total of about 1,600 feet 

of drifts, and a small stope. The largest pit (pit 1) is on the north side 

of Silver Cliff Hill and the other three (pits 2, 3, 4) are on the south 

side (pi. 1). Much of the uranium ore was produced from these pits. 

The underground workings, which approximately parallel the Silver 

Cliff fault, consist of five levels: The 5,100, 5,190, 5,243, 5,228, and 

5,220 (pi. 1). A vertical shaft, just north of pit 2, is reported to be 

285 feet deep; it is now caved about 30 feet below the surface. An 

adit, the 5,100 level, was driven from the south bank of Niobrara Creek 

reportedly for 1,200 feet and connects near the bottom of the 285-foot 

shaft. The adit is accessible for 400 feet and was driven S. 20° E. 

in an altered shear zone in schist of pre-Cambrian age. Silver and 

copper ores were produced from these workings.

GEOLOGY

The rocks of Silver Cliff Hill (pi. 1) consist of southeast-dipping 

calcareous sandstone of Cambrian (?) age that unconformably overlies 

metamorphic rocks of pre-Cambrian age. On the hills south and east 

of the mapped area, the Guernsey formation, of Devonian and 

Mississippian age, unconformably overlies the sandstone.

ROCK UNITS

Pre-Cambrian rocks are exposed on the flanks of Silver Cliff Hill and 

in the saddle between it and the hill to the south. These rocks 

consist 'of highly metamorphosed rocks that have been injected by 

pegmatite dikes, and include quartzite, mica schist, actinolite schist,
granite-gneiss, and a carbonate vein. The quartzite is dark gray, fine 

grained, and locally contains some mica flakes. The mica schist 

ranges in texture from a fine-grained quartz-muscovite-biotite schist to 

a coarse-grained rock that contains abundant large flakes of biotite and 

muscovite. A 2- to 3-foot band of actinolite schist occurs in the 

footwall of the Silver Cliff (main) fault in pit 1 (pi. 1). The granitegneiss is dark gray, coarse grained, and composed of feldspar, mica, 

quartz, and hornblende. A dark-brown, 8-foot-wide siliceous carbonate vein is exposed in a prospect pit south of the main mine workings 

(pi. 1). The vein strikes N. 25° E. and dips steeply southeast.

The pegmatite dikes conform to the foliation of the metamorphic 

rocks, and range in width from a few inches to 10 feet and in length 

from several feet to 180 feet in length. Several of the larger pegmatites are shown on the map (pi. 1). By visual estimate, the 

pegmatite composition is perthite (60 percent), quartz (20 percent), 

muscovite (15 percent), and black tourmaline and andraditic garnet 

(5 percent).

The calcareous sandstone of Cambrian (?) age is described by 

Denson and Botinelly (1949) as a hard coarse-grained reddish-brown 

conglomeratic sandstone ranging in thickness from a feather edge to 60 

feet. This calcareous sandstone has been tentatively correlated with 

the basal Cambrian sandstone and quartzite in the Laramie Mountains 

to the west and with the basal sandstone of the Dead wood formation 

in the Black Hills to the north. In the vicinity of the Silver Cliff 

mine, this calcareous sandstone caps Silver Cliff Hill and several 

buttes to the north and south, and forms clifflike walls as much as 20 

feet high above the contact with the pre-Cambrian rocks. The 

calcareous' sandstone consists of well-rounded quartz grains and

feldspar fragments cemented by calcite. The quartz grains average 

0.5 mm in diameter, but in places, particularly near the base, quartz 

pebbles as much as 2 inches in diameter are abundant. Adjacent to 

the fault, the calcite cement has been partly removed, forming a 

sugary-textured sandstone. Crossbedding is well developed locally 

in the sandstone, which ranges in color from light reddish brown 

through buff to dark brown. About 15 feet above the contact between the metamorphic rocks and the sandstone of Cambrian age, a 

6.-foot bed of dense, fine-grained, dark-brown sandstone forms an

excellent markep bed. The original thickness of the calcareous sandstone in this area is not known, because erosion has removed the upper 

part, but a thickness of about 80 feet remains on Silver Cliff Hill (pi. 1). 

Petrographic study of the calcareous sandstone brings out clearly the 

granular texture of the rock. Quartz grains constitute from 80 to 

85 percent of the rock and range from 0.03 millimeter to 1 millimeter 

in diameter. They are well rounded to subangular, and most grains
GEOLOGICAL SURVEY BULLETIN 1009 PLATE 2

A. PHOTOMICROGRAPH OB' URANIUM-BEARING SANDSTONE.

Highly fractured quart/ pebbles replaced by chalcocite (black) and a mixture of uranopbane, 

malachite, and azurite (light gray). X 400.

a. PHOTOMICROGRAPH OF TYPICAL URANOPHANE-BEARING SANDSTONE. 

Hounded quart/, pebbles (tj) with the interstitial crystalline iiranophaue («), culcite (< ), and limonte

(Mack). X 400.
URANOPHANE AT SILVER CLIFF MINE, LUSK, WYO. 5

have many minute inclusions of a reddish-brown opaque material, 

probably hematite. The quartz grains show undulatory extinction 

and a few have been replaced by calcite and limonite along fractures. 

Feldspar grains, ranging from 0.2 millimeter to 0.6 millimeter in 

diameter, constitute 2 to 5 percent of the rock. They are altered and 

generally contain narrow veinlets of limonite along cleavage planes. 

Malachite, uranophane, and azurite have replaced some feldspar 

grains. Calcite and limonite, the principal cementing materials, 

constitute as much as 10 percent of the rock.

STRUCTURE

The pre-Cambrian metamorphic rocks that crop out in the mapped 

area (pi. 1) have been severely deformed. They are characterized by 

a well-developed foliation that strikes N. 15°-20° E. and dips 

65°-85° E. The calcareous sandstone overlies the pre-Cambrian 

rocks with a marked angular unconformity, strikes N. 59°-62° E 

and dips 4° SE. The relative positions of these rocks are shown in. 

plate 1. The pre-Cambrian and Cambrian (?) rocks are part of the 

eroded core of the Lusk dome, the most northerly of three similar 

domelike structures along the eastern margin of the Hartville uplift.

A high-angle reverse fault, designated by the writers as the Silver 

Cliff fault, cuts the south end of the Lusk dome; the uranium deposit 

of the Silver Cliff mine occurs along the fault. This fault has an 

average strike of N. 15° E., dips 53°-68° E., and can be traced on the 

surface from the north side of Silver Cliff Hill southward for about 

1,200 feet. Movement along the fault h<as placed pre-Cambrian rocks 

over calcareous sandstone of Cambrian (?) age; the stratigraphic throw 

is about 70 feet. In the underground workings, gouge, in places as 

much as 10 feet thick, marks the fault. In the sandstone adjacent 

to pit 1, a normal fault strikes N. 10° E., dips 54° E. and has a vertical 

displacement of 2 feet. This fault, subsidiary to the Silver Cliff fault, 

apparently had no control on the deposition of the uranium.

Two sets of joints are conspicuous in the sandstone except near the 

Silver Cliff fault where it is extensively fractured. The more conspicuous of the joint sets strikes N. 65°-75° W. and dips 55°-65° N. 

The other set strikes N. 30°-40° E. and dips 75°-85° E.

ORE DEPOSITS

Silver an<d copper ore was produced from the Silver Cliff mine before its exploitation for uranium in 1919. The silver, copper, and 

uranium minerals occur as fracture fillings in calcite veins and as 

irregular replacement bodies in either the fault zone, or the highly 

fractured calcareous sandstone of Cambrian (?) age in the foot wall of 

the Silver Cliff fault. Four uranium minerals- pitchblende, gummite,
6 CONTRIBUTIONS TO THE GEOLOGY OF URANIUM

uranophane, and metatorbernite have been identified from the Silver 

Cliff mine. They are intimately associated with native copper, nativesilver, chalcocite, malachite, azurite, cuprite, and chrysocoUa, in a 

gangue of calcite, limonite and clinozoisite. Previous mining operations for uranium were restricted to the sandstone deposits. In several prospect pits the pre-Cambrian metamorphic rocks contain, 

secondary copper minerals but no radioactive material.

URANIUM OCCURRENCE IN FAULT ZONE

Uranophane is the only uranium mineral found in the fault zone,, 

and is associated with secondary copper minerals, native silver, limonite, and calcite. In the upper mine workings the fault zone is as 

much as 6 feet wide and contains uranophane and the associated 

minerals as fracture fillings and as small replacement bodies along 

the fractures. In the 5,100 level, the fault zone is as much as 10 feet 

wide and contains abundant pyrite adjacent to the fault on the 

hanging wall side. The 10 samples of fault zone material (table 1)> 

contained from 0.001 to 0.12 percent uranium and minor quantities 

of copper and silver. The uranium is sporadically distributed throughout the fault zone as is shown by these samples. Only .one sample 

(W-17) from the fault zone contained more than 0.1 percent uranium 

and it was taken from a surface outcrop. The samples collected from 

the fault zone in the mine workings indicated no commercial quantities of uranium, this suggests that perhaps sample W-17 was enriched 

by weathering. However, the Silver Cliff fault zone, above the zone 

of oxidation, probably does not contain commercial quantities of 

uranium, although mineable quantities of primary uranium minerals 

may exist in the fault zone at depth.

URANIUM OCCURRENCE IN SANDSTONE

The uraniferous zone in the sandstone at the Silver Cliff mine lies 

in the footwall of the Silver Cliff fault. The surface outcrop of thiszone extends continuously along the fault from pit 1 on the north side

of Silver Cliff Hill, southward for 830 feet and discontinuously for

another 240 feet to the southernmost prospect pit on the fault (pi. i), 

The SOUtherD limit is not accurately known; no abnormal radioactivity, however, could be detected southward along the surface outcrop 

of the fault, nor were uranium minerals observed south of this point.. 

Inasmuch as there appears to be a radioactive zone outside the mapped

area on the southward extension of the Silver Cliff fault, examination

of surface outcrops may discover uraniferous material south of the

mapped area. The known vertical dimension of the uraniferous zone

in the sandstone extends about 60 feet from the surface to the lowest 

mine working, and probably the zone extends downward another 20
URANOPHANE AT SILVER CLIFF MINE, LUSK, WYO.

TABLE 1. Sample analyses, Silver Cliff mine, Nidbrara County, Wyo. 

[Analyses by Trace Elements Section Denver Laboratory]

Sample 

No.

W-6

W-7

W_o

W-9

W-10

W-ll

. W-12

W-13

W-14

W-15

W-16

W-17

W-18

W-19

W-20

W-21

W-22

W-23

W-24

W-25

W-26

W-51

W-52

W-53

W-54

W-55

W-56

W-57

W-58

W-59

W-61 

W-63 

W-64

W-<55

W-66

W-69

W-70

W-72

W-73

W-74

Type of material

3-foot channel across actinolite layer in fault zone..-.. 

Grab sample from dump of old Silver Cliffe mine i -.

Representative sample from uranophane-bearing sandEquivalent 

uranium ' 

(percent)

ft nni

<¥U

.005

.006

ftnfl 

001 ftfti

.029

.017 ftfu

.12

.11

.014

ft(19

.001

.002

noo

on^

.008

.001

.020

.006

.56

1.38

1.59

.007

.004

.015

.004

.006

.006 

.014 

.001

3.80

.90

.004

080

.012

.006

.003

Uranium 

(percent)

0.001

.001

(¥11

.004

.001

.001

.001

.033

.029

.004

.10

.12

.009

.001

.001

.003

.020

.004

.008

.001

.020

.003

.61

1.48

1.41

.004

.001

.008

.002

.002

.002 

.002 

.001

3.39

1.03

.001

.095

.007

.002

.002

Copper 

(percent)

0.37

10.88

2.60

1.97

.27

.02

.16

.21 .03

.03 

<.02 

<.02

2.85

2.05

<.02

Silver 

(ounces 

per ton)

0.28-

.98'

15.04

1.23:

.16

.92:

.98.

1.06

1.00'

.98

.97 

.94 

.96

1.41 .72'

.97'

1 Equivalent uranium is based on measurement of radioactivity wherein it is assumed that all of theradioactivity of a sample arises from uranium and its disintegration products, and that none of the radioactivity arizes from the thorium series or from potassium; it is assumed, furthermore, that the uranium lain radioactive equilibrium with all of its disintegration products wherein each radioactive product in the 

series is disintegrating at exactly the same rate at which it is being formed.

feet to the contact of the sandstone with the pre-Cambriaii metamorphic rocks (pi. 1). Near the south end of the mapped area, erosion has removed the sandstone. Thus the thickness of the uraniumbearing rock ranges from less than an inch to about 80 feet.

The width of the uranium-bearing zone in the sandstone on the footwall of the Silver Cliff fault is not known accurately. In the mine 

workings, uranium minerals are detectable in the sandstone for asmuch as 25 feet away from the fault; on the surface, however, no uranium minerals were found nor could abnormal radioactivity be detected in the sandstone near the fault.

Uranium occurs in two types of sandstone. The less abundant but 

more radioactive type is a heavy, compact, brown to black sandstonethat contains pitchblende, gummite, uranophane, chalcocite, second-
8 CONTRIBUTIONS TO THE GEOLOGY OF URANIUM

ary copper minerals, native silver, and limonite. This brown to black 

sandstone was found only in small pockets, as much as 3 feet across, 

along the east side of the stope (pi. 1). Thin sections (pi. 2A) studies 

of this sandstone show that all the quartz grains have been corroded 

and many are replaced by copper and uranium minerals. The uranium content of this ore ranges from 0.61 to 1.48 percent. Appreciable 

quantities of copper and silver were found in sample W-52, from the 

stope (pi. 1). This brown to black type grades into the more common 

type of uraniferous sandstone. This type is light-buff to rust-brown 

sandstone which, in contrast to the other type does not contain large 

quantities of copper and silver minerals. Uranophane is the most 

abundant uranium mineral, and characteristically occurs interstitial 

to the quartz grains, (pi. 25), as thin coatings on fracture surfaces, 

and as small replacement bodies in the sandstone. Minor quantities 

of metatorbernite, azurite, and malachite are associated with the 

uranophane. Many of the fractures are filled with calcite, and locally, 

where these fractures are incompletely filled, the uranophane forms 

:radiating crystal aggregates. The uranium content of the sandstone 

ranges from 0.001 to 3.39 percent, and is dependent upon the spacing 

of the fractures and replacement bodies.

DEPOSITION AND DISTRIBUTION OF URANIUM MINERALS

The deposition of the uranium ore, thus far mined in the sandstone 

was controlled principally by the fractures that resulted from movement along the Silver Cliff fault. The brittle sandstone was readily 

fractured during faulting and these open fractures apparently acted 

as channels for the movement of the ore solutions. The relatively 

soft and plastic gouge along the Silver Cliff fault probably dammed 

the ore solutions, restricting them to the footwall and thus prevented 

the deposition of uranium minerals in the pre-Cambrian quartzite on 

the hanging wall.

The distribution of uranium in the sandstone is irregular. In any 

set of fractures, some surfaces are coated with uranophane and the

others are completely barren. Assays of samples (table 1 and pi. 1) 

indicate considerable variation in the distribution of the uranium 

minerals.

MINERALOGY

Uranophane, the most abundant uranium mineral at the Silver Cliff 

mine, occurs in the sandstone as lemon-yellow to pale-green coatings 

as much as one-sixteenth inch thick on fracture surfaces and in irregular replacement bodies as much, as 2 inch.es across, and is estimated to

constitute as much as 2 percent of the rock. In thin section veinlets

of uranophane as much as 0.02 mm in width fill fractures in quartz
URANOPHANE AT SILVER CLIFF MINE, LUSK, WYO. 9'

grains and in calcite veins. Locally uranophane has impregnated and 

replaced the calcite of the veins and the cementing material. In 

fracture fillings and coatings uranophane is massive or in small radiate 

or stellate clusters; whereas in replacements it is restricted to radiating 

aggregates. Uranophane has been analyzed by Mary Thompson of 

the U. S. Geological Survey and contained 65.24 percent uranium 

oxide, 8.53 percent calcium oxide, 12.66 percent silica, and 13.02 percent water. The optical properties of the uranophane are: optically 

negative, with slight varying optical angle, 2VLt(32°-42°), 2VNa (37°- 

45°) and strong dispersion. The indices of refraction measured by 

the immersion method are: a(1.642), 0(1.665), and 7(1-672).

Pitchblende and gummite, both uranium oxides, were identified by 

Schaller (Larsen, Hess, and Schaller, 1926 p. 161-162) in the ore from 

the Silver Cliff mine. Pitchblende has been identified by S. J. Garde 

of the U. S. Geological Survey in ore specimens collected by the writers 

from the dump material. The pitchblende occurs as small specks in 

the calcite veins and as a replacement in the calcite cement. Some of 

the pitchblende in the calcite veins is surrounded by crystalline 

uranophane. In a polished section of black sandstone collected from 

the stope, a narrow irregular veinlet of pitchblende (?) parallels a 

chalcocite vein that cuts across the quartz grains. The pitchblende (?) 

has been partially replaced by uranophane and secondary copper 

minerals. A deep-red to orange isotropic mineral, tentatively identified as gummite, is associated with several small masses of uranophane. 

Small quantities of metatorbernite, a pale-green hydrous copper 

uranium phosphate, previously reported by Wyant,5 were found on the 

mine dump, but the mineral was not found in the mine. The metatorbernite is associated with secondary copper minerals and coats fracture surfaces in the sandstone. The primary ore minerals, other than 

pitchblende, are chalcocite and native silver in a gangue of calcite and 

minor clinozoisite. Chalcocite, the only copper sulfide noted, occurs 

as fracture fillings and as small replacement masses in the sandstone' 

The alteration products of the chalcocite are native copper, azurite. 

malachite, cuprite, and chrysocolla. These secondary copper minerals 

coat fracture surfaces in the sandstone, fill fractures in the fault zone, 

and locally replace the calcite cement. Most of the prospect pits in 

the metamorphic rocks show secondary copper minerals. Small 

quantities of native silver were found as thin coatings on fracture 

surfaces of a specimen of dark-brown radioactive sandstone from the 

stope. Analysis of ore samples (table 1) shows that silver is a common 

constituent of the ore.

Calcite is the gangue mineral in the veins and is the most abundant 

cementing material in the sandstone. The calcite cement occurs in

« Wyant, D. Q., op. cit. p. 1.
10 CONTRIBUTIONS TO THE GEOLOGY OF URANIUM

white to gray, rounded fragments that range from 0.03 to 0.07 millimeter in diameter. Most of the calcite cement contains ,many 

minute grains of limonite. The white to light-gray vein calcite is 

coarse grained and forms subhedral crystals as much as 0.4 millimeter 

in length and 0.03 millimeter in width. Several of the larger calcite 

veins contain euhedral calcite crystals perpendicular to the vein walls 

and finer grained calcite filling the center of the vein. Clinozoisite 

was found in one thin section. It is a clear, gray mineral that forms 

radiating aggregates in rounded spaces formerly occupied by quartz 

grains. Much bright-green fluorescence was noted in places, but these 

invariably showed low radioactivity, whereas the most radioactive 

areas were only slightly fluorescent. The fluorescent material is 

probably hyalite that contains enough uranium to produce fluorescence.

RADIOACTIVITY SURVEYS ON SILVER CLIFF FAULT AND lH 

ADJACENT AREAS

The accessible mine workings, prospect pits, and surface are as 

adjacent to the Silver Cliff fault were examined with a Geiger counter. 

All the mine dumps along the fault showed an abnormal amount of 

radioactivity. The highest readings in the mine workings were obtained from a black sandstone adjacent to the fault on the east side of 

the stope (pi. 1). Pitchblende was tentatively identified in specimens 

of sandstone from this area. Abnormal radioactivity was noted in the 

sandstone on the footwall of the Silver Cliff fault, but the pre-Cambrian quartzite on the hanging wall of the fault showed no abnormal 

radioactivity. Surface examination with a Geiger counter indicated 

no abnormal radioactivity in the sandstone in the footwall of the 

Silver Cliff fault, suggesting the possible leaching of uranium minerals 

by ground waters or the sporadic distribution of uranium minerals. 

At several places in the fault gouge zone, however, abnormal radioactivity was noted, and on close examination secondary uranium

minerals were found.

GRADE

Although the grade of the ore shipped in 1919 from the Silver Cliff 

inine was reported to be 3 percent uranium (Dietz, 1929, p. 17-20), 

the average grade of rock left in the mine is much less. Samples 

assayed 0.001 percent to 1.48 percent uranium. The erratic distribution of uranium minerals along fractures and in replacement bodies in

the sandstone would make bulk or open pit mining necessary, and

the average grade of the material mined would depend upon the spacing of the fractures, and the number and size of replacement bodies.
TJRANOPHANE AT SILVER CLIFF MINE, LUSK, WYO. H

'The richest sample (W-53) collected in the underground workings 

was taken from the east side of the stope (pi. 1), and contained 1.48 

percent uranium. This sample was from an area of about 3 square 

feet of dark-brown to black sandstone. The richest sample (W-65)

 collected from the dump material contained 3.39 percent uranium. 

Of the 38 samples (table 1) collected by the writers, 8 contained more 

than 0.1 percent uranium, 4 contained from 0.02 to 0.1 percent 

uranium, and the rest contained less than 0.02 percent uranium. 

West 6 reports assays ranging from 0.02 to 0.23 percent uranium for 

25 chip samples in the underground workings, and'from 0.31 to 4.65 

percent uranium from selected saniples of dump material. Slaughter 

.and Nelson 7 report the uranium content of two grab samples of

 dump material as 0.076 percent and 0.090 percent.

ORIGIN

Two theories on the origin of the uranium deposit at the Silver Cliff 

mine have been proposed. (Larson, Hess and Schaller, 1926, p. 156)

 states that the uranium minerals in part fill fractures and in part replace

 the sandstone. He believes that the solutions which deposited 

minerals followed cracks and spread from,these into sandstone. Hess, 

also states that the original uranium minerals were dispersed among the

 quartz grains, and so mixed with other minerals that pure material 

has not been found. West,8 suggested that the primary uranium 

.minerals in the sandstone were formed at the time of or after the 

formation of carbonate veins in the fault fissures. Secondary uranium 

minerals were'formed by oxidation of the primary mineral.

The writers believe the sequence of geologic events in the vicinity

 of the Silver Cliff mine to be reverse faulting, minor faulting and 

fracturing of the sandstone and deposition of the ore. Calcite,

 chalcocite, and pitchblende, the primary minerals were deposited 

in the fault zone and in the fractured sandstone on the footwall of the 

fault. The paragenesis of these minerals, has not been .definitely 

'.established; probably they are contemporaneous or have overlapping 

periods of deposition. During supergene alteration, the primary

 copper minerals were converted to cuprite, and the pitchblende was 

.altered to gummite, uranophane, and metatorbernite. The deposits

 of uraniferous sandstone probably are the result of alteration of the 

primary minerals in place, whereas those in the light buff sandstone 

result from solution and redeposition of the uranium by descending 

ground waters to form a commercial secondary uranium deposit.

 Op. cit. p. 9-11.

' Slaughter, A. L., and Nelson, J. T., op. cit. p. 22.

 West, J. F., op. cit
12 CONTRIBUTIONS TO THE GEOLOGY OF URANIUM

Ball, S. H., 1906, Copper deposits of the Hartville uplift, Wyoming: U. S. Geol.

Survey Bull. 315. 

Denson, N. M., and Botinelly, Theodore, 1949, Geology of the Hartville uplift,

eastern Wyoming: U. S. Geol: Survey Oil and Gas Inv., Prelim, map 102. 

Dietz, C. S., 1929, Developed and undeveloped mineral resources of Wyoming:

Wyo. Geol. Survey Bull., v. 21, p. 17-20. 

Larsen, E. S., Hess, F. L., and Schaller, W. T., 1926, Uranium minerals from

Lusk, Wyoming: Am. Mineralogist, v. 11, no. 6, p. 155-164. 

Lind, S. C., and Davis, C. W., 1919, A new deposit of uranium ore: Science^

new set., v. 49, p. 441-443.

O