GEOLOGICAL SURVEY CIRCULAR 291 

A URANIUM-BEARING RHYOLITIC TUFF 

DEPOSIT NEAR COALDALE 

ESMERALDA COUNTY, NEVADA 

This report concerns work done on 

behalf of the U. S. Atomic Energy 

Commission and is published with 

the permission of the Commission. 

UNITED STATES DEPARTMENT OF THE INTERIOR 

Douglas McKay, Secretary 

GEOLOGICAL SURVEY 

W. E. Wrather, Director 

GEOLOGICAL SURVEY CIRCULAR 291 

A URANIUM-BEARING RHYOLITIC TUFF DEPOSIT NEAR 

COALDALE, ESMERALDA COUNTY, NEVADA 

By D. C. Duncan 

This report concerns work done on 

behalf of the U. S. Atomic Energy 

Commission and is published with 

the permission of the Commission. 

Washington, D. C., 1953 

Fret> on apphcatH n to the GPological Survey, Washington 25, D. C. 

A URANIUM-BEARING RHYOLITIC TUFF DEPOSIT NEAR 

COALDALE, ESMERALDA COUNTY, NEVADA 

By D. C. Duncan 

CONTENTS 

Page 

Abstract...................................... 1 

Introduction . . . . . . . . • . . . . . . . . . • • . . . . . . . . . • . . . . 1 

Location............................... . 1 

Previous work . . . . . . . . . . . . . . . . . . . . . . . • . . . 1 

Geology ............•.....•.....••.......•.... 3 

Ore deposit. . . . • . . . • . . • . . . • . . • . • . . . . . . . • . . . . . 3 

Composition of tuff . • . . . . . . . • . . . . • . . . . . . . 3 

Siliceous vein material .•.... 1 •••••••••• •• 3 

Page 

Ore deposit-Continued 

Ore minerals . . . . . . . . . . . . • . . . . . . . . . . . . . . 3 

Origin of uranium. . . . . . . . . • . . . • . . . . . . . . . 5 

Radioactivity. . . . . . . . . . . . . . . . . . . . . . . . . . .. 5 

Sample data . . . . . . . • • . • . . . . . . • . . . • . . . . . . 5 

Potentialities of the deposit . . . . . . . . . . . . . . 6 

Literature cited . . . . . • • . . . . . • . . • . . . . . . . . . . . . . 6 

Unpublished report . . . . . . . . . . . . . . . . . . . . . . . • . .. 6 

ILLUSTRATIONS 

Page 

Figure 1. Map showing location of uranium deposit near Coaldale, Esmeralda County, Nev. . . . . . . . . . . . . . . . 2 

2. Geologic sketch map of uranium-bearing tuff deposit . . . . . . • . • . • . • • . • . . . . . . • . . • • . . . . . . . . . . • . . . . 4 

3. V~ew of northeastern part of the Silver Peak Mountain~ showing the location of 

uranium-bearing tuff deposit. . . . . . . . • . . • . . • • . . . . . . . . . . . . . . . . • • . . . • . . . . . . . . . . . . . . . . . . . . . .. . 5 

4. View of east side of uranium-bearing tuff deposit .....•.....................•............... - . 5 

ABSTRACT 

A small deposit of uranium-bearing rhyolitic tuff is exposed at the northern end of the Silver Peak Mountains about 

4 miles south of Coaldale inE5meralda County, Nev. The 

deposit consists of weakly mineralized welded tuff containing 

veinlets and small irregular pods of higher grade uraniumbearing rock. The conspicuous ore minerals are autunite 

and phosphurany lite, which coat some fractures and partly 

fill some feldspar crystal cavities in the tuff. Uranium is 

also present in small amounts in siliceous material that 

occurs as scattered veinlets and as matrix of a breccia pipe. 

Numerous limonite-stained joint surfaces on the welded tuff 

also contain small amounts of uranium. No identifiable uranium minerals were found in the siliceous vein material or in 

the limonite-stained fracture coatings. Several samples collected from weathered outcrops contained from 0. 002 to 

1. 86 percent uranium. 

INTRODUCTION 

In the course of examining lignitic shale deposits 

about 4 miles south of Coaldale, Nev. (fig. 1), a deposit of uranium-bearing rhyolit.:.c tuff was found and 

examined in reconnaissance in mid-October 1951 by 

N. M. Denson and D. C. Duncan. The deposit was 

found by using a portable gamma scintillation detector 

1 

to trace slightly radioactive gravels in dry washes to 

the bedrock sources of the gravel. The locations given 

in this report were made by compass intersection and 

by the use of geologic maps of the nearby lignite field 

which served as general bases. Ten representative 

samples of the deposit were analysed by the Geological 

Survey. Specimens collected for mineral identification 

were examined by Maurice Deul, and the welded tuff 

was examined petrographically by Clarence Ross1 of the 

Geological Survey. 

Location 

The uraniferous tuff deposit, accessible by 

3 miles of dirt road leading south from the junction of 

U. S. Highways 6 and 95 (fig. 1), is near the south line 

of the NEt sec. 33, T. 2 N., R. 37 E., Mount Diablo 

meridian on privately owned patented coal land. No 

evidence of previous prospecting or development of the 

uranium deposit was noted during the investigation. 

Previous work 

The general geology of the Silver Peak quadrangle 

was mapped and described by Spurr (1906). The Tertiary 
Mina 29 miles 

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MOUNTAINS _.... I 'deposit 

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19 24 

30 25 

R.37 E. 

0 2 Miles 

Figure 1. -Map showing location of uranium deposit near Coaldale, Esmeralda County, Nev. 

2 
sedimentary rocks of the region were described and 

assigned to the Esmeralda formati.on by Turner (1899). 

Lignitic shale deposits in the Tertiary rocks that crop 

out north of the uranium deposits were described by 

Hance (1913) and Toenges and others (1948), and were 

examined by V. H. Johnson and R. C. Robeck (1944) 

for the Geological Survey. These earlier reports include considerable data which are useful in interpreting 

the geology of the area but which do not include direct 

information on the uranium deposit. 

GEOLOGY 

Rocks exposed in the northern part of the Silver 

Peak Mountains are Cambrian and Ordovician marine 

sedimentary rocks unconformably overlain by Tertiary 

volcanic rocks and intermontane basin sediments 

(Spurr, 1908, p. 17-21, and pl. 1). Volcanic rocks, 

mostly of rhyolitic composition, make up the prominent 

land forms of the northeastern tip of the mountains 

(figs. 1 and 3). The Esmeralda formation of Tertiary 

age is a thick sequence of shale, tuffaceous sandstone, 

conglomerate, and a few impure lignite beds. The formation is exposed in low foothills along the north flank 

of the mountains. In this part of the Silver Peak Mountains the Esmeralda formation is in fault contact with 

the Tertiary volcanic rocks at most places, but near 

the uranium deposit (fig. 2), the basal part of the formation overlies the volcanic rocks with normal sedimentary contact. Extensive pediment "flats" southeast 

of the deposit are mostly covered by a thin veneer of 

alluvium. 

Along the northern front of the Silver Peak 

Mountains a zone of normal faults in echelon trends 

generally no,rthwest and downdrops the sedimentary 

rocks of the Esmeralda formation on the northeast 

against the volcanic rocks on the southwest. The 

faulted zone is about 2 miles long. Its general location is shown on a map by Hance (1913, pl. 23); part 

of the faulted belt near the uranium deposit is ill ustrated in greater detail by Toenges (1948, fig. 1). The 

uranium deposit (fig. 2) is located near the southeast 

end of the exposed fault zone. Although the earlier 

maps show a probable fault passing northeast of the 

deposit, the presence of such a fault was not confirmed 

in the present brief examination. About 130 feet northeast of the deposit a thin lignitic shale bed at the base 

of the Esmeralda formation overlies welded tuff in 

apparently normal .sedimentary contact. A narrow 

zone between the lignitic shale outcrop and the uraniferous tuff is poorly exposed under a rubble of essentially nonuraniferous rhyolitic tuff. Closely spaced 

joints in the uraniferous tuff deposit suggest that some 

minor displacement may have occurred, however, in 

the poorly exposed rocks just northeast of the deposit. 

ORE DEPOSIT 

Uranium is irregularly disseminated in siliceous 

veinlets, in a siliceous breccia pipe, along limonitestained joint surfaces, and as incrustations of autunite 

and phosphuranylite ic welded rhyolitic tuff at the northeastern tip of the Silver Peak Mountains (fig. 1). The 

principal exposure of the uraniferous tuff forms a knoll 

that rises about 50 feet above the adjacent flats. A 

second smaller mass crops out through alluvium to the 

southeast. These exposures indicate that the deposit 

3 

occupies a roughly elliptical area about 400 feet long 

and as much as 200 feet wide (fig. 2). The deposit is 

elongate approximately parallel to a fault that trends 

northwest from a point just north of the deposit. The 

southeastern limit of the radioactive tuff is concealed 

by alluvium, but the other better exposed boundaries 

grade into relatively nonradioactive light-gray tuff of 

apparently similar general composition. The gray 

tuff interfingers with other flat-lying red-weathering 

rhyolitic volcanic rocks, presumably both flows and 

tuffs, west of the radioactive deposit. 

The uranium-bearing part of the tuff is closely 

jointed (fig. 4); some joints contain veinlets a few 

inches to a few feet long consisting of light- to darkgray fine-grained siliceous material. Most joint surfaces are coated with a thin brown limonite veneer 

that is radioactive. 

Composition of tuff 

Specimens of tuff taken from wall rock adjacent 

to a siliceous veinlet are light gray flecked with both 

dark and light phenocrysts as much as three-fourths 

inch in diameter. In thin section, the tuff contains 

crystals of unaltered potash feldspar, phenocrysts of 

another feldspar replaced by a pale yellowish-green 

claylike mineral that possibly is sericite or alunite, 

embayed crystals of smoky quartz, and spherulites 

of cristobolite. The groundmass is mostly altered to 

claylike minerals, butinplaces it contains glass shards. 

The rock i's considered by Mr. Ross to be an altered 

welded tuff of rhyolitic composition. 

Siliceous vein material 

The siliceous vein material ranges from black 

to light gray, with gradational color changes observable 

within individual veinlets. The contacts of the veinlets 

with the enclosing welded tuff are sharp. The vein 

material is unhanded and predominantly glassy material 

containing numerous microscopic inclusions of the tuff. 

In places the veinlets contain scattered megascopically 

visible angular inclusions of tuff. A breccia pipe about 

10 feet in diameter at locality 42, figure 2, is composed 

of angular welded tuff fragments cemented with gray to 

black siliceous matrix. The dark parts of the vein material contain more uranium than the light parts. Samples 

of dark vein material contained as much as 0. 28 percent 

uranium, but no uranium minerals were detected in the 

samples examined under the microscope. 

Ore minerals 

Bright yellow powdery uranium-bearing minerals 

coat fractures and joint surfaces at one place (loc. 32, 

fig. 2) and form encrustations that partly fill some 

feldspar crystal cavities in the tuff at localities 31 and 

32, figure 2. The yellow mineral was tentatively identified as a mixture of autunite (a hydrous-calcium-uranium phosphate) and phosphuranylite (a lead-calciumuranium phosphate) by microscopic and spectroscopic 

tests. From the field relationships these minerals 

seem to be secondary. No other uranium-bearing minerals were detected in the deposit although the limonite 

stain of some joint surfaces and the black siliceous vein 

material contained as much as 0. 28 percent uranium. 
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EXPLANATION 

r071 u 

Alluvium 

§ 

Esmeralda formation 

Tuffaceous sandstone, conglomerate, and 

shale with lignitic shale beds designated 

on the map by letters 

0 

Rhyolite flows and tuffs 

Uranium-bearing 

~ rhyolitic tuff 

.., ~·.,..-

Formation boundary 

(do~hed where opproximote/y located 

dotted where concealed) 

/o" 

Fault, 

dashed where appr"xlmately located 

U, upth.rown side, D, downthrown side .. 

~ 

Coal prospect shaft 

e30 

Sample loco lit y 

Figure 2. -Geologic sketch map of uranium-bearing tuff deposit NEt, sec. 33, T. 2 N., R. 37 E., near Coaldale, Esmeralda County, Nev. 
Figure 3. - View of northeastern part ofthe Silver Peak Mountains, showing the location of uranium- be.aring tuff deposit, 

Esmeralda County, Nev. (outlined). Numbers show locations of samples referred to in text. An abandoned coal 

prospect is in right foreground. Photographed from ridge about 1 mile northeast of deposit. 

Figure 4. -View of east side of uranium-bearing tuff deposit. Welded rhyolitic tuff exposed on the knoll is closely 

jointed. Note the iron-stained joint surfaces (dark) that are strongly radioactive and areas of white tuff (light) 

that are weakly radioactive. 

5 
Origin- of uranium 

The concentrations of uranium in siliceous vein 

materials and along limonite-stained joint surfaces 

·within the deposit suggest t'"lat primary uranium mineralizaticn took place at the time of introduction of 

siliceous vein material that spread along joint surfaces 

during or after faulting. The conspicuous uranium minerals, autunite and phosphuranylite, are probably secondary alteration products of other uranium-bearing 

compounds. The source of the uranium is conjectural. 

Possibly it was originally disseminated in minute amounts in the volcanic rocks in which the deposit occurs, 

or possibly it was introduced from another source by 

hydrothermal solutions. 

Radioactivity 

The uraniferous tuff deposit was found by use of 

a portable gamma scintillation detector which showed a higher radioactivity for stream gravels (35 to 60 mr/hr) 

down drainage from the radioactive tuff as contrasted 

with the less radioactive lignite-bearing sedimentary 

Denver 

rocks of the Esmeralda formation (10 to 20 mr/hr). 

The more r·ldioactive stream gravels were followed 

upstream to their source. Measurements of radioactivity with a Geiger counter on the deposit ranged 

from 5 on the 0. 2 scale to 10 on the 20 scale. The 

light""gray rhyolitic tuff that makes up the bulk of the 

deposit averaged an estimated 0. 008 to 0. 009 percent 

equivalent uranium readings )f approximately 10 or 11 

divisions on the 0. 2 ~cale of the counter. The limonitestained joint surfaces, siliceous vein material, and 

small masses of tuff showing the yellow uranium minerals produced radioactivity estim&ted to range from 

about 0. 015 to 1 percent equivalent uranium reading 

of 17 on the 0. 2 scale to 10 on the 20 scale of the 

counter. 

Sample data 

Chip samples of the rhyolitic tuff taken at 

four places, away from visible joint or vein surfaces and considered representative of large parts of 

the deposit, yielded the following analyses: 

Equivalent Locality Uranium Laboratory Kind of material uranium no. (percent) no. (percent) 

3lb 54303 Tuff, white, rhyolitic; chips from several faces near o.oo8 0.002 

southwest margin of deposit. 

32 54304. Tuff, white, rhyolitic; chips from several faces near .029 .026 

central part of deposit. 

33 54306 Tuff, white, rhyolitlc; chips from several places from .013 .008 

34 54308 

central part of' deposit. 

Tuff, white, rhyolitic; chips from several faces along .009 .003 

northeast margin of' deposit. 

Selected samples of the siliceous vein material, 

yellow-flecked rhyolite adjacent to joints, and rock 

adjacent to brown-stained joint surfaces yielded 

the following analyses: 

Locality Denver Equivalent Uranium 

no. Laboratory Kind of material uranium (percent) no. (percent) 

30 54300 Tuff adjacent to limonite-stained joint surface; sample 0.046 0.044 

taken from 2-inch wide zone, about 1 ft long, adja54301 

cent to joint surface. 

31 Vein material, dark-gray, siliceous; sample taken from .041 .039 

3la 54302 

3-inch wide veinlet along about 1! ft of outcrop. 

Tuff flecked with yellow uranium mineral filling .14 .12 

feldspar crystal cavities; chips from weathered 

surface. 

32a 54305 Tuff adjacent to joint surface stained with yellow 1.6 1.86 

uranium mineral and flecked with same mineral filling 

feldspar crystal cavities; chips from 10-inch wide 

33a 54307 

zone, about 4 or 5 ft long. 

Veinlet, dark-gray, siliceous, and adjacent tuff; .24 .26 

42 54309 

sample ·~aken from 2-inch wide zone about 2 ft long. 

Vein material, gray, siliceous, in breccia pipe about .031 .024 

10 ft in diameter; chips from several parts of 

exposure. 

6 
These higher grade samples represent only a 

small amount of material sporadically distributed 

through the deposit. 

Potentialities of the deposit 

During the brief examination of the deposit no 

reliable geologic guides for determining the extent and 

grade of the mineralized rock below surface were recognized; more thorough physical exploration, including 

penetrations by drilling or prospect pitsJ will be required to appraise accurately the uranium content. 

The samples collected from weathered surfaces 

on the deposit contained from 0. 002 to 1. 86 percent 

uranium. Although they may not reflect correctly the 

uranium content at depth, from available data the grade 

of the deposit as a whole is interpreted to average between 

0. 005 and 0. Ol percent uranium. Smaller parts of the 

deposit, including a breccia. pipe filled with siliceous 

vein material (loc. 42, fig. 2) and a mass of tuff 

(loc. 32, fig. 2), each of which is about 10 feet in diameter, contain about 0. 025 percent uranium. Thin 

veinlets and smaller masses of adjacent wall rock containing 0. 1 to J. 86 percent uranium are sporadically 

distributed t.hrough the deposit and apparently aggregate only a small tonnage of ore. Although the deposit 

7 

is presumably too small to be considered an important source of uranium, it is of interest as a new 

locality at which a small amount of commercialgrade uranium ore is found, in a region that may contain other similar deposits. 

LITERATURE CITED 

Hance, J. H. , 1913, The Coaldale coalfield, Esmeralda 

County, Nev.: U. S. Geol. SurveyBull. 531-K, 

p. 313-322. 

Spurr, J. E. , 1906, Ore deposits oft he Silver Peak quadrangle, Nevada: U. S. Geol. SurveyProf. Paper 55. 

Toenges, A. L., and others, 1946, ·Exploration, composition, and washing, burning and gas-producer 

tests of a coal occurring near Coaldale, Esmeralda 

County, Nev.: U. S. Bur. Mines Tech. Paper 687. 

Turner, H. W., 1899, The Esmeralqa formation, a 

fresh-water lake deposit: U. S. Geol. Survey 21st 

Ann. Rept., pt. 2, p. 197-208. 

UNPUBLISHED REPORT 

Johnson, V. H., and Robeck, R. C., 1944, Geology 

of the Coaldale mining district, Esmeralda County, 

Nev.: Manuscript and maps, U. S. Geol. Survey files. 

INT.-DUP. SEC., IIASH., D.C.~H6l