GEOLOGICAL SURVEY CffiCULAR 142 

VIRGIN VALLEY OPAL DISTRICT 

HUMBOLDT COUNTY, NEVADA 

By M. H. Staatz and H. L. Bauer, Jr. 

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 

Oscar L. Chapman, Secretary 

GEOLOGICAL SURVEY 

W. E. Wrather, Director 

GEOLOGICAL SURVEY CIRCULAR 142 

VIRGIN VALLEY OPAL DISTRICT, 

HUMBOLDT COUNTY, NEVADA 

M. R. Staatz .and H. L. Bauer, Jr. 

Washington, D. C., 1951 

Free on application to the Geological Survey, Washington 26, D. C. 
VIRGIN VALLEY OPAL DISTRICT, HUMBOLDT COUNTY, NEVADA 

Abstract .. 

Introduction 

Geology .. 

Ore Deposits 

CONTENTS 

Page 

1 

2 

2 

2 

Ore Deposits-Continued 

Form of deposits . 

"Radioactivity . . . 

Suggestions for Prospecting 

ILLUSTRATIONS 

Figure l. Index map, Virgin Valley, Humboldf County, Nevada . . . . . . . . . . . . . . . . 

2. Geologic map and section, east side of Virgin Valley, Humboldt County, Nevada .. 

3. Geologic map and sections, west side of Virgin Valley, Humboldt County, Nevada. 

ABSTRACT 

The Virgin Valley opal district, Humboldt County, Nevada, is near the Oregon-Nevada border 

in the Sheldon Game Refuge. Nineteen claims owned by Jack and Toni Crane were examined, 

sampled, and tested radiometrically for uranium. 

Numerous discontinuous layers of opal are interbedded with a gently-dipping series of vitric 

tuff and ash which is at least 300 ft thick. The tuff and ash are capped by a dark, vesicular 

basalt in the eastern part of the area and by a thin layer of terrace qravels in the area along the 

west side of Virgm Valley. 

Silicification of the ash and tuff has produced a rock that ranges from partly opalized rock 

that resembles silicified shale to completely altered rock that is entirely translucent, and consists 

of massive, brown and pale-green opal. 

Carnotite, the only identified uranium mineral, occurs as fracture coatings or fine layers 

in the opal; in places, no uranium minerals are visible in the radioactive opal. 

The opal layers are irregular in extent and thickness. The exposed length of the layers 

ranges from 8 to 1, 200ft or more, and the thickness of the layers ranges from 0. 1 to 3. 9 ft. 

The uranium content of each opal layer, and of different parts of the same layer, differs 

widely. On the east side of Virgin Valley four of the seven observed opal layers, nos. 3, 4, 5, 

and 7, are more radioactive than the average; and the uranium content ranges from 0. 002 ·to 0. 12 

percent. Two samples, taken 5 ft apart across opal layer no. 7, contained 0. 003 and 0.-049 percent 

uranium. On the west side of the valley only four of the fifteen observed opal layers, nos; 9, , 10, 

14, and 15, are more radioactive than the average; and the uranium content ranges from 0. 004 to 

0. 047 percent. Material of the highest grade was found in a small discontinuous layer of palegreen opal (no. 4) on the east side of Virgin Valley. The grade of this layer ranged from 0. 027 

to 0. 12 percent uranium. 

1 

Page 

2 

5 

7 

Page 

4 

6 

3 
INTRODUCTION 

Uraniferous opal has been found in the Virgin 

Valley opal district, Humboldt County, Nev. {fig. 1). 

This district has produced some of the best gem opal 

found in the United States. The district is on the 

Virgin Valley ranch in T. 45 N. , R. 26 E. , Mt. Diablo 

principal meridian, in the Sheldon Game Refuge, 

88 miles east of Cedarville, Calif. , · and 41 miles 

west of Denio, Nev. Denio, Nev., is on the OregonNevada boundary line and is 102 miles northwest of 

Winnemucca, Nev. A road log fr·om the post office 

in Denio to the Virgin Valley Ranch house is given 

below. 

Mileage 

0.0 

2. 7• 

4.3 

5. 1 

11.4 

14.4 

15.5 

30.7 

30.9 

31. 9 

33.2 

33.3 

36.4 

37.8 

41.4 

Road log 

Description 

Post office, Denio - travel west on 

State Route 8-A. 

Junction - keep to the left. 

Junction - keep to the left. 

Junction - keep to the right. 

Junction - keep to the right. 

Junction - keep to the left. 

Enter Sheldon Game Refuge. 

Junction - turn left on the Camp 

Sheldon road. 

Junction - keep to the left. 

Junction - keep to the right. 

Camp Sheldon {Old CCC Camp). 

Junction - turn to the right. 

Gate in fence. 

Small reservoir. 

Virgin Valley ranch house. Workings 

are 200 yards northwest and 300 

yards northeast of the ranch house. 

The .area examined consists of 19 claims 

located by Jack and Toni Crane of Route 2, Oswego, 

Oreg. , during July and August 1950. These claims 

are: the Monday, Tuesday, Wednesday, Thursday, 

Friday, Saturday, and Wee Wee Marie--all on the 

west side of Virgin Valley; and the September, 

October, March, 4th of July no. 1, 4th of July no. 2, 

F ebruary, April Fool no. 1, April Fool no. 2, 

January, November, August, and December--on the 

east side of the valley. Before the discovery of . 

uranium minerals in 1950 about 20 small pits had 

been dug on the east side of Virgin Valley during 

prospecting for semiprecious opal. Several thousand 

pounds of a light green fluorescent opal and a mottled, 

variegated opal have been mined and sold to mineral 

collectors and cabochon cutters. Uranium minerals 

were discovered by Jack Crane early in the summer 

of 1950. Development work done since that time 

consists pf a 315-foot bulldozer trench and 12 small 

pits on the east side. of the valley, and 3 small pits 

on the west side of the valley. 

The properties were examined on behalf of the 

Atomic Energy Commission by M. H. Staatz and 

H. L. Bauer, Jr. , of the U. S. Geological Survey 

from November 4th to 7th, 1950. An area of 1, 000 

by 1, 200 ft on the east side of Virgin Valley (fig. 2), 

and an area of '750 by 1, 580 ft on the west side of 

Virgin V8.1ley (fig. 3) were mapped with a plane table 

and a telescopic alidade on a scale of 1: 1200. Two 2 

grab samples and 35 channel samples were analyzed 

for uranium. All opal beds in the mapped area were 

tested radiometrically along their outcrop with a 

Geiger-Mueller counter (Victoreen). · 

Jack Crane guided the party around the property 

and gave much valuable information on the area. 

GEOLOGY 

The Virgin Valley has been cut in a series of 

gently- dipping tuff and ash beds capped by a basalt 

flow and terrace gravels. The intimately interbedded 

series of tuff and ash beds is at least 300 ft. thick. 

Individual beds range in thicknes3 from 0. 1 to 10 ft; 

most of them are from 1 to 3 :t in thickness. Greenishgray, vitric tuff beds are most common, and consist 

largely of fragments between 0. 5 and 30 millimeters 

in diameter·. The tuff is porous and quite friable. In 

general the ash beds are lighter colored than the tuf~s; 

many are white. Numerous extremely fine- grained, 

chalk-white- ash beds occur on the east ·side of Virgin 

Valley. These beds, which range in thickness from 

0. 1 to 4 ft and consist in part of clay minerals, are 

probably altered feldspathic tuffs. · 

Basalt caps the hilltops on the east side of 

Virgin Valley about 400 yards east of, and 200 feet 

above, the mapped area. Along the west side of 

Virgin Valley, the, ridges in the mapped area are 

capped by a thin layer of terrace gravels, containing 

pebbles of volcanic rocks and fragments of fossilized 

wood, opal, and chert. The valley floor is covered 

with alluvium. 

ORE DEPOSITS 

The uraniferous opal occurs in discontinuous 

layers parallel to the bedding of the ash and tuff beds. 

The opal is confined to tuff beds in most places, but 

on the east side of Virgin Valley some of the opal 

layers, especially the more translucent varieties, 

occur in fine- grained, chalk-white ash beds. In the 

area mapped on the east side of Virgin Valley {fig. 2), 

four of seven opal layers were more radioactive than 

average. In the area mapped on the west side of the 

valley (fig. 3), four of fifteen opal layers were more 

radioactive than average. 

Form of Deposits 

The only uranium mineral noted is a fine, 

powdery, canary-yellow mineral, which has been 

identified as carnotite. It commonly occurs as fi~e 

coatings on parting planes and fractures. In one place, 

t.he ·carnotite occurs as fine yellow layers, and appears 

to have been deposited simultaneously with opal. In 

some opal layers, no uranium minerals are visible, 

but analyses indicate a uranium content higher than 

.average. An example of this is the pale-green opal 

on the east side of Virgin Valley, in which the uranium 

either is an integral part of the opal or is present as 

finely scattered, minute grains of carnotite. 

Hydrated silica is common in tuff and ash beds 

of the district, and many stages of silicification were 

noted. Where relatively small amounts of silica have 

been added the silic ified r ock resembles a silicified 

s hale bed, and all the original pyroclastic structure 
42°f9==============r====-~----~----:-------J-.J -r-~------li---------..1-----------

• Virgin Valley 0 Me Dermitt 

0

Tuscarora 

0 Austin Eureka 

Tonopah 

3ar-----~~~--~--~~-~-------­ MonoLoke 

' ' " ' ' 

116° 

IOOMiles 

Figure l. --Index map, Virgin Valley, Humboldt County, Nevada 

3 

36" 
100 

EXPLANATION 

[,;//1 

Opal 

D Interbedded ash 

and tuff 

\2 Dip and strike 

of beds 

Cut with dump 

58-22-QO· 0 12 

Sample number; percent uranium 

100 

Scale 

Contour Interval 20 Feet 

Datum assumed 

A' 

~~.·rA ~~~· . . ~ .. ~ . = -=- -,.__...,_-,_ ~- ~ . I ~--------------

SECTION A-A1 

Figure 2. --Geologic map and section, east side of Virgin Valley, Humboldt 

County, Nevada 

4 

200 FEET . 
has been destroyed. Thoroughly opalized layers are 

translucent. The opal in the mapped area has a 

distinctive conchoidal fracture and is gray, brown, 

tan, black, white, and pale green. Some of the opal 

opal layer on the east side of the valley, which is 

only 8 ft long. The layers mapped were from 0. 1 to 

3. 9 ft thick. 

is banded, sor:1e is uniform in color, and some is Radioactivity 

variegated. Banding is usually parallel to the bedding 

in the tuff and ash beds. Hair -line fracturing or 

"checking" is common, especially in some of the less 

opalized layers. 

The radioactivity of all opal layers was checked 

at numerous points along their strike with a GeigerMueller counter. Readings on the counter ranged 

,from two divisions on the 0. 2 scale to seven divisions 

The uraniferous opal layers. differ considerably 

in length and thickness. Two layers (no. 10 and 

on the 2. 0 scale. Nineteen samples were taken of 

no. 12) on the west side of Virgin Valley {fig. 3) were 

followed continuously for more than 1, 200ft along 

strike; others are much shorter--such as a variegated 

the opal on the east side of Virgin Valley, 16 samples 

were taken of opal on the west side, and 2 samples 

were taken of white ash adjacent to opal beds (tables 

1 and 2). 

Table 1. --Analyses of samples, .east side of Virgin Valley, Nev. 

Equivalent 

Field sample Layer uranium Uranium 

number number Description (percent) (percent) 

BS- 6.,.50 4 Pale-green opal 0.048 0.046 

BS- 7-50 5 Tan to brown opal . 004 . 005 

BS- 8-50 6 White, iron-stained opal . 002 . 002 

BS- 9-50 6 White, iron-stained opal . 002 . 003 

BS-10-50 5 Brown, variegated opal . 014 . 014 

BS-11-50 4 Brown to pale-green opal . 033 . 027 

BS-12-50 5 Brown, variegated opal . 010 . 008 

BS-13-50 6 White, iron-stained opal . 003 . 005 

BS-14-50 6 White, iron-stainedopal . 001 . 003 

BS-15-50 7 Gray, iron-stained opal . 019 . 014 

BS-16-50 7 Gray, iron-stained opal . 002 . 002 

BS-17-tiO 6 White, iron-stained opal . 003 . 004 

BS-18-50 7 White to pale-green opal . 049 '049 

BS-19-50 7 Brown opal . 002 . 003 

BS-20-50 7 Brown to tan opal . 014 . 021 

BS-21-50 - Chalk-white ash . 003 . 007 

BS-22-'50 4 Pale-green opal .11 . 12 

BS-23-50 3 Yellowish to brown opal . 005 . 005 

BS-24-50 - White to black, variegated opal . 013 . 015 

BS-25-50 3 Tan to greenish-brown opal . 014 . 015 

Table 2. --Analyses of samples, west side of Virgin Valley, Nev. 

Equivalent 

Field Sample Layer uranium Uranium 

number number Description (percent) (percent) 

BS-26-50 12 Cream to tan opal 0.002 0.002 

BS-27-50 10 Brown to white opal . 015 • 019 

BS-28-50 12 Cream to tan opal . . 003 . 003 

BS-29-50 10 Brown to gray, banded opal . 016 . 021 

BS-30-50 10 Brown, banded opal . 021 . 018 

BS-31-50 12 White to brown opal . 002 . 003 

BS-32-50 9 Gray to brown, variegated opal . 004 . 007 

BS-33-50 10 Gray, variegated opal . 024 . 033 

BS.:.34-50 9 Tan to brown opal . 032 I . 047 

BS-35-58 10 Tan to brown opal . 040 . 038 

BS-36-50 14, 15 Two tan to brown opal layers . 026 . 032 

separated by ash bed 

BS-37-50 -- Gray ash . 007 . 003 

BS-38-50 14, 15 Two tan to brown opal layers . 018 . 024 

separated by ash bed 

BS-39-50 10 Gray, iron-stained opal . 020 . 025 

BS-40-50 9 Brown, variegated opal . 004 . 004 

BS-41-50 10 White to brown, variegated . 025 . 030 

o_pal 

BS-42-50 9 Gray, iron-stained opal . 004 . 005 

5 
LAYER 

EXPLANATION 

D 

Alluvium 

t2TIJ 0 

0 

Terrace grovel 

~ 

~ 

Opal 

D Interbedded ash 

and tuff 

Inferred contact 

Strike ond dip 

· of beds 

• 0 

0 • 0 0 

0 0 0 

100 

I 

Scale 

Sample number; percent uranium Contour Interval 20 Feet 

Datum Assumed 

A 

SECTION A-A1 SECTION 8-B' 

Figure 3. --Geologic map and sections, west side of Virgin Valley, Humboldt 

County, Nevada 

6. 

200 FEET 

I 
The radioactivity and uranium content of the 

opal.differ in individual layers and along the same 

layer. Layer no. 7 on the east side of Virgin V'3.lley 

demonstrates the range of uranium content of a 

single layer. Near the center of the eastern edge 

of the area (fig. 2) white to pale-green opal is 

exposed on the east side of a small, 5-foot-wide pit, 

and brown opal is exposed on the west side. The 

uranium content of the white to pale-green opal is 

0. 049 percent; the uranium content of the brown opal 

is 0. 003 percent. 

The uranium content of opal ranges from 

0. 002 to 0. 12 percent (tables 1 and 2). Of the 15 

opal layers on the west side, nos. 9, 10, 14, and 

15 showed higher radioactivity than the average: 

The. uranium content of layer no. 9 ranged from 0. 004 

to 0. 047 percent in four samples, but only one 

sample contained over 0. 007 percent. Seven samples, 

taken over an outcrop length of 1, 220 feet in layer 

no. 10 ranged from 0. 018 to 0. 038 percent uranium. 

This layer averages 1 foot in thickness. Layers 

no. 14 and no. 15 are about half a foot thick and are 

separated by a thin layer of ash. These two opal 

layers and the intervening ash bed were sampled 

together, and two samples showed 0. 024 to 0. 032 

percent uranium. These layers are exposed for 

about 400 ft along strike. 

Of the seven layers on the east side of Virgin 

Valley, nos. 3, 4, 5, and 7 showed higher radioactivity than average. Layer no. 3 is exposed over 

an outcrop length of 400 ft in the western half of the 

mapped area (fig. 2). Two samples of this layer 

contained 0. 005 and 0. 015 percent uranium respectively. 

Layer no. 4 is made up of a series of small 

discontinuous lenses, 12 to 25 ft in outcrop length, 

in a chalk-white ash bed. It is 2 ft or more in 

thickness, and the pale -green opal has a uranium 

content of 0. 027 to 0. 12 percent. The uranium 

content of this pale -green opal is the highest in the 

district. About 300 feet southwest of the southern7. 

most exposure of pale-green opal is a small lens 

of clear to translucent variegated opal. This lens 

may be on the same horizon as the pale-green opal, 

or it may be in a higher horizon. The clear to 

translucent variegated opal and the pale-green opal 

are sold to mineral collectors and cabochon cutters 

for $0. 25 or more per pound. 

Layer no. 5 is separa,.ted by 1. 5 to 2 ft of 

tuff and ash beds from layer no. 4· and is from 0. 5 to 

1. 0 foot in thickness. It is a variegated light-brown 

to black opal with a uranium content from 0. 005 

to 0. 014 percent. 

Layer no. 7 is exposed in the bulldozer trench 

and in several trenches near the top of the ridge. It 

is characterized by wide ranges of thickness, shape, 

color, and uranium content. It is 0. 5 to 3. 9 ft thick, 

,and is commonly irr.egular, with many small 

stringers extending into the chalk-white ash from the 

main opal mass. Its color ranges from brown or 

gray to translucent white or pale green, and its 

uranium content ranges from 0. 002 to 0. 049 percent. 

In one of the pits this layer is encrusted by fine, 

powdery carnotite. 

SUGGESTIONS FOR PR.OSPFCTING 

The uranium content of the known deposits of 

uraniferous opal in the Virgin Valley opal district is 

insufficient to warrant economic development at 

present prices. The possibilities of finding economically minable material by additional exploration of 

known uraniferous layers are not considered good. 

Only a small area was studied in detail for this 

report. Because opal deposits are widely distributed 

in the Virgin Valley area, however, further prospecting may discover additional deposits of radioactive cpal. Prospecting should be directed toward 

the discovery of opal layers of greater thickness and 

regularity, and of higher uranium content than those 

described in this report. 

INT.-DUP. SEC., !lASH., D.C .l095l