Mineral Resources of the Mount Limbo 

Wilderness Study Area, Pershing County, 

Nevada

U.S. GEOLOGICAL SURVEY BULLETIN 1726-A

Chapter A

Mineral Resources of the Mount Limbo 

Wilderness Study Area, Pershing County, 

Nevada

By WILLIAM J. KEITH, ROBERT L. TURNER, 

and DONALD PLOUFF 

U.S. Geological Survey

CLAYTON M. RUMSEY 

U.S. Bureau of Mines

U.S. GEOLOGICAL SURVEY BULLETIN 1726-A

MINERAL RESOURCES OF WILDERNESS STUDY AREAS: 

HUMBOLDT AND PERSHING COUNTIES, NEVADA
DEPARTMENT OF THE INTERIOR 

DONALD PAUL MODEL, Secretary

U.S. GEOLOGICAL SURVEY 

Dallas L. Peck, Director

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1986

For sale by the

Books and Open-File Reports Section

U.S. Geological Survey

Federal Center, Box 25425

Denver, CO 80225

Library of Congress Cataloging-in-Publication Data

Mineral resources of the Mount Limbo Wilderness Study 

Area, Pershing County, Nevada.

U.S. Geological Survey Bulletin 1726-A

Bibliography: p. 4

Supt. of Docs. No.: I 19.3:1726-A

1. Mines and mineral resources Nevada Mount Limbo

Wilderness. 2. Geology Nevada Mount Limbo

Wilderness. 3. Mount Limbo Wilderness (Nev.) I. Keith,

William J., 1933- . II. Series.

QE75.B9 No. 1726-A 557.3 s 86-600348

[TN24.N3] [553'.09793'53l
STUDIES RELATED TO WILDERNESS

Bureau of Land Management Wilderness Study Area

The Federal Land Policy and Management Act (Public Law 94-579, October 21, 1976) 

requires the U.S. Geological Survey and the U.S. Bureau of Mines to conduct mineral 

surveys of certain areas to determine the mineral values, if any, that may be present. 

Results must be made available to the public and be submitted to the President and the 

Congress. This report presents the results of a mineral survey of part of the Mount 

Limbo Wilderness Study Area (NV-020-201). Pershing County, Nevada.

CONTENTS

Su m mary

Abstract Al

Character and setting 1

Identified resources 1

Mineral resource potential 1 

Introduction 3

Area description 3

Previous and present investigations 3

Acknowledgments 3 

Appraisal of identified resources 3

Methods 3

Mining history 3

Sites studied 6

Conclusions 6 

Assessment of potential for undiscovered resources 6

Geological studies 6

Geochemical studies 6

Geophysical studies 7

Mineral and energy resources 7 

References cited 7

Appendix 1. Definition of levels of mineral resource potential and certainty of assessment 10 

Geologic time chart 11

Figures

1. Index map showing the location of the Mount Limbo Wilderness Study Area, Pershing County, Nevada A2

2. Geologic map of the Mount Limbo Wilderness Study Area showing prospects, mineralized areas, and 

areas of mineral resource potential 4

3. Major elements of mineral resource potential/certainty classification 10

Tables

1. Descriptions of prospects and sinter or tufa sites in the Mount Limbo Wilderness Study Area 9

MINERAL RESOURCES OF WILDERNESS STUDY AREAS: HUMBOLDT AND PERSHING COUNTIES, NEVADA

Mineral Resources of the Mount Limbo 

Wilderness Study Area, Pershing County, Nevada

By William J. Keith, Robert L. Turner, and Donald Plouff 

U.S. Geological Survey

Clayton M. Rumsey 

U.S. Bureau of Mines

SUMMARY

Abstract

At the request of the Bureau of Land 

Management, 12,900 acres of the Mount Limbo 

Wilderness Study Area (NV-020-201) were studied. In 

this report, the area studied is referred to as "the 

wilderness study area," or simply "the study area." 

The study area is located at the southern end of the 

Selenite Range in Pershing County, Nev. Geological, 

geochemical, geophysical, and mineral surveys were 

conducted by the U.S. Geological Survey and the U.S. 

Bureau of Mines from 1983 to 1986 to assess the 

mineral resource potential of the study area. No 

resources were identified in the area; however, the 

results of these surveys indicate the existence of a 

zone with moderate potential for the southern end of 

the study area. Elsewhere within the study area, the 

potential for gold and silver resources is low. 

Potential for geothermal resources is low in the entire 

study area. The granitic and volcanic rocks found in 

the study area are unfavorable for the accumulation of 

oil and gas.

Character and Setting

The study area is located at the southern end of 

the Selenite Range, approximately 20 mi southeast of 

Gerlach. Nev. (fig. 1). The topography is typical of the

Basin and Range geomorphic province, with northsouth trending fault-bounded valleys and ranges. It is 

an area of high relief, ranging from 4,000 ft at the 

valley floor to 8.237 ft at the summit of Kumiva 

Peak. The study area comprises a granodiorite pluton 

of Cretaceous age (63 to 138 million years before 

present, or Ma) (see Geologic Time Chart, last page of 

report) bounded on the east and west by high-angle 

normal faults. Small felsic to mafic, aplitic to 

pegmatitic dikes intrude the pluton. Alluviated valleys 

flank the study area.

Identified Resources

Four prospects are located in the study area, but 

it contains no mines or active claims. Past interest 

centered on locally mineralized quartz veins, but they 

contained no identified mineral resources. Small tufa 

(calcareous sinter) deposits in the southwest part of 

the study area contain low concentrations of gold and 

silver and may be significant. Stone, sand, and gravel 

are present in the study area but are distant from 

anticipated markets.

Mineral Resource Potential

The southernmost part of the study area has 

moderate resource potential for gold and silver (fig. 

2). Stream-sediment samples from this part of the 

study area contain anomalous concentrations of a suite 

of elements (arsenic, antimony, bismuth, cadmium, and

Al
MOUNT LIMBO

WILDERNESS STUDY

AREA (NV-020-201)

Figure 1. Index map showing the location of the Mount Limbo Wilderness Study Area, Pershing County, Nevada.

A2
silver) that typically occur in gold and silver vein 

deposits. The presence of faults and veins in this area 

also facilitates this type of mineralization. Most of 

the observed alteration consists of thin (several inches 

thick) argillic selveges around veins and oxidation 

halos adjacent to minor faults and fractures.

In the rest of the study area the rarity of veins 

and absence of extensive alteration indicate low 

potential for gold and silver resources.

The potential for geothermal resources in the 

study area is low. although a known geothermal 

resource area (KGRA), with water temperatures as 

high as 95°C. is located in the San Emidio desert (fig. 

1, KGRA) 4 mi west of the study area.

The granitic and volcanic rocks found in the 

study area are not favorable host rocks for the 

accumulation of hydrocarbons.

The U.S. Bureau of Mines studied the known 

prospects and claims within and adjacent to the study 

area as a means of developing a model that would 

predict the grade and tonnage of mineral resources 

that might occur in the study area (Rumsey, 1986).

Acknowledgm ents

The U.S. Geological Survey and the U.S. Bureau 

of Mines received assistance from the following: V. 

Dunn. U.S. Bureau of Land Management geologist, 

provided information on locations of mineralized 

areas; Helen Thrasher, owner, and Harold Playford. 

lessee of the Thrasher mine, provided local history and 

directions; R. Baker, C. Erdman, K. Greene, J. Lewis, 

and N. Logue assisted in the fieldwork.

INTRODUCTION

Area Description

At the request of the Bureau of Land 

Management, 12,900 acres of the Mount Limbo 

Wilderness Study Area (NV-020-201) were studied. In 

this report, the area studied is referred to as "the 

wilderness study area," or simply "the study area." 

The study area consists of rugged, sparsely vegetated 

terrain bordering the northeast edge of Winnemucca 

Dry Lake in western Pershing County, Nev. (fig. 1). It 

is located in the southernmost part of the Selenite 

Range and is flanked by broad (2 mi wide on the west. 

10 mi wide on the east) alluviated valleys. The area is 

accessible by gravel roads on all sides, but the interior 

is accessible only by helicopter, horseback, or on 

foot. Altitudes range from 4,000 ft at the southern tip 

of the study area to 8,237 ft at Kumiva Peak near the 

north end.

Previous and Present Investigations

A geologic map of the study area prepared by the 

U.S. Geological Survey in 1984 (fig. 2) at a scale of 

1:62,500 provided a basis for the interpretation of 

geochemical. geophysical and mining claim data. 

Supplemental geologic data for the north end of the 

study area was taken from a Master's thesis by R.A. 

Crewdson (1974).

Studies by Smith and others (1971). Johnson 

(1977), and the U.S. Bureau of Land Management 

(1983) also provided geologic data for this report.

Geochemical data were obtained from a 

geochemical-geostatistical study done for the U.S. 

Bureau of Land Management by Barringer Resources, 

Inc. (1982) and from analyses of stream-sediment and 

rock-chip samples collected by the U.S. Geological 

Survey in 1984 (Day and others. 1986).

Earlier geophysical data, which consisted of a 

regional gamma-ray survey (Geodata International. 

Inc., 1978) and aeromagnetic maps (U.S. Geological 

Survey, 1972). were supplemented in 1984 by a gravity 

study and a regional gravity compilation conducted by 

the U.S. Geological Survey (unpub. data).

APPRAISAL OF IDENTIFIED RESOURCES

By_Clayton M. Rumsey, 

U.S. Bureau of Mines

Methods

From 1984 to 1986. the U.S. Bureau of Mines 

(USBM) conducted a library search and examined U.S. 

Bureau of Land Management (BLM), USBM, and 

Pershing County claim and production records in order 

to determine the level of mining and prospecting 

activity in the study area. The field study included 

inspections of all prospects and mineralized localities 

within the study area. The Thrasher and Stormy Day 

mines, and three nearby prospects, were examined to 

determine whether mineralized zones might extend 

into the study area. In addition, ground and air 

reconnaissance was done in areas of obvious rock 

alteration. Sixteen prospects and mineralized sites in 

and near the study area were examined and 69 samples 

were collected, including chip samples collected across 

mineralized zones and grab samples from stockpiles of 

mineralized rock. All samples were assayed for gold 

and silver by the combined fire assay-inductively 

coupled plasma method. The presence and 

concentration of other elements were determined by 

atomic-absorption analysis. Selected samples from 

sites with anomalous radioactivity were analyzed for 

uranium. Gold and silver concentrations are reported 

in parts per million (ppm); the content of other 

elements is reported in ppm or percent. Detection 

limits were as follows: gold, 0.007 ppm; silver, 0.3 

ppm; uranium. 0.5 ppm; and lead. 30 ppm. Conversion 

factors are 1 ppm = 0.0292 oz/ton, and 10,000 ppm = 1 

percent.

Mining History

In 1941 tungsten was discovered in the southern 

part of the Hooker mining district, which extends to 

within 1 mile of the north end of the study area. 

Between 1944 and 1957, the Stormy Day mine 

produced 19.523 tons of ore containing 1.66 percent 

tungsten trioxide. The neighboring Thrasher mine 

produced an unknown amount of tungsten ore (Johnson,

A3
40°22'30'

Qvf

geo, Au, Ag [ 1 ]

Qvf

APPROXIMATE BOUNDARY 

OF

STUDY AREA

M/C Au, Ag[l

14

ftT \15

16'

Figure 2. Geologic map of the Mount Limbo Wilderness Study Area showing prospects, mineralized 

areas, and areas of mineral resource potential.

A4
EXPLANATION

AREA WITH MODERATE MINERAL RESOURCE POTENTIAL-Certainty level C (M/C). 

See Appendix 1 and figure 3 for definition of mineral resource potential and 

certainty of assessment

AREA WITH LOW MINERAL AND GEOTHERMAL RESOURCE POTENTIAL-Certainty 

level C (L/C)

COMMODITIES 

Au Gold 

Ag Silver 

geo Geothermal

[ ] TYPES OF DEPOSITS AND OCCURRENCES 

1 Epithermal precious metal deposits

CORRELATION OF MAP UNITS

-QUATERNARY

Qvf

Qt

Tr

Tb

Ta

Ttu

[- Holocene

I- Pleistocene

-TERTIARY

Unconformity

JCRETACEOUS

TRIASSIC

GEOLOGIC MAP UNITS

Qvf VALLEY FILL (HOLOCENE)

Qt TUFA (PLEISTOCENE)

Tr RHYOLITE DIKES (TERTIARY)

Tb BASALT (TERTIARY)

Ta ANDESITE DIKES AND PLUG (TERTIARY)

Ttu UNDIFFERENTIATED TUFF (TERTIARY)--Tuff of

latitic to dacitic composition (Crewdson, 1974) 

Kgd GRANODIORITE (CRETACEOUS) 

"Ems METASILTSTONE (TRIASSIC)~Metamorphosed calcareous

siltstone (Crewdson, 1974)

MAP SYMBOLS 

 CONTACT

FAULT-Dashed where approximately located, dotted where hidden.

Bar and ball on downthrown side

STRIKE AND DIP OF FOLIATION IN PLUTONIC ROCKS-Number indicates dip angle 

DIKES-Composition indicated by symbol 

PROMINENT JOINTS 

PROSPECT-Number refers to table 1 

SINTER OR TUFA DEPOSITS SAMPLED-Number refers to table 1

Figure 2. Continued.

A5
1977. p. 58). Both mines are now inactive. There is no 

record of mining or production within the study area, 

and the study area contained no active claims at the 

time of the study (1984).

Sites Studied

The 16 prospects and mineralized sites examined 

(fig. 2) consist of calcareous sinter or tufa deposits and 

quartz veins in granodiorite (table 1). Quartz veins at 

prospects in the study area are small and have low 

mineral concentrations. Although a relatively high 

concentration of silver (580 ppm) was obtained from a 

sample collected at one prospect (fig. 2. no. 9), silver 

concentrations in other samples are relatively low. 

Sinter or tufa deposits near Winnemucca Lake contain 

low, yet anomalous, concentrations of gold, silver, and 

uranium (table 1).

Conclusions

Although precious-metal resources may occur at 

depth, none were identified at the surface in the study 

area. Although there are sites of anomalous uranium, 

the conditions are not favorable for uranium deposits 

and no resources are indicated.

Tungsten deposits at the Thrasher and Stormy 

Day mines (fig. 2) do not extend into the study area. 

Stone, sand, and gravel, although present, are not 

considered resources because they are too far from 

anticipated markets.

the pluton. probably are segregations in the pluton 

rather than xenoliths.

Mafic dikes found in the study area have a wide 

range of compositions and fill joints and faults in the 

pluton. The smaller dikes (1- to 24-in. wide), 

containing fine-grained granular hornblende, biotite, 

plagioclase, and quartz, occur throughout the pluton 

but are more numerous in the southern part. One dike 

composed almost entirely of hornblende was observed 

on the east side of the study area. Andesite dikes (fig. 

2. unit Ta), ranging from 1 to 15 ft in width, occur 

sporadically throughout the pluton. These dikes are 

aphanitic to porphyritic with sparse phenocrysts 

(xenocrysts?) of plagioclase and (or) hornblende.

Felsic dikes occur throughout the study area as 

pegmatites of predominantly potassium feldspar and 

(or) quartz and as aplites. Locally (north-central ridge 

crest), 1- to 3-ft-wide dikes of aphanitic rhyolite (fig. 

2. Tr) trend approximately N. 70 E.

Dendritic and cellular tufa (fig. 2, unit Qt), 

reportedly of algal origin (Morrison, 1964), drape parts 

of the granodiorite. Cavities in the tufa typically are 

filled with detrital biotite, hornblende, and lesser 

amounts of other detrital minerals making chemical 

analysis of the unit quite variable. The tufa is 

confined to the area below the 4,400-ft contour 

(approximate upper level of Pleistocene Lake 

Lahontan, IV-prison, 1964). Some of the authors of this 

report believe that the tufa (calcareous sinter) 

deposits along the southwest slopes of the study area 

are of thermal-spring origin, and others believe that 

they precipitated from chemically saturated lake 

water.

ASSESSMENT OF POTENTIAL FOR UNDISCOVERED 

RESOURCES

By William J. Keith. Robert L. Turner, and

Donald Plouff

U.S. Geological Survey

Geological Studies

The Selenite Range (fig. 1) is a north-trending 

mountain range in the Basin and Range physiographic 

province. The wilderness study area, at the southern 

tip of the range, contains a fault-bounded granodioritic 

body (fig. 2, unit Kgd) of Cretaceous age (Smith and 

others, 1971) that was intruded by mafic, felsic, and 

aplitic to pegmatitic dikes. Much of the faulting in 

the study area is of the high-angle basin-range type. 

Alteration consists largely of oxidized fringes on the 

faults and dikes and argillic fringes on some of the 

veins.

The granodiorite pluton consists of plagioclase, 

potassium feldspar, quartz, hornblende, biotite, and 

sphene, and has a texture that ranges from generally 

equigranular in the southern part of the study area to 

porphyritic in the north. A north-trending plane of 

foliation is less intense to the north. The northern 

part of the pluton appears to be younger than the 

southern part, and younger phases of the pluton locally 

intrude the older. Inclusions, consisting of hornblende, 

biotite, plagioclase, and quartz, that occur throughout

Geochemical Studies

Geochemical studies of the study area were 

conducted at two levels: a regional reconnaissance 

survey (Barringer Research Inc., 1982) with an average 

sample density of one sample per square mile, and a 

more detailed study (Day and others. 1986) of an area 

shown by the Barringer study to contain relatively high 

concentrations of arsenic.

In the Barringer study, 54 stream-sediment 

samples were collected from sites along the range 

front. Analytical results of this survey show relatively 

high concentrations of arsenic, cadmium, copper, 

fluorine, strontium, uranium, and zinc. The 

distribution of these elements may reflect enrichment 

or depletion in older and younger parts of a pluton 

(Hildreth, 1981).

Twenty-nine sites were sampled (both streamsediment and rock samples) along a 4-mile zone in the 

southern part of the study area. Analyses of the 

nonmagnetic part of heavy-mineral concentrates in the 

stream-sediment samples show anomalous 

concentrations of lead, scandium, and thorium (Day 

and others, 1986). Sphene, which contains minor 

amounts of thorium, was identified in all but two 

samples and is the probable source of the thorium. 

The occurrence of anomalous lead and scandium, each 

at a single site, is not significant. Chemical analyses 

of rock samples show concentrations of antimony, 

arsenic, bismuth, cadmium, and silver that are greater 

than the normal range in fresh granodiorite from the

A6
area (Day and others, 1986). This suite of elements is 

characteristic of precious-metal deposits (Boyle, 

1984).

Geophysical Studies

Concentrations of potassium and equivalent 

thorium and uranium were estimated for the study 

area by examining composite-color maps of gammaray spectrometric data (J.S. Duval, written commun., 

1985). The data were obtained from radiometric 

profiles compiled by Geodata International, Inc. 

(1978). Based on criteria discussed by Duval (1983), 

the study area has moderate radioactivity with values 

of 1.5 to 2.5 percent potassium, 6 to 12 ppm equivalent 

thorium, and 2.0 to 3.5 ppm equivalent uranium. There 

is no indication of anomalous concentrations of 

radioelements.

In 1984, the U.S. Geological Survey established 

47 gravity stations in and near the study area to 

compile a regional gravity map (D. Plouff, unpub. 

data). Although lacking coverage along the crest of 

the Selenite Range, the Bouguer gravity-anomaly map 

shows a gravity high over the range that exceeds 10 

milliGals relative to the surrounding valleys. The 

thickness of Quaternary sediments east and west of 

the range are believed to be insufficient to entirely 

account for the magnitude of the gravity high. 

Prominent gravity gradients along the east and west 

flanks of the range suggest that block-faulting 

occurred along inferred major faults (fig. 2) that 

separate the crystalline core of the range from the 

surrounding, less dense basement rocks.

An aeromagnetic map of the region shows a 

distinct magnetic high located over the Selenite Range 

(U.S. Geological Survey, 1972). The shape of the 

anomaly closely conforms to topography; hence, it 

reflects moderate magnetization of the granodiorite. 

The aeromagnetic survey was flown at a constant 

barometric elevation of 9,000 ft above sea level. 

Consequently, the level of magnetic intensity is about 

100 nanoteslas lower along a flightline near Mount 

Limbo (7.312 ft) than along a flightline near Kumiva 

Peak (8,237 ft). The shape of the magnetic high is 

similar to that of the gravity high, but the magnetic 

gradient along the west flank of the range is more 

linear and extends farther south than the gravity 

gradient, and the magnetic gradient along the east 

flank is less pronounced. The small scale of the 

aeromagnetic map (1:250,000), the wide spacing of 

flightlines (2 mi), and strong topographic effects limit 

interpretation to defining the margin of the 

granodiorite of Mount Limbo.

MINERAL AND ENERGY RESOURCES

Anomalous concentrations of arsenic, antimony, 

bismuth, cadmium, and silver, coupled with permissive 

host rock and the presence of faults and veins in an 

area in the southern part of the Mount Limbo 

Wilderness Study Area (fig. 2), indicate that the 

mineral resource potential for gold and silver at depth 

is moderate with a C certainty level. See fig. 3 and 

appendix 1 for definitions of levels of mineral resource

potential and certainty of assessment. No estimate of 

depth to or size of the body of mineralized rock can be 

made from the available data.

The rest of the study area has a low mineral 

resource potential (certainty level C) for gold and 

silver. Plutonic rock commonly is the host for baseand precious-metal deposits, but the lack of 

hydrothermal alteration and the sporadic, isolated 

distribution of the geochemical anomalies indicate 

that the probability of the occurrence of a vein or 

disseminated type of mineral deposit in this area is 

low.

Geothermal energy resource potential is low 

(certainty level C) throughout the study area. A 

known geothermal resource area four mi west of the 

study area in the San Emidio Desert (fig. 1, KGRA) has 

water temperatures as high as 95°C (Muffler, 1978), 

but there is no evidence that a hydrothermal system 

extends into the study area.

The predominant rock type in the study area is 

granodiorite, which is an unfavorable host for 

petroleum and natural gas accumulation.

REFERENCES CITED

Barringer Resources, Inc., 1982 , Geochemical and 

geostatistical evaluation of wilderness study 

areas, Winnemucca district, northwest Nevada, 5 

v.: Golden, Colo. available from NTIS, U.S. 

Department of Commerce, Springfield, VA 22161, 

or call Information Center and Bookstore (202) 

377-0365 .

Crewdson, R.A., 1974, The geology and mineral 

deposits of part of the Selenite Range, Pershing 

County, Nevada: Golden, Colo., Colorado School 

of Mines, M.S. Thesis, 65 p.

Day, Gorden W., Turner, R.L., Conklin, Nancy N., and 

Briggs, Paul, 1986, Analytical results and sample 

location map of heavy-mineral concentrates and 

rock samples from the Mount Limbo Wilderness 

Study Area (NV 020-201), Pershing County, 

Nevada: U.S. Geological Survey Open-File Report 

86-378 in press .

Duval, J.S., 1983, Composite color images of aerial 

gamma-ray spectrometric data: Geophysics, v. 

48, no. 6, p. 722-735.

Geodata International, Inc., 1978, Aerial radiometric 

and magnetic survey, Lovelock national 

topographic map, Nevada: U.S. Department of 

Energy Open-File Report GJBX-125 (78), v. 2, 69

PHildreth, W., 1981, Gradients in silicic magma

chambers: Implications for lithospheric

magmatism: Journal of Geophysical Research, v.

86, no. Bll, p. 10153-10192. 

Johnson, M.G., 1977, Geology and mineral deposits of

Pershing County, Nevada: Nevada Bureau of

Mines and Geology Bulletin 89, 115 p. 

Morrison, R.B., 1964, Lake Lahontan: Geology of

southern Carson Desert, Nevada: U.S. Geological

Survey Professional Paper 401, 156p. 

Muffler, L.J.P., ed., 1978, Assessment of geothermal

resources of the United States 1978: U.S.

Geological Survey Circular 790, 163 p. 

Rumsey, C.M., 1986, Mineral resources of the Mount

A7
Limbo Wilderness Study Area and vicinity, U.S. Bureau of Land Management, 1983, Mount Limbo

Pershing County, Nevada: U.S. Bureau of Mines Wilderness Study Area, in Winnemucca

Open-File Report 35-86, 11 p. Wilderness Technical Report, WNP T830094332, p.

Smith, J.G., McKee, E.H., Tatlock, D.B., and Marvin, 131-146.

R.F., 1971, Mesozoic granitic rocks in U.S. Geological Survey, 1972, Aeromagnetic map of

northwestern Nevada a link between the Sierra parts of the Lovelock, Reno, and Millet 1° by 2°

Nevada and Idaho Batholiths: Geological Society quadrangles, Nevada: U.S. Geological Survey

of America Bulletin, v. 82, p. 2933-2944. Open-file Report, 1 pi., scale 1:250,000.

A8
-Descriptions of prospects and sinter or tufa sites in the Mt. Limbo Wilderness Study Area 

[Asterisk indicates locations outside the study area]

Map 

No. Summary Workings and production Sample and resource data

Pinto prospect 

(silver-uranium)

Unnamed prospect 

(silver-uranium)

Unnamed prospect 

(uranium)

Sinter or tufa site 

(uranium)

Unnamed prospect 

(uranium)

Unnamed prospect 

(silver-uraniumgold)

Sinter or tufa site 

(silver-uranium)

Sinter or tufa site 

(gold-uranium)

A 5-ft-thick limonite-stained quartz 

vein and float "occur" along a rangefront fault that parallels a contact 

between granodiorite and volcanic 

rocks. The fault strikes N. 10° E., 

dips 65° NW., and can be traced for 

6 mi.

A 6-ft-thick, iron-oxide-stained 

smoky quartz vein with an attitude of 

N. 45° W., 35° HE. is exposed for 

7 ft in granodiorite.

Iron-oxide-stained contact between 

granodiorite and rhyolite trends 

generally easterly.

About 80 percent of the granodiorite 

in a 500- by 2,200-ft area is coated 

with calcareous sinter or tufa that 

is 0.5 to 4 ft thick.

Altered rhyolite tuff and 

granodiorite "occur" along a contact 

that strikes N. 85° E. and dips 

45° N.

A 0.5-ft-thick, iron-oxide-stained 

quartz vein "occurs" in altered 

granodiorite

About 60 percent of a 170- by 85-ft 

area of granodiorite is coated with 

calcium carbonate 1 ft thick.

Calcareous sinter or tufa at a former 

thermal spring coats about 25 percent 

of underlying granodiorite to a 

thickness of 1.5 ft, 50 to 300 ft 

from the spring.

One prospect pit.

One prospect pit.

Two 150- by 2-ft-deep 

trenches, and a prospect 

pit.

A 90-ft trench

Two 30-ft trenches

Eight samples: one chip sample 

collected across the quartz vein 

contained 3.532 ppm silver; one grab 

sample of quartz from the prospect 

pit contained no gold or silver; one 

of six other grab samples from along 

the fault contained 1.041 ppm silver 

and two each had 0.97 ppm l^Og.

One chip sample contained 9.19 ppm 

silver and 4.0 ppm l^Og.

Three chip and two grab samples: chip 

samples of granodiorite contained 2.8 

to 5.1 ppm 1)300; a grab sample of 

concentrated black sand had 544 ppm 

U^Og; and a grab sample from a dump 

contained 3.5 ppm U30g.

Three chip samples and one grab 

sample of sinter or tufa contained 

between 1.8 and 2.8 ppm U3 0g.

Five chip samples taken along the 

contact contained from 2.8 to 7.0 ppm 

U,0fl .

Three chip samples contained 1.047 to 

1.209 ppm silver and 2.7 to 4.0 ppm 

U30g; one had 0.04 ppm gold.

Four chip samples contained from 1.00 

to 1.142 ppm silver; three contained 

3.4 to 3.7 ppm U30g.

Four chip samples contained from 1.5 

to 6.4 ppm U3 0 8 ; one had 0.008 ppm 

gold.

Unnamed prospect 

(silver-leaduranium)

Sinter or tufa site 

(silver-uraniumgold)

Sinter or tufa site 

(silver-gold)

Sinter or tufa site 

(silver-uranium)

Unnamed prospect 

(silver-uranium)

Limbo group

(uranium-silvergold)

Sinter or tufa site 

(uranium)

Unnamed prospect 

(uranium)

A 1,600-ft-wide zone of leached and 

iron-oxide-stained quartz veins 

trends northeasterly in granodiorite.

Calcareous sinter or tufa, at least 

0.5 ft thick, overlies granodiorite 

520 ft from former thermal springs.

Calcium carbonate sinter or tufa of 

undetermined thickness coats 

granodiorite over a 50-ft diameter 

area.

Calcium carbonate sinter or tufa 0.1 

to more than 10-ft-thick coats about 

80 percent of the granodiorite within

thermal springs.

Two 1-ft-thick aplite dikes enclose 

0.3-ft-thick veins of iron-oxidestained quartz. The dikes are 5 ft 

apart, exposed for 50 ft, trend 

northward, and dip 10° eastward in 

granodiorite.

A 4-ft-thick, iron-oxide-stained 

quartz vein which strikes N. 30° W. 

and dips 60° NE., is exposed for 200 

ft in granodiorite.

Calcium carbonate sinter or tufa 1 to

5 ft thick near a former thermal

spring coats basalt on a mound that

is 100 ft wide and 50 ft high.

Three 2-ft-thick, aplite dikes within 

400 lateral feet strike between due 

north and N. 30° W. and dip 

vertically in altered granodiorite.

About an acre of the ridge 

top was bulldozed.

One 18-ft adit.

Three chip and two grab samples: one 

chip sample from a small vein 

contained 0.01 ppm gold, 580 ppm 

silver, 2.19 percent lead, and 59 ppm 

U3 0g. The other chip samples 

contained 0.8 and 2.966 ppm silver, 

and 33 and 50 ppm lead. One grab 

sample contained 3.22 ppm silver, 

0.019 ppm lead, and 2.2 ppm U30g.

Three chip samples contained from 

0.989 to 1.191 ppm silver and 3.4 to 

5.1 ppm U3 0g; two had 0.024 and 0.058 

ppm gold.

Three chip samples contained from 

1.042 to 1.217 ppm silver; two had 

0.018 and 0.124 ppm gold.

Four chip samples contained from 2.4 

to 3.8 ppm UjOg; 

1. 106 ppm silver.

.

to 3.8 ppm UjOg; three had 0.997 to 

ilv

Minor trenches and topsoil 

bulldozed from a 40 by 100 

ft area.

Two chip samples contained 1.175 and 

1.336 ppm silver, and 5.9 and 6.5 ppm 

U30g.

Ten samples of the quartz vein 

contained from 3.2 to 153 ppm U30g. 

Two samples had 0.007 and 0.013 ppm 

gold, and nine contained from 1.149 

to 5.023 ppm silver.

Two chip samples contained 1.3 and 

4.1 ppm U3 0g.

Five samples: four had from 1.8 to 

7.5 ppm U30g.

A9
APPENDIX 1. Definition of levels of mineral resource 

potential and certainty of assessment

Mineral resource potential is defined as the 

likelihood of the presence of mineral resources in a 

defined area; it is not a measure of the amount of 

resources or their profitability.

Mineral resources are concentrations of naturally 

occurring solid, liquid, or gaseous materials in such 

form and amount that economic extraction of a commodity from the concentration is currently or potentially feasible.

Low mineral resource potential is assigned to 

areas where geologic, geochemical, and geophysical 

characteristics indicate a geologic environment where 

the existence of resources is unlikely. This level of 

potential embraces areas of dispersed mineralized rock 

as well as areas having few or no indications of 

mineralization. Assignment of low potential requires 

specific positive knowledge; it is not used as a catchall 

for areas where adequate data are lacking.

Moderate mineral resource potential is assigned 

to areas where geologic, geochemical, and geophysical 

characteristics indicate a geologic environment 

favorable for resource occurrence, where interpretations of data indicate a reasonable chance for resource 

accumulation, and where an application of genetic and 

(or) occurrence models indicates favorable ground.

High mineral resource potential is assigned to 

areas where geologic, geochemical, and geophysical 

characteristics indicate a geologic environment 

favorable for resources, where interpretations of data 

indicate a high likelihood for resource accumulation, 

where data support occurrence and (or) genetic models 

indicating presence of resources, and where evidence 

indicates that mineral concentration has taken place. 

Assignment of high resource potential requires positive 

knowledge that resource-forming processes have been 

active in at least part of the area; it does not require 

that occurrences or deposits be identified.

Unknown mineral resource potential is assigned 

to areas where the level of knowledge is so inadequate 

that classification of the area as high, moderate, or

low would be misleading. The phrase "no mineral 

resource potential" applies only to a specific resource 

type in a well-defined area. This phrase is not used if 

there is the slightest possibility of resource 

occurrence; it is not appropriate as the summary 

rating for any area.

Expression of the certainty of the mineral 

resource assessment incorporates a consideration of (1) 

the adequacy of the geologic, geochemical, geophysical, and resource data base available at the time of 

the assessment, (2) the adequacy of the occurrence or 

the genetic model used as the basis for a specific 

evaluation, and (3) an evaluation of the likelihood that 

the expected mineral endowment of the area is, or 

could be, economically extractable.

Levels of certainty of assessment are denoted oy 

letters, A-D (fig. 3).

A. The available data are not adequate to 

determine the level of mineral resource potential. 

Level A is used with an assignment of unknown mineral 

resource potential.

B. The available data are adequate to suggest 

the geologic environment and the level of mineral 

resource potential, but either evidence is insufficient 

to establish precisely the likelihood of resource occurrence, or occurrence and (or) genetic models are not 

known well enough for predictive resource assessment.

C. The available data give a good indication of 

the geologic environment and the level of mineral 

resource potential, but additional evidence is needed 

to establish precisely the likelihood of resource occurrence, the activity of resource-forming processes, or 

available occurrence and (or) genetic models are 

minimal for predictive applications.

D. The available data clearly define tiie geologic 

environment and the level of mineral resource 

potential, and indicate the activity of resource 

forming processes. Key evidence to interpret the 

presence or absence of specified types of resources is 

available, and occurrence and (or) genetic models are 

adequate for predictive resource assessment.

U/A

UNKNOWN 

POTENTIAL

H/B

HHjH POTENTIAL

M/B

MODERATE POTENTIAL

L/B

LOW POTENTIAL

H/C

HIGH POTENTIAL

M/C

MODERATE POTENTIAL

L/C

LOW POTENTIAL

H/D

HIGH POTENTIAL

M/D

MODERATE POTENTIAL

L/D

LOW POTENTIAL

N/D

NO POTENTIAL

B C

LEVEL OF CERTAINTY

Figure 3. Major elements of mineral resource potential/certainty classification.

A10
GEOLOGIC TIME CHART 

Terms and boundary ages used by the U.S. Geological Survey in this report

EON

Phanerozoic

Proterozoic

Archean

pre -Arc

ERA

Cenozoic

Mesozoic

Paleozoic

Late Proterozoic

Middle Proterozoic

Early Proterozoic

Late Archean

Middle Archean

Early Archean 

hean 2

PERIOD

Quaternary

Neogene 

Subperiod

Tertiary 

Paleogene 

Subperiod

Cretaceous

Jurassic

Triassic

Perm an

Pennsylvanian 

Carboniferous

Mississippian

Devonian

Si urian

Ordovician

Cambrian

 -(3800?)- -

EPOCH

Holocene

Pleistocene

Pliocene

Miocene

Oligocene

Eocene

Paleocene

Late 

Early

Late 

Middle 

Early

Late 

Middle 

Early

Late 

Early

Late 

Middle 

Early

Late 

Early

Late 

Middle 

Early

Late 

Middle 

Early

Late 

Middle 

Early

Late 

Middle 

Early

 _

AGE ESTIMATES 

OF BOUNDARIES 

(in Ma)

- 0.010 

- 1.7

- 5

- 38

- 55

_ fifi

- 96

too

one

- *>Qn

_ yi-OOn

- 360

- 410

- 500 

_. c~7m

1 cAn

- 2500

- 3000

- 3400 

AKKn

'Rocks older than 570 Ma also called Precambrian, a time term without specific rank. 

"Informal time term without specific rank.

All

GPO 785-048/48016