Mineral Resources of the North Fork of the 

Little Humboldt River W.. .mess St Area 

Humboldt County, Nevada

U.S. GEOLOGICAL SURVEY BULLETIN 1732-A

Chapter A

Mineral Resources of the North Fork 

of the Little Humboldt River 

Wilderness Study Area, Humboldt 

County, Nevada

By JOCELYN A. PETERSON and WILLIAM D. HERAN, 

U.S. Geological Survey

ANDREW M. LESZCYKOWSKI 

U.S. Bureau of Mines

U.S. GEOLOGICAL SURVEY BULLETIN 1732-A

MINERAL RESOURCES OF WILDERNESS STUDY AREAS: 

LITTLE HUMBOLDT RIVER REGION, 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

Peterson, Jocelyn A.

Mineral resources of the North Fork of the Little 

Humboldt River Wilderness study area, Humboldt 

County, Nevada

(U.S. Geological Survey Bulletin 1732-A)

Bibliography

Supt. of Docs, no.: I 19.3:1732-A

1. Mines and mineral resources Nevada Little

Humboldt River Wilderness. 2 Little Humboldt River

Wilderness (Nev.) I. Heran, William D.

II. Leszcykowski, Andrew M. III. Title. IV. Series.

QE75.B9 no. 1732-A 557.3 s 86-600046

[TN24.N3] [553'.09793'54]
STUDIES RELATED TO WILDERNESS

Bureau of Land Management Wilderness Study Areas

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 on 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 the North Fork 

of the Little Humboldt River Wilderness Study Area (NV-020-827), Humboldt County, 

Nevada.

CONTENTS

Summary Al

Abstract 1

Character and setting 1 

Mineral resource potential 1 

Introduction 1

Appraisal of identified resources 4 

Assessment of potential for undiscovered resources 4 

Geology 4 

Geochemistry 5 

Geophysics 6 

Mineral and energy resources 6

Epithermal precious-metal vein deposits 6 

Mercury deposits 7 

Uranium deposits 7 

Disseminated-gold deposits 8 

Other commodities 8 

References cited 8

Appendix 1. Definition of levels of mineral resource potential 

and certainty of assessment 10

FIGURES

1. Index map showing location of the North Fork of the Little Humboldt River 

Wilderness Study Area A2

2. Map showing mineral resource potential of the iSforth Fork of the Little Humboldt 

River Wilderness Study Area 3

3. Major elements of mineral resource potential/certainty 

classification 10

V

MINERAL RESOURCES OF WILDERNESS STUDY AREAS: 

LITTLE HUMBOLDT RIVER REGION, NEVADA

Mineral Resources of the North Fork of the 

Little Humboldt River Wilderness Study Area, 

Humboldt County, Nevada

By Jocelyn A. Peterson and William D. Heran 

U.S. Geological Survey

Andrew M. Leszcykowski 

U.S. Bureau of Mines

SUMMARY

Abstract

The Hortft Fork of the Little Humboldt River 

Wilderness Study Area (NV-020-827) includes 69,683 

acres, of which the U.S. Bureau of Land Management 

requested studies on 8,900 acres, along the North Fork 

of the Little Humboldt River in northern Nevada. 

Field work during the summer of 1984 by the U.S. 

Geological Survey and the U.S. Bureau of Mines was 

carried out to assess the mineral resource potential of 

the study area as of May 1985. No mines or prospects 

are located in the study area; however, one mercury 

prospect is located at Greeley Crossing 3 mi west of 

the north end of the study area. The rhyolite of the 

Swisher Mountain Tuff, which comprises the bedrock in 

the study area, has low potential for mercury 

resources. The base of the tuff is probably near the 

surface. Rocks beneath the tuff are similar to those in 

nearby regions that locally contain disseminated gold, 

epithermal precious-metal veins, and anomalous 

amounts of uranium. No information is available as to 

whether there are any undiscovered subsurface 

deposits in the study area. There is no potential for oil 

and gas and potential for geothermal resources is 

unknown.

Character and Setting

The North Fork of the Little Humboldt River 

Wilderness Study Area (NV-020-827) in north-central 

Nevada lies about 25 mi east of the Santa Rosa Range 

and about 20 mi east-northeast of the town of Paradise 

Valley, Nev. (fig. 1). Although a relatively gentle 

plateau topography characterizes the region, the study

area itself encloses most of the canyon of the North 

Fork of the Little Humboldt River. The canyon 

reaches a maximum depth of 600 ft. Tertiary rhyolite, 

correlated with the Swisher Mountain Tuff, underlies 

the entire area but probably does not extend far into 

the subsurface. Basalt, which is tentatively correlated 

with the Big Island Formation, forms a thin veneer 

over the rhyolite in the north (Peterson and Wong, 

1985a). No prospects are located within the study 

area, but a mercury sulfide claim is located 3 mi west 

of its northern boundary. There is also no active 

exploration for ore deposits in and near the study area.

Mineral Resource Potential

Mercury is present in very small quantities in the 

southeast corner of the study area (Connors and 

others, 1982) where one stream-sediment sample 

contained an anomalous value for mercury of 20 parts 

per billion (ppb). The mercury resource potential of 

the rhyolite is low (fig. 2) because geochemical data 

and the absence of altered rocks indicate that a 

hydrothermal system was not active in the area at the 

time of or after rhyolite deposition. Precious and base 

metals have been produced from several mining 

districts surrounding the area, but nothing indicates 

that such deposits exist inside the study area. 

However, rocks underlying the rhyolite may contain 

epithermal precious-metal vein deposits, disseminated 

gold deposits, and uranium anomalies for which there 

is no surficial evidence. Potential for geothermal 

resources is unknown and there is no potential for oil 

and gas.

INTRODUCTION

The North Fork of the Little Humboldt River

Al
1 18°00'

42°00' _ _

1 17°30' 1 17°00'

41°30' -

41°00' -

co OREGON | IDAHO QWYHEE CO _

NORTH FORK OF THE 

LITTLE HUMBOLDT RIVER 

WILDERNESS STUDY AREA 

(NV-020-827)

Paradise Valley

Chimney 

Reservoir

Figure 1. Index map showing location of the North Fork of the Little Humboldt River Wilderness Study Area.

A2
EXPLANATION

Area with low mineral resource potential, 

certainty level B (L/B). See Appendix 1 and 

Figure 3 for definition of levels of mineral 

resource potential and certainty of assessment

COMMODITIES 

Hg Mercury

TYPES OF DEPOSITS AND OCCURRENCES 

[ 1 ] Mercury veins in Tertiary volcanic terrane

CORRELATION OF MAP UNITS

 TERTIARY

DESCRIPTION OF MAP UNITS

Tb BIG ISLAND (?) FORMATION (MIOCENE)-- 

Basalt

Ts SW1SHER MOUNTAIN TUFF (MIOCENE)-- 

Low silica rhyolite

CONTACT

FAULT-Dashed where inferred

APPROXIMATE BOUNDARY OF

NORTH FORK OF THE LITTLE HUMBOLDT RIVER 

WILDERNESS STUDY AREA (NV 020827)

Geology from Pelerson ond Wong (1985c

Figure 2. Map showing mineral resource potential of the North Fork of the Little Humboldt River Wilderness 

Study Area.

A3
Wilderness Study Area (NV-020-827) encompasses 

69,683 acres, of which the U.S. Bureau of Land 

Management requested mineral surveys on 8,900 acres, 

in north-central Nevada along the southwestern 

boundary of the Owyhee Plateau (fig. 1). The surveyed 

part of the area covers 15 mi of the steep, narrow 

canyon of the North Fork of the Little Humboldt 

River. Elevations range from 4,700 to 5,600 ft at the 

bottom of the canyon and from 5,300 to 5,800 ft along 

the canyon rim, with the greatest depth of the canyon 

being about 600 ft. The bottom of the canyon can be 

reached only by hiking down tributary canyons. The 

North Fork of the Little Humboldt River is a perennial 

stream but all of its tributaries are intermittent. The 

arid climate allows the growth of desert brush, 

predominantly sagebrush, along the canyon rim; 

willows, dogwood, several grasses, and brambles grow 

along the river where water is plentiful.

The area lies about 25 mi east of the Santa Rosa 

Range, 20 mi east-northeast of the town of Paradise 

Valley, Nev., and approximately 55 mi northeast of 

Winnemucca, Nev. Maintained gravel roads lead from 

Paradise Valley to Chimney Reservoir (fig. 1). From 

there, the quality of the roads decreases gradually 

until four-wheel drive is required in places and passage 

during wet weather is unadvisable. All parts of the 

area can be reached on foot from jeep trails that 

parallel both sides of the study area.

The U.S. Bureau of Mines personnel searched the 

library and examined Humboldt County and U.S. 

Bureau of Land Management mining and mineral lease 

records to determine if the area had undergone any 

mining or prospecting. U.S. Bureau of Mines, state, 

and other production records were also searched. 

Mining activities that may not have been recorded 

were searched for by ground and air reconnaissance.

The U.S. Geological Survey mapped the geology 

of the study area during a two-week period in the 

summer of 1984 and collected rock samples for 

geochemical analysis (Peterson and Wong, 1985b). 

Reconnaissance mapping for the geologic map of 

Nevada (Stewart and Carlson, 1978) and for a report on 

mineral deposits in Humboldt County (Willden, 1964) 

did not include field work in this area. The current 

study (Peterson and Wong, 1985a) provides the only 

detailed geologic map of the study area (fig. 2).

Personnel at the U.S. Bureau of Land 

Management office in Winnemucca generously 

provided information about road access problems due 

to unexpectedly late winter storms and freely shared 

their information about the area's geology. The 

owners of the Bullhead Ranch kindly allowed us to use 

jeep trails on their property to obtain access to U.S. 

Bureau of Land Management land on the west side of 

the river canyon.

APPRAISAL OF IDENTIFIED RESOURCES

By_ Andrew M. Leszcykowski, U.S. Bureau of Mines

The U.S. Bureau of Mines investigation showed 

that there are no mines or prospects, or related 

activities in the North Fork of the Little Humboldt 

River Wilderness Study Area. The nearest mining 

activity is 20 to 25 mi from the study area in the

Potosi (tungsten and gold), Poverty Peak (mercury), 

and Buckskin (tungsten and gold) mining districts. The 

geology of those mining districts is different from that 

exposed in the study area.

Samples from small accumulations of sand and 

gravel along the river contained no detectable gold or 

other significant heavy minerals.

ASSESSMENT OF POTENTIAL FOR UNDISCOVERED 

RESOURCES

By Jocelyn A. Peterson and William D. Heran, U.S. 

Geological Survey

Geology

The North Fork of the Little Humboldt River 

Wilderness Study Area lies near the boundary between 

the Snake River Plain and Basin and Range tectonic 

provinces. The Owyhee Plateau, which includes the 

study area, forms the southernmost part of the Snake 

River Plain. Tertiary and Quaternary volcanic tuffs 

and flows with minor interbedded fluviatile and 

lacustrine sedimentary rocks characterize the Owyhee 

Plateau; however, older rocks are exposed at many 

localities on the plateau. The volcanic rocks form a 

bimodal rhyolite-basalt suite in which thick rhyolitic 

sequences are covered by a thin basaltic veneer. 

Extensional tectonic features, including block-faulted 

subparallel mountain ranges separated by broad 

alluvial valleys, characterize the Basin and Range 

province. Near the study area, the mountain ranges 

trend north to slightly northeast and are composed of a 

variety of sedimentary, volcanic, and intrusive rocks 

spanning the Phanerozoic Era (Stewart and Carlson, 

1978).

The study area incorporates characteristics of 

both tectonic provinces. The rocks of the area are 

part of the bi modal rhyolite-basalt suite from the 

Owyhee Plateau, whereas structurally the area 

resembles the Basin and Range province and is cut by 

several high-angle faults of small displacement. A 

well-defined northwest-trending lineament in central 

Nevada that coincides with magnetic highs, basaltic 

dike swarms, and faults (Mabey and others, 1978; 

Stewart and others, 1975) transects the study area. 

Although it was previously thought that a conspicuous 

alignment of mining areas in north-central Nevada 

(Roberts, 1960) corresponded to the lineament 

(Stewart and others, 1975), the two are now considered 

to be separate trends (R.J. Madrid, oral commun., 

1985). The eastern edge of a saucer-shaped 

topographic feature that may indicate a caldera (Ekren 

and others, 1984) lies about 7 mi west of the north 

boundary of the study area. Ore deposits in the Santa 

Rosa Range coincide with the western and southern 

edges of this feature (Vikre, 1985).

The Swisher Mountain Tuff, stratigraphically the 

lowest exposed rock unit of the study area, is at least 

700 ft thick. Its base is not exposed but is probably 

close to the surface at the bottom of the canyon based 

on Ekren and others' (1974) projected extent of the 

unit. Flow banding 1 to 5 in. thick is nearly ubiquitous 

and flat lying in only a few places; locally, laminations 

form tight recumbent folds. Both petrographic thin

A4
sections and hand specimens show phenocrysts of 

plagioclase, potassium feldspar, clinopyroxene, and 

opaque oxides occurring in a gray, pink, maroon, or 

lavender groundmass. The groundmass is partially to 

totally devitrified. Major-element geochemical 

analyses indicate that this tuff is a low-silica rhyolite 

(see table 1, Peterson and Wong, 1985a), and the 

absence of hornblende and biotite indicates that the 

magma had a low water content. The Swisher 

Mountain Tuff also contains minor vitrophyre and 

pumiceous tuff. A black porphyritic vitrophyre occurs 

as lenses throughout the rhyolite, most extensively 

near the top of the unit. It has the same phenocryst 

mineralogy as the rhyolite, but the groundmass 

consists of flow-banded perlitic glass containing 

feldspar microlites. In some localities the vitrophyre 

forms angular clasts in an orange tuffaceous matrix. 

This breccia is particularly common along the east rim 

of the canyon and extends to the hills farther east. At 

one location midway down the side of the canyon, an 

orange tuff about 10 ft thick contains abundant pumice 

lapilli and possible glass shards. A thin section of the 

pumiceous tuff indicates that it is non-welded. The 

rhyolite sequence suggests that the unit is an ash-flow 

tuff that melted and destroyed most of the pyroclastic 

textures and developed flow bands. Based on 

mineralogic and chemical affinities, Peterson and 

Wong (1985a) correlated this rhyolite with the Miocene 

Swisher Mountian Tuff (13.85 Ma, Ekren and others, 

1984).

Basalt flows, tentatively correlated with the 

Miocene Big Island Formation of Coats (1985), cover 

the Swisher Mountain Tuff over much of the study area 

(Peterson and Wong, 1985a). The basalt is at most 

about 400 ft thick, but individual flow sheets are only 

15 to 20 ft thick. The unit thins southward until a 

single flow comes to a narrow point along the west rim 

of the canyon (fig. 2). A dense lower part and a 

vesicular top that includes about one-third of the 

thickness of the flow sheet characterizes each flow. 

The flows are flat lying or gently undulating. Most of 

the basalt is dark gray and fine grained, although 

medium-grained phases are present locally. Thin 

sections show that phenocrysts of olivine occur in a 

groundmass of plagioclase, pyroxene, opaque oxides, 

and olivine. A diktytaxitic texture is prevalent in 

fine-grained rocks. Chemically, these rocks are 

tholeiitic high-alumina basalts (Coats, 1985).

Calcite locally fills vesicles in the basalt and 

several areas within the rhyolite contain cherty silica 

as breccia matrix or cavity fillings. Both of these 

minerals were probably deposited from ground waters 

percolating through the area and are not related to 

mineralizing systems. Neither the Swisher Mountain 

Tuff nor the Big Island(?) Formation show evidence of 

alteration in outcrops. Except for narrow weathering 

rinds, the rocks are fresh. Thin sections also do not 

indicate alteration. Phenocrysts are fresh. In the 

Swisher Mountain Tuff, many of the phenocrysts have 

been partially resorbed by the groundmass but have not 

otherwise been affected.

Information about subsurface rocks is absent. 

However, we believe that the subsurface rocks are 

most likely volcanic, possibly outflow-facies rocks 

from the basin structure to the northwest or other 

volcanic sequences related to the Snake River Plain.

These rocks probably lie close to the surface, based on 

the available information for the Swisher Mountain 

Tuff. It is these subsurface rocks that are the 

potential host rocks for most of the deposit types 

discussed in the sections below.

Geochemistry

Twenty-two rock and 32 stream-sediment 

samples were collected in and near the wilderness 

study area to assess the geochemical characteristics of 

the area. The rock samples were crushed, prepared, 

and analyzed by semiquantitative emission 

spectrographic and flame atomic-absorption methods 

at the U.S. Geological Survey in Denver, Colo. 

(Peterson and Wong, 1985b). All but two rhyolite 

samples, which contain visible introduced silica, are 

representative of the basalt and rhyolite from which 

they were collected. Barringer Resources, Inc., 

collected, prepared, and analyzed the 32 streamsediment samples (Connors and others, 1982). The 

samples were analyzed by a variety of methods, 

including induction-coupled argon-plasma emission, 

atomic absorption, colorimetry, fluorimetry, and 

carbon fusion and specific ion electrode to determine 

concentrations of oxides and trace elements. Peterson 

and Wong (1985b) give numerical data for both rock 

and stream-sediment samples.

The geochemical data show little variation 

within each rock type; however, the Swisher Mountain 

Tuff can be easily distinguished from the Big Island(?) 

Formation by the data for several elements. Even the 

two rhyolite samples, containing introduced silica show 

trace-element concentrations similar to the other 

rhyolite samples except that a 12 ppm arsenic (As) 

concentration was detected (by atomic absorption) in 

the siliceous sample from the Red Hills. The other 

rhyolite samples contain less than 5 ppm As (the lower 

determination limit). Because the measured As value 

is not associated with any other geochemically high 

values and the rocks in the Red Hills are not 

hydrothermally altered, it probably indicates a locally 

above-average background value.

The stream-sediment data show a similar lack of 

variation in reported trace-element values. There are 

four isolated above-average values, two for mercury 

(Hg), one for As, and one for fluorine (F). The two Hg 

values (20 and 30 ppb) came from samples collected 

northeast and southeast of the study area. The 20-ppm 

value for As came from a sample collected 3 mi 

southeast of the study area. As with the rock data, 

these values are so small that they represent 

background fluctuations, particularly as they occur 

some distance from one another. A 2,400-ppm value 

for F was obtained from a sample collected 8 mi 

northeast of the study area. This value is three to four 

times higher than the others in the geochemicalsampling area. Again, the isolated nature of this 

anomaly and the lack of other indicators of 

hydrothermal activity probably precludes it from 

association with a mineralizing system. Thus, the 

geochemical data for rock and stream-sediment 

samples indicate that it is unlikely that mineral 

deposits exist in the study area.

A5
Geophysics

The regional geophysical setting in north-central 

Nevada includes several prominent lineaments. The 

northern Nevada rift is a northwest-trending lineament 

crossing north-central Nevada that is characterized by 

a positive aeromagnetic anomaly produced by basaltic 

dike swarms and related volcanic flows (Mabey, 1966; 

Mabey and others, 1978; Zoback and Thompson, 

1978). The lineament also contains a northwesttrending zone of closely spaced faults cut by many 

northeast-trending basin-and-range faults.

The broad, northeast-trending Humboldt 

structural zone extends across Nevada to Yellowstone, 

Wyo. (Rowan and Wetlaufer, 1981). Many northeasttrending, left-lateral, strike-slip faults define this 

zone. The northeast-trending Midas lineament system 

lies within the Humboldt structural zone. Landsat 

images of the Midas lineament show prominent 

northeast-trending structure and topography. The 

southeastern margin of the Humboldt structural zone 

lies along a line of high gravity anomalies; the Battle 

Mountain heat-flow anomaly also corresponds to this 

northeast-trending zone. The Humboldt structural 

zone and the Midas lineament both cross the northern 

Nevada rift system.

Gravimetric data are not available for the study 

area, but we have drawn general conclusions about the 

mineral resources from the regional airborne magnetic 

data that cover parts of the Vya and McDermitt 1° by 

2° quadrangles in northern Nevada (U.S. Geological 

Survey, 1972). The aeromagnetic survey was flown at 

3,200 to 4,300 ft above the ground surface (about 9,000 

ft above sea level), and flight lines were spaced 1 mi 

apart. We examined the data covering latitudes 41°30' 

to 42° and longitudes 117° to 117°15', which includes 

the North Fork of the Little Humboldt River 

Wilderness Study Area. The contour interval is 20 

gammas and values range from 2,400 to 2,800 gammas 

with a regional gradient of 9 gammas/mi removed in 

the northeast direction.

The general trend of the magnetic anomalies in 

this area corresponds to the northwest trend of the 

northern Nevada rift magnetic anomaly. Depths to the 

source of several anomalies were estimated at 3,000 

ft, using the slope-distance rule. Thus, the data 

reflect near-surface variations in the rock types, 

because the height of the aircraft is about 3,000 ft 

above the ground.

The northwest part of the area studied shows 

higher magnetic values than the southeast part, 

reflecting either the topographic trend or a change in 

magnetization between the Big Island(?) Formation and 

Swisher Mountain Tuff. A steep magnetic gradient in 

the southern part of the area studied has traditionally 

been correlated with deep-rooted feeder dikes of the 

northern Nevada rift (Robinson, 1970; Zoback and 

Thompson, 1978).

Mineral and Energy Resources

The North Fork of the Little Humboldt River 

Wilderness Study Area is situated between a number of 

mining districts that contain base and precious metals 

in epithermal-vein deposits, mercury deposits, uranium

occurrences, and disseminated-gold deposits. The area 

also lies at the north end of two separate linear trends 

of ore deposits. One trend includes the Getchell, 

Pinson, and Preble disseminated-gold deposits in the 

Osgood Mountains south of the study area and a newly 

discovered deposit near Chimney Reservoir (W.C. 

Bagby, oral commun, 1985). The other trend includes 

the Midas, Ivanhoe, Bootstrap, and Lynn districts that 

contain epithermal-and disseminated-gold deposits 

southeast of the study area. Some of the districts lie 

outside the Owyhee Plateau tectonic province and, 

because the mineralized rocks in these districts are 

older than the Swisher Mountain Tuff, it is possible 

that the rocks were mineralized prior to the eruption 

of the Swisher Mountain Tuff. Nevertheless, the 

characteristics of several nearby deposits are 

examined here because there is a slight chance that 

similarly mineralized rocks may occur within the study 

area, particularly in subsurface rocks. Deposit types 

for which there is no surficial evidence, but which 

could possibly occur in subsurface rock units, are 

described and evaluated, but not classified as to their 

potential, because insufficient information exists 

about the subsurface rocks to give a more definite 

assessment of the potential for these types of 

deposits. This assessment was made as of May 1985 

and follows the definitions of mineral resource 

assessment and the levels of certainty outlined in 

Appendix 1.

Epithermal Precious-metal Vein Deposits

In several districts near the study area, Tertiary 

volcanic rocks contain gold- and silver-bearing quartz 

veins. These include the National district in the Santa 

Rosa Range (Roberts, 1940), the Cornucopia, Midas, 

and Jarbidge districts in Elko County, Nev., east and 

southeast of the study area (Smith, 1976), and the 

Silver City and DeLamar districts in Idaho (Asher, 

1968; Pansze, 1975; Halsor, 1983). In these districts, 

quartz veins host native gold and a wide variety of 

sulfide and sulfosalt minerals, particularly those 

containing silver. All these districts contain rocks 

that have been hydro therm ally altered to form 

bleached outcrops characterized by silicic alteration. 

In some of the districts, alteration products such as 

adularia, sericite, clay minerals, and chlorite have also 

formed. Four altered zones have been identified in the 

Silver City district; from the vein outward they are 

potassium silicate, quartz-sericite, argillic, and 

propylitic zones (Pansze, 1975). The veins in the 

DeLamar deposit formed in ring fractures suggestive 

of a caldera environment in which insufficient lavas 

were extruded to form collapse structures (Asher, 

1968). The mineral deposits in the Midas district have 

been dated at 15 Ma (Smith, 1976), those at Silver City 

at 15.6 Ma (Pansze, 1975), and those at DeLamar at 15 

Ma ago (Halsor, 1983).

The exposed rocks in the study area are not 

likely to contain base- and precious-metal epithermal 

veins similar to those outside the area for a number of 

reasons. First, the hydrothermally altered rocks that 

accompany the known epithermal veins are absent. 

Second, the Swisher Mountain Tuff and Big Island(?) 

Formation may be younger than the mineralization in

A6
the surrounding districts. Third, geochemical data do 

not indicate precious- and base-metal veins in the 

area. There is a slight possibility, however, that 

epithermal-vein deposits exist beneath the rhyolite. 

The thickness of the Swisher Mountain Tuff is 

unknown, but because the study area lies near its 

projected southern margin (Ekren and others, 1984), it 

is likely that the unit is not much thicker than the 

exposed part (about 700 ft). Subsidence within the 

basin northwest of the study area may have resulted 

from magma withdrawal that, as in the DeLamar area, 

was insufficient to produce an identifiable collapse 

structure. If this interpretation is correct, 

epithermal-vein deposits similar to those in Idaho or 

the National district may occur in volcanic rocks 

beneath the Swisher Mountain Tuff; however, the 

topographic expression of the basin suggests that the 

study area lies outside this structure, where there are 

less likely to be caldera-related vein deposits. 

Therefore, the exposed rocks have no potential for 

epithermal-vein deposits and there is not enough data 

on subsurface rocks to determine whether or not they 

contain epithermal-vein deposits, but the environment 

may be permissive.

economic significance, but if any cinnabar veins occur 

in the study area, they would probably closely 

resemble those at Silver City. There, the cinnabar 

veins occur in the massive, cherty Silver City Rhyolite 

of Asher (1968) as thin coatings with associated pyrite 

along the faces of joint planes.

Small cinnabar veins similar to those at 

McDermitt or Silver City may exist in the study area. 

Any cinnabar localities in the Swisher Mountain Tuff 

are likely to be spatially limited because the lack of 

silicified or argillized rocks indicates that there 

probably was not a significant hydrothermal system 

active in the area during or after deposition of the 

Swisher Mountain Tuff. Stream-sediment geochemical 

data do not show an enrichment of mercury in the 

unaltered rhyolite. Mercury deposits, like epithermalvein deposits, could exist in units directly underlying 

the Swisher Mountain Tuff. Again, however, because 

the study area lies outside the basin to the northwest, 

caldera-related mercury deposits are less likely to 

occur. The Swisher Mountain Tuff has a low potential 

for mercury deposits with a B certainty level and 

underlying rocks could be permissive for mercury 

deposits, but not enough information is known.

Mercury Deposits

Mercury deposits occur in the National district 

(Roberts, 1940), in the McDermitt caldera complex 

about 30 mi northwest of the study area (Rytuba and 

Glanzman, 1979), in the Ivanhoe district in Elko 

County (Smith, 1976), all in Nevada, and in the Silver 

City region of Idaho (Asher, 1968). The mercury 

sulfide cinnabar occurs in sinter deposits (National 

district), in association with epithermal veins (National 

district), in opalized veins in volcanic rocks (Ivanhoe 

and McDermitt districts), in lake-bed sediments 

(McDermitt district), and as coatings along fractures 

and joints in the host rocks (Silver City district). 

McDermitt is the only one of these districts currently 

producing and it is the best documented. In this 

district, cinnabar occurs in volcanic rocks and lake-bed 

sediments. In the volcanic rocks, cinnabar is 

associated with ring fractures related to caldera 

formation and the rocks have been silicified (primarily 

in rhyolite) and argillized (primarily in basalt). The 

unaltered rhyolites contain high concentrations of the 

elements that form the deposits and, therefore, may 

be genetically related to the mineralization. They 

contain 0.25 to 0.65 ppm Hg (Rytuba and Glanzman, 

1979) as far away as several kilometers from the 

deposits. Around the lake-bed deposits-sediments have 

been altered to a variety of zeolite and clay facies, 

which Glanzman and Rytuba (1979) attributed to 

hydrothermal alteration, while Wallace and Roper 

(1981) believed the minerals are of diagenetic origin. 

Pyrite accompanies the cinnabar (Wallace and Roper, 

1981), and uranium and lithium minerals are also 

present locally (see below). The youngest rocks in the 

area are about 13.5 m.y. old (Rytuba and Glanzman, 

1979) and the mercury mineralization probably 

occurred during the Miocene.

Cinnabar veins in the Silver City area have little

Uranium deposits

Uranium occurrences at McDermitt are closely 

associated with the ring-fracture system of the 

calderas. Uranium minerals also occur along flowboundary zones in volcanic rocks and in layers within 

lake-bed sediments (Wallace and Roper, 1981). Of 

these, only uranium concentrations associated with 

flow features of volcanic rocks could possibly occur in 

the study area. Uraninite and coffinite, the uranium 

minerals at McDermitt, are localized in the upper 

parts of altered vesicular flows and in breccia layers 

within a complex intermediate-composition volcanic 

sequence (see Roper and Wallace, 1981). Some ore is 

also associated with steep fractures. Alteration 

products, such as clay minerals, zeolites, and pyrite, 

occur in rocks adjacent to the uranium minerals. 

Hypogene and supergene processes helped concentrate 

the uranium. Roper and Wallace (1981) believed the 

uranium minerals were derived from a hydrothermal 

source genetically related to the lava that formed the 

host volcanics, where background values in unaltered 

rock are about 20 ppm uranium (U) (Rytuba and 

Glanzman, 1979).

In the study area, uranium-bearing rocks 

comparable to those at McDermitt, if they exist, are 

covered by the Swisher Mountain Tuff and Big Island(?) 

Formation. The exposed rocks in the study area show 

no evidence of argillic or zeolitic alteration, and 

uranium minerals are absent. Although streamsediment and rock geochemical data cannot be directly 

correlated, the stream-sediment values in the study 

area range from 0.2 to 3.6 ppm U (Peterson and Wong, 

1985b), which suggests that the rocks in the area are 

probably not enriched in uranium. The study area has 

no potential for uranium in exposed rock units, but 

subsurface rocks may provide a permissive 

environment for suoh deposits.

A7
Disseminated-Gold Deposits

Several disseminated-gold deposits, including the 

Getchell mine, formed in Paleozoic strata in the 

Osgood Mountains. The Getchell mine and similar gold 

mines elsewhere in Nevada are characterized by high 

gold to silver ratios, Hg and As geochemical 

anomalies, carbonaceous limy host rock, an associated 

igneous intrusion, and extensive silicic alteration 

(Berger, 1980). The gold occurs as micron-size 

disseminated grains in irregular pods of widely varying 

grade (averaging 9 to 10 ppm). Although the 

mineralization may have been associated with the 

igneous intrusion, recent zircon-dating studies suggest 

that the deposits may be as young as Miocene (Berger, 

1980). Many of the features of these deposits indicate 

that they formed in a hot-spring environment. 

Volcanic-hosted disseminated-gold deposits, including 

some of the mineralization at DeLamar, Idaho, are 

also of hot-spring origin (Berger, 1983). These deposits 

are highly silicified and are associated with fracture 

systems.

Although we do not know what kinds of rocks 

underlie the Swisher Mountain Tuff, Paleozoic 

sedimentary rocks, such as those at the Getchell mine 

and other disseminated-gold deposits, are probably 

present, but may be deeply buried. Volcanic rocks, 

possibly outflow-facies rocks from the basin structure 

to the northwest or other volcanic sequences related 

to Snake River volcanism, may form thick sequences 

beneath the Swisher Mountain Tuff. Therefore, were 

disseminated-gold deposits to occur in the study area, 

they would more likely be hosted by volcanic rocks, 

but could also be present in more deeply buried 

sedimentary strata. The Swisher Mountain Tuff and 

the Big Island(?) Formation are not silicified and are 

probably barren. Thus, they have no potential for 

disseminated gold resources. The unexposed strata 

underlying the Swisher Mountain Tuff, however, could 

be favorable for disseminated gold deposits, but we 

have no factual data about these rocks.

Other Commodities

Almost 500 acres of oil and gas leases are 

located adjacent to the study area (U.S. Bureau of 

Land Management, 1983). Most of the known 

petroliferous strata, however, are found south of a line 

between Reno and Elko, Nev., and no hydrocarbons 

have been found in areas covered by rocks of the Snake 

River Plain (Bortz, 1983). Because the area lies in the 

Battle Mountain high heat-flow region (Hyde, 1979), 

hydrocarbon accumulations are unlikely. A recent 

assessment of petroleum potential of wilderness lands 

indicates that the North Fork of the Little Humboldt 

River Wilderness Study Area has no potential for 

petroleum resources (Sandberg, 1983). We also classify 

the area as having no petroleum potential.

Warm springs occur about 15 mi west of the 

study area and have been reported in Paradise Valley 

along the Little Humboldt River southwest of the 

study area (Waring, 1965). The study area is located in 

the northeast part of the Battle IWountain high heat 

flow region but no designated geothermal areas are 

located near the study area (Hyde, 1979). No

geothermal leases are located in or adjacent to the 

area. The geothermal resource potential of the area is 

unknown.

Zeolite minerals occur in Tertiary lacustrine and 

fluviatile tuffs southwest of the study area (Sheppard 

and Gude, 1983). Because these tuffs do not crop out 

in the study area and because zeolite minerals were 

not seen in vugs in either the Swisher Mountain Tuff or 

Big Island(?) Formation, zeolite minerals probably do 

not occur. The area has no potential for zeolite 

resources.

REFERENCES CITED

Asher, R.R., 1968, Geology and mineral resources of a 

portion of the Silver City region, Owyhee County, 

Idaho: Idaho Bureau of Mines and Geology 

Pamphlet 138, 106 p.

Berger, B.R., 1980, Geologic and geochemical 

relationships at the Getchell Mine and vicinity, 

Humboldt County, Nevada: Society of Economic 

Geologists Fieldtrip Guidebook, Field Conference 

to Northern Nevada, p. 111-135.

__1983, Hot spring gold, silver, model 5.6, in Cox, 

D.P., ed., U.S. Geological Survey - INGEOMINAS 

mineral resource assessment of Colombia: Ore 

deposit models: U.S. Geological Survey Open-File 

Report 83-423, p. 38.

Bortz, L.C., 1983, Hydrocarbons in the northern Basin 

and Range, Nevada and Utah: Geothermal 

Resources Council Special Report No. 13, p. 179- 

197.

Coats, R.R., 1985, The Big Island Formation, a 

Miocene formation in northern Elko County, 

Nevada, and adjacent Idaho, including a 

consideration of its composition and petrographic 

character: U.S. Geological Survey Bulletin 1605- 

A, in press.

Connors, R.A., Robinson, M.L., Bukofski, J.F., Meyer, 

W.T., and Howarth, R.J., 1982, Geochemical and 

geostatistical evaluation, wilderness study area, 

Winnemucca District, northwest Nevada: Golden, 

Colo., Barringer Resources, Inc., 5 volumes.

Ekren, E.B., Mclntyre, D.H., and Bennett, E.H., 1984, 

High-temperature, large-volume lavalike ash-flow 

tuffs without calderas in southwestern Idaho: U.S. 

Geological Survey Professional Paper 1272, 76 p.

Glanzman, R.K., and Rytuba, J.J., 1979, Zeolite-clay 

mineral zonation of volcaniclastic sediments 

within the McDermitt caldera complex of Nevada 

and Oregon: U.S. Geological Survey Open-File 

Report 79-1668, 25 p.

Halsor, Sid, 1983, A volcanic dome complex and 

genetically associated hydrothermal system, 

DeLamar silver mine, Owyhee County, Idaho: 

Houghton, Mich., Michigan Technological 

University, M.S. thesis, 111 p.

Hyde, Joy, 1979, An overview of geothermal 

exploration and development in Nevada, in 

Newman, G.W., and Goode, H.D., eds., Basin and 

Range Symposium: Rocky Mountain Association 

of Geologists and Utah Geological Association, 

p. 381-386.

Mabey, D.R., 1966, Regional gravity and magnetic 

anomalies in part of Eureka County, Nevada:

A8
Society of Exploration Geophysicists 1 Mining 

Geophysics, Case Histories, v. 1, p. 77-83.

Mabey, D.R., Zietz, Isidore, Eaton, G.P., and 

Kleinkopf, M.D., 1978, Regional magnetic 

patterns in part of the Cordillera in the Western 

United States, ill Smith, R.B., and Eaton, G.P., 

eds., Cenozoic tectonics and regional geophysics 

of the western Cordillera: Geological Society of 

America Memoir 152, p. 93-106.

Pansze, A.J., Jr., 1975, Geology and ore deposits of 

the Silver City-DeLamar-Flint region, Owyhee 

County, Idaho: Idaho Bureau of Mines and 

Geology Pamphlet 161, 79 p.

Peterson, J.A., and Wong, George, 1985a, Geologic 

map of the North Fork of the Little Humboldt 

River Wilderness Study Area, Humboldt County, 

Nevada: U.S. Geological Survey Miscellaneous 

Field Studies Map MF-1815, scale 1:24,000 in 

press .

__1985b, Geochemical analyses of rock and streamsediment samples from the North Fork of the 

Little Humboldt River Wilderness Study Area, 

Humboldt County, Nevada: U.S. Geological 

Survey Open-File Report 85-373, 10 p.

Roberts, R.J., 1940, Quicksilver deposit at Buckskin 

Peak, National mining district, Humboldt County, 

Nevada: U.S. Geological Survey Bulletin 922-E, 

p. 115-133.

__I960, Alinement of mining districts in northcentral Nevada: U.S. Geological Survey 

Professional Paper 400-B, p. B17-B19.

Robinson, S.E., 1970, Relations between geological 

structure and aeromagnetic anomalies in central 

Nevada: Geological Society of America Bulletin, 

v. 81, p. 2045-2060.

Roper, M.W., and Wallace, A.B., 1981, Geology of the 

Aurora uranium prospect, Malheur County, 

Oregon, in Goodell, P.C., and Waters, A.C., eds., 

Uranium in volcanic and volcaniclastic rocks: 

American Association of Petroleum Geologists, 

Studies in Geology, no. 13, p. 81-88.

Rowan, L.C., and Wetlaufer, P.H., 1981, Relation 

between regional lineament systems and 

structural zones in Nevada: American Association 

of Petroleum Geologists Bulletin, v. 65, 

p. 1414-1429.

Rytuba, J.J., and Glanzman, R.K., 1979, Relation of 

mercury, uranium, and lithium deposits to the 

McDermitt caldera complex, Nevada-Oregon, in

Ridge, J.D., ed., Papers on mineral deposits of

western North America: Nevada Bureau of Mines

and Geology Report 33, p. 109-117. 

Sandberg, C.A., 1983, Petroleum potential of

wilderness lands, Nevada: U.S. Geological Survey

Miscellaneous Investigations Series Map 1-1542,

scale 1:1,000,000. 

Sheppard, R.A., and Gude, A.J., 3d, 1983, Zeolites in

Tertiary tuffs along the Little Humboldt River,

Humboldt and Elko Counties, Nevada: U.S.

Geological Survey Open-File Report 83-458, 10 p. 

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

76-56, 194 p. 

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of Nevada: U.S. Geological Survey, scale

1:500,000. 

Stewart, J.H., Walker, G.W., and Kleinhampl, F.J.,

1975, Oregon-Nevada lineament: Geology, v. 3,

no. 5, p. 265-268. 

U.S. Bureau of Land Management, 1983, Winnemucca

Wilderness Technical Report: For 18 wilderness

study areas in northwest Nevada: U.S. Bureau of

Land Management Winnemucca, Nev. , 408 p. 

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the Vya and McDermitt 1° by 2 quadrangles,

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

72-393, scale 1:250,000. 

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P.C., and Waters, A.C., eds., Uranium in volcanic

and volcaniclastic rocks: American Association of

Petroleum Geologists, Studies in Geology, no. 13,

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Geology, v. 6, p. 111-116.

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

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 the geologic 

environment and the level of mineral resource 

potential, and indicate the activity of resourceforming 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

HIGH 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

LEVEL OF CERTAINTY

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

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