Mineral Resources of the North Fork 

Owyhee River Wilderness Study 

Area, Owyhee County, Idaho

U.S. GEOLOGICAL SURVEY BULLETIN 1719-A

Chapter A

Mineral Resources of the North Fork 

Owyhee River Wilderness Study Area, 

Owyhee County, Idaho

By SCOTT A. MINOR and DONNIE SAWATZKY 

U.S. Geological Survey

ANDREW M. LESZCYKOWSKI 

U.S. Bureau of Mines

U.S. GEOLOGICAL SURVEY BULLETIN 1719-A

MINERAL RESOURCES OF WILDERNESS STUDY AREAS: 

OWYHEE RIVER REGION, IDAHO AND 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

Minor, Scott A.

Mineral resources of the North Fork Owyhee River

Wilderness Study Area, Owyhee County, Idaho.

Geological Survey Bulletin 1719-A

Bibliography: p. 10

Supt. of Docs. No.: I 19.31719-A

1. Mines and mineral resources Idaho North Fork

Owyhee River Wilderness. 2. Geology Idaho North Fork

Owyhee River Wilderness. 3. North Fork Owyhee River

Wilderness (Idaho) I. Sawatzky, Donnie. II. Leszcykowski,

Andrew M. III. Title. IV. Series.

QE75.B9 No. 1719-A 557.3s 86-600197

ITN24.I2] [553'.09796'211
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 

Owyhee River Wilderness Study Area (ID-16-40) within the southern Owyhee Mountains in 

southwest Idaho.

CONTENTS

Summary Al

Abstract 1

Character and setting 1

Identified resources 1

Mineral resource potential 1 

Introduction 2

Character and setting 2

Previous and present investigations 2 

Appraisal of identified resources 2

History and production 2

Mineral deposits 2 

Assessment of mineral resource potential 5

Geology 5

Geochemical studies 6

Geophysical studies 6

Mineral and energy resources 6 

Base and precious metals 6 

Uranium 7 

Geothermal energy 7 

Petroleum and natural gas 7 

References 7 

Appendix 1 - Definition of levels of mineral resource potential and certainty of assessment

FIGURES

1. Index map showing location of the North Fork Owyhee River Wilderness Study 

Area, Owyhee County, Idaho 3

2. Map showing mineral resource potential of the North Fork Owyhee River 

Wilderness Study Area, Owyhee County, Idaho 4

3. Major elements of mineral resource potential/certainty classification 9

MINERAL RESOURCES OF WILDERNESS STUDY AREAS: OWYHEE RIVER REGION, IDAHO AND NEVADA

Mineral Resources of the North Fork Owyhee River 

Wilderness Study Area, Owyhee County, Idaho

By Scott A. Minor and Donnie Sawatzky 

U.S. Geological Survey

Andrew M. Leszcykowski 

U.S. Bureau of Mines

SUMMARY

Abstract

The North Fork Owyhee River Wilderness Study 

Area (ID-016-040), within the southern Owyhee 

Mountains in southwest Idaho, encompasses 49,470 

acres, of which the U.S. Geological Survey and the 

U.S. Bureau of Mines were asked to study 39,745 

acres. Hereafter, the terms "study area" and 

"wilderness study area" refer only to the smaller 

acreage. Field work for this report was carried out 

between 1984 and 1985. No mines or prospects are 

located in the study area, although a few borrow pits 

exist just outside its southern boundary. The only 

identified resource is blocky basalt suitable for use as 

building or decorative stone. The study area has a low 

resource potential for gold, silver, zinc, and uranium. 

The potential for petroleum and natural gas and 

geothermal energy is unknown.

Character and Setting

The North Fork Owyhee River Wilderness Study 

Area is located in the southern Owyhee Mountains in 

southwest Idaho, about 75 mi southwest of Boise (fig. 

1). The area is situated along a stream-dissected 

plateau with canyons as deep as 400 ft. The study area 

is underlain by a thick pile of Miocene (about 5 to 24 

million years before present) volcanic flows and minor 

interbedded sedimentary rocks. A thick rhyolite ashflow tuff sheet is exposed throughout most of the 

study area; the tuff is overlain in the southern part of 

the study area by another rhyolite ash-flow tuff and a 

younger sequence of thin basalt flows and interbedded 

sedimentary rocks. The older ash-flow tuff fills a 

collapse caldera formed due to the emplacement of 

the tuff. Several small-displacement normal faults of 

various trends cut the rocks. The rhyolite ash-flow 

tuffs are also intensely fractured locally.

There are no known mines, prospects, claims, or 

mineral leases in the study area or its immediate 

vicinity. Several small borrow pits for road 

construction material exist along the southern 

boundary (fig. 2).

Identified Resources

The only identified resource in the study area is 

blocky basalt suitable for use as building or decorative 

stone. The thin, blocky slabs of basalt occur over an 

area of several square miles in the southern part of the 

study area.

Mineral Resource Potential

Gold, silver, zinc, and uranium have minor 

resource potential in the wilderness study area (fig. 

2). The potential for geothermal energy and petroleum 

and natural gas is unknown.

Two mining districts occur in the region near the 

wilderness study area: the South Mountain (silver-leadzinc) and the Silver City (gold-silver) districts. 

Mineralization in the former takes the form of silverlead replacement veins and silver-lead-zinc skarn 

within regionally metamorphosed rocks. The Silver 

City district, in particular the Delamar mine area, is 

characterized by epithermal mineralization along 

Miocene normal faults and fractures within a 

contemporaneous, inferred silicic ring complex. 

Anomalous concentrations of antimony, arsenic, 

copper, mercury, selenium, and zinc are associated 

with the gold-silver deposits.

Relatively high concentrations of arsenic, 

molybdenum, silver, and zinc have been detected in 

samples collected in the wilderness study area. A 

similar geologic environment (caldera-related silicic 

volcanic rocks and contemporaneous faults and 

fractures) and trace-element anomalies similar to 

those in the Silver City district suggest that

Al
epithermal mineralization has occurred beneath the 

study area. No mineral deposits or evidence of 

secondary hydrothermal alteration have been found in 

the area. Thus, the study area is judged to have a low 

resource potential for gold, silver, and zinc (fig. 2).

Weakly anomalous uranium concentrations occur 

in a few places in the southern part of the wilderness 

study area. The rhyolite ash-flow tuffs that underlie 

most of the study area have relatively high background 

levels of uranium and are probably the source of the 

anomalies. Therefore, the study area is considered to 

have a low resource potential for uranium (fig. 2).

No evidence of geothermal activity exists in the 

wilderness study area, and pertinent subsurface data is 

lacking to quantify the resource potential. The study 

area is considered to have an unknown potential for 

geothermal energy.

No evidence of hydrocarbons has been found in 

the study area, and pertinent subsurface data is 

lacking to quantify resource potential. The study area 

has an unknown potential for petroleum and natural 

gas.

INTRODUCTION

Character and Setting

The North Fork Owyhee River Wilderness Study 

Area (ID-016-040), located about 30 mi southeast of 

Jordan Valley, Oregon, and 75 mi southwest of Boise, 

encompasses 49,470 acres within the Owyhee 

Mountains of southwest Idaho (fig. 1). Elevations 

range from about 4,800 ft, where the North Fork of 

the Owyhee River crosses the western boundary of the 

study area, to about 6,800 ft along the northeastern 

boundary. South Mountain (7,850 ft), located 5 mi 

northwest of the study area, is the only prominent 

peak in the immediate vicinity. The rugged, 

extensively stream-dissected northern part of the 

study area contrasts greatly with the locally incised 

tablelands of the southern part. With a depth as great 

as 400 ft, the gorge of the North Fork of the Owyhee 

River is the deepest canyon in the study area. All of 

the other streams in the study area are intermittent, 

reflecting the semi-arid climate of the region. Juniper 

trees, low-lying sage and other brush, and seasonal 

varieties of grasses and wild flowers grow throughout 

most of the area. The larger stream channels support 

a variety of riparian vegetation.

An improved county-maintained gravel road that 

extends from Jordan Valley, Oregon, to Grand View, 

Idaho provides the best access to the study area. 

Several jeep trails branching off this road extend into 

the study area (fig. 1). The rugged northeastern part 

of the study area is accessible only by foot, horse, or 

helicopter.

Previous and Present Investigations

The first published geologic map of the study 

area region was a reconnaissance map (l:62,500^scale) 

accompanying the report by Bennett (1976) on the 

geology and geochemistry of the South MountainJuniper Mountain area. The only other geologic map 

covering the area is that of Owyhee County by Ekren 

and others (1981; 1:125,000-scale).

The U.S. Geological Survey conducted field 

studies in the study area during 1984 and 1985. These 

studies included geologic mapping and collection of 

rock samples for petrographic studies. In addition, a 

geologic reconnaissance of the region surrounding the 

study area was conducted as an aid to formulating oredeposit models. Linear features that appeared in 

Landsat multispectral-^scanner images were mapped by 

photogeologic interpretation for the region. 

Geochemical data used in making mineral-resource 

evaluations in this report include stream ^sediment 

data obtained from both Bennett (1976) and a U.S. 

Department of Energy National Uranium Resource 

Evaluation (NURE) report (Oak Ridge Gaseous 

Diffusion Plant, 1982).

U.S. Bureau of Mines personnel conducted a 

library search of Owyhee County, state, and U.S. 

Bureau of Land Management mining-production and 

mineral-lease records for information on mines, 

prospects, and claims within the study area. A ground 

and air reconnaissance was undertaken to check for 

undocumented mining activity. The Bureau of Mines 

conducted field studies during 1984. Rock and streamsediment samples were collected to help evaluate the 

mineral resources of the study area. Complete 

analytical data are on file at the U.S. Bureau of Mines, 

Western Field Operations Center, Spokane, 

Washington, 99202.

APPRAISAL OF IDENTIFIED RESOURCES

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

History and Production

There are no known mines, prospects, claims, or 

mineral leases in the wilderness study area. There are, 

however, two mining districts nearby. The South 

Mountain district is located 7 mi northwest of the 

study area near South Mountain (fig. 1). Silver-leadzinc ore was produced there intermittently from the 

early 1870's (Sorenson, 1927) to 1945. The ore occurs 

as replacement-vein and skarn deposits in limestone 

(Beaver, 1984). The Silver City mining district, 

located 24 mi north of the study area (fig. 1), has been 

active since 1863, most recently at the Delamar openpit mine, a large-tonnage, low-grade, gold and silver 

mine. More than $40 million of gold and silver 

(derived from epithermal veins) have been produced in 

this district.

Four small borrow pits are located along the 

county road that skirts the southern part of the study 

area (fig. 2). Material for road metal was extracted 

from the pits for use on nearby roads.

Mineral Deposits

No mineral deposits have been identified within 

the wilderness study area. The only identified 

resource is tabular, blocky basalt suitable for use as

A2
42°45'

42°30'

117°OCr ~"I

116°45'

Triangle 

Reservoir

NORTH FORK OWYHEE RIVER 

WILDERNESS STUDY AREA 

(ID-016-040) ,

IDAHO 

AREA OF AAAP

Figure 1. Index map showing location of the North Fork Owyhee River Wilderness Study Area, 

Owyhee County, Idaho.

A3
116°52'30" 

I

116°45'

APPROXIMATE BOUNDARY

OF

NORTH FORK OWYHEE RIVER WILDERNESS 

STUDY AREA (ID016 040)

Tjs

42° 

37' 

30"

f C^PLEASANT VALLEY TABLE

3 MILES

EXPLANATION

Area with low mineral resource potential. 

See Appendix 1 and Figure 3 for 

definition of levels of mineral resource 

potential and certainty of assessment.

CORRELATION OF MAP UNITS

Miocene

COMMODITIES 

Au Gold 

Ag Silver 

U Uranium 

Zn Zinc

Geology modified from Ekren and others (1981)

GEOLOGIC MAP UNITS

Tb Banbury Basalt (Miocene)

Ts Sedimentary rocks (Miocene)-

interbedded with Banbury Basalt

Rocks of Juniper Mountain volcanic center 

(Miocene)--In this area, consists of:

Tjl Lower lobes of Juniper Mountain 

Tjs Swisher Mountain Tuff

Tob Older basalt flows (Miocene)

MAP SYMBOLS

 - Contact-Dashed where approximately 

located

 * Fault-Bar and ball on downthrown side 

X Borrow pit

Figure 2. Map showing mineral resource potential of the North Fork Owyhee River Wilderness Study Area, 

Owyhee County, Idaho.

A4
building or decorative stone. It crops out over an area 

of several square miles on Nickel Creek and Pleasant 

Valley tables in the southern part of the study area 

(fig. 2). Although the basalt is in sufficient quantity to 

supply any local demand, transportation costs to get 

the stone to a major market area would be 

considerable.

ASSESSMENT OF MINERAL RESOURCE POTENTIAL

By Scott A. Minor and Donnie Sawatzky 

U.S. Geological Survey

Geology

The wilderness study area lies within the Owyhee 

volcanic field (Panze, 1975), which consists of a thick 

pile of complexly interfingering and overlapping 

Tertiary volcanic, pyroclastic, and subordinate 

sedimentary rocks. Basement rocks, consisting of 

Mesozoic intrusive and metamorphic rocks, crop out 

within the mountainous parts of the region, including 

South Mountain 5 mi northwest of the study area. The 

rocks are locally displaced along primarily northnorthwest-trending normal faults.

The study area is underlain by flat-lying, middle 

and late Miocene rhyolitic ash-flow tuffs, basalt flows, 

and minor intercalated sedimentary rocks (fig. 2). The 

oldest rock unit is the Swisher Mountain Tuff, the 

lowermost subunit of the Juniper Mountain volcanic 

center (Ekren and others, 1981; Ekren and others, 

1984). The Swisher Mountain Tuff is a rhyolitic ashflow tuff made up of several cooling units, including an 

intracaldera phase. The tuff underlies the northern 

two-thirds of the study area. Older basalts that 

disconformably underlie the Swisher Mountain Tuff are 

exposed less than 1 mi west of the study area. The 

Swisher Mountain Tuff has a minimum thickness of 

about 500 ft within the study area; its base is not 

exposed. Its rhyolitic composition is expressed, in 

part, by phenocrysts of plagioclase, potassium 

feldspar, pyroxene, and opaque iron oxides. The 

groundmass consists of gray, devitrified glass that 

weathers reddish to orangish brown. Flow foliations 

are ubiquitous and usually folded, with amplitudes as 

great as 300 ft. The tuff is characterized by outcrops 

that form jagged pinnacles and precipitous cliffs along 

major drainages. The unit is interpreted by Ekren and 

others (1984) and the senior author of this report to be 

an ash-flow tuff sheet composed of several cooling 

units. The unit's large areal extent, locally preserved 

vitroclastic texture, and the sporadic occurrence of 

black vitrophyre layers within the unit all support this 

interpretation. A geologic reconnaissance of the 

region indicates the existence of a collapse caldera, 

the western part of which includes the study area. The 

northern part of the caldera is well delineated by a 

large semicircular area devoid of rocks older than the 

Swisher Mountain Tuff. Much of this area is underlain 

by an extensive domal mass of the Swisher Mountain 

Tuff that may be a younger intracaldera phase of the 

unit; to the north, it laps onto an earlier outflow facies 

of the Swisher Mountain Tuff that is tilted and

underlain by older rocks. An inferred caldera-moatfilling sedimentary sequence laps onto the 

intracaldera-tuff phase along the northern margin of 

the caldera, north of Big Boulder and Rock Creeks 

(fig. 1). South of the study area, the caldera is buried 

by younger volcanic units. The eruption of the Swisher 

Mountain Tuff probably resulted in the formation of 

the caldera and a late phase of the tuff was ponded 

within the resulting basin, thus concealing much of the 

caldera structure; rocks in the western part of the 

study area likely conceal a western segment of the 

caldera's ring fracture.

The lower lobes of the Juniper Mountain subunit 

of the rocks of the Juniper Mountain volcanic center 

unit (Ekren and others, 1981), a rhyolitic ash-flow tuff, 

lap onto the Swisher Mountain Tuff along the southern 

boundary of the study area. The lower lobes of Juniper 

Mountain are very similar to the Swisher Mountain 

Tuff to the north in both lithology and field expression, 

the main distinction of the former being the 

occurrence of a large amount of quartz phenocrysts. 

Ekren and others (1984) identify this unit as an ashflow tuff sheet on the basis of similar evidence. Its 

name is derived from Juniper Mountain, 7 mi south of 

the study area (fig. 1), a broad dome-shaped landform 

thought to be a vent complex from which the tuff 

erupted (Ekren and others, 1981; Ekren and others, 

1984).

In the southern part of the wilderness study area, 

the ash-flow tuffs are overlain by flat-lying basalt 

flows and intercalated sedimentary rocks mapped by 

Ekren and others (1981) as the Banbury Basalt and 

unnamed interbedded sedimentary rocks. The basalt 

flows range from 25 to 50 ft in thickness and have a 

maximum cumulative thickness of about 150 ft. They 

form mesas that dominate the landscape of the 

southern study area (Pleasant Valley and Nickel Creek 

tables). The tabular upper parts of the flows 

commonly possess vesicles that are locally lined with 

calcite, clay, and (or) zeolite minerals. The basalts 

are aphyric to weakly porphyritic and contain 

phenocrysts of olivine and (or) plagioclase, as well as 

iron oxides and pyroxene. The interbedded 

sedimentary rocks have a maximum exposed thickness 

of about 100 ft along the southeastern boundary of the 

study area, where they rest directly on the tuff and 

below the basalt. To the west, the sedimentary rocks 

pinch out locally and are intercalated with a lower and 

upper set of basalt flows. The sedimentary rocks 

consist primarily of variegated vitric-ash tuff, 

tuffaceous sandstone and siltstone, and basaltic 

sandstone and conglomerate. The sedimentary rocks 

and some of basalt flows may correlate with the 

caldera-fill sequence recognized to the north.

Small amounts of Quaternary alluvium occur 

along the major stream channels and consist of 

unconsolidated silt, sand, and gravel.

Several small-displacement, widely spaced 

normal faults exist within the wilderness study area 

(fig. 2); most have less than 50 ft of throw. Those 

located in the eastern part of the study area have 

predominantly N. 30° W. trends and down-to-thesouthwest and -northeast apparent displacement. 

Faults with approximately N. 60° W. trends and 

principally down-to-the-northeast apparent 

displacement occur in the western part of the study

A5
area. Using color and infrared U-2 imagery, Bennett 

(1976) noted the existence of a zone of intense 

fracturing approximately 4 mi wide that extends east 

to west across the central part of the study area. This 

zone lies entirely within the outcrop area of the 

Swisher Mountain Tuff and is only locally accompanied 

by faulting. Ekren and others (1984) determined that 

north-northwest-trending faults in the region began to 

develop about 16 Ma and continued to form until at 

least as recently as 8 Ma, subsequent to the eruption 

of the Banbury Basalt. Panze (1975) inferred that 

west-northwest-trending faults in the region formed 

concurrently with the other fault set.

Geochemical Studies

The principal source of geochemical data 

evaluated in this report was the reconnaissance study 

by Bennett (1976); 91 stream-sediment samples 

collected within the study area were analyzed for 

copper, lead, nickel, silver, and zinc. Fourteen 

stream-sediment samples collected within and 

adjacent to the study area by the U.S. Department of 

Energy were analyzed for 33 elements (including 

copper, lead, molybdenum, nickel, silver, uranium, and 

zinc) as part of the NURE assessment of the region 

(Oak Ridge Gaseous Diffusion Plant, 1982). Four rock 

samples collected by the U.S. Bureau of Mines were 

fire-assayed for gold and silver and analyzed for 40 

elements by semiquantitative methods (Leszcykowski, 

1986). These data were evaluated along with those of 

Bennett's. Anomalous values were determined 

primarily by considering the range, distribution, and 

mean of the elemental concentrations.

Anomalously high molybdenum, silver, uranium, 

and zinc concentrations occur in several of the 

stream-sediment samples. High lead and nickel values 

were also detected, but these appear to be associated 

with rock types having primary high background levels 

in those elements, and are not further considered in 

this study. Relatively high silver concentrations (0.51- 

2 ppm) occur sporadically in sediment samples 

collected from drainages within both basalt and 

rhyolite tuff. All the high zinc concentrations (90-125 

ppm) occur in sediment samples from widely scattered 

localities within the outcrop area of the Swisher 

Mountain Tuff in the northern part of the wilderness 

study area; most of these samples were collected 

within the highly fractured zone described by Bennett 

(1976). Two sediment samples collected along the 

study area's southern boundary are weakly anomalous 

in molybdenum (10-11 ppm); their source(s) is 

uncertain. Stream-sediment samples apparently 

anomalous in uranium (4.1-5.5 ppm) were also 

collected along the southern boundary and may have 

been derived from rock units located there. The 

source of one of these samples, however, was clearly 

the Swisher Mountain Tuff.

High concentrations of arsenic (300-500 ppm) 

were detected in two of the rock samples. Both of 

these samples were collected from unaltered Swisher 

Mountain Tuff in the central part of the study area.

Geophysical Studies

An aerial gamma-ray survey was performed in 

the region as part of the U.S. Department of Energy's 

NURE program (Geodata International, 1979). One 

north-south and three east-west composite flightlines 

spaced at intervals of 3 mi traversed the wilderness 

study area. Values of equivalent uranium exceeded the 

background level over the Swisher Mountain Tuff along 

various segments of all four flightlines, principally 

over the eastern part of the study area. Values of 

equivalent uranium for rocks in the area determined 

from the aerial survey indicate that both rhyolite ashflow tuff units have comparatively high background 

levels.

Linear features in Landsat multispectral-scanner 

(MSS) images were mapped by photogeologic 

interpretation for the region of southwestern Idaho. 

Linear features are the surface expression of rock 

fracture patterns and other structural and lithologic 

lineaments. Analysis of linear features, in conjunction 

with geologic and geophysical maps, may reveal 

relationships such as structural control of 

mineralization.

Surface expression of linear features is generally 

very poor in the volcanic deposits of southwestern 

Idaho, especially in the Snake River plain. Linear 

features are well expressed in an area bounded by 

116°-117° W. longitude and 42°30'-43° N. latitude, 

which includes the wilderness study area. 

Concentrations of linear features indicate dominant N. 

20° W. and N. 60-80° E. trends. Maximum 

concentrations lie in an elongate area trending N. 20° 

W., with the N. 20° W. trend prominent at the south 

end of the area near the study area and changing 

northwestward around to the N. 80° E. trend. The 

dominant north-northwest trend is consistent with a 

major fault zone trending N. 20° W. south and east of 

the study area and another fault zone to the northeast 

trending N. 40° W. The east-northeast trend is 

probably typical of a pervasive (but unmapped) 

regional-fracture set found in the investigations of 

other linear features (Rowan and Purdy, 1984) that 

proved to be an important structural control of 

mineralization.

Mineral and Energy Resources

Base and Precious Metals

Similarities in geologic setting, age, and traceelement geochemistry of the rocks of the wilderness 

study area to the Delamar mineral deposits suggest 

that epithermal gold-silver mineralization may have 

occurred at depth. At the Delamar Silver mine, about 

25 mi north of the study area in the Silver City range, 

epithermal gold-silver deposits occur primarily as vein 

fillings along north-northwest-trending faults and 

fractures (Panze, 1975; Halsor, 1983). Rhyolitic lava 

flows and domes, apparently comprising a silicic ring 

complex, host the mineralization; the rhyolitic 

extrusive rocks are inferred to be localized along a

A6
ring fracture developed during the shallow 

emplacement of a body of magma (Halsor, 1983). Acid 

leaching and kaolinization are the most common types 

of alteration observed at Delamar. The mineralization 

(14.8-15.2 Ma) is closely associated in time with the 

rhyolitic volcanism (15.6-15.7 Ma) (Panze, 1975). 

Antimony-, arsenic-, copper-, lead-, and seleniumbearing minerals also occur in the deposits (Panze, 

1975). Anomalous concentrations of copper, gold, 

lead, mercury, silver, and zinc have been detected in 

stream-sediment samples collected near the mine 

(Bennett and Galbraith, 1975). Furthermore, there is a 

high geochemical correlation between gold and silver 

in this area. No clear evidence of secondary 

hydrothermal alteration has been observed in the study 

area, despite the existence of the anomalous 

geochemical concentrations of arsenic, molybdenum, 

silver, and zinc in many of the stream-sediment 

samples. Commonly, the rhyolite tuff exhibits 

limonite staining along small fractures in surface 

exposures, but this is most likely a weathering 

feature. The likelihood that the study area is located 

within the marginal part of a caldera suggests that a 

precollapse rhyolitic ring complex similar to that in 

the Delamar area may exist beneath the Swisher 

Mountain Tuff. Hydrothermal activity would be 

expected to have occurred within the ring complex and 

in the Swisher Mountain Tuff. The trace-element 

signatures detected in the study area likely reflect 

leakage haloes related to gold-silver epithermal 

mineralization at shallow depth. Previous regional 

investigations (Ekren and others, 1981; Panze, 1975: 

Ekren and others, 1984) and the present geologic 

reconnaissance study indicate that faulting (and 

probably localized fracturing) of rocks within the study 

area occurred almost at the same time as 

emplacement of the Swisher Mountain Tuff (that is, 

formation of the proposed caldera) and silicic 

volcanism and mineralization in the Silver City 

district. The numerous faults and the zone of intense 

fracturing within the area may have acted as conduits 

for ascending metal-bearing fluids.

The cause of the sporadic anomalous arsenic, 

molybdenum, silver, and zinc concentrations in the 

wilderness study area remains indefinite due to the 

lack of identified mineral occurrences and alteration 

zones. The entire study area has a low mineral 

resource potential, with certainty level C, for gold, 

silver,and zinc (fig. 2) (see appendix 1 and fig. 3 for 

definitions of mineral resource potential and certainty 

levels).

Uranium

Uraniferous-mineral deposits have been 

identified within the McDermitt caldera complex 100 

mi southwest of the wilderness study area and in the 

Triangle Ranch area located 10 mi to the northeast 

(fig. 1). The McDermitt deposits occur in tuffaceous 

caldera-fill sedimentary and volcanic rocks, and in 

intrusive bodies and flow domes (Rytuba and 

Glanzman, 1979). The uranium ore was deposited by 

an epithermal system and is associated with mercury 

deposits and anomalous concentrations of arsenic, 

molybdenum, and silver. The Triangle Ranch deposit

contains as much as 0.03 percent equivalent uranium 

and occurs in what are thought to be hydrothermally 

altered tuffaceous caldera-moat-fill sedimentary rocks 

(Berry and others, 1982) that overlie the Swisher 

Mountain Tuff. The sedimentary rocks inter bedded 

with basalt in the southern part of the study area (fig. 

2) are at the same stratigraphic level as the uraniumbearing sedimentary rocks at Triangle Ranch and may 

also be part of a caldera-fill sequence. The high 

equivalent-uranium values within the study area 

determined by aerial survey (Geodata International, 

1979) and the weakly anomalous uranium 

concentrations detected in samples from the study 

area may be expressions of undiscovered uranium 

mineralization. No uranium-bear ing mineral 

occurrences or secondary hydrothermal alteration have 

been identified in the study area, suggesting that the 

equivalent-uranium values are expressions of a leakage 

halo from a concealed uranium deposit(s). The 

anomalous geochemical concentrations may also 

reflect the relatively high background levels of 

equivalent uranium associated with both rhyolitic-tuff 

units (see Geophysical Studies section). The study area 

has a low mineral resource potential, certainty level B, 

for uranium.

Geothermal Energy

Evidence of geothermal activity, such as hot 

springs, recent volcanism, and geothermal wells, is 

lacking in the wilderness study area. Furthermore, a 

list of thermal springs and wells in Idaho compiled by 

Bliss (1983) did not include such features in the 

vicinity of the study area. Therefore the study area 

has an unknown potential, certainty level A, for 

geothermal-energy resources.

Petroleum and Natural Gas

The rock units exposed in the wilderness study 

area, as well as the basement rocks cropping out to the 

north, are not capable of generating hydrocarbons. It 

is remotely possible that hydrocarbons exist at depth 

within concealed stratigraphic units, but subsurface 

data is lacking. Therefore, the study area has an 

unknown potential, certainty level A, for petroleum 

and natural gas.

REFERENCES

Beaver, Dennis, 1984, Geology and mineralization of 

the Zn-Pb-Ag skarn, South Mountain, Owyhee 

County, Idaho abs. : Northwest Mining 

Association Annual Meeting, 90th, Spokane, 

Wash., 1984, Proceedings.

Bennett, E. H., 1976, Reconnaissance geology and 

geochemistry of the South Mountain-Juniper 

Mountain region, Owyhee County, Idaho: Bureau 

of Mines and Geology Pamphlet 166, 68 p.

Bennett, E. H., and Galbraith, J. H., 1975, 

Reconnaissance geology and geochemistry of the 

Silver City-South Mountain region, Owyhee 

County, Idaho: Idaho Bureau of Mines and 

Geology Pamphlet 16Z, 88 p.

A7
Berry, M. R., Castor, S. B., and Robins, J. W., 1982, 

National uranium resources evaluation, Jordan 

Valley quadrangle, Oregon and Idaho: United 

States Department of Energy, Open-File Report 

PGJ/F-132(82), 33 p. available through U.S. 

Geological Survey Branch of Open-File Services, 

Box 25425, Federal Center, Denver, Colo., 

80225 .

Bliss, J. D., 1983, Idaho: Basic data for thermal 

springs and wells as recorded in Geotherm, part 

A: U.S. Geological Survey Open-File Report 83- 

431 A, 63 p.

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

1984, High-temperature, large-volume, lava-like 

ash-flow tuffs without calderas in southwestern 

Idaho: U.S. Geological Survey Professional Paper 

1272, 76 p.

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

Malde, H. E., 1981, Geologic map of Owhyee 

County, Idaho west of longitude 116° W.: U.S. 

Geological Survey Miscellaneous Investigations 

Map 1-1256, scale 1:125,000.

Geodata International, Inc., 1979, Aerial radiometric 

and magnetic survey, Jordan Valley national 

topographic map, Oregon and Idaho: United 

States Department of Energy Open-File Report 

GJBX-95(80), 203 p., 108 fiche.

Halsor, Sid, 1983, A volcanic dome complex and 

genetically associated hydrothermal system, 

Delamar Silver Mine, Owyhee County, Idaho:

Hough ton, Mich., Michigan Technological 

University, M.S. thesis, 111 p.

Leszcykowski, A. M., 1986, Mineral resources of the 

North Fork Owyhee River Wilderness Study Area, 

Owyhee County, Idaho: U.S. Bureau of Mines 

Open-File Report 10-86, 8 p.

Oak Ridge Gaseous Diffusion Plant, 1982, 

Hydrogeochemical and stream sediment 

reconnaissance basic data for Jordan Valley 

quadrangle, Oregon and Idaho: United States 

Department of Energy, Open-File Report GJBX58(82), 97 p., 1 fiche.

Panze, A. J., Jr., 1975, Geology and ore-deposits of 

the Silver City-Delmar-Flint region, Owyhee 

County, Idaho: Idaho Bureau of Mines and 

Geology Pamphlet 161, 79 p.

Rowan, L. C., and Purdy, T. L., 1984, Analysis of 

linear features in the Wallace, Idaho-Montana 1° 

x 2° quadrangle: U.S. Geological Survey 

Miscellaneous Field Studies Map MF-1354-H, 38

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

mercury, uranium, and lithium deposits to the 

McDermitt caldera complex, Nevada-Oregon: 

Nevada Bureau of Mines Report 33, p. 109-117.

Sorenson, R. E., 1927, The geology and ore deposits of 

the South Mountain mining district, Owyhee 

County, Idaho: Idaho Bureau of Mines and 

Geology Pamphlet 22, 47 p.

A8
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 (l) 

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.

M/D

MODERATE POTENTIAL

LEVEL OF CERTAINTY

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

A9
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