Mineral Resources of the North Fork Wilderness Study Area, Owyhee County,

U.S. GEOLOGICAL SURVEY BULLETIN 1719-A

Chapter A Mineral Resources of the 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 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 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 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 There are no known mines, prospects, claims, or mineral leases in the study area or its immediate vicinity. Several small borrow pits for road Abstract construction material exist along the southern boundary (fig. 2). The North Fork Owyhee River Wilderness Study Area (ID-016-040), within the southern Owyhee Mountains in southwest Idaho, encompasses 49,470 Identified Resources acres, of which the U.S. Geological Survey and the U.S. Bureau of Mines were asked to study 39,745 The only identified resource in the study area is acres. Hereafter, the terms "study area" and blocky basalt suitable for use as building or decorative "wilderness study area" refer only to the smaller stone. The thin, blocky slabs of basalt occur over an acreage. Field work for this report was carried out area of several square miles in the southern part of the between 1984 and 1985. No mines or prospects are study area. 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 Mineral Resource Potential building or decorative stone. The study area has a low resource potential for gold, silver, zinc, and uranium. Gold, silver, zinc, and uranium have minor The potential for petroleum and natural gas and resource potential in the wilderness study area (fig. geothermal energy is unknown. 2). The potential for geothermal energy and petroleum and natural gas is unknown. Two mining districts occur in the region near the Character and Setting wilderness study area: the South Mountain (silver-lead- zinc) and the Silver City (gold-silver) districts. The North Fork Owyhee River Wilderness Study Mineralization in the former takes the form of silver- Area is located in the southern Owyhee Mountains in lead replacement veins and silver-lead-zinc skarn southwest Idaho, about 75 mi southwest of Boise (fig. within regionally metamorphosed rocks. The Silver 1). The area is situated along a stream-dissected City district, in particular the Delamar mine area, is plateau with canyons as deep as 400 ft. The study area characterized by epithermal mineralization along is underlain by a thick pile of Miocene (about 5 to 24 Miocene normal faults and fractures within a million years before present) volcanic flows and minor contemporaneous, inferred silicic ring complex. interbedded sedimentary rocks. A thick rhyolite ash- Anomalous concentrations of antimony, arsenic, flow tuff sheet is exposed throughout most of the copper, mercury, selenium, and zinc are associated study area; the tuff is overlain in the southern part of with the gold-silver deposits. the study area by another rhyolite ash-flow tuff and a Relatively high concentrations of arsenic, younger sequence of thin basalt flows and interbedded molybdenum, silver, and zinc have been detected in sedimentary rocks. The older ash-flow tuff fills a samples collected in the wilderness study area. A collapse caldera formed due to the emplacement of similar geologic environment (caldera-related silicic the tuff. Several small-displacement normal faults of volcanic rocks and contemporaneous faults and various trends cut the rocks. The rhyolite ash-flow fractures) and trace-element anomalies similar to tuffs are also intensely fractured locally. those in the Silver City district suggest that

Al epithermal mineralization has occurred beneath the Juniper Mountain area. The only other geologic map study area. No mineral deposits or evidence of covering the area is that of Owyhee County by Ekren secondary hydrothermal alteration have been found in and others (1981; 1:125,000-scale). the area. Thus, the study area is judged to have a low The U.S. Geological Survey conducted field resource potential for gold, silver, and zinc (fig. 2). studies in the study area during 1984 and 1985. These Weakly anomalous uranium concentrations occur studies included geologic mapping and collection of in a few places in the southern part of the wilderness rock samples for petrographic studies. In addition, a study area. The rhyolite ash-flow tuffs that underlie geologic reconnaissance of the region surrounding the most of the study area have relatively high background study area was conducted as an aid to formulating ore- levels of uranium and are probably the source of the deposit models. Linear features that appeared in anomalies. Therefore, the study area is considered to Landsat multispectral-^scanner images were mapped by have a low resource potential for uranium (fig. 2). photogeologic interpretation for the region. No evidence of geothermal activity exists in the Geochemical data used in making mineral-resource wilderness study area, and pertinent subsurface data is evaluations in this report include stream ^sediment lacking to quantify the resource potential. The study data obtained from both Bennett (1976) and a U.S. area is considered to have an unknown potential for Department of Energy National Uranium Resource geothermal energy. Evaluation (NURE) report (Oak Ridge Gaseous No evidence of hydrocarbons has been found in Diffusion Plant, 1982). the study area, and pertinent subsurface data is U.S. Bureau of Mines personnel conducted a lacking to quantify resource potential. The study area library search of Owyhee County, state, and U.S. has an unknown potential for petroleum and natural Bureau of Land Management mining-production and gas. mineral-lease records for information on mines, prospects, and claims within the study area. A ground and air reconnaissance was undertaken to check for INTRODUCTION undocumented mining activity. The Bureau of Mines conducted field studies during 1984. Rock and stream- sediment samples were collected to help evaluate the Character and Setting mineral resources of the study area. Complete analytical data are on file at the U.S. Bureau of Mines, The North Fork Owyhee River Wilderness Study Western Field Operations Center, Spokane, Area (ID-016-040), located about 30 mi southeast of Washington, 99202. Jordan Valley, , and 75 mi southwest of Boise, encompasses 49,470 acres within the Owyhee Mountains of southwest Idaho (fig. 1). Elevations APPRAISAL OF IDENTIFIED RESOURCES range from about 4,800 ft, where the North Fork of the Owyhee River crosses the western boundary of the By Andrew M. Leszcykowski, U.S. Bureau of Mines 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 History and Production peak in the immediate vicinity. The rugged, extensively stream-dissected northern part of the There are no known mines, prospects, claims, or study area contrasts greatly with the locally incised mineral leases in the wilderness study area. There are, tablelands of the southern part. With a depth as great however, two mining districts nearby. The South as 400 ft, the gorge of the North Fork of the Owyhee Mountain district is located 7 mi northwest of the River is the deepest canyon in the study area. All of study area near South Mountain (fig. 1). Silver-lead- the other streams in the study area are intermittent, zinc ore was produced there intermittently from the reflecting the semi-arid climate of the region. Juniper early 1870's (Sorenson, 1927) to 1945. The ore occurs trees, low-lying sage and other brush, and seasonal as replacement-vein and skarn deposits in limestone varieties of grasses and wild flowers grow throughout (Beaver, 1984). The Silver City mining district, most of the area. The larger stream channels support located 24 mi north of the study area (fig. 1), has been a variety of riparian vegetation. active since 1863, most recently at the Delamar open- An improved county-maintained gravel road that pit mine, a large-tonnage, low-grade, gold and silver extends from Jordan Valley, Oregon, to Grand View, mine. More than $40 million of gold and silver Idaho provides the best access to the study area. (derived from epithermal veins) have been produced in Several jeep trails branching off this road extend into this district. the study area (fig. 1). The rugged northeastern part Four small borrow pits are located along the of the study area is accessible only by foot, horse, or county road that skirts the southern part of the study helicopter. area (fig. 2). Material for road metal was extracted from the pits for use on nearby roads. Previous and Present Investigations Mineral Deposits The first published geologic map of the study area region was a reconnaissance map (l:62,500^scale) No mineral deposits have been identified within accompanying the report by Bennett (1976) on the the wilderness study area. The only identified geology and geochemistry of the South Mountain- resource is tabular, blocky basalt suitable for use as

A2 117°OCr 116°45' ~"I

Triangle Reservoir

42°45'

NORTH FORK OWYHEE RIVER WILDERNESS STUDY AREA (ID-016-040) ,

42°30'

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" 116°45'

I APPROXIMATE BOUNDARY OF NORTH FORK OWYHEE RIVER WILDERNESS STUDY AREA (ID016 040) Tjs

42° 37' 30"

f C^PLEASANT VALLEY TABLE

3 MILES

EXPLANATION Geology modified from Ekren and others (1981) Area with low mineral resource potential. GEOLOGIC MAP UNITS See Appendix 1 and Figure 3 for definition of levels of mineral resource Tb Banbury Basalt (Miocene) potential and certainty of assessment. Ts Sedimentary rocks (Miocene)- interbedded with Banbury Basalt CORRELATION OF MAP UNITS Rocks of Juniper Mountain volcanic center (Miocene)--In this area, consists of: COMMODITIES Tjl Lower lobes of Juniper Mountain Au Gold Tjs Swisher Mountain Tuff Ag Silver Miocene U Uranium Tob Older basalt flows (Miocene) Zn Zinc 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 underlain by older rocks. An inferred caldera-moat- of several square miles on Nickel Creek and Pleasant filling sedimentary sequence laps onto the Valley tables in the southern part of the study area intracaldera-tuff phase along the northern margin of (fig. 2). Although the basalt is in sufficient quantity to the caldera, north of Big Boulder and Rock Creeks supply any local demand, transportation costs to get (fig. 1). South of the study area, the caldera is buried the stone to a major market area would be by younger volcanic units. The eruption of the Swisher considerable. 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 ASSESSMENT OF MINERAL RESOURCE POTENTIAL caldera structure; rocks in the western part of the study area likely conceal a western segment of the By Scott A. Minor and Donnie Sawatzky caldera's ring fracture. U.S. Geological Survey 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, Geology 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 The wilderness study area lies within the Owyhee Tuff to the north in both lithology and field expression, volcanic field (Panze, 1975), which consists of a thick the main distinction of the former being the pile of complexly interfingering and overlapping occurrence of a large amount of quartz phenocrysts. Tertiary volcanic, pyroclastic, and subordinate Ekren and others (1984) identify this unit as an ash- sedimentary rocks. Basement rocks, consisting of flow tuff sheet on the basis of similar evidence. Its Mesozoic intrusive and metamorphic rocks, crop out name is derived from Juniper Mountain, 7 mi south of within the mountainous parts of the region, including the study area (fig. 1), a broad dome-shaped landform South Mountain 5 mi northwest of the study area. The thought to be a vent complex from which the tuff rocks are locally displaced along primarily north- erupted (Ekren and others, 1981; Ekren and others, northwest-trending normal faults. 1984). The study area is underlain by flat-lying, middle In the southern part of the wilderness study area, and late Miocene rhyolitic ash-flow tuffs, basalt flows, the ash-flow tuffs are overlain by flat-lying basalt and minor intercalated sedimentary rocks (fig. 2). The flows and intercalated sedimentary rocks mapped by oldest rock unit is the Swisher Mountain Tuff, the Ekren and others (1981) as the Banbury Basalt and lowermost subunit of the Juniper Mountain volcanic unnamed interbedded sedimentary rocks. The basalt center (Ekren and others, 1981; Ekren and others, flows range from 25 to 50 ft in thickness and have a 1984). The Swisher Mountain Tuff is a rhyolitic ash- maximum cumulative thickness of about 150 ft. They flow tuff made up of several cooling units, including an form mesas that dominate the landscape of the intracaldera phase. The tuff underlies the northern southern study area (Pleasant Valley and Nickel Creek two-thirds of the study area. Older basalts that tables). The tabular upper parts of the flows disconformably underlie the Swisher Mountain Tuff are commonly possess vesicles that are locally lined with exposed less than 1 mi west of the study area. The calcite, clay, and (or) zeolite minerals. The basalts Swisher Mountain Tuff has a minimum thickness of are aphyric to weakly porphyritic and contain about 500 ft within the study area; its base is not phenocrysts of olivine and (or) plagioclase, as well as exposed. Its rhyolitic composition is expressed, in iron oxides and pyroxene. The interbedded part, by phenocrysts of plagioclase, potassium sedimentary rocks have a maximum exposed thickness feldspar, pyroxene, and opaque iron oxides. The of about 100 ft along the southeastern boundary of the groundmass consists of gray, devitrified glass that study area, where they rest directly on the tuff and weathers reddish to orangish brown. Flow foliations below the basalt. To the west, the sedimentary rocks are ubiquitous and usually folded, with amplitudes as pinch out locally and are intercalated with a lower and great as 300 ft. The tuff is characterized by outcrops upper set of basalt flows. The sedimentary rocks that form jagged pinnacles and precipitous cliffs along consist primarily of variegated vitric-ash tuff, major drainages. The unit is interpreted by Ekren and tuffaceous sandstone and siltstone, and basaltic others (1984) and the senior author of this report to be sandstone and conglomerate. The sedimentary rocks an ash-flow tuff sheet composed of several cooling and some of basalt flows may correlate with the units. The unit's large areal extent, locally preserved caldera-fill sequence recognized to the north. vitroclastic texture, and the sporadic occurrence of Small amounts of Quaternary alluvium occur black vitrophyre layers within the unit all support this along the major stream channels and consist of interpretation. A geologic reconnaissance of the unconsolidated silt, sand, and gravel. region indicates the existence of a collapse caldera, Several small-displacement, widely spaced the western part of which includes the study area. The normal faults exist within the wilderness study area northern part of the caldera is well delineated by a (fig. 2); most have less than 50 ft of throw. Those large semicircular area devoid of rocks older than the located in the eastern part of the study area have Swisher Mountain Tuff. Much of this area is underlain predominantly N. 30° W. trends and down-to-the- by an extensive domal mass of the Swisher Mountain southwest and -northeast apparent displacement. Tuff that may be a younger intracaldera phase of the Faults with approximately N. 60° W. trends and unit; to the north, it laps onto an earlier outflow facies principally down-to-the-northeast apparent of the Swisher Mountain Tuff that is tilted and displacement occur in the western part of the study

A5 area. Using color and infrared U-2 imagery, Bennett Geophysical Studies (1976) noted the existence of a zone of intense fracturing approximately 4 mi wide that extends east An aerial gamma-ray survey was performed in to west across the central part of the study area. This the region as part of the U.S. Department of Energy's zone lies entirely within the outcrop area of the NURE program (Geodata International, 1979). One Swisher Mountain Tuff and is only locally accompanied north-south and three east-west composite flightlines by faulting. Ekren and others (1984) determined that spaced at intervals of 3 mi traversed the wilderness north-northwest-trending faults in the region began to study area. Values of equivalent uranium exceeded the develop about 16 Ma and continued to form until at background level over the Swisher Mountain Tuff along least as recently as 8 Ma, subsequent to the eruption various segments of all four flightlines, principally of the Banbury Basalt. Panze (1975) inferred that over the eastern part of the study area. Values of west-northwest-trending faults in the region formed equivalent uranium for rocks in the area determined concurrently with the other fault set. from the aerial survey indicate that both rhyolite ash- flow tuff units have comparatively high background levels. Geochemical Studies Linear features in Landsat multispectral-scanner (MSS) images were mapped by photogeologic The principal source of geochemical data interpretation for the region of southwestern Idaho. evaluated in this report was the reconnaissance study Linear features are the surface expression of rock by Bennett (1976); 91 stream-sediment samples fracture patterns and other structural and lithologic collected within the study area were analyzed for lineaments. Analysis of linear features, in conjunction copper, lead, nickel, silver, and zinc. Fourteen with geologic and geophysical maps, may reveal stream-sediment samples collected within and relationships such as structural control of adjacent to the study area by the U.S. Department of mineralization. Energy were analyzed for 33 elements (including Surface expression of linear features is generally copper, lead, molybdenum, nickel, silver, uranium, and very poor in the volcanic deposits of southwestern zinc) as part of the NURE assessment of the region Idaho, especially in the plain. Linear (Oak Ridge Gaseous Diffusion Plant, 1982). Four rock features are well expressed in an area bounded by samples collected by the U.S. Bureau of Mines were 116°-117° W. longitude and 42°30'-43° N. latitude, fire-assayed for gold and silver and analyzed for 40 which includes the wilderness study area. elements by semiquantitative methods (Leszcykowski, Concentrations of linear features indicate dominant N. 1986). These data were evaluated along with those of 20° W. and N. 60-80° E. trends. Maximum Bennett's. Anomalous values were determined concentrations lie in an elongate area trending N. 20° primarily by considering the range, distribution, and W., with the N. 20° W. trend prominent at the south mean of the elemental concentrations. end of the area near the study area and changing Anomalously high molybdenum, silver, uranium, northwestward around to the N. 80° E. trend. The and zinc concentrations occur in several of the dominant north-northwest trend is consistent with a stream-sediment samples. High lead and nickel values major fault zone trending N. 20° W. south and east of were also detected, but these appear to be associated the study area and another fault zone to the northeast with rock types having primary high background levels trending N. 40° W. The east-northeast trend is in those elements, and are not further considered in probably typical of a pervasive (but unmapped) this study. Relatively high silver concentrations (0.51- regional-fracture set found in the investigations of 2 ppm) occur sporadically in sediment samples other linear features (Rowan and Purdy, 1984) that collected from drainages within both basalt and proved to be an important structural control of rhyolite tuff. All the high zinc concentrations (90-125 mineralization. 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 Mineral and Energy Resources study area; most of these samples were collected within the highly fractured zone described by Bennett Base and Precious Metals (1976). Two sediment samples collected along the study area's southern boundary are weakly anomalous Similarities in geologic setting, age, and trace- in molybdenum (10-11 ppm); their source(s) is element geochemistry of the rocks of the wilderness uncertain. Stream-sediment samples apparently study area to the Delamar mineral deposits suggest anomalous in uranium (4.1-5.5 ppm) were also that epithermal gold-silver mineralization may have collected along the southern boundary and may have occurred at depth. At the Delamar Silver mine, about been derived from rock units located there. The 25 mi north of the study area in the Silver City range, source of one of these samples, however, was clearly epithermal gold-silver deposits occur primarily as vein the Swisher Mountain Tuff. fillings along north-northwest-trending faults and High concentrations of arsenic (300-500 ppm) fractures (Panze, 1975; Halsor, 1983). Rhyolitic lava were detected in two of the rock samples. Both of flows and domes, apparently comprising a silicic ring these samples were collected from unaltered Swisher complex, host the mineralization; the rhyolitic Mountain Tuff in the central part of the study area. extrusive rocks are inferred to be localized along a

A6 ring fracture developed during the shallow contains as much as 0.03 percent equivalent uranium emplacement of a body of magma (Halsor, 1983). Acid and occurs in what are thought to be hydrothermally leaching and kaolinization are the most common types altered tuffaceous caldera-moat-fill sedimentary rocks of alteration observed at Delamar. The mineralization (Berry and others, 1982) that overlie the Swisher (14.8-15.2 Ma) is closely associated in time with the Mountain Tuff. The sedimentary rocks inter bedded rhyolitic volcanism (15.6-15.7 Ma) (Panze, 1975). with basalt in the southern part of the study area (fig. Antimony-, arsenic-, copper-, lead-, and selenium- 2) are at the same stratigraphic level as the uranium- bearing minerals also occur in the deposits (Panze, bearing sedimentary rocks at Triangle Ranch and may 1975). Anomalous concentrations of copper, gold, also be part of a caldera-fill sequence. The high lead, mercury, silver, and zinc have been detected in equivalent-uranium values within the study area stream-sediment samples collected near the mine determined by aerial survey (Geodata International, (Bennett and Galbraith, 1975). Furthermore, there is a 1979) and the weakly anomalous uranium high geochemical correlation between gold and silver concentrations detected in samples from the study in this area. No clear evidence of secondary area may be expressions of undiscovered uranium hydrothermal alteration has been observed in the study mineralization. No uranium-bear ing mineral area, despite the existence of the anomalous occurrences or secondary hydrothermal alteration have geochemical concentrations of arsenic, molybdenum, been identified in the study area, suggesting that the silver, and zinc in many of the stream-sediment equivalent-uranium values are expressions of a leakage samples. Commonly, the rhyolite tuff exhibits halo from a concealed uranium deposit(s). The limonite staining along small fractures in surface anomalous geochemical concentrations may also exposures, but this is most likely a weathering reflect the relatively high background levels of feature. The likelihood that the study area is located equivalent uranium associated with both rhyolitic-tuff within the marginal part of a caldera suggests that a units (see Geophysical Studies section). The study area precollapse rhyolitic ring complex similar to that in has a low mineral resource potential, certainty level B, the Delamar area may exist beneath the Swisher for uranium. Mountain Tuff. Hydrothermal activity would be expected to have occurred within the ring complex and in the Swisher Mountain Tuff. The trace-element Geothermal Energy signatures detected in the study area likely reflect leakage haloes related to gold-silver epithermal Evidence of geothermal activity, such as hot mineralization at shallow depth. Previous regional springs, recent volcanism, and geothermal wells, is investigations (Ekren and others, 1981; Panze, 1975: lacking in the wilderness study area. Furthermore, a Ekren and others, 1984) and the present geologic list of thermal springs and wells in Idaho compiled by reconnaissance study indicate that faulting (and Bliss (1983) did not include such features in the probably localized fracturing) of rocks within the study vicinity of the study area. Therefore the study area area occurred almost at the same time as has an unknown potential, certainty level A, for emplacement of the Swisher Mountain Tuff (that is, geothermal-energy resources. formation of the proposed caldera) and silicic volcanism and mineralization in the Silver City district. The numerous faults and the zone of intense Petroleum and Natural Gas fracturing within the area may have acted as conduits for ascending metal-bearing fluids. The rock units exposed in the wilderness study The cause of the sporadic anomalous arsenic, area, as well as the basement rocks cropping out to the molybdenum, silver, and zinc concentrations in the north, are not capable of generating hydrocarbons. It wilderness study area remains indefinite due to the is remotely possible that hydrocarbons exist at depth lack of identified mineral occurrences and alteration within concealed stratigraphic units, but subsurface zones. The entire study area has a low mineral data is lacking. Therefore, the study area has an resource potential, with certainty level C, for gold, unknown potential, certainty level A, for petroleum silver,and zinc (fig. 2) (see appendix 1 and fig. 3 for and natural gas. definitions of mineral resource potential and certainty levels). REFERENCES

Beaver, Dennis, 1984, Geology and mineralization of Uranium the Zn-Pb-Ag skarn, South Mountain, Owyhee County, Idaho abs. : Northwest Mining Uraniferous-mineral deposits have been Association Annual Meeting, 90th, Spokane, identified within the McDermitt caldera complex 100 Wash., 1984, Proceedings. mi southwest of the wilderness study area and in the Bennett, E. H., 1976, Reconnaissance geology and Triangle Ranch area located 10 mi to the northeast geochemistry of the South Mountain-Juniper (fig. 1). The McDermitt deposits occur in tuffaceous Mountain region, Owyhee County, Idaho: Bureau caldera-fill sedimentary and volcanic rocks, and in of Mines and Geology Pamphlet 166, 68 p. intrusive bodies and flow domes (Rytuba and Bennett, E. H., and Galbraith, J. H., 1975, Glanzman, 1979). The uranium ore was deposited by Reconnaissance geology and geochemistry of the an epithermal system and is associated with mercury Silver City-South Mountain region, Owyhee deposits and anomalous concentrations of arsenic, County, Idaho: Idaho Bureau of Mines and molybdenum, and silver. The Triangle Ranch deposit Geology Pamphlet 16Z, 88 p.

A7 Berry, M. R., Castor, S. B., and Robins, J. W., 1982, Hough ton, Mich., Michigan Technological National uranium resources evaluation, Jordan University, M.S. thesis, 111 p. Valley quadrangle, Oregon and Idaho: United Leszcykowski, A. M., 1986, Mineral resources of the States Department of Energy, Open-File Report North Fork Owyhee River Wilderness Study Area, PGJ/F-132(82), 33 p. available through U.S. Owyhee County, Idaho: U.S. Bureau of Mines Geological Survey Branch of Open-File Services, Open-File Report 10-86, 8 p. Box 25425, Federal Center, Denver, Colo., Oak Ridge Gaseous Diffusion Plant, 1982, 80225 . Hydrogeochemical and stream sediment Bliss, J. D., 1983, Idaho: Basic data for thermal reconnaissance basic data for Jordan Valley springs and wells as recorded in Geotherm, part quadrangle, Oregon and Idaho: A: U.S. Geological Survey Open-File Report 83- Department of Energy, Open-File Report GJBX- 431 A, 63 p. 58(82), 97 p., 1 fiche. Ekren, E. B., Mclntyre, D. H., and Bennett, E. H., Panze, A. J., Jr., 1975, Geology and ore-deposits of 1984, High-temperature, large-volume, lava-like the Silver City-Delmar-Flint region, Owyhee ash-flow tuffs without calderas in southwestern County, Idaho: Idaho Bureau of Mines and Idaho: U.S. Geological Survey Professional Paper Geology Pamphlet 161, 79 p. 1272, 76 p. Rowan, L. C., and Purdy, T. L., 1984, Analysis of Ekren, E. B., Mclntyre, D. H., Bennett, E. H., and linear features in the Wallace, Idaho- 1° Malde, H. E., 1981, Geologic map of Owhyee x 2° quadrangle: U.S. Geological Survey County, Idaho west of longitude 116° W.: U.S. Miscellaneous Field Studies Map MF-1354-H, 38 Geological Survey Miscellaneous Investigations P- Map 1-1256, scale 1:125,000. Rytuba, J. J., and Glanzman, R. K., 1979, Relation of Geodata International, Inc., 1979, Aerial radiometric mercury, uranium, and lithium deposits to the and magnetic survey, Jordan Valley national McDermitt caldera complex, Nevada-Oregon: topographic map, Oregon and Idaho: United Nevada Bureau of Mines Report 33, p. 109-117. States Department of Energy Open-File Report Sorenson, R. E., 1927, The geology and ore deposits of GJBX-95(80), 203 p., 108 fiche. the South Mountain mining district, Owyhee Halsor, Sid, 1983, A volcanic dome complex and County, Idaho: Idaho Bureau of Mines and genetically associated hydrothermal system, Geology Pamphlet 22, 47 p. Delamar Silver Mine, Owyhee County, Idaho:

A8 APPENDIX 1. Definition of levels of mineral resource low would be misleading. The phrase "no mineral potential and certainty of assessment resource potential" applies only to a specific resource type in a well-defined area. This phrase is not used if Mineral resource potential is defined as the there is the slightest possibility of resource likelihood of the presence of mineral resources in a occurrence; it is not appropriate as the summary defined area; it is not a measure of the amount of rating for any area. resources or their profitability. Expression of the certainty of the mineral Mineral resources are concentrations of naturally resource assessment incorporates a consideration of (l) occurring solid, liquid, or gaseous materials in such the adequacy of the geologic, geochemical, geophysi­ form and amount that economic extraction of a com­ cal, and resource data base available at the time of modity from the concentration is currently or poten­ the assessment, (2) the adequacy of the occurrence or tially feasible. the genetic model used as the basis for a specific evaluation, and (3) an evaluation of the likelihood that Low mineral resource potential is assigned to the expected mineral endowment of the area is, or areas where geologic, geochemical, and geophysical could be, economically extractable. characteristics indicate a geologic environment where the existence of resources is unlikely. This level of Levels of certainty of assessment are denoted oy potential embraces areas of dispersed mineralized rock letters, A-D (fig. 3). as well as areas having few or no indications of mineralization. Assignment of low potential requires A. The available data are not adequate to specific positive knowledge; it is not used as a catchall determine the level of mineral resource potential. for areas where adequate data are lacking. Level A is used with an assignment of unknown mineral resource potential. Moderate mineral resource potential is assigned to areas where geologic, geochemical, and geophysical B. The available data are adequate to suggest characteristics indicate a geologic environment the geologic environment and the level of mineral favorable for resource occurrence, where interpreta­ resource potential, but either evidence is insufficient tions of data indicate a reasonable chance for resource to establish precisely the likelihood of resource occur­ accumulation, and where an application of genetic and rence, or occurrence and (or) genetic models are not (or) occurrence models indicates favorable ground. known well enough for predictive resource assessment. High mineral resource potential is assigned to C. The available data give a good indication of areas where geologic, geochemical, and geophysical the geologic environment and the level of mineral characteristics indicate a geologic environment resource potential, but additional evidence is needed favorable for resources, where interpretations of data to establish precisely the likelihood of resource occur­ indicate a high likelihood for resource accumulation, rence, the activity of resource-forming processes, or where data support occurrence and (or) genetic models available occurrence and (or) genetic models are indicating presence of resources, and where evidence minimal for predictive applications. indicates that mineral concentration has taken place. Assignment of high resource potential requires positive D. The available data clearly define the geologic knowledge that resource-forming processes have been environment and the level of mineral resource active in at least part of the area; it does not require potential, and indicate the activity of resource- that occurrences or deposits be identified. forming processes. Key evidence to interpret the presence or absence of specified types of resources is Unknown mineral resource potential is assigned available, and occurrence and (or) genetic models are to areas where the level of knowledge is so inadequate adequate for predictive resource assessment. that classification of the area as high, moderate, or

M/D

MODERATE POTENTIAL

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

A9 CD 00 -N| O.&. CO ^ 00 O O