Mineral Resources of the Rincon 

Wilderness Study Area, 

Harney County, Oregon

U.S. GEOLOGICAL SURVEY BULLETIN 1740-E

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Chapter E

Mineral Resources of the Rincon 

Wilderness Study Area, 

Harney County, Oregon

By DEAN B. VANDER MEULEN, DONALD PLOUFF, and 

HARLEY D. KING 

U.S. Geological Survey

RONALD T. MAYERLE and RICHARD L. RAINS 

U.S. Bureau of Mines

U.S. GEOLOGICAL SURVEY BULLETIN 1740

MINERAL RESOURCES OF WILDERNESS STUDY AREAS: 

STEENS MOUNTAIN-RINCON REGION, OREGON
DEPARTMENT OF THE INTERIOR 

DONALD PAUL MODEL, Secretary

U.S. GEOLOGICAL SURVEY 

Dallas L. Peck, Director

Any use of trade names and trademarks 

in this publication is for descriptive 

purposes only and does not constitute 

endorsement by the U.S. Geological Survey

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1988

For sale by the

Books and Open-File Reports Section

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Library of Congress Cataloging-in-Publication Data

Mineral resources of the Rincon Wilderness Study Area, 

Harney County, Oregon.

(U.S. Geological Survey bulletin ; 1740-E)

Bibliography: p.

Supt. of Docs, no.: I 19.3:1740-E

1. Mines and mineral resources Oregon Rincon 

Wilderness. 2. Rincon Wilderness (Or.) 

I. Vander Meulen, Dean B. II. Series: Geological 

Survey bulletin ; 1740-E.

QE75.B9 no. 1740-E 557.3s 88-600258 

[TN24.07] [553'.09795'95]
STUDIES RELATED TO WILDERNESS 

Bureau of Land Management Study Area

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

U.S. Geological Survey and the U.S. Bureau of Mines to conduct mineral surveys 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 part of the Rincon Wilderness Study Area (OR-002-082), Harney County, Oregon.

CONTENTS

Summary El

Abstract El

Character and setting El

Identified resources El

Mineral resource potential El 

Introduction E3

Area description E3

Previous and present investigations E3 

Appraisal of identified resources E3

Mines, prospects, claims, and mineralized areas E3

Appraisal of mineral resources E5 

Assessment of mineral resource potential E5

Geology E5

Geochemical studies E6

Geophysical studies E7

Mineral resource potential E7 

References cited E8 

Appendixes

Definition of levels of mineral resource potential and certainty of assessment E12

Resource/reserve classification E13

Geologic time chart E14

FIGURES

1. Index map showing location of the Rincon Wilderness Study Area, Harney County, Oregon E2

2. Map showing generalized geology and mineral resource potential of the Rincon Wilderness Study 

Area, Hamey County, Oregon E4

Contents

Mineral Resources of the 

Rincon Wilderness Study Area, 

Harney County, Oregon

By Dean B. Vander Meulen, Donald Plouff, and Harley D. King 

U.S. Geological Survey

Ronald T. Mayerle and Richard L. Rains 

U.S. Bureau of Mines

SUMMARY 

Abstract

At the request of the U.S. Bureau of Land Management, 

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

conducted field studies of 20,500 acres of the Rincon Wilderness Study Area. The Rincon Wilderness Study Area 

(OR-002-082) is located between Callow Valley and the 

Pueblo Mountains in southeastern Oregon. In this report, the 

area studied is referred to as the "wilderness study area," or 

simply the "study area." Field work was conducted by the 

U.S. Geological Survey during 1986 and 1987, and by the 

U.S. Bureau of Mines during 1986, to evaluate the identified 

mineral resources (known) and the mineral resource potential 

(undiscovered) of the study area.

No mineral resources were identified in the study area. 

However, the study indicates moderate potential for silver 

resources in a rhyolite ash-flow tuff exposed near the central 

part of the study area and high potential for sand and gravel 

resources in lake shoreline deposits along the northwest 

boundary of the study area. The entire study area has low 

potential for geothermal resources and no potential for oil 

and gas.

Character and Setting

The Rincon Wilderness Study Area is located along 

Callow Rim approximately 90 mi south of Burns, Oregon, 

and 10 mi west of Fields, Oregon (fig. 1). Callow Rim is 

a fault scarp that rises 1,000 to 1,900 ft above Callow 

Valley and forms the entire lenglh of the impressive 60-miManuscript approved for publication, June 22, 1988.

long easl valley wall. A large displacement faull at Ihe 

base of Ihe escarpment is Ihe dominanl geologic structure 

in the region and forms Ihe west margin of Ihe 30- by 90- 

mi, north-trending Sleens-Pueblo Mountains fault block. 

The area east of Ihe rim is characterized by a gently southwesl-tilied dip slope cul by several west-flowing creeks

(fig. 1).

Middle Miocene-age basalt flows form the 1,600-flhigh Callow Rim escarpment and are Ihe oldesl rocks 

exposed in Ihe sludy area. The middle Miocene ranges 

from 11.2 lo 16.6 million years before presenl, or Ma; see 

appendixes for geologic lime chart. A sequence of andesile 

flows, luffaceous sedimenlary rocks, and ash-flow luffs 

overlies Ihe basall flows. The straligraphic section is 

capped by younger andesile flows of lale Miocene and 

Pliocene age (Walker and Repenning, 1965). The western 

part of the study area extends into Callow Valley, a broad, 

irregular-shaped graben. Pleistocene shoreline deposits and 

Holocene dunes are exposed in the valley.

Identified Resources

No mines, claims, or prospects were found, and no 

mineral or energy resources were identified within or adjacent to the study area.

Mineral Resource Potential

A rhyolite ash-flow tuff near the central part of the 

study area (fig. 2) has moderate potential for silver resources. One unaltered sample of this tuff contains

Mineral Resources of the Rincon Wilderness Study Area, Harney County, Oregon E1
42

30'

42<

15'

CATLOW

119°00'

VALLEY

118° 45'

APPROXIMATE BOUNDARY

OF

RINCON

WILDERNESS STUDY AREA 

(OR-002-082)

BASQUE

V To Frenchglen, Oregon 

25 mi

E2

42° 

00'

Figure 1. Index map showing location of the Rincon Wilderness Study Area, Harney County, Oregon. 

Mineral Resources of Wilderness Study Areas: Steens Mountain-Rincon Region, Oregon
anomalously high silver, and a stream-sediment sample 

collected from the basin containing the tuff also contains 

anomalous amounts of silver.

Shoreline sand and gravel deposits form extensive 

terraces along the base of Callow Rim in the western part 

of the study area (fig. 2). Shoreline gravel deposits near the 

northwest boundary of the study area have been utilized for 

aggregate; therefore, the northwestern part of the wilderness study area has high potential for sand and gravel 

resources.

Fault zones that cross parts of the study area may be 

associated with geothermal waters at depth. Hot springs 

issue from similar fault zones 10 mi east and 20 mi south 

of the study area. The entire study area has low potential 

for geothermal resources and no potential for oil and gas.

INTRODUCTION

This mineral survey was requested by the U.S. 

Bureau of Land Management and is a joint effort by the 

U.S. Geological Survey and U.S. Bureau of Mines. An 

introduction to the wilderness review process, mineral 

survey methods, and agency responsibilities was provided 

by Beikman and others (1983). The U.S. Bureau of Mines 

evaluates identified resources at individual mines and 

known mineralized areas by collecting data on current and 

past mining activities and through field examination of 

mines, prospects, claims, and mineralized areas. Identified 

resources are classified according to the system modified 

from that described by McKelvey (1972) and the U.S. 

Bureau of Mines and U.S. Geological Survey (1980). 

Studies by the U.S. Geological Survey are designed to 

provide a reasonable scientific basis for assessing the potential for undiscovered mineral resources by determining 

geologic units and structures, possible environments of 

mineral deposition, presence of gcochemical and geophysical anomalies, and applicable ore-deposit models. Mineral 

assessment methodology and terminology as they apply to 

these surveys were discussed by Goudarzi (1984). See 

appendixes for definition of levels of mineral resource 

potential, certainty of assessment, and resource/reserve 

classification.

Area Description

The area studied encompasses 20,500 acres in the 

northern Basin and Range physiographic province of southeastern Oregon. It is located along the southeast side of 

Catlow Valley (fig. 1) approximately 35 mi south of 

Frenchglcn, Oregon. The study area is accessible by an 

improved gravel road that parallels the west side of Catlow 

Rim and by several unimproved dirt roads and jeep trails 

that approach the study area from the north and south.

Maximum elevation in the study area is approximately 

6,400 ft above sea level at the crest of Square Mountain 

(fig. 1); minimum elevation is about 4,500 ft on Catlow 

Valley floor. The climate is semiarid, and the vegetation is 

sparse with sage brush and grasses dominant at all elevations. Parts of the study area are currently used for cattle 

grazing.

Previous and Present Investigations

Previous geologic investigations that included the 

study area were reconnaissance geologic mapping of the 

Adel 1° by 2° quadrangle by Walker and Repenning 

(1965), an aeromagnetic survey by the U.S. Geological 

Survey (1972), and an aerial radiometric and magnetic 

survey by Geodata International, Inc. (1980). The U.S. 

Geological Survey conducted a combined geologic, geochemical, and geophysical survey of the wilderness study 

area during 1986 and 1987. Field investigations focused on 

correlating geochcmical and geophysical anomalies with 

rock units and geologic structures.

The U.S. Bureau of Mines investigation consisted of 

prefield and field studies (Mayerle and Rains, 1987). 

During the prefield study, mining-related data were examined. These data were gathered from the libraries and 

records of the U.S. Bureau of Mines and the U.S. Bureau 

of Land Management; state, county, and other government 

agencies; and private sources. Field studies included a 

ground reconnaissance survey to search for evidence of 

possible mineralized areas and unrecorded mining activity. 

Additional information is available from the U.S. Bureau of 

Mines, Western Field Operations Center, E. 360 Third 

Avenue, Spokane, WA 99202.

APPRAISAL OF IDENTIFIED RESOURCES

By Ronald T. Mayerle and Richard L Rains 

U.S. Bureau of Mines

Mines, Prospects, Claims, and Mineralized 

Areas

No records of mining or mineral exploration were 

found in the literature, nor did a ground search reveal any 

evidence of mining-related activity within the study area.

The nearest mining district, Pueblo-Steens, roughly 

parallels and extends to within 4 mi of the east boundary of 

the study area. The district is located in a mineralized belt 

that extends 40 mi north-northeast along the east escarpment of Pueblo and Steens Mountains. In the district, small 

amounts of mercury, gold, and copper have been mined 

from vein and replacement deposits in silicified zones

Mineral Resources of the Rincon Wilderness Study Area, Harney County, Oregon E3
42° 

12' 

30"

118° 57'30" 

 I 

H/D S. G

APPROXIMATE BOLNDARY,

OF 

RINCON

WILDERNESS STUDY AREA 

(OR-002-082)

H/D S. G

Ta

2 MILES

Figure 2. Mineral resource potential of the Rincon Wilderness Study Area, Harney County, Oregon. 

E4 Mineral Resources of Wilderness Study Areas: Steens Mountain-Rincon Region, Oregon
(Munts and Willett, 1987; L.E. Esparza and T.J. Peters, 

written commun., 1987). Other metal occurrences in the 

Pueblo-Steens mining district include uranium in veins; 

gold, copper, molybdenum, and mercury, possibly associated with a multi-phase intrusive felsic complex; and epithermal disseminated gold in silicified zones (Roback and 

others, 1987). Occurrences of perlite, zeolite, diatomite, 

dimension and decorative stone, agate, and chalcedony 

gemstones are also known. Claims in the district are being 

explored for epithermal disseminated gold deposits by the 

Freeport Minerals Corporation (Mayerle and Rains, 1987).

Appraisal of Mineral Resources

No mineral or energy resources or occurrences were 

identified in or adjacent to the Rincon Wilderness Study 

Area. One of seven alluvial samples taken from the major 

drainages in the study area contains $0.07 gold per cubic 

yard (calculated using a gold price of $400.00 per troy 

ounce). That sample was collected from a stream bed 

along the south boundary. The gold was probably derived 

from a source outside of the study area. Presently, gold 

concentrations such as this are of economic interest as

EXPLANATION

Ag 

S,G 

Geo

Qp 

Qau

Qs

Tya

Tts

Ta 

Ts

Area with high resource potential (H) 

Area with moderate resource potential (M) 

Area with low resource potential (L)

Levels of certainty of assessment

Data only suggest level of potential

Data give good indication of level of potential

Data clearly define level of potential

Commodities 

Silver

Sand and gravel 

Geothermal

Geologic map units

Playa deposits and sand dunes (Quaternary)

Alluvium, colluvium, and talus deposits; undifferentiated

(Quaternary)

Shoreline deposits (Quaternary) 

Younger andesite flows (Tertiary) 

Ash-flow tuff and sedimentary rocks (Tertiary) Includes

Devine Canyon Ash-flow Tuff, tuff of Oregon Canyon,

and tuff of Trout Creek Mountains 

Andesite flows (Tertiary) 

Steens Basalt (Tertiary)

Contact Dotted where approximate 

Fault Dotted where concealed; ball and bar on 

downthrown side

Figure 2. Continued.

indicators but not as gold-bearing gravel deposits to be 

mined. Geological environments similar to those associated with mineralization in the Pueblo-Steens mining district are not recognized in the Rincon Wilderness Study 

Area.

ASSESSMENT OF MINERAL RESOURCE 

POTENTIAL

By Dean B. Vander Meulen, Donald Plouff, and

Harley D. King

U.S. Geological Survey

Geology

The oldest rock unit exposed in the Rincon Wilderness Study Area is the middle Miocene Steens Basalt (Piper 

and others, 1939; Baksi and others, 1967), which consists 

of a thick, chemically homogeneous sequence of basalt 

flows. The base of the Steens Basalt is not exposed in the 

study area. Steens Basalt has a minimum thickness of at 

least 1,600 ft in the northern part of the study area along 

Callow Rim escarpment. The basalt flows, which are 

commonly 20 to 40 ft thick, underlie most of the northern 

part of the study area. The flows form a well-exposed, 

laterally continuous sequence along the northern part of the 

escarpment; offsets produced by northwest-trending normal 

faults are visible along the escarpment.

The Steens Basalt dips 2° to 8° southeast and is 

completely overlain by a sequence of andesite flows in the 

southern part of the study area. This relation is best revealed along Callow Rim escarpment near ihe ceniral part 

of the study area, where andesite flows form a massive 

300-fl-lhick capping unil lhal is faulled againsl and overlies 

Sleens Basall. In ihe southern part of ihe sludy area along 

Funnel Canyon (fig. 2), ihe Steens Basalt dips southeast 

below Callow Rim and is concealed by the andesile flows 

and Quaternary valley-fill sedimeni. In ihis area, ihe sequence of andesite flows is aboul 800 ft thick and forms ihe 

enlire ihickness of ihe Callow Rim escarpment The andesile unil varies from plaly and flow folialed lo vesicular 

and nonfolialed. In ihe wesi-ceniral part of ihe sludy area 

along Callow Rim, ihe andesile unil ihickens, loses mosi of 

ils flow characteristics, and grades into massive porphyriiic 

rock. The ihickness suggesls lhal eilhcr a source venl for 

ihe andesile was located nearby, or lhal ihe area was a 

paleodepression where andesile lavas ponded and cooled 

slowly.

Tuffaceous sedimentary rocks and a partly welded to 

nonwelded lilhic ash-flow luff overlie ihe andesile flows 

near ihe easi boundary of ihe sludy area. This same lilhic 

luff is exposed along ihe wcsi dip slope of Pueblo Mountains, 10 mi soulheasl of ihe sludy area (Tower, 1972;

Mineral Resources of the Rincon Wilderness Study Area, Harney County, Oregon E5
Roback and others, 1987). Younger andesite flows cap 

both the lithic tuff and sedimentary rocks near the east 

boundary of the study area. The younger andesite flows are 

thicker and more extensive near the central part of the study 

area, where they fill a small west-trending graben.

In the northern part of the study area, the S teens 

Basalt is unconformably overlain by sedimentary rocks and 

two middle Miocene rhyolite ash-flow tuffs of the McDermitt volcanic field, dated at 15 to 16.1 Ma (Rytuba and 

McKee, 1984). The tuff of Oregon Canyon and tuff of 

Trout Creek Mountains (Rytuba and others, 1983) form a 

40- to 60-ft-thick topographic bench along the south side of 

Long Hollow Canyon (fig. 2). A third ash-flow tuff, which 

is 10 to 15 ft thick, is concordant with and directly overlies 

the McDermitt tuffs. Whole-rock and trace-element data 

indicate that this overlying tuff is correlative with part of 

the Devine Canyon Ash-flow Tuff (Greene, 1973; Hildreth, 

1981), dated at 9.2 Ma (Walker, 1979). The combined 

thickness of the tuff of Oregon Canyon and Devine Canyon 

Ash-flow Tuff exceeds 160 ft where they fill a small graben 

1.5 mi south of Long Hollow Canyon. These two tuffs are 

locally included in the Tertiary ash-flow tuff and sedimentary rocks unit (fig 2).

Isolated exposures of Devine Canyon Ash-flow Tuff 

are also present in the eastern and central parts of the study 

area. Near the east boundary of the study area, exposures 

of the welded tuff typically form narrow meandering 

ridges. These meandering ridges represent paleostream 

channels that were filled by the welded tuff and later 

exposed by preferential erosion of the surrounding rock.

All four tuffs exposed in the study area represent the 

distal margins of caldera-forming ash-flow tuff eruptions. 

The tuff of Oregon Canyon was erupted from the Washburn caldera, located about 45 mi east-southeast of the 

study area. The tuff of Trout Creek Mountains was erupted 

from the Pueblo caldera, 8 mi east of the study area 

(Rytuba and McKee, 1984). The Devine Canyon Ash-flow 

Tuff erupted from a caldera located in the Harney Basin, 

about 70 mi north of the study area (Walker, 1970; Greene, 

1973).

West of Callow Rim is Callow Valley, a large, irregular-shaped graben formed by basin-and-range extensional tectonism. During the Pleistocene, the valley was 

filled by Catlow Lake. In the study area, wave-cut benches 

and shoreline deposits that formed during several lake 

levels are perched on Catlow Rim escarpment. The highest 

shoreline deposit is 4,795 ft above sea level, about 260 ft 

above the present valley floor. At least five ancient shorelines are exposed along the west face of the escarpment. 

The shorelines are delineated by beach, bar, and spit deposits and by wave-cut benches. Deltaic deposits also form 

part of the ancient shorelines, where larger streams entered 

Catlow Lake. Shoreline deposits consist of partly sorted to 

well-sorted sand and gravel. A shoreline bar near the

central part of the study area has an estimated maximum 

thickness of 50 ft.

Holocene alluvium, colluvium, and talus deposits 

cover parts of the Catlow Rim escarpment (fig. 2). Along 

the base of the escarpment, shoreline deposits are partly 

concealed by these deposits. Streams draining the western 

part of the study area have breached and eroded some of 

the shoreline benches and bars; in other locations, alluvial 

stream deposits have buried the shorelines. In the westernmost part of the study area, playa deposits and sand dunes 

are common along the valley floor. Studies by Mehringer 

and Wigand (1984) indicate that dunes 4 mi north of the 

study area migrated several times during the Holocene. 

Two substantial air-fall tuffs from Mount Mazama, dated at 

0.007 Ma (Mehringer and Wigand, 1984), are preserved in 

the dunes. Ancient artifacts are also preserved in the dunes.

Three different fault sets were mapped in the Rincon 

Wilderness Study Area. The most conspicuous set is the 

northeast-trending range-front fault zone and parallel normal faults, along which basin-and-range extensional tectonism formed the prominent Catlow Rim escarpment. A 

second set of normal faults trends N. 75° W. and forms 

several small horst and graben structures that displace the 

rim perpendicular to the range-front fault. Long Hollow 

Canyon is the largest northwest-striking graben, trending 

parallel to the north boundary of the study area (fig. 2). 

Extensional forces trending parallel to Catlow Rim escarpment, about 90° to the major extension in the region, 

formed these horst and graben structures. A third fault set 

is expressed as a series of well-preserved N. 45° W.-striking en echelon fault scarps that parallel the east-central 

boundary of the study area. These northwest-striking fault 

scarps form a linear fault zone less than 0.5 mi wide that 

extends into the north-central part of the study area (fig. 2). 

Some of the N. 75° W.-striking faults along the northcentral part of Catlow Rim are truncated by this fault zone.

Ceochemical Studies

In 1986-87, the U.S. Geological Survey conducted a 

reconnaissance geochemical study of the Rincon Wilderness Study Area. The study included the collection and 

analysis of 11 rock samples, 22 stream-sediment samples, 

and 21 nonmagnetic heavy-mineral concentrates of stream 

sediments. Stream-sediment samples, and stream sediments from which the concentrates were derived, were 

collected from active alluvium in stream channels.

Stream sediments represent composites of rock and 

soil exposed upstream from sample sites. Stream-sediment 

samples are useful in identifying basins that contain concentrations of elements possibly related to mineralized 

rock. Nonmagnetic heavy-mineral-concentrate samples 

provide information about the chemistry of a limited 

number of minerals in rock material eroded from the drainE6 Mineral Resources of Wilderness Study Areas: Steens Mountain-Rincon Region, Oregon
age basin upstream from each sample site. Many of the 

minerals found in the nonmagnetic fraction of heavy-mineral concentrates may be ore forming or ore related. Selective concentration of minerals permits determination of 

some elements that are not easily detected in bulk streamsediment samples. Most rock samples appeared fresh and 

unaltered and were collected to provide information on 

geochemical background values. A few of the rock 

samples appeared altered and possibly mineralized; these 

were collected to determine the suite of elements associated 

with the observed alteration or mineralization.

All samples were analyzed semiquantitatively for 31 

elements using direct-current arc emission spectrography 

(Grimes and Marranzino, 1968). Eight of the rock samples 

were analyzed for 43 elements using a similar method. 

Steam-sediment samples and four of the rock samples were 

also analyzed by inductively coupled argon plasma atomicemission spectroscopy for antimony, arsenic, bismuth, 

cadmium, and zinc; the same samples were analyzed by 

atomic absorption for gold and mercury (Crock and others,

1987). Analytical data are from M.S. Erickson (written 

commun., 1987).

One rock sample collected from Devine Canyon 

Ash-flow Tuff (fig. 2) near the central part of the study area 

contains an anomalous concentration of silver (8.5 parts per 

million, or ppm). The rhyolite tuff is located in a gentle 

saddle, underlies an area of about 0.2 mi2, and contains 

pumice fragments and small fractures filled with a grayblack mineral. Qualitative element identification of that 

mineral, using the scanning electron microscope, suggests 

that barite is present. However, no silver was detected in 

the mineral. The silver anomaly may reflect more mineralized rock at some unknown depth from which silverenriched fluids have moved upward, possibly along small 

fractures. A stream-sediment sample collected 1.4 mi 

southwest and downstream from the rhyolite tuff exposure 

contains an anomalous concentration of silver (1.5 ppm). 

This anomalous value likely reflects the silver detected in 

the ash-flow tuff; no other silver anomalies were detected 

in stream-sediment samples.

A heavy-mineral concentrate sample collected 

downslope from an exposure of the Devine Canyon Ashflow tuff in the Home Creek Wilderness Study Area, 8 mi 

north of the study area, also contains an anomalous concentration of silver (20 ppm) (Vander Meulen and others,

1988).

Geophysical Studies

Geophysical evaluation of the mineral resources of 

the Rincon Wilderness Study Area is based on interpretations of radiometric, aeromagnetic, and gravity surveys.

Radiometric data were compiled by Geodata International, Inc. (1980) for the National Uranium Resource

Evaluation program of the Department of Energy. Coverage in the study area consists of four east-west flightlines, 

totaling 10 mi in length. The recorded flight altitudes 

generally ranged from 200 to 400 ft above the ground. 

Recordings were made of gamma-ray flux from radioactive 

isotopes of uranium, thorium, and potassium. Abrupt shifts 

in flux level were recorded over geologic contacts, but no 

anomalous concentrations of uranium or thorium were 

indicated in the study area.

A regional aeromagnetic survey was flown over the 

study area (U.S. Geological Survey, 1972). Flightlines 

were spaced at intervals of 2 mi and were flown at a 

constant barometric elevation of 9,000 ft above sea level. 

The northern part of the study area lacks magnetic anomalies. The easternmost part of a 6- by 12-mi, east-trending 

low covers the remaining parts of the study area. The large 

size and shape of the magnetic low and the lack of an 

anomaly in the northern part of the study area suggest that 

most of the rocks in the study area have low magnetization. 

Some of the rocks within the low also may be reversely 

magnetized because (1) the crest of a similar-shaped magnetic low overlies a topographic crest 2 mi north of the 

study area, and (2) the north end of a 13-mi-long band of 

magnetic lows that overlies a north-trending topographic 

ridge is 8 mi southeast of the center of the study area. 

Therefore, the basalt flows in the study area may be equivalent in age to the older, reversely magnetized part of the 

Steens Basalt (Mankinen and others, 1985). On the west 

side of the Callow Rim, aeromagnetic contours along the 

south edge of the magnetic low are deflected slightly northward, further suggesting reversed magnetization of the 

surface rocks or shallowly buried rocks. Furthermore, the 

inferred greater thickness of reversely magnetized rocks 

along the axes of the magnetic lows in and to the north of 

the study area may be controlled by west- and northwesttrending horst and graben structures.

In 1986, the U.S. Geological Survey established 27 

gravity stations in and adjacent to the study area. A preliminary gravity map prepared from these data shows that 

the study area is located in a broad, nose-shaped gravity 

high that extends westward from a major gravity high 

centered near the crest of the Pueblo Mountains (Plouff, 

1987). Pre-Cenozoic basement rocks that underlie the 

gravity high in the Pueblo Mountains are denser than basement rocks at comparable depths. Gravity data coverage is 

too sparse west of Catlow Rim to show a gravity gradient 

across the rim that could be associated with basin-andrange block faulting.

Mineral Resource Potential

A rock sample collected from an exposure of Devine 

Canyon Ash-flow Tuff located near the center of the study 

area (fig. 2) contains an anomalous concentration of silver.

Mineral Resources of the Rincon Wilderness Study Area, Harney County, Oregon E7
An anomalous concentration of silver is also detected in a 

stream-sediment sample collected 1.4 mi downstream from 

the tuff. The area underlain by the tuff has moderate 

potential for silver resources, level C certainty.

Lake shoreline bar deposits located adjacent to the 

northwest boundary of the study area have been utilized for 

sand and gravel aggregate. Similar shoreline deposits are 

located in the western part of the study area. Therefore, the 

northwestern part of the study area has high potential for 

sand and gravel resources, level D certainty.

The Rincon Wilderness Study Area is located 10 mi 

east of the Alvord Valley Known Geothermal Resource 

Area (KGRA) (Muffler, 1979; Cleary and others, 1981). 

Hot springs in this area and 32 mi north and 20 mi south 

of the study area are aligned along recently active basinforming fault zones (Piper and others, 1939; Garside and 

Schilling, 1979). Although no hot springs are identified in 

the study area, geothermal waters may exist at depth along 

recent fault zones. The study area has low potential for 

geothermal resources, level B certainty.

Tertiary volcanic rocks underlying the Rincon Wilderness Study Area are not sources of hydrocarbons. 

Furthermore, geologic structures favorable for the production of oil and gas are not known to underlie the study area. 

Pre-Tertiary basement rocks exposed about 12 mi east of 

the study area are Mesozoic-age metamorphic and intrusive 

rocks (Walker and Repenning, 1965; Roback and others, 

1987) and are unlikely sources or reservoirs for hydrocarbons. Sedimentary rocks that could serve as a source rock 

for oil and natural gas reserves are limited, and there is no 

geologic evidence to suggest that they are of greater extent 

at depth. The absence of rocks capable of producing or 

storing hydrocarbons suggests that the study area has no 

potential for oil and gas, level D certainty.

REFERENCES CITED

Baksi, A.K., York, D., and Watkins, N.D., 1967, Age of Steens 

Mountain geomagnetic polarity transition: Journal of Geophysical Research, v. 72, p. 6299-6308.

Beikman, H.M., Hincle, M.E., Frieders, Twila, Marcus, S.M., and 

Edward, J.R.,1983, Mineral surveys by the Geological Survey 

and the Bureau of Mines of Bureau of Land Management 

Wilderness Study Areas: U.S. Geological Survey Circular 

901, 28 p.

Cleary, John, Lange, I.M., Qamar, A.L. Krouse, H.R., 1981 

Gravity, isotope, and geochemical study of Alvord Valley 

geothermal area, Oregon: Geological Society of America 

Bulletin, v. 92, no. 6, p. 319-322.

Crock, J.G., Briggs, P.H., Jackson, L.L., and Lighte, F.E., 1987, 

Analytical methods for the analysis of stream sediments and 

rocks from wilderness study areas: U.S. Geological Survey 

Open-File Report 87-84, 35 p.

Garside, L.J., and Schilling, J.H., 1979, Thermal waters of Nevada: Nevada Bureau of Mines and Geology Bulletin 91, 163 

PGeodata International, Inc., 1980, Aerial radiometric and 

magnetic survey, National Topographic Map, Adel, Oregon: 

U.S. Department of Energy Open-File Report GJBX-104 

(80), v. 2, 145 p.

Greene, R.C., 1973, Petrology of the welded tuff of Devine 

Canyon, southeastern Oregon: U.S. Geological Survey Professional Paper 797, 26 p.

Grimes, D.J., and Marranzino, A.P., 1968, Direct-current arc and 

alternating-current spark emission spectrographic field methods for the semiquantitative analysis of geologic materials: 

U.S. Geological Survey Circular 591, 6 p.

Goudarzi, G.H., 1984, Guide to preparation of mineral survey 

reports on public lands: U.S. Geological Survey Open-File 

Report 84-787, 51 p.

Hildreth, Wes, 1981, Gradients in silicic chambers; implications 

for lithospheric magmatism: Journal of Geophysical Research, 

v. 86, no. Bll, p. 10,153-10,192.

Mankinen, E.A., Prevot, Michel, and Gromme, C.S., 1985, The 

Steens Mountain (Oregon) geomagnetic polarity transition 

1. Directional history, duration of episodes, and rock magnetism: Journal of Geophysical Research, v. 90, no. B12, p. 

10,393-10,416.

Mayerle, R.T., and Rains, R.L., 1987, Mineral resources of the 

Rincon study area, Harney County, Oregon: U.S. Bureau of 

Mines Open-File Report MLA 66-87, 11 p.

McKelvey, V.E., 1972, Mineral resource estimates and public 

policy: American Scientist, v. 60, p. 32-40.

Mehringer, P.J., Jr., and Wigand, P.E., 1984, Holocene history of 

Skull Creek dunes, Catlow Valley, southeastern Oregon, 

U.S.A.: Journal of Arid Environments, v. 11, p. 117-138.

Muffler, L.J.P., ed., 1979, Assessment of geothermal resources of 

the United States, 1978: U.S. Geological Survey Circular 790, 

163 p.

Munts, S.R., and Willett, S.L., 1987, Mineral resources of the 

Pueblo Mountains study area, Harney County, Oregon, and 

Humboldt County, Nevada: U.S. Bureau of Mines Open-File 

Report MLA 8-87, 43 p.

Piper, A.M., Robinson, T.W., Jr., and Parks, C.F., Jr., 1939, 

Geology and ground-water resources of the Harney Basin, 

Oregon: U.S. Geological Survey Water-Supply Paper 841, 

189 p.

Plouff, Donald, 1987, Gravity observations by the U.S. Geological Survey in northwest Nevada, southeast Oregon, and northeast California, 1984-1986: U.S. Geological Survey OpenFile Report 87-639, 33 p.

Roback, R.C., Vander Meulcn, D.B., King, H.D., Plouff, Donald, 

Munts, S.R., and Willett, S.L., 1987, Mineral resources of the 

Pueblo Mountains Wilderness Study Area, Harney County, 

Oregon, and Humboldt County, Nevada: U.S. Geological 

Survey Bulletin 1741-A, 30 p.

Rytuba, J.J., Minor, S.A., Conrad, W.K., and Vander Meulen, 

D.B., 1983, Geologic map of the Trout Creek Canyon quadrangle, Harney County, Oregon: U.S. Geological Survey 

Open-File Report 83-281.

Rytuba, J.J., and McKee, E.H., 1984, Peralkaline ash-flow tuffs 

and calderas of the McDermitt volcanic field, southeast Oregon and north-central Nevada: Journal of Geophysical Research, v. 89, no. BIO, p. 8616-8628.

Tower, D.B., 1972, Geology of the central Pueblo Mountains

E8 Mineral Resources of Wilderness Study Areas: Steens Mountain-Rincon Region, Oregon
(Triassic-Recent), Harney County, Oregon: Corvallis, Oregon 

State University, M.S. thesis, 96 p.

U.S. Bureau of Mines and U.S. Geological Survey, 1980, Principles of a resource/reserve classification for minerals: U.S. 

Geological Survey Circular 831, 5 p.

U.S. Geological Survey, 1972, Aeromagnetic map of the Adel and 

parts of the Bums, Boise, and Jordan Valley 1° by 2° quadrangles, Oregon region: U.S. Geological Survey open-file 

Report, scale 1:250,000.

Vander Meulen, D.B., Griscom, Andrew, King, H.D., Vercoutere, 

T.L., and Moyle, P.R., 1988, Mineral resources of the Home

Creek Wilderness Study Area, Harney County, Oregon: U.S. 

Geological Survey Bulletin 1740-C, 12 p.

Walker, G.W., 1970, Cenozoic ash-flow tuffs of Oregon: The Ore 

Bin, v. 32, no. 6, p. 97-115.

 1979, Revisions to the Cenozoic stratigraphy of the Harney Basin, southeastern Oregon: U.S. Geological Survey 

Bulletin 1475, 35 p.

Walker, G.W., and Repenning, C.A., 1965, Reconnaissance geologic map of the Adel quadrangle, Lake, Harney, and Malheur 

Counties, Oregon: U.S. Geological Survey Miscellaneous 

Investigations Map 1-446, scale 1:250,000.

Mineral Resources of the Rincon Wilderness Study Area, Harney County, Oregon E9

APPENDIXES
DEFINITION OF LEVELS OF MINERAL RESOURCE POTENTIAL 

AND CERTAINTY OF ASSESSMENT

LEVELS OF RESOURCE POTENTIAL

H HIGH 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 high degree of likelihood for resource accumulation, where data support mineral-deposit models 

indicating presence of resources, and where evidence indicates that mineral concentration has taken place. 

Assignment of high resource potential to an area requires some positive knowledge that mineral-forming processes 

have been active in at least part of the area.

M 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 reasonable likelihood for resource accumulation, and (or) where an application of mineral-deposit models 

indicates favorable ground for the specified type(s) of deposits.

L LOW mineral resource potential is assigned to areas where geologic, geochemical, and geophysical characteristics 

define a geologic environment in which the existence of resources is permissive. This broad category embraces 

areas with dispersed but insignificantly mineralized rock, as well as areas with little or no indication of having 

been mineralized.

N NO mineral resource potential is a category reserved for a specific type of resource in a well-defined area.

U UNKNOWN mineral resource potential is assigned to areas where information is inadequate to assign a low, 

moderate, or high level of resource potential.

LEVELS OF CERTAINTY

A Available information is not adequate for determination of the level of mineral resource potential.

B Available information only suggests the level of mineral resource potential.

C Available information gives a good indication of the level of mineral resource potential.

D Available information clearly defines the level of mineral resource potential.

Z

LLJ

6

CL.

LLJ

U

C£

D

Oen

LLJ

o

_l

LLJ

A

U/A

UN KNOWN POTENTIAL

B

H/B

HIGH POTENTIAL

M/B

MODERATE POTENTIAL

L/B

LOW POTENTIAL

C

H/C

HIGH POTENTIAL

M/C

MODERATE POTENTIAL

L/C

LOW POTENTIAL

D

H/D

HIGH POTENTIAL

M/D

MODERATE POTENTIAL

L/D

LOW POTENTIAL

N/D

NO POTENTIAL

LEVEL OF CERTAINTY

Abstracted with minor modifications from:

Taylor, R.B., and Steven, T.A., 1983, Definition of mineral resource potential: Economic Geology, v. 78, no. 6, p. 1268-1270.

Taylor, R.B., Stoneman, R.J., and Marsh, S.P., 1984, An assessment of the mineral resource potential of the San Isabel National Forest, south-central Colorado: U.S.

Geological Survey Bulletin 1638, p. 40-42. 

Goudarzi, G.H., compiler, 1984, Guide to preparation of mineral survey reports on public lands: U.S. Geological Survey Open-File Report 84-0787, p. 7, 8.

E12 Mineral Resources of Wilderness Study Areas: Steens Mountain-Rincon Region, Oregon
RESOURCE/RESERVE CLASSIFICATION

ECONOMIC

MARGINALLY 

ECONOMIC

SUBECONOMIC

IDENTIFIED RESOURCES

Demonstrated

Measured

Rest

Indicated

jrves

Marginal 

Reserves

Demonstrated 

Subeconomic 

Resources

Inferred

Inferred 

Reserves

Inferred 

Marginal 

Reserves

Inferred 

Subeconomic 

Resources

UNDISCOVERED RESOURCES

Probability Range

Hypothetical Speculative

Major elements of mineral resource classification, excluding reserve base and inferred reserve base. Modified from McKelvey, V.E., 1972, Mineral 

resource estimates and public policy: American Scientist, v. 60, p. 32-40; and U.S. Bureau of Mines and U.S. Geological Survey, 1980, Principles 

of a resource/reserve classification for minerals: U.S. Geological Survey Circular 831, p. 5.

Mineral Resources of the Rincon Wilderness Study Area, Harney County, Oregon E13
GEOLOGIC TIME CHART

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

EON

Phanerozoic

Proterozoic

pre-Archean2

ERA

Cenozoic

Mesozoic

Paleozoic

Late Proterozoic

Middle Proterozoic

Early Proterozoic

Late Archean

Middle Archean

Early Archean

PERIOD

Neogene

Subperiod

Tertiary

Subperiod

V- 1 trld\~cv/U 3

Jurassic

Triassic

Permian

Pennsylvanian 

Carboniferous

Mississippian

Devonian

Silurian

Ordovician

Cambrian

.......... (3800?)- -

EPOCH

Holocene

Pleistocene

Pliocene

Miocene

Oligocene

Eocene

Paleocene

Late

Early

Late 

Middle 

Early

Late 

Middle 

Early

Late 

Early

Late 

Middle 

Early

Late 

Early

Late 

Middle 

Early

Late 

Middle 

Early

Late 

Middle 

Early

Late 

Middle 

Early

ACE ESTIMATES OF 

BOUNDARIES IN 

MILLION YEARS (Ma)

0.010

1.7

5

1 it

1Q

55

 66

96

1 1Q

205

-240

 290

r,-Hf1

 360

 410

 435

J/ U

 1600

JUUU

 3400

 Arm 

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

informal time term without specific rank.

E14 Mineral Resources of Wilderness Study Areas: Steens Mountain-Rincon Region, Oregon

GPO 585-045/9760
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