Mineral Resources of the Sheep Mountain 

Wilderness Study Area 

Baker County, Oregon

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Chapter B

Mineral Resources of the Sheep Mountain 

Wilderness Study Area, 

Baker County, Oregon

By ROGER P. ASHLEY, ROBERT C. ROBACK, 

ROBERT L. TURNER, and ROBERT C. JACHENS 

U.S. Geological Survey

TERRY J. CLOSE and RICHARD L. RAINS 

U.S. Bureau of Mines

U.S GEOLOGICAL SURVEY BULLETIN 1741

MINERAL RESOURCES OF WILDERNESS STUDY AREAS: 

EAST-CENTRAL 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 Catalog! ng-in-Publication Data

Mineral resources of the Sheep Mountain Wilderness Study 

Area, Baker County, Oregon.

(U.S. Geological Survey Bulletin ; 1741-B)

Bibliography: p.

Supt. of Docs. no. : I 19.3:1741-8

1. Mines and mineral resources Oregon Sheep 

Mountain Wilderness. 2. Sheep Mountain Wilderness 

(Or.) I. Ashley, Roger P. (Roger Parkmand), 1940- . 

II. Series.

QE75.B9 no. 1741-B 557.3 s 88-600233 

[TN24.07] [553'.09795'75]
STUDIES RELATED TO WILDERNESS

Bureau of Land Management Wilderness Study Area

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

U.S. Geological Survey and 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 a part 

of the Sheep Mountain Wilderness Study Area (OR-006-003), Baker County, Oregon.

CONTENTS

Summary Bl

Abstract Bl 

Character and setting Bl 

Identified mineral resources Bl 

Mineral resource potential Bl 

Introduction B2

Location and physiography B3 

Procedures and sources of data B3 

Appraisal of identified resources B3 

Assessment of mineral resource potential B4 

Geology B4

Paleozoic and Mesozoic rocks B5 

Tertiary rocks B6 

Quaternary surficial deposits B6 

Geochemistry B6 

Methods B6

Results and interpretations B6 

Geophysics B7

Gravity data B7 

Aeromagnetic data B8 

Radiometric data B8 

Mineral resource potential B8 

References cited B9 

Appendixes

Definition of levels of mineral resource potential and certainty of assessment B12 

Resource/reserve classification B13 

Geologic time chart B14

FIGURES

1. Index map showing location of the Sheep Mountain Wilderness Study Area, Baker County, 

Oregon B2

2. Map showing mineral resource potential and generalized geology of the Sheep Mountain Wilderness Study Area, Baker County, Oregon B4

Contents

Mineral Resources of the

Sheep Mountain Wilderness Study Area,

Baker County, Oregon

By Roger P. Ashley, Robert C. Roback, Robert L. Turner, and 

Robert C. Jachens 

U.S. Geological Survey

Terry J. Close and Richard L. Rains 

U.S. Bureau of Mines

SUMMARY 

Abstract

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

7,040 acres of the Sheep Mountain Wilderness Study Area 

(OR-006-003) were evaluated for identified mineral resources (known) and mineral resource potential (undiscovered). In this report, the area studied is referred to as the 

"wilderness study area" or simply "the study area;" any reference to the Sheep Mountain Wilderness Study Area refers 

only to that part of the wilderness study area for which a 

mineral survey was requested by the U.S. Bureau of Land 

Management. Fieldwork was conducted in 1985 and 1986 to 

assess the mineral resources and resource potential of the 

area. No mineral resources were identified.

A low potential exists for silver, lead, zinc, copper, and 

gold in polymetallic vein deposits in the southeastern part of 

the Sheep Mountain Wilderness Study Area. The area has no 

geothermal energy, uranium and thorium, or oil and gas 

resource potential.

Character and Setting

The Sheep Mountain Wilderness Study Area is located 

on the west side of Hells Canyon, in the eastern part of 

Baker County, Oregon, about 46 mi east of the city of 

Baker (fig. 1). Elevations in the study area range from 

about 1,800 ft near the Snake River to about 4,900 ft on the 

high ridge in the central part of the study area.

Manuscript approved for publication, June 21, 1988.

The study area is underlain mainly by basaltic lava 

flows of the Columbia River Basalt Group of Tertiary 

(Miocene) age (see appendixes for geologic time chart). 

These rocks unconformably overlie plutonic and metasedimentary rocks of late Paleozoic and early Mesozoic age 

that are exposed along the southeast boundary of the study 

area.

Identified Mineral Resources

No mineral resources or prospects were identified 

within or adjacent to the Sheep Mountain Wilderness Study 

Area. Basalt, which covers most of the study area, contains 

common opal and agate that are of limited interest to 

mineral collectors. The basalt in the study area could be 

used locally as construction material (crushed stone or fill); 

however, suitable material is widespread in the region, and 

other areas of basalt are more accessible to possible markets.

Mineral Resource Potential

Geochemical data from stream-sediment and rock 

samples suggest that the exposed plutonic and metasedimentary rocks within the study area have low potential for 

silver, lead, zinc, copper, and gold in polymetallic vein 

deposits (fig. 2). The wilderness study area has no potential for geothermal energy, uranium and thorium, or oil and 

gas.

Mineral Resources of the Sheep Mountain Wilderness Study Area, Oregon B1
INTRODUCTION

This mineral survey was requested by the U.S. Bureau 

of Land Management and is the result of a cooperative 

effort by the U.S. Geological Survey and the 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 a system that is a modification of that described by McKelvey (1972) and U.S. Bureau of Mines and 

U.S. Geological Survey (1980). U.S. Geological Survey 

studies are designed to provide a scientific basis for 

assessing the potential for undiscovered mineral resources 

by determining geologic units and structures, possible 

environments of mineral deposition, presence of 

geochemical and geophysical anomalies, and applicable 

ore-deposit models. Goudarzi (1984) discussed mineral 

assessment methodology and terminology as they apply to 

these surveys. See appendixes for the definition of levels 

of mineral resource potential and certainty of assessment 

and for the resource/reserve classification.

117°00' 116°45

45°00' -

44°45' -

APPROXIMATE BOUNDARY

OF

SHEEP MOUNTAIN 

WILDERNESS STUDY AREA

(OR-006-003)

To Carson, Oregon 

\ 3 mi

To Baker, Oregon

52 mi

To Cambridge, Idaho 

I 24 mi

Figure 1. Index map showing location of the Sheep Mountain Wilderness Study Area, Baker 

County, Oregon.

B2 Mineral Resources of Wilderness Study Areas: East-Central Oregon
Location and Physiography

The Sheep Mountain Wilderness Study Area comprises 7,040 acres near the north edge of the Payette section 

of the Columbia Plateau physiographic province (Fenneman, 1931). It is southwest of Oxbow Dam and occupies 

the divide between the Snake River and Pine Creek (fig. 1). 

The boundary of the study area follows the boundary between Federal and private land, except on the southeast 

side, where the area abuts a Federal power land withdrawal. 

The area is dominated by a nearly flat-topped ridge that 

reaches a maximum elevation of 4,940 ft and is dissected 

by rugged canyons. The lowest elevation, near the Snake 

River, is about 1,800 ft. Oregon Highway 86, located in 

the canyon of Pine Creek, passes close to the west boundary of the study area. Idaho Highway 71 extends from 

Cambridge, Idaho, to Brownlee Dam, then continues north 

on the Oregon side of the Snake River Canyon to Oxbow 

Dam, passing close to the east boundary of the study area.

The climate in the study area is semiarid. The lower 

and more exposed parts of the study area lie within the Big 

Sagebrush Vegetation Zone, characterized by big sagebrush 

(Artemisia tridentatd), grasses, and various other shrubs 

(Frenkel, 1979). The higher and more sheltered parts of the 

area, including the valleys and north-facing slopes, support 

conifers, mainly Ponderosa pine (Pinus ponder osd).

Procedures and Sources of Data

Investigations of geology and mineral resources have 

been conducted north of the study area. Between 1973 and 

1975, mines and prospects within Hells Canyon, north of 

the study area, were evaluated by Close and others (1982). 

Several reports describe the Homestead mining district and 

Iron Dyke mine (fig. 1; Oregon Department of Geology and 

Mineral Industries, 1939; Vallier and Brooks, 1970; Juhas 

and others, 1976, 1981). Vallier (1967, 1974) studied the 

geology of the Snake River Canyon, which includes only 

the northernmost part of the study area. Under a U.S. 

Bureau of Land Management contract, Fredericksen and 

Fernette (1983) conducted a GEM (geology, energy, and 

mineral resources) study of the Sheep Mountain Wilderness 

Study Area.

The U.S. Geological Survey conducted detailed field 

investigations of the Sheep Mountain Wilderness Study 

Area in the summers of 1985 and 1986. This work included geologic mapping at scales of 1:62,500 and 

1:24,000, geochemical sampling, and examining outcrops 

for evidence of mineralization. Stream-sediment samples 

were collected at nine sites, and a fine fraction and heavymineral concentrate from each site were analyzed for minor 

elements. Rock samples were collected at 38 sites, mostly 

to obtain petrographic information; 18 samples from 11 of 

these sites were also analyzed for minor elements.

Prior to fieldwork, the U.S. Bureau of Mines searched 

Oregon State, Baker County, U.S. Bureau of Land Management, and U.S. Geological Survey records and publications 

for claim and minerals-related data. Fieldwork in 1986 

involved a search for mineralized rocks that may not have 

been reported within or near the study area. Six rock and 

eight alluvium samples were taken. The rock samples were 

checked for radioactivity and fluorescence at the Western 

Field Operations Center of the U.S. Bureau of Mines and 

sent to a private laboratory for analysis. Concentrations of 

18 elements were determined by inductively coupled 

plasma analysis. Reconnaissance samples of alluvium were 

reduced in the field by hand panning and further concentrated on a Wilfley table at the Western Field Operations 

Center. The resulting concentrate was then checked for 

gold, other heavy metals, and fluorescence and radioactivity. Complete results of sample analyses are available from 

the U.S. Bureau of Mines, Western Field Operations Center, 360 E. 3rd Ave., Spokane, WA 99202.

APPRAISAL OF IDENTIFIED RESOURCES

By Terry J. Close and Richard L Rains 

U.S. Bureau of Mines

No mining claims are known to have been located in 

the study area, and no mine workings were found during 

field investigations. Quartz-calcite veins in calcareous 

mudstone were examined at two sites in the southeastern 

part of the study area. At locality 1 (fig. 2), three parallel 

veins fill fractures in metasedimentary rocks over a distance of about 400 ft. The veins are as thick as 15 ft, strike 

N. 50° to 70° E., and dip 30° to 45° NW. At locality 2 (fig. 

2), a 1-ft-thick vein strikes N. 15° E. and dips 65° SE. 

Country rock at both sites is sheared, kaolinized, and limonite-stained. Analyses of five samples from the veins, 

altered calcareous mudstone wallrock, and overlying basalt 

do not indicate the presence of metallic resources (Close 

and Rains, 1987).

Fredericksen and Fernette (1983) concluded that the 

study area has a moderate favorability for metallic resources based on the similarity of the geology to that at the 

Iron Dyke mine. The mine is in the Seven Devils Group, 

which hosts volcanogenic, Kuroko-type, silver-gold-copper 

deposits (Juhas and others, 1981; Close and others, 1982). 

However, recent studies by Morris and Wardlaw (1986) 

and Mann and Vallier (1987) indicate that rocks of the 

Seven Devils Group do not crop out or occur near the 

surface in the study area.

Eight reconnaissance samples of alluvium taken from 

drainages in and near the study area contain no gold or 

significant amounts of other heavy metals.

Mineral Resources of the Sheep Mountain Wilderness Study Area, Oregon B3
Basalt of the Columbia River Basalt Group can be used 

as construction material, including stone, crushed stone, or 

fill. Because of its low unit value and high transportation 

cost, basalt is a commodity that must have nearby markets, 

of which there are none at present. Ample supplies of 

suitable stone are available outside the study area to meet 

regional demand for rock products. Vugs and palagonite 

interbeds in the basalt contain common agate and opal of 

limited interest to mineral collectors.

There is no evidence of energy resources in the study 

area.

ASSESSMENT OF MINERAL RESOURCE 

POTENTIAL

By Roger P. Ashley, Robert C. Roback, Robert L. Turner, 

and Robert C. Jachens 

U.S. Geological Survey

Geology

The Sheep Mountain Wilderness Study Area is dominated by a thick sequence of basaltic lava flows of Miocene 

age (fig. 2). These volcanic rocks overlie an older, com116°50

44055' =

APPROXIMATE BOUNDARY

OF

SHEEP MOUNTAIN

WILDERNESS STUDY AREA

(OR-006-003)

Sheep 

Mountain

L/C Ag, Pb, Zn, 

Cu, Au

L/C Ag, Pb, 

Zn, Cu, Au

B4

Figure 2. Mineral resource potential and generalized geology of the Sheep Mountain Wilderness Study Area, Baker County, Oregon.

Mineral Resources of Wilderness Study Areas: East-Central Oregon
plex geologic terrane composed of deformed and metamorphosed intrusive igneous rocks and sedimentary rocks of 

late Paleozoic and early Mesozoic age. After deformation 

and metamorphism, the Paleozoic and Mesozoic rocks were 

eroded for a long period, and the erosion surface was 

flooded by Tertiary lavas.

Paleozoic and Mesozoic Rocks

The Paleozoic and Mesozoic rocks are exposed only 

along the southeast edge of the study area (fig. 2). They are 

part of an inlier of older rocks exposed at the bottom of the 

Snake River Canyon from Eagle Island to the mouth of 

Jacobs Ladder Creek. To obtain adequate data for characterizing these rocks and determining their mineral resource 

potential, the entire pre-Tertiary inlier was examined. 

Another small pre-Tertiary inlier northeast of the study 

area, on both sides of the Snake River at the mouth of 

Cottonwood Creek, was also examined.

The oldest rock unit is an intrusive complex consisting 

mainly of plagiogranite, leucodiorite, and leucogabbro, as 

well as minor amounts of leucotonalite and diabase. These

EXPLANATION

Area having low mineral resource potential, (L); 

data give good indication of level of potential 

(Q

Commodities

Silver

Lead

Zinc

Copper

Gold

Geologic map units

Surficial deposits (Quaternary) Consists of alluvium and landslide deposits

Basaltic dikes (Tertiary) Feeder dikes in the 

Columbia River Basalt Group

Basalt flows (Tertiary) divine-bearing lavas of 

the Columbia River Basalt Group

Sedimentary rocks (Jurassic and Late Triassic) 

Metamorphosed limestone, graywacke, argillite, and andesitic sandstone and conglomerate; in part correlative with the Martin Bridge 

Limestone and Hurwal Formation

Intrusive rocks (Early Triassic and Permian) 

Metamorphosed plagiogranite, leucodiorite, 

leucogabbro, leucotonalite, and diabase; correlative with the (informal) Oxbow complex

 Contact

 Fault

Geochemical sampling site See text for discussion

Ag 

Pb 

Zn 

Cu 

Au

Qs 

Tbd 

Tb 

Jfis

TiPi

 3

Figure 2. Continued.

intrusive rocks have been metamorphosed; typical major 

constituents are albitc, epidote, and chlorite, and typical 

minor constituents are white rnica, sphene, and opaque 

oxides. Actinolite is a constituent of diabase, and relict 

quartz is a constituent of leucotonalite. Calcite is locally 

abundant in all rock types. The mineral assemblages are 

those formed under conditions of low-grade regional metamorphism (Winkler, 1979). All of these rocks show evidence of premetamorphic crushing and shearing, which 

produced cataclastic areas that were then rehealed by 

growth of metamorphic minerals, mainly epidote. Local 

breakage and suturing of plagioclase crystals indicate postmetamorphic shearing of some rocks.

The intrusive rocks are considered part of the (informal) Oxbow complex of Vallier (1974). The Oxbow 

complex consists of amphibolite, gabbro, plagiogranite, and 

diabase and is exposed in a northeast-trending belt at 

Oxbow Dam on the Snake River, about 2 mi northeast of 

the area (Brooks and Vallier, 1967; Vallier and others, 

1977; Mann and Vallier, 1987). Radiometric dating indicates that intrusive rocks of this complex are probably 

Permian in age (Walker, 1981, 1983).

North of the intrusive complex is a belt of deformed 

sedimentary rocks that trends northeastward through the 

southeastern part of the study area. The southeastern part 

of this belt consists of interbedded limestone, argillite, 

graywacke, and volcanic sandstone and conglomerate. The 

northwestern part of the belt consists of conglomerate and 

breccia, locally including interbedded graywacke and argillite. Clasts in the conglomerates are mainly andesitic 

volcanic rocks; subordinate rock types include felsic volcanics, limestone, and chert. Beds strike northeastward to 

northward, and dip steeply to the northwest. Graded beds 

at one locality suggest the section is upright, so the strata 

are younger toward the northwest. The limestone and 

argillite part of the section contains conodonts of Late 

Triassic age (Morris and Wardlaw, 1986), and it is correlated with the Martin Bridge Limestone and Hurwal Formation (T.L. Vallier, oral commun., 1987). The volcaniclastic 

conglomerate and breccia are apparently in depositional 

contact with the limestone and argillite sequence, and 

therefore they must be younger, but have not been dated. 

They probably are correlative with one of the Triassic and 

Jurassic formations exposed to the north in the Wallowa 

Mountains and Snake River Canyon (Vallier, 1977; T. L. 

Vallier, oral commun., 1987).

The sedimentary rocks have also been metamorphosed. 

Major mineral constituents include albite, epidote or clinozoisite, chlorite, and calcite. Minor constituents include 

quartz, actinolite, white mica, hematite or opaques, and 

sphene or leucoxene. Some rocks have relict clinopyroxene, some have pumpellyite, and a few have both. The 

mineral assemblages generally reflect conditions of lowgrade metamorphism. Locally, however, they reflect conditions of very low grade metamorphism.

Mineral Resources of the Sheep Mountain Wilderness Study Area, Oregon B5
In the Sheep Mountain area, the intrusive complex is 

found only in fault contact with the deformed sedimentary 

rocks. In the Wildhorse River area, 2 mi to the southeast, 

similar plutonic rocks intrude a chert conglomerate and 

argillite sequence and are in fault contact with sedimentary 

rocks similar to those of the Sheep Mountain area (Mann 

and Vallier, 1987). The intrusive rocks also appear in the 

inlier just northeast of the area. These relations suggest 

that the Tertiary basalt section of the Sheep Mountain area 

is generally underlain by the Oxbow complex; they also 

suggest that metasedimentary rocks older than the complex 

and fault bounded metasedimentary rocks younger than the 

complex are preserved locally.

Tertiary Rocks

The basaltic lava flows that dominate the Sheep 

Mountain area are part of the Columbia River Basalt Group 

(Waters, 1961). The erosion surface covered by the flows 

ranges from an altitude of 1,750 ft along the Snake River 

to 3,150 ft about 1 mi southeast of the study area, so local 

relief on the surface is at least 1,400 ft. The flows are 

nearly flat lying but dip about 5° westward. The dip-corrected thickness of the basalt section exposed from the 

Snake River to the top of Sheep Mountain is 3,180 ft.

The basalt section consists of numerous lava flows 

ranging from about 10 ft to as much as 200 ft in thickness, 

but most flows are between about 25 and 75 ft thick. 

Individual flows typically have a vesicular base overlain by 

a dense colonnade that grades upward into vesicular flow 

breccia. Lenses of bedded palagonite tuff are present locally between some flows; they contain fine to coarse basaltic fragments and are as much as tens of feet thick. 

Microscopic textures of most of the basalts are intersertal; 

brown basaltic glass fills interstices between plagioclase, 

clinopyroxene, opaque oxide, and clay-altered olivine crystals. Less commonly, the flows are hyalophitic; glass 

surrounds the crystalline minerals. About half the flows are 

porphyritic and contain plagioclase, clinopyroxene, and 

altered olivine phenocrysts. The dense interior parts of 

some of the thicker flows have subophitic to ophitic textures. The basaltic dikes that cut the flows also have intersertal or hyalophitic textures and are mineralogically similar to the flows. They were clearly feeders for flows higher 

in the section.

Quaternary Surficial Deposits

The lower reaches of the drainages contain poorly 

sorted, unconsolidated alluvium, within 0.5 to 1 mi of the 

Snake River. Colluvial basalt debris covers the easily 

eroded tops and bottoms of flows and, in some places, 

obscures the underlying pre-Tertiary rocks as well. A few 

small landslides are found at low altitudes in the southeastern part of the area.

Geochemistry

Methods

The U.S. Geological Survey collected and analyzed 

stream-sediment samples, heavy-mineral concentrates derived from stream-sediment samples, and rock samples. 

Stream sediments were collected at 9 sites and rock 

samples were collected at 11 sites.

The stream-sediment samples were collected from the 

active alluvium in the stream channels. Each sample is a 

composite of material from several localities along a channel length of approximately 50 ft. The stream sediments 

were sieved through an 80-mesh screen and pulverized to a 

fine powder before analysis. To obtain heavy-mineral 

concentrates, the stream sediments were sieved through a 

10-mesh screen and then panned until most of the quartz, 

feldspar, clay-sized material, and organic matter were 

removed. The remaining light minerals were separated 

from the heavy minerals using a heavy liquid (bromoform). 

The magnetite and ilmenite were removed from the heavier 

fraction using an electromagnet. The nonmagnetic concentrates were ground to a fine powder before analysis.

Stream sediments represent a composite of rocks exposed in the drainage basin above the sample site. Heavymineral concentrates include most ore-forming and orerelated minerals and permit determination of some elements that are not easily detected in bulk stream sediments.

No evidence of alteration or mineralization was found 

within the area, but signs of alteration and mineralization 

were found at three localities within 0.5 mi of the wilderness study area boundary. Samples were collected from 

these localities to determine the suite of elements formed 

by alteration and mineralization. The rocks were pulverized to a fine powder before analysis.

All samples were analyzed using semiquantitative 

emission spectroscopy for 31 elements (Grimes and Marranzino, 1968; Crock and others, 1983). The rock samples 

were also analyzed for antimony, arsenic, bismuth, cadmium, and zinc using atomic-absorption spectrometry 

(O'Leary and Viets, 1986), for gold using atomic-absorption spectrometry (detection limit 0.1 parts per million, 

ppm; Thompson and others, 1968), and for mercury using 

cold-vapor atomic-absorption spectrometry (Koirtyohann 

and Khalil, 1976). A description of the analytical techniques is given by Crock and others (1987).

Results and Interpretations

Not enough data were available to establish geochemical background values by statistical methods. Analyses of 

altered basalts were compared to analyses for four unaltered basalt samples collected from three sites in the study 

area. Because basalt dominates all the drainage basins in 

the area, stream-sediment samples were also compared to

B6 Mineral Resources of Wilderness Study Areas: East-Central Oregon
the unaltered basalt samples. Analyses of altered intrusive 

rocks and one andesitic metavolcaniclastic rock were 

compared to average minor element abundances given for 

mafic, intermediate, and silicic igneous rocks by Rosier and 

Lange (1972) and Parker (1967). An element concentration 

was considered anomalous if it exceeded the appropriate 

comparison value by a factor greater than two.

The analytical data reveal that samples containing 

anomalous concentrations of various elements are limited 

to two areas. One area is defined by stream-sediment 

samples from drainages entering the Snake River between 

Black Canyon and the drainage about 1 mi northeast of 

Eagle Island Creek. These drainage basins include the 

southeastern part of the wilderness study area. One streamsediment sample from the southeastern part of the study 

area contains an anomalous concentration of 0.11 ppm 

mercury. The heavy-mineral concentrates of two samples 

contain anomalous amounts of lead (2,000 ppm and 10,000 

ppm). Except for the quartz-calcite veins described in the 

Appraisal of Identified Resources section, no notable evidence of alteration or mineralization was observed in outcrops on the west side of the Snake River in this area. On 

the east side of the river, however, altered and mineralized 

rocks are found within a 0.5 mi2 area opposite the mouth of 

Eagle Island Creek (localities 3 and 4, fig. 2). Two rock 

samples contain anomalous concentrations of arsenic (33 

and 88 ppm), bismuth (15 and 41 ppm), copper (200 and 

greater than 20,000 ppm), cadmium (0.8 and 1.0 ppm), and 

mercury (0.57 and 1.18 ppm). One of these rock samples 

contains anomalous values of silver (3 ppm) and zinc (250 

ppm); the other contains an anomalous value of molybdenum (50 ppm). A third sample contains anomalous values 

of bismuth (2 ppm), molybdenum (7 ppm), cadmium (0.3 

ppm), and mercury (0.64 ppm).

A rock sample collected at the mouth of Cottonwood 

Creek (locality 5, fig. 2) contains an anomalous concentration of mercury (0.24 ppm). This site is 0.5 mi from the 

northeast boundary of the wilderness study area.

Almost all the rock samples in both areas that contain 

anomalous values are from northeast-trending postmetamorphic shear zones in intrusive rocks of the Oxbow 

complex. Hydrothermal solutions moved along these shear 

zones, silicifying and mineralizing them, as shown by the 

presence of pyrite, limonite, and, more rarely, malachite. 

The mineralized rocks have anomalous concentrations of 

one or more of the elements arsenic, bismuth, copper, lead, 

mercury, molybdenum, cadmium, silver, and zinc.

One example of mineralized rock was found in rocks 

other than those of the Oxbow complex. Pyrite was observed along a northeast-trending postmetamorphic fault 

within the metasedimentary section at locality 6 (fig. 2). 

Amounts of cadmium (1.2 ppm) and antimony (2 ppm) in 

a sample of pyritic metavolcaniclastic rock from this fault 

zone are anomalous.

The basalt flows of the Columbia River Basalt Group 

within the study area have been altered locally. Evidence 

of alteration includes vesicle fillings of chalcedony (agate), 

opal, and zeolites; quartz stringers; and pervasive chalcedonic silica. Nine samples of altered basalt from four 

localities, however, have the same concentrations of elements as the four unaltered basalt samples collected to 

establish background abundances of elements. The lack of 

enrichment of ore-related elements suggests that the alteration was produced by circulation of groundwater through 

the basalt flows; it also suggests that the source of anomalous concentrations of elements in stream-sediment samples 

is most likely limited to the mineralized shear zones in the 

pre-Tertiary rocks.

Geophysics

Three types of geophysical data from northeastern 

Oregon (gravity, magnetic, and radiometric) were compiled 

and examined to aid in assessing the mineral resource 

potential of the Sheep Mountain Wilderness Study Area. 

The widely distributed nature of the three data sets makes 

them adequate for determining the regional structural and 

tectonic setting of the study area, but it does not permit 

detailed statements about mineral resource potential at the 

deposit scale, except in areas directly beneath detailed 

profiles.

Gravity Data

Gravity data for the region surrounding the study area 

were obtained from the National Geophysical Data Center, 

National Oceanic and Atmospheric Administration, Boulder, Colorado. Data points are scattered at 2- to 4-mi 

spacing in the region, but no data points are located within 

the study area boundary. The observed gravity data, based 

on the International Gravity Standardization Net datum 

(Morelli, 1974), were reduced to free-air gravity anomalies 

using standard formulas (Telford and others, 1976). 

Bouguer, curvature, and terrain corrections (out to a distance of 103.6 mi from each station) at a standard reduction 

density of 2.67 grams per cubic centimeter (g/cm3) were 

added to the free-air anomaly at each station to determine 

complete Bouguer gravity anomalies.

The Bouguer gravity field over the study area and 

surrounding regions reflects both shallow density contrasts 

and deep-crustal density distributions that correlate with the 

topography in a manner consistent with the concept of 

isostasy. To isolate that part of the gravity field that results 

from upper crustal density distributions, an isostatic residual-gravity map was constructed from the Bouguer gravity 

data. The method was removal of a regional gravity field 

computed from a model of the crustal-mantle interface. 

This model assumes Airy-type isostatic compensation 

(Jachens and Griscom, 1985).

Mineral Resources of the Sheep Mountain Wilderness Study Area, Oregon B7
The Sheep Mountain Wilderness Study Area lies near 

the east end of a gently curved, 50-mi-long by 15-mi-wide 

linear gravity high that trends mostly east-west. The gravity high lies a few miles south of the north edge of the preCretaceous oceanic crust terrane of Brooks (1979) and 

roughly parallels the suture zone between this terrane and 

the Wallowa-Seven Devils terrane of Brooks (1979), to the 

north. Over much of its length, the source of the gravity 

high is concealed beneath Tertiary volcanic deposits; 

however, near Sparta, Oregon, about 20 mi west of the 

study area, the gravity anomaly culminates over a pre-Late 

Triassic intrusive complex composed predominantly of 

gabbro and altered gabbro (Brooks and others, 1976). 

Similar rocks are a likely source for the entire anomaly.

The gravity high near the study area suggests that the 

pre-Cretaceous basement beneath the area contains mafic 

intrusive rocks of the oceanic crust terrane similar to those 

exposed near Sparta. The study area lies within the gentle 

gravity gradient that defines the east end of the linear high. 

This gravity gradient may reflect a gradual transition from 

mafic intrusive rocks on the west to more felsic, less dense 

intrusive rocks to the east; or it may reflect an eastward 

increasing thickness of supracrustal sedimentary and volcanic rocks of the oceanic crust terrane.

Aeromagnetic Data

An aeromagnetic survey of the Baker 1° by 2° quadrangle, Oregon and Idaho, was flown and compiled under 

contract to the U.S. Department of Energy as part of the 

National Uranium Resource Evaluation program (Geodata 

International, Inc., 1978). Total-field magnetic data were 

collected along east-west flightlines spaced approximately 

3 mi apart and at an average height of 400 ft above the terrain. Additional data were collected along north-south 

flightlines spaced approximately 12 mi apart. Corrections 

were applied to the data to compensate for diurnal variations of the Earth's magnetic field, and the International 

Geomagnetic Reference Field (updated to the months that 

the data were collected) was subtracted to produce a residual magnetic field.

Three flightlines traversed the study area. East-west 

flightlines crossed at latitudes 44°53'20" N. and 44°55'48" 

N. and a north-south flightline crossed at longitude 

116°52'4r W. Magnetic measurements were made approximately every 250 ft along the flightlines.

The residual magnetic field over the study area is 

characterized by numerous short-wavelength anomalies 

(widths of 0.5 to 1 mi) superposed on a broad magnetic 

high that is bounded on the east by a broad magnetic low 

beginning about at the Snake River. The short-wavelength 

anomalies are caused by topography of the strongly magnetic Tertiary volcanic rocks exposed in the study area. 

The broad magnetic low east of the study area is spatially 

correlated with exposures of pre-Cretaceous basement

(Mitchell and Bennett, 1979), thus indicating that the basement rocks are, for the most part, only weakly magnetic. 

Because the volcanic rocks within the broad low are 

strongly magnetic, they must be underlain at shallow depth 

by the pre-Cretaceous basement. The broad magnetic high 

over the study area probably reflects a section of Tertiary 

volcanic rocks that thickens to the west from near zero 

along the Snake River. A west-dipping surface on the preCretaceous basement beneath the study area would account 

for the thickening.

Radiometric Data

Radiometric data were collected along the same flightlines and at the same altitude as the aeromagnetic data 

(Geodata International, Inc., 1978). Recordings were made 

of gamma-ray flux from radioactive isotopes of potassium, 

bismuth, and thallium. Count rates were low along flightlines over the study area and gave no indication of anomalous amounts of radioactive elements. However, because 

the flightlines are widely spaced and because gamma rays 

are attenuated by passage through Earth materials, these 

data do not preclude the presence of anomalous amounts of 

radioactive elements between flightlines or buried a few 

feet or more beneath the surface.

Mineral Resource Potential

The evaluation of mineral resource potential is based 

mainly on geologic and geochemical data, but it takes into 

account geophysical features and the absence of existing 

mines and prospects.

The southeastern part of the Sheep Mountain Wilderness Study Area has low mineral resource potential, certainty level C, for silver, zinc, lead, copper, and gold in 

polymetallic vein deposits. Geologic conditions suitable 

for these deposits are found only in the Paleozoic and 

Mesozoic plutonic and metasedimentary rocks. Because 

aeromagnetic data indicate that the Columbia River Basalt 

Group thickens to the west, the area of mineral resource 

potential is confined to the outcrop area of Paleozoic and 

Mesozoic rocks; however, any veins discovered near the 

basalt contact could conceivably be developed for short 

distances beneath the lavas. Although no polymetallic vein 

deposits have been found in the study area, such deposits 

are found in the Cuddy Mountain mining district, 6-8 mi to 

the southeast, and in the Homestead area (included in the 

Seven Devils mining district), 4-7 mi to the northeast 

(Livingston, 1923; Cook, 1954). The veins known in these 

districts have been explored and, in some cases, mined for 

one or more commodities including lead, zinc, copper, 

silver, and gold, but all the deposits were of small tonnage.

A descriptive model for polymetallic vein deposits is 

given by Cox (1986). A grade-tonnage model based on 75 

deposits (Bliss and Cox, 1986) shows a median deposit size

B8 Mineral Resources of Wilderness Study Areas: East-Central Oregon
of 7,600 tons, and 10 percent of the deposits exceed 

200,000 tons. Median grades for commodities in the model 

are 24 troy oz/ton for silver, 0.004 troy oz/ton for gold, 9 

percent for lead, 2.1 percent for zinc, and less than 0.05 

percent for copper. The small size of similar veins in 

northeastern Oregon and western Idaho and the low median 

tonnage in the grade-tonnage model indicate that any veintype deposits found in shear zones or faults in the study 

area, although possibly high grade, are likely to contain 

only small quantities of base and precious metals (lead, 

zinc, copper, silver, and gold).

Fredericksen and Fernette (1983) concluded that the 

Sheep Mountain Wilderness Study Area is moderately 

favorable for metallic minerals, specifically in volcanogenie massive sulfide deposits, based on inferred similarity 

of the pre-Tertiary rocks to those that host such deposits in 

the Hells Canyon area to the north. As Fredericksen and 

Fernette (1983) note, however, all known deposits and 

occurrences of this type are restricted to metavolcanic and 

metavolcaniclastic rocks of the Seven Devils Group, and all 

are found north of Oxbow Dam. Paleontological data that 

were gathered during this study and by Morris and 

Wardlaw (1986) suggest that metavolcaniclastic rocks of 

the Sheep Mountain area are too young to be part of the 

Seven Devils Group. Massive sulfide deposits, which are 

characterized by conspicuous copper and zinc minerals and 

minor but often economically important amounts of precious metals, cannot be ruled out as a possible cause of 

some of the observed geochemical anomalies. We conclude, however, that no potential exists for base and precious metals in massive sulfide deposits. This conclusion 

is based on age of the metavolcanic rocks in the area and 

geologic observations that indicate all mineral occurrences 

in and near the area are of hydrothermal rather than volcanogenic origin.

The Sheep Mountain Wilderness Study Area lies 

within 20 mi of a large area in southwestern Idaho determined to be favorable for discovery and development of 

local sources of low-temperature (less than 90 °C) geothermal water (Sammel, 1979). The Blue Mountains region of 

Oregon, however, including the Sheep Mountain area, is 

characterized by normal heat-flow values (Blackwell and 

others, 1978), and the nearest known thermal spring is 

located 25 mi to the southwest (Oregon Department of 

Geology and Mineral Industries, 1982). Thus, there is no 

resource potential, certainty level D, for geothermal energy 

in the area.

No potential exists for oil and gas, certainty level D. 

This assessment is based on extensive metamorphism and 

plutonism in the Paleozoic and Mesozoic rocks and lack of 

hydrocarbon source rocks in the Tertiary volcanic section.

No potential exists for uranium and thorium, certainty 

level D. This assessment is based on lack of favorable 

igneous or sedimentary host rocks.

REFERENCES CITED

Beikman, H.M., Hinkle, 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.

Blackwell, D.D., Hull, D.A., Bowen, R.G., and Steele, J.L., 1978, 

Heat flow of Oregon: Oregon Department of Geology and 

Mineral Industries Special Paper 4, 42 p.

Bliss J.D., and Cox, D.P., 1986, Grade and tonnage model of 

polymetallic veins in Cox, D.P., and Singer, D.A., eds., 

Mineral deposit models: U.S. Geological Survey Bulletin 

1693, p. 125-129.

Brooks, H.C., 1979, Plate tectonics and the geologic history of the 

Blue Mountains: Oregon Geology, v. 41, p. 71-80.

Brooks, H.C., McLityre, J.R., and Walker, G.W., 1976, Geology 

of the Oregon part of the Baker 1° x 2° quadrangle: Oregon 

Department of Geology and Mineral Industries Geological 

Map Series GMS-7, scale 1:250,000.

Brooks, H.C., and Vallier, T.L., 1967, Progress report on the 

geology of part of the Snake River Canyon, Oregon and 

Idaho: The Ore Bin, v. 29, p. 233-266.

Close, T.J., Federspiel, F.E., Leszcykowski, A.M., and Hyndman, 

P.C., 1982, Mineral resources of the Hells Canyon study area, 

Adams and Nez Perce Counties, Idaho and Wallowa County, 

Oregon: U.S. Bureau of Mines Open-File Report MLA 41-82, 

29 p.

Close, T.J., and Rains, R.L., 1987, Mineral resources of the Sheep 

Mountain study area, Baker County, Oregon: U.S. Bureau of 

Mines Open-File Report MLA 57-87, 11 p.

Cook, E.F., 1954, Mining geology of the Seven Devils region: 

Idaho Bureau of Mines and Geology Pamphlet 97, 22 p.

Cox, D.P., 1986, Descriptive model of polymetallic veins, in Cox, 

D.P., and Singer, D.A., eds., Mineral deposit models: U.S. 

Geological Survey Bulletin 1693, p. 125.

Crock, J.G., Lichte, F.E., and Briggs, P.H., 1983, Determination 

of elements in National Bureau of Standards geological reference material SRM 278 obsidian and SRM 668 basalt by induction coupled argon plasma-atomic emission spectrometry: 

Geostandards Newsletter, No. 7, p. 335-340.

Crock, J.G., Briggs, P.H., Jackson, L.L., and Lichte, 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.

Fenneman, N.M., 1931, Physiography of the Western United 

States: New York, McGraw-Hill Book Company, 534 p.

Fredericksen, R.S., and Fernette, Greg, 1983, Geology, energy, 

and mineral (GEM) resource evaluation of the Homestead 

GRA, Oregon-Idaho, including the McGraw Creek (6-1), 

Homestead (6-2) and Sheep Mountain (6-3) Wilderness 

Study Areas: prepared by WGM, Inc., Mining and Geological 

Consultants, under Contract No. YA-553-CT2-1039 for the 

Bureau of Land Management, 54 p.

Frenkel, R.E., 1979, Vegetation, in Highsmith, R.M., and Kimerling, A.J., eds., Atlas of the Pacific Northwest (6th ed.): 

Corvallis, Oregon State University Press, 135 p.

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

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

Mineral Resources of the Sheep Mountain Wilderness Study Area, Oregon B9
Report 84-787, 51 p.

Geodata International, Inc., 1978, Area! radiometric and magnetic 

survey, Baker national topographic map, Idaho and Oregon: 

U.S. Department of Energy Open-File Report GJBX-101 

(78), v. 2, variously paged.

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.

Jachens, R.C., and Griscom, Andrew, 1985, An isostatic residual 

gravity map of California A residual map for interpretation 

of anomalies from intracrustal sources, in Hinze, W.J., ed., 

The utility of regional gravity and magnetic anomaly maps: 

Tulsa, Society of Exploration Geophysicists, p. 347-360.

Juhas, A.P., Fredericksen, R.S., and Gallagher, T.D., 1976, Precious and base metal mineralization at the Iron Dyke and Red 

Ledge deposits: Northwest Mining Association, 82nd Annual 

Convention Session Paper, 28 p.

Juhas, A.P., Freeman, P.S., and Fredericksen, R.S., 1981, Kuroko 

type copper-gold mineralization at the Iron Dyke mine, 

Oregon, in Silberman, M.L., Field, C.W., and Berry, A.L., 

eds., Proceedings of the Symposium on Mineral Deposits of 

the Pacific Northwest: U.S. Geological Survey Open-File 

Report 81-355, p. 236-245.

Koirtyohann, S.R., and Khalil, Moheb, 1976, Variables in determination of mercury by cold-vapor atomic absorption: Analytical Chemistry, v. 48, p. 136-139.

Livingston, D.C., 1923, A geologic reconnaissance of the Mineral 

and Cuddy Mountain mining districts, Washington and 

Adams Counties, Idaho: Idaho Bureau of Mines and Geology 

Pamphlet 13, 24 p.

Mann, G.M., and Valuer, T.L., 1987, Geology of the Wildhorse 

River area, western Idaho: telescoped terranes of the Blue 

Mountains province [abs.]: Geological Society of America 

Abstracts with Programs, v. 19, p. 318.

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

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

Mitchell, V.E., and Bennett, E.H., 1979, Geologic map of the 

Baker quadrangle, Idaho: Idaho Bureau of Mines and Geology 

Geologic Map Series, scale 1:250,000.

Morelli, C.E., ed., 1974, The international gravity standardization 

net 1971: International Association of Geodesy Special Publication 4, 194 p.

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Geology of the Blue Mountains region of Oregon, Idaho, and 

Washington: U.S. Geological Survey Professional Paper 

1435, p. 59-64.

O'Leary, R.M., and Viets, J.G., 1986, Determination of antimony, 

arsenic, bismuth, cadmium, copper, lead, molybdenum, silver, 

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half: Oregon Department of Geology and Mineral Industries

Bulletin 14-A, 125 p.

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Geothermal resources of Oregon: map, scale 1:500,000.

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in Fleischer, Michael, technical ed., Data of geochemistry, 6th 

ed.: U.S. Geological Survey Professional Paper 440-D, p. 

D1-D19.

Rosier, H.J., and Lange, H., 1972, Geochemical tables: Amsterdam, Elsevier Publishing Company, 468 p.

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waters in the United States, in Muffler, L.J.P., ed., Assessment of geothermal resources of the United States 1978: 

U.S. Geological Survey Circular 790, 163 p.

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1976, Applied Geophysics: New York, Cambridge University 

Press, 960 p.

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Survey Research 1968: U.S. Geological Survey Professional 

Paper 600-B, p. B130-B132.

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Geological Survey Circular 831, 5 p.

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and adjacent areas in northeastern Oregon and western Idaho: 

Corvallis, Oregon, Oregon State University unpub. Ph.D. 

dissertation, 267 p.

Vallier, T.L., 1974, A preliminary report on the geology of part of 

the Snake River Canyon, Oregon and Idaho: Oregon Department of Geology and Mineral Industries Geological Map 

Series GMS-6, scale 1:125,000.

 1977, The Permian and Triassic Seven Devils Group, 

western Idaho and northeastern Oregon: U.S. Geological 

Survey Bulletin 1437, 58 p.

Vallier, T.L., and Brooks, H.C., 1970, Geology and copper deposits of the Homestead area, Oregon and Idaho: The Ore Bin, v. 

32, p. 37-56.

Vallier, T.L., Brooks, H.C., and Thayer, T.P., 1977, Paleozoic 

rocks of eastern Oregon and western Idaho, in Stewart, J.H., 

Stevens, C.H., and Fritsche, A.E., eds., Paleozoic palegeography of the western United States: Society of Economic Paleontologists and Mineralogists, Pacific Section, Pacific Coast 

Paleogeography Symposium 1, p. 455-466.

Walker, N.W., 1981, U-Pb geochronology of ophiolitic and volcanic-plutonic arc terranes, northeastern Oregon and westernmost-central Idaho [abs.]: EOS, Transactions of the American 

Geophysical Union, v. 62, p. 1087.

 1983, Pre-Tertiary tectonic evolution of northeastern Oregon and west-central Idaho: constraints based on U/Pb ages of 

zircons [abs.]: Geological Society of America Abstracts with 

Programs, v. 15, p. 371.

Waters, A.C., 1961, Stratigraphic and lithologic variations in the 

Columbia River Basalt: American Journal of Science, v. 259, 

p. 583-611.

Winkler, H.G.F., 1979, Petrogenesis of metamorphic rocks (5th 

ed.): New York, Springer-Verlag, 348 p.

BIO Mineral Resources of Wilderness Study Areas: East-Central Oregon
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

LU

o

0_

LU 

U

U_

O

_1

LU

ULJ

A

U/A

UNKNOWN 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.

B12 Mineral Resources of Wilderness Study Areas: East-Central Oregon
RESOURCE/RESERVE CLASSIFICATION

ECONOMIC

MARGINALLY 

ECONOMIC

SUBECONOMIC

IDENTIFIED RESOURCES

Demonstrated

Measured

Rese

Indicated

jrves

Marginal 

Reserves

I

i 

Demonstrated

Subeconomic

Resources

i

Inferred

Inferred 

Reserves

Inferred 

Marginal 

Reserves

Inferred 

Subeconomic 

Resources

UNDISCOVERED RESOURCES

Probability Range

Hypothetical Speculative

1

1

1 

1

1

1 

i

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 Sheep Mountain Wilderness Study Area, Oregon B13
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

r aleogene 

Subperiod

L~ retaceou s

Jurassic

Triassic

Permian

Pennsylvanian 

Carboniferous

Mississippian

Devonian

Silurian

Ordovician

Cambrian

uof\r\7\

EPOCH

Holocene

Pleistocene

Pliocene

Miocene

Oligocene

Eocene

Paleocene

Late

Early

Late 

Middle 

Early

Late 

Middle 

Early

Late 

Early

Late 

Middle 

Early

Late 

Early

Late 

Middle 

Early

Late 

Middle 

Early

Late 

Middle 

Early

Late 

Middle 

Early

AGE ESTIMATES OF 

BOUNDARIES IN 

MILLION YEARS (Ma)

0.010

1.7

5

3fl

55

 66

96 

1 ia

205

 290 

-330

 360 

 410

j\j\j

 j/\j

y\J\J

 1600

^j\j\j

IfWl

 3400

 Accr\ 

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

informal time term without specific rank.

B14 Mineral Resources of Wilderness Study Areas: East-Central Oregon

GPO 585-045/9753
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Maps

Geologic Quadrangle Maps are multicolor geologic maps on 

topographic bases in 71/2- or 15-minute quadrangle formats (scales mainly 1:24,000 or 1:62,500) showing bedrock, surficial, or engineering geology. Maps generally include brief texts; some maps include structure 

and columnar sections only.

Geophysical Investigations Maps are on topographic or planimetric 

bases at various scales; they show results of surveys using geophysical 

techniques, such as gravity, magnetic, seismic, or radioactivity, which 

reflect subsurface structures that are of economic or geologic significance. 

Many maps include correlations with the geology.

Miscellaneous Investigations Series Maps are on planimetric or 

topographic bases of regular and irregular areas at various scales; they 

present a wide variety of format and subject matter. The series also includes 71/2-minute quadrangle photogeologic maps on planimetric bases 

which show geology as interpreted from aerial photographs. Series also 

includes maps of Mars and the Moon.

Coal Investigations Maps are geologic maps on topographic or 

planimetric bases at various scales showing bedrock or surficial geology, stratigraphy, and structural relations in certain coal-resource areas.

Oil and Gas Investigations Charts show stratigraphic information 

for certain oil and gas fields and other areas having petroleum potential.

Miscellaneous Field Studies Maps are multicolor or black-andwhite maps on topographic or planimetric bases on quadrangle or irregular areas at various scales. Pre-1971 maps show bedrock geology 

in relation to specific mining or mineral-deposit problems; post-1971 

maps are primarily black-and-white maps on various subjects such as 

environmental studies or wilderness mineral investigations.

Hydrologic Investigations Atlases are multicolored or black-andwhite maps on topographic or planimetric bases presenting a wide range 

of geohydrologic data of both regular and irregular areas; principal scale 

is 1:24,000 and regional studies are at 1:250,000 scale or smaller.

Catalogs

Permanent catalogs, as well as some others, giving comprehensive listings of U.S. Geological Survey publications are available under 

the conditions indicated below from the U.S. Geological Survey, Books 

and Open-File Reports Section, Federal Center, Box 25425, Denver, 

CO 80225. (See latest Price and Availability List)

"Publications of the Geological Survey, 1879-1961" may be purchased by mail and over the counter in paperback book form and as a 

set of microfiche.

"Publications of the Geological Survey, 1962-1970" may be purchased by mail and over the counter in paperback book form and as a 

set of microfiche.

"Publications of the U.S. Geological Survey, 1971-1981" may be 

purchased by mail and over the counter in paperback book form (two 

volumes, publications listing and index) and as a set of microfiche.

Supplements for 1982,1983,1984,1985,1986, and for subsequent 

years since the last permanent catalog may be purchased by mail and 

over the counter in paperback book form.

State catalogs, "List of U.S. Geological Survey Geologic and 

Water-Supply Reports and Maps For (State)," may be purchased by mail 

and over the counter in paperback booklet form only.

"Price and Availability List of U.S. Geological Survey Publications," issued annually, is available free of charge in paperback booklet form only.

Selected copies of a monthly catalog " New Publications of the U. S. 

Geological Survey" available free of charge by mail or may be obtained 

over the counter in paperback booklet form only. Those wishing a free 

subscription to the monthly catalog "New Publications of the U.S. 

Geological Survey" should write to the U.S. Geological Survey, 582 

National Center, Reston, VA 22092.

Note.-Prices of Government publications listed in older catalogs, 

announcements, and publications may be incorrect. Therefore, the 

prices charged may differ from the prices in catalogs, announcements, 

and publications.