Mineral Resources of the Owyhee Canyon 

Wilderness Study Area, 

Malheur County, Oregon

U.S. GEOLOGICAL SURVEY BULLETIN 1719-E

Chapter E

Mineral Resources of the

Owyhee Canyon Wilderness Study Area,

Malheur County, Oregon

By JAMES G. EVANS, JAMES D. HOFFMAN, and DOLORES M. KULIK 

U.S. Geological Survey

J. MITCHELL LINNE 

U.S. Bureau of Mines

U.S. GEOLOGICAL SURVEY BULLETIN 1719

MINERAL RESOURCES OF WILDERNESS STUDY AREAS: 

OWYHEE RIVER REGION, IDAHO AND NEVADA
DEPARTMENT OF THE INTERIOR 

DONALD PAUL MODEL, Secretary

U.S. GEOLOGICAL SURVEY 

Dallas L. Peck, Director

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1987

For sale by the

Books and Open-File Reports Section

U.S. Geological Survey

Federal Center, Box 25425

Denver, CO 80225

Library of Congress Cataloging-in-Publication Data

Mineral resources of the Owyhee Canyon Wilderness Study 

Area, Malheur County, Oregon.

U.S. Geological Survey Bulletin 1719-E

Bibliography

Supt. of Docs. No.: I 19.3:1719-E

1. Mines and mineral resources Oregon Owyhee Canyon

Wilderness. 2. Owyhee Canyon Wilderness (Or.) I. Evans,

James George, 1938- . II. Series.

QE75.B9 No. 1719-E 557.3s

[553'.09795'97]

87-600297
STUDIES RELATED TO WILDERNESS

Bureau of Land Management Wilderness Study Area

The Federal Land Policy and Management Act (Public Law 94-379, 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 

Owyhee Canyon Wilderness Study Area (OR-003-195), Malheur County, Oregon.

CONTENTS

Summary El

Abstract 1

Character and setting 1

Identified mineral resources 1

Mineral resource potential 1 

Introduction 3

Area description 3

Previous and present investigations 3 

Appraisal of identified resources 3

Methods 3

History and production 5

Mineral resources 5 

Assessment of mineral resource potential 6

Geology 6

Geochemical studies 7

Geophysical studies 8

Mineral and energy resource potential 9 

References cited 10 

Appendixes

Definition of levels of mineral resource potential and certainty of assessment 15

Resource/reserve classification 16

Geologic time chart 17

FIGURES

1. Index map showing location of the Owyhee Canyon Wilderness Study Area, 

Malheur County, Oregon E2

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

Owyhee Canyon Wilderness Study Area, Malheur County, Oregon 4

3. Complete Bouguer gravity map of the Owyhee Canyon Wilderness Study Area 

and vicinity, Malheur County, Oregon 8

TABLE

1. Prospects and mineralized areas in the Owyhee Canyon Wilderness Study Area E18

MINERAL RESOURCES OF WILDERNESS STUDY AREAS: 

OWYHEE RIVER REGION, IDAHO AND NEVADA

Mineral Resources of the

Owyhee Canyon Wilderness Study Area,

Malheur County, Oregon

By James G. Evans, James D. Hoffman, and Dolores M. Kulik 

U.S. Geological Survey

J. Mitchell Linne 

U.S. Bureau of Mines

SUMMARY 

Abstract

The part of the 180,680-acre Owyhee Canyon 

Wilderness Study Area (OR-003-195) on which mineral 

surveys were requested encompasses 15Z,040 acres in 

southeastern Malheur County, Oreg. In this report any 

reference to the Owyhee Canyon Wilderness Study 

Area refers only to that part of the wilderness study 

area on which mineral surveys were requested by the 

Bureau of Land Management. A mineral resource 

survey of the area included geologic mapping, a 

geochemical survey of stream sediments, and a study 

of mines, prospects, and mineralized areas. 

Subeconomic sand and gravel resources with a 

combined total of approximately 100,000,000 cubic 

yards were identified in three areas. Two areas have 

low potential for silver, lead, and tin, the 

northernmost one of which may be in epithermal veins 

associated with fault zones. One area has low 

potential for tin. One area has low potential for 

geothermal resources. The whole study area has 

unknown potential for oil and gas. The sand and gravel 

resources and stream gravels in the Owyhee River 

have unknown potential for gold.

Character and Setting

The Owyhee Canyon Wilderness Study area is in 

the Owyhee Plateau southwest of Jordan Valley, 

southeastern Oregon (fig. 1). The area has a total 

relief of 2,400 ft and consists of broad flats and rolling 

hills cut by spectacular canyons incised to depths of 

1,000 ft. The area is underlain by flat-lying volcanic, 

pyroclastic, and sedimentary rocks of middle Miocene 

(see appendixes for geologic time chart) through 

Holocene age. Rhyolitic welded ash-flow tuff and

tuff breccia comprise the Swisher Mountain Tuff, the 

oldest and most abundant rock unit in the study area. 

Basalt overlies the rhyolite and forms low, rounded 

buttes that are probably the remains of shield 

volcanos. In the middle part of the area, the basalt 

and rhyolite are separated stratigraphically by a thin 

wedge of conglomerate, siltstone, tuff, and lacustrine 

deposits. In the northern part, the basalt is covered 

by conglomerate and silt. High-angle normal faults, 

ranging in age from Miocene through Holocene, are 

common in the northern part of the study area.

Identified Mineral Resources

Approximately 100,000,000 yd 3 of subeconomic 

sand and gravel resources were identified. Four of six 

samples of this material contain traces of gold. No 

metallic mineral resources were identified during this 

study. Several outcrops of lacustrine tuffaceous 

sediments were analyzed for zeolites; no zeolites 

were detected. Of 126 rock samples, 51 samples, or 

about 40 percent, contain detectable levels of either 

gold, silver, arsenic, antimony, ,or sulfur. Of the 51 

samples, 13 contain gold, 33 contain silver, 13 contain 

arsenic, 1 contains antimony, and 2 contain sulfur. 

However, these values were generally low and do not 

indicate the presence of resources. Oil and gas leases 

have been let, and subsequently terminated, on parts 

of the study area, but no geophysical exploration or 

drilling has been done. Trace amounts of gold were 

found in gravel samples from the Owyhee River.

Mineral Resource Potential

Two areas of the Owyhee Canyon Wilderness 

Study Area have low mineral resource potential for 

silver, lead, and tin. These commodities are 

associated with weak epithermal zones and faults close

E1
X^Three Forks Road 

I A '

Skull Creek 

Burns Junction Road

OWYHEE CANYON

WILDERNESS STUDY AREA

(OR-003-195)

Battle Mountain/- »* 

^ _ ^Antelope nt /\

Flats \

Antelope Flats Road

Oregon Hill / OREGON ) IDAHO

Figure 1 . Index, map showing location of the Owyhee Canyon Wilderness Study Area, Malheur County, 

Oregon.

to basaltic vents that are just outside the study area 

(Grassy Mountain, Sacramento Hill, fig. 2). The area 

associated with Black Butte basaltic vent has low 

potential for tin. Deposits of subeconomic sand and 

gravel resources and stream gravels along the Owyhee 

River have unknown potential for gold. Low 

potential for geothermal resources occurs along the

Owyhee Canyon south of Three Forks where several 

warm springs flow into the Owyhee River. These 

springs appear to be connected by faults to the chain 

of basaltic vents that trend northwest from Warm 

Springs Canyon through Grassy Mountain, both have 

low potential for geothermal energy because of 

relatively low temperature.

E2
INTRODUCTION 

Area Description

The Owyhee Canyon Wilderness Study Area (OR003-195) covers 152,040 acres along the Owyhee River, 

parts of the West Little Owyhee River and Big 

Antelope Creek, and adjacent parts of the Owyhee 

Plateau southwest of Jordan Valley, Oreg. (fig. 1). The 

terrain is generally flat to gently rolling, rising 

gradually from 3,400 ft at the north end near Rome, 

Oreg., to 5,800 ft at the south end at Anderson 

Crossing. Deeply incised canyons, especially the 

spectacular Owyhee Canyon, with maximum relief of 

1,100 ft, are cut into the plateau. The arid climate 

supports sagebrush, rabbit brush, and sparse native 

grasses, replaced locally by seedings of crested 

wheatgrass (by the U.S. Bureau of Land Management). 

Scattered juniper trees grow above 5,000 ft on the 

plateau; small groves of juniper are present in the 

Owyhee Canyon near Three Forks. Aspen and 

cottonwood grow near springs and in sheltered parts of 

the canyons. Riparian areas support mostly grasses, 

rushes, and sedges. The study area is used for cattle 

grazing and also supports mule deer, pronghorn 

antelope, wild horses, bighorn sheep, cougar, coyote, 

black bear, and numerous species of birds, small 

mammals, and reptiles, including the western 

diamondback rattlesnake.

Motor vehicle access to and within the area to 

many localities on the canyon rims is by dirt roads and 

jeep trails along the study area boundary. The main 

acces° roads southward from the Rome area include 

the Skull Creek Road, an unnamed dirt road on the 

northeast rim of the Owyhee Canyon, and the Three 

Forks Road, which connects U.S. Route 95 to Three 

Forks. Access to the northeast rim of the Owyhee 

Canyon south of Three Forks is by an unnamed dirt 

road over Brown Ridge toward C Ranch near the Idaho 

State line. The southern part of the area is best 

reached by the Antelope Flats Road, which joins U.S. 

Route 95 15 mi north of McDermitt, Nev. This road 

crosses the West Little Owyhee River at Anderson 

Crossing. Several jeep trails branch from this road to 

provide access to points in the southern and 

southwestern parts of the study area.

Previous and Present Investigations

The Owyhee Canyon Wilderness Study Area 

includes parts of the following 7.5-minute 

quadrangles: Scott Reservoir, Dry Creek Rim, Little 

Grassy Mountain, Indian Fort, Skull Creek, White 

Horse Butte, Squaw Flat, Three Forks, No Crossing 

Crossing, Drummond Basin, Deacon Crossing, Rawhide 

Springs, Rawhide Pocket, Beaver Charlie Breaks, 

Guadalupe Meadows, Stoney Corral, and Oregon 

Butte. The study area is in the west half of the Jordan 

Valley 1° by 2° sheet. Reconnaissance geologic 

mapping of this area was published by Walker and 

Repenning (1966) at the scale of 1:250,000 and Walker 

(1977) at the scale of 1:500,000 and by Evans (1987) at 

the scale of 1:62,500. Other workers (Hart, 1982; Hart 

and Aronson, 1983; Hart and Mertzman, 1983; Gray 

and others, 1983; Hart and others, 1984) studied the

stratigraphy and chemistry of volcanic rocks from 

southeastern Oregon and adjacent regions, including 

the Owyhee Canyon Wilderness Study Area.

The U.S. Geological Survey carried out field 

investigations in the study area during 1984 and 

1985. This work included geologic mapping at the 

scale of 1:24,000 for later compilation at 1:62,500 and 

geochemical sampling. Rock samples were collected 

for petrographic and chemical analysis in order to 

determine if the rocks showed evidence of 

hydrothermal mineralizing systems. The analytical 

data from 202 stream-sediment and 149 heavy-mineral 

concentrate samples are in Erickson and others 

(1986). The geologic field observations and 

geochemical data were evaluated with the information 

obtained by the U.S. Bureau of Mines to assess 

identified mineral resources (known) and the mineral 

resources potential (undiscovered) in the study area.

APPRAISAL OF IDENTIFIED RESOURCES

By J. Mitchell Linne 

U.S. Bureau of Mines

Methods

Malheur County records and BLM master title 

plats were examined for mining claims in the study 

area, and USBM library materials and production 

records were studied for additional information. A 

field search was conducted for mines, prospects, or 

mineralized zones in or near the study area. Thirteen 

alluvial (placer) and 148 rock (lode) samples were 

collected. Seven grab samples of alluvium consisting 

of from one to ten 14-in. pans each were collected 

from some of the larger gravel bars on the Owyhee and 

West Little Owyhee Rivers. Six bulk placer samples 

consisting of 1.2 ft each of loose gravel were taken 

from 6-ft vertical parts of three gravel deposits; 

these samples were concentrated in a sluice and 

analyzed at the Bureau of Mines, Western Field 

Operations Center placer laboratory. None of the bulk 

placer samples were from material at bedrock. All 

placer samples were further concentrated in the 

laboratory. Gold was extracted from the concentrates 

by handpicking and amalgamation. The concentrates 

were also checked for radioactivity and fluorescence. 

Rock (lode) samples were assayed for gold and silver 

by fire assay, or by fire assay and atomic absorption. 

Concentrations of other metallic elements, including 

arsenic, antimony, mercury, molybdenum, and 

uranium, were determined by atomic absorption. 

Several, samples from each locality were analyzed for 

40 elements (Ag, Al, As, Au, B, Ba, Be, Bi, Ca, Cd, 

Co, Cr, Cu, Fe, Ga, K, La, Li, Mg, Mn, Mo, Na, Nb, Ni, 

P, Pb, Pd, Pt, Sb, Sc, Si, Sn, Sr, Ta, Te, Ti, V, Y, Zn, 

and Zr) by semiquantitative spectrography to detect 

elements of possible significance. If anomalous 

amounts of economic elements were indicated, the 

sample was analyzed by more precise methods. A 

complete description of sampling and analysis 

techniques is given in Linne (1986). Detailed 

information is available from the U.S. Bureau of

E3
117°30' 117C 00'

42°30'

42° 15'

Tsm

3

Three Forks

APPROXIMATE BOUNDARY 

OF

OWYHEE CANYON

WILDERNESS STUDY AREA

(OR-003-195)

Tsm

5 MILES

Figure 2. Mineral resource potential and generalized geology of the Owyhee Canyon Wilderness Study Area, Malheur County, 

Oregon.

E4
EXPLANATION

I I Area of low mineral resource potential-See 

 appendix for definition of levels of mineral

resource potential and certainty of assessment

^.

x Identified resource See table 1 for description

Commodities

Ag Silver

Pb Lead

Sn Tin

Geo Geothermal

Geologic map units

QfI Fluviatile and landslide deposits (Quaternary) 

Tb Basalt (Tertiary) 

Tsm Swisher Mountain Tuff (Miocene)

 Contact

 Fault Dotted where concealed

x " Prospect See table 1 for description

A Unnamed peak

Figure 2. Continued.

Mines, Western Field Operations Center, E. 360 Third 

Avenue, Spokane, WA 99202.

History and Production

No mines are located within or adjacent to the 

study area. The closest operating mine is the 

DeLamar mine approximately 30 mi northeast of the 

area. This mine has produced silver and gold since 

1977. Within the McDermitt caldera complex, 30 mi 

southwest of the study area, 15,185 flasks of mercury 

were produced from the Bretz mine between 1931 and 

1968, 12,367 flasks were produced from the Opalite 

mine between 1927 and 1944, and 114,000 flasks were 

produced from the Cordero mine between 1933 and 

1970. Until January 20, 1987 mercury was produced at 

the McDermitt mine, which has the largest deposit of 

mercury in North America (Rytuba and Glanzman, 

1979, p. 103, 113). There is no projected date for 

reopening at this time.

Several mining companies have filed claims and 

explored the Rome beds deposits 15 mi northeast of 

the study area for zeolites and fluorspar. Limited 

amounts of zeolites were mined by Norton Company 

during the 1960's (Santini and LeBaron, 1982, p. 1).

Several mining companies have conducted 

geochemical sampling near Oregon Hill (fig. 1) and in 

the study area, where anomalous concentrations of

gold, tungsten, silver, and tin were identified by Gray 

and others (1983) and Bukofski and others (1984).

Prospecting for gem-grade chalcedony occurred 

at two sites (Nos. 1 and 8, table 1, fig. 2). Neither site 

has had any recent work.

As of 1984, there were no active mining claims in 

the study area. The most recent activity (1982) was 

confined to oil and gas leases, which covered 21,000 

acres of the northern part of the study area. These 

leases were terminated by 1984. No seismic 

refraction studies have been conducted, nor have any 

wells been drilled in or near the area.

Mineral Resources

Prospects and mineralized areas examined during 

this study are shown in figure 2 and described in the 

appendixes (see also Linne, 1986). Included in these 

sites are those identified by Gray and others (1983) and 

Bukofski and others (1984) as anomalous for gold, lead, 

silver, fluorine, molybdenum, or tin. Samples of 

altered and brecciated zones, chalcedony, and opalized 

material (fig. 2, Nos. 1, 4, 5, 7, 8, and 9) were 

collected near several of these anomalies. Of 116 rock 

samples from sites 7, 8, and 9, 42 samples, or about 36 

percent, contain gold concentrations as much as 0.02 

troy ounce per ton (oz/ton), silver as much as 0.2 

oz/ton, or arsenic as much as 29 parts per million. 

Nine samples of altered opalized vitrophyre and 

vitrophyre breccia from three sites in the West Little 

Owyhee River and Big Antelope Creek areas contain 

no significant mineral values. A trace of gold was 

found in six reconnaissance placer samples from the 

Owyhee River at Five Bars (table 1, No. 6).

Tuffs and tuffaceous sediments are present in 

the northern part of the study area and the area 

around Three Forks (fig. 2, No. 3). The sediments in 

the northern part of the area are equivalent to the 

zeolite-rich, informally named Rome beds. None of 

the 23 samples from outcrops of the tuffs around 

Three Forks contain zeolites. Nine composite samples 

prepared from 20 of these samples and 1 chip sample 

were analyzed for gold, silver, arsenic, mercury, and 

molybdenum. Seven samples contain gold between 

0.017 and 0.075 ppm, six contain traces of arsenic (3-5 

ppm), and three contain silver at 10.5 oz/ton (325 

ppm), 2.5 oz/ton (77 ppm), and 2.1 ppm. (Note: the 

high silver concentrations could not be duplicated on 

these samples by the laboratory of the U.S. Geological 

Survey in Denver (J.D. Hoffman, written commun., 

1986)). No antimony, mercury, or molybdenum were 

detected.

Areas of Quaternary sediments (Walker and 

Repenning, 1966) in the northern part of the study area 

were examined to determine the quantity of sand and 

gravel present. Three major areas were identified (fig. 

2, No. 2) where a total of approximately 100 million 

yd of subeconomic resources of sand and gravel cover 

about 1,200 acres.

Platy, rhyolitic welded tuff that could be used as 

decorative building stone is present over an area of 

about 2 mi west of Black Butte at the south end of 

the study area. The rock weathers to rectangular 

plates approximately 2 in. thick with sides ranging 

from approximately 1 to 4 ft long. It has no unique

E5
properties and since abundant, better quality material 

exists closer to markets, this material is not classified 

as a resource.

ASSESSMENT OF MINERAL RESOURCE POTENTIAL

By James G. Evans, James D. Hof fman, and 

Dolores M. Kulik 

U.S. Geological Survey

Geology

The Owyhee Canyon Wilderness Study Area is 

underlain by a flat-lying sequence of rhyolitic welded 

tuff and basalt and by poorly consolidated to 

unconsolidated sedimentary and tuffaceous rocks that 

range in age from Middle Miocene to Holocene. 

Geology shown in figure 2 is simplified from Evans 

(1987).

The oldest exposed rock unit in the study area is 

a rhyolitic, welded ash-flow tuff, named the Swisher 

Mountain Tuff by Eckren and others (1984, p. 24-32). 

These rocks were also described by Walker and 

Repenning (1966), Hart (1982, p. 218-220), and Hart 

and Aronson (1983). Along the Owyhee Canyon the 

Swisher Mountain Tuff consists of at least two cooling 

units. The rock is typically purplish gray to reddish 

brown on fresh surfaces, weathers to brown or reddish 

brown, is usually finely laminated, and has 10 to 50 

percent phenocrysts as much as 0.2 in. across. The 

phenocrysts are mostly alkali feldspar (orthoclase, 

anorthoclase, sanidme) and plagioclase (An30) and 

include minor amounts of quartz, pigeonite, 

hypersthene, magnetite, and zircon. The cryptocrystalline to glassy matrix consists of glass shards and 

deformed pumice fragments. In places the rock has 

abundant cavities as much as 2 ft long and 2 in. wide 

parallel to the laminae. The cavities are partly filled 

with white chalcedony and translucent bluish-gray 

hyalite. Parts of the unit consist of dark-gray to 

black, massive or layered vitrophyre, red, black, and 

yellow welded tuff breccia, and gray massive 

rhyolite. Minimum thickness of the Swisher Mountain 

Tuff in the study area is estimated at 1,000 ft. This 

estimate is greater than the thickness inferred for this 

formation by Ekren and others (1984, pi. 2). They 

suggested that these rocks were erupted from a vent 

at Juniper Mountain, southwest Idaho. Geophysical 

evidence presented below, however, suggests that the 

Swisher Mountain Tuff was deposited as a result of 

caldera-forming eruptions. Potassium-argon ages on 

samples of the Swisher Mountain Tuff range from 12.5 

to 14.6 million years (Ma) (Hart and Aronson, 1983; 

Neill, 1975), or middle Miocene (Palmer, 1983).

A sedimentary and pyroclastic unit that is 

generally very poorly exposed lies above the Swisher 

Mountain Tuff in the study area from about 5 mi north 

to 10 mi south of Three Forks. The basal part of the 

unit is commonly a moderately to poorly consolidated 

pale-brown conglomerate containin,' 10 to 20 percent 

rounded to angular pebbles of reddish-brown rhyolitic 

tuff similar to the Swisher Mountain Tuff, and lessabundant, highly polished chert pebbles from sources 

outside the study area. Upward in the section the beds

are generally progressively less consolidated and 

consist of pale-brown to white siltstone, lacustrine 

deposits, and rhyolitic tuff. White limestone and 

opaline chert lenses are present near the top of the 

unit. North of Three Forks, rhyolitic tuff as much as 

60 ft thick is interbedded with basalt flows indicating 

that rhyolitic and basaltic extrusion overlap in time. 

The upper 10 ft of the sedimentary and pyroclastic 

unit was baked red in places, where the rocks were 

relatively dry when they were covered by basalt. In 

other places, the upper contact is marked by 

hyaloclastite where the basalts covered a wet playa. 

The unit is as much as 300 ft thick. J.P. Bradbury 

(written commun., 1986) suggested a middle to late 

Miocene fossil age for a sample of diatomaceous chert 

from the upper part of the unit. A middle Miocene age 

is most likely for this unit as the youngest potassiumargon age for the underlying welded tuff below it and 

the oldest potassium-argon age for the overlying basalt 

are also middle Miocene.

Dark-gray to black, generally vesicular basalt 

unconformably overlies the Swisher Mountain Tuff and 

the overlying sedimentary and pyroclastic unit. The 

uppermost 100 ft of basalt at the north end of the 

study area overlies part of the Rome beds, a siltstone, 

lacustrine, and tuff sequence (Walker and Repenning, 

1966). The basalts have been described by Walker and 

Repenning (1966), Hart (1982, p. 42-74), Hart and 

Mertzman (1982, 1983), and Hart and others (1984). 

They contain less than 20 percent phenocrysts of 

labradorite and olivine partly altered to iddingsite. 

Diktytaxitic texture is common. In places the basalts 

have as much as 20 percent vesicles, and locally 

contain abundant white calcareous and zeolitic 

amygdules. Hart (1982, p. 357-368) determined that 

most of the basalts in the study area are tholeiites, or 

transitional to alkaline olivine basalt. One sample is a 

low potassium-high alumina olivine basalt. The basalts 

total as much as 600 ft thick, including lenses of 

interflow sediments as much as 30 ft thick, along 

Owyhee Canyon near the north end of the study area. 

The low, rounded buttes common near the study area 

are shield volcanos from which the basalt, at least the 

younger basalt, was extruded. Nine potassium-argon 

ages of basalt from the study area and vicinity were 

determined by Hart (1982, p. 45-46) and Hart and 

Mertzman (1983, p. 17); ages range from 11.5 to 1.5 

Ma or late middle Miocene to Pleistocene (Palmer, 

1983). An older potassium-argon age of 13.8±1.5 Ma, 

or middle Miocene, was reported from basalt located 

about 0.3 mi north of the study area (Fiebelkorn and 

others, 1983). Most dated samples in the study area 

are late Miocene. Regionally, the basalt appears to 

have been extruded during three main pulses: 0 to 2.5 

Ma, 3.5 to 6 Ma, and 7 to 10 Ma (Hart, 1982, p. 93; 

Hart and others, 1984, p. 193), all of which are present 

in the study area.

Pleistocene sedimentary deposits mostly 

northwest of Skull Creek consist of gravel, silt 

(possibly loess), and alluvial fan or delta deposits that 

overlie the basalt and rhyolite. The gravel contains 

rounded to angular clasts, as much as 1 ft across, of 

rhyolitic welded tuff, basalt, and polished chert 

pebbles. Deposits are as thick as 100 ft in the study 

area and were laid down by an ancestral Owyhee River 

system before the canyon was incised.

E6
Holocene deposits include unconsolidated 

landslide debris, alluvium along the Owyhee River and 

its tributaries, and older alluvium covering remnant 

stream terraces along the Owyhee River. The 

landslide deposits, covering as much as several square 

miles of the inner Owyhee Canyon, are most common 

in the north half of the study area between Three 

Forks and Skull Creek. The landslides are mostly 

basalt boulders, but include small playas formed in 

closed depressions on the landslides. The other 

Holocene deposits consist of sand and silt containing 

pebbles and cobbles of rhyolitic welded tuff, basalt, 

and chert.

Closely spaced, generally steep, normal faults 

strike northwest to east, close to the trend of the 

Owyhee Canyon (Evans, 1987). These faults were 

influential in localizing the Owyhee River along its 

present course. Stratigraphic separations along the 

faults vary from a few feet to a few hundred feet. 

The most conspicuous offset occurs 8 mi north of 

Three Forks where a fault runs along the canyon 

bottom. The opposing canyon rims differ in elevation 

by 300 ft, but the stratigraphic separation is more than 

1,000 ft (downthrown block on the northeast). The 

southern part of the study area along the West Little 

Owyhee River and Toppin Creek (fig. 1) has few faults 

that penetrate the basaltic rim rocks. Some of the 

faulting may be related to upwarping of the terrane in 

response to emplacement of basalt prior to venting. 

In the case of the Grassy Mountain vent system, 

however, the vents may have been localized along 

preexisting northwest-trending faults. Ages of the 

faults range from middle Miocene to Pleistocene.

Geochemical Studies

Reconnaissance geochemical studies were based 

on analysis and evaluation of 202 stream sediments, 

149 heavy-mineral concentrates from stream 

sediments, and 22 rock samples. The stream sediment 

and heavy-mineral concentrate analyses represent the 

chemistry of the rock material eroded from the 

drainage basin upstream from each sample site; 

moreover, analyses of heavy-mineral concentrates 

provide data about the chemistry of heavy minerals in 

the rock. Analytical data and a description of the 

sampling and analytical techniques used are given in 

Erickson and others (1986).

All samples were analyzed for 31 elements by a 

six-step semiquantitative emission spectrographic 

method (Myers and others, 1961); the heavy-mineral 

concentrate samples were analyzed for 31 elements by 

a different six-step semiquantitative emission 

spectrographic method (Grimes and Marranzino, 

1968). In addition, all the stream-sediment samples 

were analyzed for antimony, arsenic, bismuth, 

cadmium, and zinc by inductively coupled argon 

plasma-atomic emission spectrometry (Crock and 

others, 1983) using a modification of an atomic 

absorption technique (Viets, 1978). These analyses are 

used to identify drainages with anomalous 

concentrations of metals.

Anomalous values were determined by a 

combination of inspection of histograms and 

percentiles of the data.

Anomalous values of arsenic, bismuth, cadmium, 

lead, and silver in stream sediment samples, and tin in 

panned concentrate samples are present in the study 

area in three areas: (1) near Skull Creek (bismuth, 

cadmium, lead, silver, tin); (2) along the West Little 

Owyhee River south of Big Antelope Creek (arsenic, 

cadmium, lead, silver, tin); and (3) along the West 

Little Owyhee River at the south end of the study 

area (arsenic, cadmium, tin). The anomalous values of 

arsenic, bismuth, cadmium, lead, and silver in streamsediments are statistically derived, are close to the 

lower limits of detection, and are not much greater 

than crustal abundances (arsenic, Gualtieri, 1973, p. 

53; bismuth, Hasler and others, 1973, p. 96; cadmium, 

Wedow, 1973, p. 107; lead, Morris and others, 1973, p. 

317; silver, Heyl and others, 1973, p. 584). Tin in 

heavy-mineral concentrates occurs in the mineral 

cassiterite in amounts that are large relative to the 

crustal abundance of tin (Sainsbury and Reed, 1973, p. 

641), but not unusual for concentrations from ordinary 

silicic volcanic rock sources.

Broad zones of epithermally altered rocks were 

not found during this study although slight argillic 

alteration and silicification (deposition of chalcedony) 

were observed in moderately well lithified tuff breccia 

that is present between ash-flow sheets south of 

Three Forks. Clearly, devitrification has also 

occurred, as feldspar microlites are present in 

vitrophyre of the Swisher Mountain Tuff (Eckren and 

others, 1984, fig. 10). The slight alteration and 

devitrification may have been enough to release minor 

amounts of certain metals for transport and 

distribution by weak hydrothermal systems.

The proximity of the weak geochemical 

anomolies and the basalt vents strongly suggest that 

the anomalous areas are due to weak hydrothermal 

systems developed on the margins of the basaltic 

vents, which are outside the study area: (1) Grassy 

Mountain and associated vents, (2) Sacramento Hill, 

and (3) Black Butte. Some of the weak mineralization 

in the northernmost area may be a result of leakage of 

hydrothermal fluids along fault zones. The tin at 

Grassy Mountain and Sacramento Hill could have been 

transported from a source in subsurface rhyolitic 

welded tuff during mineralization, or could have been 

reconcentrated from Pleistocene sedimentary (eolian?) 

deposits that are mostly eroded. The tin in the third 

anomaly is from the welded tuff, and may have been 

epithermally concentrated in that area.

Low-level lead anomalies in the Big Antelope 

Creek area and near Anderson Crossing appear to be 

unrelated to other mineralization. These anomalies 

could be due to bullets or lead shot, as these areas 

have easy road access and are attractive to hunters.

The anomalies inferred by Gray and others (1983) 

and by Bukofski and others (1984) were not duplicated 

in this study at current U.S. Geological Survey levels 

of detection. The anomalous values they report for 

arsenic, barium, beryllium, cobalt, copper, fluorine, 

lead, manganese, mercury, molybdenum, nickel, tin, 

tungsten, uranium, and zinc are low and generally 

approximate crustal abundance. Their anomalies 

would be largely consistent with the model of weak 

mineralization associated with the three basalt vent 

systems mentioned above and with an additional zone 

of weak mineralization near the Toppin Butte basalt 

vent.

E7
43° 00

117° 00' 116° 00'

42° 00'

APPROXIMATE BOUNDA'k 

I OF

OWYHEE CANYON 

WILDERNESS STUDY AREA 

(OR-OCfr20 MILES

EXPLANATION

(e^x Gravity contour-In milligals. Hachured in 

^ ^ direction of low gravity value

Gravity station

J\ Area of anomaly discussed in text 

H Gravity high

Outcrops of Idaho batholithic rocks

Figure 3. Complete Bouguer gravity map of the Owyhee Canyon Wilderness Study Area and vicinity, Malheur County, 

Oregon.

No evidence was found bearing on whether any of 

these weakly mineralized zones comprise leakage 

anomalies from more mineralized rock at depth.

Geophysical Studies

Gravity and magnetic studies were undertaken as 

part of the mineral resource evaluation of the study 

area. The geophysical data provide information on the 

subsurface distribution of rock masses and the 

structural framework.

Gravity data were obtained from files 

maintained by the Defense Mapping Agency of the 

Department of Defense and supplemented by 

additional data obtained in 1985. Bouguer gravity 

anomaly values were computed using the 1967 gravity 

formula (International Association of Geodesy, 1967) 

and a reduction density of 2.67 g/cm (grams per cubic 

centimeter). Terrain corrections were made by 

computer for a distance of 167 km (100 mi) from the 

station using the method of Plouff (1977).

The area of extreme southwestern Idaho and 

southeastern Oregon is characterized by intermediate

E8
Bouguer values between -150 and -175 mGals over a 

predominantly rhyolitic terrane (fig. 3). Gravity 

values decrease southeastward across the area shown 

in figure 3 due to a greater thickness of low density 

material, presumably rhyolitic tuffs and welded tuffs. 

The low culminates east of 117° long where a roughly 

circular low gravity anomaly 20 to 25 mi in diameter 

(fig. 3, A) suggests an underlying volcanotectonic 

depression that may have been a source of the 

rhyolitic pyroclastic rocks at the surface, including the 

Swisher Mountain Tuff.

Higher gravity values (fig. 3, B) are associated 

with outcrops of rocks of the Idaho batholith east of 

the study area. The increasing gravity values over the 

northern part of the study area (fig. 3, C) suggest that 

similar rocks may be present in the surface here.

Aeromagnetic data west of 117° long is shown on 

a U.S. Geological Survey map (1972). Flightlines are 

east-west at approximately 2- to 3-mi spacing and 

9,000 ft barometric elevation. The magnitude of the 

aeromagnetic anomalies, range of total magnetic 

intensity values, and wavelengths of the anomalies are 

similar over a broad area of mixed rhyolites and 

basalts in southeastern Oregon and southwestern 

Idaho. In and near the study area magnetic lows seem 

to be associated with some areas of fracturing and 

faulting, particularly in the basalt unit, and suggest 

destruction of magnetic minerals by weathering or 

weak hydrothermal alteration along faults or near 

basalt vents. Rocks below the exposed Miocene 

volcanics may contribute to the aeromagnetic 

anomalies; the anomalies are not clearly related to 

potential mineral resources.

Mineral and Energy Resource Potential

Geologic studies, geochemical sampling, and 

examination of prospects, altered rock, and placer 

deposits indicate that the Owyhee Canyon Wilderness 

Study Area contains areas of low resource potential 

for silver, lead, and tin in zones of weak epithermal 

alteration. Anomalous concentrations of arsenic, 

bismuth, and cadmium indicate zones of weak 

epithermal alteration and mineralization. The 

presence of these rare elements in high concentrations 

could accompany precious and other metallic deposits, 

t>ut would not themselves be likely to constitute a 

minable resource. Thus, two areas have low potential 

for silver, lead, and tin and one area has potential for 

tin in epithermal zones, with certainty level C.

Several warm springs (88-95 °F) with an 

estimated combined flow of 1,000 gallons per minute 

(Waring, 1965, p. 41) issue from cracks in the Swisher 

Mountain Tuff near the mouth of Warm Springs Canyon 

south of Three Forks. These springs lie along the 

northwest trend of the Grassy Mountain vent system 

and are close to faults that may have controlled the 

locations of these vents. Deep circulation of water 

along the faults in an area of slightly elevated heat 

flow would explain the occurrence of the warm 

springs. However, it seems likely that the faults near 

the springs are connected with the Grassy Mountain 

vent system. The slightly elevated heat flow may be 

from a still-cooling basaltic magma at depth. This 

relation between the springs and the vents would be an

attractive hypothesis if the basalt on Grassy Mountain 

and related flows could be shown to be very young. 

The basalt at the base of the mountain is Miocene age 

(7-10 Ma) (Hart, 1982, p. 46), but a possible remnant 

vent crater, now a shallow playa, at the top of the 

mountain, suggests that the uppermost basalt is 

millions of years younger than the basalt at its base. 

If extrusive activity at the Grassy Mountain vent 

system had continued until late Pliocene or 

Pleistocene time, the warm springs could be reflecting 

residual heat from this volcanism.

The warm springs south of Three Forks are in a 

region characterized by young silicic volcanic rocks, 

some parts of which offer likely geologic settings for 

magmatic heat reservoirs (MacLeod and others, 

1976). However, most silicic magma sources in this 

part of southeastern Oregon are much older than 2 Ma, 

and probably have cooled to near ambient 

temperatures. The warm springs in the study area may 

be a result of young basaltic volcanism or of deep 

circulation in a region of slightly elevated heat flow. 

The small basalt vents, most likely extending from 

great depth, are poor targets for geothermal heat 

production, however. Therefore, the potential for 

geothermal resources south of Three Forks is low with 

certainty level C.

North of the study area in the western Snake 

River basin of Oregon and Idaho, wells were drilled 

for oil and gas. Study of these wells and the regional 

geology led Newton and Corcoran (1963) to conclude 

that' the depositional environments of the basin 

resembled other nonmarine basins that produce oil and 

gas and that hydrocarbons formed there during the late 

Miocene and early Pliocene. Late Miocene and early 

Pliocene time in the study area, however, was 

characterized by basaltic volcanism. Warner (1980) 

suggested that southeastern Oregon is underlain by 

deposits of Mesozoic and Tertiary age that contain 

potential source material for petroleum genesis. 

However, pre-Tertiary rocks exposed in the region at 

South Mountain in Idaho and the Pueblo Mountains, in 

Nevada and Oregon, are metasedimentary and 

metavolcanic rocks that do not appear to be potential 

petroleum sources. During this study no surficial 

evidence was found for inferring the presence of 

hydrocarbons at depth. Any hydrocarbon reservoirs 

that might be present beneath the middle Miocene 

Swisher Mountain Tuff probably would have to be 

located by geophysical methods.

Surface evidence of oil or gas resources was not 

found in or near the study area, although hydrocarbon 

concentrations at depth below the Swisher Mountain 

Tuff cannot be ruled out. The potential for oil and gas 

in the entire study area is unknown.

According to Bureau of Mines' data, four of six 

samples taken from the sand and gravel deposits 

contained traces of gold in concentrations less than 

0.000125 oz/yd . Because of limitations placed on the 

sampling program by the BLM, these deposits could 

not be adequately evaluated. Therefore these areas 

have been assigned unknown potential for gold. Trace 

amounts of gold, found in gravel samples at Five Bars, 

suggest that the Owyhee River gravels have unknown 

potential for gold.

Platy welded tuff (Swisher Mountain Tuff) at the 

south end of the study area is potentially useful as

E9
decorative building stone. However, it is structurally 

inferior to abundant andesite northwest of the study 

area that has been used for this purpose and is 15 mi 

closer to a major highway. If the andesite were 

depleted, the welded tuff might be considered an 

alternative source of decorative stone.

REFERENCES CITED

Bukofski, J.F., Lovell, J.S., and Meyer, W.T., 1984, 

Mineral resource, assessment through geochemical 

studies of heavy mineral concentrates from 

wilderness study areas in the Burns, Prineville, 

and Vale districts, southeastern Oregon: 

Barringer Resources, Inc., prepared for U.S. 

Department of the Interior, Bureau of Land 

Management, Oregon State Office, Portland, 94 p.

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

Determination of elements in National Bureau of 

Standards' Geological reference materials SRM 

278 obsidian and SRM 688 basalt by inductively 

coupled argon plasma-atomic emission 

spectrometry: Geostandards Newsletter, no. 7, p. 

335-340.

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

High-temperature, large-volume, lavalike ashflow tuffs without calderas in southwestern 

Idaho: U.S. Geological Survey Professional Paper 

1272, 76 p.

Erickson, M.E., Hoffman, J.D., and King, H.D., 1986, 

Analytical results and sample locality maps of 

stream-sediment, heavy-mineral-concentrate, and 

rock from the Owyhee Canyon (OR-003-195) and 

Honeycombs (OR-003-077A) Bureau of Land 

Management Wilderness Study Areas, Malheur 

County, Oregon: U.S. Geological Survey 

Open-File Report 86-628.

Evans, J.G., 1987, Geologic map of the Owyhee 

Canyon Wilderness Study Area, Malheur County, 

Oregon: U.S. Geological Survey Miscellaneous 

Field Studies Map MF-1926, scale 1:62,500.

Fiebelkorn, R.B., Walker, G.W., MacLeod, N.S., 

McKee, E.H., and Smith, J.G., 1983, Index to K-Ar 

determinations for the state of Oregon: 

Isochron/West, no. 37, p. 3-60.

Gray, J.J., Peterson, N.N., Clayton, Janine, and 

Baxter, Gary, 1983, Geology and mineral resouces 

of 18 BLM wilderness study areas, Harney and 

Malheur Counties, Oregon: Oregon Department of 

Geology and Mineral Resources Open-file report 

OF-83-2, BLM Contact no. YA-553-CT1-1086, 108

PGrimes, D.J., and Marranzino, A.P., 1968, Directcurrent arc and alternating current spark emission 

spectrographic field methods for the semiquantitative analysis of geologic materials: U.S. 

Geological Survey Circular 591, 6 p.

Gualtieri, J.L., 1973, Arsenic, in Brobst, D.A. and 

Pratt, W.P., eds., United States mineral 

resources: U.S. Geological Survey Professional 

Paper 820, p. 51-61.

Hart, W.K., 1982, Chemical, geochronologic, and 

isotopic significance of low-K, high-alumina 

olivine tholeiite in the northwestern Great Basin,

U.S.A.: Cleveland, Ohio, Case Western Reserve 

University, Ph.D. dissertation, 410 p.

Hart, W.K., and Aronson, J.L., 1983, K-Ar ages of 

rhyolites from the western Snake River Plain 

area, Oregon, Idaho, and Nevada: Isochron/West, 

no. 36, p. 17-19.

Hart, W.K., Aronson, J.L., and Mertzman, S.A., 1984, 

Areal distribution and age of low-K, high-alumina 

olivine tholeiite magmatism in the northwestern 

Great Basin: Geological Society of America 

Bulletin, v. 95, no. 2, p. 186-195.

Hart, W.K., and Mertzman, S.A., 1982, K-Ar ages of 

basalts from south-central and southeastern 

Oregon: Isochron/West, no. 33, p. 23-26.

 1983, Late Cenozoic volcanic stratigraphy of the 

Jordan Valley area, southeastern Oregon: Oregon 

Geology, v. 45, no. 2, p. 15-19.

Hasler, J.W., Miller, M.H., and Chapman, R.M., 1973, 

Bismuth, in Brobst, D.A. and Pratt, W.P., eds., 

United States mineral resources: U.S. Geological 

Survey Professional Paper 820, p. 95-98.

Heyl, A.V., Hall, W.E., Weissenborn, A.E., Stager, 

H.K., Puffett, W.P., and Reed, B.L., 1973, Silver, 

in Brobst, D.A. and Pratt, W.P., eds., United 

States mineral resources: U.S. Geological Survey 

Professional Paper 820, p. 581-603.

International Association of Geodesy, 1967, Geodetic 

Reference System, 1967: International 

Association of Geodesy Special Publication No. 3, 

116 p.

Linne, J.M., 1986, Mineral resources of the Owyhee 

Canyon Study Area, Malheur County, Oregon: 

U.S. Bureau of Mines Mineral Land Assessment 

Open-File Report MLA-22-8 6, 14 p.

MacLeod, N.S., Walker, G.W., and McKee, E.H., 1976, 

Geothermal significance of eastward increase in 

age of upper Cenozoic rhyolitic domes in 

southeastern Oregon, in Proceedings Second 

United Nations Symposium on the development 

and use of geothermal resources, v. 1: Washington, 

D.C., U.S. Government Printing Office, p. 465- 

476.

Morris, H.T., Heyl, A.V., and Hall, R.B., 1973, Lead, in 

Brobst, D.A. and Pratt, W.P., eds., United States 

mineral resources: U.S. Geological Survey 

Professional Paper 820, p. 313-332.

Myers, A.T., Haven, R.G., and Dunton, P.J., 1961, A 

spectrochemical method for semiquantiative 

analysis of rocks, minerals, and ores: U.S. 

Geological Survey Bulletin 1084-1, p. 207-229.

Neill, W.M., 1975, Geology of the southeastern Owyhee 

Mountains and environs, Owyhee County, Idaho: 

Stanford, Calif., Stanford University, M.S. thesis, 

59 p.

Newton, V.C., and Corcoran, R.E., 1963, Petroleum 

geology of the western Snake River Basin, 

Oregon-Idaho: Oregon Department of Geology 

and Mineral Industries Oil and Gas Investigations 

no. 1, 67 p.

Palmer, A.R., 1983, The decade of North American 

geology - 1983 geologic time scale: Geology, v. 

11, no. 9, p. 503-504.

Plouff, Donald, 1977, Preliminary documentation for a 

FORTRAN program to compute gravity terrain 

corrections based on topography digitized on a 

geographic grid: U.S. Geological Survey OpenE10
File Report 77-535.

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

mercury, uranium, and lithium deposits to the 

McDermitt Caldera Complex, Nevada-Oregon: 

Nevada Bureau of Mines and Geology Report no. 

33, p. 109-117. Sainsbury, C.L., and Reed, B.L., 

1973, Tin, in_ Brobst, D.A. and Pratt, W.P., eds., 

United States mineral resources: U.S. Geological 

Survey Professional Paper 820, p. 637-651.

Santini, K.N., and LeBaron, M.R., 1982, Geology of the 

Rome zeolite deposit, Malheur County, Oregon: 

Society of Mining Engineers Preprint no. 82-392, 

10 p.

U.S. Geological Survey, 1972, Aeromagnetic map of 

the Adel and parts of the Burns, Boise, and Jordan 

Valley 1° by 2° quadrangles, Oregon: U.S. 

Geological Survey Open-File Report 72-390, scale 

1:250,000.

Viets, J.G., 1978, Determination of silver, bismuth, 

cadmium, copper, lead, and zinc in geologic 

materials by atomic absorption spectrometry with 

trieaprylylonethyl ammonium chloride: Analytical 

Chemistry, v. 50, p. 1097-1101.

Walker, G.W., 1977, Geologic map of Oregon east of 

the 121st meridian: U.S. Geological Survey 

Miscellaneous Investigations Series Map 1-902, 

scale 1:500,000.

Walker, G.W., and Repenning, C.A., 1966, 

Reconnaissance geologic map of the west half of 

the Jordan Valley Quadrangle, Malheur County, 

Oregon: U.S. Geological Survey Miscellaneous 

Investigations Series Map 1-457, scale 1:250,000.

Waring, G.A., 1965, Thermal, springs of the United 

States and other countries of the world a 

summary: U. S. Geological Survey Professional 

Paper 492, 383 p.

Warner, M.M., 1980, Southern Idaho, northern Nevada, 

southeastern Oregon a prime regional target for 

petroleum exploration: Wyoming Geological 

Association Earth Science Bulletin, v. 13, no. 1, p. 

19-40.

Wedow, Helmuth, Jr., Cadmium, in Brobst, D.A., and 

Pratt, W.P., eds., United States mineral 

resources: U.S. Geological Survey Professional 

Paper 820, p. 105-109.

E11

APPENDIXES

DEFINITION OF LEVELS OF MINERAL RESOURCE POTENTIAL 

AND CERTAINTY OF ASSESSMENT

Definitions of Mineral Resource Potential

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 

few or no indications of having been mineralized.

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 of resource accumulation, and (or) where an application of 

mineral-deposit models indicates favorable ground for the specified type(s) of deposits,

HIGH mineral resource potential is assigned to areas where geologic, geochemical, and geophysical 

characteristics indicate a geologic environment favorable for resource occurence, where interpretations of 

data indicate a high degree of likelihood for resource accumulation, where data supports 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.

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

moderate, or high levels of resource potential.

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

Levels of Certainty

t

U/A

UNKNOWN

POTENTIAL

H/B

HIGH POTENTIAL

M/B 

MODERATE POTENTIAL

L/B

LOW

POTENTIAL

H/C

HIGH POTENTIAL

M/C 

MODERATE POTENTIAL

L/C

LOW

POTENTIAL

H/D

HIGH POTENTIAL

M/D 

MODERATE POTENTIAL

L/D

LOW POTENTIAL

N/D

NO POTENTIAL

B C 

LEVEL OF CERTAINTY

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

B. Available information 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.

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.

E15
RESOURCE/RESERVE CLASSIFICATION

ECONOMIC

MARGINALLY 

ECONOMIC

SUBECONOMIC

IDENTIFIED RESOURCES

Demonstrated

(Measured Indicated

I 

Reserves

I

I 

Marginal 

Reserves

I

I

Demonstrated 

Subeconomic 

Resources.

i

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 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.

E16
GEOLOGIC TIME CHART 

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

EON

Phanerozoic

Proterozoic

Archean

pre-Arc

ERA

Cenozoic

Mesozoic

Paleozoic

Late Proterozoic

Middle Proterozoic

Early Proterozoic

Late Archean

Middle Archean

Early Archean 

hean 2

PERIOD

Quaternary

Tertiary

Neogene 

Subperiod

Paleogene 

Subperiod

Cretaceous

Jurassic

Triassic

Permian

Carboniferous 

Periods

Pennsylvanian

Mississippian

Devonian

Silurian

Ordovician

Cambrian

 -(3800?)- -

EPOCH

Holocene

Pleistocene

Pliocene

Miocene

Oligocene

Eocene

Pateocene

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 Ma)

- 0.010

- 1.7

1 A

 OQ

\J\J

cc

- 96

1 OO

one

- -240

^«/w

_ ,^oon

- 1RO ouvs 

- 410

- 4*3R HO3

- 500

_ *^, c~7m

ann

- 1600

 oRnn

- 3000

w^W

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

Informal time term without specific rank.

E17
m

00

TABLE 1. Prospects and mineralized areas in the Owyhee Canyon Wilderness Study Area

Map No. 

fig. 2) Name Summar y Workings Sample and resour

Unnamed prospect

Sand and 

gravel

Three Forks, 

tuff

Chalcedony vein as much as 1 ft thick Hand dug pits 

exposed intermittently for 1,000 ft.

Three gravel deposits covering about Bulldozer 

1,200 acres. Terraces of smooth, scrapes 

well-rounded unseated sand and gravel 

as much as 3 in. in diameter. The 

material consists of basalt, 

rhyolite, chert, and quartzite.

Light-gray, buff, green, or brown tuff None 

and tuffaceous sediments with minor 

limonite and some clay. Outcrops as 

thick as 60 ft are exposed intermittently for approximately 3 mi>

One sample contains 0.3 oz/ton silv

Chalcedony excavated from pits.

Approximately 100 million yd 3 O f san

prospects. Four of six samples co

(less than 0.000125 oz/yd 3 ).

Twenty-three samples along the 3-mi 

chip, 1 grab, and 1 select. All a

No zeolites were detected. Nine c

representing 20 chip samples were 

arsenic, antimony, mercury, and mo

composites and one chip sampled co

0.075 ppm gold and three contain 1

and 2.1 ppm silver. Six samples c

of arsenic. Of seven samples that 

five also contain gold. One sampl

and arsenic.

4 Owhyee RiverThree Forks

5 Antelope Creek

6 Five Bars

7 West Little 

Owyhee River

8 Toppin Creek

9 Upper Owyhee 

River

O

00 

Ln 

I O 

-Poo

-eo

10 Pole Creek

Altered vitrophyre, opal, tuff, and None 

exposed thickness of ^5 ft.

Altered vitrophyre and altered None 

rhyolite; exposed thickness of 50 ft.

Gravel bars 3 to 10 ft thick None

Vitrophyre breccia, rhyolite, altered None 

vitrophyre with limonite and 

chalcedony and stream gravel.

Altered vitrophyrye as thick as 5 ft Shallow bullwith pods, veinlets, and a thin dozer scrapes 

capping of chalcedony and jasperoid in chalcedony, 

geodes. Vitrophyre occurs within 

rhyolite with minor limcnite and is 

exposed for 1.5 mi.

Altered and brecciated vitrophyre and None 

rhyolite breccia with jasperoid 

geodes, overlain by partially 

opalized, tuffaceous sandstone and 

siltstone. Vitrophyre is underlain 

by and locally contains pods of 

rhyolite. Main exposure is 

approximately 150 ft thick. 

Intermittent exposure of 8 mi.

Rhyolite outcrop. None

Three chip samples contain no detec

Two chip samples contain no detecta

Six grab samples: all contain a tra

extremely fine and flat indicating 

Two grab samples of stream gravel c

Three samples of altered vitrophyr

from one site contain 0.1 oz silve

samples contain a trace gold.

Fifty-three samples: 32 chip, 18 se

select samples, one of chalcedony 

contain a trace of gold. Seven ch

vitrophyre, altered vitrophyre, an

silver; three select samples of ch

22 to 29 ppm arsenic.

Sixty samples along a 3-mi intermit

random chip, 5 select. Five chip 

altered vitrophyre and rhyolite co

gold. Nine chip samples of altere

sediments, and rhyolite; six rando

vitrophyre, rhyolite, and vitrophy

samples of altered vitrophyre, and 

silver. One 2-ft chip sample of o

17.8 ppm arsenic; one 23-ft chip s

and rhyolite contains 7 ppm arseni

chalcedony contains 15.3 PPm arsen

One sample contains H.2 ppm uranium