Mineral Resources of the Upper Leslie Gulch 

and Slocum Creek Wilderness Study Areas, 

Malheur County, Oregon

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Chapter D

Mineral Resources of the Upper Leslie Gulch 

and Slocum Creek Wilderness Study Areas, 

Malheur County, Oregon

By DEAN B. VANDER MEULEN, ANDREW GRISCOM, and 

HARLEY D. KING 

U.S. Geological Survey

JOHN R. BENHAM 

U.S. Bureau of Mines

U.S. GEOLOGICAL SURVEY BULLETIN 1741

MINERAL RESOURCES OF WILDERNESS STUDY AREAS: 

EAST-CENTRAL OREGON
DEPARTMENT OF THE INTERIOR 

MANUEL LUJAN, JR., Secretary

U.S. GEOLOGICAL SURVEY 

Dallas L. Peck, Director

Any use of trade, product, or firm names

in this publication is for descriptive purposes only

and does not imply endorsement by the U.S. Government

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1989

For sale by the

Books and Open-File Reports Section

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Library of Congress Cataloging-in-Publication Data

Mineral resources of the Upper Leslie Gulch and Slocum Creek Wilderness 

Study Areas, Malheur County, Oregon / by Dean B. Vander Meulen . . . [et 

al.]

p. cm. (U.S. Geological Survey bulletin ; 1741-D)

Bibliography: p.

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

1. Mines and mineral resources Oregon Upper Leslie Gulch Wilderness.

2. Mines and mineral resources Oregon Slocum Creek Wilderness.

3. Upper Leslie Gulch Wilderness (Or.) 4. Slocum Creek Wilderness (Or.) 

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

QE75.B9 no. 1741-D 557.3 s dc20 89-600182 

(TN24.07] [5531 .09795'97] CIP
STUDIES RELATED TO WILDERNESS 

Bureau of Land Management Study Area

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

1976) requires the U.S. Geological Survey and the U.S. Bureau of Mines to conduct mineral surveys on certain areas to determine the mineral values, if any, that may be present. 

Results must be made available to the public and be submitted to the President and the 

Congress. This report presents the results of a mineral survey of the Upper Leslie Gulch 

and Slocum Creek Wilderness Study Areas (OR-003-074 and OR-003-075), Malheur 

County, Oregon.

CONTENTS

Summary Dl 

Abstract Dl 

Character and setting Dl 

Identified resources D3 

Mineral resource potential of the Upper Leslie Gulch Wilderness Study

Area D3

Mineral resource potential of the Slocum Creek Wilderness Study Area D3 

Introduction D5

Area description D5 

Previous and present investigations D6 

Appraisal of identified resources D6

Mines, prospects, claims, and mineralized areas D6 

Appraisal of mineral resources D6 

Assessment of mineral resource potential D7 

Geology D7

Caldera evolution D7 

Stratigraphy D8 

Geochemical studies D9 

Geophysical studies DIG 

Mineral and energy resource potential of the Upper Leslie Gulch Wilderness

Study Area D12 

Mineral and energy resource potential of the Slocum Creek Wilderness Study

Area D13 

References cited D14 

Appendixes

Definition of levels of mineral resource potential and certainty of

assessment D17 

Resource/reserve classification D18 

Geologic time chart D19

FIGURES

1. Index map showing location of Upper Leslie Gulch and Slocum Creek Wilderness Study Areas, Malheur County, Oregon D2

2. Map showing mineral resource potential and generalized geology of Upper Leslie 

Gulch and Slocum Creek Wilderness Study Areas, Malheur County, 

Oregon D4

3. Bougucr gravity anomaly map of Owyhee volcanic field, eastern Oregon Dll

Contents

Mineral Resources of the Upper Leslie Gulch 

and Slocum Creek Wilderness Study Areas, 

Malheur County, Oregon

By Dean B. Vander Meulen, Andrew Griscom, and Harley D. King 

U.S. Geological Survey

John R. Benham 

U.S. Bureau of Mines

SUMMARY

Abstract

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

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

conducted field studies of 3,000 acres and 7,600 acres, respectively, in the contiguous Upper Leslie Gulch and Slocum 

Creek Wilderness Study Areas (OR-003-074 and 

OR-003-075); the areas are located east of the Owyhee 

Reservoir in eastern Oregon. In this report, the study areas 

are referred to, individually or collectively, as the "wilderness 

study area" or simply the "study area." Fieldwork was conducted by the U.S. Geological Survey and the U.S. Bureau of 

Mines during 1987 to evaluate the identified mineral resources (known) and the mineral resource potential (undiscovered) of the study area.

No energy or mineral resources were identified in either 

study area. Areas underlain by peralkaline rhyolite ash-flow 

and air-fall tuff and by intrusions associated with the Mahogany Mountain caldera have moderate potential for uranium 

and thorium resources and low potential for zinc resources. 

These tuffs are exposed in the west-central and eastern parts 

of the study area. All of the tuffs and rhyolite intrusions 

exposed in the study area have low potential for lithium 

resources. Rhyolite dikes and plugs that intrude both study 

areas have moderate potential for gold, silver, mercury, and 

zinc resources. North-trending fault zones within and adjacent to the Slocum Creek Wilderness Study Area have moderate potential for geothermal resources. The Upper Leslie 

Gulch Wilderness Study Area has unknown potential for 

geothermal resources. The Upper Leslie Gulch and Slocum 

Creek Study Areas have no potential for oil and gas resources.

Manuscript approved for publication, May 26, 1989.

Character and Setting

The Upper Leslie Gulch and Slocum Creek Wilderness 

Study Areas are located within the Mahogany Mountain 

caldera approximately 50 mi west of Boise, Idaho, and 25 

mi north of Jordan Valley, Oregon (fig. 1). The study area 

includes the southeastern part of the caldera-collapse structure north of Mahogany Mountain. This part of the caldera 

forms a topographic low characterized by north- and northwest-trending ridges and intervening, deeply eroded canyons.

The study area is underlain by a thick sequence of 

middle Miocene intracaldera ash-flow and air-fall tuff. 

Time spanned by the middle Miocene is herein considered 

to extend from approximately 11.2 to 16.6 million years 

before present, or Ma (see appendixes for geologic time 

chart). The intracaldera ash-flow and air-fall tuff erupted 

from a vent located near the center of the Mahogany 

Mountain caldera, contemporaneously with the collapse of 

the caldera. These caldera-forming tuff units (Tlga and 

Tlgi, fig. 2) are the oldest rocks exposed in the study area. 

The central-vent structure was subsequently filled by intrusions of rhyolite breccia and dikes (Trvi, fig. 2) and is 

presently exposed in the north-central part of the study 

area.

The central and eastern parts of the study area were 

uplifted, domed, and faulted by post-caldera resurgent activity. Two post-caldera ash-flow tuff units unrelated to the 

Mahogany Mountain caldera ponded in the caldera following resurgent activity. A north-trending graben cuts the 

resurgent dome near the center of the study area and is 

filled with post-caldera tuff (Tsc and Tbc, fig. 2). Basalt 

dikes, plugs, and north-trending rhyolite dikes, Tbi and Trp

Upper Leslie Gulch and Slocum Creek Wilderness Study Areas, Oregon D1
117° 30 1

43° 30 1 -

117° 15' 117° 00'

43° 15' -

43°00'

A To Oregon 

j X Hwy 20, 

i 37 miles

APPROXIMATE BOUNDARY

OF THE HONEYCOMBS 

1 VOLCANIC CENTER . 

<APPROXIMATE MARGIN

OF THREE FINGERS

CALDERA

Red 

Butte

x A 4585 

Red Butte 

prospect 

(gold)

To Oregon/ 

Hwy 78 I

X BannocR" 

prospect 

(gold)

APPROXIMATE

BOUNDARY OF SLOCUM

CREEK WILDERNESS

STUDY AREA

(OR-003-075)

APPROXIMATE BOUNDARY

OF UPPER LESLIE

GULCH WILDERNESS

STUDY AREA

(OR-003-074)

y

y A MAHOGANY

MOUNTAIN

6522

APPROXIMATE MARGIN

OF MAHOGANY

MOUNTAIN

CALDERA

Spring

Mountain

A

5998

JORDAN 

CRATERS 

LAVA FIELD

Table

Mountain

A

5050

To Jordan 

Valley, I 

5 miles '

Figure 1. Index map showing location of Upper Leslie Gulch and Slocum Creek Wilderness Study Areas, Malheur County, 

Oregon. Caldera margins and nearby prospects also shown; inferred location of caldera margins shown by queries.

D2 Mineral Resources of Wilderness Study Areas: East-Central Oregon
respectively, as shown on figure 2, intrude the calderaforming and post-caldera tuffs and are the youngest volcanic rocks in the study area.

Identified Resources

No mines, claims, or prospects were found in either 

study area. There are no identified mineral or energy resources within or adjacent to the study areas. Sand and 

gravel deposits along drainages within the study areas do 

not constitute resources because similar deposits are more 

accessible and closer to existing markets. Welded parts of 

the caldera-forming tuff exposed within the study areas are 

suitable for construction purposes; however, because similar tuff elsewhere is more accessible and closer to existing 

markets, this tuff does not constitute a resource.

Mineral Resource Potential of the

Upper Leslie Gulch Wilderness Study Area

Parts of the Upper Leslie Gulch Wilderness Study 

Area underlain by caldera-forming rhyolite ash-flow and 

air-fall tuff associated with the Mahogany Mountain 

caldera have moderate potential for uranium and thorium 

resources. The resource potential rating is based on anomalous concentrations in four rock samples and on highly 

anomalous aerial gamma-ray values for uranium and thorium reported over the study area.

Six samples of caldera-forming and post-caldera tuff 

collected 2 mi north of the study area contain high concentrations of lithium. One sample of a rhyolite dike collected 

along the western boundary of the study area also contains 

anomalous concentrations of lithium. All tuffs in the study 

area have low potential for stratiform lithium resources.

Anomalous concentrations of zinc were detected in 43 

samples of caldera-forming rhyolite tuff and 10 samples of 

vent rhyolite breccia. Approximately one-third of these 

samples were collected within the study area and the remaining samples were collected north and west of the study 

area. Therefore, parts of the study area underlain by 

caldera-forming tuff have low potential for zinc in volcanogenie disseminated deposits.

Rhyolite dikes that intrude the western part of the 

Upper Leslie Gulch Wilderness Study Area are locally 

brecciated and cut by quartz veins. Chemical analyses of 

the dikes indicate that the brecciated rhyolite contains 

anomalous concentrations of mercury, zinc, and arsenic. 

Although no gold and silver were detected in the samples 

analyzed, mercury and arsenic are pathfinders for these 

resources. Gold and silver resources have been discovered 

in similar rhyolite intrusions 6 mi north of the study area. 

Therefore, the rhyolite dikes have moderate potential for 

gold, silver, mercury, and zinc resources.

Several thermal springs and wells are located approximately 17 mi north of the study area. The springs are

underlain by a thermal water reservoir less than 3,280 ft 

below the surface. A thermal spring approximately 6 mi 

west of the study area (fig. 1) issues from a north-trending 

normal fault. No north-trending fault zones were mapped 

in the study area, although similar faults may be buried by 

alluvium along the length of Leslie Gulch. Consequently, 

the study area has unknown potential for geothermal energy.

Parts of the caldera-forming and post-caldera ash-flow 

and air-fall tuff (fig. 2) that underlies most of the study area 

are suitable for construction purposes; however, similar 

deposits elsewhere are more accessible and closer to existing markets.

There is no potential for oil and gas resources in the 

Upper Leslie Gulch Wilderness Study Area.

Mineral Resource Potential of the 

Slocum Creek Wilderness Study Area

Parts of the study area underlain by caldera-forming 

rhyolite ash-flow tuff, air-fall tuff, and rhyolite intrusions 

have moderate potential for uranium and thorium resources 

on the basis of anomalous concentrations of uranium and 

thorium contained in four samples of tuff and two samples 

of rhyolite intrusions. The tuffs and intrusions are exposed 

over approximately one-third of the study area.

Six samples of caldera-forming and post-caldera tuff 

collected 4 mi northeast of the study area contain high 

concentrations of lithium. Samples of rhyolite dikes collected in the western and northeastern parts of the study 

area also contain anomalous concentrations of lithium. All 

tuffs in the study area have low potential for lithium resources in stratiform deposits.

Anomalous concentrations of zinc were detected in 43 

samples of caldera-forming rhyolite tuff and 10 samples of 

vent rhyolite breccia. Approximately two-thirds of these 

samples were collected within the study area and the remaining samples were collected north and east of the study 

area. Therefore, parts of the study area underlain by 

caldera-forming tuff and the vent breccia have low potential for zinc in volcanogenic disseminated deposits.

Rhyolite dikes that intrude the eastern and northwestem parts of the Slocum Creek Wilderness Study Area are 

locally brecciated and cut by quartz veins. Chemical analyses of the dikes indicate that the brecciated rhyolite contains anomalous concentrations of mercury, zinc, and arsenic. Although no gold and silver were detected in the 

samples analyzed, mercury and arsenic are pathfinders for 

these resources. Gold and silver resources have been discovered in similar rhyolite intrusions 10 mi northeast of the 

study area. Therefore, the rhyolite dikes have moderate 

potential for gold, silver, mercury, and zinc resources.

Several thermal springs and wells are located approximately 16 mi north of the study area. The springs are 

underlain by a thermal water reservoir less than 3,280 ft

Upper Leslie Gulch and Slocum Creek Wilderness Study Areas, Oregon D3
sI

IS1

117°20'

APPROXIMATE BOUNDARY OF

UPPER LESLIE GULCH 

WILDERNESS STUDY AREA

(OR-003-074)

U/AGeo 

(entire study area)

Trp

IL-M/CGeo 

Tlga: 1

APPROXIMATE

BOUNDARY

OF SLOCUM

CREEK 

WILDERNESS

STUDY AREA /TSC

(OR-003-075)
EXPLANATION

AgAu 

Geo

Hg

Area having moderate resource potential (M)Area having moderate mineral resource potential (M), certainty level B, for 

Au, Ag, Hg, and Zn

Area having low (L) mineral resource potentia] 

Levels of certainty of assessment

Data not adequate for determination of potentialData only suggest level of potential

Data give good indication of level of potential

Commodities

SilverGold

Geothermal energy

Mercury

Li

Th

U

Lithium 

Thorium 

Uranium

Geologic map units

Qal Alluvium (Quaternary)

Qc Colluvium deposits (Quaternary)

Qls Landslide deposits (Quaternary)Ts Sedimentary rocks (Tertiary)Trp Rhyolite porphyry (Tertiary)

Tbi Basalt intrusions (Tertiary)

Tbc Tuff of Birch Creek (Tertiary)Tsc Tuff of Spring Creek (Tertiary)

Trvi Rhyolite vent intrusion (Tertiary)Tlga Leslie Gulch Tuff; air-fall facies (Tertiary)

Tlgi Leslie Gulch Tuff; intracaldera facies (Tertiary)

Trf Rhyolite flows (Tertiary)

Contact Dashed where approximately located

 * Fault Dashed where inferred; dotted where concealed; ball and bar on 

	downthrown side

2. Mineral resource potential and generalized geology of Upper Leslie Gulch and Slocum Creek Wilderness Study Areas, Malheur County, Oregon.

ro 

w

O 

ro c/> 

n-it 5"r-ho'

3

111

n S < =

p- 3 a 5

z

73

O

D

CO

z

00
The area is drained by several small creeks that flow north 

and northwest into Leslie Gulch and the Owyhee Reservoir. 

The climate is semiarid and the average annual precipitation is 10 in. or less. Vegetation is sparse and includes 

sagebrush and grasses at all elevations. Juniper trees grow 

along creek bottoms and in protected gulches.

Previous and Present Investigations

Previous geologic investigations that include the study 

area are a reconnaissance geologic map of the Owyhee 

region by Kittleman and others (1967) and an aeromagnetic 

survey by Boler (1979). An airborne radiometric and 

aeromagnetic survey of the study area was conducted by 

Geometries, Inc. (1979) during the U.S. Department of 

Energy's National Uranium Resource Evaluation (NURE) 

program. A preliminary evaluation of the mineral resources of the Slocum Creek Wilderness Study Area was 

made by the Oregon Department of Geology and Mineral 

Industries (DOGAMI) (Gray and others, 1983).

The U.S. Geological Survey mapped the geology of 

the study area in the summers of 1984-85 and 1987 

(Vander Meulen and others, 1987a, d). A caldera collapse 

structure, referred to as the Mahogany Mountain caldera, 

which underlies both study areas (fig. 1) was first recognized in 1984 (Rytuba and others, 1985). Field investigations during 1984 and 1985 involved detailed geologic 

mapping at scales of 1:24,000 and 1:18,000 of the resurgent 

dome and vent complex located near the center of the 

caldera (Vander Meulen, 1988; Vander Meulen, 1989). 

The U.S. Geological Survey conducted a combined geologic, geochemical, and geophysical survey of the Upper 

Leslie Gulch and S locum Creek Wilderness Study Areas 

during 1987. Field investigations focused on correlating 

geochemical and geophysical anomalies with rock units 

and geologic structures. Rock, stream-sediment, and nonmagnetic heavy-mineral concentrates of stream-sediment 

samples were analyzed for 31 elements by semiquantitative 

emission spectrography. Atomic absorption was used to 

analyze for gold and mercury, and the presence of uranium 

was determined by ultraviolet fluorescence. Earlier geophysical gravity data were supplemented by data from 

additional gravity stations within and near the study area 

(Plouff, 1987).

The U.S. Bureau of Mines investigation consisted of 

prefield and field studies (Benham, 1988). During the 

prefield study, mining-related data were examined. These 

data were gathered from the libraries and records of the 

U.S. Bureau of Mines and the U.S. Bureau of Land Management; state, county, and other government agencies; and 

private sources. Field studies included a ground reconnaissance to search for evidence of possible mineralized areas 

and unrecorded mining activity. Additional information is 

available from the U.S. Bureau of Mines, Western Field 

Operations Center, E. 360 Third Avenue, Spokane, WA 

99202.

APPRAISAL OF IDENTIFIED RESOURCES

By John R. Benham 

U.S. Bureau of Mines

Mines, Prospects, Claims, and Mineralized 

Areas

Gold placer claims were located south of Leslie Gulch 

(fig. 1) around 1906 (Scott, 1986). Other than the placer 

claims, no records of mining or mineral exploration were 

found in the literature, nor was any evidence of exploration 

or mining-related activity discovered during surface and 

aerial reconnaissance of the study area.

The nearest active mine is the Wild Horse mine located 

approximately 9 mi north of the study area (Scott, 1986, p. 

12). Picture jasper is produced from this mine on a seasonal basis. A zeolite mineral (clinoptilolite) and bentonite 

are currently being mined 18 mi northeast of the study area 

by Teague Mineral Products of Adrian. Presently, the 

nearest metal producing mine, the DeLamar mine, is 26 mi 

southeast of the study area. At this mine, gold and silver 

are produced from epithermal deposits in veins, fracture 

fillings, and veinlets within porphyritic rhyolite.

Presently, there are no mining districts in or near the 

study area. The nearest district in Oregon is the Harper 

District where diatomite was produced from 1910 to 1975 

(Moore, 1937, p. 65-71). It is located approximately 40 mi 

northwest of the study area.

According to current U.S. Bureau of Land Management plats, there are no geothermal or other energy leases 

or lease applications within the study area. No hot springs 

or other geothermal phenomena were seen within the study 

area during field investigations.

Appraisal of Mineral Resources

There are no mineral or energy resources in or adjacent 

to the Upper Leslie Gulch and Slocum Creek Wilderness 

Study Areas. Twenty-three rock samples collected from 

the study area were analyzed for 18 elements. Examination 

of the data by statistical methods revealed no unusual 

concentrations of elements defined as anomalous or economic.

During field reconnaissance, portable gamma-ray detecting scintillometers were carried and monitored to detect 

anomalous radioactivity. No anomalous radioactivity was 

detected in the study area. A detailed radiometric survey of 

the study area may be warranted if a national shortage of 

uranium were to occur.

Because the study area lies near producing zeolite 

mines, 10 samples were screened for cation exchangemolecular sieve properties using a test described by 

Helfferich (1964). The results were negative and no further 

testing was done. Also, most of the tuff in the study areas 

is indurated, unlike the disaggregated or unconsolidated

D6 Mineral Resources of Wilderness Study Areas: East-Central Oregon
tuffaceous material that is commonly the source of commercial zeolite deposits. During the upper Miocene and 

Pliocene, the depositional environment of the region was 

favorable for the accumulation of diatomite; however, careful field examination did not reveal any diatomite deposits 

in the study area. Sand and gravel are not a resource 

because similar deposits elsewhere are more accessible and 

closer to existing markets.

The study area lies 40 mi south of the Vale Known 

Geothermal Resource Area (KGRA) (Couch and Baker, 

1977) between two blocks of land classified by the U.S. 

Geological Survey as valuable for geothermal steam and 

associated geothermal resources (Muffler, 1979). No hot 

springs or other geothermal phenomena were observed 

within the study area.

An estimated 3,000 acres of stone (welded tuff) is 

present in the study area. The rock could be used locally 

for crushed stone applications. The stone is not considered 

a resource because similar deposits elsewhere are more 

accessible and closer to existing markets.

ASSESSMENT OF MINERAL RESOURCE 

POTENTIAL

By Dean B. Vander Meulen, Andrew Griscom, and

Harley D. King

U.S. Geological Survey

Geology

Caldera Evolution

The Upper Leslie Gulch and Slocum Creek Wilderness 

Study Areas lie along the northern boundary of the Basin 

and Range physiographic province, a region characterized 

by east-west extensional tectonism. The study area is 

located within the eastern part of the Mahogany Mountain 

caldera (fig. 1), a collapse structure that resulted from the 

eruption of voluminous ash-flow and air-fall tuffs. The 

caldera is expressed as a 8- by 12-mi elliptical depression. 

Erosion removed the original caldera structural wall and 

left a marginal high topographic wall encircling most of the 

caldera moat. The caldera margin (fig. 1) parallels the base 

of the topographic wall, which is well exposed along the 

north escarpment of Mahogany Mountain; the north topographic wall coincides with an escarpment 1.9 mi north of 

Leslie Gulch (fig. 1). Several mineral exploration companies are currently prospecting for disseminated gold and 

silver deposits along the margins of the Mahogany Mountain and Three Fingers calderas (Vander Meulen and others, 1989; fig. 1). The east-central part of the Mahogany 

Mountain caldera was uplifted during post-collapse resurgence, a result of renewed rise of magma (Lipman, 1984). 

Uplift of the caldera floor domed and faulted intracaldera 

rocks and formed a topographic high near the center of the

caldera. The resurgent dome includes most of the central 

and eastern parts of the study area. A north-trending graben that cuts the central part of the study area probably 

formed in response to extensional stresses across the crest 

of the resurgent dome. The faults that bound the graben are 

buried by post-caldera tuff.

The topographic margin of the caldera is, in part, 

controlled by north-trending normal faults; this indicates 

that caldera collapse may have occurred along these preexisting faults. North-trending normal faults that cut the west 

side of the map area (fig. 2) partly define the east side of 

a large north-trending graben. Most of the western margin 

of the caldera is buried by post-caldera basalt flows, sedimentary rocks, and debris slides; this suggests subsidence 

along these faults probably continued after caldera collapse.

The volcanic and structural evolution of the Mahogany 

Mountain caldera generally parallels the stages of caldera 

evolution proposed by Smith and Bailey (1968). Three 

episodes of silicic volcanism are associated with the formation of this caldera. They are (1) an extensive precaldera 

dome and multiflow rhyolite complex at Mahogany Mountain, (2) rhyolite ash-flow and air-fall tuffs that erupted 

concurrently with the collapse of the caldera, and (3) postcaldera rhyolite dikes and plugs that intrude intracaldera 

tuffs. Mineralization in the study area was associated with 

the last two stages of volcanic activity.

The precaldera rhyolite dome and flow complex located 0.5 mi south of the study area was originally mapped 

as the Jump Creek Rhyolite by Kittleman (1973), but our 

current mapping has demonstrated that those silicic rocks 

are older than the type area of the Jump Creek Rhyolite and 

are here informally named the rhyolite of Mahogany 

Mountain (Trf, fig. 2). The northern part of the dome and 

flow complex subsided with the formation of the Mahogany Mountain caldera. Remnants of the rhyolite complex 

that did not subside include the northern scarp of Mahogany Mountain and the southern topographic wall that represents the caldera margin.

Rhyolite ash-flow and air-fall tuffs erupted concurrently with the collapse of the Mahogany Mountain caldera 

and filled the resulting depression with an estimated 2,600 

ft of intracaldera tuff (Vander Meulen, 1988). The intracaldera tuff units represent the second stage of silicic volcanism and are the oldest rocks exposed in the study area. 

The intracaldera tuff units are part of the Leslie Gulch AshFlow Tuff Member of the Sucker Creek Formation of 

Kiltleman and others (1965) (see also, Kittleman, 1973). A 

potassium-argon age obtained from the tuff is 15.5±0.5 Ma 

(Vander Meulen and others, 1987d). All ages reported in 

this investigation were obtained from sanidine mineral 

separates. Major- and trace-element analyses (Vander 

Meulen, 1988) indicate that the intracaldera tuffs are 

weakly peralkaline comendites.

Silicic volcanism associated with the Mahogany 

Mountain caldera concluded with rhyolite intrusions. Two 

north-striking and one northwest-trending rhyolite dike

Upper Leslie Gulch and Slocum Creek Wilderness Study Areas, Oregon D7
systems intrude the floor of the caldera. The north-striking 

rhyolite dikes cut across the width of the caldera and are 

exposed in the western part of the Upper Leslie Gulch 

Wilderness Study Area and in the eastern part of the 

Slocum Creek Wilderness Study Area (fig. 2). The rhyolite 

dikes parallel both sides of the graben in the resurgent 

dome. A potassium-argon age obtained from one of the 

north-trending rhyolite dikes is 14.7±0.4 Ma (Vander 

Meulen and others, 1987d). The northwest-trending rhyolite dike system intrudes the northwestern part of the 

Slocum Creek Wilderness Study Area and has a potassiumargon age of 14.9±0.4 Ma (Vander Meulen and others, 

1987d). Several rhyolite domes and plugs of similar and 

younger age intrude the north and east margins of the 

caldera; this indicates an episode of ring-fracture volcanism. The rhyolite intrusions are locally brecciated, silicified, and cut by quartz veins.

Stratigraphy

The lowest stratigraphically exposed rocks in the study 

area are in the eastern uplifted part of the caldera. The 

rocks consist of an interbedded sequence of peralkaline 

rhyolite ash-flow tuff, air-fall tuff, and surge deposits that 

have a minimum thickness of 1,150 ft (Tlga and Tlgi, fig. 

2). The interbedded tuffs and surge deposits were originally mapped as a single ash-flow tuff unit named the 

Leslie Gulch Ash-Flow Tuff Member of the Sucker Creek 

Formation by Kittleman and others (1965) (see also, Kittleman, 1973). On the basis of recent mapping, the Leslie 

Gulch Ash-Flow Tuff Member is here divided into three 

facies; they are an intracaldera facies consisting of ash-flow 

and air-fall tuff geographically restricted to the caldera, an 

outflow facies consisting of ash-flow tuff exposed outside 

the caldera (outside the area of figure 2), and an air-fall 

facies exposed within and adjacent to the caldera (Vander 

Meulen and others, 1987d). All three facies were deposited 

during the collapse of the Mahogany Mountain caldera. 

The intracaldera and air-fall facies are exposed over approximately two-thirds of the study area and underlie the 

entire study area at depth.

The intracaldera facies is best exposed in the central 

uplifted part of the caldera and covers approximately 0.8 

mi2 along the north-central part of the study area (fig. 2). It 

consists of ash-flow tuff and several interbedded air-fall 

and surge deposits that have a combined minimum thickness of 490 ft. Voluminous amounts of the intracaldera tuff 

probably filled the Mahogany Mountain caldera during its 

collapse, although the tuff units are presently exposed in 

only the deeply eroded resurgent dome. Estimated total 

original thickness of the intracaldera facies is 2,000 to 

3,000 ft (Vander Meulen, 1988).

The intracaldera facies is a composite ash-flow tuff 

sheet comprised of several successive flow units that 

cooled as a single unit; the base of the tuff is not exposed. 

The intracaldera facies shows lateral and vertical variation

through the section. The ash-flow tuff is generally poorly 

sorted, nonbedded, and contains discontinuous layers of 

pumice and lithic fragments; subtle eutaxitic foliation is 

common in the lower parts of the unit where heat and 

compression were greater. The composite ash-flow tuff 

contains several intervening air-fall tuffs and surge deposits 

composed of well-bedded ash and pumice lapilli. Beds of 

air-fall tuff are laterally continuous, whereas the surge 

deposits are laterally discontinuous. Both types of intervening deposits thicken toward a central-vent complex in 

the north-central part of the study area.

The air-fall facies is made up of bedded air-fall tuff 

and interbedded ash-flow tuff and surge deposits that have 

a combined minimum thickness of 650 ft. Outcrops of this 

unit are distributed over approximately two-thirds of the 

study area. Within the study area, the air-fall facies is 

separated from the underlying intracaldera facies by a 

gradational contact; rocks stratigraphically above the contact are dominantly air-fall tuffs and rocks below the contact are dominantly ash-flow tuffs. Angular, unflattened 

orangish-brown pumice fragments and light-green rhyolite 

lithic fragments are common throughout the air-fall facies. 

Pumice fragments are typically altered to clays and form 

pitted features in the tuff. Lithic fragments have vesicular 

rims that indicate they were molten when erupted. The 

pumice and juvenile lithic fragments are typically 0.5 to 1.3 

in. in diameter. The upper part of the air-fall facies consists 

predominantly of water-laid, reworked light-brown to 

brownish-yellow ash. Blocks and bombs that are restricted 

to the lower part of the air-fall facies typically exceed 1 ft 

in length near the central-vent complex.

Near the north-central boundary of the study area, a 

0.4- by 1-mi rhyolite vent complex (Trvi, fig. 2) intrudes 

and grades into the intracaldera tuffs (Vander Meulen, 

1988). The tuffs and vent complex form part of the central 

resurgent dome of the Mahogany Mountain caldera. Radially outward dipping tuff beds surrounding the vent structure were tilted north during resurgent activity. The vent 

intrusions consist of matrix-supported autobreccia crosscut 

by dikes and plugs of flow-foliated rhyolite. Both sharp 

and gradational contacts exist between parts of the vent 

breccia and intracaldera tuff. The eastern margin of the 

vent generally forms a sharp contact with the intracaldera 

tuff. Along this margin, large chaotic blocks of intracaldera tuff as much as 35 ft across are contained in the 

vent breccia (Vander Meulen, 1988). Conversely, blocks 

of vent breccia as much as 4 ft in diameter are contained in 

the intracaldera tuff. Massive deposits of vent breccia 

merge with ash-flow luff along the central and northern 

margins of the vent, whereas bedded deposits of breccia 

interfinger with air-fall tuff along the southern margin. 

Internal flow features are absent in the vent breccia. 

Gcochemical correlations between the intracaldera tuff and 

vent intrusions, as well as facies changes in the tuff indicate 

that part of the intracaldera tuff erupted from the central 

vent (Vander Meulen, 1988). In the study area, parts of the

D8 Mineral Resources of Wilderness Study Areas: East-Central Oregon
vent complex and the adjoining tuffs are hydrothermally 

altered.

Following the collapse of the Mahogany Mountain 

caldera, two unrelated post-caldera rhyolite ash-flow tuffs 

erupted from nearby centers and ponded in the caldera 

depression. The tuff of Spring Creek (Tsc, fig. 2) first 

described by Vander Meulen and others (1987d) was the 

earliest ash-flow tuff to pond in the caldera and unconformably overlies the air-fall facies. The tuff of Spring 

Creek is a composite ash-flow tuff that erupted from the 

Three Fingers caldera (Vander Meulen and others, 1989) 

located 3 mi northeast of the Mahogany Mountain caldera. 

The tuff contains multiple ash-flow sheets, each less than 

65 ft thick, that cooled as a single unit. Inside the caldera 

and near the central part of the study area, the tuff of Spring 

Creek has a minimum thickness of 1,000 ft where it fills the 

central graben. In the southwestern part of the study area 

near the topographic wall, the tuff of Spring Creek has a 

minimum thickness of 1,300 ft where it fills the southern 

part of the caldera depression (fig. 2).

The tuff of Birch Creek (Tbc, fig. 2) first described by 

Plumley (1986) is the younger of the two post-caldera ashflow tuffs. The tuff probably vented from a source about 

9 to 16 mi southwest of the Mahogany Mountain caldera. 

In the caldera, the thickness of this tuff ranges from 30 ft 

in the central part of the study area, where it conformably 

overlies the tuff of Spring Creek, to about 130 ft along the 

southwestern boundary of the study area, where it unconformably overlies the air-fall facies of the Leslie Gulch 

Ash-Row Tuff Member. This post-caldera tuff thickens to 

an estimated 450 ft adjacent to the Owyhee River (fig. 1) 

9 mi southwest of the study area. There, it forms an 

angular unconformity with the underlying outflow facies of 

the Leslie Gulch Ash-Flow Tuff Member.

Three rhyolite dike systems and several basalt dikes 

and plugs cut the entire sequence of intracaldera tuffs and 

are the youngest rocks in the study area. These rhyolite 

dikes (Tip, fig. 2) are highly resistant to erosion and form 

steep spired ridges that rise 2,000 to 3,000 ft above the 

intracaldera tuffs. Intracaldera tuffs adjacent to the rhyolite 

dikes are densely welded due to secondary heating. Secondary welding and silicification of the adjoining tuffs further 

contribute to the resistant nature of the rhyolite dikes. 

Hydrothermal alteration and silicification are common in 

parts of the dike systems and in the adjoining tuff. The 

rhyolite dikes are 1 to 16 ft wide and typically exhibit welldeveloped sets of horizontal columnar joints.

In the study area, alluvium is restricted to the main 

drainage canyons. The thickest areas of alluvium are 

probably along the northwestern part of the study area 

where they fill the lower part of Leslie Gulch.

Geochemical Studies

In 1987, the U.S. Geological Survey conducted a reconnaissance geochemical study of the Upper Leslie Gulch

and Slocum Creek Wilderness Study Areas. Sixty-one rock 

samples, 30 stream-sediment samples, and 29 nonmagnetic 

heavy-mineral concentrates of stream sediments collected 

in and near the study area were analyzed during this study. 

Stream-sediment samples, and stream sediments from 

which the concentrates were derived, were collected from 

alluvium in active stream channels.

Stream sediments represent composites of rock and 

soil exposed upstream from sample sites. Stream-sediment 

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

rock. Nonmagnetic heavy-mineral concentrate samples 

provide information about the chemistry of a limited 

number of minerals in rock material eroded from the drainage basin upstream from each sample site. Many minerals 

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

concentration of minerals permits determination of some 

elements that are not easily detected in bulk stream-sediment samples. Most rock samples appeared fresh and 

unaltered, and they were collected to determine geochemical background values. A few rock samples appeared altered and possibly mineralized; these were collected to 

determine the suite of elements associated with the observed alteration.

All samples were analyzed semiquantitatively for 31 

elements (some for 35 elements) using the direct-current 

arc emission spectrographic method (Grimes and Marranzino, 1968). Stream-sediment and rock samples were 

also analyzed for certain elements of special interest or that 

have high lower limits of determination by the spectrographic method. Inductively coupled argon plasma-atomic 

emission spectroscopy (Crock and others, 1987) was used 

to analyze for antimony, arsenic, bismuth, cadmium, and 

zinc. Atomic absorption was used to analyze for gold 

(O'Leary and Meier, 1986) and for mercury (Crock and 

others, 1987). Uranium was determined by ultraviolet 

fluorescence using digestion method B described by 

O'Leary and Meier (1986). Analytical data are from M.S. 

Erickson (written commun., 1988).

Peralkaline rhyolite tuffs and intrusions associated 

with the Mahogany Mountain caldera have anomalously 

high thorium (18 parts per million, or ppm) and uranium (8 

ppm) contents. Aerial gamma-ray surveys (Geometries, 

Inc., 1979) that cover part of the caldera show uranium 

anomalies over the caldera. The anomalies probably reflect 

the high concentrations of thorium and uranium present in 

the intracaldera tuffs and intrusive rocks associated with the 

Mahogany Mountain caldera.

Caldera-forming tuffs adjacent to the central-vent 

complex in the northern part of the study area contain 

localized zones of hydrothermal alteration. Highly oxidized, altered zones exposed east of the vent complex 

generally follow stratified horizons in the bedded tuffs. 

Four rock samples collected from an altered zone in this 

area contain anomalous concentrations of thorium and

Upper Leslie Gulch and Slocum Creek Wilderness Study Areas, Oregon D9
uranium. The altered tuffs contain as much as 17 ppm 

thorium and 9 ppm uranium. Rock samples collected from 

the brecciated parts of the vent intrusion contain as much as 

7 ppm uranium. Local high-grade vein-type uranium 

deposits are associated with peralkaline rhyolite intrusions 

in the McDermitt caldera complex (Rytuba and Glanzman, 

1979). Similar deposits may be present at depth, adjacent 

to, or within the peralkaline rhyolite vent complex of the 

Mahogany Mountain caldera.

Two miles north of the study area, 4 rock samples 

collected from the outflow and air-fall facies of Leslie 

Gulch Ash-Row Tuff Member contain anomalous values 

of lithium (170 to more than 200 ppm). Two samples of 

the tuff of Spring Creek collected stratigraphically above 

the outflow and air-fall facies also contain high values of 

lithium (more than 200 ppm). The peralkaline rhyolite 

tuffs associated with the Mahogany Mountain caldera are 

probably the source of high initial lithium concentrations. 

Samples from rhyolite dike systems in the central and 

northwestern parts of the study area contain 51 ppm and 81 

ppm lithium, respectively. Remobilization and concentration of lithium in stratiform deposits may have occurred 

along the caldera margin during the last stages of caldera 

activity. In the study area, similar remobilization of the 

lithium may have occurred near or within the central-vent 

complex, along the rhyolite dike systems, or near any areas 

where residual heat may have initiated hydrothermal activity. Lithium deposits similar to those associated with 

peralkaline rhyolites of the McDermitt caldera (Rytuba, 

1981) may be present at depth in the Mahogany Mountain 

caldera.

Peralkaline rhyolite tuffs and intrusions associated 

with the Mahogany Mountain caldera have exceptionally 

high zinc concentrations that are probably attributable to 

chlorine enrichment of the magma (Rytuba, 1981). Fortyfour rock samples of caldera-forming ash-flow and air-fall 

tuff and 11 samples of vent breccia were analyzed for zinc 

by X-ray-fluorescence spectroscopy. Thirty-three of these 

samples were collected in the study area; zinc values of 

these samples typically range from 180 to 250 ppm and 

some values are as high as 688 ppm.

Several high-silica rhyolite plugs and dikes that are 

locally cut by quartz veins occur in the central and northwestern parts of the study area. Samples of fractured 

rhyolite and quartz veins collected 2 mi north of the study 

area contain anomalous concentrations of arsenic (30 ppm), 

mercury (2.8 ppm), and zinc (250 ppm). The high zinc 

content probably reflects normal magmatic enrichment and 

(or) concentrations derived from hydrothermal systems 

active during the close of the caldera cycle. No gold was 

detected in these samples.

Two heavy-mineral concentrate samples that were 

collected from the northwestern part of the Slocum Creek 

Wilderness Study Area contain 3,000 and 200 ppm lead. 

Both samples are probably contaminated, possibly from 

oxidized lead particles eroded from artifacts such as bullets.

Anomalous concentrations of barium were detected in 

three heavy-mineral-concentrate samples and one rock 

sample collected from widely spaced sites in the Slocum 

Creek Wilderness Study Area. The anomalous barium 

concentrations ranging from 5,000 ppm to more than 

10,000 ppm are due to the occurrence of barite in the 

samples as detected by microscopic examination.

An anomalous concentration of tin (more than 2,000 

ppm) was detected in a heavy-mineral-concentrate sample 

collected from a tributary of Slocum Creek in the northern 

part of the Slocum Creek Wilderness Study Area. The 

sample contains a few grains of cassiterite, of the variety 

known as wood tin. On the basis of the extremely small 

amount of cassiterite in the sample, we do not believe a 

significant amount of tin is present in the study area.

Geophysical Studies

Geophysical evaluation of the mineral resources of the 

Upper Leslie Gulch and Slocum Creek Wilderness Study 

Areas is based on interpretations of radiometric, aeromagnetic, and gravity surveys.

An aeromagnetic survey that includes both study areas 

was conducted in 1976 and 1977 by Oregon State University during a study of the Vale-Owyhee geothermal area 

(Boler, 1979). The aeromagnetic data were collected along 

parallel east-west flightlines spaced 1 mi apart and flown at 

a constant barometric elevation of 7,000 ft above sea level. 

The Earth's main magnetic field was subtracted from the 

data that were plotted by machine and hand-contoured at a 

scale of 1:62,500. The final residual map was drafted at a 

scale of 1:125,000.

Additional aeromagnetic data are available in the atlas 

on the Boise 1° by 2° quadrangle published for the U.S. 

Department of Energy NURE program (Geometries, Inc., 

1979). The data were taken along east-west flightlines 

spaced at intervals of 3 mi flown by helicopter at an average height of 400 ft above the ground. Two profiles 

compiled from the data cross the wilderness study area.

Variations in the Earth's magnetic field on an aeromagnetic residual map are, in general, related to the amount 

of magnetic minerals in the rocks. Magnetite is the common magnetic mineral in the rocks underlying this region. 

Magnetic minerals, where locally concentrated or absent, 

may cause a high or low magnetic anomaly that can be a 

guide to mineral occurrences or deposits. At these magnetic latitudes, the inclination of the Earth's main magnetic 

field is relatively steep (68° below horizontal); therefore, 

boundaries between magnetic and relatively less magnetic 

rock are located approximately at the steepest gradient 

along the flanks of a magnetic anomaly.

The survey aircraft collecting aeromagnetic data for 

the Boler (1979) study was generally more than 2,000 ft 

above the ground, a distance sufficient to suppress most 

short wavelength anomalies generated by rocks at or near 

the surface. The contour magnetic map (Boler, 1979)

D10 Mineral Resources of Wilderness Study Areas: East-Central Oregon
shows a 3- to 4-mi-wide linear magnetic low trending weakly magnetic ash-flow tuff that fill the Mahogany

approximately N. 35° E. across the center of the study area. Mountain caldera. Magnetic highs bound the magnetic low

The low is probably associated with thick sections of on the southeast and northwest sides. The crests of the

117°30' 117°00'

43°30' 

43°15'

Figure 3. Bouguer gravity anomaly map of Owyhee volcanic field, eastern Oregon. Heavy dotted lines, approximate margins 

of Mahogany Mountain caldera (south) and Three Fingers Caldera (north). Contour interval, 2 milligal; hachured in direction 

of gravity low. See text for discussion of anomaly A.

Upper Leslie Gulch and Slocum Creek Wilderness Study Areas, Oregon D11
highs are located beyond the borders of the study area. The 

southeastern magnetic high appears to be caused by a substantial thickness of older precaldera rhyolites (Vander 

Meulen and others, I987d) that make up Mahogany Mountain. The northwestern magnetic high has a substantial 

width (10 mi) and represents a causative rock mass at least 

5,000 ft thick. The rock mass is concealed by relatively 

nonmagnetic clastic rocks of the Sucker Creek Formation 

and the Deer Butte Formations of Kittleman and others 

(1967). These data indicate normal magnetic features for 

this complex volcanic area and do not appear to have any 

economic significance.

Gravity data for the region are available from the 

National Geophysical Data Center (1984). Most data were 

gathered from an earlier survey conducted by students of 

Oregon State University. The earlier survey was described 

and generally interpreted in a thesis by Lillie (1977). 

Additional gravity stations were located within and near the 

study area by Plouff (1987). Average spacing between 

stations within and near the study area is 2 to 3 mi. 

Complete Bouguer gravity data were contoured at an interval of 2 mGal (milligals) and a reduction density of 2.67 

grams per cubic centimeter or g/cm3 (fig. 3).

Several coalescing gravity lows are associated with 

rocks in the study area. These lows and a major gravity 

low extending 15 mi north (fig. 3) have been interpreted by 

Plouff (Rytuba and others, 1985; Vander Meulen and others, 1987c) as the expression of thick low-density ash-flow 

tuff, air-fall tuff, and sedimentary rock filling a caldera. 

Within the study area, three coalescing gravity lows are 

associated specifically with the Mahogany Mountain 

caldera. The coalescing lows of about 10 mGal partly 

coincide with the caldera. The southern edge of the gravity 

anomaly generally follows the topographic wall of the 

caldera along Mahogany Mountain but locally extends as 

much as 3 mi beyond the caldera margin to the southeast. 

The western part of the caldera has no clearly identifiable 

expression on the gravity map. The northern extension of 

the gravity low probably reflects a subsided wedge of tuffaceous sedimentary rock, and a substantial thickness of 

ash-flow and air-fall tuff that fill the Three Fingers caldera 

and Honeycombs volcanic center (Vander Meulen and 

others, 1987b)

The northern elongated gravity low that includes the 

Mahogany Mountain caldera, Three Fingers caldera, and 

Honeycombs volcanic center indicates a caldera complex 

about 25 mi long and 10 mi wide. The small closed gravity 

contour that expresses a high near the center of the eastern 

part of the Mahogany Mountain caldera (A, fig. 3) is probably associated with the resurgent dome. Either an uplifted 

central floor or a central intrusion within the resurgent 

dome may have caused local thinning of low-density intracaldera tuffs. A linear gravity gradient trending northsouth along the west side of the caldera complex is part of 

a longer feature extending on strike at least 25 mi farther

north. The linear feature is probably the gravity expression 

of a major north-trending fault zone near the western border of the Slocum Creek Wilderness Study Area.

Aerial gamma-ray-spectrometer measurements for 

uranium, thorium, and potassium taken along 3-mi-spaced 

flightlines are available in Geometries, Inc. (1979). Statistically significant uranium anomalies were detected over 

the Upper Leslie Gulch Wilderness Study Area. The 

anomalies are 2 to 3 standard deviations above the average 

values for the Boise 1° by 2° quadrangle. Anomalies near 

the southern margin of the Mahogany Mountain caldera 

and over the central part of the caldera correlate with a 

region of high potassium values. Conversely, the gravity 

low extending north to the Honeycombs volcanic center 

does not correlate with particularly high potassium concentrations. Peralkaline rhyolite tuffs and intrusions of the 

Mahogany Mountain caldera are probably the source for 

the high potassium and uranium concentrations. However, 

the potassium anomalies may be in part related to the introduction of potassium by hydrothermal alteration.

Mineral and Energy Resource Potential of the 

Upper Leslie Gulch Wilderness Study Area

Parts of the study area underlain by caldera-forming 

ash-flow and air-fall tuffs have moderate potential for 

uranium and thorium resources, level C certainty. Except 

for a few isolated outcrops of post-caldera tuff, the calderaforming tuffs underlie the entire study area (fig. 2). The 

mineral resource potential rating is based on anomalous 

concentrations of uranium and thorium detected in rock 

samples and on anomalous aerial gamma-ray values for 

these elements over the study area.

Peralkaline rhyolite tuff and intrusions associated with 

the Mahogany Mountain caldera have high concentrations 

of lithium, as do samples collected from the overlying postcaldera tuffs, though not as high as the peralkaline tuff and 

intrusions. Stratiform lithium deposits similar to those 

associated with peralkaline rhyolite of the McDermitt 

caldera (Rytuba, 1981) may be present in the study area. 

All tuffs in the study area have low potential for lithium 

resources in stratiform deposits, level C certainty.

Samples of caldera-forming tuffs have exfraordinarily 

high zinc concentrations. The high zinc concentrations are 

associated with pumice fragments in these rocks. Volcanogenie disseminated zinc deposits of this type were previously unrecognized in the world. Areas underlain by the 

tuffs are assigned low potential for zinc resources, level B 

certainty.

An exploration company Manville International 

Group is currently prospecting for gold (Au) deposits 

northeast of the study area. The Bannock gold prospect, 2 

mi northeast of the study area (fig. 1), is associated with a 

rhyolite ring-dome complex that intrudes the eastern margin of the Mahogany Mountain caldera. The Katie gold

D12 Mineral Resources of Wilderness Study Areas: East-Central Oregon
prospect, 7 mi northeast of the study area (fig. 1), is associated with rhyolite ring-domes and northwest-striking fault 

zones that cut the east margin of the Three Fingers caldera. 

Best assays of as much as 0.5 troy oz Au/short ton and 6 

troy oz Ag (silver)/short ton, accompanied by high selenium content, occur in a rhyolite dome. Mineralized zones 

consist of hairline fractures filled with quartz and black 

sulfide minerals. Rhyolite dikes and plugs exposed 3 mi 

northwest of the study area, near the north margin of the 

Mahogany Mountain caldera, are locally brecciated and cut 

by quartz veins. There, the rhyolites contain anomalous 

concentrations of arsenic, mercury (pathfinder elements for 

gold and silver deposits), and zinc (Vander Meulen and 

others, 1987c). Hydrothermal activity during the close of 

the caldera cycle probably concentrated these elements. 

Similar rhyolite dike systems extend south across the 

western part of the study area. Because of the mineral 

association in the Mahogany Mountain caldera, all rhyolite 

intrusions in the study area have moderate potential for 

gold, silver, mercury, and zinc resources, level B certainty.

An unnamed hot spring issues from a north-trending, 

high-angle normal fault about 6 mi west of the study area 

(fig. 1). Recorded temperature of the spring water is 50 °C 

(Bliss, 1983). Several thermal springs and wells are located about 17 mi north of the study area (Oregon Department of Geology and Mineral Industries, 1982). One area, 

less than 11 mi north of the study area, may be underlain 

at shallow depth (less than 3,280 ft) by thermal water of 

sufficient temperature for direct heat applications (Oregon 

Department of Geology and Mineral Industries, 1982). No 

hot springs or recent fault zones were mapped in the study 

area, although both are present in the adjoining study area. 

The Upper Leslie Gulch Wilderness Study Area has unknown potential for geothermal resources, certainty level 

A.

The Upper Leslie Gulch Wilderness Study Area is 

located within the Mahogany Mountain caldera. Postcaldera sedimentary rocks that commonly accumulate in 

caldera depressions may contain oil and gas resources. The 

study area is located over the eastern part of the caldera's 

central resurgent dome, an area that has been uplifted and 

deeply eroded. No post-caldera sedimentary rocks are 

preserved in the study area. Any petroleum or natural gas 

reservoirs trapped in precaldera sedimentary rocks would 

have been destroyed by magmatic heat accompanying 

caldera formation. Therefore, the study area has no potential for oil and gas resources, level D certainty.

Mineral and Energy Resource Potential of the 

Slocum Creek Wilderness Study Area

Samples collected from caldera-forming ash-flow and 

air-fall tuff in the Slocum Creek Wilderness Study Area 

contain anomalous concentrations of uranium and thorium. 

The caldera-forming tuff probably underlies the entire

study area but is buried by thick deposits of post-caldera 

tuff in the northwestern and southeastern parts of the study 

area (fig. 2). Samples collected from a rhyolite vent 

complex associated with the caldera also contain anomalous concentrations of uranium and thorium. The vent 

complex intrudes the central part of the caldera along the 

northeastern boundary of the study area. On the basis of 

anomalous thorium and uranium values detected in an 

aerial gamma-ray survey flown over the caldera, parts of 

the study area underlain by caldera-forming ash-flow and 

air-fall tuff and the vent complex (fig. 2) have moderate 

potential for uranium and thorium mineral resources, level 

C certainty. These elements were probably concentrated by 

hydrothermal activity during the last stages of caldera formation.

Peralkaline rhyolite tuff and intrusions associated with 

the Mahogany Mountain caldera have high concentrations 

of lithium, as do samples collected from the overlying postcaldera tuffs, though not as high as the peralkaline tuff and 

intrusions. Stratiform lithium deposits similar to those 

associated with peralkaline rhyolite of the McDermitt 

caldera (Rytuba, 1981) may be present in the study area. 

All tuffs in the study area have low potential for lithium 

resources in stratiform deposits, level C certainty.

Samples of caldera-forming tuffs and vent breccia 

intrusions contain extraordinarily high zinc concentrations. 

The high zinc concentrations are associated with pumice 

fragments in these rocks. Volcanogenic disseminated zinc 

resources of this type were previously unrecognized in the 

world. Areas underlain by the tuffs and vent breccia are 

assigned low potential for zinc resources, level B certainty.

An exploration company Manville International 

Group is currently prospecting for gold deposits northeast 

of the study area. The Bannock gold prospect, 5 mi northeast of the study area (fig. 1), is associated with a rhyolite 

ring-dome complex that intrudes the eastern margin of the 

Mahogany Mountain caldera. The Katie gold prospect, 9 

mi northeast of the study area (fig. 1), is associated with 

rhyolite ring-domes and northwest-striking fault zones that 

cut the east margin of the Three Fingers caldera. Best 

assays of as much as 0.5 troy oz Au/short ton and 6 troy oz 

Ag/short ton, accompanied by high selenium content, occur 

in a rhyolite dome. Mineralized zones consist of hairline 

fractures filled with quartz and black sulfide minerals. 

Rhyolite dikes and plugs exposed 1.5 mi north of the study 

area, near the northern margin of the Mahogany Mountain 

caldera, are locally brecciated and cut by quartz veins. The 

rhyolites contain anomalous concentrations of arsenic, 

mercury (pathfinder elements for gold and silver deposits), 

and zinc (Vander Meulen and others, 1987c). Hydrothermal systems active during the close of the caldera cycle 

likely concentrated these elements. Similar rhyolite dike 

systems extend south across the eastern and northwestern 

parts of study area. Because of the mineral association in 

the Mahogany Mountain caldera, all rhyolite intrusions in

Upper Leslie Gulch and Slocum Creek Wilderness Study Areas, Oregon D13
the study area have moderate potential for gold, silver, 

mercury, and zinc resources, level B certainty.

An unnamed hot spring that issues from a north-trending normal fault 2 mi west of the study area (fig. 1) has a 

recorded temperature of 50 °C (Bliss, 1983). Several thermal springs and wells are located about 16 mi north of the 

study area (Oregon Department of Geology and Mineral 

Industries, 1982). One area, less than 10 mi north of the 

study area, may be underlain at shallow depth (less than 

3,280 ft) by thermal water of sufficient temperature for 

direct heat applications (Oregon Department of Geology 

and Mineral Industries, 1982). Although no hot springs 

were identified in the study area, geothermal waters may 

exist at depth along recent fault zones in and adjacent to the 

western part of the study area. Areas adjacent to these fault 

zones have moderate potential for geothermal resources, 

level C certainty.

The Slocum Creek Wilderness Study Area is located 

within the Mahogany Mountain caldera. Post-caldera sedimentary rocks that commonly accumulate in caldera depressions may contain oil and gas resources. The study 

area is located over the eastern part of the caldera's central 

resurgent dome, an area that has been uplifted and deeply 

eroded. No post-caldera sedimentary rocks are preserved 

in the study area. Any petroleum or natural gas reservoirs 

trapped in precaldera sedimentary rocks would have been 

destroyed by magmatic heat accompanying caldera formation. The study area has no potential for oil and gas resources, level D certainty.

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.

Benham, J.R., 1988, Mineral resources of the Slocum Creek and 

Upper Leslie Gulch study areas, Malheur County, Oregon: 

U.S. Bureau of Mines Open-File Report MLA 22-88, 16 p.

Bliss, J.D., 1983, Oregon; basic data for thermal springs and wells 

as recorded in Geotherm: U.S. Geological Survey Open-File 

Report 83-435, 247 p.

Boler, P.M., 1979, Aeromagnetic measurements, magnetic source 

depths, and the Curie point isotherm in the Vale-Owyhee, 

Oregon, geothermal area [M.S. thesis]: Corvallis, Oregon, 

Oregon State University, 104 p.

Couch, R.W., and Baker, B., 1977, Geophysical investigations of 

the Vale-Owyhee geothermal region, Malheur County, Oregon: Oregon State University Technical Report No. 2 to the 

Geothermal Research Program of the U.S. Geological Survey, 

52 p.

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

Analytical methods for the analysis of stream sediments and 

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

Open-File Report 87-84, 35 p.

Geometries, Inc., 1979, Aerial gamma ray and magnetic survey 

Idaho project, Hailey, Idaho Falls, Elk City quadrangles of 

Idaho/Montana and Boise quadrangle, Oregon/Idaho, (contract DE-AC 13-76GJ01664, subcontract no. 79-323-5): 

U.S. Department of Energy Open-File Report GJBX-10 (80), 

[Available through U.S. Geological Survey Branch of Open 

File Services, Denver, CO 80225], 114 p.

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

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

Report 84-787, 25 p.

Gray, J.J., Peterson, J.C., Clayton, J., and Baxter, G., 1983, 

Geology and mineral resources of 18 BLM wilderness study 

areas, Harney and Malheur Counties, Oregon: Oregon Department of Geology and Mineral Industries, Open-File Report 0-83-2, 106 p.

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

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

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

Helfferich, Fred, 1964, A simple identification reaction for zeolites (molecular sieves): American Mineralogist, v. 49, p. 

1752-1754.

Kittleman, L.R., 1973, Guide to the geology of the Owyhee region 

of Oregon: University of Oregon Museum of Natural History 

Bulletin 21, 61 p.

Kittleman, L.R., Green, A.R., Hagood, A.R., Johnson, A.M., 

McMurray, J.M., Russell, R.A., and Weeden, D.A.,1965, 

Cenozoic stratigraphy of the Owyhee region, southeastern 

Oregon: University of Oregon Museum of Natural History 

Bulletin 1, 45 p.

 1967, Geologic map of the Owyhee region, Malheur 

County, Oregon: University of Oregon Museum of Natural 

History Bulletin 8, scale 1:125,000.

Lillie, R.J., 1977, Subsurface geologic structure of the Vale, 

Oregon, Known Geothermal Resource Area from the interpretation of seismic reflection and potential field data [M.S. 

thesis]: Corvallis Oregon, Oregon State University, 52 p.

Lipman, P.W., 1984, The roots of ash-flow caldera in western 

North America: Windows into the tops of granitic batholiths: 

Journal of Geophysical Research, v. 89, no. BIO, p. 

8801-8841.

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

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

Moore, B.N., 1937, Nonmetallic mineral resources of eastern 

Oregon: U.S. Geological Survey Bulletin 875, 180 p.

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

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

163 p.

Myers, A.T., Haven, R.G., and Dunton, P.J., 1961, A spectrochemical method for the semiquantitative analysis of rocks, 

minerals, and ores: U.S. Geological Survey Bulletin 1084-1, 

p. 207-229.

National Geophysical Data Center, 1984, DMA gravity file of the 

U.S.: National Oceanic and Atmospheric Administration, 

Boulder, Colo., 2 magnetic tapes.

O'Leary, R.M., and Meier, J.G., 1986, Determination of antimony, arsenic, bismuth, cadmium, copper, lead, molybdenum, 

silver, and zinc in geological materials by atomic absorption 

spectrometry using a hydrochloric acid-hydrogen peroxide 

digestion: Atomic Spectroscopy, v. 7, no. 1, p. 4-8.

D14 Mineral Resources of Wilderness Study Areas: East-Central Oregon
Oregon Department of Geology and Mineral Industries, 1982, 

Geothermal resources of Oregon [Available through Oregon 

Department of Mineral Industries, 1005 State Office Building, 

Portland, OR 97201], scale 1:500,000.

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

Plumley, P.S., 1986, Volcanic stratigraphy and geochemistry of 

the Hole in the Ground area, Owyhee Plateau, southeastern 

Oregon [M.S. thesis]: Moscow, Idaho, University of Idaho, 

161 p.

Rytuba, J.J., 1981, Relation of calderas to ore deposits in the 

western United States, in Dickinson, W.R., and Payne, W.D., 

eds., Relation of tectonics to ore deposits in the southern 

Cordillera: Arizona Geological Society Digest, v. 14, p. 

227-236.

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 33, Fifth International Association on the 

Genesis of Ore Deposits Quadrennial Symposium, p. 

109-117.

Rytuba, J.J., Vander Meulen, D.B., Plouff, Donald, and Minor, 

S.A., 1985, Geology of the Mahogany Mountain caldera, 

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

Programs, v. 17, no. 4, p. 263.

Scott, D.F., 1986, Mineral resources of the Honeycombs study 

area, Malheur County, Oregon: U.S. Bureau of Mines OpenFile Report, MLA 30-86, 26 p.

Smith, R.L., and Bailey, R.A., 1968, Resurgent cauldrons, in 

Studies in volcanology: Geological Society of America 

Memoir 116, p. 613-662.

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

Geological Survey Circular 831, 5 p.

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

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

Report 72-0390, 1 pi., scale 1:250,000.

Vander Meulen, D.B., 1988, Intracaldera ash-flow and air-fall 

tuffs of the Mahogany Mountain caldera [M.S. thesis]: San 

Jose, California, San Jose State University, 88 p.

 1989, Intracaldera ash-flow and air-fall tuffs of the Mahogany Mountain caldera: U.S. Geological Survey Open-File 

Report 89-77, 68 p., 2 pi., scale 1:72,750.

Vander Meulen, D.B., Rytuba, J.J., Grubensky, M.J., and Goeldner, C.A., 1987a, Geologic map of the Bannock Ridge 7.5' 

quadrangle, Malheur County, Oregon: U.S. Geological Survey Miscellaneous Field Studies Map MF-1903A, scale 

1:24,000.

Vander Meulen, D.B., Rytuba, J.J., Grubensky, M.J., Vercoutere, 

T.L., and Minor S.A., 1987b, Geologic map of Pelican Point 

7.5' quadrangle, Malheur County, Oregon: U.S. Geological 

Survey Miscellaneous Field Studies Map MF-1904B, scale 

1:24,000.

Vander Meulen, D.B., Rytuba, J.J., King, H.D., and Plouff 

Donald, 1987c, Mineral resources of the Honeycombs Wilderness Study Area, Malheur County, Oregon: U.S. Geological 

Survey Bulletin 1741C, 15 p.

Vander Meulen, D.B., Rytuba, J.J., Minor, S.A., and Harwood, 

C.S., 1989, Preliminary geologic map of the Three Fingers 

Rock quadrangle, Malheur County, Oregon: U.S. Geological 

Survey Open-File Report 89-344, scale 1:24,000.

Vander Meulen, D.B., Rytuba, J.J., Vercoutere, T.L., and Minor, 

S.A., 1987d, Geologic map of the Rooster Comb 7.5' quadrangle, Malheur County, Oregon: U.S. Geological Survey 

Miscellaneous Field Studies Map MF-1902D, scale 1:24,000.

Upper Leslie Gulch and Slocum Creek Wilderness Study Areas, Oregon D15

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.

o

Q_

LLJ 

U

c*.

oCO

LLJ

o

_J

LLJ 

>

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.

D18 Mineral Resources of Wilderness Study Areas: East-Central Oregon
RESOURCE/RESERVE CLASSIFICATION

ECONOMIC

MARGINALLY 

ECONOMIC

SUBECONOMIC

IDENTIFIED RESOURCES

Demonstrated

Measured

Resc

Indicated

;rves

Marginal 

Reserves

Demonstrated 

Subeconomic

Resources 

i

Inferred

Inferred 

Reserves

Inferred 

Marginal 

Reserves

Inferred 

Subeconomic 

Resources

UNDISCOVERED RESOURCES

Probability Range

Hypothetical Speculative

t

Major elements of mineral resource classification, excluding reserve base and inferred reserve base. Modified from McKefvey, 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.

Upper Leslie G ulch and Slocum Creek Wilderness Study Areas, Oregon D19
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 a I eogen e

^i il^norifvJ

C retaceou s

Jurassic

Triassic

Permian

Pennsylvanian 

Carboniferous

Mississippian

Devonian

Silurian

Ordovician

Cambrian

/T orir\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

24

10

 55

 66

96 

1 10

°05

-240

 290 

-330

 360

 410

*T J*}

 500

 '-570

 1600

J\AJ\J

 3400

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

informal time term without specific rank.

D20 Mineral Resources of Wilderness Study Areas: East-Central Oregon


SELECTED SERIES OF U.S. GEOLOGICAL SURVEY PUBLICATIONS

Periodicals

Earthquakes & Volcanoes (issued bimonthly). 

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