Mineral Resources of the 

Little Jacks Creek, Big Jacks Creek, and 

Duncan Creek Wilderness Study Areas, 

Owyhee County, Idaho

U.S. GEOLOGICAL SURVEY BULLETIN 1720-A

~**>~^jr*~

"f

Chapter A

Mineral Resources of the 

Little Jacks Creek, Big Jacks Creek, and 

Duncan Creek Wilderness Study Areas, 

Owyhee County, Idaho

By D. H. MclNTYRE, H. D. KING, 

D. M. KULIK, and D. L. SAWATZKY 

U.S. Geological Survey

R. A. WINTERS and A. M. LESZCYKOWSKI 

U.S. Bureau of Mines

D. F. KAUFFMAN and BILL BONNICHSEN 

Idaho Geological Survey

U.S. GEOLOGICAL SURVEY BULLETIN 1720

MINERAL RESOURCES OF WILDERNESS STUDY AREAS 

BRUNEAU RIVER-JACKS CREEK REGION, IDAHO
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 Little Jacks Creek, Big Jacks Creek, and 

Duncan Creek Wilderness Study Areas, Owyhee County, Idaho.

(Mineral resources of wilderness study areas Bruneau River-Jacks 

Creek region, Idaho ; ch. A)(U.S. Geological Survey Bulletin ; 1720)

Bibliography: p.

Supt. of Docs. No.: I 19.3:1720-A

1. Mines and mineral resources Idaho Little Jacks Creek 

Wilderness. 2. Mines and mineral resources Idaho Big Jacks Creek 

Wilderness. 3. Mines and mineral resources Idaho Duncan Creek 

Wilderness. 4. Little Jacks Creek Wilderness (Idaho). 5. Big Jacks 

Creek Wilderness (Idaho). 6. Duncan Creek Wilderness (Idaho). I. 

Mclntyre, David H. II. Series. III. Series: U.S. Geological Survey 

bulletin ; 1720.

QE75.B9 no. 1720 557.3 s 87-600174 

[TN24.I2] [553'.097%'21]
STUDIES RELATED TO WILDERNESS

Bureau of Land Management Wilderness Study Areas

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

1976) requires the U.S. Geological Survey and the U.S. Bureau of Mines to conduct mineral 

surveys on certain areas to determine the mineral values, if any, that may be present. Results 

must be made available to the public and be submitted to the President and the Congress. 

This report presents the results of a mineral survey of the Little Jacks Creek (ID-111-006), 

Big Jacks Creek (ID-111-007C), and Duncan Creek (ID-111-007B) Wilderness Study Areas, 

Owyhee County, Idaho.

CONTENTS

Summary 1 

Introduction 1

Investigations by the U.S. Bureau of Mines 3 

Investigations by the U.S. Geological Survey 3 

Appraisal of identified resources 3 

Assessment of potential for undiscovered resources 3 

Geology 3

Rhyolitic rocks 3

Basalt and associated sedimentary rocks and silicic ash 6 

Structure 6 

Geochemistry 6 

Methods 6

Summary of analytical results 7 

Characteristics of tin-bearing concentrates 7 

Sources for minerals in concentrates containing tin 8 

Geophysics 8

Gravity data 8 

Aeromagnetic data 8 

Remote-sensing data 10 

Mineral and energy resource potential 10 

Recommendations for future work 10 

References cited 10 

Appendix 13

FIGURES

1. Map showing location and mineral resource potential of the Little Jacks Creek, 

Big Jacks Creek, and Duncan Creek Wilderness Study Areas 2

2. Map showing generalized geology of the Little Jacks Creek, Big Jacks Creek, 

and Duncan Creek Wilderness Study Areas 4

3. Map showing sample locations and anomalous concentrations of tin, lead, antimony, and bismuth in samples from the Little Jacks Creek, Big Jacks Creek, 

and Duncan Creek Wilderness Study Areas 5

4. Complete Bouguer gravity map of the Little Jacks Creek, Big Jacks Creek, and 

Duncan Creek Wilderness Study Areas 9

5. Aeromagnetic map of the Little Jacks Creek, Big Jacks Creek, and Duncan Creek 

Wilderness Study Areas 11

TABLE

1. Number of samples collected from the Little Jacks Creek, Big Jacks Creek, and 

Duncan Creek Wilderness Study Areas 6

Contents V

MINERAL RESOURCES OF WILDERNESS STUDY AREAS BRUNEAU RIVER-JACKS CREEK REGION, IDAHO

Mineral Resources of the 

Little Jacks Creek, Big Jacks Creek, and 

Duncan Creek Wilderness Study Areas, 

Owyhee County, Idaho

By D. H. Mcjntyre, H. D. King, D. M. Kulik, and D. L. Sawatzky 

U.S. Geological Survey

R. A. Winters and A. M. Leszcykowski 

U.S. Bureau of Mines

D. F. Kauffman and Bill Bonnichsen 

Idaho Geological Survey

SUMMARY

In 1985 and 1986, the USBM (U.S. Bureau of 

Mines), the USGS (U.S. Geological Survey), and the 

Idaho Geological Survey conducted investigations to appraise the mineral resources and assess the mineral resource potential of the Little Jacks Creek (ID-111-006), 

Big Jacks Creek (ID-111-007C), and Duncan Creek (ID111-007B) Wilderness Study Areas. These investigations 

revealed no identified mineral resources and a low mineral resource potential for tin and all other metals, oil 

and gas, and geothermal energy in the three study areas.

The Little Jacks Creek, Big Jacks Creek, and Duncan Creek Wilderness Study Areas are located in southwestern Idaho, south of the Snake River Plain. These 

three contiguous study areas are underlain chiefly by Miocene rhyolitic rocks that are locally covered by a thin 

veneer of basalt and locally separated by a thin deposit 

of poorly consolidated sedimentary rocks and silicic ash. 

The rock sequence is cut by numerous northwesttrending high-angle faults of small displacement. None 

of the rocks are hydrothermally altered.

No minerals have been produced and there are no 

known mineralized areas in the wilderness study areas. 

No mines, prospects, mineralized areas, patented mining 

claims, or mineral leases were identified in the study 

areas. Sand, gravel, and industrial rock resources are in 

the study areas, but sufficient quantities are available 

elsewhere to satisfy current local needs.

Nonmagnetic heavy-mineral concentrates collected 

during the stream-sediment geochemical survey locally

contain high values for tin and lead. Very fine sand size 

to silt-size cassiterite (variety wood tin) occurs in the tinrich samples. The cassiterite probably is derived from the 

sedimentary rocks locally present between the rhyolitic 

rocks and the basalt; its ultimate source is probably outside the wilderness study areas. The high lead values are 

probably man-caused contaminants. The wilderness 

study areas have a low mineral resource potential for 

undiscovered tin associated with the sedimentary rocks 

beneath the basalt and for all other metals. The potential 

for undiscovered oil and gas and geothermal resources 

also is low.

INTRODUCTION

In 1985, the USGS and the USBM, in cooperation 

with the Idaho Geological Survey, conducted mineral resource investigations in the Little Jacks Creek, Big Jacks 

Creek, and Duncan Creek Wilderness Study Areas. These 

three contiguous wilderness study areas (fig. 1) encompass 

93,275 acres, divided as follows: Little Jacks Creek, 

34,000 acres; Big Jacks Creek, 49,875 acres; Duncan 

Creek, 9,400 acres. The areas are located on a gently 

rolling plateau into which the gorges of Little Jacks 

Creek, Big Jacks Creek, and Duncan Creek are incised; 

these gorges commonly are a thousand or more feet deep. 

State Highway 51 passes east of the wilderness study 

areas. Light-duty roads and jeep trails provide access to

Little Jacks, Big Jacks, and Duncan Creeks Wilderness Study Areas A1
42°45'

42°30

116°00'

Bruneau 

10 Miles

APPROXIMATE 

BOUNDARY

OF THE LITTLE

JACKS CREEK

WILDERNESS

STUDY AREA

(ID-111 006A)

APPROXIMATE BOUNDARY

OF THE BIG JACKS CREEK

WILDERNESS STUDY AREA

(ID-111-007C)

/APPROXIMATE

/BOUNDARY 

OF THE

DUNCAN CREEK 

WILDERNESS 

STUDY AREA 

(ID-111-007B)

EXPLANATION

L/C Geologic terrane having low mineral resource potential for tin and all 

other metals, oil and gas, and geothermal energy, certainty level C

C Certainty level Data indicate geologic environment and resource potential but do 

not establish activity of resource-forming processes

 Unpaved roads

5 MILES 

J

Figure 1. Map showing location and mineral resource potential of the Little Jacks Creek, Big Jacks Creek, and 

Duncan Creek Wilderness Study Areas, southwestern Idaho.

A2 Mineral Resources of Wilderness Study Areas Bruneau River-Jacks Creek Region, Idaho
the areas from all sides. When water conditions permit, 

parts of Big Jacks Creek and Little Jacks Creek can be 

floated by raft.

This report presents an evaluation of the mineral 

endowment (identified resources and mineral resource potential) of the study area and is the product of several 

separate studies by the USBM and the USGS. Identified 

resources are classified according to the system of the 

USBM and USGS (1980), which is shown in the Appendix of this report. Identified resources are studied by the 

USBM. Mineral resource potential is the likelihood of occurrence of undiscovered metals and nonmetals, industrial 

rocks and minerals, and of undiscovered energy sources 

(coal, oil, gas, oil shale, and geothermal sources). It is 

classified according to the system of Goudarzi (1984), 

which is shown in the Appendix of this report. Undiscovered resources are studied by the USGS.

Investigations by the U.S. Bureau of Mines

The USBM appraisal of identified resources for the 

three wilderness study areas was by R. A. Winters and 

A. M. Leszcykowski (1986a, b, c). Their work included 

library research and examination of Owyhee County and 

U.S. Bureau of Land Management mining, lease, claim, 

and land status records. Field studies in the summer of 

1985 involved searches for any mines, prospects, claims, 

or mineralized zones not on record. In addition, three rock 

samples and five samples of stream alluvium were taken 

from the three areas. The rock samples were pulverized 

and fire assayed for gold and silver at detection limits 

of 0.005 and 0.2 troy ounces per ton, respectively, and 

were analyzed for 40 elements by semiquantitative spectrography (Winters and Leszcykowski, 1986a, b, c). The 

alluvial samples, each consisting of two level panfuls, 

were first concentrated by hand panning. The concentrates 

were further reduced by a laboratory-size Wilfley table 

and inspected microscopically for gold and other valuable 

minerals.

Investigations by the U.S. Geological Survey

A new geologic map of the study areas was prepared for the USGS by Daniel F. Kauffman and Bill 

Bonnichsen, Idaho Geological Survey (Kauffman and 

Bonnichsen, in press). Harley D. King conducted a 

stream-sediment geochemical survey within the study 

areas (M. S. Erickson and others, unpub. data, 1986). 

A gravity survey was conducted by Dolores M. Kulik 

and a remote-sensing study was conducted by Don L. 

Sawatzky.

APPRAISAL OF IDENTIFIED RESOURCES

By R. A. Winters and A. M. Leszcykowski 

U.S. Bureau of Mines

No mines, prospects, or mineralized structures were 

identified within the wilderness study areas except for one 

placer claim, located in 1924 and currently inactive, 

which extends into the northernmost part of the Big Jacks 

Creek Wilderness Study Area. No economic concentrations of minerals were detected in any of the rock or 

placer samples. Parts of the three wilderness study areas 

formerly were covered by oil and gas lease applications; 

all applications were cancelled prior to 1985. No known 

exploration has occurred in the areas covered by the lease 

applications.

Sand and gravel occurrences along Little Jacks 

Creek, Duncan Creek, Big Jacks Creek, and their 

tributaries are small in volume and consist of gravel and 

sand derived from the local volcanic rocks. The small 

volume and angular nature of the gravel preclude it from 

being considered a resource.

ASSESSMENT OF POTENTIAL FOR 

UNDISCOVERED RESOURCES

By D. H. Mclntyre, H. D. King, D. M. Kulik, and

D. L. Sawatzky

U.S. Geological Survey

D. F. Kauffman and Bill Bonnichsen 

Idaho Geological Survey

Geology

The Little Jacks Creek, Big Jacks Creek, and Duncan Creek Wilderness Study Areas are underlain by a sequence of rhyolitic rocks that are best exposed in the canyons along the creeks, but also crop out widely on the 

plateau surface (fig. 2). The rhyolitic rocks are overlain, 

locally, by a thin veneer of basalt erupted mainly from 

vents within and near the study areas. A thin layer of 

poorly consolidated sedimentary rocks and silicic ash separates the rhyolitic rocks from the overlying basalt in parts 

of the study areas.

Rhyolitic Rocks

The rhyolitic rocks commonly are dark brown, reddish brown, or reddish purple, crystal poor, and contain 

sparse, small crystals of plagioclase, pyroxene, and

Little Jacks, Big Jacks, and Duncan Creeks Wilderness Study Areas A3
116M5' 116'00'

42 M5'

42°30 f

! r'BOUNDARY

ORTHE^ITTLE

i?/"$r

APPROXIMATE 

^BOUNDARY

r^ri^¥^--^iiP; >HAPPROXJiyiAIE ^Jj

' "' ' «"jC\

Base from U.S. Geological Survey, 1:250,000, 

Jordan Valley, 1970; Twin Falls, 1971

CORRELATION OF MAP UNITS

Holocene and Pleistocene

s^-Nifyr "".'-. ' A ' x'l //-,/l. ^I^T^i^^ i/ ?x~.'"^\ I r

\ ' *.'

J ^ A,53<5__£

'.-< 

4w

Geology modified from Kauffman and others (in press) 

5 MILES

Q

Ts

Tr

Pliocene and Miocene

Miocene

LIST OF MAP UNITS

Q Surficial deposits

Teh Chalk Hills Formation

Tb Basalt

Ts Sedimentary rocks

Tr Rhyolitic rocks

 Contact

^^ Fault Dotted where concealed

Figure 2. Map showing generalized geology of the Little Jacks Creek, Big Jacks Creek, and Duncan Creek Wilderness Study Areas.

A4 Mineral Resources of Wilderness Study Areas Bruneau River-Jacks Creek Region, Idaho
116°15* 116°00'

42°45'

^^l&P-vHV^!

THE LITTLE JACKS 

CREEK WILDERNESS

BOOr4DARY GF^ THE

DUNCAW -CREEK! v

WILDERNESS STUDY^

, O" Y y 

AREA i i /w\"=D

^WJ.: >%^r-i) J

v, KT\ j

42.30- V;\, ^

\ ( !^3

Base from U.S. Geological Survey, 1:250,000, 

Jordan Valley, 1970; Twin Falls, 1971

0 5 MILES

EXPLANATION 

 Stream-sediment sample locality

[ ) Greater than 2,000 ppm Sn (tin) in heavy-mineral concentrates-High values of 

Bi (bismuth) and Sb (antimony) also found in some heavy-mineral concentrates

( ) High values of Pb (lead) in heavy-mineral concentrates

Figure 3. Map showing sample locations and anomalous concentrations of tin, lead, antimony, and bismuth 

in samples from the Little Jacks Creek, Big Jacks Creek, and Duncan Creek Wilderness Study Areas.

Little Jacks, Big Jacks, and Duncan Creeks Wilderness Study Areas AS
opaque oxides; quartz and sanidine are uncommon constituents of some units. In the canyons, individual cooling 

units form ledges that extend for several miles. Black, 

glassy zones occur at the bottom and top of some units; 

some have brecciated zones at the bottom or top. Thin 

layers of poorly consolidated silicic volcanic ash separate 

some cooling units; these ash layers seldom are well exposed. None of the rhyolitic rocks are known to be hydrothermally altered.

Current studies (Kauffman and Bonnichsen, in 

press) suggest that most of the rhyolitic cooling units are 

lava flows. Past studies (Ekren and others, 1984) concluded that the rhyolitic rocks were emplaced as a sequence of extremely hot ash-flow tuffs that developed 

lava-like characteristics due to flowage prior to final chilling; these rocks were called Little Jacks Tuff by Ekren 

and others (1984).

Potassium-argon age determinations for these rocks 

within the wilderness study areas are from 11.2 to 9.6 

Ma (Mega-annum) (Armstrong and others, 1980; Hart and 

Aronson, 1983).

Basalt and Associated Sedimentary Rocks and Silicic Ash

Basalt lava, erupted from vents within and near the 

wilderness study areas, flowed over a relatively flat surface on top of the rhyolitic rock sequence. The basalt 

is commonly dark gray to black and contains small, sparse 

phenocrysts of plagioclase and olivine. The flows are fine 

to medium grained and none are hydrothermally altered.

The basalt lava is commonly seen as one or more 

thin, black flows cropping out at some canyon rims and, 

away from the canyon rims, as a widespread, rough, and 

rocky veneer on the plateau surface. The vents for the 

flows are expressed as low hills that rise above the general 

level of the plateau. No potassium-argon ages are available for these rocks; however, they are considered to be 

Pliocene or Miocene (Kauffman and Bonnichsen, in press) 

(see geologic time chart in Appendix).

Locally, poorly consolidated sedimentary rocks, 

sediments, and silicic ash crop out beneath the basalt or 

between units of the rhyolite. In many places within the 

wilderness study areas, such deposits are very thin or absent or might be concealed by debris shed from overlying 

outcrops of the basalt or rhyolite. Good exposures of these 

rocks are seldom found, and therefore our understanding 

of these deposits is incomplete. They have been characterized as white, tan, and brown lacustrine and fluvial 

deposits of silt, clay, and sand (Kauffman and Bonnichsen, in press). Locally prominent are deposits of 

silicic volcanic ash.

Basalt mantling the plateau surface was more extensive in the past than it is now. Erosion has stripped the 

basalt, and underlying sedimentary rocks where present, 

from much of the area. Where the basalt and underlying

sedimentary rocks have been removed by erosion, the 

rhyolitic rocks are exposed beneath the rather flat erosion 

surface.

Structure

The rocks in the wilderness study areas form a uniformly dipping sequence that dips at a low angle northeastward toward the Snake River Plain and is cut by 

numerous high-angle faults. Displacement on most of the 

faults is small; on many, vertical offset of the rocks is 

too small to be shown on the geologic maps. Exposed 

fault zones consist of masses of rather loose-appearing 

rubble, attesting to low confining pressures at times of 

faulting. Alteration in the fault zones is absent to very 

weak.

Geochemistry

Methods

The reconnaissance geochemical study of the Little 

Jacks Creek, Big Jacks Creek, and Duncan Creek Wilderness Study Areas included the collection, analysis, and 

evaluation of samples from each area. Table 1 shows the 

distribution of samples collected.

Table 1. Number of samples collected from the Little 

Jacks Creek, Big Jacks Creek, and Duncan Creek Wilderness Study Areas

Little Jacks Creek

Duncan Creek ........

Nonmagnetic

heavy-mineralconcentrate

samples

56

16

7

Streamsediment

samples

(fines)

59

81

17

Rock

samples

9

14

2

Analyses of the stream-sediment samples represent 

the chemistry of the rock material eroded from the drainage basin upstream from each sample site. Such information is useful in identifying those basins that contain concentrations of elements that may be related to mineral deposits. 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. The selective concentration of minerals, many of which may be 

ore related, permits determination of some elements that 

are not easily detected in stream-sediment samples.

A6 Mineral Resources of Wilderness Study Areas Bruneau River-Jacks Creek Region, Idaho
Analytical data and a description of the sampling and 

analytical techniques used are given by M. S. Erickson 

and others (unpub. data, 1986).

All samples were analyzed semiquantitatively using 

direct-current arc emission spectrographic methods. The 

rock and stream-sediment samples were analyzed using 

techniques described by Myers and others (1961) and the 

nonmagnetic heavy-mineral concentrates using techniques 

described by Grimes and Marranzino (1968). Certain elements of special interest or that have high lower limits 

of determination by emission spectrography were also 

analyzed in rock and stream-sediment samples using the 

following methods: antimony, arsenic, bismuth, cadmium, 

and zinc by ICAP-AES (inductively coupled argon 

plasma atomic emission spectroscopy, after Crock and 

others, 1983) using a modification of the digestion method 

of O'Leary and Viets (1986); gold by atomic absorption 

using a modification of the method described by 

Thompson and others (1968); and mercury by a modified 

Koirtyohann and Khalil (1976) cold-vapor atomicabsorption method.

Sample localities are shown on figure 3, along withsamples containing anomalous concentrations of tin, lead, 

antimony, and bismuth.

Summary of Analytical Results

Nonmagnetic heavy-mineral concentrates from 13 

sample sites in the Little Jacks Creek Wilderness Study 

Area contained anomalous concentrations of tin, 2,000 

ppm (parts per million) and greater. Two of the samples 

anomalous in tin also contained anomalous bismuth, 20 

and 70 ppm. One of these also contained 700 ppm antimony. Bismuth and antimony commonly are present in 

material containing abundant tin. Three samples yielded 

anomalous values for lead, ranging from 500 to 5,000 

ppm; these samples were from sites different from those 

that yielded high values for tin. No other elements were 

present in anomalous concentrations in these samples. No 

other samples collected during the geochemical survey in 

the Little Jacks Creek Wilderness Study Area contained 

anomalous concentrations of any element.

Nonmagnetic heavy-mineral concentrates from one 

sample site in the Big Jacks Creek Wilderness Study Area 

and one site a few miles northeast of the study area contained greater than 2,000 ppm tin. No other elements were 

present in anomalous concentrations in these samples. No 

other samples collected during the geochemical survey in 

the Big Jacks Creek Wilderness Study Area contained 

anomalous concentrations of any element.

Nonmagnetic heavy-mineral concentrates from one 

sample in the Duncan Creek Wilderness Study Area and 

one sample adjacent to the study area contained greater 

than 2,000 ppm tin; one of these also contained 20 ppm 

bismuth.

All the sample sites from which samples containing 

anomalous tin were collected are located in parts of the 

study areas underlain by rhyolitic rocks. The highest values are associated with streams only a mile or so long 

that drain the erosion surface on the rhyolite from which 

the basalt and associated sediments and silicic ash have 

been stripped. Samples from many nearby streams draining the area of rhyolite exposures contain little or no tin. 

Tin was not detected in any samples of the rhyolitic rocks.

The samples with high values for lead are not systematically associated with any geologic feature. Each 

sample contains high values only for lead; other, commonly associated elements are not present in high concentrations. These samples, moreover, all were from places 

near roads or places where people could reasonably have 

been expected to have gone in the past. Therefore, we 

interpret these data to indicate that the lead is caused by 

a contaminant (probably bullet fragments or birdshot); 

these wilderness study areas have long been popular with 

hunters. Unfortunately, this surmise has not been directly 

verified by detailed examination of the samples because, 

in each case, all that remains of the sample is the part 

ground to fine powder for the chemical analysis.

Characteristics of Tin-Bearing Concentrates

Visual examination of parts of the nonmagnetic 

heavy-mineral concentrates containing 2,000 ppm tin and 

greater that were set aside for reference showed that the 

tin occurs as very fine sand size to silt-size grains of cassiterite (variety wood tin). A simple chemical test for tin, 

immersion of the cassiterite in hydrochloric acid in the 

presence of metallic zinc, verified the visual identification. The bulk of most reference samples consisted of zircon, accompanied by minor pyroxene and biotite. Several 

samples contained a few grains of pyrite.

Zircon, the most abundant mineral in most of the 

nonmagnetic heavy-mineral concentrates from the three 

areas occurs chiefly as well-formed, prismatic crystals; a 

few tiny, rounded zircon grains also were present in one 

sample. Zircon does not occur in the basalt and is uncommon in the rhyolite. Those grains seen in the rhyolite 

are tiny, rounded blebs, similar only to those that are 

subordinate in the nonmagnetic heavy-mineral concentrates. The well-formed zircon crystals must have another 

source.

Pyroxene in the nonmagnetic heavy-mineral concentrates occurs as well-formed, prismatic crystals. The 

species of pyroxene present have not been determined. 

Some of this pyroxene could have been derived locally 

from either the basalt or the rhyolite.

Biotite in the nonmagnetic heavy-mineral concentrates occurs as rounded, altered thick flakes that never 

are larger than very fine sand. Biotite is unknown both 

in the basalt and rhyolite; it must have another source.

Little Jacks, Big Jacks, and Duncan Creeks Wilderness Study Areas A7
Pyrite, present in several of the samples examined, 

occurs as untarnished, subangular grains. Angularity and 

freshness of the pyrite suggests a local source; the source 

has not been found.

Sources for Minerals in Concentrates Containing Tin

Most minerals (including tin minerals) identified in 

the nonmagnetic heavy-mineral concentrates that contain 

abundant tin do not occur in the basalt and rhyolitic rocks 

within the wilderness study areas. The highest tin values 

are in samples from streams draining the erosion surface 

on the rhyolite from which the basalt and associated sediments and sedimentary rocks have been nearly or completely eroded. Lack of tin either in the basalt or the 

rhyolite suggests that the sediments and rocks locally present between the basalt and the rhyolite are probably the 

carriers of the tin. The high specific gravity and small 

grain size of the cassiterite may have favored its lodgement and concentration in irregularities on the erosion surface. Later, small streams draining this erosion surface 

on the rhyolite would have carried away and further concentrated the cassiterite. The final concentration of the 

cassiterite would have taken place during our sampling 

of these streams and the making of the nonmagnetic 

heavy-mineral concentrates. Lack of exposures of the sediments and rocks in critical areas prevents direct testing 

of this hypothesis on the origin of the tin anomalies.

Source regions for the stream deposits, which according to Kauffman and Bonnichsen (in press) account 

for much of the sediment locally exposed beneath the 

basalt, may have been sources for the tin and some of 

the other exotic minerals. Cassiterite (variety wood tin) 

is known from placer deposits in the Silver City area, 

located about 40 mi northwest of the wilderness study 

areas (Piper and Laney, 1926). Cassiterite (variety wood 

tin) also has been found in Miocene stream deposits north 

of Silver City; the headwaters of these streams were the 

Silver City area (Mclntyre, 1972). The mineralized rhyolitic rocks exposed in the Silver City area are the most 

likely source for the cassiterite found in the placers. Tin 

occurs in small amounts in hydrothermally altered rhyolitic rocks west of Silver City (Barrett, 1985). These rhyolites have been dated at about 16 Ma (Armstrong, 1975), 

distinctly older than the rhyolitic rocks exposed in the 

wilderness study areas. We propose that the Silver City 

region may have been the ultimate source for the cassiterite that is found in the wilderness study areas.

Zircon and biotite occur in a rhyolitic ash-fall deposit that rests on the rhyolitic rocks and is overlain or 

interbedded with the basalt, near the east border of the 

Duncan Creek Wilderness Study Area. This deposit, at 

least 40 ft thick, consists chiefly of glass shards and minor 

pumice. It also contains quartz, sanidine, plagioclase, pyroxene, amphibole, biotite, magnetite, and zircon. A

heavy-mineral concentrate prepared from a 6-pound bulk 

sample of the ash contained no cassiterite. The zircon in 

the ash includes many well-formed prismatic grains that 

appear similar to some of those found in the nonmagnetic 

heavy-mineral concentrates. Some of the biotite in the 

nonmagnetic heavy-mineral concentrates may also have 

been supplied by the ash.

Geophysics

Geophysical data provide information on the subsurface distribution of rock masses and the structural 

framework. Gravity, magnetic, and remote-sensing studies 

were undertaken as part of the mineral resource evaluation 

of the Little Jacks Creek, Big Jacks Creek, and Duncan 

Creek Wilderness Study Areas.

Gravity Data

Gravity data were obtained from files maintained 

by the Department of Defense, supplemented with data 

obtained by D. M. Kulik 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/cm3 . Terrain corrections were made 

by computer for a distance of 167 km from the station 

using the method of Plouff (1977). A complete Bouguer 

gravity anomaly map is given on figure 4.

The Owyhee plateau area of southwestern Idaho is 

characterized by intermediate Bouguer anomaly values between -150 and 175 milligals over a predominantly 

rhyolitic terrane. The Duncan Creek Wilderness Study 

Area and the southern parts of the Big Jacks Creek and 

Little Jacks Creek Wilderness Study Areas are at the margin of a roughly circular gravity low, 20 to 25 mi in 

diameter, that suggests an underlying volcano-tectonic depression that could be the source of some of the extrusive 

rocks exposed there. This low coincides with a broad, 

shallow structural sag centered on the source area for the 

Miocene Swisher Mountain Tuff (Ekren and others, 

1984). Toward the northeast, a relatively steep gravity 

gradient separates the circular gravity low from a gravity 

high (shown in the northeast corner of fig. 4) over the 

basalts of the Snake River Plain. The northern parts of 

the Big Jacks Creek and Little Jacks Creek Wilderness 

Study Areas lie within the gradient near the margin of 

the Snake River Plain.

Aeromagnetic Data

The aeromagnetic data are from the aeromagnetic 

map of Idaho (U.S. Geological Survey, 1978). Flight 

lines are east-west at 5-mi intervals and 12,500-ft 

barometric elevation.

A8 Mineral Resources of Wilderness Study Areas Bruneau River-jacks Creek Region, Idaho
117°00' 116°30' 116°00'

43°00'

42°30' -

42°00'

APPROXIMATE BOUNDARY 

. OF THE LITTLE JACKS 

CREEK WILDERNESS 

STUDY AREA

APPROXIMATE BOUNDARY 

OF THE BIG JACKS CREEK 

WILDERNESS STUDY AREA

APPROXIMATE BOUNDARY 

OF THE DUNCAN CREEK 

WILDERNESS STUDY AREA

5 MILES

750, EXPLANATION

Gravity contours--Dashed where approximate. Hachures indicate gravity low. 

Contour interval 5 milligals

 Gravity station- From this study 

H Gravity high

Figure 4. Complete Bouguer gravity anomaly map map of the Little Jacks Creek, Big Jacks Creek, and Duncan 

Creek Wilderness Study Areas.

Little Jacks, Big Jacks, and Duncan Creeks Wilderness Study Areas A9
The aeromagnetic values show only moderate (approximately 300 gammas) variation over the rhyolite terrane southwest of the wilderness study areas (fig. 5). The 

northeastern parts of the Big Jacks Creek and Little Jacks 

Creek Wilderness Study Areas lie along a steep magnetic 

gradient that reflects an abrupt thickening of buried basalts 

of the Snake River Plain (Mabey and others, 1983). A 

high-amplitude, short-wavelength, southwest-trending 

magnetic high in the western part of the Little Jacks Creek 

Wilderness Study Area (fig. 5) coincides with outcrops 

of Eocene volcanic rocks at the margin of the Snake River 

Plain (Mabey, 1982; Ekren and others, 1981). The steep 

gravity and magnetic gradients between the rhyolites of 

the plateau and the basalts of the Snake River Plain parallel the northwest-trending fault system mapped in the 

study areas (fig. 2), suggesting that the margin of the 

plain is fault controlled.

Remote-Sensing Data

Linear features on Landsat MSS (multispectral scanner) images were mapped by photogeologic interpretation 

in the region of southern Idaho. Linear features are the 

surface expression of rock fracture patterns and other 

structural and lithologic lineaments. Analysis of linear features in conjunction with geologic and geophysical maps 

may reveal new relationships, such as fracture control of 

mineralization.

In southwestern Idaho, expression of linear features 

is very poor in volcanic deposits, as in the Snake River 

Plains and also in the Owyhee plateau. North and west 

of the study areas linear features are well expressed in 

an area bounded by long 116°-117° W. and lat 42°30'^3° 

N. Concentrations of linear features have dominant N. 

20° W. and N. 60° E. trends. The north-northwest trend 

is consistent with a N. 40° W.-trending gradient on the 

aeromagnetic map (fig. 4) and northwest-trending highangle faults in the study area (fig. 2). The east-northeast 

trend is typical of pervasive regional fracture sets.

Mineral and Energy Resource Potential

No hydrothermal alteration was noted during 

geologic mapping in any of the three wilderness study 

areas (Kauffman and Bonnichsen, in press) nor has alteration of the rocks been noted during past studies (that is, 

Ekren and others, 1984, and references cited therein). No 

veins or other mineralized structures are known in the 

wilderness study areas. No minerals have been produced 

and no surface evidence exists for metallic-mineral or energy resources in the wilderness study areas. There are

no features present that would suggest the presence of 

metallic-mineral or energy resources at depth beneath the 

wilderness study areas.

Much of the surface in all three study areas is blanketed by basalt that has not been known to contain mineral 

deposits. The rhyolitic rocks present beneath the basalt 

in the study areas are unaltered, and are a type not known 

to be associated with mineral deposits. The rhyolitic rocks 

are characterized by low water and fluorine contents 

(Ekren and others, 1984, table 10) and an anhydrous mineral assemblage. No anomalous concentrations of any elements were detected by our analyses of these rocks.

Little is known about the sediments and sedimentary 

rocks that locally are present between the basalt and the 

rhyolite because the sediments and rocks are poorly exposed. They are the probable source for the cassiterite 

(variety wood tin) that occurs in some of the streams 

draining the erosion surface from which the basalt and 

the sediments and sedimentary rocks have been stripped. 

It is unlikely that high concentrations of cassiterite exist 

within the sediments and sedimentary rocks because the 

amount that has been detected by the stream-sediment 

geochemical survey is low. High values occur only in 

a few samples of stream sediment that underwent deliberate concentration during the collection process. The three 

study areas have low mineral resource potential for tin 

and all other metals, with certainty level C.

Accumulations of oil and gas are unlikely because 

of high temperatures produced during volcanism in the 

past and because of the numerous high-angle faults. A 

geothermal resource at depth is unlikely as there are no 

hot springs within the study areas; none are found even 

along highly permeable, rubble-filled fault zones. None 

of the high-angle faults show signs of significant alteration 

that might be attributed to past hot-spring activity. The 

resource potential for oil and gas and geothermal energy 

is low in the three wilderness study areas, with certainty 

level C.

Recommendations for Future Work

A study devoted to better characterization of the 

sediments and sedimentary rocks locally present beneath 

the basalt would add to our understanding of them and 

to the sources and mode of occurrence of the cassiterite 

(variety wood tin). The study would be difficult because 

exposures of these rocks are so scarce.

REFERENCES CITED

Armstrong, R. L., 1975, The geochronometry of Idaho: Isochron/West, no. 14, p. 1-50.

A10 Mineral Resources of Wilderness Study Areas Bruneau River-Jacks Creek Region, Idaho
117

APPROXIMATE 

BOUNDARY OF 

THE BIG JACKS 

CREEK WILDERNESS 

STUDY AREA

APPROXIMATE BOUNDARY^ fQ 

OF THE LITTLE JACKS CREEK 

WILDERNESS STUDY AREA

APPROXIMATE BOUNDARY

OF THE DUNCAN CREEK

WILDERNESS STUDY AREA

42°30f r 

5 MILES

J

EXPLANATION 

Aeromagnetic contours- -Hachures indicate aeromagnetic lows. Contour interval 100 gammas

Figure 5. Total-intensity aeromagnetic map of the Little Jacks Creek, Big Jacks Creek, and Duncan Creek Wilderness Study Areas. Data and contours from U.S. Geological Survey (1978).

Little jacks, Big jacks, and Duncan Creeks Wilderness Study Areas A11
Armstrong, R. L., Harakal, J. E., and Neill, W. M., 1980, 

K-Ar dating of Snake River Plain (Idaho) volcanic rocks 

new results: Isochron/West, no. 27, p. 5-10.

Barrett, R. A., 1985, The geology, mineralization, and 

geochemistry of the Milestone hot-spring silver-gold deposit 

near the Delamar silver mine, Owyhee County, Idaho: Moscow, Idaho, University of Idaho, M.S. thesis, 237 p.

Crock, J. G., Lichte, F. E., and Briggs, P. H., 1983, Determination of elements in National Bureau of Standards' geological reference materials SRM278 obsidian and SRM688 

basalt by inductively coupled argon plasma-atomic emission 

spectrometry: Geostandards Newsletter, v. 7, p. 335-340.

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

High-temperature, large volume, lavalike ash-flow tuffs 

without calderas in southwestern Idaho: U.S. Geological 

Survey Professional Paper 1272, 76 p.

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

H. E., 1981, Geologic map of Owyhee County, Idaho, west 

of longitude 116° W.: U.S. Geological Survey Miscellaneous Field Investigations Map 1-1256, scale 1:125,000.

Goudarzi, G. H., 1984, compiler, Guide to preparation of mineral survey reports on public lands: U.S. Geological Survey 

Open-File Report 84-787, 42 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.

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

from the western Snake River Plain area, Oregon, Idaho, 

and Nevada: Isochron/West, no. 36, p. 17-19.

International Association of Geodesy, 1967, Geodetic Reference 

System, 1967: International Association of Geodesy Special 

Publication No. 3, 116 p.

Kauffman, D. F., and Bonnichsen, Bill, in press, Geologic map 

of the Little Jacks Creek, Big Jacks Creek, and Duncan 

Creek Wilderness Study Areas, Owyhee County, Idaho: 

U.S. Geological Survey Miscellaneous Field Studies Map, 

scale 1:50,000.

Koirtyohann, S. R., and Khalil, Moheb, 1976, Variables in the 

determination of mercury by cold vapor atomic absorption: 

Analytical Chemistry, v. 48, no. 1, p. 136-139.

Mabey, D. R., 1982, Geophysics and tectonics of the Snake 

River Plain, Idaho, in Bonnichsen, Bill, and Breckenridge, 

R. M., eds., Cenozoic geology of Idaho: Idaho Bureau of 

Mines and Geology Bulletin 26, p. 139-153.

Mabey, D. R., Zietz, Isidore, Eaton, G. P., and Kleinkopf, 

M. D., 1983, Regional magnetic patterns in part of the cordillera in western United States, in Smith, R. B., and Eaton, 

G. P., eds., Cenozoic tectonics and regional geophysics of 

the western cordillera: Geological Society of America 

Memoir 152, p. 93-106.

Mclntyre, D. H., 1972, Cenozoic geology of the Reynolds 

Creek Experimental Watershed, Owyhee County, Idaho: 

Idaho Bureau of Mines and Geology Pamphlet 151, 

115 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.

O'Leary, R. M., and Viets, 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.

Piper, A. M., and Laney, F. B., 1926, Geology and metalliferous resources of the region about Silver City, Idaho: Idaho 

Bureau of Mines and Geology Bulletin 11, 165 p.

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 Open-File Report 77-535, 45 p.

Thompson, C. E., Nakagawa, H. M., and VanSickle, G. H., 

1968, Rapid analysis or gold in geologic materials, in 

Geological Survey research 1968: U.S. Geological Survey 

Professional Paper 600-B, p. B130-B132.

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, 1978, Aeromagnetic map of Idaho: 

U.S. Geological Survey Geophysical Investigations Map 

GP-919, scale 1:500,000.

Winters, R. A., and Leszcykowski, A. M., 1986a, Mineral resources of the Duncan Creek Study Area, Owyhee County, 

Idaho: U.S. Bureau of Mines Open-File Report MLA 48- 

86, 9 p.

 1986b, Mineral resources of the Big Jacks Creek Study 

Area, Owyhee County, Idaho: U.S. Bureau of Mines OpenFile Report MLA 49-86, 9 p.

 1986c, Mineral resources of the Little Jacks Creek Study 

Area, Owyhee County, Idaho: U.S. Bureau of Mines OpenFile Report MLA 50-86, 9 p.

A12 Mineral Resources of Wilderness Study Areas Bruneau River-Jacks Creek Region, Idaho
APPENDIX
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 unlikely. 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 a 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 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.

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

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

o

Q_

O c/)

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.
RESOURCE/RESERVE CLASSIFICATION

ECONOMIC

MARGINALLY 

ECONOMIC

SUBECONOMIC

IDENTIFIED RESOURCES

Demonstrated

Measured Indicated

1

Reserves

Marginal Reserves

Demonstrated 

Subeconomic Resources

Inferred

Inferred Reserves

Inferred 

Marginal Reserves

Inferred 

Subeconomic 

Resources

UNDISCOVERED RESOURCES

Probability Range

 (or) 

Hypothetical i Speculative

1 

1

1

_L1

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.
GEOLOGIC TIME CHART 

Terms and boundary ages used in this report

EON

Phanerozoic

Proterozoic

Archean 

pre-Ar(

ERA

Cenozoic

Mesozoic

Paleozoic

Late Proterozoic

Middle Proterozoic

Early Proterozoic

Late Archean

Middle Archean

Early Archean 

:hean2

PERIOD

Quaternary

Neogene 

Subperiod

Tertiary 

Paleogene 

Subperiod

Cretaceous

Jurassic

Triassic

Permian

Pennsylvanian 

Carboniferous

Periods 

Mississippian

Devonian

Silurian

Ordovician

Cambrian

 3800? -

EPOCH

Holocene

Pleistocene

Pliocene

Miocene

Oligocene

Eocene

Paleocene

Late 

Early

Late 

Middle 

Early

Late 

Middle 

Early

Late 

Early

Late 

Middle 

Early

Late 

Early

Late 

Middle 

Early

Late 

Middle 

Early

Late 

Middle 

Early

Late 

Middle 

Early

 -

BOUNDARY AGE 

IN 

MILLION YEARS

- 0.010

- 1.7

- 24

_ tft

_ AA

- 96

OrtC

-A* 240

-~ 330

oon

- 410

- 435

- ~ 570 1

- 900 

- 1600

- 2500

-3000 

- 3400

 4550 

' Rocks older than 570 m.y. also called Precambrian, a time term without specific rank. 

! Informal time term without specific rank.

*U.S. GOVERNMENT PRINTING OFFICE: 1987-0-773-047/46052