Mineral Resources of the 

Encampment River Canyon 

Wilderness Study Area, 

Carbon County, Wyoming

U.S. GEOLOGICAL SURVEY BULLETIN 1757-F

r

WYOMING

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Chapter F

Mineral Resources of the 

Encampment River Canyon 

Wilderness Study Area, 

Carbon County, Wyoming

By EDWARD A. DU BRAY, VI Kl BAN KEY, and 

RANDALL H. HILL 

U.S. Geological Survey

GEORGE S. RYAN 

U.S. Bureau of Mines

U.S. GEOLOGICAL SURVEY BULLETIN 1757

MINERAL RESOURCES OF WILDERNESS STUDY AREASSOUTHERN WYOMING
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: 1989

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 Encampment River Canyon Wilderness Study Area, 

Carbon County, Wyoming.

(Mineral resources of wilderness study areas southern Wyoming; ch. F) 

(U.S. Geological Survey bulletin ; 1757)

Bibliography: p.

Supt. of Docs, no.: I 19.3:1757F

1. Mines and mineral resources Wyoming Encampment River Canyon 

Wilderness. 2. Encampment River Canyon Wilderness (Wyo.) I. duBray, 

E.A. II. Series. III. Series: U.S. Geological Survey bulletin ; 1757. 

QE75.B9 no. 1757-F 557.3 s [553'.09787'86] 88-6007925 

[TN24.W8]
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 Encampment 

River Canyon Wilderness Study Area (WY-030-301), Carbon County, Wyoming.

CONTENTS

Abstract Fl 

Summary Fl

Character and setting Fl

Identified mineral resources F2

Mineral resource potential F2 

Introduction F4

Investigations by the U.S. Bureau of Mines F4

Investigations by the U.S. Geological Survey F4 

Appraisal of identified resources F4 

Assessment of potential for undiscovered resources F5

Geology F5

Geochemistry F6

Geophysics F7

Mineral and energy resources F8 

References cited F9 

Appendix Fll

FIGURES

1. Index map showing location of the Encampment River Canyon Wilderness Study 

Area F2

2. Map showing mineral resource potential and generalized geology for the Encampment River Canyon Wilderness Study Area F3

3. Residual aeromagnetic anomaly map of the Encampment River Canyon Wilderness Study Area and vicinity F7

4. Uranium equivalent (eU) values for the Encampment River Canyon Wilderness 

Study Area and vicinity F8

5. Thorium equivalent (eTh) values for the Encampment River Canyon Wilderness 

Study Area and vicinity F8

6. Potassium (K) values for the Encampment River Canyon Wilderness Study Area 

and vicinity F9

Contents V

MINERAL RESOURCES OF WILDERNESS STUDY AREAS-SOUTHERN WYOMING

Mineral Resources of the 

Encampment River Canyon 

Wilderness Study Area, 

Carbon County, Wyoming

By Edward A. du Bray, Viki Bankey, and

Randall H. Hill

U.S. Geological Survey

George S. Ryan 

U.S. Bureau of Mines

ABSTRACT

The Encampment River Canyon Wilderness Study 

Area (WY-030-301), which is about 4 miles south of Encampment, Wyoming, contains 4,547 acres whose 

mineral endowment was jointly studied by the U.S. 

Bureau of Mines and U.S. Geological Survey in 1987. 

There are no identified resources in the study area. 

Resource potential for all undiscovered metallic commodities and for undiscovered industrial minerals is low. 

The study area has no potential for undiscovered energy resources, including oil and gas, coal, geothermal 

resources, and uranium.

SUMMARY

Character and Setting

The USGS (U.S. Geological Survey) and the USBM 

(U.S. Bureau of Mines) studied the mineral resource 

potential of 4,547 acres in the Encampment River

Canyon Wilderness Study Area (WY-030-301), Carbon 

County, Wyo., at the request of the BLM (U.S. Bureau of 

Land Management). In this report, the area studied is 

referred to as the"wilderness study area"or the "study 

area." Field studies of the study area were conducted 

during the summer of 1987 by USBM geologists and 

USGS geologists, geochemists, and geophysicists.

The Encampment River Canyon Wilderness Study 

Area is about 4 mi (miles) south of Encampment, Wyo. 

(fig. 1). The study area, which is accessible from 

numerous graded unpaved roads that intersect Wyoming State Route 230, is on the eastern slope of the Sierra Madre, the Continental Divide in this region. 

Elevations in the study area range between 7,200 and 

8,545 ft (feet).

The study area is underlain by Archean felsic gneiss 

and Early Proterozoic quartzite (see geologic time chart 

in Appendix); both are intruded by minor Middle 

Proterozoic mafic plutonic rock. Gneiss occurs 

throughout the eastern and northwestern parts of the 

study area, whereas quartzite occurs in the western and 

southwestern parts. All lithologies are locally offset along 

high-angle faults.

Manuscript approved for publication January 11,1989.

Encampment River Canyon Wilderness Study Area F1
RIVERSIDE,

APPROXIMATE BOUNDARY 

OF ENCAMPMENT RIVER 

CANYON WILDERNESS 

STUDY AREA 

(WY-030-301)

BLACKHALL 

MOUNTAIN

Figure 1. Index map showing location of the Encampment River Canyon Wilderness Study Area, Carbon County, Wyo. Dashed lines are 

selected unpaved roads.

Identified Mineral Resources

During the early 1900's, the original Encampment 

mining district was an important base-metal producer, 

encompassing more than 2,500 mines and prospects. 

The district comprised 25,000 mi2, including the study 

area, and extended from the western slope of the Sierra 

Madre to the eastern slope of the Medicine Bow Mountains and south into Colorado. Within the district, massive suifide deposits occur in Archean rocks, whereas 

stratabound suifide deposits occur in Proterozoic rocks. 

(In this report, the term "deposit," unmodified, does not 

carry a connotation of economic value.) Most of the 

mines are located in the Sierra Madre 5-20 mi west of 

the town of Encampment, within the boundaries of what 

constitutes the presently accepted Encampment mining 

district. Total production of the district is not known, but 

it probably approached 20 million pounds of copper, 

and some zinc, lead, silver, and gold. From 1899 to 1908, 

24 million pounds of copper were produced in Wyoming, nearly all from the original Encampment district 

(Hausel, 1986).

In southern Wyoming, small mines have produced 

uranium, vermiculite, copper, silver, platinum, palladium, and gold associated with quartz or pegmatite assemblages in Precambrian metamorphic host rocks. As 

a result, surface occurrences of siliceous rocks in 

southern Wyoming have been thoroughly examined, accounting for the number of prospects in the study area. 

No production from prospects in the study area is documented. Several claim blocks adjoin and overlap the 

study area boundary.

Small amounts of sand and gravel are present in alluvial deposits of the Encampment River canyon. These 

materials do not constitute a resource because of their 

small volume; abundant quantities are available immediately north of the study area where the Encampment River enters the broad and relatively flat valley of 

the North Platte River.

Mineral Resource Potential

The geologic environment in the Encampment River 

Canyon Wilderness Study Area is favorable for the occurrence of certain types of metallic mineral deposits, 

but the insignificant quantity of mineralized rock, lack of 

major geochemical anomalies not attributable to I it hoiogy, and absence of favorable geophysical characteristics indicate that mineral resource potential in the study 

area is low for all metals (fig. 2).

Energy resources, including oil and gas, coal, 

uranium, and geothermal resources, are unknown in the 

study area. The crystalline rocks that underlie the study 

area are neither source nor reservoir rocks for oil and 

gas; nor are they coal-bearing rocks. The structural setting of the study area is not appropriate to undiscovered 

resources of these commodities. The absence of 

uranium anomalies in the geochemical database and the 

failure of the aeroradiometric survey to identify 

anomalous uranium concentrations indicate no potenF2 Mineral Resources of Wilderness Study Areas-Southern Wyoming
41*11'

41*10'

41*09'

41*08'

106*50' 106*48' 106*46

T

APPROXIMATE BOUNDARY OF 

ENCAMPMENT RIVER CANYON 

WILDERNESS STUDY AREA 

(WY-030-301)

2 MILES

L/C

N/D

EXPLANATION

Geologic terrane having low mineral resource potential,

with certainty level C, for all metals and industrial

minerals 

Geologic terrane having no resource potential, with certainty level D, for coal, oil and gas, geothermal resources, and uranium 

Alluvium (Quaternary)

Landslide deposits (Quaternary)

Pegmatite (Proterozoic)

Mafic plutonic rock (Proterozoic)

Quartzite (Proterozoic)

Felsic gneiss (Archean)

Contact

Fault Dotted where concealed

Prospect pits, adits, and shafts

Levels of certainty

Data indicate geologic environment and give good 

indication of level of resource potential but do not 

establish activity of resource-forming processes

Data clearly define geologic environment and level of 

resource potential

Figure 2. Map showing mineral resource potential and generalized geology for the Encampment River Canyon 

Wilderness Study Area, Carbon County, Wyo.

Encampment River Canyon Wilderness Study Area F3
tial for uranium deposits in the study area. The absence 

of geothermal phenomena and the old age of rocks in 

the study area indicate that there is no potential for 

geothermal energy resources. Accordingly, there is no 

potential for energy resources in the study area (fig. 2). 

Available geologic, geochemical, and geophysical 

data indicate low potential for undiscovered industrial 

mineral resources in the study area (fig. 2).

INTRODUCTION

The Encampment River Canyon Wilderness Study 

Area, about 4 mi south of Encampment, Wyo. (fig. 1), is accessible from graded unpaved roads that intersect Wyoming State Route 230. Travel in the study area is by way of a 

few rough jeep roads and on foot. The study area is located 

on the eastern slope of the Sierra Madre, part of the Continental Divide in this region, and is a gently rolling, dissected upland covered by grasses and sage; the highest 

north-facing slopes support stands of aspen and fir trees. 

The Encampment River flows south to north, through a 

1,000-ft-deep canyon, across the study area. Elevations in 

the area range from about 7,200 ft in the Encampment 

River canyon to 8,545 ft on the adjacent canyon walls.

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 U.S. 

Bureau of Mines and U.S. Geological Survey (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 concentrations of metals and nonmetals, of unappraised industrial 

rocks and minerals, and of undiscovered energy resources 

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

classified according to the system of Goudarzi (1984), 

which is shown in the Appendix of this report. The potential for undiscovered resources is studied by the USGS.

Investigations by the U.S. Bureau of 

Mines

USBM geologists searched the literature in which mining, mineral lease, and mining claim information has been 

compiled. Files at the BLM State Office in Cheyenne, 

Wyo., and the Carbon County Courthouse in Rawlins, 

Wyo., were reviewed for mining claim locations, patented 

mining claims, and oil and gas and geothermal energy 

leases and lease applications. Lessees, mine owners, and 

persons having knowledge of mineral occurrences and

mining activities in and near the study area were contacted. 

Field work by USBM personnel entailed examination of 

prospects and mines in and near the wilderness study area. 

Accessible mine workings were mapped, and mineralized 

areas and mine dumps were sampled. Detailed descriptions of analysis procedures and analytical results are 

presented by Ryan (1988), and complete analytical results 

are available for public inspection at the U.S. Bureau of 

Mines, Intermountain Field Operations Center, Building 

20, Denver Federal Center, Denver, Colo.

Investigations by the U.S. Geological 

Survey

The assessment of the mineral resource potential of 

the Encampment River Canyon Wilderness Study Area is 

based on data from many sources. Among these are 

geologic studies of the Sierra Madre and the Medicine Bow 

Mountains (Ferris, 1964; Houston and Ebbett, 1977; Graff, 

1978; Houston and others, 1978; and du Bray, this report); 

geochemical studies by the USBM (Ryan, 1988) and the 

USGS (R.H. Hill, unpub. data, 1987); and interpretations 

of geophysical data including aeromagnetic and 

aeroradiometric data (Viki Bankey, this report) and 

Landsat Thematic Mapper data (Alta Walker, written 

commun., 1988).

APPRAISAL OF IDENTIFIED 

RESOURCES

By George S. Ryan 

U.S. Bureau of Mines

Malachite, azurite, and iron and manganese oxides 

along contacts of quartz or pegmatite pods and veins within 

the metamorphic rocks and mafic intrusive rocks are 

similar to contact mineral associations and assemblages 

present at mines and prospects at nearby mining districts. 

Most workings in the study area appear to be from turn-ofthe-century operations; in many cases caving and extensive 

erosion have obliterated the source of mineralized rocks 

found in dumps. One adit was found open and was mapped 

and sampled, but no significant mineralized rock was noted 

and only minor structures were found. No claim notices 

were found in the courthouse that could be associated with 

the prospects found in the field.

Assay results for selected elements (Ryan, 1988, table 

1) indicate that copper and manganese were the most 

widespread elements of interest. Copper concentrations in 

some samples would be significant if present in large tonF4 Mineral Resources of Wilderness Study Areas - Southern Wyoming
nages; however, the restricted and limited occurrences observed indicate small tonnages. A single panned-concentrate sample was devoid of any elements of interest, and 

that absence indicates a lack of mineral concentrations in 

rocks upstream. Sixty-four chip and grab samples contained no element concentrations other than those expected as a consequence of surface observations of copper, 

iron, and manganese oxides and carbonates. Four samples 

that contained greater than 1 percent copper were from 

widely separated sample localities and associated with ubiquitous copper and iron oxide-stained rocks. Two of 

these samples contained 20.8 ppm (parts per million) silver and 1.55 ppm gold, respectively. These low concentrations and apparently restricted occurrences do not 

constitute a resource.

Although numerous prospect pits are present, no 

mineral resources were identified during the USBM investigation of the study area. Copper and manganese were 

present in samples from several widely spaced localities; 

however, these elements were not found in sufficient quantities to indicate a resource. Similarly, silver and gold were 

present in low concentrations and do not appear to represent a resource.

Metamorphic rocks in the study area are not conducive to the accumulation of hydrocarbons. Wilderness 

lands on the east flank of the Sierra Madre uplift have no 

known oil and gas potential (Spencer, 1983); nor are there 

oil and gas leases or lease applications in the study area.

Small amounts of sand and gravel are present in alluvial deposits of the Encampment River canyon, but steep, 

narrow drainage channels preclude major concentrations 

of sand and gravel deposits within the study area. Abundant material is available immediately north of the study 

area where the Encampment River enters the broad and 

relatively flat valley of the North Platte River.

ASSESSMENT OF POTENTIAL FOR 

UNDISCOVERED RESOURCES

By Edward A. du Bray, Viki Bankey, and

Randall H. Hill

U.S. Geological Survey

Geology

Existing geologic maps at various scales (Ferris, 1964; 

Graff, 1978; Houston and Ebbett, 1977; and Houston and 

others, 1978) were employed in compilation of a preliminary geologic map of the study area prior to field work by 

USGS personnel. Outcrops were examined for signs of

mineralized rock. Samples of all rock units were collected 

for petrographic studies.

The Encampment River Canyon Wilderness Study 

Area is at the southern margin of the Archean Wyoming 

craton (Hills and Houston, 1979) and is adjacent to the 

Colorado Plateau physiographic province (Bickford and 

others, 1986) along the Cheyenne belt. The Cheyenne belt 

is an inferred suture zone at the southern edge of the North 

American craton along which lithospheric material was accreted during Proterozoic time. The study area is underlain by Archean felsic gneiss and Early Proterozoic 

quartzite; both are intruded by small bodies of Middle 

Proterozoic mafic plutonic rock. Gneiss occurs throughout 

the eastern and northwestern parts of the study area, 

whereas quartzite occurs in the western and southwestern 

parts. All lithologies are locally offset along high-angle 

faults.

Quartz-rich, biotite-muscovite felsic gneiss underlies 

much of the east half of the study area. The gneiss is 

moderately well foliated, as denoted by segregation banding, and is fine to medium grained. Anhedral quartz grains 

0.004-0.1 in. in diameter and displaying strain-induced undulose extinction form about 80 percent of the gneiss. Albite-twinned, anhedral grains and some relict 

porphyroblasts of plagioclase (0.004-0.08 in. in diameter) 

form 5-30 percent of the gneiss. Light-tan to tan subhedral 

flakes of biotite (0.004-0.06 in. in diameter) form 5-10 percent of the rock. Colorless, subhedral muscovite (0.004- 

0.03 in. in diameter), stably intergrown with biotite, 

composes 3-5 percent of the rock. About 1 percent 

granular epidote (0.004-0.02 in. in diameter) was also identified. The gneiss contains trace amounts of apatite and 

sphene.

Rock that varies between quartzite and quartz-mica 

schist underlies the western third of the study area. This 

white- to buff-weathering rock is fine grained and finely 

laminated. It is composed of 90 percent anhedral quartz 

grains (0.004-0.01 in. in diameter) that display undulatory 

extinction. Subhedral, interstitial muscovite laths (less than 

0.008 in. in diameter) form accumulations that impart a 

laminated texture to this lithology. The quartzite contains 

trace amounts of gridiron-twinned microcline.

Altered, mafic plutonic rock intrudes the felsic gneiss 

and the quartzite. The mafic plutonic rock is massive to 

weakly foliated, fine grained, and equigranular. It is principally (80-90 percent) composed of pale-brownish-green 

to pale-blue-green, anhedral actinolite grains (0.02-0.08 in. 

in diameter) that are probably pseudomorphs after 

pyroxene. Quartz, in anhedral grains (less than 0.008 in. in 

diameter) that display undulatory extinction, occurs interstitially as a trace constituent, forming as much as about 15 

percent of the rock. Interstitial, albite-twinned plagioclase 

grains (0.016- 0.08 in. in diameter) form as much as 5 percent of the mafic plutonic rock. About 5 percent subhedral

Encampment River Canyon Wilderness Study Area F5
epidote (less than 0.004 in. in diameter) and trace amounts 

of sphene, apatite, and opaque oxides were also identified. 

Numerous pegmatite dikes concordantly intrude felsic 

gneiss east of the Encampment River. The dikes have massive quartz cores, and large feldspar crystals concentrated 

along dike margins have altered to clay. Because this clay 

is highly susceptible to erosion, most evidence of feldspar 

in the dikes is absent and the dikes resemble massive quartz 

veins. The dikes contain sparse, fine-grained kyanite.

Geochemistry

In September of 1987, a reconnaissance geochemical 

survey (R.H. Hill, unpub. data) was conducted in the Encampment River Canyon Wilderness Study Area to aid 

evaluation of its mineral resource potential. Rocks, stream 

sediments, and heavy-mineral concentrates of stream sediment were selected as the sample media for evaluation.

Thirteen minus-60-mesh stream-sediment samples 

collected from active stream alluvium and 14 heavymineral panned concentrates derived from stream sediment were selected as primary sample media. The 

sediments are thought to represent a composite of the 

chemistry of rock and soil exposed in the drainage basin 

upstream from the sample site. Chemical analyses of such 

stream-sediment samples provide data useful in identifying 

those basins that contain unusually high concentrations of 

elements that maybe related to mineral occurrences. In addition, studies have shown that heavy-mineral panned concentrates derived from stream sediment are useful in 

studying arid to semiarid environments or areas of rugged 

topography, where mechanical erosion predominates over 

chemical erosion (Bugrov and Shalaby, 1975; Overstreet 

and Marsh, 1981). Heavy-mineral-concentrate samples 

provide information about the chemistry of a limited number of minerals present in rock material eroded from the 

drainage basin upstream from each sample site. Preparation of heavy-mineral concentrates from stream sediments 

results in a concentration of mineral assemblages that may 

contain ore-forming and ore-related minerals. The selective concentration of these minerals permits determination 

of some elements that are not easily detected in bulk 

stream-sediment samples.

Seven fresh and unaltered rock samples were collected 

from outcrops to provide chemical and mineralogic information for rocks exposed near stream-sediment sample 

sites. In addition, a mine-dump sample was collected to 

provide information concerning the composition of 

mineralized rock present in the study area. The actual areal 

extent of the influence of each rock type on the chemistry 

of individual stream-sediment and panned-concentrate 

samples is indeterminate, though the sampling program

was designed to provide general information concerning 

the geochemical nature of the exposed lithologies.

The dry stream-sediment samples were sieved through 

60-mesh stainless-steel sieves. The minus-60-mesh 

material was retained for analysis and pulverized with 

ceramic plates to at least minus-100 mesh prior to analysis. 

To produce the heavy-mineral concentrate, bulk stream 

sediment was first sieved through a 10-mesh screen. Approximately 25-30 pounds of the minus-10-mesh sediment 

was panned to remove most of the quartz, feldspar, organic 

material, and clay- sized material. These samples were then 

air dried, passed through a 30-mesh sieve, and separated 

into light and heavy fractions by means of bromoform 

(heavy liquid, specific gravity 2.86). The light fraction was 

discarded. The material of specific gravity greater than 2.86 

was separated into three fractions (highly magnetic, weakly magnetic, and nonmagnetic) by means of a modified 

Frantz Isodynamic Magnetic Separator. The nonmagnetic 

fraction was hand ground and retained for emission 

spectrographic analysis. Rock samples were crushed and 

pulverized to at least minus-100 mesh with ceramic plates 

prior to analysis. Rock, stream-sediment, and nonmagnetic 

portions of the prepared heavy-mineral concentrate were 

analyzed for 35 elements by means of a semiquantitative, 

direct-current arc emission spectrographic method 

(Grimes and Marranzino, 1968); abundances of platinum 

and palladium in the concentrate samples were also determined by this method.

Threshold values, defined as the upper limit of normal 

background values, were determined for each element by 

inspection of frequency-distribution histograms for all 

sample media. The composite mine-dump rock sample was 

not included in construction of the histograms. A 

geochemical value greater than the threshold value is considered anomalous and worthy of consideration as a possible indicator of mineralized rock.

The primary criterion for delineating an area as 

anomalous is the presence of multi-element geochemical 

anomalies clustered within a restricted geographic region. 

Follow-up studies should, however, include isolated 

anomalous sample localities with geographically extensive, 

geochemically anomalous areas because the scale of sampling conducted for this study is so small that moderatesized deposits could be indicated by a single sample site, or 

not be apparent at all. In addition, this study is a surface 

study and may not reflect the presence of mineralized rock 

at depth.

Heavy-mineral-concentrate samples from two sites in 

the southwestern part of the study area, one from a westflowing stream and the other from a southeast-flowing 

stream (R.H. Hill, unpub. data, 1987), contained 

anomalous concentrations of beryllium (200 and 100 ppm, 

respectively), lead (300 and 200 ppm), tin (700 and 100

F6 Mineral Resources of Wilderness Study Areas - Southern Wyoming
ppm), and tungsten (500 and 200 ppm). In addition, the 

stream-sediment sample from the west-flowing stream 

contained 500 ppm chromium and 100 ppm nickel. 

Anomalous elemental abundances in the heavy-mineral 

concentrates may result from weak hydrothermal alteration and mineralization of quartzite along thrust faults 

(Graff, 1978) that could have served as conduits for migrating hydrothermal fluids. Alternatively, the anomalies may 

arise from dense, detrital accessory minerals contained 

within quartzite and quartz-pebble conglomerate exposed 

in this area. Anomalous nickel and chromium abundances 

in the stream-sediment sample from the west-flowing 

stream are believed to represent detrital accumulations of 

chromite derived by mechanical weathering of nearby 

mafic plutonic rocks.

A heavy-mineral-concentrate sample from a site approximately 1 mi southwest of the confluence of Miner 

Creek and Encampment River (R.H. Hill, unpub. data, 

1987), in a tributary of Miner Creek, contains 5,000 ppm 

barium, 100 ppm bismuth, and 150 ppm tungsten. These 

anomalous values may indicate weak metamorphism, 

hydrothermal alteration, and mineralization at the contact 

zone where quartzite is intruded by mafic plutonic rocks. 

Alternatively, the anomaly may be related to weak 

mineralization of rock in a thrust fault (Graff, 1978), which 

could have served as a conduit for migrating hydrothermal 

fluids, located upstream in the head of the drainage basin. 

Another heavy-mineral-concentrate sample from Miner 

Creek, near its confluence with Encampment River, contains 150 ppm tin and 100 ppm tungsten. These anomalous 

concentrations may indicate downstream transport of 

dense ore minerals from anomalous areas upstream.

Anomalous bismuth and tungsten abundances were 

determined for heavy-mineral-concentrate samples 

sporadically distributed in the study area. The source of 

these element anomalies is uncertain, although they may 

indicate weak metamorphism, hydrothermal alteration, 

and mineralization at the contact zone where quartzite and 

(or) felsic gneiss is intruded by mafic plutonic rocks.

Geophysics

A detailed aeromagnetic survey of the wilderness 

study area and vicinity was flown in 1979 by a fixed-wing 

aircraft at a nominal ground clearance of 400 ft. Flight 

traverses were oriented north-south and spaced 0.25 mi 

apart; these specifications provide 60 percent coverage of 

surface sources and 100 percent coverage of sources buried 

below 200 ft depth. After correction for diurnal effects and 

tie-line discrepancies, the International Geomagnetic 

Reference Field was removed. The resulting residual magnetic data were compiled and contoured by the contractor

106°50' 106°45'

Figure 3. Residual aeromagnetic anomaly map of the Encampment 

River Canyon Wilderness Study Area and vicinity, Carbon County, 

Wyo. Heavy line is approximate boundary of wilderness study area. 

Contour interval 25 nanoteslas. A, aeromagnetic anomaly discussed 

in text; hachures indicate area of closed low.

(Bendix Field Engineering Corporation, 1983) to produce 

a regional aeromagnetic map. The map is useful in geologic 

interpretations of the region that contains the study area; 

a subset of the data was used to aid interpretation of the 

geology in the study area.

Aeromagnetic data (fig. 3) are useful in delineating 

concealed intrusive rocks that could be associated with 

mineral deposits. Aeromagnetic data for the Encampment 

River Canyon Wilderness Study Area are characterized by 

a northwest grain that corresponds with the outcrop pattern of northwest-trending mafic plutonic rock and pegmatite dikes. Specifically, two northwest-trending 

magnetic highs are associated with mafic plutonic rock; 

these highs are limited to the area underlain by this rock 

type. Mafic rocks in the southwestern part of the study area 

are not associated with magnetic anomalies, which suggests 

that the mafic plutonic rocks are variably magnetized. In 

the northeastern part of the study area and north of its 

boundary, a linear magnetic low trends northwest and is associated with pegmatite outcrops. Positive magnetic 

anomaly A causes discontinuity in the magnetic low in a 

region characterized by fewer pegmatite outcrops. 

Anomaly A is probably attributable to outcropping mafic 

plutonic rocks that are abundant in this area and more intensely magnetized than those that crop out to the east. 

Neither quartzite nor felsic gneiss in the study area appear 

to contain magnetic minerals, and no other magnetic 

anomalies were detected.

Encampment River Canyon Wilderness Study Area F7
106°50' 106°45' 106°50' 106°45'

Rgure 4. Uranium equivalent (eU) values for the Encampment River 

Canyon Wilderness Study Area and vicinity, Carbon County, Wyo. 

Heavy line is approximate boundary of wilderness study area. 

Contour interval 0.5 parts per million; hachures indicate area of 

closed low.

Rgure 5. Thorium equivalent (eTh) values for the Encampment River 

Canyon Wilderness Study Area and vicinity, Carbon County, Wyo. 

Heavy line is approximate boundary of wilderness study area. 

Contour interval 1.5 parts per million; hachures indicate area of 

closed low.

Aerial gamma-ray spectroscopy is a radiometric technique that provides an estimate of the near-surface (depth 

0-20 in.) concentrations of uranium (U), thorium (Th), and 

potassium (K), and is used in exploration for uranium 

deposits. The occurrence of radioactive quartz-pebble 

conglomerate in the Medicine Bow Mountains and Sierra 

Madre necessitated a detailed aeroradiometric survey of 

the region to determine the extent and abundance of 

uranium. Aeroradiometric data for the study area (figs. 4- 

6) were obtained from traverses flown in 1979 by contractors working for the U.S. Department of Energy as part of 

the National Uranium Resource Evaluation (Bendix Field 

Engineering Corporation, 1983). The traverses are along 

north-south lines spaced 0.25 mi apart and have a nominal 

ground clearance of 400 ft. These specifications are sufficient to achieve 60 percent ground coverage. The U and 

Th values are "equivalent" values (eU and eTh) to account 

for possible disequilibrium in the decay series for those elements.

Some of the rock types in the study area, including felsic gneiss and quartzite, may have above-background 

radioactivity due to elevated abundances of the radioactive 

elements in potassium feldspar, mica, or accessory 

minerals. Uranium can become concentrated along fractures in quartz and feldspar. However, clean quartzite may 

be depleted in radioactive minerals and would approach 

background levels. In the study area none of the concentrations of uranium, thorium, or potassium is anomalous 

where quartzite crops out. Concentrations range from 0.5 

to 2.0 ppm eU, 4.0 to 13.5 ppm eTh, and 1.0 to 2.0 percent 

K. Observed radioactivity slightly exceeds background 

levels in and beyond the northern tip of the study area, 

where readings reach 2 ppm eU, 13.5 ppm eTh, and 2 percent K. The weakly anomalous radioactivity may be associated with minor segregations of radioactive minerals in 

pegmatite dikes; the radioactivity is orders of magnitude 

less intense than that associated with ore-grade radioactive 

mineral deposits.

Landsat Thematic Mapper data can be used to identify indications of mineralized or altered rock exposed at 

the Earth's surface. Data for seven spectral bands, 

analyzed individually, as composites, and as ratios, do not 

indicate the presence of mineralized or altered rock in the 

study area (Alta Walker, written commun., 1988).

Mineral and Energy Resources

Accumulated evidence suggests that there is low 

mineral resource potential, with level-C certainty, for undiscovered metallic mineral deposits in the Encampment 

River Canyon Wilderness Study Area (fig. 2). The geologic 

setting of the study area is favorable for various types of 

metalliferous deposits (including deposits of platinumF8 Mineral Resources of Wilderness Study Areas - Southern Wyoming
106 0 50' 10B°45'

Figure 6. Potassium (K) values for the Encampment River Canyon 

Wilderness Study Area and vicinity, Carbon County, Wyo. Heavy line 

is approximate boundary of wilderness study area. Contour interval 

0.5 parts per million; hachures indicate area of closed low.

group elements associated with mafic plutonic rock, 

deposits of gold and uranium in quartz-pebble conglomerate, massive and stratabound sulfide deposits, and 

pegmatite-hosted deposits), but supporting evidence and 

additional features indicative of favorability for the occurrence of deposits in the study area are absent. The quantity of mineralized rock, found at widely dispersed localities 

during field work, was minimal and the intensity of 

mineralization insignificant. Anomalous concentrations in 

the geochemical data are a function of lithologies present 

in the sampled drainage basins or reflect the presence of 

weakly, locally mineralized rock. Geophysical data do not 

indicate the presence of mineral deposits and geologic settings are not favorable for them. These findings suggest that 

the mineral resource potential for all metals in the wilderness study area is low, with level-C certainty.

Energy resources, including oil and gas, coal, uranium, 

and geothermal resources, are unknown in the study area. 

The crystalline rocks that underlie the study area are 

neither source nor reservoir rocks for oil and gas; nor are 

they coal-bearing rocks. The structural setting of the study 

area is not appropriate to undiscovered resources of these 

commodities. The absence of uranium-anomalous values 

in the geochemical database and the failure of the 

aeroradiometric survey to identify anomalous uranium 

concentrations indicate no potential for uranium deposits 

in the study area. The absence of geothermal phenomena 

and the old age of rocks in the study area indicate that there 

is no potential for geothermal resources. Accordingly, a

rating of no potential (with level-D certainty) for energy 

resources is assigned to the study area.

The geologic setting in the study area is not favorable 

for the occurrence of industrial mineral resources. Available geologic, geochemical, and geophysical data indicate 

low potential (with level-C certainty) for undiscovered industrial mineral resources.

REFERENCES CITED

Bendix Field Engineering Corporation, 1983, Aerial radiometric and 

magnetic survey, Sierra Madre Detail Area: U.S. Department of 

Energy Open-File Report GJBX-034(83), scale 1:62,500.

Bickford, M.E., Van Schmus, W.R., and Zietz, Isidore, 1986, Proterozoic 

history of the midcontinent region of North America: Geology, v. 

14, p. 492-4%.

Bugrov, V.A., and Shalaby, I.M., 1975, Geochemical prospecting in the 

eastern desert of Egypt, in Elliott, I.L., and Fletcher, W.K., eds., 

Geochemical exploration, 1974: Elsevier, p. 523-530.

Ferris, C.S., 1964, Petrology and structure of the Precambrian rocks 

southeast of Encampment, Wyoming: Laramie, University of 

Wyoming, Ph.D. thesis, 74 p.

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

public lands: U.S. Geological Survey Open-File Report 84-787, 42

PGraff, PJ., 1978, Geology of the lower part of the early Proterozoic Snowy 

Range Supergroup, Sierra Madre, Wyoming: Laramie, University 

of Wyoming, Ph.D. thesis, 85 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.

Hausel, W.D., 1986, Mineral deposits of the Encampment mining district, 

Sierra Madre, Wyoming-Colorado: Geological Survey of Wyoming 

Report of Investigations 37, 31 p.

Hills, F.A., and Houston, R.S., 1979, Early Proterozoic tectonics of the 

central Rocky Mountains, North America: University of Wyoming 

Contributions to Geology, v. 17, p. 89-109.

Houston, R.S., and Ebbett, B.E., 1977, Geologic map of the Sierra Madre 

and Western Medicine Bow Mountains, southeastern Wyoming: 

U.S. Geological Survey Miscellaneous Field Studies Map MF-827, 

scale 1:125,000.

Houston, R.S., and others, 1978, A regional study of rocks of Precambrian 

age in that part of the Medicine Bow Mountains lying in 

southeastern Wyoming, with a chapter on The relationship between 

Precambrian and Laramide structure: Geological Society of Wyoming Memoir 1,167 p.

Overstreet, W.C., and Marsh, S.P., 1981, Some concepts and techniques 

in geochemical exploration, in Skinner, B.J., ed., Economic Geology, 75th Anniversary Volume: Economic Geology Publishing 

Company, p. 775-805.

Ryan, G.S., 1988, Mineral investigation of the Encampment Wilderness 

Study Area (WY-030-301), Carbon County, Wyoming: U.S. 

Bureau of Mines Mineral Land Assessment Open File Report 

MLA17-88,17p.

Spencer, C.W., 1983, Petroleum potential of wilderness lands in Wyoming, in Miller, B.M., ed., Petroleum potential of wilderness lands in 

the Western United States: U.S. Geological Survey Circular 902- 

A-P, p. M1-M10.

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.

Encampment River Canyon Wilderness Study Area F9

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

HtGH POTENTIAL

M/D 

MODERATE POTENTIAL

L/D

LOW POTENTIAL

N/D

NO POTENTIAL

t

o

CL

O V)

LU 

GC

LU 

LU

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

I

+

1

1

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

Tertiary

Neogene 

Subperiod

Paleogene 

Subperiod

Cretaceous

Jurassic

Triassic

Permian

Carboniferous 

Periods

Pennsylvanian

Mississippian

Devonian

Silurian

Ordovician

Cambrian

__ __ 3800,- -

EPOCH

Holocene

Pleistocene

Pliocene

Miocene

Oligocene

Eocene

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

- 5

 JO

GR

- 96

1 JO 

.- 1AC

-A, 240 

- oao

-~ 330

- 360

- 410

A OC

-~ 570'

- 900 

- 1600

w 1CAA

- 3000 

- 3400

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

2 Informal time term without specific rank.

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