Mineral Resources of the 

Eagle Mountain 

Wilderness Study Area, 

Pitkin County, Colorado

U.S. GEOLOGICAL SURVEY BULLETIN 1717-B

COLORADO

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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

HJGH POTENTIAL

M/D 

MODERATE POTENTIAL

L/D

LOW POTENTIAL

N/D

NO POTENTIAL

o

Q.

(J

DC

O

LU 

QC

U.

O

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
Chapter B

MINERAL RESOURCES OF WILDERNESS STUDY AREAS 

NORTH-CENTRAL COLORADO

Mineral Resources of the 

Eagle Mountain 

Wilderness Study Area, 

Pitkin County, Colorado

By Sandra J. Soulliere and Mark A. Arnold, 

U.S. Geological Survey, and

Steven E. Kluender and Jeanne E. Zelten, 

U.S. Bureau of Mines

U.S. GEOLOGICAL SURVEY BULLETIN 1717-B
DEPARTMENT OF THE INTERIOR 

DONALD PAUL MODEL, Secretary

U.S.GEOLOGICAL SURVEY 

Dallas L. Peck, Director

Library of Congress Cataloging in Publication Data

Mineral resources of the Eagle Mountain Wilderness Study area, Pitkin County, Colorado

(U.S. Geological Survey bulletin ; 1717-B)

Bibliography p.

Supt. of Docs. No.: I 19.3:1717-6

1. Mines and mineral resources Colorado Eagle Mountain Wilderness. 2. Geology Colorado Eagle 

Mountain Wilderness. 3. Eagle Mountain Wilderness (Colo.) I. Soulliere, Sandra J. II. Series. 

QE75.B9 no. 1717-B 557.3 s [553'.9788'43] 86-600004 

[TN24.C6]

UNITED STATES GOVERNMENT PRINTING OFFICE: 1986

For sale by the Branch of Distribution 

U.S. Geological Survey 

604 South Pickett Street 

Alexandria, VA 22304
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 Eagle Mountain (CO-070-392) 

Wilderness Study Area, Pitkin County, Colorado.

CONTENTS

Summary 1 

Introduction 1

Investigation by the U.S. Bureau of Mines 1

Investigation by the U.S. Geological Survey 3

Geologic setting 3 

Appraisal of identified resources 3

Mining and mineral-exploration activity 3 

Assessment of potential for undiscovered resources 7

Geology 7

Geochemistry 7

Analytical methods 7 

Results of survey 8

Mineral and energy resources 8 

References cited 8

FIGURES

1. Map showing mineral resource potential of the Eagle Mountain Wilderness Study 

Area 2

2. Geologic map and cross section of the Eagle Mountain Wilderness Study Area 4

3. Mine and prospect map of the Eagle Mountain Wilderness Study Area 6

Contents V

MINERAL RESOURCES OF WILDERNESS STUDY AREAS NORTH-CENTRAL COLORADO

Mineral Resources of the

Eagle Mountain Wilderness Study Area,

Pitkin County, Colorado

By Sandra J. Soulliere and Mark A. Arnold 

U.S. Geological Survey and 

Steven E. Kluender and Jeanne E. Zelten 

U.S. Bureau of Mines

SUMMARY

In 1983 and 1984 the U.S. Bureau of Mines and 

the U.S. Geological Survey conducted investigations to 

appraise the identified mineral resources and assess the 

mineral resource potential of the Eagle Mountain (CO070-392) Wilderness Study Area. These investigations revealed no identified mineral resources and low mineral 

resource potential for undiscovered resources including 

base and precious metals, oil, gas, and uranium (fig. 1).

The Eagle Mountain Wilderness Study Area is 

about 10 mi (miles) west of Aspen, in Pitkin County, 

Colo., (fig. 1). Steep, mountainous topography is typical 

of the 330-acre study area, which adjoins the Maroon 

Bells-Snowmass Wilderness in the White River National 

Forest. At least 5,200 ft of Pennsylvanian to Cretaceous 

age sedimentary rocks are exposed in one-half of the 

study area (fig. 2); Quaternary glacial deposits and colluvium cover one-third. The Snowmass Creek fault zone, 

a series of arcuate faults, cuts the southern edge of the 

study area.

No minerals have been produced and no surface 

evidence exists for metallic-mineral or energy resources 

in the Eagle Mountain Wilderness Study Area. Minor but 

unknown amounts of copper and silver have been produced from mines within 0.25 mi of it. No mines, prospects, mineralized areas, patented mining claims, or 

mineral leases were identified in the study area. Sand, 

gravel, and industrial rock resources are in the study 

area, but sufficient quantities are available elsewhere to 

satisfy current local needs.

Copper, lead, silver, zinc, uranium, and vanadium 

minerals may occur below surface in favorable host 

rocks, most likely the Dakota Sandstone and Mancos 

Shale, or along faults which transect the study area. 

However, geological and geochemical evidence suggest

a low potential for these metals. No surface evidence 

of uranium mineralization was noted during the field investigation, and no uranium was detected in the 

geochemical samples. Although the Morrison Formation 

is a favorable host rock for uranium, the potential for 

the presence of this resource is low. Oil and gas potential is low. Temperatures were too high for the preservation of oil in favorable reservoir rocks, and the structural 

setting is unfavorable for gas.

INTRODUCTION

The Eagle Mountain (CO-070-392) Wilderness 

Study Area in Pitkin County, Colorado, is about 10 mi 

west of Aspen and comprises approximately 330 acres 

(fig. 1). The western boundary of the study area is the 

Maroon Bells-Snowmass Wilderness in the White River 

National Forest. The area is accessible from a county road 

which forms parts of the southern boundary.

Steep, mountainous topography is typical of the 

study area, in which elevations range from 8,280 ft (feet) 

near Snowmass Creek to 9,937 ft at the summit of Eagle 

Mountain. Several intermittent streams drain the study 

area and flow into Snowmass Creek, a tributary to the 

Roaring Fork river.

Investigation by the U.S. Bureau of Mines

In September 1983, the U.S. Bureau of Mines 

(USBM) conducted a mineral investigation of the Eagle 

Mountain Wilderness Study Area (Kluender and Zelten, 

1985). Prior to the field investigation, USBM geologists 

reviewed published and unpublished information, interEagle Mountain Wilderness Study Area, Pitkin County, Colorado B1
107°00'00" 106°'57'30"

39°12'30"

APPROXIMATE BOUNDARY OF THE

EAGLE MOUNTAIN WILDERNESS

STUDY AREA

(CO-070-392)

MAROON BELLSSNOWMASS 

WILDERNESS

EXPLANATION 

-k Inaccessible adit

L/B Geologic terrane having low mineral resource

potential with certainty level B for base and 

precious metals, uranium, and oil and gas

COLORADO

0.5MILE

Figure 1. Map showing mineral resource potential of the Eagle Mountain Wilderness Study Area, Pitkin 

County, Colorado.

B2 Mineral Resources of Wilderness Study Areas North-Central Colorado
viewed local residents and personnel from the USBM and 

U.S. Bureau of Land Management (BLM) regarding minerals in the area, and examined BLM and county records 

for locations of patented and unpatented mining claims 

and mineral leases.

Investigation by the U.S. Geological Survey

In June 1984, the U.S. Geological Survey (USGS) 

conducted an investigation to assess the potential for undiscovered mineral resources of the Eagle Mountain Wilderness Study Area. This investigation consisted of a review of previous geologic mapping, collection of streamsediment and rock samples for geochemical analyses, and 

a search for mines, prospects, and mineralized areas. 

Complete USGS analytical data are available for public 

inspection at the U.S. Geological Survey, MS 905, Building 25, Denver Federal Center, Denver CO 80225. The 

assessment of undiscovered mineral resources is based on 

the above work, the geologic map of the Highland Peak 

quadrangle (Bryant, 1972), and the USGS-USBM study 

of the Maroon Bells-Snowmass Wilderness (Freeman and 

Weisner, 1984). Bryant (1979) also was a valuable source 

of information about the mineralization in the area and 

the geology of the Snowmass Creek mine, just outside 

the study area. Mineral resource potential was classified 

according to the system of Goudarzi (1984).

Geologic Setting

The Eagle Mountain Wilderness Study Area is in 

the Middle Rocky Mountains physiographic province and 

lies within the Eagle basin between the White River uplift 

and the Sawatch uplift. The area is just north of the Colorado mineral belt and northeast of the Uncompahgre uplift. Regional geology of the middle Rocky Mountains 

consists of a number of Laramide uplifts modified by Tertiary plutons and faults (Bryant, 1979).

Sedimentary rocks of Pennsylvanian to Cretaceous 

age are exposed in the proposed wilderness (fig. 2). The 

Mancos Shale (unit Km, fig. 2), Dakota Sandstone and 

Burro Canyon Formation (unit Kdb), and the Morrison 

Formation (unit Jm) crop out in the northern part of the 

study area; the Entrada Sandstone (unit Je), the Chinle 

Formation (unit T*c), the State Bridge Formation (unit 

T*Ps), and the Maroon Formation (unit PPm) crop out 

in the southern half. Quaternary colluvium (unit Qc) and 

glacial-moraine deposits (unit Qm) cover parts of the 

study area. The Snowmass Creek fault zone, transecting 

the southeastern boundary of the study area, consists of 

a series of arcuate faults. According to Bryant (1979), 

this fault zone was associated with Tertiary intrusive activity. One prominent north-trending fault, with both leftlateral and vertical offsets, cuts the sedimentary rocks 

along the western boundary and may join the Snowmass 

Creek fault zone near the southern part of the study area.

APPRAISAL OF IDENTIFIED RESOURCES

By Steven E. Kluender and jeanne E. Zelten, 

U.S. Bureau of Mines

Mining and mineral-exploration activity

There has been no mineral production and there is 

no surface evidence of metallic-mineral or energy resources in the Eagle Mountain Wilderness Study Area, 

which is in the Snowmass mining district. A small unknown amount of copper-silver ore of unknown grade was 

shipped from the Snowmass Creek mine, 0.25 mi northeast of the study-area boundary (fig. 3) (Bryant, 1979). 

No mines, prospects, mineralized areas, patented mining 

claims, or mineral leases are in the study area. As of 

September 1984, BLM files showed 10 unpatented mining 

claims in or within 1 mi of the study area. Several 

patented mining claims are within 1 mi southwest of the 

area (fig. 3). An oil and gas lease (C36477) is 0.5 mi 

east of the study area. Three test holes drilled for oil 

and gas about 4 mi northeast of the study area were dry.

Sulfide minerals were previously identified at three 

caved workings along the Snowmass Creek fault zone in 

the Cretaceous Dakota Sandstone, 0.25 mi to the northeast 

(fig. 3). Bryant (1979) visited the workings before they 

were caved, and the following description of mineral occurrences at these and other workings in the region was 

taken from his report.

At the Snowmass Creek mine, copper and iron sulfides are associated with carbonaceous material in the 

Dakota Sandstone. Copper and silver, with associated molybdenum and zinc, were detected in samples taken by 

Bryant. He suggested a genetic relationship between the 

faulting and the sulfide deposits in the carbonaceous material in the Dakota. If this relationship is valid, additional 

copper and silver mineralized rock may be present along 

the northwestern boundary, about 0.75 mi west of the 

Snowmass Creek mine, where another fault cuts the 

Dakota. If the mineralization was controlled by the presence of carbonaceous material which created a reducing 

environment and caused precipitation of sulfide minerals 

from ground water, then mineralization could have been 

more widespread in the Dakota. No surface evidence of 

mineralization along this second fault or in the Dakota 

was noted during the investigation. The mineral occurrence at the Snowmass Creek mine is small; there is no 

surface indication of a deposit of large tonnage.

Several workings on private land southwest of the 

study area (fig. 3) are also in the Dakota. Samples taken

Eagle Mountain Wilderness Study Area, Pitkin County, Colorado B3
107°00'00" 106°57'30'

39°12'30"

HKrff

Qm

-<;.., t|r:u

***,

Km

Ql

EMto

Ql

Qf

Km

Km Kdb

Kdb

Jm

PPm

Ws,

Qf

v'itn

"""^ir

lsPs<

'" ^=C:

PPm

Jm

1m

Ql' I \ \ ^<*^

r \ ! \f^i

^EM04 _» ^

if A - 

^/* Snowma^s Creek \ 

^\ mine i /

1% ! :/ ^

'AETVIOSi / ,'

^m

 ^

i

\f JO

Qk

Qc

/

''^

/^) lm^-^*+»* ^

25

j£**~ ' ,'' -' ' ' /-! J&-- ..'i-' Qm

APPROXIMATE BOUNDARY OF THE 3 

EAGLE/^OUNTAiN WIl^ERNESS 

STUDY AREA ..,-,.. ,.,; ,T>-r 

,»»-* "***

PPm

«**"

^^-^

Kdb

Km Ql

K^b

Qm

COLORADO

Ol

Jm

Jm Tg

Figure 2. Geologic map and cross section of the Eagle Mountain Wilderness Study Area, Colorado. Geology 

modified from Bryant (1972). Base from U.S. Geological Survey Highland Peak, Colo., 1960, 1:24,000.

B4 Mineral Resources of Wilderness Study Areas North-Central Colorado
PPm

OlF the 

same age

Of the 

same age

EXPLANATION

Qf Alluvial-fan deposits (Quaternary)

Q| Landslide deposits (Quaternary)

Qm Glacial deposits (Quaternary)

Qc Colluvlum (Quaternary)

Tg Granodlorlte (Tertiary)

Km Mancos Shale (Cretaceous)

Kdb Dakota Sandstone and Burro Canyon Formation 

(Cretaceous)

Jm Morrison Formation (Jurassic)

Je Entrada Sandstone (Jurassic)

~Rc Chlnle Formation (Trlasslc)

State Bridge Formation (Triasslc and Permian) 

Maroon Formation (Permian and Pennsylvanian)

 Contact

D -, ! Fault dashed where approximately located, dotted where 

concealed. U, upthrown side; D, downthrown side. 

Arrows show direction of movement 

i Strike and dip of beds 

 K Inaccessible adit 

9 EM04 Sample locality and number 

L/B Geologic terrane having low mineral resource

potential with certainty level B for base and 

precious metals, uranium, and oil and gas

by Bryant at these workings contained lesser amounts of 

copper, molybdenum, and silver than those taken at the 

Snowmass Creek mine. These workings have no record 

of production. Samples taken by Bryant at other areas 

along the Snowmass Creek fault zone did not contain evidence of mineralization.

The Mississippian Leadville Limestone, which may 

underlie the area, has been mined for base and precious 

metals in central Colorado. If the Leadville is present beneath the study area, and if Bryant's theories of hydrothermal mineralization are correct, then there could be replacement deposits in the Leadville below the surface.

Most of the formations present in the study area 

were described by Bass and Northrop (1963) as "prospectively valuable as reservoirs for oil and gas" for an area 

15 mi to the north. The study area was designated by 

Spencer (1983) as having unknown potential because of 

insufficient subsurface data. Three test holes drilled for 

oil and gas about 4 mi northeast of the area were dry.

The Jurassic Morrison Formation, exposed in the 

northern part of the study area, has been the source of 

about 85 percent of the uranium mined in Colorado, 

mostly in the Colorado Plateau physiographic province 

(U.S. Geological Survey, 1968). The Morrison has been 

prospected for uranium in the region surrounding the 

study area, but no uranium discovery has been reported. 

No surface evidence of uranium mineralization was noted 

during the investigation.

Sand, gravel, and industrial rock resources are present. Glacial deposits and colluvium cover almost one-third

9,000' -

8,000'-

7,000' -

6,000'-

5,000'-

No vertical exaggeration 

Surficial deposits not shown

A'

-10,000'

-9,000'

-8,000'

-7,000' 

-6,000'

-5,000'

Figure 2. Continued.

Eagle Mountain Wilderness Study Area, Pitkin County, Colorado B5
I07°00'

R 86 W

EAGLE

MOUNTAIN

WILDERNESS

STUDY AREA

NOW'MASS 

'-CREEK \

MAROON BELLS - SNOWMASS

I07°00' R. 86 W.

SCALE 1-24,000 

0

CONTOUR INTERVAL 200 FEET

Figure 3. Mine and prospect map of the Eagle Mountain Wilderness Study Area, Colorado. Location of faults, min

Bryant (1972). Oil and gas lease data from U.S. Bureau of Land Mangement, September 1984. Base from U.S. Ge

Peak, 1960; and Highland Peak, 1960.
of the study area, and sandstone crops out over almost 

one-half. Sufficient quantities of sand and gravel are 

available outside the study area to satisfy current local 

needs.

ASSESSMENT OF POTENTIAL FOR 

UNDISCOVERED RESOURCES

By Sandra J. Sou I lie re and Mark A. Arnold, 

U.S. Geological Survey

Geology

Rock units in the study area were described in detail 

by Bryant (1972). The Maroon Formation (Permsylvanian 

and Permian) is composed of grayish-red, reddish-brown, 

and pale-red interbedded sandstone, siltstone, mudstone, 

and conglomerate. These beds are generally calcareous 

and micaceous. A well-sorted, crossbedded, fine-grained 

quartz sandstone occurs locally as a large lenticular layer 

at the base of the formation. The Maroon Formation is 

unconformable with the overlying State Bridge Formation 

(Permian and Triassic), which consists of pale- to darkreddish-brown calcareous and micaceous siltstone, sandstone, and conglomerate. These sedimentary rocks are 

characterized by oscillatory ripple marks, crossbedding, 

and spherical quartz grains.

The Triassic Chinle Formation unconformably overlies the State Bridge Formation and consists of brick-red 

to very dark red calcareous siltstone and minor beds of 

red to very light gray silty limestone and conglomerate 

composed of angular fragments of limestone. The Jurassic 

Entrada Sandstone unconformably overlies the Chinle Formation and consists of salmon-red to pink, crossbedded, 

medium- to coarse-grained subrounded sandstone. The 

Morrison Formation (Jurassic) is conformable with the underlying Entrada Sandstone and consists of grayish-red 

and greenish-gray siltstone and claystone grading downward into calcareous sandstone and siltstone. Yellowishgray silty limestone and calcareous sandstone beds at the 

base constitute less than one-fourth of the thickness of 

the formation.

The Cretaceous Burro Canyon Formation conformably overlies the Morrison Formation. The Burro Canyon 

consists of light-gray to white quartz sandstone with interbeds of greenish-gray claystone, silty shale, and sandy 

siltstone. The Dakota Sandstone was mapped together 

with the Burro Canyon. It consists of dark-gray siltstone 

and shale interbedded with medium-gray sandstone. The 

sandstone locally contains plant fragments. The Cretaceous Mancos Shale conformably overlies the Dakota. It 

consists of dark-gray silty shale with thin beds of siltstone 

and fine-grained sandstone. The Fort Hays Limestone 

Member of the Mancos Shale consists of dark-gray limestone in 1-2 ft thick beds.

A Tertiary hornblende granodiorite occurs as a thin 

sill and as dikes about 0.5 mi south of the study area. 

It is light greenish gray, fine grained, and generally altered. The granodiorite may be present beneath the study 

area, but no surface evidence was found. Quaternary alluvium and glacial deposits cover the area around Snowmass Creek.

The Snowmass Creek fault zone, a series of arcuate 

faults, cuts the southeastern edge of the study area. According to Bryant (1979), this fault zone had post-Dakota 

movement during the emplacement of a granodiorite pluton. One prominent north-trending fault cuts the sedimentary rocks along the western boundary of the study area 

and may join the Snowmass Creek fault zone at the southern end.

Geochemistry 

Analytical Methods

During an investigation of the Maroon Bells-Snowmass Wilderness, Freeman and Weisner (1984) sampled 

outcrops and stream sediments along the Snowmass Creek 

fault zone and found anomalous amounts of copper, silver, lead, zinc, molybdenum, cobalt, and gold. Because 

of the proximity of the Snowmass Creek fault zone to 

the Eagle Mountain Wilderness Study Area, these elements were of primary interest in our geochemical survey. 

Stream-sediment samples were collected from four sites 

(fig. 2). Three stream-sediment samples (EMO1, EMO2, 

EMO5) were collected from intermittent streams which 

drain the study area. An additional sample (EMO4) was 

collected of sediment that Was deposited in drainage from 

one of the adits of the Snowmass Creek mine; the mine 

is just outside the study area. Two types of samples were 

collected at each of these sites. A panned-concentrate 

sample was prepared for analysis for heavier metallic elements such as copper, lead, zinc, silver, and gold, and 

a fine-fraction sample of mud was collected to analyze 

for metals such as molybdenum, uranium, and arsenic, 

which typically adhere to clay minerals.

Each panned concentrate represents a composite of 

three to five grab samples of coarse stream sediment 

which was sieved in the field through a 10-mesh (0.039- 

in.) stainless-steel screen. The sediment was panned until 

only about 50 percent quartz and feldspar remained in 

the pan. In the laboratory, this material was dried, sieved 

to minus-35 mesh (0.0165 in.), processed through 

bromoform separation (specific gravity greater than 2.85) 

and magnetic separation (1.0 amp, side slope of 10° and 

forward slope of 15°) to obtain nonmagnetic heavy-mineral concentrates for analyses. The nonmagnetic heavymineral-concentrate fraction was split, then ground in a 

mortar and pestle and analyzed semiquantitatively for 31 

elements by the six-step, direct-current arc, optical-emisEagle Mountain Wilderness Study Area, Pitkin County, Colorado B7
sion spectrographic method (Grimes and Marranzino, 

1968).

A fine-fraction sample, about 25 ounces of clay and 

mud, also was collected at each sample site. In the laboratory, the fine-fraction samples were ground with a mortar 

and pestle to release clay-cementing aggregates and were 

then sieved to minus-100 mesh (0.0059 in.). The procedure for sample preparation and analysis by inductively 

coupled plasma atomic-emission spectroscopy (ICP) followed the techniques of Taggart and others (1981). Each 

sample was analyzed for 44 elements.

Two composite mineralized rock samples (EMO3A 

and B) were taken from the Snowmass Creek mine dump 

just outside the study area. In the laboratory, the rock 

samples were crushed and pulverized to less than 100 

mesh (0.0059 in.) and were analyzed by semiquantitative 

emission spectroscopy similar to the method used for 

heavy-mineral concentrates as discussed above.

Results of Survey

The heavy-mineral concentrate and stream-sediment 

samples contained no anomalous concentrations of precious or base metals. One sample (EMO4), taken just outside the study area, showed a higher amount of copper 

(190 ppm or parts per million) than other stream-sediment 

samples from the area (average 19 ppm). Uranium was 

not detected in any of the panned-concentrate samples.

Rocks containing visible metallic minerals, scattered 

about the dump at the caved Snowmass Creek mine, were 

sampled and analyzed. Visual examination of these samples revealed pyrite, chalcopyrite, bornite, and copper 

oxide minerals (malachite, azurite) in the Dakota Sandstone. The samples were also rich in plant debris. 

Semiquantitative spectrographic analyses of rock sample 

EMO3A showed greater than 2 percent copper, 1,500 

ppm silver, 200 ppm molybdenum, 1,340 ppm zinc, and 

100 ppm lead. Sample EMO3B showed greater than 2 

percent copper, 200 ppm silver, 30 ppm molybdenum, 

612 ppm zinc, and 15 ppm lead. These amounts reflect 

the high grade of the selected samples.

Mineral and Energy Resources

The mineral resource potential for the Eagle Mountain Wilderness Study Area is low for all mineral commodities (fig. 1). Available information indicates the 

geologic environment and suggests the low level of mineral resource potential; the certainty level is B (see inside 

front cover for definitions of certainty levels). Copper, 

lead, silver, and zinc minerals of the type found at the 

Snowmass Creek mine may be present in favorable host 

rocks or along faults which transect the study area. No 

metallic mineral resources were identified at the surface

in the study area, and the geochemical samples taken during this investigation contained no anomalous concentrations of base or precious metals.

At the Snowmass Creek mine, copper and iron sulfide minerals replace plant debris in the Dakota Sandstone. Similar mineralized rock has not been found elsewhere in the region (Bryant, 1979). Bryant (1979) 

suggested that low-temperature hydrothermal fluids were 

responsible for the mineralization at the Snowmass Creek 

mine. He stated that the mineralization appeared to be 

controlled by the carbonaceous beds in the Dakota Sandstone and by proximity to the Snowmass Creek fault zone. 

If the mineralization in the study area was fault controlled, 

then copper, lead, silver, and zinc minerals may be present below the surface along the western boundary of the 

study area where the Dakota Sandstone and Mancos Shale 

are cut by a north-trending fault (fig. 2).

The mineralization at the Snowmass Creek mine 

may have been controlled by carbonaceous beds, regardless of proximity to the faults. Copper and other metals 

are precipitated when ground water passes through carbonaceous beds (reducing facies) in any of the sedimentary 

rocks. If this occurred in the area, copper, silver, lead, 

zinc, and associated uranium and vanadium may be present in the unexposed carbonaceous beds of the Dakota 

Sandstone and Mancos Shale. However, no base metals, 

uranium, or vanadium have been reported in the region 

from carbonaceous beds in the Dakota or Mancos, and 

none of the metals were detected by our geochemical sampling.

The Eagle Mountain Wilderness Study Area has a 

low mineral resource potential for uranium; the certainty 

level is B. No uranium minerals were identified during 

the field investigation, and geochemical analyses revealed 

no anomalous concentrations of uranium.

The mineral resource potential for oil and gas in 

the study area is low, with certainty level B; no producing 

oil fields are nearby. According to Bass and Northrop 

(1963), some of the formations present may be favorable 

reservoir rocks for oil and gas. These formations include 

the Leadville, Belden, Maroon, Entrada, Morrison, and 

Dakota. Geothermal information (Freeman and Weisner, 

1984) indicated that temperatures were too high for preservation of oil in these formations. The structural setting 

is unfavorable for gas.

REFERENCES CITED

Bass, N. W., and Northrop, S. A., 1963, Geology of Glen wood 

Springs quadrangle and vicinity, northwestern Colorado: 

U.S. Geological Survey Bulletin 1142-J, 74 p.

Bryant, Bruce, 1972, Geologic map of the Highland Peak quadrangle, Pitkin County, Colorado: U.S. Geological Survey 

Geologic Quadrangle Map GQ-932, scale 1:24,000.

B8 Mineral Resources of Wilderness Study Areas North-Central Colorado
 1979 [1980], Geology of the Aspen 15-minute quadrangle, Pitkin and Gunnison Counties, Colorado: U.S. 

Geological Survey Professional Paper 1073, 146 p.

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S. P., Kropschot, S. J., and Dickinson, R. G., eds., Wilderness mineral potential, assessment of mineral-resource 

potential in U.S. Forest Service lands studied 1964-1984: 

U.S. Geological Survey Professional Paper 1300, v. 1, p. 

458-461.

Goudarzi, G. H., compiler, 1984, 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.

Kluender, S. E., and Zelten, J. E., 1985, Mineral investigation 

of the Eagle Mountain Wilderness Study Area, Pitkin 

County, Colorado: U.S. Bureau of Mines Open File Report 

MLA7-85, 11 p.

Spencer, C. W., 1983, Petroleum potential of wilderness lands, 

Colorado, in Miller, B. M., ed., Petroleum potential of wilderness lands in the western United States: U.S. Geological 

Survey Circular 902-E, 8 p.

Taggart, J. E., Jr., Lichte, F. E., and Wahlberg, J. S., 1981, 

Methods of analysis of samples using X-ray fluorescence 

and induction-coupled plasma spectroscopy, in Lipman, P. 

W., and Mullineaux, D. R., eds., The 1980 eruptions of 

Mount St. Helens, Washington: U.S. Geological Survey 

Professional Paper 1250, p. 683-687.

U.S. Senate Committeee on Interior and Insular Affairs, 1968, 

Mineral and water resources of Colorado: U.S. 90th Congress, 2d session, Senate Document 115, 302 p.

& U.S. GOVERNMENT PRINTING OFFICE: 1986 676-047/26,020 REGION NO. 8 Eagle Mountain Wilderness Study Area, Pitkin County, Colorado B9


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