U.S. Department of the Interior

U.S. Geological Survey

U.S. Geological Survey Bulletin 2213

Resource Potential and Geology of the

Grand Mesa, Uncompahgre, and Gunnison

(GMUG) National Forests and Vicinity, Colorado
Cover. Iron bog above Ophir, Uncompahgre National Forest, Colorado.
U.S. Department of the Interior

U.S. Geological Survey

Resource Potential and Geology of the

Grand Mesa, Uncompahgre, and Gunnison 

(GMUG) National Forests and Vicinity, Colorado

Edited by Viki Bankey

U.S. Geological Survey Bulletin 2213

This report summarizes information on known mineral resources, delineates areas 

favorable for occurrence of undiscovered mineral resources for seven types of 

metallic mineral deposits having a reasonable chance of occurring, provides an 

assessment of coal resources, describes some of the environmental effects of 

historical mining, and identifies the distribution and quality of potential sources 

of crushed stone and sand and gravel for natural aggregate in the Grand Mesa, 

Uncompahgre, and Gunnison (GMUG) National Forests, Colorado
U.S. Department of the Interior

Gale A. Norton, Secretary

U.S. Geological Survey

Charles G. Groat, Director

Version 1.0, 2004

This publication is only available online at:

http://pubs.usgs.gov/bul/b2213

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

Manuscript approved for publication July 7, 2003

Published in the Central Region, Denver, Colorado

Graphics by Viki Bankey

Photocomposition by Gayle M. Dumonceaux

Edited by L.M. Carter
Volume Contents

[Chapters are lettered]

 – Selected results, by U.S. Geological Survey....................................................................................... vi

A. Introduction, by Viki Bankey.................................................................................................................. 1

B. Review of the geology of western Colorado, by Warren C. Day and Dana J. Bove .................. 11

C. Regional sediment and rock geochemistry, by Steven M. Smith.................................................. 37

D. Geophysical studies, by Viki Bankey, Robert P. Kucks, and Kim Oshetski................................... 54

E. Distribution of mines and mineralized areas, by Anna B. Wilson and Gregory T. Spanski ....... 67

Potential for undiscovered mineral resources—Locatable minerals

F. Qualitative and quantitative mineral resource assessment methodology, by Gregory T.

Spanski and Viki Bankey .............................................................................................................. 87

G. Mineral resource assessment for porphyry molybdenum deposits, by Dana J. Bove,

Daniel H. Knepper, Jr., Viki Bankey, Gregory T. Spanski, and Steven M. Smith................ 105

H. Mineral resource potential assessment for the sandstone-hosted uranium deposit 

type, by Gregory T. Spanski, Viki Bankey, and Steven M. Smith ......................................... 124

I. Mineral resource assessment for volcanic-associated massive sulfide deposits, by

Warren C. Day, Gregory T. Spanski, Viki Bankey, Anna B. Wilson, and

Steven M. Smith .......................................................................................................................... 135

J. Mineral resource assessment for polymetallic vein deposits, by Anna B. Wilson, 

J. Thomas Nash, Gregory T. Spanski, Viki Bankey, and Steven M. Smith ......................... 150

K. Mineral resource assessment for polymetallic replacement deposits, by Anna B. 

Wilson, Gregory T. Spanski, Viki Bankey, and Steven M. Smith.......................................... 164

L. Mineral resource assessment for sediment-hosted copper deposits, by David A.

Lindsey, Viki Bankey, Daniel H. Knepper, Jr., and Gregory T. Spanski................................ 177

Mineral resources—Leasable minerals

M. Coal resources and coal resource potential, by R.D. Hettinger, L.N.R. Roberts, and

M.A. Kirschbaum......................................................................................................................... 188

Mineral resources—Salable minerals

N. Potential aggregate resources, by Daniel H. Knepper, Jr., and Viki Bankey ........................... 224

O. References cited and additional selected bibliography for the GMUG greater study 

area, compiled by Steven M. Smith, Anna B. Wilson, and M.J. Crane .............................. 232

iii
Definition of Terms

Reserves. Economically recoverable mineral-bearing material in identified deposits (Brobst 

and Pratt, 1973).

Resources. Mineral-bearing material not yet discovered, or discovered material that currently 

cannot be recovered (Brobst and Pratt, 1973).

Identified resources. Specific bodies of mineral-bearing material whose location, quality, and 

quantity are known from geologic evidence (Brobst and Pratt, 1973). These resources 

are not particularly evaluated as to feasibility of mining and can be economic, marginal, or 

subeconomic.

Undiscovered resources. Unspecified bodies of mineral-bearing material surmised to exist 

on the basis of broad geologic knowledge and theory (Brobst and Pratt, 1973). These 

bodies can occur in known mining districts or in geologic terranes that presently have no 

discoveries. These resources are also not evaluated as to feasibility of mining and can be 

economic, marginal, or subeconomic.

Mineral deposit. An occurrence of sufficient size and grade that under the most favorable 

circumstance could be considered to have economic potential (Cox and others, 1986).

Mineral occurrence. A concentration of a mineral that is considered valuable by someone 

somewhere or that is of scientific or technical interest (Cox and others, 1986).

Ore deposit. A mineral deposit that has been tested and is known to be of sufficient size, 

grade, and accessibility to be producible and yield a profit (Cox and others, 1986).

Measurement Units

The grade and tonnage curves used in this study contain grades either as grams or as percent per metric 

ton. Thus the estimated amounts of metallic resources within an undiscovered deposit are reported in 

metric tons of metal. In many situations, units of measurement are noted as they were originally reported. 

Measurements originally made and reported in feet, in miles, in square miles, and in short (2,000-lb) tons, 

for example, are included here in their original units for clarity and to avoid misstatement of precision in 

conversion. 

To convert To Multiply by

feet meters 0.3048

miles kilometers 1.61

pounds kilograms 0.45

square miles square kilometers 2.6

Equivalences useful for reading this volume include the following:

1 troy ounce (oz) 31.1 grams (g)

1 short ton 0.9072 metric ton (t)

1 troy ounce per short ton 34.285 parts per million (ppm)

1 part per million (ppm) 1 gram per metric ton (g/t)

1 percent (%) 10,000 ppm

1 metric ton (t) 32,154 troy ounces

iv
v

EON ERA PERIOD EPOCH

AGE ESTIMATES

OF BOUNDARIES

(Ma*)

Phanerozoic

Proterozoic

Archean

pre-Archean††

*Millions of years prior to A.D. 1950.

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

††Informal time term without specific rank.

Cenozoic

Mesozoic

Paleozoic

Late 

Middle

Early

Late 

Middle

Early

Late 

Middle

Early

Late 

Middle

Early

Late 

Middle

Early

Late 

Middle

Early

Late 

Middle

Early

Holocene

Pleistocene

Pliocene

Miocene

Oligocene

Eocene

Paleocene

Late 

Early

Late 

Early

Late 

Early

Late 

Middle

Early

Late 

Middle

Early

Quaternary

Tertiary

Neogene

Subperiod

Paleogene

Subperiod

Cretaceous

Jurassic

Triassic

Permian

Carboniferous

Period

Pennsylvanian

Mississippian

Devonian

Silurian

Ordovician

Cambrian

3,800?

4,550

3,400

3,000

2,500

1,600

900

≈570†

500

435

410

360

≈330

290

≈240

205

138

96

66

55

38

24

5

1.7

0.010

Geologic Time Chart—Terms and Boundary Ages by the U.S. Geological Survey in this Report.
• This study was undertaken at the request of the United 

States Department of Agriculture (USDA) Forest Service. The USGS assessment involved a team of scientists with expertise in geology, geochemistry, geophysics, 

economic geology, coal, mineral deposits, and resource 

analysis.

• The forests include part of the Colorado Mineral Belt, one of 

the most productive areas of base and precious metals in 

North America.

• Mining and mineral exploration have played a central role 

in the history of the forests since the late 1800’s; several 

world-class mines are either in or adjacent to the forests, 

and smaller mines are abundant throughout the forests.

• Large tracts within the forests, including wilderness areas, 

contain indicators that suggest the presence of metallic 

mineral deposits.

• Twenty-two metallic mineral deposit types were identified 

in the forests. Mineral resource potential was assessed 

for seven of these deposit types: granite-hosted porphyry 

molybdenum, granodiorite-hosted porphyry molybdenum, sandstone-hosted uranium, volcanic-hosted massive 

sulfide, polymetallic vein, polymetallic replacement, and 

sediment-hosted redbed copper.

• The commodities most likely to occur are gold, silver, copper, lead, zinc, molybdenum, uranium, and vanadium.

• Permissive tracts for metallic minerals are shown in Chapter 

K, figure K2; Chapter I, figure I2; Chapter G, figure G1; 

chapter H, figure H1; Chapter L, figure L2; and Chapter 

J, figure J1.

• Favorable tracts for metallic minerals are shown in Chapter 

K, figure K3; Chapter I, figure I3; Chapter G, figure G2; 

Chapter G, figure G3; Chapter H, figure H2; Chapter L, 

figure L3; and Chapter J, figure J2.

• The forests have a high coal resource potential, with 

resources estimated at about 38 billion short tons, 

in areas underlain by the Mesaverde Group or the 

Mesaverde Formation. This value does not reflect economic, environmental, technological, or geologic restrictions affecting availability and recoverability.

• Tracts for aggregate suitable for asphaltic concrete and Portland cement concrete are shown in Chapter N, figures N1 

and N2.

Selected Results

Undiscovered Mineral Resource Potential for Select Deposits in the

Grand Mesa, Uncompahgre, and Gunnison National Forests, Colorado

By U.S. Geological Survey

This summary highlights the mineral resources of the forests and is directed toward

land-use planners and other interested persons
U.S. Department of the Interior

U.S. Geological Survey

Introduction 

By Viki Bankey 

Chapter A of

Resource Potential and Geology of the Grand Mesa, Uncompahgre, 

and Gunnison (GMUG) National Forests and Vicinity, Colorado

Edited by Viki Bankey

U.S. Geological Survey Bulletin 2213– A
Contents

Background ................................................................................................................................................... 1

Acknowledgments........................................................................................................................................ 1

Geographic Setting....................................................................................................................................... 4

Geologic Setting............................................................................................................................................ 4

Previous Assessments................................................................................................................................. 4

Method for Identifying Favorable Areas for Undiscovered Mineral Resources................................ 5

Summary of Mineral Resource Assessments.......................................................................................... 6

Granite Porphyry Molybdenum Deposits......................................................................................... 6

Granodiorite Porphyry Molybdenum Deposits ............................................................................... 6

Sandstone-Hosted Uranium Deposits.............................................................................................. 7

Volcanic-Hosted Massive Sulfide (VMS) Deposits........................................................................ 7

Polymetallic Vein Deposits................................................................................................................. 7

Polymetallic Replacement Deposits................................................................................................. 8

Sediment-Hosted Copper Deposits .................................................................................................. 8

References Cited .......................................................................................................................................... 8

Figures

A1. Map showing Grand Mesa, Uncompahgre, and Gunnison (GMUG) National

Forests within greater study area...................................................................................................... 2

A2. Map showing BLM land and named BLM Resource Areas within greater study area ............ 3

Tables 

A1. Previous mineral resource assessment studies of wilderness land in or near

Grand Mesa, Uncompahgre, and Gunnison National Forests, Colo. ........................................... 5

viii
Background

The U.S. Geological Survey (USGS) provides earth-science information to the United States Department of Agriculture (USDA) Forest Service, Bureau of Land Management 

(BLM), and other land-management agencies that is used to 

address land stewardship, resource sustainability, and environmental questions on Federal lands. For public lands of the 

Grand Mesa, Uncompahgre, and Gunnison National Forests 

and vicinity, an assembled panel of USGS scientists came 

together and summarized information on known mineral 

resources; delineated areas favorable for the occurrence of 

undiscovered mineral resources for seven types of metallic 

mineral deposits that have a reasonable chance of occurring; 

provided an assessment of coal resources; described some of 

the environmental effects of historical mining; and identified 

the distribution and quality of potential sources of crushed 

stone and sand and gravel for natural aggregate. This information is intended to help Federal agencies plan for potential 

mining activities, determine values for land exchanges, derive 

information on soils and habitats, plan for engineering and 

recreational projects, evaluate remediation plans, rehabilitate historical mines, and make land management decisions 

throughout western Colorado.

This mineral resource assessment of the Grand Mesa, 

Uncompahgre, and Gunnison (GMUG) National Forests was 

produced to assist the USDA Forest Service in fulfilling the 

requirements of the Code of Federal Regulations (36CFR 

219.22) and to supply information and interpretations necessary for mineral resources to be considered along with other 

kinds of resources (such as timber, wildlife, and recreation) 

in land-use planning. This report addresses the potential for 

undiscovered mineral and coal resources in the three national 

forests and surrounding greater study area and is based on 

information available as of 1998. The undiscovered-mineral 

resource assessment was conducted for only those deposit 

types for which there is a reasonable probability of occurrence. 

The completeness of each 1998 data set is further described in 

individual chapters of this report.

Together the Grand Mesa, Uncompahgre, and Gunnison 

(GMUG) National Forests are referred to as “the forests” in 

this report. Many of the data sets used in this report extend 

beyond the boundaries of the forests and cover parts of the San 

Juan, White River, and San Isabel National Forests, intervening BLM resource areas, and national park, State, and private 

land. For this report, a rectangle was defined to standardize 

the boundaries of most data sets, from lat 37°45′ N. to 39°30′

N. and from long 106° W. to 109° W. This rectangular area 

is referred to as the “greater study area.” Within the greater 

study area, outlines of the three forests of this volume are 

shown in figure A1, and BLM lands are shown in figure A2. 

The general location of other national forests in the greater 

study area is also indicated in figure A1; however, they were 

not included in the mineral resource assessment of this volume.

The greater study area includes the BLM Uncompahgre 

Basin Resource Area and parts of the Grand Junction, San 

Juan, Gunnison, and Glenwood Springs Resource Areas. 

Where possible, the authors include data and mineral resource 

assessments for nearly 3.2 million acres (4,868 mi2

) of BLM 

land within the greater study area.

The forests lie within an area known as the Colorado 

Mineral Belt (Tweto and Sims, 1963), where mining of 

metallic commodities has historically been important. In 

the Uncompahgre Plateau and vicinity, in the western part of 

the greater study area, uranium mining was important during 

the 1950’s, 1960’s, and 1970’s. The economic importance 

of continued mining in Colorado has diminished. However, 

issues still arise from the effects of past mining, such as the 

risks from inactive mines and decreased water quality, which 

land-use planners must address. Environmental geochemistry 

of high-priority watersheds was also investigated by Miller 

(1998, 1999), Bove and Knepper (2000), and Nash (2001). 

These studies focus on water data near Tertiary-age intrusions 

and hydrothermally altered rocks, the low-temperature processes controlling mobility of metals, acid drainage associated 

with pyritic systems, and natural acid mitigation by wallrock 

alteration, carbonate rocks, and alluvium.

Acknowledgments

Many individuals contributed data, ideas, and assistance 

to this study. Margo Toth developed a nice format for earlier 

reports (for example, Toth and others, 1993), which we used 

as a general guideline. Greg Lee and Anne McCafferty helped 

Introduction

By Viki Bankey
0

0

Disappointment Creek

Gunnison River

Uncompahgre River Lake Fork Cebolla

Creek

Tomichi Creek

Taylor River

Blue MesaReservoir

CochetopaCreek

Dolores River

WHITE RIVER NF

SAN JUAN NF

RIO GRA

UNCOMPAHGRE NF

GRAND MESA NF

GUNNISON NF

GUNNISON NF

Uncompahgre Plateau

Grand Mesa

West Elk  Mtns

Elk Mtns

Sawatch R

San Juan Mtns

39o 30'

109o 

37o 45'

70

139

65

92

50

550

50

50

82

135

133

149

62 145

141

141

90

114

Grand

Junction

Delta

Montrose

Gunnison

Crested Butte

Lake City

Uravan

Telluride

Ouray

Silverton

Aspen

Figure A1. Grand Mesa, Uncompahgre, and Gunnison National Forests (green) within GMUG greater study area.
0

0

COLORADO

GreaterStudyArea

Disappointment Creek

Gunnison River

Uncompahgre River Lake Fork Cebolla

Cre

e

k

Tomichi Creek

Taylor River

Blue MesaReservoir

CochetopaCreek

Dolores River

39o 30'

109o 

37o 45'

70

139

65

92

50

550

50

50

82

2

135

133

149

62 145

141

141

90

114

Grand

Junction

Delta

Montrose

Gunnison

Crested Butte

Lake City

Uravan

Telluride

Ouray

Silverton

Aspen

Grand Junction Glenwood Springs 

Uncompahgre

San Juan

Saguach

RG

Gunnison

La Jara

Approximate outline of BLM Resource Areas

EXPLANATION

Figure A2. BLM land (purple) and named BLM Resource Areas within greater study area.
4 Resource Potential and Geology, Grand Mesa, Uncompahgre, and Gunnison National Forests, Colorado

with the technical side of GIS. Greg Green worked with the 

digital geology to compile and attribute a cohesive data set. 

Samantha Tokash helped compile information and locations 

for mines. 

Our reviewers helped to improve our ideas and presentation. They include Greg Lee (geochemistry); David Campbell and David Daniels (geophysics); Brad Van Gosen, Doug 

Nichols, and Ted Dyman (coal); David Lindsey and Robert 

Moench (geology); and Karl Evans, Donald Sweetkind, Richard Grauch, Robert Eppinger, and William Langer (mineral 

assessments).

Geographic Setting

The Grand Mesa, Uncompahgre, and Gunnison National 

Forests encompass about 3.12 million acres (4,868 mi2) in 

west-central Colorado. The forests include much of the headwater regions of the Gunnison and Uncompahgre River watersheds (fig. A1). The Grand Mesa National Forest covers the 

Grand Mesa in the north-central part of the greater study area. 

The Uncompahgre National Forest is made up of three parcels. 

The largest parcel includes the Uncompahgre Plateau, an area 

of elevated Paleozoic strata between the Uncompahgre and 

Dolores Rivers. Southeast of the Uncompahgre Plateau, the 

remaining two parcels of the Uncompahgre Forest include part 

of the rugged and remote San Juan Mountains near the towns 

of Telluride and Ouray. East of the Uncompahgre Forest lies 

the Gunnison National Forest, which includes the east half of 

the broad circular band of forest land. This forest includes all 

of the Sawatch Range west of the Continental Divide, as well 

as the Elk and West Elk Mountains.

The topography of the forests typifies the central Rocky 

Mountains of Colorado and varies from valleys and plateaus 

to steep and rugged mountains, including many of Colorado’s 

mountains exceeding 14,000 ft elevation. The forests adjoin 

the White River, San Isabel, Rio Grande, and San Juan 

National Forests (listed clockwise from north to south, fig. 

A1).

Parts of nine counties lie within the forests: Delta, Gunnison, Hinsdale, Montrose, Mesa, Ouray, Saguache, San 

Juan, and San Miguel. U.S. Highways 50 and 550 cross the 

area from northwest to southeast. Numerous other State and 

county roads extend through or to the forests. The major communities within and near the forests include the cities of Grand 

Junction, Delta, Montrose, and Gunnison. Former mining 

centers that have become small town recreational destinations 

include Crested Butte, Lake City, Telluride, and Ouray. 

Geologic Setting

The geologic setting of western Colorado is a culmination of more than 1.8 billion years of geologic processes, 

which have yielded a breathtaking landscape. The oldest 

known rocks are 1.8–1.7 billion year old metamorphic and 

intrusive rocks. Preserved are accumulations of sediments and 

volcanic rocks deposited in an island-arc environment, which 

have been intruded by granitoids and later by 1.4 billion year

old granites.

The Paleozoic Era opened with the invasion of shallow 

seas from the east that deposited the materials of sandstones, 

carbonates, and shales in fault-bounded basins. During the 

middle and late Paleozoic, regional mountain-building associated with the uplift of the Ancestral Rocky Mountains resulted 

in erosion and deposition of sediments, evaporites, and carbonates in basins adjacent to the ancient uplifts.

Erosion of the Ancestral Rocky Mountains continued into 

the early part of the Mesozoic. During the Late Triassic and 

Early Jurassic the environment changed from warm and humid 

to more arid, and great deposits of windblown sands accumulated. The continental desert environment gave way to an eastward-encroaching marine environment. Regional erosion was 

followed by a depositional environment of lakes and floodplain-deposited mud and silt. Regional compressive tectonism 

began to control paleogeography and sedimentary environment 

during the Cretaceous, with the onset of the Sevier orogeny 

throughout the Western Interior of North America.

The Laramide orogeny dominated the geologic history of 

the early part of the Cenozoic Era; regional mountain building 

and nonmarine sedimentation occurred in the Piceance and 

San Juan Basins. Tertiary stratovolcano and caldera eruptions formed the highlands of the San Juan, West Elk, and Elk 

Mountains. Rifting resulted in bimodal volcanism, and basalt 

flows extended across the Grand Mesa, San Juan volcanic 

field, and Flat Tops area. Silicic plutonism accompanied the 

regional rifting.

Miocene uplift exposed the region to erosion, forming a 

regional erosional surface that dissected earlier surfaces. Canyon cutting continued in Quaternary time, and a major climatic 

cooling brought on glaciation, which continued from about 

500,000 years ago into the Holocene. During three glacial 

periods, ice almost totally covered the higher ranges, and the 

valleys were filled with glaciers; the modern alpine topography with deep U-shaped valleys is largely a product of glacial 

erosion. Holocene alluvium and glacial deposits are present in 

drainages and fans across the forests; these deposits consist of 

gravel, sand, and silt with varying degrees of consolidation.

Previous Assessments

The forests contain several wilderness areas (listed in 

table A1). Mineral resource assessments have previously been 

conducted for these wilderness areas as well as other BLM 

wilderness areas. Mineral resource assessments have also 

been conducted for three adjacent National Forests: San Juan, 

White River, and San Isabel (Van Loenen and Gibbons, 1997; 

Toth and others, 1993; Taylor and others, 1984).
Introduction 5

This report does not attempt to reconcile any differences 

between areas and quantitative values listed in earlier reports 

with the results in this report. More similarities occur than 

differences, because the assessments were based on the same 

or similar mineral deposit models and presumably the same 

criteria for evaluation. However, the data that were evaluated are not always the same. For example, some previous 

reports did not include geophysical data. Also, for this report, 

locations of mines and minerals were updated and verified 

where possible, and we used our new definition of mineralized 

areas as a defining criterion for many deposit models. The 

use of computer-selected areas to choose criteria from various 

databases has minimized subjective decisions that may have 

been used to refine permissive or favorable areas in earlier 

reports. Finally, in that the quantitative assessment is based on 

subjective decisions by scientists, it may differ from previous 

opinions regarding the potential for undiscovered deposits. 

The quantitative assessment process is discussed further in 

Chapter F, this volume. 

Method for Identifying Favorable Areas 

for Undiscovered Mineral Resources

Mineral and coal resources are divided into three categories: locatable, leasable, and salable. Locatable minerals 

comprise all minerals for which exploration, development, and 

production are regulated under the Federal General Mining 

Law of 1872, and include most metallic resources and some 

industrial minerals. Leasable minerals are defined by the Mineral Leasing Act of 1920 to include oil, gas, coal, and several 

other minerals. Of those minerals, we assess herein only the 

undiscovered resources for coal. Salable minerals are defined 

by the Federal Materials Act of 1947 as those which have 

low unit value per ton, which are dependent on easy access to 

transportation, and which are generally used near the production site. These resources include dimension stone, aggregate, 

and sand and gravel.

The cornerstone of the USGS mineral resource assessment was preparing, applying, and disseminating a set of 

georeferenced digital data, compatible with commonly used 

Geographic Information Systems (GIS). With the cooperation 

of the Colorado Geological Survey, a digital geologic map 

has been compiled (Day and others, 1999), and locations of 

igneous intrusions associated with base- and precious-metal 

deposits were added. The geologic setting for this vast study 

area is reviewed by Day and Bove (this volume, Chapter B). 

Aeromagnetic, gravity, aeroradiometric, and Landsat Thematic 

Mapper data sets, or derivative maps from these data, were 

prepared to help identify buried intrusions and associated 

hydrothermally altered rocks. Available geochemical streamsediment data (Bove and others, 2000; Smith, this volume, 

Chapter C), and mine and mineral occurrence information and 

locations (Wilson and others, 2000) have also been compiled. 

Models of seven mineral deposit types were prepared to assess 

undiscovered mineral potential. 

Table A1. Previous mineral resource assessment studies of wilderness land in or near 

Grand Mesa, Uncompahgre, and Gunnison National Forests, Colo. 

[Bold names indicate areas within the GMUG Forests]

Study area Reference

Cannibal Plateau Roadless Area Sharp and others, 1983.

Dominguez Canyon BLM WSA1 Toth and others, 1983, 1987.

Eagle Mountain BLM WSA Soulliere and others, 1986.

Fossil Ridge WSA DeWitt and others, 1985.

Gunnison Gorge BLM WSA Armbrustmacher and others, 1989.

Handies Peak BLM WSA Sanford and others, 1987.

La Garita WSA Steven and Bieniewski, 1977.

Maroon Bells-Snowmass WSA Freeman and others, 1985.

Mt. Massive WSA Van Loenen and others, 1989.

Oh-Be-Joyful WSA Ludington and Ellis, 1983.

Powderhorn WSA Sharp and others, 1983.

Redcloud Peak BLM WSA Sanford and others, 1987.

Tabeguache Creek BLM WSA Dickerson and others, 1990.

Uncompahgre Primitive Area2 Fischer and others, 1968.

Uncompahgre Primitive Area Additions3 Steven and others, 1973, 1977.

West Elk WSA Gaskill and others, 1977.

Wilson Mountains Primitive Area4 Bromfield and others, 1972.

 1Wilderness Study Area (WSA).

 2 Currently the Big Blue Wilderness Area.

 3 Includes parts of the Big Blue Wilderness and Mt. Sneffels Wilderness Areas.

 4 Currently the Lizard Head Wilderness Area.
6 Resource Potential and Geology, Grand Mesa, Uncompahgre, and Gunnison National Forests, Colorado

In order to assess the mineral resource potential of the 

GMUG greater study area, we outlined 36 mineralized areas 

(Wilson and Spanski, this volume, Chapter E). A mineralized 

area encloses a geographic area that is defined by the presence of mines, prospects, and (or) mineralized occurrences 

that belong to one deposit type or a group of genetically 

related deposit types in a distinct geologic setting. A mineralized area may include an entire district or portions of several 

mining districts, just as a mining district may include several 

mineralized areas. Twenty-five of the areas are based on the 

occurrence of a single mineral deposit type and 11 on multiple 

deposit types. 

Mineral resource potential assessments were conducted 

for seven of the most significant deposit types within the 

GMUG greater study area. We began the assessment with an 

initial consideration of mineral types likely to be present in the 

area. Of these, we selected the most important, both historically and of future impact. These deposit types are granitehosted porphyry molybdenum, granodiorite-hosted porphyry 

molybdenum, sandstone-hosted uranium, volcanic-hosted 

massive sulfide, polymetallic vein, polymetallic replacement, 

and sediment-hosted redbed copper. From descriptive models 

for these deposit types, criteria were determined to allow us 

to generate areas of permissive and favorable tracts. A GIS 

computer program was used to select these criteria from 

various geologic, geophysical, and geochemical data bases. 

Finally, quantitative assessments were performed on four of 

the deposit types that had adequate grade and tonnage models. 

Upper Cretaceous strata are known to contain coal in the 

vicinity of the forests. The forests have a low to moderate coal 

resource potential in areas underlain by the Dakota Sandstone and a moderate to high coal resource potential in areas 

underlain by the Fruitland Formation, Mesaverde Group, or 

Mesaverde Formation. Contiguous areas of high coal resource 

potential in the Grand Mesa and Gunnison National Forests 

are estimated to have a combined coal resource of about 38 

billion short tons. This study does not estimate coal reserves 

that can be economically produced at the present time. 

Summary of Mineral Resource 

Assessments

Granite Porphyry Molybdenum Deposits

Granite porphyry molybdenum deposits are characterized 

by mineralization and by intrusion of high-silica, alkali-rich 

granite or rhyolite. Generally small and cylindrical, these 

high-silica stocks or plugs are thought to represent high-level 

cupolas that extend above large silicic plutons. The ore zones 

of the deposits are centered in or above the apical portion of 

the source granitic intrusion. More than 90 percent of the 

molybdenite is present in thin, moderately to steeply dipping 

stockwork veinlets. Hydrothermal alteration is associated with 

these deposits. Granite molybdenite systems are thought to be 

associated with the transition from compressive to extensional 

tectonism. The Mount Emmons and Redwell Basin deposits 

along with the nearby world-class Climax and Henderson 

deposits attest to the unique character of the Tertiary magmatic 

terrane that underlies the study area and its capacity to generate deposits of this type.

Areas classified as permissive are those that are underlain 

by Tertiary-age intrusions. As shown in Chapter G, figure G1, 

the permissive tract is quite extensive owing to the abundance 

of intermediate- to silicic-composition intrusions in the eastern 

3/4 of the GMUG greater study area. These intrusions are 

absent within the more tectonically stable western 1/4 of the 

study area. 

Specific characteristics of favorable tracts (Chapter G, 

fig. G2) include distinctive chemical composition of the intrusions (high-silica granite or rhyolite), anomalous geochemical values for tin, tungsten, or niobium, and the occurrence 

of molybdenite or other minerals such as fluorite. A small 

potential exists for the occurrence of one more deposit, based 

on the fact that two (Mount Emmons and Redwell Basin) out 

of the nine deposits used in the construction of the Climaxtype grade and tonnage models lie within the study area, and 

that areas within the study area have been targets of repeated 

exploration interest. The repeated shows of exploration interest indicate that other knowledgeable parties believe in the 

possible existence of additional Climax-type deposits.

Granodiorite Porphyry Molybdenum Deposits

Granodiorite molybdenum systems are associated with 

small composite stocks, late-stage batholiths, and less commonly single phase stocks. Intrusions range from quartz 

monzonite to granodiorite. Deposits are fluorine deficient and 

distinct from their fluorine-rich, granite molybdenum counterparts. North American granodiorite deposits are mostly confined to Mesozoic and Tertiary intrusive rocks. The associated 

ore bodies are cylindrical, tabular, or irregular; mineralization 

was generally confined to stockwork veinlets developed in 

or around the roof of the intrusion. These veinlets contain 

molybdenite and quartz with pyrite, biotite, and minor carbonates. No molybdenum or copper has been produced from 

granodiorite porphyry molybdenum deposits in the GMUG 

greater study area or other parts of Colorado. However, 

several subeconomic granodiorite molybenum prospects have 

been located in western Colorado. 

Areas classified as permissive are those that are underlain 

by Tertiary intrusions, identical to granitic deposits (Chapter 

G, fig. G1). Several intrusive units and correlative dikes are 

excluded from rocks characterized as permissive for mineralization based on previous mineral assessment and geologic 

studies. Specific characteristics of favorable tracts (Chapter 
Introduction 7

G, fig. G3) restrict intrusions to intermediate- to siliciccomposition intrusions, dikes, hypabyssal stocks, and plugs 

ranging from quartz monzonite to granodiorite. Other criteria 

include the presence of anomalous geochemical values for 

lead, zinc, or silver, or a mine, occurrence, or mineralized site 

with sphalerite, galena, or chalcopyrite present or containing 

elevated lead, zinc, copper, silver, or gold. The small deposit 

potential determined in Chapter G recognizes the fact that the 

magmatic terrane that underlies the study area and dominated 

geologic events during Tertiary time is unique in terms of its 

molybdenum geochemistry. As each pulse of magmatic activity evolves, it gives rise to a series of intrusive and extrusive 

events in which later events demonstrate a tendency to become 

anomalously enriched in molybdenum. Owing to their size 

and the disseminated nature of the molybdenum mineralization, any undiscovered deposit would only be of interest to 

major mining companies. Under these circumstances, exploration interest in targets of this type is expected to be virtually 

nonexistent unless a major increase in market demand and 

price for molybdenum develops.

Sandstone-Hosted Uranium Deposits

Sandstone-hosted uranium occurrences are concentrated 

in two distinct geographic areas; one borders the GMUG 

area on the west (Uravan mineral belt) and a second, smaller 

area lies in the northwest corner of Gunnison County (RubyIrwin Mineralized Area). These occurrences are genetically 

similar—uranium minerals fill intergranular pore spaces and 

replace carbonaceous material, quartz grains, and interstitial cements in clastic rocks, mainly sandstones. A variety 

of sources of the uranium has been proposed: (1) sediment 

derived from incompletely weathered felsic rocks in the highland areas, (2) clays with adsorbed uranium that are delivered to the basin, or (3) detritus deposited on the basin floor. 

Oxidizing ground water percolated through these sediments, 

leaching uranium from volcanic glass, feldspars, and clay minerals, and transporting it. Where these uranium-bearing waters 

encountered reducing conditions around organic rich sediment 

layers, uranium was precipitated. 

Permissive areas include feldspathic and carbonaceous 

sandstone units occurring interbedded with mudstones and 

shales (Chapter H, fig. H1). For favorable areas, increased 

importance is placed on evidence that mineralization has 

occurred, on anomalous geochemistry or radioactivity, or on 

evidence from previous reports (Chapter H, fig. H2). Under 

current free market conditions, sandstone-hosted uranium 

deposits in Colorado are noneconomic. Should price increase 

to dollar values of the high teens or above, expectation is 

strong that some existing mines with proven reserves would 

reopen, and exploration for new deposits of similar size and 

grade, considered highly likely to be present in the favorable 

tract areas, would resume. 

Volcanic-Associated Massive Sulfide (VMS) 

Deposits

Volcanic-associated massive sulfide (VMS) deposits are 

located in the Dubois Greenstone belt, an area of Proterozoic 

volcanic and intrusive rocks exposed within the Gunnison 

uplift. VMS deposits contain copper, lead, and zinc as primary 

ore metals, and lesser amounts of silver and gold. VMS ores 

are deposited in structurally controlled zones from fluids in 

hydrothermal systems associated with underwater volcanoes 

and rifts. Permissive areas (Chapter I, fig. I2) are defined by 

three rock types that contain metavolcanic rocks, metasedimentary rocks, or both that are mapped as Xfh, Xf, and Xb of 

Day and others (1999). Favorable areas restrict the geology 

to Xf or Xfh, and include evidence of known mineralization 

(Chapter I, fig. I3). These areas lie east of Blue Mesa Reservoir near Gunnison. As in the past, VMS deposits in the 

greater study area will likely be valued for their gold content, 

with other commodities considered as byproducts.

Polymetallic Vein Deposits

Veins rich in copper, lead, and zinc, and carrying smaller 

amounts of silver or gold, form from rising, hydrothermal 

solutions. Polymetallic vein deposits form as part of complexly zoned subvolcanic systems and are known to form 

in rocks of many ages (Precambrian to Tertiary) and many 

compositions. The fundamental requirement is that the host 

rock be brittle enough to break and stay open, thus allowing 

a vein to fill open space. The southern area (Chapter J, fig. 

J2) generally hosts larger deposits that are related to Tertiary 

volcanic rocks. The western San Juan Mountains between 

Silverton and Telluride contain some of the best endowed 

veins in the study area. Deposits in the northeastern area tend 

to be smaller and occur in Proterozoic and Paleozoic rocks. 

These veins differ from the San Juan type in that they have 

more milky “bull” quartz and a relationship to early to middle 

Tertiary granitic stocks that appear not to have generated an 

extensive volcanic edifice. 

Permissive criteria include the presence of mapped shallow, subvolcanic Tertiary intrusions or geophysical evidence 

for them (Chapter J, fig. J1). For favorable areas, increased 

importance is placed on evidence that mineralization has 

occurred, on anomalous geochemistry, and on proximity to 

caldera structures (Chapter J, fig. J2). Favorable potential 

is mainly associated with the probable existence of smaller 

undiscovered ore bodies, especially in areas adjacent to or 

directly beneath areas of currently known polymetallic vein 

occurrences. In the past, such an area would have been 

developed as a mine or as an extension to a mine. In today’s 

economic and environmental climate, large mining companies 

are unlikely to be interested in sporadic occurrences of this 

nature; however, these smaller occurrences might be attractive 

development targets for smaller entrepreneurial groups. The 
8 Resource Potential and Geology, Grand Mesa, Uncompahgre, and Gunnison National Forests, Colorado

viability of these mineralized areas is dependent on economic 

factors rather than geology.

Polymetallic Replacement Deposits

Polymetallic replacement deposits commonly contain 

lead, zinc, copper, and silver sulfide minerals and have been 

historically important contributors to the total lead, zinc, copper, silver, and manganese produced in Colorado. Substantial 

amounts of gold or silver in these smaller deposits compensate 

for their limited volume and make them commercially attractive. Polymetallic replacement deposits are hydrothermal 

accumulations of sulfide minerals hosted in limestone, dolomite, or other chemically reactive (soluble) rock, adjacent to 

intrusions. Deposits range from small pods and veins to large, 

mixed-sulfide replacement bodies; the shapes are irregular 

and structurally and stratigraphically controlled. Deposits 

are predominantly hosted by carbonates with a minor number 

hosted in sandstone, evaporite (gypsum), calcareous shale, and 

occasionally, permeable zones in volcanic rocks.

Permissive areas are within 10 km of known or inferred 

felsic Cretaceous or Tertiary plutons and are underlain by 

Paleozoic, Mesozoic, or Cenozoic sedimentary rock units that 

contain permeable and chemically reactive lithologic units 

(Chapter K, fig. K2). To determine favorable areas, increased 

importance is placed on the presence of sedimentary rock units 

having a substantial carbonate component and on evidence that 

mineralization has occurred. Favorable areas cluster around 

the towns of Ouray, Silverton, and Telluride in the south and 

near and to the east of Crested Butte in the northwestern part 

of the study area (Chapter K, fig. K3). Favorable areas large 

enough to conceal district-size deposits that have not been 

extensively explored are lacking. However, the potential is 

high for the existence of new ore bodies in areas adjacent 

to, or beneath, currently known polymetallic replacement 

mineralized rocks. In today’s economic and environmental 

climate, large mining companies are unlikely to be interested 

in occurrences of this nature; however, these smaller occurrences might be attractive development targets for smaller 

entrepreneurial groups.

Sediment-Hosted Copper Deposits

Copper occurs in sedimentary rocks of the salt anticlines 

of the Paradox Basin, the borders of the Uncompahgre uplift, 

and the Eagle Basin. Most known occurrences are small; 

however, deposits large enough to be productive occur in the 

salt anticline areas. Chapter L describes a new model for 

structurally controlled, sediment-hosted copper deposits found 

in the Paradox Basin, formed where basin brines rose through 

faults and permeable sandstone formations adjoining salt 

anticlines. Four tracts were determined to be permissive for 

the presence of sediment-hosted copper deposits; three areas in 

those tracts are further identified as favorable (Chapter L, figs. 

L2 and L3). One favorable area includes much of the Paradox 

Basin, a second encompasses the southwestern Uncompahgre 

uplift, and a third is located in and near the Eagle Basin. A 

viable near-term development potential is associated with 

these favorable areas. No quantitative assessment could be 

conducted.

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Keenan, and Brown, S.D., 1989, Mineral resources of the 

Gunnison Gorge Wilderness Study Area, Montrose and 

Delta Counties, Colorado: U.S. Geological Survey Bulletin 

1715-D, p. D1–D14.

Bove, D.J., Hon, Ken, Budding, K.E., Slack, J.R., Snee, L.W., 

and Yeoman, R.A., 2000, Geochronology and geology of 

late Oligocene through Miocene volcanism and mineralization in the western San Juan Mountains, Colorado, version 

1.0: U.S. Geological Survey Open-File Report 99-347, 35 

p., URL: http://pubs.usgs.gov/of/1999/ofr-99-0347/.

Bove, D.J., and Knepper, D.H., Jr., 2000, Surface water data 

and geographic relation to Tertiary age intrusions and 

hydrothermal alteration in the Grand Mesa, Uncompahgre, 

and Gunnison National Forests (GMUG) and intervening Bureau of Land Management (BLM) lands: U.S. 

Geological Survey Open-File Report 00-271, URL: http:

//pubs.usgs.gov/of/2000/ofr-00-0271/, computer file.

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Mineral resources of the Wilson Mountains Primitive Area, 

Colorado: U.S. Geological Survey Bulletin 1353-A, p. 

A1–A79.

Day, W.C., Green, G.N., Knepper, D.H., Jr., and Phillips, 

R.C., 1999, Spatial geologic data model for the Gunnison, Grand Mesa, Uncompahgre National Forests mineral 

assessment area, southwestern Colorado and digital data for 

the Leadville, Montrose, Durango, and the Colorado parts 

of the Grand Junction, Moab, and Cortez 1°x2° geologic 

maps: U.S. Geological Survey Open-File Report 99-427, 

CD-ROM.

DeWitt, Ed, Stoneman, R.J., Clark, J.R., and Kluender, S.E., 

1985, Mineral resource potential map of the Fossil Ridge 

Wilderness Study Area, Gunnison County, Colorado: U.S. 

Geological Survey Miscellaneous Field Studies Map MF1629-A, scale 1:50,000, 21 p.

Dickerson, R.P., Barton, H.N., Blank, H.R., Jr., and Scott, 

D.C., 1990, Mineral resources of the Tabeguache Creek 

Wilderness Study Area, Montrose County, Colorado: U.S. 

Geological Survey Bulletin 1715-E, p. E1–E15.
Introduction 9

Fischer, R.P., Luedke, R.G., Sheridan, M.J., and Raabe, R.G., 

1968, Mineral resources of the Uncompahgre Primitive 

Area, Colorado: U.S. Geological Survey Bulletin 1261-C, 

p. C1–C91.

Freeman, V.L., Campbell, D.L., King, H.D., Weisner, R.C., 

and Bieniewski, C.L., 1985, Mineral resource potential map 

of the Maroon Bells–Snowmass Wilderness and Additions, 

Gunnison and Pitkin Counties, Colorado: U.S. Geological 

Survey Miscellaneous Field Studies Map MF-1647-A, scale 

1:100,000.

Gaskill, D.L., Rosenbaum, J.G., King, H.D., Meeves, H.C., 

and Bieniewski, K.L., 1977, Mineral resources of the West 

Elk Wilderness and vicinity, Delta and Gunnison Counties, 

Colorado: U.S. Geological Survey Open-File Report 77-

751, 111 p.

Ludington, Steve, and Ellis, C.E., 1983, Map showing geology and mineral resource potential of the Oh-Be-Joyful 

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Geological Survey Miscellaneous Field Studies Map MF1582-A, scale 1:24,000.

Miller, W.R., 1998, Geochemical baselines and processes 

affecting surface water, Redcloud Peak area, Colorado: 

U.S. Geological Survey Open-File Report 98-35, 20 p.

Miller, W.R., 1999, Geochemical baselines and maps showing acid-neutralizing capacity and potential release of total 

dissolved solids of stream and spring waters from different 

rock compositional types from mountainous watersheds 

in the Gunnison, Uncompahgre, and Grand Mesa National 

Forests, Colorado: U.S. Geological Survey Open-File 

Report 99-580, 107 p.

Nash, J.T., 2002, Hydrogeochemical investigations of historic 

mining districts, central Western Slope of Colorado, including influence on surface-water quality: U.S. Geological 

Survey Digital Data Series DDS-73, CD-ROM.

Sanford, R.F., Grauch, R.I., Hon, Ken, Bove, D.J., Grauch, 

V.J.S., and Korzeb, S.L., 1987, Mineral resources of the 

Redcloud Peak and Handies Peak Wilderness Study Area, 

Hinsdale County, Colorado: U.S. Geological Survey Bulletin 1715-B, p. B1–B35.

Sharp, W.N., Martin, R.A., and Lane, M.E., 1983, Mineral 

resource potential and geologic map of the Powderhorn 

Wilderness Study Area and Cannibal Plateau Wilderness 

Roadless Area, Gunnison and Hinsdale Counties, Colorado: 

U.S. Geological Survey Miscellaneous Field Studies Map 

MF-1483-A, 6 p.

Soulliere, S.J., Arnold, M.A., Kluender, S.E., and Zelten, J.E., 

1986, Mineral resources of the Eagle Mountain Wilderness Study Area, Pitkin County, Colorado: U.S. Geological 

Survey Bulletin 1717-B, p. B1–B9.

Steven, T.A., and Bieniewski, C.L., 1977, Mineral resources of 

the La Garita Wilderness, San Juan Mountains, southwestern Colorado, with a section on Geophysical interpretation 

by G.P. Eaton: U.S. Geological Survey Bulletin 1420, 65 p.

Steven, T.A., Lipman, P.W., Fisher, F.S., Bieniewski, C.L., 

and Meeves, H.C., 1973, Mineral resources of study areas 

contiguous to the Uncompahgre Primitive Areas, San Juan 

Mountains, southwestern Colorado: U.S. Geological Survey Open-File Report 73-271, 205 p.

Steven, T.A., Lipman, P.W., Fisher, F.S., Bieniewski, C.L., 

and Meeves, H.C., 1977, Mineral resources of study areas 

contiguous to the Uncompahgre Primitive Area, San Juan 

Mountains, southwestern Colorado: U.S. Geological Survey Bulletin 1391-E, p. E1–E126.

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, with a section 

on Salable minerals by J.S. Dersch: U.S. Geological Survey 

Bulletin 1638, 42 p.

Toth, M.I., Davis, L.L., Patterson, C.G., and Seeley, J.L., 1983, 

Geology and mineral resource potential of the Dominguez 

Canyon Wilderness Study Area, Delta, Mesa, and Montrose 

Counties, Colorado (GEM phase 2): U.S. Geological Survey Open-File Report 83-797, 35 p.

Toth, M.I., Patterson, C.G., Kulik, D.M., and Schreiner, R.A., 

1987, Mineral resources of the Dominguez Canyon Wilderness Study Area, Delta, Mesa, and Montrose Counties, 

Colorado: U.S. Geological Survey Bulletin 1736-A, p. 

A1–A10.

Toth, M.I., Wilson, A.B., Cookro, T.M., Bankey, Viki, Lee, 

G.K., and Case, J.E., 1993, Mineral resource potential and 

geology of the White River National Forest and the Dillon 

Ranger District of the Arapaho National Forest, Colorado, 

with a section on Salable commodities by J.S. Dersch: U.S. 

Geological Survey Bulletin 2035, 117 p.

Tweto, Ogden, and Sims, P.K., 1963, Precambrian ancestry of 

the Colorado Mineral Belt: Geological Society of America 

Bulletin, v. 74, no. 8, p. 991–1014.

Van Loenen, R.E., and Gibbons, A.B., eds., 1997, Mineral 

resource potential and geology of the San Juan National 

Forest, Colorado, with a section on Salable minerals by 

A.G. Raby and J.S. Dersch: U.S. Geological Survey Bulletin 2127, 140 p.

Van Loenen, R.E., Lee, G.K., Campbell, D.L., and Thompson, J.R., 1989, Mineral resource appraisal of the Mount 

Massive Wilderness Area, Lake County, Colorado: U.S. 

Geological Survey Bulletin 1636, 18 p.
10 Resource Potential and Geology, Grand Mesa, Uncompahgre, and Gunnison National Forests, Colorado

Wilson, A.B., Spanski, G.T., Crane, M.J., and Woodard, M.D., 

2000, Databases and spatial data model for mineralized 

areas, mines, and prospects in the Grand Mesa, Uncompahgre, and Gunnison (GMUG) National Forests, Colorado: 

U.S. Geological Survey Open-File Report 00-298, CDROM. URL: http://pubs.usgs.gov/of/2000/ofr-00-0298/.
U.S. Department of the Interior

U.S. Geological Survey

Review of the Geology of Western Colorado

By Warren C. Day and Dana J. Bove

Chapter B of

Resource Potential and Geology of the Grand Mesa, Uncompahgre, 

and Gunnison (GMUG) National Forests and Vicinity, Colorado

Edited by Viki Bankey

U.S. Geological Survey Bulletin 2213– B
Contents

Abstract........................................................................................................................................................ 13

Introduction ................................................................................................................................................. 14

Proterozoic Eon........................................................................................................................................... 14

Regional Setting................................................................................................................................. 14

Proterozoic Rocks of Western Colorado........................................................................................ 14

Paleozoic Era............................................................................................................................................... 17

Early and Middle Paleozoic.............................................................................................................. 17

Late Paleozoic .................................................................................................................................... 19

Mesozoic Era............................................................................................................................................... 21

Triassic................................................................................................................................................. 21

Late Triassic Through Jurassic ....................................................................................................... 22

Cretaceous.......................................................................................................................................... 25

Cenozoic Era................................................................................................................................................ 26

Tertiary Sedimentary Deposits ........................................................................................................ 26

Tertiary Volcanism and Associated Sedimentation ..................................................................... 28

Western San Juan Mountains................................................................................................ 29

Central San Juan Mountains Caldera Cluster ..................................................................... 29

Late Basalts and Rhyolites of the Central San Juan Mountains Caldera Cluster.......... 30

Sawatch Range Calderas and Related Mount Princeton Batholith................................. 30

Plutonic and Volcanic Rocks in the Elk Mountains Region ............................................... 31

Late Bimodal Rocks in the Elk Mountains Region............................................................... 31

Late Tertiary and Quaternary Unconsolidated Deposits ............................................................. 31

References Cited ........................................................................................................................................ 32

Figures

B1. Generalized geologic map showing Precambrian terranes of Colorado and southern 

Wyoming .............................................................................................................................................. 15

B2. Generalized geologic map of the Proterozoic rocks in study area............................................. 16

B3. Map showing distribution of lower to middle Paleozoic and upper Paleozoic rocks

in study area ........................................................................................................................................ 18 

B4. Map showing paleogeographic setting of Ancestral Rocky Mountains highlands

and related features........................................................................................................................... 20

B5. Tectonic province map, showing distribution of uplifts and sedimentary basins

of western Colorado........................................................................................................................... 22

B6. Generalized geologic map showing distribution of Mesozoic sedimentary units

in study area ........................................................................................................................................ 24

B7. Generalized geologic map of San Juan volcanic field of southern Colorado........................... 27
Abstract 

The geology of western Colorado is a culmination of 

more than 1.8 billion years of Earth history, which has yielded 

a breathtaking landscape. This report’s review of major 

geologic events provides the context necessary to understand 

accompanying chapters of this volume that cover the mineral 

resource endowment and the environmental consequences of 

hardrock mining in the region.

The oldest known rock units of western Colorado are the 

1.8–1.7 billion-year-old Precambrian layered gneisses, schists, 

and massive intrusive rocks of the Yavapai geologic province. 

The gneisses and schists originated as sediments and volcanic 

rocks deposited in an island-arc environment, probably similar 

to the modern Indonesian archipelago. The Precambrian rocks 

were buried to depths of 10–15 kilometers, strongly deformed, 

recrystallized to metamorphic gneisses and schists, and 

intruded by ≈1.7 billion-year-old magmas. About 1.4 billion 

years ago, granite and associated intrusive rocks invaded the 

metamorphic rocks. The last episode of Precambrian activity, 

about 1.1 billion years ago, was marked by the intrusion of 

large masses of granite and associated mafic rocks.

During the billion years between formation of western 

Colorado’s Precambrian rocks and the formation of Paleozoic 

rocks, weathering and erosion stripped off Precambrian rocks 

that were once so deeply buried. In the Paleozoic, about 540 

Ma (million years ago), thin blankets of sand, carbonate, 

and clay were deposited in shallow seas on the North American continental shelf. Between about 440 and 280 Ma, the 

region was uplifted, creating the Ancestral Rocky Mountains. 

Quartz- and feldspar-rich sands, silts, and conglomerates 

eroded from these uplifted ranges filled restricted intermontane basins with thick redbed sediments as well as evaporite 

layers.

Erosion of the Ancestral Rocky Mountains continued into 

early Mesozoic time (250–230 Ma). Streams and rivers from 

the ancient highlands carried sand and mud out onto alluvial 

plains and into deltas and lakes. During Late Triassic to Early 

Jurassic time (≈210 Ma), the climate changed from warm/

humid to arid; great deserts of windblown sand formed in 

western North America. In western Colorado this episode of 

Earth’s history (≈205 Ma) is recorded in the rocks of the Glen 

Canyon Group, comprising the Wingate Sandstone, Kayenta 

Formation, and Navajo Sandstone. Subsequently, during the 

Middle Jurassic, an eastward-encroaching sea laid down 

sediments which consolidated as the Carmel Formation of 

the San Rafael Group. As the Middle Jurassic sea retreated, 

the arid desert conditions returned, bringing the environment 

recorded in the Entrada Sandstone and Wanakah Formation 

of the San Rafael Group. Much of the San Rafael Group 

was then eroded and covered by stream and lake deposits of 

the Morrison Formation. From Late Jurassic through Early 

Cretaceous time, low-gradient streams meandered across flood 

plains, mud flats, and saline basins.

During Late Jurassic and Early Cretaceous time (≈160–

96 Ma), streams and basins deposited mud and silt of the 

Morrison Formation and, in places, sands and conglomerates 

of the Lower Cretaceous Burro Canyon Formation. Where 

the Burro Canyon is not present, above the Morrison lie 

sandstones, shales, and coal seams of the Dakota Sandstone, 

one of the most extensive geologic formations in the Western 

Interior of North America. The Dakota formed in an eastwardadvancing sea, which subsequently deposited the deeper water 

sediments of the Mowry, Mancos, and Pierre Shales and sands 

of the Frontier Sandstone and Ferron Sandstone Members of 

the Mancos Shale. The great inland sea then retreated, and 

along its shores in deltas, marshes, and lagoons were deposited 

the nonmarine sand, shale, and coal deposits of the Mesaverde 

Group (or Formation).

Regional mountain-building and associated advances 

and retreats of the inland sea were the controls on the region 

during the Cretaceous. The mountain-building events consisted of two main pulses: the older, called Sevier, affected 

rocks generally outside this area, whereas the younger, called 

Laramide, produced many of the mountain ranges of western 

Colorado, including the Uncompahgre, White River, Sawatch, 

and Gunnison uplifts (as well as the Gore Range and Front 

Range). Uplifts of the ancestral Rockies were rejuvenated and 

new mountain ranges formed; basins also formed between the 

uplifts, creating centers of deposition for marine sediments 

and detritus from the uplifts. Two of these basins formed 

in western Colorado—the Piceance Basin of northwestern 

Colorado and adjacent Wyoming, and the San Juan Basin of 

southwestern Colorado and adjacent New Mexico.

Following the Laramide event came several episodes of 

volcanism (≈30–22 Ma), some catastrophic; these formed the 

highlands of the San Juan and Elk Mountains. This time saw 

emplacement of many of the mineral deposits that occur in the 

Review of the Geology of Western Colorado

By Warren C. Day and Dana J. Bove
area known as the Colorado Mineral Belt. Miocene and early 

Pliocene rifting followed the great volcanic eruptions; rifting 

was accompanied by quiet eruptions of basalt flows on what 

are now Grand Mesa, the San Juan Mountains, and the Flat 

Tops of the White River uplift. In places, small rhyolite intrusions accompanying the regional rifting are mineralized with 

molybdenum. Miocene uplift led to erosional downcutting 

through earlier Eocene surfaces; in the Pliocene, uplift may 

have accelerated, and canyon cutting apparently continued into 

the Quaternary.

About 500,000 years ago, the climate began to cool: 

three times glacial ice filled the mountain valleys and almost 

totally covered the higher ranges. Colorado’s alpine landscape 

of deep U-shaped valleys is largely a product of glacial 

erosion.

Introduction

The GMUG greater study area encompasses more than 

52,820 km2 of southwestern Colorado, of which USDA Forest 

Service lands account for approximately 12,740 km2. This 

large area of western and southwestern Colorado encompasses 

a region of numerous ecosystems and diverse physiographies. 

For example, the San Juan Mountains in the southern part of 

the area host spectacular peaks more than 4,000 m in elevation 

and contain ecosystems that range from alpine, subalpine, to 

montane as defined by Fleischer-Mutel and Emrick (1984). In 

the eastern part of the study area, the Sawatch Range also contains several peaks greater than 4,000 m and has ecosystems 

that extend from the montane up to the alpine. In the western 

part of the study area, in the lower elevations (around 1,500 m) 

along the Colorado and Gunnison Rivers, ecosystems range 

into the upper Sonoran (of Fleischer-Mutel and Emrick, 1984).

The study area’s wealth of information on the geologic 

history of Colorado is not only fascinating from an academic 

point of view but is also important to informed planning and 

land use. Mining and ranching shaped the early history of the 

Rocky Mountain region, and central and southwestern Colorado’s mineral endowment played a major role. Early European 

settlement centered on the areas rich in mineral resources as 

well as along regions with water and arable soil for ranching.

Today, an ever-increasing source of revenue for the region 

is tourism, owing in large part to its spectacular geologic 

scenery. Tourist destinations include several towns such as 

Telluride, Crested Butte, Aspen, and Vail, all of which fall 

within this study area. The resulting economic development, 

however, has placed increased pressure on the transportation and housing infrastructures to accommodate the growing 

populations.

The demands of continuing development include new 

housing, roads, schools, and places of work, all of which are 

affected by the geologic setting. Sources for aggregate vital 

for new construction are a paramount issue; construction on 

swelling soils, such as those within the Morrison Formation, 

must be engineered to minimize damage to buildings, bridges, 

and roads. The effect of abandoned mine lands on water quality is a key issue for Federal and State agencies. The results of 

mining techniques employed more than a century ago are still 

with us today.

This report summarizes the complex geologic history of 

this large area and therein provides the regional framework for 

the integrated mineral and environmental studies in subsequent 

chapters. This report also provides the context for a companion digital geologic map compilation (Day and others, 1999) 

covering southwestern Colorado.

Proterozoic Eon

Regional Setting

The Precambrian rocks of the GMUG greater study area 

are among the oldest preserved rocks in the southern Rocky 

Mountains of Wyoming, Colorado, and New Mexico. The 

Precambrian rocks of the southern Rocky Mountain region 

have been divided into three large terranes, or tectonostratigraphic provinces, that have internally consistent ages of 

formation, rock types, and mineral deposits. The Archean 

rocks of the Wyoming province make up the northernmost 

Precambrian terrane. The northeast-trending Cheyenne belt 

forms the suture between the Archean rocks of the Wyoming 

province and the Proterozoic rocks of Colorado and New 

Mexico to the south (fig. B1). The Cheyenne belt is a zone 

of pulverized, recrystallized rock (mylonite) exposed in the 

Medicine Bow and Sierra Madre Mountains of Wyoming and 

Colorado (Houston and others, 1989). The Proterozoic rocks 

of the Yavapai province of northern and central Colorado and 

the Mazatzal province of New Mexico constitute the other 

two Precambrian terranes. Rocks of the Yavapai province 

range from 1.8 to 1.7 Ga (billions of years), whereas the rocks 

within the Mazatzal province range from 1.7 to 1.6 Ga (Silver, 

1965, 1968; Van Schmus and Bickford, 1981; Reed and others, 

1987). 

Unlike the Archean-Proterozoic boundary represented 

by the Cheyenne belt, the Yavapai-Mazatzal tectonic boundary is poorly defined. Using several lines of evidence, Shaw 

and Karlstrom (1999) proposed that the boundary could only 

be narrowed to a 300 km-wide zone that trends northeastward 

through southern Colorado and northern New Mexico (fig. 

B1).

Proterozoic Rocks of Western Colorado

The Proterozoic rocks of the greater study area (map 

area, fig. B2) belong to the Yavapai province. These rocks 

are an amalgamation of 1.8–1.7 billion year old volcanic and 

associated sedimentary (supracrustal) rocks that have been 

14 Resource Potential and Geology, Grand Mesa, Uncompahgre, and Gunnison National Forests, Colorado
Review of the Geology of Western Colorado 15

metamorphosed to upper greenschist to middle amphibolite 

grade. Granitic rocks intruded the supracrustal sequences 

during three broad time windows and formed distinct 

plutonic suites: the Routt Plutonic Suite (≈1.6–1.7 Ga), the 

Berthoud Plutonic Suite (≈1.4 Ga), and rocks of the Pikes 

Peak (≈1.1 Ga) batholith. According to Tweto (1987), 

deformation accompanied the emplacement of the Routt 

Plutonic Suite, whereas the Berthoud Plutonic Suite was 

a so-called anorogenic group that was emplaced passively 

into the supracrustal sequences. However, recent structural 

studies accompanied by new radiometric data have shown 

that emplacement of some of the ≈1.4 Ga Berthoud Plutonic 

Suite rocks throughout northern and central Colorado was 

accompanied by ductile deformation along major northeasttrending shear zones (Selverstone and others, 1997). The 

Pikes Peak Granite, east of the study area, is an anorogenic 

granite inasmuch as no regional deformation was associated 

with its emplacement (Smith and others, 1999). 

The oldest Proterozoic supracrustal rocks in the study 

area can be further subdivided into low and medium metamorphic grades. Medium-grade metamorphic rocks, biotite schist, 

metapelite, amphibolite, quartz-feldspar and hornblende-biotite gneisses, form the basement of the northern part of the 

study area; they have been intruded by pegmatite, granite, and 

gabbroic dikes, sills, and plutons (Tweto, 1987). The protoliths for the biotite schist and metapelite were dirty sandstone 

and shale. The amphibolite represents metamorphosed dikes, 

sills, and (or) flows of basaltic magma. The gneissic rocks 

probably represent metamorphosed volcanic rocks and sediments. Reed and others (1987) showed that, based on U-Pb 

zircon dates, the Precambrian supracrustal rocks of the Yavapai 

province of Colorado decrease in age southward from the 

Archean-Proterozoic boundary (Cheyenne belt) in Wyoming 

(fig. B1). The northern part of the province (just south of the 

Cheyenne belt) is made up of the >1.75 Ga Green Mountain 

block, which may have formed in an island-arc setting. The 

central part of the Proterozoic rocks of Colorado belongs to a 

composite back-arc basin that was intruded by igneous rocks 

at about 1.70 Ga (and earlier). The southern part is made up 

of the Salida-Gunnison magmatic-arc complex; it contains 

plutons that range in age from 1.76 to 1.60 Ga.

The Salida-Gunnison magmatic-arc complex, described 

by Reed and others (1987), is part of the Dubois Greenstone 

belt, a package of volcanic, sedimentary, and granitic rocks 

of low metamorphic grade that crop out east and south of 

Blue Mesa Reservoir along the Uncompahgre uplift (fig. B2). 

The greenstone belt is made up of bimodal (felsic and mafic) 

volcanic rocks and associated volcaniclastic sediments interlayered with banded iron-formation, massive sulfide, and goldbearing exhalative mineral deposits (Afifi, 1981; Hedlund and 

Olson, 1981; Knoper and Condie, 1988). Bickford and others 

(1989, and references therein) have noted two periods of volcanism and plutonism in the greenstone belt: an earlier episode 

that occurred from 1,770 to 1,760 Ma followed by plutonism 

from 1,755 to 1,750 Ma and a later episode of volcanism that 

spanned from 1,740 to 1,714 Ma with emplacement of granites 

from 1,725 to 1,714 Ma.

Figure B1. Precambrian terranes of Colorado 

and southern Wyoming. Modified from Shaw and 

Karlstrom (1999). MF (dotted line), deformational 

front of the Mazatzal province; Nd, neodymium 

model age boundary between the 2.0–1.8 Ga and 

1.8–1.6 supracrustal rocks; CB, Cheyenne belt; 

GM, Green Mountain magmatic arc; CBA, composite back-arc; CMB, Colorado Mineral Belt; SG, 

Salida-Gunnison magmatic complex.

Middle Proterozoic plutons

Early Proterozoic rocks

Transition zone between Yavapai

 and Mazatzal provinces

Transition zone between Mojave

 and Yavapai provinces

EXPLANATION

Proterozoic shear zone

WY

CO

NM

CO

GM

CBA

SG

Mojave

province

Yavapai

province

Wyoming province

108o 106o

38o

40o

42o

0 100 KILOMETERS

CMB

CB

Colorado

Plateau

MF

Nd

Approximate

outline of study area
Gunnison uplift Uncompahgre Plateau Paradox Basin San Juan Mountains Grand Mesa Sawatch Mountains 

Eagle Basin 

DuBois Greenstone belt low- to medium-grade rocks medium-grade rocks Tectonic transport direction 39o 30'109o 

37o 45'

70

139

65

92

50

50 

82

135

133

149

62

145

141

141

90

114

Grand

Junction

Delta

Montrose

Gunnison

Crested Butte

Lake City

Uravan

Telluride

Ouray

Silverton

Aspen

Disappointment Creek

Gunnison River

Uncompahgre River Cebolla

Creek

Tomichi Creek

Taylor River

CochetopaCreek

Dolores River

0

0

Approximate outline of national forests in GMUG study area

EXPLANATION

550

Figure B2 (above and following page). Generalized geologic map of the Proterozoic rocks in GMUG greater study area, showing location of t
Review of the Geology of Western Colorado 17

Paleozoic Era

More than 1 billion years passed from the time the Proterozoic rocks exposed in western Colorado were deposited 

and the next major rock package was deposited during the 

Paleozoic. During this vast interval the Precambrian rocks 

were stripped off due to weathering and erosion, exposing 

rocks that were once buried more than 10 km deep in the 

Earth’s crust. The Paleozoic history of the study area can be 

broadly grouped into two distinct periods of evolution (fig. 

B3). An early set of units (Cambrian through mid-Mississippian) record several cycles of marine transgression-regression, 

deposition having been primarily in shallow epicontinental 

marine environments. Later units (Pennsylvanian and Permian) record regional uplift, erosion, and basin formation 

associated with the growth and destruction of the Ancestral 

Rocky Mountains of Colorado (fig. B4). 

Early and Middle Paleozoic

During the Paleozoic Era, what is now Colorado was 

part of the ancient continent of Laurentia. The Precambrian 

rocks, such as those in the mountain uplifts of the Rockies, 

formed the core of Laurentia. During the Paleozoic starting in 

about the Late Cambrian (≈520 Ma), shallow seas transgressed 

across the eroded Precambrian basement rocks, depositing 

sediments that became sandstone, shale, and relatively thin 

limestone and dolomite. In western Colorado, these rock units 

include the Upper Cambrian Sawatch and Ignacio Quartzites 

and overlying dolomites of the Peerless and Dotsero Formations (Haynes and others, 1972; Tweto and others, 1976; 

 Proterozoic units

Middle and Early Proterozoic

Granitic rocks— Includes intrusive units of granodiorite and granite (≈1,400 Ma age group)

Middle to Early Proterozoic granitic rocks and gneiss, undivided

Early Proterozoic

Granitic rocks— Includes intrusive units of granodiorite, monzonite, monzodiorite, 

and granite (≈1,700 Ma age group)

Felsic and mafic gneiss and schist— Interlayered metamorphosed felsic and mafic volcanic

rocks and associated volcanogenic sedimentary rocks

Biotite schist— Biotite schist with interlayers of hornblende-rich mafic rocks, iron-formation, and 

felsic schist; dominantly metamorphosed graywacke, chemical sediments,

mafic and felsic volcanogenic sedimentary rocks

Trace of tectonic boundary between the Proterozoic gneiss, schist, and granite of the

medium metamorphic grade rocks of the composite back-arc rocks of central

Colorado and the magmatic arc rocks of the Dubois Greenstone belt of low to

medium metamorphic grade— Teeth on boundary indicate area where boundary is a

south-vergent thrust fault

Anticline

Syncline

EXPLANATION
Gunnison uplift Uncompahgre Plateau Paradox Basin San Juan Mountains Grand Mesa Sawatch Mountains 

Eagle Basin 

DuBois Greenstone belt West Elk Mountains 39o 30'109o 

37o 45'

70

139

65

92

50

50 

82

135

133

149

62

145

141

141

90

114

Grand

Junction

Delta

Montrose

Gunnison

Crested Butte

Lake City

Uravan

Telluride

Ouray

Silverton

Aspen

Disappointment Creek

Gunnison River

Uncompahgre River Lake Fork Cebolla

Creek

Tomichi Creek

Taylor River

Blue MesaReservoir

CochetopaCreek

Dolores River

0

0

Approximate outline of national forests in GMUG study area

EXPLANATION

550

Figure B3 (above and following page). Generalized distribution of exposed lower to middle Paleozoic (Cambrian-Mississippian) and upper PPermian) rocks in GMUG greater study area. Precambrian basement rocks are also shown.
Review of the Geology of Western Colorado 19

Tweto and others, 1978; Tweto, 1979; De Voto, 1990). A 

period of widespread erosion affected the Cambrian units, and 

not until the Early Ordovician (≈ 500 Ma) were marine sediments deposited again. In western Colorado, these Ordovician units include the Lower Ordovician Manitou Dolomite, 

Middle Ordovician Harding Sandstone, and Upper Ordovician 

Fremont Dolomite.

As reviewed by De Voto (1990), central Colorado experienced tectonic uplift along high-angle faults active during 

the Early Ordovician, exposing the Cambrian and Lower 

Ordovician rocks to local erosion (Tweto and others, 1976). 

Subsequent to Ordovician tectonism, units of the Chaffee 

Group were deposited in shallow basins during a continentwide marine transgressive-regressive cycle upon the regional 

erosional unconformity. As exposed in the study area near 

Leadville, Colo., units of the Upper Devonian and Lower 

Mississippian Chaffee Group include the Parting Formation, 

Dyer Dolomite, and Gilman Sandstone. In the western and 

southwestern part of the study area, units deposited during this 

interval include the mudstones and dolomites of the Upper 

Devonian Elbert Formation and the fossiliferous Ouray Limestone (Steven and others, 1974; Campbell, 1996).

The Mississippian Leadville Limestone, overlying rocks 

of the Chaffee Group, represents a large-scale transgressive 

shelf carbonate sequence (Horton and Geissman, 1990), which 

in the northern part of the study area is dominantly dolomite. 

The Leadville has been divided into two units: the lower one 

is the Red Cliff Member, and the upper one is the Castle Butte 

Member (Beaty and others, 1988). The Red Cliff Member 

contains carbonate breccia composed of angular masses of 

coarse-grained dolomite in mudstone, and casts of halite and 

gypsum (Horton and De Voto, 1990), indicating deposition in 

a shallow-marine evaporitic environment much like that in the 

modern Persian Gulf. An erosional unconformity marked by 

dolomitic breccia lies at the top of the Red Cliff Member. This 

unconformity was interpreted to have resulted from subaerial 

emergence of the Leadville Limestone (De Voto, 1990). The 

Castle Butte Member consists of dominantly carbonate sand 

(packstones and grainstones) interpreted by Horton and De 

Voto (1990) to have been deposited within low-relief emergent 

areas in the ancestral Front Range–Wet Mountains area east 

of the study area. Regional karst erosion on the upper surface 

of the Leadville Limestone created sinkholes, undulating erosional surfaces, caves, and paleovalleys. These karst features 

later were important in channelization of ore-forming hydrothermal solutions that deposited Ba-Ag-Pb-Zn Sherman-type 

mineral deposits (Landis and Tschauder, 1990; Tschauder and 

others, 1990).

Late Paleozoic

The transgression-regression of shallow continental seas 

typical of early to middle Paleozoic time gave way in late 

Paleozoic time (Pennsylvanian and Permian Periods) to mountain building that resulted in the uplift and partial denudation 

of the Ancestral Rocky Mountains. Numerous fault-bounded 

structural basins formed adjacent to the uplifts, and into these 

actively subsiding basins were shed coarse boulders, gravel, 

and sand. In the study area these ancient highlands closely 

coincided with the present-day Uncompahgre uplift, Sawatch 

Mountains, and southern Mosquito Range (MacLachlan, 1972; 

Maughan, 1980; De Voto, 1980, 1990; Schenk and others, 

1987; Houck, 1997). The upper Paleozoic strata were deposited in shallow, warm, westward-deepening continental seas 

EXPLANATION

Permian and Pennsylvanian sedimentary rocks

Mississippian, Devonian, Ordovician, and Cambrian sedimentary rocks

Cambrian carbonatite intrusive rocks— Includes carbonatite, nepheline syenite, pyroxenite

ijolite, melanite-orthoclase plutonic phases and magnetite-ilmenite-perovskite dikes

Precambrian gneisses, schists, and granites

Anticline

Syncline

Paleozoic units

Precambrian units
20 Resource Potential and Geology, Grand Mesa, Uncompahgre, and Gunnison National Forests, Colorado 

that encroached eastward onto lowlands and interfingered with 

fluvial sedimentary environments adjacent to the highland 

regions. De Voto (1980) estimated that the Ancestral Rocky 

Mountains had as much as 3,000 m of relief. Houck (1997) 

has shown that the local tectonic movements along the basinbounding faults control the thickness of sequences and lateral 

distribution of deposits within the basins. Extensive sequences 

of sand, gravel, evaporite, shale, and carbonate sediment 

Figure B4. Paleogeographic setting of Ancestral Rocky Mountains highlands, approximate range of arkosic sediment shed from uplifted regions, location of the Central Colorado trough, and location of the equator during Middle 

Pennsylvanian to Early Permian time. Modified from Lindsey and Clark (1995).

0

0 200 KILOMETERS

100 200 MILES

28 o

32o

36o

40 o

110 o 106 o 102 98 o o

EXPLANATION

Highlands and other positive elements

Arkose shed from highlands

10° S.

Front

Range

Epicontinental

Sea

Apishapa 

highland

highland

Central Colorado trough

Wichita highla

Pedernal

nd

highland

Basin

Paradox

Sierra

Grande

highland

Uncompahgre

highland

EQUATOR

MARATHON–OUACHITA OROGENIC BELT
Review of the Geology of Western Colorado 21

accumulated deposits as thick as 2,745 m in the Paradox 

Basin and Eagle Basin (fig. B3). The Eagle Basin was part 

of the northwest-trending central Colorado trough (Mallory, 

1972; Schenk and others, 1987). During and after deposition, ground water oxidized much of the sediment to reddish 

and ochre colors, giving rise to the descriptive term “redbeds” 

commonly applied to these sedimentary rock sequences.

In the southwestern part of the study area in the Paradox 

Basin (fig. B4), units of Pennsylvanian age were deposited 

unconformably upon the Devonian Ouray or Mississippian Leadville Limestones. Pennsylvanian units include 

the redbeds of the Lower and Middle Pennsylvanian Molas 

Formation and evaporites as well as the fluvial sandstone and 

fossiliferous marine limestone, siltstone, black shale, and gray 

sandstone of the Middle and Upper Pennsylvanian Hermosa 

Group. The lower part of the Hermosa Group is made up of 

fluvial sandstone interbedded with marine sandstone and shale 

of the Pinkerton Trail Formation. The middle part, the Paradox Formation, comprises evaporite and limestone deposits. 

The upper member, the Honaker Trail Formation, is made up 

of fluvial sandstone, shale, and limestone (Haynes and others, 

1972; Williams, 1976; Cole and others, 1996). Within the 

Paradox Basin, the Elephant Canyon Formation lies at the top 

of the Hermosa Group and intertongues with the overlying 

Lower Permian Cutler Formation (Cole and others, 1996). In 

the San Juan Mountains the Rico Formation lies between the 

Pennsylvanian Hermosa Group and the Lower Permian Cutler 

Formation (Campbell, 1996). The Cutler Formation itself 

represents a continental sequence of finer grained near-shore 

redbeds, alternating with units of coarse-grained eolian sandstone (Haynes and others, 1972).

The Central Colorado trough (fig. B4) was a narrow, 

deep, structural and sedimentary basin (Mallory, 1972; Schenk 

and others, 1987; Houck, 1997) whose trend was northwest 

across the northern and central part of the study area. This 

fault-bounded basin was the site of Paleozoic deposition of the 

aforementioned Pennsylvanian Molas; the shale, carbonates, 

and sandstone of the Belden Formation; sandstone, grit, conglomerate, and shale of the Minturn Formation; the gypsumrich Eagle Valley Evaporite; the conglomerate, mudstone, and 

redbeds of the Pennsylvanian and Permian Maroon Formation; 

and the sand of the Schoolhouse Member of the Maroon Formation (Johnson and others, 1990).

As reviewed by De Voto (1980), more than 3,050 m of 

marine and nonmarine Pennsylvanian rocks, and as much as 

6,100 m of Pennsylvanian to Permian strata (Maroon Formation) occur in down-dropped blocks within the Central 

Colorado trough. Even-bedded shallow-marine siltstones 

and sandstones of the Lower Permian to Lower Triassic State 

Bridge Formation (Tweto and others, 1978) were deposited 

north of Aspen in the Central Colorado trough (fig. B4). This 

period represented clastic, carbonate, and evaporite deposition 

into the rapidly subsiding basin while the Ancestral Rocky 

Mountains were uplifted.

Mesozoic Era

Triassic

Shallow-marine Permian and Pennsylvanian sedimentation gave way to periods of erosion and weathering of the 

Ancestral Rocky Mountains during the Triassic. Much of 

North America emerged from the late Paleozoic seas in Early 

Triassic time to form a high continental landmass (Maughan, 

1980) and formed part of the western coast of the ancient 

continent Pangea. Dubiel (1994) pointed out that during the 

Triassic the Western Interior of North America was about at 

the latitude of the paleoequator, subjecting the west coast of 

Pangea to monsoonal circulation. Rivers swollen with these 

heavy rainfalls washed large amounts of sediment off ancestral highlands onto adjacent tidal flats and shallow-marine 

settings. In southwestern Colorado, Early Triassic sediments 

are represented by the Lower and Middle(?) Triassic Moenkopi Formation, exposed on and west of the Uncompahgre 

Plateau (figs. B5 and B6). The Moenkopi Formation contains 

chocolate-brown ripple-bedded shale, brick-red sandy mudstone, reddish-brown and chocolate-brown sandstone, purple 

and reddish-brown arkosic conglomerate, and local beds of 

gypsum (Williams, 1976). The Moenkopi represents sediments deposited in offshore-marine to continental alluvial fan 

and marginal-marine, deltaic and eolian environments (Dubiel, 

1994). In northwestern Colorado, Early Triassic sediments 

are recorded in the Permian and Lower Triassic State Bridge 

Formation, which is an orange-red to red-brown siltstone and 

sandstone (Tweto and others, 1978).

The Upper Triassic rocks of the Western Interior of North 

America are continental in origin, forming an assemblage of 

units deposited in alluvial, marsh, lacustrine, playa, and eolian 

environments (Stewart and others, 1972). In western Colorado, the Upper Triassic Chinle Formation overlies the Lower 

Triassic sedimentary rocks and is made up of red to reddishbrown siltstone interbedded with lenses of sandstone and shale 

as well as pebble conglomerate (Baars, 1972; Stewart and 

others, 1972; Dubiel, 1994). Meandering streams, point-bar, 

channel, and over-bank fluvial sediments trapped plant debris 

deposits within the Chinle. This organic material, later fossilized, created local reducing environments important for scavenging and precipitating uranium and vanadium dissolved in 

circulating ground water. Thus, the organic material became 

important for formation of the uranium and vanadium deposits on the Colorado Plateau (Baars, 1972). The finer grained 

siltstones and shales present in the upper part of the Chinle 

Formation record a period on the Colorado Plateau during the 

Late Triassic when the paleo-landscape was more subdued 

than during early Chinle time.

At approximately the same time the Chinle was being 

deposited in the Colorado Plateau region, the fluviatile, eolian, 

and lacustrine sediments forming the Upper Triassic Dolores 

Formation were being laid down. An angular unconformity 

beneath the Dolores Formation is evidence of uplift and 
22 Resource Potential and Geology, Grand Mesa, Uncompahgre, and Gunnison National Forests, Colorado 

erosion during the Early Triassic (Campbell and Brew, 1996). 

Baars (1972) reported that the much coarser clastic material typical of the Dolores compared to that in the temporally 

equivalent Chinle is a result of the Dolores being closer to the 

source from the Uncompahgre uplift. Fossil leaves and bones 

are also found in the Dolores, suggesting that the climate was 

warm and humid.

Late Triassic Through Jurassic

The Late Triassic to Early Jurassic saw a dramatic change 

in the climate of western Colorado (Dubiel, 1994). The environment shifted from warm and humid to arid. Windblown 

sand deposits dominate the formations of Late Triassic age. 

Baars (1972) compared the Late Triassic of the Western Interior to that of the present-day Sahara Desert of North Africa, 

with great windblown sand dunes drifting throughout the 

region. These windblown sands, including the Lower 

Jurassic Glen Canyon Group, were deposited conformably 

on top of the Chinle Formation throughout the western and 

southern parts of the study area (Haynes and others, 1972; 

Cashion, 1973; Williams, 1976; Berman and others, 1980). 

The oldest unit in the Glen Canyon Group is the Wingate 

Sandstone, a fine-grained, well-cemented, eolian sandstone 

with beautifully preserved crossbeds and local stream deposits, 

indicating at least seasonal variation. The Kayenta Formation, 

the middle formation of the Glen Canyon Group, is an irregularly interbedded, fine- to coarse-grained sandstone-dominant 

unit with lesser quantities of shale and siltstone; thin beds of 

limestone- and shale-pebble conglomerate are locally present 

(Haynes and others, 1972; Williams, 1976). The latest Early 

Jurassic age Navajo Sandstone (Berman and others, 1980) 

is the youngest unit in the Glen Canyon Group. It also is a 

Figure B5. Tectonic province map of Colorado (modified from Tweto, 1980), showing distribution of uplifts and sedimentary basins of the

western part of the State.
Review of the Geology of Western Colorado 23

fine-grained, cross-stratified eolian sandstone, which blankets 

much of the western part of the area. The rocks of the Glen 

Canyon Group are well exposed across the Colorado Plateaus 

province, forming prominent cliffs in the Colorado National 

Monument, steep canyon walls throughout the Uncompahgre uplift, and striking features throughout the Four Corners 

region in the southwestern part of the study area.

The continental desert environment recorded in strata of 

the Triassic and Lower Jurassic Glen Canyon Group gave way 

to an eastward-encroaching marine environment, as preserved 

in the overlying Middle Jurassic San Rafael Group (Lucas and 

Anderson, 1997). In the study area the San Rafael Group is 

made up of (from oldest to youngest) the Carmel Formation, 

Entrada Sandstone, and Wanakah Formation. The thin-bedded silty shale, siltstone, and silty sandstone of the Carmel 

Formation lie disconformably upon the Glen Canyon Group 

rocks, evidence of a period of erosion in the region during the 

transition period between the Early and Middle Jurassic. The 

Carmel extends only a short distance into western Colorado 

and is absent in the central part of the State. Haynes and others (1972) interpreted its paleoenvironment as one of lacustrine or tidal-flat origin.

The withdrawal of the Carmel sea during the Middle 

Jurassic resulted in the deposition once again of continental 

sediments, such as the eolian Entrada Sandstone. The Entrada 

is a widespread unit throughout the area, forming prominent 

cliffs of “slickrock. ” The Entrada Sandstone is a white to 

buff, crossbedded to massive sandstone. As reviewed by 

Berman and others (1980), the Entrada represents windblown 

desert sand deposits that intertongue to the north and west with 

water-laid units of the same age. Subsequent marine incursion 

during the late Middle Jurassic resulted in deposition of Curtis 

and Summerville Formations (Lucas and Anderson, 1997). In 

the central and eastern part of the study area, the seas deposited sediment forming the glauconitic sandstone and oolitic 

limestone of the Curtis Formation (Tweto and others, 1978). 

In the southern and southwestern part of the area, the Middle 

Jurassic Summerville Formation—alternating thin beds of 

gypsiferous siltstone, fine-grained sandstone, shale, and 

characteristic mudstone—is interpreted as terrestrial in origin 

(Haynes and others, 1972; Cashion, 1973; Williams, 1976). 

Peterson (1988) suggested that the unit was deposited in a 

quiet, ephemeral shallow-water environment, perhaps along a 

coastal plain. Lucas and Anderson (1997) pointed out that the 

lithology, coupled with the lack of fluvial features, suggests 

that the depositional environment for the Summerville was a 

sabkha, large shallow playa, and tidal flat, where windblown 

sand provided the clastic sediment input. The fine- to coarsegrained, crossbedded eolian sandstone of the Junction Creek 

Sandstone Member of the Morrison Formation lies conformably atop the San Rafael Group rocks in the southwestern part 

of the study area, in the Cortez 1°×2° quadrangle (Haynes and 

others, 1972). Berman and others (1980) noted that the Upper 

Jurassic Junction Creek interfingers with and overlies the 

Summerville Formation throughout the Colorado Plateau.

The San Rafael Group is overlain throughout the region 

by the Upper Jurassic Morrison Formation. In the study 

area, the Morrison contains the Tidwell Member, Salt Wash 

Member, and the overlying Brushy Basin Member. Although 

the Salt Wash and Brushy Basin Members host uranium and 

vanadium deposits, the Salt Wash Member hosts significantly 

more ore and has been a world leader in production of these 

metals. (See Spanski and others, this volume, Chapter H.)

The base of the lower Morrison Formation lies on a 

regional unconformity on the San Rafael Group rocks and the 

Junction Creek Formation (where present). The Salt Wash 

Member of the Morrison contains thick discontinuous beds of 

fine- to medium-grained fluvial sandstone interbedded with 

variegated mudstone. Thin beds of limestone occur locally 

near its base. The Salt Wash Member has all the hallmarks 

of a fluvial deposit; the Salt Wash sandstone beds are crossbedded, pebbly-conglomeratic, and quartzose. Mud, clay, 

and woody clasts and trace fossils are common (Anderson 

and Lucas, 1997). Turner-Peterson (1986) determined that 

the unit’s trough crossbeds consistently indicate a northeasterly-flowing fluvial system that changed upsection to a more 

easterly stream flow environment.

The overlying Brushy Basin Member of the Morrison 

Formation is mostly varicolored claystone and bentonitic 

mudstone (altered volcanic ash) with a few lenses of gray 

sandstone, limestone, and chert-pebble conglomerate: it has 

much less sandstone than does the underlying Salt Wash 

Basin Member. Volcanic ash-rich beds, now zeolitized, are 

found throughout the Morrison but are especially concentrated 

in the Brushy Basin Member. Turner and Fishman (1991) 

suggested that the Brushy Basin Member was deposited in a 

large, saline and alkaline lacustrine basin, which they named 

Lake T′oo′dichi′. Anderson and Lucas (1997) took umbrage 

at this concept, noting sedimentological evidence for meander 

belt channels, giving evidence for a humid subtropical climate 

instead of an arid saline alkaline playa. Regardless of these 

differences in interpretation, the Morrison seems to have been 

deposited in an environment that changed through time from 

a regime dominated by stream influx to lake and flood-plain 

sedimentation marked by the input of volcanic ash through 

time. One of the most significant features of the Morrison 

Formation is the significant accumulation of vertebrate fossils 

including several species of sauropods, allosaurids, stegosaurids, and iguanodontids, several other species of dinosaurs, 

and more than 50 species of mammals (Anderson and Lucas, 

1997).

The transition from the Late Jurassic to and through the 

Early Cretaceous in the western part of the study area along 

the Colorado Plateau region was a gradual change from the 

Jurassic lake and flood-plain-deposited mudstone and siltstone 

of the Brushy Basin Member of the Morrison upward to the 

fluvial sandstone and conglomerate interbedded lacustrine 

siltstone, shale, mudstone, and thin beds of impure limestone 

of the Burro Canyon Formation. Both the Morrison and Burro 

Canyon Formations evolved in continental environments 
Gunnison uplift Uncompahgre Plateau Paradox

Basin

San Juan

Mountains

Grand

Mesa

Sawatch Mountains

Eagle Basin

DuBois

Greenstone belt

West

Elk Mountains

39o 30'

109o 

37o 45'

70

139

65

92

50

50 

82

135

133

149

62

145

141

141

90

114

Grand

Junction

Delta

Montrose

Gunnison

Crested Butte

Lake City

Uravan

Telluride

Ouray

Silverton

Aspen

Disappointment Creek

Gunnison River

Uncompahgre River Lake Fork Cebolla

Creek

Tomichi Creek

Taylor River

Blue MesaReservoir

CochetopaCreek

Dolores River

0

0

Approximate outline of national forests in GMUG study area

EXPLANATION

550

Figure B6 (above and following page). Generalized distribution of Mesozoic sedimentary units.
Review of the Geology of Western Colorado 25

wherein low-gradient streams meandered across flood plains, 

mud flats, and saline basins.

Cretaceous

Regional compressive tectonism started to exercise 

control over the geography and sedimentary environment 

during the Cretaceous, with the onset of the Sevier orogeny. 

The Sevier orogenic belt during this time lay to the west in 

west-central Utah and was the result of crustal shortening 

experienced throughout the Western Interior of North America. 

(See Lawton, 1994.) The eastward-migrating orogenic front, 

caused by the collision of the continental North American 

plate with the Farallon oceanic plate, was marked by the 

dominantly north south trending Sevier orogenic thrust belt in 

eastern Utah and western Colorado. Foreland basins formed 

ahead and to the east of the advancing orogenic front. These 

asymmetric foreland basins were occupied by a large inland 

sea (Western Interior Basin) that extended from Alaska south 

to the modern Gulf of Mexico. Marine shale as well as terrestrially derived sediment shed from the orogenic belt was 

deposited into the basins, which were deepest along their 

western margins. The sedimentary basins thinned eastward as 

they onlapped onto the North American craton. From Early 

through Late Cretaceous, the eastward migration of the foreland basin progressively changed the depositional environment 

across the study area.

As the eastward-migrating sea advanced, blankets of 

beach sands and intervening carbonaceous shales were deposited, forming the Dakota Sandstone. The Dakota is one of the 

most extensive formations throughout the Western Interior of 

North America, representing a beachfront that transgressed 

eastward throughout Utah, across Colorado, and through 

the Great Plains. As such, its age varies depending on the 

paleogeographic location of a given point in relation to the 

advancement of the Cretaceous sea. The Dakota Sandstone is 

a quartzose sandstone interbedded with dark shale and shaly 

sandstone, and minor amounts of gray claystone, impure coal, 

and carbonaceous shale. Locally, the Dakota has a basal conglomerate. Because the unit is resistive to weathering, it forms 

a prominent stratigraphic marker horizon throughout the study 

area, forming hogbacks of buff sandstone.

As the Late Cretaceous inland sea transgressed eastward, 

deep-water marine sediments were deposited in the foreland 

basins. Within the study area, these depocenters included the 

Piceance Basin in northwestern Colorado and the San Juan 

Basin of southwestern Colorado and northwestern New Mexico as well as the Denver Basin east of the study area (fig. B5). 

Thick accumulations of carbonaceous sandstone and limestone 

as well as black shale of the Mancos Shale were deposited at 

this time. The lower part preserves thin-bedded brown sandy 

fossiliferous limestone and dark-gray shale (Juana Lopez 

Member) and sandstone (Frontier Sandstone and Ferron Sandstone Members) that give way upsection to the thick sequence 

of marine black shale of the main body (upper member) of the 

Mancos (Dyman and others, 1994). The Mancos Shale and its 

lateral and temporal (to younger) equivalent Pierre Shale to the 

east represent carbonaceous sediments that were deposited in 

stagnant, poorly oxygenated waters that preserved numerous 

marine fossils.

The continued eastward progression of the thrust front of 

the Sevier orogeny (see Lageson and Schmitt, 1994) uplifted 

and exposed the tectonic hinterland west of the study area 

in Utah to erosion, causing an influx of terrigenous sediments shed as tongues of sand within the upper part of the 

Mancos and younger sediments. The axis of the depositional 

system advanced eastward into the Denver Basin and Great 

Plains region as recorded by the nonmarine clastics, shale, 

and bituminous deposits of the Mesaverde Group (or Formation). These units were deposited in fluvial deltas, marshes, 

and lagoons that formed at the front of the advancing tongues 

of terrigenous sediments. The encroachment of the Cretaceous sea was erratic, resulting in shifting depositional facies 

that produced intertonguing of deeper water shales with the 

near-shore and fluvial sediments. In the Piceance Basin in 

the northern part of the study area (fig. B5), this shifting of 

depositional environments resulted in deposition of the Castlegate Sandstone, the Buck Tongue of the Mancos Shale (which 

represents an intertonguing of underlying deeper water facies 

of Mancos Shale with the clastic terrigenous sedimentary 

wedges of the Sego Sandstone), and the sandstone and interlayered shale deposits of the Mount Garfield, Hunter Canyon, 

Nelson, and Farrer Formations. Migration of the shoreline in 

the Piceance Basin caused changes in the depositional environment as the sandstone, shale, and coal seams of the lower 

part of the Mesaverde were deposited upon the terrigenous 

sediments. The upper part of the Mesaverde is dominated by 

sandstone that is interbedded with lesser amounts of shale and 

coal.

In the San Juan Basin the Mesaverde is composed of 

the massive marine sandstone horizons of the Point Lookout 

Sandstone, nonmarine crossbedded sandstone, claystone, and 

shale, coal seams, ironstone and limestone concretions of the 

EXPLANATION

Cretaceous rocks

Jurassic rocks

Triassic and lesser amounts of Permian

 rocks

Anticline

Syncline

Mesozoic sedimentary units
26 Resource Potential and Geology, Grand Mesa, Uncompahgre, and Gunnison National Forests, Colorado 

Menefee Formation, and the crossbedded marine sandstone 

and gray shale of the Cliff House Sandstone (Haynes and 

others, 1972). These carbonaceous sediments within the 

Mesaverde Group host important coal resources, discussed 

by Hettinger and others (this volume, Chapter M). In the 

Four Corners area of the San Juan Basin, the Point Lookout Sandstone represents accumulations of near-shore sand 

that mark one of the regressions of the Cretaceous sea. The 

overlying carbonaceous shale units of the Menefee Formation, 

which hosts coal-rich horizons, were deposited in a near-shore 

lagoonal and marshy edge of the retreating sea. Subsequent 

transgressive encroachment of the Cretaceous sea resulted in 

flooding of the near-shore environment of the Menefee Formation, laying down the beach sand of the Cliff House Sandstone.

In the San Juan Basin, transgression of the Cretaceous 

sea flooded over the Mesaverde Group, resulting in deposition 

of the marine Lewis Shale (Dyman and others, 1994). The 

Lewis is a clay-rich gray to black shale with rusty-weathering 

limestone concretions and contains thin beds of fine-grained 

sandstone near both the transitional top and base of the unit 

(Haynes and others, 1972). After deposition of the Lewis 

Shale, the shoreline of the Cretaceous sea again withdrew. 

With it the beach and near-shore facies sand deposits of the 

Pictured Cliffs Sandstone migrated across the San Juan Basin 

(Elder and Kirkland, 1994), and over it were laid the coastalplain deposits of the Fruitland Formation (Molenaar, 1983). 

Alluvial-plain deposits of the Kirtland Shale onlapped and 

covered the Fruitland Formation coastal-plain sediments as a 

result of continued regression of the Cretaceous sea.

In northern Colorado the Lewis Shale is preserved on the 

White River Plateau (fig. B5), but it is absent in the Piceance 

Basin, where its time equivalent units are the nonmarine clastic sediments of the Mount Garfield and Hunter Canyon Formations (Dyman and others, 1994). These nonmarine clastic 

units represent the incursion of terrigenous sediments into the 

foreland basin that were being shed off of the Sevier orogenic 

hinterlands from the west in central Utah (Lawton, 1994).

Cenozoic Era

During the latest Cretaceous and early Cenozoic time, 

the Western Interior Cretaceous seaway began to withdraw 

northeastward across the region (Tweto, 1975). The regional 

geology was soon dominated by the Laramide orogeny, which 

in the study area resulted from east-west-directed regional 

compression that caused uplift of many of the mountain ranges 

and intervening basins exposed today. The Cenozoic Era in 

central to west-central Colorado was marked by four major 

geologic events: nonmarine sedimentation in the Piceance and 

northern San Juan Basins (early Paleocene to latest Eocene), 

formation of the San Juan volcanic field (middle Tertiary), 

shallow-level magmatic intrusion in the Elk Mountains region 

(Oligocene), and the formation of surficial deposits during the 

Pleistocene and Quaternary Periods.

Tertiary Sedimentary Deposits

Large and deep structural basins formed concurrently 

with Laramide uplift during Late Cretaceous to Eocene time 

(Tweto, 1975). Gravel and sand deposited in these basins 

record uplift of Laramide highlands. Like the mountains, 

several of the basins inherited their structure in part from late 

Paleozoic features (Tweto, 1975). In all the basins, Upper 

Cretaceous deposits derived from distant western sources associated with the Sevier orogeny were succeeded by Laramide 

orogenic clastic sediments derived from local positive tectonic features. In most basins, the youngest marine deposits 

(Late Cretaceous age) grade upward into and intertongue with 

coal-bearing brackish- and fresh-water sandstones and shales 

deposited during the Laramide orogeny.

In the study area the two main basins that hold Cenozoic 

sediments are the Piceance Basin in the north and the San Juan 

Basin in the south. The Piceance Basin is bounded by several 

extensive Laramide tectonic features, including the Uncompahgre, White River, Sawatch, and Gunnison uplifts (figs. 

B5–B7). The axis of the basin trends northwest and southeast, 

and maximum depositional thickness is about 12 km (Ochs 

and Cole, 1981). Tertiary sedimentary rocks within the basin 

include alluvial, deltaic, and lacustrine deposits of the Fort 

Union, Wasatch, Green River, and Uinta Formations. These 

Tertiary sedimentary rocks overlie thick Cretaceous sandstones 

and shales in the Mesaverde Group. From Late Cretaceous 

through Eocene time, the Piceance Basin was a catchment for 

both the eroded detritus from previously deposited sedimentary rocks and sediment derived from volcanic, intrusive, 

and pyroclastic rocks (Johnson, 1985). Early deposition in 

the basin (early or middle Paleocene) followed a period of 

widespread beveling of basins. The early sedimentary rocks 

are mostly conglomeratic and reflect the intensity of tectonic 

activity and the composition of the source rocks. Throughout 

most of the Piceance Basin, thin basal conglomeratic sandstone was deposited on the Cretaceous-Tertiary unconformity. 

The basal conglomerate may have been derived both from the 

underlying Mesaverde Group rocks and from the surrounding 

highlands. By late Paleocene time, large shallow lakes and 

swamps covered much of the basin. Gray and carbonaceous 

shale, thin coalbeds and fossiliferous limestones, and lenticular sandstones were deposited in these lacustrine and paludal 

environments. In latest Paleocene to early Eocene, huge 

wedges of mud and sand, dominantly of fluvial and alluvial 

origin, began to prograde from basin margins inward, possibly 

reflecting a period of renewed uplift of adjacent highlands. 

The described sequence of basal conglomerates, lacustrine and 

paludal rocks, and uppermost fluvial and alluvial mudstone 

and sandstone make up the Paleocene Fort Union and Eocene 

Wasatch Formations (Ochs and Cole, 1981).

In early Eocene time, a permanent fresh-water lake 

formed in the central and northwestern part of the Piceance 

Basin (Johnson, 1985). Lacustrine deposits of the Green River 

Formation mark the earliest stage of Lake Uinta and are characterized by low-grade oil shale, limestone, sandstone, and 
Review of the Geology of Western Colorado 27

U

B

LG

SL

SR

LGS

Uncompahgre

uplift

C

Tbv

Tbv

Tbv Tbv

Tbv

Tbv

Tbv

SJ

0

0

25

20 40

50 75 100 KILOMETERS

0

0

60 MILES

COLORADO

Greater

Study

Area

area of figure

GP

MA

 SAN JUAN 

VOLCANIC FIELD

MzPz

MzPz

MzPz

MzPz

Ts

Ts

Ts

Ts Ts

P

108o

36o

38o

Tbv

106o

Ts

MzPz

P

Lower Miocene to Laramide intrusive rocks 

Miocene and younger volcanic rocks—Basalt to 

 rhyolite, contemporaneous with rifting

Paleozoic and Mesozoic sedimentary rocks— Includes some lower Tertiary rocks

Holocene to Miocene sedimentary rocks in 

 Rio Grande rift

Middle Tertiary volcanic rocks—Andesite to rhyolite

Normal fault—Showing late Cenozoic movement

Precambrian rocks

Late Cenozoic volcanic cone or vent

EXPLANATION

Tv

Tbv

Caldera boundary

kerogen-rich shale (Johnson and Keighin, 1981). The Green 

River Formation conformably overlies and intertongues with 

the older Wasatch Formation (Ochs and Cole, 1981). Lake 

Uinta expanded rapidly to cover most of the basin in late early 

to early middle Eocene. Salinity began to increase after this 

transgression and led to the precipitation of great quantities 

of saline minerals. The kerogen content of the Green River 

Formation far exceeds that of the earlier fresh water stage, and 

Figure B7. Generalized geology of San Juan volcanic field of southern Colorado. Modified from Lipman (2000). Calderas indicated by symbols are as follows: SJ, San Juan; U, Uncompahgre; LGS, La Garita South; SR, South River; 

B, Bachelor; SL, San Luis caldera complex; LG, La Garita; C, Cochetopa Park; MA, Mount Aetna; GP, Grizzly Peak.
28 Resource Potential and Geology, Grand Mesa, Uncompahgre, and Gunnison National Forests, Colorado 

resulted in the formation of the economically important oil 

shale horizons within the formation. During the middle to late 

Eocene, a delta consisting mainly of volcaniclastic debris prograded southward across the basin, mostly filling the Piceance 

Basin portion of Lake Uinta.

During the late Eocene, Lake Uinta was gradually filled, 

first by volcaniclastics from the Absaroka volcanic field in 

Wyoming to the north, and later by sediments from local 

Laramide uplifts (Johnson, 1985). A deltaic complex named 

the Uinta Formation forms the youngest stratigraphic unit 

preserved within the Piceance Basin; erosional remnants of it 

occur across most of the basin. 

Several sedimentary units in the southern portion of the 

study area were deposited along the northern margins of the 

San Juan Basin. These sedimentary units are generally the 

coarsest near the basin margins, as they were derived from the 

nearby highlands (Tweto, 1975). The uppermost Cretaceous 

and lower Paleocene Animas Formation contains coarsegrained andesitic clasts and records erosion of the major 

uplifts and volcanic centers in the northern and northeastern 

part of the San Juan Basin (La Plata Mountains area) (Cross 

and Larsen, 1935). The Nacimiento Formation, which is in 

part time-equivalent with the Animas Formation, represents 

deposition of fine-grained material in the southern part of the 

San Juan Basin by the same streams that carried the Animas in 

the north (Fassett, 1985). Although the Nacimiento Formation consists largely of shale and siltstone in the central part of 

the basin, it is typically characterized by sandstone and some 

conglomerate where it crops out in the study area (extreme 

northern part of the basin). The Eocene San Jose Formation is 

the youngest Tertiary sedimentary unit in the San Juan Basin. 

This formation consists of intertonguing sandstone, conglomerate, and shale derived from granitic rocks in a highland north 

of the basin. All the just-mentioned sedimentary units of the 

northern San Juan Basin are volumetrically minor in the study 

area and are generally less than about 30 m thick.

The Telluride Conglomerate (Eocene), which crops out 

mostly in the southwestern part of the study area, was important host for base- and precious-metal replacement ore in the 

southwest San Juan Mountains (Mayor and Fisher, 1972). The 

conglomerate was largely derived from large alluvial debris 

fans along the margins of the San Juan uplift (fig. B5) (Baars 

and Ellingson, 1984). The lower beds of this formation commonly contain fragments of red sandstone and gray limestone, 

whereas most of the clasts in the upper beds are gneiss, schist, 

and quartzite. This reversed stratigraphic sequence, as represented in the clasts, reflect the early stripping of sedimentary 

cover and later removal of the Precambrian core of the nearby 

uplift. The top of the Telluride Conglomerate is conformable 

with and grades into the tuffs and breccias of the San Juan 

volcanic field. The formation is about 300 m thick in the area 

west of Telluride and thins to about 10 m in the mountains 

west of Silverton (Baars and Ellingson, 1984).

Tertiary Volcanism and Associated 

Sedimentation 

The material forming most of the volcanic rocks in the 

southern Rocky Mountains was erupted during Oligocene 

time, and originated from a single composite volcanic field, 

the so-called Southern Rocky Mountain volcanic field of Steven (1975). Although erosion prevents accurate estimations 

of the size of this field, it probably covered most of south-central Colorado and adjacent parts of New Mexico. The onset 

of caldera-forming eruptions in the Sawatch Range (Grizzly 

Peak and Mount Aetna calderas; fig. B7) (Fridrich and others, 1991) preceded the initiation of activity in the western 

and central San Juan volcanic field by about 6 million years 

(Steven and Lipman, 1976; Bove and others, 2000). Caldera 

sources within the central and western San Juan volcanic field 

account for nearly all the ash-flow units present within the 

study area. A total of 15 major ash-flow sheets were accompanied by recurrent caldera subsidence between 28.6 and 22.9 

Ma in the western and central San Juan Mountains (Lipman, 

2000). Some details of the Tertiary volcanic history of the 

study area are given here owing to its prominence in both the 

mineral resources of the region and associated environmental 

consequences.

The San Juan volcanic field is the largest erosional remnant of the Southern Rocky Mountain volcanic field. Erosionally preserved rocks of the San Juan field now occupy an area 

of more than 25,000 km2

 and have a volume of about 40,000 

km3 (Lipman and others, 1970). Volcanic materials were 

deposited in the San Juan Mountains in the middle Tertiary 

after erosion had stripped the sedimentary cover and cut into 

Precambrian rocks elevated by the Laramide Uncompahgre–

Needle Mountains upwarp (Steven, 1975). The earliest volcanic activity in the San Juan field (about 35–30 Ma) produced 

the intermediate-composition lavas and breccias that erupted 

from scattered central volcanoes, forming the San Juan and 

Conejos Formations. Beginning about 30 Ma, the nature of 

the volcanic activity changed significantly. Tremendous volumes of ash and volcanic glass were explosively erupted about 

30–26 Ma from caldera sources. At about 28–26 Ma, volcanism shifted to a bimodal assemblage dominated by basalt and 

rhyolite, concurrently with the onset of regional extension and 

the establishment of the Rio Grande rift (Tweto, 1975; Lipman 

and others, 1978).

The calderas of the San Juan volcanic field formed within 

a cluster of precaldera stratovolcanoes. Although the detailed 

distribution of the precaldera rocks has been obscured, the 

remnants of some of these volcanic centers can be recognized 

(fig. B7). The vent regions are identified by thick sequences 

of andesitic to dacitic lavas, explosion breccias, and agglutinates intruded by stocks and radial dike swarms. Deep basins 

were filled by volcaniclastic sediments, tuffaceous conglomerates, and mudflow breccias shed from the surrounding 

stratovolcanoes (Steven and Lipman, 1976). These voluminous deposits are referred to as the San Juan Formation in the 

western San Juan volcanic field, and they are lithologically 
Review of the Geology of Western Colorado 29

analogous to the Conejos Formation in the central and eastern 

San Juan field (Luedke, 1996; Lipman, 2000).

Western San Juan Mountains

Five major calc-alkaline (dacitic to low-silica rhyolite) 

ash-flow sheets erupted from caldera sources (Ute Ridge, 

Blue Mesa, Dillon Mesa, Sapinero Mesa, and Crystal Lake 

Tuffs) in the western San Juan volcanic field from 28.6 to 27.4 

Ma (Bove and others, 2000). Even after erosion, ash-flow 

sheets associated with these calderas extend as much as 80 km 

from their sources and are present just south of the West Elk 

volcano (fig. B7). Eruption of the 29.1 Ma Ute Ridge Tuff 

produced the Ute Creek caldera, the oldest in the western San 

Juan caldera complex (fig. B7). The next caldera to form was 

the Lost Lake caldera, which produced the widespread Blue 

Mesa Tuff (28.4 Ma). Both the Lost Lake and Ute Ridge calderas were infilled by later lavas and related pyroclastics. The 

Ute Ridge and Blue Mesa Tuffs are only preserved in distal 

outflow sheets.

Collapse of the San Juan and Uncompahgre calderas took 

place about 28.4-28.2 million years ago in association with 

major ash-flow eruptions of the Sapinero Mesa Tuff and possibly early eruption of the less voluminous Dillon Mesa Tuff 

(Lipman and others, 1973; Steven and Lipman, 1976). The 

Uncompahgre caldera formed an irregular depression about 

20 km in diameter west and southwest of Lake City. The San 

Juan caldera formed nearly concurrently about 20 km west of 

the Uncompahgre caldera. The Sapinero Mesa Tuff has been 

subdivided into three major units—the main body of the Sapinero Mesa (outflow), the Eureka (intracaldera fill), and the 

Picayune Megabreccia Members (Lipman and others, 1973). 

The Picayune Megabreccia Member commonly lies stratigraphically below and intertongues with the Eureka Member 

(Lipman and others, 1976), and consists of a chaotic assemblage of precaldera rocks that slid from the oversteepened 

walls of the San Juan and Uncompahgre calderas. The Eureka 

graben, which is a downdropped keystone fault zone along 

the crest of the elliptical resurgent dome of the coalesced San 

Juan–Uncompahgre calderas, is an important host to mineralization that postdates these calderas by about 5–15 million 

years (m.y.) (Lipman and others, 1976). 

The 27.6 Ma Silverton caldera, which is nested within the 

San Juan caldera (28.2 Ma), collapsed in response to eruption of the Crystal Lake Tuff (Lipman and others, 1976; Bove 

and others, 2000). The Crystal Lake Tuff forms a relatively 

small volume ash-flow sheet in comparison with deposits 

related to other calderas in the San Juan volcanic field (Steven 

and Lipman, 1976). The Crystal Lake Tuff is mostly absent 

within the Silverton caldera undoubtedly owing to erosion 

within this relatively shallow depression. Present within the 

Silverton caldera itself is a thick sequence of finely porphyritic 

dacitic-andesitic lavas (Burbank and Luedke, 1969; Yager and 

others, 1998); these lavas are generally referred to as the Burns 

Member of the Silverton Volcanics (Lipman and others, 1973; 

Burbank and Luedke, 1969; Yager and others, 1998). This 

thick package of lavas largely erupted along the ring fracture 

zones of the earlier collapsed San Juan and Uncompahgre 

calderas, prior to collapse of the Silverton caldera (Lipman 

and others, 1976). 

Collapse of the Lake City caldera postdated all other caldera-related eruptions in the western San Juan Mountains by 

more than 4 million years. It is thought to be associated with 

bimodal magmatism related to the onset of extensional tectonism in this general region at about 25 Ma (Lipman and others, 1978). The Lake City caldera, which is nested within the 

older Uncompahgre caldera, formed in response to the eruption of the Sunshine Peak Tuff at 22.9 Ma. Accumulations 

of related ash-flow material exceeded 300 km3

, the majority 

of which ponded within the caldera along with subsidencerelated breccias. A large quartz syenite pluton was intruded 

into the intracaldera fill, causing resurgence of the caldera. 

The resurgent intrusions were derived from the same alkalic 

magma chamber that produced the caldera-forming ash-flow 

eruptions (Hon and Lipman, 1989; Hon, 1987). Continued 

volcanic activity produced a thick sequence of dacitic postcaldera-collapse lavas and intrusions that accumulated near 

the eastern margin of the caldera. These lavas and intrusions 

were host to subeconomic molybdenum-copper mineralization 

and to one of the largest alunite deposits in the Western United 

States (Bove and others, 2000). 

A large swarm of calc-alkaline intrusions was emplaced 

between about 26 and 25 Ma over a broad region of the 

western San Juan Mountains (Bove and others, 2000). These 

intrusions range from large stocks and sills at Mt. Wilson west 

of Ophir and Sultan Mountain near Silverton to smaller plugs 

intruded near Capitol City in the Lake City area (fig. B7). 

Although the overall size of the exposed intrusions decreases 

from west to east, this may be an artifact of erosion rather than 

a reflection of their actual size. These intrusions vary from 

simple to complexly zoned and in many places are associated 

with veins and disseminated and stockwork molybdenumcopper mineralized rocks (Ringrose and others, 1986; Slack, 

1980; Caskey, 1979; Pyle, 1980). Many of these intrusions 

are completely crystalline and cut through the entire ashflow sequence, suggesting that they may represent cores of 

now-eroded stratovolcanoes that fed thick sequences of lavas 

nearby (Lipman and others, 1973). Alternatively, the widespread distribution of intrusions suggests that they represent 

the uppermost portion of the underlying batholith rising 

into and consuming the earlier volcanic pile—similar to the 

emplacement of the Boulder batholith into the Elkhorn Mountains Volcanics in Montana (Lipman and others, 1976).

Central San Juan Mountains Caldera Cluster

The six calderas of the central cluster, as in the other calderas of the volcanic field, formed within a locus of precaldera 

volcanoes. Following early ash-flow eruption from the western San Juan calderas, explosive activity converged in the central San Juan region, beginning with eruption of the Masonic 

Park Tuff at 28.3 Ma and then the enormous Fish Canyon Tuff 
30 Resource Potential and Geology, Grand Mesa, Uncompahgre, and Gunnison National Forests, Colorado 

from the La Garita caldera (35×75 km; fig. B7) at 27.6 Ma 

(Lipman, 2000). Ensuing central San Juan calderas (Bachelor, 

South River, San Luis caldera complex, and Creede), which 

were sources of smaller volume and more areally restricted 

tuff sheets, were nested within the much larger La Garita and 

La Garita South calderas. 

The dacitic Fish Canyon Tuff, long recognized as the 

world’s largest ash-flow sheet, both spread widely beyond 

and ponded within its source caldera. The current volume of 

this ash-flow sheet has been estimated to exceed 5,000 km3

, 

which is nearly five times greater than the largest of the other 

22 ash-flow sheets within the San Juan field (100 to >1,000 

km3

) (Lipman, 2000). Present outcrops of Fish Canyon Tuff 

outflow extend to nearly 70 km laterally from the La Garita 

caldera.

The San Luis caldera complex (fig. B7) is now recognized as the composite source of three sizable ash-flow sheets: 

the Rat Creek, Cebolla Creek, and Nelson Mountain Tuffs. 

These ash-flow sheets are all similar in composition, ranging from dacite to rhyolite, and record three separate subsidence events. Outflow of the Nelson Mountain Tuff filled the 

inferred Cochetopa Park caldera, located about 20 km northeast of the San Luis caldera complex (fig. B7). The inferred 

Cochetopa Park caldera, which is in the Gunnison National 

Forest (Chapter A, fig. A1), is bounded by a horseshoe-shaped 

complex of faults that represents a hinged subsidence feature 

(Steven and Lipman, 1976). Thick accumulations of Nelson 

Mountain Tuff within this inferred caldera were intruded and 

overlain by thick rhyolitic flows. The eroded remnants of this 

rhyolitic mass still persist as the feature called the Cochetopa 

dome. 

The Creede caldera formed during eruption of the Snowshoe Mountain Tuff at about 26.9 Ma and is thought to be the 

youngest of the central San Juan calderas. The dacitic intracaldera tuff is as much as 2 km thick, although the outflow 

sheet is less than 100 m thick and is generally limited to the 

central San Juan caldera cluster (Lipman, 2000). Resurgent 

doming resulted in the formation of a moat area between the 

resurgent dome and outer margin walls of the caldera. Sedimentary fill within this moat basin consists largely of finely 

laminated shale and sandstone, together called the Creede 

Formation (Steven and Ratté, 1973; Lipman, 2000).

Most calderas of the central San Juan field were filled 

rapidly after subsidence by andesite to rhyolite lavas and 

domal masses interleaved with ash-flow tuffs as well as minor 

sediments. The lavas were erupted from central volcanoes 

within or on the margins of the calderas, whereas the major 

tuff units are associated with younger adjacent calderas (Lipman, 2000). Intrusions associated temporally and spatially 

with the central San Juan calderas are relatively minor in 

volume and distribution, range from granite to andesite, and 

represent late stages of caldera magmatism. The intrusive 

rocks are commonly associated with weak argillic and pyritic 

alteration products; however, no evidence of significant mineralization has yet been found (Lipman, 2000).

The overall patterns of alteration and mineralization 

related to the central San Juan calderas are highly influenced 

by north-trending faults that were recurrently active throughout this caldera cycle. Localization of major mineralization 

events in the Creede district was also probably due to sustained magmatic activity and thermal flux over central parts of 

the subvolcanic batholith related to caldera eruptions (Lipman, 

2000).

Late Basalts and Rhyolites of the Central San 

Juan Mountains Caldera Cluster

In the early Miocene, the nature of volcanism changed 

markedly. Although the Oligocene volcanics of the San Juan 

field are mostly intermediate lavas and more silicic ash-flow 

tuffs, the younger rocks are largely a bimodal assemblage of 

basalt and silicic alkalic rhyolite (Lipman and others, 1969). 

The basaltic rocks of the Hinsdale Formation are mainly alkali 

olivine basalt flows; however, andesites are also common. The 

basalt flows, which are now much eroded, cap high flat mesas 

and typically rest upon older ash-flow units. The rhyolites 

consist of small, scattered volcanic necks, plug domes, and 

the ash-flow sheet related to the 23 Ma Lake City caldera. 

Basalt and rhyolite were erupted intermittently throughout the 

Miocene and Pliocene, forming a widespread thin veneer over 

the older volcanic rocks. Miocene-age alkali basalt on Grand 

Mesa is also part of the bimodal basalt-rhyolite suite and is 

roughly equivalent to the basalts of the Hinsdale Formation. 

Basaltic rocks on Grand Mesa are largely confined to plugs 

and related feeder dikes. The maximum preserved thickness 

of the basalt flows on Grand Mesa is about 240 m (Tweto and 

others, 1978).

Sawatch Range Calderas and Related Mount 

Princeton Batholith

The ≈34 Ma Mount Aetna and Grizzly Peak calderas are 

located in the south-central Sawatch Range (fig. B7). The 

Mount Aetna caldera complex consists of three main elements: (1) the 36.6 Ma Mount Princeton pluton, (2) the 34.4 

Ma Mount Aetna caldera, and (3) chemically evolved 30 Ma 

granitic intrusions (Johnson and others, 1989). The Mount 

Princeton pluton (batholith, fig. B7) is elliptical (24×36 km), 

compositionally zoned, and flat-topped. It is interpreted to 

represent the plutonic roots of a caldera in which all evidence 

of the volcanic edifice and collapse structure has been completely removed by erosion (Johnson and others, 1989). The 

Mount Aetna caldera consists of two collapse structures (12 

and 25 km in diameter) that have been deeply eroded, exposing the precaldera floor. Pyroclastic eruptive components 

from this caldera are only preserved in the southern part of the 

caldera complex. The younger evolved granites are a subgroup of granites that are associated with significant mineralization episodes in Colorado (Johnson and others, 1989).
Review of the Geology of Western Colorado 31

The Grizzly Peak caldera is located on the crest of the 

Sawatch Range, about 30 km north of the more deeply eroded 

Mount Aetna caldera (fig. B7). Early rhyolitic volcanism 

(Grizzly Peak Tuff) culminated in the collapse and formation 

of the 17×23 km Grizzly Peak caldera at 34 Ma (Fridrich and 

others, 1991). Approximately half of the erupted tuff ponded 

within the caldera margins. Only small remnants of outflow 

Grizzly Peak Tuff have been found more than 20 km away 

from the caldera margins (Fridrich and others, 1991). Following collapse, the caldera was resurgently domed, in part 

by the emplacement of a granodiorite laccolith, now exposed 

by erosion. A belt of dacite to rhyolite dikes and small stocks 

formed across the center of the domed caldera. Late felsic 

resurgent intrusions are spatially associated with hydrothermal 

alteration and weak mineralization resembling that found in 

porphyry molybdenum deposits (Fridrich and others, 1991).

Plutonic and Volcanic Rocks in the Elk 

Mountains Region

In contrast to Tertiary igneous rocks of the San Juan 

volcanic field, which are largely volcanic, those preserved in 

the Elk Mountains region are largely epizonal plutons. The 

Elk Mountains region, as used in this report, includes the Elk 

Mountains, Ruby Range, Treasure Mountain dome, and West 

Elk Mountains. However, the age and sequence of rock types 

are very similar in these two areas (Lipman and others, 1969). 

Igneous activity in both, dominantly of intermediate composition, occurred during late Oligocene time and produced large 

volumes of intermediate-composition rocks and their comagmatic silicic differentiates. As in the San Juan volcanic field, 

small volumes of bimodal mafic and silicic rocks were erupted 

in Miocene and Pliocene time (Lipman and others, 1969).

Upper Cenozoic rocks of the Elk Mountains region are 

mostly intrusive and can be divided into granodiorite plutons 

of Oligocene age, Miocene and younger? mafic dikes, and 

highly evolved granite to rhyolite stocks, plugs, and dikes 

of Miocene age. The Oligocene stocks, laccoliths, sills, and 

dikes consist mainly of granodiorite and granodiorite porphyry 

and intrude rocks as young as the Eocene Wasatch Formation 

in the Elk Mountains region. Available age data (Mutschler 

and others, 1981; Cunningham and others, 1994) indicate that 

these intrusions were emplaced between 34 and 29 Ma.

Studies by Mutschler and others (1981) group the Oligocene Intrusive Suite of the Elk Mountains area into several 

main stages based on age, mode of emplacement, and relationship to mineralized rocks. Stage A includes large plutons 

of equigranular to porphyritic granodiorite, and includes the 

Sopris, Snowmass, and Whiterock plutons, as well as the 

Italian Mountain intrusive complex. Mineralized material 

associated with these intrusions occurs dominantly as polymetallic disseminated and vein replacements along margins of 

the intrusions. Intrusions of stage B include sills, laccoliths, 

and dikes of granodiorite porphyry composition. Contact 

metamorphism is minimal in stage B intrusions, and they are 

not known to be associated with any significant mineralization. Stage D intrusions are small andesite to granodiorite 

stocks in a northeast zone, several of which are the centers of 

radial or linear dike swarms. Stage D intrusions extend from 

the West Elk Mountains and are present along and beyond the 

crest of the Ruby Range. Products of mineralization associated with these intrusions include chalcopyrite-pyritemolybdenite deposits and other miscellaneous vein and 

replacement deposits. 

Late Bimodal Rocks in the Elk Mountains Region

Small mafic dikes cut the Oligocene granodiorites in the 

Elk Mountains regions and are thought to be Miocene and 

early Pliocene in age (Lipman and others, 1969). The composition of these mafic rocks is similar to that of the basalts 

of the Hinsdale Formation of the San Juan volcanic field, the 

alkali basalts on Grand Mesa, and basalts in the Flat Tops 

areas to the north. Silicic Miocene intrusive rocks include the 

granite of Treasure Mountain (Mutschler and others, 1981), 

rhyolite to granite intrusions related to the Mount Emmons 

and Redwell Basin molybdenum deposits (White and others, 

1981; Thomas and Galey, 1982), and other miscellaneous 

domes, dikes, and small intrusions throughout the area.

Late Tertiary and Quaternary Unconsolidated 

Deposits

A major period of uplift, erosion, and deposition started 

in early Miocene and continued through Pliocene time, significantly disrupting and dissecting an Eocene regional erosion 

surface (Epis and others, 1980). Uplifted mountain blocks 

were deeply eroded, and the resulting debris was deposited in 

basins and channels bordering the mountain ranges. Uplift 

may have accelerated during the Pliocene, cutting deep canyons that characterize the mountain flanks. Pliocene canyon 

cutting apparently continued into Quaternary time until a 

stable base was achieved. Canyon cutting and pedimentation 

continued in Quaternary time, but a major climatic cooling 

event initiated glaciation, which continued from about 500,000 

years ago into the late Pleistocene, ending approximately 

15,000 years ago (Epis and others, 1980). During three glacial 

maxima, ice almost covered the higher ranges, and the alpine 

valleys were filled with glaciers. The modern alpine topography of deep U-shaped valleys and sharp peaks and ridges is 

largely a product of glacial erosion (Mierding and Birkeland, 

1980). Glacial advances deposited numerous sheets of till and 

outwash gravel in stream valleys.

Holocene alluvium is present in drainages and fans across 

the national forests and consists of gravel, sand, and silt with 

varying degrees of consolidation. In some places, alluvium of 

Pliocene and Pleistocene(?) age is present on ridges. Landslide deposits are common in the study area; semi-steep slopes 

underlain by Mancos Shale, and the Wasatch Mesaverde 

Formation are particularly prone to landslides.
32 Resource Potential and Geology, Grand Mesa, Uncompahgre, and Gunnison National Forests, Colorado 

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Matterhorn Peak stock, Hinsdale County, Colorado: Austin, 

Tex., University of Texas M.A. thesis, 89 p.

Reed, J.C., Jr., Bickford, M.E., Premo, W.R., Aleinikoff, J.N., 

and Pallister, J.S., 1987, Evolution of the Early Proterozoic 

Colorado province—Constraints from U-Pb geochronology: 

Geology, v. 15, no. 9, p. 861–865.

Ringrose, C.R., Harmon, R.S., Jackson, S.E., and Rice, C.M., 

1986, Stable isotope geochemistry of a porphyry-style 

hydrothermal system, West Silverton district, San Juan 

Mountains, Colorado, U.S.A., in F. Earl Ingerson Festschrift—Part III: Applied Geochemistry, v. 1, no. 3, p. 

357–373.

Schenk, C.J., Johnson, S.Y., and Karachewski, J.A., 1987, 

Field guide and road log—Pennsylvanian and Permian 

depositional systems and cycles in the Eagle Basin, northwestern Colorado: U.S. Geological Survey Open-File 

Report 87-430, 29 p.

Selverstone, Jane, Hodgins, Meghan, Shaw, Colin, Aleinikoff, 

J.N., and Franning, C.M., 1997, Proterozoic tectonics of the 

northern Colorado Front Range, in Bolyard, D.W., and Sonnenberg, S.A., eds., Geologic history of the Colorado Front 

Range: Rocky Mountain Association of Geologists Guidebook, 1997 Symposium, Denver, Colo., p. 9–18.

Shaw, C.A., and Karlstrom, K.E., 1999, The Yavapai–Mazatzal 

crustal boundary in the southern Rocky Mountains: Rocky 

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Special Paper 82, p. 185–186.

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implications in Precambrian zircons from Bagdad, Arizona, 

in Abstracts for 1966: Geological Society of America Special Paper 101, p. 420.

Slack, J.F., 1980, Multistage vein ores of the Lake City district, western San Juan Mountains, Colorado: Economic 

Geology, v. 75, no. 7, p. 963–991.

Smith, D.R., Noblett, J., Wobus, R.A., Unruh, D.M., Douglass, J., Beane, R., Davis, C., Goldman, S., Kay, G., 

Gustavson, B., Saltoun, B., and Stewart, J., 1999, Petrology 

and geochemistry of late-stage intrusions of the A-type, 

mid-Proterozoic Pikes Peak Batholith (central Colorado, 

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of the southern Rocky Mountains: Geological Society of 

America Memoir 144, p. 75–94.

Steven, T.A., and Lipman, P.W., 1976, Calderas of the San 

Juan volcanic field, southwestern Colorado: U.S. Geological Survey Professional Paper 958, 35 p.

Steven, T.A., Lipman, P.W., Hail, W.J., Jr., Barker, Fred, and 

Luedke, R.G., 1974, Geologic map of the Durango quadrangle, southwestern Colorado: U.S. Geological Survey 

Miscellaneous Investigations Series Map I-764, scale 1:

250,000.

Steven, T.A., and Ratté, J.C., 1973, Geologic map of the 

Creede quadrangle, Mineral and Saguache Counties, Colorado: U.S. Geological Survey Geologic Quadrangle Map 

GQ-1053, scale 1:62,500.

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related Upper Triassic strata in the Colorado Plateau region: 

U.S. Geological Survey Professional Paper 690, 336 p.

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County, Colorado: Economic Geology, v. 77, no. 5, p. 

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Thompson, T.B., eds., Carbonate-hosted sulfide deposits 

of the central Colorado Mineral Belt: Economic Geology 

Monograph 7, p. 308–338.

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36 Resource Potential and Geology, Grand Mesa, Uncompahgre, and Gunnison National Forests, Colorado 

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uranium-bearing sandstone—A study of the Westwater Canyon Member of the Morrison Formation, San Juan Basin, 

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eds., A basin analysis case study; The Morrison Formation, 

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Kent, H.C., and Porter, K.W., eds., Colorado geology: 

Rocky Mountain Association of Geologists Guidebook, 

1980 Symposium, Denver, Colo., p. 37–46.

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1321, 54 p.

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Colorado: U.S. Geological Survey Miscellaneous Investigations Series Map I-999, scale 1:250,000.

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1976, Geologic map of the Montrose 1° × 2° quadrangle, 

southwestern Colorado: U.S. Geological Survey Mineral 

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chronology and evolution of the midcontinent region, North 

America, in Kröner, A., ed., Precambrian plate tectonics: 

Amsterdam, Elsevier, Developments in Precambrian Geology, 4, p. 261–296.

White, W.H., Bookstrom, A.A., Kamilli, R.J., Ganster, M.W., 

Smith, R.P., Rantra, D.E., and Steininger, R.C., 1981, 

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p. 270–316.

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of the Moab quadrangle, Colorado and Utah: U.S. Geological Survey Miscellaneous Investigations Series Map I-360, 

scale 1:250,000.

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J.M., and Sole, T.C., 1998, Digital geologic compilations 

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abandoned mine lands; Review of preliminary results, Denver, Colorado, February 4–5, 1998: U.S. Geological Survey 

Open-File Report 98-297, p. 27.
U.S. Department of the Interior

U.S. Geological Survey

Regional Sediment and Rock Geochemistry

By Steven M. Smith

Chapter C of

Resource Potential and Geology of the Grand Mesa, Uncompahgre, 

and Gunnison (GMUG) National Forests and Vicinity, Colorado

Edited by Viki Bankey

U.S. Geological Survey Bulletin 2213– C
Contents

Abstract........................................................................................................................................................ 39 

Source and Description of Geochemical Data ...................................................................................... 39

Sediment Data............................................................................................................................................. 39

Rock Data..................................................................................................................................................... 41

Gridding and Contouring............................................................................................................................ 41

Interpretive Maps ....................................................................................................................................... 43

Summary ...................................................................................................................................................... 49

References Cited ........................................................................................................................................ 49

Figures

C1–C4. Maps showing:

 C1. Localities of sediment samples collected in GMUG greater study area and a

contiguous buffer area......................................................................................................... 42

 C2. Localities of rock samples collected in GMUG greater study area.............................. 45

 C3. Distribution of copper in sediment samples collected in GMUG greater

study area............................................................................................................................... 47

 C4. Distribution of copper in rock samples collected in GMUG greater

study area............................................................................................................................... 48

Tables

C1. Sources of geochemical data from previous USGS studies within the greater

study area ............................................................................................................................................ 40

C2. Sources of geochemical data from National Uranium Resource Evaluation (NURE) 

Hydrogeochemical and Stream Sediment Reconnaissance (HSSR) projects and

NURE Detailed Studies within GMUG greater study area ........................................................... 41

C3. Additional sources of geochemical data for GMUG greater study area that were not 

included in this study ......................................................................................................................... 41

C4. Clarke Index values for crustal abundance of selected elements; based on

Fortescue (1992).................................................................................................................................. 46
Abstract

A geochemical data set prepared for the GMUG project 

contains analyses for 13,314 sediment samples and 5,957 rock 

samples. These data allowed calculation of baseline concentrations of elements within the GMUG region and identification of areas with relatively high or low abundances.

Source and Description of 

Geochemical Data

The USGS National Geochemical Database (Hoffman 

and Marsh, 1994; Smith, 2000) contains data for a large number of geochemical samples from within the GMUG greater 

study area. Data for sediment and rock samples were retrieved 

from the USGS National Geochemical Database within an 

area bounded by lat 37°30′ N. to 39°45′ N. and long 105°45′

W. to 109°15′ W. This region includes the GMUG greater 

study area plus an extra 15-minute-wide buffer zone that 

was added to reduce edge-effect errors produced by surficial 

modeling of geochemical data. Data for samples of unique, 

unusual, or nonrepresentative material were removed. Data 

records were also removed if the sample was not analyzed by 

total-digestion chemical methods for elements of interest to 

this study. Data for 56 stream-sediment samples, collected 

during the 1996 and 1997 field seasons to fill gaps in the geographic coverage and to evaluate the geochemical signatures 

of mineral deposit types, were added to the project geochemical data sets. The resultant GMUG project geochemical data 

sets contain analyses for 13,314 sediment samples and 5,957 

rock samples.

The data for geochemical samples retrieved from the 

USGS National Geochemical Database were derived from 

two primary sources: (1) rock and sediment samples collected 

since 1966 for various USGS projects in support of mineral 

resource assessment studies, energy resource studies, element 

distribution studies, ore deposits research, lithologic geochemistry research, and geologic mapping; and (2) sediment 

samples collected during 1976–79 for the National Uranium 

Resource Evaluation (NURE) Hydrogeochemical and Stream 

Sediment Reconnaissance (HSSR) program. Table C1 lists 

USGS projects for which the stream-sediment and rock data 

were collected and includes references for associated reports. 

Some of the rock samples could not be identified with specific 

projects. Table C2 lists the projects responsible for collection 

of the NURE sediment data that are used in this study. Other 

known geochemical data from the GMUG greater study area 

that were not used are listed in table C3. Most of these data 

were not available in digital form.

The USGS sediment and rock samples were analyzed 

by DC-arc emission spectrography (Grimes and Marranzino, 1968; Golightly and others, 1987), inductively coupled 

plasma–atomic emission spectroscopy (Lichte and others, 

1987; Briggs, 1996) or delayed neutron counting analysis 

(McKown and Millard, 1987). The number of elements analyzed and the determination limits varied with slight modifications in these analytical methods over the years represented in 

the data set.

The NURE samples were analyzed by energy dispersive 

X-ray fluorescence, DC-arc emission spectrography, delayed 

neutron counting analysis and neutron activation analysis. The 

details of these NURE analytical methods can be found in 

many of the HSSR quadrangle reports (for example, Shannon, 

1980a).

The new minus-80-mesh stream-sediment samples collected for the current study were analyzed for 40 elements by 

an inductively coupled plasma–atomic emission spectrometry 

(ICP-AES) total extraction method (Lichte and others, 1987; 

Briggs, 1996). The samples were decomposed using a mixture 

of hydrochloric, nitric, perchloric, and hydrofluoric acids at 

low temperatures as described by Crock and others (1983). 

Each digested sample was aspirated into the ICP-AES instrument and the concentrations of 40 elements were determined 

simultaneously.

Sediment Data

The sediment samples are primarily minus-80-mesh 

(USGS) or minus-100-mesh (NURE) stream sediments, 

although a few samples were collected from ponds or springs. 

Some USGS samples are listed in the USGS National Geochemical Database only as “unconsolidated sediments”; the 

exact sources of these samples are unclear. Most of the sediment samples were collected specifically to represent regional 

elemental variation. The NURE HSSR studies systematically 

Regional Sediment and Rock Geochemistry

By Steven M. Smith
40 Resource Potential and Geology, Grand Mesa, Uncompahgre, and Gunnison National Forests, Colorado 

sampled the region at an average density of about one sample 

per 10 km2

 (Sharp and Aamodt, 1978). Within this regional 

coverage, NURE detailed studies (for example, Maassen and 

others, 1981) and USGS studies of proposed Wilderness Study 

Areas (WSA) sampled selected areas at densities of as much 

as one sample per 2.6 km2

. Figure C1 shows the distribution 

of sediment samples in the GMUG project data set.

Because of the extensive coverage and the representative nature of the samples, the sediment data can be used for 

various purposes. In this study, the data have been used to 

calculate baseline concentrations of elements within the region 

and to identify areas with relatively high or low abundances. 

By interpolating element concentrations between sample sites, 

we have constructed geochemical surface models and contour 

maps to show the distribution of elements across the GMUG 

greater study area. The data were also incorporated into the 

GIS Mineral Resource Assessment models as point measurements. (See chapter on mineral resource assessments for various deposit types included in this volume.)

The sediment geochemical data were modified slightly to 

accommodate the constraints of statistical, gridding, and mapping software applications. Owing to the number of different 

analytical methods and variations of these analytical methods, 

the data contain many differing analytical determination limits. Data above and below these determination limits 

Table C1. Sources of geochemical data from previous USGS studies within GMUG greater study area.

Study area Geochemical data reference

Black Ridge Canyons BLM WSA Toth, Stoneman, and others, 1983; Bullock, others, and Fey, 1989.

Buffalo Peaks WSA Domenico and others, 1984; Nowlan and Gerstel, 1985.

Cannibal Plateau Roadless Area Sharp and Lane, 1983.

Chama-Southern San Juan Mtns. WSA Brock and others, 1985.

Collegiate Peaks WSA Fridrich and others, 1998, and unpublished data.

Dolores Project Area-Irrigation Studies Butler and others, 1995.

Dolores River Canyon BLM WSA Bullock, others, and Briggs, 1989.

Dominguez Canyon BLM WSA Toth, Davis, and others, 1983; Toth and others, 1987, and unpublished data.

Eagle Mountain WSA Soulliere and others, 1986, and unpublished data.

Flume Canyon BLM WSA Gaccetta and others, 1990.

Fossil Ridge WSA Adrian, Clark, and others, 1984; Clark and Adrian, 1984.

Geochemistry of Black Shales Vine and others, 1969.

Geochemistry of Eocene Rocks Vine and Tourtelot, 1973.

Gunnison Gorge BLM WSA Bullock, Barton, Briggs, and Roemer, 1989.

Handies Peak BLM WSA Sanford and others, 1987, and unpublished data.

Holy Cross WSA Wallace and others, 1989, and unpublished data.

Hunter-Fryingpan WSA Mosier and others, 1980; Ludington and Yeoman, 1980.

Maroon Bells-Snowmass WSA McHugh and others, 1987.

Mt. Massive WSA Van Loenen and others, 1989, and unpublished data.

Oh-Be-Joyful WSA Ludington and Ellis, 1983, and unpublished data.

Palisade BLM WSA Hovorka and others, 1983, and unpublished data.

Porphyry Mountain WSA Mosier and others, 1980.

Powderhorn WSA Sharp and Lane, 1983.

Redcloud Peak BLM WSA Sanford and others, 1987, and unpublished data.

San Juan Geologic Mapping R.G. Luedke, oral commun., 2000, and unpublished data.

San Juan NF Mineral Resource Assessment Barton and others, 1992.

Sangre de Cristo WSA1 Adrian, Arbogast, and Zimbelman, 1984; Zimbelman, 1989.

Sewemup Mesa BLM WSA Soulliere and others, 1983, and unpublished data.

Tabeguache Creek BLM WSA Bullock and others, 1990.

Uncompahgre Primitive Area2 Fischer and others, 1968.

Uncompahgre Primitive Area Additions3 Steven and others, 1973; Steven and others, 1977.

Uncompahgre Project Area-Irrigation Studies Crock and others, 1994; Butler and others, 1994; Butler and others, 1996.

Upper Arkansas River Basin Church, 1993; Church and others, 1994; Smith, 1994.

West Elk WSA Gaskill and others, 1977.

West Needle WSA1 Birmingham and Van Loenen, 1983; Van Loenen, 1985.

Westwater BLM WSA Bullock, others, and Fey, 1989.

Wilson Mountains Primitive Area4 Bromfield and others, 1972.

 1 Study from outside of the GMUG greater study area but within the contiguous 15-minute buffer area.

 2Currently the Big Blue Wilderness Area.

 3 Includes parts of the Big Blue Wilderness and Mt. Sneffels Wilderness Areas.

 4Currently the Lizard Head Wilderness Area.
Regional Sediment and Rock Geochemistry 41

(qualified values) were replaced either by real values of 0.7 

times the lower determination limit or by null values.

Rock Data

Unlike the sediment samples, rock samples were not 

collected systematically throughout the entire GMUG greater 

study area. Rocks were collected primarily around wilderness 

study areas and mining districts. Large areas of the GMUG 

greater study area were only sparsely sampled (fig. C2). Some 

rocks are representative samples of extensive geologic units, 

whereas other samples are of mineralized and altered rock 

from individual mine waste dumps. 

The rock data are not appropriate for contouring or 

determining average baseline concentrations for the region, 

owing to the poor coverage and the mixture of mineralized and 

nonmineralized samples. Therefore, the rock geochemistry 

was used only as point data within the GIS Mineral Resource 

Assessment models. Only unqualified rock data were used. 

The qualified values (data outside of the determination limits) 

were neither replaced nor used in the GIS analysis.

Gridding and Contouring 

A geochemical “surface” model was interpolated for 

each element in the sediment data set using an algorithm in 

Table C2. Sources of geochemical data from National Uranium Resource Evaluation (NURE) Hydrogeochemical and 

Stream Sediment Reconnaissance (HSSR) projects and NURE Detailed Studies within GMUG greater study area. 

[The NURE data were retrieved from Smith (2000)]

Study area Geochemical data reference

Cortez Quadrangle HSSR Maxwell, 1977; Warren and others, 1979.

Denver Quadrangle HSSR1 Bolivar and others, 1978; Shettel and others, 1981.

Durango Quadrangle HSSR Maxwell, 1977; Dawson and Weaver, 1979; Shannon, 1980a.

Grand Junction Quadrangle HSSR Langfeldt and others, 1981.

Leadville Quadrangle HSSR Planner and others, 1980.

Moab Quadrangle HSSR Maxwell, 1977; Goff and others, 1979.

Montrose Quadrangle HSSR Maxwell, 1977; Broxton and others, 1979.

Pueblo Quadrangle HSSR1 Shannon, 1978; Shannon, 1979b.

Sawatch Range Detailed Study Maassen and others, 1981.

Tallahassee Creek, Badger Creek, Castle Rock Shannon, 1979a.

Gulch, and Buffalo Gulch Detailed Study.1

Trinidad Quadrangle HSSR1 Morris and others, 1978; Shannon, 1980b.

Vallecito Creek Special Study Area1 Warren and others, 1981.

1 Study from outside of the GMUG greater study area but within the contiguous 15-minute buffer area.

Table C3. Additional sources of geochemical data for GMUG greater study area that were not included in this study. 

[Most of these data were not available in digital form during the data compilation phase of this study. —, no known samples]

Study area Rocks Sediments Geochemical data reference

American Flats–Silverton BLM Planning Unit 89 1203 Weiland and others, 1980.

Browns Canyon WSA — 121 Leibold and others, 1987.

Cortez Quadrangle NURE Evaluation 45 1657 Campbell and others, 1982a.

Denver Quadrangle NURE Evaluation1 485 301 Hills and others, 1982.

Durango Quadrangle NURE Evaluation 156 118 Theis and others, 1981.

La Garita WSA 172 253 Steven and Bieniewski, 1977.

Leadville Quadrangle NURE Evaluation 267 12 Collins and others, 1982.

Moab Quadrangle NURE Evaluation 131 — Campbell and others, 1982b.

Montrose Quadrangle NURE Evaluation 365 30 Goodnight and Ludlam, 1981.

Pueblo Quadrangle NURE Evaluation1 478 150 Dickinson and others, 1982.

San Juan Primitive Area1, 2 467 828 Steven and others, 1969.

Trinidad Quadrangle NURE Evaluation1 90 — Johnson and others, 1982.

 1 Study from outside of the GMUG greater study area but within the contiguous 15-minute buffer area.

 2Currently the Weminuche Wilderness Area.
39o 30'109o 

37o 45'

70

139

65

92

50

50 50

82

24 

135

133

149

62

145

141

141

90

114

Grand

Junction

Delta

Montrose

Gunnison

Lake City

Uravan

Telluride

Ouray

Silverton

Aspen

0

0

550
Regional Sediment and Rock Geochemistry 43

Dynamic Graphics, Inc. EarthVision software that employs a 

bi-harmonic spline of minimum tension to create a continuous grid from scattered point data. As is common to most 

gridding algorithms, the interpolation of grid values into areas 

unconstrained by actual data points occasionally produces 

unrealistic high or low values. These “edge effects” may 

occur near the margins of the data set or within “holes” in the 

data distribution. To reduce the influence of these “effects,” 

we constrained the grid values to the range of maximum and 

minimum concentration values from the input data set. We 

eliminated most “edge effects” by including data from a 15-

minute-wide buffer zone around the GMUG greater study area 

during the gridding process.

The grid files were imported into the ERDAS Imagine 

GIS package and masked to remove grid nodes outside the 

GMUG greater study area. The mean and standard deviation 

were calculated for the node values in each element grid and 

then compared with the mean and standard deviation of the 

input point data set to ensure that the grids closely modeled 

the input data. For modeling and contour display purposes, 

each grid was converted by a formula that calculates standard 

deviation units (SDU), as follows:

where X is the individual grid value, Xm is the arithmetic mean 

for all grid values, σ is the standard deviation, and SDU is the 

resultant Standard Deviation Unit. The SDU values resulting from the equation measure, in units of standard deviation, 

the difference of each concentration value from the mean. 

Element grids transformed by the equation have a mean of 

zero and a standard deviation of one. Following the conversion of each grid, we reclassified the calculated SDU values 

into discrete categories by rounding each value to the nearest 

0.5 SDU. Thus the mean SDU value of 0 actually represents 

a range from –0.250 to 0.249 SDU. (For example, for an element with a mean of 13 parts per million (ppm) and a standard 

deviation of 8, an SDU of 0 represents the concentration range 

from 11 to 14.9 ppm.) For elements with good sample and 

analytical coverage, the concentration range represented by 0 

SDU can be used as one estimate of the mean local baseline 

concentration for that element.

The advantage of plotting SDU maps is that the distribution ranges of each element can be shown by a common 

scheme that is easily interpreted. This allows the user to 

quickly compare a large number of maps and assimilate the 

information with minimal effort. In addition, the SDU transformation can facilitate combining or comparing data from 

different sample media, analytical techniques, laboratories, 

or terranes. A limitation inherent to SDU maps is that the 

element ranges are dependent solely on the populations of element concentrations from the area of interest. Large standard 

deviation values caused by extreme outliers may suppress 

variation on the maps and hide subtle anomalies. In addition, 

this local range of element concentrations may or may not 

reflect the variability that is found worldwide. Thus the “hot” 

values plotted on SDU maps can highlight geologic terranes 

with only moderate enrichment that may not be of economic 

significance. Before using an SDU map to focus additional 

studies, the actual concentration values should be checked to 

confirm that the levels are high enough to warrant the followup work.

To address the issues of whether the SDU anomalies 

are of economic or environmental significance, we plotted 

some elements on sample location maps using symbols to 

represent multiples of crustal abundance estimates (Fortescue, 1992). This type of map is also more suitable than SDU

maps for those elements with a very limited distribution of 

concentration values above analytical detection limits or for 

those sample media with limited geographical coverage, as 

in the case of GMUG rock geochemical data. The set of 

crustal abundance estimates, or Clarke Index values, used for 

this study is given in table C4. Multiples of the Clarke Index 

value are known as “Clarkes.” In a manner similar to the SDU

maps, each concentration value was converted and then classified by rounding to the nearest 1.0 Clarke. A Clarke value 

of 1 thus represents a range of element concentrations from 

0.50 to 1.49 Clarkes. (As an example, for an element with a 

crustal abundance estimate of 20 ppm, the concentration range 

represented by 1 Clarke is 10 to 29.9 ppm.) The concentration 

range represented by 1 Clarke can be used as one estimate of 

the global background range for an element. Clarke units are 

multiples of this background range.

Interpretive Maps

A large number of interpretive maps were created from 

the GMUG greater study area sediment and rock geochemical 

data, as follows: 

Figure C1(above and previous page). Localities of sediment 

samples (dots) collected within GMUG greater study area 

and a contiguous 15-minute buffer area.

(X - Xm) = SDU σ

70 KILOMETERS

40 MILES

COLORADO

Greater

Study

Area

Approximate outline of national forests in GMUG study area

EXPLANATION
44 Resource Potential and Geology, Grand Mesa, Uncompahgre, and Gunnison National Forests, Colorado 

As an example, figure C3 is an SDU map showing the 

distribution of copper in sediments. The mean concentration 

of copper in sediment samples collected from the GMUG 

greater study area is 42 ppm. This is lower than the crustal 

abundance estimate of 68 ppm (table C4) recommended by 

Fortescue (1992) but is essentially equivalent to the regional 

baseline value of 40 ppm used by Smith (1994) in studies of 

the Upper Arkansas River Basin in Colorado. A number of 

areas containing elevated concentrations of copper can be 

identified in figure C3. Most of these highs are associated 

with areas of known mineralization, and the general pattern 

follows the trend of the Colorado Mineral Belt.

The distribution of copper in rock samples is illustrated 

as a Clarke map in figure C4. Only those rock samples having a copper concentration of 2 or more Clarkes are shown. 

Because of the large percentage of rocks collected from mineralized areas, the copper concentrations in rock data highlight 

several mining districts within the GMUG greater study area.

 SDU maps

antimony copper strontium

barium gold thorium

bismuth lead tin

cadmium manganese tungsten

chlorine nickel uranium

chromium rubidium vanadium

cobalt silver zinc

 Clarke maps, sediment samples

antimony gold scandium

arsenic hafnium silver

barium lanthanum strontium

bismuth lead thorium

cadmium lithium tin

chlorine lutetium tungsten

chromium manganese uranium

cobalt molybdenum vanadium

copper nickel ytterbium

dysprosium niobium zinc

europium rubidium zirconium

samarium

 Clarke maps, rock samples

arsenic gold thorium

barium lead tin

bismuth manganese tungsten

cadmium mercury uranium

chromium molybdenum vanadium

cobalt nickel zinc

copper silver
39o 30'

109o 

37o 45'

70

139

65

92

50

50 

82

135

133

149

62

145

141

141

90

114

Grand

Junction

Delta

Montrose

Gunnison

Crested Butte

Lake City

Uravan

Telluride

Ouray

Silverton

Aspen

Disappointment Creek

Gunnison River

Uncompahgre River Lake Fork Cebolla

Creek

Tomichi Creek

Taylor River

Blue MesaReservoir

CochetopaCreek

Dolores River

0

0

Approximate outline of national forests in GMUG study area

EXPLANATION

550

Figure C2. Localities of rock samples (dots) collected within GMUG greater study area.
46 Resource Potential and Geology, Grand Mesa, Uncompahgre, and Gunnison National Forests, Colorado 

Figure C3 (left and following page). Distribution of copper 

in sediment samples collected in GMUG greater study area. 

One Standard Deviation Unit (1.0 SDU) is element concentration range that is approximately one standard deviation 

above mean concentration range (0 SDU).

Table C4. Clarke Index values for crustal abundance of selected elements; based on Fortescue 

(1992).

[Element concentrations are reported in parts per million (ppm: equivalent to micrograms/gram) unless otherwise 

noted; pct, percent]

Element Clarke Index Value Element Clarke Index Value

Al 8.36 pct Nb 20.0

Fe 6.22 pct Li 18.0

Ca 4.66 pct Pb 13.0

Mg 2.764 pct Th 8.10

Na 2.27 pct Sm 7.02

K 1.84 pct Dy 5.00

Ti 0.632 pct Yb 3.10

Mn 1,060 Hf 2.80

Ba 390 Cs 2.60

Sr 384 U 2.30

Zr 162 Eu 2.14

V 136 Sn 2.10

Cl 126 Be 2.00

Cr 122 As 1.80

Rb 78.0 Mo 1.20

Zn 76.0 W 1.20

Ni1 75.0 Lu 0.54

Ce 66.4 Sb 0.20

Cu2 40.0 Bi1 0.17 

La 34.6 Cd 0.16

Co 29.0 Ag 0.080

Sc 25.0 Au 0.0040

 1Bismuth and nickel values from Taylor (1964). Fortescue (1992) lists values of 0.0082 ppm Bi and 99 ppm Ni.

 2Copper value from Smith (1994). Fortescue (1992) lists a value of 68 ppm Cu.

EXPLANATION

SDU Concentration Range

–0.5

0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

< 12 ppm Cu

12–73 ppm Cu

73–134 ppm Cu

134–196 ppm Cu

196–257 ppm Cu

257–318 ppm Cu

318–379 ppm Cu

379–441 ppm Cu

441–502 ppm Cu

> 502 ppm Cu

Mean = 42 ppm Cu

Standard Deviation = 122.5
39o 30'

109o 

37o 45'

70

139

65

92

50

50 

82

135

133

149

62

145

141

141

90

114

Grand

Junction

Delta

Montrose

Gunnison

Crested Butte

Lake City

Uravan

Telluride

Ouray

Silverton

Aspen

Disappointment Creek

Gunnison River

Uncompahgre River Lake Fork Cebolla

Creek

Tomichi Creek

Taylor River

Blue MesaReservoir

CochetopaCreek

Dolores River

0

0

Approximate outline of national forests in GMUG study area

EXPLANATION

550
39o 30'109o 

37o 45'

70

139

65

92

50

50 

82

135

133

149

62

145

141

141

90

114

Grand

Junction

Delta

Montrose

Gunnison

Crested Butte

Lake City

Uravan

Telluride

Ouray

Silverton

Aspen

Disappointment Creek

Gunnison River

Uncompahgre River Lake Fork Cebolla

Creek

Tomichi Creek

Taylor River

Blue MesaReservoir

CochetopaCreek

Dolores River

0

0

Approximate outline of national forests in GMUG study area

EXPLANATION

550

Figure C4 (above and following page). Distribution of copper in rock samples collected in GMUG greater study area. A Clarke Index Value is 

dance concentration for the element and is used as median of background range of concentrations. Clark units are multiples of background 
Regional Sediment and Rock Geochemistry 49

Summary

The available digital geochemistry data from streamsediment and rock samples collected within the GMUG 

greater study area were compiled, modeled, and interpreted. 

These results were then extensively used in the assessments of 

various mineral resource deposit types.

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EXPLANATION

Clarke Concentration Range

 -

< 2

2 – 3

4 – 10

11 – 20

> 20

< 60 ppm Cu

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140–420 ppm Cu

420–820 ppm Cu

> 820 ppm Cu

 - Not analyzed

Clarke Index Value = 40 ppm Cu

Not detected (mult. limits)
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Irving Formation by J.E. Bair and Harry Klemic: U.S. Geological Survey Bulletin 1261-F, p. F1–F187.

Taylor, S.R., 1964, The abundance of chemical elements in the 

continental crust—A new table: Geochimica et Cosmochimica Acta, v. 28, no. 8, p. 1273–1285.

Theis, N.J., Madson, M.E., Rosenlund, G.C., Reinhart, W.R., 

and Gardner, H.A., 1981, Durango quadrangle, Colorado: 

Report of Bendix Field Engineering Corporation prepared 

for the U.S. Department of Energy, Grand Junction, Colo., 

GJQ-011(81), 48 p.

Toth, M.I., Davis, L.L., Patterson, C.G., and Seeley, J.L., 1983, 

Geology and mineral resource potential of the Dominguez 

Canyon Wilderness Study Area, Delta, Mesa, and Montrose 

Counties, Colorado (GEM phase 2): U.S. Geological Survey Open-File Report 83-797, 35 p.

Toth, M.I., Patterson, C.G., Kulik, D.M., and Schreiner, R.A., 

1987, Mineral resources of the Dominguez Canyon Wilderness Study Area, Delta, Mesa, and Montrose Counties, 

Colorado: U.S. Geological Survey Bulletin 1736–A, p. 

A1–A10.

Toth, M.I., Stoneman, R.J., Patterson, C.G., and Jackson, L.L., 

1983, Geology and mineral resource potential of the Black 

Ridge Canyon Wilderness Study Area, Mesa County, Colorado (GEM Phase 2): U.S. Geological Survey Open-File 

Report 83-794, 36 p.

Van Loenen, R.E., 1985, Geologic and geochemical maps of 

the West Needle Wilderness Study Area, San Juan and La 

Plata Counties, Colorado: U.S. Geological Survey Miscellaneous Field Studies Map MF–1632–B, scale 1:50,000.

Van Loenen, R.E., Lee, G.K., Campbell, D.L., and Thompson, J.R., 1989, Mineral resource appraisal of the Mount 

Massive Wilderness Area, Lake County, Colorado: U.S. 

Geological Survey Bulletin 1636, 18 p.

Vine, J.D., and Tourtelot, E.B., 1973, Geochemistry of lower 

Eocene sandstones in the Rocky Mountain region: U.S. 

Geological Survey Professional Paper 789, 36 p.

Vine, J.D., Tourtelot, E.B., and Keith, J.R., 1969, Element distribution in some trough and platform types of black shale 

and associated rocks: U.S. Geological Survey Bulletin 

1214–H, p. H1–H38.

Wallace, A.R., Lee, G.K., Campbell, D.L., Lundby, William, 

and Brown, S.D., 1989, Mineral resources of the Holy Cross 

Wilderness Study Area, Pitkin and Lake Counties, Colorado: U.S. Geological Survey Bulletin 1879, 22 p.

Warren, R.G., Gallimore, D.L., Martell, C.J., Sandoval, W.F., 

Garcia, S.R., Minor, M.M., and McInteer, Carlotta, 1981, 

Data release for intermediate-density hydrogeochemical and stream sediment sampling in the Vallecito Creek 

Special Study Area, Colorado, including concentrations of 

uranium and forty-six additional elements: Los Alamos, N. 

Mex., Los Alamos Scientific Laboratory Informal Report 

LA-8483-MS, U.S. Department of Energy, Grand Junction, 

Colo., GJBX-217(81), 101 p.

Warren, R.G., George, W.E., Apel, C.T., Minor, M.M., 

Steinhaus, D.W., Martell, C.J., Stein, S.L., and Cheadle, J., 

III, 1979, Uranium hydrogeochemical and stream sediment 

reconnaissance of the Cortez NTMS quadrangle, 

Colorado/Utah, including concentrations of forty-three 

additional elements: Los Alamos, N. Mex., Los Alamos 

Scientific Laboratory Informal Report LA-7505-MS, U.S. 

Department of Energy, Grand Junction, Colo., GJBX77(79), 198 p.

Weiland, E.F., Johnson, S.A., and Grauch, R.I., 1980, Streamsediment geochemical survey of the Bureau of Land 

Management’s American Flats–Silverton Planning Unit 

in southwest (Lake City area) Colorado: U.S. Geological 

Survey Open-File Report 80-541, 111 p.

Zimbelman, D.R., 1989, Maps showing trace-element geochemistry of the Sangre de Cristo Wilderness Study Area, 

south-central Colorado: U.S. Geological Survey 

Miscellaneous Field Studies Map MF-1635-C, scale 1:

125,000.
U.S. Department of the Interior

U.S. Geological Survey

Geophysical Studies

By Viki Bankey, Robert P. Kucks, and Kim Oshetski

Chapter D of

Resource Potential and Geology of the Grand Mesa, Uncompahgre, 

and Gunnison (GMUG) National Forests and Vicinity, Colorado

Edited by Viki Bankey

U.S. Geological Survey Bulletin 2213– D
Contents

Abstract........................................................................................................................................................ 56

Gravity Data ................................................................................................................................................. 56

Gravity Map Preparation .................................................................................................................. 56

Gravity Map Interpretation............................................................................................................... 56

Magnetic Data............................................................................................................................................. 59

Magnetic Map Preparation.............................................................................................................. 59

Aeromagnetic Map Interpretation.................................................................................................. 59

Aeroradiometric Data ................................................................................................................................ 61

Aeroradiometric Map Preparation ................................................................................................. 61

Aeroradiometric Map Interpretation.............................................................................................. 61

References Cited ........................................................................................................................................ 65

Figures

D1–D4. Maps showing:

 D1. Isostatic gravity values of GMUG greater study area ................................................... 57

 D2. Aeromagnetic anomaly values of GMUG greater study area ...................................... 60

 D3. Outline of Tertiary plutons inferred from aeromagnetic data ....................................... 62

 D4. Uranium values of GMUG greater study area................................................................. 63

Table

D1. Summary of stratigraphic units containing anomalous uranium as described in NURE 

reports .................................................................................................................................................. 64
Abstract

Three sets of geophysical data, comprising gravity, aeromagnetic, and radiometric maps, were compiled from previous 

studies and interpreted for the GMUG greater study area.

Gravity Data

Gravity Map Preparation

The isostatic gravity anomaly map (fig. D1) for this 

report was produced using edited gravity data from stations 

collected during the past several decades; the data were 

extracted for this study from the Defense Mapping Agency 

gravity database, available from the National Geophysical 

Data Center, Boulder, Colo. Gravity measurements were 

obtained at single stations, and contoured values were mathematically interpolated between stations. These data were 

projected using a Lambert conformal conic projection having 

a central meridian of longitude 108°W. and a base latitude of 

0°. They were gridded at a spacing of 2 km using the minimum curvature algorithm in the MINC computer program by 

Webring (1981). 

Large, broad gravity anomalies caused by regional 

geologic features can often hide small anomalies that may be 

geologically significant for mineral assessments. To focus this 

study on shallower, more local anomalies, an isostatic gravity correction was applied to the Bouguer gravity data. This 

correction was made by removing from the Bouguer gravity 

field a model of the gravity expression caused by deficiencies 

in mass (compensating mass) that support topographic loads. 

The calculation of the isostatic model used averaged digital 

topography, a crustal thickness of 30 km, a crustal density of 

2.67 g/cm3

, and a density contrast between the crust and upper 

mantle of 0.35 g/cm3

. The resulting isostatic gravity anomaly 

map (fig. D1) emphasizes anomalies produced by shallow 

sources and suppresses longer wavelength anomalies that are 

related to deep sources caused by isostatic compensation of 

mountain roots.

Gravity Map Interpretation

Gravity anomalies occur from the juxtaposition of rocks 

that have measurable density contrasts caused by structural or 

geologic features such as faults, folds, downwarps, intrusions, 

basin fill, lithologic contacts, or facies changes. The number 

and quality of gravity stations limit the accuracy of anomaly 

definition, especially in mountainous terrain where station 

spacing is often sparse. As a result, gravity stations may be 

too widely spaced to define or locate small mineral deposits, 

especially if density variations caused by a hydrothermal system are not large and the geologic setting is complex. However, on a regional scale, gravity mapping is a useful tool for 

locating structural breaks, folds, or zones of weakness, and for 

delineating intrusions. Because many of the regional structures in this area were initiated in Precambrian or Paleozoic 

times and later reactivated during the Laramide orogeny, gravity mapping can help delineate areas of long-standing crustal 

weaknesses that may have played a role in mineral formation.

Regional northeast-trending magnetic and gravity highs, 

lows, and gradients occur within and beyond the greater study 

area. The northeast-trending grain in a regional aeromagnetic 

map of Colorado has been interpreted as part of a Proterozoic 

zone or belt of en echelon shears 200 mi1

 wide that extends 

from the Grand Canyon to south of the Black Hills (Zietz and 

others, 1969). Northeast-trending shear zones and faults were 

recognized by Lovering (1935) and Tweto and Sims (1963) as 

influencing the location of Laramide intrusives and related ore 

deposits in the Colorado Mineral Belt.

Warner (1978, 1980) proposed a Middle Proterozoic 

wrench fault system of the San Andreas type that encompasses 

a zone about 100 mi wide that covers the entire forest study 

area. Warner postulated that this zone, which he named the 

Colorado lineament, can be traced from the Grand Canyon to 

Lake Superior and probably ceased as an active wrench-fault 

system about 1,700 m.y. ago. Regardless of their origin—

tilted bedding planes, shear zones, or wrench-fault systems—

these northeast-trending anomalous areas are of interest in 

mineral formation because they are probably zones of crustal 

weakness that may have provided conduits for later intrusions 

and possible mineralizing fluids.

Geophysical Studies

By Viki Bankey, Robert P. Kucks, and Kim Oshetski

 1Measurement units are given in the system in which originally reported. 

To convert miles to kilometers, multiply by 1.61. To convert feet to meters, 

multiply by 0.3048.
39o 30'

109o 

37o 45'

70

139

65

92

50

50 

82

135

133

149

62

145

141

141

90

114

Grand

Junction

Delta

Montrose

Gunnison

Crested Butte

Lake City

Uravan

Telluride

Ouray

Silverton

Aspen

Gunnison

River

Uncompahgre River

Blue MesaReservoir

0

0

Approximate outline of national forests in GMUG study area

EXPLANATION

550

Uncompahgre uplift

Uncompahgre uplift

CochetopaParkcaldera

San Luis caldera La Garita 

caldera

Lake Citycaldera

Grizzly Peakcaldera

 Colorado Mineral Belt

Colorado 

West Elk Mtns

Grand

crest of UncompahgMesa

re uplift

buried front

Paradox Valley anticline

Gypsum Valley

Grand Valley Gunnison Uplift

Piceance Basin

Elk Mountains

Sawatch Range

Cimarron fault

Red R

 fault

ocks

topographic edge of Uncompahgre Plateau

Figure D1. Isostatic gravity anomaly map of GMUG greater study area. Warm colors, high gravity values; cool colors, low gravity values.
58 Resource Potential and Geology, Grand Mesa, Uncompahgre, and Gunnison National Forests, Colorado 

Density values for rocks in the greater study area were 

compiled from reports covering this and adjacent areas, 

including Wallace and others (1988), Case and others (1992), 

and Toth and others (1993). The latter report summarizes 

eight sets of physical properties in the vicinity. For purposes 

of interpreting isostatic gravity anomalies in the greater study 

area, the following generalizations are made from the physical 

property data:

Proterozoic basement rocks in this area are heterogeneous 

in composition and in their physical properties. Proterozoic 

metamorphic and mafic igneous rocks, especially the amphibolites and gneisses, are the densest rocks in the area, varying 

from 2.70 to 2.89 g/cm3

. Proterozoic granitic and felsic rocks 

vary from average density (near 2.67 g/cm3) to slightly less 

dense (2.64 g/cm3

) than average. Proterozoic rocks as a group 

are denser than Tertiary intrusive or volcanic rocks, but their 

densities may fall within the range of values that also characterize some sedimentary rocks.

Paleozoic sedimentary rocks vary in density: porous 

sandstones and siltstones have lower densities than average 

(2.40–2.60 g/cm3

); limestones and dolomites have high densities (as much as 2.85 g/cm3

). Many Mesozoic and Tertiary 

sedimentary rocks are commonly slightly less dense (2.30–

2.50 g/cm3

) than Paleozoic sedimentary rocks.

The southernmost part of the study area lies on the north 

edge of an extensive 30–50 mGal (milligal) gravity low, called 

the Colorado Mineral Belt gravity low (Case, 1965), that 

trends southwest from the Front Range to the San Juan Mountains and cuts across many Laramide features. See the gravity 

map of Colorado (Abrams and Knepper, 1994) for a clear view 

of this gravity low. The Colorado Mineral Belt gravity low 

is attributed to a low-density, silicic, batholithic mass of Late 

Cretaceous to Tertiary age that is postulated to underlie a large 

part of the belt (Crawford, 1924; Case, 1967). An intracrustal 

origin for the gravity low, having an apex within a few thousand feet of the surface, a depth extending 40,000 ft below sea 

level, and a width averaging 15–20 mi, can be demonstrated 

by gravity models (Case, 1965; Tweto and Case, 1972; Isaacson and Smithson, 1976).

Figure D1 shows gravity highs in warm colors (yellows, 

oranges, and reds), and low values in blues, magentas, and 

purples. Regionally, gravity lows are associated with the high 

mountain areas of the West Elk Mountains, Elk Mountains, 

and the Sawatch Range in the northeast and the San Juan 

Mountains in the south. Gravity highs trend northwest-southeast across valleys in the area of figure D1 and are truncated 

by faults in many places. Although this pattern is unlike the 

more common pattern of gravity highs over mountains (cored 

by Proterozoic rocks) and gravity lows caused by low-density 

valley fill that are found elsewhere in Colorado (Abrams and 

Knepper, 1994), it is predictable from the density contrasts of 

the rocks in this area.

The low gravity values in the San Juan volcanic field 

are caused by thick, low-density pyroclastic rocks probably 

underlain by a large, concealed batholith genetically related 

to caldera formation (Plouff and Pakiser, 1972). Deep lows 

correspond to areas of mapped calderas such as the Lake City, 

San Luis, La Garita, and Cochetopa Park calderas. Additional 

geophysical interpretations of the area south of that of figure 

D1 (the San Juan National Forest) are given in McCafferty 

and others (1997).

The southwestern part of figure D1’s area is characterized by narrow, linear gravity lows that mark salt-cored (very 

low density) anticlines in the Paradox Basin. These gravity 

anomalies are distinctive indicators for these features. Anticlinal axes were inferred from these gravity lows and were added 

to mapped anticlinal axes to use as a criterion in the assessment of sediment-hosted copper.

A northwest-southeast-trending gravity gradient correlates with the inferred southwestern boundary of the late 

Paleozoic ancestral Uncompahgre uplift (Hansen, 1965) and 

primarily reflects the 16,000–20,000 ft basement structural 

relief between the Paradox Basin and the crest of the uplift. 

This gravity gradient is not spatially associated with the 

topographic edge of the present-day Uncompahgre Plateau, 

although it correlates spatially with the Ridgeway fault. Case 

and Joesting (1972) have modeled geophysical anomalies 

across this boundary and interpreted the offset between the 

topographic edge of the Uncompahgre Plateau and the gravity gradient to be the result of a change in density within the 

heterogeneous Precambrian basement. They showed that 

low-density quartz monzonite and granite predominate in the 

southwest, whereas higher density biotite gneiss, gneissic 

granodiorite, and amphibolite form the core of the Uncompahgre uplift to the northeast and are shallowly covered in Grand 

Valley between Grand Junction and Montrose. The gravity 

gradient is also steepened by the wedging-out of about 4,000 

ft thickness of low-density evaporites against the southwestern margin of the uplift. Case and others (1992) interpreted 

that the maximum gravity values indicated just west of Grand 

Junction mark the crest of the ancestral uplift. 

A gravity gradient follows the Precambrian Cimarron 

and Red Rocks faults east of Montrose. Gravity values from 

northeast of this fault to Gunnison are high, caused by shallowly buried or exposed, high-density Proterozoic basement 

rocks (mafic or biotite gneisses and granites) of the Gunnison 

uplift (Tweto, 1980). Gravity values decrease to the north in 

the West Elk volcanic field, where quartz monzonite laccoliths 

intrude sedimentary rocks, and low-density ash-flow tuffs 

and volcanic gravels predominate. Gravity values become 

even lower to the northeast, in the topographically high Elk 

Mountains and Sawatch Range. These gravity lows are the 

combined result of the postulated batholith associated with 

the Colorado Mineral Belt and shallower low-density Tertiary 

intrusive rocks, some of which are exposed in places of deepest gravity lows.

Gravity values are moderately low in the Piceance basin 

and Grand Mesa areas where sedimentary rocks, some of 

which contain low-density evaporitic rocks, predominate.

The gravity gradient between low-density rocks of 

western Grand Mesa and high-density Proterozoic rocks of the 

Uncompahgre uplift in the valley to the west follows 
Geophysical Studies 59

topography. This suggests a structural weakness that affected 

both features.

Magnetic Data

Magnetic Map Preparation

The aeromagnetic data (fig. D2) are a subset of the 

aeromagnetic compilation for the State of Colorado. The 

survey specifications, data quality, and processing methods are 

described by Oshetski and Kucks (2000). The individual grids 

were continued to 305 m (meters) above ground and merged 

into a single total field grid with a 1,000 m grid interval. 

All magnetic bodies act as secondary magnets in the 

Earth’s magnetic field and may produce positive and negative anomaly pairs (dipole anomalies). In Colorado, polarity 

effects typically show up as local lows along the north side 

of a magnetic high. In some areas, the polarity lows are too 

diffuse to be seen or are obscured by the fields of other nearby 

magnetic bodies. Polarity lows may complicate the interpretation of primary magnetic anomalies. To reduce the effects of 

polarity lows, the total field grid was reduced to the pole. The 

goal of reduction to the pole is to produce a magnetic map 

as though the area had been surveyed at the Earth’s magnetic 

north pole in order to position the anomalies closer to their 

sources. 

Aeromagnetic Map Interpretation

Aeromagnetic anomalies are caused by rocks that contain 

significant amounts of magnetic minerals (magnetite being 

the most common); these anomalies reflect variations in the 

amount and type of magnetic material and the shape and depth 

of the body of rock. In general, igneous rocks and some metamorphic rocks contain enough magnetic minerals to generate 

magnetic anomalies, whereas sedimentary and metasedimentary rocks are commonly weakly magnetic. Aeromagnetic 

anomaly maps are important tools in mapping surficial and 

buried igneous rocks. The features and patterns of aeromagnetic anomalies can also be used to delineate details of subsurface geology, including the locations of buried faults and the 

thickness of surficial sedimentary rocks.

A complicating factor in magnetic anomaly interpretation is the remanent magnetization direction of the rock, which 

may differ from the present-day magnetic field direction. If 

the remanent magnetization is sufficiently strong and in a 

different direction, the anomaly will be changed in amplitude, 

or shifted away from the source, or both. High-amplitude 

magnetic lows may indicate igneous rocks that acquired their 

magnetic properties during a period of magnetic field reversal; such magnetic lows are associated with some outcrops 

of Tertiary basaltic rocks in the San Juan volcanic field, near 

Telluride, Lake City, Ouray, and Silverton (south-central part 

of area of fig. D2). 

Aeromagnetic anomaly maps have some limitations in 

their use to locate mineral deposits. Mineral deposits without 

associated magnetite or pyrrhotite are not expected to create 

magnetic highs. Some shallow deposits associated with magnetic intrusions may be severed from that source by subsequent faulting. Other deposits may have lost their early-stage 

magnetite during subsequent hydrothermal alteration. Tertiary 

stocks that intrude magnetic Proterozoic crystalline rocks 

could create small magnetic lows or highs over the stocks or 

show no anomalies at all, depending on the relative magnetizations of both stock and surrounding rocks.

Proterozoic rocks in this area have a wide range of 

measured magnetic susceptibilities: the Proterozoic granitoid 

and gabbroic rocks are generally the most magnetic (Moss and 

Abrams, 1985). Proterozoic metamorphic rocks are generally 

moderately magnetic, although Proterozoic metasedimentary 

rocks may be relatively nonmagnetic (Daniels, 1987). 

Heterogeneous magnetite content in Proterozoic rocks 

causes many of the magnetic anomalies illustrated in figure 

D2, especially in the western part of the area, as noted by Case 

and Joesting (1972). For example, a linear string of positive 

magnetic anomalies correlates with exposed Precambrian 

granitic rocks along the southwestern margin of the Uncompahgre Plateau. Magnetic anomalies also arise from variation 

in depth to basement rocks, owing to uplifts and troughs of the 

buried basement surface. These anomalies are not significant 

for mineral assessments, except where they may indicate structures such as faults that were active in Precambrian time and 

reactivated since, and that may provide conduits for mineralizing fluids. However, gravity data provide a clearer picture of 

these structures, and gravity data were used to delineate such 

features in the assessment models (this volume).

Some Tertiary plutons are magnetic and produce conspicuous positive anomalies (Moss and Abrams, 1985; Campbell, 

1985; Daniels, 1987), but where altered, they may produce 

relative magnetic lows or plateaus in the regional magnetic 

field. Other Tertiary intrusions have low susceptibilities and 

generate no magnetic highs; they may even produce magnetic 

lows where they intrude more magnetic Proterozoic rocks 

(Moss and Abrams, 1985; Campbell and Wallace, 1986).

Each igneous and metamorphic rock type of the GMUG 

greater study area (given in Day and others, 1999, with 

descriptions and detailed location information) was spatially 

compared with the magnetic anomaly map to determine which 

units had identifiable magnetic signatures (usually appearing 

as corresponding positive anomalies of comparable amplitude). We identified six igneous units with strong magnetic 

signatures. For example, middle Tertiary intrusive rocks 

(Tmi) commonly produce positive, high-amplitude anomalies 

in the greater study area. We were then able to infer additional 

shallowly buried Tmi units from the aeromagnetic map by 

visually correlating exposed Tmi with corresponding magnetic highs. We attributed nearby magnetic highs of similar 

size and amplitude to unexposed Tmi rocks and digitized the 
Disappointment Creek Gunnison River Uncompahgre River Lake Fork Cebolla Creek Tomichi Creek Taylor River 

Blue Mesa Reservoir Dolores River Cochetopa Creek Uncompahgre uplift Uncompahgre uplift topographic edge of Uncompahgre Plateau San Juan volcanic field 39o 30'109o 

37o 45'

70

139

65

92

50

50 

82

135

133

149

62

145

141

141

90

114

Grand

Junction

Delta

Montrose

Gunnison

Crested Butte

Lake City

Uravan

Telluride

Ouray

Silverton

Aspen

0

0

Approximate outline of national forests in GMUG study area

EXPLANATION

550

Figure D2. Aeromagnetic anomaly map of GMUG greater study area. Warm colors, high magnetic intensity; cool colors, low magnetic intens
inferred locations. Similar interpretive products were prepared 

for rock units Tbb (bimodal basalt), TKi (Laramide intrusive 

rocks), Tiql (Tertiary inter-ash quartz latitic lavas), Xg (1,700 

m.y. age group granitic rocks), and Yg (1,400 m.y. age group 

granitic rocks). The map of inferred magnetic Tmi rocks was 

used in modeling the potential for molybdenum (Chapter G), 

polymetallic veins (Chapter J), and polymetallic replacement 

deposits (Chapter K). The maps for Tbb, Tki, and Tiql were 

also used in modeling polymetallic veins (Chapter J). Figure 

D3 shows the outlines of the inferred Tertiary intrusions used 

for modeling.

Aeroradiometric Data

Aeroradiometric Map Preparation

Aeroradioactivity is measured from low-flying aircraft. 

The instruments measure gamma rays emitted by isotopes 

of potassium (K), uranium (U), and thorium (Th) present in 

surficial rock and soil to about 12 in. depth. Aerial gamma-ray 

radioactivity data used in this report are from spectrometer 

surveys flown during the U.S. Department of Energy National 

Uranium Resource Evaluation (NURE) program (≈1974–

1983). NURE surveys that include parts of the forest are those 

for the Leadville (Geometrics, 1979), Montrose (Geometrics, 1979), Moab (Geometrics, 1979), Cortez (Aero Service 

Division, 1979a), Durango (Aero Service Division, 1979b), 

and Grand Junction (Geodata Int., 1981) 1°×2° quadrangles. 

Data for these quadrangles were merged for the conterminous 

United States (Phillips and others, 1993; Duval and others, 

1995), and subsets covering the greater study area were created for this report. The uranium data are shown in figure D4. 

Flightline spacing for these quadrangles is 3 mi east-west and 

12 mi north-south.

The near-surface distribution of potassium, uranium, and 

thorium generally reflects bedrock lithology and modifications due to weathering, erosion, transportation, ground-water 

movement, and hydrothermal alteration. Common rock 

types readily discriminated by aeroradioactivity measurements include (1) more radioactive (greater concentrations of 

radioactive minerals) felsic igneous rocks, arkosic sandstones, 

and most shales and (2) less radioactive (lesser concentrations) 

mafic igneous rocks, (clean) quartzose sandstones, and most 

limestones. Specific rock formations were described in the 

original NURE survey reports, listed in the previous paragraph, as having higher uranium values; these are summarized 

in table D1.

Aeroradiometric Map Interpretation

The near-surface distribution patterns of potassium, 

uranium, and thorium as displayed by aeroradioactivity maps 

of the greater study area are similar, resulting from common 

rock-type associations for these elements. Mancos Shale, 

for example, shows higher radioelement values where it is 

exposed, especially in the western part of the greater study 

area. All three data sets have a distinct northwest-trending gradient that separates high radioelement values to the 

northeast from lower values to the southwest (fig. D4). The 

boundary corresponds to the Gunnison River in Grand Valley between Grand Junction and Delta and reflects the high 

radioelement values found in Mancos Shale exposures west of 

the river. Northeast of Grand Valley, high radioelement values 

are associated with Precambrian and Tertiary igneous rocks, 

although a spatial, formation-by-formation evaluation did not 

uncover a direct correspondence that could be used to map 

certain intrusive rocks as the magnetic data could.

The grid interval for the uranium anomaly map shown 

in figure D4 is 2 km. At this grid interval, these data cannot 

accurately display individual uranium spikes that are present 

in the original flightline data. Even in the Uravan mineral 

belt, a well-known uranium mineralized area, the data are not 

diagnostic. The original NURE reports, however, provide a 

detailed evaluation of individual uranium anomalies. 

For the uranium-vanadium deposit model (Chapter H), a 

derivative calculation using thorium proved more valuable in 

determining favorable terrane than did the uranium data alone. 

Thorium generally has a more consistent distribution pattern than potassium or uranium, because thorium is the least 

mobile of these elements. We selected areas where the uranium:thorium ratio is greater than 1 standard deviation above 

its mean and where thorium is less than 1 standard deviation 

below its mean (Aero Service Division, 1979a, b). The results 

clearly delineate the Uravan mineralized area, among others, 

and are shown in Spanski and Bankey, this volume, Chapter H, 

figure H2.

Uranium mines and mineralized areas are further discussed in the chapter on sandstone-hosted uranium deposits 

(this volume, Chapter H).

Geophysical Studies 61
39o 30'109o 

37o 45'

70

139

65

92

50

50 

82

135

133

149

62

145

141

141

90

114

Grand

Junction

Delta

Montrose

Gunnison

Crested Butte

Lake City

Uravan

Telluride

Ouray

Silverton

Aspen

Disappointment Creek

Gunnison River

Uncompahgre River Lake Fork Cebolla

Creek

Tomichi Creek

Taylor River

Blue MesaReservoir

CochetopaCreek

Dolores River

0

0

Approximate outline of national forests in GMUG study area

EXPLANATION

550

Figure D3. Outline of Tertiary plutons inferred from aeromagnetic data. Red, Tiql; green, Tbb; light blue or gray, Tmi; dark blue, Tki.
Disappointment Creek

Gunnison River

Uncompahgre River Lake Fork Cebolla

Creek

Tomichi Creek

Taylor River

Blue MesaReservoir

CochetopaCreek

Dolores River

39o 30'

109o 

37o 45'

70

139

65

92

50

50 

82

135

133

149

62

145

141

141

90

114

Grand

Junction

Delta

Montrose

Gunnison

Crested Butte

Lake City

Uravan

Telluride

Ouray

Silverton

Aspen

0

0

Approximate outline of national forests in GMUG study area

EXPLANATION

550

Figure D4. Uranium values of GMUG greater study area. Cool colors, lower values; warm colors, higher values of uranium. Areas in white in 

have no data.
Table D1. Summary of stratigraphic units containing anomalous uranium as described in NURE reports.

[See Day and others (1999) for condensed information on these units]

Age Stratigraphic unit Unit symbol as Uranium content

 used in NURE reports

Quaternary (Q) Alluvium.................. Qa, Qg Especially overlying Tertiary Uinta Formation.

Tertiary (T) Intrusives, volcanics Ti, Tst, Thu, Tv, 

 such as Sunshine Tki

 Peak Tuff, Huerto Fm.

Uinta Formation......... Tu Known host for uranium.

Green River Formation.. Tg, Tgp, Tgl Known host for uranium.

Cretaceous (K) Mesaverde Group......... Kmv, Kmvl, Subeconomic in Colorado, but produces in

 Kmvu, Kh Wyoming.

Mancos Shale............ Km, Kmgs, Areally large but subeconomic uranium deposits.

 Kmu, Kml, Kfd

Dakota Sandstone...... Kd, KJdm, Kdb, Subeconomic uranium deposits.

 KJdj, KJdw, 

 KJde

Burro Canyon Formation Kbc, Kdb Subeconomic uranium deposits.

Jurassic (J) Morrison Formation..... Jm, Jms, Jmb, Major uranium deposits, especially in the Salt 

 Jmj, Jmw, Wash, Brushy Basin, and Recapture Members.

 Jmwe, Jme,

 KJdm, KJdj, 

 KJdw, KJde

Summerville Formation Jse Subeconomic uranium deposits.

Entrada Sandstone...... Je, Jwe, Jme, Subeconomic uranium deposits.

 KJde, Jmwe,

 Jse, Jmc,

 Jmd

Wanakah Sandstone.... Jwe, Jmw, Jmwe, Large, low-grade deposits, source rock for

 KJdw, Jmc, Rifle mines.

 Jmd

Navajo Sandstone...... Jn Subeconomic uranium deposits.

Kayenta Formation.... k, kw, kwc Subeconomic uranium deposits.

Triassic () Wingate Sandstone.... w, kw, wc, Subeconomic uranium deposits.

 kwc

Chinle Formation...... c, wc, Major uranium deposits, especially in the

 kwc, Jmc, conglomerate members and Moss Back

 Pcs Member.

Moenkopi Formation.. m Production noted.

Permian (P) Cutler Formation....... Pc, Pdc Subeconomic uranium deposits, especially

 in arkosic member.

Permian and Maroon Formation..... Pm, Pwm Subeconomic uranium deposits.

Pennsylvanian Rico Formation......... Prm Subeconomic uranium deposits.

 (P) 

Pennsylvanian Belden Formation....... b, mb, mbe Host rock for deposits at Marshall Pass.

 () Hermosa Formation..... h, hu, hp, Especially associated with potassium salts.

 Prm

Ordovician Harding Sandstone...... MOr Host rock for Little Indian No. 36 mine.

Precambrian Undivided granite, Yg Host rock for structurally controlled deposits

 especially fault brec- in Cochetopa Creek, Powderhorn district,

 cia, shear zones, and and Frying Pan claims.

 fracture-controlled

 subeconomic uranium

 deposits.

64 Resource Potential and Geology, Grand Mesa, Uncompahgre, and Gunnison National Forests, Colorado 
References Cited

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gradient, and terrain maps of Colorado: U.S. Geological 

Survey Geophysical Investigations Map GP-1009, scale 1:

1,000,000.

Aero Service Division, 1979a, Airborne gamma-ray spectrometer and magnetometer survey, Cortez quadrangle (Colorado, Utah); Final report, 2 Volumes: U.S. Department of 

Energy, Grand Junction, Colo., GJBX-144(79), variously 

paginated.

Aero Service Division, 1979b, Airborne gamma-ray spectrometer and magnetometer survey, Durango quadrangle (Colorado); Final report, 2 Volumes: U.S. Department of Energy, 

Grand Junction, Colo., GJBX-143(79), variously paginated.

Campbell, D.L., 1985, Gravity and aeromagnetic maps of 

the Maroon Bells–Snowmass Wilderness and Additions, 

Gunnison and Pitkin Counties, Colorado: U.S. Geological 

Survey Miscellaneous Field Studies Map MF-1647-B, scale 

1:100,000.

Campbell, D.L., and Wallace, A.R., 1986, Aeromagnetic map 

of the Holy Cross Wilderness Area, Eagle, Lake, and Pitkin 

Counties, Colorado: U.S. Geological Survey Miscellaneous 

Field Studies Map MF-1841-B, scale 1:100,000.

Case, J.E., 1965, Gravitational evidence for a batholithic mass 

of low density along a segment of the Colorado Mineral 

Belt, in Abstracts for 1964: Geological Society of America 

Special Paper 82, p. 26.

Case, J.E., 1967, Geophysical ore guides along the Colorado 

Mineral Belt: U.S. Geological Survey Open-File Report 

67-39, 13 p.

Case, J.E., and Joesting, H.R., 1972, Regional geophysical 

investigations in the central Colorado Plateau: U.S. Geological Survey Professional Paper 736, 31 p.

Case, J.E., Morin, R.L., and Dickerson, R.P., 1992, Map showing interpretation of geophysical anomalies of the northwestern Uncompahgre Uplift and vicinity, Grand County, 

Utah, and Mesa County, Colorado: U.S. Geological Survey 

Geophysical Investigations Map GP-997, scale 1:250,000.

Crawford, R.D., 1924, A contribution to the igneous geology 

of central Colorado: American Journal of Science, v. 7, p. 

365–388.

Daniels, J.J., 1987, Geophysical appraisal, in Snyder, G.L., 

Patten, L.L., and Daniels, J.J., Mineral resources of the 

Mount Zirkel Wilderness and northern Park Range vicinity, 

Jackson and Routt Counties, Colorado: U.S. Geological 

Survey Bulletin 1554, p. 215–221.

Day, W.C., Green, G.N., Knepper, D.H., Jr., and Phillips, R.C., 

1999, Spatial geologic data model for the Gunnison, Grand 

Mesa, Uncompahgre National Forests mineral assessment area, southwestern Colorado and digital data for the 

Leadville, Montrose, Durango, and the Colorado parts of 

the Grand Junction, Moab, and Cortez 1° × 2° geologic 

maps: U.S. Geological Survey Open-File Report 99-427, 

CD-ROM.

Duval, J.S., Jones, W.J., Riggle, F.R., and Pitkin, J.A., 1995, 

Potassium, uranium, and thorium aerial gamma-ray maps 

of the conterminous United States: U.S. Geological 

Survey Geophysical Investigations Map GP-1010, scale 1:

2,500,000, 7 p.

Geodata International, 1981, Aerial radiometric and magnetic 

survey, Grand Junction national topographic map, Colorado 

and Utah: U.S. Department of Energy, Grand Junction, 

Colo., GJBX-112(81), variously paginated.

Geometrics, 1979, Aerial gamma ray and magnetic survey, 

Uncompahgre Uplift Project, Salina, Utah; Moab, Utah and 

Colorado; Montrose and Leadville, Colorado quadrangles, 

Final Report, 2 Volumes: U.S. Department of Energy, 

Grand Junction, Colo., GJBX-95(79), variously paginated.

Hansen, W.R., 1965, The Black Canyon of the Gunnison 

[Colorado], today and yesterday: U.S. Geological Survey 

Bulletin 1191, 76 p.

Isaacson, L.B., and Smithson, S.B., 1976, Gravity anomalies 

and granite emplacement in west-central Colorado: Geological Society of America Bulletin, v. 87, no. 1, p. 22–28.

Lovering, T.S., 1935, Geology and ore deposits of the Montezuma quadrangle, Colorado: U.S. Geological Survey 

Professional Paper 178, 119 p.

McCafferty, A.E., Pitkin, J.A., and Ager, C.M., 1997, Geophysical studies of the San Juan National Forest, Colorado, 

in Van Loenen, R.E., and Gibbons, A.B., eds., Mineral 

resource potential and geology of the San Juan National 

Forest, Colorado, with a section on Salable minerals by 

A.G. Raby and J.S. Dersch: U.S. Geological Survey Bulletin 2127, p. 49–60.

Moss, C.K., and Abrams, Gerda, 1985, Geophysical maps 

of the Vasquez Peak Wilderness Study Area and the Williams Fork and St. Louis Peak Roadless Areas, Clear Creek, 

Grand, and Summit Counties, Colorado: U.S. Geological 

Survey Miscellaneous Field Studies Map MF-1588-D, scale 

1:50,000.

Oshetski, K.C., and Kucks, R.P., 2000, Colorado aeromagnetic and gravity maps and data; a web site for distribution of data: U.S. Geological Survey Open-File Report 

00-42; URL: http://pubs.usgs.gov/of/2000/ofr-00-0042/

colorado.htm, computer file.

References Cited 65
66 Resource Potential and Geology, Grand Mesa, Uncompahgre, and Gunnison National Forests, Colorado 

Phillips, J.D., Duval, J.S., and Ambroziak, R.A., 1993, 

National geophysical data grids—Gamma-ray, gravity, 

magnetic, and topographic data for the conterminous United 

States: U.S. Geological Survey Digital Data Series DDS-9, 

CD-ROM.

Plouff, Donald, and Pakiser, L.C., 1972, Gravity study of 

the San Juan Mountains, Colorado, in Geological Survey 

research 1972: U.S. Geological Survey Professional Paper 

800-B, p. B183–B190.

Toth, M.I., Wilson, A.B., Cookro, T.M., Bankey, Viki, Lee, 

G.K., and Case, J.E., 1993, Mineral resource potential and 

geology of the White River National Forest and the Dillon 

Ranger District of the Arapaho National Forest, Colorado, 

with a section on Salable commodities by J.S. Dersch: U.S. 

Geological Survey Bulletin 2035, 117 p.

Tweto, Ogden, 1980, Precambrian geology of Colorado, in

Kent, H.C., and Porter, K.W., eds., Colorado geology: 

Rocky Mountain Association of Geologists Guidebook, 

1980 Symposium, Denver, Colo., p. 37–46.

Tweto, Ogden, and Case, J.E., 1972, Gravity and magnetic 

features as related to geology in the Leadville 30-minute 

quadrangle, Colorado: U.S. Geological Survey Professional 

Paper 726-C, p. C1–C31.

Tweto, Ogden, and Sims, P.K., 1963, Precambrian ancestry of 

the Colorado Mineral Belt: Geological Society of America 

Bulletin, v. 74, no. 8, p. 991–1014.

Wallace, A.R., Ludington, Steve, Lovering, T.G., Campbell, 

D.L., Case, J.E., Grauch, V.J.S., and Knepper, D.H., Jr., 

1988, Leadville 1° × 2° quadrangle, Colorado—A preassessment: U.S. Geological Survey Open-File Report 

88-74, 57 p.

Warner, L.A., 1978, The Colorado lineament; A middle 

Precambrian wrench fault system: Geological Society of 

America Bulletin, v. 89, no. 2, p. 161–171.

Warner, L.A., 1980, The Colorado lineament, in Kent, H.C., 

and Porter, K.W., eds., Colorado geology: Rocky Mountain 

Association of Geologists Guidebook, 1980 Symposium, 

Denver, Colo., p. 11–21.

Webring, M.W., 1981, MINC—A gridding program based on 

minimum curvature: U.S. Geological Survey Open-File 

Report 81-1224, 41 p.

Zietz, Isidore, Bateman, P.C., Case, J.E., Crittenden, M.D., Jr., 

Griscom, Andrew, King, E.R., Roberts, R.J., and Lorentzen, 

G.R., 1969, Aeromagnetic investigation of crustal structure 

for a strip across the Western United States: Geological 

Society of America Bulletin, v. 80, no. 9, p. 1703–1714.
U.S. Department of the Interior

U.S. Geological Survey

Distribution of Mines and Mineralized Areas

By Anna B. Wilson and Gregory T. Spanski

Chapter E of

Resource Potential and Geology of the Grand Mesa, Uncompahgre, and 

Gunnison (GMUG) National Forests and Vicinity, Colorado

Edited by Viki Bankey

U.S. Geological Survey Bulletin 2213– E
Contents

Abstract........................................................................................................................................................ 70

Introduction ................................................................................................................................................. 70

Mineralized Areas ...................................................................................................................................... 70

Ruby ................................................................................................................................................. 72

Mount Bellview .................................................................................................................................. 73

Elk Mountains..................................................................................................................................... 73

Dorchester .......................................................................................................................................... 73

Forest Hill ............................................................................................................................................ 73

Spring Creek ....................................................................................................................................... 73

Tincup/Cumberland Pass.................................................................................................................. 74

Gold Brick............................................................................................................................................ 74

Quartz Creek Pegmatite.................................................................................................................... 74

Box Canyon......................................................................................................................................... 74

Whitepine............................................................................................................................................ 74

Marshall Pass..................................................................................................................................... 75

Cochetopa North................................................................................................................................ 75

Green Mountain ................................................................................................................................. 75

Cochetopa Central............................................................................................................................. 76

Cochetopa Tantalite........................................................................................................................... 76

Cochetopa South ............................................................................................................................... 76

Wolf Creek........................................................................................................................................... 76

Beaver Creek...................................................................................................................................... 76

Goose Creek ....................................................................................................................................... 76

Vulcan ................................................................................................................................................. 76

Vulcan South....................................................................................................................................... 77

Powderhorn ........................................................................................................................................ 77

Bondholder ......................................................................................................................................... 77

Carson ................................................................................................................................................. 77

Red Mountain (East).......................................................................................................................... 78

Lake City .............................................................................................................................................. 78

Golden Wonder .................................................................................................................................. 78

Henson Creek ..................................................................................................................................... 78

Uncompahgre Peak........................................................................................................................... 79

Cimarron.............................................................................................................................................. 79

Ouray ................................................................................................................................................. 79

San Juan ............................................................................................................................................. 80

Wilson Peaks...................................................................................................................................... 80

Western Uranium............................................................................................................................... 80

Sinbad ................................................................................................................................................. 80

Sunrise/Morning Glory...................................................................................................................... 81

Cashin (La Sal Creek) ........................................................................................................................ 81

References Cited ........................................................................................................................................ 81
Figure

E1. Map showing metallic mineralized areas within GMUG study area .......................................... 71

Table

E1. Metallic mineralized areas and deposit types in GMUG forests and Uravan area.................. 72
Abstract

In order to assess the mineral resource potential of the 

GMUG study area, we outlined 38 mineralized areas. A mineralized area encloses a geographic area that is defined by the 

presence of mines, prospects, and (or) mineralized occurrences 

that belong to a single deposit type or to a group of genetically 

related deposit types in a distinct geologic setting. A mineralized area may include an entire district or portions of several 

mining districts. 

Introduction

The term “mining district” as used in Colorado is an artifact of the State’s days as a territory prior to statehood in 1876. 

During this period, mining districts were created by the miners 

in a mining camp acting as an association with a charter and 

by-laws. Such an association provided a form of self-rule 

intended to bring order to mining-related activity. Boundaries 

were ill defined and subject to change at the discretion of the 

association. The discovery of a new deposit could result in the 

creation of a new district with a new charter or, if the discovery were near an existing district and the claimant agreed to 

honor the charter, inclusion within the existing district. With 

statehood and the establishment of formal county governments 

in Colorado, the need for self-rule that the mining district 

provided was no longer required. Claims were recorded with 

the county, and laws were set by the State. The mining district 

nomenclature, however, endured. Claimants continued to cite 

district names in claim location descriptions, a practice that 

continues to this day. However, Henderson (1926, p. 62) noted 

that “many of the names used represent nothing more than 

the guess or whim of the locator, and many of the commonly 

used names of local districts are carried miles away and across 

county lines.” Using the mining district as a guide to location 

or as an indicator of the types of deposits being developed and 

commodities produced became unreliable. 

To deal with these geographic ambiguities, we have 

introduced the concept of the mineralized area. A mineralized 

area encloses a geographic area that is defined by the presence 

of mines, prospects, or mineralized occurrences that belong to 

a single deposit type or a group of genetically related deposit 

types. A mineralized area may include an entire district 

or portions of several mining districts. Where practical, a 

mineralized area is given the name of one of the more prominent mining districts or mining settlements that is within its 

bounds. Where this is not feasible, a new name is assigned 

that is geographically descriptive, or the name of a productive 

or well-known mine located in the area is used. We have not 

attempted to apply mineralized areas to the placer mining districts as defined by Parker (1961) and have not included placer 

districts in the following discussion of mineralized areas. 

Many of the data used to outline the mineralized areas 

have been extracted from site records for mines, prospects, 

and occurrences contained in U.S. Geological Survey (USGS) 

and U.S. Bureau of Mines (USBM) data files. For this project, 

A.B. Wilson, M.J. Crane, and M.D. Woodard (U.S. Geological Survey) verified as many of these data records as possible 

and added many more sites from information extracted from 

references and maps during the verification process (Wilson 

and others, 2000). Locations of approximately 1,300 sites are 

believed to be accurate to within about 1⁄2 mi of their actual 

ground position. In addition, about 1,000 sites could not be 

verified from references but could be generally located using 

data (such as Township and Range or descriptive information) 

in the database records: these sites are believed to be accurate 

to within about 1 mi. About 200 of the sites in the data files 

could not be plotted owing to insufficient location data. 

Using the digital compilation of the published 1:250,000 

geologic maps by Day and others (1999) as a base, A.B. Wilson and G.T. Spanski delineated 40 mineralized areas in the 

GMUG restricted study area (fig. E1, table E1) utilizing data 

about individual mineral occurrences and regional information 

on bedrock geology, geologic setting, geochemistry, and major 

structural features. Brief descriptions for each of the mineralized areas follow, arranged in a generally clockwise direction 

with respect to figure E1. 

Mineralized Areas

The following is a brief description of the mineralized 

areas in the GMUG restricted study area, Colorado (fig. E1). 

This description includes a review of the historical names of 

the mining districts, mine names, deposit types, general geologic setting, and available production information. This background is vital for understanding the metallogeny of the region 

and also provides a solid foundation for studying the effects of 

Distribution of Mines and Mineralized Areas

By Anna B. Wilson and Gregory T. Spanski
CimarronUncompahgre Peak Henson Ck

Western

Uranium

Sinbad

Sunrise–

Morning Glory

Wilson Peaks

San Juan

Ouray

Lake City

Carson

Red

Mountain

(E)

Golden Wonder

Bondholder

Powderhorn

Goose

Ck

Wolf

Ck

Vulcan

Beaver

Ck

Vulcan

 So

Cochetopa

No

Green Mt

Cochetopa

Central

Coc

Tan

Cochetopa 

Gold

Brick

Spring Ck

Forest Hill

Dorchester Elk Mts

Ruby

Cashin

(La Sal Ck)

Mount 

Bellview

39o 30'

109o 

37o 45'

70

139

65

92

50

50 

82

135

133

149

62

145

141

141

90

114

Grand

Junction

Delta

Montrose

Gunnison

Crested Butte

Lake City

Uravan

Telluride

Ouray

Silverton

Aspen

Disappointment Creek

Gunnison River

Uncompahgre River Lake Fork Cebolla

Creek

Tomichi Creek

Taylor River

Blue MesaReservoir

CochetopaCreek

Dolores River

0

0

550

Approximate outline of national forests in GMUG study areaApproximate outline of restricted study area

EXPLANATION

Figure E1. Metallic mineralized areas (shaded areas) shown within GMUG greater study area.
72 Resource Potential and Geology, Grand Mesa, Uncompahgre, and Gunnison National Forests, Colorado 

past mining on the environment. Additional descriptions and 

references can be found in Wilson and others (2000). 

Ruby

The Ruby area is nearly synonymous with the Irwin, 

Ruby, Mount Emmons, and Redwell Basin mining districts. It 

is in the Oh-Be-Joyful (Gaskill and others, 1967), Mt. Axtell 

(Gaskill and others, 1987), and Marcellina Mountain (Gaskill 

and Godwin, 1966) quadrangles. This area hosts polymetallic veins and porphyry Climax-type molybdenum deposits. 

Well-known mines in the area are the Daisey, Keystone, 

and Standard (Micawber). This area also includes the welldocumented but never mined Mount Emmons molybdenum 

deposit. 

In the Ruby area, Oligocene felsic sills, dikes, and stocks 

intrude Cretaceous to Tertiary sandstones and shales. 

Table E1. Metallic mineralized areas and deposit types in GMUG forests and 

Uravan area.

Area name Deposit type

Beaver Creek Kuroko massive sulfide

Bondholder Polymetallic vein

Box Canyon Low-sulfide gold

Carson Polymetallic vein

Cashin (La Sal Creek) Redbed copper

Cimarron Polymetallic vein

Cochetopa central Vein U

Cochetopa north Low-sulfide gold

Cochetopa south Vein U

Cochetopa tantalite Pegmatite

Dorchester Polymetallic vein and replacement

Elk Mountains Polymetallic vein and replacement

Forest Hill/Italian Mountain unknown

Gold Brick Polymetallic vein

Golden Wonder Hot spring

Goose Creek Low-sulfide gold

Green Mountain Kuroko massive sulfide

Henson Creek Polymetallic vein

Lake City 23 m.y. barite precious metal

Marshall Pass Vein U

Mt. Bellview Cu-Mo porphyry

Ouray Polymetallic vein and replacement

Powderhorn Th veins

Quartz Creek Pegmatite Pegmatite

Red Mountain (east) Cu-Mo porphyry

Ruby Polymetallic vein

San Juan Polymetallic vein and replacement

Sinbad Redbed copper

Spring Creek Polymetallic replacement

Sunrise/Morning Glory Polymetallic veins and replacement

Tincup/Cumberland Pass Polymetallic veins and replacement

Uncompahgre Peak Vein U

Vulcan Kuroko massive sulfide

Vulcan south Kuroko massive sulfide

Western Uranium Uranium

Whitepine Polymetallic vein and replacement

Wilson Peaks Polymetallic vein

Wolf Creek Low-sulfide gold
Distribution of Mines and Mineralized Areas 73

 Especially in the southern part of the area, brilliant, redorange-yellow pyritic altered rock is adjacent to intrusions 

(Red Lady Basin, Redwell Basin). 

Silver mining in the Irwin part of the district began in 

1874 when it was “still part of the Ute Indian Reservation and 

effectively ended by 1890. The Ruby Chief and Bullion King 

mines, followed by the Forest Queen and Ruby King mines 

were the district’s early principal producers” (Ellis, 1983, p. 

4). The first two mines were obliterated by snowslides in 1882 

and 1884, respectively (Ellis, 1983, p. 4). Only the Forest 

Queen continued to operate intermittently, “reaching $1 million in production by 1915” (Socolow, 1955, p. 52–53).

Base-metal and silver ores were mined intermittently 

from fissure veins on the flanks of Mount Emmons. The 

largest producers were the Daisey, Keystone, and Standard 

(Micawber) mines. Two major molybdenum deposits were 

discovered in the 1970’s in the Mount Emmons-Redwell Basin 

areas (Thomas and Galey, 1982). Neither has been developed.

Mount Bellview

The Mount Bellview area, in the southeastern part of the 

Snowmass Mountain quadrangle (Mutschler, 1970), surrounds 

a zoned granodiorite to quartz monzonite intrusive complex. 

The intrusions are hydrothermally altered. A 1,200-m diameter hornfels aureole in Mancos Shale host rock surrounds the 

intrusive complex, and local quartz-molybdenite veinlets are 

also present (Lynch and others, 1985). The only mine in the 

area, the Silver Spruce, consists of three adits on a vein along 

the intrusion-Mancos Shale contact. The mine produced a 

small amount of silver and lead ore in 1933–1934, and molybdenum is present in most samples (Weisner and Bieniewski, 

1984). Although probably not of sufficient grade and tonnage 

to rank as a “deposit,” this area is shown as a mineralized area 

because of previous exploration interest and indications such 

as surface alteration and geochemistry which suggest that a 

mineralizing event took place.

Elk Mountains

The Elk Mountains mineralized area, about 20 mi north 

of Crested Butte, includes the historic Elk Mountain district 

and the town of Gothic. It is primarily on the Gothic quadrangle (Gaskill and others, 1991) and extends into the southern 

part of the Maroon Bells quadrangle (Bryant, 1969). Other 

workers (for example, Plumlee and others, 1995; Streufert and 

Cappa, 1994) have depicted the Elk Mountain mining district 

as three distinct areas in close proximity. We have chosen to 

consolidate the entire area into one mineralized area containing several small polymetallic vein and replacement and skarn 

deposits. The best known of these deposits is the Sylvanite 

mine. Mineralized veins in the area contain sphalerite, galena, 

and chalcopyrite. Gold and silver, in unknown forms, were 

also present. “Mineralization is widespread but the veins are 

small and irregular” (Vanderwilt, 1947, p. 101). The area 

is in Pennsylvanian to Upper Cretaceous sedimentary rocks 

intruded by the Oligocene White Rock pluton (Gaskill and 

others, 1991; Bryant, 1969). 

Dorchester

The Dorchester mineralized area is in the Pearl Pass and 

Italian Creek 71⁄2-minute quadrangles. It includes most of the 

historic Dorchester mining district (including the Taylor River 

and Taylor Park areas), about which extremely little is known 

geologically. The host rocks are Paleozoic sedimentary rocks 

overlying 1,700 Ma granitic rocks that were intruded by 33.9 

Ma tonalite of the Italian Mountain Intrusive Suite (Fridrich 

and others, 1998). Only a few small mines, the Hope, Bull 

Domingo, Clara, Star, and Ender, are mentioned in the literature (Garrett, 1950; Harrer and Tesch, 1959; Prather, 1961; 

Slebir, 1957). These mines are presumed to be polymetallic 

vein and replacement deposits. 

Forest Hill

The Forest Hill area includes a small part of the historic, 

but poorly defined Taylor Park mining district (Vanderwilt, 

1947, p. 107) in the Italian Creek 71⁄2-minute quadrangle. 

The only known mines are the Forest Hill and Paymaster, 

which are both polymetallic replacement and polymetallic 

vein deposits (Wilson and others, 2000). The Paymaster is at 

the southern margin of the Grizzly Peak caldera at the fault 

contact of the middle rhyolite subunit of the Oligocene Grizzly 

Peak Tuff inside the caldera with Early Proterozoic metasedimentary gneiss intruding the granite of Henry Mountain 

surrounding the caldera (see Fridrich and others, 1998). The 

Forest Hill mine is in these same Early Proterozoic rocks outside the caldera margin. Granite of Henry Mountain and the 

rhyolite are both iron-stained and cut by quartz-pyrite veinlets. A small Eocene felsic dike of Winfield Peak and Middle 

Mountain is present in the granite between the two mines 

(Fridrich and others, 1998).

From 1932 to 1945, as many as three lode mines were 

operating in the Taylor Park district (Vanderwilt, 1947). The 

Forest Hill and Paymaster are assumed to be two of these. 

However, because the reported production figures were 

combined with the Tincup district, only 19 oz gold, 14,726 

oz silver, 2,500 lb copper, 454,900 lb lead, and 24,400 lb zinc 

is directly attributable to Taylor Park during this time frame 

(Vanderwilt, 1947, p. 108).

Spring Creek

Spring Creek is a small area in the Matchless Mountain 

71⁄2-minute quadrangle that includes polymetallic replacement 

and replacement manganese deposits at the Doctor and Barium 

Maggie mines. Paleozoic (from Cambrian to Pennsylvanian) 

sedimentary rocks form a narrow south-trending “peninsula” 
74 Resource Potential and Geology, Grand Mesa, Uncompahgre, and Gunnison National Forests, Colorado 

in the mineralized area adjacent to 1,700 Ma granitic rocks on 

the east and west (Tweto and others, 1976; Day and others, 

1999). North-trending faults are mapped in the peninsula. 

Oligocene intrusive rocks are exposed to the northeast and 

may also be buried in the vicinity of the mine workings. 

The only known significant mine in the district is the 

Doctor mine, which produced an unknown amount of “silverbearing lead carbonate in 1880 and 1890 and at least 17,000 

tons zinc carbonate in 1917 and 1918” (Vanderwilt, 1947, p. 

106). “The last recorded production was 641 tons of sorted 

zinc carbonate from the dump in 1937 and 1938 that yielded 

203,000 pounds of zinc and 25,900 pounds of lead” (Vanderwilt, 1947, p. 107).

Tincup/Cumberland Pass

The Tincup/Cumberland Pass mineralized area includes 

most of the historic Tincup and Quartz Creek mining districts 

on the north and south flanks of Cumberland Pass on the Tincup, Cumberland Pass, and Fairview Peak 71⁄2-minute quadrangles. 1,700 Ma granitic rocks are overlain by Paleozoic 

(Cambrian to Pennsylvanian) sedimentary rocks (Tweto and 

others, 1976; Day and others, 1999). Oligocene (38–26 Ma) 

intrusive rocks are exposed throughout the area which is on 

the west margin of the Mount Aetna volcanic area at the south 

end of the Mount Princeton batholith (Toulmin and Hammarstrom, 1990). 

Much of the area contains polymetallic replacement 

deposits in the Paleozoic carbonates. In the southern part of 

the mineralized area, there are base and precious metals in 

quartz veins in Precambrian rocks (Wilson and others, 2000; 

USGS, 1999a; Dings and Robinson, 1957). Tungsten-molybdenum veins in the Cumberland Pass area were explored in 

the 1970’s (USGS, 1999a). Graphite deposits occur at the 

southeast edge of the area (Dings and Robinson, 1957). 

Total production credited to the historic Tincup mining 

district from 1901 to 1935 was 298 oz gold, 26,446 oz silver, 

177 lb copper, and 153,820 lb of lead (Vanderwilt, 1947, p. 

109). Half the tonnage came from one mine, and the remainder from seven mines. In 1932 and 1933 the district produced 

a “small amount” of ore. Production from 1938 to 1941, 

and possibly 1934–1937, was combined with Taylor Park 

and could account for as much as 82 oz gold, 7,164 oz silver, 

3,000 lb copper, 152,700 lb lead, 115,000 lb zinc (Vanderwilt, 

1947, p. 108) included in the figures credited to Taylor Park. 

From 1934 to 1943, as many as three lode mines in the historic 

Quartz Creek district produced 186 oz gold, 3,781 oz silver, 

150 lb copper, and 13,560 lb lead (Vanderwilt, 1947, p. 104). 

Gold Brick

The Gold Brick area includes only the northernmost part 

of the historic Gold Brick mining district. This area is on the 

Fairview Peak and Pitkin quadrangles and includes polymetallic vein deposits at the Carter, Raymond, Sandy Hook, 

Chronicle, Gold Links, and Grand Prize mines (Crawford 

and Worcester, 1916; Hill, 1909; Wilson and others, 2000). 

Almost the entire area is underlain by ≈1,700 Ma granitic 

rocks and interlayered felsic and hornblende gneiss (Tweto 

and others, 1976; Day and others, 1999). 

The principal ore is gold-silver-lead in veins in Precambrian granite and gneiss. In the 4 × 1 mi productive zone, 

numerous mines have produced chiefly gold with some silver, 

lead, and copper. The ore is low tonnage, but high grade, 

and all the underground workings are only a few hundred 

feet beneath the surface, except for the Carter mine, which 

extends to a depth of 1,500 ft (Crawford and Worcester, 1916). 

“Nearby sedimentary formations *** have not been productive” (Vanderwilt, 1947, p. 103). From 1932 to 1942, between 

3 and 13 lode mines produced 69,566 tons ore, which yielded 

16,395 oz gold, 45,657 oz silver, 2,350 lb copper, and 218,990 

lb lead (Vanderwilt, 1947, p. 103).

Quartz Creek Pegmatite

Although it is not a metallic mineralized area, Quartz 

Creek Pegmatite is shown on the map (fig. E1) because the 

numerous mines indicate an historical mining interest in the 

area. The area overlaps the Parlin and Pitkin quadrangles and 

includes the Brown Derby mine, a pegmatite deposit known 

for its lithium content and mineral specimens of cleavelandite 

and lepidolite (Staatz and Trites, 1955). 

Box Canyon

Box Canyon is an ill-defined area on the Whitepine 

and Pitkin quadrangles in the vicinity of the vaguely located 

Independence and Campbird mines (not to be confused with 

the Camp Bird mine in the San Juan Mountains) and prospects 

in the Precambrian rocks. Precambrian (≈1,700 Ma) granitic 

rocks and biotitic gneisses and migmatite are unconformably 

overlain by Paleozoic units and Upper Cretaceous Mancos 

Shale (Tweto and others, 1976; Day and others, 1999). 

As of 1909, the Independence had been inactive for 

several years, as the deposit was entirely exhausted (Hill, 

1909, p. 38). Although “considerable production [was] 

claimed for the early years,” 573 tons of ore yielded 69 oz of 

gold and 10 oz silver in 1932, 1938, and 1939 (Vanderwilt, 

1947, p. 98).

Whitepine

The Whitepine, or Tomichi, area overlaps the Whitepine 

and Garfield quadrangles. In the Whitepine area, Paleozoic 

sedimentary strata overlie ≈1,700 Ma granitic rocks (Tweto 

and others, 1976; Day and others, 1999). Both are intruded by 

a 39–32 Ma rhyolite and quartz monzonite to granite intrusive 

stock of the Mount Princeton batholith (Bove and Knepper, 

2000). Tertiary extrusive rocks are present along the northeast 
Distribution of Mines and Mineralized Areas 75

margin of both the intrusion and the mineralized area. All of 

the numerous small, but productive mines are in or adjacent 

to the stock: none is in the Precambrian rocks. The mines 

exploited lead, silver, zinc, and minor copper from polymetallic vein and replacement deposits (Dings and Robinson, 1957). 

Gold was important locally. The northern part of the area also 

hosts an iron skarn, but the iron ore was not of commercial 

grade (Dings and Robinson, 1957; Harder, 1909). 

Initially, oxidized silver and lead ore were the primary 

commodities. Later, primary lead and zinc were the valuable 

ores. Some gold, silver, and copper were recovered, as was a 

small amount of iron ore from a magnetite deposit. Most of 

the ore is classified as (1) replacement ores in limestone and 

dolomite, (2) contact deposits (skarn), and (3) fissure veins 

(Vanderwilt, 1947, p. 112). Of the mines in the area, the 

Akron was the most productive. Between 1901 and 1950 it 

produced nearly 100,000 tons of ore containing 724 oz gold, 

474,160 oz silver, 232,783 lb copper, 20,751,676 lb lead, and 

25,629,942 lb zinc (Dings and Robinson, 1957).

Marshall Pass

The Marshall Pass area is in the Pahlone Peak quadrangle 

(Olson, 1977). It includes the stratabound and vein uranium 

mines and occurrences in Harding Quartzite, Belden Formation, and veins in any rock occurring in the vicinity of the 

Chester fault zone. Early Proterozoic metasedimentary and 

metavolcanic rocks and pegmatitic granite are east of the fault. 

Cambrian and Pennsylvanian sedimentary rocks are west of 

the fault. At the Pitch mine, uranium occurs in brecciated 

Mississippian Leadville Limestone in the footwall of the Chester fault zone (Goodknight, 1981).

The major mines in the area produced nearly 1.3 million 

lb of uranium oxide from approximately 113,000 short tons of 

ore between 1956 and 1963 (Nelson-Moore and others, 1978; 

Nash, 1988). Numerous small mines were superseded in the 

1980’s by a large open pit at the Pitch mine, where reserves 

of 2.1 millions tons of ore containing 7.14 million lb of 0.17 

percent uranium oxide were reported (Nash, 1979, 1988; 

Ward, 1978).

Cochetopa North

The Cochetopa North area, on the Iris quadrangle, 

includes the northeastern part of the Gunnison Gold Belt and 

parts of the historic Cochetopa, Green Mountain, or Gold 

Basin mining districts (Vanderwilt, 1947, p. 100–101). The 

geologic setting for this area has been reviewed by Afifi 

(1981a, 1981b), Bickford and others (1989), Day and others 

(this volume), Drobeck (1981), Olson (1976a), and Sheridan 

and others (1981). The area is mapped as interlayered Proterozoic felsic and mafic metavolcanic rocks that are overlain by 

Jurassic Morrison Formation and Junction Creek Sandstone, 

and by Oligocene-age volcanic tuffs and breccias (Olson, 

1976a). Locally there are biotitic and migmatitic gneisses and 

granitic rocks. Mines in this area are hosted in the Proterozoic 

rocks and occur as low-sulfide shear-zone-hosted lode gold 

deposits. Examples include the Lucky Strike, Maple Leaf, 

Lubricator, and Mineral Hill mines. “The veins *** are relatively small and contain primarily gold. A small production is 

reported” (Vanderwilt, 1947, p. 193).

Workings at the Lucky Strike (at the west end of the 

mineralized area) are about 200 ft deep and 600 ft long. The 

ore vein is about 2–4 ft wide in massive iron-stained to white 

quartz containing tourmaline. Reportedly, the ore contains 

from 1 to 2 oz gold per ton. A small amount of copper and 

possibly tellurium is in the “waste pulp” (Hill, 1909, p. 

37–38). A visit to this site revealed recent activity including a 

newly roofed building, a new cyanide tank, and a fenced leach 

pond (Anna Wilson and Warren Day, unpub. field data, July 

28, 1998). 

The Maple Leaf mine (at the east end of the mineralized 

area) is developed on a free-milling massive gold-bearing 

quartz vein containing both gold and silver in east-west-striking veins in coarse diorite (Hill, 1909, p. 38). The mine was 

closed in 1908 but appears to have been worked intermittently 

since then (Anna Wilson and Warren Day, unpub. field data, 

July 28, 1998). This deposit is a low-sulfide gold-bearing 

quartz lode deposit. The geologic map of the Iris quadrangle 

(Olson, 1976a) shows this as a quartz vein in amphibolite. 

The inclined and caved adit follows a shear zone approximately 10 ft wide striking nearly due east. Chalcopyrite is 

visible in the quartz vein. 

Green Mountain

As outlined, the Green Mountain area lies within the Iris 

(Olson, 1976a) and Houston Gulch (Olson, 1976b) quadrangles and probably connects the northeasternmost part of the 

Gunnison Gold Belt (Drobeck, 1981) with the southern part of 

the historic Cochetopa Creek mining district. Felsite, felsite 

porphyry, amphibolite, and metasedimentary rocks of the 

Dubois Greenstone (Olson, 1976a,b) host several Kuroko-type 

massive sulfide deposits including the Denver City mine on 

the west end, and the Alaska and Yukon mines on the east end. 

At the Denver City mine, discovered in 1898 (USGS, 

1999a), sulfide ore (containing sphalerite, pyrite, and minor 

chalcopyrite and pyrrhotite) occurs in a stratabound lens 

parallel to the host rhyolite (felsite) (Drobeck, 1981, p. 280). 

Local gold and silver enrich the value of the deposit. Production records are not available (USGS, 1999a). Workings on 

the Alaska and Yukon mines date to the late 1800’s (USGS, 

1999a). Only five ore cars of 4–11 percent copper ore were 

produced from the Yukon, and only four ore cars of 34 percent 

zinc ore, 10 tons of 0.7 oz gold ore, and 15 tons of 11 percent 

copper ore were produced from the Alaska mine (Drobeck, 

1981, p. 281). 
76 Resource Potential and Geology, Grand Mesa, Uncompahgre, and Gunnison National Forests, Colorado 

Cochetopa Central 

Cochetopa Central is a relatively large area that overlaps 

onto the Iris (Olson, 1976a), Houston Gulch (Olson, 1976b), 

Sawtooth Mountain (Olson and Steven, 1976a), and Razor 

Creek Dome (Olson and Steven, 1976b) quadrangles and is 

roughly equivalent in its geologic and metallogenic setting to 

the Southern Cochetopa mineralized area. It includes mines 

and occurrences with vein or stratabound uranium mineralization occurring in Precambrian or Mesozoic rocks in the 

vicinity of the Los Ochos fault and related faults (Wilson and 

others, 2000; McCulla, 1980, Malan and Ranspot, 1959). 

Geologic maps (Olson, 1976a, b; Olson and Steven, 

1976a, b) show that the area lies within a deeply eroded 

Proterozoic basement complex of low-grade metamorphosed 

mafic and felsic volcanic rocks and their associated sediments, 

which were later intruded by granite. The basement is unconformably overlain by a generally flat lying sequence of Jurassic and Cretaceous sedimentary rocks and Oligocene volcanic 

rocks. Major faulting trends east-west and is steeply dipping, 

showing major movement during Laramide time.

Originally the Cochetopa region was mined for gold, 

but after 1955 uranium was the commodity of choice. Veins 

of pyrite-marcasite-pitchblende occur along Los Ochos fault 

zone; however, most ore was produced from stratabound 

deposits in sandstones in the Jurassic Morrison Formation and 

some in the Cretaceous Dakota Sandstone. Virtually all of the 

1.35 million lb of uranium oxide produced from 1956 to 1963 

came from the Los Ochos mine complex (Nelson-Moore and 

others, 1978).

Cochetopa Tantalite

Like the Quartz Creek Pegmatite area, this area, which 

is on the Sawtooth Mountain quadrangle (Olson and Steven, 

1976a), is not a metallic mineralized area. It is shown as a 

distinct area only because of its density of prospects and proximity to known metallic deposits. The area was prospected 

for pegmatite deposits in Precambrian granite, but none was 

developed (USGS, 1999a). No production has been recorded. 

Cochetopa South

This area on Sawtooth Mountain quadrangle (Olson and 

Steven, 1976a) includes vein uranium deposits in Precambrian 

granite that is overlain by younger sedimentary rocks. Originally the district was mined for gold and base metals from 

small gold-bearing veins in Precambrian rocks. Of the known 

mines, only the LaRue has recorded production. From 1954 

to1960, it produced 7 tons of ore, yielding 28 lb U3

O8 and 16 

lb V2

O5 (Nelson-Moore and others, 1978; Malan and Ranspot, 

1959; Olson, 1988, p. 19).

Wolf Creek

This tiny area, on the Carpenter Ridge quadrangle 

(Hedlund and Olson, 1973), outlines possible low-sulfide 

gold deposits (the Keezer and Lilly Belle?) (USGS, 1999a) in 

1,400 Ma alkalic and mafic rocks. This area overlaps a much 

larger area of thorium–rare-earth element (Th–REE) veins 

(see Powderhorn, following). Both properties may have been 

prospected for tungsten (scheelite) (Argall, 1943). Apparently, 

the properties have not been productive (USGS, 1999a). 

Beaver Creek

This tiny area on the Spring Hill Creek quadrangle (Olson 

and others, 1975) may be an extension of the Vulcan district. 

The two mines in this area, the Midland and Continental (Wilson and others, 2000), are Kuroko massive sulfide deposits 

very similar to those in the Vulcan area. The two mineralized 

areas are separated here, based on intervening host lithologies. 

The only reported production was gold-silver ore from the 

Continental mine in 1932 when 46 tons of ore assayed 1.28 

oz/t gold and 0.05 oz/t silver (USGS, 1999a). Zinc may be 

present (USGS, 1999b).

Goose Creek

Goose Creek is an area in the Gateview quadrangle 

(Olson and Hedlund, 1973), within the Th–REE veins of the 

Powderhorn area, that contains low-sulfide gold deposits. 

Geologically, it is similar to the Vulcan area, which is in the 

same rock units to the east (Hedlund and Olson, 1974), but 

that area hosts massive sulfide deposits. Occasional small 

shipments of lead-silver and gold-silver-copper ore were 

recorded from this area of Precambrian granite and schist 

overlain by Oligocene ash-flow tuffs. The only production 

since 1931 (in 1939 and 1940) was 30 tons of ore that yielded 

1 oz gold, 178 oz silver, 400 lb copper, and 1,400 lb lead 

(Vanderwilt, 1947, p. 104).

Vulcan

The Vulcan area, on the Powderhorn quadrangle (Hedlund and Olson, 1974), is also known as the Cebolla, Vulcan, 

Domingo, or White Earth district (Vanderwilt, 1947). This 

area includes some of the region’s more important Kuroko 

massive sulfide deposits, such as the Vulcan and Mammoth–

Good Hope. Together, these mines are an Environmental 

Protection Agency Superfund Site. These deposits are in a 

narrow greenstone belt included in the 1,700 Ma felsic and 

hornblendic gneiss unit (Tweto and others, 1976; Day and 

others, 1999). 

Gold was produced from lenses of pyrite-rich rock in 

Precambrian metavolcanic rocks, part of a submarine volcanic 

province. Much of the pyrite has little or no gold content. 
Distribution of Mines and Mineralized Areas 77

Sphalerite is the most common sulfide ore (Sheridan and 

others, 1981). Small shipments of lead, gold-silver, and 

copper-gold-silver ores were reported from other veins. Iron 

and manganese deposits have been described, but no production has been recorded. The 75 tons of ore mined in 1932, 

1933, 1934, and 1941 yielded 55 oz gold, 208 oz silver, 100 lb 

copper, and 100 lb lead (Vanderwilt, 1947, p. 100). In addition, these deposits may have produced another half million 

dollars (or about 25,000 oz gold) between 1898 and 1902, 

and 250 oz gold and 1,200 oz silver in 1919 (Drobeck, 1981). 

Most of the precious metals (gold-silver tellurides) are in 

chalcedony veinlets (Drobeck, 1981).

Vulcan South

Vulcan South is a small area outlining the Old Lot mine, 

a Kuroko massive sulfide deposit, in the Powderhorn quadrangle (Hedlund and Olson, 1974). The area is mapped almost 

entirely as 1,700 Ma felsic and hornblendic gneiss (Tweto and 

others, 1976; Day and others, 1999). Intervening Precambrian 

granite precluded connecting this area to the Vulcan area. The 

only recorded production in 1931 and 1934 totaled about 308 

oz gold and 300 oz silver (USGS, 1999a).

Powderhorn

The Powderhorn area covers a broad area with Th–REE 

veins and carbonatite prospects in the Powderhorn (Hedlund 

and Olson, 1975), Rudolph Hill (Olson, 1974), Gateview 

(Olson and Hedlund, 1973), Carpenter Ridge (Hedlund and 

Olson, 1973), and Big Mesa (Hedlund, 1974) quadrangles. It 

includes the historic Powderhorn district and part of Gunnison Gold Belt. As outlined, the area includes the Wolf Creek, 

Goose Creek, Vulcan South, and much of the Vulcan mineralized areas.

Early Proterozoic Dubois Greenstone and Powderhorn 

Granite is intruded by the latest Proterozoic or Cambrian 

Iron Hill alkalic complex (originally mapped as limestone of 

Iron Hill by Larsen, 1942). The area is cut by the northweststriking Cimarron fault (Armbrustmacher, 1980; Olson and 

Hedlund, 1981). At the core of the complex is the Iron Hill 

carbonatite stock. 

According to the MRDS database (USGS, 1999a), the 

area has been prospected since the late 1880’s, when magnetite 

was recognized. The magnetite was never valuable as iron ore 

because it contained much titanium (perovskite). “Prospectors 

either mistook magnetite-perovskite for base-metal sulfides or 

believed that it indicated sulfide presence at depth” (USGS, 

1999a). From 1935 to 1944 and again from 1958 to 1961, 

vermiculite, used in insulation, plaster, tile, and fireproofing, 

was mined; no production figures are available. Thorium was 

discovered in 1949 and prospected and studied until 1956, but 

none was ever produced. It wasn’t until 1956 that Iron Hill 

was recognized as a niobium-bearing carbonatite. Since then, 

there have been several development and feasibility studies for 

extracting titanium oxide, rare-earth elements, and niobium 

ores; however, no substantial production of any of these 

commodities is recorded.

Bondholder

The Bondholder area is roughly equivalent to the Bondholder district on the Stewart Peak and San Luis Peak (Lipman 

and Sawyer, 1988) quadrangles. It is in the center of the San 

Luis caldera in Oligocene volcanic rocks including inter-ash 

flow quartz latitic and andesitic lavas and breccias, Rat Creek 

Tuff, and quartz latite of Baldy Cinco. Several small heterogeneous hypabyssal intrusive rock units consist of equigranular 

to coarsely porphyritic gabbro, diorite, granodiorite, monzonite, and quartz monzonite emplaced during the period of 

ash-flow eruptions (Steven and others, 1974; Day and others, 

1999). Minor Quaternary glacial drift is locally exposed. 

The area may include polymetallic vein deposits in Tertiary 

volcanic rocks. Prior to Steven and Bieniewski (1977), no 

reports about the Bondholder area were available. Most of the 

previous work in the region has focused on the Creede district, 

on the same trend and several miles to the south. 

Earliest records of prospecting date to 1887 with the staking of three mining claims in the vicinity of the Cascade mine 

(Steven and Bieniewski, 1977). At the Cascade mine, three 

short tunnels follow irregular curving and branching mineralized fractures with only local concentrations of ore-grade 

material. Numerous workings in the vicinity of the Woodmansee mine have lead, zinc, and silver values approaching 

ore-grade. In the 1960’s the Allara tunnel was driven “to 

explore the ground below old workings on the hillside above” 

(Bieniewski, 1977). None of the mines was economically significant. Bieniewski (1977) attributes at most $100,000 from 

production in the entire Bondholder area in comparison with 

the approximately $81 million from Creede. 

Carson

Carson area is a small area, historically known as Carson 

Camp, on the Lake San Cristobal and Finger Mesa quadrangles. The Carson volcanic center is a 29 Ma monzonite to 

quartz monzonite plug (Bove and Knepper, 2000) intruding 

intermediate lavas and breccias and andesites and rhyolites 

of the Henson and Burns Members of the Silverton Volcanics 

(Steven and others, 1974; Day and others, 1999). Quaternary 

glacial drift, especially in Wager Gulch, and landslide material 

are present locally. 

Discontinuous and irregular “gashes and fractures” in 

the Carson volcanic center contain ore minerals in polymetallic veins in altered porphyry (Larsen, 1911). Ore containing 

silver and lead with copper (primarily in enargite, chalcopyrite, and galena) and some gold in barite gangue varies in 

these zones from a few inches to 18 in. wide (Larsen, 1911). 

Mineralized rock extends south across the divide at the head 

of Wager Creek into the head of Lost Trail Creek (Vanderwilt, 
78 Resource Potential and Geology, Grand Mesa, Uncompahgre, and Gunnison National Forests, Colorado 

1947, p. 114), and the area’s more productive mines, the St. 

Jacob and George III, are outside the GMUG Forests. Bog 

iron deposits are known in Wager Gulch (Larsen, 1911). 

Red Mountain (East)

Red Mountain (East), on the Lake San Cristobal quadrangle, is an altered 22.9 Ma complex of hydrothermally altered 

dacitic lavas and intrusions that formed on the eastern margin 

of the coeval Lake City caldera (Bove and others, 2000; Bove 

and Hon, 1990; Bove, 1988). In the 1970’s drill hole 

exploration delineated reserves of 70 million metric tons of 

alunite (Bove and others, 2000). Subeconomic porphyry 

molybdenum and copper mineralized rock is present several 

hundred meters below the surface (Bove and others, 2000). 

No development has taken place.

Lake City

The Lake City area (also known as the Lake Fork or 

Lake San Cristobal mining district) straddles the Lake City 

and Lake San Cristobal quadrangles on the northeast flank of 

the Lake City caldera. Host rocks include Oligocene Dillon Mesa Tuff, Sapinero Mesa Tuff and its Eureka Member, 

Carpenter Ridge Tuff and its Bachelor Mountain Member and 

Outlet Tunnel unit, Fish Canyon Tuff, and Henson and Burns 

Members of the Silverton Volcanics (Tweto and others, 1976; 

Day and others, 1999). The area includes 23 Ma barite-bearing precious-metal veins and slightly older quartz-base-metal 

veins (Slack, 1980; Bove and others, 2000). After the initial 

flurry of activity in the late 1800’s, most of the mines have 

been abandoned or worked only intermittently.

Notable mines in the area include the Golden Fleece 

(originally the Hotchkiss), which was discovered in 1874 and 

produced high-grade gold-telluride ore. As of 1926, it was 

credited with total production of $1.4 million (Henderson, 

1926, p. 51; Irving and Bancroft, 1911, p. 14). The Pelican 

mine produced silver ore intermittently from 1891 to 1960. 

The ore minerals apparently were freibergite (argentiferous 

tetrahedrite), pyrargyrite, and galena (Irving and Bancroft, 

1911, p. 97). Fanny Fern mine produced silver primarily from 

tetrahedrite (Brown, 1926, p. 14). In 1920–1923 and 1931, 

the mine produced at least 1,250 tons of ore yielding about 

74,000 oz of silver and 65 oz of gold (computed from MRDS 

data, USGS, 1999a). Black Crook mine operated intermittently for 12 years until 1903. In 1884, it produced 1,277 tons 

of ore valued at $124,447 (Irving and Bancroft, 1911, p. 116). 

Apparently it also produced intermittently from 1913 to 1953. 

Some production occurred from numerous other small mines 

in the area (Wilson and others, 2000; USGS, 1999a).

Golden Wonder

The Golden Wonder is a single mine, not a district. It 

is singled out from other deposits in the area because of its 

unique hot spring deposit type in an intrusive volcanic breccia 

pipe (Slack, 1980). Recorded production includes two carloads in 1906, unrecorded amounts in 1913 and 1935–1937, 

63 tons in 1939 (63 oz silver, 46 oz gold), 700 tons in 1961 

(205 oz silver, 81 oz gold), and 45 tons in 1981.

The Golden Wonder mine was recognized as unique by 

Irving and Bancroft (1911, p. 101), but it was classified by 

them as a “true replacement deposit.” Billings (1983) and 

Billings and Kalliokoski (1982) classified the deposit as a hot 

springs-type gold-telluride deposit. The ore occurs within a 

rhyolite flow-dome complex that was emplaced along the ring 

fracture zone of the Uncompahgre caldera (Billings, 1983). 

Productive portions of the vein were emplaced in a zone of 

closely spaced en echelon fractures. Two ore assemblages 

are present: gold-bearing chert (chert type) and pyrite-marcasite-sulfosalt (sulfide type). The chert type occurs in pods 

bounded by the fracture surfaces in areas where hydrothermal 

waters could pond. The sulfide type is found along the vein 

structure in between the high-grade chert-type pods where the 

vein structure was more constricted (Billings, 1983; Kalliokoski and Rehn, 1987). Two types of hydrothermal breccias 

are also present: the silicified dikes with fragments of sulfide 

and chert veins locally contain gold, whereas the argillically 

altered dikes do not (Billings, 1983).

Henson Creek

Henson Creek area includes the polymetallic vein deposits in the eastern part of the historic Galena district on the Lake 

City, Uncompahgre Peak, and Redcloud Peak quadrangles. 

This mineralized area is on the northeast edge of the 23 Ma 

Lake City caldera (Bove and others, 2000; Slack, 1980). Host 

rocks include Oligocene pre- and inter-ash flow andesitic lavas 

and breccias, intrusive rocks emplaced during the period of 

ash-flow eruptions, and Pliocene and Miocene plugs, dikes, 

and small flows of bimodal rhyolitic rocks (Tweto and others, 

1976; Day and others, 1999). There are two main groups of 

vein deposits: 26 Ma quartz-base-metal veins that are generally tangential to ring fractures of the Lake City caldera and 

contemporaneous with intrusions in the Capitol City area, 

and 23 Ma barite-precious metal veins radial to Red Mountain (Bove and others, 2000; Slack, 1980). Ore ranges from 

mediocre to extremely rich; it is primarily composed of argentiferous galena, argentiferous tetrahedrite (freibergite), native 

silver, chalcopyrite, and sphalerite. Much of the ore was the 

result of secondary enrichment and oxidation that were concentrated in the upper parts of the mines, but by 1903 the rich 

deposits were depleted and the region began its decline (Irving 

and Bancroft, 1911). 

Silver-lead ore was discovered in the Ute and Ulay veins 

in 1871 (Vanderwilt, 1947, p. 439), and production began in 
Distribution of Mines and Mineralized Areas 79

1874. Together, the veins of the Ute-Ulay mine are among 

the largest producers of silver and lead in Colorado with about 

$12 million (gross) as of 1911 (Irving and Bancroft, 1911, 

p. 14, 89). Production continued intermittently from 1918 

to 1967, and some cleanup work was done in 1980 (USGS, 

1999a). The adjacent Hidden Treasure has produced ore worth 

at least another $700,000 (Irving and Bancroft, 1911, p. 89). 

Other productive mines include the Ocean Wave, which 

was discovered in 1876 and ceased production in 1906. It 

claimed a total production of more than $115,000 (Irving and 

Bancroft, 1911, p. 86). The Yellow Medicine mine produced 

$40,000 worth of ore prior to 1896 (Irving and Bancroft, 1911, 

p. 78) and small amounts intermittently until 1952 (USGS, 

1999a). The Czar shipped ore in 1899 (Irving and Bancroft, 

1911, p. 14), but the cost to separate the lead and zinc made 

these shipments unprofitable (Irving and Bancroft, 1911, p. 

76). The property was revived briefly in the 1950’s (USGS, 

1999a). The Capitol City mine was discovered in 1882 and 

last produced ore in 1954 (USGS, 1999a). The Big Casino 

mine was discovered in 1876. Although its owners claimed 

assays ran as high as 200–412 oz silver per ton of ore (Irving 

and Bancroft, 1911, p. 81), the values for 1927, 1928, and 

1968 indicate that 100 tons of ore contained 3,167 oz silver, 

less than 2 oz gold, 56,534 lb lead, and 51,351 lb zinc (USGS, 

1999a). No other production records are available. No 

production is recorded prior to 1967 for the Pride of America 

(Sanford and others, 1986) adjacent to the Big Casino (USGS, 

1999a). In 1967–1968 and 1976–1977, more than 1,000 tons 

of ore (containing more than 13,000 oz silver, 182,000 lb lead, 

and 247,000 lb zinc) were produced (USGS, 1999a). The ore 

in both mines was galena and freibergite (Irving and Bancroft, 

1911, p. 81). 

Uncompahgre Peak

The Uncompahgre Peak area, straddling the Wetterhorn 

Peak (Luedke, 1972) and Uncompahgre Peak quadrangles, is 

on trend with regional structure in Oligocene volcanic rocks 

(Tweto and others, 1976; Day and others, 1999). It includes 

two similar vein uranium occurrences, the Beth and the Eagle 

and Mary Alice (Nelson-Moore and others, 1978; Steven and 

others, 1977; Wilson and others, 2000). 

The Beth group of unpatented claims produced 18 tons 

of ore containing 68 lb of U3

O8 ore in 1958–1961 (USGS, 

1999a). The Eagle and Mary Alice claims are inactive prospects that never produced. To the best of our knowledge, no 

mining activity has occurred and no mineral exploration interest has been shown in the area for nearly 40 years (NelsonMoore and others, 1978; Steven and others, 1977).

Cimarron

The Cimarron area also straddles the Wetterhorn Peak 

(Luedke, 1972) and Uncompahgre Peak quadrangles, north 

of the Uncompahgre Peak mineralized area. A west-northwest-trending zone of small intrusions in the northern part of 

the area are exposures of a 30 to 35 Ma quartz monzonite to 

monzonite volcanic center (Lipman and others, 1976). The 

Matterhorn Peak stock, at the southern part of the area, is 26 

Ma quartz monzonite to monzonite (Lipman and others, 1976; 

Bove and others, 2000). 

The Silver Jack, at the northeast corner of the area, is 

the only productive mine. It was last worked in 1931 and 

supposedly produced a small amount of silver and lead ore 

(probably galena), but no known records of production exist 

(USGS, 1999a). The Dix and Cimarron Chief, at the southern 

part of the area in the Matterhorn Peak stock, consist of 240 

unpatented claims (USGS, 1999a). Together they constitute 

an inactive, only slightly developed, molybdenum prospect 

consisting of several prospect trenches and pits, and one shallow shaft. 

Ouray

As shown, the Ouray area, on the Ouray quadrangle 

(Luedke and Burbank, 1962), encompasses much of the 

historic Ouray or Uncompahgre mining district (Luedke and 

Burbank, 1981). This area includes mineral deposits north of 

the San Juan volcanic field that are hosted in Paleozoic and 

Mesozoic sedimentary rocks adjacent to Laramide intrusive 

rocks (Tweto and others, 1976; Day and others, 1999). The 

area hosts polymetallic vein and replacement deposits. Major 

deposits include the American Nettie, Bachelor (including 

Wedge and Neodesha), Mineral Farm, and Pony Express.

The Bachelor mine (including the Wedge and Neodesha 

mines) was a large intermittent producer (USGS, 1999a): as 

of 1905 it was credited with $3.5 million in production (Cross 

and others, 1907, p. 17). Between 1942 and 1946 it produced 

an additional 201,000 pounds(!) silver, 2,080,000 lb lead, 

1,300,000 lb zinc, and 71,000 lb copper (USGS, 1999a). Ore 

minerals included galena, sphalerite, chalcopyrite, argentiferous tetrahedrite, and pearceite (Bastin, 1923, p. 70). Locally 

chalcocite, chrysocolla, argentite, and native silver may also 

occur (Bastin, 1923, p. 72). 

Although the American Nettie is listed as a small intermittent producer (USGS, 1999a), between 1889 and January 

1905 it produced 23,641,316 lb of ore valued at $1,464,923.35 

(Irving, 1905, p. 70). This ore averaged $123.12 per ton, or 6 

oz gold per ton of sorted rock (Irving, 1905, p. 70). When the 

mine was last worked is not known.

As of 1905, ore values in the Mineral Farm and Pony 

Express mines were described as “extremely irregular and 

*** uniformly low” (Irving, 1905, p. 73). Ore from the Pony 

Express averaged $30 per ton; the average value of the Mineral Farm ore was so low it didn’t pay to mine it (Irving, 1905, 

p. 73).

Production for this area is included with all production 

from Ouray County for the period 1946–1958. It is not possible, from published reports, to determine the production from 

the Ouray mineralized area. As of 1988 the American Nettie 
80 Resource Potential and Geology, Grand Mesa, Uncompahgre, and Gunnison National Forests, Colorado 

(Au, Ag, Pb, Zn) was in an exploration and development stage 

with one employee (Streufert and Ohl, 1989); Black Girl (Ag, 

Au, Pb, Zn) was on “standby”; and the Bachelor (Ag, Au) was 

being explored and rehabilitated. 

San Juan

The San Juan mineralized area encompasses all or part 

of numerous historic mining districts including Burrows Park, 

western Galena (Henson Creek), Eureka, S. Ouray, 

Sneffels, Telluride, Ophir; Red Mountain, and Lower San 

Miguel (Placerville) districts (Plumlee and others, 1995). The 

area overlaps four counties—Hinsdale, Ouray, San Juan, and 

San Miguel, and seven quadrangles—Mount Wilson (Bromfield and Conroy, 1963), Ophir (Luedke, 1996), Telluride 

(Burbank and Luedke, 1966), Ironton (Burbank and Luedke, 

1964), Ouray (Luedke and Burbank, 1962), Handies Peak 

(Luedke and Burbank, 1987), and Redcloud Peak. The 

mineralized area is centered on the San Juan caldera in the 

northwestern part of the San Juan volcanic field. 

Each of the mining districts has unique characteristics, 

yet overall, most of the deposits are classified as polymetallic veins and replacements for mineral resource assessment 

purposes. Numerous well-studied mines are in this area, 

including the well-known Idarado and Camp Bird mines, and 

dozens of others. The area could easily have been expanded to 

the south into the Silverton area. However, that would extend 

it outside the GMUG forest area; thus an arbitrary southern 

boundary was drawn. The references listed herein provide 

more information.

Wilson Peaks

Wilson Peaks area includes only the mineralized part 

of the Mount Wilson mining district, straddling the Dolores 

Peak (Bush and Bromfield, 1966) and Mount Wilson (Bromfield and Conroy, 1963) quadrangles. The Wilson Peak stock, 

composed mostly of granodiorite and quartz monzonite, 

intruded Mancos Shale, Telluride Conglomerate, and Tertiary 

volcanic rocks. Most of the mines and prospects in the area 

are polymetallic veins in the stock. Locally, some veins cut 

the contact between metamorphosed and sedimentary rocks 

(Bromfield and others, 1972). 

West- and southwest-striking vein systems are offset 

by thin barren north-striking veins (Varnes, 1947, p. 428). 

Productive veins were quartz-filled fissures containing pyrite, 

chalcopyrite, and arsenopyrite, with lesser amounts of galena, 

sphalerite, tetrahedrite, stibnite, and calcite. High values of 

gold occur along with chalcopyrite and galena in narrow pay 

streaks within fine-grained diorite facies of the diorite. In 

coarser grained parts of the intrusion, the gold may be associated with arsenopyrite (Varnes, 1947; Bromfield and others, 

1972). 

Except for the lack of phyllic alteration products and 

molybdenum, the zone of disseminated and vein-filling 

chalcopyrite in the quartz monzonite phase of the Wilson Peak 

stock is similar to that associated with other porphyry coppermolybdenum deposits. The exposed stock may be the deep 

part of an eroded porphyry system (Van Loenen and others, 

1997, p. 71). The small area of exposed volcanic rocks may 

have potential for Creede-type epithermal veins (Van Loenen 

and others, 1997, p. 86–87). 

Between 1882 and 1898, the Silver Pick mine produced 

6,030 tons of ore containing 94,923 oz silver and 32,442 oz 

gold (USGS, 1999a; Bromfield, 1967, p. 91). The Morning 

Star produced some ore between 1878 and 1903, 667 tons of 

ore from 1904 to 1914 (Bromfield, 1967, p. 88), and a small 

amount in 1952 (USGS, 1999a). Other figures are unknown.

Western Uranium

A number of historic mining districts that contain sandstone-hosted uranium-vanadium deposits, including Gateway 

(Vanderwilt, 1947, p. 141), Uravan, Bull Canyon, Gypsum 

Valley, Slick Rock (Chenoweth, 1981, p. 166), Paradox, and 

Uravan Mineral Belt (USGS, 1999a; Fischer and Hilpert, 

1952), have been combined to form the Western Uranium area 

(Wilson and others, 2000). In this area, the Permian to Upper 

Cretaceous sedimentary rock sequence is of predominantly 

terrestrial origin. Structures present include a series of parallel 

northwest-trending salt-cored anticlines; associated steeply 

dipping faults with small displacement cut the fold flanks 

parallel to the fold axes (Williams, 1964; Haynes and others, 

1972; Day and others, 1999). More than 1,200 mines and 

mineralized sites have been identified in the region 

(Chenoweth, 1981, p. 166).

Deposits were first mined for radium from 1898 to 1923 

(Finch, 1967). In the mid 1930’s and continuing through 

World War II, emphasis shifted to vanadium production for 

the alloy-steel industry (Chenoweth, 1996, p. 97). Interest in 

recovery of uranium started in 1942 with the Manhattan 

project. In 1947, the newly created Atomic Energy Commission contracted to buy uranium concentrates. Production 

peaked in 1960 and grew under a program of government price 

supports, which ended in 1970 (Chenoweth, 1996; 1981). 

Demand from the nuclear energy industry sustained production until 1990, when the last mill shut down (Chenoweth, 

1996, p. 95). Between 1947 and 1982, the Uravan Mineral 

Belt produced 85 million lb of uranium oxide (calculated from 

Chenoweth, 1996, p. 98; USGS, unpub. data), and 427 million 

lb of vanadium oxide was produced between 1947 and 1982 

(USGS, unpub. data). Vanadium production prior to 1947 

amounted to less than a tenth of that produced after 1947.

Sinbad

The Sinbad area includes redbed copper deposits in Sinbad Valley, including the Copper Rivet and Pyramid (Wilson 

and others, 2000). The area overlaps the Juanita Arch 
Distribution of Mines and Mineralized Areas 81

(Shoemaker, 1955), Roc Creek (Shoemaker, 1956), and Dolores Point South quadrangles and is surrounded by uranium 

deposits. The area is underlain by Pennsylvanian to Jurassic 

sedimentary rock units, especially Hermosa, Cutler, Chinle, 

and Moenkopi Formations (Williams, 1964; Day and others, 

1999). The copper deposits are structurally controlled, in 

veins and disseminated zones or horizons along cross-faults on 

the flank of a salt anticline.

As of 1921, there had been “considerable prospecting” at 

the Pyramid and Copper Rivet properties (Coffin, 1921). In 

1940 and 1942 the area shipped 30 tons of ore containing 9 

percent copper and 4 oz silver per ton (Vanderwilt, 1947, p. 

142). The small tonnage of low-grade copper ore was deemed 

unsuitable for acid leaching and not adaptable to open-cut 

mining operations (Traver, 1947, p. 491). 

Sunrise/Morning Glory

This area includes known redbed copper deposits on the 

Roc Creek (Shoemaker, 1956) quadrangle. The area is 

underlain by Triassic Chinle, and Jurassic Kayenta and Wingate Formations on the flank of a salt anticline (Williams, 

1964; Day and others, 1999). The veins and disseminated 

deposits are structurally controlled along the northern extension of the Cashin fault (Coffin, 1921, p. 220). 

Very little ore has been shipped from any of the properties in the West Paradox Valley. The Sunrise mine, on a N. 22° 

E.-trending fault, “produced 12 cars of ore assaying better than 

30% copper and containing from 6 to 10 ounces of silver per 

ton” (Coffin, 1921, p. 220). The Fairview claim “encountered 

a little ore,” and the Morning Glory was a prospect (Coffin, 

1921, p. 220).

Cashin (La Sal Creek)

he Cashin area, on the Paradox quadrangle (Withington, 1955), is roughly equivalent to the La Sal Creek mining 

district. Its redbed copper deposits are concentrated along 

northeast-trending faults in Permian to Jurassic sedimentary 

rock units on the flank of the Paradox salt anticline (Williams, 

1964; Day and others, 1999). Major deposits in the area 

include the Cashin and Cliffdweller (USGS, 1999a).

The ore is concentrated along two intersecting fault 

fissures in the Dolores Formation (Vanderwilt, 1947, p. 151). 

Adjacent to the fissures, chalcocite impregnates sandstone. 

Native copper with some native silver is found in breccia 

zones. Copper sulfides in many places occur higher in the fissures than the metallic copper. The fissures have been developed by several hundred feet of tunnels and winzes (Vanderwilt, 1947, p. 151). Before 1920, an unspecified amount of 

ore was shipped that contained 35–50 percent copper with 

8–10 oz silver per ton (Vanderwilt, 1947, p. 154). From 1937 

to 1945 between one and three lode mines, presumably including the Cashin and Cliffdweller, produced 97 oz gold, 59,537 

oz silver, and 1,462,200 lb copper (Vanderwilt, 1947, p. 153). 

Exploration drilling has revealed a reserve of 10.9 million 

tons of 0.55 percent copper at the Cashin Mine (Anonymous, 

1995).

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U.S. Department of the Interior

U.S. Geological Survey

Qualitative and Quantitative Mineral Resource 

Assessment Methodology

By Gregory T. Spanski and Viki Bankey

Chapter F of

Resource Potential and Geology of the Grand Mesa, Uncompahgre,

and Gunnison (GMUG) National Forests and Vicinity, Colorado

Edited by Viki Bankey

U.S. Geological Survey Bulletin 2213– F
Contents

Abstract........................................................................................................................................................ 89

Mineral Deposit Types in the GMUG Greater Study Area.................................................................... 89

Delineation of Permissive and Favorable Mineral Deposit Potential Tracts .................................... 89

Quantitative Assessment of Locatable Mineral Resource Potential ................................................. 94

Grade and Tonnage Models...................................................................................................................... 94

Estimation of Undiscovered Deposits ..................................................................................................... 96

Estimation of Endowments........................................................................................................................ 96

References Cited ........................................................................................................................................ 96

Appendix F1. Mark3 Mineral Resource Endowment Estimates......................................................... 97

Figures

F1. Diagram bitmapping favorable criteria .................................................................................. 95

F2. Graph showing example of a cumulative frequency plot of ore endowment

estimates from a Mark3 simulation ........................................................................................ 99

 F3–F7. Graphs showing simulated Mark3 endowment distributions occurring in

undiscovered deposits in permissive tracts in GMUG greater study area:

 F3. Climax-type porphyry molybdenum deposits: ore and molybdenum .................... 100

 F4. Porphyry molybdenum, low fluorine deposits: ore and molybdenum................... 101

 F5. Precambrian Kuroko-type massive sulfide deposits: ore and copper.................. 102

 F6. Precambrian Kuroko-type massive sulfide deposits: zinc and lead ..................... 103

 F7. Precambrian Kuroko-type massive sulfide deposits: silver and gold................... 104

Tables

 F1. Deposit types compatible with geologic environments in GMUG forests ............................... 90

 F2. Areas and percentages calculated for permissive and favorable tracts for

 mineral resource assessments ....................................................................................................... 93
Abstract

We conducted an assessment of mineral resource potential for a select number of deposit types within the GMUG 

greater study area, beginning with an initial consideration 

of mineral types likely to be present in the area. Of these, 

we selected the most important, both those historically and 

those having potential for future development. Included for 

assessment are granite- and granodiorite-hosted porphyry 

molybdenum, sandstone-hosted uranium, volcanic-associated 

massive sulfide, polymetallic vein, polymetallic replacement, 

and sediment-hosted redbed copper deposits. From descriptive models for these deposit types, we determined criteria to 

allow us to identify areas of permissive and favorable resource 

potential. Finally, quantitative assessments were performed on 

four of the deposit types that had adequate information available (grade and tonnage models).

Mineral Deposit Types in the GMUG 

Greater Study Area

Information about the geoenvironmental and physical 

characteristics of mineral deposit types has been assembled 

in descriptive models. For examples, see Erickson, 1982; 

Eckstrand, 1984; Cox and Singer, 1986; Roberts and Sheahan, 

1988; Bliss, 1992; Hoover and others, 1992. The information 

in a descriptive model is used to identify areas where deposits 

are likely to occur, to judge the degree of that likelihood, and 

to classify known mineral occurrences. Identification is based 

on how closely geologic conditions in an area agree with those 

in the descriptive model or on the presence of mineral occurrences that represent the deposit type.

Mineral deposit types initially considered in the GMUG 

area are listed in table F1. They were identified through 

comparisons of geoenvironmental settings observed in the 

study area and mineral occurrences. The geologic conditions 

used to qualify each deposit type for consideration are listed 

with examples of known mineral occurrences where available. 

Model numbers correspond to those used in Cox and Singer 

(1986).

The mineral resource potential was assessed for seven 

deposit types: granitic and granodioritic porphyry molybdenum, sandstone-hosted uranium, volcanic-associated massive 

sulfide, polymetallic vein, polymetallic replacement, and 

sediment-hosted redbed copper. Their selection is based on 

both the prominent role that these deposit types have played in 

the area’s mining history and the expectation that these are the 

most likely deposit types for development in the foreseeable 

future. Results are detailed in the following chapters of this 

volume.

In the case of the other 15 deposit types considered (table 

F1), available information is insufficient to support either a 

belief in a more-than-negligible probability for undiscovered 

deposits or an expectation for significant near-term exploration or development under any foreseeable economic scenario, 

with one exception. That exception is the vein- and sedimenthosted uranium occurrences that occur in the Marshall Pass 

and Cochetopa areas. Several deposits, the Pitch mine and the 

Thornburg (Los Ochos) mine, have produced 1.2 and 1.4 million lb respectively of U3

O8 (Goodknight and Ludlam, 1981). 

However, these deposits are not amenable to quantitative evaluation. Controversy surrounding their genesis (Olson, 1988) 

prevents their identification with a specific deposit model, and 

in the absence of a descriptive model, formal resource potential tracts cannot be delineated. It is reasonable to expect that 

interest in exploration for similar deposits will occur during 

periods of favorable market conditions for uranium. The focus 

on where that interest might occur depends upon the genetic 

model on which the exploration philosophy is based. Insights 

on areas of future interest regarding these deposits are summarized by Goodknight and Ludlam (1981).

Delineation of Permissive and 

Favorable Mineral Deposit

Potential Tracts

A mineral deposit potential tract defines an area where 

the known or inferred geologic conditions at the surface or in 

the shallow subsurface suggest that the probability of one or 

more undiscovered deposits existing is more than negligible. 

Qualitative and Quantitative Mineral Resource 

Assessment Methodology

By Gregory T. Spanski and Viki Bankey
Table F1. Deposit types compatible with geologic environments in GMUG forests. Deposit type USGS Model Terrane favorability characteristics2 

 No.1 Granite-hosted 16 High-silica granite (rhyolite) to alkalic plutonic and hypabyssal intrusions asociated with con- Elk Mtn. (Gothic

 porphyry Mo. temporaneous extensional tectonism. Polymetallic vein mineralization with accessory fluorite, Quartz Creek dist

rhodochrosite, high-temperature tungstates, or Climax Mo type mineralization. Propylitic Ruby dist./ Mt. E

 alteration zones with anomalous levels of Cu, Pb, Mo, Sn, F, U, Rb, W, Nb, Ta, and (or) Zn. well Basin prosp

 Multi-stage intrusive history of development in a high-silica granite or alkalic plutonic system. Volcanic vent or caldera complex exhibiting multiple resurgent events. Granodiorite-hosted 21a Porphyritic granodioritic to quartz monzonitic (calc-alkaline) intrusions emplaced in a convergent Capitol City area

 porphyry Mo. plate boundary environment. Polymetallic vein and (or) polymetallic replacement and (or) skarn The Blowout. mineralization. Anomalous levels of Cu, Mo, Au, Ag, W, B, Sr, Pb, Zn, As, Sb, Se, Te, Mn, Co, Matterhorn Cente

 Ba, and (or) Rb in soil/stream sediment/rock. Sandstone-hosted U-V 30c* Feldspathic alluvial to fluvial sandstone with localized reduced facies zones or felsic tuffaceous Ruby (Irwin) dist

 volcaniclastic sandstone. Sandstone-hosted U mineralization. Anomalous levels of background Uravan Mineral B

 radioactivity in soils and ground water. Anomalous levels of U, V, Mo, Se, Cu, and Ag in soil/ stream-sediment samples. Volcanic-associated 28a Presence of Proterozoic bimodal (mafic or felsic) metavolcanic or associated metasedimentary rocks. Gold Basin (Gree

 massive sulfide. Areas proximal to felsic volcanic centers. Regions within or adjacent to known mineralized areas Iris dist./ Denver 

 containing VMS deposits and prospects. Anomalous enrichments in Zn, Cu, and Au in stream- Cochetopa dist./ A

 sediment samples. Goose Creek dist

 Vulcan dist./Good

Polymetallic veins (PMV) 22c Presence of intermediate to felsic (calc-alkaline) shallow, subvolcanic Tertiary intrusions or geo- Elk Mtn. (Gothic

 physical evidence for them. Polymetallic vein, polymetallic replacement, skarn, and (or) Goldbrick dist./ C

porphyry mineralization. Geochemically anomalous concentrations of Cu, Pb, Zn, or detectable mines. Ag or Au. Dominant fractures such as those related to calderas and caldera-related structures or Tincup dist./ Jimmzones of extensional tectonic activity. Hydrothermal alteration minerals and zones of propylitic mines. alteration and (or) silicification of carbonate rocks. Tomichi (Whitepi

 mines. Quartz Creek dist

 mines. Taylor Park dist. Lake Fork (Lake 

Quartz mine. Galena (Henson C

 America, Vermo

mines. Cimarron dist./ S

Burrows Park (W

mine. Sherman dist. Carson Camp dis

Larson Center are

Matterhorn Cente

Upper Cow Creek 

Eureka (Mineral P
Table F1. Deposit types compatible with geologic environments in GMUG forests.—Continued Deposit type USGS Model Terrane favorability characteristics2 Kno

 No.1

Polymetallic 19a Permeable or chemically reactive rocks, especially carbonates, proximal to known or inferred Tertiary Elk Mtn. (Gothic replacement. or Cretaceous felsic intrusive (plutonic or calc-alkaline hypabyssal) rocks. Polymetallic replacement, Tincup dist./ Tinc

 polymetallic vein, skarn, and (or) porphyry mineralization. Anomalous levels of Ag, Pb, or Zn Tomichi (Whitepi

 in stream-sediment or rock samples. Localized areas of dolomitization or silicification of limestones Nest Egg, Morn and (or) jasperoid or calc-silicate alteration of carbonate rocks and (or) argillic-propylitic alteration Quartz Creek dist

 of carbonate rocks and (or) argillic-propylitic alteration of igneous rocks. mines.

Spring Creek dist

Sediment-hosted 30b.2* Clastic sedimentary rocks containing permeable stratigraphic intervals. Salt-cored anticlines and La Sal dist./ Cash redbed Cu. associated prominent faults near axial zones of anticlines. Evidence of mineralizing activity 

 commonly associated with Cu-U-V.

Thorium-rare earth veins 11d Alkalic plutonic rocks (inclusive of carbonatites). Thorite- and (or) monazite-bearing quartz veins. Powderhorn area/ 

 Anomalous levels of Th-oxide in alkalic rocks and Th and REE in soil/ stream sediment. Badger 1, Little 

 mines, Whitney 

Carbonatite 10 Carbonatite and associated alkalic plutonic rocks. Zones of fenitized and brecciated rocks. Anomalous Powderhorn (Wh

 levels of Nb and (or) REE in soil/ stream sediment.

Epithermal quartz- 25e Felsic volcanogenic and hypabyssal calc-alkaline rocks. Areas of pervasive acid-sulfate alteration or Red Mountain are alunite veins. advanced argillic alteration with alunite, pyrophyllite, kaolinite and cristobalite. Massive opaline or

 chalcedonic silicification of permeable rocks. Porphyry and (or) polymetallic replacement mineral-

 ization. Anomalous levels of Au, As, Cu, base metals, Te(?) and W in soil/ stream sediment. Prominent

 through-going faults and fractures.

Epithermal quartz- 25c+ Subaerial accumulations of volcanogenic rocks of intermediate to felsic calc-alkaline to alkaline compos- Lake Fork (Lake  adularia veins. 25d ition. Caldera complex exhibiting resurgent intrusive activity. Precious-metal-bearing quartz-adularia- Fleece, Gold Qu

 calcite vein mineralization. Quartz-alunite vein or hot spring mineralization. Prominent through-going

 faults or fractures. Silicic, argillic to advanced argillic, phyllic, and propylitic zoned alteration with

 low sulfide content. Gold placer mineralization. Anomalous levels of Hg, Au, As, Sb, Cu, Se, Te, Mo,

 Zn, and (or) Pb in soil/ stream sediment.

Creede epithermal 25b Intermediate to felsic calc-alkaline plutonic and hypabyssal rocks or bimodal volcanic rocks. Caldera Bondholder dist./  veins. complex exhibiting resurgent intrusive activity. Creede epithermal vein mineralization. Epithermal mines.

 quartz-alunite vein, polymetallic replacement and (or) placer gold mineralization. Prominent

 through-going faults or fractures and (or) fractures and faults associated with doming and caldera

 evolution. Anomalous levels of Au, As, Sb, Hg, Cu, Ag, Pb, and (or) Zn in soil/ stream sediment.

Placer Au-PGE 39a Tertiary to recent terrace and alluvial deposits. Anomalous levels of Au, Ag, As, Hg, Sb, Cu, Fe, Gunnison River p

 and (or) S in stream sediment. Gold placer, porphyry, polymetallic vein and replacement, quartz Elk Mtn. placers.

 adularia and alunite vein, low-sulfide gold-quartz vein or other gold-bearing mineralization. Goldbrick placers

Powderhorn place

Taylor Park place

Tincup placers.

Union Park place

Unaweep placers

LaSal Creek plac

Naturita placers.

Uncompahgre pla

Eureka placers.

Lower San Migue
Table F1. Deposit types compatible with geologic environments in GMUG forests.—Continued Deposit type USGS Model Terrane favorability characteristics2 

 No.1 Fe skarn 18d Carbonate or carbonate-bearing sedimentary rocks near the margins of intermediate to felsic Elk Mtn. dist./ Iro

 plutonic rocks. Polymetallic vein and (or) replacement mineralization. Tomichi (Whitepi

Tincup dist./ Cum

Low-sulfide Au- 36a Metavolcanic and metasedimentary (greenschist to middle amphibolite facies) rocks. Low-sulfide Box Canyon dist.

 qtz veins. gold-quartz vein, placer gold and (or) Kuroko volcanogenic massive-sulfide mineralization. mines. Anomalous levels of As, Ag, Pb, Zn, and (or) Cu in soil/ stream sediment. Prominent compressional Wolf Creek area/ 

 high-angle faults and shears. Areas of carbonate alteration proximal to quartz and calcite veins. Goose Creek dist

Willow Creek dis

Vulcan dist./ Con

Cochetopa dist./ B

 Leaf mines. Gold Basin (Gree

 Lucky Strike, Lu

Cebolla dist./ Cas

Distal disseminated 19c Permeable sedimentary and clastic volcanogenic rocks occurring within 20 km of intermediate to Ouray area/ Dako Ag-Au. felsic plutonic or hypabyssal intrusive rocks. Distal disseminated, porphyry Cu, skarn, and (or) stock. polymetallic vein or replacement mineralization. Areas of silicification and (or) argillization. Anomalous levels of Hg, As, Sb, with or without Au in soil/ stream sediment. Hot spring Au-Ag 25a Felsic plutonic and hypabyssal intrusions and (or) rhyolitic volcanogenic centers. Geothermal hot Lake Fork (Lake 

 springs. Areas of chalcedonic sinter, massive silicification, stockworks, or quartz-adularia veins Wonder, Golden 

 and (or) breccias cemented by quartz. Epithermal quartz vein and (or) hot spring Hg mineralization. Anomalous levels of Au, As, Sb, Hg, and Te in soil/ stream sediment. Hot spring Hg 27a Mafic to intermediate hypabyssal intrusions and (or) basaltic to andesitic volcanic rocks. Geothermal Cochetopa dist./ M

 hot springs. Areas of siliceous sinter with minor pyrite or Fe-oxides. Hot spring mineralization. Anomalous levels of Au, As, Sb, Hg, and Te in soils/ stream sediment. Volcanogenic U 25f High-silica rhyolite extrusive or hypabyssal rocks. Volcanogenic U vein mineralization. Anomalous Uncompahgre Pe

 levels of Li, Hg, As, Sb, F, Mo, W, and REE in soil/ stream sediment. Pegmatite U N.A.4 Basement complex of uranium-bearing pegmatite, granite gneiss, quartz monzonite and granite rock. Harry Creek area/ 

 Anomalous levels of Pb, Zn, Th, V, and Y in rock/ soil/ stream sediment. Areas of strong argillic Grp., Marshall P

 alteration. Sediment-hosted V N.A. Intraformational unconformities in permeable sandstone overlain by limestone and localized in a zone Placerville dist./ O

 marginal to the depositional edge of the capping limestone. Sediment-hosted, stratiform chromium- mines. bearing micaceous and (or) sediment-hosted roscoelite mineralization. Sediment- and vein- N.A. Prominent, deep crustal thrust faults along which crystalline and sedimentary rocks are juxtaposed. Marshall Pass dis

 hosted U. Sedimentary carbonate and carbonaceous sandstone, siltstone, and shales unconformably overlain mines. by intermediate to felsic volcanoclastic rocks. Hypabyssal rhyolite intrusive rock. Cochetopa dist./ L

Tomichi (Whitepi

Akron tunnel, B

Jacks Cabin area/ 

*No grade or tonnage models available.

1Modified from Cox and Singer, 1986; Bliss, 1992.2Geo-environmental or physical features suggesting deposit-type compatibility.3Names of district or area / mineralized site(s).4N.A., not available.
Qualitative and Quantitative Mineral Resource Assessment Methodology 93

Singer (1993) has suggested that negligible in this context 

equates with a deposit occurrence probability of between 1 in 

100,000 and 1 in 1,000,000. Land in the study area where that 

probability is greater than the latter value is classified permissive. Areas within a permissive tract that are believed to have 

a probability that is significantly greater than negligible are 

defined as favorable.

Mineral potential classifications of permissive or favorable are subjective interpretations based on geologic and 

mineral occurrence information available at the time that 

indicates deposit presence. Descriptive models provide guidance on what kinds of information, such as mineral occurrence, regional geologic, geotectonic, petrologic, geochemical, 

and geophysical data, indicate the presence of a deposit. We 

gain a sense of the relative importance of each data set and its 

meaningful data threshold values from looking at conditions 

in other areas where these deposit types are found. Minimum 

conditions are established for each deposit type and expressed 

as delineating criteria. For example, the presence of carbonate rocks is a delineating criterion for defining areas as permissive for replacement deposit types. Lateral buffers (radial 

distances) are applied to point, line, and area data to include 

shallowly buried extensions of a feature and to minimize 

errors caused by mislocation or inadequate sampling of an 

area that has a non-negligible potential.

In general, permissive lands are defined using criteria 

that discriminate between areas that could host a deposit and 

ones that could not. Where the condition is absent, potential 

for existence of the related deposit type is considered negligible. Mapped geology is commonly the primary delineating 

criterion. Favorable lands are defined using delineating criteria that attest to the probable existence of deposit-generating 

processes and the intensity of that activity. Examples would 

be the presence of distinctive alteration mineral assemblages, 

specialized sedimentary depositional environments, anomalous trace-element geochemistry, or known occurrences of the 

deposit type or a genetically related deposit type. For each of 

the seven deposit types assessed in this study, the delineation 

criteria used to identify permissive and favorable tracts are 

listed in a table within each chapter. Area (in square miles) of 

permissive and favorable tracts are listed in table F2.

After the delineation criteria are determined, permissive and favorable tracts are computer generated using the 

commercial IMAGINE geographic information system (GIS) 

Table F2. Areas (in square miles) and percentages calculated for permissive and favorable tracts for mineral resource 

assessments (Chapters G–L). 

[Total area includes all public and private lands; USFS (USDA Forest Service) area includes only area within the GMUG forests; BLM area 

includes only area managed by the Bureau of Land Management. Areas and percentages rounded to the nearest whole number]

Assessment model name Total area (mi2)

[percent of area]

USFS area (mi2)

[percent of area]

BLM area (mi2) [percent 

of area]

GMUG study area, no model 19,800 4,868 5,092

porphyry molybdenum - permissive 3,144

[16]

1,371

[28]

320

[6]

porphyry molybdenum - granitic 

 - favorable

2,832

[14]

1,242

[26]

295

[6]

porphyry molybdenum - granodioritic 

- favorable

737

[4]

324

[7]

95

[2]

uranium - permissive 6,656

[34]

1,628

[33]

2,650

[52]

uranium - favorable 1,352

[7]

94

[2]

955

[19]

massive sulfide - permissive 1,373

[7]

220

[5]

288

[6]

massive sulfide - favorable 558

[3]

176

[4]

163

[3]

polymetallic vein - permissive 6,880

[35]

2,374

[49]

950

[19]

polymetallic vein - favorable 2,973

[15]

1,200

[25]

334

[7]

polymetallic replacement - permissive 5,133

[26]

1,832

[38]

739

[15]

polymetallic replacement - favorable 1,674

[8]

618

[13]

150

[3]

sediment-hosted copper - permissive 3,128

[16]

806

[17]

1,360

[27]

sediment-hosted copper - favorable 441

[2]

195

[4]

247

[5]
94 Resource Potential and Geology, Grand Mesa, Uncompahgre, and Gunnison National Forests, Colorado 

software from Erdas, Inc. Boolean logic statements (commonly AND or OR) that express each delineation criterion are 

used to combine data taken from gravimetric, magnetic, and 

radiometric surveys, Landsat imagery, rock and sediment geochemistry, mineralized areas, mine and prospect locations, and 

tectonic, stratigraphic, and structural analyses. These data are 

stored in vector or raster format in theme layers. For example, 

stratigraphic, lithologic, and structural data are stored in a 

geologic map theme layer. Delineation criteria with applied 

buffers (for example, 500 m surrounding a mineral location of 

gold) are used to identify areas within the theme layer where 

required conditions exist. Permissive areas are determined by 

selecting and combining delineating criteria from pertinent 

theme layers (for example, either Tertiary intrusions from the 

geology theme layer or positive aeromagnetic anomalies from 

the aeromagnetic theme layer, or both). In this report, areas 

are shown as either permissive for the selected deposit type or 

not permissive.

The process we used to combine theme layer areas to 

determine favorability tracts is known as bitmapping and is 

illustrated in figure F1. In bitmapping, a unique bit value 

from a power of 2 series (1, 2, 4, 8, 16, 32, etc.) is assigned to 

selected areas within a rasterized theme layer. When theme 

layers are combined using the GIS program, bit values are 

added where theme areas are superposed, producing derivative 

areas having bit values that are unique for the combination. 

For example, a sum of 19 can only result from the superposition of areas with bit values of 16, 2, and 1. The sum, therefore, describes each of the input criteria that makes the area 

favorable. Whereas the permissive maps only show where, the 

favorable maps also show why.

Note, however, that bitmap values do not rank areas of 

mineral potential: an area with a sum bit value of 18 does not 

necessarily possess a potential that is greater than an area with 

a sum bit value of 15. Theme combinations wholly determine 

potential, and for tract delineation purposes, only two meaningful levels of potential are distinguished: permissive and 

favorable. By definition, a favorable area must also be defined 

first as permissive. Therefore, a mask is used on the derivative 

bitmap to eliminate areas that are not permissive.

Complex relationships among the criteria can be displayed using color variations on the figures showing favorable tracts. For example, if anomalous geochemistry is only 

a delineating criterion in combination with other criteria, 

favorable areas will not be colored where geochemistry is 

the only criterion. Where a large number of theme layers 

are combined, the potential for complexity escalates rapidly. 

Three theme layers can generate as many as 7 unique derivative areas, and five theme layers can give rise to 31 derivative 

areas. In such cases, the figures are simplified by displaying 

similar criteria in the same color, and this will be indicated in 

the explanation.

Quantitative Assessment of Locatable 

Mineral Resource Potential

Quantitative mineral resource potential of an area is 

expressed in estimates of the probable numbers of undiscovered economic to marginally economic deposits believed 

likely to exist within the area and estimates of the amounts 

(endowments) of ore and recoverable commodities likely to 

occur in those deposits. We used the “three-part” assessment 

methodology (Singer, 1975) developed by the USGS in the 

1970’s. It is subjective and assumes that the undiscovered 

deposits represent one or more distinct deposit types. Each 

deposit type is characterized by a set of physical and genetic 

attributes common to a group of known mineral deposits that 

represent the deposit type. Where data characterizing the size 

(tons of ore) and commodity grades of economic to marginally 

economic deposits are available as grade and tonnage models, 

the number of undiscovered deposits can be estimated. The 

latter information can in turn be used to estimate the ore and 

commodity endowments likely to be associated with these 

undiscovered deposits. Quantitative results are presented in 

a probabilistic format to emphasize the uncertainties inherent 

in the assessment process. The three parts of the methodology are discussed in greater detail in the following section. 

A comprehensive discussion of the three-part assessment 

methodology and the procedure used to estimate endowments 

is found in Singer (1993) and Root and others (1992).

Grade and Tonnage Models

Grade and tonnage models are used to define the size and 

grade of deposit-size occurrences, where deposit refers to only 

those occurrences that have been commercially exploited or 

are believed to have a potential to be commercially developed 

in the future. These models must be available before opinions 

concerning the probability of additional deposits occurring can 

be formulated. The mean and variance of the size and grade 

distributions of these deposit populations are used for that purpose. The models are also critical in quantifying the endowment aspects of an undiscovered deposit population.

In this study, grade and tonnage models in Cox and 

Singer (1986) or modified versions from it have been used to 

estimate undiscovered deposit populations and ore and commodity endowments for four of the seven deposit types we 

assessed. These were the two molybdenum deposit types, the 

volcanic massive sulfide type, and the polymetallic replacement type. In the absence of grade and tonnage models for 

polymetallic vein deposits, sandstone-hosted uranium-vanadium deposits, and sediment-hosted redbed copper deposits, 

provisional minimum deposit size and grade parameters were 

used for the sole purpose of tract delineation. Provisional 

values are established from values that, given the commodities 

involved and development considerations, would likely define 
Qualitative and Quantitative Mineral Resource Assessment Methodology 95

11 2 3

4

2

1

3

6 5

4

7

2

1

2

3

6 5

4

7

1 2 4

bit values are added

and combined onto one layer

Each delineation criterion (such as

a rock type, structure, or buffered 

geochemical sample site) is given

a unique bit value on a theme layer 

Mask of permissive areas,

white = permissive

Favorable bitmap areas masked by permissive areas

Bitmap of favorable areas

Figure F1. Bitmapping favorable criteria. 
96 Resource Potential and Geology, Grand Mesa, Uncompahgre, and Gunnison National Forests, Colorado 

the minimum grade and tonnage for occurrences considered to 

be marginally economic at the time of this study. Provisional 

values cannot be used to estimate numbers of undiscovered 

deposits or to estimate ore and commodity endowments. 

Estimation of Undiscovered Deposits

The estimation of undiscovered deposits allows us to 

quantify the resource potential of an area for the deposit types 

that are permissive in the geologic environment present. The 

deposits being estimated are presumed to possess grade and 

tonnage characteristics consistent with those used to construct 

the grade and tonnage models. Where available data do not 

support that assumption, quantification of resource potential 

was not included. Without new size and grade data and with 

inappropriate existing grade and tonnage models, no valid 

basis exists for estimating undiscovered deposits. For the 

volcanic-hosted massive sulfide deposit type, the grade and 

tonnage data were modified to include only Precambrian 

deposits. We believe that these models portray a range of 

grades and tonnages that might be present in the study area. 

Model median and population variance values are used as 

guides in conceptualizing the undiscovered deposits.

The estimates of deposits are generated by a team of 

individuals with an understanding of the geology and metallogeny of an area and the genesis of the deposit types being 

assessed. Each estimate reflects what the team members agree 

is the largest number of undiscovered deposits believed likely 

to be present. For each deposit type assessed, five estimates 

are generated at five levels of confidence including a high 

level of confidence (90 percent), intermediate estimates at the 

50 percent, 10 percent, and 5 percent, and a highly speculative degree of confidence at the 1 percent confidence level. 

Estimation certainty is reflected in the magnitude of change 

occurring between estimates in the five-tier sequence. For 

example, levels of confidence percentages of 90, 50, 10, 5, and 

1 may result in respective numbers of undiscovered deposits 

of 1, 1, 2, 5, and 11. These numbers of undiscovered deposits 

indicate that the assessors are fairly certain that one or two 

undiscovered deposits are present; however, the large spread 

in values at the low level of confidence indicates much greater 

uncertainty. The overall evaluation process is subjective but is 

based on the best professional opinions given the data available. A wide variety of approaches is used to predict numbers 

of undiscovered deposits (Singer, 1993). Team members are 

free to employ any procedure of which they are confident as 

long as deposit model size and grade consistency are honored. 

Locations of undiscovered deposits within the study area 

are not directly addressed in the assessment process. The 

same data used to delineate mineral potential tracts are used 

to estimate numbers of undiscovered deposits, and we might 

assume that they would likely be within favorable areas as 

opposed to merely permissive areas. However, it should not be 

assumed that any predetermined percentage of an 

undiscovered deposit population would or should necessarily 

occur in a given mineral potential tract.

Estimation of Endowments

Resource endowments are measures of the total quantities of ore and recoverable commodities that are associated 

with an undiscovered deposit population. Like the undiscovered deposit estimates on which their measure depends, the 

estimates are reported in a probabilistic format. Undiscovered 

deposit estimates along with the appropriate grade and tonnage 

model data are input into a Monte Carlo simulation routine 

(Mark3 Simulator). The computer-based simulator (Root 

and others, 1992) generates 4,999 hypothetical undiscovered 

deposit scenarios that are statistically consistent with the 

deposit estimates and the grade and tonnage models. The ore 

and commodity endowment results for each scenario are used 

to construct cumulative frequency distributions from the smallest endowment to the largest for each commodity and for ore. 

Results can be used in economic analyses, land-use planning, 

or designing remediation and (or) mitigation of environmental 

impacts that might occur from future mineral exploration or 

development. A few demonstrative endowment values are 

summarized in tables in this volume: see Chapter I, table I2, 

and Chapter G, tables G3 and G6. Included are the estimated 

commodity and ore endowments at the 90th, 50th, and 10th 

percentiles of the frequency distributions and the mean endowments. The probabilities of these mean endowments are also 

included, because the means can be highly skewed and misleading where the estimates of undiscovered deposits reflect a 

high degree of uncertainty at the low confidence levels. These 

tables also list the probabilities for each of the possible deposit 

scenarios including no deposits. More complex endowment 

summaries are presented in graphical format in Appendix F1 

of this chapter, together with guidelines on how the graphs 

may be interpreted.

References Cited

Bliss, J.D., ed., 1992, Developments in mineral deposit modeling: U.S. Geological Survey Bulletin 2004, 168 p.

Cox, D.P., and Singer, D.A., eds., 1986, Mineral deposit models: U.S. Geological Survey Bulletin 1693, 379 p.

Eckstrand, O.R., ed., 1984, Canadian mineral deposit types—

A geological synopsis: Geological Survey of Canada 

Economic Geology Report 36, 86 p.

Erickson, R.L., ed., 1982, Characteristics of mineral deposit 

occurrences: U.S. Geological Survey Open-File Report 

82-795, 253 p.
Qualitative and Quantitative Mineral Resource Assessment Methodology 97

Goodknight, C.S., and Ludlam, J.R., 1981, Montrose quadrangle, Colorado: Report of Bendix Field Engineering 

Corporation prepared for the U.S. Department of Energy, 

Grand Junction, Colo., GJQ-010(81), 91 p.

Hoover, D.B., Heran, W.D., and Hill, P.L., eds., 1992, The 

geophysical expression of selected mineral deposit models: 

U.S. Geological Survey Open-File Report 92-557, 129 p.

Olson, J.C., 1988, Geology and uranium deposits of the 

Cochetopa and Marshall Pass districts, Saguache and Gunnison Counties, Colorado: U.S. Geological Survey Professional Paper 1457, 44 p.

Roberts, R.G., and Sheahan, P.A., eds., 1988, Ore deposit 

models: Geoscience Canada Reprint Series 3, 194 p.

Root, D.H., Menzie, W.D., and Scott, W.A., 1992, Computer 

Monte Carlo simulation in quantitative resource estimation: 

Nonrenewable Resources, v. 1, no. 2, p. 125–138.

Singer, D.A., 1975, Mineral resource models and the Alaskan 

mineral resource assessment program, in Vogely, W.A., ed., 

Mineral materials modeling—A state-of-the-art review: 

Baltimore, Md., Johns Hopkins University Press, p. 370–

382.

Singer, D.A., 1993, Basic concepts in three-part quantitative 

assessments of undiscovered mineral resources: Nonrenewable Resources, v. 2, no. 1, p. 69–81.

Appendix F1. Mark3 Mineral Resource 

Endowment Estimates

Estimated ore and commodity endowments for 

undiscovered deposit populations for three types of deposits 

expected to be present within the GMUG greater study area 

have been simulated using the U.S. Geological Survey Mark3 

Simulator (Root and others, 1992). Input to the simulator 

for each deposit type includes the estimates of numbers of 

undiscovered deposits, estimated at the 90th, 50th, 10th, 5th 

and 1st percentile levels of confidence, and the grade and 

tonnage model data. The undiscovered deposit estimates 

are fit to a frequency distribution model that calculates 

an occurrence probability for each of the possible deposit 

populations that fall within the range from zero up to the 

number of deposits estimated at the 1st percentile. A Monte 

Carlo simulation methodology is used to compute theoretical 

ore and commodity endowments that would be associated with 

each of 4,999 hypothetical deposit scenarios. In each scenario, 

a number representing the number of deposits expected to be 

present is chosen, followed by the selection of a tonnage and 

commodity grades for each deposit. Commodity endowments 

are calculated for each deposit and summed for that scenario. 

The frequency with which any given grade, tonnage, or 

number of deposits is used in a scenario is determined by 

their respective frequency distribution models. The ore 

and commodity endowment estimates resulting for the 

4,999 scenarios are sorted in order of increasing value and 

displayed in cumulative frequency graphs. To assist the user 

in interpreting these graphs, a brief explanation of the display 

format used follows.

Explanation of Graphical Display of Mark3 

Output

The 4,999 hypothetical ore and commodity endowment 

estimates resulting from a Mark3 Simulator run are sorted in 

order of increasing value and graphically displayed in a log 

linear plot of ore or commodity endowment versus proportion 

of simulations. The cumulative frequency plot is used because 

probabilistic conclusions concerning the ore and metal endowment potential of an area can be drawn directly from these 

displays. 

Key interpretive elements of a typical plot of Mark3 

results (fig. F2) are cross referenced by letter to the following 

descriptive explanations.

 A Title—Identifies the mineral deposit type, the 

undiscovered deposit estimates input (in parentheses) 

and endowment (ore or commodity) plotted. In the 

example, ore endowment estimates are displayed for 

a hot-spring Au-Ag deposit type where the estimates 

of undiscovered deposits input is 2, 4, 9, 15 and 25 

at the 90th, 50th, 10th, 5th and 1st confidence levels 

respectively.

 B Vertical axis (left)—Linear scale of proportion of 

simulation scenarios, graduated in 0.1 increments.

 C Horizontal axis—Logarithmic scale of endowments 

in metric tons. In the example, ore endowment 

ranges from 6,300 to 6,300,000,000 t (metric tons) 

and is expressed in millions of metric tons.

 D Endowment value—Open circles denote endowment 

values occurring at 5 percentile intervals. The values 

are also listed in a table appearing to the right of the 

graph (figs. F3–F7). In the example, the ore endowment at the 35th percentile is 72 million metric tons. 

 E Minimum endowment—Solid circle denotes the minimum non-zero endowment value simulated. Annotation includes the value of the endowment and the 

proportion of simulations producing a zero endowment, noted on the flattened extension of the endowment curve to the left of the symbol. In the example, 

the minimum ore endowment is 170,000 t of ore, and 

0.04 or 4 percent of the simulations contained no ore 

endowment.

 F No endowment field—A shaded field denotes that 

portion of the simulation scenarios that had no 
98 Resource Potential and Geology, Grand Mesa, Uncompahgre, and Gunnison National Forests, Colorado 

endowment. This field will always be present in that 

every deposit distribution includes some finite probability of there being no deposits. Zero ore endowments result where a no-deposit condition is modeled 

in a scenario. Zero commodity endowments result 

where either a no-deposit scenario is modeled or the 

commodity grade model shows that the commodity 

occurs in only a portion of the deposit of a deposit 

type. In the example, 4 percent of the simulation scenarios were run under the assumption that no deposits 

were present. The remaining 96 percent were run 

with the assumption that from 1 to 25 deposits were 

present.

 G Maximum endowment—Denoted with an open circle. 

Annotated with the value of the largest ore or 

commodity endowment simulated. In the example, 

the largest ore endowment is equivalent to 1,800 

million t.

 H Median endowment—Denoted with an open circle 

and annotated with an endowment value. Half of the 

4,999 simulation scenarios have smaller endowments 

and half have larger endowments. In a probabilistic 

sense, given the simulation conditions, the probability 

of an endowment being larger or smaller is 50 percent 

or equal. In the example, the median ore endowment is equal to 110 million t. The probability of an 

endowment greater or smaller than 110 million t is 

equal.

 I,J Endowments at the 10th and 90th percentiles—

Denoted with open circles and annotated with 

endowment values. Values commonly reported in 

the past assessments as defining the upper and lower 

limits of the endowment. Highlights the simulated 

endowment scenarios that are symmetric about the 

median excluding the more erratic values occurring 

in the tails of the endowment distributions. In the 

example, the ore endowment at the 10th percentile 

is 18 million t and the 90th percentile 470 million t. 

One may therefore assume that there is an 80 percent 

probability K of an ore endowment between 18 and 

470 million t in size being present.

 L Mean endowment—An open square denotes the 

mean endowment. It is annotated with an endowment value and the proportion of scenarios that have 

endowments smaller than the mean. In the example, 

67 percent (0.67) of the simulation scenarios M had 

ore endowments smaller than the 190 million t mean, 

or a 67 percent probability exists that the ore endowment is less than 190 million t. 

 N Exceedance probability—Linear scale indicating 

the probability of a given endowment value being 

exceeded. The example shows that the probability of 

the presence of an endowment larger than the mean 

endowment of 190 million t is 33 percent probability. 

Any endowment can be tested by selecting an

endowment on the horizontal axis and extending 

a line vertically to the endowment curve and then 

extending a horizontal line to the exceedance probability scale.

Figures F3–F7 enlarge on this theme and give

additional data from the GMUG greater study area. 
Qualitative and Quantitative Mineral Resource Assessment Methodology 99

0.17

4% chance of no ore endowment

 0.0063 0.025 0.1 0.4 1.6 6.3 25 100 400 1,600 6,300 

1.0

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

0.0

18 110

 190

1.0

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

0

B

C

D

F

G

H

I

J

K

L

E

1,800 0.04

0.67

470

N

33 %

A

M

PROPORTION OF SIMULATIONS

HOT-SPRINGS AU-AG (2,4,9,15,25)

Ore endowments

EXCEEDANCE PROBABILITY 

ORE ENDOWMENT, IN MILLIONS OF METRIC TONS

Figure F2. Example of a cumulative frequency plot of ore endowment estimates from a Mark3 simulation of undiscovered hot-spring gold-silver type deposits. Letters in circles are keyed to the preceding 

explanatory text.
100 Resource Potential and Geology, Grand Mesa, Uncompahgre, and Gunnison National Forests, Colorado 

1.0

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

0.0

0

10

20

30

40

50

60

70

80

90

100

0.929

0.933

6.7%

3.2

0.0016 0.0063 0.025 0.1 0.4 1.6 6.3 25 100 400 1,600 

0.016

0.04

1.0

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

0.0

0

10

20

30

40

50

60

70

80

90

100

0.929

0.933

6.7%

 0.0063 0.025 0.1 0.4 1.6 6.3 25 100 400 1600 6300 

21

1140 9.2

92.9% probability of no

 ore endowment 

92.9% probability of no

 Mo endowment

MOLYBDENUM ENDOWMENT, IN MILLIONS OF METRIC TONS

PROPORTION OF SIMULATIONS

EXCEEDANCE PROBABILITY PERCENTAGE

 ORE ENDOWMENT, IN MILLIONS OF METRIC TONS

PROPORTION OF SIMULATIONS

EXCEEDANCE PROBABILITY PERCENTAGE

 0

 0

 0

 0

 0

 0

 0

 0

 0

 0

 0

 0

 0

 0

 0

 0

 0

 0

 0

 120

1,140

Ore

 0

 5

10

15

20

25

30

35

40

45

50

55

60

65

70

75

80

85

90

95

100

Percentile

 0

 0

 0

 0

 0

 0

 0

 0

 0

 0

 0

 0

 0

 0

 0

 0

 0

 0

 0

0.23

 3.2

Moly.

 0

 5

10

15

20

25

30

35

40

45

50

55

60

65

70

75

80

85

90

95

100

Percentile

Figure F3. Climax-type porphyry molybdenum deposits: simulated Mark3 endowment distributions for ore and molybdenum occurring in undiscovered deposits in permissive tracts in GMUG 

greater study area.
Qualitative and Quantitative Mineral Resource Assessment Methodology 101

1.0

0.000025 0.0001 0.0004 0.0016 0.0063 0.025 0.1 0.4 1.6 6.3 250

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

0.0

0

10

20

30

40

50

60

70

80

90

100

0.0087

0.0022

0.961

0.963

1.66

1.0

 0.1 0.4 1.6 6.3 25 100 400 1,600 6,300 25,000 100,000

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

0.0

0

10

20

30

40

50

60

70

80

90

100

11

2.4

0.961

0.962 3.8%

1950

3.7%

 MOLYBDENUM ENDOWMENT, IN MILLIONS OF METRIC TONS

PROPORTION OF SIMULATIONS

EXCEEDANCE PROBABILITY PERCENTAGE

 ORE ENDOWMENT, IN MILLIONS OF METRIC TONS

PROPORTION OF SIMULATIONS

EXCEEDANCE PROBABILITY PERCENTAGE

 0

 0

 0

 0

 0

 0

 0

 0

 0

 0

 0

 0

 0

 0

 0

 0

 0

 0

 0

 0

1,950

Ore

 0

 5

10

15

20

25

30

35

40

45

50

55

60

65

70

75

80

85

90

95

100

Percentile

 0

 0

 0

 0

 0

 0

 0

 0

 0

 0

 0

 0

 0

 0

 0

 0

 0

 0

 0

 0

1.66

Moly.

 0

 5

10

15

20

25

30

35

40

45

50

55

60

65

70

75

80

85

90

95

100

Percentile

96.1% probability of no

 Mo endowment 

96.1% probability of no 

 ore endowment

Figure F4. Porphyry molybdenum, low fluorine deposits: simulated Mark3 endowment distributions for ore and molybdenum occurring in undiscovered deposits in permissive tracts in 

GMUG greater study area.
102 Resource Potential and Geology, Grand Mesa, Uncompahgre, and Gunnison National Forests, Colorado 

ORE ENDOWMENT, IN MILLIONS OF METRIC TONS

PROPORTION OF SIMULATIONS

EXCEEDANCE PROBABILITY PERCENTAGE

COPPER ENDOWMENT, IN MILLIONS OF METRIC TONS

PROPORTION OF SIMULATIONS

EXCEEDANCE PROBABILITY PERCENTAGE

 0

 0

 0

 0

 0

 0

 0

 0

 0

 0

 0

 0

 0

 0

 0

 0

 0

 0

 0

 0.37

 110

Ore

 0

 5

10

15

20

25

30

35

40

45

50

55

60

65

70

75

80

85

90

95

100

Percentile

 0

 0

 0

 0

 0

 0

 0

 0

 0

 0

 0

 0

 0

 0

 0

 0

 0

 0

 0

0.0048

2.6

Copper

 0

 5

10

15

20

25

30

35

40

45

50

55

60

65

70

75

80

85

90

95

100

Percentile

92.9 % probability of no

 copper endowment 

92.9 % probability of no

 ore endowment 

1.0

 0.000025 0.0001 0.0004 0.0016 0.0063 0.025 0.1 0.4 1.6 6.3 25

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

0.0

0.00015 0.0051 2.6

0.929

0.951 4.9%

0

10

20

30

40

50

60

70

80

90

100

92.9 % probability of no

copper endowment

1.0

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

0.0

0.014

0.37

110

0.929

0.951 4.9%

0

10

20

30

40

50

60

70

80

90

100

92.9 % probability of no

ore endowment

 0.0016 0.0063 0.025 0.1 0.4 1.6 6.3 25 100 400 1,600

Figure F5. Precambrian Kuroko-type massive sulfide deposits: simulated Mark3 endowment 

distributions for ore and copper occurring in undiscovered deposits in permissive tracts in 

GMUG greater study area.
Qualitative and Quantitative Mineral Resource Assessment Methodology 103

1.0

 0.0001 0.0004 0.0016 0.0063 0.025 0.1 0.4 1.6 6.3 25 100

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

0.0

0.0004 0.013 10.4

0.946

3.7%

0

10

20

30

40

50

60

70

80

90

100

1.5% 1.0

0.000025 0.0001 0.0004 0.0016 0.0063 0.025 0.1 0.4 1.6 6.3 25

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

0.0

0.00021

0.00072

0.82

0.983

0.985

0

10

20

30

40

50

60

70

80

90

100

0.963

ZINC ENDOWMENT, IN MILLIONS OF METRIC TONS

PROPORTION OF SIMULATIONS

EXCEEDANCE PROBABILITY PERCENTAGE

LEAD ENDOWMENT, IN MILLIONS OF METRIC TONS

PROPORTION OF SIMULATIONS

EXCEEDANCE PROBABILITY PERCENTAGE

94.6 % probability of no

zinc endowment 

98.3 % probability of no

lead endowment 

 0

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Lead

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 0

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0.0018

 10.4

Zinc

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Figure F6. Precambrian Kuroko-type massive sulfide deposits: simulated Mark3 endowment distributions for zinc and lead occurring in undiscovered deposits in permissive tracts 

in GMUG greater study area.
104 Resource Potential and Geology, Grand Mesa, Uncompahgre, and Gunnison National Forests, Colorado 

1.0

0.9

0.8

0.7

0.6

0.5

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0.3

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0.1

0.0

0.12

18

12,000

0.953

3%

0

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30

40

50

60

70

80

90

100 0.025 0.1 0.4 1.6 6.3 25 100 400 1,600 6,300 25,000

1.0

0.9

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0.7

0.6

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0.4

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0.2

0.1

0.0

0.00024

0.54

310

0.958

2.6% 0

10

20

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40

50

60

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80

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100

0.0001 0.0004 0.0016 0.0063 0.025 0.1 0.4 1.6 6.3 25 100 400 1,600 

0.974

0.97

SILVER ENDOWMENT, IN METRIC TONS

PROPORTION OF SIMULATIONS

EXCEEDANCE PROBABILITY PERCENTAGE

GOLD ENDOWMENT, IN METRIC TONS

PROPORTION OF SIMULATIONS

EXCEEDANCE PROBABILITY PERCENTAGE

95.3 % probability of no

silver endowment 

95.8 % probability of no

gold endowment

 0

 0

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12,000

Silver

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Percentile

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310

Gold

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Percentile

Figure F7. Precambrian Kuroko-type massive sulfide deposits: simulated Mark3 endowment distributions 

for silver and gold occurring in undiscovered deposits in permissive tracts in GMUG greater study area.
U.S. Department of the Interior

U.S. Geological Survey

Mineral Resource Assessment for Porphyry

Molybdenum Deposits

By Dana J. Bove, Daniel H. Knepper, Jr., Viki Bankey, Gregory T. Spanski,

and Steven M. Smith

Chapter G of

Resource Potential and Geology of the Grand Mesa, Uncompahgre,

and Gunnison (GMUG) National Forests and Vicinity, Colorado

Edited by Viki Bankey

U.S. Geological Survey Bulletin 2213– G
Contents

Abstract...................................................................................................................................................... 107

Granite Porphyry Molybdenum Deposits.............................................................................................. 107

Genetic Model for Granite Porphyry Molybdenum Deposits ................................................... 107

Description of the Areas Containing Known Granite Porphyry Molybdenum Deposits ...... 108

Application of the Deposit Model for a Mineral Resource Assessment of Granite

Porphyry Molybdenum Deposits ..................................................................................... 108

Permissive Tracts............................................................................................................................. 108

Favorable Tracts............................................................................................................................... 108

Undiscovered Deposit and Endowment Potential...................................................................... 111

Granodiorite Porphyry Molybdenum Deposits .................................................................................... 114

Genetic Model for Granodiorite Porphyry Molybdenum Deposits.......................................... 114

Description of the Areas Containing Known Granodiorite Porphyry Molybdenum

Deposits ............................................................................................................................... 115

Application of the Deposit Model for a Mineral Resource Assessment of

Granodiorite Porphyry Molybdenum Deposits.............................................................. 117

Permissive Tracts............................................................................................................................. 118

Favorable Tracts............................................................................................................................... 118

Undiscovered Mineral Deposit and Endowment Potential....................................................... 120

References Cited ...................................................................................................................................... 121

Figures

G1–G3. Maps of GMUG greater study area showing:

 G1. Permissive tracts for porphyry molybdenum deposits ............................................... 110

 G2. Favorable tracts for granite porphyry molybdenum deposits ................................... 112

 G3. Favorable tracts for granodiorite porphyry molybdenum deposits .......................... 116

Tables

G1. Delineation criteria for granite porphyry molybdenum deposits in GMUG greater 

study area .......................................................................................................................................... 109

G2. Granite porphyry molybdenum tracts in GMUG greater study area ........................................ 111

G3. Summary of results of resource endowment potential assessment for undiscovered

Climax-type porphyry molybdenum deposits within GMUG greater study area.................... 114

G4. Delineation criteria for granodiorite porphyry molybdenum deposits in GMUG

greater study area ............................................................................................................................ 118

G5. Granodiorite porphyry molybdenum tracts in GMUG greater study area ............................... 119

G6. Summary of results of resource endowment potential assessment for undiscovered

porphyry molybdenum, low fluorine deposits within GMUG greater study area................... 121
Abstract

Porphyry molybdenum deposits that are economical to 

produce are large tonnage and bulk-mined by underground 

and open-pit operations. Molybdenite (MoS2

) is present in 

stockwork veinlets within variable host rocks that have been 

hydrothermally altered in a pattern roughly concentric to a 

complex of nested intrusions. Genetic models describing 

these deposits emphasize an important relation to the chemical 

composition of the related intrusive rocks. Deposits or mineral 

occurrences within the GMUG greater study area group into 

two main types: (1) high-silica granite and (2) granodiorite or 

quartz monzonite. In this chapter, we describe the geologic, 

geochemical, and geophysical characteristics of both types and 

discuss the likelihood of undiscovered deposits.

Granite Porphyry Molybdenum 

Deposits

Genetic Model for Granite Porphyry 

Molybdenum Deposits

Granite systems in North America, exemplified by the 

Climax, Henderson, and Mount Emmons deposits in Colorado 

(White and others, 1981; Carten and others, 1993), are present 

almost exclusively within the western United States and are 

the most significant of the porphyry molybdenum deposits in 

regards to ore-grade tonnage, and production history. Most 

granite molybdenum deposits contain between 50 and 1,000 

million t (metric tons) of molybdenum ore (White and others, 1981), with a median of 200 million t (Cox and Singer, 

1986). The average ore body ranges from 0.1 percent MoS2

 at 

the margins to average internal grades of 0.3 to 0.45 percent 

MoS2

 (White and others, 1981). Tin and tungsten are important byproducts in some deposits, whereas copper content is 

relatively low: Cu:Mo ratios range from 1:100 to 1:50 (White 

and others, 1981). 

Granite porphyry molybdenum deposits are characterized 

by episodic mineralization and penecontemporaneous 

intrusion of high-silica (>74 percent SiO2), alkali-rich granite 

or rhyolite. Generally small and cylindrical, these high-silica 

stocks or plugs are thought to represent high-level cupolas that 

extend above large silicic plutons (White and others, 1981). 

These small granitic plugs or stocks are highly differentiated 

and are characterized by the following general geochemical 

signature (Ludington, 1981):

F (>0.1 percent) Rb (>250 ppm) Sr (<50 ppm) 

Cs (>10 ppm) Nb (>50 ppm) Ta (>10 ppm) 

Ba (<300 ppm) Sn (>5 ppm) (U >10 ppm) 

La (<50 ppm) Y (>50 ppm) 

All dated deposits in the western U.S. are less than 50 

Ma, whereas the five major deposits in Colorado (Climax, 

Henderson, Urad, Mount Emmons, and Redwell Basin) 

range from 33 to 17 Ma, and are mostly < 25 Ma (White and 

others, 1981). Granite molybdenite systems are thought to 

be associated with the transition from compressive to extensional tectonism (White and others, 1981; Ludington, 1981; 

Mutschler, Wright, and others, 1981; Carten and others, 1993). 

The ore zones of granite molybdenum deposits are centered 

in or above the apical portion of the source granitic intrusion. 

Generally elliptical in plan view and concave downward in 

cross section, the ore bodies have thicknesses of ≈130–330 

m and vertical dimensions of about 330–660 m (Mutschler, 

Wright, and others, 1981; White and others, 1981). More 

than 90 percent of the molybdenite is present in thin, moderately to steeply dipping stockwork veinlets along with quartz, 

fluorite, and traces of biotite, potassium feldspar, pyrite, and 

sericite. Tungsten is generally concentrated in discrete zones 

in or adjacent to the molybdenite ore bodies, whereas pyrite 

and base-metal sulfide zones extend to higher levels and more 

laterally than the molybdenum and tungsten bodies. The basemetal sulfide zone consists of veins and veinlets containing 

galena, sphalerite, and pyrite with minor chalcopyrite, rhodochrosite, and fluorite; these veins typically extend outward into 

the peripheral host rocks. Although spatially associated with 

molybdenite ore bodies, the base-metal polymetallic veins 

typically postdate molybdenite mineralization and related 

hydrothermal alteration (Thomas and Galey, 1982; White and 

others, 1981; Wallace and others, 1968). However, 

isotopic studies (Stein and Hannah, 1985) indicate that lead 

within base-metal veins is predominantly derived from the 

Mineral Resource Assessment for Porphyry 

Molybdenum Deposits

By Dana J. Bove, Daniel H. Knepper, Jr., Viki Bankey, Gregory T. Spanski, and Steven M. Smith
108 Resource Potential and Geology, Grand Mesa, Uncompahgre, and Gunnison National Forests, Colorado 

Tertiary-age stock(s) related to molybdenite mineralization, 

whereas only a minor component of the lead is scavenged 

from nearby host rocks. 

Hydrothermal alteration associated with these deposits is 

consistent with the classic assemblages and patterns inherent 

to all molybdenum and copper porphyry deposits as described 

by Lowell and Guilbert (1970). In general, alteration changes 

from a potassic zone (secondary potassium feldspar and 

biotite) near the core of the deposit outward and upward into 

quartz-sericite-pyrite (QSP), argillic (quartz, kaolinite, smectite), and finally into propylitic alteration (primary minerals, 

chlorite, epidote, sericite, and calcite). The QSP zone, which 

contains as much as 10 volume percent pyrite (White and 

others, 1981), is an important signature in the exploration for 

these types of deposits. 

Description of the Areas Containing Known 

Granite Porphyry Molybdenum Deposits

Three granite porphyry molybdenum deposits have been 

discovered beneath the slopes of Mount Emmons, which is 

about 6 to 8 km northwest of Crested Butte, Colo. (fig. G1). 

Two of these deposits are located deep (>600 m) beneath a 

high cirque basin north of Mount Emmons known as Redwell 

Basin. The third deposit is on the south side of Mount 

Emmons beneath the western rim of Red Lady Basin and is 

referred to as the Mount Emmons deposit. 

The Mount Emmons deposit is a contact-related stockwork of quartz veinlets containing molybdenite along with 

fluorite, pyrite, and minor huebnerite that is draped over the 

top of an 18–16 Ma granite stock (Thomas and Galey, 1982; 

White and others, 1981). As defined by a 0.2 percent MoS2

boundary, the deposit is a nearly circular 90 m thick ring in 

plan view that has an outside diameter of about 670 m. Ore 

reserves have been calculated at about 141 million t of rock 

with an average grade of 0.44 percent MoS2 (Thomas and 

Galey, 1982). The Mount Emmons deposit is much richer and 

larger than the Redwell deposits and lies at a shallower depth 

(Thomas and Galey, 1982). 

The uppermost molybdenite deposit in Redwell Basin 

is at a depth of about 730 m, where it is associated with a 

small intrusion of rhyolite porphyry. This cupola is surficially 

expressed as an intrusion breccia complex that was fed via a 

crackled zone of hornfels present between the deep rhyolite 

intrusion and the base of the breccia body. As defined by a 0.1 

percent MoS2

 boundary, the upper deposit contains about 17 

million t of rock that average 0.18 percent MoS2

 (Thomas and 

Galey, 1982). Rhyolite porphyry in the deep intrusion grades 

downward into a granite porphyry stock. The lower molybdenite deposit is about 300 m below the upper contact of the 

granitic stock. 

Other known granite porphyry molybdenum occurrences 

are present throughout the GMUG greater study area (map 

area, fig. G1). References regarding the nature of these 

occurrences are found in the following section on application 

of the deposit model for a mineral resource assessment.

Application of the Deposit Model for a 

Mineral Resource Assessment of Granite 

Porphyry Molybdenum Deposits

The criteria used to define permissive and favorable tracts 

for granite porphyry molybdenum deposits are listed in table 

G1. Many of these criteria are summarized in the Climax 

molybdenum deposit model in Cox and Singer (1986; model 

16) and were derived from numerous detailed studies of economic granite molybdenite deposits. (See White and others, 

1981; Wallace and others, 1968; Thomas and Galey, 1982; 

Ludington, 1981; Mutschler, Wright, and others, 1981; Stein 

and Hannah, 1985.) Areas classified as permissive are those 

that are underlain by Tertiary-age intrusions as mapped in the 

database compiled by Bove and Knepper (2000) and Day and 

others (1999) or interpreted from aeromagnetic survey data 

(Bankey and others, this volume, Chapter D, fig. D3). Several intrusive units, which are not known to host any significant mineralization or related hydrothermal alteration, were 

excluded from the permissive tracts. Each of the intrusions 

included in the criteria was surrounded by a 1 km buffer to 

allow for the presence of covered deposits at depth. 

Various combinations of criteria (table G1) were applied 

in delineation of favorable tracts for the GMUG study area. 

Distinctions between Criteria 2 and 3 intrusions were based 

on previous mineral assessment and geologic studies (Bove 

and others, 2000; DeWitt and others, 2000; Fridrich and others, 1998; Van Loenen and Gibbons, 1997; Cunningham and 

others, 1994; Sanford and others, 1987; Hon, 1987; Mutschler, 

Wright, and others, 1981; Mutschler, 1980). Specific characteristics of favorable tracts include distinctive chemical composition of the intrusions, the presence of diagnostic alteration 

minerals and assemblages, and the occurrence of molybdenite 

or other minerals such as fluorite. 

Permissive Tracts

In the GMUG greater study area, 1,371 mi2 is classified 

as permissive for the occurrence of granite porphyry molybdenum deposits (fig. G1). As shown in figure G1, the permissive 

tract is quite extensive, owing to the abundance of intermediate to silicic composition intrusions in the eastern 3/4 of the 

GMUG greater study area. These intrusions are absent in the 

more tectonically stable western part of the study area.

Favorable Tracts

In the GMUG greater study area, 1,242 mi2 is classified 

favorable for the occurrence of granite porphyry molybdenum 

deposits (fig. G2, table G2). These lands represent about 90 

percent of the permissive tract. The critical criteria used for 
Mineral Resource Assessment for Porphyry Molybdenum Deposits 109

delineating favorable tract areas for granite porphyry molybdenum deposit occurrences are listed in table G1. In addition 

to meeting the permissive criterion requirement, these lands 

also meet one or more of the conditions listed in the favorable 

criteria. Tracts are grouped in broadly defined areas, and the 

criteria used to classify each of the tracts are identified (table 

G2).

West Elk Mountains Area (Area 1, fig. G2): This area covers a large part of the West Elk Mountains. Criteria used to 

delineate this tract include Miocene age (criterion 4), highsilica granite or rhyolite intrusions (criterion 3) in various 

combination with magnetic anomalies (criterion 7) (generally 

associated with more mafic, Oligocene plutons), local element 

enrichments in rocks or stream-sediment samples (Sn ≥20 

ppm, W ≥25 ppm, or Nb ≥40 ppm) (criterion 6), and anomalous geochemical enrichments in rocks or stream-sediment 

samples for Mo (>15 ppm) or mines with molybdenum in orerelated materials as reported in the MRDS and MAS databases 

(USGS, 1999a) (criterion 5).

Specific subareas of interest in area 1 (fig. G2) include 

(A) the 12 Ma granite at Treasure Mountain (criteria 3 and 4) 

with associated molybdenite mineralization (criterion 5); (B) 

altered and mineralized Miocene age rhyolite dikes and a narrow intrusion (criteria 3 and 4) that cuts the Whiterock pluton 

northeast of Crested Butte; geochemical anomalies of 

criteria 5 and 6 are mostly associated with replacement and 

vein mineralization at the margins of the Oligocene age 

Whiterock granodiorite pluton (Mutschler, Ernst, and others, 

1981), (C) the Mount Emmons and Redwell Basin molybdenum deposits, just west of Crested Butte, and (D) the Hunter 

Peak–Cataract Creek mineralized and altered area (Miller and 

Ficklin, 1976; Bryant, 1971) with anomalous concentrations 

of Sn, W, or Nb in stream sediments or rock samples (criterion 

6) and anomalous Mo in stream sediment or rock samples or 

within mines (criterion 5).

Grizzly Peak Caldera Area (Area 2, fig. G2): This area is in 

and adjacent to resurgent granitic intrusions of the 35–33 Ma 

Grizzly Peak caldera and associated with the Winfield felsic stocks and plugs (≈39 Ma), the latter mostly within and 

adjacent to the Twin Lakes pluton. The resurgent intrusions 

of the Grizzly Peak caldera meet criterion 2, have significant 

magnetic anomalies (criterion 7), and are hydrothermally 

altered. The entire tract has an anomalous scattering of Sn, 

W, or Nb in stream sediments or rock samples (criterion 6). 

Areas surrounding the Winfield felsic stocks meet criterion 3, 

owing to the evolved nature of these intrusions, and have other 

criteria including magnetic anomalies (criterion 7), and dense 

clustering of molybdenum geochemical anomalies in stream or 

rock samples or within mines (criterion 5).

Fossil Ridge (Area 3, fig. G2): This tract in the Fossil Ridge 

area (DeWitt and others, 2002) is underlain by intrusions 

and dikes of highly evolved Oligocene rhyolite and granite 

Table G1. Delineation criteria for granite porphyry molybdenum deposits in GMUG greater study area.

Diagnostic criterion for permissive tract delineation

1. Presence of Tertiary or very late Cretaceous hypabyssal intrusions or dikes (2 km buffer) as mapped by Bove and Knepper (2000), or 

 inferred intrusions (2 km buffer) as interpreted from aeromagnetic data (this volume, Chapter D, fig. D3). Several intrusive units and

 correlative dikes are excluded from the permissive criteria based on previous mineral assessment and geologic studies (Bove and others, 

 2000; DeWitt and others, 2000; Fridrich and others, 1998; Van Loenen and Gibbons, 1997; Cunningham and others, 1994; Sanford and

 others, 1987; Hon, 1987; Mutschler, Wright, and others, 1981; Mutschler, 1980). Criterion met by map units Tbgt, Tbrh, Tbrhd, Tbdr,

 Tdp, Tlrh, Tlsy, Trh, Tqm, Tegd, Term, Termd, Tgm, Tiyg, TmiA, Twfm, Tiys, TmiD, TmiDd, Teqm, Teqmd, Tea, Tead of

 Bove and Knepper (2000).

Diagnostic criteria for favorable tract delineation (in addition to criterion 1)

2. Tertiary age felsic intrusions and dikes generally known to be granitic or rhyolitic (2 km buffer). Previous geologic and mineral assessment 

 studies indicate that these intrusions are either unlike Climax-type intrusions (White and others, 1981) in their composition or mode of

 emplacement, or did not undergo mineralization or related hydrothermal alteration. Criterion met by map units Tbdr, Tlrh, Trh, and unit

 Tiyg as mapped by Bove and Knepper (2000).

3. Tertiary age high-silica, high-Nb, alkali rhyolite or granite intrusions or dikes (2 km buffer). Many are similar in composition and mode of 

 emplacement to Climax-type rhyolites or granites (White and others, 1981), which are associated with major economic stockwork

 molybdenite deposits. Many of these intrusive units are associated with molybdenite mineralization or hydrothermal alteration indicative

 of granite porphyry molybdenum mineralization. Map units include Tbgt, Tbrh, Tbrhd, Term, Termd, Tgm, and Twfm (Bove and 

 Knepper, 2000).

4. Criterion 2 or 3 and less than 24 Ma.

5. Surficial rock or stream-sediment samples contain Mo concentration exceeding 15 ppm (parts per million) (500 m buffer) or mines or

 occurrences with molybdenum in ore-related materials (1 km buffer) as reported in the MRDS and MAS databases (USGS, 1999a, 1999b).

6. Elevated concentrations of any of the following elements in stream sediments or rock samples: Sn ≥20 ppm, W ≥25 ppm, or Nb ≥40 ppm 

 (500 m buffer) or mine, occurrence, or mineralized site with huebnerite, fluorite, wolframite, scheelite, tungsten, or topaz present (1 km 

 buffer) or containing elevated F, Sn, W, Nb, Ta, Li, Be, or Rb (1 km buffer) as reported in the MRDS and MAS databases (USGS, 1999a, 

 1999b).

7. Magnetic anomalies interpreted to be Tertiary age intrusions as defined in criteria 2 and 3 (2 km buffer).
39o 30'109o 

37o 45'

70

139

65

92

50

50 

82

135

133

149

62

145

141

141

90

114

Grand

Junction

Delta

Montrose

Gunnison

Crested Butte

Lake City

Uravan

Telluride

Ouray

Silverton

Aspen

Disappointment Creek

Gunnison River

Uncompahgre River Lake Fork Cebolla

Creek

Tomichi Creek

Taylor River

Blue Mesa Reservoir

CochetopaCreek

Dolores River

0

0

Approximate outline of national forests in GMUG study area

EXPLANATION

550

Figure G1. GMUG greater study area, showing permissive tracts (gray shade) for porphyry molybdenum deposits. 
Mineral Resource Assessment for Porphyry Molybdenum Deposits 111

(criterion 3). These intrusions have anomalous concentrations 

of Sn, W, or Nb in stream sediments or rock samples (criterion 

6), and anomalous Mo in stream-sediment or rock samples or 

within mines (criterion 5). Molybdenum geochemical anomalies are very densely clustered in the vicinity of Cumberland 

Pass and Green Mountain (fig. G2). Most hydrothermal 

alteration in this tract, as detected by broad-band remote sensing, took place in the area surrounding Cumberland Pass and 

Green Mountain. The Tomichi dome, a Miocene (criterion 4) 

domal rhyolite intrusion (criterion 3) lies just outside the south 

end of this tract. However, unlike the Fossil Ridge intrusions 

to the north, this intrusion is unaltered and no occurrences of 

mineralized rock are reported. 

Lake City Caldera Area (Area 4, fig. G2): This tract generally encompasses the Miocene age Lake City caldera, which 

is nested with the older Uncompahgre caldera (Oligocene). 

Intrusions within this tract are young (criterion 4) and meet 

criterion 2, but they lack characteristics such as evidence of 

molybdenite mineralization, diagnostic chemical composition, 

hydrothermal alteration, or mode of emplacement (Sanford 

and others, 1987; Hon, 1987) typical of criterion 3. Anomalous molybdenum in rocks, sediment, or mines in the vicinity 

of the Lake City caldera (criterion 5) is largely due to high Mo 

concentrations in the intracaldera Sunshine Peak Tuff, anomalous concentrations in polymetallic veins (as much as 1,000 

ppm), and sparse dissemination around intrusions (Hon, 1987; 

Sanford and others, 1987). Magnetic anomalies (criterion 7) 

mostly coincide with intrusions related to resurgence of the 

Lake City caldera. 

Favorable criteria in two smaller areas within this tract 

warrant further discussion. Molybdenum and associated 

geochemical anomalies or occurrences (criteria 5 and 6) in the 

vicinity of Handies Peak (fig. G2) are related to polymetallic 

vein mineralization (Sanford and others, 1987). A 17.1 Ma 

(criterion 4) high-silica rhyolite dike in the Cuba Gulch area 

meets criterion 3 and is associated with anomalous concentrations of Sn, W, or Nb in stream-sediment or rock samples 

(criterion 6), and anomalous Mo in stream-sediment or rock 

samples or within mines (criterion 5). Field studies indicated 

that this dike is associated with mineralized pebble dikes, 

fluorite, and sparse molybdenum (Hon, 1987).

Silverton Caldera Area (Area 5, fig. G2): This large tract covers much of the Silverton caldera area. It contains numerous 

small intrusions and dikes of Miocene high-silica rhyolite (criteria 3 and 4). In addition, anomalous concentrations of Sn, 

W, or Nb occur in stream-sediment or rock samples (criterion 

6), and anomalous Mo in stream-sediment or rock samples 

and in mine samples (criterion 5). Molybdenum mineralization in the Red Mountain Pass and Anvil Mountain areas (fig. 

G2) was related to 23 Ma dacitic intrusions (discussed in the 

section on granodiorite molybdenum models). Large centers 

of pervasive hydrothermal alteration are centered in the Red 

Mountain Pass and Anvil Mountain areas but are generally 

related to the older dacitic intrusions (Bove and others, 2000). 

A Miocene rhyolite intrusion (criteria 3 and 4) in the Horseshoe Bend area (fig. G2) is associated with anomalous Mo in 

stream-sediment or rock samples (criterion 5), but molybdenite mineralization proved to be subeconomic during exploratory drilling in the 1970’s (Van Loenen and Gibbons, 1997). 

Intense quartz-sericite-pyrite alteration is zoned around this 

intrusion.

Undiscovered Deposit and Endowment Potential

Results of the undiscovered deposit and endowment 

potential assessment are given in table G3. The five-fold 

estimation of numbers of undiscovered deposits at the 90th, 

50th, 10th, 5th and 1st levels of confidence of 0, 0, 0, 1, 1 

indicates that the presence of additional Climax molybdenum 

type porphyry deposits, having grade and tonnage characteristics similar to those depicted by Cox and Singer (1986), is not 

very likely. However, the estimate of one deposit at the 5 and 

1 percent confidence levels suggests a small yet measurable 

potential for one more deposit occurring somewhere within a 

kilometer of the surface and within the bounds of the permissive and favorable tract areas. Inclusion of this small deposit 

potential acknowledges the fact that two (Mount Emmons and 

Redwell Basin) out of the nine deposits used in the construction of the Climax-type grade and tonnage models occur 

within the study area and that areas within the study area have 

been targets of repeated exploration interest for this type of 

deposit. The Mount Emmons and Redwell Basin deposits 

Table G2. Granite porphyry molybdenum tracts in GMUG greater study area.

Tract No.a Tract name Delineation criteria

P1 Permissive for granite molybdenum deposits 1

F1 West Elk Mountains 1, 3, 4, 5, 6, 7

F2 Grizzly Peak caldera 1, 2, 3, 5, 6, 7

F3 Fossil Ridge 1, 3, 5, 6

F4 Lake City caldera 1, 2, 3, 4, 5, 6, 7

F5 Silverton caldera 1, 3, 4, 5, 6

 aP denotes a permissive tract, F, a favorable tract.
1 A Cumberland Pass

4 5 

C B D E Anvil Mtn.Handies Peak Horseshoe Bend Red Mtn. Pass 39o 30'109o 

37o 45' 70 139 65 92 50 50 

82 

135 133 149 62 145 141 141 90 114 

Grand Junction Delta Montrose Gunnison Crested Butte Lake City Uravan Telluride Ouray Silverton Aspen 

Disappointment Creek Gunnison River Uncompahgre River Lake Fork Cebolla Creek Tomichi Creek Taylor River 

Blue Mesa Reservoir Cochetopa Creek Dolores River

0

0

Approximate outline of national forests in GMUG study area

EXPLANATION

550

Figure G2 (above and following page). GMUG greater study area, showing favorable tracts for granite porphyry deposits. 1, West Elk MountTreasure Mountain; B, dikes and intrusion that cut Whiterock pluton; C, Mount Emmons and Redwell Basin molybdenum deposits; D, Hunter P

mineralized and altered area; 2, Grizzly Peak caldera area; 3, Fossil Ridge; 4, Lake City caldera area; 5, Silverton caldera area.
Mineral Resource Assessment for Porphyry Molybdenum Deposits 113

Criterion 3— Tertiary age high-silica, high Nb, alkali rhyolite or granite

 intrusions or dikes (2 km buffer) 

Criterion 3 and less than 24 Ma (criterion 4)

Criterion 2 (Tertiary rhyolite or granite intrusions excluding criterion 3)

 and Mo anomalies or occurrences (criterion 5)

Criteria 2 or 3 and 5; with or without 4

Criteria 3, 4, and 5

Criteria 3 and 6

EXPLANATION 

Criteria 2 or 3, 4, and 6

Criteria 5 and 6

Criteria 3, 5, and 6

Criteria 3 and 7 (magnetic anomalies inferred to be Tertiary age intrusions) 

Criteria 3, 4 or 6, and 7

Criteria 6 and 7

Criteria 4, 6, 7, and 1 or 2

Criteria 1, 7, 6, and 5
114 Resource Potential and Geology, Grand Mesa, Uncompahgre, and Gunnison National Forests, Colorado 

along with the nearby world-class Climax and Henderson 

deposits attest to the unique character of the Tertiary magmatic terrane that underlies the study area and its capacity to 

generate deposits of this type. The repeated shows of exploration interest indicate that other knowledgeable parties believe 

the existence of additional Climax-type deposits may be a 

possibility. If present, they are more likely to reside in the 

favorable tract areas and deeper regions of the 1,000 m zone of 

consideration, where exploration has been less thorough.

The low expectations for the existence of undiscovered 

deposits are reflected in the molybdenum and ore endowment simulation results summarized in table G3. The results 

show that within the permissive and favorable tract areas for 

Climax-type deposits, a 92.8 percent probability of no deposits 

is present, along with a 7.2 percent probability of one deposit; 

a no-deposit scenario is nearly 13 times more likely than a 

one-deposit scenario. For the Climax-type tracts, the Mark3 

simulation indicates that at the 90, 50, 10 percent probability 

levels, both the molybdenum and ore endowments attributable 

to undiscovered deposits are likely to be zero. Expressed in 

terms of mean endowments, the mean molybdenum endowment for the area is expected to be about 40,000 t (metric tons) 

and the mean ore endowment, 21 million t. 

A more meaningful understanding of the economic 

importance of the Climax-type deposit potential can be gained 

from looking at the endowment frequency plots in Spanski and 

Bankey, this volume, Chapter F, Appendix F1 (see fig. F3). 

In these plots it can be seen that there is less than a 7 percent 

probability for occurrence of a molybdenum endowment equal 

to or larger than the 40,000 t mean. Within the study area 

the known deposits at Redwell Basin and Mount Emmons 

have molybdenum endowments of 240,000 and 390,000 t, 

respectively; they have not been commercially developed. 

The median molybdenum endowment for the deposit population used to create the Climax-type grade and tonnage models 

is 388,000 t. The determination of commercial value of the 

Mount Emmons deposit is complicated by local considerations, and the Redwell Basin deposit may be too small for its 

depth of burial; however, to expect that a deposit containing 

380,000 t of molybdenum would be near the development 

threshold is not unreasonable. The probability of an undiscovered deposit being present possessing a molybdenum 

endowment of more than 380,000 t is indicated to be less than 

3.6 percent. In spite of this low probability, that a history 

of recurring interest in exploring for these types of deposits 

exists, driven by market fluctuations, is unlikely to change in 

the near future. 

Granodiorite Porphyry Molybdenum 

Deposits

Genetic Model for Granodiorite Porphyry 

Molybdenum Deposits

Granodiorite molybdenum systems are exemplified by 

deposits and prospects at Buckingham, Nevada; White Cloud, 

Little Boulder Creek, and Thompson Creek, Idaho; and 

Boss Mountain, Endako, and Lime Creek, British Columbia 

(Soregaroli and Sutherland-Brown, 1976). The median size 

of these deposits is 94 million tons of ore averaging 0.085 

percent MoS2

 (Cox and Singer, 1986; model 26b), which 

is slightly below the average ore grade of granite porphyry 

deposits. Byproducts, when present, include tungsten, copper, 

gold, silver, lead, and antimony. Some granodiorite systems 

are transitional to true granite porphyry deposits having Cu:

Mo ratios of 1:10 to 1:30, whereas near end-member 

Table G3. Summary of results of resource endowment potential assessment for undiscovered Climaxtype porphyry molybdenum deposits within GMUG greater study area.

Mark3 inputs— Undiscovered deposit estimates:

Estimation confidence 90% 50% 10% 5% 1%

Deposits 0 0 0 1 1

Mark3 outputs— Deposit occurrence probability:

Number of deposits 0 1

Probability of occurrence 92.9% 7.1%

Resource endowment estimates (minimums):

Resource Probability Mean (probability)

90% 50% 10%

Molybdenum 0 0 0 40,000 (6.7%)

Ore 0 0 0 2,200,000 (6.7%)

Endowment given in metric tons.
Mineral Resource Assessment for Porphyry Molybdenum Deposits 115

granodiorite deposits have Cu:Mo ratios ranging from 1:1 to 1:

10 (Soregaroli and Sutherland-Brown, 1976; White and others, 

1981). 

Granodiorite molybdenum systems are associated with 

small composite stocks, late-stage batholiths, and less commonly single phase stocks. Intrusions range from quartz monzonite to granodiorite in composition; whole rock and traceelement chemistry is more similar to Cordilleran porphyry 

copper deposits (Lowell and Guilbert, 1970) than to granite 

molybdenum systems. Theodore (1982) classified granodiorite deposits as fluorine-deficient and considered this an 

important distinction from their fluorine-rich, granite molybdenum counterparts. North American granodiorite deposits 

are mostly confined to Mesozoic and Tertiary age intrusive 

rocks (Soregaroli and Sutherland-Brown, 1976). 

Most ore-related intrusive bodies are cylindrical (<1,500 

m in diameter) and elliptical to circular in plan view (Soregaroli and Sutherland-Brown, 1976; White and others, 1981). 

The associated ore bodies are cylindrical, tabular, or irregular 

in cross section; mineralization was generally confined to producing stockwork veinlets developed in or around the roof of 

the intrusion. These veinlets contain molybdenite and quartz 

with pyrite, biotite, and minor carbonates. Unlike the composition of veins within the granite molybdenum ore bodies, 

fluorite and fluorine-bearing minerals are absent. The central 

portion of most ore bodies contains high concentrations of 

tungsten, mostly in the form of scheelite. A halo of hypogene 

copper sulfides is typically situated above the central molybdenite zone and grades into peripheral zones of Pb-Zn and 

Au-Ag (Westra and Keith, 1981). Hydrothermal alteration 

related to granodiorite systems is similar to that of the granite 

molybdenum deposits (Lowell and Guilbert, 1970: Theodore, 

1982). However, greisen and zones of pervasive silicification, 

which are common in granite molybdenum deposits (White 

and others, 1981), have not been reported. Other distinctions 

from granite systems include the lack of fluorine and tin-bearing minerals, and a more weakly developed potassic assemblage (Westra and Keith, 1981). 

Description of the Areas Containing Known 

Granodiorite Porphyry Molybdenum Deposits

No molybdenum or copper has been produced from 

granodiorite porphyry molybdenum deposits in the GMUG 

greater study area or other parts of Colorado. However, several subeconomic granodiorite molybdenum prospects have 

been located in western Colorado. In the Middle Fork–Ophir 

Pass, Capitol City, Iron Beds, and Matterhorn Peak areas 

(fig. G3), a close spatial association is documented between 

subeconomic porphyry Mo-Cu mineralized rocks, base-metal 

sulfide veins, and 26–25 Ma monzonite to quartz monzonite 

intrusions (Bove and others, 2000; Ringrose and others, 1986; 

Slack, 1980; Caskey, 1979; Pyle, 1980). These mineralized 

and altered intrusions are part of a swarm of calc-alkaline 

intrusions emplaced between 26 and 25 Ma over a broad 

region of the western San Juan Mountains (Bove and others, 

2000). 

A weakly mineralized Mo-Cu porphyry system present in 

the Middle Fork–Ophir Pass area, west of Silverton (fig. G3), 

is temporally and genetically related to a late quartz monzonite 

phase (25 Ma) of the Sultan Mountain stock (Ringrose and 

others, 1986). At this locality, quartz-molybdenite stockwork 

veins associated with intense quartz-sericite-pyrite (QSP) 

altered rock are cut by molybdenite-bearing base-metal 

veins present mostly on the margins of the porphyry system 

(McCusker, 1982). 

Widespread hydrothermally altered rocks in the Capitol 

City area (fig. G3) are also spatially zoned with respect to 

several small 26 Ma monzonite to monzogranite porphyry 

stocks. In some areas, however, alteration does not correspond 

spatially to intrusive rock outcrops, suggesting the presence of 

concealed intrusions at shallow depth. In addition to pyrite, 

disseminations of microscopic chalcopyrite are relatively 

abundant within the pervasively altered rock. Base-metal 

veins are also spatially associated with the intrusions and 

contain elevated concentrations of Sb, As, Bi, Cd, and Mo 

(Sanford and others, 1987). 

Intrusions in the Iron Beds area (fig. G3) are related to 

areas of locally strong propylitization, some of which are overprinted by intense hydrothermal alteration with slight Cu, Mo, 

and Zn anomalies (Caskey, 1979). Areas of intense alteration 

typically grade from an inner zone of silicified rock outward 

into argillic, weak argillic, and finally into propylitized rock. 

No obvious concentric zoning of more intensely altered rock 

is apparent around the Iron Beds intrusions, indicating that the 

more intense alteration may have been related to deeper seated 

intrusion(s). Most quartz veins in the Iron Beds area are barren with no visible sulfides at the surface (Caskey, 1979). 

The Matterhorn Peak stock, located about 2 km north of 

the Iron Beds area (fig. G3), comprises at least three separate 

intrusions that range in composition from monzonite to quartz 

monzonite porphyry (Pyle, 1980). A hydrothermal alteration halo zoned outward from silicic to QSP, and to propylitic 

assemblages is distributed concentrically around the westernmost of these intrusions. Molybdenum values range as high 

as 150 ppm in silicified zones, and visible molybdenite can be 

recognized in scarce veinlets (Pyle, 1980). Elevated copper 

concentrations (as much as 130 ppm) are generally restricted 

to argillic-altered rock, which also contains sparse disseminated chalcopyrite.

A quartz monzonite phase of the Wilson Peak stock contains a large zone of disseminated and vein-filling chalcopyrite 

(Bromfield and others, 1972). However, the stock lacks welldeveloped QSP alteration and molybdenum; associated veins 

are rich in gold rather than base metals.

Dacite porphyry intrusions dated 23 Ma at Red Mountain near Lake City, Colo., are associated with vertically and 

horizontally zoned hydrothermally altered rock and subeconomic Mo and Cu mineralization (Bove and Hon, 1992). The 

Red Mountain area is also known to host one of the largest 

replacement alunite deposits in the United States (Bove and 
39o 30'109o 

37o 45' 70 139 65 92 50 50 

82 

135 133 149 62 145 141 141 90 114 

Grand Junction Delta Montrose Gunnison Crested Butte Lake City Uravan Telluride Ouray Aspen 

Disappointment Creek Gunnison River Uncompahgre River Lake Fork Cebolla Creek Tomichi Creek Taylor River 

Blue Mesa Reservoir Cochetopa Creek Dolores River

0

0

Approximate outline of national forests in GMUG study area

EXPLANATION

550

8

10

3

7

5

9 4

6

2

1

Wilson Peak Silverton

Figure G3 (above and following page). GMUG greater study area, showing favorable tracts for granodiorite porphyry molybdenum deposits. 

area; 3, Cow Creek–Larson area; 4, Capitol City–Matterhorn Peak and Iron Beds; 5, East Red Mountain; 6, Lake City caldera; 7, Carson; 8, West caldera; 10, Sneffels Peak.
Mineral Resource Assessment for Porphyry Molybdenum Deposits 117

Hon, 1992). A weakly mineralized quartz monzonite intrusion in nearby Alpine Gulch is genetically associated with the 

Red Mountain intrusions (Bove and others, 2000). The Alpine 

Gulch intrusion has features associated with Cu and (or) Mo 

mineralization including fragmental dikes, tourmaline breccias, magnetite veinlets, and quartz veins containing pyrite, 

hematite, sphalerite, galena, and chalcopyrite (Bove and 

others, 2000). However, alteration haloes are conspicuously 

absent around this intrusion.

Application of the Deposit Model for a Mineral 

Resource Assessment of Granodiorite Porphyry 

Molybdenum Deposits

The criteria used to define permissive and favorable tracts 

for granodiorite porphyry molybdenum deposits are listed 

in table G4. Many of these criteria are summarized in the 

porphyry molybdenum low fluorine deposit model (Theodore, 

1982; model 21b), and were derived from numerous detailed 

studies of economic granodiorite molybdenite porphyry 

deposits. Areas identified in the permissive model are those 

that are underlain by Tertiary-age intrusions as mapped in the 

GIS database compiled by Bove and Knepper (1999) and Day 

and others (1999) or inferred from aeromagnetic survey data 

(Chapter D, fig. D3). Several intrusive units, which are not 

known to host any significant mineralization or related hydrothermal alteration, are excluded from the permissive tract. 

Each of the intrusions included in the criteria was surrounded 

by a 1 km buffer to include a two-dimensional approximation 

to allow for the presence of covered deposits at depth. 

Various combinations of all the “favorable” criteria (table 

G4) were applied to areas in the permissive tract to delineate favorable tracts in the GMUG study area. Distinctions 

Criterion 5— Mo mines, geochemistry, mineralogy, buffered

Criteria 4 (hydrothermally altered) and 5

Criteria 4, 5, and 7 (magnetic anomaly inferred to be Tertiary intrusion)

Criteria 3 (known mineralized type of Tertiary intrusive) and 4

Criteria 2 or 3, and 5 

Criteria 2 and 4

EXPLANATION

Criteria 2, 4, and 5

Criteria 2, 4, and 6 (elements associated with Mo in mines or geochemistry,

 buffered)

Criteria 2, 4, 5, and 7

Criteria 3 and 6

Criteria 3, 4, and 7
118 Resource Potential and Geology, Grand Mesa, Uncompahgre, and Gunnison National Forests, Colorado 

between intrusions described in favorable criteria 2 and 3 were 

based on previous mineral assessment and geologic studies (Bove and others, 2000; Van Loenen and Gibbons, 1997; 

Cunningham and others, 1994; Sanford and others, 1987; Hon, 

1987; Mutschler, Wright, and others, 1981; Mutschler, 1980). 

Permissive Tracts

In the GMUG greater study area, 1,371 mi2 is classified 

as permissive for the occurrence of granodiorite porphyry 

molybdenum deposits (fig. G1). As explained in table G4, 

these areas are coincident with Late Cretaceous to Tertiary 

intrusions that were given a 1 km buffer to include a twodimensional approximation to allow for the presence of covered deposits at depth. As shown in figure G1, the permissive 

tract is extensive owing to the abundance of intermediate to 

silicic composition intrusions in the eastern 3/4 of the GMUG 

study area. These intrusions are absent in the tectonically 

stable western part of the study area.

Favorable Tracts

In the GMUG greater study area, 324 mi2 is classified 

favorable for the occurrence of granodiorite porphyry 

molybdenum deposits (fig. G3, table G5). These lands represent about 24 percent of the permissive tract. The critical 

criteria used for delineating regions favorable for hosting 

granodiorite porphyry molybdenum deposits are listed in table 

G4. These criteria are more restrictive than those used to 

delineate the permissive tracts.

Whiterock Area (Area 1, fig. G3): This tract covers the Whiterock and Italian Mountain granodiorite plutons in the West Elk 

Mountains. Assessment criteria applicable within this tract 

include granodiorite plutons (criterion 3) that have locally 

undergone hydrothermal alteration (criterion 4). In addition, 

this tract is characterized by local base-metal geochemical 

anomalies in stream-sediment or rock samples (Pb ≥100 ppm, 

Zn ≥250 ppm, or Ag ≥1 ppm) (criterion 6) and anomalous 

geochemical enrichments in stream-sediment or rock samples 

for Mo (criterion 5) or mines with molybdenum in ore-related 

materials as reported in the MRDS and MAS databases 

(USGS, 1999a, 1999b). Magnetic anomalies (criterion 7) are 

also associated with these plutons. Disseminated pyrite, chalcopyrite, and molybdenite showings are present near intrusive 

contacts with sedimentary rocks (Mutschler, Ernst, and others, 

1981) but have not been used as a factor in assessing deposit 

potential in the area.

Table G4. Delineation criteria for granodiorite porphyry molybdenum deposits in GMUG greater study area.

Diagnostic criterion for permissive tract delineation

1. Presence of Tertiary or very late Cretaceous hypabyssal intrusions or dikes (2 km buffer) as mapped by Bove and Knepper (2000), or inferred 

 intrusions (2 km buffer) as interpreted from aeromagnetic data (this volume, Chapter D, fig. D3). Several intrusive units and correlative

 dikes are excluded from the permissive criteria based on previous mineral assessment and geologic studies (Bove and others, 2000; DeWitt and 

 others, 2000; Fridrich and others, 1998; Van Loenen and Gibbons, 1997; Cunningham and others, 1994; Sanford and others, 1987; Hon, 1987; 

 Mutschler, Ernst, and others, 1981; Mutschler, 1980). Criterion met by map units Tdp, Tlsy, Tqm, Tiys, TmiD, TmiDd, Teqm, Tea, Tead, 

 Tegd, and TmiA of Bove and Knepper (2000).

Diagnostic criteria for favorable tract delineation (in addition to criterion 1)

2. Tertiary age intermediate to silicic composition intrusions and dikes (1 km buffer). Previous geologic and mineral assessment studies indicate that 

 these intrusions are related to hydrothermal alteration or sparse mineralization. Criterion met by map units Tlsy, Tiys, Tea, Tead, Tegd,

 TmiA as mapped by Bove and Knepper (2000).

3. Tertiary age hypabyssal stocks, plugs, and dikes ranging from quartz monzonite to granodiorite in composition (1 km buffer). May represent 

 cores of eroded volcanos or may be the uppermost portion of underlying calc-alkaline batholiths (Bove and others, 2000). Some intrusions are 

 the centers of radial or linear dike swarms. Intrusions may range from simple to complexly zoned and in many places are associated with

 pervasive hydrothermal alteration with base-metal veins and subeconomic Mo-Cu mineralization as determined by previous geologic and

 mineral assessment studies (Bove and others, 2000; Van Loenen and others, 1997; Cunningham and others, 1994; Sanford and others, 1987; 

 Hon, 1987; Mutschler, Ernst, and others, 1981; Mutschler, 1980). Map units include Tdp, Tqm, TmiD, TmiDd, and Teqm (Bove and

 Knepper, 2000).

4. Hydrothermally altered rock as interpreted using broad-band spectroscopy (Bove and Knepper, 2000) spatially coincident with intrusions.

5. Surficial rock or stream-sediment samples contain Mo concentration exceeding 15 ppm (parts per million) (500 m buffer) or mines or occurrences 

 with molybdenum in ore-related materials (1 km buffer) as reported in the MRDS and MAS databases (USGS, 1999a, 1999b), or the

 area defined as a mineralized area containing granodiorite molybdenum (Wilson and Spanski, this volume, Chapter E).

6. Elevated concentrations of any of the following elements in stream sediments or rock samples: Pb ≥100 ppm, Zn ≥250 ppm, or Ag ≥1 ppm (500 

 m buffer) or mine, occurrence, or mineralized site with sphalerite, galena, chalcopyrite present (1 km buffer) or containing elevated Pb, Zn, Cu,

 Ag, or Au (1 km buffer) as reported in the MRDS and MAS databases (USGS, 1999a, 1999b).

7. Magnetic anomalies inferred to be caused by Tertiary age intrusions as defined in criteria 2 and 3, where associated with surficial rock or stream- 

 sediment samples containing Mo concentrations exceeding 15 ppm or mines or occurrences with molybdenum in ore-related materials as

 reported in the MRDS and MAS databases (USGS, 1999a, 1999b) (2 km buffer).
Mineral Resource Assessment for Porphyry Molybdenum Deposits 119

Ruby Area (Area 2, fig. G3): This tract encompasses a 

north- to northeast-trending band of small stocks and plugs of 

andesite to granodiorite (criterion 3)—some of which are the 

centers of radial or linear dike swarms—extending from the 

West Elk Wilderness along the crest of the Ruby Range (Ruby 

Peak, Mount Owen, Afley, Augusta, and Paradise Peak stocks) 

to the Elk Range and Mount Bellview (fig. G3). The southernmost stock, which is located in the West Elk Wilderness, is 

andesitic and associated with localized hydrothermal alteration 

(criterion 4). Gaskill and others (1977) reported geochemical 

anomalies associated with this altered intrusion; however, no 

metallic minerals other than pyrite were observed. 

The small granodiorite stocks of the Ruby Range (criterion 3) are associated with hydrothermal alteration (criterion 

4) and zoned metallic mineralization (Mutschler, Ernst, and 

others, 1981). The Paradise Pass stock at the north end of the 

Ruby Range is associated with hydrothermal alteration (criterion 4), local base-metal anomalies in stream-sediment or rock 

samples (criterion 6), and anomalous Mo in stream-sediment 

or rock samples or within mines (criterion 5). The intrusion is 

cut by a stockwork of QSP veins that locally contain molybdenite and chalcopyrite (Mutschler, Ernst, and others, 1981); 

pervasive quartz-sericite-pyrite alteration is locally developed, 

especially along the margins of the stock.

Mount Bellview is the site of a zoned granodiorite to 

quartz monzonite intrusive complex (criterion 3). The intrusions are hydrothermally altered (criterion 4) with local basemetal anomalies in stream-sediment or rock samples (criterion 

6), and anomalous Mo in stream-sediment or rock samples 

or within mines (criterion 5). A 1,200 m diameter hornfels 

aureole in Mancos Shale host rock surrounds the intrusive 

complex, and local quartz-molybdenite veinlets are also present (Lynch and others, 1985).

Cow Creek–Larson Area (Area 3, fig. G3): This tract encompasses three eroded Oligocene age volcanic centers (from 

Larson, Cimarron, and Cow Creek volcanos) and associated 

intermediate composition stocks and dikes (criterion 3). The 

composite stocks are hydrothermally altered (criterion 4) and 

have local base-metal anomalies in stream-sediment or rock 

samples (criterion 6), and anomalous Mo in stream-sediment 

or rock samples or within mines (criterion 5). Weak disseminated mineralization and alteration are related to emplacement 

of these stocks (Hon and others, 1986). Veins and mineralized 

shears are present along the margins of dikes and in radial 

fractures, and these also appear to be related to the volcanic 

complexes (Hon, 1987). 

Capitol City–Matterhorn Peak (Area 4, fig. G3): This tract 

contains hydrothermally altered (criterion 4), Miocene age 

monzonite to quartz monzonite stocks and plugs (criterion 3) that extend northward from the Capitol City area to 

Matterhorn Peak (fig. G3). These intrusions coincide with 

anomalous Mo in stream-sediment or rock samples or within 

mines (criterion 5) and local base-metal anomalies in streamsediment or rock samples (criterion 6). Scattered magnetic 

anomalies are also present within this tract and overlap with 

geochemical anomalies of criteria 5 and 6, further reinforcing 

the favorable designation. However, some magnetic anomalies 

are related to more silicic intrusions, which are discussed in 

the previous section on granite molybdenum systems, and for 

that reason the presence of a magnetic anomaly alone is not 

sufficient cause for applying a favorable classification.

East Red Mountain (Area 5, fig. G3): Tract includes hydrothermally altered (criterion 4) dacite porphyry intrusions 

(criterion 3) in the vicinity of Red Mountain (near Lake City) 

and an unaltered dacite to quartz monzonite intrusion (criterion 3) in nearby Alpine Gulch. Anomalous concentrations 

of base metals in stream-sediment or rock samples (criterion 

6), and anomalous Mo in stream-sediment or rock samples 

or within mines (criterion 5) are present in the area around 

Red Mountain. The Alpine Gulch intrusion is associated with 

anomalous base-metal concentrations in stream-sediment or 

rock samples (criterion 6). Details on mineralization related 

to these intrusions are found in the previous section describing 

areas containing known deposits.

Lake City caldera (Area 6, fig. G3): Includes quartz syenite 

intrusions (criterion 2) associated with the resurgence of the 

Miocene age Lake City caldera. Hydrothermal alteration is 

commonly present in or adjacent to these intrusions (criterion 

Table G5. Granodiorite porphyry molybdenum tracts in GMUG greater study area.

Tract No.a Tract name Delineation criteria

P1 Permissive for granodiorite molybdenum deposits 1

F1 Whiterock-Italian Mountain 1, 2, 4, 5, 6, 7

F2 Ruby 1, 3, 4, 5, 6

F3 Cow Creek-Larson 1, 3, 4, 5, 6

F4 Capitol City-Matterhorn Peak 1, 3, 4, 5, 6

F5 East Red Mountain 1, 3, 4, 5, 6

F6 Lake City caldera 1, 3, 4, 5, 6

F7 Carson 1, 3, 4

F8 West Silverton 1, 3, 4, 5, 6

F9 Silverton caldera 1, 3, 4, 5, 6, 7

F10 Sneffels Peak 1, 2, 4, 5, 6

 aP denotes a permissive tract; F, a favorable tract.
120 Resource Potential and Geology, Grand Mesa, Uncompahgre, and Gunnison National Forests, Colorado 

4). Anomalous concentrations of base metals in stream-sediment or rock samples (criterion 6), and anomalous Mo in 

stream-sediment or rock samples or within mines (criterion 

5) are present in the vicinity of these intrusions. Convective 

circulation of large volumes of meteoric water during resurgence of the Lake City caldera appears to have scavenged 

base metals and molybdenum from related pyroclastic units 

with subsequent deposition into veins and fractures above and 

adjacent to these intrusions (Bove and others, 2000). 

Carson (Area 7, fig. G3): Tract includes intermediate composition intrusive core of the Oligocene age Carson volcano 

(criterion 3); the intrusion and surrounding area is hydrothermally altered (criterion 4). Previous studies indicate that the 

central intrusion and adjacent lavas are irregularly altered and 

cut by fracture zones with quartz-barite-sulfide veins (Lipman 

and others, 1976). 

West Silverton (Area 8, fig. G3): Within this large tract are 

hydrothermally altered (criterion 4), Oligocene age quartz 

monzonite to granodiorite stocks and plutons (criterion 3) that 

extend from Howardsville (east of Silverton) west to Mount 

Wilson (fig. G3). The surface rocks, stream sediment, mine 

occurrences, and mineralized sites near these intrusions contain anomalous concentrations of base metals (criterion 6) and 

anomalous levels of Mo (criterion 5). Areas of notable mineralization include the Middle Fork–Ophir Pass and Wilson Peak 

areas, discussed in a previous section. 

Silverton caldera (Area 9, fig. G3): Tract contains Miocene 

age stocks and dikes of dacite porphyry in the vicinity of 

Houghton Mountain, Engineer Pass, and in the Red Mountain 

area, north of Silverton (fig. G3). Intrusions in these areas 

have undergone hydrothermal alteration (criterion 4) and 

locally coincide with geochemical anomalies of criteria 5 and 

6, as well as magnetic anomalies (criterion 7). The dacite 

intrusions within this tract are similar in age, texture, and 

composition to alunitized and mineralized dacite porphyry 

intrusions at Red Mountain, near Lake City, and at the Summitville mine in the eastern San Juan Mountains (Bove and 

others, 2000). Combined geochemical anomalies (criteria 5 

and 6) in the absence of criterion 2 or 3 intrusions are clustered in the areas of Mineral Creek and the Red Mountain Pass 

area; however, these anomalies are likely related to polymetallic vein and breccia deposits (see previous section describing 

areas containing known deposits). 

Sneffels Peak (Area 10, fig. G3): This tract includes Oligocene 

Sneffels Peak granodiorite stock and similar intrusions (criterion 2) several kilometers to the west. These intrusions have 

been hydrothermally altered (criterion 4); however, only the 

area around Mount Sneffels has an associated anomalous basemetal and molybdenum geochemical signature (criteria 5 and 

6). Mineralization in the vicinity of Mount Sneffels took place 

later than the emplacement of the stock (Miocene) and predominantly resulted in vein and replacement types of deposits 

(Lipman and others, 1976). Anomalous concentrations of base 

metals in stream-sediment or rock samples (criterion 6), and 

anomalous Mo in stream-sediment or rock samples or within 

mines (criterion 5) are present in the vicinity of these intrusions. Several aeromagnetic anomalies are related to these 

intrusions (criterion 7).

Undiscovered Mineral Deposit and Endowment 

Potential

Results of the undiscovered mineral deposit and endowment potential assessment are given in table G6. The estimation of numbers of undiscovered deposits at the 90th, 50th, 

10th, 5th, and 1st levels of confidence of 0, 0, 0, 0, 2 indicates 

that the presence of additional granodiorite porphyry molybdenum, low fluorine-type deposits, having grade and tonnage characteristics similar to those depicted by Menzie and 

Theodore (1986), is not very likely. However, the estimate of 

two deposits at the 1 percent confidence level implies that a 

small yet measurable potential for up to two more deposits 

occurring within a kilometer of the surface exists. The small 

deposit potential recognizes the fact that the magmatic terrane that underlies the greater study area and that dominated 

geologic events during Tertiary time is unique in terms of its 

molybdenum geochemistry. As each pulse of magmatic activity evolved, it gave rise to a series of intrusive and extrusive 

events in which the later events demonstrated a tendency to 

become anomalously enriched in molybdenum. This process 

is clearly demonstrated by the younger granite systems and 

their associated Climax-type mineralization, and it is believed 

that the earlier evolving granodiorite systems also possessed 

this same intrinsic molybdenum geochemistry. Although there 

are no known examples of granodiorite porphyry molybdenum, low fluorine deposits, prospects of this type are present. 

Where deposits are present, they are more likely to occur in 

the favorable rather than the permissive tract areas and in the 

deeper regions of the 1,000 m zone of consideration or where 

a host terrane is buried by younger surficial materials.

Results of the Mark3 molybdenum and ore endowment 

simulations are also summarized in table G6. The results 

show that within the permissive and favorable tract areas for 

the granodiorite porphyry molybdenum, low fluorine-type 

deposits there is a 96.1 percent probability of no deposits 

being present, a 1.9 percent probability of one deposit and a 

2 percent probability of two deposits. A no-deposit scenario 

is nearly 48 times more likely to occur than either a one- or 

a two-deposit scenario. The Mark3 simulation indicates 

that at the 90, 50, and 10 percent probability levels, both the 

molybdenum and ore endowments attributable to undiscovered 

deposits are zero. Expressed in terms of mean endowments, 

the mean molybdenum endowment is expected to be about 

8,700 t and the mean ore endowment, 11 million t. 

A better appreciation of the exploration and development potential associated with the deposit type can be gained 

through looking at the endowment frequency plots in Spanski 

and Bankey, this volume, Chapter F, Appendix F1 (see figs. F3 

and F4). These plots show less than a 4 percent probability of 

any molybdenum endowment; however, the probability drops 
Mineral Resource Assessment for Porphyry Molybdenum Deposits 121

even lower when economic viability is considered. There 

are no examples in the greater study area or in Colorado to 

use as an economic gauge; however, it is reasonable to apply 

the same parameters used for the Climax-type deposits. If a 

380,000 t molybdenum endowment is used, the probability of 

that or a larger endowment occurring is only one half of 1 percent, and the simulation does not clarify whether that endowment is associated with one or two deposits. The 380,000 t 

constraint may be too large in light of the fact that a deposit of 

this type is being actively mined at Thompson Creek in Idaho, 

which has a reported endowment of 250,000 t of molybdenum 

(Cox and Singer, 1986). At this reduced level, the probability 

of occurrence increases only to 1 percent. Owing also to the 

size and the disseminated nature of the molybdenum mineralization, a deposit of this type would only be of interest to 

major mining companies. Under these circumstances, exploration interest in targets of this type will be virtually nonexistent 

unless market demand and price for molybdenum increase.

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hydrothermal alteration in the Grand Mesa, Uncompahgre, 

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Carten, R.B., White, W.H., and Stein, H.J., 1993, High-grade 

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origin, in Kirkham, R.V., Sinclair, W.D., Thorpe, R.I., and 

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Caskey, D.J., 1979, Geology and hydrothermal alteration of 

the Iron Beds area, Hinsdale County, Colorado: Austin, 

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Cox, D.P., and Singer, D.A., eds., 1986, Mineral deposit models: U.S. Geological Survey Bulletin 1693, 379 p.

Cunningham, C.G., Naeser, C.W., Marvin, R.F., Luedke, R.G., 

and Wallace, A.R., 1994, Ages of selected intrusive rocks 

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U.S. Geological Survey Bulletin 2109, 31 p.

Day, W.C., Green, G.N., Knepper, D.H., Jr., and Phillips, R.C., 

1999, Spatial geologic data model for the Gunnison, Grand 

Mesa, Uncompahgre National Forests mineral assessment area, southwestern Colorado and digital data for the 

Leadville, Montrose, Durango, and the Colorado parts of 

the Grand Junction, Moab, and Cortez 1° × 2° geologic 

maps: U.S. Geological Survey Open-File Report 99-427, 

CD-ROM.

Table G6. Summary of results of resource endowment potential assessment for undiscovered porphyry 

molybdenum, low fluorine deposits within GMUG greater study area.

Mark3 inputs— Undiscovered deposit estimates:

Estimation confidence 90% 50% 10% 5% 1%

Deposits 0 0 0 0 2

Mark3 outputs— Deposit occurrence probability:

Number of deposits 0 1 2

Probability of occurrence 96.1% 1.9% 2%

Resource endowment estimates (minimums):

Resource Probability Mean (probability) 90% 50% 10%

Molybdenum 0 0 0 8,700 (3.7%)

Ore 0 0 0 11,000,000 (3.7%)

Endowment given in metric tons.
122 Resource Potential and Geology, Grand Mesa, Uncompahgre, and Gunnison National Forests, Colorado 

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MF–1949, scale 1:24,000.

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Flats Wilderness Study Area, western San Juan Mountains, 

Colorado: U.S. Geological Survey Open-File Report 86-

431, 37 p.

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Luedke, R.G., and Steven, T.A., 1976, Multiple ages of midTertiary mineralization and alteration in the western San 

Juan Mountains, Colorado: Economic Geology, v. 71, no. 

3, p. 571–588.

Lowell, J.D., and Guilbert, J.M., 1970, Lateral and vertical 

alteration-mineralization zoning in porphyry ore deposits: 

Economic Geology, v. 65, no. 4, p. 373–408.

Ludington, Steve, 1981, The Redskin Granite; evidence for 

thermogravitational diffusion in a Precambrian granite 

batholith, in Granites and rhyolites: Journal of Geophysical 

Research, B, v. 86, no. 11, p. 10423–10430.

Lynch, W.C., Beaty, D.W., Gonzalez-Urien, Eliseo, and 

Reisbick, Fred, 1985, The Mt. Bellview, Colorado, igneoushydrothermal-breccia complex—A calc-alkaline molybdenite occurrence: Geological Society of America Abstracts 

with Programs, v. 17, no. 4, p. 253.

Menzie, W.D., and Theodore, T.G., 1986, Grade and tonnage 

model of porphyry Mo, low-F, in Cox, D.P., and Singer, 

D.A., eds., Mineral deposit models: U.S. Geological Survey 

Bulletin 1693, p. 120–122.

Miller, W.R., and Ficklin, W.H., 1976, Molybdenum mineralization in the White River National Forest, Colorado: U.S. 

Geological Survey Open-File Report 76-711, 29 p.

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Treasure Mountain Dome, Colorado, and the genesis of 

stockwork molybdenite deposits, in Tectonics and mineral 

resources of southwestern North America: New Mexico 

Geological Society Special Publication 6, p. 199–205.

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1981, Igneous rocks of the Elk Mountains and vicinity, Colorado—Chemistry and related ore deposits, in Epis, R.C., 

and Callender, J.F., eds., Western Slope Colorado—Western 

Colorado and eastern Utah: New Mexico Geological Society Guidebook, 32nd Field Conference, p. 317–324.

Mutschler, F.E., Wright, E.G., Ludington, Steve, and Abbott, 

J.T., 1981, Granite molybdenite systems: Economic Geology, v. 76, no. 4, p. 874–897.

Pyle, P.F., 1980, Geology and hydrothermal alteration of the 

Matterhorn Peak stock, Hinsdale County, Colorado: Austin, 

Tex., University of Texas M.A. thesis, 89 p.

Ringrose, C.R., Harmon, R.S., Jackson, S.E., and Rice, C.M., 

1986, Stable isotope geochemistry of a porphyry-style 

hydrothermal system, West Silverton district, San Juan 

Mountains, Colorado, U.S.A., in F. Earl Ingerson Festschrift—Part III: Applied Geochemistry, v. 1, no. 3, p. 

357–373.

Sanford, R.F., Grauch, R.I., Hon, Ken, Bove, D.J., Grauch, 

V.J.S., and Korzeb, S.L., 1987, Mineral resources of the 

Redcloud Peak and Handies Peak Wilderness Study Area, 

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469–474.

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resource potential and geology of the San Juan National 

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A.G. Raby and J.S. Dersch: U.S. Geological Survey 

Bulletin 2127, 140 p.

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W.B., Bookstrom, A.A., and Surface, V.E., 1968, Multiple 

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Graton–Sales Volume: New York, American Institute of 

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p. 270–316.
U.S. Department of the Interior

U.S. Geological Survey

Mineral Resource Potential Assessment for the 

Sandstone-Hosted Uranium Deposit Type

By Gregory T. Spanski, Viki Bankey, and Steven M. Smith

Chapter H of

Resource Potential and Geology of the Grand Mesa, Uncompahgre,

and Gunnison (GMUG) National Forests and Vicinity, Colorado

Edited by Viki Bankey

U.S. Geological Survey Bulletin 2213–H
Contents

Abstract...................................................................................................................................................... 126

Introduction ............................................................................................................................................... 126

Model for Sandstone-Hosted Uranium Mineral Deposits ................................................................. 126

Description of Sandstone-Hosted Uranium Deposits in the GMUG Greater Study Area............. 127

Delineation of Mineral Resource Potential Areas for Sandstone-Hosted Uranium Deposits..... 129

Permissive Tracts............................................................................................................................. 130

Favorable Tracts............................................................................................................................... 131

Undiscovered Deposit and Endowment Potential............................................................................... 133

References Cited ...................................................................................................................................... 133

Figures

H1. Map of GMUG greater study area showing permissive tracts for sandstone-hosted

uranium deposits .............................................................................................................................. 128

H2. Map of GMUG greater study area showing favorable tracts for sandstone-hosted

uranium deposits .............................................................................................................................. 132

Tables

H1. Delineation criteria for sandstone-hosted uranium deposits in GMUG greater

study area .......................................................................................................................................... 130

H2. Sandstone-hosted uranium tracts in GMUG greater study area.............................................. 131
Abstract

Sandstone-hosted uranium deposits occur in the Uravan 

mineral belt in the extreme western portion of the GMUG 

greater study area. From the late 1940’s to the mid 1970’s, 

the area has served as the source for the production of more 

than 63 million pounds of U3O8 and 330 million pounds of 

V2O5. In this study, an area of 6,700 square miles is identified 

as permissive for the presence of deposits containing endowments of more than 2.3 metric tons of U3

O8. Approximately 

1,500 square miles of these lands fall within the GMUG 

National Forests and 2,650 square miles are in land managed 

by the Bureau of Land Management. Data for anomalous 

levels of radioactivity, anomalous levels of uranium in stream 

sediments and water, the presence of existing occurrences, and 

previous assessment results suggest that some 1,350 square 

miles of the permissive area is believed to have substantially 

higher probability of hosting these deposits and is delineated 

as favorable. However, owing to the lack of grade and tonnage 

information about existing deposits, no estimates concerning 

the possible numbers of undiscovered deposits that might be 

present could be conducted. Past experience has shown that 

exploration and development activity related to these deposits 

are extremely sensitive to price fluctuations, which suggests 

that future increases in price above $15 per pound level would 

be a sufficient stimulus.

Introduction

Uranium hosted in sedimentary rocks occurs in five 

geographically distinct areas associated with the GMUG 

greater study area in western Colorado: the Uravan mineral 

belt, Placerville area, Marshall Pass area, Cochetopa area, 

and Ruby-Irwin area. Their existence is attributed to at 

least three different modes of origin. The Placerville 

occurrences represent mineralization that appears to have been 

controlled by a lacustrine shoreline effect. These deposits 

are predominantly vanadium rich: recorded production is 

13 million lb of V2

O5 with only minor byproduct uranium 

production (31 thousand lb of U3

O8). The Marshall Pass 

and Cochetopa areas have a history of significant production 

(nearly 8 million lb) of U3

O8; however, the origins of these 

deposits remain in question and have variously been attributed 

to either intrusion-related magmatic-hydrothermal activity 

(Malan and Ranspot, 1959) or supergene enrichment (Olson, 

1988). Occurrences in the Uravan mineral belt and the 

Ruby-Irwin area are genetically similar and descriptively 

match the characteristics of the deposit model for sandstonehosted uranium deposits (Turner-Peterson and Hodges, 

1986). Deposits in the Uravan mineral belt have produced 

approximately 63 million lb of U3

O8 and 330 million lb of 

V2O5 through the early 1970’s (Nelson-Moore and others, 

1978), whereas only occurrences have been found in the 

Ruby-Irwin area with no production. The sandstone-hosted 

uranium deposits have historically been the dominant 

producers of uranium and vanadium in the region and the more 

probable source of any future production. By comparison, 

the production from the other sediment-hosted deposits in 

the Placerville, Marshall Pass, and Cochetopa areas has been 

of minor interest, and in the absence of appropriate deposit 

models their potential cannot be assessed. This assessment 

effort is therefore wholly focused on evaluating the potential 

of the sandstone-hosted uranium deposits. 

The U.S. Geological Survey has conducted an intensive 

long-term program of internal and contractual investigations of 

uranium occurrences in the United States, which culminated in 

the early 1980’s with the publication of a series of reports of 

the National Uranium Resource Evaluation (NURE) contract 

study. Those reports summarized the then current understanding of how uranium deposits form, where they occur, and 

where new resources are likely to be found. Subsequent studies have not substantially altered the findings and conclusions 

in those reports. The discussion and delineation of assessment 

tracts that follow are largely based on the content of the NURE 

reports for the Moab (Campbell and others, 1982a), Cortez 

(Campbell and others, 1982b), Durango (Theis and others, 

1981), Montrose (Goodknight and Ludlam, 1981), and Leadville (Collins and others, 1982) 1°×2° quadrangles and the 

Butler and Fischer (1978) review of uranium and vanadium 

resources in the Moab quadrangle.

Model for Sandstone-Hosted Uranium 

Mineral Deposits

A significant number of descriptive and genetic models 

have been proposed over the last half-century to explain the 

existence of sandstone-hosted uranium deposits. Examples 

Mineral Resource Potential Assessment for the

Sandstone-Hosted Uranium Deposit Type

By Gregory T. Spanski, Viki Bankey, and Steven M. Smith
Mineral Resource Potential Assessment for the Sandstone-Hosted Uranium Deposit Type 127

of models include those of Fischer and Hilpert (1952), Finch 

(1967), Fischer (1968), Shawe (1976), Austin and D’Andrea, 

Jr. (1978), Mickle and Mathews (1978), Ruzicka and Bell 

(1984), and Turner-Peterson and Hodges (1986). Although 

differing in detail, such as ore genesis processes, the models are consistent on the broader descriptive aspects of these 

deposits and the geologic environments in which the deposits 

may be expected to occur. 

Uranium in these deposits occurs in a low-valent form 

as primary uraninite (pitchblende) and coffinite with pyrite 

and sporadic marcasite. Organic debris is commonly plentiful. The uranium minerals fill intergranular pore spaces and 

replace carbonaceous material, quartz grains, and interstitial 

cements in clastic rocks, mainly sandstones. Individual mineralized zones may vary in form from tabular bodies concordant 

with formation bedding to discordant C- or S-shaped bodies 

that cut across bedding. The original sand bodies are generally immature, possessing a large proportion of lithic-tuffaceous and (or) feldspathic compositional grains, and occur 

as layers or lenses in interbedded sequences or interfingering 

with mudstone. Permeability is highly variable, caused by 

rapid changes in grain size or localized variation in the content 

of clay minerals, intergranular cements, or carbonaceous 

detritus. The original sand bodies were deposited in an arid 

to semi-arid, terrestrial (continental) environment, on a stable 

platform, foreland-interior basin, or shelf-margin tectonic setting, where stream gradients were low and channel meandering and formation of backswamp environments were dominant 

processes. These environments were coupled with an adjacent 

highland area in which erosion of felsic intrusive and pyroclastic rocks contributed sediment to the basin; nearby volcanic 

activity periodically deposited widespread air-fall tuffaceous 

detritus over the basin surface. 

A variety of sources of the uranium has been proposed: 

(1) feldspathic detrital sediment derived from incompletely 

weathered felsic rocks in the highland areas, (2) clays with 

adsorbed uranium that are delivered to the basin, or (3) devitrifying tuffaceous detritus deposited on the basin floor. Oxidizing, bicarbonate-charged meteoric ground water percolating 

through these sediments leached uranium from diagenetically 

altering volcanic glass, feldspars, and clay minerals, and 

transported it in its high-valent (+6) oxidation state. Where 

these uranium-bearing waters encountered reducing conditions, uranium was precipitated as the low-valent (+4) oxide 

(uraninite) or silicate (coffinite). Possible agents responsible 

for producing the reducing conditions include buried organic 

or carbonized material, entrapped H2

S or methane gases produced by anaerobic bacterial activity, humic acids, or sulfide 

minerals, mainly pyrite. In the arid to semi-arid environment 

described, forest vegetation would be restricted to highland 

areas and lower gradient areas near the basin center or coastal 

plain. The overall low density of vegetation would be consistent with episodes of high runoff, flooding and rapid cutting 

and filling of basin sediments, and consequent rapid burial of 

vegetation carried in flood water. Reducing conditions would 

be preserved in the low-permeability, organic-rich muds 

accumulating in backswamp areas during the early stages 

of diagenesis by persistent high water table conditions and 

presence of entrapped bacterially generated gases. Oxidizing 

conditions would be restricted to the adjacent higher permeability, coarser grained channel-fill sands through which 

uranium-bearing ground water flowed basinward. Primary 

uranium mineralization occurred where the reducing and 

oxidizing conditions interfaced at the margins of organic-rich 

sediment layers and organic detritus buried in more permeable 

sand layers.

Description of Sandstone-Hosted 

Uranium Deposits in the GMUG 

Greater Study Area

Within the GMUG greater study area (fig. H1) and surrounding environs, significant deposits of the sandstone-hosted 

uranium deposit type occur in the Chinle, Morrison, and Cutler Formations; and minor occurrences are found in the upper 

member of the Hermosa Formation, Wingate Sandstone, Ohio 

Creek Member of the Mesaverde Formation, and Wasatch 

Formation. In addition the Burro Canyon, Rico, Moenkopi, 

Dolores, and Dakota Formations are reported to host occurrences of this type in areas adjacent to the GMUG area (Finch, 

1967). Rock units containing uranium as described in NURE 

reports are listed in table D1 (chapter D, this volume). 

Sandstone-hosted uranium type occurrences are concentrated in two distinct geographic areas; one lies west of the 

Uncompahgre National Forest (Uravan mineral belt) and a 

second, smaller area is in the northwest corner of Gunnison 

County (Ruby-Irwin area). 

The Uravan mineral belt, initially defined by Fischer and 

Hilpert (1952) is the oldest uranium mining area in the United 

States. It forms a rough arc that trends in a southerly direction 

from just south of Gateway through Uravan to the Slick Rock 

area, a distance of approximately 115 km, and extends on 

into Utah. Bordering the east edge of a more broadly defined 

Uravan area, it is defined by the presence of nearly 1,200 

documented occurrences and deposits (Nelson-Moore and others, 1978), hosted in sandstones and conglomerates of the Salt 

Wash and Brushy Basin Members of the Morrison Formation. 

Individual zones of mineralization in the Uravan mineral belt 

are typically elongated—either in a tabular shape conforming 

with bedding in the host sandstone (peneconcordant) or as 

less common roll-type deposits, which exhibit a C- or S-like 

cross section that cuts vertically across bedding. Size varies 

from a few metric tons of rock in a single occurrence to more 

than 1,000,000 t (metric tons), where one or more zones occur 

closely spaced in clusters. 

Vanadium is also prevalent in these deposits. Vanadiumuranium ratios systematically increase in the mineral belt from 

north to south, from near 3:1 in the Gateway area to approximately 8:1 in the Slick Rock area. Primary (unoxidized) ores 
39o 30'109o 

37o 45'

70

139

65

92

50

50 

82

135

133

62

145

141

141

90

114

Grand

Junction

Delta

Montrose

Gunnison

Crested Butte

Lake City

Uravan

Telluride

Ouray

Silverton

Aspen

Disappointment Creek

Gunnison River

Uncompahgre River Lake Fork Cebolla

Creek

Tomichi Creek

Taylor River

Blue MesaReservoir

CochetopaCreek

Dolores River

0

0

Approximate outline of national forests in GMUG study area

EXPLANATION

550

149

Figure H1. GMUG greater study area, showing permissive tracts (shaded) for sandstone-hosted uranium deposits.
Mineral Resource Potential Assessment for the Sandstone-Hosted Uranium Deposit Type 129

contain uranium oxide (uraninite) and silicate (coffinite) and 

the vanadium oxide (montroseite) and vanadium-bearing 

silicates (mica, chlorite and clay). In the near-surface oxidized 

zone, uranium vanadates (carnotite and tyuyamunite) are abundant. Ore minerals coat sand grains, fill pores, and in some 

cases replace sand grains, interstitial clays, calcite cement, and 

carbonaceous debris. Primary mineralization is restricted to 

reduced zones in the host sandstone, which are light gray to 

buff in color and contain disseminated pyrite and carbonized 

material. Where mineralized zones are weathered, the pyrite 

imparts a yellowish cast to the rock’s color. 

The host sandstones in the Salt Wash and lower part of 

the overlying Brushy Basin Members of the Morrison Formation occur as layers and lenses interbedded with mudstones. 

Thicker intervals of uniform sandstone are devoid of significant mineralized uranium. The depositional environment 

is interpreted to have been a large alluvial fan complex that 

spread out to the east into western Colorado from a source 

in east-central Utah. The Uravan mineral belt overlies the 

arcuate distal front of the fan complex where gradients flattened and streams began to meander and braid before entering 

either a marginal marine or a lacustrine environment nearer 

the basin center. Coarse sand collected in fluvial channels, 

and fine sand and clay-rich sediment accumulated in adjacent backswamp and fresh-water lake (lacustrine) flood-plain 

environments. Vegetation swept down in floods or growing 

on adjacent flood plains became commingled with, and buried 

in, the rapidly accumulating sediments. A shallow water 

table preserved and aided in the carbonization of entrapped 

organic matter. The long axes of the uranium deposits in the 

mineral belt are roughly normal to the trend of the belt and 

parallel to the expected direction of flow of the distributive 

streams and mineralizing ground water. The sediment source 

is hypothesized to have been a highland area to the southwest. 

Sandstones are feldspathic and mudstones are bentonitic, 

suggesting the presence of a significant component of volcanic ash in the distal part of the alluvial fan system. Ash is a 

suspected source of the uranium, along with vanadiferous and 

uraniferous heavy minerals present in the juvenile sandstones. 

Contemporaneous with deposition, flowage of evaporite 

deposits underlying the late Paleozoic Paradox basin superimposed an alternating pattern of slowly subsiding synclinal 

basins and slowly rising diapiric anticlinal ridges that dammed 

streams and produced temporary lakes in the synclinal areas. 

These conditions persisted during the early sedimentation of 

the Brushy Basin Member of the Morrison, where uranium 

deposits are also found. 

Uranium host rocks are characterized by the presence 

of broad, continuous sandstone lenses and lenticular channel sandstones and conglomerates separated laterally and 

vertically by mudstone layers of variable thickness. Organic 

matter, ranging from tree-trunk size down to fine debris, was 

preserved and carbonized, where protected by high water 

tables. Uranium and vanadium were likely leached from feldspars, heavy minerals in sandstones, and tuffaceous horizons 

in mudstones and transported in ground water under oxidizing 

conditions, possibly at different times, basinward through the 

more porous and permeable channel sandstones. On meeting localized reducing conditions, uranium was deposited as 

oxides and silicates and vanadium as an oxide, coating sand 

grains, filling pore spaces, and replacing carbonized plant 

matter. Vanadium also invaded clay mineral structures, 

producing vanadium-rich clays. Mineralization is believed to 

have occurred early in the period of consolidation from sediment to sedimentary rock.

Uranium occurrences in the Ruby-Irwin area are hosted 

in carbon-bearing, fluvial channel-fill sands in thick alluvial-fan and alluvial-plain deposits (Upper Cretaceous Ohio 

Creek Member of Mesaverde Formation and Tertiary Wasatch 

Formation) that built to the west into the Piceance Basin from 

the rising Sawatch uplift during the Late Cretaceous and early 

Tertiary. The source of the uranium in these occurrences may 

have been the abundant volcanic debris that is found in the 

lower part of the Wasatch Formation. No uranium minerals have been identified, and the uranium is likely bound by 

adsorption in the carbonaceous debris found in the channel 

sands. Vanadium is present only in trace amounts. 

Delineation of Mineral Resource 

Potential Areas for Sandstone-Hosted 

Uranium Deposits

The permissive and favorable mineral resource potential 

tracts, delineated in this study for sandstone-hosted uranium 

deposits, identify areas where there is believed to be more than 

a trivial probability of additional occurrences of deposit size 

existing. Their delineation is complicated by the confidentiality that surrounds grade and tonnage data for existing deposits 

and precludes the development of traditional deposit-based 

grade and tonnage models that are normally used to characterize the size and grade range for deposits. However, two 

studies do provide some insights that can be used to characterize deposit size for these deposits. In the NURE studies, geographic areas were delineated wherein the geologic 

environment was such that a minimum aggregated endowment 

of 100 tons (91 t) of U3

O8 in rocks having an average grade not 

less than 100 ppm U3

O8 could be presumed to be present. The 

U3

O8 endowment of individual occurrences was not addressed. 

W.I. Finch and C.T. Pierson (written commun., 1992) proposed using clustered deposit and occurrence data in place 

of single values, thereby preserving confidentiality. Average 

grade and aggregate ore tonnage values for clusters of closely 

spaced ore bodies that had been or would likely be mined as a 

unit in any future mining effort were used to construct a set of 

grade and tonnage models. Using data for 64 clusters occurring in central Utah in geologic environments similar to those 

in the Uravan mineral belt, they obtained a value of 29,000 t 

and a grade of 0.18 percent U3

O8 for a median cluster, which 

equates to a median endowment of 52 t of U3O8. Clusters 
130 Resource Potential and Geology, Grand Mesa, Uncompahgre, and Gunnison National Forests, Colorado

range in size from 1,100 t to 7.38 million metric tons, and 

endowments range between 0.9 and 17,710 t of contained 

U3

O8. Given these models, tracts are delineated for the potential existence of deposits that contain more than 2,000 t of ore 

and a U3

O8 endowment of more than 2.3 t. 

The tract delineation criteria listed in table H1 are largely 

based on those used in the NURE studies of the Moab (Campbell and others, 1982a), Cortez (Campbell and others, 1982b) 

and Montrose (Goodknight and Ludlam, 1981) quadrangles, 

the observed associations of uranium with sandstone summarized by Finch (1967), and the descriptive deposit model 

of Turner-Peterson and Hodges (1986). Criterion 1 is used to 

identify permissive tracts, in which the probability for the existence of deposits is more than negligible. Criteria 2 through 4 

are applied in various combinations to the permissive terrane 

to highlight areas that have a higher probability for hosting 

deposits—the favorable tracts. Each criterion is supported by 

data that are widely available at 1:250,000 scale. Criterion 5 is 

included to ensure that the assessment results from the NURE 

program and the expertise that went into producing them 

were not overlooked in the current effort. Several additional 

“favorable criteria” listed would be appropriate for identifying 

favorable tracts, provided the data were available; they could 

prove useful in refining the boundaries of favorable areas in 

localized areas. 

Buffers are used with point and area data to compensate for location inaccuracy, at 1:250,000 scale, to indicate 

the probable area of influence attributable to a class of point 

information, and to make allowances for lateral extrapolation 

of non-point information where control data are lacking. The 

values used are based on empirical precedent. Where several 

buffers may be applicable, the largest is used. 

Permissive Tracts

The affinity that uranium occurrences and deposits have 

demonstrated for the feldspathic and carbonaceous sandstone 

units occurring interbedded with mudstones and shales in continental sedimentary sequences has long been known (Finch, 

1967; Turner-Peterson and Hodges, 1986). In the study area 

occurrences and deposits are found in sandstone intervals of 

more than a dozen units (Finch, 1967; Nelson-Moore and others, 1978). However, to resolve each suitable sandstone unit 

for display at a scale of 1:250,000 is impractical; therefore, 

stratigraphic intervals containing several suitable sandstone 

intervals are used to define a permissive area. 

In the GMUG study area a noncontiguous area of nearly 

6,700 mi2

 is identified as having a permissive potential for 

sandstone-hosted uranium deposits through the application 

Table H1. Delineation criteria for sandstone-hosted uranium deposits in GMUG greater study area.

[Map symbols in parentheses are those shown at 1:250,000-scale mapping (Day and others, 1999) that include the specified unit. Bold indicates the principal 

symbol for the lithologic unit]

Diagnostic criterion for permissive tract delineation

1. Feldspathic and (or) tuffaceous-bearing clastic sedimentary rock deposited in a predominantly continental (nonmarine) basin environment 

 (includes all Permian through Jurassic age sedimentary rock units, the Cretaceous Dakota Sandstone and Ohio Creek Member of the

 Mesaverde Formation, and the Tertiary Wasatch Formation) known or inferred to be present within 1,500 m of the surface.

Diagnostic criteria for favorable tract delineation (in addition to criterion 1)

2. Presence of anomalous radioactivity as indicated by anomalous uranium:thorium ratio equal to or greater than one standard deviation above 

 its mean and a thorium signal at least one standard deviation below its mean.

3. Presence of an anomalous level of uranium in stream-sediment (>10 ppm) or water (>10 ppm) sample (500 m buffer).

4. Presence of sandstone-hosted uranium occurrence or deposit (500 m buffer).

5. Area designated as favorable for subclass 243 or 244 type sediment-hosted uranium type deposits in the Morrison or Cutler Formations in 

 the National Uranium Resource Evaluation (Moab, Cortez, or Montrose) quadrangle reports.

Other favorable tract criteria

6. Lithologies characterizing deposition in a lower (distal) alluvial-plain, a near-shore (nonmarine), or a delta-plain environment. Includes

 permeable, lenticular, lacustrine or lagoonal, feldspathic or arkosic, fine-grained sandstones, or channel-fill sandstones and (or) conglom- 

 erates, containing concentrations of carbonaceous matter interbedded with impermeable, tuffaceous siltstones and mudstones, known to

 be present within 1,500 m of the surface:

 A. Salt Wash Member of Morrison Formation (Jms, Jm, Jmw, Jmwe, Jmj, Jmce, Jmc)

 B. Brushy Basin Member of Morrison Formation (Jmb, Jm, Jmw, Jmj, Jmwe)

 C. Moss Back Member of Chinle Formation (c, wc, kwc)

 D. Cutler Formation (Pc)

 E. Ohio Creek Formation (Two)

 F. Wasatch Formation (Two, Tw)

7. Presence of gray bentonitic (swelling) clay, or noncalcareous lacustrine mudstone interbedded with sandstone.

8. Presence of uranyl vanadate, phosphate, and (or) silicate mineralogy in oxidized surface exposures (500 m buffer).
Mineral Resource Potential Assessment for the Sandstone-Hosted Uranium Deposit Type 131

of criterion 1 (table H1). These lands (fig. H1) are underlain 

by sedimentary rocks formed for the most part under continental (nonmarine) depositional conditions, which persisted 

throughout western Colorado from the Late Pennsylvanian 

into the Cretaceous and again in the early Tertiary. The name, 

Western Slope–Intermountain–Piceance Basin Area, applied 

to the tract reflects the diversity of the area of coverage. The 

effective depth of consideration for classification purposes was 

extended to 1,500 m to achieve parity with that used in the 

NURE investigations. Approximately 1,630 mi2

 of the permissive tract lands falls within GMUG National Forest boundaries 

and an additional 2,650 mi2

 underlies land managed by the 

Bureau of Land Management. 

 Favorable Tracts

Approximately 20 percent (1,352 mi2) of the lands in 

the permissive sandstone-hosted uranium tracts are classified 

as favorable through the application of criteria 2 through 5 in 

table H1. Less than 100 mi2

 occurs within forest bounds and 

955 mi2 underlies BLM land. The lands occur in two distinct 

areas, the Uravan Area and the Ruby-Irwin Area. Figure H2 

and table H1 show what criterion or combinations of criteria 

were used and the relative impact each has on determining the 

final configuration of the favorable tracts. Criterion 4, singularly or in combination with 2 and (or) 3, is used to categorize 

approximately 300 mi2 of the permissive terrane as favorable. 

Application of criterion 5 (favorable areas in NURE reports) 

categorizes an additional 1,054 mi2

 as favorable, increasing 

the area by nearly 350 percent. The presence of criterion 2 or 

3 alone is not judged to be singularly sufficient justification to 

invoke a favorable classification. Little new information has 

been gathered on these deposits since the early 1980’s, and the 

conclusions reached in the NURE investigations by Campbell 

and others (1982a, 1982b) and Goodknight and Ludlam (1981) 

remain valid. The interpretive approach and data support used 

to define favorable areas in the NURE studies equate well with 

those used in the present assessment. 

Uravan Area: This area contains more than 98 percent of 

lands designated as favorable. The favorable determination 

is largely based either on the presence of a distal alluvialplain lithofacies, which equates stratigraphically with the Salt 

Wash and Brushy Basin Members of the Morrison Formation, 

or on the presence of a sequence of arkosic sandstones and 

Table H2. Sandstone-hosted uranium tracts in GMUG greater study area.

Tract No.a Tract nameb Delineation criteriac

P30c.1 Western Slope-Intermountain-Piceance Basin Area 1

F30c.1 Uravan Area 1, 2, 3, 4, 5

F30c.2 Ruby-Irwin Area 1, 5

a

P denotes a permissive tract, F a favorable tract followed by a deposit type model number.

b

Area of permissive tracts includes the area of the favorable tracts lying within.

c

Indicates the criteria used to delineate the lands in the tract. See table H1.

EXPLANATION (fig. H1, next page) 

Criterion 5— Previously identified favorable areas in the Morrison or Cutler Formations

Criterion 4— Uranium mines and prospects, buffered

Criteria 4 and 5

Criterion 4 or 5 or both and criterion 3— Anomalous geochemistry where U > 10 ppm

Criterion 4 or 5 or both and criterion 2— Anomalous radioactivity ratio

Criteria 2 and 3 and criterion 1 or 2 or both 
39o 30'109o 

37o 45' 70 139 65 92 50 50 

82 

135 133 149 62 145 141 141 90 114 

Grand Junction Delta Montrose Gunnison Crested Butte Lake City Uravan Telluride Ouray Silverton Aspen 

Disappointment Creek Gunnison River Uncompahgre River Lake Fork Cebolla Creek Tomichi Creek Taylor River 

Blue Mesa Reservoir Cochetopa Creek Dolores River

0

0

Approximate outline of national forests in GMUG study area

EXPLANATION

550

COCHETOPA

RUBY-IRWIN

PLACERVILLE

URAVAN

MINERAL

BELT

Figure H2 (above and previous page). GMUG greater study area, showing favorable tracts for sandstone-hosted uranium deposits. Dashed known uranium districts.
Mineral Resource Potential Assessment for the Sandstone-Hosted Uranium Deposit Type 133

shales, deposited in an alluvial-fan environment dominated by 

meandering streams occurring in the upper part of the Cutler 

Formation. In fact, the Cutler and Salt Wash units overlap in a 

portion of the south half of the area. 

Ruby-Irwin Area: This small area contains less than 2 percent 

of the favorable land; it lies to the west of Crested Butte (fig. 

H2). Its inclusion as favorable is based largely on the fact that 

it had been designated favorable in a NURE report (Goodknight and Ludlam, 1981). The classification is based on the 

presence of a stratigraphic interval of interbedded, fluvial, 

arkosic sands, conglomerates, siltstones, and volcanic debris 

deposited in a transitional environment between an alluvial fan 

and an alluvial flood plain. The interval includes the upper 

part of the Oak Creek Sandstone and lower part of the Wasatch 

Formation.

Marshall Pass, Cochetopa, and Placerville Areas: These 

three areas (fig. H2) are locations of known uranium deposits; 

however, we determined that these types of uranium deposits 

do not adequately fit the sandstone-hosted model studied in 

this report.

Undiscovered Deposit and Endowment 

Potential

The potential for new sandstone-hosted uranium deposits 

and quantitative estimation of their uranium and vanadium 

endowments cannot be assessed using the three-part methodology described in Chapter F of this volume. Although the 

deposit type has been descriptively modeled (Turner-Peterson 

and Hodges, 1986), the grade and tonnage characteristics of 

that fraction of occurrences that would be considered deposits 

have not been modeled. The cluster-based grade and tonnage 

models proposed by Finch and Pierson (1992) have never been 

formally recognized and should not be used for estimating 

resources in undiscovered deposits. In the absence of a recognized set of models, the numbers of undiscovered deposits 

cannot be estimated. 

However, historical precedent and numerous scientific 

investigations conducted since the early 1950’s suggest that, 

within the bounds of the permissive and favorable tracts, a 

significant uranium and vanadium endowment is associated 

with the sandstone-hosted type of deposit. By 1999 more than 

63 million lb of uranium oxide and 330 million lb of vanadium 

oxide have been produced from sandstone-hosted uranium 

deposits occurring in Permian, Triassic, and Jurassic age rocks 

in the Colorado portion of the Colorado Plateaus province. 

Most of that production occurred during a period from 1947 

to 1968 when market pricing was subsidized and again from 

1973 to 1987 when there was a rapid expansion of the nuclear 

energy industry; high prices and demand made lower grade 

ores economic. By 1990, however, all mining activity had 

ceased, not as a result of resource depletion, but rather in 

response to steep declines in demand and market price for 

uranium (Chenoweth, 1996). These declines were precipitated 

by the transition from a subsidized to a free market economy 

for uranium and associated contraction of the nuclear energy 

industry in the 1980’s. Under current free market conditions the sandstone-hosted uranium deposits in Colorado 

have become non-economic. This sensitivity to market price 

is demonstrated by the case of the Sunday mine in the Big 

Gypsum Valley of San Miguel County: this mine reopened in 

1997 at a time when uranium oxide prices rose above $15/lb 

(Cappa, 1998). The mine closed again in 1999 after prices 

dropped back into the $10–$11/lb range (Cappa and Carroll, 

2000). Future interest in development and exploration will be 

totally a function of market dynamics. At current price levels, 

development potential is extremely low. Should price increase 

into the high teens or above, there is a strong expectation that 

some existing mines with proven reserves will reopen, and 

exploration for new deposits of similar size and grade, which 

are highly likely to be present in the favorable tract areas, will 

resume. 

References Cited

Austin, S.R., and D’Andrea, R.F., Jr., 1978, Sandstone-type 

uranium deposits, in Mickle, D.G., and Mathews, G.W., 

eds., Geologic characteristics of environments favorable for 

uranium deposits: Report of Bendix Field Engineering Corporation prepared for the U.S. Department of Energy, Grand 

Junction, Colo., GJBX-067(78), p. 87–119.

Butler, A.P., Jr., and Fischer, R.P., 1978, Uranium and vanadium resources of the Moab 1°×2° quadrangle, San Juan 

County, Utah and Montrose County, Colorado: U.S. Geological Survey Professional Paper 988–B, p. B1–B22.

Campbell, J.A., Franczyk, K.J., Lupe, R.D., and Peterson, 

Fred, 1982a, Cortez quadrangle, Colorado and Utah: Report 

of U.S. Geological Survey prepared for the U.S. Department 

of Energy, Grand Junction, Colo., PGJ/F-051(82), 65 p.

Campbell, J.A., Franczyk, K.J., Lupe, R.D., and Peterson, 

Fred, 1982b, Moab quadrangle, Colorado and Utah: Report 

of U.S. Geological Survey prepared for the U.S. Department 

of Energy, Grand Junction, Colo., PGJ/F-056(82), 68 p.

Cappa, J.A., 1998, Annual review 1997—Mining and exploration overviews—Colorado summary: Mining Engineering, 

v. 50, no. 5, p. 77–79.

Cappa, J.A., and Carroll, C.J., 2000, Annual review 1999—

Mining and exploration overviews—Colorado summary: 

Mining Engineering, v. 52, no. 5, p. 64–67.

Chenoweth, W.L., 1996, The uranium industry in the Paradox 

Basin, in Huffman, A.C., Jr., Lund, W.R., and Godwin, 

L.H., eds., Geology and resources of the Paradox Basin: 

Utah Geological Association Guidebook 25, p. 95–108.
134 Resource Potential and Geology, Grand Mesa, Uncompahgre, and Gunnison National Forests, Colorado

Collins, D.B., Graham, D.C., and Hornbaker, A.L., 1982, 

Leadville quadrangle, Colorado: Report of Bendix Field 

Engineering Corporation prepared for the U.S. Department 

of Energy, Grand Junction, Colo., PGJ/F-027(82), 81 p.

Day, W.C., Green, G.N., Knepper, D.H., Jr., and Phillips, 

R.C., 1999, Spatial geologic data model for the Gunnison, Grand Mesa, Uncompahgre National Forests mineral 

assessment area, southwestern Colorado and digital data for 

the Leadville, Montrose, Durango, and the Colorado parts 

of the Grand Junction, Moab, and Cortez 1°×2° geologic 

maps: U.S. Geological Survey Open-File Report 99-427, 

CD-ROM.

Finch, W.I., 1967, Geology of epigenetic uranium deposits 

in sandstone in the United States: U.S. Geological Survey 

Professional Paper 538, 121 p.

Fischer, R.P., 1968, The uranium and vanadium deposits of the 

Colorado Plateau region, in Ridge, J.D., ed., Ore deposits of 

the United States, 1933–1967; The Graton–Sales Volume: 

New York, American Institute of Mining, Metallurgical, and 

Petroleum Engineers, Inc., v. I, p. 735–746.

Fischer, R.P., and Hilpert, L.S., 1952, Geology of the Uravan 

Mineral Belt: U.S. Geological Survey Bulletin 988–A, p. 

1–13.

Goodknight, C.S., and Ludlam, J.R., 1981, Montrose quadrangle, Colorado: Report of Bendix Field Engineering 

Corporation prepared for the U.S. Department of Energy, 

Grand Junction, Colo., GJQ-010(81), 91 p.

Malan, R.C., and Ranspot, H.W., 1959, Geology of the uranium deposits in the Cochetopa mining district, Saguache 

and Gunnison Counties, Colorado: Economic Geology, v. 

54, no. 1, p. 1–19.

Mickle, D.G., and Mathews, G.W., eds., 1978, Geologic characteristics of environments favorable for uranium deposits: 

Report of Bendix Field Engineering Corporation prepared 

for the U.S. Department of Energy, Grand Junction, Colo., 

GJBX-067(78), 250 p.

Nelson-Moore, J.L., Collins, D.B., and Hornbaker, A.L., 1978, 

Radioactive mineral occurrences of Colorado and 

bibliography: Colorado Geological Survey Bulletin 40, 

1054 p.

Olson, J.C., 1988, Geology and uranium deposits of the 

Cochetopa and Marshall Pass districts, Saguache and Gunnison Counties, Colorado: U.S. Geological Survey Professional Paper 1457, 44 p.

Ruzicka, Vlad, and Bell, R.T., 1984, Sandstone uranium, 

chapter 6.4, in Eckstrand, O.R., ed., Canadian mineral 

deposit types—A geological synopsis: Geological Survey 

of Canada Economic Geology Report 36, p. 28.

Shawe, D.R., 1976, Sedimentary rock alteration in the Slick 

Rock district, San Miguel and Dolores Counties, Colorado: U.S. Geological Survey Professional Paper 576-D, p. 

D1–D51.

Theis, N.J., Madson, M.E., Rosenlund, G.C., Reinhart, W.R., 

and Gardner, H.A., 1981, Durango quadrangle, Colorado: 

Report of Bendix Field Engineering Corporation prepared 

for the U.S. Department of Energy, Grand Junction, Colo., 

GJQ-011(81), 48 p.

Turner-Peterson, C.E., and Hodges, C.A., 1986, Descriptive 

model of sandstone U, in Cox, D.P., and Singer, D.A., eds., 

Mineral deposit models: U.S. Geological Survey Bulletin 

1693, p. 209–210.
U.S. Department of the Interior

U.S. Geological Survey

Mineral Resource Assessment for

Volcanic-Associated Massive Sulfide Deposits

By Warren C. Day, Gregory T. Spanski, Viki Bankey, Anna B. Wilson, and

Steven M. Smith

Chapter I of

Resource Potential and Geology of the Grand Mesa, Uncompahgre,

and Gunnison (GMUG) National Forests and Vicinity, Colorado

Edited by Viki Bankey

U.S. Geological Survey Bulletin 2213– I
Contents

Abstract...................................................................................................................................................... 137

Introduction ............................................................................................................................................... 137

Model for Volcanic-Associated Massive Sulfide Mineral Deposits ................................................ 137

Description of the Volcanic-Hosted Massive Sulfide Deposits in the Study Area ........................ 138

Application of the Deposit Model for a Mineral Resource Assessment of VolcanicAssociated Massive Sulfide Deposits ..................................................................................... 140

Permissive Tracts............................................................................................................................. 141

Favorable Tracts............................................................................................................................... 142

Undiscovered Deposit and Endowment Potential............................................................................... 145

References Cited ...................................................................................................................................... 148

Figures

I1–I3. Maps of GMUG greater study area showing:

 I1. Location, deposit type, and commodity recovered for selected Proterozoic

 mineral deposits...................................................................................................................... 139

 I2. Permissive tracts for hosting Kuroko-type VMS copper-zinc±gold deposits .............. 143

 I3. Favorable tracts for hosting Kuroko-type VMS copper-zinc±gold deposits ................ 144

 I4. Graphs showing grade and tonnage models for 187 Precambrian Kuroko-type

massive sulfide deposits............................................................................................................. 147

Tables

I1. Delineation criteria for tracts of land with potential for undiscovered volcanicassociated Kuroko-type VMS copper-zinc±gold deposits in GMUG greater

study area .......................................................................................................................................... 141

I2. Summary of results of resource endowment potential assessment for undiscovered

Kuroko-type volcanic massive sulfide deposits within GMUG greater study area............... 146
Abstract

Precambrian volcanic rocks of the GMUG greater study 

area have historically produced copper, gold, silver, and lead 

in Kuroko-type volcanic-associated massive sulfide deposits, 

located primarily in the Dubois Greenstone belt, south of Gunnison, Colorado. Commonly, these massive sulfide deposits 

originally formed within bimodal (felsic and mafic) metavolcanic and associated metasedimentary rock packages that are 

proximal to felsic volcanic centers in clusters adjacent to one 

another. In areas where the base- and precious-mineral-rich 

rocks are exposed, the resulting erosion yields stream sediments with elevated concentrations of base and precious metals. We describe herein the characteristics of the Kuroko-type 

volcanic-associated massive sulfide deposits, briefly discuss 

their mode of origin, give an overview of the geologic setting 

for regions known to host such deposits, present the criteria 

used in the mineral resource assessment to highlight areas both 

permissive and favorable for such deposits, and discuss the 

results of the mineral resource assessment. 

The robustness of the mineral resource assessment is 

limited to the quality and thoroughness of the data available at 

the regional scale. As such, the main diagnostic tools used for 

delineating tracts favorable for hosting Kuroko-type volcanic-associated massive sulfide deposits were the existing 1:

250,000-scale geologic maps, mine and prospect location and 

mineral production information, regional stream-sediment 

data, all of which were augmented by new geochemical data 

on bedrock and mine dumps as well as onsite geologic observations made throughout the region specific for these deposits. 

Information lacking at the regional scale that would enhance 

the assessment would include maps of mineral alteration 

assemblages, detailed electromagnetic geophysical maps, and 

closely spaced stream-sediment, soil, and bedrock geochemical sampling. However, we identified several tracts that are 

favorable for hosting Kuroko-type volcanic-associated massive 

sulfide deposits within the GMUG greater study area.

Introduction

The GMUG greater study area hosts several volcanicassociated massive sulfide (VMS) mineral deposits within 

the Precambrian basement rocks. The VMS deposits are 

located within the Dubois Greenstone belt, which is a belt of 

Proterozoic volcanic and intrusive rocks exposed within the 

Gunnison uplift between Cochetopa Creek and the Lake Fork 

River in Gunnison and Saguache Counties, Colorado. The 

general character of the VMS deposits within the GMUG area 

is similar to that described in the Kuroko-type massive sulfide 

deposit model presented by Singer (1986) as well as models 

for volcanic-associated massive sulfide deposits presented by 

Hutchinson (1982), Franklin (1993), and Franklin and others 

(1981, 1998).

Model for Volcanic-Associated

Massive Sulfide Mineral Deposits 

Volcanic-associated massive sulfide (VMS) deposits contain copper, lead, and zinc as their primary ore metals and can 

carry lesser albeit important amounts of silver and gold. They 

occur as lenses, layers, and (or) disseminations of base-metalbearing sulfide minerals within sequences of marine volcanic 

and associated sedimentary rocks. The host volcanic successions range from bimodal sequences of mafic and felsic rocks 

(Kuroko-type) to andesite-dominated sequences (Norandatype). In both types the host rocks are generally subaqueous flows, tuffs, pyroclastic deposits, and volcanic 

breccias; many host successions are proximal to felsic 

volcanic domes. Worldwide, VMS deposits range in age from 

Archean to Cenozoic. They form both in island-arc and in 

mid-ocean ridge tectonic settings; however, the majority occur 

in island-arc sequences (Franklin and others, 1998) and 

especially within arc-related rift settings (Sawkins, 1990).

VMS ores are deposited from metalliferous fluids generated within thermally driven convective hydrothermal systems 

associated with subaqueous volcanism and rifting (Franklin 

and others, 1981, 1998; Hutchinson, 1982; Seyfried and Janecky, 1985; Poulsen and Hannington, 1996; Franklin, 1993). 

Cool seawater is circulated through piles of volcanic (basaltic) rocks adjacent to areas of active subaqueous volcanism 

and local subvolcanic intrusions, which heat the pore water, 

forming a hydrothermal convection system. Within such a 

hydrothermal system, the relatively cool hydrothermal fluids 

Mineral Resource Assessment for

Volcanic-Associated Massive Sulfide Deposits

By Warren C. Day, Gregory T. Spanski, Viki Bankey, Anna B. Wilson, and Steven M. Smith
138 Resource Potential and Geology, Grand Mesa, Uncompahgre, and Gunnison National Forests, Colorado

circulate deep within the volcanic pile. The steep geothermal gradient within the rift system, caused in part by shallow 

subvolcanic intrusions, heats the deeper circulating hydrothermal fluids. As they become hotter, the fluids leach base 

and precious metals along their journey through the volcanic 

rocks. The hot hydrothermal metal-bearing fluids then rise 

through the volcanic pile, ascending along structural pathways 

(faults and caldera margins), which focus the fluid flow, and 

form alteration pipes. At the termination of the convective 

system, the metalliferous fluids are debouched on the sea 

floor in hydrothermal vent zones (submarine hot springs). 

These metal-laden, hot, hydrothermal fluids (approximately 

200°–300°C) mix with cool seawater and precipitate base- and 

precious-metal-bearing sulfide minerals. Discharge of the 

metalliferous hydrothermal fluids must be focused along a 

relatively small structurally controlled zone through a prolonged period of time to produce the high concentrations of 

metal-rich precipitants found in large VMS mineral deposits. 

Metals can be precipitated above the sediment-water interface 

as layered massive sulfide horizons and (or) as replacement 

deposits below the sediment-water subsurface.

Description of the Volcanic-Associated 

Massive Sulfide Deposits in the

Study Area

Through an understanding of the geologic setting, character, and mode of origin of known VMS deposits, we can establish critical criteria to help us identify additional areas with 

potential to host undiscovered VMS deposits throughout the 

GMUG greater study area (map area, fig. I1). Known Kurokotype VMS copper and zinc mineral deposits are exposed in 

the Dubois Greenstone belt (fig. I1). The greenstone belt is 

made up of bimodal (mafic and felsic) metavolcanic rocks and 

associated sedimentary rocks that have been variously intruded 

by synvolcanic to posttectonic granodiorite to granite plutons. 

The volcanic rocks were deposited in submarine environments, 

inasmuch as pillow structures are preserved in the basalts, and 

the felsic volcanic rocks and associated epiclastic sediments 

show subaqueous reworking (Hedlund and Olson, 1981). Afifi 

(1981) noted that volcanism was episodic, with intervals of 

quiescence marked by deposition of layered intervals of 

EXPLANATION (fig. I1, next page)

Cochetopa area

Graflin mine (VMS; Cu, Zn, and native Au)

Denver City mine (VMS; Zn, Cu, Ag, Au, Pb, Sb, 

and Te)

Yukon mine (VMS; Cu, Zn, Ag, Au, and Pb)

Lulu mine (low-sulfide quartz/gold; Au)

Buzzard mine; Mineral Hill mine (low-sulfide quartzgold; Au)

Lucky Strike (low-sulfide quartz-gold; Au)

Powderhorn area

Headlight mine (VMS; Cu, Ag, Au, and Zn)

Gunnison mine (Au-bearing chert, Ag, Cu, Pb, Te, 

and Se)

Champion shaft (Au-bearing chert)

Iron Cap mine (VMS and Au-bearing chert, Ag, Cu, 

Zn, and Pb)

Copper King mine (VMS; Au, Ag, and Cu)

Anaconda mine (VMS and Au-bearing chert, Ag, 

Cu, and Zn)

Old Lot mine (VMS; Au, Ag, Zn, Cu, and Pb)

Vulcan mine area

Vulcan/Good Hope mines (VMS; Au, Ag, Te, Cu, 

Zn, Sb, V, Bi, and As)

Midland mine (VMS; native Au, Ag, Pb, and Zn)

1

2

3

5

4

6

7

8

9

10

11

12

13

14

15
39o 30'

109o 

37o 45'

70

139

65

92

50

50 

82

135

133

149

62

145

141

141

90

114

Grand

Junction

Delta

Montrose

Gunnison

Crested Butte

Lake City

Uravan

Telluride

Ouray

Silverton

Aspen

Disappointment Creek

Gunnison River

Uncompahgre River Lake Fork Cebolla

Creek

Tomichi Creek

Taylor River

Blue MesaReservoir

CochetopaCreek

Dolores River

0

0

Approximate outline of national forests in GMUG study area

EXPLANATION

550

Bondholder

Goose

Ck

Vulcan

Beaver

Ck

Cochetopa No

Green Mt

Cochetopa

Central

Cochetopa

Tantalite

Cochetopa

So 

Box Cany

Revenue

Quartz

Ck Pegmatite

"76"

Gold

Brick

Powderhorn

+

+++++ + +

+ +

+

+++

1

2

3

15 4 5

6

7

8 910,11

12 13

14

Figure I1 (above and previous page). Location, deposit type, and commodity recovered for selected Proterozoic mineral deposits in GMUG g
140 Resource Potential and Geology, Grand Mesa, Uncompahgre, and Gunnison National Forests, Colorado

ferruginous chert and lean banded iron-formation. The Dubois 

Greenstone belt differs from most of the Proterozoic basement 

rocks of Colorado in that it has experienced a relatively low 

grade of metamorphism (upper greenschist to lower amphibolite facies), whereas most of the remaining Precambrian rocks 

are of higher grade (middle to upper amphibolite facies). In 

addition, although rocks within the belt have experienced at 

least two phases of regional deformation (Afifi, 1981; Knoper 

and others, 1991; Hetherington, 1994), they still have primary 

sedimentary and volcanic rock textures preserved (Hedlund 

and Olson, 1981). 

The Dubois Greenstone belt is composed of two separate 

volcanic successions (Bickford and others, 1982; Knoper and 

Condie, 1988; Knoper and others, 1991; Wortman, 1991), 

both of which host VMS deposits. The older succession lies 

in the western part of the greenstone belt, in the Cebolla Creek 

area; it is made up of bimodal volcanic rocks (dominantly 

basalt) with horizons of ferruginous chert (some of which is 

gold bearing) as well as pyroclastic and epiclastic sedimentary 

rocks, all of which are intruded by granite. These rocks range 

in age from 1,780 to 1,750 Ma, and the granite intrusions are 

1,755 to 1,751 m.y. old. The younger succession crops out in 

the eastern part of the greenstone belt, in the Cochetopa Creek 

area; it is also made up of bimodal volcanic rocks (Bennett 

and others, 1984) and is dominated by the felsic volcanic and 

volcaniclastic sediment component of the bimodal succession. Ages of the felsic volcanic rocks that host VMS mineral 

deposits in the eastern part of the Dubois Greenstone belt cluster between 1,741 and 1,730 Ma (Wortman and others, 1990). 

Metamorphism and peak deformation occurred prior to 1,713 

Ma, which is the age of the late synkinematic tonalite of Gold 

Basin that crosscuts the deformational fabrics developed in the 

volcanic rocks (Wortman and others, 1990).

The mafic rocks in both volcanic successions are similar 

in composition to modern basalts that form in island-arc environments (Knoper and Condie, 1988). The textural features 

developed in the volcanic and sedimentary rocks indicate that 

the environment of deposition was submarine. The combination of coeval mafic and felsic volcanism and pretectonic granitoid plutonism indicates that the tectonic setting of the Dubois 

Greenstone belt is consistent with that of a rifted island-arc 

environment. This setting is similar to others known to host 

VMS deposits (Sawkins, 1990).

Two general classes of Precambrian-aged mineral 

deposits occur in the Dubois Greenstone belt: (1) syngenetic 

Kuroko-type VMS massive sulfide and associated gold-bearing ferruginous chert (exhalative) deposits, and (2) epigenetic 

low-sulfide gold-quartz vein deposits. Although there are 

several small examples of epigenetic low-sulfide gold-quartz 

vein deposits in the Dubois Greenstone belt, no significant 

deposit has been identified at the time of this study. Because 

evaluation of the mineral resource potential for epigenetic 

low-sulfide gold-quartz vein deposits is not being conducted 

in this report, discussion is limited herein. The Kuroko-type 

VMS deposits occur within piles of dacitic to rhyolitic lava 

flows and tuffs (Afifi, 1981; Drobeck, 1981; Sheridan and 

others, 1981). For example, in the western part of the Dubois 

Greenstone belt, the Headlight, Old Lot, Copper King, and 

Vulcan/Good Hope mines are in copper-zinc-bearing Kurokotype VMS deposits hosted in dacitic to rhyolitic porphyritic 

and lapilli tuffs (fig. I1). The Vulcan/Good Hope Kuroko-type 

VMS deposit was the largest producer in the greenstone belt 

(Drobeck, 1981). Several gold-bearing ferruginous chert 

(exhalative) deposits, such as at the Gunnison mine and the 

Champion shaft, also occur in the western part of the greenstone belt (Nelson and Riesmeyer, 1983). The Anaconda and 

the Iron Cap mines (fig. I1) are hosted primarily within mafic 

metavolcanic rocks (Nelson and Riesmeyer, 1983) that have 

characteristics typical of both Kuroko-type VMS and exhalative gold deposits, inasmuch as they contain thin bedding-parallel stringers of auriferous ferruginous chert interlayered with 

massive sulfide horizons. Epigenetic low-sulfide gold-quartz 

vein deposits occur in the western part of the greenstone belt 

in the Powderhorn mineral area (Wilson and Spanski, this volume, Chapter E, fig. E1; this report, fig. I3) where fairly small, 

shear-zone-hosted systems are present in the mafic metavolcanic sequence in the Lake Fork River and Cebolla Creek areas 

(Olson and Hedlund, 1973; Hedlund and Olson, 1975).

The most notable syngenetic Kuroko-type VMS and ferruginous chert (exhalative) gold deposits in the eastern part of 

the Dubois Greenstone belt are those in the Iris district (Denver City and Graflin mines; fig. I1) described by Afifi (1981) 

and Drobeck (1981). These deposits are hosted in dacitic to 

rhyolitic tuffs and are intervals of stratabound lenses of sulfide 

minerals (chalcophyrite, sphalerite, pyrite, pyrrhotite, and 

accessory gold) that are parallel to the dip of the host rocks 

and have undergone all of the same phases of deformation as 

the host rocks.

Epigenetic low-sulfide gold-quartz vein deposits do occur 

in the eastern part of the greenstone belt in the Cochetopa area 

(fig. I1) and are represented by the Buzzard, Mineral Hill, 

Lulu, and Lucky Strike mines, as well as numerous other small 

occurrences (Olson, 1976; Afifi, 1981). 

Application of the Deposit Model for a 

Mineral Resource Assessment of

Volcanic-Associated Massive

Sulfide Deposits

The criteria listed in table I1 were used for this reconnaissance mineral resource assessment to identify tracts of land 

with potential for undiscovered Kuroko-type VMS copperzinc+gold deposits. These criteria were developed based on 

the genetic model presented previously, on the viable rock 

types within the GMUG greater study area, and on known 

styles of mineralization. Regional databases available for this 

GIS-based assessment include the digital geologic map data 

(Day and others, 1999), mines and prospect locations (Wilson 

and others, 2000), outlines of mineralized regions (Wilson 
Mineral Resource Assessment for Volcanic-Associated Massive Sulfide Deposits 141

and Spanski, this volume, Chapter E), and the regional NURE 

stream-sediment database (Smith, 2000).

The digital geologic map and spatial data model used for 

the mineral resource assessments for the GMUG project are 

those of Day and others (1999). They generated the geologic 

data model using six published 1°×2° geologic quadrangle 

maps (Leadville, Montrose, Durango, Grand Junction, Moab, 

and Cortez). The six original maps were digitized, merged, 

clipped to the study area boundary (lat 37°45′–39°30′ N., long 

106°–109° W.), and projected into a Lambert conformal conic 

projection. The spatial geologic data model was created from 

the resultant maps by developing common geologic map units 

for each map to be used throughout the area and attributing 

these common units for age, lithologic descriptions, rock type, 

economic geology, and natural aggregate characteristics.

A detailed set of assessment criteria could be developed 

that would include more data than are available in the regional 

databases assembled for this project. For instance, alteration mineral assemblages maps, detailed stream-sediment 

geochemical data, geophysical data such as electromagnetic 

(E-M) and ground magnetic studies would greatly enhance 

this type of assessment. However, the regional databases do 

permit outlining broad areas of both permissive and favorable 

natures that warrant further investigation for these types of 

deposits. These broad tracts also help identify for the Federal 

land-management agencies (USDA Forest Service and U.S. 

Bureau of Land Management) regions that may be impacted 

by future mineral resource development.

Permissive Tracts

Areas identified as “permissive” in the GMUG greater 

study area are those that meet the conditions set forth in 

criterion 1 (table I1); these are areas in which rock types are 

present that are consistent in terms of their petrology and environments of formation with those outlined in the Kuroko-type 

VMS model. These include the bimodal metavolcanic rock 

packages mapped as Xfh, the felsic metavolcanic rocks in unit 

Xf, and the metasedimentary rocks whose protolith could have 

been associated with Kuroko-type VMS copper-zinc+gold 

mineralization (unit Xb) (Day and others, 1999). Map unit 

Xb is dominantly biotite schist but locally contains both mafic 

and felsic volcaniclastic metasedimentary horizons, which 

are rock types known to host VMS deposits. Only parts of 

map unit Xb would qualify as rocks typically associated with 

VMS deposits, but all of the unit is included herein because 

the original protolith was not delineated in the source regional 

geologic maps used by Day and others (1999).

In addition to possibly hosting Kuroko-type VMS 

deposits, unit Xb is a candidate for hosting Besshi-type VMS 

deposits. The Besshi-type deposits form as lenses of sulfide 

minerals, hosted in sequences of mafic metavolcanic rocks, 

pelitic schist, quartz schist, and phyllite (Sawkins, 1990). 

Within the study area, map unit Xb locally contains horizons 

of mafic metavolcanic rocks and amphibolite (sills?), metagraywacke, pelitic schist, quartz-rich horizons, and phyllite. 

However, neither Besshi-type VMS deposits nor examples of 

Table I1. Delineation criteria for tracts of land with potential for undiscovered volcanic-associated Kuroko-type VMS copper-zinc±gold

deposits in GMUG greater study area.

[Details of the map units whose symbols appear here can be found in Day and others (1999)]

Diagnostic criterion for permissive tract delineation

1. Presence of Proterozoic bimodal (mafic and (or) felsic) metavolcanic and (or) associated metasedimentary rocks. Criteria met by map units 

 Xfh, Xf, and Xb of Day and others (1999)

Diagnostic criteria for favorable tract delineation 

2. Evidence of Proterozoic terranes dominated by metavolcanic rocks of bimodal (mafic and felsic) composition and proximal to felsic

 volcanic centers as characterized by thick accumulations of felsic metavolcanic and volcaniclastic metasedimentary rocks. Criteria met by

 map units Xfh and Xf of Day and others (1999).

3. Districts or mineralized regions containing VMS deposits and prospects.

4. Anomalous geochemical enrichments in stream-sediment samples for copper of greater than 100 parts per million and (or) zinc greater than 

 250 parts per million in areas permissive for hosting Kuroko-type VMS copper-zinc±gold deposits. Anomalous sample sites were given 

 a 0.5 km buffer as an estimate for the zone of influence for the potential that a Kuroko-type VMS deposit may have on the stream-

 sediment signature.

5. Occurrence of base-metal Kuroko-type VMS mineralization products (1 km buffer). Differs from 2 above in that an occurrence may not be 

 within a known mineral district.

Other favorable tract criteria, not used in this study because 

comprehensive regional data are not available

6. Alteration assemblage maps for prospective Proterozoic host terranes that would include the minerals sericite, calcite, epidote, chlorite, and 

 (or) pyrite.

7. Detailed electromagnetic (E-M) geophysical maps showing relative conductivity that would highlight zones of sulfide mineralization that 

 would typically result in highly conductive horizons in volcanic sequences.

8. Closely spaced stream-sediment, soil, and bedrock geochemical sampling across zones of known and inferred mineralization to identify 

 zones with potential of hosting Kuroko-type VMS mineralization.
142 Resource Potential and Geology, Grand Mesa, Uncompahgre, and Gunnison National Forests, Colorado

Besshi-type mineralization have been identified in or adjacent 

to the GMUG study area. In light of this, the area was not 

assessed for Besshi-type VMS deposits.

Areas permissive for hosting Kuroko-type VMS copperzinc+gold deposits, outlined in figure I2, are treated as a single 

discontinuous tract underlain by Proterozoic units Xfh, Xf, 

and Xb (table I1). Major parcels are exposed in a belt trending westward from the western flank of the Sawatch Mountains for a distance of approximately 125 km. The breadth 

of exposure narrows to the west and its northern boundary 

approximates the course of the Gunnison and Taylor Rivers. 

The rock units present were deposited under subaqueous conditions adjacent to the Dubois Greenstone belt, which formed 

in a rifted island-arc environment.

Favorable Tracts

The criteria used for delineating regions “favorable” for 

hosting Kuroko-type VMS copper-zinc+gold deposits are 

listed in table I1. The criteria are conditionally more restrictive than those for permissive areas and are applicable only 

within areas designated “permissive.” Proximity to felsic centers (map unit Xf) and (or) presence of bimodal volcanism and 

associated metasedimentary rocks (map unit Xfh) (criterion 2, 

table I1) are geologic conditions that show a strong correlation 

with known VMS deposits in the GMUG area. Field evidence 

also shows that the Kuroko-type VMS deposits discovered 

at the time of this study cluster within distinct regions (criterion 3, table I1). NURE stream-sediment data may be used 

to identify regions containing anomalous concentrations of 

the ore metals copper and zinc (criterion 4, table I1). Values 

greater than 100 parts per million for copper and 250 parts per 

million for zinc in stream-sediment samples can be interpreted 

as suggesting that somewhere within the watershed Kurokotype VMS mineralization occurred. The stream-sediment 

samples are point data that represent a sampling of the rocks 

exposed within a given drainage basin. In consideration of the 

scale at which this assessment is being conducted, a buffer of 

0.5 km was applied for each geochemically anomalous sample 

site to represent the area of influence attributed to the sample. 

Criterion 5 (table I1) deals with specific occurrences of basemetal VMS mineralization. Its effect in areas where VMS 

occurrences are clustered is somewhat redundant with that of 

criterion 3 (table I1); however, it does capture the importance 

of isolated VMS occurrences. A 1 km buffer is used around 

each site. The assessment technique of bit mapping employed 

herein (Spanski and Bankey, this volume, Chapter F) to delineate “favorable” lands does not weight the criteria as they are 

utilized. Therefore, areas underlain by rocks that meet more 

than one of the criteria for “favorable” designation receive the 

same final value as those underlain by only one such criterion.

The areas identified as being “favorable” for hosting 

Kuroko-type VMS mineral deposits are shown in figure I3. 

The region underlain by felsic metavolcanic rocks (map unit 

Xf) and bimodal metavolcanic rocks (map unit Xfh) is more 

limited than that outlined in the “permissive” assessment (fig. 

I2). Figure I3 shows the areas that satisfy the four individual 

critical criteria (table I1) as well as various combinations 

thereof.

The majority of the areas identified as being favorable 

for hosting Kuroko-type VMS deposits can be correlated with 

areas where mineralization is known to have occurred. For 

example, the westernmost area identified in figure I3 with 

criterion 2 (gray shade) lies within the Powderhorn, Goose 

Creek, Vulcan mine, and Beaver Creek mineral areas. The 

Vulcan mine area contains several tracts with various combinations of criteria (table I1), including areas within 1 km of a 

known Kuroko-type VMS deposit (criterion 5; green shade), 

within mineralized areas (criterion 3; dark blue shade), and 

coincidences of criteria 2, 3, 4 and 5 (red shade). The Beaver 

Creek area contains combinations of criteria 2, 3, and 5 (yellow shade), which corresponds to the area near the Midland 

mine, a VMS deposit that contained native gold as well as 

silver, lead, and zinc sulfide minerals (fig. I1). This western 

area (Powderhorn and Beaver Creek mineral areas) is identified as a region that warrants further exploration, owing to its 

favorable rank for hosting undiscovered Kuroko-type VMS 

mineral deposits.

The Cochetopa area, which is made up of the Cochetopa 

north, central, and south mineral areas as well as the Green 

Mountain mineral area (fig. I1) is also identified as being 

favorable for containing undiscovered Kuroko-type VMS 

deposits. The Cochetopa area contains the Graflin, Denver 

City, and Yukon mines, which are VMS deposits hosted in 

felsic volcanic rocks.

Several areas are identified as having the favorable bedrock types but lie outside of historical mining districts (units 

Xf and Xfh; criterion 2, table I1). Favorable Proterozoic rock 

units are mapped north and east of Tomichi Creek. Criterion 

4 (NURE stream-sediment samples with anomalous concentrations of copper and (or) zinc; table I1) is met by several 

samples throughout this region. Note that several NURE 

stream-sediment sample sites with anomalous concentrations 

of copper and (or) zinc correspond to known mineral areas that 

have post-Precambrian age mineral deposits (this report, fig. 

I3; Chapter E, fig. E1). For example, the Tincup/Cumberland 

Pass, Gold Brick, and Quartz Creek Pegmatite mineral areas 

have anomalous stream-sediment sample sites. The area 

east of the Tincup/Cumberland Pass area has several such 

anomalous stream-sediment sample sites that may be associated with polymetallic vein-type mineralization (see Wilson 

and others, this volume, Chapter J), or with undiscovered 

Kuroko-type VMS mineral potential. Another area that meets 

critical criteria 2 and 3 (table I1) lies northeast of the Marshall 

Pass mineral area and south of U.S. Highway 50 (fig. I3). 

The anomalous stream-sediment sites do not correspond to 

a mine or prospect in the database assembled by Wilson and 

others (2000), nor to any historical mineral area. As such, 

this area may be favorable for hosting undiscovered Kurokotype VMS copper-zinc+gold deposits. Further exploration is 

recommended for these areas that do not correspond to areas 
39o 30'

109o 

37o 45'

70

139

65

92

50

50 

82

135

133

149

62

145

141

141

90

114

Grand

Junction

Delta

Montrose

Gunnison

Crested Butte

Lake City

Uravan

Telluride

Ouray

Silverton

Aspen

Disappointment Creek

Gunnison River

Uncompahgre River Lake Fork Cebolla

Creek

Tomichi Creek

Taylor River

Blue MesaReservoir

CochetopaCreek

Dolores River

0

0

Approximate outline of national forests in GMUG study area

EXPLANATION

550

Figure I2. GMUG greater study area, showing permissive tracts (shaded) for hosting Kuroko-type VMS copper-zinc±gold deposits.
Powderhorn Goose Ck Vulcan Beaver Ck Vulcan So Cochetopa No Green MtCochetopa Central Cochetopa Tantalite Cochetopa So 

Revenue Quartz Ck Pegmatite "76"Gold Brick 

39o 30'109o 

37o 45'

70

139

65

92

50

50 

82

135

133

149

62

145

141

141

90

114

Grand

Junction

Delta

Montrose

Gunnison

Crested Butte

Lake City

Uravan

Telluride

Ouray

Silverton

Aspen

Disappointment Creek

Gunnison River

Uncompahgre River Lake Fork Cebolla

Cre

e

k

Tomichi Creek

Taylor River

Blue MesaReservoir

CochetopaCreek

Dolores River

0

0

Approximate outline of national forests in GMUG study area

EXPLANATION

550

Figure I3 (above and following page). GMUG greater study area, showing favorable tracts for hosting Kuroko-type VMS copper-zinc±gold d
Mineral Resource Assessment for Volcanic-Associated Massive Sulfide Deposits 145

of known Kuroko-type VMS mineralization to determine the 

feasibility of undiscovered mineral resources.

Undiscovered Deposit and Endowment 

Potential

The undiscovered deposit and endowment potential is 

assessed using a modified version (Spanski and Bankey, this 

volume, Chapter F, Appendix F1) of the global grade and 

tonnage models developed by Singer and Mosier (1986) for 

Kuroko-type massive sulfide deposits. The modified models 

are constructed exclusively with data for the Precambrian 

age deposits in the global model to produce a suite of models that are more consistent with the Proterozoic age setting 

that is present in the study area. The Precambrian model has 

a median deposit size of 1.2 million t (metric tons) of ore as 

opposed to 1.5 million t in the global model. The proportions 

of deposits that contain recoverable copper, zinc, and silver are 

similar at 100, 75, and 66 percent respectively. However, the 

Precambrian deposits are only half as likely to contain recoverable lead, 26 percent versus 43 percent in the global model, 

and approximately 62 percent of the Precambrian deposits 

contain extractable quantities of gold, versus 56 percent in the 

global model. The median grades for the Precambrian grade 

models are 1.3 percent copper, 2.6 percent zinc, 0.42 percent 

lead, and 23 and 0.59 g/t for silver and gold, respectively. 

No examples of deposits from Colorado are in the models; 

however, an unpaired t-test of the production data available for 

deposits in the Grape Creek and Cebolla districts (Long and 

others, 1998) shows that the lead, zinc, and silver grades and 

size (ore tonnages) of these deposits are not inconsistent with 

those in the Precambrian models. The gold and copper grades 

are, however, anomalous, being high in gold and low in copper. An assessment panel (Day, Spanski, Bankey, Wilson, and 

Smith) estimated the numbers of undiscovered deposits to be 

0,0,0,1,1 at the 90th, 50th, 10th, 5th, and 1st levels of confidence respectively (see table I2). The estimates suggest that 

the panel believe the probability of the presence of additional 

Kuroko-type deposits, with grades and tonnages similar to 

those of deposits in the Precambrian model, to be very low. 

The estimate of one deposit at the 5 and 1 percent confidence 

levels does, however, indicate that the panel recognized a 

small yet measurable potential that one additional deposit 

occurs somewhere within a kilometer of the surface within 

the bounds of the permissive and favorable tract areas. The 

latter is an acknowledgment of the fact that the majority of the 

exploration and development activity occurred in the first half 

of the 1900’s and was confined to shallow depths. Inasmuch 

as Kuroko-type VMS deposits are often clustered, the panel 

saw a potential for deposits at depth that the exploration methods used in the early 1900’s would not have revealed. 

The commodity and ore endowments resulting from the 

Mark3 simulations are summarized in table I2. The results 

show that within the permissive and favorable tract areas for 

Kuroko-type VMS deposits there is a 92.9 percent probability 

that no additional deposits are likely to be present and a 7.1 

percent probability of one deposit; a no-deposit scenario is 

nearly 13 times more likely than a one-deposit scenario. 

EXPLANATION

Criterion 2— Area underlain by either felsic or bimodal metavolcanic rocks

Criterion 3— Mineralized area known to contain Kuroko-type VMS mines or prospects 

Criterion 4— NURE stream-sediment sample site with anomalous concentrations of copper and (or) zinc

Criterion 5— Area within 1 km of a Kuroko-type VMS mine or prospect

Criteria 2, 3, and 5— Area within 1 km buffer of a Kuroko-type VMS mine or prospect and within a mineralized area 

 containing Kuroko-type VMS mines or prospects

Criteria 2, 4, and 5— Area containing a NURE stream-sediment sample with anomalous concentrations of copper and (or)

 zinc and within 1 km of a Kuroko-type VMS mine or prospect

Criteria 2, 3, 4, and 5— Area within mineralized area known to contain Kuroko-type VMS mines or prospects, adjacent to

 NURE stream-sediment sample site with anomalous concentrations of copper and (or) zinc, and

 within 1 km of a Kuroko-type VMS mine or prospect
146 Resource Potential and Geology, Grand Mesa, Uncompahgre, and Gunnison National Forests, Colorado

On the basis of these probabilities, the Mark3 simulation 

indicates that at the 90, 50, 10 percent probability levels the 

endowments of ore, copper, zinc, lead, silver, and gold that 

might be present in undiscovered deposits would be zero. The 

mean endowment values are averages for all scenarios run 

and include the no-deposit and one-deposit simulations. The 

variation in reported probabilities of mean endowments for the 

five commodities is largely due to the fact that in the grade and 

tonnage models not all the metals are recovered from every 

deposit. For example, copper is present in every deposit in the 

model; however, only 26 percent of the deposits in the model 

have reported lead grades. Therefore in three out of every four 

scenarios where a deposit is assumed to be present, no lead 

endowment will be calculated. The effect on the simulation 

results is to reduce the probability of the existence of lead and 

its mean. This effect is best seen in the endowment frequency 

plots, figure I4.

A more meaningful understanding of the economic 

importance of the Kuroko-type VMS deposit endowment 

potential can be gained by looking at the endowment frequency plots (Chapter F, Appendix F1). As in the past, 

Kuroko-type VMS deposits in the study area will likely be 

valued for their gold content with other commodities considered as byproducts. The endowment frequency plot for gold 

endowment (Chapter F, Appendix F1) indicates only a 4.2 

percent (100 minus 95.8 percent probability of no endowment) 

likelihood of the occurrence of a gold endowment, and that its 

size would range between 240 g and 310 t. The probability 

that a gold endowment equal to or greater than the mean of 

0.54 t could occur is only 2.6 percent. Although these are not 

encouraging scenarios, the 5 g/t gold grade reported for the 

Cebolla district (Long and others, 1998) would place it in the 

99th percentile (Chapter F, Appendix F1) in terms of the gold 

grade for Precambrian Kuroko-type VMS deposits. If this high 

gold content is indicative of VMS mineralization in the study 

area, then the simulation results are open to some reinterpretation. It could indicate that the local VMS deposits are abnormally enriched in gold, and that the 66 percent of deposits 

with gold reported may also be low and may under-represent 

the deposit population in the study area. As a result, the very 

low endowment threshold of 240 g of gold would be increased 

significantly, as would the upper limit on the endowment 

range. The 4.2 percent probability of the existence of a gold 

endowment would increase to approach 7.1 percent. These 

subtle differences give reason to believe that a rising gold 

market could renew exploration interest which would target 

the favorable Kuroko-deposit VMS tract areas. In addition to 

being of possible interest for future exploration, these deposits 

are of concern from an environmental standpoint. Although 

the median size is a relatively modest 1.2 million t, these 

deposits are sulfide-rich systems and are capable of generating 

large volumes of acid effluents when exposed to surface conditions during exploration or development.

Table I2. Summary of results of resource endowment potential assessment for undiscovered 

Kuroko-type volcanic massive sulfide deposits within GMUG greater study area.

Endowments given in metric tons.

Mark3 inputs— Undiscovered deposit estimates:

Estimation confidence 90% 50% 10% 5% 1%

Deposits 0 0 0 1 1

Mark3 outputs— Deposit occurrence probability:

Number of deposits 0 1

Probability of occurrence 92.9% 7.1%

Resource endowment estimates (minimums):

Resource Probability Mean

 90% 50% 10% (probability)

Copper 0 0 0 5,100 (4.9%)

Gold 0 0 0 0.54 (2.6%)

Silver 0 0 0 18 (3%)

Lead 0 0 0 720 (1.5%)

Zinc 0 0 0 13,000 (3.7%)

Ore 0 0 0 370,000 (4.9%)
Figure I4. Grade and tonnage models for 187 Precambrian Kuroko-type massive sulfide deposits (modified from Singer, 1986). Cebolla deposit 

study area.

1.0

 0.032 0.056 0.1 0.18 0.32 0.56 1.0 1.8 3.2 5.6 10

0.90.80.70.60.50.40.30.20.10.0 0.5 1.4 3.9

1.0

0.0016 0.0063 0.025 0.1 0.4 1.6 6.3 25 100 400 1,600 

0.90.80.70.60.50.40.30.20.10.0 0.13 1.19 11

1.0

 0.01 0.025 0.063 0.16 0.4 1.

0.90.80.70.60.50.40.30.20.10.0

1.0

 0.01 0.025 0.063 0.16 0.4 1.0 2.5 6.3 16 40 100 

0.90.80.70.60.50.40.30.20.10.0

0.58

1.0

 0.01 0.025 0.063 0.16 0.4 1.0 2.5 6.3 16 40 100 

0.90.80.70.60.50.40.30.20.10.0

0.18 1.9

1.0

 0.4 1.0 2.5 6.3 16 4

0.90.80.70.60.50.40.30.20.10.0

9.2 

 COPPER GRADE, IN PERCENT

PROPORTION OF DEPOSITS

 ORE, IN MILLIONS OF METRIC TONS

PROPORTION OF DEPOSITS

 ZINC GRAD

PROPORTION OF DEPOSITS

 LEAD GRADE, IN PERCENT

PROPORTION OF DEPOSITS

 GOLD GRADE, IN GRAMS PER METRIC TON

PROPORTION OF DEPOSITS

 SILVER GRADE, IN GR

PROPORTION OF DEPOSITS

n = 187 n = 187 

n = 187 n = 187 

Cebolla

Cebolla

C

Cebolla
148 Resource Potential and Geology, Grand Mesa, Uncompahgre, and Gunnison National Forests, Colorado

References Cited

Afifi, A.M., 1981, Stratigraphy, petrology, and structure of 

Precambrian metavolcanic rocks in the Iris area, Gunnison 

and Saguache Counties, Colorado, in Epis, R.C., and Callender, J.F., eds., Western Slope Colorado—Western Colorado and eastern Utah: New Mexico Geological Society 

Guidebook, 32nd Field Conference, p. 287–292.

Bennett, G.S., Bickford, M.E., and Girty, G.H., 1984, Geochemistry and petrotectonic setting of bimodal volcanic 

and volcaniclastic rocks, Cochetopa Canyon area, central 

Colorado: Geological Society of America Abstracts with 

Programs, v. 16, no. 4, p. 214.

Bickford, M.E., Bowring, S.A., and Gray, J.E., 1982, Chronology of igneous events in the Proterozoic of central 

Colorado: Geological Society of America Abstracts with 

Programs, v. 14, no. 6, p. 303.

Day, W.C., Green, G.N., Knepper, D.H., Jr., and Phillips, R.C., 

1999, Spatial geologic data model for the Gunnison, Grand 

Mesa, Uncompahgre National Forests mineral assessment area, southwestern Colorado and digital data for the 

Leadville, Montrose, Durango, and the Colorado parts of 

the Grand Junction, Moab, and Cortez 1° × 2° geologic 

maps: U.S. Geological Survey Open-File Report 99-427, 

CD-ROM.

Drobeck, P.A., 1981, Proterozoic syngenetic massive sulfide 

deposits in the Gunnison Gold Belt, Colorado, in Epis, R.C., 

and Callender, J.F., eds., Western Slope Colorado—Western 

Colorado and eastern Utah: New Mexico Geological Society Guidebook, 32nd Field Conference, p. 279–286.

Franklin, J.M., 1993, Volcanic-associated massive sulfide 

deposits, in Kirkham, R.V., Sinclair, W.D., Thorpe, R.I., and 

Duke, J.M., eds., Mineral deposit modeling: Geological 

Association of Canada Special Paper 40, p. 315–334.

Franklin, J.M., Hannington, M.D., Jonasson, I.R., and Barrie, C.T., 1998, Arc-related volcanogenic massive sulfide 

deposits: British Columbia Geological Survey Short Course 

Notes, Open File 1998-5, Section B, p. N1–N32.

Franklin, J.M., Lydon, J.W., and Sangster, D.F., 1981, Volcanic-associated massive sulfide deposits, in Skinner, B.J., 

ed., Economic Geology 75th Anniversary Volume: El Paso, 

Tex., Economic Geology Publishing Co., p. 485–627.

Hedlund, D.C., and Olson, J.C., 1975, Geologic map of the 

Powderhorn quadrangle, Gunnison and Saguache Counties, 

Colorado: U.S. Geological Survey Geologic Quadrangle 

Map GQ–1178, scale 1:24,000.

Hedlund, D.C., and Olson, J.C., 1981, Precambrian geology 

along parts of the Gunnison uplift of southwestern Colorado, in Epis, R.C., and Callender, J.F., eds., Western 

Slope Colorado—Western Colorado and eastern Utah: New 

Mexico Geological Society Guidebook, 32nd Field Conference, p. 267–272.

Hetherington, E.D., 1994, Structural analysis of lithotectonic 

boundaries in Proterozoic rocks near Gunnison, Colorado: 

Geological Society of America Abstracts with Programs, v. 

26, no. 6, p. 18.

Hutchinson, R.W., 1982, Syn-depositional hydrothermal 

processes and Precambrian sulfide deposits, in Hutchinson, 

R.W., Spence, C.D., and Franklin, J.M., eds., Precambrian 

sulfide deposits, H.S. Robinson Memorial Volume: Geological Association of Canada Special Paper 25, p. 761–791.

Knoper, M.W., and Condie, K.C., 1988, Geochemistry and 

petrogenesis of early Proterozoic amphibolites, west-central 

Colorado, U.S.A.: Chemical Geology, v. 67, no. 3–4, p. 

209–225.

Knoper, M.W., Condie, K.C., and Bickford, M.E., 1991, Tectonic evolution of early Proterozoic terranes in west-central 

Colorado: Geological Society of America Abstracts with 

Programs, v. 23, no. 4, p. 39.

Long, K.R., DeYoung, J.H., Jr., and Ludington, S.D., 1998, 

Database of significant deposits of gold, silver, copper, 

lead, and zinc in the United States: U.S. Geological Survey 

Open-File Report 98-206A & B, 33 p., 1 diskette.

Nelson, C.J., and Riesmeyer, W.D., 1983, Geology of the Anaconda–Gunnison mine area, Gunnison County, Colorado, in

Handfield, R.C., ed., Gunnison Gold Belt and Powderhorn 

Carbonatite field trip guidebook: Denver Region Exploration Geologists Society, p. 8–14.

Olson, J.C., 1976, Geologic map of the Iris quadrangle, Gunnison and Saguache Counties, Colorado: U.S. Geological Survey Geologic Quadrangle Map GQ–1286, scale 1:

24,000.

Olson, J.C., and Hedlund, D.C., 1973, Geologic map of the 

Gateview quadrangle, Gunnison County, Colorado: U.S. 

Geological Survey Geologic Quadrangle Map GQ–1071, 

scale 1:24,000.

Poulsen, K.H., and Hannington, M.D., 1996, Volcanic-associated massive sulfide gold, in Eckstrand, O.R., Sinclair, 

W.D., and Thrope, R.I., eds., Geology of Canadian mineral 

deposits types: Geological Society of America Decade 

of North American Geology, v. P-1, Geological Survey of 

Canada Geology of Canada no. 8, p. 183–196.

Sawkins, F.J., 1990, Metal deposits in relation to plate tectonics: New York, Springer-Verlag, 461 p.

Seyfried, W.E., Jr., and Janecky, D.R., 1985, Heavy metal and 

sulfur transport during subcritical and supercritical 
Mineral Resource Assessment for Volcanic-Associated Massive Sulfide Deposits 149

hydrothermal alteration of basalt—Influence of fluid pressure and basalt composition and crystallinity: Geochimica 

et Cosmochimica Acta, v. 49, no. 12, p. 2545–2560.

Sheridan, D.M., Raymond, W.H., and Cox, L.J., 1981, Precambrian sulfide deposits in the Gunnison region, Colorado, 

in Epis, R.C., and Callender, J.F., eds., Western Slope Colorado—Western Colorado and eastern Utah: New Mexico 

Geological Society Guidebook, 32nd Field Conference, p. 

273–277.

Singer, D.A., 1986, Descriptive model of Kuroko massive 

sulfide, in Cox, D.P., and Singer, D.A., eds., Mineral deposit 

models: U.S. Geological Survey Bulletin 1693, p. 189–197.

Singer, D.A., and Mosier, D.L., 1986, Grade and tonnage 

model of Kuroko massive sulfide, in Cox, D.P., and Singer, 

D.A., eds., Mineral deposit models: U.S. Geological Survey 

Bulletin 1693, p. 190–197.

Smith, S.M., 2000, National Geochemical Database; 

Reformatted data from the National Uranium Resource 

Evaluation (NURE) Hydrogeochemical and Stream Sediment Reconnaissance (HSSR) Program, Version 1.20: U.S. 

Geological Survey Open-File Report 97-492. URL: http:

//pubs.usgs.gov/of/1997/ofr-97-0492/, [unpaginated].

Wilson, A.B., Spanski, G.T., Crane, M.J., and Woodard, M.D., 

2000, Databases and spatial data model for mineralized 

areas, mines, and prospects in the Grand Mesa, Uncompahgre, and Gunnison (GMUG) National Forests, Colorado: 

U.S. Geological Survey Open-File Report 00-298, CDROM. URL: http://pubs.usgs.gov/of/2000/ofr-00-0298/.

Wortman, G.L., 1991, Time relationships among deformation, 

metamorphism, and plutonism in the Early Proterozoic of 

Colorado: Lawrence, Kans., University of Kansas M.S. 

thesis, 64 p.

Wortman, G.L., Coleman, D.S., and Bickford, M.E., 1990, 

Timing of arc accretion and deformation in Early Proterozoic volcanogenic rocks, central Colorado: Geological 

Society of America Abstracts with Programs, v. 22, no. 7, 

p. 262.
U.S. Department of the Interior

U.S. Geological Survey

Mineral Resource Assessment for

Polymetallic Vein Deposits

By Anna B. Wilson, J. Thomas Nash, Gregory T. Spanski, Viki Bankey, and

Steven M. Smith

Chapter J of

Resource Potential and Geology of the Grand Mesa, Uncompahgre,

and Gunnison (GMUG) National Forests and Vicinity, Colorado

Edited by Viki Bankey

U.S. Geological Survey Bulletin 2213– J
Contents

Abstract...................................................................................................................................................... 152

Introduction ............................................................................................................................................... 152

Genetic Model for Polymetallic Veins ................................................................................................... 152

Description of Mining Areas Containing Known Polymetallic Veins ............................................... 153

Application of the Deposit Model for a Mineral Resource Assessment of Polymetallic

Vein Deposits ............................................................................................................................... 154

Permissive Tracts............................................................................................................................. 156

Favorable Tracts............................................................................................................................... 156

Undiscovered Deposit and Endowment Potential............................................................................... 161

Discussion.................................................................................................................................................. 161

References Cited ...................................................................................................................................... 161

Figures

J1. Map showing GMUG restricted study area and locations of polymetallic vein or

polymetallic replacement deposits and the mineralized areas adjacent to or

containing them ................................................................................................................................ 155

J2. Map of GMUG greater study area showing permissive tracts for polymetallic vein

deposits .............................................................................................................................................. 157

J3. Map of GMUG greater study area showing favorable tracts for polymetallic vein

deposits .............................................................................................................................................. 159

Tables

J1. Delineation criteria for polymetallic vein deposits in GMUG restricted study area .............. 154

J2. Polymetallic vein tracts in GMUG restricted study area ............................................................ 156
Abstract

Polymetallic veins, rich in copper, lead, and zinc, with 

smaller but economically important amounts of silver or 

gold, form from rising, hydrothermal solutions. Location of 

the veins is determined by local structural features. Known 

polymetallic vein deposits containing silver, gold, lead, 

zinc, and copper were locally important producers in two 

areas of the GMUG National Forest and in areas adjacent 

to the GMUG Forests. Vein deposits in the San Juan area 

are generally large and related to Tertiary volcanic rocks, 

whereas deposits in the northeastern area are mostly smaller 

and occur in Paleozoic and Proterozoic rocks. Approximately 

6,880 square miles of the GMUG Forest study area, located 

within 10 kilometers of known or inferred shallow subvolcanic Tertiary intrusions, is classified as “permissive” for the 

occurrence of undiscovered polymetallic vein deposits. Of 

this “permissive” area, 2,973 square miles in the vicinity of 

Ruby and Elk Mountains, Dorchester and Forest Hill, Tincup/

Cumberland Pass, Gold Brick, and Whitepine, Bondholder, 

Lake City, Henson Creek, Cimarron, Carson, San Juan, 

Ouray, and Wilson Peaks mineralized areas are also classified 

as “favorable.” The favorable areas are within 3 kilometers 

of a known polymetallic replacement or polymetallic vein 

occurrence or mineralized area known or inferred to host those 

types of deposits, are within 500 meters of a stream-sediment 

or rock sample containing anomalous levels of copper, lead, 

zinc, or detectable silver or gold, and are within 10 kilometers 

of a caldera or caldera-related structure. Because mining these 

deposits involved development of several related types of 

deposits, grade and tonnage records of only polymetallic veins 

were not available. Therefore, a quantitative assessment was 

not performed. 

Introduction

Veins containing silver, gold, lead, zinc, and copper in a 

quartz-carbonate gangue have been mined or explored at many 

places in the GMUG study area (fig. K1). Economically, 

silver was the most important metal, although some veins were 

rich in gold, and locally base metals added value. Except close 

to the surface in the weathering zone, the dominant minerals 

are sulfides such as galena (PbS), sphalerite (ZnS), chalcopyrite (CuFeS2

), arsenopyrite (FeAsS), and pyrite (FeS2). Most 

of the silver is contained in the galena, but it also occurs 

in minerals such as tetrahedrite ((Cu,Fe,Ag)12(Sb,As)4

S13), 

enargite (Cu3AsS4), and silver sulfosalts (such as stephanite, 

pyrargyrite, polybasite, proustite, and pearceite). Gold occurs 

in its native state and locally in tellurides (such as hessite, 

calaverite, petzite, krennerite, and sylvanite). Electrum (native 

gold containing more than 20 percent silver) is reported in 

only one mine (Golden Fleece, Lake City mineralized area) in 

the GMUG Forests, but in several in the Silverton area. 

Economic geologists classify these vein deposits in many 

ways (Cox and Singer, 1986; Guilbert and Park, 1986; Panteleyev, 1988), but the unifying features are the vein geometry 

and polymetallic composition, hence the term polymetallic 

vein (PMV) deposit. We recognize that the broad definition 

used here encompasses many deposit types used by others, 

and also that the vein portion of the deposit commonly grades 

outward into disseminated or replacement types of ores. For 

instance, in the large Idarado mine, veins tended to be rich 

in quartz-adularia-gold at high elevations, but they became 

increasingly rich in chalcopyrite and galena at depth, graded 

laterally into a replacement zone where the vein crossed the 

Telluride Conglomerate, and formed skarn-type ore in deeper 

limestone beds (Mayor and Fisher, 1993). For simplicity, we 

will include all these kinds of mineralization styles in one 

descriptive model in this report.

Genetic Model for Polymetallic Veins

Veins rich in copper, lead, and zinc, and carrying smaller 

but economically important amounts of silver or gold, form 

from rising, hydrothermal solutions with a temperature of 

about 250° to 350° C. Vein features are generally those of the 

“epithermal” class of deposits, but some deeper vein portions may be better characterized by the archaic term “mesothermal” of Lindgren (1933). Several of the deposits in and 

Mineral Resource Assessment for

Polymetallic Vein Deposits

By Anna B. Wilson, J. Thomas Nash, Gregory T. Spanski, Viki Bankey, and Steven M. Smith
Mineral Resource Assessment for Polymetallic Vein Deposits 153

near the study area have been studied in some detail in the past 

25 years to increase understanding of ore-forming processes. 

(See, for example, Casadevall and Ohmoto, 1977; Nash, 1975; 

Slack, 1980; Fisher, 1990; Krasowski, 1976; Rosenlund, 1984; 

Jefferson, 1985; Herald, 1981; Neff, 1988; Earley, 1987.) 

Geometry of the veins is determined chiefly by the structural 

framework of faults and local brittle character of host rocks; 

non-brittle or reactive rocks such as shale and limestone commonly host replacement zones adjacent to the source vein. 

Geochronologic studies (Lipman and others, 1976; Bove 

and others, 2000) demonstrate that the veins formed a million 

or more years after the host rocks, and in some places they 

are contemporaneous with small intrusive bodies. Measured 

ages for minerals associated with ore, and ages inferred from 

geologic relations, are in the range of 30 to 5 Ma. Ages as old 

as early Tertiary are likely in the eastern part of the study area, 

and in theory, we are not aware of any reason to rule out even 

older times of formation. 

Stable isotope studies demonstrate that sulfur is derived 

from igneous rocks, but water is probably derived from both 

magmatic and near-surface sources (Casadevall and Ohmoto, 

1977; Taylor, 1997; Ohmoto and Goldhaber, 1997). In places, 

fluid inclusions indicate that the hydrothermal fluids boiled 

(Nash, 1975). This process marks the uppermost levels (the 

upper depth limit) of ore formation, but for the large veins 

as at Sunnyside and Idarado, boiling is not indicated (Nash, 

1975). Mineral textures and vein fabrics suggest that the 

larger vein systems formed deeper than typical hot springs 

type deposits (currently important sources of gold and silver in 

Nevada). Determining the source of heat and metals for these 

deposits is difficult, although clear answers to this seemingly 

academic question would greatly improve the spatial analysis 

of where the deposits could be expected.

Polymetallic vein deposits form as part of complexly 

zoned subvolcanic systems (Silberman and Berger, 1985). 

The zoning is helpful to specialists during exploration, but also 

is confusing. The top of the system, which in many places is 

in volcanic lavas and tuffs, is very different in chemical and 

mineralogical composition from the deeper part 2,000 to 3,000 

ft (about 600–900 m) below. In some parts of the western 

United States, mining has shown that veins of this type extend 

down into the plutonic rocks that were related to the source. 

The top may be rich in gold and associated with fine-grained 

silica and adularia, whereas the deeper parts have more copper 

and lead sulfide minerals, often with tungsten and bismuth; 

alteration products are sericite, epidote, and other minerals 

reminiscent of parts of porphyry copper systems (Guilbert 

and Park, 1986). Vertical variability of composition and ore 

grade complicates classification of small prospects and our 

interpretation of descriptions of deposits in reports: we must 

consider whether the prospect is the top of a larger deposit at 

depth, the bottom of a mostly eroded deposit, or some other 

type of deposit. Vertical relief in the San Juan Mountains can 

be of help in this evaluation, and also in the practical aspects 

of mining, but other parts of the study area lacking much relief 

do not have this benefit.

Description of Mining Areas

Containing Known Polymetallic Veins

Polymetallic vein deposits are prominent in two regions 

within the GMUG restricted study area: in the south extending 

from the Wilson Peaks to the Bondholder mineralized areas 

and in the northeast extending from the Ruby to the Whitepine 

(also known as Tomichi) mineralized areas (fig. J1). The 

southern area generally hosts larger deposits that are related 

to Tertiary volcanic rocks. Deposits in the northeastern area 

are mostly smaller, and most occur in Paleozoic and Proterozoic rocks. Understanding the geologic setting, character, and 

mode of origin of known polymetallic vein deposits allows us 

to formulate critical criteria that can help identify additional 

areas with potential to host similar deposits.

The western San Juan Mountains between Silverton and 

Telluride contain some of the best endowed veins in the study 

area. Here, deposits were mined underground with hundreds 

of miles of interconnected tunnels. Years of detailed work 

by many geologists (for example, Burbank, 1930, 1933, 

1951; Burbank and Luedke, 1969; Kelley, 1946; Luedke and 

Burbank, 1981), underground and on the surface, document 

a regional fracture pattern that radiates out from the elliptical 

structure of the Silverton caldera margin. Major preexisting graben structures (Eureka graben) appear to control ore 

distribution within the caldera. Aided by miles of exposure 

in mines, geologists recognized that the deposits extend 

downward for thousands of feet, in fact far below the deepest 

levels of mining, which were generally defined by the elevation of deep tunnels that served as haulages for ore and drains 

for water. Deep drilling at Idarado (Mayor and Fisher, 1993) 

intersected skarn-like ore 1,000 to about 3,000 feet below the 

mined veins, but this ore has not been pursued because of the 

expense of mining at those depths (ore would have to be lifted, 

and water pumped). Some of the vein deposits were in preTertiary sedimentary rocks, and those were, in general, less 

productive. The Telluride Conglomerate, at the base of the 

volcanic section, contains reactive clasts of limestone that host 

replacement ores adjacent to larger veins. 

Ore in the San Juan Mountains area was mined from 

small shafts prior to 1900. In later years, consolidation of 

mining properties permitted large mine complexes to be developed. Ore was transported to mills through long tunnels at 

elevations below treeline that were sheltered from avalanches 

and severe winter conditions. A combination of depressed 

metal prices and high underground mining costs forced almost 

all of the mines and mills in the western San Juan Mountains 

to close by the mid-1980’s. (The Sunnyside mine, south of the 

study area, remained open until 1991.) Total production from 

the Idarado mine was almost 24 million short tons as of 1976 

(Mayor, 1978), and from the Camp Bird purportedly about 13 

million short tons (USGS, unpub. data). 

Elsewhere in the study area, polymetallic vein deposits 

were generally much smaller in size. The volcanic-hosted 

veins near Lake City produced some extremely rich ore in the 
154 Resource Potential and Geology, Grand Mesa, Uncompahgre, and Gunnison National Forests, Colorado

1880’s and ′90’s valued at more than $10 million at the time of 

mining (Irving and Bancroft, 1911), but production in the 20th 

century was less than 1 million tons. The Lake City area never 

came close to the production of the nearby Eureka, Red Mountain (West), and Telluride areas (all in the San Juan mineralized area; see Wilson and Spanski, this volume, Chapter E). 

Vein deposits in the northeastern part of the study area 

are commonly somewhat different from the classic San Juan 

type. The area is on the west flank of the Sawatch uplift and 

contains Tertiary intrusive rocks (primarily granodiorite), 

Paleozoic carbonate and clastic rocks, and mines known (or 

suspected) to have produced ore from polymetallic vein type 

ore bodies. These veins differ from the San Juan type in that 

they have more milky “bull” quartz and a relationship to early 

to middle Tertiary granitic stocks that appear not to have generated an extensive volcanic edifice. Even the most productive 

mines in veins in sedimentary host rocks in the Ruby mineralized area near Crested Butte produced well under a million 

tons of ore. The veins mined in the Gold Brick and Tincup/

Cumberland Pass mineralized areas were locally important, 

but relative to veins in the San Juan area were at least an order 

of magnitude less productive. (See Crawford and Worcester, 1916; Vanderwilt, 1947; Fisher, 1990; Henderson, 1926, 

among others.) 

Application of the Deposit Model for 

a Mineral Resource Assessment of 

Polymetallic Vein Deposits

The criteria listed in table J1 are those used in this reconnaissance mineral resource assessment to define “permissive” 

and “favorable” mineral deposit potential tracts for undiscovered polymetallic vein deposits. The criteria are based on the 

genetic model presented herein and the data available for the 

GMUG study area. The applicable regional databases available for this GIS-based assessment include digital geologic 

map data (Day and others, 1999), mine and prospect locations (Wilson and others, 2000), outlines of mineralized areas 

(Wilson and Spanski, this volume, Chapter E), regional NURE 

stream sediment geochemical data (S.M. Smith, unpub. data, 

2001), a map of intrusions based on aeromagnetic survey data 

interpretation (Bankey and others, this volume, Chapter D), 

and maps showing detailed age and composition of intrusions 

(Bove and others, 2000; D.J. Bove and others, unpub. data, 

2001).

In this chapter, GMUG restricted study area refers only 

to GMUG National Forests and the lands they roughly surround from Grand Junction to the area east of Gunnison (fig. 

J1). It also includes the western slope in the Uravan area, but 

the restricted study area does not include lands in adjacent 

National Forests: White River, San Isabel, Rio Grande, or 

San Juan, even though they are within the “greater study area” 

boundary. Any resource potential indicated outside of this 

restricted study area is based on incomplete data, especially 

for mines, prospects, and mineralized areas. Many more areas 

in the adjacent lands, including national forests, may be permissive, or even favorable. For instance, Aspen and Leadville, 

both large productive districts, contain polymetallic vein and 

polymetallic replacement deposits, yet neither is shown in figure J3. The square miles indicated as permissive and favorable 

are therefore minimums for the entire “greater study area.”

The polymetallic vein deposit model used for assessment 

purposes in the restricted study area is a highly generalized 

model that embraces at least five deposit models described 

in Cox and Singer (1986). Although individual examples of 

hot-spring gold-silver (Berger, 1986), polymetallic vein (Cox, 

1986), Creede epithermal vein (Mosier and others, 1986), 

polymetallic replacement (Morris, 1986), and base- and 

precious-metal skarn deposits can be cited, for the most part 

these deposit types commonly occur spatially intermingled. 

As a consequence of this clustering or nesting of deposit types, 

Table J1. Delineation criteria for polymetallic vein deposits in GMUG restricted study area.

Diagnostic criterion for permissive tract delineation

1. Presence of mapped shallow, subvolcanic Tertiary intrusions or geophysical evidence for them. We include a 10 km buffer because known 

 deposits occur approximately this far outward from intrusive centers.

Diagnostic criteria for favorable tract delineation 

2. Proximity to known or suspected polymetallic deposits, both vein and replacement. We use a buffer of 3 km to allow space for

 undiscovered deposits or deposits that are known but not in our database of deposits.

3. Within 3 km of a mineralized area known or inferred to host polymetallic vein or polymetallic replacement deposits.

4. Geochemically anomalous concentrations of copper, lead, zinc, or detectable silver or gold. This geochemical association is the same as 

 discussed for polymetallic replacement deposits. The buffer used is 500 m. Other elements, such as bismuth, cadmium, molybdenum,

 arsenic, antimony, or tellurium, may be useful in theory, but available information is either nonexistent or inconsistent.

5. Proximity to calderas and caldera-related structures; we include a buffer of 10 km inside and outside calderas to cover caldera-related

 structures that are known to control ore deposits.

6. Permissive terrane (see criterion 1) minus the areas underlain by Quaternary sediments or young Tertiary basalts.
39o 30'

109o 

37o 45'

70

139

65

92

50

50 

82

135

133

149

62

145

141

141

90

114

Grand

Junction

Delta

Montrose

Gunnison

Crested Butte

Lake City

Uravan

Telluride

Ouray

Silverton

Aspen

Disappointment Creek

Gunnison River

Uncompahgre River Lake Fork Cebolla

Creek

Tomichi Creek

Taylor River

Blue MesaReservoir

CochetopaCreek

Dolores River

0

0

550

Approximate outline of national forests in GMUG study areaApproximate outline of restricted study area

EXPLANATION

++++

+++

+

++++++++++++++++++++ +++++++++++

++++ +++

+

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++

+

++

+

+ ++ +

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+ + ++++++++

+ + 

+++

+

+ + + ++ ++++

+

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+

+

+

+

++++

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+

+ + +

++

+

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+

+

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+++++++

+

+ +++

+++ +

+

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+

+

+ + ++

+

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+

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+

++

+

++++ + +++

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+

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Cimarron

Henson

Ck

Sunrise–

Morning Glory

Wilson Peaks

San Juan

Ouray

Carson

Bondholder

Gold

Brick

Spring Ck

Dorchester Elk Mts

Ruby

Lake City

Figure J1. GMUG restricted study area, showing locations of polymetallic vein or polymetallic replacement deposits (plus signs) and the min

or containing them (red outlines).
156 Resource Potential and Geology, Grand Mesa, Uncompahgre, and Gunnison National Forests, Colorado

early mining, particularly where production was significant, 

required development of several types of deposits. These 

occurrences are now identified in terms of what is believed 

to have been the dominant deposit type present. However, 

for making subjective estimations of numbers of remaining 

deposits, this type of count is inadequate. The prominence 

of subordinate deposit types is under-represented, and the 

size and grade characteristics of the various deposit types 

involved, reflected in production records, cannot be properly 

modeled. Historically, production was reported for mills 

and might reflect the output from a single large mine or from 

several mines of varying size. In addition, production was in 

many cases further consolidated and reported by county. To 

reconstruct the size and grades of the mines by deposit types 

or to obtain an accurate count of the numbers of deposits of 

each deposit type that have so far been identified is virtually 

impossible. Given this situation, the general polymetallic vein 

model was adopted for the current assessment.

The set of delineation criteria outlined in table J1 is by 

no means exhaustive. It is limited by the availability of supporting data sets that are both fairly complete and geographically comprehensive at 1:250,000 scale. Additional criteria 

could have been considered for tract assessment if the digital 

databases had been more complete. Examples of other data 

that could have been used to classify and evaluate the mineral 

resource potential of the area include:

• occurrence of small veins or prospects containing 

minerals such as stibnite or manganese oxides, or pathfinder 

elements such as mercury;

• distribution of hydrothermal alteration minerals, such 

as chlorite, sericite, quartz, with or without pyrite or barite;

• anomalous concentrations of copper, tungsten, gold, 

arsenic, antimony, bismuth, barium, manganese, iron, or magnesium in bedrock, altered rock, or stream-sediment samples;

• presence of structural features, such as faults or zones 

of extensional tectonic activity;

• presence of small porphyritic dikes and stocks and 

associated zones of hydrothermal alteration and breccia pipes;

• presence of Cretaceous or Tertiary age porphyry 

(model 17, Cox and Singer, 1986), skarn (18b, c, d, Cox and 

Singer, 1986), or evidence of polymetallic replacement 

mineralization;

• detailed aeroradiometric maps showing Th/U ratios 

of less than 4:1 suggesting that uranium depletion may have 

resulted from hydrothermal activity; 

• local ground electromagnetic surveys with sufficient 

detail to map veins containing high concentrations of sulfide 

minerals; and 

• detailed maps of calderas and related structures located 

outside the study area that may have an influence inside the 

study area, such as the Mount Aetna caldera (potentially influencing the Tincup/Cumberland Pass and Whitepine areas), or 

locations of nonresurgent calderas, such as the Cochetopa Park 

caldera (which does not appear to be associated with Tertiary 

intrusions or mineralization).

Permissive Tracts

In the GMUG greater study area, 6,880 mi2 is classified 

“permissive” for the occurrence of polymetallic vein deposits (fig. J2; table J2). Polymetallic vein deposits are known 

to form in rocks of many ages (Precambrian to Tertiary) and 

compositions. The fundamental requirement is that the host 

rock be brittle enough to break and stay open, thus allowing a 

vein to fill open space. Even this broad rule is violated: veins 

also occur locally in more ductile rocks like shales, and change 

to more diffuse replacement zones in reactive carbonate rocks. 

Therefore, there are few geologic restraints on where these 

veins might form. Veins could be concealed under Quaternary 

sedimentary deposits, so even these sediments are included 

in the permissive tracts. Because of the generally accepted 

association with shallow, subvolcanic intrusions, we include a 

general stipulation that those rocks, or geophysical evidence 

for them, be present within 10 km.

Favorable Tracts

In the GMUG greater study area, 2,973 mi2 is classified 

“favorable” for the occurrence of polymetallic vein deposits 

(fig. J3). Some of this area is outside the restricted study 

area, and this value should be considered a minimum for the 

“greater study area” for reasons mentioned in the “Application 

Table J2. Polymetallic vein tracts in the GMUG restricted study area.

Tract No.a Tract name

P1 Permissive for polymetallic veins 1

F1 Ruby and Elk Mountains 2, 3, 4, 6

F2 Dorchester and Forest Hill 2, 3, 5, 6

F3 Tincup/Cumberland Pass, Gold Brick, and Whitepine 2, 3, 4, 6

F4 Bondholder 2, 6

F5 San Juan area 2, 3, 4, 5, 6

a

P, permissive tract; F, favorable tract.

Unnamed variable

Delineation criteria
39o 30'

109o 

37o 45'

139

9

141

90

Grand

Junction

Montrose

d Butte

ray

Aspe

Disa poi tme t C eek

Gunniso

ke

Fo

r

k

ebo

0

0

Approximate outline of national forests in GMUG study area

EXPLANATION

Figure J2. GMUG greater study area, showing permissive tracts (shaded) for polymetallic vein deposits. 
158 Resource Potential and Geology, Grand Mesa, Uncompahgre, and Gunnison National Forests, Colorado

of the Deposit Model for Mineral Resource Assessment of 

Polymetallic Vein Deposits” section. These “favorable” lands 

represent about 43 percent of the “permissive” tract. Most of 

the favorable lands are in five well-defined areas, discussed 

later. The critical criteria used for delineating regions “favorable” for hosting polymetallic vein type deposits appear in 

table J1. These criteria are more restrictive than those used 

to delineate the permissive terrain. Increased importance is 

placed on the presence of evidence that mineralization has 

occurred, on anomalous geochemistry, and on proximity to 

caldera structures. 

Analysis of the known deposits suggests five additional geologic and geochemical criteria for spatial modeling of favorable areas: proximity (3 km) to areas of known 

or suspected polymetallic vein or polymetallic replacement 

deposits; proximity (3 km) to individual polymetallic vein or 

replacement deposits; geochemically anomalous copper, lead, 

zinc, or detectable silver or gold (500 m); proximity (10 km) 

to calderas or caldera-related structures; and omitting areas 

underlain by young Tertiary basalts and Quaternary sediments. 

These are mostly empirical criteria and are consistent with the 

genetic model described earlier. Most of the genetic criteria 

such as isotopic values or fluid inclusion numbers are either 

not available for much of the area or are not amenable to spatial modeling. Proximity to faults was studied and modeled, 

but deleted as a favorable criterion. Proximity to faults alone 

was considered insufficient evidence to classify the ground as 

favorable, and elsewhere the ground may already be included 

using other criteria.

To be considered favorable for polymetallic veins (fig. 

J3), an area must have been classified as permissive (fig. J2) 

and meet at least one additional criterion (table J1). Areas 

meeting at least three criteria are clustered in the northeastern 

and southern parts of the study area. In the northeastern part 

of the study area these include (F1) Ruby and Elk Mountains, 

(F2) Dorchester and Forest Hill, and (F3) Tincup/Cumberland 

Pass, Gold Brick, and Whitepine mineralized areas. In the 

southern part of the study area they include (F4) Bondholder, 

and (F5) Lake City, Henson Creek, Cimarron, Carson, San 

Juan, Ouray, and Wilson Peaks mineralized areas (fig. J3). 

The Ruby and Elk Mountains tract (fig. J3, area F1) 

contains known polymetallic vein deposits that appear to 

be related to Tertiary plutons. Most of the veins are hosted 

in Cretaceous Mesaverde Formation consisting of interbedded sandstone, shale, and coal, or in Ohio Creek Member of 

the Mesaverde Formation and Tertiary Wasatch Formation 

consisting of conglomerates, sandstones, and shales. Veins 

related to the Afley, Augusta, and Mt. Owen stocks are weakly 

mineralized (Ellis, 1983, p. 9). Most of the mineralization 

is attributed to the younger felsite plugs related to the Mount 

Emmons porphyry molybdenum deposit. Mineralogically, the 

veins carry sulfides, such as galena and sphalerite, the primary 

sources of silver, as well as pyrite, minor chalcopyrite, and 

possibly tetrahedrite. Farther from the intrusions, the veins 

may contain native and ruby silver, and arsenopyrite. 

According to Ellis (1983), mining began in 1874 from 

silver-rich base-metal veins such as the Forest Queen, Ruby 

Chief, Ruby King, and Bullion King. Several of these mines 

produced intermittently until the early 1900’s. In the 1950’s–

1960’s mining resumed at the Keystone, Micawber (Standard), 

and Daisy mines. After this, the emphasis shifted to molybdenum discoveries in Redwell (the Mount Emmons deposit) and 

Red Lady Basins. Little production information is available. 

Ellis (1983) credited the area with at least 24,000 oz gold, 5.2 

million oz silver, 6.6 million lb copper, 30.9 million lb lead, 

and 55.2 million lb zinc between 1901 and 1969. 

These areas meet criteria 1, 3, 6, and locally, 2 and 4 

(tables J1 and J2). Additional, similarly small polymetallic vein deposits are likely to be present in the vicinity or as 

extensions of known deposits. 

Little is known about the deposits in the Dorchester and 

Forest Hill mineralized areas (fig. J3, area F2) (Garrett, 1950; 

Prather, 1964; Slebir, 1957). None of the deposits in these 

areas is classified as a polymetallic vein deposit, yet because 

these deposits are intimately associated with polymetallic 

replacement deposits, at least some of the deposits are likely to 

fit the category. These deposits are on the flank of the Grizzly 

Peak caldera and may be associated with that structure. This 

caldera is older than the calderas of the San Juan volcanic field 

(Day and Bove, this volume, Chapter B). The combination of 

a mineralized area suspected of containing polymetallic vein 

or replacement deposits (criteria 2 and 3) and a high density 

of anomalous geochemical values (criterion 4) within 10 km 

of the Grizzly Peak caldera (criterion 6) places this tract in the 

favorable category. 

Tincup/Cumberland Pass and Whitepine mineralized 

areas (fig. J3, area F3) are noted more for their polymetallic replacement than vein deposits. Reactive carbonate rocks 

were favored for replacement ore zones adjacent to veins 

(faults), as discussed for the polymetallic replacement deposits (Wilson and others, this volume, Chapter K). However, 

because the two deposit types are closely related both spatially 

and genetically, and many of the deposit descriptions (Dings 

and Robinson, 1957; Worcester, 1919; Trammel, 1961; Rosenlund, 1984; Goddard, 1936; Hill, 1909) mention veins, they 

are considered together. Additional small polymetallic vein 

deposits are likely to be present in the vicinity or as extensions 

of known deposits. 

Deposits in the Gold Brick district are fissure-vein 

deposits (Rugg, 1956; Crawford and Worcester, 1916, p. 80). 

Locally, deposits hosted by veins in schist and gneiss are widened up to a foot and a half by replacement. Much of the ore 

consists of sulfide minerals, but there is also a concentration 

of oxidized ores. Galena appears to have been the primary ore 

mineral containing gold and silver. In addition, similar small 

polymetallic vein deposits are likely to be present in the vicinity or as extensions of known deposits. 

These three mineralized areas, Tincup/Cumberland 

Pass, Whitepine, and Gold Brick, combined (fig. J3, area F3) 

have at least three characteristics favorable for the presence 

of polymetallic vein deposits. Appropriate intrusions and 
39o 30'

109o 

37o 45'

70

139

65

92

50

50 

82

135

133

149

62

145

141

141

90

114

Grand

Junction

Delta

Montrose

Gunnison

Crested Butte

Lake City

Uravan

Telluride

Ouray

Silverton

Aspen

Disappointment Creek

Gunnison River

Uncompahgre River Lake Fork Cebolla Creek

Tomichi Creek

Taylor River

Blue MesaReservoir

CochetopaCreek

Dolores River

0

0

Approximate outline of national forests in GMUG study area

EXPLANATION

550

Approximate outline of area assessed for polymetallic vein deposits

F1

F2

F3

F4

F5

Figure J3 (above and following page). GMUG greater study area, showing favorable tracts for polymetallic vein deposits. F1, Ruby and Elk M

and Forest Hill; F3, Tincup/Cumberland Pass and Whitepine; F4, Bondholder; F5, San Juan Mountains tracts. Descriptions in text.
160 Resource Potential and Geology, Grand Mesa, Uncompahgre, and Gunnison National Forests, Colorado

host rocks are present (criteria 1 and 6), as well as known 

polymetallic vein deposits (criterion 2) or mineralized areas 

containing polymetallic vein or replacement deposits (criterion 

3). Locally, the mineralized tract may contain geochemically 

anomalous areas (criterion 4). 

The small veins in the Bondholder district (fig. J3, area 

F4) are somewhat enigmatic. This district has seen minimal 

production. Only a study by Steven and Bieniewski (1977) 

mentioned the mines in the area and concluded that the area 

has low economic potential “because of the small size of the 

veins and the erratic distribution of valuable metals along 

them” (p. 33). There are very few geochemical anomalies, but 

this could be due to insufficient sample density in the databases used for this national forest assessment. The presence of 

a mineralized area (criterion 2) within a caldera (criterion 6), 

places this tract in the favorable area. 

The San Juan Mountains tract (fig. J3, area F5) contains 

the Wilson Peaks, San Juan, Ouray, Cimarron, Henson Creek, 

Lake City, and Carson mineralized areas (fig. J1). This tract 

contains a slightly different type of polymetallic vein from the 

rest of the study area, the most striking distinction being their 

relatively large size. Some of the San Juan Mountains area 

veins contain millions of tons of ore. All but the veins on the 

periphery, in Wilson Peaks and Ouray areas, almost certainly 

have a genetic link to the caldera-related structures (criterion 

5). However, radiometric age determinations show that the 

ores are significantly younger than the caldera events and 

probably synchronous with younger shallow plugs that were 

Criterion 2 or 3 or both— Within 3 km of a deposit or mineralized area 

 containing polymetallic vein (PMV) or polymetallic replacement 

 (PMR) deposits

Criterion 4— Within 500 m of anomalous geochemical sample

Criteria 2 or 3 or both, and 4— Within 3 km of a deposit or mineralized 

 area containing polymetallic vein (PMV) or polymetallic 

 replacement (PMR) deposits and within 500 m of anomalous 

 geochemical sample

Criterion 5— Within 10 km of a caldera boundary

Criteria 2 or 3 or both, and 5— Within 3 km of a deposit or mineralized 

 area containing polymetallic vein (PMV) or polymetallic 

 replacement (PMR) deposits and within 10 km of a caldera boundary 

Criteria 4 and 5— Within 500 m of anomalous geochemical sample 

 and within 10 km of a caldera boundary 

Criteria 2 or 3 or both, and 4 and 5— Within 3 km of a deposit or mineralized 

 area containing polymetallic vein (PMV) or polymetallic 

 replacement (PMR) deposits and within 500 m of anomalous geochemical 

 sample and within 10 km of a caldera boundary 

EXPLANATION
Mineral Resource Assessment for Polymetallic Vein Deposits 161

intruded along caldera structures (Lipman and others, 1976; 

Bove and others, 2000). 

By 1945, this area produced $345 million worth of ore, 

nearly 44 percent of it in gold (summarized from Burbank, in 

Vanderwilt, 1947, p. 404–405). About 7 million oz of gold 

accounted for about $150 million (gold was valued at approximately $20.67 per oz until 1934). Silver accounted for 30 

percent of the value, lead for 16 percent, and copper and zinc 

5 percent each. As of the mid 1970’s, the Idarado and Camp 

Bird mines alone had produced more than 37 million tons of 

ore.

This tract is classified as favorable because it contains the 

appropriate host rocks (criteria 1 and 6), contains mineralized 

areas with polymetallic vein or replacement deposits (criterion 3) or individual deposits (criterion 2), is within 10 km of 

a caldera (criterion 5), and has many geochemical anomalies 

(criterion 4). 

Undiscovered Deposit and Endowment 

Potential

Estimation of the probable numbers of additional deposits 

and simulation of the potential mineral resource endowment 

that could be associated with those deposits requires that grade 

and tonnage models for those deposit types be available. In 

the absence of such models for the polymetallic vein deposit 

type used, no quantitative assessment was conducted. In its 

deliberations, however, the USGS assessment panel (Wilson, 

Spanski, Bankey, Nash, D.A. Lindsey, Smith, and W.C. Day) 

believed that a secondary, nonquantifiable potential is associated with polymetallic vein type mineralization in the study 

area. That potential is associated with the probable existence 

of smaller undiscovered ore bodies that, in the past, would 

have been developed as a mine or as an extension to a mine. 

The panel believed that the potential is highest for these ore 

bodies in areas adjacent to, or directly beneath, the areas of 

currently known polymetallic vein occurrences. Their numbers, size, and endowment characteristics cannot be estimated 

because size and grade models are not available. In today’s 

economic and environmental climate, large mining companies 

are not likely to be interested in sporadic occurrences of this 

nature; however, these smaller occurrences might be attractive 

development targets for smaller entrepreneurial groups. 

Discussion

What is the outlook for exploration and mining of 

polymetallic vein deposits in the next 10 or 20 years? From a 

geological standpoint, we can say with confidence that large 

tonnages of polymetallic mineralized rock exist in the study 

area both in former mining areas and as undiscovered deposits. The vital question is not one of geology but of economics. 

Economic factors are complex, and we are not experts in this 

field, but we would consider the following as impediments to 

future mining: (1) lack of a mining infrastructure, including 

mills and mining expertise; (2) depressed prices for silver and 

other metals in recent years, with few signs of recovery; and 

(3) societal resistance to mining and fear of environmental pollution from mining. 

Some deposits were mined with high profits in the 

1880’s, but the mines were closed during the silver crash 

of the 1890’s and never reopened. Many of these mining 

operations had, in fact, taken out the richest ore; and probably 

insufficient high quality ore remained to allow mining during 

periods of higher silver prices in the 20th century. Exploration 

has been intense, if not exhaustive, in the study area, reducing the likelihood of future discoveries of bonanza-type ores. 

Most of the production from polymetallic vein deposits has 

been from large mines since 1920, when changes in milling technology and the advent of selective floatation allowed 

profitable mining of “low grade” ores. The new technology 

allowed zinc, which formerly drew a smelter penalty and was 

considered a contaminant, to be recovered for profit (Burbank 

and Luedke, 1964). Future changes in mining and milling 

technology are possible, of course, but are not expected to be 

enough to offset the economic advantage of lower cost mining, 

milling, and smelting in other parts of the world. The paradox 

here is that for mining of polymetallic vein deposits to produce 

a significant tonnage, a large mining and milling operation is 

mandated, and this is most at odds with considerations 1 and 

3 in the preceding paragraph. Small-scale mining of local 

pockets of rich ore may be viable, particularly if the operators 

can find a way to mill and smelt their ore.

The viability of these mineralized areas, again, is 

dependent on economic factors rather than geology. Geology 

is favorable beyond the known deposits, and only detailed 

studies including drilling can determine if additional deposits 

exist. We expect that any future discoveries would be within 

or adjacent to the colored areas in figure J3. However, new 

geologic information, or new concepts of ore formation, could 

lead to the discovery of ore outside of the favorable areas 

shown in figure J3 but within the permissive area shown in 

figure J2.

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district, Gunnison County, Colorado: Ft. Collins, Colo., 

Colorado State University M.S. thesis, 106 p.

Ohmoto, Hiroshi, and Goldhaber, M.B., 1997, Sulfur and carbon isotopes, in Barnes, H.L., ed., Geochemistry of hydrothermal ore deposits, 3rd Edition: New York, John Wiley, p. 

517–612.

Panteleyev, A., 1988, A Canadian Cordilleran model for epithermal gold-silver deposits, in Roberts, R.G., and Sheahan, 

P.A., eds., Ore deposit models: Geoscience Canada Reprint 

Series 3, p. 31–43.

Prather, T.L., 1964, Stratigraphy and structural geology of the 

Elk Mountains, Colorado: Boulder, Colo., University of 

Colorado Ph. D. dissertation, 153 p.

Rosenlund, G.C., 1984, Geology and mineralization of the 

Cumberland Pass area, Gunnison County, Colorado: Ft. 

Collins, Colo., Colorado State University M.S. thesis, 113 p.

Rugg, E.S., 1956, Geology of the Carter mine, Gunnison 

County, Colorado: Golden, Colo., Colorado School of 

Mines M.S. thesis, 54 p.

Silberman, M.L., and Berger, B.R., 1985, Relationship of 

trace-element patterns to alteration and morphology in 

epithermal precious-metal deposits, in Berger, B.R., and 

Bethke, P.M., eds., Geology and geochemistry of epithermal 

systems: Reviews in Economic Geology 2, p. 203–232.

Slack, J.F., 1980, Multistage vein ores of the Lake City district, western San Juan Mountains, Colorado: Economic 

Geology, v. 75, no. 7, p. 963–991.

Slebir, E.J., 1957, The geology of the North Cement Creek 

area, Gunnison County, Colorado: Golden, Colo., Colorado 

School of Mines M.S. thesis, 93 p.

Steven, T.A., and Bieniewski, C.L., 1977, Mineral resources of 

the La Garita Wilderness, San Juan Mountains, southwestern Colorado, with a section on Geophysical interpretation 

by G.P. Eaton: U.S. Geological Survey Bulletin 1420, 65 p.

Taylor, H.P., Jr., 1997, Oxygen and hydrogen isotope relationships in hydrothermal mineral deposits, in Barnes, H.L., ed., 

Geochemistry of hydrothermal ore deposits, 3rd Edition: 

New York, John Wiley, p. 229–302.

Trammell, J.W., 1961, Geology of the Cumberland Pass area, 

Gunnison County, Colorado: Boulder, Colo., University of 

Colorado M.S thesis, 109 p.

Vanderwilt, J.W., 1947, Mineral resources of Colorado: 

Denver, Colo., State of Colorado Mineral Resources Board, 

547 p.

Wilson, A.B., Spanski, G.T., Crane, M.J., and Woodard, M.D., 

2000, Databases and spatial data model for mineralized 

areas, mines, and prospects in the Grand Mesa, Uncompahgre, and Gunnison (GMUG) National Forests, Colorado: 

U.S. Geological Survey Open-File Report 00-298, CDROM. URL: http://pubs.usgs.gov/of/2000/ofr-00-0298/.

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notes on the molybdenum industry: Colorado Geological 

Survey Bulletin 14, 131 p.

Mineral Resource Assessment for Polymetallic Vein Deposits 163
U.S. Department of the Interior

U.S. Geological Survey

Mineral Resource Assessment for

Polymetallic Replacement Deposits

By Anna B. Wilson, Gregory T. Spanski, Viki Bankey, and Steven M. Smith

Chapter K of

Resource Potential and Geology of the Grand Mesa, Uncompahgre,

and Gunnison (GMUG) National Forests and Vicinity, Colorado

Edited by Viki Bankey

U.S. Geological Survey Bulletin 2213– K
Contents

Abstract...................................................................................................................................................... 166

Introduction ............................................................................................................................................... 166

Genetic Model for Polymetallic Replacement Deposits .................................................................... 166

Description of the Areas Containing Known Polymetallic Replacement Deposits ....................... 168

Application of the Deposit Model for a Mineral Resource Assessment of Polymetallic

Replacement Deposits ............................................................................................................... 169

Permissive Tracts............................................................................................................................. 170

Favorable Tracts............................................................................................................................... 170

Undiscovered Deposit and Endowment Potential............................................................................... 175

References Cited ...................................................................................................................................... 175

Figures

K1. Map showing GMUG restricted study area and locations of polymetallic vein or

polymetallic replacement deposits and the mineralized areas adjacent to or

containing them ................................................................................................................................ 167

K2. Map of GMUG greater study area showing permissive tracts for polymetallic

replacement deposits ...................................................................................................................... 171

K3. Map of GMUG greater study area showing favorable tracts for polymetallic

replacement deposits ...................................................................................................................... 172

Tables

K1. Delineation criteria for polymetallic replacement deposits in GMUG study area ................. 170

K2. Polymetallic replacement tracts in GMUG restricted study area............................................. 174
Abstract

Polymetallic replacement deposits are hydrothermal, epigenetic accumulations of sulfide minerals in bedded deposits 

(mantos), massive lenses, pipe-shaped bodies, and associated 

veins hosted in limestone, dolomite, or other chemically reactive (soluble) rock, adjacent to porphyritic intrusions. In the 

GMUG study area, polymetallic replacement deposits containing lead, zinc, copper, silver, and manganese were locally 

important producers in the Tincup/Cumberland Pass, Whitepine, Ouray, and San Juan mineralized areas. Approximately 

5,133 square miles of the GMUG Forest study area, located 

within 10 kilometers of known or inferred Tertiary or Cretaceous felsic intrusive rocks, within 2 kilometers of permeable 

or chemically reactive sedimentary rocks, or in areas known 

or inferred to be underlain by Paleozoic sedimentary rocks 

beneath volcanic rocks, are classified as “permissive” for the 

occurrence of undiscovered polymetallic replacement deposits. 

Of this “permissive” area, 1,676 square miles in the vicinity 

of the Ruby, Elk Mountains, Dorchester, Forest Hill, Spring 

Creek, Tincup/Cumberland Pass, Gold Brick, Whitepine, 

Cimarron and Henson Creek, San Juan east, Ouray and San 

Juan west mineralized areas are also classified as “favorable.” 

The favorable areas contain known polymetallic replacement 

or polymetallic vein occurrences, are within 2 kilometers of 

a known polymetallic replacement-type occurrence, contain 

known or inferred carbonate rocks, and are within 500 meters 

of a stream-sediment or rock sample containing anomalous 

levels of silver, lead, or zinc. The assessment team estimated that the probability of even one undiscovered deposit 

occurring within the “permissive” and “favorable” tracts for 

polymetallic replacement deposits fell below the range of 

estimation confidence; therefore, a quantitative assessment 

was not performed. 

Introduction

Polymetallic replacement deposits have been historically 

important contributors to the total quantity of lead, zinc, copper, silver, and manganese produced in Colorado, especially 

in the Leadville, Gilman, Alma, Rico, and Tenmile areas, all 

outside the GMUG restricted study area (fig. K1). Within the 

restricted study area, in the Tincup/Cumberland Pass, Whitepine (also known as Tomichi), and Ouray mineralized areas, 

polymetallic replacement deposits have produced smaller, but 

locally significant, quantities of ore. Substantial amounts of 

gold or silver in these smaller deposits compensated for their 

limited volume and made them commercially attractive. 

Most of these deposits have similar physical characteristics and occur in the same geologic environments as deposits 

included in the descriptive model of polymetallic replacement deposits (Morris, 1986, model 19a). Mosier and others 

(1986) compiled grade and tonnage data for 52 deposits of 

this type worldwide. Although none of the Colorado deposits 

is included in the model, grade and tonnage of the GMUG 

deposits fit the general distribution of the deposits from all 

over the world used to develop the model. Had they been 

included, deposits at Leadville, Gilman, and Alma, all close to 

the GMUG restricted study area, would rank among the five 

largest producers in one or more of the five commodities (lead, 

zinc, copper, silver, and manganese) most frequently recovered 

from this type of deposit. 

Genetic Model for Polymetallic 

Replacement Deposits

Polymetallic replacement deposits are hydrothermal, epigenetic accumulations of sulfide minerals in bedded deposits 

(mantos), massive lenses, pipe-shaped bodies, and associated 

veins hosted in limestone, dolomite, or other chemically reactive (soluble) rock, adjacent to porphyritic calc-alkaline intrusions. Occasionally, deposits are distant from an intrusion. 

The mineralization and intrusive activity are contemporaneous. Worldwide, replacement deposits may be of any age. 

Deposits range from small pods and veins to large, 

mixed-sulfide, replacement bodies; the shapes are irregular 

and structurally and stratigraphically controlled. Ore bodies 

are localized by faults, vertical beds, bedding planes, and breccia zones. Limestones below contacts with shale can be especially productive. Vein or pipe structures serve as feeders and 

also may contain ore. Base-metal skarns, polymetallic veins, 

and porphyry copper deposits are genetically and spatially 

Mineral Resource Assessment for

Polymetallic Replacement Deposits

By Anna B. Wilson, Gregory T. Spanski, Viki Bankey, and Steven M. Smith
39o 30'

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149

62

145

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90

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Grand

Junction

Delta

Montrose

Gunnison

Crested Butte

Lake City

Uravan

Telluride

Ouray

Silverton

Aspen

Disappointment Creek

Gunnison River

Uncompahgre River Lake Fork Cebolla

Creek

Tomichi Creek

Taylor River

Blue MesaReservoir

CochetopaCreek

Dolores River

0

0

550

Approximate outline of national forests in GMUG study areaApproximate outline of restricted study area

EXPLANATION

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Cimarron

Henson

Ck

Sunrise–

Morning Glory

Wilson Peaks

San Juan

Ouray

Carson

Bondholder

Gold

Brick

Spring Ck

Dorchester Elk Mts

Ruby

Lake City

Figure K1. GMUG restricted study area, showing locations of polymetallic vein or polymetallic replacement deposits (plus signs) and the min

or containing them (red outlines).
related to these deposits, and boundaries between these deposit 

types may be gradational. Many deposits are enriched by 

supergene processes; some of the deposits have been oxidized 

and lack sulfide minerals.

Polymetallic replacement deposits commonly contain 

lead, zinc, copper, and silver sulfide minerals; tungsten, manganese, bismuth, and trace amounts of gold may also occur. 

Primary ores consist principally of sphalerite and galena; commonly chalcopyrite, silver-bearing tetrahedrite, silver minerals, 

bismuth minerals, manganese minerals, and gold are present. 

Secondary oxidized ores typically include cerussite (lead carbonate), smithsonite (zinc carbonate), and cerargyrite (silver 

chloride). Pyrite (iron sulfide), siderite (iron carbonate), barite 

(barium sulfate), and quartz are the principal gangue minerals. At district scale, polymetallic replacement deposits may 

show mineralogical and compositional zonation from Cu-Au 

(± Bi) nearer the intrusive source, to Pb-Ag, to Zn-Mn at the 

periphery. Closer to a source intrusion (pluton), polymetallic 

replacement deposits may grade into skarn deposits.

Alteration may be extensive in the rocks surrounding a 

deposit. Carbonate rocks may be dolomitized and silicified. 

Shale and igneous rocks may be chloritized and argillized. 

Pyrite is locally abundant.

Polymetallic replacement deposits are deposited from 

aqueous metalliferous fluids separated from an intrusive 

magma during crystallization. The metals carried in solution are primarily derived from the magma, but some may be 

derived from the country rock where connate water in convecting cells leaches metals and mixes with magmatically derived 

fluids. Replacement is most efficient at high temperatures but 

typically occurs in a range of 200°–400°C. Limestones and 

permeable calcareous sedimentary rocks are most likely to 

host ore. Polymetallic replacement deposits are predominantly 

hosted by carbonates (limestone and dolostone); some are 

hosted in sandstone, evaporite (gypsum), and calcareous shale, 

and a few in permeable zones in volcanic rocks. 

The precise location of a deposit is the result of a complex mix of physical, chemical and structural interactions. 

Replacement occurs in situ (in place): the host is replaced by 

ore, particle by particle (volume for volume), preserving most 

aspects of the structure of the host rock. 

Mosier and others (1986) prepared grade and tonnage 

models based on data for 52 areas where production from 

polymetallic replacement ore deposits predominated. However, historically reported production for the mines in these 

areas was aggregated and the models therefore characterize 

deposits that are district-size. Available data are insufficient 

to characterize the individual ore bodies or closely spaced 

clusters of ore bodies that would more properly fit a minebased definition of a deposit. The models are also biased, 

because districts generating less than 100,000 t (metric tons)1

of ore were not included, and in several cases the ore values in 

the model were calculated from production and estimates of 

commodity grades. Given these limitations, the models characterize a population of district-size deposits that ranges in ore 

tonnage from 0.1 to 69 million t of ore, averaging 5.6 million 

t; average commodity grades are 8.1 percent lead, 6.3 percent 

zinc, 0.28 percent copper, and 260 g (grams) of silver and 1.4 

g of gold per metric ton. 

Description of the Areas Containing 

Known Polymetallic Replacement 

Deposits

Several mineralized areas in the GMUG restricted study 

area contain examples of deposits that fit the descriptive 

polymetallic replacement model (Morris, 1986). An understanding of the geologic setting, character, and mode of origin 

of known polymetallic replacement deposits allows us to 

formulate criteria that can be used to identify additional areas 

with the potential to host these types of deposits. 

The northeastern part of the GMUG restricted study area, 

extending from Tincup/Cumberland Pass to Whitepine mineralized areas, is on the west flank of the Sawatch uplift. This 

area contains Tertiary intrusive rocks, primarily granodiorite, 

Paleozoic carbonate rocks, and mines known (or suspected) to 

have produced ore from polymetallic replacement ore bodies. 

Presence of polymetallic base-metal vein deposits in this same 

area further attests to base-metal-rich hydrothermal mineralization in the area. Most replacement deposits in the area are 

hosted in limestone and dolomite of the Manitou Dolomite, 

Dyer Dolomite (Chaffee Group), and Leadville Limestone; the 

larger occurrences are bedded replacement (blanket) deposits 

or irregular deposits along premineral faults and fractures 

(Dings and Robinson, 1957). Locally, they grade into one 

another and the contact between ore and wall rock is very 

irregular. 

In the Tincup/Cumberland Pass area, bedded replacement deposits (Gold Cup, Silver Cup, Tincup, Robert E. Lee, 

Drew, El Capitan, and West Gold Hill mines) were locally 

important producers of silver and lead (Dings and Robinson, 

1957). Most of the ore was mined along the contact of a gray 

limestone and overlying dolomite, within and stratigraphically 

about 150 ft below the top of the Leadville Limestone.2

 The 

largest of these ore bodies, a fairly continuous mineralized 

body, extends about 1,000 ft along the strike of the beds and 

down dip for 800 ft in the Gold Cup mine. Some important 

replacement deposits are clearly related to premineral faults 

(Dings and Robinson, 1957). Ore in the Maid of Athens, 

Citizen, and Ben Franklin mines (Tincup/Cumberland Pass 

area) probably is in the sedimentary beds near or adjacent to 

the Athens fault. 

1

Models are based on metric units for grade (grams) and tonnage (metric 

tons or megagrams).

2

These data are given in the units originally measured and published. To 

convert feet to meters, multiply by 0.3048.

168 Resource Potential and Geology, Grand Mesa, Uncompahgre, and Gunnison National Forests, Colorado
In the Whitepine area, the principal ore bodies are in the 

upper part of the Leadville Limestone at or near the contact 

with the overlying Belden Formation (Erie and Eureka-Nest 

Egg mines). Some important replacement deposits are clearly 

related to premineral faults (Dings and Robinson, 1957). Most 

such deposits are small lenses or pods, but ore shoots in the 

Akron mine in the sedimentary rocks (primarily Manitou 

Dolomite) along the west side of the Star fault are as much as 

300 ft long, 50 ft wide, and 8 ft thick (Dings and Robinson, 

1957). 

The southern part of the GMUG study area, from Ouray 

to Ophir (fig. K1), contains intrusive and sedimentary rocks 

known to be favorable host rocks for replacement deposits at 

Rico and in the Idarado mine. Favorable units include calcareous strata (such as Lower Mississippian Leadville Limestone 

and Middle and Upper Pennsylvanian Hermosa Group) where 

they occur below known vein deposits (as in the Idarado 

mine) and peripheral to Tertiary stocks emplaced along the 

ring fracture zone of the Silverton caldera. These strata are 

about 3,000–4,000 ft below the elevation of Red Mountain 

Pass. Polymetallic base-metal vein deposits occurring near the 

surface in Tertiary volcanic rocks are a strong indication that 

base-metal-rich fluids passed upward through the underlying 

favorable host rocks along structurally controlled channelways. Deep drilling from within the Idarado mine (Mayor and 

Fisher, 1993) tested about 3,000 ft of Paleozoic strata and late 

Proterozoic rocks, intersecting Pb-Zn-Cu mineralized calcsilicate skarn zones in calcareous rocks. The holes did not 

encounter the probable Tertiary intrusion responsible for the 

high-temperature alteration and mineralization. 

Classic examples of polymetallic replacement mineralization are found in mines such as the American Nettie, 

Mineral Farm, and Wanakah in the Ouray mineralized area, 

and the Idarado, Saratoga, Baltic, Portland, and Crown Point 

in the San Juan mineralized area. Many of these deposits are 

in limestone units that are not exposed at the surface, such 

as the manto or channel deposits in Leadville Limestone and 

Molas Formations at the Mineral Farm mine (Burbank, 1940, 

p. 205–206, 238; King and Allsman, 1950, p. 51). Host-rock 

permeability and structure play secondary roles in controlling 

localization of deposits. Permeable, bedded channel deposits 

in Dakota Sandstone (metamorphosed to quartzite) localized 

ore at the American Nettie mine (Burbank, 1940, p. 205–206, 

223–225, 229; King and Allsman, 1950, p. 50–51). Deposits 

in the upper Dakota zone on the east side of the Uncompahgre 

Valley and 2,000–4,000 ft north of the Laramide-age Blowout 

intrusive center are localized in minor folds or terrace-like 

warps superimposed on the generally north-northeast-dipping 

regional host. 

Minor iron and manganese replacement deposits rich 

in magnetite occur in limestone and dolomite beds at or near 

contacts with intrusive rocks. In the Tincup/Cumberland Pass 

area, the Cumberland mine produced iron ore from a layer of 

limestone in the Belden Formation between a quartz diorite 

porphyry body and the Tincup porphyry (Dings and Robinson, 1957). In the Whitepine area, the Iron King mine occurs 

in metamorphosed limy beds of the Belden Formation at the 

north end of the Morning Glim fault, where the fault is cut off 

by the Mount Princeton batholith (Dings and Robinson, 1957). 

Application of the Deposit Model for 

a Mineral Resource Assessment of 

Polymetallic Replacement Deposits

The criteria listed in table K1 are those used in this reconnaissance mineral resource assessment to define “permissive” 

and “favorable” mineral deposit potential tracts for undiscovered polymetallic replacement deposits. The criteria are based 

on the descriptive model (Morris, 1986), grade and tonnage 

models (Mosier and others, 1986), and data available for the 

GMUG study area. The applicable regional databases available for this GIS-based assessment include the digital geologic 

map data (Day and others, 1999), mines and prospect locations (Wilson and others, 2000), outlines of mineralized areas 

(Wilson and others, 2000), regional NURE stream-sediment 

geochemical data (Smith, 2000), a map of igneous intrusions 

based on interpretation of aeromagnetic survey data (Bankey 

and others, this volume, Chapter D), and detailed maps showing age and composition of intrusions (Day and Bove, this 

volume, Chapter B, and D.J. Bove, unpub. data, 2000). 

In this chapter, GMUG restricted study area refers only 

to GMUG National Forests and the land they roughly surround from Grand Junction to the area east of Gunnison (fig. 

K1). It also includes the western slope in the Uravan area, 

but the restricted study area does not include lands in adjacent National Forests—White River, San Isabel, Rio Grande, 

or San Juan—even though they are within the “greater study 

area” boundary. Any favorable resource potential indicated 

outside of this restricted study area is based on incomplete 

data, especially for mines, prospects, and mineralized areas. 

Many more areas in the adjacent lands, including national 

forests, may be permissive, or even favorable. For instance, 

Aspen and Leadville, both large productive districts, contain 

polymetallic vein and polymetallic replacement deposits, yet 

neither is shown in figure K3. The square miles indicated as 

permissive and favorable are, therefore, minimums for the 

entire “greater study area.”

The delineation criteria in table K1 are not exhaustive; 

they are limited by the availability of supporting data sets that 

are both fairly complete and geographically comprehensive at 

1:250,000 scale. Additional criteria could have been considered for tract assessment if the digital databases had been more 

complete. Examples of other criteria that could have been 

used to classify and evaluate the mineral resource potential are 

the following:

• occurrence of manganese oxide minerals in veins or 

disseminated in carbonate rocks;

• distribution of zones of silicification or dolomitization 

in limestone, with accompanying pyrite or barite;

Mineral Resource Assessment for Polymetallic Replacement Deposits 169
• presence of jasperoid or calc-silicate alteration of carbonate rocks or argillic-propylitic alteration of igneous rocks; 

• anomalous concentrations of copper, tungsten, gold, 

arsenic, antimony, bismuth, barium, manganese, iron, or magnesium in bedrock, altered rock, or stream-sediment samples;

• presence of lithologic shale-limestone unit interfaces;

• presence of major structural features, such as major 

faults and zones of extensional tectonic activity;

• presence of small porphyritic dikes and stocks;

• presence of Cretaceous or Tertiary age porphyry 

(model 17), skarn (18b, c, d), or polymetallic vein (22c) mineralization;

• detailed aeroradiometric maps showing Th/U ratios of 

less than 4:1, suggesting uranium depletion may have resulted 

from hydrothermal activity;

• local ground electromagnetic surveys.

Permissive Tracts

In the GMUG greater study area, 5,133 mi2 is classified 

“permissive” for the occurrence of polymetallic replacement 

deposits (fig. K2; Spanski and Bankey, this volume, Chapter F, 

table F1). These areas are within 10 km of known or inferred 

felsic Cretaceous or Tertiary plutons and are underlain by 

Paleozoic, Mesozoic, or Cenozoic sedimentary rock units that 

contain permeable and chemically reactive lithologic units 

(table K1, criterion 2). Exposures of plutonic rocks, with a 1 

km internal buffer, are not considered permissive for replacement deposits. 

In most of the eastern part of the permissive tract in the 

GMUG restricted study area, the plutons and Paleozoic and 

Mesozoic carbonate rocks are exposed at the surface. In most 

of the central part of the GMUG area, the plutons are inferred 

(buried) based on interpretations of geophysical data. In the 

San Juan Mountains, roughly west of long 107°30′, carbonatebearing sedimentary rocks may be present locally, within 1 km 

of the surface beneath the volcanic rocks.

Favorable Tracts

In the GMUG study area, 1,676 mi2 is classified “favorable” for the occurrence of polymetallic replacement deposits (fig. K3). These lands represent about 33 percent of the 

“permissive” tract. The criteria used for delineating regions 

“favorable” for hosting polymetallic replacement deposits 

are listed in table K1. These criteria are more restrictive than 

those used to delineate the “permissive” terrain. Increased 

importance is placed on the presence of sedimentary rock units 

having a substantial carbonate component and on evidence 

that mineralization has occurred. 

In order to be considered favorable, an area had to display 

evidence of mineralization. Because polymetallic vein and 

Table K1. Delineation criteria for polymetallic replacement deposits in GMUG study area.

Diagnostic criteria for permissive tract delineation

1. Located within a 10-km-wide zone peripheral to known or inferred Tertiary or Cretaceous felsic intrusive (plutonic) rocks; qualifying units 

 include being within 10 km of units Tui, Tmi, Tsi, Tiy, Tio, Ti, or TKi of Day and others (1999), and other Tertiary intrusive units identi- 

 fied on smaller scale maps (D.J. Bove, unpub. data, 2000), and inferred intrusions as interpreted from aeromagnetic survey data (Bankey 

 and others, this volume, Chapter D). (Intrusion contact is buffered 1 km into the intrusion, except for the inferred intrusions, which are 

 not buffered, to accommodate contact mislocation errors in 1:250,000-scale mapping.)

2. Presence of permeable or chemically reactive sedimentary rock (2 km external buffer on surface contacts).

 A. Paleozoic units containing Ignacio, Manitou, and Fremont Formations; Chaffee Group; Dyer, Elbert, Ouray, Leadville, Molas, 

Belden, and Minturn Formations; Hermosa Group; Eagle Valley and Rico Formations (including map units Mli, Mr, Or, MOr, 

MDr, hu, ee, e, h, mb, b, m, Pm, Prm (from Day and others, 1999)).

 B. Mesozoic and Cenozoic units: includes map units Jmd, KJde, KJdw, KJdj, KJdm, Jme, Jwe, Jmw, Jmwe, Kdb, Kbc, Kd, 

Kml, Kmu, Km, Kmvl, Kmvu, Kmv, Tsbt, Tkec (from Day and others, 1999).

3. Location in areas known or inferred to be underlain by Paleozoic sedimentary rocks beneath volcanic rocks in the San Juan Mountains.

Diagnostic criteria for favorable tract delineation (in addition to 1–3)

4. Presence of a mineralized area known to host polymetallic replacement or polymetallic vein deposits or occurrences.

5. Within 2 km of a known polymetallic replacement occurrence.

6. Known or inferred presence of carbonate rocks exhibiting a high affinity for polymetallic replacement mineralization within 1 km of the 

 surface exposure (units from Day and others, 1999).

 A. Paleozoic units containing Ordovician Manitou Dolomite or Fremont Dolomite; Devonian Ouray Limestone or Elbert Formation; 

Devonian-Mississippian Dyer Dolomite of the Chaffee Group; Mississippian Leadville Limestone; Pennsylvanian Hermosa, Minturn, 

or Belden Formations; and Pennsylvanian-Permian Rico Formation (Mli, Mr, Or, MOr, MDr, Doe, hu, h, mb, b, 

m, PDre, Pr, Ml).

 B. Mesozoic and Cenozoic units containing Middle Jurassic Wanakah Formation or Eocene Telluride Conglomerate (Jmd, Jwe, 

Jmw, Jmwe, KJdw, TKec, Tsbt).

7. Within 500 m of a stream-sediment or rock sample site containing anomalous levels of silver, lead, or zinc (Ag>1 ppm, Pb>100 ppm, 

 Zn>250 ppm).

170 Resource Potential and Geology, Grand Mesa, Uncompahgre, and Gunnison National Forests, Colorado
39o 30'

109o 

37o 45'

70

139

65

92

50

50 

82

135

133

149

62

145

141

141

90

114

Grand

Junction

Delta

Montrose

Gunnison

Crested Butte

Lake City

Uravan

Telluride

Ouray

Silverton

Aspen

Disappointment Creek

Gunnison River

Uncompahgre River Lake Fork Cebolla

Creek

Tomichi Creek

Taylor River

Blue MesaReservoir

CochetopaCreek

Dolores River

0

0

Approximate outline of national forests in GMUG study area

EXPLANATION

550

Figure K2. GMUG greater study area, showing permissive tracts (shaded) for polymetallic replacement deposits.
39o 30'109o 

37o 45'

70

139

65

92

50

50 

82

135

133

149

62

145

141

141

90

114

Grand

Junction

Delta

Montrose

Gunnison

Crested Butte

Lake City

Uravan

Telluride

Ouray

Silverton

Aspen

Disapp ointment Creek

Gunnison River

Uncompahgre River Lake Fork Cebolla

Creek

Tomichi Creek

Taylor River

Blue MesaReservoir

CochetopaCreek

Dolores River

0

0

Approximate outline of national forests in GMUG study area

EXPLANATION

550

Approximate outline of area assessed for polymetallic replacement deposits

Ophir

F1

F2

F3 F4

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F7 F

F9

F10

F11
Figure K3 (above and previous page). GMUG greater study area, showing favorable tracts for polymetallic replacement depositstains; F3, Dorchester; F4, Forest Hill; F5, Spring Creek; F6, Tincup/Cumberland Pass; F7, Gold Brick; F8, Whitepine; F9, Cimarron 

eastern part of San Juan; F11, Ouray and western part of San Juan tracts. Descriptions in text.

Criterion 3-— Mineralized area containing known polymetallic replacement (PMR) depositsCriteria 3 + 5— Mineralized area containing PMR deposits + carbonate host rock (Criterion 5)Criterion 4— Within 2 km of known PMR depositCriteria 3 + 4— Mineralized area containing PMR deposits + within 2 km of known PMR depositCriteria 3 + 5— Mineralized area containing PMR deposits + carbonate host rockCriteria 3 + 4 + 5— Mineralized area containing PMR deposits + within 2 km of known PMR deposit + carbonate host rockCriterion 6— Within 500 m of elevated Ag, Pb, or Znvalue (buffered geochemistry)

EXPLANATION

Criteria 3 + 6— Mineralized area + buffered geochemistry

Criteria 5 + 6— Carbonate host 

Criteria 3 + 5 + 6— Mineralized + carbonate host rock + buffered Criteria 4 + 6— Within 2 km of kbuffered geochemistryCriteria 3 + 4 + 6— Mineralized + within 2 km of known PMR deCriteria 4 + 5 + 6— Within 2 km host rock + buffered geochemistrCriteria 3 + 4 + 5 + 6— Mineradeposits + within 2 km of known host rock + buffered geochemistr
174 Resource Potential and Geology, Grand Mesa, Uncompahgre, and Gunnison National Forests, Colorado

polymetallic replacement deposits are so closely related, any 

area with evidence of either type of deposit is considered 

favorable if it also meets the permissive criteria. The field 

evidence shows that polymetallic replacement deposits tend to 

occur in clusters (table K1, criterion 4); therefore, proximity to 

known examples of polymetallic replacement deposits is given 

greater importance (criterion 5). Evidence of a known or suspected polymetallic replacement deposit alone is an indication 

of favorable terrain, especially where detailed geologic information is lacking, possibly owing to the scale of the collected 

data and mapping. A 2-km buffer was given around each mine 

or prospect known to be a polymetallic replacement deposit 

(criterion 5). Criteria 4 and 5 are partially redundant; however, criterion 5 becomes important in capturing polymetallic 

replacement mineralization that occurs at isolated sites beyond 

the boundaries of recognized mineralized areas. 

Although any rock can host a polymetallic replacement, 

most deposits occur in carbonate rocks. For this reason we 

have further restricted the potential host rock units (see criterion 2) to only those with a substantial carbonate component 

(criterion 6). 

NURE stream-sediment geochemical data (Smith, 2000) 

are useful in identifying areas where anomalous levels of metals are concentrated in these deposits (criterion 7). Anomalous 

threshold values were determined to be 100 parts per million 

(ppm) for copper, 250 ppm for zinc, and 1 ppm for silver (see 

Smith, this volume, Chapter C). Inasmuch as these samples 

are composites of materials derived from all rocks exposed in 

a drainage basin, they are subject to the effect of dilution; a 

500 m buffer has been applied to each geochemically anomalous site to represent the area of influence for the sample. 

Inasmuch as the assessment technique of bitmapping 

employed herein (Spanski and Bankey, this volume, Chapter F) does not rank areas, no importance is attached to the 

number of criteria that are met at a given location in terms of 

classification. An area is classified “favorable” if it is within a 

“permissive“ tract and meets either the conditions of criteria 4 

or 5, or any combination of criteria 4 through 7. Criteria 6 and 

7 are not used singularly to establish a “favorable” status. 

The areas identified as being “favorable” for hosting 

polymetallic replacement deposits are shown in figure K3 and 

listed in table K2. (Sunrise/Morning Glory, Wilson Peaks, 

Carson, Lake City, and Bondholder mineralized areas (fig. K1) 

are not favorable for polymetallic replacement deposits.) All 

of the areas identified as being favorable correlate with areas 

with known mineralization. 

Ruby tract (fig. K3, area F1) is a known mineralized area 

containing polymetallic vein occurrences (criterion 4) and 

local geochemical anomalies (Ag>1 or Pb>100 or Zn>250 

ppm) (criterion 7). Polymetallic replacement occurrences are 

suspected in some of the many mines and prospects, but none 

is documented.

Elk Mountains tract (fig. K3, area F2) contains a known 

mineralized area of polymetallic vein and polymetallic 

replacement mineralization (criterion 4) coupled with favorable carbonate formations (criterion 5). Polymetallic replacement occurrences are suspected in some of the mines and 

prospects, but none is documented.

Dorchester tract (fig. K3, area F3), a known mineralized 

area, contains polymetallic replacement occurrences (criterion 

4), known polymetallic replacement occurrences (criterion 

5), favorable carbonate formations (criterion 6), and local 

geochemical anomalies (criterion 7). Little is known about 

the mineral deposits in the Bull Domingo mine, the most 

productive mine in the area, nor about the scattered deposits in 

Star Basin (Garrett, 1950; Cunningham, 1976). They appear 

to have been mined for silver and lead from replacement ore 

bodies in limestone adjacent to 33 Ma intrusions of the Italian 

Mountain Intrusive Suite.

Table K2. Polymetallic replacement tracts in GMUG restricted study area.

Cretaceous–early Tertiary tracts:

Tract No.a Tract name Delineation criteria

P1 Permissive for polymetallic replacements 1, 2, 3

F1 Ruby 1, 2, 3, 4, 7

F2 Elk Mountains 1, 2, 3, 4, 5

F3 Dorchester 1, 2, 3, 4, 5, 6, 7

F4 Forest Hill 1, 2, 3, 4

F5 Spring Creek 1, 2, 3, 4, 5, 6, 7

F6 Tincup/Cumberland Pass 1, 2, 3, 4, 5, 6, 7

F7 Gold Brick 1, 2, 3, 4, 6, 7

F8 Whitepine 1, 2, 3, 4, 5, 6, 7

F9 Cimarron and Henson Creek 1, 2, 3, 4, 7

F10 San Juan east 1, 2, 3, 4, 6, 7

F11 Ouray and San Juan west 1, 2, 3, 4, 5, 6, 7

a

P, permissive tract; F, favorable tract.
Mineral Resource Assessment for Polymetallic Replacement Deposits 175

Forest Hill (fig. K3, area F4), a known mineralized area, 

contains suspected polymetallic replacement occurrences (criterion 4). The Forest Hill mine, for which the area is named, 

is inferred, but not known, to be a polymetallic replacement 

mine; therefore, it is not shown in figure K1.

Spring Creek tract (fig. K3, area F5) overlaps a known 

mineralized area (fig. K1) containing polymetallic replacement 

occurrences (including replacement manganese) (criterion 

4), known polymetallic replacement occurrence (criterion 5), 

favorable carbonate formations (criterion 6), and local geochemical anomalies (criterion 7). The Doctor mine reportedly 

was primarily a lead (cerussite) and zinc (smithsonite) deposit 

with some copper (Meissner, 1954); manganese was distal to 

the main occurrence. Sulfides such as galena and sphalerite 

are rare, and no pyrite was observed.

Tincup/Cumberland Pass tract (fig. K3, area F6) includes 

a known mineralized area containing polymetallic vein and 

replacement occurrences (criterion 4), known polymetallic 

replacement mines (criterion 5), favorable carbonate formations (criterion 6), and local geochemical anomalies (criterion 

7). Tincup was well known for its polymetallic replacement 

occurrences such as the Gold Cup mine (Dings and Robinson, 

1957). (See earlier section, “Description of the Areas Containing Known Polymetallic Replacement Deposits.”) 

Gold Brick tract (fig. K3, area F7) is a known mineralized area containing polymetallic vein occurrences (criterion 

4), favorable carbonate formations (criterion 6), and, locally, 

geochemical anomalies (criterion 7). Additional scattered 

geochemical anomalies (criterion 7) lie to the west in favorable carbonate formations (criterion 6).

Whitepine tract (fig. K3, area F8), a known mineralized 

area, contains polymetallic vein and replacement occurrences 

(criterion 4), known polymetallic replacement mines (criterion 5), favorable carbonate formations (criterion 6), and local 

geochemical anomalies (criterion 7).

Cimarron and Henson Creek tract (fig. K3, area F9) 

contains two known mineralized areas (Cimarron and Henson 

Creek, fig. K1) hosting polymetallic vein occurrences (criterion 4), and, locally, geochemical anomalies (criterion 7). 

Polymetallic replacement mineralization is suspected in some 

of the occurrences, but none is documented.

The eastern part of San Juan mineralized area (fig. K3, 

area F10) contains polymetallic vein and replacement occurrences (criterion 4), favorable carbonate formations (criterion 

6), and, locally, geochemical anomalies (criterion 7). 

Ouray and western part of San Juan mineralized area tract 

(fig. K3, area F11) are in a known mineralized area containing polymetallic vein and replacement occurrences (criterion 

4), known polymetallic replacement occurrences (criterion 5), 

favorable carbonate formations (criterion 6), and, locally, geochemical anomalies (criterion 7). At Ophir, the area includes 

the Crown Point polymetallic replacement deposit, and near 

Ouray, the Portland, Mineral Farm, Wanakah, and American 

Nettie deposits, which are at least partly polymetallic 

replacements.

Undiscovered Deposit and Endowment 

Potential

The assessment of endowment potential and the probability for the existence of undiscovered polymetallic replacement 

deposits are based on the grade and tonnage models of Mosier 

and others (1986). Our examination of the grade and tonnage 

figures suggests that no significant difference exists between 

the lead, silver, and gold grades and size distribution of districts in the model population and in seven major replacement 

districts in Colorado. We estimated that the probability of even 

one undiscovered deposit occurring within the “permissive” 

and “favorable” tracts for polymetallic replacement deposits 

fell below the range of estimation confidence. The determination was strongly influenced by the rather large median size 

(1.8 million t of ore) and grades (5 percent lead, 3.9 percent 

zinc, 0.23 percent copper, and 0.75 g gold and 175 g silver per 

metric ton) that are associated with the district-size deposit 

model population. Also, areas of favorable terrain large 

enough to conceal district-size deposits that have not been 

extensively explored are lacking. 

We believe a secondary, nonquantifiable potential is 

associated with polymetallic replacement mineralization in 

the study area. That potential is associated with the existence 

of smaller undiscovered ore bodies that, in the past, as either 

single or tightly clustered bodies, would have been developed 

as a mine or an extension to a mine. Potential cannot be quantified because individual mines lack the production records 

needed to characterize their grades and tonnage. However, 

we think that the potential is high for the existence of new ore 

bodies in areas adjacent to or beneath currently known areas of 

polymetallic replacement mineralization. In today’s economic 

and environmental climate, large mining companies would 

likely not be interested in occurrences of this nature; however, 

these smaller occurrences could be attractive development 

targets for smaller entrepreneurial groups. Some fragmentary 

production records for mines in the Whitepine (Tomichi) and 

Monarch districts suggest that ores with silver grades in excess 

of 2.5 times the model median grade of 175 g/t occurred 

locally, and ore bodies with gold grades ranging from 17 to 

124 g/t were encountered in the Gold Cup mine in the Tincup 

district (Dings and Robinson, 1957).

References Cited

Burbank, W.S., 1940, Structural control of ore deposition in 

the Uncompahgre district, Ouray County, Colorado, in Contributions to economic geology 1938–39: U.S. Geological 

Survey Bulletin 906–E, p. 141–261.

Cunningham, C.G., Jr., 1976, Petrogenesis and post-magmatic 

geochemistry of the Italian Mountain intrusive complex, 

eastern Elk Mountains, Colorado: Geological Society of 

America Bulletin, v. 87, no. 6, p. 897–908.
176 Resource Potential and Geology, Grand Mesa, Uncompahgre, and Gunnison National Forests, Colorado

Day, W.C., Green, G.N., Knepper, D.H., Jr., and Phillips, R.C., 

1999, Spatial geologic data model for the Gunnison, Grand 

Mesa, Uncompahgre National Forests mineral assessment area, southwestern Colorado and digital data for the 

Leadville, Montrose, Durango, and the Colorado parts of 

the Grand Junction, Moab, and Cortez 1° × 2° geologic 

maps: U.S. Geological Survey Open-File Report 99-427, 

CD-ROM.

Dings, M.G., and Robinson, C.S., 1957, Geology and ore 

deposits of the Garfield quadrangle, Colorado: U.S. 

Geological Survey Professional Paper 289, 110 p.

Garrett, H.L., 1950, The geology of Star Basin and Star mine, 

Gunnison County, Colorado: Golden, Colo., Colorado 

School of Mines M.S. thesis, 45 p.

King, W.H., and Allsman, P.T., 1950, Reconnaissance of metal 

mining in the San Juan region, Ouray, San Juan, and San 

Miguel Counties: U.S. Bureau of Mines Information Circular 7554, 109 p.

Mayor, J.N., and Fisher, F.S., 1993, Skarn-hosted mineralization in Paleozoic rocks beneath the Idarado mine, northwest 

San Juan Mountains, Colorado: U.S. Geological Survey 

Open-File Report 93-183, 16 p.

Meissner, F.F., 1954, The geology of Spring Creek Park, Gunnison County, Colorado: Golden, Colo., Colorado School 

of Mines M.S. thesis, 150 p.

Morris, H.T., 1986, Descriptive model of polymetallic replacement deposits, in Cox, D.P., and Singer, D.A., eds., Mineral 

deposit models: U.S. Geological Survey Bulletin 1693, p. 

99–100.

Mosier, D.L., Morris, H.T., and Singer, D.A., 1986, Grade 

and tonnage model of polymetallic replacement deposits, in 

Cox, D.P., and Singer, D.A., eds., Mineral deposit models: 

U.S. Geological Survey Bulletin 1693, p. 101–104.

Smith, S.M., 2000, National Geochemical Database; Reformatted data from the National Uranium Resource Evaluation (NURE) Hydrogeochemical and Stream Sediment 

Reconnaissance (HSSR) Program, Version 1.20: U.S. 

Geological Survey Open-File Report 97-492. URL: http:

//pubs.usgs.gov/of/1997/ofr-97-0492/, [unpaginated].

Wilson, A.B., Spanski, G.T., Crane, M.J., and Woodard, M.D., 

2000, Databases and spatial data model for mineralized 

areas, mines, and prospects in the Grand Mesa, Uncompahgre, and Gunnison (GMUG) National Forests, Colorado: 

U.S. Geological Survey Open-File Report 00-298, CDROM. URL: http://pubs.usgs.gov/of/2000/ofr-00-0298/.
U.S. Department of the Interior

U.S. Geological Survey

Mineral Resource Assessment for

Sediment-Hosted Copper Deposits

By David A. Lindsey, Viki Bankey, Daniel H. Knepper Jr., and Gregory T. Spanski

Chapter L of

Resource Potential and Geology of the Grand Mesa, Uncompahgre,

and Gunnison (GMUG) National Forests and Vicinity, Colorado

Edited by Viki Bankey

U.S. Geological Survey Bulletin 2213– L
Contents

Abstract...................................................................................................................................................... 179

Introduction ............................................................................................................................................... 179

Descriptive and Genetic Models for Sediment-Hosted Copper Deposits....................................... 179

Description of the Areas Containing Sediment-Hosted Copper Deposits ...................................... 180

Application of the Deposit Model for Mineral Resource Assessment ............................................ 181

Permissive Tracts............................................................................................................................. 182

Favorable Tracts............................................................................................................................... 185

Undiscovered Deposit and Endowment Potential............................................................................... 186

References Cited ...................................................................................................................................... 186

Figures

L1. Cross section showing geologic model of structurally controlled sediment-hosted

copper deposits in Paradox Basin, Colorado Plateau................................................................ 180

L2. Map of GMUG greater study area showing permissive tracts for structurally

controlled sediment-hosted copper deposits.............................................................................. 183

L3. Map of GMUG greater study area showing areas favorable for sediment-hosted

copper deposits ................................................................................................................................ 184

Tables

L1. Delineation criteria for structurally controlled sediment-hosted copper deposits

in GMUG greater study area........................................................................................................... 181

L2. Two Landsat scenes used to map bright rocks in eastern part of Paradox Basin in

GMUG greater study area ............................................................................................................... 182

L3. Structurally controlled sediment-hosted copper tracts in GMUG greater study area ......... 185
Abstract

Copper occurs in sedimentary rocks of the salt anticlines of 

the Paradox Basin, the borders of the Uncompahgre uplift, and 

the Eagle Basin. Most known occurrences are small, but deposits large enough to produce occur in the salt anticline terrane. 

All such deposits and occurrences are classified as “sedimenthosted,” but they vary considerably in geologic setting, origin, 

form, and size.

Introduction

Sediment-hosted copper deposits, aggregated in a generic 

global model by Cox (1986), have been classified into three 

models: reduced-facies, redbed, and Revett. Each model has 

different geologic features, grades and tonnages, and anticipated 

environmental impacts when mined and processed (Lindsey 

and others, 1995). Deposits of the reduced-facies model are 

hosted in widespread black-shale formations, are relatively high 

tonnage, and have been mined mostly underground. Deposits of 

the redbed model occur in local areas of reduced rocks in redbed sequences, are low-tonnage, and have been mined near the 

surface by open-pit and small underground operations. Redbed 

deposits have not been a major source of copper. Deposits of 

the Revett model (Spanski, 1992), based on deposits restricted 

to the Mesoproterozoic Revett Formation of the Belt Supergroup of Montana and Idaho, are intermediate in tonnage and 

have been mined entirely underground. 

Although sediment-hosted copper deposits in the assessment area include some typical of the redbed model, the largest 

deposits are not. The primary difference is that they are structurally controlled. Although these were previously considered 

as a variant of the redbed model (Lindsey, 1996), they are more 

fully described here and assigned to a new model, equal in rank 

to existing sediment-hosted copper models. These structurally 

controlled deposits share some features of other sedimenthosted models, including the redbed and reduced-facies models, 

which are commonly associated with salt deposits. 

Descriptive and Genetic Models for 

Sediment-Hosted Copper Deposits

Deposits of the reduced-facies model are found where 

continental clastic sedimentary rocks are overlain by regionally extensive marine or lacustrine shales or carbonates, rich 

in organic material, that act as traps for mineral deposition 

(for example, Johnson, 1976; Ensign and others, 1968). 

Host rocks may be shale or adjacent limestone, sandstone, 

or conglomerate. Evaporite deposits overlie, or are believed 

to have once overlain, copper deposits of the reduced-facies 

model. Deposits of the redbed model occur in the same geologic setting as do deposits of the reduced-facies model, but 

they lack regionally extensive reduced strata. In Devonian 

and later strata, copper commonly replaces local accumulations of fossil plant matter. Redbed copper deposits may 

occur in rifts or intracratonic basins. Deposits of the Revett 

model occur in thick beds of reduced (pyritic) quartzite 

(properly, metasandstone) near pre-ore oxidation-reduction 

fronts (Hayes, 1990). Ore bodies may be stacked, especially 

near faults. Copper is not associated with solid organic matter in Revett deposits, but may have been deposited as the 

result of reactions between a copper-bearing ore fluid and a 

transient gas reductant generated by decay of organic matter.

Structurally controlled sediment-hosted copper deposits 

of the Paradox Basin (fig. L1) share some of the characteristics of all three models. Like redbed and Revett deposits, the 

Paradox deposits are in permeable sandstones, commonly 

located between impermeable beds. The Paradox deposits 

consist of veins in faults and disseminated bodies adjacent 

to faults (Schmidt, 1967; Morrison and Parry, 1986). Some 

disseminated ore bodies are stacked. As in redbed deposits, 

but not as in Revett deposits, some ore replaces plant matter. 

Like reduced-facies and some redbed deposits, but unlike 

Revett deposits, the Paradox deposits are associated with 

salt (gypsum and halite). In the Paradox Basin, structurally 

controlled deposits overlie salt-cored anticlines and diapirs. 

As in many redbed deposits, supergene minerals such as 

chalcocite, malachite, and azurite are the principal ore 

Mineral Resource Assessment for

Sediment-Hosted Copper Deposits

By David A. Lindsey, Viki Bankey, Daniel H. Knepper, Jr., and Gregory T. Spanski
180 Resource Potential and Geology, Grand Mesa, Uncompahgre, and Gunnison National Forests, Colorado

minerals. The distribution of ore in all models may be controlled by redox reactions, temperature gradients, and solubility of dissolved metal- and metal-complex ions during mixing 

of ascending basinal brines with descending ground water.

Structurally controlled deposits of the Paradox Basin 

formed where warm saline basin brines rise through faults and 

permeable sandstone formations on the flanks and crests of 

salt anticlines, possibly during Tertiary time (fig. L1) (Morrison and Parry, 1986). Salt, the source of saline brines that 

leach and transport copper, was deposited in the Pennsylvanian 

Paradox Formation. Salt diapirs, including salt-cored anticlines, formed from Pennsylvanian to Jurassic time and were 

reactivated during the Late Cretaceous to Eocene Laramide 

orogeny. Ground-water recharge areas formed within the 

uplifted Uncompahgre uplift during the Laramide orogeny 

from Late Cretaceous to Eocene time (Dickinson and others, 

1988) and within the La Sal Mountains, which were formed 

by intrusion of igneous stocks and laccoliths in Oligocene and 

early Miocene time (Hunt, 1958; Nelson and others, 1992). 

Thereafter, ground water entered the Paradox Basin from 

recharge areas, became saline and warm within the basin, and 

rose through faults and permeable strata in salt anticlines. The 

warm saline brines leached copper from source rocks such as 

redbeds, traveled updip along permeable zones and faults, and 

deposited copper during mixing with cool, oxidizing surface 

fluids. Copper deposition by reduction in organic matter is not 

required by the model, but probably occurred locally. Nearsurface oxidation in the weathering zone may further concentrate copper.

Description of the Areas Containing

Sediment-Hosted Copper Deposits

In the assessment area, structurally controlled sedimenthosted copper deposits are probably the most important type. 

The Lisbon mine in Utah and the Cashin mine in Colorado 

are the most important deposits known to be structurally 

controlled (Fischer, 1936; Morrison and Parry, 1986). The 

Lisbon mine produced more than 134,000 t (metric tons) of 

1.4 percent copper, and the Cashin mine produced 732,740 lb 

copper and 363,778 oz silver (Morrison and Parry, 1986). In 

a report by Summo Minerals Corp. to the U.S. Securities and 

Exchange Commission (SEC Archives, 1997), production for 

the Cashin mine is given as 20,670 t of about 4 percent copper 

and 18.5 oz/ton silver, slightly higher than reported by Morrison and Parry (1986). Production at both mines was from 

veins and replacements along faults. Exploration by Summo 

Minerals Corp. in the 1990’s has revealed large lenses of 

disseminated ore at both mines (Anonymous, 1995). Lenses 

of disseminated ore are stacked in sandstone beds at more 

Figure L1. Cross section showing geologic model of structurally controlled sediment-hosted copper deposits in Paradox Basin, 

Colorado Plateau. Barbs on fault show direction of relative movement. No scale. Modified from Morrison and Parry, 1986.

Copper minerals

Igneous intrusive rocks

Evaporite salts

 (gypsum and halite)

EXPLANATION

Fluid flow path

Paleozoic and

Precambrian rocks

Mesozoic sandstones and shales

Faulted

salt-cored

anticline

Igneous intrusion

Evaporite salts

Paleozoic sandstones

Copper deposited

from cooling brines

mixing with surface water

Ground-water

recharge area

Basin brines leach

copper from sandstone

Descending water heated

by geothermal gradient,

leaches salt, forms warm brine
Mineral Resource Assessment for Sediment-Hosted Copper Deposits 181

than one stratigraphic level; at the Lisbon mine, disseminated 

and replacement ore is concentrated in coaly intervals. Ore 

minerals replace coalified plant fossils and fill voids. Host 

rocks include permeable sandstone formations of Permian age 

or younger, especially the Cretaceous Dakota Sandstone and 

the Jurassic Wingate Sandstone. Veins were also explored and 

mined at the Cliffdweller, Sunrise, and Copper Rivet mines 

(Fischer, 1936). 

In the mines just listed, copper minerals in structurally 

controlled deposits include chalcocite, minor amounts of other 

copper sulfides, and abundant malachite and azurite. Malachite 

and azurite occur in oxidized zones near the surface. Oxidized 

zones give way to chalcocite at depth, typically several hundred meters below the surface. Large bodies of disseminated 

chalcocite ore are of economic interest because solvent extraction and electrowinning can recover copper. No smelting, with 

attendant environmental problems, is required. Minor gangue 

minerals include pyrite, iron oxides and hydroxides, and manganese oxides. Silver was an important product of the Cashin 

mine, and minor amounts of silver are present in some other 

structurally controlled copper deposits (Fischer, 1936).

In addition to copper and gangue minerals, other indications of mineralizing activity in the structurally controlled 

environment include bleached zones in sandstone (Conel and 

Alley, 1984); anomalous quantities of copper, lead, zinc, and 

silver in rocks, soils, and stream sediments; and copper associated with uranium deposits and anomalously radioactive rock. 

Deposits of the redbed model are represented only by 

small occurrences in the assessment area. They occur in two 

stratigraphic intervals (Lindsey, 1996): (1) the Upper Triassic 

Chinle Formation, and (2) the Middle and Upper Pennsylvanian Hermosa Group and Lower Permian Cutler Formation. 

Copper deposits in Upper Triassic rocks are generally associated with concentrations of organic plant remains (such as 

logs and leaves) in permeable sandstone. The White Canyon 

district, Utah, located west of the assessment area in the Upper 

Triassic Shinarump Member of the Chinle Formation, is the 

largest representative of the redbed model; it produced 

530,000 t at 0.75 percent copper (Finch, 1959). Deposits and 

occurrences in the Hermosa Group and Cutler Formation are 

small lenticular bodies in reduced gray sandstone, siltstone, 

and shale preserved in redbeds. Most host rocks contain 

organic plant remains, and some contain pyrite. Anomalous 

concentrations of uranium are commonly present.

Application of the Deposit Model for 

Mineral Resource Assessment

Structurally controlled sediment-hosted copper deposits 

are the principal focus of this assessment. Criteria used to 

identify tracts that have a more-than-negligible probability 

of hosting structurally controlled copper deposits include the 

presence of (1) clastic sedimentary rocks containing permeable stratigraphic intervals, (2) salt-cored anticlines, (3) 

prominent faults near axial zones of salt-cored anticlines, (4) 

evidence of mineralizing activity commonly associated with 

Cu-U-V deposit formation, and (5) favorable intervals containing highly permeable sedimentary features, such as channels 

(table L1).

For identification of tracts, criteria were divided into 

those that would be useful for identifying permissive tracts 

Table L1. Delineation criteria for structurally controlled sediment-hosted copper deposits in GMUG greater study area.

Diagnostic criteria for permissive tract delineation

1. Presence of clastic sedimentary rocks containing permeable stratigraphic intervals (where present in the Pennsylvanian Hermosa Group; 

 Pennsylvanian and Permian Maroon Formation; Permian Cutler Formation; Triassic Shinarump Member of Chinle Formation; Jurassic 

 Glen Canyon Group including Wingate Sandstone, Kayenta Formation, and Navajo Sandstone; Jurassic Entrada Sandstone; Jurassic Salt 

 Wash Member of Morrison Formation; Cretaceous Dakota Sandstone and Mesaverde Group or Formation) known or inferred to occur 

 within 0.2 km of the surface.

2. Presence of anticlines (10 km lateral buffer normal to the fold axis).

Diagnostic criteria for favorable tract delineation

3. Presence of prominent faults near axial zones of anticlines (0.5 km lateral buffer normal to the surface trace of the fault).

4. Evidence of mineralizing activity commonly associated with Cu-U-V deposits.

 A. Bleached zones detected by remote sensing survey.

 B. Occurrence of mines and prospects with Cu mineralized rock, with or without U and V (10 km lateral buffer to location of mine or 

prospect).

Other criteria (not used to identify favorable tracts in this assessment)

 C. Radiometric (U) anomalies detected by remote sensing surveys.

 D. Anomalous Cu, U, and (or) V geochemical signature in rock, soil, and stream sediments.

5. Presence of certain favorable intervals containing highly permeable sedimentary features, for example, stream channels filled with lenses 

 of permeable conglomerate and sandstone containing carbonaceous material, such as fossil logs and other plant matter (includes the 

 Shinarump Member of Chinle Formation and the Salt Wash Member of Morrison Formation) known or inferred to occur within 0.2 km of 

 the surface.
182 Resource Potential and Geology, Grand Mesa, Uncompahgre, and Gunnison National Forests, Colorado

and those useful for identifying favorable tracts; other criteria 

considered, but not used in tract identification, are also listed 

(table L1). Permissive tracts (fig. L2) require both the presence of permeable clastic sedimentary rocks and the presence 

of anticlines. Ideally, anticlines should be salt-cored, but it 

was not possible to specify which structures were underlain by 

evaporite salt and which were not, so all anticlines were used 

to define permissive tracts. Within permissive tracts, favorable 

tracts were identified by the presence of one or more additional criteria (favorable criteria). Favorable criteria include 

faults and evidence for the activity of mineralizing fluids. 

Color coding is used to distinguish the criterion or combination of criteria that apply to each tract (fig. L3). A large 

portion of the favorable tracts is defined by the presence of 

faults and a buffer zone placed around each fault; these areas 

are shown by the color “pink” on the tract map. Other parts 

of favorable tracts are shown by color codes that represent the 

presence of bleached strata and copper minerals, either singly 

or in combination (fig. L3).

Some criteria, although indicative of the presence of 

mineralized rocks, were not applied because data sets for these 

criteria were lacking at the scale of assessment. The NURE 

(National Uranium Resource Evaluation) geochemical data 

set, for example, is reconnaissance in nature; the spacing of 

NURE samples was too wide to permit reliable detection of 

mineralized areas only a few kilometers across. The data of 

the NURE aeroradioactivity surveys are also widely spaced 

and better suited for detection of uranium occurrences than for 

the detection of structurally controlled copper deposits, many 

of which do not have a radiometric signature.

The bleaching of the host rocks during sedimentary copper mineralization provides an excellent criterion for locating 

areas of potential mineralization. The bleached host rocks, 

primarily sandstones, are devoid of the iron oxides that commonly give them their color, and light-colored clay minerals 

have formed from the alteration of feldspars and the original 

interstitial clays. The result of this process is primarily the 

formation of quartz sandstones, with or without a clay matrix, 

that are highly reflective (high albedo) and appear bright in all 

parts of the visible spectrum.

Landsat Thematic Mapper (TM) data provide an ideal 

means for locating exposures of these bright rocks over large 

regions. Portions of two Landsat scenes were required to 

cover the Paradox Basin part of the GMUG greater study area 

(table L2). The data for the two scenes were georeferenced 

and projected to the common Lambert conformal projection 

used in the overall study. The scenes were then placed in a 

digital mosaic and the data for the Paradox Basin extracted.

The data for the visible and near infrared bands of the 

Paradox Basin data set were used to compute the second 

principal component for the data. Principal components 

is a method for identifying the variability in the raster data 

(Sabins, 1986, p. 262). The first principal component, the 

major source of variability, is variability caused by topographic factors. The second principal component corresponds 

to albedo or the overall reflectance of the materials exposed at 

the Earth’s surface in the visible portion of the spectrum. An 

image of the second principal component was calculated for 

the Paradox Basin Landsat TM data subset and a threshold 

was visually established to isolate only the brightest rocks and 

soils (highest albedos) by increasing the albedo cut-off until 

areas of known bleached sandstones were included and nearby 

unbleached sandstones were not. This procedure established 

that bleached sandstone occurs in 6 percent of the mosaic area 

in the eastern part of the Paradox Basin.

Bleached sandstones, however, are not the only bright 

rocks in the eastern part of the Paradox Basin. Carbonate 

strata and rocks containing abundant gypsum also form very 

bright natural outcrops in the Paradox Basin and are included 

in the bright rocks image layer. Nevertheless, by requiring 

that the bright rocks be sandstones in the structurally controlled copper model, the bright rocks data contribute to the 

definition of areas favorable for potential deposits.

Permissive Tracts

Four tracts that are considered to be permissive for 

occurrence of sediment-hosted copper deposits were identified 

from the distribution of permeable intervals of sandstone and 

anticlines (table L3; fig. L2). The largest tract (P1) encompasses the salt anticlines of the Salt, Paradox, and Lisbon 

valleys and the adjacent Uncompahgre uplift, which is a broad 

regional anticline. Thick intervals of permeable sandstone of 

Pennsylvanian, Permian, Triassic, Jurassic, and Cretaceous age 

underlie much of tract P1. Tracts P2 and P3, called “Cretaceous cliffs” for their tendency to form escarpments in the 

vicinity of Grand Junction, Colo., consist mainly of sandstone 

in the Cretaceous Mesaverde Group. Tract P4, consisting of 

the Eagle Basin, Grand Hogback, and mountains to the south, 

is underlain by Pennsylvanian, Permian, Triassic, Jurassic, and 

Cretaceous sandstone; the Eagle Basin contains gypsum- and 

halite-bearing diapirs of the Pennsylvanian Eagle 

Table L2. Two Landsat scenes used to map bright rocks in eastern part of Paradox Basin in GMUG greater 

study area.

Scene ID LT5035034008722810 LT5036033008616810

Path 35 35

Row 34 33

Date acquired 08/16/87 09/14/86
P1

P3

P2

P4

UNCOMPAHGRE UPLIFT

PARADOX BASIN

GRAND HOGBACK EAGLE BASIN 39o 30'

109o 

37o 45'

70

139

65

92

50

50 

82

135

133

149

62

145

141

141

90

114

Grand

Junction

Delta

Montrose

Gunnison

Crested Butte

Lake City

Uravan

Telluride

Ouray

Silverton

Aspen

Disappointment Creek

Gunnison River

Uncompahgre River Lake Fork Cebolla

Creek

Tomichi Creek

Taylor River

Blue MesaReservoir

CochetopaCreek

Dolores River

0

0

Approximate outline of national forests in GMUG study area

EXPLANATION

550

Figure L2. GMUG greater study area, showing permissive tracts (shaded) for structurally controlled sediment-hosted copper deposits. P1, s

Cretaceous cliffs 1; P3, Cretaceous cliffs 2; P4, Eagle Basin–Grand Hogback.
F1 F2 F3 39o 30'109o 

37o 45' 70 139 65 92 50 50 

82 

135 133 149 62 145 141 141 90 114 

Grand Junction Delta Montrose Gunnison Crested Butte Lake City Uravan Telluride Ouray Silverton Aspen 

Disappointment Creek Gunnison River Uncompahgre River Lake Fork Cebolla Creek Tomichi Creek Taylor River 

Blue Mesa Reservoir Cochetopa Creek Dolores River

0

0

Approximate outline of national forests in GMUG study area

EXPLANATION

550

UNCOMPAHGRE UPLIFT

PARADOX BASIN

GRAND HOGBACK EAGLE BASIN

Figure L3 (above and following page). GMUG greater study area, showing areas favorable for sediment-hosted copper deposits. F1, salt an

uplift; F3, Eagle Basin–Grand Hogback.
Mineral Resource Assessment for Sediment-Hosted Copper Deposits 185

Valley Evaporite (Mallory, 1971). The tract also contains 

large domes formed by intrusion of Tertiary igneous rocks. 

Since their formation, domes may have provided ground water 

and hydrostatic pressure for basin brines. As is the case for 

the Paradox Basin, brines in the Eagle Basin may have leached 

copper from clastic rocks and deposited copper along faults.

A large part of the assessment area was not included 

because it failed to include both permissive criteria; that is, 

either permeable clastic sedimentary rocks were present or 

anticlines could be present, but not both. In addition, the 

presence of permeable clastic rocks within each stratigraphic 

interval varies with facies changes across the assessment area, 

so that permeable clastic rocks within an interval may not be 

present everywhere. Descriptions of each stratigraphic interval differ somewhat among 1°×2° geologic quadrangle maps 

within the assessment area, and the identification of permissive areas for this assessment is dependent on the descriptive 

information from each map. The effects of facies changes and 

attendant descriptions of map units are evident where tract 

boundaries (for example, tract P3) are more linear than circular. Such straight boundaries are artifacts of the data source 

but nevertheless give a general location of the tract.

Favorable Tracts

Within the permissive tracts, three tracts were identified 

as favorable for the occurrence of sediment-hosted copper 

deposits (table L3; fig. L3). In addition to the presence of 

permeable sandstone and anticlinal structures, favorable tracts 

contain faults and evidence of mineralizing fluids, such as 

bleached zones, copper-mineralized areas in sedimentary rock, 

or copper mines and occurrences. Each of these features was 

assigned numerical values, so that their presence singly or 

in combination yielded a unique value that identifies which 

features are present. Areas associated with each numerical 

value were assigned colors on the map (fig. L3). Tract F1, a 

series of linear areas oriented parallel to faults, occurs over 

salt-cored anticlines. In some parts of tract F1, bleached sandstone, copper-mineralized areas, and copper mines record the 

passage of mineralizing fluids. Tract F1 contains the Cashin 

mine, where recent exploration has revealed a large mineralized zone; the tract extends west to include the copper mines 

of Lisbon Valley, Utah, outside the assessment area. Tract F2

is located mostly on the flanks of the Uncompahgre uplift, 

where bleached areas and faults in sandstone are the principal 

indicators of a favorable terrane. Tract F3 is located in and 

near the Eagle Basin, where salt diapirs, copper, and bleached 

rock are indications of favorable conditions for sedimenthosted copper deposits. Some of the copper occurrences in 

tract F3 might be of hydrothermal origin, but such information 

Criterion 3— Faults, buffered 0.5 km

Criterion 4A— Bleached zones

Criterion 4B— Cu mines and prospects, buffered

Criteria 4A and 4B

Criteria 3 and 4A

Criteria 3 and 4B

Criteria 3, 4A, and 4B

EXPLANATION

Table L3. Structurally controlled sediment-hosted copper tracts in GMUG greater study area.

Tract Tract name Delineation Tract area

 No.a criteria (km2

)

b

P1 Salt anticlines 1 and 2

P2 Cretaceous cliffs 1 1 and 2 

P3 Cretaceous cliffs 2 1 and 2

P4 Eagle Basin-Grand Hogback 1 and 2

F1 Salt anticlines 1, 2, 3, 4A, 4B

F2 Uncompahgre highland 1, 2, 3, 4A, 4B 2,480

F3 Eagle Basin-Grand Hogback 1, 2, 3, 4A, 4B

a

P, permissive tract; F, favorable tract.

b

Area of permissive tracts includes the area of the favorable tracts lying within.

8,100
186 Resource Potential and Geology, Grand Mesa, Uncompahgre, and Gunnison National Forests, Colorado

was not available for this analysis. More research is needed to 

verify the presence of structurally controlled sediment-hosted 

copper in the Eagle Basin.

Undiscovered Deposit and Endowment 

Potential

The resource potential for structurally controlled sediment-hosted copper was not quantitatively assessed. The 

descriptive and grade and tonnage models used in previous 

assessments (Ludington and others, 1996) to assess redbed 

copper deposit potential are not deemed to be appropriate at 

the scale used in this assessment. Those models embrace a 

suite of deposits that is intended to characterize the diversity in sediment-hosted copper deposits present in a broad 

region covering four States. By necessity the models were 

required to encompass a fairly broad array of ore controls and 

deposit grades and tonnages. The copper resource potential 

of structurally controlled copper deposits, however, is largely 

restricted to deposits that exhibit a mix of characteristics, some 

common to deposits included in the redbed copper model and 

some associated with deposits in the reduced-facies copper 

model. Size and grade data for structurally controlled deposits are insufficient to determine whether the redbed copper 

deposit grade and tonnage models adequately represent them. 

The data are also insufficient to produce a new set of grade 

and tonnage models. The use of the existing redbed copper 

deposit models would introduce unwarranted uncertainty into 

the estimates of undiscovered structurally controlled deposits. 

Although the magnitude of the resource potential cannot be estimated quantitatively, activity over the last 5 years 

suggests that structurally controlled sediment-hosted copper mineralization produced deposits that are of commercial 

interest as a potential source of copper. Exploration drilling 

in the mid-1990’s identified a reported geologic resource of 

11.9 million t (metric tons) of mineralized rock grading 0.496 

percent copper occurring in a 2,500-acre area that includes 

the historic Cashin and Cliffdweller mines (SEC Archives, 

1997). During this same period, a deposit containing proven 

and probable reserves of 31.8 million t of ore grading 0.464 

percent copper and containing 147,000 t of recoverable copper was outlined on a 5,900 acre site in the Lisbon Valley 30 

km southwest of the Cashin site. Development of the fully 

permitted Lisbon Valley site is contingent on the currently 

(2001) depressed market price for copper rebounding to the 

level of $0.90/lb (SEC Archives, 1997). These examples suggest a viable near-term development potential associated with 

structurally controlled sediment-hosted copper deposits, which 

is supported by the environmentally benign character of the 

extraction and refining methods used to recover metal from 

these ores. The surface-mined, oxidized ore is treated with 

acid to put the metal into solution from which the metal is 

removed by electrowinning. Primary management issues for 

the near term would be expected to include exploration-related 

activity and mine site reclamation; development would likely 

be restricted to areas within the “favorable” tracts.

References Cited

Anonymous, 1995, Feasibility gives Lisbon Valley the passing 

grade: The Northern Miner, v. 81, no. 28, Sept. 11, 1995, 

pages unknown.

Conel, J.E., and Alley, R.E., 1984, Lisbon Valley, Utah, 

uranium test site report, in Abrams, M.J., Conel, J.E., 

Lang, H.R., and Paley, H.N., eds., The Joint NASA/Geosat 

Test Case Project, Final Report—Part 2, Volume 1: Tulsa, 

Okla., American Association of Petroleum Geologists, p. 

8.1–8.158.

Cox, D.P., 1986, Descriptive model of sediment-hosted Cu, in 

Cox, D.P., and Singer, D.A., eds., Mineral deposit models: 

U.S. Geological Survey Bulletin 1693, p. 205.

Dickinson, W.R., Klute, M.A., Hayes, M.J., Janecke, S.U., 

Lundin, E.R., McKittrick, M.A., and Olivares, M.D., 1988, 

Paleogeographic and paleotectonic setting of Laramide 

sedimentary basins in the central Rocky Mountain region: 

Geological Society of America Bulletin, v. 100, no. 7, p. 

1023–1039.

Ensign, C.O., Jr., White, W.S., Wright, J.C., Patrick, J.L., 

Leone, R.J., Hathaway, D.J., Trammell, J.W., Fritts, J.J., and 

Wright, T.L., 1968, Copper deposits in the Nonesuch Shale, 

White Pine, Michigan, in Ridge, J.D., ed., Ore deposits of 

the United States, 1933–1967; The Graton–Sales Volume: 

New York, American Institute of Mining, Metallurgical, and 

Petroleum Engineers, Inc., v. I, p. 460–488.

Finch, W.I., 1959, Geology of uranium deposits in Triassic 

rocks of the Colorado Plateau region: U.S. Geological 

Survey Bulletin 1074–D, p. 124–164.

Fischer, R.P., 1936, Peculiar hydrothermal copper-bearing 

veins of the northeastern Colorado Plateau: Economic 

Geology, v. 31, no. 6, p. 571–599.

Hayes, T.S., 1990, A preliminary study of thermometry and 

metal sources of the Spar Lake strata-bound copper-silver 

deposit, Belt Supergroup, Montana: U.S. Geological 

Survey Open-File Report 90-484, 30 p.

Hunt, C.B., 1958, Structural and igneous geology of the La 

Sal Mountains, Utah: U.S. Geological Survey Professional 

Paper 294-I, p. 305–364.

Johnson, K.S., 1976, Permian copper shales of southwestern 

Oklahoma, in Johnson, K.S., and Croy, R.L, eds., Stratiform 

copper deposits of the Midcontinent region, a symposium: 

Oklahoma Geological Survey Circular 77, p. 3–14.
Mineral Resource Assessment for Sediment-Hosted Copper Deposits 187

Lindsey, D.A., 1996, Rationale for Colorado Plateau tracts 

C65, C66B, and C78A, in Ludington, Steve, Cox, D.P., and 

McCammon, R.B., eds., Data base for a National Mineral 

Resource Assessment of undiscovered deposits of gold, 

silver, copper, lead, and zinc in the conterminous United 

States: U.S. Geological Survey Open-File Report 96-96, 

file COPL, CD-ROM.

Lindsey, D.A., Woodruff, L.G., Cannon, W.F., Cox, D.P., 

and Heron, W.D., 1995, Sediment-hosted Cu deposits, in 

du Bray, E.A., ed., Preliminary compilation of descriptive 

geoenvironmental mineral deposit models: U.S. Geological 

Survey Open-File Report 95-831, p. 214–224.

Ludington, Steve, Cox, D.P., and McCammon, R.B., eds., 

1996, Data base for a National Mineral Resource Assessment of undiscovered deposits of gold, silver, copper, lead, 

and zinc in the conterminous United States: U.S. Geological Survey Open-File Report 96-96, CD-ROM.

Mallory, W.W., 1971, The Eagle Valley Evaporite, northwest 

Colorado—A regional synthesis, in Contributions to general 

geology 1969: U.S. Geological Survey Bulletin 1311–E, p. 

E1–E31.

Morrison, S.J., and Parry, W.T., 1986, Formation of carbonatesulfate veins associated with copper ore deposits from 

saline basin brines, Lisbon Valley, Utah—Fluid inclusion 

and isotopic evidence: Economic Geology, v. 81, no. 12, p. 

1853–1866.

Nelson, S.T., Heizler, M.T., and Davidson, J.P., 1992, New 40Ar/39Ar ages of intrusive rocks from the Henry and La Sal 

Mountains, Utah: Utah Geological Society Miscellaneous 

Publication 92-2, 24 p.

SEC Archives, 1997, Summo Minerals Corporation: U.S. 

Securities and Exchange Commission File Number 0-

27272, available at http://www.sec.gov/Archives/edgar/data/

941230/0001001348-97-000048.txt.

Sabins, F.F., Jr., 1986, Remote sensing, Second Edition: New 

York, W.H. Freeman, 449 p.

Schmidt, L.J., 1967, Uranium and copper mineralization in the 

Big Indian Wash–Lisbon Valley mining district, southeastern Utah: New York, Columbia University Ph. D. dissertation, 149 p.

Spanski, G.T., 1992, Quantitative assessment of future development of copper/silver resources in the Kootenai National 

Forest, Idaho/Montana—Part 1, Estimation of the copper 

and silver endowments: Nonrenewable Resources, v. 1, no. 

2, p. 163–183.
U.S. Department of the Interior

U.S. Geological Survey

Coal Resources and Coal Resource Potential

By R.D. Hettinger, L.N.R. Roberts, and M.A. Kirschbaum

Chapter M of

Resource Potential and Geology of the Grand Mesa, Uncompahgre, and 

Gunnison (GMUG) National Forests and Vicinity, Colorado

Edited by Viki Bankey

U.S. Geological Survey Bulletin 2213– M
Contents

Abstract...................................................................................................................................................... 191

Introduction ............................................................................................................................................... 191

Purpose and Scope ......................................................................................................................... 191

Location............................................................................................................................................. 193

Acknowledgments........................................................................................................................... 193

Upper Cretaceous and Tertiary Rocks .................................................................................................. 193

Dakota Sandstone Coal in the Grand Mesa, Gunnison, and Uncompahgre National Forests .... 196

Grand Mesa National Forest.......................................................................................................... 196

Gunnison National Forest............................................................................................................... 199

Uncompahgre National Forest ...................................................................................................... 199

Fruitland Formation Coal in the Uncompahgre National Forest........................................................ 199

Mesaverde Group and Mesaverde Formation Coal in the Grand Mesa and Northwestern

Part of the Gunnison National Forests (Area 1) ..................................................................... 200

Data ............................................................................................................................................... 200

Coal Geology..................................................................................................................................... 200

Black Diamond Coal Group ................................................................................................... 200

Cameo-Fairfield Coal Group.................................................................................................. 200

Cameo-Wheeler Coal Zone (West of Long 107°15’ W.) ........................................... 203

South Canyon Coal Zone (West of Long 107°15’ W.)................................................ 203

Coal Ridge Coal Zone (West of Long 107°15’ W.) ..................................................... 203

Lower, Middle, and Upper Coal Zones (East of Long 107°15’ W.) .......................... 210

Coal Quality....................................................................................................................................... 210

Coal Resources ................................................................................................................................ 210

Methods ................................................................................................................................... 210

Results ...................................................................................................................................... 211

Cameo-Wheeler Coal Zone.......................................................................................... 216

South Canyon Coal Zone .............................................................................................. 216

Coal Ridge Coal Zone .................................................................................................... 217

Coal Resources of the Cameo-Fairfield Coal Group East of Long 107°15’ W....... 217

Coal Production................................................................................................................................ 218

Summary of Coal Resource Potential in the Grand Mesa, Uncompahgre, and Gunnison

National Forests .......................................................................................................................... 218

Coal Resource Potential of the Dakota Sandstone in the Grand Mesa and

Uncompahgre National Forests....................................................................................... 218

Coal Resource Potential of the Fruitland Formation in the Uncompahgre

National Forest ................................................................................................................... 221

Coal Resource Potential of the Mesaverde Group and Mesaverde Formation in the

Grand Mesa and Gunnison National Forests ................................................................ 221

References Cited ...................................................................................................................................... 221
Figures

 M1–M4. Maps showing:

 M1. GMUG greater study area, showing location of Grand Mesa, Uncompahgre,

 and Gunnison National Forests, and southern Piceance Basin coal

 assessment unit (Area 1)......................................................................................... 192

 M2. Location of major structural features in GMUG greater study area

 and their relationship to Grand Mesa, Uncompahgre, and Gunnison

 National Forests........................................................................................................ 194

 M3. Location of coal fields in GMUG greater study area........................................... 195

 M4. Areas in Grand Mesa, Uncompahgre, and Gunnison National Forests

 underlain by the Dakota Sandstone, Fruitland Formation, Mesaverde

 Formation, or Mesaverde Group ............................................................................ 197

 M5. Chart showing stratigraphic nomenclature used for the Mesaverde Group and 

 Mesaverde Formation in southern part of Piceance Basin.......................................... 198

 M6. Map showing location of data points used to assess coal resources of Area 1 ...... 201

 M7. Chart showing stratigraphy of continental and marine rocks in the Upper

 Cretaceous Mesaverde Group and Mesaverde Formation along cross section

 A–A’ , in Area 1...................................................................................................................... 204

 M8–M12. Isopach maps of net coal in:

 M8. Cameo-Fairfield coal group in Area 1.................................................................... 206

 M9. Cameo-Wheeler coal zone in Area 1..................................................................... 207

 M10. South Canyon coal zone in Area 1 ......................................................................... 208

 M11. Coal Ridge coal zone in Area 1 ............................................................................... 209

 M12. Cameo-Fairfield coal group east of long 107°15’ W., in Area 1 ......................... 211

M13–M16. Isopach maps of overburden on base of:

 M13. Cameo-Wheeler coal zone in Area 1..................................................................... 212

 M14. South Canyon coal zone in Area 1 ......................................................................... 213

 M15. Coal Ridge coal zone in Area 1 ............................................................................... 215

 M16. Cameo-Fairfield coal group east of long 107°15’ W., in Area 1 ......................... 216

 M17. Map showing location of coal mines that have produced from Cameo-Fairfield

 coal group in vicinity of Area 1.......................................................................................... 219

 M18. Maps showing coal resource potential in Grand Mesa, Uncompahgre, and

 Gunnison National Forests................................................................................................. 220

Tables

M1. Summary of Cretaceous strata in Grand Mesa, Uncompahgre, and Gunnison (GMUG) 

National Forests................................................................................................................................ 196

M2. Drill hole and outcrop data in Area 1............................................................................................. 202

M3. Ash yield, sulfur content, and calorific values of coal in Cameo-Fairfield coal group in 

vicinity of Area 1, southern part of the Piceance Basin, Colo................................................... 210

M4. Original coal resources and other occurrences of non-resource coal in Cameo-Wheeler 

coal zone, Area 1 .............................................................................................................................. 214

M5. Original coal resources and other occurrences of non-resource coal in South Canyon

coal zone, Area 1 .............................................................................................................................. 214

M6. Original coal resources and other occurrences of non-resource coal in Coal Ridge

coal zone, Area 1 .............................................................................................................................. 217

M7. Original coal resources in Cameo-Fairfield coal group located east of long 107°15’ W.,

Area 1.................................................................................................................................................. 217
Abstract

Upper Cretaceous strata are known to contain coal in 

the vicinity of the Grand Mesa, Uncompahgre, and Gunnison 

(GMUG) National Forests, Colorado, and these coal-bearing 

rocks extend under some areas of the forests. Forest areas 

are assigned a high, moderate, or low coal resource potential 

where coal-bearing strata have less than 6,000 feet of overburden. Areas of high potential have nearby outcrop or drill hole 

data that substantiate the presence of coal. Areas of moderate 

potential do not have drill hole or outcrop data to substantiate 

the presence of coal; however, data in adjacent areas indicate 

that coal is likely to be present. Areas of low potential have 

no information to substantiate the presence of coal; however, the presence of coal is inferred from regional data. The 

Uncompahgre National Forest has a low to moderate coal 

resource potential in areas underlain by the Dakota Sandstone, 

and it also has a moderate to high coal resource potential in 

areas underlain by the Fruitland Formation. The Grand Mesa 

National Forest has a low coal resource potential where it 

is underlain by the Dakota Sandstone, and it has a high coal 

resource potential in areas underlain by the Mesaverde Group 

and Mesaverde Formation. The Gunnison National Forest also 

has a high coal resource potential in areas underlain by the 

Mesaverde Group and Mesaverde Formation.

Introduction

Purpose and Scope

Upper Cretaceous rocks in the GMUG greater study area 

(map area, fig. M1) contain coal-bearing strata that extend 

under parts of the Grand Mesa, Uncompahgre, and Gunnison 

(GMUG) National Forests, Colorado. The coal-bearing strata 

are in the Upper Cretaceous Dakota Sandstone, Fruitland 

Formation, Mesaverde Formation, and Mesaverde Group. 

Although some of the coal has been mined since the late 

1800’s, only the West Elk mine is currently operating within 

the three national forests of this study area (herein called the 

GMUG forests). The purpose of this chapter is to summarize 

the coal geology, and assess the coal resource potential for 

the GMUG forests. We estimate coal resources in the GMUG 

forest areas underlain by economically significant deposits of 

coal in the Mesaverde Group and Mesaverde Formation, and 

this main coal assessment unit is referred to as Area 1 (fig. 

M1) in this chapter. We also describe less significant deposits 

of coal that underlie other parts of the GMUG forests. 

The areas of high coal resource potential in the Grand 

Mesa and Gunnison National Forests are contiguous, and they 

are estimated to have a combined coal resource of about 38 

billion short tons, as determined in this study. That tonnage 

is reported for all beds of coal more than 1 ft thick and having 

less than 6,000 ft of overburden. This study does not attempt 

to estimate coal reserves that are the subset of the resource 

which can be economically produced at the present time. The 

coal resource is in the regionally extensive Cameo-Fairfield 

coal group of the Mesaverde Formation and Mesaverde Group. 

The Cameo-Fairfield has as much as 97 ft of net coal, and 

individual beds are as much as 30 ft thick. The Grand Mesa 

and Gunnison National Forests contain an additional 26 billion 

short tons of non-resource coal that is also in the Cameo-Fairfield coal group at depths greater than 6,000 ft.1

The large coal resource reported for the Grand Mesa 

and Gunnison National Forests must be regarded with caution because the figure does not take into account economic, 

land-use, environmental, technological, and geologic restrictions that affect the coal’s availability and recoverability. The 

coal would have to be mined using underground methods, 

and technological and economical constraints generally limit 

current longwall mining to depths of less than 3,000 ft, beds 

more than 3.5 ft thick, and strata inclined by less than 12°; 

additionally, only about 14 ft of coal can be mined even if 

the bed is of greater thickness (Timothy J. Rohrbacher, U.S. 

Coal Resources and Coal Resource Potential 

By R.D. Hettinger, L.N.R. Roberts, and M.A. Kirschbaum

1

Measurements originally made and reported in feet, in miles, in square 

miles, and in short (2,000-lb) tons are included here in their original units for 

clarity and to avoid misstatement of precision in conversion. To convert feet 

to meters, multiply by 0.3048; to convert short tons to metric tons, multiply by 

0.91; to convert miles to kilometers, multiply by 1.61; to convert square miles 

to square kilometers, multiply by 2.6.
Gunnison  National Forest 

Grand Mesa  National Forest 

Uncompahgre  National Forest 

National forest boundary Road 

City 

Area 1 boundary

EXPLANATION

 

Area 1

Area 1

Nucla

Norwood

Montrose

Gunnison 

Delta

Cedaredge

Grand Junction

Fruita 

Aspen

RIO 

GRAND

DOLORES SAN 

JUAN

MINERAL 

HINSDALE SAN MIGUEL

OURAY

MONTROSE

DELTA

GUNNISON

MESA

PITKIN

GARFIELD EAGLE 

109o 108o 107o 

39o

38o

37o 45'

38o 30'

39o 30'

0 

Figure M1. Location of Grand Mesa, Uncompahgre, and Gunnison National Forests, and southern Piceance Basin coal 

within GMUG greater study area. Study area is located in western Colorado (inset) between lat 37°45′ and 39°30′ N. and 
Coal Resources and Coal Resource Potential 193

Geological Survey, oral commun., 1996). Only an estimated 

37 percent of the coal resource estimated for the Cameo-Fairfield coal group in the Grand Mesa and Gunnison National 

Forests meets favorable underground mining criteria regarding 

depth of burial (less than 3,000 ft). Furthermore, only a fraction of that coal could be mined economically because many 

beds are either less than 3.5 ft thick or more than 14 ft thick 

and because many localities are steeply inclined. Additional 

coal would also be restricted from mining because it might be 

in beds that are discontinuous, left in the ground as pillars for 

roof support, or bypassed due to mining of adjacent strata.

Location

The greater study area and GMUG forests are located 

in western Colorado (fig. M1, index) between lat 37°45’ and 

39°30’ N. and long 106° and 109° W. (map area, fig. M1). 

The study area is situated on and adjacent to the northeastern part of the Colorado Plateau, and major structural features include the Sawatch and Uncompahgre uplifts, and the 

Piceance and Paradox Basins (fig. M2). The Uncompahgre 

uplift separates the Piceance Basin from the Paradox Basin to 

the south.

The GMUG forests contain lands located within or adjacent to several coal fields of western Colorado (fig. M3). The 

coal field boundaries have been variously defined by Landis 

(1959), Hornbaker and others (1976), and Tremain and others (1996); and the boundaries shown in figure M3 represent 

a best-fit approximation of their various descriptions. The 

Grand Mesa National Forest extends across part of the Grand 

Mesa coal field and lies in close proximity to the Book Cliffs 

and Somerset coal fields. The northwestern part of the Gunnison National Forest extends across the Carbondale, Crested 

Butte, and Somerset coal fields. Farther south, parts of the 

Uncompahgre National Forest lie within the Tongue Mesa coal 

field and adjacent to the Nucla-Naturita coal field. The geology and resources of each coal field were described in Landis 

(1959) and are updated annually by the Colorado Geological 

Survey (for example, Hornbaker and others, 1976; Tremain 

and others, 1996).

Acknowledgments

We thank Ted Dyman, Tom Judkins, and Brad Van Gosen 

for their thorough reviews of the manuscript.

Upper Cretaceous and Tertiary Rocks

The GMUG forests are underlain by coal-bearing strata in 

the Dakota Sandstone, Mesaverde and Fruitland Formations, 

and Mesaverde Group (table M1). These Upper Cretaceous 

rocks were deposited in continental and nearshore marine settings along the western margin of the Western Interior seaway. 

Shoreline positions and depositional systems during the Late 

Cretaceous are shown in Roberts and Kirschbaum (1995). 

Although the Dakota Sandstone has a wide distribution 

throughout the GMUG forests, the Mesaverde Formation and 

Mesaverde Group are confined to areas where the Gunnison 

and Grand Mesa National Forests extend across the Piceance 

Basin, and the Fruitland Formation is confined to an isolated 

area where the Uncompahgre National Forest extends across 

the Tongue Mesa coal field (fig. M4).

The Dakota consists of conglomerate, sandstone, 

mudrock, carbonaceous shale, and coal deposited in alluvial and coastal plain settings during the initial incursion of 

the Western Interior seaway during the Cenomanian Stage 

of the Cretaceous Period. The Dakota is about 30–200 ft 

thick (Young, 1960, his fig. 16) and is overlain by the Mancos Shale. The Mancos consists of about 4,000–5,000 ft of 

mudrock deposited in an offshore marine environment that 

persisted from the Cenomanian through Campanian in the 

study area, when the shoreline was located in Utah.

As the shoreline moved back into the study area during 

the late Campanian, strata were deposited in a complex system 

of continental, coastal plain, and shoreface environments. At 

the Tongue Mesa coal field (fig. M3), about 200 ft of Upper 

Cretaceous coal-bearing strata is assigned to the Fruitland Formation by Dickinson (1987a, 1987b, 1988) and Hornbaker and 

others (1976). These rocks are part of a 1,000-ft thick stratigraphic interval that was referred to as the Mesaverde Formation by Landis (1959). In the southern part of the Piceance 

Basin, about 2,100–5,600 ft of strata has been assigned to the 

Mesaverde Group and Mesaverde Formation. The Mesaverde 

has been assigned group status in the Book Cliffs, Grand Hogback, and Carbondale coal fields, but is considered a formation 

in the Crested Butte and Grand Mesa coal fields. In the Book 

Cliffs coal field, the Mesaverde Group was divided into (in 

ascending order) the Castlegate Sandstone, Sego Sandstone, 

Mount Garfield Formation, and Hunter Canyon Formation 

(Erdmann, 1934; Fisher and others, 1960). In the Grand Hogback and Carbondale coal fields, the Mesaverde Group was 

divided into (in ascending order) the Iles and Williams Fork 

Formations (Collins, 1976). The stratigraphy and nomenclature of the Mesaverde are shown in figure M5; a more detailed 

discussion of Mesaverde stratigraphy is provided by Johnson 

(1989) and Hettinger and others (2000).

Depositional systems of continental origin prevailed 

throughout the study area from the latest part of the Cretaceous Period to the middle part of the Eocene Epoch of the 

Tertiary Period. The later part of the Tertiary was characterized by basalt flows and intrusions of igneous stocks, dikes, 

sills, and laccoliths. Volcanic activity was especially prevalent 

along the southeastern flank of the Piceance Basin and in the 

San Juan volcanic field (fig. M2).
RIO DOLORES GRAN

SAN JUAN MINERAL HINSDALE SAN MIGUEL OURAY 

MONTROSE 

DELTA GUNNISON MESA PITKIN 

GARFIELD EAGLE 

109o 108o 107o 

39o 38o 38o 30' 39o 30' PICEANCE BASIN SAN JUAN VOLCANIC FIEL

UNCOMPAHGRE UPLIFT P ARADOX BASIN S

0 10 20 30 MILES Figure M2. Location of major structural features in GMUG greater study area, and their relationship to Grand Mesa

Gunnison National Forests. GMUG forest boundaries are shown as red dashed lines, and the forests are identified i
Figure M3. Location of coal fields in GMUG greater study area. County line Boundaries of Grand Mesa, Gunnison, and Uncompahgre National Forests 

Book Cliffs

coal field

Grand Mesa

coal field Somerset coal field

Crested Butte

coal field

Carbondale coal field

Grand Hogback coal field

(southern part)

Tongue Mesa

coal field

NuclaNaturita

coal field

Uncompahgre National Forest 

Uncompahgre National Forest 

Uncompahgre National Forest 

Gunnison Na

Forest 

Grand Mesa

National 

Forest 

Uncompahgre National Forest 

Gunnison

National 

Forest 

EXPLANATION

109o 108o 107o 

39o

38o

38o 30'

39o 30'

0 1
196 Resource Potential and Geology, Grand Mesa, Uncompahgre, and Gunnison National Forests, Colorado

Dakota Sandstone Coal in the Grand 

Mesa, Gunnison, and Uncompahgre 

National Forests

Geologic investigations by Young (1960,1973) indicate 

that the Dakota Sandstone is widely distributed throughout 

much of the greater study area. It underlies all of the Grand 

Mesa National Forest and is present within parts of the 

Uncompahgre and Gunnison National Forests (fig. M4). The 

Dakota is gently dipping where it is exposed along the flanks 

of the Uncompahgre uplift; it abuts the San Juan volcanic field 

to the south and is locally disrupted by Tertiary intrusions in 

the Gunnison area.

Coal beds in the Dakota are generally thin and discontinuous, and they contain numerous partings of carbonaceous 

and coaly shale. Beds as thick as 7.7 ft are found locally in the 

study area, but they also contain many partings (Eakins, 1986). 

Numerous studies have been conducted to evaluate the coal in 

the Dakota; the best and most current summary of Dakota coal 

is by Eakins (1986). Dakota coal is being produced currently 

at the New Horizon mine in the Nucla-Naturita coal field (fig. 

M3) and burned at the Nucla power plant. The power plant 

uses a fluidized-bed combustion process and can therefore 

burn a lower quality coal than is used at most power plants 

(Eakins, 1986). The Dakota coals are high-volatile B and 

C bituminous in apparent rank (Murray, 1981). Coal in the 

Nucla-Naturita coal field has an ash yield from 6.1 to 12.8 

percent and a sulfur content from 0.5 to 1.1 percent on an 

as-received basis (Murray, 1981). Haines (1978) analyzed 21 

coal samples from three beds in the Nucla-Naturita coal field 

and reported an ash yield of about 11–28 percent and sulfur 

content of about 0.3–0.7 percent, with calorific values between 

7,370 and 11,550 Btu/lb.

Grand Mesa National Forest

The Dakota Sandstone does not crop out within the 

Grand Mesa National Forest, but it is widespread in the 

subsurface (fig. M4). Most of the Dakota is buried at depths 

greater than 4,000 ft, based on its stratigraphic position below 

younger units within the forest. Dakota coals crop out 6–10 

mi south and west of the forest, between the towns of Grand 

Junction and Delta, Colo. These coals were measured by 

Table M1. Summary of Cretaceous strata in Grand Mesa, Uncompahgre, and Gunnison (GMUG) National Forests.

Age Group or 

Formation

Thickness

(ft)

Description

Late

Cretaceous

Mesaverde

Group and

Mesaverde

Formation

and

Fruitland

Formation

2,150-

5,600

200

Sandstone, mudrock, carbonaceous shale, and coal. Sandstone is very fine grained to 

medium grained, and locally coarse grained. Upper part is fine grained to coarse grained 

and conglomeratic. Lower part intertongues with Mancos Shale. The Mesaverde Group 

or Mesaverde Formation underlies the Grand Mesa and Gunnison National Forests and 

is exposed in the Book Cliffs, Carbondale, Crested Butte, Grand Hogback, Grand Mesa, 

and Somerset coal fields (figs. M3 and M4). In the Book Cliffs coal field, the Mesaverde 

Group is divided into the Castlegate Sandstone, Sego Sandstone, Mount Garfield Formation, and Hunter Canyon Formation. In the Grand Hogback and Carbondale coal fields, 

the Mesaverde Group is divided into the Iles and Williams Fork Formations. Coeval 

strata are assigned to the Mancos Shale and Mesaverde Formation in the Grand Mesa and 

Crested Butte coal fields. Stratigraphic correlations are shown in figures M5 and M7.

The Fruitland Formation underlies areas in the Uncompahgre National Forest, and it is 

exposed in the Tongue Mesa coal field (figs. M3 and M4).

Mancos

Shale

4,000-

5,000

(maximum)

Dark-gray shale with minor sandstone and siltstone; includes thin lenses of limestone, 

sandy limestone, and limy shale. The Mancos intertongues with the lower part of the 

Mesaverde Group and Mesaverde Formation.

Dakota

Sandstone

30-

200

Light-gray and tan, fine- to coarse-grained sandstone or quartzite; minor interbeds of darkgray shale, shaly sandstone, conglomeratic sandstone, and thin and lenticular beds of coal.
Area underlain by the Dakota Sandstone Area underlain by the Mesaverde Group or Formation Area underlain by the Fruitland Formation 

EXPLANATION

109o 108o 107o 

39o

38o

38o 30'

39o 30'

0 

Figure M4. Areas in Grand Mesa, Uncompahgre, and Gunnison National Forests underlain by the Dakota SandstoneMesaverde Formation, or Mesaverde Group. Dakota Sandstone also underlies areas covered by the Fruitland and Mrocks in Gunnison National Forest might also be underlain by the Dakota Sandstone or Mesaverde Formation. Nationidentified in figure M1.
198 Resource Potential and Geology, Grand Mesa, Uncompahgre, and Gunnison National Forests, Colorado

Alluvial, coastal plain and mire; 

coal zones are in dark gray Depositional environments Upper shoreface 

and estuarine

Lower shoreface and 

offshore marine

Crested Butte coal field

lower coal zone

Rollins Ss 

Mbr

middle coal zone

upper coal zone

Mesaverde Formation (part) Mancos Shale (part)

Black Diamond 

coal group Cameo-Fairfield coal group

Cameo-Fairfield 

coal group

Book Cliffs coal field Grand Mesa and Somerset coal fields Carbondale coal field 

B

Sego Ss

Corcoran Mbr

Cozzette Mbr 

Rollins Ss Mbr

middle ss

upper ss

Cameo cz

Carbonera cz

Coal Ridge cz

South Canyon cz

Wheeler cz

Anchor cz

Palisade cz

Mancos Shale (part)

Mesaverde Gp (pt) or Formation (pt) and tongues of the Mancos Sh

Chesterfield cz 

top of Coal Ridge cz

Crested Butte coal field

Southeast

lower 

coal zone

middle 

coal zone

upper coal zone

Bowie 

Shale 

Mbr

Paonia 

Sh 

Mbr 

(part)

Mancos 

Shale 

(part)

Rollins 

Ss

Mbr

Mesaverde Formation (part)

Bowie 

Shale 

Mbr

Paonia 

Shale 

Mbr 

(part)

Williams Fork Fm (part) Iles Formation

Mancos 

Shale 

(part)

Rollins 

Ss Mbr

tongue of Mancos 

Shale

Mesaverde Group

Castlegate Ss

 Sego 

Ss

Mancos Sh

(part)Mount Garfield and Hunter Canyon Formations (part)

Mesaverde Gp (pt) and tongues of the Mancos Shale

Cozzette Ss Mbr

Book Cliffs coal field Grand Mesa and Somerset coal fields Carbondale coal field

Mesaverde Fm

(part)

Mesaverde Fm.

(part)

Coalbearing 

mbr

Barren mbr (part)

Upper part of the Sego Ss

Anchor Mine Tongue of Mancos Shale

Buck Tongue of Mancos Shale

Corcoran Mbr

Cozzette

Mbr. 

Rollins Ss Mbr

Rollins Ss. Mbr.

middle ss

upper ss

Cameo cz

Carbonera cz

Coal Ridge cz

South Canyon cz

Wheeler cz

Anchor cz

Palisade cz

Chesterfield cz 

Lower part of the Sego Ss

A West

Index

Book Cliffs

Grand Mesa Somerset

Carbondale

Crested 

Butte

Approximate line of section

Figure M5. Stratigraphic nomenclature used for the Mesaverde Group and Mesaverde Formation in southern part of Piceance Basin, 

Colo.; modified from Hettinger and others (2000). A, Facies relationships along a line of section that is perpendicular to depositional 

strike. Index shows line of section in relation to coal fields. B, Stratigraphic position and nomenclature used in this report for coal 

groups and coal zones in the Mesaverde Group and Mesaverde Formation. Line of section is shown in A. Ss, ss, sandstone; Mbr, mbr, 

member; cz, coal zone; Sh, Shale; Gp, Group; Fm, Formation; pt, part.
Coal Resources and Coal Resource Potential 199

Woodruff (1912) and Lee (1912). The thickest single bench of 

coal measured was 20 in. thick; at another locality 6 ft of coal 

was described within 11 ft of coal-bearing strata (Woodruff, 

1912). The poor quality and thin discontinuous nature of the 

coal precluded development in the area (Woodruff, 1912). The 

presence of coal within the forest is unknown, but any coal 

that might be present is likely to be of similar poor quality, 

quantity, and character.

Gunnison National Forest

Although the Dakota Sandstone is widely distributed 

in the Gunnison National Forest (fig. M4), data by Gaskill 

and Godwin (1966a, 1966b), Gaskill and others (1967, 1986, 

1987), and Godwin (1968) suggest that the Dakota lacks coal 

in the eastern and southern parts of the forest. Young (1960, 

his fig. 6) showed a thin carbonaceous interval within the 

Dakota at localities east of Delta, Colo., but he did not indicate 

that this interval contains coal.

Uncompahgre National Forest

The Dakota Sandstone crops out in the Uncompahgre 

National Forest; however, it is generally poorly exposed, 

concealed by thick vegetation, or covered by Quaternary landslide deposits. No published reports list precise thicknesses 

of Dakota coal in the forest; however, a 2.1-ft thick coal bed 

was measured in the forest about 12 mi northeast of the town 

of Nucla (fig. M1) (W.W. Boyer, USGS, unpub. data, 1926). 

Landis (1959) evaluated the Dakota coal as part of a statewide 

compilation. His generalized maps and descriptions indicate that Dakota coal beds in the forest are likely to be thin, 

impure, and discontinuous, but that minable reserves might be 

found locally.

Examples of coal deposits in the Dakota Sandstone are 

provided from two areas located 5–6 mi outside of the forest. One area is located near the town of Norwood (fig. M1); 

the other area is less than 5 mi from the town of Nucla in the 

Nucla-Naturita coal field (figs. M1 and M3). Coal beds in 

the Norwood area are about 2–11 ft thick including partings 

(Eakins, 1986). Eleven small mines operated 1–2 mi west 

of Norwood. The mines worked in beds that were 2.6–5.5 

ft thick, and about 25,000 short tons of coal was produced 

between 1925 and 1979. Coal beds in the Nucla area are 

reported to be 1.3 to 9.0 ft thick; they contain numerous 

partings, and they can only be mapped over short distances 

(Eakins, 1986). Landis (1959) estimated that a 15 mi2

 part of 

the Nucla-Naturita coal field contained about 114 million short 

tons of coal. Another small area in the NW1⁄4 sec. 31, T. 47 

N., R. 15 W., near the town of Nucla, was estimated to contain 

about 278,900 short tons of coal (Haines, 1978). Twelve small 

underground mines and one strip mine operated within 4 mi 

of Nucla, and they produced more than 2 million short tons of 

coal between 1915 and 1983 (Eakins, 1986). Currently, the 

New Horizon strip mine supplies the Nucla Power Plant with 

coal, and about 400,000 short tons of coal was mined in 1995 

(G. Sullivan, written commun., 1997, compiled from Mine 

Safety and Health Administration data). The New Horizon 

mine is about 1 mi west of Nucla.

Fruitland Formation Coal in the 

Uncompahgre National Forest

Approximately 200 ft of coal-bearing strata is present in a 

small part of the Uncompahgre National Forest at the Tongue 

Mesa coal field (figs. M3 and M4). The coal-bearing rocks 

were assigned to the Fruitland Formation by Hornbaker and 

others (1976) and Dickinson (1987a, 1987b, 1988), and they 

are part of a 1,000-ft thick interval that was originally thought 

to be equivalent to the Mesaverde Formation by Landis (1959). 

Both Landis (1959) and Dickinson (1987a, 1987b, 1988) 

described the coal-bearing interval as being concealed by 

heavy vegetation, landslides, talus, and glacial deposits. Coal 

in the Tongue Mesa coal field is reported to have an ash yield 

of 6.7–8.4 percent, a sulfur content of 0.5–0.9 percent, and a 

calorific value of 9,350–10,200 Btu/lb on an as-received basis 

(Hornbaker and others, 1976). The apparent rank of the coal 

is subbituminous B (U.S. Bureau of Mines, 1937, p. 110–111) 

and subbituminous C (Dickinson, 1987a, 1987b, 1988). Some 

of the coal is reported to be oxidized and bony (Hornbaker and 

others, 1976).

The geology of the Tongue Mesa coal field was mapped 

in the vicinity of the Uncompahgre National Forest at a 1:

24,000 scale by Dickinson (1987a, 1987b, 1988). Dickinson’s 

maps show the Fruitland cropping out at only a few small and 

widely spaced localities, and depth to the top of the formation ranges from 0 to 2,500 ft within the forest. The Fruitland 

contains one laterally extensive coal bed that is about 20–40 ft 

thick, and three to five coal beds that are about 5–13 ft thick. 

The beds of coal are gently inclined and disrupted by numerous faults; however, the precise location and displacement of 

the faults cannot be determined from surface mapping because 

the area is extensively covered by landslide debris. The faulting and landslide cover have also made the coal resources difficult to assess (Dickinson, 1987a, 1987b). Dickinson stated 

that the coal-bearing strata were drilled extensively for Federal 

permits and leases, but the drilling data had not been released 

at the time of his publications.

Some minor underground mining took place in the 

Tongue Mesa coal field intermittently between the 1890’s and 

1940’s (Murray, 1981). The Lou Creek, Economy, Tyler, and 

Kennedy mines operated within the forest in T. 46 N., R. 7 

W.; the Lou Creek mined a 40-ft thick bed, and the Economy 

and Tyler each mined a 30-ft thick bed (Dickinson, 1987a, 

1988). Additionally, four small mines operated less than 3 mi 

from the forest and produced from beds that were 6–23 ft thick 

(Dickinson, 1987a).

Landis (1959) estimated the 58 mi2 Tongue Mesa coal 

field to contain a coal resource of about 2,355 million short 
200 Resource Potential and Geology, Grand Mesa, Uncompahgre, and Gunnison National Forests, Colorado

tons. Hornbaker and others (1976) thought the resources 

could be as high as 4,000 million short tons, apparently on the 

basis of core drilling information available to them. However, the forest lands are only partially within the coal field 

(fig. M3), and no estimate is available for the portion of this 

resource that is within the forest.

Mesaverde Group and Mesaverde 

Formation Coal in the Grand Mesa and 

Northwestern Part of the Gunnison 

National Forests (Area 1)

Coal-bearing strata in the Mesaverde Group and 

Mesaverde Formation underlie approximately 620 mi2

 of the 

Gunnison National Forest and 520 mi2

 of the Grand Mesa 

National Forest (fig. M4). These forest lands form a contiguous region that is designated as Area 1 in this report (fig. M1). 

The coal-bearing Mesaverde Group and Mesaverde Formation 

extend throughout the subsurface of the Piceance Basin (fig. 

M2) and are exposed in the Book Cliffs, Carbondale, Crested 

Butte, Grand Hogback, Grand Mesa, and Somerset coal fields 

(fig. M3). Numerous mines have produced from these coal 

fields since the late 1800’s, and several mines are currently 

operating near the southern forest boundaries (see section, 

“Coal Production”). Some of the coal is also considered to be 

an important source for natural gas (Johnson, 1989). Because 

of the ongoing economic interest, the coal resources of Area 1 

are evaluated in this report.

Data

The evaluation of Area 1 is based primarily on data and 

digital files used by Hettinger and others (2000) to describe 

the geology and estimate coal resources in the southern part 

of the Piceance Basin, an area included in the USGS National 

Coal Resource Assessment. The digital files were manipulated 

in a Geographic Information System (GIS) using ARC/INFO 

software to report coal resources within various parameters in 

Area 1. With the exception of files of national forest boundaries, all digital files were prepared in-house or imported from 

the existing public domain, and they have been made available 

by Biewick and Mercer (2000). Methods regarding the generation and use of the digital files have been provided by Biewick 

and Mercer (2000), Hettinger and others (2000), and Roberts 

and others (2000).

Lithologic and stratigraphic data used to assess coal 

resources in Area 1 are from 94 drill holes and outcrops 

located in Area 1 (table M2; fig. M6). Additional data were 

also used from a much larger data base by Hettinger and others (2000), and those data points are also shown in figure M6. 

Lithologic interpretations were made using a combination of 

responses from natural-gamma (gamma ray), density, 

resistivity, neutron, spontaneous potential, and caliper logs. 

Coal bed thicknesses were rounded to the nearest foot, and 

beds less than 1 ft thick were not included in the assessment. 

Because coal thicknesses were rounded, we used a minimum 

thickness of 1 ft rather than the 14-in. cutoff for bituminous 

coal as suggested by Wood and others (1983).

Geologic coverages used to assess Area 1 include (1) a 

geologic map that shows outcrops of rock units, (2) a structure 

contour map of the base of each coal resource interval, and (3) 

isopach maps that show the thickness, net coal, and overburden for each coal resource interval. Outcrops of rock units in 

Area 1 were obtained from a digital geologic map of Colorado 

by Green (1992) that was compiled from the 1:500,000-scale 

geologic map of the State of Colorado by Tweto (1979). 

Structure contour and isopach maps were prepared using lithologic and stratigraphic information gathered from drill holes 

and outcrops. These spatial data were gridded using Earth 

Vision [Dynamics Graphics, Inc.], and the resulting contour 

lines were then converted into ARC/INFO polygon coverages 

(Roberts and others, 2000).

Coal Geology

 In Area 1, the Mesaverde Group and Mesaverde Formation contain coal within the Black Diamond and Cameo-Fairfield coal groups as referred to by Hettinger and others (2000) 

(fig. M5). The stratigraphic distribution of Mesaverde coal in 

Area 1 is demonstrated on cross section A–A’ (fig. M7). The 

cross section is oriented nearly perpendicular to shorelines of 

the Cretaceous Western Interior seaway. The datum used for 

the cross section is a bentonite bed located stratigraphically 

near the base of a tongue of Mancos Shale that underlies the 

Rollins Sandstone Member of the Mount Garfield, Mesaverde, 

and Iles Formations.

Black Diamond Coal Group

The Black Diamond coal group is located stratigraphically below the Rollins Sandstone Member, and contains (in 

ascending order) the Anchor, Palisade, and Chesterfield coal 

zones (fig. M5). Individual beds of coal are generally less 

than 6 ft thick where they are exposed in the Book Cliffs and 

Grand Hogback coal fields, and they pinch out southeast of 

those localities. The Black Diamond coal group underlies 

Area 1 in Tps. 7, 8, 9 S., Rs. 94, 95, 96 W.; drill hole data 

show that the coal group lies 3,500–10,500 ft deep and has 

less than 6 ft of net coal in those areas. Resources were not 

estimated for the Black Diamond coal group in Area 1 because 

the coal beds are too thin and too deep to be economically 

significant.

Cameo-Fairfield Coal Group

The Cameo-Fairfield coal group overlies the Rollins 

Sandstone Member and contains the thickest and most 
Coal Resources and Coal Resource Potential 201

extensively mined coals in the Piceance Basin; the coal group 

is also an important source for natural gas (Johnson, 1989). 

The coal group is about 1,000 ft thick in the northeastern part 

of Area 1, and it is less than 200 ft thick in the southwestern and southeastern parts of Area 1. The Cameo-Fairfield 

extends throughout most of the subsurface of Area 1, and it is 

exposed near the forest boundaries in the Carbondale, Crested 

Butte, Grand Mesa, and Somerset coal fields (fig. M3). 

Exploratory coal drilling in the Grand Mesa and Somerset 

coal fields has been reported by Eager (1978, 1979), Dunrud 

(1989a, 1989b), Johnson (1948), and Toenges and others 

(1949, 1952). Exploratory coal drilling and outcrop measurements in the southern part of the Carbondale coal field have 

been reported by Collins (1976), Donnell (1962), Dunrud 

(1989a), Ellis and others (1988), and Kent and Arndt (1980a, 

1980b). Coal bed thicknesses in the Crested Butte coal field 

have been reported in geologic maps by Lee (1912), Gaskill 

and Godwin (1966a, 1966b), Godwin (1968), and Gaskill and 

others (1967, 1986). References to coal zones and coal bed 

thicknesses in those areas are based on our interpretations of 

their data.

Following the nomenclature of Hettinger and others 

(2000), the Cameo-Fairfield group contains (in ascending 

order) the Cameo-Wheeler, South Canyon, and Coal Ridge 

coal zones in areas located west of long 107°15′ W. (figs. M5, 

M7; table M1). East of long 107°15′ W., the Cameo-Fairfield 

group is simply divided into the lower, middle, and upper coal 

zones (figs. M5, M7, and table M1). Coal zone nomenclature 

was not extended across long 107°15′ W., owing to structural 

and stratigraphic complexities, and a paucity of data east of 

the longitudinal line. Coal-bearing strata in the southern part 

of the Carbondale and Crested Butte coal fields are poorly 

exposed, steeply inclined, displaced by numerous faults, and 

intruded by sills, dikes, and laccoliths. Additionally, coal beds 

underlie many of the laccoliths.

Net coal in the Cameo-Fairfield coal group ranges from 

about 50 to 97 ft in a 20- to 30-mi wide belt that extends north 

to south across the central part of Area 1 (fig. M8). Net coal 

decreases to less than 50 ft in the remaining parts of Area 1. 

Coal distribution in the Cameo-Wheeler, South Canyon, 

and Coal Ridge coal zones is shown in a series of net coal 

isopach maps in figures M9, M10, and M11, respectively. 

Figure M6. Location of data points used to assess coal resources of Area 1. Area 1 is located where the Mesaverde 

Formation and Mesaverde Group underlie the Gunnison and Grand Mesa National Forests (figs. M1 and M4). Labeled 

data points in Area 1, and near its eastern boundary, are referenced in table M2.

Area 1 Data point and reference number 37 Data point referenced in Hettinger and others (2000)

EXPLANATION

0 10 20 MILES

DELTA

GUNNISON

PITKIN

MESA 

GARFIELD

 86 W 89 88

 96 W 95 94 93 92 91

 R 97 W

13 S

11 S

10 S

9 S

T 8 S

12 S

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94

93

92

91

90

88 89

87

86

85

84

83

82

81

80

79

78

77

76

75

74

73 71

72

70

69

68

67

66

65

64

62 63

61

60

59

58

57

56 55

54

53

52

51

50

49

48

46 47

45

44

43 42

41

40

39 38

37

36

35

34

33

32

31

30

29

28

27

26

25

24

23

22

21

20

19

18

17

16

15

14

13

12

11

10

9

8

7

6

5

4 2 3

1 39o

107o 108o
202 Resource Potential and Geology, Grand Mesa, Uncompahgre, and Gunnison National Forests, Colorado

Table M2. Drill hole and outcrop data in Area 1.

[Information is provided for each data point shown in Area 1 in figure M6. Map number (Map No.) refers to the data point in figure M6. Point Identification (ID) is 

the 10-digit American Petroleum Information (API) number of an oil or gas hole, or the original number of a coal exploration hole or measured section. Type refers 

to oil and gas hole (O&G); measured section (MS); lithologic log (LL); rotary hole (ROT); rotary and core hole (R/C); undefined (UND). Source refers to the lease 

operator, well name, data collector, or a U.S. Geological Survey (USGS) or U.S. Bureau of Mines (USBM) publication. The type of publication is an Open-File 

Report (OF), Bulletin (Bull), Professional Paper (PP), Geologic Quadrangle Map (GQ), or Technical Paper (TP). Latitude and longitude of data point are in decimal 

degrees. Elevation is measured in feet above sea level to the Earth’s surface (or to Kelly bushing of the drill rig) at the data point. The total thickness of coal (Total 

coal) is provided for the Cameo-Fairfield coal group and includes all coal beds in the group that are more than 1 ft thick. The total thickness of coal and number of 

coal beds (# beds) are also provided for Cameo-Wheeler, South Canyon, and Coal Ridge coal zones, which are within the Cameo-Fairfield group. Total coal and 

number of beds includes all coal beds that are more than 1 ft thick. Blank space indicates (1) the coal zones had no coal at the data point, (2) data were not provided 

(map numbers 31 and 34), or (3) the coal zones were not identified at location of data point (map numbers 86 through 94)]

 Data point identification Data point location Net coal Cameo-Wheeler South Canyon Coal Ridge

in Cameo- coal zone coal zone coal zone

Map Elevation Fairfield Total # Total # Total #

No. Point ID Type Source Long W. Lat N. Sec. Township Range (ft) group* (ft) coal (ft) beds coal (ft) beds coal (ft) beds

1 IP-77-1 ROT USGS OF 78-540 108.24233 38.99503 21 12 S 97 W 7230 31.0 31 8

2 IP-77-2A ROT USGS OF 78-540 108.14739 38.88108 32 13 S 96 W 8700 15.5 15.5 6

3 IP-77-3 ROT USGS OF 78-540 108.13039 38.87767 33 13 S 96 W 8450 15.0 15 5

4 HK-77-1 ROT USGS OF 78-540 108.11172 38.88131 34 13 S 96 W 8030 34.5 34.5 9

5 05-077-05011 O&G PACIFIC-SMITH 1-C 108.04617 39.06228 30 11 S 95 W 10191 31.0 31 4

6 05-077-05017 O&G WESTERN FRNTR-BIG CRK 1 107.90175 39.12242 4 11 S 94 W 9464 39.0 39 10

7 05-029-05031 O&G APACHE-MACHELSON 2 107.88661 39.06178 34 11 S 94 W 10133 61.0 61 6

8 05-077-08188 O&G EXXON-OLD MAN MTN #1 107.87972 39.13839 33 10 S 94 W 10043 43.0 43 12

9 05-077-08598 O&G FUELCO-FEE E-22-10-94-S 107.87486 39.17894 22 10 S 94 W 7929 48.0 48 12

10 05-077-08043 O&G MOBIL-BEAR CRK 1-25 107.83967 39.33339 25 8 S 94 W 8430 74.0 55 6 14 6 5 3

11 05-077-07360 O&G ALPINE OIL-GOVT 1 107.83089 39.18153 24 10 S 94 W 8512 71.0 68 9 3 1

12 05-077-05026 O&G EL PASO-LEON CRK 1 107.79828 39.16478 29 10 S 93 W 9062 37.0 35 5 2 1

13 05-077-08295 O&G EXXON-KENNY CREEK #1 107.7875 39.11528 9 11 S 93 W 9577 50.0 50 5

14 05-077-08395 O&G BV-CARLTON CURRIER 2-1 107.73472 39.22083 2 10 S 93 W 8052 70.0 57 5 9 4 4 3

15 05-077-05001 O&G SUNRAY-COLO FED C-1 107.69861 39.01908 8 12 S 92 W 9825 81.0 81 10

16 05-077-05097 O&G EL PASO CONOCO GOVT 1 107.69756 39.28253 8 9 S 92 W 7665 89.0 70 3 15 3 4 1

17 05-077-05108 O&G APACHE CORP-RUSHMORE 1 107.69633 39.29686 5 9 S 92 W 7617 87.0 63 2 18 4 6 2

18 05-077-07352 O&G NORDON-GOVT 1 107.692 39.261 20 9 S 92 W 7610 66.0 54 7 5 3 7 3

19 05-077-08435 O&G COORS-USA 1-16 SC 107.67528 39.27861 16 9 S 92 W 7850 91.0 68 4 13 3 10 5

20 05-077-08545 O&G CELERON PORTER MT FD35-1 107.62722 39.23453 35 9 S 92 W 8267 95.0 55 3 24 8 16 2

21 05-077-08575 O&G AMOCO-RUTH MTN #1 107.61647 39.18703 13 10 S 92 W 8711 97.0 60 7 21 7 16 5

22 05-029-05000 O&G UNION-OVERLAND GOV'T 1 107.61003 39.106 13 11 S 92 W 9421 66.0 44 3 14 4 8 2

23 05-029-06001 O&G PAN AM-USA MARVIN WOLF 1 107.60042 39.12897 6 11 S 91 W 8899 82.0 57 5 25 3

24 05-077-05074 O&G MTN STATES-28-1 GOV'T 107.56033 39.24308 28 9 S 91 W 9180 94.0 57 6 35 5 2 1

25 05-029-05083 O&G VICTOR DRLG-GOVT #1 107.55767 39.13117 4 11 S 91 W 8538 94.0 62 4 27 5 5 1

26 05-029-06069 O&G AMOCO-ELECTRIC MTN U.#1 107.51167 39.09444 13 11 S 91 W 8216 73.0 32 5 29 5 12 4

27 05-051-06043 O&G PETRO-FED 1-25-10-91 107.49297 39.15547 25 10 S 91 W 8195 74.0 45 3 27 8 2 1

28 05-051-06008 O&G RALSTON-PETROLWS 11-90-7 107.48417 39.12028 7 11 S 90 W 8181 74.0 40 6 22 6 12 5

29 05-051-06009 O&G RALSTON-FED 31 107.47939 39.14411 31 10 S 90 W 7901 73.0 45 3 23 4 5 2

30 LEE-MS-47 MS USGS BULL 510 107.4775 38.86167 5 14 S 90 W 54.0 27 7 10 1 16 2

31 Not provided

32 DUN-40 LL USGS MAP C-115 107.47111 38.86278 5 14 S 90 W 8109 76.0 33 6 29 2 14 2

33 05-051-06024 O&G PIUTE-COAL BSN10-8-11-90 107.46944 39.11742 8 11 S 90 W 7918 78.0 40 7 27 6 11 4

34 Not provided

35 05-051-06035 O&G AMOCO-SUMERSET #2 107.46544 39.02875 8 12 S 90 W 8515 85.0 40 4 29 4 16 3

36 05-051-06007 O&G RALSTON-PETRLWS 11-90-17 107.46533 39.09867 17 11 S 90 W 8101 55.0 26 8 16 4 13 5

37 DUN-41 LL USGS MAP C-115 107.46 38.85611 9 14 S 90 W 8256 77.0 32 5 29 2 16 3

38 LH-2-33 R/C USBM TP 721 107.45947 38.87931 33 13 S 90 W 7504 74.9 27.6 4 34.9 6 9.7 2

39 LH-5-33 R/C USBM TP 721 107.45758 38.87164 33 13 S 90 W 7636 64.5 25.1 5 28.9 3 6.3 1

40 05-051-06021 O&G AMOCO-SUMERSET #1 107.45756 39.03128 9 12 S 90 W 8441 87.0 39 4 30 4 18 2

41 05-051-06010 O&G RALSTON-RALSTON 10-90-32 107.45744 39.14203 32 10 S 90 W 7711 9 2 23 4 6 4

42 LH-1-33 R/C USBM TP 721 107.45739 38.87164 33 13 S 90 W 7643 73.7 27.7 5 33.3 6 7.3 3

43 LH-3-4 R/C USBM TP 721 107.45667 38.86558 4 14 S 90 W 7765 50.0 13.6 4 23 4 12.3 4

44 05-051-05007 O&G POOL-HENDERSON 1 107.4485 39.11806 9 11 S 90 W 7532 35.0 30 6 5 1

45 DUN-42 LL USGS MAP C-115 107.44667 38.83944 16 14 S 90 W 7678 20 3 17 2

46 DUN-52 UND USGS Map C-115 107.44331 38.89461 28 13 S 90 W 7092 81.0 31 4 34 5 16 4

47 DUN-51 UND USGS Map C-115 107.44272 38.90539 21 13 S 90 W 6765 71.0 35 4 24 2 12 2

48 DUN-43 LL USGS MAP C-115 107.4425 38.83167 15 14 S 90 W 8442 79.0 43 3 26 2 10 2

49 DUN-44 LL USGS MAP C-115 107.4425 38.82056 22 14 S 90 W 8484 78.0 34 3 33 3 11 2

50 05-051-06011 O&G RALSTON-FED33 (10-90-33) 107.43794 39.14511 33 10 S 90 W 7561 55.0 31 5 16 3 8 4

51 05-051-06013 O&G RALSTON-HENDRSN 11-90-10 107.43764 39.11386 10 11 S 90 W 7550 31.0 25 8 6 2

52 DUN-53 UND USGS Map C-115 107.43675 38.88839 27 13 S 90 W 7597 62.0 21 3 28 4 13 3

53 05-051-06023 O&G PIUTE-RAGGED MTN FED16-4 107.43533 39.18561 16 10 S 90 W 8238 44.0 34 5 6 2 4 2

54 05-051-06014 O&G RALSTON-10-90-34-SW 107.43203 39.14431 34 10 S 90 W 7490 51.0 30 4 15 1 6 4

55 DUN-57 LL USGS MAP C-115 107.43167 38.86 3 14 S 90 W 8292 47.0 13 2 27 3 7 2

56 LH-4-10 R/C USBM TP 721 107.431 38.854 10 14 S 90 W 8041 46.2 10.9 4 18.6 3 15.1 5

57 DUN-56 LL USGS MAP C-115 107.42917 38.87139 34 13 S 90 W 8051 77.0 31 6 35 4 11 2

58 DUN-58 LL USGS MAP C-115 107.42889 38.85167 10 14 S 90 W 8156 62.0 22 5 20 2 20 3

59 DUN-54 UND USGS Map C-115 107.42544 38.88636 27 13 S 90 W 7955 57.0 18 2 20 2 19 5

60 DUN-55 LL USGS MAP C-115 107.42539 38.87856 34 13 S 90 W 8114 65.0 18 3 33 3 14 2

61 05-051-06001 O&G SUNRAY-S.WOLF CRK UNIT 1 107.41792 39.22683 34 9 S 90 W 9170 48.0 30 4 12 4 6 3

62 DUN-76 LL USGS MAP C-115 107.36131 39.14925 31 10 S 89 W 9060 47.0 21 2 18 2 8 1

63 DUN-78 LL USGS MAP C-115 107.35 39.15222 32 10 S 89 W 9530 51.0 21 3 24 3 6 1

64 LH-11-9 R/C USBM B-501 107.34992 38.92189 9 13 S 89 W 6385 18.2 12 4 2.4 1 0 0

65 KENT MS-9 MS USGS OF 80-709 107.34989 39.19494 17 10 S 89 W 54.1 23.1 3 15 2 16 9

66 Lee-MS-71/69/68 MS USGS BULL 510 107.34833 38.7775 4 15 S 89 W 56.4 29.2 5 21.2 2 6 1
Coal Resources and Coal Resource Potential 203

Data used to construct the maps are identified in table M2 and 

figure M6.

Cameo-Wheeler Coal Zone (West of Long 107°15′ W.)

The Cameo-Wheeler coal zone (fig. M5) underlies a 

925 mi2

 area that includes all parts of Area 1 west of long 

107°15′ W. The coal zone overlies the Rollins Sandstone 

Member and is about 100–400 ft thick. The Cameo-Wheeler 

coal zone has approximately 5–80 ft of net coal, and net coal 

exceeds 50 ft throughout the central part of Area 1 (fig. M9). 

Near the southern boundary of Area 1, in the Grand Mesa and 

Somerset coal fields, the Cameo-Wheeler has 10–70 ft of net 

coal in as many as 15 beds that are 1–30 ft thick. Principal 

coals in the Somerset coal field include the Old King Coal 

(A) bed, Somerset (B) bed, Bear (C) bed, and Orchard Valley 

(D) bed (Dunrud, 1989a, 1989b). Near the eastern boundary 

of Area 1, in the Carbondale coal field, the Cameo-Wheeler 

contains about 7–27 ft of net coal in one to three beds that are 

3–18 ft thick. Principal coal beds in the southern part of the 

Carbondale field are the Coal Basin A, B (Somerset), and C 

(Bear) (Dunrud, 1989a; Ellis and others, 1988).

South Canyon Coal Zone (West of Long 107°15′ W.)

The South Canyon coal zone underlies a 530 mi2 region 

in Area 1. This coal zone overlies and intertongues with the 

middle sandstone of the Bowie Shale Member of the Williams 

Fork Formation (fig. M7). It extends west from long 107°15′

W. and pinches out along a sinuous line that trends about N. 

20° W. from sec. 31, T. 12 S., R. 92 W. to sec. 30, T. 8 S., R. 

95 W. (fig. M10). The coal zone is 1–200 ft thick and contains 

1–30 ft of net coal (fig. M10). Net coal exceeds 20 ft along 

a 5- to 10-mi wide belt that trends N. 20° W. throughout the 

central part of Area 1. In the Somerset coal field, the South 

Canyon has 15–35 ft of net coal in two to five beds that are 

1–25 ft thick, and important coal beds include the Oliver (D), 

D-1, and D-2 beds (Dunrud 1989a). In the southern part of the 

Carbondale field, at Coal Basin, the South Canyon contains 

the 3–20 ft thick Dutch Creek coal bed (Collins, 1976; 

Dunrud, 1989a).

Coal Ridge Coal Zone (West of Long 107°15′ W.)

The Coal Ridge coal zone overlies and intertongues with 

the upper sandstone in the Bowie Shale Member of the Williams Fork Formation (fig. M7), and the coal zone occupies 

about the same area as the underlying South Canyon coal 

zone. The Coal Ridge is 100–400 ft thick near the line of long 

107°15’ W., is less than 100 ft thick throughout most of its 

west half, and pinches out near the same line as the underlying 

South Canyon coal zone (fig. M11). The Coal Ridge generally 

has less than 10 ft of net coal, although a small area with about 

20 ft of net coal is located near the Somerset coal field (figs. 

M3 and M11). In the Somerset coal field, the Coal Ridge coal 

zone contains 10–26 ft of net coal in two to seven beds that are 

1–10 ft thick; important beds include the Hawksnest (E) and 

E-2 (Dunrud, 1989a). In the southern part of the Carbondale 

coal field, the Coal Ridge coal zone has 2–10 beds of coal that 

are 1–23 ft thick, and named beds include the Placita, Sunshine, North Rim, and Lake Ridge coal beds (Ellis and others, 

1988).

Table M2. Drill hole and outcrop data in Area 1.—Continued 

*Cameo-Fairfield group = Cameo-Fairfield coal group of the Mesaverde Group or Formation. The coal group contains the Cameo-Wheeler, South Canyon, and Coal Ridge coal zones 

west of long 107° 15′ W.

 Data point identification Data point location Net coal Cameo-Wheeler South Canyon Coal Ridge

in Cameo- coal zone coal zone coal zone

Map Elevation Fairfield Total # Total # Total #

No. Point ID Type Source Long W. Lat N. Sec. Township Range (ft) group* (ft) coal (ft) beds coal (ft) beds coal (ft) beds

67 LH-6-16 R/C USBM B-501 107.34275 38.9275 16 13 S 89 W 6358 24.4 18.5 4 4.2 2 0 0

68 DUN-79 LL USGS MAP C-115 107.33717 39.15242 33 10 S 89 W 9640 36.0 19 4 13 2 4 1

69 LH-19-28 R/C USBM B-501 107.33597 38.89428 28 13 S 89 W 6560 18.2 15.6 3 2.4 1

70 LH-16-21 R/C USBM B-501 107.33397 38.91314 21 13 S 89 W 6440 19.0 16 2 0 0

71 LH-9-16 R/C USBM B-501 107.33381 38.92189 16 13 S 89 W 6400 17.7 16 3 1.5 1

72 LH-18-21 R/C USBM B-501 107.33353 38.90431 21 13 S 89 W 6500 18.5 14.8 3 3.1 1

73 LH-14-15 R/C USBM B-501 107.32722 38.91856 15 13 S 89 W 6534 23.2 17.6 3 1.5 1 2.4 2

74 DUN-68 LL USGS MAP C-115 107.32167 38.8625 3 14 S 89 W 7236 24.5 13.5 3 11 2

75 LH-15-22 R/C USBM B-501 107.31953 38.91344 22 13 S 89 W 6650 19.7 18.1 2 0 0

76 LH-20-15 R/C USBM B-501 107.316 38.924 15 13 S 89 W 7635 21.9 17.2 2 2.6 2 1 1

77 LEE-MS-64 MS USGS BULL 510 107.31278 38.81083 26 14 S 89 W 40.5 16.1 3 6 1 18.4 3

78 DUN-69 LL USGS MAP C-115 107.31111 38.84917 11 14 S 89 W 7600 20.0 11 5 9 2

79 LH-17-23 R/C USBM B-501 107.30906 38.90922 23 13 S 89 W 6740 24.7 18.8 3 2.2 1 0 0

80 ELLIS-MS-129 MS USGS MAP C-97-B 107.30806 39.07333 26 11 S 89 W 13.0 13 4

81 LH-25-14 R/C USBM B-501 107.304 38.922 14 13 S 89 W 7540 22.2 15.2 2 5.3 3 0 0

82 DUN-71 LL USGS MAP C-115 107.30167 38.79861 35 14 S 89 W 8562 20.5 13.5 4 3 1 4 1

83 DUN-72 LL USGS MAP C-115 107.3 38.78917 35 14 S 89 W 8854 26.0 13 2 5 2 8 3

84 LH-21-23 R/C USBM B-501 107.29878 38.90811 23 13 S 89 W 6940 18.7 14.7 2 2.1 2 0 0

85 ELLIS-MS-128 MS USGS MAP C-97-B 107.29 39.07667 25 11 S 89 W 5.0 5 3

86 GASKILL-MS-1 MS USGS MAP GQ-1604 107.24917 38.99333 29 12 S 88 W 7.1

87 LEE-MS-125 MS USGS BULL 510 107.07139 38.93917 12 13 S 87 W 3.0

88 LEE-MS-123 MS USGS BULL 510 107.03722 38.90333 20 13 S 86 W 4.0

89 LEE-MS-122 MS USGS BULL 510 107.02583 38.90056 29 13 S 86 W 6.0

90 LEE-MS-127 MS USGS BULL 510 107.02167 38.91472 20 13 S 86 W 11.0

91 GASKILL-MS-4 MS USGS MAP GQ-1604 107.00667 38.775 4 15 S 86 W 14.0

92 LEE-MS-121 MS USGS BULL 510 107.00444 38.89139 28 13 S 86 W 10.0

93 LEE-MS-115 MS USGS BULL 510 106.97778 38.84972 11 14 S 86 W 26.8

94 LEE-MS-117 MS USGS BULL 510 106.97528 38.85611 11 14 S 86 W 23.7
Figure M7 (above and following page). Stratigraphy of continental and marine rocks in the Upper Cretaceous Mesaverde Group and Mesaverde Fosection A–A’ , in Area 1. Location of cross section A–A’ is shown in index.

Datum

05-077-08271

Somerville #1

11S-97W-26

KB Elev. 10,009 ft

05-077-05011

Smith 1-C

11S-95W-30

KB Elev. 10,191 ft

05-029-06068

Youngs Creek, Scott 19-1

12S-94W-19

KB Elev. 8,485 ft

05-029-05031

Machelson-2

11S-94W-34

KB Elev. 10,133 ft

05-029-05001

Colorado Federal C-1

12S-92W-08

KB Elev. 9,825 ft

05-029-06027

Bowie Unit #1

12S-91W-15

KB Elev. 7,699 ft

05-051-06003

Hotchkiss 3-11

12S-90W-11

KB Elev. 7,043 ft

05-051-05006

Spatafore 1

11S-90W-27

KB Elev. 7,459 ft

3428226189313226

3222

15

2

1222

3

4347345

10

33

Coa

mo

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3525

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6 4500

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1213

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21237 4

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1

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22

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41

8

?

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Rollins 

Ss Mbr

Mesaverde 

Formation

 (part)

Coal Ridge coal zone

South Canyon 

coal zone

CameoWheeler

coal zone

A

8 mi 7 mi 5 mi 10 mi 9 mi 6 mi 3 mi 8 mi

tongue of the 

Mancos Shale

Mesaverde Fm 

(part) Main body of Mancos Shale 

1 2 3 4

5 6 7 8 

Cozzette 

Mbr

0 10 20 MILES

89 88 86 W

 96 W 95 94 93 92 91

 97 W

13 S

11 S10 S

9 S8 S

l 12 S

l l

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66

39°

108° 107°

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7

8

5 6

4

3

1 2 A

A'

Index—Location of drill holes and measured section used to construct cross section A-A'. Transect A-A' is shown by red line. Area 1 is shown by blue 

line

9
Coal Resources and Coal Resource Potential 205

Coastal plain and alluvial— 

 Coal zones are light green

Upper shoreface 

 and estuarine

Lower shoreface and 

 offshore marine

Depositional environments 

(indicated by background pattern) Rock type (indicated by color in drill hole column)

Sandstone

Siltstone or interbedded 

 sandstone and 

 mudrock

Mudrock

Bentonite— Queried 

 where uncertain

Coal

EXPLANATION

05-077-08271

Somerville #1

11S-97W-26

KB Elev. 10009 ft

2

Lithologic and depositional interpretations

Lithology not interpreted— 

 Interval does not 

 contain coal

4 mi

Distance between 

 drill holes and 

 measured sections

Vertical scale (ft)

0

100

200

300

400

500

Drill hole information

Scale

Abbreviations

Drill hole identification number as shown on cross section 

American Petroleum Information (API) number

8

10

7

900

Location (township, range, section)

Drill hole name

Columnar section showing selected rock types penetrated by 

drill hole; down-hole depths are reported in 500 ft intervals on 

right side of column. Rock types were interpreted from 

geophysical logs. Thickness of coal (black) is shown in feet on 

left side of column

Drill hole elevation (Kelly bushing) 

Fm Formation

Mbr Member

Ss Sandstone 

Sh Shale

pt part

Facies contact that rises 

 stratigraphically— Dashed 

 where approximate

Facies contact— Dashed where 

 approximate

Datum; dashed where approximate— 

 Represents a surface of maximum 

 marine transgression

Bentonite marker bed

Contacts
Figure M8. Isopach map of net coal in Cameo-Fairfield coal group in Area 1. Net coal values represent all beds of coa

Data point location Outcrop of Cameo-Fairfield coal group

Tertiary laccolith or stock

Area 1

Thickness of net coal (in feet) in Cameo-Fairfield coal group: No coal 50–100 

1–50

Isoline showing thickness (in feet) of net coal in Cam Fairfield coal group— Contour interval 25 ft

EXPLANATION

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Figure M9. Isopach map of net coal in Cameo-Wheeler coal zone in Area 1. Net coal values represent all coal beds moWheeler coal zone is defined only for areas located west of long 107°15′ W. EXPLANATIONData point location Tertiary laccolith or stock Area 1 Range of net coal thickness (in feet): 1–20 20–40 40–60 Boundary of Cameo-Wheeler coal zone Isoline showing net thickness (in feet) of coal b

thick in Cameo-Wheeler coal zone— ContouHachured for closed low 60–80 

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Figure M10. Isopach map of net coal in South Canyon coal zone in Area 1. Net coal values represent all coal beds moCanyon coal zone is defined only for areas located west of long 107°15′ W.

0 10 20 MILES

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outcrop of top of Rollins 

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outcSand

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Range of thickness (in feet) of net coal in South Canyon coal zone:

Outcrop of South Canyon  coal zone 10 Isoline showing thickn

 interval 10 ft; 1 ft con

Data point location

Tertiary laccolith or stock

Area 1

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EXPLANATION

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Figure M11. Isopach map of net coal in Coal Ridge coal zone in Area 1. Net coal values represent all coal beds more coal zone is defined only for areas located west of long 107°15′ W.

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outcrop of top of Rollins 

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Tertiary laccolith or stock Data point locationArea 1

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EXPLANATION

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210 Resource Potential and Geology, Grand Mesa, Uncompahgre, and Gunnison National Forests, Colorado

Lower, Middle, and Upper Coal Zones (East of Long 

107°15′ W.)

East of long 107°15′ W., the Cameo-Fairfield coal group 

is divided into the lower, middle, and upper coal zones. The 

collective coal zones have about 1–30 ft of net coal (fig. M12) 

in one to five beds, and individual beds are 1–25 ft thick.

The lower coal zone overlies a basal marine sandstone 

that was considered to be equivalent to the Rollins Sandstone 

Member by Gaskill and Godwin (1966a, 1966b), Godwin 

(1968), and Gaskill and others (1967, 1986, 1987). The lower 

coal zone contains only one or two coal beds that were measured locally along outcrops in the Crested Butte coal field. 

The only important coal in the lower zone is the 0–4.0 ft thick 

A bed, which is located 7–10 mi south of the town of Crested 

Butte in the Ohio Creek district (T. 15 S., R. 86 W.) (Gaskill 

and others, 1987). 

The middle coal zone overlies a second marine sandstone 

that is about 100–200 ft stratigraphically above the Rollins 

equivalent sandstone. The middle coal zone contains two to 

six coal beds that range from 1 to 25 ft thick. Included in the 

middle zone are four beds near the town of Crested Butte; 

these are bed I (1.5–6.5 ft thick), bed II (5.0–10.0 ft thick), 

bed III (2.0–25.0 ft thick), and bed IV (0–6.0 ft thick) (Gaskill 

and others, 1986). Other important beds include the B bed, 

which is 5.6–8.6 ft thick in the Ohio Creek district, and several 

unnamed beds that have been mined on Anthracite Mesa in T. 

13 S., R. 86 W. (Gaskill and others, 1967).

The upper coal zone is about 300 ft stratigraphically 

above the Rollins equivalent sandstone, and it contains several 

lenticular coal beds in the Crested Butte coal field. Important 

beds include the C bed, which is about 5 to 6 ft thick in the 

Ohio Creek district, and a 3.5-4.5 ft thick anthracite bed that 

has been mined 7 mi southwest from the town of Crested Butte 

(Gaskill and others, 1987).

Coal Quality

The Cameo-Fairfield coal group has an ash yield of 

1.9–29.9 percent, a sulfur content of 0.3–3.2 percent, and 

calorific values of 8,160–15,190 Btu/lb, based on values in the 

Grand Mesa, Somerset, Carbondale, and Crested Butte coal 

fields (table M3). The coal has an apparent rank that varies 

from subbituminous A to anthracite in the southern part of 

the Piceance Basin (Hornbaker and others, 1976). The coal’s 

apparent rank generally increases to the southeast along the 

basin’s southern and eastern flanks owing to the increase in 

depth of burial (Johnson, 1989), and it also increases near 

igneous intrusions owing to local heating (Hornbaker and 

others, 1976). The apparent rank of coal is subbituminous A 

to high volatile B bituminous along the basin’s southern flank, 

and high volatile C bituminous to medium volatile bituminous 

along the basin’s eastern flank; some beds have been metamorphosed to semianthracite and anthracite in the Carbondale 

and Crested Butte coal fields. Coal with coking properties has 

been identified in the eastern part of the Somerset coal field, 

the southern part of the Carbondale coal field, and the Crested 

Butte coal field (Hornbaker and others, 1976; Murray and 

others, 1977).

Coal Resources

Methods

Coal resources were estimated using the methodology of 

Wood and others (1983). Coal quantities reported as resources 

represent, as accurately as data allow, all coal in the ground 

in beds greater than 1 ft thick and under less than 6,000 ft of 

overburden. The term “original resource” refers to coal in the 

ground prior to mining. More deeply buried coal is reported 

as other occurrences of non-resource coal. This study does 

not attempt to estimate coal reserves which are that subset of 

the resource that can be economically produced at the present 

time. Coal resources were estimated by multiplying the volume of coal by the average density of coal (Wood and others, 

1983, p. 36). For this study, we used an average density of 

1,800 short tons per acre-ft for bituminous coal.

Coal tonnages were reported within overburden categories of 0–500, 500–1,000, 1,000–2,000, 2,000–3,000, and 

3,000–6,000 ft. Overburden was determined by subtracting 

Table M3. Ash yield, sulfur content, and calorific values of coal in Cameo-Fairfield coal group in vicinity of Area 1, 

southern part of the Piceance Basin, Colo. 

[Coal field locations are shown in figure M3. Modified from Hettinger and others (2000). Values are based on ranges of proximate and 

ultimate analyses summarized by Hornbaker and others (1976), Murray and others (1977), and Tremain and others (1996); values in the U.S. 

Geological Survey USCHEM database provided by R.H. Affolter (written commun., 1998), and include values summarized by Toenges and 

others (1949, 1952) for the Somerset coal field. Coal from the C.M.C. mine had an ash yield of 23.3 percent and was included in the Book 

Cliffs coal field by Tremain and others (1996); we included that ash value in the Grand Mesa coal field because the C.M.C mine was located 

in the Grand Mesa coal field as defined by Landis (1959)]

Coal field Ash (pct) Sulfur (pct) Btu/lb

Grand Mesa

Somerset

Crested Butte

Carbondale

2.1–23.3

2.4–29.9

3.2–9.1

1.9–16.2

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8,300–13,490

8,160–14,380

11,080–14,440

10,160–15,190
Coal Resources and Coal Resource Potential 211

elevations at the base of the specified coal interval from surface elevations; the difference therefore represents the maximum overburden on the specified coal interval. Elevations 

at the base of the Cameo-Fairfield coal group and CameoWheeler coal zone were determined from a structure contour 

map of the top of the Rollins Sandstone Member (Hettinger 

and others, 2000). Similarly, elevations at the base of the 

South Canyon and Coal Ridge coal zones were determined 

from structure contour maps that represent the base of those 

respective coal zones. Maximum overburden thicknesses 

on the Cameo-Wheeler, South Canyon, and Coal Ridge coal 

zones are shown in figures M13, M14, and M15, respectively, 

and the maximum overburden thickness on the base of the 

Cameo-Fairfield coal group east of long 107°15′ W. is shown 

in figure M16.

Coal tonnages are also reported by identified and hypothetical reliability categories as defined by Wood and others 

(1983). Identified resources are located less than 3 mi from a 

coal measurement (data point), and hypothetical resources are 

located more than 3 mi from a coal measurement.

Results

Area 1 has an original coal resource of about 38 billion 

short tons in the Cameo-Fairfield coal group. That resource 

represents coal beds more than 1 ft thick and under less than 

6,000 ft of overburden. The resource figure does not include 

coal folded over the flanks of laccoliths or buried beneath 

laccoliths. Approximately 32 percent of the resource is in the 

Grand Mesa National Forest, and 68 percent of the resource is 

in the Gunnison National Forest. Area 1 also contains about 

34 billion short tons of non-resource coal in the Cameo-Fairfield group that is covered by 6,000–11,500 ft of overburden. 

Approximately 76 percent of the non-resource coal is in the 

Grand Mesa National Forest, and 24 percent is in the Gunnison National Forest. Coal tonnages are reported by reliability 

and overburden categories for each coal zone in the CameoFairfield group where it is located west of long 107°15′ W. 

(tables M4, M5, and M6, respectively), and tonnages are 

reported for the entire Cameo-Fairfield coal group where it is 

located east of long 107°15′ W. (table M7).

Figure M12. Isopach map of net coal in Cameo-Fairfield coal group east of long 107°15′ W., in Area 1. Net coal values represent all 

coal beds more than 1 ft thick. 

0 10 20 MILES

 Data point location

Location of Cameo-Fairfield coal group east of long 

107o

15' W. (green area)

Tertiary intrusive

1–10

No coal

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Isoline showing thickness of net 

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 isoline is also shown 

Outcrop of base of Cameo-Fairfield 

 coal group east of long 107°15' 

 W.

38o45' 

39o

107o

107o

15' 

Area 1

39o

107o 108o 107o

15'

EXPLANATION

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Figure M13. Isopach map of overburden on base of Cameo-Wheeler coal zone in Area 1. Cameo-Wheeler coal zone located west of long 107°15′ W.

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Tertiary laccolith or stock Data point locationArea 1

39o

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DELTA

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EXPLANATION

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Figure M14. Isopach map of overburden on base of South Canyon coal zone in Area 1. South Canyon coal zone is definewest of long 107°15′ W.

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Tertiary laccolith or stock Data pointArea 1

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107o15'

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1000 Isoline showing thickness of overburden— 

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Ou 

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EXPLANATION

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214 Resource Potential and Geology, Grand Mesa, Uncompahgre, and Gunnison National Forests, Colorado

Table M4. Original coal resources (A) and other occurrences of non-resource coal (B) in Cameo-Wheeler coal 

zone, Area 1. 

[Coal tonnages were rounded to two significant figures, and categories that show total tonnage may not equal the sum of the components because of independent rounding]

A. Original coal resources (in millions of short tons) in Cameo-Wheeler coal zone, Area 1.

Forest Reliability Overburden (ft) Total

0-500 500-1,000 1,000-2,000 2,000-3,000 3,000-6,000

Grand Mesa Identified 140 130 420 940 5,500 7,100

 78 94 290 440 3,900 4,800

Grand Mesa Total 210 220 710 1,400 9,300 12,000

Gunnison Identified 940 820 2,200 2,600 8,100 15,000

Hypothetical 80 15 0.058 200 1,800 2,100

Gunnison Total 1,000 830 2,200 2,800 9,900 17,000

Grand Total 1,200 1,100 2,900 4,100 19,000 29,000

B. Other occurrences of non-resource coal (in millions of short tons) in Cameo-Wheeler coal zone at depths greater 

than 6,000 ft in Area 1.

Forest Reliability Overburden (ft) Total

6,000-10,000 >10,000

Grand Mesa Identified 16,000 790 17,000

Hypothetical 4,400 450 4,900

Grand Mesa Total 21,000 1,200 22,000

Gunnison Identified 5,400 0.00 5,400

Hypothetical 1,300 0.00 1,300

Gunnison Total 6,700 0.00 6,700

Grand Total 27,000 1,200 28,000

Hypothetical

Table M5. Original coal resources (A) and other occurrences of non-resource coal (B) in South Canyon coal 

zone, Area 1. 

[Coal tonnages were rounded to two significant figures, and categories that show total tonnage may not equal the sum of the components because of independent rounding]

A. Original coal resources (in millions of short tons) in the South Canyon coal zone, Area 1. 

Forest Reliability Overburden (ft) Total

0-500 500-1,000 1,000-2,000 2,000-3,000 3,000-6,000

Grand Mesa Identified 0.00 0.00 0.00 0.00 0.25 0.25

Hypothetical 0.00 0.00 0.00 0.47 26 26

Grand Mesa Total 0.00 0.00 0.00 0.47 26 27

Gunnison Identified 180 350 840 740 2,500 4,600

Hypothetical 0.2 2.5 20 59 410 490

Gunnison Total 180 350 860 790 2,900 5,100

Grand Total 180 350 860 790 2,900 5,100

B. Other occurrences of non-resource coal (in millions of short tons) in South Canyon coal zone at depths greater 

than 6,000 ft in Area 1. 

Forest Reliability Overburden (ft) Total

6,000-10,000 >10,000

Grand Mesa Identified 2,000 100 2,100

Hypothetical 300 48 340

Grand Mesa Total 2,300 150 2,500

Gunnison Identified 1,100 0.00 1,100

Hypothetical 170 0.00 170

Gunnison Total 1,300 0.00 1,300

Grand Total 3,600 150 3,800
Figure M15. Isopach map of overburden on base of the Coal Ridge coal zone in Area 1. Coal Ridge coal zone is defined o

of long 107°15′ W. R 95 W 94 93 92 91 89 88 W 90

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108o 107o15'

EXPLANATION

C

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Tertiary laccolith or stock Data point locationArea 1

0–1,000 1,000–3,000 3,000–6,000 6,000–10

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216 Resource Potential and Geology, Grand Mesa, Uncompahgre, and Gunnison National Forests, Colorado

The large coal resource figure reported for Area 1 must 

be regarded with caution because it does not reflect economic, 

land-use, environmental, technological, and geologic restrictions that affect the availability and recoverability of coal. The 

coal would have to be mined using underground methods, 

and technological and economical constraints generally limit 

current longwall mining to (1) depths of less than 3,000 ft, (2) 

beds more than 3.5 ft thick, and (3) strata inclined by less than 

12°; additionally, only about 14 ft of coal can be mined even 

if the bed is of greater thickness (Timothy J. Rohrbacher, oral 

commun., 1996). These overburden and bed thickness limits 

are supported by a summary of 81 longwalls operating in the 

United States by 30 companies (Merritt and Fiscor, 1995, p. 

32–38). Only an estimated 14 billion short tons of coal in 

Area 1 meets favorable underground mining criteria regarding depth of burial (less than 3,000 ft), and only a fraction of 

that coal could be mined economically because many beds 

are either less than 3.5 ft thick or more than 14 ft thick, and 

because many localities in the vicinity of the Crested Butte 

and Carbondale coal fields are steeply inclined. Additional 

coal would also be restricted from mining because it might be 

in beds that are discontinuous, left in the ground as pillars for 

roof support, or bypassed due to mining of adjacent strata.

Cameo-Wheeler Coal Zone

The Cameo-Wheeler zone has an original coal resource 

of 29 billion short tons in Area 1 (table M4A). The resource 

is distributed across 560 mi2

 where the coal is covered by less 

than 6,000 ft of overburden (fig. M13). Approximately 9.3 

billion short tons is under less than 3,000 ft of overburden, and 

5.2 billion short tons is under less than 2,000 ft of overburden. 

The Cameo-Wheeler contains an additional 28 billion short 

tons of non-resource coal in Area 1 (table M4B). The nonresource coal is covered by 6,000–11,500 ft of overburden.

South Canyon Coal Zone

The South Canyon zone has an original coal resource of 

approximately 5.1 billion short tons in Area 1 (table M5A). 

The resource is distributed across a 320 mi2

 area where the 

Figure M16. Isopach map of overburden on base of Cameo-Fairfield coal group east of long 107°15′ W., in Area 1. 

0 10 20 MILES

EXPLANATION

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Location of Cameo-Fairfield coal group east of long 107o15' 

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Tertiary intrusive

Outcrop of base of Cameo-Fairfield coal group 

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38o45' 

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107o

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Area 1

39o

107o 108o 107o15'

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Coal Resources and Coal Resource Potential 217

coal is covered by less than 6,000 ft of overburden (fig. 

M14). Approximately 2.1 billion short tons is under less than 

3,000 ft of overburden, and 1.4 billion short tons is under less 

than 2,000 ft of overburden. The South Canyon contains an 

additional 3.8 billion short tons of non-resource coal in Area 

1. The non-resource coal is covered by 6,000–11,200 ft of 

overburden (table M5B).

Coal Ridge Coal Zone

The Coal Ridge coal zone has an original coal resource 

of approximately 3.4 billion short tons in Area 1 (table M6A). 

The resource is distributed across 360 mi2

 where the coal 

is covered by less than 6,000 ft of overburden (fig. M15). 

Approximately 1.7 billion short tons is under less than 3,000 

ft of overburden, and 1.1 billion short tons is under less than 

2,000 ft of overburden. The Coal Ridge contains an additional 

1.8 billion short tons of non-resource coal in Area 1. The 

non-resource coal is covered by 6,000–11,000 ft of overburden 

(table M6B).

Coal Resources of the Cameo-Fairfield Coal Group East of 

Long 107°15′ W.

Area 1 has an original resource of 980 million short tons 

of coal in the Cameo-Fairfield coal group where it is located 

east of long 107°15′ W. (table M7). The resource is 

Table M7. Original coal resources in Cameo-Fairfield coal group located east of long 107°15’ W., Area 1. 

[All of these coal resources are within the Gunnison National Forest. Coal tonnages were rounded to two significant figures, 

and categories that show total tonnage may not equal the sum of the components because of independent rounding]

Reliability Overburden (ft) Total

0– 500 500– 1,000 1,000– 2,000 2,000– 3,000 3,000– 6,000

Identified

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Table M6. Original coal resources (A) and other occurrences of non-resource coal (B) in Coal Ridge coal zone, Area 1. 

[Coal tonnages were rounded to two significant figures, and categories that show total tonnage may not equal the sum of the components 

because of independent rounding]

A. Original coal resources (in millions of short tons) in Coal Ridge coal zone, Area 1.

Forest Reliability Overburden (ft) Total

0-500 500-1,000 1,000-2,000 2,000-3,000 3,000-6,000

Grand Mesa Identified 0.00 0.00 0.00 0.27 0.00 0.27

Hypothetical 0.00 0.00 0.18 5.8 27 33

Grand Mesa Total 0.00 0.00 0.18 6.1 27 34

Gunnison Identified 170 230 670 540 1,400 3,000

Hypothetical 0.96 0.82 0.22 38 330 370

Gunnison Total 170 230 670 580 1,700 3,300

Grand Total 170 230 670 580 1,700 3,400

B. Other occurrences of non-resource coal (in millions of short tons) in Coal Ridge coal zone at depths greater than 6,000 ft in

Area 1.

Forest Reliability Overburden (ft) Total

6,000-10,000 >10,000

Grand Mesa Identified 1,200 20 1,200

Hypothetical 300 11 310

Grand Mesa Total 1,500 31 1,500

Gunnison Identified 230 0.00 230

Hypothetical 32 0.00 32

Gunnison Total 260 0.00 260

Grand Total 1,700 31 1,800
218 Resource Potential and Geology, Grand Mesa, Uncompahgre, and Gunnison National Forests, Colorado

distributed across 220 mi2 and is in the lower, middle, and 

upper coal zones. This resource figure is tenuous because of 

the complex geology and paucity of coal measurements in the 

area. Additionally, the resource figure does not include coal 

that is folded over the flanks of laccoliths or that is buried 

beneath laccoliths in the region. Maximum overburden on the 

Cameo-Fairfield coal group east of long 107°15′ W. is shown 

in figure M16. Approximately 910 million short tons of coal 

is under less than 3,000 ft of overburden, and 760 million short 

tons is under less than 2,000 ft of overburden.

Coal Production

About 150 million short tons of coal has been mined 

since the late 1800’s from the Cameo-Fairfield coal group in 

the Carbondale (southern part), Crested Butte, Grand Mesa 

(eastern part), and Somerset coal fields. About 99 million 

short tons was mined in Gunnison County, 30 million tons was 

mined in Pitkin County, and 21 short million tons was mined 

in Delta County (Eakins and Coates, 1998). The coal was 

produced from about 60 mines; the mine areas are shown in 

figure M17. Mining activity prior to 1977 was compiled by 

Murray and others (1977), and mining activity from January 

1977 to December 1997 was summarized by Hettinger and 

others (2000). About 83 million short tons has been extracted 

from 21 mines that operated at various times between January 

1977 and December 1997. Only four mines were producing 

coal at the end of 1997; all four mines are in the Somerset coal 

field, and they are the Bowie No. 1 (Orchard Valley mine), 

Bowie No. 2 mine, Sanborn Creek, and West Elk (Mt. Gunnison) mines. In 1997, the Sanborn Creek and West Elk (Mt. 

Gunnison) mines produced 1.6 million and 5.6 million short 

tons of coal, respectively.

About 19 mines have produced coal from the Grand 

Mesa and Gunnison National Forests (fig. M17), and most of 

the mines are located in the Crested Butte and Somerset coal 

fields. We did not attempt to determine production from the 

Grand Mesa and Gunnison National Forests because production records generally reflect operations conducted within and 

adjacent to the forest lands. Coal production data by Gaskill 

and others (1986, 1987) show that none of the coal mines in 

the Crested Butte coal field have operated since the 1950’s. 

Only the West Elk mine in the Somerset coal field was operating within the Gunnison National Forest at the time of this 

publication.

Summary of Coal Resource Potential 

in the Grand Mesa, Uncompahgre, and 

Gunnison National Forests

The three GMUG forests are considered to have coal 

resource potential in areas where underlying strata (1) are 

likely to have accumulated in a coal-forming environment, 

and (2) the potential coal-bearing rocks are less than 6,000 

ft deep (fig. M18). As summarized in this report, coal-bearing strata are either known or are likely to be in the Dakota 

Formation, Fruitland Formation, Mesaverde Formation, or 

Mesaverde Group. Areas of high coal resource potential have 

nearby outcrop or drill hole data that substantiate the presence 

of coal. Areas of moderate coal resource potential do not have 

drill hole or outcrop data to substantiate the presence of coal; 

however, data in adjacent areas indicate that coal is likely to be 

present. Areas of low coal resource potential have no information to substantiate the presence of coal; however, the presence 

of coal is inferred from regional data. 

Coal Resource Potential of the Dakota Sandstone in the Grand Mesa and Uncompahgre 

National Forests

There are two problems in trying to determine the coal 

resource potential of the Dakota Sandstone. The first problem 

is that few data are available for Dakota coal in the GMUG 

forests. The presence of coal in the Dakota must therefore 

be inferred from adjacent areas where the Dakota has been 

described. The second problem is that the Dakota Sandstone 

and underlying Jurassic strata have been mapped as a single 

unit at many localities in the Gunnison and Uncompahgre 

National Forests, and presence of the Dakota is not certain in 

those areas. Based on published geologic maps, the Dakota is 

definitely present where mapped separately from the underlying Burro Canyon Formation, and it is likely to be present 

below areas where younger sedimentary rocks have been 

mapped at the surface.

The GMUG forests have either a moderate, low, or no 

resource potential for coal in the Dakota Sandstone (fig. 

M18A). The Uncompahgre National Forest has a low to moderate coal resource potential in areas underlain by the Dakota 

Sandstone. Although few data are available to substantiate the 

presence of coal in the forest, the occurrence of minable coals 

outside of the forest (near the towns of Nucla and Norwood) 

indicates that isolated deposits of minable coal might also be 

in the forest. The Dakota Sandstone has a low coal resource 

potential in a small part of the Grand Mesa National Forest. 

The Dakota is 5,000 and 6,000 ft deep in that area, and its 

low resource potential is based on outcrop data that show the 

Dakota to contain a few thin coal beds about 10 mi outside the 

forest along the Gunnison River. Any Dakota coal that might 

be present in the Grand Mesa National Forest would not have 

current mining potential because it is at depths that exceed the 

physical or economic limits of present-day mining techniques. 

The Dakota Sandstone has no coal resource potential in the 

remaining part of the Grand Mesa National Forest because it 

is more than 6,000 ft deep. Available data indicate that the 

Dakota does not contain coal where it is exposed in the vicinity of the Gunnison National Forest, and therefore this forest is 

not considered to have resource potential for Dakota coal.
Figure M17. Location of coal mines that have produced from Cameo-Fairfield coal group in vicinity of Area 1. Mines that have produced coal since 1

Mines that became inactive prior to 1977 are not named. Mines that have operated within the forest are shown in list.

0 10 20 MILES

EXPLANATION

X Approximate mine location

Hawks Nest

5

Mine location— Named mines have operated since 1977.  Numbers refer to mines within GMUG forests, and are  listed at right

Area 1, Grand Mesa and Gunnison  National Forests (part)

Area underlain by Cameo-Fairfield coal  groupOutcrop of base of Cameo-Fairfield coal  group

DELTA

MESA 

GARFIELD

GUNNISON

PITKIN

13 S

11 S

10 S

9 S

T 8 S

89 88

 96 W 95 94 93 92 91

 R 97 W

107o 108o

39o

XX

X

X

XX

X

X

X

X X

X

XX

X

Tomahawk

Red Canon Orchard

Valley

Blue Ribbon

Somerset

Bear No. 3

Hawks Nest

Ohio Creek No. 2 (O.C Coal)

Bear Creek

Coal Basin

L.S. Wood

Dutch Creek No. 1 and No. 2

West Elk 

(Mt. Gunnison)

Bowie 

No. 1

Cyprus 

Orchard Valley

Bowie 

Sanborn Creek

Thompson Creek No. 3Thompson Creek No. 1

1

2

3

4

15 12 13 14

11

10

9

7

8

6

5

16

17

18

19

Mines that have produced coaMines are located by numbers 

MAP NUMBER A

1 Kannah Creek2 Blue Ribbon3 Bear No. 34 West Elk (Mt. Gunnison)5 Castle Rock6 Hinkle 7 K-D (Kozy Draw); S.L. Staple9 Robinson (Kochevar)8 Comstock

10 Richardson; Richardson Coa Coal Co.; Gomer Dollard and 

11 Mt. Carbon Anthracite Coal  Coal Co., and W. Hinds

12 Ruby-Anthracite (Floresta No Union Pacific Coal Co., and 

13 Unknown

14 Wheatstone

15 Verzuh16 Crested Butte, Bulkley No. 1 

17 Peanut, Elk Mountain, and H18 Bear Creek 19 Coal Basin
220 Resource Potential and Geology, Grand Mesa, Uncompahgre, and Gunnison National Forests, Colorado

109o 108o 107o 106o

39o

38o

38o30'

39o30'

Fruitland 

Formation

Mesaverde Formation 

and Mesaverde Group

B

109o 108o 107o 106o

39o

38o

38o30'

39o

30'

0 30 MILES

A

20

0 20 30 MILES

Moderate Low to high High Low to 

moderate Coal resource potential: None

County line National forest boundary

EXPLANATION
Coal Resources and Coal Resource Potential 221

Coal Resource Potential of the Fruitland 

Formation in the Uncompahgre National Forest

The Uncompahgre National Forest has a moderate to high 

resource potential for coal where it is underlain by the Fruitland Formation in the Tongue Mesa coal field (fig. M18B). 

The area is given a high resource potential because it is known 

to contain thick beds of subbituminous coal; the area is also 

assigned a moderate resource potential because coal bed continuity could not be determined, owing to poor exposure and 

structural complexities. Coal beds were mined locally in the 

Tongue Mesa coal field between the 1890’s and 1940’s (Dickinson, 1987a, 1987b, 1988), and there has been some interest 

to develop the coal since that time (Hornbaker and others, 

1976; Dickinson, 1987a, 1987b, 1988). Although the area has 

a moderate to high resource potential, Hornbaker and others 

(1976) thought that the coal in the Tongue Mesa area could not 

compete with better coal in the Somerset field. 

Coal Resource Potential of the Mesaverde 

Group and Mesaverde Formation in the Grand 

Mesa and Gunnison National Forests

The Grand Mesa and Gunnison National Forests have a 

high coal resource potential where the Cameo-Fairfield coal 

group is at depths of less than 6,000 ft (fig. M18B). This 

regionally extensive coal group is in the Mesaverde Group and 

Mesaverde Formation; it contains as much as 97 ft of net coal, 

and has individual coal beds as thick as 30 ft within the forest 

areas. Cameo-Fairfield coal has been mined at several coal 

fields located in and adjacent to the forests. About 150 million 

short tons has been produced since the late 1800’s, and the 

West Elk mine is currently operating in the Gunnison National 

Forest.

The area of high coal resource potential in the Grand 

Mesa and Gunnison National Forests (fig. M18B) is estimated 

to contain about 38 billion short tons of coal in the CameoFairfield coal group, as determined for Area 1 in this study. 

This large resource figure does not represent minable reserves, 

which are a subset of the resource that could be economically 

produced at the present time. Coal in the Cameo-Fairfield 

would have to be mined using underground methods, and 

technological and geologic restrictions preclude much of the 

resource from being economically mined. For example, only 

37 percent of the coal resource is at depths (less than 3,000 ft) 

favorable for longwall mining. Some coal would be precluded 

from mining because the beds are too thin, thick, or steeply 

inclined. Additional coal would also be restricted from mining 

because the beds might be discontinuous, left in the ground as 

pillars for roof support, or bypassed due to mining of adjacent 

strata.

References Cited

Biewick, L.H., and Mercier, T.J., 2000, The Colorado Plateau 

Geographic Information System (GIS); An introduction to 

the ArcView project and data library, in Kirschbaum, M.A., 

Roberts, L.N.R., and Biewick, L.H., eds., Geologic assessment of coal in the Colorado Plateau; Arizona, Colorado, 

New Mexico, and Utah: U.S. Geological Survey Professional Paper 1625–B, Chapter D, CD-ROM.

Boyer, W.W., unpublished [submitted for U.S. Geological 

Survey review, 1926], Coal in the Dakota (?) Formation in 

southwestern Colorado and eastern Utah: U.S. Geological 

Survey Field Records Library, Denver, Colo., item NO8009; figures in folder F-4047.

Collins, B.A., 1976, Coal deposits of the Carbondale, Grand 

Hogback, and southern Danforth Hills coal fields, eastern 

Piceance Basin, Colorado: Colorado School of Mines 

Quarterly, v. 71, no. 1, 138 p.

Dickinson, R.G., 1987a, Geologic map of the Buckhorn Lakes 

quadrangle, Gunnison, Montrose, and Ouray Counties, 

Colorado: U.S. Geological Survey Geologic Quadrangle 

Map GQ–1642, scale 1:24,000.

Dickinson, R.G., 1987b, Geologic map of the Washboard 

Rock quadrangle, Gunnison, Montrose, and Ouray Counties, Colorado: U.S. Geological Survey Geologic Quadrangle Map GQ–1643, scale 1:24,000.

Dickinson, R.G., 1988, Geologic map of the Courthouse 

Mountain quadrangle, Gunnison, Hinsdale, and Ouray 

Counties, Colorado: U.S. Geological Survey Geologic 

Quadrangle Map GQ–1644, scale 1:24,000.

Donnell, J.R., 1962, Geology and coal resources of the 

Carbondale area, Garfield, Pitkin, and Gunnison Counties, 

Colorado: U.S. Geological Survey Open-File Report (62-

38), 2 sheets.

Dunrud, C.R., 1989a, Geologic map and coal stratigraphic 

framework of the Cedaredge area, Delta County, Colorado: 

U.S. Geological Survey Coal Investigations Map C–116, 

scale 1:50,000.

Dunrud, C.R., 1989b, Geologic map and coal stratigraphic 

framework of the Paonia area, Delta and Gunnison Counties, Colorado: U.S. Geological Survey Coal Investigations 

Map C–115, scale 1:50,000.

Figure M18 (previous page). Coal resource potential in Grand 

Mesa, Uncompahgre, and Gunnison (GMUG) National Forests. 

GMUG forests are identified in figure M1. A, Coal resource potential for GMUG forest areas underlain by the Dakota Sandstone. B, 

Coal resource potential for GMUG forest areas underlain by the 

Fruitland Formation, Mesaverde Formation, or Mesaverde Group. 

GMUG forest areas intruded by volcanic rock were not assessed.
Eager, P.G., 1978, Geophysical logs of coal test drill holes in 

the Grand Mesa coal field, Delta and Mesa Counties, 

Colorado: U.S. Geological Survey Open-File Report 78-

540, 97 p.

Eager, P.G., 1979, Geophysical logs of coal test drill holes in 

the Grand Mesa coal field, Delta County, Colorado: U.S. 

Geological Survey Open-File Report 79-327, 21 p.

Eakins, Wynn, 1986, Coal resources of the Dakota Sandstone, 

southwestern Colorado: Colorado Geological Survey OpenFile Report 86-1A, 77 p.

Eakins, Wynn, and Coates, M.M., 1998, Focus—Colorado 

Coal: Colorado Geological Survey Rock Talk, v. 1, no. 3, 

6 p.

Ellis, M.S., Freeman, V.L., and Donnell, J.R., 1988, Cross 

sections showing coal beds and coal zones in the Mesaverde 

Formation in the Carbondale 30’ × 60’ quadrangle, westcentral Colorado: U.S. Geological Survey Coal Investigations Map C–97–B, scale 1:100,000.

Erdmann, C.E., 1934, The Book Cliffs coal field in Garfield 

and Mesa Counties, Colorado: U.S. Geological Survey Bulletin 851, 150 p.

Fisher, D.J., Erdmann, C.E., and Reeside, J.B., Jr., 1960, Cretaceous and Tertiary formations of the Book Cliffs, Carbon, 

Emery, and Grand Counties, Utah, and Garfield and Mesa 

Counties, Colorado: U.S. Geological Survey Professional 

Paper 332, 80 p.

Gaskill, D.L., Colman, S.M., DeLong, J.E., Jr., and Robinson, 

C.H., 1986, Geologic map of the Crested Butte quadrangle, 

Gunnison County, Colorado: U.S. Geological Survey Geologic Quadrangle Map GQ–1580, scale 1:24,000.

Gaskill, D.L., DeLong, J.E., Jr., and Cochran, D.M., 1987, 

Geologic map of the Mt. Axtell quadrangle, Gunnison 

County, Colorado: U.S. Geological Survey Geologic Quadrangle Map GQ–1604, scale 1:24,000.

Gaskill, D.L., and Godwin, L.H., 1966a, Geologic map of the 

Marble quadrangle, Gunnison and Pitkin Counties, Colorado: U.S. Geological Survey Geologic Quadrangle Map 

GQ–512, scale 1:24,000.

Gaskill, D.L., and Godwin, L.H., 1966b, Geologic map of the 

Marcellina Mountain quadrangle, Gunnison County, Colorado: U.S. Geological Survey Geologic Quadrangle Map 

GQ–511, scale 1:24,000.

Gaskill, D.L., Godwin, L.H., and Mutschler, F.E., 1967, 

Geologic map of the Oh-Be-Joyful quadrangle, Gunnison 

County, Colorado: U.S. Geological Survey Geologic Quadrangle Map GQ–578, scale 1:24,000.

Godwin, L.H., 1968, Geologic map of the Chair Mountain 

quadrangle, Gunnison and Pitkin Counties, Colorado: U.S. 

Geological Survey Geologic Quadrangle Map GQ–704, 

scale 1:24,000.

Green, G.N., 1992, The digital geologic map of Colorado in 

ARC/INFO format: U.S. Geological Survey Open-File 

Report 92-507, 15 disks.

Haines, D.V., 1978, Core-hole drilling and coal analysis report 

for nine holes drilled during 1977 in the Nucla coal field, 

Montrose County, Colorado: U.S. Geological Survey OpenFile Report 78-899, 37 p.

Hettinger, R.D., Roberts, L.N.R., and Gognat, T.A., 2000, 

Investigations of the distribution and resources of coal in the 

southern part of the Piceance Basin, Colorado, in Kirschbaum, M.A., Roberts, L.N.R., and Biewick, L.H., eds., Geologic assessment of coal in the Colorado Plateau; Arizona, 

Colorado, New Mexico, and Utah: U.S. Geological Survey 

Professional Paper 1625–B, Chapter O, CD-ROM.

Hornbaker, A.L., Holt, R.D., and Murray, K.D., 1976, Summary of coal resources in Colorado, 1975: Colorado Geological Survey Special Publication 9, 17 p.

Johnson, R.C., 1989, Geologic history and hydrocarbon potential of Late Cretaceous-age, low-permeability reservoir, 

Piceance Basin, western Colorado: U.S. Geological Survey 

Bulletin 1787-E, p. E1–E51.

Johnson, V.H., 1948, Geology of the Paonia coal field, Delta 

and Gunnison Counties, Colorado: U.S. Geological Survey 

Preliminary Map, scale 1:48,000.

Kent, B.H., and Arndt, H.H., 1980a, Geology of the Carbondale coal mining area, Garfield and Pitkin Counties, Colorado, as related to subsurface hydraulic mining potential: 

U.S. Geological Survey Open-File Report 80-709, 94 p.

Kent, B.H., and Arndt, H.H., 1980b, Geology of the Thompson 

Creek coal mining area, Pitkin County, Colorado, as related 

to subsurface hydraulic mining potential: U.S. Geological 

Survey Open-File Report 80-507, 81 p.

Landis, E.R., 1959, Coal resources of Colorado, in Contributions to economic geology 1957: U.S. Geological Survey 

Bulletin 1072–C, p. 131–232.

Lee, W.T., 1912, Coal fields of Grand Mesa and the West Elk 

Mountains, Colorado: U.S. Geological Survey Bulletin 

510, 237 p.

Merritt, Paul, and Fiscor, Steve, 1995, Longwall census: Coal 

Magazine, v. 100, no. 2, p. 30–39.

Murray, D.K., 1981, Upper Cretaceous (Campanian) coal 

resources of western Colorado, in Epis, R.C., and Callender, 

J.F., eds., Western Slope Colorado—Western Colorado and 

eastern Utah: New Mexico Geological Society Guidebook, 

32nd Field Conference, p. 233–239.

222 Resource Potential and Geology, Grand Mesa, Uncompahgre, and Gunnison National Forests, Colorado
Murray, D.K., Fender, H.B., and Jones, D.C., 1977, Coal and 

methane gas in the southeastern part of the Piceance Creek 

Basin, Colorado, in Veal, H.K., ed., Exploration frontiers of 

the central and southern Rockies: Rocky Mountain Association of Geologists Guidebook, 1977 Symposium, Denver, 

Colo., p. 379–405.

Roberts, L.N.R., Brownfield, M.E., Hettinger, R.D., and Johnson E.A., 2000, Methodology for calculating coal resources 

for the Colorado Plateau, U.S. Geological Survey National 

Coal Assessment, in Kirschbaum, M.A., Roberts, L.N.R., 

and Biewick, L.H., eds., Geologic assessment of coal in the 

Colorado Plateau; Arizona, Colorado, New Mexico, and 

Utah: U.S. Geological Survey Professional Paper 1625–B, 

Chapter C, CD-ROM.

Roberts, L.N.R., and Kirschbaum, M.A., 1995, Paleogeography of the Late Cretaceous of the Western Interior of middle 

North America—Coal distribution and sediment accumulation: U.S. Geological Survey Professional Paper 1561, 

116 p.

Toenges, A.L., Dowd, J.J., Turnbull, L.A., Davis, J.D., Smith, 

H.L., and Johnson, V.H., 1949, Reserves, petrographic and 

chemical characteristics, and carbonizing properties of coal 

occurring south of Dry Fork of Minnesota Creek, Gunnison 

County, near Paonia, Colorado, and the geology of the area: 

U.S. Bureau of Mines Technical Paper 721, 48 p.

Toenges, A.L., Turnbull, L.A., Davis, J.D., Reynolds, D.A., 

Parks, B.C., Cooper, H.M., and Abernethy, R.F., 1952, Coal 

deposit, Coal Creek district, Gunnison County, Colorado—

Reserves, coking properties, and petrographic and chemical 

characteristics: U.S. Bureau of Mines Bulletin 501, 83 p.

Tremain, C.M., Hornbaker, A.L., Holt, R.D., Murray, D.K., 

and Ladwig, L.R., 1996, Summary of coal resources in 

Colorado [1995]: Colorado Geological Survey Special 

Publication 41, 19 p.

Tweto, Ogden, 1979, Geologic map of Colorado: U.S. Geological Survey Special Geologic Map, scale 1:500,000.

U.S. Bureau of Mines, 1937, Analyses of Colorado coals: 

U.S. Bureau of Mines Technical Paper 575, 327 p.

Wood, G.H., Jr., Kehn, T.M., Carter, M.D., and Culbertson, 

W.C., 1983, Coal resource classification system of the U.S. 

Geological Survey: U.S. Geological Survey Circular 891, 

65 p.

Woodruff, E.G., 1912, The coal resources of Gunnison Valley, 

Mesa and Delta Counties, Colorado, in Campbell, M.R., ed., 

Contributions to economic geology 1910—Part II, Mineral 

fuels: U.S. Geological Survey Bulletin 471, p. 565–573.

Young, R.G., 1960, Dakota Group of Colorado Plateau: 

American Association of Petroleum Geologists Bulletin, v. 

44, no. 2, p. 156–194.

Young, R.G., 1973, Depositional environments of basal 

Cretaceous rocks of the Colorado Plateau, in Fassett, J.E., 

ed., Cretaceous and Tertiary rocks of the southern Colorado Plateau: Four Corners Geological Society Memoir, p. 

10–27.

Coal Resources and Coal Resource Potential 223
U.S. Department of the Interior

U.S. Geological Survey

Potential Aggregate Resources 

By Daniel H. Knepper Jr., and Viki Bankey

Chapter N of

Resource Potential and Geology of the Grand Mesa, Uncompahgre,

and Gunnison (GMUG) National Forests and Vicinity, Colorado

Edited by Viki Bankey

U.S. Geological Survey Bulletin 2213– N
Contents

Abstract...................................................................................................................................................... 226

Introduction ............................................................................................................................................... 226

Aggregate Sources .................................................................................................................................. 226

Evaluating and Modeling Potential Aggregate Resources................................................................ 226

Limited Assessment of Landslide Hazards for Four Geologic Units................................................. 229

References Cited ...................................................................................................................................... 231

Figures

N1–N3. Maps of GMUG greater study area showing:

 N1. Quality and location of bedrock aggregate resources................................................ 227

 N2. Quality and location of unconsolidated aggregate resources................................... 228

 N3. Some potential areas of landslide risk........................................................................... 230

Table

N1. Areas calculated for bedrock and unconsolidated material for potential

aggregate sources .......................................................................................................................... 229
Abstract

This chapter presents the results of potential aggregate 

resources that are suitable for use in asphaltic and Portland 

cement concrete, evaluating both the exposed bedrock and the 

unconsolidated sedimentary deposits. A limited assessment 

of landslide hazards for Mancos Shale, Brushy Basin Member 

of the Morrison Formation, Wasatch Formation, and sedimentary deposits identified as landslide-produced is also included 

herein.

Introduction

Almost any rock or unconsolidated deposit can be used 

by the construction industry for some purpose, such as back 

fill or road base. However, the specifications for aggregate 

used in asphaltic concrete or Portland cement concrete are 

extremely high and rigid, and the material used for aggregate 

must pass specific tests of the American Society for Testing 

and Materials (2000). The consequences of the rigid ASTM 

tests are that even in a region rich with fresh, exposed bedrock 

and sand and gravel deposits such as western Colorado, the 

amount of potential concrete aggregate may be substantially 

less than might be presumed.

Because aggregate that meets specifications for concrete 

applications is also useful for most other construction applications, this study deals exclusively with the assessment of 

potential aggregate sources that are suitable for use in asphaltic and Portland cement concrete.

Aggregate Sources

Two primary sources of potential concrete aggregate 

exist: bedrock and gravel. Both boulders and large cobblestones from bedrock or gravel can be crushed to form appropriately sized aggregate particles. In fact, aggregate for use in 

asphaltic concrete must be composed of particles that have all 

fractured surfaces. Crushed stone is the major source of natural aggregate in the eastern United States, and it is becoming 

more and more important in the West as available supplies of 

alluvial sand and gravel are depleted or otherwise preempted 

by urbanization and other alternative land uses. Appropriately sized gravel particles obtained by screening and washing 

of alluvial gravel deposits are preferred for use in Portland 

cement concrete, but crushed stone can be used as well.

Both the exposed bedrock and the unconsolidated sedimentary deposits in the GMUG greater study area were evaluated for potential suitability as a natural aggregate resource, 

and maps were prepared showing the distribution of these 

potential resources (figs. N1, N2).

Evaluating and Modeling Potential 

Aggregate Resources

There is no substitute for a geologist standing on an 

outcrop for evaluating the physical and chemical properties of 

a rock unit or gravel deposit for its potential as a natural aggregate resource. Nevertheless, a great deal about the quality of 

a potential resource can be inferred from the lithology and age 

of the deposit alone (Langer and Knepper, 1998). For example, lithologic units composed primarily of shale, siltstone, salt 

or gypsum, or friable sandstone most likely do not have the 

hardness and durability required for high-quality aggregate. 

Similarly, pebbles, boulders, and cobbles in gravel deposits of 

Tertiary age in the GMUG greater study area are commonly 

highly weathered and crumble under a minimum of stress. 

Silicic volcanic and shallow intrusive rocks, although having 

excellent physical properties for natural aggregate, are highly 

likely to contain microcrystalline quartz (cristobalite, tridymite) that reacts adversely with the alkali in Portland cement 

and significantly weakens the resulting concrete. Langer and 

Knepper (1998) presented a more complete description of the 

common rock types that in many places provide suitable natural aggregate. Their report also described the general physical 

and specific deleterious chemical properties of common rocks 

Potential Aggregate Resources

By Daniel H. Knepper, Jr., and Viki Bankey
39o 30'

109o 

37o 45'

70

139

65

92

50

50 

82

135

133

149

62

145

141

141

90

114

Grand

Junction

Delta

Montrose

Crested Butte

Lake City

Uravan

Telluride

Ouray

Silverton

Aspen

Disappointment Creek

Gunnison River

Uncompahgre River

Taylor River

Dolores River

0

0

Approximate outline of national forests in GMUG study area

EXPLANATION

550

Figure N1. GMUG greater study area, showing quality and location of bedrock aggregate resources. Pink, satisfactory, deleterious; blue, sat
39o 30'109o 

37o 45'

70

139

65

92

50

50 

82

135

133

149

62

145

141

141

90

114

Grand

Junction

Delta

Montrose

Gunnison

Crested Butte

Lake City

Uravan

Telluride

Ouray

Silverton

Aspen

Disappointment Creek

Gunnison River

Uncompahgre

River Lake

Fo

r

k

Cebolla

Creek

Tomichi Creek

Taylor River

Blue Mesa

CochetopaCreek

Dolores River

0

0

COLORADO

GreStuAre

Approximate outline of national forests in GMUG study area

EXPLANATION

550

Figure N2. GMUG greater study area, showing quality and location of unconsolidated aggregate resources. Blue, satisfactory, innocuous.
Potential Aggregate Resources 229

and minerals that should be considered in evaluating potential 

natural aggregate resources.

Using the lithologic and mineralogic criteria of Langer 

and Knepper (1998), each of the map units on the geologic 

map of the GMUG greater study area (Day and others, 1999) 

was rated for its physical and chemical properties as a potential natural aggregate source, drawing heavily on a previous 

rating of each map unit on the geologic map of Colorado 

(Knepper and others, 1999). Physical properties were rated 

either satisfactory, fair, poor, or unsuitable. Based on the ratings of the lithologic units, a simple model was constructed to 

identify potential sources of natural aggregate in the GMUG 

greater study area.

Figure N1 shows the results for bedrock, where pink 

areas show potential aggregate sources that are satisfactory 

and deleterious, and blue areas show potential aggregate 

sources that are satisfactory and innocuous. Figure N2 shows 

similar results for unconsolidated material. No areas fit the 

satisfactory and deleterious category; blue areas show potential aggregate sources that are satisfactory and innocuous. 

Table N1 lists areas of modeled results, in square miles.

Limited Assessment of Landslide

Hazards for Four Geologic Units

At the request of the USDA Forest Service, a limited 

assessment of landslide hazards was generated by calculating slope from topographic data and combining it with the 

geologic data set (Day and others, 1999). The four geologic 

units selected for this limited assessment were Mancos Shale, 

Brushy Basin Member of the Morrison Formation, Wasatch 

Formation, and sedimentary deposits identified as landslideproduced. These were chosen because they are known to 

cause landslide problems in the study area.

Figure N3 shows the results of the limited assessment 

of landslide hazards. A combination of color and intensity 

is used to display information about geologic unit and slope. 

Mancos Shale is shown in shades of red/pink, Brushy Basin 

Member in shades of blue, Wasatch Formation in green, and 

landslide sediments in yellow/gold. Darker colors indicate 

steeper slopes, ranging from 15° to 20° for light colors, from 

20° to 25° for medium colors, and greater than 25° for darkest 

colors.

Table N1. Areas calculated for bedrock and unconsolidated material for potential aggregate 

sources.

[Total area includes all public and private lands; forest area includes only area within the Grand Mesa, Uncompahgre, 

and Gunnison National Forests; BLM area includes only area managed by the Bureau of Land Management. Areas 

rounded to the nearest whole number]

 Total area (mi2) Forest area (mi2) BLM area (mi2)

GMUG study area, no model 19,800 4,868 5,092

Bedrock—satisfactory, deleterious 1,450 335 347

Bedrock—satisfactory, innocuous 4,681 1,636 736

Unconsolidated—satisfactory, 0 0 0

deleterious

Unconsolidated—satisfactory, 1,313 38 146

innocuous 
39o 30'109o 

37o 45' 70 139 65 92 50 50 

82 

135 133 149 62 145 141 141 90 114 

Grand Junction Delta Montrose Gunnison Crested Butte Lake City Uravan Telluride Ouray Silverton Aspen 

Disappointment Creek Gunnison River Uncompahgre River Lake Fork Cebolla Creek Tomichi Creek Taylor River 

Blue Mesa Reservoir Cochetopa Creek Dolores River

0

0

Approximate outline of national forests in GMUG study area

EXPLANATION

550

Figure N3. GMUG greater study area, showing some potential areas of landslide risk. Red/pink, Mancos Shale; yellow/gold, landslide sedime

Member of Morrison Formation; green, Wasatch Formation. Darker values of the same color indicate steeper slopes.
Potential Aggregate Resources 231

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