Mineral Resource Potential and Geology

of the Routt National Forest and the

Middle Park Ranger District of the

Arapaho National Forest, Colorado

U.S. Department of the Interior

U.S. Geological Survey

U.S. Geological Survey Professional Paper 1610
M A R C H 3, 18 4 9

U.S. DEPARTMENT OF THE INTERIOR

Mineral Resource Potential and Geology

of the Routt National Forest and the

Middle Park Ranger District of the

Arapaho National Forest, Colorado

Edited by Viki Bankey, Sandra J. Soulliere, and Margo I. Toth

U.S. Geological Survey

With a section on Salable Minerals

By John S. Dersch

U.S. Forest Service 

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON: 2000

U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1610

Supersedes U.S. Geological Survey Open-File Report 96-82
For sale by U.S. Geological Survey, Information Services

Box 25286, Federal Center

Denver, CO 80225

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

Library of Congress Cataloging-in-Publication Data

U.S. DEPARTMENT OF THE INTERIOR

BRUCE BABBITT, Secretary

U.S. GEOLOGICAL SURVEY

Charles G. Groat, Director

Mineral resource potential and geology of the Routt National Forest and the Middle Park 

Ranger District of the Arapaho National Forest, Colorado / edited by Viki Bankey,

Sandra J. Soulliere, Margo I. Toth ; with a section on Salable Minerals by John S. Dersch.

p. cm. — (U.S. Geological Survey professional paper ; 1610)

Includes bibliographical references.

1. Mines and mineral resources—Colorado—Arapaho National Forest. 

2. Geology—Colorado—Arapaho National Forest. I. Bankey, Viki. II. Soulliere, 

Sandra J. III. Toth, Margo I. IV. Series. 

TN24.C6 M56 2000

553′.09788′65—dc21

99-056293

CIP
III

CONTENTS

Summary ......................................................................................................................... 1

Character and Geologic Setting .............................................................................. 1

Mineral Resources................................................................................................... 2

Energy Resources.................................................................................................... 2

Undiscovered Mineral and Energy Resources ........................................................ 3

Locatable Minerals.......................................................................................... 3

Leasable Minerals ........................................................................................... 8

Salable Minerals.............................................................................................. 8

Quantitative Assessment of Metallic Resources Using Grade-Tonnage Models ... 8

Introduction..................................................................................................................... 9

Geographic Setting.................................................................................................. 9

Methods for Identifying Favorable Areas for Undiscovered Mineral Resources ... 9

Previous Assessments ............................................................................................. 11

Acknowledgments................................................................................................... 12

Geology........................................................................................................................... 13

By Margo I. Toth and Sandra J. Souilliere

Geologic History ..................................................................................................... 13

Description of Rock Units....................................................................................... 13

Proterozoic Rocks ........................................................................................... 13

Paleozoic Rocks .............................................................................................. 14

Mesozoic Rocks ............................................................................................. 14

Middle Tertiary Rocks .................................................................................... 15

Late Cretaceous and Tertiary Rocks ............................................................... 15

Quaternary and Pliocene(?) Unconsolidated Deposits.................................... 15

Colorado Mineral Belt............................................................................................. 15

Structural Geology .................................................................................................. 17

Geochemistry ................................................................................................................. 19

By Steven M. Smith

Geochemical Surveys.............................................................................................. 19

Method of Study...................................................................................................... 19

Results..................................................................................................................... 21

Hahns Peak Mining District............................................................................ 21

Cam Claims Area ............................................................................................ 21

Henderson-Urad Molybdenum Area............................................................... 21

Mt. Cumulus.................................................................................................... 21

Vasquez Peak Molybdenum Anomalous Area ............................................... 21

Bear Creek–Lone Pine Creek Anomalous Area.............................................. 21

Elkhorn Mine Area.......................................................................................... 21

Red Canyon–Crystal Mining District.............................................................. 22

Spring Claims Area......................................................................................... 22

Diamond Park Area......................................................................................... 22

Beaver Creek Area (Farwell Mountain Mining District)................................ 22

Dailey (Atlantic) Mining District.................................................................... 22

King Solomon Mine Area (Farwell Mountain Mining District)..................... 22

La Plata Mining District.................................................................................. 22

Teller Mining District ..................................................................................... 22

Red Elephant Mountain Area.......................................................................... 23

Beaver Creek–Norris Creek Area ................................................................... 23
IV CONTENTS

Beaver Creek Area.......................................................................................... 23

Spencer Heights Area ..................................................................................... 23

Greenville Mine Area ..................................................................................... 23

Lower Slavonia Mining District ..................................................................... 23

Upper Slavonia Mining District...................................................................... 23

Pearl Mining District....................................................................................... 24

Poison Ridge Area (A.O. Porphyry Copper Deposit)..................................... 24

Peak 9731–Elkhorn Complex Area ................................................................ 24

Big Creek Area ............................................................................................... 24

Continental Divide–Fish Creek Reservoir Anomalous Area.......................... 24

Morrison Creek–Beaver Creek Anomalous Area ........................................... 24

Other Occurrences .......................................................................................... 24

Geophysical Studies........................................................................................................ 27

By Viki Bankey and James A. Pitkin

Previous Studies...................................................................................................... 27

Gravity Data............................................................................................................ 27

Aeromagnetic Data ................................................................................................. 27

Physical Properties of Rocks .................................................................................. 32

Interpretations of Gravity and Magnetic Data ........................................................ 35

Regional Features............................................................................................ 35

Park and Gore Ranges and the Mt. Zirkel Wilderness Area........................... 36

Mt. Zirkel and Northern Park Range .............................................................. 37

Southern Park Range and Gore Range............................................................ 38

Medicine Bow Mountains and Rawah Wilderness Area ................................ 39

Rabbit Ears Range and Never Summer Mountains ........................................ 39

Comanche-Neota-Never Summer Study Areas .............................................. 39

Williams Fork Mountains, Vasquez Peak, and St. Louis Peak Areas ............ 40

Flat Tops ......................................................................................................... 41

Aerial Gamma-Ray Radioactivity........................................................................... 41

Introduction..................................................................................................... 41

Discussion ....................................................................................................... 41

Interpretation................................................................................................... 43

Analysis of Radioelement Data by NURE Aerial Contractor......................... 47

Mineral Resources—Locatable Minerals........................................................................ 49

By Sandra J. Souillere and Margo I. Toth

Mining and Exploration History ............................................................................. 49

Potential for Undiscovered Mineral Resources—Locatable Minerals ................... 49

Stockwork Molybdenum (A) .......................................................................... 49

Commodities, By-Products, and Trace Metals ....................................... 49

Host Rocks .............................................................................................. 50

Structural Control.................................................................................... 50

Age.......................................................................................................... 50

Deposit Description ................................................................................ 50

Geochemical Signature ........................................................................... 50

Geophysical Signature ............................................................................ 50

Known Deposits...................................................................................... 50

Assessment Criteria ................................................................................ 50

Assessment.............................................................................................. 50

Economic Significance ........................................................................... 52

Porphyry Copper-Molybdenum (B)................................................................ 52

Commodities, By-Products, and Trace Metals ....................................... 52

Host Rocks .............................................................................................. 52
CONTENTS V

Structural Control ................................................................................... 52

Age.......................................................................................................... 52

Deposit Description ................................................................................ 52

Geochemical Signature........................................................................... 52

Geophysical Signature............................................................................ 52

Known Deposits...................................................................................... 53

Assessment Criteria ................................................................................ 53

Assessment ............................................................................................. 53

Economic Significance ........................................................................... 53

Polymetallic Veins (C) ................................................................................... 53

Commodities........................................................................................... 53

Host Rocks.............................................................................................. 53

Structural Control ................................................................................... 53

Age.......................................................................................................... 53

Deposit Description ................................................................................ 53

Geochemical Signature........................................................................... 53

Geophysical Signature............................................................................ 54

Known Deposits...................................................................................... 55

Assessment Criteria ................................................................................ 55

Assessment ............................................................................................. 56

Economic Significance ........................................................................... 56

Massive Sulfides (D) ..................................................................................... 57

Commodities........................................................................................... 57

Host Rocks.............................................................................................. 57

Structural Control ................................................................................... 57

Age.......................................................................................................... 57

Deposit Description ................................................................................ 57

Geochemical Signature........................................................................... 57

Geophysical Signature............................................................................ 57

Known Deposits...................................................................................... 57

Assessment Criteria ................................................................................ 57

Assessment ............................................................................................. 57

Economic Significance ........................................................................... 59

Fluorspar Veins (E)......................................................................................... 59

Commodity ............................................................................................. 59

Host Rocks.............................................................................................. 59

Structural Control ................................................................................... 59

Age.......................................................................................................... 59

Deposit Description ................................................................................ 59

Geophysical Signature............................................................................ 59

Geochemical Signature........................................................................... 59

Known Deposits...................................................................................... 59

Assessment Criteria ................................................................................ 59

Assessment ............................................................................................. 59

Economic Significance ........................................................................... 61

Vein Uranium (F) ........................................................................................... 61

Commodities, By-Products, and Trace Metals....................................... 61

Host Rocks.............................................................................................. 61

Structural Control ................................................................................... 61

Age.......................................................................................................... 61

Deposit Description ................................................................................ 61

Geochemical Signature........................................................................... 61

Geophysical Signature............................................................................ 61
VI CONTENTS

Known Deposits ................................................................................. 61

Assessment Criteria............................................................................ 61

Assessment ......................................................................................... 61

Economic Significance....................................................................... 61

Sandstone Uranium-Vanadium (G)............................................................ 62

Commodities and By-Products........................................................... 62

Host Rocks ......................................................................................... 62

Structural Control............................................................................... 62

Age ..................................................................................................... 62

Deposit Description............................................................................ 63

Geochemical Signature....................................................................... 64

Geophysical Signature........................................................................ 64

Known Deposits ................................................................................. 64

Assessment Criteria............................................................................ 64

Assessment ......................................................................................... 64

Economic Significance....................................................................... 64

Placer Gold (H) .......................................................................................... 64

Commodities ...................................................................................... 64

Host Rocks ......................................................................................... 64

Structural Control............................................................................... 64

Age ..................................................................................................... 64

Deposit Description............................................................................ 64

Geochemical Signature....................................................................... 65

Geophysical Signature........................................................................ 65

Known Deposits ................................................................................. 65

Assessment Criteria............................................................................ 65

Assessment ......................................................................................... 65

Economic Significance....................................................................... 66

Platinum-Group Elements in Ultramafic Rocks (I).................................... 67

Commodities, By-Products, and Trace Metals................................... 67

Host Rocks ......................................................................................... 67

Structural Control............................................................................... 67

Age ..................................................................................................... 67

Deposit Description............................................................................ 67

Geophysical Signature........................................................................ 67

Geochemical Signature....................................................................... 67

Known Deposits ................................................................................. 67

Assessment Criteria............................................................................ 67

Assessment ......................................................................................... 67

Economic Significance....................................................................... 67

U-Th-REE in Pegmatites (J)....................................................................... 67

Commodities ...................................................................................... 67

Host Rocks ......................................................................................... 67

Structural Control............................................................................... 68

Age ..................................................................................................... 68

Deposit Description............................................................................ 68

Geophysical Signature........................................................................ 69

Geochemical Signature....................................................................... 69

Known Deposits ................................................................................. 69

Assessment Criteria............................................................................ 67

Assessment ......................................................................................... 67

Economic Significance....................................................................... 67

Minor Occurrences..................................................................................... 70
CONTENTS VII

Mineral Resources—Leasable Minerals ........................................................................ 73

Coal Geology and Resources ................................................................................. 73

By Laura N. Robinson Roberts and Carol L. Molnia

Introduction .................................................................................................... 73

Area 1 ............................................................................................................. 74

Methods Used to Estimate Coal Resources............................................................ 75

Mesaverde Group Coals................................................................................. 78

Lance Formation Coals .................................................................................. 85

Fort Union Formation Coals .......................................................................... 85

Limitation of Data .......................................................................................... 88

Potential for Other Undiscovered Coal Resources................................................. 89

Areas 2, 4, and 6............................................................................................. 89

Area 3 ............................................................................................................. 89

Area 5 ............................................................................................................. 89

Oil, Gas, and Coal-Bed Methane Resources.......................................................... 89

By Craig J. Wandrey, Ben E. Law, Charles W. Spencer, and Charles E. Barker

Introduction .................................................................................................... 89

Conventional Hydrocarbon Accumulations................................................... 89

Subthrust Hydrocarbon Accumulations ......................................................... 90

Unconventional Hydrocarbon Accumulations............................................... 92

Coal-Bed Methane.................................................................................. 92

Basin-Centered Gas Accumulation ........................................................ 93

Oil in Fractured Shales........................................................................... 93

Geothermal Resources.................................................................................................... 97

By Sandra J. Soulliere

Mineral Resources—Salable Minerals........................................................................... 99

By John S. Dersch, U.S. Forest Service

Mineral Resource Potential—Salable Minerals ..................................................... 99

Crushed Aggregate......................................................................................... 99

Dimension Stone ............................................................................................ 100

Sand and Gravel ............................................................................................. 100

Probabilistic Assessment of Undiscovered Metallic Endowment Using

Grade-Tonnage Models.................................................................................................. 101

By Theresa M. Cookro

Introduction ............................................................................................................ 101

Assessment Method................................................................................................ 101

Delineation of Permissive Areas.................................................................... 101

Grade-Tonnage Models and Estimation of Undiscovered Deposits .............. 101

MARK3 Computer Program .......................................................................... 102

Calculation of the Mean and the Median ....................................................... 102

Results .................................................................................................................... 103

Massive Sulfide or Sierran Kuroko Deposits................................................. 103

Porphyry Copper Deposits ............................................................................. 103

Placer Gold Deposits...................................................................................... 104

Summary of Environmental Studies .............................................................................. 105

By Margo I. Toth, Sandra Souillere, and Steven M. Smith

Mines and Mineralized Areas ................................................................................ 105

Water-Quality and Atmospheric Studies................................................................ 105

References Cited ............................................................................................................ 106
VIII CONTENTS

PLATE

[Plate is in pocket]

1. Mineral resource potential and geology of the Routt National Forest and the Middle Park Ranger District

of the Arapaho National Forest, Colorado

FIGURES

1. Map summarizing potential for undiscovered locatable minerals................................................................................ 6

2. Map summarizing potential for leasable minerals ....................................................................................................... 7

3. Index map showing the location of the Routt National Forest and Middle Park Ranger

District of the Arapaho National Forest, Colorado ...................................................................................................... 10

4. Index map of studied public lands in or adjacent to the Routt National Forest and

Middle Park Ranger District of the Arapaho National Forest, Colorado ..................................................................... 11

5. Index map showing mining districts and mineralized areas......................................................................................... 16

6. Map showing areas of previous geophysical studies.................................................................................................... 28

7. Map showing location of aeromagnetic surveys used to compile aeromagnetic map ................................................. 29

8. Complete Bouguer gravity anomaly map..................................................................................................................... 30

9. High-pass filtered Bouguer gravity anomaly map........................................................................................................ 31

10. Residual magnetic anomaly map.................................................................................................................................. 32

11. Potassium aeroradioactivity gray-scale contour map................................................................................................... 42

12. Uranium aeroradioactivity gray-scale contour map ..................................................................................................... 43

13. Thorium aeroradioactivity gray-scale contour map ..................................................................................................... 44

14. Uranium:thorium aeroradioactivity gray-scale contour map ....................................................................................... 45

15. Potassium:thorium aeroradioactivity gray-scale contour map ..................................................................................... 46

16. Mineral resource map for stockwork molybdenum deposits (A)................................................................................. 51

17. Mineral resource map for stockwork copper-molybdenum deposits (B)..................................................................... 54

18. Mineral resource map for polymetallic vein deposits (C)............................................................................................ 55

19 Mineral resource map for stratabound massive sulfide deposits (D) ........................................................................... 58

20. Mineral resource map for fluorspar veins and dissemination deposits (E) .................................................................. 60

21. Mineral resource map for vein uranium (F) ................................................................................................................. 62

22. Mineral resource map for sandstone uranium-vanadium deposits (G) ........................................................................ 63

23. Mineral resource map for placer gold (H).................................................................................................................... 66

24. Mineral resource map for PGE in ultramafic rocks (I)................................................................................................. 68

25. Mineral resource map for U-Th-REE (J) ..................................................................................................................... 69

26. Index map of coal fields in and adjacent to the Routt National Forest ........................................................................ 74

27. Structural setting of Area 1 in the southeastern part of the Sand Wash Basin of the

Greater Green River Basin ........................................................................................................................................... 76

28. Generalized geologic map showing Upper Cretaceous sedimentary rocks and Tertiary

sedimentary, volcanic, and intrusive rocks in and near Area 1 .................................................................................... 77

29. Stratigraphic units in and near Area 1 in the eastern part of the Sand Wash Basin...................................................... 78

30. Spontaneous potential and resistivity log signatures of Upper Cretaceous

stratigraphic units in Area 1.......................................................................................................................................... 79

31. Spontaneous potential and resistivity log signatures of Upper Cretaceous and

Tertiary stratigraphic units in Area 1 ........................................................................................................................... 80

Appendix 1. Geologic Time Chart.................................................................................. 114

Appendix 2. Definitions of Levels of Mineral Resource Potential and Certainty of

Assessment................................................................................................. 115

Appendix 3. Coal Resource Classification ..................................................................... 116

Appendix 4. Definition of Terms.................................................................................... 117

Appendix 5. Units of Measurement ................................................................................ 117
CONTENTS IX

32. A. Isopach map of cumulative coal in the Iles Formation in and near Area 1 ............................................................ 81

B. Isopach map of overburden; datum is top of Iles Formation in Area 1.................................................................. 81

33. A. Isopach map of cumulative coal in the Williams Fork Formation in and near Area 1........................................... 84

B. Isopach map of overburden; datum is top of Williams Fork Formation in Area 1................................................. 84

34. A. Isopach map of cumulative coal in the Lance Formation, in and near Area 1 ....................................................... 86

B. Isopach map of overburden; datum is base of Lance Formation (top of Fox Hills

Sandstone) in Area 1. ............................................................................................................................................. 86

35. A. Isopach map of cumulative coal in the Fort Union Formation, in and near Area 1 ............................................... 87

B. Isopach map of overburden; datum is top of Fort Union Formation in Area 1...................................................... 87

36. Map showing coal resource potential of areas underlain by coal-bearing Upper Cretaceous

and Tertiary stratigraphic units .................................................................................................................................... 88

37. Map showing oil fields, gas fields, and wells .............................................................................................................. 91

38. Energy resource map for conventional and subthrust oil and gas accumulations (L) ................................................. 92

39. Energy resource map for coal-bed methane gas accumulations.................................................................................. 94

40. Energy resource map for basin-centered gas accumulations (N) ................................................................................ 95

41. Energy resource map for fractured shale oil accumulations (O) ................................................................................. 96

TABLES

1. Resource potential of lands in the Routt National Forest and the Middle Park Ranger District of the

Arapaho National Forest, Colorado, classified according to type of deposit.............................................................. 3

2. Description of areas of locatable and leasable resources in the Routt National Forest and the

Middle Park Ranger District of the Arapaho National Forest, Colorado .................................................................... 4

3. Previous studies that cover parts of the Routt National Forest and the Middle Park Ranger District of the

Arapaho National Forest, Colorado ........................................................................................................................... 12

4. Threshold values used to divide concentration ranges of elements in different sample media into

five classes: low background, high background, slightly anomalous, moderately anomalous, and

highly anomalous ........................................................................................................................................................ 20

5. Average susceptibility and density values for rocks in the Routt National Forest and the Middle Park

Ranger District of the Arapaho National Forest, Colorado ........................................................................................ 34

6. References for magnetic anomaly prefixes on figure 10 ............................................................................................. 36

7. Estimated hypothetical coal resources of Area 1 (in millions of short tons) according to overburden category ........ 75

8. Location data for drill holes shown in figures 32–35 .................................................................................................. 82

9. Location and coal thickness data of measured outcrop sections shown on figures 32–35.......................................... 83

10. Mean number of deposits and probability of zero deposits for quantitative estimates in the Routt National

Forest and the Middle Park Ranger District of the Arapaho National Forest, Colorado ............................................ 103

11. Quantitative estimates of undiscovered resources of massive sulfide, porphyry copper-molybdenum,

and placer gold deposits in the Routt National Forest and the Middle Park Ranger District of the

Arapaho National Forest, Colorado ........................................................................................................................... 104
1

Mineral Resource Potential and Geology

of the Routt National Forest and the

Middle Park Ranger District of the

Arapaho National Forest, Colorado

Edited by Viki Bankey, Sandra J. Soulliere, and Margo I. Toth

With a section on SALABLE MINERALS

By John S. Dersch

U.S. Forest Service

SUMMARY

The assessment of the mineral resource potential of the

Routt National Forest and the Middle Park Ranger District of

the Arapaho National Forest, Colorado (referred to as “the

Forest” in this report) was made to assist the U.S. Forest Service in fulfilling the requirements of Title 36, Chapter 2, part

219.22, Code of Federal Regulations, and to supply resource

information so that the mineral resources of the Forest can be

considered along with other resources in land-use planning.

The Middle Park Ranger District of the Arapaho National

Forest is included in this report on the Routt National Forest

because the two areas are administered by the staff of the

Routt National Forest and are included in a single planning

document.

This report discusses the potential for as yet undiscovered mineral and energy resources within the Forest. All

available information regarding mineral deposits or occurrences and energy resources, as of June 1994, was assembled

to assess the mineral and energy potential. Geological maps

were compiled at a scale of 1:250,000 (plate 1). Mineral and

energy potential maps were also created for deposit types

within the Forest.

Although most of the Forest is north of the productive

Colorado Mineral Belt, the Forest has a history of mineral

exploration and some mining development. The molybdenum mine at Henderson is located just outside the southeastern boundary of the Forest, and the Northgate district, in the

northeastern part of the Forest, was an important producer of

fluorspar. Deposits of gold, silver, lead, zinc, copper, fluorspar, uranium, and vanadium have been exploited from

mines and prospects within the Forest. Some production of

the industrial minerals mica, vermiculite, crushed and lightweight aggregate, and sand and gravel has also occurred.

Coal beds and oil and gas reservoirs have been identified

within and adjacent to the Forest, and geothermal waters

from hot springs in the area have been used for recreational

purposes.

CHARACTER AND GEOLOGIC SETTING

The Routt National Forest and the Middle Park Ranger

District of the Arapaho National Forest cover about 1.2

million acres in north-central Colorado. Seven mountain

ranges and parts of six counties are within the Forest. The

Forest consists primarily of forested land at higher elevations

separated by rolling hills, sage-covered valleys, and broad

meadows. Elevations range from about 6,800 ft at Mad

Creek to more than 12,900 ft at Clark Peak in the Medicine

Bow Mountains. Within the Forest, the Continental Divide
2 MINERAL RESOURCE POTENTIAL AND GEOLOGY, ROUTT NATIONAL FOREST, COLORADO

trends north-south through the Park Range, turns east-west

to follow the crestline of the Rabbit Ears Range, and then

turns southward toward the Vasquez Mountains in the southern part of the Forest. North- and east-flowing streams drain

into the Platte River, and south- and west-flowing streams

drain into the Colorado River.

The geology of the Forest varies considerably in age

and character; rocks range in age from nearly 2-billion-yearold granitic and mafic intrusive and metamorphic rocks to

modern stream gravels (see geologic time chart in Appendix

1). In the northwestern part of the Forest, the Elkhead

Mountains contain Upper Cretaceous through Tertiary sedimentary rocks crosscut by Upper Tertiary intrusions, dikes,

and sills. Within this area, the Miocene Hahns Peak stock

has particular importance because ore has been mined from

the surrounding breccia (Gale, 1906; Vanderwilt, 1947).

Small patches of volcanic lava flows and breccias also overlie the sedimentary rocks locally.

The central part of the Forest consists of a north-trending belt of mountains that includes the southern part of the

Sierra Madre Mountains, the Park Range, and the northern

part of the Gore Range. The core of these ranges is made up

of Proterozoic metamorphic and granitic rocks, whereas the

flanks of the ranges are composed of Paleozoic sedimentary

rocks. The Sierra Madre Mountains and the Park Range

consist of Proterozoic gneisses, amphibolite, migmatite, and

younger mafic and granitic plutons. The northern Gore

Range consists mainly of Proterozoic granite with isolated

outcrops of Paleozoic sedimentary rocks.

The northeastern part of the Forest is in the Medicine

Bow Mountains, and the rocks are mainly Proterozoic granite and hornblende gneisses. Just outside the Forest boundary in this area, the Independence Mountain thrust fault

separates Paleozoic and Tertiary sedimentary rocks from the

Proterozoic rocks. At the southern extension of the range,

the Never Summer Mountains consist of Proterozoic biotite

gneiss and migmatite intruded by shallow-level Tertiary

stocks and dikes. Tertiary sedimentary rocks are exposed

west of the Never Summer thrust fault.

West of the Never Summer Mountains, the Rabbit Ears

Range separates the North Park Basin from the Middle Park

Basin. In the Rabbit Ears Range, Tertiary sedimentary and

volcanic rocks are intruded by small Tertiary intermediatecomposition porphyries.

The southeastern part of the Forest includes parts of the

Williams Fork and Vasquez Mountains. These ranges are

primarily Proterozoic granitic rocks and gneisses with

Tertiary sedimentary rocks exposed in the valley between

the ranges. Faulting is prevalent in this area and cuts across

all rock types. The Williams Fork thrust fault at the western

boundary of the Forest juxtaposes Proterozoic granite and

gneiss over Cretaceous sedimentary rocks. 

MINERAL RESOURCES

In this report, mineral resource information is given in

terms of mineral deposit types and their geologic settings.

Mineral deposit types are defined by geologic characteristics of known deposits that may occur within or near the

Forest. Each deposit type may be represented by a known

mine or mining district. For the discussion that follows,

each mineral deposit type has been assigned a letter designation (A, B, or C, etc.) for distinction on figures and

tables within the text. Definitions of terms used in the

assessment of potential are summarized in Appendix 2. All

available information was assembled and analyzed according to the procedures outlined by Shawe (1981) and Taylor

and Steven (1983). This study is based primarily on

information from published sources but also includes

unpublished data from previous studies.

The Forest contains several mines and mining districts

and includes part of the productive Colorado Mineral Belt.

No major quantities of metallic minerals have been produced

from mines within the Forest (Neubert, 1994). Minor production occurred from the late 1800's though the early

1970's, and exploration for metals continues today. Areas

within the Forest displaying substantial evidence of metallic

mineralization, but only minor production, include the Hahns

Peak, Pearl, Teller, and La Plata-Dailey mining districts, the

Greenville mine area, and the Parkview and Poison Ridge

intrusive centers (Neubert, 1994).

Three areas within or adjacent to the Forest have

records of major production. The Northgate district, an

important fluorite producer, is in the northeastern part of

the Forest. This area was the second largest producer of

fluorite in Colorado and accounted for approximately 32

percent of the total fluorspar production in Colorado

before it closed in the early 1970's (Brady, 1975). To the

southwest of Northgate, the Crystal district also produced

fluorite until the 1970's. The Henderson mine, located just

outside the southeastern part of the Forest, was a major

producer of molybdenum.

ENERGY RESOURCES

Areas in the Forest underlain by Paleozoic through Tertiary sedimentary rocks have been explored intermittently for

oil and gas since 1928. Three oil fields have been discovered

in the Elkhead Mountains and Flat Tops area. Although no

coal production has been recorded from the Forest, major

mines operate near the Forest, and parts of two major coal

fields extend into the Forest. Geothermal springs are known

in the town of Steamboat Springs and several miles to the

north at Strawberry Park Hot Springs.
SUMMARY 3

UNDISCOVERED MINERAL AND ENERGY 

RESOURCES

Mineral and energy resources are classified into three

types: locatable, leasable, and salable. Tables 1 and 2 summarize the mineral potential for each area. Figure 1 shows

areas of potential for all locatable minerals, and figure 2

shows areas of potential for all leasable minerals.

LOCATABLE MINERALS

Locatable minerals include most metals and industrial

minerals categorized by the General Mining Law of 1872.

Ten principal types of deposits of locatable minerals were

considered in this assessment and are listed below and in

tables 1 and 2. Each summary of the deposit type includes a

brief description of the geologic setting and associated metals, and the location of principal areas favorable to host these

resources. Each deposit type is assigned a letter designation

for distinction on figures and tables within this report.

A. Stockwork molybdenum.—Formed in the upper

parts of granite bodies and in adjacent country rock; deposits

are valuable mainly for molybdenum but also contain

tungsten, tin, and bismuth. The area around Hahns Peak and

a small area in the Never Summer Mountains have high

potential for molybdenum in small stockwork deposits. An

elongate area in the southeastern part of the Forest, near the

Henderson mine, has moderate potential for undiscovered

small stockwork deposits of molybdenum.

B. Porphyry copper-molybdenum.—Formed in shattered portions of granitic intrusions and surrounding country

rock; deposits contain copper and molybdenum with

byproduct gold, tungsten, and tin, and traces of silver, lead,

and zinc. Two small areas in the Rabbit Ears Range have

high potential for stockwork copper-molybdenum deposits.

A large east-west-trending area, consisting of several shallow plutons, along the crest of the Rabbit Ears Range has

moderate potential for stockwork copper-molybdenum.

C. Polymetallic veins.—Related to Proterozoic(?),

Laramide, and Tertiary igneous activity; deposits contain

lead, zinc, silver, copper, and gold with minor molybdenum,

tin, tungsten, bismuth, and antimony. Major areas favorable

for this deposit type include the Williams Fork Mountains

area, the northern part of the Park Range, and the Never

Summer Mountains in the eastern part of the Forest.

D. Massive sulfides.—Deposited in volcanic and sedimentary rocks in a marine environment during Proterozoic

time; later metamorphism converted the volcanic and sedimentary rocks to amphibolite, calc-silicate, and felsic

gneisses. These deposits contain lead, zinc, silver, copper,

Table 1. Resource potential of lands in the Routt National Forest and Middle Park Ranger District

of the Arapaho National Forest, Colorado, classified according to type of deposit.

[The Routt National Forest and Middle Park Ranger District of the Arapaho National Forest contain a total of

about 2,028 mi 2. Figures in columns under each category of resource potential are in mi2 and are rounded to the

nearest mi 2. Letters preceding the deposit type reference the deposit type as discussed in the text] 

Type of deposit Resource potential

High Moderate

Locatable resources

A. Stockwork molybdenum ..................................................... 14 59

B. Porphyry copper-molybdenum ........................................... 1 79

C. Polymetallic vein ............................................................... 159 5

D. Massive sulfides .................................................................. 123 364

E. Fluorspar veins.................................................................... 51 0

F. Vein uranium....................................................................... 1 6

G. Sandstone uranium-vanadium............................................. 7 32

H. Placer gold .......................................................................... 5 6

I. Platinum group elements in ultramafic rocks ..................... 1 0

J. U-Th-REE in pegmatites..................................................... 4 2

Total Locatable Resources .................................................. 366 547

Leasable resources

K. Coal..................................................................................... -- -- 

L. Conventional and subthrust oil and gas .............................. 0 1,120

M. Coal-bed methane ............................................................... 0 225

N. Basin-centered gas .............................................................. 0 0

O. Oil in fractured shales ......................................................... 0 188

 Total Leasable Resources.................................................... 0 1,622
4 MINERAL RESOURCE POTENTIAL AND GEOLOGY, ROUTT NATIONAL FOREST, COLORADO

Table 2. Description of areas of locatable and leasable resources in the Routt

National Forest and Middle Park Ranger District of the Arapaho National Forest,

Colorado.

[Level of resource potential and certainty explained in Appendix 2. Areas of potential are shown

on figures 16-25, 36, and 38-41 and plate 1. --do-- indicates the entry is the same as the one

above it]

Map Resource Commodities

area potential

Stockwork molybdenum

A1 H/C Mo, Cu, Pb, Zn, Ag, Au

A2 H/C Mo, Cu, Pb, Zn, Ag, Sn, Nb

A3 M/C Mo, Cu, Pb, Zn, Ag, Sn, Au

Porphyry copper-molybdenum

B1 M/C Cu, Mo, Pb, Zn, Ag, Au

B2 H/C Cu, Mo, Pb, Zn, Ag

B3 H/C Cu, Mo, Pb, Zn, Ag, Au, As

Polymetallic veins

C1 H/C Ag, Au, Cu, Pb, Zn

C2 H/C Pb, Zn, Ag, Mo

C3 M/B Cu, Pb, Mo, Ag, W

C4 H/C Cu, Pb, An, Mo, As, Cd, Sb

C5 M/B Ag, As, V Zn, Co, Sn, W

C6 H/C Cu

C7 H/C Cu, Mo, Pb, As, Ag, W, Zn, Sn

Massive sulfides

D1 H/C Cu, Pb, Zn

D2 M/B --do--

D3 H/C --do--

Fluorspar veins

E1 H/C F

E2 H/C --do--

E3 H/C --do--

Vein uranium

F1 M/B U

F2 H/C --do--

Sandstone uranium-vanadium

G1 M/C U, V

G2 H/C --do--

G3 M/B --do--

G4 H/C --do--

Placer gold

H1 M/C Au

H2 H/B --do--

H3 H/C --do--

H4 H/C --do--

H5 H/C --do--

H6 M/B --do---
SUMMARY 5

Table 2. Description of areas of locatable and leasable resources in the Routt

National Forest and Middle Park Ranger District of the Arapaho National Forest,

Colorado—Continued.

[Level of resource potential and certainty explained in Appendix 2. Areas of potential are shown

on figures 16–25, 36, and 38–41 and plate 1. --do-- indicates the entry is the same as the one

above it]

Map Resource Commodities

area potential

Sandstone uranium-vanadium

G1 M/C U, V

G2 H/C --do--

G3 M/B --do--

G4 H/C --do--

Placer gold

H1 M/C Au

H2 H/B --do--

H3 H/C --do--

H4 H/C --do--

H5 H/C --do--

H6 M/B --do---

H7 M/B --do---

Platinum group elements in ultramafic rocks

I1 H/C Pt, Pd

I2 H/C --do--

U-Th-REE in pegmatites

J1 M/C U, Th, REE

J2 H/C --do--

J3 H/C --do--

J4 M/B --do--

J5 H/C --do--

Coal

K1 H/B Coal

K2 M/B --do--

K3 L/C --do--

Conventional and subthrust gas accumulations

L1 M/B Gas

L2 M/B --do--

L3 M/B --do--

L4 L/C --do--

L5 L/B --do--

L6 L/B --do--

Coal-bed methane

M1 M/C Coal-bed methane

M2 L/D --do--

Basin-centered gas

N1 L/C Gas

Oil in fractured shales

O1 M/D Oil

O2 M/D --do--

O3 L/C --do--
6 MINERAL RESOURCE POTENTIAL AND GEOLOGY, ROUTT NATIONAL FOREST, COLORADO

107°30' 107°00' 106°30' 106°00'

41°00'

40°30'

40°00'

39°30'

COLORADO

Craig

Steamboat

Springs

Eagle

Vail

Kremmling

Walden

Lake

Granby

Rabbit Ears Range

Williams Fork Mountains

Flat

Tops

Gore Range

Gore Range

Park Range

Hahns Park Range

Peak

Mt

Zirkel Medicine Bow Mountains

Rocky

Mountain

National

Park

Colorado River

Colorado River

0 5 10 MILES

WYOMING

COLORADO

i

70

Approximate boundary of the

Routt National Forest and the

Middle Park Ranger District

of the Arapaho National Forest

Approximate boundary of the

Routt National Forest and the

Middle Park Ranger District

of the Arapaho National Forest

Approximate boundary of the

Routt National Forest and the

Middle Park Ranger District

of the Arapaho National Forest

EXPLANATION

RESOURCE POTENTIAL

High

Moderate

and gold. A large northeast-trending zone in the northern

part of the Park Range has high potential for massive sulfide

deposits associated with calc-silicate and amphibolite host

rocks. A large area of moderate resource potential encloses

this high potential area. In the southeastern part of the Forest,

a medium-sized area has high potential for massive sulfides;

this area has several mines and prospects containing copper,

lead, and zinc.

E. Fluorspar veins.—Formed from hot solutions associated with igneous intrusions; deposits contain fluorspar and

minor amounts of barite. Three areas within the Forest have

high potential for vein fluorspar: the northern part of the

Forest, near the Northgate deposits; the Crystal mining

district, on the eastern side of the Park Range; and the

Delaney Butte area east of the Forest in North Park.

F. Vein uranium.—Formed from hot solutions associated with igneous intrusions; deposits contain uranium and

other trace elements. A small area in the southeastern part of

the Forest near Jones Pass has high potential for uranium in

veins associated with fractures in granitic rock. On the

Figure 1. Map summarizing potential for undiscovered locatable minerals in the Routt National Forest and Middle Park Ranger 

District of the Arapaho National Forest, Colorado. Where areas of potential overlap, only the highest degree of potential is shown.
SUMMARY 7

107°30' 107°00' 106°30' 106°00'

41°00'

40°30'

40°00'

39°30'

COLORADO

Craig

Steamboat

Springs

Eagle

Vail

Kremmling

Walden

Lake

Granby

Rabbit Ears Range

Williams Fork Mountains

Flat

Tops

Gore Range

Gore Range

Park Range

Park Range

Hahns

Peak

Mt

Zirkel Medicine Bow Mountains

Rocky

Mountain

National

Park

Colorado River

Colorado River

0 5 10 MILES

WYOMING

COLORADO

i

70

Approximate boundary of the

Routt National Forest and the

Middle Park Ranger District

of the Arapaho National Forest

Approximate boundary of the

Routt National Forest and the

Middle Park Ranger District

of the Arapaho National Forest

Approximate boundary of the

Routt National Forest and the

Middle Park Ranger District

of the Arapaho National Forest

EXPLANATION

RESOURCE POTENTIAL

High

Moderate

southwest side of the Gore Range, near Morrison Creek, a

small area has moderate potential for vein uranium

associated with granitic rock.

G. Sandstone uranium-vanadium.—Formed during

diagenesis of sandstone units; deposits contain uranium,

vanadium, and minor amounts of copper. Two areas have

high potential for sandstone-hosted uranium and vanadium:

a north-south-trending elongate area near Rabbit Ears Pass

and the Troublesome mining district northeast of

Kremmling. Two small areas have moderate potential for

sandstone-hosted uranium and vanadium: along Norris

Creek on the east side of the Park Range and a north-southtrending elongate area near Rabbit Ears Pass.

H. Placer gold.—Deposited in streams or slope washes

that traversed or eroded gold-bearing rock; deposits contain

gold and minor quantities of silver. Three small areas near

Hahns Peak have high potential for gold in placer deposits,

and one small area has moderate resource potential. On

Independence Mountain, just outside the Forest east of Mt.

Zirkel, one area has moderate potential and one area has high

Figure 2. Map summarizing potential for leasable minerals in the Routt National Forest and Middle Park Ranger District of the 

Arapaho National Forest, Colorado. Where areas of potential overlap, only the highest degree of potential is shown.
8 MINERAL RESOURCE POTENTIAL AND GEOLOGY, ROUTT NATIONAL FOREST, COLORADO

resource potential for small placer deposits. An area north of

the Rabbit Ears Range on the Forest boundary has moderate

potential for gold.

I. Platinum-group elements in ultramafic

rocks.—Formed from gravity settling and convection

processes in intrusive basaltic magmas. Near Elkhorn

Mountain in the northernmost part of the Park Range, a

small area within the Elkhorn complex has high potential

for platinum-group elements; a small area between Bear

and Lone Pine Creeks on the east side of the Park Range

also has high potential for platinum-group elements.

J. U-Th-REE in pegmatites.—Formed from crystallization of minerals from residual melts of granitic bodies. Three

areas have high potential and two areas have moderate potential for U, Th, and REE (rare earth elements) in pegmatites.

These areas occur within the north-central part of the Forest

in granitic rocks of the Park Range.

LEASABLE MINERALS

The first major change in the Mining Law of 1872 came

with the passage of the Mineral Leasing Act of 1920. The

1920 Act placed the following minerals under the leasing

law: oil, gas, coal, oil shale, sodium, potassium, phosphate,

native asphalt, bituminous rock, and sulfur. Geothermal

energy was added to the list of leasable minerals by the Geothermal Steam Act of 1970. The principal leasable minerals

in the Forest are listed below, and areas favorable for

resources are briefly described. Hot springs are present in the

town of Steamboat Springs and about 8 mi to the north at

Strawberry Park Hot Springs. The extent of these springs has

been outlined in previous studies, and there is no potential for

undiscovered springs in the remaining parts of the Forest. 

K. Coal.—Formed from the decomposition and alteration of organic remains in deltaic environments. One area

in the northwestern part of the Forest has high potential for

coal. An area of moderate potential is in the southwestern

part of the Forest in the Flat Tops, and an area of low potential is in the eastern part of the Forest in the Rabbit Ears

Range.

L. Conventional oil and gas.—Formed in near-shore

and coastal-plain environments from the decay of hydrocarbons in source rocks. Three large areas in the Forest have

moderate potential, and five large areas have low potential

for further conventional discoveries of oil and gas.

M. Coal-bed methane.—Generated during the maturation process of coal. One small area in the northwest part of

the Forest in the Elkhead Mountains has moderate potential,

and one small area in the southwest part of the Forest in the

Flat Tops has low resource potential for coal-bed methane

accumulations.

N. Basin-centered gas.—Formed in near-shore and

coastal-plain environments from the decay of hydrocarbons

in source rocks. A small area in the western part of the

Elkhead Mountains has low resource potential for basincentered gas.

O. Fractured shale-oil accumulations.—Formed in

near-shore and coastal-plain environments from the decay of

hydrocarbons in source rocks. Shale is both the source rock

and the reservoir rock; oil can be produced where shale is

thermally mature and fractured. Three areas in the western

part of the Forest have moderate potential for fractured shaleoil accumulations, and one large area in the eastern part of

the Forest has low potential.

SALABLE MINERALS

Salable minerals, as defined in the Materials Act of

1947, include petrified wood, sand, dimension stone, gravel,

pumice, cinders, perlite, and some clay. Salable minerals in

the Forest include crushed aggregate, dimension stone, sand,

and gravel.

Crushed aggregate.—Numerous sources of crushed

aggregate are present in the Forest in the Elkhead Mountains,

the Park and Medicine Bow Ranges, and the Flat Tops.

Aggregate includes sandstone, volcanic rock, granite, basalt,

landslide material, and glacial drift. Uses are roadway building, concrete, railroad ballast, rip rap, and fill.

Dimension stone.—Some decorative dimension stone is

produced locally in the Forest in the Park Range, Elkhead

Mountains, and Flat Tops. Moss- or lichen-covered granite

and sandstone are used for interior or exterior facing in

homes or buildings.

Sand and gravel.—Numerous deposits of sand and

gravel are located along the Elk and Colorado Rivers and

their major tributaries within the Forest. Uses include concrete work and products, fill material, plastering sands, and

snow and ice control.

QUANTITATIVE ASSESSMENT OF METALLIC 

RESOURCES USING GRADE-TONNAGE 

MODELS

At the request of the U.S. Forest Service, the U.S. Geological Survey has provided subjective probabilistic estimates of undiscovered mineral resources that might exist

within the Forest. Based on the geology, geophysics,

geochemistry, and production records of known deposits in

the Forest, deposit types were defined and compared to other

similar deposits worldwide. The number of undiscovered

deposits of median size on the grade-tonnage curve likely to

be present in the Forest was estimated at the 90th, 50th, 19th,

5th, and 1st percentiles; the first percentile is the likelihood

of occurrence with a 1 percent probability. Using the computer program MARK3, tonnages for undiscovered deposits

in the Forest were estimated from known tonnages and

grades of deposits worldwide. Estimates do not consider any

of the economics involved in extracting the metals.
INTRODUCTION 9

Only three mineral deposit types have sufficient grade

and tonnage information for assessment using MARK3:

massive sulfide deposits, porphyry copper-molybdenum

deposits, and placer gold deposits. All remaining deposits

known in the Forest lack sufficient data for the quantitative

assessment.

For massive sulfide deposits, the number of undiscovered deposits was estimated to be 0, 0, 1, 1, and 2 at the 90th,

50th, 10th, 5th, and 1st percentiles, respectively (estimates

for the remaining deposits are presented in the same order of

percentiles). Estimates of mean metal content in undiscovered massive sulfide deposits in the Forest are: 3,200 tons of

copper, 1,900 tons of zinc, 180 tons of lead, 0.05 tons of

gold, and 2.9 tons of silver in 48,000 tons of total ore. For

porphyry copper-molybdenum deposits, the number of

undiscovered deposits was estimated to be 0, 0, 1, 1, and 2.

Estimated mean metal contents are 109,000 tons copper,

1,600 tons molybdenum, 2.6 tons gold, and 17 tons silver in

a total of 20,000,000 tons of ore. For placer gold, the number

of undiscovered deposits was estimated at 0, 0, 1, 1, and 1.

Estimated mean metal content is 0.6 tons of gold and 0.003

tons of silver in 3,900,000 tons of ore.

INTRODUCTION

This report presents an assessment of the mineral and

energy potential of the Routt National Forest and the Middle

Park Ranger District of the Arapaho National Forest,

referred to as “the Forest” in this report. The Middle Park

Ranger District of the Arapaho National Forest is included in

this report because the two areas are administered by the

staff of the Routt National Forest and are included in a single

planning document. The Middle Park Ranger District is east

of the Routt National Forest and includes parts of the Gore

and Rabbit Ears Ranges and the Williams Fork and Vasquez

Mountains. For simplicity, only figure 3 distinguishes the

Middle Park Ranger District; in all other plates and figures,

the district is included within the Forest boundary.

This mineral resource assessment was produced to

assist the U.S. 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 in land-use planning. This report addresses the

potential for undiscovered mineral and energy resources in

the Forest and is based upon information available as of May

1994. The identified, or known, mineral and energy

resources of the Forest were studied by the U.S. Bureau of

Mines (Neubert, 1994).

 “ Ton,” as used in this report, indicates “metric ton” unless 

specified otherwise.

GEOGRAPHIC SETTING

The Routt National Forest and the Middle Park Ranger

District of the Arapaho National Forest cover approximately

1.2 million acres in north-central Colorado. Parts of six

counties are within the Forest: Routt, Jackson, Grand, Moffat, Rio Blanco, and Garfield. Five separate parcels comprise the Forest (fig. 3) and consist of primarily forested land

at higher elevations separated by rolling hills and valleys of

brush and meadows. The largest parcel borders Wyoming

and includes the Elkhead Mountains and the Park and Gore

Ranges. Small scattered parcels along the crest of the Medicine Bow Mountains comprise the northeastern part of the

Forest. The central parcel borders Rocky Mountain National

Park and includes the Rabbit Ears Range and part of the

Never Summer Mountains. The two southern parcels share

borders with the White River National Forest: the southwestern parcel includes part of the Flat Tops Primitive Area and

the southeastern parcel includes the Williams Fork and

Vasquez Mountains.

The Continental Divide lies within the Forest along the

crests of the Park and Rabbit Ears Ranges and forms part of

the Forest boundary in the Never Summer and Vasquez

Mountains. Elevation in the Forest ranges from about 6,800

ft at Mad Creek to more than 12,900 ft at Clark Peak in the

Medicine Bow Mountains. The Forest also partially

encloses two topographic basins, North Park and Middle

Park, located along the eastern slopes of the Park and Gore

Ranges. The Rabbit Ears Range divides the mostly flatlying, sage-covered North Park from the hills and valleys of

Middle Park.

Most of the tributaries that drain the Forest flow into the

Colorado River, with the exception of the tributaries that

drain into North Park and flow into the North Platte River.

Numerous alpine lakes dot the topography, particularly in

the Mount Zirkel Wilderness Area—and Steamboat Lake

and Lake Catamount are located just outside the Forest (pl.

1). Access to the Forest is provided by several improved and

unimproved roads from U.S. Highway 40 and State

Highways 9, 125, and 131. The major communities near the

Forest are Steamboat Springs, Walden, Kremmling, Craig,

and Yampa.

METHODS FOR IDENTIFYING FAVORABLE 

AREAS FOR UNDISCOVERED MINERAL 

RESOURCES

Mineral and energy resources include three categories:

locatable resources, leasable resources, and salable

resources. Areas favorable for the occurrence of locatable

and leasable resources are summarized on figures 1 and 2.

Areas within the Forest that were rated as favorable for

specific types of as yet undiscovered resources are similar
10 MINERAL RESOURCE POTENTIAL AND GEOLOGY, ROUTT NATIONAL FOREST, COLORADO

107°30' 107°00' 106°30' 106°00'

41°00'

40°30'

40°00'

39°30'

COLORADO

Craig

Steamboat

Springs

Eagle

Vail

Winter

Park

Granby

Yampa

State Bridge

Kremmling

Wolcott

Walden

Lake

Granby

Rabbit Ears Range

Williams Fork Mountains

Flat

Tops

Gore Range

Gore Range

Vasquez Mtns

Park Range

Park Range

Madre

Sierra

Hahns

Peak Sand Wash

Basin

Elkhead

Mountains

Independence

Mountain

Mt

Zirkel Medicine Bow Mountains

Rocky

Mountain

National

Park

Colorado River

Colorado River

0 5 10 MILES

WYOMING

COLORADO

i

70

i

70

Approximate

boundary of the

Routt National

Forest

Approximate

boundary of the

Routt National

Forest

Approximate boundary of the

Routt National Forest 

Approximate boundary of the

Routt National Forest 

Approximate boundary of the

Middle Park Ranger District

of the Arapaho National Forest

Middle

Park

Never Summer

 Mtns

40

34

40

40

40

125

14

14

131

131

134

Yampa River

Yampa

ivR

er

to those that host productive deposits elsewhere in the

Forest or in other parts of the United States.

Mineral and energy resources are specified in terms of

deposit types and their geologic settings. Deposit types are

based on geologic characteristics of known deposits within

or near the Forest. Most of the mineral deposit types are

exposed in mines or prospects. The boundaries of favorable areas are based on a combination of geologic,

geochemical, and geophysical criteria. A letter designation

(A, B, C, etc.) is used to represent the deposit type in the

text, on figures, and on the plates. Definition of terms used

in this assessment of mineral potential for each deposit type

are summarized in Appendix 2.

All available information was assembled and analyzed

according to the procedures outlined by Shawe (1981) and

Taylor and Steven (1983). Mineral and energy resource

potential information is presented in detail on plate 1 and is

summarized on figures 1 and 2. This study is based

primarily on published literature but includes unpublished

data from studies in progress.

Figure 3. Index map showing the location of the Routt National Forest and Middle Park Ranger District of the Arapaho National 

Forest, Colorado.
INTRODUCTION 11

107°30' 107°00' 106°30' 106°00'

41°00'

40°30'

40°00'

39°30'

COLORADO

Craig

Steamboat

Springs

Eagle

Vail

Granby

Yampa

Kremmling

Walden

Lake

Granby

Rabbit Ears Range

Williams Fork Mountains

Flat Tops

Gore Range

Park Range

Mt

Zirkel Medicine Bow Mountains

Colorado River

Colorado River

0 5 10 MILES

WYOMING

COLORADO

Yampa River

Yampa

River

Platte River

Adjacent

Never

Summer

Mountains 

Neota

Rawah

Approximate boundary of the

Routt National Forest and the

Middle Park Ranger District

of the Arapaho National Forest

Approximate boundary of the

Routt National Forest and the

Middle Park Ranger District

of the Arapaho National Forest

Approximate boundary of the

Routt National Forest and the

Middle Park Ranger District

of the Arapaho National Forest

St. Louis Peak

Vasquez

Peak

Williams

Fork

Service Creek

PREVIOUS ASSESSMENTS

Parts of the Forest have been previously assessed for

mineral and energy resources by the U.S. Geological

Survey. These areas include the Flat Tops Primitive Area,

the Mount Zirkel Wilderness, the Williams Fork Roadless

Area, the Service Creek Roadless Area, and the Never

Summer, Rawah, and Vasquez Peak Wilderness Study

Areas (fig. 4). In addition, a small Bureau of Land

Management Wilderness Study Area that adjoins the Forest

(Platte River Adjacent Wilderness Study Area) was also

studied (fig. 4) (Dickerson and McDonnel, 1989).

Published reports describe the geology and evaluate the

mineral potential of each area. References for each report

are shown in table 3.

Figure 4. Index map of studied public lands in or adjacent to the Routt National Forest and Middle Park Ranger District of the 

Arapaho National Forest, Colorado.
12 MINERAL RESOURCE POTENTIAL AND GEOLOGY, ROUTT NATIONAL FOREST, COLORADO

Table 3. Previous studies that cover parts of the Routt National Forest and Middle Park Ranger District of the Arapaho National

Forest, Colorado.

[NURE, National Uranium Resource Evaluation]

 Study Area Reference

Flat Tops Primitive Area........................................................Mallory and others (1966)

Williams Fork Roadless Area................................................Theobald and others (1985); Barton (1985b)

St. Louis Peak Roadless Area ...............................................Theobald and others (1985); Barton (1985a)

Vasquez Peak Wilderness Study Area ...................................Theobald and others (1985); Barton (1985c)

Mt. Zirkel Wilderness Study Area.........................................Snyder and others (1981); Snyder (1987b)

Service Creek Roadless Area ................................................Schmidt and others (1984)

Rawah Wilderness Study Area ..............................................Motooka and others (1979); Pearson and others (1982)

Neota-Flat Tops Wilderness Study Areas..............................Pearson and others (1981)

Never Summer Wilderness Study Area.................................Pearson and others (1981)

Craig 1° × 2° NURE..............................................................Bolivar and others (1979); Craig and others (1982)

Rawlins 1° × 2° NURE .........................................................Weaver and others (1978)

Cheyenne 1° × 2° NURE ......................................................Trexler (1978)

Greeley 1° × 2° NURE..........................................................Bolivar and others (1978); Youngquist and others (1981)

Denver 1° × 2° NURE...........................................................Bolivar and others (1978); Shettel and others (1981)

Leadville 1° × 2° NURE .......................................................Planner and others (1981)

Craig 1° × 2° and Rawlins 1° × 2° detailed NURE.............Shannon and others (1981)

Hahns Peak and Pearl districts ..............................................Allen (1982)

ACKNOWLEDGMENTS

Several individuals contributed data and assistance to

this study. George Snyder provided valuable information

on the geology of north-central Colorado and this manuscript benefited greatly from his input. Rusty Dersch of the

U.S. Forest Service provided information regarding Forest

boundaries and salable minerals. Anna Wilson lent

assistance in digitizing areas of potential. 
13

GEOLOGY

By Margo I. Toth and Sandra J. Soulliere

The geology of the Forest was compiled at a scale of

1:250,000 from published geologic maps. Most of the geology was modified from the Craig 1°×2° quadrangle (Tweto,

1976), but parts of the Denver (Bryant and others, 1981),

Leadville (Tweto and others, 1978), and Greeley (Braddock

and Cole, 1978) 1°×2° quadrangles were also used. Detailed

geology is given in other publications (Snyder, 1980a, 1980b,

1980c and 1980d; Braddock and Cole, 1990). 

GEOLOGIC HISTORY

The geologic history of the Forest encompasses almost

2 billion years and is characterized by complex structural and

rock-forming events. Excellent papers by Tweto (1980),

Wallace (1990), De Voto (1990), and Reed and others (1993)

describe the geologic setting and tectonic history of Colorado. Hedge and others (1986) describe the Proterozoic era

of the Rocky Mountain region, and Tweto (1987) describes

the nomenclature of Proterozoic rocks in Colorado. Much of

the following discussion is drawn from these sources. 

Within the area of the Forest, the oldest rocks are Early

Proterozoic. Volcanic and sedimentary formations were

accreted onto the margin of the Archean Wyoming craton and

metamorphosed to gneiss, schist, and migmatite at about 1.7

Ga (billion years ago). During Early Proterozoic (1.7 Ga) and

Middle Proterozoic (1.4 Ga) time, these rocks were intruded

by large granitic plutons. Within the Forest, Proterozoic

rocks make up the core of most of the major mountain ranges

(plate 1). The Proterozoic rocks are cross-cut by welldefined, northeast-trending faults and shear zones of the Colorado lineament, which was initially formed in the Proterozoic (Warner, 1978) and was later reactivated. The northern

margin of this lineament, known as the Mullen Creek–Nash

Fork shear zone, traverses southeastern Wyoming.

The next major geologic event occurred during early and

middle Paleozoic time when a thick sequence of marine and

nonmarine sediments was deposited upon the Proterozoic

rocks (Berg, 1960). In late Paleozoic time, two elements of

the Ancestral Rocky Mountains, the Uncompahgre highland

and the Ancestral Front Range, were formed. The Ancestral

Front Range occupied much of the area currently encompassed by the Forest. Parts of the Ancestral Rocky Mountains

may have attained altitudes of as much as 5,000 to 10,000 feet

above sea level (Mallory, 1971). Uplift of the Ancestral

Rockies caused older sedimentary rocks to be eroded, and in

places the Proterozoic basement was partially exposed. Sedimentary rocks were deposited in a large basin that formed

between the Uncompahgre and Front Range and also locally

on newly exposed Proterozoic basement rocks.

During Mesozoic time, mountain-building decreased

and inland seas covered the area, depositing marine and nonmarine sediments. Clastic sediments were deposited in early

Mesozoic time as erosion of the highlands continued. Continental, marginal-marine, and complex intertonguing marine

and nonmarine sediments were deposited throughout the rest

of the Mesozoic.

The final exit of the sea marked the beginning of the

Laramide orogeny, which produced most of the present primary mountain ranges in central Colorado. During this orogeny, plutonic rocks were emplaced along a northeast-trending

zone in central Colorado coincident with major Proterozoic

structures. Streams eroded older sedimentary rocks, and

these deposits accumulated in structural basins formed during

Late Cretaceous and Tertiary time.

During late Cenozoic time, crustal extension associated

with formation of the Rio Grande rift to the south caused

development of the present basins and ranges. Extension was

accompanied by intrusion of a wide variety of granitic rocks

28 to 23 Ma, by eruption of volcanic rocks 33 to 30 Ma, and

by intrusion and eruption of rhyolite and basalt 20 Ma to 7.6

Ma. 

A major period of glaciation began in the area of the Forest about 500,000 years ago and recurred as recently as

12,000 years ago. During the height of glaciation, ice almost

totally covered the higher ranges, and the valleys were filled

with glaciers. The modern alpine topography, with deep Ushaped valleys that is seen today, is largely a product of

glacial erosion.

DESCRIPTION OF ROCK UNITS 

PROTEROZOIC ROCKS

The most widespread rocks in the Forest are Proterozoic

metamorphic and plutonic rocks, exposed in the core of most
14 MINERAL RESOURCE POTENTIAL AND GEOLOGY, ROUTT NATIONAL FOREST, COLORADO

of the major mountain ranges. The rocks fall into five lithologic groups and are of two ages: 1.7 Ga and 1.4 Ga.

Two of the groups of metamorphic rocks occur in the

Forest: hornblende gneiss, amphibolite, and calc-silicate

gneiss (Xfh, plate 1), and biotite gneiss and migmatite (Xb,

plate 1). These rocks are typically metamorphosed to upper

amphibolite or sillimanite facies and are structurally and

stratigraphically complex. Hornblende gneiss, amphibolite,

and calc-silicate gneiss are dominant in the northern part of

the Forest in the Park Range. Smaller amounts are present in

the Medicine Bow Mountains, Never Summer Mountains,

and in the southeastern part of the Forest in and near the

Vasquez Mountains. Before metamorphism, the hornblende

gneiss and amphibolite rocks were probably mafic lava

flows, near-surface intrusions, or layers of basaltic ash that

were deposited in seawater; the carbonate-rich layers and

calc-silicate gneisses likely were carbonate sediments.

Biotite gneiss and migmatite rocks crop out in small isolated

areas in the Park Range and Gore Range, in large parts of the

Never Summer Mountains, and as a large, continuous body

in the southeastern part of the Forest in the Williams Fork

Range and Vasquez Mountains. Before metamorphism, the

gneiss and migmatite probably were marine deposits of

graywacke, shale, and felsic volcanic rocks.

The Routt and Berthoud Plutonic Suites make up the

remaining Proterozoic rocks in the Forest. Rocks of the

Routt Plutonic Suite are 1.7 Ga in age and consist of a mafic

and a granitic suite of plutons; rocks forming the Berthoud

Plutonic Suite are 1.4 Ga in age and consist solely of granitic

plutons.

The 1.7-Ga mafic plutons in the northern part of the

Park Range (Xm, plate 1) are small, homogeneous, and consist of biotite-hornblende quartz diorite. The largest, the

Elkhorn Mountain pluton, extends from the northwestern

part of the Park Range across the Wyoming border. The

mafic plutons grade into 1.7-Ga granitic rocks, but elsewhere, angular inclusions of mafic plutons occur in granitic

rocks, indicating that they are distinctly older than the granite

(Snyder, 1987b).

The 1.7-Ga granitic rocks include foliated quartz

monzonite and granodiorite rocks (Xg, plate 1) of the

Rawah and Boulder Creek batholiths (Tweto, 1987). The

largest continuous outcrop of these rocks is found in the

core of the Gore Range, but significant amounts also occur

in the northern Park Range closely associated with calcsilicate gneisses, in the Medicine Bow Mountains, and in

the southeastern part of the Forest in the Williams Fork

Mountains. The granitic plutons are generally concordant

with the enclosing gneisses and contain biotite and (or)

hornblende. The bodies were intruded during and immediately following folding and metamorphism of the gneiss

and migmatite complex.

The 1.4-Ga intrusive rocks are composed of massive to

gneissic-biotite granite, quartz monzonite, and granodiorite

(Yg, plate 1). These bodies have the following occurrences:

large continuous bodies of the Sherman batholith in the

Medicine Bow Mountains; one northeast-elongate,

continuous body of the Mt. Ethel pluton in the central part of

the Gore Range; and fault slivers of the Silver Plume

batholith in the Williams Fork Mountains. Contacts with

the country rock range from sharp to diffuse. Rocks from

these units lack metamorphic foliation, and most are discordant with the enclosing gneisses. This plutonic event was

largely anorogenic.

PALEOZOIC ROCKS

Lower Paleozoic rocks in the Forest (plate 1) include the

following sedimentary units: Leadville Limestone (Lower

Mississippian), Gilman Sandstone (Mississippian or Devonian), Dyer Dolomite (Mississippian? and Devonian), Parting Sandstone (Devonian), and the Sawatch Quartzite (Upper

Cambrian). The units are only present as isolated outcrops

on the southwestern flank of the Gore Range. The maximum

combined thickness of the lower Paleozoic rocks is less than

350 ft.

Middle and upper Paleozoic rocks in the Forest include

the State Bridge Formation (Lower Triassic to Lower Permian), Goose Egg Formation (Permian), Satanka Shale (Permian), Maroon Formation (Lower Permian to Middle

Pennsylvanian), Weber Sandstone (Lower Permian to Middle Pennsylvanian), the Eagle Valley Evaporite (Middle and

Upper Pennsylvanian), and the Minturn Formation (Middle

Pennsylvanian). These formations contain conglomerate,

sandstone, mudstone, shale, gypsum, anhydrite, and minor

amounts of limestone. Many of the formations are characteristically maroon or red. These formations either pinch out or

are truncated against the western flank of the Gore Range;

only small, isolated outcrops are present in the Forest.

MESOZOIC ROCKS

Mesozoic rocks in the Forest include the following sedimentary units: Lance Formation (Upper Cretaceous), Fox

Hills Sandstone (Upper Cretaceous), Lewis Shale (Upper

Cretaceous), Mesaverde Group (Upper Cretaceous), Pierre

Shale (Upper Cretaceous), Mancos Shale (Upper Cretaceous), Colorado Group (Upper Cretaceous), Dakota Sandstone (Upper and Lower Cretaceous), Morrison Formation

(Upper Jurassic), Curtis Formation (Middle Jurassic), Sundance Formation (Middle Jurassic), Entrada Sandstone

(Middle Jurassic), Nugget Sandstone (Lower Jurassic), Glen

Canyon Sandstone (Lower Jurassic), Chinle Formation (Triassic), and the Chugwater Group (Triassic), (Ku, Ju, and

TrPu, plate 1). The lower of these units consists dominantly

of sandstone and conglomerate; shale becomes more prevalent in the upper part of the section. Mesozoic rocks crop out

in small, elongate, north-trending areas along the flanks of
GEOLOGY 15

the Gore and Park Ranges. Extensive outcrops of Cretaceous rocks are found west of the Gore Range in the Flat

Tops, but only the Mancos Shale crops out within the Forest.

MIDDLE TERTIARY ROCKS

Middle Tertiary rocks are moderately widespread in the

Forest except in the Park and Gore Ranges. Four groups

crop out in the Forest: (1) Oligocene Rabbit Ears volcanic

rocks, (2) Oligocene and late Miocene calc-alkaline

hypabyssal intrusive rocks, (3) late Miocene compositionally diverse intrusive and extrusive rocks, and (4) Miocene

basalt and basaltic andesite flows.

The oldest of the Tertiary igneous rocks in the Forest are

those of the Rabbit Ears volcanic field, which extends southward from the crest of the Rabbit Ears Range into Middle

Park (Tv, plate 1). The Rabbit Ears volcanic rocks were

erupted 30–33 Ma ago (Izett, 1966; Naeser and others, 1973).

The volcanic rocks range in thickness from 0–100 ft in the

southern areas to as much as 800 ft thick in the northern outcrop area. The Rabbit Ears volcanic rocks consist of a complex interlayered sequence of breccias, lahars, tuffs, and a

few thin, discontinuous trachyandesite and trachybasalt lava

flows; the tuffs are generally rhyolitic and are slightly

welded to nonwelded. Fragments in breccia vary widely in

color and texture and range in composition from silicic to

intermediate. Fragments in the south are less than 2 ft in

diameter. In the north, blocks as much as 5 ft in diameter are

common, suggesting a northern source area. Trachyandesite

lava flows are interlayered in the upper part of the sequence,

whereas trachybasalt lavas have been found only at the base.

Most of the Oligocene and late Miocene hypabyssal

intrusive rocks in the Forest crop out in an east-westtrending belt in the Rabbit Ears Range that extends from

Baker Mountain in the western Rabbit Ears Range to the

Never Summer Mountains (Ti, plate 1). Two other occurrences of these intrusive rocks are in or near the Forest: a

medium-sized pluton at the southern end of the Gore Range

and the small Red Mountain plug (associated with molybdenum), just outside the southeastern boundary of the Forest

(pl. 1). The rocks range in age from 22.7 to 28.8 Ma (Izett,

1966; Naeser and others, 1973), although most of them are

Oligocene in age (older than 24 Ma). Compositions range

from dacite to rhyolite. Most of the intrusive rocks are fine

to medium grained and are strongly porphyritic in texture.

 Compositionally diverse late Miocene rocks crop out in

two fairly limited areas: west of the Park Range in the Elkhead Mountains and high in the Park Range in the area

around Walton Peak and Rabbit Ears Pass (To, plate 1).

Most of the rocks in the Elkhead Mountains are 7.6 to 11.5

Ma and occur as hypabyssal stocks, sills, and dikes. Intrusive rocks in the Elkhead Mountains range in composition

from basalt to rhyodacite and include alkalic varieties of

these compositions; the more felsic rocks are concentrated in

the central area around Hahns Peak. Near Walton Peak and

Rabbit Ears Pass, a large area of trachybasaltic volcanic

rocks (Tv, plate 1) is intruded by small, intermediatecomposition porphyries. One porphyry just west of Rabbit

Ears Pass area has an age of 17.0 Ma (Snyder, 1980b); the

trachybasalt flows were erupted between 17 and 20 Ma.

Miocene basaltic and basaltic andesite flows are located

in the southwestern part of the Forest, predominantly in the

area of the Flat Tops (Tb, plate 1). The basalts are part of a

bimodal assemblage that includes small rhyolitic dikes and

flows on the eastern side of the Flat Tops. The basaltic rocks

are dense, black, and alkalic and form flows 5 to 200 ft thick;

they include interbedded tuffs and volcanic conglomerates.

LATE CRETACEOUS AND TERTIARY ROCKS

Late Cretaceous and Tertiary rocks in the Forest include

the following Formations: Middle Park Formation

(Paleocene and late Cretaceous?), Wasatch Formation

(Eocene and Paleocene), Coalmont Formation (Eocene and

Paleocene), Fort Union Formation (Paleocene), White River

Formation (Oligocene), Troublesome Formation (Miocene

and Oligocene), Browns Park Formation (Miocene), and

North Park Formation (Miocene) (Tks, plate 1). These units

were deposited in local structural basins and small grabens

that formed during Tertiary time. The rocks consist of claystone, siltstone, limestone, sandstone, and conglomerate;

locally they contain beds of volcanic ash. Extensive

outcrops are present to the west of the Park Range in the

Sand Wash Basin and east of the Park and Gore Ranges in

North and Middle Parks.

QUATERNARY AND PLIOCENE(?) 

UNCONSOLIDATED DEPOSITS

Holocene alluvium in drainages and fans across the

Forest consists of gravel, sand, and silt. Extensive landslides

of Holocene and Pleistocene age occurred along the east side

of the Gore Range and in the Elkhead Mountains. The landslide deposits consist of shaly material with variable

amounts of boulders of sandstone and basalt. Pleistocene

glacial till and outwash in the Gore Range consist of

boulders, gravel, and sandy deposits. Pliocene(?) gravel

deposits are found on Gravel Mountain in the eastern part of

the study area near the Continental Divide. 

COLORADO MINERAL BELT

In Colorado, most of the important hydrothermal

mineral deposits are part of an elongate zone known as the

Colorado Mineral Belt (Tweto and Sims, 1963) (fig. 5),

which extends from the San Juan Mountains in southwestern
16 MINERAL RESOURCE POTENTIAL AND GEOLOGY, ROUTT NATIONAL FOREST, COLORADO

107°30' 107°00' 106°30' 106°00'

41°00'

40°30'

40°00'

39°30'

COLORADO

Craig

Steamboat

Springs

Eagle

Vail

Yampa

Kremmling

Walden

Lake

Granby

Rabbit Ears Range

Williams Fork Mountains

Gore Range

Park Range

Medicine Bow Mountains

Colorado River

Colorado River

0 5 10 MILES

WYOMING

COLORADO

Yampa River

Yampa

ivR

er

Approximate boundary of the

Routt National Forest and the

Middle Park Ranger District

of the Arapaho National Forest

Approximate boundary of the

Routt National Forest and the

Middle Park Ranger District

of the Arapaho National Forest

Approximate boundary of the

Routt National Forest and the

Middle Park Ranger District

of the Arapaho National Forest

Colorado Mineral Belt

1PK 2

3 4 DP

6

BL7

5

BG BN

FC

BC

SC

MB

SC

8

SH

10CM

CC

9 PM

11

12

13 VP

RE

EXPLANATION

 Elkhorn Mountain

 Pearl

 Hahns Peak

 Farwell Mountain

 Greenville

 Upper and lower Slavonia

 Crystal

 Northgate

 Poison Ridge

 Teller

 La Plata

 Dailey

 Henderson-Urad 

1

2

3

4

5

6

7

8

9

10

11

12

13

Beaver Creek

Big Creek

Bear Creek–Lone Pine Creek 

Beaver Creek–Norris Creek

Cam Claims

Mt. Cumulus

Diamond Park

Fish Creek Reservoir

Morrison Creek–Beaver Creek

Peak 9731

Parkview Mtn

Service Creek

Spencer Heights

Vasquez Peak

BC

BG

BL

BN

CC

CM

DP

FC

MB

PK

PM

SC

SH

VP

Mining districts Mineralized areas

Figure 5. Index map showing mining districts and mineralized areas within or adjacent to the Routt National Forest and Middle

Park Ranger District of the Arapaho National Forest, Colorado.
GEOLOGY 17

Colorado to the eastern margin of the Front Range, northwest of Denver. The belt contains a large number of mineral

deposits associated with numerous felsic to intermediate

intrusive and volcanic rocks of Late Cretaceous to late Tertiary age. The Colorado Mineral Belt cuts across both preCretaceous structural trends and the present north-south

topographic grain of the southern Rocky Mountains. Only

the southeastern part of the Forest falls within the Colorado

Mineral Belt.

STRUCTURAL GEOLOGY

Rocks in the Forest have a complex structure that

reflect events in Proterozoic, Paleozoic, early Mesozoic,

and late or post-Miocene time. Many of the major structures were affected by recurrent movements and intrusion

of Proterozoic and Tertiary plutons.

Three major systems of Proterozoic faults and shear

zones are recognized in the Forest (pl. 1) (Tweto, 1980). A

fault system trending north-northwest is widespread; some

elements of this system originated before the emplacement

of the 1.7-Ga plutons, but other elements postdate emplacement of the 1.4-Ga granitic plutons. The Gore fault on the

west side of the Gore Range is typical of these structures

(pl. 1).

A system trending northeast is expressed mainly by

strongly developed shear zones that cut across the trend of

major mountain ranges. This system was active principally

during and following intrusion of the 1.4-Ga granitic suite,

although it may have originated earlier (Tweto, 1980). The

shear zone in the northern Park Range between Soda Creek

and North Fork Fish Creek is one of the most well developed of these zones in the Forest (pl. 1). Rocks in these

zones are typically ground or “mylonitized” into very fine

grained rocks.

The third system of faults is an east-west-trending

fault system that parallels major lithologic contacts in

basement rock; this system is only present in the northernmost part of the Forest close to the Wyoming border. To

the north of the Forest in Wyoming, a similar structure

called the Mullen Creek–Nash Fork shear zone separates

Archean basement rocks north of the zone from Early

Proterozoic rocks south of the zone. The shear zone is

interpreted as a collision boundary between the Archean

craton to the north and Proterozoic island-arc terrane to the

south (Hills and Houston, 1979).

Uplift of the Ancestral Rocky Mountains reactivated

Proterozoic structures, such as the Gore fault, and formed

new faults. The Gore fault bounded the western side of the

Ancestral Front Range; aprons of coarse arkosic sediment

were deposited next to the fault in the central Colorado

trough.

Tectonic uplift, folding, and faulting were also associated with the Laramide orogeny in latest Cretaceous time.

Several of the major Laramide uplifts occupy the sites of

late Paleozoic uplifts. The Medicine Bow, Park, and Gore

Ranges occupy the site of the late Paleozoic Front Range

highland (Tweto, 1980). Most of the uplifts were formed

by reactivation of late Paleozoic and Proterozoic faults, but

new faults were also formed, such as the Williams Fork

Range thrust fault (pl. 1) . The thrust faults were originally

interpreted to be near-surface expressions of steep faults

formed by vertical forces (Tweto, 1980), but recent

research indicates that many thrust faults flatten beneath

mountain uplifts and were formed by horizontal forces

(Erslev and Rogers, 1993). Large, deep structural basins

formed concurrently with the Laramide uplifts, and sediments deposited in the basins are a principal record of the

Laramide orogeny. North Park, Middle Park, and Sand

Wash Basin are examples of these basins (pl. 1).

Laramide tectonism waned in the Eocene and was

replaced by erosion, sedimentation, uplift, local folding,

and normal faulting related to crustal extension. Block

faulting reactivated many Laramide and Proterozoic faults,

renewing uplift of many of the mountain ranges. Several

faults of late Miocene age occur along the western side of

the Park Range. The Steamboat Springs fault has as much

as 900 ft of movement (Izett, 1975). Extension formed the

north-northwest-trending Rio Grande rift zone, which may

extend as far north as Wyoming (Tweto, 1979).
19

GEOCHEMISTRY

By Steven M. Smith

GEOCHEMICAL SURVEYS

Geochemical data used in the assessment of the Forest

were obtained primarily from samples collected and analyzed

for previous mineral assessments. Sample data are in the U.S.

Geological Survey National Geochemical Database

(described by Hoffman and Marsh, 1994). These data were

derived from USGS analyses of 1,701 rock, 2,460 streamsediment, and 475 heavy-mineral-concentrate samples, taken

for previous resource assessments of Wilderness Areas (table

3), and National Uranium Resource Evaluation (NURE)

project analyses of 3,256 stream-sediment samples taken for

uranium resource assessments of 1°×2° quadrangles (table

3). Data for another 206 stream-sediment samples from the

Hahns Peak and Pearl mining districts (Allen, 1982) also

were added; altogether, 8,098 samples are represented. The

NURE data were combined from the Cheyenne, Craig, Denver, Greeley, Leadville, and Rawlins 1°×2° quadrangle

reconnaissance studies and from detailed follow-up studies in

the Craig and Rawlins 1°×2° quadrangles (table 3). Published data were included from USGS mineral resource

assessments of the Flat Tops Primitive Area, the St. Louis

Peak, Service Creek, and Williams Fork Roadless Areas, the

Neota–Flat Top, Never Summer, Mt. Zirkel, Rawah, and

Vasquez Peak Forest Service Wilderness Study Areas (table

3). Unpublished data were included from analyses performed

in support of USGS geologic mapping throughout the region.

METHOD OF STUDY

The data contain analytical values for 62 different elements, although no single sample was analyzed for all of

these elements. Eighteen elements commonly associated

with mineral deposits were selected for primary use in this

study: antimony, arsenic, bismuth, cadmium, chromium,

cobalt, copper, gold, lead, mercury, molybdenum, nickel, silver, tin, tungsten, uranium, vanadium, and zinc.

The use of three separate sample media, analyzed by

various techniques at different laboratories, precluded simply combining raw data into one interpretable database. For

each sample medium, data were separated by project (table

3) and analytical method, and were divided into five classes;

low background, high background, slightly anomalous,

moderately anomalous, and highly anomalous. The four

threshold values (table 4) between these five classes were

determined by examining data frequency histograms in conjunction with crustal-abundance data (Parker, 1967; Fortescue, 1992), spatial distribution patterns, multielement

correlations, and geology. In most cases, the four threshold

values were the same or similar for samples from projects

analyzed by the same laboratory and method. Exceptions

are noted in table 4.

Data that fall within each class were assigned a

“weight” value from 0 to 4—low background (0), high

background (1), slightly anomalous (2), moderately anomalous (3), and highly anomalous (4). A new data set, composed of the assigned weight values for each element and

sample medium, was created. Point-plot maps were made

from the new data set for all 18 elements. Areas containing

samples with “weight” values of 2, 3, or 4 were identified as

geochemically anomalous. This method allowed interpretations to be made from geochemical maps that simultaneously displayed all available element data. Multielement

suite maps also were created by plotting the sums of

selected element weights. These maps were used to distinguish geochemically anomalous areas that have mineral

potential from areas that have anomalies unrelated to mineral deposits.

Most of the samples containing elevated concentrations

of base and precious metals and related elements are associated with known mining districts and mineral deposits. The

following section describes geochemical anomalies within

and proximal to the Forest. Most descriptions are given

with respect to areas of known deposits; other anomalies are

referenced to local prominent geographic features. The general locations of the anomalous areas are shown on figure 3,

and mining districts are shown on figure 5; more detailed

location information is on plate 1.
Table 4. Threshold values used to divide concentration ranges of elements in different sample media into five classes: low backgrou

anomalous, moderately anomalous, and highly anomalous. [Values in parts per million. (a), threshold of high background class (lowest high background value); (b), threshold of slightly anomalous class (lowe

threshold of moderately anomalous class (lowest moderately anomalous value): (d), threshold of highly anomalous class (lowest highly anomalous valclasses below the lower detection limits of the analytical method; aa, threshold values for additional analyses by atomic absorption spectroscopy] NURE stream- USGS stream- Other stream- Heavy-mineral- Element sediment samples sediment samples sediment samples1 concentrate samples 

 (a) (b) (c) (d) Ag ...... -- -- 2 10 0.5 1 5 10 0.5 1 5 10 -- 2 3 50 0

As ...... 12 20 30 50 10 20 30 40 n.a. -- 140 200 1000 -Au ...... -- -- 0.04 0.5 -- -- 0.02 0.5 n.a. -- -- -- 50 -

0.03 0.05 0.5 2 aa 0

Bi ...... n.a. -- -- -- -- n.a. -- 14 20 100 -Cd ...... -- -- 5 7 -- -- -- -- n.a. -- -- -- -- -Co2 ..... 16 25 50 75 30 50 70 100 12 18 30 70 50 100 300 700 2Cr2 ..... 100 200 300 800 200 500 1000 5000 40 70 100 500 200 500 1000 5000 10Cu ...... 40 60 100 500 40 60 100 500 40 60 100 500 30 50 100 500 5Hg ...... n.a. n.a. n.a. n.a. 0Mo3 ..... -- -- -- 4 3 5 10 20 n.a. 10 50 100 500 Ni ...... 35 65 100 500 70 100 200 500 30 70 100 500 30 70 200 700 7Pb2 ..... 20 35 100 200 35 55 100 200 20 35 100 200 200 500 1000 2000 5Sb ...... -- 2 4 9 -- -- -- -- n.a. -- -- 140 200 -Sn ...... -- 10 16 30 -- 10 16 30 n.a. 20 50 100 300  U ...... 7 15 30 60 n.a. n.a. n.a.  V ...... 120 160 250 400 200 300 500 700 n.a. 200 500 700 1000 15 W ...... -- 15 20 40 -- -- 30 50 n.a. 70 150 500 1000 3Zn ...... 150 175 250 500 90 150 250 500 90 150 250 500 -- 300 700 2000 7

1 Stream-sediment samples from Allen (1982) 2 Co, Cr, and Pb thresholds used for USGS stream-sediment samples from the Service Creek Roadless Area are 50-70-100-150 (Co); 300-700-1000-5000 (Cr); and 100-13 Mo thresholds used for USGS stream-sediment samples from the Rawah and Never Summer Wilderness Areas are 3-7-15-20. (a) (b) (c) (d) (a) (b) (c) (d) (a) (b) (c) (d) (a
GEOCHEMISTRY 21

RESULTS

HAHNS PEAK MINING DISTRICT

Samples from the Hahns Peak mining district (fig. 5)

and surrounding gold placers contained anomalous

concentrations of antimony, arsenic, lead, molybdenum,

silver, and zinc (for locations, see Snyder and others,

1981). Silver and arsenic were found at highly anomalous

concentrations in stream-sediment samples; zinc, lead,

molybdenum, and antimony also were determined to be

moderately anomalous in at least one sample. One rock

sample had moderately anomalous concentrations of

arsenic and antimony. The anomalies are probably related

to disseminated silver-lead-zinc mineral deposits associated

with the Tertiary Hahns Peak porphyry stock (Young and

Segerstrom, 1973).

CAM CLAIMS AREA

Samples from this area in the Never Summer Mountains contained anomalous concentrations of arsenic, bismuth, molybdenum, and zinc (for locations, see Pearson

and others, 1981). One stream-sediment sample contained

500 parts per million (ppm) zinc, which is considered

highly anomalous. Two rock samples contained anomalous

concentrations of arsenic and molybdenum; bismuth and

zinc also were found at moderately anomalous concentrations in rock samples. The Cam claims are in exposed

mineralized Proterozoic gneiss.

HENDERSON-URAD MOLYBDENUM AREA

Highly anomalous concentrations of lead, tin, tungsten,

and zinc and moderately anomalous concentrations of gold

and copper are in stream-sediment samples around the Henderson and Urad molybdenum deposits (fig. 5) (for locations,

see Theobald and others, 1985; Barton, 1985c). Each of the

stream-sediment samples contained zinc in concentrations

greater than 1,000 ppm and are among the highest streamsediment zinc values in the study area. These deposits are

classic examples of Climax-type molybdenum porphyry

deposits (White and others, 1981).

MT. CUMULUS

Stream-sediment and rock samples collected on the

west side of Mt. Cumulus in the Never Summer Mountains

contained elevated concentrations of arsenic, bismuth, lead,

molybdenum, tin, silver, and zinc (for locations, see Pearson and others, 1981). Zinc concentrations were anomalously high in most of the stream-sediment samples, and

one rock sample contained greater than 1 percent Zn.

Stream-sediment samples were characterized by an

association of moderately to highly anomalous concentrations of zinc, lead, molybdenum, and tin plus slightly

anomalous silver. Geochemically anomalous rock samples

were distinguished by zinc-cadmium-lead, tin-arsenic,

molybdenum-arsenic, or lead-silver-bismuth associations.

The anomalies in the area are thought to be associated with

the Tertiary Mt. Cumulus stock.

VASQUEZ PEAK MOLYBDENUM ANOMALOUS AREA

The area of the Vasquez Mountains (fig. 3) is

characterized by high background concentrations of molybdenum and tin in rock and stream-sediment samples with

moderately to highly anomalous concentrations of arsenic,

bismuth, gold, lead, molybdenum, silver, and tin (for

locations, see Theobald and others, 1985; Barton, 1985c).

Slightly to moderately anomalous concentrations of molybdenum are in several rock and heavy-mineral-concentrate

samples. One rock contained an anomalous concentration

of tin, and several heavy-mineral-concentrates contained

moderately to highly anomalous tin. Slightly anomalous

concentrations of lead were determined in rock and heavymineral-concentrate samples. Arsenic was highly

anomalous in two rock samples. Several heavy-mineral

concentrates contained moderately to highly anomalous

concentrations of silver, gold, or bismuth. One concentrate

sample contained the highest gold and silver values (300

ppm Au and 200 ppm Ag) in samples from the Forest. A

single stream-sediment sample in the same area contained

0.48 ppm gold. The anomalies are probably associated

with a Tertiary stock in Proterozoic granite.

BEAR CREEK–LONE PINE CREEK ANOMALOUS AREA

Three rock samples in this area on the northeast side of

the Park Range contained anomalous concentrations of

cobalt, copper, nickel, and chromium (for locations, see

Snyder and others, 1981). Another rock had anomalous

concentrations of bismuth, copper, chromium, and silver.

These anomalies are associated with an exposure of

Proterozoic peridotite.

ELKHORN MINE AREA

The anomaly at the Elkhorn mine just south of the

Wyoming border in the Park Range (Elkhorn Mountain

mining district; fig. 5) is characterized by elevated

concentrations of silver, gold, cadmium, copper, mercury,

lead, antimony, and zinc in six rock samples (for locations,

see Snyder and others, 1981). The analyzed rocks

contained as much as 500 ppm cadmium, 3,000 ppm silver,
22 MINERAL RESOURCE POTENTIAL AND GEOLOGY, ROUTT NATIONAL FOREST, COLORADO

1,000 ppm antimony, 1 percent copper, 1 percent lead, and

20 percent zinc. Several stream-sediment samples have

been collected in the Elkhorn mine area; none of these

samples contained anomalous metal concentrations. The

anomalies are associated with mineralized pods and veins in

Proterozoic gabbro in the vicinity of the Elkhorn mine.

RED CANYON–CRYSTAL MINING DISTRICT

Ten rock samples from the Red Canyon–Crystal district

(fig. 5) have moderately to highly anomalous concentrations

of arsenic, molybdenum, and mercury (for locations, see

Snyder and others, 1981) . These anomalies are probably

related to fault zones in Proterozoic granite that have been

prospected for fluorite and uranium.

SPRING CLAIMS AREA

Two spring-sediment samples collected for the National

Uranium Resource Evaluation program east of the Park

Range in North Park contained highly anomalous concentrations of uranium, including the maximum value (385 ppm U)

in the Forest geochemical database (for locations, see Bolivar and others, 1979; Craig and others, 1982). The Spring

claims cover an area with uranium enrichment in peat.

DIAMOND PARK AREA

Geochemical samples from the Diamond Park area in

the northern part of the Park Range contained anomalous

concentrations of bismuth, copper, lead, molybdenum,

silver, and tungsten (for locations, see Snyder and others,

1981). Bismuth, copper, lead, and molybdenum values were

moderately to highly anomalous in rock samples. Silver and

tungsten were anomalous in rock and stream-sediment

samples. Known deposits in the area consist of small

mineralized quartz veins associated with fault zones in

Proterozoic gneiss.

BEAVER CREEK AREA (FARWELL MOUNTAIN

MINING DISTRICT)

Geochemical samples from the Beaver Creek area of the

Farwell Mountain mining district just east of Hahns Peak

contained elevated concentrations of arsenic, bismuth,

copper, gold, lead, molybdenum, tin, tungsten, silver, and

zinc (for locations, see Snyder and others, 1981). Rock

samples were characterized by moderately to highly anomalous concentrations of copper, bismuth, tin, molybdenum,

gold, silver, and tungsten, and low concentrations of lead and

zinc. Several stream-sediment samples were characterized

by anomalous zinc, lead, and silver concentrations. Molybdenum was also anomalous in two stream-sediment samples.

The anomalies are probably associated with mineralized

Proterozoic pegmatite, schist, and gneiss.

DAILEY (ATLANTIC) MINING DISTRICT

Geochemical anomalies of copper, lead, molybdenum,

silver, tin, and zinc are in the Dailey mining district (fig. 5)

(for locations, see Theobald and others, 1985; Barton,

1985c). Rock samples contained weakly anomalous concentrations of molybdenum, lead, and silver, and isolated

moderately anomalous concentrations of lead, tin, and zinc.

One heavy-mineral-concentrate sample was highly anomalous in copper, tin, and zinc and moderately anomalous in

silver, molybdenum, and lead. Other heavy-mineralconcentrate samples contained moderate to high singleelement anomalies of silver, copper, lead, or zinc. These

geochemical anomalies are associated with mineralized

shears and faults in Middle Proterozoic Silver Plume

Granite.

KING SOLOMON MINE AREA (FARWELL MOUNTAIN 

MINING DISTRICT)

Three rock samples from the King Solomon mine area

just east of Hahns Peak contained moderately to highly

anomalous concentrations of copper, bismuth, zinc, lead,

and silver (for locations, see Snyder and others, 1981).

One stream-sediment sample also contained anomalous

silver. The King Solomon mine exposes mineralized

Proterozoic gneiss.

LA PLATA MINING DISTRICT

Highly anomalous concentrations of antimony, arsenic,

cadmium, copper, lead, molybdenum, silver, tin, and zinc are

in several rock samples from this district (fig. 5) (for locations, see Theobald and others, 1985; Barton, 1985b). Three

rocks contained 1 to 1.5 percent lead, five rocks had greater

than 1 percent zinc, five rocks had 1 percent or greater

arsenic, and two rocks contained high silver (1,000 and 1,500

ppm Ag). Heavy-mineral-concentrate samples collected in

this area have slightly to moderately anomalous concentrations of copper, molybdenum, and tin. The anomalies are

associated with mineralized faults and shear zones in Middle

Proterozoic Silver Plume Granite.

TELLER MINING DISTRICT

Numerous geochemical anomalies are in the Teller

mining district (fig. 5) (for locations, see Pearson and others,

1981). Elements with elevated concentrations include

antimony, arsenic, cadmium, copper, gold, lead,
GEOCHEMISTRY 23

molybdenum, silver, tin, and zinc. Silver was present in

moderately to highly anomalous concentrations in rock and

stream-sediment samples. The district also contains

moderately to highly anomalous molybdenum in rock

saples; one sample contained greater than 2,000 ppm. Analyses of five rocks showed arsenic concentrations that were

greater than 10 percent. Two rock samples contained greater

than 500 ppm cadmium. Concentrations of antimony range

as high as 1,500 ppm in rock samples. Lead was highly

anomalous in several rock samples (four determinations

were greater than 1.5 percent) and in stream-sediment samples. Six rock samples contained zinc in quantities from 1 to

12 percent; several stream-sediment samples were also

highly anomalous in zinc. Gold, copper, and tin were at

anomalous concentrations in rock samples. Most streamsediment samples from the district contained anomalous

concentrations of silver, lead, and zinc; scattered streamsediment samples also contained anomalous concentrations

of arsenic, cobalt, copper, and molybdenum. The Teller district mines and prospects were excavated in mineralized

fracture zones and veins in Proterozoic granite and schist.

Silver was probably the primary target of exploration

(Pearson and others, 1981).

RED ELEPHANT MOUNTAIN AREA

One rock sample collected from this area in the northeast part of the Park Range near Mt. Zirkel contained anomalous concentrations of bismuth, copper, gold, lead, and

silver (for locations, see Snyder and others, 1981). None of

the stream-sediment samples from this area were anomalous

in metals. The anomalous rock came from a mineralized

zone in steeply dipping layered gneiss (Patten, 1987).

BEAVER CREEK–NORRIS CREEK AREA

Anomalous concentrations of molybdenum and

uranium were determined in stream-sediment samples from

this area on the east side of the Park Range (for locations, see

Snyder and others, 1981). No rock samples with anomalous

metals were analyzed from the area. The anomalies are

probably associated with local uranium enrichment in the

Upper Jurassic Morrison Formation.

BEAVER CREEK AREA

Elevated uranium and molybdenum concentrations

were found in rocks from the Beaver Creek area southeast of

Kremmling (for locations, see Snyder and others, 1981).

The maximum uranium value of the entire data set was at this

location (1180 ppm U). Four other rock samples from this

area contained anomalous concentrations of uranium. 

Molybdenum was found at moderately to highly

anomalous concentrations in five rock samples. The anomalies are probably associated with exposures of uraniumenriched Upper Cretaceous(?) and Paleocene Middle Park

Formation.

SPENCER HEIGHTS AREA

Rock samples from the Spencer Heights area east of the

Forest in the Cache la Poudre contained highly anomalous

concentrations of uranium, as much as 872 ppm (for locations, see Pearson and others, 1981). The anomaly is associated with uranium mineralization in a broad shear zone and

pegmatite dikes cutting granitic gneiss.

GREENVILLE MINE AREA

Several geochemical samples from the Greenville

mine and vicinity (Greenville mining district; fig. 5) contained elevated concentrations of antimony, bismuth, cadmium, copper, lead, silver, tin, tungsten, and zinc (for

locations, see Snyder and others, 1981). Copper, lead, and

zinc were each in rock samples at concentrations greater

than 10 percent. Moderately and highly anomalous concentrations of silver, bismuth, cadmium, antimony, and tin

were in selected rock samples. A few rocks contained

anomalous concentrations of gold, and almost every rock

contained molybdenum in slightly anomalous concentrations. Thirteen stream-sediment samples were collected by

Snyder (1987a) downstream from the Greenville mine. At

a distance of about 1 km downstream from the mine, the

concentrations of copper, lead, and silver in these samples

dropped from highly anomalous to low background levels;

zinc was still slightly anomalous 2 km downstream. This

suggests, at least within the Park Range, that streamsediment anomalies are locally derived. The anomalies at

the Greenville mine are associated with a metamorphosed

massive sulfide deposit (Snyder, 1987a).

LOWER SLAVONIA MINING DISTRICT

This area in the headwaters of Gilpin Creek in the Park

Range contains geochemical anomalies of antimony,

bismuth, cadmium, copper, gold, lead, silver, tin, and zinc

(for locations, see Snyder and others, 1981). Rock samples

contained highly anomalous concentrations of each of these

elements. Only one stream-sediment sample from the area

had anomalous copper concentrations.

UPPER SLAVONIA MINING DISTRICT

The Upper Slavonia mining district in the headwaters of

Gilpin Creek in the Park Range contains enriched
24 MINERAL RESOURCE POTENTIAL AND GEOLOGY, ROUTT NATIONAL FOREST, COLORADO

concentrations of antimony, bismuth, cadmium, copper,

gold, lead, mercury, molybdenum, silver, tin, and zinc (for

locations, see Snyder and others, 1981). Rock samples contained anomalously high values for many of these elements;

greater than 500 ppm cadmium, 2 percent copper, 1.5 percent

lead, 19 percent zinc, and greater than 10 ppm mercury.

Three stream-sediment samples had anomalous concentrations of copper, lead, or zinc.

PEARL MINING DISTRICT

Geochemical samples from the Pearl mining district

(fig. 5), and the associated mineralized area on Independence

Mountain, contained anomalous concentrations of bismuth,

cadmium, copper, gold, lead, mercury, silver, tin, tungsten,

and zinc (for locations, see Snyder and others, 1981). Most

of the rock samples from the area were highly anomalous in

copper, lead, zinc, silver, gold, and bismuth. A few rock

samples were moderately or highly anomalous in cadmium,

mercury, or tungsten. Stream-sediment samples were

slightly to moderately anomalous in zinc and silver, with a

few samples having slightly anomalous concentrations of

copper, lead, and tungsten.

POISON RIDGE AREA (A.O. PORPHYRY

COPPER DEPOSIT)

No geochemical samples from the Poison Ridge area

(Poison Ridge mining district; fig. 5) were in the database.

Kinney and others (1968) report anomalous molybdenum,

lead, and copper in rock samples as well as very anomalous

concentrations of copper, lead, zinc, and molybdenum in

stream-sediment samples. The anomalies are related to the

A.O. porphyry copper deposit, associated with a Tertiary

quartz latite porphyry stock. Karimpour (1982) reported that

rock samples from exploration drill holes contained anomalous concentrations of copper (as much as 4,400 ppm),

molybdenum (140 ppm), gold (0.45 ppm), silver (2.74 ppm),

and tungsten (10 ppm).

PEAK 9731–ELKHORN COMPLEX AREA

This area near the Wyoming border in the Park Range

contains anomalous concentrations of cobalt, chromium, and

nickel (for locations, see Snyder and others, 1981). Rock and

stream-sediment samples show the same geochemical signature of anomalous chromium and cobalt associated with

slightly anomalous nickel. The anomaly is associated with

gabbros and peridotite in the Elkhorn igneous complex.

BIG CREEK AREA

A weak geochemical anomaly of slightly elevated concentrations of antimony, arsenic, cadmium, copper, gold,

mercury, tungsten, vanadium, and zinc was found in the Big

Creek area on the western side of the Park Range (for locations, see Snyder and others, 1981). Two rock samples

have anomalous concentrations of vanadium, cadmium, and

zinc. Stream-sediment samples were characterized by

slightly anomalous concentrations of arsenic, gold, copper,

mercury, antimony, tungsten, and zinc. The anomalies are

probably related to massive sulfide mineralization in

Proterozoic schist and layered gneiss.

CONTINENTAL DIVIDE–FISH CREEK RESERVOIR 

ANOMALOUS AREA

A single rock sample collected on or near the Continental Divide west of Fish Creek Reservoir contained highly

anomalous concentrations of gold, cobalt, molybdenum, and

vanadium; moderately anomalous concentrations of silver

and arsenic; and slightly anomalous concentrations of copper

and nickel (for locations, see Snyder and others, 1981). No

additional information about the source of this mineralized

sample or the type of rock analyzed is available in the

database.

MORRISON CREEK–BEAVER CREEK ANOMALOUS 

AREA

Two stream-sediment samples from the vicinity of Morrison and Beaver Creeks, (8 mi west of Yampa, Colo.) contained highly anomalous concentrations of silver, arsenic,

vanadium, and zinc with moderately anomalous concentrations of cobalt, tin, and tungsten (for locations, see Schmidt

and others, 1984). The concentration of silver here is one of

the highest values for the Forest study area (69 ppm). No

sources for these anomalies are known.

OTHER OCCURRENCES

Silver.—The maximum silver value (79 ppm) in the

stream-sediment data set was in a NURE sample from the

Service Creek drainage within the Service Creek Roadless

Area. Two more samples in the Silver Creek drainage,

within the Service Creek Roadless Area, contained anomalous silver values. The source of these anomalies is not

known. Three NURE stream-sediment samples with anomalous concentrations of silver were collected on the northeast

flank of Independence Mountain, west of State Highway 125

and the Forest boundary. Possibly, silver may have come

from the conglomerates on the crest of Independence Mountain, which may also have been the source for gold placers on

the southwest flank (Hail, 1965).

Arsenic.—Two stream-sediment samples with anomalous arsenic values were collected in the Pagoda Peak–Sand
GEOCHEMISTRY 25

Peak area northwest of the Flat Tops Primitive Area. No

sources for the anomalies are known.

Bismuth.—One rock sample, from the north side of Buffalo Mountain in the Soda Creek drainage basin, contained

anomalous concentrations of bismuth, copper, and mercury.

The source of the metal anomaly is unknown. Several

claims and prospects in this area are assumed to have been

located for uranium exploration (Snyder, 1987a).

Cadmium.—Localities of stream-sediment samples

with slightly anomalous values of cadmium are scattered

widely throughout the Forest. Almost none of these localities coincide with areas of known mineral deposits or with

localities containing anomalous cadmium in rock samples.

The anomalous stream-sediment samples were analyzed by

the same laboratory and method. The anomalous cadmium

sample distribution pattern and the lack of correlation with

known cadmium occurrences suggests that these anomalies

are random and possibly due to analytical variation near the

lower determination limit of the analytical method.

Cobalt.—A broad cobalt anomaly in NURE streamsediment samples occurs in the Flat Tops Primitive Area.

These samples are associated with Tertiary basalt flows that

cover the area (Tweto, 1976; Tweto and others, 1978). A

similar cobalt anomaly is associated with exposures of the

Proterozoic Elkhorn igneous complex in the northern part of

the Forest. Mafic rock types, including the basalts in the Flat

Tops Primitive Area and the gabbro and peridotite sequences

in the Proterozoic Elkhorn igneous complex, are commonly

enriched in cobalt, chromium, nickel, and vanadium. A

cobalt anomaly over exposures of these rock types may not

be related to unusual or economic concentrations of cobalt or

associated elements but merely represents high background

levels of cobalt. Several rock samples in the area of Simpson

Mountain between Service Creek and Silver Creek within

the Service Creek Roadless Area contained slightly anomalous concentrations of cobalt. The source of these anomalies

is not known.

Chromium.—The distribution of elevated chromium in

stream-sediment samples is similar to that of cobalt, nickel,

and vanadium and correlates with exposures of Tertiary

basalt in the Flat Tops Primitive Area and exposures of the

Elkhorn complex. Chromium values also were slightly to

moderately anomalous in rock samples in the Simpson

Mountain area. These anomalous chromium values are

attributed to high background in mafic rocks.

Copper.—One rock sample with a highly anomalous

copper concentration and a moderately anomalous mercury

concentration was collected from the vicinity of the Continental Divide just west of Round Mountain near the southern boundary of the Mt. Zirkel Wilderness. The source of

these anomalies is not known. Another rock sample, collected on the north side of Buffalo Mountain in the Soda

Creek drainage basin contained anomalous concentrations

of copper, bismuth, and mercury. The source of the

anomalous rock is unknown. Another rock sample with

anomalous copper concentration was collected from the

Kelly Lake area just west of the Rawah Wilderness.

Pearson and others (1982) describe an outcrop of copperbearing rock at this locality. Copper Ridge, just north of

Steamboat Springs, is the source of an additional rock

sample with anomalous copper.

Gold.—The maximum value for gold in the streamsediment data set (7.42 ppm Au) was determined in a sample

from the drainage basin just south of Threemile Creek on the

northeast flank of Independence Mountain. The source of

this gold is probably the same conglomerates thought to be

responsible for the placers on the southwest side of Independence Mountain (Hail, 1965). Another stream-sediment

sample that contained a highly anomalous concentration of

gold (1.06 ppm) was collected outside of the Forest in the

Soda Creek drainage basin, 1 mi north of Steamboat Springs.

The source of this gold is not known. Two stream-sediment

samples from the northeast flank of Bear Mountain and just

outside the Mt. Zirkel Wilderness contained anomalous gold

values. The source for these samples may be mineralized

rock similar to that in the Upper Slavonia district just west of

the Continental Divide. Two stream-sediment samples, collected southwest of Rand and in the Willow Creek drainage

basin just north of the Forest boundary, contained anomalous

concentrations of gold. The source of this gold is probably

in the upper Willow Creek drainage basin and may lie within

the Forest.

Lead.—A cluster of anomalous lead concentrations in

heavy-mineral-concentrate samples was found in the headwaters of Keyser Creek within the St. Louis Peak Roadless

Area. These samples were also anomalous in tungsten and

zinc. Theobald and others (1985) suggests that these anomalies are associated with small massive sulfide deposits.

Mercury.—One rock sample from the Continental

Divide, just west of Round Mountain, contained moderately

anomalous concentrations of mercury and copper. A group

of four rock samples from the north side of Buffalo Mountain in the Soda Creek drainage basin contained slightly to

moderately anomalous concentrations of mercury. One of

these samples was also anomalous for copper. The sources

of these anomalies are unknown.

Molybdenum.—The entire southern half of the Rawah

Wilderness contains slightly to moderately anomalous concentrations of molybdenum in stream-sediment samples.

Pearson and others (1982) suggested that the Proterozoic

granites in the Wilderness Area contained elevated concentrations of molybdenum. Four stream-sediment samples

with anomalous concentrations of molybdenum were collected in the Silver Creek drainage basin within the Service

Creek Roadless Area. No source is known for these anomalies. One rock sample in the Northgate fluorite mining district (fig. 5) was highly anomalous in molybdenum. Similar

molybdenum enrichment was associated with the fluorite

deposits in the Red Canyon–Crystal mining district (fig. 5).
26 MINERAL RESOURCE POTENTIAL AND GEOLOGY, ROUTT NATIONAL FOREST, COLORADO

Nickel.—Almost every stream-sediment or rock sample

with elevated nickel concentrations collected in the Forest

region is associated with the common mafic rock geochemical signature of cobalt-nickel-chromium-vanadium. The

Tertiary basalts in the Flat Tops Primitive Area and the

Elkhorn complex are delineated by anomalous nickel concentrations in stream-sediment samples. The Simpson

Mountain area also has slightly anomalous nickel concentrations in rock samples.

Tin.—A number of stream-sediment samples collected

in Big and Little Red Parks northwest of Hahns Peak (fig.

5) contained anomalous tin values. The Fish Creek area

west of Steamboat Springs contains three stream-sediment

sample localities and one rock sample locality with anomalous tin. Several rock samples and two stream-sediments

samples that were collected in the Service Creek Roadless

Area between Service Creek and Silver Creek contained

anomalous concentrations of tin. No sources are known for

these anomalies.

Uranium.—Five rock samples collected along the west

side of lower Troublesome Creek contained anomalous

concentrations of uranium and molybdenum. These rocks,

from the upper Oligocene and Miocene Troublesome

Formation of Middle Park, may reflect sandstone-hosted

uranium enrichment. One stream-sediment sample containing anomalous concentrations of uranium and zinc was in

the Agua Fria area. The uranium is associated with a

pegmatite in Proterozoic quartz monzonite (Snyder, 1987a).

One rock sample, containing 896 ppm uranium, was collected just north of U.S. Highway 40 before the descent into

the Harrison Creek drainage. This anomaly is unrelated to

known sources. A stream-sediment sample in the Service

Creek drainage also contained anomalous concentrations of

uranium without a known source.

Vanadium.—The distribution of vanadium is similar to

that of cobalt, chromium, and nickel as discussed above. In

addition to slight enrichment in rock samples near Simpson

Mountain, nearby stream-sediment samples in Silver Creek

contained anomalous concentrations of vanadium. Several

rocks in the Byers Peak region of the St. Louis Peak Roadless

Area contained anomalous concentrations of vanadium. The

sources of these anomalies are not known.

Tungsten.—Heavy-mineral-concentrate samples collected from the upper Keyser Creek drainage basin were

anomalous in tungsten, lead, and zinc. Small massive sulfide

deposits may be responsible for the anomaly (Theobald and

others, 1985). Several stream-sediment samples from the

Rawah Wilderness Area were highly anomalous in tungsten.

This same region is also high in molybdenum. The elevated

tungsten and molybdenum values may be related to the

granitic terrain.

Zinc.—Anomalous zinc was associated with anomalous

lead and tungsten in heavy-mineral-concentrate samples

collected from the Keyser Creek region.
27

PREVIOUS STUDIES

Geophysical studies were made of six Forest Service

Wilderness Study Areas within or adjacent to the Forest: the

Mt. Zirkel Wilderness and northern Park Range vicinity

(Daniels, 1987); the Rawah Wilderness (Pearson and others,

1982); the Comanche–Big South, Neota–Flat Top, and

Never Summer Wilderness Study Areas (Pearson and others,

1981); the Indian Peaks Wilderness Study Area (Pearson and

U.S. Bureau of Mines, 1980); the Gore Range–Eagle’s Nest

Wilderness (Tweto and others, 1970); and the Vasquez Peak

Wilderness and vicinity (Moss and Abrams, 1985). Other

geophysical studies in the area were made by Behrendt and

others (1969) and Johnson and others (1984). Figure 6 shows

areas included in these published reports that provide both

maps and interpretations of geophysical anomalies.

Three sets of geophysical data, comprising gravity,

aeromagnetic, and radiometric maps, were compiled from

previous studies (figs. 7 and 8) and interpreted for the area of

the Forest.

GRAVITY DATA

The complete Bouguer gravity anomaly map (fig. 8)

was produced using edited gravity data from 4,200 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 UTM projection having a central meridian

of longitude 107°W. and a base latitude of 39°. These data

were gridded at a spacing of 1.2 mi (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

on shallower, more local anomalies, a derivative gravity map

(fig. 9) was calculated from the Bouguer gravity grid using

the computer program FFTFIL (Hildenbrand, 1983) to

remove or filter anomaly wavelengths longer than about 42

mi (70 km). The filter was selected to eliminate 100 percent

of the wavelengths greater than 48 mi (80 km), to pass 100

percent of the wavelengths less than 36 mi (60 km), and to

pass a linear percentage of the wavelengths between these

values. This “high-pass” derivative map emphasizes anomalies produced by shallow sources and suppresses longer

wavelength anomalies that are related to deep sources. 

The horizontal gradient of the gravity field was calculated using the method of Cordell and Grauch (1985), which

results in high gradient values where the gravity field

changes intensity over short distances across the map. The

maximum gradient trends are plotted on the high-pass gravity map (fig. 9) as dashed white, somewhat discontinuous

lines. These sinuous lines of maximum gradient commonly

follow geologic boundaries resulting from measurable density contrasts. The method best reflects the surface projection of vertical boundaries between shallow units;

boundaries dipping less than 90° will be offset from the maximum gradient (Blakely and Simpson, 1986). These inaccuracies are less apparent at regional scales (Grauch and

Cordell, 1987).

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 limits 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.

AEROMAGNETIC DATA

Figure 7 is a reference map for previous aeromagnetic

surveys showing location, flight-line spacing and direction, and original flight elevation of surveys. A magnetic

anomaly map of the Forest (fig. 10) was produced from

GEOPHYSICAL STUDIES

By Viki Bankey and James A. Pitkin
28 MINERAL RESOURCE POTENTIAL AND GEOLOGY, ROUTT NATIONAL FOREST, COLORADO

107°30' 107°00' 106°30' 106°00'

41°00'

40°30'

40°00'

39°30'

COLORADO

Eagle

Lake

Granby

Colorado River

Colorado River

0 5 10 MILES

WYOMING

COLORADO

i

70

Pearson and

others (1982)

Pearson and

others (1981)

Pearson and

Johnson (1980)

Moss and

Abrams (1985)

Tweto and

others (1970)

Daniels (1987)

Behrendt and

others (1969)

profile

Johnson and others (1984)

these surveys. Data from these surveys were projected

using a UTM projection having a central meridian of longitude 107°W. and a base latitude of 39°N. The data were

initially gridded at a spacing of 1/3 to 1/4 the flight-line

spacing, then were regridded to 1 km. The Definitive

International Geomagnetic Reference Field (DGRF),

updated to the date and elevation of each survey, was

removed before merging, using a program by Sweeney

(1990). The total-intensity magnetic anomaly map (fig.

10) shows regional magnetic anomalies; for more detail,

refer to the original magnetic maps referenced in figure 7

that are plotted at a larger scale. 

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 and

metamorphic rocks contain enough magnetic minerals to

Figure 6. Map showing areas of previous geophysical studies. See References Cited section of this report for complete citations.
29

107°30' 107°00' 106°30' 106°00'

41°00'

40°30'

40°00'

39°30'

COLORADO

Lake

Granby

Colorado River

Colorado River

0 5 10 MILES

WYOMING

COLORADO

i

A

B

C

D

E

400 a.t.

400 a.t.

12,000 bar.

13,000 bar.

14,000 bar.

Letter

on map

Elevation

(ft)

Line spacing

(mi)

Dir. Survey

area

Year

flown

Reference

3 mi

2 mi

2 mi

1 mi

1 mi

E-W

E-W

E-W

N-S

N-S

Craig NURE

North Park west

North Park central

Front Range NURE

Wolcott-Boulder

1978

1972

1965

1977

1967

LKB Resources (1979)

USGS (1978)

USGS (1978)

Geometrics (1979a, 1979b)

USGS (1968)

EXPLANATION

70

A

B

C

D

E

Figure 7. Map showing location of aeromagnetic surveys used to compile aeromagnetic map.

[Line spacing refers to distance between flight lines; Dir. refers to the direction the survey was flown; Elevation refers to the elevation of the survey

flown, either at constant barometric elevation (bar.) or constant clearance above terrain (a.t.)]

GEOPHYSICAL STUDIES
30 MINERAL RESOURCE POTENTIAL AND GEOLOGY, ROUTT NATIONAL FOREST, COLORADO

107°30' 107°00' 106°30' 105 106°00' °30'

41°00'

40°30'

40°00'

39°30'

COLORADO

Craig

Steamboat

Springs

Eagle

Vail

Kremmling

Walden

Lake

Granby

Colorado River

Color ado River

COLORADO

i

70

Park Range

Rabbit Ears Range

Gore Range

Medicine Bow Range

Williams Fork Mtns

Flat Tops

Rocky

Mountain

National

Park

0 15 30 MILES

0 25 50 KILOMETERS

generate magnetic anomalies, whereas sedimentary rocks

are commonly nonmagnetic.

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

northern side of a magnetic high. In some cases, 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. Another 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-intensity 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 on

the White River uplift. Reversals in older intrusive rocks

(such as Proterozoic rocks) may no longer cause a magnetic low because the magnetization of the rocks tends to

decay over time and eventually will align itself with the

direction of the present-day Earth’s magnetic field.

Aeromagnetic anomaly maps are important tools in

mapping surficial and buried igneous rocks. Aeromagnetic

data can be used to locate and estimate depths to igneous

intrusions that may be related to possible mineral deposits. Rings of magnetic highs with central or reentrant lows

may indicate porphyry systems in which hydrothermal

alteration has destroyed preexisting magnetic minerals.

30 MINERAL RESOURCE POTENTIAL AND GEOLOGY, ROUTT NATIONAL FOREST, COLORADO

Figure 8. Complete-Bouguer gravity anomaly map of the Routt National Forest and Middle Park Ranger District of the 

Arapaho National Forest, Colorado. Darker grays indicate lower gravity values; lighter grays indicate higher gravity values. White blocks show large areas of no data.
31

0 15 30 MILES

0 30 60 KILOMETERS

COLORADO

107°30' 107°15' 107°00' 106°45' 106°30' 106°15' 106°00' 105°45'

41°00'

40°45'

40°30'

40°15'

40°00'

39°45'

Elkhead

Mountain

Hahn's

Peak Mt. Zirkel

Mt.

Werner

Park Range

Mt. Ethel pluton

Soda Creek-Fish Creek

shear zone

Blacktail

Mountain

Flat Tops

North Park Syncline

Independence Mtn

fault

Rawah

Batholith

Medicine Bow Mtns

Gore Range

NorthPark

Rabbit Ears Range

Skin Gulch

shear zone

Mt

Cumulus

Never Summer Mtns

Shadow Mountain

Reservoir

Williams Range thrust fault

Williams Fork Mtns

Frasier Valley

Lake shear zone

Red

Mtn

Straight Creek

fault zone

Berthoud Pass

shear zone

Walden Syncline

Local magnetic highs may exist where hydrothermal

alteration or contact metamorphism has created secondary

magnetic minerals, as for example in a magnetite-bearing

ore body.

Aeromagnetic anomaly maps have some limitations in

locating mineral deposits. Some of the known mineral

deposits in the Forest have no distinctive aeromagnetic

expression—the Henderson molybdenum deposit is one

example. 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.

GEOPHYSICAL STUDIES

Figure 9. High-pass filtered Bouguer gravity anomaly map of the Routt National Forest and Middle Park Ranger District of 

the Arapaho National Forest, Colorado. Darker grays indicate lower gravity values; lighter grays indicate higher gravity 

values. Dashed white lines trace high gravity gradients (see text). White blocks show large areas of no data.
32 MINERAL RESOURCE POTENTIAL AND GEOLOGY, ROUTT NATIONAL FOREST, COLORADO

0

0

5 10 20 MILES

5 10 15 30 KILOMETERS

107°45' 107°30' 107°15' 107°00' 106°45' 106°30'

41°00'

40°45'

40°30'

40°15'

40°00'

39°45'

39°30'

VR6

VR9A

VR9C

VR8

VR9B

VRG10

VRG7

BP3

BP1

DZ2

DZ4

DZ3

DZ1

DZ6

VR5

VR1VR2

VRG4

VRG3

BPG2

PHYSICAL PROPERTIES OF ROCKS

Earlier studies in the Forest and vicinity that provide

measurements of density and susceptibility of various rock

types are summarized in table 5.

Proterozoic rocks in and near the Forest have a wide

range of measured magnetic susceptibilities and densities.

The Proterozoic granitoid and gabbroic rocks are generally

the most magnetic (Moss and Abrams, 1985); Proterozoic

metamorphic rocks are generally moderately magnetic,

MINERAL RESOURCE POTENTIAL AND GEOLOGY, ROUTT NATIONAL FOREST, COLORADO

Figure 10 (above and facing page). Residual shaded-relief magnetic anomaly map of the

Routt National Forest and Middle Park Ranger District of the Arapaho National Forest,

Colorado. Sun angle is 80 degrees, sun direction from the northwest. Magnetic anomalies

are identified in text.
33

106°30' 106°15' 106°00' 105°45' 105°30'

0

0

5 10 20 MILES

5 10 15 30 KILOMETERS

41°00'

40°45'

40°30'

40°15'

40°00'

39°45'

39°30'

BPG2

FR7 FR3

FR6

FR4

FR2

FR1

FC5

FC1

FC3B

FC3A

BPG4

VRG10

BP1

VRG7

MV2

MV3

MV4 MV7

MV6

MV5

MV1B

MV1A

VR11

Lake Shear Zone

although Proterozoic metasediments may be relatively nonmagnetic (Daniels, 1987). Proterozoic migmatites and

biotite gneisses appear to be more magnetic than granites in

some parts of the Gore Range in the southern part of the

Forest (Daniels, 1987).

In and near the Forest, amphibolites and gabbros are the

densest of the common Proterozoic rocks, whereas some

granites and felsic metamorphic rocks have lower densities

(Behrendt and others, 1969; Moss and Abrams, 1985). As a

group, the Proterozoic rocks are significantly denser than

Tertiary intrusive rocks (Case, 1967; Behrendt and others,

1969; Brinkworth, 1973; Moss and Abrams, 1985).

Oligocene intrusives are among the least dense rocks in the

study area (Moss and Abrams, 1985).

GEOPHYSICAL STUDIES
34 MINERAL RESOURCE POTENTIAL AND GEOLOGY, ROUTT NATIONAL FOREST, COLORADO

Table 5. Average susceptibility and density values for rocks in the Routt National Forest and Middle Park

Ranger District of the Arapaho National Forest and vicinity, Colorado.

[n.a. not available]

Rock type Number of Susceptibility Density

samples (cgs units) (g/cm3)

Daniels, 1987 (Mt. Zirkel area)

1.8-Ga gabbro and mafic intrusions 8 1.8 × 10-3 n.a.

1.7-Ga quartz monzonite - Buffalo Mtn. 2 1.8 × 10-3 n.a.

1.7-Ga quartz monzonite - other 9 0.1 × 10-3 n.a.

1.4-Ga quartz monzonite - Marguerite 2 4.1 × 10-3 n.a.

1.4-Ga quartz monzonite - other 3 0.46 × 10-3 n.a.

Proterozoic pelitic schists - chloritized 1 8.6 × 10-3 n.a.

Proterozoic pelitic schists 3 0.02 × 10-3 n.a.

Proterozoic gedrite gneiss 1 6.2 × 10-3 n.a.

Proterozoic hornblende gneiss 2 0.65 × 10-3 n.a.

Proterozoic metasedimentary and 12 0.33 × 10-3 n.a.

 metavolcanic rocks, undifferentiated

Tertiary Browns Park Formation, altered 4 0.024 × 10-3 n.a.

Tertiary Browns Park Formation, altered 4 0.114 × 10-3 n.a.

Tertiary intrusives, olivine-bearing 4 2.68 × 10-3 n.a.

Tertiary intrusives 6 0.732 × 10-3 n.a.

Moss and Abrams, 1985 (Vasquez Peak and vicinity)

Tertiary intrusive rocks

 rhyolite (altered) n.a. 0.35 × 10-3 2.38

 quartz monzonite n.a. 0.5 × 10-3 n.a.

 unaltered average n.a. n.a. 2.62

Cretaceous sedimentary rocks

 Pierre Shale n.a. nonmagnetic 2.61

 Dakota Sandstone n.a. nonmagnetic 2.52

 Niobrara Formation n.a. nonmagnetic 2.66

Proterozoic intrusive rocks

 Silver Plume Granite n.a. 0.6 × 10-3 2.67

 Boulder Creek Granodiorite n.a. 1.2 × 10-3 2.66

 gabbro n.a. 10.3 × 10-3 3.03

Proterozoic metamorphic rocks

 hornblende gneiss n.a. 1.6 × 10-3 2.89

 biotite gneiss n.a. 0.5 × 10-3 2.74

 sillimanite gneiss n.a. 0.9 × 10-3 2.76

 amphibolite and

 calc-silicate gneiss n.a. 1.0 × 10-3 2.80

Case, 1967 (Colorado Mineral Belt)

Proterozoic granitic rocks 35 n.a. 2.65

Proterozoic metamorphic rocks 46 n.a. 2.79

Tertiary porphyritic rocks 64 n.a. 2.65

Tweto and Case, 1972 (Leadville 30-minute quadrangle)

Proterozoic rocks n.a. 0.32 × 10-3–4.5 × 10-3 2.75

Paleozoic sandstones and

 quartzites n.a. 0 2.63

Paleozoic dolomites and

 limestones n.a. 0 2.80

Paleozoic, upper

 (undifferentiated) n.a. 0 2.50

Cretaceous and Tertiary

 intrusive rocks n.a. 0.46 × 10-3–2.67 × 10-3 2.63
35

Density measurements of Mesozoic and Tertiary sedimentary rocks to the west and southwest in the central Colorado Plateau (Plouff, 1961) vary from 2.3 to 2.6 grams/cubic

centimeter (g/cm3). Mesozoic and Tertiary rocks in the study

area are lithologically similar (Behrendt and others, 1969)

and may have similar densities. The few available measurements of magnetic susceptibility of these rocks indicate that

they are virtually nonmagnetic.

Some Tertiary plutons are magnetic and produce conspicuous positive anomalies (Moss and Abrams, 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).

No measurements of remanent magnetizations for the

rocks in the Forest and vicinity are available. Pearson and

U.S. Bureau of Mines (1980) suggest that the midProterozoic Silver Plume granite may be reversely magnetized, but the amount of remanence is unknown. On the magnetic anomaly map (fig. 10), the Silver Plume granite appears

less magnetic than surrounding rocks where it crops out in

the study area.

INTERPRETATIONS OF GRAVITY AND 

MAGNETIC DATA

REGIONAL FEATURES

Regional northeast-trending magnetic and gravity

highs, lows, and gradients occur across the study area and

beyond and have been noted previously. The northeasttrending grain in a regional aeromagnetic map of the area has

been interpreted as part of a Proterozoic zone or belt of en

echelon shears 200 mi wide that extends from the Grand

Canyon to south of the Black Hills (Zietz and others, 1969).

Pratt and Zietz (1973) interpreted the alignment of magnetic

anomalies from Rangely, Colo., to 6 mi south of Julesburg,

Colo., as a major basement structural discontinuity that may

have controlled the location of the Tertiary volcanic centers

of the Rabbit Ears Range and Never Summer Mountains.

Prodehl and Lipman (1989) recognized that the dominant

structural grain of Proterozoic rocks parallels accretion

boundaries, primarily the Mullen Creek–Nash Fork zone of

continental suturing of Archean and Proterozoic crust in

southeastern Wyoming. Major Proterozoic shear zones, such

as the Soda Creek–Fish Creek, Homestake, Berthoud Pass,

and Idaho Springs–Ralston shear zones, parallel this trend.

These northeast-trending zones were recognized by Lovering

Table 5. Average susceptibility and density values for rocks in the Routt National Forest and Middle Park

Ranger District of the Arapaho National Forest and vicinity, Colorado—Continued.

[n.a. not available]

Rock type Number of Susceptibility Density

samples (cgs units) (g/cm3)

Isaacson and Smithson, 1976 (Sawatch Range, Elk and West Elk Mountains)

Proterozoic rocks 36 n.a. 2.71

Tertiary granitic rocks 27 n.a. 2.63

Campbell and Wallace, 1986 (Holy Cross Wilderness Area and vicinity)

Proterozoic intrusive rocks n.a. 0.68 × 10-3 n.a.

Proterozoic metamorphic rocks n.a. 0.3 × 10-3 n.a.

Proterozoic granites n.a. 0.5 × 10-3 n.a.

Cretaceous-Tertiary 

 intrusive rocks n.a. 0.58 × 10-3 n.a.

Behrendt and others, 1969 (Park Range, North Park); Plouff, 1961 (Roberts Tunnel)

Proterozoic metamorphic rocks n.a. n.a. 2.84

Proterozoic granitic rocks n.a. n.a. 2.64

Mesozoic sedimentary rocks n.a. n.a. 2.60

Cenozoic sedimentary rocks n.a. n.a. 2.40

GEOPHYSICAL STUDIES
36 MINERAL RESOURCE POTENTIAL AND GEOLOGY, ROUTT NATIONAL FOREST, COLORADO

(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 the area between the Mullen Creek–Nash Fork shear

zone at its northern boundary and the Homestake shear

zone—an area about 100 mi wide that covers the entire

Forest study area. Warner postulates that this zone, which he

named the Colorado lineament, can be traced from the Grand

Canyon to Lake Superior and probably ceased as a wrenchfault system about 1,700 m.y. ago. Regardless of the

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 preexisting conduits for later intrusions and possible mineralizing

fluids.

Zietz and others (1969) also notes an east-west trend in

the aeromagnetic data, especially in the western part of the

State, that includes the east-west-trending Independence

Mountain fault (within the Forest) and Proterozoic fold axes

in the Front Range (east of the Forest).

Tweto (1987) has postulated that the Rawah batholith

encompasses a much larger area than is mapped in the Medicine Bow Mountains. He has included outcropping granitic

rocks in the Gore and Park Ranges as part of this batholith

and cites boreholes that penetrated similar rock in North

Park and southwest of the Park Range as evidence of continuity beneath cover. Rock composition within this batholith

varies from granite, quartz diorite, and quartz monzonite,

with numerous inclusions of more mafic igneous rocks. This

variation in rock composition results in varying shapes and

intensities of magnetic anomalies.

The southernmost part of the study area lies on the

northern 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.

This 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 Colorado Mineral

Belt (Crawford, 1924; Case, 1967). An intracrustal origin

Table 6. References for magnetic anomaly prefixes in figure 10.

Prefix Author Area Reference

DZ Daniels Mt. Zirkel Wilderness Daniels (1987)

BP Behrendt North Park and vicinity Behrendt and others (1969)

FR Flanagan Rawah Wilderness Pearson and others (1982)

FC Flanagan Comanche Pearson and others (1982)

MV Moss Vasquez Peak Moss and Abrams (1985)

VR Bankey Routt National Forest This report

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). Because this gravity low

does not continue northward into the Forest, we can predict

that mineralization of the type associated with the lowdensity batholith and the related Colorado Mineral Belt will

not be present in the study area.

For the purposes of discussing local geophysical features, the Forest is divided into five areas: the Park and Gore

Ranges, including the Mt. Zirkel Wilderness Area; the

Medicine Bow Mountains; the Rabbit Ears and Never

Summer Mountains; the Williams Fork Mountains, including the Vasquez Wilderness Area; and the Flat Tops (fig. 4).

Geophysical anomalies show poor correlation with mapped

rocks in the northwestern part of the Forest near the Elkhead

Mountains, and this area is not included in detailed interpretation. Magnetic anomaly numbers on figure 10 have been

assigned as shown in table 6.

PARK AND GORE RANGES AND THE MT. ZIRKEL 

WILDERNESS AREA

Behrendt and others (1969) conducted a geophysical

study of the North Park Basin and surrounding mountains

using gravity, aeromagnetic, and seismic data. Their

discussion of gravity and magnetic anomalies in the Park

Range and Medicine Bow Mountains is summarized here

and augmented with more detailed or current interpretation

where appropriate.

Gravity anomalies arise in this area from contrasts

between Proterozoic metamorphic and plutonic rocks, and

between Cenozoic sedimentary and older sedimentary rocks.

The Independence Mountain thrust sheet north of North Park

is associated with a gravity high that increases northward to

the Wyoming border. This gravity high is bifurcated by a

north-south-trending gravity low (fig. 9) that may indicate a

sedimentary basin below the thrust plate or may be caused by

low-density igneous rocks. Gravity lows caused by lowdensity basin-fill are at the deepest parts of the North Park

syncline and the Walden syncline, outside the Forest.
37

High-pass filtering of the Bouguer gravity data (fig. 9)

shows a northeastern continuation of the Mt. Ethel pluton

gravity low beyond the mapped extent of the pluton. Magnetization contrasts are not associated with this pluton. The

gravity low encompasses the Walden syncline, reaches

lower values just southwest of the Medicine Bow range-front

fault, and continues across the northernmost Medicine Bow

Mountains where granitic rocks are mapped. A gravity

model of the Mt. Ethel pluton shows a granitic body with a

density contrast of 0.2 g/cm3 between the granite and surrounding rocks (Behrendt and others, 1969). Although low

gravity values in North Park partially result from a syncline

filled with low-density rocks, the trend and extent of the

gravity low and correlation with magnetic gradients seem to

confirm Behrendt and others’ conclusion that buried granitic

rocks are a contributing cause of this broad gravity low.

Behrendt and others (1969) concluded that positive

magnetic anomalies in the Park Range and Medicine Bow

Mountains are caused by Proterozoic rocks of varying composition, some having estimated magnetite content of as

much as 1 percent. From geophysical evidence, they

inferred the presence of high-density gneisses and schists,

but later geologic mapping shows intermixing of unmetamorphosed igneous rocks (quartz monzonites and diorites)

with crystalline metavolcanic and metasedimentary rocks

(Snyder 1980a, 1980b, 1980c, 1980d). Because the magnetic variation within these units may be greater than the

variation between units, determination of rock type based on

magnetic mapping is not possible in this area.

A northeast-trending magnetic high (BP1; fig. 10, table

6) extends from the eastern Park Range, North Park, and the

Medicine Bow Mountains. Behrendt and others (1969)

modeled two gravity and magnetic profiles along this magnetic feature. Magnetic models show a complex basement

terrane composed of blocks of varying amounts of mafic

material, consistent with Tweto’s (1987) hypothesis that the

Rahwah batholith underlies this region and trends northeastsouthwest. Gravity modeling suggests that rocks in the Medicine Bow Mountains have higher densities (2.90 g/cm3)

than rocks in the Park Range (2.80 g/cm3). The maximum

thickness of the sediment fill in North Park is 3.2 mi (5.3

km), and vertical uplift is at least 4.0 mi (6.7 km) for the

Medicine Bow Mountains and 3.8 mi (6.4 km) for the Park

Range, relative to North Park.

North-south gravity profiles through the Park Range

and Medicine Bow Mountains by Johnson and others (1984)

show that the Proterozoic island-arc crust south of the

Mullen Creek–Nash Fork shear zone is thicker than the

Archean Wyoming cratonic crust north of the zone, in agreement with recent seismic studies (Mooney, 1991). A density

of 2.85 g/cm3 for pelitic schists and gneisses (Johnson and

others, 1984) is equivalent to high-density schists and

gneisses of assumed density of 2.90 g/cm3 (Behrendt and

others, 1969). South of lat 40°30'N., a gravity high is attributed to both pelitic schist and gneiss and a body of Tertiary

volcanic rocks having a density of 2.90 g/cm3 (Johnson and

others, 1984). The volcanic rocks do not appear magnetic on

figure 10. Johnson and others (1984) suggest that the density

of the Mt. Ethel granite may be 2.65 g/cm3. The granite body

is bounded on the south by the northeast-trending Soda

Creek–Fish Creek mylonitic shear zone (Snyder, 1980b), a

recurrently reactivated zone of movement that controlled the

emplacement of the pluton (Tweto, 1987).

MT. ZIRKEL AND NORTHERN PARK RANGE

In the northern Park Range, magnetic anomalies that

appear shallow (using anomaly size and intensity) may be

caused by Proterozoic granites and metamorphic rocks,

which are associated with polymetallic veins, or they may

indicate areas of Tertiary intrusive rocks such as the Hahn’s

Peak intrusion, which are associated with disseminated

silver-lead-zinc mineral deposits ( fig. 16). Tertiary intrusions in proximity to Mt. Ethel batholithic rocks are also

linked elsewhere in the Forest to fluorspar veins. Other evidence, such as geochemical anomalies, must be used to

determine the significance of positive magnetic anomalies in

this area.

Daniels (1987) interpreted aeromagnetic anomaly data

for the Mt. Zirkel Wilderness Area and vicinity, and his work

is summarized here with additional comments where applicable. Anomalies DZ1-DZ6, shown on figure 10, can be

modelled as buried or exposed Proterozoic rocks, but

Daniels (1987) also suggested that they could be caused by

subsurface Tertiary intrusions, indicating a higher mineral

potential in areas of magnetic highs. However, anomaly

DZ4 is not a likely Tertiary intrusion as reinterpreted here.

Positive anomaly DZ4 (fig. 10) is attributed to the

highly magnetic Elkhorn gabbro and the anomaly continues

west and southwest of the exposed pluton, indicating a buried

extension about equal to its exposed mass. A small positive

anomaly northeast of magnetic high DZ4 is associated with

ultramafic rocks where there is a high potential for platinumgroup elements (see fig. 24). A large gravity high is also

associated with this gabbro. High-pass filtering of the gravity data prepared for this report (fig. 9) shows this positive

gravity anomaly to be centered on the outcrop, with positive

values continuing northwest into Wyoming and southwest as

a positive gravity ridge. Although small outcrops of Xm

rocks are found near magnetic anomalies DZ3 and DZ6, the

main gravity and magnetic anomalies do not continue eastward in the Forest, and we conclude that this mafic body does

not have a significant eastward buried extent.

Positive magnetic anomalies DZ1 and DZ2 are located

on the western edge of the Mount Ethel pluton (fig. 10). The

grain size and composition at the western edge of the pluton

changes from medium-grained biotite granite and quartz

monzonite to a swarm of coarse-grained pink aplite and

leucogranite porphyry dikes (Snyder, 1987b), but this

GEOPHYSICAL STUDIES
38 MINERAL RESOURCE POTENTIAL AND GEOLOGY, ROUTT NATIONAL FOREST, COLORADO

compositional change in itself would not account for positive

magnetic anomalies. Quartz monzonite porphyry or more

mafic granodioritic rocks are likely sources of anomalies

DZ1, DZ2, and perhaps DZ3 (Daniels, 1987), whose model

across anomalies DZ1 and DZ2 suggests a burial depth of

nearly 0.6 mi (1 km) to the top of each intrusion. No gravity

anomalies are associated with magnetic anomalies DZ1-DZ3

(fig. 9).

Positive magnetic anomaly DZ5, at Big Agnes Mountain near Mt. Zirkel (fig. 10), has a source modeled below 1

mi (1.5 km) (Daniels, 1987). Felsic gneiss, pelitic schist, and

granite, with small intrusions of pegmatite and ultramafic

rock exposed at the surface cannot account for anomaly DZ5;

thus, an ultramafic source at depth is required. A gravity

high associated with magnetic anomaly DZ5 supports the

presence of a more mafic, dense body at depth.

Positive magnetic anomalies DZ6 and VR5 are located

at and southeast of Farwell Mountain (fig. 10). These anomalies have gravity highs associated with them, which suggests a different source than anomalies DZ1–DZ3. Gabbro

crops out just east of anomaly DZ6. The lack of physical correlation between the gabbro outcrop and the anomaly might

be explained by destruction of magnetite by mineralization

in the nearby Farwell Mountain mining district.

Positive magnetic anomalies form a northeast-trending

ridge, VR1, north of and parallel to the Mount Ethel pluton

(fig. 10). A gravity high correlates with this ridge. Massive

sulfide deposits are associated with the eastern and western

parts of the ridge; and the ridge of high magnetic and gravity

values, coupled with the preferred northeast direction of the

anomalies, indicates the possible extent of such deposits.

The actual source of the anomalies is undetermined: either a

structural high or a variation in rock type could account for

this feature.

A north-south-trending ridge of magnetic highs (VR2)

is associated with rocks on the upper plate of a thrust

mapped just east of the Elkhorn gabbro, in an area of outcropping quartz monzonite and nonmagnetic metavolcanics

(fig. 10). Throughout the northern Park Range, correlation

of magnetic anomalies with Proterozoic igneous rocks is

variable: some positive and negative magnetic anomalies

reflect variations in the magnetization of quartz monzonites

and granitic rocks, whereas other magnetic anomalies may

indicate small buried intrusions of unknown composition.

Without other confirming evidence of intrusions that may

be associated with mineral deposits, magnetically anomalous areas such as this ridge must be regarded as suggestive

of intrusions but cannot be confirmed. In this case, a northsouth-trending gravity low correlates with the magnetic

ridge, a pattern that elsewhere resembles anomalies caused

by magnetic Tertiary intrusions, which are frequently less

dense than Proterozoic rocks. Behrendt and others (1969)

attributed this gravity low to a sedimentary basin below the

thrust plate, but granitic rocks also have low densities and

could cause the gravity low.

The horizontal gradient of the magnetic field was calculated, and this gradient map shows two northeast-trending

magnetic gradients that cross the northern part of the Forest:

VRG3 extends from Glen Eden in the southwest to the northeastern corner of the Forest, and VRG4 parallels VRG3

across the Park Range and approximately 15 mi south. These

gradients encompass a magnetically low area that includes

the Elk River valley (and thus is partially topographically

controlled) and continues northeastward to include the Independence thrust sheet. These trends could reflect unmapped

shear zones or structural grains in Proterozoic rocks.

SOUTHERN PARK RANGE AND GORE RANGE

Magnetic gradient BPG2 follows the southern boundary

of the Mount Ethel pluton and is associated with the Soda

Creek–Fish Creek shear zone (fig. 10). South of the gradient,

magnetic low BP3 is located just east of Mt. Werner. This

low is not topographically related and has no associated

gravity expression. Gneissic rocks (Xfh) mapped in the area

correlate poorly with the low, because the rocks are widespread elsewhere over the Park Range with no corresponding

intense low. Tweto (1987) has inferred from borehole data

that a northeast-trending wedge of largely metasedimentary

rock (Xb) underlies this area. The range of rock types within

this Xb unit (biotite gneiss, migmatite, marble, and calcsilicate rocks) vary widely in susceptibility ranges, and presumably a non-mafic type predominates in this area.

Another possible source is a buried, reversely magnetized,

mafic intrusion of similar density to Proterozoic diorite or

monzonite (but not Mt. Ethel pluton-type rocks, which are

less dense). Precambrian basement north of this zone is primarily felsic and hornblendic gneiss (Xfh), whereas south of

the zone basement is granitic rock of the Routt Plutonic Suite

(Tweto, 1987). 

Two small, northeast-trending positive anomalies

(labeled VR6, fig. 10) at and northeast of Blacktail Mountain

are superimposed upon generally high magnetic values of

Proterozoic granitic rocks. The limited aerial extent and

sharp gradients of anomalies VR6 suggest shallow sources,

possibly intrusions. One of the anomalies is associated with

a mapped Tertiary basalt; however, other similar outcrops of

basalt have no related positive magnetic anomalies. Alternatively, anomalies VR6 could indicate more highly magnetic

areas of Proterozoic rocks.

In the Gore Range, an abrupt east-west gradient

(VRG7) separates high magnetic values to the north from

lower magnetic values to the south (fig. 10). Negative magnetic anomaly VR8 is part of a northeast-trending, magnetically low zone that continues through the Rabbit Ears Range

and the Never Summer Range (including anomaly FC4),

where its southern boundary is associated with the Skin

Gulch shear zone. It is part of a major magnetic feature that

crosses the Routt National Forest study area and beyond,
39

traversing the Flat Tops, following the northwest border of

the Rabbit Ears Range, cutting through Cameron

Pass–Specimen Mountain, and continuing across the northern part of the Front Range along the Cache la Poudre River

to Fort Collins. This magnetic zone is more than 120 mi (200

km) long and between 12 mi (20 km) and 30 mi (50 km)

wide. This zone is a major basement discontinuity and, as

previously mentioned (Pratt and Zietz, 1973; Brinkworth,

1973), may have controlled emplacement of the Tertiary volcanic rocks in the Rabbit Ears Range and elsewhere along its

length. A northeast-trending gradient, VRG10, follows the

southern boundary of this zone of weakly magnetic rocks.

Positive magnetic anomalies VR9A-VR9C are shallow,

local features that fall within the larger zone of magnetic low

values (fig. 10). Anomaly VR9B correlates with mapped

Tertiary mafic intrusive rocks, which are the likely sources

of all three of these positive magnetic anomalies. A positive

gravity anomaly on the high-pass filtered map (fig. 9) correlates with this area of magnetic anomalies, reflecting the

denser mafic rocks found here.

MEDICINE BOW MOUNTAINS AND RAWAH 

WILDERNESS AREA

Clusters of magnetic highs in the southern Medicine

Bow Mountains are similar in pattern and trend to magnetic

highs to the southeast in the Front Range (Pearson and others, 1982). These magnetic highs are separated by a

northeast-trending, deep magnetic low described earlier that

here follows major faults in the Medicine Bow Mountains.

The magnetic highs in the Medicine Bow Mountains

are intensified where magnetic rocks are in high topographic relief, but analysis of the topography (Pearson and

others, 1982) demonstrates that many anomalies are not

solely caused by topographic changes. The dominant granitic rocks of the Rawah batholith are moderately magnetic,

based on measurements of similar-age granites (table 5)

and on the generally high magnetic values here, but the

intensity of the associated magnetic field appears to vary

locally. Localized magnetic highs (FR1, FR2, FR3, FR6,

and FR7, on fig. 10) may arise from more mafic rocks at a

depth of a few kilometers, enhanced by clusters of

shallower inclusions.

A northeast-trending magnetic low (FR4) is postulated

(Pearson and others, 1981) as the result of a combination of

weakly magnetic inclusions, oxidation of magnetite along

faults, and a topographic low. Magnetic low FR4 (fig. 10) is

centered on mapped polymetallic vein deposits of Cu and Ag

and has been used to help define the favorable terrane for

polymetallic veins in this area, on the assumption that it may

be associated with either a buried intrusion of low magnetization or with hydrothermal alteration that destroyed

magnetic material.

A north-south-trending magnetic low, FR5, parallels

the Laramie River valley but lies east of and above the valley

floor (fig. 10). This anomaly, located 1 mi west of a mapped

reverse fault, is interpreted as a wedge of Phanerozoic sedimentary rock beneath Proterozoic crystalline rocks thrust

over it (Pearson and others, 1982).

RABBIT EARS RANGE AND NEVER SUMMER 

MOUNTAINS

The section of the Forest that includes the Rabbit Ears

Range and Never Summer Mountains lies within the broad,

northwest-trending magnetic low described in the section on

the Gore Range. Within this broad area of low magnetic values, short-wavelength magnetic highs in the Rabbit Ears

Range correlate with both volcanic and intrusive rocks.

Magnetic highs of similar extent and amplitude along the

Continental Divide show that intrusive rocks are also shallowly buried beneath Cenozoic sedimentary rocks. Mapped

volcanic rocks south of the Divide and in the Never Summer

Mountains do not have associated magnetic anomalies, due

in part to topographic effects of lower elevations in this area.

Gravity values are poorly correlated to mapped rock units,

including the exposed volcanic rocks.

The lack of gravity expression and the reduced amplitudes of magnetic anomalies in the Never Summer Mountains indicate shallow depth to the bases of the source bodies

of the magnetic anomalies as a result of the thin Never Summer thrust plate overlying a structural trough of low-density

sedimentary rocks (Behrendt and others, 1969). North-south

gradients in the magnetic data (anomaly BPG4) and gravity

data, with increasing values to the east, suggest that the

Never Summer Mountains are separated from the Front

Range by a north-trending fault that parallels the Laramie

River fault (FR5) to the north.

COMANCHE-NEOTA-NEVER SUMMER STUDY AREAS

Pearson and others (1981) studied gravity and magnetic

data of the Comanche–Big South, Neota–Flat Top, and

Never Summer Wilderness Study Areas (fig. 4), and his

work is reviewed here.

Positive magnetic anomaly FC1 (fig. 10) is caused by

the Tertiary Mount Richthofen granodiorite, bounded on the

south by the Mt. Cumulus rhyolite-porphyry stock. The

Mount Richthofen stock generates no gravity anomaly,

whereas a prominent gravity low where Mt. Cumulus rocks

crop out is evident on the gravity map (fig. 9) and shows a

buried east-west extension of the Mt. Cumulus stock. The

Teller mining district of polymetallic veins is associated with

the western flank of the gravity low, and a Climax-type

molybdenum occurrence is associated with the gravity minimum. Positive magnetic anomalies FC3A and FC3B are

GEOPHYSICAL STUDIES
40 MINERAL RESOURCE POTENTIAL AND GEOLOGY, ROUTT NATIONAL FOREST, COLORADO

located south and north respectively of the Mt. Cumulus

stock, suggesting that the stock is nonmagnetic or is

reversely magnetized. A regional gravity map ( Pearson and

others, 1981), which was made using a slightly different

method of anomaly filtering than that used to create figure 9,

shows local gravity lows at Mt. Cumulus, at Jack Creek, and

west of Specimen Mountain. These anomalies may reflect

shallow plutons that are connected at depth. The aeromagnetic data used for this report (fig. 10) are of higher quality

than those used by Flanagan, and they reveal a greater number of shallow magnetic highs and lows on the Never Summer thrust plate that may be associated with unexposed

Tertiary intrusions and possible mineralization.

Negative magnetic anomaly FC2, east of and parallel to

positive magnetic anomaly FC1 (fig. 10), is thought to be a

southward continuation of the Laramie River fault marked

by anomaly FR5 to the north. This discontinuous northsouth anomaly can be traced as far south as Shadow Mountain Reservoir.

A northeast-trending magnetic low reaches its lowest

values at FC4 (fig. 10). This magnetic low was described

earlier in the sections on the Gore Range and the Rabbit Ears

Range. This low has been interpreted as the result of

destruction of magnetic minerals along fault zones (Pearson

and others, 1981), which may account for a small part of the

magnetic low, but the extent and intensity is much larger

than expected if due solely to alteration.

Local positive anomaly FC5 is within the regional

magnetic low. The source is not exposed but may be a

small intrusive Tertiary(?) plug, a local area of more magnetic facies of Proterozoic(?) granite, or a dike swarm of

unknown age.

WILLIAMS FORK MOUNTAINS, VASQUEZ PEAK,

AND ST. LOUIS PEAK AREAS

Moss and Abrams (1985) studied the gravity and magnetic fields of the Williams Fork Roadless Area, in the southeasternmost part of the Forest (fig. 3), and the Vasquez Peak

Wilderness Study Area and St. Louis Peak Roadless Area

northeast of the Williams Fork Roadless Area. Rocks

exposed in this area include Proterozoic intrusive and metamorphic rocks. Measurements of rock properties (table 5)

show that the Proterozoic Silver Plume granite is less dense

and less magnetic than the metasedimentary gneisses and

schists.

Moss and Abrams (1985) selected six small areas of

low gravity values and magnetic highs or lows that might

indicate either buried Oligocene stocks or varying magnetic

mineral content of Proterozoic intrusive or metamorphic

rocks. Positive magnetic anomalies MV1A and MV1B (fig.

10) may reflect more mafic rocks within a sequence of Proterozoic metasedimentary rocks. Anomalies MV3 and MV4

are associated with Proterozoic gabbro outcrops. Positive

magnetic anomalies MV5 and MV6 may arise from Boulder

Creek Granodiorite. Magnetic high MV8 was interpreted by

Brinkworth (1973) as caused by an early Tertiary intrusive;

Brinkworth attributes a similar buried intrusive to magnetic

high MV2.

An east-northeast-trending gravity low on the gravity

map (fig. 9) is bounded by the Lake shear zone on the northwest and the Straight Creek fault zone on the southeast. The

gravity low marks the presumed exposed and buried extent

of the Silver Plume batholith. The gravity high northwest of

this low is associated with denser, more mafic metamorphic

rocks.

Two deep, northeast-trending magnetic lows dominate

the magnetic anomaly map in the Williams Fork region. The

northernmost of the two (labelled MV7, fig. 10) follows St.

Louis Creek. The southernmost magnetic low (VR11)

crosses topography and roughly correlates with the northern

part of the Silver Plume batholith. A northeast-trending positive magnetic ridge separates the two lows, and the St. Louis

Lake shear zone corresponds to the magnetic gradient

between this positive ridge and magnetic low VR11. This

ridge corresponds to a gravity gradient that Moss and

Abrams (1985) postulated was the contact between Silver

Plume Granite and metamorphic rocks; however, this does

not explain the northern magnetic low. Two possible causes

for the northern low are Silver Plume granitic rocks lie

beneath St. Louis Creek, or the source is a nonmagnetic

phase of Proterozoic rocks unidentified in this area. A gravity low on the high-pass gravity map (fig. 9) correlates with

the eastern part of magnetic anomaly MV7, suggesting granite as a source; alternatively this could indicate less-dense

sediments filling Frasier Valley.

A northwest-trending gravity low correlates with

decreased magnetic values in the northwestern Williams

Fork Mountains and suggests a wedge of sedimentary rocks

beneath the Williams Fork thrust plate. Magnetic anomalies

are continuous and are not offset along the Williams Fork

Mountains thrust. The source of these anomalies is probably

basement rocks in the shallow footwall.

Tertiary intrusive rocks, not exposed in this area, are

believed responsible for base- and precious-metal mineralization, such as the subvolcanic Oligocene stock associated

with the Henderson molybdenum deposit southeast of the

Forest. Brinkworth (1973) studied the aeromagnetic and

gravity maps of the Climax area and the Front Range.

Because of the importance of molybdenum in the southern

part of the study area, his work comparing the Climax and

Red Mountain (Henderson mine) intrusive complexes is

included here.

The Oligocene stocks associated with nearby molybdenum deposits have the lowest density of all rocks in the area

and may be delineated by gravity lows (Brinkworth, 1973).

However, gravity data are sparse in this remote region, and

the gravity field near the large Henderson molybdenum

deposit a few miles east of the Williams Fork Roadless Area
41

shows only a slight negative deflection in the Bouguer gravity field (fig. 8). High-pass filtering, however, has removed

the strong regional gravity low caused by the batholith associated with the Colorado Mineral Belt, and a residual gravity

low just east of the Forest boundary attributed to the Silver

Plume batholith also correlates with the Henderson mine

(fig. 9). Part of this gravity low could be caused by the subsurface Tertiary intrusive associated with mineralization of

subvolcanic rhyolite (averaging 2.49 g/cm3) and altered

country rocks. The negative gravity anomaly continues to

the northeast and suggests that any related mineralized areas

are more probably east or north, outside the Forest.

The Colorado Mineral Belt gravity low encompasses

the area of plutons related to molybdenum enrichment,

including the Red Mountain stock and other outlying rhyolitic plutons such as the Montezuma, Leavenworth, and

Cabin Creek stocks and the Revenue Mountain and Handcart

Gulch stocks, south of the Forest (Brinkworth, 1973). All of

these stocks have common volcanic origins; they have components associated with separate intrusive events and hydrothermal episodes; and they are located near major

Proterozoic shear zones that may have localized igneous

activity at preexisting zones of crustal weakness. However,

each intrusion is eroded to a different level; Brinkworth

(1973) calculated that the Climax stock is 3,000 ft more

deeply eroded than the Red Mountain stock. Neither the

Climax nor the Red Mountain stocks have much magnetic

expression, but the flight lines are at least 0.5 mi away from

the stock outcrops.

FLAT TOPS 

The magnetic field of the Flat Tops is characterized by

several high-frequency, small positive anomalies over some

basalt outcrops. These small anomalies are superimposed on

long-wavelength, deep-source anomalies that are probably

caused by magnetic contrasts within the Proterozoic basement. Insufficient gravity and magnetic data limits further

interpretation in the southwestern part of the Forest.

AERIAL GAMMA-RAY RADIOACTIVITY

INTRODUCTION

Aerial gamma-ray radioactivity data for the Forest are

from spectrometer surveys flown during the U.S. Department of Energy National Uranium Resource Evaluation

(NURE) program (ca. 1974–1983). NURE surveys that

include parts of the Forest are those for the Craig (LKB

Resources, Inc., 1979), Denver (Geometrics, 1979a),

Greeley (Geometrics, 1978), and Leadville (Geometrics,

1979b) 1°×2° quadrangles. Aerial gamma-ray data

(aeroradioactivity) from these surveys were used to prepare

an aeroradioactivity database for the Forest. Other

compilations of NURE data that include the Forest are

Phillips and others (1993) and Duval and others (1995).

Aeroradioactivity is the measurement of terrestrial

radioactivity with instruments operated in low-flying aircraft. The source of the radioactivity measured is the

near-surface rock and soil (to 12-inch depth) where the primary gamma-ray emitting isotopes are from the natural radioelements potassium (K), uranium (U), and thorium (Th).

NURE aerial systems were quantitatively calibrated at sites

of known radioelement concentrations, permitting quantitative reporting of survey data in percent for K and parts per

million (ppm) for U and Th (assuming equilibrium in the

respective decay series). The near-surface distribution of K,

U, and Th generally reflects bedrock lithology and modifications due to weathering, erosion, transportation, groundwater 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.

Aerial flight-line spacing for the Forest database is

3-mi east-west and 12-mi north-south for the Craig and

Leadville 1°×2° quadrangles and 1-mi east-west and 4-mi

north-south for the Greeley and Denver 1°×2° quadrangles.

A minimum-curvature algorithm (Webring, 1981) was

applied to the flight-line data, producing K, U, and Th

1.8-mi-square grids, which comprise the Forest database.

Most of the Forest is within the Craig 1°×2° quadrangle,

and the bulk of the database is derived from data for that

quadrangle; hence the choice of 1.8-mi for grid-cell size.

The grids were used to prepare K, U, and Th color and

black-and-white maps at 1:250,000 and 1:500,000 scales

for use in the assessment and gray-scale maps at

1:1,000,000 scale for inclusion in this report. Grids of the

ratios U:Th and K:Th were also prepared.

DISCUSSION

K, U, Th, U:Th, and K:Th aeroradioactivity gray-scale

contour maps of the Forest are shown (respectively) in figures 11, 12, 13, 14, and 15. The Forest boundary is shown

on each map, and geographic locations shown on the maps

are described in the figure captions. Bodies of water, such as

Lake Granby and Green Mountain Reservoir, have no measurable aeroradioactivity. However, the grids used to make

figures 11–15 were not masked to show areas of no data for

lakes and reservoirs, and any discernable gray-scale values

for any bodies of water should be ignored.

The near-surface distribution patterns of K, U, and Th

as displayed by aeroradioactivity maps are often similar,

resulting from common rock-type associations for these

GEOPHYSICAL STUDIES
42 MINERAL RESOURCE POTENTIAL AND GEOLOGY, ROUTT NATIONAL FOREST, COLORADO

107°30' 107°15' 107°00' 106°45' 106°30' 106°15' 106°00' 105°45'

41°00'

40°45'

40°30'

40°00'

40°15'

39°45'

0 15 30 MILES

0 30 60 KILOMETERS

5 10

10 20

0.8–1.0

1.0–1.2

1.2–1.4

1.4–1.6

1.6–1.8

1.8–2.0

2.0–2.2

POTASSIUM, IN PERCENT

co no

wa

cr ss

ra

lg

gr

kr

gm

COLORADO

WYOMING

elements. However, discontinuities in the patterns can reflect

significant mineralogic discontinuities, such as the contrasting properties of felsic and mafic igneous rocks. Th generally

has a more consistent distribution pattern than K or U, likely

resulting from Th being the least mobile of these elements.

For this reason, Th is used as the stable denominator in U:Th

and K:Th ratios, thereby highlighting subtle variations in U

and K distribution. Of particular interest are variations from

the 0.25 ratio on figure 14. The ratio for normal crust is 1:4

or, in the case of figure 14, 0.25. Values on figure 14 greater

than 0.25 suggest relative enrichment of U, values less than

0.25 suggest relative depletion of U.

Figure 11. Potassium aeroradioactivity gray-scale contour map of the Routt National Forest and Middle

Park Ranger District of the Arapaho National Forest, Colorado. Values in percent K. Geographic locations:

co = Columbine, cr = Craig, gr = Granby, kr = Kremmling, no = Northgate, ra = Rand, ss = Steamboat

Springs, wa = Walden. 
43

COLORADO

WYOMING

0 15 30 MILES

0 30 60 KILOMETERS

5 10

10 20

107°30' 107°15' 107°00' 106°45' 106°30' 106°15' 106°00' 105°45'

41°00'

40°45'

40°30'

40°00'

40°15'

39°45'

co

no

wa

cr ss ra

lg

gr

kr

gm

URANIUM, IN PPM

0.6–1.0

1.0–1.4

1.4–1.8

1.8–2.2

2.2–2.6

2.6–3.0

3.0–3.4

3.4–3.9

INTERPRETATION

Natural radioelement distribution for the Forest, as

demonstrated in the grey-scale contour maps (figs. 11–15)

has a varied pattern that reflects the diverse geology of the

Forest. 

Areas of notably higher radioactivity within the Forest

include a sizeable area of 1.8 to 2.0 percent K, 2.6 to 3.9 ppm

U, and 10 to 13 ppm Th northeast of Steamboat Springs that

reflects the presence of 1.4-Ga granite of the Mt. Ethel

pluton, a smaller area of 1.8 to 2.0 percent K, 2.6 to 3.0 ppm

U, and 11 to 13 ppm Th north of Northgate at the Forest

GEOPHYSICAL STUDIES

Figure 12. Uranium aeroradioactivity gray-scale contour map of the Routt National Forest and Middle

Park Ranger District of the Arapaho National Forest, Colorado. Values in ppm U. Geographic locations: co

= Columbine, cr = Craig, gr = Granby, kr = Kremmling, no = Northgate, ra = Rand, ss = Steamboat

Springs, wa = Walden.
44 MINERAL RESOURCE POTENTIAL AND GEOLOGY, ROUTT NATIONAL FOREST, COLORADO

0 15 30 MILES

0 30 60 KILOMETERS

5 10

10 20

THORIUM, IN PPM

15–16

14–15

13–14

12–13

11–12

10–11

9–10

8–9

7–8

6–7

5–6

2–5

107°30' 107°15' 107°00' 106°45' 106°30' 106°15' 106°00' 105°45'

40°45'

40°30'

40°15'

40°00'

39°45'

41°00'

co no

wa

cr

ss ra

lg

gr

kr

gm

COLORADO

WYOMING

boundary that also relates to 1.4-Ga granite, and an area of

2.0 to 2.2 percent K, 3.4 to 3.9 ppm U, and 11 to 13 ppm Th

southeast of Rand where the source rocks are Tertiary arkosic

sedimentary rocks of the Coalmont and Middle Park Formations. The area of higher radioactivity that includes the Mt.

Ethel pluton extends farther north on the U map compared

Figure 13. Thorium aeroradioactivity gray-scale contour map of the Routt National Forest and Middle Park

Ranger District of the Arapaho National Forest, Colorado. Values in ppm Th. Geographic locations: co =

Columbine, cr = Craig, gr = Granby, kr = Kremmling, no = Northgate, ra = Rand, ss = Steamboat Springs,

wa = Walden.
45

co no

wa

cr ss ra

lg

gr

kr

gm

0 15 30 MILES

0 30 60 KILOMETERS

5 10

10 20

URANIUM:THORIUM

0.075–0.2

0.2–0.225

0.225–0.25

0.25–0.275

0.275–0.3

0.3–0.325

0.325–0.35

0.35–0.375

0.375–0.4

0.4–0.425

0.425–0.45

0.45–0.9

107°30' 107°15' 107°00' 106°45' 106°30' 106°15' 106°00' 105°45'

41°00'

40°45'

40°30'

40°15'

40°00'

39°45'

COLORADO

WYOMING

U:Th values. In North Park, between the western and eastern

parts of the Forest, the distinct pattern of varied and frequently

higher radioactivity (2.0 to 2.4 percent K, 3.0 to 3.9 ppm U,

with the same feature on the K and the Th maps, suggesting

relatively enhanced U in the adjoining 1.7-Ga metamorphic

rocks, a lithologic occurrence substantiated by higher (>0.34)

Figure 14. Uranium:thorium aeroradioactivity gray-scale contour map of the Routt National Forest and

Middle Park Ranger District of the Arapaho National Forest, Colorado. Values in ppm U per ppm Th. Geographic locations: co = Columbine, cr = Craig, gr = Granby, kr = Kremmling, no = Northgate, ra = Rand,

ss = Steamboat Springs, wa = Walden.

GEOPHYSICAL STUDIES
46 MINERAL RESOURCE POTENTIAL AND GEOLOGY, ROUTT NATIONAL FOREST, COLORADO

107°30' 107°15' 107°00' 106°45' 106°30' 106°15' 106°00' 105°45'

41°00'

40°45'

40°30'

40°00'

40°15'

39°45'

0 15 30 MILES

0 30 60 KILOMETERS

5 10

10 20

co no

wa

cr ss ra

lg

gr

kr

gm

COLORADO

WYOMING

POTASSIUM:THORIUM

0.1–0.125

0.125–0.15

0.15–0.175

0.175–0.2

0.2–0.225

0.225–0.25

0.25–0.275

0.275–0.3

0.3–0.325

0.325–0.35

0.35–0.375

0.375–0.4

lithologies—from west of Rand and Northgate to the Forest

boundary, the U:Th (>0.35) and K:Th (>0.30) ratios include

strong positive, often non-coincident, features that indicate

U- and K-dominant radioactive lithologies.

11 to 14 ppm Th) relates mostly to arkosic sedimentary rocks

of the Tertiary Coalmont Formation. The U:Th and K:Th

ratios for the area between Walden and Rand show relatively

mundane patterns indicating relatively similar radioactive

Figure 15. Potassium:thorium aeroradioactivity gray-scale contour map of the Routt National Forest and

Middle Park Ranger District of the Arapaho National Forest, Colorado. Values in K per ppm Th. Geographic

locations: co = Columbine, cr = Craig, gr = Granby, kr = Kremmling, no = Northgate, ra = Rand, ss =

Steamboat Springs, wa = Walden.
47

Forest, distinct U relative highs of as much as 3.4 ppm and

U:Th of >0.35 are suggestive of U deposits in Cretaceous

sedimentary rocks.

The southwestern part of the Forest has undistinguished

radioelement expression, with mostly low concentrations of

0.7 to 1.2 percent K, 0.7 to 1.8 ppm U, and 1.5 to 7 ppm Th

for Cretaceous and Tertiary sedimentary rocks and Tertiary

basalt. The previously discussed U high (lat 40°07'N., long

107°28'W.) is just outside the Forest in Cretaceous rocks.

Other occurrences of 1.4-Ga granite include areas where

one radioelement is more distinct than the other two, reflecting radioactive lithologic differences between similar rock

types. One is a K-distinct (1.8 to 2.2 percent) feature about

12 mi northeast of Northgate, outside the Forest. Another is

the Th-distinct (12 to 14 ppm) feature with associated 1.8 to

2.0 percent K and 2.6 to 3.0 ppm U at the southeast corner of

the Middle Park Ranger District of the Arapaho National Forest that is related to the Silver Plume batholith and includes

the Urad and Henderson Mo orebodies. 

That part of the Forest on the east side of North Park has

varied radioactivity expression, reflecting source rocks with

different radioactive lithologies. At Baker Mountain (not

labeled on figs 11–15) about 16 mi east-southeast of Rand

and outside of the Forest, a Th-dominant feature (11 to 13

ppm) with coincident K (1.8 to 2.0 percent) and U (2.2 to 3.0

ppm) highs reflects plutonic rocks with potential for Mo porphyry deposits.

Southeast of Kremmling, radioelement data for the Middle Park Ranger District of the Arapaho National Forest has

northwest trends reflecting the Williams Range thrust fault.

Radioelement concentrations vary appreciably, with the previously discussed Th feature of the 1.4-Ga Silver Plume

batholith being the most prominent. A notable feature is an

area of >0.20 K:Th east of Green Mountain Reservoir where

the source rocks are 1.7-Ga metamorphic and granitic rocks.

ANALYSIS OF RADIOELEMENT DATA BY

NURE AERIAL CONTRACTOR

U.S. Department of Energy (DOE) NURE program procedures for each 1°×2° quadrangle included analysis of the

aerial survey radioelement data to determine the possibility

of U deposit occurrence. The radioelement data (U, K, Th),

collected along flight-line profiles, and their ratios (U:K,

U:Th, Th:K) were analyzed statistically with surficial geologic data to calculate standard deviations from the mean

“per radioelement quantity” per geologic unit. Areas along

flight lines that fit statistical (generally at least one standard

deviation above the mean for U, U:K, U:Th) and geologic criteria were termed significant anomalies; areas along flight

lines that had U, U:K, and U:Th anomalies (generally) at least

two deviations above the mean, and had geologic characteristics that were appropriate for U deposits, were termed

preferred anomalies. For differences in anomaly definition

The radioelement data show a distinct northeast trend

that includes the Mt. Ethel pluton, extends past Northgate to

the northeast, and extends southwest to include an isolated U

high of 3.0 to 3.4 ppm (lat 40°07'N., long 107°28'W.) in Cretaceous sedimentary rocks at the southwest corner of the Forest. Radioelement concentrations for the trend range from

low to high, include a variety of rock types, and parallel the

strong northeast trend of bedrock outcrops and topography

south of the Mt. Ethel pluton. The trend is most pronounced

in the U data, but it is also apparent in the K and Th

data—coincidence of the three radioelements is demonstrated by lack of expression in the U:Th and K:Th ratios.

In the Forest north of the area of higher concentrations

that include the Mt. Ethel pluton, relatively low values of 0.7

to 1.4 percent K, 0.7 to 1.4 ppm U, and 1.5 to 6 ppm Th characterize 1.7-Ga metasedimentary, metavolcanic, and plutonic

rocks. This area includes several K-distinctive features,

including a zone of 1.6 to 2.0 percent K in metamorphic and

plutonic rocks that trends northeast from Round Mountain

(lat 40°30'N., long 106°54'W. ) through Pearl (lat 40°59'N.,

long 106°33'W.). North of Columbine, a distinct low of 0.7

to 1.0 percent K, 0.7 to 1.0 ppm U, and 1.5 to 5 ppm Th

reflects the low radioelement concentrations of the Elkhorn

Gabbro. West of Columbine, the Forest has moderate values

of 1.0 to 2.0 percent K and 1.4 to 2.6 ppm U and low values

of 1.5 to 7 ppm Th for Cretaceous sedimentary rocks and Tertiary sedimentary, volcaniclastic, and intrusive rocks. Several of the intrusives have distinctive K of 1.4 to 1.8 percent

and K:Th of >0.28. In the northwest part of the Forest, in the

western Elkhead Mountains west of Columbine, low values

of 0.7 to 1.2 percent K, 1.0 to 1.8 ppm U, and 1.5 to 6 ppm

Th characterize sedimentary and volcaniclastic rocks.

In the Forest immediately south of the Mt. Ethel pluton,

relatively low concentrations of 0.7 to 1.4 percent K and 1.5

to 6 ppm Th and moderate values of 1.4 to 2.6 ppm U for an

area of 1.7-Ga metamorphic and plutonic rocks indicate

radioactive lithologies similar to those for similar rocks north

of the Mt. Ethel pluton. Farther south in the Forest, and

including the northern part of the Gore Range, radioelement

concentrations are generally moderate to low with some

K-distinctive areas for mostly 1.7-Ga granitic rocks. Two

3-element relative highs occur southeast of Steamboat

Springs in the Forest in 1.7-Ga granitic rocks, and have possible polymetallic vein deposit significance. One is about 17

mi southeast of Steamboat Springs in the Service Creek

drainage at about lat 40°17'N., long 106°40'W., where radioelement concentrations are 1.8 to 2.2 percent K, 2.6 to 3.0

ppm U, 9 to 11 ppm Th, no U:Th expression, and distinct

K:Th of <0.18. The other is 20 mi south-southeast of Steamboat Springs between the Morrison and Silver Creek drainages at about lat 40°12'N., long 106°45'W., where

concentrations are 1.8 to 2.0 percent K, 2.2 to 2.6 ppm U, 11

to 12 ppm Th, no U:Th expression, and distinct K:Th of

<0.18. East and northeast of the Service Creek feature and 15

to 25 mi north-northwest of Kremmling, partly within the

GEOPHYSICAL STUDIES
48 MINERAL RESOURCE POTENTIAL AND GEOLOGY, ROUTT NATIONAL FOREST, COLORADO

between quadrangles, the appropriate report (in the DOE

GJBX series) should be consulted. K and Th statistics were

used to help understand the geology in the vicinity of possible U deposits.

The Craig 1°×2° quadrangle (LKB Resources, Inc.,

1979) includes most of the Forest and 2 of 53 preferred U

anomalies determined for the quadrangle occur within the

Forest. Both are in the northern part of the Forest, in the Park

Range. One is about 2 mi east-southeast of Columbine in an

area of 1.4 to 1.8 ppm U where it is associated with the Tertiary felsic intrusion of Hahns Peak. The anomaly is of relatively limited dimension and is not apparent in the

small-scale radioelement maps of this report. It is in an area

of relatively higher U:Th and K:Th because Th is relatively

low (1.5 to 5 ppm).

The other preferred anomaly within the Forest is about

13 mi west of Northgate in an area of 2.2 to 2.6 ppm U where

the source rock is Proterozoic felsic and hornblendic gneiss.

It is in an area of relatively higher U:Th (>0.35) because of

low Th (1.5 to 5 ppm); K is low (1.0 to 1.2 percent) and K:Th

is moderate and obscure.

As many as 11 preferred U anomalies are near or adjacent to the Forest, none of which are associated with known

U occurrences. (In the following text, the U, K and Th features discussed are from the NURE statistical analysis;

some are apparent in maps of this report, some are not.)

Eight of these anomalies have Cretaceous shale (Mancos

primarily) as the source rock; all have good expression in U

and its ratios because of slight or no expression in K and

Th; locations includes sites northwest of Steamboat

Springs, north and south of the southwest part of the Forest,

and in several locales in Middle Park along the contact with

the Gore Range. The other three anomalies are also well

expressed in U and (generally) not in K and in Th and are in

the Kremmling-Granby area; one is just northeast of

Whitely Peak where the source rock is Tertiary intrusive

igneous rock intruded into Pierre Shale; one is in intermediate volcanic rocks of Tertiary age; the third is in the Middle

Park Formation.

NURE procedures culminated in evaluation of the

favorability of specific 1°×2° quadrangles for the occurrence

of U deposits, published in the Department of Energy (DOE)

PGJ/F series. Geological, geochemical, geophysical, and

mining data were used, although tight deadlines on occasion

resulted in aerial gamma-ray survey or geochemical data not

being used because of not being available at the time the

evaluation was done. The evaluation of the Craig 1°×2°

quadrangle utilized both the aerial survey and geochemistry

and resulted in one area, which includes part of the Forest,

being judged favorable for U deposits. This area includes the

approximate western half of the southwest part of the Forest

and is described as follows (Craig and others, 1982, p. 1):

Area D is in the Salt Wash Member of the Morrison Formation (Upper Jurassic) in the southwestern part of the Craig quadrangle and uranium

deposits are classed as nonchannel-controlled

peneconcordant sandstone-type deposits (subclass

244). The favorable area is essentially a projection

of the Meeker mining district based on the regional

stratigraphy of the Salt Wash Member and the

presence of relatively thick, high-energy sandstone

beds containing carbonaceous trash. No airborne

radiometric, water or stream-sediment, or

drill-hole anomalies support the favorable

assignment.

The Salt Wash Member of the Morrison Formation does

not crop out in the southwestern part of the Forest—the

favorable judgment is based on subsurface stratigraphic

extrapolation.
49

MINERAL RESOURCES—LOCATABLE MINERALS

By Sandra J. Soulliere and Margo I. Toth

Locatable minerals include all minerals for which

exploration, development, and production are regulated

under the Federal General Mining Law of 1872. Most

metallic minerals and a large group of nonmetallic or industrial minerals are included in this group. The U.S. Bureau of

Mines has identified and described all the known mines and

prospects in the Forest, and some of the following is taken

from that report (Neubert, 1994).

MINING AND EXPLORATION HISTORY

The earliest known mining in the area occurred in prehistoric times, as evidenced by a quartzite quarry near Rabbit

Ears Pass. Archaeologists have recently discovered the

remains of a stone-age quarry mined by nomadic tribes for

quartzite as early as 8,000 years ago and as late as 500 years

ago (Rocky Mountain News, 1993). The remains of nearly

200 quarry pits are found within the Forest at Windy Ridge,

about 15 mi southeast of Steamboat Springs.

Modern mining from areas within or near the Forest

began in the late 1800’s with the discovery of gold placer

deposits below Hahns Peak. Major quantities of metallic

minerals have not been produced from the Forest (Neubert,

1994). Minor mineral production was recorded between

1860 and the early 1970’s from several mines and mining districts within and adjacent to the Forest. These areas include

the Elkhorn Mountain, Hahns Peak, Pearl, Crystal, Northgate, Teller, and La Plata-Dailey mining districts, and the

Greenville mine area (fig. 5). Areas explored for metallic

minerals and areas where mining has occurred but production

is unknown include the following: Slavonia, Fish Creek, Blue

Ridge, and Slater mining districts, and the Parkview and Poison Ridge intrusive centers in the Rabbit Ears Range.

Although exploration for mineral deposits is ongoing, the last

major production in the area was recorded in 1973 from fluorite deposits in the Northgate mining district.

The molybdenum mine at Henderson (fig. 5) is located

just outside the southeastern boundary of the Forest. The

Northgate mining district (fig. 5), the second major fluorite

producer in Colorado, includes part of the northeastern Forest. Some of the largest concentrations of mineral deposits in

the Rocky Mountain region are just south of the Forest in an

area known as the Colorado Mineral Belt. The Colorado

Mineral Belt is an elongate zone of hydrothermal mineral

deposits that extends from the San Juan Mountains in southwestern Colorado to the Front Range northwest of Denver

(Tweto and Sims, 1963). A large concentration of mineral

deposits and numerous intrusive and volcanic rocks of late

Cretaceous to late Tertiary age characterize the belt. The belt

extends into the southeastern part of the Forest (fig. 5).

POTENTIAL FOR UNDISCOVERED MINERAL 

RESOURCES—LOCATABLE MINERALS

The Routt National Forest and the Middle Park Ranger

District of the Arapaho National Forest were evaluated for 10

locatable mineral deposit types (pl. 1, fig. 1). In the following

sections, the characteristics of the known deposits in the Forest are briefly summarized, and assessment criteria are established. The assessment criteria were used to evaluate areas

within the Forest for the potential for each deposit type. Specific areas within the Forest that met the criteria were delineated and assigned a rating of either moderate or high

potential; areas of low potential were not outlined for metallic resources (Gourdarzi, 1984). These ratings refer to the

likelihood for the occurrence of a given deposit type. Levels

of certainty, labeled A through D, were also assigned to qualify the data; level A indicates the least amount of supporting

data and level D the greatest. Definitions and explanations of

the mineral resource rating system are in Appendix 2. On figures 1–2, 16–25, and plate 1, areas of high potential are

shown in dark gray and areas of moderate potential are shown

in light gray. Each mineral deposit type was assigned a letter

and number designation to more easily distinguish each type

on a map or figure in the report. These letter-number designations are shown on the figures, on plate 1, and have been

previously described in the section “Undiscovered Mineral

and Energy Resources.”

STOCKWORK MOLYBDENUM (A)

COMMODITIES, BY-PRODUCTS, AND TRACE METALS

The commodity is molybdenum; by-products are

commonly tungsten, bismuth, and tin. 
50 MINERAL RESOURCE POTENTIAL AND GEOLOGY, ROUTT NATIONAL FOREST, COLORADO

HOST ROCKS

The deposits are in or associated with Tertiary granitic

plutons. Ore may also occur in the country rocks, including

Proterozoic crystalline rocks and Paleozoic and Mesozoic

sedimentary rocks.

STRUCTURAL CONTROL

Stockwork veins form in fractures produced by the

intrusion of a small stock at very shallow crustal levels. The

joint pattern may be controlled by joint patterns within the

host pluton or by bedding, joints, or faults outside the pluton.

Stockwork veins can occur in any brittle rock that can be

shattered repeatedly, including the host pluton and favorable

country rocks.

AGE

In Colorado, stockwork molybdenum deposits are Oligocene and younger in age.

DEPOSIT DESCRIPTION

Stockwork molybdenum deposits occur in epizonal

granitic plutons. Stocks emplaced during multiple phases of

intrusion and alteration are the most favorable for hosting

stockwork deposits. Favorable plutons have porphyritic textures, associated intrusive breccias and pebble dikes, and

commonly have radial dikes. Source rocks have greater than

76 percent SiO2, high trace amounts of fluorine, rubidium,

yttrium, and niobium; and low trace amounts of barium,

strontium, and zirconium.

Ore minerals occur in a complex network of stockwork

quartz veins, but disseminated flakes of molybdenite are also

present in the host granite. Quartz, molybdenite, potassium

feldspar, pyrite, fluorite, and phlogopite are the dominant

vein minerals; a variety of other minerals may also be

present, including topaz, cassiterite, and magnetite. The ore

zone occurs above the central igneous complex and also

overlaps it; the general morphology of the zone is an

inverted bowl. Peripheral veins contain lead, zinc, silver, and

gold. Copper may be present but is rare in this type of stockwork system (White and others, 1981).

Wallrocks exhibit pervasive hydrothermal alteration.

Assemblages of alteration minerals are zoned outward from

potassium feldspar in the system center to quartz-sericitepyrite and argillic alteration to propylitic assemblages along

the outer margins. Oxidized pyrite in the phyllic zone commonly produces a red halo above and around the deposit.

Early potassic alteration in intrusions was generated by fluids

derived by magmatic processes, and later quartz and sericite

formed by mixing with fluids of meteoric origin (Hannah and

Stein, 1986).

GEOCHEMICAL SIGNATURE

Stream-sediment samples typically contain anomalous

concentrations of molybdenum, lead, tungsten, silver, tin,

and gold, although some of these elements are more

abundant in peripheral vein systems. Rock samples contain

anomalous concentrations of molybdenum, silver, tungsten,

bismuth, and fluorine.

GEOPHYSICAL SIGNATURE

Plutons are expected in the area of low gravity of the

Colorado Mineral Belt, which is associated with a large,

unexposed batholith. Within this area, favorable terrane may

be associated with a smaller, residual gravity low in the

southeast part of the Forest.

Proterozoic granitic host rocks are often the most magnetic rocks in the Forest and commonly have associated magnetic highs. Tertiary granitic host rocks may or may not be

magnetic and are less dense than Proterozoic rocks,

especially where extensively altered. Gravity lows, with or

without associated magnetic highs, may indicate Tertiary

intrusive rock.

Host plutons are notably radioactive because of the

elevated concentrations of K, U, and Th that characterize

granitic/felsic igneous rocks, and consequently are detectable by radioactivity measurements, dependent on surface

exposure.

KNOWN DEPOSITS

There are no known deposits in the Forest. However, a

deposit is present at Red Mountain, just outside the southeastern border of the Forest near Berthoud Pass.

ASSESSMENT CRITERIA

1. Presence of a Tertiary, fluorine-rich, high-silica

pluton.

2. Multistage igneous activity.

3. Stocks emplaced at very shallow crustal levels in a 

Tertiary extensional environment.

4. Extensive hydrothermal alteration within and around

the stock.

5. Presence of molybdenite in veins and stockwork

veinlets and as disseminated flakes.

6. Anomalous molybdenum concentrations in rocks and 

stream-sediment samples.

ASSESSMENT

Area A1.—A small target area around Hahns Peak in the

north-central part of the Forest has high resource potential for

molybdenum in stockwork deposits (pl. 1, fig.16), with certainty level C. In this area, a composite laccolithic intrusion

of latite and quartz latite is present and includes a central

cone-shaped body of intrusive breccia. Hydrothermal alteration consists of early albitization, followed by a later phase

of propylitic, argillic, phyllic, and advanced argillic mineral

assemblages zoned around the breccia cone sheet (Cascaceli,

1984). Stockwork veins of silica, pyrite, and molybdenite

are present, and galena, sphalerite, chalcopyrite occur
MINERAL RESOURCES—LOCATABLE MINERALS 51

107°30' 107°00' 106°30' 106°00'

41°00'

40°30'

40°00'

39°30'

COLORADO

Craig

Steamboat

Springs

Eagle

Vail

Kremmling

Walden

Lake

Granby

Rabbit Ears Range

Williams Fork Mountains

Flat

Tops

Gore Range

Gore Range

Park Range

Hahns Park Range

Peak

Mt

Zirkel Medicine Bow Mountains

Rocky

Mountain

National

Park

Colorado River

Colorado River

0 5 10 MILES

WYOMING

COLORADO

i

70

Approximate boundary of the

Routt National Forest and the

Middle Park Ranger District

of the Arapaho National Forest

Approximate boundary of the

Routt National Forest and the

Middle Park Ranger District

of the Arapaho National Forest

Approximate boundary of the

Routt National Forest and the

Middle Park Ranger District

of the Arapaho National Forest

EXPLANATION

RESOURCE POTENTIAL

High

Moderate

A1

A2

A3

locally. Minor amounts of auriferous pyrite, argentiferous

tetrahedrite, and trace covellite are also locally present.

Area A2.—A small target area in the northeastern part

of the Forest in the Never Summer Mountains has high mineral resource potential for molybdenum in stockwork deposits (pl. 1, fig. 16), with certainty level C. Within this area,

the Tertiary Mount Cumulus stock has intruded highly fractured Proterozoic igneous and metamorphic rocks and

younger sediments. The stock is rhyolitic in composition

and contains molybdenite and pyrite in miarolitic cavities

and along joints (Pearson and others, 1981). Anomalous

concentrations of fluorine, lead, zinc, silver, tin, and niobium

are in sediment samples from surrounding drainages. The

body occupies a distinctive magnetic high and a gravity low,

although the gravity low could be due to the effects of

thrusting. The stock has expression in the K, U, and Th

aeroradioactivity maps (figs. 11-15), reflecting its rhyolitic

composition and probable surface expression. Although no

large molybdenum deposits have been found, the unexplored

interior of the stock is permissive for a deposit.

Figure 16. Mineral resource map for stockwork molybdenum deposits (A) in the Routt National Forest and Middle Park Ranger

District of the Arapaho National Forest, Colorado.
52 MINERAL RESOURCE POTENTIAL AND GEOLOGY, ROUTT NATIONAL FOREST, COLORADO

Area A3.—In the southeastern part of the Forest, a

northeast-trending area has moderate potential for

molybdenum in stockwork deposits (pl. 1, fig. 16), with certainty level C. Rock and stream-sediment samples from this

area contain anomalous concentrations of molybdenum,

lead, zinc, silver, and minor copper, tin, and gold. Pyrite,

fluorite, and molybdenite are present in many of the samples

(Theobald and others, 1985). The area occurs within the

Colorado Mineral Belt and is adjacent to the Henderson

mine, a known stockwork molybdenum system with previous metal production. The northeast part of the area has relatively elevated values in the K, U, and especially the Th

data (figs. 11–13), and the Henderson mine (outside the

Forest) has a distinct Th character.

ECONOMIC SIGNIFICANCE

The United States currently exports molybdenum and

has almost half the world’s identified resources, most of

which occur in deposits of this type (U.S. Bureau of Mines,

1993). Resources of molybdenum are adequate to supply

world needs for the foreseeable future.

PORPHYRY COPPER-MOLYBDENUM (B)

COMMODITIES, BY-PRODUCTS, AND TRACE METALS

Commodities are copper and molybdenum; byproducts are gold, tungsten, and tin; trace metals include

silver, lead, and zinc.

HOST ROCKS

Host rocks include late Cretaceous and Tertiary granitic

rocks, Proterozoic crystalline rocks, and some Mesozoic

sedimentary rocks.

STRUCTURAL CONTROL

Stockwork veins form in fractures produced by the

intrusion of a small stock at very shallow crustal levels. The

joint pattern may be controlled by joint patterns within the

host pluton or by bedding, joints, or faults outside the pluton.

Stockwork veins can occur in any brittle rock that has been

shattered repeatedly including the host pluton and favorable

country rocks. Favorable plutons occur along intersections

of regional fault systems.

AGE

Porphyry copper-molybdenum deposits are late

Cretaceous to late Eocene in age.

DEPOSIT DESCRIPTION

Porphyry copper-molybdenum deposits are a combination of disseminations and stockwork veins that occur in the

shattered portions of an intrusive and in surrounding country

rocks. The upper and outer margins of the intrusive have

been shattered from adjustment during cooling or from the

high vapor pressure of late mineralizing fluids. Granitic

plutons that have multiple phases of intrusion are the most

favorable for these types of deposits. Compositions of these

plutons range from monzogranite to granite in the Forest.

Stocks are most commonly porphyritic; they have radial

dikes and associated breccias.

Disseminated ore is most common in the core of the

intrusion, and veinlets of ore are dominant toward the outer

margins of the intrusive and in the country rock. The richest

ore zone occurs where disseminations are still dominant over

veinlets (Lowell and Guilbert, 1970). The primary sulfides

consist of pyrite, chalcopyrite, bornite, and molybdenite.

Other minerals that may be present include sphalerite,

galena, gold and silver minerals, wolframite, and cassiterite.

A pyrite-rich shell usually occurs just outside of the main ore

zone. Erosion and weathering of the metal-bearing portions

of the intrusions releases copper, giving rise to a zone of

supergene sulfide enrichment. The oxidation of pyrite in the

argillic zone commonly produces a large red-colored halo

above and around the deposit.

Porphyry copper-molybdenum deposits show pervasive hydrothermal effects that extend into surrounding wallrocks. Assemblages of alteration minerals exhibit

systematic spatial and temporal relationships with respect to

one another. Characteristic alteration assemblages are zoned

outward from potassium feldspar in the system center to

quartz-sericite-pyrite and argillic alteration; propylitic

assemblages occur along the outer margins.

GEOCHEMICAL SIGNATURE

The deposits have anomalous concentrations of copper,

molybdenum, zinc, lead, silver, and local tungsten, boron,

and strontium anomalies. In panned concentrates, tin, tungsten, molybdenum, and fluorine may be present in anomalous concentrations.

GEOPHYSICAL SIGNATURE

Plutons are expected in the gravity low of the Colorado

Mineral Belt, which is associated with a large, unexposed

batholith. Within this area, favorable terrane may be associated with a smaller, residual gravity low in the southeast part

of the Forest.

Proterozoic granitic host rocks are often the most magnetic rocks in the Forest and commonly have associated

magnetic highs. Tertiary granitic host rocks may or may not

be magnetic and are less dense than Proterozoic rocks, especially where extensively altered. Gravity lows, with or without associated magnetic highs, may indicate Tertiary

intrusive rock. Granitic host rocks can be distinctive in

aeroradioactivity maps, dependent on surface exposure,

because of relatively higher radioelement (K, U, Th)

concentrations.
MINERAL RESOURCES—LOCATABLE MINERALS 53

KNOWN DEPOSITS

One known deposit is in the Forest at Poison Ridge in

the Rabbit Ears Range. The Poison Ridge deposit has been

estimated to contain 51 million tons of rock averaging 0.22

percent copper in the hypogene zone, and 3.3 million tons

averaging 0.70 percent copper in the supergene zone

(Karimpour, 1982). The Anaconda Company estimated

36–45 million tons averaging 0.15–0.20 percent copper,

possibly an additional 90 million tons averaging 0.05 percent

copper, and trace amounts of gold for the Poison Ridge

deposit (Anaconda Document Collection, file 93304.01,

1981).

ASSESSMENT CRITERIA

1. Presence of a late Cretaceous to Tertiary 

monzogranite to granite intrusion.

2. Extensive hydrothermal alteration in and around the 

pluton.

3. Presence of chalcopyrite and molybdenite.

4. Anomalous copper and molybdenum concentrations 

in rock and stream-sediment samples.

ASSESSMENT

Area B1.—A large east-west-trending area along the

crest of the Rabbit Ears Range has moderate resource potential for copper and molybdenum in porphyry copper deposits, (pl. 1, fig. 17), with certainty level C. Several Tertiary

hypabyssal plutons crop out within this area, including the

one at Poison Ridge that contains identified copper

resources. Some of the plutons have alteration pattern

assemblages typical of porphyry copper-molybdenum

deposits and have anomalous concentrations of copper, lead,

zinc, silver, molybdenum, and (or) gold in rocks and streamsediment samples (Karimpour and Atkinson, 1983; Spicker,

1973). Magnetic data indicate that several plutons of similar

size are buried beneath the Rabbit Ears volcanics at the western edge of the area of potential. The plutons are along an

east-west trend that is likely to be a Proterozoic lineament

and a zone of weakness.

Area B2.—A small target area on Parkview Mountain

has high mineral resource potential for copper and molybdenum in copper porphyry deposits (pl. 1, fig. 17), with certainty level C. Parkview Mountain is within the area of

moderate potential outlined in B1, but veinlets and disseminations of copper, lead, zinc, silver, and molybdenite are

present (Spicker, 1973). Alteration is moderate to intense

and consists of locally pervasive quartz-sericite-pyrite with

local zones of silica stockworks.

Area B3.—In the headwaters of the Illinois River, a

small target area surrounding an intrusive body has high

resource potential for copper and molybdenum in porphyry

copper deposits (pl. 1, fig. 17), with certainty level C. In

this location, a bedded breccia pipe of quartz latite composition contains anomalous concentrations of copper, lead,

zinc, gold, silver, molybdenum, and arsenic (Metzger,

1974) Alteration is pervasive and consists of chlorite and

sericite. The breccia pipe lies near the intersection of two

major linear intrusive trends: the Rabbit Ears intrusives to

the west and the intrusives of the Never Summer Range to

the north.

ECONOMIC SIGNIFICANCE

The United States currently has almost half the world’s

identified resources of molybdenum and had a very small (3

percent) import reliance for copper in 1992 (U.S. Bureau of

Mines, 1993).

POLYMETALLIC VEINS (C)

COMMODITIES

Commodities include lead, zinc, silver, copper, and

gold with minor molybdenum, tin, tungsten, bismuth, and

antimony.

HOST ROCKS

Host rocks vary in lithology. Most deposits occur in

faults, fractures, or shear zones in Proterozoic igneous and

metamorphic rocks. Other host rocks include Tertiary

granitic plutons, dikes, and Paleozoic sedimentary rocks.

Brittle or easily fractured rocks are particularly susceptible

to mineralization.

STRUCTURAL CONTROL

Location of vein deposits is controlled by permeability

of host rock. Most deposits are near or adjacent to faults,

fractures, fault intersections, and breccia or shear zones

although some occur near intrusive contacts. 

AGE

Ages of mineralization are late Cretaceous and Tertiary.

Host rocks may be of any pre-intrusive age.

DEPOSIT DESCRIPTION

Veins occur in a wide variety of host rocks and are

related to late Cretaceous and Tertiary granitic plutons. Vein

and vein systems are controlled by the distribution and size

of fractures and are commonly modified by movement along

fractures both during and after vein filling. Wallrock alteration adjacent to the vein is common, with the most intense

alteration a few feet from the vein.

GEOCHEMICAL SIGNATURE

Geochemical anomalies vary with the composition of

the individual vein. Lead, zinc, copper, silver, gold, arsenic,

antimony, manganese, and barium are most commonly in
54 MINERAL RESOURCE POTENTIAL AND GEOLOGY, ROUTT NATIONAL FOREST, COLORADO

107°30' 107°00' 106°30' 106°00'

41°00'

40°30'

40°00'

39°30'

COLORADO

Craig

Steamboat

Springs

Eagle

Vail

Kremmling

Walden

Lake

Granby

Williams Fork Mountains

Flat

Tops

Gore Range

Gore Range

Park Range

Hahns Park Range

Peak

Mt

Zirkel Medicine Bow Mountains

Rocky

Mountain

National

Park

Colorado River

Colorado River

0 5 10 MILES

WYOMING

COLORADO

i

70

Approximate boundary of the

Routt National Forest and the

Middle Park Ranger District

of the Arapaho National Forest

Approximate boundary of the

Routt National Forest and the

Middle Park Ranger District

of the Arapaho National Forest

Approximate boundary of the

Routt National Forest and the

Middle Park Ranger District

of the Arapaho National Forest

EXPLANATION

RESOURCE POTENTIAL

High

Moderate

B1

B2

Rabbit Ears B3

Range

stream-sediment and rock samples. Element dispersal from

individual veins may not be great enough to produce a

detectable anomaly in stream-sediment samples.

GEOPHYSICAL SIGNATURE

Most veins are too small to be detected by regional

gravity and magnetic surveys. Some intrusives and major

structures exhibit prominent anomalies and steep gradient

zones in the magnetic- and gravity-anomaly data. Many

areas of known veining and wallrock alteration are

characterized by magnetic highs; however, delineating these

highs requires very detailed geophysical surveys. Electrical

resistivity and other site-specific ground surveys are

successful at locating veins. However, no such data were

available for this assessment. Host rocks can have distinct

aeroradioactivity expression, dependent on lithology,

surface expression, and detail of aerial survey.

Figure 17. Mineral resource map for stockwork copper-molybdenum deposits (B) in the Routt National Forest and Middle Park

Ranger District of the Arapaho National Forest, Colorado.
MINERAL RESOURCES—LOCATABLE MINERALS 55

107°30' 107°00' 106°30' 106°00'

41°00'

40°30'

40°00'

39°30'

COLORADO

Craig

Steamboat

Springs

Eagle

Vail

Kremmling

Walden

Lake

Granby

Rabbit Ears Range

Williams Fork Mountains

Flat

Tops

Gore Range

Gore Range

Park Range

Park Range

Hahns

Peak

Mt

Zirkel Medicine Bow Mountains

Rocky

Mountain

National

Park

Colorado River

Colorado River

0 5 10 MILES

WYOMING

COLORADO

i

70

Approximate boundary of the

Routt National Forest and the

Middle Park Ranger District

of the Arapaho National Forest

Approximate boundary of the

Routt National Forest and the

Middle Park Ranger District

of the Arapaho National Forest

Approximate boundary of the

Routt National Forest and the

Middle Park Ranger District

of the Arapaho National Forest

EXPLANATION

RESOURCE POTENTIAL

High

Moderate

C1

C2 C3

C5

C4

C6

C7

KNOWN DEPOSITS

Known vein deposits include the La Plata mine in the

La Plata mining district, the Endomile mine in the Teller

district, an area of small claims in the Slater district, the

Elkhorn mine in the vicinity of the Encampment district,

the Service Creek claims area east of Rabbit Ears Pass,

Buffalo Pass and Buffalo Ridge area in the Mt. Zirkel

Wilderness, and the Hahns Peak/Farwell Mountain district.

ASSESSMENT CRITERIA

1. Presence of vein deposits in the vicinity.

2. Presence of base or precious metals in rock or 

stream-sediment samples.

3. Presence of faults, breccia or shear zones, fault intersections, or intrusive contacts.

4. Presence of plutonic or hypabyssal igneous bodies of 

Late Cretaceous or Tertiary age.

Figure 18. Mineral resource map for polymetallic vein deposits (C) in the Routt National Forest and Middle Park Ranger District

of the Arapaho National Forest, Colorado.
56 MINERAL RESOURCE POTENTIAL AND GEOLOGY, ROUTT NATIONAL FOREST, COLORADO

ASSESSMENT

Area C1.—In the northwest part of the Forest, the area

around Elkhorn Mountain has high mineral resource potential for metals in polymetallic veins (pl. 1, fig. 18), with

certainty level C. Within this area, the Elkhorn mine contains

elevated amounts of silver, gold, cadmium, copper, mercury,

lead, antimony, and zinc in rock samples. Stream-sediment

samples from the area did not contain anomalous concentrations of metals. The host rock is the Elkhorn gabbro, and

metals are found in pods and veins. The entire area is outlined by a magnetic high. The Elkhorn gabbro is within a

sizeable aeroradioactivity low (figs. 11–13), as mafic rocks

are relatively deficient in K, U, and Th concentrations.

Area C2.—In the northwest part of the Forest, the area

around Farwell Mountain has high mineral resource potential for metals in polymetallic veins (pl. 1, fig. 18), with certainty level C. Several workings in this area include trenches

that expose quartz veins and veinlets in gneiss. Rock samples from this area contain anomalous to highly anomalous

concentrations of copper, bismuth, tin, molybdenum, gold,

silver, and tungsten; lead and zinc are conspicuous for their

lack of anomalous values in the rock samples. Stream-sediment samples have anomalous concentrations of lead, zinc,

silver, and molybdenum. Host rock is pegmatite, schist, and

gneiss.

Area C3.—A small area in the northern part of the Park

Range at Diamond Park has moderate mineral resource

potential for metals in polymetallic veins (pl. 1, fig. 18), with

certainty level B. Rock samples from this area have anomalous concentrations of bismuth, copper, lead, and molybdenum, silver, and tungsten. The metals are found in small

quartz veins associated with fault zones in Proterozoic

gneiss.

Area C4.—In the northeastern part of the Forest, the

Teller mining district and surrounding area has high mineral

resource potential for metals in polymetallic veins (pl. 1, fig.

16), with certainty level C. Numerous geochemical anomalies are in this area, including antimony, arsenic, cadmium,

copper, gold, lead, molybdenum, silver, tin, and zinc. Silver

was present in anomalous to highly anomalous amounts in

rock samples (as much as 700 ppm Ag) and in streamsediment samples. The district also contained anomalous to

highly anomalous molybdenum in rock samples; one sample

contained greater than 2,000 ppm Mo. Five rocks analyzed

contained more than 10 percent arsenic. Two rock samples

contained greater than 500 ppm cadmium. As much as 1,500

ppm Sb was found in rock samples. Lead was found to be

highly anomalous in several rock samples (four were greater

than 1.5 percent Pb) and in stream-sediment samples. Six

rock samples contained zinc in quantities from 1 to 12 percent Zn; stream-sediment samples were also highly anomalous in zinc. Gold, copper, and tin were also found at

anomalous levels in rock samples. Isolated stream-sediment

samples contained anomalous concentrations of arsenic,

cobalt, copper, and molybdenum in addition to ubiquitous

silver, lead, and zinc. In the Teller district, mines and prospects were located in mineralized fracture zones and veins in

Proterozoic granite and schist.

Area C5.—Moderate mineral resource potential for

metals in polymetallic veins (pl. 1, fig. 18), with certainty

level B, exists in a small area on the southwest side of the

Gore Range in and near Morrison Creek. Two streamsediment samples from this vicinity contain highly anomalous concentrations of silver, arsenic, vanadium, zinc,

cobalt, tin, and tungsten. The value for silver (69 ppm) is

one of the highest values in the Forest. The host rock in

this area is Proterozoic granitic rock. The granite is a distinct, although moderate, feature on the aeroradioactivity

maps (figs. 11–13).

Area C6.—At the north end of the Williams Fork

Mountains, a small area on Copper Mountain has high

mineral resource potential for metals in polymetallic veins

(pl. 1, fig. 18), with certainty level C. Host rocks in the area

are isolated pegmatites lenses in gneiss. Rock samples from

the area have highly anomalous concentrations of copper,

but other metals are present only in low concentrations

(Neubert, 1994).

Area C7.—In the southeasternmost part of the Forest, a

large northeast-trending area at the southern extent of the

Williams Fork Mountains has high mineral resource potential for metals in polymetallic veins (pl. 1, fig. 18), with certainty level C. This area encompasses the Atlanta and La

Plata mining districts. Rock samples contain anomalous

concentrations of molybdenum, lead, arsenic, and silver, and

isolated anomalous values of lead, tin, and zinc. Heavymineral-concentrate samples have anomalous concentrations of copper, tin, zinc, silver, molybdenum, lead. The area

occupies several magnetic lows and is on the edge of the

Colorado Mineral Belt (fig. 5). The Straight Creek fault

zone extends through the center of the area of potential.

Metals are associated with shears and faults in the Proterozoic Silver Plume Granite. 

ECONOMIC SIGNIFICANCE

Polymetallic vein deposits have produced copper, lead,

and zinc, and the precious metals gold and silver. The

United States has a net import reliance of 8 percent lead, 3

percent copper, and 34 percent zinc; gold and silver are currently exported (U.S. Bureau of Mines, 1993). Porphyry copper mines in the Southwestern United States produce the

major portion of this metal in the United States, whereas the

Midcontinent region produces most of the Nation’s lead and

zinc from large replacement deposits. A major change in

demand or price for these commodities would be needed to

support operation of small mines required to exploit vein

deposits, although the presence of silver and gold would certainly enhance the possibility of development. Because

veins the Forest are likely to be small, production from them

is likely to have only local economic impact.
MINERAL RESOURCES—LOCATABLE MINERALS 57

MASSIVE SULFIDES (D)

COMMODITIES

Commodities include lead, zinc, silver, copper, and

gold.

HOST ROCKS

Deposits are found in Proterozoic calc-silicate and

hornblende gneiss, amphibolite, and felsic gneiss units.

Within the Forest, major areas of these rock types occur west

of the Williams Fork thrust fault, within the Never Summer

Mountains, northwest of the Independence Mountain thrust

fault, and along the Park Range in the largest parcel of the

Forest.

STRUCTURAL CONTROL 

No regional scale structural control is evident; locally

small-scale folds may have concentrated ore. Deposits

occur as stratabound layers, pods, or lenses within the host

rock; they tend to cluster and follow specific stratigraphic

horizons.

AGE

Deposits have the same Proterozoic age as the enclosing host rock.

DEPOSIT DESCRIPTION

The term “massive” refers to the mineralization composed entirely of sulfides and does not carry any size or textural connotation (Sangster, 1972). Principal ore minerals

found in these deposits are sphalerite, chalcopyrite, and

galena, with minor amounts of silver and gold. Gahnite (zinc

spinel), magnetite, and silicates make up the matrix for the

sulfide minerals. The sulfides occur in small to large lenses

and in laterally extensive zones of disseminated sulfides; all

are generally conformable to the layering in the enclosing

gneisses and amphibolite. The sulfide ore minerals weather

to produce oxide, carbonate, and sulfate minerals.

Although most of the original textures and structures

have been destroyed by intense regional metamorphism and

deformation, the stratabound deposits are generally considered to be metamorphosed volcanic- or sedimentary-hosted

sulfide deposits (Sheridan and others, 1990). In these

deposits, the sulfide minerals were syngenetically deposited

with felsic and mafic volcanic and sedimentary rocks in a

submarine environment. Deposition of the metals probably

occurred as the result of exhalative discharge of hydrothermal vents in the sea floor. Later metamorphism converted

the volcanic rocks to amphibolite, calc-silicate, and felsic

gneisses. It is this later metamorphism, along with deformation, that makes interpretation of the origin of these

deposits difficult. Varying interpretations concerning the

origin and paleotectonic setting of the deposits in northern

Colorado and southern Wyoming have been proposed

(Tweto, 1968; Giles, 1976, 1987; Sheridan and others,

1990; Klipfel, 1992). The Forest therefore has potential for

several types of massive sulfide deposits. Recent work by

Klipfel (1992) concluded that some stratabound sulfide

deposits in the northern Park Range were sediment-hosted

exhalative (Sedex) deposits related to volcanism that produced nearby Besshi-type and mixed Besshi-Sedex (Broken

Hill) type deposits. According to Klipfel, deposition

occurred in an ensialic-rift setting rather than an island-arc

setting as is currently accepted for this area. The area also

may host Kuroko-type (Franklin and others, 1981) volcanogenic massive sulfide deposits.

GEOCHEMICAL SIGNATURE

Geochemical signatures include anomalous concentrations of copper, lead, and zinc in stream-sediment and rock

samples.

GEOPHYSICAL SIGNATURE

Magnetic and gravity data are of minimal use in identifying favorable terranes. Magnetite-rich zones can be

detected with magnetic surveys, although most zones in this

terrane are too small to be detected by regional surveys.

Deposits associated with felsic volcanic rocks (Kuroko type)

would have an aeroradioactivity signature, dependent on

surface expression and detail of aerial survey.

KNOWN DEPOSITS

Several stratabound massive sulfide deposits and

related occurrences are within or near the Forest, mainly in

the northern Park Range and southern Sierra Madre. Mines

in the Pearl district, near the community of Pearl, and the

Greenville mine, north of Steamboat Springs, are considered

by Sheridan and others (1990) to be examples of stratabound

massive sulfide deposits in amphibolite-facies terrane.

ASSESSMENT CRITERIA

1. Presence of Proterozoic gneiss, especially amphibolite and calc-silicate rocks.

2. Presence of geochemical anomalies of base and 

precious metals.

3. Metals conformable with layers.

4. Island-arc tectonic setting at the time of mineralization.

ASSESSMENT

Area D1.—A large northeast-trending zone in the

northern part of the Park Range has high mineral resource

potential for massive sulfide deposits (pl. 1, fig. 19), with

certainty level C. This area is underlain by favorable calcsilicate and amphibolite host rocks and stream-sediment

samples have anomalous concentrations of copper, lead,
58 MINERAL RESOURCE POTENTIAL AND GEOLOGY, ROUTT NATIONAL FOREST, COLORADO

107°30' 107°00' 106°30' 106°00'

41°00'

40°30'

40°00'

39°30'

COLORADO

Craig

Steamboat

Springs

Eagle

Vail

Kremmling

Walden

Lake

Granby

Rabbit Ears Range

Williams Fork Mountains

Flat

Tops

Gore Range

Gore Range

Park Range

Hahns Park Range

Peak

Mt

Zirkel

Medicine Bow Mountains

Rocky

Mountain

National

Park

Colorado River

Colorado River

0 5 10 MILES

WYOMING

COLORADO

i

70

Approximate boundary of the

Routt National Forest and the

Middle Park Ranger District

of the Arapaho National Forest

Approximate boundary of the

Routt National Forest and the

Middle Park Ranger District

of the Arapaho National Forest

Approximate boundary of the

Routt National Forest and the

Middle Park Ranger District

of the Arapaho National Forest

EXPLANATION

RESOURCE POTENTIAL

High

Moderate

D1

D2

D3

and zinc. Several mines and prospects are located in the

area, and rock samples contain chalcopyrite, galena, and

sphalerite. Anomalous amounts of silver, bismuth, cadmium, antimony, gold, molybdenum, and tin are also

present in some rock samples. A ridge of gravity and magnetic high extends from the Greenville mine on the west to

the Slavonia mine in the central part of the area. The area

of potential was extended farther to the northeast to include

the Pearl mine and to account for the general northeast

trend of geophysical data.

Area D2.—Enclosing the area of high potential D1, a

large area of moderate potential (pl. 1, fig. 19), certainty

level B, is defined by outcrop of calc-silicate and amphibolite rock. Isolated stream-sediment samples within this

area also contain anomalous concentrations of copper,

lead, and zinc.

Figure 19. Mineral resource map for stratabound massive sulfide deposits (D) in the Routt National Forest and Middle Park

Ranger District of the Arapaho National Forest, Colorado.
MINERAL RESOURCES—LOCATABLE MINERALS 59

Area D3.—In the southeast part of the Forest, a

medium-sized area has high resource potential for massive

sulfides (pl. 1, fig. 17), with certainty level C. This area is

underlain by favorable calc-silicate and amphibolite host

rocks, and stream-sediment samples have anomalous concentrations of copper, lead, and zinc. Pods and small masses

of gossan derived from massive pyrite bodies are common in

the calcic metamorphic rocks, and some gossan retain

geochemically anomalous amounts of silver, lead, and zinc

(Theobald and others, 1985). 

ECONOMIC SIGNIFICANCE

The United States has a net import reliance of 8 percent lead, 3 percent copper, and 34 percent zinc; gold and

silver are currently exported (U.S. Bureau of Mines,

1993).

FLUORSPAR VEINS (E)

COMMODITY

 The commodity is fluorspar.

HOST ROCKS

Dominantly 1.4-Ga Proterozoic granitic rock; lesser

amounts of the White River Formation.

STRUCTURAL CONTROL

Deposits are concentrated along Laramide and riftrelated late Tertiary(?) extensional faults.

AGE

Deposits are of two ages: Laramide and late Tertiary(?).

Only the younger deposits have been productive.

DEPOSIT DESCRIPTION

The following description is from Steven (1960). Fluorite veins and stringers occur along faults in 1.4-Ga granitic

rocks. The fluorite consists typically of botryoidal layers

that formed as successive encrustations along open fractures

or as finely granular aggregates replacing and cementing

fault gouge and the White River Formation. The veins contain numerous open spaces ranging from small pores to large

cavities as much as 20 ft in width and several hundred feet in

diameter. Fluorite is the principal vein material, and fragments of country rock and chalcedony or finely granular

quartz constitute the chief impurities. As much as 2 percent

fine-grained pyrite is locally disseminated through the fluorspar and adjacent wallrock, although generally it is very

sparse. Rounded fragments of wallrocks are in some of the

open cavities along the vein. There is a lack of any notable

wallrock alteration.

Fluorite was deposited from dilute aqueous solution,

predominantly meteoric water, circulating though broken

rock. Rounded fragments of wallrocks in open cavities of

some veins indicate that movement of the mineralizing solution was sufficient to move and abrade these fragments.

Mineral textures of some veins indicate that, locally, mineralization occurred both below and above the water table.

The temperature of deposition ranged between 100°C and

150°C.

In the Forest, all veins occur within 1.4-Ga granite of

the Mt. Ethel pluton and in Cenozoic sediments overlying

the granite. The Mt. Ethel pluton contains interstitial fluorite. Most of the fluorite is magmatic and was deposited as

the pluton crystallized, but some has been mobilized after

first deposition (Snyder, 1987b). Magmatic fluorite in the

Mt. Ethel pluton may have been mobilized and redeposited

as veins or breccias during Tertiary time. Alternatively,

the source of the fluorite may be a buried intrusion of

Tertiary age.

This report only addresses vein fluorite of Tertiary age.

Although magmatic fluorite is present in the Mt. Ethel

pluton, the potential for undiscovered resources of fluorite of

this type of occurrence is low. 

GEOPHYSICAL SIGNATURE

Geophysical data may be used to infer the presence of

buried plutons that could be the source for the fluorite.

Granitic host rocks will be prominent on aeroradioactivity

(K, U, Th) maps, dependent on surface expression.

GEOCHEMICAL SIGNATURE

Anomalous concentrations of fluorine in rocks and

stream-sediment samples.

KNOWN DEPOSITS

The Northgate mining district is in the northeastern part

of the Forest. Most of the production has come from the

Fluorspar-Gero-Penbar and Fluorine–Camp Creek vein

zones. Since 1922, at least 109,700 tons was produced

(Steven, 1960).

ASSESSMENT CRITERIA

1. Presence of late(?) Tertiary, rift-related extensional 

faults.

2. Presence of 1.4-Ga granitic or other hard rocks in 

which open fractures can be maintained; presence of

extremely permeable rock.

3. Anomalous concentrations of fluorine in rocks.

ASSESSMENT

Area E1.—A large area in the northern part of the Forest

has high resource potential for additional deposits of fluorite,

with certainty level C (pl. 1, fig. 20). This area encompasses
60 MINERAL RESOURCE POTENTIAL AND GEOLOGY, ROUTT NATIONAL FOREST, COLORADO

107°30' 107°00' 106°30' 106°00'

41°00'

40°30'

40°00'

39°30'

COLORADO

Craig

Steamboat

Springs

Eagle

Vail

Kremmling

Walden

Lake

Granby

Rabbit Ears Range

Williams Fork Mountains

Flat

Tops

Gore Range

Gore Range

Park Range

Hahns Park Range

Peak

Mt

Zirkel Medicine Bow Mountains

Rocky

Mountain

National

Park

Colorado River

Colorado River

0 5 10 MILES

WYOMING

COLORADO

i

70

Approximate boundary of the

Routt National Forest and the

Middle Park Ranger District

of the Arapaho National Forest

Approximate boundary of the

Routt National Forest and the

Middle Park Ranger District

of the Arapaho National Forest

Approximate boundary of the

Routt National Forest and the

Middle Park Ranger District

of the Arapaho National Forest

EXPLANATION

RESOURCE POTENTIAL

High

E1

E2 E3

the Northgate mining district, a past producer of fluorite. In

this area, fluorite is present as veins and stringers along

faults in 1.4 Ga-granitic rocks and Cenozoic sediments.

Area E2.—On the eastern side of the Park Range, the

Crystal district has high resource potential for additional

deposits of fluorite, with certainty level C (pl. 1, fig. 20).

The district includes rocks of the 1.4-Ga Mt. Ethel pluton,

and fluorite is present in fault breccias and veins (Snyder,

1987b). The Crystal district occupies a magnetic and gravity

high and may be underlain by a younger pluton that provided

the source for the fluorite.

Area E3.—A small area on Delaney Butte east of the

Park Range in North Park has high resource potential for fluorite, with certainty level C (pl. 1, fig. 20). Vein fluorite is

present in the Proterozoic granite (Snyder, 1987b), an extension of the 1.4-Ga Mt. Ethel pluton.

Figure 20. Mineral resource map for fluorspar veins and dissemination deposits (E) in the Routt National Forest and Middle Park

Ranger District of the Arapaho National Forest, Colorado.
MINERAL RESOURCES—LOCATABLE MINERALS 61

ECONOMIC SIGNIFICANCE

The United States has a significant reliance upon import

sources of fluorspar. In 1992, about 87 percent of the fluorspar was imported (U.S. Bureau of Mines, 1993). Within the

United States, deposits in southern Illinois accounted for the

majority of production. The Northgate district accounted for

about 32 percent of Colorado’s production through 1973 and

at one point accounted for about 10 percent of the Nation’s

annual production (Brady, 1975). Production ceased in the

Northgate district in the Forest in 1973. Most of the nearsurface, easily minable fluorspar at Northgate has been

removed, and expensive underground mining would be

required to extract the remaining ore (Neubert, 1994).

VEIN URANIUM (F)

COMMODITIES, BY-PRODUCTS, AND TRACE METALS

Uranium deposits, with associated trace amounts of

gold, silver, antimony, lead, zinc, and molybdenum occur

along fractures and faults in Proterozoic granites.

HOST ROCKS

Host rocks are Proterozoic granitic rock, gneiss, and

pegmatite.

STRUCTURAL CONTROL

Deposits occur within or along fractures or breccia

zones.

AGE

Probably Late Cretaceous to Tertiary.

DEPOSIT DESCRIPTION

This type of deposit occurs in silicified and brecciated

veins along fault zones in 1.7-Ga Proterozoic granitic plutons. Not much is known about the genesis of these deposits. 

Opinions have been divided between hydrothermal and

supergene mechanisms (Nash and others, 1981), but fluidinclusion studies seem to suggest hydrothermal processes,

with later supergene enrichment. Comparison to similar

deposits suggest that the high concentrations of uranium and

other metals in the granites probably reflect uraniumenriched supracrustal rocks. This deposit is in contrast to the

Schwartzwalder-type of deposit found along the eastern side

of the Front Range, where uranium deposits are found solely

in Proterozoic metamorphic rocks (Wallace and Karlson,

1985).

GEOCHEMICAL SIGNATURE

A geochemical signature commonly associated with

these deposits is uranium, mercury, arsenic, antimony, fluorine, molybdenum, and tungsten.

GEOPHYSICAL SIGNATURE

There is no significant magnetic signature. Gammaray spectrometer surveys are appropriate for exploration.

Surficial deposits (less than 20 inches deep) would generate

anomalies on radiometric maps; deeper deposits may not

be evident with this method. Aeroradioactivity (K, U, Th)

data will highlight host granites and can locate vein systems, dependent on surface expression and detail of aerial

survey.

KNOWN DEPOSITS

The only known occurrence of uranium-bearing

veins, the Ray claims, occur about 0.5 mi outside the

southeastern boundary of the Forest near Jones Pass, in

the Vasquez Mountains. The eastern edge of the Vasquez

Peak Wilderness Area was rated as having low potential

for uranium in veins in faulted granites (Theobald and

others, 1985).

ASSESSMENT CRITERIA

1. Evidence of uranium minerals.

2. Presence of faults, fractures, joints, or shear zones.

3. Presence of quartz veins or silicified faults.

4. Anomalous uranium radioactivity.

ASSESSMENT

Area F1.—On the southwest side of the Gore Range in

the vicinity of Morrison Creek, a small area has moderate

resource potential for vein uranium (pl. 1, fig. 21), with

certainty level B. The host rock in this area is Proterozoic

granitic rock. Aeroradioactivity data show moderate values

of K, U, and Th (figs. 11–13) and geochemical data indicate

a high U/Th value in a few samples. A small magnetic high

is also present within the area.

Area F2.—In the southeastern part of the Forest just

southwest of Jones Pass, a small area has high resource

potential for vein uranium (pl. 1, fig. 21), with certainty level

C. The uranium minerals autunite and uranophane are

present along with abundant quartz veins in Proterozoic granitic rocks in the area of the Ray claims. Faults, fractures,

and joints are abundant and silicified. The area has high aeroradioactivity (K, U, Th) values (figs. 11–13).

ECONOMIC SIGNIFICANCE

The number of producing uranium mines in the United

States is relatively small and is likely to remain in this condition for the near future. Exploration and development of

any uranium resources in the Forest is unlikely in the foreseeable future due to the depressed state of the uranium

industry. Import and export figures for uranium are

proprietary (U.S. Bureau of Mines, 1993). 
62 MINERAL RESOURCE POTENTIAL AND GEOLOGY, ROUTT NATIONAL FOREST, COLORADO

107°30' 107°00' 106°30' 106°00'

41°00'

40°30'

40°00'

39°30'

COLORADO

Craig

Steamboat

Springs

Eagle

Vail

Kremmling

Walden

Lake

Granby

Rabbit Ears Range

Williams Fork Mountains

Flat

Tops

Gore Range

Gore Range

Park Range

Hahns Park Range

Peak

Mt

Zirkel Medicine Bow Mountains

Rocky

Mountain

National

Park

Colorado River

Colorado River

0 5 10 MILES

WYOMING

COLORADO

i

70

Approximate boundary of the

Routt National Forest and the

Middle Park Ranger District

of the Arapaho National Forest

Approximate boundary of the

Routt National Forest and the

Middle Park Ranger District

of the Arapaho National Forest

Approximate boundary of the

Routt National Forest and the

Middle Park Ranger District

of the Arapaho National Forest

EXPLANATION

RESOURCE POTENTIAL

High

Moderate

F1

F2

SANDSTONE URANIUM-VANADIUM (G)

COMMODITIES AND BY-PRODUCTS

Commodities are uranium and vanadium with copper

by-product.

HOST ROCKS

Host rocks include Paleozoic sedimentary rocks, in

particular the Morrison Formation, the Dakota Sandstone,

the Troublesome Formation, and the Middle Park

Conglomerate.

STRUCTURAL CONTROL

Structural control is variable.

AGE

Mesozoic age.

Figure 21. Mineral resource map for vein uranium (F) in the Routt National Forest and Middle Park Ranger District of the

Arapaho National Forest, Colorado.
MINERAL RESOURCES—LOCATABLE MINERALS 63

107°30' 107°00' 106°30' 106°00'

41°00'

40°30'

40°00'

39°30'

COLORADO

Craig

Steamboat

Springs

Eagle

Vail

Kremmling

Walden

Lake

Granby

Rabbit Ears Range

Williams Fork Mountains

Flat

Tops

Gore Range

Gore Range

Park Range

Hahns Park Range

Peak

Mt

Zirkel Medicine Bow Mountains

Rocky

Mountain

National

Park

Colorado River

Colorado River

0 5 10 MILES

WYOMING

COLORADO

i

70

Approximate boundary of the

Routt National Forest and the

Middle Park Ranger District

of the Arapaho National Forest

Approximate boundary of the

Routt National Forest and the

Middle Park Ranger District

of the Arapaho National Forest

Approximate boundary of the

Routt National Forest and the

Middle Park Ranger District

of the Arapaho National Forest

EXPLANATION

RESOURCE POTENTIAL

High

Moderate

G1

G2

G3

G4

DEPOSIT DESCRIPTION

Deposits form as microcrystalline uranium oxides,

phosphates, carbonates, vanadates, and silicates and are

deposited during diagenesis in locally reduced environments

in fine- to medium-grained sandstone beds (Turner-Peterson

and Fishman, 1986). Some uranium minerals are also deposited during redistribution at the interface between oxidized

and reduced areas by ground water. Interbedded shale and

mudstone sequences are the source for the ore-related fluids.

Fluvial channels, braided-stream deposits, continental basin

margins, and stable coastal plains are the most characteristic

settings for these deposits.

Deposits are usually massive and tabular in shape and

orebodies are nearly concordant with gross sedimentary features of the host sandstone. Deposits may also occur as rollfront bodies that are crescent shaped and discordant to bedding in cross section. Tabular uranium occurs as lenses

within reduced sandstone, and roll-front deposits occur at

interfaces between oxidized and reduced ground.

Figure 22. Mineral resource map for sandstone uranium-vanadium deposits (G) in the Routt National Forest and Middle Park

Ranger District of the Arapaho National Forest, Colorado.
64 MINERAL RESOURCE POTENTIAL AND GEOLOGY, ROUTT NATIONAL FOREST, COLORADO

GEOCHEMICAL SIGNATURE

Uranium, vanadium, molybdenum, selenium, silver,

and copper are present in anomalous concentrations.

GEOPHYSICAL SIGNATURE

No expression was recognized in regional aeromagnetic

or gravity data. U aeroradioactivity data can be a primary

indicator, dependent on surface expression and detail of survey.

KNOWN DEPOSITS

Known deposits occur in the Beaver Creek–Troublesome district near Kremmling, near Norris Creek on the east

side of the Mt. Zirkel Wilderness, and at Rabbit Ears Pass.

ASSESSMENT CRITERIA

1. Presence of Morrison Formation, Dakota Sandstone, 

Troublesome Formation, or Middle Park

Conglomerate.

2. Alteration and oxidation of host rocks.

3. Presence of carbonaceous beds or other reductants 

that cause deposition of uranium.

4. Presence of uranium in favorable units.

5. Anomalous uranium radioactivity, dependent on 

surface expression

ASSESSMENT

Area G1.—An area on the east side of the Park Range

along and near Norris Creek has moderate resource potential

for sandstone-hosted uranium and vanadium, with certainty

level C (pl. 1, fig. 22). The host rock is the Morrison Formation, and background radioactivity readings are above normal (Snyder, 1987a). No uranium minerals were identified.

Area G2.—A north-south-trending elongate area in the

vicinity of Rabbit Ears Pass has high resource potential for

sandstone-hosted uranium and vanadium, with certainty

level C (pl. 1, fig. 22). Uranium is present mostly as carnotite

within the uppermost 25 ft of the Dakota Sandstone; carnotite

forms grain coatings, concentrations replacing and surrounding carbonaceous material, and fracture coatings (Malan,

1957).

Area G3.—An elongate area extending south from area

G2 has moderate mineral resource potential for sandstonehosted uranium and vanadium, with certainty level B (pl. 1,

fig. 22). This area encompasses the upper portion of the

Dakota Sandstone, favorable for uranium in area G2.

Geochemical data shows favorable U/Th ratios in this area.

U (fig. 12) and U/Th (fig. 14) aeroradioactivity data are relatively anomalous.

Area G4.—The Troublesome district, north of Highway

40 between Kremmling and Hot Sulphur Springs has high

mineral resource potential for additional sandstone-hosted

uranium and vanadium, with certainty level C (pl. 1, fig. 22).

Uranium occurs in the basal portions of the Tertiary Troublesome Formation in sandstone lenses. Carnotite, autunite,

schroeckingerite, and unidentified vanadium minerals are

present (Malan, 1957). Anomalous radioactivity is present in

widely scattered localities.

ECONOMIC SIGNIFICANCE

The number of producing uranium mines in the United

States is relatively small and is likely to remain in this condition for the near future. Exploration and development of any

uranium resources in the Forest is unlikely in the foreseeable

future due to the depressed state of the uranium industry.

Import and export figures for uranium are proprietary (U.S.

Bureau of Mines, 1993). 

PLACER GOLD (H)

COMMODITIES

Gold with minor silver.

HOST ROCKS

Host rocks include modern to Pleistocene-age alluvium

in stream sediments, colluvium, or slope-wash deposits; and

glacial-fluvial gravels and tills. Placers originate from

erosion of veins in Proterozoic and Paleozoic bedrock,

although some placers may have been derived from the

reworking of paleoplacers, in particular the basal conglomerates of the Dakota Sandstone and the Browns Park Formation.

STRUCTURAL CONTROL

Structure has little influence on the location or deposition of placer deposits. Deposits are usually downstream

from known precious-metal lode deposits, mostly related to

veins in various rock types. Irregular flow patterns of the

streams separated light from heavy components of stream

sediment. Gold and other heavier minerals tend to be concentrated on the inside of oxbows and in pools cut into bedrock. Gold is not generally dispersed throughout and

constitutes only a small part of the gravels.

AGE

Pliocene(?) or early Pleistocene deposition (Parker,

1961; Parker, 1974).

DEPOSIT DESCRIPTION

Placers are irregularly shaped accumulations of heavy

minerals near the bottom of gravel deposits. They range in

size from a few feet to a few acres along stream beds or

within landslide deposits. Gold is concentrated in ribbons or

streaks in individual channels that are normally 180 to 400 ft

wide (Parker, 1974). Associated heavy-mineral black sands
MINERAL RESOURCES—LOCATABLE MINERALS 65

contain magnetite, chromite, ilmenite, hematite, monazite,

pyrite, zircon, garnet, and rutile.

Gold in placer deposits formed in modern alluvium and

colluvium and in Pleistocene fluvial sediments. Deposits are

found at the base of stream gravels deposited along presentday streams (Willow and Beaver Creeks), in terrace gravels

related to these streams, in colluvium or slope-wash deposits

(Little Red Park), alluvial fan deposits (Poverty Bar), and in

Pleistocene glacial-fluvial deposits. There are no placer

deposits formed strictly by glacial activity; however, glaciation can modify or destroy previously formed placer deposits (Parker, 1974). The placers in the Forest have been

worked almost exclusively for gold, although the Bear Paw

claims along the South Fork of the Michigan River were

staked to test for platinum-group elements. No platinumgroup elements have been found at this location.

GEOCHEMICAL SIGNATURE

Anomalous concentrations of gold, silver, and locally

bismuth are present.

GEOPHYSICAL SIGNATURE

Regional geophysical surveys are of little use, but local

seismic surveys could be used to determine the thickness of

placer gravels. Ground magnetometers could be used to

detect magnetite and ilmenite that may be concentrated in

the gold placers and to help locate shallowly buried placers.

U and Th aeroradioactivity from monazite, zircon, and other

radioactive minerals can be an indicator, dependent on

surface expression and detail of survey.

KNOWN DEPOSITS

Areas within the Forest that have produced gold from

placer deposits include the following: the area along the

South Fork of the Michigan River, between the confluence

of Porcupine Creek and the west boundary of the Never

Summer Wilderness, which has been intermittently claimed

(Bear-Paw group claims) for gold and PGE (Neubert,

1994); gravels along Beaver Creek, Ways Gulch and Deep

Creek (between 8,000 and 8,500 ft elevation) in the Hahns

Peak mining district (Parker, 1974); the Willow Creek

Canyon area; hill-slope placers at Little Red Park, north of

Hahns Peak (Parker, 1974); on Sawmill Creek, southwest

of Walden; and the Independence Mountain placer (Parker,

1974, p. 31). Poverty Bar, on the east side of Deep Creek

just west of the village of Hahns Peak, was the most productive placer in the Hahns Peak district. Total production

from placer deposits in the Hahns Peak district has been

estimated to be from $500,000 (Parker, 1974) to

$2,200,000 (Lakes, 1909).

ASSESSMENT CRITERIA

1. Presence of alluvial or colluvial deposits downstream 

from known precious-metal deposits.

2. Presence of alluvial or colluvial gravels.

3. Anomalous concentrations of base and precious 

metals in panned-concentrate samples.

ASSESSMENT

Area H1.—A small area on the north slope of Hahns

Peak in Little Red Park was mined for placer deposits

(Parker, 1974) and is assigned a moderate resource potential

for additional small, undiscovered placer deposits, with a

certainty level of C (pl.1, fig. 23). The source of the gold

may have been from disseminations in the porphyry at

Hahns Peak. The gold at Little Red Park is distinctive for its

high silver content (Desborough, 1970).

Area H2.—Poverty Bar, on the south slope of Hahns

Peak and west of the village of Hahns Peak, was mined for

placer deposits and is assigned a high potential for additional, small undiscovered placer deposits, with a certainty

level of B (pl. 1, fig. 23). Most of the placer gold production

from the Hahns Peak district came from the Poverty Bar

placer (Neubert, 1994), and the source of the gold may also

have been from the Hahns Peak porphyry.

Area H3.—A small area south of Hahns Peak, which

includes part of the Beaver Creek drainage and Ways Gulch,

has been mined for placer deposits. This area is assigned a

high resource potential for additional, small, undiscovered

placer deposits, with a certainty level of C (pl. 1, fig. 23).

During reconnaissance studies in the 1960’s, gold was

recovered from placers in Ways Gulch (Neubert, 1994), and

small localized deposits may still host gold.

Area H4.—The Willow Creek Canyon area was mined

for placer deposits but production is unknown (Snyder,

1987a). The area has a high resource potential for additional,

small, undiscovered placer deposits, with a certainty level of

C (pl. 1, fig. 23). Fine flakes of gold were found in samples

collected from placer material on or near bedrock (Snyder,

1987a).

Area H5.—Several drainages along Independence

Mountain, just outside the Forest, have produced gold from

placer deposits and are included in the small outlined area

(pl. 1, fig. 23). This area has a high resource potential for

additional undiscovered small placer deposits, with a certainty level of C. Small placer operations were attempted at

the head of Threemile Creek, along Placer and Lawrence

Draws, and along an unnamed draw between Placer Draw

and California Gulch (Hail, 1965, Parker, 1974). The source

of the gold is unknown.

Area H6.—A small area south of Threemile Creek, on

the northeast slope of Independence Mountain has a moderate resource potential for small placer deposits, with certainty level B (pl. 1, fig. 23). Alluvial material is present in

stream drainages and stream-sediment samples from this

area contained anomalous concentrations of gold.

Area H7.—A small area just outside the Forest boundary, near Rand, has a moderate resource potential for small
66 MINERAL RESOURCE POTENTIAL AND GEOLOGY, ROUTT NATIONAL FOREST, COLORADO

107°30' 107°00' 106°30' 106°00'

41°00'

40°30'

40°00'

39°30'

COLORADO

Craig

Steamboat

Springs

Eagle

Vail

Kremmling

Walden

Lake

Granby

Rabbit Ears Range

Williams Fork Mountains

Flat

Tops

Gore Range

Gore Range

Park Range

Park Range

Hahns

Peak

Mt

Zirkel Medicine Bow Mountains

Rocky

Mountain

National

Park

Colorado River

Colorado River

0 5 10 MILES

WYOMING

COLORADO

i

70

Approximate boundary of the

Routt National Forest and the

Middle Park Ranger District

of the Arapaho National Forest

Approximate boundary of the

Routt National Forest and the

Middle Park Ranger District

of the Arapaho National Forest

Approximate boundary of the

Routt National Forest and the

Middle Park Ranger District

of the Arapaho National Forest

EXPLANATION

RESOURCE POTENTIAL

High

Moderate

H1

H4

H3

H2

H6

H5

H7

placer deposits, with certainty level B (pl. 1, fig. 23).

Stream-sediment samples from the area have anomalous

concentrations of gold and silver.

ECONOMIC SIGNIFICANCE

Gold is a commodity in high demand. Unlike lodegold deposits, small placer deposits can usually be worked

at a relatively low cost, making them attractive to individuals or small operations. Although most of the known

deposits have already been exploited, there is likely to

always be an interest in deposits within the Forest. Areas of

potential in the Forest are small, have already been well

explored, and may only be exploited for recreational panning or mining operations. The United States currently

exports gold (U.S. Bureau of Mines, 1993).

Figure 23. Mineral resource map for placer gold (H) in the Routt National Forest and Middle Park Ranger District of the

Arapaho National Forest, Colorado.
MINERAL RESOURCES—LOCATABLE MINERALS 67

PLATINUM-GROUP ELEMENTS IN

ULTRAMAFIC ROCKS (I)

COMMODITIES, BY-PRODUCTS, AND TRACE METALS

The commodities are the platinum-group elements

(PGE): osmium, iridium, ruthenium, rhenium, platinum, and

palladium. Other associated metals include chromium,

nickel, and cobalt.

HOST ROCKS

Host rock for these types of deposits is altered

peridotite.

STRUCTURAL CONTROL

None.

AGE

Early Proterozoic.

DEPOSIT DESCRIPTION

In the Forest, platinum-group minerals, and associated

chromium, cobalt, and nickel, occur in pods and layers in

ultramafic rocks (Snyder, 1987a). These layers form from

gravity settling and convection processes in intrusive

basaltic magmas. Early-formed crystals in these magmas

settle to the bottom of the magma chamber, forming continuous layers and pods of minerals. These bodies generally

range in composition from peridotite (olivine and pyroxene)

at the base, through gabbro (olivine, pyroxene, and plagioclase), to anorthosite (mostly plagioclase) at the top; not all

compositions are always present. PGE minerals, and chromium, cobalt, and nickel-bearing minerals crystallize from

the melt in the early stages and accumulate in the lower portions of these bodies in the peridotite layers. Platinum-group

elements occur as sulfide, arsenide, and sulfide-arsenide

minerals, commonly closely associated with copper-sulfide

minerals. The precipitation of sulfide minerals from a silicate melt is the most important phenomena in localizing and

concentrating PGE; the mechanisms of this process are

poorly understood.

GEOPHYSICAL SIGNATURE

Ultramafic plutons are commonly associated with

magnetic and gravity highs and unusually low K, U, and Th

radioactivity.

GEOCHEMICAL SIGNATURE

Elevated concentrations of cobalt, chromium, and

nickel are the most common. Silver, gold, cadmium, copper,

mercury, lead, antimony, and zinc are also present.

KNOWN DEPOSITS

There are no known deposits in the Forest. However,

about 16 mi due north of the northeasternmost part of the

Forest in Wyoming, the New Rambler mine operated from

1900 to 1918 and produced more than 6,000 tons of Cu, Ag,

Au, Pt, and Pd ore (McCallum and Orback, 1968).

ASSESSMENT CRITERIA

1. Presence of peridotite.

2. Anomalous concentrations of PGE, Co, Cr, Ni, and 

(or) Cu, Au, and Ag in soils, rocks, or heavy-mineralconcentrate samples.

ASSESSMENT 

Area I1.—Near Elkhorn Mountain in the northernmost

part of the Park Range a small area of peridotite within the

Elkhorn gabbro complex has high potential for PGE, with

certainty level C (pl. 1, fig. 24). The peridotite is a volumetrically minor part of the gabbro mass, but is well-exposed on

Peak 9,731 on the Continental Divide. Rock samples from

this area contain anomalous concentrations of platinum and

palladium; one sample contains as much 0.03 ppm platinum

plus palladium (Snyder, 1987a).

Area I2.—A small area between Bear Creek and Lone

Pine Creek on the east side of the Park Range has high potential for PGE elements, with certainty level C (pl. 1, fig. 24).

This locality contains a long, linear peridotite intrusive.

Rock samples from this area contain anomalous concentrations of platinum and palladium (Snyder, 1987a); one

sample contained as much as 0.048 ppm platinum plus

palladium.

ECONOMIC SIGNIFICANCE

The United States has a net import reliance of 94 percent for the platinum-group metals (U.S. Bureau of Mines,

1993). The only mining of PGE ore in the United States in

1992 was in Montana at the Stillwater complex. World

resources are estimated at 100 million kilograms; United

States resources are estimated at 9 million kilograms. Any

deposits within the Forest are likely to be small and of little

economic import.

U-TH-REE IN PEGMATITES (J)

COMMODITIES

The commodities are uranium, thorium, and the rareearth elements (REE).

HOST ROCKS

Pegmatites in Proterozoic granitic rocks.
68 MINERAL RESOURCE POTENTIAL AND GEOLOGY, ROUTT NATIONAL FOREST, COLORADO

107°30' 107°00' 106°30' 106°00'

41°00'

40°30'

40°00'

39°30'

COLORADO

Craig

Steamboat

Springs

Eagle

Vail

Kremmling

Walden

Lake

Granby

Rabbit Ears Range

Williams Fork Mountains

Flat

Tops

Gore Range

Gore Range

Park Range

Hahns Park Range

Peak

Mt

Zirkel Medicine Bow Mountains

Rocky

Mountain

National

Park

Colorado River

Colorado River

0 5 10 MILES

WYOMING

COLORADO

i

70

Approximate boundary of the

Routt National Forest and the

Middle Park Ranger District

of the Arapaho National Forest

Approximate boundary of the

Routt National Forest and the

Middle Park Ranger District

of the Arapaho National Forest

Approximate boundary of the

Routt National Forest and the

Middle Park Ranger District

of the Arapaho National Forest

EXPLANATION

RESOURCE POTENTIAL

High

I1

I2

STRUCTURAL CONTROL

U, Th, and REE minerals occur within pegmatites.

There is no structural control except for pegmatites that are

emplaced in fault zones.

AGE

U-Th-REE pegmatites are of two ages: 1.4 Ga and 1.7

Ga. The younger pegmatites contain more U-Th-REE than

do the older ones, with a few rare exceptions.

DEPOSIT DESCRIPTION

U-Th-REE minerals occur in pegmatites within granitic

rocks. Pegmatites contain the minerals that crystallized from

the residual melt of a slowly cooling, deeply buried granitic

magma body. This residual melt is commonly enriched in

gaseous constituents such as H2O, P, Cl, F, and S as well as

the rare-earth elements, U, and Th.

Reported radioactive minerals include uraninite, zircon,

xenotime, allanite, fergusonite, euxentie, carnotite, black

Figure 24. Mineral resource map for PGE in ultramafic rocks (I) in the Routt National Forest and Middle Park Ranger District of

the Arapaho National Forest, Colorado.
MINERAL RESOURCES—LOCATABLE MINERALS 69

107°30' 107°00' 106°30' 106°00'

41°00'

40°30'

40°00'

39°30'

COLORADO

Craig

Steamboat

Springs

Eagle

Vail

Kremmling

Walden

Lake

Granby

Rabbit Ears Range

Williams Fork Mountains

Flat

Tops

Gore Range

Gore Range

Park Range

Hahns Park Range

Peak

Mt

Zirkel Medicine Bow Mountains

Rocky

Mountain

National

Park

Colorado River

Colorado River

0 5 10 MILES

WYOMING

COLORADO

i

70

Approximate boundary of the

Routt National Forest and the

Middle Park Ranger District

of the Arapaho National Forest

Approximate boundary of the

Routt National Forest and the

Middle Park Ranger District

of the Arapaho National Forest

Approximate boundary of the

Routt National Forest and the

Middle Park Ranger District

of the Arapaho National Forest

EXPLANATION

RESOURCE POTENTIAL

High

Moderate

J1 J2

J3

J4

J5

chalcedony or dark-purple fluorite, as well as secondary

alteration products such as autunite, uranophane, or gummite

(Snyder, 1987a).

GEOPHYSICAL SIGNATURE

U and Th aeroradioactivity will be anomalous, depending on surface expression and detail of the survey.

GEOCHEMICAL SIGNATURE

Anomalous concentrations of U, Th, or REE in rocks

and stream-sediment samples.

KNOWN DEPOSITS

There are no known deposits within the Forest.

Figure 25. Mineral resource map for U-Th-REE (J) in the Routt National Forest and Middle Park Ranger District of the Arapaho

National Forest, Colorado.
70 MINERAL RESOURCE POTENTIAL AND GEOLOGY, ROUTT NATIONAL FOREST, COLORADO

ASSESSMENT CRITERIA

1. Presence of pegmatites

2. Anomalous concentrations of U, Th, or REE in rocks 

or stream-sediment samples

3. Anomalous uranium and thorium radioactivity

ASSESSMENT

Six areas have moderate or high resource potential for

U, Th, and REE in pegmatites. All of these areas fall within

the north-central part of the Forest in the granitic rocks of the

Park Range. The following information is from Snyder

(1987a).

Area J1.—A small area on Farwell Mountain has moderate resource potential for U, Th, and REE in pegmatites,

with certainty level B (pl. 1, fig. 25). Samples of pegmatites

in this area contained as much as 18.5 ppm thorium and 12.4

ppm uranium. Pegmatite muscovite has a reported K-Ar age

of 1.9 Ga (Segerstrom and Young, 1972).

Area J2.—Pegmatites in Mica Basin have high resource

potential for U, Th, and REE, with certainty level C (pl. 1,

fig. 25). A few large specimens of uraninite or euxenite are

reported from a mica-beryl pit in pegmatite. One sample

from the area assayed 86 percent uranium.

Area J3.—Pegmatites around Agua Fria Lake have high

mineral resource potential for U, Th, and REE, with certainty

level C (pl. 1, fig. 25). Samples from this area contain greater

than 20 percent rare-earth elements and yttrium, as much as

15,000 ppm Th, and 7,000 ppm uranium. The area is within

the prominent U and Th aeroradioactivity high (figs. 12 and

13) of the Mt. Ethel pluton.

Area J4.—An elongate zone along the Soda Creek shear

zone has moderate potential for U, Th, and REE, with certainty level B (pl. 1, fig. 25). Several samples from the area

contain elevated values of uranium and thorium, ranging

from 6.1 to 16.9 ppm and 12.6 to 34.4, ppm respectively.

The area is within the prominent U and Th aeroradioactivity

high (figs. 12 and 13) of the Mt. Ethel pluton.

Area J5.—In the North Fork of Fish Creek, a northeasttrending area has high resource potential for U, Th, and REE

in pegmatites, with certainty level C (pl. 1, fig. 25). Samples

from this area contained 3,000–7,000 ppm uranium (Beroni

and McKeown, 1952; Beroni and Derzay, 1955). Beroni and

McKeown (1952) reported uranium analyses of nine samples

from the area (20–540 ppm) and constructed an isoradioactivity map showing pod-shaped radioactive zones 1 to 20 ft

in length. The Fair-U claims encompass much of this area.

The area is within the prominent U and Th aeroradioactivity

high (figs. 12 and 13) of the Mt. Ethel pluton.

ECONOMIC SIGNIFICANCE

The uranium market in the United States is relatively

depressed and is likely to remain in this condition for the near

future. Import and export figures are proprietary (U.S.

Bureau of Mines, 1993). Import data for thorium is not available, but large deposits of thorium occur elsewhere in the

United States in beach and stream placers, veins, and carbonatites. The United States exported rare-earth metals in 1992

but, previous to that, had a net import reliance of 25 percent

(U.S. Bureau of Mines, 1993). Total undiscovered resources

of rare earths are believed to be very large relative to

expected demands.

MINOR OCCURRENCES

Industrial mica.—Mica Basin, on the northeast side of

Big Agnes Mountain in the Mt. Zirkel Wilderness, was the

site of many recorded mining claims in the late 1940’s (Snyder, 1987a). Many pegmatites, associated with gneiss and

schist, in this area contain large mica crystals of muscovite

and biotite. The mica crystals contain parting planes that

make the material scrap grade (Snyder, 1987a). There is no

recorded production of mica from this area or from other

sites in the Forest. Although pegmatites are fairly common

in the Forest, most do not contain minable quantities of mica

or other commercially important minerals.

Uranium in wetlands.—Wetlands have the capacity for

extracting metals, particularly uranium, from ground and

surface waters containing only very dilute concentrations of

metals (Owen and others, 1992). Plutonic and volcanic rocks

of the Forest may contain uranium concentrations high

enough to feed into local wetlands. Uranium oxides dissolve

readily in the oxidizing, carbonate-bearing waters characteristic of most surface waters and near-surface ground waters,

and they precipitate when these waters encounter reductants

in the subsurface (Owen and others, 1992). Organic-rich

sediments found in wetlands act as reductants and may concentrate uranium locally.

Wetland uranium normally does not have associated

detectable gamma-ray radioactivity because (1) about

500,000 years are needed for uranium to decay and produce

sufficient gamma-ray-emitting daughters to result in measurable gamma-ray anomalies, and (2) the water present can

absorb most gamma-rays emitted by uranium daughter

products.

Owen and others (1992) sampled wetland areas

within the Forest and found that uranium concentrations

ranged between 2.4 and 62 ppm and are considered to be

weakly to moderately enriched. Twenty-two samples

were collected from wetlands in and adjacent to the Mt.

Zirkel Wilderness.

At the Spring claims, about 11 mi west of Walden,

uranium-bearing peat beds in the vicinity of uranium-bearing springs have been reported (Malan, 1957). The claims

are located on the lower eastern slope of Sheep Mountain,

about a mile outside the Forest boundary. Uranium-bearing

meteoric waters reportedly issue from flat-lying clay,
MINERAL RESOURCES—LOCATABLE MINERALS 71

alluvium, and peat. The peat beds provide a reducing environment that effectively precipitates uranium from the mineralized meteoric waters. The peat beds appear to be a

local feature; no uranium-bearing peat beds have been

mined at this location.

The only uranium deposit in the United States that is

mined from surficial wetlands is on the north fork of Flodelle

Creek in northeastern Washington (Johnson and others,

1987). No production has been recorded from wetlands or

peat bogs within the Forest.
73

The Mineral Leasing Act of 1920 amended the General

Mining Law of 1872 and set up regulations requiring prospect permits and leases to control the exploration, development, and production of specified minerals. The Mineral

Leasing Act placed the following under the leasing law: oil,

gas, coal, oil shale, sodium, potassium, phosphate, native

asphalt, bitumen, and bituminous rock. Geothermal energy

was added to the list of leaseable minerals by the Geothermal

Steam Act of 1970.

Known occurrences of leaseable minerals were studied

by the U.S. Bureau of Mines and are described in Neubert

(1994). The potential for undiscovered deposits of these

minerals is addressed in the following sections of this report.

Areas within the Forest were assigned a rating of no, low,

moderate, or high potential for the occurrence of undiscovered deposits. Levels of certainty, labeled A through D,

were also assigned to qualify the data—level A indicates the

least amount of supporting data and level D the greatest.

Definitions and explanations of the mineral resource rating

system may be found in Appendix 2. On figures 2 and 36,

figures 38–41, and plate 1, areas of high potential are shown

in dark gray and areas of moderate potential are shown in

light gray. For ease in reading the plate, areas of low potential are only shown on the page-size figures and, in these

cases, are shown in gray. Coal, oil, gas, coal-bed methane,

and geothermal resources have been identified within or

adjacent to the Forest. No potash, sodium, asphalt, or phosphate resources are known in the Forest.

COAL GEOLOGY AND RESOURCES 

By Laura N. Robinson Roberts and Carol L. Molnia

INTRODUCTION

This chapter in part reviews the coal geology of the Forest as described in previous studies and presents new

resource estimates for coal in the northwest part of the Forest

(Area 1, fig. 26) based on information obtained from wells

drilled since earlier resource estimates were made (Bass and

others, 1955; Landis, 1959). A summary of coal resources

compiled from previous work for areas in and adjacent to the

Forest is given in Neubert (1994). A new estimate of total

hypothetical coal resources of Area 1 under less than 3,000

feet of overburden is about 3.5 billion short tons (table 7).

The resource estimates provided in this report are highly

speculative, and a great deal of additional information is

required to increase the reliability of these figures. No information is available for the vast majority of the area supposedly underlain by coal; major assumptions are made in terms

of continuity of coal-bearing intervals, both in terms of

stratigraphic and structural setting.

The Forest covers portions of several counties in the

northwestern part of Colorado. Because the Forest consists

of isolated parcels, number designations are assigned to its

different parts for ease of reference (fig. 26). Area 1, the

main area of emphasis for this chapter, is where new hypothetical coal resources were estimated. It is in the northwestern part of the Forest and includes the Elkhead Mountains.

Area 2 is the central part of the Forest and includes parts of

the Park and Gore Ranges; Area 3 is the southwestern part

of the Forest and includes the Beaver, Dunkley and Little

Flat Tops; Area 4 is the narrow northwest-southeasttrending Medicine Bow Mountains in the northeastern part

of the Forest; Area 5 is the east-central part of the Forest and

includes the Rabbit Ears Range; and Area 6 is in the southeastern part of the Forest and includes the Williams Fork

Mountains.

Parts of two major coal regions extend into the Forest

(U.S. Geolgical Survey and Colorado Geological Survey,

1977). The Green River region extends into Area 1 from

Wyoming and from points west in Colorado (fig. 26) and

also extends into the northwestern corner of Area 3. Virtually all of the coals mined to date in the Colorado part of the

Green River region have come from the Yampa coal field,

which is Colorado’s greatest producer of coal (Keystone

Coal Industry Manual, 1993). More than 13 million short

tons of coal were produced during 1990 from the Mesaverde

Group. Most of the coal is burned at electric generating

plants in Craig and Hayden; however, several million short

tons are shipped annually by railroad to power plants as far

away as Texas and Nebraska. Although major mines are

currently operating in the Yampa coal field, no coal production has been recorded from within the bounds of the Forest.

The most recent discussion of the history of coal mining

activity in the Yampa coal field is given by Neubert (1994).

The North Park region covers most of Area 5 (fig. 26).

The North Park and Middle Park coal fields of this region are

separated by the generally east-west-trending Rabbit Ears

Range. The North Park coal field contains significant coal

MINERAL RESOURCES—LEASEABLE MINERALS
74 MINERAL RESOURCE POTENTIAL AND GEOLOGY, ROUTT NATIONAL FOREST, COLORADO

Colorado River

Lake

Granby

Colorado River

Medicine

Bow

Mountains

Flat

Tops

Area 5

Area 6

Area 2

Area 4

107°30' 107°00' 106°30' 106°00'

41°00'

40°30'

40°00'

39°30'

COLORADO

0 5 10 MILES

Eagle

Kremmling

Walden

Williams Fork M

Gore Range

ountains

Gore Range

Park Range

Park Range

Craig Steamboat

Springs

Middle

Park

coal

field

Hahns

Peak

Mt

Zirkel

Elkhead Mountains

Coalmont

district

Rabbit Ears Range

Rocky

Mountain

National

Park

Yampa

coal field

North Park

coal field

Area 1

Area 3

GREEN RIVER

REGION

NORTH PARK

REGION

70z

resources in the Paleocene Coalmont Formation in the Coalmont district (fig. 26) (Erdmann, 1941); however, minable

beds probably do not extend into Area 5. The Middle Park

coal field contains rocks equivalent to the coal-bearing Coalmont Formation in the North Park coal field (the Middle Park

Formation), but lack of data precludes a resource estimate in

this area. Published geologic maps that cover Area 5 of the

Forest include Izett (1968); Izett and Barclay (1973); Hail

(1968); Kinney (1970); and Kinney and Hail (1970).

AREA 1

Area 1 occupies the southeastern part of the northwestsoutheast-trending Sand Wash Basin (fig. 27). Structure

contours drawn on the top and base of the Williams Fork Formation in the eastern Sand Wash Basin indicate that strata

dip generally less than 2° to the west (Tyler and Tremain,

1993), except at the eastern margin of the basin where Bass

and others (1955) measured dips of as much as 23° in rocks

Figure 26. Index map of coal fields in and adjacent to the Routt National Forest. Forest areas are bounded by heavy line. Note number

designations for the five areas of the Forest discussed in the text. Shaded pattern represents named coal regions and fields (U.S. Geological

Survey and Colorado Geological Survey, 1977). In this report, new resource estimates are presented for Area 1 (cross-hatched area).
75

of the Mesaverde Group in the southeastern part of Area 1.

Figure 28 shows the surface distribution of Upper Cretaceous and Tertiary rocks and a generalized cross section of

those rocks from northwest to southeast across Area 1.

Upper Cretaceous coal-bearing strata include the Iles

and Williams Fork Formations of the Mesaverde Group, and

the Lance Formation (fig. 29). Reports that discuss the distribution, stratigraphy, and depositional environments of

these formations in and near Area 1 include Fenneman and

Gail (1906), Bass and others (1955), Sharps (1962), Bader

(1983), Irwin (1986), Siepman (1986), Honey and Hettinger

(1989), Roehler and Hansen (1989), and Hamilton (1993).

The Iles Formation is about 1,600 ft thick and consists

of interbedded light-brown, light-gray and white sandstone,

gray sandy mudstone, gray shale and coal (Bass and others,

1955). The contact between the Iles and Williams Fork is the

top of the fine-grained, massive, 100-ft-thick Trout Creek

Sandstone Member (fig. 30). This member, which can be

traced for many miles across the eastern Sand Wash Basin,

was deposited in a nearshore marine environment. Strata of

the Williams Fork Formation are about 1,200 ft thick and are

similar to the those of the Iles Formation, including interbedded sandstone, mudstone, and coal.

The youngest coal-bearing formation of Cretaceous age

is the Lance (fig. 29), which is about 1,200 ft thick and consists of continental deposits, including interbedded finegrained sandstone, gray mudstone, and a few coal beds. In a

study of Upper Cretaceous and lower Tertiary stratigraphy in

the Sand Wash Basin, Honey and Hettinger (1989) placed

the regional Cretaceous-Tertiary unconformity within a

thick coarse-grained sandstone unit (their “unnamed

Cretaceous and Tertiary sandstone unit”), based on pollen

dates. This unit is distinguishable on electric logs of the few

wells that penetrate it within or adjacent to the Forest (fig.

31). However, for this report, and for convenience, the

Lance-Fort Union (Cretaceous-Tertiary) contact is placed at

the top of this unit.

The Tertiary coal-bearing unit, the Paleocene Fort

Union Formation (fig. 29), is about 1,000 ft thick and is composed chiefly of brown and gray sandstone and gray mudstone of fluvial origin and coal. Information on the Fort

Union Formation in this area can be found in Bass and others

(1955), Beaumont (1979), Bader (1983), Irwin (1986),

Honey and Hettinger (1989), and Tyler and McMurry

(1993).

METHODS USED TO ESTIMATE COAL 

RESOURCES

Area 1 is the only area where coal resources were estimated for this study. Figures 32–35 show locations of drill

holes and measured sections used in resource estimates in

Area 1; tables 8 and 9 list sources of these data. The new

coal resources for Area 1 are here classified as “hypothetical” (refer to Appendix 3). Hypothetical resources are

defined by Wood and others (1983) as tonnage estimates (1)

for regions where tonnage estimates are based on knowledge

of the geologic character of coal (for example, thickness

trends based on knowledge of depositional environments),

and (2) for areas beyond a radius of 3 mi from a point of

measurement (for example, drill hole or outcrop). Although

MINERAL RESOURCES—LEASABLE MINERALS

Table 7. Estimated hypothetical coal resources of Area 1 (in millions of short tons) according to overburden category. 

[In millions of short tons, values rounded to two significant figures]

Overburden thickness categories

Total Total Total

Formation 0-1,000 ft 1-2,000 ft 2-3,000 ft 0-3,000 ft 3-6,000 ft 0-6,000 ft

Subbituminous- beds 2.5 ft thick and greater

Fort Union 71 120 290 480 120 600

Lance (base) 340 230 250 820 860 1,700

subtotal 410 350 540 1,300 980 2,300

Bituminous- beds 1.2 ft thick and greater

Williams Fork 540 290 610 1,400 2,700 4,100

Iles 350 190 220 760 1,900 2,600

subtotal 890 480 830 2,200 4,600 6,700

Total 1,300 830 1,400 3,500 5,600 9,000
76 MINERAL RESOURCE POTENTIAL AND GEOLOGY, ROUTT NATIONAL FOREST, COLORADO

WYOMING

WYOMING

COLORADO

COLORADO

UTAH

MOXA ARCH

UINTA MTNS

41°

41°

109°

42°

43°

110° 108°

AXIAL BASIN ARCH

UINTA MOUNTAINS

Uinta thrust

RANGE

Moffat County

Routt County

Craig

UTAH

0 10 miles

108° 107°

Colorado

Wyoming

Utah

CHEROKEE ARCH

CHEROKEE ARCH

PARK 

Greater

Green

River

Basin

GREEN RIVER

BASIN

GREAT DIVIDE

BASIN

WASHAKIE

BASIN

WASHAKIE

BASIN

WAMSUTTER ARCH

ROCK

SPRINGS

UPLIFT

SAND WASH

BASIN

SAND WASH BASIN

Area 1

0 20 miles

approx.

10

Top of Lance subcrop

Figure 27. Structural setting of Area 1 in the southeastern part of the Sand Wash Basin of the Greater Green River Basin.

Sawteeth on upper plate of thrust fault.
77

A

A'

R. 86 W.

T. 10 N.

T. 11 N.

T. 9 N.

T. 8 N.

R. 90 W. R. 89 W. R. 88 W. R. 87 W. Moffat County Routt County

107°22'30" 107°15' 107°07'30"

40°52'30"

40°45'

40°37'30"

Lewis Shale

Lance Formation (coal-bearing) 

Fort Union Formation

 (coal-bearing)

Wasatch Formation

Browns Park Formation

TERTIARY ROCKS UPPER CRETACEOUS ROCKS

0 10 MILES 2 4 6 8

Tertiary volcanics and intrusives Williams Fork Formation

 (coal-bearing)

Iles Formation

 (coal-bearing)

Mancos Shale

Mesaverde

Group

Fault—bar and ball on

 downthrown side

 

20,000 ft

2,000 ft

A A'

NW SE

Browns Park and

Wasatch Formations, undivided

Fort Union Formation

Lance and Fox Hills Sandstone (at base) Lewis Shale

Williams Fork Formation

Iles Formation

Mancos Shale

Browns Park Formation

MINERAL RESOURCES—LEASABLE MINERALS

Figure 28. Generalized geologic map showing Upper Cretaceous sedimentary rocks and Tertiary sedimentary, volcanic, and intrusive

rocks in and near Area 1. (Modified from Tweto, 1976). Generalized cross section A-A' of Upper Cretaceous through Tertiary sedimentary rocks in Area 1. An unconformity lies at the base of the Wasatch and Fort Union Formations. See figure 26 for location of Area 1.
78 MINERAL RESOURCE POTENTIAL AND GEOLOGY, ROUTT NATIONAL FOREST, COLORADO CRETACEOUS

Upper

Paleocene

Eocene

TERTIARY

MESOZOIC CENOZOIC Era

System

Series or

Stage

Browns Park Formation and

Wasatch Formation,

undivided

Fort Union Formation

Iles Formation

Williams Fork Formation

Mancos Shale

Lance Formation

Fox Hills Sandstone

Lewis Shale

Mesaverde

Group

SAND WASH BASIN

EXPLANATION

Coal-bearing

Unconformity

points of measurement exist within the Forest that would

make it possible to estimate resources for categories of

higher reliability (measured, indicated, and inferred categories; see Wood and others, 1983), it is impractical to separate

out these categories because there are so few points of measurement and because they are so widely scattered.

By definition, hypothetical coal resources include coal

beds to a depth of 6,000 ft that are 1.2 ft or more thick for

bituminous coal and 2.5 ft or more thick for subbituminous

coal. For this report, the approach used to estimate hypothetical resources within the Forest is a modification of the

“extrapolated coal zone method” described in Wood and others (1983). It is a modification because, for these estimates,

the entire coal-bearing formation takes the place of “coal

zone.”

Resource estimates were calculated and reported for

coal in each of the formations of interest: Iles, Willliams

Fork, Lance, and Fort Union Formations. Tonnage estimates were derived by multiplying a weighted average

thickness value of cumulative coal in the formation by the

area underlain by the formation. The weighted average

thickness values and the areas were calculated with the aid

of the Interactive Surface Modeling (ISM) computer program developed by Dynamic Graphics, Incorporated.

Because the Mesaverde Group coals are bituminous in

rank, coal beds thinner than 1.2 feet are excluded from

resource consideration in that Group, and because Fort

Union and Lance coals are subbituminous in rank, coal

beds thinner than 2.5 feet are similarly excluded in those

formations (Wood and others, 1983).

MESAVERDE GROUP COALS

Three different groups of coal beds occur in the Mesaverde Group (Bass and others, 1955; Fenneman and Gale,

1906). It is possible to identify these coal groups, in a general way, in well logs examined for this study. The stratigraphic position of these coal groups are shown on figure 30.

The lower coal group consists of coal beds of the Iles Formation. The middle coal group consists of coal beds between

the base of the Williams Fork Formation and the Twentymile

Sandstone Member. The upper coal group consists of coal

beds above the Twentymile Sandstone and the top of the

Williams Fork Formation.

Wells drilled on the Forest that penetrated the lower

coal group (Iles Formation coals) are 1–4 (fig. 32). The

thickest single bed in this group within the Forest is 5 ft

thick, and the average is about 3 ft. These thicknesses are

consistent with those Iles Formation coal beds that are

exposed within and adjacent to the Forest boundary in T. 9

N., R. 86 W. (Bass and others, 1955 ) (fig. 32; table 9). Total

cumulative coal thickness, determined from the available

data, for this group within the Forest ranges from 22 to 37 ft.

The number of coal beds greater than 1.2 ft thick ranges from

7 to 12. The isopach map (fig. 32A) shows distribution and

trends of coal thickness, and the overburden map (fig. 32B)

shows the depth to the top of the Iles Formation.

The Iles Formation (includes coal of the “lower coal

group” of Bass and others, 1955) contains an estimated 2.7

billion short tons of bituminous coal under less than 6,000 ft

of overburden in beds greater than 1.2 feet thick (table 7).

This total represents about 29 percent of the total of the four

formations. However, more than 70 percent of Iles Formation coal in Area 1 is deeper than 3,000 ft. Near the eastern

margin of the basin, where the Iles Formation is exposed at

the surface, the dip of the strata containing coal is as much as

23° (Bass and others, 1955); therefore, there is only a narrow areal distribution of shallow coal (fig. 32B) away from

which the overburden rapidly thickens to the west.

The middle coal group (near the base of the Williams

Fork Formation) contains the thickest and most extensive

coal beds of the three groups (fig. 33). The Wadge coal bed,

which is currently mined in the eastern part of the Yampa

coal field, is in the lower part of this group. Within the

Forest, wells that were drilled through these coals are 1, 2

Figure 29. Stratigraphic units in and near Area 1, in the eastern

part of the Sand Wash Basin.
79

LEWIS SHALE

WILLIAMS FORK

FORMATION

ILES FORMATION

ILES FORMATION

MANCOS SHALE

Twentymile

Sandstone

Trout Creek

Sandstone

(includes coals

of the "lower

coal group")

Tow Creek

Sandstone

5000

stratigraphic

horizon of

"upper coal group"

5500

6500

6000

coal

coal

coal

coal

coal

coal

coal

coal

coal

coal

coal

coal

coal

coal

coal

"middle coal group"

S.P.

(-) (+)

Resistivity

Increasing

#4

1 MADERA-FEDERAL

Sec. 16, T. 9 N., R. 88 W.

MINERAL RESOURCES—LEASABLE MINERALS

Figure 30. Spontaneous potential (S.P.) and resisitivity log signatures of Upper Cretaceous stratigraphic units in Area 1, showing

the “lower coal group” of the Iles Formation and the “middle coal group” and “upper coal group” of the Williams Fork Formation

(terminology of Bass and others, 1955). In addition to these logs, gamma ray and sonic logs were used to identify coal beds. Log

depths are in feet.
80 MINERAL RESOURCE POTENTIAL AND GEOLOGY, ROUTT NATIONAL FOREST, COLORADO

S.P.

(-) (+)

Resistivity

Increasing

#5

PAN AMERICAN PETROLEUM

1-U.S.A. DAVIS

Sec. 14, T. 10 N., R. 90 W.

WASATCH

FORMATION

LEWIS

SHALE

FOX HILLS

SANDSTONE

LANCE

FORMATION

? ?

FORT UNION

FORMATION

LANCE

FORMATION

UPPER CRETACEOUS TERTIARY

coal

coal

coal

coal

coal

2500

3000

3500

4500

5000

4000

Figure 31. Spontaneous potential (S.P.) and resistivity log signatures of Upper Cretaceous and Tertiary stratigraphic units in Area 1,

showing coal beds in the Lance Formation and in the Fort Union Formation. Shaded interval is the “unnamed Cretaceous-Tertiary

sandstone unit” of Honey and Hettinger (1989). In addition to these logs, gamma ray and sonic logs were used to identify coal beds. Log

depths are in feet.
81

30

25

30

30

25

35

6000

10002000

3000

1

3

2

4

7

9

10

11

14

1

3

4 2

7

9

10

11

14

148

149

150

152 153

142

144 145

148

149

150

152 153

142

144 145

A

B

107°22'30" 107°15' 107°07'30"

40°52'30"

T. 10 N.

40°45'

T. 9 N.

T. 8 N.

40°52'30"

T. 10 N.

T. 9 N.

T. 8 N.

40°45'

107°22'30" 107°15' 107°07'30"

R. 90 W. R. 89 W. R. 88 W. R. 87 W. R. 86 W.

R. 90 W. R. 89 W. R. 88 W. R. 87 W. R. 86 W.

Inferred outcrop

top of Iles Formation

Inferred

outcrop

top of Iles

Formation

Moffat County

Routt County

Moffat County

Routt County

0 2 4 6 MILES

0 2 4 6 MILES

Figure 32. A, Isopach map of cumulative coal in the Iles Formation in and near Area 1, including coals of the

lower coal group (terminology of Bass and others, 1955). Thickness values are in feet. B, Isopach map of overburden; datum is top of Iles Formation in Area 1. Thickness values are in feet. Drill holes represented by ( ) and

number, measured outcrop sections by (×) and number. See table 8 for location of drill holes; see table 9 for the

location, thickness, and source data for measured sections.

MINERAL RESOURCES—LEASABLE MINERALS
Table 8. Location data for drill holes shown in figures 32-35. Well Ground ID elev (ft) T. (N.) R. (W.) Sec. Quarter Well Name O 1 8533 10 87 26 ne se se 1 Grizzly-Federal Su 2 8380 9 87 24 sw nw nw 4-24 California-Federal Bu 3 8800 9 86 29 nw nw nw 1-29 Chevron Federal Ch 4 7831 9 88 16 se nw 1 Madera-Federal G 5 8729 10 90 14 se sw 1 USA-Davis Pa 7 6852 8 88 14 nw ne sw 1-14 Murphy Ranch-Govt Ch 8 7307 10 90 28 sw sw 1 Welba Peak Unit Be 9 8427 8 87 3 nw sw se 1-3 Hayden Public Library D10 9040 8 87 11 nw nw se 1-11 Hayden Public Library D11 6999 9 89 35 c se sw 1 Federal M12 7090 9 89 19 se sw se 1-19 E. Van Tassel Co13 7958 9 90 3 ne se ne 3-9-90 Villard A14 8351 11 87 33 c ne sw 1-B Colo-Federal Su15 7714 9 90 23 se nw nw 1 Federal Belco O16 8150 10 90 26 se se sw 1 Villard M17 7886 10 90 27 sw sw ne A-1 Black Mountain A
MINERAL RESOURCES—LEASABLE MINERALS 83

Table 9. Location and coal thickness data of measured outcrop sections (Bass and others, 1955) shown on

figures 32-35.

Meas T. (N.) R.(W) Sec. Coal bed Number of beds

sec. ID thickness (ft) (if more than 1)

Iles Formation, lower coal group

i148 9 86 20 5.0

i149 9 86 29 2.5

i150 9 86 32 4.7

i152 9 86 32 3.5

i153 9 86 32 4.8

i142 8 86 5 3.1

i144 8 86 6 3.0

i145 8 86 6 4.6 2

Williams Fork Formation, middle and upper coal group

w221 9 87 24 3.5

w151 9 86 32 3.5

w146 8 86 6 4.7 2

w143 8 86 6 7.7

Lance Formation coal

L220 9 87 19 6.5

L260 9 88 24 2.8

L261 9 88 25 5.3 2

L262 9 88 26 3.7

L222 9 87 26 21.2 4

L201 8 87 7 4.0

Fort Union coal

F343 8 89 2 4.5

and 4 (fig. 33). The thickness of individual beds within the

group ranges from 2 to 22 ft, and total cumulative thickness

for the group reaches a maximum of 40 ft. Coal-bed thicknesses as thick as 7.7 ft were measured on the steeply dipping

outcrops in and adjacent to the Forest boundary (table 8).

The stratigraphic horizon where the upper coal group is

expected to occur was logged in hole 4 within the Forest; no

coal beds were discernible from that log. This is also the

case for holes 14 and 11, just northeast and southwest of the

Forest boundary, respectively (fig. 33). However, holes 7

and 9, outside the Forest boundary to the southeast, do show

coal beds at this horizon. Unfortunately, no data for this

zone was recorded in the other wells drilled within the Forest

(holes 1 and 2) because the logged interval started below the

coal zone. The cumulative thickness of both coal zones in

the Williams Fork Formation is represented on the isopach

map (fig. 33A).

The Williams Fork Formation (includes coal of the

“middle” and “upper coal group” of Bass and others, 1955)

contains almost half of all the coal within this part of the

Forest. An estimated 4.1 billion short tons of bituminous

coal may occur within 6,000 ft of the surface in beds 1.2 ft

thick or greater (table 7). However, of this total, over 65

percent of the coal is deeper than 3,000 ft. The isopach

map of overburden (fig. 33B) shows that relatively shallow

coals may exist along the eastern margin of the basin but

only in a narrow strip. Overburden increases dramatically

in a short distance to the west as the strata dip steeply

toward the basin axis. It should be noted that two of the

three drill holes within the Forest that contain data on Williams Fork coals were logged starting below the upper coal

zone, so they do not have data for the horizon that might

include these coal beds. For this reason, the isopachs (fig.

33B) may represent a minimum cumulative thickness of

coal along the eastern edge of Area 1.

Most of the coal of the Mesaverde Group is highvolatile C bituminous in apparent rank but ranges from subbituminous B to high-volatile B bituminous (Bass and

others, 1955; Landis, 1959; Khalsa and Ladwig, 1981;

Boreck and others, 1981). Coal in the extreme eastern edge

of the Yampa coal field is locally metamorphosed to anthracite by igneous intrusions (Bass and others, 1955). For

analyses of 21 coal samples from the Yampa coal field, the

as-received sulfur values range from 0.4 to 0.9 percent; ash

yields range from 4.0 to 20.0 percent, with a geometric mean

of 8.1 percent; and calorific values range from 9,870 to

12,010 Btu/lb, with a geometric mean of 11,130 Btu/lb

(Khalsa and Ladwig, 1981).
84 MINERAL RESOURCE POTENTIAL AND GEOLOGY, ROUTT NATIONAL FOREST, COLORADO

40

35

35

35

30

25

40

6000

3000

2000

1000

1

2

4

7

9

11

14

1

2

4

7

9

10

11

14

221

151

143

146

221

151

143

146

Inferred

outcrop

top of

Williams

Fork Formation

Moffat County

Routt County

Moffat County

Routt County

0 2 4 6 MILES

Inferred

outcrop

top of

Williams

Fork Formation

10

0 2 4 6 MILES

107°22'30" 107°15' 107°07'30"

40°52'30"

T. 10 N.

R. 90 W. R. 88 W. R. 87 W. R. 86 W.

107°22'30" 107°15' 107°07'30"

R. 90 W. R. 88 W. R. 89 W.

R. 89 W.

R. 87 W. R. 86 W.

40°45'

T. 9 N.

T. 8 N.

40°52'30"

T. 10 N.

40°45'

T. 9 N.

T. 8 N.

A

B

Figure 33. A, Isopach map of cumulative coal in the Williams Fork Formation in and near Area 1, including

coals of the middle coal group and of the upper coal group (terminology of Bass and others, 1955). Thickness

values are in feet. Hachures indicate area of local thinning. B, Isopach map of overburden; datum is top of

Williams Fork Formation in Area 1. Thickness values are in feet. Drill holes represented by ( ) and number,

measured outcrop sections by (×) and number. See table 8 for location of drill holes; see table 9 for the location,

thickness, and source data for measured sections.
MINERAL RESOURCES—LEASABLE MINERALS 85

The Mesaverde Group coals (Iles and Williams Fork

Formations) have high coal resource potential with certainty

level C (K1, fig. 36). This qualitative assessment is based on

the proximity of known coal resources (Yampa coal field)

and the known distribution, thickness, depth, quantity and

quality of coal in these strata. Although Mesaverde Group

coal underlies most of Area 1, most of it is buried too deeply

to be considered an economic resource.

LANCE FORMATION COALS

Only holes 4 and 5 provide data for Lance coals within

the Forest, but several holes outside the Forest added control

points for cumulative coal thickness and overburden isopachs (fig. 34). The majority of and the thickest coal beds of

the Lance Formation occur within less than 200 ft of the base

of the formation (fig. 31). Within the Forest, the thickest coal

bed encountered in drill holes is 6 ft thick, but most of the

beds are 3–4 ft thick. Lance Formation coal beds are typically thin, lenticular, and difficult to trace for any distance.

On the outcrop in sec. 26, T. 9 N., R. 87 W., a cumulative

thickness of 21.2 ft of coal in 4 beds near the base of the

Lance was measured in an interval of 75 ft (table 9).

The Lance Formation contains an estimated total of

more than 1.7 billion short tons of subbituminous coal in

beds 2.5 feet thick or greater and under less than 6,000 ft of

overburden (table 7). This total represents about 18 percent

of the total of the four formations. About 50 percent of the

Lance coal is deeper than 3,000 ft. In contrast to the method

used for the other formations to determine overburden thickness, structure contours were drawn on the base of the Lance

rather than on the top. Because the Lance is about 1,200 ft

thick, and because most of the Lance coals occur near the

base of the formation, it is a better representation of

overburden thickness (fig. 34B).

In the early 1920’s Lance coal was obtained from small

wagon mines northeast of Craig and in T. 7 N., R. 90 W. and

T. 6 N., R. 89 W. (Bass and others, 1955). Lance coal is of

subbituminous B and C apparent rank (Murray, 1980) and

has a calorific value, as mined, of about 9,700 Btu/lb (Bass

and others, 1955). On an as-received basis, average sulfur

content and ash yields for 6 mine and outcrop samples in the

Yampa coal field are 0.5 and 4.8 percent, respectively (Bass

and others, 1955).

Based on the available information, the Lance Formation in Area 1 has a high potential for coal resources, with

certainty level B (K1, fig. 36). Although coal in this formation underlies most of Area 1, much of it is is thin and lenticular in nature and buried too deeply to be considered an

economic resource.

FORT UNION FORMATION COALS

There is almost no information about the Fort Union

Formation as it occurs within the Forest boundary. The

Fort Union is exposed only in a small area in T.10 N., R. 88

W. and is otherwise completely covered by younger sediments of the Wasatch and Browns Park Formations (fig.

28). Drill hole 5 (fig. 35) is the only drill hole within the

Forest that provides data on this unit, but, as with the Lance

Formation, wells to the west and south of the boundary provide control for maps that is necessary to make projections

into the area with no data. Several studies of Fort Union

stratigraphy in the Sand Wash Basin include isopach maps

of cumulative thickness of coal in the Fort Union Formation (Boreck and others, 1981; Tyler and McMurry, 1993;

Beaumont, 1979; Honey and Hettinger, 1989). Isopachs

were projected into the Forest area based on these previous

interpretations; cumulative thickness of coal in the Fort

Union Formation appears to be thinning to the east toward

the basin margin (fig. 35A). Almost all of the coal beds

occur in the lower part of the Fort Union Formation. Coal

beds range in thickness from less than 3 ft to a maximum of

17 ft, with an average thickness of about 5 ft.

The Fort Union Formation contains an estimated total

of about 600 million short tons of subbituminous coal

under less than 6,000 ft of overburden in beds 2.5 feet thick

or greater (table 7). About 20 percent of this total is deeper

than 3,000 ft. The Fort Union Formation coal resource tonnage represents only seven percent of the total of the four

formations. Seventy-one million short tons are estimated to

lie within 1,000 feet of the surface. This may be a very

high estimate. The isopach trends (fig. 35A, 35B) indicate

that the coal and overburden are thinning toward the east;

therefore, it is possible that the Fort Union Formation is

barren of coal where it comes within mining distance of the

surface. It is not known how much of the Fort Union Formation may have been removed by erosion prior to deposition of the overlying Wasatch Formation. More

information is needed to understand which geologic controls may have affected this resource.

Fort Union coals range from subbituminous B or C in

apparent rank in outcrops along the southern, eastern, and

northwestern margins of the basin to probably highvolatile A bituminous in the deeper parts of the Sand Wash

Basin (Murray, 1980; Scott, 1993). There are no analyses

of Fort Union coal in Forest Area 1, but analyses of five

samples nearby in Routt and Moffat Counties (of uncertain

reliability due to thin overburden and probable weathering)

indicate that, on an as-received basis, ash yields are about 5

percent and sulfur values are about 0.3 percent, with a calorific value of about 9,850 Btu/lb (Bass and others, 1955,

their table 9).

Fort Union Formation coals have high coal resource

potential, with certainty level B (K1, fig. 36). The geologic

characteristics of Fort Union coal beds indicate that their

cumulative thickness tends to thin to less than 5 ft thick

toward the east in Area 1 as they approach mining depth.
86 MINERAL RESOURCE POTENTIAL AND GEOLOGY, ROUTT NATIONAL FOREST, COLORADO

20

15

10

10

5

1000

2000

3000

4

5

13

15

17

4

5

11

12

13

15

16

17

222

220

262

201

260 261

222

220

262

201

260

261

Moffat County

Routt County

Moffat County

Routt County

0 2 4 6 MILES

0 2 4 6 MILES

Outcrop

base of

Fox Hills

Sandstone

Outcrop

base of

Fox Hills

Sandstone

107°22'30" 107°15' 107°07'30"

40°52'30"

T. 10 N.

40°45'

T. 9 N.

T. 8 N.

R. 90 W. R. 89 W. R. 88 W. R. 87 W. R. 86 W.

107°22'30" 107°15' 107°07'30"

40°52'30"

T. 10 N.

40°45'

T. 9 N.

T. 8 N.

R. 90 W. R. 89 W. R. 88 W. R. 87 W. R. 86 W.

11

12

16

A

B

Figure 34. A, Isopach map of cumulative coal in the Lance Formation, in and near Area 1. Thickness values are

in feet. B, Isopach map of overburden; datum is base of Lance Formation (top of Fox Hills Sandstone) in Area 1.

Thickness values are in feet. Drill holes represented by ( ) and number, measured outcrop sections by (×) and

number. See table 8 for location of drill holes; see table 9 for the location, thickness, and source data for

measured sections.
87

30

25

20

15

10

5

1000

2000 3000

12

15

343

5

5

13

15

16

17

343

Inferred outcrop

top of Fort Union

Formation

Inferred outcrop

top of Fort Union

Formation

Moffat County

Routt County

0 2 4 6 MILES

Moffat County

Routt County

0 2 4 6 MILES

17

16

13

12

B

40°52'30"

T. 10 N.

T. 9 N.

T. 8 N.

40°45'

107°22'30" 107°15' 107°07'30"

R. 90 W. R. 89 W. R. 88 W. R. 87 W. R. 86 W.

A

40°52'30"

T. 10 N.

T. 9 N.

T. 8 N.

40°45'

107°22'30" 107°15' 107°07'30"

R. 90 W. R. 89 W. R. 88 W. R. 87 W. R. 86 W.

MINERAL RESOURCES—LEASABLE MINERALS

Figure 35. A, sopach map of cumulative coal in the Fort Union Formation, in and near Area 1. Thickness

values are in feet. B, Isopach map of overburden; datum is top of Fort Union Formation in Area 1. Thickness

values are in feet. Drill holes represented by ( ) and number, measured outcrop sections by (×) and number.

See table 8 for location of drill holes; see table 9 for the location, thickness, and source data for measured

sections.
88 MINERAL RESOURCE POTENTIAL AND GEOLOGY, ROUTT NATIONAL FOREST, COLORADO

Colorado River

Lake

Granby

Colorado River

Rocky

Mountain

National

Park

Flat

Tops

Area 2

Area 3

Area 6

Area 4

Area 1

107°30' 107°00' 106°30' 106°00' 41°00'

40°30'

40°00'

COLORADO 0 5 10 MILES

EXPLANATION

RESOURCE POTENTIAL

High

Moderate

Low

Craig

Steamboat

Springs

Walden

Kremmling

Gore Range

Park Range

Park Range

Hahns

Peak

Mt

Zirkel

Williams Fork Mountains

Medicine Bow Mountains

Area 5

LIMITATION OF DATA

It is clear on the maps (figs. 32–35) that drill-hole and

outcrop data are sparse at best. Of the 16 wells used for

this study, only five are within the Forest boundary, and, of

those five, only one (hole 4) provides data for three of the

four coal-bearing formations. The other holes provide

complete data for only one or two of the coal-bearing

formations. Therefore, these estimates of coal resources

are highly speculative.

The contacts between the Wasatch and the Fort Union

and between the Fort Union and the Lance Formations are

difficult to distinguish in well logs because these strata are

fluvial deposits mapped on the surface using characteristics

that do not appear in subsurface geophysical logs, such as

color and fairly subtle differences in grain size. Also, picking a contact between the marine and marginal marine units

(i.e., Mancos/Iles, Williams Fork/Lewis, and Lewis/Fox

Hills) is somewhat arbitrary because these units intertongue.

However, stratigraphic correlations of the coal-bearing formations across the study area must be attempted in order to

assign coal zones to the correct formation and in order to

assign a contact between formations for the purpose of

constructing structure contour maps. Documented stratigraphic information (Boreck and others, 1981; Bader, 1983;

Honey and Hettinger, 1989) was extrapolated into the Forest

from south and west of the study area.

The cumulative coal thickness of the formations also

had to be projected across the area with sparse data from

areas where documented coal-thickness information exists.

Figure 36. Map showing coal resource potential of areas underlain by coal-bearing Upper Cretaceous and Tertiary stratigraphic units

in and adjacent to the Routt National Forest and Middle Park Ranger District of the Arapaho National Forest, Colorado.
MINERAL RESOURCES—LEASABLE MINERALS 89

Measurements of coal at the outcrop were generally used to

delineate the distribution of the coal and as a minimum

thickness value for the isopachs. Outcrop measurements are

considered minimum values because they usually only

represent a single coal bed in the formation, whereas the isopachs represent cumulative coal thickness in the entire formation. For the Iles and Williams Fork Formations (figs.

32A, 33A), general isopach trends were drawn based on the

estimated orientation of the shoreline that was present at the

time of deposition of the peat that formed the coal (Lillegraven and Ostresh, 1990; Hamilton, 1993; Cobban and

others, 1994). Isopach trends for the Lance and Fort Union

Formation coals (figs. 34A, 35A) were extrapolated into the

Forest from trends determined in previous studies from data

to the west of the Forest (Boreck and others, 1981; Tyler

and McMurry, 1993; Beaumont, 1979).

Isopach maps of overburden (depths to the top of the

Iles, Willliams Fork, and Fort Union Formations; depth to

the base of the Lance Formation) were constructed by subtracting the elevation above sea level of the top or base of

the coal-bearing formation from the elevation of the

ground surface. It must be stressed that the overburden

isopachs on the top of the Iles, Williams Fork, and Fort

Union Formations (figs. 32B, 33B, 35B) are absolute minimums because coal beds in these formations commonly are

near the base of the formation. Therefore, the overburden

thickness to the stratigraphically highest coal in these formations is actually greater than appears on the maps.

Owing to the thickness of the formations, this difference

could be as much as 1,000 ft. The thickness of overburden

to the base of the Lance Formation (fig. 34B) was used

because the coals are all within 200 ft of the base of the

formation. Therefore the overburden thickness on the

highest coal is actually less than appears on the map.

POTENTIAL FOR OTHER UNDISCOVERED 

COAL RESOURCES

AREAS 2, 4, AND 6

These areas have no coal resource potential based on

the presence of predominantly crystalline rocks and on the

lack of coal-bearing rocks, with certainty level D (fig. 36).

AREA 3

The Iles Formation is the only coal-bearing formation

in Area 3 and it occurs only in the northwestern part; all

younger coal-bearing rocks are eroded (Sharps, 1962). The

Iles Formation has moderate resource potential for coal,

with certainty level B (K2, fig. 36). Although much of the

coal is burned along the outcrop, Sharps (1962) did report a

coal bed 3.5 ft thick in this area.

OIL, GAS, AND COAL-BED

METHANE RESOURCES 

By Craig J. Wandrey, Ben E. Law, Charles W. Spencer, and

Charles E. Barker

INTRODUCTION

More than half the area within Routt National Forest

and the Middle Park Ranger District of the Arapaho National

Forest is underlain by Proterozoic metamorphic and igneous

rocks of the Park, Gore, Rabbit Ears, Front Range, and the

Never Summer Mountains (plate 1)—this area has no potential for oil and gas production except beneath the Independence and Williams Fork thrusts. The remaining lands of the

Forest where Paleozoic through Tertiary sedimentary rocks

occur have been intermittently explored for oil and gas since

1928. This activity resulted in the discovery of one oil field

in the Elkhead Mountains and two oil fields in the Flat Tops

areas of the Forest (fig. 37). These fields are the California

Park field, discovered in 1983 and abandoned in 1984; the

Pinnacle field, discovered in 1956 (presently shut-in); and

the Scott Hill field, with three producing wells.

In the following discussion, the potential of conventional and unconventional hydrocarbon accumulations are

independently discussed. Conventional hydrocarbon accumulations are discrete oil and gas deposits that occur in

structural, stratigraphic, and combination traps. In contrast,

unconventional hydrocarbon accumulations are regionally

extensive and cut across structural and stratigraphic boundaries. They also lack down-dip water contacts. Unconventional accumulations in this region include basin-centered

gas, coal-bed methane, and oil in fractured shale. Most of

the discussion regarding reservoirs and source rocks applies

to most of northwest Colorado, or the North Park and Middle

Park Basins.

CONVENTIONAL HYDROCARBON ACCUMULATIONS

Reservoirs.—The principal conventional reservoirs on

the Forest lands include the Middle Pennsylvanian Minturn

AREA 5

Resource potential for coal in the Coalmont and Middle

Park Formations is largely unknown because of the lack of

sufficient data; however, the potential is probably low, with

certainty level C (K3, fig. 36). Published geologic maps of

Area 5 did not report the occurrence of coal in these formations. Although operating coal mines are located north of

this area, the geologic characteristics of the coal-bearing

units indicate that the coal beds thin and pinch out southward

in the direction of Area 5.
90 MINERAL RESOURCE POTENTIAL AND GEOLOGY, ROUTT NATIONAL FOREST, COLORADO

Formation; the Permian and Pennsylvanian Weber Sandstone and associated formations, the Upper Triassic

Shinarump Sandstone and Moenkopi Formation; the Middle

Jurassic Entrada Sandstone and Upper Jurassic Morrison

Formation; the Lower Cretaceous Dakota and Lakota Sandstones; and the Upper Cretaceous Frontier Formation,

Nobrara Formation, Morapos Sandstone Member of the

Mancos Shale, Pierre Shale, Mesaverde Group including the

Almond Formation, and the Lewis Shale. Tertiary reservoir

rocks are absent through most of the area and, where present,

are not considered to have any potential. Porosity in the

principal conventional reservoirs ranges from less than 10 to

20 percent and permeability ranges from less than 0.1 to 300

millidarcies (mD). Reservoir thickness ranges from 8 to 65

ft. Drilling depths to these reservoirs range from less than

1,000 to 10,000 ft.

Source rocks and geochemistry.—Possible hydrocarbon source rocks include the Middle Pennsylvanian Belden

Shale (Nuccio and Schenk, 1986; Waechter and Johnson,

1986), the Permian Phosphoria Formation, the Upper Cretaceous Mowry Shale (Fillmore, 1986), Niobrara Shale, Mancos Shale, Pierre Shale, and Lewis Shale. In addition, the

coal beds contained in the Upper Cretaceous Mesaverde

Group are likely sources of gas. The Belden Shale may be

thermally overmature west of the Gore and Park Ranges and

is absent from the stratigraphic section east of the ranges.

The Phosphoria and Cretaceous rocks are mature to undermature with respect to hydrocarbon generation. The Paleozoic reservoirs may contain oil with sulfur and sour gas.

Mesozoic oil is low sulfur and the gas is sweet. Produced

water may vary from salty at depths of more than 5,000 ft

and fresh at depths less than 2,000 ft.

Timing of generation and migration.—The present-day

levels of thermal maturity were probably achieved during

Oligocene time. Most structural traps were most likely

formed during the Laramide orogeny; however, some traps

could have formed as early as Pennsylvanian time west of

the Gore and Park Ranges. Consequently, the temporal relationships between hydrocarbon generation, migration, and

development of structural traps were favorable.

Traps and seals.—Although reservoirs such as the

Weber, Entrada, Shinarump, Morrison, Dakota, and Frontier

Formations potentially have stratigraphic traps, all conventional fields in northwest Colorado and North Park Basin

produce from structural traps. Consequently, there is no

compelling reason to expect the discovery of significant

stratigraphic accumulations. Structural traps could include

small, tightly folded anticlines or faulted anticlines. In

North and Middle Parks, anticlinal traps produce near the

basin margins. The numerous low-permeability shales in

Paleozoic and Mesozoic rocks could provide adequate seals.

Exploration status and resource potential.—The area is

relatively maturely explored. However, the area is structurally complex and has a long history of structural deformation

dating back to Proterozoic time. Some older structures may

have been overlooked due to concealment by younger,

Tertiary rocks. 

The area in the Elkhead Mountains is underlain by

Cretaceous and Tertiary sedimentary rock and has moderate

potential for gas resources, with certainty level B (L1, fig.

38). A certainty level of B is required because much of the

area is covered by Tertiary sedimentary rocks that may

conceal older structures in the underlying rock units.

In the Flat Tops area (fig. 37), there are two small

accumulations in structural traps; the Pinnacle field, which

produced from the Shinarump and Dakota, and the Scott Hill

field, which produced from the Weber, Dakota, and Frontier.

Untested northwest-trending surface structures in the Flat

Tops have moderate potential for gas resources, with

certainty level B (L2, fig. 38). Based on the cumulative production from the Pinnacle and Scott Hill fields (less than 150

thousand barrels of oil (MBO)), it is unlikely that new

discoveries in this area will exceed 1 million barrels of oil

equivalent (MMBOE).

The area from the Rabbit Ears Range south to Corral

Peaks between North and Middle Park Basins (L3, fig. 38) is

underlain by Cretaceous and Tertiary sedimentary rocks covered and intruded by Tertiary volcanic rocks. The volcanic

rocks may conceal structural traps. Immature to marginally

mature source rocks in this area have low oil potential, but

several test wells have had gas shows. The area has moderate

potential for gas resources, with certainty level B. It is

unlikely that new discoveries in this area will exceed 6

billion cubic feet (BCF) or 1 million barrels of oil equivalent

(MMBOE).

The areas within the Forest on the southwest side of the

Gore Range, along the eastern side of the Park Range, and on

the northwestern corner of the Williams Fork Mountains (L4,

fig. 38) have relatively thin sedimentary sections that are less

conducive to gas generation and trapping. These areas have

low potential for gas resources, with certainty level C.

SUBTHRUST HYDROCARBON ACCUMULATIONS 

Reservoirs.—The most prospective reservoir rocks are

the Jurassic Entrada Sandstone and Morrison Formation, and

the Cretaceous Dakota and Lakota Sandstones, Frontier Formation, and Niobrara and Pierre Shales. Porosities range

from 10 to 20 percent, and permeabilities range from less

than 0.1 to 300 mD. Reservoir thicknesses are generally

thinner in these basin-margin settings and range from less

than 8 ft to 60 ft.

Source rocks.—The Cretaceous Mowry Shale has probably the best source-rock potential (Fillmore, 1986) and has

been identified as a primary source rock in the adjacent

Greater Green River Basin. Thermal maturities are potentially higher in the subthrust due to increased heating because

of greater burial depths.
91

Independence Mountain

thrust fault

0 4 8 MILES

0 4 8 KILOMETERS

107°30' 107°15' 107°00' 106°45' 106°30' 106°15' 106°00'

41°00'

41°45'

41°30'

41°15'

40°00'

39°45'

Craig

Steamboat

Springs

Walden

Kremmling

EXPLANATION

Well location

National Forest boundaries

WYOMING

COLORADO

JACKSON COUNTY

GRAND COUNTY

SUMMIT

COUNTY

CLEAR

CREEK

COUNTY

GARFIELD COUNTY EAGLE COUNTY

RIO BLANCO

COUNTY

ROUTT

COUNTY

MOFFAT COUNTY

NATIONAL

FOREST

ROUTT

GRAND

LAKE

Blue River Valley

Gore Range

Gore Range

Vasquez Mountains

Williams Fork

thrust fault

Middle

Park

Corral Peaks

Rabbit Ears Range

North Park

Coalmont

Butler

Delaney Creek

Butte

Lone

Pine

Michigan

River

Battleship

Canadian River

South McCallum

North

McCallum

Sierra M

Medicine Bow Mountains

adre

Park Range

Focus

Ranch

Slater

Elk

Head

Mountains

California

Park

North

Craig

Curtis

Buck

Peak

Tow

Creek

Grassy

Creek

Sage

Creek

Oak

Creek

Pinnacle

Pagoda

Scott Hill

Flat Tops

Exploration status and resource potential.—Thrust

faults having a heave sufficient to create or preserve traps of

commercial size are the Independence Mountain thrust,

where Proterozoic rocks are thrust south as much as 12 mi

over the Paleocene and Eocene Coalmont Formation of

North Park (Blackstone, 1977), and the Williams Fork thrust

on the east side of the Blue River Valley, where Proterozoic

rocks of Williams Fork Mountains are thrust westward over

the Pierre Shale. The rocks of the subthrust have not been

tested at either the Independence Mountain or Williams Fork

thrusts. The potential for finding a 1 MMBOE field under

Timing of generation and migration.—Burial histories

for North and Middle Park Basins indicate that maximum

burial and thermal maturity occurred during Oligocene time

(Fillmore, 1986; Maughan, 1988) with migration updip over

short distances.

Traps and seals.—Preexisting traps (anticlines and

faulted anticlines) may be preserved and enhanced beneath

mountain-front thrusts. Proterozoic rocks in the hanging

wall of the thrust may also act as a trap in conjunction with

an underlying thick, low-permeability shale seal.

Figure 37. Map showing oil and gas fields in and adjacent to the Routt National Forest and Middle Park Ranger District of the Arapaho

National Forest, Colorado.

MINERAL RESOURCES—LEASABLE MINERALS
92 MINERAL RESOURCE POTENTIAL AND GEOLOGY, ROUTT NATIONAL FOREST, COLORADO

107°30' 107°00' 106°30' 106°00'

41°00'

40°30'

40°00'

39°30'

COLORADO

Craig

Steamboat

Springs

Eagle

Vail

Kremmling

Walden

Lake

Granby

Rabbit Ears Range

Williams Fork Mountains

Flat

Tops

Gore Range

Gore Range

Park Range

Park Range

Hahns

Peak

Mt

Zirkel Medicine Bow Mountains

Rocky

Mountain

National

Park

Colorado River

Colorado River

0 5 10 MILES

WYOMING

COLORADO

i

70

Approximate boundary of the

Routt National Forest and the

Middle Park Ranger District

of the Arapaho National Forest

Approximate boundary of the

Routt National Forest and the

Middle Park Ranger District

of the Arapaho National Forest

Approximate boundary of the

Routt National Forest and the

Middle Park Ranger District

of the Arapaho National Forest

EXPLANATION

RESOURCE POTENTIAL

Moderate

Low

L1

L2 L3

L4

L4

L4

L4

L5

L6

UNCONVENTIONAL HYDROCARBON 

ACCUMULATIONS

COAL-BED METHANE 

The coal-bearing units within the Forest include the

Upper Cretaceous Iles, Williams Fork, Almond, and Lance

the Williams Fork thrust is low, with certainty level B (L5,

fig. 38). The potential is also low, with certainty level B (L6,

fig. 38) for the Independence Mountain thrust because only a

small area of the subthrust extends into the Forest. Resource

estimates are based on source-rock potential and volume of

potential reservoir rocks within the Forest (subthrust boundaries are highly speculative).

Figure 38. Energy resource map for conventional and subthrust oil and gas accumulations (L) in the Routt National Forest and

Middle Park Ranger District of the Arapaho National Forest, Colorado.
93

Elkhead Mountains has moderate potential for coal-bed

methane resources, with certainty level C (M1, fig. 39). A

small area outlined by outcrop of the Mesaverde in the

northwestern part of the Flat Tops has low resource potential

for coal-bed methane resources, with certainty level D (M2,

fig. 39). This rating is assigned because of very shallow

depths and limited presence of coal-bearing rocks.

BASIN-CENTERED GAS ACCUMULATIONS

Gas in basin-centered accumulations (also referred to

as “tight gas reservoirs”) in the Forest include the Lower

Cretaceous Dakota Sandstone and Upper Cretaceous

Frontier Formation. For the most part, the potential for

basin-centered gas accumulations in the Forest is low

because the source rocks associated with low-permeability reservoirs in these units are immature to slightly

mature. Studies of basin-centered gas accumulations in

the Greater Green River Basin of Wyoming, Colorado,

and Utah by Law (1984) have indicated that, at presentday depths of at least 8,000 ft and at levels of thermal

maturity of 0.8 percent Ro, there is a high probability of

encountering basin-centered gas accumulations. For

detailed descriptions of basin-centered gas accumulations,

see Law and Spencer (1993). The western part of the

Elkhead Mountains area has some tight reservoir potential and thus has low potential for gas resources, with certainty level C (N1, fig. 40). It is unlikely that there will

be any discoveries within the Forest that will exceed 1

MMBOE. Cretaceous tight reservoirs generally produce

sweet gas and they generally do not produce significant

amounts of water.

OIL IN FRACTURED SHALES

Fractured shales in the Upper Cretaceous Mancos

Shale in northwest Colorado produce oil. The shales

(including siltstone, calcareous shale, and limestone) are

both the source rock and reservoir rock. Oil production may

occur in areas where the shales are thermally mature and

fractures are present, such as in areas of maximum flexure

along the crests of anticlines and monoclines or in highly

faulted areas. Because the production is fracture-related,

well productivity is highly variable and unpredictable. The

oil is low sulfur, and wells do not produce water. The California Peak field, located in the Elkhead Mountains area of

the Forest (fig. 37), is the only fractured shale field in the

Forest. This field is very small and produced about 1,748

barrels of oil (BO) and 471 thousand cubic feet of gas

(MCFG) through 1991 (Colorado Oil and Gas Conservation

Commission, 1992). Examples of fields outside the Forest

producing oil from fractured shales include Buck Peak,

Grassy Creek, Tow Creek, and Coalmont (fig. 37). The

Formations and the Paleocene Fort Union Formation. Coal

beds also occur in the Lower Cretaceous Dakota Sandstone

and Upper Cretaceous Frontier Formation; however, the

coal beds in these units are so lenticular and thin that they are

not considered to have any potential for economic methane

production. In order of decreasing coal-bed methane potential, the coal-bearing units are Williams Fork, Almond, Iles,

Lance, and Fort Union. The factors considered in this ranking include number of coal beds, cumulative thickness of

coal beds, quality of coal, coal rank, and gas content. The

principal studies of coal-bed methane resources in the region

include those by Boreck and others (1981), McCord (1984),

Tremain (1990), Kaiser and others (1993), and Kaiser

(1993). Coal-bed methane is sweet (low in sulfur), and water

may be fresh to brackish with moderate bicarbonate content.

Coal beds in these Cretaceous and Tertiary rocks were

deposited in environments that include fluvial, delta-plain,

and back-barrier depositional systems. The thicker and more

continuous coal beds in northwest Colorado occur in intervals or zones 100 to 1,200 ft thick. There are as many as 30

coal beds in any single coal zone, but, more commonly, there

are four to eight coal beds greater than 2 ft thick. Individual

coal beds are as much as 40 ft thick.

Based on analyses of coal beds from the Sand Wash

Basin, the rank of coal beds in the various zones ranges from

sub-bituminous B to high-volatile bituminous B (0.45–0.75

percent Ro, reflectance in oil). The coal is composed of

humic organic matter; vitrinite is the main coal maceral.

Cleat development is good and is considered normal for subbitumuinous and bituminous coal. The gas content of the

coal ranges from 100 to 541 standard cubic feet per short ton

(scf/ton), and the gas typically has large amounts of methane

with lesser amounts of ethane and heavier hydrocarbons. In

some areas of the Sand Wash Basin, coal-bed methane contains carbon dioxide in amounts as high as 25 percent.

In the absence of specific test data, it can be assumed

that all coal beds in the Forest contain gas; however, the

presence of large amounts of water in coals commonly precludes economic rates of gas production. In areas like the

San Juan Basin of New Mexico and Colorado, dewatering

programs have been successful, and structural or stratigraphic traps are not necessary for production. However, in

other areas, where there are large amounts of water associated with the coal that cannot be economically dewatered,

structural traps are necessary for economic rates of gas production (Rice and Law, 1993).

Although no coal-bed methane wells have been drilled

in the Forest, regional data from coal-bed methane tests in

the Sand Wash Basin indicate the presence of good gas contents, and all of the wells have encountered large volumes of

water. In the absence of any conflicting data, the coal-bed

methane potential within the Forest is considered low to

moderate because of the probable presence of large volumes

of water. It is unlikely that there will be any discoveries

larger than 6 BCF of gas (1 MMBOE). The area of the

MINERAL RESOURCES—LEASABLE MINERALS
94 MINERAL RESOURCE POTENTIAL AND GEOLOGY, ROUTT NATIONAL FOREST, COLORADO

107°30' 107°00' 106°30' 106°00'

41°00'

40°30'

40°00'

39°30'

COLORADO

Craig

Steamboat

Springs

Eagle

Vail

Kremmling

Walden

Lake

Granby

Rabbit Ears Range

Williams Fork Mountains

Flat

Tops

Gore Range

Gore Range

Park Range

Park Range

Hahns

Peak

Mt

Zirkel Medicine Bow Mountains

Rocky

Mountain

National

Park

Colorado River

Colorado River

0 5 10 MILES

WYOMING

COLORADO

i

70

Approximate boundary of the

Routt National Forest and the

Middle Park Ranger District

of the Arapaho National Forest

Approximate boundary of the

Routt National Forest and the

Middle Park Ranger District

of the Arapaho National Forest

Approximate boundary of the

Routt National Forest and the

Middle Park Ranger District

of the Arapaho National Forest

EXPLANATION

RESOURCE POTENTIAL

Moderate

Low

M1

M2

Figure 39. Energy resource map for coal-bed methane gas accumulations (M) in the Routt National Forest and Middle Park

Ranger District of the Arapaho National Forest, Colorado.

producing interval in these fields may be as thick as 50 ft.

The source rocks are marine shales that contain 1 to 4 percent

organic matter. The oil was probably generated in Oligocene

time, during maximum burial. The oil is trapped by less brittle, unfractured shale.

The presence of small, tightly folded anticlines and

faults in the Elkhead Mountains and Flat Tops areas of the

Forest present favorable conditions for the existence of fractured shales. Both of these areas have moderate resource

potential for oil, with certainty level D (O1 and O2, fig. 41).

Favorable conditions may also exist for the Niobrara Shale in

the area of th Rabbit Ears Range and southward into Middle

Park (O3, fig. 41); this area has low resource potential for oil,

with certainty level C. It is unlikely that any discoveries will

exceed 1 MMBO, but future exploration may include the

drilling of horizontal and inclined wells that have a better

ability to intersect open fractures. Only a few wells have

been drilled in northwest Colorado using these methods.
95

107°30' 107°00' 106°30' 106°00'

41°00'

40°30'

40°00'

39°30'

COLORADO

Craig

Steamboat

Springs

Eagle

Vail

Kremmling

Walden

Lake

Granby

Rabbit Ears Range

Williams Fork Mountains

Flat

Tops

Gore Range

Gore Range

Park Range

Hahns Park Range

Peak

Mt

Zirkel Medicine Bow Mountains

Rocky

Mountain

National

Park

Colorado River

Colorado River

0 5 10 MILES

WYOMING

COLORADO

i

70

Approximate boundary of the

Routt National Forest and the

Middle Park Ranger District

of the Arapaho National Forest

Approximate boundary of the

Routt National Forest and the

Middle Park Ranger District

of the Arapaho National Forest

Approximate boundary of the

Routt National Forest and the

Middle Park Ranger District

of the Arapaho National Forest

EXPLANATION

RESOURCE POTENTIAL

Low

N1

Figure 40. Energy resource map for basin-centered gas accumulations (N) in the Routt National Forest and Middle Park Ranger

District of the Arapaho National Forest, Colorado.

MINERAL RESOURCES—LEASABLE MINERALS
96 MINERAL RESOURCE POTENTIAL AND GEOLOGY, ROUTT NATIONAL FOREST, COLORADO

107°30' 107°00' 106°30' 106°00'

41°00'

40°30'

40°00'

39°30'

COLORADO

Craig

Steamboat

Springs

Eagle

Vail

Kremmling

Walden

Lake

Granby

Rabbit Ears Range

Williams Fork Mountains

Flat

Tops

Gore Range

Gore Range

Park Range

Hahns Park Range

Peak

Mt

Zirkel Medicine Bow Mountains

Rocky

Mountain

National

Park

Colorado River

Colorado River

0 5 10 MILES

WYOMING

COLORADO

i

70

Approximate boundary of the

Routt National Forest and the

Middle Park Ranger District

of the Arapaho National Forest

Approximate boundary of the

Routt National Forest and the

Middle Park Ranger District

of the Arapaho National Forest

Approximate boundary of the

Routt National Forest and the

Middle Park Ranger District

of the Arapaho National Forest

EXPLANATION

RESOURCE POTENTIAL

Moderate

Low

O1

O2 O2

O3

Figure 41. Energy resource map for fractured shale oil accumulations (O) in the Routt National Forest and Middle Park Ranger

District of the Arapaho National Forest, Colorado.
97

No known geothermal springs are within the Forest.

However, several geothermal springs are located near the

Forest, including those at Steamboat Springs along the

Yampa River, Brand’s Ranch artesian well west of

Walden, and Routt Hot Springs (also called Strawberry

Park Hot Springs) 8 mi north of the town of Steamboat

Springs. These thermal springs occur in an area of high

regional geothermal gradient in faulted rocks ranging from

Proterozoic to Tertiary in age. The presence of hot springs

is independent of rock type but is dependent on both structure and the presence of hydrothermal systems. The areal

extent of possible hot springs has been estimated for the

Steamboat Springs vicinity (Pearl, 1979) and may include

land within the Forest. 

The town of Steamboat Springs, just outside the Forest

boundary, takes its name from one of the several mineral hot

springs within the town limits, along the Yampa River. The

temperatures of these springs ranges from a low of 68°F to

a high of 102°F (Barrett and Pearl, 1978). Heart Spring,

located at the southeast end of town, is the only developed

spring, and its waters are used in the community pool.

Extrapolation from geologic mapping and geophysical studies suggests that these springs are fault controlled (Christopherson, 1979; Pearl, 1979). Pearl and others (1983)

estimated that the Steamboat Springs reservoir has an areal

extent of 0.52 mi2. 

GEOTHERMAL RESOURCES 

By Sandra J. Soulliere

A group of thermal springs known as the Routt Hot

Springs, or Strawberry Park Springs, are located approximately 8 mi north of Steamboat Springs. These springs are

used commercially for recreational bathing and are located

on private land within the Forest. Five springs in this area

discharge water between 124°F and 147°F, at rates of 2 to 50

gallons per minute (Barrett and Pearl, 1978). The springs are

fault and fracture controlled and may be part of the Steamboat Springs geothermal system. Pearl and others (1983)

estimated that the Routt Hot Springs reservoir has an areal

extent of 0.5 to 0.75 mi2. 

Brand’s Ranch artesian well, located just west of

Walden, was originally an 800-ft-deep oil test hole. Two

estimates of the reservoir’s extent were made by Pearl

(1979). One estimate measured the extent to be limited to

0.36 mi2 around the well. The other estimate extended the

reservoir to include the projected faults south and approximately 1.3 mi north of the well. This system encompasses an

area of 1.5 mi2.

All of the geothermal springs in the vicinity of the

Forest have been identified and their extent estimated.

Thorough investigations by the Colorado Geological Survey

(Barrett and Pearl, 1978; Pearl, 1979; Pearl and others,

1983) have not identified additional geothermal resources

near or within the Forest. Therefore, there is no potential

for undiscovered geothermal resources within the Forest,

with certainty level C.
99

MINERAL RESOURCES—SALABLE MINERALS

By John S. Dersch, U.S. Forest Service

The Federal Minerals Act of 1947, as amended by the

Multiple Surface Use Act of 1955, removed petrified wood,

common varieties of sand and gravel, stone, pumice, volcanic

cinders (including scoria), and some clay from acquisition by

either location or lease. These minerals may be acquired

from the U.S. Government only by purchase and are referred

to as salable minerals. Several exceptions to the salable category are block pumice, perlite, and forms of dimension

stone such as travertine and high-quality marble. Determination that a particular mineral is a salable mineral must be

reviewed on case-by-case basis in light of past legal decisions. Salable minerals generally have a low unit value

(value per ton); their exploitation is dependent on easy access

to transportation, and generally they are used near the

production site.

Numerous crushed aggregate sites are within the Forest,

some of which are currently active. Needs are met by quarrying bedrock such as sandstone, volcanic rocks, granite, and

basalt. Dimension stone needs are met from moss- or lichencovered granite and sandstones.

Sand and gravel resources are available from most

major drainages, landslide deposits, and glacial drift. Uses

range from road fill, aggregate for concrete, macadam, mortar, and other purposes. Schwochow (1981) noted nine sand,

sand and gravel, rubble, and borrow-material sites within the

Forest.

The Forest is included in District 1, a Bureau of Mines

designated area for the northwestern counties of Colorado.

The sand-and-gravel production in this area for 1992 was

560,000 short tons worth $19,600. The crushed aggregate

production in 1991 was 43,000 short tons valued at $144,000.

MINERAL RESOURCE 

POTENTIAL—SALABLE MINERALS

The mineral resource potential for salable materials is

evaluated in this section. Income from the sale of these minerals will vary according to accessibility, unit cost and value,

and production. Environmental factors have not been

considered in this assessment.

CRUSHED AGGREGATE

Aggregate sources can be found in the Elkhead Mountains, the Park and Medicine Bow Ranges, and the Flat

Tops. The Elkhead Mountains consist of a sedimentary

rock section intruded by subvolcanic Tertiary igneous

rocks. The Park and Medicine Bow Ranges include a granitic rock core intermixed with felsic and hornblende

gneisses. The Flat Tops are a northerly dipping sedimentary package partially covered in the south by Tertiary

basalts. Glacial drift can be found at higher elevations in

the Park Range.

Sandstone.—Sandstone occurs as distinct ridges and

caprock in Tertiary, Triassic, and Cretaceous rocks. The

Browns Park Formation is a siltstone, claystone, and conglomerate that is flat lying to gently dipping (<10°). It is

easily mined, crushed, and sorted by sizes. Uses in the Forest are predominantly roadway construction, but also

includes sand and gravel for concrete work. The White

River Formation is a tuffaceous siltstone and claystone that

is easily crushed and is used for sand and gravel needs or as

binder in roadway construction and general building needs.

The Wasatch Formation is an arkosic sandstone, mudstone,

and conglomerate. These rocks are used for building roadway and may satisfy general building and concrete needs

after crushing if sufficient fines are generated during production. The Upper Triassic Chinle and Chugwater Formations are sandstones with intermixed conglomerates,

siltstones, mudstones, and shales. All are easily crushed

providing a wide range of sizes. Production would exclude

the mudstone and shale units. Uses include road building

and concrete manufacture.

Volcanic rocks.—Numerous volcanic necks can be

found on the northeast corner of the Flat Tops. Numerous

Tertiary dikes or sills are found in the Elkhead Mountains

east of Bears Ears Mountain. They can both be crushed and

used for base materials, particularly roadways and railroad

ballast. The necks, dikes, and sills are found as topographically resistant features that can be removed to the ground

surface with little visual impact.
100 MINERAL RESOURCE POTENTIAL AND GEOLOGY, ROUTT NATIONAL FOREST, COLORADO

Granite.—The Proterozoic granites within the Park

Range are crushed for variously sized aggregates that are

used in roadway construction, rip rap, and railroad ballast.

Sufficient fines are usually produced as binder if needed.

Local rock glaciers and talus slopes are a source of material for crushing.

Scoria.—Tertiary basalt flows are found capping sedimentary units on the northeast corner of the Flat Tops. The

volume of material is probably limited. These flows can be

used for lightweight aggregate and concrete needs.

Landslide material.—These materials, usually consisting of sedimentary rocks, are found at the northern end of

the Flat Tops. They can be crushed for general aggregate

needs, such as roadway building, railroad ballast, rip rap,

and fill.

Glacial drift.—Quaternary glacial drift is found in

the Park Range and requires screening only to remove

large-sized fragments. The large sizes are easily crushed

to usable sizes. Reserves are local and limited. Uses

would include roadway building, railroad ballast, rip rap,

and fill.

DIMENSION STONE

Some dimension or decorative stone work occurs

locally through free use and mineral sale contracts. Moss or

lichen-covered granite is used specifically for interior or

exterior facing in homes or buildings. In addition to the

organic veneer, the color, texture, shape, and crystal size of

the granite are important. Hard, resistant sandstone can be

cut and trimmed for building facing if the bedding, texture,

and colors are pleasing. Most use is local and involves small

quantities. Granitic rocks are found in the Park Range, and

sandstones can be found in either the Elkhead Mountains or

the Flat Tops.

SAND AND GRAVEL

Sand and gravel deposits can be found along the Elk

and Colorado Rivers and their major tributaries within the

Forest. The deposits must be screened and tested for specific

uses. Uses include concrete work and products, fill material,

plastering sands, and snow and ice control.
101

PROBABILISTIC ASSESSMENT OF UNDISCOVERED METALLIC 

ENDOWMENT USING GRADE–TONNAGE MODELS

By Theresa M. Cookro

INTRODUCTION

Subjective probabilistic estimates of undiscovered

deposits and metal endowment are provided by the U.S. Geological Survey for undiscovered mineral deposits and metal

endowment within the Forest. The estimates are used to

present geologic information in language that can be used by

economists and in land-use planning. The probabilistic estimates involve three parts: (1) delineation of areas that are

favorable for the types of mineral resources indicated by the

geological, geophysical, and geochemical information; (2)

subjective estimation of the size and number of undiscovered

deposits, which is indicated by historical occurrence and production data and a comparison with grade and tonnage distribution curves of the mineral deposit models; and (3) MARK3

computer program estimation of metal endowment based on

grade and tonnage distributions from the known deposits

(Singer, 1993; Singer and Cox, 1987; Singer and Ovenshine,

1979; Menzie and Singer, 1990; Root and others, 1992). 

Before the assessment could be undertaken, a comprehensive inventory of data from the Forest was compiled. The

data included the nature of the existing deposits, their locations, and any available production records (Neubert, 1994).

The inventory is used to determine if there are sufficient data

to identify the types of mineral deposits that occur within the

Forest and to determine whether undiscovered resources

could be estimated. The data are combined with geologic,

geochemical, and geophysical information to accurately

delineate areas within the Forest that are favorable for the

occurrence of undiscovered resources.

Although ten types of deposits were identified in the

Forest, three grade and tonnage models were used for the prediction of undiscovered deposits: Sierran Kuroko, porphyry

copper, and placer gold (Cox and Singer, 1986; Singer,

1992). Estimates for other deposit types were not possible

because of insufficient data or the lack of a mineral deposit

model in the computer program. The resulting estimates of

metal endowment are a useful but imprecise measurement

and should not be considered exact. The compilation of production data for individual deposits in the study area and for

the deposits used in the grade-tonnage curves is quantitative.

Some of the resource estimation methods are subjective, and

verification of the results is impossible until the area, in years

to come, is thoroughly explored.

Definitions of terms are presented in Appendix 4, and

units of measurement are described in Appendix 5.

ASSESSMENT METHOD

DELINEATION OF PERMISSIVE AREAS

For the three-part assessment process, an assessment

team was assembled consisting of project geoscientists (S.J.

Soulliere, M.I. Toth, V. Bankey, S.M. Smith, J.A. Pitkin, and

T.M. Cookro). Compiling geological, geochemical, geophysical and mineral resource data for the Forest was the first

step in the assessment process. Deposit types that had characteristics that matched the characteristics defined by individual deposit models were used to determine the probable

mineral endowment. Areas were delineated within the Forest

as having high or moderate potential for undiscovered mineral resources. These areas predictably may contain a deposit

of the type being evaluated. The areas of moderate to high

potential correspond to areas where a number of characteristics, geologic, geophysical, geochemical, and structural elements overlap. Areas of mineral potential were considered to

extend to a reasonable depth (frequently 0.5 km) depending

on the type of deposit and therefore do not always follow map

units.

GRADE-TONNAGE MODELS AND ESTIMATION OF 

UNDISCOVERED DEPOSITS

Mineral deposit models and grade and tonnage curves

comprise information from mineral deposits throughout the

world that have the specific characteristics found in the definition of each model. Because the models have ore distributions (tonnage and grade) from many deposits of a specific

type, they can be applied locally to predict grades and

tonnages of undiscovered deposits. Some deposit types
102 MINERAL RESOURCE POTENTIAL AND GEOLOGY, ROUTT NATIONAL FOREST, COLORADO

cannot be used because not enough background data is available to use the models. The MARK3 computer program can

be used to determine undiscovered mineral endowment if

the deposit model’s grade and tonnage frequency distribution is similar to the distribution of the estimated undiscovered ore deposits in the study area (Orris and Bliss, 1991;

Bliss, 1992; Singer, 1992; Menzie and Singer, 1990; Singer

and Cox, 1987; Cox and Singer, 1986; Bliss, 1992; Orris and

Bliss, 1991). A test for the applicability of the method is

whether 80 percent of the expected number of deposits in the

Forest lie between the 90th and 10th percentile of the grade

and tonnage curves. If not, the estimates cannot be made,

and the mineral resource potential cannot be reported in

terms of endowment.

After the scientific team delineated areas favorable for

the occurrence of mineral deposits, the team’s expert judgment was then used to subjectively estimate the number of

undiscovered deposits. Economics is not a factor in this

determination. The members of the team who estimated the

highest or lowest number of unknown deposits were asked

to explain their rationale for estimation and a consensus was

attained.

The number of undiscovered deposits was estimated at

the following levels of probability: 90th, 50th, 10th, 5th, and

1st percentile. A 90th percentile would correspond to 90

percent probability that one or more deposit are present. The

least speculative of the five levels of confidence is the 90th

percentile. The number of deposits estimated for the 90th

percentile is influenced by the number of known deposits of

a particular type in the Forest and the extent of past exploration for those known deposits (Singer and Ovenshine, 1979).

A number of factors can be used for the estimates,

including the presence and amount of unconsolidated surficial deposits, areas that have a greater number of geologic

conditions (surficial or at depth) typical of a particular

deposit type, the number of genetically related known

deposits in the area, geochemical anomalies, and the presence of favorable alteration zones. The lower the percentile

estimate, the more speculative the data. Half of the estimated number of deposits at each confidence level (90 percent, 50 percent, 10 percent, 5 percent , 1 percent) is

expected to be larger than the median tonnage for the specific grade and tonnage model (Cox and Singer, 1986;

Singer, 1993).

MARK3 COMPUTER PROGRAM

The estimated number of undiscovered deposits for

each probability (90 percent, 50 percent, 10 percent, 5 percent, 1 percent) are entered into a Monte Carlo simulator

(MARK3) computer program. The MARK3 program

randomly selects values from grade and tonnage frequency

distributions (created from a database of known deposits in

each model) to produce a probability distribution (or

probability curve) of the metal tonnages for the expected

deposits (Drew and others, 1986; Root and Scott, 1988; Root

and others, 1992). The MARK3 selects metal tonnages that

would likely be present together in the undiscovered deposits. The selection process consists of 4,999 iterations using

a random number generator, and the results are sorted to permit reporting of ore and metal tonnages at the percentiles that

were originally estimated (90, 50, 10, 5, and 1).

MARK3 assumes the grade and tonnage distributions

are representative of all deposits with similar geologic, geophysical, and geochemical attributes. Grade and tonnage

variables are made dependent by treating them as individually normal or jointly bivariate normal (Root and Scott,

1988), even though they are actually independent and not

normal. The independent data cannot be used to predict

grades and tonnages of deposits with similar attributes. In

order to make predictions the data are artificially made

dependent by treating the median and mean of the sampling

distribution as equal to the median and mean of the grade and

tonnage values. The mean and the product of the grade and

tonnage values are then set equal to the mean of the metal

content. Thus, the two variables, grade and tonnage, are

essentially brought into a cumulative distribution function

that is jointly bivariate (Drew and others, 1986; Root and

Scott, 1988; Root and others, 1992).

CALCULATION OF THE MEAN AND THE MEDIAN

The mean and median are both measures of central tendency. The mean is the arithmetic average of the total estimated tonnage divided by the 4,999 tonnage estimations that

were performed. The median is the midpoint of the tonnage

distribution. The mean is useful because it is additive, so the

mean amount of gold for all expected deposits is the sum of

gold means from all of the models (Sierran Kuroko, copper

porphyry, and placer gold). The median is not additive, but

it is useful because it reflects the shape of the curve of tonnage frequency distributions, or the ranking in a particular

set of 4,999 iterations. The mean is strongly influenced by a

few large deposits (for example, a world-class deposit) that

are in the data sets, whereas the median values are not. The

median is a more conservative estimation of undiscovered

resources. In our estimations, the mean is a larger number

than the median, because grade and tonnage frequency distributions are asymmetric, with a tail toward larger values. A

reason for this asymmetry is that the lowest values (grades

and tonnages for mineral occurrences) in the frequency distribution are not included because they are small and do not

merit extensive exploration. Deposit models realistically

cannot contain all mineral-occurrence data because of their

subeconomic nature. This grade and tonnage data is rarely

recorded in the literature, but it would fall on the lower part

of the tonnage frequency distribution curves. Because of the

lack of data, each deposit model has a specified minimum
PROBABILISTIC ASSESSMENT OF UNDISCOVERED METALLIC ENDOWMENT USING GRADE–TONNAGE 103

cutoff ore grade and minimum deposit size. Hence, the mean

is a higher value, and the difference in the mean and median

is a measure of the skewness of the curve. It is therefore

important to pay attention to both mean and median because

together they help to understand the projected metal

endowment. 

RESULTS

Although several types of deposits were identified in the

Forest, it was determined that three deposit types are sufficiently understood to compare them with available deposit

models and estimate undiscovered metallic resources. These

models are: Sierran Kuroko (massive sulfide) in Singer

(1992), porphyry copper in Singer and others (1986), and

placer gold in Orris and Bliss (1986). The remaining deposit

types in the Forest lack pertinent data for quantitative assessment. The expected amount of contained metal for each

metallic commodity associated with the deposit type and the

total metric tons of ore are reported in terms of five probability levels (90 percent, 50 percent, 10 percent, 5 percent, 1

percent). The 90th percentile means there is a 90 percent

chance that undiscovered deposits will contain at least the

reported metric tons of metal and total ore. In order to determine the value of minable undiscovered resources, the

results must be subjected to an economic analysis. 

MASSIVE SULFIDE OR SIERRAN KUROKO DEPOSITS

The Sierran Kuroko grade and tonnage curves (Singer,

1992, p. 30–32) were modified for this analysis by using the

lower half of the curve for the MARK3 simulation because

the expected range of grades and tonnages of the undiscovered deposits were comparable to only those grades and tonnages on the lower half of the curve. Eighty percent of the

expected number of deposits in the Forest were considered be

less than 310,000 metric tons, 1.4 percent copper, 1.3 grams

per metric ton gold, and 32 grams per metric ton silver. This

is below the midpoint of the curve, so it is more appropriate

to use the lower half of the curve. The assessment team estimated that there is a 90 percent chance of no undiscovered

deposits, a 50 percent chance of no undiscovered deposits, a

10 percent chance for one deposit, a five percent chance for

one deposit, and a 1 percent chance that there are two deposits. These estimates imply a mean of 0.33 undiscovered

deposits. The resulting assessment (tables 10 and 11)

indicates that there is a 10 percent chance that there are

190,000 total metric tons of undiscovered Sierran Kuroko

(massive sulfide) ore in the Forest. From this there could be

a production of 4,200 metric tons copper, 6,600 metric tons

zinc, 0.14 metric tons of gold, and 8.9 metric tons silver. A

five percent chance exists for 250,000 total metric tons of

undiscovered ore containing 6,100 metric tons of copper,

13,000 metric tons of zinc, 870 metric tons of lead, 0.33 metric tons of gold, and 21 metric tons of silver (table 11). A one

percent chance exists for 360,000 total metric tons of undiscovered ore that would produce 10,000 metric tons of copper,

27,000 metric tons of zinc, 5,000 metric tons of lead, 0.92

metric tons of gold, and 46 metric tons of silver. The median

of the total estimated ore and metal content is zero, and the

mean is 48,000 metric tons of ore containing, in metric tons,

3,200 copper, 1,900 zinc, 180 lead, 0.05 gold, 2.9 silver. The

results of the MARK3 simulation are reported to two significant figures.

PORPHYRY COPPER DEPOSITS

The porphyry copper grade and tonnage curves (Singer

and others, 1986, p. 80–81) were modified for this analysis

(only the lower half of the curves were used for the MARK3

simulation). As was used for the Sierran Kuroko model, it

was also more appropriate to use the lower half of the curve

for the MARK 3 simulation because 80 percent of the

expected number of deposits in the Forest fell below the midpoint of the curve (or less than 140 million metric tons that

contain 56 percent copper). The assessment team estimated

a 90 percent chance that there are no porphyry copper deposits, a 50 percent chance that there are no deposits, a 10 percent chance for one deposit, a 5 percent chance for one

deposit, and a 1 percent chance of 2 deposits. These estimates imply a mean of 0.33 undiscovered deposits. The

results of the simulation (tables 10 and 11) suggest that there

is a 10 percent chance that 79,000,000 total metric tons of ore

occurs within the Forest, from which there could be a production of 380,000 metric tons of copper, 2,000 metric tons of

molybdenum, and 3.2 metric tons of gold. A five percent

chance exists for 110,000,000 total metric tons of undiscovered ore containing 590,000 metric tons of copper, 8,900

metric tons of molybdenum, 19 metric tons of gold, and 120

metric tons of silver. And finally, a one percent chance exists

for 180,000,000 total metric tons of undiscovered ore that

could contain 1,400,000 metric tons of copper, 35,000 metric

PROBABILISTIC ASSESSMENT OF UNDISCOVERED METALLIC ENDOWMENT

Table 10. Mean number of deposits and probability of zero deposits for quantitative estimates. 

Massive Sulfide Porphyry Copper- Placer Gold

Molybdenum

Expected mean number of deposits 0.33 0.33 0.6

Probability of zero deposits 70% 70% 70%
104 MINERAL RESOURCE POTENTIAL AND GEOLOGY, ROUTT NATIONAL FOREST, COLORADO

tons of molybdenum, 57 metric tons of gold, and 410 metric

tons of silver. The median of the estimated total ore and

metal content is zero, and the mean is 20,000,000 metric tons

of ore that could contain, in metric tons, 109,000 copper,

1,600 molybdenum, 2.6 gold, 17 silver. The results of the

MARK3 simulation are reported to two significant figures.

PLACER GOLD DEPOSITS

The placer gold grade and tonnage curves (Yeend,

1986; Orris and Bliss, 1986, p. 263–264) were modified for

this analysis. Only the lower half of the curves were used for

the MARK3 simulation because the expected deposits were

considered small enough that 80 percent of the expected

number of deposits in the Forest were below the midpoint of

the curve (or less than 1.1 million metric tons of ore containing 0.2 grams per metric ton of gold)(tables 10 and 11). The

assessment team estimated a 90 percent chance for no undiscovered placer gold deposits, a 50 percent chance for no

deposits, a 10 percent chance for one deposit, a 5 percent

chance for one deposit, and a 1 percent chance for one

deposit. These estimates imply a mean of 0.6 undiscovered

deposits. Silver is reported, even though silver was not historically produced from placers, because it is present in some

of the placers and is a resource that may be produced. The

results of the simulation (tables 10 and 11) suggest a 10 percent chance that the Forest may have 7,600,000 total metric

tons of ore containing one metric ton of gold and no silver.

A five percent chance exists for 21,000,000 total metric tons

of undiscovered ore containing 3.1 metric tons of gold, and

0.009 metric tons of silver. And finally, a one percent

chance exists for 97,000,000 total metric tons of undiscovered ore containing 13 metric tons of gold and 0.59 metric

tons of silver. The median of the total estimated placer ore

and contained metals is zero, and the mean is 3,900,000

metric tons of ore containing, in metric tons, 0.6 gold and

0.003 silver. The results of the MARK3 simulation are

reported to two significant figures.

Table 11. Quantitative estimates of undiscovered resources of massive sulfide, porphyry copper-molybdenum, and

placer gold deposits in the Routt National Forest and Middle Park Ranger District of the Arapaho National Forest and

vicinity, Colorado.

[Values in metric tons. Percentages in column headings refer to confidence levels for estimates of undiscovered deposits occurring in

the Forest]

Metal 90% 50% 10% 5% 1% mean

 median

Massive Sulfide

Number of deposits 0 0 1 1 2

Total Metric Tons 0 0 190,000 250,000 360,000 48,000

Contained metal for:

Copper 0 0 4,200 6,100 10,000 3,200

Zinc 0 0 6,600 13,000 27,000 1,900

Lead 0 0 0 870 5,000 180

Gold 0 0 0.14 0.33 0.92 0.05

Silver 0 0 8.9 21 46 2.9

Porphyry Copper-Molybdenum

Number of deposits 0 0 1 1 2

Total metric tons 0 0 79,000,000 110,000,000 180,000,000 20,000,000

Contained metal for:

Copper 0 0 380,000 590,000 1,400,000 109,000

Molybdenum 0 0 2,000 8,900 35,000 1,600

Gold 0 0 3.2 19 57 2.6

Silver 0 0 0 120 410 17

Placer Gold

Number of deposits 0 0 1 1 1

Total metric tons 0 0 7,600,000 21,000,000 97,000,000 3,900,000

Contained metal for:

Gold 0 0 1.0 3.1 13.0 0.6

Silver 0 0 0 0.009 0.59 0.003
105

SUMMARY OF ENVIRONMENTAL STUDIES 

By Margo I. Toth, Sandra J. Soulliere, and Steven M. Smith

MINES AND MINERALIZED AREAS

Metals in mineral deposits or mineralized areas are commonly contained in sulfide complexes. When oxygen-rich

waters react with these sulfide minerals, they often result in

highly acidic waters that contain potentially toxic elements

such as zinc, copper, cadmium, and arsenic (Plumlee and others, 1993). The presence of carbonate minerals such as calcite can help reduce the acidity and concentrations of some

but not all of these metals. Naturally occurring sulfide-rich

rocks, underground and open-pit mine workings, and mine

waste dumps and tailings can be major sources for acid

waters and metals in the environment. The acid waters and

toxic metals can adversely affect water quality, aquatic life,

wildlife, and agriculture.

Plumlee and others (1993) have shown that different

mineral deposit types have characteristic environmental signatures that are a function of geology, geochemical processes, climate, and mining method. The geology or mineral

content of a deposit and the geochemical processes that affect

the minerals control the acidity and amount of dissolved metals in waters that drain mineralized areas. The local climate

and mining methods used affect the rate of acid generation

and the extent of dispersion of potentially toxic metal elements. Environmental-geology models, based upon these

factors, can be used to predict the likely pre-mining and postmining environmental signatures associated with a deposit

(Plumlee and others, 1993). Such an appraisal can help identify and prioritize the study of existing hazardous mine sites

and help predict and plan for the environmental effects of any

possible future development.

The major mines and mineralized areas within the Forest

are shown in figure 5. Based on geologic and geochemical

considerations, only the following mining districts have any

significant likelihood of producing high-acid or high-metal

waters: Hahns Peak, Pearl district, Greenville mine, Teller

district, La Plata-Dailey districts, and Poison Ridge. All of

these areas share some or all of the following criteria: extensive alteration, high pyrite content, significant metal concentration, and lack of a carbonate buffer. None of these areas

are currently known to produce high-metal or high-acid

waters, but only one of these areas, the Greenville mine, was

investigated during this study. Studies of waters draining

some of the tailings at the Greenville mine showed nearneutral waters and low conductivity, indicating low dissolved

solids. Additionally, red iron-oxide precipitates have been

reported in streams draining Poison Ridge (Kinney and others, 1968) and the La Plata-Dailey mining districts (R.G.

Eppinger, oral commun., 1994). These precipitates are commonly, but not exclusively, found in streams affected by acid

drainage from mineralized areas. Further study may be warranted in these areas.

The Colorado Water Quality Control Division (1989)

has identified several tributaries of the Yampa River that

contain metal concentrations in water that exceed agricultural, water-supply, or aquatic-life standards. The headwaters of two of these tributaries are in the Forest. Elk River,

from the South Fork to the Yampa River, has lead concentrations in water that exceed basic standards for aquatic life.

The source of this lead was attributed to subsurface mining,

although no specific mines or districts were specified as the

source. Trout Creek originates on the northeast side of Pyramid Peak and contains cadmium, copper, and mercury

above basic aquatic life standards from the Rio Blanco

County line to Foidel Creek; below Foidel Creek to the

Yampa River, concentrations of copper, iron, mercury, and

zinc exceed basic standards for aquatic life, and manganese

exceeds the water-supply standard. The source of these

metals appears to be coal mines just outside the Forest

boundary.

WATER-QUALITY AND ATMOSPHERIC 

STUDIES

Ongoing work by John Turk of the U.S. Geological Survey (Turk and others, 1992, 1993; Turk and Campbell, 1987)

has shown a high acidity in snow in the north-central part of

the Forest in the Mt. Zirkel Wilderness. Although the cause

has not been identified, the increase may be an effect of coalfired power plants to the west of the mountain range in Craig

and Hayden. The U.S. Forest Service and U.S. Geological

Survey have undertaken a study of lichens and mosses to

determine whether the coal plants are the sources of pollutants and associated decreasing visibility within the range

(Rocky Mountain News, 9/1/94, 9/6/94; Jackson and others,

1996). 
106 MINERAL RESOURCE POTENTIAL AND GEOLOGY, ROUTT NATIONAL FOREST, COLORADO

REFERENCES CITED

Allen, M.S., 1982, An evaluation of stream sediment and heavy

mineral concentrate geochemistry as aids in geologic mapping

and mineral exploration in northern Colorado: Kingston,

Ontario, Canada, Queen’s University M.S. thesis, 243 p. (v. 1)

and 222 p. (v. 2).

Bader, J.W., 1983, Section A-A', Subsurface correlations of some

Upper Cretaceous and Tertiary rocks from the Cherokee Ridge,

Wyoming, to the southeast flank of the Sand Wash Basin,

Colorado: U.S. Geological Survey Open-File Report 83-362,

2 sheets.

Barrett, J.K., and Pearl, R.H., 1978, An appraisal of Colorado’s

geothermal resources: Colorado Geological Survey Bulletin

39, 224 p.

Barton, H.N., 1985a, Geochemical maps showing the distribution

and abundance of selected elements in heavy-mineral concentrates derived from stream sediments and ridgetop soils from

the upper Keyser Creek basin in the St. Louis Peak Roadless

Area, Grand County, Colorado: U.S. Geological Survey Miscellaneous Field Studies Map MF-1588-H, scale 1:50,000.

———1985b, Geochemical maps showing the distribution and

abundance of selected elements in heavy-mineral concentrates

from ridgetop soils from the Williams Fork Roadless Area,

Clear Creek, Grand, and Summit Counties, Colorado: U.S.

Geological Survey Miscellaneous Field Studies Map MF1588-I, scale 1:50,000.

———1985c, Geochemical maps showing the distribution and

abundance of selected elements in heavy-mineral concentrates

of stream sediments from 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 MF1588-G, scale 1:50,000.

Bass, N.W., Eby, J.B. and Campbell, M.R., 1955, Geology and mineral fuels of parts of Routt and Moffat Counties, Colorado:

U.S. Geological Survey Bulletin 1027-D, p. 143–250.

Beaumont, E.A., 1979, Depositional environments of Fort Union

sediments (Tertiary, northwest Colorado) and their relation to

coal: American Association of Petroleum Geologists Bulletin,

v. 63, p. 194–217.

Behrendt, J.C., Popenoe, P., and Mattick, R.E., 1969, A geophysical

study of North Park–Middle Park Basin, Colorado: Geological Society of America Bulletin, v. 80, p. 1523–1538.

Berg, R.R., 1960, Cambrian and Ordovician history of Colorado, in

Weimer, R.J., and Haun, J.D., eds., The Geology of Colorado:

Denver, Geological Society of America, Rocky Mountain

Association of Geologists, and Colorado Scientific Society,

p. 10–16.

Beroni, E.P., and Derzay, R.C., 1955, The uranium deposits of the

Fish Creek district, Colorado, in Guidebook to the Geology of

Northwest Colorado: Salt Lake City, Utah, Intermountain

Association of Petroleum Geologists, 6th Annual Field Conference, Rocky Mountain Association of Geologists, Annual Field

Conference, 1955, p. 123.

Beroni, E.P., and McKeown, F.A., 1952, Reconnaissance for uraniferous rocks in northwestern Colorado, southwestern

Wyoming, and northeastern Utah: U.S. Geological Survey

Trace Elements Investigations Report TEI-308-A, p. 41.

Blackstone, D.L., 1977, Independence Mountain thrust fault, North

Park Basin, Colorado, in University of Wyoming Contributions

to Geology, v. 16, no. 1, p. 1–16.

Blakely, R.J., and Simpson, R.W., 1986, Approximating edges of

source bodies from magnetic or gravity anomalies: Geophysics, v. 51, no. 7, p. 1494–1498.

Bliss, J.D., ed., 1992, Developments in mineral deposit modeling:

U.S. Geological Survey Bulletin 2004, 168 p.

Bolivar, S.L., Broxton, D.E., and Olsen, C.E., 1978, Uranium

hydrogeochemical and stream sediment reconnaissance of the

Denver and Greeley NTMS quadrangles, Colorado: Los

Alamos, NM, Los Alamos Scientific Laboratory informal

report LA-7177-MS, Grand Junction, CO, U.S. Department of

Energy, GJBX-60(78), 138 p.

Bolivar, S.L., Hill, D.E., Sandoval, W., Trujillo, L., Bunker, M.E.,

Waterbury, G.R., Talcott, C.L., Minor, M.M. and Cheadle, J.,

III, 1979, Uranium hydrogeochemical and stream sediment

reconnaissance of the Craig NTMS quadrangle, Colorado,

including concentrations of forty-three additional elements:

Los Alamos, NM, Los Alamos Scientific Laboratory informal

report LA-7506-MS, Grand Junction, CO, U.S. Department of

Energy, GJBX-76(79), 238 p.

Boreck, D.L., Tremain, C.M., Sitowitz, L., and Lorenson, T.D.,

1981, The coal bed methane potential of the Sand Wash Basin,

Green River coal region, Colorado: Colorado Geological Survey Open-File Report 81-6, 24 p.

Braddock, W.A., and Cole, J.C., 1978, Preliminary geologic map of

the Greeley 1°×2° quadrangle, Colorado and Wyoming: U.S.

Geological Survey Open-File Report 78-532, scale 1:250,000.

———1990, Geologic map of Rocky Mountain National Park and

vicinity, Colorado: U.S. Geological Survey Miscellaneous

Investigation Series Map I-1973.

Brady, B.T., 1975, Map showing fluorspar deposits in Colorado:

U.S. Geological Survey Mineral Investigations Resource Map

MR-70, 10 p.

Brinkworth, G.L., 1973, A geophysical investigation of the Colorado Front Range mineral belt: Boulder, University of Colorado, Ph.D. dissertation, 251 p.

Brobst, D.A., and Pratt, W.P., 1973, Introduction, in Brobst, D.A.,

and Pratt, W.A., eds., United States Mineral Resources: U.S.

Geological Survey Professional Paper 820, p. 1–8.

Bryant, B., McGrew, L.W., and Wobus, R.A., 1981, Geologic map

of Denver 1°×2° quadrangle, north-central Colorado: U.S.

Geological Survey Miscellaneous Investigations Map I-1163

(2 sheets), scale 1:250,000. 

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.

Casaceli, R.J., 1984, The geology and mineral potential of the

Hahns Peak intrusive porphyry, Routt County, Colorado: Corvallis, Oregon State University, unpublished M.S. thesis,

223 p.

Case, J.E., 1965, Gravitational evidence for a batholithic mass of

low density along a segment of the Colorado Mineral Belt:

Geological Society of America Special Paper 82, p. 26.

———1967, Geophysical ore guides along the Colorado Mineral

Belt: U.S. Geological Survey Open-File Report 67-039, 13 p.
REFERENCES CITED 107

Christopherson, K.R., 1979, A geophysical study of the Steamboat

Springs, Colorado, geothermal systems: Boulder, University of

Colorado, M.S. thesis, 107 p.

Cobban, W.A., Merewether, E.A., Fouch, T.D., and Obradovich,

J.D., 1994, Some Cretaceous shorelines in the Western Interior

of the United States, in Caputo, M.V., Peterson, J.A., and

Franczyk, K.J., eds., Mesozoic Systems of the Rocky Mountain

Region, USA: Rocky Mountain Section, Society of Economic

Paleontologists and Mineralogists (Society for Sedimentary

Geology), p. 393–414.

Colorado Oil and Gas Conservation Commission, 1992, Oil and

Gas Statistics for the State of Colorado: Colorado Oil and Gas

Conservation Commission.

Colorado Water Quality Control Division, 1989, Colorado nonpoint

assessment report, p. 166–173.

Cordell, Lindrith, and Grauch, V.J.S., 1985, Mapping basement

magnetization zones from aeromagnetic data in the San Juan

Basin, New Mexico, in Hintze, W.J., ed., The Utility of

Regional Gravity and Magnetic Anomaly Maps: Society of

Exploration Geophysics, p. 181–197.

Cox, D.P., Barton, P.B., and Singer, D.A., 1986, Mineral deposit

models, in Cox, D.P., and Singer, D.A., eds., 1986, Mineral

Deposit Models: U.S. Geological Survey Bulletin 1693, p.

1–10.

Cox, D.P., and Singer, D.A., eds., 1986, Mineral deposit models:

U.S. Geological Survey Bulletin 1693, 379 p.

Craig, L.C., Hail, W.J., Jr., Luft, S.J., Boudette, E.L., and Snyder,

G.L., 1982, National Uranium Resource Evaluation, Craig

quadrangle, Colorado: U.S. Department of Energy, Grand

Junction Office Report PGJ/F-017(82), 106 p., 13 maps, scale

1:500,000.

Crawford, R.D., 1924, A contribution to the igneous geology of

central Colorado: American Journal of Science, 5th ser., v. 7,

p. 365–388.

Daniels, J.J., 1987, Geophysical appraisal of mineral resources of

the Mount Zirkel Wilderness and northern Park Range vicinity,

Jackson and Routt Counties, Colorado: U.S. Geological Survey

Bulletin 1554-D, p. 217–221.

Desborough, G.A., 1970, Silver depletion indicated by microanalysis of gold from placer occurrences, Western United States:

Economic Geology, v. 65, no. 3, p. 304–311.

De Voto, R.H., 1990, Paleozoic stratigraphy, tectonism, thermal history, and basin evolution of central Colorado, in, Beaty, D.W.,

Landis, G.P., and Thompson, T.B., eds., Carbonate-Hosted Sulfide Deposits of the Central Colorado Mineral Belt: Economic

Geology Monograph 7, p. 29–44.

Dickerson, R.P., and McDonnell, J.R., Jr., 1989, Mineral resources

of the Platte River Adjacent Wilderness Study Area, Jackson

County, Colorado: U.S. Geological Survey Open-File Report

89-154, 12 p.

Drew, L.J., Bliss, J.D., Bowen, R.W., Bridges, N.J., Cox, D.P., De

Young, J.C., Jr., Houghon, J.C., Ludington, Steve, Menzie,

W.D., Page, J.J., Root, D.H., and Singer, D.A., 1986, Quantitative estimation of undiscovered mineral resources: A case

study of U.S. Forest Service wilderness tracts in the Pacific

mountain system: Economic Geology, v. 81, p. 80–88.

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, 3 maps, scale

1:2,500,000, 7 p. text.

Erdmann, C.E., 1941, Preliminary report on the geology of the

Coalmont District, Jackson County, Colorado: U.S. Geological

Survey Open-File Report 705, 207 p.

Erslev, E.A., and Rogers, J.L., 1993, Basement-cover geometry of

Laramide fault-propogation folds: Geological Society of

America Special Paper 280, p. 125–146.

Fenneman, N.M., and Gale, H.S., 1906, The Yampa coal field, Routt

County, Colorado: U.S. Geological Survey Bulletin 297, 96 p.

Fillmore, B.J., 1986, Stratigraphy and source-rock potential of the

Mowry Shale (Lower Cretaceous), North Park, Colorado: Colorado School of Mines, M.S. thesis, 144 p.. 

Fortescue, J.A.C., 1992, Landscape geochemistry: Retrospect and

prospect 1990: Applied Geochemistry, v. 7, p. 1–53.

Franklin, J.M., Sangster, D.M., and Lydon, J.W., 1981, Volcanicassociated massive sulfide deposits: Economic Geology, 75th

Anniversay Volume, p. 485–627.

Gale, H.S., 1906, The Hahns Peak gold field, Colorado: U.S. Geological Survey Bulletin 285-H, p. 28–34.

Geometrics, 1978, Aerial gamma ray and magnetic survey, Rock

Springs, Rawlins, and Cheyenne quadrangles, Wyoming, and

Greeley quadrangle, Colorado: U.S. Department of Energy,

Grand Junction Office Report GJBX-17(78), 8 vols.

———1979a, Aerial gamma ray and magnetic survey, Denver and

Pueblo quadrangles, Colorado: U.S. Department of Energy,

Grand Junction Office Report GJBX-49(79), 8 vols. 

———1979b, Aerial gamma ray and magnetic survey, Uncompahgre uplift project, Salina, Utah, Moab, Utah and Colorado,

Montrose and Leadville, Colorado, quadrangles: U.S Department of Energy, Grand Junction Office Report GJBX-95(79), 8

vols.

Giles, D.L., 1976, Precambrian mineralization in the southern

Rocky Mountain region, in Tectonics and Mineral Resources of

Southwestern North America: New Mexico Geological Society

Special Publication no. 6, p. 127–131.

———1987, Proterozoic framework and metallogeny: Southern

Rocky Mountain Province of Colorado and New Mexico [abs]:

Geological Society of America Abstracts with Programs, p.

676.

Gourdarzi, G.H., compiler, 1984, Guide to preparation of mineral

survey reports on public lands: U.S. Geological Survey OpenFile Report 84-0787, p. 7–8.

Grauch, V.J.S., and Cordell, Lindrith, 1987, Limitations of

determining density or magnetic boundaries from the horizontal gradient of gravity or pseudogravity data: Geophysics, v.

52, no. 1, p. 118–121.

Hail, W.J., Jr., 1965, Geology of northwestern North Park,

Colorado: U.S. Geological Survey Bulletin 1188, 133 p.

———1968, Geology of southwestern North Park, Colorado: U.S.

Geological Survey Bulletin 1257, 119 p.

Hamilton, D.S., 1993, Stratigraphy and coal occurrence of the

Upper Cretaceous Mesaverde Group, San Wash Basin, in

Kaiser, W.R., Scott, A.R., Hamilton, D.S., Tyler, R., McMurry,

R.G., Zhou, N., and Tremain, C.M., eds., Geologic and Hydrologic Controls on Coal-Bed Methane—Sand Wash Basin,

Colorado and Wyoming: Gas Research Institute Topical Report

GRI-92/0420, p. 79–106.

Hannah, J.L., and Stein, H.J., 1986, Oxygen isotope compositions

of selected Laramide-Tertiary granitoid stocks in the Colorado 
108 MINERAL RESOURCE POTENTIAL AND GEOLOGY, ROUTT NATIONAL FOREST, COLORADO

Mineral Belt and their bearing on the origin of Climax-type

granite-molybdenum systems: Contributions to Mineralogy

and Petrology, v. 93, no. 3, p. 347–358.

Hedge, C.E., Houston, R.S., Tweto, Ogden., Peterman, Z.E.,

Harrison, J.E., and Reid, R.R., 1986, The Precambrian of the

Rocky Mountain region, in, Harrison, J.E., and Peterman, Z.E.,

eds., Correlation of Precambrian Rocks of the United States

and Mexico: U.S. Geological Survey Professional Paper 1241-

D, 17 p.

Hildenbrand, T.G., 1983, FFTFIL—A filtering program based on

two-dimensional Fourier analysis on geophysical data: U.S.

Geological Survey Open-File Report 83-237, 61 p.

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

the central Rocky Mountains, North America: Contributions to

Geology, v. 17, p. 89–109

Hoffman, J.D., and Marsh, S.P., 1994, National geochemical data

base [abs.], in Carter, L.M.H., Toth, M.I., and Day, W.C., eds.,

USGS Research on Mineral Resources—1994, Part A, Programs and Abstracts: Ninth V.E. McKelvey Forum on Mineral

and Energy Resources, U.S. Geological Survey Circular 1103-

A, p. 47–48.

Honey, J.G., and Hettinger, R.D., 1989, Cross section showing correlations of Upper Cretaceous Fox Hills Sandstone and Lance

Formation, and lower Tertiary Fort Union and Wasatch Formations, southeastern Washakie Basin, Wyoming, and eastern

Sand Wash Basin, Colorado: U.S. Geological Survey Miscellaneous Investigations Series Map I-1964.

Irwin, C.D., 1986, Upper Cretaceous and Tertiary cross sections,

Moffat County, Colorado, in Stone, D.S., and Johnson, K.S.,

eds., New Interpretations of Northwest Colorado Geology:

Rocky Mountain Association of Geologists, p. 151–156.

Isaacson, L.B., and Smithson, S.B., 1976, Gravity anomalies and

granite emplacement in west-central Colorado: Geological

Society of America Bulletin, v. 87, p. 22–28.

Izett, G.A., 1966, Tertiary extrusive volcanic rocks in Middle Park,

Grand County, Colorado: U.S. Geological Survey Professional Paper 550-B, p. 42–46.

———1968, Geology of the Hot Sulphur Springs quadrangle,

Grand County, Colorado: U.S. Geological Survey Professional

Paper 586, 79 p.

———1975, Late Cenozoic sedimentation and deformation in

northern Colorado and adjoining areas, in Curtis, B.F., ed.,

Cenozoic History of the Southern Rocky Mountains: Geological Society of America Memoir 144, p. 179–209.

Izett, G.A., and Barclay, C.S.V., 1973, Geologic map of the Kremmling quadrangle, Grand County, Colorado: U.S. Geological

Survey Geologic Quadrangle Map GQ-115.

Jackson, L.L., Geiser, L., Blett, T., Gries, C., and Haddow, D.,

Biogeochemistry of lichens and mosses in and near Mt. Zirkel

Wilderness, Routt National Forest, Colorado: U.S. Geological

Survey Open-File Report 96-295, 162 p.

Johnson, R.A., Karlstrom, K.E., Smithson, S.B., and Houston, R.S.,

1984, Gravity profiles across the Cheyenne belt, a Precambrian

crustal suture in southeastern Wyoming: Journal of Geodynamics, v.1, no. 3–5, p. 445–472.

Johnson, S.Y., Otton, J.K., and Macke, D.L., 1987, Geology of the

Holocene surficial uranium deposit of the north fork of

Flodelle Creek, northeastern Washington: Geological Society

of America Bulletin, v. 98, p. 77–85.

Kaiser, W.R., 1993, Abnormal pressure in coal basins of the western

United States, in Proceedings of the 1993 International Coalbed Methane Symposium, p. 173–186.

Kaiser, W.R., Scott, D.S., Hamilton, D.S., Tyler, R., McMurry,

R.G., and Zhou, N., 1993, Geologic and hydrologic controls on

coalbed methane: Sand Wash Basin, Colorado and Wyoming:

GRI Topical Report GRI-92/0420, 151 p.

Karimpour, M.H., 1982, Petrology, geochemistry, and genesis of

the A.O. porphyry copper complex in Jackson and Grand

Counties, northwestern Colorado: Boulder, University of

Colorado, M.S. thesis, 251 p.

Karimpour, M.H., and Atkinson, W.W., Jr., 1983, Petrogenesis of

the A.O. porphyry copper complex in Jackson and Grand

Counties, northwestern Colorado: Global Tectonics and

Metallogeny, v. 2, p. 15–27.

Keystone Coal Industry Manual, 1993: Chicago, Ill., MacLeanHunter Publishing Company, 1004 p.

Khalsa, N.S., and Ladwig, L.R., 1981, Colorado Coal Analyses

1976–1979: Colorado Geological Survey Information Series

10, 364 p.

Kinney, D.M., 1970, Preliminary geologic map of the Rand 15'

quadrangle, North and Middle Parks, Jackson and Grand

Counties, Colorado: U.S. Geological Survey Open-File Report

70-183, 1 sheet.

Kinney, D.M., and Hail, W.J., 1970, Preliminary geologic map of

the Hyannis Peak quadrangle, North and Middle Parks, Jackson and Grand Counties, Colorado: U.S. Geological Survey

Open-File Report 70-184, 1 sheet.

Kinney, D.M., Izett, G.A., King, R.U., and Taylor, R.B., 1968, The

Poison Ridge volcanic center and related mineralization,

Grand and Jackson Counties, Colorado: U.S. Geological

Survey Circular 594, 8 p.

Klipfel, P.D., 1992, Geology and metallogeny of the southern

portion of the Encampment district, Wyoming: Golden,

Colorado School of Mines, Ph.D. thesis, T-4080, 244 p.

LKB Resources, Inc., 1979, NURE aerial gamma-ray and magnetic

reconnaissance survey, Colorado-Arizona area, Craig NK 13-

10 quadrangle: U.S. Department of Energy, Grand Junction

Office Report GJBX-153(79), 2 vols., scale 1:500,000.

Lakes, Arthur, 1909, The Hahns Peak mining region, Routt County,

Colorado: The Mountain States Mineral Age, July, 1927, p.

10–14.

Landis, E.R., 1959, Coal resources of Colorado, in Contributions to

Economic Geology: U.S. Geological Survey Bulletin 1072-C,

p. 131–232.

Law, B.E., 1984, Relationships of source-rock, thermal maturity,

and overpressuring to gas generation and occurrence in lowpermeability Upper Cretaceous and lower Tertiary rocks, greater Green River Basin, Wyoming, Colorado, and Utah, in Woodward, J., Meissner, F.F., and Clayton, J.L., eds., Hydrocarbon

Source Rocks of the Greater Rocky Mountain Region: Rocky

Mountain Association of Geologists, p. 469–490.

Law, B.E., and Spencer, C.W., 1993, Gas in tight reservoirs—An

emerging major source of energy, in Howell, D.G. ed., The

Future of Energy Gases: U.S. Geological Survey Professional

Paper 1570, p. 233–252.

Lillegraven, J.A., and Ostresh, L.M., Jr., 1990, Late Cretaceous

(earliest Campanian/Maastrichtian) evolution of western

shorelines of the North American Western Interior Seaway in

relation to known mammalian faunas, in Bown, T.M., and
REFERENCES CITED 109

Rose, K.D., eds., Dawn of the Age of Mammals in the Northern

Part of the Rocky Mountain Interior: Geological Society of

America Special Paper 243, p. 1–30.

Lovering, T.S., 1935, Geology and ore deposits of the Montezuma

quadrangle, Colorado: U.S. Geological Survey Professional

Paper 178, 119 p.

Lowell, J.D., and Guilbert, J.M., 1970, Lateral and vertical

alteration-mineralization zoning in porphyry ore deposits:

Economic Geology, v. 65, p. 373–408.

Malan, R.C., 1957, Geology of uranium occurrences in North and

Middle Parks, Colorado, in Guidebook to the Geology of North

and Middle Parks Basin, Colorado: Rocky Mountain Association of Geologists, 11th Annual Field Conference: p. 126–136.

Mallory, W.W., 1971, The Eagle Valley evaporite, northwest Colorado—A regional synthesis: U.S. Geological Survey Bulletin

1311-E, 30 p.

Mallory, W.W., Post, E.V., Ruane, P.J., Lehmbeck, W.L., and

Stotelmeyer, R.B., 1966, Mineral resources of the Flat Tops

Primitive Area, Colorado: U.S. Geological Survey Bulletin

1230-C, 30 p.

Maughan, E.K., 1988, Geology and petroleum potential, Colorado

Park Basin province, north-central Colorado: U.S. Geological

Survey Open-File Report 88-450 E, 35 p.

McCallum, M.E., and Orback, C.J., 1968, The New Rambler

copper-gold-platinum district, Albany and Carbon Counties,

Wyoming: Geological Survey of Wyoming Preliminary Report

no. 8, 12 p.

McCord, J.P., 1984, Geologic overview, coal, and coalbed methane

resources of the Greater Green River coal region—Wyoming

and Colorado, in Rightmire, C.T., Eddy, G.E., and Kirr, J.N.,

eds., Coal-Bed Methane Resources of the United States: American Association of Petroleum Geologists Studies in Geology

Series no. 17, p. 271–293.

Menzie, W.D., and D.A. Singer, 1990, A course on mineral resource

assessment: Proceedings of the International symposium on

Mineral Exploration—The case of artificial intelligence: Tokyo

and Tsukuba, Japan, Mining and Materials Processing Institute

of Japan, p. 177–188.

Metzger, C.W., 1974, Geology, mineralization, and geochemistry of

the upper Illinois River drainage basin, Grand and Jackson

Counties, Colorado: Golden, Colorado School of Mines, M.S.

thesis, 92 p.

Mooney, W.D., 1991, Continental crustal evolution during the

Archean and Proterozoic; evidence from crustal seismology:

Geophysical Journal International, v. 104, no. 3, p. 669.

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.

Motooka, J.M., Curtis, C.A., McDougal, C.H., Pearson, R.C., and

McCallum, M.E., 1979, Geochemical analyses of rocks and

stream sediments of the Rawah Wilderness and nearby Wilderness Study Areas, Jackson and Larimer Counties, Colorado:

U.S. Geological Survey Open-File Report 79-1502, 79 p.

Murray, D.K., 1980, 1979 Summary of coal resources in Colorado:

Colorado Geological Survey Special Publication 13, 24 p.

Naeser, C.W., Izett, G.A., and White, W.H., 1973, Zircon

fission-track ages from some middle Tertiary igneous rocks

in northwestern Colorado [abs.]: Geological Society of

America Abstract with Programs, p. 498.

Nash, J.T., Granger, H.C., and Adams, S.S., 1981, Geology and concepts of genesis of important types of uranium deposits, in

Skinner, B.J., ed., Economic Geology, 75th Anniversary

Volume: El Paso, Texas, Economic Geology Publishing

Company, p. 63–116.

Neubert, J.T., 1994, Mineral appraisal of Routt National Forest,

Colorado with a section on Industrial minerals by J.S. Dersch,

U.S. Forest Service: U.S. Bureau of Mines Open-File Report

MLA 13-94, 103 p., 2 plates.

Nuccio, V.F., and Schenk, C.J., 1986, Thermal maturity and hydrocarbon source-rock potential of the Eagle Basin, northwestern

Colorado, in Stone, D.S., ed., New Interpretations of Northwest

Colorado Geology: Rocky Mountain Association of Geologists, p. 259–264.

Orris, G.J., and Bliss, J.D., 1986, Grade and tonnage model of

placer Au-PGE, in Cox, D.P., and Singer, D.A., eds., Mineral

Deposit Models: U.S. Geological Survey Bulletin 1693, p.

261–264.

———1991, Grade and tonnage models for industrial mineral

deposits: U.S. Geological Survey Open File Report 91-11B.

70 p.

Owen, D.E., Otton, J.K., Hills, F.A., and Schumann, R.R., 1992,

Uranium and other elements in Colorado Rocky Mountain wetlands–A reconnaissance study: U.S. Geological Survey Bulletin 1992, 33 p.

Parker, B.H., Jr., 1961, The geology of the gold placers of Colorado:

Golden, Colorado School of Mines, Ph.D. dissertation, 578 p.

———1974, Gold placers of Colorado: Quarterly of the Colorado

School of Mines, v. 69, no. 4, book 2, 224 p. 

Parker, R.L., 1967, Composition of the Earth’s crust, chap. D, in

Fleischer, Michael, ed., Data of Geochemistry, Sixth Edition:

U.S. Geological Survey Professional Paper 440-D, p. D1–D19.

Patten, L.L., 1987, Mine 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-B,

p. 58–107.

Pearl, R.H., 1979, Colorado’s hydrothermal resource base—An

assessment: Colorado Geological Survey Resource Series 6,

144 p.

Pearl, R.H., Zacharakis, T.G., and Ringrose, C.D., 1983, Geothermal resource assessment of the Steamboat–Routt Hot Springs

area, Colorado: Colorado Geological Survey Resource Series

22, 86 p.

Pearson, R.C., and U.S. Bureau of Mines, 1980, Mineral resources

of the Indian Peaks Study Area, Boulder and Grand Counties,

Colorado, with a section on Interpretation of aeromagnetic data

by Gordon Johnson: U.S. Geological Survey Bulletin 1463,

109 p.

Pearson, R.C., Braddock, W.A., Flanigan, V.J. and Patten, L.L.,

1981, Mineral resources of the Comanche–Big South,

Neota–Flat Top, and Never Summer Wilderness Study Areas,

north-central Colorado: U.S. Geological Survey Open-File

Report 81-578, 73 p.

Pearson, R.C., and Johnson, G., 1980, Mineral resources of the

Indian Peaks Study Area, Boulder and Grand Counties,

Colorado: U.S. Geological Survey Bulletin 1463, 109 p.
110 MINERAL RESOURCE POTENTIAL AND GEOLOGY, ROUTT NATIONAL FOREST, COLORADO

Pearson, R.C., McCallum, M.E., Griswold, M.L., and Patten, L.L.,

1982, Mineral resources of the Rawah Wilderness, Larimer

County, Colorado, with a section on Interpretation of aeromagnetic data by V.J. Flanigan: U.S. Geological Survey Open-File

Report 82-376, 27 p.

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, 1 CD-ROM.

Planner, H.N., Apel, C.T., Fuka, M.A., George, W.E., Hansel, J.M.,

Hensley, W.K., and Pirtle, June, 1981, Uranium hydrogeochemical and stream sediment reconnaissance data release

for the Leadville NTMS quadrangle, Colorado, including concentrations of forty-two additional elements: Los Alamos,

NM, Los Alamos Scientific Laboratory informal report LA8013-MS, Grand Junction, CO, U.S. Department of Energy,

GJBX-13(81), 185 p.

Plouff, Donald, 1961, Gravity profile along the Roberts tunnel,

Colorado: U.S. Geological Survey Professional Paper 424-C,

p. C263–C265.

Plumlee, G.S., Smith, S.M., Toth, M.I., and Marsh, S.P., 1993,

Integrated mineral resource and mineral environmental assessments of public lands: Applications for land management and

resource planning: U.S. Geological Survey Open-File Report

93-571, 18 p.

Pratt, W.P., and Zietz, Isidore, 1973, Geologic interpretation of Colorado aeromagnetic map, in U.S. Geological Survey Research

1973: U.S. Geological Survey Professional Paper 850, p. 50.

Prodehl, Claus, and Lipman, P.W., 1989, Crustal structure of the

Rocky Mountain region, in Pakiser, L.C., and Mooney, W.D.,

eds., Geophysical Framework of the Continental United States:

Geological Society of America Memoir 172, p. 249–284.

Reed, J.C., Bryant, Bruce, and Sims, P.K., 1993, Geology and

mineral resources of central Colorado: Colorado School of

Mines Quarterly Review, v. 93, no. 1, p. 7–16.

Rice, D.D., Law, B.E., and Clayton, J.L., 1993, Coal-bed gas—An

undeveloped resource, in Howell, D.G., ed., The Future of

Energy Gases: U.S. Geological Survey Professional Paper

1570, p. 389–404.

Rocky Mountain News, November 28, 1993, Evidence of ancient

miners abounds on ridge near Steamboat, p. 33A.

———September 1, 1994, Visibility study launched, p. 24A.

———September 6, 1994, Scientists study lichens for signs of

pollution, p. 70A.

Roehler, H.W., and Hansen, D.E., 1989, Surface and subsurface

correlations showing depositional environments of the Upper

Cretaceous Mesaverde Group and associated formations, Cow

Creek in southwest Wyoming to Mt. Harris in northwest

Colorado: U.S. Geological Survey Miscellaneous Field Studies

Map MF-2077.

Root, D.H., and Scott, W.A., 1988, User manual for mineral simulation program: U.S. Geological Survey Open File Report 88-

15, 64 p.

Root, D.H., Menzie, W.D., and Scott, W.A., 1992, Computer Monte

Carlo simulation in quantitative resource estimation: Journal of

Nonrenewable Resources, p. 125–138.

Sangster, D.F., 1972, Precambrian volcanogenic massive sulphide

deposits in Canada: A review: Geological Survey of Canada

Paper 72-22, 44 p.

Schmidt, P.W., Lovering, T.G., and Kleunder, S.E., 1984, Geologic

and mineral resource potential map of the Service Creek

Roadless area, Routt County, Colorado: U.S. Geological

Survey Miscellaneous Field Studies Map MF-1639, scale

1:48,000.

Schwochow, S.D., 1981, Inventory of non-metallic mining and

processing operations in Colorado: Colorado Geological

Survey Map Series 17, scale 1:250,000, 39 p.

Scott, A.R., 1993, Coal rank, gas content, and composition and

origin of coalbed gases, Fort Union Formation, Sand Wash

Basin, in Kaiser, W.R., Scott, A.R., Hamilton, D.S., Tyler, R.,

McMurry, R.G., Zhou, N., and Tremain, C.M., eds., Geologic

and Hydrologic Controls on Coal-Bed Methane—Sand Wash

Basin, Colorado and Wyoming: Gas Research Institute Topical

Report GRI-92/0420, p. 107–113.

Segerstrom, Kenneth, and Young, E.J., 1972, General geology of

the Hahns Peak and Farwell Mountain quadrangles, Routt

County, Colorado, with a discussion of Triassic and Jurassic

rocks, by G.N. Pipiringos: U.S. Geological Survey Bulletin

1349, 63 p.

Shannon, S.S., Jr., Gallimore, D.L., Martell, C.J., Hensley, W.K.,

Jackson, C.K., and Macdonell, C.J., 1981, Detailed uranium

geochemical stream sediment survey data release for the

selected portions of the Craig NTMS quadrangle, Colorado,

and Rawlins NTMS quadrangle, Wyoming, including concentrations of forty-five additional elements: Los Alamos, NM,

Los Alamos Scientific Laboratory informal report LA-8481-

MS, Grand Junction, CO, U.S. Department of Energy, GJBX216(81), 132 p.

Sharps, S.L., 1962, Geology of the Pagoda quadrangle, northwestern Colorado: Boulder, Colo., University of Colorado, Ph.D.

thesis, 282 p.

Shawe, D.R., 1981, U.S. Geological Survey workshop on non-fuel

mineral resource appraisal of Wilderness and CUSMAP areas:

U.S. Geological Survey Circular 845, 18 p.

Sheridan, D.M., Raymond, W.H., Taylor, R.B., and Hasler, J.W.,

1990, Metallogenic map of stratabound exhalative and related

occurrences in Colorado: U.S. Geological Survey Miscellaneous Investigation Series Map I-1971, scale 1:1,000,000.

Shettel, D.L., Jr., Langfeldt, S.L., Youngquist, C.A., D’Andrea,

R.F., Jr., Zinkl, R.J., Garcia, S.R., Hanks, D., George, W.E. and

Bolivar, S.L., 1981, Uranium hydrogeochemical and stream

sediment reconnaissance data release for the Denver NTMS

quadrangle, Colorado: U.S. Department of Energy, Grand

Junction, CO, GJBX-263(81), 153 p.

Siepman, B.R., 1986, Facies relationships in Campanian wavedominated coastal deposits in Sand Wash Basin, in Stone, D.S.,

and Johnson, K.S., eds., New Interpretations of Northwest

Colorado Geology: Rocky Mountain Association of

Geologists, p. 157–164.

Singer, D.A., 1992, Grade and tonnage model of Sierran Kuroko

deposits, U.S. Geological Survey Bulletin 2004, p. 29–32.

———1993, Basic concepts in three-part quantitative assessments

of undiscovered resources: Nonrenewable Resources v. 2, no.

2, p. 69–81.

Singer, D.A., and Cox, D.P., 1987, Applications of mineral deposit

models to mineral resource assessments: U.S. Geological

Survey Yearbook, Fiscal Year 1987, p. 55–57.
REFERENCES CITED 111

Singer, D.A., and Mosier, D.L., 1986, Applications of mineral

deposit models to resource assessments: U.S. Geological

Survey Yearbook, Fiscal Year 1987 p. 55–57. 

Singer, D.A., Mosier, D.L., and Cox, D.P., 1986, Grade and tonnage

model of Porphyry Cu, in Cox, D.P., and Singer, D.A. eds.,

Mineral Deposit Models, U.S. Geological Survey Bulletin,

1693, p. 77–81.

Singer, D.A., and Ovenshine, A.T., 1979, Assessing mineral

resources in Alaska: American Scientist, v. 67, p. 582–589.

Snyder, G.L., 1980a, Generalized map showing Precambrian rocks

and metamorphic facies, keyed to aluminosilicate minerals in

meta-shales with excess alumina, northern Park Range,

Colorado: U.S. Geological Survey Open-File Report 80-787.

———1980b, Geologic map of the central part of the northern Park

Range, Jackson, and Routt Counties, Colorado: U.S. Geological Survey Miscellaneous Investigations Series Map I-1112,

scale 1:48,000.

———1980c, Geologic map of the northernmost Gore Range and

the southernmost northern Park Range, Grand, Jackson, and

Routt Counties, Colorado: U.S. Geological Survey Miscellaneous Investigations Series Map I-1114.

———1980d, Geologic map of the northernmost Park Range and

southernmost Sierra Madre, Jackson and Routt Counties, Colorado: U.S. Geological Survey Miscellaneous Investigations

Series Map I-1113.

———1987a, Evaluation of surface metallic and nonmetallic

resource potential, 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-C,

p. 109–213.

———1987b, Geologic 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-A, p. 13–55.

Snyder, G.L., Hopkins, R.T., Jr., Domenico, J.A., Frisken, J.G., and

Mitchell, J., 1981, Analyses of rock and stream sediment

samples from the Mount Zirkel Wilderness and northern Park

Range vicinity, Jackson and Routt Counties, Colorado: U.S.

Geological Survey Open-File Report 81-186, 85 p.

Spicker, F.A., 1973, The geology, alteration, and mineralization of

Parkview Mountain, Grand and Jackson Counties, Colorado:

Fort Collins, Colorado State University, M.S. thesis, 96 p.

Steven, T.A., 1960, Geology and fluorspar deposits, Northgate

district, Colorado: U.S. Geological Survey Bulletin 1082-F,

422 p.

Sweeney, R.E., 1990, IGRFGRID—A program for creation of a

total magnetic field (International Geomagnetic Reference

Field) grid representing the Earth’s main magnetic field: U.S.

Geological Survey Open-File Report 90-45A-B.

Taylor, R.B., and Steven, T.A., 1983, Definition of mineral resource

potential: Economic Geology, v. 78, no. 6, p. 1268–1270.

Taylor, R.B., Stoneman, R.J., and Marsh, S.P., 1984, An assessment

of the mineral resource potential of the San Isabel National Forest, south-central Colorado, with a section on salable minerals

by J.S. Dersch: U.S. Geological Survey Bulletin B-1638, 42 p.

Theobald, P.K., Bielski, A.M., Eppinger, R.G., Moss, C.K.,

Kreidler, T.J., and Barton, H.N., 1985, Mineral resource potential map 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-A, scale

1:50,000.

Tremain, C.M., 1990, Coalbed methane development in Colorado:

Colorado Geological Survey, Information Series 32, 35 p.

Trexler, P.K., 1978, Uranium hydrogeochemical and stream sediment reconnaissance of the Cheyenne NTMS quadrangle,

Wyoming: Los Alamos, NM, Los Alamos Scientific Laboratory informal report LA-7237-MS, Grand Junction, CO, U.S.

Department of Energy, GJBX-106(78), 67 p.

Turk, J.T., and Campbell, D.H., 1987, Estimates of acidification of

lakes in the Mt. Zirkel Wilderness Area, Colorado: Water

Resources Research, v. 23, p. 1757–1761.

Turk, J.T., Campbell, D.H., Ingersoll, G.P., and Clow, D.A., 1992,

Initial findings of synoptic snowpack sampling in the Colorado

Rocky Mountains: U.S. Geological Survey Open-File Report

92-645, 6 p.

Turk, J.T., Campbell, D.H., and Spahr, N.E., 1993, Use of chemistry

and stable sulfur isotopes to determine sources of trends in

sulfate of Colorado lakes: Water, Air and Soil Pollution, v. 67,

p. 415–431.

Turner-Peterson, C.E., and Fishman, N.S., 1986, Geologic synthesis and genetic models for uranium mineralization, Grants

uranium region, New Mexico, in, Turner-Peterson, C.E., and

Santos, E.S., eds., A Basin Analysis Case Study—The

Morrison Formation, Grants Uranium Region, New Mexico:

American Association of Petroleum Geologists Studies in

Geology, no. 22.

Tweto, Ogden, 1968, Geologic setting and interrelationships of

mineral deposits in the mountain province of Colorado and

south-central Wyoming, in Ridge, J.D., ed., Ore Deposits of the

United States 1933–1967: New York, New York, American

Institute of Mining, Metallurgical, and Petroleum Engineers, v.

1, p. 551–558.

———1976, Geologic map of the Craig 1°× 2° quadrangle, northwestern Colorado: U.S. Geological Survey Miscellaneous

Investigations Map I-972, scale 1:250,000.

———1979, The Rio Grande rift system in Colorado, in Riecker,

R.E., ed., Rio Grand Rift: Tectonics and Magmatism:

Washington D.C., American Geophysical Union, p. 33–56.

———1980, Summary of Laramide orogeny in Colorado, in Kent,

H.C., and Porter, K.W., Colorado Geology: Rocky Mountain

Association of Geologists, p. 129–134. 

———1987, Rock units of the Precambrain basement in Colorado:

U.S. Geological Survey Professional Paper 1321-A, 54 p.

Tweto, Ogden, Bryant, Bruce, and Williams, F.E., 1970, Mineral

resources of the Gore Range–Eagles Nest Primitive Area and

vicinity, Summit and Eagle Counties, Colorado: U.S. Geological Survey Bulletin 1319-C, 127 p. 

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,

31 p.

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

Colorado Mineral Belt: Geological Society of America Bulletin, v. 74, p. 991–1014.

Tweto, Ogden, Moench, R.H., and Reed, J.C., Jr., 1978, Geologic

map of the Leadville 1°×2° quadrangle, northwestern
112 MINERAL RESOURCE POTENTIAL AND GEOLOGY, ROUTT NATIONAL FOREST, COLORADO

Colorado: U.S. Geological Survey Miscellaneous Investigations Series Map I-999, scale 1:250,000.

Tyler, R., and McMurry, R.C., 1993, Stratigraphy and coal occurrence of the Paleocene Fort Union Formation, Sand Wash

Basin, in Kaiser, W.R., Scott, A.R., Hamilton, D.S., Tyler, R.,

McMurry, R.G., Zhou, N., and Tremain, C.M., eds., Geologic

and Hydrologic Controls on Coal-Bed Methane—Sand Wash

Basin, Colorado and Wyoming: Gas Research Institute Topical

Report GRI-92/0420, p. 79–106.

Tyler, R., and Tremain, C.M. 1993, Tectonic evolution , stratigraphic setting, and coal fracture patterns of the Sand Wash Basin, in

Kaiser, W.R., Scott, A.R., Hamilton, D.S., Tyler, R., McMurry,

R.G., Zhou, N., and Tremain, C.M., eds., Geologic and Hydrologic Controls on Coal-Bed Methane—Sand Wash Basin,

Colorado and Wyoming: Gas Research Institute Topical

Report GRI-92/0420, p. 3–19.

U.S. Bureau of Mines, 1993, Mineral Commodity Summaries,

202 p.

U.S. Geological Survey, 1968, Aeromagnetic map of the WolcottBoulder area, north-central Colorado: U.S. Geological Survey

Open-File Report 68-295, scale 1:125,000.

———1978, Aeromagnetic map of North Park and vicinity,

Colorado: U.S. Geological Survey Open-File Report 78-112,

scale 1:125,000.

U.S. Geological Survey and Colorado Geological Survey, 1977,

Energy resources of Colorado: U.S. Geological Survey

Miscellaneous Investigations Series Map I-1039.

Vanderwilt, J.W., 1947, Mineral resources of Colorado: Denver,

State of Colorado Mineral Resources Board, 547 p.

Waechter, N.B., and Johnson, W.E., 1986, Pennsylvanian-Permian

paleostructure and stratigraphy as interpreted from seismic

data in the Piceance Basin, northwest Colorado: Rocky

Mountain Association of Geologists, p. 51–64.

Wallace, A.R., 1990, Regional geologic and tectonic setting of the

central Colorado Mineral Belt, in, Beaty, D.W., Landis, G.P.,

and Thompson, T.B., eds., Carbonate-Hosted Sulfide Deposits

of the Central Colorado Mineral Belt: Economic Geology

Monograph 7, p. 19–28.

Wallace A.R., and Karlson, R.C., 1985, The Schwartzwalder uranium deposit, I: Geology and structural controls on mineralization: Economic Geology, v. 80, p. 1842–1857.

Warner, L.A., 1978, The Colorado lineament: A middle Precambrian wrench fault system: Geological Society of America

Bulletin, v. 89, p. 161–171.

———1980, The Colorado lineament: Colorado Geology, 1980

Symposium of the Rocky Mountain Association of Geologists,

p. 11–21.

Weaver, T.A., Morris, W.A., and Trexler, P.K., 1978, Uranium

hydrogeochemical and stream sediment reconnaissance of the

Rawlins NTMS quadrangle, Wyoming: Grand Junction, CO,

U.S. Department of Energy, GJBX-81(78), 73 p.

Webring, M.W., 1981, MINC—A gridding program based on

minimum curvature: U.S. Geological Survey Open-File

Report 81-1224, 41 p.

White, W.H., Bookstrom, A.A., Kamilli, R.J., Ganster, M.W.,

Smith, R.P., Ranta, D.E., and Steininger, R.C., 1981, Character and origin of Climax-type molybdenum deposits, in

Skinner, B.J., ed., Economic Geology 75th Anniversary

Volume: El Paso, Texas, Economic Geology Publishing

Company, p. 270–316.

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.

Yeend, W.E., 1986, Descriptive model of placer Au-PGE, in Cox,

D.P., and Singer, D.A., eds., 1986, Mineral Deposit Models:

U.S. Geological Survey Bulletin 1693, p. 261.

Young, E.J., and Segerstrom, Kenneth, 1973, A disseminated silverlead-zinc sulfide occurrence at Hahns Peak, Routt County,

Colorado: U.S. Geological Survey Bulletin 1367, 33 p.

Youngquist, C.A., D’Andrea, R.F., Jr., Zinkl, R.J., Shettel, D.L., Jr.,

Langfeldt, S.L., Hensley, W.K., Thomas, G.J., Martell, C.J. and

Maassen, L.W., 1981, Uranium hydrogeochemical and stream

sediment reconnaissance of the Greeley NTMS quadrangle,

Colorado: Grand Junction, CO, U.S. Department of Energy,

GJBX-265(81).

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, p. 1703–1714.

Published in the Central Region, Denver, Colorado

Manuscript approved for publication January 29, 1999

Edited by Richard W. Scott, Jr.

Graphics prepared by authors and Gayle M. Dumonceaux

Photocomposition by Gayle M. Dumonceaux
APPENDIXES
114 MINERAL RESOURCE POTENTIAL AND GEOLOGY, ROUTT NATIONAL FOREST, COLORADO

EON ERA PERIOD EPOCH

AGE ESTIMATES

OF BOUNDARIES

(Ma*)

CENOZOIC

MESOZOIC

PALEOZOIC

LATE PROTEROZOIC

MIDDLE PROTEROZOIC

EARLY PROTEROZOIC

LATE ARCHEAN

MIDDLE ARCHEAN

EARLY ARCHEAN

PROTEROZOIC

ARCHEAN

PRE-ARCHEAN

PHANEROZOIC

QUATERNARY

TERTIARY

CRETACEOUS

JURASSIC

TRIASSIC

PERMIAN

Neogene

subperiod

Paleogene

subperiod

CARBONIFEROUS

PERIODS

PENNSYLVANIAN

MISSISSIPPIAN

DEVONIAN

SILURIAN

ORDOVICIAN

CAMBRIAN

Holocene

Pleistocene

Pliocene

Miocene

Oligocene

Eocene

Paleocene

Late

Early

Late

Middle

Early

Late

Middle

Early

Late

Middle

Early

Late

Middle

Early

Late

Early

Late

Middle

Early

Late

Middle

Early

Late

Middle

Early

Late

Early

3,800?

0.010

1.7

5

24

38

55

66

96

138

205

240

290

330

360

410

435

500

570

900

1,600

2,500

3,000

3,400

4,550

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

Rocks older than 5 m.y. also called Precambrian, a time term without specific rank.

Informal time term without specific rank.

APPENDIX 1—GEOLOGIC TIME CHART

Terms and boundary ages used by the U.S. Geological Survey in this report
115

UA H/B H/C H/D

M/B M/C M/D

L/B L/C L/D

LEVEL OF RESOURCE POTENTIAL

N/D

LEVEL OF CERTAINITY 

A B C D

LEVELS OF CERTAINITY

Unknown

potential

High potential

Moderate potential Moderate potential Moderate potential

High potential High potential

Low potential Low potential

Low potential

No potential

DEFINITIONS OF MINERAL RESOURCE 

POTENTIAL

LOW mineral resource potential is assigned to areas where

geologic, geochemical and geophysical characteristics

define a geologic environment in which the existence

of resources is unlikely. This broad category embraces

areas with dispersed but insignificantly mineralized

rock as well as areas with few or no indications of

having been mineralized.

MODERATE mineral resource potential is assigned to areas

where geologic, geochemical, and geophysical characteristics indicate a geologic environment favorable for

resource occurrence, where interpretations of data

indicate a reasonable likelihood of resource accumulation, and (or) where an application of mineral-deposit

models indicates favorable ground for the specified

type(s) of deposits.

HIGH mineral resource potential is assigned to areas where

geologic, geochemical, and geophysical characteristics

indicate a geologic environment favorable for resource

occurrence, where interpretations of data indicate a

high degree of likelihood for resource accumulation,

where data support mineral-deposit models indicating

presence of resources, and where evidence indicates

that mineral concentration has taken place. Assignment of high resource potential to an area requires

some positive knowledge that mineral-forming

processes have been active in at least part of the area.

UNKNOWN mineral resource potential is assigned to areas

where information is inadequate to assign low, moderate, or high levels of resource potential.

NO mineral resource potential is a category reserved for a

specific type of resource in a well-defined area.

A. Available information is not adequate for determination of the level of mineral resource potential.

B. Available information suggests the level of mineral

resource potential.

C. Available information gives a good indication of the

level of mineral resource potential.

D. Available information clearly defines the level of

mineral resource potential.

Abstracted with minor modifications from:

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

survey reports on public lands: U.S. Geological Survey

Open-File Report 84-787, p. 7–8.

Taylor, R.B., and Steven, T.A., 1983, Definition of mineral

resource potential: Economic Geology, v. 78, no. 6, p.

1268–1270.

Taylor, R.B., Stoneman, R.J., and Marsh, S.P., 1984, An assessment of the mineral resource potential of the San Isabel

National Forest, sout-central Colorado, with a section on

salable minerals by J.S. Dersch: U.S. Geological Survey

Bulletin 1638, p. 40–42.

APPENDIX 2—DEFINITIONS OF LEVELS OF MINERAL RESOURCE POTENTIAL 

AND CERTAINTY OF ASSESSMENT

APPENDIX 2
116 MINERAL RESOURCE POTENTIAL AND GEOLOGY, ROUTT NATIONAL FOREST, COLORADO

CUMULATIVE

PRODUCTION

ECONOMIC

MARGINALLY

ECONOMIC

SUBECONOMIC

RESERVES

MARGINAL

RESERVES

INFERRED

RESERVES

INFERRED

MARGINAL

RESERVES

INFERRED

SUBECONOMIC

RESOURCES

SUBECONOMIC

RESOURCES

OTHER

OCCURRENCES INCLUDES NONCONVENTIONAL MATERIALS

DEMONSTRATED

MEASURED INDICATED

INFERRED

PROBABILITY RANGE

HYPOTHETICAL SPECULATIVE (or)

IDENTIFIED RESOURCES UNDISCOVERED RESOURCES

RESOURCES OF COAL

AREA: (mine, district, field, state, etc.) UNITS: (short tons)

CUMULATIVE

PRODUCTION

ECONOMIC

MARGINALLY

ECONOMIC

SUBECONOMIC

OTHER

OCCURRENCES

DEMONSTRATED

MEASURED INDICATED

INFERRED

PROBABILITY RANGE

HYPOTHETICAL SPECULATIVE

IDENTIFIED RESOURCES UNDISCOVERED RESOURCES

RESOURCES OF COAL

AREA: (mine, district, field, state, etc.) UNITS: (short tons)

A portion of reserves or any resource category may be restricted from extraction by laws or regulations.

A portion of reserves or any resource category may be restricted from extraction by laws or regulations.

(or)

INCLUDES NONCONVENTIONAL MATERIALS

RESERVE BASE

INFERRED RESERVE BASE

SUBECONOMIC

RESOURCES

INFERRED

SUBECONOMIC

RESOURCES

APPENDIX 3—COAL RESOURCE CLASSIFICATION
117

APPENDIX 4—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 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 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 circumstances could be considered to have economic potential (Cox and others,

1986).

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

APPENDIX 5—UNITS OF MEASUREMENT

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

percent per metric ton. Thus the estimated amounts of metallic resources within an undiscovered deposit are reported in metric tons of metal. Other units of measurement are

noted as they are reported. Conversion factors useful for this report include the following:

1 troy ounce 31.1 grams

1 short ton 0.9072 metric ton

1 troy ounce per short ton 34.285 parts per million

1 part per million (ppm) 1 gram per metric ton

1 percent (%) 10,000 ppm

1 metric ton 32,154 troy ounces

APPENDIX 5