Geology and Ore Deposits of the Uncompahgre (Ouray) 

Mining District, Southwestern Colorado 

Professional Paper 1753 

U.S. Department of the Interior 

U.S. Geological Survey 
On the cover: View looking south along the Uncompahgre River valley, one of the 

few passageways into and through the mountainous terrain of the western San Juan 

Mountains region of southwestern Colorado. In the foreground is a broad, gently 

south-sloping bench along this part of the river valley; the bench surface is about 200 

to 300 feet above the ranch lands beyond and to the right in the flat-bottomed, 

glacier-carved valley. The town of Ouray (about 7,800 feet altitude), just visible 

through the narrow, cliff-edged, V-shaped gap in the valley, is situated in a picturesque mountain park at the confluence of the Uncompahgre River and Canyon Creek 

valleys and is almost surrounded by mountains towering to as much as 13,500 feet. 

Because of its setting and scenic splendor, the town and environs are aptly referred 

to as the “Switzerland of America.” The Uncompahgre River can be traced southward through the narrow, deep gorge and continues to the left of Abrams Mountain 

(12,801 feet) in the distant center. U.S. Highway 550, known locally as the “Million 

Dollar Highway” south of Ouray, parallels the Uncompahgre River valley to connect 

Ouray with Montrose to the north and extends through the gorge and other canyons 

and mountain passes to connect Ouray with Silverton and Durango to the south. The 

many mines and prospects that constitute the Uncompahgre mining district are 

located mostly within the cliffy slopes surrounding the town of Ouray. From Atwood, 

W.W., and Mather, K.F., 1932, U.S. Geological Survey Professional Paper 166, plate 

34B; photograph attributed to Whitman Cross, August 1904.
Geology and Ore Deposits of the 

Uncompahgre (Ouray) Mining 

District, Southwestern Colorado

By Wilbur S. Burbank and Robert G. Luedke

Professional Paper 1753

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

DIRK KEMPTHORNE, Secretary

U.S. Geological Survey

Mark D. Myers, Director

U.S. Geological Survey, Reston, Virginia: 2008

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Although this report is in the public domain, permission must be secured from the individual copyright owners to 

reproduce any copyrighted materials contained within this report.

Suggested citation:

Burbank, W.S., and Luedke, R.G., 2008, Geology and ore deposits of the Uncompahgre (Ouray) mining district, southwestern Colorado: U.S. Geological Survey Professional Paper 1753, 107 p., 7 pls. in pocket.

ISBN 978–1–4113–2085–7

Library of Congress Cataloging-in-Publication Data

Burbank, W. S. (Wilbur Swett), 1898-1975.

Geology and ore deposits of the Uncompahgre (Ouray) mining district, southwestern Colorado / by Wilbur S. Burbank 

and Robert G. Luedke.

p. cm. -- (Professional paper ; 1753)

Includes bibliographical references.

ISBN 978-1-4113-2085-7 (alk. paper)

1. Geology--Colorado--Ouray County Region. 2. Geology, Stratigraphic. 3. Ore deposits--Colorado--Ouray County 

Region. 4. Orogeny--Colorado--Ouray County Region. I. Luedke, Robert G. II. Title. 

QE92.O83B87 2008

553.409788’22--dc22

2008010055
iii

Preface

Field studies in the Uncompahgre mining district were started by Wilbur S. Burbank in 1928 

and a preliminary, incomplete manuscript prepared by the mid-1930s. Because the mining 

district was part of a larger regional study of the western San Juan Mountains in southwestern 

Colorado, field studies progressed without hesitation into the adjacent areas. This manuscript 

was postponed by commitments to studies on the adjacent areas, strategic mineral studies 

during and after World War II, and various administrative duties. Consequently, only a few short 

reports pertinent to the district were released during this time. I joined Wilbur in 1956, and 

we continued the studies of the adjacent areas, although we did publish the geologic map of 

the Ouray 7.5-min quadrangle. Shortly before his death in 1975, Wilbur asked me to help him 

complete the mining district report. Unfortunately, I had other commitments in addition to completing studies of the adjacent areas for the next few years. But finally, I added a few chapters 

and edited the entire manuscript, which necessitated some modification and (or) elimination 

of material as being incomplete or out-of-date. However, Wilbur’s observations and interpretations have been retained as much as possible and are presented here to show the breadth of his 

understanding of the problems at the time of his study and the first, original writing. The basis 

of this report, dominantly Wilbur’s work, essentially now represents our joint efforts.

Robert G. Luedke

U.S. Geological Survey

Reston, VA 20192

January 2008

v

Contents

Abstract...........................................................................................................................................................1

Introduction.....................................................................................................................................................2

History and Production .................................................................................................................................5

Stratigraphy ..................................................................................................................................................10

Proterozoic Metamorphic Rocks .....................................................................................................10

Uncompahgre Formation..........................................................................................................10

Paleozoic Sedimentary Rocks ..........................................................................................................11

Elbert Formation.........................................................................................................................11

Ouray Limestone ........................................................................................................................14

Leadville Limestone...................................................................................................................14

Molas Formation ........................................................................................................................15

Hermosa Formation ...................................................................................................................16

Cutler Formation.........................................................................................................................16

Mesozoic Sedimentary Rocks ..........................................................................................................17

Dolores Formation......................................................................................................................18

Entrada Sandstone ....................................................................................................................18

Wanakah Formation ..................................................................................................................18

Pony Express Limestone Member .................................................................................19

Bilk Creek Sandstone Member.......................................................................................19

Unnamed Upper Member................................................................................................19

Morrison Formation...................................................................................................................19

Tidwell Member.................................................................................................................20

Salt Wash Member...........................................................................................................21

Brushy Basin Member.....................................................................................................21

Dakota Sandstone .....................................................................................................................21

Mancos Shale.............................................................................................................................23

Tertiary Rocks......................................................................................................................................23

Telluride Conglomerate.............................................................................................................23

San Juan Formation ..................................................................................................................23

Quaternary Deposits...........................................................................................................................26

Intrusive Rocks.............................................................................................................................................27

Diabase.................................................................................................................................................27

Porphyritic Granodiorite ....................................................................................................................28

Altered Porphyritic Rocks .................................................................................................................32

Clastic Dikes ........................................................................................................................................33

Porphyritic Andesite and Quartz Latite ...........................................................................................34

Structure........................................................................................................................................................36

Proterozoic Structure.........................................................................................................................36

Paleozoic Structure............................................................................................................................39

Southern Block...........................................................................................................................40

Northern Block...........................................................................................................................41

Mesozoic Structure............................................................................................................................43
vi

Folding and Tilting......................................................................................................................44

Faulting and Fracturing.............................................................................................................44

Structure of Intrusive Bodies...................................................................................................46

Dikes and Sills ...................................................................................................................46

Stocks and Plugs ..............................................................................................................47

Laccoliths...........................................................................................................................49

Ore Deposits .................................................................................................................................................50

Distribution and General Relations..................................................................................................50

Late Cretaceous to Early Tertiary Ore Deposits.............................................................................51

Structural Classification and Control .....................................................................................54

Northern Sector................................................................................................................56

Western Sector.................................................................................................................58

Southern Sector................................................................................................................59

Eastern Sector...................................................................................................................61

Late Tertiary Ore Deposits.................................................................................................................61

Rock Alteration.............................................................................................................................................62

Alteration of Sedimentary Rocks .....................................................................................................63

Alteration of Igneous Rocks..............................................................................................................64

Wall-Rock Alteration of High-Temperature Veins .........................................................................65

Wall-Rock Alteration of Other Ore Deposits ..................................................................................66

Source of Altering Fluids ...................................................................................................................67

Rock Alteration Compared With Late-Stage Igneous-Rock Crystallization.....................68

Rock Alteration Compared With Clastic Dike Formation ....................................................70

General Conclusions ..........................................................................................................................70

Paragenesis of Primary Ores.....................................................................................................................71

Normal Mineral Sequence................................................................................................................71

Hydrothermal Leaching......................................................................................................................72

Open-Space Filling..............................................................................................................................74

Sulfide Paragenesis............................................................................................................................75

Carbonate Paragenesis .....................................................................................................................77

Descriptions of Mines.................................................................................................................................77

Mineral Farm Mine .............................................................................................................................77

Location, History, and Development.......................................................................................77

Production...................................................................................................................................79

Geology........................................................................................................................................80

Mineralogy and Origin of the Orebodies................................................................................82

Source of Mineralizing Solutions and Zoning Along the Ore Channel in Depth .............83

American Nettie Group of Mines .....................................................................................................83

Location, History, and Development.......................................................................................83

Geology........................................................................................................................................86

Structure......................................................................................................................................86

Form and Origin of the Orebodies ...........................................................................................88

Extension of the Ore Channel at Depth ..................................................................................91

Bachelor Group of Mines ..................................................................................................................92

Location, History, and Development.......................................................................................92
vii

Production and Grade ...............................................................................................................94

Geology ........................................................................................................................................94

Sequence of Vein Formation ....................................................................................................96

Localization and Form of Ore Shoots ......................................................................................97

Calliope Mine .......................................................................................................................................98

Location, History, and Development .......................................................................................98

Geology ........................................................................................................................................98

Localization and Form of Ore Shoots ......................................................................................99

Mineralogy of the Vein ..............................................................................................................99

Newsboy Mine ..................................................................................................................................100

Location, History, and Development .....................................................................................100

Production .................................................................................................................................100

Geology ......................................................................................................................................101

Mineralogy ................................................................................................................................101

Señorita Mine ....................................................................................................................................101

Location, History, and Development .....................................................................................101

Production .................................................................................................................................102

Geology ......................................................................................................................................102

Mineralogy ................................................................................................................................102

References Cited........................................................................................................................................103

Plates

[in pocket]

 1. Geologic sections in Uncompahgre mining district 

 2. Plan maps and sections of American Nettie group of mines

 3. Plan and structure contour maps of American Nettie group of mines

 4. Plan map and sections of Bachelor group of mines

 5. Maps and sections of Calliope mine

 6. Plan maps and sections of Newsboy mine

 7. Plan map and sections of Señorita mine

Figures

 1. Index map of southwestern Colorado. ......................................................................................3

 2. Photograph showing environs of town of Ouray .....................................................................4

 3. Graph of metal-production values ...........................................................................................10

 4. Generalized geologic map of area ...........................................................................................12

 5–6. Photographs showing—

 5. Angular unconformity on Proterozoic rocks .................................................................13

 6. Angular unconformity in The Amphitheater ..................................................................17
viii

7. Sketch showing gypsum in Pony Express Limestone Member of Wanakah 

Formation .....................................................................................................................................20

8. Lithologic profiles of Dakota Sandstone.................................................................................22

9. Photograph of sawn slab of San Juan Formation.................................................................24

10. Total alkali-silica variation diagram.........................................................................................26

11. Photograph of The Amphitheater.............................................................................................27

12. Outcrop distribution map of intrusive rocks in area .............................................................30

13. Ternary plot of intrusive-rock analyses ..................................................................................32

14. Generalized cross section of Dexter Creek laccolith...........................................................32

15. Photographs of sawn slabs of intrusive clastic dikes..........................................................35

16. Section showing probable direction of Bachelor clastic dike injection ...........................36

17. Photograph looking along Calliope dike in Dexter Creek valley.........................................37

18. Generalized structural features map of area.........................................................................38

19. Cross sections in southern part of district .............................................................................40

20. Photograph of west wall of Uncompahgre canyon ..............................................................43

21. Location map of mines, prospects, mills, and hot spring in the district ............................52

22. Section showing structural control of ore deposition in American NettieWanakah flexure.........................................................................................................................56

23. Sections showing ore deposition in veins south of Dexter Creek......................................58

24. Plan and section of the Rock of Ages mine............................................................................60

25. Photographs of sawn slabs of mineralized rock in Dexter Creek area .............................67

26. Diagram showing paragenetic relations of minerals in porphyritic granodiorite ...........69

27. Camera lucida drawing of polished section...........................................................................76

28. Plan and section of Mineral Farm mine and orebody...........................................................78

29. Section through Mineral Farm mine ore channel .................................................................80

30. Plan of Mineral Farm mine ore channel .................................................................................81

31. Sketch of American Nettie mine on canyon wall..................................................................83

32. Generalized section of cliff face at American Nettie mine .................................................84

33. Diagrammatic sketch of splitting of Calliope vein.................................................................98

Tables

1. Ouray County production.............................................................................................................6

2. Uncompahgre district production..............................................................................................9

3. Chemical analyses of carbonate rocks...................................................................................13

4. Partial chemical analyses of carbonate rocks......................................................................14

5. Chemical analyses of volcanic rocks from the San Juan Formation.................................25

6. Analyses of hot-spring deposits and water...........................................................................28

7. Chemical analyses of granodiorite..........................................................................................31

8. Calculated mineralogical analyses of granodiorite..............................................................32

9. Chemical analyses of altered porphyritic rocks....................................................................33

10. Chemical and spectrographic analyses of Tertiary dikes....................................................37

11. Late Cretaceous to early Tertiary principal ore and gangue minerals ..............................53

12. Late Tertiary principal ore and gangue minerals ..................................................................62

13. Carbonate mineral analyses.....................................................................................................77
ix

14. Early production values at Mineral Farm mine......................................................................79

15. Early production values at American Nettie mine ................................................................84

Conversion Factors

Multiply By To obtain

Length

inch (in.) 2.54 centimeter (cm)

inch (in.) 25.4 millimeter (mm)

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

Area

square foot (ft2) 0.09290 square meter (m2)

square mile (mi2) 2.590 square kilometer (km2) 

Mass

pound, avoirdupois (lb) 0.4536 kilogram (kg) 

ton, short (2,000 lb) 0.9072 metric ton (t) 

Temperature in degrees Celsius (°C) may be converted to degrees Fahrenheit (°F) as follows:

°F=(1.8×°C)+32

Temperature in degrees Fahrenheit (°F) may be converted to degrees Celsius (°C) as follows:

°C=(°F-32)/1.8

Geology and Ore Deposits of the Uncompahgre (Ouray) 

Mining District, Southwestern Colorado

By Wilbur S. Burbank1 and Robert G. Luedke

Abstract

The Uncompahgre mining district, part of the Ouray 

mining district, includes an area of about 15 square miles 

(mi2

) on the northwestern flank of the San Juan Mountains 

in southwestern Colorado from which ores of gold, silver, 

copper, lead, and zinc have had a gross value of $14 to 15 

million.

Bedrock within the district ranges in age from Proterozoic to Cenozoic. The oldest or basement rocks, the 

Uncompahgre Formation of Proterozoic age, consist of 

metamorphic quartzite and slate and are exposed in a small 

erosional window in the southern part of the district. Overlying those rocks with a profound angular unconformity are 

Paleozoic marine sedimentary rocks consisting mostly of limestones and dolomites and some shale and sandstone that are 

assigned to the Elbert Formation and Ouray Limestone, both 

of Devonian age, and the Leadville Limestone of Mississippian age. These units are, in turn, overlain by rocks of marine 

transitional to continental origin that are assigned to the Molas 

and Hermosa Formations of Pennsylvanian age and the Cutler 

Formation of Permian age; these three formations are composed predominantly of conglomerates, sandstones, and shales 

that contain interbedded fossiliferous limestones within the 

lower two-thirds of the sequence.

The overlying Mesozoic strata rest also on a pronounced 

angular unconformity upon the Paleozoic section. This thick 

Mesozoic section, of which much of the upper part was 

eroded before the region was covered by rocks of Tertiary age, 

consists of the Dolores Formation of Triassic age, the Entrada 

Sandstone, Wanakah Formation, and Morrison Formation all 

of Jurassic age, and the Dakota Sandstone and Mancos Shale 

of Cretaceous age. These strata dominantly consist of shales, 

mudstones, and sandstones and minor limestones, breccias, 

and conglomerates.

In early Tertiary time the region was beveled by erosion 

and then covered by a thick deposit of volcanic rocks of midTertiary age. These volcanic rocks, assigned to the San Juan 

Formation, are chiefly tuff breccias of intermediate composition, which were deposited as extensive volcaniclastic aprons 

around volcanic centers to the east and south of the area.

The Ouray area, in general, exhibits the typical effects 

of a minimum of three major uplifts of the ancestral San Juan 

Mountains. The earliest of these uplifts, with accompanying 

deformation and erosion, occurred within the Proterozoic, and 

the other two occurred at the close, respectively, of the Paleozoic and Mesozoic. The last event, known as the Laramide 

orogeny, locally was accompanied by extensive intrusion of 

igneous rocks of dominantly intermediate composition. Domal 

uplifts of the ancestral mountains resulted in peripheral monoclinal folds, plunging anticlines radial to the central core of the 

mountain mass, faults, and minor folds.

The principal ore deposits of the Uncompahgre district 

were associated with crosscutting and laccolithic intrusions 

of porphyritic granodiorite formed during the Laramide (Late 

Cretaceous to early Tertiary) orogeny. The ores were deposited 

chiefly in the Paleozoic and Mesozoic sedimentary strata having an aggregate thickness of about 4,500 feet (ft) and occur 

beneath the early Tertiary unconformity, which in places truncated some of the uppermost deposits. A few ore deposits of 

late Tertiary age occur also in the sedimentary rocks near the 

southern margin of the district, but are restricted mostly to the 

overlying volcanic rocks. Ore deposits in the Uncompahgre 

district range from low-grade, contact-metamorphic through 

pyritic base-metal bodies containing silver and gold tellurides 

and native gold to silver-bearing lead-zinc deposits, and are 

zoned about the center of intrusive activity, a stock in an area 

referred to as The Blowout.

Ore deposition within the Uncompahgre district was 

largely controlled by structural trends and axes of uplift established mainly in the late Paleozoic phase of deformation, but 

also in part by structural lines established in Proterozoic time. 

Movement on these older structural lineaments was renewed 

and accentuated by deformation during the Laramide orogeny at the time of the igneous intrusions. There are two main 

structural axes in the district: (1) a north-northwest-trending 

axis of uplift, called the Uncompahgre axis, which extends 

outward from the margin of the mountains, and (2) an intrusive axis of northeastward trend which roughly parallels that 

of the Paleozoic and Mesozoic mountain-front monoclinal 

uplifts, respectively, within and north of the district. The two 

axes intersect near the center of eruptive activity and divide 

the district into four structural sectors, north, west, south, 

and east, within which conditions of mineralization varied in 1

Deceased.
accordance to their relation to the structural axes and to the 

intrusive center.

Sources of the ore-forming fluids lie along the northeasttrending intrusive zone and appear to be related genetically to 

the igneous rocks. The fluids evidently rose from depth along 

the line of the intrusive zone toward the Uncompahgre axis of 

uplift, and then moved laterally outward into the sedimentary 

formations in paths that generally conformed to the dip of the 

strata. Along the Uncompahgre axis the fluids gained access 

to shallow tensional breaks that permitted their escape more 

directly upward toward the surface. Most of the channels of 

mineralization along bedding in the sedimentary rocks and in 

fractures trend eastward or somewhat diagonal to the strike 

and appear to have been controlled, in general, by the predominant directions of fracturing or flexing of the rocks.

Contact-metamorphic deposits and most of the pyritic 

gold-bearing deposits lie in the central parts of the district, 

within a mile or less of the crossing of the axes and of the 

main crosscutting eruptive vent. The silver-lead-zinc deposits, which are from 1 to 4 mi from the crossing of the axes, 

were found chiefly in the northern and southern sectors of the 

district, inasmuch as the bulk of the fluids that formed this 

type of ore followed the most direct east-west paths from the 

high-pressure zones along the intrusive axis to the low-pressure zones along the Uncompahgre axis of uplift. The eastern 

and western sectors of the district do not contain appreciably 

large deposits of silver-lead-zinc ore. In addition to the above, 

the ore deposits also include both tabular bodies and shoots of 

less regular shape that lie more or less conformably within the 

bedding and fractures. Different forms of deposits are closely 

associated in many places.

Ore deposition was controlled by nearness to the intrusive contacts, nearness to zones of shallow tensional fractures, 

prevailing directions of faults, folds, and fractures in the 

sedimentary rocks, and the permeability and other physical or 

chemical properties of the rocks.

The largest and highest grade deposits, most of which 

are in the northern sector of the district, were either mostly 

developed or nearly exhausted at the time of our studies; these 

higher grade deposits occurred in a zone near the Uncompahgre axis of uplift. The Uncompahgre River valley, which follows this axial line, also exposed the mineralized rocks to the 

best advantage for mining operations. The northern and southern sectors of the district are the most likely to be the sites of 

any discoveries of new ore channels and to disclose any longer 

extensions of known ore channels. Future exploration consequently should be directed toward the probable sources of the 

mineralizing fluids and away from the Uncompahgre axis into 

zones in which higher temperature mineralization would have 

prevailed and lower grade deposits were formed.

The ore deposits of late Tertiary age are chiefly silverlead-zinc veins, principally in the volcanic rocks. Their related 

centers of mineralization are to the south of the Uncompahgre district. Sedimentary rocks along the south margin of the 

district, however, do contain a few ore deposits of late Tertiary 

age, but in general, the younger veins are absent in the northern parts of the district.

Introduction

The Uncompahgre district is a part of the more extensive 

Ouray mining district and is located at the northwestern edge 

of the rugged San Juan Mountains, a dominantly volcanic 

mountain mass occupying much of southwestern Colorado. 

The district covers an area of about 15 mi2 in the south-central 

part of Ouray County (fig. 1) that includes the valley of the 

Uncompahgre River at and north of the town of Ouray, located 

in the southern part of the district. The region enclosing the 

district, near the northwest escarpment of the mountains bordering the adjacent Colorado Plateau country to the north and 

west, is characterized by high peaks, precipitous slopes, and 

narrow, deeply incised valleys.

Ouray, the county seat and commercial center for the 

Uncompahgre and nearby mining districts to the south, is 

nestled in a mountain park at the junction of the Uncompahgre 

River and Canyon Creek at an altitude of 7,800 ft (fig. 2). At 

the southwest edge of town, both streams above their confluence are entrenched in deep, vertical-walled canyons; that in 

Canyon Creek is a local scenic attraction called Box Canyon. 

Cliffs tower above the town but recede to the east into The 

Amphitheater, a large cirque-like or steeply backed bowlshaped basin. Because of its picturesque mountain setting, the 

town and environs have appropriately been referred to as the 

“Switzerland of America.” U.S. Highway 550, which south 

of town in the Uncompahgre River gorge is known as the 

Million Dollar Highway, is the principal access route into and 

out of the area; this highway connects Ouray with Montrose 

to the north and, across the San Juan Mountains via Silverton, 

with Durango to the south. A few secondary roads and trails 

provide access to the remainder of the district. Now serviced 

by motor freight, Ouray was formerly the southern terminus 

of a narrow-gauge branch line of the Denver and Rio Grande 

Western Railroad that extended southward from Montrose.

Commercial development of the mining district was 

controlled to a very large extent by the physical features of the 

valley of the Uncompahgre River, one of the major drainage 

systems in the western San Juan region, which traverses northwestward through the heart of this mineralized area. The river 

valley to the north is large and broadly U-shaped in profile, 

characteristic of many valleys in the glaciated western San 

Juan Mountains region. However, approaching Ouray from 

the north, the valley narrows and the precipitous canyon walls 

carved into sedimentary rocks rise about 1,400 ft above the 

gravelly river bottom as cliffy, step-like slopes to an altitude 

of about 9,000 ft; the valley sides above that altitude consist 

generally of somewhat more gentle slopes that are surmounted 

by still higher pinnacled and jagged cliffs. Ranging from an 

altitude of about 7,260 ft in the river bottom at the north edge 

to 12,275 ft on the eastern edge, total relief within the district 

2 Geology and Ore Deposits of the Uncompahgre Mining District, Southwestern Colorado
Introduction 3

Figure 1. Index map showing the location of the Uncompahgre (Ouray) mining district (shaded) within the Ouray 7.5-min quadrangle 

and surrounding region in the western San Juan Mountains of southwestern Colorado.
Figure 2. View looking south 

at the town of Ouray (average 

altitude 7,800 ft) nestled in a 

mountain park at the base of 

Hayden Mountain and at the 

confluence of the canyons of 

the Uncompahgre River (left) 

and Canyon Creek (right). 

Rocks exposed at town level 

and on the lower slopes are 

predominantly sedimentary; 

those in the higher precipitous 

mountains surrounding the 

town site are mainly volcanic. 

Altitude of the mountain crests 

on the skyline ranges from 

12,500 to more than 13,000 ft.

is about 5,000 ft. Altitudes just beyond the area to the west, 

south, and east reach 13,500 ft.

The district is subject to a wide variation in climatic conditions because of its location on the flank of the mountains 

and because of the considerable topographic relief within the 

region. Summers are short and relatively cool, whereas winters 

are long but seldom severe. Minimum temperatures of several 

degrees below 0°F may occur in winter months, but not for 

long duration, and maximum temperatures in the summer 

rarely exceed 90°F. Typical of mountainous areas, precipitation occurs as rain in the summer and snow, sometimes heavy, 

in the winter; afternoon thunderstorms are common in July 

and August and cease with the first heavy frost or snowstorm, 

which usually occurs before the end of September. Ice and 

snow in the winter, snowslides in early spring, and flash floods 

or debris flows (Jochim, 1986) in the summer are seasonal 

weather hazards.

Timber and vegetation growth vary as might be expected 

in accordance with variations of location and altitude. Scrub 

oak, piñon, and juniper are common on lower slopes of the 

district, especially in the northern part. Cottonwood and willow grow along the waterways, and aspen, pine, fir, and spruce 

on the higher mountain slopes. Most of the district is below 

timberline, which, depending upon exposure, averages about 

11,500 ft in altitude.

Although limited geologic examinations of the region 

were made by the earlier Hayden Surveys, not until the late 

1800s and early 1900s were more comprehensive geologic 

studies made. These include the Ouray 15-min quadrangle 

(Cross and others, 1907). The areal geology, with revisions 

and at a smaller scale, was incorporated into a preliminary 

report on the entire San Juan Mountains region (Cross and 

Larsen, 1935) and later into a more complete geologic and 

petrologic report on the same region (Larsen and Cross, 

1956). A resurvey of the mineral resources in areas and (or) 

mining districts, including the Uncompahgre, of the western San Juan Mountains region was begun in the late 1920s 

by Burbank (1930, 1936, 1940, 1941, 1947), and in the late 

1930s by Kelley (1946). Much of that material was included 

as part of a later study, begun in 1956, of quadrangle mapping in the northwestern part of the San Juan Mountains. The 

geologic map of the Ouray 7.5-min quadrangle at a scale of 

1:24,000 (Luedke and Burbank, 1962) was the first of those 

maps prepared in cooperation with the Colorado State Metal 

Mining Fund Board. A more detailed geologic map of only 

the Uncompahgre mining district at a scale of 1:12,000 was 

subsequently completed and published (Luedke and Burbank, 

1981). Copies of these two maps are recommended for at-hand 

reference, but if not available, the illustrations included herein 

should suffice in showing locations, distribution of major rock 

units, structure, and other features pertinent to the discussion. A later geologic and mineral resources appraisal of the 

mining district and surrounding Uncompahgre Primitive Area 

was made by Fischer and others (1968) and Steven and others 

(1977) to determine the area’s suitability for incorporation into 

the National Wilderness Preservation System.

Acknowledgment is gratefully extended to property owners and mine operators for permitting access to mines and use 

of data, and to the many individuals in the area for their generous cooperation in providing information on previous and 

current mining operations. Assistance in the field was given by 

M.G. Barclay, F.G. Wells, R.A. Yund, and T.H. Foss. D. Paul 

Mathieux provided invaluable instruction and assistance in the 

digital preparation of the illustrations.

4 Geology and Ore Deposits of the Uncompahgre Mining District, Southwestern Colorado
History and Production

Limited prospecting was done throughout the San Juan 

Mountains region beginning about 1860, but not until the 

spring of 1874, upon completion of negotiations of a treaty 

between the Ute Indians and the Federal Government, was 

the western part of the region officially opened to prospecting and settlement (Burbank and Luedke, 1968). In the rush 

to the western San Juans, prospectors came into the Ouray 

area supposedly not from the north along the more easily 

accessible Uncompahgre River valley but from the south 

through the rugged mountains. The lure of mineral wealth 

resulted in a rapid increase in population, so that the need for 

a community to service and supply the area was soon realized. A permanent mining camp was established in 1875, and 

within a very short time the camp became a town and was 

platted and soon incorporated. Aside from conflicting information concerning who founded the town, it was first called 

Uncompahgre City; the name was later changed to Ouray in 

honor of Chief Ouray, leader of the Uncompahgre Ute Indians. By 1880, the town of Ouray had about 900 inhabitants 

and was an important supply center to the adjacent mining 

regions (Strahan, 1881). The town continued to prosper and 

grow through its early years and, typical of most mining 

communities, had its “boom and bust” periods. Today, with 

mining activities at an ebb, it serves as an important hub for 

tourism. For more comprehensive discussions on the early 

history of the town and its environs, consult the accounts 

of Wolle (1949), Dorset (1970), Benham (1976), and Rice 

(1980).

Ouray County, organized in 1883 and reportedly the 

first county created following Colorado statehood (Dawson, 1954), is divided into several mining districts. The 

Uncompahgre mining district is part of the larger, allinclusive Ouray district. The principal ore deposits of the 

Uncompahgre district have been found in the sedimentary 

strata exposed along the walls of the Uncompahgre River 

valley to a maximum altitude of about 10,000 ft, and east 

and west of the main valley in several tributary canyons; 

the more productive deposits have all been within 4 mi of 

town, mostly to the north. Most mine workings are entered 

by adits situated on the slopes or steep walls of the canyons. 

The local high relief favored this type of mine development 

and, also, the use of aerial tramways to transport ore to the 

valley bottoms from cliffy locations otherwise inaccessible 

except by pack-animal trails. The local physical relief, as it 

affected accessibility, as well as the regional climatic conditions were very important locally in the evaluation of a mine 

or prospect. Production data of precious and base metals for 

both Ouray County and the Uncompahgre mining district 

are given in tables 1 and 2, respectively, and are compared 

geographically in figure 3.

The Trout and Fisherman claim near Box Canyon, one of 

the earliest mining properties in the area, was staked by men 

who had come into the area with the original intent to hunt and 

fish. During the summer of 1875 (Henderson, 1926, p. 54), the 

Grand View claim north of Ouray was located, registered, and 

patented. Soon, other gold-bearing lodes were located in the 

canyon walls within sight of town, and, by 1877, silver-lead 

deposits were discovered in the limestone benches southwest 

of town. Although most prospecting was limited by weather 

to late spring and summer, many claims had been staked and 

mines started by the late 1870s and early 1880s. Progress associated with mining in the district was slow at first “...because 

of its remoteness from competing markets, want of local 

facilities for reduction of ores, and extravagantly high rates for 

freight. No ores yielding less than $150 to $200 per ton will 

pay for shipping...” (U.S. Bureau of the Mint, 1882, p. 419). 

Only a few mines were shipping because of the normally low 

grade of ore.

During the 1880s, a few deposits of high-grade gold and 

high-grade silver-lead ores were discovered and subsequently 

mined. Among mines in this group was the Calliope in the 

valley of Dexter Creek, which was brought into production in 

1887 and, prior to 1890, yielded ore valued at several hundred 

thousand dollars. The discovery of very rich gold ore high on 

the north wall of The Blowout at the American Nettie mine in 

1889 resulted in rapid development of this and adjoining gold 

properties, which together yielded subsequently the bulk of the 

gold production of the district. The principal body of silver ore 

within the Bachelor mine, also located in the valley of Dexter 

Creek, was not discovered until 1892, but by about 1895 at 

the peak of its activity, the production and profit over a period 

of about two years had exceeded that of any other mine in the 

district; during that period the mine had a total production of 

more than $3 million.

The mining industry in the Uncompahgre district kept 

pace with that of the neighboring districts in attempts to treat 

lower grade base-metal ores, and to search for additional 

shoots of high-grade gold- and silver-bearing ores. Gradual 

improvements in milling techniques through concentration and 

amalgamation and the operation of several local smelters all 

facilitated a more profitable handling of the lower grade ores. 

A number of mines had their own mills, but a considerable 

proportion of the base-metal ores of the district was treated at 

several different custom mills. One of the more important of 

these after 1930 was the 120-ton-per-day Banner American 

custom flotation mill north of Ouray, which permitted a continuous small production of silver-lead-zinc ore to be treated 

through the period of World War II.

In the early days of mining, pack animals were used to 

carry most of the equipment and supplies to the surrounding 

mines and the ore down to the smelters in Ouray. With time, 

trails were widened and a few roads were built that lowered 

transportation costs; because the ore was brought out by 

wagons instead of by pack animals, lower grade ores could be 

handled more profitably. By 1882, the narrow-gauge Denver 

and Rio Grande Western Railroad had been constructed to 

within 30 mi of Ouray and was expected to reach the town the 

following summer, but that extension was not completed until 

1887. Otto Mears was responsible for extending the railroad 

History and Production 5
Table 1. Ouray County mine production of gold, silver, copper, lead, and zinc in terms of recovered metals, 1878–1986, inclusive.

[Production figures, some estimated, were calculated for 1878–1923 and are from Henderson (1926, p. 186); for 1924–1931 from “Mineral Resources of the 

United States” (U.S. Bureau of Mines, 1927–1934); and for 1932–1986 from “Minerals Yearbook” (U.S. Bureau of Mines, 1933–1988). No data for 1957; data 

for 1967, 1970, and 1979–1985 are withheld to avoid disclosing company proprietary data. s.t., short tons; --, zero]

Year

Gold Silver Copper Lead Zinc Total

value

(dollars)

Quantity1

(ounces)

Value

(dollars)

Quantity1

(ounces)

Value

(dollars)

Quantity2

(s.t.)

Value

(dollars)

Quantity2

(s.t.)

Value

(dollars)

Quantity2

(s.t.)

Value

(dollars)

1878 242 5,000 38,672 44,473 -- -- -- -- -- -- 49,473

1879 411 8,500 38,672 43,313 -- -- 99 8,150 -- -- 59,963

1880 411 8,500 69,610 80,052 -- -- 100 10,000 -- -- 98,552

1881 2,661 55,000 85,078 96,138 50 18,200 115 11,040 -- -- 180,378

1882 3,386 70,000 77,344 88,172 250 95,500 115 11,270 -- -- 264,942

1883 967 20,000 386,719 429,258 200 66,000 585 50,310 -- -- 565,568

1884 508 10,500 572,344 635,302 182 47,206 1,500 111,000 -- -- 804,008

1885 484 10,000 900,000 963,000 200 43,200 2,200 171,600 -- -- 1,187,800

1886 1,270 26,241 993,867 983,928 200 44,400 1,604 147,568 -- -- 1,202,137

1887 1,106 22,853 952,255 933,210 333 91,908 1,334 120,066 -- -- 1,168,037

1888 1,175 24,289 789,396 742,032 290 97,289 1,630 143,436 -- -- 1,007,046

1889 1,279 26,436 913,254 858,459 199 53,704 2,352 183,466 -- -- 1,122,065

1890 17,083 353,133 2,791,626 2,931,207 333 103,858 2,114 190,296 -- -- 3,578,494

1891 23,160 478,750 2,273,054 2,250,323 433 110,726 2,084 179,262 -- -- 3,019,061

1892 6,709 138,688 754,114 656,079 319 74,109 4,006 320,509 -- -- 1,189,385

1893 9,136 188,854 1,221,155 952,501 300 64,800 4,000 296,000 -- -- 1,502,155

1894 8,617 178,138 995,153 626,946 300 57,000 2,211 145,926 -- -- 1,008,010

1895 8,355 172,697 1,515,693 985,200 300 64,200 2,874 183,904 -- -- 1,406,001

1896 6,824 141,046 2,371,912 1,612,900 109 23,469 3,300 197,974 -- -- 1,975,389

1897 26,746 552,840 2,776,394 1,665,836 1,093 262,210 3,892 280,232 -- -- 2,761,118

1898 41,246 852,555 1,420,330 837,995 518 128,410 1,400 106,398 -- -- 1,925,358

1899 82,000 1,694,940 2,346,194 1,407,716 153 52,185 3,778 340,037 -- -- 3,494,878

1900 69,565 1,437,909 1,985,267 1,230,866 176 58,493 4,739 417,061 10 880 3,145,209

1901 74,810 1,546,323 1,633,725 980,235 326 109,040 3,952 339,903 -- -- 2,975,501

1902 117,113 2,420,726 789,855 418,623 264 64,238 2,131 174,745 -- -- 3,078,332

1903 105,056 2,171,508 417,343 225,365 190 52,116 1,675 140,724 -- -- 2,589,713

1904 105,194 2,174,361 294,028 170,536 216 55,174 1,022 87,915 3 256 2,488,242

1905 112,882 2,333,282 758,107 462,445 262 81,775 2,674 251,368 24 2,848 3,131,718

1906 48,001 992,179 916,256 623,054 331 127,787 2,861 326,131 5 633 2,069,784

1907 116,838 2,415,049 352,519 232,663 454 181,735 1,803 191,155 15 1,794 3,022,396

1908 98,147 2,028,698 415,070 219,987 510 134,584 1,517 127,401 -- -- 2,510,670

1909 147,306 3,044,825 345,815 179,824 492 127,955 1,407 120,999 10 1,034 3,474,637

1910 106,234 2,195,847 414,250 223,695 310 78,770 2,002 176,208 -- -- 2,674,520

1911 94,483 1,952,958 512,800 271,784 282 70,534 1,975 177,742 -- -- 2,473,018

1912 50,778 1,049,590 545,177 335,284 200 66,091 1,495 134,507 70 9,706 1,595,178

1913 46,414 959,377 537,634 324,731 250 77,551 1,090 95,946 100 11,224 1,468,829

1914 58,635 1,211,993 594,289 328,642 427 113,587 1,060 82,663 22 2,275 1,739,160

1915 54,089 1,118,016 576,621 292,347 432 151,174 995 93,562 4 903 1,656,002

1916 23,763 491,175 803,461 528,677 222 109,244 1,170 161,393 35 9,248 1,299,737

1917 4,491 92,831 868,097 715,312 90 49,018 1,016 174,728 266 54,345 1,086,234

1918 5,208 107,645 801,359 801,359 77 37,820 1,294 183,742 20 3,576 1,134,142

1919 4,467 92,338 627,659 702,978 56 20,867 891 94,492 12 1,704 912,379

6 Geology and Ore Deposits of the Uncompahgre Mining District, Southwestern Colorado
Table 1. Ouray County mine production of gold, silver, copper, lead, and zinc in terms of recovered metals, 1878–1986, inclusive.—Continued

[Production figures, some estimated, were calculated for 1878–1923 and are from Henderson (1926, p. 186); for 1924–1931 from “Mineral Resources of the 

United States” (U.S. Bureau of Mines, 1927–1934); and for 1932–1986 from “Minerals Yearbook” (U.S. Bureau of Mines, 1933–1988). No data for 1957; data 

for 1967, 1970, and 1979–1985 are withheld to avoid disclosing company proprietary data. s.t., short tons; ­­, zero]

Year

Gold

Quantity1 Value

(ounces) (dollars)

Silver

Quantity1 Value

(ounces) (dollars)

Copper

Quantity2 Value

(s.t.) (dollars)

Lead

Quantity2 Value

(s.t.) (dollars)

Zinc

Quantity2 Value

(s.t.) (dollars)

Total 

value 

(dollars)

1920

1921

1922

1923

1924

1925

1926

1927

1928

1929

1930

1931

1932

1933

1934

1935

1936

1937

1938

1939

1940

1941

1942

1943

1944

1945

1946

1947

1948

1949

1950

1951

1952

1953

1954

1955

1956

1958

1959

1960

1961

1962

1,634

3,543

6,094

2,864

659

675

2,121

1,311

12,909

17,831

7,226

9,760

12,478

9,255

8,159

12,520

8,979

19,176

12,761

12,586

11,565

10,790

6,782

4,937

3,212

3,126

8,504

4,536

2,466

2,824

5,000

2,168

2,443

3,224

2,688

3,266

2,647

201

13

185

16,569

12,560

33,777

73,229

125,960

59,207

13,625

13,962

43,853

27,106

266,853

368,598

149,384

201,764

257,949

191,323

285,163

438,201

314,272

356,167

446,621

440,510

404,775

377,650

237,370

172,795

112,420

109,410

297,640

158,760

86,310

98,840

175,000

75,880

85,505

112,840

94,080

114,310

92,645

7,035

455

6,475

579,915

439,600

465,577

730,970

1,226,670

840,044

166,345

35,934

111,181

79,291

48,007

20,683

18,122

40,586

47,780

53,323

109,853

226,183

406,244

182,389

213,382

158,798

249,930

159,186

141,411

91,679

143,325

176,926

194,297

150,886

172,713

206,667

238,021

129,216

95,871

91,975

68,249

48,450

56,475

1,866

117

17,763

482,792

339,886

507,479

730,970

1,226,670

688,836

111,451

24,938

69,377

44,958

28,084

11,024

6,977

11,770

13,474

18,663

71,016

162,569

314,636

141,078

137,944

107,790

177,728

113,199

100,559

65,194

101,920

125,814

156,992

136,552

156,314

187,044

215,421

116,947

86,768

83,242

61,769

43,850

51,113

1,689

106

16,076

446,336

368,776

43

43

29

22

5

100

102

43

25

27

13

37

45

81

106

127

125

242

362

179

203

128

71

72

82

84

100

135

167

173

190

256

197

254

250

181

126

1

­­

35

1,548

1,356

15,986

10,970

7,850

6,497

1,279

28,258

28,658

11,228

7,306

9,358

3,380

6,673

5,670

10,336

16,936

20,999

23,046

58,443

70,903

37,274

45,765

30,208

17,097

18,720

22,140

22,680

32,238

56,658

72,478

68,162

79,040

123,904

95,348

145,796

147,500

127,566

106,675

579

­­

22,181

928,800

835,235

617

604

742

769

406

191

306

145

51

51

86

184

157

140

216

429

705

481

426

327

399

319

400

540

972

896

871

1,092

1,472

1,521

1,100

1,211

1,161

1,360

1,131

639

527

9

3

260

5,545

4,363

106,766

54,378

81,649

107,662

64,913

33,234

48,976

18,315

5,875

6,478

8,550

13,618

9,420

10,375

15,947

34,288

64,860

56,758

39,192

30,757

39,900

36,309

53,553

80,925

155,520

154,026

189,987

314,539

526,976

480,636

297,000

419,006

373,842

356,320

309,894

190,422

165,588

2,153

644

60,758

1,142,322

802,773

­­

­­

­­

­­

­­

42

67

47

­­

­­

­­

­­

4

­­

­­

11

­­

13

­­

7

34

19

354

327

534

572

623

642

1,260

1,374

909

1,101

957

864

712

465

588

­­

3

222

7,391

5,924

­­

­­

­­

­­

­­

6,308

10,050

5,952

­­

­­

­­

­­

240

­­

­­

968

­­

1,625

­­

676

4,221

2,850

65,900

70,632

121,752

131,560

152,134

155,485

335,160

340,752

258,156

400,764

317,724

198,720

153,792

114,390

161,098

­­

794

57,173

1,699,884

1,362,647

664,008

869,547

1,442,129

862,202

191,268

106,700

200,914

107,559

308,118

395,458

168,291

233,825

286,753

230,697

389,062

657,025

716,814

614,071

694,660

617,007

672,389

560,216

474,479

408,266

513,752

543,490

828,991

821,994

1,177,238

1,175,434

1,024,617

1,136,501

959,187

896,918

767,035

590,538

577,119

11,456

1,999

162,663

4,797,257

3,809,031

History and Production 7
8 Geology and Ore Deposits of the Uncompahgre Mining District, Southwestern Colorado

Table 1. Ouray County mine production of gold, silver, copper, lead, and zinc in terms of recovered metals, 1878–1986, inclusive.—Continued

[Production figures, some estimated, were calculated for 1878–1923 and are from Henderson (1926, p. 186); for 1924–1931 from “Mineral Resources of the 

United States” (U.S. Bureau of Mines, 1927–1934); and for 1932–1986 from “Minerals Yearbook” (U.S. Bureau of Mines, 1933–1988). No data for 1957; data 

for 1967, 1970, and 1979–1985 are withheld to avoid disclosing company proprietary data. s.t., short tons; --, zero]

Year

Gold Silver Copper Lead Zinc Total

value

(dollars)

Quantity1

(ounces)

Value

(dollars)

Quantity1

(ounces)

Value

(dollars)

Quantity2

(s.t.)

Value

(dollars)

Quantity2

(s.t.)

Value

(dollars)

Quantity2

(s.t.)

Value

(dollars)

1963 10,154 355,390 313,453 400,944 1,141 702,548 4,043 873,375 5,499 1,264,713 3,596,970

1964 6,279 219,765 134,757 174,241 453 295,160 1,713 448,714 2,325 632,318 1,770,198

1965 6,009 210,315 119,537 154,561 413 292,510 1,833 571,818 2,436 711,210 1,940,414

1966 4,292 150,220 211,248 273,144 500 361,844 2,059 622,557 2,954 856,791 2,264,556

1968 2,689 105,570 194,036 416,130 525 439,561 1,976 522,085 2,773 748,616 2,231,962

1969 2,665 111,000 163,100 292,000 445 423,000 1,560 465,000 2,032 593,000 1,884,000

1971 2,400 99,001 129,484 200,181 579 601,952 3,079 849,788 3,997 1,287,147 3,038,069

1972 3,430 200,998 173,004 291,512 878 898,744 5,118 1,538,537 6,462 2,293,865 5,223,476

1973 2,701 264,185 157,853 403,789 842 1,001,907 4,041 1,316,676 5,684 2,348,737 5,335,294

1974 2,148 343,122 134,090 631,563 618 955,269 3,475 1,563,861 5,127 3,681,393 7,175,208

1975 1,764 284,868 214,145 946,521 779 999,840 3,516 1,511,754 5,510 4,297,620 8,040,603

1976 1,433 179,583 165,025 717,858 485 674,564 3,067 1,417,157 4,748 3,513,688 6,502,850

1977 1,259 186,722 69,115 319,311 183 244,849 1,286 789,441 2,151 1,479,849 3,020,172

1978 576 111,485 19,768 106,747 73 107,474 618 458,746 1,031 704,144 1,488,596

1986 576 212,106 644 3,523 1 977 3 1,287 -- -- 217,893

Total 2,007,872 45,857,154 49,110,490 41,667,585 26,609 14,645,946 148,253 26,676,549 78,391 30,644,924 159,492,158

1

Expressed as fine ounces for 1878–1957; as troy ounces for 1958–1986. 2Figures in pounds recalculated to short tons for 1879–1948.

around the west side of the San Juan Mountains from Durango via Rico and Telluride to Ouray (fig. 1); he originally 

had intended to come to Ouray from the south as he had with 

a toll road in 1881, now U.S. Highway 550. This so-called 

Mears system of roads and railroads, built mostly in the 1880s 

and early 1890s throughout the San Juans, greatly facilitated 

development of the entire region. Over the years, improvement 

in transportation and in treatment of ores continued to reduce 

the costs of ore production.

The first recorded production of ores from Ouray County 

was in 1878 and consisted of gold and silver (table 1). Production of lead was first reported in 1879 and copper in 1881. 

Zinc was not recovered by local smelters, and its presence in 

ores resulted in a penalty charge during the early years of mining. Its first reported production was in 1900; it was reported 

again in 1904 and then sporadically until the late 1930s, after 

which production was reported almost every year. County production figures for gold, silver, copper, lead, and zinc for the 

years prior to 1904 were obtained from agents’ reports, Director of the Mint reports, and smelter and mint receipts, and for 

the years 1904 through 1923 from mine reports to Federal and 

State agencies (Henderson, 1926); production figures for 1924 

through 1931 are reported in the annual volumes in “Mineral Resources of the United States” (U.S. Bureau of Mines, 

1927–1934) and for 1932 to 1986 in “Minerals Yearbook” 

(U.S. Bureau of Mines, 1933–1988). Production figures for 

these five metals from the Uncompahgre mining district were 

reported separately only for the years 1905–24 and 1932–55 

(table 2). The total production for the district compared with 

that for Ouray County for the years 1905–55 is shown in 

figure 3. During that time, the combined values for gold and 

silver accounted for most of the district’s total except for the 

periods 1916–18 and particularly 1944–51, when the values 

of base-metal production far exceeded that of the precious 

metals. Production figures for the county obtained during the 

period from 1878 to about 1980, for gold, silver, copper, lead, 

and zinc, were mostly from ore deposits of late Tertiary age 

and from other districts within the county. The Uncompahgre 

district’s total value, gained from Late Cretaceous to early Tertiary ores, is estimated to have accounted for 8 to 10 percent 

of the county total, or about $14–15 million. The exact value 

is not known because the early records are incomplete. Almost 

all of the gold and silver produced in the district came from 

lode deposits; the very small amount obtained from placer 

operations in the Uncompahgre River valley was of little economic significance.

Several siliceous travertine deposits containing iron, 

manganese, tungsten, and other metals in trace amounts are 

associated with hot springs within the city limits of Ouray. 

The travertine apron at Ouray Hot Spring (also called Pavillion Spring) near Box Canyon in the southwest corner of town 

was quarried for its manganese content during World War II. 

Waters from the several hot springs have been utilized for 

heating and in local bathing and recreational establishments.
Table 2. Uncompahgre district mine production of gold, silver, copper, lead, and zinc in terms of recovered metals, 1905–1924 and 

1932–1955, inclusive.

[Source: “Mineral Resources of the United States” (U.S. Geological Survey, 1906–1927, and U.S. Bureau of Mines, 1927–1934) and “Minerals Yearbook” (U.S. 

Bureau of Mines, 1933–1988). lb, pounds avoirdupois; oz, fine ounces; --, zero]

Year Producing 

lode mines

Ore sold or treated 

(short tons)

Gold1

(oz)

Silver

(oz)

Copper 

(lb)

Lead

(lb)

Zinc

(lb)

Total value 

(dollars)

1905 6 -- 1,484 26,569 16,972 66,195 46,767 55,237

1906 11 -- 1,064 178,994 47,819 265,105 10,377 166,902

19072 -- -- -- -- -- -- -- -19083 -- -- -- -- -- -- -- 67,499

1909 15 2,646 1,936 38,548 47,454 61,605 19,148 69,922

1910 12 3,482 1,664 30,257 18,750 70,946 -- 56,230

1911 16 22,154 6,616 48,049 128,421 211,844 -- 187,802

1912 26 23,021 9,890 135,071 61,618 192,356 45,262 309,434

1913 22 27,601 10,691 115,624 124,535 90,387 68,553 317,942

1914 30 40,241 17,118 154,362 563,631 160,589 14,962 521,174

1915 27 37,860 7,981 155,607 552,114 43,659 7,282 343,435

1916 31 6,060 869 101,748 156,809 507,623 41,090 164,016

1917 28 5,708 381 137,273 26,055 496,500 -- 170,809

1918 24 3,787 947 124,790 19,449 468,338 -- 183,445

1919 11 1,618 196 62,634 3,091 120,848 -- 81,189

1920 12 751 287 35,554 2,402 21,538 -- 46,856

1921 5 170 4 21,676 1,667 4,088 -- 22,149

1922 8 5,310 2,501 14,087 6,459 23,693 -- 67,955

1923 4 239 129 7,695 1,470 31,127 -- 11,373

1924 4 120 159 4,609 1,152 10,788 -- 7,381

1932 3 1,588 753 13,305 1,300 35,000 8,000 20,694

19334 6 990 1,206 7,286 1,400 15,300 -- 28,129

1934 6 10,895 619 49,144 7,700 111,350 -- 58,137

1935 6 16,348 1,083 144,836 26,400 422,700 16,000 161,810

19365 5 19,604 552 340,793 44,500 1,046,000 -- 335,482

1937 6 7,676 257 86,892 18,600 356,600 -- 99,487

1938 8 6,784 543 83,479 13,500 168,300 -- 82,036

1939 6 4,159 751 68,120 16,700 204,100 -- 83,854

1940 6 7,721 549 122,815 26,200 325,600 57,000 129,382

1941 11 6,227 368 38,167 13,400 176,000 34,000 54,184

1942 8 3,369 159 27,997 6,000 85,700 -- 31,942

1943 4 2,044 66 5,362 3,900 160,000 37,000 22,626

1944 5 4,251 139 27,197 12,800 652,200 143,000 94,411

1945 3 6,443 89 72,038 17,000 624,500 244,000 138,404

19466 -- -- -- -- -- -- -- -19476 -- -- -- -- -- -- -- -1948 4 9,501 113 57,042 36,000 1,004,000 547,000 315,860

1949 4 3,365 72 17,555 18,000 529,000 244,000 135,792

1950 5 2,136 23 8,268 14,600 281,700 107,600 64,633

1951 4 428 9 2,022 2,000 80,000 11,200 18,507

1952 3 856 28 4,886 3,000 132,000 52,600 36,112

19532 -- -- -- -- -- -- -- -1954 1 22 3 292 300 6,400 -- 1,334

19552 -- -- -- -- -- -- -- -1

Figures were calculated for 1909–1924. 2No production recorded. 3Total value, only, reported in 1909. 4Includes two placer operations with 7 oz gold 

and 1 oz silver. 5Includes two placer operations with 27 oz gold and 80 oz silver. 6Combined with Sneffels district production.

History and Production 9
10 Geology and Ore Deposits of the Uncompahgre Mining District, Southwestern Colorado

Figure 3. Graph of metalproduction values (gold, silver, 

copper, lead, and zinc) for the 

Uncompahgre mining district 

compared with that for Ouray 

County, 1905–1955, inclusive.

Stratigraphy

The Uncompahgre district is underlain by a great variety 

of metamorphic, sedimentary, and volcanic rocks ranging in 

age from Proterozoic to Tertiary (fig. 4). Much of the area is 

blanketed by surficial deposits of Quaternary age.

The stratigraphy in the western San Juan Mountains has 

been discussed in the literature over the past 40 years so that 

an extensive description of the local units is not deemed necessary. However, the strata are briefly described and discussed 

here in order to consolidate and update pertinent literature, to 

establish a base for understanding the structural features, and 

to present the close relation of the rocks to alteration and ore 

deposition. Detailed measured sections of the different formations are available elsewhere (Armstrong and Mamet, 1976; 

Burbank, 1930; Franczyk, 1993; Luedke and Burbank, 1994; 

O’Sullivan, 1992).

Proterozoic Metamorphic Rocks

Only limited exposures of metasedimentary rocks of late 

Paleoproterozoic (Proterozoic Y) to early Mesoproterozoic 

(Proterozoic X) age (Plumb, 1991) are found in the southern 

part of the Uncompahgre district (fig. 4). Other metamorphic 

and igneous rocks similar to those in the Needle Mountains 

region to the south (fig. 1), although not exposed in this area, 

may underlie the northern and eastern parts of the district. 

Metasedimentary gneisses and schists were reported at moderate depths in several wells only 15 to 20 mi to the north 

(O’Sullivan, 1992; Tweto, 1987, pl. 1).

Uncompahgre Formation

The Uncompahgre Formation of Proterozoic age, named 

for the exposures in the Uncompahgre River gorge south of 

Ouray, was described by Cross, Howe, and Ransome (1905, 

p. 3), Cross and others (1907, p. 2–3), and Kelley (1946, p. 

296–297, 305–306). Exposures of the formation in the Needle 

Mountains were described in detail by Barker (1969), who 

determined its age to be between 1,720 Ma and 1,460 Ma. Further extensive stratigraphic revision and structural reinterpretation of the Proterozoic complex has been done by Tewksbury 

(1985, 1989), Harris (1990), and Harris and Eriksson (1990).

The metasedimentary rocks in the Uncompahgre Formation consist of interlayered units, each several hundreds of feet 

thick, of quartzite or slate; quartzite is the dominant rock type. 

Within the mining district, the formation is estimated to have 

a minimum thickness of 3,000 ft in an exposed area of less 

than a square mile (fig. 4) that is terminated on the north by a 

major (Ouray) fault. Cumulative exposures of the formation 

within the district and in the river canyon for about 2 mi south 

of the district indicate the formation has a probable thickness 

in excess of 7,800 ft, with neither the bottom nor the top of the 

formation exposed.

The quartzite is massive to thin bedded, strongly jointed, 

and white to gray but locally red or black from the presence of 

minute flakes of iron oxides; weathered outcrops are commonly buff colored. The quartzite beds have some thin partings of argillaceous material. No definite progression was recognized in the nature of the commonly well-defined quartzite 

bedding within any one unit, but in some units, individual beds 

appear to be thinner near the top or bottom of the unit rather 
than in the middle. For the most part, the beds are composed 

of well­indurated, fine­ to medium­grained quartz relatively 

free of impurities; minor amounts of mica and carbonaceous 

material occur locally. Within the thick unit of quartzite near 

the south edge of the district are a few pink to brown beds 

composed of subangular to rounded, closely packed quartz 

grains, 0.5 to 3.0 millimeters (mm) in size, cemented by iron 

oxide; this rock tends to fracture on grain boundaries, with the 

larger clear to blue quartz grains appearing to have a frosted 

surface. A few beds are conglomeratic and contain wellrounded quartz, jasper, and chert pebbles averaging about a 

centimeter in diameter. However, the quartzite is characterized 

by the sparseness rather than the abundance of conglomerate. 

Just south of and adjacent to the Ouray fault, the quartzite 

grades into a narrow belt or zone of finely contorted mica 

schist. Crossbedding and ripple marks are common, and aid in 

the determination of tops and bottoms of beds where involved 

in complex structure. A few thin beds of quartzitic sandstone 

occur locally near the contact of the quartzite and slate units. 

The lower contact of quartzite with slate is generally even and 

sharp, whereas the upper contact is gradational through several 

feet, which also shows the younging direction.

The slate units are dark green, brown, and gray to black 

and consist of laminated thin to thick beds that in a few places 

grade either into argillite or phyllite. Thin, lenticular metasiltstone to fine­grained quartzite layers commonly are interbedded in the lower part of the slate units. Although locally the 

slate beds are contorted, the beds in many places are readily 

cleavable into sheets about 0.25 inch (in.) thick; where slaty 

cleavage is less perfectly developed an irregular jointing 

produces blocky fracturing. Prominent cleavage surfaces often 

are coated with fine mica. Mud cracks and ripple marks, the 

latter often with considerable amplitude, are common in some 

beds. The slates are dense, very fine­grained aggregates of 

quartz, sericite, iron oxides, opaque matter, and chlorite, with 

accessory rutile, apatite, and tourmaline; sparse andalusite and 

staurolite were noted as imperfect grains.

The upper contact of the Uncompahgre Formation is 

everywhere a profound angular unconformity (fig. 5). Following an erosional period of long duration, the surface 

upon which the Paleozoic rocks lie is remarkably even with 

very gentle undulations, as shown by the trace of the erosion 

surface where the Paleozoic rocks wedge out south of Ouray 

on the west side of the Uncompahgre River canyon. However, 

after the early Tertiary regional erosion, the Tertiary rocks 

overlap exposed basement rocks with a more marked unevenness. On the east side of the river gorge, local relief on this 

erosion surface is as much as 350 ft.

The Uncompahgre Formation as a whole has yielded 

less in mineral wealth in proportion to its areal extent than 

the younger rocks. Generally north­ and west­oriented joints 

and fractures, probably related to Late Cretaceous to early 

Tertiary deformation (Laramide orogeny), are small and 

usually carry only barren quartz veins. Mineral deposits 

of minor economic interest may occur in the veins of late 

Tertiary age.

Stratigraphy 11

Paleozoic Sedimentary Rocks

Sedimentary rocks of Paleozoic age constitute a large 

amount of the rocks cropping out within the district (fig. 4), 

but they have been the source of only a minor proportion of 

the mineral wealth of the district. These rocks form the base 

and lower slopes of the river valley throughout the area except 

south of the Ouray fault. They singularly and collectively were 

subjected to folding, faulting, and erosion prior to deposition 

of the overlying Mesozoic rocks. The Ignacio Quartzite of 

Late Cambrian age, present elsewhere in the western San Juan 

Mountains region, and reported in the Cow Creek area to the 

northeast (Cross and others, 1907, p. 3), is thought to have been 

deposited locally but later eroded from the Ouray area prior to 

Late Devonian time. No Ordovician, Silurian, or Lower and 

Middle Devonian rocks have been recognized in the western 

San Juans. Paleozoic rocks, deposited during the Late Devonian 

through Permian, rest upon the erosion surface of the Proterozoic rocks and total more than 3,900 ft in thickness. The lower 

part of the exposed Paleozoic section consists mostly of marine 

carbonate rocks that grade upward into marine clastic rocks and 

minor carbonate rocks. The upper part of the section consists of 

dominantly terrestrial clastic rocks.

Elbert Formation

The Elbert Formation, of Late Devonian age, is exposed 

only south of the Ouray fault from Box Canyon southward 

along the benches west of the river gorge and is considered 

to be representative of the formation. A very small exposure 

occurs also south of the fault on the south side of The Amphitheater just beneath the base of the Tertiary volcanic rocks.

The formation is green, buff, and gray and consists of 

thin-bedded calcareous shales, dense sandy limestones, and 

impure sandstones. The limestones have thin shaly partings 

and are sandy and dolomitic, as indicated by the silica and 

magnesia contents, determined by rapid-rock methods (Shapiro, 1967), of a typical limestone bed from the Elbert (table 3, 

sample no. 1). In general, the carbonate beds are lenticular and 

wedge out or grade laterally into calcareous shales or sandstones. The sandstones are fine to coarse grained and have thin 

shale and limestone partings. Locally interbedded in the lower 

part of the formation are a few thin lenticular conglomeratic 

beds. Occasional casts after salt crystals are found within 

the shaly beds. Lateral lithologic changes in the Elbert occur 

within short distances throughout the formation. The local 

rock types differ from those at the type locality in the southern 

San Juan Mountains (Cross, 1904) in the amount of contained 

detrital material and the lack of fossils.

The Elbert Formation ranges in thickness from 30 to 50 

ft, and appears to grade into the overlying Ouray Limestone. 

At some places, the contact between these formations is difficult to pick, but at others is marked by a thin sandy or conglomeratic limestone at the base of the Ouray Limestone.
12 Geology and Ore Deposits of the Uncompahgre Mining District, Southwestern Colorado

Figure 4. Generalized geologic map, excluding surficial deposits, of the Ouray 7.5-min quadrangle and the Uncompahgre mining 

district, southwestern Colorado (modified from Luedke and Burbank, 1962, 1981).
Stratigraphy 13

Figure 5. Angular unconformity between the underlying nearly 

vertical Proterozoic metamorphic rocks of the Uncompahgre 

Formation and the overlying nearly horizontal Paleozoic sedimentary 

rocks of the Elbert Formation, Ouray Limestone, and Leadville 

Limestone at Box Canyon (see fig. 4 for location) near the southwest 

corner of the town of Ouray. Photograph by A.K. Armstrong.

Table 3. Chemical analyses of carbonate rocks in the western 

San Juan Mountains region, southwestern Colorado.

[Analyses by U.S. Geological Survey. Major oxide data, in weight percent, from rapid rock methods by Samuel Botts; selected trace element data, in parts per million, from semiquantitative spectrographic methods by J.L. Harris. N, 

not detected at limit of detection; ls., Ls., limestone]

Sample no. 1 2 3 4 5 6

Field no. 69S10 69S11 69S12 69S13 69W1 69S14

Lab no. W173383 W173384 W173385 W173386 W173388 W173387

SiO2 10.30 6.00 3.00 1.60 0.78 13.00

Al2

O3 1.50 1.30 0.46 0.10 0.08 0.54

Fe2

O3 0.50 0.31 0.52 0.11 0.08 0.22

FeO 1.00 0.44 0.16 0.08 0.00 0.20

MgO 18.00 18.40 17.40 0.54 0.32 0.83

CaO 29.60 30.70 33.00 53.60 54.90 46.80

Na2

O 0.10 0.00 0.00 0.03 0.33 0.00

K2

O 0.10 0.26 0.16 0.10 0.10 0.13

H2

O+ 1.50 0.95 0.61 0.42 0.48 0.51

H2

O– 0.20 0.15 0.03 0.00 0.00 0.04

TiO2 0.08 0.06 0.00 0.03 0.00 0.02

P2

O5 0.02 0.00 0.00 0.00 0.00 0.02

MnO 0.20 0.04 0.00 0.08 0.00 0.04

CO2 37.30 42.30 46.00 43.50 43.50 38.20

Total 100 101 101 100 101 101

Powder density 2.84 2.88 2.88 2.72 2.72 2.72

Bulk density 2.78 2.8 2.76 2.7 2.69 2.69

Ba 30 30 7 7 3 30

Cu 20 3 2 5 1 5

Pb 1 1 5 1 3 N

Sr 100 70 70 100 100 700

V 10 10 5 5 3 7

Sample descriptions and locations:

1. Gray sandy dolomite, Elbert Formation, Molas Pass area; 37°45’46” N., 

107°40’38” W.

2. Gray dolomite, Ouray Ls., Molas Pass area; 37°45’46” N., 107°40’40” W.

3. Gray impure dolomitic ls., Leadville Ls., Molas Pass area; 37°45’45” N., 

107°40’42” W.

4. Gray ls., Leadville Ls., Molas Pass area; 37°45’08” N., 107°40’32” W.

5. Gray ls., Leadville Ls., Cow Creek area; 38°06’03” N., 107°36’42” W.

6. Gray sandy ls., Hermosa Formation, Molas Pass area; 37°44’57” N., 

107°41’38” W.
No ore deposits of commercial importance are known to 

exist in the formation in the Ouray area. The few veins that cut 

the formation near the south edge of the district contain hematite, chlorite, rhodochrosite, and quartz; those minerals have 

locally replaced some beds, as have minor amounts of pyrite, 

sphalerite, and galena.

Ouray Limestone

The Ouray Limestone of Late Devonian age was named 

for exposures on the benches southwest of the town of Ouray, 

but as originally defined, included rocks of both Devonian and 

Mississippian ages (Spencer, 1900). Kirk (1931) restricted the 

name Ouray Limestone to the lower thin- and medium-bedded 

sequence (Devonian age), and assigned the upper massive part 

to the Leadville Limestone (Mississippian age).

The Ouray Limestone, with the same distribution as the 

underlying Elbert Formation, consists predominantly of gray, 

buff, and white dense to medium-grained limestone, locally 

dolomitic, and dolomite, and has a few thin shaly partings. 

Dolomite constitutes the bulk of the formation, ranges in 

texture from sublithographic to crystalline, and is nonfossiliferous. Some of the more earthy, fine-grained beds consist 

of calcareous cemented dolomite rhombs, 0.02 to 0.06 mm in 

size, as well as minor quantities of insolubles including quartz 

and clay (table 4, sample no. 2). About 15 to 20 ft above the 

base of the formation in the cliffs about a mile southeast of 

Box Canyon, a pink crystalline limestone bed, 4 ft thick, 

contains numerous invertebrate fossils characteristic of Late 

Devonian fauna (Burbank, 1930, p. 159). A chemical analysis 

of a typical dolomitic limestone from this formation is given 

in table 3 (sample no. 2). Most beds, particularly those of 

limestone, cannot be traced throughout the district. Within the 

district, however, the formation has a fairly uniform thickness 

of about 60 to 70 ft.

The base of the Ouray Limestone is marked in a few 

places by a sandy or conglomeratic limestone containing 

crinoid fragments (table 4, sample no. 1), but in other places 

it was determined by the amount of contained sandstone and 

shale (rocks that are more characteristic of the underlying 

Elbert). The top of the Ouray is marked in a few places by a 

sharp change of lithic type that perhaps indicates a time of 

some erosion, but otherwise appears to grade into the overlying Leadville Limestone. This contact was placed arbitrarily 

at the base of a bluish-gray, non-magnesian, fine-grained, 

thin-bedded limestone that commonly contains black chert 

nodules. In places this chert-bearing limestone gives way to 

an inconspicuous limestone breccia or, as at Box Canyon, to 

thin calcareous sandstone layers. Difficulty in determining the 

upper contact, not only here but throughout the western San 

Juan Mountains, was noted by others (Armstrong and Mamet, 

1976; Baars and See, 1968).

The Ouray Limestone does not host ore deposits in the 

Uncompahgre district. Small scattered blebs of base-metal 

sulfides were seen in the more limy beds on the benches and 

along the cliffs overlooking Uncompahgre Canyon and lower 

Canyon Creek, but nowhere did they show more than an 

extremely feeble mineralization. Dolomitic beds were even 

less favorable to replacement.

Leadville Limestone

The Leadville Limestone of Early Mississippian age 

(Kirk, 1931) constitutes the upper massive part of the former 

Ouray Limestone of Spencer (1900). Knight and Baars (1957) 

divided the formation into two members: the lower dolomitic 

Table 4. Partial chemical analyses of carbonate rocks from the Uncompahgre mining district, Ouray County, 

southwestern Colorado.

[Analyses by U.S. Geological Survey. Chemical data, in weight percent, by standard rock methods. Analyses by J.J. Fahey. --, zero]

Sample no. 1 2 3 4 5 6

Field no. O–3602 O–36205–9 O–36203–8 O–36203–D O–2613 O–2610

Insolubles 3.10 1 13.00 2.02 0.42 0.36 2 36.66

CaCO3 96.61 53.43 97.07 99.68 99.54 61.74

MgCO3 0.54 31.66 1.07 0.42 0.42 0.52

FeCO3 -- 1.32 -- -- -- 0.97

MnCO3 -- -- -- -- -- -Total 100.25 99.41 100.16 100.52 100.32 99.89

1

Includes 7.76 percent SiO2 and 2.28 percent Al2O3

2

Includes 30.58 percent SiO2 and 3.86 percent Al2O3

Sample descriptions (sample locations are on sedimentary benches south of Ouray):

1. Ouray Limestone: Coarse crystalline crinoid-bearing limestone bed near base of formation.

2. Ouray Limestone: Impure dolomitic limestone bed about 49 feet (ft) above base of formation.

3. Leadville Limestone: Dense black limestone near base of formation.

4. Leadville Limestone: Limestone bed in middle of formation.

5. Leadville Limestone: Limestone about 150 ft above base of formation.

6. Leadville Limestone: Impure limestone bed at top of formation.

14 Geology and Ore Deposits of the Uncompahgre Mining District, Southwestern Colorado
beta member, which is absent at Ouray, and the upper alpha 

member, a limestone. South of the Ouray fault the Leadville 

Limestone has the same distribution and general attitude as the 

underlying Devonian formations. North of the fault, it crops 

out south of the town of Ouray, along the south edge of The 

Amphitheater, and at the east edge of town. The Leadville also 

occurs in the workings of the Portland mine near the head of 

The Amphitheater.

The formation locally ranges from 180 to 235 ft in thickness and may be roughly divided into two parts: a lower massive, cliff-forming part and an upper step-like, bench-forming 

part consisting of interbedded layers of crystalline limestone 

and more easily eroded shale. Reported thicknesses vary 

dependent upon placement of the contacts and assignment of 

bedded units to the Ouray, Leadville, or Molas formations. 

Armstrong and Mamet (1976) measured a thickness of 214 

ft in their section near Box Canyon, whereas Kindle (1909) 

measured only about 75 ft in the same locality.

In general, the lower 55 ft of unfossiliferous locally dolomitic limestone (table 3, sample no. 3) is massive to evenly 

bedded and dark bluish-gray or brown, and contains some 

interbedded gray crystalline limestone and sandy limestone 

layers. Chert-bearing limestone, limestone breccia, or calcareous sandstone is locally present at the base. Sample no. 3 

(table 4) from a dense limestone within 8 ft of the base of the 

formation contains only a small amount of insolubles. This 

unfossiliferous lower part is included in the Leadville because 

the beds are similar lithologically to the overlying fossiliferous 

strata rather than to that in the underlying Ouray.

The generally thicker upper part of the formation consists 

of gray to brownish-gray, coarsely crystalline limestone in 

thick beds, which commonly contain poorly preserved fossils. 

Near the top of this upper part a few of the beds are composed almost entirely of fossils or fossil fragments. The purest 

limestones of the formation occur in this part (table 4, sample 

nos. 4, 5). Alternating with these massive beds in the upper 

part of the formation are limestone breccias; these breccias, 

containing red shaly seams, become more abundant upward. 

Also, some of the limestone beds are more siliceous and 

contain thin layers or stringers of chert nodules. Insolubles, 

particularly quartz sand, increase in the uppermost beds as 

indicated by sample no. 6 (table 4). On the whole, chemical 

analyses (tables 3 and 4) from the Ouray area, from the Molas 

Pass area about 4 mi south of Silverton (fig. 1), and from the 

Cow Creek area about 6 mi north-northeast of Ouray, indicate 

that the Leadville in the western San Juan Mountains consists 

dominantly of limestone.

The uppermost beds, included in the Leadville but quite 

similar to beds in the overlying Molas Formation, illustrate the 

greatest lateral compositional change in the formation. This 

sequence of beds, 50 to 75 ft thick, consists of siliceous and 

dolomitic limestones that grade laterally into clastic limestones separated by thin red shale partings and limestone breccias, or into ferruginous limy shale and thinly layered chert. In 

most exposures this uppermost shaly part, particularly above 

Box Canyon, appears to have been eroded before deposition of 

the overlying Molas Formation. The upper contact is everywhere a very irregular erosional unconformity.

No deposits of commercial importance are known to occur 

in the lower unfossiliferous beds of the Leadville Limestone. 

Manto-type orebodies formed at the time of the Laramide orogeny, known principally at the Mineral Farm mine located on the 

benches southwest of Ouray, appear to occur about equally in 

the very uppermost beds of the Leadville and in the basal chert 

conglomerates and red shales of the overlying Molas Formation. In the Portland mine near the head of The Amphitheater, 

there has been some replacement of the brecciated limestone 

walls immediately adjacent to the late Tertiary vein. The greater 

part of the massive beds in the Leadville, however, probably 

was unfavorable to easy replacement.

Molas Formation

The Molas Formation of Early and Middle Pennsylvanian 

age has about the same distribution in the district as the underlying Leadville Limestone. It is poorly exposed in most places 

because of its rock type and its clayey cementing material. 

Merrill and Winar (1958) divided the Molas Formation into 

three members in the Needle Mountains area (fig. 1). Baars 

and others (1967) suggested that particularly the lower part 

of the formation was in part an ancient regolith or paleosol of 

possibly Late Mississippian to Early Pennsylvanian age, and 

that it formed a karst surface on the Leadville Limestone. Only 

the middle member and a small thickness of the lower member 

crop out in the Ouray area.

The formation is composed of thin to thick lenticular 

beds of reddish-brown and red shale and conglomerate containing minor amounts of red sandstone and green and gray 

shale. All of the sandstones and shales are calcareous. The 

conglomerates contain black and gray chert fragments, ranging 

from 0.25 to 6 in. in diameter, cemented in a reddish clay and 

sand matrix. Just north of Canyon Creek, downstream of Box 

Canyon, the formation rests upon the uppermost shale in the 

Leadville Limestone, which appears to have been removed by 

erosion elsewhere in the district.

Throughout the district the formation ranges from 40 to 

60 ft in thickness. The differences in thickness probably are 

the result of irregularities on the underlying erosion surface. 

Deposition of the Molas beds was preceded by considerable 

erosion, as shown by the many chert and limestone clasts 

derived from the Leadville Limestone that are included in the 

basal beds of the Molas Formation. The upper contact of the 

Molas is conformable and apparently gradational with the 

overlying Hermosa Formation.

The uppermost part of the Leadville and the lower chertbearing shales and conglomerates of the Molas constitute the 

horizon within which most of the ore was deposited at the 

Mineral Farm mine. Also, the lower beds of the Molas, at a 

few places in The Amphitheater, seem to have influenced the 

localization of ore adjacent to fractures of Tertiary age.

Stratigraphy 15
Hermosa Formation

The Hermosa Formation of Middle and Upper Pennsylvanian age crops out as a series of greenish-gray and red ledges 

and cliffs west of Ouray and in the valley of Canyon Creek, 

north from within The Amphitheater around into Skyrocket 

Creek, and in the lower slopes above the benches at the north 

end of Hayden Mountain south of Ouray. One small block 

is exposed north of the Ouray fault on the south side of The 

Amphitheater. No attempt was made to separate or to correlate the formation as mapped here to the units or subdivisions 

proposed by Wengerd and Matheny (1958), Wengerd (1962), 

Baars and others (1967), and Spoelhof (1976) for nearby areas 

to the south and west.

The Hermosa Formation is composed of thin to massive beds and is about 1,450 ft thick. Throughout the district, 

correlation of individual beds was found to be impracticable 

because of the lenticularity of beds or lateral lithologic gradation. For descriptive purposes only, the Hermosa Formation 

here is divided into three parts.

The lower part, about 450 ft thick, consists predominantly 

of green, gray, and red calcareous sandstone, siltstone, and 

shale and contains interbedded greenish-gray sandy limestone. 

A chert-pebble conglomerate is at the base. The sandstones, 

many of which are pink grits, are fine to coarse grained; some 

of the sandstones are conglomeratic. Most of the siltstones and 

shales are micaceous, and some of the shales are fossiliferous. 

Thin black carbonaceous shale is interbedded with dark, dense 

limestone near the base. The limestones are dense, but locally 

may be gnarly and contain siliceous concretions; many of the 

limestones are fossil bearing. Sample no. 6 (table 3) from the 

Molas Pass area, about 4 mi south of Silverton, is an example 

of dense sandy limestone from the lower part of the formation. 

The limestones occur in beds that typically are 1 to 5 ft thick 

but that may be as much as 25 ft thick.

Pink to red grit, sandstone, siltstone, and sandy shale, 

in 50- to 80-ft-thick massive beds, constitute the 700-ft-thick 

middle part of the Hermosa Formation, and are interbedded 

with thin red and gray, fossiliferous shale and gnarly limestone. Many of the sandstones are feldspathic and conglomeratic, and some of the shales are very micaceous.

The thin- to thick-bedded upper part of the Hermosa is 

composed of about 300 ft of pink to red arkosic coarse-grained 

sandstone and conglomerate containing intercalated shale and 

limestone. The shale is gray to red, micaceous, and generally 

silty or sandy; the limestone is gray, dense, and fossiliferous. 

The upper part of the Hermosa Formation more closely resembles lithologically the overlying Cutler Formation than the 

underlying lower and middle parts of the Hermosa in having 

more conglomeratic lenses and a greater content of feldspathic 

material in the sandstones. However, the shales in this upper 

part are not sandy like the shales in the immediately overlying 

beds of the Cutler. The upper part also contains fossil-bearing 

beds that have a greater proportion of mollusks to brachiopods 

as compared to fauna in the lower and middle parts of the 

formation. A similar ratio of mollusks to brachiopods is typical 

of the Rico Formation of Pennsylvanian age, a transitional 

unit between marine Hermosa below and continental Cutler 

above, as mapped in the Rico area about 25 mi to the southwest (McKnight, 1974). Comparison of the fauna from the 

two areas found that each contained a considerable number of 

species in common, but also that each had a considerable number of species peculiar to itself (Burbank, 1930, p. 165). The 

similar faunal ratio and the similar lithology suggest, however, 

that the upper part of the Hermosa Formation at Ouray may be 

an approximate equivalent of the Rico Formation.

The Hermosa Formation apparently grades into the conformable overlying Cutler Formation. The upper contact was 

placed arbitrarily about 130 ft above the uppermost fossiliferous horizon where the succeeding beds are unfossiliferous and 

correspond more closely in lithologic character to the red beds 

in the thick Cutler Formation above. There is little assurance 

that the contact is everywhere at the same horizon.

In the vicinity of Ouray the Hermosa Formation does not 

contain replacement deposits of the blanket type. Some thin 

limestones and associated black limy shales, however, contain 

small pyritic replacement deposits along some mineralized 

fractures or veins near the stock in The Blowout. These deposits have not proved commercially valuable except where they 

were later enriched by late gold-bearing solutions that formed 

small ore shoots. The lack of large replacement deposits in 

this formation is due either to unfavorable lithic characteristics 

of its beds or to some unfavorable structural conditions. It is 

significant, however, that the major massive-sulfide replacement ore deposits characteristic of the middle Hermosa in the 

Rico mining district are not present at Ouray (Ransome, 1901; 

McKnight, 1974).

Cutler Formation

The Cutler Formation of Early Permian age was named for 

the red beds that crop out at the mouth of Cutler Creek (Cross 

and others, 1907, p. 5) and along the Uncompahgre River valley 

from the town of Ouray northward. The formation crops out 

typically as benches and cliffs because of the variation in resistance of the beds to erosion. It consists of thin to massive beds 

that are commonly lenticular and exhibit distinct vertical and 

lateral gradations in lithology; many of the beds are cross-stratified and show cut-and-fill deposition. For descriptive purposes 

only, this formation is divided into two parts.

The lower part consists of interbedded shale, siltstone, 

and sandstone. The rocks are generally pink, red, and reddish 

brown, but a few are gray. Selective bleaching has altered 

the color of some beds to white, yellow, or green, and has 

imparted a mottled appearance to others. The shales and 

siltstones are calcareous, sandy, and micaceous, and some 

shale contains limestone nodules. The sandstones are micaceous and arkosic and are fine to very coarse grained; many 

are slightly to very conglomeratic, containing rounded pebbles 

and cobbles as much as 4 in. in diameter. These fragments 

were derived from all the underlying rocks, but fragments 

of Precambrian granite, gneiss, schist, slate, and quartzite 

16 Geology and Ore Deposits of the Uncompahgre Mining District, Southwestern Colorado
predominate. Conglomeratic beds increase in number upward. 

The thickest measurement obtained in the area for the lower 

part of the formation was about 1,450 ft.

The upper part of the Cutler Formation is sandstone that 

contains subordinate shale and mudstone, as well as several 

very coarse conglomeratic beds as much as 40 ft thick. These 

conglomeratic beds, which contain very little interstitial 

material, are composed of well-rounded pebbles, cobbles, and 

boulders that are as much as 10 in. in diameter but which average about 4 in. Some conglomerates consist of predominantly 

metamorphic and igneous rock fragments, whereas others 

consist mainly of limestone fragments. Especially in the upper 

part of the formation, beds cannot be traced laterally for any 

great distance, in part because of the gradation between rock 

types and in part because of slope cover. The thickest measurement obtained in the area for the upper part of the formation 

was about 550 ft.

The upper contact of the Cutler Formation with the 

overlying Dolores Formation is everywhere an angular 

unconformity; this angular contact is particularly evident on 

the north wall of The Amphitheater (fig. 6) where the Dolores 

Formation transgresses the Cutler Formation to lie on beds of 

the Hermosa Formation. Therefore the total thickness of the 

Cutler Formation ranges from zero to about 2,000 ft.

Very few beds in the Cutler Formation were favorable to 

mineral deposition. Most of the sandstones consist of poorly 

sorted grains and are too shaly or micaceous to be permeable. 

A few thick, presumably more permeable, sandstone beds 

near the center of mineralization (in the area of The Blowout) 

contain small gold-bearing replacement veins where the ore 

shoots were restricted mainly in the sandstone walls. The same 

veins in the more shaly beds commonly had little or no value.

Mesozoic Sedimentary Rocks

Sedimentary rocks of Mesozoic age constitute only a 

small part of the rocks exposed within the district (fig. 4), but 

account for a major proportion of the district’s ore deposits. 

The strata have been subjected to folding and faulting both 

separately as an age group or together with the underlying 

Stratigraphy 17

Figure 6. View looking northeast from Hayden Mountain at cliffs on the north side of The Amphitheater, east of the town of Ouray, 

showing the angular unconformity (marked by the long-dashed line) at the base of the nearly horizontal Mesozoic beds overlying 

inclined northwest-dipping Paleozoic strata. The back of The Amphitheater (to the right) and the upper slopes behind are Tertiary 

volcaniclastic rocks; its floor in the foreground and mostly to the right of the road (bottom center) is covered by glacial and landslide 

debris. The light-colored, somewhat circular area in the next gulch to the north, known as The Blowout (outlined by short-dashed 

curved line), consists of extremely altered Paleozoic and Mesozoic sedimentary rocks surrounding an intruded, partly altered igneous 

stock; the top of the altered rocks and the base of overlying unaltered Tertiary volcanic rocks (indicated by the straight segment of the 

short-dashed line) was a factor in recognizing and defining this alteration-and-mineralization episode (related to the Laramide orogeny) 

within the Uncompahgre mining district.
Paleozoic units. The entire stratigraphic section was eroded 

extensively prior to deposition of the Cenozoic rocks. The 

Mesozoic rocks, of Triassic through Cretaceous age, total 

more than 2,000 ft in thickness and are composed dominantly 

of terrestrial clastic rocks.

Dolores Formation

The Dolores Formation of Late Triassic age generally 

crops out in steep rubble-covered slopes or cliffs and benches 

along the west side of the Canyon Creek and Uncompahgre 

River valleys as well as along the east side of the Uncompahgre 

and its tributary canyons from The Amphitheater northward. 

The formation is extremely variable in rock type both horizontally and vertically and consists predominantly of sandstone 

and siltstone and minor amounts of mudstone, limestone, and 

conglomerate. For the most part it consists of thin to massive 

even beds, but in places it is irregularly bedded, lenticular, and 

cross-stratified; locally it shows cut-and-fill structures. The 

upper one-half to two-thirds of the formation generally is thin 

bedded, whereas the lower part consists of thick and massive 

beds; the latter commonly weathers to a knobby surface and 

contains hackly or subconchoidal fractures. Poorly preserved 

bone, teeth, and plant fragments have been found locally.

The sandstone is very fine to coarse grained and contains 

mudstone partings and pebble-size conglomerate streaks. 

Laminated thin beds of sandstone and siltstone are generally 

reddish brown, but thick or massive beds are often mottled 

red and white. The mudstone, in places fissile, is red, reddish brown, green, and yellow. Everywhere at the base of 

the formation is a pebble conglomerate ranging in thickness 

from 6 in. to 8 ft. At most places this conglomerate is white, 

pinkish white, or light gray, but locally, near Ouray, it is reddish brown. The subangular to rounded pebbles are set in a 

calcareous sandy matrix and consist of quartz, quartzite, chert, 

mudstone, and limestone as much as 2 in. across. A few red, 

white, or gray dense or silty limestone and limestone-pebble 

conglomerate beds, averaging about 1 ft thick, occur in the 

middle and upper parts of the formation. Pebbles of the limestone and calcareous mudstone, often slightly flattened, are 

as much as an inch across; a few quartz and quartzite pebbles 

occur sporadically. Locally, the uppermost 2 or 3 ft of the 

formation is altered greenish gray and yellow. Alternating thin 

beds of mottled, limy mudstone and fine-grained sandstone 

occur within a scour at the top of the formation, west of Black 

Lake at the northwest edge of the mining district (fig. 4). 

These beds are similar to and are believed to be an erosional 

remnant of formerly extensive beds more characteristic of the 

Chinle Formation and the Wingate Sandstone, both of Late 

Triassic age, in the Colorado Plateau region to the west.

As previously stated, the base of the Dolores Formation 

is marked by a very important angular unconformity. At the 

east side of The Amphitheater the Dolores rests directly upon 

the Hermosa; contact with older formations was not seen at 

that locality, but structural conditions suggest that the Dolores 

possibly may have lain across the entire section of Paleozoic 

beds. The contact with the overlying Entrada Sandstone is 

an erosional unconformity. Erosion preceding and following 

deposition of the Dolores, as well as possibly intra-formational 

erosion, accounts for the variance in thickness of the formation, which ranges from 40 to 130 ft and averages about 80 ft.

Few beds in the Dolores Formation appeared favorable to 

mineral deposition. Locally some beds do contain minor vein 

deposits.

Entrada Sandstone

The white to buff Entrada Sandstone of Middle Jurassic 

age crops out in a steep to rounded cliff along the Uncompahgre River valley walls. It is thick bedded to massive and typically characterized by sweeping crossbeds. At a few places, 

however, the upper part is evenly bedded.

The friable and calcareous sandstone is fine grained and 

consists of rounded clear and frosted quartz grains. Characteristic of and scattered throughout the sandstone are larger single, 

rounded, frosted quartz grains (so-called “Entrada berries”) 

averaging 1 mm in diameter. Red angular fragments of the 

underlying Dolores Formation are locally incorporated near the 

base of the sandstone. Thin streaks of very fine-laminated black 

shale or mudstone are common near the top of the unit.

The Entrada Sandstone ranges from 45 to 80 ft in 

thickness. The difference in thickness primarily is due to 

the erosional unconformity at its base. The lower and upper 

contacts are sharp because of the contrasting composition of 

the Entrada and the underlying and overlying formations. To 

all appearances, the upper contact is even and in a few places 

appears to grade into the overlying formation, but in other 

places it is somewhat irregular.

The Entrada Sandstone is made up mostly of windblown 

sand of relatively fine grain size and consequently is not 

highly permeable; the formation was not a favorable host to 

ore deposition. However, locally there are a few vein deposits 

of minor value.

Wanakah Formation

The Wanakah Formation of Middle Jurassic age crops out 

along the walls of the Uncompahgre River valley and generally forms a cliff in the lower part and a mostly covered gentle 

to steep slope, with some ledges, in the upper part.

Named for exposures at the Wanakah mine north of 

Ouray (Burbank, 1930, p. 172), the Wanakah Formation consists of three distinct units: a lower limestone and breccia unit 

(Pony Express Limestone Member), a middle sandstone unit 

(Bilk Creek Sandstone Member), and an upper limy mudstone 

and siltstone unit (unnamed marl member). The unnamed 

upper unit, generally referred to as the “marl” member (Bush 

and others, 1959; Vhay, 1962; Bromfield, 1967), is now informally called “beds at Sawpit” (O’Sullivan, 1992, p. 11). The 

formation ranges from 85 to 125 ft in thickness.

18 Geology and Ore Deposits of the Uncompahgre Mining District, Southwestern Colorado
Pony Express Limestone Member

The Pony Express Limestone Member, known from its 

occurrence at the Pony Express mine, consists generally of 

two parts: a lower limestone and limy shale and an upper, 

not always present, calcareous shaly breccia, hereinafter 

referred to as limestone breccia. At only a few localities 

along the Uncompahgre River valley north of the district, 

the Pony Express consists of a lower limestone and shale, a 

middle thin breccia, and an upper shaly mudstone containing interstitial gypsum nodules. Underground in some mines, 

the Pony Express consists entirely of thin layers of shaly 

sandstone.

The lower part of the member, ranging from 1 to 5 ft in 

thickness, is predominantly laminated to thin-bedded, darkgray to black, dense limestone interbedded with black bituminous shale. At one locality the lower part consists of a single 

limestone bed about 1 ft thick. The limestone beds are contorted, crumpled, or crinkled and somewhat broken (pseudobreccia) but healed by coarse crystalline calcite or by white 

fibrous gypsum. A freshly broken exposure of the limestone 

emits a petroliferous odor. This lower part is present wherever 

the Pony Express is exposed.

The upper part ranges from zero to 20 ft thick and usually 

consists of a very porous breccia composed of grayish-black, 

very thin but angular shale and limestone fragments cemented 

by calcium carbonate. The contact between the Pony Express 

Limestone Member and the overlying member is commonly 

sharp and conformable, but at a few places appears to be 

uneven.

A few localities on both the west and east sides of the 

Uncompahgre River valley north of the mining district are particularly instructive as to the origin of the breccia in the upper 

part. At these localities, the upper part contains somewhat 

flattened, oval to round white gypsum nodules surrounded by 

thin layers of dark calcareous mudstone. This part may reach a 

maximum of 70 ft in thickness, but laterally thins rapidly to a 

porous breccia. Upon dissolution of the gypsum by circulating 

ground waters, the layers of mudstone sagged, broke into fragments or small plates, and were gradually compacted to form 

the porous breccia of a much lesser thickness (fig. 7).

Dissolution of most of the gypsum probably occurred in 

Late Jurassic time; several sections measured near one of the 

gypsum localities indicate that the overlying Bilk Creek Sandstone Member is of uniform thickness whereas the uppermost 

part of the Wanakah Formation and lower part of the Morrison 

Formation are thinned and arched slightly over the remaining 

gypsiferous mass.

The Pony Express Limestone Member, including all the 

beds, was known locally to the miners as the “Pony Express 

beds,” “Pony Express contact,” or “Iron Clad contact;” it was 

an important ore-bearing horizon in the sedimentary sequence 

and of considerable economic interest. Where cut by veins, 

the breccia was mineralized, and bedded deposits were formed 

along many of the ore channels throughout the district and in 

adjacent areas particularly to the north and west.

Bilk Creek Sandstone Member

The Bilk Creek Sandstone Member, named by Goldman 

and Spencer (1941, p. 1750) for exposures at Bilk Creek to 

the west, is rarely well exposed. It is a greenish-gray to buff, 

silty, very fine- to fine-grained, calcareous sandstone and is 

very soft and friable. In a few of the better exposures, some 

thin greenish-gray shale partings were seen in the upper part. 

The beds are mostly thin to thick and planar but locally are 

somewhat irregular and lenticular. This member ranges from 

14 to 25 ft in thickness.

The uppermost part of the Bilk Creek is marked by a 

1-ft-thick, comparatively hard and resistant, greenish-gray, 

medium-grained sandstone bed containing grains of jasper or 

red chert. This bed is the “carnelian sandstone” described in 

neighboring areas (Goldman and Spencer, 1941). O’Sullivan 

(1992, p. 10) suggested this sandstone marker bed could be 

a reworked zone and should be considered the basal part of 

the overlying strata, but he retained it (fig. 3B; p. 22) as the 

capping, uppermost part of the Bilk Creek Sandstone Member. 

The upper contact of this member where seen is sharp and 

even.

Although not of major importance, the sandstone in 

the Bilk Creek is generally mineralized where the underlying Pony Express Limestone Member is mineralized. Ore 

deposits in the sandstone exist both as fracture fillings and as 

bedded replacement types. The latter often were associated 

with deflections in the dip of a vein, locally known as “rolls” 

in a vein, where the vein broke through the underlying Pony 

Express.

Unnamed Upper Member

The upper unit, or marl member, of the Wanakah Formation is lithologically not marl, but consists of irregular, 

thin-bedded, reddish-brown calcareous mudstone and siltstone 

ranging from 40 to 75 ft in thickness. Interbedded with the 

mudstone and siltstone are a few thin, lenticular, gray and 

brown, very fine-grained sandstone and nodular limestone 

beds. The limestone is finely crystalline, locally dolomitic, 

and commonly vuggy. The sandstone, locally, and a very silty 

limestone near the top of the member contain grains of jasper 

and a green chert or chalcedony. Stratigraphic and lateral gradations are common among the lithic types that throughout the 

unit are laced with seams and stringers of crystalline calcite. 

Typically the uppermost 1 to 2 ft is bleached olive to yellowish gray. The upper contact everywhere is sharp and apparently flat, but locally is irregular, having a few inches of relief. 

This member is commonly void of mineralization.

Morrison Formation

The Morrison Formation of Late Jurassic age crops out in 

cliffs, ledges, and steep slopes high along the walls of Canyon 

Creek and the Uncompahgre River valley. Although mapped 

as a single unit, the formation within the district was recogStratigraphy 19
Figure 7. Generalized 

sketch of two nearby 

separate measured sections 

of the Wanakah Formation 

high on the east side of the 

Uncompahgre River valley, 

about 1.5 mi north of the mining 

district in the vicinity of Ouray’s 

Cedar Hill Cemetery (see fig. 

4), where the thicknesses 

and compositions of the Pony 

Express Limestone Member 

of the Wanakah Formation 

are exposed. The thicker 

section, representing the unit 

as originally deposited and 

containing gypsum nodules, is 

compared with the common, 

normally exposed thinner 

section consisting of porous, 

angular breccia resulting from 

the dissolution of the gypsum 

and the subsequent collapse, 

breakage, and cementation of 

the thin calcareous mudstone 

sheaths that had surrounded 

individual gypsum nodules.

nized to consist of two members, the lower Salt Wash Member 

and the upper Brushy Basin Member (Burbank, 1930; Craig 

and others, 1955; Luedke and Burbank, 1962, 1981). The 

contact between the members is neither sharply defined nor 

everywhere at the same horizon. Subsequent to these studies, 

O’Sullivan (1992) recognized and correlated a third member 

within the Morrison Formation throughout western Colorado 

and eastern Utah. He subdivided the Salt Wash Member by 

restricting the upper part to the Salt Wash Member and the 

lower part to the Tidwell Member of the Morrison Formation.

As a whole, the Morrison Formation consists of mainly 

sandstone and some interbedded mudstone in the lower part, 

and mostly mudstone and siltstone containing minor amounts 

of sandstone in the upper part; also, throughout the formation 

are minor amounts of limestone and conglomeratic sandstone. 

The character of the beds varies laterally, with few of the beds 

persistent for any distance other than the prominent basal 

sandstone. The Morrison Formation ranges from 620 to 750 ft 

in thickness; it averages about 700 ft thick.

A few ostracodes (Steele, 1985, p. 12) and other poorly 

preserved and indeterminable invertebrate fossils have been 

found in limestones near the base, and, in addition, fragmentary plant and bone specimens were found in mudstones and 

sandstones stratigraphically higher in the formation. A fossilized dinosaur footprint is exposed in the prominent basal sandstone near the boarding house of the American Nettie mine.

Other than the basal sandstone unit, most of the Morrison 

Formation consists of relatively impermeable rock types and 

was not mineralized except where some veins cutting through 

sandstone layers formed small, locally productive deposits.

Tidwell Member

The Tidwell Member is 49 to 72 ft thick and consists of 

a prominent light-colored, ledge-forming sandstone unit at the 

20 Geology and Ore Deposits of the Uncompahgre Mining District, Southwestern Colorado
base that is overlain by slope-forming red and gray siltstone 

and limestone interbedded with some mudstone and sandstone. The dense to crystalline limestone beds, averaging 1 to 

1.5 ft thick, are not everywhere present, nor are they persistent 

laterally.

The resistant, mostly evenly bedded to massive quartzitic 

sandstone at the base, “bed A” of O’Sullivan (1992, p. 12), 

here ranges from 14 to 29 ft in thickness and averages 20 ft. 

It is yellowish white to buff, usually speckled with limonite, 

and locally stained by desert varnish. The sandstone consists 

of fine to medium, subangular to well-rounded, frosted and 

clear quartz grains cemented generally by carbonate or, near 

intrusive and mineralized zones, by silica. Because of its 

original permeability and susceptibility to mineralization, the 

sandstone has been altered to quartzite in the central part of 

the district. This sandstone unit, an important horizon marker 

in the stratigraphic section, contains a large number of small 

bedded ore deposits and was known locally to the miners 

as the “lower quartzite” to distinguish it from the “upper 

quartzite” (Dakota Sandstone of Cretaceous age) about 700 ft 

stratigraphically above. Locally the unit is capped by a limy 

mudstone or limestone bed seldom as much as 3 ft thick; this 

limestone bed, which has been replaced and mineralized in 

and near the Wanakah mine, was known to the miners as the 

“Bright Diamond contact” or “upper contact” in that area.

Salt Wash Member

The Salt Wash Member consists predominantly of 

light-colored, fine- to medium-grained sandstone interbedded with red and gray mudstone. The sandstones are yellowish white, yellowish brown, buff, and pink, and locally are 

speckled with limonite; at places, the surfaces are stained by 

bluish-black desert varnish. Composed mainly of quartz but 

including some chert and feldspar, the subangular to wellrounded grains are cemented by calcareous or siliceous material. The sandstones form lenticular thin to thick beds that 

pinch and swell along strike. Bedding locally is accentuated 

by included flakes or semi-flattened pellets of mudstone. 

Crossbedding is common in individual sandstone beds, and 

occasionally ripple marks are seen. The tops of individual 

sandstone beds are gradational, whereas the bottoms are typically sharp and irregular. Joint surfaces are commonly coated 

with carbonate. A few streaks or thin lenses of pebble-size 

conglomerate occur throughout the sandstones. Interstratified with the sandstones are a few sandy shale partings and 

thin- to thick-lensing beds of gray, red, green, and variegated 

sandy and silty mudstone.

The Salt Wash Member ranges from about 225 to 400 ft 

in thickness. The number and thickness of mudstone layers 

increase upward everywhere in the formation, thus making 

it difficult to pick a contact between this and the overlying 

Brushy Basin Member. Although not shown on the map, this 

arbitrary contact, not always at the same horizon, was usually 

placed at the top of a massive cliff-forming sandstone 20 to 30 

ft thick wherever that sandstone was present.

Brushy Basin Member

The Brushy Basin Member consists predominantly of 

variegated, generally calcareous, well-indurated mudstone 

and subordinate amounts of sandstone and limestone. The 

calcareous sandstone and silty limestone are similar to those 

described in the Tidwell Member.

The Brushy Basin Member averages about 275 ft in 

thickness. Although generally covered by slide rock, the contact between this member and the overlying Dakota Sandstone 

locally appears sharp and conformable. However, the contact 

is undoubtedly an erosional unconformity as indicated by 

ancient scours—as much as 25 ft deep and 100 ft wide and 

filled with yellowish-brown sandstone and conglomerate—

that are exposed in the west wall of the Uncompahgre River 

valley northwest of the district, and that are interpreted to be 

erosional remnants of the Burro Canyon Formation of Early 

Cretaceous age, found in the Colorado Plateau region to the 

west.

Dakota Sandstone

The Dakota Sandstone of Early(?) and Late Cretaceous age forms prominent cliffs high on the walls of the 

Uncompahgre River valley and tributary valleys, and ranges 

from about 140 to 175 ft in thickness except where extensively 

eroded in early Tertiary time. It consists locally of one to three 

different lithologic units throughout the area, each unit having 

differing physiographic expression and varying compositions 

of quartzite and shale layers (fig. 8). The quartzose sandstones 

within the Dakota are commonly well indurated so that the 

term quartzite is locally appropriate, especially in the vicinity 

of metalliferous deposits.

The lower unit of the Dakota Sandstone consists of interbedded quartzite and shale above a basal chert-pebble conglomerate. This basal bed and some overlying shale beds may 

be absent locally or may not be recognized because of lateral 

facies changes, nondeposition, or intraformational erosion.

The middle unit of the Dakota, consisting of thick permeable quartzite, some quartzitic sandstone, and a few shale 

partings of variable thickness, is persistent throughout the 

area. Quartzite in the middle unit occurs as thin to massive 

lenticular beds that have both graded bedding and torrential 

cross-stratification, and contains some 2- to 4-in.-thick partings of mudstone and shale, and lenses or streaks of pebble 

conglomerate. These beds are gray, yellowish tan, white, 

and buff, and commonly are freckled by limonite specks 

or stained by bluish-black desert varnish. The sandstone is 

quartzose, fine to coarse grained, and consists mainly of 

clear and etched subangular to well-rounded grains mostly 

cemented by silica but locally by clayey or calcareous material. Shale in the middle unit is gray to black, thin bedded, 

somewhat calcareous, and very carbonaceous; also, the 

proportion of shale to quartzite increases upward in this unit. 

Pebbles in the conglomerate lenses consist of chert, quartz, 

quartzite, and clay, and may be as much as 1 in. in diameter 

Stratigraphy 21
22 Geology and Ore Deposits of the Uncompahgre Mining District, Southwestern Colorado

Figure 8. Generalized 

diagrams showing lithic 

variations in the Dakota 

Sandstone and their respective 

topographic expressions. 

See figure 4 for approximate 

locations of sections. A, Mill 

Creek section in the valley 

of Dexter Creek; B, Gold Hill 

section north of Ouray; C, 

Cascade Creek section east 

of Ouray and north of The 

Amphitheater. Km, Mancos 

Shale (Upper Cretaceous); Kd, 

Dakota Sandstone (Upper and 

Lower (?) Cretaceous); Jm, 

Morrison Formation (Upper 

Jurassic).

but usually average 0.25 in. The formation contains a few 

carbonized plant remains.

The upper unit of the Dakota consists of gray carbonaceous shale interbedded with dark-colored quartzite and is 

compositionally similar to the lower unit. This upper unit is not 

everywhere present because of erosion in early Tertiary time.

The contact between the Dakota Sandstone and the 

overlying Mancos Shale is poorly exposed. It is conformable 

and gradational except near Ouray, where the Mancos appears 

to lie unconformably on the Dakota. Although not everywhere 

at the same horizon, this contact was arbitrarily picked either 

at the top of the uppermost prominent quartzite bed or at the 

change from carbonaceous shale to calcareous shale.

The Dakota Sandstone was known to the miners as the 

“upper quartzite.” Several important ore-bearing horizons or 

“contacts” occur within the formation. As previously stated, 

near mineralized zones the sandstone was converted to quartzite, and, near the larger ore channels, beds were completely 
recrystallized, hydrothermally leached, and replaced by rich 

ore in the resulting cave-like open spaces.

Mancos Shale

Small erosional remnants of Mancos Shale of Late Cretaceous age are exposed within the district in small patches 

high on the cliffs west of Ouray and between The Blowout 

northward into the canyon of Dexter Creek. Most of the Mancos and younger formations of Cretaceous age that originally 

covered the area were removed by erosion probably during the 

later phases of the Laramide orogeny. Within the district, the 

Mancos ranges from zero to about 325 ft in thickness, but in 

the ridges to the west and northwest, it is about 2,000 ft thick.

The Mancos consists dominantly of marine, dark-gray 

to black, fissile shale and platy mudstone and contains a few 

thin buff sandstone and gray limestone lenses in the lower part. 

Locally the shale is sandy, carbonaceous, calcareous, or a knobby 

aggregate of shale concretions. In places, the shale has seams and 

crosscutting veinlets of coarsely crystalline calcite as much as 0.5 

in. thick. Near igneous intrusive bodies, the shale has been metamorphosed to argillite and slate, and may be impregnated with 

pyrite. Limy parts of the Mancos contain numerous invertebrate 

fossils, and the basal sandy shale contains many shark teeth.

Shale in the lower part of the Mancos, and possibly 

shale in the underlying upper part of the Dakota, where present, forms the cap that essentially marks the upper limit of 

mineralized sedimentary beds in the district. This horizon in 

the section also had considerable influence upon the igneous 

intrusive activity related to the Laramide orogeny, because at 

or near this horizon the magma intruded outward to form sills 

and laccoliths away from the intrusive center in The Blowout. 

The blanketing effect of the shale is clearly reflected in the 

character of the ore deposits related to that intrusive activity.

Tertiary Rocks

Rocks of Tertiary age throughout the district rest upon a 

profound erosional surface and, except for limited outcrops 

of an early Tertiary sedimentary unit, consist principally of a 

thick section of extrusive rocks. There are also a few intrusive 

dikes of mid-Tertiary age (fig. 4).

The volcanic rocks within the district are part of the 

so-called early intermediate-composition volcanic rocks 

(Steven and Hail, 1989) as mapped throughout much of the 

western San Juan Mountains. These rocks are composed of 

and characteristic of outflow rocks (volcaniclastic facies) that 

accumulated as coalescing marginal aprons of numerous scattered stratovolcanoes rather than as the near-source rocks (vent 

facies). Extensive younger Tertiary lava flows and welded 

ash-flow tuffs occur to the west, south, and east of the district 

but are not pertinent to the discussion herein.

There are abundant Pleistocene and Holocene surficial 

deposits of considerable extent and variety throughout the 

district.

Telluride Conglomerate

The Telluride Conglomerate of Eocene age (Steven and 

Hail, 1989) is exposed within the Uncompahgre district only 

on Hayden Mountain south of the town of Ouray, at the back 

of The Amphitheater, and at two localities high on the canyon 

wall southeast of Ouray. These few scattered remnants are 

preserved in shallow depressions on the underlying erosional 

surface. The formation formerly overlay a broad erosional 

surface that transgressed all older metamorphic, sedimentary, 

and igneous rocks and covered or lapped onto hills formed by 

the more resistant granodioritic intrusive bodies that existed on 

the surface in the current vicinity of Ouray. Post-depositional 

erosion of the formation accounts for the paucity of outcrops. 

Known to have a thickness in excess of 200 ft elsewhere in the 

western San Juan Mountains, it here is less than 50 ft thick.

In general, the Telluride is composed of gray to red, 

well-indurated conglomerate having a silty to sandy matrix. 

The rounded cobbles and boulders of the conglomerate vary 

in size but have a maximum diameter of about 1 ft. They 

consist of metamorphic and igneous rocks mostly from the 

Needle Mountains area to the south (fig. 1); they also consist 

of numerous angular to rounded fragments of granodiorite 

derived locally. Fragments of some of the more resistant sedimentary rocks are commonly incorporated within the lower 

few feet of the formation. One of the two mentioned small 

outcrops on the east side of the Uncompahgre River canyon 

southeast of Ouray, located east of U.S. Highway 550 and just 

north of Bear Creek (fig. 4), differs from the other exposures 

of the Telluride in consisting of alternating thin beds of very 

limy siltstone and conglomerate composed mainly of small 

plates of slate in a silty matrix.

San Juan Formation

The San Juan Formation of Oligocene age forms steep 

rounded slopes and cliffs and is prominently exposed high on 

the valley walls east and west of the Uncompahgre River. Its 

thickness varies within and near the map area in part because 

of pre- and post-depositional erosion and in part because of 

presumed distances from volcanic sources for the debris. At 

the back of The Amphitheater the formation is more than 

3,000 ft thick. Based upon a few radiometric ages determined 

elsewhere in the western San Juan Mountains region, the 

rocks of dominantly volcanic origin were deposited probably 

between 35 and 30 Ma (Lipman and others, 1970).

Southwest of the map area in figure 4, along the valley of 

Canyon Creek, a twofold division by Burbank (1930) of the 

formation into the lower Canyon Creek Member and the upper 

Sneffels Member can be extended into the map area only with 

some uncertainty. In general, the lower part of the formation 

is from 300 to 1,200 ft thick, comprises the most important 

cliff-forming part of the formation, and differs from the upper 

part in being a more heterogeneous mixture of volcanic debris 

with interbedded conglomerate beds; the conglomeratic layers 

are characterized by containing distinctly rounded, water-worn 

Stratigraphy 23
volcanic fragments that impart a crude layering or bedding to 

the formation when viewed from a distance. The lower hundred feet also includes foreign fragments of pre-Tertiary rocks 

similar to those in the underlying Telluride Conglomerate. The 

upper part of the formation is about 1,500 ft thick, is more 

uniform in textural composition (more finely porphyritic lava 

containing numerous feldspar phenocrysts), and with fewer 

conglomeratic beds than in the lower part.

The San Juan Formation has been propylitically altered 

and thus is characteristically gray to greenish gray, grading 

locally to shades of red and purple depending on the degree of 

alteration. The formation consists of a chaotic accumulation 

of moderately indurated, mostly water-reworked sandy tuffs 

or air-fall ash beds, tuff conglomerates, tuff breccias (fig. 9), 

some agglomerates, flow breccias, and a few lava flows. The 

fragments range in size from microscopic to blocks several 

inches, locally a foot or more, across. Mudflow (lahar) deposition probably accounts for the major part of the pyroclastic 

detritus constituting the San Juan Formation where exposed in 

the district. Sources of the debris are not fully established, but 

most of it within the district probably came from a volcano or 

cluster of volcanoes to the east or south.

Volcanic rocks in the San Juan Formation are predominantly intermediate in composition (table 5), as classified 

according to the International Union of Geological Sciences (IUGS) system (Le Bas and others, 1986), and range 

in composition from basaltic andesite to dacite and trachyte 

(fig. 10). Most of the available chemical rock analyses (table 

5) show that the composition of the volcaniclastic rocks 

clusters on the boundary of the trachyandesite-andesite 

fields, and that the rocks are dominantly subalkaline according to Irvine and Baragar (1971). The two samples that plot 

within the basaltic andesite field are from lava flows. Most 

of the rock fragments are finely porphyritic; the phenocrysts 

average 1 mm in size, and consist of plagioclase, hornblende, 

pyroxene, and sparse biotite in an aphanitic groundmass. 

Labradorite or andesine phenocrysts predominate; they are 

euhedral to anhedral and commonly zoned and twinned, and 

have an anorthite content averaging about An40. Hornblende 

phenocrysts are more common than either those of pyroxene 

or biotite; however, pyroxene is the principal ferromagnesian 

mineral in the lava flows. The groundmass is cryptocrystalline, fine-grained or felted, and consists of quartz, potassic 

feldspar or sodic plagioclase, apatite, and specks of iron 

ore; where felted, the microlites are sanidine. Relict perlitic 

structure in the fine-grained felsitic mixtures of quartz and 

orthoclase indicates that some of the rocks were originally 

glassy. The mafic minerals typically are altered, whereas the 

plagioclase may or may not be. Alteration products are carbonate minerals, chlorite, epidote, sericite, clay, secondary 

silica, iron oxides, and pyrite. In addition to the formation 

having been weakly to moderately propylitized throughout 

24 Geology and Ore Deposits of the Uncompahgre Mining District, Southwestern Colorado

Figure 9. Photograph of 

polished rock slab of tuff 

breccia from the San Juan 

Formation showing the typical 

chaotic texture of subrounded 

to angular, porphyritic 

volcanic rock fragments in 

a dense matrix of sandy tuff 

and pulverized volcanic rock. 

Sample RM39–133 from Red 

Mountain district south of the 

Uncompahgre mining district.
Table 5. Chemical analyses of volcanic rocks from the San Juan Formation near the Uncompahgre mining district, southwestern Colorado.

[Analyses by U.S. Geological Survey. Major-oxide data, in weight percent, are from rapid-rock methods by P.D.L. Elmore, S. Botts, G. Chloe, L. Artis, J. Glenn, 

H. Smith, and J. Kelsey. Trace-element data, in parts per million, are from semiquantitative spectrographic methods by H.C. Worthing, J.C. Hamilton, and J.L. 

Harris. N, not detected, at limit of detection; L, detected, but below limit of determination; --, no data]

 Lava flow Tuff breccia

Sample no. 1 2 3 4 5 6 7 8 9 10 11 12 13 14

Field no. 58I51 60I45 67W1 70W2 63T4 66W25 66W22 66C2 59SJ5 63SJ123 63SJ124 66SJ8 66SJ2 63SJ125

Lab no. W157572 W157673 W169359 W175091 W174233 W175608 W175607 W175592 W157570 W174219 W174220 W175594 W175593 W174221

SiO2 52.80 52.80 61.90 61.00 59.20 61.40 57.30 55.50 59.60 61.90 59.00 60.40 59.90 62.00

Al2O3 16.10 15.70 16.40 17.10 17.40 17.40 17.60 16.70 16.60 16.50 17.30 17.70 16.40 17.00

Fe2O3 5.60 4.30 3.30 3.00 5.20 3.30 7.00 5.60 5.30 3.40 5.30 4.40 4.30 5.80

FeO 4.20 6.10 1.70 1.90 2.00 2.10 0.76 1.60 0.62 1.50 0.96 1.60 2.50 0.60

MgO 4.10 3.60 0.73 1.80 0.90 1.80 1.50 3.80 1.70 1.70 1.90 0.93 2.70 0.92

CaO 8.20 8.60 4.90 5.10 3.20 4.60 5.70 5.00 5.10 4.60 5.90 0.35 5.80 4.50

Na2O 2.50 2.30 3.70 3.60 4.00 3.80 3.90 3.30 3.10 2.60 3.50 3.80 3.30 3.60

K2O 1.80 0.97 3.00 3.20 2.20 3.20 2.60 3.10 2.50 3.50 3.00 5.10 1.40 3.10

H2O 2.20 2.30 1.74 2.21 2.50 1.45 2.30 3.20 3.70 3.19 2.00 1.45 2.53 1.10

TiO2 1.20 1.10 0.60 0.65 1.00 0.61 0.83 0.74 0.65 0.56 0.73 0.68 0.85 0.79

P2O5 0.39 0.39 0.24 0.32 0.39 0.27 0.37 0.40 0.32 0.26 0.33 0.26 0.20 0.33

MnO 0.16 0.16 0.07 0.08 0.05 0.08 0.13 0.13 0.08 0.08 0.16 0.08 0.10 0.04

CO2 0.16 2.10 1.50 0.05 2.00 0.05 0.05 0.20 0.00 0.05 0.05 0.05 0.05 0.05

Total 99 100 100 100 100 100 100 99 99 100 100 100 100 100

Powder density 2.80 -- 2.65 2.52 2.76 2.68 2.68 2.68 2.57 2.52 2.60 2.68 2.68 2.64

Bulk density -- -- 2.60 2.53 2.73 2.49 2.48 2.42 -- 2.40 2.58 2.02 2.52 2.43

Ba 700 300 1,000 1,500 1,000 1,000 1,000 1,000 150 1,000 1,000 1,000 1,000 1,000

Be N N 1 N 2 1 N N N 2 2 N N 2

Ce N N 200 200 200 300 100 100 N 300 200 100 100 200

Co 30 30 15 10 10 10 10 15 15 10 15 10 15 15

Cr 30 15 7 30 5 10 10 20 3 10 20 20 20 20

Cu 30 150 30 15 5 15 10 10 15 10 20 10 5 15

Ga 7 7 10 15 15 15 15 15 7 15 15 15 15 15

La 30 N 70 70 70 100 70 70 30 100 70 70 70 70

Mo N N 3 5 3 5 N N N 3 N 3 3 3

Nb 15 N 5 10 10 10 10 7 N 10 10 10 10 7

Ni 15 30 N 10 N 5 3 20 N N N 7 10 L

Pb 15 300 70 7 20 10 5 5 15 10 10 5 7 7

Sc 30 7 7 15 30 10 15 15 7 10 15 10 20 10

Sr 1,500 1,500 700 1,500 1,500 1,500 2,000 2,000 1,500 1,000 1,500 2,000 1,000 1,500

V 300 70 100 100 150 100 100 100 70 50 70 70 150 50

Y 30 7 50 50 30 30 30 30 30 30 30 30 30 30

Yb 3 N 5 5 3 3 3 N 3 3 3 N N 3

Zr 150 30 200 150 150 150 150 150 150 300 150 150 150 200

Sample descriptions and (or) locations:

1. Near Bimetallist mine on southeast slope of Potosi Peak, Ouray County; 37°58’48” N., 107°44’10” W.

2. Bench at north end of Hayden Mountain, altitude 11,637 feet (ft), Ouray County; 37°59’16” N., 107°40’26” W.

3. Altered rock; near head of upper basin of Wildhorse Creek, Ouray County; 38°01’16” N., 107°35’06” W.

4. Valley bottom of West Fork of Cimarron River, Hinsdale County; 38°04’52” N., 107°32’43” W.

5. Altered rock; lower east side of Bridal Veil basin, San Miguel County; 37°54’43” N., 107°46’09” W.

6. Saddle in ridge between Middle and East Forks of Cimarron River, Hinsdale County; 38°04’55” N. 107°30’20” W.

7. Altered rock; ridgetop south of Courthouse Mountain, Hinsdale County; 38°06’51” N., 107°33’42” W.

8. East slope of Cimarron Ridge south of Courthouse Mountain, altitude 11,200 ft, Hinsdale County; 38°08’00” N., 107°34’05” W.

9. Altered rock; east side of Chimney Rock, altitude 11,000 ft, Hinsdale County; 38°08’49” N., 107°34’02” W.

10. Near top of Owl Creek Pass, west side of Cimarron Ridge, Ouray County; 38°09’48” N., 107°34’01” W.

11. Altered rock; West Fork of Cimarron River, Gunnison County; 38°11’22” N., 107°31’36” W.

12. Altered rock; East side of northern Dike Ridge, Gunnison County; 38°12’14” N., 107°31’36” W.

13. West side of northern Dike Ridge, Gunnison County; 38°12’20” N., 107°32’42” W.

14. Altered rock; East Fork of Cimarron River, Gunnison County; 38°13’24” N., 107°30’52” W.

Stratigraphy 25
Figure 10. Total alkalisilica variation diagram, with 

the International Union of 

Geological Sciences (IUGS) 

classification grid (Le Bas and 

others, 1986), showing the 

composition of rock samples 

from the San Juan Formation 

(table 5) and of the Tertiary 

dikes (table 10). Alkalinesubalkaline division is that 

of Irvine and Baragar (1971). 

Major-element oxides were 

recalculated 100 percent 

volatile free. •, San Juan 

Formation; ▲, Tertiary dikes.

the district (although to a lesser degree in the northeastern 

part), it also was sericitized in the southern part.

The profound erosional unconformity at the base of 

the San Juan Formation represents a lengthy time interval 

as suggested by the extreme variation in the exposed thicknesses throughout the area, particularly where it overlies the 

buried laccolithic hills. These relations contribute important 

evidence as to the age and time involving emplacement of 

the granodioritic intrusions. At two localities, one near Twin 

Peaks northwest of Ouray and the other high on the south 

side of Dexter Creek valley, finely comminuted debris of the 

formation are seen draped over and into joint cracks in the 

spheroidally weathered granodiorite that has as much as a 1.5in.-thick weathered rind.

A few veins and associated rock alteration of mid- to late 

Tertiary age occur within the San Juan Formation within the 

district, such as at the Portland mine located in The Amphitheater. The majority of important ore deposits of this age are 

located to the south, southeast, and southwest of the district.

Quaternary Deposits

The Uncompahgre district has extensive and varied 

deposits of Pleistocene and Holocene ages that consist of 

several types of material and several modes of origin. Most of 

the deposits consist of unsorted and unconsolidated fragmental material derived from the Uncompahgre River drainage 

area within a radius of about 6 mi south of the Ouray 7.5-min 

quadrangle (fig. 4).

The Pleistocene Series is represented by landslide deposits, glacial drift, and lakebed deposits. Landslide deposits 

consist of locally derived bedrock materials, and have typical hummocky and broken surfaces and flowage features. All 

gradations exist between small earthflows and larger slumps 

and block slides. The majority of landslides probably occurred 

during Pleistocene time, but slumping and sliding on a minor 

scale have continued to the present.

The isolated masses of glacial drift are considered to be 

of Wisconsin age, and are not further differentiated according to the stages of Atwood and Mather (1932) or Richmond 

(1954) because of inconclusive correlation and insufficient 

data. Most masses of the drift are till, composed of unconsolidated to semiconsolidated, unsorted material ranging in size 

from clay to boulders; however, some masses locally may 

include outwash gravels.

The morphology and development of The Amphitheater 

east of Ouray (fig. 11), which resembles a glacial cirque, 

and of the large surficial mass considered to be glacial drift 

occupying the bottom of the basin, is interpreted to be the 

result of the confluence of two major trunk glaciers where 

the town is now located. One flowed northeastward down the 

valley of Canyon Creek, and the other north down the upper 

Uncompahgre River valley. The angle of impact and the pressure, owing to the massive weight from change in altitude, 

exerted by the glacier from the southwest pushing into the 

26 Geology and Ore Deposits of the Uncompahgre Mining District, Southwestern Colorado
other glacier may have caused a gradual sapping, or plucking, 

of bedrock on the opposite side of the juncture to create the 

bowl shape. The mass of debris within the bowl, subsequent 

to the retreat of the glaciers, was later subjected to considerable surface slumping and sliding; also, some debris from 

the oversteepened slopes slumped into the basin. A landslide 

origin is believed improbable for formation of the basin and its 

contained mass of debris.

Remnants of former lakebeds were found on the lower 

part of the valley walls near the mouth of Dexter Creek. They 

consist of semiconsolidated silt and fine sand that were deposited in a lake formed by the damming of Dexter Creek valley 

by the lateral moraine of the glacier occupying the Uncompahgre River valley.

Talus deposits composed of angular rock fragments of 

varying size form cones, aprons, and a general slope cover commonly at or near the base of many cliffs. With the increase of 

silt and sand, the talus grades into unsorted and unconsolidated 

colluvial deposits on flatter surfaces, and at the mouths of many 

gulches and tributary stream valleys into lobate­shaped alluvial 

fan and cone deposits. Alluvial deposits, found in most stream 

valleys, are extensive in the Uncompahgre River valley.

Several travertine deposits are associated with active hot 

springs within the city limits of Ouray. The principal deposit 

is near Box Canyon and known as the Ouray or Pavillion 

Hot Spring (see figure 21, no. 33); it contains considerable 

amounts of metals, especially iron and manganese (table 6). 

A semiquantitative spectrographic analysis of this manganese 

oxide­rich material indicates that it contains more than 50 

percent manganese and more than 1 percent each of lead and 

tungsten (Hewett and Fleischer, 1960, p. 41). Bastin (1923, p. 

68) cited four water analyses from a hot spring near the old 

zinc concentrator in Ouray that had water temperatures ranging from 132°F to 152°F and a neutral pH; the waters, with 

similar solute compositions consisting dominantly of sodium 

and calcium sulfates and minimal iron and manganese, were 

being deposited as porous masses of gypsum. He concluded 

that those analyses failed to indicate conclusively the origin of 

the spring waters.

Intrusive Rocks

Intrusive activity occurred within and close to a mile or two 

beyond the Uncompahgre mining district (fig. 12) during three 

and probably more distinct periods, ranging in age from possibly 

Proterozoic through late Tertiary. These mostly igneous intrusive 

rocks have a great range in texture and composition, and their 

mode of emplacement forms both concordant and discordant 

bodies that cut rocks in all parts of the stratigraphic section. The 

intrusive rocks of the Late Cretaceous to early Tertiary Laramide 

orogeny are the most important within the mining district relative 

to the emplacement of mineral deposits. Unusual intrusive clastic 

dikes are closely related to the igneous rocks of the Laramide 

orogeny.

Diabase

A Proterozoic(?) diabase dike, which intrudes only the 

Uncompahgre Formation of Proterozoic age, ranges in width 

from 39 to 150 ft and averages 50 ft. It is vertical to steeply 

Intrusive Rocks 27

Figure 11. View looking 

east into the bowl-shaped 

Amphitheater, east of the 

town of Ouray, that was 

formed by glacial processes 

at the confluence of two 

major glaciers moving 

northward (to the left along 

the Uncompahgre River valley) 

during the Pleistocene. At left 

center are exposed Paleozoic 

and Mesozoic sedimentary 

rocks; exposed in the back 

are volcaniclastic rocks of the 

Oligocene San Juan Formation. 

The light-colored rocks in the 

center and right foreground 

(roadcuts) consist of 

Mississippian limestone faulted 

against Proterozoic quartzite 

and slate. The bottom of The 

Amphitheater is covered by 

glacial debris exhibiting slump 

and slide features.
28 Geology and Ore Deposits of the Uncompahgre Mining District, Southwestern Colorado

Table 6. Analyses of travertine deposits and water from the Ouray (Pavillion) Hot Spring within the town’s city limits, Uncompahgre 

mining district, southwestern Colorado.

[Modified from Burbank and Luedke, 1961, 1969. gpm, gallons per minute; --, not looked for. Samples nos. 1–8: semiquantitative spectrographic analyses, in 

percent; the following elements were below these limits of sensitivity, in percent: Ag, 0.0002; B, 0.001; Cd, 0.005; Ga, 0.0005; Bi, 0.001; Sc, 0.001; Ge, 0.002; 

In, 0.001; Ta, 0.005; Ni, 0.0005; Co, 0.0005; Sn, 0.001. Analyses by U. Oda and E.F. Cooley, U.S. Geological Survey; samples collected by D.E. White and 

R.W. Lakin, U.S. Geological Survey. Sample no. 9: chemical analysis, in parts per million. Analysis by H.C. Whitehead, U.S. Geological Survey; sample collected by D.E. White and R.W. Lakin, U.S. Geological Survey]

Ouray Hot Spring Deposits Ouray Hot Spring Water

Sample no. 1 2 3 4 5 6 7 8 Sample no. 9

Lab no. 58–1138S 58–1139 58–1140 58–1141 58–1142 58–1143 58–1144 58–1145 Lab no. 850

Fe 0.1 0.7 0.3 0.7 0.2 0.7 0.5 7 SiO2 49

Mg 0.07 0.07 0.15 0.1 0.07 0.07 0.03 0.15 Fe 0.42

Mn 50 30 7 3 3 3 2 15 Mn 0.92

Ba 0.7 0.7 0.07 0.03 0.03 0.05 0.03 0.7 Ca 376

Be 0.03 0.03 0.001 0.001 0.0005 0.001 0.001 0.015 Mg 6.1

Cr 0.007 0.007 0.001 0.0005 0.0005 0.0005 0.0005 0.0015 Na 111

Cu 0.05 0.03 0.0007 0.0005 0.001 0.0005 0.0005 0.0015 K 8

La 0.01 0.01 0.005 0.005 0.005 0.005 0.005 0.005 Li 1

Mo 0.003 0.003 0.001 0.001 0.0015 0.001 0.001 0.001 HCO3 128

Pb 1 0.7 0.015 0.005 0.003 0.005 0.003 0.07 SO4 1,030

Sb 0.5 0.3 0.02 0.02 0.02 0.02 0.02 0.02 Cl 45

Sr 0.5 0.5 0.2 0.2 0.15 0.1 0.1 0.5 F 3

Ti 0.003 0.002 0.005 0.002 0.002 0.007 0.001 0.03 PO4 0.06

V 0.015 0.01 0.001 0.002 0.001 0.0015 0.001 0.002 B 0.23

W 1 1 0.07 0.03 0.03 0.03 0.03 0.2 H2

S -Y 0.003 0.003 0.001 0.001 0.001 0.002 0.002 0.002 Solids 1,760

Zn 0.3 0.2 0.02 0.02 0.02 0.02 0.02 0.02 pH 6.8

Zr 0.0005 0.0005 0.0005 0.0005 0.0005 0.0005 0.0005 0.003 Temperature, in °F 143

Discharge, in gpm 15

southwest dipping, and although somewhat irregular in trend, 

having many angular bends, strikes northwest (see figs. 4, 12).

The diabase is dark greenish gray to black and weathers 

brownish gray. It is fine to medium grained and is composed 

of labradorite and augite in a subophitic to ophitic (diabasic) 

texture. Interstitial accessory minerals are magnetite and 

apatite. Both the labradorite and augite are partly altered to 

calcite; the augite also is partly altered to penninite. Where the 

rock is highly altered, its composition of labradorite, quartz, 

carbonate, and secondary biotite partly altered to chlorite, 

together with a somewhat granular-appearing texture, is suggestive of a lamprophyre similar in composition to a kersantite 

described by Cross, Howe, Irving, and Emmons (1905) in the 

Needle Mountains area (fig. 1).

Like the enclosing Uncompahgre Formation, the dike has 

been sharply truncated by erosion and is overlain by nearly 

horizontal Paleozoic strata. It therefore was interpreted to be 

of Proterozoic(?) age. However, other diabase dikes within 

Proterozoic terrane in and adjacent to the San Juan Mountains 

are now suggested to be of Cambrian or Ordovician age, as 

based upon a Rb-Sr whole-rock date of 510±60 Ma obtained 

from a diabase dike in the Black Canyon of the Gunnison 

River, more than 30 mi northeast of Ouray (Hansen and Peterman, 1968).

Porphyritic Granodiorite

The most intense intrusive activity within the Uncompahgre mining district was during the Laramide orogeny of Late 

Cretaceous to early Tertiary time and consisted of the intrusion 

of concordant and discordant igneous bodies in a zone near 

Ouray. Most prominent are the laccolithic domes intruded at 

or near the Dakota Sandstone-Mancos Shale contact. These 

plutons surround and were connected to a central conduit 

in The Blowout that now is occupied by a small stock; this 

approximately 2,000- by 2,000-ft, square-shaped, intrusivefilled channel, with two western prongs, is somewhat eccentrically located with respect to the outcrop area of the laccolithic bodies. A few thin dikes and sills that intruded into 

the Paleozoic and Mesozoic sedimentary rocks were possible 

channels for magma supplied to the laccoliths, particularly 

the laccolith-like mass between Angel Creek and Canyon 

Creek (fig. 12). The thickness of the larger laccolithic bodies 

exceeded 1,500 ft, and, at the time of intrusion, it is estimated 

that they were overlain by about a mile of Upper Cretaceous 

impervious shales. Early during emplacement of the laccoliths, 

the shales served as a blanket effectively preventing the escape 

of magmatic emanations; internal alteration concurrent with 

crystallization thus was greater within the laccolithic bodies 
than if they had been intruded into a more permeable environment. Thermal metamorphism of the country rock adjacent to 

most of the laccoliths was slight. The stock in The Blowout, 

however, is surrounded by a contact metamorphosed and 

hydrothermally altered zone varying from several hundred to a 

couple thousand feet in width.

The intrusive rocks are remarkably uniform in composition and the few chemical analyses (table 7), all from different 

parts of the individual intrusive bodies, show little variation. 

The rocks are granodiorite (fig. 13), which is characteristically 

porphyritic with phenocrysts in a light-gray, fine- to mediumgrained groundmass. Petrographic examination of numerous 

thin sections indicated only minor textural, mineralogical, and 

alteration differences. Phenocrysts constitute about 45 percent 

of the rock and consist of plagioclase, hornblende, biotite, and 

quartz. Both plagioclase and hornblende phenocrysts are as 

much as a centimeter long and, particularly near contacts, have 

a linear parallelism. The plagioclase is dominantly andesine 

(An40); it is commonly twinned and many grains exhibit oscillatory zoning. In one thin section examined, at least ten major 

oscillations were counted in addition to other minor ones. 

The groundmass consists of quartz, orthoclase, and minor 

sodic plagioclase and biotite, with common accessory apatite, 

sphene, and iron oxides. Typical alteration products within the 

rock are chlorite, sericite, calcite, quartz, clay minerals, pyrite, 

and a little epidote.

The laccolithic bodies were emplaced probably by a 

single injection of magma of remarkably uniform composition, as inferred partly from internal structures within the 

laccolithic bodies as well as from their relations to the main 

feeding channel in The Blowout. Additional magma, probably 

from deeper sources, was emplaced as vertical dikes and small 

bodies within the limits of the feeder channel.

A detailed petrographic and chemical study by Burbank 

(l936) of the emplaced laccolithic body connected to and 

extending northward to northeastward from The Blowout 

to Dexter Creek determined its composition to be a sodic 

granodiorite. Chemical analyses (table 7, samples 3 and 4) 

and calculated mineralogical compositions and modes (table 

8), however, show that any change in the chemical character of the granodiorite was not the result of regional sodic 

enrichment due in part to a widespread, uniform hydrothermal 

alteration of rocks of the same composition and of contemporaneous origin closely following the intrusive and extrusive 

period (Cross and others, 1907, p. 10) but instead was almost 

entirely the consequence of arrested crystallization in different 

stages of this magmatic history. In conjunction with the course 

of crystallization, Burbank (1936, p. 237–238) established 

the fact that the most pronounced changes in composition of 

the magma took place during the later stages of crystallization. With the exception of the oxides of iron, a comparatively 

high ratio of sodium to calcium, and an intermediate potassium content, little difference between the other constituents 

was indicated in the chemical compositions of these three 

rock samples. From near the chilled margin of the so-called 

American Nettie sill, which is really the southern tapered 

edge of the Dexter Creek laccolith, sample 3 (fig. 14) shows 

that the formation of modal oligoclase in the laccolith was 

entirely the result of a more complete reaction and attainment 

of equilibrium between the earlier formed phenocrysts and the 

residual magmatic liquid. Crystallization in the so-called sill 

was arrested at an earlier stage than in the thicker and larger 

part of the laccolithic body with the consequence that this rock 

(sample 3) contained only about 15 percent by volume of plagioclase phenocrysts as compared to a rather uniform content 

of 40 percent in the part of the laccolith indicated by sample 

4. The plagioclase phenocrysts in sample 3 are zoned from 

labradorite (An55) to oligoclase (An25), whereas this zoning 

elsewhere in the laccolithic body was nearly all destroyed. A 

reaction, within the sill, between the labradorite and oligoclase cores of the crystals and the magmatic liquid presumably formed orthoclase that affected 5 to 10 percent of their 

volume. Also, within this part, some but not all of the original 

hornblende and biotite was unaltered, but in the central part of 

the laccolith the ferromagnesian minerals reacted completely 

with the magmatic liquid to form sodic plagioclase, magnetite, 

orthoclase, epidote, apatite, chlorite, rutile, and other minor 

reaction products. As deduced from the modal calculations 

(table 8), there was no introduction of sodium after crystallization.

Alteration of the porphyritic granodiorite in the laccolithic bodies is interpreted to be in part the result of magmatic crystallization beneath an impervious shale blanket that 

nearly surrounded the laccoliths and lessened the escape of 

residual magmatic fluids, although it is thought that the stock 

later vented to the surface. The effect of those fluids upon the 

intratelluric crystals thus was much greater in the earlier stages 

of crystallization than would have occurred in a body intruded 

into a more permeable rock environment. The later stages of 

crystallization consequently correspond more closely to and 

overlap the early stages of mineralization. This fact, to be discussed later, can be clearly shown by comparing of processes 

of contact metamorphism and mineralization that took place 

near the magma conduit (The Blowout) with the later stages of 

crystallization within the laccolithic bodies.

Few age data are available for the porphyritic granodioritic rocks. Isotopic ages have been determined only for the 

laccolithic body between Angel and Canyon Creeks and for 

the intrusive body in The Blowout. Armstrong (1969) and 

Cunningham and others (1994), respectively, obtained a K-Ar 

age of 50.6 Ma and apparent fission-track ages on zircon of 

40.7±6.9 and 54.1±4.0 Ma for the Canyon Creek granodioritic 

body. Billings (1980) reported fission-track data on zircon and 

apatite; the zircon ages ranged from 27±3 to 47.5±3 Ma for 

the Canyon Creek body and 31±3 to 49±4 Ma for the stock in 

The Blowout, and the apatite ages were too young and erratic. 

We consider all of these ages to be too young; possibly they 

represent a localized occurrence of alteration and mineralization or younger heating events. Several lines of field evidence 

mentioned earlier (p. 26) prove beyond a doubt that the intrusives are older and were formed during the Laramide orogeny. 

Other isotopic data, obtained and reported by Taylor (1974) on 

Intrusive Rocks 29
30 Geology and Ore Deposits of the Uncompahgre Mining District, Southwestern Colorado

Figure 12. Generalized geologic map showing distribution of intrusive rocks in a stock, dikes, laccoliths, and irregular-shaped masses 

within the Uncompahgre mining district and the Ouray 7.5-min quadrangle, southwestern Colorado (modified from Luedke and Burbank, 

1962, 1981).
Table 7. Chemical analyses of porphyritic granodiorite of Late Cretaceous to early Tertiary age in and near the Uncompahgre mining 

district, southwestern Colorado.

[Analyses by U.S. Geological Survey. Major-oxide data, in weight percent, are from standard and rapid-rock methods by J.G. Fairchild (samples 1, 2, and 4), 

C. Milton (sample 3), J.J. Fahey (sample 5), and P.L.D. Elmore, I.H. Barlow, S.D. Botts, G. Chloe, L. Artis, J. Glenn, H. Smith, and J. Kelsey (samples 6, 7, 8, 

and 9). --, no data]

Sample no. 1 2 3 4 5 6 7 8 9

Field no. O35–55 O–16337 -- O34–20 O–07238 O–57–105 66D1 67W7 67W8

Lab no. -- -- -- -- -- 157568 169345 173362 169362

SiO2 63.15 67.34 65.38 65.02 65.35 65.20 64.90 57.30 59.10

Al2O3 16.78 16.59 15.41 15.83 16.09 16.50 15.90 15.70 16.40

Fe2O3 3.39 1.54 1.97 2.13 2.69 2.00 2.80 3.70 3.10

FeO 2.79 1.86 2.19 1.56 1.40 1.50 1.00 2.60 2.80

MgO 1.83 0.75 1.43 1.14 1.33 1.30 0.80 1.90 1.70

CaO 4.27 3.71 2.87 3.32 2.52 3.00 3.70 5.30 4.10

K2O 2.55 2.96 3.42 3.42 3.34 3.00 2.50 2.60 2.70

H2O- 0.24 0.11 0.09 0.44 0.14 2.80 0.9 0.81 1.50

H2O+ 1.50 0.95 1.29 1.24 1.26 -- 3.10 2.60 1.90

TiO2 0.56 0.26 0.88 0.40 0.39 0.32 0.33 0.75 0.62

CO2 0.15 0.20 0.27 0.15 0.19 0.06 0.05 4.30 1.50

P2O5 0.06 0.08 0.09 0.12 0.06 0.18 0.17 0.38 0.39

MnO 0.10 0.04 0.23 1.23 0.58 0.10 0.13 0.08 0.18

BaO 0.08 0.08 0.09 0.03 -- -- -- -- -Cr2

O3 -- -- -- -- 0.03 -- -- -- -SO3 -- -- -- -- 0.04 -- -- -- -FeS2 -- 0.62 -- -- -- -- -- -- -Total 100.39 100.65 99.74 100.06 99.85 99 100 100 99

Sample descriptions and locations:

1. Small crosscutting body on south side of Skyrocket Creek, altitude about 8,750 feet (ft); 38°02’07” N., 107°39’51” W.

2. Stock in lower part of The Blowout, near Horsethief Trail, altitude about 8,500 ft; 38°02’12” N., 107°40’14” W.

3. Laccolith (sill) above the American Nettie mine, altitude about 9,700 ft; 38°02’46” N., 107°40’16” W.

4. Near base of laccolith south side of Dexter Creek 0.75 miles above the Bachelor mine (Khedive adit), altitude about 9,100 ft; 38°03’27” N., 107°38’47” W.

5. Near top of laccolith, south side of Dexter Creek about 5,000 ft northeast of sample no. 3, altitude about 10,125 ft; 38°03’11” N., 307°39’19” W.

6. Laccolith, ridge top on west side of Cow Creek northeast of the mining district, altitude about 9,200 ft; 38°07’29” N., 107°38’03” W.

7. Ramshorn Ridge intrusive mass on east side of Cow Creek northeast of mining district, altitude about 8,100 ft; 39°08’19” N., 108°08’11” W.

8. Dike in Red Creek northeast of the mining district, altitude about 8,400 ft; 38°07’23” N., 107°36’54” W.

9. Dike in Red Creek northeast of the mining district, altitude about 8,600 ft; 38°07’20” N., 107°36’40” W.

Intrusive Rocks 31
Figure 13. Ternary variation diagram of normative compositions 

of quartz (Q), alkali feldspars (A), and plagioclase (P), using the 

diagram of the International Union of Geological Sciences (IUGS) 

classification and nomenclature (Streckeisen, 1976), of silicic 

plutonic rocks (table 7) in the Uncompahgre mining district and 

Ouray 7.5-min quadrangle, southwestern Colorado.

the Canyon Creek intrusive body, were δ18O of +3.2 and δD of 

–145; he states that both numbers are low and are indicative of 

large-scale transport and interaction between circulating ground 

waters and the igneous intrusions. Taylor (1997) later included 

the Ouray sample location, based upon values determined on 

hypogene clay, with the Lake City and Silverton localities as 

having a δ18O = –6 to 0 and δD = –150 to –130.

Altered Porphyritic Rocks

The group of altered porphyritic igneous rocks, as classified in the field (see Luedke and Burbank, 1962), consists 

of several dikes, sills, and irregular-shaped bodies that intrude 

the Paleozoic and Mesozoic strata along the Uncompahgre 

River valley mostly in the northern part of the mining district, 

particularly near the junction with Dexter Creek valley (fig. 

12). Most of the dikes average about 6 ft in width and occupy 

vertical, west-trending fractures. The circular to irregularshaped bodies at Lake Lenore and north of Dexter Creek, 

respectively, appear to be upward enlargements or sheet-like 

offshoots of dikes at depth. The igneous mass west of Lake 

Lenore, about 195 ft in width, has very irregular contacts and 

tapers at both the east and west ends into dikes having nearly 

vertical walls. Also included in this group are a dike in Angel 

Creek, southwest of Ouray, and a dike on the south side of The 

Amphitheater.

This nondescript group of rocks is somewhat porphyritic 

with very small phenocrysts in a dark-gray or green aphanitic 

groundmass. Feldspar phenocrysts are mostly or completely 

altered; the mafic phenocrysts are completely altered. The 

latter, judging from their crystal outlines, were probably 

amphibole, pyroxene, biotite, and possibly olivine in that 

order of abundance. The few tabular phenocrysts commonly 

Table 8. Calculated mineralogical compositions and modal 

analyses of porphyritic granodiorite, samples 3 and 4 from table 7.

[After Burbank (1936, p. 238) and used with permission of American Geophysical Union]

Sample no. 3 4

Mineralogical compositions

Quartz 22.98 21.00

Orthoclase 15.11 17.29

Albite 34.58 33.50

Anorthite 12.04 13.53

Biotite1 9.17 6.90

Hornblende1 1.44 2.92

Magnetite 2.28 1.65

Hematite 0.20 0.90

Apatite 0.62 0.35

Total 98.42 98.04

Approximate modal analyses

Phenocrysts 26.00 51.10

Quartz 0.50 2.50

Orthoclase 0.50 0.60

Plagioclase 15.00 (An55-25) 40.00 (An25-30) 2

Ferromagnesians1 10.00 8.00

Groundmass 74.00 48.90

Quartz 22.00 19.00

Orthoclase 14.60 16.50

Plagioclase 31.00 (An20) 7.00 (An25)

Accessories3 6.40 6.40

 Total 100.00 100.00

1

Proportions of hornblende and biotite from modal analyses; these minerals 

largely resorbed to magnetite and chlorite.

2

Includes some albite.

3

Accessories include magnetite, hematite, chlorite, epidote, rutile, titanite, 

apatite, and calcite.

Figure 14. Generalized geologic cross section showing relation 

of Dexter Creek laccolith to feeding channel of the granodioritic 

magma in The Blowout, Uncompahgre mining district, southwestern 

Colorado. Numbers show relative positions of analyzed samples 3, 

4, and 5 in table 7 (no. 3 lies 2,500 ft north of plane of section). Veins 

strike about east. Modified from Burbank (1936, p. 241) and used 

with permission of American Geophysical Union.

32 Geology and Ore Deposits of the Uncompahgre Mining District, Southwestern Colorado
are aligned near the margins of the dikes. The groundmass is a 

cryptocrystalline to fine-grained aggregate of felsic minerals, 

or a felted to pilotaxitic aggregate of feldspar laths. Interstitial material is mostly quartz, apatite, and alteration products, 

the latter consisting mostly of sericite, chlorite, calcite, iron 

oxides, and clay minerals and smaller amounts of quartz, 

leucoxene, and sulfides. Most of these rocks are vesicular; the 

separate body at Lake Lenore contains, in addition, a large 

number of sedimentary rock inclusions. Chemical analyses 

(table 9) of these altered bodies suggest a granodioritic to 

quartz monzodioritic composition (fig. 13), but because these 

rocks are so intensely altered, any rock name must be stated 

with reservation.

A particularly unusual feature of the dikes in the north 

part of the mining district is that they appear to terminate 

upward in the Morrison Formation of Jurassic age. The dike 

on the south side of The Amphitheater is cut off at the early 

Tertiary erosion surface and is overlain by the San Juan 

Formation. The age of these intrusives is uncertain, but based 

on rock characteristics, structural relations, and associated 

ore deposits, they are thought to be Late Cretaceous to early 

Tertiary rather than late Tertiary.

Clastic Dikes

Closely associated with the granodioritic intrusives are 

several dikes of intrusive origin, but of clastic or non-igneous 

composition, occupying east-trending fractures. At least eight 

of these dikes crop out within the mining district and intrude 

the stratigraphic section beneath the early Tertiary erosion 

surface. The best known dikes are those in the Bachelor-Pony 

Express group of mines and in the American Nettie mine.

The clastic dikes average about 4 ft thick but pinch and 

swell, ranging from a few inches to 10 ft in width. Contact 

with the unaltered wall rock is sharp. The dikes are composed 

of angular to subrounded fragments of pre-Tertiary metamorphic, igneous, and sedimentary rocks, some undoubtedly of 

deep origin, in a gray to green matrix of similar but pulverized 

material (fig. 15A, C). Fragmental quartz, feldspar, biotite, 

chlorite, and calcite are the common constituents of the 

matrix. The dikes have textures ranging from that of an angular breccia to that of a rock closely resembling conglomerate. 

Size and shape of the fragments vary within a single dike as 

well as between different dikes; the clasts average about an 

inch in diameter but may be as large as 5 in., and the shape 

(roundness) depends upon the amount of particle attrition during intrusion.

Fragmentation is thought to have been contemporaneous 

with the forceful intrusion along a concurrently established 

fracture system as a result of a sudden escape of vapor from 

a buried intrusive center. Injection under pressure is indicated 

not only by the roundness of the fragments and, locally, by a 

mortar-and-pestle grinding effect from fragmental abrasion, 

but also by alignment of platy fragments (fig. 15B, D) parallel 

to the dike wall and by small-scale piecemeal stoping. Orientation of many of the fragments in the dikes in the Bachelor 

and Calliope mines indicates there were both vertical and 

horizontal components of movement; the latter in some cases 

was equal to or greater than the former. In the upper levels of 

the Bachelor mine, the clastic dike along strike contains many 

angular dark shale fragments derived from the Dakota Sandstone and perhaps the Mancos Shale (fig. 15B), whereas in the 

lower levels of the mine dark shale fragments are sparse. The 

position of the dike with shale fragments in the upper levels of 

the Bachelor mine suggests that the fragments were injected 

from east to west (fig. 16); the regional eastward dip indicates 

the shale beds are deeper to the east than at the west.

The origin of these clastic dikes has been variously interpreted. Ransome (1901), in describing the clastic dike in the 

upper mine workings of the Bachelor group, recognized that 

some of the included rocks were from the fissile black shales 

of Cretaceous age and not shales in older beds stratigraphically lower. Therefore, he postulated the dikes to have been 

fractures, filled chiefly by rock fragments from above, that 

were later aligned by fault movement. Downer and DeCou 

(1901) briefly describe these unusual dikes in their discussion of the Bachelor ore deposits, but only questioned Ransome’s interpretation without further speculation upon their 

Table 9. Chemical analyses of altered intrusive bodies of Late 

Cretaceous to early Tertiary age from the Uncompahgre mining 

district, southwestern Colorado.

[Analyses by U.S. Geological Survey. Major-oxide data in weight percent; 

analyses are from standard rock methods by J.G. Fairchild]

Sample no. 10 11 12

Field no. O34–40 O34–35 O–06219

SiO2 44.02 60.92 64.81

Al2

O3 16.94 15.84 15.73

Fe2

O3 4.19 2.20 1.37

FeO 3.54 3.24 1.79

MgO 1.94 1.84 0.94

CaO 9.27 3.55 4.01

Na2

O 2.03 3.65 3.52

K2

O 6.36 3.36 2.54

H2

O- 0.48 0.32 0.49

H2

O+ 2.90 2.12 1.87

TiO2 0.81 0.62 0.26

CO2 7.01 2.32 2.81

P2

O5 0.31 0.25 0.12

MnO 0.32 0.12 0.14

BaO 0.13 0.05 0.03

Total 100.25 100.4 100.43

Sample descriptions and locations:

10. Extremely altered rock; dike about 4,000 ft north of Dexter Creek; 

38°04’12” N., 107°41’04” W.

11. Intrusive plug on north side of Dexter Creek valley northeast of Lake 

Lenore, altitude about 9,353 ft; 38°04’02” N., 107°40’35” W.

12. Intrusive body on peninsula in Lake Lenore; 38°03’27” N., 107°40’57” W.

Intrusive Rocks 33
origin. Irving (1905) also briefly described the same dikes, and 

considered them to be a consolidated mass of black shale and 

other country rock fragments that formed as a friction breccia 

within the fracture, later reopened, and locally mineralized. 

In 1923, Spurr described the clastic dikes in the Bachelor and 

Pony Express mines as sand or breccia dikes formed by the 

opening of a fracture that was filled by an upwelling mass 

of mud composed of detritus of the underlying rocks; this 

mud was then dammed in its ascent by shale beds, but with 

continued upward pressure, the mud burst the so-called dam 

to continue its ascent and, in addition, gather fragments of the 

shales, which were then aligned parallel to the fracture walls.

An important association exists between the clastic 

intrusions and hydrothermal activity. Mineralizing solutions 

followed a course similar, in part, to that of the mostly earlierformed clastic injections, moved along the same fractures, and 

locally deposited ore in veins and blanket deposits.

Porphyritic Andesite and Quartz Latite

The youngest intrusive rocks are dikes emplaced in 

mid- to late Tertiary time in the western San Juan Mountains 

(fig. 12). The Germania dike at the head of The Amphitheater, the Bridalveil dike in the valley of the same name, the 

Calliope dike in Dexter Creek valley (fig. 17), and several 

unnamed dikes northeast and east beyond the limits of the 

mining district, all intrude the Oligocene San Juan Formation. 

These dikes are related to, and occupy fractures radial to, the 

mid-Tertiary Cow Creek intrusive center about 5 mi to the 

east (Luedke, 1972; Steven and Hail, 1989). The central stock 

in the similar Cimarron intrusive center, another center 7 mi 

farther northeast, has a zircon fission-track age of 30.2 Ma 

(Steven and Hail, 1989). Dikes exposed west of the mining 

district (fig. 12) are related to the mid-Tertiary Mt. Sneffels intrusive center to the west-southwest that has K-Ar and 

fission-track ages respectively of 32.7±1.0 and 32.5±4.6 Ma 

(Lipman and others, 1976). Many of these vertical or nearly 

vertical younger dikes, locally more resistant than the country 

rock, are extensive and average about 10 ft in thickness.

These dikes vary in composition (fig. 10) and texture, but 

for descriptive purposes were classified in the field as either 

andesite or quartz latite. All of these dike rocks are slightly to 

moderately porphyritic, having phenocrysts 2 or 3 mm in size 

in a dense aphanitic groundmass.

Dikes cropping out mostly west of the district (fig. 12) 

and classified as porphyritic andesites (Luedke and Burbank, 

1962) have phenocrysts of augite, hypersthene, labradorite, 

and minor olivine in a felted to pilotaxitic groundmass of plagioclase microlites. Interstitial materials are pyroxene, apatite, 

quartz, and iron oxides. Locally, the pyroxene phenocrysts 

occur in clusters that impart a glomeroporphyritic texture to 

the rock. Alteration products are calcite, chlorite, epidote, clay 

minerals, iron ores, celadonite, and quartz. The rock is dark 

greenish gray and weathers brownish gray.

Figure 15 (facing page). Photographs of polished rock slabs 

of intrusive clastic dikes in the Uncompahgre mining district, 

southwestern Colorado, showing a variety of compositions and 

textures. A, Dike exposed at creek level in Canyon Creek valley 

(fig. 12) contains rock fragments of varying shapes and sizes 

from the underlying Proterozoic basement of quartzite (qe) and 

slate (sl), with Paleozoic sedimentary red-bed mudstone (ms) 

and sandstone (ss) in a matrix of small fragments of altered 

rhyolite (r) and pulverized materials. Sample O35–92. B, Dike 

at the breast of drift on the Syracuse tunnel level (east end), 

north of the Pony Express vein, in the Bachelor mine located 

east of the Uncompahgre River valley and south of Dexter Creek 

valley (pl. 4). Platy, mostly black shale (sh) fragments with a 

somewhat subparallel orientation are interspersed with lighter 

colored, round to angular, very fine-grained sandstone (ss) and 

quartz (q) fragments. Included also are rounded fragments of 

schistose greenstone (gs) derived from the underlying Proterozoic 

basement. Sample O–06224. C, Short dike exposed high on 

east side of the Uncompahgre River valley, southeast of the 

Pony Express dike (fig. 12). Clasts with a variety of shapes and 

compositions indicate the entire geologic section, exluding that 

of Tertiary age, was traversed. Granite (gr), gneiss (gn), schist 

(sch), slate (sl), sedimentary (sed), quartz (q), and clay (c) occur 

in a matrix of pulverized to finely ground rock fragments. Sample 

O34–48. D, Oriented specimen of the Bachelor dike from a short 

drift east from the Syracuse tunnel level in the Bachelor mine, 

located east of the Uncompahgre River valley and south of Dexter 

Creek valley (pl. 4). Platy black shale fragments of Mancos Shale 

(Cretaceous) in a gray pulverized matrix suggest both horizontal 

and vertical components of movement (from east to west; see 

fig. 16). Fragments were presumably derived downdip at a shallow 

depth. Sample O35–38.

34 Geology and Ore Deposits of the Uncompahgre Mining District, Southwestern Colorado

12) are tan to light-brownish-gray, moderately porphyritic 

dacite or trachyte (table 10, samples B–D; fig. 10) that contain 

phenocrysts of hornblende, biotite, quartz, and plagioclase 

(oligoclase-andesine) in a groundmass of varying textures. 

In some dikes, the groundmass is cryptocrystalline to finegrained aggregates of potassic feldspar, sodic plagioclase, 

and quartz, whereas in others, the groundmass is felted or 

pilotaxitic orthoclase and sodic plagioclase laths with interstitial quartz, biotite, and apatite. Alteration products consist of 

calcite, chlorite, chalcedony, and iron oxides.

The Germania dike, a dark greenish-gray, more mafic 

variety (table 10, sample A), was classified as trachyandesite 

(fig. 10). It weathers brownish gray. The rock consists of phenocrysts of hornblende, andesine, and sparse augite in a felted 

to pilotaxitic groundmass of plagioclase microlites. Interstitial 

materials include apatite, quartz, and iron oxides; alteration 

products are calcite and chlorite.

Most of the dikes east and northeast of the district (fig. 
Intrusive Rocks 35
Figure 16. Longitudinal section along the plane of the Bachelor mine workings, vein, and clastic dike indicating the probable course 

(arrows) of the clastic dike injection. Line A–B is the junction of the Bachelor fracture and the south split or Pony Express fracture. Pc, 

Cutler Formation; ^d, Dolores Formation; Je, Entrada Sandstone; Jmw, Wanakah and Morrison Formations, undifferentiated; Kd, Dakota 

Sandstone; Km, Mancos Shale; TKig, intrusive granodiorite; Tsj, San Juan Formation; Qs, landslide debris; es, early Tertiary erosion 

surface. Modified from Burbank (1930, p. 198).

Structure

Structures within the Uncompahgre mining district, and 

the Ouray area in general (fig. 18), are typical of the western 

border region of the San Juan Mountains and are characteristic 

of deformation closely associated with mountain building. 

At least three different periods of deformation are visibly 

marked by major unconformities within the area; the oldest 

period undoubtedly occurred well within its timeframe, but the 

middle and younger periods more-or-less represent the closing 

phases of major divisions of geologic time. Each period differed in certain respects from the others: in general, the intensity of deformation decreased through time, whereas associated igneous activity increased. Rocks of Proterozoic age, 

only, were faulted and folded by strong compressive deformation; this deformation was accompanied or followed locally by 

minor intrusive activity. In late Paleozoic time (Rocky Mountain orogeny), domal uplift of the western San Juan Mountains 

region resulted in folds and faults apparently with little or no 

accompanying igneous activity. Deformation in late Mesozoic to early Cenozoic time (Laramide orogeny) resulted in a 

renewal of domal uplift and additional folds and faults that in 

part paralleled the earlier structural trends. This deformation, 

however, was accompanied locally by considerable magmatic 

activity and an early period of mineralization, making the area 

a part of the southwest extension of the Colorado Mineral Belt 

(Tweto and Sims, 1963). Each of these three different periods 

of deformation was followed by extensive and widespread erosion prior to deposition of the overlying strata. Also, numerous 

crustal disturbances between these major periods of deformation are indicated by erosional unconformities and gaps within 

the stratigraphic sequence.

Deformation during mid- and late Cenozoic time, representing a fourth major period, occurred south of the area and 

has minor significance locally. This period was distinguished 

by widespread volcanic activity that resulted in formation 

of the San Juan and Silverton calderas and related intrusive 

centers, and that was accompanied by multiple episodes of 

alteration and mineralization.

Proterozoic Structure

Rocks of Proterozoic age exposed in the Uncompahgre River canyon south of Ouray (figs. 4, 18) afford only an 

incomplete picture of the regional structure formed during this 

timeframe. This limited exposure of rocks is bounded by the 

west-northwest-trending Ouray fault, of unknown extent, presumably related to ancient and deep crustal structures of eastwest trend in the western San Juan Mountains region (Tweto, 

1987, p. A46 and pl. 1). However, within the Rocky Mountains of southwestern Colorado, the Ouray area is regionally 

located along or close to a major northeast-trending shear zone 

and crustal province boundary (1.65 Ga deformation) established in Proterozoic time (Karlstrom and Humphreys, 1998; 

CD-ROM Working Group, 2002).

36 Geology and Ore Deposits of the Uncompahgre Mining District, Southwestern Colorado
Figure 17. View looking west along the Tertiary (Oligocene) 

Calliope dike, in the valley of Dexter Creek, toward the 

Uncompahgre River valley (benches formed on sedimentary beds 

along the west side) and the Uncompahgre Plateau (skyline) to the 

northwest. The dike, radiating out from a volcanic center located 

in the Difficulty Creek-upper Cow Creek area about 3 mi to the 

east, strikes about N. 80° W. and dips from vertical to 79° S. It is 

seen here intruded into volcaniclastic rocks of the Oligocene San 

Juan Formation.

The metamorphic rocks of the Uncompahgre Formation 

are conveniently divided on the basis of the prevailing strike 

and dip of beds into two blocks separated by an arbitrary line 

marked by a zone of crumpled and faulted beds that extends 

across the canyon a little south of the diabase dike. In the narrower and smaller northern block, the beds strike west-northwest and dip from steeply south through vertical to steeply 

overturned to the north; in the larger southern block, the beds 

strike east-northeast and dip 55°–70° north. The rocks in the 

northern block, in general, are more intensely metamorphosed 

than those in the southern block. A few thin beds of slate 

within the quartzite layer in the northern block were metamorphosed locally to a contorted mica schist owing to intrusion of 

the diabase dike.

Table 10. Chemical and spectrographic analyses of Tertiary 

porphyritic igneous dikes of mafic to intermediate composition in 

and adjacent to the Uncompahgre mining district, southwestern 

Colorado.

[Analyses by U.S. Geological Survey. Major-oxide data, in weight percent, 

from rapid-rock methods by P.L.D. Elmore, I.H. Barlow, S.D. Botts, and G. 

Chloe. Trace-element data, in parts per million, from semiquantitative spectrographic methods by K.S. Hazel. N, looked for but not found]

Sample A B C D

Field no. O–57–129 O–57–41 O–57–38 O–57–123

Lab no. 154555 154553 154552 154554

SiO2 56.80 62.00 62.60 65.20

Al2

O3 16.70 15.80 15.60 15.60

Fe2O3 5.40 3.60 1.90 3.50

FeO 2.20 0.53 0.42 0.60

MgO 3.60 1.10 0.58 0.73

CaO 4.80 3.50 5.20 2.50

Na2O 3.40 3.20 3.60 2.60

K2O 2.90 3.40 3.60 5.20

H2O 3.20 3.50 2.80 2.20

TiO2 0.64 0.44 0.24 0.44

P2O5 0.40 0.22 0.13 0.20

MnO 0.17 0.16 0.14 0.08

CO2 0.06 1.40 2.30 0.62

Total 100 100 99 99

Ag N 2 3 N

As 100 150 100 100

Ba 700 700 700 700

Be 2 2 3 2

Co 150 15 7 15

Cr 3 3 N 3

Cu 70 150 150 70

Ga 15 7 7 7

La 30 N 30 30

Mo 3 3 7 3

Nb 3 3 3 3

Ni 15 7 7 7

Pb 15 15 30 150

Sc 15 7 7 7

Sn 7 N N 15

Sr 300 300 300 150

V 150 70 30 70

Y 15 15 15 15

Yb 2 2 2 2

Zr 300 300 300 300

Sample descriptions and locations:

A. Germania dike (trachyte) near head of The Amphitheater; 38°01’57” N., 

107°37’56” W. 

B. East dike (dacite) on ridge southeast of Mount Baldy; 38°05’27” N., 

107°37’51” W.

C. West dike (dacite) on ridge southeast of Mount Baldy; 38°05’26” N., 

107°37’53” W.

D. Dike (trachyandesite) in upper Cascade Creek basin; 38°02’10” N., 

107°37’39” W.

Structure 37
38 Geology and Ore Deposits of the Uncompahgre Mining District, Southwestern Colorado

Figure 18. Generalized structure map of the Uncompahgre mining district and the Ouray 7.5-min quadrangle, southwestern Colorado 

(modified from Burbank, 1940, and Luedke and Burbank, 1962, 1981).
Structure 39

A fault zone, exposed in the inner gorge east of the 

Uncompahgre River, strikes about N. 80° W. and dips steeply 

to the north. Striations on the fault surface plunge 70° W. but 

probably indicate only the direction of the last movement on 

the fault. The drag and apparent displacement of the quartzite 

beds near the fault suggest a relative upward movement of the 

northern hanging-wall block, but the evidence is not conclusive. On the other hand, faulting within the overlying Paleozoic 

strata along the west side of the valley, on the extension of this 

fault zone, suggests that renewed activity resulted in downward 

movement of the hanging wall. This later movement tended to 

offset the earlier displacement so that the net result negates the 

reverse displacement. Whatever the relative displacement, this 

fault separates Proterozoic blocks of sharply discordant attitudes, and provides evidence for faulting in Proterozoic time.

Along the fault line, the layers of quartzite and slate have 

been tightly compressed into a west-plunging syncline, the 

axis of which approximately coincides with the major fault 

zone east of the river, but turns westward through the faultbounded slate block (fig. 19A) west of the river. Cleavage and 

drag folds in the slate west of the river suggest the syncline 

plunges about 40° W. This section (fig. 19A) was drawn to 

include the Ouray fault, which bounds the northern outcrop 

of the Proterozoic rocks; extensions of the layers and faults 

in depth are shown chiefly to illustrate the general relations 

between the different faults and the synclinal axis, and are 

not considered to be quantitatively accurate. However, field 

evidence favors the interpretation that the break along the axis 

of the synclinal fold originated in Proterozoic time.

The diabase dike is presumed to have been intruded into 

the Proterozoic rocks subsequent to deformation and metamorphism during Proterozoic time; it lacks any effects of the 

dynamic metamorphism typical of the enclosing beds of the 

Uncompahgre Formation. The dike occupies a jagged tension 

fracture, thus suggesting that strong tensional stresses acted 

upon the Proterozoic rocks prior to Paleozoic time. The dike 

cuts across bedding in the northern block, which implies, but 

does not offer conclusive evidence of, later reactivation of 

inherited Proterozoic breaks. The dike, with the vertical beds 

of the Uncompahgre Formation, was truncated by erosion. 

Rocks of Devonian age deposited unconformably upon that 

erosion surface provide no conclusive evidence for the dike’s 

age being either Neoproterozoic or early Paleozoic (see discussion p. 28).

Paleozoic Structure

The Paleozoic strata in the western San Juan Mountains 

are exposed in a belt peripheral to the west side of the mountain front. In the Ouray 7.5-min quadrangle, near the northern 

extremity of this belt, these strata with structures indicative 

of uplift are exposed for a distance of about 8 mi (figs. 4, 18). 

Deformation of late Paleozoic age within the Uncompahgre 

district resulted in relatively simple structures that include 

monoclinal and anticlinal folds and faults (fig. 18). This period 

of deformation is readily recognized by means of the local, but 

profound, unconformity between the Paleozoic and Mesozoic 

strata, which establishes the age as neither older than Early 

Permian nor younger than Late Triassic.

The lower part of the Paleozoic section (Devonian and 

Mississippian) is exposed mainly south of Ouray along the 

west side of the Uncompahgre River canyon. The upper part of 

the Paleozoic section (Pennsylvanian and Permian) crops out 

in the vicinity of Ouray, northward along the Uncompahgre 

River valley for about 6 mi, and southwestward along Canyon 

Creek for about 2 mi. For a short distance along the valley at 

and north of Ouray, the Paleozoic strata dip moderately north to 

northwest; the strata there are more or less horizontal to where 

they dip beneath the level of the river valley in a sharp monoclinal fold of late Mesozoic age. About 8 mi to the southwest of 

Ouray, upper Paleozoic strata reappear in the canyon of the San 

Miguel River at Telluride (Burbank and Luedke, 1966). Also, 

about 7 mi northeast of Ouray, a very limited exposure of lower 

and upper Paleozoic rocks crops out in the valley of Cow Creek 

(Luedke and Burbank, 1962; Luedke, 1972).

In the general description of the stratigraphy, all Paleozoic units in the Uncompahgre district were noted to be more 

or less conformable, so that structural features assignable 

to late Paleozoic deformation collectively apply to all of 

the units. With Ouray as a focus, the Paleozoic strata dip to 

the west, northwest, and north (fig. 18; pl. 1). Ouray thus is 
Figure 19. Structural control in the area south of Ouray, 

Uncompahgre mining district, southwestern Colorado (modified 

from Burbank, 1940). A, Idealized section, of which the eroded 

parts of the lower Paleozoic limestones were restored, showing 

relation of faults and folds in the Proterozoic metamorphic rocks 

to faults and flexures in the immediately overlying Paleozoic beds. 

*m, Molas Formation; DM, Devonian and Mississippian rocks, 

undifferentiated; s (slate) and qtzte (quartzite), Uncompahgre 

Formation. B, Section approximately through the New Mineral 

Farm mine on the northwest slope of Hayden Mountain 

showing the approximate position of the concealed syncline 

in the Proterozoic metamorphic basement rocks; tunnel on 

the north-striking vein is nearly parallel to plane of section. 

Ql, landslide mass; Tsj, San Juan Formation; Mz, Mesozoic 

rocks, undifferentiated; *P, Pennsylvanian and Permian rocks, 

undifferentiated; DM, Devonian and Mississippian rocks, 

undifferentiated; s (slate) and qtzte (quartzite), Uncompahgre 

Formation.

located on a nose formed during the Paleozoic uplift of the 

ancestral San Juan Mountains, perhaps representing some 

swing or sharp turn along the western flank of the ancient 

landmass. The surrounding cover of Mesozoic and Cenozoic 

rocks, however, effectively hides the nature of this feature. On 

the cross sections (pl. 1), the base of the Triassic Dolores Formation, where not actually transected, has been projected onto 

the plane of the section in order to help differentiate the effects 

of the Paleozoic deformation from those of later periods. This 

erosion surface was presumably a horizontal, featureless plain 

prior to Mesozoic sedimentation; it is used primarily as a 

datum to estimate the amount of tilting of the Paleozoic strata. 

From this datum, the Paleozoic strata on the flanks of the San 

Juan Mountains uplift declined between 1,000 and 1,200 ft/

mi. Using Ouray as the focal point, the beds north of town 

flattened to almost horizontal within a distance of less than 2 

mi, and to the southwest toward Telluride, the beds likewise 

flattened within about 3 mi. The original extent of these outer 

flat structural shelves that border the Paleozoic uplift at Ouray 

is unknown, because Paleozoic strata are not exposed beyond 

the limits described.

The tilted strata on this northwestern flank of the San 

Juan uplift do not dip away uniformly but in a series of 

relatively steep flexures between which the beds have less 

than average dip. Weimer (1980) referred to these flexures as 

“paleodrape” folds. These local flexures and the Ouray fault, 

a major fault which strikes N. 70° W. across the canyon of the 

Uncompahgre River just south of town, are certainly the more 

important late Paleozoic structures within the district. Paleozoic strata adjacent to the fault are locally and sharply flexed, 

whereas the fault has only slightly affected the overlying 

Mesozoic strata along strike to the northwest. The Paleozoic 

strata decline about 5,400 ft from the southern edge of the 

map area to their deepest position in the valley north of Ouray. 

About 4,000 ft of this amount are the result of tilting, monoclinal folding, and faulting during late Paleozoic time. Most 

of the Paleozoic displacement occurred within 3 to 5 mi of the 

faulted margin of the Proterozoic basement rocks. This fact is 

of significance in interpreting the geologic history of the San 

Juan Mountains.

Details concerning Paleozoic structures within the 

Uncompahgre district are conveniently divided between two 

blocks that are separated by the Ouray fault and the intervening prong-like outcrop of Proterozoic rocks (figs. 4, 18).

Southern Block

Devonian and Mississippian rocks on the bench along the 

west side of the Uncompahgre River canyon, south of Ouray, 

are poorly exposed because of a mantle of talus and a covering of soil and timber. Where exposed in gulches and prospect 

tunnels, the limestone beds exhibit local variations in attitude 

because of small faults and disordered crumpling where they 

rest on slate in the underlying Uncompahgre Formation. Most 

of these minor irregularities are related to faults of Tertiary 

age, which also cut the overlying Telluride Conglomerate and 

San Juan Formation. Disregarding these minor variations, 

the average strike of the strata for about a mile along the 

high bench is nearly north, with dips of 10°–15° W. (figs. 4, 

18). Although the outcrop width on the bench is not great, a 

40 Geology and Ore Deposits of the Uncompahgre Mining District, Southwestern Colorado
westward dip of about 12° suffices to carry the basal Paleozoic 

units to the top of the quartzite outcrops on the cliff face on 

the east side of the canyon several thousand feet distant (pl. 

1, section B–B′). North of the southwest-trending fault that is 

located west of the river and that cuts the overlying sedimentary beds, there is a moderate steepening of the westward dip 

of the beds; near Canyon Creek, 4,000 ft to the northwest, the 

dips are as much as 20°.

Above the bench on the east slope of Hayden Mountain 

the Hermosa Formation is exposed just beneath the Telluride 

Conglomerate, but on the west slope it is almost completely 

covered by landslide deposits. In a few adits and shafts the 

Hermosa beds dip 12°–13° W. as far as the bed of Canyon 

Creek. At Angel Creek, a tributary of Canyon Creek, the dips 

have flattened to between 4° and 10° W. These dips are only 

a few degrees greater than those of the overlying Triassic 

beds (pl. 1, section B–B′), which dip in the same direction 

and hence probably are a result in part of a late Mesozoic 

component of tilting. The Hermosa Formation dips beneath 

the level of Canyon Creek a short distance to the southwest 

in the Ironton 7.5-min quadrangle (Burbank and Luedke, 

1964). Southwestward from that location, the canyon bottom 

is underlain by the Cutler Formation, which dips 15°–18° SW., 

apparently in a series of flexures. At the southwest termination 

of the Paleozoic outcrops in Canyon Creek, the Cutler dips 

only 2°–3°.

Sections A–A′ and D–D′ (pl. 1), parallel to and across 

Canyon Creek valley, respectively, illustrate the important 

structural features of a broad anticlinal nose plunging northwest in the southern block. In this valley southwest of section 

A–A′ (pl. 1), the thickness of the Cutler Formation beneath 

the Triassic unconformity is not known because of inadequate 

exposures to permit correlation with the type Cutler section; 

the lithology of the beds in the Cutler indicates that they 

belong to the lower third of the formation. The southwest part 

of section A–A′ thus was based mainly upon structural readings, but the errors in the section are considered minimal. It 

should be noted that our interpretation of the structure and designation of the Paleozoic strata differ from that of early maps 

(Cross, Howe, and Ransome, 1905; Cross and others, 1907), 

in which the valley of Canyon Creek is shown to be underlain 

by the Hermosa Formation. Our interpretation estimates the 

depth to the Paleozoic limestone units to be roughly 2,500 to 

3,000 ft beneath the base of the Triassic. These limestone units 

consequently are too deep to be of much immediate interest 

for mineral exploration.

Northern Block

The Ouray fault, which separates the blocks, was traced 

over a distance of about 2 mi from the base of the cliffs on the 

south side of The Amphitheater northwestward to the slopes 

above Box Canyon (fig. 18). The fault is essentially vertical. 

An accurate amount of displacement along the fault is difficult 

to determine, but Paleozoic strata crop out on both sides at 

each end of the fault and thus permit estimation of the net 

vertical displacement. At its eastern end, a small wedge of the 

Elbert Formation and basal part of the overlying Ouray Limestone are exposed in the south wall of the fault; the north wall 

consists of the Leadville Limestone and Hermosa Formation. 

Estimation of the exact displacement here is complicated by 

sharp changes in the dips near the fault and by minor crossfaulting, but the total vertical component of movement on 

the fault was not less than 400 ft and may have exceeded this 

amount by 100 ft or more. At the western end of the fault near 

Box Canyon, the Molas and Hermosa Formations are in contact with Proterozoic rocks, indicating displacement there was 

not less than 400 ft and possibly was as much as 600 ft. Considering drag and any flexing of beds along the fault, the net 

displacement between the blocks on the Ouray fault probably 

was 600 to 700 ft. There is little evidence for any horizontal 

component of movement along the fault; striations on the fault 

surfaces are vertical.

In the slopes west of Box Canyon, which are covered 

partly by glacial drift and talus, continuation of the main 

Ouray fault is difficult to trace to the northwest. Cross and 

others (1907, p. 13) thought the fault was Tertiary in age and 

displaced Mesozoic strata. Traverses of level lines about 1,500 

ft long were made along the exposures of the Dolores Formation and the Entrada Sandstone on the mountain slope west of 

Box Canyon, but the bases of these two formations were found 

to have been displaced less than 50 ft by either faulting or 

flexing. The minimal, but consistent displacement, within the 

Paleozoic units was at least 400 ft along the entire fault length. 

This establishes that the principal movement on the fault was 

pre-Dolores.

West of Box Canyon the thickness of the Permian Cutler 

Formation exposed beneath the angular unconformity provides 

additional evidence concerning the age of faulting. Its thickness on the north side of the fault is much greater than on the 

south side. Also, the strike of beds in the Hermosa and Cutler 

Formations is about N. 65°–80° E. on the north side compared 

to N. 30° E. on the south side. This consistent relation of strike 

direction to the projection of the fault and the absence of 

any appreciable change of bedding attitudes in the overlying 

Mesozoic strata strongly suggest pre-Mesozoic deformation.

A deceptive but unreal appearance of displacement of 

several hundreds of feet can be seen along the Ouray fault 

because of the granodiorite sills within the Cutler Formation farther upslope west of Box Canyon. The sills rise in the 

stratigraphic section to cut across beds along vertical breaks 

or splits in the westward trace of the fault, so that the upper 

sill is between the Entrada Sandstone and Wanakah Formation, a change in position clearly due to some diversion during 

intrusion, not to faulting of the sill. A similar diversion of 

exposed sills can be seen along the northwest-trending fault 

about 1,500 ft southwest of the Ouray fault. Undoubtedly 

pre-intrusion structures were responsible in part for minor 

changes in altitude and stratigraphic position of the sills on 

this fault, but the two sills on the north side are not more than 

50 ft thick, thus diversion of a sill in stratigraphic position 

across the unconformity at the base of the Mesozoic section 

Structure 41
does not appreciably affect computation of the displacement. 

Furthermore, both sills thicken immediately south of this fault, 

giving the appearance, in effect, of more offset than the actual 

downthrow of 50 ft or less north of the fault. Thus the earlier 

conclusion that the fault was Tertiary in age is not supported 

by detailed mapping.

What appears to have been renewed movement at the east 

end of the Ouray fault on the south side of The Amphitheater 

was the result of minor downthrow of a narrow wedge of San 

Juan Formation on the south side of the fault. This movement 

was opposite to the principal Paleozoic displacement along the 

main fault and suggests that only minor movement occurred 

as the result of readjustments during Tertiary time. Renewal 

of fault activity in Tertiary time, likewise, does not affect the 

conclusions reached as to the time of the principal displacement on the Ouray fault.

The attitude of the Leadville Limestone changes near the 

Ouray fault; the Leadville forms the north wall of the fault 

along much of its exposure in the central part of the canyon, so 

that where flexed the bedding more or less parallels the fault 

in strike and dips steeply to moderately north. This relation 

exists within a narrow wedge from zero to 700 ft wide and 

about 4,000 ft long (fig. 18). North of this wedge, the beds 

resume their strike of N. 50°–70° E. As shown on the geologic 

map (Luedke and Burbank, 1981), the wedge is separated from 

the main body of northwest-dipping rocks by a fault or zone 

of flexing that diverges northward from the main trend of the 

Ouray fault. As this disturbed zone diverges farther from the 

main fault, the intensity of deformation within the Leadville 

gradually decreases; this deformation dies out westward, or at 

least decreases markedly in intensity, at about the presumed 

location where the Ouray fault crosses a buried contact between 

slate and quartzite. The relative competency of the underlying rocks traversed by the fault possibly affected the type of 

structures formed in the overlying beds. This may account for 

the minor monoclinal flexing in the upper Paleozoic strata along 

the Ouray fault west of Canyon Creek (pl. 1, section C–C′) as 

compared with the lower Paleozoic strata along the fault on the 

south side of The Amphitheater (pl. 1, section F–F′). This, in 

turn, possibly may have some bearing on arguments as to the 

age of the Ouray fault. Absence of a broad zone of monoclinal 

flexing in the walls of the fault at and west of Canyon Creek is 

no reason to infer an abrupt decrease in its magnitude. Furthermore, the lack of displacement of the basal Mesozoic strata and 

the lack of monoclinal flexing of the Cutler Formation west of 

Canyon Creek is not necessarily a result of the fault dying out in 

a westward direction. Other supporting evidence is needed.

Rocks in the block north of the Ouray fault (pl. 1, section 

A–A′) appear to have been uplifted with respect to the steeply 

flexed band of rocks directly in contact with the fault. This 

uplift requires either some reversal of movement along parts 

of the Ouray fault and some auxiliary faults, or some mechanism in which graben-like blocks of steeply dipping beds 

formed during the Paleozoic activity of the fault. No independent evidence exists to determine relative ages of the different Paleozoic faults and flexures involved in these Paleozoic 

structures. However, there is evidence provided by the Mesozoic structures that show the entire block of Paleozoic and 

Mesozoic strata east of the Uncompahgre River valley was 

tilted eastward, probably prior to or coincident with the intrusions of granodiorite. It is thought that this tilting reactivated 

certain faults and flexures in the north wall of the Ouray fault 

and thus accounted, in part, for these particular structures. 

This later deformation does not appear to have appreciably 

affected beds adjacent to the wall of the Ouray fault west of 

the Uncompahgre River valley.

Structures more clearly of Paleozoic age are readily 

apparent in cross section F–F′ (pl. 1). A sharp monocline in 

the Leadville Limestone along the south wall of The Amphitheater strikes about N. 45° E. and dips 30°–40° NW. At the 

head of The Amphitheater in the workings of the Portland 

mine, the Leadville Limestone strikes N. 60° E. and dips 

10°–12° NW., which conforms with the attitude of beds in the 

Hermosa Formation exposed in the cliffs along the north side 

of The Amphitheater. Thus, it is assumed that the sharp monocline along the south wall flattens abruptly under the surficial 

cover and the beds conform to the attitude of those exposed at 

the entrance of The Amphitheater and in the Portland mine.

About a mile north of Ouray a sharp monocline (fig. 20) 

in the Cutler Formation is well exposed in the cliffs south of 

Corbett Creek. Its axis trends west-northwest, nearly parallel 

to the Ouray fault to the south; the dip on the steep north limb 

is greater than 40°. A monocline can be seen on the opposite 

side of the Uncompahgre River valley near Skyrocket Creek, 

but the dip of the beds there is slightly steeper, and possibly 

they were rotated by faulting and tilting related to intrusion 

of granodiorite. However, the local steep dips are not due 

primarily to the igneous intrusions, which occurred during the 

Laramide orogeny, because the basal Mesozoic strata on either 

side of the valley are unconformable on the upturned Paleozoic strata. Projecting the fold axes across the valley indicates 

that the steepest parts of the flexures are not in line. This may 

have been caused in part by rotation of the fault blocks on the 

east side of the valley during later intrusive activity, but possibly the late Paleozoic flexures may have formed an overlapping en echelon pattern of folds, such as the flexures that 

form the north wall of the Ouray fault. These flexures could 

have resulted from movement on faults in rocks beneath and 

within the Uncompahgre Formation; the stress was transmitted upward into the overlying Paleozoic strata, resulting in 

irregular or en echelon folds because of differences in the 

competency of the underlying interbedded quartzite and slate. 

This is best illustrated by flexures related to the Ouray fault, 

which were most prominent where limestone beds overlay 

incompetent slate beds (fig. 19A).

Inasmuch as the Hermosa Formation dips beneath the 

valley floor in the area of the intrusive bodies, the Paleozoic 

structures north of the point where Bridalveil Creek joins the 

Uncompahgre River were more difficult to determine. Tracing 

gently dipping key horizons in the Cutler Formation, a broad, 

very gentle east- to northeast-trending anticline is indicated 

42 Geology and Ore Deposits of the Uncompahgre Mining District, Southwestern Colorado
near Cutler Creek (fig. 18). This flexure is truncated by the 

overlying Mesozoic strata.

The base of the Paleozoic section is estimated to be about 

2,600 ft beneath the floor of the Uncompahgre River valley 

at the north edge of the mining district compared to about 

3,000 ft near Lake Lenore (pl. 1, section F–F′). Most of the 

difference in depth between these points is due to the rise in 

the valley floor being steeper on the average than the rise in 

attitude of the rocks. These estimates, however, do not consider possible changes in thickness of the Hermosa and Cutler 

Formations toward the northwest.

Mesozoic Structure

Mesozoic structures of the Uncompahgre mining district 

include features related to uplift of the San Juan Mountains 

region and the locally intruded suite of igneous plutons 

with related ore deposits in late Mesozoic to early Cenozoic 

(Laramide orogeny) time. The district is included as part of 

the southwest extension of the Colorado Mineral Belt (Tweto 

and Sims, 1963). Late Mesozoic uplift of the western San Juan 

Mountains region renewed deformation that originated in late 

Paleozoic time. This uplift, which began late in the Cretaceous 

Period, continued almost certainly into the Eocene Epoch. 

Here, as elsewhere in Colorado, the deformation very likely 

occurred in a number of stages that extended over a considerable amount of geologic time. Granodioritic magma intruded 

and locally arched the Upper Cretaceous beds, beneath which 

the magma spread out in sills and laccoliths. Tilting of the 

larger crustal blocks was accompanied by renewed movement 

on some of the older faults and by additional fracturing. Some 

small blocks close to the intrusive center were upthrown and 

tilted, but not all structural features can be directly correlated 

with this major period of deformation. This deformation was 

in turn followed by a period of very extensive erosion prior to 

deposition of the Telluride Conglomerate of early Tertiary age.

The pre-Telluride erosion surface allows the recognition and assignment of some of the more important structural 

features to Late Cretaceous to early Tertiary time. The most 

important features, dated by means of this unconformity, are 

the igneous intrusions, the regional uplift, and the folding and 

faulting of the Mesozoic and older strata. Many veins and 

faults were terminated at the Telluride erosion surface; however, some did not penetrate entirely through the Mesozoic 

strata to the unconformity.

The Telluride erosion surface is a pediment sloping north 

and west at less than 50 ft/mi to as much as 400 ft/mi. Within 

the Ouray area, several prominent residual hills or monadnocks above the general level of this surface are the laccoliths 

from which the upbowed upper Mesozoic strata were mostly 

removed by erosion. The most pronounced doming, resulting 

from the intrusions, can only be inferred by reconstruction of 

the eroded beds. All of the laccoliths intruded at or near the 

base of the Upper Cretaceous shales, except one large body 

emplaced near the base of Jurassic strata.

In the canyon walls of the Uncompahgre River and its 

tributaries, the laccoliths now exposed lie for the most part 

beneath the level of maximum local uplift and constitute the 

roots of a laccolithic mountain group. In this respect, the 

Figure 20. View looking northwest at the west wall of the Uncompahgre River valley between Oak and Corbett Creeks showing a 

monocline involving upper Paleozoic strata. The Paleozoic rocks are unconformably overlain (dashed white line) by nearly flat to gently 

north-dipping Mesozoic strata. Source is photographs nos. 239 and 240 by E. Howe, from U.S. Geological Survey Geologic Atlas, Folio 

153 (Cross and others, 1907).

Structure 43
structure within the district is similar to that of many other laccolithic mountain groups of the western San Juan Mountains 

and nearby Colorado Plateau, which for the most part reveal 

only the superstructure of the domed areas. In the root zone of 

the local group, however, it is difficult to distinguish whether 

uplift resulted only or mainly from regional deformation or 

from a deeper unexposed spreading of magma. North of the 

Uncompahgre district, a gentle monocline in the Mesozoic 

rocks may possibly represent the margin of basement uplift 

of the San Juan Mountains. Southward through the district 

toward the center of the mountain mass, the Mesozoic strata 

rise gradually in altitude, but what part of this elevation is 

due to local igneous intrusion and what part to the regional 

deformation cannot be accurately determined. For this reason, 

details of the Mesozoic structure are discussed below under 

three headings: (1) folding and tilting, (2) faulting and fracturing, and (3) structure of intrusive bodies, particularly the 

superstructure and roots of the laccolithic group.

Folding and Tilting

The regional structure of the northwest flank of the San 

Juan Mountains has been described by Whitman Cross and 

his associates (Cross and Purington, 1899; Cross and Spencer, 

1900; Cross and others, 1907; Larsen and Cross, 1956). The 

Mesozoic strata in the Ouray region and the adjacent region 

to the west and south dip gently north to northwest with a 

gradient ranging from about 250 ft/mi to 400 ft/mi. Tilting 

preserved on the flanks of the Mesozoic uplift is somewhat 

less than that which developed on the flanks of the Paleozoic 

(ancestral Rockies) uplift. In general, the width of the deformation zone during the Mesozoic (about 9–12 mi) appears to 

have been considerably greater than that during the Paleozoic 

(about 3–5 mi). This moderate tilt away from the center of the 

western San Juan Mountains is broken here and there, just as it 

is in the Paleozoic rocks, by abrupt monoclines.

Two major Mesozoic monoclines can be seen in the 

Uncompahgre River valley. One, shown in figure 18, is about 

2 mi north of the mining district, and is a comparatively gentle 

fold that takes the Paleozoic strata beneath the valley floor. 

The other fold is about 5 mi north of the mining district and 

also is a comparatively gentle fold; however, within the short 

distance of about 3 mi it has taken all the Mesozoic strata, 

with the exception of the Mancos Shale, beneath the valley 

floor. The declination of the formations to the north-northwest 

as a result of these monoclines is somewhat in excess of 2,500 

ft, and is nearly the same as that of the several monoclines of 

late Paleozoic age within the Uncompahgre district. These two 

monoclines apparently merge into a single fold elsewhere in 

the western San Juan Mountains region; they can be traced 

laterally along the mountain front from the Uncompahgre 

River valley, with short distances covered by younger Tertiary volcanic rocks, about 5 mi northeast to the Cow Creek 

drainage and about 10 mi southwest to the San Miguel River 

drainage near Telluride (fig. 1), south to Ophir, thence to other 

valleys south and southeast. The concealed trend of the fold 

axis west of the Uncompahgre River valley swings to about 

S. 40° W. around the northwest nose of the San Juan uplift. At 

Telluride, downwarping of the strata appears to be less, about 

1,300 ft, but near Ophir it is again in excess of 2,500 ft. The 

length of this fold and its conformity to the general concaveeast outline of the mountain mass clearly indicates that it is a 

product of regional deformation, perhaps of a deeper plutonic 

origin, not the result of igneous intrusion at any particular center. The total declination of Mesozoic strata in the Uncompahgre sector of uplift, about 4,500 ft from the south to north, is 

the combined result of this monocline and local tilting. This 

is about the same order of magnitude as that estimated for the 

combined folding, faulting, and tilting credited to late Paleozoic deformation. The Mesozoic decline did exceed that of the 

Paleozoic by about 500 ft, a difference caused, perhaps in part, 

by later tilting of Tertiary age.

Within the Uncompahgre district the Mesozoic strata are 

gently tilted, and nowhere except near an intrusive body do 

the beds dip 10° or more. The strata east of the Uncompahgre 

River valley tilt north to northeast, in contrast to west of the 

valley and in the valley of Canyon Creek, where the strata tilt 

3°–4° north to northwest. This difference in attitudes of strata 

on the two sides of the valley indicates that the so-defined east 

and west blocks probably are separated by a zone of faulting 

or flexing, or both, coincident with a radial axis of uplift; this 

anticlinal axis (Uncompahgre axis) trends north-northwest 

approximately parallel to the river valley (fig. 18). Dislocation 

of the east and west blocks described above is of more than 

academic interest as it may account, in part, for the distribution and origin of mineral deposits in the sedimentary rocks, 

particularly on the east side of the valley.

Faulting and Fracturing

It has been shown above that there is a considerable 

discordance in the direction and amount of tilt of blocks on 

the east and west sides of the Uncompahgre River valley as far 

north as Dexter Creek. The more competent beds have been 

distorted and fractured along the boundary zone now marked 

by the river valley; also locally, faulting dislocated some of the 

beds. The north-south and east-west components of tilt in the 

block east of the valley differ from those in the block west of 

the valley, suggesting the blocks were the result of torsional 

stresses rather than of compression.

The greatest distortion was in a zone extending from 

Ouray northward (downvalley) for about 2 mi (fig. 18). The 

valley walls diverge to the north and the strata on either 

side become less tilted; any discordance, therefore, cannot 

have been the result of comparatively gentle bending of the 

beds. South of town, the interpretation of and differentiation 

between structures of Paleozoic and Mesozoic age are more 

difficult. Between the Ouray fault and Portland Creek, minor 

diverging faults, fractures, and reversals in the dip of beds 

in the Leadville Limestone may be in part younger than the 

Ouray fault. In this area, the limestone is traversed by numerous minor north- to north-northwest-striking veinlets and 

44 Geology and Ore Deposits of the Uncompahgre Mining District, Southwestern Colorado
fractures along which the beds were recrystallized and minor 

amounts of vein quartz and sulfides deposited. This minor 

fracturing was of late Mesozoic age, as was the mineralization. 

In addition, other faults illustrated in section A–A′ (pl. 1) may 

have been reactivated by any local movement along the Ouray 

fault in late Mesozoic time.

Evidence that a concealed fault (Uncompahgre “break”), 

or possibly a flexure of beds, beneath the town of Ouray (fig. 

19A; pl. 1, section A–A′) is furnished by an outcrop of the 

Leadville Limestone exposed at the eastern edge of town. 

Extensive deposits of travertine at this locality indicate a 

long-continued issuance of modern hot-spring waters, and the 

altitude of the exposed limestone suggests the existence of the 

fault shown hypothetically in sections A–A′ and C–C′ (pl. 1).

At one locality, almost due south of town, the Ouray fault 

has been broken by a minor north-striking fault, which disappears in the narrow slate layer south of the limestone and fault; 

adjacent quartzite on strike with this minor fault is not broken. 

Other minor cross faults cut the Ouray fault where it crosses 

the Uncompahgre canyon as well as at its extreme southeastern exposure.

Rocks cropping out in the Uncompahgre River valley 

walls within 2 to 3 mi north of the Ouray fault afford evidence 

for sharp flexing or minor dislocation formed in conjunction 

with the northeastward tilt of the block bordering the valley on 

the east. Southward, this flexing appears to fan out into minor 

breaks and to die out for the most part at the slate layer in the 

south wall of the Ouray fault. Toward the north, this deformational feature is assumed to pass into a gentle anticline or very 

gentle monocline. The position of this deformation zone and 

the apparent connection between abnormal tilting east of the 

valley and crosscutting intrusive bodies suggest the resultant 

forces are of local origin, and probably, in part, are the direct 

result of intrusive activity. This zone probably constituted a 

vent or “leak” that extended upward through the Mesozoic 

strata, and thus tended to concentrate the lateral migration of 

gases and mineralizing solutions in the surrounding beds.

A series of north-northwest-striking veins, fractures, and 

minor faults parallels the concealed zone of flexing in the 

Uncompahgre River valley. These features are present on both 

sides of the valley (fig. 18), especially on the east as far north 

as Gold Hill, about 1.5 mi north of Ouray; beyond that point 

they are more widely spaced or inconspicuous. One of the 

largest faults of this trend is found on the west side of the valley just north of Oak Creek. This fault strikes N. 30° W. and 

has a displacement of about 50 ft, with the east or upthrown 

side having movement the same as supposed for the main 

valley break. Another fault, probably of this same system, can 

be seen crossing the western spur of Cascade Mountain, just 

north of Cascade Creek, on the east side of the valley. Its east 

side was upthrown for only a few feet. Several fractures and 

minor faults on the slopes of Cascade Mountain parallel this 

fault. The distribution and strike of these fractures suggest 

they are the result of stresses associated with the local tilting 

of the strata. In some mines, this fracture system is complemented by other, less conspicuously developed systems, in 

particular the competent quartzitic sandstone beds. The course 

of the Uncompahgre River valley might have been influenced 

by this structural weakening of the rocks along this trend.

The strongest group of fractures in the mining district 

includes a few dikes and is most prominent just north of the 

town of Ouray. Both the fractures and dikes are closely spaced, 

and swing in trend from N. 55° E. at the southwestern edge of 

the zone to N. 80° E., or nearly due east, at the northeastern 

edge. Farther north, individual breaks are more widely separated 

and consist of a few east-trending dikes and veins. At some 

places, the dikes and veins appear to constitute two systems, one 

striking northeast and the other more nearly due east; however, 

this division does not appear to be significant. One of the most 

prominent faults strikes about N. 55° E. across the Uncompahgre River valley, trending up the spur of Cascade Mountain (fig. 

18); to the west, it is well exposed where it crosses the mouth of 

Oak Creek canyon and the Ouray fault to parallel the valley of 

Canyon Creek. The fault is downthrown to the north. Displacement on this fault at Oak Creek is about 150 ft and at Cascade 

Mountain about 100 ft; the displacement appears to decrease 

both to the southwest and to the northeast. About 2,000 ft to 

the north, a nearly parallel fault, with similar displacement, 

terminates to the east in the intrusive bodies of The Blowout 

area. This fracture system is conjectured to be connected to the 

deeper crustal deformation that permitted invasion of the granodioritic magma, but there is no indication that the local structure 

is related to possible structures in the underlying Precambrian 

basement. The average trend of the fracture system, however, is 

similar to that of the local northeast-trending margin of the San 

Juan Mountains uplift.

A number of minor, but locally important, fractures and 

faults possibly of late Mesozoic age cut the Proterozoic and 

Paleozoic rocks south of the Ouray fault. One fault striking 

about S. 45° W. displaced the overlying Paleozoic beds, and 

perhaps the Tertiary volcanic rocks on the spur of Hayden 

Mountain. It is uncertain whether this fault was first reactivated in late Mesozoic time, because the unconformity 

between the Paleozoic or Mesozoic and Cenozoic strata is not 

preserved here. On the benches south of Ouray and west of 

the Uncompahgre River, a series of northeast- and northweststriking minor faults were important in localizing the mineral 

deposits; they are considered to be of Late Cretaceous to early 

Tertiary age rather than late Tertiary. The zone of faulting follows the east-northeasterly trend of the quartzite-slate contact 

in the underlying Proterozoic rocks, and probably reflects 

minor differential movements between the slate and quartzite 

beds. Although evidence is lacking, the pronounced tilting 

of the Mesozoic strata east of the Uncompahgre River valley 

probably reactivated most pre-existing fault lines. The resulting displacement, however, was very small.

Some displacement occurred on faults subsidiary to the 

Ouray fault in the Mesozoic strata north of Canyon Creek, 

but the absence here of beds of Tertiary age prevents distinguishing how much movement occurred during the Laramide 

orogeny and how much during later Tertiary time. At the 

southeast end of the Ouray fault, there was definite movement 

Structure 45
in late Tertiary time because the faults extend into the overlying San Juan Formation. The evidence suggests the Ouray and 

associated faults were reactivated during all major periods of 

deformation.

Structure of Intrusive Bodies

Intrusive rocks are an important part of the structural history in the Uncompahgre mining district, and provide data as 

to the age of certain structural features. Structure of the intrusive bodies is considered on the basis of (1) their root zone and 

(2) their internal structure and relation to their sedimentary 

superstructure.

Dikes and Sills

Intrusive bodies in the root zone include dikes, sills, and 

other crosscutting bodies that occur, with few exceptions, 

within the east-northeast-trending to northeast-trending zone 

of fracturing (intrusive axis) just north of the town of Ouray. 

This zone, having a width of about 1.5 mi where it crosses the 

Uncompahgre River valley, has at least 15 dikes exposed in 

the steep walls over a vertical range of more than 2,000 ft; less 

than half that number crop out north of this zone. The dikes in 

this zone tend to converge toward the large crosscutting body 

in The Blowout (fig. 12). The individual dikes, with a few 

important exceptions, trend from N. 55° E. to due east, and are 

vertical or dip very steeply southeast. Most of the dikes cut 

the Paleozoic and Mesozoic strata beneath the horizon of the 

Mancos Shale, but a few penetrated the laccolithic bodies, and 

one cuts entirely through the laccolith into the overlying shale. 

Many of the dikes in this root zone merge with concordant 

intrusions or pass upward into sheets or sills.

Some of the dikes appear to be feeders formed from the 

earliest magma injected into the sedimentary rocks. One of the 

larger of these northeast-trending feeder dikes, known locally 

as the Speedwell dike (fig. 12), crops out on the west side of 

the canyon just northwest of Ouray (Luedke and Burbank, 

1981). The dike is locally 50 ft or more in width, and is irregular in shape, in part, because it was injected along intersecting 

fractures and, in part, because of the tendency of the sedimentary beds to split along stratification planes. This dike extends 

northeastward to merge with the large crosscutting body of 

granodiorite in The Blowout. Directly southwest along the 

vein-fault extension of this dike, about 3,000 ft and 5,000 ft 

from the river, two short dike segments crop out within the 

Mesozoic strata higher on the north wall of Oak Creek canyon 

(fig. 12). These dike segments suggest that the main break 

probably extends southwestward for some distance beneath 

the less competent mudstones in the upper part of the Morrison Formation. Each dike segment abuts against a fracture 

trending N. 30° W.

This N. 30° W.-trending fracture system east of the 

Uncompahgre River valley is thought to be local and the result 

of northeastward tilting of the strata. Only a few dikes occupy 

this fracture system; they are short or discontinuous but were 

seen within mines on Gold Hill and Cascade Mountain. Here 

also, east of the valley, some dikes of the dominantly eastnortheast-trending system terminate against this northwesttrending system of faults and fractures. The northwest-trending system probably formed before the younger rocks were 

invaded by magma; it very likely was of comparatively shallow origin, owing to the fact that tensional stresses produced 

by stretching above axes of bending or of doming increase 

toward the surface. The northwest-trending fractures were 

sufficiently open only in the shallower and more competent 

strata that did not yield by stretching, whereas the east-northeast-trending fractures were thought to be of very deep-seated 

origin and undoubtedly the main feeding channels of magma 

from depth. These conclusions bear directly upon the influence that the two fracture systems had upon localizing mineralization. The east-northeast-trending system contains the 

most persistent veins in the district that are mineralized to the 

greatest depth, whereas the northwest-trending system, serving 

as local channels for ore solutions only in the competent strata, 

rarely contains veins of much economic importance.

The few east-trending dikes cropping out farther to the 

north within the district, in the vicinity of Dexter Creek (fig. 

12), differ as described (p. 32) from the granodioritic dikes 

to the south. These east-trending dikes, including several 

crosscutting plugs of altered intrusive rock near the mouth of 

Dexter Creek canyon, from Lake Lenore north (Luedke and 

Burbank, 1981) are not found in any mines or much east of 

their mapped surface terminations. Their trend and associated 

breccia and (or) clastic injections, however, clearly suggest 

that they formed during the same period of igneous activity. 

Hence, the more northerly fractures of the east-trending system seem to contain only a minor amount of igneous material 

that perhaps was fed steeply upward from a source beneath the 

east side of the valley; contained linear rock fragments, plunging 10°–65° E. in these dikes, support this interpretation.

A few dikes, including the prominent one known locally 

as the “Jonathan dike” that trends N. 30° E., have cut into or 

through the larger, overlying laccoliths on Cascade Mountain 

and on Gold Hill (fig. 12). These younger intruding and apparently less common dikes do not, however, differ appreciably in 

texture and mineralogical composition from those constituting 

the main granodioritic dike swarm.

Most clastic dikes (fig. 12) occupy fractures of the easttrending system and most, if not all, postdate the igneous 

dikes. Some clastic dikes are exposed only locally; others, 

such as the Nettie clastic dike on Gold Hill, or the BachelorPony Express clastic dikes south of Dexter Creek, have been 

traced for 1 to 2 mi in both surface and mostly underground 

exposures. Linear and platy rock fragments in the clastic dikes 

cropping out east of the Uncompahgre River valley indicate 

the dikes were injected upward from the east (fig. 16) at 

angles ranging between about 10° and 55° above the horizontal. Where the dikes were restricted or confined by overlying 

barriers of impenetrable or poorly fractured rock, the linear 

features nearly always parallel the dip of the local bedding 

planes. A few dikes contain east-plunging linear elements 

46 Geology and Ore Deposits of the Uncompahgre Mining District, Southwestern Colorado
even where apparently not confined by immediately overlying obstructions. The Abbey clastic dike (fig. 12), cropping 

out about 1,750 ft south of Dexter Creek, terminates in the 

Mancos Shale, but most of the other clastic dikes failed to 

rise that high in the stratigraphic section. The Bachelor clastic 

dike, traced underground, clearly terminated at several places 

within the upper part of the Dakota Sandstone, or at the base 

of the Mancos Shale in a zone of churned soft shales. On the 

basis of these features and the eastward extension of the clastic 

dikes in mine workings, it was concluded their energy source 

originated farther east of the Uncompahgre River valley at a 

different and probably somewhat shallower depth than that for 

the altered porphyritic dikes in the vicinity of Dexter Creek. 

The projected intersection of their contained linear features 

with those of the east-northeast-trending dike swarm may 

perhaps indicate their ultimate sources.

No internal linear features were seen in the few clastic 

dikes on the west side of the Uncompahgre River valley. They 

are thought to have terminated not far to the west, and probably were injected vertically, although their origin may have 

been deep to the east.

An exception to the general distribution of dikes, as 

outlined, is the presence of an altered porphyritic dike and a 

clastic dike exposed in the valley of Angel Creek and Canyon Creek, respectively, about 1.5 and 1.25 mi southwest of 

Ouray (fig. 12). Outcrops of these dikes are mostly concealed 

by glacial debris; however, both dikes were seen almost side 

by side in an adit near the stream level of Canyon Creek. The 

dikes trend N. 70° E. and are vertical. The clastic dike contains 

a considerable amount of altered rhyolite (fig. 15A) in addition to fragments of Proterozoic metamorphic and Paleozoic 

sedimentary rocks. Structures within the clastic dike suggest 

that it was injected almost vertically. The linear alignment of 

hornblende crystals in the igneous dike at Angel Creek indicates that it plunges vertically to steeply northeast. The dikes 

suggest the main intrusive zone’s southwest continuation at 

depth across the Ouray fault, at least as far as Angel Creek, but 

the exposed expression has greatly diminished in that direction. These two dikes, otherwise, are the only evidence favoring any possibility of an additional magmatic source beneath 

the laccolith exposed in Canyon Creek.

Sills associated with the east-northeast-trending dikes are 

few. The larger sills, without exception, occur in the southern 

part of the dike swarm. This distribution is probably related 

to structure and to the cover of Paleozoic strata, which thins 

southward toward the mountain uplift. The decreased lithostatic load would allow a greater penetration of magma into the 

sedimentary rocks. Sills penetrating northward along a given 

bedding plane within the Paleozoic strata would have had to 

lift a much greater thickness of overlying beds.

A few sills of limited extent on the east side of the 

Uncompahgre River valley penetrated the steeply inclined 

beds of the Hermosa Formation near Skyrocket Creek. One 

thin sill extends south of the main intrusive zone for nearly a 

mile along the north side of The Amphitheater to terminate at 

the base of the Entrada Sandstone. The largest sills are in the 

Cutler Formation on the west side of the valley (pl. 1, sections C–C′, D–D′), and they extend almost unbroken toward 

the southwest, where they join the Canyon Creek laccolith. 

Similar to the sill in The Amphitheater east of the valley, some 

of the larger sills rise stratigraphically from their presumed 

sources by cutting across bedding. Where interrupted by 

older, northwest-trending faults and fractures, they tend to be 

diverted along these breaks to a higher position; this structural 

feature is well shown where the sills cross the older Ouray and 

associated faults (Luedke and Burbank, 1981; fig. 12). The 

sills diverge from the Speedwell dike at an altitude of about 

8,500 ft and gradually thicken southwestward, reaching a 

thickness of 75 ft before joining the Canyon Creek laccolith at 

an altitude of about 9,000 ft. Their increase in altitude suggests 

the magma moved to the southwest; however, their thickening 

to the southwest suggests also that their source may have been 

either a concealed extension of the main dike swarm or one of 

the larger dikes. Unlike the other laccolithic mountain centers 

of the western San Juan Mountains region, the Rico and La 

Plata Mountains centers contain many moderately thick sills; 

fewer sills occur in the Ouray center and are presumed to be of 

little significance to uplift in the root zone.

Stocks and Plugs

About a mile north of Ouray, a roughly square-shaped 

intrusive body, or stock, occupies an area of extremely altered 

rocks between Bridalveil and Skyrocket Creeks, and is known 

and labeled as The Blowout in figure 12. Near the head of 

Skyrocket Creek, the eastern and southern contacts of this 

body are exposed vertically more than 1,000 ft beneath the 

Tertiary unconformity through the Mesozoic section and some 

underlying Paleozoic beds. Its northwestern contact is exposed 

through a lesser vertical range of sedimentary rocks, partly 

because of the tapered connection to the laccolith to the north. 

Two west-projecting prongs of the intrusive body taper westward into dikes. The larger prong is thought not to be entirely 

crosscutting but, as shown in section F–F′ (pl. 1), is probably 

a semi-concordant body whose roof was lifted or tilted by 

magma injection. The main body, or stock, probably does not 

extend far eastward; however, its extent, particularly to the 

northeast beneath the overlying Tertiary San Juan Formation, 

is indeterminable. The basal part of the San Juan Formation 

there contains many fragments of both altered porphyritic 

granodiorite and sedimentary rocks. The sedimentary rocks 

surrounding the stock have been intensely metamorphosed, 

altered, and mineralized. Several large blocks of sedimentary 

rocks are enclosed within the larger eastern part of the stock 

(Luedke and Burbank, 1981), but their exact position and attitude are unknown. As previously stated, the stock is thought to 

represent the magmatic plug within a former volcanic vent.

The localized structure attributed in part to intrusion of 

the stock is the convex fault that forms the south boundary of 

The Blowout area (fig. 18). This fault can be traced eastward 

from near the mouth of Skyrocket Creek (Luedke and Burbank, 1981), where it strikes about N. 70° E. and is occupied 

Structure 47
by a dike (fig. 12) for more than 500 ft to a point where it is 

then concealed for about 2,000 ft by talus and slide debris. 

East of this cover the fault strikes about N. 55° E. through a 

small intrusive body to where it connects to the east contact of 

the stock (fig. 18). North of the fault, near the mouth of Skyrocket Creek, beds in the Hermosa Formation dip 50°–65° N. 

and have been injected by minor sills and dikes. Farther east 

and north of the fault, the sedimentary rocks appear to have 

been thrust upward because fossiliferous limestone beds in 

the Hermosa are exposed along Skyrocket Creek to an altitude 

of 8,700 ft. As shown in section F–F′ (pl. 1), the local position and attitude of the Hermosa beds suggest the block was 

thrust up 400 to 500 ft. The direction of dip or tilt of the beds 

north of the fault varies from 12° NE. to 20° NW., but was 

not entirely due to rotation of the block as a result of igneous 

intrusion; this also is the position where the north-dipping 

Paleozoic monoclinal fold crosses the valley. Other small 

blocks west and north of the stock indicate minor thrusting.

The rock in the stock differs somewhat from that in the 

nearby laccoliths; it is less porphyritic and contains more 

quartz and orthoclase than the rocks of the laccoliths. Rock 

alteration and erosion in The Blowout area mostly obliterated 

any evidence of renewed magma emplacement. The relative 

distribution, mineralogy, and degree of alteration of the different facies of intrusive rock, although vague, support such an 

interpretation. The most siliceous igneous facies occupies the 

central part of the stock and is generally less altered than the 

granodiorite of the stock margins or the laccoliths. Microscopically, the protoclastic structure suggests the rock crystallized under dynamic conditions. This facies is also thought to 

have been intruded late, but whether it or an earlier intrusion 

resulted in the upthrust along the south edge cannot be determined.

The small intrusive body of non-porphyritic granodiorite, which appears to be cut by the fault forming the south 

boundary of The Blowout area, is possibly the youngest in 

this area; this intrusive body invaded the Cutler Formation 

along the south side of Skyrocket Creek, is mostly unaltered, 

and has extremely irregular, gradational-appearing contacts. 

Its small size, the minimal structural disturbance, the intense 

recrystallization of the surrounding sedimentary rocks, and the 

fault apparently engulfed by it suggest comparatively static 

and quiet conditions and the possibility that it formed partly 

by extreme metasomatism. This is the only intrusive body 

here in which hematite crystallized contemporaneously during 

its emplacement. Nearby intrusive bodies also are veined by 

hematite and epidote.

Several small intrusive bodies or plugs of altered porphyry, located near the mouth of Dexter Creek canyon and at 

Lake Lenore (fig. 12), are related to east-trending dikes and 

(or) fractures in the northern part of the mining district where 

fractures are fewer and farther apart. These fractures formed 

early, and largely controlled the first invasion of magma. The 

intrusive bodies are structurally less important, but illustrate 

how the small necks or plugs were emplaced. Rocks in these 

small plugs are similar in chemical composition to the granodiorite of the stock in The Blowout, but are finer grained, have 

trachytic textures, and are commonly altered. Flow foliation 

and lineation are particularly characteristic of the rocks in 

both the larger of two bodies on the north side of lower Dexter 

Creek valley and in two small intrusive bodies near Lake 

Lenore. North of Dexter Creek, the eastern and smaller of 

the two plugs is composed chiefly of coarse intrusive breccia 

consisting of sedimentary and igneous rock fragments. A small 

amount of intrusive breccia occurs with the western and larger 

plug as well as with the associated dikes. The two intrusive 

bodies near Lake Lenore contain an unusually large amount of 

sedimentary rock inclusions in the dacitic matrix.

The bodies north of Dexter Creek are clearly related to 

dikes of the east-trending system. A dike forms the northeast 

prong or extension of the larger body and is exposed underground in a tunnel about 200 ft below ground surface; in this 

tunnel, there is an additional parallel dike that does not crop 

out on the surface. These two dikes are about 65 ft apart, strike 

N. 75°–80° E., and are vertical. Also underground, a breccia 

or clastic dike occurs adjacent to the northern dike. Linear 

structures on the walls of the larger southern dike plunge about 

50° E.; the south wall of this dike flares outward and upward. 

The plunge of the linear structures indicates roughly the direction of the magma movement. Linear structures in the dikelike prong at the northeast end rake 15°–20° E. Just east of 

this dike-like prong, there is a short dike composed of breccia 

containing igneous and sedimentary rock fragments, some of 

which are quartzite and shale of local origin.

The larger igneous body north of Dexter Creek was possibly localized, in addition, by the intersection of fractures of 

the east-trending system with those of the northwest-trending 

system; the west contact of this body is parallel to fractures of 

the latter system. At its northwest corner, the igneous body is 

cut by small breccia dikes having northwestward and eastward trends that parallel its internal flow structure. The flow 

structure dips steeply and generally is parallel to the contacts, 

except in the sill-like apophyses where it is more nearly horizontal. The north and west contacts are steep; the south contact 

near the east end is in part horizontal where the magma spread 

into shale beds near the top of the Pony Express Limestone 

Member of the Wanakah Formation. Emplacement of the body 

probably was accomplished in part by pushing aside the shale 

and sandstone beds at intersecting fractures and in part by 

fragmentation of the rocks, the fragments of which were carried along in the viscous melt.

Most of the breccia dikes and masses closely followed the 

injection of magma, but in a few instances may have preceded 

or accompanied injection. Any earlier-formed breccias must 

have been pushed upward or incorporated into the magma. 

Evidence that local explosive action did precede or accompany formation of an igneous body can be seen in the smaller 

ovate plug about 1,000 ft east of the larger body (Luedke and 

Burbank, 1981). It is composed chiefly of fragments of rock 

similar to that of the larger body but, in addition, contains 

blocks of crossbedded sandstone 3 to 4 ft across, resembling 

Pennsylvanian or Permian rocks, as well as abundant smaller 

48 Geology and Ore Deposits of the Uncompahgre Mining District, Southwestern Colorado
fragments of quartzite and shale. Whether this breccia forms 

the top of a filled vent or constitutes the bottom of an upwardflaring funnel could not be determined, as all exposed contacts 

are nearly vertical. Analogous with the other large clastic dikes 

of the mining district, which commonly contain fragments 

of Proterozoic rocks that have been carried many thousands 

of feet upward, this breccia body is interpreted to extend to a 

considerable depth. The abundance of fragments in some of 

the intrusive bodies in the northern part of the district, such 

as those near Lake Lenore, suggests that some fragmentation, 

whether or not of explosive origin, preceded and accompanied 

injection of the magma.

Laccoliths

The laccoliths are at a stratigraphically higher position 

than the intrusions previously described. Erosion cut deeply 

into the superstructure of the laccolithic center in early Tertiary 

time, before it was buried by local erosional debris and the 

younger volcanic rocks, so that the relation between individual 

laccoliths or between them and their respective feeder channels is preserved only in fragmentary form. Remnants of the 

laccoliths related to the center at The Blowout extend about 5 

mi to the southwest and northwest, about 2 mi to the northeast, 

and about 1.5 mi to the southeast. With the exception of the 

Canyon Creek body, which was intruded near the base of the 

Morrison Formation of Jurassic age, all the other laccoliths were 

intruded near the base of the Mancos Shale or into the upper 

part of the Dakota Sandstone, both of Cretaceous age (Luedke 

and Burbank, 1962). Except for the exposed central vent (stock) 

and dike swarm in the Uncompahgre River canyon, no other 

feeding channels for these bodies are recognized; the magma 

moved radially outward along the channels, forming tonguelike masses. These conclusions are based chiefly upon (1) the 

composition and stratigraphic positions of the laccoliths and the 

tendency for the bases of the laccolithic bodies to rise stratigraphically away from the Uncompahgre center, (2) the internal 

flow structures in some of the bodies, and (3) the apparent 

asymmetric distribution of the bodies and the structures beneath 

the level of concordant injection.

Three main laccolithic bodies exposed on the east side of 

the Uncompahgre canyon (fig. 12) are (1) the Gold Hill body 

prominently exposed high in the cliffs facing the Uncompahgre 

River valley and north of The Blowout, (2) the Dexter Creek 

body along the south side of that valley, and (3) the Cascade 

Mountain body. The Gold Hill and Dexter Creek bodies are 

separated by a screen of Mancos Shale, with their presumed 

connection covered by the shale and by volcaniclastic rocks of 

the San Juan Formation (Luedke and Burbank, 1981). Chemical 

analyses of the porphyritic granodiorite from these two bodies are similar in composition (table 7). The base of the Dexter 

Creek body is exposed only for a short distance along the bed 

of the creek; further evidence is needed to prove positively that 

most of this body is a laccolith and not a northeastward extension of the crosscutting intrusive body in The Blowout. However, the limited exposure of its floor at the northern margin, the 

absence of the siliceous protoclastic intrusive facies present in 

The Blowout, and the lack of intense alteration so prominently 

displayed in The Blowout area strongly suggest that the body is 

a laccolith. Flow structures are indeterminate within the Dexter 

Creek body where the granodiorite is massive or where the 

hornblende crystals are indistinct because of resorption. However, lineation near the margins of the body in its eastern part 

consistently plunge zero to 20° in an eastward direction; also, 

near the top and bottom, flow structure exhibited by feldspar 

and other tabular crystals roughly parallels the contacts of the 

body. At the easternmost exposures of the granodiorite along 

Dexter Creek valley this foliation dips between 10° and 25° 

N., somewhat steeper than the supposed dip of the concealed 

underlying sedimentary rocks. At the extreme west end of the 

body where it thins to a wedge, the horizontal lineation strikes 

north. Between altitudes of 10,000 and 10,300 ft on a ridge 

above and south of the large landslide mass along Dexter Creek 

canyon, the lineation strikes northeast, almost directly away 

from the northeastern edge of the stock in The Blowout. The 

internal structures, therefore, strongly suggest that the Dexter 

Creek body was fed from the Uncompahgre center. They further 

suggest the magma flowed north to northeast from the center as 

far as the exposed margins of the body in Dexter Creek valley. 

The eastern part of this body is quite thick at the point where it 

is covered by volcanic rocks; because there is little change in 

attitude of the flow structures here, the body may extend 1,000 

to 2,000 ft to the east beyond the exposure.

In addition to the flow structures, several prominent joint 

sets developed in all parts of the Dexter Creek body. The most 

prominent of the joint sets strike roughly north and east, and 

commonly are vertical or dip steeply in either direction. Foliation, lineation, and jointing in the highest outcrops of the body, 

at an altitude of about 10,500 ft, suggest the former intrusive 

margin (top) of the laccolith was near the surface now exposed. 

Exhumed outcrops of granodiorite, in the eastern part of the 

body, have a blocky weathered surface that is plastered with tuff 

breccia and has joints filled with tuff of the San Juan Formation.

A few lineation measurements in the southern part of the 

Gold Hill body trend west to northwest. The small, narrow 

intrusive mass between the Gold Hill and Dexter Creek bodies 

has a north-south horizontal lineation indicating this exposed 

part had penetrated northward, strongly suggesting it connects 

with the two large bodies. The small body above the Calliope 

mine, at Mill Creek north of Dexter Creek (fig. 12), is an erosional outlier of the main Dexter Creek body. Other than the 

northernmost tip of the Gold Hill body, the exposures of the 

northern margin of the Dexter Creek body are within the Mancos Shale from 25 ft to several hundred feet above its base. 

The intrusive bodies near The Blowout are mostly at the top 

of the quartzite in the Dakota Sandstone, with less than a few 

inches to a few feet of shale parting. The Cascade Mountain 

body, now separated by erosion from the intrusive center, was 

undoubtedly fed from this center.

There are two laccoliths west of the Uncompahgre River 

valley in the mining district (fig. 12): the Twin Peaks body, 

exposed mostly west and northwest of the district (Luedke 

Structure 49
and Burbank, 1962), and the Canyon Creek body, exposed 

mostly southwest of the map area (Burbank, 1930, pl. 1). The 

Canyon Creek laccolith is about 1,000 ft thick and more than 

1.5 mi long on the northwest wall of Canyon Creek valley 

southwest of Angel Creek. Of these two laccoliths, only the 

Canyon Creek body was examined closely for structural data. 

Evidence has been presented to show that sills joining the 

laccolith were fed southwestward from the dike swarm in the 

Uncompahgre River valley; it was shown also that the base of 

the sills rise stratigraphically away from the intrusive center. 

The base of the laccolith varies little in position at the base 

of the Morrison Formation from its rise of several hundred 

feet above the Dolores Formation at the northeast edge. Flow 

lineation (aligned hornblende crystals) is conspicuous along 

the base of the body. Random lineation measurements are 

consistent within the central part of the laccolith. The average trend of 15 measurements in the laccolith from the base 

to several hundred feet above the base is N. 40° E., and is in 

almost direct alignment with the dike swarm to the northeast. 

The lineation trend in the upper sill connecting to the laccolith, 

where the sill crosses Oak Creek (fig. 12), is about N. 50° E. 

Thus, the flow structures support the suggestion that both the 

sills and the laccolith were fed from the dike swarm and that 

magma in the concordant injections flowed radially outward 

from the center of intrusive activity. Little evidence exists to 

suggest the presence of another crosscutting body of appreciable size beneath the Canyon Creek laccolith. Therefore, 

a concealed center of intrusion and mineralization near this 

locality is unlikely. The lack of strong alteration in or around 

the laccolith further suggests the absence of another intrusive 

center here.

The part of the laccolith below the Twin Peaks is within 

the Dakota Sandstone. The total extent and original thickness of 

the laccolith cannot be determined because of cover by the San 

Juan Formation and landslide deposits (Luedke and Burbank, 

1962). Outcrops along the north side of Oak Creek valley west 

of Twin Peaks, as previously mentioned, have joints filled with 

volcanic debris; spheroidally weathered joint blocks have up to 

1.5-in.-thick weathered rinds and have been plastered by volcanic debris, indicating a former erosion surface. Large blocks of 

granodiorite are contained in the capping outlier of tuff breccia 

forming Twin Peaks. The laccolith extends more than a mile 

west of Twin Peaks, forming the ridge between the upper parts 

of Oak and Corbett Creeks, and reaches an altitude of about 

11,200 ft (Luedke and Burbank, 1962). Near the head of Corbett 

Creek valley west of the district, the outcrops also are at an 

altitude of 11,000 ft; this altitude may represent the probable 

former height attained by the laccolithic body. However, at this 

place the base of the body is thought to have penetrated the bedding several hundred feet above the base of the Mancos Shale, 

a feature apparently typical of the laccolithic bodies, which is 

to migrate upward as they spread away from the Uncompahgre 

intrusive center. The original total thickness of this laccolith was 

probably about 1,500 ft, a figure that was apparently a limiting thickness for all of the larger bodies associated with the 

intrusive center in the district. In no case, however, is it possible 

to determine how much rock was eroded from the current high 

parts of the laccolithic bodies.

Ore Deposits

Distribution and General Relations

The principal metallogenic epoch of primary ore deposition in the Uncompahgre mining district is the most important locally, and is closely related to the intrusive activity 

that accompanied regional domal uplift in Late Cretaceous 

to early Tertiary (Laramide orogeny) time. The intrusive 

activity resulted in dikes, sills, and laccoliths that crystallized 

under about a mile of Upper Cretaceous sedimentary rocks 

(Burbank, 1930, fig. 1). This sedimentary cover is thought to 

have been penetrated by a conduit in The Blowout area, now 

occupied by a small stock. Venting is indicated by volcanic 

rocks within an Upper Cretaceous formation that is exposed 

in the Cimarron Ridge area to the north-northeast (Dickinson 

and others, 1968; Steven and Hail, 1989). Extensive early 

Tertiary erosion removed most of the sedimentary cover 

in the area, resulting in a surface of low relief surmounted 

only by hills composed of the more resistant granodioritic 

laccoliths. Undoubtedly during this long period of erosion, 

some mineral deposits were partly exposed and subjected to 

weathering, and others were destroyed. The Eocene Telluride Conglomerate, particularly that part locally derived 

and deposited upon this erosion surface north of the mining 

district, has a very minor concentration of placer gold that 

proved to be of no economic value. During the Oligocene 

and Miocene, eruptions from volcanic centers south and east 

of the district formed an extensive volcanic field that covered 

the western San Juan Mountains region. This later igneous 

activity was accompanied by multiple episodes of major 

mineralization and alteration, most of which is of minor 

importance in the Uncompahgre district.

A few local ore deposits, whose age cannot be proved 

directly but which have the same structural character and 

mineral composition as those of proved (Late Cretaceous to 

early Tertiary) Laramide mineralization, are found entirely 

within pre-Tertiary rocks and are not related to a single 

important fracture or dike traceable into overlying Tertiary 

rocks. However, a few Laramide fractures that were reopened 

and extended into the overlying Tertiary rocks are barren 

or contain only minor gangue minerals; these veins cut the 

basal unaltered Tertiary volcanic rocks directly overlying the 

intensely altered and fractured rocks in and near The Blowout.

Ore deposits definitely correlating with the Laramide 

mineralization surround the Uncompahgre center of eruptive 

activity (The Blowout). The most productive ore deposits 

were within an area (fig. 21) extending from near Ouray 

northward to Cutler Creek, a distance of 4 to 5 mi, and from 

the Uncompahgre River valley eastward, a distance of about 2 

mi. A narrow fringe of deposits occurs along the west side of 

50 Geology and Ore Deposits of the Uncompahgre Mining District, Southwestern Colorado
the valley, but the intensity of mineralization there decreases 

abruptly westward. The few deposits on both sides of the 

valley that extends about 2 mi south from the intrusive center 

were less productive than those north of the center.

Geologic conditions that favored laccolithic intrusions 

during the Laramide orogeny also favored the formation of 

“blanket” replacement ore deposits. Fractures surrounding the 

stock penetrated upward into the overlying sedimentary beds 

only short distances, but some of the more permeable beds 

served as channels for the lateral diversion of the ore-bearing 

fluids and, particularly, where overlain by impermeable shale 

beds, were especially favorable to the diversion of the fluids laterally, sometimes for great distances. Ore deposits of 

the “blanket” type were formed in several different permeable beds in the sedimentary section. Some gradation exists 

between the vein and blanket deposits; a single deposit may 

include both types. Contact-metamorphic deposits were 

formed near the stock in The Blowout where higher temperatures prevailed.

The ores, containing gold, silver, lead, zinc, and minor 

amounts of copper, may be divided on the basis of their metal 

content into four groups: (1) magnetite-pyrite ores containing 

a little copper and gold; (2) pyritic ores containing copper and 

gold; (3) pyritic base-metal ores containing native gold plus 

gold and silver tellurides; and (4) siliceous and baritic ores 

containing silver, lead, zinc, commonly some copper, and a 

little gold. The productivity of the different groups increases 

in about the order stated, but group 3, because of its relatively 

rich deposits, returned the most value on small investments.

Deposits of the first two groups, on both sides of the 

Uncompahgre River valley, are clustered chiefly north and 

south (fig. 21) within a mile’s distance of the intrusive center 

(The Blowout). Deposits of the third group, richest in gold, 

are east of the valley and north within about half a mile of the 

intrusive center. These three groups constitute the so-called 

gold belt. Deposits of the fourth group, valuable chiefly for 

silver and lead ores and composing the so-called silver belt, lie 

outside the zones of highly pyritic ores both north and south of 

the intrusive center; the deposits in the northern area near Dexter Creek were by far the most productive. As stated, the gold 

content is low in the siliceous and baritic ores, and conversely, 

the lead content is low in the pyritic ores; zinc, common in 

both ore types, is more abundant in the outlying deposits. 

Silver-bearing minerals of the gray copper group, present in 

both the silver-lead ores and pyritic base-metal ores, are the 

most widely distributed valuable minerals.

Ore deposits of late Tertiary age within the Uncompahgre 

mining district are related, with few exceptions, to centers of 

igneous intrusion and mineralization that are several miles outside the district, mainly to the south and east (Burbank, 1941; 

Kelley, 1946; Vanderwilt and others, 1947, p. 419–443). The 

few veins of this age found around the margins of the Uncompahgre district, containing gold, silver, lead, zinc, and copper 

ores similar to those deposited earlier, are divided into three 

groups: (1) base-metal ores containing manganiferous carbonates and small to moderate amounts of gold and silver; (2) 

baritic silver-lead ores having a low to moderately high silver 

content; and (3) siliceous low-grade gold-silver ores consisting 

of native gold in quartz. The first two groups, which in places 

are filled compound veins containing both types of ore, are 

found along the southern margin of the Uncompahgre district 

as extensions of mineralized zones into the area from the 

south. In addition to these groups, there are fluorite-bearing 

veins in the Hayden Mountain area southwest of Ouray (fig. 

21). The only Tertiary deposits that do not appear to be related 

to the centers of mineralization south of the district are associated with the Cow Creek center about 5 mi to the east. These 

are siliceous gold-silver veins in volcanic rocks of the San 

Juan Formation in upper Dexter Creek valley to the northeast 

of Ouray, and a few similar veins east of Ouray.

Placer deposits of Holocene age along the Uncompahgre 

River valley northward from Ouray have proved to be of little 

economic consequence.

It is neither possible nor our intent within the limits of 

this report to describe the many individual deposits in the 

Uncompahgre mining district in great detail. Typical of mining 

districts, many deposits were not accessible at the time of our 

investigations (W.S. Burbank, late 1920s to mid-1930s; R.G. 

Luedke, late 1950s). Published descriptions have dealt with the 

general and detailed character of many of the ore deposits. Our 

intent, herein, is to point out certain relations of the ore deposits to the local structure during mineralization, with emphasis 

principally related to the Laramide orogeny, and to detail the 

evidence upon which the existence of the ore deposits was 

determined.

Late Cretaceous to Early Tertiary Ore Deposits

The principal ore deposits of the Uncompahgre mining 

district were formed after emplacement of the intrusive rocks 

of Late Cretaceous to early Tertiary time (Laramide) and are 

genetically related to them. It is inferred from the local structure 

and stratigraphy of the sedimentary rocks that the somewhat 

shallower deposits formed at a depth of a mile or less differed 

from those formed at slightly greater depths. This premise is 

based upon the presence of clastic dikes and the emplacement of 

orebodies that filled caves in the sedimentary rocks.

The clastic dikes provide evidence of the close tie between 

igneous and mineralizing processes. The clastic dike associated 

with the Bachelor silver vein is typical of this kind of eruptive 

activity. Its composition, characteristics, and relation to the ores 

suggest that the dike is a true injection of clastic material torn 

from the walls of the fracture at depth and propelled upward 

(diagonally or laterally in places) by expanding gases genetically related to the igneous and mineralizing activity. A number 

of examples found in the district suggest that some clastic dikes 

were closely associated with the intrusion of igneous rocks, 

whereas others were locally independent of them. Some dikes 

clearly formed between the early and late stages of vein deposition and thus were closely associated with formation of the ore 

deposits. These dikes are thought to have been formed by the 

Ore Deposits 51
52 Geology and Ore Deposits of the Uncompahgre Mining District, Southwestern Colorado

Figure 21. Generalized geologic map of the Uncompahgre mining district and vicinity within the Ouray 7.5-min quadrangle showing the 

approximate location (numbered positions) of the principal mines or groups of mines, prospects, mills, and hot spring (see list). Modified 

from Burbank (1930, 1940) and from Luedke and Burbank (1962, 1981).
sudden expansion of mineralizing solutions during reopening of 

the veins. A typical mineralized fracture, therefore, was filled by 

an igneous dike, then by major amounts of premineral injected 

clastic material, and finally by vein material and minor amounts 

of intramineral clastic material. This is the normal sequence 

throughout the district, but it was not complete in all veins.

In the central part of the mining district near intrusive 

contacts, the mineralization sequence began locally with the 

development of contact-metamorphic silicates, minor amounts 

of magnetite, and pyrite. However, in most parts of the district 

the silicate stage was lacking and mineralization began with 

the formation of sericite, ankerite, siderite, chlorite, calcite, 

and pyrite through metasomatic replacement of the wall rocks. 

Both of these processes were interrupted by local reopening 

of the fractures and the deposition of specularite, by local 

intramineral clastic injections, and by an interval during which 

cavities were formed in the rocks by hydrothermal leaching. 

Deposition of the principal sulfide and gangue minerals followed (table 11).

Large cavities formed locally by hydrothermal leaching along the bedding planes in both sandstone and limestone 

served as receptacles for ore and gangue minerals. Some 

cavities, however, were never completely filled and now 

contain only thin crusts of mineral crystals; others were partly 

or completely filled with higher grade ores while the adjacent 

wall rocks apparently were partly replaced by lower grade 

ores. Two noteworthy examples are the bedding-channel 

deposits in the Dakota Sandstone in the American Nettie 

mine (fig. 21, no. 17) and the bedded or channel deposits in 

the Leadville Limestone and Molas Formation at the Mineral 

Farm mine (fig. 21, no. 35). Irving (1905, 1911a) recognized 

that some of the orebodies in the American Nettie mine were 

formed by cavity filling, although he considered replacement to have been the dominant process. If the entire body of 

altered rock and the lower grade pyritic bodies in the American Nettie ore channel are considered, the total volume of 

replaced rock greatly exceeded the open space that was filled 

with ore. Regardless of the processes involved, the original 

fluids were sufficiently undersaturated with rock constituents 

to effect a considerable solution of the channel walls, and were 

responsible for the leaching activity. The fact that silica was 

carried away in considerable volume suggests the presence of 

a condensed medium of some proportion. Saturation probably was rapidly attained, as a certain proportion of the silica 

dissolved from the sandstone was precipitated in pores of the 

rock short distances from the main channels of mineralization. 

The enclosing walls of these channels became converted to 

hard, massive quartzite, which was very much less permeable than the original sandstone. From experience, the miners 

considered the hardness or toughness of the quartzite a good 

indicator of the grade of ore likely to be encountered, as well 

as a good indicator of the nearness to an ore channel. Induration of the sandstones, or their conversion to quartzite, and 

silicification of limestones preceded the main period of sulfide 

formation.

Table 11. Principal primary ore and gangue minerals and some 

more-prominent secondary minerals, classified according to the 

principal groups, for Late Cretaceous to early Tertiary deposits in 

the Uncompahgre mining district, southwestern Colorado.

[Modified from Burbank, 1940]

Contactmetamorphic 

deposits

(magnetite-pyrite)

Pyritic deposits

(copper-gold and basemetal, with gold and silver 

tellurides)

Siliceous and 

baritic deposits

(silver-lead-zinc)

Primary (hypogene) metallic minerals

Magnetite Pyrite Galena

Pyrite Chalcopyrite Sphalerite

Chalcopyrite Sphalerite Pyrite

Specularite Galena Chalcopyrite

Tetrahedrite Tetrahedrite

Tetradymite Pearceite

Hessite Polybasite

Benjaminite Pyrargyrite

Gold Proustite(?)

Primary (hypogene) gangue minerals

Andradite Sericite Quartz

Hornblende Quartz Chalcedony

Actinolite Chlorite Chert

Epidote Calcite Jasper

Quartz Siderite Barite

Calcite Barite Rhodochrosite

Thuringite Ankerite

Stilpnomelane Calcite

Chlorite

Apatite

Rutile

Siderite

Secondary (supergene) minerals

Limonite Limonite Limonite

Clay minerals Goethite Azurite

Goethite Jarosite Malachite

Jarosite Copiapite(?) Chrysocolla

Chalcocite Clay minerals Native silver

Covellite Chalcocite Argentite

Malachite Malachite Chalcocite

Azurite Azurite Covellite

Cerussite Sulfur

Gold(?) Calcite or aragonite

Hydrous sulfates of iron, 

copper, and other metals

Gypsum

Sulfur

Native copper

Ore Deposits 53
In the gold belt, quartz, ankerite or siderite, and sericite 

were the principal gangues deposited with the early sulfides. 

The general order of mineral deposition was pyrite, chalcopyrite, sphalerite, tetrahedrite (with additional chalcopyrite), 

tetradymite, benjaminite, galena, hessite, and native gold. 

Although hessite was the principal telluride, small amounts 

of petzite and possibly calaverite(?) were deposited during 

the telluride stage of mineralization. Barite, sparingly present 

except in the cave deposits, formed later than the cupriferous 

minerals of the gold belt and was most closely associated with 

galena-bearing ores, or was deposited in relatively late barren 

veins with quartz. Late scalenohedral calcite also was deposited in the cavities.

In the silver belt, jasperoid and early quartz formed in the 

first mineral stage, which was preceded or accompanied by 

hydrothermal leaching of limestones or other easily attacked 

beds; shale and sandstone beds were not as strongly altered as 

those in the gold belt, but locally they were leached. Ankerite 

or ferruginous rhodochrosite, sericite, and pyrite formed by 

replacement in the second mineral stage. Barite deposition 

preceded the bulk of the base-metal sulfides that were deposited in order as pyrite, sphalerite, chalcopyrite and tetrahedrite, 

pearceite and galena, and locally pyrargyrite. Some rhodochrosite or manganiferous calcite accompanied the late basemetal sulfides. In a comparison of this sulfide sequence with 

that of the cupriferous or pyritic ores of the gold belt, notable 

differences are the lack of a prominent chalcopyrite stage 

between the deposition of pyrite and sphalerite, and the usual 

position of barite early in the sulfide sequence. In the silver 

belt, quartz, a little barite, and scalenohedral calcite formed as 

late barren gangues.

Zoning within the orebodies or shoots in many deposits 

conforms to the general sequence of mineralization: pyritic 

copper and zinc ores are nearest the sources, high-grade 

tetrahedrite-bearing silver ores are in an intermediate position, 

and baritic ores of low sulfide content are farthest from the 

possible sources of mineralization. Overlapping of the different sequences, beginning with the igneous dikes and extending 

through the clastic injections to the mineral stages, suggests 

that mineralization began in the inner pyritic or gold belt and 

that sulfide ores of the outer or silver zone were formed later. 

Thus, the ultimate sources of the ore-forming fluids may have 

migrated with time from their earliest position at the apex of 

the intrusive bodies near the crossing of the structural axes to 

positions very much deeper in the crust along extensions to the 

northeast or southwest of the intrusive axis. This possibility 

bears directly upon the question of whether zoning at depth 

corresponds with the lateral or areal zoning of the deposits. We 

think that the areal and depth zones do not correspond exactly; 

therefore, it is unlikely the most distant deposits will change 

in depth to those now found in the inner zone surrounding the 

shallower intrusive bodies.

The most favorable host rocks for the ore deposits in the 

Uncompahgre district are those possessing physical or chemical properties that in combination with the structural features 

tended to yield a maximum of available open space. Formation 

of open spaces, however, was not dependent solely upon fracturing of the rocks, but also upon the chemical aggressiveness 

of the mineralizing fluids. All conditions being equal, the more 

permeable host rocks appear to have been most susceptible 

to leaching; however, the rocks most susceptible to leaching appear to have been the most susceptible to replacement. 

Replacement appears to have been less selective near the 

eruptive center, but the grade of the ore was low. The minerals 

that replaced rocks most extensively included all the pre-ore 

alteration products such as carbonates, sericite, quartz, and 

chlorite minerals. Of the sulfides, pyrite and sphalerite most 

commonly accompanied those alteration products, but the 

later sulfides had a distinct preference for open-space filling or 

for the replacement of the earlier-formed sulfides. Where the 

earlier-formed sulfides, particularly pyrite, replaced the host 

rocks so as to form massive sulfide bodies, these bodies in turn 

acted as hosts to later fluids. Some of the highly pyritic, lowgrade sulfide replacement bodies in limy beds near the center 

of mineralization were later enriched locally by gold-bearing 

fluids that followed zones of fracturing.

In the Uncompahgre district, the less permeable beds that 

capped and underlay the sedimentary rocks tended to localize 

mineral deposition by restricting the circulation of ore-forming 

fluids to channels nearly conformable with bedding. The 

term “channel” or “ore channel” as used here applies to the 

main conduits along which ore shoots are found. Ore shoots 

in the favorable sedimentary rocks, where form and distribution were controlled by bedding or permeability of the rocks, 

are referred to as bedded deposits; most but not all of these 

conform to the bedding. In many ore channels, vein and bedded deposits occur together and have a small vertical range. 

As there are gradations between and variations in the number 

of vein to bedded deposits within the same mine, many ore 

deposits cannot be classified readily according to form. If the 

controlling break is a crosscutting fracture and the rock has 

clean fractures and a minimum of gouge, vein deposits are 

formed; if the controlling structure is a simple flexure, competent massive sandstone beds become jointed and fracture channels are formed. Descriptions of the favorable rocks indicate 

that in many places the rocks immediately above or below 

an unconformity had properties favorable for the formation 

of bedded deposits. The thicker formations in the district, in 

general, contained fewer favorable beds.

Structural Classification and Control

The classification of ore deposits presented here is based 

on the major and minor structural features, the outstanding 

structural feature being the crossing of the northeast-trending 

intrusive axis and the north-northwest-trending Uncompahgre 

axis of uplift (Burbank, 1940). This crossing divided the mining district into four parts, each of which is characterized by 

minor structural features (figs. 18, 21) that largely controlled 

the distribution, size, and shape of the orebodies. The four 

major parts, separated by the two axes and although slightly 

54 Geology and Ore Deposits of the Uncompahgre Mining District, Southwestern Colorado
skewed, are designated the northern, southern, eastern, and 

western sectors.

The major intrusive axis has a trend of about N. 55°–60° 

E. and is a zone about 2,000 ft wide, perhaps narrowing 

somewhat southwestward where it crosses the Uncompahgre 

River valley; this zone contains several dikes and irregular 

bodies of porphyritic granodiorite and related rocks in Paleozoic and Mesozoic sedimentary rocks. Both to the northeast 

and southwest the zone is concealed by overlying Tertiary 

volcanic rocks, but there is little doubt that rupturing of the 

rocks within and along this zone was of major magnitude and 

extended deep into the crust. The trend and position of the 

intrusive zone is perhaps related particularly to the northeasttrending Paleozoic and Mesozoic fronts of the ancestral San 

Juan Mountains north of Ouray.

The major structural axis trends N. 25°–30° W. along the 

Uncompahgre River valley, cutting diagonally across the trend 

of the intrusive zone. This axis marks the trend of several 

parallel or slightly divergent components dating from both 

the late Paleozoic and the late Mesozoic uplifts. Two of these 

components are radial axes of uplift and folding (fig. 18) that 

are evidently related in their position and trend to a change in 

direction of the Paleozoic and Mesozoic mountain fronts, and 

a third, the so-called Uncompahgre “break,” an axis of crustal 

extension indicated by faults or abrupt changes in the direction 

and amount of tilting of the sedimentary strata as shown on the 

geologic section across the valley (pl. 1, section C–C′). South 

of the intrusive zone, this “break” appears to feather out into 

the north wall of the Ouray fault; north of the intrusive zone it 

passes into a gentle anticlinal axis in the Mesozoic strata (see 

pl. 1, section E–E′).

The main feeding channel of the laccoliths is on the east 

side of the “break;” near it and the intersection of the axes, the 

sedimentary rocks have been intensively altered in The Blowout area by mineralizing fluids. These fluids passed upward 

toward an ancient land surface along strong fractures or a 

fracture zone that breached the summit of the domed intrusive 

area. Elsewhere, the Upper Cretaceous shales and the laccoliths formed a thick blanket that was relatively impermeable to 

the upward passage of mineralizing fluids. Field studies of the 

mineral deposits near the axial intersection suggest that mineralizing fluids moved, in general, toward the Uncompahgre axis 

principally from the east side of the valley. In the northern part 

of the district, at places more distant from the main axes, many 

mineralized channels were slightly inclined, about the same as 

the enclosing strata; zoning within those deposits suggests that 

fluids moved, in general, from east to west. Associated clastic 

dikes also were intruded along paths that inclined upward 

toward the west. Thus it appears that the intrusive zone contained an easterly source for high-pressure mineralizing fluids 

and that the Uncompahgre axis or “break” was the principal 

low-pressure zone toward which the fluids moved.

Of the four major sectors, the northern was by far the 

most productive, having yielded more than 80 percent of 

the district’s mineral output. The exact production from the 

other sectors is not known, but the relative differences are 

not significant. The important feature is that, compared to 

the northern sector, the productivity of deposits in the eastern 

and western sectors decreased within comparatively short 

distances from the intrusive zone. The productivity of the 

southern sector was intermediate between these extremes. 

The differences in productivity between the sectors evidently 

resulted, in part, from the dominant directions of secondary 

structural control.

There are several systems of fractures and dikes throughout the mining district, most of which appear to be secondary 

features related to the main structural axes. In both the northern and western sectors the principal system of veins and dikes 

trends roughly east, and intersects the northeast trend of the 

intrusive zone at an angle of about 30°. Most of the fractures 

are vertical or dip steeply toward the intrusive zone, and many 

contain igneous and clastic dikes as well as vein matter. Possibly the fractures formed by extension resulting from the lateral 

shifting in the rock masses on either side of the intrusive zone. 

If the north side moved east relative to the south side, the 

fractures would be comparable to tension or feather joints. The 

practical importance of the system is that it appears to have 

furnished the main long, deep-seated fractures that tapped both 

the igneous and ore reservoirs.

A prominent fracture system present in all sectors of the 

mining district trends more or less parallel to the Uncompahgre axis; these individual, steeply dipping fractures strike north 

to N. 40° W. In contrast to the east-trending system, these 

fractures are discontinuous, locally are en echelon, and except 

for those within or near the intrusive zone, do not commonly 

contain igneous dikes. Some are locally mineralized, but many 

contain only late barren gangues. South of the intrusive zone 

a few contain vein matter of late Tertiary age. The presence 

of this system along the Uncompahgre axis, the discontinuity 

of individual fractures, and the lack of dikes except near the 

intrusive zone suggest that it was formed at relatively shallow 

depths by stretching along the crest of the Uncompahgre arch. 

In most places the fractures failed to tap the primary sources 

of mineralizing fluids. Because a few fractures near the intrusive zone do contain dikes, the system must have formed early, 

but it also was reactivated in late Tertiary time.

In the southern and eastern sectors of the district, the 

attitudes of the fractures are more diverse than those of the 

two systems described above. This may have been the result, 

in part, from interfingering of the Laramide fractures by those 

of late Tertiary age, the latter becoming more prevalent to the 

south and east. The effect of structural control of the basement rocks also is increasingly apparent, especially in the 

southern sector. There, some of the fractures and, in addition, 

flexures parallel the general easterly trend of the folds in the 

Proterozoic basement. Other structural features of diverse 

trend formed possibly by renewed movement on some of the 

older faults. An increase of inherited structural lines in the 

southern sector also is in part a result of exposure of the older 

formations and renewed movement along the hinge line of the 

Paleozoic uplift.

Ore Deposits 55
Flexures and associated complex fractures were important in controlling the form and distribution of some orebodies. 

In the northern sector, the two most pronounced flexures trend 

northeast but somewhat more easterly than the intrusive zone, 

so that if continuous, they would intersect the intrusive zone 

at a more acute angle than those of the east-trending fracture 

system. Along these flexures the strata were warped downward toward the intrusive zone, resulting in either a reversal of 

their normal northward dip or a local flattening of the normal 

dip (fig. 22). The more competent beds of sandstone, quartzite, or limestone were complexly fractured and thus provided 

either potential bedding channels for mineralizing fluids or 

zones of relatively low pressure connecting to the Uncompahgre axis on the west. The most productive orebodies in the 

northern sector of the district are on these flexures. Flexures in 

the southern sector of the district were related to old lines of 

weakness in the Proterozoic basement and affected chiefly the 

lower Paleozoic strata.

The Uncompahgre and intrusive axes, in conjunction with 

the main east-trending systems of fracturing and flexing, controlled to a large degree the distribution of the deposits between 

the district’s four sectors. The bulk of the mineralizing fluids 

originating within the intrusive zone sought the most direct 

paths from deep sources to the lower-pressure zone along the 

Uncompahgre axis, traveling westward in the northern sector 

and eastward in the southern sector along the east-west lines of 

fracturing and flexing. The eastern and western sectors appear 

to have been deprived of the main bulk of the fluids, except near 

the crossing of the axes, where the existing pressures tended 

to force the fluids to rise steeply within or along the border of 

the intrusive zone until permeable channels or fractures were 

encountered that permitted direct escape toward the Uncompahgre “break” or axis. In the eastern sector the mineralizing fluids 

in part appear to have traveled southward along fractures or 

dikes paralleling the Uncompahgre axis until they intersected 

other fractured zones that trended west toward the axis. Because 

the fractures related to the Uncompahgre axis system are comparatively shallow extensional breaks, the largest deposits in 

the eastern sector are in the Dakota Sandstone, which forms the 

uppermost competent unit of the sedimentary sequence. Deposits stratigraphically lower in favorable beds were confined to 

positions very near the intrusive zone.

Large deposits of silver-lead ores have not, as yet, been 

found in the eastern and western sectors but are confined to 

the northern and southern sectors. As noted in discussion 

of the mineral zoning, fluids that deposited silver-lead ore 

evidently originated at places distant from the crossing of the 

two main axes, and hence the bulk of these fluids appear to 

have traversed east-west channels mainly in the northern and 

southern sectors. If other avenues of egress existed for these 

fluids, there is little probability that they will be discovered 

because they must lie at considerable distances to the northeast 

or southwest of the intersection of the axes, where the older 

strata are deeply buried beneath the Tertiary volcanic rocks. 

Confinement of most of the known silver-lead deposits to the 

northern rather than to the southern sector suggests some lack 

Figure 22. Generalized section through the American NettieWanakah flexure (localized flattening of dip) and ore shoots (solid 

areas) showing the types of structural control of ore deposition. 

Modified from Burbank (1940). Tsj, San Juan Formation; 

TKig, Laramide porphyritic granodiorite laccolith; Km, Mancos 

Shale; Kd, Dakota Sandstone; Jm, Morrison Formation; 

Jw, Wanakah Formation, which includes the Pony Express 

Limestone Member (pe); Je, Entrada Sandstone; ^d, Dolores 

Formation; Pc, Cutler Formation; ncd, Nettie intrusive clastic dike 

No. 2; v, vein; id, Laramide intrusive granodiorite dike. Arrows, 

where shown, indicate known relative movement.

of symmetry in the igneous or structural framework. Possibly 

rocks on the northern side of the intrusive zone were more 

strongly fractured by the east-west system, or more likely, the 

intrusive zone was more strongly developed to the northeast 

than to the southwest.

Northern Sector

The northern sector contains the largest gold-bearing and 

silver-bearing deposits. The gold-bearing deposits are in the 

southern part of this sector, mostly within a mile of the intrusive 

zone, and include both bedding replacement and vein types. 

Within or near the intrusive zone, the smaller gold-bearing 

deposits are veins that contain mainly pyrite and chalcopyrite 

with lesser amounts of sphalerite and other sulfides; in addition 

to these sulfides, a few veins also contain magnetite and hematite. However, the larger, pyritic base-metal-type deposits of the 

gold-bearing group lie mainly within the bedding of favorable 

sedimentary rocks in both the basal zone of the Upper Jurassic rocks and in the basal zone of the Upper Cretaceous rocks. 

These deposits not only contain more sulfides, but the richer of 

them, such as the American Nettie and adjoining properties in 

the upper of the two ore zones, also contain gold and silver tellurides and silver-bearing tetrahedrite.

Some gold-bearing deposits within and bordering the 

intrusive zone are of the contact-metamorphic type that occur 

within favorable beds ranging stratigraphically from the upper 

part of the Hermosa Formation to the lower part of the Mor56 Geology and Ore Deposits of the Uncompahgre Mining District, Southwestern Colorado
rison Formation. Owing to location near the central intrusive 

body, the beds range widely in dip; some are steeply tilted, 

and have been cut by numerous dikes, fractures, and faults (pl. 

1, section F–F′). The principal deposits of economic interest, 

however, occur in the slightly to moderately inclined beds in 

the basal Jurassic strata. The contact-metamorphic deposits 

contain the characteristic suite of calc-silicate minerals, magnetite, and hematite plus sulfides that were introduced later 

and constitute the ore. These deposits, which were enriched by 

late gold-bearing fluids, were found mainly in highly fractured 

limestones. The gold in these deposits was not of contactmetamorphic origin, but appears to be more closely related to 

the pyritic base-metal phase of deposition.

The best examples of contact-metamorphic deposits were 

those in the Wanakah group of mines (fig. 21, no. 19; pl. 2, 

section B–B′) consisting of slightly inclined orebodies that 

conformed to the bedding in the Upper Jurassic strata. The 

two horizons most favorable to replacement were (1) the Pony 

Express Limestone Member of the Wanakah Formation (socalled “Iron Clad contact”) and (2) the limestone and enclosing 

shale beds (so-called “Bright Diamond contact”) about 10 to 15 

ft above the basal quartzite in the Tidwell Member of the Morrison Formation. Some gold-bearing sulfide masses also occurred 

in the upper part of the quartzite, but they were not typical of 

contact-metamorphic deposits. Limy shales, interbedded with 

and above these two horizons, contain considerable quantities 

of silicate minerals. The underlying Dolores Formation contains 

deposits of lodestone and minor amounts of sulfides.

The principal silicate minerals are andradite, hornblende, 

actinolite, epidote, thuringite, stilpnomelane, and chlorite. Magnetite and hematite are commonly associated with the contactmetamorphic silicates, minor apatite, rutile, and titanite, as well 

as the dominant sulfide, pyrite. Ore and gangue minerals were 

deposited in several fairly well defined stages (table 11). The 

earliest stage began with the formation of andradite, hornblende, 

and actinolite, and ended with the introduction of iron oxides. 

Before or early in the sulfide stage, fracturing was renewed with 

the addition of hematite, epidote, and thuringite. Sulfides were 

deposited in the normal order of the gold belt, namely pyrite, 

chalcopyrite, sphalerite, and galena. Finally, native gold was 

deposited in late-formed fractures.

Outlines of the lower or “gold channel” in the Pony 

Express beds in the Wanakah mine shown in plate 3A extend 

northeastward diagonally across the sulfide replacement body. 

The northwest boundary of the channel grades into barren beds, 

whereas the southeast boundary is an indefinite assay wall 

within the sulfide replacement mass. The upper channel is about 

120 ft above the lower channel, and is essentially parallel to 

it. Along the south side of the gold channel, particularly at the 

west, gold ore was closely associated with contact-metamorphic 

silicates. To the northeast and toward the apparent source of 

the gold-bearing fluids, the ore channel is bordered by altered 

rock including the minerals chlorite, thuringite, quartz, sericite, 

siderite, and calcite rather than the higher-temperature contactmetamorphic silicates. The gold appears to have been concentrated in fractures or minor faults that are parallel or diagonal to 

the main channel. At the extreme northeast end of the channel, 

a pocket of high-grade gold ore was found along the southwest 

side of the northwest-trending fault, locally known as the Iron 

dike (pl. 3A), but exploration northeast of the fault found only 

weakly mineralized ground, which did not constitute ore grade. 

Massive pyrite bodies within the channel contained very little 

gold, commonly no more than 0.05 ounces (oz) to the ton; 

according to early records, however, gold-enriched pockets of 

ore from the channel averaged about 0.5 oz of gold to the ton, 

and the ore pocket along the Iron dike contained from 3 to 5 oz 

of gold to the ton. A similar pocket of gold ore was found near 

the Jonathan dike at the northeast end of the American Nettie 

ore channel in the Dakota Formation (pl. 3A). The location of 

these two pockets of high-grade ore, relative to their location 

along the dikes, suggests that the gold-bearing fluids had a 

source to the east or northeast of the dike barriers.

Silver-lead deposits in the northern sector occurred from a 

mile to more than 3 mi north of the crossing of the Uncompahgre and intrusive axes. Most were veins that had some associated bedded and wall-rock replacement deposits, but the richest 

ore was in the veins. The main channels of mineralization, 

closely associated with clastic and igneous dikes, were confined 

mostly to a lower zone in the basal Upper Jurassic strata and 

an upper zone in and just below the Dakota Sandstone. Veins 

along and south of Dexter Creek valley were mineralized in 

both the upper and lower zones, but the fractures north of the 

mouth of the valley (fig. 21) were mineralized mainly in the 

lower zone. Almost all of the silver-bearing veins strike east. 

Those that contained ore shoots or local pockets of enriched 

ore occur 0.5 to 2 mi east of the Uncompahgre axis, but farther 

east, the ores were more depleted in silver and lead and enriched 

in iron and zinc. The ore-forming fluids apparently originating 

from sources near the intrusive axis, here about 3 mi east of the 

Uncompahgre axis, moved upward and westward along paths 

nearly conformable to the eastward-dipping beds. The principal 

valuable sulfides in these veins include galena, tetrahedrite, and 

other silver-bearing minerals such as pearceite and pyrargyrite. 

The amount of pyrite and sphalerite in these deposits commonly 

increased eastward, especially in the veins along and south of 

Dexter Creek.

The position of an individual ore shoot in the eaststriking veins was dependent on the character of the enclosing 

rocks, the strike and dip of the fracture, and certain deflections 

in the dip of the fracture locally known as “rolls.” Within alternating sandstone and shale beds, the more massive sandstones 

maintained open fractures, whereas in the interbedded shales, 

fractures tended to choke or tighten; fractures deflected from 

the vertical in certain more favorable horizons were diverted 

10 to 40 ft or more laterally upon passing from a massive 

sandstone into shale or limy shale. Along these rolls, the ore 

spread out in the flatter parts between the diverted more-vertical segments of the vein; some ore in these flats was continuous for long distances along the strike of the vein. Where the 

beds were locally flexed, the rolls were wide and the permeability of some beds, such as the breccia in the Pony Express 

Limestone Member of the Wanakah Formation, favored the 

Ore Deposits 57
development of bedded deposits. Deflection of the fracture 

may be the result of dip change in the course of the fracture 

so that minor faulting along it may have changed the dip, 

resulting in the creation of favorable openings. Most fractures 

are remarkably straight, and most strike changes were not sufficient to appreciably affect the distribution of the ore.

An important structural feature coincident with ore 

shoots in the group of veins south of Dexter Creek, such as in 

the Pony Express and Bachelor veins, was the reversal in the 

normal northward dip of the country rock. This dip reversal 

was strongest along the line of the Pony Express vein at the 

extreme west end where beds dip locally 15°–20° S. (fig. 23A, 

B). In the easternmost workings on the Bachelor vein, the 

greatest dip reversal occurred on the north side of the vein and 

extended north of Dexter Creek into the trend of the Calliope 

ore shoot about 1,200 ft north of the Bachelor vein. There, the 

beds locally dip as much as 30o

 S. or SE., but only for short distances. In most veins north of Dexter Creek, there are no pronounced flexures. Ore shoots lie either to one side or the other 

of cross breaks that are essentially parallel to the Uncompahgre 

axis. These cross breaks are interpreted to be gentle extensional 

fractures that formed along the crest of the arch and did not 

extend to depths great enough to tap the sources of mineralization. Few of these cross breaks, which extend northward from 

the intrusive axis to beyond Cutler Creek, contain ore except 

in the south nearer the intrusive zone, where they also may 

contain igneous dikes. Fractures of this trend are not continuous for long distances, but appear to be en echelon.

Western Sector

The larger deposits of the gold-bearing veins in the western sector are located along the west side of the Uncompahgre 

River canyon in a narrow zone near, but west of the intrusive 

and Uncompahgre axial intersection (fig. 21). Most of the 

veins strike nearly east, but a few follow the more northeasterly 

trend of the few associated porphyritic granodiorite dikes. All 

have cut the Cutler Formation and overlying Mesozoic strata. 

The ore is mostly of the pyritic gold-bearing type that also 

contains minor amounts of chalcopyrite, sphalerite, galena and 

other base-metal sulfides; quartz, sericite, and chlorite are the 

principal gangue minerals, but locally a little calcite and barite 

are present. Some veins contain specular hematite, partly associated with and partly independent of sulfides. The gold ore 

here always seems to contain some chalcopyrite, which is not 

necessarily true in ores in the other sectors. In addition to fracture fillings that contained the better grade ore, the ore locally 

replaced the more permeable sandstone beds near the fractures.

Where the east-trending veins and dikes intersected 

favorable beds in the basal Upper Jurassic strata, some bedding-replacement deposits were formed either in breccia in the 

Pony Express Limestone Member of the Wanakah Formation, 

such as that in the Rock of Ages mine (fig. 21, no. 28; fig. 

24) or in some of the more massive sandstones. The quartzite 

of the Tidwell Member of the Morrison Formation, which 

in the western sector contains some bedded deposits, shows 

Figure 23. Generalized sections showing types of structural 

control of ore deposition in veins south of Dexter Creek (modified 

from Burbank, 1940). A, Bachelor and Pony Express veins and 

flexure. Km, Mancos Shale; Kd, Dakota Sandstone; Jmw, Morrison 

and Wanakah Formations, undifferentiated; pe, Pony Express 

Limestone Member of the Wanakah Formation; Je, Entrada 

Sandstone; bcd, Bachelor intrusive clastic dike. B, Pony Express 

vein and bedding channel of lower ore zone (enlargement from 

section A above). Jm, Morrison Formation; lq, “lower quartzite” of 

miners (Tidwell Member of the Morrison Formation); Jw, Wanakah 

Formation; pe, Pony Express Limestone Member of the Wanakah 

Formation; Je, Entrada Sandstone; ^d, Dolores Formation; 

Pc, Cutler Formation.

58 Geology and Ore Deposits of the Uncompahgre Mining District, Southwestern Colorado

evidence of leaching and cavity formation similar to that in 

the American Nettie mine; the caves, however, are typically 

small and decrease westward in size and number, suggesting 

that the mineralizing solutions moved eastward toward the 

Uncompahgre axis. Accessible mine workings along veins on 

the west side of the Uncompahgre River canyon are mostly 

less than 1,000 ft in length; the ore appears to have been 

slightly impoverished in the westernmost workings. Most of 
the bedded deposits lie close to the crest of the arch along the 

Uncompahgre axis and, in general, they were less productive 

than those in the northern sector east of the axis.

Some of the mines in the western sector were developed 

on veins in the Cutler Formation. The massive sandstones and 

arkosic sandstones interbedded with commonly tight shales 

were more favorable for the formation of small ore shoots, 

particularly where the alternations of the sandstones and shales 

caused deflections of the fractures so that changes in strike and 

dip, as well as in the nature of the rocks, apparently influenced 

the formation of ore shoots. Most of the veins are narrow and 

the ore shoots are small.

Although most of the deposits in this sector are near the 

intrusive axis, some are a mile or two to the west and north 

(fig. 21). To the west in the Oak Creek area, the deposits consist of gold-bearing veins in the Dakota Sandstone, and to the 

northwest near Corbett Creek they consist in part of similar 

gold, silver, and lead vein deposits in the Dakota Sandstone 

and in part of bedded deposits in the Pony Express Limestone 

Member. The latter are essentially the western extensions of 

the silver-lead-zinc channel deposits of the northern sector. 

Most ore shoots here were of little economic importance. The 

total production of the outlying deposits in the western sector 

was comparatively small.

Southern Sector

The southern sector contains chiefly vein and bedded 

deposits of silver-lead-zinc ores. Except for small pyritic 

deposits close to the south margin of the intrusive zone, most 

of the deposits are in Paleozoic limestone beds close to or south 

of the Ouray fault southwest of the town of Ouray (fig. 21). 

Rocks of Mesozoic age have been eroded, except for a narrow 

strip along the south side of the intrusive zone on the north 

side of Canyon Creek valley. Formations of Pennsylvanian and 

Permian age, exposed along both sides of Canyon Creek valley, 

overlie the carbonate rocks of Devonian and Mississippian age 

near the junction of Canyon Creek and Uncompahgre River 

valleys. Because the Molas Formation is dominantly shale, most 

of the silver-lead deposits within the bedding of the Leadville 

Limestone occur just beneath those impermeable shaly beds. 

Production from this sector has come mainly from one beddingchannel deposit, the Mineral Farm mine (fig. 21, no. 35), and 

from several smaller combined bedded and vein deposits. Production of silver-bearing ores of Laramide origin in this sector 

was probably less than $1 million.

The silver-bearing ores are similar to those of the northern sector, and include baritic or siliceous lead-zinc sulfide 

ore that contained chalcopyrite, tetrahedrite, pearceite, and 

ruby silver (pyrargyrite). The ore contained little gold. These 

deposits differ from the nearby late Tertiary silver-lead veins 

of Hayden Mountain by the latter’s typical association of pearceite and tetrahedrite in an ankeritic carbonate gangue.

For purposes of description, the southern sector is conveniently divided by the Ouray fault. The area south of the fault 

was the more important commercially. The area north of the 

fault and south of the town of Ouray contains a few pyritic 

deposits close to the intrusive zone and some silver-bearing 

veins in the limestone just north of the fault.

Deposits related to fractures, flexures, and faults of generally easterly trend parallel the strike of the underlying Proterozoic rocks. Differences in competency of the steeply dipping to 

vertical, alternating thick quartzite and slate layers and the probable presence of very old lines of weakness along their contacts 

strongly influenced the younger deformation. Because fracturing was parallel to the underlying steeply inclined bedding, the 

Paleozoic beds lying discordantly above were flexed or slightly 

faulted, and the fractures in them became feeder channels 

for the relatively few mineral deposits found in the favorable 

limestone beds. Weak fractures and flexures commonly died out 

in the shaly beds at the base of the Pennsylvanian strata. Near 

Hayden Mountain, where the strata are thin, a few of the stronger fractures extended through and into the overlying volcanic 

rocks, suggesting that renewed movement along established 

fault lines continued to late Tertiary time.

South of the Ouray fault, mineralizing fluids evidently 

originated to the west along the intrusive axis and moved up 

along the trend of the formations toward the Uncompahgre 

River valley and the Uncompahgre “break.” The main channels for the mineralizing fluids were the fractures in the south 

wall of the Ouray fault and certain minor flexures associated 

with the east-trending faults. Ore in the silver-lead veins of the 

southern sector is somewhat similar mineralogically to that in 

the northern sector.

At the Mineral Farm mine, which is the principal mineral deposit in the southern sector, leaching appears to have 

preceded and partly accompanied silicification of the country 

rocks. Leaching of the Leadville Limestone along fracture 

channels, just beneath shale in the overlying Molas Formation, formed caves and cave breccias in which most of the 

ore was deposited. Individual ore shoots were controlled, in 

part, by fractures of northwest and southwest trends, but the 

main ore channel appears to parallel a fault in the underlying 

Proterozoic rocks striking N. 75o

 W.; this fault is between 

layers of quartzite and slate but several hundred feet north of 

the deposit. Numerous small ore deposits in this general area 

are mainly veins, but locally the ore extended into favorable 

layers to form bedded deposits in the adjoining sedimentary 

rocks. Solution channels were leached in the limestone near 

or along the fractures. High-grade silver ore was found only 

locally.

The area north of the Ouray fault, extending to the 

intrusive and Uncompahgre axes, is underlain by Paleozoic 

formations, and is mostly covered by surficial debris. The 

most productive deposit in this part of the southern sector was 

the Trout and Fisherman vein (fig. 21, no. 32), in the Leadville Limestone and Molas Formation near the Ouray fault. 

Fractures in this area have a northerly trend and are probably 

extension fractures parallel to the Uncompahgre axis. Sulfides, 

including pyrargyrite, were deposited mostly in solution channels leached in the limestone near and along the fractures; the 

Ore Deposits 59
60 Geology and Ore Deposits of the Uncompahgre Mining District, Southwestern Colorado

Figure 24. Plan map and partial vertical projection of underground workings, Rock of Ages mine (fig. 21, no. 28). Compiled from 

descriptions and field sketches of W.S. Burbank.
ore-forming fluids presumably reached the area along fracture 

channels that dip west along the Ouray fault.

Eastern Sector

The eastern sector contains small contact-metamorphic 

deposits along the south margin of the intrusive zone and 

pyritic gold-bearing deposits that extend for about 4,000 ft 

southeast of the intrusive zone. The northern part of the sector, 

between the latitude of Ouray and the intrusive zone, is underlain by both Paleozoic and Mesozoic rocks; in the southern 

part, the Mesozoic and part of the Paleozoic rocks have been 

eroded. The Amphitheater area east of Ouray is covered by 

glacial and landslide deposits.

South of the faults that limit the southern margin of the 

intrusive zone, a few favorable beds contain contact-metamorphic deposits. The deposits of the Skyrocket group of mines 

(fig. 21, no. 38), in the limestone of the Pony Express Member 

of the Wanakah Formation, consist of highly pyritic ores that 

replaced the altered rocks. In this area the beds have been 

cut by an igneous plug, dikes, and sills, and numerous small 

fractures, features that guided the mineralizing fluids into the 

more favorable rocks. The deposits, however, have not been 

extensively developed because of their low content of gold 

and silver.

Gold deposits in this sector extend about as far from the 

intrusive zone as do those in the northern sector, but all of the 

deposits explored were much smaller and their production was 

very much less. As in the western sector, no commercial leadsilver deposits were formed during the Laramide mineralization. A few lead-silver veins, however, were emplaced east and 

southeast of Ouray during the late Tertiary.

Pyritic ores along the south side of the intrusive axis are 

related to fractures and dikes that trend either east or north to 

northeast. They resemble deposits in the northern sector in 

their stratigraphic position and mineral content, and they range 

from low-grade contact-metamorphic deposits in limestones 

of the Hermosa Formation and basal Upper Jurassic strata to 

higher grade gold deposits in the Dakota Sandstone. Sandstones in the Dakota were altered to quartzite, which was 

irregularly warped and faulted along north- and northwesttrending fractures that acted as feeding channels for mineralizing fluids to enter and deposit ores. The fluids were in part 

diverted to west-trending fractures along which some deposits 

were formed by either replacement of beds or filling of solution cavities.

Late Tertiary Ore Deposits

Ore deposits of late Tertiary age, Miocene and perhaps 

some Pliocene, in and near the Uncompahgre district are 

related principally to a major center of mineralization, the 

Silverton caldera area to the south. The principal deposits of 

late Tertiary age are mainly in volcanic rather than sedimentary rocks and are related to structures of deep-seated volcanic 

origin. Ores of late Tertiary age are similar to those deposited 

during the Late Cretaceous to early Tertiary time of mineralization in their content of gold, silver, lead, zinc, and copper, 

but they differ somewhat in their mineralogic character (tables 

11 and 12). The principal gold deposits formed during this 

younger stage consist of native gold in quartz veins in contrast 

to the pyritic gold deposits formed during the earlier stage. 

Silver-bearing veins also appear to differ somewhat in details 

of their mineralogy.

Near Ouray, the late Tertiary ore deposits are mostly 

veins and were only of minor interest. They include the veins 

of The Amphitheater in the San Juan Formation and underlying sedimentary rocks as well as those along the Uncompahgre River canyon in the Proterozoic quartzites and slates. 

Some veins strike north to northwest; others strike more nearly 

east. The north-trending system appears to be the northward 

termination of the fractures and veins in the Bear Creek area 

adjacent to and just southeast of the Uncompahgre district, 

where the San Juan Formation has been cut by a strong system 

of north-striking fractures.

Northeast of Ouray, the east- and north-striking fractures 

are weak or absent. A conjugate system of northeast- and 

northwest-striking fractures, some of which contain veins, is 

present within the San Juan Formation immediately east and 

north of The Blowout area. Many of these veins are barren 

quartz and sparse barite, but some contain small amounts of 

base-metal sulfides and are reported by miners to have been 

prospected for their gold content. In this same area, however, 

a few fractures and faults having easterly and northerly trends 

appear to represent the reopening of Laramide fractures in late 

Tertiary time; most of these fractures contain either barren 

quartz and barite or barren carbonates.

Veins in the walls of The Amphitheater east and southeast 

of Ouray strike north to N. 30°W. and dip steeply east and 

west. The intensity of late Tertiary fracturing in the San Juan 

Formation here was undoubtedly controlled by the underlying 

Proterozoic rocks at the south and the northward-thickening 

wedge of Paleozoic and Mesozoic rocks between the volcanic 

and Proterozoic rocks. This feeble system of younger, northstriking fractures was weakly mineralized to the north; the few 

veins were mostly barren quartz or barite containing small, 

widely scattered pockets of galena.

On Hayden Mountain south of Ouray, and partly south 

of the map area (Luedke and Burbank, 1981), the San Juan 

Formation is cut by two prominent fracture systems, one striking N. 15° W. and the other N. 50°–70° E. These late Tertiary 

fracture systems were more strongly mineralized than those 

northeast of Ouray; however, the few veins that were developed have not proved to be of economic value.

A few dikes and veins of late Tertiary age extend into 

the Uncompahgre district from the upper Cow Creek center 

several miles to the east (Steven and others, 1977; Steven and 

Hail, 1989). The southernmost dike, the Germania dike in the 

east wall of The Amphitheater (fig. 21), reportedly contained 

pockets of complex base-metal ore; some gold was found 

along the dike walls. The northernmost east-striking dike, 

known locally as the Calliope (fig. 17), parallels the CalliOre Deposits 61
Table 12. Principal primary ore and gangue minerals and some 

more-prominent secondary minerals, classified according to the 

principal groups, for late Tertiary deposits in the Uncompahgre 

mining district, southwestern Colorado.

[Modified from Burbank, 1940]

Base metal 

deposits

Baritic lead and 

silver deposits

Siliceous silvergold deposits1

Primary (hypogene) metallic minerals

Pyrite Pyrite Pyrite

Galena Sphalerite Sphalerite

Sphalerite Galena Gold

Chalcopyrite Chalcopyrite Proustite

Specularite Tetrahedrite

Stephanite

Polybasite

Primary (hypogene) gangue minerals

Quartz Quartz Quartz

Sericite Barite Sericite

Calcite Rhodochrosite Beidellite(?)

Rhodochrosite Calcite

Sericite

Chlorite

Secondary (supergene) minerals

Limonite Limonite Limonite

Wad Wad

Malachite

Azurite

1

Dexter Creek area.

ope vein of Laramide origin, but is about 500 ft north of the 

vein. Walls of the dike were locally fractured but only weakly 

mineralized. The parallelism of the Calliope dike and the vein 

is thought to be accidental and controlled here by the tendency 

for fracturing in the basement rocks in this direction.

Geologic conditions during the late Tertiary stage of 

mineralization did not normally favor the formation of 

replacement deposits similar to those of the earlier Laramide 

mineralization. Mineralizing fluids apparently were confined to the late Tertiary fractures, and accordingly, did not 

form extensive lateral replacement deposits in sedimentary 

rocks beneath the San Juan Formation, providing that either 

those rocks were not removed by early Tertiary erosion or 

they were too deeply buried at the time of ore deposition. 

The Portland vein, at the head of The Amphitheater, is a late 

Tertiary vein in this area. The Portland mine (fig. 21, no. 

42) contained complex base-metal and precious-metal ore 

in veins that cut the San Juan Formation and the immediately underlying Paleozoic rocks. The ore spread laterally 

short distances near the contact and replaced basal layers 

of tuff in the San Juan Formation and brecciated limestones 

in the Paleozoic rocks. At and near the base, the tuff here is 

locally calcareous and contains some more easily replaced 

thin lenses or beds of limestone. Most of the production 

from this mine came from shallow stopes. The ore minerals 

were pyrite, sphalerite, galena, and chalcopyrite in a gangue 

of silicified limestone, quartz, barite, and rhodochrosite or 

manganiferous carbonate; a later generation of comb quartz, 

pyrite, and kaolin possibly constituted the gold-bearing vein 

material. The last barren stage of mineralization consisted of 

abundant drusy quartz and coarse calcite.

Rock Alteration

The most important period of rock alteration within the 

Uncompahgre district, excluding any changes to the rocks as 

a result of igneous activity during Proterozoic or Paleozoic 

times, was related to the Late Cretaceous to early Tertiary 

Laramide orogeny. At that time, the most pronounced 

changes were the result of heat and related hydrothermal 

effects of the contemporaneous igneous activity, which 

locally indurated and altered many of the adjacent sedimentary units.

As a result of that igneous activity, the more important 

types of rock alteration occurred in several ways: (1) from the 

effects of direct heating of the rocks, which varied according to their composition and moisture content, (2) from direct 

permeation of the rocks by hydrothermal emanations given off 

by the igneous masses, and (3) from the heating of circulating ground waters on the borders of the local uplift. These 

processes undoubtedly were integrated and (or) combined. 

The highly altered rocks near the main intrusive center (The 

Blowout) were unquestionably the result of direct heating and 

hydrothermal emanations.

Discussion of rock alteration has been arbitrarily divided 

between the early effects near the main center of igneous 

activity, which are correlated with the end-phases of igneous 

rock crystallization, and the effects produced in the immediate 

walls of ore deposits within the mining district. Some overlap 

exists between the different phases of alteration and related 

mineralization so classified, but this division facilitates the 

comparison of alteration resulting directly from local igneous 

activity with alteration in which the genetic relations are less 

clearly defined.

The earlier discussion of the porphyritic granodioritic 

rocks pointed out the important mineralogical changes that 

occurred in the laccoliths during the later stages of crystallization. When the magmas intruded comparatively impermeable 

strata, the changes resulted entirely from interaction of the 

pyrogenic minerals and the constituents concentrated in or 

released from the residual fluids of the magma. Those conclusions are supported by the bulk compositions of the rocks, 

which show little change other than a progressive oxidation 

of ferrous to ferric iron. Most of the capping shales were 

eroded from the larger laccoliths, but a few shale remnants 

found adjacent to those bodies were only moderately baked 

and altered by heat or by emanations from the intruding igneous rocks. In sharp contrast to this relatively minor alteration 

62 Geology and Ore Deposits of the Uncompahgre Mining District, Southwestern Colorado
near contacts of the concordant intrusions, a zone of very 

strong alteration and mineralization occurs near the central 

vent through which the large volume of magma was intruded. 

Alteration in The Blowout area not only strongly affected 

the surrounding sedimentary rocks over a distance of several 

thousand feet from the center, but also resulted in alteration of 

the porphyritic igneous rocks that filled the narrow neck at the 

cessation of intrusive activity.

Rock alteration as a result of widespread volcanism and 

intrusive igneous activity during Oligocene and Miocene time, 

while extremely important in much of the western San Juan 

Mountains region, was minor within the Uncompahgre district 

except in the walls of a few veins.

Recrystallization of rocks and the deposition of minerals 

near the conduits of the modern thermal springs in the district 

are most likely the result of meteoric waters of deep circulation. Alteration effects of ordinary weathering and shallow 

ground-water circulation were for the most part of relatively 

minor importance, and are considered only as phases associated with the supergene enrichment of ores.

Alteration of Sedimentary Rocks

Sedimentary beds in The Blowout area include about a 

2,000-ft thickness of rocks ranging in stratigraphic position 

from the upper part of the Hermosa Formation (Pennsylvanian) to the base of the Mancos Shale (Cretaceous). A great 

variation exists locally in the kind and degree of alteration 

found in the different rocks, which depended upon composition, relative permeability, proximity to the stock, and the 

rocks’ position with respect to channels that allowed the 

circulation of fluids. The most intense alteration occurred in 

the rocks beneath the mudstones of the Morrison Formation, 

and included many of the calcareous shales and limestones of 

all the formations, the breccia in the Pony Express Limestone 

Member of the Wanakah Formation, and the limy beds of the 

Dolores Formation. The basal beds of the Wanakah Formation 

constitute the most widely altered and mineralized strata in 

the mining district. Dense limestones of the Hermosa Formation, of which only a few crop out, were slightly metamorphosed; the coarser sandstones and grits of the Hermosa and 

Cutler Formations, although strongly bleached, retained their 

sedimentary textures. Most rock units are easily recognizable 

except locally or within the central, highly altered zone of 

The Blowout, where in places it was difficult to distinguish 

between sedimentary or igneous rocks.

On the south side of The Blowout, ordinarily red calcareous shale and limestone-pebble conglomerate beds of the 

Dolores Formation were converted into (1) color-banded or 

mottled green and gray rocks, or (2) dark-gray, dense calcsilicate hornfels. The original color banding of the rocks 

was partly preserved by color alternations or by alternations 

of finer and coarser textured metamorphic minerals. Within 

the hornfels, the coarser grained minerals occur as irregular 

lenticular patches or nodular segregations. Microscopically, 

this rock has a felted texture owing to numerous interlacing prismatic crystals of amphibole. This amphibole has a 

yellow to blue-green pleochroism and an extinction angle 

of 26°–27° corresponding to the variety pargasite; the crystals are very small, mostly less than 0.1 mm long. Scattered 

grains of other minerals large enough to identify were quartz, 

orthoclase, albite, labradorite (An65), emerald-green epidote, 

brownish-green biotite, titanite, rutile, chlorite, hematite, 

and magnetite. Local, finer grained crystalline aggregates 

contain a few embedded larger grains ranging from less than 

0.01 to 0.03 mm in size, and consist largely of intergrowths 

of quartz, orthoclase, and albite. Cordierite, which might be 

expected in rocks of this type, was not identified, but if present, must occur as extremely minute grains. The great variation in texture from almost submicroscopic to coarser, nodular 

aggregates suggests that equilibrium between the different 

metamorphic minerals had not been attained; this suggestion is 

emphasized by presence in the rock of both albite and anorthite that were definitely identified both in thin section and by 

oil immersion studies. Minor amounts of alkali feldspars may 

have resulted from recrystallization of the micaceous constituents of the shales. Epidote, commonly associated with clusters 

of magnetite and hematite crystals, definitely formed later than 

the plagioclase (andesine/labradorite) and amphibole. Both 

epidote and the iron minerals appear to have been introduced 

into coarser textured, nodular segregations. These rocks probably formed by the incipient recrystallization of the original 

constituents followed by the introduction of iron-bearing fluids. The original layers of these rocks were not as rich in iron 

as those that were altered.

A color-banded and mottled green and gray altered 

mudstone from the Wanakah Formation at the same locality 

contains highly silicified layers composed of quartz and sodic 

feldspar; this quartz-rich mosaic, having grains less than 0.1 

mm in size, has been peppered with numerous small grains 

of garnet, epidote, rutile, chlorite, and calcite. Iron oxides are 

not evident, thus the ferric iron presumably is, in part, in the 

garnet, which is nearly pure andradite. Coarse layers in the 

rock contain andradite, epidote, and calcite in grains as large 

as 1.0 mm. The borders of the nodular aggregates are marked 

with albite crystals having both Carlsbad and albite twinning. Apatite, rutile, and zircon crystals are scattered in the 

coarser quartz. Feldspar and garnet apparently formed early 

because epidote replaces garnet, and quartz-epidote-hematite 

intergrowths crosscut the other minerals. Chlorite and calcite 

formed late.

Bleached impure sandstones of the Cutler Formation 

unconformably underlie the rocks described above. Rock 

textures were preserved both megascopically and microscopically. The larger grains of quartz and feldspar remain 

mostly unaltered except for replacement locally by scattered growths of hornblende, epidote, and secondary quartz. 

Where recrystallization was most advanced, rounded quartz 

granules and larger quartz mosaics were formed together 

with crystals of epidote and hornblende. Hornblende veinlets 

also cut the rock. The fine-grained matrix of the sandstone 

Rock Alteration 63
has been recrystallized to intergrowths of various metamorphic minerals and some calcite. Sericite is present, but is not 

conspicuous. Paragenetic relations between these minerals 

are obscure.

The most highly altered sandstone beds are found near 

the central part of The Blowout area, either in the large block 

of sedimentary rock enclosed by the intrusive body or in sandstone beds bordering the edges of the stock; recrystallization 

of both sedimentary and igneous rocks here is far advanced. In 

a sandstone from the north branch of Skyrocket Creek, quartz 

was segregated or recrystallized into rounded grains, about 0.5 

mm in diameter, which have sutured or scalloped edges, and 

have been embedded in larger orthoclase crystals or intergrown in a granular mosaic with the orthoclase. The texture, 

except for the large size of the quartz grains, resembles that 

developed in the groundmass of the porphyritic granodiorite of 

the laccoliths where recrystallization was farthest advanced. A 

few larger crystal outlines may have been originally plagioclase, but the shape suggests they were probably porphyroblasts rather than original detrital constituents. Other minerals 

of the rock are sericite, stilpnomelane, chlorite, epidote, albite, 

and pyrite. Orthoclase has replaced and veined the sericite, 

and chlorite was converted to brown stilpnomelane. These 

relations suggest a compound metamorphism in which the 

rocks had earlier been partly sericitized and pyritized. This 

reversal in the normal crystallization sequence of these minerals, also found in certain high-temperature replacement veins 

in this same area, suggests either rising temperature during a 

single stage of metamorphism, or possibly two separate injections of granodioritic magma.

On the north side of The Blowout, alteration in the 

lower part of the Mesozoic strata, particularly the more 

permeable beds, was characterized by a conspicuous introduction of iron oxides and by the presence of tabular bodies 

of sulfides. A bed of sandy calcareous shale in the Dolores Formation was converted into an aggregate of quartz, 

actinolite, epidote, and calcite. Small grains of quartz were 

partly recrystallized, but the original texture of the rock 

was not destroyed. Epidote formed later than actinolite, and 

calcite replaced both minerals. With further replacement by 

calcite and chlorite, however, the actinolite-bearing rock 

was converted to a mottled dark gray and green layered 

rock containing prominent cleavage plates of calcite more 

than an inch across. The darker colored layers are chiefly 

composed of quartz and some apatite, biotite, and orthoclase, or of epidote and a little calcite and chlorite; the latter 

minerals have a relict fibrous texture presumably inherited 

from replacement of earlier-formed actinolite or hornblende. 

Biotite and orthoclase probably were also early-formed 

minerals. The lighter colored layers are composed of quartz, 

calcite, and chlorite. Again, both calcite and chlorite have 

inherited fibrous textures like that of the actinolite, although 

none of that mineral remains. Replacement by calcite and 

chlorite occurred later than the formation of epidote in both 

the dark and light layers. The typical epidote in rocks of 

The Blowout area has abnormally high refractive indices; 

however, some common epidote was recognized. Perhaps 

an additional element, such as titanium, may cause the high 

refractive indices; megascopically the mineral has neither 

the reddish-brown or pink pleochroism of manganiferous 

epidotes nor the black of rare earth-bearing epidotes. From 

the mineral association, it is suggested that the ferric iron 

content increased during the intermediate epidote stage, but 

not in a sufficient amount to form iron oxides. The described 

sequences of mineral crystallization suggest several pulsations of replacement in which the intensity of each successive pulse was sufficient to partly or completely destroy the 

effects of earlier ones.

Some layers of dark siliceous rocks in other strata 

within The Blowout area were locally converted to massive 

lodestone. These rocks are mottled by epidote and quartz in 

interlacing veinlets; thuringite, stilpnomelane, and epidote 

having high refractive indices are common in these rocks. The 

magnetite masses (lodestone) embedded in quartz are bordered 

in some places by sheaf-like growths of a highly birefringent 

chlorite-like mineral having the optical properties of stilpnomelane. The quartz has been charged with small crystals of 

apatite, rutile or other titanium minerals, epidote, and some 

zircon. Magnetite apparently was one of the earlier minerals to 

replace the sandstone, and was accompanied by some apatite, 

rutile, and zircon. The rock then was fractured; epidote and 

hematite were formed, accompanied by additional veining and 

recrystallization of the quartz and, possibly, introduction of 

more titanium minerals. Calcite is present, and pyrite occurs in 

minor amounts. In other rocks from the same locality, thuringite rather than stilpnomelane was the predominant mineral 

deposited following or accompanying the epidote stage. The 

general order of mineral formation was fibrous hornblende, 

andradite, magnetite, pyrite, hematite, epidote, thuringite, and 

finally calcite as well as additional pyrite and other sulfides of 

the orebodies.

Alteration of Igneous Rocks

The igneous rocks in The Blowout area are generally 

much less altered than the enclosing sedimentary rocks, 

but compared with the igneous rocks in the laccoliths, have 

equally striking mineralogical changes. For this reason, it 

is thought necessary to attempt to distinguish between true 

endomorphic alteration and deuteric alteration resulting 

during the later stages of consolidation of these rocks. Many 

minerals are common to both types of alteration so that mineralogical changes alone are insufficiently diagnostic, but, as 

suggested by the mineralogical changes that occurred in the 

laccoliths, the state of oxidation of the iron and the extent of 

reaction in the plagioclase crystals are fairly accurate indicators of the degree of alteration. The chemical analysis (table 

7, sample 2) of granodiorite from The Blowout and the sample containing magnetite crystals suggest a comparatively 

minor oxidation of the iron. This rock is similar to that of the 

rapidly chilled margins of the laccoliths where only minor 

64 Geology and Ore Deposits of the Uncompahgre Mining District, Southwestern Colorado
deuteric alteration occurred. The lack of late magmatic reaction is further indicated within the plagioclase phenocrysts, 

despite local endomorphic alterations, by the retention of 

their pronounced zoning. In some instances, although the 

plagioclase crystals in the igneous rocks of The Blowout are 

completely replaced by secondary minerals, the magnetite 

crystals appear to have been little affected by endomorphic 

processes of alteration.

The igneous rocks subjected to the highest grade of 

metamorphism, and more or less pyritized, are found within 

the main crosscutting intrusion in The Blowout. As in the 

sedimentary rocks, the extremes of alteration are confined to 

narrow zones of high-temperature mineralization. Granodiorite 

from the east-central part of this stock, in which the magnetite 

was not oxidized, retained most of its original texture. Plagioclase phenocrysts are comparatively fresh and the zoning 

was unaffected, but a few contain a little sericite; some quartz 

phenocrysts were recrystallized into a granular mosaic. Ferromagnesian minerals, however, were completely altered and 

their outlines indicated only by a dusting of magnetite crystals 

or by aggregates of biotite intergrown with quartz, magnetite, 

rutile, calcite, and chlorite. Biotite, the most prominent secondary mineral, has been scattered throughout the rock in clusters 

or in small flakes; it differs from ordinary pyrogenic biotite 

chiefly by its texture and mode of crystallization. Chlorite has 

replaced part of the biotite, and calcite has partly replaced all 

minerals in the rock.

In other parts of the stock, alteration similar to that in 

the surrounding feldspathized sedimentary rocks consisted 

mostly of calcite and sericite. Within the igneous rocks, the 

ferromagnesian minerals were altered to aggregates of quartz, 

chlorite, magnetite, and calcite. Orthoclase phenocrysts were 

little altered except for some replacement by late calcite. The 

groundmass of the intrusive rocks has been largely recrystallized and contains coarse patches or lentils of a mosaic of 

quartz and orthoclase grains. Veinlets of quartz and orthoclase 

traverse the rocks and formed later than the sericite in the plagioclase. Pyritization probably occurred both before and after 

the veining by quartz and orthoclase.

The alteration, so far mentioned, was comparatively 

weak and applies chiefly to the more massive parts of the 

stock. Where this intrusive body was highly fractured or 

near contacts with sedimentary rocks, it was more extremely 

altered, and granular rocks consisting chiefly of aggregates of 

quartz, orthoclase, and sericite were formed. As stated above, 

it was not always possible in the zones of extreme alteration to 

distinguish clearly between originally sedimentary or igneous 

rocks. Typically, little evidence existed as to which rock type 

was most subject to alteration, but it is thought that the igneous rocks were somewhat less permeable to altering fluids. 

The more extreme alteration thus was confined at the contacts 

or to fractures in the igneous rocks.

The effects of epidotization are inconspicuous or lacking 

in rocks of the stock. However, epidote is more conspicuous than biotite in the larger, west-extending prong from this 

igneous body (fig. 12). The principal alteration products are 

epidote, chlorite, and calcite. All original biotite and hornblende have been altered to aggregates of chlorite, epidote, 

and commonly magnetite and rutile; plagioclase has been 

replaced partly by epidote and calcite but otherwise was little 

altered. Joint surfaces of these igneous rocks are coated with 

pyrite or with epidote, hematite, and calcite. The chemical 

analysis of porphyritic granodiorite from this area (table 7, 

sample 2), differed very little from the other analyses, except 

for the addition of a minor amount of pyrite and possibly a 

slight leaching of magnesia. Pyrite is restricted to coatings 

on joint surfaces, suggesting that, except where fractured, the 

intrusive rock was less susceptible to the alteration than were 

the sedimentary rocks.

Wall-Rock Alteration of High-Temperature Veins

Both sedimentary and igneous rocks in The Blowout 

area were cut by small, so-called high-temperature veins. 

These veins occur in tight, minor shear zones or along irregular, narrow fractures as a result of replacement rather than 

open-space filling. In a few places, they were prospected or 

mined for their gold content. Their mineralogy is intermediate between that of high-temperature feldspathization in 

the sedimentary and intrusive rocks and that of the contactmetamorphic replacement deposits north of The Blowout. 

The ore minerals are mainly pyrite, chalcopyrite, and a little 

sphalerite and galena. These veins are notable because of the 

recrystallization of orthoclase in the vein walls as well as 

veinlets or seams of plumose or massive magnetite. Orthoclase forms granular masses, and the grains, intergrown with 

quartz, are usually not more than a fraction of a millimeter 

in size. Minor accessory minerals formed in the vein walls 

include apatite, epidote, hematite, thuringite, and less commonly rutile and zircon.

In the Great Western mine (fig. 21, no. 21), both orthoclase and quartz have replaced and veined the sheared and 

crushed sandstones in the Hermosa Formation; there were 

several phases of shearing and fracturing. Early orthoclase 

and magnetite were veined by pyrite, again by magnetite and 

hematite, and then followed by quartz, thuringite, chalcopyrite, and sphalerite. Comparatively abundant apatite, in minute 

crystals, probably was introduced along with magnetite and 

hematite. Thuringite commonly accompanied chalcopyrite, 

and in places was intergrown with it. A few small grains of 

yellow to deep-brown pleochroic epidote, resembling allanite, 

formed during this stage of mineralization. Calcite was later 

than all the other minerals.

Somewhat similar veins have cut both sedimentary rocks 

and porphyritic granodiorite near the central part of the main 

intrusive body in The Blowout. One vein, cutting the block of 

altered sedimentary rock enclosed within this body, is composed chiefly of quartz, sericite, and orthoclase and lesser 

stilpnomelane, pyrite, magnetite, and hematite. Sericite formed 

early in the altered porphyritic igneous rock surrounding 

the sedimentary block, and was later veined and replaced by 

Rock Alteration 65
orthoclase and quartz. Veins of magnetite and stilpnomelane 

cut the early quartz and other minerals. Some pyrite, probably 

associated with the early sericite, was commonly rimmed by 

altered iron oxides or a ferric silicate mineral. Late-formed 

pyrite was disseminated in all rocks in this area; chalcopyrite 

was not detected. These same mineral relations were noted in 

the contact-metamorphic ores of the Bright Diamond mine in 

the Wanakah group of mines (fig. 21, no. 19) on the northwest 

side of The Blowout.

Similar to but separate and distinct from the so-called 

high-temperature veins are the so-called hematite veins (Burbank, 1936, p. 241–243). Immediately west of the central 

intrusive body (figs. 12, 14), numerous small, dominantly 

hematite-bearing veins have cut all the rocks; hematite also 

locally coats the joint faces in the porphyritic granodiorite. 

On the west side of the Uncompahgre River valley, some 

of the hematite veins are a foot thick. These veins typically followed pre-existing zones of weakness such as the 

walls of igneous dikes or pyrite veins; in places, however, 

they independently filled other fractures, and contain only 

hematite or chiefly hematite and minor amounts of quartz, 

thuringite, epidote, and calcite. Some sulfide minerals are 

present wherever there are quartz and other silicate minerals. 

The paragenetic relation of hematite following pyrite, where 

hematite veins parallel walls of pyrite veins, suggests that the 

hematite veins corresponded to the ferric-iron stage evident in all sequences of rock alteration, and were emplaced 

following widespread, renewed fracturing or reopening of 

earlier-formed fractures. Hematite veining was less conspicuous in tight shear zones and replacement deposits. The 

high-ferric-iron hypothermal stage of mineralization, represented by stilpnomelane, epidote, thuringite, and secondgeneration magnetite, paragenetically corresponds to or 

immediately followed the hematite vein-forming episode of 

mineralization. In the orthoclase-bearing veins of the Great 

Western mine, the hematite episode of mineralization clearly 

represented a period of fracturing that followed deposition 

of early magnetite and pyrite and preceded deposition of 

thuringite, chalcopyrite, and other sulfides.

Wall-Rock Alteration of Other Ore Deposits

This category is differentiated on the basis of the predominant mineralogical composition of alteration products resulting from the processes of sericitization and chloritization, 

silicification, and carbonatization. The mineral assemblages 

duplicate, in some instances, those already described in the 

rocks of the intrusive center. Most or all of the comparatively 

high-temperature minerals, however, are less conspicuous or 

are absent in the walls of veins and replacement deposits in 

areas outside the zone of contact metamorphism. Alteration 

generally was confined to the immediate walls of veins, or was 

more or less coextensive with the orebodies in the replacement 

deposits.

Sericitization and chloritization were the predominant 

processes of alteration found in the walls of pyritic veins of the 

mining district. The formation of much pyrite was in almost 

every instance accompanied by sericite both in vein walls and 

in quartz gangue. Chlorite commonly was more conspicuous 

where the vein walls are intrusive rocks; chlorite typical of 

zones of higher temperature alteration is commonly absent. 

Alteration of intrusive wall rocks therefore ranged from more 

highly sericitized and pyritized selvages near fracture walls to 

weaker propylitic types in which sericite, chlorite, and calcite 

were the predominant alteration minerals. The largest bodies of more-or-less completely sericitized rock are those in 

the pyritic replacement deposits in quartzite best seen in the 

American Nettie and Jonathan mines (fig. 21, nos. 17 and 18). 

Sericite and recrystallized quartz are the predominant minerals, but minor amounts of chlorite and grains of rutile, anatase, 

and apatite were found.

Sericitization and chloritization of the rocks in walls of 

the silver-lead-zinc veins are less conspicuous, but apparently become more so with an increase in pyrite in the veins. 

Intrusive clastic dikes along some of these veins are generally 

characterized by a sericitized, chloritized, and carbonatized 

matrix; some fragments within the dikes may be pyritized. It 

thus appears that the fluids involved in propelling the clastic 

material were sufficiently heated to produce recrystallization 

of the finer grained matrix. Pyritized fragments also suggest 

some alteration occurred near the initial sources of the dike 

material prior to its injection. The walls of these dikes are 

seldom, if at all, altered unless they were reopened and subsequently mineralized. Apparently, the occluded fluids were 

so cooled and attenuated by expansion during injection of the 

dikes that their temperature and volume after condensation 

tended to minimize alteration of the walls.

Silicification is a more conspicuous type of alteration in 

the outlying deposits of the mining district than it is nearer the 

center. The lower grade silver-lead-zinc replacement deposits, 

particularly in the Pony Express breccia beds, are characterized by silicification. The breccia may locally be highly silicified for hundreds of feet along its outcrop and converted into 

a porous, clinker-like rock. Any accompanying mineralization, 

however, was weak. The limestone fragments were replaced 

by light- to dark-colored, fine-grained quartz (fig. 25A), and 

the cavities of the breccia were lined with drusy coatings of 

small calcite crystals. Nearer the orebodies, fine crystals of 

barite were found in the cavities of the breccia (fig. 25B). The 

massive limestone and red shaly beds in the Molas Formation 

were strongly silicified in the Mineral Farm mine (fig. 21, no. 

35) in the southern part of the district. There, the shaly beds 

were especially converted into brown jasper, which formed a 

casing to the pipe-like ore shoots.

Carbonatization was more or less active in all parts of the 

district. Carbonate minerals were disseminated in noncarbonate wall rocks, and carbonatization aided the recrystallization 

of the limestone and breccia in the Pony Express Limestone 

Member of the Wanakah Formation. These beds, completely 

recrystallized in some areas into layered carbonate rock, 

66 Geology and Ore Deposits of the Uncompahgre Mining District, Southwestern Colorado
Rock Alteration 67

Figure 25. Photographs 

of polished slabs of typical 

altered and mineralized wall 

rock of veins in or near Dexter 

Creek, north of The Blowout 

area. A, Matrix of silicified 

limestone breccia composed 

of angular, loosely compacted 

platy fragments (br) in the Pony 

Express Limestone Member 

of the Wanakah Formation, 

showing thin seams of 

unidentified carbonate material 

and cavities (some developed 

by leaching) filled or lined with 

small calcite (c) crystals; dark 

centers in two cavities near 

right center of photograph are 

voids into which scalenohedral 

calcite crystals project. From 

Pony Express mine (fig. 21, 

no. 15). B, Silicified limestone 

breccia from the Pony Express 

Limestone Member of the 

Wanakah Formation showing 

vugs lined with crystalline 

quartz (q), barite (b), and 

calcite (c). From vein roll in the 

Señorita mine (fig. 21, no. 1).

Source of Altering Fluids

Various aspects of both the foregoing and following 

discussion were published by Burbank (1936). The views and 

conclusions expressed in that paper, although dated but based 

upon valid observations, have been summarized in order to 

present some of his interpretations concerning the relation of 

have cavities lined with small to large crystals of calcite. An 

intermediate-temperature type of alteration probably resulted 

in the development of iron- and manganese-bearing carbonate minerals near orebodies. The most common carbonate 

minerals are (1) manganiferous ankerite, such as that found in 

replaced limestone at the Mineral Farm mine, and (2) ferruginous rhodochrosite that was found in many lead-zinc deposits 

in the northern part of the district.
the magma solidification to the altering and mineralizing processes as applied to rock alteration and ore deposition in the 

Uncompahgre mining district. They, for their historic value, 

are included here to show the breadth of his understanding of 

the problems at that time (1930s).

The localization of zones of contact metamorphism, 

intense alteration, and mineralization of rocks within a few 

thousand feet of the central intrusive channel in The Blowout 

area suggests a genetic relation between these processes and 

the igneous intrusions. Numerous studies of similar occurrences have considered the source of altering and mineralizing 

fluids to be crystallizing magma. The intensity of the alteration 

and the extent of recrystallization in the granodiorite of the 

laccoliths during late stages of crystallization and the so-called 

hydrothermal conditions existing at that time make it appear 

unnecessary to require the existence of a large stock immediately beneath the central part of The Blowout area.

Alteration and mineralization in The Blowout area thus 

are thought to have been the result of superheated residual 

fluids from the later stages of the granodioritic magma crystallization. The bulk of the fluids’ original constituents were 

acquired from within the magma. There is little or no evidence 

of a discontinuity in the physical state of the fluids controlling 

the end phases of magma crystallization or the corresponding 

hypothermal phases of mineralization and alteration in the 

surrounding rocks. However, there seem to have been episodic 

breaks in the continuity of the mineralizing processes, marked 

by the formation of hematite veins. Structural conditions 

attending the formation of hematite veins suggest a relation 

between widespread fracturing and a consequent sudden 

release of pressure upon the crystallizing igneous bodies.

Rock Alteration Compared With Late-Stage 

Igneous-Rock Crystallization

The described principal stages of rock alteration are 

compared with those of igneous-rock crystallization in figure 

26. Correlation between different columns is indicated by the 

placement of the minerals (oldest at the top). The only stage 

for which there is evidence of an approximate time equivalence is indicated by the connecting dashed line across the four 

columns. All minerals above this dashed line may or may not 

be correlated correctly with respect to the time of their formation because it is evident that local reactions between the host 

rocks and mineralizing fluids may have displaced members 

within a reaction stage. Different minerals, belonging to one 

or another of the columns in the figure, formed products in a 

more or less definite order or pseudo reaction-series that have 

a considerable range of stability whereas other minerals have 

identified and limited positions in the series; some minerals 

occupy a definite position herein referred to as a discontinuity.

The crystallization sequence of hematite in all columns, 

thought to have followed a sudden structural adjustment such 

as shearing, fracturing, brecciation, or intrusion of some clastic 

dikes, and followed by the sequence of ferric iron-bearing 

silicates (epidote, chlorite, and thuringite) and metal sulfides, 

is probably a true time correlation. This interpretation is supported by observed field relations in both veins and mineralized 

rocks. The fact that hematite, an oxide, is locally the only or the 

principal mineral in some veins and that it has entered into a 

sulfide sequence suggests a break in the physical and chemical conditions of mineralization. The ferric iron-oxide stage 

of mineralization is thought to represent a discontinuity in the 

series resulting from an oxidation stage during late crystallization of the parent igneous bodies. The acidic emanations supposedly had large enough volume to dissolve and carry away 

much material to be deposited elsewhere. Hematite veins are in 

fact better developed west of the central zone of mineralization. 

Veins containing orthoclase and other magmatic minerals are 

more common close to the eruptive vent. Hence, the mineralogy of the highly altered rocks nearer the center of intrusive 

activity more closely resembles that of the intrusive body.

Conditions of the late recrystallization in the laccolithic 

bodies must have differed in some respects from those that 

existed during rock alteration and the emplacement of the hightemperature veins. The laccoliths presumably crystallized under 

conditions in which most of the residual fluids were retained 

until very late stages of crystallization, whereas the altered 

zones and veins in The Blowout area formed under conditions 

in which the mineralizing fluids were moving and reacting with 

the rocks they traversed. The physical and chemical conditions 

in the granodiorite in the laccoliths, not yet completely crystallized, were inferred from their later stages of crystallization 

history on the basis of the comparatively late episode of ferrous 

to ferric iron oxidation. This is suggested by chemical analyses 

(table 7, samples 4 and 5) of the rocks, and is indicated microscopically in the fairly well crystallized groundmass before 

oxidation of the embedded magnetite crystals took place. The 

most advanced stages of recrystallization in these rocks appear 

to correlate with a high ratio of ferric to ferrous oxide. These 

late stages resulted in complete recrystallization of feldspars, 

both as phenocrysts and in the groundmass, and in development 

of abundant secondary minerals such as orthoclase, albite, epidote, hematite, magnetite, calcite, and chlorite. Minor accessory 

minerals formed or recrystallized during these stages include 

apatite, rutile, and zircon(?). Sulfides such as pyrrhotite, pyrite, 

and chalcopyrite in addition to calcite were segregated in the 

interstices of the crystallizing rocks. These minerals are exactly 

the same as those formed in the altered and metamorphosed 

sedimentary rocks and the high-temperature veins.

In contrast to the granodiorite in the stock, the small 

intrusive body on the south side of Skyrocket Creek, within 

The Blowout area (fig. 12), was little altered. Its contacts are 

both gradational and interfingering; adjacent beds of shale, 

along bedding planes or joints, were recrystallized and contain 

porphyroblasts of plagioclase. This body intruded the sedimentary beds with no evident disturbance of the bedding and 

local structure. Composition of this equigranular rock (table 7, 

sample 1) is slightly less siliceous than the granodiorite, and 

contains slightly less potash and soda and more iron, magnesia, 

and alumina. The plagioclase is much less strongly zoned and 

68 Geology and Ore Deposits of the Uncompahgre Mining District, Southwestern Colorado
Rock Alteration 69

Figure 26. Paragenetic 

relations of minerals in 

porphyritic granodiorite, in 

sedimentary rocks in the 

exomorphic zone, and in veins. 

Modified from Burbank (1936, p. 

242) and used with permission 

of American Geophysical 

Union.

has only trace amounts of sericite. The only ferromagnesian 

mineral, brown biotite, has been altered partly to chlorite and 

partly to green biotite with a high birefringence; some magnetite and apatite are clustered around the partly altered biotite. 

Very minute flakes of chlorite are scattered sparsely throughout 

the rock. Accessory minerals include small grains of epidote, 

titanite, zircon, and hematite. Quartz was clearly the last mineral 

to crystallize; it cuts across grains or fills grain boundaries. 

Magnetite crystals, unlike those in the granodiorite stock in 

The Blowout, contain intergrowths of hematite similar to that 

in the moderately recrystallized and altered granodiorites in 

the laccoliths. The mineral composition and texture (grain size 

1–2 mm in diameter) of the rock in this small intrusive body, 

as well as the lack of strong reaction and alteration compared 

with the other granodiorites, suggest that it crystallized at a 

higher temperature; equilibrium was more closely maintained 

between melt, the volatile constituents, and the already crystallized minerals throughout the crystallization process. The strong 

recrystallization and feldspathization of the surrounding shales 

indicates that they have been granitized. Similar relations, but 

on a much larger scale, have been described by Eckel (1949) in 

the La Plata Mountains to the southwest. Plagioclase porphyroblasts, formed earlier than the epidote, chlorite, magnetite, 

and hematite in the surrounding beds, suggest that the rock in 

this small body was not intruded in a molten state; it perhaps 

crystallized during the metamorphism of the surrounding rocks. 
Oxidation of magnetite to hematite and formation of small 

amounts of epidote were contemporaneous with and genetically 

related to the ferric oxide stage of alteration. The lack of more 

advanced alteration and recrystallization of minerals in the rock, 

compared with those in the granodiorite of the laccoliths, was 

then the result of a slightly higher temperature of crystallization 

and a lower content of volatile constituents in the residual fluids 

during late stages of crystallization of this small intrusive body.

Rock Alteration Compared With Clastic Dike 

Formation

The sequence of alteration and mineralization observed 

near The Blowout is less applicable away from the igneous 

center. Correlating the timing of events in the central area with 

those in the outer areas is difficult. There is a suggestion of 

timing, however, as indicated by the hematite vein formation 

and clastic dike intrusion. Deposition of hematite, following a 

renewal of fracturing (fig. 26), is a fairly definite correlation 

in the mineralogical sequence. This ferric oxide discontinuity 

has not been recognized beyond about a mile north or about 

1.5 mi west of the intrusive center. Clastic dikes, fairly common throughout the Uncompahgre mining district, appear to 

have preceded deposition of ore, but in the central part of The 

Blowout area it was difficult, because of the intense alteration, to differentiate between clastic dikes or breccia masses 

formed by other means. A clastic dike on the northwest side 

of The Blowout contains fragments of shale and quartzite plus 

inclusions of highly sericitized and pyritized rock; this dike 

probably was intruded following the first period of sericitization. This occurrence, in conjunction with evidence found in 

the Ben Hur property (fig. 21, no. 24) on the west side of the 

Uncompahgre River valley near Corbett Creek, shows that 

some small clastic dikes formed more or less contemporaneously with the hematite veins, and that their occurrence with 

barite, galena, and manganiferous calcite marks a transition to 

the silver-lead deposits. However, in other parts of the district, 

clastic dikes and the mineralogical characteristics of the silverlead ores did not necessarily follow the discontinuity in the 

central area. Fragments of pyritized or sericitized rock were 

found locally among breccia fragments, but at most places, the 

immediate walls were little altered prior to breccia injection. 

Clastic dikes in intermediate to outer areas of the district also 

are commonly much larger than any found near The Blowout.

General Conclusions

The mineralogical interrelation of the high-temperature 

veins, contact deposits, hematite veins, and pyritic copperbearing veins and the fact that all are closely clustered about 

the central intrusive vent suggest that the mineralizing fluids 

carrying copper and iron were derived from a comparatively shallow igneous body beneath the central part of the 

Uncompahgre mining district along the line of the east-northeasterly dike swarm. The processes occurring in later stages 

of crystallization of this igneous body were responsible for 

the segregation of the mineralizing emanations and their metal 

contents. As stated earlier, igneous rocks in the area of the 

central vent and surrounding laccoliths probably crystallized 

under a cover of about a mile of relatively impermeable rocks, 

mostly shale, of Late Cretaceous age. The environment within 

the central vent area and its influence on the later stages of 

crystallization of the rocks thus are thought to be responsible 

for the relatively greater concentrations of copper and iron in 

the high-temperature veins and replacement deposits compared to the amounts of those metals in outlying deposits of 

the district. This zonal arrangement of metallization probably 

resulted from an increase in depth and change in position of 

the loci of segregation as crystallization and consolidation of 

the magma progressed. Solidification of the hydrous magma 

and related hydrothermal fluids may have a bearing upon 

conditions favoring disseminated concentrations of copper and 

iron, with an affinity to a porphyry copper deposit, in the igneous rocks beneath the central eruptive vent area.

Factors controlling metal concentrations may depend to a 

considerable degree on the manner in which energy contained 

within the magma was expended during crystallization. This 

seems to be illustrated by the local intrusive bodies and other 

mineralized rocks as related to copper and iron concentration. 

However, a detailed analysis of the magmatic processes was not 

undertaken concerning the distribution of energy in the system 

locally represented by the crystallizing igneous body and the 

surrounding zone of altered and mineralized rocks, because it is 

thought that metamorphism of the surrounding rocks required 

an expenditure of energy of which most was derived from the 

crystallizing body of igneous rock. The loci of energy release 

would have begun at a shallow depth in the top of the igneous 

body and retreated downward as the body crystallized.

In the Uncompahgre mining district, no exposed rock in 

The Blowout area is suggestive of concealed, recrystallized 

rock forming a porphyry copper deposit, but perhaps disseminated concentrations of copper could be present at moderately 

greater depths. Concentrations of copper were found in the La 

Plata Mountains (Eckel, 1949), a laccolithic center in the San 

Juan Mountains similar to and southwest of the Uncompahgre 

district (fig. 1).

Copper and iron concentrations were less in deposits in 

the northern and more outlying parts of the district because 

the source rocks were at a much greater depth. The lack 

of copper concentration was probably due, in part, to the 

structural relations of the crystallizing igneous sources from 

which the silver-lead-zinc deposits were derived. However, 

clastic dikes associated with many of the lead-bearing veins 

would have acted similarly to the hematite veins in releasing 

pressure, reducing fractionation, and promoting rapid oxidation of the iron and sulfur in the residual solutions. Rocks 

that crystallized at greater depths would presumably have 

had less tendency to concentrate iron in the later stages of 

crystallization. The pyrite and sphalerite content in the leadzinc ores increases slightly eastward from the Uncompahgre 

River valley, but massive pyritic ores, in general, are not 

70 Geology and Ore Deposits of the Uncompahgre Mining District, Southwestern Colorado
characteristic of the area. Lead-zinc veins closest to the central part of the district contained greater proportions of pyrite 

and chalcopyrite. The total iron content also was considerably less where barite is prominent because most iron in the 

sulfide fraction had been previously eliminated by oxidation.

Paragenesis of Primary Ores

Paragenesis of the high-temperature veins and of the 

minerals formed in the narrow aureole of metamorphism in 

The Blowout area was described briefly in connection with the 

discussions on both crystallization of the granodioritic rocks 

(p. 28–32) and wall-rock alteration of high-temperature veins 

(p. 65–66). Burbank (1936) described in considerable detail the 

close relation between the igneous rocks and the mineralizing 

processes, and documented the gradual passage from the later 

stages of crystallization of the porphyritic igneous rocks to the 

introductory stages of high-temperature mineralization. Here, 

the discussion is concerned with the minerals constituting the 

bulk of the ore deposits that form a continuation of the hightemperature veins and contact deposits. The paragenesis of the 

ores must also take into account three equally abrupt “breaks” 

or distinguishable discontinuities: (1) clastic injections of both 

premineral and intramineral age, (2) hematite veins or deposits 

and their associated mineral assemblages, and (3) processes of 

leaching and solution of the wall rocks or of previously formed 

mineral assemblages.

As previously discussed, it was concluded that the clastic 

injections, constituting the premineral clastic dikes, resulted 

primarily from the sudden releases of pressure upon the magma 

chamber(s) of the igneous rocks. The potential gas-forming 

constituents of the fluids expanded violently in the manner of 

gaseous volcanic eruptions, resulting in an injection of clastic debris carried along by the expanding medium. The most 

prominent clastic injections in the Uncompahgre mining district 

are premineral in age. Later intermineral clastic injections, 

commonly containing fragments of ore minerals and rocks of 

definitely local origin, resulted evidently from the local expansions of gases in the vein-forming fluids ranging from those 

of high temperature to those that deposited the sulfide minerals. Later explosive interruptions of the mineralizing processes 

definitely are tied to renewed fracturing, or to reopening of veins 

as the clastic dikes penetrated the walls of the original fractures 

or along newly formed fractures. The emplacement of the clastic 

dikes is thus part of the deformational history of the ore deposits.

Hematite mineralization of high-temperature type was 

associated with renewed fracturing of the rocks near the central intrusive vent. The release of pressure on the crystallizing 

igneous rocks perhaps resulted in boiling of the residual fluids 

and consequent production of vapor phases that removed the 

ferric iron from the igneous rocks and deposited it in the open 

fractures above. This was merely a milder form of gaseous 

expansion and, in certain instances, appeared to have been 

more or less contemporaneous with the more violent bubbling 

of the fluids that produced the local clastic injections and the 

hydrothermal leaching.

Hydrothermal leaching represents merely a negative phase 

of mineral formation. Dissolving of wall rocks or resolution of 

gangues in some ore deposits in the district tended to enrich the 

ore grade enough to be of commercial interest. This process, 

however, is not everywhere sharply distinguished from that 

of metasomatic replacement. Where hydrothermal leaching 

resulted in the formation of solution cavities in the rock, the 

cavities subsequently were more or less filled by minerals 

precipitated from solution, but the final result of open filling 

and replacement under some conditions may simulate that of 

metasomatic replacement. However, the chief concern here is 

hydrothermal leaching as related to the unfilled or partly filled 

solution cavities common in and near the ore deposits.

In many bedded deposits, such as those in quartzite at the 

American Nettie mine and in limestone at the Mineral Farm 

mine, the orebodies were localized in relatively minor fractures 

and cavities or along the bedding. In other places, such as within 

the permeable limestone breccia beds of the Pony Express 

Limestone Member of the Wanakah Formation where intersected by veins, bedded deposits are more common near veins 

than elsewhere. A correlation of mineral paragenesis in veins 

with that in the bedded deposits suggests that certain effects 

of hydrothermal leaching were concurrent with the reopening 

of fractures and injections of clastic material, and thus were 

apparently consequent upon the release of pressure. A similar 

correlation of hydrothermal leaching involving pressure release 

upon vein-forming solutions, following an effective distillation 

of volatile acid constituents, was postulated by Lasky (1936) for 

veins of the Virginia mining district in New Mexico.

Leaching discontinuities in the mineralization sequence 

in the Uncompahgre district, like the hematite veins and clastic 

injections, appear to have been related to pressure changes 

controlled by structural disturbances. The temporary leaching 

power of the fluids can be attributed to desaturation and a pH 

increase in acidity resulting from vaporization or from the socalled distillation column of Lasky (1936). Some hydrothermal leaching in the district also appears to have resulted from 

wall-rock reactions to hydrothermal fluids, dependent upon 

existing pressures and temperatures, in the ore channels as 

discussed by Holland and Malinin (1979, p. 465–471). Fluids 

passing from tight fractures into highly fractured or open permeable beds would have been affected by a drop in pressure, 

especially in the more open parts of the channel that were connected by conduits or other fractures to the land surface.

Normal Mineral Sequence

A recognizable mineralization sequence can be seen in 

many of the deposits despite numerous irregularities and local 

complexities. This sequence, omitting the silicate minerals in 

the contact-metamorphic deposits, is as follows:

(1) Fracturing and (or) premineral clastic injection.

Paragenesis of Primary Ores 71
(2) Sericitization, pyritization, chloritization, and carbonatization—chiefly or entirely metasomatic processes—followed, or accompanied in places, by the hydrothermal leaching of the wall rock or by renewed fracturing.

(3) Open-space filling, or replacement in part, by pyrite, 

sphalerite, chalcopyrite, and possibly small amounts of other 

sulfides. Quartz, the principal gangue of this early mineral 

aggregate, was deposited in open spaces; fine-grained quartz 

or jasperoid was fairly common in the wall rock.

(4) Mineralization in this stage was the one of greatest economic significance in all parts of the Uncompahgre 

mining district. Early mineralization may or may not have 

been interrupted by fracturing, intermineral clastic injection, 

or leaching, but it was nearly always followed by a coppersilver or copper-gold mineralization. Characteristic minerals 

are tetrahedrite and chalcopyrite together with other coppersilver sulfantimonides or sulfarsenides. In certain types of 

gold ores, such as in the American Nettie mine, tetradymite, 

copper-lead sulfobismuthides, and tellurides are also present. Varying amounts of the common base-metal sulfides 

nearly always accompanied these other minerals, and may 

be either new acquisitions of material from deep sources or 

mineral matter redissolved and reprecipitated by preceding 

intermineral leaching processes. The gangue was commonly 

quartz, but barite may have appeared late in the sequence or 

may have accompanied minimal galena deposited during this 

stage. The bulk of the galena, however, was always later than 

the copper minerals, but minor amounts of chalcopyrite and 

other copper-silver minerals may have replaced or veined 

galena locally. Calcite or manganiferous calcite occurred with 

the later minerals of this stage. Native gold likewise appeared 

late, either by a breakdown of the previously precipitated tellurides or by some process of local “dumping,” such as that 

which occurred in the American Nettie deposit. The occurrence of very late-formed, large nugget-like masses of native 

gold in this ore and the localization of ore shoots in the large 

pre-ore cavities leached in the quartzite, suggest that localized 

throttling processes (Burbank, 1950, p. 300) were responsible 

for the precipitation of this ore. In general, these rich, highly 

complex silver-gold ores probably formed by both open-space 

filling and metasomatic replacement. Especially during the 

later stages, metasomatism was almost entirely confined to the 

sulfide series, suggesting that either the fluids were saturated 

with all the common wall-rock constituents at this time or their 

energy was insufficient to cause much of an interchange with 

the more refractory gangue minerals.

(5) Refracturing and local clastic injection may have 

initiated the final stage. Galena with lesser amounts of other 

base-metal sulfides commonly was deposited following the 

copper-silver or copper-silver-gold mineralization closing 

the main period of sulfide deposition Galena ores are more 

characteristic of the silver-bearing deposits than of the goldbearing deposits. In the higher temperature pyritic veins in the 

central parts of the district, late galena is sparse or lacking. 

Both barite and quartz are present as gangue, and barite was 

commonly deposited after the galena, so that barren baritic or 

very low-grade galena-barite deposits are found in the silver 

veins, or more especially, in the veins in the outermost parts 

of the district. Carbonate minerals such as manganoan calcite 

accompany galena in places, and rhombohedral and scalenohedral calcite, containing minor amounts of dolomite and ankerite, locally fill vugs in the sulfide ore or in the barite masses. 

Banded or delicate drusy quartz was commonly deposited on 

the carbonate minerals, and the carbonate material was then 

leached to leave a skeleton of quartz. Very locally, particularly 

in limestone wall rock, the post-ore stage of leaching attained 

considerable prominence, with the result that the carbonate 

gangue surrounding the sulfide ore was extensively leached 

and formed caves in which fragments of sulfide masses and 

residual clay were deposited.

Hydrothermal Leaching

The most striking examples of hydrothermal leaching are 

found near the central part of the Uncompahgre mining district, where massive sandstone beds are extensively leached, 

forming pre-ore caves. The most permeable sandstones, or 

those that were strongly jointed, were more susceptible to 

leaching; capping or parting shales localized the horizons of 

strongest leaching. Pre-ore cavities appear to have been most 

closely associated with those stages of mineralization that 

immediately followed or accompanied the early sericitization 

and carbonatization.

The sandstones are intensely indurated near the ore 

channels, and conversion of the permeable sandstones to 

quartzite was widespread throughout the district. This is true 

particularly of (1) the massive sandstone beds of the Dakota 

Sandstone known by miners as the “upper quartzite,” (2) some 

sandstone beds in the uppermost Morrison Formation, and (3) 

the basal sandstone of the Tidwell Member of the Morrison 

Formation, known by miners as the “lower quartzite.” In the 

central part of the district, in addition, small scattered crystals 

of pyrite were found in the recrystallized beds for a considerable distance from the principal concentrations of sulfide 

minerals; closer to the orebodies, minor amounts of sericite 

accompanied the pyrite. Silicification of the more permeable 

sandstones definitely correlates with this particular phase 

of the paragenetic sequence. It is significant that the most 

indurated beds are also those that were the most leached and 

pyritized. The “upper quartzite” sandstones contain the largest 

caves; some caves are referred to as “pear shaped” (Cross and 

others, 1907, p. 18), are as much as 20 ft wide and 30 ft long, 

and are commonly aligned on fractures. The “lower quartzite” 

sandstone contains the next largest caves. The permeability of 

the beds and the volume of pore space must have been important factors controlling the degree of hydrothermal leaching. 

Furthermore, the silica that dissolved near the ore channels 

migrated mostly outward into the permeable beds with the 

later sulfide solutions that deposited the pyrite. Additional 

dissolved silica was carried along the main open channels and 

72 Geology and Ore Deposits of the Uncompahgre Mining District, Southwestern Colorado
was deposited as incrustations of crystalline quartz and banded 

material on the walls of both fractures and cavities.

In the quartzitic beds in the central part of the mining 

district, the processes of supersaturation and undersaturation 

of silica apparently were not separated by any recognized 

event and evidently were closely related in time and space. In 

the early phases of hydrothermal leaching, solution exceeded 

deposition, and the cavities gradually were enlarged until a 

balance was reached. Locally, this balance was not attained 

until after the beginning of sulfide deposition, but in other 

places, incrustations of quartz formed before the beginning of 

sulfide deposition. In the latter case, the sulfides were deposited in lesser amounts, and a saying among miners was “...

as soon as one finds crystals in the quartzite there will be no 

more ore” (Irving, 1905, p. 68). Where this balance was not 

reached until after the beginning of sulfide deposition, the sulfides were deposited immediately on a smooth wall of impervious quartzite, or they graded outward into a narrow zone of 

metasomatic replacement or as fillings of small openings. This 

gradation zone commonly was called “honeycombed quartzite.” In zones close to the main feeding channels of the ore 

fluids, especially along tight fractures, or where the fractures 

passed from less permeable rocks into quartzite, metasomatic 

replacement appears to have been the dominant process from 

the beginning to the end of mineralization, and there is little or 

no evidence of hydrothermal leaching or the formation of open 

cavities.

In the bedded deposits in limestone or limestone breccia, 

the processes of hydrothermal leaching were marked by the 

formation of cave breccias or of very extensive open brecciated ground in which the ore was only locally deposited. 

Cave breccia formed by hydrothermal leaching in limestone 

is best illustrated by the orebody at the Mineral Farm mine. 

There, the ore channel lies in the topmost part of the Leadville 

Limestone immediately beneath the capping lower shales of 

the Molas Formation. In cross section, the channel is a brecciated mass of silicified and otherwise altered shale occupying a linear depression in the underlying limestone. The ore 

has filled quartz-lined openings or spaces in porous, silicified 

breccia chiefly in the central parts of the channel, but also 

occurs in smaller masses formed by replacement in the surrounding altered shale and limestone. Mostly barren, quartzlined caves also are found in the shale and limestone, but they 

are not as extensive, beyond the limits of ore deposition, as 

those in quartzite. Paragenesis of the minerals is complex and 

evidence of the actual beginnings of the mineralizing processes is obscure. Structural conditions indicate, however, that 

the caves and cave breccia were of early origin and preceded 

or formed concurrently with silicification. Conditions preceding and following formation of the caves in the limestone and 

shale were more complex than in the quartzite because the two 

kinds of country rock differed widely in chemical reactivity. 

Open cavities formed mostly in space previously occupied by 

the limestone, hence the solution of limestone was certainly 

much more rapid than the solution of shale. As the openings 

in the limestone enlarged, the shale caved into them. The 

following or overlapping wave of silicification, on the other 

hand, advanced much more intensively into the shale than 

into the underlying limestone. Shales and sandy beds capping 

the ore channel were silicified through a thickness of 5 to 10 

ft, whereas the underlying limestone was chiefly recrystallized and, if silicified, recrystallized only near the ore. Ore in 

the limestone may have only a fraction of an inch to several 

inches of a silicified casing. Silicification was followed by 

veining and replacement of all the rocks, whether previously 

altered or not, by sericite, pyrite, ankerite, and quartz. The 

chemical character of the solutions introducing the stage of 

sulfide mineralization was the same as that which deposited 

ore in the quartzitic rocks previously described. Compared 

to the quartzite, the solution of limestone did not attain a balance with deposition until the beginning of the sulfide stage, 

whereas in the shale, silica was deposited comparatively early 

in the sequence. The lack of extensive silicification in the 

limestone presumably indicates that leaching overbalanced 

deposition and that the solutions were undersaturated with 

calcium carbonate. Overlying shale beds, because of sagging, slumping, and consequent fracturing, thereby permitted 

greater penetration and condensation of the solutions and their 

siliceous material. The solutions appear to have begun their 

leaching when they were nearly saturated with silica, in contrast to conditions nearer the sources such as those resulting in 

the quartzitic deposits.

The limestone breccia in the Pony Express Limestone 

Member of the Wanakah Formation was also hydrothermally 

leached. This unit, as described, possessed an original permeability and porosity far exceeding that of any other rocks in 

the mining district. Leaching and solution of the sedimentary 

nodular gypsum took place considerably prior to mineralization; it was not hydrothermal but the result of ground-water 

circulation. The breccia thus formed is a collapse type composed of the broken and compacted, relatively insoluble shale 

and limestone layers of the original shaly limestone-gypsum 

deposit. Conditions were favorable for leaching where this 

breccia bed was traversed by fractures that served as conduits 

for the hydrothermal solutions. Evidence indicative of the 

hydrothermal leaching process is obscure, but at many places 

near the ore deposits, the partly mineralized breccia is locally 

much more porous than the original breccia. This is particularly evident along the immediate borders of the ore channels, 

where the rock now consists of a porous, clinkery mass of 

fine-grained dark quartz, in which the original bedded nature 

of the breccia has been locally preserved. In other places, the 

original bedding was completely lost and only a jumbled breccia of silicified ribs or fragments remains. The latter condition 

appears to have been brought about by successive stages of 

leaching, local slumping and fracturing, and silicification. This 

is locally illustrated in the Pony Express vein and in several 

other silver-lead veins that transected this breccia horizon. 

Stages of leaching and silicification are correlated in part with 

the stages of deposition of a similar but fine-layered colloform 

silica on the walls of the feeding veins. This type of silica was 

commonly deposited upon the walls of veins immediately 

Paragenesis of Primary Ores 73
following a stage of fracturing or reopening of the fractures, in 

part preceding the deposition of the early sulfides and in part 

between the stages of sulfide deposition. Intermineral stages 

of silicification also were tied closely to clastic dike injection. 

Fragments of silicified material are found within the clastic 

debris, and the fractured walls of the clastic dikes are locally 

cemented by similar siliceous material.

Quantitatively important hydrothermal leaching seems to 

have been confined to permeable sandstones or to limestone. 

Less pervious rocks also have been leached along fractured 

zones or near other openings. The fine-grained silty sandstone 

of the Bilk Creek Sandstone Member of the Wanakah Formation that overlies the Pony Express Limestone Member has 

been leached at a number of places. In the Seaburg tunnel, 

beneath the American Nettie mine workings, this sandstone 

was fractured and local zones of solution breccia were formed. 

The sandstone was locally silicified, or chloritized and sericitized, evidently prior to leaching. The cavities formed by 

leaching were lined with clear quartz. Some hematite, chlorite, 

pyrite, sphalerite, and calcite were deposited in the openings, but for the most part, the cavities were not filled. Along 

the north side of The Blowout, near the Skyrocket group of 

mines, this sandstone was locally brecciated, apparently as a 

result of slumping brought about by leaching of the underlying 

breccia in the Pony Express Limestone Member. The cavities 

were filled with either sulfides or hematite, chlorite, calcite, 

and other minerals. This is the only locality where there is 

evidence that silicification of the rocks preceded hydrothermal 

leaching. The breccia fragments contain garnet and outlines 

of other silicate minerals and associated hematite, all of which 

appear to have been formed prior to brecciation.

Open-Space Filling

The structural and textural evidence for open-space 

filling in bedded deposits in both quartzite and limestone is 

summarized as follows:

1. Open or partly filled cavities were crustified by quartz in 

the intermediate and outer zones parallel to the trend of 

the ore channels in quartzite, limestone, or shale.

2. The downward-bulging capping shale, where in direct 

contact with unoxidized ore of the massive sulfide bodies, 

can be seen in the host rocks in the downdip and central 

part of the ore channels. This feature was noted by Lakes 

(1901) from the quartzite-shale associations exhibited in 

the Dakota Sandstone in the American Nettie deposit.

3. Breccia clasts, consisting of detached fragments of shale, 

fell below their normal stratigraphic position to the floors 

of cavities or filled caves. They now are found only in the 

outer parts of the ore channel. This feature is especially 

well preserved in the shale-capped limestone in the Mineral Farm mine. It is seldom found in quartzite.

4. Crystal faces of minerals project into the vugs in the sulfide 

orebodies, suggesting that solution commonly exceeded 

deposition in the late stages of ore deposition to an extent 

incompatible with true metasomatic replacement.

Both large and small open cavities were crustified by 

comb-quartz crystals and sulfides in layers ranging from one 

layer deep to essentially a complete filling. This is the most 

direct evidence for the existence of voids prior to ore deposition, and was noted by Irving (1905, p. 68), who examined the 

deposits during their early mining development. Regarding 

formation of the cavities, he stated, “There can be no question 

that they were produced by solution.” These features can still 

be seen along the unmined borders of the original orebodies. 

Remnants of the crustified linings of the caves also can still be 

seen in the central channel of the orebody, but essentially all 

the sulfide ore was removed from the caves during mining. In 

some centrally located caves, surfaces were not crustified but 

were very smooth and had the type of intersecting, concaveoutward surfaces so common in solution-formed limestone 

caverns. This feature was originally reported by Irving (1905), 

but it also was found in true replacement deposits (Bastin 

and others, 1931, p. 602), so that it may not necessarily be 

evidence of prior cavity formation.

Fragments of shale detached from the walls of the 

cavities also were seen along the borders of the ore shoot, 

but crustification was sufficiently well preserved to furnish 

independent evidence of the opening. The most significant reference to possible caving of the shale from the roofs of some 

of the larger orebodies was made by Lakes (1901, p. 243) who 

noted that “...when there is a bulge or roll of the shale down 

into the quartzite ... locally large bodies of ore are likely to be 

found.” These downward “rolls” of the shale may have been 

similar to those now seen in the Mineral Farm mine where 

solution cavities in the Leadville Limestone resulted in slumping or “rolling” of shale from the overlying Molas Formation.

The foregoing discussion concerns many of the ore 

deposits in the Uncompahgre mining district that provided 

strong evidence for open-space filling, particularly at a time 

when the concept of metasomatic replacement (essentially 

“molecule by molecule” with negligible open space available) was becoming more readily recognized as discussed by 

Lindgren (1933, p. 91–92, 173–174). Evidence that indicates 

the effects of metasomatic replacement processes locally in the 

formation of the ore deposits is as follows:

1. Recrystallization of host rocks in walls of the orebodies and their conversion to sericite, quartz or jasper, and 

carbonate minerals or mixtures of these minerals with 

disseminated sulfides, as well as the preservation of the 

original internal features of the host rock.

2. Massive sulfide bodies, lacking preserved textures but 

conformable to bedding surfaces or localized by fractures 

or dike contacts, that exist where continuous open space 

was precluded by structural conditions. As suggested by 

the wide extent of orebodies beneath incompetent capping beds, or by the lack of caving or slumping of these 

74 Geology and Ore Deposits of the Uncompahgre Mining District, Southwestern Colorado
capping beds, these orebodies consist of both contact 

metamorphic ores and associated pyritic ore masses.

3. Massive ores, examined under a reflecting microscope, 

showing that a complex series of metasomatic interchanges took place between host rocks and earlier formed 

gangues, different sulfide minerals, and the later mineralbearing fluids. This evidence consists of preserved relict 

mineral textures, formation of pseudomorphs, and other 

commonly accepted microscopic criteria (Bastin and others, 1931, p. 590–606).

Quartz and calcite were the principal minerals dissolved, but this apparent selectivity is thought to result from 

mainly structural and physical properties in the sandstone 

and, perhaps, from chemical properties in the limestone. The 

clean quartz sandstones were the most permeable rocks other 

than the limestone breccia in the Pony Express Limestone 

Member of the Wanakah Formation. These rocks tended to 

permit clean or open fractures. The permeability of the limestone breccia was probably a factor in promoting low pressure conditions favorable to solution. If the solutions tended 

toward acidity, resulting from the process of distillation, the 

susceptibility of limestone to attack is easily accountable. 

Lasky (1936, p. 163–164) reported that in the Virginia district 

of New Mexico, calcite, sericite, and chlorite were dissolved. 

In the Uncompahgre district, it was not possible to establish 

with equal certainty the solution of other mineral aggregates. 

Some sericitized and chloritized rocks have abundant small 

solution cavities lined with quartz crystals; these furnish the 

most positive evidence of leaching of other minerals. Solution 

cavities in shales in the Molas Formation, near the Mineral 

Farm mine orebody, probably were the result of the solution of 

broken or shattered shale. The volume of other mineral matter 

dissolved is apparently subordinate to that from sandstone and 

limestone. Sulfide minerals also appear to have been dissolved 

locally, leaving casts or ribs of siliceous material.

Hydrothermal leaching of the quartzite was more or less 

concurrent with early sericitization and carbonatization in 

deposits such as those at the American Nettie mine. Perhaps 

the quartzite was first replaced with an aggregate of sericite 

and calcite that was dissolved afterward, leaving the outlines 

of the replacement body. However, little evidence favors this 

interpretation; the quartzite surrounding the cavities in the 

outer parts of the ore channel supposedly would be expected 

to contain more of the metasomatic alteration products in the 

walls of the cavities. During the beginning stages of hydrothermal leaching near the more central parts of the channel, 

some sericite and calcite may have been dissolved, but evidence is lacking because later deposition of massive sulfides 

obscures the early stages of the process.

Sulfide Paragenesis

The sulfide minerals have a common paragenetic 

sequence in most of the ore deposits. Pyrite is the earliest 

formed mineral in all deposits. Nearest the intrusive body in 

The Blowout area, pyrite was followed by hematite, and in 

turn by chalcopyrite, quartz, and thuringite. Sphalerite and 

galena are not abundant, but where present, were formed later 

than this introductory sequence.

A moderate distance from the restricted high-temperature zone of The Blowout area, chalcopyrite followed more 

directly after pyrite, and both pyrite and chalcopyrite were 

fractured and veined by sphalerite. Pyrite appears to have been 

selectively and extensively replaced by chalcopyrite. In the 

massive sulfide and telluride ores of the American Nettie mine 

in this principal gold-bearing zone, the order of crystallization sequence, as determined by microscopic examination of 

ore specimens, was pyrite, chalcopyrite, sphalerite, tennantite 

and minor additional chalcopyrite, tetradymite, benjaminite(?), galena, hessite, and native gold. According to descriptions of the large orebodies from which ore of this type was 

mined (Endlich, 1889; Lakes, 1901; Irving, 1911a), barite also 

was contained in the massive sulfide ores. Its position in the 

sequence, however, is indeterminate as most ore of this type 

had already been mined; where seen by W.S. Burbank, barite 

was definitely later than pyritic copper ore but was commonly 

coated with pyrite. Similar ore types in nearby mines indicate 

that barite was deposited late with galena. In ore from the Ben 

Hur vein, copper minerals were followed by barite, manganiferous calcite, and galena.

The complex ores of the American Nettie mine, from 

which the above mineral sequence was determined, have a 

roughly, concentrically layered texture, particularly along 

the margins of sericitized and pyritized gangue fragments. 

The pyrite was reduced mostly to minute, rounded fragments 

engulfed in the chalcopyrite, which evidently replaced it. 

Tetradymite is the only mineral in the crystallization sequence 

that has well-developed crystal outlines.

In the silver-lead veins farther to the north from the 

intrusive center, pyrite together with sphalerite formed early. 

Chalcopyrite commonly followed rather than preceded sphalerite; chalcopyrite, however, is a subordinate mineral in the 

zinc-lead-silver ores, suggesting that the common sulfides 

were deposited successively more or less in the order of their 

abundance. Silver deposition followed the early base-metal 

sulfides. Chalcopyrite and silver-bearing tetrahedrite were 

deposited more or less contemporaneously, and they veined or 

replaced the sphalerite. Small veinlets of chalcopyrite and tetrahedrite in sphalerite and quartz were truncated by galena. In 

most mines in this zone, the late origin of galena was clearly 

shown; ore consisting chiefly of pyrite, sphalerite, and a little 

chalcopyrite, often brecciated, was veined by the galena.

Pearceite or polybasite is closely associated with tetrahedrite or with tetrahedrite and chalcopyrite, and as noted and 

shown by Bastin (1923, p. 70) there was little doubt that pearceite is primary (fig. 27); its deposition overlaps that of galena. 

Bastin (1923, p. 71–72) did describe late veinlets of pearceite, 

chalcopyrite, and calcite veining all of the earlier sulfides, but 

considered them to be probably of supergene origin. In view 

of the solution and redeposition of recognizable non-sulfide 

Paragenesis of Primary Ores 75
76 Geology and Ore Deposits of the Uncompahgre Mining District, Southwestern Colorado

Figure 27. Camera lucida drawing of a polished thin section 

of an ore specimen showing primary pearceite intergrown 

with tetrahedrite, chalcopyrite, and galena, Bachelor mine, 

Uncompahgre mining district, southwestern Colorado (modified 

from Bastin, 1923).

minerals and of small stringers of late fine-grained pyrite and 

chlorite noted in some ores, we think that the late veining by 

certain sulfides such as chalcopyrite, pearceite, and pyrargyrite 

was a primary or hypogene process rather than a supergene 

process. Veinlets of this kind seem to be no more common in 

ores indicative of definite supergene deposition than in ores in 

which supergene mineralization was greatly restricted.

Repetition of the sulfide crystallization sequence is common in the silver ores. Ore from a prospect on the north slope 

of Dexter Creek valley consists of a breccia of early pyrite, 

sphalerite, and galena in a matrix of sandstone fragments and 

fine-grained material; this breccia was replaced and veined 

by a second generation of pyrite, sphalerite, chalcopyrite, 

and galena and some freibergite and pyrargyrite. The second 

generation of pyrite and sphalerite consists in part of concentric colloform layers deposited on the earlier minerals. In ore 

from the James V. Dexter vein, deposited near a granodiorite 

sill, the crystallization succession was early quartz, pyrite, and 

sphalerite followed by hematite, second-generation pyrite, 

chalcopyrite, and galena. The early sphalerite, where brecciated, was healed and cemented by a second-generation 

sphalerite having a slightly lighter color. Second-generation 

sphalerite is typical in the sphalerite ore; it is thought to have 

resulted principally from the solution and redeposition of 

first-generation sphalerite. The second-generation pyrite has 

an unusual texture in that it has a much finer granular form, 

and many of the grains are anisotropic. Ore from the Dexter 

vein, however, contained some gold and is not typical of the 

high-grade silver-lead ores. This vein locally provides evidence for deposition at a higher temperature, based both on 

the percentage of base metals present and the recurrence of 

the hematite discontinuity, which is not commonly recognized 

in ores several miles from the intrusive center. The hematite 

discontinuity here was very weakly developed. Because the 

Dexter vein strikes north-northeast across the silver-bearing 

vein of the Calliope mine, there is a possibility that the Dexter 

vein tapped local, but somewhat different, sources to the south 

where the pyritic gold veins occur. Ore fluids forming the Calliope vein came chiefly from the east.

The association of minor repetitions in the deposition 

of the early minerals with discontinuities, especially of the 

type noted in the Dexter vein and in the breccia ore described 

from the north slope of Dexter Creek valley, indicates that 

fracturing and fluctuations within the hydrothermal fluids 

dissolved and reprecipitated some of the earlier minerals. 

Repetitions in the sulfide sequence probably were also the 

result of the tapping of deeper or new sources of the mineralizing fluids.

Economically important stages in the paragenesis of 

the silver-bearing veins, mostly to the north of the intrusive 

center, were (1) formation of pyrite and sphalerite as early 

base-metal ore, (2) formation of chalcopyrite and tetrahedrite 

as cupriferous silver ore, and (3) formation of galena ore 

containing relatively low amounts of silver; barite accompanied both the second (copper) and third (lead) ore stages. 

These three stages were in many places sufficiently well 

separated by breaks in the mineral deposition sequence, 

caused by clastic injections and refracturing, that recognition 

of the vein structure is of value in mine development. This 

is best illustrated in the Bachelor group of mines (pl. 4). The 

events that occurred there were (1) premineral clastic injection (the Bachelor clastic dike), (2) base-metal mineralization consisting chiefly of pyrite and sphalerite, (3) interdeposition fracturing and clastic injection, and (4) deposition 

of quartz and barite accompanied by tetrahedrite and galena, 

to be followed by more-or-less barren quartz and barite. At 

the western termination of the veins in this group of mines, 

the last barite and galena were locally deposited in fractures 

different from those containing the high-grade silver ore. At 

the eastern termination of the veins toward the sources of the 

mineralizing fluids, depositional stages were somewhat less 

distinct owing to the ore containing less silver and galena 

and more pyrite and sphalerite. Ore in the veins can be 

roughly divided into zones corresponding to the three stages 

of ore formation mentioned above, suggesting the fractures 

were gradually filled from east to west, in that order. Later 

structural disturbances, such as intermineral clastic injection 

or refracturing, were less effective in producing recognizable breaks within the vein formation as distance was gained 

along the fractures from their easterly mineralization source. 

The mineral succession thus seems related both to zoning in 

individual veins and to regional zoning in a broad sense.
Carbonate Paragenesis

The carbonate gangue minerals associated with the ores 

have a general sequence of deposition paralleling the zonal 

distribution of ore minerals in the Uncompahgre mining 

district. Siderite is locally present in the higher temperature 

replacement deposits nearer the intrusive center, and has been 

reported from the American Nettie mine and in the pyritic ores 

of the Wanakah group of mines. It seems to have been deposited early in the sequence and was associated with the early 

stages of sericitization and pyritization.

The Pony Express vein in the lead-silver zone, at the 

eastern end of the Syracuse tunnel level in the Bachelor group 

of mines (pl. 4), farther north of the intrusive center, contains 

ferruginous rhodochrosite associated with the early deposited 

pyrite and sphalerite. An analysis of this mineral is given in 

table 13. The later galena ore stage in the same vein system contains manganocalcite as a gangue. This mineral, as determined 

by its optical properties, is closer to calcite than to rhodochrosite 

in composition. The final stage of carbonatization in nearly 

all the veins is either rhombohedral or scalenohedral calcite. 

Locally, minor amounts of dolomite and ankerite encrusted vugs 

in the ore. These carbonate minerals have no consistent paragenetic relations except that they are always late in any particular 

sequence. In some places, the earlier carbonate minerals were 

replaced by later ones, but in other places, only casts of the earlier carbonate minerals remain; these minerals were first coated 

or veined by quartz and then dissolved.

In contrast to the low magnesia content of the early 

rhodochrosite in the Pony Express vein, an analysis of early 

ankerite from the Mineral Farm lead-silver ore in the southern part of the district is given for comparison in table 13. 

This deposit occurs at the top of a limestone and dolomite 

sequence, suggesting that additional magnesia and lime may 

have been derived from solution of these beds at depth rather 

than from an igneous source. A single example, however, is 

Table 13. Chemical analyses of carbonate mineral samples 

associated with ore deposits in the Uncompahgre mining district, 

southwestern Colorado.

[Analyses by U.S. Geological Survey. Chemical data, in weight percent, are 

from standard methods by K.J. Murata]

Sample no. 1 2

Field no. O-3636 O-06397

CaCO3 54.67 2.18

MgCO3 13.98 0.91

MnCO3 10.54 62.59

FeCO3 13.32 31.77

Insolubles 7.20 2.38

Total 99.71 99.83

Sample descriptions:

1. Ankerite, Mineral Farm mine; mixed manganiferous carbonate replacing 

upper part of Leadville Limestone.

2. Ferruginous rhodochrosite, Bachelor mine group; Pony Express vein on 

Syracuse tunnel level.

not conclusive. With this exception, the predominant carbonate minerals of the district are, in general, of iron, manganese, 

and lime varieties rather than of the magnesia variety.

Descriptions of Mines

As previously stated, descriptions have been published in 

varying degrees of detail that concern the general and detailed 

history, development, geology, and mineralogy of some of the 

mines and mine groups and their respective ore deposits in the 

Uncompahgre mining district. However, it is neither possible 

nor practical, within the limits of this report, to describe in 

detail all the mines in the district. The mines discussed vary in 

size, amount of development, extent of information available, 

and accessibility at the time of field examination. Certain differences and unique features of these mines are described with 

respect to the developed ore deposits as distinguished by the 

local conditions of fracturing and the local sources of mineralizing fluids. Only mines that exploited the older Late Cretaceous to early Tertiary (Laramide orogeny) ore deposits are 

included. Unless otherwise specified, much of the historical 

and mine data are attributed to W.S. Burbank, who, during his 

field investigations from the late 1920s to mid-1930s, verbally 

acquired this information from knowledgeable, long-time area 

residents and the operators of a few, then-active mines.

Mineral Farm Mine

Location, History, and Development

The Mineral Farm mine (fig. 21, no. 35), at an altitude 

of about 8,500 ft, is located about a half-mile southwest of 

the town of Ouray on the bench at the north base of Hayden 

Mountain, and is accessed from the Canyon Creek road. The 

original group of four claims, located in 1875 or 1876 by 

Augustus W. Begole and John Eckles (Echols?) and worked 

only during the summers until the fall of 1878, was among the 

first to be located in this part of Colorado, and was, as stated 

by Fossett (1879, p. 529), “...one of the wonders of this part 

of the state.” Through the next few years the original property 

was consolidated and enlarged with the addition at different 

times of more claims including the Miser’s Dream, Miser’s 

Desire, and Midnight.

The first ore taken from the property was packed or 

hauled by bull teams 150 mi across the San Juan Mountains 

to Alamosa, the nearest railhead, for shipment to Denver; 

transportation costs are said to have been $40 to $50 or more 

per ton, which seems low compared to costs previously quoted 

(see p. 5). This ore was obtained principally from stripping 

and quarrying operations from small, shallow pits and shafts 

sunk into the surface of the bench that here is a dip slope on 

the top of the Leadville Limestone. The property was sold in 

the early 1880s for a sum stated to be $75,000 (questionably 

$7,500 in Rice, 1980) to a Virginia-based company (NorDescriptions of Mines 77
78 Geology and Ore Deposits of the Uncompahgre Mining District, Southwestern Colorado

m, Molas Formation; Ml, h, Hermosa Formation; 

* Plan and section of the Mineral Farm mine and orebody (modified from Burbank, 1930, p. 221). Qc, colluvium; 

*

, downthrown side. , upthrown side of fault; D , winze; U , raise; w

Figure 28. Leadville Limestone; s, shaft; r
folk & Ouray Reduction Co.), which, in addition to mining, 

constructed and operated a nearby smelter to treat the ores, 

thereby reducing hauling costs. Production continued intermittently until the spring of 1886 when, according to a news item, 

the mine was sold to parties under the name of the Mineral 

Farm Mining Co. Mining operations continued for at least 

two years following construction of the railroad to Ouray in 

1887; most of the ore was from open cuts and old workings. 

R.J. Lucas purchased the property, then known as the Mineral 

Farm Consolidated Mining Co., consolidated additional claims 

to it and built a new mill to treat the ores; the property was 

mined through an inclined shaft. This operation continued for 

several years with varying degrees of success, but was interrupted now and then by shutdowns because of the lack of ore, 

or of water for the mill, until temporarily closing in the late 

1890s. B.H. DuPraw worked the property under lease for a 

year or so with little success, then turned it back to the owners. 

Retaining DuPraw as superintendent, the company retimbered an old crosscut located 400 to 500 ft west of the eastern 

workings, sank a vertical shaft and discovered a new orebody, 

which created considerable interest and resulted in the staking 

of adjoining ground in the hope that a large blanket deposit 

had been discovered; new investments in mining and milling 

equipment were made during development of this orebody. 

The mill operated until the fall of 1907, when operations were 

again discontinued. Within that timeframe, exploration was 

done below the level of and near the western end of the eastern ore shoot by means of an 80-ft-deep shaft (fig. 28). J.C. 

Ingersol, who examined those working about 1910, reported 

two drifts had been extended from the bottom of the shaft; one 

intersected a cross vein that was drifted on for 95 ft. This vein 

was in some places a gouge-filled slip and in other places an 

open fracture containing reddish-brown clay and calcite crystals. Various lessees apparently operated the property intermittently over the next few years. According to Rice (1980, p. 

38), J. VanDaam operated the property about 1915, and built 

a small smelter to treat his and custom ores. Minor production 

is reported for the property in 1916, but apparently after 1918 

only small lots of crude ore were shipped. James Braden next 

acquired the property during World War II, cleaned up the old 

machinery, and shipped it for scrap iron. King and Allsman 

(1950, p. 51) reported the mine was operated through 1947 by 

A.E. Alexander, of Ouray, but the operation was suspended 

early in 1948. More recently one of the adits has been cleaned 

up and converted to a tourist attraction.

Production

Production records are incomplete and do not exist for 

many of the years the mine was in operation. Before 1886, 

production under somewhat adverse conditions could not have 

been large. About $40,000 supposedly was received for ore 

mined after the arrival of the railroad. Combined reports of 

Fossett (1879) and of the annual volumes of the U.S. Bureau 

of the Mint (1882–1893) in table 14 indicate production values 

of more than $137,000, much of which was for rich silver ore 

Table 14. Values of early production, in dollars, of gold, silver, 

and lead from the Mineral Farm mine.

Year Gold Silver Lead Total

18771 -- -- -- 3,000

18782 -- -- -- 8,000

18813 -- -- -- -18824 -- -- -- -18875 -- 5,836 2,967 8,803

18886 -- -- -- -18907 6,649 42,926 13,050 62,625

18918 429 17,608 -- 18,037

18929 400 34,314 2,175 36,889

1

Fossett (1879, p. 512): Smelter production, Ouray County, 30 tons treated, 

possibly from Mineral Farm mine property.

2

Fossett (1879, p. 512): Norfolk & Ouray Works, Ouray.

3

U.S. Bureau of the Mint (1882, p. 419): “...Mineral Farm...producing from 10 to 12 tons of ore per day, of a value that exceeds any ore yet 

extracted in this region. It is gray copper, brittle silver, and galena, averaging 

from 300 to 400 ounces, mill run.” 4

U.S. Bureau of the Mint (1883, p. 510): Description of Mineral Farm 

deposit; no production.

5

U.S. Bureau of the Mint (1888, p. 176): Silver estimated at $1.29 per fine 

ounce and lead calculated at 4.5 cents per pound for 1887.

6

U.S. Bureau of the Mint (1889, p. 117): Listed, no production recorded.

7

U.S. Bureau of the Mint (1891, p. 137): Silver reported at $1.29 per ounce, 

gold at $20 per ounce, and lead at $87 per ton for year 1890.

8

U.S. Bureau of the Mint (1892, p. 182).

9

U.S. Bureau of the Mint (1893, p. 128).

Descriptions of Mines 79

from the Miser’s Dream claim. Production claimed for the orebody developed through the inclined shaft and the one through 

the vertical shaft was $299,000 and $226,000, respectively. 

These claims as well as incomplete records for ore shipped 

since 1902 suggest the total production from the Mineral Farm 

mine perhaps could have been about $700,000.

The ore reportedly was classified as either copper ore or 

lead ore, and apparently there was little admixture of the two 

types in the ore shoots. The surface and eastern ore shoot, developed by pits, adits, and the inclined shaft, contained mostly 

high-grade copper-silver ore, although the northwestern prong 

(open stope at surface?) of this orebody was lead-zinc ore (fig. 

28). Early records do not report the copper content of the ores, 

but a rough idea was obtained from assayed samples from the 

old workings. Eight samples from the eastern ore shoot, below 

the Miser’s Dream crosscut, averaged about 2.5 percent copper, 

4 percent lead, and 109 oz of silver to the ton. The western and 

deeper ore shoot, developed by a vertical shaft, was reported 

to be lead-zinc ore. Twenty-eight assayed samples from the 

western workings averaged 1.1 percent copper, 13.3 percent 

lead, and 17.4 oz of silver to the ton. Also, the zinc content was 

higher in this ore; two of the assays indicated a ratio of zinc to 

lead somewhat more than two to one. According to these assays, 

galena and sphalerite were more abundant at depth, suggesting a possibly telescoped sulfide zoning in this part of the ore 
80 Geology and Ore Deposits of the Uncompahgre Mining District, Southwestern Colorado

Figure 29. Generalized section through the Mineral Farm mine 

ore channel, at the contact between the Molas Formation (*m) 

and the Leadville Limestone (Ml), showing the probable structural 

control of ore deposition (modified from Burbank, 1940).

channel. The gold content of all the ores was low, ranging from 

a trace to one ounce, and probably averaged about 0.06 oz to 

the ton. Lovering and others (1968, p. B112) reported that a jasperoid sample from the Mineral Farm mine contained 0.2 ppm 

gold. The highest grade ore taken from the mine came from the 

upper part of the eastern ore shoot, where it is said that large 

masses of “solid gray copper” ore were mined that carried about 

20 percent copper and 800 oz of silver to the ton.

Geology

The shape of the eastern or discovery orebodies of the 

Mineral Farm mine was long and narrow; the orebodies were 

found chiefly in the basal beds of the Molas Formation and the 

uppermost beds of the Leadville Limestone. These formations have a fairly regular dip of 15°–18° W. On this dip slope, 

east of the inclined shaft, the Molas was largely removed by 

erosion, so the orebodies in the Leadville were mined from 

open cuts. The contact between the Molas and the Leadville is 

a surface characterized by numerous irregularities, of which 

some possibly represent the original weathered surface of the 

limestone where residual clay, sand, and chert accumulated 

and where evidently there are warps or rolls in the dip of the 

chert beds and narrow elongated troughs that inlay patches of 

chert-bearing shaly beds in the underlying limestone.

The basal beds of the Molas Formation, where unaltered, 

consist of deep-red, noncalcareous, somewhat sandy shale 

containing many angular to subangular fragments of chert 

derived from weathering and erosion of the uppermost beds of 

the Leadville Limestone. These beds, although of unusual texture and composition, are unquestionably of sedimentary origin because they occur not only at the base of the unit but also 

are interbedded with conglomerates, sandstones, and shales 

above the base. Near the mine these red beds range from 5 to 

10 ft in thickness. Near the eastern orebody, the red beds have 

been bleached light gray to yellowish brown. Both these beds 

and the immediately overlying shales and sandstones have 

been highly silicified.

Very small remnants of ore consist of disseminated 

specks in the low-grade siliceous, locally jasperoid (Lovering, 1972, p. 101) casing or in the limestone near the eastern 

ore shoot. The mined ore consisted in part of disseminated ore 

and in part of solid masses of sulfide ore, the latter consisting 

largely of silver-bearing tetrahedrite. Specimens of this type 

of ore and of the massive chalcopyrite ore would suggest that 

deposition in the central parts of the ore shoot was effected, in 

part, by the filling of open cavities or honeycombed siliceous rock because the sulfides were formed upon or were 

intergrown with terminated quartz crystals. The orebodies, 

evidently a result of combined open-space filling and replacement, were from 20 to 40 ft wide and 6 to 10 ft high. Descriptions of the orebodies and statements by those familiar with 

the mine refer to the existence of “knife-edge” cracks that 

paralleled the elongation of the ore shoots and supposedly 

controlled the shape and position of the orebodies. Where the 

ore has been removed, there is little evidence of these cracks; 

also, they probably were confined chiefly to the ore shoots. 

There are, however, along the silicified borders of the channel 

casing a few prominent guiding fractures and many crisscrossing systems of joints (fig. 29).

Two sets of fractures occur in the rocks: one strikes east 

to northeast and is locally curved; the other strikes northwest and appears to be the stronger set in the mine. From the 

general trend of the ore shoots and from details seen in the 

pits and accessible mine workings, these fracture sets together 

with the bedding controlled the trends, dimensions, and shapes 

of the different orebodies. No dikes or major faults and folds 

are exposed near the mineralized zone. Basement structure 

beneath the Paleozoic rocks may have been a factor in the 

localization of the ore channels.

Silicification advanced through the rock in an irregular 

manner and was clearly influenced by the fractures, but it also 

transgressed fractures without regard to their presence. Silicified masses of altered chert-bearing shale were offset in places 

along the fractures. These relations are interpreted to suggest 

that the fractures were intermittently active or, being formed 

during early stages of alteration, preceded the introduction 

of the massive sulfide ores. Minor fractures do not in many 

places extend into the underlying limestone and cannot be 

traced far either horizontally or vertically. They are thought to 

be of local origin, and hence only of secondary importance in 

the control of mineralization.

Primary structures localizing the ore shoots, as stated previously, are thought to have been the major fracture sets that 

bounded the walls of the silicified casing in some parts of the 

mine. These are simple, steeply inclined breaks along which 

there was minor faulting or warping of the beds. Changes in 

dip or local oversteepening of the beds may also have been a 

factor of some importance.

Structural effects noted along the fractures of both the 

northeast- and northwest-striking sets include (1) local thickening of Molas chert beds along one wall of a fracture, which 

partly compensated thinning in the other wall, (2) sudden 

changes in the apparent displacement of the contact between 

the chert beds and the underlying limestone along the strike 

of a fracture, (3) linear trough-like depressions along certain 
Figure 30. Plan of the 

Mineral Farm mine’s presumed 

original ore channel showing 

two en echelon ore shoots 

(cross-hatched). Position of 

developed ground is shown 

relative to the subsurface 

contact of the Proterozoic 

quartzite and slate units and 

to dikes at the Contact tunnel 

(Legal Tender mine (fig. 21, 

no. 36)). Closed dashed lines 

within ore channel represent 

mined-out surface orebodies 

modified from Burbank (1940). 

Ml, Leadville Limestone; *m, 

Molas Formation; *h, Hermosa 

Formation; Ql, glacial and 

landslide debris.

Descriptions of Mines 81

fractures caused by the caving of Molas chert beds into the 

underlying limestone, (4) local sharp rolls, or distortions in the 

dip, of the chert beds involved with these different structures, 

and (5) local destruction of bedding in the shale. The best 

explanation for these features as well as for the structural characteristics of the Mineral Farm ore shoots was the dissolution 

of the Leadville Limestone at the base of the Molas Formation and the concurrent slumping of the shale into the void 

thus formed. The produced effects varied because a solution 

would work outward principally along one wall of a fracture 

at one place and from the opposite wall at another place, 

thereby creating changes in the apparent displacement along 

the strikes of the fractures. If a large amount of limestone was 

removed from beneath the shale, the latter would gradually 

be forced down into the opening by the weight of overlying 

rock, thus producing sharp rolls of the shale beds along the 

edges of the trough formed. Beds along borders of the trough 

consequently would have been thinned, especially if the 

shale was sufficiently plastic to flow laterally into the cave. 

This explanation satisfactorily accounts for the distortion or 

apparent noticeable destruction of bedding in the chert-bearing 

shale near the ore shoots or along certain barren or silicified 

fracture zones. Minor fractures of short vertical and horizontal 

extent, noticeable in the altered beds near the ore shoots, are 

thought to have resulted chiefly, if not entirely, from this local 

slumping and not from regional strain. Sharp, steep rolls in the 

“contact” near the orebodies resulted, in part, from an unconformity formed by solution and slumping. Large-amplitude 

folds and major changes in dip, however, probably occur and 

these, rather than the simple fractures, may have localized the 

cave formation in some places. Where the changes in dip of 

the limestone are small, detection is difficult because there are 

no easily recognizable horizons in the upper beds of the Leadville. It seems that such changes in dip might have resulted in 

a certain amount of slip along the contact with strata having 

different physical characteristics from those involved. If the 

strain had a strike-slip component, local fractures would have 

formed in one or both formations. In many instances the origin 

of the primary structures producing the solution channels in 

the contact zone is very obscure. As evident from the location 

of the ore, the shales of the Molas Formation rather than limestone in the Leadville Limestone provided the most permeable 

channel for the introduction and migration of the ore-forming 

fluids. Limestone immediately beneath the ore was, in general, 

only weakly mineralized, although it locally was recrystallized or was replaced by ankerite and sulfides. These relations 

suggest that the deformation producing the permeability of the 

channels was localized in the relatively insoluble shale rather 

than in the limestone, and support the concept that the channels were formed by caving.

As shown in figure 30, the northeast-trending ore channel 

consists of two en echelon ore shoots. The present extent of 

the workings indicates two possible trends to the ore channel. 

The northeast trend of the individual shoots has generally been 

assumed to be that of the ore-bearing zone, but the western 

trend, suggested by alignment of the southern boundaries of 

the two en echelon shoots, perhaps may be closer to the true 

trend of the channel. Figures 19A and 21, and the geologic 

map of the mining district (Luedke and Burbank, 1981), show 

that the mine lies only a few hundred feet above the base of 

the Paleozoic rocks; the trend of the mine workings appears 

to parallel the faulted west-northwest-trending northern limb 

of the compressed syncline of slate in the underlying Proterozoic Uncompahgre Formation. As previously pointed 

out, displacements along this trend were transmitted upward 

from the underlying slate layer either by bedding-plane faults 

and distortion of the slate beds, or by differential and lateral 

strike-slip movements within both the quartzite and slate along 
the contacts of these units. These displacements from renewed 

movement in the underlying Proterozoic rocks account satisfactorily for the deformation in the overlying limestones and 

shales and for the apparent west-northwest trend of the deeper 

parts of the ore channel in the Mineral Farm property.

Mineralogy and Origin of the Orebodies

Specimens from the two ore shoots are mineralogically 

similar, but it has been reported that the copper-silver ore 

from the eastern shoot was very rich in gray copper and silver, 

whereas the lead-zinc ore from the deeper western shoot 

contained only moderate amounts of chalcopyrite and silver. 

In addition to the two main ore shoots, small amounts of 

low-grade pyritic or baritic lead-zinc ore were in the limestone along the north side of the eastern shoot and near the 

northeastern end of the open pit. The wall rocks of the surface 

workings were extensively silicified.

The order of events and deposition of the minerals forming 

the altered rock and the orebodies was as follows: (1) solution 

of the Leadville Limestone beneath the Molas beds and local 

slumping of the chert-bearing shales, (2) silicification of the 

shale beds adjoining the channels and additional slumping and 

fracturing, (3) sericitization, ankeritization, and pyritization of 

both the silicified beds and the surrounding less-altered limestone and shale, (4) deposition of barite accompanied by quartz, 

pyrite, sphalerite, chalcopyrite, and galena, (5) deposition or 

recrystallization of quartz and deposition of chalcopyrite, galena, 

tetrahedrite, pearceite or polybasite, and calcite, and (6) additional solution of the soluble limestone beds along fractures and 

surrounding the highly altered bodies, and the formation of small 

caves lined with crystals of quartz and scalenohedral calcite.

The early solution of the rocks was followed by replacement with silica, from the altering fluids, that was substituted 

for the alumina, iron, potash, and other non-siliceous constituents of shales; the limestones were only weakly attacked 

by these fluids. Silica under-saturation of the early pre-ore 

fluids, which gradually became saturated with silica, suggests 

that at greater depths siliceous rocks may have been dissolved 

or replaced by other minerals. This supposition is based on 

similarity to observations made in the mineralized rocks of 

the Gold Hill area, north of The Blowout (fig. 21), where the 

quartzite was attacked and cavities were formed.

The earliest sulfide solutions then penetrated and replaced 

rocks near the silicified central channel, and also appear to have 

bypassed the main channel locally to form low-grade sericitized and sulfide replacement zones along the north side of the 

channel. As the unsilicified rocks surrounding the channel were 

replaced by sericite, pyrite, and ankerite, open spaces were 

maintained within the channel to be filled later by higher grade 

silver-bearing chalcopyrite-tetrahedrite ore. The silicified walls 

of the channel were attacked, but to a lesser extent because they 

had been replaced locally by some pyrite, sericite, and local 

masses of barite and sulfides. Possibly, the open channels were 

initially too choked by silicified cave breccia to accommodate 

the volume of the early-stage mineralizing fluids that traversed 

the channel for a long distance at depth; these early fluids probably deposited much of the minerals at a higher temperature 

before reaching the shallower positions of the channel.

Small amounts of ankerite containing a relatively high 

magnesia content (table 13) replaced unsilicified limestone 

north of the Mineral Farm ore channel. The high magnesia 

content possibly was derived from dolomitic limestone beds at 

depth in the channel.

The barite and main sulfide deposition followed that of 

the early gangue, but with some overlap because there was no 

sharp break in the mineral sequence. Examination of specimens of the extremely high-grade, massive tetrahedrite ore 

that filled voids in the silicified channel suggest that the later 

stages of sulfide deposition represent chiefly open-space filling. Both the later chalcopyrite and tetrahedrite are intergrown 

with terminated quartz crystals near the edge of the sulfide 

masses. Thus the latest ore filled large openings in the channel 

and the honeycombed masses of silicified breccia.

The rocks were fractured subsequent to ore deposition by 

northwest-striking fractures that allowed solutions capable of 

dissolving the limestone to flow through these fractures and 

around previously mineralized ground. An attempt to outline 

the orebody in the deeper workings encountered several caves 

in the less altered and weakly mineralized limestone beds that 

appeared to nearly surround the orebody. The caves had formed 

both parallel to the bedding and along vertical fractures. In some 

fractures and caves, particularly south of the orebody, crystals of 

both quartz and calcite were reported. The quartz may have been 

formed much earlier than the calcite; cavities in the limestone 

containing quartz also commonly contained traces of sulfides. It 

is possible, although not proved, that the earlier quartz-bearing 

caves formed soon after ore deposition and were the result of the 

dying stages of mineralization. Some scalenohedral calcite probably was deposited at this time. Some scattered masses of sphalerite or galena, previously deposited in the unsilicified parts of 

the limestone, were freed of their surrounding limy gangue and 

were found as detached blocks or boulders in the clayey material 

on the cave floors adjacent to the deeper orebody.

Oxidation of the ores by supergene solutions appeared 

to have been of minor importance other than possibly in the 

orebodies that were exposed on the bench east of the open cut. 

Chalcocite, covellite, malachite, azurite, and oxides of iron 

and manganese were the principal supergene minerals noted. 

Other than staining of the clay deposits in the caves by iron 

oxide, this suite of minerals has not been specifically referred 

to as characteristic of the cave deposits.

A ground-water origin for the Mineral Farm pre-ore caves 

is not considered a possible interpretation to extend the ore channel in depth, becase the beds dip 15°–18° or more west beneath 

the level of Canyon Creek. It is doubtful that caves formed by 

meteoric waters would extend to the depths thought to exist 

under a few thousand feet of overlying strata, particularly if 

those caves and associated ore deposits were formed during the 

Laramide orogeny as interpreted. Therefore, it is unlikely that a 

gently dipping ore channel would continue to great depth.

82 Geology and Ore Deposits of the Uncompahgre Mining District, Southwestern Colorado
Source of Mineralizing Solutions and Zoning 

Along the Ore Channel in Depth

In the N. 80° W. direction assumed for the extension of 

the ore channel, the most likely source of the ore-forming 

fluids would appear to be the concealed southwest extension 

of the intrusive zone (fig. 18). The south edge of the zone 

should intersect the ore channel downdip of the beds about 

2,500 to 3,000 ft west of Canyon Creek, a little more than a 

mile from the outcrop of the ore channel. The upper part of the 

Leadville Limestone there is estimated to be about 1,500 to 

2,000 ft lower than the outcrop near the mine. If the ores of the 

Mineral Farm channel were derived from the same sources of 

mineralization as those in the other parts of the Uncompahgre 

mining district, which is likely, then the zoning of the ores in 

the other parts of the district may be applied to the Mineral 

Farm channel area. Such ore channels are characterized by 

the concentration of all stages of mineralization in a comparatively narrow body, as a result of overlapping or telescoping 

of the ores along the channel. Thus exploration might find a 

more complete mineralogical sequence in a channel deposit 

than would be expected in a vein. The Mineral Farm ore 

contains many of the mineral associations found in the central 

part of the district, but the proportions of the different sulfides 

correspond more closely to the lead-silver-zinc deposits of the 

northern sector or outer zones of the district. The development 

of sericite and the amount of chalcopyrite found in parts of the 

Mineral Farm orebody are possibly more typical of the pyritic 

ores of the Gold Hill type, and may be indicative of a change 

in depth to ores of the pyritic type that contain chalcopyrite 

and perhaps gold. However, chalcopyrite is found commonly 

in ores of both the deep and shallow zones of mineralization.

Pyritic or cupriferous gold-bearing ores may occur at 

somewhat greater depth along the Mineral Farm ore channel based on local conditions and comparison with other area 

deposits. Whether they would be economic or not can be 

Descriptions of Mines 83

determined only by exploration. In general, the Mineral Farm 

deposit at its present depth of exploration has proven to be too 

small to support mining of disseminated low-grade ore. The 

size and form of an ore channel at depth is important. There are 

several explanations as to why the form or cross section of an 

ore channel formed by rising hot solutions might change with 

depth. Walker (1928) cited two reasons as to why such channels 

might enlarge in depth: (1) the increase in corrosive power of 

the pre-ore cave-forming solutions toward their source, and (2) 

a tendency for increased deposition of silica toward the egress, 

which would have insulated the upper parts of the channel from 

further enlargement by later solutions. An increase in the corrosive power of solutions nearer their source, a concept supported 

by the conditions in the American Nettie mine, shows where 

quartzite was dissolved prior to the deposition of the ore. The 

effects of insulation of the channel by silicification are illustrated 

in the Mineral Farm mine, where much of the ore was low grade 

because of the high silica content. Apparently silicification in the 

channel was restricted to the later ore-bearing solutions, resulting in deposition chiefly in open spaces rather than by replacement. The central channel also might have changed in form at 

depth by the choking or clogging of the caves as suggested by 

the Mineral Farm channel, which was apparently choked with 

cave breccia, so that the earlier-stage sulfide fluids bypassed the 

central silicified channel to deposit sulfides by replacement in 

the surrounding unsilicified limestone or shale margin.

American Nettie Group of Mines

Location, History, and Development

The American Nettie group, including the O & N, Schofield, and Jonathan mines together with a few other adjoining 

claims, is located in the so-called Ouray “gold belt” (Endlich, 

1889; Lakes, 1901), and is conveniently discussed as a unit; 

Figure 31. Sketch of view 

looking northwest showing the 

American Nettie mine buildings 

located high on the east wall 

of the Uncompahgre River 

canyon, north of Ouray (from 

Lakes, 1901).
84 Geology and Ore Deposits of the Uncompahgre Mining District, Southwestern Colorado

Figure 32. Generalized cross section of the cliff face at the 

American Nettie mine showing stratigraphic relations and the 

ore shoots (dark), or filled solution cavities, formed within the 

quartzite mostly beneath impervious barriers or partings of shale 

as suggested by the flat tops (from Irving, 1905, fig. 4).

mine development tended gradually toward the consolidation 

of claims. Openings to these mines (Luedke and Burbank, 

1981) are mostly within the Dakota Sandstone or uppermost 

part of the Morrison Formation at altitudes ranging from 

9,400 to 9,500 ft on the east wall of the Uncompahgre River 

valley about 1.5 mi north of Ouray (figs. 21, 31; pls. 2, 3). 

Also included here and briefly discussed is the Wanakah 

mine group, located about 700 ft stratigraphically below and 

developed in the Wanakah Formation and the lower part of the 

Morrison Formation (fig. 21; pls. 2, 3).

Discovery of rich gold ore in quartzite on the American 

and Nettie claims, made in 1889, was followed by the rapid 

development of these properties. By 1900 or 1901 the great 

bulk of the ore, parts of which were phenomenally rich, had 

been mined out. The lower part of the Dakota Sandstone, in 

which most of the ore was found, crops out as a bluff 50 ft or 

more in height, bounded below by the talus-covered slopes of 

the Morrison Formation and above by the receding but steep, 

step-like slopes of alternating sandstone and shaly sandstone of 

the Dakota Sandstone and shale of the overlying Mancos Shale. 

Above this sedimentary interval (fig. 32) the canyon walls are 

composed of a porphyritic granodiorite sheet that forms cliffs 

and largely inaccessible slopes to an altitude of more than 

10,000 ft. Discovery of the rich ores at the American Nettie 

mine is said to have been made near one of the many crosscutTable 15. Production data from the American Nettie mine for the 

years 1897 and 1898.

[ , zero] -Year 18971 18982

Output, in tons 522 754

Average net value, in dollars, per ton 152 133

Total net value, in dollars 79,344 100,282

Profit, in dollars, per ton 79 75

Receipts, in dollars:

Ore and mining 79,563 100,090

Smelter 564 811

Interest 559 626

 Total receipt per ton 154 135

 Total 80,686 101,527

Expenses, in dollars:

Ore and mining 37,527 42,275

Office, taxes, etc. 1,788 1,780

Purchase claims -- 986

 Total expenses per ton 75 60

 Total 39,315 45,041

1

American Gold Mining Co. annual report, October 1, 1897.

2

American Gold Mining Co. annual report, September 15, 1898.

ting fractures and dikes prominently exposed along the quartzite 

cliff. Upon breaking into the quartzite face, prospectors discovered open cavities within the rock, which were partly filled with 

iron oxides containing native gold and oxidized silver ore. Such 

ore, in an easily removable form and already concentrated by 

natural processes, financed development of the properties.

In May 1889, the American Nettie mine, then consisting 

of three claims (American, Nettie, and Schofield), was sold to 

the newly organized American Gold Mining Co. Production 

of the early mined high-grade ore, reported in July 1889 by 

the company, amounted to seven cars, each holding 20,000 

pounds (lb), that was shipped to Pueblo and netted $23,314.40. 

By the end of the fiscal year, the company is reported to have 

paid $150,000 in dividends. Endlich (1889, p. 335), who 

briefly examined the mine in the fall of 1889, reported that “...

nearly 200 tons of ore have been shipped by this time, which 

ran from 16 to 40 ounces in gold by the car, and carried some 

silver besides.” Production continued during 1890 and 1891, 

although dividend payments were less than those of the first 

year. A mill was not constructed prior to 1891.

In 1892 the company contracted with the Edison Electric 

Co. to construct a hydroelectric plant to furnish power and 

lighting for the mine; this plant was in operation late that year. 

Nearly 100 men were employed by the company in 1894, but 

the workforce was reduced to 65 in 1895, and was reduced 

further in October of that year. Only a few men worked on 

development and mining of ore during the following winter 

and the first part of 1896. New orebodies apparently were 

developed and the workforce increased in the fall of 1896. 

The newly found ore was in caves in the uppermost part of the 

quartzite and consisted of stiff yellow mud that had to be han­
dled several times in the process of drying before shipment. 

Judging from its similarity to the ore-bearing muds found in 

the O & N property adjoining on the north, it probably came 

from near the northern boundary of the main ore shoot. Dividend payments resumed in 1897. A summary statement issued 

for the period May 16, 1889, to October 1, 1897, reported the 

company had mined 6,687 tons of ore having an average value 

of about $140 per ton. During this period, the total receipts 

were $934,998 and dividends were $279,000. Annual statements issued by the American Gold Mining Co. for 1897 and 

1898 are listed in table 15.

In October 1898, the company stated it was working a 

large body of sulfide ore having a value of $1,000 to $3,000 

per car of 10 tons capacity; 6 to 8 carloads per month were 

being shipped to the Pueblo smelter. Smelter charges were 

about $7 per ton and freight about $8 per ton. This high-grade 

sulfide ore containing tellurides presumably came from the 

northeastern part of the ore shoot in the area commonly known 

as the “Bull Pen.” About 35 men were employed at that time.

Production continued steadily in the first part of 1899. 

About 11 carloads were shipped monthly, but in December 

the property was temporarily leased to several local parties at 

a 50 percent royalty. The last dividend payment reported was 

in March 1899, and amounted to $0.07 per share on 300,000 

shares, bringing the grand total of dividend payments to about 

$446,000.

The period of operation just reported clearly indicates 

considerable depletion of the high-grade orebodies. In January 

1900, the company announced it would erect a concentrating plant to treat the dump and large bodies of not-yet-broken 

second-class ore. This new mill, operated by hydroelectric 

power (Brady, 1901), had a capacity of 60 tons per day and 

was in operation by September of that year. The automatic 

tramway from the mine to the mill across the Uncompahgre 

River valley descended 1,820 ft within a horizontal distance of 

about 3,400 ft; the single span across the valley from the last 

tram tower on the east side to the mill on the west side was 

2,100 ft in length. During 1901, 25 to 50 men were employed; 

at full operation, 50 tons of ore daily were carried to the mill. 

The ore, as it came from the mine, was sorted in a room near 

the mine entrance and the highest grade material sacked for 

direct shipment; the remainder was sent to the mill, which had 

Wilfley tables and 20 stamps, as well as plates to catch the 

native gold. Operations continued from 1901 through 1903, 

although they were temporarily interrupted by reconstruction 

of the tramway and by milling difficulties. In July 1904 the 

stockholders voted to issue and sell treasury stock in order 

to finance improvement of the milling facilities, but production subsequent to 1906, other than the year 1928 when 4,000 

tons of ore was milled by the Mutual Gold Mines Co., was 

on a small scale or sometimes nil. The value of $470,000 

for recovered metals from 1905 to 1937, added to the figure 

of $1,464,923.35 (Cross and others, 1907, p. 18) that was 

the total production value from 1889 to 1905, gives a total 

production from the mine to the end of 1937 of approximately 

$1,935,000.

The American Nettie mine workings were developed by 

gentle inclines, drifts, and crosscuts, and total more than 5 mi 

along the bedding of the quartzite. The main drifts were driven 

along the larger fractures and dikes; the method of exploring 

was to drive laterals along fractures and veins that intersected 

the main drifts. The mine map (pl. 2) gives a comprehensive 

picture of the fracture pattern in the quartzite. The minor fractures and veins locally enlarged into small pockets, or led to 

large caves that could be entered by the miners. Oxidized ore 

that filled the lower parts of the caves and encrusted parts of 

the walls was removed with long iron scrapers; the walls were 

brushed and any hard crusts of ore still adhering to the walls 

were then barred (pried) off. The ore was immediately sacked 

for shipment. As depth was gained into the hill, the mining 

methods were changed to allow the removal of the more massive sulfide ores. Where ore entirely filled some voids in the 

quartzite, drifts were accordingly enlarged into rooms for mining the sulfide ore. Because the ore pockets were not confined 

to a single horizon within the quartzite, inclines were driven 

beneath parts of the main ore zone, raises and winzes were 

extended, and exploration drifts were driven along the larger 

productive fractures from different levels. The aggregate drifting was large in view of the comparatively small area of the 

mine. The resulting labyrinth of workings was in places confusing, even to one familiar with the mine. Downer (1901, p. 

106) stated, “A map of the mine has the appearance of a maze 

or puzzle of some kind, so honey-combed is the quartzite by 

old drifts ... the workings aggregate nearly six miles, and 

are practically on one level... .” The main or Nettie incline 

penetrates about 1,600 ft eastward into the hill, and considerably beyond the limits of the main ore shoot. Development 

toward the northeast first centered about Incline No. 39, which 

branches from the Nettie incline about 475 ft from the portal. 

During later development at the time that the concentrating 

plant was built in 1900, the compressor plant was placed in an 

offset of the Nettie incline about 1,000 ft from the portal. The 

mine was lighted by incandescent lamps after the construction 

of the first hydroelectric plant, and it was recorded in a news 

item of June 15, 1895, that the company had started the use of 

electric drills, a venture which met with little or no success.

The O & N Tunnel Co., which owned several claims 

north of the American Nettie mine, was organized soon after 

the American Gold Mining Co., and in 1893 announced that 

a proposed tunnel of about 2,000 ft in length was projected to 

cut the extension of the ore shoot already partly developed on 

adjoining Nettie property. Development of the O & N mine 

does not seem to have gotten well under way until 1895, but in 

1896 the workings encountered the extension of the American 

Nettie ore channel (pls. 2, 3) where the O & N Tunnel Co. discovered very rich pockets of ore in the upper part of the massive quartzite beds. The ore was partly oxidized and consisted 

of a yellow mud, some of which was so thin as to necessitate 

removal in buckets. Some stoping was also done on mineralized fractures in the quartzite. During 1897 and 1898, the 

property had a remarkable record of high-grade production, 

but by December 1898 news reports stated that the mine had 

Descriptions of Mines 85
been worked out and all effects had been sold off. No new ore 

discoveries had been made in the previous several months. 

Prior to its final closing, an incline was driven eastward to a 

point about 1,800 ft east of the portal into ground stated to 

have been the primary objective of the tunnel. This exploration did not locate new ore shoots, and as developments on the 

American Nettie had already more or less defined the eastern 

limits of profitable ore, there was little incentive to continue 

the exploration.

The portal of the O & N main tunnel (crosscut) is in talus 

near or just below the base of the Dakota Sandstone, about 550 

ft northwest of the Nettie incline portal (pl. 2). The principal 

high-grade orebodies were developed from several raises into 

the upper part of the massive quartzite beds. This tunnel and 

lower workings were inaccessible during this investigation.

About 1912 a lower crosscut, the Seaburg tunnel (pl. 

2), was driven beneath the American Nettie mine to explore 

the quartzitic rocks about 700 ft stratigraphically below the 

orebodies in the Dakota Sandstone. Although small bodies of 

low-grade pyritic gold ore were encountered, this enterprise 

failed to develop commercial orebodies.

Development of the Jonathan mine was not as extensive 

as that of the American Nettie and O & N mines. Nevertheless 

the Jonathan mine produced high-grade ore both from pockets in the quartzite and along numerous small east-trending 

veins. Owing to the trend of the main ore zone more or less 

parallel to a dike and diagonal to the outcrop of the quartzite, 

the orebodies did not extend as far into the hill. The Jonathan 

mine was operated intermittently into the 1930s.

From 1928 to 1930, the Mutual Gold Mines, Inc., which 

was milling dump material and ores from the American Nettie, 

Iron Clad, and Bright Diamond mines, instituted an exploration program in Jonathan property in an attempt to find orebodies east of the Jonathan dike. While some ore was developed and mined along or near the Schofield vein and along 

lower contacts in the Dakota Sandstone and upper Morrison 

Formation, exploration farther to the east encountered only 

weakly mineralized rock.

The finding of the small, flat orebodies and vertical 

replacements in and near the Jonathan dike in the lower horizons of the Dakota Sandstone and the uppermost sandstone 

beds of the Morrison Formation, led to exploration in the 

1930s of the eastern end of the adjacent Puzzle claim located 

near the richest parts of the original American Nettie ore shoot. 

A shaft was sunk along the west wall of the Jonathan dike, and 

exploratory tunnels were driven across and parallel to the dike 

near the base of the Dakota and in the uppermost Morrison 

sandstones, but this effort failed to find ore. A few years later, 

a small, flat orebody overlooked during the original development of the mine was discovered by W. McCullough and 

Withrow in the back of the old stopes a short distance to the 

south of the above-mentioned shaft. This small body contained 

some very high-grade ore that had been concealed by a “false 

back” in the original stope. After this discovery, there was 

little further development on the properties to 1938. Reduced 

operations until the mid-1940s were restricted to the shipment 

of small tonnages of gold-silver-copper ore and treatment of 

old mill tailings. Since then, there was minimal activity by lessees in the late 1950s and early 1960s.

Geology

The blanket-like orebodies in the American Nettie and 

adjoining mines were confined mostly to the upper 20 to 30 

ft of massive quartzite beds in the lower part of the Dakota 

Sandstone. The miners, in general, restricted the term “upper 

quartzite” to this lower part that contains only a few thin 

shale partings; the entire formation includes, in addition, 50 

ft or more of overlying interbedded carbonaceous shale and 

quartzite. The base of this overlying upper shale essentially 

forms the roof of the ore-bearing zone. There are two principal 

horizons at which the orebodies were most commonly found: 

one is just beneath the capping beds of carbonaceous shale, 

and the other is at or just beneath thin shale partings 20 to 30 ft 

lower in the massive quartzite. The relative distribution of the 

ore within the entire zone is inadequately known for the mine 

as a whole. Judging from the mined-out stopes examined, the 

upper contact contained the larger orebodies, particularly in 

parts of the eastern or downdip sections of the mine. In the 

updip sections and near the outcrop, the ore was perhaps more 

common in occurrence and thicker near the parting shales. The 

rocks generally dip 5°–12° N. to NE.

In these deposits, the term “ore shoot” is used to refer to 

an aggregate of orebodies that were individually small compared to the extent of mineralized ground. The principal ore 

shoot in the American Nettie and O & N mines (pl. 3A) was 

scarcely more than 300,000 ft2 in plan. In outcrop, the mineralized channel that contained the ore shoot was 400 to 500 ft 

wide and trended N. 70° E. into the hill; the trend of the beds 

downdip in the ore shoot turned to N. 40° E. and terminated 

near the wall of the Jonathan dike, a distance of about 1,100 

to 1,200 ft from the outcrop. Its form or shape is essentially 

that of a swelling, bulbous outline. Taking into consideration 

ground including the scattered small ore pockets and veins in 

the easternmost workings of the American Nettie mine and 

adjoining properties, the area of mineralization extended more 

than 1,200 ft along the outcrop of the quartzite and covered 

several times the area of the principal shoot of the American 

Nettie and O & N mines. The aggregate of small orebodies 

may be considered as separate shoots, but ore in the eastern 

workings of the American Nettie mine is perhaps an appendage or enlargement of the main shoot.

Structure

The quartzite in the American Nettie mine is cut by many 

joints, fractures, and minor faults, as well as several dikes. 

These, in conjunction with the parting and capping shale 

beds, controlled the localization of the orebodies. Strike and 

dip variations in the quartzite beds indicate the existence of 

terrace-like warpings and other irregularities in the regional 

86 Geology and Ore Deposits of the Uncompahgre Mining District, Southwestern Colorado
attitude of the strata north of Ouray (pl. 1, sections C–C′ and 

F–F′). These structures were significant in forming the fracture and joint sets, and were partly responsible for the location 

and trend of the ore shoots.

The Jonathan dike is exposed in all the mines (pl. 2) over 

a distance of more than 3,000 ft and strikes N. 25°–35° E. 

Except where altered, it is a dark-colored, massive porphyritic 

dike that differs little in composition or texture from the more 

common east-trending granodioritic dikes of the district. It is 

25 to 35 ft thick and has an average dip of 80° E.; however, 

along strike in the distance between Nettie dikes Nos. 1 and 

2, it is locally vertical or dips steeply west. Locally the dike 

mushroomed or bulged at the contact of the quartzite and 

overlying carbonaceous shales and sent out small sill-like apophyses. Surface exposure on the valley wall above the mines 

indicates that the dike cuts through the overlying Mancos 

Shale, the 500-ft-thick granodiorite sheet, and 15 to 20 ft more 

of overlying Mancos Shale (figs. 12, 21). The Jonathan dike is 

one of the few dikes within the district that penetrated all the 

pre-Tertiary formations. Like all of the older igneous rocks, 

it was cut by the unconformity at the base of the Tertiary San 

Juan Formation; at that level it is 5 to 10 ft thick. Within the 

mines, one or both walls of the dike commonly have been 

fractured and mineralized, but the ore generally is of lower 

grade than that found in parallel breaks or sheeted ground a 

short distance from the walls. The grade of the replacement 

orebodies in the quartzite immediately adjacent to the dike 

was reported to have decreased within the Jonathan mine. The 

richest ore occurred in pockets or flat bodies east and west of 

the dike but not in direct contact with it. This also was probably true in the northern part of the American Nettie–O & N 

ore shoot near its termination, because the largest and highestgrade sulfide orebodies at its downdip termination lay 15 to 

20 ft west of the dike at the intersection of parallel and cross 

fractures; these orebodies apparently did not make immediate 

contact with the dike. It is thought that this may have resulted 

from local pre-ore reduction in permeability of the wall rocks 

immediately in contact with the large dikes. More significantly, however, the greater part of the high-grade ore came 

either from the quartzite west of the Jonathan dike or from 

near the contact with its walls. The dike appears to have acted 

as a barrier to lateral east-west migration of the ore-forming 

fluids. Fractures adjacent to the dike walls may have provided some of the principal feeding channels from depth and 

between horizons as well as laterally.

Another porphyry dike, the Nettie dike No. 1 (see pl. 3A), 

crops out on the face of the quartzite cliff not far from the 

main portal of the American Nettie mine, and within the southern part of the main ore shoot. The dike strikes S. 50°–60° E. 

diagonally across the trend of the ore shoot and has an average 

dip of 85° S. Its thickness probably does not exceed 8 ft. It is 

cut but not displaced by the Jonathan dike, and continues to 

the east where it was intersected by the Schofield tunnel of the 

Jonathan mine about 900 ft from the portal. A possible southeast continuation is an outcrop of a dike striking S. 30°–40° E. 

(pl. 2). If the latter is the Nettie dike No. 1, it has changed 

strike, perhaps resulting from “jumping” of the dike from one 

fracture to another. There are fractures in the walls of the Nettie dike No. 1 that have been stoped intermittently along the 

dike to some distance east of the Jonathan dike. Mineralization 

along the dike walls was comparatively weak and noncommercial within shale beds of the Morrison Formation where the 

dike was cut by the Jonathan tunnel. Where cut 800 ft below 

in the workings of the Seaburg tunnel, the Nettie dike No. 1 

has some ore along its walls, but it is pyritic and only spottily 

enriched by gold.

Along the Nettie incline, the Nettie dike No. 2 (pls. 2, 3) 

is a clastic dike that trends almost due east and has an average 

dip of about 60° S. Like the other clastic dikes, it is of variable 

width and is composed of rock fragments that were forcibly 

injected into the fracture. Some of the dike matter is doubtless 

of local origin; in places, the bulk of it definitely was locally 

derived. The dike maintains its clastic characteristics to a 

depth of 800 ft or more beneath the American Nettie workings 

to exposures in the workings of the Seaburg and West View 

tunnels. In the Seaburg tunnel, the dike consists in part of 

coarse quartz and orthoclase containing much apatite, rutile, 

and sericite; there, it closely resembles a pegmatite. This dike, 

like the igneous Nettie dike No. 1, also cuts diagonally across 

the main ore shoot, but the distribution of the workings relative to the main Nettie incline suggests that the dike was only 

locally effective as a baffle or guide for the lateral migration of 

the ore-forming fluids. The quartzite parallel to the dike walls 

is strongly fractured; this fracturing clearly controlled the 

shapes of some stoped orebodies not far from the dike walls. 

Near the portal of the main incline, stopes in both walls of the 

dike are less extensive than those between 500 and 800 ft from 

the portal where the main shoot appears to cross the dike with 

but little deflection. The dike here was not an effective barrier 

to the lateral migration of fluids within the quartzite, probably 

because of its relative softness, its variation in thickness, and 

the more effective control by the open fractures in the quartzite along the principal trend of the ore channel. According to 

Cross and others (1907, p. 18), “It is the impression of the 

miners that the ore emanates from this fault breccia and makes 

out from it into the country rock...,” but they concluded there 

was no connection between the dike and the ore. Displacement 

along the fracture was only a few feet, and at the portal, the 

downthrow was toward the south.

Far to the northeast in the American Nettie mine workings, the Hultona dike and an unnamed granodiorite dike were 

cut by the Chipeta tunnel (pl. 2). The walls of these dikes were 

weakly pyritized and mineralized, but the dikes had no apparent bearing on the ore deposition in the American Nettie mine.

Additional structures are the steeply inclined fractures 

in the quartzite. The larger or master fractures such as the 

Jonathan vein extend through the strata for a considerable 

distance vertically; in some places, these fractures where 

mineralized were stoped below the Dakota Sandstone for a 

short distance, but deeper into the underlying shaly rocks were 

tight and unproductive. Nearly all smaller fractures and joints 

in the massive quartzite were confined to more competent or 

Descriptions of Mines 87
brittle rocks and did not extend into the shales either below or 

above the Dakota, and many did not extend through thin shale 

partings within the Dakota. Drifts in the quartzite, presumably driven along small guiding “knife-edge” cracks, were 

the customary method of prospecting. These cracks were not 

seen where the back of the old drift is a parting shale, yet at 

the drift face, were readily visible as an open fracture or even 

a large cavity. Because of the concentration of ore-bearing 

ground beneath shale partings or beneath the capping shales, 

many of the old drifts were thus unsatisfactory for observations of structural detail which could have been obtained 

during operation of the property. Except for the larger or 

master fractures and faults, the structural details shown on 

plates 2 and 3 were based on the drift patterns in many parts of 

the American Nettie mine complex. This is justified because 

many early references cite the guidance of small fractures in 

exploratory drifting. As may be seen on plate 3A, the master 

fractures gradually change from an approximate eastward 

trend in the southern part to a N. 40° W. trend in the northeastern part of the workings. Some fractures of the eastward trend 

are found throughout the range of workings as these belong to 

the district pattern as a whole rather than to local patterns.

To determine the possible relations of the fracture patterns to the structure of the sedimentary formations, approximate structure contour maps (pl. 3B) were prepared both on 

the top of the massive quartzite bed in the Dakota Sandstone 

and on the top of the basal beds at or near the so-called “Iron 

Clad contact” of miners (beds of the Pony Express Limestone 

Member of the Wanakah Formation). Altitudes were determined in Incline No. 39 of the American Nettie mine to the top 

of the winze connecting to the Chipeta tunnel, and in some of 

the lower workings connecting to the Seaburg tunnel and parts 

of the Wanakah mine workings. Base altitudes were taken 

from mine maps by Richard Whinnerah of Ouray, who kindly 

supplied us with the mine data at his disposal. Most of the 

underground altitudes were determined using an alidade, plane 

table and stadia rod, and are only of approximate accuracy. At 

best, the altitude data are too scattered to permit the construction of accurate contour maps, but the general relations are 

apparent, and were controlled on the outcrops by the topographic base map on which the geology was mapped (Luedke 

and Burbank, 1981). A series of terrace-like warps were superimposed upon the general northward tilt of the formations 

in this part of the district. These warps are diagonal to the 

regional strike of the rocks, and appear to trend northeast more 

or less parallel to the general trend of the American Nettie ore 

shoot and to the trend of the deeper ore shoots in the Bright 

Diamond (Tidwell Member of the Morrison Formation) and 

Iron Clad (Pony Express Limestone Member of the Wanakah 

Formation) “contacts” 800 ft stratigraphically below. Flexing 

of this type was produced perhaps by both rotation and extension that could account for fracturing of the more competent 

beds. Relations between fracture trends and the structure of 

the beds are not readily apparent. Possibly the cross flexures 

resulted from local uplift and subsidence roughly parallel to 

the margins of the northeast-trending intrusive zone that lies 

a few thousand feet south of the mineralized area (fig. 21; pl. 

1, section F–F′). Many small faults, downthrown to the south, 

further indicate that a slight subsidence followed the initial 

upward surge of the intrusive bodies. The manner in which the 

main ore shoots in both the American Nettie and the Wanakah 

mines have cut diagonally across the intersecting dikes and 

other structures is an argument in favor of a control of permeability in the beds resulting from a linear fracture channel.

Form and Origin of the Orebodies

As already noted in the historical summary, the ore was 

found concentrated for the most part in replacement bodies 

or cave fillings along the bedding of the quartzite, and to a 

minor extent in vertical fractures or near the walls of dikes. As 

there were no orebodies of appreciable extent remaining in the 

mines, especially of the larger “cave types,” current knowledge of the nature of the ore in place and of its relation to the 

enclosing rocks was derived chiefly from older reports and 

descriptions of the mine. At the time of the American Nettie 

mine’s major development (1889 to early 1900s), it attracted 

much attention and consequently many observers recorded 

their observations in mining and geologic journals. The early 

records, together with what was gleaned from a somewhat 

unsatisfactory examination of the gutted workings (early 

1930s), furnished additional evidence concerning the form and 

origin of the orebodies. Some pertinent observations of the 

earlier writers are reviewed.

F.M. Endlich (1889, p. 335), geologist with the Hayden 

Survey, visited the mine soon after its discovery in 1889. He 

observed that the ore occurred in vugs, nests, and caves that 

ranged in dimensions from small “bug holes” to caverns 7 or 

8 ft in height and 20 to 30 ft in lateral extent. The caves were 

found chiefly within 20 to 30 ft of the “bituminous shales” 

overlying the upper quartzite bed, and many showed a “distinct shelving.” Near the caves the quartzite was found to have 

been cut by numerous vertical “cleavage fissures,” many of 

which were only cracks, but others were as much as a foot or 

more in width and extended below the horizon of the caves. 

He also noted that nearly all caves that contained appreciable 

quantities of ore were directly connected to vertical fractures. 

The ore consisted of a yellow or brown auriferous clay that 

filled the bottoms of the caves; the sides and bottoms of some 

caves were also encrusted with anglesite, cerussite, sulfides 

and their various oxidation products. Barite was reported to be 

mixed with the ore or to line some of the fractures. The gold 

was found native in wires of varying thickness.

Richard Pearce (1890, p. 453) described the ore that he 

examined as being associated with silica and mixed with an 

ocherous material, which had a large amount of “sesquisulphate of iron.” He also detected the presence of bismuth in 

nearly all samples of the ore; in one sample, he distinguished 

crystals of what he thought to be the bismuth telluride “wehrlite.” We presume the “sesquisulfate of iron” to be copiapite 

and the bismuth mineral tetradymite. The latter mineral has 

since been detected in the massive unoxidized sulfide ore from 

88 Geology and Ore Deposits of the Uncompahgre Mining District, Southwestern Colorado
near the root zone of the orebody. These observations establish 

that the bismuth telluride and gold-bearing minerals, formed in 

late stages of mineralization, occupied the entire length of the 

central highly mineralized parts of the ore channel.

Downer (1901) examined the American Nettie mine in 

the late 1890s or early 1900s after the mine workings had been 

considerably developed. The greatest width of ore at that time 

was said to lie in most cases at or below the junction of the 

vertical “slips” (iron-stained cracks), with the shale parting 

about 30 or 40 ft below the top of the massive quartzite. Ore, 

in places, extended into the overlying shale, and was found 

to be wholly oxidized in one pocket and entirely sulfide in 

another. He listed the minerals occurring in the mine as follows: (1) in the native state—gold, silver, copper, and sulfur; 

(2) as sulfides—iron, zinc, lead, copper, bismuth, and silver; 

(3) as sulfates—iron, lead, copper, barium, and calcium; (4) 

as carbonates—iron, copper, manganese, and calcium; and (5) 

as oxides—iron, copper, and manganese. Near the top of the 

main ore shoot, in one or two pockets of oxidized ore, there 

was a porous black silver sulfide consisting of 71 to 75 percent 

silver associated with native sulfur; the sulfur assayed 10 oz 

gold to the ton.

Certain observations by Lakes (1901, p. 243–244) are 

thought to be particularly significant in regard to the origin 

of the orebodies. He noted that ore was rarely found either in 

the black shale above the quartzite or at its contact with the 

quartzite, but where the shale bulged down into the quartzite, 

locally large bodies of ore were likely to be found. Also, where 

the quartzite was locally less siliceous, it was more likely 

to be barren, but where the quartzite was “semi-vitreous,” it 

was likely to have ore. He did not think that the cavities were 

formed exactly like those at Leadville, by substitution or 

metasomatic interchange of ore for rock, because there was 

no gradual “fading off” of the ore into the country rock; to 

the contrary, the ore was bounded by the walls of the cavity 

and when the ore, whether sulfide or oxide, was removed the 

surface of the cavity was round and smooth. He concluded 

that the cavities were formed prior to being filled by the ore, 

or were filled immediately on forming by the same fluids that 

formed them.

Although Lakes’ observation that the smooth contact 

indicates solution and fill would not now be considered evidence in favor of his argument, there is additional evidence 

on the shapes of the cavities bearing on this point. Shelving of 

the caves as noted by Endlich (1889, p. 335) can still be seen 

in the mine; these cuspate forms of the intersecting shelving 

surfaces, which are convex outward, are known to result from 

solution cavities.

Lakes’ (1901, p. 243) description of the mine mentioned 

the downward bulges of the shale over the larger orebodies 

lying immediately beneath, but he advanced no arguments 

regarding possible relations of the downward bulges to incipient caving into cavities. It is noteworthy that the large sulfide 

orebodies, which occurred at positions well downdip of the 

orebody, were unoxidized or relatively little oxidized, and that 

the unoxidized ore lay immediately in contact with the capping shale.

Irving (1905) favored the view that the orebodies of the 

mine were formed partly by replacement and partly by filling 

of previously formed caves. He observed, as did others, that 

the larger orebodies or shoots were found along the course of a 

fracture just beneath the black shale at the top of the quartzite, 

and extended outward laterally from the fracture sometimes as 

much as 30 ft. Near the supplying fractures, the ore formed a 

solid mass, but outward only partly filled the cavity, or often 

only lined the open space. On the extreme outer limit of the 

shoot, the empty cavity was lined with nothing but quartz 

crystals. Some of the empty cavities contained no ore whatsoever. According to Irving (1905, p. 68), “It is a saying among 

the miners that as soon as one finds crystals in the quartzite 

there will be no more ore. These empty cavities have all the 

characteristics of cavities produced by solution in limestone 

and can in no wise be distinguished from them except for the 

lithological character of the parent rock. There can be no question that they were produced by solution.”

Irving (1905, p. 69) further stated:

“Often a lining of honeycombed quartz is observable, as much as 4 or 5 inches in thickness, either 

lining the cavity or completely filling it. The celllike spaces in this quartz are sometimes irregular, 

but often have the form of pyrite crystals, showing clearly that they are caused by the oxidation 

and removal of former masses and crystals of iron 

sulphide. Much of the oxidized ore was shoveled 

directly from the cavities into ore sacks and shipped 

without further sorting. More careful sorting is now 

necessary... .

“On penetrating farther into the hill the ore gradually changes to pyrite. This is either massive, with 

occasional cavities into which complete crystals of 

pyrite project, or is composed of crystalline grains of 

pyrite embedded in a matrix of gray and white secondary quartz. Large admixtures of chalcopyrite and 

galena and other sulphides are found, among which 

are sphalerite, telluride of gold and silver (perhaps 

hessite, found usually in the Jonathan mine), molybdenite, and gray copper (probably argentiferous 

tetrahedrite).”

He further noted that barite frequently occurred in the 

sulfide ore. Fibrous bands of gypsum were found between the 

ore and the overlying black shales, and the sulfide ore also 

was more common directly beneath the black shales. The 

contact between the ore and the quartzite was usually sharp 

and showed an irregular and undulating surface often seen in 

replacement deposits.

Irving (1911b) recognized that the differences between 

the contact surfaces produced by replacement and those 

produced by solution were not always readily distinguishable. 

He thought that the intersecting concave depressions separated by sharp or slightly rounded ridges, common to solution 

Descriptions of Mines 89
cavities in limestone, were indicative of solution if they were 

sufficiently developed to form a striking characteristic of the 

openings, and that replacement orebodies rarely showed such 

intersecting concave surfaces. Concave surfaces of this kind, 

however, in the open caverns believed to have been produced 

by solutions in the quartzite, were observed in the American 

Nettie mine.

Irving (1911a, p. 553–554) regarded either a heavy black 

shale or a thinner, interbedded layer of clay shale an effective 

impervious barrier to quartzite replacement in the American 

Nettie deposit. In addition to the replacement masses in the 

quartzite, the filled solution cavities, and the orebodies usually 

surrounded by solution cavities, all showed the concave surfaces and other features characteristic of openings formed by 

solution. The main orebodies, however, seemed to have been 

formed by replacement.

The descriptions and conclusions of those who had the 

opportunity to see the orebodies firsthand have been combined 

somewhat diagrammatically in vertical sections across the 

American Nettie ore channel (pl. 2, sections A–A′ and B–B′). 

These approximate scale drawings of the structural relations 

have been supplemented with observations made in the stopes 

by W.S. Burbank. These combined conclusions suggest the 

relations between replacement and open-cavity filling processes in the formation of the orebodies in the American Nettie 

mine.

The structural or textural evidence favoring open-space 

filling may be summarized as follows: (1) crustification 

of cavities lying in the intermediate and outer zones of the 

section across the trend of the ore channel; (2) intersecting 

concave solution surfaces, found in mined caves in essentially 

all parts of the channel except in the central orebodies in the 

downdip parts of the channel, where mining and subsequent 

caving has obscured the original relations of ore and enclosing rock; (3) downward bulges or rolls of the capping shale, 

as reported by Lakes (1901), where the massive sulfide bodies 

were in direct contact with the shale in the downdip central 

parts of the channel; and (4) porosity of the massive sulfide 

bodies shown by the projection of crystal faces into vuggy 

openings. This does not show necessarily that the orebody as 

a whole was formed by open-space fillings, but only that, in 

general, filling predominated during later stages of deposition.

Evidence favoring replacement is as follows: (1) 

concave-convex or irregular undulating surfaces of contact 

between the sulfides and the quartzite or shale, as noted by 

Irving (1911b) and seen also by W.S. Burbank, occur in the 

downdip parts of the orebodies near the Jonathan dike, and 

at contacts between ore and granodiorite, shale, or quartzite; (2) masses of quartzite, shale, and porphyry, alongside 

walls, fractures, or bedding contacts have been converted into 

massive sericite containing some quartz, pyrite, and other 

sulfides, especially noticeable along the Jonathan dike in the 

Jonathan mine, in the lower mineralized zones in the Dakota 

Sandstone or in the uppermost beds of the Morrison Formation; (3) quartzite surrounding the cavities in the intermediate 

or outer parts of the main ore channel contains disseminated 

crystalline pyrite or blebs or small masses of tetrahedrite and 

other sulfides; and (4) textures of the massive ores examined 

under the reflecting microscope show that a complex series of 

metasomatic interchanges have occurred between the earlier 

formed gangue and sulfide minerals and the later sulfidebearing fluids.

From these structural relations and from the study of 

polished and thin sections of the altered rocks and ores, 

the general sequence of processes forming the orebodies is 

determined to be as follows: (1) solution of the quartzite, 

forming open cavities along the linear fracture channel, which 

was produced by structural disturbances connected with the 

emplacement or subsidence of the main intrusive bodies to the 

south of the mine; (2) formation of sericite, ankerite, or siderite, together with smaller amounts of rutile, anatase, apatite, 

titanite, and chlorite, as metasomatic replacement of quartzite, 

shale, or granodiorite, but principally along rock contacts or 

tight fractures in the downdip parts of the ore channel or along 

the walls of the granodiorite and clastic dikes; (3) pyritization 

of the altered rocks overlapping or occurring as a part of the 

preceding mineralization; and continuing with (4) deposition of sulfides in this order: pyrite, sphalerite, chalcopyrite, 

galena and lesser barite, tennantite or tetrahedrite, additional 

chalcopyrite locally, tetradymite, benjaminite(?), hessite, and 

native gold. Late quartz and calcite occur locally. The position in the sequence of the abundant barite to which Irving 

(1905, p. 69) refers, is not definitely known. Massive sulfide 

ore containing barite was entirely mined, but barite veinlets 

are known to have cut the early pyrite and probably accompanied the sphalerite and galena as they did in the nearby veins 

and in similar small deposits in quartzite beds on the opposite 

side of the Uncompahgre River valley. Fragmentation of the 

early metasomatic pyrite near igneous dike walls and along 

the Nettie clastic dike suggests that there was some structural 

disturbance after the beginning of mineralization. Structural 

evidence, with regard to the time of the clastic dike injection, 

is inconclusive because of the strong local alteration of both 

the dike walls and the fracture filling. A thin section of the 

clastic material from the dike indicates that it is composed, in 

part, of rock fragments that were evidently sericitized prior to 

fragmentation. In the Seaburg tunnel, the Nettie clastic dike 

consists, locally, of coarse-grained and partly sericitized quartz 

and orthoclase containing much apatite and titaniferous aggregates, suggestive of the early stages of the high-temperature 

replacement veins of The Blowout area that were formed 

immediately prior to and following the early pyritization. It 

is concluded that the fracture was reopened, probably during 

concurrent formation of this vein at deeper levels, or at about 

the time of the earliest sericitization. The clastic injections 

resulted, in part, from the explosive expansion of these earliest 

mineralizing fluids and perhaps from an explosive escape of 

gases from the deeper intrusive body. Minor late movements 

also occurred during pyritization.

The formation of the metasomatic replacement deposits 

and of the orebody in the central channel is interpreted as follows: fluids rising along the tight feeder fractures from depth 

90 Geology and Ore Deposits of the Uncompahgre Mining District, Southwestern Colorado
replaced the wall rock by metasomatic interchange, but upon 

entering the relatively open-fractured channel in the quartzite, 

the fluids apparently entered a zone of relatively low pressure 

and probably expanded, forming a gaseous differentiate and 

a liquid that attacked and dissolved cavities in the quartzite. 

This episode was one of “hydrothermal” leaching that preceded the sulfide mineralization at many places in the mining 

district. As the temperature of the fluids decreased, or as the 

channel was plugged updip by the transported siliceous matter, 

the filling of the cavities by hydrothermal fluids with gangue 

and ore commenced. Continued constriction of the channels 

caused by passing fluids and reaction between the fluids and 

the unattacked parts of the quartzite resulted in replacement 

of the walls of the previous deposits formed. The open channels down the dip of the beds nearest the egress of the fluids 

became constricted early by the rapid precipitation when the 

fluids first entered the openings. This area showed the most 

pronounced effects of metasomatic replacement. Thus the walls 

of the orebodies deeper in the channel or nearer its center show 

the characteristics more of metasomatic replacement, as noted 

by Irving (1911a). It does not necessarily follow, however, 

that the bulk of these bodies were formed by replacement. The 

updip or outer zones of the channel contain much less of this 

type of mineralization, as the open spaces were never completely filled. The outermost zones of the channel also were 

less mineralized because the later sulfide-bearing fluids had 

less penetrating strength than those that did the leaching. In 

addition, the feeding fractures became more constricted with 

time as mineralization advanced. The more central parts of the 

channel probably would have received the greatest volume of 

migrating fluids because this was the most direct path from 

their entrance to their exit up the dip of the quartzite.

These conclusions are proposed with regard to the distribution of ore in the open breccias and to the shapes of the 

orebodies formed along the channel. The absence of ore at 

one place in an open breccia is not necessarily an indication 

that the breccia as a whole was unmineralized. The outward 

bulging shape of the American Nettie ore channel near the 

entrance of the fluids along the walls of the Jonathan dike and 

from nearby fractures appears to have been the natural result 

of local constrictions of the original open spaces and of the 

tendency for the fluids to have spread laterally around those 

early constrictions into the outer zones of the channel. Reaction between the quartzite and fluids in the downdip positions 

and the precipitation of minerals from the fluids became less 

reactive with respect to the quartzite as they passed upward 

into the more open spaces. Hence, filling predominated over 

replacement in the up-channel open spaces as well as in the 

outer zones. The outer zones, as noted, are characterized by 

quartz fillings of the early-stage mineralization that doubtless represent in part the silica previously dissolved during 

leaching and metasomatic exchange, and which was then 

precipitated outward into the open spaces. In contrast to the 

early comb-quartz filling of the outer zones of the channel, the 

inner or central zones in the updip positions are described as 

having smooth and relatively polished walls against which the 

ore lay. Minor metasomatic replacement by recrystallized and 

honeycombed quartz was noticeable in those walls but not in 

the amount found nearer the Jonathan dike.

Considerable metasomatic interchange occurred between 

the sulfide-bearing fluids and the already deposited minerals and gangues. The rising fluids constantly changed their 

metal content, demonstrating that they were more-or-less 

out of equilibrium with the previously precipitated sulfides. 

The composition of the gangues transported with the metals, 

however, apparently did not change in a corresponding degree; 

metasomatic replacement of wall rock or gangues by sulfides 

was less pronounced in the later stages of mineralization.

Extension of the Ore Channel at Depth

It can be seen on plate 3A that the principal ore channel 

in the American Nettie mine terminated downdip from the 

Dakota Sandstone outcrop in a somewhat bulbous enlargement 

enclosing the area along the walls of part of the Jonathan and 

Nettie No. 2 dikes and both parallel- and cross-fracture systems. The northwest-trending cross fractures were particularly 

well developed near the downdip termination of the central 

massive sulfide ore shoots, and cut through the Jonathan dike 

into the quartzite on its east wall where they were apparently 

predominant in the now-inaccessible deeper parts of the mine 

workings. These fractures were comparatively tight in the 

rocks beneath the level of the Dakota and contained replacement deposits very small in volume and low in grade. In the 

Jonathan mine, along the Dakota outcrop south of the American Nettie mine, the ore was likewise concentrated near the 

Jonathan dike and, in some places, the replacement bodies 

along the walls were minable down to the upper sandstone 

beds of the Morrison Formation. The decreasing amount of ore 

down the dip of the Dakota east of the Jonathan dike suggests 

that the bulk of the mineralizing fluids gained access to the 

beds along the line of the Jonathan dike.

The distribution of the ore shoots suggests access for two 

possible feeder sources of ore from depth (pls. 2, 3A). One 

source might have been along the walls of the Jonathan dike 

and the strong fractures north of and beneath the south-dipping 

Nettie clastic dike, and another might have been upward along 

the intersection of the Jonathan and Nettie igneous dikes. 

Rocks in the exploratory eastern workings of the Seaburg 

tunnel (pl. 2, section B–B′) and near the Nettie dike No. 1, 

stratigraphically 800 ft below the Dakota Sandstone, contained 

some early-stage mineralization. The sandstone of the basal 

Morrison Formation was converted locally into an opensolution breccia and the walls of the igneous dike were locally 

mineralized by replacement. The open breccia contained 

only minor amounts of quartz, chlorite, and carbonate and 

small amounts of sulfides and hematite, which were generally 

perched on the walls of the cavities. It appears, therefore, that 

the later fluids generated after those that leached the fractured 

rocks were for the most part diverted from these feeder fractures during later stages of the mineralization, or that the fluids 

passed through the fractures without precipitating ores. The 

Descriptions of Mines 91
main feeder fractures, if such existed, would have lain somewhere to the northeast down the dip of the Dakota Sandstone. 

The downward projection of the apparent ore-feeding fractures 

in the Dakota, along with the evidence found at deeper levels, 

suggests that they were somewhere near or east of the Jonathan dike.

On plate 3A the positions of the Jonathan, Nettie No. 1, 

and Nettie No. 2 dikes have been projected to intersect the 

Entrada Sandstone and the Pony Express Limestone Member 

of the Wanakah Formation. All three dikes intersect at nearly 

the same point on the Pony Express horizon, a little southeast of and almost beneath the intersection of the Jonathan 

and the Nettie No. 2 dikes on the Dakota Sandstone horizon. 

Thus again, the main feeding fractures at depth are suggested 

to presumably lie north of these intersections and east of the 

Jonathan dike. Those rocks, however, were not explored by 

any of the earlier deep workings.

Bachelor Group of Mines

Location, History, and Development

The Bachelor group, consisting of the Bachelor, Wedge, 

Neodesha, and Pony Express mines, is on the east side of the 

Uncompahgre River valley and in Dexter Creek valley about 

2.5 mi north of Ouray (fig. 21). All were operated at one time 

as separate properties. In later years, the Bachelor and Wedge 

mines were operated by the Bachelor Consolidated Mining 

Co., and subsequently, parts of the properties, other than the 

Neodesha, were operated by the Banner American Mining and 

Milling Co. Because the geologic features are closely connected, these four mines are described together; the eastern 

mine workings are on the single Bachelor vein, and west of 

the vein split the mine workings are on both the Bachelor 

vein and the Pony Express vein (pl. 4). The veins and bedded 

deposits of this mine group are typical of the silver-lead-zinc 

type within the mining district.

The Bachelor group of mines (pl. 4) were developed by 

both adits and shafts on the east side of the Uncompahgre 

River valley and just along the south side of Dexter Creek 

valley. The principal deep openings consist of the Syracuse 

tunnel (fig. 21, no. 14) at an altitude of 8,040 ft on the lower 

slope of the Uncompahgre River valley, and the Khedive adit 

(Bachelor on fig. 21, no. 11) at an altitude of 8,595 ft in Dexter 

Creek valley about a mile east of the Uncompahgre valley. 

The Wedge mine (fig. 21, no. 12) was first operated through a 

shaft at an altitude of 9,216 ft on the flat-topped ridge between 

the two valleys. Openings of the Neodesha mine (fig. 21, 

no. 13) are on the slope above the Syracuse tunnel between 

altitudes of 8,600 and 9,100 ft. These workings (pl. 4) are 

now connected by underground shafts, drifts, and stopes, and 

extend about 8,500 ft east of the lower tunnel entrances in 

the Uncompahgre valley; the maximum vertical range of the 

workings is about 1,200 ft.

The earliest patent on the mine group was issued on the 

Pony Express claim (fig. 21, no. 15) in 1887 (Rice, 1980, p. 

4); both this property and the Neodesha, on which a patent was 

issued in 1896, produced ore in the late 1880s. Undoubtedly, 

the first work on the Pony Express deposit had been started 

in the prominent Pony Express breccia beds cropping out on 

the east side of the Uncompahgre valley at an even earlier 

date. The richest silver ore mined from the group came from 

parts of the Bachelor, Wedge, and Neodesha mines; these were 

blind ore shoots in veins concealed by cappings of Mancos 

Shale and (or) granodiorite that form the top part of the ridge 

between the Uncompahgre River and Dexter Creek valleys. In 

1892, C.A. Armstrong of Ouray, who had driven a tunnel into 

the hillside on the Bachelor claim, made the first strike of the 

high-grade silver orebodies. The Bachelor discovery tunnel 

(pl. 4), at an altitude of about 8,930 ft on the south slope of 

Dexter Creek valley, was started in the upper mudstones and 

shales of the Morrison Formation and encountered the Bachelor vein in the Dakota Sandstone about 550 ft from the portal. 

The Bachelor clastic dike and associated rich silver vein were 

cut in the sill of the tunnel, but were not exposed in the back 

because of a capping of shale on the quartzite at this level. 

Shortly thereafter, Armstrong, J.F. Sanders, and G.R. Hurlburt, all bachelors, formed the Bachelor Mining Co., a closed 

corporation, for the development of their claims which lay to 

the east of the Wedge claim.

Between 1892 and 1897, the Bachelor Mining Co. produced regularly from a shoot of rich silver ore that was mostly 

in quartzite beds of the Dakota Sandstone. During the height of 

production in 1895, the company was shipping about 30 cars of 

crude ore monthly; the production and profit sustained during this short period exceeded that of any other property in the 

Uncompahgre district. By 1896, the operations on the property 

had been gradually transferred to lower tunnels that were driven 

to intersect the vein at more convenient working levels. A few 

years later essentially all operations were conducted through the 

lower tunnel in Dexter Creek valley known as the Khedive adit 

(pl. 4). By 1899, the bulk of the high-grade ore had been mined 

out above the Khedive level and west of the adit at this level. 

As the grade of the ore became lower, a concentrating mill was 

constructed at the Khedive adit site to treat material from the 

dumps that had been discarded earlier. Prior to that time, most 

of the crude ore was shipped directly to the smelters; only a 

small amount of ore from both the mine and older dumps had 

been treated in a neighboring mill. Mill output was increased as 

the ore mined from near the Khedive level contained increasing 

quantities of zinc and, according to company assays, a decreasing silver content of about 9 oz to the ton. In 1902, the monthly 

output was about 15 to 25 carloads of concentrates; in addition, 

shipments of sorted, high-grade crude ore continued to be sent 

directly to the smelters. The company mill was equipped with 

crushers and rolls, jigs, Bartlett tables, a Wilfley table, slime 

settling tanks, and canvas slime tables, but according to Irving 

(1905, p. 63) a considerable loss of silver could not be avoided. 

As was common in mills of that time, difficulty was encountered in saving the silver values because of the tendency of gray 

copper (argentiferous tetrahedrite) to be converted into slime 

even with the most careful crushing techniques. From 1904 to 

92 Geology and Ore Deposits of the Uncompahgre Mining District, Southwestern Colorado
1907, the Bachelor mine was idle except for the work of a few 

lessees; this timeframe is considered to mark the end of the 

early period of development. The mine had employed, under 

normal conditions, from 50 to 75 men, and more temporarily. 

During the years of operation subsequent to 1900, the Bachelor 

shaft was sunk about 500 ft below the Khedive level to develop 

the downward extension of some ore shoots, but the amount 

of favorably mineralized ground decreased considerably in the 

alternating shales and sandstone of the Morrison Formation 

underlying the Dakota Sandstone.

From the time of its discovery until about 1903, the 

Bachelor mine, exclusive of the Wedge, Neodesha, and Pony 

Express mines, had a production of nearly $2 million, more 

than any other mine of the district. It was reported in news 

items for 1895 that the property had a net profit of $35,000 per 

month, but operations in the years of declining grade and falling silver prices materially diminished the profits.

Development of the Wedge property was started not long 

after the discovery on the Bachelor, but according to news 

items, an extension of the rich Bachelor ore shoot was not 

encountered until 1896. This development was started by a 

vertical shaft that was sunk about 200 ft; some drifting was 

done before encountering the main ore shoot which extended 

to within about 130 ft of the surface near the shaft. In 1898, 

the Wedge shaft was connected with the Khedive level of 

the Bachelor mine (pl. 4). Operations by the Wedge Mines 

Co., formed by J.B. Farrish, E. Richards, and L.W. Ross 

in December 1898, after purchase of the property from the 

former owners for a sum said to have been $40,000, continued 

until January 1900, when the mine was closed. The company 

regularly employed between 50 and 75 men, and during 1899, 

nearly 100. Production from the Wedge is not known, but it 

was considerably less than that from the Bachelor during its 

first period of operation. Based upon the stoped area of the 

Wedge mine and upon the length of its operations compared 

to the Bachelor, production during this period is estimated to 

have been approximately $1 million. The high-grade ore was 

similar to that in the western part of the Bachelor mine.

The Neodesha property, on the western extension of the 

Wedge vein, apparently contained only a small part of the 

high-grade ore shoot. The rake of the ore shoot toward the east 

brought its base much higher stratigraphically on Neodesha 

ground; development on the vein in the Morrison Formation 

below the Dakota Sandstone does not appear to have found 

orebodies. This property was purchased and operated by the 

Florence Mining and Milling Co. in 1889, but exploitation by 

this company was unsuccessful. In 1899, a good orebody was 

said to have been found; the property then was operated under 

lease, employed about 20 men, and produced about 7 tons 

of ore daily. No record of the total production exists, but it 

undoubtedly was small compared to that from the Wedge and 

Bachelor properties.

The vein in the Pony Express mine is a southern split 

of the Bachelor vein (pl. 4); the bulk of the ore from this 

mine was obtained from bedded deposits in or near the Pony 

Express Limestone Member of the Wanakah Formation, stratigraphically lower than the deposits in the Bachelor, Wedge, 

and Neodesha mines. Shipments of ore from this property, 

first recorded in 1888, were made over a considerable period 

of time. Between 1893 and 1896, occasional shipments of 5 to 

8 carloads per week were reported, but the production apparently not was continuous. The property was equipped with a 

mill by 1897. The Pony Express vein narrowed into the hill 

and the ore locally decreased both in quantity and grade at 

cross faults; little development was done beyond 600 ft from 

the outcrop.

The next important period of development of these properties, not so much for their productivity as for delimitation of 

the mineralized ground, began in 1924 after the consolidation 

of the Bachelor and Wedge properties. The Syracuse tunnel, 

intended for development of the mines at depth and as a main 

haulage way, was started in October 1924 and eventually was 

connected to the Bachelor shaft by a crosscut and raise (pl. 4). 

Ore was blocked out along the Pony Express vein and in the 

roll or bedded deposit in the Pony Express breccia as well as 

in the Bachelor vein above the tunnel level. However, production from these deposits had to be postponed because of 

unfavorable metal prices as well as the small size and moderate grade of the blocked-out orebodies. Work was suspended 

in 1929.

G.A. Franz, Jr., and associates, who operated the Banner American mill (fig. 21, no. 16) between 1931 and 1938, 

extended some of the mine workings higher in the upper 

Bachelor Consolidated property through the Khedive adit. The 

Khedive level also was extended about 800 ft east beyond its 

previous heading, and crosscuts were driven both north and 

south to explore for parallel veins in this mine (pl. 4). Some 

ore was produced both in the Bachelor vein and in some of 

the parallel veins, but owing to a large amount of shale in the 

Dakota Sandstone at this horizon, the veins contained only 

small and irregular ore shoots. This work, being mainly of an 

exploratory nature, was abandoned finally in favor of pushing 

the development on the Pony Express vein at lower levels. 

From 1933 to 1937, when operations were suspended, the 

Banner American Milling Co. essentially mined out the ore on 

the Syracuse tunnel level east of the Pony Express property in 

ground previously developed by the Bachelor Consolidated 

Co. The steep vein in this part of the property ranged from 6 

in. to 3 ft in thickness; ore in the roll was about 4 ft thick in 

most places. Silver values in this vein decreased from west to 

east, from a maximum of 50 oz to about 5 oz per ton of ore; 

lead and zinc content ranging from 3 to 10 percent occurred 

in about equal amounts. The ore mined during this period 

was locally of good grade and width. The shoot at the contact 

narrowed into the hill and was locally impoverished near the 

large cross faults at the eastern end of Pony Express ground 

(pl. 4). Ore in the vertical vein was narrow and stoping was 

limited vertically to the massive sandstones at this horizon. 

Under these conditions and because of the extensive distance required to reach and handle the ore in proportion to its 

volume, mining costs rose prohibitively. The modern flotationtype mill of 100-ton capacity made a good recovery according 

Descriptions of Mines 93
to the operators. The main product was a lead concentrate that 

contained the bulk of the silver.

The mining property was worked by J.R. Sonza from 

1942 to 1946, and then by the American Zinc, Lead & Smelting Co. during 1947 (King and Allsman, 1950). From mid1951 to mid-1952, the Bachelor mine was operated by the 

Bachelor Development Co. of Ouray (J.R. Sonza and K.M. 

Quirk) through the Khedive adit under a Defense Minerals 

Administration contract. The extent of mining developments 

within this mine group subsequent to 1952 are unknown, 

except to note that some development took place at the Bachelor and Pony Express mines during the mid- to late 1950s.

Production and Grade

The total production from the Bachelor, Wedge, Neodesha, and Pony Express mines prior to about 1905 was estimated by local authorities to be between $3,500,000 and $5 

million. It is thought, however, that the lower figure is more 

nearly correct. As already noted, the combined production 

from the Bachelor and Wedge mines prior to this time was 

estimated to have been roughly $3 million. Production from 

the Neodesha mine was small, but most of the ore was high 

grade. The production from the Pony Express mine was almost 

entirely of lower grade ore, and the volume of stoped ground 

markedly decreased eastward into the hill. It is probable, 

therefore, that the production from these two properties prior 

to 1905 was between $500,000 and $700,000.

The grade of the early mined silver ore from the Bachelor 

and Wedge mines ranged from $20 to $75 per ton (Irving, 

1905, p. 62); locally, however, extremely rich pockets of 

nearly pure ruby silver (pyrargyrite) were encountered. The 

value of ore in the eastern and lower parts of the Bachelor ore 

shoot was closer to $10 per ton. Ore from the Pony Express 

mine was said to have averaged nearly $30 per ton, but this 

value apparently did not hold up throughout the ore shoot. 

Some ground blocked out above the Syracuse tunnel level 

contained ore, but milling returns suggested that only with 

considerable selection of the ore and sacrifice of tonnage 

could the mill feed be kept near the values mentioned.

Geology

The rocks enclosing the ore deposits in this group of mines 

range from the upper part of the Cutler Formation (Permian) to 

the basal part of the Mancos Shale (Cretaceous), a thickness of 

about 1,200 ft. The most productive parts of the veins were in 

the quartzitic beds in the Dakota Sandstone and the upper 300 

ft of the underlying Morrison Formation. Another productive 

zone, only 100 to 200 ft thick, occurred in the basal Morrison Formation, Wanakah Formation, Entrada Sandstone, and 

Dolores Formation; it is estimated that this zone yielded about 

one-quarter of the total value of production.

The strata dip as much as 20°–30° S., but average about 

5°–10° ESE. to locally south within a narrow zone along the 

east-striking vein system in which the regional northerly dip of 

the beds has been flattened to reversed. The average component 

of dip within the plane of the vein, however, varies between 3° 

and 8° E. In addition, the south side of the plane was slightly 

down-faulted. This local flexing probably resulted from subsidence related to intrusive igneous activity, such as noted in the 

area of the American Nettie deposit. These bedding attitudes, 

as well as changes in the physical characteristics of the rocks, 

were important in the localization of the ore and in the rake of 

the ore shoots. The upper Bachelor-Wedge-Neodesha ore shoot 

rakes eastward more steeply than the dip of the beds because of 

the local change in dip and certain structural features of the vein 

resulting from the deflections in dip. The ore shoots in the Pony 

Express vein system plunge essentially with that of the rocks. 

The lower ore shoots were chiefly at and near the intersection of the vein with the limestone breccia in the Pony Express 

Limestone Member of the Wanakah Formation, a bed of exceptional permeability. Except very locally in parts of the Wedge 

and Bachelor mines, shale walls were generally unfavorable 

or much less favorable to the deposition of ore in the fractures 

than where the walls were of more competent sandstone. On 

the other hand, shale beds and partings influenced the localization of rolls or deflections in the attitudes of the fractures, thus 

producing structures locally favorable for ore deposition.

In the eastern part of the workings on the upper or Khedive level, the main Bachelor vein strikes N. 80°–83° E., and 

dips from nearly vertical to 80° S. (pl. 4). Between the Bachelor and Wedge shafts, the vein is essentially vertical according to Irving (1905, p. 61), and at places dips about 80° N. 

according to Ransome (1901, p. 227). No detailed level maps 

of the old workings were available and as these workings were 

not accessible, local changes in the dip of the vein in relation 

to the stopes could not be determined. In the extreme western 

part of the Neodesha property, the main vein crops out on the 

slope of the Uncompahgre River valley and dips from vertical to 80° S. About midway between the Bachelor and Wedge 

shafts, as traced in the upper levels (pl. 4), the fracture system 

splits, forming two branches. The south branch, the Pony 

Express vein, strikes about N. 78°–80° E. and dips between 

50° and 80° S. The north branch, or so-called BachelorWedge-Neodesha “split,” consists of a series of gradually 

diverging breaks that forms two veins separated by about 50 

to 70 ft near the Wedge shaft and widens to a fractured zone 

about 200 ft in width at the extreme western end of the Neodesha property. Here, also, the two main branches of the vein are 

separated by about 200 ft (pl. 4) on the Syracuse tunnel level.

The main fracture includes the north branch over nearly 

its entire length, from the easternmost workings to its western 

outcrop on the Uncompahgre River valley wall, and is occupied by the Bachelor clastic dike that was described by Ransome (1901), Irving (1905), and Spurr (1923). The Bachelor 

dike where seen underground ranges from a narrow seam to 3 

or 4 ft in thickness. The walls of the dike are sharp particularly 

on the Syracuse tunnel level beneath the horizon where the 

dike was mineralized. In the uppermost workings, the dike as 

well as the enclosing fracture pinched out in the Mancos Shale 

94 Geology and Ore Deposits of the Uncompahgre Mining District, Southwestern Colorado
and terminated abruptly in a zone of extremely contorted and 

broken shale. As previously discussed (p. 33–34), the dike was 

formed by explosive injection from depth and contained debris 

both from shales in the Dakota Sandstone as well as from 

rocks at greater depths (fig. 16). The Bachelor dike extends 

beneath the level of the deepest mine workings. Westward 

beyond Lake Lenore the fracture has been filled by an intrusive igneous dike (fig. 12).

There are differences of opinion as to when the Bachelor 

clastic dike formed in relation to the vein filling. Both Ransome (1901) and Irving (1905) concluded that the dike was 

emplaced prior to the formation of the Bachelor vein. Irving 

(1905, p. 61) stated that the vein formed subsequent to the 

solidification of the clastic dike, as it broke across the dike 

from one side to the other or was enclosed entirely within it. 

Also, the faulting on the vein, about 7 ft, was observed only 

along certain parts of the vein.

Spurr (1923, p. 843–844), thinking he was dealing with 

an inverted vein-filling sequence, had the clastic dikes injected 

between stages of his so-called metalliferous veindikes. He 

first filled the vein fracture with silver-barite ore—the Bachelor vein—then split it open again to be filled with upwelling dark mud derived from the underlying formations. What 

formed the fluid part of the dike was the mineralizing solutions of his first ore stage because some parts of the dike were 

impregnated by sulfides and other parts pierced by veinlets 

of ore identical in character with the principal vein-filling; 

the mineralization antedated intrusion of the persistent black 

clastic dike, known to the miners as the Bachelor dike. Spurr 

(p. 844–846) further noted breccia injections of the same age 

elsewhere in this group of mines were gray instead of black 

because they lacked the black shale fragments characteristic 

of the Bachelor dike. The gray breccia is hard, siliceous, and 

occurs frequently, but not always, alongside the veins; its habit 

shows that it is not a fault breccia, but an injection breccia. 

From the breccia dike, sheets intrude bedding planes of the 

country rock. This was noted especially in connection with 

the Neodesha vein where the breccia is 3 to 10 ft wide. The 

breccia is barren, but does contain fragments of the baritesilver ore of the first vein period; it also contains fragments of 

vein-quartz. He concluded that the gray siliceous breccia and 

the black shaly breccia of the Bachelor dike were probably the 

same injection. An attempt was made to re-examine Spurr’s 

conclusion that the Bachelor dike must have been injected 

after the Bachelor and Wedge ore shoots were formed, but 

all evidence that can now be seen is decidedly to the contrary 

and instead confirms the conclusions of Ransome (1901) and 

Irving (1905). Because most of the ore had been mined from 

the Bachelor and Wedge ore shoots at the time of Spurr’s 

examination in 1909, he had less opportunity to observe the 

geologic relations than Ransome and Irving, both of whom 

examined the deposits when they were being mined.

The gray siliceous breccias seen by Spurr in the Neodesha and Pony Express mines were apparently different from 

the black breccias of the Bachelor dike because of the differences in composition, general color, and texture as well as 

in time of injection. The actual observations of the several 

authorities, therefore, need not lead to conflicting interpretations. W.S. Burbank examined the Bachelor dike where it was 

unmineralized both on the Syracuse tunnel level and at places 

in the eastern parts of the Bachelor mine on the Khedive level; 

he also examined the dikes in parts of the Pony Express and 

Neodesha workings and concluded that not one but several 

injections of clastic debris occurred. The Bachelor dike, wherever seen, was very distinctive in appearance, and the size and 

arrangement of the fragments varied little from its exposures 

in the north workings of the Neodesha mine to those 800 

or 900 ft below and several thousand feet to the east on the 

Syracuse tunnel level. At no place were fragments of previously formed vein material found other than sparse pyritized 

or silicified fragments that might have had a very deep-seated 

origin. Fragments of schist and other rocks of deep-seated 

origin, however, were common. Many gray to black shale 

fragments that have been broken across the cleavage were 

also conspicuous. Had this dike been intruded subsequent to 

the formation of the silver-bearing veins, there is no question 

that fragments of the previously formed veins should be fairly 

common, because the Bachelor and Wedge ore shoots had a 

significant longitudinal and vertical extent along the path of 

the injection. This injection, from great depth, has the same 

age as many other pre-mineral clastic dikes in the district.

The Pony Express vein, as seen both along the Syracuse 

tunnel level and in the eastern parts of the Pony Express mine, 

contains definite siliceous gray clastic dikes corresponding in 

appearance to those described by Spurr (1923, p. 843–849). 

They range from mere stringers in the vein walls in the eastern 

workings to dikes several feet in width in the western workings. The dikes were not seen in contact with sulfide ore 

because mining had obliterated their relations to the former 

orebodies in the westernmost Pony Express workings. Dikes 

exposed on the Syracuse tunnel level have features suggesting that they formed during intermineralization epochs. At 

one place, abundant fragments of silicified wall rock and 

sphalerite were included in the breccia, whereas galena and 

tetrahedrite, late minerals in the normal vein sequence, were 

deposited between the fragments or partly replaced the matrix 

of the fragmental material. Spurr’s sketches (1923, p. 845, 

figs. 157 and 158) of relations in the Pony Express mine show 

(1) fragments of a clastic dike included in baritic vein matter, 

and (2) a 2-ft-thick clastic dike cutting a barite vein containing chalcopyrite and tetrahedrite. Where the dike was seen 

by W.S. Burbank in this mine, fragments of quartz, silicified 

rock, and locally derived shale and sandstone were abundant 

in contrast to that of the true Bachelor dike; it was thought that 

most of this material had been torn from the walls at, immediately below, or east of the places where observed. The dike 

lay directly against the wall of hard, siliceous, low-grade vein 

matter and, as a consequence, contained few if any recognizable fragments of the massive vein sulfides. Those observations, combined with those of Spurr (1923), Ransome (1901), 

and Irving (1905), suggest strongly that the Bachelor dike was 

injected prior to the main period of vein formation. Explosive 

Descriptions of Mines 95
pulsations of the mineralizing fluids, however, continued intermittently into the later stages of vein formation and formed 

the intermineral clastic injections, of which all of the clastic 

material was of relatively local origin. These intermineral 

clastic dikes apparently attained considerable thickness toward 

the west, 3 to 10 ft in the Neodesha mine and 2 ft in the Pony 

Express mine, according to Spurr (1923, p. 846 and fig. 158), 

whereas the dikes scarcely exceeded 1 ft in the eastern Pony 

Express mine and are scattered in occurrence farther west on 

the Khedive and Syracuse tunnel levels. It may be concluded 

that the westernmost dike accumulations were gradually built 

up by a series of pulsations. The possible factors that produced this condition are as follows: (1) the vein fractures were 

gradually filled from the east toward the west by the mineralizing fluids, (2) reopenings of the fractures, connecting to the 

surface, released the pressure on the mineralizing fluids that 

resulted from more feeble structural disturbances with lapse of 

time, and (3) the temperatures of the later vein-forming fluids 

were lower and consequently the positions of sudden pressure 

release migrated toward the egress of the fluids with lapse of 

time.

Sequence of Vein Formation

The general sequence in the formation of the dikes and 

veins as determined from structural relations and crustification 

was (1) opening of the Bachelor main fracture and the injection of the pre-mineral Bachelor clastic dike, (2) reopening 

of the Bachelor dike and formation of the Pony Express vein 

split, with beginning of mineralization in somewhat variable 

order, (3) pyritization, sericitization, and formation of ferroan 

rhodochrosite (mostly in the easternmost parts of the vein 

system) followed by quartz and sphalerite (with some galena, 

chalcopyrite, and tetrahedrite), (4) intermineral clastic injections followed by chalcopyrite, tetrahedrite, pearceite, and 

galena overlapping in deposition with barite or quartz gangue, 

(5) additional chalcopyrite with some pearceite and calcite 

veining earlier sulfides, and finally pyrargyrite(?), and (6) barren barite, quartz, and rhombohedral and scalenohedral calcite 

and locally barren quartz. The principal barite gangue was 

deposited during stage 4 and commonly was associated with 

either copper-silver minerals or galena, chiefly in the westernmost workings. As noted above, stage 4 also was interrupted 

by one or more clastic injections that formed the siliceous orebearing clastic dikes. The most prominent dike in the eastern 

Pony Express vein on the Syracuse tunnel level followed the 

major deposition of pyrite and sphalerite that contained minor 

chalcopyrite, tetrahedrite, and galena. It in turn was followed 

by the bulk of the galena and additional chalcopyrite and 

tetrahedrite.

It was not possible to confirm Spurr’s conclusion (1923, 

p. 843) that the sequence of mineral deposition in the Bachelor 

and Pony Express mines was inverted, the higher grade silver 

ore being early and followed by pyrite, sphalerite, and galena, 

although barite was most abundant in the late rather than early 

stages. Bastin (1923, p. 70–73) did not mention evidence 

for an inverted sequence; he considered that pearceite and 

tetrahedrite, which are intimately intergrown (see fig. 27), 

were essentially contemporaneous with galena, sphalerite, and 

chalcopyrite and therefore were primary. He also noted that 

the primary ore was cut by small veinlets of simultaneously 

deposited pearceite, calcite, and chalcopyrite.

Further evidence for the repetition and inversion of 

mineral deposition was supplied by a crustified vein on the 

lower levels of the Pony Express mine, which in the westernmost workings had the following sequence from the wall 

inward to the open vuggy center: (1) the silicified sandstone 

wall, (2) a layer of quartz crystals and sphalerite, chalcopyrite, and galena, (3) a layer of dark-banded fine-grained 

quartz that was deposited upon and cemented the partly 

brecciated layer 2, (4) a thin layer of chalcopyrite, tennantite, and galena merging with massive intergrown galena and 

barite, and (5) some late barren barite in the open central 

vugs. The bulk of the barite and galena here were later than 

the main silver mineralization, but here as elsewhere, the 

first-stage sphalerite together with associated chalcopyrite 

and galena formed earlier than the silver-bearing minerals. 

It appears, therefore, that there were at least two principal as 

well as possibly other minor repetitions of deposition in the 

veins. The earliest base-metal sulfides were apparently more 

abundantly deposited in the eastern part of the mine where 

the ore was sphalerite-bearing and lower grade, whereas the 

later mineral sequences were more abundant in the westernmost parts of the fracture system. This, as previously 

stated, suggests that the veins were progressively filled 

from the east to the west and that certain minerals were 

dissolved and reprecipitated. Other reprecipitated minerals consisted of minute crystals of nearly all the common 

sulfides found coating vugs in the ores or open spaces in the 

barren fracture. The general sequence, disregarding details, 

is (1) early base-metal sulfides with pyrite and sphalerite 

predominant, (2) copper-silver minerals predominant and 

minor base-metal sulfides, with or without barite, and (3) 

barite and galena with other minerals in minor amounts. The 

emphasis of the copper-silver deposition during the intermediate stages of mineralization is characteristic of most of 

the silver deposits in the northern part of the Uncompahgre 

district, and accounts in part for the intermediate position of 

the rich silver-copper ore in the ore shoots of the BachelorWedge vein that was bounded on both sides and below by 

lower grade ores. In veins in the central area of the mining 

district, gold rather than silver was the more valuable metal 

deposited with the copper, except in the American Nettie 

mine where both gold and silver were concentrated. Thus, 

as distance was gained away from the centers of mineralization and as the ores were derived from deeper and deeper 

sources, silver became the predominant precious metal 

of the intermediate stage of mineralization, almost to the 

exclusion of gold. The deposition of barite following the 

release of pressure is also thought to have resulted from the 

oxidation of sulfur at or near the sources of the mineralizing 

fluids.

96 Geology and Ore Deposits of the Uncompahgre Mining District, Southwestern Colorado
Localization and Form of Ore Shoots

The principal ore shoots were localized in the vein within 

the more competent rocks. The highest grade ores from the 

upper part of the Bachelor-Wedge ore shoot (indicated as 

stoped) were found in the Dakota Sandstone, but this shoot 

narrowed downward into the upper parts of the Morrison Formation eastward toward the Bachelor shaft. Structural control 

of this shoot in shale of the upper Morrison could not be determined because that part of the mine was inaccessible. It is possible that the pre-mineral Bachelor dike and the steep dip of 

the vein in this part of the mine may have been factors contributing to an open fracture. The vein along the dike in the upper 

workings ranged from 6 in. to 2 ft wide, but in places the ore 

was more or less mixed throughout the dike over a width of 

3 or 4 ft. Eastward in the upper parts of the mine above the 

Khedive level, the distribution of ore became more and more 

irregular owing, in part, to the increase in the amount of shale 

in the Dakota Sandstone. The easternmost mine workings 

were too high in the stratigraphic section to find continuous or 

large bodies of ore although some small bodies of relatively 

good grade were encountered. It is presumed that the fracture 

along which the ore-bearing fluids gained access to the main 

Bachelor-Wedge ore shoot plunged to the east and connected 

to lower shoots along beds in the Pony Express Limestone 

Member of the Wanakah Formation. The feeding fractures 

between the upper and lower mineralized horizons probably 

extended for some distance east of the Khedive shaft.

The Pony Express channel, as noted, was localized 

chiefly by the relatively high permeability of the limestone 

breccia horizon, which as discussed on page 19, was due to 

dissolution of primary gypsum and the subsequent collapse, 

breakage, and cementation of the clay sheaths that surrounded 

each gypsum nodule, thereby forming a very permeable breccia. The lateral spread of the fluids resulted in ore shoots, 

as bedded deposits that varied from 20 ft to over 100 ft in 

width and from zero to 15 ft in thickness. A large part of the 

ore in the shoot lies along the bedding of the Pony Express 

Limestone Member. The Pony Express vein, however, west 

of where it split from the Bachelor vein, was mineralized 

above and below the limestone breccia beds. Ore shoots in 

the steeper vein are comparatively narrow, ranging in width 

from less than 6 in. to a maximum of 2 to 3 ft. In the bedded 

deposits, the ore channel apparently increased markedly in 

width west of the nearly vertical cross-fault that intersects the 

beds west of the junction of the Syracuse tunnel drift with the 

Pony Express vein (pl. 4). To the west of this fault, most of 

the ore came from the bedded deposit, whereas to the east, the 

bedded deposit was comparatively narrow and most of the ore 

came from the steeper vein. The grade of the ore throughout 

the lower shoot was less than that in the western and upper 

parts of the Bachelor-Wedge ore shoot. At the east heading of 

the Syracuse tunnel drift on the Pony Express vein, the Pony 

Express Limestone Member contains a low-grade, rhodochrosite-bearing siliceous ore that is relatively high in zinc and 

low in silver; the vein was evidently choked by the precipitation of early-stage minerals within the breccia. There also was 

considerable replacement at this horizon, which in general 

appears to have increased in importance compared to fracture 

filling toward the east.

In addition to the localization of ore in steep fractures 

and permeable strata, ore was concentrated in a special type 

of vein structure, known by the miners as a “roll”; ore formed 

as a bedded deposit at several horizons in the upper ore shoot. 

Three prominent rolls in the Morrison Formation, one 50 ft 

or less below the base of the Dakota Sandstone in interbedded shales and sandstones and two at about 250 ft and 350 ft, 

respectively, beneath the upper roll, are shown on some early 

maps of the mine workings. The exact stratigraphic horizons 

of the two lower rolls is not known, but by projecting the 

generalized section of the Morrison Formation upon the mine 

section it would appear that they lie in shales just above or 

below thick sandstone beds. A roll is commonly present in the 

Pony Express beds. Within the Bachelor and Wedge mines, the 

clastic dike whether accompanied by a vein or a barren fracture was offset at the rolls. These structures, shown in sections 

A–A′ and B–B′ (pl. 4) and in figure 23, resulted primarily from 

the lateral offsetting of the steep fracture generally at contacts 

between competent and incompetent beds such as sandstone 

and shale or within shale beds. According to those familiar 

with development of the mine, these structures were nearly 

continuous for distances of 2,000 to 3,000 ft in the Bachelor 

and Wedge mines. Between the offset ends of the vertical parts 

of the vein the beds were fractured, and the ore filled the flat 

fractures or replaced the fractured beds. According to Irving 

(1905, p. 62) ore in the rolls mainly filled open spaces.

The rolls in the veins and dikes must have formed 

under special conditions as they are not common in veins 

that traversed horizontally bedded deposits. Because of the 

reversal of dip of the beds to the south along the east-trending Bachelor fracture system, in contrast to the regional dip 

to the north, the upper part of the vertical fracture was offset 

updip to the north or relative to the lower part. The amount 

of the offset ranges from 10 to 20 ft in the Bachelor vein and 

is more than 50 ft locally in the Pony Express vein. The bedding fault or fracture between the offset parts of the vertical 

fracture does not appear to connect with other bedding faults 

or fractures above or below in nearby beds. The amount of 

offset may vary considerably within short distances longitudinally. The vertical fracture, in effect, was refracted by its 

traverse through rocks of different physical characteristics. 

The apparent deformation and concurrent bending of the 

beds to the south perhaps aided in producing offsets of the 

magnitude found.

The Bachelor dike does not commonly follow the rolls 

in the Bachelor mine; also, the top of the dike appears to 

have been cut off sharply at the bottom of the roll as if by a 

fault. There was very little ore in the Bachelor-Wedge ore 

shoot beyond the two ends of the vertical dike, other than ore 

in small stringers. The vertical vein immediately above and 

below the rolls commonly contained lenses of high-grade ore 

but was not much thicker than normal.

Descriptions of Mines 97
At places in the Pony Express beds, particularly at its 

extreme western termination, ore extended beyond the limits of 

the vertical vein, especially on the updip sides. The ore channel there was 100 to 120 ft wide, and alteration extended even 

farther. The fracture apparently was not offset more than 50 

to 60 ft (pl. 4, section A–A′; fig. 23B). The moderate reversal 

in dip of the beds probably contributed to the width of the roll 

as well as to the local fracturing of the limestone breccia beds. 

The permeability of the limestone breccia beds provided open 

space in addition to that resulting from the fracturing. Replacement appears to have been more intense at this horizon than in 

the steeper vein, but the bulk of the replacement material was 

mostly barren gangue rather than ore. The parts of the breccia in 

the Pony Express Limestone Member that escaped much of the 

sulfide mineralization consisted of an open network of silicified 

fragments which contained small crystals of late quartz deposited in the openings; this material was at least as porous as the 

original limestone breccia. This suggests that early in the mineralization stages the limestone breccia fragments were incipiently 

silicified, but that the process was apparently interrupted by the 

leaching of the remaining limestone, forming a skeletal siliceous 

breccia. Brecciation and leaching most likely occurred during 

a period of pressure release such as during the intermineral 

clastic injections or at the very beginning of mineralization. The 

dark-colored, fine-grained quartz of the porous breccia is similar 

to that deposited in the fractures during the pre-ore stages of 

mineralization that accompanied or immediately followed the 

periods of intermineral clastic injections, such as stage 3 of the 

sequence in the lower levels of the Pony Express mine. Movements on fractures or cross faults may have accounted locally 

for the barren open breccia by locally diverting the movements 

of fluids. There is barren rock in the eastern parts of the Pony 

Express mine near some of the larger cross faults.

Calliope Mine

Location, History, and Development

The Calliope mine is north-northeast of Ouray (fig. 21, 

no. 8) near the confluence of Dexter and Mill Creeks about 2 

mi east of the Uncompahgre River valley. The lower or Iowa 

Chief tunnel level (pl. 5) is at an altitude of 8,620 ft on the 

north side of Dexter Creek, opposite the Khedive adit of the 

Bachelor mine. The Calliope vein contained the most important orebodies of this mine, which was brought into production 

by Adam Herzinger in the 1880s and prior to 1890 produced 

high-grade silver ore having a value of several hundred thousand dollars as reported in Henderson (1926, p. 185). In 1889, 

this property and the James V. Dexter mine were consolidated 

and the Calliope Mining Co. was organized. After formation 

of the company, development of the Calliope mine until about 

1900 was said to have been somewhat erratic; no reliable 

production records exist for that period. A mill, constructed in 

1896 to treat the lower grade ores, was operated during the last 

few years of the company’s ownership. In 1900, the original 

Calliope interests were sold to a group of men from Chicago 

under an agreement to extend development of the property.

Early development of the property was from levels 

above the Iowa Chief tunnel; the latter was extended eastward 

beneath the upper workings in 1909 and completed in 1911. 

Raises were connected to the upper levels, and ore was then 

stoped from the eastern parts of the vein. The lower workings 

were eventually extended farther east, but records or maps of 

the extent of these workings were unavailable and the workings were inaccessible. Ore shipments made between 1916 and 

1919 were principally from the eastern extension, according 

to smelter records. Subsequently, the property was worked by 

lessees from time to time, but by the late 1950s the tunnels 

were caved and the buildings torn down with little likelihood 

of further activity.

Geology

The Calliope vein strikes between N. 85° E. and 

S. 85° E., dips on the average about 60° S., and occupies a 

fault with the south side downthrown only a few feet. Ore 

shoots within the vein were chiefly in the Dakota Sandstone, 

locally converted to quartzite, and partly in the irregularly 

98 Geology and Ore Deposits of the Uncompahgre Mining District, Southwestern Colorado

Figure 33. Diagrammatic sketch showing a possible 

interpretation of the splitting, with depth, between the mine’s 

tunnel levels in the lower part of the steeply south-dipping 

Calliope vein. The rolls in the vein were accompanied by 

contortion and lateral injection of the carbonaceous shale 

interbeds. Cross-hatched bands represent ore-bearing vein and 

mineralized seams; solid bands represent ore-enriched part of the 

vein.
shaped sill-like body of porphyritic granodiorite that had 

intruded into the basal part of the overlying Mancos Shale. 

The vein in the Morrison Formation below the quartzitic 

sandstone beds of the Dakota was commonly tight and yielded 

little ore. Strata in the vicinity of the mine workings dip 

7°–10° E. to SE., but locally near the veins there are short 

south-dipping flexures of 25°–30° or more.

The Calliope vein lies along a broad, generally easttrending flexure that is about 3,000 ft in width and includes 

the parallel Bachelor and Wedge veins, 1,300 to 1,500 ft to 

the south. Beds within this flexure dip southeast toward the 

intrusive axis to the south, a reversal of the regional north dip 

away from the San Juan Mountains uplift.

The early Tertiary erosion surface is at an altitude of 

slightly more than 9,000 ft near the outcrop of the Calliope 

vein, so that in places the ore-bearing beds have been truncated. 

The ore in the porphyritic granodiorite sill above the quartzite 

was stoped up to the unconformity at the base of the San Juan 

Formation, but the vein did not extend into the overlying Tertiary volcanic rocks. This fact and the abrupt termination of the 

stopes, particularly those in the granodiorite, at the base of the 

volcanic rocks (pl. 5) clearly indicate a pre-San Juan age for the 

Calliope mineralization. This is one of the few localities within 

the Uncompahgre mining district where these relations clearly 

are shown. In many of the district’s mines, intervening beds of 

Mancos Shale between the productive parts of the veins and 

the erosion surface show little or no fracturing within the shale 

beds, thus providing inconclusive evidence in this matter.

The James V. Dexter vein, which intersects the Calliope 

vein, strikes N. 25°–30° E. and consists of a series of relatively tight, minor steeply-dipping fractures within a zone of 

60 ft or more in width (pl. 5). Ore shoots in the Dexter vein 

were not as regular in distribution along the fracture zone as 

those in the Calliope vein; the larger shoots were replacement 

bedded deposits near the top of the Dakota Sandstone or in 

sills and irregular offshoots of the porphyritic granodiorite 

body. The ore in this vein was reported to have contained more 

lead and zinc than that of the Calliope vein, but other than the 

small orebodies mined near the portal of the Dexter tunnel, 

there was little development of these cross veins. Where the 

N. 25°–30° E. fractures cross the Calliope vein, they tended to 

split but cut through it sufficiently to indicate that they underwent some minor late movement. North of the Calliope vein, 

the Dexter fractures are barren or only weakly mineralized; 

also, the intensity of fracturing decreases in that direction. The 

only known reported production from the Dexter vein is about 

$69,000 for the year 1887.

Localization and Form of Ore Shoots

The beds adjacent to the Calliope vein were locally 

sharply flexed, evidently during formation of the main fracture, but in most places have only moderate to slight displacement. The Dakota Sandstone consists here of several alternating quartzite and shale layers, so that where the fractures pass 

through the shale interbeds in the raise above the Iowa Chief 

tunnel level, the wall rocks are sharply flexed and the vein 

is tightened, as shown diagrammatically in figure 33. The 

carbonaceous shale beds have been highly distorted, and comminuted shale appears locally to have been forcibly injected 

between the thinner quartzite layers.

The fractures tend to “roll” where they pass through 

the different rock layers, and at these rolls the fractures are 

split, so that at several places there are two roughly parallel 

veins (fig. 33), locally called the north and south veins. At the 

easternmost end of the accessible workings on the Iowa Chief 

tunnel level, there are two well-defined, south-dipping veins; 

according to notations on a few old mine maps, north and 

south veins also were indicated in the upper levels. It was not 

possible to determine if these veins were continuous between 

levels, but it is doubtful they were continuous for great distances either vertically or longitudinally from one part of the 

mine to another.

Clastic injections along the walls of the south vein on 

the Iowa Chief tunnel level consist of a highly silicified and 

indurated mass of quartzite and shale fragments. Just as in the 

Bachelor and Pony Express veins, these clastic injections were 

produced by explosive expansions of the pre-ore mineralizing 

fluids. Their effect in the Calliope vein, especially in and near 

the shale interbeds, was to plug the openings and locally divert 

the later ore-forming fluids to more open splits of the vein. 

Replacement ore in the clastic material or in the tight, shaly 

parts of the vein was commonly low grade.

The largest bodies of ore appear to have been localized 

beneath or in the porphyritic granodiorite body that was sufficiently competent to maintain open fractures above the Dakota 

Sandstone beds. The eastern limit of the upper orebody is 

assumed to mark approximately the contact between the Mancos Shale and the granodiorite body, as the configuration of the 

stoped block is somewhat similar to the surface exposure just 

south of the mine workings (pl. 5). More definitive conclusions 

about the localization of the ore cannot be made because the 

only accessible workings were the main levels and parts of the 

raise at the east end of the section shown on plate 5.

There is, however, a suggestion based on the mine workings (pl. 5) that the N. 85° E. strike of the fracture near the 

shaft (altitude 8,915 ft) at Mill Creek curves slightly toward 

the east to south of east, thus resulting in a more favorable 

trend of the vein. This curve in the strike, forming a southfacing concavity in the plane of the Calliope vein, may have 

been controlled to some extent by the local east-southeastward 

dip of the flexure, the S. 60° dip, and the small vertical displacement. Horizontal displacement of the fracture walls is 

not considered necessary to produce openings along the trend 

of the plane of the fracture, whereas vertical changes within it 

may have accounted for the more productive stretches along 

the vein.

Mineralogy of the Vein

Early ore produced from the Calliope vein was reported 

to consist of galena, brittle silver (stephanite), and native 

Descriptions of Mines 99
silver. Although it is possible that brittle silver may have been 

present in the near-surface ore, the silver-bearing samples 

obtained and examined from accessible lower workings 

consisted of tetrahedrite and polybasite associated with pyrite, 

sphalerite, chalcopyrite, and galena. The polybasite was found 

only in microscopic amounts. Tests identified both antimony 

and arsenic, the former predominating; a moderate amount of 

copper usually was detectable. Polybasite, or pearceite, was 

probably the principal silver mineral, the same as was found in 

the Bachelor and Pony Express veins. Where silver occurred 

in the upper levels, stephanite perhaps was formed and the ore 

there enriched in silver; the reported native silver was probably of supergene origin. Other than minor repetitions within 

the mineral sequence, ores of the Calliope vein, like those of 

the Dexter vein, had no unusual features. Quartz and a small 

amount of brownish rhodochrosite accompanied by pyrite 

and sphalerite were early in the paragenesis, followed by the 

principal introduction of chalcopyrite, tetrahedrite, and galena. 

The quartzitic wall rock of the Calliope vein was recrystallized 

and locally replaced by pyrite, sericite, and sphalerite; some 

late carbonate accompanied the introduction of galena. Both 

quartz and calcite formed late veinlets in the ore.

Newsboy Mine

Location, History, and Development

The Newsboy mine (fig. 21, no. 4) is a consolidation of 

the older Slide and Newsboy properties on the east side of 

the Uncompahgre River valley about 3.5 mi north of Ouray, 

between altitudes of 8,400 and 9,300 ft. The main productive zone of the mine was between the altitudes of 8,400 

and 8,700 ft, in beds ranging from the base of the Entrada 

Sandstone upward through the Wanakah Formation into the 

lower part of the Morrison Formation. The Scotsman claim, 

oldest of the Slide group, was located in October 1880 and 

patented in 1883; the Slide and several other claims were 

patented in 1891. This property was worked profitably by 

Frank Butterfield until the slump in silver prices in 1893 

that contributed to its shutdown (Rice, 1980, p. 7). The 

Slide group was sold in the 1890s to an eastern syndicate 

which formed the Slide Mining Co. M.S. Corbett, owner 

of the Newsboy claim, which included an extension on 

the main veins east of the end line of the Scotsman claim, 

started development higher on the slope in beds in and 

beneath the base of the Dakota Sandstone. This work failed 

to disclose sufficiently large orebodies to justify extensive operations. Later development of both properties was 

continued through the lower tunnels, and in 1900 or 1901, 

the properties were consolidated under the name of the 

Newsboy Mining and Milling Co. Details of the reorganization and development of the properties were not available 

for this report, nor were the properties investigated; production records prior to 1895 also were not available. It was 

reported that at times 15 to 30 men were employed on each 

property, and that an appreciable production of high- and 

low-grade silver-lead ores was made from both properties 

prior to consolidation.

There are three principal development tunnel levels (pl. 

6): the lower or Emerson tunnel at an altitude of 8,445 ft, the 

Contact tunnel at 8,515 ft, and the Ward tunnel about 140 ft 

above the Contact tunnel. The lower tunnel, in the Entrada 

Sandstone, is about 2,700 ft long according to the records of 

Mineral Survey 17331, made in September 1904. The Contact 

tunnel is about 1,650 ft long according to the records of J.H. 

Tumbach, lessee, and the Ward tunnel is possibly 750 to 800 ft 

in length. A shaft sunk on the Newsboy claim by Corbett, 180 

to 200 ft deep according to different records and in stratigraphically higher beds in about the middle of the Morrison Formation, reportedly failed to discover ore. About 400 to 500 ft of 

workings both in and just below the Dakota Sandstone opened 

only small bodies of ore. In addition to the larger tunnels, 

there are a few shorter tunnels or cuts both at intermediate 

levels and beneath the lower or Emerson tunnel. These include 

the 350-ft-long tunnel of the Army-Navy mine (fig. 21, no. 

3), about 600 ft lower in beds in the Cutler Formation, which 

had a recorded production of 5 tons of ore valued at $1,091 

(Steven and others, 1977, p. E75–E76).

The Newsboy Mining and Milling Co. continued mining 

and development for several years subsequent to consolidation, but in 1905 it was unable to meet expenses and a lease of 

the property was granted to Fred Herzinger. Herzinger’s work 

was confined to extracting ore from the Contact tunnel level 

and reopening the lower Emerson tunnel. In 1906, he assigned 

this lease to J.H. Tumbach who, according to his report to 

the secretary of the company in 1909, extended the Contact 

level 228 ft during the term of the lease. Tumbach’s additional 

development work above this level found a body of ore lying 

east of the old workings that extended up into the miners’ 

“lower quartzite” bed. The extent of this work is shown on 

plate 6, but the position of the new workings is given relative 

to a 76-ft raise above and near the breast of the Contact tunnel 

as it existed when company work ceased, rather than relative 

to the tunnel portal; for this reason, the true position of the 

workings may be slightly different than as shown on plate 6. 

Tumbach reported that about 400 tons of ore were mined in 

the 1910s, which had an average value of $37.50 per ton after 

freight and treatment charges. He later extended one raise 

in an attempt to find a continuation of the ore shoot, but this 

development failed.

The uppermost mine workings, just below and within the 

Dakota Sandstone, failed to disclose appreciable bodies of ore, 

suggesting that perhaps the main ore channels in this part of 

the Uncompahgre mining district occur mostly lower within 

the basal and middle Upper Jurassic beds.

Production

Complete records of production for this mining property are not available; however, production estimates extend 

from $400,000 upward. With the exception of only a very few 

years, ore even in minor amounts was produced from 1895 to 

100 Geology and Ore Deposits of the Uncompahgre Mining District, Southwestern Colorado
the early 1920s, and then sporadically into the early 1940s. 

According to Steven and others (1977, p. E75), the official 

recorded production from the Newsboy mine was 1,895 tons 

of ore valued at $71,595, but company records showed considerably more ore had been mined, as stated above.

Geology

As shown by Luedke and Burbank (1981) and on plate 6, 

the two principal veins of this group of mines are the southern or Newsboy vein and the northern or Scotsman vein. The 

Newsboy vein has an average strike of N. 83° E. and dips 

nearly vertical to steeply north. The Scotsman vein, evidently 

productive only in the westernmost workings, there dips 

65°–85° S. toward the Newsboy vein. Both veins possibly may 

be splits from one main fracture at depth. West of and beneath 

the mine workings, the northern fracture within the Cutler 

Formation also is occupied by a fine-grained latite dike. In the 

now accessible few hundred feet of the westernmost workings, 

the most productive parts of both veins extend from the lower 

part of the Entrada Sandstone upward for somewhat more than 

200 ft into the lower part of the Morrison Formation. In general, the most productive horizon was the “lower quartzite,” 

here a 25- to 35-ft-thick indurated quartzitic sandstone in the 

Tidwell Member at the base of the Morrison Formation. The 

formations dip only a few degrees to the east, so that the entire 

Contact tunnel is within the breccia beds in the Pony Express 

Limestone Member at the base of the Wanakah Formation (pl. 

6). The most productive horizon of the veins was within these 

beds, owing to a tendency for the fracture to roll slightly and 

to the formation of bedding deposits in the permeable breccia. 

As shown on plate 6, both veins have rolled to the south, the 

Scotsman vein more so than the Newsboy vein. Toward the 

east the Newsboy vein was found to roll near the top of the 

“lower quartzite” bed and the ore was cut off sharply at this 

roll. In addition, according to Tumbach, the upward extension of the fracture was entirely lost in the eastern workings. 

A crosscut driven north from one of the raises within the 

Tumbach lease area (pl. 6) intercepted a fracture 52 ft north of 

the Newsboy vein above the “lower quartzite” bed. Tumbach 

believed this to be one of several parallel fractures, such as the 

Scotsman, but it was barren.

Several cross fractures, such as those shown near the 

portals of the tunnels, diagonally cut across the western part of 

the Newsboy vein (pl. 6). These cross fractures appear to have 

locally influenced distribution of the ore in the different rocks 

as noted previously in the Pony Express and Neodesha mines. 

Tumbach reported that one cross fracture about 1,400 ft from 

the portal of the Contact tunnel appeared to have cut off the 

main ore shoot above this level; his subsequent work found a 

small body of ore farther east. In the lower or Emerson tunnel, 

it was reported that ore was mined both east and west of this 

same cross fracture in the Entrada Sandstone. The vein ranges 

from a few inches to several feet in width, averaging less than 

a foot; the ore was of higher grade than that found along the 

bedding channels or rolls.

Where the fractures rolled in and near the Pony Express 

beds, the bending and resultant fracturing of the beds tended to 

widen the zone of fractured and mineralized rock. In addition, 

the ore “made out” into the limestone breccia beds from 15 

to 20 ft or more from the main fractures; however, within the 

accessible mine workings extending inward for about 400 ft, 

the width of ore in the flat part of the channel was less.

Mineralogy

Ore in the Newsboy vein consisted primarily of pyrite, 

sphalerite, galena, tetrahedrite, and chalcopyrite in a gangue of 

barite and quartz. Associated with the tetrahedrite was a small 

amount of the silver-bearing minerals polybasite or pearceite, 

or both, containing antimony, arsenic, and a trace amount of 

copper as determined by microchemical tests. Ankerite, manganocalcite, and calcite were the common carbonate minerals 

recognized. Within the mineral sequence, pyrite and sphalerite 

were deposited first; galena accompanied and followed barite. 

Polished sections of the ore show that these early minerals 

were locally veined by late chalcopyrite and tetrahedrite. Still 

later, covellite and chalcocite, evidently of supergene origin, 

were formed in the ore nearer the surface. However, other than 

for ore mined near the surface, secondary enrichment appeared 

to have been negligible. Quartz, sericite, ankerite and some 

barite constitute the earlier formed gangues, and in places 

preceded the bulk of the sulfides. Quartz and barite, however, 

also formed late in cavities in the ore, especially along the 

main bedding channel. Sericite was associated mostly with the 

pyritic ore. Limestone breccia and sandstone beds were more 

intensively silicified, particularly near the better grade ore. 

The most common oxidation products of the minerals, found 

in the bedded deposits as much as several hundred feet from 

the surface openings, were limonite, malachite, azurite, and 

manganese oxides.

Señorita Mine

Location, History, and Development

The Señorita mine (fig. 21, no. 1) is about 4 mi north of 

Ouray on the east side of the Uncompahgre River valley, at an 

altitude of about 8,600 ft. Like the Newsboy and Black Girl 

(fig. 21, no. 2) mines, the principal or only important ore zone 

lies in the Jurassic rocks extending upward from the Entrada 

Sandstone into the lower part of the Morrison Formation. 

Little historical information is available concerning this property. According to Rice (1980, p. 7), the property had various 

owners over the years including A.W.F. Warde, who “...made 

a living but never grew wealthy;” his daughter, Mrs. Huffnagle, of Ridgway (fig. 1), inherited the property and granted 

a number of leases over the years. All of the mining activities 

apparently were limited. Three development tunnels are shown 

on plate 7, but only the main or intermediate level in the 

breccia beds of the Pony Express Limestone Member of the 

Descriptions of Mines 101
Wanakah Formation was examined in detail during this study. 

This tunnel is at an altitude of 8,589 ft and extends about 770 

ft eastward along the mineralized zone.

Production

Prior to 1900, ore was reported to have been mined in 

1893 from the large stope in the limestone and breccia beds of 

the Pony Express Limestone Member that extends from 200 to 

365 ft east of the portal (pl. 7), and the ore mined had a value 

of $40,000 or $50,000. Production was sporadic after 1900 but 

continued to the 1960s.

Geology

The Señorita vein consists of a series of discontinuous 

mineralized fractures, strikes almost due east, and dips steeply 

north. Where the fractures cut the Pony Express beds, the 

roll is gently downward toward the north (pl. 7), opposite in 

direction to that of the more steeply-dipping rolls in the veins 

in mines to the south. Regionally, the strata dip gently north 

away from the San Juan uplift, and as exposed within the mine 

workings, which are on the east side of the Uncompahgre axis, 

have little or no eastward tilt. The beds, however, have minor 

warps where the roll’s trend was intersected by cross fractures 

and minor faults. The larger orebodies were localized at the 

level of the roll along the limestone and breccia beds of the 

Pony Express Limestone Member, but the steeper part of the 

fractures below and above this level locally contained vein 

deposits.

Within the “lower quartzite” (in the Tidwell Member) at 

the base of the Morrison Formation where seen in the upper 

tunnel level, the Señorita vein strikes N. 80°–85° E. and dips 

68° N. The ore there consisted of galena and barite similar to 

that found in the intermediate-level workings, but the vein was 

much narrower.

The lower tunnel, which is mostly in morainal debris, 

was not examined. Fractures within the Cutler Formation 

below this level are relatively weak and difficult to trace 

westward. About 2,000 ft west of the mine, in a lower cliff 

exposure, there is a fracture that strikes about N. 78° W., parallel to the lower tunnel, and dips nearly vertical; this fracture 

contains a few quarter-inch-thick stringers of barren quartz 

and barite, but the north side was displaced down about 7 ft. 

Parallel to and about 90 ft to the south of this fault, there is a 

zone of vertical calcite stringers. Still farther to the west, this 

break was not seen in outcrops of the Cutler Formation.

On the steeper slope of the hill above and east of the 

mine, moderate soil cover, dense piñon-juniper forest, and 

local slide debris obscure any possible extension of the 

Señorita vein; a few hundred feet above the mine is a large 

landslide mass composed of the Dakota Sandstone and San 

Juan Formation. It is doubtful that the vein extended upward 

and eastward or was mineralized. Furthermore, the history 

of development of other mines in this area (known locally 

as Carbonate Hill or Newsboy Hill) suggests that the only 

productive part of the Señorita fracture zone was along, just 

above, and mostly below the base of the Morrison Formation. 

The ore channel thus had a comparatively short vertical extent 

and, similar to the other ore channels in this area, followed the 

inclination of the beds for an indefinite distance to the east. 

The sources of the ore fluids evidently lay toward the east, as 

suggested by the regional distribution of ore deposits in the 

mining district.

Plate 7 illustrates, by position of the larger stopes, the 

localization of the ore mainly in beds of the Pony Express 

Limestone Member of the Wanakah Formation but also in the 

overlying Bilk Creek Sandstone Member and in the underlying Entrada Sandstone. Fractures above the line of the roll 

tended to feather-out locally, but did not contain sufficient ore 

to warrant stopes between the main and upper levels. Ore in 

the Entrada Sandstone, however, was of high-grade tetrahedrite type and locally was stoped along the vein for 15 to 20 

ft below the main level. The productive part of the vein was 

narrow, being commonly less than a foot in width, although 

the enclosing sandstone was silicified and locally impregnated 

with sulfides near the vein walls. The vein matter was barite 

and quartz with some tetrahedrite and other sulfides. These 

features are similar to those reported for the vein in the Black 

Girl mine to the south, where the high-grade tetrahedrite 

ore also was found mostly along the vein within the Entrada 

Sandstone.

Along the roll, the ore impregnated the limestone breccia 

in the Pony Express Limestone Member for a maximum width 

of 25 to 30 ft. For the entire width and length of the workings 

shown on plate 7, the breccia (fig. 25B) was silicified and small 

vugs were lined with quartz, barite, and calcite crystals as well 

as with scattered crystals of sulfide minerals. In the breast of 

the tunnel, ore in the roll was evidently of a lower grade than 

that taken from the western stope at this horizon; it also was 

very siliceous but did contain some barite. Massive barite and 

some associated ankerite, partly oxidized, were exposed along 

the down dip of the roll, and where the beds were brought up to 

the level of the drift, the minerals were deposited in the highly 

silicified Entrada Sandstone. This contact suggests that some 

disturbance separated the two stages of mineralization; the 

baritic ore contains fragments of the silicified material. This 

may have resulted either from renewed movement along the 

fracture zone or from local slumping of the beds incident to processes of hydrothermal leaching and development of cavities. 

In the silicified rocks in the face, there are impregnations and 

small veinlets of quartz, chalcopyrite, and tetrahedrite. Galena 

and some sphalerite were associated with the baritic ore; this 

ore is said to average 4 or 5 oz of silver. The bulk of the deposit 

contained sparse sulfide minerals.

Mineralogy

Microscopic examination of polished sections of the ore 

reveal no common hypogene minerals other than those that 

can be recognized with the naked eye, except for minute blebs 

102 Geology and Ore Deposits of the Uncompahgre Mining District, Southwestern Colorado
of polybasite or pearceite with the tetrahedrite. Some minute 

crystals of pyrite were embedded in some of the chalcopyrite 

and also were associated with sphalerite. Microscopic relations 

suggest the galena crystallized late, supporting the concepts 

noted elsewhere in the mine. The ore contained a little covellite and chalcocite. Barite and quartz were intergrown with all 

the sulfide minerals, which other than pyrite, had grown on 

barite or quartz crystals in drusy cavities of the ore. The associated carbonate minerals include some ankerite, manganocalcite, and rhombohedral and scalenohedral calcite. Even near 

the breast of the workings, the manganiferous carbonates have 

been oxidized to brownish or blackish sooty oxides of iron and 

manganese, especially in the more porous or vuggy parts of 

the ore throughout the mine.

References Cited

Armstrong, A.K., and Mamet, B.L., 1976, Biostratigraphy and 

regional relations of the Mississippian Leadville Limestone 

in the San Juan Mountains, southwestern Colorado: U.S. 

Geological Survey Professional Paper 985, 25 p.

Armstrong, R.L., 1969, K-Ar dating of laccolithic centers of 

the Colorado Plateau and vicinity: Geological Society of 

America Bulletin, v. 80, no. 10, p. 2081–2086.

Atwood, W.W., and Mather, K.F., 1932, Physiography and 

Quaternary geology of the San Juan Mountains, Colorado: 

U.S. Geological Survey Professional Paper 166, 176 p.

Baars, D.L., and See, P.D., 1968, Pre-Pennsylvanian stratigraphy and paleotectonics of the San Juan Mountains, southwestern Colorado: Geological Society of America Bulletin, 

v. 79, no. 3, p. 333–350.

Baars, D.L., Parker, J.W., and Chronic, John, 1967, Revised 

stratigraphic nomenclature of Pennsylvanian system, Paradox Basin: American Association of Petroleum Geologists 

Bulletin, v. 51, no. 3, pt. 1, p. 393–403.

Barker, Fred, 1969, Precambrian geology of the Needle 

Mountains, southwestern Colorado: U.S. Geological Survey 

Professional Paper 644–A, p. A1–A35.

Bastin, E.S., 1923, Silver enrichment in the San Juan Mountains, Colorado, in Contributions to economic geology 

1922, Part I, Metals and nonmetals except fuels: U.S. Geological Survey Bulletin 735–D, p. 65–129.

Bastin, E.S., Graton, L.C., Lindgren, Waldemar, Newhouse, 

W.H., Schwartz, G.M., and Short, M.N., 1931, Criteria 

of age relations of minerals, with especial reference to 

polished sections or ores: Economic Geology, v. 26, no. 6, 

p. 561–610.

Benham, Jack, 1976, Ouray: Ouray, Colo., Bear Creek Publishing Co., 64 p.

Billings, Patty, 1980, Fission track annealing related to vein 

mineralization and hydrothermal alteration, Ouray County, 

Colorado: Golden, Colo., Colorado School of Mines, 

unpublished M.S. thesis, 42 p.

Brady, F.W., 1901, An electric plant for a mountain mine: 

Engineering and Mining Journal, v. 71, no. 22, p. 689–690.

Bromfield, C.S., 1967, Geology of the Mount Wilson quadrangle, western San Juan Mountains, Colorado: U.S. Geological Survey Bulletin 1227, 100 p.

Burbank, W.S., 1930, Revision of geologic structure and stratigraphy in the Ouray district of Colorado, and its bearing 

on ore deposition: Colorado Scientific Society Proceedings, 

v. 12, no. 6, p. 151–232.

Burbank, W.S., 1936, A source of heat-energy in crystallization of granodiorite magma, and some related problems of 

volcanism: American Geophysical Union Transactions, 17th 

Annual Meeting, pt. 1, p. 236–255.

Burbank, W.S., 1940, Structural control of ore deposition in 

the Uncompahgre district, Ouray County, Colorado, with 

suggestions for prospecting: U.S. Geological Survey Bulletin 906–E, p. 189–265.

Burbank, W.S., 1941, Structural control of ore deposition in 

the Red Mountain, Sneffels, and Telluride districts of the 

San Juan Mountains, Colorado: Colorado Scientific Society 

Proceedings, v. 14, no. 5, p. 141–261.

Burbank, W.S., 1947, Early Tertiary ore deposits, Uncompahgre (Ouray) district, Ouray County, in Vanderwilt, J.W., 

ed., Mineral resources of Colorado: Denver, Colo., Mineral 

Resources Board, p. 409–414.

Burbank, W.S., 1950, Problems of wall-rock alteration in shallow volcanic environments, in Van Tuyl, F.M., and Kuhn, 

T.H., eds., Applied geology; a symposium: Colorado School 

of Mines Quarterly, v. 45, no. 1B, p. 287–319.

Burbank, W.S., and Luedke, R.G., 1961, Origin and evolution 

of ore and gangue-forming solutions, Silverton caldera, San 

Juan Mountains, Colorado: U.S. Geological Survey Professional Paper 424–C, p. C7–C11.

Burbank, W.S., and Luedke, R.G., 1964, Geology of the Ironton quadrangle, Colorado: U.S. Geological Survey Geologic 

Quadrangle Map GQ–291, scale 1:24,000.

Burbank, W.S., and Luedke, R.G., 1966, Geologic map of the 

Telluride quadrangle, southwestern Colorado: U.S. Geological Survey Geologic Quadrangle Map GQ–504, scale 

1:24,000.

Burbank, W.S., and Luedke, R.G., 1968, Geology and ore 

deposits of the western San Juan Mountains, Colorado, 

in Ridge, J.D., ed., Ore deposits of the United States, 

1933–1967, the Graton-Sales Volume: American Institute of 

Mining, Metallurgical, and Petroleum Engineers, v. 1, pt. 5, 

chap. 34, p. 714–733.

References Cited 103
Burbank, W.S., and Luedke, R.G., 1969, Geology and ore 

deposits of the Eureka and adjoining districts, San Juan 

Mountains, Colorado: U.S. Geological Survey Professional 

Paper 535, 73 p.

Bush, A.L., Bromfield, C.S., and Pierson, C.T., 1959, Areal geology of the Placerville quadrangle, San Miguel County, Colorado: U.S. Geological Survey Bulletin 1072–E, p. 299–384.

CD–ROM Working Group, 2002, Structure and evolution of the 

lithosphere beneath the Rocky Mountains; Initial results from 

the CD–ROM experiment: GSA Today, v. 12, no. 3, p. 4–10.

Craig, L.C., Holmes, C.N., Cadigan, R.A., Freeman, V.L., 

Mullens, T.E., and Weir, G.W., 1955, Stratigraphy of the 

Morrison and related formations, Colorado Plateau region; 

a preliminary report: U.S. Geological Survey Bulletin 

1009–E, p. 125–168.

Cross, Whitman, 1904, A new Devonian formation in Colorado: American Journal of Science, 4th ser., v. 18, no. 106, 

p. 245–252.

Cross, Whitman, and Larsen, E.S., 1935, A brief review of the 

geology of the San Juan region of southwestern Colorado: 

U.S. Geological Survey Bulletin 843, 138 p.

Cross, Whitman, and Purington, C.W., 1899, Description of 

the Telluride quadrangle [Colorado]: U.S. Geological Survey Geologic Atlas, Folio 57, 18 p.

Cross, Whitman, and Spencer, A.C., 1900, Geology of the 

Rico Mountains, Colorado: U.S. Geological Survey, 21st 

Annual Report, Part II, p. 7–165.

Cross, Whitman, Howe, Ernest, and Irving, J.D., 1907, 

Description of the Ouray quadrangle [Colorado]: U.S. Geological Survey Geologic Atlas, Folio 153, 20 p.

Cross, Whitman, Howe, Ernest, and Ransome, F.L., 1905, 

Description of the Silverton quadrangle [Colorado]: U.S. 

Geological Survey Geologic Atlas, Folio 120, 34 p.

Cross, Whitman, Howe, Ernest, Irving, J.D., and Emmons, 

W.H., 1905, Description of the Needle Mountains quadrangle [Colorado]: U.S. Geological Survey Geologic Atlas, 

Folio 131, 14 p.

Cunningham, C.G., Naeser, C.W., Marvin, R.F., Luedke, R.G., 

and Wallace, A.R., 1994, Ages of selected intrusive rocks 

and associated ore deposits in the Colorado Mineral Belt: 

U.S. Geological Survey Bulletin 2109, 31 p.

Dawson, J.F., 1954, Place names in Colorado: Denver, Colo., 

The J. Frank Dawson Publishing Co., 54 p.

Dickinson, R.G., Leopold, E.B., and Marvin, R.F., 1968, Late 

Cretaceous uplift and volcanism on the north flank of the 

San Juan Mountains, Colorado, in Epis, R.C., ed., Cenozoic 

volcanism in the southern Rocky Mountains: Colorado 

School of Mines Quarterly, v. 63, no. 3, p. 125–148.

Dorset, P.F., 1970, The new Eldorado; the story of Colorado’s 

gold and silver rushes: New York, Macmillan, 434 p.

Downer, R.H., 1901, Ore deposits of the American-Nettie 

mine, Ouray, Colorado: Colorado School of Mines Bulletin, 

v. 1, no. 2, p. 104–107.

Downer, R.H., and DeCou, R.E., 1901, A description of the 

working mines of Ouray County, Colorado: Colorado 

School of Mines Bulletin, v. 1, no. 3, p. 242–259.

Eckel, E.B., 1949, Geology and ore deposits of the La Plata 

district, Colorado: U.S. Geological Survey Professional 

Paper 219, 179 p.

Endlich, F.M., 1889, The origin of the gold deposits near 

Ouray, Colorado: Engineering and Mining Journal, v. 48, 

no. 16, p. 335.

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

1968, Mineral resources of the Uncompahgre Primitive 

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

91 p.

Fossett, Frank, 1879, Colorado, its gold and silver mines, 

farms and stock ranges, and health and pleasure resorts; 

tourist’s guide to the Rocky Mountains: New York, C.G. 

Crawford, Printer and Stationers, 540 p.

Franczyk, K.J., 1993, Measured section of the Pennsylvanian 

Hermosa Group near Ouray, Colorado: U.S. Geological Survey Open-File Report 93–712, 25 p.

Goldman, M.I., and Spencer, A.C., 1941, Correlation of Cross’ 

La Plata sandstone, southwestern Colorado: American 

Association of Petroleum Geologists Bulletin, v. 25, no. 9, 

p. 1745–1767.

Hansen, W.R., and Peterman, Z.E., 1968, Basement-rock 

geochronology of the Black Canyon of the Gunnison, Colorado, in Geological Survey Research 1968: U.S. Geological 

Survey Professional Paper 600–C, p. C80–C90.

Harris, C.W., 1990, Polyphase suprastructure deformation in 

metasedimentary rocks of the Uncompahgre Group; remnant 

of an Early Proterozoic fold belt in southwest Colorado: Geological Society of America Bulletin, v. 102, no. 5, p. 664–678.

Harris, C.W., and Eriksson, K.A., 1990, Allogenic controls on 

the evolution of storm to tidal shelf sequences in the Early 

Proterozoic Uncompahgre Group, southwest Colorado, 

USA: Sedimentology, v. 37, no. 2, p. 189–213.

Henderson, C.W., 1926, Mining in Colorado: U.S. Geological 

Survey Professional Paper 138, 236 p.

Hewett, D.F., and Fleischer, Michael, 1960, Deposits of the 

manganese oxides: Economic Geology, v. 55, no. 1, p. 1–55.

Holland, H.D., and Malinin, S.D., 1979, The solubility and 

occurrence of non-ore minerals, in Barnes, H.L., ed., Geochemistry of hydrothermal ore deposits (3d ed.): New York, 

John Wiley and Sons, p. 461–508.

104 Geology and Ore Deposits of the Uncompahgre Mining District, Southwestern Colorado
Irvine, T.N., and Baragar, W.R.A., 1971, A guide to the chemical classification of the common volcanic rocks: Canadian 

Journal of Earth Sciences, v. 8, no. 5, p. 523–548.

Irving, J.D., 1905, Ore deposits of the Ouray district, Colorado, in

Contributions to Economic Geology 1904; Series A, Economic 

geology: U.S. Geological Survey Bulletin 260, p. 50–77.

Irving, J.D., 1911a, Replacement ore-bodies and the criteria 

for their recognition, Part I: Economic Geology, v. 6, no. 6, 

p. 527–561.

Irving, J.D., 1911b, Replacement ore-bodies and the criteria 

for their recognition, Part II: Economic Geology, v. 6, no. 7, 

p. 619–669.

Jochim, C.L., 1986, Debris-flow hazard in the immediate 

vicinity of Ouray, Colorado: Colorado Geological Survey 

Special Publication 30, 63 p.

Karlstrom, K.E., and Humphreys, E.D., 1998, Persistent influence of Proterozoic accretionary boundaries in the tectonic 

evolution of southwestern North America; interaction of 

cratonic grain and mantle modification events: Rocky 

Mountain Geology, v. 33, no. 2, p. 161–179.

Kelley, V.C., 1946, Geology, ore deposits, and mines of the Mineral Point, Poughkeepsie, and Upper Uncompahgre districts, 

Ouray, San Juan, and Hinsdale Counties, Colorado: Colorado 

Scientific Society Proceedings, v. 14, no. 7, p. 289–466.

Kindle, E.M., 1909, The Devonian fauna of the Ouray Limestone: U.S. Geological Survey Bulletin 391, 60 p.

King, W.H., and Allsman, P.T., 1950, Reconnaissance of metal 

mining in the San Juan region, Ouray, San Juan, and San 

Miguel Counties, Colorado: U.S. Bureau of Mines Information Circular 7554, 109 p.

Kirk, Edwin, 1931, The Devonian of Colorado: American 

Journal of Science, 5th ser., v. 22, no. 129, p. 222–240.

Knight, R.L., and Baars, D.L., 1957, New developments on 

age and extent of Ouray limestone: American Association of 

Petroleum Geologists Bulletin, v. 41, no. 10, p. 2275–2283.

Lakes, Arthur, 1901, The American Nettie: Mines and Minerals, v. 21, no. 6, p. 241–245.

Larsen, E.S., Jr., and Cross, Whitman, 1956, Geology and 

petrology of the San Juan region, southwestern Colorado: 

U.S. Geological Survey Professional Paper 258, 303 p.

Lasky, S.G., 1936, Hydrothermal leaching in the Virginia mining district, New Mexico: Economic Geology, v. 31, no. 2, 

p. 156–169.

Le Bas, M.J., Le Maitre, R.W., Streckeisen, A., and Zanettin, 

B., 1986, A chemical classification of volcanic rocks based 

on the total alkali-silica diagram: Journal of Petrology, 

v. 27, no. 3, p. 745–750.

Lindgren, Waldemar, 1933, Mineral deposits (4th ed.): New 

York, McGraw-Hill, 930 p.

Lipman, P.W., Steven, T.A., and Mehnert, H.H., 1970, 

Volcanic history of the San Juan Mountains, Colorado, as 

indicated by potassium-argon dating: Geological Society of 

America Bulletin, v. 81, no. 8, p. 2329–2352.

Lipman, P.W., Fisher, F.S., Mehnert, H.H., Naeser, C.W., 

Luedke, R.G., and Steven, T.A., 1976, Multiple ages of 

mid-Tertiary mineralization and alteration in the western 

San Juan Mountains, Colorado: Economic Geology, v. 71, 

no. 3, p. 571–588.

Lovering, T.G., 1972, Jasperoid in the United States; its characteristics, origin, and economic significance: U.S. Geological Survey Professional Paper 710, 164 p.

Lovering, T.G., Lakin, H.W., and Hubert, A.E., 1968, Concentration and minor element association of gold in orerelated jasperoid samples, in Geological Survey Research 

1968: U.S. Geological Survey Professional Paper 600–B, 

p. B112–B114.

Luedke, R.G., 1972, Geologic map of the Wetterhorn Peak 

quadrangle, Colorado: U.S. Geological Survey Geologic 

Quadrangle Map GQ–1011, scale 1:24,000.

Luedke, R.G., and Burbank, W.S., 1962, Geology of the Ouray 

quadrangle, Colorado: U.S. Geological Survey Geologic 

Quadrangle Map GQ–152, scale 1:24,000.

Luedke, R.G., and Burbank, W.S., 1981, Geologic map of the 

Uncompahgre (Ouray) mining district, southwestern Colorado: U.S. Geological Survey Miscellaneous Investigations 

Series Map I–1247, scale 1:12,000.

Luedke, R.G., and Burbank, W.S., 1994, Measured stratigraphic sections in the Ouray area, western San Juan Mountains, Colorado: U.S. Geological Survey Open-File Report 

94–583, 28 p.

McKnight, E.T., 1974, Geology and ore deposits of the Rico 

district, Colorado: U.S. Geological Survey Professional 

Paper 723, 100 p.

Merrill, W.M., and Winar, R.M., 1958, Molas and associated formations in San Juan Basin-Needle Mountains area, 

southwestern Colorado: American Association of Petroleum 

Geologists Bulletin, v. 42, no. 9, p. 2107–2132.

O’Sullivan, R.B., 1992, The Jurassic Wanakah and Morrison 

Formations in the Telluride-Ouray-western Black Canyon 

area of southern Colorado: U.S. Geological Survey Bulletin 

1927, 24 p.

Pearce, Richard, 1890, The association of gold with other 

metals in the West: American Institute of Mining Engineers 

Transactions, v. 18, p. 447–457.

References Cited 105
Plumb, K.A., 1991, New Precambrian time scale: Episodes, 

v. 14, no. 2, p. 139–140.

Ransome, F.L., 1901, A peculiar clastic dike near Ouray, 

Colorado, and its associated deposit of silver ore: American 

Institute of Mining Engineers Transactions, v. 30, p. 227–236.

Rice, F.A., 1980, The mines of Ouray County: Ouray, Colo., 

Bear Creek Publishing Co., 56 p.

Richmond, G.M., 1954, Modification of the glacial chronology of the San Juan Mountains, Colorado: Science, v. 119, 

no. 3096, p. 614–615.

Shapiro, Leonard, 1967, Rapid analysis of rocks and minerals 

by a single-solution method, in Geological Survey Research 

1967: U.S. Geological Survey Professional Paper 575–B, 

p. B187–B191.

Spencer, A.C., 1900, Devonian strata in Colorado: American 

Journal of Science, 4th ser., v. 9, no. 50, p. 125–133.

Spoelhof, R.W., 1976, Pennsylvanian stratigraphy and paleotectonics of the western San Juan Mountains, southwestern 

Colorado, in Epis, R.C., and Weimer, R.J., eds., Studies in 

Colorado field geology: Colorado School of Mines Professional Contributions No. 8, p. 159–179.

Spurr, J.E., 1923, The ore magmas; a series of essays on ore 

deposition: New York, McGraw-Hill, v. 1, p. 1–430, and 

v. 2, p. 431–915.

Steele, B.A., 1985, Preliminary report on and measured sections of the middle Jurassic Entrada Sandstone and Wanakah 

Formation near Placerville, southwestern Colorado: U.S. 

Geological Survey Open-File Report 85–446, 55 p.

Steven, T.A., and Hail, W.J., Jr., 1989, Geologic map of the 

Montrose 30′ × 60′ quadrangle, southwestern Colorado: 

U.S. Geological Survey Miscellaneous Investigations Series 

Map I–1939, scale 1:100,000.

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

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

contiguous to the Uncompahgre Primitive Area, San Juan 

Mountains, southwestern Colorado, with a section on Interpretation of aeromagnetic data, by Peter Popenoe and R.G. 

Luedke: U.S. Geological Survey Bulletin 1391–E, 126 p.

Strahan, R.E., 1881, Gunnison and San Juan, southwestern Colorado: Omaha, Nebr., The New West Publishing Co., 48 p.

Streckeisen, Albert, 1976, To each plutonic rock its proper 

name: Earth-Science Reviews, v. 12, no. 1, p. 1–33.

Taylor, H.P., Jr., 1974, Oxygen and hydrogen isotope evidence 

for large-scale circulation and interaction between ground 

waters and igneous intrusions, with particular reference to 

the San Juan volcanic field, Colorado: Carnegie Institute of 

Washington Publication 634, p. 299–324.

Taylor, H.P., Jr., 1997, Oxygen and hydrogen isotope relationships in hydrothermal mineral deposits, in Barnes, H.L., ed., 

Geochemistry of hydrothermal ore deposits (3d ed.): New 

York, John Wiley and Sons, p. 229–302.

Tewksbury, B.J., 1985, Revised interpretation of the age of 

allochthonous rocks of the Uncompahgre Formation, Needle Mountains, Colorado: Geological Society of America 

Bulletin, v. 96, no. 2, p. 224–232.

Tewksbury, B.J., 1989, Proterozoic geology of the Needle 

Mountains; a summary, in Grambling, J.A., and Tewksbury, B.J., eds., Proterozoic geology of the southern Rocky 

Mountains: Geological Society of America Special Paper 

235, p. 65–73.

Tweto, Ogden, 1987, Rock units of the Precambrian basement 

in Colorado: U.S. Geological Survey Professional Paper 

1321–A, 54 p.

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

the Colorado Mineral Belt: Geological Society of America 

Bulletin, v. 74, no. 8, p. 991–1014.

U.S. Bureau of Mines, 1927–1934, Mineral resources of the 

United States 1924–1931: Washington, D.C., U.S. Government Printing Office, variously paginated.

U.S. Bureau of Mines, 1933–1988, Minerals yearbook 

1932–33 to 1986: Washington, D.C., U.S. Government 

Printing Office, variously paginated. (Also available online 

at http://minerals.usgs.gov/minerals/pubs/usbmmyb.html.)

U.S. Bureau of the Mint, 1882, Report of the Director of the 

Mint upon the statistics of the productions of the precious 

metals in the United States [for the year 1881]: Washington, 

D.C., U.S. Government Printing Office, p. 419.

U.S. Bureau of the Mint, 1882–1893, Report of the Director of 

the Mint upon the [statistics of the] production of the precious metals in the United States [for the years 1881–1892]: 

Washington, D.C., U.S. Government Printing Office, variously paginated.

U.S. Geological Survey, 1906–1927, U.S. Geological Survey 

mineral resources of the United States 1905–1924: Washington, D.C., U.S. Government Printing Office, variously 

paginated.

Vanderwilt, J.W., and others, 1947, Mineral resources of Colorado: Denver, Colo., Mineral Resources Board, 547 p.

Vhay, J.S., 1962, Geology and mineral deposits of the area 

south of Telluride, Colorado: U.S. Geological Survey Bulletin 1112–G, p. 209–310.

Walker, R.T., 1928, Deposition of ore in pre-existing limestone 

caves: The American Institute of Mining and Metallurgical 

Engineers Technical Publication 154, 43 p.

106 Geology and Ore Deposits of the Uncompahgre Mining District, Southwestern Colorado
Weimer, R.J., 1980, Recurrent movement on basement faults, 

a tectonic style for Colorado and adjacent areas, in Kent, 

H.C., and Porter, K.W., eds., Colorado geology: Rocky 

Mountain Association of Geologists, Symposium on Colorado Geology, Denver, Colo., 1980, p. 23–35.

Wengerd, S.A., 1962, Pennsylvanian sedimentation in Paradox 

Basin, Four Corners region, in Branson, C.C., ed., Pennsylvanian system in the United States, a symposium: Tulsa, Okla., 

American Association of Petroleum Geologists, p. 264–330.

Wengerd, S.A., and Matheny, M.L., 1958, Pennsylvanian 

system of Four Corners region: American Association of 

Petroleum Geologists Bulletin, v. 42, no. 9, p. 2048–2106.

Wolle, M.S., 1949, Stampede to Timberline; the ghost towns 

and mining camps of Colorado: Boulder, Colo., M.S. Wolle, 

544 p.

References Cited 107