Mineralogy, Mineral Chemistry, and 

Paragenesis of Gold, Silver, and Base-Metal 

Ores of the North Amethyst Vein System, 

Sari Juan Mountains, Mineral County, 

Colorado

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Mineralogy, Mineral Chemistry, and 

Paragenesis of Gold, Silver, and Base-Metal 

Ores of the North Amethyst Vein System, 

San Juan Mountains, Mineral County, 

Colorado

By NORA K. FOLEY, STANTON W. CADDEY, CRAIG B. BYINGTON, and 

DAVID M. VARDIMAN

U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1537

A study of progressive vein-related 

mineralization of structures in volcanic rocks 

of the San Luis and Bachelor calderas of the 

San Juan volcanic field of southwestern 

Colorado and its bearing on the economic 

potential of the area

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON: 1993
U.S. DEPARTMENT OF THE INTERIOR 

BRUCE BABBITT, Secretary

U.S. GEOLOGICAL SURVEY 

Dallas L. Peck, Director

Any use of trade, product, or firm names in this publication is for

descriptive purposes only and does not imply endorsement by the

U.S. Government

Library of Congress Cataloging in Publication Data

Mineralogy, mineral chemistry, and paragenesis of gold, silver, and base-metal ores of the North Amethyst vein system, San 

Juan Mountains, Mineral County, Colorado / by Nora K. Foley . . . [et al.]. 

p. cm. — (U.S. Geological Survey professional paper ; 1537) 

Supt.ofDocs.no.: 119.16:1537 

1. Gold ores—Geology—Colorado—Mineral County. 2. Ore deposits—Colorado—Mineral County. I. Foley, N.K. II.

Series.

QE390.2.G65M56 1993

553.41 '09788'38-dc20 92-40595

CIP

For sale by U.S. Geological Survey, Map Distribution

Box 25286, Bldg. 810, Federal Center

Denver, CO 80225
CONTENTS

Page

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

Introduction....................................................................... 1

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

Geologic Setting................................................................. 3

Age of Mineralization .................................................... 4

Previous Studies................................................................. 5

Style of Mineralization......................................................... 5

Character of the Ore Zones ............................................ 6

Nature of the Veins ...................................................... 6

Mineral Textures.......................................................... 6

Vein Mineralogy and Compositional Ranges ............................ 7

Ag-Sulfide and Ag-Cu-Sulfide Minerals ............................ 7

Barite......................................................................... 7

Bournonite.................................................................. 7

Carbonate Group Minerals ............................................. 9

Chalcopyrite................................................................ 11

Chlorite ...................................................................... 12

Electrum..................................................................... 13

Fluorite...................................................................... 15

Galena........................................................................ 15

Gypsum...................................................................... 16

Hematite..................................................................... 16

Magnetite.................................................................... 17

Manganese Silicate Minerals........................................... 17

Marcasite.................................................................... 18

Polybasite................................................................... 18

Potassium Feldspar....................................................... 18

Pyrargyrite-Proustite.................................................... 18

Pyrite......................................................................... 19

	Page 

Vein Mineralogy and Compositional Ranges—Continued

Quartz........................................................................ 19

Silver......................................................................... 19

Sphalerite.................................................................... 19

Stephanite................................................................... 20

Tetrahedrite-Tennantite................................................. 20

Uytenbogaardtite......................................................... 20

Discussion of Mineral Equilibria............................................ 21

Gold and Silver Sulfides................................................. 21

Carbonate Minerals....................................................... 21

Manganese Silicate Equilibria......................................... 21

Mineral Associations and Assemblages ................................... 22

Alteration and Pre-Ore Vein Assemblages ........................ 22

Sericitic Alteration.................................................. 22

Potassium Metasomatism and Bleaching..................... 23

Manganese-Gold Association........................................... 25

Stage Alpha........................................................... 26

Stage Beta ............................................................ 26

Transitional Breccias and Sediments ................................ 28

Breccia-2............................................................... 28

Base-Metal-Silica Association......................................... 29

Stage 1................................................................. 30

Stage 2................................................................. 32

Stage 3................................................................. 32

Correlation of Mineral Assemblages With Other Vein

Systems......................................................................... 32

Conclusions........................................................................ 37

References Cited................................................................ 37

ILLUSTRATIONS

FIGURE 1. Map of the San Juan volcanic field, southwestern Colorado .......................................................................................... 2

2. Generalized geologic map of Creede mining district and vicinity ................................................................................... 3

3. Plan map showing the levels of the North Amethyst vein system ................................................................................. 5

4. Photographs of North Amethyst Mn- and Au-bearing vein assemblages......................................................................... 8

5. Photographs of North Amethyst base-metal-bearing vein assemblages........................................................................... 10

6. Cross-sectional view of mine workings, North Amethyst vein system............................................................................ 11

7. Ternary diagram showing compositions of carbonates for three stages of the North Amethyst vein mineralization................ 16

8. CaC03-MnC03 binary diagram showing proposed solvii from experimental studies and from studies of naturally

occurring carbonates ....................................................................................................................................... 17

9. Histogram of gold content of electrum occurring in stage-beta assemblages.................................................................... 17

10. Ternary diagram showing compositions of all pyroxmangite and rhodonite analyzed for stage-alpha mineralization................ 18

11-22. Diagrams showing:

11. Progression of events in the development of the North Amethyst ores.............................................................. 23

12. Composite paragenesis for the North Amethyst mineralization......................................................................... 24

in
IV CONTENTS

	Page

13. Early vein assemblages ................................................................................................................................ 25

14. Stages alpha and beta .................................................................................................................................. 27

15. Two periods of Mn-carbonate and Mn-silicate mineralization of stage alpha........................................................... 28

16. Relation between beta-1 and beta-2 substages.................................................................................................. 29

17. Breccia and breccia fragments ....................................................................................................................... 30

18. Relation of stage-1 minerals to all preceding stages and breccias......................................................................... 31

19. Local development of breccia-3 ...................................................................................................................... 32

20. Reopening of vein and deposition of stage-2 minerals on preexisting mineral stages and breccias.............................. 33

21. Development of breccia cemented by pink calcite.............................................................................................. 34

22. Development of stage-3 breccia...................................................................................................................... 35

TABLES

TABLE 1. Radiometric ages of lavas, ash-flow sheets, and mineralized veins of the central caldera cluster of the San Juan

	Mountains, Colo ............................................................................................................................................... 4

2. Stages of mineralization and brecciation events of the North Amethyst vein system.......................................................... 6

3. Assemblages and minerals of the North Amethyst vein system...................................................................................... 12

4. Locations and descriptions of samples from the North Amethyst vein system analyzed by electron microprobe...................... 13

5. Representative electron microprobe analyses of carbonate minerals from the North Amethyst vein system........................... 14

6. Representative electron microprobe analyses of electrum from the North Amethyst vein system......................................... 14

7. Representative electron microprobe analyses of Mn-silicate minerals from the North Amethyst vein system ......................... 15

8. Representative electron microprobe analyses of sphalerite from the North Amethyst vein system ....................................... 16

9. Districtwide correlation of mineral assemblages and brecciation events............................................................................ 36
MINERALOGY, MINERAL CHEMISTRY, AND PARAGENESIS OF GOLD,

SILVER, AND BASE-METAL ORES OF THE NORTH AMETHYST VEIN

SYSTEM, SAN JUAN MOUNTAINS, MINERAL COUNTY, COLORADO

By NORA K. FOLEY/ STANTON W. CADDEY,2 CRAIG B. BYINGTON,S and DAVID M. VARDIMAN*

ABSTRACT

Gold-rich adularia-sericite-type mineralization occurs near the southern margin of the San Luis caldera, at the intersection of the Equity 

fault and the northern extension of the Amethyst fault system. 

Mineralized rock is confined primarily to steeply dipping structures in 

silicified rhyolite and dacite. Intense sericitic alteration occurs at 

higher levels in the vein system, and wall rock adjacent to some veins 

is bleached. The ores are multiply brecciated, and vein filling locally 

shows sedimentary textures.

Textural, mineralogical, and chemical criteria indicate that there are 

at least two partially coextensive associations of mineral assemblages 

separated by periods of brecciation and sedimentation. An older 

gold-bearing association consists of two fine-grained ore stages, both of 

which contain electrum, uytenbogaardtite, tetrahedrite, silver sulfosalts, silver sulfides and base-metal sulfides, and a manganese-rich 

stage containing the assemblages (1) manganese silicate + manganese 

carbonate minerals + quartz and (2) magnetite + hematite + pyrite + 

quartz. A younger crosscutting association contains calcite, adularia, 

fluorite, and quartz, plus the assemblages (1) coarse-grained basemetal sulfides and (2) hematite + chlorite + quartz. Quartz, 

manganese-rich calcite, and trace pyrite line late-stage vugs.

Mineralogic, lead-isotopic, and fluid-inclusion characteristics of the 

younger association are similar to those of ores of the southern and 

central parts of the Creede mining district. In contrast, the gold and 

manganese-silicate assemblages of the older association are rare to 

absent in the southern and central parts of the district. The local and 

early occurrence of the manganese and gold assemblages may indicate 

that they formed in a small hydrothermal cell that predated the 

extensive hydrothermal system from which ores of the central and 

southern parts of the Creede district are proposed to have been 

deposited (Bethke, 1988). If similar early-stage cells were present in 

the southern and central parts of the district, they may have been 

replaced or overprinted by later assemblages, and they may remain to 

be discovered. In the latter case, mineral assemblages that formed at 

early stages in the paragenesis hold the most promise for gold 

exploration.

INTRODUCTION

The North Amethyst vein system is located just south 

of the Continental Divide along the southern margin of

Manuscript approved for publication October 14, 1992. 

1U.S. Geological Survey.

2Homestake Mining Company, Golden CO 80401. 

3Homestake Mining Company, Lead, SD 57754.

the San Luis caldera in Mineral County, Colo. (fig. 1). 

The precious- and base-metal-bearing veins fill structures of the east-west-trending reverse Equity fault and 

steeply dipping structures related to an extension of the 

north-northwest-trending normal Amethyst fault system 

(fig. 2). These structures cut rhyolitic and rhyodacitic 

volcanic tuffs and flows of the central caldera cluster of 

the San Juan volcanic field. The veins lie just inside the 

northern boundary of the historic Creede mining district.

Homestake Mining Company discovered the Aubearing ores and delineated an extensive high-grade 

zone adjacent to old workings of the near-surface Equity 

mine. Available published geologic descriptions of the 

mineralized rocks of the North Amethyst vein system 

are limited mainly to historical reports on the Equity 

mine (Emmons and Larsen, 1913; 1923) and some preliminary studies of the newly discovered ores (S.W. 

Caddey and C.B. Byington, oral commun., 1988; Plumlee 

and others, 1989).

This paper, part of a detailed investigation of the 

petrology of the complex ores of the North Amethyst 

vein system, concentrates on the mineralogy of the 

Au-rich areas exposed by underground workings. The 

mineralogy, mineral chemistry, and paragenesis of the 

ore of the North Amethyst veins, with limited petrologic 

interpretation, are presented here. A comparison is 

made with Ag- and base-metal-rich mineralization of the 

southern Amethyst, Bulldog Mountain, P, and OH veins 

of the main Creede mining district. Only a limited 

amount of petrologic interpretation is included in this 

paper; instead, the paper focuses on the mineralogy and 

stage assemblages. The petrology, fluid-inclusion thermometry, and lead-isotopic geochemistry of the vein 

system are described by Foley (1990).

ACKNOWLEDGMENTS

Homestake Mining Company discovered the veins that 

constitute the North Amethyst system in 1983 as the
ORES OF THE NORTH AMETHYST VEIN SYSTEM, COLORADO 

108° 107°

38° -

10

I

20

I

30 40

I I

50 KILOMETERS

FIGURE 1. —The extent of the major ash-flow tuffs and associated calderas in the San Juan volcanic field (SJVF) in 

southwestern Colorado. From Plumlee (1989), after Steven and Lipman (1976), and incorporating changes suggested 

by Lipman and Sawyer (1988). The box in the center outlines the area of figure 2, a map of the Creede mining district 

and vicinity.

result of an extensive drilling program (S.W. Caddey and 

C.B. Byington, oral commun., 1988; Plumlee and others, 

1989). Homestake Mining Company generously provided 

access to exploratory workings of the North Amethyst 

vein and the opportunity to study some aspects of the 

newly discovered mineralization. Stanton W. Caddey, 

Craig B. Byington, and David M. Vardiman, Homestake 

Mining Company geologists who discovered and developed the veins, contributed their time and their knowledge of the underground workings. They provided access 

to samples collected throughout the development of the 

underground workings and therefore improved the interpretations of hand-sample textures and vein relations.

The senior author would like to thank Paul B. Barton, 

Jr., and Philip M. Bethke of the U.S. Geological Survey 

(USGS) for suggesting that she undertake this project

and for providing an initial set of samples. Geoffrey S. 

Plumlee of the USGS also contributed samples and took 

some of the underground photographs. Careful reviews 

of this manuscript by Robert A. Ayuso (USGS), Paul B. 

Barton, Jr., Philip M. Bethke, James R. Craig (Viginia 

Polytechnic Institute and State University), Geoffrey S. 

Plumlee, and Gilpin R. Robinson, Jr. (USGS), have 

improved both its content and presentation. Much of this 

manuscript constitutes a part of the senior author's 

Ph.D. dissertation. Conversations with H.E. Belkin, 

J.M. Hammarstrom, D.O. Hayba, Pamela Heald, J.S. 

Huebner, M.J.K. Flohr, P.M. Okida, J.F. Slack, and 

D.S. Sweetkind, all colleagues at the USGS, were invaluable throughout the duration of the study. J. J. McGee 

and J.V. Emery provided guidance in use of the electron 

microprobe.
GEOLOGIC SETTING

37° 55' —

37° 50' —

North Amethyst 

vein system San Luis 

caldera

EXPLANATION

Quartz latite intrusive 

Quartz latite flows 

Creede Formation 

Disseminated mineralization

Amethyst fault

Fault-dashed where inferred; 

Midwest-Knoess vein stippled where mineralized

Bachelor \

caldera

Solomon-Holy 

Moses vein

Mountain veins

Alpha-Corsair 

vein

Creede 

caldera

FIGURE 2.—Generalized geology of the Creede mining district and vicinity. The graben that is formed by the faults is within the Bachelor 

caldera and links the San Luis and Creede calderas. The North Amethyst vein system is at the intersection of the Equity fault and the 

Amethyst fault. From Plumlee (1989); geology after Bethke and Lipman (1987) and Lipman and Sawyer (1988).

GEOLOGIC SETTING

The San Juan volcanic field is an erosional remnant of 

a much larger volcanic field that covered part of the 

southern Rocky Mountains in the middle Tertiary 

(Steven, 1975; Lipman and others, 1978). The present 

extent of the major ash-flow tuffs and their associated 

calderas is shown in figure 1. The volcanic field rests 

upon an eroded base consisting of crystalline metavolcanic, metasedimentary, and granitiod rocks of Proterozoic age, and, possibly, remnants of Paleozoic, Mesozoic, 

and Tertiary sedimentary rocks (summarized by Steven 

and Lipman, 1976; Lipman and others, 1978; Baars and 

Stevenson, 1984; Gries, 1985; Bickford, 1988). The composite volcanic field consisted dominantly of andesitedacite lavas and associated volcaniclastic rocks erupted 

from precaldera-stage volcanoes in early Oligocene time, 

mainly between 35 and 30 Ma. A large negative Bouguer 

gravity anomoly underlies much of the San Juan volcanic 

field and is believed to reflect a large composite batholith 

emplaced beneath the field (Plouff and Pakister, 1972). 

From about 28 to 26 Ma (table 1), the central San Juan 

volcanic field experienced widespread silicic ash-flow 

eruptions and related caldera subsidence (Lipman and 

others, 1970, 1978; Lipman and Sawyer, 1988; Lanphere, 

1988). The mineral deposits of the Creede district are 

located in a complex set of nested calderas formed by the 

pyroclastic eruptions.

The most volumetrically important sediments that 

accumulated within the San Juan area during the middle
OEES OF THE NOETH AMETHYST VEIN SYSTEM, COLORADO

TABLE l.—Radiometric ages of lavas, ash-flow sheets, and mineralized veins of the central caldera cluster of the San Juan Mountains, Colo.,

given in generalized chronological order

Volcanic rock or vein mineral Age (Ma) Caldera Reference

OH vein illite-smectite............................... 24.8±1.3.

North Amethyst vein adularia....................... 25.1±0.1.

OH vein adularia .................................... 25.1±0.6.

OH vein sericite ..................................... 25.3±1.4.

OH vein sericite ..................................... 25.5±0.8.

OH vein sericite ..................................... 25.7±1.2.

Quartz latite of Baldy Cinco, equivalent to 25.7±0.1..

volcanics of Stewart Peak. 

Nelson Mountain Tuff................................ 26.1±0.1..

Rat Creek Tuff...................................... 26.4±0.1.,

Regional K-metasomatic event....................... 26.4±1.3.,

Snowshoe Mountain Tuff............................. 26.8±0.1.,

Wason Park Tuff .................................... 27.1±0.2.,

Campbell Mountain welding zone of Bachelor 27.2±0.2.,

Mountain Member of Carpenter Ridge Tuff. 

Carpenter Ridge Tuff................................ 27.6±0.2..

Fish Canyon Tuff.................................... 27.8±0.2.,

Masonic Park Tuff................................... 28.2±0.2..

Bachelor. 

San Luis. 

Bachelor. 

Bachelor. 

Bachelor. 

Bachelor. 

San Luis.

San Luis. 

San Luis.

Creede......

South River 

Bachelor....

Bachelor.

La Garita... 

Mount Hope

Vergo (1987).

Lanphere (1987, 1988).

Bethke and others (1976). *

Bethke and others (1976). l

Bethke and others (1976). l

Bethke and others (1976). 1

M. Lanphere (written commun., 1989).

Lanphere (1987, 1988). 

Lanphere (1987, 1988). 

Vergo (1987). 

Lanphere (1987, 1988). 

Lanphere (1987, 1988). 

Vergo (1987).

Lanphere (1987, 1988); Hurford and

Hammerschmidt (1985). 

Lanphere (1987, 1988). 

Lanphere (1987, 1988).

1Revised by Hon and Mehnert (1983) using new IUGS constants.

Tertiary are those that were deposited in the moat of the 

Creede caldera and in channels incised in the caldera 

wall. They constitute the Creede Formation and consist 

of stream-channel gravels, landslide breccias, travertines, bedded lacustrine sediments, and water-laid tuffs 

(Steven and Ratte, 1965). Thin, layered sediments interbedded with volcanic units have also been found in the 

north-central part of the district by Chevron Resources, 

Inc., and Homestake Mining Company (James O'Brient, 

written commun., 1982; S.W. Caddey and C.B. Byington, oral commun., 1983). Within the San Luis caldera, 

fluvial and lakebed sediments (a few tens of meters thick) 

that accumulated in small depositional basins overlie the 

Equity Member of the Nelson Mountain Tuff.

The complex history of movement and reactivation 

along faults of the Creede graben has been described in 

detail by Steven and Ratte (1965). Normal faults are the 

dominant structural element controlling the location of 

mineralized rock in the central San Juan area, and most 

are related to evolution of the Oligocene calderas (fig. 2). 

The reverse Equity fault, which formed by fault-bounded 

uplift of a triangular block presumably related to a 

shallow underlying intrusion (Lipman and Sawyer, 

1988), is also mineralized. The dominant structural features are the well-formed caldera expressions and the 

north-northwest-trending Creede graben, which initially 

formed as a keystone graben in the Bachelor caldera (fig. 

1). The major faults from west to east include the AlphaCorsair, Bulldog Mountain, Amethyst, Midwest-Knoess, 

and Solomon-Holy Moses faults. The OH, P, and other 

minor faults cut the central block of the graben, are

nearly vertical, and had primarily strike-slip motion 

(S.W. Caddey, written commun., 1985).

AGE OF MINERALIZATION

A single radiometric age determination (table 1) on 

adularia (40Ar/39Ar incremental heating ages) from drillcore from the North Amethyst vein system indicates an 

age of 25.1 ±0.1 Ma (M.A. Lanphere, written commun., 

1987). The adularia is found with base-metal minerals of 

a late-stage hydrothermal event that occurred along the 

North Amethyst vein system. The Au-bearing ores 

predate this base-metal and vein adularia stage (Foley 

and Vardiman, 1988), so their age falls in the interval 

between that of the youngest host rock, which is the 

Nelson Mountain Tuff, at 26.1 Ma, and the vein adularia, 

at 25.1 Ma. At least five volcanic units were deposited as 

part of the San Luis caldera complex after deposition of 

the oldest host rock, which is the Carpenter Ridge Tuff, 

at 27.6 Ma. These include the Rat Creek Tuff at 26.4 Ma, 

the Nelson Mountain Tuff at 26.1 Ma, tuff of Cathedral 

Peak (undated), rhyolite of Mineral Mountain (undated), 

and volcanic rocks of Stewart Peak (undated). The 

youngest volcanic rock in the area is quartz latite of 

Baldy Cinco (25.7 Ma), a unit equivalent to the volcanic 

rocks of Stewart Peak in the San Luis quadrangle 

(Lipman and Sawyer, 1988). Radiometric ages of gangue 

associated with ore minerals from the main Creede 

mining district indicate that those minerals were deposited at about 25 Ma (table 1); this deposit is generally 

concurrent with base-metal mineralization of the North 

Amethyst veins.
STYLE OF MINERALIZATION

10250L

Northern Amethyst fault

N

Equity fault, 10250L

10880L

11024L Equity fault

Portal

10000

10750L

Northern Amethyst fault 10325sL

J0565L J

25m

25m

1009011 10120L

FIGURE 3.—Plan map showing the levels of the North Amethyst vein system along the northern Amethyst and Equity faults. Sample

localities are shown by solid dots; localities off the vein are on the decline.

PREVIOUS STUDIES

Mineralized rock in the central part of the San Juan 

area has been the subject of numerous reports and 

papers since the area was first mined in the early 1880's. 

The relationship of the mineralization to regional geologic setting was first described in detail by Steven and 

Ratte (1960, 1965) and Steven (1968). The area in and 

around the Creede mining district has been the subject of 

continuing research since the late 1950's by the USGS 

and by several universities. Their work has been published in a series of papers on the Creede district (Steven 

and Eaton, 1975; Bethke and others, 1976; Roedder, 

1977; Barton and others, 1977; Wetlaufer, 1977; Bethke 

and Rye, 1979; Hayba and others, 1985; Plumlee, 1989; 

Foley, 1990; Foley and others, 1990). Historic accounts of 

ores of the Equity mine and a summary of preliminary

reports of the more recently discovered North Amethyst 

ores are described in Foley (1990).

STYLE OF MINERALIZATION

The vertical and lateral extent, and overall shape, of 

ore occurrences of the North Amethyst vein system (fig. 

3) is not well delineated because of the complex nature of 

the mineralized rock and the complex structural character of the veins. A detailed structural analysis of the 

North Amethyst vein system is necessary to resolve 

composite vein and structure relations. Some general 

observations, however, can be made relative to ore zones 

of the Bulldog and Amethyst vein systems of the main 

mining district at Creede as described by Plumlee (1989) 

and to ore zones of the OH and P veins.
ORES OF THE NORTH AMETHYST VEIN SYSTEM, COLORADO

TABLE 2.—Stages of mineralization and brecciation events of the 

North Amethyst vein system

[Abbreviations: », much greater than; >, greater than; =, approximately 

equal; { }, distribution unknown, local occurrence; [ ], molds]

Stage North Amethyst vein mineral assemblage

Base-metal-silica association:

3 Mn-calcite » quartz > pyrite

Breccia-4 Mn-calcite cement, fragments of earlier assemblages, breccias and minerals [wall rock]

2b Quartz > calcite > rhodocrosite > adularia > 

fluorite > chlorite = pyrite

Breccia-3 Quartz + chlorite cement, fragments of wall rock, 

and coarse stage-1 sulfides

Ib Quartz > sphalerite > galena > pyrite > chalcopyrite > tetrahedrite

la Quartz > pyrite = hematite > chlorite

Breccia-2 Quartz cement, fragments of stages alpha and beta, 

wall rock, sedimented structures, black quartz 

breccia

Mn-Au association:

Beta-2 Sphalerite > galena > chalcopyrite > pyrite > 

tetrahedrite > Ag- and Au-bearing sulfide minerals > electrum {magnetite, hematite}

Breccia-1 Quartz cement, fragments of stages alpha and beta

Beta-1 Sphalerite > galena > chalcopyrite > pyrite > 

tetrahedrite > Ag- and Au-bearing sulfide minerals > electrum {magnetite, hematite}

Alpha Rhodocrosite = quartz > Mn-calcite = rhodonite 

> K-feldspar » sphalerite > pyrite > galena > 

[barite]

Mineralized rock of the North Amethyst vein system 

fills structures related to the northern Amethyst and 

Equity faults. The North Amethyst veins are mineralogically complex, and the assemblages can be divided 

into two contrasting associations: a Mn-Au association 

and a base-metal-silica association. The minerals found 

in each association are grouped in table 2 according to the 

stages in which they are typically found and are discussed in greater detail herein. Both mineral associations 

occur along almost the entire extent of the underground 

workings, a distance of roughly 600 m along strike of the 

vein. The base-metal association is displayed over a 

vertical extent of about 150 m, whereas minerals of the 

Mn-Au association occur over an apparently larger vertical interval of 600 m.

CHARACTER OF THE ORE ZONES

Ore zones of the main mining district typically are of 

much greater horizontal than vertical extent. For example, the ore zone comprising the Bulldog Mountain vein 

system was mined for approximately 3 km laterally and 

200 to 300 m vertically (Plumlee, 1989). The combined 

productive ore zones of the central and southern Amethyst veins were of similar vertical extents and of even 

greater lateral extents. Mineralized zones of the OH and 

P structures delineated by mine workings are approximately 2,100 by 275 m and 975 by 225 m, respectively.

Only the dominantly base-metal-rich assemblages are 

found in core drilled just south of the North Amethyst 

underground workings. Farther to the south, the basemetal assemblages also occur, at least discontinuously, 

along much of the Amethyst fault (D.S. Sweetkind, 

written commun., 1990). If these assemblages are 

coeval, the base-metal minerals of the northern Amethyst structure may have lateral and vertical extents 

more like those of the main mining district.

In contrast to the base-metal-rich assemblages, the 

complete Au-bearing assemblages of the North Amethyst system are not found in drillcore to the south of the 

North Amethyst workings, nor are they reported to 

occur in underground workings. (As discussed in the 

section "Electrum," isolated occurrences of electrum 

have been reported in ore of the main mining district.) 

These findings suggest that the Au-bearing assemblages 

may have a more restricted lateral extent compared to 

ore zones of the main mining district.

NATURE OF THE VEINS

The veins of the North Amethyst system vary in width 

and mineral content because of their complex structural 

character and the high degree of structural activity 

contemporaneous with, and occurring after, mineralization. Veins can vary from a few centimeters to greater 

than 2 m in width, and they contain multiple assemblages 

that record different stages of mineralization.

Most vein assemblages are brecciated, and some vein 

fillings show sedimentary textures. Fracturing events 

are recorded in crosscutting vein relations and in breccias that developed at sites of intense structural activity. 

Multiple periods of movement resulted in many generations of breccias with characteristic fragment types and 

cements; these can be used to demonstrate timing relations.

MINERAL TEXTURES

Minerals were deposited in a number of stages that are 

characterized by distinctive textural styles. Extensive 

brecciation and recrystallization overprint the earlier 

stages, but a striking textural contrast between stages of 

the earlier, Mn-Au association and those of the later, 

base-metal association is retained in many veins.

The minerals of the Mn-Au association were deposited 

in two stages that show a variety of textures (fig. 4), The 

earliest stage of sulfide-bearing mineralization, desig-
VEIN MINERALOGY AND COMPOSITIONAL RANGES

nated here as stage alpha, was deposited primarily in 

open fractures, in some cases around preexisting barite 

and quartz crystals. The cryptocrystalline material 

shows a faint but clearly visible mineralogic banding on 

aged surfaces that is due to manganese oxidation. Stage 

alpha also shows some evidence for replacement of 

volcanic wall rock. Embayed remnant feldspar grains are 

caught up in a fine-grained "frozen mush" consisting 

dominantly of rhodocrosite, manganese pyroxenoids and 

other carbonate minerals, and quartz. The second stage 

of the Mn-Au association, stage beta, consists of finegrained precious- and base-metal-sulfide-rich seams that 

cut and recrystallized the stage-alpha minerals.

The later, base-metal-silica association minerals filled 

open spaces in fractures and breccias (fig. 5). Although 

many veins filled completely, some large vugs lined with 

crystals of the latest stages remained open. Minerals of 

this association are generally coarser grained than those 

of the Mn-Au association and show crustification 

sequences. Dissolution and replacement textures are 

also common.

VEIN MINERALOGY AND COMPOSITIONAL 

RANGES

Detailed descriptions of ore and gangue minerals are 

given here for common or economically important minerals and for those used to constrain conditions of ore 

formation. The minerals are listed in alphabetical order 

and grouped by type for convenience; sample locations 

are shown in figure 6. Table 3 describes the relative 

abundance and distribution of minerals in each assemblage and includes all minerals reported for the deposit. 

Compositional ranges for carbonate minerals, electrums, 

manganese silicates, and sphalerites described in table 4 

are listed in tables 5 through 8. Sample distribution, 

selection method, analytical details, and analyses are 

described by Foley (1990). Ore minerals were identified 

on the basis of descriptions presented by Ramdohr (1969) 

and Craig and Vaughan (1981).

Ag-SULFIDE AND Ag-Cu-SULFIDE MINERALS

Four Ag- and Cu-bearing sulfide minerals occur in the 

Au-rich stage of mineralization: acanthite, jalpaite, 

mckinstryite, and, possibly, stromeyerite. These phases 

were identified on the basis of optical properties and 

analysis by an energy dispersive system but were not 

examined by X-ray diffraction. Where possible, the 

compositions were estimated by energy dispersive analysis and electron microprobe analysis. The Cu-Ag-S 

minerals do not yield reliable temperatures of specific 

conditions of ore deposition because of their exceedingly 

fast reequilibration rates (Skinner, 1966) but are

included in this discussion because of their economic 

importance.

Acanthite (Ag2S)-Argentite (Ag2S).—Small (<0.03 

mm), irregularly shaped grains of acanthite occur alone 

in quartz gangue and at the margins of. galena, pyrargyrite, and chalcopyrite grains and as inclusions in pyrite 

and galena. The Ag sulfide is contemporaneous with 

chalcopyrite and pyrargyrite. It is also found both alone 

and with polybasite, in association with galena, with 

chalcopyrite and electrum, and rimming and enveloping 

rounded grains of galena. Some small (<0.01 mm), 

rounded grains are contained within galena; however, no 

clearcut exsolution textures were observed.

Jalpaite (AggCuS^.—Soft, light-gray, Ag- and Cubearing sulfide mineral grains, 10 to 25 (xm in width, are 

intergrown with or are enclosed by acanthite that is in 

turn bordered by chalcopyrite and galena. This mineral is 

tentatively identified as jalpaite on the basis of an 

absence of Sb or As, high Ag relative to Cu, good polish, 

and distinct anisotropism without the striking blue-violet 

tints of stromeyerite. The mineral also occurs in aggregates with acanthite and Ag-bearing tetrahedrite that 

are enclosed by gangue.

Mckinstryite (Ag13Cu07S).—Ai\hedra\ crystals of 

mckinstryite (15 (xm wide) are intergrown locally with 

acanthite, jalpaite, uytenbogaardtite, and, possibly, 

stromeyerite. The mineral also occurs as small <20-|jLmwide anhedral grains in aggregates intergrown with 

chalcopyrite and acanthite, and with wire silver the 

mckinstryite fills cavities and microfractures that cut 

sulfide-rich zones. Although optical properties of this 

phase closely resemble those of stromeyerite, the two 

Cu-Ag sulfides can be distinguished optically when intergrown. Identification was confirmed by electron microprobe where possible.

BARITE

Barite (BaS04) was not found in samples collected 

from underground workings. However, tabular molds up 

to 3 cm in length resembling the crystal habit of barite do 

occur in samples from the lower elevations in the workings, especially to the south. The mineral is found in 

drillcore samples from the southern end of the underground workings and in core drilled to the south of the 

exploratory workings. Tabular and bladed crystals of 

colorless to pale-cream barite ranging in length from 1 to 

10 mm occur in veinlets with quartz, calcite, and minor 

pyrite.

BOURNONITE

A gray-white phase (PbCuSbS3) occurs with galena 

rimming corroded sphalerite and as small aggregates of
ORES OF THE NORTH AMETHYST VEIN SYSTEM, COLORADO
VEIN MINERALOGY AND COMPOSITIONAL RANGES 9

equant to slightly platy grains (<100 jxm) associated with 

acanthite, electrum, tetrahedrite, chalcopyrite, and 

galena. Twinning, distinctive color adjacent to galena, 

very slight anisotropy, and lack of internal reflections are 

indicative of bournonite.

CARBONATE GROUP MINERALS

Carbonate minerals are abundant in all stages of the 

North Amethyst paragenesis, and each stage is characterized by carbonate minerals of different composition, 

color, and texture. The earlier, Mn-Au association is 

characterized by three fine-grained carbonate minerals: 

a manganoan calcite, kutnahorite,4 and a calcian rhodocrosite. The later, base-metal-silica association contains 

coarser grained manganoan calcite, manganosiderite (oligonite), and a calcian rhodocrosite. Each mineral has a 

range of compositions. Representative analyses are 

given in table 5; all analyses are plotted in figures 7 

and 8.

Calcite (rCa 75_piMn 26_ 0pC03>).—Three calcites are 

distinguished on the basis of paragenetic position, color, 

and grain size; however, they overlap in part in composition. Calcite associated with the Mn-Au stages is very 

fine grained (crystals <50 jxm in width) and is intergrown with microcrystalline Mn-silicates, kutnahorite, 

rhodocrosite, and quartz. Calcite of this stage shows a 

range in manganese contents from about 10 to about 30 

mole percent MnC03.

Calcite that occurs in the early stages of the basemetal-silica association is colorless to pale yellow and 

generally has lower manganese contents than other 

analyzed carbonate minerals (9 to 15 mole percent

4Capobianco and Navrotsky (1987) recommend that the name kutnahorite be 

applied to the 50:50 phase having dolomite-type ordering and that naturally 

occurring phases lacking dolomite-type ordering be termed pseudokutnahorite. 

The fine-grained nature of these samples hindered their characterization; hence, 

ordering (or the lack of ordering) is not implied by the use of the name 

kutnahorite.

FIGURE 4.—Photographs of North Amethyst Mn- and Au-bearing 

vein assemblages. A, Vein of stage-alpha Mn-carbonates, Mnsilicates, and quartz cutting volcanic wall rock. Field of view is 1 

m. Arrow points up. B, Closeup of bleached wall rock, sphalerite 

grains (s), and stage-alpha material. C, View of Au-rich stagebeta sulfide vein cutting fine-grained Mn-carbonates and 

Mn-silicates of stage alpha. Photograph by G.S. Plumlee. D, 

Closeup of Au-bearing stage-beta sulfide vein cutting fine-grained 

banded Mn-carbonates, Mn-silicates, and quartz. Scale bar equals 

1 cm. E, Equigranular sulfide assemblage of stage beta. Scale bar 

equals 25 (Jim. F, Closeup of electrum (A), sphalerite (s), and 

chalcopyrite (c) of stage beta. Scale bar equals 15 (Jim. G, View of 

sedimented vein. Photograph by G.S. Plumlee. H, Graded bedding in siliceous sediment. Note small offsets in bedding, fine-tocoarse texture, clasts of altered volcanics, rounded quartz crystals, and small sulfide aggregates.

MnC03). The crystals range in size from a few millimeters to 2 to 3 cm and are intergrown with 0.1- to 

1.0-mm-wide grains of oligonite, the manganoan siderite. 

The calcite contains virtually no magnesium and generally less than 1 mole percent FeC03

The last stage of the base-metal-silica association is 

composed dominantly of a pink to pale-cream manganoan 

calcite, especially in the northern part of the ore zone. 

Thick crusts of crystalline carbonate, up to 15 cm in 

thickness, are found mainly lining late-stage vugs and 

open spaces in the vein. The crusts are dominantly pale 

cream in color and contain thin bands of darker tan to 

rust-colored carbonate. The compositional range for this 

calcite is similar to calcite of the Mn-Au stages.

Kutnahorite (Ca^7Mn 52C03).—A Mn- and Cabearing carbonate mineral having a composition equivalent to kutnahorite, the manganese analog of dolomite, 

has been identified intergrown with fine-grained rhodrocrosite, Mn-calcite, and Mn-silicates of the Mn-Au association (table 5). X-ray diffraction analyses of the mixture suggest primarily quartz, rhodonite, pyroxmangite, 

and carbonate minerals having a wide range of Ca-Mn 

solid solution. Microprobe analyses of the carbonate 

minerals yield some compositions in the range Cc46Rc54 

to Cc53Rc47 with less than 1 percent (Fe,Mg)C03. The 

material is too fine grained to permit single-crystal X-ray 

study to determine if the kutnahorite is ordered; longrange cation ordering of kutnahorite is not detectable by 

conventional powder X-ray diffraction techniques (Peacor and others, 1987).

Rhodocrosite (Mn 86_ 76Ca 13_^C03).—ln the Mn-Au 

association, rhodocrosite is the most abundant carbonate 

mineral and occurs in a microcrystalline (<50 |jim) admixture composed mainly of rhodonite, quartz, and Mncalcite. The admixture is zoned mineralogically on the 

scale of hundreds of micrometers. Coarser rhodocrosite 

(up to 0.5 mm in width) occurs in some of the zones and 

in patches that cut across zones; the patches consist of 

mats of intergrown curls that resemble poodle hair. 

Compositions of the poodle-hair rhodocrosites are similar 

to, and as calcic as, those of the finer grained material (up 

to Cc24Rc76).

In the base-metal-silica association, rhodocrosite is 

associated with amethystine quartz and fluorite. 

Rhodocrosite also occurs in most later stages of the 

North Amethyst paragenesis in small (<5 mm wide) 

veinlets, and as 1- to 5-mm-wide, near-gem-quality crystals that line small vugs and are perched on minerals 

from earlier stages.

Siderite, manganoan.— Fe- and Mn-bearing oligonite 

(Fe 54Mn 32Ca 12Mg 02C03) is found in one assemblage of 

the base-metal-silica association. The mineral occurs as 

discrete crystals 0.1 to 1.0 mm wide within a coarser 

calcite. The carbonate minerals are intergrown primarily
10 ORES OF THE NORTH AMETHYST VEIN SYSTEM, COLORADO
VEIN MINERALOGY AND COMPOSITIONAL RANGES 11

Surface

10120L

MM 30m

30m

FIGURE 6.—Cross-sectional view of mine workings, North Amethyst vein system. Locations of samples used in the electron microprobe study are

labeled. See table 4 for sample descriptions.

with chlorite, quartz, hematite, and minor adularia and 

sphalerite and were deposited just after a major period of 

base-metal mineralization.

Wetlaufer (1977), in a study of carbonate minerals 

occurring in the southern part of the Creede district, 

documented significant compositional variations in 

rhodocrosite and siderite. She found that Bulldog Mountain rhodocrosite compositions from the early pre-ore

•< FIGURE 5.—Photographs of North Amethyst base-metal-bearing 

vein assemblages. A, Meter-wide vein of coarse base-metal sulfides of stage 1, base-metal-silicate association. Arrow points up. 

Photograph by G.S. Plumlee. B, Closeup of base-metal sulfide of 

stage 1 and quartz, calcite, and chlorite of stage 2. C, Reflectedlight photograph of sphalerite (S), galena (G), and quartz (Q) of 

the stage-1 assemblage. Scale bar equals 5 mm. D, Transmittedlight photograph of sphalerite. Note color banding paralleling 

growth zones defined by fluid inclusions. Scale bar equals 1 cm. E, 

2- to 3-cm-long stage-2 quartz crystals coated by stage-3 calcite; 

calcite is dusted with tiny pyrite cubes. F, Brecciated stage-1 

sulfides coated with fluorite (F) of stage 2. Note pockets of quartz 

sediment (s) between fluorite and calcite of stage 3. G, 15-cm-wide 

crusts of cream- to pink-colored Mn-rich calcite of stage 3 lining 

late vugs. Photograph by G.S. Plumlee. H, Pink calcite (light) and 

pyrite (dark) of stage 3.

stage lie along the FeC03-MnC03 join and typically 

contain 68 to 93 mole percent MnC03 . CaC03 contents 

vary from 2 to 15 mole percent, and MgC03 contents are 

generally less than 4 mole percent. A later intra-ore 

siderite-manganosiderite stage was found to contain 33 

to 94 mole percent FeC03 , 6 to 67 mole percent MnC03 , 

1 to 13 mole percent CaC03 and 1 to 10 mole percent 

MgC03.

CHALCOPYRITE

Chalcopyrite (CuFeS2) is abundant in three sulfidebearing stages of mineralization and is not found in 

stages composed dominantly of gangue. Size is highly 

variable; diameters of the irregular grains range from a 

few micrometers to a few millimeters. Earlier stage 

occurrences are generally finer grained than later stage 

occurrences.

Chalcopyrite is generally formed later than and 

replaces sphalerite. Extensive chalcopyrite disease textures (Barton and Bethke, 1987) are present in all 

sphalerite-bearing stages. The disease consists of
12 ORES OF THE NORTH AMETHYST VEIN SYSTEM, COLORADO

TABLE 3.— Assemblages and minerals of the North Amethyst vein system

[Abbreviations: x, present; -, not present; A, wall-rock alteration; ?, previously reported by Homestake Mining Company (C.B. Byington, S.W. Caddey, D.M.

Vardiman, oral commun., 1987) but not found in this study]

Mineral Pre-ore Alpha Beta 1 '.

Sericite................ x x -

K-feldspar ............. A x

Quartz................. x x x

Barite ................. ? ?

Pyrite ................. x x x

Hematite ..............

Magnetite. ............. - x x

Chlorite................ -

Rhodocrosite. .......... - x

Rhodonite ............. - x

Mn-calcite ............. - x

Kutnahorite. ........... - x

Alleganyite ............ - ?

Bustamite ............. - ?

Pyroxmangite. ......... x

Inesite ................. - x

Sphalerite ............. - x x

Galena................. - x x

Chalcopyrite ........... - - x

Marcasite .............. - -

Acanthite .............. - - x

Pyrargyrite ............ - - x

Electrum .............. - - x

Uytenbogaardtite ...... - - x

Polybasite ............. - - ?

Tetrahedrite ........... x

Jalpaite................ - - x

Stephanite ............. - -

Bournonite. ............ - - -

Siderite ................

Fluorite. ............... -

Stromeyerite .......... - -

Nativp ACT — _ —

Gypsum. ...............

micrometer-sized chalcopyrite grains that are rounded to 

rod shaped; they are generally concentrated at the rims 

of sphalerite crystals. 

In earlier stages, chalcopyrite is found in sphalerite 

and as somewhat larger separate grains bordering and 

corroding sphalerite. Xenomorphic grains of chalcopyrite 

are intergrown with galena, tetrahedrite, silver minerals, and electrum, and together these minerals fill interstices around crystals of sphalerite and pyrite (fig. kE 

andF). 

In the later stages, chalcopyrite occurs as anhedral 

grains bordering sphalerite and as common intergrowths 

with galena. It also occurs as isolated grains that are 

entirely surrounded by quartz and carbonate gangue. 

Oxidation of the chalcopyrite to chalcocite and covellite 

has been reported for some samples of the North

Stage

Beta 2 1-a 1-b 2-a 2-b 3 Post-or

- X X X - - -

- - X X -

X X X X X X -

X X X - X X -

X XX---

X _____ _

X X X - -

_ _ _ _ x - -

_ V _ __ _ _ _

- - X - X

_

_

_____ x

X _ x - - -

X _ x - - -

X X

X _ _ _ _ x

X _____

x _____

X _____

X _____

x _____ _

X _ x - - -

v _ _ _ _ _ _.

X _____

X _____

_ _ _ x -

- - X - X

_____ x

_____ x

_ _f __ __ _ __ v

Amethyst ores (S.W. Caddey and D.M. Vardiman, ora 

commun. , 1988); this type of oxidation was not observec 

in any of the samples examined in this study.

CHLORITE

In the North Amethyst veins, chlorite (A^Z^j 

(OH)8 , where A=Al,Fe2+ ,Fe3+ ,Mg,Mn and Z=Al,Fe3+ 

Si) occurs primarily in the later, base-metal-silica asso 

ciation where it is intergrown with hematite, quartz, am 

pyrite. Assemblages containing the green, fine-grainec 

(<100-(jLm-wide flakes) layer silicate formed just prior to 

and concurrently with, the period of major base 

metal-sulfide mineralization in the later association 

Some chlorite is also intergrown with quartz and calcit< 

deposited after the base-metal sulfides.

e

1 

I

0

>

I 

I

>

^
VEIN MINERALOGY AND COMPOSITIONAL RANGES 13

TABLE 4.— Locations and descriptions of samples from the North Amethyst vein system analyzed by electron microprobe 

[Samples collected by: DMV, D.M. Vardiman, Homestake Mining Co.; GSP, G.S. Plumlee, U.S. Geological Survey; NKF, N.K. Foley, U.S. Geological Survey]

Sample no. Location Description

DMV-1............. 10250 level, E-126+75'N

DMV-3 ............ .10325 level, 2' south,

GSP-MM-10-86 .... 10000 level, E-125..,

GSP-MN-87........ 10000 level, E-125+35'N

GSP-MO-2-86..... .10000 level, E-126+50'N, in scram

off vein.

NKF-A-1-87....... 10250 level, E-127+140'N

NKF-K-11-87...... 10250 level, E-85 .........

NKF-M-13-87...... 10090 level, E-120........

NKF-Q-17-87...... 10000 level, 25 ft from north face

NKF-R-18-87...... 10000 level, E-126+25'S.

NKF-X-24-87...... 10000 level, below IS orebody, 30'N.

NKF-AA-24-87 ... .10000 level, E-125...................

NKF-BB-54-87 ... .10250 level...........................

NKF-BK-63f-87.... 10250 level, No. 2 dogleg ............

NKF-BM-65-87.... 10250 level, S2 raise

. Coarse-grained (up to 1 cm wide) stage-1 lemon-yellow crystals of 

sphalerite intergrown with galena and chalcopyrite in veinlets 2 

mm to 1 cm wide cutting bleached volcanic rock.

. Seam of fine-grained electrum-bearing sulfide at contact of volcanic wall rock with breccia of stage beta.

.20-cm-wide seam of gold-bearing fine-grained sulfides of stage 

beta in fracture cutting vein of stage alpha. Rounded grains of 

sphalerite intergrown with chalcopyrite, galena, electrum, and 

other precious- and base-metal minerals of stage beta.

.Large (up to 1 cm wide) color-banded crystals of sphalerite of 

stage 1 intergrown with quartz and chlorite, coated by stage-3 

pink calcite.

Coarse veins of honey-brown sphalerite, galena, chalcopyrite, 

pyrite, and quartz of stage 1 cutting a meter-wide brecciated 

vein of Mn-carbonate, Mn-silicate, and quartz.

.Coarse- to medium-grained sulfides of Zn, Pb, and Cu of stage 1; 

quartz and calcite of stage 2.

.Fluorite at base overlain by thick crust of calcite color banded 

from pink to pale-cream.

.Small (1 to 2 mm wide) rounded sphalerite grains of stage alpha 

rimmed with chalcopyrite disease, contained in fine-grained 

admixture of rhodonite, rhodocrosite, and quartz.

.Meter-wide vein of Mn-carbonates and Mn-silicates containing 

disseminated galena (stage alpha), sulfides, quartz, and 

rhodocrosite.

. Banded hematite, magnetite, sulfides, quartz, and electrum (stage 

alpha). Cut by fractures containing coarse-grained sulfides of 

stage 1.

.Coarse-grained sulfides (stage 1), quartz, and carbonate on chlorite + pyrite alteration. Yellow to red-brown sphalerite intergrown with other stage-1 minerals, overgrown by quartz, 

fluorite, and calcite of stage 2.

. 15-cm-wide seam of high-grade ore in vein of Mn-carbonates and 

Mn-silicates. Galena taken from Au-bearing zone (stage beta).

. Fine-grained carbonate, Mn-silicate, and quartz material of stage 

alpha.

.Disseminated sulfides in recrystallized Mn-carbonates and Mnsilicates. Fine-grained minerals of stage alpha cut by veins 

containing red-brown to yellow sphalerite of stage 1, pyrite, 

chalcopyrite, and quartz and calcite gangue.

. Rounded cobble of fine-grained sulfide containing gold and sphalerite (stage beta) in breccia.

No quantitative compositional data are available for 

North Amethyst chlorites. They are iron rich like other 

Creede chlorites (Barton and others, 1977) and contain 

substantial manganese (by energy dispersive analysis).

ELECTRUM

Electrum (Au0 3_o 5Ag0 7_o 5) is characteristic of the 

early, Mn-Au association of the North Amethyst paragenesis. The mineral is intricately intergrown with a 

variety of base- and precious-metal sulfides and generally formed late with respect to the base-metal sulfides 

(fig. 4F). The whitish-yellow Ag-rich grains occur most

frequently as small rounded inclusions (3 to 25 fjtm) in 

chalcopyrite and uytenbogaardtite, although they are 

also found in acanthite, pyrargyrite, and galena. More 

rarely, electrum occurs as small (3 to 30 fjtm) separate 

grains in quartz gangue and occurs adjacent to sphalerite 

aggregates rimmed with galena and chalcopyrite. Larger 

grains, up to 75 fjtm in diameter and visible in hand 

specimen, occur with pyrargyrite, chalcopyrite, and 

galena. They fill interstices between, and in part replace, 

sphalerite grains at lower elevations in the mine. Representative electrum compositions are given in table 6 and 

plotted in figure 9. Analyzed electrum contained entirely 

within pyrite (Nos. 6, 7, table 6) has slightly higher Au
14 ORES OF THE NORTH AMETHYST VEIN SYSTEM, COLORADO

TABLE 5.—Representative electron microprobe analyses of carbonate minerals from the North Amethyst vein system 

[Values in weight percent unless otherwise indicated; m%, mole percent; collected by: NKF, Nora K. Foley; GSP, Geoffrey S. Plumlee]

CaO .............

MgO .............

FeO .............

MnO .............

SrO..............

BaO .............

C02 ..... .........

Total ........

Ca...... .........

Mg....... .......

Fe ...............

Mn. ..............

Sr ...............

Ba.. .............

CO*.. ............

m% Ca. ..........

m% Mg ..........

m% Fe. ..........

m% Mn ..........

i

46.90

.25

.65

8.64

.14

.08

42.91

99.57

1.72

.01

.02

.25

.00

.00

2.00

85.77

.63

.93

12.49

2

24.08

.00

.59

34.63

.01

.00

39.53

98.84

.94

.00

.02

1.05

.00

.00

2.00

46.81

.00

.91

52.26

3

11.75

.01

.77

47.20

.01

.09

38.49

98.32

.47

.00

.02

1.51

.00

.00

2.00

23.31

.03

1.23

75.35

4

6.10

.00

.83

53.16

.03

.08

38.31

98.51

.25

.00

.03

1.72

.00

.00

2.00

12.50

.00

1.33

86.09

5

48.31

.01

.23

8.25

.08

.09

43.24

100.21

1.75

.00

.01

.24

.00

.00

2.00

87.68

.02

.33

11.84

6

5.83

.86

34.36

19.93

.04

.12

38.98

100.12

.23

.05

1.08

.63

.00

.00

2.00

11.73

2.40

54.00

31.73

7

39.13

.11

.50

17.21

.09

.01

41.41

98.46

1.46

.01

.01

.52

.00

.00

2.00

73.14

.30

.62

25.85

1. NKF-M-13-87, fine-grained pale-pink calcite from stage alpha-1.

2. NKF-BB-54-87, fine-grained "kutnahorite" from stage alpha-2.

3. NKF-BB-54--87, medium-grained "poodle-hair" rhodocrosite from stage alpha-2.

4. NKF-BB-54--87, medium-grained pink rhodocrosite of stage alpha-1.

5. GSP-MN-24--87, colorless to pale-cream calcite of stage 2 (intergrown with no. 7).

6. GSP-MN-1-87, yellowish manganoan siderite of stage 2 (intergrown with no. 6).

7. NKF-K-11-87, true pink manganoan calcite of stage 3.

TABLE 6.—Representative electron microprobe analyses of electrum from the North Amethyst vein

system 

[Values in weight percent; collected by: NKF, Nora K. Foley; GSP, Geoffrey S. Plumlee]

Ag..............

Au.......... ..

Total .......

Ag+1 . .......... Au+1

i

. . . . 57.52

.... 40.74

. . . . 98.26

.72

.28

2 3 4

49.33 42.77 36.76 

49.31 56.02 61.31

98.64 98.79 98.07 

Atomic proportion

.65 .58 .52

.35 .42 .48

5

38.80 

59.65

98.45

.54 

.46

6

33.40 

64.34

97.74

.49 

.51

7

32.90 

65.73

98.63

.48 

.52

1. NKF-AA-27-87, silver-rich electrum grain contained in matrix of siliceous quartz.

2. NKF-AA-27-87, electrum contained in chalcopyrite.

3. NKF-AA-27-87, electrum rimmed by acanthite and uytenbogaardtite.

4. NKF-BM-65-87, electrum grain intergrown with sphalerite, galena, and chalcopyrite.

5. NKF-BM-65-87, electrum rimmed by silver minerals including acanthite and uytenbogaardtite.

6. GSP-MM-lOa, gold-rich electrum grain entirely rimmed by pyrite.

7. DMV-3, gold-rich electrum grain contained in pyrite.
VEIN MINERALOGY AND COMPOSITIONAL RANGES 15

TABLE 7 .—Representative electron microprobe analyses of Mn-silicate minerals from the North Amethyst vein system 

[Values in weight percent; -, not detected; collected by: NKF, Nora K. Foley; GSP, Geoffrey S. Plumlee; EQ, David M. Vardiman]

Si02 ...........

A1203 ..........

Fe203 ..........

FeO ...........

MnO ...........

MgO ...........

CaO ...........

1

. . . . . 45.41

..... .13

. . . . . .00

..... 1.91

. . . . . 47.95

. . . . . .42

. . . . . 3.65

2

45 fi4

.27

.00

5.87

47.01

.31

1.51

3

45.35

.13

.00

2.70

47.79

.56

1.73

4

47.10

55

.00

2.21

45.22

.39

6.02

5

47.06

.20

.00

2.98

43.23

.71

6.32

6

46.49

.25

.00

1.03

41.79

.03

10.59

7

45.00

.32

1.73

37.48

.47

6.78

Total. 99.47 100.61 98.26 101.49 100.50 

Number of ions on the basis of 6 or 36 oxygen

100.18 91.78

Si IV ...........

A1IV...........

T site ...........

Al VI ...........

Fe+3 ............

Fe+2 ............

Mn+2 ...........

Mg.............

Ca..............

M1,M2. .........

1.98

.01

1.98

, . . . . -

, . . . . -

.07

1.77

.03

.17

2.04

1.98

.01

1.99

-

-

.21

1.72

.02

.07

2.03

2.00

.00

2.00

.00

-

.10

1.78

.04

.08

2.00

1.99

.01

2.00

.01

.00

.08

1.61

.02

.27

2.00

2.00

.00

2.00

.01

-

.11

1.55

.04

.29

2.00

1.98

.01

1.99

-

-

.04

1.51

.00

.48

2.03

9.88

.08

10.00

-

-

.32

6.97

.15

1.60

9.00

1. NKF-AA-27-2-87, colorless to pale-pink crystals of rhodonite having relatively low calcium contents.

2. NKF-AA-27-2-87-3M, pyroxmangite crystal adjacent to no. 1.

3. NKF-AA-27-2^87-3M, pyroxmangite crystal adjacent to no. 1.

4. EQ-16-1, fine crystals of rhodonite intermixed with rhodocrosite, calcite, and quartz.

5. NKF-Q-17-87, colorless to pink crystals of rhodonite adjacent to vuggy quartz.

6. NKF-M-13^87, calcian rhodonite.

7. Inesite from the Equity vein, Equity mine; FeO determined colorimetrically, MnO by XRF, and all 

other elements by gravimetrics (Van Loenen, 1980).

contents than electrum rimmed by chalcopyrite or Au-Ag 

sulfides.

FLUORITE

Large cubes of fluorite (CaF2) up to 13 mm on a side 

are found lining vugs, veins, and cavities in the North 

Amethyst vein system. The crystals are most frequently 

green; purple and colorless varieties are less common. 

No clear-cut relation between color and paragenetic 

position was discerned. Fluorite generally forms late in 

the paragenesis and usually grows on quartz; however, 

the fact that the bases of many late-stage quartz crystals 

contain numerous small octahedra of colorless to palegreen fluorite indicates near-contemporaneous deposition. Fluorite crystals are sometimes dusted with finegray quartz sediment that is overgrown by coarse creamto-pink, manganese-rich calcite and (or) pyrite (fig. 5F). 

In small vugs, fluorite may follow quartz and adularia.

GALENA

Galena (PbS) occurs in both of the principal sulfide 

stages of the North Amethyst paragenesis. It generally

is intergrown with chalcopyrite and is similar to chalcopyrite in size and texture where they are found together.

In the Mn-Au association, galena is found as both 

isolated anhedral grains and aggregates bordering and 

corroding sphalerite and pyrite in a gangue of finegrained Mn-carbonate minerals, Mn-silicates, and 

quartz. In sulfide-rich assemblages, galena is usually in 

mutual contact with chalcopyrite, pyrargyrite, acanthite, uytenbogaardtite, and polybasite. Bournonite rims 

are present on some galena. Micrometer-wide rounded 

grains of galena occur along fractures in the outer zones 

of sphalerite crystals, and larger grains (10 to 100 jim 

wide) occur in an almost equigranular mix of sphalerite 

and chalcopyrite.

In the base-metal-mineral association, anhedral grains 

and aggregates of 2- to 3-mm-wide galena grains are 

found bordering sphalerite or enclosed by gangue. The 

galena is often intergrown with chalcopyrite or tetrahedrite. The generally xenomorphic texture of galena and 

the occurrence along cracks in sphalerite crystals suggest that it may have formed later than did sphalerite 

and pyrite. Skeletal crystals of galena also grow on 

sphalerite and project into quartz- and calcite-filled vugs.
16 ORES OF THE NORTH AMETHYST VEIN SYSTEM, COLORADO

TABLE 8.—Representative electron microprobe analyses of sphalerite from the North Amethyst vein system 

[Values in weight percent; collected by: NKF, Nora K. Foley; GSP, Geoffrey S. Plumlee]

Fe+2

Cd+2

Mn+2

s-2 ... ........

i

...... 0.22

...... .32

...... .19

...... 32.69

...... 66.86

2

0.80

.34

.80

33.11

64.99

3

0.60

.31

63

32.88

65.98

4

1.58

.28

.69

33.03

64.08

5

0.67

.53

.24

32.77

66.56

6

0.59

.34

.06

33.10

66.90

7

4.78

.43

.57

32.96

61.27

8

2.12

.34

.56

32.73

63.48

9

0.78

.33

.16

32.59

66.67

Total 100.28 100.04 100.40 99.66 100.77 100.99 100.01 ).23 100.53

Fe +2 ... ..........

Cd +2 . ............

Mn +2 ............

S .................

Zn +2 .............

Total .........

.00

.00

.00

.99

.99

1.98

.01

.00

.01

1.01

.97

2.00

.01

.00

.01

00

.98

1.99

.03

.00

.01

1.01

.96

2.01

.01

.00

.00

.98

.98

1.97

.01

.00

.00

.99

.99

1.99

.08

.00

.01

.99

.90

1.98

.04

.00

.01

1.00

.95

2.00

.01

.00

.00

.98

.98

1.97

1. NKF-M-13-87, core of sphalerite crystal occurring in sulfide stringer in fine-grained admixture of rhodocrosite, rhodonite, 

and quartz.

2. NKF-AA-27-87, core of sphalerite crystal occurring in sulfide stringers in fine-grained admixture of rhodocrosite, 

rhodonite, and quartz.

3. NKF-BM-65-87, sphalerite crystal occurring in fine-grained gold-bearing sulfide seam containing pyrite.

4. NKF-BM-67-87, sphalerite crystal occurring in fine-grained gold-bearing sulfide seam containing pyrite + magnetite + 

hematite.

5. NKF-X-24-87, coarse-grained lemon-yellow sphalerite crystal of late base-metal mineralization.

6. NKF-R-18--87, colorless sphalerite in coarse-grained banded crystal occurring in veinlet cutting earlier gold-bearing sulfide 

seam.

7. NKF-R-18-87, red-brown sphalerite in coarse-grained banded crystal occurring in veinlet cutting earlier gold-bearing 

sulfide seam.

8. NKF-R-18-87, sphalerite from earlier gold-bearing seam containing magnetite + hematite + pyrite, cut by veinlet 

containing coarser sphalerite (nos. 6 and 7).

9. GSP-MN-87, coarse crystals of late-stage yellow sphalerite rimmed with minor chalcopyrite.

GYPSUM

Large curls of gypsum (CaS04 -2H20) ranging in size 

from a few to tens of centimeters are found in open vugs 

and cavities along the vein structures. Gypsum having a 

fine-grained spongelike texture also occurs in vugs and 

cavities. These crystals certainly formed later than all of 

the ore-stage minerals and probably are of recent (modern) age.

HEMATITE

Hematite (Fe203), the most abundant iron oxide mineral, occurs in both mineral associations of the North 

Amethyst paragenesis. It is intergrown with preciousand base-metal sulfides and other iron oxides in the 

earlier association and with quartz, chlorite, and basemetal sulfides in the later association. Much of the 

hematite is coated with iron oxides; microprobe analyses 

of visually unoxidized grains indicate that they contain 

less than 1 weight percent combined MnO + Si02 + 

A1203 + MgO.

CaCO

EXPLANATION 

a Stage alpha 

• Stage 2 \

FeC03 + MgC03 MnCO

FIGURE 7. —Ternary diagram showing compositions of carbonates for 

three stages of the North Amethyst vein mineralization. Bars show 

range of compositions for carbonates present in more than one stage. 

Possible tie-lines are dashed.
VEIN MINERALOGY AND COMPOSITIONAL RANGES 17

5 -

300

Mole Fraction MnC03 in (CaMn)C03

FIGURE 8. — CaC03-MnC03 binary diagram of the proposed solvii from 

experimental studies (solid lines) of Goldsmith and Graf (1957) and 

deCapitani and Peters (1981) and from studies of naturally occurring 

carbonates (dashed lines) by Peacor and others (1987). Compositions 

of carbonates from stage-alpha mineralization are plotted as bands at 

330°C, the temperature estimated for coexisting Mn-silicate minerals 

(see text for discussion). This figure is after Peacor and others (1970) 

and includes analyses from the following areas: SN, Serra do Navio 

(Brazil) (Scarpelli, 1973); S, Scerscen (Bernina, Switzerland); A, 

Alagna (Italy); MP, Muretta Pass (Switzerland) (Peters and others, 

1978); B, Buritirama (Parana, Brazil) (Peters and others, 1977); BK, 

Bald Knob (North Carolina, U.S.A.) (Winter and others, 1981); AI, 

Andros Island (Greece); and SH, Sterling Hill (New Jersey, U.S.A.) 

(Peacor and others, 1987).

Blades of hematite, 0.1 to 1.0 mm long, in electrumbearing samples of the earlier association are intergrown 

with rounded anhedral magnetite grains (<50 |jim in 

diameter) and cubes of pyrite. In some sections, the 

hematite is replaced by magnetite, while in other areas, 

the hematite has been altered to fine-grained iron oxides. 

Abundant hematite colors some ore samples a deep red 

hue.

Thin, transparent-red hematite flakes having welldeveloped hexagonal outlines occur in the later association as fine- to medium-grained crystals (<60 |jim wide) 

intergrown with quartz, chlorite, pyrite, sphalerite, 

galena, and chalcopyrite. The flakes are also included in 

sphalerite and quartz crystals, where they frequently are 

clustered along colored growth zones.

MAGNETITE

Magnetite (FeFe204) is found in the earlier mineral 

association of the North Amethyst paragenetic sequence

E 3

2

48 samples

0.27 0.32 0.37 0.42 

Atomic fraction Au +

0.47 0.52

FIGURE 9.—Histogram of gold content of electrum occurring in stagebeta assemblages.

and is primarily associated with hematite and pyrite in a 

quartz-rich gangue. It occurs as equant grains (<0.5 mm 

wide) intergrown with euhedral pyrite and laths of 

hematite, and as a replacement of hematite. Some magnetite contains blebs of electrum several micrometers 

wide. Electron microprobe analyses of magnetite intergrown with hematite indicate virtually no substitution of 

MnO, A1203 , Si02, ZnO, or MgO. Magnetite is not found 

with chlorite. In general, magnetite appears to be 

restricted to the earlier mineral association, whereas 

chlorite characterizes the later, base-metal association.

MANGANESE SILICATE MINERALS

At least four Mn-silicate minerals occur in the earliest 

stage of the North Amethyst ores. Rhodonite is the most 

abundant, and pyromangite, caryopilite(?), and inesite 

occur in much smaller quantities. The complete assemblages include rhodocrosite and quartz. Kutnohorite and 

Mn-calcite are also present in small amounts.

Caryopilite (Mn6Si4 010 (OH)8).—Fine-grained intergrown aggregates of a pleochroic yellow to brown 

hydrated layer silicate (~<0.01x0.05 mm) are found in 

small, matted, 2-mm-wide clumps in the early, Mn-Au 

association. The mineral fills in around earlier formedquartz crystals and rims pyrite, chalcopyrite, and sphalerite stringers in a rhodonite + quartz admixture. The 

mineral clearly formed late, probably by retrograde 

alteration of rhodonite, pyroxmangite, and inesite. The 

mineral has been tentatively identified as caryopilite on 

the basis of petrographic characteristics as described in 

Abrecht (1989) and energy dispersive analysis.

Inesite (Ca2Mn7Si10 028(OH)2 '5H20).—A.cicu\ar crystals of inesite (table 7) up to 2 cm in length form radiating
18 ORES OF THE NORTH AMETHYST VEIN SYSTEM, COLORADO

80

70

FIGURE 10. —Compositions of all pyroxmangite and rhodonite analyzed 

for stage-alpha mineralization. See table 4 for sample descriptions 

and text for discussion. Possible tie-lines are dashed. Stability fields 

shown are consistent with well-characterized, naturally occurring 

pyroxenoids in metamorphic environments (estimates at 650±100°C 

for all occurrences); presented in Peacor and others (1978), after 

Ohashi and others (1975).

sheaths that fill fractures cutting silicified wall rock and 

early Mn-carbonate- and Mn-silicate-bearing assemblages in higher levels of the North Amethyst workings. 

Minerals intergrown with the coarse pale-pink to creamcolored crystals include quartz, rhodocrosite, Mn-calcite, 

galena, sphalerite, pyrite, and pyrargyrite.

Pyroxmangite (Ca 04_ . 02Mn 86_ 92Fe,Mg ^_ 06Si03). — 

Pyroxmangite (table 7, fig. 10) is present in the earliest 

stage of mineralization. The mineral ranges in size from 

a few micrometers to, at most, 100 jxm in length and is 

tentatively identified by X-ray diffraction analysis, petrographic study, and electron microprobe analysis but 

has not been confirmed by single crystal techniques. It is 

intergrown primarily with rhodocrosite, rhodonite, and 

quartz. Microprobe analyses of the mineral (table 7) show 

a small variation in calcium, iron, and manganese.

Rhodonite (Ca 08_,2Fe,Mg0_ 08Mn 92_ 76Si03). — Rhodonite is the most abundant Mn-silicate present in the 

early stages of the paragenesis (table 7, fig. 10). The 

mineral occurs primarily in a cryptocrystalline admixture of quartz, carbonate minerals, and other Mnsilicates and as coarser crystals edging small quartz-filled 

vugs. The coarser grained crystals range up to 50 jxm in 

width and 200 jxm in length.

MARCASITE

Characteristic reflection pleachroism and strong anisotropy distinguish marcasite (FeS2) from other iron

sulfides. The mineral occurs primarily as 50- to 100-jxmwide tabular or carrot-shaped crystals growing on pyrite 

and, at times, is coated with chalcopyrite. Anhedral 

marcasite is found coating and filling cracks in pitted 

crystals of pyrite. Marcasite generally appears to have 

formed late in the base-metal-silica association, as 

carrot-shaped crystals resting on pyrite, as anhedral 

fillings, and as small fragments caught up in a fine quartz 

sediment matrix. Distribution of marcasite within the 

veins is difficult to define because of the fine grain size 

and scarcity of this mineral.

POLYBASITE

Polybasite ((Ag,Cu) 16Sb2Sn) is most commonly associated with acanthite and generally formed late with 

respect to other Ag-bearing minerals. The sulfosalt is 

admixed with galena and sphalerite and intergrown with 

chalcopyrite; it also replaces sphalerite. The softness of 

the mineral, light etching, and darker color adjacent to 

galena help to distinguish it from pyrargyrite.

POTASSIUM FELDSPAR

Feldspar (K 98_. 92Na.o2-.o8-A-lSi308) occurs as anhedral 

to subhedral grains extensively replaced by fine-grained 

Mn-carbonate, rhodonite, and other Mn-silicates and by 

quartz in the earliest Mn-rich vein stage. The feldspars 

vary in composition from Or93 to Or96 and have barium 

contents that vary from —0.1 to 2.4 weight percent BaO. 

These feldspars are compositionally similar to wall-rock 

phenocrysts of sanidine or orthoclase adjacent to the 

veins but locally have higher barium contents. The 

feldspars may have been incorporated into the vein 

during formation of the earliest stages of mineralization 

by metasomatic alteration of the volcanic wall rock.

Potassium feldspar also occurs as 0.1- to 1.0-mm-long 

euhedral barium-rich crystals of adularia intergrown 

with amethystine quartz and filling small vugs, cavities, 

and fractures in most vein material. The adularia is 

associated with minerals of the base-metal association 

and is relatively common throughout the ore zone. Adularia ranges in content from Or92 to Or98 and contains 0.4 

to 1.6 weight percent BaO.

PYRARGYRITE-PROUSTITE

Pyrargyrite (Ag3SbS3)-proustite (Ag3AsS3) primarily 

occurs alone in small (>10 jxm) anhedral grains scattered 

through a silicious gangue and in aggregates with acanthite that border galena and sphalerite. It also is intergrown with acanthite and native silver in aggregates that 

border or are enclosed by chalcopyrite. Other grains are
VEIN MINERALOGY AND COMPOSITIONAL RANGES 19

intergrown with acanthite and uytenbogaardtite and rim 

chalcopyrite, tetrahedrite, and galena. Subhedral prismatic to anhedral crystal aggregates are enclosed by 

rhodochrosite and quartz and occur along cracks in 

broken pyrite cubes.

PYRITE

Pyrite (FeS2) is the most abundant and ubiquitous 

sulfide mineral, as it occurs in almost all stages of both 

mineral associations. It is primarily associated with 

either quartz-chlorite-hematite assemblages or other 

base-metal sulfides. The pyrite shows idiomorphic habit, 

and cubes are the most common shape, although some 

overgrowths and aggregates are anhedral. Thin botryoidal crusts of pyrite occur late in the paragenetic sequence 

in places.

In the early association, subhedral equant pyrite crystals are bordered by sphalerite, rimmed by chalcopyrite 

and galena, and contain inclusions of all three sulfides. 

The crystals range in size up to a millimeter on edge; 

however, most are in the range 0.1 to 0.3 mm. Some of 

these crystals also contain rounded blebs of electrum. 

Grain margins of the pyrite crystals are often corroded 

and replaced by sphalerite and galena.

Some crystals are isotropic to faintly anisotropic and 

twinned. There is often a zonal arrangement with sphalerite, acanthite, and pyrargyrite crystals, which suggests that pyrite began to crystallize early, and its 

deposition continued as other sulfides and sulfosalts 

began to form. Later pyrite overgrowths on these crystals do not show inclusions. Rarely, marcasite crystals sit 

on pyrite. Pyrite also occurs as isolated equant cubes in 

quartz and carbonate gangue.

In the later association, euhedral pyrite is found in 

mixtures of quartz, hematite, and chlorite and is also 

intergrown with euhedral sphalerite and anhedral galena 

and chalcopyrite. The pyrite crystals of the late association are much larger in size, and cubes up to a centimeter 

on edge do occur. However, most pyrite crystals in this 

association are a millimeter or less on a side. Pyrite 

crystals in the size range 0.1 to 0.5 mm dust singly 

terminated quartz crystals and coarse crystalline calcite 

(fig. 5E") in the latest stage. They also occur as 

millimeter-thick layers within crusts of carbonate minerals. At times, 50- to 100-(xm-sized carrot-shaped marcasite crystals overlie this latest stage pyrite.

QUARTZ

Quartz (Si02) occurs in all mineralogical stages and 

shows a variety of textures ranging from cryptocrystalline interlocking grains to large, terminated euhedral

crystals. Quartz also occurs as a chalcedonic cementing 

medium in many breccias.

In the early stages of mineralization, quartz <50 jxm in 

size is intergrown in a microcrystalline mix of Mnsilicate and Mn-carbonate minerals. It fills barite molds 

and small vugs that occur within the mix.

In later stages, quartz occurs most often as larger (0.2 

cm to 5 cm) crystals intergrown with base-metal sulfides, 

fluorite, and calcite. Where they project into vugs, the 

crystals are euhedral and terminated. Much of the quartz 

is amethystine, especially the outer parts of crystals. The 

larger crystals contain solid inclusions, including fluorite 

octahedra, laths of rhodonite(?), and carbonate crystals. 

Fluid and solid inclusions often are found decorating 

acicular structures that radiate from the bases of the 

quartz crystals.

Quartz also forms the cement for many of the breccias 

and sediments found within the ore zones. The siliceous 

cements are generally gray and chalcedonic. Coarser 

areas of cement show a granular texture with overgrowths of quartz. In some samples, the quartz has 

sutured grain boundaries. One of the breccias has a 

distinctive black quartz cement; the color apparently 

results from fine sulfides and electrum admixed with the 

quartz. Some vein sediment areas consist of fine parallel 

laminations distinguished by subtle variations in color 

and grain size. The chalcedonic laminations have a mosaic 

texture, with sutured grain boundaries between fine 

crystalline quartz, that may reflect recrystallized amorphous silica sediment.

SILVER

Native silver (Ag°) occurs primarily as scarce, lone 10- 

to 100-jxm-sized grains in fractures and as millimeterlong wire aggregates in cavities or resting on broken 

surfaces. It is also found as scarce grains bordering 

galena that are enclosed by gangue and are rarely 

bordered by acanthite. In some samples, native silver 

borders sphalerite in aggregates that include chalcopyrite, jalpaite, and possibly stromeyerite. Textural relations indicate that native silver occurs much later in the 

paragenetic sequence than most other silver minerals.

SPHALERITE

Sphalerite ((Zn,Fe,Mn,Cd)S) is one of the most abundant sulfide minerals in the deposit. The mineral is found 

in two stages of the early, Mn-Au association and in one 

stage of the later, base metal-silica association. Sphalerites in different stages vary in grain size, color, and 

composition.
20 ORES OF THE NORTH AMETHYST VEIN SYSTEM, COLORADO

Stage alpha.—Small (<2 mm) rounded grains of 

intensely chalcopyritized sphalerite occur in small stringers within the cryptocrystalline admixture of 

Mn-silicates, carbonate minerals, and quartz that 

make up the earliest stage. The sphalerite is intergrown 

with cubes of pyrite and anhedral galena and contains 

generally low and subequal amounts of iron, cadmium, 

and manganese (table 8, Nos. 1 and 2).

Stage beta. —Abundant sphalerite is present also in the 

precious- and base-metal seams of the Mn-Au association. It is often rimmed and replaced by electrum, 

pyrargyrite, acanthite, chalcopyrite, uytenbogaardtite, 

and galena. Sphalerite corrodes and sometimes replaces 

pyrite. Small equant grains also occur in siliceous 

gangue, with sparse micrometer-sized exsolution blebs 

of chalcopyrite. Sphalerite is bordered and corroded by 

all of the other sulfides, especially galena and chalcopyrite, and by tetrahedrite. All sphalerite of the 

Au-bearing stage is extensively chalcopyritized, and 

most crystals (table 8, No. 3) have a composition similar 

to that of the sphalerite described above. An important 

exception to this is that sphalerite intergrown with 

magnetite and hematite has a more iron rich composition 

(table 8, Nos. 4 and 8), generally containing around 2 

percent iron by weight.

Stage 1. — Sphalerite crystals of the later, base-metal 

stage range in width from about a millimeter to a 

maximum of about 2 cm and range in hue from colorless 

to greenish to pale yellow to dark red-brown. The 

subhedral, equant crystals form aggregates that are 

intergrown with galena, euhedral pyrite, chalcopyrite, 

and minor tetrahedrite. All sphalerite of this association 

is chalcopyritized; it contains numerous microscopic 

inclusions of chalcopyrite that are especially abundant at 

the rims. Some grains also contain abundant minute 

blebs of tetrahedrite that are concentrated at grain 

boundaries.

While most grains are uniform in color, many larger 

crystals of sphalerite are color banded on a simple scale. 

The colors fall into three groups—red-brown, yellow, 

and colorless—that show only minor color variation. On 

the scale of individual crystals, the contact between 

colors is sharp rather than gradational.

The majority of the banded samples studied have 

colorless or yellow cores and red-brown rims (thinsection thicknesses ranging from 60 |xm to 1 mm). Rims, 

in shades ranging from moderate red-brown to deep 

red-brown, are more finely banded than cores; however, 

they still do not show the beautifully well-developed 

banding of samples typical of the OH vein in the main 

Creede district as described by Barton and others (1977) 

and Roedder (1977). Other samples show an alternating 

set of color bands, from red-brown to yellow to redbrown, but these are less common. No attempt was made

to correlate color banding in the sphalerite, because 

possible marker horizons lacked continuity.

Analyzed sphalerite from the later, sulfide-rich stage 

displays higher and more variable iron contents (up to 8 

mole percent FeS) than earlier sphalerites (table 8, Nos. 

5-8). Some of the darkest sphalerites were not analyzed, 

so the true range may be higher. Measured iron contents 

are not as high as those of sphalerite from the OH vein of 

the main mining district (Barton and others, 1977), which 

contains up to 16 mole percent FeS. However, the most 

typical iron contents of North Amethyst sphalerite (~1 

mole percent FeS) are similar to the most typical iron 

contents of OH sphalerite (~1 to 2 mole percent FeS).

STEPHANITE

A silver sulfosalt resembling stephanite (Ag5SbS4) is 

intergrown with pyrargyrite and polybasite in aggregates in a siliceous matrix. It is also found alone filling 

cavities and fractures in the siliceous matrix.

TETRAHEDRITE-TENNANTITE

Tetrahedrite-tennantite ((Cu,Fe,Ag,Zn)12 (Sb,As)4S13) 

occurs in all sulfide-bearing stages, usually in association 

with sphalerite, galena, pyrite, and chalcopyrite. 

Reported to be Ag-rich (S.W. Caddey, C.B. Byington, 

and D.M. Vardiman, written commun., 1988), energy 

dispersive analysis confirms that much of the tetrahedrite contains appreciable amounts of silver and arsenic.

In the early association, tetrahedrite is most often 

adjacent to sphalerite and galena and also is in aggregates with chalcopyrite and acanthite. Scarce anhedral 

grains of tetrahedrite occur marginal to, and intergrown 

with, chalcopyrite and acanthite. Tetrahedrite also 

occurs with polybasite, intergrown with pyrargyrite and 

acanthite, alone, and intergrown with galena and chalcopyrite in cavity fillings.

In the later association, 50- to 70-|xm-sized grains of 

tetrahedrite most often are with contemporaneous chalcopyrite and galena in aggregates that border sphalerite. 

Rounded micrometer-sized grains of tetrahedrite occur 

in sphalerite in a texture similar to that of chalcopyrite 

disease.

Growth banding was not observed in any of the 

samples, possibly because of their small grain size. 

Growth banding in tetrahedrite from the Bulldog vein of 

the Creede district is described by Plumlee (1989). He 

found that the growth bands reflected compositional 

variations in Ag, As, and Sb.

UYTENBOGAARDTITE

Uytenbogaardtite (Ag3AuS2) most often is intergrown 

with acanthite containing micrometer-sized electrum
DISCUSSION OF MINERAL EQUILIBRIA 21

(Au 3_ 4Ag 7_ 6) grains. When intergrown with acanthite, 

uytenbogaardtite can be distinguished readily from the 

acanthite, and from many other silver minerals, by its 

extremely rapid light etching. Small and irregularly 

shaped anhedral grains of uytenbogaardtite also occur 

alone or in contact with other minerals such as pyrargyrite, and, especially, chalcopyrite.

DISCUSSION OF MINERAL EQUILIBRIA

GOLD AND SILVER SULFIDES

Experimental work by Barton (1978) demonstrated 

that uytenbogaardtite is stable at relatively low temperatures (<200°C) with Au-bearing argentite but that only 

below 113°C can uytenbogaardtite coexist with acanthite. Barton (1978) also showed that uytenbogaardtite is 

stable with relatively Au-rich electrum compositions 

(0.8-0.9 atomic fraction Au). The intergrowth textures 

described in this study for uytenbogaardtite, acanthite, 

and electrum are similar to those described by Barton 

and others (1978) for uytenbogaardtite from the Cornstock lode. For that deposit, they suggested that the 

textures may reflect exsolution during cooling of a higher 

temperature Au-bearing argentite + Ag-rich electrum 

assemblage. Upon cooling, the argentite would exsolve 

uytenbogaardtite and leave the original Ag-rich electrum 

with acanthite and uytenbogaardtite. Thus, the Au- and 

Ag-sulfide equilibria for the North Amethyst ores most 

likely reflect subsolidus reequilibration at quite low 

temperatures and, like the Cu-Ag-sulfides, cannot be 

used to yield reliable temperatures of specific conditions 

of ore deposition because of their fast reaction rates.

CARBONATE MINERALS

Peacor and others (1978, 1987) have suggested possible solvii between calcite and kutnahorite and kutnahorite and rhodocrosite to account for the compositions of 

some naturally occurring carbonate minerals. Comparison of those solvii with the North Amethyst carbonate 

data suggests temperatures of formation above 350°C for 

the calcite-kutnahorite compositions of the early assemblage and temperatures in excess of 425°C, and possibly 

in excess of 500°C, for rhodocrosite intergrown with 

kutnahorite in the early assemblage (fig. 8). The most 

Ca-rich rhodocrosite of the earliest carbonate-bearing 

stage has a composition of Cc23Rc75 and contains less 

than 2 mole percent combined FeC03 + MgC03. This 

composition is somewhat less calcic than the critical point 

of the CaC03-MnC03 solvus as reported by deCapitani 

and Peters (1981) at 540°C and a composition of Cc32Rc68. 

(Experiments were run at 2 and 10 kbar, and no significant differences due to pressure were detected.) These

temperatures are exceedingly high and, as discussed 

below, unreasonable for this environment.

Temperatures above 300°C are unusual for mineralization occurring in the typically shallow epithermal environment (Heald and others, 1987) where depths are 

generally on the order of 500 m. However, for the North 

Amethyst vein, about 1,500 m of volcanic and volcaniclastic rock may have overlain the base of Au-rich 

mineralized rock (Lipman and Sawyer, 1988; Foley, 

1990). A simple analysis based on temperature-depth 

calculations of Haas (1971) indicates that the boiling point 

of a nonsaline fluid at 1,500 m is about 320°C; fluids that 

are more saline would boil at higher temperatures. 

Temperatures as high as those suggested by carbonate 

mineral equilibria are unlikely, however, because they 

would require formation at unreasonably great depths. 

The excessively high temperatures suggest that compositions of rapidly deposited carbonates of the early stage 

may be metastable.

Rhodocrosites and calcites from all later stages of 

mineralization have more restricted compositional 

ranges and lie outside experimental solvii (Goldsmith and 

Graf, 1957; diCapitani and Peters, 1981) at temperatures 

of less than 400°C.

MANGANESE SILICATE EQUILIBRIA

Pyroxmangite and rhodonite are polymorphs of 

MnSi03. Rhodonite is the high-temperature form and is 

stable above about 350°C at pressures of 1 to 2 kbar, 

whereas pyroxmangite of MnSi03 composition is the 

stable phase at low pressures below 350°C (Maresch and 

Mottana, 1976). The presence of both rhodonite and 

pyroxmangite can be used to constrain temperature if 

the compositions do not depart markedly from MnSi03 

and if pressure is known. In this case, because up to 11 

weight percent CaO and 2 weight percent FeO + MgO 

replaces MnO, and because pressures are not well constrained (Foley, 1990), the assemblage can be used only 

qualitatively. To a first approximation based on ionic 

radii, the substitution of Mg+ and Fe+ would stabilize 

pyroxmangite at higher temperatures, whereas the substitution of Ca+ should stabilize rhodonite to lower 

temperatures (Maresch and Mottana, 1976).

If an ideal ionic activity model is used as a first 

approximation to describe the activity of MnSi03 in 

rhodonite and pyroxmangite and the phase equilibria 

data of Maresch and Mottana (1976) are used, the free 

energy of reaction can be written as:

AGr = A#r - ASr + PAFr + RTlnKr = 1347(±100) 

- 2.069(±0.2)7X°K) + 0.0418(±0.025)P(bars) 

+ 8.31437X K)ln(AMnjrhodonite/AMnjpyroxmangite)
22 ORES OF THE NORTH AMETHYST VEIN SYSTEM, COLORADO

where AGy = Gibbs free energy of formation for the

reaction

&Hr = enthalpy of formation of the reaction 

ASr = entropy of formation of the reaction 

P = pressure

A Vr = change in volume of the reaction 

R = gas constant 

T = temperature of the reaction 

Kr = equilibrium constant for the reaction 

X = mole fraction

This can be further recast to define temperature:

T(or. = 162(±12) - 5.027(±3.0)P(kbars) 

^ ^ 0.2488(±0.025) - ln(Zrhodonite/Zpyroxmangite)

-273

When data for coexisting rhodonite-pyroxmangite pairs 

(P = 0.5 kbar) are used, an approximation to the 

temperature of equilibrium can be calculated:

pyroxmangite rhodonite (estimated)

0.89±0.01 

0.86

0.875±0.01 

0.85

327±58°C 

339±58°C

Temperatures at the middle to lower range of this 

approximation to the temperature of equilibrium 

(~330±60°C) are in general agreement with mineral 

assemblages, fluid-inclusion data, and other geologic 

considerations for the Mn-Au association of the North 

Amethyst vein system.

MINERAL ASSOCIATIONS AND ASSEMBLAGES

The ore and gangue minerals of the North Amethyst 

vein system are paragenetically complex; they fall into 

two mineralogically, texturally, and chemically distinct 

associations. The associations overlap spatially but were 

separated in time by a period of extensive brecciation 

and sedimentation. The earlier association is characterized by two mineralogic stages, whereas the second, 

later association is subdivided into three stages on the 

basis of mineralogy and textural relations. Successive 

stages in each association are generally marked by 

relatively sharp rather than gradational changes in mineralogy and by periods of movement, brecciation, sedimentation, or replacement. In only a few instances are 

the transitions characterized by intergrowths or interbanding between mineral assemblages of adjacent 

stages.

Figure 11 describes the sequence of events relating to 

formation of mineralized rock in the North Amethyst 

area. The general sequence of mineral deposition is 

shown in figure 12. Temporal and spatial relationships

between mineralization, brecciation, and sedimentation 

are shown in a series of diagrams beginning with figure 

13.

ALTERATION AND PRE-ORE VEIN ASSEMBLAGES

Several types of wall-rock alteration have been 

observed in the North Amethyst vein system. These 

include intense sericitic alteration at higher levels in the 

system, two stages of potassium metasomatism of the 

wall rock at depth, and a bleaching of the wall rock 

adjacent to some vein structures at all levels (figs. 11, 

13). A detailed examination of the alteration assemblages 

was beyond the scope of this study and is unwarranted 

because of existing regional studies of alteration relations in the Creede area. However, certain observations 

can be made on the basis of field relations, hand-sample 

study, geochemical variations, and some X-ray diffraction analyses.

SERICITIC ALTERATION

Intense sericitic alteration characterized by illitesmectite clay minerals is generally restricted to above 

the 10400 level of the North Amethyst vein workings 

(see fig. 6). Geochemical analysis of selected material by 

the WDXRF and graphite AA techniques outlined in 

Baedecker (1987) (N.K. Foley, unpub. data, 1990) shows 

that the most intense sericitically altered rock consists of 

about 90 percent Si02 + A1203, 4 percent K20, and 2.5 

percent combined Fe203 + FeO + MgO; measured 

precious-metal values (~5 ppm Au) are much higher than 

those of typical unaltered volcanic rock but lower than 

those of substantially less-altered wall rock adjacent to 

veins lower in the workings (20 to 40 ppm Au). 5

The light-colored sericitic alteration is cut by seams 

of fine-grained dark-colored clay (mixed illite-smectite 

by X-ray diffraction). Quartz veins also cut the sericitic 

material; sharp contacts suggest that the veins formed 

when the rock was more competent, prior to the peak 

alteration event. Clear-cut paragenetic relations 

between ore mineralization and this intense alteration 

were not found; however, the alteration zone appears to 

overlie the zone of base-metal-rich mineralized rock and 

cuts across the zone of Au-bearing mineralized rock. Auand Mn-silicate-bearing assemblages occur at elevations 

above and below the intense alteration zone, but the 

base-metal-rich later stages are only below, as is true for 

veins in the main Creede district. No information is 

available on the presence or absence of alteration assem5Average gold values are notoriously difficult to determine because of the effect 

of a single nugget of electrum on a bulk analysis, so these values are given only 

for general discussion. They do not reflect actual modes or means.
MINERAL ASSOCIATIONS AND ASSEMBLAGES 23

Process

Mineralization 

K-metasomatism 

Bleach

Early quartz/ 

carbonate veins

Stage alpha 

Stage beta-1 

Stage beta-2 

Stage 1 

Stage 2 

Stage 3 

Supergene

Fracturing and 

vein movement

Brecciation 

Sedimentation

Leaching and 

replacement

Early Association (Time- Late Association

25.1 Ma

Breccia-1 -2 -3 -4

FIGURE 11. —Progression of events in the development of the North Amethyst ores. See text for description of stages and named

breccias. Arrow locates interval containing dated adularia.

blages above the highest elevations of gold mineralization. Rocks cropping out on the hillside above the North 

Amethyst portal are stained by iron oxides all along the 

trace of the Equity fault.

Well-developed clay alteration zones cap vein structures throughout much of the central and southern 

Creede district (Steven and Eaton, 1975; Barton and 

others, 1977; Horton, 1983). Barton and others (1977) 

and Bethke and Rye (1979) proposed that the caps 

formed primarily as a result of reaction of the wall rock 

with acidic fluid that resulted from condensation of ore 

fluid that boiled at depth. An age date on illite-smectite 

from the cap material in the OH vein of the southern part 

of the district is in agreement with dates measured on 

vein adularia and sericite (table 1). A similar interpretation could explain the relation between overlying 

alteration-zone and base-metal mineralization in the 

North Amethyst area; however, any relation to gold 

mineralization is left unexplained.

POTASSIUM METASOMATISM AND BLEACHING

Wall rocks of the North Amethyst vein are potassium 

metasomatized at depth. A general increase in potassium 

content of the wall rock with depth is reported by

Sawyer and others (1989), who suggest that the intensity 

of potassium metasomatism may increase with depth in 

the vein system. This pattern may be influenced by type 

and distribution of veins within the wall rock, because 

bleached wall rock has elevated potassium contents over 

adjacent unbleached wall rock at all depths (N.K. Foley, 

unpub. data, 1990). Moreover, the barium contents of 

potassium feldspar in the bleached wall rock are higher 

than barium contents of unbleached wall rock, a fact 

suggesting that the bleaching event was chemically 

distinct from the regional pre-ore potassium metasomatic event (N.K. Foley, unpub. data, 1990). Sharp 

contacts between bleached and unbleached wall rock, 

coupled with the chemical distinctions cited above, suggest that the bleaching event postdated the potassium 

metasomatism; however, this needs to be substantiated 

by studies that delineate the extent of the metasomatism 

within the Nelson Mountain Tuff and that determine 

mass transfer among the alteration zones and vein mineral assemblages. Within the central and southern 

Creede district, a bleaching event confined to wall rock 

adjacent to a vein does postdate the regional potassium 

metasomatism of the Carpenter Ridge Tuff (P. Barton 

and P. Bethke, oral commun., 1989).
24 ORES OF THE NORTH AMETHYST VEIN SYSTEM, COLORADO

Association 

early ] [ late ] [ supergene ]

Sericite

K-feldspar

Quartz

Barite

Pyrite

Hematite

Magnetite

Chlorite

Rhodocrosite

Rhodonite

Mn-calcite

Sphalerite

Galena

Chalcopyrite

Acanthite

Pyrargyrite

Electrum

Uytenbogaardtite

Ag-sulfosalts

Tetrahedrite

Mn-siderite

Calcite

Fluorite

Native Ag

Gypsum

Stages Alpha Beta Stage 1 Stage 2 Stage 3

FIGURE 12.—Composite paragenesis diagram for the North Amethyst mineralization. Mineral deposition is shown in black. Leaching 

is not shown because of its local character. The thickness of a bar for a given mineral is proportional to its abundance. Dashed 

vertical lines show temporal boundaries for stages. Most transitions are defined by brecciation (see fig. 11).

The bleaching event (fig. 13A) resulted in a color 

change from gray and gray-tan to off-white of widths 

ranging from a few centimeters to tens of centimeters in 

wall rock adjacent to some veins. The event apparently 

predates all mineralization because vein minerals of both 

associations fill structures that cut both bleached and 

unbleached wall rock. In drillcore away from the underground workings, similar bleached fractures are filled 

with a fine-grained mixture of pyrite, quartz, and, at 

times, hematite and (or) chlorite.

Unbleached Nelson Mountain Tuff forms the wall for 

many of the mineralized veins. In general, the rock is 

hard and brittle, medium to dark gray, and retains 

volcanic textures consisting of phenocrysts (<2 mm) and 

flattened pumice in a very fine groundmass. The brittle 

nature indicates that the Nelson Mountain Tuff is silicifled adjacent to veins, although chemical analyses of the 

rock show that Si02 and total alkali contents are not

much elevated over values typical for volcanic rocks of 

dacitic-rhyolitic composition, on the basis of the IUGS 

classification scheme (LeMaitre, 1989; N.K. Foley, 

unpub. data, 1990). In the northern part of the workings, 

the veins cut slightly coarser grained rocks, possibly 

intrusive dacitic Nelson Mountain Tuff (Equity Member; 

Lipman and Sawyer, 1988), that are propyllitized; they 

contain epidote, calcite, and abundant chlorite.

Most of the wall rock visible underground is cut by 

small (<3 cm) fractures that are crosscut in turn by 

minerals of the two main sulfide-bearing associations— 

the Mn-Au and base-metal-silica. These fractures are 

filled primarily with quartz and calcite and cut both 

bleached and unbleached wall rock. In drillcore away 

from the intensely mineralized area, bleached fractures 

contain mainly quartz, calcite, and pyrite, and occasionally barite and minor amounts of base-metal sulfides. In 

drillcore south of the exploratory workings, barite has
MINERAL ASSOCIATIONS AND ASSEMBLAGES 25

A

Bleached' -. / 

wall rock

Precursor vein,

calcite, quartz, and barite

FIGURE 13. —A, Bleached and unbleached wall rock adjacent to early pyrite + chlorite + 

quartz vein; these veins are found in drillcore away from main ore zones. B, Relation 

between precursor veins and bleached and unbleached wall rock within the main ore zone.

also been identified in similar fractures (D. Sweetkind, 

written commun., 1990). Barite was not found in samples 

collected underground and examined in this study, 

although it has been reported (S.W. Caddey, C.B. 

Byington, D.M. Vardiman, oral, commun., 1987). As 

described earlier, possible barite molds have been identified in samples of early assemblages collected from the 

underground workings.

MANGANESE-GOLD ASSOCIATION

The Mn-Au association is so named because of the 

presence of electrum and Au-Ag sulfide in one stage and

because of the abundance of Mn-bearing minerals in the 

other stage. The occurrence and proportion of these two 

elements and the textural character of the minerals 

distinguish this association from the later, base-metalrich association of the North Amethyst vein system and 

from other mineralized veins found throughout the 

Creede district.

The first stage, alpha, is characterized by Mncarbonate minerals, Mn-silicates, and minor base-metal 

sulfides. The second stage, beta, has abundant preciousand base-metal minerals and minor quartz and carbonate 

gangue. Each stage has two mineralogically similar
26 ORES OF THE NORTH AMETHYST VEIN SYSTEM, COLORADO

substages identified by crosscutting relations. The 

sulfide-rich substages were separated by a period of 

fracturing and the development of a breccia having 

characteristic fragments.

STAGE ALPHA

The first vein-filling event that is directly related to 

ore development in the North Amethyst area occurred 

during faulting and fracturing of the volcanic wall rock. 

This event introduced Mn-rich fluids that deposited a 

fine-grained, almost cryptocrystalline, banded mineral 

mixture consisting predominantly of rhodocrosite, manganoan calcite, rhodonite, pyroxmangite, and quartz 

(figs. 11, 14A). Locally, this fine-grained material of 

stage alpha coated a platy, tabular mineral (barite?) that 

was subsequently leached; the molds are filled with 

coarser adularia and quartz (probably stage-2 minerals; 

see below). Minor sulfides, primarily pyrite, sphalerite, 

and galena, occur in stringers in the Mn-carbonate, 

Mn-silicate, and quartz matrix. Retrograde alteration of 

this material produced a complex suite of hydrated Mnsilicate and Mn-carbonate minerals.

Vein widths of stage alpha vary from a few centimeters to more than a meter. Features that characterize 

this stage are best preserved in the narrower veins; 

wider veins show multiple fracturing events and recrystallization of minerals.

Alpha-1 substage veins cut bleached and unbleached 

wall rock and precursor veins, indicating that both the 

bleaching event and the development of precursor veins 

predated this stage. In general, contacts of vein and wall 

rock are sharp; however, the occurrence of embayed and 

partially replaced grains of potassium feldspar in the 

Mn-rich matrix suggests that some metasomatic replacement of wall rock may have occurred.

Deposition appears to have progressed from the walls 

in toward the center of the veins, leaving open spaces in 

the center. The walls of the spaces have a rounded, 

bulbous texture and were filled later with 1- to 2-mmlong adularia rhombs, coarse amethystine quartz (~2 to 

3 cm long), rhodocrosite (1 to 5 mm wide), and, in places, 

fluorite (1 to 2 mm wide) and sphalerite (1 to 2 mm wide; 

probably stage-2 minerals).

The alpha-2 substage consists of a fine-grained mixture of quartz, rhodocrosite, and Mn-silicate minerals 

deposited around fragments of alpha-1 substage and wall 

rock (fig. 15). This substage is more quartz rich than the 

earlier alpha substage and has a greater proportion of 

base-metal sulfides. Definition of this stage and its exact 

position in the paragenesis is complicated by evidence 

that most fluids of later stages reacted with or recrystallized stage alpha to some extent. Alpha-2 substage is 

probably a product of the local reaction of later stage 

fluids with fragments of the alpha-1 substage; however,

a second influx of Mn-rich fluids may have contributed to 

the mineralization.

STAGE BETA

Continued movement of the North Amethyst vein and 

fracturing of the stage-alpha assemblage caused the vein 

system to open up again. Precious- and base-metal-rich 

fluids invaded the fractures and precipitated a complex 

suite of Au-, Ag-, Cu-, Zn-, Pb-, and Fe-bearing sulfides 

and sulfosalts (figs. 12, 14B). The location of the sulfiderich seams may have been controlled in part by the 

structurally weak vuggy zones in the core of stage-alpha 

vein material. Seam widths vary from a few millimeters 

to tens of centimeters, although most samples studied 

had widths of 2 to 3 cm. The ore seams are reasonably 

intact in some areas, but most show minor offsets. In 

other areas, the seams are stretched and broken, and 

sulfide-rich pods are more strongly deformed.

Stage-beta fluids appear to have irregularly altered 

stage-alpha minerals from a dominantly Mn-carbonate, 

Mn-silicate, quartz assemblage to a Mn-carbonate and 

quartz assemblage. During the retrograde alteration, 

hanging-wall sides of the sulfide-rich seams are often 

recrystallized to a coarse Mn-calcite, whereas foot walls 

are altered to a dusty fine-grained mixture of rhodocrosite and quartz (fig. 4D).

The complex suite of precious- and base-metal sulfides 

precipitated by stage-beta fluids is listed in table 3, and 

the paragenetic sequence for the most common stagebeta minerals is shown in figure 12. The intergrowth of 

gangue, sulfides, and sulfosalts shows that, in general, 

deposition within stage beta was initiated by precipitation of pyrite and sphalerite, followed by galena, chalcopyrite, electrum, and other Au and Ag minerals. 

Although the depositional sequence was interrupted by 

minor periods of dissolution (for example, replacement of 

pyrite crystals; chalcopyrite, and tetrahedrite disease in 

sphalerite; corroded sphalerite crystals), stage-beta precipitation was generally continuous. Native silver, stromeyerite, and other minerals are found mainly in tiny 

late-stage veinlets that cut the stage-beta seams or as 

rims on earlier phases, and may be supergene.

Stage-beta seams show a mineralogic zonation that is 

discontinuous along strike of the seams and irregularly 

distributed across the vein widths. Zn, Fe, and Pb 

minerals occur across the width of all seams, while 

chalcopyrite and other Cu-bearing minerals are concentrated toward the center of the seams. In general, 

electrum and Au- and Ag-sulfides and sulfosalts occur 

with, or are generally formed later than, chalcopyrite.

The iron-oxide and -sulfide assemblage of stage beta 

consists of magnetite, hematite, and pyrite. This relatively oxidizing assemblage occurs locally along the
MINERAL ASSOCIATIONS AND ASSEMBLAGES 27

Amethystine 

quartz

Stage alpha

Rhodocrosite, rhodonite,

quartz, calcite, and

K-feldspar

Rhodocrosite

v \ Barite 

* •* /" molds (?)

Adularia

Sphalerite, galena, and other 

stage-beta minerals

Stage alpha

Wall rock

Reaction zone,

microfractures,

and small offsets

Chalcopyrite, gold, 

and other preciousmetal minerals

FIGURE 14. —A, Relation among stage alpha, precursor vein, and bleached and unbleached wall rock. Inset shows banded 

nature of fine-grained Mn-carbonates, Mn-silicates, quartz, sulfides. B, A stage-beta sulfide seam. The location of the 

seam was probably controlled by a structurally weak, vuggy zone at the core of stage alpha.

stage-beta seams and is most frequently found at depth 

in the system. The irregular distribution of the ironoxide minerals, in contrast to the pervasive occurrence of 

pyrite, may indicate local change from pyrite equilibria 

to pyrite-hematite equilibria or a pyrite-magnetitehematite buffer by an oxidative process such as boiling 

(Foley, 1990).

The pyrite, hematite and magnetite are intergrown 

with fine-grained Zn-, Pb-, and Cu-sulfides and abundant 

quartz. Electrum is the only precious-metal mineral 

commonly found with the iron-rich assemblage; it is 

intergrown with magnetite and pyrite and compositionally is similar to electrum in other assemblages. Euhedral magnetite is intergrown with pyrite and hematite in
28 ORES OF THE NORTH AMETHYST VEIN SYSTEM, COLORADO

Disseminated

base-metal sulfides

in stage alpha-2

Stage alpha-2

Unbleached 

wall rock

Bleached 

wall rock

Alpha-1

FIGURE 15.—Two periods of Mn-carbonate and Mn-silicate mineralization of stage alpha are shown by the occurrence of one in the 

other.

apparent equilibrium, and magnetite also replaces hematite blades; the contrasting textures support a change 

from pyrite to pyrite-hematite-magnetite and locally to 

pyrite-magnetite or magnetite equilibria, as noted previously. Hematite appears, in some cases, to be altered 

to limonite; this alteration probably is supergene in 

origin.

The occurrence of rounded cobbles of stage beta in 

breccias and sedimented zones cut by later competent 

seams of stage-beta sulfides was the basis for defining 

two substages (beta-1 and beta-2). The two have essentially identical mineralogies and differ only in their 

occurrence (fig. 16). Lead-isotopic compositions of galenas from the two substages are identical within analytical 

error (Foley, 1990). Electrum compositions and sphalerite compositions are also virtually identical for the two 

substages (tables 6 and 8).

Breccia-1.— Precious-metal minerals were deposited 

in two pulses separated by a period of brecciation and 

sedimentation along parts of the vein. The breccia that 

developed between the two beta substages is characterized by an unsorted mix of subangular to rounded pieces 

of fine-grained Au-bearing ore, angular fragments of 

bleached and unbleached wall rock, and fragments of 

stage alpha held together with a microcrystalline, dominantly quartz cement (fig. 16).

TRANSITIONAL BRECCIAS AND SEDIMENTS

The transition between deposition of the Mn- and 

Au-rich assemblages and assemblages of the basemetal-silica association is marked by extensive brecciation of, and sedimentation in, the veins. The movement 

along the veins may have been a continuation, on a much 

larger scale, of the fracturing that formed breccia-1. 

Most of the breccias and sediments formed during this 

period are lumped together under the category 

breccia-2, with the exception of a matrix-supported, 

quartz-cemented breccia referred to as the "black quartz 

breccia." The different breccias types record a short 

history of the transition period from the initial dynamic 

hydrology that reworked fragments and deposited them 

as fragment-supported breccias in large shoots and sediment "dumps" to the more quiescent hydrologic setting 

that resulted in graded bedding and chemically 

cemented, matrix-supported breccias.

BRECCIA-2

Breccias that are lumped together in this group have 

fragments that range in width from a few millimeters to 

15 cm and in shape from angular to rounded (fig. 17). The 

fragments consist of all earlier stages of mineralization 

and wall rock; most fragments were derived from points 

close to the site of deposition.

Large breccia zones, resembling "shoots" of sedimented material with local graded bedding and quartz 

cement, formed as the fluids dumped their sediment 

load. The unsorted material forming the breccia zones 

consists mainly of subangular to rounded, bleached and 

unbleached fragments of volcanic wall rock, rounded and 

altered pieces of stage alpha, rounded to subrounded 

cobbles up to 15 cm in width of sulfide-rich stage beta, 

and fragments of earlier breccias (fig. 17A,5). The pieces 

are held together with quartz-rich cement. Interstitial 

sediment between larger fragments has graded bedding, 

soft-sediment deformation, and other textures indicating 

that the material settled out of solution after the larger 

cobbles and blocks were dumped (fig. 175).

In another area, silt- to gravel-sized fragments settled 

out in large graded beds of at least four cycles that 

extend for 30 m along a vein wall and are greater than 3 

m in height (tops and bottoms were not visible; fig. 4G, 

17C). The water-deposited beds show minor softsediment deformation and an occasional sulfide-bearing 

cobble (up to 8 cm in width) that has fallen into finely 

graded sediment (fig. 4#). The graded beds are truncated at one end by a mineralized linking fault, and at the 

other end they appear to drape against hanging-wall 

volcanic rocks.

Black quartz breccia.—Late in this transition period, 

veins of "black quartz breccia" developed along planes of 

movement (fig. 17.D). The black quartz breccias consist of 

50 to 90 volume percent microcrystalline quartz-rich 

matrix and 10 to 50 percent fragments of stage-beta and 

stage-alpha material, earlier breccias, and wall rock. The
MINERAL ASSOCIATIONS AND ASSEMBLAGES 29

.Stage beta-2

Beta-1

Fine-grained 

quartz cement

Wall rock

Wall-rock 

fragments

Alpha-1 

fragment

Beta-1 fragments

Beta-1 cobble

Alpha 

fragment

Bleached 

wall rock

Wall rock' Beta -1 

cobble

FIGURE 16.—Relation between beta-1 and beta-2 substages. Insets show development of breccia-1, which contains cobbles

of beta-1 substage and is cut by a seam of beta-2 substage.

cement is composed dominantly of microcrystalline 

quartz having a granular texture with sutured grain 

boundaries. The cements grade in color from light gray 

to black, apparently in relation to the amount of fine 

sulfides and electrum contained therein. It has not been 

possible to ascertain whether the sulfide aggregates are 

clastic or if they formed as precipitates, but the shapes of 

the grains suggest that they were broken and ground. 

They may be finely ground dust from earlier stage-beta 

sulfide seams.

BASE-MET AI^SILICA ASSOCIATION

This association has three mineralogical stages: (1) an 

interval consisting dominantly of base-metal sulfides, 

iron oxides, and quartz; (2) an interval of Ca-, Mn-, and 

Fe-bearing carbonate minerals, fluorite, and quartz; and 

(3) a Mn-calcite, quartz, and pyrite interval. The association is named after the most abundant constituents, 

base-metal sulfides and quartz. Complete mineralogical 

listings of all stages are given in table 3.
30 ORES OF THE NORTH AMETHYST VEIN SYSTEM, COLORADO

Beta-1 cobble

Fine quartz 

cement

Wall-rock 

fragments

Beta-1 cobble

Breccia-1

Breccia-1

Sulfide clots

Breccia-1

Quartz cement with 

fine dark sulfides

Fine

Quartz cement

with fine dark

sulfides

Wall rock 

Breccia-1

Coarse quartz sediment

Wall rock

Breccia-1 

Stage beta

Breccia-2

FIGURE 17. —A, Breccia containing wall rock and sulfide cobbles cemented with a quartz slurry. B, Detail showing bedding in 

fine-grained quartz cement surrounding breccia fragments. C, Graded bedding from fine-grained quartz cement to coarser breccia 

fragments. D, Black quartz breccia (see text); breccia fragments in quartz colored by sulfide dust.

STAGE 1

The first stage of the base-metal-silica association (fig. 

18) is characterized by abundant base-metal sulfides, 

argentian tetrahedrite, hematite, chlorite, and quartz. 

Two mineralogically distinct substages are recognized: a 

chlorite, hematite, and quartz assemblage was deposited 

prior to, and concurrently with, the major period of basemetal mineralization that resulted in a crustified 

sequence of sphalerite, galena, pyrite, chalcopyrite, and 

tetrahedrite.

The fine-grained chlorite, hematite, and quartz assemblage was deposited on blocks of earlier breccias and in 

fractures cutting earlier assemblages of the Mn-Au association. Isolated crystals of sphalerite appear in the 

siliceous mixture. Some of the siliceous mixture was 

fragmented and recemented before the fluids began 

depositing relatively coarse base-metal sulfides. In other 

areas, the sulfides are intergrown with chlorite, hematite, and quartz without obvious breaks, so a gradual 

shift in dominant mineralogy is suggested.
MINERAL ASSOCIATIONS AND ASSEMBLAGES 31

Breccia-2

Chlorite, pyrite, 

and quartz

Stage-1 

minerals

Breccia-1 

fragments

Stage-1 sphalerite 

crystals

Sphalerite, galena 

pyrite, chalcopyrite, 

and quartz

Skeletal 

galena

Stage-alpha 

fragment

Beta-1 

fragment

Skeletal 

galena

Sphalerite crystals 

in quartz matrix

Cryptocrystalline quartz,' 

chlorite, and Mn-calcite

Vuggy zone

FIGURE 18.— Relation of stage-1 minerals to all preceding stages and breccias. Insets show textures typical of stage 1. See figure 5A-D.

The coarse-grained sphalerite, galena, pyrite, and 

chalcopyrite assemblage of stage 1 contains interstitial 

quartz, chlorite, and hematite. Hematite flakes also are 

included in the sphalerite crystals. No clear-cut paragenetic relations are apparent among the minerals, with 

the exception that fluid reacted with sphalerite along 

fractures and grain boundaries to form chalcopyrite- and 

tetrahedrite-disease textures (Barton and Bethke, 1987). 

Sphalerite crystals are corroded, and periods of dissolution and reprecipitation are identifiable through studies 

of thin sections. Late-forming galena of stage 1 occurs as 

skeletal crystals in vugs that were subsequently filled 

with minerals of stage 2 (fig. 18).

Breccia-3.— Continued local fracturing of stage-1 minerals formed a breccia that is characterized by fragments 

of coarse base-metal sulfide and chloritized wall rock 

cemented by a slurry of cryptocrystalline quartz, chlorite, and pyrite (figs. 5F, 19). The chlorite gives the
32 ORES OF THE NORTH AMETHYST VEIN SYSTEM, COLORADO

Stage beta-2

Stage-beta fragment

Breccia-3, base-metal

sulfide fragments, 

quartz, and chlorite cement

Relatively unbroken 

Stage 1

Breccia-3

Breccia-2

FIGURE 19.—Local development of breccia-3, characterized by fragments of stage-1 sulfides and a green, chlorite-rich cement.

cement a green hue. Minor amounts of base-metal sulfides were deposited on this brecciated material. Fragments of the base-metal sulfide are angular to subangular, and the wall-rock fragments are rounded. The 

breccia is most frequently found where stage-1 vein 

widths are greatest.

STAGE 2

The second stage of mineralization in the base-metalsilica association (fig. 20) is characterized by coarsegrained quartz, calcite, rhodocrosite, and fluorite, all 

intergrown with minor amounts of adularia, chlorite, and 

pyrite. Locally, manganosiderite occurs within the calcite. Stage 2 was deposited on breccia-3 and stage 1. The 

minerals are coarse grained; quartz crystals grew up to 

40 mm in length, while green fluorite cubes attained 13 

mm on a side. Smaller octahedra of fluorite (<3 mm) are 

colorless or pale green or purple. In general, quartz, 

chlorite, and calcite were deposited before fluorite and 

additional quartz. Quartz crystals are zoned from colorless and white to amethystine and contain solid inclusions 

of calcite, adularia, and fluorite. Occasionally, a small 

amount of fine gray quartz sediment was deposited in 

vugs on crystals of stage 2.

Quartz and adularia that fill barite(?) molds in stage 

alpha probably crystallized from stage-2 fluids. The 

minerals are texturally identical to stage-2 minerals and 

have fluid-inclusion salinities and homogenization temperatures that are similar to those of other stage-2 

minerals (Foley, 1990). Rhodonite crystals (up to 0.5 mm 

in length) that form selvages and project into some of the

quartz-filled molds, and are intergrown with some 

rhodocrosite, may have formed from recrystallization of 

finer grained stage-alpha material.

Breccia-4,.— This breccia contains angular fragments 

of all earlier assemblages, some breccias, and broken 

crystals and is cemented with a pale- to light-pink 

calcium-rich carbonate having a composition of Cc95Rc5 

(fig. 21). It is especially prevalent in the north parts of 

the vein system. The fragile and vuggy nature of the 

breccia suggests a late formation, probably concurrent 

with the development of stage 3.

STAGE 3

The last clearly defined mineralization in the paragenetic sequence was deposition of quartz or manganocalcite, with pyrite, along the North Amethyst structure 

(fig. 22). Movement along the structure created vugs, 

cavities, and brecciated zones that cut all of the earlier 

stages. This movement was accompanied by, or followed 

by, leaching of earlier vein material along the entire 

length of the North Amethyst workings.

In the northern part of the ore zone, extensive crusts 

of pink to pale-yellow Mn-bearing calcite, interlayered 

with thin layers of crustiform pyrite, were deposited in 

the vugs and on material leached earlier. The carbonate 

crusts ranged in total thickness from millimeters to a 

meter. The carbonate minerals were selectively leached 

and now contain large, open vuggy zones.

Leaching of earlier stages, especially the extensive 

breccias, appears to have been more extensive in the 

southern workings. Samples of breccia-2 exhibit gradations in the degree of leaching ranging from bleaching of 

fragments to alteration of phenocrysts to clay minerals to 

samples that consist entirely of a foamlike siliceous 

material containing molds possibly of leached blocks of 

wall rock and rare specks of dark sulfides. Geochemical 

analysis of the most altered and leached material indicates that, of the major elements, only Si02 (—94 percent), and minor A1203 (~3 percent) and K20 (~1 percent) remain. Relatively high values of base and precious 

metals also persist (—90 ppm Au, 30 ppm Ag). This 

leached material is overgrown by large terminated crystals of quartz in the southern part of the ore zone. The 

quartz crystals are included in stage 3 because they 

overgrow the leached base; however, their formation 

may predate the manganocalcite. Minute cubes of pyrite 

dust the quartz surfaces; timing of this pyrite relative to 

the crustiform occurrence within manganocalcite is not 

clear.

CORRELATION OF MINERAL ASSEMBLAGES 

WITH OTHER VEIN SYSTEMS

The relation of the North Amethyst ores to those of 

the main Creede district 5 km to the south is of great
CORRELATION OF MINERAL ASSEMBLAGES WITH OTHER VEIN SYSTEMS 33

Quartz

Stage beta-2

Skeletal 

galena

Sphalerite 

and galena

Calcite

Breccia-3 Quartz

Mn-calcite

Quartz 

sediment

Fluorite

Wall rock

Base-metal sulfide 

fragments

Quartz and chlorite 

cement

1.0cm

FIGURE 20.—Reopening of vein and deposition of stage-2 minerals on preexisting mineral stages and breccias. Insets show

textures of stage-2 minerals.

importance in assessing the nature, extent, and timing of 

mineralization. The mineralogy, textures, and paragenetic sequences established in this study can be used to 

compare the North Amethyst veins to veins of the main 

mining district at Creede. Paragenetic relations for the 

North Amethyst vein system and some veins of the main 

mining district are summarized in table 9. Mineral stages

defined for the North Amethyst vein system can be 

correlated, in part, with mineral stages defined in previous studies for the southern Amethyst vein (Guidice, 

1981; Robinson, 1981; and Robinson and Norman, 1984), 

the Bulldog Mountain vein system (Plumlee, 1989), and 

the OH and P veins (Barton and others, 1977; Bethke and 

Rye, 1979; P.B. Barton, Jr., oral commun., 1989). Most
34 ORES OF THE NORTH AMETHYST VEIN SYSTEM, COLORADO

Vug space

Wall rock

Mn-calcite cement 

Alpha

Fluorite octahedra 

Breccia-2

Highly fractured base

of wall rock, breccia, and

vein material

FIGURE 21.—Development of breccia cemented by pink calcite just 

prior to, or concurrently with, stage-3 mineralization.

of the corresponding stages are mineralogically similar 

and occur throughout the district; however, there are 

some important exceptions.

At present, stages alpha and beta of the Mn-Au 

association have been found only in the North Amethyst 

area. The assemblages have not been described elsewhere within the boundaries of the Creede mining district. Mn-silicates and uytenbogaardtite are not found 

south of the North Amethyst workings, although rare 

occurrences of electrum have been reported for a couple 

of localities. An occurrence of dark-yellow gold with a 

high degree of fineness contained in a gangue of manganese oxides has been described by Emmons and Larsen 

(1913, 1923) from the Happy Thought and Amethyst 

mines of the Amethyst vein. The gold occurred in cracks 

and veinlets cutting older ore and was said to be a 

common supergene occurrence in near-surface workings. 

Gold also has been reported as inclusions in pyrite, 

galena, sphalerite, and other minerals (Emmons and 

Larsen, 1923); however, no additional details are available. P.M. Bethke and P.B. Barton, Jr. (oral commun., 

1989), note the occurrence of abundant finely disseminated gold in early chalcopyrite in a single sample from 

the OH vein (their stage A or B, table 9). Galenas from 

North Amethyst gold-bearing stage beta are distinct in 

their lead-isotopic character from galenas of stage 1 of 

the North Amethyst veins and from most veins of the 

main Creede mining district (Foley, 1990; Foley and 

Ayuso, in press); this distinctness lends support to their 

unique occurrence in the north.

The three stages of the base-metal-silica association of 

the North Amethyst vein system can be correlated

directly with mineralogically similar stages of the OH 

and P veins (table 9; defined as stages A through E). 

Stage A of the northern OH and P veins consists 

primarily of massive chalcedonic quartz intergrown with 

rhombs of adularia. This stage is not found in the North 

Amethyst ores; however, in the interval of brecciation 

and sedimentation between the two sulfide-rich North 

Amethyst stages (stage beta and stage 1), quartz was 

deposited as chalcedonic cement in breccia. Although not 

defined as a vein-filling stage, the quartz cement may 

correlate with OH vein stage-A quartz, and this correlation would indicate a districtwide period of silica deposition.

Stage B of the OH and P veins is dominated by 

fine-grained base-metal sulfides—sphalerite, galena, 

chalcopyrite, and lesser amounts of Ag-rich tetrahedrite—contained in a chlorite, pyrite, hematite, and 

quartz gangue assemblage. This stage correlates directly 

with stage 1 of the North Amethyst vein (table 9). 

Sphalerites of the North Amethyst, OH, and P vein 

assemblages show extensive chalcopyrite- and minor 

tetrahedrite-disease textures. Zoned sphalerites from 

the OH stage B and North Amethyst stage 1 are 

compositionally similar; light zones have generally low 

iron contents (<2 mole percent), and dark bands are iron 

rich (up to 8 mole percent for North Amethyst stage 1, 

up to 16 mole percent for OH stage B). Galenas (stage 

IB) from all three veins also have similar lead-isotopic 

compositions (Foley, 1990; Foley and Ayuso, in press). 

The composition of OH and P vein tetrahedrite is variable but generally is silver rich (L.B. Wiggins and T.L. 

Woods, unpub. data, 1980), similar to that of the North 

Amethyst ore.

Stage 2 of the North Amethyst vein correlates mineralogically with stage C of the OH and P veins (table 9). 

Both stage 2 and stage C contain fluorite, quartz, and 

carbonate minerals. The North Amethyst stage-2 carbonate minerals consist of intergrown calcite and manganosiderite, whereas the OH and P veins contain only 

manganosiderite (Wetlaufer, 1977).

OH and P vein stage D ores are apparently absent in 

the North Amethyst vein system (table 9). Stage D 

consists largely of coarsely crystalline sphalerite, galena, 

quartz, chalcopyrite, and hematite. D-stage sphalerite is 

free of chalcopyrite disease and is color zoned from core 

to rim with yellow-white to red-brown to yellow-white 

zones having iron contents of less than 3 mole percent 

FeS (Barton and others, 1977).

Stage 3 of the North Amethyst vein system can be 

correlated with stage E of the OH and P veins mainly on 

the basis of the occurrence of pyrite in all of these stages 

(table 9). Pyrite in North Amethyst stage 3 is volumetrically minor compared to stage E, and quartz and
CORRELATION OF MINERAL ASSEMBLAGES WITH OTHER VEIN SYSTEMS 35

Beta 1 Precursor vein

Stage 3, Mn-calcite, 

pyrite, and vug

Mn-calcite

Stage 3, Mn-calcite 

filling earlier vugs

Stage 1

Stage 2

FIGURE 22. —Stage 3 is marked by deposition of thick crusts of a coarse pink calcite, pyrite, and quartz. Inset shows calcite and pyrite

occurrence.
36 ORES OF THE NORTH AMETHYST VEIN SYSTEM, COLORADO

TABLE 9.—Districtwide correlation of mineral assemblages and brecciation events

[Stage designations for each vein are shown in parentheses. For example, (II) is stage II of Plumlee (1989). Abbreviations: Ac, acanthite; Ad, adularia; Ag, silver; Au, 

electrum; Bar, barite; Cc, calcite; Chl, chlorite; Cpy, chalcopyrite; Fl, fluorite; Gn, galena,; Hm, hematite; Mn, manganese; Pear, pearcite; Px, pyroxmangite; 

Py, pyrite; Pyr, pyrargyrite; Qz, quartz; Re, rhodocrosite; Rh, Rhodonite; Sp, sphalerite; Sid, siderite; Stib, stibnite; Td, tetrahedrite-tennantite; Uyten, 

uytenbogaardtite]

North Amethyst vein 

(Foley and Vardiman, 1988; 

Foley, 1990)

(3) Mn-Cc + Qz + Py 

Breccia-4 

Absent

OH and P veins 

(Bethke and Rye, 1979)

(E) Py + Stib 

Absent? 

(D) Sp + Gn + Qz +

Bulldog vein 

(Plumlee, 1989)

(V) Py + Stib + Pyr 

Absent? 

(IV) Sp + Gn + Cpy +

a J.T. A ii. i • Disseminated ore, OH Southern Amethyst vein , . , , , ' . m u- it\ai\ and Amethyst veins (Robinson, 1981) (Guidice, 1981)

Absent? 

Absent? 

Absent?

(2) Py + Qz. 

Absent? 

Absent?

(2) Qz + Mn-Cc + Fl + 

Re + Mn-sid + Chl + 

Py + Ad

Breccia-3

(1) Sp + Gn + Py + Cpy + 

Qz + Hm + Chl + Td

Breccia-2

(Beta-2) Gn + Sp + 

Cpy + Py + Td + 

Ac + Uyten + Au

Breccia-1

(Beta-1) Gn + Sp + 

Cpy + Py + Td + 

Ac + Uyten + Au

(Alpha) Re + Qz + 

Mn-Cc + Rh + Px + 

Sp + Gn + Py + Ad

Cpy -i-Hm 

(C) Fl + Qz + Mn-sid

Absent?

(B) Qz + Sp + Chl + 

Gn + Hm + Td + Au

(A) Qz + Ad 

Absent?

Absent? 

Absent?

Absent?

Td + Pear + Ac

(III) Qz + Fl + Mn-sid + 

Sp + Gn

Absent?

(II) Bar + Qz + Sp + 

Gn + Td

(I) Re + Qz + Sp + 

Gn + Ad

Absent?

Absent? 

Absent?

Absent?

Absent?

Absent?

(2) Qz + Sp + Gn + 

Py + Td + Hm + 

Ac + Ag

(1) Re + Qz + Bar + 

Cc + Sp + Gn + 

Py + Cpy

Absent?

Absent? 

Absent?

Absent?

Included in (1)?

Absent?

(1) Sp + Gn + Py + Cpy + 

Chl + Re + Bar + 

Ag + Td + Ac

Absent?

Absent?

Absent? 

Absent?

Absent?

manganoan calcite occur in relatively greater proportions.

The sequence of mineral stages defined for the Bulldog 

Mountain vein system has been correlated with mineral 

stages of the OH and P veins by Plumlee (1989). The 

comments and ideas presented in that study may hold 

true for comparing the North Amethyst and Bulldog 

Mountain paragenetic sequences because of the mineralogical similarities between the OH and North Amethyst 

assemblages (table 9). As in the case of the OH vein, 

chalcedony of Bulldog Mountain stage I may correspond 

to chalcedonic cements of North Amethyst breccia-2. 

The North Amethyst stage 2 is more like the sphaleriteand galena-rich, Ag-poor assemblages of the northern 

Bulldog Mountain stage II than the barite and Ag-rich 

assemblages of the southern half of the Bulldog system. 

However, in contrast to the North Amethyst stage 2, 

northern Bulldog Mountain stage II does contain some 

barite and does not have abundant chlorite-pyritehematite-quartz gangue. Compositions of carbonate minerals of North Amethyst stage 2 (table 5) are similar to 

those of Bulldog Mountain stage III (Wetlaufer, 1977).

Thus, Bulldog Mountain stage III may correlate with 

North Amethyst stage 2. Bulldog Mountain stage IV, 

which correlates with OH stage D, is not found in the 

North Amethyst area. Pyrites appear in both Bulldog 

Mountain stage V and North Amethyst stage 3.

Correlations between mineralogical stages of the 

North Amethyst vein system and mineralized segments 

of the southern part of the Amethyst vein are difficult 

because paragenetic relations for some of those areas are 

less well known. Two main stages of mineralization along 

the southern Amethyst vein system have been defined 

by Robinson (1981), and he has correlated the two 

assemblages with the A and B stages defined by Bethke 

and Rye (1979) for the OH vein. Minerals of southern 

Amethyst vein stage 2 correlate mineralogically with 

stage 1 of the North Amethyst vein (table 9). Galenas 

from the southern Amethyst stage-2 assemblage have 

lead-isotopic values that overlap the field defined by 

lead-isotopic compositions of galenas from the North 

Amethyst stage 1 (Foley, 1990).

Disseminated ore present in the hanging wall of the 

Amethyst fault at the southern end of the mineralized
REFERENCES CITED 37

structure, in the vicinity of an on-strike projection of the 

OH vein, has been examined by Guidice (1981). He has 

described two main hypogene stages and a supergene 

stage for disseminated minerals in volcanic rock located 

between the two veins. The first hypogene stage of 

mineralization may correlate with a combination of 

stages 1 and 2 of the North Amethyst vein and, as 

described by Plumlee (1989), with a combination of 

stages A through C of the OH vein and stages I through 

III of the Bulldog Mountain vein. This lumping of stages 

clearly reflects a lack of clear-cut paragenetic relations in 

the disseminated ore. The second hypogene stage for the 

disseminated ore correlates well with stage 3 of the 

North Amethyst vein.

The detailed paragenetic studies (table 9) demonstrate 

that base-metal minerals of the North Amethyst stage 1, 

OH and P vein stage B, and Bulldog Mountain stage II 

constitute the most widely distributed and persistent 

mineralogical stage, occurring in some form in all veins. 

Later stages of mineralization are more local and were 

interrupted by periods of dissolution. The North Amethyst vein retains the most complete record of mineralogical stages deposited anywhere in the district (table 9). 

In some of the other veins, only certain mineralogical 

stages are well developed, and many stages are absent or 

unrecognized. Thus, the North Amethyst vein probably 

has the most complete record of the changing chemical 

composition of the hydrothermal fluids.

CONCLUSIONS

1. The paragenesis of ore minerals of the North Amethyst vein system can be divided into two multistage 

associations separated by periods of intense brecciation. The earlier association has two stages, the first 

of which is characterized by abundant manganese 

minerals and the second by high gold content. The 

later association has three stages that consist primarily of base-metal sulfides, chlorite, quartz, carbonate 

minerals, and fluorite, in varying proportions.

2. Complete assemblages of the earlier Mn-Au stages of 

the North Amethyst system are not found elsewhere 

in the central San Juan region. In contrast, mineralogical stages and breccias identified in the basemetal-silica association can be correlated with paragenetic stages defined for veins of the central and 

southern Creede mining district.

3. The local and early occurrence of the Mn and Au 

assemblages may indicate that they either formed in a 

small hydrothermal cell that predated the extensive 

hydrothermal system from which ores of the Creede 

district are proposed to have been deposited (Bethke, 

1988) or are remnants of a larger early hydrothermal

system. If other early hydrothermal cells with similar 

characteristics were present in structures of the central and southern parts of the district, they may have 

been replaced or overprinted by later assemblages, or 

they remain to be discovered. Early-stage paragenetic mineral assemblages may hold the greatest 

potential for vein-related gold mineralization in structures of the central caldera cluster of the San Juan 

Mountains.

4. The correlations of mineral assemblages described 

above have important implications for hydrologic 

modeling of the northern, central, and southern parts 

of the Creede district. The extensive correlation of 

base-metal-, silver-, and silica-rich assemblages suggests a similar source of fluids and metals for the ore 

mineralization and implies that the minerals were 

deposited in the veins by similar mechanisms and 

under similar conditions of ore formation.

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Baars, D.L., and Stevenson, G.M., 1984, The San Luis uplift, Colorado 

and New Mexico—An enigma of the ancestral Rockies: The 

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Baedecker, P.A., ed., 1987, Methods for geochemical analysis: U.S. 

Geological Survey Bulletin 1770, 132 p.

Barton, Mark, 1978, The Ag-Au-S system: Blacksburg, Va., Virginia 

Polytechnic Institute and State University, unpublished M.S. 

thesis, 75 p.

Barton, M.D., Kieft, C., Burke, E.A.J., and Oen, I.S., 1978, Uytenbogaardtite, a new silver-Au sulfide: Canadian Mineralogist, v. 16, 

pt. 4, p. 651-657.

Barton, P.B., Jr., and Bethke, P.M., 1987, Chalcopyrite disease in 

sphalerite: Pathology and epidemiology: American Mineralogist, v. 

72, p. 451^67.

Barton, P.B., Jr., Bethke, P.M., and Roedder, Edwin, 1977, Environment of ore deposition in the Creede mining district, San Juan 

Mountains, Colorado; Part III, Progress toward interpretation of 

the chemistry of the ore-forming fluid for the OH vein: Economic 

Geology, v. 72, no. 1, p. 1-24.

Bethke, P.M., 1988, The Creede, Colorado ore-forming system: A 

summary model: U.S. Geological Survey Open-File Report 88-403, 

29 p.

Bethke, P.M., Barton, P.B., Jr., Lanphere, M.A., and Steven, T.A., 

1976, Environment of ore deposition of the Creede mining district, 

San Juan Mountains, Colorado; Part II, Age of mineralization: 

Economic Geology, v. 71, no. 6, p. 1006-1011.

Bethke, P.M., and Lipman, P.W., 1987, Deep environment of volcanogenie epithermal mineralization: Proposed research drilling at 

Creede, Colorado: Eos, Transactions, American Geophysical 

Union, v. 68, no. 13, p. 177, 187-189.

Bethke, P.M., and Rye, R.O., 1979, Environment of ore deposition of 

the Creede mining district, San Juan Mountains, Colorado; Part 

IV, Source of fluids from oxygen, hydrogen, and carbon isotope 

studies: Economic Geology, v. 74, no. 8, p. 1832-1851.
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Bickford, M.E., 1988, The accretion of Proterozoic crust in Colorado: 

Igneous, sedimentary, deformational, and metamorphic history, in 

Ernst, W.G., ed., Metamorphism and crustal evolution of the 

Western United States: Prentice Hall, Rubey Volume 7, p. 

411^30.

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Foley, N.K., and Vardiman, D.M., 1988, Paragenesis and mineral 

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Hayba, D.O., Bethke, P.M., Heald, Pamela, and Foley, N.K., 1985, 

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volcanic units in the San Juan Mountains, Colorado: Recalculated 

K-Ar age determinations using IUGS constants: U.S. Geological 

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Horton, D.G., 1983, Argillic alteration associated with the Amethyst 

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Hurford, A.J., and Hammerschmidt, K., 1985, ^Ar/^Ar and K/Ar 

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SELECTED SERIES OF U.S. GEOLOGICAL SURVEY PUBLICATIONS

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