Major Styles of Mineralization and 

Hydrothermal Alteration and Related 

Solid- and Aqueous-Phase Geochemical 

Signatures

By Dana J. Bove, M. Alisa Mast, J. Bradley Dalton, Winfield G. Wright, and 

Douglas B. Yager

Chapter E3 of

Integrated Investigations of Environmental Effects of Historical 

Mining in the Animas River Watershed, San Juan County, Colorado

Edited by Stanley E. Church, Paul von Guerard, and Susan E. Finger

Professional Paper 1651

U.S. Department of the Interior

U.S. Geological Survey
Contents

Abstract .......................................................................................................................................................165

Introduction.................................................................................................................................................165

Methods.......................................................................................................................................................166

Outline of Major Styles and Ages of Mineralization and Hydrothermal Alteration 

within the Animas River Watershed Study Area .....................................................................168

Pre-Ore Regional Propylitic Alteration Assemblage ...........................................................................169

CEC Group ..........................................................................................................................................169

CIC Group ...........................................................................................................................................170

Geochemistry of Propylitically Altered Rocks .............................................................................173

Background Surface Water Chemistry .........................................................................................178

26–25 Ma Molybdenum–Copper Porphyry Mineralization and Alteration .......................................178

Mount Moly Area Ore Deposits ......................................................................................................178

Hydrothermal Alteration in the Mount Moly Area ..............................................................179

Alteration Zones Near the Southeast Margin of the Silverton Caldera ..................................181

Geochemistry of Altered Rocks ......................................................................................................181

Mine Dump Compositions ...............................................................................................................182

Mine Water Chemistry .....................................................................................................................182

Background Surface Water Chemistry .........................................................................................184

23 Ma Acid-Sulfate Alteration and Mineralization ...............................................................................192

Red Mountains Area Ore Deposits ................................................................................................197

Ohio Peak–Anvil Mountain Mineralization and Ore Deposits ...................................................199

Hydrothermal Alteration ..................................................................................................................199

Acid-Sulfate Alteration Assemblages ..................................................................................201

Silicification .....................................................................................................................201

Quartz-Alunite Alteration ...............................................................................................201

Quartz-Pyrophyllite and Argillic Assemblages ..........................................................203

Hydrothermal Sericitic Assemblages ...................................................................................203

Quartz-Sericite-Pyrite Assemblage .............................................................................203

Weak Sericite-Pyrite Assemblage ...............................................................................204

Origin, Age, and Relation to Intrusions .........................................................................................204

Geochemistry of Altered Rocks in the Red Mountains and Ohio Peak–Anvil 

Mountain Areas ...................................................................................................................204

Mine Dump Compositions ...............................................................................................................205

Red Mountains Area ...............................................................................................................205

Ohio Peak–Anvil Mountain Area ...........................................................................................205

Mine Water Chemistry .....................................................................................................................206

Red Mountains Area ...............................................................................................................206

Ohio Peak–Anvil Mountain Area ...........................................................................................206

Background Surface Water Chemistry .........................................................................................207

Red Mountains Area ...............................................................................................................207

Ohio Peak–Anvil Mountain Area ...........................................................................................208
18–10 Ma Polymetallic Vein Systems—Northeast-Trending Veins ...................................................208

Eureka Graben Area .........................................................................................................................208

Eureka Gulch, Treasure Mountain, and Placer Gulch Gold-Rich Veins ..........................209

Base-Metal Veins with Late-Stage Manganese Silicates ................................................211

Mineral Point ...................................................................................................................211

California Gulch ...............................................................................................................211

Zoning of Manganese Silicate Gangue Material ...............................................................212

Vein-Related Hydrothermal Alteration .................................................................................212

Rhyolites and Associated Mineralization ............................................................................212

Geochemistry of Altered Rock and Vein-Adjacent Zones ................................................214

Mine Dump Compositions ......................................................................................................214

Mine Water Chemistry ............................................................................................................214

Background Surface Water Chemistry ................................................................................215

Aqueous Geochemical Signatures of High-Silica Rhyolite Intrusions 

and Late-Stage Vein Assemblages (F, W, Mo) ......................................................215

<18 Ma Polymetallic Vein System—Northwest- and Northerly-Trending Veins ............................217

South Silverton Area ........................................................................................................................217

Western Shear System ...........................................................................................................219

Eastern Shear System .............................................................................................................219

Vein-Related Hydrothermal Alteration .................................................................................220

Geochemistry of Altered Rocks ......................................................................................................220

Mine Dump Compositions ...............................................................................................................220

Mine Water Chemistry .....................................................................................................................221

Background Surface Water Chemistry .........................................................................................221

Discussion and Summary .........................................................................................................................222

Mount Moly Mineralization and Hydrothermal Alteration .........................................................222

Red Mountains and OPAM Acid-Sulfate Areas ...........................................................................222

Eureka Graben Area .........................................................................................................................224

South Silverton Area ........................................................................................................................224

Acid-Neutralizing Potential (ANP) of Minerals and Relationship to Alteration 

Assemblages ........................................................................................................................225

Selected References .................................................................................................................................226

Figures

 1. Map showing five areas that represent major styles and ages of mineralization 

and alteration in the study area .............................................................................................167

 2. AVIRIS image of propylitic assemblage minerals in Boulder Gulch–Howardsville 

area .............................................................................................................................................171

 3. AVIRIS images showing propylitic assemblage minerals west of Mineral Creek, 

in Niagara Gulch area, and in Kendall Mountain area .......................................................172

 4. Box plots of outcrop rock chemistry data from alteration assemblages in Mount 

Moly, combined Red Mountains–OPAM, and Eureka Graben areas ...............................178

 5. Alteration map of the Mount Moly alteration area .............................................................180
 6. Box plots comparing surface and subsurface rock chemistry data from acidsulfate and quartz-sericite-pyrite zones in Red Mountains–OPAM and Mount

Moly alteration areas ...............................................................................................................181

 7. Box plots comparing mine dump data from the five mineralized areas ..........................184

 8. Box plots comparing chemistry of all mine water samples to only those 

with pH <4.5 ...............................................................................................................................185

 9. Graphs of metals versus pH for all mine waters from the five mineralized areas .........188

 10. Box plots comparing low-pH mine water data from the five mineralized areas ............192

 11. Box plots comparing geochemistry of background waters draining different 

alteration assemblages ...........................................................................................................196

 12. Graph of dissolved sulfate versus calcium plus strontium in propylitic water 

in Mount Moly alteration area ................................................................................................197

 13. Map showing zone of economically important breccia-hosted ore deposits 

in Red Mountains area .............................................................................................................198

 14. Alteration map of Ohio Peak–Anvil Mountain (OPAM) area .............................................200

 15. Alteration map of Red Mountains area .................................................................................202

 16. Photograph showing highly silicified quartz-alunite-pyrophyllite breccia zone 

along near-horizontal plane ....................................................................................................203

 17. Generalized geologic and alteration map of Eureka Graben area ...................................209

 18. Diagram showing paragenetic stages of vein mineralization from Eureka Graben 

area and adjacent areas .........................................................................................................210

 19. Map of Eureka Graben area showing manganese silicate zoning trends ......................213

 20. Map showing fluoride data from mainstem streams and miscellaneous inflows 

superimposed on generalized alteration map of study area .............................................216

 21. Map of South Silverton mineralized area .............................................................................218

 22. Graph showing mean values of dissolved magnesium versus pH in background 

waters from the five mineralized areas ................................................................................225

Tables

 1. Point count data in volume percent for propylitic samples ...............................................169

 2. Analytical data for various mineral species .........................................................................170

 3. Summary of rock geochemical data from the Mount Moly, Red Mountains, 

OPAM, and Eureka Graben areas ..........................................................................................175

 4. Summary of mine dump geochemistry from five different mineralized areas ................183

 5. Summary of mine water geochemistry from five different mineralized areas ...............186

 6. Ranges and geometric means of mine and background water geochemistry 

from five different mineralized areas ....................................................................................189

 7. Stable isotope data for vein and alteration minerals present within the 

study area ..................................................................................................................................190

 8. Summary of background waters from five different mineralized areas ..........................193
Abstract

The Animas River watershed study area was subdivided 

into five discrete areas that represent the major styles and ages 

of mineralization and alteration in this area: (1) Mount Moly 

area—26–25 Ma molybdenum–copper porphyry mineralization, (2) Red Mountains and (3) Ohio Peak–Anvil Mountain 

areas (OPAM)—sites of 23-Ma acid-sulfate mineralization, 

(4) Eureka Graben and (5) South Silverton areas—18–10 Ma 

northeast- and northwest-trending polymetallic veins, respectively. Each of these five areas was affected by calcite-rich, 

regional propylitic alteration that predated the mineralizing 

episodes by 5–15 million years.

Combined geologic and aqueous geochemical studies 

within these five areas have led to a better understanding of 

the major sources of anthropogenic and natural acidity and 

metals in the watershed. Detailed geologic mapping has shown 

how the degree of bedrock alteration controls the variability 

of water composition within each of these mineralized source 

areas. Aqueous geochemical signatures, quite distinctive in 

some of these areas, can generally be explained by the varying 

geologic and mineralogic characteristics. Some of the more 

diagnostic geochemical signatures have been useful in our 

identifying the origin of unidentified discharge distal to these 

sources.

Introduction

The Animas River watershed study area is characterized 

by several highly contrasting styles and ages of mineralization and related hydrothermal alteration. As suggested in 

Plumlee and others (1999), the geology and mineralogy of 

each of these types of mineralized or altered rocks can exert 

an important and predictable control on the environmental 

signatures that result from mining as well as on the signatures 

of natural processes. Important geologic features, such as 

the acid-producing or acid-neutralizing potential of minerals 

in deposits and related alteration assemblages, influence the 

chemical response of the deposits to weathering. The mineralogy and trace-element compositions of the deposits and the 

altered wallrocks also importantly affect the chemical and 

physical response to weathering and environmental dispersion (Plumlee and Logsdon, 1999; Plumlee, 1999; Plumlee 

and others, 1999). That such effects occur is confirmed by 

integrated geologic and aqueous geochemical studies conducted in other parts of the San Juan Mountains (Miller and 

McHugh, 1994; Bove and others, 1996; Kirkham and others, 

1995; Miller, 1999; Bove and Knepper, 2000), which document how different lithologies or types of mineralization and 

related zoned alteration sequences produce relatively distinct 

aqueous geochemical signatures. These studies were grounded 

in an understanding of the geologic setting, structural controls, 

mineralogy and geochemistry of the deposits and mine wastes, 

and the character of the hydrothermal alteration zones. Thus, 

using these studies as a model, we chose a number of subbasins representative of the major styles and ages of mineralization in the study area for integrated geologic and aqueous 

geochemical studies.

Details of the ore deposits and mineralization history 

were obtained from many excellent sources covering this classic area (for example, Ransome, 1901; Kelley, 1946; Varnes, 

1963; Burbank and Luedke, 1969; Casadevall and Ohmoto, 

1977; Langston, 1978; Ringrose, 1982). Although information 

regarding the distribution of hydrothermal alteration assemblages is available in some areas (Burbank and Luedke, 1969; 

Fisher and Leedy, 1973; Casadevall and Ohmoto, 1977), the 

level of detail was generally insufficient to clarify specific 

water-rock interactions. For this reason, we carefully mapped 

alteration assemblages and important structural features in 

several key mineralized areas by field and by remote methods 

(Airborne Visible/Infrared Imaging Spectrometer or AVIRIS; 

Dalton and others, this volume, Chapter E2); a detailed 

hydrothermal alteration map was also compiled for the entire 

watershed. Mineralogic and geochemical data from altered 

rocks, mineralized samples, and mine dumps were compiled to 

characterize the signatures of source material and to assess the 

dispersion of associated elements into surface water.

Chapter E3

Major Styles of Mineralization and Hydrothermal 

Alteration and Related Solid- and Aqueous-Phase 

Geochemical Signatures

By Dana J. Bove, M. Alisa Mast, J. Bradley Dalton, Winfield G. Wright, and Douglas B. Yager
The aqueous geochemical data utilized in this report 

are part of a broader study (Mast and others, this volume, 

Chapter E7) that characterized the water quality from nonanthropogenic sources in the entire study area. As part of that 

study, 221 streams and water from mines were sampled from 

1995 to 1999 in the Animas River watershed study area (Mast 

and others, this volume; Mast, Evans, and others, 2000).

The structure of this report reflects two of the major 

objectives of this study: (1) to provide the framework for our 

understanding of the complex character of mineralization, 

hydrothermal alteration, and mining activity in the study area, 

and (2) utilization of this framework data, wherever possible, 

to examine the chemical and physical responses to weathering 

of these variably altered and mineralized rocks and multiple 

types of mineral deposits. Some of the most detailed investigations for this study were conducted in the Red Mountains 

and OPAM acid-sulfate areas (fig. 1), where contributions 

of acidity and metals to the watershed from both mined and 

unmined sources are substantial. Although our work in these 

areas focused on water-quality issues, it has also led to a better 

understanding of the genesis, character, and geometry of these 

massive and complex acid-sulfate systems.

The integration of the geologic studies with results of the 

aqueous geochemical data has produced these results:

• Definition of geochemical signatures of both solid and 

aqueous phases within the major mineralized areas in 

the watershed

• Understanding of the relationship between specific 

alteration assemblages, other structural and mineralogic features, and local water chemistry

• Effective comparison of the compositional signatures 

of mining versus non-mining-related water within each 

mineralized area

• Better overall understanding of the types of geologic 

and hydrothermal systems in the watershed

• Source of information for geoenvironmental comparison to other large acid-sulfate systems of different 

genesis and climatic setting.

Methods

The boundaries of the five mineralized areas discussed 

in this report (fig. 1) were delineated based on the distribution 

and zoning of hydrothermally altered rock, the age and type 

of mineralization and hydrothermal alteration, and trends or 

patterns of veins and faults. These areas were differentiated in 

order to characterize and contrast the geochemical signatures 

of solids and waters in a variety of mineralized areas. In this 

report, we assigned various geochemical data to one of the 

mineralized areas if they were collected within the boundary 

of that area.

Hydrothermal alteration assemblage maps were compiled from the following sources: (1) detailed field mapping 

(1:12,000 scale) and associated X-ray diffraction (XRD) 

analysis, (2) mineral maps by remote spectroscopy (AVIRIS) 

(Dalton and others, this volume), and (3) mapping by aerial 

photography at a scale of 1:24,000. Mapping by Ringrose 

(1982) was also used in this compilation. Field and remote 

spectroscopic identification of mineral phases was confirmed 

by whole rock XRD. XRD studies of more than 200 samples 

were performed using standard clay XRD techniques (Moore 

and Reynolds, 1989). These results are in the relational database (Sole and others, this volume, Chapter G). We obtained 

major- and minor-element analyses of discrete mineral phases 

using a JEOL 8900 electron microprobe at the U.S. Geological 

Survey (USGS), Denver, Colo. Sample analysis was performed on polished thin sections with machine conditions of 

15 kV accelerating voltage with a 15–20 μm (micrometer) 

beam width. The analyses were corrected using on-line ZAF 

correction procedures (Goldstein and others, 1992). Replicate 

analyses of secondary standards indicated a relative analytical 

precision of better than ±1 percent (1 σ) for major elements. 

For trace elements, analytical error is less than that for counting statistics where counting error is equal to one sigma or ± 

the square root of n.

One hundred twenty-six samples of variably altered rock 

and mineralized material were analyzed for 40 elements by 

inductively coupled plasma–atomic emission spectroscopy 

(ICP-AES) using the procedures of Briggs (1996). In addition, 

25 rock samples and several mineral separates were analyzed 

by inductively coupled plasma–mass spectroscopy (ICP-MS) 

(Lamothe and others, 1999) at USGS laboratories in Denver, 

Colo. These analytical results are in the relational database 

(Sole and others, this volume). Analytical data for samples 

of waste rock from mine dumps discussed in this report were 

part of a larger data set presented by Fey and others (2000); 

analytical methods and sampling procedures are discussed in 

that report, and the data and field relationships are discussed 

in Nash and Fey (this volume, Chapter E6). All rock and 

mine dump geochemical data collected by the USGS during 

the course of the study (Sole and others, this volume) were 

included in this report. However, mill tailings were excluded 

from the mine dump data set. Several additional geochemical data sets with well-documented methodologies were also 

used in this study. These data include 12 outcrop samples 

(Ringrose, 1982), 153 drill core splits (McCusker, 1982), and 

15 mine dump composites (McCusker, 1983) from the Mount 

Moly area (fig. 1), and 82 surface rock samples (Langston, 

1978) from the Eureka Graben area (fig. 1). Two of these 

non-USGS data sets contain mostly metals concentration 

data; analyses of major elements were generally not conducted. Where both major ion and metal concentration data 

are available, they will be evaluated in the related discussions. 

We grouped rock geochemical data according to alteration 

assemblage on the basis of field mapping and mineralogical 

descriptions.

166 Environmental Effects of Historical Mining, Animas River Watershed, Colorado
Mineralization, Alteration, and Geochemical Signatures 167

Minerals analyzed for stable isotopes were separated 

by various physical and chemical methods and analyzed in 

the USGS laboratory of R.O. Rye in Denver, Colo. Analytical methods used in these various analyses are described in 

Wasserman and others (1992).

Water-quality data were collected from 75 mine and 

146 “background” sites during summer low-flow conditions 

(Mast and others, this volume). The term “background” as 

used in this report refers to sites where dissolved constituents 

in surface water are derived from weathering processes rather 

than mining-related sources. Classification of waters into 

mining-affected and background categories was based on 

criteria discussed in Mast and others (this volume). According 

to this classification scheme, mining-affected sites fall under 

categories III and IV, whereas background sites are classified as category I or II. Background sites were also grouped 

by the dominant alteration type upstream of the sampling site 

(“end-member waters”) on the basis of alteration mapping 

Figure 1. Five areas that represent major styles and ages of mineralization and alteration in the study area. 

Generalized map of hydrothermal alteration assemblages is superimposed. Solid triangle, prominent peak. 

Mount Moly is an informal name used herein to identify the area centered on peak 3,792 m.

Houghton Mt.

California Mt.

Kendal Mt.

Silverton

Red Mtn. 3

Anvil Mt.

Ohio Pk.

Mt. Moly

Red Mtn 3

Kendall Mt

Houghton Mt

California Mt

EUREKA GRABEN

SOUTH SILVERTON

OHIO PEAK–ANVIL 

MOUNTAIN (OPAM)

RED MOUNTAINS

MOUNT MOLY

Cunningham 

Creek

Eureka Gulch

 3,792 m

Ohio Pk

Anvil Mt

EXPLANATION

Hydrothermal Alteration Assemblages

Vein-related quartz-sericite-pyrite (V-QSP)

Weak sericite-pyrite (WSP)

Quartz-sericite-pyrite (QSP)

Acid-sulfate 

Regional propylitic 

0 1 2 KILOMETERS

37°

56'

26"

107°44'49" 107°41'06" 107°35'02"

37°

48'

43"
of the watershed. Samples from localities draining mixtures 

of alteration types (“mixed waters”) were not included in the 

statistical tabulations of these end-member waters. In addition, 

sites that drain end-member alteration assemblages but that 

are also downstream of large mineralized or vein structures 

(unmined), were also placed into separate groups. The locations of streams, springs, and mine sites, along with related 

chemical results, are in the database (Sole and others, this 

volume); sampling and analytical methods appear in Mast and 

others (this volume). All geochemical data, with the exception 

of water-quality data from the Red Mountains area (fig. 1), fall 

within the boundaries of the Animas River watershed study 

area (fig. 2 in von Guerard and others, this volume, Chapter B). 

Ten mine and background water samples included in the Red 

Mountains data set (30 total samples) were collected less 

than 0.5 km outside the study area (Bove and Knepper, 2001; 

Neubert, 2000). These data were included to better represent 

the geochemical signature of waters draining acid-sulfate 

altered rock, which underlies a large percentage of the Red 

Mountains mineralized area. Only three such waters were 

located and sampled within the study area by the USGS during 

the course of this study (Mast, Evans, and others, 2000).

Statistical comparison of water and solids geochemical data was performed using the nonparametric Wilcoxon 

Signed-Ranks tests. Concentrations or abundances were considered statistically different if values of α were ≤0.01.

Outline of Major Styles and Ages 

of Mineralization and Hydrothermal 

Alteration within the Animas River 

Watershed Study Area

Most mineralization as well as associated hydrothermal 

alteration in the study area was temporally and genetically 

associated with three major episodes of high-level magmatism 

between about 27 and 10 Ma (Bove and others, 2001). These 

events postdated the collapse of the San Juan, Uncompahgre, 

and Silverton calderas (28–27 Ma) (see Yager and Bove, 

this volume, Chapter E1, pl. 1) by about 5–15 Ma (Lipman 

and others, 1976; Bove and others, 2001; Yager and Bove, 

this volume). Caldera collapse and resurgent doming created 

a favorable structural environment for later mineralization 

and hydrothermal activity. In general, deuteric activity was 

temporally associated with caldera development and caused 

regional propylitization, which characterizes a major portion of the study area. Rocks affected by this alteration type 

contain abundant calcite, epidote, and chlorite, which contribute 

to the intrinsic acid-neutralizing capacity of this alteration 

assemblage (Desborough and others, 1998; Bove and others, 

2000).

Because of the large size of the study area, we have delineated five major areas that represent the major styles and ages 

of mineralization and alteration as just outlined. As shown 

in figure 1, these areas include (1) Mount Moly1

—26–25 Ma 

molybdenum-copper porphyry mineralization, (2) Red 

Mountains—23 Ma acid-sulfate system, (3) Ohio Peak–Anvil 

Mountain—23 Ma acid-sulfate mineralization, (4) Eureka 

Graben—northeast-trending polymetallic veins (<18 Ma), and 

(5) South Silverton—north- to northwest-trending polymetallic 

veins (<18 Ma). Owing to the widespread nature of the earlier 

formed regional propylitic event, we discuss the characteristics 

of this related assemblage as it occurs throughout the entire 

study area.

The earliest important mineralizing and related hydrothermal event, at about 26–25 Ma, was related to a large 

swarm of calc-alkaline intrusions emplaced over a broad 

region of the western San Juan Mountains (Bove and others, 

2001). The Mount Moly area, centered around peak 3,792 m 

between South and Middle Forks Mineral Creek (fig. 1) is the 

only known mineralized hydrothermal system of this type and 

age in the study area.

Remnants of two large acid-sulfate systems are centered in the Red Mountains and Ohio Peak–Anvil Mountain 

(OPAM) areas (fig. 1). The Red Mountains system encompasses a roughly 22 km2

 area that was host to economically 

important silver-copper-lead breccia-pipe and fault-hosted 

mineralization (Burbank and Luedke, 1969; Bove and 

others, 2000). The OPAM system (21 km2

), although similar 

in size and genesis, was generally devoid of economically 

important mineralization, as shown by the paucity of mines in 

the area (Church, Mast, and others, this volume, Chapter E5). 

Both of these altered and mineralized systems formed at 

23 Ma and are genetically linked to high-level dacite porphyry 

intrusions.

Economically important, post-18 Ma vein mineralization 

in the western San Juan Mountains is closely associated with 

intrusion of high-silica alkali rhyolite (Lipman and others, 

1973; Bartos, 1993). Veins are mostly a polymetallic variety 

(silver, lead, zinc, copper, ±gold) and formed as fracture- or 

fissure-fillings. The veins within the San Juan and inner nested 

Silverton calderas are either concentric or radial to the ring 

fracture zone, as in the South Silverton area (fig. 1) or trend 

parallel to graben faults related to resurgent doming of the 

San Juan–Uncompahgre calderas (for example, the Eureka 

Graben area) (Burbank and Luedke, 1969). Zones of hydrothermally altered rock related to the veins consist of narrow 

envelopes that are superimposed over regional propylitically 

altered rock (Ransome, 1901; Burbank and Luedke, 1969).

Because these five major mineralized areas cover only a 

portion of the study area, some mineralized features or mineral 

occurrences are not addressed in this report. However, many of 

these details can be found in other chapters of this volume, or 

within the wealth of previously published literature pertaining 

to the study area.

1

Mount Moly is the informal name for the peak labeled in figure 1 as 

3,792 m. Its use in this report is confined to giving a name to the altered and 

mineralized area around that peak and does not indicate a formal geographic 

name for peak 3,792 m.

168 Environmental Effects of Historical Mining, Animas River Watershed, Colorado
Mineralization, Alteration, and Geochemical Signatures 169

Pre-Ore Regional Propylitic Alteration 

Assemblage

Nearly all the rocks in the study area were affected by 

low-grade regional metamorphism or propylitization resulting 

from thermal events related to the San Juan–Uncompahgre 

and later Silverton calderas. The timing of this alteration event 

is roughly constrained to about 28.2–27.5 Ma—the ages of 

these respective calderas—and preceded most ore mineralization by 5–15 Ma (Lipman and others, 1976; Bove and others, 

2001). The propylitic mineral assemblage contains varying 

amounts of chlorite, epidote, calcite, and illite, in the presence 

of fresh to weakly altered primary feldspar crystals. Mass balance studies indicate that elements within the protolith were 

mostly conserved during the propylitic event with only minor 

additions of carbon dioxide and water (Burbank and Luedke, 

1969; Fisher and Leedy, 1973). However, the common presence of epidote, calcite, and chlorite within fractures indicates 

that some of the major base chemical elements were locally 

redistributed.

Field mapping in conjunction with AVIRIS mineral maps 

(Dalton and others, this volume) indicates that the propylitic 

alteration assemblage can be broadly differentiated into two 

major subgroups: (1) chlorite-epidote-calcite dominant (CEC), 

and (2) chlorite-illite-calcite dominant (CIC). Mapped distributions of these assemblages reflect several important variables including degree of alteration, nature and composition of 

protolith, and stratigraphic position or relative depth of burial.

CEC Group

Rocks of the CEC propylitic assemblage are characteristically green to pistachio green in color due to the presence of 

epidote and chlorite. Plagioclase phenocrysts, most notably 

the calcic cores, show significant replacement by mixtures of 

epidote, chlorite, calcite, and illite, whereas the groundmass 

is altered to fine aggregates of these secondary minerals. 

The appearance of epidote denotes a higher degree of alteration, although calcite instead of epidote can predominate 

within these altered grains. As shown in table 1, percentages 

of epidote, chlorite, and calcite within the CEC group vary 

significantly: total abundances range from about 30 to more 

than 50 volume percent of the rock. Limited point count studies suggest that although calcite and epidote coexist in some 

rocks, these minerals more typically occur to the exclusion 

of one another. Calcite and chlorite can be quite abundant in 

these rocks (table 1); however, chlorite is almost everywhere 

present, whereas calcite is locally absent. Percentages of illite, 

although not determined quantitatively, are estimated to range 

as high as 10–20 volume percent in some samples. Chemical 

compositions of these mineral phases are in table 2.

The degree or intensity of propylitic alteration, as noted 

by systematic variations in the propylitic mineral assemblage, was strongly influenced by lithologic characteristics. 

For example, lavas of the Burns Member of the Silverton 

Volcanics were generally affected by a much higher degree 

of propylitic alteration than were lavas and volcaniclastics 

of the overlying Henson Member. (For maps showing contacts between Burns and Henson Members, see Burbank 

and Luedke, 1964; Luedke and Burbank, 1989; Luedke and 

Burbank, 2000.) This difference in alteration intensity is 

well illustrated north of the Animas River in the vicinity of 

Boulder Gulch (fig. 2), where the strong CEC signature of the 

underlying Burns Member contrasts sharply with the weakly 

altered (epidote-poor) Henson Member. The lack of epidote in 

the Henson Member probably represents an incipient phase of 

the CEC propylitic assemblage. This assemblage is marked by 

relatively fresh plagioclase, chlorite-illite-leucoxene-altered 

ferromagnesian minerals, and hematite along microfractures 

and disseminated after primary opaque grains. A decrease in 

overall permeability in the upper Henson Member is probably 

an important factor that influenced this general upward transition from stronger to weaker propylitic assemblages. Such permeability contrasts are documented by studies in the Prospect 

Gulch area (D.J. Bove and others, unpub. data, 2002) and 

around the Sunnyside vein system (Langston, 1978), where the 

movement of hypogene hydrothermal fluids was also similarly 

restricted at the Burns Member–Henson Member interface. 

There are many exceptions, however, to this generalized 

alteration pattern, and localized zones of strongly CEC-altered 

rock extend into the upper Henson Member. These irregularities likely correspond to areas of increased fracture density 

or local changes in lithology, which would have effectively 

increased permeability to propylitic-altering fluids (Bove and 

others, 2000).

Table 1. Point count data in volume percent for propylitic 

samples.

[All samples with exception of ODY9735 (CIC group) from CEC group; 

500 points counted for each sample]

Sample No. Epidote Chlorite Calcite

HDY9840 2.8 26.3 9.5

IDB7297 26.4 13.5 0

IDB6597 27.6 6.7 0

ODY9735 0 45.6 0.4

ODY9801A 17 25.2 19.2

ODY9801B 18.6 11.3 18.2

ODY9802 13.5 35.3 0.3

SDB34 5.4 35.5 0.7

SDB35A 0.2 21.1 33.7

SDY9752 1.4 33.5 16.9

SDY9754 0.2 32.1 15.8

SDY9626 0 11.9 37

SDY9767 0 8.6 50.6

SDY9837 27 46.9 0.3

SJ98740 41.7 12.7 7.9

SJ9848 21.8 19.4 0
CIC Group

Mapping by AVIRIS remote sensing (Dalton and others, this volume) has been very useful in differentiation of 

propylitically altered rocks dominated by combinations 

of illite and chlorite (CIC group) versus the CEC mineral 

assemblage. However, field studies have shown that the 

CIC group as mapped by AVIRIS actually represents two 

notably different types and styles of alteration: (1) superimposition of the regional propylitic assemblage by a later 

calcite-poor, weak “hydrothermal” mineral assemblage, and 

(2) an epidote-poor, calcite-rich assemblage formed during 

the regional propylitic event (chlorite-illite-calcite, CIC). Illite 

predominates over chlorite within rocks superimposed by the 

weak sericite2

-pyrite “hydrothermal” alteration suite (illitechlorite-pyrite with metastable feldspars). Epidote is generally 

subordinate or absent in this assemblage. In contrast, chlorite, 

which formed mostly during the regional propylitic event, is 

typically metastable, showing partial replacement by illite. 

Further discussion of the hydrothermal “weak sericite-pyrite” 

suite is found in later sections on hydrothermal alteration 

assemblages.

The regional CIC propylitic assemblage is especially 

characteristic of the San Juan Formation west of Mineral 

Creek (fig. 3A), and within the Eureka Member of the 

Sapinero Mesa Tuff (Eureka Tuff) in the southeast and east 

portions of the study area (fig. 3B). The San Juan Formation 

west of Mineral Creek is notably epidote poor and illite rich. 

Plagioclase within these intermediate-composition rocks 

is generally altered to a mixture of illite, calcite, and lesser 

chlorite, whereas biotite, pyroxene, and hornblende are mostly 

altered to leucoxene and illite. Matrix and pumice within a 

nearby ash-flow unit, however, are typically chloritic-altered, 

and plagioclase phenocrysts are weakly altered to illite, 

chlorite, and calcite. These rocks are locally weakly silicified and contain as much as 1 volume percent pyrite owing 

to the high density of the northwest-trending veins cutting 

this region. Calcite is still mostly stable within these altered 

zones.

2

Petrographic term used for fine-grained, highly birefringent muscovite 

(Srodon and Eberl, 1984). Used interchangeably with illite in this report.

Table 2. Analytical data for various mineral species; values in percent. 

[n.d., not determined] 

Element plag1 chlorite1 diopside1 illite2 epidote1 pyrite3 pyrite4 alunite5 gypsum6 calcite7 calcite8

Na2O 11.20 0.01 0.37 0.10 0.01 0.00 0.00 4.41 0.01 0.03 0.01

MgO 0.00 18.46 15.26 1.38 0.00 0.00 0.00 0.00 0.02 0.03 0.10

Al2O3 19.87 19.76 0.81 33.00 24.13 0.00 0.00 39.03 0.02 0.02 0.04

SiO2 67.37 28.09 53.36 48.90 37.47 0.00 0.00 0.00 0.00 0.00 0.00

TiO2 0.01 0.13 0.19 0.33 0.08 0.00 0.00 0.00 0.00 0.00 0.00

FeO 0.11 21.03 8.37 <0.08 10.61 46.40 n.d. 51.60 0.08 0.09 0.04

MnO 0.00 0.73 0.54 0.00 0.75 0.00 0.00 0.00 0.01 0.30 1.42

K2

O 0.19 0.09 0.00 9.32 0.00 0.00 0.00 4.26 0.00 0.00 0.00

CaO 0.81 0.08 21.87 <0.03 21.97 0.00 0.00 0.00 32.20 57.40 57.40

BaO 0.01 0.00 0.02 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

S 0.00 0.00 0.00 0.00 0.00 53.51 n.d. 0.00 0.00 0.00 0.00

SO2 0.00 0.00 0.00 0.00 0.00 n.d. n.d. 0.00 0.00 0.00 0.00

Cu 0.00 0.00 0.00 0.00 0.00 0.02 <.01 0.00 0.00 0.00 0.00

Pb 0.00 0.00 0.00 0.00 0.00 0.00 0.04 0.00 0.00 0.00 0.00

Zn 0.00 0.00 0.00 0.00 0.00 0.04 0.05 0.00 0.00 0.00 0.00

As 0.00 0.00 0.00 0.00 0.00 0.01 <.01 0.00 0.00 0.00 0.00

Co 0.00 0.00 0.00 0.00 0.00 0.06 <.01 0.00 0.00 0.00 0.00

Sr n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. 0.30 0.04 0.25

H2

O n.d. n.d. n.d. n.d. n.d. n.d. n.d. 13.35 n.d. n.d. n.d.

1

Albitized plagioclase from Mount Moly and OPAM areas; chlorite, epidote, and diopside from miscellaneous locations; all by electron microprobe; 

this study.

2

Red Mountains and Eureka Graben areas (Eberl and others, 1987).

3

Disseminated pyrite from Prospect Gulch, electron microprobe; this study.

4

Disseminated pyrite, Mount Moly area by ICP-MS; this study.

5

Alunite from National Belle (Hurlburt, 1894).

6

Gypsum near Paradise mine (AMLI # 168); ICP-AES, this study.

7

Calcite from regional propylitic assemblage; ICP-AES, this study.

8

Sparry calcite from vein, South Silverton area, ICP-AES, this study.

170 Environmental Effects of Historical Mining, Animas River Watershed, Colorado
Mineralization, Alteration, and Geochemical Signatures 171

A distinct contrast between CIC-altered Sapinero Mesa 

Tuff (Eureka and Picayune Megabreccia Members) and overlying CEC-altered Burns Member lavas is observed in the area 

west of Niagara Gulch (fig. 3B). The CIC-altered tuffs are 

characterized by waxy, light-green, illite-altered pumice, 

whereas the groundmass is conspicuously whitish green due 

to replacement by chlorite, illite, and calcite. Plagioclase phenocrysts are variably altered to illite, with lesser chlorite, and 

calcite. Biotite phenocrysts are typically replaced by chlorite 

and fine opaque minerals.

There are notable exceptions to these simple lithologyspecific associations. For example, immediately south of 

Silverton (outside the boundary of the Silverton caldera but 

within the San Juan caldera), both the Eureka and Picayune 

Megabreccia Members and overlying Burns Member are altered 

to the CEC assemblage (fig. 3C). Similarly, near Animas Forks 

(Yager and Bove, this volume, pl. 1), exposures of both the 

Picayune Megabreccia Member and the Burns lavas are distinctly CEC-altered. However, this assemblage changes upward 

into the CIC suite at about 3,750 m elevation, which dominates 

Figure 2. Propylitic assemblage minerals as mapped by AVIRIS (Dalton and others, this volume) in 

Boulder Gulch–Howardsville area. Burns Member is generally present south of white dashed line and has 

undergone relatively more intense propylitic alteration with abundant epidote. Henson Member is north of 

dashed line and is typically less altered. White triangle, peak.

0 0.5 1 KILOMETER

Boulder

Gulch

Storm Peak

Animas

River

Howardsville

Chlorite with sericite

Sericite

Sericite with epidote and (or) calcite

Chlorite

Epidote with calcite and chlorite

Epidote

Epidote with calcite 

Calcite with epidote and chlorite

Calcite with epidote

Calcite 

EXPLANATION 

N
in all lithologies westward toward Cinnamon Pass. A similar 

trend towards CIC dominance with higher elevation is observed 

in the Burns lavas west of Denver Lake (3,660 m), up to an 

elevation of 3,960 m near Engineer Pass (in Ouray County just 

north of the area of pl. 1).

Thus, from these examples, style of propylitic alteration appears to be correlated with lithology as well as with 

topographic elevation. The difference in alteration patterns 

within the Sapinero Mesa members at Silverton (CEC; for 

example, figs. 2 and 3C) and west of Eureka (CIC; for example, fig. 3B) could be explained by their relative topographic 

elevations or depth of burial following asymmetric collapse 

of the Silverton caldera (Yager and Bove, this volume) within 

the earlier subsided San Juan caldera. Rocks were most deeply 

buried near the southwest margin of the Silverton caldera, 

where subsidence reached a maximum of about 600 m (Varnes, 

1963; Lipman and others, 1973). In contrast, near the Silverton 

caldera’s hinged eastern margin, caldera subsidence was 

Figure 3 (above and following pages). AVIRIS mapping images, showing A, dominance of CIC 

propylitic assemblage in volcanic sequence west of Mineral Creek. White triangle, peak. B, Propylitic 

assemblage minerals in the Niagara Gulch area, showing alteration changes specific to lithologies. 

CEC-altered Burns Member lavas (Tsb) overlie the CIC-altered Sapinero Mesa Tuff (Eureka and 

Picayune Megabreccia Members; Tse). Both rock types have undergone moderate to intense 

propylitization. C, Alteration assemblage minerals in Kendall Mountain area, immediately south of 

Silverton. Both the Eureka and Picayune Megabreccia Members (Tse) and overlying Burns Member 

(Tsb) are altered to the CEC assemblage. 

0 0.25 0.5 KILOMETERS

Chlorite with sericite

Sericite

Sericite with epidote and (or) calcite

CIC GROUP

EXPLANATION N

MINERAL

BASIN

Porphyry

Gulch

Mineral

Creek

County

Line

Red Mountain

Pass

Chattanooga

 Bend

172 Environmental Effects of Historical Mining, Animas River Watershed, Colorado
Mineralization, Alteration, and Geochemical Signatures 173

minimal to nonexistent. The correlation of the CEC-dominant 

assemblage in the most deeply subsided areas of the caldera 

and changes from CEC to CIC alteration with increasing topographic elevation are consistent with mineral zoning patterns 

that can develop with the establishment of temperature gradients. These temperature gradients and the resultant mineral 

zoning patterns (for example, increasing epidote abundance 

with higher temperature) probably reflect differential burial 

depths during the regional propylitic event (Sillitoe, 1984).

Geochemistry of Propylitically Altered Rocks

Geochemical data from 72 propylitically altered rocks 

from the Mount Moly, combined Red Mountains–OPAM, and 

Eureka Graben areas (fig. 1) are summarized in table 3 and 

figure 4. Comparisons of data from these areas show the 

abundances of manganese, lead, zinc, and copper to be 

analytically indistinguishable, with the exception of slightly 

lower concentrations of lead and zinc in the Eureka Graben 

Figure 3—Continued. AVIRIS mapping images. B, Propylitic assemblage minerals in the Niagara Gulch 

area, showing alteration changes specific to lithologies. CEC-altered Burns Member lavas (Tsb) overlie 

the CIC-altered Sapinero Mesa Tuff (Eureka and Picayune Megabreccia Members; Tse). Both rock types 

have undergone moderate to intense propylitization. Solid white line is geologic contact between units 

Tse and Tsb. White triangle, peak.

Animas

River

Eureka Gulch

Niagara

Gulch

Eureka

Tse

Tsb

0 0.25 0.5 KILOMETERS

Chlorite with sericite

Sericite

Sericite with epidote and (or) calcite

Chlorite

Epidote with calcite and chlorite

Epidote

Epidote with calcite 

Calcite with epidote and chlorite

Calcite with epidote

Calcite 

CEC GROUP CIC GROUP

N

EXPLANATION
area data set. Mean3

 concentrations, as calculated for each 

of these areas, range from 777 to 946 ppm manganese, 10 to 

36 ppm lead, 67 to 110 ppm zinc, and 33 to 50 ppm copper. 

Total iron abundances in the Mount Moly and Eureka Graben 

area samples also overlap statistically and have a mean of 

3 weight percent. In contrast, total iron concentrations in the 

combined Red Mountains–OPAM area samples are markedly 

higher than in the other two areas, with a mean of 4.8 weight 

percent. The differences in total iron may represent both differences in original bulk rock composition and varying abundances of volumetrically minor pyrite. Although pyrite does 

not appear as an essential product of this alteration assemblage 

(Burbank and Luedke, 1969), it is widely disseminated, owing 

to the high density of veins and fracturing within the entire 

study area. In the following discussions on the chemistry of 

rocks from various hydrothermally altered and mineralized 

systems, these basin-wide “baseline” or unmineralized values 

will provide useful comparisons with data from mineralized 

and more intensely altered rocks.

Figure 3—Continued. AVIRIS mapping images. C, Alteration assemblage minerals in Kendall Mountain 

area, south of Silverton. Both the Eureka and Picayune Megabreccia Members (Tse) and overlying 

Burns Member (Tsb) are altered to the CEC assemblage. White triangle, peak; white line, contact.

0 0.25 0.5 KILOMETERS

Chlorite with sericite

Sericite

Sericite with epidote and (or) calcite

Chlorite

Epidote with calcite and chlorite

Epidote

Epidote with calcite 

Calcite with epidote and chlorite

Calcite with epidote

Calcite 

CEC GROUP CIC GROUP

N

EXPLANATION

KENDALL MOUNTAIN

Tse

Tse

Tsb

Tsb

Swansea

Gulch

3

The term “mean” as used in this manuscript refers to geometric mean 

calculations.

174 Environmental Effects of Historical Mining, Animas River Watershed, Colorado
Mineralization, Alteration, and Geochemical Signatures 175

Table 3. Summary of rock geochemical data from the Mount Moly, Red Mountains, OPAM, and Eureka Graben areas. [n, number; QSP, quartz-sericite-pyrite; V-QSP, vein-related quartz-sericite-pyrite; gmean, geometric mean; < , less than; n.d., not determined]

Al Ca Fe K Mg Na P Ti Mn Ag As Ba Cu Mo Pb Sr Zn

percent ppm

Mount Moly area

Surface Outcrop Samples:Hydrothermal-propylitic (n=9)1

:

25th percentile 7.87 1.43 2.52 3.27 1.33 2.40 0.13 0.39 800 n.d. n.d. 578 23 1 30 570 6875th percentile 8.25 2.36 3.38 3.52 1.42 2.86 0.14 0.46 1,300 n.d. n.d. 693 61 3 41 707 91gmean 8.13 1.78 3.04 3.11 1.27 2.55 0.13 0.42 946 n.d. n.d. 624 34 2 36 666 81Weak sericite-pyrite (n=5)2

:

25th percentile 7.76 0.21 2.10 3.15 0.58 2.28 0.07 0.31 175 n.d. n.d. 720 37 9 17 290 4675th percentile 8.03 0.40 2.20 4.00 1.08 2.54 0.11 0.38 650 n.d. n.d. 770 72 16 50 520 63gmean 7.98 0.29 2.48 3.14 0.82 2.38 0.09 0.36 313 n.d. n.d. 819 59 12 58 413 45all USGSQSP (n=12)3

:

25th percentile 8.42 0.08 0.43 3.43 0.26 0.32 0.09 0.44 34 n.d. 3 563 14 3 27 123 2075th percentile 9.23 0.12 1.30 3.49 0.41 0.54 0.12 0.55 64 n.d. 10 890 31 16 95 310 36gmean 8.71 0.09 0.70 3.47 0.32 0.32 0.11 0.50 54 n.d. 6 624 23 8 49 205 257 USGS, 5 Ringrose, 1982Drill Core Metals Data—3 Meter Sample Splits4

:

QSP-unoxidized (n=30)25th percentile n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. 260 14 20 n.d. 7975th percentile n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. 560 40 33 n.d. 130gmean n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. 374 22 28 n.d. 109QSP oxidized (n=73)25th percentile n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. 8 5 12 n.d. <575th percentile n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. 30 23 25 n.d. 11gmean n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. 15 11 18 n.d. 8Hydrothermal-propylitic (n=50):25th percentile n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. 130 12 20 n.d. 4275th percentile n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. 280 48 34 n.d. 58gmean n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. 194 24 28 n.d. 35
Table 3. Summary of rock geochemical data from the Mount Moly, Red Mountains, OPAM, and Eureka Graben areas.—Continued

Al Ca Fe K Mg Na P Ti Mn Ag As Ba Cu Mo Pb Sr Zn

percent ppm

Combined Red Mountains–OPAM areas2

Propylitic (n=15):25th percentile 8.13 1.86 4.62 1.94 1.40 1.86 0.15 0.47 935 2 12 519 41 2 20 238 8375th percentile 8.59 3.30 5.26 2.96 1.98 2.49 0.16 0.57 1,230 2 30 958 64 2 33 673 110gmean 8.38 1.92 4.83 2.31 1.52 1.66 0.14 0.50 927 2 19 718 50 2 26 396 110QSP (n=20):25th percentile 7.16 0.04 0.29 2.38 0.23 0.02 0.03 0.43 23 2 10 290 3 2 29 58 1075th percentile 8.33 0.12 2.30 3.33 0.40 0.09 0.11 0.61 59 2 29 1,063 16 5 113 188 37gmean 7.42 0.07 0.87 2.82 0.21 0.05 0.06 0.46 35 2 18 530 8 3 62 106 20Silicified or vein structure (n=16):25th percentile 0.23 0.01 0.50 0.04 0.01 0.01 0.01 0.04 6 4 50 56 12 2 220 67 875th percentile 1.63 0.06 8.23 2.20 0.01 0.03 0.19 0.25 18 51 2,419 221 291 17 3,740 796 32gmean 0.76 0.03 2.05 0.30 0.01 0.01 0.05 0.09 12 14 260 122 55 6 870 177 18Acid-Sulfate Assemblage:Argillic (n=8)

25th percentile 6.68 0.14 0.38 0.04 0.01 0.01 0.14 0.43 4 2 10 457 6 2 70 372 675th percentile 8.40 0.19 1.90 1.01 0.09 0.03 0.21 0.51 17 2 37 904 28 3 571 787 19gmean 7.90 0.14 0.86 0.18 0.02 0.02 0.16 0.46 9 2 25 607 11 3 237 309 9Quartz-pyrophyllite (n=13):25th percentile 4.40 0.09 0.09 0.04 0.01 0.01 0.12 0.19 4 2 10 375 3 2 99 332 575th percentile 6.50 0.16 0.54 0.90 0.02 0.06 0.18 0.51 14 2 20 1,020 27 2 396 1,100 13gmean 4.17 0.10 0.26 0.18 0.01 0.04 0.11 0.34 8 3 20 470 7 2 255 512 8Quartz-alunite (n=13):25th percentile 6.80 0.05 0.21 2.00 0.01 0.24 0.13 0.17 4 2 10 99 3 2 123 567 475th percentile 9.00 0.15 1.20 3.20 0.01 0.47 0.22 0.29 12 2 14 247 20 2 1,130 976 15gmean 7.53 0.09 0.54 2.45 0.01 0.25 0.13 0.23 9 2 13 162 8 2 358 550 8Acid-sulfate-drill core (n=34):25th percentile 6.10 0.11 4.40 1.45 0.01 0.04 0.08 0.11 4 2 10 133 36 2 48 434 575th percentile 7.55 0.14 5.95 2.45 0.06 0.52 0.12 0.25 30 2 16 288 128 3 674 838 30gmean 6.68 0.12 4.77 1.51 0.02 0.16 0.10 0.17 11 2 14 195 79 3 141 535 13

176 Environmental Effects of Historical Mining, Animas River Watershed, Colorado
Mineralization, Alteration, and Geochemical Signatures 177

Table 3. Summary of rock geochemical data from the Mount Moly, Red Mountains, OPAM, and Eureka Graben areas.—Continued

Al Ca Fe K Mg Na P Ti Mn Ag As Ba Cu Mo Pb Sr Zn

percent ppm

Eureka Graben area5

Propylitic (n=48):25th percentile n.d. n.d. 2.8 n.d. n.d. n.d. n.d. n.d. 660 <0.5 n.d. n.d. 26 n.d. 7 n.d. 5975th percentile n.d. n.d. 4.0 n.d. n.d. n.d. n.d. n.d. 900 <0.5 n.d. n.d. 54 n.d. 14 n.d. 7495th percentile n.d. n.d. 4.7 n.d. n.d. n.d. n.d. n.d. 1,330 <0.5 n.d. n.d. 73 n.d. 20 n.d. 96gmean n.d. n.d. 3.2 n.d. n.d. n.d. n.d. n.d. 777 <0.5 n.d. n.d. 33 n.d. 10 n.d. 67QSP-vein (V-QSP) (n=36):25th percentile n.d. n.d. 1 n.d. n.d. n.d. n.d. n.d. 160 <0.5 n.d. n.d. 16 n.d. 4 n.d. 2575th percentile n.d. n.d. 2 n.d. n.d. n.d. n.d. n.d. 1,430 1 n.d. n.d. 48 n.d. 14 n.d. 7195th percentile n.d. n.d. 3 n.d. n.d. n.d. n.d. n.d. 4,200 4 n.d. n.d. 152 n.d. 155 n.d. 106gmean n.d. n.d. 1.5 n.d. n.d. n.d. n.d. n.d. 497 0.7 n.d. n.d. 28 n.d. 11 n.d. 33

1Two samples analyzed at USGS labs by ICP-MS and ICP-40 element-AES; seven samples analyzed by XRF; data from Ringrose (1982).2USGS labs by ICP-40 element-AES.3Seven samples analyzed at USGS labs by ICP-MS and ICP-40 element-AES; five samples analyzed by XRF; data from Ringrose (1982).4Mo by colorimetric method and atomic absorption (AA), AMAX and Skyline Labs, Denver Colo.; Cu, Pb, Zn, by AA, AMAX lab; data from McCusker (1983). 5Atomic absorption (AA); data from Langston (1978).
Background Surface Water Chemistry

A discussion of the geochemistry of surface waters that 

interacted with all propylitically altered rocks in the study 

area is presented in Mast and others (this volume). Results 

of that study demonstrate that streams and springs draining 

areas of propylitically altered rock had moderate to high pH 

values (5.74–8.49), measurable alkalinity (1.4–68 mg/L), 

low dissolved metal concentrations (zinc=20–237 μg/L; 

copper=4–73 μg/L), and low concentrations of dissolved iron 

(30–310 μg/L) and aluminum (40–140 μg/L). Specifics on the 

relationship between propylitically altered rocks and related 

waters within each of the four major mineralized areas, however, will be discussed in their respective sections following. 

(Red Mountains and Ohio Peak–Anvil Mountain are discussed 

together.)

26–25 Ma Molybdenum–Copper 

Porphyry Mineralization and Alteration

Mount Moly Area Ore Deposits

More than 19 km2 of intensely altered and pyritized rock 

reflects the presence of a subeconomic molybdenum–copper 

porphyry hydrothermal system in the Mount Moly area 

(fig. 1). Mineralization and hydrothermal alteration in this 

area are temporally and genetically related to a late (25 Ma) 

quartz monzonite phase of the 26 Ma Sultan Mountain stock 

(Ringrose and others, 1986; Bove and others, 2001). The main 

zone of molybdenum-copper enrichment is coincident with 

intense QSP-(quartz-sericite-pyrite) altered rock, high-density 

Figure 4. Box plots of outcrop rock chemistry data from alteration assemblages in the Mount Moly area, 

combined Red Mountains–OPAM area, and Eureka Graben area. See text for description of alteration 

assemblages. AS, combination of all AS zones from surface samples in table 3. No molybdenum data are 

available from the Eureka Graben area. All vertical scales are in parts per million (ppm) except iron (in 

percent); bdl, below detection limit.

Iron (percent) Manganese (ppm) Lead (ppm)

Zinc (ppm) Copper (ppm) Molybdenum (ppm)NO DATA

10

100

0.01

0.1

1

Quartz-sericite-pyrite (QSP)

Propylitic (PROP)

Weak sericite-pyrite (WSP)

QSP-vein-related

10

100

1,000

10,000

Mount Moly

Red Mtns

 OPAM

Eureka

Graben

10

100

1,000

10,000

Mount Moly

Red Mtns

 OPAM

Eureka

Graben

10

100

 Mount Moly

Red Mtns

 OPAM

Eureka

Graben

10

100

1,000

1,000

10,000

 Mount Moly

Red Mtns

 OPAM

Eureka

Graben

1

10

100

bdl

 Mount Moly

Red Mtns

 OPAM

Eureka

Graben

1 1

1 1

Acid-sulfate, all zones (AS)

EXPLANATION

 Mount Moly

Red Mtns

 OPAM

Eureka

Graben

95th 

percentile

75th 

Median 

25th 

5th 

178 Environmental Effects of Historical Mining, Animas River Watershed, Colorado
Mineralization, Alteration, and Geochemical Signatures 179

quartz stockwork veining, and as much as 5 volume percent 

sulfides (McCusker, 1982). Disseminated sulfides in this 

zone consist mainly of pyrite, lesser chalcopyrite, and traces 

of molybdenite and bornite. The sulfides occur as tiny grains 

replacing feldspars and other phenocrysts and disseminated 

iron oxides in the groundmass.

Quartz-sulfide veins, most of which are within a central core of QSP-altered rock (fig. 5), form a stockwork of 

narrow (<1.3 cm), discontinuous veinlets. Veinlet density 

reaches a maximum (0.3–1.5/m2

) in a roughly 0.9 km2 area 

centered around peak 3,792 m (fig. 5) (McCusker, 1982). 

The veinlets are typically filled with gray-blue translucent 

quartz along with scattered grains or discontinuous bands 

of pyrite with minor chalcopyrite, molybdenite, and bornite. 

Two sericite samples from wallrock adjacent to the stockwork yielded concordant K-Ar ages of about 25 Ma, close to 

the same age as biotite from nearby quartz monzonite intrusions (Ringrose, 1982; Bove and others, 2001). Drill hole 

data (McCusker, 1982) and our recent field studies indicate 

that the highest concentrations of copper and molybdenum 

and the most intense QSP alteration are distributed near the 

margins of the quartz monzonite intrusions. Detailed rock 

geochemical sampling delineates depletions of the trace elements zinc, manganese, and strontium within the innermost 

intensely altered zones and enrichments of these elements 

around the periphery of the hydrothermal system (Ringrose, 

1982). Drill hole data (McCusker, 1982) indicate that oxidation and supergene leaching are present to depths as much as 

100 m beneath the surface. Secondary minerals within this 

oxidized zone include hematite, goethite, ferrimolybdenite, 

and minor secondary copper sulfides including chalcocite, 

digeonite, and covellite. The secondary copper minerals are 

present mostly as thin films and as partial replacements after 

pyrite.

Relatively narrow (0.3–1 m), quartz base-metal veins 

are present on the periphery of the porphyry system, mostly 

on the northeast flank of peak 3,792 m. Mines include 

the Bonner—probably the largest producer in this area, 

Independence, Magnet, and Ruby Trust (Church, Mast, and 

others, this volume; fig. 5), whose veins are generally oriented north-south (Ringrose, 1982; McCusker, 1983). These 

veins contain silver and some gold in sulfide ores consisting principally of galena, sphalerite, and lesser pyrite, with 

minor fine-grained tetrahedrite-tennantite and chalcopyrite 

(Ringrose, 1982; McCusker, 1983). The primary gangue mineral is quartz, with lesser barite and carbonate minerals, and 

minor manganese-silicate minerals. Galena is mostly coarse 

grained and intergrown with irregular patches of subhedral 

sphalerite. Quartz is typically milky white, massive, and fine 

grained, although euhedral, vug-filling crystals are also present. Pyrite, tetrahedrite, and barite and carbonate gangue were 

generally precipitated after sphalerite and galena. Several 

of these polymetallic veins are molybdenite-bearing and cut 

quartz-molybdenite stockwork veins associated with the central QSP zone in the Mount Moly area (McCusker, 1982). The 

orientation, presence of molybdenite, and proximity of many 

of these polymetallic veins to the inner zone of QSP alteration 

suggest that some are contemporaneous with the low-grade 

molybdenum-copper porphyry system (26–25 Ma). In addition, δ18O and δD values of sphalerite inclusions from peripheral base-metal veins resemble the magmatic water compositions of minerals within the central zone of QSP alteration and 

related quartz-molybdenite stockwork (Ringrose, 1982).

Hydrothermal Alteration in the Mount Moly Area

The detailed map of hydrothermal alteration assemblages in the Mount Moly area (fig. 5) was compiled from 

three principal sources: (1) field mapping by Ringrose (1982), 

(2) field mapping by the USGS for this study (1996–99), 

and (3) remotely sensed mapping by AVIRIS spectroscopy 

(Dalton and others, this volume). Pervasive QSP-alteration 

grades outward into weak sericite-pyrite and hydrothermal 

propylitic assemblages, and finally into the regional propylitic 

assemblage. For simplicity, the hydrothermal propylitic and 

weak sericite-pyrite assemblages have been combined into a 

generalized “weak sericite-pyrite” assemblage, as shown in 

figures 1 and 5.

The large area of QSP-altered rock (3.5 km2

) is centered 

roughly on the summit of peak 3,792 m and is coincident with 

steep, highly colorful hematite and jarosite-stained slopes. In 

this zone, intrusive and volcaniclastic rocks as well as flow 

rocks are intensely altered and are characterized by complete 

replacement of feldspars and groundmass by coarse-grained 

sericite, which is readily discerned with a hand lens. Secondary quartz is also commonly present as a blue-gray microcrystalline “flooding” throughout the groundmass and within 

dense networks of thin stockwork veinlets. Fine-grained pyrite 

(≤1–2 mm) is disseminated throughout rock and is present in 

fractures and veinlets; abundances typically range from 2 to 

5 volume percent. Geochemical data show that both disseminated pyrite and fracture-filling pyrite have low trace-metal 

abundances (table 2) and are similar in composition to QSPrelated pyrites analyzed from other parts of the study area. 

Fracture-controlled zones of QSP-altered rock are also common on the margins of the main QSP alteration assemblage, 

cutting broad zones of more weakly altered rock.

Weak sericite-pyrite altered rock (WSP, fig. 5) is characterized by incipient to partial replacement of plagioclase by 

sericite; biotite and pyroxene are altered to chlorite, sericite, 

and fine opaques. These rocks are generally less silicified 

than their QSP counterparts and contain fewer quartz-sulfide 

stockwork veinlets. Hydrothermal propylitic alteration zones 

(combined in WSP assemblage, fig. 5), which formed contemporaneously with the porphyry hydrothermal system, are 

composed of fresh to weakly altered feldspar, chlorite- and 

epidote-altered biotite and pyroxene, and abundant veins and 

fractures filled with pyrite, chlorite, magnetite, and quartz. 

Rocks affected by this alteration are present along the outer 

margins of the porphyry system. An earlier formed regional 

propylitic assemblage characterizes about 25 percent of the 

rock within the Mount Moly area. Although this assemblage 
Figure 5. Alteration map of the Mount Moly area, centered around peak 3,792 m.

107o46'51"

37o51'20"

107o42'51"

Bonner

Mine

Independence

Mine

Ruby Trust

Mine

Paradise

Portal

Magnet

Mine

unnamed

mine

37o47'48"

 3,792 m

HYDROTHERMAL ALTERATION ASSEMBLAGES

QU Quaternary undivided; lacks spectral signature (AVIRIS)

QQAS Coarser surficial deposits with strong spectral signature.

 Dominantly quartz-alunite-pyrophyllite (QAP)

 and less quartz-sericite-pyrite (QSP)

VQSP-P Vein-related QSP with 

 propylitic (PROP)

WSP Weak sericite-pyrite 

QSP-WSP Mixed QSP and WSP

QSP Quartz-sericite-pyrite 

Acid-Sulfate Suite

QAP-S Mixed QA and QAP with less QSP

PROP-AS Mixed PROP and Acid-Sulfate

 Suite (AS)

PROP Propylitic alteration undivided

V-WSP Vein-related (WSP)

QQSP Coarser surficial deposits with strong spectral

 signature. Dominantly QSP Fault or major fracture zone

Adit

QAm Mixed quartz-alunite (QA) and 

 quartz-alunite-pyrophyllite (QAP)

EXPLANATION

Mineral

Creek

Middle

Fork

Red

tributary

South

Fork

0 0.5 1 KILOMETER

180 Environmental Effects of Historical Mining, Animas River Watershed, Colorado
Mineralization, Alteration, and Geochemical Signatures 181

grossly resembles the Mount Moly area hydrothermal propylitic assemblage, it is distinguished largely by the absence 

of pyrite, quartz, and chlorite veinlets, and the scarcity of 

finely disseminated pyrite. The distribution of calcite within 

both regional and hydrothermal propylitically altered rocks 

is important because of calcite’s inherent acid-neutralizing 

capacity (Mast and others, this volume).

Relations in drill core show that QSP alteration overprinted secondary biotite alteration at depth beneath peak 

3,792 m (McCusker, 1982). Disseminated aggregates and flakes 

of secondary biotite and coarse-grained selvages surrounding 

quartz-molybdenite veins were encountered near the bottom 

of a 155-m drill hole collared approximately 305 m below the 

peak’s summit. The secondary biotite assemblage is accompanied by vein and disseminated anhydrite and associated sericite. 

Gypsum, which is present within vein and fractures along with 

disseminated sulfides, was also reported locally within several 

exploratory drill holes (McCusker, 1982). Coarse-grained gypsum crystals are also present in mineralized fractures along the 

periphery of the Mount Moly altered area.

Alteration Zones Near the Southeast Margin 

of the Silverton Caldera

Although the most obvious area of weak sericite-pyrite 

alteration and mineralization is in the Mount Moly area, one of 

the five major areas discussed in this report, several large zones 

of weak sericite-pyrite to locally QSP-altered rock are roughly 

coincident with an elongate quartz monzonite intrusive body 

exposed along the projected southeastern ring-fracture zone of 

the Silverton caldera. These altered rocks discontinuously crop 

out in a 7,000 m long by 1,800 m wide zone on the east side of 

the upper Animas River, extending from Cunningham Creek to 

Eureka Gulch (fig. 1). Although these rocks contain elevated 

abundances of finely disseminated pyrite, AVIRIS mapping 

does not detect extensive zones of jarosite or goethite oxidation, which are generally coincident with more intensely altered 

and pyritic rock (Dalton and others, this volume). Limited grab 

sampling of altered rocks from this zone shows trace to minor 

amounts of gold and silver (Varnes, 1963). However, it is possible that these subdued metal anomalies are related to younger 

(< 18 Ma) northwest-striking veins that intersect this altered 

area. As shown in figure 1, areas of propylitically altered rock 

are intermittently interspersed throughout this zone.

Geochemistry of Altered Rocks

Geochemical data as grouped by alteration types in the 

Mount Moly area are summarized in table 3 and figure 4. 

Analyses of surface or outcrop samples (26 samples, table 3) 

show that magnesium, calcium, sodium, and manganese 

generally decrease with increasing alteration intensity from 

propylitic through weak sericite-pyrite to QSP-altered rocks. 

In contrast, potassium and aluminum abundances are most elevated in QSP-altered rocks but are not statistically different in 

the propylitic and WSP-altered groups. The relatively low iron 

content of QSP-altered samples, as shown in figure 4, results 

from the weathering of pyrite and loss of iron. Zinc and copper 

concentrations are generally higher in propylitic samples than 

in QSP and WSP samples, whereas lead and molybdenum 

are higher in QSP and WSP samples compared to propylitic 

samples.

Comparison of oxidized versus unoxidized QSP-altered 

rock from drill cores in the Mount Moly area (153 samples) 

(table 3 and fig. 6) depicts marked contrasts in zinc and copper 

10

100

0.1

1

Manganese (ppm)

10

100

1,000

10,000

Lead (ppm)

10

100

1,000

Zinc (ppm)

10

100

1,000

10,000

Copper (ppm)

1

10

100

Molybdenum (ppm)

QSP-Unoxidized Zone n=30

QSP-Oxidized Zone n=73

AS-Drill Core n=34

AS-Surface n=34

ALL DRILL CORE

Mount Moly

Red Mtns

 OPAM

Red Mtns

OPAM

Red Mtns

OPAM

Mount 

Moly

Red Mtns

 OPAM

Red Mtns

 OPAM

 Mount Moly

Red Mtns

 OPAM

 Mount Moly

10

100

1,000

10,000

Iron (ppm)

EXPLANATION

95th 

percentile

75th 

Median 

25th 

5th 

Figure 6. Box plots comparing surface and subsurface (drill core) rock chemistry data from acid-sulfate (AS) and 

quartz-sericite-pyrite (QSP) zones in the Red Mountains–OPAM and Mount Moly alteration areas. All vertical scales 

in parts per million (ppm); n, number of samples.
concentration, along with minor variations in molybdenum. 

Drill core logs indicate that zones of strong oxidation and 

pervasive bleaching are present to depths as much as 100 m 

below the ground surface. The mean copper concentration 

in unoxidized drill core intercepts of QSP-altered rock is 

374 ppm, compared to 15 ppm in oxidized QSP-altered rocks. 

Similarly, mean zinc concentration changes from 109 ppm to 

8 ppm in unoxidized versus oxidized QSP-altered core samples. Molybdenum concentrations, however, show only a slight 

decrease in the oxidized rocks (mean of 11 versus 24 ppm 

molybdenum). The higher copper concentration in unoxidized 

samples reflects the presence of secondary copper enrichment 

beneath the upper oxidized zone. That secondary enrichment 

has occurred is also supported by the partial replacement 

of pyrite by chalcocite, digeonite, and covellite within this 

deeper zone of enrichment (McCusker, 1983). In contrast, 

lead concentrations are not significantly different between the 

oxidized and unoxidized QSP core samples, reflecting the low 

solubility of lead-sulfide minerals (White and Brantley, 1995). 

The above-mentioned differences in metal concentrations as 

observed in oxidized and unoxidized rocks give a general portrayal of the relative mobility of some of these metals in lowpH surface water. Referred to as supergene oxidation, these 

processes postdate hydrothermal alteration and have been 

especially active from the time of Neogene uplift to the present (Steven and others, 1999; Yager and Bove, this volume).

Mine Dump Compositions

Analytical data summarizing the geochemistry of 

15 mine dumps in the Mount Moly area are in table 4. Figure 7 

compares these data to similar analyses from the four other 

mineralized areas discussed herein. Manganese abundance in 

mine dump samples from this area (262 ppm geometric mean; 

table 3) is relatively low compared to three of the mineralized 

areas shown in figure 7. This value is two times lower than the 

mean manganese concentration of surrounding propylitically 

altered rocks. These low manganese values reflect the paucity 

of manganese silicate and manganese carbonate gangue minerals in Mount Moly area vein deposits. Mean lead and zinc 

concentrations both exceed 1,500 ppm. Mean lead concentration is lower than that in any of the other four mineralized 

groups, whereas mean zinc is similar to the concentration of 

the other groups (fig. 7). Both lead and zinc abundances in the 

mine dump samples are >30 times that of all unmined rock 

samples collected in this area. Mean copper (192 ppm) and 

arsenic (171 ppm) are substantially lower in Mount Moly area 

dump samples relative to copper-arsenic-rich dumps such as 

those in the Red Mountains area (fig. 7). These low combined 

copper and arsenic values reflect the presence of relatively 

small quantities of tetrahedrite-tennantite within these ores 

(Ringrose, 1982; McCusker, 1983). Mount Moly area mine 

dump samples, however, have somewhat lower mean copper 

concentrations than do unoxidized rocks intercepted in exploratory drill holes (see subsurface metals data; table 3). Mean 

molybdenum concentration of mine dump samples is 36 ppm, 

with higher values exceeding 105 ppm. Along with dumps 

from the Eureka Graben area, Mount Moly area dumps contain 

the highest molybdenum concentrations of the five mineralized areas (fig. 7). These data support reports of molybdenite 

within the vein assemblage (McCusker, 1983) and suggest that 

some of these veins may be genetically linked to the 26–25 Ma 

Mount Moly Mo-Cu hydrothermal system.

Mine Water Chemistry

Water-quality data from eight mine-discharge samples 

in the Mount Moly area are shown in figure 8 and summarized 

in table 5. The chemistry of these samples is highly variable: 

pH ranges from 3.00 to 6.51 and concentrations from 1 to 

22,000 μg/L aluminum, 530 to 5,900 μg/L manganese, 39 

to 2,300 μg/L zinc, and 2 to 629 μg/L copper. As shown in 

figure 9, these mine samples, like those from the other four 

mineralized areas, have a roughly bimodal distribution of pH 

values. The chemistry of low-pH waters is particularly useful 

for this study, because metals, which can be used as geochemical signatures to discriminate between different types 

of mineral deposits, generally remain in solution. Where pH 

exceeds 4.5, reduced solubility results in the precipitation or 

loss of many metals. In addition, many of the higher pH waters 

may represent a substantial mixing of acidic waters with more 

alkaline waters from unaltered or nonmineralized sources.

Four mine discharge samples in the Mount Moly area 

have pH from 5.01 to 6.51, wide ranges in alkalinity (from 0 

to 70 mg/L) and zinc concentration (39 to 620 μg/L), and very 

low concentrations of lead and copper (sum <13 μg/L). Manganese, which is more soluble at higher pH than most metals 

(Smith and Huyck, 1999), is widely variable, ranging from 

530 to 5,900 μg/L. Three of these samples have high combined concentrations of calcium (>100 to 380 mg/L), strontium (>950 to 2,600 μg/L), and sulfate (>80 to 1,300 mg/L) 

compared to other samples from the study area (table 6). 

Aqueous δ34S data, which have been used in this study to help 

constrain the sources of dissolved sulfate, range from +4.7 to 

+5.2 per mil in two of these high calcium-strontium-sulfate 

waters (Nordstrom and others, this volume, Chapter E8). 

These values are significantly heavier than those measured in 

sulfide vein minerals (–0.2 to +2.0 per mil) and disseminated 

pyrite (–3.1 to +2.5 per mil) from the surrounding alteration 

assemblages (table 7). These data indicate that waters may 

have interacted with isotopically heavier gypsum or anhydrite4

, which have δ34S compositions larger than +14 per mil 

throughout the watershed. These data are supported by the 

presence of gypsum gangue material on several of these mine 

dumps. Two of the high calcium-strontium-sulfate waters 

(Paradise mine; mine # 168, Church, Mast, and others, this 

4

Upon oxidation or dissolution of minerals there is little or no fractionation 

of sulfur isotopes, so the resulting isotopic signature of surface waters should 

be representative of the geologic source. Once dissolved in surface water, the 

sulfur in sulfate can be fractionated by sulfate-reducing bacteria (Krouse and 

others, 1991), but only in instances of very low dissolved oxygen.

182 Environmental Effects of Historical Mining, Animas River Watershed, Colorado
Mineralization, Alteration, and Geochemical Signatures 183

Table 4. Summary of mine dump geochemistry from five different mineralized areas.[n, number of samples; n.d., not determined; gmean, geometric mean; ppm, parts per million]

Al Ca Fe K Mg Na P Ti Ag As Ba Cd Cu Mn Mo Ni Pb Sr Zn

percent ppm

Mt. Moly area (n=15)1

:

25th percentile n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. 53 158 n.d. n.d. 98 140 17 n.d. 570 220 35575th percentile n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. 500 235 n.d. n.d. 363 345 105 n.d. 3,500 320 7,750gmean n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. 149 171 n.d. n.d. 192 262 36 n.d. 1,580 266 1,860Red Mountains area (n=20)2

:

25th percentile 4.7 0.05 3.4 0.53 0.02 0.06 0.10 0.01 5 120 290 2 140 7 4 6 600 355 26075th percentile 7.6 0.10 10.3 2.55 0.45 0.15 0.17 0.45 59 9,200 470 20 26,000 175 6 18 14,300 993 5,330gmean 5.7 0.07 6.1 0.96 0.11 0.09 0.12 0.06 24 943 368 6 1,930 39 5 9 1,870 602 935Ohio Peak–Anvil Mountain area (n=6)2

:

25th percentile 7.8 0.11 3.3 3.0 0.55 0.21 0.11 0.47 22 73 308 7 331 441 11 5 5,720 295 2,76075th percentile 8.6 0.20 5.2 3.5 0.67 0.30 0.19 0.58 35 100 648 21 613 1,040 25 7 12,750 319 4,880gmean 8.3 0.15 3.3 3.3 0.60 0.22 0.16 0.54 27 87 479 12 382 679 17 6 7,550 320 3,070Eureka Graben area (n=18)2

:

25th percentile 6.7 0.05 2.5 3.3 0.31 0.06 0.07 0.15 10 83 383 2 100 425 22 4 1,950 76 51875th percentile 8.0 0.19 4.6 3.8 0.62 0.19 0.18 0.32 56 210 848 68 423 3,580 155 9 15,000 168 15,400gmean 6.9 0.13 3.4 3.4 0.46 0.13 0.12 0.20 18 123 591 13 189 1,500 50 6 3,330 112 2,110South Silverton area (n=8)2

:

25th percentile 4.8 0.15 4.0 2.4 0.35 0.12 0.06 0.05 24 24 298 6 738 2,710 17 6 3,550 51 75575th percentile 6.2 0.98 5.7 2.9 0.61 0.22 0.10 0.16 99 94 593 39 2,100 6,000 46 14 13,500 66 9,530gmean 5.3 0.37 4.9 2.6 0.47 0.16 0.07 0.05 43 49 450 13 1,510 3,440 27 9 6,390 60 2,450

1Mo by colorimetric method and atomic absorption (AA), AMAX and Skyline Labs, Denver, Colo.; Cu, Pb, Zn, by AA, AMAX lab; remainder by XRF, Skyline and AMAX labs; data from McCusker (1983).2Data from Fey and others (2000).
volume, Chapter E5; and nearby prospects; fig. 5) are characterized by anomalously high aluminum concentrations 

(8,600 μg/L and 22,000 μg/L) and are associated with thick 

aluminum hydroxide precipitates where they discharge over 

high-angle mine tailings (Peter Butler, Robert Owen, and 

William Simon, Unpublished report to Colorado Water Quality 

Control Commission, Animas River Stakeholders Group, 

2001; Wright and others, this volume, Chapter E10). The precipitation of aluminum hydroxide at the pH range of the mine 

portal discharge—based on constraints on aluminum solubility with respect to pH—necessitates a mixing of acidic waters 

(pH <5.5) containing high dissolved aluminum with more 

alkaline waters (Nordstrom and Ball, 1986). The resulting rise 

in pH above 5.5 at or near the mine portals—the lower solubility range for aluminum—would then promote the precipitation of aluminum hydroxide from solution, as observed on the 

Paradise mine dumps (Nash and Fey, this volume, fig. 20). In 

addition to causing the precipitation of aluminum, the mixing of more alkaline waters with the acidic waters may have 

reduced the overall zinc concentrations of these waters, further 

obscuring the primary metal signatures of the vein deposits 

(Nordstrom and Ball, 1986).

As just discussed, a compilation of the low-pH mine 

water geochemistry from the Mount Moly area (pH <4.5) may 

help us distinguish the aqueous geochemical signatures of 

these mineral deposits and compare them to analogous data 

from the other mineralized areas. Geochemical data from four 

mine discharge samples with low pH in the Mount Moly area 

are summarized in table 5 and compared to similar analyses 

from the other mineralized areas in figure 10. Manganese 

concentration is moderately elevated compared to manganese in samples from the other four mineralized areas in the 

study area (fig. 10; table 5) and may reflect the presence of 

minor amounts of manganese-bearing gangue minerals in 

the associated veins. The mean concentrations of lead (21 

μg/L), zinc (505 μg/L), copper (148 μg/L), and arsenic (<1 

μg/L) are among the lowest of the acidic mine waters from the 

other mineralized areas (table 5). Mean sulfate and strontium 

concentrations are 254 mg/L and 208 μg/L, respectively. Aqueous δ34S data from the Bonner and Independence mines (mine 

# 172 and # 171, respectively, Church, Mast, and others, this 

volume) (fig. 5) range from +5.6 to +6.4 per mil (Nordstrom 

and others, this volume). These values are heavier than those 

measured in sulfide vein minerals (–0.2 to +2.0 per mil) and 

disseminated pyrite (–3.1 to +2.5 per mil) from the surrounding alteration assemblages (table 7). As discussed previously, 

these waters may also have interacted with isotopically heavier 

gypsum, which is present along pyrite-bearing fractures on the 

periphery of the Mount Moly area.

Background Surface Water Chemistry

Geochemical data from all “background” streams and 

springs that interacted with “end-member” alteration assemblages (waters that dominantly drain a single alteration type) 

in the Mount Moly area are summarized in table 8. As shown 

in figure 11, the chemistry of water draining these different 

alteration assemblages varied, although variation within alteration assemblage was much less than variation across assemblages. Streams and springs draining QSP-altered rock were 

the most acidic (mean pH=3.22), having the highest mean 

concentrations of aluminum (18,400 μg/L), iron (22,200 μg/L), 

1

10

100

1,000

10,000

100,000

Mn ppm Pb ppm Zn ppm Cu ppm As ppm Sr ppm Ca ppm

South Silverton

Mount Moly Eureka Graben

OPAM

Red Mountains

n=15

n=20

n=6

n=18

n=8

Mo ppm

nd

EXPLANATION

95th 

percentile

75th 

Median 

25th 

5th 

Figure 7. Box plots comparing mine dump data from the five mineralized areas. All vertical scales in parts per million (ppm); 

n, number of analyses; nd, not determined.

184 Environmental Effects of Historical Mining, Animas River Watershed, Colorado
Mineralization, Alteration, and Geochemical Signatures 185

Figure 8. Box plots comparing chemistry of all mine water samples to only those with pH <4.5. Note: South Silverton discharge 

samples (11 of 12 samples > pH 4.5) shown only in figure 9; g/L, grams per liter; mg/L, milligrams per liter; μg/L, micrograms per 

liter; nsd, not sufficient data.

0.1

1

10

100

1,000

10,000

100,000

1,000,000

1

2

3

4

5

6

7

8

9

10

Al (μg/L) Fe (μg/L) Mn (μg/L) Pb (μg/L) Zn (μg/L) Cu (μg/L) As (μg/L) Sulfate (mg/L) pH

Mt Moly

Al (μg/L) Fe (μg/L) Mn (μg/L) Pb (μg/L) Zn (μg/L) Cu (μg/L) As (μg/L) Sulfate (mg/L)

0.1

1

10

100

1,000

10,000

100,000

1,000,000

pH

Red Mountains

nsd

OPAM

Eureka Graben

Low pH

All samples

n=4 

n=12 

Low pH

All samples 

n=9

n=10

1

2

3

4

5

6

7

8

9

10

0.1

10

100

1,000

10,000

100,000

1,000,000

1

2

3

4

5

6

7

8

9

10

Al (μg/L) Fe (μg/L) Mn (μg/L) Pb (μg/L) Zn (μg/L) Cu (μg/L) As (μg/L) Sulfate (mg/L) pH

Low pH

All samples

n=5

n=13

0.1

1

10

100

1,000

10,000

100,000

1,000,000

1

2

3

4

5

6

7

8

9

10

Al (μg/L) Fe (μg/L) Mn (μg/L) Pb (μg/L) Zn (μg/L) Cu (μg/L) As (μg/L) Sulfate (mg/L) pH

Low pH

All samples

n=8

n=21

95th 

percentile

75th 

Median 

25th 

5th 
Table 5. Summary of mine water geochemistry from five different mineralized areas.

[SC, specific conductance; μS/cm, microsiemens per centimeter; μg/L, micrograms per liter; mg/L, milligrams per liter; gmean, geometric mean; n, number of samples; n.d., not determined]

SC

μS/cm pH

Alkalinity

mg/L as 

CaCO3

SO4 Ca Mg Na K Si Al As* Ba Be Cd Cr Cu Fe Mn Mo Ni Pb Sr# Zn

mg/L μg/L

Mt Moly area:pH <4.5 (n=4)1

25th percentile 560 3.12 n.d. 177 33.5 4.9 4.69 0.58 19.2 4,700 <1 3 <1 1.4 <15 53 4,770 1,140 <10 14 21 77 22475th percentile 900 3.14 n.d. 363 95.3 8.8 6.73 0.84 22.4 6,970 1 13 1 13.4 15 629 6,940 2,870 15 14 21 404 2,280gmean 693 3.13 n.d. 254 59.0 6.5 5.81 0.67 21.0 5,670 <1 8 <1 3.4 <15 148 5,790 1,600 <10 14 21 208 505all samples (n=8)125th percentile 488 3.13 n.d. 153 34.1 4.8 4.48 0.50 16.1 288 <1 8 <1 0.9 <15 3 5,381 1,226 <10 11 1 125 24575th percentile 1,165 5.87 18.5 572 172.1 14.8 7.00 0.81 24.0 7,379 1 14 1.5 2.3 <15 64 23,763 3,430 <10 14 21 1,388 599gmean 680 4.60 n.d. 272 82.9 8.8 5.53 0.64 19.7 1,073 <1 10 1.1 1.7 <15 16 8,335 2,013 <10 <10 6 407 355Red Mountains area:pH <4.5 (n=9)225th percentile 935 2.40 n.d. 367 3.5 1.2 0.56 0.62 11.5 5,870 51.0 2 <1 43.5 <15 1,640 60,100 208 <10 16 42 39 9,77075th percentile 1,840 2.79 n.d. 1,000 6.0 2.3 1.28 0.92 27.0 21,900 167.0 18 <1 170.2 <15 4,900 177,000 603 18 50 545 48 28,900gmean 1,020 2.63 n.d. 514 5.5 2.3 0.85 0.87 16.6 7,870 103.2 7 <1 50.2 <15 2,340 63,300 401 14 28 177 37 10,600all samples (n=10)325th percentile 614 2.40 n.d. 143 3.5 1.2 0.56 n.d. 8.5 1,849 30.0 2 <1 10.0 <15 638 11,252 205 <10 14 30 38 1,70075th percentile 1,838 2.79 n.d. 1,000 7.2 3.3 1.43 n.d. 27.0 21,879 167.0 21 <1 170.2 <15 4,904 176,760 603 18 48 545 155 28,882gmean 913 2.93 n.d. 446 7.2 2.3 0.90 n.d. 13.4 4,205 35.2 7 <1 33.7 <15 1,107 25,976 266 13 25 136 96 5,268Ohio Peak–Anvil Mt area:pH <4.5 (n=5)125th percentile 897 2.67 n.d. 416 25.2 10.3 1.23 n.d. 22.0 12,600 <1 6 <1 4.4 <15 204 10,200 2,960 n.d. 28 27 n.d. 2,17075th percentile 1,620 3.62 n.d. 1,010 69.9 27.3 2.58 n.d. 35.0 29,500 11.3 19 4 35.3 <15 1,190 107,000 9,500 n.d. 70 233 n.d. 7,010gmean 950 3.02 n.d. 498 43.4 15.4 1.86 n.d. 20.4 14,400 2.7 10 2 10.9 <15 314 23,900 3,750 n.d. 41 62 n.d. 3,640all samples (n=8)125th percentile 1,055 3.21 n.d. 645 29.6 12.5 1.55 n.d. 17.5 250 <1 6.5 <1 1.35 <15 6 7,900 1,750 n.d. 14 11 280 72975th percentile 1,783 6.70 47.5 1,025 239.5 20.9 7.63 n.d. 30.9 19,000 8.5 11 2 21.6 <15 632 53,378 8,325 n.d. 45 29 2,547 4,960gmean 1,148 4.27 0.4 643 94.4 15.1 3.52 n.d. 19.6 1,129 2.0 9.4 1.2 4.94 <15 51 17,084 3,364 n.d. 25 16 706 751

186 Environmental Effects of Historical Mining, Animas River Watershed, Colorado
Mineralization, Alteration, and Geochemical Signatures 187

Table 5. Summary of mine water geochemistry from five different mineralized areas.—Continued

SC

μS/cm pH

Alkalinity

mg/L as 

CaCO3

SO4 Ca Mg Na K Si Al As* Ba Be Cd Cr Cu Fe Mn Mo Ni Pb Sr# Zn

mg/L μg/L

Eureka Graben area:pH <4.5 (n=8)4

25th percentile 455 3.16 n.d. 164 24.0 3.5 0.37 0.63 8.6 1,480 <1 7 2 20.4 n.d. 218 1,910 5,700 <10 14 73 64 4,18075th percentile 706 3.64 n.d. 363 59.1 6.2 1.61 0.94 12.9 5,250 7.1 12 4 106.0 n.d. 2,550 16,950 18,900 <10 22 278 254 19,200gmean 541 3.37 n.d. 224 36.9 4.0 0.85 0.87 9.8 3,040 <1.2 8 2 42.1 n.d. 633 5,050 8,100 <10 18 170 124 9,570all samples (n=13)525th percentile 378 3.36 0.0 147 26.3 2.8 0.5 0.6 4.2 116 <1 9.5 <1 7.8 n.d 10 121 1,223 <10 14 21 87 2,30575th percentile 680 6.21 8.0 269 59.6 6.1 3.3 1.4 10.3 3,275 7.1 14 3 50.6 n.d. 1,409 11,519 7,339 <10 21 182 740 9,427gmean 484 4.25 0.1 194 43.1 4.0 1.2 1.0 6.6 755 1.2 11 1 19.6 n.d. 120 842 3,307 <10 16 62 239 4,067South Silverton area (n=12)6

:

25th percentile 64 6.20 13 13.1 0.5 0.29 0.10 1.0 40 <1 6 <1 0.6 <15 5 3 6 n.d. <10 1 n.d. 6075th percentile 322 7.86 144 66.4 2.8 2.42 0.38 5.1 100 <1 15 <1 9.4 <15 98 130 503 n.d. <10 114 n.d. 1,750gmean 146 6.58 47 24.0 1.3 0.76 0.16 2.6 95 1.1 10 1 2.2 <15 27 26 63 n.d. 11 10 n.d. 313

1Mast and others, 2000; this volume.2Seven samples from Mast and others, 2000; this volume; one from Colorado Department of Public Health and Education (CDPHE); Bove and Knepper, 2000; one sample from Division of Mines and 

Geology (DMG), Unpublished Cement Creek feasibility report, CDMG, 1998.

3Eight samples from Mast and others, 2000; this volume; one from Colorado Department of Public Health and Education (CDPHE); Bove and Knepper, 2000; one sample from Division of Mines and 

Geology (DMG), Unpublished Cement Creek feasibility report, CDMG, 1998.

4Four samples from Mast and others, 2000; four samples from Unpublished Cement Creek feasibility report, CDMG, 1998.5Nine samples from Mast and others, 2000; four samples from Unpublished Cement Creek feasibility report, CDMG, 1998.6Allsamples from Division of Mines and Geology (DMG), Unpublished Cement Creek feasibility report, CDMG, 1998.*Arsenic data represents six analyses in both the Red Mountains and Eureka Graben areas.

#Red Mountains and Eureka Graben strontium data represent analysis of five and four samples, respectively. 
potassium (1 mg/L), magnesium (10 mg/L), silicon (20 mg/L), 

zinc (247 μg/L), and copper (45 μg/L). The mean abundances 

of aluminum and iron were more than 2.5 times greater than 

in low-pH mine waters from this area, whereas ranges in zinc, 

copper, and sulfate were generally similar to those of low-pH 

mine waters. Although QSP-altered rocks have the lowest 

abundances in magnesium and manganese compared to other 

altered rocks in the Mount Moly area, mean concentrations 

of these elements in Mount Moly background QSP waters 

are among the highest of all water types. The relatively high 

magnesium and manganese concentrations in QSP waters 

is probably due to higher solubility of most base cations at 

these lower pH ranges and the availability of cations within 

islands of less-altered rock preserved within the larger QSP 

alteration zone. Mean sulfate concentration in these waters is 

very high (309 mg/L) compared to that of other background 

Cd 

0

1

10

100

1,000

2345678

Cu 

1

10

100

1,000

10,000

100,000

2345678

Mt Moly area

Red Mtns

OPAM

Eureka Graben

South Silverton

Zn 

1

10

100

1,000

10,000

100,000

2345678

Pb 

1

10

100

1,000

10,000

2345678

Al 

0.1

1.0

10.0

100.0

1,000.0

10,000.0

100,000.0

2345678

Mn 

0.1

1.0

10.0

100.0

1,000.0

10,000.0

100,000.0

2345678

Figure 9. Graphs of metals versus pH for all mine waters from the five mineralized areas. All metal concentrations in micrograms per 

liter (y axes); pH, in standard units (x axes). Dashed vertical line is rough divide between low and high pH, as discussed in text.

188 Environmental Effects of Historical Mining, Animas River Watershed, Colorado
Mineralization, Alteration, and Geochemical Signatures 189

Table 6. Ranges and geometric means of mine and background water geochemistry from five different mineralized areas.

[gmean, geometric mean; SC, specific conductance, in microsiemens per centimeter; nd, not determined; n, number of samples]

SC

μS/cm pH

Alkalinity

mg/L as 

CaCO3

SO4 Ca Mg Na K Si

mg/L

All mine waters (n=77)

gmean 442 4.84 1.00 182 30.5 3.5 1.37 0.64 7.7

min 17 2.17 <1 3 2.0 0.2 0.07 0.64 1.0

max 3,140 8.17 122.00 5,000 400.0 42.0 10.00 0.64 38.0

All background waters (n=111)

gmean 244 4.99 0.92 80 19.6 2.5 1.26 0.56 6.2

min 12 2.66 0.00 1 0.8 0.1 0.10 0.11 0.9

max 1,845 8.49 69.00 1,200 420.0 27.8 13.00 2.80 53.0

Al As Ba Be Cd Cr Cu Fe Mn Mo Ni Pb Sr Zn

μg/L

All mine waters (n=77)

gmean 672 2.0 10 1 7.4 7 85 1,272 698 6 15 269 28 1,255

min 1 0.1 1 1 0.6 1 2 3 3 1 2 23 1 6

max 51,863 580.0 54 5 278.7 20 22,000 120,000 71,600 42 110 4,351 5,606 52,370

All background waters (n=111)

gmean 518 nd 23 1 1.7 9 10 235 88 9 16 17 189 66

min 1 nd 1 1 0.3 1 1 1 1 1 1 1 6 20

max 54,020 nd 101 65 84.0 50 372 76,652 74,669 70 43 141 5,700 14,383

samples from the study area (table 6), whereas corresponding 

calcium (25 mg/L) and strontium (172 μg/L) are relatively low. 

δ34S values from three QSP-related waters range from +1.7 

to +3.9 per mil (Nordstrom and others, this volume, table 1), 

which is close to the range of all sulfides analyzed from the 

area (–3.1 to +2.5 per mil; table 7). These combined data 

suggest that much of the dissolved sulfate was derived from 

the oxidation of fine-grained pyrite in the QSP-altered rocks. 

These waters with sulfate that is likely related, for the most 

part, to the oxidation of pyrite contrast markedly with those 

that likely had more interaction with vein-filling gypsum or 

anhydrite within weakly sericitized rocks (WS-gyp; fig. 11). 

Waters likely influenced by the dissolution of isotopically 

heavy gypsum or anhydrite (WS-gyp) have measurable alkalinity, greater than two times the mean sulfate (702 mg/L), 

and more than nine times the calcium (251 mg/L mean) and 

strontium (2,370 μg/L) concentration of QSP waters. δ34S values of three of the WS-gyp-related waters were substantially 

heavier (δ34S= +7.0 to +8.0 per mil) than the QSP waters, 

due to interaction with gypsum or anhydrite (δ34S= +14.6 to 

+18.0 per mil in the entire Animas River watershed study area; 

table 7; Casadevall and Ohmoto, 1977). WS-gyp waters have 

the highest manganese concentrations (1,490 μg/L mean) of all 

background streams and springs in the Mount Moly area. This 

may indicate dissolution of manganese silicate or manganese 

carbonate gangue minerals occurring with gypsum or anhydrite along fracture zones.

Streams and springs draining calcite-bearing propyliticaltered rock have the highest mean values of pH and alkalinity 

(6.74 and 16.4 mg/L, respectively) and the lowest mean dissolved metal concentrations (13 mg/L aluminum; 5 μg/L iron; 

26 μg/L zinc; 3 μg/L copper) of all Mount Moly area water 

samples. Sulfate concentrations for these 10 propylitic samples 

(table 8) have a bimodal distribution (fig. 12); six samples 

from a low-sulfate group have concentrations ranging from 

19 to 77 mg/L, whereas the remaining samples have values 

ranging from 210 to 263 mg/L. Sulfate shows a strong positive 

correlation with strontium and calcium, both of which also are 

bimodally distributed within this sample set (fig. 12). δ34S data 

from three of these waters show fairly predictable associations, 

with the lightest (–2.8 and –0.8 per mil) and heaviest (+7.1 per 

mil) values correlated with the low and high sulfate-calciumstrontium groups, respectively. As discussed previously, the 

WS-gypsum-related waters show similar associations between 

anomalously high sulfate-calcium-strontium and heavy 

δ34S values. Therefore, we suggest that sulfate in the propylitic waters is derived both by pyrite oxidation and gypsum or 

anhydrite dissolution.

Samples draining rocks of the weak sericite-pyrite 

assemblage (WSP) have a mean pH of 4.52 and fall compositionally between QSP and PROP samples, reflecting the 

transition between the two alteration assemblages (table 8; 

fig. 11). Alkalinity within most WSP samples is measurable and ranges up to 10 mg/L; calcite was still detected in 

many of these altered rocks. In contrast, the low pH of QSP 
Sample Mineral Vein Mineralogical notes δ18O δ18OTotal δ18OSO4 δ18OOH δ34S sulfide δ34S sulfate δD

Mount Moly area

SDY3997 pyrite Fine blebs of 1-2 mm dissem pyrite in QSP zone -- -- -- -- 2.5 -- --SDY05A97 pyrite Fine blebs of 1-2 mm dissem pyrite in QSP zone -- -- -- -- 0.7 -- --SDPH01PY pyrite Medium-grained pyrite, fracture in hyd prop zone -- -- -- -- 0.9 -- --SDY10B98 pyrite x Medium-grained pyrite in qtz vein in central QSP zone -- -- -- -- –3.1 -- --SDY0398 barite x Vein in QSP-altered rock -- -- -- -- 17.4 -- --BONNERPY pyrite x Late-stage pyrite in 3 mm vein -- -- -- -- 2.0 -- --BONNERGA galena x Coarse mid-stage vein galena -- -- -- -- –0.2 -- --BONNERSP sphalerite x Coarse mid-stage vein sphalerite -- -- -- -- 0.3 -- --PPSO4-1 gypsum x White, fine-granular, vein margin at Paradise Portal -- -- 4.7 -- -- 14.6 --

Red Mountains area

Table 7.

IDB61B pyrite Finely disseminated pyrite, QSP zone Prospect Gulch -- -- -- -- –1.5 -- --IDB40 pyrite Finely disseminated pyrite, QSP zone Prospect Gulch -- -- -- -- –1.7 -- --IDB98 pyrite Finely disseminated pyrite, QSP zone Prospect Gulch -- -- -- -- –4.7 -- --IDB04 PY pyrite Finely disseminated pyrite, quartz-pyrophyllite-alunite zone Red Mtn #3 -- -- -- -- –5.0 -- --HMT-804-602PY pyrite Finely disseminated pyrite, quartz-alunite zone 602' beneath upper Lark mine -- -- -- -- –6.9 -- --IDB1496 alunite Fine-grained replacement alunite, Red Mtn #3 -- -- -- -- -- 10.0 --IDB0696 alunite Fine-grained replacement alunite, Red Mtn #3 -- -- -- -- -- 14.6 --IDB1296 alunite Fine-grained replacement alunite, Red Mtn #3 -- 5.8 11.1 -- -- 17.5 –63IDB9997 alunite Fine-grained replacement alunite, Prospect Gulch breccia -- -- -- -- -- 15.4 --IDB10097 alunite Coarse-grained vein alunite, Prospect Gulch -- 2.7 11.1 -- -- 27.0 –94LS998 PY pyrite x Fine-grained vein pyrite, Lark mine stage 3b (see text) -- -- -- -- –3.6 -- --LS2B PY pyrite x Fine-grained vein pyrite, Lark mine stage 3b (see text) -- -- -- -- –3.9 -- --GQWSTPY pyrite x Fine-grained vein pyrite, Prospect Gulch, above Galena Queen, stage 3b (see text) -- -- -- -- –2.8 -- --LS998 SPH sphalerite x Coarse vein sphalerite, Lark mine, stage 2a (see text) -- -- -- -- –3.9 -- --GQ498SPH sphalerite x Coarse vein sphalerite, Galena Queen mine, stage 2a (see text) -- -- -- -- –4.7 -- --GQ398 GA galena x Coarse vein galena, Galena Queen mine, stage 2a -- -- -- -- –3.6 -- --

 Stable isotope data for vein and alteration minerals present within the study area.

[Samples not taken from five main mineralized areas are shown in italics (American tunnel, S. Silverton). Data in per mil values. x, vein sample; --, not meaningful or not determined]

190 Environmental Effects of Historical Mining, Animas River Watershed, Colorado
Mineralization, Alteration, and Geochemical Signatures 191

Sample Mineral Vein Mineralogical notes δ18O δ18OTotal δ18OSO4 δ18OOH

δ34S

sulfide

δ34S

sulfate δD

Red Mountains area—Continued

GQ498 PY pyrite x Fine-grained vein pyrite, Galena Queen mine, stage 3b (see text) -- -- -- -- –3.4 -- --104SP sphalerite x Coarse vein sphalerite, Henrietta mine, stage 2a? -- -- -- -- –1.1 -- --HENBAR-1 barite x Late-stage vein barite, Henrietta mine dump -- -- -- -- 31.5 -- --IDB01 dickite Replacement after plagioclase, waxy 4.6 -- -- -- -- -- –94GQWEST dickite Waxy vein-related gangue 5.2 -- -- -- -- -- –90NBELLE-1 kaolinite Soft, white, along 0.5 cm veinlet, dump sample at National Belle; post-sulfides –3.7 -- -- -- -- -- –123IDB98 illite 2m1 /1m polytype, after breccia in upper Prospect Gulch, QSP zone –0.3 -- -- -- -- -- --

PDB7B illite Poorly crystalline illite in weak sericite-pyrite zone (WSP) in upper Prospect 

Gulch

–3.5 -- -- -- -- -- --

Ohio Peak–Anvil Mountain area

SDB39 pyrite Finely disseminated pyrite, QSP zone, mid Topeka Gulch -- -- -- -- –2.3 -- --SDB38 pyrite Finely disseminated pyrite, QSP zone, mid Topeka Gulch -- -- -- -- –5.4 -- --TG-GNA galena x Coarse-grained, mid stage-vein galena, lower Topeka Gulch -- -- -- -- –0.8 -- --TG-1 gypsum x Coarse, clear vein? gypsum; mine dump lower Topeka Gulch -- -- 3.6 -- -- 17 --TG-BAR barite x Late-stage, vein barite, lower Topeka Gulch -- -- -- -- -- 28.9 --SDB4196 alunite Fine-grained replacement alunite, upper Topeka Gulch -- -- -- -- -- 24.0 --

American tunnel

B1-1918PY gypsum x Fine-grained pyrite in vein with anhydrite, 1,918 ft below A.T. -- -- -- -- –1.6 -- --B1-1685 gypsum x Coarse, clear vein gypsum, 1,685 ft below A.T. -- -- –1.9 -- -- 16.7 --B1-1918AN anhydrite x Vein anhydrite, 1,918 ft below A.T. -- -- –3.3 -- -- 17.3 --B1-1900 anhydrite x Vein anhydrite, 1,900 ft below A.T. -- -- 1.1 -- -- 18.0 --

Near South Silverton

Table 7.

NSM-1 gypsum x Coarse, clear vein gypsum; adjacent to QSP margin -- -- 1.1 -- -- 15.2 --

 Stable isotope data for vein and alteration minerals present within the study area.—Continued
Figure 10. Box plots comparing low-pH mine water data from the five mineralized areas. Note that South Silverton data represent 

per liter (mg/L); n, number of analyses.

Al (μg/L) Fe (μg/L) Mn (μg/L) Pb (μg/L) Zn (μg/L) Cu (μg/L) As (μg/L) Sulfate (mg/L)

0.1

1

10

100

1,000

10,000

100,000

1,000,000

1E7

* *

*

*

*

*

*

*

South Silverton

Eureka Graben

Mount Moly

OPAM

Red Mountains

Mine waters with pH < 4.5 All mine waters *

n=4

n=8

n=5

n=8

n=12

EXPLANATION

95th 

percentile

75th 

Median 

25th 

5th 

samples reflects the scarcity of calcite in the large volume of 

rock from which these waters drain. The paucity of calcite in 

QSP-altered rocks is attributed to the low pH of the ancient 

hydrothermal fluids that formed this mineral assemblage (Rose 

and Burt, 1979; Ringrose, 1982). Along with the changes in pH 

and alkalinity in these varied water types, note that dissolved 

magnesium increases from a mean of 2 to 3 mg/L in PROP and 

WSP waters, respectively, to 10 mg/L in QSP waters (table 8). 

These changes must be related to a marked increase in the rate 

of chlorite dissolution in these more acidic waters, as other 

magnesium-bearing minerals are absent in the associated altered 

rocks. These data confirm laboratory leach experiments (Jambor 

and others, 2000) documenting accelerated rates of chlorite dissolution in highly acidic solutions (pH <3.5). Although experimental data predict that chlorite dissolution in low-pH water 

has some acid-neutralizing potential (Lawrence and Wang, 

1997; Jambor and others, 2000), the low pH of the QSP waters 

suggests that the degree of chlorite breakdown has done little to 

neutralize all the acid generated by pyrite oxidation.

As a group, all water samples collected in the Mount 

Moly area have higher concentrations of sodium compared 

to the other four mineralized areas, with upper concentrations 

ranging from 7 to 13 mg/L. Similar enrichments in sodium are 

only observed in two other areas within the entire study area—

Chattanooga bend and Topeka Gulch (Yager and Bove, this 

volume, pl. 1). As discussed in Mast and others (this volume), 

waters in both areas were derived by interaction with rocks 

that were enriched in secondary albite, probably during the 

regional episode of propylitic alteration. Although not apparent in summary tables of end-member background waters 

(table 8), seven samples in the entire Mount Moly background 

data set have substantially elevated molybdenum concentrations (21–70 μg/L). In combination with elevated copper 

and secondary copper mineralization in this subeconomic 

porphyry system.

23 Ma Acid-Sulfate Alteration 

and Mineralization

The study area hosts two of the largest acid-sulfate 

hydrothermal systems in the western United States. The 

Red Mountains system (fig. 1) encompasses an area of roughly 

22 km2

 and was host to economically important silver-copperlead breccia-pipe and fault-hosted mineralization (Burbank and 

Luedke, 1969; Bove and others, 2000). The Ohio Peak–Anvil 

Mountain (OPAM) acid-sulfate system is very similar both in 

size (21 km2

) and character of alteration; however, unlike its 

northern counterpart, it is largely devoid of important mineral 

deposits. Both areas are located along the structurally complex northwest and southwest margins of the 27 Ma Silverton 

caldera.

192 Environmental Effects of Historical Mining, Animas River Watershed, Colorado

(25–372 μg/L), these data reflect the presence of molybdenite 

high-pH samples as only one sample had pH below 4.5. Vertical scales in micrograms per liter (μg/L) except for sulfate, in milligrams 
Mineralization, Alteration, and Geochemical Signatures 193

Table 8. Summary of background waters from five different mineralized areas.1[PROP, propylitic; PROP-V, propylitic with vein-related alteration; WSP, weak sericite-pyrite; WSP-gyp, WSP with gypsum in fractures; SC, μS/cm, specific conductance, in microsiemens per centimeter; n.d., not determined; nsd, not sufficient data for statistical summary; gmean, geometric mean]

SC

(μS/cm) pH

Alkalinity

mg/L as 

CaCO3

SO4 Ca Mg Na K Si Al Ba Be Cd Cu Fe Mn Mo Pb Sr Zn

mg/L μg/L

Mt Moly area:PROP (n=10) 25th percentile 136 6.52 9.9 30 21.3 1.3 1.73 0.3 4.2 3 20 1 1.0 1 <3 <1 <10 <30 140 <3

75th percentile 493 6.86 29.0 210 86.5 4.7 3.00 0.4 7.0 52 30 1 2.0 4 16 30 <10 <30 873 32

gmean 237 6.74 16.4 75 39.6 2.3 1.98 0.3 4.7 13 24 1 1.5 3 5 6 <10 <30 292 26

WSP (n=10) 25th percentile 235 3.82 <0.5 92 24.5 2.6 3.68 0.4 11.8 480 19 1 2.0 4 312 260 <10 <30 178 20

75th percentile 458 4.85 <0.5 178 55.9 4.6 5.08 0.7 18.9 2,300 25 1 2.0 4 814 490 18 <30 386 51

gmean 301 4.52 <0.5 120 35.7 3.1 4.06 0.5 13.2 843 19 1 1.7 5 457 296 <10 <30 251 30

WSP-gyp (n=6) 25th percentile 1,173 4.72 1.1 600 207.5 6.6 6.08 0.6 12.8 414 9 1 1.0 8 402 642 <10 <30 1,570 20

75th percentile 1,791 6.01 29.3 1,150 405.0 17.0 7.53 0.8 18.0 1,630 12 1 1.8 11 1,650 2,450 <10 <30 4,780 98

gmean 1,413 5.06 2.9 788 286.2 9.8 7.13 0.7 15.3 343 10 1 1.5 13 826 1,180 <10 <30 2,660 40

QSP (n=10) 25th percentile 493 3.05 n.d. 185 16.1 4.9 5.01 0.9 16.2 8,710 16 1 2.0 21 15,300 822 <10 <30 70 253

75th percentile 1,172 3.37 n.d. 658 81.1 26.1 6.12 1.1 24.6 49,900 36 1 2.0 131 54,900 1,379 <10 <30 640 351

gmean 759 3.22 n.d. 333 27.7 9.8 3.73 1.0 20.2 18,400 22 1 2.1 45 22,200 775 <10 <30 181 247

Red Mountains area:PROP (n=4) 25th percentile 48 5.80 8.2 7 6.2 1.0 0.32 0.2 1.1 <1 34 1 2.0 4 30 5 <10 <30 289 <3

75th percentile 101 6.19 27.8 27 14.4 2.2 1.00 0.2 1.7 <1 62 1 2.0 5 39 25 23 <30 601 <3

gmean 62 6.04 11.1 10 8.3 1.3 0.54 0.2 1.4 <1 46 1 2.0 4 37 9 16 <30 298 <3

PROP-V (n=4) 25th percentile 243 3.09 <0.5 108 12.9 2.0 0.34 n.d. 3.4 120 9 1 2.0 4 37 420 <10 <30 125 74

75th percentile 671 5.18 4.5 197 35.1 8.3 1.08 n.d. 11.1 6,460 49 1 2.0 15 19,900 845 <10 <30 473 402

gmean 251 4.12 0.6 52 12.3 2.8 0.50 n.d. 6.2 788 18 1 2.0 8 853 208 <10 <30 128 137

WSP (n=4) 25th percentile 93 3.98 <0.5 29 8.8 1.3 0.41 0.2 1.7 102 31 1 2.0 4 36 62 15 <30 152 <3

75th percentile 130 5.41 7.6 52 15.8 2.2 0.78 0.4 2.6 662 37 1 2.0 6 174 233 22 <30 482 <3

gmean 106 4.63 0.9 35 9.3 1.5 0.49 0.3 2.1 229 35 1 2.0 5 82 115 17 <30 144 <3
Table 8. Summary of background waters from five different mineralized areas.—Continued

SC

(μS/cm) pH

Alkalinity

mg/L 

as CaCO3

SO4 Ca Mg Na K Si Al Ba Be Cd Cu Fe Mn Mo Pb Sr Zn

mg/L μg/L

Red Mountains area:QSP (n=5) 25th percentile 287 3.25 n.d. 118 6.3 4.1 0.22 0.5 6.1 1,760 27 1 2.0 12 461 469 12 <30 65 123

75th percentile 654 3.72 n.d. 238 33.8 7.4 0.32 0.5 10.3 8,840 44 1 2.0 45 5,212 1,160 21 38 285 156

gmean 348 3.33 n.d. 137 15.4 4.7 0.32 0.5 7.3 4,500 32 1 2.2 22 1,706 576 16 33 168 117

AS-all samples (n=13) 25th percentile 48 3.20 nsd 10 1.3 0.3 0.25 0.7 3.8 204 21 nsd 0.30 4 81 12 nsd <30 nsd 20

75th percentile 233 3.67 nsd 57 4.0 0.7 0.39 0.7 7.7 4,400 39 nsd 0.60 16 790 71 nsd <30 nsd 45

gmean 110 3.50 nsd 26 2.3 0.5 0.30 0.7 5.3 1,150 32 nsd 0.53 11 300 33 nsd 35 nsd 37

AS-subgroupsfresh sulfides (n=7) 25th percentile 206 3.03 n.d. 45 1.4 0.6 0.26 0.7 5.0 3,400 20 n.d. 0.3 14 565 46 n.d. <30 n.d. 39

75th percentile 286 3.40 n.d. 72 6.0 1.2 0.40 0.9 10.3 4,750 30 n.d. 0.5 45 1,100 145 n.d. 44 n.d. 106

gmean 283 3.17 n.d. 61 3.0 0.9 0.32 0.8 6.9 4,080 24 n.d. 0.4 27 1,140 79 n.d. 41 n.d. 64

oxidized sulfides (n=6) 25th percentile 30 3.49 nsd 8 1.4 0.2 0.26 0.7 2.7 163 35 nsd 0.3 4 33 9 nsd <30 nsd <3

75th percentile 57 4.40 nsd 10 2.0 0.3 0.30 0.7 5.6 323 75 nsd 2.0 4 83 17 nsd <30 nsd <3

gmean 37 3.92 nsd 10 1.7 0.3 0.28 0.7 3.8 262 47 nsd 0.8 4 63 12 nsd <30 nsd <3

Ohio Peak–Anvil Mountain area:PROP (n=2) min 146 6.99 30.5 41 23.0 1.9 1.15 n.d. 2.4 <1 42 1 2.0 4 30 3 <10 <30 363 <3

max 168 7.46 34.0 53 24.4 4.2 1.24 n.d. 3.7 63 93 1 2.3 4 37 13 <10 <30 405 <3gmean 157 7.22 32.2 47 23.7 2.8 1.19 n.d. 3.0 50 62 1 2.1 4 33 6 <10 <30 383 <3

QSP (n=9) 25th percentile 500 3.21 n.d 173 15.9 3.1 1.88 0.7 21.3 6,600 16 1 1.0 7 2,950 332 <10 <30 38 76

75th percentile 673 3.56 n.d 370 55.0 7.6 6.00 2.5 47.0 16,000 29 2 2.0 41 13,000 1,300 16 <30 114 206

gmean 396 3.52 n.d 132 21.0 3.9 2.96 1.3 24.1 5,150 16 1 1.4 20 4,370 539 13 <30 44 142

AS-oxidized sulfides (n=2) min 17 3.88 n.d 4 1.5 0.3 0.97 0.3 8.1 74 39 1 2.0 4 33 3 <10 <30 13 <3

max 70 5.53 2.6 21 4.2 0.7 1.12 0.4 10.1 599 78 1 3.1 4 211 46 <10 <30 34 29gmean 34 4.63 n.d. 9 2.5 0.5 1.04 0.3 9.1 211 55 1 2.5 4 83 12 <10 <30 21 24

194 Environmental Effects of Historical Mining, Animas River Watershed, Colorado
Mineralization, Alteration, and Geochemical Signatures 195

Table 8. Summary of background waters from five different mineralized areas.—Continued

SC

(μS/cm) pH

Alkalinity

mg/L 

as CaCO3

SO4 Ca Mg Na K Si Al Ba Be Cd Cu Fe Mn Mo Pb Sr Zn

mg/L μg/L

Eureka Graben area:PROP (n=3) min 87 6.92 10.0 28 12.8 0.8 0.43 0.5 1.2 <1 12 1 2.0 4 30 3 <10 30 81 <3

max 287 6.97 34.5 113 41.8 5.0 1.37 0.5 2.6 <1 34 1 2.0 4 30 4 14 30 349 237gmean 179 6.95 17.6 60 27.3 2.1 0.64 0.5 1.6 <1 19 1 2.0 4 30 3 11 30 145 46

PROP-V (n=13) 25th percentile 109 5.81 1.0 44 19.5 1.5 0.70 0.3 1.6 <1 18 1 2.0 4 30 9 <10 30 100 20

75th percentile 254 7.16 21.2 103 37.9 3.1 1.14 0.5 3.1 112 25 1 2.0 5 30 922 <10 30 254 470

gmean 184 6.24 5.1 69 27.8 2.4 0.82 0.4 2.3 142 22 1 3.0 9 31 53 <10 30 163 143

V-QSP (n=3) min 609 3.26 n.d. 313 22.5 4.3 0.92 0.4 7.6 4,230 7 3 2.0 4 30 2,670 <10 30 60 640

max 793 4.55 n.d. 430 76.6 11.5 2.98 2.2 10.6 39,700 15 65 84.0 309 5,690 74,700 <10 30 229 14,400gmean 711 3.88 n.d. 366 49.1 8.0 1.90 1.1 9.0 16,900 10 13 9.0 48 172 10,900 <10 30 113 3,530

South Silverton area:PROP (n=4) min 232 8.13 50.0 55 39.7 1.9 1.96 0.5 2.7 <1 20 1 2.0 4 30 3 <10 30 422 <3

max 260 8.42 59.0 81 44.6 3.2 2.51 0.5 3.1 77 48 1 2.0 4 30 7 <10 30 585 <3gmean 225 8.15 52.7 61 37.6 2.4 2.02 0.5 2.8 52 21 1 2.0 4 30 5 13 30 358 21

1Values cited in the text for individual samples are available in the database CD-ROM; see Sole and others, this volume, Chapter G.
Figure 11. Box plots comparing geochemistry of background waters draining different alteration assemblages. South 

Silverton data are not presented because only PROP waters were sampled. See text for details on alteration assemblages; 

n, number of samples; Alk, alkalinity as CaCO3

; bdl, below detection limit. PROP, propylitic; PROP-V, vein-related propylitic; 

WSP, weak sericite-pyrite; WS-gyp (see text); QSP, quartz-sericite-pyrite; V-QSP, vein-related quartz-sericite-pyrite; 

AS, acid-sulfate.

0.01

0.1

1

10

100

1,000

10,000

100,000

bdl bdl bdl bdl bdl

bdl bdl

2.5

3.5

4.5

5.5

6.5

PROP-V

AS

WSP

QSP

PROP

n=4

n=4

n=13

n=4

n=5

Red Mountains

nsd

Al (μg/L) Fe (μg/L) Mn (μg/L) Pb (μg/L) Zn (μg/L) Cu (μg/L) CaCO3 (mg/L) Sr (μg/L) pH

1.5

1.5

Sulfate (mg/L)

0.01

0.1

1

10

100

1,000

10,000

100,000

1

2

3

4

5

6

7

8

9

10

Eureka Graben 

PROP-V

V-QSP

PROP

n=13

n=3

n=3

bdl

bdl

bdl bdl

0.01

0.1

1

10

100

1,000

10,000

100,000

bdl bdl bdl

bdl bdl

N=3

2.5

3.5

4.5

5.5

6.5

7.5

8.5

AS

QSP

PROP n=2

n=2

n=9

OPAM

Al (μg/L) Fe (μg/L) Mn (μg/L) Pb (μg/L) Zn (μg/L) Cu (μg/L) Sulfate (mg/L) Sr (μg/L) pH

Al (μg/L) Fe (μg/L) Mn (μg/L) Pb (μg/L) Zn (μg/L) Cu (μg/L) Sulfate (mg/L) Sr (μg/L) pH

CaCO3 (mg/L)

CaCO3 (mg/L)

0.01

0.1

1

10

100

1,000

10,000

100,000

Al (μg/L) Fe (μg/L) Mn (μg/L) Pb (μg/L) Zn (μg/L) Cu (μg/L) Sulfate (mg/L) CaCO3 (mg/L) Sr (μg/L) pH

2

3

4

5

6

7

8

WS-gyp

WSP

QSP

PROP

n=10

n=6

n=10

n=10

Mt Moly area

95th percentile

75th

Median

25th

5th

196 Environmental Effects of Historical Mining, Animas River Watershed, Colorado
Mineralization, Alteration, and Geochemical Signatures 197

200

400

600

800

1,000

1,200

1,400

0

0

50 100 150 200 250 300

-0.8

-2.8

+7.1

SULFATE, IN MILLIGRAMS PER LITER

CALCIUM +STRONTIUM, IN MILLIGRAMS PER LITER

Figure 12. Dissolved sulfate versus calcium plus strontium in propylitic water in 

Mount Moly alteration area; shows corresponding values of δ34S values for three 

of these samples. (See Nordstrom and others, this volume, table 1.) The calcium/

strontium-rich waters that are also rich in sulfate probably have interacted with 

gypsum or anhydrite.

Red Mountains Area Ore Deposits

The majority of the economically important brecciahosted deposits in the Red Mountains area lies within a belt 

trending N. 20°–30° E., roughly 6 km long and 2 km wide 

(fig. 13). This zone is bounded by Ironton Park on the north, 

the Carbon Lake mine area on the south, Red Mountain–

Mineral Creeks on the west, and Red Mountains #2 and #3 

on the east. It marks the intersection of the northeastern ring 

fracture zone of the Silverton caldera and a 6 km-wide zone 

of veins and faults trending N. 40°–45° W. (Yager and Bove, 

this volume, pl. 1). Here, many 23 Ma dacite porphyry stocks 

were intruded at high crustal levels (Bove and others, 2001). 

Other contemporaneous mines and prospects, with ores similar 

to that in the zone just described, are present at the heads 

of Prospect, Dry, Georgia, and Corkscrew Gulches (fig. 13) 

(Ransome, 1901; Bove and others, 2000).

Mines of the Red Mountains area were active between 

1882 and 1900, whereas since 1900 there has been only minor 

mining activity (Hillebrand and Kelley, 1959). The deposits 

were noted for rich silver-lead-copper ores; individual deposits 

typically contained from 50 to a few hundred ounces silver and 

15–40 percent lead and copper, and minor gold. Total production for these mines was between 10 and 20 million dollars, 

which was limited due to the small size and irregular shape of 

the orebodies, and the extremely corrosive nature of the mine 

waters (Hillebrand and Kelley, 1959). The area experienced 

a resurgence of exploration activity for gold in the late 1970s 

and early 1980s, as reflected by the myriad of drill roads near 

Red Mountain #3 and above Corkscrew Gulch (fig. 13).

In the vicinity of Red Mountain Pass (fig. 13), the breccia 

bodies are surficially expressed as prominent silicified knolls 

(45–150 m wide) that rise from 8 to 60 m above the valley 

bottoms (Ransome, 1901; Hillebrand and Kelley, 1959). The 

entire breccia masses are nearly vertical, irregularly cylindrical bodies (90–600 m long by 30–425 m wide) with crescent 

to ellipsoidal cross sections, surrounded by a zone of silicified 

and highly pyritic rock (Ransome, 1901). Some of the breccia 

bodies transition into relatively unbroken wallrock, whereas 

some of the smaller masses have abrupt boundaries. The better 

delineated breccia bodies are encircled by a zone of vertically 

sheeted jointing and appear to be localized at intersections of 

multiple fractures or faults.

East of Red Mountain Pass, in the headwaters of Prospect, 

Dry, and Corkscrew Gulches (fig. 13), hydrothermal breccias 

are also common; however, classic mineralized “breccia bodies,” such as those observed in the Red Mountain Pass area, 

are not as well exposed at the surface. Instead, mineralized 

hydrothermal breccias were generally localized along preexisting fissures and occur in irregular, anastomosing zones emanating outward from the fissure breccias (Bove and others, 2000). 

Several of the more classic-looking polymetallic vein deposits 

on the periphery of the Red Mountain acid-sulfate system (for 

example the Carbonate King in Corkscrew Gulch, the Henrietta 

mine, in south Prospect Gulch (mine # 84), and the Kansas 

City mine in Georgia Gulch (mine # 145; Church, Mast, and 

others, this volume, fig. 3) have associated breccias and acidsulfate-related gangue minerals. The presence of these features 

indicates that the deposits are genetically linked to the mineralized breccias within the interior of the acid-sulfate system.
Although records are incomplete—because many of the 

mines were closed before geological studies could be made—

Ransome (1901) reconstructed a vertical mineral zonation 

throughout the breccia deposits. The upper parts of the 

orebodies consist of silver-bearing galena, with sphalerite and 

pyrite. These ores grade downward into silver-rich copper ores 

of enargite, tetrahedrite, stromeryite, bornite, and chalcopyrite, 

which gradually diminish downward with increasing amounts 

of low-grade pyrite. The following general paragenetic 

sequence was ascertained by Nash (1975) from fragments of 

ore on mine dumps: (1) early acid-sulfate-related silicification 

with fine pyrite and high-temperature clays; (2) galena overlapped by sphalerite; (3) enargite and other copper minerals; 

and (4) barite, calcite, fluorite, bornite, and minor gold. Paragenetic studies of ore material from the Lark, Galena Queen, 

and Hercules mines in Prospect Gulch (D.J. Bove, unpub. data, 

2002) confirm Nash’s observations in the early ore-bearing 

stages, and also reveal complex interrelationships between 

repeated episodes of hydrothermal brecciation, ore mineralization, and acid-sulfate gangue deposition. Compositions of the 

major ore minerals from these Prospect Gulch mines (Wirt 

and others, 1999; D.J. Bove, unpub. data, 2002) are comparable to those in the Red Mountain Pass area, just to the west 

(see various analytical data in Ransome, 1901). The presence 

of abundant copper-arsenic-antimony-rich ores containing 

enargite and tetrahedrite-tennantite, as well as copper ores of 

chalcocite, bornite, and covellite, distinguish these and similar 

breccia ores from more typical polymetallic vein deposits 

within and outside the Red Mountains area.

In some mines such as the National Belle, Grand Prize 

(fig. 13), Enterprise, and Vanderbilt, oxidized ores were present in the upper 100 m of the deposit, either attached to the 

walls of caves, as broken detached masses within the caverns, 

or as bedded deposits with clayey mud and sand filling the 

caves (Schwarz, 1883). The ore-bearing caves, which were 

present throughout the upper silicified zones, can still be seen 

today along the cliff base and margins of the silicified outcroppings. Chambers are as much as 15 m in diameter and are 

interconnected by irregular rounded passages that branch out 

toward the surface and pinch and intersect at depth. Oxidized 

lead ores mined in these upper levels accounted for most of the 

early production. These ores consisted mostly of lead and iron 

carbonates and sulfates with galena cores, with various iron 

oxides and arsenates (Schwarz, 1883); the entire ore masses 

were encrusted by limonite and goethite. Botryoidal masses of 

sphalerite were also found within the caverns along with the 

highly oxidized galena. Intermingled with the cave ores were 

abundant fine “spongy” quartz, crystalline dickite/kaolinite, 

diaspore, pyrophyllite, and alunite.

Most of the Red Mountains breccia ores are unoxidized 

and generally formed beneath the shallow oxidized ore zone 

(Schwarz, 1883). These so-called “primary” ores (Ransome, 

1901) generally begin where the cave ores cease. However, in 

many deposits, such as the Congress, St. Paul, Galena Queen, 

Yankee Girl, Guston, and Silver Bell (fig. 13), either the 

upper cavernous oxidized zone was eroded, or it never existed 

(Ransome, 1901). In these mines, unoxidized ores were commonly present near ground surface. Although the unoxidized 

Figure 13. Zone of economically important breccia-hosted ore deposits (red 

line) in Red Mountains area. Congress and St. Paul mines (1); Grand Prize mine 

(2); Longfellow mine (3).

0 1 KILOMETER

Red Mt

#2

Red Mt

#3

{

{

Study area b oundary

107o38'42"

37o52'30"

107o45' 

37o56'42"

Silver Bell

Mine

Guston

Mine

Genessee

 Mine

National

Belle Mine

Koehler

 tunnel

Mineral

Creek

Prospect 

Gulch

Cement

Creek

Creek

Mountain

Red

IRONTON 

 PARK

Red 

Mountains 

area

1

2

Georgia Gulch

Dry

Gulch

Cork Gulch screw

3

Carbon 

 Lake area

Red Mt

Pass

198 Environmental Effects of Historical Mining, Animas River Watershed, Colorado
Mineralization, Alteration, and Geochemical Signatures 199

orebodies are typically massive and solid aggregates, ore specimens such as enargite, pyrite, and polybasite preserved from 

several mines are distinctly botryoidal or stalactitic in nature. 

Ores of this nature were reported to have grown within caverns 

and open spaces in the unoxidized ore zone (Ransome, 1901). 

The most common “primary” ores include enargite, galena, 

chalcopyrite, pyrite, chalcocite, bornite, tetrahedrite-tennantite, 

sulfobismuthites, and stromeryite (Schwarz, 1883). The upper 

unoxidized galena ores typically carried a few tens to 100 oz 

silver per ton and as much as 60 percent lead (Ransome, 1901). 

Deeper copper-bearing ores were almost always richer, with 

grades of 100 oz/ton to sometimes more than 1,500 oz silver/

ton and 15–25 percent copper. Grades diminished in the lower 

pyrite-rich ores that contained less than 20 and 5 ounces silver 

and copper, respectively, per ton.

Ohio Peak–Anvil Mountain Mineralization 

and Ore Deposits

In contrast to the Red Mountains area, relatively few 

mines and prospects are associated with the OPAM acidsulfate system (Church, Mast, and others, this volume, fig. 1). 

The best known of these is the Zuni mine (mine # 177, 

Church, Mast, and others, this volume) (fig. 14), although 

its importance was mostly associated with the occurrence of 

zunyite and guitermanite (mixture of obscure lead-bismuth 

minerals) (Ransome, 1901). Ore minerals in this deposit were 

zoned downward from anglesite into unoxidized guitermanite, 

changed gradually to a silver- and enargite-rich ore, and then 

into massive pyrite with low metal content. Similar to the Red 

Mountain deposits, some ore masses lined the walls of small 

caves and vugs in brecciated rock. Ransome (1901) reported 

several smaller prospects of the general type of the Zuni mine 

present on Anvil Mountain (Church, Mast, and others, this 

volume, fig. 2), but he also remarked on the limited size and 

grade of these deposits.

The May Day (mine # 181, Church, Mast, and others, this 

volume) and several unnamed mines in the Ohio and Topeka 

Gulch areas (fig. 14) mark the eastern margin of the OPAM 

acid-sulfate system. Ore minerals on associated waste piles 

from these mines include pyrite, galena, anglesite, and sphalerite, and ferberite (Fey and others, 2000). No copper-arsenic 

minerals were identified at these sites. The apparent scarcity 

in silver-bearing enargite ore minerals within this acid-sulfate 

system contrasts markedly with the Red Mountains area and 

explains the lack of mining and ore production therein. Considering the absence of the characteristic copper-arsenic ores, 

and without geochronologic data, it is difficult to constrain 

the age of mineralization from this area. However, the presence of acid-sulfate gangue material within some of the veins 

(S. Sutley, unpub. data, 2001), coupled with their close proximity to the main acid-sulfate system (fig. 14), suggests that 

some of the related ore deposits could have been contemporaneous with the acid-sulfate event. However, the occurrence 

of some late-stage tungsten-fluorite mineralization products 

within veins of this general area (Belser, 1956; Waegli, 

1979), also suggests that some of this mineralization could be 

younger than 18 Ma. (See section, “Rhyolites and Associated 

Mineralization.”)

Hydrothermal Alteration

Detailed field and laboratory studies in conjunction with 

AVIRIS remote spectroscopy (Dalton and others, this volume) 

are the basis for a compilation of maps showing the complex 

zonation of hydrothermal alteration assemblages and related 

structural features around the Red Mountains and OPAM acidsulfate systems (figs. 14 and 15). More comprehensive details 

on the mineralogy and genesis of these hydrothermal systems 

are found in Bove and others (2001). The acid-sulfate mineral 

assemblage formed by replacement of the original volcanic 

host rocks by low-pH, sulfate-rich solutions (Rye and others, 

1992; Bove and others, 2000). These solutions were localized 

along fault and hydrothermal breccia zones and permeated 

outward throughout intensely fractured wallrock (Bove and 

others, 2000; Dalton and others, 2000). Although these fluids 

must have originally flowed along steeply inclined conduits 

(Bove and others, 2000), they were also commonly localized 

along subhorizontal zones parallel to bedding and foliation 

within the lavas and volcaniclastics (fig. 16). These subhorizontal zones of high silicification could potentially act as partial baffles to ground water due to sharp permeability contrasts 

with overlying and more weakly altered rocks.

Surface mapping and careful paragenetic studies demonstrate that centers of acid-sulfate-altered rock are internally 

zoned and transition outward from quartz-alunite into pyrophyllite, dickite, and further into smectite, and (or) propylitically altered rock. However, as noted in some key localities, 

the acid-sulfate assemblage (quartz-alunite, pyrophyllite, 

dickite assemblages) grades directly outward into propylitically altered rock, without an intervening QSP zone. Similar 

observations were noted in studies of the Red Mountain Pass 

breccias by Burbank and Luedke (1969), who also noted 

the absence of a QSP assemblage within this laterally zoned 

sequence. Nonetheless, broad expanses of QSP-altered rock, 

such as that mapped on the southern margins of the Red 

Mountains area (fig. 15), are present between individual acidsulfate zones. Although these mapped QSP zones extend from 

3,250 m to as high as 3,780 m, the overall acid-sulfate zones 

generally dominate at higher elevations. As surmised from the 

lateral alteration sequences (no QSP zone present in horizontal 

sequence), it is likely that the QSP and acid-sulfate assemblages formed during alternating time intervals.

Relatively pervasive zones of QSP-altered rock are particularly well exposed along the deeply incised margins of the 

composite acid-sulfate zones in the Red Mountains (Prospect 

Gulch) and OPAM (Topeka and Ohio Gulches) areas (figs. 14 

and 15). Detailed mapping and clay XRD studies show that 
CC128

Topeka

Gulch

Ohio

Gulch

Porcupine Gulch

Cement

Creek

Ohio Peak

ANVIL

MOUNTAIN

Niagara Gulch

Zuni

Gulch

Mineral

Creek

Mineral

Creek

Zuni

Mine

Irene

Mine

May Day

Mine

107o40'29"

37o48'40"

107o43'53"

37o52'19"

0 0.5 1 KILOMETER

200 Environmental Effects of Historical Mining, Animas River Watershed, Colorado
Mineralization, Alteration, and Geochemical Signatures 201

the QSP zone on the southwest margin of the Red Mountains 

acid-sulfate system (Prospect Gulch area; fig. 15) grades 

outward into a weak sericite-pyrite assemblage (Bove and 

others, 2000). In addition, mineralogical data (discussed later) 

indicate that a similar transition may be present on the eastern 

margins of the OPAM hydrothermal system. Propylitically 

altered rocks surround the Red Mountains and OPAM hydrothermal systems. Although most of this surrounding propylitic 

assemblage formed during a regional hydrothermal event 

(>26 Ma), some local propylitic alteration formed at ≈23 Ma, 

peripheral to mineralized and altered faults and fractures. 

Although this younger assemblage generally resembles the 

regional propylitic assemblage, it is distinguished largely by 

the presence of relatively coarse grained pyrite (1 mm cubes) 

disseminated and along fractures, and by quartz, chlorite, and 

magnetite within veinlets.

Mineralogical data from three exploratory drill holes in 

the vicinity of Red Mountain #3 (fig. 15) and adjacent Prospect 

Gulch (Bove and others, 2000) demonstrate that the acid-sulfate 

alteration zones have roots extending at least 300 m beneath the 

ground surface. Deeper exploratory drilling in the headwaters 

of Corkscrew Gulch (fig. 13) revealed that the pervasive zones 

of QSP-altered rock, at least in this area, are zoned upward and 

away from a deeper, potassic alteration assemblage (Gilzean, 

1984). The potassic assemblage, as observed in these deep 

drill holes, is characterized by secondary potassium feldspar, 

biotite, fluorite, and andalusite. Abundant veins and veinlets of 

anhydrite with associated quartz, sericite, and pyrite are present 

within the deeper alteration assemblages, but are absent within 

the acid-sulfate zones (Gilzean, 1984).

Acid-Sulfate Alteration Assemblages

Silicification

Zones of silicification, which are spatially associated 

with the inner quartz-alunite-pyrophyllite zones, generally 

formed as replacements along mineralized pipelike to tabular 

hydrothermally brecciated masses and along brecciated faults 

and strong fracture zones (D.J. Bove, unpub. data, 2002). 

Examples of these silicified breccias include the previously 

discussed Red Mountain Pass mineralized breccia bodies, a 

large, moderately dipping breccia body near Red Mountain #3 

(fig. 15) (Bove and others, 2001), and a similarly shallowdipping mass centered near Ohio Peak (fig. 14). Disseminated 

pyrite (10–25 percent), enargite, and lesser covellite and 

galena are present throughout the entire silicified breccia at 

Red Mountain #3. However, sulfides are mostly oxidized in 

the uppermost 90 m of the entire mineralized zone. Silicified 

volcanic rocks and breccias range from light gray to highly 

bleached granular aggregates and dark jasperoidal masses 

composed of a microcrystalline mosaic of quartz (>75 percent), 

pyrite (5–25 percent), leucoxene (trace), and minor dickite, 

alunite, pyrophyllite, and diaspore (5–20 percent).

Quartz-Alunite Alteration

Similar to the silicic assemblage, quartz-alunite alteration was localized along hydrothermally brecciated fault and 

fissure zones (10–40 m across) as well as in areas of more 

massive hydrothermal brecciation (as much as 200–400 m 

wide). The quartz-alunite assemblage is defined by the presence of quartz, alunite, natroalunite, pyrophyllite, dickite, 

Figure 14 (above and facing page). Alteration map of Ohio Peak–Anvil Mountain (OPAM) area. Text gives detailed description 

of alteration types.

EXPLANATION

HYDROTHERMAL ALTERATION ASSEMBLAGES

QU Quaternary undivided; lacks spectral signature (AVIRIS)

QQAS Coarser surficial deposits with strong spectral

 signature. Dominantly quartz-alunite-pyrophyllite (QAP)

 and less quartz-sericite-pyrite (QSP)

VQSP-P Vein-related QSP with propylitic (PROP)

WSP-PROP Mixed WSP and PROP

WSP Weak sericite-pyrite 

QSP Quartz-sericite-pyrite 

Acid-Sulfate Suite

QAm Mixed quartz-alunite (QA) and quartz-alunite-pyrophyllite (QAP)

QAP-S Mixed QA and QAP with less QSP

ARG Argillic 

PROP-AS Mixed (PROP) and acid-sulfate suite (AS)

PROP Propylitic alteration undivided

SMECT Smectite with other clays 

V-WSP Vein-related weak sericite-pyrite (WSP)

QQSP Coarser surficial deposits with strong spectral

 signature. Dominantly QSP

SILIC Silicified zone 

Fault or major fracture zone; all approximately located

Adit

Shaft

Spring water sample site
Figure 15. Alteration map of Red Mountains area.

Prospect

Red Mt #3

Gulch

Georgia

Gulch

Dry

Gulch

Lark

Mine

Joe and

Johns Mine

Galty Boy

Mine

Kansas

City

Mine

+

+

+

0 0 0

0

0

0

0 0.25 0.5 KILOMETERS

37o53' 

37o54'22"

107o41'24" 107o39'41"

EXPLANATION

HYDROTHERMAL ALTERATION ASSEMBLAGES

QU Quaternary undivided; lacks spectral signature (AVIRIS)

QQAS Coarser surficial deposits with strong spectral

 signature. Dominantly QAP and less QSP

V-QSP Vein-related quartz-sericite-pyrite (QSP)

WSP-PROP Mixed weak sericite-pyrite (WSP) and propylitic (PROP)

WSP Weak sericite-pyrite alteration 

QSP-PROP Mixed QSP and PROP

QSP Quartz-sericite-pyrite 

MPY Massive pyrite zone

Acid-Sulfate Suite

QAm Mixed quartz-alunite (QA) and quartz-alunite-pyrophyllite (QAP)

QAP-S Mixed QA and QAP with less QSP

ARG Argillic 

PROP-AS Mixed PROP and acid-sulfate (AS) suite 

PROP Propylitic alteration undivided

QLS Landslide

Fault or major fracture zone

Adit

Spring water sample site

SILIC Silicified zone 

CC129

202 Environmental Effects of Historical Mining, Animas River Watershed, Colorado
Mineralization, Alteration, and Geochemical Signatures 203

diaspore, and pyrite, with minor to trace amounts of zunyite, 

sulfur, and tridymite. These highly silicified rocks contain 

between 30 and 80 percent fine-grained quartz, with a mean 

of 50 percent. Pyrite, which is present as disseminations and 

present along tiny fracture networks, typically ranges from 

20 to 30 weight percent within these altered rocks. Alunitized 

rocks are characterized by the breakdown of all primary rock 

constituents with the exception of zircon and resorbed quartz 

phenocrysts, where present. Biotite, hornblende, and primary 

opaque minerals are altered to leucoxene; hematite and jarosite 

formed as near-surface oxidation products of pyrite.

Compositional data as determined by AVIRIS remote 

spectroscopy (Dalton and others, this volume) indicate that alunite within the Red Mountains and OPAM areas ranges mostly 

from a nearly pure potassium end-member through intermediate potassium and sodium compositions. However, natroalunite 

has been identified in some localities by remote spectroscopic 

and XRD methods. An individual chemical analysis of alunite 

from the Red Mountain Pass area (Hurlburt, 1894) shows the 

presence of potassium and sodium in roughly equal proportions 

(table 2).

Quartz-Pyrophyllite and Argillic Assemblages

Quartz-pyrophyllite-altered rocks contain between 15 

and 40 percent quartz, abundant pyrophyllite, and lesser alunite, sericite, diaspore, and finely disseminated pyrite. These 

rocks are difficult to distinguish from quartz-alunite-altered 

rocks in hand specimen and have many textural and morphologic similarities in thin section, as well. In contrast, rocks 

affected by argillic alteration are generally quite distinctive 

due to the presence of a soft waxy clay pseudomorphous after 

feldspar and along fractures. These bleached argillic zones, 

which form the broad outer margins around the quartz-alunite, 

quartz-pyrophyllite, and silicic zones, are characterized by 

the presence of dickite and quartz with varying amounts of 

pyrite. These rocks are poorly to only moderately indurated 

and contain between a trace and 40 percent fine-grained quartz. 

Zones of smectitic alteration are similar in appearance to the 

argillic zones but are characterized by the presence of smectite, 

chlorite, and variably altered feldspar grains.

Hydrothermal Sericitic Assemblages

Quartz-Sericite-Pyrite Assemblage

The quartz-sericite-pyrite (QSP) assemblage is characterized by the complete replacement of plagioclase, potassium feldspar phenocrysts, and groundmass by roughly 

25–55 percent fine-grained pervasive quartz (40 percent 

average), abundant highly crystalline illite (sericite), and 

10–20 percent finely disseminated and fracture-filling pyrite. 

Kaolinite may be present in varying abundances in some 

Figure 16. Highly silicified quartz-alunite-pyrophyllite breccia zone (SILIC+QAP BXA) along nearhorizontal plane; base is subparallel to bedding of lavas. Near-vertical zones probably controlled by 

high-angle fractures. Area of photo is about 2 km across.

QA Pre-alteration fault ?

SILIC + QAP BXA
localities. Abundant white, fine-grained kaolinite has also 

been observed in veinlets postdating ore mineralization near 

the National Belle mine, in the Red Mountain Pass area 

(fig. 13). Altered rocks of the QSP assemblage typically have 

a bleached appearance and are variably oxidized. Thin stockwork quartz veinlets are locally present.

Weak Sericite-Pyrite Assemblage

The weak sericite-pyrite assemblage, as mapped on the 

margins of the Red Mountains area (WSP; fig. 15), is characterized by the presence of weakly to unaltered plagioclase and 

metastable chlorite; these minerals are typically absent in the 

adjacent QSP assemblage. WSP rocks are less silicified than 

their QSP counterparts, containing between 15 and 25 percent 

and an average of 20 percent microcrystalline quartz. Illite 

(sericite) is also abundant along with more than 5–10 volume 

percent finely disseminated pyrite.

Origin, Age, and Relation to Intrusions

Geologic relations and isotopic dating indicate that 

acid-sulfate and QSP alteration, hydrothermal brecciation, 

and silver-copper-arsenic ore deposition in the Red Mountains 

area were intimately associated with episodic intrusion of 

dacite porphyry magma at about 23 Ma (Lipman and others, 

1976; Gilzean, 1984; Bove and others, 2000; Bove and others, 

2001). The contemporaneity of these events is verified by 

the following: (1) coeval K-Ar age determinations of about 

23 Ma on both alunite and dacite porphyry intrusions in the 

Red Mountains area (Lipman and others, 1976; Gilzean, 1984; 

Bove and others, 2000; Bove and others, 2001), (2) geologic 

evidence for repeated episodes of alternating acid-sulfate 

alteration, silver-copper-arsenic ore deposition, and dacite 

intrusion (Lipman and others, 1976; Fisher and Leedy, 1973), 

and (3) paragenetic relations showing dikes of fresh dacite 

porphyry cutting QSP and QA-altered hydrothermal breccias that contain clasts of earlier intruded dacite porphyry. No 

outcrops of dacite porphyry have been identified in the OPAM 

acid-sulfate area. However, similarities in mineralogy, alunite 

isotopic composition (table 7), and geometry to that in the Red 

Mountains area suggest that both systems were contemporaneous and associated with similar intrusions.

The Red Mountains and OPAM acid-sulfate systems 

are highly analogous to magmatic hydrothermal acid-sulfate 

systems at Summitville, Colo., Lake City, Colo., and Julcani, 

Peru (Rye and others, 1992; Bove and others, 2002). All 

these localities have acid-sulfate alteration zones in nearsurface epithermal deposits, which are characterized by deep 

vertical as well as horizontal zoning, the presence of coeval 

pyrite, phosphate analogs of alunite, and zunyite, and later 

gold, enargite, and other copper-arsenic-antimony-sulfide 

minerals (Rye and others, 1992). Our detailed geologic and 

stable isotope studies in the Red Mountains and OPAM areas 

(D.J. Bove, unpub. data, 2002) demonstrate that the alunite 

and pyrite formed from vapor plumes rich in H2S and SO4 due 

to the disproportionation of magmatic sulfur dioxide.. Geologic 

and geochronologic constraints, as outlined in the beginning 

of this section, and stable isotope data suggest that these vapor 

plumes emanated from the high-level dacite porphyry intrusions and had very high H2

S:SO4 ratios. Paragenetic studies 

(D.J. Bove, unpub. data, 2002) demonstrate that acid-sulfate 

alteration and ore mineralization occurred in alternating 

episodes. Detailed ore genesis studies of similar acid-sulfate 

systems at Summitville, Colo., and Julcani, Peru (Stoffregen, 

1987; Rye and others, 1992) suggest that these alternations 

may be related to changes in magmatic fluid composition. 

These compositional changes could be explained by episodic 

pulses of dacite intrusion coupled with intermittent cycling of 

the magmatic vapor plumes.

The coexistence of both H2

S and SO4 in the fluids that 

formed the Red Mountains and OPAM acid-sulfate systems 

explains the abundance of coexisting pyrite and alunite in the 

acid-sulfate mineral zones. The spatial association of pyrite 

and alunite at depth and at high levels in this hydrothermal 

system contrasts sharply with what appears in other large 

acid-sulfate systems in the Rocky Mountains (magmatic 

steam-heated systems such as Marysvale, Utah; Rye and 

others, 1992; Cunningham and others, 2005), where pyrite 

formed only in deeper zones beneath the alunite horizons. 

These inherent differences in deposit type have important 

implications with regard to pyrite exposure to surficial weathering and subsequent production of acidic runoff. On the one 

hand, magmatic-hydrothermal acid-sulfate systems, such as 

those observed in the study area, are generally associated with 

low-pH waters owing to more ubiquitous presence of pyrite, 

regardless of erosion depth (Bove and others, 1996; Bove 

and others, 2000). In contrast, surficial pyrite weathering and 

related acidic runoff appear only to be a factor in the magmatic 

steam-derived acid-sulfate systems, where the underlying 

pyritic zone is exposed from erosive processes (Cunningham 

and others, 2005). Thus, knowledge regarding the type or genesis of the particular acid-sulfate system can be used to predict 

the likelihood of impact from acidic drainage. This can be 

useful in preliminary assessments of acid-sulfate-altered areas 

where no water quality data are available, or in desert environments where surficial runoff is limited.

Geochemistry of Altered Rocks in the Red 

Mountains and Ohio Peak–Anvil Mountain 

Areas

The combined geochemical data for 69 surface samples 

from the Red Mountains and Ohio Peak–Anvil Mountain 

acid-sulfate areas are shown in table 3 and figure 4. These 

data were combined due to strong similarities in altered rock 

lithology, and because of poor sample representation of the 

OPAM acid-sulfate assemblage. Although the data represent 

most major alteration assemblages differentiated by geologic 

204 Environmental Effects of Historical Mining, Animas River Watershed, Colorado
Mineralization, Alteration, and Geochemical Signatures 205

mapping (figs. 14 and 15), samples from the weak sericitepyrite assemblage were not presented because of low sample 

numbers. Major-element trends are similar to that of the 

Mount Moly area with substantially lower calcium, magnesium, and sodium in QSP and acid-sulfate (AS) versus propylitically altered rocks. Manganese content decreases from 

a mean of 927 ppm in propylitically altered rocks, to 35 ppm 

in QSP samples, to <12 ppm in AS-altered rocks. Lead 

abundance is highest in the AS assemblage (278 ppm mean) 

and decreases from a mean of 62 ppm in QSP-altered rocks 

to 26 ppm in propylitically altered rocks (table 3 and fig. 4). 

In contrast, mean zinc and copper abundances are highest in 

propylitically altered rocks (110 and 50 ppm, respectively) 

with substantially lower mean values in QSP and AS-altered 

outcrops. Molybdenum values in all altered rocks sampled 

are near analytical detection limits. Lower iron content within 

QSP and AS versus propylitically altered rocks is probably 

due to surficial oxidation of pervasive pyrite within the QSP 

and AS zones. Svanbergite [(Sr, Ca)Al3

(SO)4(PO)4(OH)6] has 

been identified by XRD in acid-sulfate-altered rocks, which 

may account for the relatively high concentrations of strontium and phosphorus within this alteration suite (table 3). 

The formation of svanbergite is related to the release of 

H3

PO4 and strontium into solution due to the breakdown of 

primary apatite and plagioclase by acid-sulfate-altering fluids 

(Stoffregen and Alpers, 1987; Bove and Hon, 1992).

Analyses of 34 drill core samples of unoxidized acidsulfate-altered rock are summarized in table 3 and compared 

to outcrop data in figure 6. Iron abundance within unoxidized 

AS-altered drill core samples (4.77 percent mean) is statistically indistinguishable from that of surface samples of propylitically altered rock (4.83 percent mean) (table 3); however, 

rocks from corresponding AS-altered outcrops (0.86 percent 

mean) have notably lower iron content than AS drill core 

samples (fig. 6), probably due to pyrite dissolution. Lead abundances statistically overlap in oxidized surface and unweathered drill core samples (AS assemblage), reflecting the relative 

immobility of lead in oxidizing surface waters (Smith and 

Huyck, 1999). Similar to iron, differences in copper in surface 

(8 ppm mean) relative to unoxidized subsurface AS samples 

(79 ppm mean), are also likely due to sulfide oxidation in the 

near-surface environment (fig. 6). Copper values in unoxidized 

AS drill core samples (80 ppm mean) are analytically indistinguishable from those of samples of propylitically altered rock 

(50 ppm mean).

As shown in figure 6, copper and molybdenum abundances from the Red Mountains area in unoxidized drill core 

samples (AS-drill core) are notably lower than in unoxidized 

core samples from the Mount Moly area (QSP-unoxidized 

zone). However, studies by Gilzean (1984) documented 

subeconomic but substantial copper and molybdenum mineralization in deeper zones of potassic-altered rock in the Red 

Mountains area. No specifics regarding these data are included 

in the report by Gilzean (1984), however.

Mine Dump Compositions

Red Mountains Area

Analytical data summarizing the geochemistry of 20 mine 

dumps from the Red Mountains area are shown in table 4 and 

figure 7. The geochemical signature of these dumps is distinctive in comparison to data from the other four mineralized 

areas. Mean manganese concentration of 39 ppm is from 6 to 

50 times lower than mean values from the other areas. Although 

mean abundances of lead (1,870 ppm) and zinc (935 ppm) are 

comparable to values in the other four areas, the mean concentrations of copper (1,930 ppm) and arsenic (943 ppm) are 

substantially higher. These concentration patterns reflect the 

characteristic presence of enargite and tetrahedrite-tennantite, 

which are commonly associated with ores from this area. In 

contrast, mean molybdenum concentration (5 ppm) is the lowest 

of the five groups.

Although manganese gangue minerals are largely absent 

in the Red Mountains ore deposits, pyroxmangite has been 

identified at the Kansas City mine in Georgia Gulch, which is 

hosted in propylitic-dominant terrain on the periphery of the 

acid-sulfate system (fig. 15). Veins of the Kansas City mine 

are along continuations of north- to northeast-trending faults 

that were exploited for acid-sulfate-style mineral deposits 

along the south side of Prospect Gulch (fig. 15). For this 

reason, mineralization at the Kansas City mine is thought to be 

contemporaneous with the 23 Ma acid-sulfate event. However, 

later episodes of ore mineralization may have also occurred 

along these structures. Mean manganese concentration within 

propylitic-altered rocks in the Red Mountains area is 927 ppm, 

whereas the large expanses of highly altered rock hosting most 

of the ore deposits are relatively manganese poor (<35 ppm 

mean). These data reflect that manganese solubility within 

hydrothermal fluids is strongly pH dependent (Casadevall and 

Ohmoto, 1977; Langston, 1978), and gangue ores may have 

only precipitated on the margins of the hydrothermal system 

where pH was higher. While mean calcium in these dumps 

(700 ppm mean) is the lowest among the five areas, mean 

strontium abundance (602 ppm) is the highest. These low 

calcium:strontium ratios can be attributed to the paucity of calcite, gypsum, and anhydrite within the entire Red Mountains 

acid-sulfate system, and the presence of discrete strontiumphosphate-sulfate mineral phases formed during acid-sulfate 

hydrothermal alteration (Stoffregen and Alpers, 1987; Bove 

and Hon, 1992).

Ohio Peak–Anvil Mountain Area

Analytical data from six mine dumps from the Ohio 

Peak–Anvil Mountain area are presented in table 4 and 

figure 7. Unfortunately, none of the dumps from the Zuni mine 

(fig. 14) and nearby prospects with enargite-bearing ores were 

sampled due to steep terrain and because of access restrictions. 

Data from the six mine dumps included in this study show 

mean abundances of 679 ppm manganese; 7,550 ppm lead; 
3,070 ppm zinc; 382 ppm copper; 87 ppm arsenic; 17 ppm 

molybdenum; 27 ppm silver; and 320 ppm strontium. The 

range of manganese concentrations falls roughly in the middle 

of the five groups, but the range is significantly higher than 

in dumps from the Red Mountains acid-sulfate area. Higher 

manganese values in the OPAM area probably reflect that 

most of the mines are near the propylitic-dominated margins 

of this southern acid-sulfate system. Although late-stage 

manganese silicate and manganese carbonate gangue could 

be present in some of these vein deposits, they have not been 

observed in any significant quantity within associated dump 

material (D.J. Bove, unpub. data, 2002). However, huebnerite 

(MnWO4

), which is common in a number of the mines along 

Cement Creek (Belser, 1956), could contribute to the elevated 

manganese abundances. Ranges of lead and zinc abundance 

overlap with ranges from the other four mineralized areas 

(fig. 7), whereas copper and arsenic abundances, although substantially lower than those from dumps of the Red Mountains 

area, fall within the range of the other three groups. The low 

arsenic abundances within these dumps (87 ppm mean) contrast markedly with those of the Red Mountains area, where 

the mine dumps have a mean arsenic abundance of 943 ppm. 

In addition, mean copper abundance is nearly five times lower 

than in the Red Mountains mine waste material. These data 

reflect the overall scarcity of enargite and related ore minerals 

in mines of the OPAM area (Ransome, 1901), whereas these 

minerals are generally present in many of the Red Mountains 

ores. The lack of these minerals within the OPAM deposits is 

probably the explanation for the relatively small number of 

mines and prospects and the lack of ore production in this part 

of the Animas River study area.

Mine Water Chemistry

Red Mountains Area

Geochemical data from 10 mine discharge samples in the 

Red Mountains area are shown in figure 8 and summarized in 

table 5. Nine of these water samples had pH values <4.0 and 

were characterized by relatively high metal concentrations 

(cadmium+lead+zinc+copper=13,100 μg/L mean) compared 

to all mine discharge samples from the five mineralized areas. 

Only one sample had a high pH (6.86) and had low dissolved 

metal concentrations (cadmium+lead+zinc+copper=<21 μg/L). 

The composition of this high-pH mine sample can be explained 

by the dominance of propylitically altered rocks nearby, and 

the lack of sulfide minerals in the mine dump.

The compositions of the eight low-pH mine waters from 

the Red Mountains area are probably the most distinctive 

of all such waters in the study area (table 5; fig. 10). Along 

with Ohio Peak–Anvil Mountain mine discharge (discussed 

later), mean aluminum (14,400 μg/L), iron (63,300 μg/L), and 

sulfate (514 mg/L) concentrations are markedly higher than in 

low-pH waters from the other mineralized areas. In contrast, 

manganese and strontium concentrations are relatively low 

compared to other areas, perhaps due to the large volumes of 

acid-sulfate-altered rock. Highly elevated copper (2,340 μg/L 

mean), arsenic (103 μg/L mean), and zinc (10,600 μg/L 

mean) in association with elevated lead (177 μg/L mean), and 

cadmium (50 μg/L mean), are the most diagnostic signatures 

of the Red Mountains low-pH mine discharge (table 5; fig. 9). 

These metals probably reflect the characteristic occurrence of 

enargite and other copper-antimony-arsenic-bearing minerals within this mineralized system. Previous studies indicate 

that before recent reclamation, some mine waters just outside 

the study area (near Red Mountain Pass; fig. 13) (data not 

included in this study) had concentrations of arsenic ranging from 600 to 1,400 μg/L, and copper between 5,000 and 

36,000 μg/L5

 (Bove and Knepper, 2000).

Low-pH mine waters representing acid-sulfate type mineralization on the periphery of the Red Mountains hydrothermal system differ geochemically from mine waters nearer the 

interior of the acid-sulfate system. Samples near the periphery 

of the acid-sulfate system have appreciably higher manganese, strontium, and base cation concentrations (Jim Herron, 

Bruce Stover, and Paul Krabacher, Unpublished Cement Creek 

reclamation feasibility report, Upper Animas River Basin, 

Colorado Division of Minerals and Geology, 1998; results not 

included in table 5) than mine samples collected nearer the 

interior of the hydrothermal system (table 5). This difference 

is likely the result of the availability of calcite, plagioclase, 

and chlorite within the surrounding propylitically altered 

wallrock.

δ34S values for six low-pH mine waters from the Red 

Mountains region range from –4.4 to –0.2 per mil (Nordstrom 

and others, this volume, table 1). These data largely are within 

the compositional range of both ore and disseminated sulfides 

(–6.9 to –1.1 per mil) (table 7), indicating that sulfate in the 

mine waters was largely to entirely derived from the oxidation 

of these isotopically light sulfide minerals.

Ohio Peak–Anvil Mountain Area

Geochemical data from 13 mine waters in the OPAM 

area are shown in figure 8 and summarized in table 5. The geochemistry of these mine waters is highly variable with pH ranging from 2.17 to 6.95 and concentrations of 7 to 51,900 μg/L 

aluminum, 445 to 16,000 μg/L manganese, <20 to 7,400 μg/L 

zinc, 3 to 1,760 μg/L copper, and 1 to 12 μg/L arsenic. As 

previously discussed, these mine waters, like those from the 

other four mineralized areas, can be generally divided into two 

groups—low and high pH—based on variability of dissolved 

metal concentrations (fig. 9). Three of the mine waters are 

circumneutral (pH 6.54 to 6.95), have wide ranges in alkalinity (20 to 137 mg/L) and dissolved zinc (<20 to 3,050 μg/L), 

and have low dissolved lead (<21 μg/L) and copper (<5 μg/L) 

concentrations. Manganese concentrations range from 1,100 to 

9,300 μg/L.

5

Waters were sampled for the Colorado Department of Health and Public 

Education (CDHPE) between 1989 and 1990 (compiled in Bove and Knepper, 

2000). Mine reclamation took place from 1996 to 1998.

206 Environmental Effects of Historical Mining, Animas River Watershed, Colorado
Mineralization, Alteration, and Geochemical Signatures 207

Two of the three circumneutral-pH waters have high 

combined concentrations of calcium (>193 mg/L), strontium 

(>1,950 μg/L), and sulfate (>860 mg/L) compared to all mine 

waters in the five areas (table 6). Aqueous δ34S values for two 

of these high calcium-strontium-sulfate waters are +8.0 and 

+10.5 per mil (Nordstrom and others, this volume). These 

values are significantly heavier than those measured in sulfides 

from veins or disseminated pyrite from the surrounding alteration assemblages (–5.4 to –0. 8 per mil; table 7). These data 

indicate that some sulfate in these waters was derived from 

isotopically heavier gypsum or anhydrite, which have δ34S 

compositions larger than +14 per mil throughout the watershed. These data are supported by the presence of gypsum 

gangue material (+17 per mil; table 7) on both of these mine 

dumps. Both of these high calcium-strontium-sulfate waters 

are characterized by high iron concentrations (8,600 μg/L and 

22,000 μg/L), whereas one sample has moderately elevated 

aluminum (484 μg/L). Similar to the Paradise mine water in 

the Mount Moly area, these data suggest mixing of acidic and 

higher pH water somewhere within the mine workings. As 

indicated in previous discussions, the mixing of more alkaline 

water with the acidic water may have reduced the overall zinc 

concentrations of these waters, further obscuring the primary 

metal signatures of the vein deposits (Nordstrom and Ball, 

1986).

As shown in figure 10, low-pH mine water from the 

OPAM and Red Mountains areas has similarly elevated abundances of aluminum, iron, lead, and sulfate; zinc is moderately 

elevated in OPAM low-pH discharge relative to low-pH mine 

waters in the other mineralized areas (tables 5, 6), although 

mean concentration (3,640 μg/L) is about half that of the Red 

Mountains low-pH mine waters. Mean copper concentration 

in OPAM low-pH mine water (314 μg/L) is nearly double 

that of Mount Moly low-pH mine discharge, but nearly six 

times lower than that measured in Red Mountains discharge 

(2,340 μg/L). Likewise, arsenic concentrations within OPAM 

low-pH water (2.7 μg/L) are low compared to Red Mountains acidic mine drainage, which has a mean concentration 

of 103 μg/L. Low concentrations of arsenic in both low-pH 

mine discharge and dump material are consistent with geologic reports (Ransome, 1901) and studies of dump material 

(S. Sutley, unpub. data, 2000) indicating the absence of enargite in all but a few mines in this area. Mean manganese (3,750 

μg/L) and magnesium (15 mg/L) concentrations in OPAM 

acidic mine discharge are also elevated relative to concentrations in low-pH mine waters from all but one of the other 

mineralized areas. The elevated manganese and magnesium 

concentrations could reflect that these associated mines are 

hosted within propylitically altered wallrock on the periphery 

of the acid-sulfate system. Similar compositions are observed 

in mine water on the periphery of the Red Mountains acidsulfate system (Unpub. Cement Creek reclamation feasibility 

report, CDMG, 1998), as discussed in the previous section. 

However, these elevated manganese concentrations could also 

be related to the dissolution of huebnerite (MnWO4

), which 

is reported on dumps in this area (Belser, 1956). δ34S values 

for three of the five low-pH OPAM mine waters range from 

–4.9 to –0.5 per mil (Nordstrom and others, this volume, table 

1) and are within the range of ore and disseminated sulfides 

for the area (–5.4 to –0. 8 per mil; table 7); these data indicate 

little to no interaction with isotopically heavier sulfate from 

gypsum or anhydrite.

Background Surface Water Chemistry

Red Mountains Area

The chemistry of background springs and streams in the 

Red Mountains area shows a wide range of compositions that 

are related to the intensity of hydrothermal alteration (table 8; 

fig. 11). Waters interacting with QSP-altered rocks have the 

lowest pH values and the highest mean sulfate, aluminum, 

iron, silicon, and metal concentrations of all water samples 

collected in this area. Although both the QSP and acid-sulfate 

water (AS) have similarly low pH values, mean calcium, 

magnesium, and manganese concentrations are at least four 

times higher in the QSP waters. One possible explanation is 

the partial replacement of wallrock during QSP alteration, 

which effectively stranded “islands” of unaltered to weakly 

altered rock that did not undergo significant leaching of these 

elements during the hydrothermal process. In contrast, the 

acid-sulfate alteration zones are more homogeneous and rarely 

contain intervals with unaltered feldspar, chlorite, or calcite. 

This relative homogeneity is probably the result of more efficient dispersion of the ancient vapor-dominant hydrothermal 

fluid throughout these highly fractured and brecciated rocks. 

Although mean calcium concentration varies widely within 

the five water types (15.4 mg/L and 2.3 mg/L in QSP and AS 

waters, respectively) (table 8), sodium abundance is quite low, 

ranging from 0.3 to 0.5 mg/L (geometric mean). These data 

contrast markedly with the more sodium-rich samples in the 

Mount Moly area (as much as 13 mg/L), and may indicate a 

scarcity of secondary albite in the Red Mountains area.

As shown in table 8, AS waters were separated into two 

subgroups based on water geochemistry and inferences from 

field data. The first subgroup (fresh sulfides) is characterized 

by a mean iron concentration of 1,140 μg/L and is thought to 

represent waters draining AS-altered areas containing fresh 

pyrite. Waters of this group have been observed in areas of 

unoxidized pyritic rock exposed in deeply incised drainages. 

In high areas unaffected by steep downcutting, these unoxidized zones are generally present at depths >60 m beneath the 

ground surface. In contrast, the second subgroup of AS waters 

(oxidized sulfides) has a mean of 63 μg/L dissolved iron. 

These waters have been observed to drain high-elevation areas 

dominated by surficial deposits, which are largely depleted 

in unoxidized pyrite due to weathering. Samples from water 

draining rocks with fresh sulfides have lower pH and significantly higher concentrations of aluminum, iron, manganese, 

zinc, copper, and sulfate than the oxidized waters (table 8). 

Differences in iron and metals in these waters mirror similar differences in the rocks in which they interact (fig. 6; for 

example, AS-drill core versus AS-surface).
Whereas mean sulfate concentration within Red 

Mountains background waters can be moderately high 

(for example, 137 mg/L in QSP waters), strontium is generally low (168 μg/L) compared to waters that have interacted 

with gypsum or anhydrite (Nordstrom and others, this volume, 

table 1). These data correspond with the light δ34S values that 

are typical of all the Red Mountains water, ranging from –4.9 

to –0.6 per mil. These values are well within the range of 

various sulfide minerals from this area (–6.9 to –1.1 per mil; 

table 7), which indicates that gypsum, anhydrite, and alunite 

were not significant sources of the dissolved aqueous sulfate. 

These data are important because they indicate that background water had little to no interaction with isotopically 

heavy CaSO4

 phases that are only present deep beneath the 

acid-sulfate roots of this hydrothermal system. Furthermore, 

these data suggest that dissolution of alunite (δ34S from 

+10 to +27 per mil; table 7) was limited within the pH range 

of these background waters (table 8).

Streams and springs interacting with propylitically 

altered rock have the highest mean values of pH and alkalinity (6.04 and 11.1 mg/L, respectively) and the lowest 

mean dissolved metal concentrations (aluminum=<1 μg/L; 

iron=37 μg/L; zinc=<3 μg/L; copper=4 μg/L) of all water 

samples collected in the Red Mountains area. Mean sulfate 

concentration for the propylitic waters is 10 mg/L, whereas 

calcium and strontium means were 8.3 mg/L and 298 μg/L, 

respectively. Similar to the Mount Moly area, water draining 

the weak sericite-pyrite assemblage (WSP) falls compositionally between QSP and PROP water, reflecting the transition 

between the two alteration assemblages (table 8; fig. 11). 

Background waters influenced by vein or fault structures 

(PROP-V) have highly variable compositions: pH ranges from 

3.0 to 6.5, mean sulfate ranges from 1 to 351 μg/L, and mean 

zinc ranges from 20 to 1,070 μg/L. The origin of each of the 

PROP-V waters is circumstantially unique and is discussed in 

detail by Bove and others (2000).

Ohio Peak–Anvil Mountain Area

As shown in table 8 and figure 11, background water 

in the Ohio Peak–Anvil Mountain area also varies according 

to the alteration assemblage that it drains. Water interacting 

with the QSP assemblage in the OPAM area (mean pH=3.5, 

sulfate=132 μg/L, and zinc=142 μg/L) is compositionally 

very similar to QSP water in the Red Mountains area, with 

the exception of mean sodium and silicon, which are higher 

in concentration in the background OPAM waters (3.0 and 

24.1 mg/L, respectively). Instead, these sodium and silicon 

concentrations are very similar to Mount Moly area QSPrelated waters, suggesting that reactions involving secondary 

albite were also important in the vicinity of Ohio Peak–

Anvil Mountain. δ34S values from six QSP-related water 

samples range from –6.7 to –1.5 per mil (Nordstrom and 

others, this volume, table 1), which are within the range of 

three sulfide mineral samples analyzed from this area (–5.4 

to –0.8 per mil; table 7). These data indicate that much of the 

sulfate was derived from the interaction with disseminated 

pyrite within the QSP-altered rocks. Like the AS (oxidized 

sulfides) waters of the Red Mountains area (table 8), these 

samples have intermediate pH values (3.88 and 5.50) and 

relatively low sulfate (4 and 21 μg/L) and low metal (<3 and 

29 μg/L Zn) concentrations compared with other background 

waters (table 8). The two propylitic waters in the OPAM 

area are similar to those in the Red Mountains area with high 

pH (6.99 and 7.46), measurable alkalinity (31 and 34 mg/L), 

relatively low calcium (23 and 24 mg/L) and strontium (363 

and 405 μg/L), and very low metal concentrations (table 8).

18–10 Ma Polymetallic Vein Systems—

Northeast-Trending Veins

Eureka Graben Area

A significant portion of mineral production within the 

western San Juan Mountains was from vein deposits localized along collapse and resurgent structures associated with 

the San Juan, Uncompahgre, and Silverton calderas (Burbank 

and Luedke, 1969; Lipman and others, 1976). Related structures include faults and subsidiary fractures of the northeasttrending Eureka graben system. These graben structures host 

many of the most economically important vein deposits in 

the Animas River watershed (Jones, this volume, Chapter C). 

Veins of the Eureka graben area are defined by a broad zone 

from Wood Mountain southwest to the Bonita fault and are 

bounded by Tuttle Mountain to the north and the Toltec fault 

on the south (fig. 17). Nearly all the important gold mines 

within the Silverton caldera lie within a relatively narrow belt, 

roughly within the center of the Eureka graben area (fig. 17). 

Near the center of this narrow gold zone are the large producing mines in the Eureka district that included the Sunnyside, 

Gold Prince, and Silver Queen mines in Eureka and the head 

of Placer Gulch, and the Sound Democrat and Scotia and 

Vanderbilt mines near Treasure Mountain. The Gold King and 

the Golden Fleece mines border this zone on the east and west, 

respectively (fig. 17). Mines in the Mineral Point, California 

Gulch, and upper Cement Creek areas also developed veins of 

the Eureka graben system. Although similar in overall mineralogy and gross paragenesis, these veins are more typical 

of polymetallic vein deposits in the Silverton area as they are 

conspicuously deficient in gold-rich ore (Ransome, 1901; 

Burbank and Luedke, 1969; Casadevall and Ohmoto, 1977; 

Langston, 1978; Standen and Kyle, 1985; Koch, 1990; Bartos, 

1993).

208 Environmental Effects of Historical Mining, Animas River Watershed, Colorado
Mineralization, Alteration, and Geochemical Signatures 209

A relatively small data set of K-Ar, fission-track, and 

Rb/Sr dates broadly constrains the age of these polymetallic veins to about 18–10 Ma (Lipman and others, 1976; Bove 

and others, 2001). Previous workers have demonstrated that this 

polymetallic vein mineralization was coeval and perhaps some 

was genetically related to the emplacement of numerous small 

intrusions of high-silica rhyolite and granite (Lipman and others, 1976; Bartos, 1993). Data presented in this study (see section, “Rhyolites and Associated Mineralization”) further refine 

these observations and indicate that at least one important stage 

of this mineralization is related to these intrusions.

Eureka Gulch, Treasure Mountain, and Placer 

Gulch Gold-Rich Veins

The Sunnyside mine (fig. 17) was a major gold producer 

(Casadevall and Ohmoto, 1977), and until closure in 1991, produced more than 800,000 ounces of gold and 14 million ounces 

of silver (Bartos, 1993; Jones, this volume). Typical ore grades 

were 0.16 oz gold/t, 1.57 oz silver/t, 2 percent lead, 0.2 percent 

copper, and 3 percent zinc (Casadevall and Ohmoto, 1977). 

In the later years of mining, the 3,050 m American tunnel 

(fig. 17) provided the primary access to workings that extended 

Figure 17. Generalized geologic and alteration map of Eureka Graben area.

EXPLANATION

Intrusive Units

Dacite porphyry (~23 Ma)

Hydrothermal Alteration Assemblages

Vein-related quartz-sericite-pyrite (V-QSP)

Weak sericite-pyrite (WSP)

Quartz-sericite-pyrite (QSP)

Acid-sulfate 

Regional propylitic 

Fault, vein, or major fracture zone

Mine site

1 Golden Fleece

2 Silver Queen

3 Sound Democrat

4 Gold Prince

5 Mountain Queen

6 Mogul

7 Queen Anne

8 Columbus

Houghton Mt

California

 Mt

Wood Mt

Tuttle Mt

California

Gulch

Eureka

South Fork

Gulch

Cement

Creek

Animas

Forks

Animas River

Mineral 

Point

Picayune Gulch

TOLTEC

FAULT

BONITA

FAULT

SUNNYSIDE

GOLD KING

1

2 3 4

5

6

7

8

MINE

MINE

Scotia-Vanderbilt

vein system

Treasure Mt

Denver Hill

Engineer Mt

37.958o

37.875o

107.657o 107.541o

Gulch Placer

American

tunnel

0 1 2 KILOMETERS

H gh-silica rhyolite (<18 Ma) i
2,100 m laterally and 610 m vertically (Casadevall and Ohmoto, 

1977). Two types of veins were distinguished based on outcrop 

character. Veins of the first type form bold, manganese-stained 

outcrops at the surface and yielded most early production in 

the area. Other veins have little to no surface expression and 

were worked in the subsurface (Casadevall and Ohmoto, 1977). 

Vein mineralization of the Sunnyside mine was generally 

complex (fig. 18), and as many as six major stages have been 

recognized (Casadevall and Ohmoto, 1977; Langston, 1978). 

Early to intermediate stages include quartz-pyrite and massive 

sulfide ores, whereas ores of gold-telluride-quartz, manganese, 

and quartz-fluorite-carbonate-sulfate formed later. Ores of the 

massive sulfide stage (stage 3; fig. 18) consist largely of massive anhedral aggregates of intergrown sphalerite and galena, 

with lesser pyrite, chalcopyrite, and tetrahedrite (Casadevall 

and Ohmoto, 1977). The majority of gold deposition followed 

the massive sulfide stage and preceded the manganese ore 

stage. Deposition of manganese ores, which occur in thick 

(3 m average) tabular bodies subparallel to veins, followed 

gold-quartz mineralization. These late-stage ores are characterized by light-pink masses of fine-grained pyroxmangite 

(MnSiO3

), lesser rhodochrosite, and quartz. Manganese ores 

within the Sunnyside mine average 20 volume percent of the 

entire vein material (Casadevall and Ohmoto, 1977). Similar 

veins throughout the Eureka Gulch, Treasure Mountain, and 

Placer Gulch areas (fig. 17) are also highly endowed in this latestage ore, with weighted averages of 6 percent manganese taken 

across 3 m vein widths (calculated from data in Belser, 1956).

Vein and fracture-filling anhydrite and gypsum with 

quartz, sericite, pyrite, and other sulfides are also associated with the manganese ore stage. These veins have been 

observed at the American tunnel level (3,260 m elevation) and 

as much as 600 m deeper in exploration drill holes. The latest 

stage of mineralization is marked by vug and veinlets filled 

with quartz, fluorite, calcite, huebnerite, and minor sulfides 

(Casadevall and Ohmoto, 1977; Langston, 1978).

The Scotia-Vanderbilt vein system on the southeast side 

of Treasure Mountain (fig. 17) is along a continuation of the 

strong N. 40° E. vein set present at the Sunnyside mine. The 

Scotia-Vanderbilt veins are exposed along strike for nearly 

2 km and have widths up to 30 m (Standen and Kyle, 1985). 

The vein system consists of three major sets of mineralized 

Figure 18. Paragenetic stages of vein mineralization from Eureka Graben area and adjacent areas. Note occurrence of late vein 

stages with fluorite and huebnerite, which are commonly associated with geochemical enrichments in molybdenum.

2.

Sunnyside

Casadevall and Ohmoto (1977)

Gold King Yukon ScotiaVanderbilt Mineral Point Koch (1990) Waegli (1979)

Standen and Kyle (1985) Bartos (1993)

1.

2.

3.

4.

5.

6.

2.

2.

1.

1.

3.

3.

1.

4.

3.

Pyrite-quartz

Banded quartz-sulfide

Base-metal sulfide

Gold-telluride-quartz

Manganese

Quartz-fluorite-carbonate-sulfate

Pyrite-quartz

Pyrite-quartz

Pyrite-quartz

Base-metal sulfide

1.

2.

3.

4.

Pyrite-quartz

Base-metal sulfide

Base-metal sulfide-manganese

Base-metal sulfide

Precious metals

Quartz-fluorite-huebnerite

Huebnerite-fluorite

Quartz-silver sulfosalts

Quartz-calcite-fluorite

Manganese

3a.Manganese

210 Environmental Effects of Historical Mining, Animas River Watershed, Colorado
Mineralization, Alteration, and Geochemical Signatures 211

fractures. The first averages a trend of about N. 50° E. and 

includes the Scotia and Vanderbilt veins. A second set of 

shorter veins is nearly perpendicular to the first set, and trends 

about N. 55° W. A third system of nearly transverse veins, represented by the notably gold-rich Golden Fleece vein (fig. 17), 

averages about N. 75°–80° E., and appears to postdate the 

other two sets (Standen and Kyle, 1985). As typical of other 

areas, the volume of ore minerals is generally greatest near 

the intersections of these prominent structures (Casadevall 

and Ohmoto, 1977; Burbank and Luedke, 1969). The veins 

are composed mainly of quartz, pyroxmangite, and calcite 

with sulfides of pyrite, sphalerite, galena, and chalcopyrite in 

decreasing order of abundance. Ore also may contain freegold, sulfobismuthites of lead and silver, and small amounts 

of tetrahedrite, molybdenite, native silver, and native copper 

(Ransome, 1901). The Scotia-Vanderbilt vein system can be 

generally subdivided into four main episodes (Standen and 

Kyle, 1985), which are roughly similar in character to those 

described within the Sunnyside deposit (Casadevall and 

Ohmoto, 1977) (fig. 18). Electron microprobe studies from 

vein minerals show some substitution of arsenic, gold, and 

silver within chalcopyrite, as much as 0.2 percent silver in 

galena, and 0.2 percent silver and 1.6 percent arsenic in pyrite 

(Standen and Kyle, 1985).

Other mines in the gold belt (discussed in the preceding section), such as the Sound Democrat, Silver Queen, 

Sunnyside Extension, and San Juan Queen (fig. 17) have 

ore mineralogies and stages that closely resemble that of the 

Sunnyside and Scotia-Vanderbilt mines (Ransome, 1901). 

Most are prominent veins with bold outcrops that are generally stained black due to the oxidation of pyroxmangite. In the 

subsurface, these veins are characterized by alternating bands 

of ore and barren pyroxmangite. Orebodies range from a few 

centimeters to 10 m in thickness. The ore minerals include 

roughly subequal proportions of sphalerite and galena, which 

together generally exceed abundances of other ore minerals. 

Free-gold along with quartz was reported in all of the justmentioned mines, and silver occurred commonly within lead 

sulfobismuthites (Ransome, 1901). Gangue minerals included 

quartz, pyroxmangite, and locally fluorite and calcite. The 

pyroxmangite stage is always poor in ore minerals, but small 

amounts may occur with the richest ore.

Some of the gold-rich mines, including the Golden Fleece, 

Lead Carbonate, and Gold King (fig. 17), are reported to be 

lacking in the intermediate base-metal stages (Ransome, 1901; 

Langston, 1978; Koch, 1990). Instead, these mines are noted for 

the occurrence of free-gold commonly associated with pyritequartz in small quartz veinlets, and vuggy quartz and carbonate 

gangue (Langston, 1978; Koch, 1990). The Golden Fleece mine 

contained a small, tight vein that produced rich free-gold ore 

(Ransome, 1901). The vein, which trends N. 75° E., ranges in 

thickness from about 1 to 20 cm and is accompanied by clay 

gouge. Ore is free dendritic gold in a gangue of quartz and rare 

rhodochrosite. Quartz occurring with the gold ore is very dark 

owing to abundant inclusions of pyrite, galena, sphalerite, and 

possibly other ore minerals. Massive sulfides and pyroxmangite were absent. The Gold King–Davis vein system consists of 

two subparallel veins that merge downward into a root zone of 

one major, northeast-trending vein (Koch, 1990). These veins 

formed along subsidiary faults and fractures of the Eureka graben that also host one of the important veins of the Sunnyside 

mine. The Gold King–Davis veins can be traced for more than 

1 km along strike and have maximum widths of 5 m. Mineralization at the Gold King mine formed deposits in four discrete 

stages: (1) massive pyrite and milky quartz, (2) base-metal 

sulfides of chalcopyrite, galena, and sphalerite, (3) the preciousmetal suite of sulfosalts, free-gold, and gold and silver tellurides, and (4) quartz, fluorite, huebnerite with minor manganese 

silicates (fig. 18) (Koch, 1990). Total production from the mine 

included 603,738 metric tons of ore averaging 0.6 oz gold/t, 

2.9 oz silver/t, 0.7 percent lead, and 0.5 percent copper (Koch, 

1990).

Base-Metal Veins with Late-Stage Manganese 

Silicates

Mineral Point

Mineral Point, located about 5 km north of the Sunnyside 

mine (fig. 17), covers about 10 km2. More than 100 veins are 

exposed in this area, although only about 20 of these had some 

production. Incomplete mill figures suggest small amounts of 

hand-sorted silver-gold ore were mined, mostly prior to 1900, 

with assays as high as 0.9 oz gold/t, and 56 oz silver/t (Bartos, 

1993). About 20,000 tons of ore was produced from 1901 to 

1941 with general grades of 0.01 oz gold/t, 1.9 oz silver/t, 

0.1 percent copper, 2.3 percent lead, and 0.8 percent zinc 

(Kelley, 1946). Post-World War II production was minimal.

Mineral Point veins are also mostly along northeasttrending Eureka graben structures, but some are along eastwest and northwesterly orientations. Veins cut dacite intrusions 

at Houghton Mountain (fig. 17), suggesting their age to be 

<23 Ma. However, similarities in mineralization style suggest they are within the same general age span as many of the 

gold-rich veins. (See previous section.) Vein widths range from 

0.3 to 3 m, averaging 1 m, and are typically simple in texture 

and mineralogy (Bartos, 1993). However, some veins are 

complex, exhibiting multiple opening and filling of fissures. 

Three general stages of mineralization (fig. 18) have been 

identified in Mineral Point veins: (1) quartz-pyrite, (2) banded 

quartz-sulfide, and (3) manganese-carbonate (Bartos, 1993). In 

contrast to many of the veins to the west, these veins lack an 

intermediate to late precious-metal stage (fig. 18), and stage 3 

manganese is generally only present within large, continuous, 

northeast-trending veins. Some of these veins contain a latestage mineral assemblage containing huebnerite (Belser, 1956) 

and are enriched in molybdenum.

California Gulch

The California Gulch mining area includes the main 

glacial valley and adjacent ridges of California Gulch above 

Animas Forks (fig. 17). The most significant mining and prospecting occurred along the north wall of the valley, spanning 
the Mountain Queen mine (mine # 41) at the top of the basin 

to the Columbus mine at Animas Forks (fig. 17). Ore production from the principal mines—Mountain Queen, Little Ida 

(mine # 15), Burrows (mine # 16), Vermillion (mine # 17), 

Frisco tunnel (mine # 19), and Columbus (mine # 23)—totaled 

about 19,000 tons from 1880 to 1950 (Burbank and Luedke, 

1969), averaging <0.01 oz gold/t, 0.2 oz silver/t, 0.1 percent 

copper, 9 percent lead, and 5 percent zinc. After 1950, ventures in this area were largely exploratory. Most veins filled 

prominent northeast-trending Eureka graben structures (such 

as the Vermillion, Columbus, Dakota veins; pl. 4 in Burbank 

and Luedke, 1969), although some in the upper basin are along 

strong east-west fissure zones (for example, the Mountain 

Queen, Custer, Indian Chief, Belcher; pl. 4 in Burbank and 

Luedke, 1969), and a few near Animas Forks filled northwesterly structures. The largest orebodies formed along the intersections of northeast-and northwest-trending veins (Burbank 

and Luedke, 1969). Veins are quartz-rich, ranging from 0.3 to 

60 m across, and many can be traced along strike for several 

kilometers. The ore mineralogy and paragenetic stages are 

generally very similar to those of veins in the Mineral Point 

area, although late-stage manganese silicate gangue may 

be more abundant (fig. 19). The abundance of manganese 

silicate gangue is reflected in the presence of manganesecemented conglomerates (of probable Holocene age) within 

the upper reaches of California Gulch (Verplanck and others, this volume, Chapter E15; Church, Fey, and Unruh, this 

volume, Chapter E12). As indicated by production figures and 

geologic reports (Ransome, 1901; Kelly, 1946; Burbank and 

Luedke, 1969), these veins were mostly rich in lead and zinc 

sulfides, marginal in copper, and largely devoid of intermediate 

to late-stage gold mineralization. However, late vein-related 

fluorite and huebnerite (Ransome, 1901; Belser, 1956) and 

associated molybdenum, fluorine, and tungsten anomalies, 

as observed in various sample media (D.J. Bove and others, 

unpub. data, 2002), are present in upper California Gulch, 

north of California Mountain.

Zoning of Manganese Silicate Gangue Material

Manganese silicates are abundant in veins of the north 

Eureka Graben area, and a definite central zone is present 

away from which they tend to diminish (fig. 19). This center is localized roughly along the Sunnyside-Mastodon vein 

trend. Calculations from assay analyses indicate that 10–14 m 

widths of these 20–60 m wide veins are composed entirely 

of normative pyroxmangite (Young, 1966; Langston, 1978). 

From this central area, the manganese content diminishes 

greatly to the west towards the Ross Basin area (Young, 1966). 

This westward decrease in manganese continues toward the 

Red Mountains area. Manganese silicates are also generally 

rare within veins west of the Bonita fault and decrease north 

of Ross Basin toward the Poughkeepsie Gulch area. Pyroxmangite also is rare within precious-metal veins to southwest 

of the central zone of manganese enrichment (Langston, 

1978), and decreases in abundance similarly to the northeast 

towards Mineral Point and Engineer Pass.

Vein-Related Hydrothermal Alteration

X-ray diffraction and petrologic studies indicate that 

vein-related alteration in the Eureka Graben area consists of a 

proximal quartz-sericite-pyrite-zunyite assemblage and a distal 

sericite-kaolinite facies with increasing chlorite and calcite 

at the transition with the propylitized wallrock (Standen and 

Kyle, 1985). Nearest the vein, wallrock is typically altered to 

an envelope of fine-grained quartz, which grades into the completely recrystallized assemblage of quartz-sericite-zunyitepyrite (Casadevall and Ohmoto, 1977). Towards the distal 

sericite-kaolinite facies, tiny euhedral grains of primary apatite 

appear, and relicts of feldspars and mafics can be recognized. Widths of the alteration envelope vary with vein width, 

although fault gouge locally seals the wallrock from the associated vein alteration. The proximal alteration zone typically 

ranges from half to one times the vein width; however, width 

of the distal alteration zone varies considerably, but generally 

is one to two times the vein width (Standen and Kyle, 1985). 

In some localities, intersections of multiple veins are marked 

by coalesced alteration zones as much as 0.3 km wide in area.

Rhyolites and Associated Mineralization

A series of small, high-silica rhyolite intrusions and 

dikes crop out along a 5 km long zone trending roughly 

N. 30° E. from California Mountain to Denver Hill (fig. 17). 

This zone of rhyolites extends another 5 km northeast of the 

study area and includes dikes in the Engineer Pass area and 

an extrusive dome complex at Dolly Varden Mountain (Hon 

and others, 1986). The rhyolites within the study area are 

generally aphanitic and finely flow banded. They are locally 

vesicular and contain rare fine-grained quartz phenocrysts. 

Geochemical data indicate that these rocks are enriched in 

large ion lithophile (LIL) elements (Lipman and others, 1982; 

P.J. Bartos, unpub. data, 2001) and have many compositional 

similarities (SiO2

>75 percent; K2O>4.5 percent; Rb>300 ppm) 

to rhyolites associated with well-known molybdenite deposits 

(Ludington, 1981; White and others, 1981; Hildreth, 1979). 

However, key elements such as niobium (20–50 ppm), tungsten (3–9 ppm), and fluorine (0.1 percent), although highly 

elevated, fall at the lower limits of or below the diagnostic 

levels of deposit-associated rhyolites. Rhyolitic intrusions 

at Houghton and California Mountains are associated with 

0.75–1 km2

 zones of pervasive QSP-altered rock containing 

a dense network of hairline quartz veinlets (fig. 17). Pyrite 

within these zones is typically altered to iron oxides and 

jarosite, and kaolinite probably formed due to related supergene processes (Bartos, 1993). The rhyolite near Denver Hill 

(fig. 17) is generally very weakly altered; however, it is locally 

silicified and pyritized, especially near its brecciated margins. 

Rare molybdenite mineralized material has been reported 

in breccia zones in the rhyolites near Denver Hill (Lipman 

and others, 1982), and molybdenum concentrations in all the 

altered rhyolites typically range from 20 to 50 ppm, along 

with 2–5 ppm copper, and 20–50 ppm lead (D.J. Bove, unpub. 

data, 2002). Molybdenum is also anomalously concentrated 

212 Environmental Effects of Historical Mining, Animas River Watershed, Colorado
Mineralization, Alteration, and Geochemical Signatures 213

in mine dumps surrounding the Denver Hill, Houghton 

Mountain, and California Mountain rhyolites, ranging as 

high as 100–200 ppm. In addition, vein-related fluorite and 

huebnerite, which are paragenetically later than most gold and 

base-metal mineralization in the area (fig. 18), have also been 

reported in mines near all three of these intrusions (Ransome, 

1901; Belser, 1956). Fluorite, huebnerite, and elevated molybdenum are also characteristic of a late-stage vein assemblage 

observed at the Sunnyside and adjacent mines and in a roughly 

3 km wide zone extending from Kendall Mountain along 

Cement Creek to just north of Gladstone (Belser, 1956).

A prominent elliptical aeromagnetic high (about 

1,500 m across) was delineated near the Gladstone area 

along the N. 30° E. trend of the previously mentioned 

rhyolites (Smith and others, this volume, Chapter E4). 

Although no rhyolite intrusions have been exposed in this 

area, the nearby American tunnel (fig. 17) exposes numerous veins containing late-stage fluorite and some huebnerite 

(Thomas Casadevall, oral commun., 2000). In addition to 

the aeromagnetic data, evidence of a buried intrusion in this 

area is suggested by mineralogic data from two deep (610 m) 

exploratory holes drilled from the main level of the American 

tunnel. Rocks near the bottom of these holes were cut by 

intensely sericitized veins and associated breccias characterized by an assemblage of anhydrite, pyrite, and fine-grained 

andradite. Fluid inclusions within a quartz-sericite-andradite 

veinlet from this deep core interval contained a daughter mineral tentatively identified as halite (Casadevall and Ohmoto, 

1977). The occurrence of halite within these inclusions 

indicates a minimum salinity of 26.5 equivalent weight percent 

NaCl—in marked contrast to salinities <3.6 equivalent weight 

percent from all other fluid inclusions measured from the 

Sunnyside vein system (Casadevall and Ohmoto, 1977). The 

presence of such high salinities in conjunction with sericiticaltered breccias and veins, hydrothermal anhydrite and garnet, 

and the aeromagnetic anomaly suggests the possibility of a 

genetic relationship to an underlying intrusion. Similar mineralogic assemblages and high-salinity fluids are documented 

within greisen zones related to rhyolite-granite intrusions and 

molybdenum porphyry systems (White and others, 1981).

Figure 19. Eureka Graben area showing manganese silicate zoning trends. Center of 

zone is along Sunnyside-Mastodon vein trend (SMVT). Data compiled from Langston 

(1978), Young (1966), and this study.

Houghton Mt

California Mt

Wood Mt

Tuttle Mt

Animas

Forks

Picayune Gulch Treasure Mt

x

x

x

Engineer Mt

Ross Basin

SMVT

37.958o

37.875

107.657o 107.541o

Direction of decreasing manganese silicates in veins (relative to 

 central SMVT high manganese zone)

o

Area where veins contain manganese silicate zone > 3 m thick

EXPLANATION

California

Gulch

Eureka Gulch

Animas River

SUNNYSIDE

MINE

Mineral 

Point

Denver Hill

GOLD KING

American

tunnel

x

x

BONITA

FAULT

TOLTEC

FAULT 0 1 2 KILOMETERS
The association between high-silica rhyolites and latestage fluorite, huebnerite, and molybdenum in polymetallic 

veins in the study area bears some similarities to Climax-type 

porphyry molybdenum systems. Detailed studies of Climaxtype deposits have documented the genetic relationship 

between LIL-enriched, high-silica rhyolite-granite intrusions 

and late veins radial to these intrusions, which contain quartz, 

base metals and precious metals, and later fluorite, huebnerite, 

and molybdenite (Bookstrom, 1989). Probably one of the best 

local examples of such relationships is in the Cuba Gulch area, 

about 4 km east of Eureka, and just outside the study area. At 

this locality, a 17.1 Ma high-silica rhyolite dike is associated 

with anomalous concentrations of tin, tungsten, niobium, and 

molybdenum in adjacent rock and stream-sediment samples 

(Bove and others, 2001). Field studies indicate that this dike is 

associated with mineralized tourmaline-bearing pebble dikes, 

fluorite, and sparse molybdenite (Hon, 1987a).

Geochemistry of Altered Rock and 

Vein-Adjacent Zones

Geochemical data from regional propylitic and QSP veinrelated rock (V-QSP) from the Eureka Graben area (Langston, 

1978) are summarized in table 3 and figure 4. Comparisons of 

these data indicate that iron is statistically higher in propylitized rocks, whereas values of lead, manganese, and silver 

(not shown) in the upper concentration ranges are considerably higher in V-QSP-altered rocks. However, zinc and copper 

are analytically indistinguishable between these two groups 

of altered rocks. The marked decrease in iron in the V-QSPaltered rocks probably reflects near-surface pyrite weathering 

and mobilization of iron. The decrease in iron and the sporadically high concentrations of silver, lead, and copper in some 

samples suggest that the V-QSP zones, prior to oxidation, 

may have been enriched in these metals. This is substantiated 

by petrographic studies (see section on vein-related alteration in <18 Ma polymetallic vein systems), which indicate an 

abundance of fine-grained base-metal sulfides in alteration 

envelopes adjacent to polymetallic veins. A wide range in 

manganese concentrations (<160 ppm to >4,200 ppm) is noted 

for samples from the V-QSP zones. The presence of manganese silicate gangue material (some samples taken above 

productive ore zones; Langston, 1978) interspersed throughout 

the V-QSP zone probably accounts for the high range of manganese concentrations. Chlorite, epidote, and thin manganese 

surface coatings in the altered wallrock likely contribute to the 

lower manganese abundances. It is notable that manganese, 

lead, zinc, and copper are statistically indistinguishable in 

propylitic rocks from the Eureka Graben, Mount Moly, and 

Red Mountains–OPAM areas. Pyrite from regional propylitically altered samples of the Eureka Graben area, like their disseminated counterparts in the Red Mountains and Mount Moly 

areas (table 2), are similarly devoid of metals and arsenic 

(Standen and Kyle, 1985).

Mine Dump Compositions

Mean manganese abundance (1,500 ppm) in dumps from 

the Eureka Graben area ranks second highest among the five 

mineralized areas (table 4; fig. 7). However, many of the mine 

dumps from the zone of highest manganese abundance, as 

delineated in figure 19, could not be sampled due to access 

restrictions on private land. Ranges of lead and zinc (mean 

of 3,330 ppm and 2,110 ppm, respectively) generally overlap 

with those from dumps in the other four mineralized areas 

(fig. 7), whereas the range of copper (189 ppm mean) is much 

lower than in samples from the Red Mountains and South 

Silverton areas. Arsenic concentrations, which overlap statistically with those of the Mount Moly area samples, are substantially higher than those in OPAM and South Silverton samples, 

but are much lower in comparison to the Red Mountains 

data. Like the Mount Moly area, ores are known to contain 

tetrahedrite-tennantite and some chalcopyrite rather than 

enargite. Molybdenum concentration is statistically higher 

than in all other mineralized areas with a mean of 50 ppm. Six 

of eight Eureka Graben-area dumps with the highest molybdenum concentrations (110–300 ppm) are clustered in the 

Houghton Mountain and Denver Hill areas (fig. 17). The other 

two dumps with anomalously elevated molybdenum are on the 

southeastern periphery of Houghton Mountain. The remaining 10 dumps from other parts of the Eureka Graben area have 

molybdenum abundances <47 ppm. These data reflect the 

association between young high-silica rhyolite intrusions such 

as at Houghton Mountain and Denver Hill, and a late-stage 

assemblage within related veins containing fluorite, huebnerite, and elevated molybdenum.

Mine Water Chemistry

Geochemical data from 13 mine waters in the Eureka 

graben area are shown in figure 8 and summarized in table 5. 

The geochemistry of these waters is highly variable: pH ranges 

from 2.63 to 7.03, and dissolved concentrations range from 

28 to 9,170 μg/L aluminum, 36 to 71,600 μg/L manganese, 

27 to 49,800 μg/L zinc, 3 to 6,450 μg/L copper, and <19 μg/L 

arsenic. As shown in figure 9, these mine waters, like those 

from the other four mineralized areas, can be generally divided 

into low- and high-pH groups.

Five of the thirteen mine waters have near-neutral pH 

(6.21 to 7.01), whereas one sample was borderline acidic 

(pH 4.69). These samples had a wide range in alkalinity (<0.1 

to 122 mg/L), and they contained dissolved zinc ranging from 

27 to 6,540 μg/L, <21 μg/L lead, and 3 to 84 μg/L copper. 

Manganese concentrations were widely variable, ranging from 

36 to 6,860 μg/L, whereas both aluminum (3 to 373 μg/L) and 

iron (<21 to 226 μg/L) concentrations were relatively low. 

Calcium (59 to 113 mg/L), strontium (683 to 780 μg/L), and 

sulfate (300 to 540 mg/L) were moderately elevated in three of 

these waters (table 5). However, these concentrations are markedly lower than the subset of high-calcium-strontium-sulfate 

waters in the Mount Moly and OPAM areas, which had more 

than double the concentration of each of these elements.

214 Environmental Effects of Historical Mining, Animas River Watershed, Colorado
Mineralization, Alteration, and Geochemical Signatures 215

The overall geochemical signature of eight low-pH 

mine waters from the Eureka Graben area is distinctive in 

comparison to low-pH discharge from mines in the other mineralized areas (fig. 10). Although aluminum, iron, and sulfate 

concentrations plot roughly in the middle of low-pH mine 

water data from the other areas, mean manganese concentration (8,100 μg/L) is 2 to 34 times higher than that of low-pH 

mine waters from the other mineralized areas. Manganese 

concentrations of 71,600 μg/L have been reported in discharge from the Silver Queen mine (Jim Herron, Bruce Stover, 

Paul Krabacher, and Dave Bucknam, Unpublished Mineral 

Creek feasibility investigations report, Upper Animas River 

Basin, Colorado Division of Minerals and Geology, 1997), and 

concentrations as high as 92,400 μg/L (see database, Sole and 

others, this volume) have been measured in discharge from the 

American tunnel—the main haulage tunnel to the Sunnyside 

and other important Eureka graben veins (fig. 17). These 

manganese-rich waters reflect the abundance of manganese 

minerals within many of the Eureka graben vein structures. In 

addition to manganese, these mine waters have high concentrations of copper and zinc (mean of 633 and 9,570 μg/L, 

respectively), and intermediate levels of copper (633 μg/L 

mean) and lead (170 μg/L mean) compared to low-pH mine 

waters from the other mineralized areas. Although base-metal 

concentrations are elevated, mean arsenic abundance was low 

(<1 μg/L mean), reflecting the absence of enargite or other 

arsenic-bearing minerals within this mineralized area.

Although no stable isotope data are available for either 

low or higher pH mine discharge within the Eureka Graben 

area, a δ34S value of +7.9 was measured for mine water issuing 

from the American tunnel portal. This value is intermediate between vein and disseminated sulfides (–6.3 to +2.7 per 

mil) and gypsum and anhydrite phases (+15.3 to +19.0 per 

mil) associated with the Sunnyside mine workings (table 4 

in Casadevall and Ohmoto, 1977; table 7, this study). These 

data suggest that sulfate within waters draining the extensive 

Sunnyside mine workings was derived by dissolution of both 

light sulfide and heavy sulfate minerals. The presence of latestage gypsum and anhydrite within many of the Eureka graben 

veins suggests that other mine waters within this mineralized 

area could have similar intermediate δ34S compositions.

Background Surface Water Chemistry

Background water within the Eureka Graben area 

contrasts sharply with background samples from the previously described mineralized areas due to the relatively small 

volume of intensely altered rock—nearly 75 volume percent 

of area is propylitically altered—and the strong interplay 

with mineralized vein structures (table 6; fig. 17). Field 

studies indicate that three QSP-related waters sampled in the 

Houghton Mountain, California Mountain, and South Eureka 

Gulch areas (fig. 1) (table 3 in Mast, Evans, and others, 2001) 

derived metals from interaction with unmined vein structures along their flow path (V-QSP waters). The composition of these waters shows the distinct influence of both the 

surrounding QSP-altered rocks (pH=3.26 to 4.55) and vein 

structures with which they interact (Mn=2,670–74,700 μg/L; 

Cu=4–309 μg/L; Zn=640–14,400 μg/L). In contrast, propylitic waters influenced by veins and other mapped structures 

(PROP-V) have a mean pH of 6.52 and a mean zinc concentration of 143 μg/L (table 8). Several of these samples contained anomalously high concentrations of manganese (1,210 

to 19,800 μg/L), copper (23 to 189 μg/L), and zinc (154 to 

4,240 μg/L), values similar to metal abundances in the V-QSP 

waters. The maximum manganese (74,700 μg/L) and zinc 

(14,400 μg/L) concentrations from V-QSP and PROP-V waters 

are more than 17 times greater than the highest abundance 

of these elements in any of the non-vein-related waters from 

the Mount Moly, Red Mountains, or OPAM areas. These 

data suggest that these distinctly metal rich waters (V-QSP 

or PROP-V) must have interacted with some mineralized 

vein material. Specifics regarding flow paths for these types 

of water/mineralized rock interactions, however, have yet to 

be resolved. Although metal concentrations are quite high in 

some of these waters (V-QSP or PROP-V), contributed metal 

loads would be minimal as discharge is at most 0.2 ft3

/s.

Relative to the volume of propylitically altered rock in 

this area, associated water samples are very poorly represented 

(n=3). Compared to other background samples discussed 

in this study (table 6), these samples have relatively high 

pH (6.9) and low to moderate alkalinity (10 to 35 mg/L), 

strontium (81 to 349 μg/L), and sulfate concentrations (28 to 

113 mg/L) (table 8). One of these three samples has anomalously elevated zinc concentration (237 μg/L), whereas zinc 

abundances in the other samples are near analytical detection limits. It is probable that the sample with elevated zinc 

interacted with some mineralized vein material, although no 

mineralized structures were noted nearby. δ34S values from 

three water samples from the Eureka Graben area range from 

–2.4 to 0 per mil (Nordstrom and others, this volume, table 1). 

These values are generally within the range of sulfides in the 

study area (table 7). These data augment aqueous geochemical data (table 8) showing a mean strontium concentration 

of 151 μg/L for the entire Eureka Graben area data set. As 

pointed out in a previous discussion, such low strontium concentrations are indicative of at most, very limited interaction 

with gypsum or anhydrite.

Aqueous Geochemical Signatures of HighSilica Rhyolite Intrusions and Late-Stage Vein 

Assemblages (F, W, Mo)

Studies regarding the geochemistry of high-silica 

rhyolites and the late fluorine-tungsten-molybdenumbearing assemblage (see section, “Rhyolites and Associated 

Mineralization”) were conducted to investigate potential 

dispersion of the diagnostic LIL-elements into local surface 

waters. Preliminary study results show that waters anomalously enriched in fluoride (>2 mg/L), potassium (>1.5 mg/L), 

beryllium (>5 μg/L), and to a lesser extent molybdenum, 

correlate strongly with exposures of high-silica rhyolite 
intrusions and related late-stage vein assemblages containing 

fluorite, huebnerite, and elevated molybdenum (D.J. Bove and 

others, unpub. data, 2002). This association is particularly well 

illustrated in the California Gulch area, where inflows with 

high fluoride concentrations (2.8 to 22 mg/L) (Kimball and 

others, this volume, Chapter E9) closely bracket the extent of a 

small, QSP-altered rhyolite intrusion near California Mountain 

(fig. 20). Local occurrences of fluorite and huebnerite have 

also been documented in this vicinity (Ransome, 1901; Belser, 

1959), and related mine discharge also contains characteristically elevated concentrations of fluoride, beryllium, and 

potassium (Peter Butler, Robert Owens, and William Simon, 

Unpublished report to Colorado Water Quality Control 

Commission, Animas River Stakeholders Group, 2001).

Houghton Mt

California Mt

Kendall Mt

Silverton

Red Mtn 3

Anvil Mt

Ohio Pk

3,792 m

tailings pond #4

Mayflower

campground

springs

0 1 2 KILOMETERS

rhyolite

intrusion

MOGUL MINE

North Fork

(Gold King)

American 

tunnel

California

Gulch

ANGLO SAXON

MINE 

Spring

x MINNESOTA MINE

NORTH STAR

MINE 

rhyolite

intrusion

Mainstems

Inflows

2-5 mg/L

1-2 mg/L

0-1 mg/L

2-5 mg/L

1-2 mg/L

0-1 mg/L

5-25 mg/L

EXPLANATION FLUORIDE DATA

Hydrothermal Alteration Assemblages

Vein-related quartz-sericite-pyrite (V-QSP)

Weak sericite-pyrite (WSP)

Quartz-sericite-pyrite (QSP)

Acid-sulfate 

Regional propylitic 

Mineralized vein

107.797o

37.874o

107.747o 107.685o 107.584o 

37.935o

37.955o

Figure 20. Fluoride data from mainstem streams and miscellaneous inflows superimposed on generalized alteration 

map of study area. Fluoride data from Kimball and others (this volume, Chapter E9), D.J. Bove and K. Walton-Day (unpub. 

data, 2002), Unpub. report to Colorado Water Quality Control Commission, ARSG, 2001.

216 Environmental Effects of Historical Mining, Animas River Watershed, Colorado
Mineralization, Alteration, and Geochemical Signatures 217

Ten springs, seeps, mines, or smaller tributaries sampled 

outside of the California Gulch area also contain fluoride concentrations in excess of 2 mg/L (fig. 20). Potassium (>1 mg/L) 

and beryllium (>3 μg/L) also show corresponding enrichments 

in most of these waters (Unpublished report to Colorado 

Water Quality Control Commission, ARSG, 2001). Seven of 

these localities (American tunnel, Gold King via North Fork 

Cement Creek, Anglo Saxon mine (mine # 183) and nearby 

spring, small mine in the South Fork of Eureka Gulch, the 

Longfellow mine/Koehler tunnel (# 77 and # 75), and a spring 

along the Minnesota vein extension) (fig. 20) correlate with 

mines or springs influenced by vein systems containing latestage fluorite and huebnerite. However, the elevated fluoride 

concentrations in the remaining three waters are more difficult 

to explain. One of these waters, which was collected from 

the Mogul mine in 1999, had the highest fluoride concentrations in the entire study (21.8 mg/L) (all sample data based 

on post-1996 sampling) (Walton-Day and others, this volume, 

Chapter E24; Sole and others, this volume; D.J. Bove and others, unpub. data, 2002). The high fluoride concentration of the 

Mogul mine discharge contrasts sharply with the low fluoride 

concentrations at the nearby Queen Anne (mine # 34) and 

Grand Mogul mines (mine # 35) (<1 mg/L fluoride), which 

exploited similarly mineralized and parallel vein structures. 

Furthermore, fluorite has not been reported as a vein constituent at the Mogul mine nor has it been observed on the associated waste dumps (Ransome, 1901; Burbank and Luedke, 

1969; T. Hennis, private mine owner, oral commun., 2001). 

However, dramatic increases in fluoride concentration in upper 

Cement Creek (above the North Fork, fig. 20) between 1996 

and 1999 (0.6 to 2.4 mg/L), suggest that fluoride concentrations at the Mogul mine may have been much lower prior to 

1996 (Briant Kimball, written commun., 2001). The extrapolated increase in fluoride concentration at the Mogul mine over 

this 3-year time interval was accompanied by large increases 

in discharge as well as manganese, sulfate, and metal loads 

(Walton-Day and others, this volume). Studies by Walton-Day 

and others (this volume) suggest that these changes may be 

related to the 1996 plugging of the American tunnel (fig. 20), 

which caused the rise of its mine pool and resulted in the 

influx of this water into the Mogul mine workings. The 

extrapolated four-fold increase in fluoride concentration in 

the Mogul mine discharge between 1996 and 1999 is consistent with the scenario postulated by Walton-Day and others 

(this volume), as it reflects the uniquely high fluoride concentration of the American tunnel mine water (3 to 18 mg/L 

fluoride; 9 to 85 μg/L beryllium; 1 to 1.3 mg/L potassium; 

30 to 90 mg/L manganese; database in Sole and others, this 

volume).

Two metal-rich seeps (2,430 and 8,310 μg/L zinc; 50 

and 80 mg/L manganese) near the south Silverton campground 

(fig. 20) also have high fluoride (3.9 and 4.6 mg/L), potassium 

(3.9 to 5.2 mg/L), and elevated beryllium (1.3 and 2 μg/L) 

concentrations. The 45,000 m2

 Mayflower tailings pond #4 

(site # 510, Church, Mast, and others, this volume), located 

less than 400 m upgradient, represents a potential source for 

these high-fluoride and metal-rich seeps. Historical records 

indicate that this tailings pond, which was erected in 1976, is 

composed entirely of Sunnyside mine waste (William Jones, 

oral commun., 2002). Considering the characteristically high 

fluoride, beryllium, potassium, and manganese concentrations 

of waters interacting with Sunnyside vein material (such as the 

American tunnel mine discharge), tailings pond #4 is likely 

to be the primary metal source for these seeps. Although the 

nearby Aspen mine (fig. 20) contains some fluorite gangue 

material (Ransome, 1901), the associated mine waters contain 

<1 mg/L fluoride, no detectable beryllium or potassium, and 

<15 μg/L manganese (Unpublished report to Colorado Water 

Quality Control Commission, ARSG, 2001). Thus it is apparent that fluoride in combination with beryllium, potassium, 

and perhaps molybdenum can be utilized as natural occurring 

pathfinder elements for correlating water chemistry to key 

geologic features and mineralized source areas in the Animas 

River watershed.

<18 Ma Polymetallic Vein System—

Northwest- and Northerly-Trending 

Veins

South Silverton Area

The South Silverton area (figs. 1 and 21) includes the 

alpine ridges and the deep glacial cirques and valleys southeast of Silverton. Ore deposits are principally within simple 

fissure-filling veins that average 1–5 m wide, but are as much 

as 10 m wide (Ransome, 1901; Varnes, 1963). With exception 

of the gold-rich Shenandoah-Dives mine, ores were predominantly low grade and base metal rich, with low gold:silver 

ratios (Bartos, 1993; Ransome, 1901; Varnes, 1963). Areas 

of hydrothermally altered rock are largely confined to narrow 

zones surrounding the veins and some broad but still localized 

areas related to 26 Ma quartz monzonite intrusions. More than 

90 volume percent of South Silverton area is characterized by 

regional propylitically altered rock. These regionally altered 

rocks are mostly of the CEC subgroup—as described in detail 

in the section on regional propylitic alteration—and contain 

abundant epidote, chlorite, and calcite.

Veins were localized along two principal fracture 

systems: the first group, or the “western shear system” 

(fig. 21; west of Cunningham Creek), includes mineralized 

northwest-trending fractures, a complementary or conjugate 

set of weakly to unmineralized northeast-trending shear 

fractures, and a very productive set of northerly-trending tensional fractures. The second major group of fractures, or the 

“eastern shear system,” is characterized by a group of mineralized northwest-trending veins in the eastern part of the South 

Silverton area (fig. 21; east of Cunningham Creek).
Figure 21. South Silverton mineralized area. Western shear zone is west of Cunningham Creek; eastern shear zone comprises Cunningham Creek and area to the east. 

Modified from Varnes (1963).

2 3 4 5

R I N G

-

F A U L T Z O N E

Mineral

Creek

m

i

nA

s

a

River Cunningh

a

m

C

re

e

k

37o51'

37o49'

37o47'

107o40' 107o36'

EXPLANATION

Intrusive rocks Sedimentary rocks

Volcanic rocks Precambrian rocks

0 1 2 MILES

Fault, vein, or dike—Dashed where approximately located

S T O C K

2

4

1

3

5

Silverton

Howardsville

Highland

Mary

Vein

Titusville Vein

Scranton City Vein

Silver Lake Vein

Melville Vein

Royal Tiger Vein

North Star Vein

Big Giant Vein Shenandoah

Dives

Vein

Syste Dmives Bas ni

1 New York Vein Iowa Vein Black Diamond Vein Black-Prince-Gold Lake Vein North Star Mine

Mayflower dike

Arrastra Dike

Magnolia Dike Kendall

Peak #2 Kendall

Peak #3

Little

Giant Peak

Kendall Gu cl h

Arrastr a Creek Sw ansea Gulc h Bl air Gulch

LITTLE

NATION

MINE

OSCEOLA

MINE

PRIDE OF THE

WEST MINE

GREEN

MOUNTAIN

MINE

HIGHLAND

MARY MINE

ROYAL

TIGER MINE

KINGSOLOMANMINE

ASPEN

MINE

SCRANTON CITY MINE

TITUSVILLE MINE

MELVILLEMINE

SHENANDOAH

DIVES MINE

218 Environmental Effects of Historical Mining, Animas River Watershed, Colorado
Mineralization, Alteration, and Geochemical Signatures 219

Western Shear System

The northwest-trending shear fractures of the western shear system host some of the most prominent veins or 

dike-filled veins in the South Silverton district, including 

the Shenandoah-Dives, Titusville, Silver Lake–Royal Tiger, 

Aspen, Big Giant, and the Magnolia dike (fig. 21). This fracture system appears to be the southern continuation of a similarly mineralized, northwest fracture zone that is prominent 

west of the Red Mountains area (Yager and Bove, this volume, 

pl. 1). Earlier studies postulated that the northwest fracture 

zone west of the Red Mountains was related to emplacement 

of the Stoney Mountain stock (Lipman and others, 1976). 

However, another possibility, as suggested by regional fracture 

patterns, is that these northwest fracture systems are part of an 

overall regional structural trend. The associated fractures in 

the South Silverton area have undergone considerable rightlateral and normal vertical displacement and generally dip to 

the northeast at moderate angles. Early reports state that ore 

minerals within these veins consisted chiefly of galena accompanied by sphalerite, chalcopyrite, pyrite, and minor tetrahedrite (Ransome, 1901). Gangue minerals were mostly quartz 

with calcite, barite, and local fluorite and gypsum. Silver values were said to be significantly low and considered relatively 

unimportant compared to that of lead (Ransome, 1901). The 

occurrence of free-gold with late quartz and (or) sphalerite and 

chalcopyrite was observed at that time only in the Royal Tiger 

and North Star veins, respectively.

Later production records, however, indicate that the South 

Silverton area was one of the principal gold producers in the 

western San Juan Mountains. The bulk of this later production was from the Shenandoah-Dives mine, which recovered 

roughly 4.5 million tons of ore with high gold:silver ratios 

rivaling that of Sunnyside mine in the Eureka Graben area 

(Bartos, 1993). The N. 40°–50° W.-trending ShenandoahDives vein system—one of the major faults of the western 

shear system—can be traced from Arrastra Creek southeastward over the high ridges and cirque basins for a distance 

of about 3,700 m to its terminus marked by the Highland 

Mary mine (fig. 21). Partly along this interval the vein follows along the andesitic Mayflower dike. The veins of the 

Aspen mine (fig. 21) may mark the northwest extension of the 

Shenandoah-Dives vein system (Varnes, 1963); however, talus 

cover in lower Arrastra Creek obscures this possible connection. Individual holdings along this vein complex, including 

the Mayflower mine in Arrastra Creek, the North Star mine on 

Little Giant Peak, and the Dives mine in Dives Basin (Varnes, 

1963, pl. 2), were consolidated into the Shenandoah-Dives 

Mining Co. after 1926 (Varnes, 1963). Descriptions from these 

individual mines by Ransome (1901) suggest that ores were 

zoned from galena to silver-bearing tetrahedrite and gold-rich 

chalcopyrite with depth. Anglesite was abundant in the highest 

levels of the Shenandoah-Dives mine and represents oxidation 

at very high vein levels (Varnes, 1963).

The North Star mine and Empire group on Sultan 

Mountain are along northwest-trending veins that are likely 

part of the northwest-trending shear set. The mineralogy of 

these veins is similar and comprises galena, sphalerite, tetrahedrite, chalcopyrite, and pyrite with a gangue of quartz and 

some barite (Ransome, 1901). Late fluorite and huebnerite 

have also been reported in the North Star and Empire lodes.

The north-trending tensional fractures were the last of 

the “western” shear group to form, and they probably resulted 

as a consequence of continued shearing along the northwesttrending group (Burbank, 1933). As a group, these fractures 

strike N. 10°–30° W. and trend diagonally across the long 

narrow blocks bounded by the northwest-oriented shear fractures; dips are much steeper than those of the northwest veins, 

averaging about 80°. As the veins that filled the north-trending 

fractures approach the northwestern lodes, they tend to deflect 

and show a significant decrease in ore metals. Generally, 

the northerly veins were some of the most productive of the 

district, and include the Melville, New York, Iowa, Royal, 

and Black Diamond (fig. 21). The associated orebodies were 

reported to be more regular and of higher grade than many of 

the northwest-trending veins. Like the northwest veins, galena 

was also the dominant ore mineral; however, chalcopyrite and 

pyrite were typically more abundant in the northerly-trending 

veins and gold values were generally higher, running several 

ounces/ton (Ransome, 1901; Varnes, 1963).

Eastern Shear System

The veins of the “eastern shear system” (Varnes, 1963) 

are mostly located within or east of the Cunningham Creek 

area (fig. 21). These veins have filled northwest-trending shear 

fractures and a set of “radial” fractures that trend normal to 

the margins of the Silverton and San Juan calderas. The radial 

fractures are thought to have formed during resurgence of the 

San Juan caldera (28.0 Ma) prior to collapse of the Silverton 

caldera (27.5 Ma) (Hardwick, 1984); the associated fractures 

in this group show no lateral displacement. Also included in 

the eastern shear system are a number of weakly mineralized, 

arcuate granite porphyry dikes and related northerly-trending 

fractures and veins.

The Green Mountain and Little Nation mines (fig. 21) 

are characteristic of the northwest-trending veins of the 

eastern shear system. The Green Mountain vein, which in 

part is hosted in Precambrian schist, trends N. 40°–45° W. 

with dips near vertical. Total production (to 1948) has been 

estimated at 35,000 tons averaging 0.03 oz gold/t, 3 oz 

silver/t, 0.2 percent copper, 4 percent lead, and 3 percent zinc 

(Hardwick, 1984). The ore minerals consisted of abundant 

sphalerite and galena, with some chalcopyrite and pyrite, in 

a quartz gangue (Ransome, 1901). The Little Nation mine, 

located about 1,000 m southwest of Howardsville (fig. 21), 

is one of the few mines situated near the ring-fault zone of the 

Silverton caldera. The workings followed several discontinuous veins that trend about N. 50° W. and were characterized 

by lead-silver ores.

The Pride of the West, Osceola (fig. 21), and Little Fanny 

mines worked a system of parallel vein structures along one 

of the “radial” fracture systems. The associated veins are 
located outside the structural margin of the Silverton caldera, 

and just inside of the earlier formed San Juan caldera (Yager 

and Bove, this volume, pl. 1). The ore deposits formed near 

the intersection of the radial fracture system with the buried 

structural margin of the San Juan caldera. Production of more 

than $10 million worth of base and precious metals has been 

recorded from these mines (Hardwick, 1984). Weighted average grades from this production were 0.08 oz gold/t, 5.3 oz 

silver/t, 0.3 percent copper, 6.8 percent lead, and 2.1 percent 

zinc (as calculated from table 1 in Hardwick, 1984). The 

main metallic minerals in order of approximate abundance are 

galena, sphalerite, pyrite, chalcopyrite, and minor magnetite 

and hematite. Native gold was reported locally at the Pride of 

the West and Osceola mines, and native silver and tetrahedrite 

were observed at the Pride of the West and Little Fanny mines 

(Hardwick, 1984). The gangue mineral assemblage consists 

primarily of quartz with lesser pyroxmangite, calcite, and 

barite. Calcite is also present as late-stage vein-fillings that 

crosscut main stage ore and gangue minerals and as cavityfillings (Hardwick, 1984). While all three mines contain 

typical fissure-filling vein deposits, the Pride of the West and 

Osceola mines contain both vein and carbonate replacement 

deposits. The replacement deposits are present exclusively 

where large megabreccia blocks of Paleozoic-age carbonate 

rocks (collapse from the topographic margin of the San Juan 

caldera) are intersected by metal-bearing veins. The replacement ore assemblage is very similar in bulk composition to the 

vein ores and varies from a massive variety to “zebra” textures 

that consist of metallic bands alternating with quartz gangue 

(Hardwick, 1984).

Vein-Related Hydrothermal Alteration

Only narrow zones of alteration surround the veins in 

the South Silverton area, with alteration envelopes characteristically three to five times the vein width (Ransome, 1901; 

Hardwick, 1984). A detailed alteration study was conducted 

by Ransome (1901) in Silver Lake Basin. His study showed 

that rocks greater than 15 m away from the vein were only 

altered to the regional propylitic assemblage and were not 

affected by the vein hydrothermal solutions. At 15 m from the 

vein, the rock in hand sample appears similar to the propylitically altered samples, but under the microscope, feldspars 

have been changed to an aggregate of calcite and sericite, 

while augite and biotite have been altered to chlorite, calcite, 

and some leucoxene. The groundmass of these rocks is altered 

to a mixture of quartz, chlorite, sericite, and minor leucoxene. 

At about 0.6 m away from the vein, minor disseminations of 

galena are noted megascopically, and the rock is mostly converted to a mixture of quartz, sericite, and pyrite (Ransome, 

1901). As observed in thin section, pyroxene and plagioclase 

in these rocks are altered to varying percentages of quartz, 

sericite, calcite, and chlorite, whereas biotites are mostly 

altered to sericite, with some chlorite, calcite, and leucoxene 

(Ransome, 1901). The groundmass is completely altered to 

a fine mixture of dominantly quartz and sericite, with lesser 

chlorite. At or near the vein, the rock becomes lighter in color, 

contains fine pyrite and minor galena, and is cut by numerous thin quartz veinlets. As observed petrographically, both 

phenocrysts and groundmass have been completely altered to 

quartz and sericite; calcite and chlorite are completely absent 

(Ransome, 1901).

Studies in the Cunningham Creek area document similar 

alteration envelopes around the veins that consist of quartz, 

sericite, pyrite, and (or) kaolinite (Hardwick, 1984). Within 

this halo, sericite has selectively altered feldspar phenocrysts 

along grain boundaries and microfractures and replaced the 

fine-grained groundmass of the volcanic wallrocks. The intensity of alteration increases inward toward the vein, and the 

wallrock adjacent the vein is almost completely silicified.

Geochemistry of Altered Rocks

Rock sample data from the South Silverton area is generally scant. Because these data are few and mostly represent 

fresh to propylitically altered rocks, they were not included in 

this study.

Mine Dump Compositions

Analytical data summarizing the geochemistry of eight 

mine dumps in the South Silverton area are presented in 

table 4. A comparison of these data to similar analyses from 

the four other mineralized areas divided in this report is shown 

in figure 7. Manganese abundance in the South Silverton samples is relatively high compared to the other four mineralized 

areas (fig. 7), with a mean of 3,440 ppm. As shown in figure 7, 

the range of these data overlaps with samples from the Eureka 

Graben area, which is noted for veins with high proportions 

of manganese-silicate gangue material. However, the South 

Silverton data are strongly influenced by two dump samples 

from mines along the Titusville vein (fig. 21) on the south side 

of the district, which contains 12,000 and 21,000 ppm manganese. Material from these two dumps is heavily manganese 

stained due to the presence of rhodochrosite, which is especially abundant within this particular vein system (Ransome, 

1901; Varnes, 1963). Other than the Titusville structure, 

only a few veins in the South Silverton area contain notable 

quantities of manganese silicate or manganese carbonate 

gangue material (Ransome, 1901; Hardwick, 1984). These 

veins, which are mostly located in the Cunningham Creek 

area (eastern shear system, fig. 21), are reported to contain 

pyroxmangite and manganese-rich calcite gangue. Several of 

the veins of the western shear system (fig. 21) also contain 

late-stage, sparry calcite that carries as much as 11,000 ppm 

manganese. Mean lead and zinc abundances within the South 

Silverton samples are 6,390 and 2,450 ppm, respectively. 

As shown in figure 7, the range of lead abundance generally overlaps with that of samples from the Red Mountains, 

OPAM, and Eureka Graben areas but is notably higher than 

in the Mount Moly area mine samples. Galena is one of the 

dominant base metals in many of the South Silverton veins. 

220 Environmental Effects of Historical Mining, Animas River Watershed, Colorado
Mineralization, Alteration, and Geochemical Signatures 221

The range and mean abundance of zinc are generally similar 

to those of the other four areas, with exception of the Red 

Mountain area dumps, which contain less than half the mean 

zinc abundance of the South Silverton area dumps. Copper, 

with a mean of 1,510 ppm, is similar in abundance to the Red 

Mountains dumps (1,930 ppm), and is substantially higher 

than in the other three mineralized areas (fig. 7). However, 

mean arsenic abundance (49 ppm), which is the lowest of the 

four areas, is nearly 20 times less than in the Red Mountains 

dumps (940 ppm). These data reflect the relative abundance 

of chalcopyrite in many of these veins along with some 

tetrahedrite-tennantite, as well (Ransome, 1901; Varnes, 1963; 

Hardwick, 1984). However, the South Silverton ores, unlike 

those of the Red Mountains area, lack enargite and other similar ore minerals.

The South Silverton mine dumps have the highest 

calcium:strontium ratios of the five mineralized areas (mean 

of 70). Similarly high ratios (mean of 130) are commonly 

observed either in mine dumps with notable quantities of 

calcite gangue or with vein material derived from mines 

hosted in Paleozoic limestones (Fey and others, 2000). In 

contrast, dumps that contain gypsum gangue minerals have 

substantially lower calcium:strontium ratios (mean of 4). 

These data suggest that calcite is the principal calcium-bearing 

gangue mineral on these dumps. This hypothesis is supported 

by preliminary geochemical studies on calcite, gypsum, and 

anhydrite from the study area (table 2) that show similarly 

contrasting ratios for these carbonate and sulfate minerals. 

Field studies (Ransome, 1901; Varnes, 1963; Hardwick, 1984; 

Fey and others, 2000) and AVIRIS mapping (Dalton and 

others, this volume) have also documented that gangue and 

wallrock-related calcite is relatively abundant within the South 

Silverton area.

Mine Water Chemistry

Mine discharge in the South Silverton area (table 5; 

fig. 9) was characteristically high in pH (range of 3.2 to 8.30; 

6.75 mean), with measurements below 4.5 recorded at only 

1 of the 12 sample sites. Eight of the twelve mine waters 

had measurable alkalinity, ranging from 5 to 73 mg/L. Most 

waters had relatively low aluminum (95 μg/L mean), iron 

(26 μg/L mean), and sulfate (47 mg/L mean) concentrations 

(table 5), which probably reflects the low solubility of these 

elements at these higher pH values. Manganese concentration, 

which ranges from 3 to 5,420 μg/L, is also typically low, as 

evidenced by a mean of 63 μg/L. However, these data do not 

include drainage from mines along the Titusville vein, one 

of the few structures in the area with manganese-rich gangue 

material (Nash and Fey, this volume, Chapter E6). The South 

Silverton mine waters have distinctively elevated base-metal 

concentrations when compared to higher pH waters from the 

other four mineralized areas (fig. 9). Zinc concentrations are 

highly variable, ranging from 12 to 11,300 μg/L; their mean 

is 313 μg/L. Although dissolved lead (<5 μg/L) and copper 

concentrations (<45 μg/L) were low in 9 of the 12 mine water 

samples, they were highly elevated in discharge samples 

from the Royal Tiger and Iowa mines (fig. 21), located in 

upper Arrastra Basin (lead 393 to 5,610 μg/L; copper 251 to 

1,390 μg/L). Note that both these mines were dominated by 

silver-lead ores, and together accounted for 20–25 percent of 

the total lead production in the South Silverton mining area 

(Varnes, 1963). The high elevation of these mines suggests 

that they exploited oxidized ores such as anglesite in their 

upper levels (Ransome, 1901; Varnes, 1963). Lead and copperbearing sulfates and salts, which were probably more prevalent 

in the oxidized portion of these veins, have higher kinetic rates 

of dissolution than their lead and copper sulfide counterparts 

(White and Brantley, 1995). However, other geochemical 

parameters, such as pH, dissolved sulfate and iron concentrations (Smith, 1999; Nordstrom and Alpers, 1999), and anion 

complexing, could also control the concentration of lead and 

copper dissolved within these mine waters. (See following section, “Discussion and Summary.”)

The lack of low-pH mine waters in the South Silverton 

area readily distinguishes this area from the other four areas 

subdivided in this report. As just discussed, the higher pH 

probably limits the dissolved concentration of aluminum, 

iron, sulfate, and metals in the majority of these mine waters. 

The low amount of acidity in these waters is probably due to 

neutralization by calcite, which is abundant in wallrock and 

vein-related gangue material. Greater than 90 volume percent 

of the rocks within this mineralized area have been affected by 

calcite-rich propylitic alteration, whereas pervasive zones of 

pyritic-bearing rock are relatively minor. Paleozoic limestones, 

which crop out on the southern margins of the South Silverton 

area (fig. 21; Yager and Bove, this volume, pl. 1), are a likely 

source for some of the carbonate that has been redistributed 

into propylitically altered rocks and many of the veins. Despite 

the high metal concentrations of some of these mine waters, 

the mainstem streams draining the South Silverton area are 

relatively low in dissolved metal concentrations in their lower 

reaches (pH 7.51 to 8.1; zinc 10 to 300 μg/L ) (Unpublished 

Report to the Colorado Water Quality Control Commission, 

ARSG, 2001).

Background Surface Water Chemistry

End-member background waters from the South Silverton 

area are limited to five samples from the propylitic alteration 

group (table 8; fig. 11). Fortunately, however, these waters represent an alteration assemblage that covers the vast majority of 

this mineralized area (≈92 percent). The five propylitic-related 

waters have mean pH and alkalinities of 8.15 and 52.7 mg/L, 

respectively. These measurements are substantially higher 

than from propylitic samples in the other four mineralized 

areas, which may be related to near-proximity to pre-Tertiary 

limestones and an abundance of propylitic-related calcite. 

Other than these parameters, the South Silverton propylitic 

waters are generally similar to low-strontium (non-gypsum 

or anhydrite-related) propylitic waters from the other four 

mineralized areas.
Discussion and Summary

The Animas River watershed study area and vicinity 

were divided into five different areas (fig. 1) based on age 

and type of mineralization, associated styles of hydrothermal 

alteration, and structural setting. The major mineralized areas 

include (1) Mount Moly area, site of a 26 Ma Mo-Cu porphyry 

system (19.5 km2

); (2) Red Mountains, 23 Ma acid-sulfate 

system (22.4 km2

); (3) Ohio Peak–Anvil Mountain, 23 Ma 

acid-sulfate area (20.6 km2); (4) Eureka Graben area, <18 Ma 

polymetallic vein mineralization (44.4 km2

); and (5) South 

Silverton, <18 Ma polymetallic veins (72.3 km2). Combined 

geologic and aqueous geochemical studies within these five 

areas have led to a better understanding of the major sources 

of anthropogenic and natural acidity and metals in the watershed. Detailed mapping has shown how the degree of bedrock 

alteration controls the variability of water compositions within 

each of these source areas. Aqueous geochemical signatures 

can be quite distinctive in some of these areas and can generally be explained by the varying geologic and mineralogic 

characteristics. Some of these more diagnostic geochemical 

signatures have been useful in identification of the origin of 

unidentified metal-bearing discharge distal to these sources.

Mount Moly Mineralization and Hydrothermal 

Alteration

The Mount Moly area is characterized by subeconomic 

molybdenum-copper mineralization related to late-phase, 

quartz monzonite intrusions of the Sultan Mountain stock. 

Zones of oxidation extend to depths of about 100 m below 

the ground surface, beneath which secondary copper was 

enriched. Relatively narrow, silver-bearing base-metal veins 

are present mostly on the periphery of the hydrothermal 

system. Hydrothermal alteration assemblages change outward from a central QSP zone into a weak sericite-pyrite 

(WSP) zone, and finally into regional propylitic-altered rock. 

As we have shown, waters draining the inner QSP alteration zones are strongly acidic due to the weathering of finely 

disseminated pyrite, and they have high dissolved sulfate, 

aluminum, and iron concentrations. Because disseminated 

pyrite in the Mount Moly area is deficient in trace metals, 

most of the dissolved metals in these waters (copper 45 μg/L 

mean; zinc 247 μg/L mean; table 8) must have been derived 

from other sulfide minerals in surface and subsurface altered 

zones (mean of 374 ppm copper, 28 ppm lead, and 109 ppm 

zinc in unoxidized drill core; table 3), and from associated 

base-metal veins (mean of 192 ppm copper, 1,580 ppm lead, 

and 1,860 ppm zinc in mine dumps; table 4). Although mean 

copper abundance is 374 ppm in rocks of the deeper zone of 

secondary enrichment (>100 m below the ground surface), 

values are greatly diminished in the overlying QSP oxidized 

rocks (15 ppm mean). Thus, the low dissolved copper concentrations in most QSP waters (45 μg/L mean) can be explained 

by either (1) shallow weathering and minimal interaction with 

the deeper copper-enriched zone, or (2) mixed contributions 

from the upper copper-deficient and the lower copper-enriched 

zones. The Mount Moly mineralized area is the only one 

of the five where background waters with elevated copper 

concentrations (>11 to 200 μg/L) are devoid of dissolved 

zinc (<10 μg/L). In addition, a subset of these waters has 

molybdenum concentrations exceeding 25 μg/L. This small 

population of background waters with high copper:zinc ratios 

may be the best representation of weathering within the deeper 

zone of secondary enriched copper. The lack of zinc in these 

copper-bearing waters is consistent with the relatively high 

copper:zinc ratios in rocks in the mineralized core of this 

porphyry system. Thus, the moderately elevated mean zinc 

concentration (247 μg/L) as calculated for the overall QSP 

water data set (table 8) may primarily represent zinc derived 

from polymetallic veins, which are concentrated toward the 

perimeter of the porphyry system. This is consistent with 

rock geochemical dispersion patterns (zinc, manganese, and 

strontium enrichment on the periphery of the hydrothermal 

system; Ringrose, 1982) and increases of zinc and manganese 

concentrations in background waters towards the perimeter of 

the hydrothermal system (Mast and others, this volume).

The mean concentrations of the metals lead (21 μg/L), zinc 

(505 μg/L), and copper (148 μg/L) in Mount Moly area low-pH 

mine waters (table 5) are among the lowest abundances in lowpH mine discharge from the five mineralized areas (fig. 10). 

Arsenic (<1 μg/L) is also very low in these waters, whereas 

manganese is only moderately elevated (1,600 μg/L). Compared 

even to these acidic mine waters (pH <4.5), QSP background 

waters (table 8) contain more than 2.5 times more aluminum 

and iron, whereas concentration ranges in zinc, copper, and 

sulfate are generally similar. Mean sulfate and strontium concentrations in QSP waters are 333 and 181 μg/L, respectively, 

which are roughly equivalent to low-pH mine discharge.

Red Mountains and OPAM Acid-Sulfate Areas

Two large centers of acid-sulfate-altered and mineralized 

rock in the Red Mountains and OPAM areas (fig. 1) are related 

to high-level dacite intrusions emplaced at ≈23 Ma. Geologic 

and stable isotope studies indicate that alunite and pyrite within 

these systems formed contemporaneously from vapor-dominant, 

magmatic hydrothermal fluids with high H2

S:SO4 ratios. The 

abundance and wide vertical range in which pyrite forms in 

these types of hydrothermal systems are important owing to 

the exposure of pyrite to weathering processes, regardless of 

erosion depth. The abundance of silver-bearing, copper-arsenic 

ores containing enargite, tetrahedrite-tennantite, as well as copper ores of chalcocite, bornite, and covellite, distinguishes the 

ores of the Red Mountains area from more typical polymetallic vein deposits within and outside the Red Mountains area. 

These distinctive ores are typically hosted in brecciated and 

highly silicified zones that resulted from processes related 

to the vapor-dominant, high-sulfidation fluids. Despite the 

similarity between the Red Mountains and OPAM acid-sulfate 

hydrothermal systems, silver- and copper-arsenic mineralization 

was apparently scarce in the OPAM area, as noted by the very 

limited production and small number of mines.

222 Environmental Effects of Historical Mining, Animas River Watershed, Colorado
Mineralization, Alteration, and Geochemical Signatures 223

The acid-sulfate assemblages in the Red Mountains 

and OPAM areas are very deeply rooted (>300 m beneath the 

surface) and grade from quartz-alunite outward into quartzpyrophyllite, dickite, and further into smectite, and (or) propylitically altered rock. Pervasive QSP-altered rock is commonly 

exposed up to the tops of many high ridges, as observed in the 

Red Mountains area, and grades laterally into a weak sericitepyrite (WSP) alteration assemblage.

Background waters in the Red Mountains and OPAM areas 

have a wide range of chemical compositions that correspond 

to the degree of hydrothermally altered rock that they drain. 

Whereas pH values in waters draining AS and QSP altered 

rocks are similarly low (mean pH=3.54), mean calcium, magnesium, and manganese concentrations are at least four times less 

in AS versus QSP waters. These data reflect the near absence of 

acid-neutralizing mineral phases within the acid-sulfate altered 

rock due to highly effective dispersion of the vapor-dominant 

hydrothermal fluids; in contrast, remnants of more weakly 

altered rock remain in the QSP alteration zones, probably due 

to less efficient circulation by lower temperature hydrothermal 

waters. Although dissolved aluminum (2,060 μg/L mean), iron 

(852 μg/L mean), and sulfate (53 mg/L mean) reach relatively 

high concentrations in AS and QSP waters in these two areas, 

mean copper (14 μg/L mean) and zinc (67 μg/L mean) concentrations are low to moderately elevated relative to other 

background waters from combined Red Mountains and OPAM 

data sets.

Much of the acidity and high loadings of sulfate, aluminum, and iron in a well-studied drainage typical of the entire 

Red Mountains area (Prospect Gulch; see fig. 15) is derived 

from weathering of the pervasive AS and QSP alteration zones 

(Wirt and others, 2001). In contrast, the majority of zinc and 

copper loading is mostly attributable to mining-related 

sources. Low zinc concentrations in background waters in 

the Red Mountains area reflect the very low zinc abundance 

within the large expanse of altered rock in this area; in fact, the 

highest zinc values are associated with relatively unmineralized, propylitic rock (110 ppm mean). Only two background 

waters in the Red Mountains and OPAM areas—both spring 

sites—have zinc concentrations above 450 μg/L. Studies by 

Bove and others (2000) suggest that one of these springs 

(site CC129; database in Sole and others, this volume), which 

discharges near the mouth of Prospect Gulch (1,072 μg/L 

zinc) (fig. 15), may represent interaction of QSP waters 

with unmined, vein-related material. The other spring site 

(CC128), which is located in the upper reaches of Topeka 

Gulch area (998 μg/L zinc) (fig. 14), is more poorly understood. However, low abundances of zinc in rocks or dissolved 

in other background waters within this drainage suggest that 

zinc in this spring water may also have been largely derived 

from vein-related material. The low but detectable dissolved 

copper concentrations observed in background waters of the 

Red Mountains area may have been derived from acid-sulfate 

altered rock, which has the highest copper abundances of all 

alteration assemblages in this area (19 to 859 ppm, with a 

mean of 79 ppm). Drill core studies indicate that elevated copper abundances within these acid-sulfate zones are related to a 

trace to as much as several percent disseminated covellite and 

enargite, and rare mineralized veins or breccias with massive 

pyrite, enargite, and minor galena. The relatively low copper 

concentrations in these background waters may reflect weathering and oxidation to depths extending to 100 m beneath the 

ground surface, which left the rocks bleached and depleted of 

copper (fig. 6). Mass balance studies from the Prospect Gulch 

area (Bove and others, 2000) indicate that essentially no dissolved metals within these background waters were derived 

from the weathering of disseminated pyrite.

Compared to the other four mineralized areas, mine 

discharge (table 5) samples from the Red Mountains area 

show pronounced enrichments in copper (2,340 μg/L mean) 

and arsenic (103 μg/L mean) in association with highly 

elevated lead (177 μg/L mean) and zinc (10,600 μg/L mean). 

The recognition of these distinct geochemical signatures in 

seeps down gradient of mines in the upper Prospect Gulch 

area (copper >4,900 μg/L, arsenic >51 μg/L; zinc >9,770 μg/L; 

W.G. Wright, unpub. data, 2002) has helped to discriminate 

these waters from those dominated by non-mining-related 

source contributions, which are nearly devoid of dissolved 

arsenic (Bove and Knepper, 2000; Neubert, 2001) and have 

low copper and low to moderate zinc concentrations compared 

to mine discharge samples (table 6).

Although copper, lead, and zinc concentrations in 

OPAM mine discharge samples are moderately high compared 

to low-pH samples from the other mineralized areas, arsenic 

abundance is low. These data combined with the relatively 

low levels of arsenic within OPAM mine dumps confirm 

geologic reports citing the absence of enargite in all but a few 

mines in this area. Mine discharge samples from the OPAM 

area and those on the periphery of the Red Mountain system 

have appreciably higher manganese, strontium, and other base 

cation abundances than mine discharge samples from the core 

of the Red Mountains altered area. In OPAM samples and 

those peripheral to the Red Mountains area, this is most likely 

related to availability of calcite, plagioclase, and chlorite due 

to near proximity to large volumes of propylitically altered 

rock. In contrast, most mine discharge samples collected 

toward the interior of the Red Mountains area drain mineralized workings within acid-sulfate-dominant rock characterized by low manganese and other base-element abundances. 

Alternatively, anomalously high manganese concentration in 

OPAM mine water samples could be related to dissolution of 

huebnerite (MnWO4

), which is common in many mines in the 

corridor around Cement Creek.

Stable isotope studies focusing on sources of dissolved 

aqueous sulfate have perhaps proved most useful in the Red 

Mountains area. Results show that the dissolved sulfate in 

mining and background waters may be largely derived from 

sulfide mineral phases. The light δ34S values of these waters 

are indicative of no to little interaction with isotopically 

heavier gypsum or anhydrite, which are present only beneath 

the upper deeply rooted, acid-sulfate alteration zones. These 

data combined with preliminary tritium studies (Bove and 

others, 2000; Wirt and others, 2001) suggest that these waters 

mostly represent recent, shallow-level recharge.
Eureka Graben Area

The Eureka Graben area (fig. 1) is characterized by 

polymetallic veins that formed along structures related to 

resurgence of the San Juan–Uncompahgre calderas. Many of 

the most economically important vein deposits within the study 

area are present here, such as the Sunnyside mine, historically one of the largest gold mines in the United States, having 

produced more than 800,000 ounces of gold and 14 million 

ounces of silver. Typical ores of the Sunnyside and related 

veins also contained as much as 2–3 percent lead and zinc, 

and as much as 0.3 percent copper. Similar veins in the Eureka 

Gulch, Treasure Mountain, and Placer Gulch areas, like the 

Sunnyside, were highly endowed in late-stage manganese ores, 

with weighted averages of 6 percent manganese over a 3-m 

wide vein. As shown in figure 19, manganese concentrations 

are highest near the Sunnyside-Mastodon vein systems and 

diminish variably in all directions away from this center.

As much as 75 percent of the rocks within the Eureka 

Graben area is propylitically altered. In contrast, QSP and 

WSP-altered areas make up only about 20 percent of the area. 

Vein-related QSP zones, which extend at most to two times the 

vein width, approximate nearly 5 percent of the area. The low 

pyrite abundance and high acid-neutralizing potential of these 

propylitic-dominant rocks explains the distribution of mostly 

higher pH in streams draining this area (Wright, Simon, and 

others, this volume, Chapter E10, fig. 10). However, the 

influence of mineralized veins on these propylitic waters is 

observed in a number of stream and spring samples. Veinrelated background waters draining propylitic-dominant rocks 

(PROP-V, fig. 11) have a mean pH of 6.52 and measurable 

alkalinity but show markedly elevated zinc and a wide range 

in manganese concentrations. Water-quality data (table 8, this 

report; Kimball and others, this volume) suggest that Eureka 

graben background waters interacting with QSP-altered rock 

and large mineralized veins along their flow path produce lowpH and highly metal rich waters. Three such springs (V-QSP, 

fig. 11) discharging near the base of large QSP-altered zones 

in California Gulch, Houghton Mountain, and South Fork 

Eureka Gulch (fig. 17) have pH values <4.55 and concentrations of greater than 640 μg/L zinc (14,400 μg/L maximum) 

and 2,670 μg/L manganese (74,700 μg/L maximum) (table 8).

One of the more important questions regarding derivation 

of these low-pH and metal-rich waters is the initial water composition prior to reaction along their flow path with unexploited 

vein minerals. Because no water draining QSP-dominant rock 

was sampled in this area, we can only speculate that initial compositions may be analogous to QSP waters in the Mount Moly, 

Red Mountains, and OPAM areas (pH from 3.2 to 4.0 and zinc 

ranging from 100 to 300 μg/L) (table 5). Although these initial 

QSP waters were probably low in metals, their high acidity 

undoubtedly played an important role in promoting metal dissolution from the mineralized veins.

Mine discharge from the Eureka Graben area had the 

highest manganese concentrations (8,100 μg/L mean) of 

the five mineralized areas, reflecting the high percentage of 

manganese gangue minerals within these veins (fig. 19). These 

manganese-rich waters also ranked among the highest of the 

five areas in mean copper (633 μg/L), zinc (9,570 μg/L), and 

lead (170 μg/L) concentrations; however, due to the absence of 

enargite ores, arsenic concentrations were low (1 μg/L).

South Silverton Area

Ore deposits of the South Silverton area (fig. 1) mostly 

occur as low-grade and base-metal rich veins that filled simple 

fissures averaging 1–5 m in width. Veins were localized along 

two principal fracture systems (fig. 21). The first group, the 

“western shear system,” includes a complex of mineralized 

northwest- and northerly-trending fractures that may be the 

southern continuation of a mineralized, northwest fracture 

zone west of the Red Mountains area (Yager and Bove, this 

volume, pl. 1). The second major group of fractures, or the 

“eastern shear system,” is characterized by northwest-trending 

shear fractures and a set of “radial” fractures that trend normal 

to the margins of the Silverton and San Juan calderas. A 

vertical zoning from galena to silver-bearing tetrahedrite and 

gold-rich chalcopyrite with depth was recognized in some 

veins of the western shear system. Anglesite was abundant 

in the highest levels of the Shenandoah-Dives vein system 

and represents oxidation of ore above the upper limits of 

Pleistocene glacial downcutting. Several deposits of the eastern shear system contain both vein and carbonate replacement 

ores. The replacement deposits are present exclusively where 

large caldera-collapse blocks of Paleozoic-age carbonate rocks 

were intersected by metal-bearing veins. The replacement ore 

assemblage is very similar in bulk composition to the vein 

ores. Areas of hydrothermally altered rock are mostly confined 

to narrow zones surrounding the veins; more than 90 percent 

of the South Silverton area is characterized by regional propylitically altered rock.

Eleven of twelve mine discharge samples in the South 

Silverton area had relatively high pH and low mean concentrations of aluminum, iron, sulfate, and manganese compared 

to all other mine samples from the five mineralized areas. 

Although mean concentrations of lead, copper, and zinc rank 

among the lowest in the five mineralized areas, these metals 

were highly elevated in discharge from the Royal Tiger and 

Iowa mines (fig. 21), located in upper Arrastra Basin. The 

relatively high lead concentrations are somewhat unusual 

at the higher pH ranges of these waters. Factors that could 

contribute to these high dissolved lead concentrations are 

(1) the presence of oxidized lead ores such as anglesite, which 

are more soluble than lead sulfides at these higher pH ranges 

(Nordstrom and Alpers, 1999), (2) mixtures of metal-rich 

acidic waters and circumneutral waters within the mine, which 

had not reached equilibrium at the sample site, and (3) lack of 

iron oxyhydroxides due to low dissolved iron concentrations, 

which limits sorption effects and results in higher dissolved 

metal concentrations (Smith, 1999). The low alkalinity of the 

Royal Tiger and Iowa mine waters (<6 mg/L) suggests that 

metal complexing with carbonate anions was not a factor in 

the high dissolved lead concentrations.

224 Environmental Effects of Historical Mining, Animas River Watershed, Colorado
Mineralization, Alteration, and Geochemical Signatures 225

The paucity of low-pH mine waters (table 5) distinguishes 

the South Silverton area from the other four areas in this report. 

The higher pH is probably controlled by an abundance of 

calcite in wallrock and vein-related gangue material. Greater 

than 90 percent of the rocks in this mineralized area have been 

affected by calcite-rich propylitic alteration, whereas pervasive 

zones of pyritic-bearing rock are relatively minor. As a result, 

the five propylitic-related water samples from this area have 

mean pH and alkalinities of 8.15 and 52.7 mg/L, respectively, 

and zinc concentrations less than 30 μg/L. Despite the high 

metal concentrations of some of these mine waters, mixing 

with high-pH background waters results in relatively high pH 

and low dissolved metal concentrations at the mouths of all 

mainstem streams that drain the South Silverton area (pH 7.51 

to 8.1; zinc 10 to 300 μg/L) (Unpub. report to Colorado Water 

Quality Commission, ARSG, 2001; Wright, Simon, and others, 

this volume, fig. 10).

Acid-Neutralizing Potential (ANP) of Minerals 

and Relationship to Alteration Assemblages

All five mineralized areas discussed in this report were 

affected by a chlorite- and calcite-rich regional propylitic 

alteration assemblage that formed 5-15 million years prior to 

the major episodes of mineralization in the area. Propylitically 

altered rocks make up about 90 percent of the entire study 

area. As shown in a petrographic study of 16 regional propylitic samples (table 1), calcite content is highly variable, ranging from 0 to more than 20 volume percent of the rock (mean 

of 2 volume percent), whereas chlorite, with a minimum 

content of 7 volume percent, is much more homogeneously 

distributed (mean of 21 volume percent). Non-mining-affected 

streams and springs weathering propylitically altered rocks 

are characterized by generally good water quality with moderate to high pH values, measurable alkalinity, and low dissolved 

metal concentrations. These higher pH and metal-deficient 

waters are particularly prevalent in the Eureka Graben and 

South Silverton areas (fig. 1), which are underlain by about 70 

and 90 percent propylitically altered rock, respectively. However, probably of greater importance—in contrast to the Mount 

Moly, Red Mountains, and OPAM areas—these two areas lack 

expanses of pervasively altered and highly pyritic rock that 

naturally weathers to produce low-pH water with high iron, 

aluminum, sulfate, and low to moderate metal concentrations 

(fig. 11).

Although calcite is well recognized for its strong acidneutralizing potential (ANP) (Jambor and others, 2000), 

empirical data on chlinochlore—the dominant chlorite phase 

in the study area—suggests that it also has low to moderate 

acid-neutralizing potential when reacted with acidic solutions 

(Lawrence and Wang, 1997; Jambor and others, 2000). However, extrapolating these experimental results to the wide pH 

range of natural waters in the study area appears problematic. 

Despite the abundance of chlorite in regional propylitic and 

weak sericite-pyrite altered rocks (WSP), background water 

in the study area with pH higher than about 4.5 (for example 

WSP and PROP waters, table 8) has very low magnesium concentrations (fig. 22); higher magnesium concentrations would 

be expected if chlorite weathering was more prominent. These 

data along with inverse geochemical modeling studies (Bove 

and others, 2000; Mast and others, this volume) demonstrate 

that acid neutralization in these waters is mostly controlled by 

calcite, due to its abundance and high rates of dissolution; in 

contrast, the ANP role of chlorite in these higher pH ranges is 

limited because of its low kinetic rates of dissolution.

Figure 22. Mean values of dissolved magnesium versus pH from quartz-sericite-pyrite (QSP), weak sericitepyrite (WSP), and propylitic (PROP) background waters from the five mineralized areas.

1.0

10.0

23456789

Mount Moly area

Red Mountains

OPAM

Eureka Graben

South Silverton

QSP

WSP

PROP

Faster rate of chlorite dissolution,

reduced availability of calcite

Slow rate of chlorite dissolution,

high availability of calcite

Slow rate of chlorite

dissolution, moderate

availability of calcite

 MAGNESIUM, 

IN MILLIGRAMS PER LITER

pH, IN STANDARD UNITS

EXPLANATION
However, the degree of chlorite breakdown appears to 

increase during interaction with more acidic background waters 

(QSP waters, fig. 22), which confirms laboratory leach experiments (Lawrence and Wang, 1997; Jambor and others, 2000) 

documenting accelerated rates of chlorite dissolution in highly 

acidic solutions (pH <3.5). Although experimental data predict 

that chlorite dissolution in low-pH water has some potential 

to neutralize acidity (Lawrence and Wang, 1997; Jambor and 

others, 2000), the low pH of the QSP waters suggests that the 

degree of chlorite breakdown has done little to neutralize all 

the acid generated by pyrite oxidation. Although chlorite is 

commonly present in “islands” within and adjacent to these 

larger QSP alteration zones, calcite is generally absent and thus 

affords little to no ANP. The paucity of calcite in QSP-altered 

rocks is attributed to the low pH of the ancient hydrothermal 

fluids that formed this mineral assemblage (Rose and Burt, 

1979; Ringrose, 1982).

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