Geologic Framework

By Douglas B. Yager and Dana J. Bove

Chapter E1 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 .......................................................................................................................................................111

Introduction.................................................................................................................................................112

Geographic Setting ....................................................................................................................................112

Previous Geologic Investigations ...........................................................................................................113

Regional Tectonic Setting .........................................................................................................................113

Pre-Oligocene Rocks of the Animas River Watershed Study Area and Vicinity .............................115

Precambrian Rocks ..........................................................................................................................115

Paleozoic Rocks ................................................................................................................................117

Mesozoic Rocks ................................................................................................................................117

Tertiary Sedimentary Rocks ............................................................................................................119

Mid-Tertiary Volcanic Rocks ....................................................................................................................121

Precaldera Rocks..............................................................................................................................121

San Juan–Uncompahgre Caldera and Silverton Caldera Rocks ..............................................121

Tertiary Structures and Veins of the San Juan–Uncompahgre and Silverton Calderas ...............126

Caldera Ring Faults ...........................................................................................................................126

San Juan Caldera Topographic Margin ........................................................................................126

Eureka Graben Faults and Formation of Radial Fracture Pattern .............................................127

Mineralized Faults and Fractures ..................................................................................................128

Implications of Caldera Faults and Veins as Flow Paths ............................................................130

Alteration Types .........................................................................................................................................130

Regional Propylitic Alteration .........................................................................................................130

Weak Sericite-Pyrite Alteration .....................................................................................................133

Vein-Related Quartz-Sericite-Pyrite Alteration ...........................................................................133

Quartz-Sericite-Pyrite Alteration ...................................................................................................133

Acid-Sulfate Alteration ....................................................................................................................133

Acid-Generating Capacity of Non-Mining-Affected Rocks ................................................................133

Acid-Neutralizing Capacity of Study Area Rocks .................................................................................133

Metamorphic Rock Acid-Neutralizing Capacity ..........................................................................133

Sedimentary Rock Acid-Neutralizing Capacity ...........................................................................134

Tertiary Volcanic Rock Acid-Neutralizing Capacity ....................................................................134

Surficial Deposit Acid-Neutralizing Capacity ..............................................................................135

Late Tertiary Erosion ..................................................................................................................................135

Glaciation Events .......................................................................................................................................135

Pleistocene to Holocene Fluvial Geomorphologic Conditions ...........................................................136

Summary......................................................................................................................................................136

References Cited........................................................................................................................................137
Plates

[Plates in this report are on accompanying CD-ROM]

 1. Generalized geologic map of part of the Animas River watershed and vicinity, 

Silverton, Colorado

 2. Ferricrete, manganocrete, and bog iron occurrences with selected sedge 

bogs and active iron bogs and springs in part of the Animas River watershed, 

San Juan County, Colorado

Figures

1. Regional tectonic map of southwest Colorado and northwest New Mexico .................114

 2. Generalized regional geologic map of Animas River watershed study area .................116

 3. Stratigraphic column for rocks in Animas River watershed study area .........................118

 4. Map showing geographic feature names and mining districts in Animas River 

watershed study area ..............................................................................................................120

 5. Diagrams showing total alkali versus silica classification ................................................122

 6. Regional map showing calderas of San Juan volcanic field and generalized 

rock types ...................................................................................................................................123

 7–9. Diagrams showing:

 7. Total alkali versus silica, alkaline-subalkaline classification .....................................124

 8. Potassium versus silica classification ...........................................................................124

 9. Strontium versus silica classification ............................................................................125

 10. Total alkali, total iron and magnesium ternary diagram showing calc-alkaline 

and tholeiitic classification fields ..........................................................................................126

 11. Photograph of fractured and mineralized Silverton caldera structural margin 

along Mineral Creek .................................................................................................................127

 12. Landsat image showing erosion along structural margin of Silverton caldera 

by Mineral Creek and Animas River ......................................................................................128

 13. Generalized north-south cross section across Silverton caldera structural 

margin .........................................................................................................................................129

 14. Generalized alteration map of Animas River watershed study area ...............................131

 15. Photomicrographs of propylitized intermediate-composition volcanic rocks ...............132

 16. Photomicrograph of calcite and chlorite microveinlet in intermediatecomposition Silverton Volcanics lava flow...........................................................................134

Tables

 1. Major vein minerals of the Sunnyside mine workings .......................................................130

 2. Minor (<0.5 volume percent) vein minerals of the Sunnyside mine workings ................130

Abstract

This geologic summary and accompanying maps of part 

of the Animas River watershed, western San Juan Mountains, 

Colorado, provide a basic geologic framework for this volume. 

The Animas River headwater region of this volume consists 

of a Precambrian crystalline basement overlain by Paleozoic 

and Mesozoic sedimentary rocks and by a Tertiary volcanic 

cover.

The Tertiary is marked by deposition of the Eocene 

Telluride Conglomerate on an extensively eroded Eocene 

surface. The Telluride Conglomerate is host to Oligocene 

to Miocene polymetallic replacement mineral deposits. A 

sequence of Tertiary volcanic rocks overlying the Telluride 

Conglomerate comprises the majority of rocks exposed in the 

Animas River watershed study area. Tertiary volcanism commenced in the study area about 35 million years ago with the 

eruption of intermediate-composition lava flows and deposition 

of volcaniclastic mudflow deposits of the San Juan Formation. 

Caldera-related volcanism began shortly after the early San Juan 

Formation volcanism ceased, with the formation of the 28.2 Ma 

San Juan caldera, source of the Sapinero Mesa Tuff, and nearly 

contemporaneous Uncompahgre caldera, source of the Dillon 

Mesa Tuff. Post-caldera collapse volcanism consisted of lower 

Silverton Volcanics intermediate-composition lava flows and 

volcaniclastic rocks that filled the San Juan caldera. An elliptical 

dome formed between the San Juan and Uncompahgre calderas, 

and extensional fracturing over the resurgent dome of the intracaldera fill resulted in Eureka graben formation. Faults of the 

Eureka graben were extensively mineralized, primarily within 

the intermediate-composition Silverton Volcanics lavas. The 

Silverton caldera collapsed in response to eruption of the Crystal 

Lake Tuff at 27.6 Ma and is nested within the older San Juan 

caldera.

The San Juan–Uncompahgre and Silverton calderas are 

characterized by an extensive system of faults and veins. A 

highly fractured, arcuate, caldera ring fault zone is exposed 

along parts of Mineral Creek and along the Animas River 

southeast of Silverton. Radial faults that are tangential to the 

ring fault zone formed in response to broad resurgence that 

affected a region extending well beyond the San Juan caldera 

margin. The northwest-trending radial faults are apparently 

more highly mineralized when compared with radial faults 

with other orientations or with caldera ring faults. Resurgent 

doming within the central core of the San Juan–Uncompahgre 

calderas also resulted in the formation of a northeast-trending 

graben that was extensively mineralized. The primary host 

for base metals and precious metals in the study area is the 

intermediate-composition Silverton Volcanics lavas, where 

this unit is cut by graben-related and radial structures. Faults 

associated with the San Juan–Uncompahgre calderas localized 

post-Silverton-caldera, late-stage dacitic to rhyolitic intrusions and associated post-caldera hydrothermal mineralizing 

fluids that leached acid-neutralizing bedrock. These faults may 

be possible ground-water flow paths for trace-element- and 

major-element-rich water.

Five general alteration types formed in response to 

hydrothermal alteration events, which affected the Animas 

River watershed study area and followed caldera formation 

by 1 million to several million years. The five alteration types, 

listed in increasing order of intensity, are regional propylitic 

alteration, weak sericite-pyrite alteration, vein-related quartzsericite-pyrite alteration, quartz-sericite-pyrite alteration, 

and acid-sulfate alteration. The propylitic alteration event, 

which affected most if not all volcanic rocks in the study 

area, introduced the assemblage of quartz-epidote-chloritecalcite-pyrite-iron oxides. Locally, volcanic rocks were 

overprinted with higher grades of alteration. The incidence 

of sericite along with the propylitic assemblage is recognized 

in areas of densely spaced veins and adjacent to areas of the 

more intensely altered quartz-sericite-pyrite assemblage. 

The quartz-sericite-pyrite assemblage is indicative of more 

intensely altered areas that formed closer to a hydrothermal 

heat source and is characterized by the primary minerals of 

the original host rock being altered to a fine-grained rock 

of quartz, sericite, and pyrite. The most intense alteration, 

the acid-sulfate assemblage, is characterized by the mineral 

assemblages of quartz-alunite and (or) quartz-pyrophyllite, 

with some argillic alteration zones.

Many of the geologic units contain primary and secondary mineral assemblages that have some limited potential to 

mitigate acidic drainage from mine sites or unmined mineralized rocks. A host rock or surficial deposit that contains 

calcium carbonate, or the assemblage chlorite-calcite-epidote, 

has a high acid-neutralizing capacity. The neutralizing 

Chapter E1

Geologic Framework

By Douglas B. Yager and Dana J. Bove
capacity of rocks locally, and in some cases over large subbasin areas, is limited because intense hydrothermal alteration 

associated with mid- to late-Tertiary volcanism has replaced 

acid-neutralizing mineral assemblages with acid-generating 

sulfide minerals. Rocks that have high acid-neutralizing 

mineral assemblages in the Animas River watershed include 

Precambrian amphibolites, schists, and gneisses with the 

chlorite-epidote-calcite assemblage, and Paleozoic and 

Mesozoic limestones, dolomites, mudstones, and calcareous sandstones. Among the Tertiary sedimentary rocks, the 

Telluride Conglomerate locally contains limestone clasts. The 

younger Silverton Volcanics lavas are voluminous and extensively propylitized to calcite, chlorite, and epidote; within 

the study area, this unit likely has the most significantly high 

acid-neutralizing capacity, where it is not intensely altered by 

hydrothermal mineralization events. Weathering of bedrock 

in headwater subbasins has resulted in deposition of surficial 

deposits. Quaternary surficial deposits provide ground-water 

flow paths throughout as much as 27 percent of the headwater 

regions of the Animas River watershed study area, which facilitate water-rock interaction when infiltrated by ground water. 

The surficial deposits are acid generating or acid neutralizing 

depending on the mineralogic assemblages from which they 

were eroded.

During the late Miocene to Pliocene, when the San Juan 

Mountains were differentially uplifted relative to the Rio 

Grande Rift basin to the southeast, accelerated weathering 

and erosion exposed volcanogenic sulfides to surface water 

and ground water and increased acid drainage in the Animas 

River watershed. Pleistocene glaciation scoured the region, 

further exposing mineralized bedrock to weathering. During the height of the Wisconsin glaciation, a 1-km thick ice 

sheet covered 20 percent of the western San Juan Mountains. 

Easily erodable cirque basins with oversteepened slopes were 

exposed after glacier retreat.

Ferricrete, which consists of surficial deposits cemented 

in place by iron oxyhydroxide, occurs throughout the watershed study area. The surficial deposits serve as flow paths and 

precipitation sites for iron-rich water derived from the oxidation of pyrite. They are an indicator of acidic conditions that 

have existed in the watershed before, during, and after mining.

Introduction

This chapter and the geologic map (pl. 1) summarize the 

geologic setting for the Animas River watershed study area and 

provide a geologic framework for assessing acid rock drainage 

issues in the reports that follow. The Animas River watershed 

study area is located in the western San Juan Mountains near 

Silverton, Colo. This chapter includes discussions of

• The tectonic setting of the western San Juan Mountains

• Principal rock types in the study area and vicinity 

with mention of the lithologies that have a high acidneutralizing capacity

• Caldera-related structures that are potential groundwater flow paths

• Major alteration types

• Late Tertiary erosion events and Pleistocene glaciation, 

which have exposed bedrock to weathering.

The elements of the geologic map (pl. 1) that are important to 

the study include

• Numerous rock types, wherein chemical constituents 

of minerals that compose the rocks interact with surface water and ground water and affect water quality

• Igneous bedrock units that are hosts for Tertiary mineralization and are locally intensely altered and regionally weakly altered

• Distribution of the caldera-related veins, which have 

been extensively mined for base-metal sulfides and 

precious metals

• Distribution of permeable surficial deposits that are 

conduits for shallow ground water systems.

The Animas River watershed study area is located in the 

western part of the San Juan volcanic field, which is one of the 

largest erosional remnants of a volcanic province that covers 

much of the southern Rocky Mountain region (Steven and 

Epis, 1968; Steven, 1975). Of great importance to the Animas 

River watershed geologic history are the volcanotectonic and 

mineralization events that occurred from 35 to 10 Ma. Sulfide 

minerals that formed during mineralization events between 

22 and 10 Ma, where exposed by weathering and erosion or 

by mining, have generated acid water and introduced major 

elements and trace elements to surface water. Tertiary-age 

mineralized fault zones and veins were extensively mined 

in this region from the late 19th century through most of the 

20th century (Bove and others, this volume, Chapter E3; Nash 

and Fey, this volume, Chapter E6), which exposed the sulfide 

system to weathering.

Geographic Setting

A discussion of the geographic setting is important in a 

geologic context, because geography strongly influences the 

geoenvironmental weathering processes affecting the watershed. The western San Juan Mountains are rugged and steep, 

their terrain further emphasized by Pleistocene glaciation and 

Holocene erosion. Topographic relief in the study area reaches 

1,000 m with a maximum elevation exceeding 4,000 m above 

sea level. Precipitation, falling mainly as snow during the 

fall, winter, and spring seasons, is as much as 60 cm (Gillam, 

1998). Snowmelt is strongly controlled by slope and aspect 

(orientation in relation to the sun’s azimuth), in addition to 

time of year and elevation. On steep north-facing slopes, 

112 Environmental Effects of Historical Mining, Animas River Watershed, Colorado
Geologic Framework 113

snow remains even in the summer and fall months, following 

a winter with normal precipitation. In contrast, south-facing 

slopes that receive direct sunlight are usually snow free from 

late spring to early fall. Thus, the volume of acid rock drainage will vary for each subbasin throughout the year, which in 

turn will influence the rate and amount of water-rock interaction. Mean annual ambient air temperature in the study area 

is approximately 2°C (Gillam, 1998). Bedrock is subject to 

strong mechanical weathering by exposure to frequent freezethaw temperature fluctuations, which in turn expose freshly 

broken surfaces to continued water-rock interaction (Hoch 

and others, 1999).

The western San Juan volcanic field is part of the 

Southern Rocky Mountain steppe ecoregion (Bailey, 1995). 

Only sparsely vegetated alpine zone tundra is present at high 

elevation. Where altered bedrock and steep slopes coincide, 

the fragile vegetation patterns cannot stabilize the easily 

erodable outcrops, which constantly contribute mineral-laden 

sediment to streams below them. The subalpine zone is delimited by Engelmann spruce and subalpine fir. At lower elevations, riparian vegetation consists of grasses, sedge grasses, 

mosses, and willows.

Previous Geologic Investigations

The Animas River watershed investigations follow the 

work of past geologists whose focus and scope of study have 

evolved over a 130-year period to meet changing scientific and 

social needs. The discovery and exploitation of economically 

important base and precious metals in the northern part of 

the Animas River watershed during the 1870s spawned early 

geologic investigations. Today, we are interested in the combined effect that weathering both of altered and mineralized 

bedrock and of mine waste has on water quality and aquatic 

ecosystems.

Early investigators attempted to decipher the general 

stratigraphy and mineral deposits of the area near Silverton, 

Colo. (Ransome, 1901; Cross and others, 1905). These works 

were the foundation for later descriptions of the regional 

aspects of ore deposits in the San Juan Mountains (Burbank 

and Luedke, 1968; Steven and others, 1969). Focused mineral deposit studies were accomplished in the highly profitable Eureka mining district (Burbank and Luedke, 1969; 

Casadevall and Ohmoto, 1977). Reports and maps of a more 

general nature that include information on the geologic history and regional stratigraphy of the San Juan Mountains, but 

which are not referred to in other parts of this chapter, include 

the works of Kelley (1946), Larsen and Cross (1956), and 

Steven and others (1974).

The Tertiary volcanotectonic history of the region is 

the emphasis of several reports. Caldera-related research in 

the western San Juan Mountains was initiated in the 1960s 

(Luedke and Burbank, 1968) and has continued as geologists 

have further refined their concepts about how the San Juan 

volcanic field has evolved (Ratté and Steven, 1967; Smith 

and Bailey, 1968; Lipman, 1976a,b; Steven and Lipman, 1976; 

Lipman, 1984, 2000). Isotopic dating of rocks in and near the 

Animas River watershed study area has aided in understanding 

the timing of volcanism in relation to ore deposition (Lipman 

and others, 1970; Lipman and others, 1976; Bove and others, 

2001). The volcanic stratigraphy is discussed in Lipman and 

others (1973) and Lipman (1976b), and better constrained by 

isotopic dating (Bove and others, 2001).

Interpretations of the Quaternary glacial history and 

physiography of the western San Juan Mountains were first 

provided in the classic work of Atwood and Mather (1932). 

Modern interpretations of the late Cenozoic geologic history 

of the region involve discussions of Pleistocene glaciation and 

variables that have influenced stream terrace formation near 

Durango, Colo. (Gillam, 1998).

Regional Tectonic Setting

The western San Juan Mountains are situated near the 

eastern margin of the Colorado Plateau. The Colorado Plateau 

is a broad region of the southwestern United States that is 

bordered on the west by the Basin and Range province in 

Utah and Nevada, on the north by the east-west-trending Uinta 

uplift of Utah, on the east by the southern Rocky Mountains of 

Colorado and New Mexico, and on the south by the Mogollon 

Rim of Arizona.

Southwest Colorado and the western San Juan Mountains 

have undergone several periods of deformation that date back to 

the Precambrian. Evidence for two major cycles of Precambrian 

deposition, folding, metamorphism, and pluton emplacement 

are recorded in the geologic record of the Precambrian rocks 

of the Needle Mountains uplift that borders the southern part 

of the Animas River watershed study area. The first cycle of 

deposition, metamorphism, and intrusion occurred between 

1,800 and 1,700 Ma; the second cycle occurred between 1,650 

and 1,450 Ma (Barker, 1969). Intense, generally north to northeast trending folding accompanied the first Precambrian cycle, 

whereas isoclinal folding to the east and southeast occurred 

during the second cycle (Barker, 1969). Continental-scale 

northwest-striking faults formed in the San Juan Mountains at 

about 1,600 Ma (Baars and Ellingson, 1984). These northwesttrending faults bound Precambrian blocks of the Grenadier 

Range, located south of Silverton, and the Sneffels horst, 

located in the vicinity of Ouray (fig. 1). An apparent conjugate 

set of northeast-striking faults that demarcate the Colorado 

mineral belt appear to offset the northwest-striking faults (Baars 

and Ellingson, 1984). The Precambrian basement faults were 

rejuvenated periodically through the Laramide.

In addition to the major tectonic events, there were at 

least four episodes of Precambrian intrusive events. The effects 

of Precambrian intrusions, tumescence, and subsequent erosion continued into the Cambrian (Barker, 1969). The source 

rocks for the Cambrian Ignacio Quartzite are Precambrian 
Regional

tectonic 

map

San 

Luis Sneffels horst

Grenadier horst

San Juan dome

COLORADO

NEW MEXICO

Ouray

Silverton

Durango

Needle Mountains

uplift

San Juan Basin

Approximate 

boundary

39°30''

37°

109° 107°

COLORADO

NEW MEXICO

0 16 32 64 MILES

30 60 90 KILOMETERS

Colorado Mineral Belt

Uncompahgre uplift

Figure 1. Regional tectonic map of southwest Colorado and northwest New Mexico. Structures bounding Sneffels 

and Grenadier horsts have been periodically reactivated since the Precambrian. Quartz-rich sediments were shed 

from horst of San Luis uplift from Cambrian to Pennsylvanian, whereas Uncompahgre uplift was a source of feldspathic 

sediments to western San Juan Mountains in the Pennsylvanian. The Laramide, broad domal uplift “San Juan dome” 

is centered near Needle Mountains south of Silverton. Approximate region of Colorado mineral belt (COMB) modified 

from Tweto and Sims (1963), shown in a stippled pattern. Current political county boundaries are provided as frame of 

reference. Modified from Baars and Ellingson (1984).

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

0
Geologic Framework 115

crystalline rocks that were shed from the northwest-southeasttrending Grenadier and Sneffels horst blocks (fig. 1) (Baars 

and Ellingson, 1984), which were uplifted high above sea level 

during the Late Cambrian and supplied coarse material to their 

flanks.

A major mountain-building episode that affected the 

San Juan Mountains is associated with the Pennsylvanian to 

Permian San Luis–Uncompahgre uplifts (fig. 1) and formation 

of the Ancestral Rockies. Late Paleozoic cratonic deformation 

in western North America in Pennsylvanian time is attributed 

to the Ouachita-Marathon orogeny, which was likely caused by 

the collision of South America and Africa with the south edge 

of the North American landmass (Kluth and Coney, 1981). 

Fault blocks that bound the Grenadier and Sneffels horst were 

reactivated and uplifted during the Early Pennsylvanian. These 

uplifted blocks were sources for quartzose sediments that were 

deposited in the Ignacio Quartzite, the oldest Paleozoic unit in 

the study area. Uplift and erosion of the San Luis highland are 

recorded in deposition of Pennsylvanian Hermosa Formation 

quartzose sediments, whereas the source of Permian-age 

Cutler Formation arkosic sediments is the Uncompahgre highland (Baars and Ellingson, 1984). The several blocks of the 

San Luis–Uncompahgre uplifts were active into the Mesozoic 

Era and were sediment source areas for the Upper Triassic 

Dolores Formation, which was unconformably deposited on 

the Cutler Formation.

Eastward-directed compressive stresses generated during 

the Laramide orogeny at about 65 Ma were accommodated by 

Precambrian basement structures (Baars and Ellingson, 1984). 

This orogeny is inferred to have resulted from east-northeastdirected compression caused by west-southwest convergence 

of the North American and Farallon plates across the newly 

rifted North Atlantic (Chapin and Cather, 1981). A relatively 

symmetrical, basement-cored domal uplift referred to as the 

“San Juan dome” attained its present proportions during this 

time (Baars and Ellingson, 1984) and is roughly centered in an 

area to the south of Silverton in the Needle Mountains (fig. 1). 

The uplifted and eroded San Juan dome surface rises from an 

elevation of about 2,800 m just north of Ouray, to 4,000 m in 

the area between Silverton and Lake City and to more than 

4,600 m in the Needle Mountains. Late Cretaceous intrusive 

magmatism accompanied the Laramide episode of tectonism 

in the western San Juan Mountains. Intrusive igneous rocks 

emplaced during the Laramide consist of granodiorite dikes, 

sills, laccoliths, and small plugs located west and north of the 

study area (Varnes, 1963). A period of erosion followed the 

mountain-building episode of the Laramide orogeny, forming the extensive Eocene erosion surface. The Eocene erosion surface is a vast western North America peneplain that 

formed following a period of both basement-cored uplifts of 

the Laramide Rocky Mountains and monoclinal uplifts of the 

Colorado Plateau (Epis and Chapin, 1975; Chapin and Cather, 

1981). The Eocene Telluride Conglomerate was deposited 

during this time; its source rocks were derived from sediments 

that were shed from the San Luis uplift east of the study area.

Pre-Oligocene Rocks of the Animas 

River Watershed Study Area and 

Vicinity

The western San Juan Mountains preserve a relatively 

complete geologic record from Precambrian to Cenozoic 

time. Within the Animas River watershed study area, however, multiple Tertiary volcanotectonic and intrusive events 

have obliterated much of the geologic record. In general, 

Animas River watershed study area stratigraphy consists of 

a Precambrian crystalline basement overlain by Paleozoic, 

Mesozoic, and Eocene-age sedimentary rocks, and a 1- to 

2-km thick, Oligocene- to Miocene-age volcanic cover 

(generalized in fig. 2; see also pl. 1). Figure 3 is an Animas 

River watershed stratigraphic column, and figure 4 is an 

index map for geographic feature names in the watershed.

Precambrian Rocks

In the context of this investigation, the Precambrian 

rocks are important because they contain mineral phases 

whose high acid-neutralizing capacity influences waterrock interaction in the south part of the study area, near the 

headwater region of Cunningham Creek and along the Animas 

River south of Silverton. Precambrian basement rock structures have also influenced the structural history of the entire 

San Juan Mountains throughout Paleozoic to Cenozoic time 

(Tweto and Sims, 1963; Baars and Ellingson, 1984).

Largely due to later Tertiary volcanotectonic events, the 

Precambrian rocks crop out near the periphery of the Animas 

River watershed study area—principally along the Animas 

River downstream of Silverton, near Cunningham Creek, 

near the eastern margin of the watershed, east of the historical 

Animas Forks townsite, and north of Abrams Mountain (fig. 4; 

pl. 1). According to Barker (1969) and Luedke and Burbank 

(2000), the Precambrian rocks along the Animas River downstream of Silverton and at the headwaters of Cunningham 

Creek are equivalent to the Irving Formation in the Needle 

Mountains farther to the south. Irving Formation rocks consist of interlayered fine-grained, dark-gray to greenish-gray 

banded amphibolite with various fine-grained, gray to green, 

plagioclase-quartz-biotite-bearing gneisses and minor sericitebiotite-chlorite schists (Barker, 1969). Amphibolite is the 

predominant rock type in the Irving Formation and consists of 

blue-green hornblende, calcic oligoclase, and epidote; varieties 

with biotite, chlorite, and quartz are not uncommon (Barker, 

1969). In many places, plagioclase-quartz-biotite-bearing 

gneisses contain one or more of the following phases: chlorite, 

epidote, microcline, garnet, calcite, magnetite, and pyrite. 

Regional uplift, tilting to the west, and subsequent retrograde 

metamorphism (Steven and others, 1969) resulted in formation 

and concentration of the acid-neutralizing mineral assemblage 

of chlorite-epidote-calcite (Desborough and Driscoll, 1998; 

Desborough and Yager, 2000).
Silverton

?

107o45' 107o40' 107o35' 107o30' 107o50'

38o00'37o55'37o50'37o45'

EXPLANATION

Late Tertiary intrusive rocksTertiary volcanic rocksTertiary and Late Cretaceous intrusive rocksMesozoic and Paleozoic sedimentary rocksPrecambrian rocksContactFaults and vein structures— Dashed where approximately locatedSilverton caldera structural margin—Dotted where concealedSan Juan caldera topographic margin—Approximately located; queried where extent uncertain

0 6 MILES

0 9 KILOMETERS

Anim

as

R

ne me C Ct r

i M en ral Cr

EUREKA GRABEN

Figure 2. Generalized regional geology of Animas River watershed study area and surrounding regions near Silverton, Colo. Major tributaries to Animas River are shown in addition to major Tertiary volcanotectonic structures related to Silverton and San Juan calderas. Modified from Casadevall and Ohmoto (1977).

116 Environmental Effects of Historical Mining, Animas River Watershed, Colorado
Geologic Framework 117

Paleozoic Rocks

Paleozoic sedimentary rocks are primarily exposed 

south of Silverton and generally dip to the west (fig. 2; pl. 1) 

(Luedke and Burbank, 2000). The thickest outcrops are preserved along the Animas River downstream of Silverton and 

in the basins draining South Fork Mineral Creek.

The Cambrian Ignacio Quartzite represents the oldest unit of the Paleozoic section. The basal Ignacio Quartzite 

consists of a quartzite conglomerate, which grades upward to 

sandstone, siltstone, mudstone, and thinly bedded dolomite 

(Baars and Ellingson, 1984). Ignacio Quartzite rests unconformably on Precambrian basement along the Animas River 

downstream of Silverton. Ordovician, Silurian, and Lower and 

Middle Devonian units are absent from the western San Juan 

Mountains, probably owing to a combination of nondeposition and a mid-Devonian erosional event (Baars and Ellingson, 

1984).

A variable 77-m thick sequence of Upper Devonian 

Elbert Formation shale and limestone and Upper Devonian 

Ouray and Mississippian Leadville Limestones overlies 

the Ignacio Quartzite. The Ouray Limestone is commonly 

brown, micritic, and dolomitized (Baars and Ellingson, 1984). 

The overlying cliff-forming Leadville Limestone is locally 

dolomitized and chemically weathered.

A dark-red Molas Formation paleosol developed on 

a karst-modified, chemically weathered paleosurface on 

top of the Leadville Limestone. The Pennsylvanian Molas 

Formation consists of calcareous shale, mudstone, sandstone, 

and conglomerate averaging about 50 m thick. The overlying 

Pennsylvanian Hermosa Formation, as much as 550 m thick, 

consists of a stratigraphically complex, alternating sequence 

of near-shore silty shale to arkosic sandstone with interbedded marine limestone characteristic of transgressive-regressive 

cyclic deposition. The Hermosa and overlying Permian Cutler 

Formations are the most volumetrically extensive Paleozoic 

units in the Animas River watershed study area.

The Cutler Formation consists of feldspar-rich sandstone, 

red shale, mudstone, conglomerate, and rare, thin limestone 

beds. To the west of the Animas River downstream of 

Silverton, Permian Cutler Formation red beds as much as 

300 m thick terminate the Paleozoic section. Cutler Formation 

rocks in the South Fork Mineral Creek subbasin have locally 

been contact metamorphosed to hornfels by Tertiary stocks 

clustered near peak 3,792 m (fig. 4; Ringrose, 1982).

Additional discussion of the Paleozoic history can be 

found in Burbank (1930), Atwood and Mather (1932), Kelley 

(1957), Varnes (1963), Burrell (1967), Baars and See (1968), 

and Baars and Ellingson (1984).

Mesozoic Rocks

Mesozoic-age rocks are best exposed near the Animas 

River watershed margins, outside the Tertiary calderas in 

the headwater subbasins of South Fork Mineral Creek (pl. 1; 

Luedke and Burbank, 2000), where they record continued continental deposition through the Jurassic Period. The generally 

west dipping Late Triassic-age reddish-brown sandstone, 

siltstone, shale, and limestone of the Dolores Formation 

overlie the Permian Cutler Formation. As with the upper 

Paleozoic rocks, the source for these terrestrial deposits 

is the San Luis–Uncompahgre uplifts. The light-colored, 

crossbedded, cliff-forming, eolian Entrada Sandstone overlies 

the Dolores Formation. The Wanakah Formation is a slopeforming unit that overlies the Entrada Sandstone and consists 

of calcareous mudstone, brown to green shale, greenish-gray 

sandstone with gypsum nodules and stringers, and minor limestone near its base. The Morrison Formation, which caps the 

Jurassic sequence, is generally a slope-forming unit that consists of a lower ledge-forming sandstone (Salt Wash Member) 

and an upper gray to green mudstone (Brushy Basin Member) 

deposited in a fluvial to lacustrine environment. Members of 

the Morrison Formation have been mined for their uranium 

content throughout areas within and bordering the Colorado 

Plateau (Galloway and Hobday, 1983). In the headwater 

basins that flow to South Fork Mineral Creek, a 120-m thick 

sequence of Mesozoic-age rocks of the Entrada, Wanakah, 

and Morrison Formations is exposed in tributaries that drain 

into South Fork Mineral Creek.

The Cretaceous section becomes more important from 

a water-rock interaction standpoint farther south of the study 

area near Durango, Colo., where a more complete Cretaceous 

section attains a thickness of nearly 2,300 m (Baars and 

Ellingson, 1984). Near Durango, the Dakota Sandstone is 

composed of medium- to coarse-grained conglomeratic 

sandstone and shale that unconformably overlie the Jurassic 

Morrison Formation. Overlying and conformable with the 

Dakota Sandstone are the dark-gray to black, organic-rich, 

fossiliferous shales of the marine Mancos Shale. Along the 

Piedra River, located southeast and outside the study area, 

the cliff-forming Mesaverde Formation sandstones intertongue with and overlie the Mancos Shale. The Mesaverde 

Formation consists of buff to gray sandstone ledges separated by interbedded gray shale (Kottlowski, 1957). The 

organic-rich Lewis Shale intertongues with and overlies the 

Mesaverde Formation sandstones south of Durango. Fluvial 

sandstones and shales represented by the multicolored 

Pictured Cliffs Sandstone, Fruitland Formation, and Kirtland 

Shale cap the Cretaceous sequence and were deposited 

near the regressing Cretaceous seaway (Baars and Ellingson, 

1984).

Cretaceous rocks in the study area consist of the 

Dakota Sandstone and overlying Mancos Shale. These rocks 

crop out in the headwaters of South Fork Mineral Creek and 

have a combined maximum thickness of about 300 m. Late 

Cretaceous magmatism in the western San Juan Mountains 

produced granodiorite dikes, sills, laccoliths, and small intrusive plugs that were emplaced west and north of the study 

area and accompanied a period of tectonism related to the 

Laramide orogeny (Varnes, 1963).
Cutler Formation: 0–300 m thick

Hermosa Formation: 0–550 m thick

Molas Formation: 0–50 m thick

Leadville and Ouray Limestones:

0–65 m thick

TERTIARY

CRETACEOUS

JURASSIC

TRIASSIC

PERMIAN

PENNSYLVANIAN

MISSISSIPPIAN

DEVONIAN

CAMBRIAN

PRECAMBRIAN

Crystal Lake Tuff: 0–70 m thick

Fish Canyon Tuff: 0–200 m thick

Silverton Volcanics: 0–1,000 m thick

Sapinero Mesa and 

Dillon Mesa Tuff: 0 to > 800 m thick

Blue Mesa Tuff: 0–250 m thick

Ute Ridge Tuff: 0–150 m thick

San Juan Formation: 0–600

m thick

Telluride Conglomerate: 0–60

m thick

Mancos Shale: 0–200 m thick

Dakota Sandstone: 0–100 m thick

Morrison Formation: 0–30 m thick

Wanakah Formation: 0–30 m thick

Entrada Sandstone: 0–20 m thick

Dolores Formation: 0–40 m thick

Elbert Formation: 0–12 m thick

Ignacio Quartzite: 0–12 m thick

intrusion

Irving Formation

generalized on pl. 1

118 Environmental Effects of Historical Mining, Animas River Watershed, Colorado
Geologic Framework 119

Tertiary Sedimentary Rocks

The Tertiary section begins with the Eocene Telluride 

Conglomerate, which was deposited on an extensive Eocene 

erosion surface (Mayor and Fisher, 1972). Lower layers 

within the Telluride Conglomerate contain rounded to subangular cobbles of Mesozoic and Paleozoic red sandstone 

and limestone. The upper layers contain mostly rounded 

to subangular clasts of Precambrian gneiss, schist, and 

quartzite, with minor quantities of granitic, volcanic, and 

sedimentary rocks in an arkosic, iron-oxide-stained, calcitecemented matrix. The depositional sequence reflects an 

inverted paleostratigraphy owing to progressively deeper 

levels of erosion of Mesozoic through Precambrian basement rocks (Baars and Ellingson, 1984). To the west of the 

study area near Mount Wilson, the unit is more than 300 m 

thick; to the northeast, the unit thins to a maximum thickness of approximately 65 m, and it is absent in some areas 

over the San Juan dome. Within the Animas River watershed 

study area and vicinity, the Telluride Conglomerate overlies 

Paleozoic and Mesozoic sedimentary rocks in the South Fork 

Mineral Creek basin, and it overlies Paleozoic Hermosa and 

Cutler Formations above and west of the Animas River south 

of Silverton.

Crystal Lake Tuff (Oligocene)—Rhyolitic ash-flow tuff 

erupted from the Silverton caldera

Fish Canyon Tuff (Oligocene)—Dacitic ash-flow tuff 

erupted from the La Garita caldera east of study area

Silverton Volcanics (Oligocene)—Lava flows of 

intermediate composition and related volcaniclastic 

sedimentary rocks in addition to minor silicic lavas 

and ashflows

Sapinero Mesa Tuff (Oligocene)—Rhyolitic ash-flow 

tuff erupted from the San Juan and Uncompahgre 

calderas

Dillon Mesa Tuff (Oligocene)—Rhyolitic ash-flow tuff 

probably derived from the Uncompahgre caldera

Blue Mesa Tuff (Oligocene)—Rhyolitic ash-flow tuff 

probably derived from the Lost Lake caldera

Ute Ridge Tuff (Oligocene)—Dacitic ash-flow tuff 

derived from the Ute Creek caldera

San Juan Formation (lower Oligocene)— Intermediatecomposition lava flows and volcaniclastic deposits 

consisting mostly of mudflow breccias 

Telluride Conglomerate (Eocene)—Conglomerate 

and sandstone. Consists of Precambrian, Paleozoic, 

and Mesozoic clasts in a silty to sandy matrix 

Mancos Shale (Upper Cretaceous)—Marine shale

Dakota Sandstone (Upper Cretaceous)—Sandstone with 

interbedded siltstone and carbonaceous shale

Morrison Formation (Upper Jurassic)—Sandstone with lenses 

of mudstone. Upper part is primarily calcareous mudstone

Entrada Sandstone (Middle Jurassic)—Crossbedded sandstone 

Wanakah Formation (Upper Jurassic)—Calcareous mudstone 

and siltstone with thin limestone and sandstone interbeds near 

top

Dolores Formation (Upper Triassic)—Siltstone and sandstone 

with interbeds of limestone and conglomerate

Cutler Formation (Lower Permian)—Shale, arkosic sandstone, 

and conglomerate, locally calcareous

Hermosa Formation (Upper and Middle 

Pennsylvanian)—Interbedded silty shale to arkosic sandstone 

with interbedded marine limestone

Molas Formation (Middle and Lower Pennsylvanian)— 

Calcareous shale, sandstone, and conglomerate

Leadville and Ouray Limestones (Lower Mississippian and 

Upper Devonian)—Altered, mineralized, and recrystallized in 

and near study area

Elbert Formation (Upper Devonian)—Dolomitic limestone and 

interbedded calcareous shale and sandstone

Ignacio Quartzite (Upper Cambrian)—Quartzite, sandstone, 

and siltstone; locally conglomeratic

Irving Formation (Precambrian)—Interlayered banded 

amphibolite, gneiss, and minor sericite-biotite-chlorite schists 

EXPLANATION

Figure 3 (above and facing page). Stratigraphic column for rocks in Animas River watershed study area. See also plate 1 for more 

detailed description of map units.
Figure 4. Geographic features, and mining districts referred to in this chapter. Shaded areas are generalized boundaries of three of the 

major mining districts. RM, Red Mountain mining district; ED, Eureka mining district; SSD, South Silverton mining district. Crossed pick 

and shovel, inactive mine discussed in report.

RM ED

SSD

former

Lake

Emma

Elk Park

Mineral Creek

Animas

River

Chattanooga

Eureka

Silverton

SUNNYSIDE

Sunnyside

Mill No. 1

Sunnyside

Flotation

Mill

Animas

Forks

Mayflower

Mill Howardsville

Bo

ulder Gulch

So uth For k

M iddle Fork

3,792 m

Ohio Peak

Sultan Mt

Blair Gulch

Cement

Creek

Prospect

Gulch

T ope

G

ka ulch

O

G hio u lch

Cunning ham Creek Highland

Mary

Anvil Mt Soda

Gulch

Mt Wilson 

Red Mtn

No 2 

Red Mtn

No 3 Red Mtn

Pass 

North

F ork Tiger G ulch

California

Gulch

Pla cer G ulch

South Fork

Abrams

Mountain

Eure ka

Gulc h

Pic ayune

Gulch

Gladstone

0

0 1

1 2 MILES

2 KILOMETERS

107o 50'30" 107o 45' 107o 37'30" 107o 30

37o

52'

 30" 

 37o 

45'

 37o 

58'

Animas River

watershed 

study area

C O L O R A D O

Durango

120 Environmental Effects of Historical Mining, Animas River Watershed, Colorado
Geologic Framework 121

The upper Telluride Conglomerate includes intermediatecomposition volcanics and volcaniclastic rocks and records the 

onset of early, mid-Tertiary volcanism. Hydrothermal polymetallic replacement ores containing copper-lead-gold-silverzinc formed in the Telluride Conglomerate along northwesttrending caldera-related veins, dikes, and structures. These 

deposits were extensively mined beginning in 1948 (Mayor 

and Fisher, 1972; Jones, this volume, Chapter C).

Mid-Tertiary Volcanic Rocks

The Tertiary volcanic rocks in the San Juan Mountains 

blanketed more than 25,000 km2

, forming one of the great 

epicontinental volcanic piles of intermediate- to siliciccomposition volcanic rocks. In general, onset of mid-Tertiary 

volcanism records construction of stratovolcanoes and 

volcanic shields built up by thick accumulations of andesite 

to rhyolite flows and breccias that were followed by several 

eruption cycles of more silicic ash flows. The eruption of ash 

flows led to caldera formation and local subsidence.

Precaldera Rocks

Mid-Tertiary volcanism in the San Juan Mountains commenced between 35 and 30 Ma with the eruption and deposition of voluminous calc-alkaline, intermediate-composition 

(52–63 percent SiO2

) lava flows, flow breccias, volcaniclastics, 

minor mafic tuffs, and abundant mudflows of the San Juan 

Formation (Lipman and others, 1973; Steven and Lipman, 

1976). The San Juan Formation lavas are typically porphyritic 

and consist of plagioclase, clinopyroxene, hornblende, and 

opaque oxide primary phases. Within the study area, volcanic 

rocks are typically propylitically altered, and replacement of 

amphibole, pyroxenes, and plagioclase by secondary chlorite, 

epidote, calcite and locally sericite is pervasive.

Different geologists working in different parts of the 

western San Juan Mountains have assigned several formation names to time-equivalent San Juan Formation units. In 

the western San Juan region, intermediate-composition rocks 

of the Lake Fork Formation (Larsen and Cross, 1956) and 

the Picayune Formation (Burbank and Luedke, 1969) are all 

temporally equivalent to the San Juan Formation (Lipman and 

others, 1973; Lipman, 1976b). San Juan Formation is the name 

used herein. Collectively, the San Juan Formation units along 

with the other early intermediate-composition lava flows of the 

San Juan volcanic field make up the largest volume of eruptive products in the San Juan Mountains (Lipman, 1984). The 

35 to 30 Ma San Juan and Picayune lavas are predominantly 

andesites and basaltic andesites; one single analysis plots in 

the trachyandesite field (fig. 5).

San Juan Formation volcanic rocks in the study area crop 

out near the northern, western, and southern topographic margin 

of the San Juan caldera (pl. 1) and attain a thickness of several 

hundred meters. Southwest of Silverton, in subbasins above 

South Fork Mineral Creek, the San Juan Formation unconformably overlies the Telluride Conglomerate and the Cutler 

Formation. Farther to the south, the San Juan Formation unconformably rests on Paleozoic strata and Precambrian rocks.

San Juan–Uncompahgre Caldera and 

Silverton Caldera Rocks

From 30 to 23 Ma, numerous eruptions formed multiple 

calderas (fig. 6), and related volcanic units were deposited 

throughout the San Juan volcanic field (Lanphere, 1988; 

Lipman and others, 1997; Bove and others, 2001). Two 

caldera events took place in the study area: the 28.2 Ma 

San Juan–Uncompahgre caldera complex and the younger, 

nested 27.6 Ma Silverton caldera (fig. 6; pl. 1) (Bove and 

others, 2001). The San Juan caldera in the study area is the 

southwest half of the roughly dumbbell shaped San Juan–

Uncompahgre caldera complex (fig. 6). Prior to caldera 

formation, magma chambers were likely emplaced at shallow 

(from 9 to 5 km) depths in the crust (Johnson and Rutherford, 

1989; Lipman and others, 1997) and differentiated to erupt 

relatively silicic (68 and 72 percent SiO2

) pyroclastic flows. 

Tertiary calderas in the western San Juans produced more than 

1,000 km3 of explosive volcanic products. For comparison, the 

volume of magma ejected from the San Juan–Uncompahgre 

caldera complex was >1,000 km3

, whereas the smaller, nested 

Silverton caldera ejected between 50 and 100 km3 of magma 

(Lipman, 2000).

San Juan caldera eruptives include the intracaldera 

Eureka Member of the Sapinero Mesa Tuff and outflow 

Sapinero Mesa Tuff. The Sapinero Mesa Tuff is rhyolitic to 

dacitic in composition (fig. 5), containing phenocrysts of 

plagioclase, with lesser amounts of sanidine, biotite, and trace 

augite. The outflow sheet thickness averages 50 m; maximum 

thickness is 100 m. Outflow Sapinero Mesa Tuff is preserved 

on scattered high peaks to the west and northwest of the study 

area. In contrast, the San Juan caldera fill contains at least 

a 700-m section of densely welded and highly propylitized, 

intracaldera Eureka Member (Lipman and others, 1973). The 

Eureka Member is compositionally variable, has a lower silica 

content, and generally has a higher crystal content than the 

outflow equivalent Sapinero Mesa Tuff (fig. 5).

The Uncompahgre caldera, located northeast of the study 

area, formed in response to eruption of the Dillon Mesa Tuff; 

eruption of this tuff may have slightly preceded that of the 

San Juan caldera-related tuffs (Lipman and others, 1973). The 

Dillon Mesa Tuff, preserved mainly west of the study area, is 

relatively thin and is a crystal-poor rhyolite that is mineralogically similar to the rhyolitic phase of the outflow Sapinero 

Mesa Tuff.

Resurgent doming of the San Juan–Uncompahgre caldera 

complex began prior to infilling of the area by intermediatecomposition lava flows (Lipman, and others, 1973). However, 
Na2O + K2O, IN WEIGHT PERCENT

SiO2, IN WEIGHT PERCENT 

0

2

4

6

8

10

12

14

16

35 40 45 50 55 60 65 70 75

Phonolite

Trachyte

Rhyolite

Trachydacite

Trachyandesite

Phonotephrite

Tephrite

Basanite

Foidite

Basalt

Picrobasalt

Basaltic

andesite Andesite

Dacite

Basaltic

trachyandesite Trachybasalt

Tephriphonolite

Na2O + K2O, IN WEIGHT PERCENT

SiO2, IN WEIGHT PERCENT 

0

2

4

6

8

10

12

14

16

35 40 45 50 55 60 65 70 75

Phonolite

Trachyte

Rhyolite

Trachydacite

Trachyandesite

Phonotephrite

Tephrite

Basanite

Foidite

Basalt

Picrobasalt

Basaltic

andesite

Andesite

Dacite

Basaltic

trachyandesite Trachybasalt

Tephriphonolite

A

B

Figure 5. Total alkali versus silica classification (Le Bas and others, 1986). A, 

Intermediate-composition lava flows; solid circle, Silverton Volcanics, Burns Member; 

solid square, Silverton Volcanics, pyroxene andesite member; solid triangle, Eureka 

Member of the Sapinero Mesa Tuff; open square, undifferentiated lava flows; open 

triangle, San Juan Formation lava flow. B, Primarily silicic intrusives (GIS and Access 

database, Sole and others, this volume, Chapter G).

122 Environmental Effects of Historical Mining, Animas River Watershed, Colorado
Geologic Framework 123

NEW MEXICO

UTAH

COLORADO

109° 106° 39°

37°

Montrose

Durango

Ouray

Creede

Gunnison

Silverton

COLORADO

0 50 100 KILOMETERS

0 50 MILES

COLORADO

Area of

index map

b

co

g sl

c

h

ba

sm

p

u ll

un

lc

sr

s

sj b

Quaternary-Tertiary sediments

Tertiary intrusions

Tertiary volcanic rocks

Mesozoic and Paleozoic sedimentary rocks

Precambrian rocks

Calderas of San Juan volcanic field—

 b, Bonanza; ba, Bachelor; c, Creede;

 co, Cochetopa; g, La Garita;

 h, Mount Hope; lc, Lake City; ll, Lost

 Lake; p, Platoro; s, Silverton; sj, San Juan;

 sl, San Luis; sm, Summitville;

 sr, South River; u, Ute Creek;

 un, Uncompahgre

EXPLANATION

Figure 6. Regional map showing calderas of San Juan volcanic field and generalized rock types. Modified from Casadevall 

and Ohmoto (1977).

extensional fracturing during formation of the Eureka graben 

may have accommodated continued late-stage uplift of the 

San Juan–Uncompahgre resurgent dome. Eureka Member 

outcrops are exposed in the Eureka graben and southeast of 

Silverton along the margins of the caldera on the flanks of 

Kendall Mountain (pl. 1).

San Juan caldera activity culminated with ring fracture 

volcanism, the products of which constitute the Silverton 

Volcanics. These rocks fill the caldera depression to a thickness exceeding 1 km. Lavas are porphyritic and typically 

contain phenocrysts of plagioclase, amphibole, pyroxene, 

opaque oxides, and minor biotite in a fine-grained to aphanitic 

groundmass. Minor rhyolite to dacite ash-flow tuff consisting of quartz, biotite, potassium feldspar, plagioclase, and 

amphibole is preserved within the upper part of the Silverton 

Volcanics such as on Abrams Mountain, north of the Animas 

River watershed (Luedke and Burbank, 1961).

Most Silverton Volcanics lavas are subalkaline (fig. 7) 

as indicated by 58 samples examined during this study. With 

the exception of four samples, which plot as medium potassium 

andesites, the Silverton Volcanics lavas have high potassium 

concentrations (fig. 8). The intermediate- to silicic-composition 

Silverton Volcanics are divided into three interfingering 

members. The Burns Member is generally characterized 

by propylitically altered lavas and dacite to rhyolite tuffs. The 

pyroxene andesite member, which generally overlies the Burns 

Member, tends to be somewhat less altered. The volcaniclastic 

sedimentary rocks of the Henson Member interfinger with 

both the Burns and the pyroxene andesite members (Lipman, 

1976b). The Burns Member has a trend toward higher silica 

compositions when compared with the commonly overlying, 

younger pyroxene andesite and Henson Member lavas. This 

relatively higher silica concentration trend in Burns Member 

lavas is shown in the strontium versus silica variation diagram 

(fig. 9).

Deposition of the Silverton Volcanics was closely followed 

by eruption of the Crystal Lake Tuff at 27.6 Ma, which resulted 

in formation of the Silverton caldera. The Crystal Lake Tuff is a 

crystal-poor, low-silica rhyolite (72 percent SiO2

) that contains 

phenocrysts of sanidine and plagioclase, biotite, and minor 

augite (Lipman and others, 1973). Outflow Crystal Lake Tuff is 

preserved outside of the Silverton caldera; accumulations of 25 

to 50 km3

 have been recognized in regions toward the northeast, 

in the moat of the Uncompahgre caldera, and to the eastsoutheast in the upper Rio Grande drainage basin (Lipman and 

others, 1973). Intracaldera Crystal Lake Tuff has not been recognized within the Silverton caldera, and outflow remnants in 

the immediate vicinity outside the caldera are also quite small. 

The problem remains whether this unit was entirely removed 

within the Silverton caldera during subsequent erosion or if the 

magma chamber was largely evacuated and most if not all of the 

Crystal Lake Tuff was deposited as outflow. More detailed mapping of the Silverton caldera is needed to resolve this issue.
SiO2, IN WEIGHT PERCENT 

K2O, IN WEIGHT PERCENT

0

1

2

3

4

50 55 60 65

Low-K

Medium-K

High-K

ACID

BASIC

Figure 8. Potassium versus silica classification (Gill, 1981). Solid circle, Silverton 

Volcanics, Burns Member lava flows; solid square, Silverton Volcanics, pyroxene 

andesite member lava flows; open square, undifferentiated lava flows; open triangle, 

San Juan Formation lava flows (GIS and Access database, Sole and others, this 

volume, Chapter G). K, potassium. Na2O + K2O, IN WEIGHT PERCENT

SiO2, IN WEIGHT PERCENT 

 40 50 60 70

0

5

10

Subalkaline

Alkaline

Figure 7. Total alkali versus silica, alkaline-subalkaline classification (Irvine and 

Baragar, 1971). Solid circle, Silverton Volcanics, Burns Member lava flows; solid 

square, Silverton Volcanics, pyroxene andesite member lava flows; open square, 

undifferentiated lava flows; solid triangle, Eureka Member of the Sapinero Mesa Tuff; 

open triangle, San Juan Formation lava flows (GIS and Access database, Sole and 

others, this volume, Chapter G).

124 Environmental Effects of Historical Mining, Animas River Watershed, Colorado
Geologic Framework 125

50 60 70 80

2,000

1,500

1,000

500

0

Sr, IN PARTS PER MILLION

SiO2, IN WEIGHT PERCENT 

Figure 9. Strontium (parts per million) versus silica classification. Solid circle, 

Silverton Volcanics, Burns Member lava flows; solid square, Silverton Volcanics, 

pyroxene andesite member lava flows; open square, undifferentiated lava flows; solid 

triangle, Eureka Member of the Sapinero Mesa Tuff; open triangle, San Juan Formation 

lava flows (GIS and Access database, Sole and others, this volume).

Beginning approximately 1 million years after Silverton 

caldera formation, numerous stocks, dikes, and other silicic 

intrusions formed along the Silverton caldera ring fracture zone. 

The oldest postcaldera granitic stocks (monzonite, monzodiorite, granodiorite, and monzogranite) are about 26.6 Ma 

and intruded along the southern part of the Silverton caldera 

ring fracture (Lipman and others, 1976; Ringrose, 1982; Bove 

and others, 2001). These porphyry stocks contain plagioclase, 

potassium feldspar, quartz, biotite, clinopyroxene, and opaque 

oxides. This late-stage igneous activity was accompanied by 

hydrothermal alteration and formation of a contemporaneous 

low-grade, molybdenum-copper porphyry deposit and cogenetic polymetallic vein deposits (Ringrose, 1982; Musgrave 

and Thompson, 1991). Concentric and radial caldera-related 

structures were paths of least resistance for the later mineralizing fluids (Varnes, 1963; Casadevall and Ohmoto, 1977; Bove 

and others, this volume, Chapter E3). Most granitic intrusives 

are altered to varying degrees, and mafic minerals such as 

biotite and pyroxene are replaced by chlorite. Late-stage, 23 

to 10 Ma dacitic to rhyolitic intrusions emplaced along the 

north and northwest structural margin of the Silverton caldera 

were concomitant with hydrothermal alteration and related 

mineralization (Bove and others, this volume). Dacitic intrusions dated at 23 Ma in the Red Mountain mining district are 

coarsely porphyritic, containing large (1 cm) phenocrysts of 

potassium feldspar, plagioclase, quartz, and biotite in a finegrained groundmass. Younger, circa 10 Ma rhyolite intrusions 

are generally finely crystalline, porphyritic to aphanitic, and 

consist primarily of quartz and potassium feldspar. These 

granitic intrusions and younger late-stage dacitic to rhyolitic 

composition intrusions of the Red Mountain mining district 

are mostly calc-alkalic (fig. 10).

Intrusions in the Red Mountain district and elsewhere 

appear to be geographically correlated with a high fracture 

density (Bove and others, this volume), and they have brecciated margins that are possibly more permeable than nonbrecciated surrounding country rocks where not filled with quartz 

and other gangue minerals. Therefore, the brecciated intrusive 

margins may strongly influence ground-water flow where a 

higher fracture density coincides with possible permeability contrasts between intruded country rocks and brecciated 

margins.

Mineralized breccia pipes are closely associated with 

some of the silicic intrusions in the Red Mountain mining 

district (Fisher and Leedy, 1973; Nash, 1975). The geology and geochemistry of the breccia pipe ores are important 

because the ores have high sulfur concentrations and contain silver-bearing copper and lead sulfosalts of arsenic and 

antimony and base-metal sulfides (Fisher and Leedy, 1973; 

Bove and others, this volume). These deposits and associated 

mine waste weather to produce some of the highest arsenic 

concentrations in stream sediments in the watershed study 

area (Bove and others, this volume; Church, Fey, and Unruh, 

this volume, Chapter E12).
Tertiary Structures and Veins of the 

San Juan–Uncompahgre and Silverton 

Calderas

Structures that developed coincident with volcanic 

activity were utilized as flow paths by later mineralizing solutions, and the resulting mineralized areas were economically 

mined for base-metal sulfides and precious metals (Varnes, 

1963; Casadevall and Ohmoto, 1977; Bove and others, 

this volume). The structures related to the San Juan–

Uncompahgre and Silverton calderas are pervasive features 

that reflect the complex volcanotectonic history of the study 

area.

Five dominant structural elements are related to formation of the San Juan–Uncompahgre and Silverton calderas: 

(1) circular and likely overlapping caldera ring fault zones, 

(2) a topographic margin associated with the San Juan caldera 

that marks the topographic expression generally found 

outward from the structural margin, (3) a northeast-trending 

graben (Eureka graben) formed during resurgent doming 

of the central core of the San Juan–Uncompahgre caldera, 

(4) radial faults and vein structures located principally near 

the northwest and southeast periphery of the San Juan 

and Silverton calderas, and (5) thousands of mineralized 

and barren veins and vein structures, many of which lack 

surface expressions. These structural elements related to the 

San Juan–Uncompahgre and Silverton calderas are discussed 

as follows.

Caldera Ring Faults

The 28.2 Ma San Juan and nearly contemporaneous 

Uncompahgre calderas form a dumbbell-shaped (fig. 6) 

double caldera complex separated by a segment of Precambrian 

basement. The caldera complex has an elongate dimension 

of 48×24 km and formed along a northeast-southwest trend 

parallel to the Colorado mineral belt. It has been proposed 

that the northeast-southwest alignment of the long axis of the 

San Juan and Uncompahgre volcanic depression coincides with 

Precambrian basement structures of the Colorado mineral belt, 

which were perhaps reactivated during the episode of Tertiaryage volcanism (Tweto and Sims, 1963; Burbank and Luedke, 

1969). A ring fault system developed simultaneously with 

collapse of the San Juan–Uncompahgre caldera system. The 

San Juan caldera ring fault is observed in underground workings 

of the Highland Mary, Green Mountain, and Pride of the West 

mines (Church, Mast, and others, this volume, Chapter E5) 

where Precambrian Irving Formation rocks are in vertical 

contact with Eureka Member of the Sapinero Mesa Tuff along 

the south margin of the caldera (Luedke and Burbank, 2000). 

When the San Juan–Uncompahgre caldera collapsed along the 

approximately 24 km diameter piston-shaped ring fault zone, 

the Sapinero Mesa Tuff was erupted and deposited outside the 

caldera as outflow. Simultaneous with caldera collapse, the 

Eureka Member of the Sapinero Mesa Tuff ponded within the 

interior of the San Juan caldera to a thickness of 700 m with no 

base exposed (Lipman and others, 1973).

The 27.6 Ma Silverton caldera is nested within the older 

San Juan caldera. The Silverton caldera is one of the least 

studied calderas in the region. Its collapse is evidenced by 

(1) the intensely faulted and fractured ring fracture zone, well 

exposed along the Mineral Creek basin (fig. 11) and east of 

Silverton along the Animas River (pronounced erosion along 

Mineral Creek and along the Animas River upstream from 

Silverton accentuates the semicircular drainage system that 

follows the Silverton caldera ring fault zone (fig. 12)), and 

(2) several hundred meters of downfaulting of both the Eureka 

Member and the Silverton Volcanics along the Silverton caldera ring fracture (Burbank, 1933; Varnes, 1963; Lipman and 

others, 1973) (fig. 13).

San Juan Caldera Topographic Margin

The geomorphic expression of the San Juan caldera is 

indicated by a highland topographic margin that defines the 

arcuate headwater regions of the Animas River in upper Maggie 

Gulch, Cunningham Creek, and along Deer Park Creek south of 

Kendall Mountain (pl. 1). Small to large, meso- to megabreccia blocks of older basement rocks and Eureka Member have 

caved inward from the steep topographic walls. North-dipping 

Paleozoic limestone units south of Silverton and megabreccia limestone blocks at the headwaters of Cunningham Creek 

(pl. 1) near the Highland Mary mine delineate the southern 

FeO*

Alk MgO

Tholeiitic

Calc-alkaline

Figure 10. Total alkali, total iron and magnesium, tholeiitic versus 

calc-alkaline classification. Solid circle, dacite porphyries; open 

square, granitic to diabase intrusions; triangle, rhyolite intrusion 

(GIS and Access database, Sole and others, this volume).

126 Environmental Effects of Historical Mining, Animas River Watershed, Colorado
Geologic Framework 127

topographic margin of the San Juan caldera. Rocks along the 

margin of the San Juan–Uncompahgre caldera have caved 

into the caldera during and after eruption, forming an inwardsloping surface of the topographic margin. The San Juan 

caldera topographic margin in the study area is located near the 

headwater regions of the South Silverton mining district where 

Precambrian, Paleozoic, and older Tertiary rocks are in contact 

with the Eureka Member of the Sapinero Mesa Tuff. Meso- to 

megabreccia blocks that have caved into the caldera also are 

present near the eastern San Juan caldera topographic margin 

near the mouth of Picayune Gulch along the upper Animas 

River above Eureka.

Eureka Graben Faults and Formation 

of Radial Fracture Pattern

Collapse of the San Juan–Uncompahgre caldera complex 

was followed closely by resurgence. Contemporaneous with 

resurgence, an elliptical dome 15 km wide by 30 km long 

formed between the two calderas, and extensional fracturing 

over the resurgent dome resulted in Eureka graben formation. The Sunnyside and Toltec faults define the northeastern 

part of the Eureka graben; the Ross Basin and Bonita faults 

that formed in the southwestern part of the graben border 

the west-northwest-trending part of the graben (pl. 1). Each 

of the Eureka graben faults has lateral and vertical extents 

of thousands of meters. Eureka graben structures were later 

extensively mineralized with base and precious metals. Late 

Silverton caldera resurgence has been postulated, to have 

been facilitated by reactivation of Eureka graben faults and 

by regional concentric and radial fractures (Smith and Bailey, 

1968). However, Lipman and others (1973) suggested that 

it is difficult to distinguish any Silverton caldera resurgence 

from the broad resurgent doming that occurred over the earlier 

and much larger calderas.

An intricate system of radial fractures developed that 

are especially abundant in the northwest and southeast sides 

of the San Juan caldera. This radiating fracture system likely 

formed during resurgent doming, and thus the area affected 

by resurgence extended beyond the San Juan caldera margin (Lipman and others, 1973). Many of the structures of 

the Eureka graben and radiating fracture system were later 

mineralized and became economic exploration targets for 

base-metal sulfides and precious metals. Nonetheless, the 

continued reactivation of the Eureka graben after San Juan 

caldera resurgence is one cause of the extensive mineralization 

of these structures. Some of the most metal-rich veins fill the 

Eureka graben faults. These are the Ross Basin, Sunnyside, 

Bonita, and Toltec vein systems (pl. 1).

Figure 11. Highly fractured and weakly mineralized intermediate-composition Silverton Volcanics lavas 

along Silverton caldera structural margin, near confluence of Mineral Creek and Middle Fork Mineral Creek.
N

South Fk Mineral Cr

A nim as

R

M

ni eral Cr

 Cem

e

n

t Cr

Silverton

10 0 10 MILES

10 0 10 KILOMETERS

Figure 12. Erosion along structural margin of Silverton caldera by Mineral Creek and Animas River has resulted in an arcuate, 

semicircular drainage pattern that accentuates area occupied by the Silverton caldera. Nonvegetated areas north of South Fork 

Mineral Creek and between Mineral and Cement Creeks are areas of hydrothermal alteration. (Remote sensing processing by 

R.R. McDougal.)

Mineralized Faults and Fractures

Thousands of veins occur at and (or) below the surface in 

the vicinity of the nested San Juan and Silverton calderas. Vein 

orientation in the mining districts near Silverton appears to 

have a strong control on whether veins are highly mineralized. 

In the South Silverton area, located southeast of Silverton, 

veins that are parallel to the caldera ring faults are only weakly 

mineralized (Varnes, 1963). It is the radial, northwest-oriented 

veins that are the most highly mineralized features. Veins 

in the study area, such as those of the Shenandoah-Dives 

deposits, have lateral extents exceeding 3 km and were mined 

through a vertical extent of greater than 300 m. Similarly, it is 

the northwest-trending veins near the northwest periphery of 

the Silverton caldera near the Camp Bird mine that are also 

highly mineralized (Hutchinson, 1988). In contrast, structures 

related to the Eureka graben had a strong influence in controlling ore deposition in the Sunnyside mine (Casadevall and 

Ohmoto, 1977). Sunnyside mine vein systems are focused 

near the intersection of the Sunnyside and Bonita faults. The 

Sunnyside mine workings beneath Eureka Gulch exposed mineralized vein structures that are parallel to the Eureka graben 

(fig. 2; pl. 1), which contain strongly developed northeast- to 

northwest-trending mineralized veins. The workings of the 

Sunnyside vein deposits were mined through a lateral and 

vertical extent of 2,100 m and 610 m respectively. Common 

vein minerals identified in vein systems of the Sunnyside mine 

workings are listed in table 1; vein minerals of the Sunnyside 

mine workings that occur with less than 0.5 percent abundance 

are listed in table 2 (Casadevall and Ohmoto, 1977).

128 Environmental Effects of Historical Mining, Animas River Watershed, Colorado
Geologic Framework 129

peu

Tsj Tse

Tse

Tsv

Tsv

Tsv Tsj Tsv

Tsj

Qu

Qu

FEET

14,000

12,000

10,000

8,000

6,000

METERS

4,000

3,000

2,000

Boulder Gulch

Animas River Qu

Hazelton 

Mountain

Kendall

Peak Arrastra

subbasin

SILVERTON CALDERA

Qu

Tsv

Tse

 Quaternary sediments

 Silverton Volcanics lavas

Eureka Member of Sapinero

 Mesa Tuff

Tsj San Juan Formation intermediate- composition laharic breccias and lavas

Precambrian rocks, undifferentiated

Fault— Dashed where approximately

 located; barbs show relative movement where known or inferred

EXPLANATION

SOUTH NORTH

Silverton caldera 

ring fault zone

0 0.5 1 MILE0 1 KILOMETER

Figure 13. Generalized cross section (modified from Luedke and Burbank, 2000), showing structural offset along ring fault zone of Silverton caldera. Note Eureka Member of the Sapinero Mesa Tuff exposed above valley floor to the south of Animas River; no Eureka Member is exposed in vicinity of Boulder Gulch.
Implications of Caldera Faults and Veins 

as Flow Paths

Caldera ring faults and associated veins of the Eureka 

graben and radial vein structures near the margin of the 

nested San Juan and Silverton calderas (pl. 1) are laterally and 

vertically continuous. Thus, these features may be important 

ground-water flow paths. Veins in the study area have widths 

that range from less than 1 m to 30 m. The wider veins are 

best described as vein zones that consist of an anastomosing 

network of multiple veins. Not all vein systems, however, are 

open fluid flow paths for ground water today. Several veins that 

were mined as part of the Sunnyside mine workings, which 

are exposed at the surface in Eureka and Placer Gulches, are 

annealed with quartz and reveal little open space for fluid 

migration. However, a vein that is filled at the surface can be 

heterogeneous: it can display open cavities at depth that could 

provide flow paths for ground water if such cavities were 

interconnected and continuous. Multiple fractures also crosscut 

veins in many places, which allows for penetration of water 

and thus exposes minerals within vein interiors to weathering.

The inward-sloping surface of the arcuate San Juan 

caldera topographic margin is an angular unconformity that 

should also be considered as a potential ground-water flow 

path. This is especially true in the region near the South 

Silverton mining district, where the topographic surface likely 

dips toward the north in the subsurface toward the center of 

the Silverton caldera. The chaotic and fragmental nature of 

deposits along the topographic margin in the subsurface could 

provide a permeable flow path for ground water, where the 

related deposits are not completely sealed with gangue minerals, such as quartz, or with ore minerals.

In order to test the degree to which bedrock fractures and 

caldera-related structures influence water quality, conductivity 

geophysical surveys were used to map structures that contain 

conductive metal-rich water. The electromagnetic geophysical 

survey shows a conductive arcuate feature that coincides with 

the Silverton caldera ring fracture (Smith and others, this volume, Chapter E4). The importance of veins, lineaments, and 

structures as pathways for conductive, metal-rich waters, along 

with the correlation with geophysical signatures, is discussed 

further in later chapters (McDougal and others, this volume, 

Chapter E13; Smith and others, this volume).

Alteration Types

The effects of a long-lived cycle of volcanotectonic 

activity and multiple mineralization events that span more than 

20 million years have resulted in five major alteration types 

in the study area (Bove and others, this volume). The multiple 

alteration and coincident mineralization events postdate caldera 

collapse, taking place between 26 and 10 Ma (Bove and others, 

2001). For a detailed discussion of the alteration types, see 

Bove and others (this volume).

Airborne Visible and Infrared Imaging Spectrometer 

(AVIRIS) data (Dalton and others, this volume, Chapter E2) 

and detailed field map data were combined to produce regional 

and detailed alteration maps (Bove and others, this volume; 

Dalton and others, this volume). X-ray diffractometry was 

completed on whole-rock hand samples to independently 

verify alteration assemblages identified via AVIRIS and field 

mapping. These techniques are useful, especially when applied 

together to decipher alteration assemblages across broad 

regions, such as at the watershed scale.

The five major alteration types identified in the region 

are listed in increasing order of weakest to most intense, as 

follows: (1) propylitic, (2) weak sericite-pyrite, (3) vein-related 

quartz-sericite-pyrite, (4) quartz-sericite-pyrite, and (5) acidsulfate (fig. 14). Those assemblages with a high acid-generating 

capacity include the weak sericite-pyrite, quartz-sericite-pyrite, 

vein-related quartz-sericite-pyrite, and acid-sulfate assemblages. 

A summary of the alteration types follows.

Regional Propylitic Alteration

Regional propylitic alteration identified by Burbank 

(1960) as a “pre-ore propylitization” event has affected most 

rocks in the vicinity of the San Juan–Uncompahgre and 

Silverton caldera complex. The mineralogy of this assemblage, 

Table 1. Major vein minerals of the Sunnyside mine workings.

[Minerals listed in decreasing volume percent abundance]

Vol. pct

quartz (SiO2

) 30–35

sphalerite (ZnS) 10–15

galena (PbS) 10–15 

pyroxmangite (MnSiO3) 10–15

pyrite (FeS2

) 6–8

rhodochrosite (MnCO3

) 5–8

chalcopyrite (CuFeS2) 3–5

tetrahedrite (Cu12Sb4

S13) 1–4

fluorite (CaF2

) 1

calcite (CaCO3) 1

Table 2. Minor (<0.5 volume percent) vein minerals of the 

Sunnyside mine workings.

hematite (Fe2O3) gold (Au)

petzite (AuAg3Te2) calaverite (AuTe2)

alabandite (MnS) huebnerite (MnWO4

)

tephroite (Mn2

–SiO4) friedelite (Mn8Si6O18)

helvite (Mn4

(Be3Si3O12)S) anhydrite (CaSO4)

sericite (KAl2 (AlSi3O10)(OH2) aikinite (PbCuBiS3)

bornite (Cu5FeS4) barite (BaSO4)

gypsum (CaSO4(2H2O))

130 Environmental Effects of Historical Mining, Animas River Watershed, Colorado
Geologic Framework 131

although variable, consists of quartz, chlorite, epidote, calcite, 

albite, pyrite, and opaque oxides in the presence of unaltered 

to slightly altered primary feldspar crystals (fig. 15). This 

alteration type is thought to have formed during the period 

28.2 Ma to about 27.6 Ma, as the nearly 1 km sequence of 

Silverton Volcanics lavas that infilled the San Juan caldera 

cooled, degassed, and literally stewed in their own volatiles. In 

the process of degassing, large quantities of CO2

, perhaps the 

primary rock-altering volatile component (among others such 

as H2

O and SO2), were released, altering the original minerals and matrix of the country rock (Burbank, 1960). Rocks 

that have experienced this type of alteration commonly have 

a green tinge in outcrop owing to the presence of chlorite and 

epidote. The subsequent, more intense alteration and mineralization events in several places overprint the regionally 

pervasive propylitic alteration assemblage.

Propylitic

107°45' 107°37'30"

37°52'30"

37°47'30"

EXPLANATION

Animas River

Animas River

Cement Creek

Mineral Creek

South Fo rk

Middl e Fork

0

0 5 MILES

8 KILOMETERS

Alteration Types

Increasing intensity of alteration

Acid-sulfate

Quartz-sericite-

 pyrite

Vein-related 

 quartz-sericite-

 pyrite

Weak sericite-

 pyrite

Figure 14. Generalized alteration map of Animas River watershed study area (Bove and others, this volume). Hillshaded, 10-m 

resolution digital elevation model used as base image.
A B

C D

E F

epidote

chlorite

calcite- and 

quartz-filled 

vesicle 

plagioclase

cpx

plagioclase

plagioclase

epidote

plagioclase

calcite- and 

quartz-filled 

vesicle 

cpx

plagioclase

plagioclase

Figure 15. Photomicrograph pairs (crossed polars on left, plane light on right) of intermediate-composition volcanic rocks sampled 

from the study area. Each frame is about 3×4 mm. Letters A–B, Weakly propylitized andesite porphyry of the San Juan Formation 

collected near headwaters of Middle Fork Mineral Creek. Rocks are relatively fresh (note mottled but intact plagioclase); however, 

vesicles and groundmass are filled with chlorite, epidote, and calcite. C–D, Propylitized intermediate-composition lava flows from 

the Silverton Volcanics, sampled near Sunnyside mine. Rock is porphyritic and composed of plagioclase, quartz, clinopyroxene, and 

opaque oxides. Greenish-gray hue of groundmass is typical of propylitic assemblage and is caused by microcrystalline chlorite and 

epidote. E–F, More strongly propylitized Silverton Volcanics sampled about 300 m from C, D. Note more highly altered plagioclase 

(faint polysynthetic twinning) and felty matrix consisting of chlorite and epidote. Most if not all study area rocks have been affected 

by some degree of propylitic alteration; higher grades of alteration locally obliterate all primary mineral phases.

132 Environmental Effects of Historical Mining, Animas River Watershed, Colorado
Geologic Framework 133

Weak Sericite-Pyrite Alteration

The weak sericite-pyrite assemblage is found in areas 

of densely spaced veins and adjacent to areas of the more 

intensely altered quartz-sericite-pyrite assemblage. Quartz, 

sericite (synonymous with illite for fine muscovite), and pyrite 

characterize this assemblage, as do weakly altered to unaltered 

plagioclase and chlorite. One of the largest areas of weak 

sericite-pyrite altered rock is present near the periphery of a 

low-grade copper-molybdenum porphyry system centered near 

peak 3,792 m between South Fork Mineral Creek and Middle 

Fork Mineral Creek (fig. 4) (Ringrose, 1982; Yager and others, 

2000). The more weakly altered rocks affected by this alteration appear similar to the propylitic assemblage in outcrop, 

but they can be distinguished by the abundance of pyrite along 

dense fractures and somewhat lesser presence of magnetite, 

epidote, and chlorite along fractures.

Vein-Related Quartz-Sericite-Pyrite Alteration

Vein-related zones of quartz-sericite-pyrite alteration have 

formed adjacent to mineralized single veins or vein zones, such 

as those adjacent to and within the Eureka graben. A northeastsouthwest-trending 2.4 km wide by 6.6 km long area of relatively densely spaced vein-related quartz-sericite-pyrite altered 

rock occurs between Houghton Mountain on the northeast and 

Minnesota Gulch to the southwest. This vein swath is generally centered near and may be related to fractures that formed 

in the resurgent core of the San Juan caldera. Other areas of 

vein-related quartz-sericite-pyrite alteration have formed along 

radial fractures outside the San Juan caldera margin.

Quartz-Sericite-Pyrite Alteration

Generally, all the primary minerals that constituted 

the original host rock are altered in the quartz-sericite-pyrite 

assemblage throughout relatively large areas. Primary feldspars and mafic minerals are altered to a fine-grained rock 

of quartz, sericite, and pyrite. In outcrop, rocks are bleached 

white; fresh pyrite is observed on freshly broken, unweathered 

surfaces. Sericite resembles very fine muscovite with a sugary 

texture. Where this assemblage has been oxidized, jarosite 

and goethite coat weathered surfaces and stain the outcrops 

light yellowish green to various shades of brown. Only 

weathered pyrite casts along with sericite and quartz remain 

in oxidized outcrops. Some areas that are highly affected by 

quartz-sericite-pyrite alteration include that of peak 3,792 m 

(fig. 4), the region near Ohio Peak, Houghton and California 

Mountains, and upper Prospect Gulch.

Acid-Sulfate Alteration

This alteration assemblage is characterized by the mineral 

assemblages of quartz-alunite and quartz-pyrophyllite, with 

some argillic alteration zones. Alunite in some outcrops near 

Red Mountain is fine grained, pinkish white, and relatively 

soft; it has replaced quartz and rectangular phenocrysts as 

much as 1 cm across. All primary minerals have been obliterated and replaced by the mineral assemblages mentioned. 

There are two regions of acid-sulfate alteration. One broad 

area of acid-sulfate alteration is centered near Red Mountain 

Nos. 1–3; another formed along Anvil Mountain between the 

headwaters of Ohio Gulch and a location about 0.6 km to the 

southeast of Zuni Gulch north of Silverton. Outcrops are commonly bleached white or stained brown by adjacent oxidized 

mineralized veins.

Acid-Generating Capacity 

of Non-Mining-Affected Rocks

One excellent example of the strong influence that 

alteration has on water quality is observed near the low-grade 

molybdenum-copper porphyry deposit near peak 3,792 m 

(fig. 4). The acid-generating capacity of non-mining-affected 

rocks related to the weak sericite-pyrite and quartz-sericitepyrite assemblages is clearly documented in the east- to 

northeast-flowing tributary that enters Middle Fork Mineral 

Creek to the north of peak 3,792 m (fig. 4). The headwaters of 

this tributary originate in San Juan Formation rocks and have 

a pH that ranges from 6.5 to 6.8. In contrast, farther downstream, the acid-generating capacity of these rocks increases 

dramatically as surface water interacts with the weak sericitepyrite and quartz-sericite-pyrite altered San Juan Formation 

rocks and porphyry intrusive rocks: pH drops below 3.5 (Mast 

and others, 2000; Mast and others, this volume, Chapter E7; 

Wright, Simon, and others, this volume, Chapter E10). This 

is one excellent example among many others in the study area 

where alteration has a strong influence on water quality (Bove 

and others, this volume; Mast and others, this volume; Wright, 

Simon, and others, this volume).

Acid-Neutralizing Capacity 

of Study Area Rocks

Hydrothermal alteration and mineralization have obliterated phases that aid in mitigating acid rock drainage in several 

regions of the study area. However, sedimentary rocks that 

contain calcite, and volcanic and other igneous rocks that have 

been altered to the propylitic assemblage chlorite-epidotecalcite, have a moderate to high acid-neutralizing capacity.

Metamorphic Rock Acid-Neutralizing Capacity

The Precambrian Irving Formation, consisting of amphibolite, gneiss, and schist, is important for its potentially high 

acid-neutralizing capacity. Regional uplift, tilting to the west, 

and subsequent retrograde metamorphism of Irving Formation 
rocks (Steven and others, 1969) resulted in formation and 

concentration of the acid-neutralizing mineral assemblage 

of chlorite-epidote-calcite (Desborough and Driscoll, 1998; 

Desborough and Yager, 2000). The Precambrian Irving 

Formation, although not compiled separately from other 

Precambrian units in this study, is exposed in the southern part 

of the area of plate 1. Areas where surface water interacts with 

Precambrian Irving Formation rocks include the headwaters 

of Cunningham Creek, the Animas River downstream of 

Silverton at the mouth of Deadwood Gulch, and other tributaries downstream including the area drained by Deer Park 

Creek.

Sedimentary Rock Acid-Neutralizing Capacity

Both the Ouray and Leadville Limestone units are 

important for their high acid-neutralizing capacity in the upper 

Animas River canyon where sediment weathered from these 

rocks reaches the Animas River. The Hermosa and overlying Permian Cutler Formations are the most volumetrically 

extensive Paleozoic units in the upper part of the watershed. 

These units are important because they are in some areas 

calcite bearing (Burrell, 1967). They therefore have a high 

acid-neutralizing capacity that is realized where weathering 

and erosion contribute reworked Cutler and Hermosa sediment to South Fork Mineral Creek and to the Animas River 

downstream of Silverton. The Telluride Conglomerate commonly contains abundant limestone clasts and locally may 

provide a high acid-neutralizing capacity. However, this unit 

is volumetrically minor in comparison with other calcareous 

sedimentary units; on a regional scale it likely does not have a 

large acid-neutralizing effect in the study area.

Tertiary Volcanic Rock Acid-Neutralizing 

Capacity

Quantitative investigations by Desborough and Yager 

(2000) on rocks of the watershed study area attest to the 

relatively high acid-neutralizing capacity of igneous bedrock. 

Most if not all of the volcanic rocks in this area were subjected to a regional propylitic alteration event that replaced 

the primary magmatic mineral phases amphibole, pyroxene, 

and plagioclase with secondary chlorite, epidote, and calcite. 

Sericite is locally pervasive. Calcite fills vesicles and microfractures (figs. 15, 16), floods the groundmass in lava flows 

and in volcaniclastic rocks (Bove and others, this volume), 

and thereby contributes to the high acid-neutralizing capacity 

of these rocks (Desborough and Yager, 2000).

Regionally, the large volume of San Juan Formation 

volcanic rocks available for water-rock interaction has a 

strong influence on water quality, depending on the amount 

of secondary propylitic minerals and textural characteristics. 

calcite and chlorite

microveinlet

epidote

Figure 16. Photomicrograph of propylitized intermediate-composition Silverton Volcanics lava flow, with 

calcite and chlorite microveinlet. Full frame is about 1 mm by 2 mm.

134 Environmental Effects of Historical Mining, Animas River Watershed, Colorado
Geologic Framework 135

The enhanced water-rock interaction of this formation results 

from its large volume of porous and permeable volcaniclastic 

rocks, with only relatively minor and highly fractured lava 

flows.

The Silverton Volcanics, in the core of the San Juan 

and Silverton calderas, is very important both volumetrically and for its high acid-neutralizing capacity. Where not 

intensely altered, most of the Silverton Volcanics have experienced propylitic alteration. Propylitization of the Silverton 

Volcanics is thought to have occurred as volatiles such as 

CO2

, SO2, and H2O degassed from the cooling, 1-km thick 

volcanic pile (Burbank, 1960). Silverton Volcanics lavas that 

lack calcite but contained abundant chlorite were still found to 

have a 10 weight percent (calcite equivalent) acid-neutralizing 

capacity (Desborough and Yager, 2000). Locally, the Silverton 

Volcanics have the strongest influence on water quality, 

depending on the amount of secondary propylitic minerals 

that impart a high acid-neutralizing capacity (Desborough 

and Yager, 2000), the degree of hydrothermal alteration, and 

textural characteristics. Lava flow foliation in the vicinity of 

Boulder Gulch and along Mineral Creek follows the dip of the 

central resurgent dome toward the west and south. The flow 

foliation facilitates fluid transport and promotes water-rock 

interaction. Surface water that passes through propylitically 

altered volcanics in Boulder Gulch has near-neutral pH (Mast 

and others, this volume, Chapter E7). However, Silverton 

Volcanics host the majority of mineral deposits mined in 

the Silverton region (Bejnar, 1957; Casadevall and Ohmoto, 

1977), and in areas where late-stage hydrothermal alteration 

introduced acid-generating minerals, such as pyrite in Prospect 

Gulch, acid-neutralizing capacity is eliminated and acidic 

drainage can have a pH as low as 2.6 (Bove and others, this 

volume; Mast and others, this volume).

Postcaldera intrusions of the Sultan Mountain and peak 

3,792 m granitic stocks also underwent propylitic alteration, 

altering the primary mafic minerals such as biotite to chlorite. 

Acid-neutralization experiments performed on these propylitically altered intrusives indicate an acid-neutralizing capacity 

as high as that of the Silverton Volcanics lavas that lack calcite 

(Desborough and Yager, 2000).

Surficial Deposit Acid-Neutralizing Capacity

Weathering of acid-sulfate mineralized bedrock and 

subsequent deposition of weathered material in downslope 

surficial deposits will result in acid generation, as ground 

water infiltrates and reacts with pyrite and other acidgenerating minerals. In contrast, surface water infiltrating 

surficial deposits containing abundant propylitic rocks with 

the mineral assemblage chlorite-epidote-calcite will probably 

undergo some reduction in acidity (Desborough and Yager, 

2000; Yager and others, 2000; Wirt and others, this volume, 

Chapter E17).

Late Tertiary Erosion

Neogene erosion denuded much of the San Juan 

Mountains volcanic cover. In places canyon cutting has 

amounted to as much as 1,520 m (Smith and Bailey, 1968). 

This period of erosion has long been referred to as the “canyon 

cycle of erosion” (Lee, 1917). The high potential for erosion 

developed during this time owing to differential displacement 

of the San Juan Mountains relative to the incipient, adjacent 

Rio Grande Rift basin to the east and post-mid-Tertiary extensional tectonic terrain. Chapin and Cather (1994) estimated 

Precambrian bedrock offset in adjacent mountain ranges and 

basins to gauge the amount of late Tertiary relative displacement. Using this technique they determined that an average 

of 5 km of relative displacement of Precambrian rocks has 

occurred. They further estimated from basin stratigraphy 

that from 7 to 30 times more sediment was supplied to the 

Rio Grande Rift basin during the Miocene to Pliocene than 

in the Pleistocene. Clearly, late Tertiary erosion was important in exposing large surface areas of mineralized bedrock 

and metalliferous veins to weathering, in the Animas River 

watershed study area. Steven and others (1995) suggested that 

regional uplift and tilting from 5 to 2 Ma, concurrent with continued active rifting in the Rio Grande Rift to the east, further 

accelerated erosion and canyon cutting throughout the uplifted 

area.

Molinar and England (1990) proposed an alternative 

hypothesis for the increased erosion rates. They suggested that 

increased erosion rates during the late Cenozoic may be due to 

climate change. A trend toward lower temperatures and stormier climate could greatly contribute to increased erosion in the 

San Juan Mountains and subsequent deposition of alluvial fans 

adjacent to mountain ranges.

Glaciation Events

Pleistocene, Wisconsin-age glaciation greatly altered 

the San Juan landscape (Atwood and Mather, 1932; Gillam, 

1998). The San Juan ice sheet was one of the largest (second only to the Yellowstone-Absaroka) to form south of the 

upper-mid-latitude Laurentide ice sheet (Atwood and Mather, 

1932; Leonard and others, 1993). The San Juan ice sheet 

covered approximately 5,000 km2

, was as much as 1,000 m 

thick, covered all but the highest peaks, and formed piedmont 

glaciers that extended to the adjacent foothill areas (Atwood 

and Mather, 1932).

Unstable slope conditions developed after glacier retreat. 

Weathering processes in glaciated cirque basins and mountain valleys have resulted in accumulation of large volumes 

of talus, debris cone, and landslide deposits. The mineral 

constituents present in the surficial deposits will influence the 

acid-generating or neutralizing capacity of these deposits.
Pleistocene to Holocene Fluvial 

Geomorphologic Conditions

The numerous surficial deposits in the watershed study 

area serve as porous and permeable pathways for surfacewater infiltration and ground-water flow. From GIS analysis 

of the geology polygon coverage (Sole and others, this volume, Chapter G), Yager and Bove have estimated that 27 percent of the bedrock in the watershed is covered by between 

1 and 5 m or more of Pleistocene to Holocene sedimentary 

deposits (pl. 1). These deposits formed as a result of mass 

wasting of weathered bedrock outcrops and accumulated in 

cirque basins and on slopes and in valleys to form talus, landslides, and colluvium. Alluvial processes have formed multiple 

generations of stream terraces (Gillam, 1998) and caused 

deposition of alluvial fans and flood-plain sediments along 

main drainages and their tributaries (Blair and others, 2002).

The surficial deposits are important in the context of 

abandoned mine land investigations because deposits that 

formed prior to mining record the geochemical baseline of 

sediments that were deposited by natural processes. These 

have been analyzed and compared with deposits that formed 

after the onset of mining to determine how the geochemical 

baseline has changed once mining commenced (Church and 

others, 1997; Vincent and others, 1999; Blair and others, 2002; 

Church, Fey, and Unruh, this volume).

Surficial deposits are also the principal precipitation sites 

for iron-rich water that is initially derived from oxidation of 

pyrite by acidic water and then precipitated to form ferricrete 

deposits. Ferricrete deposits are iron oxyhydroxide cemented 

clastics, or nonclastic bog iron (Verplanck and others, this volume, Chapter E15, and this report, pl. 2). Radiocarbon dates 

on logs preserved in ferricrete deposits range from modern to 

9,000 yr B.P. (Verplanck and others, this volume). A relationship exists between upslope sources of acidic drainage, mineralized bedrock, structures, and the Holocene surficial deposits 

where ferricrete has formed that substantiates the inferred link 

between acidic water and ferricrete deposits (Vincent and others, this volume, Chapter E16; Wirt and others, this volume; 

this report, pl. 2).

Current studies indicate that geologic processes affecting erosion prior to mining have not changed dramatically 

in the past 15,000 years (Vincent and others, this volume). 

Around the end of the 19th century, however, mill wastes 

were dumped into streams and transported throughout the river 

system (Nash, 2000; Vincent and others, 1999; Nash and Fey, 

this volume). This affected the Animas River’s hydrology. 

For example, mill waste that was reworked by floods below 

Eureka town between 1900 and 1930 resulted in sediment 

aggradation along a low-gradient reach of the upper Animas 

River (Blair and others, 2002). These fluvial tailings included 

some concentrated metal-rich stamp mill tailings, as well as 

extensive tailings deposits from the Sunnyside Eureka Mill 

(Jones, this volume; Church, Mast, and others, this volume). 

The effect of dumping mill tailings into the streams was 

a dramatic impact on vegetation health, and especially on 

bank-stabilizing willows that were characteristic of the upper 

Animas River riparian zone prior to mining (Vincent and others, 1999; Vincent and Elliott, this volume, Chapter E22).

Elsewhere, the fluvial geomorphology in basins such as 

Cement Creek appears to have been minimally affected by 

mining (Vincent and others, this volume). This is not to say, 

however, that mining has not altered historical water compositions. Tracer injection studies along Cement Creek have identified the principal trace- and major-element loading sources 

(Kimball and others, this volume, Chapter E9).

Although the overall geomorphology of the present 

Cement Creek basin has not been greatly modified, sudden 

storms can cause rapid movement of large volumes of mineralbearing and acid-generating sediment. These events occur 

frequently and confound watershed characterization efforts. 

On August 26, 1997, one such storm upstream of Silverton 

sent thousands of cubic meters of sediment pouring into 

Cement Creek, principally from Topeka and Ohio Gulches. 

Furthermore, geochemical analyses of debris-flow sediments 

collected from the August 26 event indicated high concentrations of iron (6.8 weight percent) and aluminum (9.0 weight 

percent). Laboratory passive leach experiments indicated that 

these deposits also had a high potential to generate acidity. 

The following year, the 1998 spring stream runoff transported 

any sediment that remained along the banks of Topeka Gulch 

from the previous summer’s storm event, into Cement Creek.

Summary

One goal of this abandoned mine lands study was to 

investigate the influence that the diverse volcanic lithology and 

alteration assemblages have on the geochemical baseline and 

on water quality in the Animas River watershed. A 1-km thick, 

weakly to intensely altered and mineralized, intermediate- to 

felsic-composition volcanic cover largely determines waterrock interaction and water quality in the core of the Animas 

River watershed study area. Subsequent regional, low-grade 

propylitic alteration resulted in formation and concentration of 

the acid-neutralizing mineral assemblage of chlorite-epidotecalcite. Replacement of primary mineral phases by the propylitic acid-neutralizing mineral phases is pervasive in the core 

and areas surrounding the Silverton and San Juan calderas. 

Later caldera structures associated with the Tertiary volcanics 

served as conduits for late-stage hydrothermal mineralizing 

fluids, which overprinted the earlier regional propylitic alteration event. Mineralization changed the primary mineral assemblage of host rocks and either eliminated or improved their 

acid-neutralizing capacity, depending on the intensity and type 

of alteration. Faults of multiple ages and orientations, bedding 

dip, and fracture density and orientation further dictate fluid 

flow and influence the surface- and ground-water drainage 

from the alpine basins to the adjacent valleys.

136 Environmental Effects of Historical Mining, Animas River Watershed, Colorado
Geologic Framework 137

Miocene to Pliocene erosion and Pleistocene glaciation 

exposed the Animas River watershed study area to extensive 

weathering and erosion to form surficial deposits that constitute as much as 27 percent of surface area exposures. Where 

these surficial deposits are derived from sulfatarically altered 

and mineralized bedrock, they serve as porous and permeable pathways that permit ground water to react with acidgenerating minerals to produce acid water and load soluble 

major and trace elements to streams. In contrast, where these 

surficial deposits are derived from bedrock of the propylitic 

assemblage, they permit ground water to react with acidneutralizing minerals and hence have the potential to neutralize acid drainage.

The red, yellow, and white precipitates from water 

draining hydrothermally altered rock and many mine sites 

are a strong visual indication of the effects that geology and 

mining have had on water quality in some tributary basins 

near Silverton, Colo. The environment is that of an extensively fractured and altered, mountainous watershed and 

water-storage system, which can supply either acid-generating 

or acid-neutralizing water, depending on the composition of 

minerals in the transport media (for example, fractured bedrock 

aquifer, porous surficial deposit, mine-waste pile, and (or) 

fluvial environment) and the amount of surface-water and 

ground-water interaction that transpires along a flow path in 

any of these transport media.

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