U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2019–3026

June 2019

Figure 1. Three-dimensional (3D) view west across upper Cement Creek (lower left) in Silverton, Colorado, study area. Mapped alteration 

from Bove and others (2007) draped on terrain. Alteration mapped in green are weakly altered rocks with acid neutralizing capacity in some 

places because of calcite. Alteration mapped in red, yellow, pink, and blue are areas of intense alteration, often containing pyrite and having 

acid-generating potential. Intensely altered areas are sometimes associated with economic mineralization. Conceptual cutaway of mountain 

(lower right) shows an example of the complexity of geologic and mining features that influence hydrologic flow paths that geophysics in this 

study will be used to interpret and map in 3D.

EXPLANATION

Near surface fractures 

 Deep fractures 

Groundwater flow, small

 arrow (shallow)

 large arrow (deep)

Intrusion (with brecciated margin)

vein (yellow) vein along fault (red)

Breccia pipe

Mine tunnel with tailings pile 

 at surface 

Mine adit discharge

Cement Creek

study

area

COLORADO

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0 .50 1 MILE

Prepared in collaboration with U.S. Forest Service, Bureau of Land Management, U.S. Environmental Protection Agency, Colorado Division 

of Reclamation Mining and Safety, Colorado Department of Public Health and Environment, and Animas River Stakeholders Group

This study integrates geological and geophysical data important for developing a three-dimensional (3D) model 

of the San Juan-Silverton caldera complex. The project aims to

• apply state-of-the-art geophysical data processing techniques to legacy data; 

• map subsurface lithologies, faults, vein structures, and surficial deposits that may be groundwater flow paths;

• better understand geophysical response of mineral systems at depth; and 

• provide geological and geophysical frameworks that will inform mining reclamation decisions and mineral 

resource assessments.

Introduction

The San Juan-Silverton caldera complex located near 

Silverton, Colorado, in the Southern Rocky Mountains volcanic 

field is an ideal natural laboratory for furthering the understanding of shallow-to-deep volcanic-related mineral systems. 

Recent advances in geophysical data processing and threedimensional (3D) model construction will help to characterize 

Geological and Geophysical Data for a Three-Dimensional View—

Inside the San Juan and Silverton Calderas, Southern Rocky 

Mountains Volcanic Field, Silverton, Colorado

shallow properties important for understanding surface water 

and groundwater quality issues and will also improve knowledge of deep geological structures that may have been conduits 

for hydrothermal fluids that formed mineral deposits (fig. 1). 

The study has general applications to mineral resource assessments in other areas of the world and to identifying possible 

groundwater flow paths and associated geochemistry important 

in abandoned mine lands cleanup.
Figure 2. View south of Animas River and Cement Creek confluence. 

Orange hue in Cement Creek is from abundant iron in Gold King Mine 

water released on August 5, 2015. (Photograph by Andrea Kron, parttime resident of Silverton, Colorado)

Ring

Fault

Collapsed

Roof

Ring

Fault

Structural Margin

Magma Chamber

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Silverton, Colorado, is the site of a spectacular succession 

of igneous rocks that formed beginning about 35 million years 

ago (Ma). Base metals (copper, lead, and zinc) and precious 

metals (silver and gold) mined from the late 1870s to 1991 owe 

their existence to a 25-million-year cycle of igneous activity. 

The presence of economic, base, and precious metal deposits 

within a complex geological setting were largely responsible 

for stimulating studies by the U.S. Geological Survey (USGS) 

conducted during the early 20th century. The focus of investigations in the late 20th and 21st centuries have broadened 

in scope to include abandoned mine lands (AML) investigations. The legacy of hard rock mining in headwater catchment 

areas caused environmental challenges for local communities 

and downstream water resource users. The Gold King Mine, 

located a few kilometers north of Silverton, illustrates the 

potential environmental effects of abandoned mines. On August 

5, 2015, during reclamation efforts at the Gold King Mine, a 

breach of collapsed workings sent approximately 3 million gallons of acidic and metal-rich mine water into the upper Animas 

River, a tributary to the Colorado River Basin (fig. 2). Miningrelated sources of metals and acidity add to geological sources 

of metals in surface water and groundwater. Weathering 

processes of altered and mineralized rock have been a source of 

acid rock drainage that have been ongoing for millennia.

What is a Caldera?

Silverton, Colorado, is located within two nested calderas (San Juan and Silverton calderas). Calderas are collapsed 

volcanoes that result when magma is erupted along a ring fault 

zone at a volcanic margin (fig. 3). The ring fault zone, also 

called a structural margin, can be circular to elliptical, greater 

than 15 kilometers in diameter, and extend several kilometers 

into the crust. Rocks overlying the magma chamber (caldera 

roof) that were at the surface of the volcano prior to eruption, 

collapse into the void created during eruption of the magma 

chamber. The volcano–tectonic upheaval during caldera formation causes rocks adjacent to the volcano to be highly fractured 

and faulted. Upwelling or resurgence of new magma beneath 

the intracaldera volcanic fill, pushed up and domed the center 

of the caldera area and caused additional fractures and faults to Figure 3. Conceptual cross section of caldera-forming eruption.

Data For Developing a 3D Model

Geology and Alteration Maps

Legacy and new datasets will be important for developing a 3D geological and geophysical model. Digital geologic and alteration maps of the study area were completed as part of the USGS AML study conducted between 1996 and 2001 (Bove and others, 

2007; von Guerard and others, 2007; Yager and Bove, 2007). These maps along with other geologic mapping are useful to interpret 

geophysical signatures of geologic units, alteration types, faults, vein structures, and mineralized systems. In addition, geologic 

cross sections and structural data are used to construct a 3D model of geology and alteration.

form. The caldera-related volcano-tectonic structures were flow 

paths for hot fluids that deposited base and precious metals.
N

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Mineral Creek

Cement Creek

Anvil

Mountain

Ohio

Peak

Storm

Peak

N

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0 1 MILES

Mineral Creek

Cement Creek

Anvil

Mountain

Ohio

Peak

Storm

Peak

Figure 4. Apparent conductivity from airborne electromagnetic 

survey. Bright colors are conductive areas. Solid and dashed black 

lines (faults); dashed red lines (veins).

Figure 5. Magnetics from airborne electromagnetic survey. Bright 

colors are magnetic areas. Solid and dashed black lines (faults); 

dashed red lines (veins).

Airborne Geophysical Data

An airborne electromagnetic (AEM) and magnetic survey was flown in 1999 as part of the AML study (Smith and others, 

2007; Anderson and others, 2018). New and improved interpretations of these legacy data are now possible using state-of-art 

geophysical data processing methods that were previously unavailable. Reprocessing these data will aid in mapping subsurface 

lithologies and interpreting electrical conductivity signatures and magnetic properties of rocks, sediments, and geologic structures 

in relation to mine workings and associated mineral deposits. The magnetic and electromagnetic (EM) response of the mineral 

systems can be measured and modeled in 3D in our project to help characterize mineral deposits in this and similar geologic 

settings around the world.

Electromagnetic Data

Electromagnetic properties of rocks and sediments vary 

depending on minerals present, moisture content, and percentage 

of clay minerals. Rocks and sediments that have sulfide minerals can be electrically conductive. Dry bedrocks lacking sulfide 

minerals tend to have low conductivity. Wet bedrocks and clays 

containing water in their crystal structure can exhibit intermediate conductivity because of conduction through pore fluids and 

along clay mineral surfaces. Electrical properties of mineralized systems, such as those that formed in the Silverton caldera 

complex, can be imaged using EM methods. The Silverton AEM 

survey was flown using a helicopter that towed an EM transmitter and receiver, which measured the conductivity of the material 

being traversed (fig. 4). The maximum depth of penetration of 

the EM survey was about 150 meters.

Magnetic Data

Magnetic properties of rocks vary depending on minerals present, mostly magnetite, a common accessory mineral in 

igneous rocks. In a caldera setting, magnetite can be produced 

or destroyed as hydrothermal fluids alter the surrounding rock. 

Aeromagnetic anomalies are caused by two kinds of magnetism: induced and remanent. The induced component occurs 

when magnetic material is affected by a primary magnetic field. 

The remanent component is inherited in a rock on removal of a 

magnetic field and is dependent on the thermal, mechanical, and 

magnetic history of the rock. The induced component is mostly 

determined by the magnetic susceptibility of a rock, which is 

a quantitative measure of how well a material can be magnetized in the presence of an applied magnetic field. In general, 

rocks with higher magnetite content have higher magnetic 

susceptibility. Thus, the rocks and mineral systems in the San 

Juan-Silverton caldera complex can be expected to produce 

aeromagnetic highs and lows because of rocks with contrasting magnetic susceptibility. An aeromagnetic map of part of the 

study area is shown in figure 5. In this environment, these data 

are sensitive to depths less than 5 kilometers.
ISSN 2327-6916 (print)

ISSN 2327-6932 (online)

https://doi.org/10.3133/fs20193026

Magnetotelluric Data

We began collecting new magnetotelluric (MT) data 

in 2018 (fig. 6). The MT method is a passive groundbased electromagnetic technique that investigates 

subsurface electrical conductivity at depths of tens of 

meters to tens of kilometers. It does so by measuring 

time variations in Earth’s electric and magnetic fields. 

Worldwide lightning activity and geomagnetic micropulsations from solar wind-charged particles interacting 

with Earth’s magnetosphere provide the main source 

of signal used by the MT method. EM properties of 

rocks and sediments for the MT method are the same 

as described in the Electromagnetic Data text box. The 

MT data help to characterize the electrical conductivity of mineralized caldera structures and adjacent rocks 

to much greater depths but that overlap depths imaged 

by the AEM data. The MT information is also useful 

in interpreting causes of high conductivity signatures 

where groundwater flow through volcanic rocks might 

have higher concentrations of total dissolved solids 

attributed to bedrock weathering processes.

Summary

Integration of geological and geophysical data will enable 

construction of a 3D geological and geophysical model of 

the caldera complex permitting new interpretations about 

economic mineralization that are applicable in similar geological 

environments in other parts of the world. The model will also 

allow interpretation of electrically conductive structures and 

deposits that are permissive groundwater flowpaths and conduits 

for acidic and metal-rich water important for mine reclamation 

decisions and abandoned mine cleanup.

References

Anderson, E.D., Deszcz-Pan, M., Yager, D.B., and Smith, 

B.D., 2018, High resolution helicopter-borne magnetic 

and electromagnetic survey Eureka Graben area, Colorado 

September 1999: U.S. Geological Survey data release, 

https://doi.org/10.5066/F75D8R1W.

Bove, D.J., Yager, D.B., Mast, M.A., and Dalton, J.B., 2007, 

Alteration map showing major faults and veins and associated 

water-quality signatures of the Animas River watershed headwaters near Silverton, southwest Colorado: U.S. Geological 

Survey Scientific Investigations Map 2976, 18-p. pamphlet, 

1 plate, scale 1:24,000.

Smith, B.D., McDougal, R.R., Deszcz-Pan, M., and Yager, D.B., 

2007, Helicopter electromagnetic and magnetic surveys, chap. 

E4 of Church, S.E., von Guerard, P., and Finger, S.E., eds., 

Integrated investigations of environmental effects of historical 

mining in the Animas River watershed, San Juan County, 

Colorado: U.S. Geological Survey Professional Paper 1651, 

p. 231–254.

von Guerard, P., Church, S.E., Yager, D.B., and Besser, J.M., 

2007, The Animas River watershed, San Juan County, 

Colorado, chap. B of Church, S.E., von Guerard, P., and 

Finger, S.E., eds., Integrated investigations of environmental 

effects of historical mining in the Animas River watershed, 

San Juan County, Colorado: U.S. Geological Survey 

Professional Paper 1651, p. 17–38. 

Yager, D.B., and Bove, D.J., 2007, Geologic framework, 

chap. E1 of Church, S.E., von Guerard, P., and Finger, S.E., 

eds., Integrated investigations of environmental effects of 

historical mining in the Animas River watershed, San Juan 

County, Colorado: U.S. Geological Survey Professional 

Paper 1651, p. 107–140.

Figure 6. Magnetotelluric field team installing a coil sensor. 

(Photograph by Douglas Yager, USGS).

By Douglas B. Yager, Eric D. Anderson, Brian D. Rodriguez, 

Maria Deszcz-Pan, and Bruce D. Smith