Helicopter Electromagnetic and 

Magnetic Surveys

By Bruce D. Smith, Robert R. McDougal, Maryla Deszcz-Pan, and 

Douglas B. Yager

Chapter E4 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 .......................................................................................................................................................235

Introduction.................................................................................................................................................235

Purpose and Scope ..........................................................................................................................236

Geophysical Surveys .................................................................................................................................236

Magnetic System and Data Processing .......................................................................................236

Electromagnetic System and Data Processing ...........................................................................240

Map Compilation ........................................................................................................................................240

Magnetic and Electrical Properties ........................................................................................................240

Magnetic Responses........................................................................................................................240

Electromagnetic Responses ...........................................................................................................241

Discussion ...................................................................................................................................................241

Magnetic Maps .................................................................................................................................241

Magnetic Features ..................................................................................................................241

Apparent Conductivity Maps ..........................................................................................................246

Apparent Conductivity Features ............................................................................................246

Hydrologic Implications ...................................................................................................................251

Conclusions.................................................................................................................................................252

References Cited........................................................................................................................................253

Plates

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

 3. Total-field magnetic map of the Animas River watershed study area, Colorado

 4. Apparent conductivity map of the Animas River watershed study area, Colorado

Figures

 1. Total-field magnetic map of a region surrounding Animas River watershed 

study area ..................................................................................................................................237

 2. Map showing generalized regional geology of Silverton and Lake City 

calderas, southwest Colorado ................................................................................................238

 3. Map showing generalized geology of Animas River watershed study area, 

and survey flightlines ...............................................................................................................239

 4. Color shaded-relief reduced-to-pole total-field magnetic map of Animas River 

watershed study area ..............................................................................................................242

 5. Color shaded-relief high-pass filtered RTP map for Animas River watershed 

study area ..................................................................................................................................243

 6. Color shaded-relief low-pass filtered RTP map for Animas River watershed 

study area ..................................................................................................................................244

 7. RTP magnetic map showing selected anomalous features ..............................................245
8–10. Color shaded-relief apparent conductivity maps of Animas River watershed 

study area:

 8. At 36,930 Hz .........................................................................................................................247

 9. At 4,310 Hz ...........................................................................................................................248

 10. At 935 Hz ..............................................................................................................................249

 11. Apparent conductivity map showing selected anomalous features ...............................250

Table

 1. Frequency and coil orientation for the electromagnetic system ......................................240

Abstract

A helicopter electromagnetic and magnetic survey over 

the Animas River watershed study area has greatly increased 

the resolution of airborne geophysical data for the study area. 

The main objective of this chapter is to present geophysical 

data and maps that can be compared to other data sets and 

maps of the study area. The results of the survey are useful 

in mapping surface and subsurface lithology and structure. A 

brief discussion of selected geophysical anomalies and trends 

is given. The total-field magnetic and apparent conductivity maps from the helicopter survey are used to interpret 

near-surface and deep geologic and hydrologic features that 

can influence ground-water flow. This qualitative discussion 

supplements a more detailed study using quantitative predictive modeling to characterize structures in the study area.

The total-field reduced-to-pole magnetic map is given 

at a scale of 1:48,000 superimposed on a topographic base. 

Derivative maps are given in color shaded relief at a scale 

of 1:100,000 and include high- and low-pass filtered maps. 

Qualitatively, a central magnetic low with peripheral magnetic 

highs caused by younger intrusives characterizes the Silverton 

caldera. Magnetic lows may be associated with alteration that 

has destroyed magnetic minerals and (or) may be associated 

with reverse magnetization.

Helicopter-borne measurements from five frequencies 

and two system geometries were used to compute apparent 

electrical conductivities. The pre-processed electromagnetic 

data have high noise along flightlines due to helicopter flying 

limitations caused by the high rugged terrain. The apparent 

conductivity map for the intermediate frequency (and depth of 

investigation) is given at 1:48,000 and again on a topographic 

base. The apparent conductivity is also presented as color 

shaded-relief maps at a scale of 1:100,000 for three frequencies and one system configuration. These maps reflect variations of the subsurface electrical conductivity ranging in depth 

from a few meters to on the order of 60 meters. The Silverton 

caldera is generally characterized by conductivity lows in the 

central part ringed by conductivity highs. Conductivity highs 

can be related to electrically conductive surficial rocks and 

sediments as well as deeper bedrock alteration both in narrow 

zones along drainages and in broad areas of hydrothermal 

alteration.

High apparent conductivities near one mine-waste pile 

suggest near-surface flow paths and a source for high dissolved solids where high-sulfide mill tailings have been 

removed after the helicopter survey. The youngest daciterhyolite intrusives show different types of magnetic and electrical properties that may be associated with different types 

of lithologic and ground-water regimes. Apparent conductivity maps suggest a northwest-trending structural zone along 

Cement Creek and extending toward Ohio Peak that may 

influence ground-water flow.

Introduction

An airborne electromagnetic and magnetic survey was 

flown over portions of the Animas River watershed to supplement other geologic, geochemical, and hydrologic studies that 

were carried out for the collaborative study. New airborne 

geophysical data were needed because the only available data 

for the study area were total-field magnetic and radiometric 

data with a flightline spacing of 2 miles (Oshetsky and Kucks, 

2000). In general airborne geophysical methods provide 

data to interpret subsurface lithology, structure, and in some 

cases electrically conductive ground water. Smith and others 

(2000) have described how airborne geophysical surveys can 

be applied to abandoned mine land studies. McCafferty and 

others (2004) have demonstrated the application of quantitative class modeling of airborne electromagnetic and magnetic 

survey data in predictive mapping of acid-neutralizing terrains 

in the Boulder River watershed, Montana.

One main focus of the Animas River watershed AML 

study has been the source, transport, and fate of acid water 

derived from mining activities and from areas that have 

not been mined. Though much of the ground-water flow 

is through streams, an unknown amount flows through the 

subsurface colluvium and bedrock of the study area. Little 

specific information about the nature of this subsurface flow 

exists, although Huntley (1979) and Caine (2003) have made 

some generalizations on a watershed scale. Ground and 

airborne geophysical studies provide data that can be used to 

interpret subsurface heterogeneities that can be related to the 

origin and flow of acid water.

Chapter E4

Helicopter Electromagnetic and Magnetic Surveys

By Bruce D. Smith, Robert R. McDougal, Maryla Deszcz-Pan, and Douglas B. Yager
Purpose and Scope

The purpose of this investigation is to present the results 

of a helicopter electromagnetic and magnetic (HEM) survey 

flown over the Animas River watershed study area. The results 

of the survey are useful in mapping surface and subsurface 

lithology and structure. Geophysical maps from the airborne 

geophysical surveys are given in plates 3 and 4 of this volume. 

The objectives of the study are to:

• Present the airborne geophysical data in maps that can 

be compared to other maps from the AML study

• Describe data collection and reduction procedures for 

the airborne survey

• Present derivative maps that can be used in interpretation of subsurface geophysical features

• Provide some interpretation of geophysical anomalies 

in terms of lithology, structure, and possible hydraulic 

features.

Geophysical Surveys

A regional airborne survey of the study area was flown 

as part of the National Uranium Resource Evaluation (NURE) 

with E.-W. flightlines 400 feet1

 above ground with 2 mile spacing (Aero Service, 1979). Oshetsky and Kucks (2000) used 

these data and other airborne surveys to compile a statewide 

total magnetic field grid at 1,000 feet above ground level with 

a cell size of 500 m square. Figure 1 shows a portion of this 

regional airborne magnetic map for the northwestern part of the 

San Juan volcanic field with the main drainages of the Animas 

River watershed study area (from west to east, Mineral Creek, 

Cement Creek, and Animas River). Locations of three detailed 

helicopter geophysical surveys that have been flown in the 

Silverton and Lake City caldera areas are shown as boxes in 

figure 1 and in figure 2 with more detail. The detailed surveys 

are located over the Lake City caldera (Red Cloud and Lake 

City surveys in fig. 2, not discussed here) and the Silverton 

caldera (Animas survey in fig. 2; see following discussion). 

The regional airborne magnetic survey shows that the Lake 

City caldera is generally characterized by magnetic highs, in 

contrast to the older Silverton caldera, which is characterized 

by a central magnetic low. Grauch (1987a) observed that the 

magnetic expression of the Silverton Volcanics (postcaldera 

volcanic flows and volcaniclastic sediments) is low within the 

Silverton caldera and high within the Lake City caldera. The 

contrasting magnetic expression within this unit is due to variable natural remanent magnetization and to alteration.

The Red Cloud survey (fig. 2) was flown as part of the 

NURE program (High Life Helicopters, 1983) with N.–S. lines 

spaced 0.25 mi (miles) using spectral radiometric and totalfield magnetic systems. The Lake City helicopter survey flown 

(fig. 2) as part of USGS programs (High Life Helicopters, 

1981) used 0.3 mi flightline spacing, N.–S. lines, 400 ft above 

ground. Grauch (1987a, b) has interpreted the Lake City survey 

with emphasis on effects of the rugged terrain on the magnetic 

data.

The Animas helicopter electromagnetic and magnetic 

(HEM) survey was conducted over part of the Animas River 

watershed (fig. 3) during late May 1999 (Smith and others, 

2000, 2004) under a contract to the U.S. Geological Survey. 

The objective of this survey was to map subsurface lithology 

and structure that could be important in geologic, structural, 

and hydrologic studies related to the Animas River AML 

study. The survey flightline spacing was 200 m and 400 m, 

as shown in figure 3. Closer flightline spacing was used in the 

western part of the survey area in order to improve resolution 

in the area of Mineral and Cement Creeks. In addition, flightlines were flown along South Fork Mineral Creek, Mineral 

Creek, Cement Creek, and the upper Animas River. These 

flightlines were done to obtain more detailed data along the 

drainages where tracer studies were planned (Kimball and others, this volume, Chapter E9). They were flown at a more constant elevation than was possible with the north-south flightlines. The stream valleys are deep and narrow, which makes it 

difficult for a helicopter to maintain constant elevation when 

crossing the terrain. Four east-west flightlines were flown as 

tie lines for processing the magnetic field measurements.

The nominal helicopter flight height was specified to 

be 60 m above terrain. In practice the flight elevation varied 

between 60 and 90 m due to flight safety considerations in the 

extremely rugged terrain. The geophysical electromagnetic 

(EM) and magnetic system was towed in an 8 m long torpedoshaped “bird” at a distance of about 30 m below the helicopter. 

The instrumentation included a high-precision GPS system 

that provided a positional accuracy of about 5 m. Barometric 

and radar altimeters provided additional data on the elevation. The position of the flight path provided by the GPS was 

checked with video records from the flight path video camera.

Magnetic System and Data Processing

A cesium split-beam total-field magnetic sensor was used 

for the magnetic field measurements. The in-flight sensitivity 

of the system was 0.001 nanotesla (nT) with a recording rate of 

0.1 second. The sensor tolerated gradients up to 10,000 nT/m, 

and the dynamic range was 20,000 to 100,000 nT. A heading 

test after system installation and prior to surveying showed a 

maximum variation of ±0.92 nT. A total-field magnetic base 

station placed within the survey area used a sensor with the 

same specifications as the airborne sensor. No magnetic storms 

occurred during the survey.

1

Measurements originally made and reported in feet and miles are retained 

here in their original units for clarity and to avoid misstatement of precision in 

conversion. To convert feet to meters, multiply by 0.3048; to convert miles to 

kilometers, multiply by 1.61.

236 Environmental Effects of Historical Mining, Animas River Watershed, Colorado
Helicopter Electromagnetic and Magnetic Surveys 237

The in-flight magnetic field data were corrected for diurnal variations using the base station records. The International 

Geomagnetic Reference Field (IGRF) was computed and 

removed along each flightline. The magnetic data were gridded using minimum curvature methods with a 40 m square cell 

size for the western part of the survey area and an 80 m cell 

size for the eastern part of the survey (fig. 1). The eastern grid 

was resampled to a 40 m cell size and merged with the western 

grid.

The IGRF-corrected total-field magnetic grid was filtered 

using a reduction-to-pole (RTP) filter implemented in the 

USGS potential field programs (Phillips, 1997; Phillips and 

others, 2003). This filter transforms the magnetic field so 

that anomalies are positioned directly above the causative 

body. This filtering will produce a correct positioning of the 

magnetic responses as long as (1) the causative source of the 

magnetic anomaly does not have a strong natural remanent 

magnetization and (2) the magnetization is predominantly all 

induced in the direction of the current main magnetic field. 

Although some rock units in the Silverton caldera exhibit 

remanent magnetization (Grauch and Hudson, 1987), the magnetization intensities and directions are low and within a range 

collinear with the present-day magnetic field.

The RTP magnetic grid was also filtered to separate 

short and long spatial wavelengths through a process termed 

matched filtering (Syberg, 1972). USGS computer field 

programs by Phillips (1997) were used to do this processing. 

The short wavelength anomalies approximate sources that are 

Figure 1. Total-field magnetic map of a region surrounding the Animas River watershed study area. Data are a subset from 

the compilation of Colorado aeromagnetic data by Oshetski and Kucks (2000). Warmer colors (reds), magnetic highs; cooler 

colors (blues), lows. The digital grid has a cell size of 500 m square. Boxes indicate areas where detailed helicopter surveys 

were done (see fig. 2 for scale). Blue lines, main drainages of the study area (see fig. 3), from west to east, South Fork 

Mineral Creek, Mineral Creek, Cement Creek, and Animas River.

0 20 KILOMETERS

COLORADO

MAP LOCATION

N
Figure 2. Generalized regional geology of Silverton and Lake City calderas, southwest Colorado. Heavy solid line, boundary of detailed aeromagnetic 

surveys flown in area. Magnetic maps from USGS Animas survey are discussed here. Geologic index map modified from Bove and others (2001). Geologic map for the Animas survey area is in Yager and Bove (2002) and plate 1 of this volume.

121212

MzPz

Late to middle Tertiary volcanic rocks

Mafic lava (16–21 m.y.)Rhyolite-dacite intrusions (10–23 Ma)Lake City caldera fill (22.9 Ma)

Postcaldera intrusions (25–26 Ma)

Uncompahgre-San Juan and Silverton caldera complex (27.6–28.4 Ma)Regional ash-flow sheets and minor  lavas (27–30 Ma)

Prevolcanic rocks

Mesozoic and Paleozoic sedimentary rocks

Contact

Proterozoic rocks (1.4–1.8 Ga)

Gunnison River

Te

Tof

Tof

Te Te

Tof

Te

Te

Tof

Te

Tof

Te

Tof

Engineer Pass

MatterhornPeak stock

Ouray

Tof

Capitol City

Henson Creek

LakeCity

Iron Bedsintrusions

Red MtnLake Fork

Sherman

Eureka 

Te

Silverton

Gladstone

Ophir

Ophir stock

Telluride

Sneffelsstock

107 30'

107 45'

37 45'

Tof

Te

EXPLANATION

Fault—Bar and ball on downthrown side

Animas R

Uncompahgre Peak

Lake City

caldera

graben 

o

38 00' o

o

o

Lavas, breccias, and related volcaniclastic rocks

Silverton caldera

0 10 20 KILOMETERS

NURE REDCLOUD SURVEY

USGS LAKE CITY SURVEY

USGS ANIMAS RIVER SURVEY

238 Environmental Effects of Historical Mining, Animas River Watershed, Colorado
Helicopter Electromagnetic and Magnetic Surveys 239 Figure 3. Generalized geology of Animas River watershed study area, southwest Colorado. Thin north-south black lines, flight path of helicopter in main part of survey area. East-west lines, tie lines. Yellow lines are flightlines flown along selected drainages. Major tributaries to the Animas River are shown in addition to major Tertiary volcano-tectonic structures related to the Silverton and San Juan calderas. Figure is modified from Casadevall and Ohmoto (1977). Silverton Cement Cr Animas R.

Mineral Cr

?

?

107 45' 107 40' 107 35' 107 30' 107 50'

38 00'

37 55'37 50'

37 45'

EXPLANATION

Late Tertiary intrusive rocksTertiary volcanic rocksTertiary and Late Cretaceous intrusive rocksMesozoic and Paleozoic sedimentary rocksPrecambrian rocksContactFaults and vein structures—Dashed where

approximately located

Silverton caldera structural margin—Dotted

where concealed

San Juan caldera topographic margin—

Approximately located; queried where

extent uncertain

0 6 MILES0 9 KILOMETERS

o

oooo

o o o o o
in the upper 200 m beneath the topographic surface. The map 

produced from this data is termed a “high-pass” magnetic 

anomaly map and should correspond most closely with the 

sources of anomalies from the electromagnetic survey.

A matched filter was also applied, using the same programs (Phillips, 1997; Phillips and others, 2003), to enhance 

long wavelength anomalies that could represent sources at 

depths of more than 1,300 m. This map is termed a “low-pass” 

magnetic anomaly map.

Electromagnetic System and Data Processing

The EM system consisted of five coil pairs operating at 

different frequencies. Three of the coil pairs were oriented in 

a horizontal coplanar (CPL) configuration and the other two in 

a vertical coaxial (CXL) configuration. Table 1 gives the frequencies for each of the coil configurations. The frequencies 

were set before the survey was started and were maintained 

during flight operations.

Data from the CXL configuration, normally used in 

exploration for dike-like conductors, are very susceptible to 

in-flight noise and have not been used in this study. The CPL 

coil pairs are best suited to mapping of subsurface electrical 

conductivities.

Care was taken to establish calibration levels for the 

electromagnetic system both before and during the survey. 

However, in post-processing by the USGS, absolute levels of 

the EM channels were found to be unreliable. A number of 

different approaches (Fitterman, 1999) were used to adjust 

the levels to produce agreement with ground electromagnetic 

measurements. None of these methods could entirely correct the data. The data presented here have not been adjusted 

beyond the leveling done by the contractor who conducted 

the airborne survey. Quantitative interpretations beyond the 

qualitative interpretation presented here will require additional 

leveling of the HEM survey data.

The contractor used the EM measurements at each 

frequency to calculate apparent resistivity using the method 

described by Fraser (1978). The method estimates the resistivity of a homogeneous half space based on the EM measurements, hence the term apparent resistivity. For the purposes 

of this study, the apparent resistivity (ρ, ohm meters) was 

converted to apparent conductivity (σ, millisiemens per meter) 

where σ=1,000/ρ. Maps of apparent conductivity presented 

here show high conductivities in warmer colors in order to 

emphasize such features.

Map Compilation

The digital raster graphic topographic base map was 

scanned from a green-line topographic base. The base is the 

same digital map as used for the geologic map of the Animas 

River watershed study area and vicinity (Yager and Bove, 

2002, and Yager and Bove, this volume, Chapter E1, pl. 1). All 

of the digital data were projected to the Universal Transverse 

Mercator (UTM zone 13) geographic projection. The 

digital database used a 2.5 m resolution and was resampled 

to 8 m for the digital version of the base. All of the airborne 

geophysical data used this same projection. The positional 

accuracy of the HEM survey is on the order of 5 m in the 

horizontal position and 10 m in the vertical position based 

on the post-processing of the differential global positioning 

system (GPS).

Magnetic and Electrical Properties

Magnetic Responses

The magnetic response of the volcanic rocks is controlled 

by the amount of magnetic minerals, primarily magnetite and 

to some degree ilmenite, maghemite, and titanomagnetite 

present in the rocks (Telford and others, 1978). Pyrrhotite is a 

magnetic iron sulfide mineral that is common in some sulfide 

mineral deposits but does not occur in abundance in the mineralized systems in the study area. The amount of magnetic minerals in rocks determines the magnetic susceptibility and thus 

its strength of magnetization. Rocks with a high susceptibility 

will produce a strong dipolar magnetic response (Telford and 

others, 1978). Natural remanent magnetization (NRM) also 

affects the amplitude of observed magnetic anomalies depending on its direction and intensity. Often the direction of the 

NRM is opposite that of the induced magnetization, resulting 

in a lowering of magnetic response or reversal of the dipolar 

signature. Grauch and Hudson (1987) and Gettings and others 

(1994) have examined the NRM and magnetic susceptibility 

of the volcanic rocks in the Silverton and Lake City calderas. 

The nature of NRM has been discussed previously in regard 

to processing of the total-field magnetic data. As is typical of 

many volcanic terrains, both NRM and magnetic susceptibility 

are highly variable both within and between stratigraphic and 

lithologic units. Basic igneous rocks have high magnetization, 

with granite, andesite, and rhyolite having generally lower 

magnetization. Volcanic rocks in general have higher magnetization than sediments unless the sediments contain magnetite. 

Hydrothermal alteration of volcanic rocks is likely to reduce 

their magnetite content (Telford and others, 1978). Several 

different types of alteration have been observed and mapped in 

the study area (Bove and others, 2000; McDougal and others, 

this volume, Chapter E13).

Table 1. Frequency and coil orientation for the electromagnetic 

system.

 Frequency 

(Hz) Configuration

886.00 Coaxial

984.00 Coplanar

4,836.00 Coaxial

4,310.00 Coplanar

32,960.00 Coplanar

240 Environmental Effects of Historical Mining, Animas River Watershed, Colorado
Helicopter Electromagnetic and Magnetic Surveys 241

Electromagnetic Responses

Several factors control the electrical conductivity of 

rocks (Telford and others, 1978; Olhoeft, 1985), and these 

can be generalized for the geologic setting of the study area. 

In general, introduction of clay minerals (including zeolites) 

tends to increase the electrical conductivity of rocks. Both 

hydrothermal alteration and weathering processes of volcanic 

rocks in the area can produce clay minerals. Metallic veins, 

particularly massive sulfides such as pyrite, can drastically 

increase the electrical conductivity. Ground water having high 

dissolved solids can increase measured electrical conductivity. Sediments in valley fill may be much more conductive 

than bedrock due to silt (particularly with high organics) and 

clay mineral content and water with high dissolved solids. 

However, the processes that create valley fill may also weather 

the bedrock under the valleys, reducing the electrical contrast 

between rock units. Some types of glacial sediments, such as 

moraines, may have high clay content and thus high electrical 

conductivity.

Areas of low conductivity (high resistivity) can be 

caused by alteration that causes silicification of porous media. 

McDougal and others (this volume) have analyzed the electromagnetic data to map areas of low conductivity that may 

be associated with fracture and fault systems. In general, any 

process that decreases the permeability and porosity of rocks 

tends to increase the resistivity or lower the conductivity. This 

variation is related to observation that the amount and nature 

of pore fluids have an enormous effect on the electrical resistivity of rocks (Olhoeft, 1985). Thus rocks above the water 

table generally have a low conductivity.

Discussion

Geophysical maps from the helicopter magnetic 

(pl. 3) and electromagnetic (pl. 4) survey are given at scales 

(1:48,000) comparable with the geologic map of the study area 

(Yager and Bove, 2002, and pl. 1, this volume). The purpose 

of the following section is to illustrate interpretations of specific anomalous features that are relevant to the Animas River 

watershed AML study. A detailed discussion of the airborne 

geophysical data is given by McDougal and others (this volume) in terms of quantitative predictive modeling designed to 

characterize structural and lithologic terrains.

Magnetic Maps

Plate 3 gives the color total-field magnetic map for 

the Animas River watershed study area superimposed on a 

topographic base. The 1:48,000-scale total-field reducedto-pole (see processing section) magnetic map can be compared directly to the geologic map of the study area given 

by Yager and Bove (2002, and this volume, pl. 1). The color 

shaded-relief map is shown for the total-field magnetic 

survey data in figure 4. The shading is shown with a sun 

angle from N. 45° E., which emphasizes trends in the conjugate (N. 45° W.) direction.

The reduced-to-pole map (pl. 3 and fig. 4) generally 

shows that the central part of the Silverton caldera is defined 

by magnetic lows relative to the margins. The high-pass magnetic map (fig. 5) shows features that should be from sources 

at depths similar to the EM data. The low-pass magnetic map 

(fig. 6) shows longer wavelength magnetic responses that are 

caused by sources that are deep or broad and shallow. Specific 

features of each map are discussed in following paragraphs 

and also by McDougal and others (this volume).

Magnetic Features

Magnetic features are shown in figure 7, where letters 

identify linear features and numbers identify anomalies.

Anomaly 10 in figure 7 is a discrete magnetic low. Narrow 

high magnetic anomalies that occur in the east and north part 

of anomaly 10 may be dikes. The magnetic low may extend 

farther to the east beyond the dikes. Rocks that are either 

reversely polarized or intensely altered (where the magnetic 

minerals have been destroyed) are possible sources for the 

magnetic low.

Linear feature P (fig. 7) is located on the southeast of 

magnetic anomaly 10. Interpreted linear features Q, R, and 

S in the magnetic map (fig. 7) may indicate structures that 

cross the caldera boundaries. Such structures could influence 

ground-water flow directions in the caldera ring fault system.

The magnetic high anomalies labeled 15 (fig. 7) are 

associated with the Sultan Mountain stock (Yager and Bove, 

2002; this volume, pl. 1), one of the Tertiary intrusives in the 

San Juan caldera complex. The strong magnetization suggests 

that this late Tertiary intrusive complex is unaltered.

The area of peak 3,792 m, between Middle and South 

Forks Mineral Creek (Yager and Bove, this volume; Bove 

and others, this volume), is characterized by a magnetic high 

(anomaly 16, fig. 7). The magnetization is not as intense as 

the magnetization of the Sultan Mountain stock.

Magnetic high anomalies occurring at the south edge of 

the Silverton caldera are labeled 11 (fig. 7). The magnetic highs 

correlate with topographic highs and are likely due to the relatively flat lying Henson Member of the Silverton Volcanics. The 

magnetic rocks of the Sultan Mountain stock have been mapped 

just north of the Animas River; so some of the high magnetic 

anomalies just to the northwest of the junction of the Animas 

River and Cement Creek may be due to this source.

Several small isolated magnetic high anomalies are 

labeled 12 in figure 7. Farther east, in the Lake City caldera, 

this type of magnetic anomaly is associated with late Tertiary 

intrusives, which have been discussed in detail by Bove and 

others (2001). Anomaly 13 (fig. 7) is a magnetic low in the 

upper Animas River basin. This magnetic low is not associated with nonmagnetic valley fill, but is more likely caused by 

volcanics that have very little magnetite (perhaps due to alteration) or are reversely polarized.
-322 -28

8 -266 -250 -236 -222 -205 -18

8 -171 -155 -141 -128 -116 -105

-94

-83

-70

-55

-40

-23

-7

10

26

41

57

73

89 105 124 146 170 196 226 260 300 356 435 587

NanoTesla

" 03' 25° 73

107°45' 107°37 3' 0"

5 KILOMETERS 1 .5 0 1 2 3 4

4 MILES 1 0 1 2 3 1/ 2

Figure 4. Color shaded-relief reduced-to-pole total-field magnetic map of Animas River watershed study area. Color scale bar shown at right. Sun direction is from northeast. Black line, flightline flown along selected drainage as shown in figure 3. Figure is of same area as shown in the 1:48,000 reduced-to-pole magnetic map (pl. 3).

242 Environmental Effects of Historical Mining, Animas River Watershed, Colorado
Helicopter Electromagnetic and Magnetic Surveys 243

-5

6

-4

2

-34

-29

-2

5

-2

2

-20

-18

-16

-15

-13

-12

-11

-10

-9

-8

-7

-6

-5

-4

-2

-1

-

9

7

5

4

2

1

0

11

13

16

19

2

4

29

37

49

74

NanoTesla

" 03' 25° 73

107°45' 107°37 3' 0"

5 KILOMETERS 1 .5 0 1 2 3 4

4 MILES 1 0 1 2 3 1/ 2

Figure 5. Color shaded-relief high-pass filtered RTP map for Animas River watershed study area. Sun direction is from northeast. Black line, flightline flown along selected drainage as shown in figure 3. Figure is of same area as shown in the 1:48,000 reduced-to-pole magnetic map (pl. 3).
-1

NanoTesla

96 -183 -172 -161 -15

2 -14

2 -132 -12

2 -113 -104

-94

-87

-80

-74

-6

4

-5

2

-39

-2

5

-1

2

1

15

2

6

35

4

6

5

6

67

77

89 101 119 139 158 171 190 210 239 273 4

4

2

" 03' 25° 73

107°45' 107°37 3' 0"

5 KILOMETERS 1 .5 0 1 2 3 4

4 MILES 1 0 1 2 3 1/ 2

Figure 6. Color shaded-relief low-pass filtered RTP map for Animas River watershed study area. Sun direction is from northeast. Black line, flightline flown along selected 

drainage as shown in figure 3. Figure is of same area as shown in the 1:48,000 reduced-to-pole magnetic map (pl. 3).

244 Environmental Effects of Historical Mining, Animas River Watershed, Colorado
Helicopter Electromagnetic and Magnetic Surveys 245

10

11

11

12

12

12

12

13

15

15

15 15

16

P

Q

R

"

0

3

'

2

5

°

7

3

107°45' 107°37 3' 0"

S

15

5 KILOMETERS 1 .5 0 1 2 3 4

4 MILES 1 0 1 2 3 1/ 2

Figure 7. RTP magnetic map showing selected anomalous features. Linear features (bold letters and black lines) are selected from a number of features identified by McDougal and others (this volume). Magnetic anomalies (numbers) have been selected to characterize general geologic features. Color shaded relief from figure 4 is shown as background. Curving solid black line, flightline along selected drainage. Heavy dashed line is general outline of Eureka graben.
Apparent Conductivity Maps

Plate 4 shows apparent conductivity in the Animas River 

watershed study area. The 1:48,000-scale color apparent 

conductivity map can be compared directly to geologic maps 

of the study area (Yager and Bove, 2002; Yager and Bove, this 

volume, pl. 1). The intermediate frequency (4,310 Hz) has 

been used for the main map because it best represents general 

characteristics of the electrical structure of the study area.

Color shaded-relief (CSR) maps are commonly used to 

emphasize trends. Figures 8–10 show CSR maps of apparent 

conductivity for the three horizontal coplanar coil pairs from 

high to low frequency. The sun illumination angle is N. 45° E. 

with an inclination of 45°. Thus, features trending northwest 

are emphasized. In these maps, the high conductivities are 

shown in warm colors (reds) that correspond to low resistivities. The general structural trends described by Burbank and 

Luedke (1969) are northeast, north, and northwest. Maps using 

several different illumination angles were examined, and the 

northeast illumination direction was qualitatively the best to 

emphasize trends discussed herein. However, this illumination 

angle does not emphasize the northeast-trending structures 

described by Casadevall and Ohmoto (1977), such as the 

Eureka graben. The use of particular color scales (resistivity or 

conductivity) depends on the type of geophysical features that 

are being highlighted. In this discussion, conductive features 

are highlighted using warmer colors. The color scale for each 

frequency is stretched to yield a consistent scale between 

maximum and minimum values of apparent conductivity. Note 

that this results in specific colors on each map representing different apparent conductivity.

The highest frequency (fig. 8) has the greatest spatial 

frequency variation due to more system noise from in-flight 

altitude variations and also due to large variations in shallow 

geology. At this frequency, the depth of investigation is only 

a few meters. Areas of high anomalous conductivity (fig. 8) 

are generally located in the northwest, between Cement and 

Mineral Creeks, and in the area of peak 3,792 m. The south 

end of the Eureka graben (fig. 3) is associated with a shallow high conductivity anomaly (fig. 8). A high conductivity 

anomaly also lies along the northern part of the Animas River.

The intermediate-frequency apparent conductivity map 

(fig. 9) has less high spatial frequency variations due to less 

system noise, greater spatial averaging, and deeper penetration. The depth of penetration at this frequency is on the order 

of 30–40 m. High apparent conductivities (fig. 9) are generally in the same areas as for the higher frequency (fig. 8). 

One difference between figures 8 and 9 is that high apparent 

conductivities occur along a longer part of the Animas River at 

the intermediate frequency (fig. 9) beginning east of Silverton 

and continuing to upstream of Eureka. This difference suggests that shallow valley fill sediment is not the sole source of 

high conductivities. The bedrock is likely more weathered and 

fractured along the Animas River part of the Silverton caldera 

structural margins. Another line of evidence that the valley fill 

is not thick is that the magnetic field map (pl. 3) does not have 

a pronounced magnetic low along the river basin. A magnetic 

low would be indicative of thick nonmagnetic sediment.

The CSR map for the lowest frequency is given in 

figure 10. The apparent conductivity for this frequency is 

10 times lower than that for the intermediate frequency shown 

in figure 9. Apparent conductivity anomalies in this map are 

smoother than the next higher frequency because again there 

is greater averaging of the signature of subsurface electrical 

features. The depth of penetration at the lowest frequency is 

on the order of 40–50 m. Again there is greater correlation of 

high apparent conductivities along the upper Animas River 

drainage than other drainages in the study area. This is likely 

due to alteration of bedrock along the caldera ring fractures 

that form the southeastern margin of the Silverton caldera. It 

is interesting that this is the only part of the caldera structure 

that has an electrical signature. This suggests that clay minerals are not a large part of the altered rocks along other areas 

of the caldera margin.

Apparent Conductivity Features

Conductivity anomaly 1 (fig. 11) is an apparent conductivity high with a low at the center. The apparent conductivity high is one of the highest in the survey. The anomaly is 

prominent in the mid and low frequencies (figs. 9 and 10), 

suggesting that the surface expression may not be obvious. 

The general shape is suggestive of an alteration halo around a 

buried intrusive. The high conductivity (red colors) is situated 

in an area of moderately high (yellows) apparent conductivity located to the north of linear feature A. This linear feature 

trends northwest to southeast and correlates well with a fault 

zone shown by Yager and Bove (2002; Yager and Bove, this 

volume, pl. 1). In one interpretation of this setting, a silicified 

fault zone is present that has acted as a barrier to hydrothermal waters. The impounded hydrothermal waters could have 

caused alteration to the northeast of the structure (D.J. Bove, 

oral commun., 2002). A high apparent conductivity is correlated with this alteration. This structural zone may also control 

modern ground-water flow (Wright, Kimball, and Runkel, this 

volume, Chapter E23).

Linear feature B has a similar trend to linear feature A

and passes through the area of the May Day mine (MA, fig. 11). 

The feature is present on the intermediate- and low-frequency 

apparent conductivity maps (figs. 9 and 10). That these trends 

are not as clear on the high-frequency map suggests that the surface expression may not be obvious. This may be why surface 

geologic maps (Yager and Bove, 2002; Yager and Bove, this 

volume, pl. 1) do not indicate a prominent structure in this area. 

Smith and others (2001) suggested that linear feature B may 

be a fracture system that could control the ground-water flow. 

Local surficial ground-water flow interaction with the May 

Day mine dump is also documented (Smith and others, 2001; 

Wright, Kimball, and Runkel, this volume). Thus this area may 

be a good example of both surface (mine waste) and subsurface 

(fracture controlled) ground-water paths. Linear features C and 

D may indicate similar structures extending to the northwest 

from linear feature B. These trends are also not as clear on the 

high-frequency apparent conductivity map (fig. 8).

246 Environmental Effects of Historical Mining, Animas River Watershed, Colorado
Helicopter Electromagnetic and Magnetic Surveys 247 0.08 0.11 0.13 0.15 0.18 0.21 0.24 0.29 0.33

0.39

0.46

0.53

0.62

0.74

0.88

1.09

1.45

2.10

3.56

Conductivity

millisiemens

"

0

3

'

2

5

°

7

3

107°45' 107°37 3' 0"

5 KILOMETERS 1 .5 0 1 2 3 4

4 MILES 1 0 1 2 3 1/ 2

Figure 8. Color shaded-relief apparent conductivity map at 36,930 Hz of Animas River watershed study area. Sun direction is from northeast. Black line, flightline flown 

along selected drainage as shown in figure 3. Figure is of same area as shown in the 1:48,000 apparent conductivity map (pl. 4).
Figure 9. Color shaded-relief apparent conductivity map at 4,310 Hz of Animas River watershed study area. Black line, flightline flown along selected drainage as shown 

in figure 3. Figure is of same area as shown in the 1:48,000 apparent conductivity map (pl. 4).

" 03' 25° 73

107°45' 107°37 3' 0"

0.06

0.09

0.13

0.18

0.26

0.37

0.53

0.75

1.08

1.55

2.21

3.17

4.54

6.50

9.31

13.32

19.08

27.31

39.11

Conductivity

millisiemens

5 KILOMETERS 1 .5 0 1 2 3 4

4 MILES 1 0 1 2 3 1/ 2

248 Environmental Effects of Historical Mining, Animas River Watershed, Colorado
Helicopter Electromagnetic and Magnetic Surveys 249 Conductivity millisiemens

0.67

0.30

0.18

0.13

0.10

0.09

0.08

0.07

0.06

0.05

"

0

3

'

2

5

°

7

3

107°45' 107°37 3' 0"

5 KILOMETERS 1 .5 0 1 2 3 4

4 MILES 1 0 1 2 3 1/ 2

Figure 10. Color shaded-relief apparent conductivity map at 935 Hz of Animas River watershed study area. Black line, flightline flown along selected drainage as shown 

in figure 3. Figure is of same area as shown in the 1:48,000 apparent conductivity map (pl. 4).
Figure 11. Apparent conductivity map showing selected anomalous features. Curving solid gray line, flightline flown along selected drainage as shown 

in figure 3. Figure is same area as shown in the 1:48,000 apparent conductivity map with the same color scale (pl. 4). Letters identify linear features (straight black lines); MA, May Day mine; MF; Mayflower tailings. Numbers indicate high conductivity anomalies.

A

B

C

D

E

F

MA

1

2

3

4

5

6

MF

"

0

3

'

2

5

°

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107°45' 107°37 3' 0"

5 KILOMETERS 1 .5 0 1 2 3 4

4 MILES 1 0 1 2 3 1/ 2

250 Environmental Effects of Historical Mining, Animas River Watershed, Colorado
Helicopter Electromagnetic and Magnetic Surveys 251

High apparent conductivities located in the area labeled 2

in fig. 11 correlate with a mapped landslide (Yager and Bove, 

2002; Yager and Bove, this volume, pl. 1). The high apparent 

conductivity is present at all frequencies, which suggests that 

the source extends from surface to depth. Clay minerals, associated with alteration of the Silverton Volcanics, are likely the 

source of the anomalous conductivity. The clay-rich lithology 

is favorable for the development of landslides and may also 

control local ground-water flow.

Anomaly 3, located along Cement Creek, is in the 

area where an iron-rich bog described in several other studies (Wirt and others, this volume; Kimball and others, this 

volume; Stanton, Yager, and others, this volume, Chapter E14) 

has formed. This high conductivity anomaly has an expression at the lower frequency and a very limited expression at 

the highest frequency. One hypothesis concerning the genesis 

of the bog is that it could be located on a fracture system that 

is acting to focus the flow of ground water. The electrical 

signature is not suggestive of a linear fracture system. In addition, ground electrical geophysical surveys on the bog surface 

do not indicate any depth extent to the high dissolved solids 

observed at the surface. The localized conductivity high is due 

to downcutting of the valley in this part of Cement Creek that 

has forced ground water to the surface (Vincent and others, 

this volume, Chapter E16).

Linear feature F (fig. 11), located at the head of Cement 

Creek, is defined as a boundary between low and high 

apparent conductivities. The general trend of linear feature 

F correlates with the Bonita fault and the western part of the 

Sunnyside fault (Yager and Bove, 2002; Yager and Bove, this 

volume, pl. 1). The specific location of this linear feature may 

indicate a part of the fault system that is important in controlling local ground-water flow.

High conductivity anomaly 4 (fig. 11) along the Animas 

River is located at the Mayflower Mill mine-waste piles (MF) 

and adjacent flood plain along the river. Ground geophysical 

surveys in the flood plain along this part of the drainage in 

2001 showed localized shallow-subsurface very high conductivity between the waste piles and the Animas River. This 

high conductivity was correlated with sulfide tailings that 

were removed in 2003 by Sunnyside Gold, Inc. The high conductivity anomaly, in general, is likely to have resulted from 

the high dissolved solids in ground water and the presence 

of conductive sediment in tailings. There is also a large fault 

zone exposed in the bank of the Animas River that may be the 

source of some of the high conductivity on the south side of 

the river east of anomaly 4.

High conductivities are associated with much of the 

Animas River northeast of Silverton. Conductive anomaly 5

is present on all frequencies (figs. 8, 9, and 10). The source 

of high conductivities is likely both valley fill and altered bedrock along the valley. The extension of the conductive features 

to depth could be an indication of structures bounding this 

part of the Silverton caldera. As a point of interest, note that 

other major drainages (Mineral Creek, South Fork Mineral 

Creek, Cement Creek) do not have as extensive areas of high 

conductivity, even though there is fill along their valleys. The 

implication is that the ring fault structure is more developed 

and altered with depth along the Animas River drainage 

than in the other drainages. Thus both surficial and fracturecontrolled ground-water flow is particularly important along 

this drainage.

Conductivity anomaly 6 and linear feature E (fig. 11) 

are correlated with the mineralized system at peak 3,792 m 

(Yager and Bove, this volume; Bove and others, this volume). 

The high conductivity anomaly is correlated in general with 

mapped quartz-sericite-pyrite lithology. No single mapped 

structural feature correlates with linear feature E southwest 

of this mineralized system (fig. 11).

Hydrologic Implications

Variations in electrical conductivity can be generalized for 

different types of ground-water systems for the Animas River 

watershed study area. Water-bearing units in the study area 

can be conceptually grouped as (1) near surface, (2) weathered 

surficial bedrock, and (3) deeper bedrock. Ground-water flow 

within water-bearing units can be associated with and influenced by lithologic and structural heterogeneities that can have 

contrasting physical properties (electrical conductivity and magnetization). These contrasts in physical properties are shown 

by the heterogeneous responses in the airborne geophysical 

maps. Predicting subsurface ground-water flow is important 

in understanding the transport of contaminants from historical 

mine sites.

Ground-water flow exists in the Holocene and Pleistocene 

deposits of clay to boulder-size sediments that are mapped as 

alluvium, alluvial fans, talus slopes, colluvium, landslides, 

glacial deposits, and bog deposits (Blair and others, 2002; 

Yager and Bove, 2002; Yager and Bove, this volume). Areas of 

silt-size sediment may contain clay minerals that cause a high 

electrical conductivity. These areas may be associated with 

alluvium, colluvium, and landslides. Alluvial valley fill may 

also contain organics (peat type deposits) that can increase the 

electrical conductivity. Talus slopes and gravel deposits without either clay or water containing high dissolved solids will 

have low electrical conductivity or be resistive. Rock glaciers 

that consist of ice cores with sand and boulders are likely to 

be areas of very high electrical resistivity (very low conductivity). The magnetization of most of the surficial deposits is low 

except where sediment was derived in place from magnetic 

bedrock.

The Silverton Volcanics of the Animas River watershed 

study area are described by Yager and Bove (this volume) as 

consisting of a basal unit (Burns Member), an overlying (but 

locally interfingering) pyroxene andesite member, and the 

Henson Member. The older Burns Member of altered lavas 

and tuffs has the highest silica content and occupies the central 

part of the Silverton caldera. The corresponding magnetic low 

and moderated apparent conductivities are consistent with this 

lithology, although NRM and hydrothermal alteration may 
play a role in causing the low magnetic anomalies. The youngest Henson Member consists of interfingering volcaniclastics 

that are marked by relatively high magnetic anomalies and low 

conductivities (high resistivity). Magnetic highs at the south 

side of the Silverton caldera and just north of the Animas 

River are correlated with the topographic highs (ridge crests) 

that are mapped as Henson Member (Lipman and others, 

1973).

As noted in the discussion of the apparent resistivities 

at different frequencies, a progression occurs from higher 

conductivities at the highest frequency (shallowest penetration) to lower conductivity at the lower frequencies (greatest 

penetration). Near-surface crystalline bedrock has permeability attributed to a three-dimensional volume of intense fracturing (Hurr and Richards, 1974; Caine and Tomusiak, 2003). 

Conceptually for the study area, in this zone of weathered and 

intensely fractured bedrock, seasonal fluctuations of recharge 

and discharge take place (Caine, 2003). High electrical conductivities may be developed if the ground water contains high 

dissolved solids characteristic of many contaminants. Weathering processes may also lead to the development of secondary 

minerals, in particular clay or zeolite minerals. Both of these 

conditions will contribute to near-surface high conductivities 

underlain by low conductivity bedrock. An electromagnetic 

anomalous signature of this setting would be high conductivities at high frequencies and lower conductivities at the deeper 

penetrating lower frequencies (fig. 10). A contributing factor 

to the decreasing conductivity with depth is, conceptually, 

the decreasing fracturing with depth. As lithostatic pressures 

increase, open fractures should decrease.

Porosity and permeability of volcanic rocks can be 

associated with primary depositional features and fractures 

developed at various scales. Bedrock ground-water flow may 

exist in highly permeable volcanic rock such as tuffs and 

breccias. There have been no systematic laboratory or borehole 

measurements of porosity and permeability of the volcanic 

rocks in the study area. Huntley (1979) and Caine (2003) have 

suggested that the San Juan Mountains in general have high 

permeability due to both fracturing and their lithology (tuffs 

and breccias, for example). Primary deposition features are 

the extensive lahars, volcaniclastic deposits, and interbedding between flows. The electrical conductivity of porous and 

permeable sediments is directly controlled by the conductivity 

of the pore fluid (Olhoeft, 1985). Consequently, large areas 

(tens of meters) containing fluids with high total dissolved 

solids can produce high conductivity signatures in the HEM 

data.

Caine and others (1996) described a conceptual model 

of the controls of ground-water flow by faults and fault zones. 

Faults and fractures within aquifers and water-bearing rocks 

can act as both conduits and barriers to water flow (Caine 

and others, 1996). Robinson (1978) and Caine and Tomusiak 

(2003) have given good descriptions of the hydrologic setting of crystalline rock in terms of the importance of faults 

and fractures. On the scale of geologic time, fault zones and 

fractures have certainly served as conduits that have focused 

hydrothermal fluids to form mineral deposits (Casadevall and 

Ohmoto, 1977). The same hydrothermal processes that have 

developed and concentrated mineralization also developed 

extensive siliceous vein systems that have essentially annealed 

or plugged the fracture system that existed at the time. Massive siliceous vein systems are likely to have low electrical 

conductivity (be resistive) (Olhoeft, 1985). Unless these are 

fractured, they are likely barriers to ground-water flow. In 

contrast, high conductivity may be due to development of clay 

in tabular alteration zones along faults that act as conduits.

Evidence for specific ground-water flow paths in the 

watershed is limited and is summarized as follows. The geophysical features extending from the May Day mine toward 

Ohio Peak (figs. 7 and 11) suggest a structure that influences 

ground-water flow over a broad scale (Smith and others, 

2001). Arcuate magnetic and conductive features that predominantly follow the Animas River suggest that the ring fracture 

system (Yager and Bove, this volume) influences ground-water 

flow. Wirt and others (this volume) suggest that development 

of bogs or precipitates near Fairview and Placer Gulches (middle part of Cement Creek, pls. 3 and 4) is partly due to fracture 

system-controlled springs. Neither area is near identified linear 

geophysical anomalies, but both are near anomalous electrical 

conductivity lows in the intermediate and low electromagnetic 

frequencies (figs. 9 and 10). The high conductivity anomaly 

near the Mayflower mine-waste piles is in part associated 

with a localized flow path that transported high metals from a 

sulfide-rich mine waste that has been removed. Thus in general the airborne geophysical surveys can be used in the study 

of both large- and small-scale ground-water flow paths.

Conclusions

The airborne electromagnetic and magnetic maps for 

the Animas River watershed study area contain features that 

selectively emphasize specific lithology and structure and offer 

clues in understanding the ground-water flow regime. The 

lithology of the caldera fill is associated with magnetic lows 

and conductivity highs. Younger intrusives ringing the caldera 

have distinctive geophysical anomalies reflecting differing 

lithologies.

The geologic maps of the study area show numerous 

structures, but only a few of these are associated with identified geophysical anomalies. Conversely, data on the geophysical maps suggest structures that have not been previously 

mapped. Geophysical maps identify areas of near-surface and 

subsurface electrical and magnetic physical property contrasts 

that suggest structural features. These structures possibly 

influence ground-water flow.

Electrical and magnetic signatures can be associated 

with particular lithologies. Only qualitative relationships have 

been suggested here. McDougal and others (this volume) have 

used predictive models to quantitatively relate geophysical 

signatures to acid-neutralizing terrains and terrains favorable 

for the occurrence of veins and structures. Physical property 

252 Environmental Effects of Historical Mining, Animas River Watershed, Colorado
Helicopter Electromagnetic and Magnetic Surveys 253

differences between Tertiary intrusives enable them to be 

classified on the basis of their geophysical signature. Low 

conductivity and magnetic intrusives may have more restricted 

ground-water flow than those that have been altered producing 

magnetic lows and a conductive signature.

Near-surface high conductivity anomalies associated 

with some mine-waste piles may indicate ground water containing high dissolved solids or a source of metal loading to the 

ground water. Other areas of intense hydrothermal alteration 

may be a surface source for ground water containing high dissolved solids.

The geophysical data have been used to identify areas 

possibly influencing surface and bedrock ground-water flow. 

Location of subsurface ground-water flow paths is a critical 

part of abandoned mine land studies.

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Yager, D.B., and Bove, D.J., 2002, Generalized geologic map 

of part of the upper Animas River watershed and vicinity, 

Silverton, Colorado: U.S. Geological Survey Miscellaneous 

Field Studies Map MF–2377, scale 1:48,000.

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