U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2008–5063

Net Acid Production, Acid Neutralizing Capacity,

and Associated Mineralogical and Geochemical

Characteristics of Animas River Watershed Igneous

Rocks Near Silverton, Colorado
Cover. Photograph of Eureka Member of Sapinero Mesa Tuff with high acid neutralizing capacity. Green chlorite (after pumice 

fiamme) in center of photograph; calcite stained with alizarin red (after phenocrysts and groundmass). Sample is 3 cm across.
Net Acid Production, Acid Neutralizing 

Capacity, and Associated Mineralogical 

and Geochemical Characteristics of 

Animas River Watershed Igneous Rocks 

Near Silverton, Colorado

By Douglas B. Yager, LaDonna Choate, and Mark R. Stanton

Prepared in cooperation with the U.S. Bureau of Land Management

Scientific Investigations Report 2008–5063

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

DIRK KEMPTHORNE, Secretary

U.S. Geological Survey

Mark D. Myers, Director

U.S. Geological Survey, Reston, Virginia: 2008

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Suggested citation:

Yager, D.B., Choate, LaDonna, and Stanton, M.R., 2008, Net acid production, acid neutralizing capacity, and

associated mineralogical and geochemical characteristics of Animas River watershed igneous rocks

near Silverton, Colorado: U.S. Geological Survey Scientific Investigations Report 2008–5063, 63 p.

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iii

Contents

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

Introduction.....................................................................................................................................................2

Previous Work ................................................................................................................................................2

New Work for this Study...............................................................................................................................2

Study Area.......................................................................................................................................................3

Geologic Setting.............................................................................................................................................4

Mid-Tertiary Volcanism........................................................................................................................4

Mineralization and Subsequent Erosion Events..............................................................................4

Rock Types of Interest for Potential Acid Neutralization Capacity .......................................................5

Precambrian Rocks ..............................................................................................................................5

Paleozoic Sedimentary Rocks ............................................................................................................6

Mesozoic Sedimentary Rocks ............................................................................................................6

Tertiary Sedimentary Rocks ................................................................................................................6

Tertiary Igneous Rocks ........................................................................................................................6

Regional Propylitic Alteration.............................................................................................................6

Quaternary Surficial Deposits ............................................................................................................7

Acid-Generating and Acid-Neutralizing Minerals ..........................................................................7

Acid-Generating Mineral Reactions..................................................................................................7

Acid-Neutralizing Minerals.................................................................................................................7

Methods...........................................................................................................................................................7

Sample Preparation and Analyses ....................................................................................................7

Net Acid Production Test...................................................................................................................18

Acid Neutralization Capacity Test (Acid Titration) ........................................................................18

Sequential Extraction Procedure.....................................................................................................19

Rock Physical Properties (Magnetic Susceptibility) ....................................................................20

Results ...........................................................................................................................................................20

Net Acid Production...........................................................................................................................20

Sulfide-Bearing Samples...................................................................................................................23

Nonsulfide-Bearing(?) Samples .......................................................................................................23

Acid Neutralizing Capacity................................................................................................................23

Eureka (Unit Tse) and Picayune Megabreccia (Unit Tsemb) Members.....................................24

San Juan Formation (Unit Tsj)...........................................................................................................24

Burns Member (Unit Tb) ....................................................................................................................25

ANC and NAP Unit Comparisons .....................................................................................................25

Sequential Leachate Results (Whole Rocks)..........................................................................................27

Magnesium ..........................................................................................................................................27

Aluminum..............................................................................................................................................27

Potassium.............................................................................................................................................29

Calcium.................................................................................................................................................29

Titanium ................................................................................................................................................31

Iron ........................................................................................................................................................31

Manganese..........................................................................................................................................31

Nickel....................................................................................................................................................33

Copper...................................................................................................................................................33
iv

Zinc........................................................................................................................................................33

Arsenic..................................................................................................................................................35

Strontium..............................................................................................................................................35

Molybdenum........................................................................................................................................35

Cadmium...............................................................................................................................................35

Barium...................................................................................................................................................37

Lead.......................................................................................................................................................37

Sequential Leachate Results (Grand Mogul Mine Waste) ...................................................................37

Magnesium ..........................................................................................................................................37

Aluminum..............................................................................................................................................37

Calcium.................................................................................................................................................39

Titanium ................................................................................................................................................39

Iron ........................................................................................................................................................39

Manganese..........................................................................................................................................39

Nickel....................................................................................................................................................41

Copper...................................................................................................................................................41

Zinc........................................................................................................................................................41

Arsenic..................................................................................................................................................41

Strontium..............................................................................................................................................41

Molybdenum........................................................................................................................................41

Cadmium...............................................................................................................................................41

Barium...................................................................................................................................................41

Lead.......................................................................................................................................................41

Rock Physical Properties (Magnetic Susceptibility) .............................................................................41

Summary........................................................................................................................................................41

ANC and NAP ......................................................................................................................................41

Burns Member (Unit Tb) and Picayune Megabreccia Member (Unit Tsemb).................41

San Juan Formation (Unit Tsj)..................................................................................................43

Sultan Mountain Stock (Unit Tig) ............................................................................................43

Sequential Leachate Studies (Whole Rocks)..........................................................................................43

Major Elements ...................................................................................................................................43

Mineralogic Control on Leachate Compositions ...........................................................................44

Trace-Element Leachate Abundances............................................................................................44

San Juan Formation (Unit Tsj)..................................................................................................44

Burns Member (Unit Tb) ...........................................................................................................44

Pyroxene Andesite Member (Unit Tpa)..................................................................................44

Mine-Waste-Leachate Compositions.......................................................................................................44

Magnetic Susceptibility and Corresponding ANC, NAP, and Leachate Rock Properties................44

Pyroxene Andesite Member (Unit Tpa)...........................................................................................44

Burns Member (Unit Tb) ....................................................................................................................44

Eureka Member (Unit Tse).................................................................................................................44

Sultan Mountain Stock (Unit Tig) .....................................................................................................44

San Juan Formation (Unit Tsj)...........................................................................................................45

Conclusions...................................................................................................................................................45

Acknowledgments.......................................................................................................................................45

References Cited..........................................................................................................................................45
v

Plate

[in pocket]

1. Map showing acid neutralizing capacity and net acid production of Animas River watershed 

rocks near Silverton, Colorado.

Figures

1. Location map and geographic features....................................................................................3

2. Generalized geology.....................................................................................................................5

3. Generalized map of alteration types in the Animas River watershed .................................8

4. Boxplots of NAP results.............................................................................................................23

5–7. Acid titration curves for:

5. Unit Tsemb sample HWDY090543.............................................................................................26

6. Unit Tsj samples ODY090552, ODY090553, and IDY090537....................................................26

7. Unit Tsj sample SDY090535A.....................................................................................................26

8. Boxplots of ANC results for unit Tsj.........................................................................................27

9. Acid titration curves for unit Tb................................................................................................28

10. Boxplots of ANC results for unit Tb .........................................................................................28

11. Boxplots of ANC and NAP for units Pz(?), Tb, Tig, Tse, Tsemb, and Tsj.............................29

12–27. Histogram of whole-rock leachate results for:

12. Mg.................................................................................................................................................30

13. Al....................................................................................................................................................30

14. K.....................................................................................................................................................31

15. Ca...................................................................................................................................................32

16. Ti....................................................................................................................................................32

17. Fe...................................................................................................................................................33

18. Mn.................................................................................................................................................34

19. Ni...................................................................................................................................................34

20. Cu...................................................................................................................................................35

21. Zn...................................................................................................................................................36

22. As...................................................................................................................................................36

23. Sr....................................................................................................................................................37

24. Mo.................................................................................................................................................38

25. Cd...................................................................................................................................................38

26. Ba..................................................................................................................................................39

27. Pb...................................................................................................................................................40

28–30. Histogram of Grand Mogul mine waste leachate results for:

28. Ca, Cu, Mg, Mn, P, and Ti ...........................................................................................................40

29. Al, Fe, Pb, and Zn ........................................................................................................................42

30. As, Ba, Cd, Mo, Ni, and Sr.........................................................................................................42

31. Boxplots of volume magnetic susceptibility...........................................................................43
vi

Tables

1. Geologic unit and locality information for samples studied..................................................9

2. Major element oxide wavelength dispersive X-ray fluorescence (WDXRF)

analyses .......................................................................................................................................11

3. Mineralogy determined by X-ray diffractometry (XRD) analysis........................................12

4. Weight percent CO2, carbon from carbonate, total carbon, and total sulfur

analyses .......................................................................................................................................13

5. Inductively coupled plasma–mass spectroscopy analyses................................................14

6. Procedure used to determine NAP..........................................................................................18

7. Procedure used to determine ANC..........................................................................................19

8. Major- and trace-element abundances for whole rocks selected for sequential 

leachate analysis........................................................................................................................20

9. Seven-step sequential extraction procedure........................................................................21

10. Nine-step sequential extraction procedure...........................................................................21

11. Volume magnetic susceptibility ...............................................................................................22

12. Net acid production results ......................................................................................................24

13. Acid neutralizing capacity.........................................................................................................25

Appendixes—Sequential Leachate Results

A. Magnesium...........................................................................................................................................48

B. Aluminum ..............................................................................................................................................49

C. Potassium..............................................................................................................................................50

D. Calcium..................................................................................................................................................51

E. Titanium..................................................................................................................................................52

F. Iron ..........................................................................................................................................................53

G. Manganese...........................................................................................................................................54

H. Nickel.....................................................................................................................................................55

I. Copper.....................................................................................................................................................56

J. Zinc.........................................................................................................................................................57

K. Arsenic...................................................................................................................................................58

L. Strontium................................................................................................................................................59

M. Molybdenum........................................................................................................................................60

N. Cadmium ...............................................................................................................................................61

O. Barium ...................................................................................................................................................62

P. Lead ........................................................................................................................................................63
Abstract

This report presents results from laboratory and field 

studies involving the net acid production (NAP), acid neutralizing capacity (ANC), and magnetic mineralogy of 27 samples 

collected in altered volcanic terrain in the upper Animas River 

watershed near Silverton, Colo., during the summer of 2005. 

Sampling focused mainly on the volumetrically important, 

Tertiary-age volcanic and plutonic rocks that host base- and 

precious-metal mineralization in the study area. These rocks 

were analyzed to determine their potential for neutralization of 

acid-rock drainage.

Rocks in the study area have been subjected to a regional 

propylitic alteration event, which introduced calcite, chlorite 

(clinochlore), and epidote that have varying amounts and rates 

of acid neutralizing capacity (ANC). Locally, hydrothermal 

alteration has consumed any ANC and introduced minerals, 

mainly pyrite, that have a high net acid production (NAP). 

Laboratory studies included hydrogen peroxide (H2

O2) acid 

digestion and subsequent sodium hydroxide (NaOH) titration 

to determine NAP, and sulfuric acid (H2SO4) acid titration 

experiments to determine ANC. In addition to these environmental rock-property determinations, mineralogical, chemical, 

and petrographic characteristics of each sample were determined through semiquantitative X-ray diffractometry (Rietveld method), optical mineralogy, wavelength dispersive X-ray 

fluorescence, total carbon-carbonate, and inductively coupled 

plasma–mass spectrometric analysis.

An ANC ranking was assigned to rock samples based 

on calculated ANC quantity in kilograms/ton (kg/t) calcium 

carbonate equivalent and ratios of ANC to NAP. Results 

show that talus near the southeast Silverton caldera margin, 

composed of andesite clasts of the Burns Member of the 

Silverton Volcanics, has the highest ANC (>100 kg/t calcium 

carbonate equivalent) with little to no NAP. The other units 

found to have moderate to high ANC include (a) andesite 

lavas and volcaniclastic rocks of the San Juan Formation, 

west and northwest of the Silverton caldera, and (b) the 

Picayune Megabreccia Member of Sapinero Mesa Tuff along 

the western San Juan caldera margin. Sultan Mountain stock, 

composed of granitoid intrusive rocks, was shown to have low 

ANC and moderate NAP. 

Sequential leachate analyses on a suite of whole-rock 

samples from the current and a previous study indicate that 

host rock composition and mineralogy control leachate 

compositions. The most mafic volcanic samples had high 

leachate concentrations for Mg, Fe, and Ca, whereas silicic 

volcanic samples had lower ferromagnesiun compositions. 

Samples with high chlorite abundance also had high leachable Mg concentrations. Trace-element substitution, such 

as Sr for Ca in plagioclase, controls high Sr concentrations 

in those samples with high plagioclase abundance. High Ti 

abundance in leachate was observed in those samples with 

high magnetite concentrations. This is likely due to samples 

containing intergrown magnetite-ilmenite. Whole rocks having high trace-element concentrations have relatively high 

leachate trace-element abundances. Some lavas of the San 

Juan Formation and Burns Member of the Silverton Volcanics had elevated Zn-, Cd-, and Pb-leachate concentrations. 

Manganese was also elevated in one San Juan Formation 

sample. Other San Juan Formation and Burns Member lavas 

had low to moderate trace-element abundances. One sample 

of the pyroxene andesite member of the Silverton Volcanics had elevated concentrations for As and Mo. Most other 

pyroxene andesite member samples had low leachate traceelement abundances. 

Mine-waste-leachate analyses indicated that one minewaste sample had elevated concentrations of Cu (1.5 orders 

of magnitude), Zn (1 order of magnitude), As (1 order of 

magnitude), Mo (1.5 to 2 orders of magnitude), Cd (1 to 2 

orders of magnitude), and Pb (2 to 3 orders of magnitude) 

compared to whole rocks. These data indicate the importance 

of whole-rock geochemistry or leachate analyses prior to 

using igneous outcrops or talus for mine-waste remediation 

projects. Also, if ANC units have modest trace contaminant 

abundances compared with mine waste targeted for cleanup, 

then these ANC materials could still be useful to consider for 

remediation projects.

Net Acid Production, Acid Neutralizing Capacity,

and Associated Mineralogical and Geochemical

Characteristics of Animas River Watershed Igneous

Rocks Near Silverton, Colorado

By Douglas B. Yager, LaDonna Choate, and Mark R. Stanton
2 Geochemical Characteristics of Animas River Watershed Igneous Rocks Near Silverton, Colorado

New magnetic susceptibility measurements indicate that 

the high ANC pyroxene andesite member of the Silverton 

Volcanics (previous study) also has the highest median volume 

magnetic susceptibility. Other units with representative high 

ANC outcrops or talus (Burns Member lavas, tuffs of Eureka 

Member of Sapinero Mesa Tuff, Picayune Megabreccia 

Member, and San Juan Formation) have variable but generally moderate to low magnetic susceptibility. Sultan Mountain 

stock has a moderate, mean magnetic susceptibility.

Introduction

Land management agencies, in partnership with mine 

owners, and other local, State, and Federal stakeholders in 

several watersheds located in the western United States are 

involved in the cleanup of abandoned hard-rock metal mines 

and mine-related features such as waste piles or tailings ponds. 

These cleanup efforts are necessary because many abandoned 

mine sites have acid-generating minerals, predominantly 

pyrite, that in the presence of oxygen and water produce sulfuric acid that is toxic to aquatic life. In addition, acidic water 

leaches major and trace elements from rocks in concentrations 

that can also be toxic to aquatic life.

Mine-site remediation project managers must consider 

multiple variables in order to achieve the goal of reducing the 

impact that acid-mine drainage has on surface and ground 

water (Hutchison and Ellison, 1992). Mine-site configuration 

in relation to topography, hydrology, possible attenuation of 

metals and acidity by surficial deposits and bedrock, geologic 

structures and hazards, and transportation logistics are but a 

few examples of issues that need consideration when designing and implementing a mine-site remediation project.

A little studied but potentially important part of minewaste remediation is the environmental rock properties of net 

acid production (NAP) and acid neutralizing capacity (ANC) 

of local bedrock. This study addresses the NAP and ANC 

of igneous rocks that are host to base- and precious-metal 

mineralization in areas adjacent to the historic mining town 

of Silverton, Colo. The area has more than a 100-year legacy 

of hard-rock mining. Federal land management agencies and 

the Animas River Stakeholders Group are involved in current 

mine-waste remediation efforts near Silverton. In addition to 

NAP and ANC, we examine magnetic susceptibility, geochemistry, and mineralogy of samples that were analyzed for NAP 

and ANC. Our data are useful to land managers and stakeholders interested in locally available acid-neutralizing materials 

for use in mine-cleanup operations. 

Previous Work

This report is a continuation of ongoing research on the 

environmental rock properties NAP and ANC, and magnetic 

susceptibility of Silverton Volcanics. In a previous study 

(Yager and others, 2005), we focused on igneous rocks near 

Silverton, Colo., and identified several igneous units that have 

high, instantaneous ANC in laboratory tests. We also measured magnetic susceptibility of rocks for which NAP and 

ANC were determined. The magnetic susceptibility measurements were used to ground-truth airborne magnetic survey 

data and to test if rocks with corresponding NAP and ANC 

properties could be mapped by airborne geophysical methods. 

The pyroxene andesite member of the Silverton Volcanics 

was shown to have the highest overall ANC and also has, on 

average, a high magnetic susceptibility. Other units that had 

one or more samples with a moderate to high ANC include 

the San Juan Formation (unit Tsj), Eureka Member of the 

Sapinero Mesa Tuff (unit Tse), Burns Member of the Silverton 

Volcanics (unit Tb), and Sultan Mountain stock (unit Tig). 

New Work for this Study

Our current research is based on supplemental samples 

collected on units that were sparsely sampled in our previous study. Additional sampling of these units was warranted 

because at least one or more samples from these units had a 

high ANC, and we needed a more statistically representative 

sample to determine the ANC rock properties of the units. In 

September 2005, 27 samples were collected from the following units: Tsj, Tig, Tse, and Silverton caldera wall sediments. 

In addition, sampling focused on unit Tb lava outcrop and 

adjacent talus clasts that are logistically accessible to land 

management agencies for use in mine-waste remediation projects. The talus, located in Minnie Gulch (fig. 1), is composed 

of both unit Tb and subordinate lava clasts of the pyroxene 

andesite member of the Silverton Volcanics (unit Tpa). 

The emphasis of this report is three-fold. First, we 

address the environmental rock properties net acid production 

(NAP) and acid neutralizing capacity (ANC) of igneous rocks 

that are host to most of the base-metal and precious-metal 

mineralization in the study area. Net acid production and acid 

neutralizing capacity data can be used to understand the relative acid generating and neutralizing capacity of waste rock 

that is commingled in mine-waste piles. Net acid production 

data can be used to identify subbasins that are likely to generate acidity. Conversely, acid neutralizing capacity data can 

be used to identify subbasins that might attenuate acidity and 

metals, and delineate local bedrock that could be used in minewaste remediation projects. 

A second focus of this research involves measuring 

geophysical properties, specifically, magnetic susceptibility 

for rocks in which NAP and ANC were also determined. We 

use these data to identify correlations between geophysical properties, NAP, and ANC. These data are also used in 

ground-truth verification of airborne electromagnetic (EM) 

and magnetic survey data, which can be used to regionally 

map rocks with varying NAP and ANC properties. In research 

subsequent to the Yager and others (2005) study, resistivity 

studies by McCafferty and others (2006) have shown a trend 
Study Area 3

between the magnetic susceptibility of a rock unit, its resistivity, and ANC. For example, unit Tpa has, on average, a high 

ANC, high magnetic susceptibility, and in some areas a low 

resistivity. These ANC and geophysical rock property data on 

unit Tpa, for example, enable regional EM survey information to be used in combination with regional maps that have 

unit Tpa delineated, to help predict where this unit is likely to 

have a high ANC.

Finally, a third focus of this study is to investigate the 

major and trace elements that can be leached from ANC 

rocks and from a mine-waste site that is targeted for remediation in the study area. These studies involve sequential 

extractions of leachate derived as the ANC rocks and mine 

waste are reacted with solutions of varying and buffered 

pH. Geochemical analyses of leachate are used to test (in 

controlled lab experiments) potential contaminants that may 

go into solution when mine water reacts with ANC rocks and 

mine waste in a field scenario. This information is important 

to land managers who might use ANC rocks in mine-waste 

remediation projects.

Study Area

The Animas River watershed near the historic mining 

town of Silverton, Colo. (fig. 1), is an excellent area in which 

to study the ANC of local rocks. A more than 100-year history of mining, primarily for silver, gold, and base metals 

(copper-lead-zinc), has left behind hundreds of abandoned 

mine-waste piles and associated mining-related features and 

deposits. Many mining-related features are located on lands 

managed by the U.S. Forest Service and U.S. Bureau of Land 

Animas River watershed

study area (box corresponds

to neatline)

Silverton

C hattanooga

E ureka

S ilverton

S UNNYS IDE

MINE

Animas

F orks

Howardsville

3,792 m

Ohio Peak

Sultan Mt

Highland

Mary

Mine

B

B

Anvil Mt

Mt Abrams

G ladstone

0

0 1

1 2 MILE S

2 K ILOME TE R S

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

37°

52'

 30"

 37o

45'

 37o

58'

S ilver C loud

Mine

C lear Lake

B

Figure 1. Location map and geographic features referred to in this and previous ANC studies.
4 Geochemical Characteristics of Animas River Watershed Igneous Rocks Near Silverton, Colorado

Management, which require Federal involvement in any 

cleanup efforts. The Animas River Stakeholders Group that is 

based in Silverton, Colo., is also currently involved in several 

mine-cleanup efforts that could benefit from detailed information on the ANC of local rocks for application in remediation 

projects. If it can be demonstrated that nearby geologic units 

have significant ANC, then more local source materials could 

be used in remediation projects, thereby reducing the amount 

of limestone that would need to be hauled from long distances.

The town of Silverton, Colo., is relatively remote, nestled 

between two mountain passes, and located 50 miles from 

Durango, Colo., the nearest large town. Elevations in the area 

are generally greater than 9,000 ft (2,743 m). Thus, utilization 

of local source materials could help reduce remediation costs 

associated with transportation of ANC rocks from other areas.

Geologic Setting

A brief discussion of the geology of the study area is 

important because it provides context for the various rock 

types that were studied in our ANC experiments. The western 

San Juan Mountains record a relatively complete geologic 

record from Precambrian to Cenozoic time. However within 

the upper Animas River watershed near Silverton, multiple 

Tertiary volcano-tectonic and intrusive events have obliterated 

much of the geologic record and have left thick accumulations of silicic igneous rocks. The stratigraphy of the Animas 

River watershed study area consists of Precambrian crystalline 

basement overlain by Paleozoic-, Mesozoic-, and Eoceneage sedimentary rocks, and a 1- to 2-km-thick Oligocene- to 

Miocene-age volcanic cover (fig. 2) (Yager and Bove, 2002; 

Yager and Bove, 2007).

Mid-Tertiary Volcanism

Rocks related to the episode of mid-Tertiary volcanism 

and later alteration is of principal interest in this study. 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 silicic-composition volcanic rocks. Onset of 

mid-Tertiary volcanism records the 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 silicic ash flows. The eruption of ash flows led to caldera 

formation and localized subsidence. 

Mid-Tertiary volcanism in the San Juan Mountains began 

between 35 and 30 Ma with the eruption and deposition of 

voluminous 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). From 30 to 23 

Ma, multiple calderas formed and related volcanic units were 

deposited throughout the San Juan volcanic field (Lanphere, 

1988; Lipman and others, 1997; Bove and others, 2001). Two 

caldera-forming events took place in the upper Animas River 

watershed: the 28.2-Ma San Juan-Uncompahgre caldera complex and the younger, nested, 27.6-Ma Silverton caldera (Bove 

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

within the southwest half of the roughly dumbbell shaped San 

Juan-Uncompahgre caldera complex. San Juan caldera eruptives include the outflow units of the Sapinero Mesa Tuff and 

the intracaldera Eureka and Picayune Megabreccia Members. 

San Juan caldera activity culminated with ring-fracture 

volcanism, the products of which constitute the Silverton 

Volcanics. These rocks, which are primarily intermediate composition lava flows and volcaniclastic sedimentary rocks, fill 

the caldera depression to a thickness exceeding 1 km and are 

perhaps the most important rocks in this study area as they can 

have a high ANC in some places and are host to the majority 

of mineral deposits mined (Bejnar, 1957).

Beginning approximately 1 million years after Silverton 

caldera formation, numerous stocks, dikes, and other silicic 

intrusions formed along the Silverton caldera ring-fracture 

zone and are known as the Sultan Mountain stock. The oldest 

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

along the southern part of the Silverton caldera ring fracture 

(Lipman, 1976; Ringrose, 1982; Bove and others, 2001).

Mineralization and Subsequent Erosion Events

The Tertiary volcano-tectonic activity was followed by 

multiple silicic igneous intrusion and mineralization events 

that span more than 20 million years and have resulted in five 

major alteration types in the study area (Bove, Yager, and 

others, 2007; Bove, Mast, and others, 2007). 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, Mast, and others (2007).

Neogene erosion denuded much of the San Juan Mountains volcanic cover. The high potential for erosion developed 

during this time due to differential displacement of the San 

Juan Mountains relative to the incipient, adjacent Rio Grande 

rift basin to the east and post-middle Tertiary extensional 

tectonic activity. Chapin and Cather (1994) estimated Precambrian bedrock offset in adjacent mountain ranges and basins 

in order to approximate the amount of late Tertiary relative 

displacement. Using this technique, they determined an average relative displacement of Precambrian rocks of 5 km. They 

further estimated from basin stratigraphy that 7–30 times more 

sediment was supplied to the Rio Grande rift basin during 

the Miocene and Pliocene than in the Pleistocene. Clearly, 

late Tertiary erosion was important in exposing large surface 

areas of mineralized terrain and also exposing large areas of 

igneous bedrock that may have a high ANC. 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.
Rock Types of Interest for Potential Acid Neutralization Capacity 5

Weathering processes in glaciated cirque basins and 

subalpine to alpine mountain valleys have resulted in accumulation of large volumes of talus, debris-cone, and landslide 

deposits. The mineral constituents present in these unstable 

surficial deposits may influence their acid generating or acid 

neutralizing capacities.

Numerous surficial deposits in the upper Animas 

River watershed serve as porous and permeable pathways 

for surface-water infiltration and ground-water flow (Blair 

and others, 2002). From GIS analysis of geology coverage 

(Yager and Bove, 2007), we estimate that 27 percent of the 

bedrock in the watershed is covered by 1 to ≥5 m of porous 

Pleistocene to Holocene sedimentary deposits (Yager and 

Bove, 2007).

Rock Types of Interest for Potential 

Acid Neutralization Capacity

Multiple lithologic units throughout the stratigraphic 

section in or near the study area, from Precambrian 

basement rocks to Quaternary surficial deposits, have a 

potentially moderate to high ANC. These rock types are 

discussed below.1

Precambrian Rocks

Precambrian rocks crop out near the periphery of the 

Animas River watershed study area—principally along the 

Animas River below Silverton, near Cunningham Creek, near 

the eastern margin of the watershed, east of the historical 

Animas Forks townsite, and north of Mount Abrams. Precambrian rocks along the Animas River below Silverton and at the 

headwaters of Cunningham Creek (sampled in this study) are 

equivalent to the Irving Formation (Barker, 1969; Luedke and 

Burbank, 2000). Irving Formation rocks consist of interlayered 

fine-grained, dark-gray to greenish-gray banded amphibolite 

with various fine-grained, gray to green, plagioclase-, quartz-, 

and biotite-bearing gneisses and minor sericite-biotite-chlorite 

schists (Barker, 1969). Amphibolite is the predominant rock 

type in the Irving Formation and consists of blue-green 

hornblende, calcic plagioclase, and epidote; varieties with 

1

 Paleozoic, Mesozoic, and Eocene sedimentary rocks were not investigated 

for ANC as part of this study.

Figure 2. Generalized geology of study area and surrounding regions near Silverton, Colo. Major tributaries to Animas River are shown 

in addition to major Tertiary volcano-tectonic structures related to Silverton and San Juan calderas. Modified from Casadevall and 

Ohmoto (1977).
6 Geochemical Characteristics of Animas River Watershed Igneous Rocks Near Silverton, Colorado

biotite, chlorite, and quartz are not uncommon (Barker, 1969). 

In many places, plagioclase-, quartz-, and 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 possible acid-neutralizing mineral 

assemblage of chlorite-epidote-calcite (Desborough and 

Driscoll, 1998; Desborough and Yager, 2000).

Paleozoic Sedimentary Rocks

Paleozoic sedimentary units are primarily exposed south 

and west of Silverton along the Animas River and in the South 

Fork Mineral Creek subbasin (Luedke and Burbank, 2000; 

Yager and Bove, 2007). Several of the Paleozoic sedimentary 

units likely have a high ANC due to the presence of limestone 

(CaCO3

) interbeds, calcareous units such as mudstone, and the 

presence of either limestone clasts or calcite-cemented matrix. 

Calcareous-bearing units include the Upper Devonian Elbert 

Formation and Ouray Limestone, Mississippian Leadville 

Limestone, Pennsylvanian Molas Formation, Pennsylvanian 

Hermosa Formation, and Permian Cutler Formation. The 

thickest section of Paleozoic strata is preserved along the 

Animas River below Silverton and in the subbasins that drain 

South Fork Mineral Creek.

Mesozoic Sedimentary 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 (Luedke and 

Burbank, 2000; Yager and Bove, 2007). As with the Paleozoic 

section, several units in the Mesozoic sedimentary section 

contain calcareous-bearing units such as the Triassic Dolores 

Formation and Jurassic Wanakah and Morrison Formations.

Tertiary Sedimentary Rocks

Tertiary sedimentary rocks in the Animas River watershed crop out in the western and southern part of the study 

area and are composed of the Eocene Telluride Conglomerate. 

Lower layers within the Telluride Conglomerate contain 

rounded to subangular cobbles of Mesozoic and Paleozoic red 

sandstone and limestone. The Telluride Conglomerate is an 

important unit because of its high ANC potential.

Tertiary Igneous Rocks

Rocks of the San Juan Formation represent the oldest 

Tertiary units sampled and are exposed outward from the 

Silverton caldera structural margin in the Mineral Creek basin 

and near the San Juan caldera margin south of Silverton in the 

vicinity of Cunningham Creek and Deer Park Creek, as well as 

within other Animas River subbasins. The San Juan Formation 

is primarily composed of intermediate composition volcaniclastic sedimentary rocks; however, minor intercalated andesitic lavas are typically porphyritic and consist of plagioclase, 

clinopyroxene, hornblende, and opaque oxide primary phases. 

This unit along with all other igneous units studied is altered 

to a secondary propylitic mineral assemblage (see below).

Subsequent to deposition of the San Juan Formation, ash 

flows were deposited in response to collapse of both the San 

Juan and younger nested Silverton calderas and are sources of 

the Eureka Member of the Sapinero Mesa Tuff and the Crystal 

Lake Tuff, respectively. The Eureka Member of the Sapinero 

Mesa Tuff is the intracaldera component of the Sapinero 

Mesa Tuff, is dacitic to rhyolitic in composition, and contains 

phenocrysts of plagioclase, with lesser amounts of sanidine, 

biotite, and trace augite. The Crystal Lake Tuff is volumetrically minor in comparison to the Sapinero Mesa Tuff and thus 

was not sampled.

During formation of the San Juan caldera, limestone units 

of the Leadville and Ouray Limestones caved inward toward 

the central core of the caldera forming megabreccia. One such 

megabreccia block crops out near the Highland Mary mine in 

the Cunningham Creek subbasin.

San Juan caldera activity culminated with ring fracture 

volcanism, the products of which constitute the Silverton 

Volcanics. The Burns Member of the Silverton Volcanics is 

characterized by propylitically altered lavas. The pyroxene 

andesite member, which generally overlies the Burns Member, 

is mainly composed of intermediate composition lava flows 

with a trend toward more mafic compositions (Yager and 

Bove, 2007) and tends to be somewhat less altered than the 

Burns Member. The volcaniclastic sedimentary rocks of the 

Henson Member of the Silverton Volcanics interfinger with 

both the Burns and the pyroxene andesite (Lipman, 1976). 

In general, lavas of the Silverton Volcanics are porphyritic 

and typically contain phenocrysts of plagioclase, amphibole, 

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

aphanitic groundmass.

Late-stage (23–10 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, Yager, and others, 2007; 

Bove, Mast, and others, 2007). 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 fine-grained groundmass.

Regional Propylitic Alteration

Regional propylitic alteration identified by Burbank 

(1960) and described further in Bove and others (2007) as 

a “pre-ore propylitization” event has affected most igneous 

rocks in the vicinity of the San Juan-Uncompahgre and 

Silverton caldera complex (fig. 3). The mineralogy of this 

assemblage, although variable, consists of quartz, chlorite, 
Methods 7

epidote, calcite, secondary potassium feldspar, albite, pyrite, 

and opaque Fe-oxides in the presence of unaltered to slightly 

altered primary feldspar crystals. This alteration type is 

thought to have formed from 28.2 Ma to about 27.6 Ma as the 

nearly 1-km-thick sequence of lavas of Silverton Volcanics 

that infilled the San Juan caldera cooled and degassed volatile 

constituents. Large quantities of carbon dioxide (CO2), perhaps the primary rock-altering volatile component (among others are water (H2

O) and sulfur dioxide (SO2)), were released 

during cooling, altering the original minerals and matrix of the 

country rock (Burbank, 1960; Bove, Yager, and others, 2007; 

Bove, Mast, and others, 2007). Rocks that have undergone this 

type of alteration commonly have a greenish hue in outcrop 

owing to the presence of chlorite and epidote. Where pyrite is 

either sparse or absent, the propylitic assemblage can supply 

some ANC. The subsequent, more intense alteration and 

mineralization events that were commonly focused along vein 

structures related to San Juan and Silverton caldera formation 

in several places overprint the regionally pervasive propylitic 

alteration assemblage and locally eliminate any ANC of the 

propylitic assemblage (fig. 3).

Quaternary Surficial Deposits

Weathering of mineralized bedrock and subsequent 

deposition of weathered material in downslope surficial 

deposits will result in acid generation as water infiltrates and 

reacts with pyrite and other acid-generating sulfide minerals. In contrast, 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, 2002).

Acid-Generating and Acid-Neutralizing 

Minerals

Complex weathering reactions of geochemical constituents of minerals in rocks determine if a sample will produce 

or consume acid. Reactions at the water-mineral interface 

involving oxidation, adsorption, desorption, ion exchange, 

dissolution, and surface area control the processes that ultimately result in the elements that are released into solution or 

are chemically bound or sequestered (Davis and Kent, 1990). 

The kinetic rates at which these reactions occur in place are 

variable and are thus difficult to reproduce or simulate in a 

laboratory setting. It is, therefore, important to emphasize that 

our laboratory data provide an estimate of readily available 

neutralizing or acid-generating capacity.

Important oxidation-weathering reactions that contribute 

to acid generation in some samples studied include pyrite, 

the principal acid-generating phase (Salmon and Malmström, 

2004), chalcopyrite and sphalerite (Salmon and Malmström, 

2004), and galena (Blowes and others, 2003). Weathering 

reactions are given below.

Acid-Generating Mineral Reactions

•	 Pyrite oxidation (FeS2) (ferric iron reaction path)

FeS2(s) + 14Fe3+ + 8H2O → 15Fe2+ + 2SO4

2- + 16H +

•	 Chalcopyrite oxidation (CuFeS2) (ferric iron reaction 

path; trace abundances in some samples)

CuFeS2(s) + 16Fe3+ + 8H2O → 17Fe2+ + Cu2+ + 

2SO4

2- + 16H +

•	 Spahalerite oxidation (ZnFeS) (ferric iron reaction 

path; trace abundances) in some samples)

ZnS(s) + 8Fe3+ + 4H2O → Zn2+ + SO4

2- + 8Fe2+ + 8H +

•	 Galena (PbS) (ferric iron reaction path; trace abundances in some samples)

PbS(s) + 8Fe3+ + 4H2O → + Pb2+ + SO4

2- +8Fe2+ + 8H +

Weathering reactions for the high ANC minerals with 

relatively fast dissolution rates for calcite (Jambor, 2003), and 

moderate ANC and intermediate dissolution rates for chlorite 

(Salmon and Malmström, 2004), are given below.

Acid-Neutralizing Minerals

•	 Calcite (CaCO3)

2CaCO3(s) + 4H+→ 2Ca2+ + 2H2CO3

-

•	 Chlorite (MgFeAl)AlSi3O10(OH)2)

Additional phases locally present in Animas River 

watershed rocks that were shown in other studies to have some 

ANC are epidote (Ca2(Fe3+,Al)3(SiO4)

3

(OH)) and hornblende 

(Ca2(Mg,Fe)4Al(Si7Al)O22(OH,F)2) (Jambor and others, 

2002). Other silicate phases that are thought to have low reactivity and minimal ANC include the sodic feldspars, potassium 

feldspar, and sericite (Jambor, 2003; Jambor and others, 2002).

Methods

Sample Preparation and Analyses

Twenty-seven composite rock samples were collected in 

September 2005 for NAP, ANC, and geochemical, mineralogical, and rock physical property studies (table 1). Splits 

of all samples were first jaw-crushed to pebble-size using 

steel plates, then pulverized to approximately <200 mesh (70 

μm) in an agate shatterbox for wavelength dispersive X-ray 

fluorescence (WDXRF) analyses (table 2), x-ray diffractometry (XRD) semi-quantitative mineralogy using the Rietveld 

method (Young, 1993; Raudsepp and Pani, 2003) (table 3), 

(Mg4.5Fe0.2Fe0.2A1)A1Si3O10(OH)8(s) + 16H+  

 4.5Mg2+ + 0.2Fe2+ + 0.2Fe3+ + 2Al3+ + 3SiO2(s)

 + 12H2O

II III
8 Geochemical Characteristics of Animas River Watershed Igneous Rocks Near Silverton, Colorado

Figure 3. Generalized map of alteration types in the Animas River watershed study area. Data in Bove , Yager and others (2007) 

and Yager and others (2007).

total carbon-carbonate analyses (table 4), inductively coupled 

plasma–mass spectrometric analyses (ICP–MS) (table 5), 

and net acid production (Lapakko and Lawrence, 1993). 

Additional splits were crushed and sieved to less than 2 mm 

for ANC acid titration experiments. None of the samples 

was subjected to grinding during any preparation process. 

Grinding could lead to surface exposure and water–mineral 

reactions on layered chlorite surfaces not commonly found 

in natural weathering processes. Such surfaces may not 

be exposed during normal weathering processes involving 

freeze–thaw cycles, rainfall precipitation events, and grainto-grain abrasion that occurs during fluvial transport.
Table 1. Geologic unit and locality information for samples studied. [Geologic unit abbreviations for tables 1, 3, 8, 11, 12, and 13 are as follows: pe, Precambrian Irving Formation; Pz(?), Paleozoic(?) rocks; TTig, Sultan Mountain stock; Tsemb, Picauyune Megabreccia Member of the Sapinero Mesa Tuff; Tse, Eureka Member of the Sapinero Mesa 

of the Silverton Volcanics; Tpa, pyroxene andesite member of the Silverton Volcanics] 

Field sample number Unit Rock type Longitude Latitude Locality descript SDY090532 Tig Propylitically altered granitic intrusion –107.74424 37.81690 West of highway 550 alon SDY090533 Tig Weakly sericitized lava –107.70367 37.80735 West of highway 550 alon SDY090534A Tig Propylitically altered granitic –107.70413 37.80832 West of highway 550 alon

intrusion (clast in colluvium) 

 SDY090534B Tig Propylitically altered granitic –107.70413 37.80832 West of highway 550 alon

intrusion (clast in colluvium) 

 SDY090535A Tsj Propylitically altered andesite –107.72785 37.86369 West of highway 550 belo SDY090535B Tsj Propylitically altered, dacite –107.72785 37.86369 West of highway 550 belo SDY090536 Tsj Propylitically altered weakly –107.73027 37.86173 West of highway 550 abov

sericitized, dacite Imogene mine 

 IDY090537A Tsj Propylitically altered dacite –107.73569 37.87625 West of Chattanooga adja

mine portal 

 IDY090537B Tsj Propylitically altered basalt –107.73569 37.87625 West of Chattanooga adja

mine portal 

 IDY090537C Tsj Propylitically altered dacite –107.73569 37.87625 West of Chattanooga adja

mine portal 

 IDY090538E Tsj Quartz-sericite-pyrite altered lava –107.74138 37.87210 West of Chattanooga east 

mine 

 IDY090538G Tsj Propylitically altered dacite –107.74138 37.87210 West of Chattanooga east 

mine 

 SDY090539 Tig Propylitically altered granitic intrusion –107.80741 37.68138 Northwest of Highway 55 HWDY090540 Unknown Propylitically altered pre-caldera –107.56105 37.86170 Minnie Gulch 

sedimentary rocks 

 HWDY090541A Tb Propylitically altered andesite –107.53239 37.85848 Minnie Gulch  HWDY090541B Tb Propylitically altered andesite –107.53239 37.85848 Minnie Gulch 
Table 1. Geologic unit and locality information for samples studied—Continued. Field sample number Unit Rock type Longitude Latitude Locality descript HWDY090542A Tb Propylitically altered andesite –107.53812 37.85436 Minnie Gulch  HWDY090542B Tb Propylitically altered andesite –107.53812 37.85436 Minnie Gulch  HWDY090543 Tsemb Propylitically altered dacitic –107.53295 37.84766 Minnie Gulch 

megabreccia 

 SDY090545 Tb Propylitically altered basaltic andesite –107.72210 37.85645 Near mouth of Browns Gu SDY090548 Tse Propylitically altered rhyolite –107.66163 37.79294 Deer Park trail 

ash-flow tuff 

 SDY090549 Tse Propylitically altered trachydacitic –107.56049 37.88902 Near Eureka townsite wes

ash-flow tuff in roadcut 

 HPDY090550 Tse Propylitically altered trachydacitic –107.55782 37.90268 Along Picayune Gulch roa

ash-flow tuff 

 ODY090552A Tsj Propylitically altered dacitic –107.77485 37.82190 Clearlake road, south of la

lahar outcrop 

 ODY090552B Tsj Propylitically altered basaltic –107.77485 37.82190 Clearlake road, south of la

lahar (float) 

 ODY090552C Tsj Propylitically altered basalt –107.77485 37.82190 Clearlake road, south of la

(float) 

 ODY090553 Tsj Propylitically altered dacite –107.76957 37.81340 Clearlake road, south of la

(float) 
Table 2. Major element oxide wavelength dispersive X-ray fluorescence (WDXRF) analyses.1 Field sample no. SiO2 Al2O3 FeTO3 MgO CaO Na2O K2O TiO2 P2O5 MnO LOI 

 SDY090532 64.3 16.8 2.86 0.66 2.63 4.39 5.84 0.28 0.24 0.07 0.81 SDY090533 57.3 16.1 8.30 3.52 2.73 2.70 2.18 0.96 0.71 0.25 5.57  SDY090534A 61.7 15.8 6.48 2.38 4.71 3.46 3.99 0.79 0.42 0.12 0.36  SDY090534B 64.5 15.2 5.37 1.93 3.97 3.25 4.02 0.64 0.32 0.10 0.52  SDY090535A 58.7 15.3 6.87 2.78 7.87 1.84 1.36 0.69 0.39 0.39 3.76  SDY090535B 62.4 15.8 5.73 2.06 4.84 2.52 1.80 0.76 0.39 0.19 2.59  SDY090536 65.4 14.5 5.82 1.97 2.75 2.80 1.16 0.64 0.36 0.10 3.76  IDY090537A 62.4 15.3 5.25 0.79 4.20 3.05 3.00 0.60 0.32 0.06 4.69  IDY090537B 44.4 18.4 9.54 4.67 7.10 4.69 1.15 1.15 0.57 0.20 7.73  IDY090537C 60.4 15.3 5.19 1.22 4.78 2.31 2.86 0.69 0.37 0.13 5.46  IDY090538E 69.1 16.1 3.29 0.85 0.49 <0.15 4.71 0.87 0.35 0.07 3.79  IDY090538G 59.9 16.8 7.56 2.44 4.97 2.26 2.52 0.82 0.38 0.15 1.65  SDY090539 72.1 13.4 2.41 0.45 0.91 2.59 6.40 0.28 0.17 0.03 1.11  HWDY090540 65.1 17.7 3.08 1.48 0.33 1.76 6.68 0.47 0.25 0.11 2.16  HWDY090541A 58.3 15.7 6.50 2.50 3.67 3.99 3.97 0.78 0.37 0.11 3.52  HWDY090541B 56.9 15.6 6.32 2.31 5.21 3.11 3.40 0.78 0.37 0.16 5.78  HWDY090542A 58.6 15.7 6.64 2.51 5.28 3.28 3.45 0.79 0.39 0.13 2.97  HWDY090542B 56.9 15.4 6.40 2.65 3.71 3.00 4.42 0.80 0.38 0.18 5.77  HWDY090543 65.7 13.0 2.99 0.89 4.45 1.29 4.90 0.39 0.17 0.14 5.20  SDY090545 54.2 16.4 10.1 3.27 6.35 2.90 1.85 0.99 0.54 0.21 2.59  SDY090548 69.2 15.0 2.06 0.65 1.72 2.36 6.48 0.33 0.16 0.05 1.20  SDY090549 67.9 15.2 3.05 1.06 2.65 2.81 5.09 0.45 0.22 0.09 1.29  HPDY090550 65.3 15.5 3.49 1.23 1.72 2.64 5.72 0.49 0.23 0.12 2.27  ODY090552A 59.1 14.5 5.61 2.65 5.31 2.43 2.73 0.63 0.28 0.18 6.13  ODY090552B 46.7 18.6 9.96 3.75 10.6 3.06 0.56 1.01 0.44 0.21 4.43  ODY090552C 53.7 15.5 10.0 3.65 7.12 2.05 2.42 1.13 0.52 0.18 2.71  ODY090553 64.6 14.9 4.96 1.97 3.65 2.94 2.40 0.56 0.30 0.09 3.48 

1WDXRF analyses provided by Peter M. Theodorakos, U.S. Geological Survey. 
Table 3. Mineralogy determined by X-ray diffractometry (XRD) analysis (values reported in weight percent). [Rietveld XRD analyses provided by Stephen J. Sutley, USGS. Accuracy and precision using the Rietveld method were shown to vary based on phase abund(Raudsepp, and others, 1999), it was demonstrated that in complex mixtures of varying proportions of skarn minerals that included wollastonite as the major ranged from 1.3 to 6 percent containing wollastonite in mixtures of 30 to 90 weight percent and 6 weight percent respectively. See table 1 for unit abbreviatiField sample no. Unit Calcite Clinochlore Epidote Hematite Hornblende Magnetite Plagioclase Quartz Mica Potas

(sericite1) felds

SDY090532 Tig 3 8 2 36 10 2 4

SDY090533 Tsj 20 0.5 34 24 4 1SDY090534A Tig 2 1 4 30 31 20 1

SDY090534B Tig 2 8 3 30 24 20 1

SDY090535A Tsj 7 15 11 32 30 5 SDY090535B Tsj 12 11 36 35 5 SDY090536 Tsj 23 5 22 39 10 IDY090537A Tsj 10 4 6 29 32 18 IDY090537B Tsj 20 17 6 42 10 4 IDY090537C Tsj 9 14 3 25 37 12 IDY090538E Tsj 2 52 42 2 

IDY090538G Tsj 21 21 42 3 1SDY090539 Tig 5 0.5 23 39 4 2HWDY090540 Pz(?) 9 20 32 20 1HWDY090541A Tb 7 10 3 45 16 2 1HWDY090541B Tb 11 12 tr. 2 38 21 8 9 HWDY090542A Tb 4 12 2 4 40 19 1HWDY090542B Tb 9 15 2 2 36 17 5 1HWDY090543 Tsemb 12 5 3 17 42 11 1SDY090545 Tb 15 28 22 tr. 1SDY090548 Tse 4 2 29 31 3 3SDY090549 Tse 5 2 30 32 4 2

HPDY090550 Tse 6 1 4 29 28 7 2ODY090552A Tsj 13 6 6 31 31 13 ODY090552B Tsj 10 15 18 42 8 6 ODY090552C Tsj 5 16 4 3 6 32 18 3 1ODY090553 Tsj 5 4 6 27 42 7 8 1Sericite is used throughout this report. The term sericite is a field and petrographic term used to describe samples containing micaceous min

definitively identified by x-ray diffractometry or by another analytical method. Samples containing sericite in this study are most similar to fmuscovite but could contain illite. 
Table 4. Weight percent CO2, carbon from carbonate, total carbon, and total sulfur 

analyses.1

Field sample no. CO2 (%) C (%) Total carbon (%) Total sulfur SDY090532 0.05 0.01 0.02 <0.05 

 SDY090533 <0.01 <0.003 <0.01 2.83 

 SDY090534A <0.01 <0.003 <0.01 <0.05 

 SDY090534B <0.01 <0.003 <0.01 <0.05  SDY090535A 1.83 0.5 0.61 0.37  SDY090535B 0.02 0.01 0.06 0.98  SDY090536 0.06 0.02 0.1 1.05 

 IDY090537A 2.82 0.77 0.8 <0.05 

 IDY090537B 4.25 1.16 1.25 <0.05  IDY090537C 3.44 0.94 0.97 <0.05  IDY090538E <0.01 <0.003 <0.01 1.77 

 IDY090538G 0.07 0.02 0.09 <0.05 

 SDY090539 0.12 0.03 0.03 0.32 

 HWDY090540 <0.01 <0.003 <0.01 0.14  HWDY090541A 1.59 0.43 0.45 <0.05 

 HWDY090541B 3.56 0.97 1 <0.05 

 HWDY090542A 0.85 0.23 0.26 <0.05 

 HWDY090542B 2.32 0.63 0.67 0.06 

 HWDY090543 3.58 0.98 1.03 <0.05  SDY090545 0.12 0.03 0.06 0.29 

 SDY090548 0.18 0.05 0.06 <0.05 

 SDY090549 0.15 0.04 0.04 <0.05 

 HPDY090550 0.84 0.23 0.26 <0.05 

 ODY090552A 3.71 1.01 1.04 <0.05 

 ODY090552B 1.59 0.43 0.47 <0.05  ODY090552C 1.17 0.32 0.37 <0.05  ODY090553 1.38 0.38 0.41 <0.05 

1Total carbon, carbonate carbon, and total sulfur analyses provided by Zoe Ann Brown, USGS. 
Table 5. Inductively coupled plasma–mass spectroscopy analyses (ICP–MS). [ICP-MS analyses provided by Monique Adams, USGS] Field sample no. Al (wt. %) Ca (wt. %) Fe (wt. %) K (wt. %) Mg (wt. %) Na (wt. %) P (wt. %) Ti (wt. %) Ag (ppm) 

SDY090532 11.1 2.07 2.18 5.37 0.494 4.05 0.0814 0.236 < 2 SDY090533 8.98 1.98 6.13 1.84 2.25 2.21 0.319 0.643 < 2 SDY090534A 9.83 3.6 4.89 3.59 1.67 3.12 0.17 0.681 < 2 SDY090534B 9.57 2.94 4.19 3.56 1.37 2.97 0.13 0.551 < 2 SDY090535A 9.52 6.19 5.13 1.19 1.88 1.68 0.15 0.4 < 2 SDY090535B 9.43 3.63 4.44 1.56 1.4 2.2 0.156 0.546 < 2 SDY090536 8.31 2.05 4.59 1.02 1.37 2.35 0.148 0.391 < 2 IDY090537A 9.54 3.17 3.62 2.7 0.524 2.74 0.132 0.35 < 2 IDY090537B Nr 5.39 6.38 1.09 3.29 4.12 0.269 0.299 < 2 IDY090537C 10.3 3.74 4.08 2.76 0.888 2.32 0.175 0.226 < 2 IDY090538E < 50 < 100 < 50 < 20 < 6 < 20 < 5 < 40 < 2 IDY090538G 11.2 4 6.3 2.5 1.87 2.26 0.175 0.747 < 2 SDY090539 9.27 0.666 2.09 6.36 0.35 2.7 0.0455 0.142 < 2 HWDY090540 13.3 0.264 2.66 6.96 1.26 1.94 0.101 0.275 < 2 HWDY090541A 11.6 3.03 5.55 4.11 2.11 4.13 0.193 0.731 < 2 HWDY090541B 11.6 4.09 5.19 3.4 1.88 3.33 0.186 0.402 < 2 HWDY090542A 11.8 4.24 5.55 3.56 2.16 3.47 0.192 0.764 < 2 HWDY090542B 9.12 2.76 4.81 4 1.85 2.69 0.163 0.467 < 2 HWDY090543 9.81 3.47 2.44 4.97 0.775 1.56 0.0591 0.172 < 2 SDY090545 10.3 5.02 8.13 1.76 2.41 2.68 0.275 0.856 < 2 SDY090548 11.2 1.37 1.71 6.68 0.539 2.63 0.0508 0.25 < 2 SDY090549 11.7 2.08 2.54 5.35 0.93 3.14 0.0807 0.35 < 2 HPDY090550 12.3 1.44 2.96 6.03 1.18 3.07 0.101 0.269 < 2 ODY090552A 7.8 3.92 3.41 2.32 1.63 1.92 0.104 0.249 < 2 ODY090552B 9.96 8.02 7.4 0.489 2.28 2.47 0.179 0.584 < 2 ODY090552C 8.18 5.29 7.19 2.08 2.26 1.67 0.216 1 < 2 ODY090553 8.07 2.68 3.31 2.08 1.22 2.37 0.0981 0.413 < 2 
Table 5. Inductively coupled plasma–mass spectroscopy analyses (ICP–MS)—Continued. 

Field sample no. Ba (ppm) Be (ppm) Bi (ppm) Cd (ppm) Ce (ppm) Co (ppm) Cr (ppm) Cu (ppm) Ga (ppm) 

 SDY090532 1,810 1.8 0.36 0.34 91.8 3.6 2.4 39.5 23.6  SDY090533 870 1.8 0.1 0.05 95 18.5 26.1 144 20.5  SDY090534A 1,010 2.1 < 0.06 0.18 101 17.3 12.8 66.2 21.2  SDY090534B 865 2.1 < 0.06 0.12 96.6 14 9.9 39.4 19.3  SDY090535A 440 1.3 < 0.06 0.26 99 11.4 20 33 20.6  SDY090535B 498 1.6 < 0.06 0.18 95.6 14.9 22.8 64 20.9  SDY090536 393 1.1 1.54 0.06 83.4 6.6 3.7 84.8 18.7  IDY090537A 850 1.2 < 0.06 0.05 78.9 8.2 5.7 5.2 17.6  IDY090537B 544 1.3 < 0.06 0.14 121 24.1 1.4 7.2 22.4  IDY090537C 1,240 1.3 < 0.06 0.2 102 7.4 1.8 6 19.6  IDY090538E < 0.2 < 0.03 < 0.06 < 0.007 < 0.1 < 0.03 < 0.5 < 2 < 0.02  IDY090538G 1,090 1.2 < 0.06 0.17 91.6 16.6 3 6.8 20.3  SDY090539 1,090 1.8 0.48 0.12 272 4.6 5 69.4 18  HWDY090540 2,750 3 0.15 0.03 167 7.1 5.3 71.3 26.7  HWDY090541A 1,290 1.6 < 0.06 0.08 97.4 19.2 16.6 53.2 20.9  HWDY090541B 847 2.1 < 0.06 0.09 93.8 17.2 15 50 21.5  HWDY090542A 1,050 1.7 0.19 0.09 96.7 20 15.6 67.8 20.5  HWDY090542B 1,340 2 < 0.06 0.05 88.9 17.3 13.8 50.4 19.4  HWDY090543 1,860 1.4 0.06 0.1 104 6.5 5.5 11.4 14.2  SDY090545 1,480 1.6 1.21 0.12 99.4 10.5 4.3 51.6 22  SDY090548 1,580 2.1 0.06 0.16 120 4.1 4.7 7.4 19.4  SDY090549 1,280 2.2 0.14 0.12 103 6.2 9.1 20.2 18.6  HPDY090550 1,500 2.4 0.19 0.27 120 9.3 10.6 23.2 19.8  ODY090552A 917 1.4 0.06 0.08 69.7 13 15.6 4.9 17.1  ODY090552B 272 1.1 < 0.06 0.1 83.7 26.6 14.8 17.8 22.3  ODY090552C 872 1.4 0.06 0.1 90.6 26.6 41.2 31.2 18.9  ODY090553 1,310 1.6 0.07 0.11 76.2 11.7 12.7 18.6 18 
Table 5. Inductively coupled plasma–mass spectroscopy analyses (ICP–MS)—Continued. Field sample no. La (ppm) Li (ppm) Mn (ppm) Mo (ppm) Nb (ppm) Ni (ppm) Pb (ppm) Rb (ppm) Sb (ppm) 

SDY090532 63.3 5 595 1.8 43 1.7 57.3 129 1 SDY090533 51.5 6.6 1,960 2.8 20 17.4 36.8 41.3 0.96 SDY090534A 56.1 17.3 979 1.8 25 11.2 26 118 0.08 SDY090534B 53.2 20.6 847 1.1 29 9.5 24.7 132 0.2 SDY090535A 56.4 26.7 3,310 0.32 19 10.8 9.14 46.7 0.75 SDY090535B 53 21.1 1,620 0.2 22 10.4 11 63.1 0.53 SDY090536 43.9 18.1 832 0.2 21 3.9 15.9 45 0.67 IDY090537A 41.5 16.4 439 0.27 14 2.8 10.3 94.2 0.38 IDY090537B 66.2 71.9 1,640 0.2 14 2.5 11.9 42.2 0.33 IDY090537C 55.3 28.3 1,170 0.1 13 2 8.03 91.8 0.26 IDY090538E < 0.05 < 0.3 <0.7 < 0.05 < 0.1 <0.3 <0.4 0.02 <0.04 IDY090538G 50 34.7 1,360 0.1 21 3 19.4 65.5 0.75 SDY090539 151 10.7 271 3.6 16 3.7 37.6 183 0.37 HWDY090540 94.5 36.9 1,010 0.38 20 4.9 53.2 241 0.34 HWDY090541A 54.3 56.5 993 0.39 26 13.4 9.92 151 0.68 HWDY090541B 52.2 56.2 1,460 1.1 12 12.8 13 155 0.34 HWDY090542A 53.7 69.2 1,200 0.32 28 13.2 20.1 118 1.6 HWDY090542B 51 65.9 1,450 1.5 18 11.8 15 192 0.45 HWDY090543 58.5 23 1,260 0.37 17 3.6 16.9 269 0.66 SDY090545 54.2 20.8 1,800 1.1 40 7.3 10.5 50.9 2.5 SDY090548 73 11.2 412 0.74 36 6.9 21.6 182 0.88 SDY090549 61 22.9 809 0.36 30 8.4 25.6 204 0.88 HPDY090550 70.8 24.5 1,130 0.2 26 8.6 35.6 262 1.4 ODY090552A 37.1 27.2 1,430 0.25 12 8.5 14.7 72.4 0.68 ODY090552B 47.8 41 1,660 0.28 16 8.2 17.2 16.3 0.42 ODY090552C 50.4 20.6 1,450 1.3 28 12.8 11.9 39.6 0.1 ODY090553 40.9 16 683 0.46 24 6.8 18.1 59.1 0.2 
Table 5. Inductively coupled plasma–mass spectroscopy analyses (ICP–MS)—Continued. 

Field sample no. Sr (ppm) Th (ppm) Tl (ppm) U (ppm) V (ppm) Y (ppm) Zn (ppm) 

 SDY090532 1,100 22.9 1.01 7.01 88 16.9 65.9  SDY090533 761 13.4 0.83 4.01 213 26.2 156  SDY090534A 832 21.1 0.59 4 150 28.8 97.7  SDY090534B 622 24.6 0.73 3.76 116 27.8 89.2  SDY090535A 657 13.3 0.68 3.82 149 29.7 133  SDY090535B 548 13.6 0.78 3.89 168 25.3 96.6  SDY090536 765 5.8 1.24 0.86 95.7 23.8 85.7  IDY090537A 498 7.14 0.81 0.76 67.4 20.8 70.2  IDY090537B 1,090 5.22 0.47 1.11 135 32.2 166  IDY090537C 498 5.04 0.96 1.19 82.3 24.5 88.6  IDY090538E < 0.8 < 0.1 <0.08 < 0.02 <0.2 < 0.05 < 3  IDY090538G 763 8.62 1.05 0.94 176 34.6 136  SDY090539 224 80 1.19 11.7 27.1 22.4 47.7  HWDY090540 290 28.6 1.83 4.88 48.3 30.1 95.4  HWDY090541A 627 10.8 1.98 3.52 152 23.5 93  HWDY090541B 343 7.85 2.14 2.56 152 20.8 83.4  HWDY090542A 690 12.3 1.09 3.52 154 24.2 90.5  HWDY090542B 277 10.8 3.03 3.44 146 21.6 88.6  HWDY090543 195 20.2 3.99 4.17 56.6 20.8 47.3  SDY090545 1,020 5.8 1.59 1.56 263 38.1 85.4  SDY090548 521 25.4 1.35 6.83 31 24.7 43.5  SDY090549 566 19 2.71 5.99 56.6 21 83  HPDY090550 376 20.1 4.26 6.93 67.9 23.8 102  ODY090552A 485 5.45 0.67 1.46 92.1 19.4 119  ODY090552B 1,040 5.38 0.17 1.21 209 29 122  ODY090552C 719 6.63 0.26 1.8 273 35.8 121  ODY090553 808 6.11 0.5 1.53 102 21.3 90 
18 Geochemical Characteristics of Animas River Watershed Igneous Rocks Near Silverton, Colorado

Net Acid Production Test

The net acid production (NAP) test was developed as 

a screening tool to determine the acid-generating potential 

of mine-waste materials (Lapakko and Lawrence, 1993). 

This method is a static laboratory bench test that utilizes 

hydrogen peroxide (H2O2) to oxidize sulfide minerals in a 

sample, creating sulfuric acid that subsequently reacts with 

other phases present. Because enhanced sulfide oxidation is

achieved, this test provides an upper limit for acid-generating 

potential. A modified version of the Lapakko and Lawrence 

(1993) NAP method was used for this study. The steps used 

for this procedure are listed in table 6.

Net acid production is calculated in calcium carbonate 

equivalent units (kg/t) that would be necessary to neutralize a 

sample to the final pH of 7.00. This calculation is based on the 

following formula:

Net acid production in kg/t CaCO3

 = 50 × mlb × Nb/

sample weight,

where mlb = total volume of NaOH used during titration 

to pH 7.00 from step nine (table 6), and Nb = normality of 

NaOH used.

Thus, progressively higher NAP values would require 

more calcite to neutralize a solution to pH 7.00.

Table 6. Procedure used to determine NAP. Steps are listed in sequential order. 

Step Procedure 

 1 Powder rock in shatterbox to less than 200 mesh. 

 2 Place 1 g representative split of rock powder from step 1 in 250 ml beaker 

 3 Incrementally add 100 ml 30 percent H2

O2. Watch for energetic reaction with 

sulfides and record relative fizz response (low-moderate-high). 

 4 After reactions have subsided and all 100 ml of H2

O2 is added, place beaker on hot 

plate and heat to 94°C (near boiling) for 1 hour. Remove from hot plate and 

cool for 5 minutes. Initial pH is recorded prior to titration to pH 7. Note that 

some samples have no NAP and will have a neutral pH of 7 or greater at the 

completion of this step. 

 5 Add another 50 ml of H2

O2 and heat to 94oC for 30 minutes. Remove from hot plate 

and cool for 15 minutes. This step is used to help ensure (but not guarantee) 

complete oxidation. If oxidation is incomplete, the NAP calculated will be a 

minimum value. 

 6 Add 1 ml of 0.016 M Cu nitrate solution and boil at 110oC for 10 minutes. 

 7 Cool solution and remove solids by filtration with filter paper and thistle funnel. 

 8 Rinse solids in filter paper with 1M CaCl2. 

 9 Titrate solution by burette to pH 7 with reagent-grade 0.1N NaOH. A magnetic 

stirrer and Teflon stir bar is used to continuously mix the solution between pH 

measurements. 

Acid Neutralization Capacity Test

(Acid Titration) 

The ANC method used in this study is similar to that 

described in Shaw and others (2002) and initially described 

by Robertson Geoconsultants (written commun., 2000) and 

provides an estimate of the instantaneous and readily available 

ANC of rock and mineral particles. The procedure used to 

determine ANC is listed in table 7.

Acid neutralization capacity is calculated in calcium 

carbonate equivalent mass units (kg/t) with the following 

formula, where total acid added in this example is 50 ml, N is 

the normality of acid used, grams (g) of sample = 50, mol = 

moles, formula weight for calcite (CaCO3) is 100 g:

H2SO4 (0.05 liters) × 0.1 N H2SO4 = 0.005 equivalents CaCO3

(0.005 mol CaCO3)

0.005 mol CaCO3 × 100 g CaCO3

 = 0.50 g CaCO3 1mol CaCO3

0.50 g CaCO3 × 1,000 g × 1,000 kg CaCO3 × 1 kg CaCO3

30 g sample 1 kg 1 t 1,000 g

≈17 kg/t CaCO3
Methods 19

splitter. One mine-waste sample was collected from the 

Grand Mogul (GM) mine located in upper Ross basin. The 

mine-waste pile was sampled using a 3-m grid-cell spacing. Surface samples were collected from each cell in 5-gal 

plastic buckets using a shovel. All samples were composited and mixed on-site and randomly replaced into the 

plastic buckets. An additional composite mixing procedure 

similar to the one used on-site was repeated in the lab; 

although during this step cobble-size clasts were excluded 

from further analysis. The mine waste was subsequently 

split using a Jones splitter and approximately 1 g of dry 

sample was obtained for extraction and analysis.

Duplicates of three of the samples were performed for 

comparison. A seven-step extraction procedure (table 9) 

was used to determine the metals released from specific 

phases and total elements liberated. A separate nine-step 

extraction procedure (table 10) having two pH extractions 

were added to determine possible metals released from 

chlorite species. Chlorite species were specifically targeted 

in sequential extraction studies because in a previous study 

(Yager and others, 2005) clinochlore and chamosite in 

propylitically altered volcanic rocks were found to have 

This calculation assumes that all of the ANC is attributable to calcite. We know that this is not the case in our study 

as many samples contain clinochlore, which continues to 

react after calcite is consumed. Some samples lack calcite 

and yet have measurable ANC (Yager and others, 2005).

Sequential Extraction Procedure

Sequential extractions were performed on eight volcanic 

rocks and one mine-waste sample. Rock samples from the 

previous study by Yager and others (2005) and from new 

samples collected in 2005 were chosen for sequential leachate 

analysis. Samples selected represent moderate to high ANC 

(9 to 123 kg/t calcium carbonate equivalent). In addition, 

samples were chosen that have a wide range of whole-rock 

and major- and trace-element concentrations (table 8). Colors 

used in table 8 and elsewhere throughout the report (figures 4, 

8, 10, 11, and 31; and tables 11, 12, and 13) represent specific 

geologic units.

Volcanic rocks were prepared using the same method 

as described above in the sample preparation section. A 

< 2-mm split from each rock was obtained using a Jones 

Table 7. Procedure used to determine ANC. Steps are listed in sequential order. 

Step Procedure 

 1 Rock sample is crushed using jaw crusher to approximately 2 mm. Samples are not 

ground to avoid heating and preferential orientation of chlorite 001 crystal 

lattice planes. 

 2 Material from step 1 is sieved using a stainless steel sieve; all material less than 2 

mm is saved for acid titration. 

 3 A representative |30-g split from step 2 is weighed and the starting mass recorded. 

 4 The 30-g sample is added to a 100-ml beaker with 30-ml deionized water and stirred; 

the initial pH and titration start time is recorded. 

 5 Sample is stirred continuously for the duration of the experiment using a mechanical 

mixer with a stainless steel shaft and Teflon propeller, and the starting pH is 

recorded. Nitrogen gas is bubbled through titrant to reduce effects of 

atmospheric CO2 on pH during titration. 

 6 Reagent-grade 0.1N H2

SO4 titrant is added incrementally to samples using a pH stat 

controller that allows pH set points to be pre-set from the starting titration pH to 

the ending pH 2. In this study, whole pH set-point intervals were used; for 

example, pH 7, 6, 5, 4, 3, and 2. The pH stat controller measures and records 

milliliter increments of titrant added to achieve titration set-point stability. 

Stability is reached when the pH stat controller detects that a pH set point is 

achieved and stops adding acid. Total acid added is summed for each pH set 

point and the cumulative total for all set points; the cumulative total is used to 

determine ANC for a sample. Note that titrant addition by burette is used on 

some samples with high ANC and the quantity added by burette is added to the 

total added by the pH stat controller. Completed titration duration times range 

from a few minutes (low ANC) to several hours (high ANC). 
20 Geochemical Characteristics of Animas River Watershed Igneous Rocks Near Silverton, Colorado

high ANC. After each extraction step, the samples were 

centrifuged at 15,000 rpm for 15 min then decanted. Five 

hundred micro liters of concentrated ultra pure nitric acid 

was added to the decantate for the following fractions: (1) 

water soluble, (2) exchangeable, (3) carbonate, (4) pH 4, 

(5) pH 2, (6) amorphous Fe- and Al-hydroxides, and (7) 

amorphous and crystalline Mn-oxides. With the exception of the water soluble fraction, all other fractions were 

diluted with MiliQ, ultra-pure water, to a final volume of 

50 mL. All extractions are analyzed by ICP–MS. A total 

digest was also performed on the nine samples and three 

duplicates using a four-acid digest procedure.

Rock Physical Properties

(Magnetic Susceptibility)

All samples collected in 2005 and from the previous 

Yager and others (2005) study were analyzed for magnetic susceptibility (MS) using a Sapphire instrument. This instrument 

is more accurate and sensitive compared to the Bison instrument that was used in the previous 2005 study (especially at 

lower susceptibility values; Joseph Rosenbaum, USGS, unpub.

lished data, 2007). The Sapphire instrument consists of a coil 

(solenoid) that is part of an alternating-current (a-c) circuit. In 

reporting susceptibility measurements, it is usual to indicate 

the magnitude of the a-c field in the coil and its frequency (600 

hertz). The field amplitude in the Sapphire is about 0.1 milliTesla. The measurement coil is part of an oscillating circuit. 

The frequency of oscillation is measured for an empty coil 

and then with a sample in the coil. The difference in frequency 

is linearly proportional to the MS reading. For most samples 

(ignoring sample anisotropy), two things affect an MS reading: 

(1) the magnetic susceptibility of the sample and (2) the size 

and shape of the sample.

For analysis in the Sapphire instrument, samples are 

placed entirely within the solenoid and do not extend close to 

the ends of the coil (that is, the entire sample is in a volume 

of uniform field). Shape can usually be ignored. If sample A 

is twice the size of sample B and they are made of the same 

material (that is, they have the same magnetic susceptibility), then the MS reading for A will be twice as large as that 

for B. We record the readings without making any correction 

for sample volume or mass. Magnetic susceptibility readings 

were subsequently divided by volume (in cm3

) and reported 

as volume susceptibility in the centimeter-gram-second 

(CGS) system of units (table 11).

Results

Net Acid Production

The NAP results are reported in table 12. Box plots of 

NAP results are shown in figure 4. The NAP value is reported in 

kg/t CaCO3 equivalent, and can be interpreted as the amount of 

CaCO3 that is required to neutralize a sample to pH 7. Therefore, the higher NAP values indicate a higher acid-generating 

potential. Low or nondetectable NAP values are suggestive that 

a sample may have ANC. Of the 27 total samples analyzed, 

NAP values range from not detectable (14 samples) to 182 kg/t 

CaCO3

 equivalent (table 12). The mean NAP value is 26 kg/t 

CaCO3

 equivalent. We discuss NAP in relation to the abundance 

of sulfide mineral species in the following sections.

Table 8. Major- and trace-element abundances for whole 

rocks selected for sequential leachate analysis. 

[Elements determined by ICP–MS for samples IDY090537B and 

HWDY090542A (this study); all other samples determined by ICP–

AES in study by Yager and others (2005). Al and Fe in weight percent; 

all other elements in parts per million. Colors represent specific 

geologic units. See table 1 for unit abbreviations; n.d., not detected] 

Unit Field sample no. SiO2 Mn Fe Cu Zn As Pb 

Tsj IDY090537B 44.4 1,640 6.4 7.2 166 1.6 12 

Tsj IDY0329 61.2 2,270 3.4 17 1,100 n.d. 51 

Tpa HDY0317 57.5 826 5.1 25 86 n.d. 17 

Tpa SDY0326 58.2 834 5.0 53 91 14 29 

Tb HWDY090542A 58.6 1,200 5.5 67.8 90 3.5 20 

Tb HWDY090542B 56.9 1,450 4.8 50.4 89 1.0 15 

Tb SDY0310 56.5 3,420 6.4 72 266 n.d. 16 

Tse HDY0314 67.6 645 2.0 5 50 n.d. 34 
Results 21

Table 9. Seven-step sequential extraction procedure applied to nine samples. 

 Phase Description of procedure 

Water soluble 1. One gram of unground sample is extracted with 25 mL of MiliQ water 

and agitated for 30 minutes at room temperature. 

Exchangeable (Tessier and 

others, 1979) 

2. The residue from 1 is extracted with 25 mL of sodium acetate at pH 8.2 

and agitated at room temperature for 1 hour. 

Carbonate (Tessier and 

others, 1979) 

3. The residue from 2 is extracted with 25 mL sodium acetate adjusted to a 

pH of 5 with acetic acid and agitated for 5 hours at room temperature. 

Amorphous Fe- and Alhydroxides and amorphous 

and crystalline Mn-oxides 

(Piatak and others, 2007) 

4. The residue from 3 is extracted with 25 mL 0.25M hydroxylamine 

hydrochloride in 0.1M HCl for 30 minutes in a 50°–54°C water bath with 

occasional stirring. 

Crystalline Fe- and Al-oxides 

(Chao and Sanzolone, 1998) 

5. The residue from 4 is extracted with 25 mL HCl heated in a boiling 

(95o

C) water bath for 45 minutes with occasional shaking. 

Sulfide (Chao and Sanzolone, 

1977) 

6. The residue from 5 is extracted by adding 0.5g KClO3 and 10 mL 

concentrated HCl; the solution is mixed and let to stand for 30 minutes. 

After 30 minutes, 10 mL of MiliQ water is added, and solution is 

centrifuged and decanted. The residue is again extracted with 10 mL 4M 

HNO3

 and heated in boiling water bath for 20 minutes. 

Silicates (Briggs, 2001) 7. The residue from 6 is transferred to Teflon vessels and extracted using 

the four-acid digest procedure. 

Table 10. Nine-step sequential extraction procedure applied to nine samples. 

 Phase Description of procedure 

Water soluble 1. One gram of unground sample is extracted with 25 mL of MiliQ water 

and agitated for 30 minutes at room temperature. 

Exchangeable (Tessier and 

others, 1979) 

2. The residue from 1 is extracted with 25 mL of sodium acetate at pH 8.2 

and agitated at room temperature for 1 hour. 

Carbonate (Tessier and 

others, 1979) 

3. The residue from 2 is extracted with 25 mL sodium acetate adjusted to a 

pH of 5 with acetic acid and agitated for 5 hours at room temperature. 

Chlorite at pH 4 4. The residue from 3 is extracted with 25 mL of pH 4 HCl and agitated 

for 45 minutes at room temperature. 

Chlorite at pH2 5. The residue form 4 is extracted with 25 mL of pH 2 HCl and agitated 

for 45 minutes at room temperature. 

Amorphous Fe- and Alhydroxides and amorphous 

and crystalline Mn-oxides 

(Piatak and others, 2007) 

6. The residue from 5 is extracted with 25 mL 0.25M hydroxylamine 

hydrochloride in 0.1M HCl for 30 minutes in a 50°–qC water bath with 

occasional stirring. 

Crystalline Fe- and Al-oxides 

(Chao and Sanzolone, 1998) 

7. The residue from 6 is extracted with 25 mL 4M HCl heated in a boiling 

(95o

C) water bath for 45 minutes with occasional shaking. 

Sulfide (Chao and Sanzolone, 

1977) 

8. The residue from 7 is extracted by adding 0.5 g KClO3 and 10 mL 

concentrated HCl; the solution is mixed and let to stand for 30 minutes. 

After 30 minutes, 10 mL of MiliQ water is added, solution is centrifuged, 

and decanted. The residue is again extracted with 10 mL 4M HNO3 and 

heated in boiling water bath for 20 minutes. 

Silicates (Briggs, 2001) 9. The residue from 8 is transferred to Teflon vessels and extracted using 

the four-acid digest procedure. 
22 Geochemical Characteristics of Animas River Watershed Igneous Rocks Near Silverton, Colorado

Table 11. Volume magnetic susceptibility in the CGS system. Measurements determined 

by a Sapphire instrument. 

[Data in this table supersedes magnetic susceptibility measurements reported in Yager and others (2005); 

Sapphire instrument used to make susceptibility measurements reported here is more accurate than in our 

previous report. Measurements provided by Ank Webbers and Joseph Rosenbaum (USGS, Denver, Colo.). 

See table 1 for geologic unit abbreviations; ser, sericitic; prop, propylitic; q-s-p, quartz-sericite-pyrite] 

Geologic unit Sample Alteration Sapphire Mass Volume Volume 

type instrument (g) (cm3

) susceptibility 

reading (CGS) 

(CGS) 

Tpa HDY0317 Prop 3.56E-02 22.95 8.51 0.00418 

Tpa IRDY0203 Prop 5.27E-02 28.62 10.54 0.005 

Tpa SDY0326 Prop 6.50E-02 31.24 11.58 0.00562 

Tpa SDY0312 Prop 4.49E-02 31.86 11.56 0.00388 

Tpa IDY0328 Weak ser 5.00E-04 28.17 10.63 0.000047 

Tpa SDY0327 Weak ser 2.20E-04 13.48 5.12 4.31E-05 

Tpa IRDY0103 q-s-p 5.10E-05 22.15 8.55 3.64E-07 

Tb SDY0310 Prop 1.09E-03 31.41 11.57 9.41E-05 

Tb SDY0325 Prop 8.76E-04 26.99 10.18 0.000086 

Tb IRDY0403 Prop 2.60E-03 25.06 9.55 0.000272 

Tb SDY0307 Prop 5.06E-04 44.13 16.56 3.05E-05 

Tb SDY0308 Prop 7.84E-04 18.37 6.91 0.000113 

Tb SDY0309 Prop 5.45E-03 32.41 12.35 0.000441 

Tb SDY0311 Prop 1.78E-03 27.49 10.41 0.000171 

Tb HDY0313 Prop 6.27E-04 18.75 7.11 8.83E-05 

Tb SDY0306 Prop 1.93E-02 21.39 8.02 0.0024 

Tb SDY0324 Weak ser 3.69E-02 18.98 6.90 0.00534 

Tb IRDY0303 Argillic 2.94E-05 19.85 8.31 3.54E-06 

Tb HWDY090541A Prop 2.27E-02 20.97 8.67 0.00261 

Tb HWDY090541B Prop 1.00E-02 11.566 5.3 0.001901 

Tb HWDY090542A Prop 1.69E-02 15.29 6.00 0.00282 

Tb HWDY090542B Prop 1.61E-02 18.94 7.36 0.00219 

Tse HDY0314* Prop 1.15E-03 28.83 11.23 0.000102 

Tse HDY0316 Prop 1.83E-04 31.16 12.94 1.42E-05 

Tse HDY0318 Prop 2.85E-02 27.96 10.47 0.00272 

Tsemb HDY0315 Prop 7.71E-03 29.54 11.10 0.000694 

Tsemb HWDY090543 Prop 2.80E-03 8.117 3.9 0.000717 

Tse HPDY090549 Prop 1.53E-02 60.529 30.0 0.00051 

Tsj IDY0329 Prop 3.29E-04 36.38 13.77 2.39E-05 

Tsj ODY0331 Prop 6.47E-03 22.33 8.44 0.000766 

Tsj SDY0330 Prop 4.19E-02 31.09 11.48 0.00365 

Tsj SDY090535A Prop 1.14E-04 7.037 3.7 3.08E-05 

Tsj SDY090535B Prop 1.63E-04 11.480 5.0 3.26E-05 

Tsj IDY090537C Prop 1.03E-04 10.515 7.0 1.46E-05 

Tsj IDY090538G Prop 3.29E-02 14.449 6.4 0.005179 

Tsj IDY090538E q-s-p 6.19E-05 46.950 23.6 2.62E-06 

Tsj ODY090552B Prop 4.87E-03 9.870 5.7 0.00085 

Tsj ODY090552C Prop 1.83E-02 9.479 3.9 0.004669 

Tsj ODY090552D Prop 1.06E-04 14.302 6.1 1.75E-05 

Tig HWDY0322 Prop 1.41E-02 35.20 13.08 0.00108 

Tig SDY0319 Weak ser 5.30E-02 26.67 9.71 0.00546 

Tig SDY320 Weak ser 3.02E-05 11.17 6.43 4.69E-06 

Tig SDY090532 Prop 1.65E-02 12.747 6.9 0.002413 

Tig SDY090533 Weak ser 1.02E-04 10.353 5.5 1.84E-05 

Tig SDY090534A Prop 1.15E-01 60.70 22.29 0.00517 

Tig SDY090534B Prop 3.21E-02 21.70 8.18 0.00393 

Tig SDY090539 Prop 9.26E-03 14.179 7.8 0.001181 

Td IDY0321 Ser 1.23E-05 36.08 15.05 1.03E-05 

Pe HWDY0323 Prop 9.20E-06 18.613 7.04 1.64E-05 
Results 23

Sulfide-Bearing Samples

Pyrite above trace amounts is observed in 4 of the 27 

samples studied (table 3). Those samples containing pyrite 

concentrations above trace amounts to 5 weight percent 

lack calcite and have NAP values that range from 13 to 182 

kg/t CaCO3 equivalent (table 12). Pyrite-bearing samples 

(SDY090533, IDY090538E, IDY090538G, and SDY090545) 

have the highest average NAP (80 kg/t CaCO3 equivalent).

One sample (HWDY090541B) contains possible trace 

pyrite (table 3) in addition to abundant calcite (11 weight percent) and has no detectable NAP. The large quantity of calcite 

is apparently neutralizing any NAP generated during H2O2

reaction with trace amounts of pyrite.

Nonsulfide-Bearing(?) Samples

Anomalously high NAP results were obtained for four 

samples (SDY090535B, SDY090536, SDY090548, and 

HPDY090550; table 12) that contain no detectable sulfide 

minerals based on X-ray diffraction analyses.

Several possible scenarios could be involved to help 

explain relatively high NAP determinations for samples that 

lack sulfide phases. The most likely scenarios involve the 

presence of sparse sulfide (<1 weight percent sulfide) that is 

not detectable based on XRD analyses, or hydrolysis reactions 

of ferric iron and water. Hydrolysis reactions involving ferric 

iron cause the water molecule to be split while also generating hydrogen ions. The Lapakko method is designed as a 

T b T ig

T se T semb T sj

P z (? )

200

150

100

 50

 0

*

200

150

100

50

0

18

3

36

43

0

31

Figure 4. Boxplots of NAP results for units Pz(?), n = 1; Tb, n = 5; Tig, n = 4; Tse, n = 3; Tsemb, n = 1; and Tsj, n = 15. 

Mean values adjacent to box; outliers indicated by asterisk.

nonconservative technique to estimate NAP as sulfur species 

are targeted for complete dissolution during boiling in H2O2. 

Thus, in this vigorous dissolution process, ferric iron could be 

liberated from a number of iron-bearing mineral species such 

as clinochlore, epidote, and hematite.

An example of the type of hydrolysis reaction that 

involves ferric iron is as follows:

4 Fe3+ + 12 H2O → 4 Fe(OH)3 ̄ + 12 H+

Another possibility is the dissolution of finely disseminated sulfate salts that have a high surface area, readily 

dissolve, and thus have a high NAP. The presence of sulfate 

salts, while frequently abundant on surface coatings in mine 

waste, was not observed in rocks studied.

Acid Neutralizing Capacity

In our previous study (Yager and others, 2005), three units 

had at least one sample with high ANC in acid titration tests. 

Unit Tse had the highest ANC in acid titration experiments of 

any sample studied (122 kg/t calcium carbonate equivalent). 

The additional units with high ANC were Tsj (44 kg/t calcium 

carbonate equivalent) and Tig (14 kg/t calcium carbonate 

equivalent). However, units Tse, Tsj, and Tig were only sparsely 

sampled in the previous study, thus a more statistically representative sample was needed to verify ANC rock properties for 

these units. Most ANC data discussed in this section is for units 

Tse, Tsj, Tig, and Tb. Additional samples of unit Tb were collected from a talus slope in Minnie Gulch that is accessible to 
24 Geochemical Characteristics of Animas River Watershed Igneous Rocks Near Silverton, Colorado

land managers for possible use in remediation projects. Twentyseven samples were analyzed for ANC in this study (table 13). 

A map showing the ANC ranking for this and the Yager and 

others (2005) study is shown on plate 1.

Eureka (Unit Tse) and Picayune Megabreccia 

(Unit Tsemb) Members

Three unit Tse samples (SDY090548, SDY090549, and 

HPDY090550) were collected from the southern and western 

parts of the San Juan caldera. Areas sampled include Deer 

Park trail south of Silverton, at the historical Eureka townsite, 

and along Picayune Gulch road. One sample of megabreccia, 

unit Tsemb, was sampled in Minnie Gulch.

No significant ANC was determined for unit Tse (table 

13). The mean ANC calculated to reach pH 4 and pH 2 respectively, was 0.2 and 0.3 kg/t calcium carbonate equivalent.

A relatively high ANC was determined for one sample of 

unit Tsemb (sample HWDY090543) collected in upper Minnie 

Gulch southeast of Silverton (fig. 5). The ANC calculated to 

reach pH 4 and pH 2 respectively, was 0.8 and 26 kg/t calcium 

carbonate equivalent. Most of the acid added during ANC 

titration was below pH 4, which could be due to chlorite species reacting at lower pH; however, calcite continuing to react 

below pH 4 cannot be ruled out because we do not achieve 

equilibrium in our acid titrations.

San Juan Formation (Unit Tsj)

Eleven unit Tsj samples were analyzed for ANC. 

Sampling of unit Tsj was focused along the western margin of the Silverton and San Juan calderas in the following 

areas: near Clear Lake, north of South Fork Mineral Creek 

(samples ODY090552 and ODY090553) (fig. 6), near the 

Table 12. Net acid production (NAP) results. 

[Underline indicates trace amounts to 5 weight percent pyrite determined by Rietveld X-ray diffraction 

analysis. See table 1 for unit abbreviations; n/a indicates that after H2

O2 digestion, the initial pH was > pH 7 

and required no NaOH base for neutralization] 

Unit Field sample no. Initial pH Total volume Weight Ending CaCO3

NaOH (ml) (g) pH kg/ton 

Tig SDY090532 4.69 4.0 1.034 6.98 18 

Tsj SDY090533 2.38 40.0 1.021 7.27 182 

Tig SDY090534A 4.97 11.5 1.025 6.99 52 

Tig SDY090534B 4.82 13.0 1.028 7.00 59 

Tsj SDY090535A <10 n/a 1.030 n/a n/a 

Tsj SDY090535B 2.50 16.0 1.025 7.10 73 

Tsj SDY090536 2.55 17.0 1.018 7.11 78 

Tsj IDY090537A <11 n/a 1.020 n/a n/a 

Tsj IDY090537B <11 n/a 1.028 n/a n/a 

Tsj IDY090537C <10 n/a 1.033 n/a n/a 

Tsj IDY090538E 2.41 12.0 1.016 6.98 55 

Tsj IDY090538G 5.59 16.0 1.032 6.98 72 

Tig SDY090539 3.55 3.0 1.023 7.06 14 

Pz(?) HWDY090540 3.90 4.0 1.012 7.13 18 

Tb HWDY090541A <10 n/a 1.018 n/a n/a 

Tb HWDY090541B <11 n/a 1.022 n/a n/a 

Tb HWDY090542A <10 n/a 1.023 n/a n/a 

Tb HWDY090542B <11 n/a 1.016 n/a n/a 

Tsemb HWDY090543 <11 n/a 1.019 n/a n/a 

Tb SDY090545 4.30 3.0 1.030 7.14 13 

Tse SDY090548 5.84 16.0 1.019 6.93 73 

Tse SDY090549 8.64 n/a 1.026 n/a n/a 

Tse HPDY090550 5.57 12.0 1.024 6.96 54 

Tsj ODY090552A <11 n/a 1.017 n/a n/a 

Tsj ODY090552B <11 n/a 1.029 n/a n/a 

Tsj ODY090552C <10 n/a 1.031 n/a n/a 

Tsj ODY090553 10.92 n/a 1.018 n/a n/a 
Results 25

Imogene mine west of highway 550 (sample SDY090535A) 

(fig. 7), and below the Silver Cloud mine west of Chattanooga 

(sample IDY090537) (fig. 6). The mean ANC calculated for 

all unit Tsj samples to reach pH 4 and pH 2, respectively was 

9 and 29 kg/t calcium carbonate equivalent. Only five of the 

eleven Tsj samples analyzed were found to have high ANC 

(figure 8).

Burns Member (Unit Tb)

Four unit Tb samples were collected from talus in Minnie Gulch located southeast of Silverton. All unit Tb samples 

were found to have moderate to high ANC (figs. 9 and 10). 

One unit Tb sample (HWDY090541B) has the highest ANC 

rank of any sample studied thus far, and two other unit Tb 

samples (HWDY090541A and HWDY090542B) are ranked 

in the top 10 (plate 1).

ANC and NAP Unit Comparisons

Boxplots comparing ANC and NAP properties are 

shown in figure 11. These data indicate that unit Tb sampled 

from the Minnie Gulch talus has the highest ANC and lowest 

NAP in this study. Unit Tsj also has several samples with 

moderate to high ANC; however, this unit also tends to have 

a high NAP in some localities. Units Tig and Tse tend to 

have low ANC and high NAP. One unit Tsemb sample has a 

moderate ANC and low NAP suggesting that more samples 

are warranted to determine the overall ANC properties of this 

unit.

Table 13. Acid neutralizing capacity calculated in terms of calcium carbonate equivalent 

(kg/t) to reach pH values of 4 and 2 respectively, during 0.1 N H2SO4 titration. NAP values 

shown for comparison. 

[n.d., no NAP detected; n.a., not analyzed. Field sample number suffix (ca,ch) contains calcite and or 

chlorite respectively; see table 1 for unit abbreviations] 

Field sample no. Unit 

CaCO3 equivalent 

units calculated 

in kg/t for 

titration to pH | 4 

CaCO3

 equivalent 

units calculated in 

kg/t for titration to 

pH | 2 

NAP 

SDY090532ch Tig 0.2 0.4 18 

SDY090533ch Tsj n.a. 1 182 

SDY090534Ach Tig n.a. 1 52 

SDY090534Bch Tig 0.4 0.5 59 

SDY090535Aca,ch Tsj 1 19 n.d. 

SDY090535Bch Tsj n.a. n.a. 73 

SDY090536ch Tsj n.a. 1 78 

IDY090537Aca,ch Tsj 10 24 n.d. 

IDY090537Bca,ch Tsj 29 67 n.d. 

IDY090537Cca,ch Tsj n.a. 36 n.d. 

IDY090538Ech Tsj n.a. 1 55 

IDY090538Gch Tsj 0.2 0.5 72 

SDY090539ch Tig 0.5 1 14 

HWDY090540ch Pz(?) n.a. n.a. 18 

HWDY090541Aca,ch Tb 83 119 n.d. 

HWDY090541Bca,ch Tb 102 146 n.d. 

HWDY090542Aca,ch Tb 6 20 n.d. 

HWDY090542Bca,ch Tb 34 83 n.d. 

HWDY090543ca,ch Tsemb 0.88 26.47 n.d. 

SDY090545ch Tb 0.2 0.4 13 

SDY090548ch Tse 0.3 0.4 73 

SDY090549ch Tse 0.1 0.2 n.d. 

HPDY090550ch Tse 0.2 0.4 55 

ODY090552Aca,ch Tsj 10 30 n.d. 

ODY090552Bca,ch Tsj n.a n.a n.d. 

ODY090552Cca,ch Tsj n.a n.a n.d. 

ODY090553ca,ch Tsj 5.71 110.11 n.d. 
0

2

4

6

8

10

0 20 40 60 80 100

0.1 N H2SO4

pH

(ml)

Figure 5. Acid titration curve for unit Tsemb sample HWDY090543. 

0

2

4

6

8

0 10 20 30 40 50 60 70

0.1 N H2SO4

pH

0.1N H2SO4 (ml)

0

2

4

6

8

10

12

0 50 100 150 200 250 300 350

O.1 N H2SO4

pH

ODY090553

IDY090537B

IDY090537A ODY090552

0.1N H2SO4 (ml)

Figure 6. Acid titration curves for unit Tsj samples ODY090552, ODY090553, and IDY090537A 

and B.

Figure 7. Acid titration curve for unit Tsj sample SDY090535A.

26 Geochemical Characteristics of Animas River Watershed Igneous Rocks Near Silverton, Colorado
Sequential Leachate Results (Whole Rocks) 27

Sequential Leachate Results 

(Whole Rocks)

A suite of seven major elements (Mg, Al, P, K, Ca, Ti, 

and Fe) and ten trace elements (Mn, Ni, Cu, Zn, As, Sr, Mo, 

Cd, Ba, and Pb) were investigated as part of the sequential 

leaching study. These elements are of interest because of possible contaminants that could be released by ANC rocks when 

the materials are used in mine-waste remediation projects.

The elements released during leaching can be used to 

evaluate the minerals that might participate in weathering 

reactions. Each sequential step is designed to react with specific mineral or amorphous phases. In the following section, 

sequential steps are referred to as “phases.” The term “phase” 

in this context refers to the sequential step that has been 

developed to target a specific mineral species or group, such as 

carbonates or silicates. Major elements are discussed below in 

order of increasing atomic number. Note that all samples were 

leached using the four-acid step described in Briggs (2001) to 

determine totals for each element. Disparities between totals 

measured from the four-acid step and totals from the sevenand nine-step methods are likely caused by sample inhomogeneity, as only 2-g splits were analyzed by each method.

Magnesium

Most of the leachable magnesium is produced during dissolution of the crystalline iron and aluminum oxide phase in step 

six. The phase designed to react with silicates and sulfides are of 

secondary importance in extracting magnesium (appendix A). 

The highest leachable magnesium concentrations (2.51 weight 

percent) are observed in the most mafic, lowest silica content 

sample (IDY090537B) (fig. 12). This is the only basalt (44.4 

weight percent SiO2

) analyzed during the sequential leachate 

study. In addition, samples containing the highest abundances 

of the Mg-bearing phase chlorite (samples IDY090537B and 

SDY0310) also tend to have higher leachable magnesium concentrations (see table 3 in Yager and others (2005); table 3, this 

study). Conversely, the less mafic, dacitic lava and rhyolitic ashflow tuff samples (IDY0329 and HDY0314) have lower magnesium concentrations (ranging from 0.46 to 0.48 weight percent, 

respectively); these samples also have lower chlorite abundances.

Aluminum

The majority of leachable aluminum is concentrated 

in solution in the silicate phase, step nine with subsidiary 

amounts in the crystalline iron and aluminum oxide phase 

120

100

80

 60

 40

 20

0

ANC _4 ANC _2

Q1, 27

Median, 36

Q3, 88

Q1, 7

Median, 10

Q3, 24

Figure 8. Boxplots of calculated ANC to reach pH 4 (ANC_4) and pH 2 (ANC_2) during acid titrations for high 

ANC unit Tsj samples, n = 5. Samples were collected near Clear Lake north of South Fork Mineral Creek, and west 

of Chattanooga adjacent to Silver Cloud mine portal. First quartile (Q1), median, and third quartile values labeled.
28 Geochemical Characteristics of Animas River Watershed Igneous Rocks Near Silverton, Colorado

 60

 40

 20

0

ANC _4 ANC _2

Q1, 13

Median, 58

Q3, 97

 80

 100

 120

 140

 160

Q3, 139

Q1, 36

Median, 101

0.00

1.00

2.00

3.00

4.00

5.00

6.00

7.00

8.00

9.00

10.00

0 100 200 300 400 500

0.1N H2SO4

pH

HWDY090541B

HWDY090542A HWDY090542B HWDY090541A

0.1 N H2SO4 (ml)

Figure 9. Acid titration curves for unit Tb samples from outcrop and talus in Minnie Gulch (sample HWDY090541A 

and B, outcrop; sample HWDT090542A and B, talus).

Figure 10. Boxplots of calculated ANC to reach pH 4 (ANC_4) and pH 2 (ANC_2) during acid titrations for high ANC 

unit Tb bedrock and talus samples in Minnie Gulch, n = 4. First quartile (Q1), median, and third quartile values labeled.
Sequential Leachate Results (Whole Rocks) 29

(fig. 13). The more mafic samples tend to have higher aluminum concentrations, with the basalt sample (IDY090537B) 

having the highest leachable aluminum in the silicate phase 

(6.32 weight percent). Lower aluminum abundances are 

observed in the dacitic lava and rhyolitic ash-flow tuff 

samples (IDY0329 and HDY0314) in the silicate phase, 

and range from 5.28 to 5.31 weight percent, respectively 

(appendix B). The probable sources for leachable aluminum 

are the primary mineral plagioclase and secondary illite, a 

potassium-aluminum-silicate mineral that is introduced during propylitic alteration and replaces primary plagioclase, 

potassium feldspar, and groundmass.

Potassium

Potassium in solution is concentrated in the silicate phase 

(fig. 14) with the greatest abundance (3.29 weight percent) 

observed in the rhyolitic ash-flow tuff sample (HDY0314) 

(appendix C). The source for potassium in the ANC rocks is 

primary potassium feldspar and secondary illite that is common in propylitically altered and weakly sericitized rocks, in 

addition to potassium in the groundmass.

Calcium

Calcium is sourced from multiple phases (fig. 15). The 

total leachable calcium is a result of complex reactions involving 

calcite introduced initially during propylitic alteration that 

replaced primary silicate minerals and groundmass, in addition 

to calcium derived from calcium-bearing aluminum silicates.

Similar to Fe, Mg, and Al, there is a trend for more mafic 

samples to also have higher calcium abundances, with the 

basalt sample (IDY090537B) having the highest calcium concentration (4.34 weight percent, four-acid leach) (appendix D). 

The most silicic sample (HDY0314) has the lowest total calcium (1.61 weight percent, total-sum four-acid leach).

Samples that lack or have low whole -rock calcite 

abundances based on x-ray diffraction analyses in the previous Yager and others (2005) study (samples SDY0310 and 

SDY0326) and a sample from this study (HWDY090542A) 

with relatively low calcite (4 weight percent calcite) also have 

a higher percentage of leachable calcium from the silicate 

phase. Other samples that have a higher percentage of calcite 

(ranging from 5 to 20 weight percent) have a lower proportion 

of calcium derived from silicates, a corresponding increase 

in calcium derived from the carbonate pH 5 acetic acid 

phase, and a larger proportion of calcium from the crystalline 

Fe- and Al-oxides phase (for example, samples IDY090537B, 

HDY0317, and IDY0329).

Other leachate phases that are involved in liberating 

calcium, but commonly to a lesser extent to those mentioned 

above, include, in relative decreasing order of abundance, (a) 

hydroxides and amorphous and crystalline Mn-oxides, (b) sulfide, (c) HCl pH 2, (d) exchangeable pH 8.2, and (e) HCl pH 4.

*

*

*

T b T ig

T se T semb T sj

P z (? )

ANC _4 ANC _2 NAP

ANC _4 ANC _2 NAP

ANC _4 ANC _2 NAP

200

150

100

50

0

18.0

0.2 0.3

42.7

0.9

26.5

0

45.0

73.7

2.6 1.6 1.8

37.7

30.7

29.0 9.4

Figure 11. Boxplots of calculated ANC to reach pH 4 (ANC_4) and pH 2 (ANC_2) during acid titrations, 

and NAP for units Pz(?), n = 1; Tb, n = 5; Tig, n = 4; Tse, n = 3; Tsemb, n = 1; and Tsj, n = 11 (ANC) and n = 15 

(NAP). Median values adjacent to box; outliers indicated by asterisk.
30 Geochemical Characteristics of Animas River Watershed Igneous Rocks Near Silverton, Colorado

Figure 12. Histogram of whole-rock leachate results for Mg. Total digestion shown by solid blue bar; next histogram bars 

for each sample are for seven- and nine-step methods respectively.

Figure 13. Histogram of whole-rock leachate results for Al. Total digestion shown by solid blue bar; next histogram bars 

for each sample are for seven- and nine-step methods respectively.
Sequential Leachate Results (Whole Rocks) 31

Figure 14. Histogram of whole-rock leachate results for K. Total digestion shown by solid blue bar; next histogram bars 

for each sample are for seven- and nine-step methods respectively.

Titanium

Titanium is mainly observed in the silicate phase 

(fig. 16, appendix E). This is inconsistent with the mineralogy 

of the rocks because the majority of titanium likely occurs as 

ilmenite intergrowths with magnetite (see Yager and others, 

2005, their fig. 20). The higher titanium abundance in the 

silicate phase, however, is likely due to the refractory nature 

of the magnetite-ilmenite intergrowths that do not release 

significant titanium until the four-acid digest procedure is 

employed. There is an observed trend of samples having both 

high magnetite and titanium abundance (for example, samples 

HDY0317, IDY090537B, and HWDY090542A) (Yager and 

other, 2005; this study, table 3).

Iron

Iron has a similar trend as that observed for magnesium, 

with the most abundant iron present in the crystalline Fe- and 

Al-oxide phase, followed by the silicate and sulfide leachate 

phases (fig. 17; appendix F).

Small quantities of iron (<0.05 weight percent) are 

present in the amorphous Fe- and Al-hydroxide and amorphous and crystalline Mn-oxide phases. The majority of 

leachable iron is derived from the most mafic lavas (samples 

IDY090537B and SDY0310), which have the highest iron 

abundances (6.4 weight percent).

The primary minerals that are sources for leachable 

iron are (a) the magnesium-iron-aluminum silicates chlorite 

(clinochlore) formed after pyroxene, hornblende, plagioclase, 

and groundmass during propylitic alteration, and (b) magnetite 

and or titanomagnetite.

Manganese

Manganese abundance is apportioned into several 

leachate phases (fig. 18). The crystalline Fe- and Al-oxide 

phase hosts the majority of manganese. One of the rocks 

with the highest chlorite (clinochlore) abundances (sample 

SDY0310) also has the highest leachable manganese concentration (0.3 weight percent; appendix G). Four other 

samples (IDY090537B, IDY0329, and HWDY090542A 

and B) that have relatively high leachable manganese also 

average approximately 13 weight percent clinochlore (Yager 

and others, 2005, table 3; table 3, this study). This finding is 

consistent with the manganese being derived from chlorite, as 

manganese substitutes for iron in the chlorite crystal lattice 

(Halferdahl, 1961). Other leachate phases in which manganese occurs, listed in relative order of abundance, include the 
32 Geochemical Characteristics of Animas River Watershed Igneous Rocks Near Silverton, Colorado

Figure 16. Histogram of whole-rock leachate results for Ti. Total digestion shown by solid blue bar; next histogram bars for 

each sample are for seven- and nine-step methods respectively.

Figure 15. Histogram of whole-rock leachate results for Ca. Total digestion shown by solid blue bar; next histogram bars 

for each sample are for seven- and nine-step methods respectively.
Sequential Leachate Results (Whole Rocks) 33

following: silicate; hydroxide, amorphous, and crystalline 

Mn-oxides; and sulfide leachate phases. Note that in some 

instances, manganese in the sulfide phase predominates over 

the hydroxide, amorphous, and crystalline Mn-oxide phase 

(for example, andesite lava sample SDY0310). Several samples indicate relatively small quantities of manganese being 

sourced from Mn-bearing carbonates (samples IDY090537B, 

IDY0329, HDY0317, SDY090326, HWDY090542A and B, 

and HDY0314). No Mn-bearing carbonates, however, were 

identified in X-ray diffraction analysis. 

Nickel

Nickel concentrations are highest in the crystalline Feand Al-oxide phases. The silicate and sulfide phases are of 

secondary importance (fig. 19; appendix H). Minor amounts of 

nickel are leached during the hydroxide, amorphous, and crystalline Mn-oxides phase, which are all part of one leachate step 

(tables 9 and 10, steps 4 and 6, respectively); which represents 

one leachate step. The lavas of the Burns Member of the 

Silverton Volcanics are relatively high in leachable nickel 

when compared to San Juan Formation and Eureka Member 

of the Sapinero Mesa Tuff samples. The highest nickel concentrations are observed in the Burns Member andesite lava 

sample (SDY0310). This could be due to the presence of trace 

quantities (<1 weight percent) of chalcopyrite (CuFeS2

) in this 

Figure 17. Histogram of whole-rock leachate results for Fe. Total digestion shown by solid blue bar; next histogram bars 

for each sample are for seven- and nine-step methods respectively.

sample (Yager and others, 2005). Nickel is a transition metal 

with a 2+ charge and similar ionic radius that may substitute 

for copper in chalcopyrite. Unit Tpa sample SDY0326 also 

has elevated Ni concentrations compared to the other unit Tpa 

sample HDY0317.

Copper

Copper is derived from several leachate phases indicating 

that this element is mobile at multiple acidic pH intervals. The 

sample with the highest copper concentration (0.0054 weight 

percent, appendix 1I) is SDY0310, which was found to contain 

trace chalcopyrite in the study by Yager and others (2005). 

Unit Tpa sample SDY0326 also has elevated Cu concentrations compared to the other unit Tpa sample HDY0317. The 

leachate phases that produce the most copper, while highly 

sample dependent, include silicate, sulfide, and crystalline Feand Al-hydroxide and crystalline Mn-oxide phases (fig. 20; 

appendix I).

Zinc

The crystalline Fe- and Al-oxide, and the amorphous Feand Al-hydroxide, and amorphous and crystalline Mn-oxide 

phases, are host to the majority of zinc (fig. 21; appendix J). 
34 Geochemical Characteristics of Animas River Watershed Igneous Rocks Near Silverton, Colorado

Figure 18. Histogram of whole-rock leachate results for Mn. Total digestion shown by solid blue bar; next histogram bars 

for each sample are for seven- and nine-step methods respectively.

Figure 19. Histogram of whole-rock leachate results for Ni. Total digestion shown by solid blue bar; next histogram bars 

for each sample are for seven- and nine-step methods respectively.
Sequential Leachate Results (Whole Rocks) 35

The silicate and sulfide phases are of secondary importance 

in most samples. Unit Tsj sample IDY0329 has the highest 

leachable zinc concentrations (between 0.14 and 0.15 weight 

percent) and a correspondingly high whole -rock zinc concentration (0.1 weight percent, table 8).

Arsenic

Arsenic tends to be concentrated in solution in the silicate 

and sulfide phases (fig. 22; appendix K). The crystalline Feand Al-oxide and the amorphous Fe- and Al-hydroxide and 

amorphous and crystalline Mn-oxide phases are of secondary 

arsenic abundance. The sample with the highest leachable 

arsenic (IDY0326) is exposed near the relatively high arsenicacid-sulfate mineralized system centered near Red Mountain 

Pass and on Anvil Mountain.

Strontium

The majority of strontium leached was produced in the 

silicate phase (fig. 23; appendix L). This is consistent with 

strontium being leached from plagioclase and epidote. Strontium commonly substitutes for calcium in plagioclase and 

Figure 20. Histogram of whole-rock leachate results for Cu. Total digestion shown by solid blue bar; next histogram 

bars for each sample are for seven- and nine-step methods respectively.

epidote. An additional source for leachable strontium could be 

from calcite; although, strontium was not detected in calcite 

by SEM analysis (Sharon Diehl, written commun., 2007). The 

highest leachable strontium was observed in the basalt sample 

IDY090537B.

Molybdenum

Molybdenum concentrations are low and mostly in the 

silicate phase (fig. 24; appendix M). This is possibly due to the 

molybdenum being leached from stockwork micro-veinlets that 

are commonly associated with granitoid intrusions along the 

southern margins of the Silverton caldera (Ringrose, 1982).

Cadmium

As with zinc and lead, the highest cadmium is observed 

from sample IDY0329 (fig. 25; appendix N) and is derived 

principally from the amorphous Fe- and Al-hydroxide and 

amorphous and crystalline Mn-oxide phase. The carbonate 

pH 5 acetic acid phase and crystalline Fe- and Al-oxide phases 

were also important leachate phases for sample IDY0329. 

Most other samples, with the exception of sample SDY0310, 

have relatively low cadmium abundances.
36 Geochemical Characteristics of Animas River Watershed Igneous Rocks Near Silverton, Colorado

Figure 22. Histogram of whole-rock leachate results for As. Total digestion shown by solid blue bar; next histogram 

bars for each sample are for seven- and nine-step methods respectively.

Figure 21. Histogram of whole-rock leachate results for Zn. Total digestion shown by solid blue bar; next histogram bars 

for each sample are for seven- and nine-step methods respectively.
Sequential Leachate Results (Grand Mogul Mine Waste) 37

Barium

Barium is mostly in the silicate phase (fig. 26; 

appendix O). This is consistent with barium being derived 

from plagioclase as barium substitutes for calcium in plagioclase. More complex leachate histograms are observed 

for samples IDY0329 and HDY0317. In the case of sample 

IDY0329, barite was detected in SEM analyses (Sharon 

Diehl, written commun., 2007), which helps explain barite 

dissolution at higher pH phases. Barite was not identified in 

sample HDY0317, so it is unclear what is causing the more 

complex Ba leachate histogram for this sample.

Lead

The phases producing leachable lead are highly variable, 

with no clear trend being consistently observed between 

samples (fig. 27; appendix P). Sample IDY0329 produced 

the highest total lead (between 70 and 80 μg/l) and has a high 

pyrite abundance (3 weight percent) (Yager and others, 2005, 

their table 3). No other sulfide minerals were observed in SEM 

analyses that could be used to identify the source of the lead. 

However, sample IDY0329 does have a high illite concentration (27 weight percent) (Yager and others, 2005, their table 

3), and it is possible that there has been lead substitution for 

potassium in the illite lattice. More work is needed to determine the mineral residence of lead. Whole-rock concentrations 

are high (51 ppm) in sample IDY0329 compared with other 

samples, and leachable lead is approximately proportional 

with whole-rock lead abundances.

Sequential Leachate Results 

(Grand Mogul Mine Waste)

Magnesium

The magnesium abundance, concentrated mainly 

in the silicate phase, is low when compared with ANC 

volcanic rocks that are host to the mineral deposits (fig. 28; 

appendix A). The lower magnesium concentrations, along with 

lower abundances of other major elements (P, Ca, and Ti), 

are a result of dilution of these elements by ore and gangue 

material in the mine waste.

Aluminum

Aluminum is concentrated mainly in the silicate phase, 

and its abundance is four times greater (3.2 weight percent, 

> 32,000 μg/l) in the mine waste compared to the highest 

aluminum concentrations observed in ANC volcanic rocks 

(fig. 29; appendix B). The likely source of aluminum is illite 

Figure 23. Histogram of whole-rock leachate results for Sr. Total digestion shown by solid blue bar; next histogram bars 

for each sample are for seven- and nine-step methods respectively.
38 Geochemical Characteristics of Animas River Watershed Igneous Rocks Near Silverton, Colorado

Figure 24. Histogram of whole-rock leachate results for Mo. Total digestion shown by solid blue bar; next histogram 

bars for each sample are for seven- and nine-step methods respectively.

Figure 25. Histogram of whole-rock leachate results for Cd. Total digestion shown by solid blue bar; next histogram 

bars for each sample are for seven- and nine-step methods respectively.
Sequential Leachate Results (Grand Mogul Mine Waste) 39

(K-Al-Si-OH) that is prevalent in quartz-sericite-pyrite alteration in upper Ross basin. Another possible aluminum source 

is chlorite that is weathering from ANC rocks that are commingled with ore and gangue material. Since the magnesium 

concentration is depressed in mine waste compared with ANC 

rocks, the principal source for aluminum is likely illite.

Calcium

Calcium is mainly concentrated in the silicate phase in 

addition to the crystalline Fe- and Al-oxide phase (fig. 28; 

appendix D). Calcium abundances are four times lower in 

mine-waste material compared to the ANC mineral deposit host 

rocks. As with magnesium, this is a result of acid leaching of 

primary silicate minerals and dilution by calcium-poor gangue 

and ore.

Titanium

Titanium is almost exclusively concentrated in the silicate 

phase (fig. 28; appendix E) and is nearly seven times lower in 

abundance in mine-waste material compared with ANC rocks. 

The lower titanium abundance is a result of the titanium host 

(intergrown magnetite-ilmenite) being destroyed along the 

quartz-sericite-pyrite veins during hydrothermal alteration.

Iron

Iron displays more heterogeneity compared with other 

major elements and is concentrated primarily in the silicate, 

sulfide, crystalline Fe- and Al-oxide, and amorphous Feand Al-hydroxide and amorphous and crystalline Mn-oxide 

phase (fig. 29; appendix F). The mine waste has three times 

the quantity of leachable iron in the amorphous Fe- and 

crystalline Mn-oxide phase (0.15 weight percent) compared 

with ANC rocks (about 0.05 weight percent), even though 

the host ANC rocks have as much as 6 to 7 weight percent 

total leachable iron. This suggests that the mine waste and 

derived leachate may be the primary source of iron in iron 

flocculent.

Manganese

Manganese is concentrated mainly in the silicate and 

sulfide phases (fig. 28; appendix G). The abundance of 

manganese in mine waste is mostly lower (0.05 to about 

0.1 weight percent) compared with ANC rocks (> 0.05 to 

nearly 0.35 weight percent) (fig. 18). This suggests that 

manganese has been diluted by quartz-sericite-pyrite altered 

rocks that are commingled with ANC rocks in the mine 

waste.

Figure 26. Histogram of whole-rock leachate results for Ba. Total digestion shown by solid blue bar; next histogram bars 

for each sample are for seven- and nine-step methods respectively.
40 Geochemical Characteristics of Animas River Watershed Igneous Rocks Near Silverton, Colorado

Figure 27. Histogram of whole-rock leachate results for Pb. Total digestion shown by solid blue bar; next histogram 

bars for each sample are for seven- and nine-step methods respectively.

Figure 28. Histogram of Grand Mogul mine waste leachate results for Ca, Cu, Mg, Mn, P, and Ti. Total digestion shown 

by solid blue bar; next histogram bars for each element are for seven- and nine-step methods respectively.
Rock Physical Properties (Magnetic Susceptibility) 41

Nickel

Nickel was not observed in mine-waste leachate indicating this is not a metal of concern from a remediation standpoint at the GM mine-waste pile.

Copper

Leachable copper concentrations are elevated in GM 

mine waste (0.05 weight percent) compared with ANC rocks 

(<0.008 to about 0.001 weight percent) (figs. 28 and 20, 

respectively; appendix I). Most copper is concentrated in the 

sulfide and crystalline Fe- and Al-oxide phases with small 

abundances being concentrated in the silicate phase. Chalcopyrite is the likely source of leachable copper in the mine 

waste.

Zinc

Zinc concentrations are highly elevated in the mine waste 

(> 1.5 weight percent) (fig. 29; appendix J). Zinc is primarily 

concentrated in the sulfide phase with the mineral sphalerite 

(ZnS) being the source of zinc. The other two leachate phases 

where zinc was concentrated are the silicate and crystalline 

Fe- and Al-oxide phase.

Arsenic

Arsenic abundances are higher by a factor of 6 to 25 in 

GM mine waste compared with ANC rocks. Arsenic concentration in mine-waste leachate is about 0.005 weight percent 

(fig. 30); arsenic leachate abundances in ANC rocks range 

from about 0.0001 to 0.0008 weight percent (fig. 22; appendix K). The sulfide and crystalline Fe- and Al-oxide phases 

is host to most of the arsenic. The source mineral for arsenic 

is not known.

Strontium

Strontium concentrations in the GM mine waste overlap those observed in ANC rock leachate and range from 

about 0.025 to nearly 0.05 weight percent (fig. 30; appendix 

L). The distribution of strontium is heterogeneous and is 

observed in multiple leachate fractions, but is principally 

concentrated in the crystalline Fe- and Al-oxide phases.

Molybdenum

Molybdenum abundances are higher by a factor of 

50 to 100 in GM mine waste compared with ANC rocks. 

Molybdenum concentration in GM mine waste is about 

0.005 weight percent and is primarily concentrated in 

crystalline Fe- and Al-oxide and sulfide phases (fig. 30; 

appendix M).

Cadmium

Cadmium is also concentrated by a factor of 10 to 

100 times in GM mine waste compared with ANC rocks. 

Cadmium concentration in GM mine waste is about 0.005 

to 0.009 weight percent. The crystalline Fe- and Al-oxide 

and sulfide phases are host to most of the cadmium (fig. 30; 

appendix N).

Barium

Similar to strontium, barium abundances overlap those 

observed in ANC rocks. Barium concentration in GM mine 

waste is about 0.0250 weight percent and is primarily concentrated in silicate and crystalline Fe- and Al-oxide phases 

(fig. 30; appendix O).

Lead

Lead is concentrated by a factor of 100 to 1,000 in GM 

mine waste compared to ANC rocks. Lead attains a concentration of 1 weight percent in GM mine waste and is concentrated primarily in the crystalline Fe- and Al-oxide and sulfide 

phases; although a significant proportion is also observed in 

the hydroxides and amorphous and crystalline Mn-oxide phase 

(fig. 29; appendix P).

Rock Physical Properties

(Magnetic Susceptibility)

A box plot of volume susceptibility is shown in figure 31. 

The highest median value was observed in unit Tpa; the lowest 

median value was identified in unit Tb. Units Tsj and Tse have 

similar susceptibility values that are relatively low compared 

to the other units. Unit Tig has a moderate magnetic susceptibility. Unit Tsemb also has a relatively moderate susceptibility; however, only two samples are represented.

Summary

ANC and NAP

Burns Member (Unit Tb) and Picayune 

Megabreccia Member (Unit Tsemb)

In this study, two units (Tb and Tsemb) were identified 

as having moderate to high ANC and no NAP. One Tb sample 

(HWDY090541B) has the highest ANC of any sample studied. 

Unit Tb in the previous study (Yager and others, 2005) did 

not have a high ANC (notably sample SDY0310, with some 

exceptions), and most of those unit Tb samples were collected 

within the core of the Silverton caldera.
42 Geochemical Characteristics of Animas River Watershed Igneous Rocks Near Silverton, Colorado

Figure 29. Histogram of Grand Mogul mine waste leachate results for Al, Fe, Pb, and Zn. Total digestion shown by solid 

blue bar; next histogram bars for each element are for seven- and nine-step methods respectively.

Figure 30. Histogram of Grand Mogul mine waste leachate results for As, Ba, Cd, Mo, Ni, and Sr. Total digestion shown 

by solid blue bar; next histogram bars for each element are for seven- and nine-step methods respectively.
Sequential Leachate Studies (Whole Rocks) 43

San Juan Formation (Unit Tsj)

Unit Tsj lavas and volcaniclastic sedimentary rocks 

were thought to have a potentially high ANC based on sparse 

samples collected during the Yager and others (2005) study. 

The median ANC for 11 unit Tsj samples studied is relatively 

high (29 kg/t calcium carbonate equivalent); however, the 

median NAP for this sample suite is also high (30 kg/t calcium 

carbonate equivalent).

Sultan Mountain Stock (Unit Tig)

Unit Tig was found to have minimal ANC and moderate 

NAP. Sparse fir trees, grasses, and other vegetation are 

observed on the North Star mine waste pile west of Silverton, 

which is composed of Tig clasts. This is an indication that 

the minimal ANC that unit Tig does supply at the North Star 

mine waste, could be neutralizing some acidity and aiding 

vegetation growth at the North Star Mmine site.

Sequential Leachate Studies

(Whole Rocks)

Major Elements

Major-element constituents produced during four-acid total 

digestion and from the silicate and crystalline Fe- and Al-oxide 

sequential phases are consistent with the original whole-rock 

compositions. Mafic rocks such as basalt sample IDY090537B 

and andesite sample SDY0310 produced high Mg, Fe, and Ca. 

More silicic samples, for example HDY0314, had lower ferromagnesian and higher potassium concentrations.

Figure 31. Boxplots of volume magnetic susceptibility for units Tb, n = 15; Tig, n = 8; Tpa, n = 7; Tse, n = 4; Tsemb, 

n = 2; Tsj, n = 11. Median value is represented by horizontal line in each box.

T b T ig T pa T se T semb T sj

0.000

0.001

0.002

0.003

0.004

0.005

0.006

G eologic Unit
44 Geochemical Characteristics of Animas River Watershed Igneous Rocks Near Silverton, Colorado

Mineralogic Control on Leachate Compositions

Silicate mineralogy strongly controls leachate compositions 

for some elements. For example, strontium commonly substitutes 

for calcium in plagioclase due to similar ionic radius and charge. 

The mafic to intermediate composition samples IDY090537B 

and SDY0326 have high plagioclase abundances and high 

calcium and strontium abundances. Chlorite (clinochlore) 

abundances also influence the total magnesium abundance. Unit 

Tb lavas (samples SDY0310 and HWDY090542A and B) have 

several weight-percent chlorite and correspondingly high total 

magnesium leachate concentrations. Titanium leachate abundance appears to be influenced by magnetite concentration. High 

magnetite and titanium abundances in samples IDY090537B, 

HWDY090542A and B, and HDY0317 are possibly due to dissolution of intergrown ilmenite in the magnetite crystal lattice 

(Yager and others, 2005, figure 20).

Trace-Element Leachate Abundances

San Juan Formation (Unit Tsj)

Whole rocks with the highest whole-rock trace-element 

abundances, for example unit Tsj sample IDY0329, have 

high total sequential leachate abundances for the elements 

Zn, Cd, Pb, and Mn. While pyrite is the only sulfide 

detected thus far in sample IDY0329 (3 weight percent 

pyrite, from XRD) other sulfides could be present in trace 

quantities that are responsible for the relatively high traceelement concentrations.

Burns Member (Unit Tb)

Unit Tb sample SDY0310 also has high Mn, Ni, Cu, Zn, 

and Cd abundances. This sample has trace quantities of both 

pyrite and chalcopyrite (Yager and others, 2005). Unit Tb samples HWDY090542A and B have moderately elevated leachate 

concentrations of Cu and Ni, and low to moderate Mo.

Pyroxene Andesite Member (Unit Tpa)

One sample (SDY0326) of unit Tpa has relatively high 

As and Mo. More work is needed to determine the source of 

As in this sample.

Mine-Waste-Leachate Compositions

The Grand Mogul mine waste was subjected to the fouracid total-digestion and the seven- and nine-step sequentialleaching methods for comparison with whole-rock sequentialleachate data. Several trace elements are elevated in mine waste 

compared to the volcanic rocks: Cu (1 order of magnitude), Zn 

(1 order of magnitude), As (1 order of magnitude), Mo (1.5 to 2 

orders of magnitude), Cd (1 to 2 orders of magnitude), Pb (2 to 

3 orders of magnitude). Iron is also elevated in the mine-waste 

amorphous Fe- and crystalline Mn-oxide phase compared to 

volcanic rocks. Elevated iron in the amorphous-leachate phase 

may shed light on the primary source of iron flocculent when 

water, mine waste, and ANC rocks interact. These data suggest 

that mine waste may be the principal source of amorphous iron 

floc; however more data is needed to answer this question.

Magnetic Susceptibility and 

Corresponding ANC, NAP, and

Leachate Rock Properties

Pyroxene Andesite Member (Unit Tpa)

New magnetic susceptibility measurements made during 

this study indicate that unit Tpa has the highest median 

magnetic susceptibility of all igneous units. Unit Tpa also 

has corresponding high ANC and low to no NAP. In addition, 

whole-rock-leachate analyses for unit Tpa sample HDY0317 

have the lowest trace contaminant abundances. 

Burns Member (Unit Tb)

Unit Tb (talus) has a high magnetic susceptibility compared to Tb samples studied previously (Yager and others, 

2005) and low NAP. Talus of unit Tb also has relatively high 

Cu, Mo, and Ni leachate abundances and a relatively low 

median magnetic susceptibility. 

Eureka Member (Unit Tse)

Sample HDY0314 of unit Tse has moderate magnetic 

susceptibility, high ANC, and low NAP, and has corresponding 

low trace-contaminant abundances, with the possible exception 

of As (3 ppm), in sequential leachate studies.

Sultan Mountain Stock (Unit Tig)

Unit Tig has a relatively moderate magnetic 

susceptibility, low ANC, and high NAP. Field observations 

of mine-waste piles where unit Tig clasts compose a significant part of the pile indicate that fir trees, willows, grasses, 

and other plants can be established, and thus, the NAP 

properties of this unit are not too adverse to prevent some 

acid tolerant plant growth. The total organic carbon content 

(0.7 weight percent) of mine waste with high proportions of 

unit Tig clasts is an indicator that this unit can weather to 

provide at least some chemical properties beneficial for soil 

development and plant growth. This is in contrast to unreclaimed, high NAP mine waste (for example, Grand Mogul 

mine) that has nearly 0 weight percent total organic carbon 

and no established vegetation (Yager and others, 2007).
Conclusions 45

San Juan Formation (Unit Tsj)

One unit Tsj sample (IDY090537B) has high ANC, 

no NAP, and moderate magnetic susceptibility. Sample 

IDY090537B also has low trace contaminants in leachate 

analyses. Other unit Tsj samples studied for ANC and NAP in 

this and previous studies suggest that in some places this unit 

has properties that may be favorable for remediation projects.

Conclusions

We have augmented sampling of units that were sparsely 

sampled in our previous study (Yager and others, 2005). Units 

Tsj, Tig, Tse, and Tsemb were targeted for additional sampling 

because one or more samples in the previous study had high 

ANC. In addition, talus from unit Tb was sampled in the present study. 

Results of this study suggest that unit Tsj can in some 

places provide high ANC, low NAP, and low leachable trace 

contaminants. Due to the variable magnetic susceptibility and 

ANC of unit Tsj, each outcrop targeted for potential use in 

remediation would need to be studied for environmental rock 

properties ANC and NAP.

Unit Tig was found to have low ANC and moderate NAP. 

No sequential leachate analyses were done on this unit. Fir 

trees, willows, shrubs, grasses, and other plants were observed 

growing on mine waste composed largely of unit Tig clasts. 

This is an indicator that the NAP properties of this unit are not 

too adverse to prevent some acid-tolerant plant growth. 

Unit Tse has a low ANC and low to moderate NAP in 

several places. Unit Tsemb and unit Tse near the San Juan 

caldera margin appear to have higher ANC, low NAP, and low 

leachable trace contaminants (one sample).

Unit Tb talus in Minnie Gulch, also near the Silverton 

caldera margin, was found to have the highest ANC of any 

sample studied. The talus also has low to moderate sequential leachate abundances for trace contaminants Cu, Ni, As, 

and Mo suggesting this material could be favorable for use in 

mine-waste remediation projects.

This study suggests that proximity to caldera margins may 

control the high ANC of some units (Tsj, Tse, and Tsemb). 

Caldera structural margins could have functioned as permeable 

conduits during propylitic alteration that focused fluids that 

enhanced formation of high ANC rocks in some areas.

Acknowledgments

Joe Rosenbaum’s expertise and guidance in updating the 

magnetic susceptibility data is greatly appreciated. Reviews 

by Sharon Diehl and Philip Hageman improved this report. 

Ank Webbers provided technical support. Thanks to Alison 

Burchell for scientific discussions. Discussions with William 

Simon (Animas River Stakeholders Group) are appreciated. 

Stephanie Odell, San Juan Forest Abandoned Mine Lands 

Coordinator; Rob Robinson, BLM, Lakewood, Colo., under 

U.S. Bureau of Land Management interagency agreement 

number 1422CAI060047; and the U.S. Geological Survey 

Mineral Resources Program supported our research.

References Cited

Barker, Fred, 1969, Precambrian geology of the Needle 

Mountains, southwestern Colorado: U.S. Geological Survey 

Professional Paper 644-A, p. A1–A35.

Bejnar, Waldemere, 1957, Lithologic control of ore deposits in 

the southwestern San Juan Mountains, in Kottlowski, F.E., 

and Baldwin, Brewster, eds., Guidebook of southwestern San 

Juan Mountains, Colorado: New Mexico Geological Society 

8th Annual Field Conference Guidebook, p. 162–173.

Blair, R.W., Jr., Yager, D.B., and Church, S.E., 2002, Surficial 

geologic maps along the riparian zone of the Animas River 

and its headwater tributaries, Silverton to Durango, Colorado, 

with upper Animas River watershed gradient profiles: U.S. 

Geological Survey Digital Data Series 071, 1 CD-ROM.

Blowes, D.W., Ptacek, C.J., and Jurjovic, Jasna, 2003, Mill 

tailings: hydrogeology and geochemistry: Mineralogical Association of Canada Short Course Series, v. 31, 

p. 95–116.

Bove, D.J., Hon, Ken, Budding, K.E., Slack, J.F., Snee, L.W., 

and Yeoman, R.A., 2001, Geochronology and geology of 

late Oligocene through Miocene volcanism and mineralization in the western San Juan Mountains, Colorado: U.S. 

Geological Survey Professional Paper 1642, 30 p.

Bove, D.J., Mast, M.A., Dalton, J.B., Wright, W.G., and 

Yager, D.B., 2007, Major styles of mineralization and 

hydrothermal alteration and related solid- and aqueousphase geochemical signatures, chap. E3 of Church, S.E., 

von Guerard, Paul, 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. 161–230.

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, Colorado: U.S. Geological 

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

1 pl., scale 1:24,000.

Briggs, P.H., 2001, The determination of forty elements in 

geological and botanical samples by inductively coupled 

plasma–atomic emission spectrometry, in Taggart, J., ed., 

Analytical methods for chemical analysis of geologic and 

other materials: U.S. Geological Survey Open-File Report 

02-223, p. G-3.
46 Geochemical Characteristics of Animas River Watershed Igneous Rocks Near Silverton, Colorado

Burbank, W.S., 1960, Pre-ore propylitization, Silverton caldera, Colorado, in Geological Survey Research 1960: U.S. 

Geological Survey Professional Paper 400-B, p. B12–B13.

Casadevall, Thomas, and Ohmoto, H., 1977, Sunnyside mine, 

Eureka mining district, San Juan County, Colorado—

Geochemistry of gold and base metal ore deposition 

in a volcanic environment: Economic Geology, v. 92, 

p. 1,285–1,320.

Chapin, C.E., and Cather, S.M., 1994, Tectonic setting of the 

axial basins of the northern and central Rio Grande rift: 

Geological Society of America Special Paper 291, p. 5–25.

Chao, T.T., and Sanzolone, R.F., 1977, Chemical dissolution 

of sulfide minerals: U.S. Geological Survey Journal of 

Research, v. 5, no. 4, p. 409–412.

Chao, T.T., and Sanzolone, R.F., 1998, Fractionation of soil 

selenium by partial dissolution: Soil Science Society of 

America Journal, v. 53, no. 2, p. 385–392.

Davis, J.A., and Kent, D.B., 1990, Surface complexation modeling in aqueous geochemistry, in Hochella, M.F., Jr., and 

White, A.F., eds., Mineral-water interface geochemistry: 

Mineralogical Society of America Reviews in Mineralogy, 

v. 23, p.177–260.

Desborough, G.A., and Driscoll, R.L., 1998, Mineralogical 

characteristics and acid-neutralizing potential of drill core 

samples from eight sites considered from metal-mine 

related waste repositories in northern Jefferson, Powell, 

and Lewis and Clark Counties, Montana: U.S. Geological 

Survey Open-File Report 98-0790, 6 p.

Desborough, G.A., and Yager, D.B., 2000, Acid-neutralizing 

potential of igneous bedrocks in the Animas River headwaters, San Juan County, Colorado: U.S. Geological Survey 

Open-File Report 00-0165, 14 p.

Halferdahl, L.B., 1961, Chloritoid: Its composition, X-ray 

and optical properties, stability and occurrence: Journal of 

Petrology, no. 2 , p. 49–135.

Hutchison, I.P.G., and Ellison, R.D., eds., 1992, Mine waste 

management: a resource for mining industry professionals, 

regulators and consulting engineers: Boca Raton:, Florida, 

Lewis Publishers, Inc., 672 p.

Jambor, J.L., 2003, Mine-waste mineralogy and mineralogical perspectives of acid-base accounting, in Jambor, J.L., 

Blowes, D.W., and Ritchie, A.I.M., eds., Environmental 

aspects of mine wastes: Mineralogical Association of 

Canada Short Course Series, v. 31, p. 117–145.

Jambor, J.L., Dutrizac, J.E., Groat, L.A., and Raudsepp, Mati, 

2002, Static tests of neutralization potentials of silicate and 

aluminosilicate minerals: Environmental Geology, v. 43, 

p. 1–17.

Lanphere, M.A., 1988, High resolution 40Ar/39Ar chronology of Oligocene volcanic rocks, San Juan Mountains, 

Colorado: Geochimica et Cosmochimica Acta, v. 52, 

p. 1,425–1,434.

Lapakko, Kim, and Lawrence, R.W., 1993, Modification of the 

net acid production (NAP) test: Proceedings of the Seventeenth Annual British Columbia Mine Reclamation Symposium, Port Hardy, British Columbia, 1993, p. 145–159.

Lipman, P.W., 1976, Geologic map of the Lake City caldera 

area, western San Juan Mountains, southwestern Colorado: 

U.S. Geological Survey Miscellaneous Investigation Series 

Map I-962, scale 1:48,000.

Lipman, P.W., Dungan, Michael, Bachmann, Olivier, 1997, 

Comagmatic granophyric granite in the Fish Canyon 

Tuff, Colorado—Implications for magma-chamber processes during a large ash-flow eruption: Geology, v. 25, 

p. 915–918.

Lipman, P.W., Steven, T.A., Luedke, R.G., and Burbank, W.S., 

1973, Revised volcanic history of the San Juan, Uncompahgre, Silverton, and Lake City calderas in the western San 

Juan Mountains, Colorado: U.S. Geological Survey Journal 

of Research, v. 1, p. 627–642.

Luedke, R.G., and Burbank, W.S., 2000, Geologic map of 

the Silverton and Howardsville quadrangles, southwestern 

Colorado: U.S. Geological Survey Geologic Investigation 

Series Map I-2681, scale 1:24,000.

McCafferty, A.E., Yager, D.B., Horton, R.J., and Diehl, S.F., 

2006, Magnetic properties, acid neutralization capacity, and 

net acid production of rocks in the Animas River watershed, Silverton, Colorado: Environmental and Engineering 

Geophysical Society, 2006 Symposium on the Application 

of Geophysics to Engineering and Environmental Problems, 

10 p.

Piatak, N.M., Seal, R.R., II, Sanzolone, R.F., Lamothe, P.J., 

Brown, Z.A., and Adams, M., 2007, Sequential extraction 

results and mineralogy of mine waste and stream sediments 

associated with metal mines in Vermont, Maine, and New 

Zealand: U.S. Geological Survey Open-File Report 2007-

1063, 34 p.

Raudsepp, Mati, and Pani, Elisabetta, 2003, Application of 

Rietveld analysis to environmental mineralogy: Mineralogical Association of Canada Short Course Series, v. 31, 

p. 165–180. 

Raudsepp, Mati, Pani, Elisabetta, and Dipple, G.M., 1999, 

Measuring mineral abundance in skarn I: the Rietveld 

method using X-ray powder diffraction data: Canadian 

Mineralogist, v. 37, p. 1–15.
References Cited 47

Ringrose, C.R., 1982, Geology, geochemistry, and stable isotope 

studies of a porphyry-style hydrothermal system, west Silverton 

district, San Juan Mountains, Colorado: Aberdeen, Scotland, 

University of Aberdeen, Ph. D. dissertation, 257 p., 19 pls.

Salmon, U.S., and Malmström, M.E., 2004, Geochemical 

processes in mill tailings deposits: modeling of groundwater 

composition: Applied Geochemistry, v. 19, p. 1–17.

Shaw, Shannon, Wels, Christoph, Robertson, Andy, and 

Lorinczi, Geyza, 2002, Physical and geochemical characterization of mine rock piles at the Questa mine, New Mexico, 

in Tailings and mine waste 2002: Proceedings of the Ninth 

International Conference on Tailings and Mine Waste, Fort 

Collins, Colo., USA, 27–30 January 2002, p. 447–458.

Steven, T.A., Hon, Ken, and Lanphere, M.A., 1995, Neogene 

geomorphic evolution of central San Juan Mountains near 

Creede, Colorado: Miscellaneous Investigation Series Map 

I-2504, scale 1:100,000.

Steven, T.A., and Lipman, P.W., 1976, Calderas of the San 

Juan volcanic field, southwestern Colorado: U.S. Geological 

Survey Professional Paper 958, 35 p.

Steven, T.A., Schmitt, L.J., Jr., Sheridan, M.J., Williams, F.E., 

Gair, J.E., and Klemic, H., 1969, Mineral resources of the 

San Juan primitive area, Colorado: U.S. Geological Survey 

Bulletin 1261-F, 187 p.

Tessier, A., Campbell, P.G.C., and Bisson, M., 1979, Sequential extraction procedure for the speciation of particulate 

trace metals: Analytical Chemistry, v. 51, no. 7, p. 844–851.

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.

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

E1 of Church, S.E., von Guerard, Paul, 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.

Yager, D.B., Burchell, Alison, Robinson, Rob, Odell, Stephanie, Dick, R.P., Johnson, C.A., Hidinger, Jack, and Rathke, 

David, 2007, Natural terrestrial sequestration potential of 

high plains prairie to subalpine forest and mined-lands soils 

derived from weathering of Tertiary volcanics: Eos, Transactions of the American Geophysical Union, Fall Meeting, 

Suppl., Abstract, B23D-1,588.

Yager, D.B., McCafferty, A.E., Stanton, M.R., Diehl, S.F., 

Driscoll, R.L., Fey, D.L., and Sutley, S.J., 2005, Net acid 

production, acid neutralizing capacity, and associated geophysical, mineralogical, and geochemical characteristics of 

Animas River watershed rocks near Silverton, Colorado: 

U.S. Geological Survey Open-File Report 2005-1433, 

75 p.

Young, R.A., 1993, The Rietveld method: New York, Oxford 

University Press, International Union Crystallography, 

298 p.
Appendix A. Sequential leachate results for magnesium. [Magnesium (ppm); BDL, below detection limit (<2 ppm); total, seven-step, and nine-step leachate reported for each sample] Sample Total four- acid Water soluble Exchangeable pH 8.2 Carbonate pH 5 acetic acid HCl pH 4 HCl pH 2 Amorphous Fe- and Al-hydroxides and amorphous and crystalline Mn- oxides Crystalline Fe- and Al- oxides IDY090537B 25,100 IDY090537B 13 16 41 203 9,460 IDY090537B 18 20 56 25 79 294 14,200 IDY0329 4,590 IDY0329 4 6 13 39 2,675 IDY0329 4 6 13 4 12 54 2,815 HDY0317 5,580 HDY0317 5 15 20 67 2,920 HDY0317 5 15 22 6 25 75 2,970 SDY0326 14,100 SDY0326 7 10 45 248 8,040 SDY0326 7 9 47 24 123 213 9,040 HWDY090542A 13,700 HWDY090542A 14 14 18 148 10,750 HWDY090542A 10 11 20 15 56 170 10,500 HWDY090542B 11,900 HWDY090542B 3 5 9 32 6,610 HWDY090542B 3 5 9 6 11 36 6,980 SDY0310 17,000 SDY0310 7 26 8 137 12,800 SDY0310 8 24 11 6 38 143 12,600 HDY0314 4,850 HDY0314 9 8 13 41 2,085 HDY0314 5 8 12 4 18 40 2,560 Grand Mogul 1,620 Grand Mogul 38 5 BDL BDL 33 Grand Mogul 27 5 BDL BDL BDL BDL 30 
Appendix B. Sequential leachate results for aluminum. [Aluminum (ppm); detection limit (<0.4 ppm); total, seven-step, and nine-step leachate reported for each sample] 

Sample 

Total 

fouracid Water soluble Exchangeable pH 8.2 

Carbonate 

pH 5 acetic 

acid 

HCl 

pH 

4 

HCl 

pH 2 

Amorphous Feand Alhydroxides and 

amorphous and 

crystalline Mnoxides 

Crystalline 

Fe- and Aloxides Su

IDY090537B 78,600 IDY090537B 21.9 6.9 3.9 250 10,200.0 4,IDY090537B 38.1 12.8 17.0 40.8 59.4 455 14,800.0 4,4

IDY0329 60,000 IDY0329 22.2 12.1 7.3 214 5,900.0 9IDY0329 26.1 7.9 11.9 13.4 22.0 376 6,015.0 8

HDY0317 65,400 HDY0317 24.4 6.4 3.2 206 5,910.0 1,2HDY0317 32.0 10.7 12.6 18.0 52.6 272 6,030.0 1,4

SDY0326 64,600 SDY0326 6.7 1.9 0.6 157 5,400.0 1,7SDY0326 6.8 1.8 1.6 1.9 90.8 162 6,160.0 1,

HWDY090542A 66,200 HWDY090542A 20.0 6.7 4.5 337 11,300.0 3,0HWDY090542A 14.9 4.3 5.0 15.1 134.0 351 11,200.0 2,6

HWDY090542B 61,900 HWDY090542B 5.9 0.9 2.6 74 7,350.0 2,HWDY090542B 4.6 1.2 2.0 4.3 29.1 74 7,810.0 2,4

SDY0310 63,600 SDY0310 14.0 3.7 2.4 336 15,800.0 3,2SDY0310 16.2 2.0 6.6 2.8 96.9 319 15,700.0 3,7

HDY0314 56,450 HDY0314 35.3 4.9 4.9 198 2,415.0 6HDY0314 13.5 3.4 4.6 7.2 77.8 185 2,815.0 6

Grand Mogul 32,500 Grand Mogul 67.0 11.7 8.2 51 710.0 1Grand Mogul 50.4 2.0 3.2 4.6 23.7 46 845.0 3
Appendix C. Sequential leachate results for potassium. [Potassium (ppm); BDL, below detection limit (<6 ppm); total, seven-step, and nine-step leachate reported for each sample] Sample Total four- acid Water soluble Exchangeable pH 8.2 Carbonate pH 5 acetic acid HCl pH 4 HCl pH 2 Amorphous Fe- and Al- hydroxides and amorphous and crystalline Mn-oxides Crystallin

Fe- and A

oxides 

IDY090537B 8,880 IDY090537B 31 57 30 30 85 IDY090537B 40 49 33 9 18 30 99 

IDY0329 26,950 IDY0329 77 107 71 68 322 IDY0329 78 104 80 15 39 90 339 

HDY0317 28,700 HDY0317 49 141 70 72 234 HDY0317 53 112 69 17 32 66 290 

SDY0326 20,200 SDY0326 9 17 14 26 152 SDY0326 9 10 22 6 16 13 215 

HWDY090542A 25,400 HWDY090542A 18 19 10 25 39 HWDY090542A 17 11 13 5 14 17 42 

HWDY090542B 31,800 HWDY090542B 20 27 27 26 59 HWDY090542B 21 18 23 10 8 13 84 

SDY0310 23,700 SDY0310 27 48 25 27 86 SDY0310 25 31 36 14 18 16 83 

HDY0314 33,350 HDY0314 51 70 41 66 210 HDY0314 35 58 43 12 32 48 234 

Grand Mogul 17,600 Grand Mogul BDL 17 38 46 897 Grand Mogul BDL BDL 31 6 14 40 746 
Appendix D. Sequential leachate results for calcium.

[Calcium (ppm); BDL, below detection limit (<40 ppm); total, seven-step, and nine-step leachate reported for each sample] 

Sample 

Total 

fouracid Water soluble Exchangeable pH 8.2 

Carbonate 

pH 5 

acetic acid 

HCl 

pH 4 

HCl 

pH 2 

Amorphous Feand Al-hydroxides 

and amorphous 

and crystalline Mnoxides 

Crystalline 

Fe- and Aloxides 

IDY090537B 43,400 IDY090537B 149 516 5,560 9,120 16,400 IDY090537B 205 692 7,440 449 4,250 7,640 6,950 

IDY0329 23,900 IDY0329 137 403 3,945 6,005 7,765 IDY0329 122 413 4,035 331 2,385 5,370 6,005 

HDY0317 29,400 HDY0317 139 729 4,880 6,760 9,820 HDY0317 147 773 5,500 235 2,770 6,200 7,000 

SDY0326 35,800 SDY0326 108 283 2,630 3,090 4,320 SDY0326 99 273 2,900 252 1,480 2,520 3,810 

HWDY090542A 33,550 HWDY090542A 119 394 3,095 2,670 4,210 HWDY090542A 111 404 2,890 238 1,250 1,650 2,960 

HWDY090542B 21,300 HWDY090542B 93 263 2,140 2,950 5,000 HWDY090542B 69 281 2,030 196 1,220 2,860 4,710 

SDY0310 32,000 SDY0310 BDL 232 151 1,130 1,610 SDY0310 BDL 226 188 45 554 1,020 1,190 

HDY0314 16,100 HDY0314 126 414 3,645 3,295 2,515 HDY0314 104 418 3,535 269 1,510 2,520 1,615 

Grand Mogul 891 Grand Mogul 38 BDL BDL BDL 101 Grand Mogul BDL BDL BDL BDL BDL BDL 135 
Appendix E. Sequential leachate results for titanium. [Titanium (ppm); BDL, below detection limit (<0.1 ppm); total, seven-step, and nine-step leachate reported for each sample] Sample Total four- acid Water soluble Exchangeable pH 8.2 Carbonate pH 5 acetic acid HCl pH 4 HCl pH 2 Amorphous Fe- and Al-hydroxides and amorphous and crystalline Mn- oxides Crystalline Fe- and Al- oxides 

IDY090537B 2,790 IDY090537B 0.2 BDL BDL 0.3 26.2 IDY090537B 0.4 BDL BDL 1.9 BDL 1.1 40.0 

IDY0329 1,530 IDY0329 0.2 0.1 BDL 0.5 14.1 IDY0329 0.3 BDL 0.2 0.1 0.1 1.2 15.0 

HDY0317 5,030 HDY0317 0.2 BDL BDL 0.4 19.9 HDY0317 0.4 BDL BDL 0.2 0.3 1.1 24.8 

SDY0326 4,500 SDY0326 BDL BDL BDL 1.3 39.0 SDY0326 0.2 BDL BDL BDL 0.2 3.1 57.1 

HWDY090542A 4,705 HWDY090542A 0.4 0.3 0.2 1.3 104.9 HWDY090542A 0.4 0.1 BDL 0.7 0.2 2.6 121.5 

HWDY090542B 3,150 HWDY090542B BDL BDL BDL 0.4 19.5 HWDY090542B 0.6 BDL BDL BDL BDL 0.8 32.4 

SDY0310 4,640 SDY0310 0.5 0.1 BDL 1.2 97.7 SDY0310 0.7 BDL BDL BDL BDL 1.8 107.0 

HDY0314 1,825 HDY0314 0.4 BDL BDL 0.6 20.5 HDY0314 0.1 BDL BDL BDL BDL 1.2 30.0 

Grand Mogul 1,640 Grand Mogul 0.2 BDL BDL 1.0 13.9 Grand Mogul 0.2 BDL BDL BDL 0.3 1.4 16.4 
Appendix F. Sequential leachate results for iron.

[Iron (ppm); BDL, below detection limit (<10 ppm); total, seven-step, and nine-step leachate reported for each sample] 

Sample 

Total 

fouracid Water soluble Exchangeable pH 8.2 

Carbonate 

pH 5 acetic 

acid 

HCl 

pH 4 

HCl 

pH 

2 

Amorphous Fe- and 

Al-hydroxides and 

amorphous and 

crystalline Mnoxides 

Crystalline 

Fe- and Aloxides 

IDY090537B 61,000 IDY090537B 16 BDL BDL 332 13,400 IDY090537B 21 BDL 13 30 74 546 20,900 

IDY0329 31,000 IDY0329 16 BDL BDL 366 14,150 IDY0329 15 BDL BDL 8 17 530 14,250 

HDY0317 46,000 HDY0317 BDL BDL BDL 300 24,200 HDY0317 10 BDL BDL 11 65 400 25,600 

SDY0326 44,000 SDY0326 BDL BDL 16 577 17,800 SDY0326 BDL BDL 22 BDL 234 659 21,600 

HWDY090542A 44,500 HWDY090542A 22 BDL BDL 450 28,000 HWDY090542A 15 BDL BDL 28 101 518 27,300 

HWDY090542B 39,000 HWDY090542B BDL BDL BDL 147 17,200 HWDY090542B BDL BDL BDL BDL 38 164 18,300 

SDY0310 61,000 SDY0310 13 BDL BDL 462 28,400 SDY0310 15 BDL BDL BDL 81 454 28,200 

HDY0314 18,500 HDY0314 15 BDL BDL 423 4,315 HDY0314 BDL BDL BDL BDL 45 239 4,850 

Grand Mogul 22,000 Grand Mogul 283 25 15 1,550 15,600 Grand Mogul 199 BDL BDL 21 142 1,270 10,700 
Appendix G. Sequential leachate results for manganese. [Manganese (ppm); detection limit <0.04 ppm; total, seven-step, and nine-step leachate reported for each sample] Sample Total four- acid Water soluble Exchangeable pH 8.2 Carbonate pH 5 acetic acid HCl pH 4 HCl pH 2 Amorphous Fe- and Al-hydroxides and amorphous and crystalline Mn- oxides Crystalline Fe- and Al- oxides 

IDY090537B 1,380 IDY090537B 0.60 1.90 105.00 196.00 622.00 IDY090537B 0.80 3.70 146.00 3.70 94.60 168.00 606.00 

IDY0329 1,910 IDY0329 1.80 5.25 230.00 620.50 1,085.00 IDY0329 1.45 5.55 236.50 12.85 175.50 524.00 1,070.00 

HDY0317 797 HDY0317 0.30 3.40 142.00 232.00 414.00 HDY0317 0.40 4.80 174.00 5.30 82.90 220.00 349.00 

SDY0326 766 SDY0326 0.30 8.50 108.00 143.00 286.00 SDY0326 0.30 7.60 108.00 11.20 60.10 95.20 286.00 

HWDY090542A 1,060 HWDY090542A 0.70 4.30 124.50 102.50 553.00 HWDY090542A 0.45 4.55 128.50 9.15 82.35 74.55 521.00 

HWDY090542B 1,220 HWDY090542B 0.40 5.20 54.70 77.50 585.00 HWDY090542B 0.20 5.00 50.30 4.10 33.10 78.20 603.00 

SDY0310 3,130 SDY0310 0.80 2.20 10.10 84.00 2,160.00 SDY0310 1.00 1.90 10.20 2.50 14.50 57.70 2,120.00 

HDY0314 592 HDY0314 1.35 4.25 136.50 158.00 201.50 HDY0314 0.40 4.20 122.00 7.60 64.15 134.00 208.00 

Grand Mogul 600 Grand Mogul 28.10 1.40 0.90 5.30 29.90 Grand Mogul 21.30 4.00 1.70 0.40 0.70 1.00 27.70 
Appendix H. Sequential leachate results for nickel. [Nickel (ppm); BDL, below detection limit (<0.08 ppm); total, seven-step, and nine-step leachate reported for each sample] 

Sample 

Total 

fouracid Water soluble Exchangeable pH 8.2 

Carbonate 

pH 5 acetic 

acid 

HCl 

pH 4 

HCl 

pH 2 

Amorphous Fe- and 

Al-hydroxides and 

amorphous and 

crystalline Mnoxides 

Crystalline 

Fe- and Aloxides 

IDY090537B 2.8 IDY090537B BDL BDL BDL BDL BDL IDY090537B BDL BDL BDL BDL BDL BDL 0.60 

IDY0329 2.8 IDY0329 BDL BDL BDL BDL 1.05 IDY0329 BDL BDL BDL BDL BDL BDL 1.30 

HDY0317 5.8 HDY0317 BDL BDL BDL BDL 3.80 HDY0317 BDL BDL BDL BDL BDL BDL 2.90 

SDY0326 10.6 SDY0326 BDL BDL BDL 0.10 4.50 SDY0326 BDL BDL BDL BDL 0.09 0.20 5.50 

HWDY090542A 12.4 HWDY090542A BDL BDL BDL 0.20 8.35 HWDY090542A BDL BDL BDL BDL 0.20 0.14 8.45 

HWDY090542B 10.7 HWDY090542B BDL BDL BDL BDL 4.80 HWDY090542B BDL BDL BDL BDL BDL BDL 5.20 

SDY0310 21.3 SDY0310 BDL BDL BDL 0.30 14.60 SDY0310 BDL BDL BDL BDL BDL 0.20 14.70 

HDY0314 3.0 HDY0314 BDL BDL BDL 0.35 1.45 HDY0314 BDL BDL BDL BDL 0.09 BDL 1.55 

Grand Mogul 0.4 Grand Mogul 0.20 BDL BDL BDL 0.30 Grand Mogul BDL BDL BDL 0.20 BDL BDL 0.09 
Appendix I. Sequential leachate results for copper. [Copper (ppm); BDL, below detection limit (<0.1 ppm); total, seven-step, and nine-step leachate reported for each sample] Sample Total four- acid Water soluble Exchangeable pH 8.2 Carbonate pH 5 acetic acid HCl pH 4 HCl pH 2 Amorphous Fe- and Al-hydroxides and amorphous and crystalline Mn- oxides Crystalline Fe- and Al- oxides 

IDY090537B 10 IDY090537B BDL 0.1 0.2 0.8 3.3 IDY090537B BDL 0.3 0.1 BDL 0.3 1.3 2.5 

IDY0329 30 IDY0329 BDL 0.4 1.6 21.1 8.4 IDY0329 BDL 0.5 1.7 2.5 20.6 7.0 

HDY0317 39 HDY0317 BDL BDL 0.3 2.7 12.2 HDY0317 BDL 0.2 0.3 0.1 0.4 4.0 13.2 

SDY0326 55 SDY0326 BDL 0.1 0.2 2.0 11.2 SDY0326 BDL 0.4 0.1 BDL 1.2 1.9 13.2 

HWDY090542A 45 HWDY090542A 0.2 0.3 0.8 14.2 24.0 HWDY090542A 0.2 0.3 0.8 BDL 5.6 9.6 18.2 

HWDY090542B 48 HWDY090542B BDL 0.2 0.3 4.0 13.0 HWDY090542B BDL 0.2 0.4 BDL 1.4 5.0 14.3 

SDY0310 54 SDY0310 BDL 0.3 1.0 23.1 20.6 SDY0310 0.1 0.3 1.0 BDL 7.8 18.1 16.2 

HDY0314 8 HDY0314 BDL 0.2 0.3 1.5 1.8 HDY0314 BDL 0.2 0.3 BDL 0.6 1.0 1.3 

Grand Mogul 650 Grand Mogul 18.1 0.9 0.6 9.2 355.0 Grand Mogul 13.4 0.5 0.4 0.5 4.3 7.6 251.0 
Appendix J. Sequential leachate results for zinc. [Zinc (ppm); BDL, below detection limit (<0.1 ppm); total, seven-step, and nine-step leachate reported for each sample] 

Sample 

Total 

four -

acid 

Water 

soluble 

Exchangeable 

pH 8.2 

Carbonate 

pH 5 

acetic 

acid 

HCl 

pH 

4 

HCl 

pH 2 

Amorphous Fe- and Alhydroxides and 

amorphous and 

crystalline Mn-oxides 

Crystalline 

Fe- and Al

oxides 

IDY090537B 161 IDY090537B BDL BDL BDL 2.0 62.1 IDY090537B 0.1 BDL BDL 0.2 0.7 2.0 92.6 

IDY0329 1,230 IDY0329 1.6 BDL 38.9 584.0 703.5 IDY0329 1.5 BDL 42.9 1.5 150.0 760.0 483.5 

HDY0317 91 HDY0317 BDL BDL BDL 1.0 46.9 HDY0317 BDL BDL BDL 2.1 0.4 1.0 46.8 

SDY0326 88 SDY0326 BDL BDL BDL 1.0 17.0 SDY0326 BDL BDL BDL BDL 0.4 0.6 20.1 

HWDY090542A 86 HWDY090542A BDL BDL BDL 1.5 55.3 HWDY090542A BDL BDL BDL 0.2 0.7 1.5 54.8 

HWDY090542B 79 HWDY090542B BDL BDL BDL 0.4 33.6 HWDY090542B BDL BDL BDL 0.1 0.2 3.4 35.7 

SDY0310 230 SDY0310 BDL BDL 2.0 40.5 134.0 SDY0310 0.2 BDL 2.0 33.2 16.5 29.4 131.0 

HDY0314 56 HDY0314 0.2 BDL 1.0 8.7 21.8 HDY0314 BDL BDL 0.9 BDL 4.3 4.3 26.7 

Grand Mogul 14,000 Grand Mogul 638.0 10.3 28.9 49.8 463.0 Grand Mogul 460.0 35.3 39.5 7.7 33.7 33.5 1,410.0 
Appendix K. Sequential leachate results for arsenic. [Arsenic (ppm); BDL, below detection limit (<0.2 ppm); total, seven-step, and nine-step leachate reported for each sample] Sample Total four- acid Water soluble Exchangeable pH 8.2 Carbonate pH 5 acetic acid HCl pH 4 HCl pH 2 Amorphous Fe- and Al-hydroxides and amorphous and crystalline Mn- oxides Crystalline Fe- and Al- oxides 

IDY090537B 1.8 IDY090537B BDL BDL BDL 0.2 0.3 IDY090537B BDL BDL BDL BDL BDL 0.3 BDL 

IDY0329 1 IDY0329 BDL BDL BDL 0.4 0.4 IDY0329 BDL BDL BDL BDL BDL 0.4 0.4 

HDY0317 2 HDY0317 BDL BDL BDL 0.2 BDL HDY0317 BDL BDL BDL BDL BDL 0.2 BDL 

SDY0326 7 SDY0326 BDL BDL BDL 0.2 0.9 SDY0326 BDL BDL BDL BDL BDL BDL 1.0 

HWDY090542A 3 HWDY090542A BDL BDL BDL 0.3 0.2 HWDY090542A BDL BDL BDL BDL BDL 0.2 0.4 

HWDY090542B 1 HWDY090542B BDL BDL BDL BDL BDL HWDY090542B BDL BDL BDL BDL BDL BDL BDL 

SDY0310 2 SDY0310 BDL BDL BDL 0.3 0.4 SDY0310 BDL BDL BDL BDL BDL 0.3 0.2 

HDY0314 2 HDY0314 BDL BDL BDL 0.6 1.5 HDY0314 BDL BDL BDL BDL 0.2 0.6 0.6 

Grand Mogul 62 Grand Mogul BDL BDL BDL 1.0 42.1 Grand Mogul BDL BDL BDL BDL BDL 2.0 28.6 
Appendix L. Sequential leachate results for strontium. [Strontium (ppm); detection limit <0.1 ppm; total, seven-step, and nine-step leachate reported for each sample] 

Sample 

Total 

fouracid Water soluble Exchangeable pH 8.2 

Carbonate 

pH 5 acetic 

acid 

HCl 

pH 

4 

HCl 

pH 

2 

Amorphous Fe- and 

Al-hydroxides and 

amorphous and 

crystalline Mnoxides 

Crystalline 

Fe- and Aloxides 

IDY090537B 1,080 IDY090537B 1.6 8.8 38.9 66.2 133.0 IDY090537B 2.2 10.8 53.2 3.1 28.1 59.0 60.6 

IDY0329 268.5 IDY0329 1.4 6.7 29.9 45.3 64.0 IDY0329 1.2 6.9 30.6 2.8 17.2 40.8 50.1 

HDY0317 298.0 HDY0317 1.4 14.4 29.7 38.6 57.3 HDY0317 1.5 15.0 33.7 1.6 14.3 36.4 43.5 

SDY0326 958.0 SDY0326 0.8 4.1 29.0 29.0 46.8 SDY0326 0.8 4.1 29.4 2.4 12.0 23.8 45.7 

HWDY090542A 636.0 HWDY090542A 0.3 2.0 4.0 4.4 8.0 HWDY090542A 0.3 2.0 3.9 0.3 1.8 3.2 6.6 

HWDY090542B 256.0 HWDY090542B 0.2 1.3 6.5 8.7 17.1 HWDY090542B 0.2 1.4 6.3 0.6 3.2 8.3 16.6 

SDY0310 436.0 SDY0310 0.1 2.6 1.0 4.3 7.8 SDY0310 0.1 2.6 1.3 0.3 1.8 4.1 6.6 

HDY0314 306.5 HDY0314 0.6 3.1 8.6 7.6 7.2 HDY0314 0.5 3.3 7.9 0.6 3.1 5.7 4.8 

Grand Mogul 285.0 Grand Mogul 17.4 29.2 6.0 4.9 200.0 Grand Mogul 14.9 62.3 30.8 7.5 10.0 20.1 287.0 
Appendix M. Sequential leachate results for molybdenum. [Molybdenum (ppm); BDL, below detection limit (<0.4 ppm); total, seven-step, and nine-step leachate reported for each sample] Sample Total four- acid Water soluble Exchangeable pH 8.2 Carbonate pH 5 acetic acid HCl pH 4 HCl pH 2 Amorphous Fe- and Al-hydroxides and amorphous and crystalline Mn- oxides Crystalline Fe- and Al- oxides 

IDY090537B 0.27 IDY090537B BDL BDL BDL BDL BDL IDY090537B BDL BDL BDL BDL BDL BDL BDL 

IDY0329 0.22 IDY0329 BDL BDL BDL BDL BDL IDY0329 BDL BDL BDL BDL BDL BDL BDL 

HDY0317 0.45 HDY0317 BDL BDL BDL BDL 0.7 HDY0317 BDL BDL BDL BDL BDL BDL BDL 

SDY0326 1.40 SDY0326 BDL BDL BDL BDL BDL SDY0326 BDL BDL BDL BDL BDL BDL BDL 

HWDY090542A 0.36 HWDY090542A BDL BDL BDL BDL BDL HWDY090542A BDL BDL BDL BDL BDL BDL BDL 

HWDY090542B 1.40 HWDY090542B BDL BDL BDL BDL BDL HWDY090542B BDL BDL BDL BDL BDL BDL BDL 

SDY0310 0.26 SDY0310 BDL BDL BDL BDL BDL SDY0310 BDL BDL BDL BDL BDL BDL BDL 

HDY0314 0.21 HDY0314 BDL BDL BDL BDL BDL HDY0314 BDL BDL BDL BDL BDL BDL BDL 

Grand Mogul 37.40 Grand Mogul BDL BDL BDL BDL 33.7 Grand Mogul BDL BDL BDL BDL BDL BDL 24.0 
Appendix N. Sequential leachate results for cadmium. [Cadmium (ppm); BDL, below detection limit (<0.004 ppm); total, seven-step, and nine-step leachate reported for each sample] 

Sample 

Total 

fouracid Water soluble Exchangeable pH 8.2 

Carbonate 

pH 5 

acetic acid 

HCl 

pH 4 

HCl 

pH 2 

Amorphous Feand Al-hydroxides 

and amorphous 

and crystalline Mnoxides 

Crystalline 

Fe- and Aloxides 

IDY090537B 0.140 IDY090537B BDL BDL 0.030 0.040 0.080 IDY090537B BDL BDL 0.040 BDL 0.020 0.040 0.040 

IDY0329 4.400 IDY0329 0.008 0.135 1.660 2.085 1.345 IDY0329 0.007 0.135 1.705 0.030 0.565 1.625 1.110 

HDY0317 0.070 HDY0317 BDL BDL 0.020 0.020 0.030 HDY0317 BDL BDL 0.020 BDL 0.009 0.020 0.020 

SDY0326 0.060 SDY0326 BDL BDL 0.010 0.020 0.020 SDY0326 BDL BDL 0.010 BDL 0.006 0.009 0.020 

HWDY090542A 0.080 HWDY090542A BDL 0.004 0.013 0.009 0.015 HWDY090542A BDL BDL 0.004 BDL BDL 0.005 0.010 

HWDY090542B 0.060 HWDY090542B BDL BDL 0.007 0.009 0.020 HWDY090542B BDL BDL 0.008 BDL 0.004 0.009 0.020 

SDY0310 0.470 SDY0310 BDL 0.030 0.060 0.270 0.170 SDY0310 BDL 0.020 0.060 0.020 0.090 0.180 0.120 

HDY0314 0.095 HDY0314 BDL 0.007 0.050 0.040 0.020 HDY0314 BDL 0.008 0.045 BDL 0.020 0.030 0.020 

Grand Mogul 61.900 Grand Mogul 2.920 0.260 0.100 0.090 1.870 Grand Mogul 2.090 0.430 0.140 0.020 0.050 0.080 5.070 
Appendix O. Sequential leachate results for barium. [Barium (ppm); BDL, below detection limit (<0.04 ppm); total, seven-step, and nine-step leachate reported for each sample] Sample Total four- acid Water soluble Exchangeable pH 8.2 Carbonate pH 5 acetic acid HCl pH 4 HCl pH 2 Amorphous Fe- and Al-hydroxides and amorphous and crystalline Mn- oxides Crystalline Fe- and Al- oxides 

IDY090537B 480.0 IDY090537B 0.21 6.31 5.19 3.44 6.35 IDY090537B 0.40 6.13 3.07 0.35 0.69 2.47 3.46 

IDY0329 998.0 IDY0329 2.94 44.95 72.40 45.45 96.65 IDY0329 2.30 43.85 74.45 3.84 12.15 50.35 95.20 

HDY0317 740.0 HDY0317 0.41 21.40 10.60 6.38 6.95 HDY0317 0.49 22.90 13.90 0.47 1.00 8.53 9.14 

SDY0326 833.0 SDY0326 0.05 0.59 0.54 0.87 4.27 SDY0326 0.06 0.58 0.85 0.06 0.55 0.66 6.30 

HWDY090542A 810.5 HWDY090542A 0.10 2.17 1.23 1.48 1.54 HWDY090542A 0.10 2.13 1.30 0.10 0.70 0.89 1.37 

HWDY090542B 1,140.0 HWDY090542B BDL 0.45 0.36 0.55 0.80 HWDY090542B BDL 0.44 0.40 0.09 0.20 0.46 0.83 

SDY0310 701.0 SDY0310 0.10 4.79 1.74 2.18 1.41 SDY0310 0.10 4.61 1.92 0.20 0.68 1.23 1.23 

HDY0314 876.0 HDY0314 0.37 6.76 4.56 3.43 2.97 HDY0314 0.20 6.95 4.45 0.23 1.23 2.93 2.82 

Grand Mogul 252.0 Grand Mogul 1.94 4.17 4.23 7.03 59.70 Grand Mogul 1.40 4.65 2.94 0.51 2.19 3.22 54.80 
Appendix P. Sequential leachate results for lead. [Lead (ppm); BDL, below detection limit (<0.01 ppm); total, seven-step, and nine-step leachate reported for each sample] 

Sample 

Total 

fouracid Water soluble Exchangeable pH 8.2 

Carbonate 

pH 5 acetic 

acid 

HCl 

pH 4 

HCl 

pH 2 

Amorphous Fe- and Alhydroxides and 

amorphous and 

crystalline Mn-oxides 

Crystalline 

Fe- and Al

oxides 

IDY090537B 13.1 IDY090537B BDL BDL 0.20 1.20 2.60 IDY090537B BDL BDL 0.40 BDL 0.04 2.00 0.99 

IDY0329 67.0 IDY0329 0.15 0.08 6.40 45.10 15.80 IDY0329 0.10 0.07 7.00 0.07 0.53 48.00 14.00 

HDY0317 9.6 HDY0317 BDL BDL 0.10 0.83 3.10 HDY0317 BDL BDL 0.10 0.02 BDL 1.40 2.40 

SDY0326 21.3 SDY0326 BDL BDL 0.02 0.50 BDL SDY0326 BDL BDL BDL BDL 0.06 0.20 BDL 

HWDY090542A 19.2 HWDY090542A BDL BDL BDL 0.77 0.94 HWDY090542A 0.01 BDL 0.02 0.01 0.12 0.71 0.45 

HWDY090542B 16.2 HWDY090542B BDL BDL 0.56 1.40 3.30 HWDY090542B BDL BDL 0.20 BDL 0.20 1.70 5.60 

SDY0310 17.0 SDY0310 0.01 BDL 0.65 6.50 1.00 SDY0310 0.02 BDL 0.20 0.30 0.40 2.80 BDL 

HDY0314 25.5 HDY0314 0.11 BDL 1.01 5.25 3.40 HDY0314 0.06 BDL 0.81 0.01 0.51 6.20 3.50 

Grand Mogul 9,480.0 Grand Mogul 25.00 108.00 208.00 712.00 7,580.00 Grand Mogul 35.10 489.00 828.00 60.50 738.00 934.00 8,630.00 
Publishing support provided by:

Denver Publishing Service Center, Denver, Colorado

Manuscript approved for publication, April 23, 2008
Yager and others—Geochemical Characteristics of Animas River Watershed Rocks, Colorado—U.S. Geological Survey Scientific Investigations Report 2008–5063

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