Geochemical and Hydrologic Processes 

Controlling Formation of Ferricrete

By Laurie Wirt, Kirk R. Vincent, Philip L. Verplanck, Douglas B. Yager, 

Stanley E. Church, and David L. Fey

Chapter E17 of

Integrated Investigations of Environmental Effects of Historical 

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

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

Professional Paper 1651

U.S. Department of the Interior

U.S. Geological Survey
Contents

Abstract .......................................................................................................................................................779

Introduction.................................................................................................................................................779

Purpose and Scope ...................................................................................................................................780

Methods.......................................................................................................................................................782

Water-Quality Sampling ...................................................................................................................782

Water-Chemistry Analysis ......................................................................................................782

Solid-Phase Samples .......................................................................................................................782

Solid-Phase Chemistry Analysis and Mineralogy ..............................................................783

Aqueous Geochemistry of Iron-Rich Ground Water ............................................................................783

Solid-Phase Geochemistry of Iron-Cemented Deposits and Stream Sediment ..............................797

Deposit Type ......................................................................................................................................797

Mineralogy .........................................................................................................................................799

Age of Iron Minerals.........................................................................................................................799

Proximity to Mineral Assemblages, by Reach .............................................................................806

Spatial Associations of Ferricrete and Bog Iron with Ground-Water pH, Geomorphology, 

and Alteration Assemblages ......................................................................................................807

Cement Creek ....................................................................................................................................810

Upper Reach, Gladstone to Fairview Gulch ........................................................................811

Middle Reach, Fairview Gulch to Hancock Gulch ..............................................................814

Lower Reach, Hancock Gulch to Cement Creek Gauge near Silverton .........................816

Mineral Creek between Middle Fork and South Fork .................................................................817

California Gulch, a Headwater Tributary of the Upper Animas River .......................................817

Trace-Element Content of Naturally Occurring Acid-Sulfate Ground Water as a Geochemical 

Baseline for Mine-Affected Drainage ........................................................................................818

Conclusions.................................................................................................................................................820

References Cited........................................................................................................................................820

Figures

 1. Map showing selected reaches in Cement Creek, Mineral Creek, and California 

Gulch, major hydrothermal alteration areas, and water sample localities .......................781

 2. Plot of log activity of iron versus pH for iron-rich ground water in Animas River 

watershed study area ...............................................................................................................795

 3. Phase diagram for Fe-S-K-O-H system showing pe versus pH at 25°C, and ironrich ground-water samples from Animas River watershed study area .............................798

 4. Box plots showing selected solid-phase elements, grouped by ferricrete 

deposit type ...............................................................................................................................800

 5. Box plots showing selected solid-phase elements, grouped by mineralogy .................802

 6. Photograph of iron-cemented stream gravel underlying ≥4,500-year-old bog iron 

with schwertmannite and secondary goethite .....................................................................805

 7. Histogram showing age constraints of iron-cemented deposits in Cement Creek, 

grouped by dominant iron-oxide mineralogy ........................................................................806
 8. Box plots showing selected major and trace solid-phase elements, grouped 

by stream reach ........................................................................................................................808

 9. Schematic diagram of inflow discharge and pH in Cement Creek during lowflow conditions ..........................................................................................................................812

 10. Photograph of fracture-controlled iron spring in lower Prospect Gulch ........................814

 11. Photograph of gaining reach of Cement Creek near the upper bog, 

characterized by numerous iron-rich seeps ........................................................................816

 12. Graphs of sum of metals versus pH for ground-water and tributary inflows, 

and iron-rich ground water compared with mining-affected samples ...........................819

Tables

 1. Chemical analyses of water samples collected in contact with iron and 

manganese precipitates in the Animas River watershed study area ..............................784

 2. Summary of solid-phase sample attributes, including reach, deposit type, 

age of host sediments, and dominant iron mineralogy ......................................................786

 3. Characteristics of ferricrete sampled in this study .............................................................794

 4. Saturation indices (log activities) for iron oxyhydroxide minerals and 

crystobalite in iron-rich ground water in Cement and Mineral Creeks ...........................796

 5. Mapped exposures of hydrothermal-alteration mineral assemblages by 

subbasin, Animas River watershed study area ...................................................................810

 6. Characteristics of selected stream reaches with respect to abundance 

of ferricrete ................................................................................................................................815

Abstract

Ferrricrete and the acidic water from which it forms 

provide geochemical baselines of premining conditions in 

highly mineralized geologic settings. Ferricrete and bog iron 

deposits composed primarily of goethite, schwertmannite, 

and amorphous iron oxyhydroxide minerals yield important 

clues regarding the release of trace elements from weathering 

of iron sulfides, subsequent transport, and storage in shallow 

terrestrial and aqueous environments. Goethite and schwertmannite are the two most common iron-oxide minerals present 

in ferricrete and are commonly saturated in iron-rich ground 

water. Goethite forms throughout the full range of field pH 

conditions (pH 2.5 to 7), whereas optimal precipitation of 

schwertmannite occurs from pH 3 to 4.5, which is analogous 

to geochemical conditions for acidic mine drainage. Schwertmannite is amorphous and usually converts quickly to goethite, but it may be preserved in some geochemical environments. Radiocarbon dating of wood and charcoal encased 

in schwertmannite-bearing ferricrete ranges from modern to 

9,000 cal. years B.P., indicating that schwertmannite formed in 

the premining past as well as in the modern postmining period. 

Preserved prehistorical schwertmannite provides mineralogical 

evidence for paleo ground-water pH.

Ferricrete forms down gradient from subbasins with large 

exposures of acid-generating rock types (as much as 60 percent 

areal extent) containing pyrite and other sulfide minerals. Ferricrete and bog iron form when reduced, acidic ground water 

discharges near the surface and becomes oxidized. Iron-oxide 

deposits provide a sink for potentially mobile iron and sulfate, 

and some trace elements such as zinc, copper, and arsenic that 

are toxic to aquatic life. We compare acid-sulfate-type ground 

water from active ferricrete deposits with both mine-affected 

drainage and ground-water samples from other parts of the 

watershed using a plot of pH versus the sum of metals. Mineaffected drainage tends to be more acidic and elevated in the 

concentrations of trace elements such as copper and zinc than 

acid-sulfate-type water, by as much as two orders of magnitude. Such comparisons can be used as a geochemical baseline 

for acid-sulfate-type ground water draining hydrothermally 

altered Tertiary-age volcanic rock in a temperate alpine setting.

Introduction

This chapter focuses on geochemical processes that produce bog iron and ferricrete, describes environmental settings 

where bog iron and ferricrete form, and compares the water 

chemistry of non-mining-affected acid drainage and miningaffected acid drainage.

Ferricrete and bog iron deposits are the byproducts of 

near-surface weathering of iron-sulfide minerals, a natural 

process that generates acid drainage. Geologists have long 

known that weathered alpine exposures of mineralized terrains 

can produce acid drainage (Ransome, 1901; Harder, 1915; 

Lovering, 1929; Theobald and others, 1963; Hanshaw, 1974). 

More recently, researchers have recognized that ferricrete and 

the acidic water from which it forms provide geochemical 

baselines of premining conditions in highly mineralized geologic settings (Bassett and others, 1992; Furniss and Hinman, 

1998; Furniss and others, 1999; Miller and McHugh, 1994; 

Miller and others, 1999). When reducing iron-rich acidic 

ground water is exposed to atmosphere or oxygenated surface 

water, impure iron oxhydroxides precipitate that can take the 

form of bog iron (stratified iron oxyhydroxide deposits that 

are relatively clast free) or ferricrete (clastic sedimentary 

conglomerates cemented by iron oxyhydroxide matrix) as 

defined by Verplanck and others (this volume, Chapter E15) 

and Furniss and Hinman (1998). Iron-oxide deposits provide 

evidence of acid drainage preserved in the geologic record. 

Until now, however, no watershed-scale investigation of the 

solid-phase geochemistry of bog iron and ferricrete deposits 

has taken place that is directly linked to the mineralogy and 

hydrogeology of the area.

Although natural in origin, many iron-oxide minerals 

found in bog iron deposits and ferricrete have also been identified in acidic mine drainage. For the following reasons, the 

study of ferricrete provides a natural analog for such drainage:

1. Ferricrete deposits provide evidence for acidic baseline 

conditions prior to mining in the area.

2. The geochemical and hydrologic processes involved in 

ferricrete formation are analogous to those involved in 

acidic mine drainage, having implications for rates of 

Chapter E17

Geochemical and Hydrologic Processes Controlling 

Formation of Ferricrete

By Laurie Wirt, Kirk R. Vincent, Philip L. Verplanck, Douglas B. Yager, Stanley E. Church, and David L. Fey
transport and storage of iron, sulfate, and potentially toxic 

base metals such as copper, zinc, arsenic, lead, cadmium, 

and nickel in a near-surface environment.

3. Ferricrete and bog iron deposits may serve as a solidphase sink or long-term repository for base metals that 

pose toxicity to aquatic life if mobile in water.

4. A systematic study of solid-phase chemistry of ferricrete 

and the ground water from which it forms could be used 

to develop a site-specific geochemical baseline for “worstcase” ground-water chemistry resulting from the weathering of sulfide mineral deposits.

What is ferricrete? Ferricrete is iron-cemented clastic 

sediment. Bog iron is a subcategory of ferricrete that consists 

of stratified, impure iron oxides with relatively few clasts, that 

have formed in bogs, iron springs, and other shallow wetlands 

(for a classification of ferricrete categories, see Verplanck and 

others, this volume; Yager and Bove, this volume, Chapter E1, 

pl. 2). Bog iron typically has relatively low porosity and forms 

near the ground surface. Organic debris such as algal filaments, evergreen needles, and branches are common trace 

fossils replaced by iron oxides in bog deposits. Conglomeratetype ferricrete deposits are dominated by sand and gravel 

clasts and contain variable amounts of iron cement, thus having variable porosity. They may also contain fossil branches 

and logs, typically from flood or avalanche debris. Both bog 

iron and ferricrete conglomerate are abundant in shallow 

valley fill and along the streambanks of Cement Creek and 

Mineral Creek in the Animas River watershed study area.

Two conditions are required for the formation of ferricrete. The first is a large, long-term source of dissolved iron, 

which is generally produced by weathering of hydrothermally 

altered rock containing pyrite that has little acid-neutralizing 

capacity. Areas of acid-generating rock types are exposed in 

the Red Mountains, Ohio Peak–Anvil Mountain, and peak 

3,792 m area between Middle and South Forks Mineral Creek 

(fig. 1). Both the Cement Creek and Mineral Creek basins contain numerous iron springs, iron bogs, and stream-deposited 

ferricrete that are spatially associated with the hydrothermally 

altered mineral assemblages in the headwaters and tributary 

drainages of these basins. Dating of ferricrete in the Animas 

River watershed study area using 14C methods and age constraints of sediment host deposits indicates a broad age range 

spanning the late Holocene (Vincent and others, this volume, 

Chapter E16).

The second condition is when reduced acid-sulfate 

ground water mixes with oxygen to drive the precipitation of 

iron minerals. This condition is found in a variety of hydrologic settings in which low-pH and low-Eh ground water 

comes in contact with the atmosphere or with oxygen generated by bacteria or plants (Stanton, Yager, and others, this 

volume, Chapter E14) or mixes with shallow oxygenated 

water. Environmental settings where ferricrete typically forms 

include iron springs, iron bogs (which are also iron fens), and 

stream terraces of low-pH streams.

Iron bogs and springs commonly occur where the water 

table intersects the land surface, frequently at the topographic 

break in slope along the valley sides. Bog iron is deposited 

in relatively low gradient wetland environments, generally 

on flood plains, or superimposed on stream-terrace deposits 

along the valley floor. In contrast to iron springs and bogs, 

conglomerate-type ferricrete and manganocrete require a 

framework of permeable clastic material for the precipitation of iron-oxide cement. Iron- and manganese-rich ground 

water tends to cement surficial deposits, such as hillslope 

colluvium, alluvial fans, talus cones, and avalanche debris. 

Bog iron accumulating in a stream terrace or iron precipitating 

within a gravel deposit is likely to be protected from erosion. 

Iron cementation can occur in a wide variety of geomorphic 

settings, including alluvial, colluvial, and stream settings. The 

length of time needed for cementation to occur is unknown 

and is thought to vary considerably with respect to the individual environmental setting.

Purpose and Scope

Chapters E15 and E16 provide a geologic framework 

for (1) the distribution, classification, and morphology of 

iron- and manganese-rich deposits (Verplanck and others, this 

volume), and (2) the surficial geomorphology and relative age 

of unconsolidated deposits in Cement Creek in relation to the 

occurrence and distribution of ferricrete (Vincent and others, 

this volume). In addition, plate 2 in Yager and Bove (this volume) maps the distribution of bog iron and ferricrete deposit 

types throughout the Animas River watershed study area. The 

objectives of this chapter are to:

• Describe aqueous-phase geochemistry of acid-sulfate 

ground water that precipitates iron oxyhydroxides, 

including pH relations and saturation state of common 

iron mineral species

• Describe mineralogy, age of host deposits, and solidphase geochemistry of iron bog deposits, iron-spring 

deposits, and alluvial and colluvial ferricrete deposits

• Compare major- and trace-element concentrations of 

iron deposits with those for manganocrete and local 

stream-deposited sediment

• Characterize spatial relations among acid-generating 

rock types, ground-water pH, and the occurrence and 

distribution of iron- and manganese-cemented deposits

• Compare the water chemistry of the acid-sulfate 

ground water that produces ferricrete with that of 

mine-affected water to provide a worst-case geochemical baseline of the acid drainage and trace-metal 

concentrations prior to mining.

780 Environmental Effects of Historical Mining, Animas River Watershed, Colorado
Processes Controlling Formation of Ferricrete 781 Figure 1. Selected reaches in Cement Creek, Mineral Creek, and California Gulch, major hydrothermal alteration provinces, and water sample localities. Hydrothermal alteration assemblages are from Bove and others (this volume, Chapter E3). Ferricrete deposits in Cement Creek basin were emphasized in this investigation. Water chemistry in table 1.

EXPLANATION

AS, acid-sulfate

QSP, quartz-sericite-pyrite

V-QSP, vein-related quartz-sericite-pyrite

WSP, weak sericite-pyrite

Cement Creek basin boundary

Mining-affected spring or adit

Iron-rich spring

Mn-rich spring

YUKON

May Day tailings

lowerbog

Gauging station

SUNNYSIDE

Dischargefrom mine

ANGLO-SAXON

Middle 

 Fork

Silverton

South Fork 

Mineral

Creek

Red Mtn 3

Houghton 

Mtn

CaliforniaMtn

Kendall Mtn

Ohio Peak

Anvil Mtn

 3,792 m

Red Mtn

Red Mtn

So da Gulch

Dry Gulch

Prospect Gulch

Tiger Gulch

Georgia Gulch

Fairview 

Gulch

Porcupine Gulch

Ohio Gulch

Topeka Gulch

Cascade Gulch

Henrietta

Gulch

California

Gulch

M ni nesota Gulch

Ill ni o si Gulch

H an cock Gu cl h

N ai gara Gulch

Grassy Gulch

Eureka

Gulch

LA3 JJP

H7A

H7A

North Fork

Middle Fork

South Fork

Cement

Creek

Animas

River

 Peak

 Inactive mine

Gladstone

99VMF12

99VMF13

99VMF10

99VMF68

99VMF67

99VMF34

99VMF72 99VMF03

99VAF01

MN-1

CGS

PG fan

YS-LB

YS-RB

Gas Spring

UB-1

Ren Spring

GT-1 to GT-8

UB-1

99VAF01

99VMF92

Animas Riverwatershed study area

C O L O R A D O

Durango

00 1

1 2 MILES

2 KILOMETERS

107°50'30" 107°45' 107°37'30" 107°30'

37°

58'

37°

52'30"
Methods

Water-Quality Sampling

Water-quality samples were collected from 19 iron-rich 

springs, 3 manganese-rich springs, and 5 mine-affected sites 

that appeared to be forming iron oxyhydroxide precipitates, 

or in a few localities, manganese-oxide precipitates or coatings. Most of these water samples were in contact with 

fine-grained iron precipitates that were either suspended 

or settled, and likely contained iron colloids. Every effort 

was made to collect water samples at the point where the 

spring emerged from the ground with the greatest flow, in an 

attempt to collect water before equilibration with atmosphere. 

In areas of diffuse seepage, we selected the sampling location by surveying with field meters to find areas with the 

lowest pH and dissolved oxygen. Field parameters including 

pH, temperature, specific conductance, and dissolved oxygen 

were measured on site. Each sample was filtered on site using 

either a small pump with a Gelman 0.45-μm (micrometer) 

capsule or syringe-mounted 0.45-μm Gelman Acro-disc, 

and then acidified. One aliquot was acidified with ultrapure 

hydrochloric acid for analysis of Fe+2 speciation and total iron. 

A separate aliquot was acidified with ultrapure nitric acid to 

a pH less than 2.0 for analysis of dissolved major and trace 

elements.

Water-Chemistry Analysis

Filtered, acidified water samples were analyzed 

for dissolved major and trace cations and sulfate. Major 

elements were determined by inductively coupled plasma–

atomic emission spectrometry (ICP-AES; Briggs and Fey, 

1996), and trace elements and sulfate were measured by 

inductively coupled plasma–mass spectroscopy (ICP-MS; 

Lamothe and others, 1999). Concentrations of the anions 

fluoride, chloride, and nitrate were determined by ion 

chromatography in filtered, unacidified samples (d’Angelo 

and Ficklin, 1996). Most samples had pH values less than 

4.5, and for these samples the presence of bicarbonate as an 

anion was assumed to be negligible. Four samples in Mineral 

Creek had pH greater than 4.5, and alkalinity was measured 

using a HACH titration kit to determine bicarbonate concentration. Analytical detection limits by laboratories used 

in this study are reported in Lamothe and others (1999) and 

d’Angelo and Ficklin (1996). In 1999, iron (II) redox species 

were determined using a modification of the FerroZineTM 

colorimetric method (Stookey, 1970; To and others, 1999) 

with a Hewlett Packard 8453TM diode array UV/VIS spectrophotometer. Between 2000 and 2002, the phenanthroline colorimetric method was used (Hach Company, 1992). 

Water-chemistry data for the ground-water samples is in 

table 1 and in the project database (Sole and others, this 

volume, Chapter G).

Solid-Phase Samples

Bog iron and ferricrete samples were collected in concert 

with the mapping of ferricrete deposits in the Animas River 

watershed study area (Verplanck and others, this volume; 

Vincent and others, this volume; Yager and Bove, this volume, 

pl. 2). We collected representative hand specimens as grab 

samples, using a rock hammer. Iron-rich solid-phase samples 

were collected from bog iron deposits, iron springs, and ironcemented conglomerates containing both alluvial (rounded) 

rock clasts deposited by fluvial processes and colluvial (angular) rock clasts deposited by hillslope processes. Eighty-six 

solid-phase samples were analyzed for solid-phase chemistry in 

this study (tables 2 and 3). These include 26 iron-bog, 15 ironspring, 21 alluvial-ferricrete, 16 colluvial-ferricrete, and 8 manganocrete samples. In some localities the sample environment 

was transitional between two or more lithologic facies, but for 

the purposes of sample coding, a dominant facies was assigned 

on the basis of the field descriptions. In addition, solid-phase 

analytical results were compared with 39 premining streamdeposited sediment and 10 modern stream-deposited sediment 

samples from Cement Creek described by Church, Fey, and 

Unruh (this volume, Chapter E12). Attributes such as stream 

reach, deposit type, age of host sediment, and the most abundant iron oxyhydroxide minerals for the solid-phase samples 

are also given. Chemical analyses and sample descriptions are 

in the project database (Sole and others, this volume).

Many iron-oxide specimens exhibited a wide color range 

from yellow to orange brown to reddish brown to brown, representing both crystalline and amorphous iron oxyhydroxide 

phases. Results from radiocarbon dating and geomorphological constraints for the host deposits indicate a maximum age 

of ferricrete deposition (table 2), and are further described in 

Vincent and others (this volume). No credible evidence was 

found for ferricrete deposits older than the last glacial advance, 

and we see it as improbable that these surficial deposits would 

have survived glacial scouring. The samples included both 

postglacial and modern iron-rich deposits that include bogs, 

alluvial ferricrete, and colluvial ferricrete deposits.

The majority of the solid-phase samples were collected 

from the Cement Creek basin because of the nearly continuous 

occurrence of bog iron deposits and ferricrete in its upper and 

lower reaches, in particular between the confluence of Cement 

Creek with the North Fork Cement Creek and Fairview Gulch 

tributaries (Yager and Bove, this volume, pl. 2). The sample 

density in the Cement Creek basin allows for spatial correlation with hydrothermal alteration mapping by Bove and 

others (this volume, Chapter E3), detailed geomorphological mapping by Vincent and others (this volume), and tracer 

injection and synoptic sampling by Kimball and others (this 

volume, Chapter E9). Additional iron oxyhydroxide samples 

were collected along Mineral Creek, and manganocrete samples were collected from the headwaters of the upper Animas 

River basin in the Eureka and California Gulch subbasins 

near California Mountain (Yager and Bove, this volume).

782 Environmental Effects of Historical Mining, Animas River Watershed, Colorado
Processes Controlling Formation of Ferricrete 783

Solid-Phase Chemistry Analysis and Mineralogy

Methods for physically separating the solid-phase matrix 

from the ferricrete and manganocrete samples are described 

in Verplanck and others (this volume), as are X-ray diffraction 

techniques for determining the mineralogy of the matrix samples. During sample preparation of ferricrete samples, the ironrich matrix was separated from the lithic clasts. We dislodged 

the cement matrix using the rocking motion of a ceramic 

hammer on the sample. The sample was placed on a ceramic 

plate and then sifted through an 80-mesh (180-micrometer) 

sieve. Solid-phase samples were prepared for analysis of major 

and trace elements using mixed-acid total digestion, followed 

by inductively coupled plasma–atomic emission spectroscopy 

(ICP-AES) (Briggs and Fey, 1996). Weight percent of sulfur 

was analyzed by SGS (Société Génerale de Surveillance) 

Minerals Laboratories, formerly known as X-Ray Assay 

Laboratories (XRAL), using high-temperature combustion in a 

stream of oxygen in the presence of elemental iron. This combustion produces SO2

 gas whose concentration was determined 

using an infrared detector. Results of analyses for major oxide 

elements and sulfur (in weight percent; wt. %); trace elements 

(in parts per million; ppm), and the presence of major iron 

minerals and rock-forming minerals are reported in the project 

database (Sole and others, this volume).

Aqueous Geochemistry of Iron-Rich 

Ground Water

In areas underlain by hydrothermally altered rock 

with abundant pyrite and poor acid-neutralizing capacity, 

the pH values of springs and low-flow streams are commonly 

≤4.5, and sulfate supplies most of the anionic charge. In areas 

underlain by propylitically altered rock, calcium is the dominant cation, followed by magnesium and silica. Calcium and 

magnesium are mostly derived from minerals produced by 

regional propylitic alteration such as clinochlore chlorite, epidote, calcite, or gypsum, or from sedimentary carbonate rock 

outside the southeast margin of the caldera (Yager and Bove, 

this volume, pl. 1). In propylitized areas, calcite in fractures 

and veins provides a degree of acid neutralization to low-pH 

waters.

In a steep-gradient alpine environment, water-rock interaction is the primary mechanism that oxidizes and dissolves 

disseminated pyrite in hydrothermally altered rock and veins. 

Most of the annual precipitation that falls at higher elevations 

leaves the watershed as snowmelt and rainfall runoff; however, 

a fraction of the surface water travels below the water table 

through saturated soil, talus, and fractured bedrock. Dissolved 

oxygen is consumed by reactions (1) through (3) following 

(Kleinman and others, 1981; Bigham and others, 1990), resulting in increasing levels of dissolved iron (Fe2+) and acidity:

 FeS2

 + 3.5O2 + H2O Æ Fe2+ + 2H+ + 2 SO4

2– (1)

 Fe2+ + 0.25O2

 + H+ Æ Fe3+ + 0.5H2O (2)

 FeS2

 + 14Fe3+ + 8 H2O Æ 15Fe2+ + 2 SO4

2– + 16 H+

 (3)

Surficial exposures of pyrite-bearing rock (typically unsaturated) produce brightly colored alteration halos of jarosite and 

hematite, particularly at higher elevations. Bog iron and ferricrete deposits, predominantly found down gradient at lower 

elevations and in water-saturated environments, are composed 

mainly of schwertmannite, goethite, and amorphous ferrihydrite. The type of iron oxyhydroxide mineral formed depends 

on the chemistry and pH of the hydrologic environment and 

the oxidation state of the iron.

The greater the distribution of iron-sulfide minerals 

(commonly pyrite), the greater the degree of acidic weathering, 

and the more likely that undiluted ground water in fractured 

rock will accumulate H+, Fe+2, SO4

2–, and trace elements with 

distance along the flow path. Acidic, anaerobic conditions 

favor reduced, ferrous (Fe+2) iron. Divalent iron is soluble and 

thus mobile in ground water. In contrast, more alkaline and 

aerobic conditions favor formation of oxidized, ferric (Fe+3) 

iron, which has a lower solubility and low mobility in ground 

water. At lower pH, iron bacteria promote the precipitation of 

iron oxyhydroxide minerals in near-surface environments such 

as bog iron deposits and springs (Stanton, Yager, and others, 

this volume, Chapter E14). Photoreduction may also play a role 

in iron oxidation, affecting diurnal rates of iron precipitation. 

In a small acidic Colorado stream, McKnight and others (1988) 

reported that daytime oxidation of ferrous iron was almost four 

times as great as nighttime oxidation of ferrous iron.

In the presence of oxygen, aqueous ferrous iron is 

converted to ferric iron (precipitate) according to the following reaction (Kleinman and others, 1981; Bigham and others, 

1990):

 Fe2+ + 2.5H2

O + 0.25O2 Æ Fe(OH)3(s) + 2H+ (4)

Ground water emerging from wet ferricrete was sampled from 

19 locations where iron oxyhydroxide appeared to be actively 

forming. Half these sites were in upper Cement Creek, and 

the remainder in Mineral Creek. Results for major dissolved 

cations and selected trace elements are in table 1. Three manganese seeps from California Gulch and drainage from three 

mine adits in Prospect Gulch and two mine-affected springs in 

Mineral Creek were also sampled. Sampling occurred during 

low-flow conditions in August to September 1999–2002, and 

thus year-to-year seasonal variations in water chemistry cannot 

be determined.

In upper Cement Creek, the pH of iron-rich ground-water 

samples ranged from 3.0 to 4.6. The pH of ground-water 

samples in Mineral Creek tended to be higher than in Cement 

Creek, with all but one pH value ranging between 4.0 and 6.6. 

The lowest pH value of 2.5 was measured at a mine adit in 

Prospect Gulch. The pH of manganese-rich water in California 

Gulch ranged between 4.5 and 4.8. Dissolved oxygen values 

ranged between 0.01 and 4.4, indicating that the ground water 

had not yet fully equilibrated with atmosphere.
Table 1. Chemical analyses of water samples collected in contact with iron and manganese precipitates in the Animas River 

watershed study area.

[Dissolved chemistry for major and trace elements by ICP-AES, except for bold values by ICP-MS. Anion concentrations (chloride, fluoride, and sulfate) by ion 

chromatography (IC), except as noted. Sample values are in milligrams per liter except for specific conductance (SC), in microsiemens per centimeter; temperature (T), in degrees Celsius; dissolved oxygen (DO); <, less than; ND, not determined]

Sample ID Reach Date SC pH T DO Ca Mg Na K Al Sr Mn Cl F SO4

Iron-rich springs

99VMF10* Mineral Cr 8/15/1999 1,070 6.6 6.6 ND 250 6.9 10.5 0.85 <0.01 1.97 1.48 2.1 0.6 560

99VMF34 Mineral Cr 8/24/1999 19 5.8 13.9 ND 2.0 0.34 1.7 0.26 0.2 0.02 <0.001 1.8 <0.1 4

99VMF67 Mineral Cr 8/24/1999 127 4.2 2.3 ND 11 1.8 3.9 0.43 0.9 0.10 0.23 1.9 0.2 42

99VMF68 Mineral Cr 8/24/1999 130 4.0 4.1 ND 11 2.0 4.1 0.46 0.9 0.10 0.28 1.9 0.3 44

99VMF92 Mineral Cr 9/1/1999 305 4.4 9.0 ND 24 2.7 3.9 1.4 0.9 0.21 0.20 2.3 0.3 84

99VMF72 Mineral Cr 8/25/1999 341 3.3 6.6 ND 28 3.2 3.8 1.2 2.7 0.20 0.36 4.6 0.3 110

00VMF01 Mineral Cr 9/6/2000 473 4.3 3.5 ND 34 6.2 4.6 0.58 7 0.29 0.66 0.8 0.57 220

00VMF03 Mineral Cr 8/31/2000 777 4.1 8.7 ND 130 4.5 7.1 0.51 1.2 1.30 0.30 0.8 0.23 381

PG fan U.Cement Cr 8/19/2002 579 3.3 10.9 2.7 24 7.5 1.5 3.9 16 0.29 0.69 0.4 0.7 240

YS-LB U.Cement Cr 10/25/2000 734 3.5 5.3 0.9 75 4.9 2.7 1.0 4.1 0.56 0.58 2.7 0.9 250

YS-RB U.Cement Cr 10/25/2000 1,219 3.2 7.1 0.3 180 8.4 5.4 2.0 5.1 1.10 1.40 0.6 1.6 630

Gas Spring U.Cement Cr 8/23/2002 568 4.2 7.1 0.1 50 8.1 2.8 2.9 9 0.57 1.10 0.6 0.9 250

Ren Spg U.Cement Cr 8/23/2002 922 3.6 7.9 0.3 120 7.4 5.2 2.0 12 0.54 0.67 0.6 1.4 430

UB-1 U.Cement Cr 10/25/2000 1,468 3.2 18.5 4.4 210 11 2.6 3.0 13 1.70 1.90 0.6 1.7 760

GT-1 U.Cement Cr 9/24/1999 1,172 3.2 12.2 8.2 180 17 4.9 1.6 9.0 0.83 4.00 2.2 1.0 560

GT-2 U.Cement Cr 9/27/1999 1,613 3.0 10.8 1.2 270 11 4.7 2.1 8.3 1.20 1.80 2.2 1.2 820

GT-3 U.Cement Cr 9/27/1999 1,462 4.6 13.1 0.0 280 11 5.3 2.4 6.1 1.40 2.00 0.6 1.5 780

GT-4 U.Cement Cr 9/27/1999 1,801 3.8 13.6 0.4 330 12 5.4 2.5 7.7 1.50 2.20 2.1 1.3 1,000

GT-7 U.Cement Cr 9/27/1999 1,578 3.3 16.4 4.1 290 11 5.0 2.1 6.2 1.40 1.90 2.1 1.2 790

GT-8 U.Cement Cr 9/27/1999 523 3.6 12.6 4.4 65 6.5 2.8 1.5 2.8 0.41 1.70 0.5 0.5 220

Manganese-rich springs

99VAF01 California Gulch 8/5/1999 703 4.5 2.6 ND 55.9 9.3 3.2 1.9 24 0.08 57 2.1 30 360

MN-1 California Gulch 10/25/2000 856 4.8 1.8 7.9 44 21 2.2 1.6 24 0.07 48 0.4 3.8 470

CGS California Gulch 8/21/2002 390 4.5 2.9 <4.5 28 3.7 3.0 1.7 14 0.04 28 0.3 15 150

Mining-affected springs or adits

L3A (adit) Prospect Gulch 9/29/1999 228 4.1 1.1 3.3 16 0.9 2.6 2.2 4.0 0.13 0.22 0.4 0.1 83

JJP (adit) Prospect Gulch 9/29/1999 1,010 2.7 3.2 7.5 2.7 1.6 0.4 1.6 13 0.05 0.19 0.5 0.4 290

H7A (adit) Prospect Gulch 9/29/1999 2,450 2.5 2.0 3.6 49 7.5 0.8 1.4 23 0.27 0.91 0.7 1.3 1,100

99VMF13 Mineral Cr 8/23/1999 481 5.8 5.5 ND 66 6.8 6.1 0.68 0.9 0.56 2.46 3.8 1.4 220

99VMF12 Mineral Cr 8/23/1999 344 5.6 4.3 ND 47 3.4 5.7 0.49 0.4 0.29 0.90 3.8 0.7 140

*

Possibly mining affected.

784 Environmental Effects of Historical Mining, Animas River Watershed, Colorado
Processes Controlling Formation of Ferricrete 785

Fe Si Zn Cu Ni Co Zn+Cu+Ni+Co Fe(II) Fe(III) Remarks or methods

Iron-rich springs

6.8 15 0.026 <0.010 <0.010 0.010 0.036 6.91 6.9 Samples run by FerroZine method for iron total and iron II

0.1 4.8 <.001 <0.010 <0.010 <0.010 0.000 0.051 <0.05 Samples run by FerroZine method for iron total and iron II

<0.02 13.4 <.001 <0.010 <0.010 <0.010 0.000 <0.01 <0.01 Samples run by FerroZine method for iron total and iron II

<0.02 14.8 0.012 <0.010 <0.010 <0.010 0.012 <0.01 <0.01 Samples run by FerroZine method for iron total and iron II

5.8 16 0.021 <0.010 <0.010 <0.010 0.021 8.4 8.0 Samples run by FerroZine method for iron total and iron II

1.1 15 0.037 <0.010 <0.010 <0.010 0.037 1.2 0.3 Samples run by FerroZine method for iron total and iron II

31 26 0.120 0.002 0.007 0.019 0.148 30.2 0.5 ICP-MS

4.7 12.2 0.150 0.002 0.005 0.008 0.165 4.2 0.5 ICP-MS

56 26 0.677 0.027 0.021 0.025 0.750 38.1 18.1 bold = ICP-MS HACH for Fe (III)

11 8 0.220 <0.010 0.012 0.019 0.251 20.0 11 Phenanthroline Chemettes for Fe (II)

29 12 0.500 0.150 0.020 0.030 0.700 20.0 9.0 Phenanthroline Chemettes for Fe (II)

39 26 0.467 0.011 0.010 0.015 0.503 5.3 33.6 bold = ICP-MS HACH for Fe (III)

28 22 0.127 0.004 0.011 0.012 0.154 12.6 15.5 bold = ICP-MS HACH for Fe (III)

45 12 1.500 <0.010 0.038 0.050 1.588 20.0 45 Phenanthroline Chemettes for Fe (II)

0.34 23 0.990 0.080 0.024 0.038 1.132 0.15 0.29 Samples run by FerroZine method for iron total and iron II

20 20 0.780 <0.010 0.025 0.031 0.836 16.7 9.3 Samples run by FerroZine method for iron total and iron II

39 20 0.780 <0.010 0.027 0.035 0.842 43.0 0.0 Samples run by FerroZine method for iron total and iron II

50 21 0.930 <0.010 0.033 0.036 0.999 66.7 0.7 Samples run by FerroZine method for iron total and iron II

28 19 0.790 <0.010 0.027 0.029 0.846 22.7 12.2 Samples run by FerroZine method for iron total and iron II

0.30 15 0.440 0.041 0.010 0.017 0.508 0.32 0.00 Samples run by FerroZine method for iron total and iron II

Manganese-rich springs

<0.02 8.7 4.040 <0.010 0.022 <0.010 4.062 <0.01 <0.01 Samples run by FerroZine method for iron total and iron II

0.5 9.3 14.0 0.140 0.0120 0.013 14.165 0.10 0.37

30.4 8.5 1.74 4.60 0.0072 0.002 6.349 0.18 30.22 bold = ICP-MS HACH for Fe (III)

Mining-affected springs or adits

23 8.1 2.20 0.160 0.0110 0.010 2.381 22.2 0.40

62 27 11.0 0.640 0.0280 0.039 11.707 0.412 61.59

291 6.7 8.2 0.590 0.1100 0.170 9.070 23.0 268.00

12 14.5 0.401 <0.010 <0.010 0.024 0.425 11.7 11.6 Samples run by FerroZine method for iron total and iron II

6.0 14.1 0.114 <0.010 <0.010 0.019 0.114 5.84 5.8 Samples run by FerroZine method for iron total and iron II
Table 2. Summary of solid-phase sample attributes, including reach, deposit type, age of host sediments, and dominant iron mineralogy.—Continued

Field No. Latitude N. (degrees) Longitude W. (degrees) Reach Deposit type Wet/Dry Dominant iron mineral1

Geomorphologic

unit2

(Vincent and 

others, this 

volume)

Age range

of host deposit

(per 1,000 years)

Site description Munsell Color code (Goddard and others, 1948)

00ABFC-231 37.879650 107.674545 Upper Cement Alluvial ferricrete alluvial 1 p 0.5 to 0 Alluvial ferricrete forming in streambed 

at upper bog, clasts subrounded, lots of algae growing on cement (from Cement Creek above Georgia Gulch, in streambed).

7.5YR-4/6 strong brown

99ABFC-102A 37.889351 107.668389 Upper Cement Alluvial ferricrete spring 1 T 6 to 0.5 Laminar iron springs deposit overlying 

alluvial ferricrete, 5 ft above low-flow water level, sample collected from north bank of Cement Creek above confluence with Dry Gulch.

5YR-3/4 dk reddish brown

99ABFC-108 37.879650 107.668333 Upper Cement Alluvial ferricrete alluvial 2 bed .5 to 0 Alluvial ferricrete forming in streambed 

at upper bog, clasts subrounded, lots of algae growing on cement (from Cement Creek above Georgia Gulch, in streambed).

5YR-4/6 yellowish red

99ABFC-110 37.888660 107.668159 Upper Cement Alluvial ferricrete dry 2 F or T 10 to .5 Alluvial ferricrete, clasts 6–8 in. down to 

fine sand, subrounded to subangular, poorly sorted, 2 ft above low-flow water level, sample from south bank Cement Creek below confluence with South Fork Cement Creek.

7.5YR-4/6 strong brown

99ABFC-111 37.888386 107.656769 Upper Cement Alluvial ferricrete dry 2 Fo or T 10 to .5 Alluvial ferricrete, clasts 6–8 in. down to 

fine sand, subrounded to subangular, moderately sorted, 1 ft above lowflow water level, sample from Cement Creek above confluence with Dry Gulch.

10YR-3/6 dk yellowish brown

99ABFC-118B 37.883068 107.650864 Upper Cement Alluvial ferricrete dry 2 T or p 6 to 0 Alluvial ferricrete, clasts 1–1.5 in. down 

to fine sand, well sorted, sample from Cement Creek downstream from Dry Gulch confluence.

5YR-4/6 yellowish red

99ABFC-119 37.882576 107.668800 Upper Cement Alluvial ferricrete dry 2 T 6 to .5 Alluvial ferricrete, clasts 3 in. down 

to fine sand, moderately sorted, 6 ft above low-flow water level, sample from Cement Creek downstream from Dry Gulch confluence.

10YR-4/6 dk yellowish brown

99ABFC-120 37.882236 107.675308 Upper Cement Alluvial ferricrete dry 2 Fy 6 to 0 Alluvial ferricrete, clasts 12 in. down to 

sand, subangular, poorly sorted, 2 ft above low-flow water level, sample from Cement Creek just downstream from Prospect Gulch confluence.

10YR-5/8 yellowish brown

99ABFC-124A 37.877666 107.666283 Upper Cement Alluvial ferricrete alluvial 2 bed .5 to 0 Actively forming ferricrete matrix forming on bed of Cement Creek at Tiger 

Gulch.

7.5YR-5/8 strong brown

786 Environmental Effects of Historical Mining, Animas River Watershed, Colorado
Processes Controlling Formation of Ferricrete 787

Table 2. Summary of solid-phase sample attributes, including reach, deposit type, age of host sediments, and dominant iron mineralogy.—Continued

Field No. Latitude N. (degrees) Longitude W. (degrees) Reach Deposit type Wet/Dry Dominant iron mineral1

Geomorphologic

unit2

(Vincent and 

others, this 

volume)

Age range

of host deposit

(per 1,000 years)

Site description Munsell Color code (Goddard and others, 1948)

99ABFC-128A 37.856407 107.667870 Middle Cement Alluvial ferricrete dry 2 T 6 to 0.5 Alluvial ferricrete, clasts 6 in. down to 

fine sand, subrounded, moderately sorted, sample from east bank of Cement Creek below confluence with Porcupine Gulch.

5YR-3/4 dk reddish brown

99ABFC-128B 37.856407 107.667870 Middle Cement Alluvial ferricrete spring 2 T 6 to .5 Iron spring deposit, fine-grained 

hydrous iron oxides overlying sample 99ABFC-128A.

7.5YR-3/4 dk brown

99ABFC-136A 37.835297 107.675392 Middle Cement Alluvial ferricrete dry 2 T or p 6 to 0 Alluvial ferricrete, clasts 8 in. down to 

sand, subrounded, moderately sorted, 1 ft above low-flow water level, from unnamed tributary on east side of Cement Creek above Hancock Gulch confluence.

10YR-4/6 dk yellowish brown

99ABFC-138 37.833183 107.675316 Lower Cement Alluvial ferricrete dry 2 T 6 to .5 Alluvial ferricrete, clast size 8 in. to 

sand, subrounded, moderately sorted, sample from west bank of Cement Creek below confluence with Hancock Gulch.

7.5YR-4/6 strong brown

99ABFC-141C 37.823231 107.675743 Lower Cement Alluvial ferricrete dry 2 Tg 10 to 6 Ferricrete, clasts 2 in. to sand, subrounded, moderately sorted. 7.5YR-4/6 strong brown

99ABFC-151 37.859333 107.677597 Middle Cement Alluvial ferricrete dry 2 T? 6 to .5 Alluvial ferricrete, clast size 10 in. to 

fine sand, subangular, moderately sorted, sample from west bank Cement Creek above confluence with Porcupine Gulch.

10YR-4/6 dk yellowish brown

99ABFC-154D 37.878017 107.676430 Upper Cement Alluvial ferricrete alluvial 5 p .5 to 0 Alluvial ferricrete, clast size 6 in. to fine 

sand, subrounded, moderately sorted.

7.5YR-3/4 dk brown

99ABFC-155 37.889320 107.672768 Upper Cement Alluvial ferricrete alluvial 2 p .5 to 0 Alluvial ferricrete, clasts 6 in. down to 

fine sand, subangular to subrounded, poorly to moderately sorted, sample from Cement Creek below confluence with South Fork Cement Creek.

7.5YR-4/6 strong brown

99ABFC-176A 37.895859 107.645218 Upper Cement Alluvial ferricrete dry 5 T 2.2 to 0 Alluvial ferricrete, clasts 1 ft down to 

sand, subangular, moderately sorted, grades vertically into bog iron deposit, sample from Cement Creek above confluence with South Fork Cement Creek.

7.5YR-4/6 strong brown

99VMS-29 37.860157 107.705559 Mineral Cr Alluvial ferricrete dry 2 - - Red/brown matrix from ferricrete outcrop, Browns Gulch. no data

99VMS-33 37.844917 107.725212 Mineral Cr Alluvial ferricrete dry 5 - - Brown matrix from 100 ft high ferricrete 

outcrop, Middle Fork Mineral Creek. 

5YR-3/3 and 3/4 dark reddish brown
Table 2. Summary of solid-phase sample attributes, including reach, deposit type, age of host sediments, and dominant iron mineralogy.—Continued

Field No. Latitude N. (degrees) Longitude W. (degrees) Reach Deposit type Wet/Dry Dominant iron mineral1 Geomorphologic unit2 (Vincent and others, this volume) Age range of host deposit (per 1,000 years) Site description Munsell Color code (Goddard and others, 1948)

ODY99-09 37.844189 107.669304 Mineral Cr Alluvial ferricrete dry 2 - - Alluvial ferricrete deposit consists of 

subangular to subrounded cobbles with some boulder-size clasts in a granular, iron-cemented matrix (along Middle Fork Mineral Creek).

7.5YR-4/6 strong brown

99ABFC-115 37.886402 107.656769 Upper Cement Colluvial ferricretes dry 2 Fo or P 10 to 0 7.5YR-4/4 brown99ABFC-131 37.851616 107.667870 Middle Cement Colluvial ferricretes dry 5 F 10 to 0 Ferricrete, large boulders to sand, 

angular to subrounded clasts, poorly sorted, sample from Cement Cr immediately below Ohio Gulch.

7.5YR-4/6 strong brown

99ABFC-133 37.850750 107.668449 Middle Cement Colluvial ferricretes dry 2 F 10 to 0 Ferricrete, angular clasts from Illinois 

Gulch, 1 ft to sand, poorly sorted, below Yukon tunnel.

7.5YR-4/6 strong brown

99ABFC-134 37.845634 107.669815 Middle Cement Colluvial ferricretes dry 2 Fo 10 to 6 Ferricrete, clasts 1 ft to sand, mostly 

subrounded, sample from old Topeka Gulch fan.

5YR-4/6 yellowish red

99ABFC-153 37.873817 107.677628 Upper Cement Colluvial ferricretes dry 2 T? 6 to .5 Alluvial ferricrete, clast size 3 in. to fine 

sand, subrounded, moderately sorted, grades upward into deposit angular and poorly sorted clasts, sample from Cement Creek above confluence with Fairview Gulch. 

10YR-4/6 dk yellowish brown

99ABFC-170 37.909893 107.650520 Upper Cement Colluvial ferricretes dry 2 colluvium 10 to 0 Colluvial ferricrete from above entrance 

to Mogul mine, fine-grained matrix sampled.

5YR-4/6 yellowish red

99ABFC-175B 37.902031 107.649010 Upper Cement Colluvial ferricretes colluvial 2 T or p 6 to 0 Colluvial ferricrete, manganese-rich 

hydrous iron oxide, east side of Cement Creek just above Red and Bonita mine.

no data

99ABFC-185C 37.864086 107.643883 Upper Cement Colluvial ferricretes dry 2 colluvium 10 to .5 Ferricrete, clasts 1 ft down to sand, 

angular to subangular, poorly sorted, grades vertically into bog iron deposit, sample from Dry Gulch.

7.5YR-4/6 strong brown

99VMS-09 37.823612 107.643753 Mineral Cr Colluvial ferricretes dry 2 - - Matrix from Zuni Gulch angular 

ferricrete, sample of float.

5YR 4/6 yellowish red

99VMS-12 37.862469 107.646286 Mineral Cr Colluvial ferricretes colluvial 5 - - Matrix from ferricrete in outcrop above 

Imogene mine.

10R-3/2 dusty red

ODY98-31B 37.833069 107.615082 Mineral Cr Colluvial ferricretes spring 5 - - Colluvial ferricrete deposit consists 

of angular to subangular pebblesize clasts in a granular iron oxide cemented matrix (near active iron spring, along north-draining tributary to Middle Fork Mineral Creek).

7.5YR-4/4 brown

788 Environmental Effects of Historical Mining, Animas River Watershed, Colorado
Processes Controlling Formation of Ferricrete 789

Table 2. Summary of solid-phase sample attributes, including reach, deposit type, age of host sediments, and dominant iron mineralogy.—Continued

Field No. Latitude N. (degrees) Longitude W. (degrees) Reach Deposit type Wet/Dry Dominant iron mineral1 Geomorphologic unit2 (Vincent and others, this volume) Age range of host deposit (per 1,000 years) Site description Munsell Color code (Goddard and others, 1948)

ODY98-32b 37.833076 107.612541 Mineral Cr Colluvial ferricretes spring 5 - - Middle of 8-in. long core in active iron 

spring consists of angular pebble-size clasts in a sand-size light-reddishbrown matrix (north-draining tributary to Middle Fork Mineral Creek).

5YR-4/6 yellowish red

SDY00-17A2 37.849567 107.753159 Middle Cement Colluvial ferricretes dry 2 colluvium 10 to 0 Colluvial ferricrete deposit consists of 

angular to subangular cobbles and pebbles, some boulder-size clasts in a granular iron oxide cemented matrix (sampled above SDY00-17A1 in Topeka Gulch).

no data

SDY00-2 37.840351 107.758644 Middle Cement Colluvial ferricretes dry 2 colluvium 10 to 0 Colluvial ferricrete deposit consists of 

angular cobbles cemented by a finely layered iron oxide cement, sampled near Niagara Gulch.

7.5YR-4/6 strong brown

SDY00-28 37.834534 107.758644 Mineral Cr Colluvial ferricretes dry 5 - - Colluvial ferricrete deposit consists of 

subangular to subrounded pebbles and cobbles, with several boulder-size clasts in a granular, iron-cemented matrix (west of and above Mineral Creek between Middle and South Fork confluences).

7.5YR-5/8 strong brown

SDY99-14 37.841110 107.688698 Mineral Cr Colluvial ferricretes dry 2 - - Colluvial ferricrete deposit consists of 

subangular to subrounded pebbles and cobbles, some boulder-size clasts in a granular, iron-cemented matrix (sampled on river right bank near northerly draining tributary to Middle Fork Mineral Creek).

10YR-4/6 dk yellowish brown

99ABFC-122C1 37.880260 107.658249 Upper Cement Iron bog bog 1 T or p? 4 to 0 Iron bog, fine-grained hydrous iron 

oxides, east side Cement Creek below confluence with Prospect Gulch (sample dried before mineralogical analysis).

2.5YR-4/6 red

99ABFC-122D1 37.880260 107.662880 Upper Cement Iron bog bog 1 T 4 to .5 Iron bog, fine-grained hydrous iron 

oxides, west side Cement Creek above upper bog (sample dried before mineralogical analysis).

7.5YR-5/6 strong brown

99ABFC-126 37.865639 107.666283 Middle Cement Iron bog dry 2 Fy 6 to 0 Iron bog deposit, fine-grained iron 

oxides, sample from west bank of Cement Creek above confluence with Minnesota Gulch in alluvial fan.

5YR-3/4 dk reddish brown

99ABFC-141B 37.823231 107.676201 Lower Cement Iron bog dry 1 Tg 4.5 Bog iron, fine-grained hydrous iron 

oxides at top of thick ferricrete section, sample from road cut on east side Cement Creek just above Silverton.

7.5YR-4/6 strong brown
Table 2. Summary of solid-phase sample attributes, including reach, deposit type, age of host sediments, and dominant iron mineralogy.—Continued

Field No. Latitude N. (degrees) Longitude W. (degrees) Reach Deposit type Wet/Dry Dominant iron mineral1 Geomorphologic unit2 (Vincent and others, this volume) Age range of host deposit (per 1,000 years) Site description Munsell Color code (Goddard and others, 1948)

99ABFC-161 37.883244 107.669373 Upper Cement Iron bog dry 2 bog on T or p 6 to 0 Bog iron deposit, fine-grained iron oxides, 

sample from South Fork Cement Creek.

5YR-3/4 dk reddish brown

99ABFC-162A 37.869652 107.669373 Upper Cement Iron bog dry 1 bog on T or p 6 to 0 Bog iron deposit, fine-grained iron oxides, 

sample from head of South Fork Cement Creek.

5YR-3/4 dk reddish brown

99ABFC-173 37.904663 107.650108 Upper Cement Iron bog dry 2 T or p 6 to 0 Bog iron deposit, fine-grained hydrous 

iron oxides, west side of Cement Creek downstream from Mogul mine.

5YR-4/6 yellowish red

99ABFC-176B 37.895859 107.644287 Upper Cement Iron bog dry 2 T 2.2 to 0 Bog iron deposit, fine-grained iron oxides 

with wood fragments.

2.5YR-4/6 dk red

99VMS-14 37.870556 107.611313 Mineral Cr Iron bog mine 1 - - Ferricrete soil from hillside above (west 

side) Chattanooga beaver ponds.

5YR-4/6 yellowish red

99VMS-92 37.816113 107.725830 Mineral Cr Iron bog bog 1 - - Iron bog area along Mineral Creek below 

Columbine mine.

5YR-3/3 dk reddish brown

IDY00-14 37.894455 107.726669 Upper Cement Iron bog dry 1 ? 10 to 0 Bog iron deposit consists entirely of iron 

oxide matrix with finely layered texture (near mouth of North Fork Cement Creek).

7.5YR-4/6 strong brown

IDY00-29A 37.883202 107.697220 Upper Cement Iron bog dry 1 colluvium? 10 to 0 Banded bog iron deposit consists of 

alternating 3 mm to 10 mm thick iron oxide matrix layers, dark-brown bands in hand sample (near lower Prospect Gulch spring). 

5YR-4/6 yellowish red

IDY00-29B 37.883202 107.566333 Upper Cement Iron bog dry 1 colluvium? 10 to 0 Banded bog iron deposit consists of alternating 3 mm to 10 mm thick iron oxide matrix layers, yellow bands in hand sample 

(near lower Prospect Gulch spring).

10YR-5/8 yellowish brown

SDY00-17A1 37.849567 107.669304 Middle Cement Iron bog dry 2 colluvium 10 to 0 Bog iron deposit consists entirely of iron 

oxide matrix with finely layered texture (sample from Topeka Gulch).

7.5YR-3/4 dk brown

95-ABS-117 37.879667 107.671776 Upper Cement Iron bog bog 1 T or p 6 to 0 Upper iron bog, Cement Creek (total 

digestion analysis).

no data

95-ABS-119 37.897034 107.612030 Upper Cement Iron bog dry 2 ? 10 to 0 Bog iron deposit exposed on upper Cement 

Creek below Red and Bonita mine (partial digestion analysis).

no data

97ABS-318a 37.816307 107.668800 Mineral Cr Iron bog bog 1 - - Fine-grained iron bog deposit, depth 1.25 in. 

below surface, north bank South Fork Mineral Creek.

2.5YR-3/4 dk reddish brown

97ABS-318b 37.816307 107.675377 Mineral Cr Iron bog bog 1 - - Fine-grained iron bog deposit, depth 4.75 in. 

below surface, north bank of South Fork Mineral Creek.

5YR-4/6 yellowish red

97ABS-318c 37.816307 107.736839 Mineral Cr Iron bog bog 1 - - Fine-grained iron bog deposit, depth 7.25 in. 

below surface, north bank South Fork Mineral Creek.

2.5YR-2.5/4 dk reddish brown

97ABS-323a 37.879667 107.729752 Upper Cement Iron bog bog 1 T or p 6 to 0 Fine-grained iron bog deposit, depth 0.5 in., 

west bank of Cement Creek, upper bog below confluence with Prospect Gulch.

5YR-4/6 yellowish red

790 Environmental Effects of Historical Mining, Animas River Watershed, Colorado
Processes Controlling Formation of Ferricrete 791

Table 2. Summary of solid-phase sample attributes, including reach, deposit type, age of host sediments, and dominant iron mineralogy.—Continued

Field No. Latitude N. (degrees) Longitude W. (degrees) Reach Deposit type Wet/Dry Dominant iron mineral1 Geomorphologic unit2 (Vincent and others, this volume) Age range of host deposit (per 1,000 years) Site description Munsell Color code (Goddard and others, 1948)

97ABS-323b 37.879667 107.729752 Upper Cement Iron bog bog 1 T or p 6 to 0 Fine-grained iron bog deposit, depth 

4.25 in., west bank of Cement Creek upper bog below confluence with Prospect Gulch.

2.5YR-2.5/4 dk reddish brown

97ABS-323c 37.879667 107.729752 Upper Cement Iron bog bog 1 T or p 6 to 0 Fine-grained iron bog deposit, depth 

6.75 in., west bank of Cement Creek upper bog below confluence with Prospect Gulch.

5YR-4/6 yellowish red

97ABS-325a 37.879667 107.729752 Upper Cement Iron bog bog 1 T or p 6 to 0 Fine-grained iron bog deposit, depth 2 in., 

east bank of Cement Creek upper bog below confluence with Prospect Gulch.

5YR-4/6 yellowish red

97ABS-325b 37.879667 107.729752 Upper Cement Iron bog bog 1 T or p 6 to 0 Fine-grained iron bog deposit, depth 4 in., 

east bank of Cement Creek upper bog below confluence with Prospect Gulch.

5YR-4/6 yellowish red

97ABS-325c 37.879667 107.736639 Upper Cement Iron bog bog 1 T or p 6 to 0 Fine-grained iron bog deposit, depth 

11.75 in., east bank of Cement Creek upper bog below confluence with Prospect Gulch.

5YR-4/6 yellowish red

97ABS-325d 37.879667 107.736444 Upper Cement Iron bog bog 1 T or p 6 to 0 Fine-grained iron bog deposit, depth 11 in., 

east bank of Cement Creek upper bog below confluence with Prospect Gulch.

5YR-4/6 yellowish red

99ABFC-102B 37.889351 107.668333 Upper Cement Iron spring dry 2 T or p 6 to 0 Laminar iron springs deposit overlying 

alluvial ferricrete, 1.5 ft above low-flow water level, sample collected from north bank of Cement Creek above confluence with Dry Gulch.

5YR-4/6 yellowish red

99ABFC-139 37.831554 107.675316 Lower Cement Iron spring dry 2 T or p 6 to 0 Iron spring deposit, fine-grained hydrous 

iron oxides, sample from east bank of Cement Creek above confluence with Soda Gulch.

7.5YR-4/6 strong brown

99ABFC-146A 37.883030 107.676003 Upper Cement Iron spring spring 1 Fo 10 to 6 Iron spring deposit, fine-grained hydrous 

iron oxides from lower spring on Prospect Gulch.

7.5YR-3/4 dk brown

99ABFC-154A1 37.878017 107.673035 Upper Cement Iron spring spring 1 p .5 to 0 Iron spring deposit, laminated iron oxides 

in dormant mound, east side of Cement Creek above confluence with Tiger Gulch.

2.5YR-3/6 dk red

99ABFC-154A2 37.878017 107.671127 Upper Cement Iron spring spring 1 p .5 to 0 Iron spring deposit, active at base. no data 99ABFC-154B1 37.878017 107.664719 Upper Cement Iron spring spring 1 p .5 to 0 Iron springs deposit,1 ft above low-flow 

water level, east side of Cement Creek (sample dried prior to mineralogical analysis).

5YR-4/6 yellowish red

99ABFC-154B2 37.878017 107.664719 Upper Cement Iron spring spring 1 p .5 to 0 Iron springs deposit,1 ft above low-flow 

water level, east side of Cement Creek (wet sample preserved for mineralogical analysis).

no data 
Table 2. Summary of solid-phase sample attributes, including reach, deposit type, age of host sediments, and dominant iron mineralogy.—Continued

Field No. Latitude N. (degrees) Longitude W. (degrees) Reach Deposit type Wet/Dry Dominant iron mineral1 Geomorphologic unit2 (Vincent and others, this volume) Age range of host deposit (per 1,000 years) Site description Munsell Color code (Goddard and others, 1948)

99ABFC-154C 37.878017 107.666595 Upper Cement Iron spring spring 5 p .5 to 0 Iron springs deposit, 1 ft above low-flow 

water level, west side of Cement Creek.

7.5YR-4/6 strong brown

99ABFC-159A 37.887234 107.669373 Upper Cement Iron spring dry 5 T 6 to .5 Large iron spring deposit, finely laminated 

hydrous iron oxides with plant material, twigs appear very young, sample from talus slope on south side of South Fork Cement Creek above confluence.

5YR-4/6 yellowish red

99ABFC-164A 37.870098 107.655220 Upper Cement Iron spring spring 2 colluvium 10 to 0 Colluvial ferricrete, fine-grained matrix 

sampled, probably an iron spring deposit, head of South Fork Cement Creek.

5YR-4/6 yellowish red

99VMS-37 37.849445 107.727798 Middle Cement Iron spring spring 1 - - Brown, organic-rich iron spring deposit, 

saddle along ridge between Mineral and Cement Creeks, south of Ohio Peak.

 no data

99VMS-65A 37.865276 107.725830 Mineral Cr Iron spring dry 2 - - Brown iron from outcrop of paleo spring 

just south of Chattanooga beaver ponds.

no data 

99VMS-65B 37.865276 107.725830 Mineral Cr Iron spring dry 1 - - Light-brown iron from outcrop of paleo 

spring just south of Chattanooga beaver ponds.

no data 

ODY98-32a 37.833076 107.609062 Mineral Cr Iron spring spring 2 - - Top of 8-in. long core in active iron spring 

consists of angular pebble-size clasts in a medium-brown sand-size matrix (north-draining tributary to Middle Fork Mineral Creek). 

5YR-3/4 dk reddish brown

ODY98-32c 37.833076 107.646027 Mineral Cr Iron spring spring 1 - - Bottom of 8-in. long core in active iron 

spring consists of fine silt to sand-size sediment, dark brown in hand sample (north-draining tributary to Middle Fork Mineral Creek).

2.5YR-3/6 dk red

00ABFC-230 37.919400 107.664452 Cal Gulch manganocrete Mn 5 - - Alluvial deposit, small rounded gravel 

clasts with manganese-oxide rich matrix (sampled in California Gulch).

no data 

00CGS-1150 37.923161 107.650688 Cal Gulch manganocrete Mn 5 - - Black matrix of subangular clasts 

conglomerate.

no data 

99VAS-01 37.919548 107.637695 Cal Gulch manganocrete Mn 5 - - Upper California Gulch, black precipitate 

as apron at spring, matrix supported.

no data 

99VMS-13A1 37.868332 107.646286 Mineral Cr manganocrete Mn 5 - - Black matrix from outcrop of draining 

adit above Chattanooga beaver ponds, Ferricrete mine.

no data 

HPDY00-13 37.910595 107.710800 Cal Gulch manganocrete Mn 5 - - Colluvial manganocrete deposit consists 

of subangular cobbles and pebbles, some boulder-size clasts in a granular, managanese and iron-cemented matrix (in Placer Gulch below Gold Prince mine).

7.5YR-2.5/2 very dk brown

792 Environmental Effects of Historical Mining, Animas River Watershed, Colorado
Processes Controlling Formation of Ferricrete 793

Table 2. Summary of solid-phase sample attributes, including reach, deposit type, age of host sediments, and dominant iron mineralogy.—Continued

Field No. Latitude N. (degrees) Longitude W. (degrees) Reach Deposit type Wet/Dry Dominant iron mineral1 Geomorphologic unit2 (Vincent and others, this volume) Age range of host deposit (per 1,000 years) Site description Munsell Color code (Goddard and others, 1948)

HPDY00-16 37.904644 107.730995 Animas manganocrete Mn 5 - - Alluvial manganocrete deposit consists 

of subround to subangular pebble-size clasts, in a sand-size, manganese-rich cemented matrix (base of alluvial fan in Eureka Gulch).

7.5YR-3/3 dk brown

HPDY00-4 37.923977 107.701668 Cal Gulch manganocrete Mn 5 - - Alluvial manganocrete deposit consists 

of subangular pebble- to cobble-size clasts in a sand-size manganese-rich cemented matrix (base of alluvial fan in California Gulch).

10YR-4/3 brown

HPDY00-6 37.906002 107.726669 Animas manganocrete Mn 5 - - Colluvial manganocrete deposit consists 

of angular to subangular pebble- and cobble-size clasts in a granular, sandsize manganese and iron-cemented matrix (at head of Eureka Gulch).

7.5YR-2.5/3 very dk brown

00ABM-275 37.844223 107.676445 Mineral Cr mine-impacted mine 1 mine ppt 0 Bonner mine precipitate. 2.5YR-3/6 dk red99VMS-10 37.851429 107.644257 Mineral Cr mine-impacted spring 5 mine ppt 0 Iron floc from seep in colluvium 30 ft 

upstream of Burro Bridge, east side of Mineral Creek; possibly from caved adit(?).

no data 

99VMS-13A2 37.868332 107.647652 Mineral Cr mine-impacted dry 5 mine ppt 0 Brown matrix from outcrop of draining 

adit above Chattanooga beaver ponds, Ferricrete mine.

5Y-3/4 dk reddish brown

99VMS-13B 37.868332 107.679100 Mineral Cr mine-impacted mine 1 mine ppt 0 Brown floc from effluent of adit above 

Chattanooga beaver ponds, Ferricrete mine.

2.5YR-4/8 red

99ABFC-102Ar 37.889351 107.668333 Upper Cement rep exclude 1 T 6 to .5 Replicate sample. 5YR-3/4 dk reddish brown99ABFC-122D2 37.880260 107.665688 Upper Cement rep exclude 2 T 4 to .5 Replicate sample (wet sample preserved 

for mineralogical analysis).

no data

1Mineralogy: 1=schwertmannite; 2=goethite; 3=jarosite; 4=pyrite; 5=amorphous iron oxides.2p = flood plain, T = terrace; bed =streambed; Fo = old fan; Fy = young fan; Tg = gravel terrace; “-” = not determined.
Table 3. Characteristics of ferricrete sampled in this study.

Type of

ferricrete

No. of

samples

Typical

iron-oxide

mineralogy

Typical

sedimentary

features

Depositional

environment

Observed

range in

water pH

Major-element

chemistry

Precipitate-type (commonly wet)

Bog iron 26 Schwertmannite> Few clasts; Shallow wetlands <2 wt. % Al

Iron spring 15 goethite> sometimes banded. Sides of valleys 40–60 wt. % Fe

Total 41 ferrihydrite 3.0 to 4.5 1–3.5 wt. % S

Conglomerate-type (commonly dry)

Alluvial 21 Goethite> Rounded clasts or Stream terraces 3–5 wt. % Al

Colluvial 16 schwertmannite> subangular clasts. Hillslope deposits 25–35 wt. % Fe

Total 37 ferrihydrite 3.0 to 6.6 <1.25 wt. % S

Manganocrete (wet or dry)

(predominantly the

colluvial-type was

sampled)

8 Manganese-rich

ferrihydrite.

Rounded clasts or 

subangular clasts.

Colluvial or alluvial 

deposits.

4.5 to 4.8 6–9 wt. % Al

<8 wt. % Fe

<0.5 wt. % S

Sampling bias must be considered because ground water 

could not be collected at depth. Samples were only collected 

where seeps and springs emerge at the ground surface. Small 

differences in sampling approach by different coauthors 

also could contribute to bias. In Cement Creek, the precise 

sampling location was selected on the basis of low dissolvedoxygen content, whereas in Mineral Creek, dissolved oxygen 

was not measured and other field criteria, such as pH and conductivity, were used to select the sampling site. Water samples 

collected for this study represent a variety of non-equilibrium 

surficial conditions, and thus may not collectively represent 

the pH and mineral solubilities of deeper circulating ground 

water. While recognizing the potential for bias, we attribute 

the large spatial differences in ground-water chemistry primarily to differences in pH that correlate spatially with the degree 

of iron-sulfide mineralization and the irregular distribution of 

saturated fractures.

The geochemical modeling program PHREEQCI 

(version 2.7.1, Parkhurst and Appelo, 1999) was used to 

determine speciation and state of saturation with respect to 

secondary mineral phases for the water samples in table 1. 

A log activity of –18.0 for schwertmannite as determined by 

Bigham, Schwertmann, and others (1996), was added to the 

WATEQ4F thermodynamic database (Ball and Nordstrom, 

1991) used with PHREEQE. Iron solubility was calculated for 

common iron solid-phase species including amorphous ferrihydrite [Fe(OH)3

 (a)], goethite (FeOOH), potassium or K-jarosite 

(KFe3

(SO4)

2

(OH)6), and schwertmannite (Fe8O8(OH) 4.5

(SO4

)

1.75). In Mineral Creek, Fe(OH)+2 was the most abundant 

dissolved species of Fe+3 for all but one sample, whereas in 

upper Cement Creek, FeSO4

–

 was the dominant dissolved-iron 

species for 9 out of 12 samples. For the most part, FeSO4

–

 was 

the dominant aqueous iron species whenever pH <4.0, which 

was more common in Cement Creek. Spatial differences in 

iron speciation and pH are dynamic, and variability may be 

attributed in part to the rock chemistry, length of the flow path, 

degree of mixing, and to diurnal and seasonal variations in 

hydrologic conditions.

Plots of pH versus the log activity of iron (pFe) are a 

conventional way of representing the solubility of minerals, 

assuming log activity of sulfate (pSO4

) is constant. Figure 2 is 

a plot of pFe versus pH for the minerals ferrihydrite, goethite, 

potassium jarosite, and schwertmannite. Mineral species most 

likely to precipitate have positive or near-zero log activities, 

and thus plot in the positive or lower half of the graph. Data in 

figure 2 and table 4 indicate that all of the sampled iron-rich 

ground water is saturated with goethite, more than half of the 

water is saturated with schwertmannite and potassium jarosite, 

and slightly less than half the samples are saturated with ferrihydrite (Fe(OH)3

). Bigham, Schwertmann, and others (1996) 

and Yu and others (1999) have shown that schwertmannite 

solubility is intermediate between the solubilities of potassium 

jarosite and goethite. Many of these water samples are not 

fully equilibrated with oxygen, which may account for some 

of the undersaturated values. Complete saturation with oxygen 

would tend to further increase saturation (precipitation) of iron 

hydroxide minerals, and decrease the remaining iron in solution. The data show that iron saturation is common and occurs 

in a wide range of pH values. Iron saturation occurs in both 

mining-impacted and non-impacted ground water discharging 

to acidic springs throughout the Cement Creek and Mineral 

Creek watersheds.

In addition to the iron hydroxide minerals shown in 

figure 2, iron-rich ground water was highly saturated or near 

saturation with respect to amorphous SiO2

 (crystobalite; 

table 4), sulfate minerals including gypsum (CaSO4

•2H2O), 

anhydrite (CaSO4

), celestite (SrSO4), melanterite 

(FeSO4

•7H2O); and with hydroxide minerals alunite (KAl3

(SO4

)2(OH)6), gibbsite (Al(OH)3), and amorphous Al(OH)3. 

In contrast, manganese-rich ground water was undersaturated 

794 Environmental Effects of Historical Mining, Animas River Watershed, Colorado
Processes Controlling Formation of Ferricrete 795

Figure 2. Plot of log activity of iron (pFe) versus pH for iron-rich ground water in Animas River watershed study area. A positive 

pFe value indicates that the mineral is likely to precipitate from solution.

MINERAL SOLUBILITY

Fe(OH)3

Mineral Creek

Mine adit

Cement Creek

Mineral Creek

Mine adit

Cement Creek

Mineral Creek

Mine adit

Cement Creek

K-jarosite Goethite

Mineral Creek

Mine adit

Cement Creek

Schwertmannite

pFe

pH, IN STANDARD UNITS

234567

28

24

20

16

12

8

4

0

-4

-8

-12

-16

-20

-24

-28

-32

-36
Table 4. Saturation indices (log activities) for iron oxyhydroxide minerals and crystobalite in iron-rich ground water in Cement and Mineral Creeks.

[See figures 1 and 2. Activity data calculated in PHREEQCI 2.7.1 using modified WATEQ4F database with log activity of –18.0 for schwertmannite, based on Bigham, Schwertmann, and others (1996). A positive saturation indices value (italic) indicates that the solution is saturated with respect to that mineral]

Site Date Reach pH pe1 Eh1

Fe(II) Fe(III) Saturation indices

(μg/L) Ferrihydrite Fe(OH)3(a)2 Goethite FeOOH K-jarosite KFe3(SO4

)2 (OH)6

Schwertmannite

Fe O8 8(OH)4.5

(SO4

)1.75

Crystobalite

SiO2(a)

Iron-bearing springs:99VMF10* 8/15/1999 Mineral Creek 6.57 6.98 0.388 6.9 6.9 4.16 9.35 5.16 26.79 0.2699VMF34 8/24/1999 Mineral Creek 5.81 5.15 0.294 0.05 <0.05 –0.72 4.76 –10.58 –12.64 –0.36

99VMF67 8/24/1999 Mineral Creek 4.16 8.76 0.479 <0.01 <0.01 –0.17 4.84 –7.81 –6.59 0.27

99VMF68 8/24/1999 Mineral Creek 3.99 11.95 0.657 <0.01 <0.01 –2.34 2.75 –8.70 –17.61 0.28

99VMF92 9/1/1999 Mineral Creek 4.41 10.98 0.614 8.4 8.0 1.90 7.19 4.20 15.34 0.2499VMF72 8/25/1999 Mineral Creek 3.33 11.87 0.659 1.2 0.3 –1.42 3.77 –2.57 11.89 0.2400VMF01 9/6/2000 Mineral Creek 4.26 9.85 0.541 30 0.54 0.70 5.77 0.83 6.84 0.5300VMF03 8/31/2000 Mineral Creek 4.12 10.60 0.593 4.2 0.49 0.25 5.53 0.66 4.04 0.13PG fan 8/19/2002 Cement Creek 3.32 11.89 0.670 38 18.1 0.15 5.52 3.42 5.69 0.42YS-LB 10/25/2000 Cement Creek 3.51 12.03 0.665 20 11 0.46 5.60 2.80 7.56 0.00YS-RB 10/25/2000 Cement Creek 3.22 11.89 0.661 20 9 –0.64 4.58 1.49 0.40 0.15Gas Spring 8/23/2002 Cement Creek 4.15 11.59 0.644 26 34 2.28 7.49 6.78 19.67 0.48Ren Spg 8/23/2002 Cement Creek 3.55 12.19 0.680 13 16 0.73 5.97 4.18 9.67 0.40UB-1 10/25/2000 Cement Creek 3.15 12.04 0.697 20 45 –0.22 5.43 3.89 3.74 –0.02GT-1 9/24/1999 Cement Creek 3.20 12.35 0.699 0.15 0.29 –2.16 3.26 –2.84 –11.85 0.35

GT-2 9/27/1999 Cement Creek 3.00 11.85 0.668 17 9.3 –1.30 4.06 0.56 –4.07 0.31GT-3 9/27/1999 Cement Creek 4.62 4.88 0.277 43 0 –3.00 2.45 –9.11 –23.30 0.28

GT-4 9/27/1999 Cement Creek 3.79 9.65 0.549 67 0.7 –0.46 5.04 1.11 –0.06 0.29GT-7 9/27/1999 Cement Creek 3.33 11.56 0.664 23 12 –0.41 5.16 2.72 1.90 0.21GT-8 9/27/1999 Cement Creek 3.59 8.40 0.477 0.32 0.004 –4.47 0.96 –11.51 –32.22 0.16

Mining-affected springs or adit of inactive mine:L3A (adit) 9/29/1999 Prospect Gulch 4.14 10.09 0.549 22 0.4 1.50 6.47 7.82 15.80 1.86JJP (adit) 9/29/1999 Prospect Gulch 2.73 14.86 0.815 0.41 62 –0.84 4.21 1.19 –0.15 0.56H7A (adit) 9/29/1999 Prospect Gulch 2.52 13.55 0.740 23 268 –1.17 3.84 1.43 –1.36 –0.0399VMF13 8/23/1999 Mineral Creek 5.75 8.62 0.476 12 12 3.72 8.86 5.51 25.55 0.2599VMF12 8/23/1999 Mineral Creek 5.62 9.07 0.500 5.8 5.8 3.28 8.38 4.25 22.49 0.25

1Calculated on the basis of relative concentrations of iron species (Fe+2/Fe+3).2(a) = amorphous.*Possibly mining affected (table 1).

796 Environmental Effects of Historical Mining, Animas River Watershed, Colorado
Processes Controlling Formation of Ferricrete 797

with respect to alunite and gibbsite, although saturated with 

respect to amorphous Al(OH) 3 and amorphous SiO2

. Water 

samples from California Gulch were highly saturated with 

respect to manganese-bearing minerals including manganite 

(MnOOH), rhodochrosite (MnCO3

), and pyrolusite (MnO2) as 

well as with carbonate minerals, commonly calcite and aragonite (CaCO3

), and dolomite (CaMg(CO3)2). The occurrence of 

manganese-rich ground water is consistent with dissolution of 

pyroxmangite ((Mn,Fe,Ca,Mg)SiO3) and rhodochrosite from 

ore veins throughout the Eureka Gulch area.

Figure 3 is a plot of pe versus pH and a phase diagram 

for the Fe-S-K-O-H system calculated using the solubility 

product (Ksp = –18.0) for schwertmannite, as determined by 

Bigham, Schwertmann, and others (1996). Potential energy 

(pe—referring to the redox states of ion pairs) as a function 

of Eh was calculated in PHREEQCI, on the basis of total 

Fe+2 and Fe+3 concentrations. Over the pH range of 3 and 4.5, 

schwertmannite is the dominant phase for most of the ironrich ground water in Cement Creek and Mineral Creek. For 

the two mine adit samples with pH <3, jarosite is metastable 

with respect to schwertmannite and goethite. Above a pH of 5, 

ferrihydrite is the dominant form, but it too has an overlapping 

stability field with schwertmannite at lower pH and goethite 

at lower pe. Only two Mineral Creek samples plot in the 

overlapping schwertmannite-goethite stability field and one 

Mineral Creek sample (pH=5.8) was in the schwertmanniteferrihydrite transitional field. Based on other studies (Bigham, 

Schwertmann, and others, 1996; Bigham, Schwertmann, and 

Pfab, 1996; Yu and others, 1999), goethite either precipitates directly or forms by hydrolysis from schwertmannite or 

ferrihydrite. Samples from this study plot predominantly in 

the non-overlapping part of the schwertmannite field. This is 

consistent with mineralogical analyses presented in the next 

section, showing both schwertmannite and goethite as the most 

common iron oxyhydroxide minerals in Mineral and Cement 

Creek basins.

Solid-Phase Geochemistry of IronCemented Deposits and Stream 

Sediment

The major-element and trace-element chemistry of the 

solid-phase samples (table 2) is evaluated with respect to 

deposit type, mineralogy, relative age of the host deposit, spatial location, and whether the deposit appeared to be actively 

precipitating iron.

Deposit Type

Alluvial and colluvial ferricrete, bog iron, iron spring, 

and manganocrete deposits were sampled to identify major 

chemical characteristics for each deposit type. As a point of 

reference, the solid-phase chemistry of iron and manganese 

cemented deposits is compared with that of premining and 

modern stream sediment collected by Church, Fey, and Unruh 

(this volume).

The distribution of major and trace elements among the 

solid-phase ferricrete and stream-deposited sediment varied 

substantially. The range in selected major elements (Al, Ca, 

Fe, K, Mg, and S) and trace elements (As, Cd, Cu, Mn, Pb, 

and Zn) is depicted by a series of box plots (fig. 4) that are 

grouped by ferricrete deposit type. The most obvious difference between the deposit types among the major elements is 

the iron content (fig. 4). Bog iron, iron spring, and miningaffected precipitates had nearly twice as much iron by weight 

percent as did alluvial and colluvial ferricrete. The iron 

content by weight in the matrix of most alluvial and colluvial ferricrete samples was between about 25 and 35 percent. 

Assuming that most of the iron cement is present as FeOOH 

(goethite), this accounts for about 40–55 percent of the total 

weight of the digested matrix. In contrast, the iron content 

of most bog iron and spring deposits is considerably higher, 

ranging from about 40 to 60 percent Fe—or 63 to as much 

as 95 percent goethite, by weight. If one alternately assumes 

that a major fraction of the iron is amorphous schwertmannite 

(Fe8

O8(OH) 4.5 (SO4)1.75), the weight fraction in some of the 

samples can exceed 100 percent, consistent with X-ray diffraction results indicating that more than one form of iron oxide is 

present.

We attribute the greater weight percentages of aluminum, calcium, potassium, and magnesium in the alluvial and 

colluvial ferricrete samples (fig. 4) to a bias introduced by the 

sample-preparation method. Whereas the bog iron and spring 

samples consist of relatively pure iron-oxide precipitate, it was 

virtually impossible to completely disaggregate the chemically 

precipitated matrix from sand and silt derived from the depositional environment. Thus, the matrix of alluvial and colluvial 

ferricrete samples tended to have a greater weight percent of 

common rock-forming minerals than did the mining-affected 

precipitates, bog iron, and iron spring samples.

Of the major elements reported by ICP-MS, aluminum 

had the highest concentration, with the exception of iron. Alluvial and colluvial ferricrete had between 3 and 5 percent aluminum by weight, and bog iron and spring deposits generally 

had considerably less than 2 percent by weight. Manganocrete 

samples had the highest aluminum concentrations, generally 

between 6 and 9 percent. In contrast with the iron-rich samples, 

manganocrete contained less than 5 percent iron by weight but 

had the most magnesium, aluminum, and potassium of all the 

deposit types, with the exception of premining and modern 

stream sediment. Manganocrete samples also had the largest 

concentrations of cadmium, copper, lead, zinc—and (of course) 

manganese. Iron springs and bogs generally had between 1.0 

and 3.5 weight percent sulfur, compared with <1.25 percent for 

colluvial-type ferricrete and manganocrete samples. The larger 

sulfur content corresponds with greater acidity and a predominance of schwertmannite as the dominant iron mineral in bog 

iron and spring samples. Bog iron and spring deposits have 

the largest individual occurrences of arsenic and are generally 

lower in copper, lead, and manganese.
Figure 3. Phase diagram for Fe-S-K-O-H system showing pe versus pH at 25°C (modified from Bigham, 

Schwertmann, and others, 1996), and iron-rich ground-water samples from Animas River watershed study area. 

Log activity defined as pe=Eh/59.2; total log activities of Fe+2=–3.47; Fe3+=–3.36 or –2.27; SO4

2–=–2.32; K+

=–3.78. 

Solid-phase iron minerals include Jt, K-jarosite; Sh, schwertmannite; Fh, ferrihydrite; Gt, goethite; Py, pyrite. Line 

equations are Gt(pe=17.9–3pH); Jt(pe=16.21–2pH); Fh(pe=21.50–3pH); Sh(pe=19.22–2.6 pH), and Py (pe=5.39–1.14pH). 

Field of goethite metastability shown by dashed line. Dot pattern demonstrates expansion of K-jarosite and 

ferrihydrite fields if lower log K’s are selected.

Sh

Jt

Gt

Fh

Py

Dissolved 

 species

-9.21 (Jt) -12.51 (Jt) 4.5 (Fh) 5.0 (Fh)

O

 H O

H O

H

2

 2

 2

 2

2468

pH, IN STANDARD UNITS

-12

-8

-4

0

4

8

12

16

20

pe

Mineral Creek

Upper Cement Creek

Adit of inactive mine

Iron-bearing springs and ground water

EXPLANATION

798 Environmental Effects of Historical Mining, Animas River Watershed, Colorado
Processes Controlling Formation of Ferricrete 799

The distribution of major elements was consistent with 

the composition of major rock-forming minerals identified 

by X-ray diffraction. Stream-deposited sediment represented 

a composite of the major rock-forming minerals in the study 

area. Quartz was by far the most abundant rock-forming 

mineral in the cement from alluvial and colluvial ferricrete 

samples, followed by albite and muscovite. Trace levels of 

clinochlore, rutile, orthoclase, and montmorillonite were also 

present in some samples. The same minerals were also found 

in bog iron and spring deposits, albeit in lesser amounts, and 

probably represent fine-grained fluvial or windblown particles 

trapped by the bogs and springs. Likewise, minerals identified 

in the manganocrete and in the over-bank sediment samples 

primarily included quartz, followed by albite and muscovite, 

with few other trace minerals. Gangue and ore-bearing veins 

in the Sunnyside mine described by Casadevall and Ohmoto 

(1977) commonly included quartz (30–35 percent), sphalerite (10–15 percent), galena (10–15 percent), pyroxmangite 

(MnSiO3

) (10–15 percent), pyrite (6–8 percent), rhodochrosite (5–8 percent), chalcopyrite (3–5 percent), tetrahedrite 

(1–4 percent), calcite (1 percent), fluorite (1 percent); plus a 

variety of minerals with typically less than 0.5 percent abundance, including hematite, gold, anhydrite, sericite, bornite, 

barite, and gypsum.

In comparison to iron springs and bogs, conglomeratetype ferricrete had similar ranges in mean concentration for 

cadmium and zinc, but this sample group tended to be lower 

in sulfur and elevated in concentrations of copper, manganese, 

and lead. Generally, acidic pH favors the adsorption of anion 

species such as arsenic and sulfate, whereas near-neutral pH 

favors the adsorption of cations such as copper, manganese, 

and lead (Dzombak and Morel, 1987, 1990). Cadmium and 

zinc are common in ore minerals and thus common in water 

affected by mine drainage, but concentrations of these two 

trace elements did not vary substantially among ferricrete 

deposit types, despite probable differences in pH environment.

Mineralogy

Iron mineralogy varied substantially among the deposit 

types (fig. 5). For the alluvial and colluvial ferricrete samples, 

the most abundant iron mineral is goethite, although commonly 

in combination with schwertmannite. For two-thirds of the bog 

iron and iron spring samples, the major iron mineral identified was schwertmannite, usually with secondary goethite. The 

dominant iron mineral for the remaining third of bog iron and 

spring samples was goethite.

Fifteen of the eighty-six solid-phase samples were not 

included in figure 5 because they did not form statistically 

viable sample groups (n ≤ 5) or had possible mining influences 

(as indicated by Mast and others, this volume). Amorphous 

iron oxide was the only iron mineral identified in the five manganocrete samples, which also were not included in figure 5. 

Two iron spring samples and five alluvial and colluvial ferricrete samples had ferrihydrite as the dominant mineral. Also 

not included were two samples of wet alluvial ferricrete from 

the Gladstone to Fairview reach of Cement Creek, which had 

schwertmannite as the dominant iron mineral.

As mentioned in the previous section, sulfur in schwertmannite’s mineral lattice accounted for greater concentrations of sulfur in the bog iron and iron spring deposit types. 

Schwertmannite is a poorly crystalline, yellowish-brown mineral formed by the oxidation of Fe (II) in acid-sulfate systems 

(Bigham and others, 1990; Bigham and others, 1994; Bigham, 

Schwertmann, and others, 1996; Bigham, Schwertmann, and 

Pfab, 1996). It has a high specific-surface area, usually in the 

range of 100–200 m2

/g; with a fibrous or “pin-cushion” morphology (Bigham, Schwertmann, and Pfab, 1996). Bigham, 

Schwertmann, and Pfab (1996) observed the gradual transformation of schwertmannite by hydrolysis to goethite in the 

laboratory over a period of 543 days via the overall reaction:

 Fe8O8(OH) 4.5 (SO4)1.75(s) + 2.5H2O(l) Æ 8FeOOH(s)

 + 2.5H+

(aq) + 1.25SO4

2–

(aq) (5)

Yu and others (1999) have shown that metastability for 

schwertmannite is significantly affected by water temperature 

and by variations in the activity of dissolved sulfate.

At several localities we have identified banded specimens of schwertmannite with goethite and (or) ferrihydrite 

or jarosite. At least three sites had alternate banding of thin 

layers of yellow schwertmannite and dark-brown goethite. 

The banding is presumably caused by seasonal differences in 

environmental conditions such as pH and Eh conditions, the 

degree of seasonal variability at the time of precipitation such 

as wetting and drying, and (or) subsequent degree of surficial 

exposure and aging.

Iron spring and bog iron deposits dominated by schwertmannite had similar levels of trace-element concentrations as 

the iron spring and bog iron deposits dominated by goethite, 

which suggests that the aging transformation to goethite does 

not produce a net release of trace elements to the environment. 

Trace elements may sorb to the mineral surfaces or be present as a solid-series phase within the crystalline lattice. Either 

process could allow for relatively high concentrations of trace 

elements in the iron-oxide cements.

Age of Iron Minerals

Understanding the age of iron cementation is important 

for two reasons. First, some amorphous or poorly crystalline minerals are thought to undergo a transformation to a 

relatively more structured crystalline lattice with time—

schwertmannite or jarosite to goethite, for example. Second, 

speciation of iron oxyhydroxide minerals such as schwertmannite and jarosite is pH sensitive, and thus the type of iron 

mineral may indicate pH and Eh conditions during the time the 

mineral formed. Such information helps to establish the pH of 

geochemical baselines prior to mining.
Modern sediment (10)

Manganocrete (8)

Iron spring (15)

Iron bog (26)

Colluvial ferricrete (16)

Alluvial ferricrete (21)

Premining sediment (39)

EXPLANATION

90th percentile

10th percentile

75th percentile

25th percentile

Median

The information displayed in box plots 

 is summarized in the graphic below.

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no sediment 

analyses for 

comparison

A

CONCENTRATION, IN WEIGHT PERCENT

 ALUMINUM

 CALCIUM

POTASSIUM

MAGNESIUM

IRON SULFUR CONCENTRATION, IN WEIGHT PERCENT

800 Environmental Effects of Historical Mining, Animas River Watershed, Colorado
Processes Controlling Formation of Ferricrete 801

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CONCENTRATION, IN PARTS PER MILLION

ARSENIC

CADMIUM

COPPER

MANGANESE

LEAD

ZINC

CONCENTRATION, IN PARTS PER MILLION

Figure 4 (above and facing page). Box plots showing selected solid-phase elements grouped by ferricrete deposit type. A, major 

elements in weight percent; B, trace elements in parts per million. Order of box plots is same as sequence in explanation.
0

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ACONCENTRATION, IN WEIGHT PERCENT

 ALUMINUM

 CALCIUM

POTASSIUM

MAGNESIUM

IRON SULFUR CONCENTRATION, IN WEIGHT PERCENT

Schwertmannite

Iron bogs (20)

Iron springs (8)

 Goethite

Alluvial ferricrete (16)

Colluvial ferricrete (11)

Iron bogs (6)

Iron springs (5) 

 Ferrihydrite

Colluvial ferricrete (5)

EXPLANATION

90th percentile

10th percentile

75th percentile

25th percentile

Median

The information displayed in box plots 

 is summarized in the graphic below.

802 Environmental Effects of Historical Mining, Animas River Watershed, Colorado
Processes Controlling Formation of Ferricrete 803

Figure 5 (above and facing page). Box plots showing selected solid-phase elements, grouped by mineralogy. Included are 71 out of 

86 iron-cemented samples in table 2 where groups of 5 or more samples were possible. A, major elements in weight percent; B, trace 

elements in parts per million. Order of box plots is same as sequence in explanation.

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CONCENTRATION, IN PARTS PER MILLION

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LEAD

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CONCENTRATION, IN PARTS PER MILLION

B
The predominance of schwertmannite in bog iron and 

iron spring samples is important because optimum pH for 

precipitation of naturally occurring schwertmannite is between 

2.8 and 4.5 (Bigham, Schwertmann, and others, 1996; Carlson 

and others, 2002). Hence, the presence of schwertmannite in a 

dated premining deposit would be evidence for acid conditions 

predating mining activity, as would the presence of jarosite. 

Schwertmannite flocculent was observed at the Silver Ledge 

mine portal (AMLI site # 73) with a pH of 6.1; however, this 

was the only known occurrence with pH >4.5 identified within 

the study area.

Jarosite forms in mine-drainage precipitate at a pH of 2.6, 

and Bigham, Schwertmann, and others (1996) have calculated 

a jarosite stability field for which pH <3. Although weathering 

halos of jarosite were commonly observed in the study area 

on unsaturated talus where pyrite was abundant, jarosite in 

ferricrete was only detected in a few localities. It was present in alternating layers with schwertmannite and goethite in 

two samples from the iron spring on lower Prospect Gulch 

(IDY00-29A and IDY00-29B; locations in table 2). Jarosite 

and amorphous iron oxide were primary cement minerals in 

a manganocrete sample from Eureka Gulch (HPDY00-16; 

location in table 2). Trace amounts of jarosite were also 

identified in an alluvial ferricrete in Browns Gulch in Mineral 

Creek basin (99VMS-29; location in table 2). Schwertmannite 

and jarosite were also identified in an augered core from an 

alluvial fan near peak 3,792 m described in Yager and others (2000). The relative lack of jarosite in natural ferricrete, 

manganocrete, and bog iron samples is attributed to few of 

these depositional settings having a pH less than 3.0. The lowest measured pH values were generally at mine adits, or poorly 

buffered seeps emerging from pyrite-bearing rocks at higher 

altitudes (Mast and others, this volume).

Dating of ferricrete cementation is challenging, and 

two approaches exist. The better approach is to find and 

date wood or charcoal encased in lamina of iron compounds, 

because the cement covering the organics must have precipitated shortly after the organic matter was deposited (an 

approach taken by Furniss and others, 1999). Only two such 

samples were obtained in our studies. A 500-year-old piece 

of wood (99ABFC-185D) was found encased in bog iron 

deposits in Dry Gulch, and a 4,500-year-old piece of charcoal 

(99ABFC-141A) was found encased in bog iron deposits near 

the top of the iron-cemented glacial outwash exposure shown 

in figure 6. (See Vincent and others, this volume, table 2.) 

Most of the ferricrete samples in this study are cemented 

clast-supported gravel containing little organic material. 

Vincent and others (this volume, fig. 7) used 14C dating of 

encased charcoal, and stratigraphic and geomorphic relations 

to bracket the ages of clastic deposition, and thus constrain 

the age of cementation. Note that young clastic deposits had 

to have been cemented recently, whereas old clastic deposits could have been cemented either recently or at any time 

following deposition of the clasts—unless other information 

rules out the latter possibility. That other information includes 

special situations (for example, the toes of old alluvial fans) 

where the zone of cementation was permanently dewatered 

by stream incision. No such sites, however, were targeted for 

chemical analysis.

As discussed previously, poorly crystalline schwertmannite is considered metastable with respect to goethite (Bigham, 

Schwertmann, and others, 1996; Bigham, Schwertmann, and 

Pfab, 1996; Yu and others, 1999), and the results in figure 7 

support this in general. The majority of ferricrete samples have 

goethite as the dominant iron-oxide mineral, and most of the 

host sediment was deposited prior to mining (>500 yr B.P.); 

the same may or may not be true for the cement. All but one 

of the schwertmannite samples are described as wet, and thus 

the ferricrete may be actively forming even if the host deposits 

are old. Of the 11 samples whose host deposits are less than 

500 yr B.P., more than half are dominated by schwertmannite 

(6) or ferrihydrite (3).

Although the majority of iron-cemented samples 

(table 2) containing schwertmannite are coded as “wet” 

deposits that are usually found at stream level, at five sites 

schwertmannite is apparently inactive, as indicated by “dry” 

outcrops. Many of these dry or unsaturated deposits are 

potentially older than 500 yr B.P. and have been previously 

described as having a thinly laminated or banded texture, 

alternating from dark-brown to yellow bands in hand specimen (see sample pairs IDY0029A and IDY0029B, schwertmannite and jarosite, 0–10,000 years; and 99ABFC102A and 

99ABFC102Ar, schwertmannite and goethite, 500–6,000 years; 

table 2). Alternating layers may also consist of schwertmannite and amorphous iron oxide (sample pairs 99ABFC122D1 

and 99ABFC122D2, 500–4,000 years; and 99VMS65A and 

99VMS65B, age unknown, table 2). More work is required to 

understand what these anomalously old ages represent.

The one other confirmed exception is schwertmannite 

identified in cemented glacial outwash in the lower reach of 

Cement Creek. Sample pair 99ABFC141B (schwertmannite) 

and 99ABFC141C (goethite) were collected in fresh exposures 

near the top of the iron-cemented deposit that was dated at 

4,500 yr B.P. (table 2; fig. 6). The schwertmannite in this outcrop could be thousands of years old unless that sample was in 

an open system and water recently percolated into the sample 

to precipitate schwertmannite, although the sample was “dry” 

at the time it was collected. Another possible explanation is 

that the schwertmannite layer formed rapidly and was sealed 

off from both ground water and atmosphere by a goethite rind 

before complete transformation to goethite could occur. If we 

assume that the system is closed to circulating ground water, 

then iron cementation of the glacial outwash occurred following deposition when the deposit was saturated, but before 

Cement Creek began to incise sharply after 4,500 yr B.P. 

(Vincent and others, this volume) and the water table would 

have dropped along with the stream. Now exposed by a road 

cut, the massive iron-cemented outwash is perched ≈30 m 

804 Environmental Effects of Historical Mining, Animas River Watershed, Colorado
Processes Controlling Formation of Ferricrete 805

Figure 6. Iron-cemented stream gravel underlying ≥4,500-year-old bog iron containing schwertmannite and secondary goethite. 

Ages were determined by radiocarbon dating and geomorphological constraints, expressed as calendar years before present 

(cal. yr B.P.). The presence of old schwertmannite is evidence that weathering of hydrothermally altered rocks produced acid 

drainage thousands of years before mining occurred in the upper Animas River basin.

bog iron

section 

containing 

4,500 cal. yr B.P.

charcoal 

sample 

(99ABFC-141A&B)

stream gravel 

deposited before

6,000 cal. yr B.P.,

and perhaps as 

early as 12,000

cal. yr B.P.

(map unit g of 

plate 5, Vincent and 

others, this volume)

above the stream channel. If the system were closed, the 

schwertmannite may be evidence that acidic conditions were 

prevalent in Cement Creek thousands of years before mining 

occurred.

Because schwertmannite with banding is preserved in at 

least four deposits that are thought to be pre-modern in age, 

some iron-oxide minerals considered “transitional,” such as 

ferrihydrite, schwertmannite, and jarosite, could persist in 

near-surface environments if shielded by a rind of goethite. 

Schwertmannite may be more stable in certain depositional 

environments and persist longer than indicated by other 

laboratory studies (Bigham, Schwertmann, and others, 1996; 

Bigham, Schwertmann, and Pfab, 1996; Yu and others, 1999). 

Better understanding is needed to determine the geochemical 

conditions necessary for preserving schwertmannite in the 

geologic record, especially in view of the potential value of 

schwertmannite, and jarosite, as indicators of pH conditions 

in paleo-ground water.
Proximity to Mineral Assemblages, by Reach

Variations in solid-phase chemistry of ferricrete and manganocrete were also compared with respect to geographical 

proximity to exposures of acid-sulfate rock. Box plots (fig. 8) 

show the ranges in concentration of major and trace elements, 

grouped by stream reach. The stream reaches were selected on 

the basis of their relative abundance (or lack) of ferricrete or 

manganocrete.

In general, the greatest solid-phase concentrations for 

most trace elements were in manganocrete deposits from the 

headwaters of the upper Animas River. Manganocrete samples 

from Eureka Gulch and California Gulch contain one to four 

orders of magnitude more manganese (24,000–60,000 ppm) 

than ferricrete samples from Mineral and Cement Creeks 

(2–4,900 ppm) (fig. 8; Sole and others, this volume). The 

manganese is derived from local weathering of manganesebearing vein gangue minerals in the Eureka Gulch area (fig. 1) 

(Casadevall and Ohmoto, 1977; Bove and others, this volume). 

Manganese is soluble at low Eh and pH conditions, but once 

in contact with the atmosphere, it will usually precipitate as 

manganese oxide at a pH ≥4.5 (Hem, 1963, 1964, 1992). Manganese oxide is known for its scavenging ability to co-precipitate with and adsorb other trace elements (Jenne, 1968, 1975; 

Dzombak and Morel, 1987, 1990). The greatest concentrations 

of cadmium (as much as 39 ppm), lead (8,000 ppm), copper 

(3,400 ppm), and zinc (3,800 ppm) were measured in manganocrete from the California Gulch and Eureka Gulch subbasins (n=8). Several mines and prospects are nearby, although 

none directly up gradient, consistent with observations that 

the high concentrations result from close spatial proximity 

to mineralized veins. In addition to trace elements shown in 

figure 8, high manganese concentrations in the Eureka graben 

area also correlate with high concentrations of beryllium (as 

much as 16 ppm), cerium (as much as 310 ppm), lithium (as 

much as 54 ppm), and silver (as much as 20 ppm). Outside 

of California Gulch and Eureka Gulch subbasins, two other 

manganese-rich ferricretes were identified in Mineral Creek 

basin near Chattanooga and in uppermost Cement Creek basin 

near the Red and Bonita mine (AMLI site # 99; Verplanck and 

others, this volume).

02468 10

AMORPHOUS

FERRIHYDRITE

 (n = 4)

SCHWERTMANNITE

 (n = 35)

GOETHITE

 (n = 39)

AGE (1,000 YEARS)

Figure 7. Histogram showing age constraints of iron-cemented deposits in Cement Creek, grouped by dominant 

iron-oxide mineralogy. Maximum age is age of host deposit. Range in age of iron-oxide cement is the full range 

indicated by the length of the bar.

806 Environmental Effects of Historical Mining, Animas River Watershed, Colorado
Processes Controlling Formation of Ferricrete 807

Spatial proximity to mineralized veins also accounts 

for the highest arsenic concentrations, ranging from 400 

to 2,100 ppm, which appear as outlier values in the upper 

Cement Creek arsenic box plot (fig. 8). These samples were 

collected from bog iron deposits near the iron spring on lower 

Prospect Gulch. Adsorption by hydrous iron oxide is an important inorganic process that can maintain arsenic at very low 

levels in water (Pierce and Moore, 1980; Hem, 1992). These 

samples were collected directly down gradient from the Red 

Mountain hydrothermal alteration assemblages, which contain 

enargite (Cu3

AsS4) in veins. The ground water supplying these 

bog iron deposits appears to circulate along fractures in the 

intensely altered volcanic rock (Wirt and others, 2001). Notably, these samples also contain between 350 and 1,000 ppm 

of vanadium (Sole and others, this volume). Like arsenic, 

the aqueous geochemistry of vanadium is complex, possessing three oxidation states and a low solubility under reducing 

conditions (Hem, 1977, 1992).

Differences in the pH of ground water do not fully account 

for differences in trace-element concentrations among conglomerate deposits from upper Cement Creek (pH ≤3.5) versus those 

in the upper Animas River tributaries (pH ≥4.5). Trace-element 

concentrations in ferricrete where the pH is similar to California Gulch—such as ferricrete in Mineral Creek (pH ≥4.5)—

are not substantially different than those for samples from 

Prospect Gulch. Both Mineral and Cement Creeks, however, 

receive drainage from the Red Mountain and Ohio Peak–Anvil 

Mountain (OPAM) areas (fig. 1). Although Mineral Creek also 

receives drainage from the peak 3,792 altered area of Bove 

and others (this volume) to the west, this source produces 

acid drainage similar to that of the Red Mountain and OPAM 

areas.

The distribution and weathering of primary and secondary rock-forming minerals are fundamental in determining 

the distribution of manganocrete versus ferricrete, as well 

as the pH of ground water. Manganocrete samples generally have the largest concentrations of most major and trace 

elements, except for iron, sulfate, vanadium, and arsenic 

(fig. 8); however, this may have more to do with proximity 

to nearby veins than with pH, mineralogy of dominant iron 

species, or ferricrete subdeposit type. Within the ferricrete 

subdeposit types, spatial differences in ground-water pH and 

chemistry probably had a strong influence on the mineralogy 

of the dominant iron-oxide species. The deposit types, such as 

conglomerate-type deposits versus bog iron and spring deposits, are largely determined by the environmental setting at the 

location where ground water emerges at the ground surface 

and becomes oxidized. Although the largest differences in 

the distribution of major elements are most strongly associated with the ferricrete subdeposit type owing to the chemical influence of sediment clasts, differences in trace-element 

concentrations of copper, manganese, and lead appear to relate 

to pH. Differences in the concentration of arsenic, sulfate, 

iron, and manganese appear related to close spatial proximity 

to mineralized veins. For these elements, factors that are typically important in other settings, such as sorption and pH, may 

be secondary to availability in this highly mineralized environment. The most important factor determining the acidity and 

concentration of solid-phase trace elements for both ferricrete 

and manganocrete appears to be the underlying geology and 

proximity down gradient from a mineralized source area. The 

trace-element chemistry of ferricrete and manganocrete deposits largely predates the influence of mining, and thus may 

serve as a geochemical record of iron-rich or manganese-rich 

precipitates produced by long-term weathering of acid-sulfate 

and ore-bearing minerals up gradient from a given site.

Spatial Associations of Ferricrete 

and Bog Iron with Ground-Water 

pH, Geomorphology, and Alteration 

Assemblages

Ferricrete deposits are exposed nearly continuously 

along some reaches of Cement Creek and Mineral Creek 

(Yager and Bove, this volume, pl. 2; Vincent and others, this 

volume). Elsewhere, ferricrete is notably less prevalent, for 

example, in the middle reach of Cement Creek. Ferricrete is 

almost entirely absent along the Animas River upstream from 

the confluence with Cement Creek at Silverton, although 

manganocrete deposits are found in the Eureka, Placer, and 

California Gulch subbasins. This section describes how spatial 

patterns in the distribution of ferricrete and manganocrete 

relate to the mapped extent of acid-sulfide mineral assemblages up gradient from the deposits. Weathering of acidsulfate minerals affects the pH and geochemistry of ground 

water, which in turn results in the formation of iron and 

manganese-oxide precipitates.

The areal extent of major mineral assemblages was calculated for selected subbasins in Cement and Mineral Creeks 

(table 5). The percentages used following are the areal coverage in percent of the total subbasin area. The areas covered 

by Quaternary alluvium were subtracted from the total area 

in calculating the percentage of each mineral assemblage type. 

The major alteration assemblages mapped for this study are 

the acid-sulfate (AS), quartz-sericite-pyrite (QSP), vein-related 

quartz-sericite-pyrite (V-QSP), and weak sericite-pyrite (WSP) 

rock types (Bove and others, this volume; Yager and Bove, this 

volume, pl. 2). Regional alteration assemblages which have 

undergone less intense hydrothermal alteration are referred 

to as propylitized (PROP) rock.

Statistical descriptions of 300 springs and streams for this 

study (Mast and others, this volume) indicate distinct waterquality characteristics for each mineral assemblage. For the 

purposes of this discussion, the water chemistry for each water 

type is briefly summarized here. PROP rock contains as much 

as 30 percent calcite, and water chemistry is controlled primarily by dissolution of calcite and some pyrite. Water samples 

in contact with PROP rock tend to have neutral pH (>6) and 

measurable alkalinity, are relatively dilute, and commonly 
Lower Cement Creek (4)

Middle Cement Creek (12)

Upper Cement Creek (43)

Mineral Creek (20)

Upper Animas (7)

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EXPLANATION

 ALUMINUM

 CALCIUM

POTASSIUM

MAGNESIUM

IRON SULFUR

A

90th percentile

10th percentile

75th percentile

25th percentile

Median

The information displayed in box plots 

 is summarized in the graphic below.

CONCENTRATION, IN WEIGHT PERCENT

808 Environmental Effects of Historical Mining, Animas River Watershed, Colorado
Processes Controlling Formation of Ferricrete 809

Figure 8 (above and facing page). Box plots showing selected solid-phase elements, grouped by stream reach. A, major elements in 

weight percent; B, trace elements in parts per million. Order of box plots is same as sequence in explanation.

1

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B

CONCENTRATION, IN PARTS PER MILLION
have trace-metal concentrations below detection. In contrast, 

QSP waters are highly acidic (pH <4) and have high sulfate 

concentrations, which result from pyrite weathering as the 

dominant reaction. QSP rocks commonly have >10 percent 

pyrite by weight. In addition, the high acidity appears to 

promote dissolution of calcite and aluminosilicate minerals 

such as kaolinite that are commonly present. Oxidized QSP 

water is highly saturated in ferrihydrite, causing precipitation 

of iron. AS-associated waters have low pH values similar to 

those produced in QSP exposures, but are comparatively much 

more dilute. Only four AS water samples were collected, and 

the collection sites were relatively high in elevation; thus, 

short travel paths with less water-rock interaction probably 

occurred for these samples. Water in contact with V-QSP 

(and in contact with PROP assemblages containing mineralized 

veins) is generally intermediate in pH and in concentrations 

of major cations and trace elements. All the water types were 

super-saturated in ferrihydrite, although the QSP water had 

the highest iron concentrations (as much as 77 mg/L). Based 

on this classification by Mast and others (this volume), the 

iron spring water presented in table 1 has the characteristics 

of acid-sulfate or QSP type waters, depending upon the amount 

of sulfate. Factors affecting variations in water chemistry 

include the amount of iron sulfides, grain size and degree of 

crystalline structure, degree of water-rock interaction, and differing redox conditions in ground water.

In the following discussion of spatial associations, we 

focus primarily on Cement Creek because of its extensive ferricrete exposures, and to a lesser extent on the middle reach of 

Mineral Creek, and on California Gulch in the headwaters of 

the upper Animas River.

Cement Creek

Extra attention is given to Cement Creek from Gladstone to Silverton because of the abundance of ferricrete, as 

well as detailed information for this reach from other studies 

presented in this volume. For the purposes of this chapter, we 

have subdivided the area into an upper, a middle, and a lower 

reach, referred to as the Gladstone to Fairview Gulch, Fairview 

Gulch to Hancock Gulch, and Hancock Gulch to Silverton 

Table 5. Mapped exposures of hydrothermal-alteration mineral assemblages (fig. 1) by subbasin, Animas River watershed study area.

[Based on data from Sole and others, this volume; blank, alteration type not present, percent (pct.) not applicable]

Basin

Area Acid-sulfate

(AS)

Quartz-sericite-pyrite

(QSP)

Vein-related 

quartz-sericite-pyrite

(V-QSP)

km2 km2 Pct. tot. area km2 Pct. tot. area km2 Pct. tot. area

Upper Cement Creek (west side):

Prospect Gulch 3.3 0.62 18.7 0.98 29.9

Georgia Gulch 1.3 0.01 0.5 0.01 0.7 0.07 5.5

Fairview Gulch 0.8 0.001 0.07

Middle Cement Creek (west side):

Minnesota Gulch 1.7 0.00 0.2 0.16 9.5

Porcupine Gulch 0.8 0.04 5.5 0.09 11.6

Ohio Gulch 1.2 0.32 26.0 0.28 22.4

Topeka Gulch 1.4 0.17 12.3 0.23 17.0

Niagara Gulch 2.2 0.92 41.9 0.41 18.6

Henrietta Gulch 0.7 0.29 44.3

Lower Cement Creek (west side):

Unnamed south of Henrietta Gulch 0.3 0.04 14.0 0.00 1.2

Unnamed north of Soda Gulch 0.9 0.28 31.1 0.04 4.3

Soda Gulch 0.5 0.01 2.2

Upper Cement Creek (east side):

Unnamed north of Cascade Gulch 0.8 0.08 10.1

Middle Cement Creek (east side):

Cascade Gulch 1.5 0.04 3.0

Grassy Gulch 0.4

Unnamed south of Grassy Gulch 0.5

Illinois Gulch 1.5 0.04

Lower Cement Creek (east side):

Hancock Gulch 1.8

Unnamed south of Hancock Gulch 1.3 0.01 0.5 0.01 1.0

Mineral Creek (Middle Fork):

Red tributary of the Middle Fork 3.0 0.54 17.6 0.001 0.04

810 Environmental Effects of Historical Mining, Animas River Watershed, Colorado
Processes Controlling Formation of Ferricrete 811

Basin

Weak sericite-pyrite

(WSP)

Propylitic

(PROP)

Quaternary

(QU)

Altered areas mapped

(less QU)

km2 Pct. tot. area km2 Pct. tot. area km2 Pct. tot. area km2 Pct. tot. area

Upper Cement Creek (west side):

Prospect Gulch 0.06 1.75 1.47 44.6 0.17 5.1 3.13 94.9

Georgia Gulch 0.04 3.21 1.07 80.8 0.12 9.3 1.20 90.7

Fairview Gulch 0.00 0.29 0.80 97.1 0.02 2.5 0.80 97.5

Middle Cement Creek (west side):

Minnesota Gulch 1.51 89.0 0.02 1.2 1.67 98.8

Porcupine Gulch 0.48 59.3 0.19 23.6 0.61 76.4

Ohio Gulch 0.49 39.1 0.16 12.5 1.09 87.5

Topeka Gulch 0.93 68.9 0.03 1.8 1.33 98.2

Niagara Gulch 0.73 33.1 0.14 6.4 2.05 93.6

Henrietta Gulch 0.35 53.0 0.02 2.7 0.65 97.3

Lower Cement Creek (west side):

Unnamed south of Henrietta Gulch 0.22 84.8 0.25 100.0

Unnamed north of Soda Gulch 0.58 64.6 0.90 100.0

Soda Gulch 0.46 97.2 0.003 0.6 0.47 99.4

Upper Cement Creek (east side):

Unnamed north of Cascade Gulch 0.71 84.4 0.05 5.5 0.79 94.5

Middle Cement Creek (east side):

Cascade Gulch 0.93 63.3 0.50 33.8 0.97 66.2

Grassy Gulch 0.40 98.2 0.01 1.8 0.40 98.2

Unnamed south of Grassy Gulch 0.18 38.3 0.29 61.7 0.18 38.3

Illinois Gulch 0.93 63.3 0.50 33.8 0.97 66.2

Lower Cement Creek (east side):

Hancock Gulch 0.00 0.06 0.91 50.4 0.89 49.5 0.91 50.5

Unnamed south of Hancock Gulch 0.04 2.93 0.68 51.3 0.58 44.3 0.74 55.7

Mineral Creek:

Red tributary of the Middle Fork 1.23 40.47 0.40 13.1 0.87 28.7 2.17 71.3

reaches. These informal subdivisions were based on the 

abundance (or relative lack) of exposed ferricrete as shown by 

Vincent and others (this volume). Mapping of surficial deposits between Gladstone and Silverton at 1:6,000 scale (Vincent 

and others, this volume, pl. 5) allows for a detailed consideration of spatial relations of ferricrete (1) with pH and waterchemistry data from both tributary and ground-water sources 

(Kimball and others, this volume), and (2) with the distribution 

of mineral assemblages mapped by Yager and Bove (this volume) and Bove and others (this volume). Ferricrete abundance 

is spatially associated with the degree of alteration and the 

areal extent of various acid-sulfate mineral assemblages in the 

tributary subbasins that supply ground water.

Upper Reach, Gladstone to Fairview Gulch

The greatest distribution of bog iron and ferricrete 

deposits occurs along Cement Creek from its confluence with 

the South Fork Cement Creek at Gladstone to the mouth of 

Fairview Gulch (Yager and Bove, this volume, pl. 2; Vincent 

and others, this volume, pl. 5). In order to characterize the 

hydrologic processes associated with ferricrete formation, 

we describe the surface-water and ground-water hydrology 

of this reach. Important surface-water sources contributing to 

Cement Creek at Gladstone include South Fork Cement Creek, 

discharge from the treatment system for Sunnyside mine 

(AMLI site # 96), and North Fork and mainstem of Cement 

Creek (fig. 9). At present, roughly half of Cement Creek’s 

discharge at Gladstone is released from the Sunnyside mine 

treatment system during summer months. Treated effluent 

includes adit flow from the American tunnel combined with 

stream water from both the headwaters of Cement Creek and 

the North Fork. The pH of the treated flow is neutral (>6.5; 

neutralized with lime; Besser and others, this volume, Chapter D), whereas the pH of untreated flow during the remainder of the year ranges from 3.5 to 4.5 (Wright, Simon, and 

others, this volume, Chapter E10). South Fork contributes an 

approximately equal volume of flow with a pH between 3.5 

and 4.5 (≈90 L/s in September 1996; Kimball and others, this 

volume). Large seasonal variations in stream quality occur in 

response to the accumulation of a deep seasonal snow pack 

and the annual hydrologic cycle (Leib and others, 2003; Wirt 
0

2

4

6

8

10

DISTANCE, IN KILOMETERS

8 4 0 4 8

Spring or ground-water seepage (pH < 4.5)

Cement Creek from Gladstone to Silverton

Ohio Gulch

Prospect Gulch

Tiger Gulch

Georgia Gulch

< 5 liters per second

> 5 and <15

>15 and <25

>25 liters per second

Fairview Gulch

Cascade Gulch

Minnesota Gulch

Porcupine Gulch

Illinois Gulch

Topeka Gulch

Niagara Gulch

lower bog

Hancock Gulch

Spring or ground-water seepage (pH > 4.5) upper reach middle reach lower reach

Cement Creek South Fork

pH UNITS

WEST EAST

Relative discharge of inflows Types of inflows

USGS gauge near Silverton

EXPLANATION

Tributary (pH < 4.5)

Tributary (pH > 4.5)

upper bog

Dry Gulch

Thickness of line correlates with discharge; color of line correlates with pH; dashed line indicates spring

Figure 9. Schematic diagram of inflow discharge and pH from tributaries and ground water in the upper, middle, and lower 

reaches of Cement Creek during low-flow conditions.

812 Environmental Effects of Historical Mining, Animas River Watershed, Colorado
Processes Controlling Formation of Ferricrete 813

and others, 2000). Iron cementation of the exposed stream 

banks from Gladstone to Fairview Gulch is nearly continuous 

(Vincent and others, this volume, pl. 5), and stream cobbles 

are coated or stained with iron oxides.

Between Gladstone and Fairview Gulch (fig. 9), discharge 

increased more than 70 percent from about 180 to 310 L/s, as 

measured during low-flow conditions in September 1996 by 

Kimball and others (this volume). Three-fourths of this 130 L/s 

gain is attributed to small, diffuse ground-water sources and 

overland flow from sedge wetlands. Contributions from three 

tributaries, Georgia Gulch, Prospect Gulch, and Tiger Gulch, 

account for the remaining 30 L/s (fig. 9).

Sedge wetlands cover most of the low-lying flood-plain 

and stream-terrace deposits throughout the reach, interrupted 

only by alluvial fans emanating from major tributaries. The 

sedge wetlands, discharging acidic waters, are elevated above 

the stream channel, generally by a meter or so. Diffuse ground 

water predominantly originates from the sides of the valley, 

as opposed to flowing parallel to the stream through gravel 

deposits in the center of the valley (Vincent and others, this 

volume). Cement Creek generally flows from north to south, 

but the Gladstone to Fairview reach curves from the east to 

the southwest, receiving most of its drainage from the west 

rather than from the east, as evidenced by the larger number 

of tributaries and larger drainage area (figs. 1 and 9). Eighty to 

ninety percent of the areal extent of the wetlands is on the west 

side of Cement Creek. Dry ferricrete deposits are exposed in 

stream cuts on both sides of Cement Creek, but this does not 

necessarily indicate the source-direction of water that formed 

those precipitates, because the lateral position of the creek at 

the time of precipitation is not known. “Wet” ferricrete (field 

exposures that were wet) was observed on both stream banks, 

but predominantly on the west bank. These observations indicate that the source of iron and acidity is mostly from the west 

side, down slope from the Red Mountain hydrothermal altered 

area (fig. 1).

In the Gladstone to Fairview reach, the areal extent of 

QSP, AS, and WSP assemblages is much greater west of 

Cement Creek than on the east. The dominant acid-producing 

mineral assemblages in the Prospect Gulch subbasin include 

about 30, 19, and 1.8 percent of QSP, AS, and WSP rock 

exposures, respectively—roughly half of the surface area of 

the Prospect Gulch subbasin. In contrast, the unnamed subbasin on the east side of Cement Creek (fig. 1 and table 5, subbasin north of Cascade Gulch and upstream of Fairview Gulch) 

has only about 10 percent of its area mapped as V-QSP. Propylitized rock (PROP) that underwent a less intense regional 

alteration underlies about 90 percent of this unnamed subbasin. 

Rock on the east side of Cement Creek provides considerably 

more acid-neutralizing capacity than rock exposed on the west 

side of this reach (Desborough and Yager, 2000).

The pH of wetlands, bog iron deposits, seeps, and springs 

in this reach is highly variable, but predominantly acidic. 

At the interface between a sedge wetland and iron bog, one 

can measure many different pH values ranging from 3.2 to 

6.8—all within a distance of just a few meters (Yager and 

Bove, this volume, pl. 2, fig. 14). Most of the ground water 

entering Cement Creek, however, has low pH. Twenty-three 

inflows sampled during the 1996 tracer study had a mean 

pH of 3.7, which accounts for the decrease in pH from 6.5 to 

4.0 from Gladstone to Fairview Gulch (Kimball and others, 

this volume). The near-surface values for dissolved oxygen 

in wetland areas also tended to be highly variable, ranging 

from about 0.1 to 7 mg/L. Specific conductance was also 

highly variable: measurements of various sedge wetlands, 

iron bogs, and iron seeps and springs ranged between 89 and 

1,100 μS/cm (microsiemens per centimeter). The large spatial 

variations in these field parameters indicate that the wetlands 

and permeable host sediment served as mixing zones between 

the deeper circulating, low-pH ground water and either the 

atmosphere or relatively shallow, more oxygenated water.

The upper reach has several examples of iron precipitating where ground-water flow emerges from fractured bedrock. Near the mouth of Fairview Gulch, two springs, one on 

each bank, are aligned along the trend of a prominent fault or 

fracture system that intersects Cement Creek (Yager and Bove, 

this volume, pl. 1). Another example of acidic water emerging 

from fractures in bedrock is the iron spring in lower Prospect 

Gulch, which is located at a break in slope at the base of a 

pronounced linear feature in the topography (fig. 10; Wirt and 

others, 2001). The pH for all three of these iron-precipitating, 

fracture-controlled springs is between 2.9 and 3.5.

Fracture flow is the likely source of the many low-pH 

inflows in this reach. For 12 iron-rich springs sampled in this 

reach (tables 1 and 6), pH ranged from 3.2 to 4.6, specific 

conductance ranged from 523 to 1,800 μS/cm, and dissolved 

sulfate concentrations ranged from 240 to 1,000 mg/L. In contrast, Tiger Gulch, a dilute tributary inflow, had a pH value of 

6.4, specific conductance of 90 μS/cm, 23 mg/L of sulfate, and 

12 mg/L of bicarbonate. These dilute inflows have little acidneutralizing capacity and react quickly, having little effect on 

the water quality of Cement Creek or other acid-sulfate waters. 

Exposures of fractures that carry acidic ground water are typically concealed by colluvium and stream-terrace deposits. In 

addition, many colluvial and alluvial deposits are cemented 

with ferricrete or covered with sedge wetlands. Precise directions of ground-water flow paths are poorly constrained. In 

general, ground water flows from the mountains towards the 

center of the valley, and then down the center of the valley 

beneath and adjacent to Cement Creek.

Ferricrete and bog iron are actively forming at many iron 

springs and bogs, and numerous streambank locations in the 

upper reach of Cement Creek. This is further indicated by 

the frequency of mapped occurrences of wet ferricrete along 

the exposed stream channel (Vincent and others, this volume, 

fig. 6). In addition, ferricrete forms on the bottom and sides 

of Cement Creek in gaining reaches, although the rates and 

volumes of deposition are variable and difficult to measure. 

Loose reddish- or orange-brown flocculent is present on the 

stream bottom and coats stream cobbles in many locations, 

although the fine-grained flocculent tends to be washed away 

at higher flows. Desborough and others (2000) observed 
orange-brown precipitates (ferrihydrite and schwertmannite, 

as determined by X-ray diffraction) accreting onto a wooden 

stake that was left in the gaining reach upstream from Tiger 

Gulch and Georgia Gulch for several weeks, although the 

stake never became firmly cemented to the substrate. In this 

study, we placed a 2-in. diameter metal pipe in the stream and 

anchored it in place on the bed with angular rocks. During a 

3-year period, no cementation of the pipe or the rock holding 

it on the streambed took place.

The streambed is strongly cemented, however, in the 

gaining reach between the upper bog and Tiger Gulch and 

Georgia Gulch, where two large, opposing alluvial fans cause 

Cement Creek and associated flood-plain deposits to narrow 

(fig. 11). Numerous iron-rich seeps, resulting from the shallow 

water table and an abrupt change in stream gradient, occur 

on both sides of the channel upstream and downstream from 

the two fans near their interface with lateral stream terraces. 

Freshly precipitated cement, coating the sides and bottom of 

the stream, is modifying the channel from year to year. This 

seemed an ideal location to sample actively forming ferricrete 

within the channel.

Using a sledgehammer, we collected cement beneath 

the water from the bottom and sides of the stream channel 

near the upper bog. Chemical analyses and X-ray diffraction 

of strongly cemented sections of the low-flow streambed 

(fig. 11; sample 00ABFC-231 in table 2) indicate that precipitate is composed of at least 50 percent iron-oxide minerals 

and one-third crystobalite (amorphous quartz). Crystobalite 

is supersaturated in nearly all of the iron-rich ground water 

sampled (table 4), and precipitates along with the iron-oxide 

minerals under similar geochemical conditions. Ferricrete with 

a high content of crystobalite is more resistant to mechanical 

erosion and in this case forms small stairstep-sized waterfalls 

at several locations in the upper reach. The rate at which the 

ferricrete is accumulating is unknown, although the stream 

incised to its present level about 500 years ago, placing a 

maximum constraint on the age of the deposit.

Middle Reach, Fairview Gulch to Hancock Gulch

In contrast with the Gladstone to Fairview Gulch reach, 

relatively few ferricrete deposits occur between Fairview 

Gulch and Hancock Gulch (Vincent and others, this volume, 

pl. 5). This relative absence is largely attributed to a greater 

percentage of surface runoff from the east side of the Cement 

Creek basin that is dominated by weakly propylitized volcanic 

rock (Bove and others, this volume), resulting in a higher pH 

for many of the tributary and ground-water inflows.

Figure 10. Fracture-controlled iron spring in lower Prospect Gulch.

814 Environmental Effects of Historical Mining, Animas River Watershed, Colorado
Processes Controlling Formation of Ferricrete 815

Table 6. Characteristics of selected stream reaches with respect to abundance of ferricrete.

Reach Subreach Selected inflows

Relative

abundance

of ferricrete

Discharge

liters/s1

Major

mineral

assemblages2

Base flow

pH 

Sulfate

concentration

(mg/L)3

Cement Creek

Upper (Gladstone to Fairview) Continuous 180–310 mixed 4.0–6.5 

(sometimes 

treated)

410–500

Iron springs (12) Continuous diffuse QSP/AS 3.2–4.6 240–1,000

Tiger Gulch None 3.1 PROP 6.4 23

Adits in Prospect Gulch 

(3)

Not determined <1 mine affected 2.5–4.1 83–1,100

Middle (Fairview to Niagara) Intermittent 310–600 mixed

West side tributaries (5) Intermittent 9.7–13.5 AS/QSP/PROP 3.0–4.9 147–600

East side tributaries (4) None PROP 6.8–7.6 30–56

Mine adits (3) Not determined >50 E mine affected 5.7–7.3 530–870

Lower (Niagara to Silverton) Continuous 600–700 mixed 3.7 150–480

Lower bog Continuous 50 AS/QSP 3.0 550

Inflows near Hancock (2) None 30 PROP/landslide 7.3–7.6 29–53

Mineral Creek

Middle Fork to South Fork Continuous 21–32.5 AS/QSP 5.7–4.9 134–220

Middle Fork Continuous 18.1 AS/QSP 4.7 290

Small seeps Continuous diffuse AS/QSP 3.6–5.0 56–390

Upper Animas

California Gulch to mixing cascade No ferricrete; 5.0–50 PROP/AS 5.2–7.0 125–175

Small seeps manganocrete diffuse vein PROP 4.3–5.3 150–470

is abundant

1

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

2

QSP, quartz-sericite-pyrite; AS, acid-sulfate; PROP, propylitic.

3

Data in Sole and others (this volume, Chapter G).

Through this reach, low-flow discharge doubled from 

approximately 310 L/s to 607 L/s (Kimball and others, this volume). At least 110 L/s of this gain came from seeps, springs, 

and tributaries on the west side of the valley. About 65 L/s was 

attributed to visible sources on the east side, and 48 L/s could 

not be attributed to any observed source. On the west side, 

Minnesota, Porcupine, Ohio, Topeka, and Niagara tributaries 

each contributed between 9.7 and 13.5 L/s, and the Elk tunnel 

(AMLI site # 147) and Anglo-Saxon mine (AMLI site # 183) 

adits contributed about 15 and 14 L/s, respectively. The mean 

pH for these five west-bank tributaries was 4.4, and the pH of 

the three mine inflows ranged from 5.7 to 7.3 (fig. 9). Despite 

the relatively high pH of the mine-related inflows, sulfate 

concentrations were high (760–868 mg/L). In contrast, several 

inflows draining propylitic rock on the east side of Cement 

Creek had a mean pH value of 7.3 and mean sulfate concentration of 35 mg/L (Cascade Gulch, two large seeps, and a large 

unnamed spring draining a glacial moraine deposit upstream 

from Ohio Gulch). Illinois Gulch (which includes a large contribution from the Yukon tunnel; AMLI site # 186) contributed 

the most inflow at 33 L/s, having a pH of 7.6 and 533 mg/L 

sulfate. The pH of Cement Creek decreased slightly from about 

4.0 to 3.8 from Ohio Gulch to Niagara Gulch (Kimball and others, this volume).

Most volcanic rock east of Cement Creek is propylitically altered, consisting of quartz, chlorite, epidote, and calcite 

(plus or minus plagioclase, amphibole, biotite, and opaque 

oxides)—but little pyrite, except for that associated with veins 

or finely disseminated. This mineral assemblage represents an 

early regional alteration event that was largely unaffected by 

the subsequent acid-sulfate altering fluids in the Red Mountain 

area (Bove and others, 2001; Bove and others, this volume). At 

high elevations some remnants of primary mineral assemblages 

are still preserved, including plagioclase, quartz, amphibole, 

and some biotite. The pH of springs in contact with propylitized rock tended to be neutral (>5) in contrast with low-pH 

background water (<5) draining more intensely altered, pyritebearing rock (Mast and others, this volume). Propylitically 

altered rock also occurs on the west side of Cement Creek 

south of the Red Mountain hydrothermal altered area (fig. 1). 
The Georgia, Fairview, and Minnesota Gulch subbasins have 

<10 percent of AS and QSP alteration types, although V-QSP 

is locally important in Minnesota Gulch.

Farther south, subbasins draining the Ohio Peak–Anvil 

Mountain hydrothermal altered area (Porcupine, Ohio, Topeka, 

and Niagara subbasins) have between 17 and 60 percent 

acid-generating rock types exposed (table 5). High-elevation 

colluvial ferricrete is present in the Topeka and Niagara subbasins, as well as bog iron in upper Topeka Gulch. The AS 

and QSP alteration assemblages in the OPAM area are similar 

to those in the Red Mountain area, although the OPAM area 

tends to be lower in the concentrations of copper, lead, arsenic, 

and other trace elements (Bove and others, this volume). The 

Ohio, Niagara, and Henrietta subbasins have more acid-sulfate 

alteration in comparison to Prospect Gulch—with 26 to 

44 percent exposures of AS (table 5). In addition, Porcupine, 

Ohio, Topeka, and Niagara subbasins contained from 12 to 

22 percent exposures of QSP. These mineral assemblages are 

highly friable and tend to be easily eroded, as evidenced by 

the large alluvial fan at the mouth of Ohio Gulch (Vincent and 

others, this volume).

The lower abundance of ferricrete deposits in this reach 

is attributed to higher pH for most tributary inflows (fig. 9), 

and a larger fraction of inflow from propylitically altered 

subbasins relative to the upper reach (table 5). The middle 

reach also receives proportionately more inflow from perennial tributaries and less from ground-water discharge (roughly 

15 percent of new water for the middle reach versus 75 percent for the upper reach). There are distinct differences in 

the pH of water on different sides of the valley, with the 

east-side inflows relatively neutral (6.8–7.6; table 6), and the 

west-side inflows more acidic (mean=4.4). The two subbasin 

inflows with the largest areal extent of AS and QSP mineral 

assemblages—Ohio Gulch and Niagara Gulch—had pH values 

of 3.0 and 3.6, respectively. A corresponding decrease in the 

pH of Cement Creek from about 4.0 to 3.8 occurs between 

these two tributaries, and ferricrete deposits are present along 

the channel near Niagara Gulch.

Lower Reach, Hancock Gulch to Cement Creek 

Gauge near Silverton

Ferricrete deposits are exposed almost continuously 

along the lower reach of Cement Creek from the mouth of 

Hancock Gulch downstream to the Cement Creek gauge near 

Silverton. Low-flow discharge in September 1996 increased 

from about 600 to 700 L/s (Kimball and others, this volume). 

Figure 11. Gaining reach of Cement Creek (left channel) near upper bog (foreground), characterized by 

numerous iron-rich seeps. View is downstream toward Georgia Gulch and Tiger Gulch alluvial fans (background). 

Streambed of Cement Creek adjacent to upper bog is cemented with ferricrete.

816 Environmental Effects of Historical Mining, Animas River Watershed, Colorado
Processes Controlling Formation of Ferricrete 817

The mean stream pH for the reach is 3.7. The greatest tributary inflows—Hancock Gulch and an unnamed subbasin 

to the south—contributed a net inflow of ≈30 L/s with pH 

values of 7.3 and 7.6, respectively. The source area for these 

inflows is along the base of a large landslide deposit east of 

Cement Creek (Blair and others, 2002; Vincent and others, this 

volume). The greatest ground water inflow (50 L/s) is in the 

vicinity of the lower bog on Cement Creek (figs. 1, 9), which 

had a pH measurement of 3.0.

Discharge from the lower bog includes diffuse inflow 

from wetlands on both banks, just downstream from an old 

railroad trestle. A small prospect along an altered vein zone 

could have some impact on the water quality of the wetland 

system. The alteration zone is a prominent feature in geophysical data (Smith and others, this volume, Chapter E4). Ground 

water supplying the wetland is likely derived from a large 

fracture system; however, evidence of ground water emerging from fractured rock is concealed by steep forested terrain 

on the west bank and by landslide debris on the east bank of 

Cement Creek.

Hydrothermal alteration in the lower reach is considerably less than that of upstream reaches of Cement Creek. 

Hancock Gulch and the unnamed tributary directly south, for 

example, contain little AS or QSP, as does Soda Gulch on 

the west side. The largest exposure of acid-generating rock 

occurs on the west side in an unnamed subbasin north of Soda 

Gulch. This unnamed tributary, with 31 and 4.3 percent AS 

and QSP exposures, respectively, drains the east flank of Anvil 

Mountain and joins Cement Creek near the lower bog. Where 

alteration does occur, at least one or more small prospects 

have been developed on either side of this reach. Wet ferricrete exposed in channel banks is present where the lower 

bog enters Cement Creek. Because the lower bog merges with 

Cement Creek so far downstream, this source has proportionately less influence on stream pH and chemistry than if it were 

located farther upstream, owing to dilution effects (Kimball 

and others, this volume). Cementation of streambed clasts 

through most of this reach is attributed to mixing of the lowpH and iron-rich ground water from the lower bog with more 

oxygen-rich water in the stream and underlying hyporheic 

zone.

Mineral Creek between Middle Fork 

and South Fork

Ferricrete is exposed continuously along Mineral Creek 

between its Middle and South Fork tributaries (Yager and 

Bove, this volume, pl. 2). Water chemistry in this reach is 

dominated by low-pH water from Middle Fork Mineral Creek, 

although small, acidic, iron-rich springs discharge from both 

banks through this reach. Discharge from Mineral Creek and 

from Middle Fork Mineral Creek was roughly equal during low-flow conditions (20.6 and 18.1 L/s, respectively, 

September 14, 1996; Kimball and others, this volume). Middle 

Fork Mineral Creek had a total-recoverable iron concentration 

of nearly 40 mg/L with a pH of 4.7. The addition of Middle 

Fork Mineral Creek to the mainstem caused pH to decrease 

from 5.7 to 4.8, whereas total recoverable iron increased 

more than three-fold in Mineral Creek, from 1.5 to 5 mg/L. 

The slight gain of 5 L/s between the Middle and South Fork 

Mineral Creek tributaries is attributed to small acidic springs, 

which have little effect on in-stream chemistry or trace-metal 

loads. We attribute the acidic nature of these small inflows to 

large exposures of altered rock on both sides of Mineral Creek. 

The subeconomic porphyry copper-molybdenum deposit of 

peak 3,792 m is west of this reach and the OPAM area is east 

(Bove and others, this volume). Bog iron deposits occur nearly 

continuously along both banks. In-stream pH and iron concentrations remain fairly constant throughout this reach to the 

mouth of South Fork Mineral Creek. Inflow from South Fork 

is neutral (7.2), and the pH downstream from its confluence is 

6.8 during low-flow conditions. The distribution of ferricrete 

deposits is no longer continuous downstream from the mouth 

of South Fork Mineral Creek.

Middle Fork Mineral Creek contributes the greatest loads 

of aluminum, iron, manganese, and sulfate to Mineral Creek 

(Kimball and others, this volume). The loads are derived from 

weathering of QSP alteration assemblages surrounding the 

subeconomic porphyry copper-molybdenum deposit at peak 

3,792 m and from mining (Mast and others, this volume). An 

unnamed north-flowing tributary subbasin, the major tributary inflow to Middle Fork Mineral Creek, has about 18 and 

40 percent areal extent of QSP and WSP rock types, respectively (table 5). Ferricrete is present along the QSP and WSP 

reaches of the tributary, which contributes about 20 percent of 

the zinc load and 24 percent of the copper load to Middle Fork 

Mineral Creek (Mast and others, this volume). Mast and others 

have estimated that comparable loads of zinc (24 percent) and 

copper (19 percent) can be attributed to mining-related sources 

elsewhere in the Middle Fork Mineral Creek basin.

California Gulch, a Headwater Tributary 

of the Upper Animas River

Springs and streams in the east half of the study area, 

drained by the upper Animas River upstream of Silverton, 

are substantially higher in pH (generally >6.5; Mast and others, 

this volume; Wright, Simon, and others, this volume), and ferricrete deposits and bog iron are correspondingly sparse (Yager 

and Bove, this volume, pl. 2). The neutral pH is attributed to 

the presence of carbonate minerals in the propylitically altered 

rocks throughout most of this basin. In several locations, 

alluvial deposits are cemented with blue-black manganeseoxide cement, here referred to as manganocrete (Verplanck 

and others, this volume; Yager and Bove, this volume, pl. 2). 

Similar to ferricrete in their genesis, these sedimentary conglomerates are less abundant than the iron-cemented deposits 

in Mineral and Cement Creeks, occurring along the flanks 
of California and Houghton Mountains (in the California, 

Placer, and Eureka Gulch subbasins, fig. 1) and as black rock 

varnish forming a surficial coating on stream cobbles and 

cementing historical gravel deposits below the old mining 

and milling town of Eureka (Vincent and Elliott, this volume, 

Chapter E22). The cement coatings are derived from the weathering of manganese-rich vein minerals in the Eureka graben 

area described by Casadevall and Ohmoto (1977) and also from 

mill tailings derived from ore production that were discharged 

to the Animas River at Eureka.

Naturally occurring manganese-rich springs were 

sampled along the lower end of manganocrete-cemented 

alluvial fans flanking the north side of California Mountain, 

in this study and also by Kimball and others (this volume). 

These springs are low in dissolved oxygen, range in pH from 

4.3 to 4.8, and have higher concentrations of dissolved sulfate, 

manganese, iron, and aluminum than the adjacent downstream reach of California Gulch (Kimball and others, this 

volume). Manganese springs (see samples 99VAF01, MN-1, 

and CGS, table 1) had among the greatest concentrations of 

dissolved aluminum (as much as 24 ppm), copper (as much as 

4.6 ppm), and zinc (as much as 14 ppm). For further information on manganese-oxide precipitates, see Hem (1963, 1964), 

Carpenter and Hayes (1980), Robinson (1993), Stollenwerk 

(1994), and Lind and Hem (1993).

Trace-Element Content of Naturally 

Occurring Acid-Sulfate Ground Water 

as a Geochemical Baseline for MineAffected Drainage

Successful remediation strategies for acidic mine 

drainage require a detailed understanding of mineralization 

and hydrology surrounding the mines in order that realistic 

geochemical baselines for water quality can be developed. 

The term “baseline” is used herein to refer to water that is 

unaffected by mining, milling, or weathering of excavated 

waste rock. We propose that for the Animas River watershed 

study area the acidic ground water associated with ferricrete, 

bog iron, and manganocrete deposits serves as an aqueousphase analog for ground water affected by acidic mine drainage. Acid-sulfate-type ground water bearing high concentrations of iron and manganese represents extreme near-surface 

conditions, and these are among the worst-case concentrations 

that could be expected from the weathering of a mineral 

deposit. The acid-sulfate-type water serves as a geochemical 

baseline for what undiluted ground water draining a mineral 

deposit may have been like prior to mining.

In a typical stream, water at any given point is a composite of ground water and surface-water runoff from a variety of 

sources, and thus a contribution of acid-sulfate-type ground 

water would almost always be diluted by other contributions. 

The spring water sampled in this study therefore represents 

a bias toward the extreme end and should not be used to set 

remediation goals for surface-water streams.

In figure 12, the sum of most common trace elements 

(zinc, copper, cobalt, and nickel in μg/L), or “sum of metals” 

is plotted against pH for different types of water that were collected throughout the Animas River watershed study area. The 

upper graph (A) shows the range of results for ground-water, 

spring, and seep samples from the project database (Sole and 

others, this volume). Sample descriptions indicate that these 

samples were not obviously affected by mining activity. Base 

flow during late-summer conditions from tributaries, largely 

from the tracer studies by Kimball and others (this volume), 

is also plotted, although many of the samples in this group are 

from subbasins where mining occurred, and are likely affected 

by historical mining to some degree. Both ground-water and 

base-flow tributary samples plot across a wide range of pH, 

with trace-element concentrations reflecting a wide range 

of conditions, from springs fed by dilute snowmelt to older 

ground water with as much as 1,200 μg/L for sum-of-metals 

concentration. The upper graph (A) provides a watershedscale picture covering a broad range of pH and sum-of-metal 

conditions that were sampled in the duration of the 5-year 

investigation by the numerous coworkers on the project. The 

lower graph (B) compares iron-rich ground water collected 

in this study (table 1) with mine-affected samples from the 

project database (Sole and others, this volume). In accordance 

with the criteria of Mast and others (this volume), the ironrich ground water samples in graph B are not significantly 

impacted by mining activities. In contrast, the acidity and sumof-metal content of the naturally occurring iron-rich water are 

exceeded by a majority—although not all—of the database 

samples coded as affected by mining. The lower graph (B) 

gives us a clear indication of how the most extreme mineaffected samples compare with the most extreme ground water 

from unmined areas strongly affected by weathering of acidsulfate rock (this study, table 1). Based on the lower graph, a 

reasonable geochemical baseline for acid-sulfate-type ground 

water from the Animas River watershed study area is the maximum concentration of the sum of metals (Co, Cu, Ni, and Zn) 

for the iron-rich ground water. A reasonable sum-of-metals 

limit for a ground-water discharge leaving a mine-affected 

site (an extreme condition) could be set at 1,200 μg/L for this 

study area—a hydrothermally altered Tertiary-age volcanic 

rock that has naturally weathered in a temperate alpine setting. 

Limits for selected trace-element concentrations that are of 

particular concern to aquatic life and human health, such as 

copper and zinc, can be determined in a similar fashion. In 

upper Cement Creek, the maximum values measured for ironrich ground water were 0.080 μg/L copper and 1,500 μg/L zinc 

(table 1). Local guidelines could be developed on a site-by-site 

basis for a given reach, depending on the hydrologic setting 

and degree of hydrothermal alteration.

818 Environmental Effects of Historical Mining, Animas River Watershed, Colorado
Processes Controlling Formation of Ferricrete 819

Figure 12. Sum of trace metals (Zn+Co+Cu+Ni), versus pH for A, ground-water and tributary inflows, and 

B, iron-rich ground water (this study) compared with mine-affected samples. Metals data include both filtered 

and unfiltered water samples from database (Sole and others, this volume). Note log scale on y-axis.

A

B

2345678

pH, IN STANDARD UNITS

Iron-bearing ground

water (this study, 

table 1)

Mine-impacted

(database)

 (Zn + Cu + Co + Ni) 

IN MICROGRAMS PER LITER

2345678

pH, IN STANDARD UNITS

10

100

1,000

10,000

100,000

1,000,000

Other ground

water (database)

Tributary inflows

(database)

10

100

1,000

10,000

100,000

1,000,000
Conclusions

Comparison of ground-water pH conditions in the 

Animas River watershed study area with mapped occurrences of ferricrete indicates that active ferricrete and bog iron 

deposits are most likely found in areas where ground-water 

pH is consistently less than 4.5. An exception to this finding is 

that older, inactive ferricrete and bog iron deposits are locally 

preserved above modern stream terraces (and the presentday water table), such as the cemented paleo-stream gravel 

deposits in Cement and Mineral Creeks. The greatest sources 

of acid-sulfate water are the Red Mountain and OPAM areas 

draining east to Cement Creek and west to Mineral Creek, 

and the area surrounding the unnamed peak 3,792 m between 

Middle Fork and South Fork Mineral Creek (fig. 1). The 

alteration assemblages most commonly associated with acid 

drainage are the acid-sulfate, quartz-sericite-pyrite, V-QSP, 

and weak sericite-pyrite alteration zones. Ferricrete occurs 

extensively along the valley floors and banks of Cement Creek 

and Mineral Creek, directly down gradient from the greatest occurrences of acid-generating rock types. The aqueous 

and solid-phase chemistry of ferricrete environments can be 

used as a geochemical baseline for an extreme but natural 

condition—acid-sulfate-type ground water draining hydrothermally altered volcanic rock in a given climatic setting.

In this chapter we have investigated aqueous chemistry 

of ground water in contact with ferricrete deposits, evaluated 

solid-phase chemistry and mineralogy of ferricrete deposits, 

and considered spatial relations with altered mineral assemblages and typical environments of ferricrete and bog iron 

deposition. Lastly, we have developed geochemical baselines 

for major and trace elements in a most extreme, but natural 

condition—that of acid-sulfate-type ground water unaffected by mining in close proximity to hydrothermally altered 

mineral assemblages. The hydrologic processes, mineralogy, 

and major-element chemistry governing the formation of ferricrete have many analogies to the formation of iron precipitates resulting from acidic mine drainage. The prevalence of 

schwertmannite in a wide range of sedimentary deposits down 

gradient from acid-sulfate rock assemblages is evidence that 

pH of ground water in these localities was probably between 

about 3.5 and 4.5 before mining began.

Despite the poor quality of these naturally occurring 

acid-sulfate-type ground waters, mine-affected water samples 

are commonly even more acidic with higher trace element 

concentrations, some by as much as two orders of magnitude 

(fig. 12). The most heavily mine affected water samples have 

pH <3, which increases the solubility of iron, aluminum, and 

most trace elements such as zinc, copper, nickel, cobalt, and 

arsenic, resulting in sum-of-metals concentrations that commonly exceed 1,200 μg/L. Representative sampling of ferricrete deposits and acid-sulfate ground water provides a needed 

strategy for our understanding of worst-case water-quality 

conditions that are natural in origin. The water chemistry of 

iron- and manganese-rich springs can be used as a premining 

baseline of pH and trace-element concentrations in ground 

water adjacent to mined deposits.

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