Formation and Geochemical Significance 

of Iron Bog Deposits

By Mark R. Stanton, Douglas B. Yager, David L. Fey, and Winfield G. Wright

Chapter E14 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 .......................................................................................................................................................693

Introduction and Background ..................................................................................................................693

Field and Analytical Methods ..................................................................................................................694

Study Area, Geology and Alteration Types, and Sample Localities .........................................694

Solid Phase Chemistry .....................................................................................................................696

Sample Collection, Preparation, and Analysis ....................................................................696

Mineralogy ................................................................................................................................696

Element Abundances ..............................................................................................................697

Carbon Abundance, Isotopic Dating (14C), and Composition ............................................697

Aqueous Chemistry...........................................................................................................................697

Sample Collection, Preparation, and Analysis ....................................................................697

Reconnaissance Study ..................................................................................................697

Detailed Study of the Upper Bog of Cement Creek ...................................................698

Biological Characterization .............................................................................................................698

Iron and Sulfur Oxidizing Bacteria ........................................................................................698

Photosynthetic Organisms .....................................................................................................699

Geochemical Modeling ....................................................................................................................699

Results .........................................................................................................................................................699

Solid Phase Chemistry of Iron Oxyhydroxide ...............................................................................699

Location and Size of Iron Deposits .......................................................................................699

Mineralogy ................................................................................................................................700

Major and Trace Elements in Solids .....................................................................................700

Core Samples ..................................................................................................................700

Bonner Waste Pile ..........................................................................................................700

Carbon Abundance, Ages, and Composition ......................................................................701

Aqueous Chemistry...........................................................................................................................702

Water Composition—All Sites ...............................................................................................702

Surface Water Versus Ground Water—All Sites ...............................................................702

Surface Water Versus Ground Water—Upper Bog of Cement Creek ............................706

Ratio of Fe2+ to Fetot ..................................................................................................................706

Dissolved Oxygen as an Indicator of Ground Water ..........................................................707

Trace-Element Chemistry and Alteration Type ...................................................................708

Microbial Populations ......................................................................................................................708

Geochemical Modeling ....................................................................................................................710

Discussion ...................................................................................................................................................711

Geochemical Evolution of Meteoric and Surface Water to Iron Bog Ground Water ............711

Geochemical Evolution of Iron Bog Ground Water to Iron Bog Surface Water ....................711

Iron Oxidation ...........................................................................................................................713

Formation of Iron Oxyhydroxide and Schwertmannite ......................................................713

Mineralogy of Iron Bog Solids ........................................................................................................714

Location of Precipitation of Solid Phases .....................................................................................714

Solid Phase Chemistry Versus Alteration Type ............................................................................715

Trace-Element Composition of Iron Oxyhydroxide .............................................................715

Water Chemistry Versus Alteration Type ......................................................................................715
Accumulation of Iron Oxyhydroxide ..............................................................................................716

Formation of Modern “Waste-Pile Ferricrete” Deposits ..................................................716

Role of Photosynthetic Organisms ........................................................................................717

Estimated Rates of Iron Oxyhydroxide Deposition .............................................................718

Carbon Preservation within Iron Bogs .................................................................................718

References Cited........................................................................................................................................718

Figures

 1. Map of Animas River watershed study area, showing generalized rock units 

and geologic contacts, areas of dominant alteration types, major drainages, 

sample site localities, and types of samples collected at each locality .........................695

 2. Photograph showing outlet at south end of lower bog in Cement Creek basin .............697

 3. Downhole plot of major- and trace-element abundances, Core 97ABS327 from 

the red spring of Prospect Gulch ...........................................................................................701

 4. Downhole plot of major- and trace-element abundances, Core 999291 from 

South Fork Mineral Creek ........................................................................................................703

 5–7. Box plots of major- and trace-element concentrations, and water chemistry 

parameters in the following:

 5. Surface and ground water, all iron depositional sites ................................................705

 6. Surface and ground water, upper bog of Cement Creek ............................................706

 7. Zero DO (ground) water, upper bog of Cement Creek, and nonzero DO 

 (surface) water from all other sites ................................................................................708

 8. Photograph of Leptothrix film on water surface at lower bog of Cement Creek ...........711

 9. Graph showing postulated evolution of ground water to surface water using 

data from upper bog of Cement Creek ..................................................................................712

 10. Graph showing relationship of aqueous concentrations of iron and aluminum 

in all iron depositional environments ....................................................................................716

 11. Photograph showing moss layer and relict algal filaments retaining iron 

oxyhydroxide near Cascade Gulch adit, Cement Creek .....................................................717

Tables

 1. Examples of X-ray diffraction results from iron bog or spring samples, Animas 

River watershed study area, showing types of iron-bearing, nonsilicate minerals ..........700

 2. Summary statistics for major- and trace-element abundances in core and minewaste iron oxyhydroxide samples ...........................................................................................702

 3. Sample site and location, deposit and alteration type, water type, major- and 

trace-element concentration, water chemistry parameter, Fe2+/Fetot ratio, and 

range (minimum, maximum) of detectable concentrations for all water samples ...........704

 4. Median values of major-element concentrations, trace-element concentrations, 

and water chemistry parameters according to separation of water types where 

iron is actively being deposited ...............................................................................................704

 5. Summary statistics of major- and trace-element concentrations, and water 

chemistry parameters in surface and ground water, all iron depositional sites .............705

 6. Summary statistics of major- and trace-element concentrations, and water 

chemistry parameters in surface and ground water (based on positive hydraulic 

head), and nonzero DO and zero DO water, upper bog Cement Creek ..............................707

 7. Results of Most Probable Number (MPN) culturing for Thiobacillus species 

in iron bogs, Animas River watershed study area ..............................................................710

Abstract

Aqueous and solid phase geochemistry of actively forming iron bogs in the Animas River watershed study area near 

Silverton, Colorado, suggests that all iron bogs (whether active 

or inactive) in the study area formed in a similar manner. 

Geochemical modeling of analytical data demonstrates that 

oxidation of ferrous iron (transported in low-pH, low dissolved 

oxygen ground water) to ferric iron is the mechanism common 

to the formation of these iron deposits. The change in oxidation state causes precipitation of fine-grained iron oxyhydroxide solids near the point of ground-water discharge. Local 

geology, parent rock mineralogy, alteration type, ground-water 

composition, geochemically active microbes and plants, and 

depositional-site geochemistry are among the major factors 

that influence the formation, growth, and persistence of the 

iron bogs.

According to this genetic model, ground water circulating through the regional rock mass acquires dissolved ferrous 

iron via dissolution of abundant sulfide minerals (primarily 

pyrite) in the country rock. This acid weathering produces 

high concentrations of aqueous species such as aluminum and 

calcium from host silicate minerals, and copper and zinc from 

metal-rich sulfide minerals. Maximum concentrations of aqueous iron, aluminum, and sulfate are 140, 22.1, and 910 milligrams per liter, respectively. Maximum concentrations of the 

trace elements copper, lead, zinc, and arsenic are Cu 18,000, 

Pb 110, Zn 53,000, and As 4,300 micrograms per liter.

Upon emergence at the surface, ferrous iron is oxidized 

to ferric iron by dissolved atmospheric or biogenic molecular 

oxygen [O2

(aq)], or iron-oxidizing microbes. Ferric iron then 

reacts to form fine-grained, texturally homogeneous crystalline 

phases and noncrystalline (amorphous) chemical precipitates. 

In iron bogs, the amorphous material generally constitutes 

more than 50 weight percent of the solids.

The crystalline iron oxyhydroxides are mainly goethite 

(α-FeOOH) and schwertmannite (Fe8O8(OH)6SO4). The 

mixed crystalline-amorphous solids are iron rich with varying concentrations of aluminum, sulfate, and trace elements. 

Iron and aluminum reach concentrations as high as 46.0 and 

12.1 weight percent, respectively. Trace-element contents are 

dependent on the parent rock mineralogy and alteration type 

drained by ground water. Maximum concentrations of copper, 

lead, zinc, and arsenic in different iron bog solids are Cu 110, 

Pb 60, Zn 660, and As 5,000 parts per million. Thus, iron bogs 

have the capacity to act as reservoirs and perhaps sources of 

several major and trace elements.

The largest volumetric accumulations of iron oxyhydroxide are produced by iron bog and iron spring deposits; seeps 

and mine adit/waste-pile drainages form smaller accumulations. Iron mobilization and deposition in iron bogs in the 

Animas River watershed has been occurring for about the last 

9,000 years, a fact supported by carbon-14 (14C) dating on 

relict carbon from the iron bogs.

Introduction and Background

Fine-grained, mostly clast free iron oxyhydroxide deposits are present in several areas of the Animas River watershed 

study area in southwestern Colorado. These deposits have 

been mapped by D.B. Yager and others (Yager and Bove, this 

volume, Chapter E1, pl. 2); most are classified as iron bogs 

because of their low-pH water and their location in watersaturated, relatively level terrain such as stream valley bottoms 

(Gore, 1983). Other iron-depositing environments, such as 

springs and seeps, and mine adit or waste-pile drainages, also 

are found throughout the area. These sites of iron deposition 

are usually located where ground water may have reached 

the surface, as, along structures or fractures, or where ground 

water intersects surface flows (streams).

An understanding of the origin, composition, and evolution of the variety of iron-rich deposits has several applications. First, because iron bogs are potential sinks or sources of 

large concentrations of many major and trace metals (Church 

and others, 1997), it is important to examine how these deposits form and persist (or degrade) in the Animas River watershed. Second, knowledge of how the deposits form can yield 

Chapter E14

Formation and Geochemical Significance of 

Iron Bog Deposits

By Mark R. Stanton, Douglas B. Yager, David L. Fey, and Winfield G. Wright
useful information concerning geochemical processes related 

to acid-rock and acid-mine drainage generation, and the 

potential effect these processes have on water quality. Finally, 

geochemical processes operating in modern-day iron bogs 

may be analogous to those that form the iron oxide cement 

of clast-bearing ferricrete found in the area (Wirt and others, 

this volume, Chapter E17; Verplanck and others, this volume, 

Chapter E15).

One process fundamental to an understanding of iron 

bog formation and geochemistry is precipitation of the solid 

phase. The oxidation of Fe2+ to Fe3+ by dissolved atmospheric 

oxygen is slow under acidic conditions, having half-times 

as long as 2–3 years (Stumm and Morgan, 1985; Singer and 

Stumm, 1970). The concentration of dissolved iron in iron bog 

water (as much as 70 mg/L) is such that chemical oxidation by 

dissolved O2

(aq) alone will not convert all Fe2+ to Fe3+ rapidly 

(a few days). Thus, a suitable oxidant in addition to O2

(aq)

is probably involved in forming iron oxyhydroxide deposits 

in low-pH water (Carlson and Kumpulainen, 2000). Bacteria 

have been implicated in a number of studies as important oxidants of ferrous iron (Bigham and others, 1990).

Bacteria of the genus Thiobacillus are potential agents 

for accelerating the rate of iron and sulfur oxidation in low-pH 

environments (Nordstrom and Southam, 1997; Bigham, 1994). 

The two prominent species are Th. ferrooxidans, a ferrous 

iron-oxidizing bacterium, and Th. thiooxidans, a reducedsulfur oxidizing bacterium. Leptothrix is another bacterium 

that is an indicator of iron oxidation, but whether it truly oxidizes Fe2+ under acidic conditions has not yet been established 

(van Veen and others, 1978).

A variety of iron minerals can be formed via oxidation 

of ferrous iron under acidic conditions present in the iron bogs. 

Schwertmannite (iron oxyhydroxysulfate; Fe8

O8(OH)6SO4) 

typically forms over the pH range 3 to 4. Goethite (iron 

oxyhydroxide; α-FeOOH) is known to form over a wider pH 

range (3.5 to 8.0). At sulfate concentrations less than 1,000 

mg/L and pH less than 6.0, goethite can form directly from 

solution (Carlson and Kumpulainen, 2000). Goethite also 

forms via recrystallization and drying of initially hygroscopic, 

amorphous and crystalline iron oxyhydroxides such as ferrihydrite (Fe(OH)3

). For simplicity, the crystalline (goethite, 

schwertmannite, ferrihydrite) and amorphous iron-aluminumsulfate-rich solids in iron bogs will be collectively referred 

to as iron oxyhydroxide in this chapter. Hematite (iron oxide, 

Fe2O3) may form by dehydration and recrystallization of 

precursor iron phases such as goethite or ferrihydrite; it is 

the thermodynamically stable iron oxide in iron bog environments. However, several kinetic and physicochemical factors 

control whether hematite, goethite, or other iron-bearing solids 

will form and remain stable in the iron bogs.

Iron bogs are considered active when water is present 

on the surface, issues from the base or sides of the deposit, or 

in some cases, collects in moderate-sized pools. (Most of the 

active bogs can be considered “iron fens”; however, because 

other depositional sites had been subject to surface water contributions, inactive (“dry”) for more than a year, or influenced 

by anthropogenic activities, a more encompassing term of 

“iron bog” was employed to describe these sites.) An inactive 

iron bog is one with a dry surface and no water issuing from 

the base or sides. Plants on an inactive bog may be dormant, 

dead, or completely absent depending on the length of time 

the bog has been inactive. The designation of an iron bog as 

active or inactive carries no precise age significance, only that 

water was present or absent from the site when visited. At 

some sites, an active iron bog is adjacent to an inactive one as 

a result of migration of water that reaches the site. Because 

aqueous data were not obtainable from inactive bogs, this 

chapter focuses on the characteristics of active, modern iron 

bog environments.

The geochemical, mineralogical, microbiological, 

and depositional characteristics are used to develop genetic 

models that can be compared to help identify formation 

mechanisms. Our model indicates that the iron oxyhydroxide 

component in all types of iron-bearing deposits forms via 

the same fundamental mechanism. However, some aspects 

of this model remain uncertain, such as the origin and 

evolution of the ferrous iron-bearing ground water, magnitude 

and types (regional versus local) of ground-water flow paths, 

and specific rates of ground-water flow and iron deposition. 

The results illustrate that iron oxyhydroxide deposition has 

been occurring in the region for several thousands of years. 

The end result of the depositional process is the formation 

of iron bogs that can act as short- and long-term reservoirs 

(or sources) of major and trace elements in the Animas River 

watershed study area.

Field and Analytical Methods

Study Area, Geology and Alteration Types, 

and Sample Localities

The Animas River watershed study area (fig. 1) covers 

approximately 500 km2

 (25 km E.-W. by 20 km N.-S.); the 

area is subdivided into three drainage basins: Mineral Creek, 

Cement Creek, and upper Animas River. The study area of 

this chapter (dotted rectangle, fig. 1) includes Cement Creek 

from 5 km below its headwaters in Ross Basin to its confluence with the Animas River, South Fork Mineral Creek west 

and outside of the Silverton caldera margin, and Mineral 

Creek from its confluence with South Fork Mineral Creek to 

its headwaters near Red Mountain Pass. Figure 1 shows the 

sites where samples described in this chapter were collected.

Figure 1 (facing page). Animas River watershed study area, 

showing generalized rock units and geologic contacts, areas of 

dominant alteration types, major drainages, sample site localities, 

and types of samples collected at each locality. Dotted rectangle, 

area of study of this chapter. Drainage map modified from Church 

and others (1997).

694 Environmental Effects of Historical Mining, Animas River Watershed, Colorado
Formation and Geochemical Significance of Iron Bog Deposits 695

RP

RP

107°45' 107°37'30"

37°52'30"

37°48'45"

37°56'15"

3,929

4,206

4,075

3,958 4,075 4,107

4,111

Howardsville

Silverton

Eureka

1

2

3

4

5

6

7

8

9 10

11

12

14

17

16

Ross

Basin Red Mtn Pass

Red Mtn

No. 3

Prospect Gulch

Sultan Mtn

RP

RP

RP

RP

RP

QSP

WSP

QSP

RP

RP

QSP

QSP

Silverton

Volcanics

San Juan Formation

volcanics

Sultan Mtn

stock

Pennsylvanian-Jurassic

sedimentary rocks Precambrian

rocks

Burns Member of

Silverton Volcanics

Approximate boundary of

Silverton caldera

AS

AS

COLORADO

Durango

Grand

Junction

DENVER

Animas River watershed

study area

13

WSP

3,792

15

Site No. Drainage Site name and (or) description Samples collected

1 SFMC Iron bog *S, *GW, SW

2 SFMC Wetland (organic) S, GW, SW, M

3 CC Seep S, SW

4 CC Upper iron bog *S, *GW, *SW, *M

5 MC Imogene adit drainage S, SW, M

6 MC Bonner mine waste pile drainage S, *GW, *SW, M

7 AR Mine-waste pile (dry) S

8 MC Junction mine adit, iron bog *S, GW, *SW, *M

9 CC Topeka Gulch spring deposit S, SW, M

10 MC Seep, iron bog S, GW, *SW

11 SFMC Iron bog *S(c), *GW, *SW, *M

12 CC Seep N. of Prospect Gulch SW

13 CC Red spring iron bog, Prospect Gulch *S(c), *GW, *SW, *M

14 CC Seep SE. of Prospect Gulch SW

15 CC Cascade Gulch adit ---

16 CC Lower iron bog ---

17 CC Yukon tunnel ---

SFMC, South Fork Mineral Creek; MC, Mineral Creek; CC, Cement Creek; AR, Animas River.

S, sediment; S(c), sediment core; GW, ground water; SW, surface water; M, microbiological.

(*), multiple samples of this type taken at site; (---), no samples collected at site.

4,206 Mountain peak

and elevation (m)

Sample site Alteration type

and number

11

Regional propylitic

Quartz-sericite-pyrite

Weak sericite-pyrite

Acid-sulfate

Geologic contact

RP

WSP

QSP

AS

0123 MILES

01234 KILOMETERS

Min eral Creek

Middle Fork

South

Fork

Cem

e

nt

C reek

A ni m a s

R ive r
Hydrothermal alteration associated with mineral 

deposits in the area produced suites of minerals that represent gradational differences in intensity of alteration (Bove 

and others, this volume, Chapter E3). Regional propylitization (RP) is the weakest grade, and minerals are primarily altered feldspars with little or no sulfides except pyrite. 

Weak sericite-pyrite (WSP) is low-grade alteration where 

the pyrite has low metal contents. Quartz-sericite-pyrite 

(QSP) is medium- to high-grade alteration; these minerals 

have metal abundances generally higher than do WSP alteration minerals (but variable). The highest grade alteration 

is acid-sulfate (AS), characterized by abundant metal-rich 

sulfide minerals. Vein-related quartz-sericite-pyrite altered 

minerals (V-QSP alteration; Bove and others, this volume) 

are present on an areally restricted, smaller scale compared 

to the other four alteration assemblages; therefore, V-QSP 

alteration is not considered separately here. Tetrahedrite, 

galena, sphalerite, and chalcopyrite are the primary ore 

minerals in the deposits in the region (Bove and others, 

this volume). A highly generalized depiction of rock types, 

areas of dominant alteration types, and geologic contacts 

is in figure 1.

South Fork Mineral Creek drains several alteration types: 

WSP/QSP-altered rocks of the subeconomic porphyry coppermolybdenum deposit on and near peak 3,792 m (hosted by 

lower Oligocene San Juan Formation volcanics) to the north,

unaltered Pennsylvanian-Permian redbeds through Jurassic 

sedimentary rocks south and west of peak 3,792 m, and 

altered and unaltered San Juan Formation volcanics farther 

west (Yager and Bove, this volume, pl. 1). Across South Fork 

Mineral Creek (southeast of peak 3,792 m) is the porphyritic 

Sultan Mountain stock (Oligocene age), an intrusive approximating quartz monzonite-diorite composition (Yager and 

Bove, this volume).

The Mineral Creek basin also drains multiple alteration 

types, from unaltered RP and WSP/QSP type rocks along its 

western border to dominantly AS and QSP altered rocks to 

the east and north.

Cement Creek is dominated on the west and north by 

acid-sulfate and QSP-altered rocks of the Silverton Volcanics 

(Oligocene) that make up the Silverton caldera, whose 

margins are approximated by the large circular outline in 

figure 1. Rocks east of Cement Creek are mainly propylitically 

altered with scattered zones of WSP and QSP alteration.

Along the upper Animas River are rocks of the Burns 

Member of the Silverton Volcanics, which are dominantly 

propylitically altered with small areas of WSP east of the 

river and QSP west of the river. The Silverton Volcanics are 

host rocks for most of the mineral deposits in the Animas 

River watershed study area north and east of the South Fork of 

Mineral Creek area (Lipman and others, 1973). See Yager and 

Bove (this volume, pl. 1), and Bove and others (this volume) 

for a detailed summary of the geology and alteration types in 

the area.

Solid Phase Chemistry

Sample Collection, Preparation, and Analysis

Sediment (iron oxyhydroxide; core or surface), ground 

water, surface water, and microbiological samples were collected at each site (when possible) in the study area (fig. 1). 

The active iron-deposition sites include iron bogs (fig. 2), 

springs, seeps, and mine drainages (adits or waste piles). 

Organic-rich wetlands can be sites of limited iron deposition but are compositionally different from iron deposits; 

they are described in Stanton, Fey, and others (this volume, 

Chapter E25).

Surficial unindurated iron oxyhydroxide samples 

(<1–5 cm thick) were removed with a spatula; indurated 

surface samples were dislodged using a cold chisel and small 

sledgehammer. Subsurface samples most commonly were 

taken as cores by driving a 0.5 to 1.5 m length of 5-cm 

diameter PVC pipe into the sediment. Cores were measured 

for recovery and then closed off from the air by sealing the 

ends with PVC caps. In instances where coring was unworkable (highly indurated sediment), a shallow trench (<1 m) 

was dug to obtain subsurface material. Grab samples were 

stored in polyethylene bags until processed in the lab approximately 2 weeks after collection.

Subsamples from cores were taken at 1 cm intervals to 

provide adequate analytical coverage. If present, large clasts 

were hand-separated; all sediment was sieved to less than 

63 mesh to remove sand-sized grains and macroscopic organic 

matter. Iron oxyhydroxide was generally fine grained and 

homogeneous, but detrital grains smaller than 63 mesh were not 

removed by sieving. Sediment was air-dried if damp, split, then 

powdered to <230 mesh (57 μm (micrometers)) using a ceramic 

mortar and pestle or shatterbox for use in subsequent analyses.

Mineralogy

X-ray diffractometry (XRD) was used to examine mineralogy of iron oxyhydroxide. A packed powder mount prepared 

from the <230 mesh sample was scanned using Ni-filtered Cu 

Kα radiation. The detection limit of a mineral with this method 

is approximately 5 weight percent.

The crystalline phase designations of major, minor, and 

trace, and estimations of amorphous material, were obtained 

by using the background intensity of the diffractograms relative to the intensities of the crystalline phases in the unknown 

sample (for amorphous material), and to the intensities of pure 

phases relative to their intensities in the unknown sample (for 

crystalline material). The designations (in weight percent) of 

major (>25), minor (≥5 to ≤25), and trace (<5) abundances are 

derived from the difference of 100 percent crystalline material 

minus the estimated amorphous material. Major, minor, and 

trace designations indicate only qualitative amounts and relative abundances of crystalline or amorphous solids detected.

696 Environmental Effects of Historical Mining, Animas River Watershed, Colorado
Formation and Geochemical Significance of Iron Bog Deposits 697

Element Abundances

A split of the <230 mesh sample was analyzed by 

inductively coupled plasma–atomic emission spectrometry 

(ICP-AES) for 40 major and trace elements. Sample preparation involves sequential digestion using hydrochloric, nitric, 

perchloric, and hydrofluoric acids. Details of the analytical 

technique, apparatus, and detection limits can be found in 

Briggs (2002).

Carbon Abundance, Isotopic Dating (14C), 

and Composition

To see whether 14C dating could be applied to determine the age of the deposits, aliquots of the <230-mesh solid 

samples were analyzed for total carbon (C(tot)), organic carbon 

(C(org)), and carbonate (mineral) carbon (C(min)) by induction 

furnace methods (Brown and Curry, 2002; Curry, 1990). Most 

iron oxyhydroxide samples did contain sufficient organic 

carbon to employ 14C dating and carbon isotopic composition (δ13C) methods that were later performed at a commercial laboratory. The uppermost layers (approximately 0.5 m) 

of iron bogs with visible, recently deposited organic matter 

were not dated, as they would show only modern dates. The 

ages are important to constrain the time of formation of the 

iron bogs, but an extensive discussion of the role of carbon in 

the geochemical evolution of the iron bogs is not within the 

scope of this chapter. Tabulation of the 14C dating results are 

in Verplanck and others, this volume; Vincent and Elliott, this 

volume, Chapter E22; and Vincent and others, this volume, 

Chapter E16.

Aqueous Chemistry

Sample Collection, Preparation, and Analysis

Reconnaissance Study

Reconnaissance water sampling at multiple sites was 

undertaken to determine the aqueous geochemistry of different 

environments where iron deposition was actively occurring. 

To adequately compare and contrast the different environments, 26 water samples from 11 localities were collected and 

analyzed. Sample sites include four iron bogs, three adit or 

waste-pile drainages, seven seeps (three associated with the 

upper bog of Cement Creek), and two spring deposits (fig. 1). 

Figure 2. Outlet at south end of lower bog in Cement Creek basin. Surface dimensions of 

this iron bog are approximately 60 m long by 6 m wide. In common with other active iron bogs, 

this deposit is characterized by abundant fine-grained iron oxyhydroxide, living organisms (for 

example, the mosses near the base and on the surface), relict organic matter, and a lack of 

large clasts. Visible part of steel spatula is approximately 10 cm long.
One organic bog (wetland; site 2) was included in the sampling (but not in the statistical calculations) to allow comparison of the iron-rich deposits to a low-iron system. Three other 

sites were visited and described but no samples were collected 

(sites 15, 16, and 17).

Samples were collected from water flowing across the 

surface, issuing from the base or sides of larger deposits, 

from pools in larger bogs, or where fresh iron oxyhydroxide was visible (for example, seeps along Cement Creek). 

At some localities, water was collected using a “sniffer,” a 

simple well-point device equipped with Teflon tubing that can 

extract ground water and indicate if hydraulic head is present 

(Wanty, 2000). After the device was driven through the solids 

into the iron bog subsurface (about 1 m maximum depth), the 

area penetrated was allowed to stabilize for approximately 

5 minutes (as evidenced by clearing of the water); the well 

was developed by drawing five volumes (20 mL each) of water 

through the sample collection loop and discarding, and then 

the ground or surface water sample was collected. Where head 

measurements could not be made, designation of water type 

was based on whether the discharging water contained detectable dissolved oxygen (surface water) or no detectable dissolved oxygen (ground water). Lack of measurable dissolved 

oxygen was later found to be characteristic of ground water.

Samples were drawn into a 60-mL (milliliter) lowdensity polyethylene syringe, then filtered through a 0.45 μm 

polycarbonate filter into a hydrochloric acid (HCl)-washed, 

deionized-water-rinsed, high-density polyethylene bottle. For 

cations, the filtered samples were acidified to pH <2 with concentrated nitric acid (HNO3

); for anions, no further treatment 

after filtration was necessary. Alkalinities were not measured, 

as all but three samples had pH <5.0, and so no carbonate species should be present. The cation aliquots were analyzed for 

44 elements by inductively coupled plasma–mass spectrometry (ICP-MS; Lamothe and others, 2002).

Total iron (Fetot) and ferrous iron (Fe2+) were determined 

on-site within 15 minutes of filtration using a portable spectrophotometer operating at 510 nm (nanometer) wavelength 

(Theodorakos, 2002); ferric iron (Fe3+) was determined by 

difference (Fe3+ = Fetot – Fe2+). These values were used to 

calculate the ratio of ferrous to total iron (Fe2+/Fetot). Aqueous 

sulfate (SO4

2–) was determined at eight sites using the Hach 

method (Hach Chemical Company, 1996) and on all samples 

2 weeks later by ICP-MS. The field and laboratory sulfate 

analyses agreed to within ±10 percent (data not shown).

Dissolved oxygen (DO, ppm), pH (standard units), temperature (°C), and specific conductance or conductivity (SpC; 

microsiemens/centimeter, μS/cm) were measured using meters 

calibrated with standards 5–10 minutes before field sampling. 

Measurements of pH on filtered and unfiltered aliquots to 

check the possible effect of colloidal iron showed no significant differences between the two sample types. The pH values 

agreed within ±5 percent for both measurements (data not 

shown).

Detailed Study of the Upper Bog of Cement Creek

To examine short-term (≤24 hours) changes in aqueous 

geochemistry, the upper bog along Cement Creek was studied 

in detail (site 4, fig. 1). The upper bog is an areally extensive 

(approximately 30 m long by 6 m wide) iron bog on the west 

bank of Cement Creek southeast of Prospect Gulch. During 

a day-long operation and in a subsequent sampling, numerous samples were collected along the long dimension (parallel to stream flow) where ground water would be most likely 

discharging into the bog. Based on the presence of a positive 

hydraulic head in the sniffer device, samples from the upper 

bog were separated into ground (head present) or surface (no 

head present) water. However, because the head was often 

small (2.5–7 cm), it could not be considered an infallible indicator of ground or surface water.

Sample collection, preparation, and analysis were the 

same as in the reconnaissance study. All samples were collected in July and August, the active time of snowmelt, rainfall, and runoff processes. During these months, biogeochemical processes and shallow and deep ground-water flow that 

could influence iron mobility are most likely to be occurring.

Biological Characterization

Iron and Sulfur Oxidizing Bacteria

Isolation and enumeration by Most Probable Number 

(MPN) culturing techniques (Gerhardt and others, 1994) 

was undertaken at eight sites to determine populations of 

Thiobacillus species. A 1.0 g (gram) raw sample of wet iron 

oxyhydroxide sediment was placed into a tube with 9 mL of 

aqueous medium and disaggregated by shaking for 5 minutes. 

This inoculation was defined as the first in a series of dilution 

tubes (100). One milliliter from this tube was used to inoculate 

other dilution tubes (from 10–1 to 10–5) containing the aqueous 

medium specifically formulated to enhance the growth of the 

microbes (Atlas, 1995). For Th. ferrooxidans, a reduced iron 

source (FeSO4

) was used; for Th. thiooxidans, a reduced sulfur 

source (thiosulfate, S2

O3

2–) was used. Tubes were inoculated 

on-site, then incubated at 37°C upon return to the lab. After 

2 weeks, 1 month, and 2 months growth, the tubes were scored 

for the presence (increased turbidity) or absence (no increase 

in turbidity) of growth. Stained slide preparations were made 

from tubes that showed positive signs of growth to confirm the 

presence of bacteria and examine their morphology.

Geochemical reactions attributed to iron-related bacteria (IRB) were determined at selected sites using standard 

test kits (Biological Activity Reagent Tests or BART’s; Hach 

Chemical Co., 1997). The tests do not distinguish among different genera of iron oxidizers but can be used as confirmatory 

evidence of their presence. One bacterial genus that would be 

among those determined by these tests is Leptothrix. At each 

site, the presence or absence of Leptothrix was noted, but no 

698 Environmental Effects of Historical Mining, Animas River Watershed, Colorado
Formation and Geochemical Significance of Iron Bog Deposits 699

attempts were made to isolate or enumerate this microbe by 

culture techniques. The medium was inoculated with a 1 mL 

unfiltered aqueous sample from the iron bog, incubated at 

room temperature (25°C), and checked daily over a 2-week 

period for either a (+) result (reddening of the originally clear 

medium) or a (–) result (no color change). Both sets of results 

(MPN and BART) are qualitative measures of the numbers of 

bacteria present at a site.

Photosynthetic Organisms

Associated with most iron-depositing environments are 

photosynthetic macro and micro organisms which grow on 

the surface, in pooled water, and adjacent to the boundaries of 

the deposit. These are mosses, cyanobacteria, algae, and small 

vascular plants (Yager and Bove, this volume, pl. 2, figs. 7 and 

14). Observations of these different organisms at several sites 

were made to look for systematic similarities or differences in 

the flora. The organisms might act as physical or geochemical 

agents by stabilizing the growing iron oxyhydroxide layers, 

acting as traps or sorbents for iron oxyhydroxide, or providing 

dissolved oxygen (molecular O2

) as an oxidant of aqueous ferrous iron. In addition, the relict forms of carbon, particularly 

organic carbon, measured in these bogs probably originated 

from these organisms. Therefore, the different organisms are 

potentially significant to the growth and geochemical evolution of the iron bogs.

Riparian zone plants also were cataloged at eight sites 

during the study of the iron deposits. These include conifers, 

deciduous trees, woody shrubs, grasses, and sedges. The 

riparian zone plant data are not presented here but can be 

found in the database chapter (Sole and others, this volume, 

Chapter G).

Geochemical Modeling

Inverse geochemical modeling was employed to help us 

understand processes responsible for the formation of the iron 

bogs. Inverse modeling attempts to assess chemical evolution 

of ground and surface water, and any associated solid phases, 

along a flow path. The water compositions and parameters 

(pH, specific conductance (SpC), temperature) of the active 

upper bog of Cement Creek were first used with NETPATH 

(Plummer and others, 1992) to examine the possible distributions of major elements within solid phases that could form 

along a flow path beginning in ground water and ending in 

surface water. The NETPATH results were then used as data 

for PHREEQEC (Parkhurst and Plummer, 1995; Allison and 

others, 1991) to examine precipitation processes that may be 

taking place in the iron bog water. (Thermodynamic data for 

the schwertmannite modeling were provided by D.L. Parkhurst 

and Laurie Wirt, written commun., 2002.)

Modeling results were used to help constrain the type of 

water that favors formation of the minerals observed by X-ray 

diffraction. Model comparison with actual data was based 

primarily on mineralogical compositions determined by X-ray 

diffraction. That is, if a mineral predicted by the model results 

was detected by X-ray diffraction, this was considered supportive evidence that the model was fundamentally correct for 

that mineral.

Results

Solid Phase Chemistry of Iron Oxyhydroxide

Location and Size of Iron Deposits

Some iron deposits form where springs or seeps discharge on slopes or at breaks in slope and cause local cementation of material. These deposits form by continued accumulation of iron oxyhydroxide with little added clastic content, 

except the material immediately underneath the layer(s) of 

iron oxyhydroxide. Often, this growth results in the formation of long, wide iron oxyhydroxide “ribbons” in gullies or 

“curtains” adhering to steep slopes or even cliff faces. In this 

chapter, these iron oxyhydroxide ribbons and curtains are 

considered spring deposits.

Spring and seep deposits appear to form in a similar manner; the primary difference between these two environments 

is that seeps have lower discharge volumes and produce much 

smaller accumulations of iron solids than bogs or springs. Iron 

oxyhydroxide also accumulates where mine adit or wastepile discharges reach the surface. Based on the absence of 

reworked and (or) transported clasts, spring, seep, and adit/pile 

discharge deposits have been classified as subtypes of iron bog 

deposits (Verplanck and others, this volume).

The surficial and volumetric accumulations of the different iron deposits have a wide range. For example, seeps 

produce small thin accumulations of iron oxyhydroxide at the 

surface (<1 m2

; 0.1 to <1 cm thick); mine adit/pile drainage 

deposits generally contain moderate accumulations (1–100 m2

; 

0.5 cm–1.0 m thick); most iron bogs provide major accumulations (≤180 m2

; ≤2 m thick; fig. 2). Spring deposits display the 

widest range in accumulations: some cover surfaces of a few 

hectares and have thicknesses estimated at 3–5 m. Site 10 on 

Mineral Creek (fig. 1) was one of these large spring deposits, 

yet it had only a few small volumes of water discharging to the 

surface when sampled.

A distinctive type of deposit, referred to as “waste pile 

ferricrete,” forms when mined waste rock becomes cemented 

by iron oxyhydroxide. Such deposits are important indicators 

of modern, rapid formation of iron oxyhydroxide cement and 

are discussed later in this chapter.
Mineralogy

Table 1 shows example results for iron minerals and 

their relative abundances from different iron bog or spring 

samples. (These samples were collected at a variety of locations in the area and do not necessarily coincide with the sites 

shown in fig. 1.) Most samples were mixtures of crystalline 

and amorphous material, but in some cases, the solids were 

either all crystalline or all amorphous material. For example, 

in several fresh surface samples, no crystalline iron-bearing 

phases were detected (sample SV-FFC; table 1). Crystalline 

mineral phases identified by X-ray analysis generally make 

up 0–25 weight percent of the solids; the remaining amorphous material that cannot be identified by X-ray diffraction 

is iron and sulfate rich (Desborough and others, 1999). Additional X-ray diffraction results are in the database (Sole and 

others, this volume).

Goethite and schwertmannite were the dominant ironbearing minerals in both active and inactive iron bogs. The 

two minerals were generally present as minor (≥5 to ≤25 wt. 

percent) components of the total solids; in many samples, 

one mineral was relatively more abundant than the other. Of 

six iron bog samples, goethite was the dominant iron-bearing 

mineral in three (and the only crystalline phase in two of these 

samples), whereas schwertmannite was dominant in the other 

three (table 1).

Schwertmannite was the primary crystalline component of iron precipitates on streambed and iron bog surfaces, 

and was present in varying abundance at depth in iron bogs 

(≤3 cm). Hematite was not present in any sample.

Quartz was the major rock-forming (silicate) mineral. 

Other silicates included kaolinite, orthoclase, and muscovite, 

and secondary (weathered) clinochlore and montmorillonite.

Major and Trace Elements in Solids

The median and range (minima, maxima) are the principal measures used for comparison of major- and traceelement concentrations and physiochemical parameters in 

sediment and water. Geochemical data generally do not exhibit 

normal distributions (Till, 1974), and in our data, lognormal 

or bimodal distributions were seen. In such distributions, 

samples with very high (Junction mine; site 8, fig. 1, AMLI 

mine site # 77) or very low (Topeka Gulch; site 9, fig. 1) concentrations would exaggerate mean values.

Core Samples

Data from Core 97ABS327, from the red spring at the 

lower end of Prospect Gulch (site 13, fig. 1), show iron and 

aluminum concentrations typical of active iron bogs. In 

individual samples from this core, iron ranges from 6.4 to 

46.0 wt. percent and aluminum ranges from 0.4 to 6.1 wt. 

percent (fig. 3; table 2), with median values of 36.0 wt. 

percent iron and 1.8 wt. percent aluminum over the entire 

core.

Trace-element ranges (table 2) show copper above 

the detection limit of 1 ppm in only 14 of 69 samples. However, in the interval of 20–30 cm, eight contiguous samples 

had relatively high copper. Lead was detectable in 61 of 

68 samples, and zinc and arsenic were detected in every sample. Of particular interest is the high median arsenic content 

of 2,700 ppm.

Core 999291 was taken on South Fork Mineral Creek 

about 1.3 km before it meets main Mineral Creek (site 11, 

fig. 1). Iron and aluminum ranges were 2.9–41.0 and 

2.2–7.5 wt. percent, respectively (fig. 4). Respective median 

values for iron and aluminum over the entire core were 27.0 

and 4.2 wt. percent.

Trace elements in core 999291 show a marked contrast 

to the samples in core 97ABS327 from Prospect Gulch. 

Copper was detectable in all but 5 samples and zinc was 

detected in all samples, but lead was detected in only 11 

samples and arsenic in only 8 samples. These two cores demonstrate that although iron and aluminum concentrations tend 

to be similar in iron bog solids, trace-element enrichment can 

differ widely.

Bonner Waste Pile

The sample of iron oxyhydroxide cement from the 

Bonner mine waste pile (AMLI mine site # 172; table 2) 

shows iron present at the same maximum abundance (46.0 wt. 

percent) as in the red spring core. However, aluminum is 

markedly lower (0.05 wt. percent) in the Bonner sample than 

in either of the two cores. Copper, lead, zinc, and arsenic were 

all at detectable levels, which again points out the variability 

of iron oxyhydroxide enrichment with respect to depositrelated elements.

Table 1. Examples of X-ray diffraction results from iron bog 

or spring samples, Animas River watershed study area, showing 

types of iron-bearing, nonsilicate minerals.

[Rock-forming minerals such as quartz were present in most samples. Iron 

minerals or solids are listed in approximate order from highest to lowest 

relative abundance]

Sample No. Sample type Iron minerals

IDY0014 Iron bog Schwertmannite, goethite.

IDY00-29A Iron bog Schwertmannite, goethite, 

jarosite.

SDY0017A Iron bog Goethite.

95-ABS-119 Iron bog Goethite.

99ABFC-141B Iron bog Schwertmannite, goethite.

99VMS-65A Iron spring, inactive Goethite, schwertmannite.

97-ABS-318C Iron bog, 3 cm depth Schwertmannite, goethite.

99ABFC-154C Iron spring, active Amorphous iron 

oxyhydroxide.

SV-FFC “Filamentous” iron 

oxide attached to 

algal mass.

Amorphous iron 

oxyhydroxide.

700 Environmental Effects of Historical Mining, Animas River Watershed, Colorado
Formation and Geochemical Significance of Iron Bog Deposits 701

Carbon Abundance, Ages, and Composition

Iron oxyhydroxide samples taken from below about 0.5 m 

contained low concentrations of all forms of carbon; in these 

deeper samples, total carbon (the sum of organic plus mineral carbon) usually did not exceed 1 wt. percent. However, 

organic carbon in samples above 0.5 m from active iron bogs 

routinely exceeded 30 wt. percent.

The 14C ages of all iron deposits (including ferricretes)

range from approximately 9,200 years before present (yr B.P.) 

to 1,200 yr B.P. (Verplanck and others, this volume; Vincent 

and others, this volume). However, we do not know whether 

organic matter in iron oxyhydroxide represents all “old” 

carbon originally deposited in the bog, or a mixture of modern 

and old carbon deposited as the bog formed. The 14C dates do 

signify that old carbon has been deposited and (or) preserved 

Figure 3. Downhole plot of major- (Al, Fe, Ca (wt. percent)) and trace- (Cu, Pb, Zn, As (ppm)) element abundances, 

Core 97ABS327 from red spring of Prospect Gulch.

80

70

60

50

40

30

20

10

0

0.001 0.01 0.1 1 10 100 1,000 10,000

As ppm

Zn ppm

Pb ppm

Cu ppm

Ca %

Al %

Fe %

EXPLANATION

CONCENTRATION, IN WEIGHT PERCENT OR PARTS PER MILLION

DEPTH, IN CENTIMETERS

Core 97ABS327

(Cu values below

detection are

plotted as 1 ppm)
in the iron deposits, rather than having been completely 

oxidized and removed. The presence of relict wood and algal 

structures preserved in many iron oxyhydroxide samples suggests that old organic carbon was preserved in the matrix of 

the iron precipitates.

Carbon isotopic analyses that show most δ13C values fall 

within the expected range for terrestrial plants (–270/

00 (per 

mil) to –340

/

00) such as those found in the iron bogs. However, 

several values range from –220

/

00 to –260

/

00,suggesting an algal 

component (–120

/

00 to –230

/

00; Hoefs, 1980). The amount, if 

any, of fresh (modern) organic carbon that has been added to 

the preserved woody and algal materials is not known.

Aqueous Chemistry

An examination of differences in water compositions is 

a useful tool in differentiation and description of iron depositional environments. We follow this sequence of description:

1. All samples collected in this study

2. Ground water and surface water from all sites where iron 

is being deposited

3. Ground water and surface water from iron bogs only 

(excludes seeps and springs, and adit/pile drainages)

4. Ground water and surface water only from the upper bog 

of Cement Creek, a site of large-scale iron deposition and 

accumulation.

Comparisons discussed in this section are between ground 

water and surface water and within each type, and they include 

(1) concentrations of major elements, primarily Al, Fe, and S 

(as SO4

) but also Ca, Si, and Mn; (2) concentrations of four 

environmentally important trace elements (Cu, Pb, Zn, As); 

and (3) water chemistry parameters (pH, SpC, DO). Table 3 

summarizes several characteristics of the aqueous samples.

Water Composition—All Sites

Median aqueous concentrations of water sampled from 

all active iron depositional sites show elevated total iron (Fetot; 

40 mg/L), aluminum (15 mg/L), and sulfate (410 mg/L), and 

acid pH (4.2; table 4). Dissolved oxygen ranged from 0 to 

7 ppm, and median specific conductance was moderately high 

at 870 μS/cm. Exceptions to the high dissolved iron, aluminum, and sulfate concentrations were samples from the Topeka 

Gulch site (site 9, fig. 1), which also had high dissolved oxygen 

(5.2 ppm), the Mineral Creek spring deposit (site 10, fig. 1), 

and the Imogene adit (site 5, fig. 1, AMLI mine site # 136). In 

contrast to these dilute water samples, the Junction mine sample 

(site 8, fig. 1) had very high aqueous concentrations of iron and 

the four trace elements. Ground and surface water issuing from 

or feeding the active iron depositional sites contained differing 

concentrations of other major elements such as calcium, silica, 

and manganese, and trace elements such as copper, lead, zinc, 

and arsenic (table 3).

Table 4 summarizes median concentrations of major 

species (iron, aluminum, sulfate) and trace elements in seven 

water types separated as all water from all iron depositional 

sites, surface water from all sites, ground water from all sites, 

surface water from all iron bogs, ground water from all iron 

bogs, surface water from the upper bog of Cement Creek, and 

ground water from the upper bog of Cement Creek. As table 4 

shows, ground and surface water compositions from all iron 

bogs are very similar to those of the upper bog of Cement 

Creek. Therefore, the discussion of iron bog water chemistry 

will focus on samples from the upper bog.

Surface Water Versus Ground Water—All Sites

The box plot in figure 5 allows for comparison of 

major- or trace-element concentrations in ground or surface 

water. Outliers (samples above the 90th percentile or below 

the 10th percentile) are shown as open circles. The lower 

Table 2. Summary statistics for major- (Al, Fe, Ca (wt. percent)) and trace- (Cu, Pb, Zn, As (ppm)) element abundances in core 

and mine-waste iron oxyhydroxide samples.

[ND*, not determined because 70 percent or more of samples were below detection. <LLD, below lower limit of detection]

Al Fe Ca Cu Pb Zn As

Core 97ABS327, red spring of Prospect Gulch

n 68 68 46 14 61 68 68

Minimum 0.4 6.4 0.005 <LLD 4.0 35 180

Maximum 6.1 46.0 0.2 70 61 660 5,000

Median 1.8 36.0 0.02 ND* 16 130 2,700

Core 999291, South Fork Mineral Creek

n 35 35 35 30 11 35 8

Minimum 2.2 2.9 0.02 1.0 <LLD 25 <LLD

Maximum 7.5 41.0 3.0 110 24 430 29

Median 4.2 27.0 0.1 9.5 ND* 40 ND*

Bonner mine-waste pile, Middle Fork Mineral Creek

One sample 0.05 46.0 <LLD 61 15 46 54

702 Environmental Effects of Historical Mining, Animas River Watershed, Colorado
Formation and Geochemical Significance of Iron Bog Deposits 703

and upper lines of the rectangular box represent the 25th 

and 75th percentiles; the horizontal line through the box is 

the median (50th percentile). For any particular variable, a 

greater vertical extent of the entire box plot indicates greater 

variation, whereas a short compact box plot indicates less 

variation.

The concentrations of dissolved species are more distinct 

when ground water and surface water are considered separately. 

Median aluminum, iron, and sulfate in ground water are nearly 

twice the concentrations of surface water (fig. 5; table 4). 

Specific conductance of ground water is higher than that of 

surface water, consistent with the higher concentrations of 

elements indicative of weathering of sulfide and silicate minerals observed in ground water (fig. 5; table 5). Median dissolved 

oxygen is lower in ground water than in surface water.

The Junction mine site is responsible for the high traceelement concentration outliers in figure 5. Water draining 

the adit at this site was considered ground water because it 

was derived from within the barricaded mine through a 30 m 

length of PVC pipe. Because dissolved oxygen could not be 

measured at the pipe outlet, the measurement was made on 

water at the iron bog 10 m downstream of the pipe.

Figure 4. Downhole plot of major- (Al, Fe, Ca (wt. percent)) and trace- (Cu, Pb, Zn, As (ppm)) element abundances, 

Core 999291 from South Fork Mineral Creek.

60

50

40

30

20

10

0

DEPTH, IN CENTIMETERS

0.01 0.1 1 10 100

As ppm

Zn ppm

Pb ppm

Cu ppm

Ca %

Al %

Fe %

CONCENTRATION, IN WEIGHT PERCENT OR PARTS PER MILLION

Core 999291

EXPLANATION

(As values below

detection are

plotted as 10 ppm 

and Pb as 5 ppm)
Table 3. Sample site and location, deposit and alteration type, water type, major- (mg/L) and trace-element concentration (μg/L), water

chemistry parameter, Fe2+/Fetot ratio, detection limit, and range (minimum, maximum) of detectable concentrations for all water samples.

[SpC, specific conductance, in microsiemens per centimeter; DO, dissolved oxygen; NA, not analyzed or not applicable; LLD, lower limit of detection; <, less 

than; hydraulic head: P, positive; N, not observed; ND, not determined]

Site

No.

Sample site 

or location

Deposit

type

Alteration

type1

Water

type2

Fe

mg/L

 Al

mg/L

SO4

mg/L

Ca

mg/L

Cu

μg/L

Pb

μg/L

Zn

μg/L

As

μg/L

pH

units

SpC

μS/cm

DO

ppm

Fe2+/

Fetot

Hydraulic

head

4 Upper bog Fe bog AS g 71.9 15.9 910 270 14.5 18.5 1,740 77.1 4.30 1,510 0.0 1.00 P

4 Upper bog Fe bog AS g 72.3 16.6 810 280 14.9 10.4 1,720 67.6 4.27 1,520 0.0 1.00 P

4 Upper bog Fe bog AS s 69.8 17.4 900 238 9.4 3.9 1,620 49.2 4.00 1,380 3.3 .93 N

4 Upper bog Fe bog AS s 69.2 17.5 820 242 13.7 33.0 1,630 44.4 4.17 1,380 ND .99 N

4 Upper bog Fe bog AS g 67.2 18.6 710 205 12.3 6.3 1,680 10.7 4.15 1,280 0.0 .99 P

4 Upper bog Fe bog AS s 50.6 18.3 720 204 28.4 23.8 1,710 4.2 3.46 1,360 0.0 .94 N

4 Upper bog Fe bog AS s 50.2 17.8 780 235 14.4 75.4 1,760 13.1 3.40 1,430 5.0 .62 N

4 Upper bog Fe bog AS g 73.2 20.0 820 238 8.0 9.2 1,690 4.6 4.11 1,260 0.0 .98 P

4 Upper bog Fe bog AS s 52.4 20.0 750 226 82.4 100. 1,710 3.0 3.41 1,410 7.0 .89 N

4 Upper bog Fe bog AS g 71.6 22.1 580 134 13.6 75.5 1,620 76.3 4.47 900 0.0 .99 P

4 Upper bog Fe bog AS s 31.8 6.4 390 102 26.3 28.0 590 3.0 3.19 850 7.0 .76 N

10 Mineral Creek Spring WSP s 7.4 5.0 80 12.7 2.1 .83 190 2.0 4.26 210 3.5 .99 ND

3 Cement Creek Seep AS g 34.5 8.7 250 49 10.6 16.2 660 3.0 4.30 440 0.0 1.00 P

3 Cement Creek Seep AS s 30.4 8.3 340 51.1 11.6 29.7 630 <LLD 3.92 520 3.9 .95 N

3 Cement Creek Seep AS s 2.3 1.1 380 148 18.3 2.9 650 <LLD 5.94 770 6.2 .95 ND

12 Cement Creek Seep AS g 38.1 8.8 350 93.1 20.3 16.4 650 10.3 4.27 670 0.0 .99 P

12 Cement Creek Seep AS g 37.6 8.7 350 91.0 8.8 37.5 640 11.2 4.26 700 0.0 .98 ND

14 Cement Creek Seep AS g 35.9 9.4 270 53.0 10.1 34.9 630 11.9 4.27 520 0.0 .97 P

14 Cement Creek Seep AS s 35.8 8.7 260 52.1 14.2 27.5 650 11.4 4.19 550 ND .99 ND

1 So. Fk. Mineral Fe bog RP s 30.0 16.0 230 23.0 40.0 .70 240 7.0 4.74 320 ND ND ND

9 Topeka Gulch Spring AS s 0.2 1.8 110 39.0 7.0 <LLD 60 5.0 3.35 240 5.2 ND ND

13 Red spring Fe bog AS g 45.0 20.0 300 32.0 <LLD .50 1,100 38.0 3.47 680 5.1 ND ND

4 Upper bog Fe bog AS g 42.0 15.0 710 200 6.0 4.0 1,500 7.0 3.37 1,310 5.8 ND N

5 Imogene adit Adit RP s 9.1 1.3 220 70.0 10.0 <LLD 420 7.0 5.87 460 ND ND ND

6 Bonner mine Adit/pile QSP s 10.0 6.1 430 120.0 100.0 11.0 2,300 8.0 3.17 990 5.1 ND ND

8 Junction mine Adit/Fe bog AS g 140.0 19.0 870 76.0 18,000 110 53,000 4,300 2.68 1,900 *5.3 ND ND

2 **Organic bog Wetland RP s 0.5 0.9 100 39.1 8.0 <LLD 50 5.0 3.25 890 6.5 ND ND

LLD <0.03 <0.1 <0.3 <0.05 <0.05 <0.05 <0.2 <2 NA NA 0.1 NA

n 26 26 26 26 25 25 26 24 26 26 21 NA

Min 0.2 1.1 80 13 2.1 0.5 60 2.0 2.7 210 0 NA

Max 140.0 22.1 910 280 18,000 110 53,000 4,300 5.9 1,900 7.0 NA

1

RP, regional propylitic; WSP, weak sericite-pyrite; QSP, quartz-sericite-pyrite; AS, acid-sulfate.

2

g, ground water; s, surface water; n, number of samples.

*

Measurement taken in iron bog downstream from adit/pipe.

**Sample not used in calculation of statistical parameters.

Table 4. Median values of major-element concentrations (Al, Fe, S (as SO4); mg/L), trace-element concentrations (Cu, Pb, Zn, As; 

μg/L), and water chemistry parameters according to separation of water types where iron is actively being deposited.

[SpC, specific conductance (μS/cm; microsiemens per centimeter); DO, dissolved oxygen (ppm; parts per million)]

Water type n Al Fe SO4 Cu Pb Zn As pH SpC DO

All water 26 15.4 40.0 410 14 17 1,300 11 4.2 870 3.3

All surface water 15 8.5 31.1 390 14 26 650 7 4.0 810 5.0

All ground water 11 16.2 56.1 650 12 16 1,560 12 4.3 1,080 0

All iron bog surface water 7 17.6 50.4 730 26 26 1,620 10 3.5 1,370 5.0

All iron bog ground water 8 18.6 71.6 710 13 9 1,680 2 4.2 1,280 1.8

Upper bog surface water 6 17.6 51.5 760 20 30 1,670 9 3.4 1,380 5.0

Upper bog ground water 6 17.6 71.8 760 13 10 1,680 39 4.2 1,290 0

704 Environmental Effects of Historical Mining, Animas River Watershed, Colorado
Formation and Geochemical Significance of Iron Bog Deposits 705

Table 5. Summary statistics of major- (Al, Fe, Ca, Si, S (as SO4), Mn; mg/L) and trace- (Cu, Pb, Zn, As; μg/L) element concentrations, 

and water chemistry parameters in surface and ground water, all iron depositional sites.

[SpC, specific conductance (μS/cm; microsiemens per centimeter); DO, dissolved oxygen (ppm)]

Surface water Ground water

n Minimum Maximum Median n Minimum Maximum Median

Al 15 1.1 20.0 8.7 11 8.7 22.1 16.6

Fe 15 0.2 69.8 33.8 11 34.5 140.0 67.2

Ca 15 13 242 111 11 32 280 93

Si 15 7.8 92 27 11 25 78 27

SO4 15 80 900 390 11 80 910 580

Mn 15 0.2 3.0 1.9 11 0.8 6.2 1.8

Cu 15 2.1 100 14 10 8.0 18,000 13

Pb 13 0.7 100 28 11 0.5 110 16

Zn 15 60 2,300 650 12 630 53,000 1,620

As 13 2 49 7 12 3 4,300 12

pH 15 3.2 5.9 4.0 12 2.7 4.5 4.3

SpC 15 210 1,430 810 12 440 1,900 900

DO 10 0 7.0 5.0 11 0 5.1 0

0.01

0.1

1

10

100

1,000

10,000

100,000

Al

mg/L

Fe

mg/L

SO4

mg/L

 Cu

μg/L

 Pb

μg/L

 Zn

μg/L

 As

μg/L

pH

units

SpC

μS/cm

DO

ppm

Surface water

Ground water

AQUEOUS SPECIES OR PARAMETER

CONCENTRATION OR UNITS

Figure 5. Box plots of major- (mg/L) and trace- (μg/L) element concentrations, and water chemistry parameters in surface 

and ground water, all iron depositional sites. An arbitrary lower limit of 0.1 ppm DO (dissolved oxygen) has been used for 

plotting purposes.
0.01

0.1

1

10

100

1,000

10,000

Al

mg/L

Fe

mg/L

SO4

mg/L

Cu

μg/L

Pb

μg/L

Zn

μg/L

As

μg/L

pH

units

SpC

μS/cm

DO

ppm

AQUEOUS SPECIES OR PARAMETER

CONCENTRATION OR UNITS

Surface water

Ground water

Figure 6. Box plots of major- (mg/L) and trace- (μg/L) element concentrations, and water chemistry parameters in surface 

and ground water, upper bog of Cement Creek. An arbitrary lower limit of 0.1 ppm DO (dissolved oxygen) has been used for 

plotting purposes.

Surface Water Versus Ground Water—Upper 

Bog of Cement Creek

The presence or absence of measurable head was a criterion that we used to separate ground water (head present) from 

surface water (head absent). By this criterion, upper bog surface 

water shows higher calcium and manganese, identical aluminum and sulfate, and lower iron and silica compared to ground 

water (fig. 6; table 6). In surface water, median pH is lower 

while specific conductance and dissolved oxygen are higher.

The similar aluminum and sulfate concentrations in 

surface and ground water suggest either that both water types 

arise from a similar source or that the two water types are 

evolving towards the same composition. Because samples 

were collected under near-base-flow conditions, ground water 

and surface water likely originate from the same source. As 

is evident, iron is the one element that distinguishes ground 

water from surface water in the upper bog of Cement Creek.

Ratio of Fe2+ to Fetot

For samples where iron species measurements were made, 

the ratio of Fe2+ to Fe3+ also proved useful in distinguishing 

ground water from surface water. Using the separation based 

on the head measurement, upper bog ground-water samples 

had high Fe2+/Fetot ratios, all from 0.98 to 1.0 (table 3), whereas 

surface-water samples had lower ratios (about 0.95 or less). For 

example, the lowest ratio from upper bog samples in table 3 was 

0.62; this sample also had correspondingly lower total iron. 

With one exception, when head was not detected (surface 

water), the highest ratio was 0.94.

Ground and surface water in the immediate vicinity of 

an active iron bog did not appear to be significantly different with respect to most dissolved elements except iron. This 

similarity probably reflects multiple ground-water sources 

of slightly differing composition that emerge and mix in the 

bog.

706 Environmental Effects of Historical Mining, Animas River Watershed, Colorado
Formation and Geochemical Significance of Iron Bog Deposits 707

Dissolved Oxygen as an Indicator of 

Ground Water

At the upper bog, the most useful parameter besides iron 

used to distinguish between ground water and surface water 

is the dissolved oxygen concentration, especially when “zero” 

(undetectable) dissolved oxygen was taken as the defining 

characteristic of ground water. When upper bog water with 

zero dissolved oxygen (fig. 7A; table 6) is compared to surface 

water from all other sites that are not iron bogs (fig. 7B), the 

narrow ranges of major-element concentrations in iron bog 

ground water become more evident. Dissolved iron, aluminum, and sulfate in upper bog ground water (zero dissolved 

oxygen) are present at approximately twice the concentration 

compared to surface water (nonzero dissolved oxygen) from 

all other sites.

In ground water, the lack of dissolved oxygen is apparent 

and pH is higher by about 0.8 units. The higher pH should aid 

formation of solid iron oxyhydroxides, as these begin precipitating around pH 3.5 (Carlson and Kumpulainen, 2000). 

Median conductivities are similar, but the higher maximum 

conductivity of iron bog ground water (based on zero dissolved oxygen) indicates that it contains more solutes than 

does surface water. The slightly lower maximum conductivity 

of bog surface water suggests that ground water contains fewer 

solutes as a result of precipitation of solids containing iron and 

other species.

When zero dissolved oxygen is used to define ground 

water, median copper and zinc are at about the same concentration in both water types, whereas lead is about two times 

lower and arsenic is about three times higher compared to 

nonzero dissolved oxygen (surface) water (table 6).

Table 6. Summary statistics of major- (Al, Fe, Ca, Si, S (as SO4), Mn; mg/L) and trace- (Cu, Pb, Zn, As; μg/L) element concentrations, 

and water chemistry parameters in surface and ground water (based on positive hydraulic head), and nonzero DO and zero DO water, 

upper bog Cement Creek.

[SpC, specific conductance (μS/cm; microsiemens per centimeter); DO, dissolved oxygen (ppm)]

Surface water, upper bog Ground water, upper bog

n Minimum Maximum Median n Minimum Maximum Median

Al 6 6.4 20.0 17.6 6 15.0 22.1 17.6

Fe 6 31.8 69.8 51.5 6 42.0 73.2 71.8

Ca 6 102 242 230 6 134 280 221

Si 6 17 27 26 6 25 78 27

SO4 6 390 900 760 6 580 910 760

Mn 6 1.5 2.4 2.4 6 1.6 2.6 2.2

Cu 6 9.4 82.4 20.4 6 6.0 15 13

Pb 6 3.9 100 30.5 6 4.0 75 9.8

Zn 6 590 1,760 1,670 6 1,500 1,740 1,680

As 6 3.0 49 8.7 6 4.6 77 39

pH 6 3.2 4.2 3.4 6 3.4 4.5 4.2

SpC 6 850 1,430 1,380 6 900 1,520 1,290

DO 5 0 7.0 5.0 5 0 5.8 0

Other nonzero DO (surface) water Zero DO (ground) water, upper bog

n Minimum Maximum Median n Minimum Maximum Median

Al 19 1.1 20.0 8.8 6 15.9 22.1 18.5

Fe 19 0.2 140.0 35.9 6 50.6 73.2 71.8

Ca 19 23 242 91 6 134.0 280 221

Si 19 7.8 92.0 27.4 6 25.2 27 27

SO4 19 110 900 350 6 580 910 760

Mn 19 0.2 6.2 1.5 6 1.6 2.6 2.3

Cu 5 9 82 14 6 8.0 28 14

Pb 5 3.9 100 33 6 6.3 75 14

Zn 5 590 1,760 1,630 6 1,620 1,740 1,700

As 5 3.0 49 13 6 4.2 77 39

pH 5 3.2 4.2 3.4 6 3.5 4.5 4.2

SpC 5 850 1,430 1,380 6 900 1,520 1,320

DO 4 0 7.0 6.0 5 0 0 0
Trace-Element Chemistry and Alteration Type

The common feature of all sites is the presence of iron 

oxyhydroxide. However, water feeding the iron bogs exhibits 

wide ranges of aqueous trace-element concentrations (table 3), 

and the supply of trace elements depends primarily on alteration type drained by ground water.

The Imogene adit (site 5, fig. 1) drains propylitically 

altered rock, and aqueous trace-metal concentrations are the 

lowest of all alteration types. Even iron and aluminum are low 

(both <10 mg/L) compared to water from higher grade alteration types; still, a thin (<5 mm) but areally extensive (6 m by 

3 m) iron oxyhydroxide deposit has formed downstream from 

the outflow.

In contrast, water from acid-sulfate alteration (Junction 

mine, site 8, fig. 1) shows the highest trace element concentrations (table 3) of all water sampled. At this site, an areally large 

but thin (100 m2 by 1 cm) accumulation of iron oxyhydroxide 

was present. The red spring and upper bog also drain acidsulfate altered rocks and show correspondingly higher trace 

element concentrations in both water and sediment. Water 

draining weak sericite-pyrite and quartz-sericite-pyrite altered 

areas generally show trace-metal concentrations between these 

two extremes, in both the aqueous and the solid phases (Bove 

and others, this volume).

Microbial Populations

The MPN (Most Probable Number) results for eight 

sites are shown in table 7. Ranges of cell numbers per 

100 mL are <2–500 for Th. ferrooxidans and <2–440 for 

Th. thiooxidans, calculated according to the formula derived 

by Thomas (1942). Whereas not all samples produced 

0.1

1

10

100

1,000

10,000

CONCENTRATION OR UNITS

Al

mg/L

Fe

mg/L

SO4

mg/L

Cu

μg/L

Pb

μg/L

Zn

μg/L

As

μg/L

pH

units

SpC

μS/cm

DO

ppm

AQUEOUS SPECIES OR PARAMETER

A

Figure 7 (above and facing page). Box plots of major- (mg/L) and trace- (μg/L) element 

concentrations, and water chemistry parameters in A, Zero DO (ground) water, upper bog 

of Cement Creek, and B, Nonzero DO (surface) water from all other sites.

708 Environmental Effects of Historical Mining, Animas River Watershed, Colorado
Formation and Geochemical Significance of Iron Bog Deposits 709

Thiobacillus isolates of both species, at least one species 

and Leptothrix sp. were usually present at all iron bogs. The 

wetland of South Fork Mineral Creek is not an iron bog but 

is included here to compare Thiobacillus numbers from an 

organic-rich environment versus the iron-dominated sites. 

There were some positive cultures for Th. thiooxidans from 

the wetland.

The spring deposit in Topeka Gulch showed no positive cultures for either Thiobacillus or iron-related bacteria 

(Leptothrix). This is believed to be the result of our sampling 

this deposit near the end of a day-long rainstorm, which was 

preceded by heavy rain the previous day and night. Nearly 

all chemical concentrations from this site were low, suggesting that this deposit was being flushed by the 2-day rainfall. 

Because of difficult access to this site, it was not practical 

to sample again at a later time. Despite the lack of positive 

cultures, this deposit was similar to other iron deposits with a 

thick accumulation of iron oxyhydroxide covered by mosses, 

algae, and cyanobacteria.

One of the more interesting results was the high number 

of positive cultures (13 of 18 inoculations) at the Junction 

mine site (site 8, fig. 1). This area receives drainage from a 

mine adit into a large shallow pool whose bottom surface was 

covered with iron oxyhydroxide at the time of sampling. The 

multiple positive cultures may reflect active microbial oxidation of available sulfides inside the mine adit and nearby 

country rocks.

A contradictory result is the lack of Th. ferrooxidans

at the upper bog of Cement Creek. Except for the two undiluted inoculations, which showed only slight positive growth, 

no iron oxidizers were detected. This seems inconsistent 

with the high percentage of Fe2+ in water (≥60 percent of 

Fetot), which would appear ideal for promoting growth of 

0.1

1

10

100

1,000

10,000

CONCENTRATION OR UNITS

Fe

mg/L

SO4

mg/L

Cu

μg/L

Pb

μg/L

Zn

μg/L

As

μg/L

pH

units

SpC

μS/cm

DO

ppm

AQUEOUS SPECIES OR PARAMETER

Al

mg/L

B
microbes that prefer Fe2+. Reasons for the lack of iron oxidizers at this site are not readily apparent. One possibility is that 

Th. thiooxidans was able to adapt to act upon iron as well as 

sulfur, which is a common trait of these chemolithotrophic 

organisms (Schippers and Sand, 1999).

All iron-related bacteria BART inoculations from active 

iron bogs produced a (+) result. Color development in some 

samples required the entire 2-week period. Because the 

results from the iron-related bacteria tests are nonspecific, 

the presence of particular iron-oxidizing bacteria cannot be 

determined. However, the results confirm the presence of 

one or more microbes whose metabolism is related to iron oxidation and demonstrate that iron-related microbes are present 

in all iron bogs.

Leptothrix was visible at several sites where pooled 

water was present and formed an iridescent, oil-like sheen on 

surface water (fig. 8). Although Leptothrix may not actively 

oxidize iron, it is usually encrusted with a several-micrometerthick coating of iron oxyhydroxide (an “iron accumulator”), 

and its presence is considered strongly indicative of active iron 

oxidation (van Veen and others, 1978).

Geochemical Modeling

Minerals that are saturated or oversaturated in upper 

bog water (saturation index (SI)>0) include the iron minerals goethite, schwertmannite, and hematite, and the sulfate 

minerals alunite (KAl3

(SO4)

2

(OH)6) and barite (BaSO4). 

Of these, goethite and schwertmannite were the dominant iron minerals detected by X-ray diffraction. Jarosite 

(KFe3

(SO4)2(OH)6), a crystalline phase often associated with acid-rock drainage (Bigham and Nordstrom, 

2000), was predicted to form in low-pH (<3.0) modeling 

simulations that did not employ the measured composition 

of upper bog water. This mineral was detected by X-ray 

diffraction in only five iron bog samples (table 1; not all 

jarosite-containing samples are listed). Other sulfate minerals that approach saturation (saturation index (SI) > –1.0 to 0) 

are gypsum (CaSO4

•2H2O), anhydrite (CaSO4), and celestite 

(SrSO4

).

Quartz and chalcedony (amorphous silica) are saturated 

in the upper bog water. In all iron bogs, few detrital quartz 

grains were observed macroscopically. Even after visible 

clasts and grains were removed, quartz was detected as a 

major phase by X-ray diffraction. The presence of quartz 

with amorphous silica suggests that most quartz is microcrystalline and newly precipitated within the iron bogs as water 

moved through the iron oxyhydroxide deposit. This result is 

consistent with the findings of Desborough and others (2000) 

that silica gels were forming on filters during filtration of 

water samples from Cement Creek. In addition, Wirt and others (this volume) found that ferricrete actively forming on the 

Cement Creek streambed near its boundary with the upper bog 

was cemented by quartz.

Kaolinite is another aluminosilicate mineral predicted 

in some modeling simulations, but it is not a stable mineral in acid-sulfate water (Bigham and Nordstrom, 2000). 

Although detected by X-ray analysis, it is most likely a 

detrital phase derived from feldspar alteration. Other detrital and secondary silicates identified by X-ray diffraction 

likely were deposited in the iron bogs by eolian or fluvial 

processes.

Table 7. Results of Most Probable Number (MPN) culturing for 

Thiobacillus species in iron bogs, Animas River watershed study 

area.

[Ratings based on relative increase in turbidity of inoculated tubes after 1 month 

elapsed time, as follows: ---, negative; vs+, very slight positive; s+, slight positive; +, positive. Results shown are for tubes 1–3 (100

–10–2) for each species, as 

there was no detectable change in tubes at higher dilutions (10–3 to 10–5)]

Dilution Th. ferrooxidans Th. thiooxidans

Tube 1 Tube 2 Tube 3 Tube 1 Tube 2 Tube 3

Junction mine

100 + + + s+ s+ s+

10–1 + + + s+ --- ---

10–2 + + + --- --- ---

Imogene adit

100 --- --- + --- + ---

10–1 s+ s+ --- s+ + ---

10–2 --- + --- --- --- vs+

Topeka Gulch spring deposit

100 --- --- --- --- --- ---

10–1 --- --- --- --- --- ---

10–2 --- --- --- --- --- ---

Bonner mine waste pile

100 s+ s+ --- s+ + s+

10–1 --- s+ s+ --- s+ s+

10–2 --- --- s+ --- --- s+

Iron bog, South Fork Mineral Creek

100 --- --- --- s+ s+ ---

10–1 --- --- --- --- vs+ vs+

10–2 --- --- --- + s+ s+

Upper bog, Cement Creek

100 vs+ vs+ --- + + +

10–1 --- --- --- + + +

10–2 --- --- --- --- s+ s+

Wetland, South Fork Mineral Creek

100 --- --- --- vs+ + +

10–1 --- --- --- s+ + ---

10–2 --- --- --- --- s+ ---

Red spring, Prospect Gulch

100 vs+ --- --- --- --- +

10–1 --- --- --- vs+ --- ---

10–2 --- --- --- s+ s+ s+

710 Environmental Effects of Historical Mining, Animas River Watershed, Colorado
Formation and Geochemical Significance of Iron Bog Deposits 711

Discussion

Geochemical Evolution of Meteoric and Surface 

Water to Iron Bog Ground Water

Infiltrating meteoric water can undergo a variety of 

changes as it evolves to ground water along a flow path 

(Hem, 1989). Dissolved O2

 is consumed by weathering reactions such as oxidation of sulfide minerals (for example, FeS2

) 

to form SO4, which is elevated in ground water. Although 

ground water has low dissolved oxygen, hydrogen sulfide 

(H2S) is not present; thus species such as sulfate form, remain 

stable, and persist along the flow path (Hanor, 2000). The 

dominance of Fe2+ over Fe3+ in upper bog ground water suggests that O2

 is first consumed to oxidize sulfur rather than 

Fe2+. Bigham and Nordstrom (2000) noted that during pyrite 

weathering, sulfur in pyrite is oxidized more quickly than iron, 

producing water enriched in ferrous iron, sulfate, and protons 

(H+

).

The dominance of Fe2+ also suggests that recharge of O2

to ground water that feeds the iron bogs must be slow or nonexistent. During winter there is little or no infiltration of O2

-

bearing surficial water into the subsurface (Vaughn and others, 

1999). And because of the acidic nature of the water, Fe2+ is 

only slowly oxidized, if at all, during its residence in ground 

water. Similarly, iron-oxidizing bacteria are aerobic (requiring 

oxygen) and not likely to inhabit an O2-deficient subsurface 

regime, and therefore, would be unavailable to oxidize iron.

Geochemical Evolution of Iron Bog Ground 

Water to Iron Bog Surface Water

Ground water discharging into the upper bog shows narrower ranges of dissolved major-element concentrations with 

higher concentrations (fig. 7A; table 3) than surface water. 

In addition, a lack of dissolved oxygen and higher values for 

Fe2+/Fetot ratios (>0.95), Fetot concentrations, and pH are characteristics that distinguish ground water from surface water at 

the upper bog.

Surface water exhibits broader ranges of dissolved 

element concentrations with lower concentrations (fig. 7B; 

table 3) compared to ground water. Surface water has dissolved oxygen greater than zero, and lower values for Fe2+/Fetot

ratios (0.6–0.95), Fetot concentrations, and pH compared to 

parent ground water that presumably originated farther up 

hydrologic gradient.

These observations, including higher median iron and 

sulfate concentrations in ground water, are all consistent with 

an oxidation-precipitation process taking place that leads to 

a lowering of major constituent concentrations in (former) 

ground water that becomes surface water in the iron bog.

Figure 8. Leptothrix film on water surface at lower bog of Cement Creek. Width of pond 

(from left to right) is approximately 1 meter.
Dilution or mixing of bog water by Cement Creek 

does not appear to be a likely mechanism for producing the 

characteristics of the bog surface water. During all sampling 

periods, Cement Creek was not spilling into the bog, and in fact, 

has been isolated from the bog by a bench of iron oxyhydroxide 

(≈25 cm high) that has formed along the bog-stream interface. 

Physical mixing was limited to the downstream (south) end of 

the bog where impounded bog water entered the stream.

Several geochemical processes occur during the evolution of the influent bog ground water to bog surface water. 

The first is oxidation of ferrous iron to ferric iron, which 

leads to saturation of certain ferric-iron-bearing solids. 

Second, hydrolysis of oxidized iron and (or) precipitation 

of ferric iron and sulfate-bearing phases leads to an increase 

in H+

 in solution (lower pH) and loss of iron and sulfate (and 

other species) to the solid phase. Finally, the newly evolved 

bog surface water undergoes precipitation reactions on the 

bog surface and (or) subsequently farther downstream. The 

end result is that the characteristics of the influent ground 

water become altered to those of a surface water. Figure 9 

illustrates the evolution of ground water to surface water 

using data from the upper bog of Cement Creek. A moderate portion (≤40 percent) of the dissolved iron becomes 

oxidized and potentially sequestered in the solid phase at the 

bog; some sulfate is also incorporated into the solids when 

sulfate-bearing phases form.

Figure 9. Graph showing postulated evolution of ground water to surface water using data from upper bog of Cement 

Creek. Note that as bog ground water evolves to bog surface water, both pH and aqueous Fe2+ concentration decrease.

0.5

0.6

0.7

0.8

0.9

1

Fe2+ / Fetot

3.0 3.2 3.4 3.6 3.8 4.0 4.2 4.4 4.6

pH, IN STANDARD UNITS

Ferrous iron oxidation

Goethite formation

Schwertmannite formation

Surface water

Ground water

712 Environmental Effects of Historical Mining, Animas River Watershed, Colorado
Formation and Geochemical Significance of Iron Bog Deposits 713

Iron Oxidation

Dissolved oxygen in the iron bog originates from the 

atmosphere and from the activity of photosynthetic plants 

and microorganisms. Ferrous iron oxidation rendered by 

atmospheric oxygen or aquatic organisms is a passive process 

(Canfield and Des Marais, 1993), whereas iron-oxidizing bacteria employ this oxygen to actively accelerate the oxidation. 

In both the upper bog and the red spring, aquatic plants were 

heavily encrusted with a light-orange floc of iron oxyhydroxide, indicating that some ferrous iron oxidation occurs near the 

point of O2

(g) release from the plants. As the floc accumulates 

on the plant surfaces, portions fall to the bottom of the iron 

bog to become incorporated into the sediment.

Mosses, cyanobacteria, and algae growing on the bog 

surface and in small pools and flows were also encrusted with 

iron oxyhydroxide. All photosynthesizers appear to play dual 

roles as oxygen producers and thus passive iron oxidizers, and 

as surfaces to which iron oxyhydroxide can sorb or bind.

The combined high number of positive results from the 

MPN and IRB tests indicates that microbes are available to act 

as additional oxidizers of Fe2+ in iron bog water. Thiobacillus numbers suggest that at mined sites (Junction mine, for 

example), the populations are relatively higher than in “undisturbed” sites. We surmise that mining has exposed additional 

fresh pyrite (FeS2) and other sulfide minerals to oxidation, and 

that greater numbers of microbes are taking advantage of the 

available sources of energy (that is, reduced iron and sulfur). 

Thus, the increase in thiobacilli cell numbers likely mirrors the 

increased availability of reduced forms of iron and sulfur and 

increased oxidation mediated by the microbes. The presence 

of Leptothrix at most sites (fig. 8) was indicative of active, 

ongoing iron oxidation. Although microbial oxidation of Fe2+

was not directly observed in our study, that iron and sulfur 

oxidizing microbes inhabit the iron-depositing environments 

is likely because of the plentiful supply of energy-yielding 

reduced iron and (perhaps) sulfur.

Formation of Iron Oxyhydroxide and Schwertmannite

Despite variability in water composition at different irondepositing sites, all water becomes saturated with respect to 

iron oxyhydroxide; the mechanism that leads to initial saturation is oxidation of Fe2+ to Fe3+. Once this occurs, ferrihydrite, 

goethite, and schwertmannite may precipitate from solution, 

depending on the solubility (saturation state) of each mineral, 

redox potential (approximated by dissolved oxygen concentration), and pH. Modeling indicated that slight changes over 

the pH range 4.0–4.5 can affect which of the three solids will 

form. As ground water undergoes these precipitation reactions, 

it acquires more of the surface water characteristics, the most 

important being more-acidic pH via production of hydrogen 

ion (H+

).

One process that could reduce pH to more-acidic values is the oxidation of ferrous iron to ferric iron followed by 

hydrolysis to form ferrihydrite:

 Fe3+ + 3H2

O → Fe(OH)3 + 3H+ (1)

The formula for ferrihydrite is considered uncertain 

(Bigham and Nordstrom, 2000); ferrihydrite may consist of 

both crystalline and amorphous material depending on conditions at the time of formation. Protons (H+

) are generated in 

solution and iron is removed as Fe(OH)3 drops out as a solid. 

As there is little or no acid-neutralizing capacity in the water, 

it continues to evolve towards bog surface water with lower 

pH and lower concentrations of total and ferrous iron. At the 

iron and sulfate concentrations and slightly higher pH (>4.0) 

of some upper bog ground-water samples, goethite would be 

the preferred phase (Carlson and Kumpulainen, 2000).

 Fe3+ + 2H2

O → FeOOH + 3H+ (2)

The Eh-pH diagram of Bigham and others (1996) shows 

that increasingly positive redox potentials at low pH (<4.5) 

favor formation of schwertmannite over goethite. As bog 

ground water evolves to bog surface water, the increase in 

dissolved oxygen concentration coupled with production of H+

via ferric iron hydrolysis or goethite formation could lower pH 

and favor formation of schwertmannite.

 8Fe3+ + SO4

2– + 14H2

O → Fe8O8(OH)6SO4 + 22H+ (3)

The upper bog of Cement Creek displays a variety of 

redox potentials, a range of pH values (3.19–4.47) that encompasses the stability fields of schwertmannite (pH 3.0–4.5) and 

goethite (pH 3.5–8.0) (Bigham and others, 1996; Bigham and 

Nordstrom, 2000), and varying iron and sulfate concentrations. 

It is these differing geochemical conditions that allow both 

iron minerals to form. Schwertmannite can convert to goethite 

via the reaction:

 Fe8O8(OH)6SO4 + 2H2O → 8FeOOH + 2H+ + SO4

2– (4)

The release of sulfate in this conversion may be the reason 

why iron bog ground water and surface water do not differ greatly with respect to sulfate concentrations (table 4), 

whereas iron concentrations vary more because all minerals 

that form consume iron.

The formation of other sulfate minerals such as jarosite, 

which was predicted to precipitate in some modeling simulations, would also cause decreased SO4

 in water that becomes 

surface water. Barite, which was predicted by modeling but 

not detected by X-ray diffraction, could potentially form. 

However, it would not change the pH as there is no production 

or consumption of protons.

 (Ba2+ + SO4

2– → BaSO4

) (5)
In summary, acidity is generated by precipitation of iron 

sulfate minerals such as schwertmannite and iron minerals 

such as goethite. These precipitation reactions are key processes that produce and maintain the acid character of iron 

bog surface water.

Mineralogy of Iron Bog Solids

Goethite and schwertmannite are the most abundant 

iron-bearing minerals in iron bog solids (table 1). The presence of schwertmannite at depth in the iron bogs suggests that 

there are kinetic or thermodynamic factors that permit this 

mineral to persist as a metastable form rather than convert to 

goethite. In contrast, fresh schwertmannite precipitates that 

formed early in the summer in stream beds and on the surfaces 

of several iron bogs (Desborough and others, 2000) did not 

persist year-round. Schwertmannite was not detected later in 

the fall in most streams, so these precipitates apparently dissolved (or perhaps were transformed to goethite) when water 

levels in the streams dropped (Desborough and others, 2000). 

These results suggest differing stabilities of schwertmannite 

and goethite in iron bogs compared to open areas such as 

streams.

Bigham and others (1990) reported that schwertmannite 

is the preferred iron phase to form at pH 3.5–4.0 in acidsulfate water (similar in composition to that at the upper bog 

of Cement Creek). At slightly higher pH (>4.0) or sulfate 

concentration (as high as 1,000 mg/L), goethite is favored over 

schwertmannite. Carlson and Kumpulainen (2000) noted that 

if iron is oxidized by Thiobacillus rather than molecular oxygen, schwertmannite is the dominant solid phase that forms. 

Assuming the aqueous chemistry varies temporally (daily to 

seasonally), then to expect both phases to be produced in the 

iron bogs is not unreasonable.

The iron bog mineralogy is in agreement with the 

predicted (by modeling) crystalline solids. That is, goethite 

and schwermannnite were predicted as the main crystalline 

iron-bearing phases and were observed. Hematite was predicted to form as it is the stable (equilibrium) iron mineral, 

but it was not present in the iron bogs. Goethite may transform 

to hematite under some conditions, but most research suggests 

that “coarse-grained” goethite (>1 μm size) is kinetically 

stable to hematite at below-neutral pH values (Fischer and 

Schwertmann, 1975). Given the highly acid pH values in the 

iron bogs, hematite is not likely to form via the dehydration 

and aging of goethite.

Goethite and hematite form from a common ferrihydrite 

precursor but along separate pathways, and kinetics and pH 

are major controls on which phase actually forms and remains 

stable (Fischer and Schwertmann, 1975). The rate of aggregation of (amorphous) ferrihydrite particles seems especially 

important in controlling which iron mineral initially forms. 

Hematite formation requires rapid aggregation of ferrihydrite 

followed by hematite crystallization and aging at higher pH 

(7–8), whereas goethite forms when ferrihydrite aggregation 

is slow. Dissolution of the ferrihydrite precursor and precipitation of goethite will be dominant at low pH (Fischer and 

Schwertmann, 1975).

Additionally, laboratory studies indicate the goethitehematite conversion to be extremely slow, requiring years 

under optimal conditions to perhaps thousands of years under 

non-ideal conditions (Langmuir and Whittemore, 1971). 

The bog deposits are geologically young, having formed in 

approximately the last 9,000 years, and most are still active, 

so the extremely slow conversion process (that is, attainment 

of equilibrium) has not occurred.

Jarosite forms at lower pH values (<3.0; Bigham and others, 1996) and was detected in only five samples, so it appears 

that only rarely did pH permit jarosite precipitation. Barite 

precipitation might be occurring near the ground water–

surface water interface, as indicated by lower dissolved barium 

in ground water compared to surface water, but barite was not 

detected as a solid phase by X-ray analysis.

Location of Precipitation of Solid Phases

Newly evolved surface water moves onto the bog surface 

where residual ferrous iron can undergo oxidation. Because 

the water is now at a lower pH and higher redox potential, 

schwertmannite is the preferred phase to precipitate. This may 

explain why fresh precipitates on bog surfaces are dominantly 

schwertmannite rather than goethite.

Field observation indicates that some iron precipitation 

happens within the region between the shallow bottom surface, 

close to the point of ground-water discharge, and the top surface 

of growing bogs that contain pooled water. Iron bogs appear to 

reconstitute from the interior and below water level after their 

surfaces have been penetrated or disrupted by sampling, rather 

than by deposition from the surface downward.

The geochemical modeling results also suggest that 

the iron deposits accumulate, at least partially, from the bog 

interior (or bottom) towards the surface. The models indicate 

that changes in water chemistry and formation of solid iron 

oxyhydroxide occur at the point of ground-water discharge 

at the base of the bog. However, because of additional mixing and iron oxidation after ground-water discharge into the 

iron bog, it is not possible to determine the precise location(s) 

where iron precipitates from solution.

The presence of >60 percent ferrous iron in all upper 

bog water samples indicates that most Fe2+ delivered by 

ground water passes through the site and continues downstream 

in Cement Creek to be oxidized and precipitated. Kimball 

and others (this volume, Chapter E9) noted that the Prospect 

Gulch–upper bog area was a major contributor to the metal load 

of Cement Creek. The lack of complete oxidation at the bog 

site results in a source of ferrous iron to the stream and helps 

explain the presence of fresh silica-bearing schwertmannite precipitates (Desborough and others, 2000) in streambed reaches 

where ground-water inflow was not observed.

714 Environmental Effects of Historical Mining, Animas River Watershed, Colorado
Formation and Geochemical Significance of Iron Bog Deposits 715

Solid Phase Chemistry Versus Alteration Type

Trace-Element Composition of Iron Oxyhydroxide

Trace-element concentrations in iron bog sediment show 

broad ranges that are affected primarily by the type of alteration 

and mineralogy of the terrain drained by ground water. Bove 

and others (this volume) found distinct aqueous geochemical 

signatures originating from each alteration type.

Water draining the propylitic alteration type (Imogene 

adit, site 5, fig. 1) contains among the lowest dissolved 

major- and trace-element concentrations of all water 

samples. By contrast, water from the Junction mine acidsulfate site (site 8, fig. 1) shows remarkably high dissolved 

element concentrations, including 4,300 mg/L arsenic. The 

site of core 97ABS327 (site 13, fig. 1; located 0.6 km east 

and 700 m downslope from the Junction mine) drains this 

same acid-sulfate zone, and solid-phase arsenic is at the 

highest level of any of the iron bogs. The higher dissolved 

arsenic in zero dissolved oxygen upper bog water (which 

also drains the acid-sulfate zone) can be explained by the 

fact that under mildly reducing conditions, the reduced 

form of arsenic [As(III)] is generally the more mobile and 

more soluble species (Smedley and Kinniburgh, 2002). In 

the iron bog sediment, arsenic becomes strongly associated with iron oxyhydroxide through oxidation to As(V) 

and coprecipitation with ferric iron (Smedley and Kinniburgh, 

2002).

Copper is present in the iron sediment from the site of 

core 97ABS327 (site 13, fig. 1; red spring), yet the aqueous 

sample did not show detectable copper. In the interval from 

approximately 10 to 30 cm (fig. 3), copper was elevated (to 

as high as 70 ppm) compared to other intervals. This suggests either episodic contribution and retention of copper from 

ground water or continued accumulation of minute levels of 

copper through time.

Lead had the lowest maximum concentration in the 

solid phase. Normalization of other element data to the lead 

concentration produces the following “enrichment” factors in 

core 97ABS327: Fe = 7,500×; Cu=1.2×; Pb = 1×; Zn = 11×; 

As = 83×. These values indicate that large amounts of iron 

oxyhydroxide must be precipitated to retain small amounts of 

trace elements. The relatively low copper, lead, and zinc values 

are likely the result of reduced adsorption and (or) coprecipitation of these metals with iron oxyhydroxide at low pH (Zanker 

and others, 2002), whereas arsenic is more strongly adsorbed 

(Smedley and Kinniburgh, 2002).

In contrast to core 97ABS327, solids from core 999291 

(site 11, fig. 1) show low but consistent copper concentrations of 10–30 ppm throughout the core; and in fact, copper 

is detectable in all but two samples. This site, located on the 

southern flanks of the subeconomic copper moly-porphyry 

deposit at peak 3,792 m, receives ground water from higher 

slopes. Although the drainage area is dominantly unmineralized propylitically altered Paleozoic sedimentary rocks and 

quartz-sericite-pyrite altered rocks with low abundances 

of deposit-related trace elements such as arsenic and lead, 

the influence of ground water from the peak’s WSP alteration 

suite is reflected in the composition of the iron bog sediment.

The modern iron oxyhydroxide cement from the Bonner 

waste pile (site 6, fig. 1) has little or no aluminum in the solid 

phase, but substantial trace-element concentrations compared 

to those of iron bog solids. This mine waste ferricrete thus 

appears to be capable of retaining most of the same elements 

as iron bogs with the exception of aluminum. The most likely 

reason that aluminum is not retained in these solids is that the 

pH of ground water (exiting the adit) was one of the lowest 

measured (3.17). At low pH, aluminum is a conservative species that remains in the aqueous phase and does not precipitate.

Water Chemistry Versus Alteration Type

Water samples from each environment that we examined show differing concentrations of iron and aluminum that 

reflect combined influences of local rock type (mineralogy), 

alteration type, and depositional conditions. Water falls into 

four major groups identifiable by iron and aluminum concentrations and depositional environment (fig. 10). Iron bogs contain the highest aqueous iron and aluminum compared to water 

from other iron-depositing environments (fig. 10; table 4). 

Seeps are intermediate in composition, having approximately 

30–40 mg/L iron but lower aluminum, 5–10 mg/L (fig. 10). 

Even more dilute is surface water (including the Imogene 

and Bonner drainages, and Mineral Creek and Topeka Gulch 

springs), which does not exceed about 10 mg/L iron and 

5 mg/L aluminum (fig. 10). The fourth group (not shown in 

fig. 10) is made up of water from wetlands primarily along 

South Fork Mineral Creek. These show the lowest aqueous 

concentrations, generally less than 10 mg/L iron and 5 mg/L 

aluminum (Stanton, Fey, and others, this volume). The low 

values in wetlands may partly reflect the ability of these systems to remove dissolved species from influent water through 

adsorption onto organic matter or precipitation of authigenic 

minerals such as metal sulfides. However, the low iron and 

aluminum concentrations most likely reflect the lack of intense 

alteration and sustainable acidic weathering of the Paleozoic 

sedimentary rocks in the South Fork Mineral Creek drainage.

Iron bogs represent environments where acid weathering 

has allowed accumulation of high concentrations of dissolved 

iron, aluminum, and trace elements along the ground-water 

flow path. Many of these elements become fixed in the solid 

iron oxyhydroxide sediment that forms as a result of geochemical changes at the depositional site. Thus, iron bogs are 

sites of deposition that display the end result of deep, intense 

weathering in the rocks. In contrast, the lower concentrations 

of aqueous iron and aluminum in the other depositional environments (seeps, springs, adits) suggest either that weathering 

is less intense or complete, or that depositional conditions 

are not as favorable for production and preservation of large 

accumulations of iron-rich solids.
Accumulation of Iron Oxyhydroxide

Formation of Modern “Waste-Pile Ferricrete” 

Deposits

The formation of iron oxyhydroxide cement in iron-rich 

rocks can be rapid with respect to geologic time, on the order 

of 50 to 100 years. This is evidenced by the presence of ironcemented “anthropogenic” deposits in several mine-waste rock 

piles in the area.

At the Yukon tunnel on Cement Creek (site 17, fig. 1), 

a large block of mine-waste material has been iron-cemented 

and differentially weathered so that the ferricrete now stands 

out in relief against the rest of the surrounding pile. The 

earliest mining in the region began in the 1870s. The Yukon 

operations began around 1900 and continued intermittently 

until around 1950 when the mine became inactive (San Juan 

Historical Society Archives, 1998); succeeding decades saw 

only occasional activity.

Similarly, another waste-pile ferricrete was observed 

on the Bonner mine waste pile (site 6, fig. 1). (This pile has 

since been remediated so the ferricrete no longer exists.) The 

relationship of water flow to the location of this ferricrete was 

clear. Water originating in the upper levels of the pile flowed 

underground and down into the waste pile, then resurfaced 

along an old roadbed in the middle of the pile; a 6 m long 

by 1 m thick section of this roadbed had been cemented 

by iron oxyhydroxide. The Bonner mine began operations 

around 1910 and ceased in 1955 (San Juan Historical Society 

Archives, 1998). Thus, in a span of about 50 years, a small 

deposit of ferricrete formed.

Approximately 50 vertical feet downhill from the roadbed 

ferricrete, the area surrounding a flowing adit had also been 

cemented by iron oxyhydroxide. This deposit was well indurated and similar in hardness to ferricrete cement, but contained few clasts and was dominantly fine grained iron oxyhydroxide. The adit appeared to flow perennially, allowing it 

to deposit a large volume and thickness of iron oxyhydroxide 

5

0

10

15

20

25

IRON, IN MILLIGRAMS PER LITER

0 10 20 30 40 50 60 70 80

Iron bogs

Seeps

Springs, adits, and

surface water

ALUMINUM, IN MILLIGRAMS PER LITER

Surface water

Ground water

Figure 10. Relationship of aqueous concentrations of iron and aluminum in all iron depositional environments.

716 Environmental Effects of Historical Mining, Animas River Watershed, Colorado
Formation and Geochemical Significance of Iron Bog Deposits 717

around the adit discharge area. Below the adit discharge was a 

level bench of waste rock that was now encrusted with several 

centimeters of fine-grained iron oxyhydroxide and covered 

with living mosses, very similar in appearance to a typical bog 

such as the lower bog of Cement Creek (fig. 2).

Mine adit drainages are another location where small iron 

deposits have formed in recent times. Near the Imogene adit, 

a thin (≤2 mm) but areally extensive (10 m by 2 m) homogeneous accumulation of fine-grained iron oxyhydroxide had 

formed across a dirt road downstream of the flowing adit. 

However, this layer was ephemeral—on a return visit to this 

site 8 months after the first encounter, most of the iron oxyhydroxide layer had dissolved. But 3 m downstream on a slope 

(4 m high) just below the road level, the iron oxyhydroxide 

accumulation was 2–4 cm thick. Once again, the solids were 

retained among a thick growth of photosynthetic macroorganisms. The situation at this adit illustrates that even on a local 

scale, some areas are more favorable for the permanent accumulation of iron oxyhydroxide than others.

Below the Junction mine on Mineral Creek, a layer of 

iron oxyhydroxide several millimeters thick accumulated in 

just a few years since the site was remediated, and an incipient 

iron bog was being established. Near the Cascade Gulch adit, 

iron oxyhydroxide (fig. 11) nearly 1 m thick has accumulated; 

it has taken no longer than 130 years for this thick deposit to 

form. Similarly, much of the east upper bog (20 m east of the 

upper bog of Cement Creek, site 4, fig. 1) has been deposited 

since railroad ties were first placed there in the late 1890s, as 

evidenced by the ties being encrusted and partially buried with 

several centimeters of iron oxyhydroxide. Thus, in a span of 

time ranging from about 50 to 130 years, several “modern” 

iron deposits have formed.

These more recent iron deposits are able to form rapidly 

because the higher porosity of the angular cobbles and boulders (compared to competent country rock) enhances surfacewater percolation and ground-water flow through the waste 

pile. The supply of ferrous iron is likely increased because of 

the greater available surface area of the crushed waste metalbearing sulfides. The larger surface area enhances the rate of 

Fe2+ solubilization and delivery to the depositional site (for 

example, the bottom of the Bonner waste pile), which in turn 

helps to accelerate iron cement formation. In contrast, natural 

iron bogs are limited by slower processes of recharge, groundwater movement, weathering (smaller mineral surface areas 

for dissolution), iron solubilization, and iron delivery to the 

depositional site.

Role of Photosynthetic Organisms

As shown in figures 2, 8, and 11, plants, mosses, algae, 

and cyanobacteria are abundant in and on the iron bogs. In 

every instance, one or more of these organisms was found in 

active iron bogs. Mosses in particular appear to stabilize the 

growth and accumulation of the bog through retention of iron 

oxyhydroxide precipitate.

The Cascade Gulch adit on Cement Creek (site 15, fig. 1) 

provided some insight into the role of organisms in the accumulation of iron. Here, both mosses and algae were growing in 

Figure 11. Moss layer (bright green) and relict algal filaments (vertical thin 

black strands) enclosing a 1-cm thick layer of iron oxyhydroxide near Cascade 

Gulch adit, Cement Creek. The layer has slumped and broken apart, but moss 

was once continuous across a thick layer of iron oxyhydroxide.
the iron deposit and acted to retain and stabilize the accumulating sediment. Figure 11 illustrates a vertical sequence of 

fresh and relict moss layers and algal strands enclosing a 1-cm 

thick layer of iron oxyhydroxide that accumulated during (one 

or more) growing seasons. In deeper (older) layers of some 

iron bogs, it was clearly evident that relict mosses were in the 

process of being replaced by iron oxyhydroxide.

Cyanobacteria and algae were seen on the surfaces of 

most active bogs, with distinct cyanobacteria layers alternating 

with iron oxyhydroxide layers in the upper core samples of all 

large iron bogs. This is likely the result of one season’s growth 

of cyanobacteria and algae, which are subsequently covered 

by the iron oxyhydroxide that precipitates during the following season(s). These layers are not well preserved in deeper 

parts of the iron bog, presumably owing to advancing (but 

incomplete) oxidation and destruction of organic matter as the 

material gets older. Nonetheless, iron-oxyhydroxide-replaced 

structures resembling fine algal or cyanobacterial filaments 

were seen in the deeper layers of several inactive and active 

bogs.

Estimated Rates of Iron Oxyhydroxide Deposition

Samples from two iron bogs on South Fork Mineral 

Creek provide a rough estimate of the rate of accumulation of 

iron oxyhydroxide in that drainage. Sample 97ABS319, a peat 

from a depth of 20.3 cm in an iron bog, produced a 14C age of 

1,480(±40) yr B.P. Assuming a constant rate of iron oxyhydroxide deposition with no removal through erosion or dissolution, an estimated accumulation rate of 0.13 m/1,000 years 

is obtained. Similarly, peat sample 97ABS320 from a depth of 

30.5 cm in another iron bog had a 14C age of 1,920(±190) yr 

B.P., or an accumulation rate of 0.16 m/1,000 years. The thickness of iron bogs in the Animas River watershed study area 

ranges from about 1 to 2 meters. At the higher depositional 

rate, it would require about 6,250 years to deposit a 1-m thick 

layer of iron oxyhydroxide. This estimated rate falls within the 

time range delimited by 14C dates, which show that the iron 

deposits (including ferricrete) formed within approximately 

the past 9,000 years (Vincent and others, this volume).

Although the 14C dates provide only a minimum age of 

deposition, they do show that the iron deposits have formed 

since the last glaciation in the area (Hanshaw, 1974). This 

seems a reasonable result because it is unlikely that the iron 

deposits could form when stream beds and valleys were occupied by ice, and both glacial action and subsequent erosion 

may have removed previous iron accumulations.

Although these depositional/accumulation rate estimates 

are imprecise, they help to illustrate that iron bog growth is 

indeed a slow process. Other factors can strongly affect the 

depositional rate in individual iron bogs, such as migrating ground-water flow paths, episodic or cyclical changes in 

precipitation and ground-water discharge, and variations in 

the rate of iron supply (weathering) and oxidation. The large 

spring deposit on Mineral Creek (site 10) may be an example 

of episodic deposition. Currently, surface discharges of ground 

water are small and isolated, but in the past, ground-water flow 

and iron deposition likely were much higher to produce this 

thick, widespread accumulation of iron oxyhydroxide. In contrast to the estimated accumulation rates in South Fork Mineral 

Creek, the large Iron Springs deposit on the west side of the 

basin divide (west of Ophir Pass) had a relatively rapid accumulation rate of 0.9 m/1,000 years (Hanshaw, 1974).

Carbon Preservation within Iron Bogs

To have found relict organic carbon in the iron bogs is 

noteworthy because these bogs are oxidizing environments. 

The dominance of Fe3+ in the solids suggests that any organic 

matter should be completely oxidized and not preserved. However, as noted previously, much of the aqueous reduced Fe2+

remains in that state and is not immediately nor completely 

oxidized to Fe3+. As a result of the slow, incomplete oxidation 

of the pool of ferrous iron and therefore deposition of iron 

oxyhydroxide, the iron bogs have slow growth and accumulation, which is further evidenced by the 14C ages of many of the 

deposits.

The reduction of dissolved oxygen to zero values by high 

aqueous Fe2+ is one reason that organic carbon is able to survive. Preservation of organic matter within these geologically 

young systems is possible—and is occurring. Relict organic 

carbon was readily visible in the upper iron oxyhydroxide 

layers of active iron bogs but not macroscopically visible in 

deeper layers.

Delivery of Fe2+ and oxidation to Fe3+ are processes that 

can occur at differing rates and would depend on factors such 

as ground-water flow rates, diffusion of O2

 into bog water, 

or microbial kinetics. Diurnal effects were not examined but 

could be important, as daytime iron oxidation rates may be 

four times greater than at nighttime (McKnight and others, 

1988). Thus, accumulation of iron bog solids may be limited 

by processes that occur during specific periods of the day or 

seasons of the year.

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