Ferricrete Classification, Morphology, 

Distribution, and Carbon-14 Age 

Constraints

By Philip L. Verplanck, Douglas B. Yager, Stanley E. Church, and Mark R. Stanton

Chapter E15 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 .......................................................................................................................................................723

Introduction.................................................................................................................................................723

Purpose and Scope ..........................................................................................................................723

Background........................................................................................................................................724

Mapping of Ferricretes .............................................................................................................................725

Iron Spring and Bog Deposits .........................................................................................................726

Colluvial Ferricrete............................................................................................................................726

Alluvial Ferricrete..............................................................................................................................727

Manganiferous Ferricretes .............................................................................................................728

Composition of Deposits ...........................................................................................................................728

Mineralogical Results ......................................................................................................................729

Chemical Results...............................................................................................................................732

Age of the Deposits ...................................................................................................................................733

Distribution of 14C Ages ....................................................................................................................733

14C Ages Determined in “Unidentified Organic Carbon” ............................................................737

Basin-Wide Implications .................................................................................................................739

Distribution of Ferricrete Deposits ..........................................................................................................740

Conclusions.................................................................................................................................................741

References Cited........................................................................................................................................742

Figures

 1. Map showing study area .........................................................................................................725

 2–7. Photographs showing:

 2. Travertine-like iron spring deposit ..................................................................................727

 3. Vertical lamination of porous spring deposit ................................................................728

 4. Poorly to moderately well cemented colluvial ferricrete ...........................................729

 5. Alluvial ferricrete ...............................................................................................................730

 6. Recent avalanche debris on Red tributary, partially cemented by iron 

 oxyhydroxide precipitates ................................................................................................731

 7. Manganocrete in spring deposit in California Gulch ...................................................731

 8. Graph showing FeO and MnO of ferricrete and manganocrete matrix 

samples ....................................................................................................................................732

 9–11. Histograms showing:

 9. Distribution of 14C ages by basin .....................................................................................737

 10. Distribution of 14C by deposit type ...................................................................................738

 11. Distribution of all 14C ages ................................................................................................739

Tables

 1. Classification of ferricrete deposits in the Animas River watershed study area ..........726

 2. Radiocarbon data for organic carbon in ferricrete samples collected from the 

Mineral Creek, Cement Creek, and upper Animas River basins, and for wood 

and peat samples collected from the Mineral Creek basin ...............................................734
Abstract

As part of the U.S. Geological Survey Abandoned Mine 

Lands Initiative Animas River watershed investigation, we 

mapped the distribution and characterized the physical properties of ferricrete deposits to build a framework for determining 

the processes responsible for their formation and preservation. 

Ferricretes are stratified iron oxyhydroxide deposits, or clastic 

sedimentary deposits cemented by stratified iron oxyhydroxide. Three classes of ferricrete were distinguished and 

mapped: those that contained essentially no clasts (spring and 

bog iron deposits), those that contained predominantly angular 

clasts, most commonly monolithologic (iron-cemented colluvial deposits), and those that contained subangular to rounded 

clasts of several lithologies (iron-cemented alluvial deposits). 

In addition, deposits that have a black matrix were mapped as 

manganocrete.

To characterize the deposits, the mineralogy and chemistry of ferricrete matrix were determined. Organic material 

from 71 samples was analyzed by the 14C dating technique 

to evaluate the temporal distribution. These ages range from 

9,150±50 years B.P. to modern; most ages fall between 

5,000 years B.P. and the present.

Ferricrete deposits are not evenly distributed within the 

study area but crop out in conjunction with specific geologic 

and geomorphologic features. Ferricrete and manganocrete 

commonly form in surficial deposits on or down gradient 

from sulfide-rich bedrock, fractures, or veins. Most ferricrete 

deposits are found within or down gradient from parts of the 

watershed where hydrothermal alteration was most intense. 

Manganocrete deposits crop out primarily along the Animas 

River and its tributaries upstream of the town of Eureka. 

Polymetallic veins in this portion of the watershed contain 

abundant manganese minerals which, when oxidized, result 

in manganese mobilization.

Introduction

Ferricretes are present in many mining districts. They 

have been used as exploration guides and as indicators of 

paleo-weathering conditions. However, no systematic investigations of their morphology and distribution within a watershed have been published. As part of the U.S. Geological 

Survey (USGS) Abandoned Mine Lands Initiative Animas 

River watershed investigation, we mapped the distribution and 

characterized the physical properties of ferricrete deposits to 

build a framework for determining the processes responsible 

for their formation and preservation. The Animas River watershed study area is ideally suited for a detailed study of ferricretes because of the presence of numerous ferricrete outcrops, 

an abundance of iron-rich springs, a wide variety of alteration 

types, rugged terrain with deeply incised valleys, and relatively thin vegetative cover. The deeply incised valleys and 

relatively thin vegetative cover provide an opportunity to view 

many sections where ferricrete deposits are located. Thus, the 

setting provides the opportunity to conduct a multidisciplinary 

study of the distribution of ferricrete in both time and space 

and to evaluate the processes responsible for the formation of 

ferricrete deposits.

Purpose and Scope

The objectives of this study are:

• To present a descriptive classification for ferricrete 

deposits within the Animas River watershed study area 

on the basis of clast type, degree of rounding, distribution, and morphology. Physical parameters (on the basis 

of field measurements), mineralogy, chemistry, and 

geochronology were determined from many localities 

within the watershed. This classification scheme was 

also used to produce a map of the ferricrete distribution 

(Yager and Bove, this volume, Chapter E1, pl. 2).

Chapter E15

Ferricrete Classification, Morphology, Distribution, 

and Carbon-14 Age Constraints

By Philip L. Verplanck, Douglas B. Yager, Stanley E. Church, and Mark R. Stanton
• To evaluate the temporal and spatial distribution of 

ferricrete within the watershed throughout the late 

Pleistocene and Holocene.

• To provide the framework for the more detailed 

investigations on the geomorphologic, hydrologic, and 

geochemical processes controlling ferricrete formation 

(Vincent and others, this volume, Chapter E16; Wirt 

and others, this volume, Chapter E17).

Background

Initially, the term ferricrete was used to describe conglomerates that were cemented by iron oxides, analogous to 

the usage of the term calcrete (Lamplugh, 1902). Ferricrete 

has also been used to describe iron-rich soil zones that form 

by the replacement of argillaceous or sandy argillaceous 

matrix by iron oxyhydroxides (Mason and others, 1959; 

Nahon, 1991). Soil scientists have attempted to relate these 

relict paleo-weathering profiles to climatic variations (BattiauQueney, 1996). In historical mining districts in the Rocky 

Mountain region and in Australia, the term ferricrete has 

been used to describe a wide range of iron oxyhydroxide-rich 

material including conglomerates, soils, and paleo-iron bog 

deposits. In addition, the terms ferrosinter and ferroconglomerate have been used to describe iron-rich spring and alluvial 

deposits (Kirkham and others, 1995). We adopt the definition 

from Furniss and Hinman (1998), which states that ferricretes 

are stratified iron oxyhydroxide deposits or clastic sedimentary 

deposits cemented by stratified iron oxyhydroxide.

Descriptions of iron-rich deposits date back to some of 

the earliest geologic investigations in the study area. Springtype limonite deposits and iron-cemented alluvial deposits 

were observed along Cement and Mineral Creeks during 

the initial surveys of the geology and mineralized rocks 

of San Juan County (Comstock, 1883; Ransome, 1901). 

Figure 1 displays locations of most creeks, peaks, and towns 

referred to in this report. Geologists working in the San Juan 

Mountains of Colorado during the late 19th century described 

numerous iron-oxide deposits associated with iron bogs and 

springs along valley floors, as well as iron-cemented creek 

beds (Comstock, 1883; Cross and Purington, 1899; Ransome, 

1901; Cross and others, 1905). Comstock (1883, p. 169) 

noted, “Large deposits of bog iron have been made along the 

banks, especially in portions of Cement and Mineral Creeks.” 

Comstock’s observations are direct evidence that ferricrete 

deposits and iron precipitates in Cement and Mineral Creeks 

were part of the local landscape prior to hard rock mining in 

the Silverton area. Ransome (1901) concluded from his studies 

in the district that as meteoric spring water came in contact 

with the atmosphere, the iron oxidized and accumulated near 

the spring to form bog iron deposits.

The first study of ferricrete in the western San Juan 

Mountains (Hanshaw, 1974) examined the bog iron deposit 

at the Iron Springs placer claim in Ophir Valley, a headwater 

subbasin adjacent to and west of Middle Fork Mineral Creek 

subbasin. This bog iron deposit is the largest known goethite 

deposit in Colorado. Approximately 91,000 metric tons of ore 

was mined for use as smelter flux and paint pigment (Cross and 

Purington, 1899; Harrer and Tesch, 1959). Vhay (1962, p. 247) 

surmised that the iron oxide in the Iron Springs deposit was 

likely “derived by the weathering of the pyrite disseminated 

through the large mass of hydrothermally altered rock that 

forms the whole mountain to the north of the valley.” Chemical analyses of different iron-bearing lithologies within the 

deposit had a range in iron concentration (as Fe2

O3) from 58.8 

to 73.8 weight percent. The samples also contained substantial 

concentrations of zinc ranging from 700 to 1,500 ppm, and 

lesser concentrations of copper ranging from 50 to 200 ppm 

and lead ranging from 15 to 50 ppm (Hanshaw, 1974).

A wood fragment located 1 m above the base of the 

deposit gave a radiocarbon age of 8,250 yr B.P. (Hanshaw, 

1974). This sample was about 7.3 m from the top of the 

deposit, yielding an average accumulation rate of 0.9 mm/yr. 

Other wood fragments from different stratigraphic levels within 

the deposit yielded accumulation rates ranging from 0.6 to 

1.6 mm/yr (Hanshaw, 1974).

Investigations to determine premining metal loading 

in historical mining districts have tried to utilize ferricrete 

deposits as proxies for paleo-weathering conditions (Plumlee 

and others, 1995; Logsdon and others, 1996; Furniss and 

others, 1999; Miller and others, 1999). At the Summitville 

mine, Plumlee and others (1995) used aerial photographs 

taken prior to open-pit mining to map ferricretes formed 

by natural springs. The ferric oxide minerals that formed the 

ferricretes were precipitated by acidic, iron-rich ground water 

as it flowed from springs and were interpreted to delineate 

the locations of naturally acidic springs that flowed prior to 

mining (Plumlee and others, 1995). The authors noted that 

ferricrete deposits tended to follow major structural features or 

were located along contacts between lithologies with differing 

hydrologic characteristics.

Furniss and others (1999) dated and analyzed ferricretes 

at the New World mining district of south-central Montana, 

concluding that the ferricretes are evidence that discharge of 

acidic, metal-bearing water predated mining activity. Twentytwo radiocarbon dates on wood fragments in the ferricrete 

deposits, ancient stream terraces and aprons around natural 

springs, ranged from 8,840±50 yr B.P. to the present. Chemical compositions of modern and ancient iron oxyhydroxides 

were compared at specific sites to gain insight into premining 

conditions. Furniss and Hinman (1998) concluded that within 

the New World mining district, similar chemical conditions 

exist today in natural iron-rich springs as have existed since 

the exposure of the sulfide-rich mineral deposit.

More recently, preliminary mapping of ferricrete along 

the valley bottoms in the Animas River watershed study area 

(T.J. Casadevall and D.J. Sofield, written commun., 1996) 

delineated ferricretes in some parts of the region. Field examination of these outcrops demonstrated the need for refinement 

of this early mapping and development of a more complete 

map of ferricrete throughout the watershed (Yager and Bove, 

this volume, pl. 2).

724 Environmental Effects of Historical Mining, Animas River Watershed, Colorado
Ferricrete Classification, Morphology, Distribution, and Age Constraints 725

Mapping of Ferricretes

In this study, the Mineral and Cement Creek basins were 

mapped in detail to delineate the extent and lithologic variation 

of ferricretes. However, because of time limitations, the upper 

Animas River basin upstream of the town of Silverton was 

mapped only in reconnaissance. To unravel the complex nature 

of these sedimentary deposits, we recorded a set of physical 

properties at each outcrop and compiled them to create a classification scheme (table 1). Important field observations include 

the presence or absence of clasts, degree of rounding of clasts 

and their lithologies, grain size, degree of sorting, whether 

clasts are matrix- or clast-supported, porosity, color, presence of small-scale structures within the deposit, orientation 

of layering, and dimensions of outcrop. In addition, we noted 

whether the outcrop was wet or dry, to determine whether the 

ferricrete deposit might be active. Because of temporal variations in ground-water flow, this observation was not always a 

definitive determination of recent ground-water activity.

We were able to distinguish and map three primary 

classes of ferricrete: ferricretes that contained essentially no 

clasts (that is, spring and bog iron deposits), ferricretes that 

contained predominantly angular clasts, usually of a single 

rock type (that is, iron-cemented colluvial deposits), and ferricretes that contained subangular to subrounded or rounded 

clasts of several lithologies (that is, iron-cemented alluvial 

deposits). In addition, ferricrete deposits were identified that 

have a black matrix. These deposits contain elevated amounts 

of manganese in the iron cement and were mapped as manganese-rich ferricrete deposits or manganocretes.

37 

45’

o

Animas River

37 

52’

30”

o

107 45’ o 107 37’30” o

o

o

o

o

3,792 m

Middle Fork

Red Mt

No. 3

Ohio Peak

0

0

1

1

2

2

3

3

Base from U.S. Geological Survey

1:100,000 Digital Line Graphs

Albers Equal-Area Conic projection

Standard parallels 37o

30' and 40o30'

Central meridian 37o

00'

Study area boundary

Red tributary

Prospect

Gulch

Houghton Mt

Anvil Mt

Silverton

Blair Gulch

Boulder Gulch

Chattanooga Eureka

former Lake

Emma

Eureka Gulch

Animas

Forks

California

Gulch

Gulch

Placer

Gulc

Pic

h

ayune

Topeka

Gulch

Tiger

Gulch

Ohio

Gulch

x

x x

x

South Fork

Mineral Creek

Cement Creek

South Fork

North Fk

Gladstone

x

4 MILES

4 KILOMETERS

Animas River

watershed 

study area

C O L O R A D O

Durango

Figure 1. Rivers, creeks, peaks, and towns in the Animas River watershed study area.
Table 1. Classification of ferricrete deposits in the Animas River watershed study area.

Class Description Location

Alluvial Iron oxyhydroxide-cemented conglomerate. Clast supported 

with subangular to subrounded, multi-lithologic clasts coated 

with iron oxyhydroxides, poor to moderately sorted. Fluvial 

sedimentary structures and subhorizontal layering observed 

in some outcrops.

Along stream beds and terraces. Steep valley 

sides of lower portions of Mineral and Cement 

Creeks contain exposures as much as 30 m 

above stream level.

Colluvial Iron oxyhydroxide cemented, clast supported with angular clasts 

of limited lithologic variation, poorly to moderately sorted, 

massive to minor layering subparallel to hillslope.

Steep tributaries, avalanche chutes, and hillslopes. 

Iron spring and bog Few or no clasts, generally contains fine, horizontal lamination 

with some vertical lamination in feeder zones. 

Iron bogs found within valley floors and iron 

springs feed some bogs, also found along hillslopes near bedrock contacts or fracture zones. 

Manganocrete Term used for ferricretes with black to dark-gray matrix. Includes 

both clast-supported and matrix-supported deposits. 

Manganocrete deposits mapped in upper Animas 

River basin upstream of Eureka townsite. 

Iron Spring and Bog Deposits

Clast-absent, iron-rich ferricretes observed along sections of Mineral and Cement Creeks tend to have two types of 

morphology. Both these morphological types are designated as 

bog iron on the ferricrete map (Yager and Bove, this volume, 

pl. 2). One type consists of dense, laminated iron oxyhydroxide, and forms aprons surrounding active springs (fig. 2). The 

second type is more porous (as much as 30 percent void space) 

and crops out along slopes near valley floors (fig. 3).

Active iron spring deposits along valley floors may form 

iron bogs. In the Mineral Creek basin, large iron bogs are 

found in beaver ponds at Chattanooga, along the lower portion 

of a north-flowing tributary of Middle Fork Mineral Creek 

referred to as Red tributary in Mast, Verplanck, and others 

(2000); along the east bank of Mineral Creek at the confluence 

with Middle Fork Mineral Creek; and in beaver ponds along 

the north bank of South Fork Mineral Creek upstream of the 

confluence with Mineral Creek (Yager and Bove, this volume, 

pl. 2). In Cement Creek, iron bog areas include large areas of 

the flood plain both upstream and downstream of the confluence with South Fork Cement Creek, south of North Fork 

Cement Creek, upstream of the confluence with Ohio Gulch, 

and an area on the west side of Cement Creek upstream of 

Soda Gulch (Yager and Bove, this volume, pl. 2).

Large bog iron deposits (inactive iron bogs) are found 

along the hillslope southwest of the active Chattanooga bogs 

on the west side of upper Mineral Creek, in the upper portion 

of Cement Creek along the east bank upstream of the confluence with North Fork Cement Creek, and in the upper portion of Boulder Gulch (Yager and Bove, this volume, pl. 2). 

Historically, some of these bog iron deposits were mined and 

used for flux in smelters (Harrer and Tesch, 1959).

Iron springs that discharge along valley floors form 

travertine-like terraces that are sporadically coated with 

algal material. Hydrogeochemical studies of these deposits are 

described in Stanton, Fey, and others (this volume, Chapter E25). 

Spring water flows as a thin film over the terraces toward 

the creek. The terraces consist of iron oxyhydroxides with 

varying amounts of organic material, including conifer cones 

and needles, leaf litter, mosses, algae, twigs, and logs. Casts 

of organic matter preserve fine detail, including bark, wood 

grain and structure of logs, and algal and moss textures. In 

cross section, these deposits are horizontally to subhorizontally 

laminated, some with strong color banding. Sand- to silt-sized 

rock fragments are incorporated in some laminae and are matrix 

supported. Some low-lying iron-rich springs have more diffuse 

outflows, which form iron bogs. These iron bogs often contain 

a wide variety of algae, and the water has highly variable pH, 

ranging from 3.5 to near 7. Organic-rich, wet, spongy ground 

and a distinct flora that includes mosses and sedges characterize 

these bogs.

The more porous, clast-absent ferricrete deposits have 

been mapped along the lower sections of the valley sides. 

Excavation for road building or mineral prospecting has 

exposed several of these deposits. Spectacular examples crop 

out on the hillslope west of the Chattanooga bogs (99VM65 on 

plate 2 of this volume) and in the upper portion of South Fork 

Cement Creek. (Note that solid sample names from a given 

site have an “S” (99VMS65) and water sample names from a 

given site have an “F” (99VMF65).) In contrast to valley-floor 

deposits, banding in these more porous ferricretes tends to be 

vertical to subvertical (fig. 3). In the upper section of South 

Fork Cement Creek, vertically layered deposits crosscut horizontally layered deposits (this volume, fig. 11 of pl. 2). Laminae may be of varying colors, including light and dark brown 

and yellow. Rock fragments are generally absent. These 

deposits appear to be paleo-feeder zones to iron-rich springs.

Colluvial Ferricrete

Colluvial ferricretes are clast supported, and the clasts 

are angular to subangular (fig. 4). These deposits commonly 

are exposed on hillslopes and may grade into iron bog and 

spring deposits down gradient where the slope angle decreases. 

Crude layering defined by alignment of clasts, typical of soil 

creep, is observed in most outcrops and is subparallel to the 

726 Environmental Effects of Historical Mining, Animas River Watershed, Colorado
Ferricrete Classification, Morphology, Distribution, and Age Constraints 727

Figure 2. Travertine-like iron spring deposit, east bank of Mineral Creek near 

confluence with Middle Fork Mineral Creek (site 99VM72).

topography. Iron oxyhydroxides coat the rock fragments and 

fill void spaces, in places displaying a layered texture. Within 

a single outcrop, the degree of cementation can vary considerably. Many deposits are poorly sorted, containing pebble- to 

cobble-sized clasts, which are mainly monolithologic. The 

range in composition of the clasts is limited to the upslope 

lithologic variations. Some outcrops are damp, with water 

coating clasts; ground-water flow is limited to drips from 

the outcrop. Organic material is limited to soil zones, which 

generally contain only roots and mosses; however, wood that 

has been replaced by iron oxide is observed in colluvial ferricretes in Topeka Gulch (SDY 017B), Cement Creek north 

of Tiger Gulch (99ABFC123), and South Fork Cement Creek 

(99ABFC163). Good examples of colluvial ferricrete can be 

seen in the North Fork Cement Creek valley (site IDY0014, 

this volume, pl. 2).

Alluvial Ferricrete

A third class of ferricrete, alluvial ferricrete, was mapped 

within and along much of Mineral and Cement Creeks. Lesser 

amounts of alluvial ferricrete are exposed in the valley of 

the Animas River upstream of the confluence with Cement 

Creek. In general, alluvial ferricretes are iron oxide-cemented 

conglomerates, but in some outcrops, lenses of well-sorted 

or graded silts and sands are present. The alluvial ferricrete 

consists of subangular to subrounded or rounded clasts with 

iron oxides coating the grains and filling void space to varying 

degrees. Clasts typically are pebble- to cobble-sized with a few 

boulders. Spaces between clasts are generally filled with finer 

grained sediment. Some outcrops display clast imbrication that 

dips upstream. Within an outcrop, variable cementation can 

create ledges that range from horizontal to subparallel with 

the valley floor. Cementation appears to be independent of 

grain size or sedimentary structures.

Some deposits are poorly sorted and may be avalanche 

or flood deposits. These deposits commonly contain logs that 

are randomly oriented, along with occasional buried stumps in 

growth position on the flood plain. At the confluence with tributary streams, alluvial ferricretes generally thicken to incorporate 

alluvial fan material from the tributaries, such as at the confluence of Prospect Gulch with Cement Creek (Vincent and others, 

this volume). The percent of void space in alluvial ferricretes is 

generally low, resulting in rocks that are highly resistant to erosion, but which can be undercut by channel erosion and break 

off as large coherent blocks from the stream banks.

Outcrops as much as 30 m in height are found along 

the lower portion of Middle Fork Mineral Creek and along 

Mineral Creek between the confluences of Middle Fork and 

South Fork Mineral Creek (fig. 5). Alluvial ferricretes of similar scale are exposed along the valley walls of lower Cement 

Creek and at the mouth of Blair Gulch on the upper Animas 

River. Sections of the creek bed in Cement Creek, lower 

Middle Fork Mineral Creek, Mineral Creek downstream of the 

confluence with South Fork Mineral Creek, and the Animas 

River downstream of the confluence with Cement Creek 

consist of alluvial ferricrete. Much of the town of Silverton is 

underlain by alluvial ferricrete; it is often the competent rock 

upon which building foundations rest (Steve Fearn, consulting mining engineer, oral commun., 1998; this volume, fig. 8 

of pl. 2). Alluvial ferricrete is actively forming in Red tributary on Middle Fork Mineral Creek (Mast, Verplanck, and 

others, 2000) at the base of a talus slope (Yager and others, 
Figure 3. Vertical lamination of porous spring deposit, west side of Mineral Creek upstream 

of Chattanooga (site 99VM65). Approximately 1 m in height.

2000). This location contains recent avalanche debris that has 

been cemented by iron oxyhydroxides (fig. 6). Ferricrete is 

also actively forming in Cement Creek near the confluence of 

Ohio and Tiger Gulches.

Manganiferous Ferricretes

In the upper Animas River basin upstream of Silverton, 

outcrops of black oxide-cemented deposits—some clast 

supported and some matrix supported—are present (fig. 7). 

Because of the distinct color, these were mapped as manganeserich ferricrete deposits, herein referred to as manganocretes. 

Ore deposits in the Eureka graben in this section of the watershed are manganese rich, containing primary pyroxmangite 

((Mn, Fe, Ca, Mg)SiO3

) and rhodochrosite (MnCO3) with 

minor friedelite (Mn8

Si6O18), tephroite (Mn2SiO4), and helvite 

(Mn4

(Be3Si3O12)S) (Casadevall and Ohmoto, 1977). Weathering 

of the deposits mobilizes manganese; subsequent manganese 

redeposition results in the formation of manganese oxides. The 

black color of these deposits is derived from the cement having 

a significant portion of manganese oxide in it. A suite of these 

samples was collected and analyzed to determine the relative portions of iron and manganese, as well as trace-element 

concentrations.

Composition of Deposits

Solid samples of matrix material from the ferricrete and 

manganocrete deposits were analyzed to determine the mineralogy and chemistry of the various cements. Matrix material 

was separated from clast-rich samples using a ceramic plate 

and roller that breaks the cement away from the clast material, thereby minimizing incorporation of rock fragments in 

the sample to be analyzed. The fine-grained matrix material 

was concentrated by sieving to <100 mesh. The fine-grained 

detritus could not be completely removed from the samples, 

728 Environmental Effects of Historical Mining, Animas River Watershed, Colorado
Ferricrete Classification, Morphology, Distribution, and Age Constraints 729

and some matrix samples still contained detritus that diluted 

the iron content of the matrix material. Samples were analyzed 

using mixed-acid total digestion (Briggs, 1996) followed by 

inductively coupled plasma–atomic emission spectroscopy 

(ICP-AES). Results are in the database (Sole and others, this 

volume, Chapter G; D.L. Fey, analyst). Sulfur was analyzed 

commercially by XRAL Laboratories using high-temperature 

combustion in a stream of oxygen in the presence of elemental 

iron. This combustion produces SO2

 gas whose concentration 

was determined using an infrared detector.

Mineralogy of the matrix samples was determined by 

X-ray diffraction. Powder patterns were obtained using a 

Shimadzu diffractometer with Ni-filtered CuKα radiation scanning from 4° to 60° 2θ. The X-ray spectra were not backgroundcorrected because samples containing weakly crystalline 

materials such as schwertmannite and poorly ordered goethite 

cannot be identified if background corrections are made to the 

spectra. All samples were run blind (Rhonda Driscoll, analyst). 

Mineralogical results are presented in terms of major, minor, 

or trace constituents on the basis of the intensity of the major 

peak in the X-ray spectra. The intensity scale has not been 

calibrated to determine the percent of the mineral present in the 

sample. Amorphous iron oxides were presumed to be present 

in all iron-matrix samples. Where no discrete iron oxide phases 

were identified in the X-ray diffraction patterns, amorphous 

iron oxides were indicated as the major iron oxide phase in the 

database (Sole and others, this volume).

Mineralogical Results

Mineralogical results for the 74 samples analyzed were 

divided into two categories, iron oxide minerals and rockforming minerals. Iron oxide minerals included goethite 

(α-FeO(OH)), schwertmannite (Fe8

IIIO8

(SO4)1.25(OH)5.5), and 

amorphous iron oxyhydroxide. Minor or trace amounts of 

jarosite (KFe3

III(SO4

)2(OH)6) were found in three samples; 

trace amounts of pyrite (FeS2

) were observed in one sample. 

No discrete manganese minerals were detected in the manganocrete samples. Minerals present as fine-grained detrital 

phases in the matrix included albite, muscovite, orthoclase, 

and quartz. Identified layer silicates included clinochlore (the 

magnesium chlorite), montmorillonite, and saponite. Rutile 

was observed in four samples and boehmite in one sample.

The mineralogy of the active iron spring and bog samples 

(32 samples) consisted primarily of amorphous iron oxides 

with identifiable schwertmannite and goethite. Both minerals are common precipitates from acidic drainage; however, 

schwertmannite is the predominant form when the pH is less 

than 4.5 and is indicative of more acidic waters (Bigham and 

others, 1996). Jarosite was observed in a banded ferricrete 

sample from the soil zone adjacent to the large spring in lower 

Prospect Gulch. Quartz was common, whereas albite and muscovite were often detected but usually in only minor amounts. 

Dissolved silica present in the stream waters produced silica 

gel when filtered (G.A. Desborough, oral commun., 2001). 

Figure 4. Poorly to moderately well cemented colluvial ferricrete, north side of Middle Fork Mineral Creek 

subbasin (site 99VM34).
Thus, quartz could be detrital or authigenic. Other silicate 

minerals in these deposits are rare and were found only in trace 

amounts. The silicate minerals and rutile were likely detrital 

lithics that were washed or blown into the bogs or springs.

The mineralogy of the inactive iron spring and bog iron 

deposits (12 samples) did not differ significantly from that 

just described except that only about half the samples contained identifiable schwertmannite. The mineralogy of the 

matrix of colluvial ferricrete (10 samples) primarily consisted 

of goethite and amorphous iron oxides. Quartz was common, and albite and muscovite were present in some samples. 

Clinochlore and rutile were present in some active colluvial 

ferricrete samples, and saponite was detected in one sample.

The mineralogy of the alluvial ferricrete deposits 

(20 samples) differed from that of the active iron spring and 

bogs in that goethite or amorphous iron oxides were commonly 

found. Only one sample contained schwertmannite, another 

jarosite, and a third pyrite. Quartz was ubiquitous. Albite and 

muscovite were common, and trace amounts of orthoclase and 

clinochlore each were detected in one sample. Montmorillonite 

was detected in 6 of the 20 samples analyzed. These minerals 

probably represented small contributions from lithic fragments 

that were not completely separated from the matrix. Boehmite, 

Al(OH)3

, was detected in one sample.

Identifiable iron minerals were absent from the manganocrete deposits (eight samples); amorphous iron oxides were 

presumed to be present. No manganese-bearing minerals, 

including oxides, were detected in these deposits; thus, amorphous manganese was also presumed to be present. Quartz 

was ubiquitous. Many samples contained fine-grained clastic 

material, as shown by the presence of identifiable albite and 

muscovite in all samples. Clinochlore was identified in four 

of the eight manganocrete samples, and other rock-forming 

minerals were identified in minor or trace amounts in the manganocrete samples.

In a laboratory, each sample was dried and compared, 

under uniform lighting, with Munsell soil color charts 

(Rhonda Driscoll, analyst). A few general observations can be 

gleaned from the mineralogical data. In general, the color of 

the samples varied with mineralogy. Ferricrete matrix samples 

that contained schwertmannite as a major phase had Munsell 

soil color indices of 2.5YR to 5YR. In these samples goethite 

was also a major phase. The one sample in which schwertmannite was the only major phase had a color index of 2.5YR 3/6 

(dark red). Most of the ferricrete matrix samples that had 

goethite as the only major iron phase had color indices of 

7.5YR to 10YR. The range in goethite color is the same range 

that Bigham and others (1992) noted.

Figure 5. Alluvial ferricrete, 30 m thick, south bank of Middle Fork Mineral Creek near confluence with 

mainstem.

730 Environmental Effects of Historical Mining, Animas River Watershed, Colorado
Ferricrete Classification, Morphology, Distribution, and Age Constraints 731

Figure 6. Recent avalanche debris on Red tributary, partially cemented by iron oxyhydroxide precipitates 

(site SDY9914).

Figure 7. Manganocrete, in spring deposit in California Gulch (site 99VA01).
Schwertmannite was primarily observed in active ferricretes, and goethite was observed in both active ferricretes 

and paleo-ferricretes. These observations are consistent with 

previous studies on the formation and preservation of schwertmannite. Experiments by Bigham and others (1996) demonstrated that schwertmannite is a metastable phase and transforms to goethite. For natural schwertmannite in an aqueous 

solution, the transformation was complete in 1,739 days; and 

for synthetic schwertmannite, the transformation was complete 

in 543 days (Bigham and others, 1996).

Chemical Results

Solid-phase chemical analyses of ferricrete and manganocrete matrix samples are tabulated in Wirt and others (this 

volume) along with chemical results from corresponding water 

samples. The major-element chemical results from matrix 

samples are compatible with the mineralogical results in that 

iron oxide was the most abundant constituent in most samples 

(FeO ranged from 5.7 to 77.2 weight percent). The black 

matrix samples had the highest manganese concentrations 

(MnO greater than about 3 weight percent) and FeO:MnO≈1; 

only two samples had FeO:MnO<1 (fig. 8).

Two samples (99ABFC170 and 175B) had MnO concentrations greater than 1 percent but were not manganocretes 

because of their brown color. These samples were from deposits on the east side of Cement Creek upstream of Gladstone 

(Yager and Bove, this volume, pl. 2). This part of the basin 

is within the western portion of the Eureka graben, which 

hosts manganese-rich veins that contain pyroxmangite and 

rhodochrosite (Casadevall and Ohmoto, 1977). Three other 

samples (99VMS13A–C) plotted in a field just above the other 

ferricrete samples because of slightly greater MnO concentrations (fig. 8). These samples were collected along the lower 

portion of the hillslope west of Mineral Creek at Chattanooga 

(this volume, pl. 2). Two of these samples were ferricrete 

matrix exposed above a draining adit, and the third was iron 

floc from the adit drainage. Although no manganese-rich mineral deposits are known in this part of the watershed, Miocene 

to Oligocene rhyolitic dikes and quartz latite stocks crop out 

upgradient, and units with similar ages and compositions are 

also present in the upper portion of the Animas River valley, upstream of Animas Forks (Luedke and Burbank, 1987). 

Bove and others (this volume, Chapter E3) have postulated 

that the Miocene to Oligocene rhyolitic dikes and quartz latite 

stocks are associated with some of the manganese-rich mineral 

deposits in the Eureka graben region.

10 20 30 40 50 60 70 80 90 100

0

1

2

3

4

5

6

7

8

9

10

99VMS13A-C

Hillslope west of Chattanooga 

ferricrete matrix and adit floc

 99ABFC170

upper Cement Creek

Field for black 

matrix samples

1:1 FeO:MnO

 99ABFC175B

upper Cement Creek

T

N

E

C

R

E

P

T

H

GI E

W

NI , O

n

M

FeO, IN WEIGHT PERCENT 

0

Figure 8. FeO and MnO of ferricrete and manganocrete matrix samples.

732 Environmental Effects of Historical Mining, Animas River Watershed, Colorado
Ferricrete Classification, Morphology, Distribution, and Age Constraints 733

Age of the Deposits

More than seventy 14C ages were determined on various materials during the course of our studies in the watershed. The data are compiled and presented in table 2 herein, 

table 1 in Vincent and others (this volume, Chapter E16), 

and table 1 in Vincent and Elliott (this volume, Chapter E22). 

The distribution of ages by basin (fig. 9) and that by material 

type (fig. 10) are biased, because they reflect the intensity 

of studies in specific areas. More detailed work was done 

on both distribution and ages of alluvial ferricrete deposits in 

the Cement Creek basin than in the other two basins. Wood in 

alluvial ferricrete deposits was the primary sample type dated. 

The distribution of peat deposits in the Cement Creek basin 

was also important in deciphering of the Holocene geomorphological development of Cement Creek (Vincent and others, 

this volume). The distribution of ages by type of ferricrete 

deposit is also biased (fig. 10) by limited sampling. As our 

focus in the Cement Creek basin was on alluvial deposits, 

we have more age data on alluvial ferricrete than on colluvial 

ferricrete.

Samples from 34 stumps in growth position or fallen logs 

were collected for 14C age determinations. We avoided obvious 

tree roots where their relationship to the surface of the ferricrete or terrace deposit was uncertain. Log samples collected 

were encased in the ferricrete deposits rather than adhering to 

or cemented to the side of the outcrop. If an outer decomposed 

portion was present, we removed it to avoid potential contamination by young carbon. The outer 10–20 rings of large 

log and stump samples were systematically used for 14C age 

determinations. Logs and stumps in growth position in alluvial 

ferricrete were buried during sedimentation, and thus they give 

the approximate age of deposition of the clastic deposit. Logs 

that were incorporated in avalanche debris were randomly 

oriented in the plane of flow within the deposit and most likely 

were from trees uprooted by the avalanche, thus yielding 

an age of clastic deposition. Logs in these deposits may be 

slightly older than the deposit if they died before entrainment 

in the avalanche debris. In both cases, the 14C ages give the 

maximum age of cementation and ferricrete formation.

We contracted two commercial laboratories to analyze 

three different material types: wood or charcoal, cellular 

matter from peat, and unidentified organic carbon. Methods 

of sample preparation, analysis, and data reduction are in footnotes in the 14C data tables referred to previously.

Distribution of 14C Ages

Dated wood fragments in alluvial ferricrete have ages 

that extend from about 5,000 yr B.P. to the present. In contrast, ages of samples from unconsolidated terrace gravel are 

young, generally less than 500 yr B.P., as expected (fig. 10). 

The two old stumps in growth position in terrace gravel that 

were dated were rooted in ferricrete. Presumably, these terrace 

gravel deposits were not cemented until some time after tree 

growth began. Dendrochronology of trees on unconsolidated 

terraces also has shown that uncemented young terraces in 

the Animas River watershed are common, dating back several 

hundred years (Church and others, 2000). A significant 

number of young 14C ages have come from wood fragments 

cemented in alluvial conglomerates by iron oxides (fig. 10).

Cementation of alluvial deposits may have been a highly 

variable process. Some outcrops of alluvial ferricrete along 

both Cement and Mineral Creeks have acid springs containing iron floc. These springs primarily emanate from the base 

of alluvial deposits along the bedrock contact, but some 

small seeps emanate from within the deposit, from 1 to 5 m 

above the creek level. This suggests that some ground water 

enters the creek near creek level, but that the upper portion of the ferricrete deposit may contain permeable zones 

where ground-water flow and cementation are occurring. 

This process can complicate the interpretation of 14C ages on 

iron-oxide cements. Conversely, the relatively high number of 

modern ages of logs in ferricrete demonstrates that, in some 

environments, cementation occurs soon after deposition. 

In some tributaries, we observed avalanche debris from the 

previous spring that was coated with iron precipitates the next 

summer, and which by the second field season was partially 

cemented.

Twenty-one samples of peat or wood from four different sites within the watershed were dated. In South Fork 

Mineral Creek peat is actively forming today and has ages 

ranging from at least 2,000 yr B.P. to the present (table 2). 

Peat samples dated from the Forest Queen site (Vincent and 

Elliott, this volume) have ages ranging from at least 3,000 yr 

B.P. to present. Stanton, Fey, and others (this volume) discuss 

the wetlands present at the Forest Queen where peat is forming 

today. A peat deposit at Gladstone has ages as old as 5,000 yr 

B.P., whereas the peat section studied from near Ohio Gulch 

has ages as old as 2,500 yr B.P. (Vincent and others, this volume, table 1).

Iron springs and bog iron deposits have a more limited 

range in age (about 5,000 yr B.P. to present) than the alluvial 

or colluvial ferricrete deposits (about 9,000 yr B.P. to present; 

fig. 10). Further, the ages of the iron springs/bog iron, and the 

colluvial ferricrete deposits are fewer in number than those 

from alluvial ferricrete. Of the 14C ages in the colluvial ferricrete deposits, only the youngest and the oldest ages are from 

wood samples. Of the 14C ages from the spring/bog iron deposits, the three youngest ages are from wood samples. All the 

other ages from colluvial ferricrete and iron spring/bog iron 

deposits are from unidentified organic carbon extracted from 

a matrix sample that had a wavy texture, which was identified 

in the field as a possible algal structure, or from goethite casts 

containing organic material such as a log.
Table 2. Radiocarbon data for organic carbon in ferricrete samples collected from the Mineral Creek, Cement Creek, and upper Animas River basins, and for wood and peat 

samples collected from the Mineral Creek basin.

[Note that “B.C.” and “A.D.” are both placed before dates in this table—for better readability and quicker recognition]

Field No. Lab No.1 δ13CPDB

o

/oo

2

14C age

years B.P.3

Calibrated ages4

Comments

Locality

(lat N.,

long W.) Intercept(s)5 1-sigma range6 2-sigma range6

Mineral Creek

Organic carbon from ferricrete deposits on west side Mineral Creek:SDY9827C GX-26687-AMS –23.3 4,200±40 B.C. 2876 B.C. 2882–2701 B.C. 2895–2624 Samples from large alluvial ferricrete deposit on west side of 

Mineral Creek between Middle and South Forks. Stratigraphic order is A–C (top to bottom).

37.8375° N.107.7276° W.

SDY9827A GX-26685-AMS –21.1 4,460±40 B.C. 3097 B.C. 3328–3028 B.C. 3346–2924 37.8375° N.

107.7275° W.

SDY9827B GX-26686-AMS –22.9 5,600±40 B.C. 4452–4404 B.C. 4460–4360 B.C. 4518–4347 37.8375° N.

107.7275° W.

99VMS65 GX-26691 –24.4 4,820±360 B.C. 3640 B.C. 3980–3100 B.C. 4360–2600 Iron spring deposit, upstream from 

Middle Fork Mineral Creek.

37.8653° N.107.7267° W.

Middle Fork Mineral Creek

Samples of wood in terrace deposits:00ABFC201 B-147007 –25* 790±70 A.D. 1260 A.D. 1190–1280 A.D. 1050–1100

A.D. 1140–1300

8 in. diameter log in alluvial ferricrete about 2.5 ft above low-flow 

water level.

37.8448° N.107.7318° W.

00ABFC202A B-147008 –25* 870±50 A.D. 1180 A.D. 1060–1230 A.D. 1030–1270 8 in. diameter log in colluvial 

ferricrete at low-flow water level.

37.8441° N.107.7290° W.

00ABFC200 B-147006 –25* 4,520±70 B.C. 3340–3200 B.C. 3360–3090 B.C. 3500–3460

B.C. 3380–3000B.C. 2980–2940

Wood in alluvial ferricrete 4 ft 

above low-flow water level.

37.8451° N.107.7323° W.

Organic carbon from ferricrete deposits:99VMS33 GX-26690-AMS –23.8 2,760±40 B.C. 902 B.C. 969–834 B.C. 1001–825 Ferricrete matrix sample from 40 ft 

up in glacial outwash deposit, lower Mineral Creek.

37.8449° N.107.7310° W.

ODY9815A GX-26688-AMS –24.9 2,790±30 B.C. 966–921 B.C. 995–901 B.C. 1004–835 Colluvial ferricrete matrix sample 

from Red tributary. 

37.8350° N.107.7532° W.

SVFFC GX-27519-AMS –23.2 4,390±30 B.C. 3017–2940 B.C. 3082–2921 B.C. 3097–2914 Filamentous ferricrete matrix sample 

downstream from Bonner mine. 

37.8448° N.107.7310° W.

South Fork Mineral Creek

Samples of peat:97ABS321B GX-27696 –26.5 710±40 A.D. 1287 A.D. 1287–1298 A.D. 1258–1386 Peat from 12 in. depth in core 

through active peat bog.

37.8143° N.107.7367° W.

97ABS322A GX-27697 –28.0 1,060±130 A.D. 990 A.D. 780–1160 A.D. 680–1260 Peat from 21 to 31 in. depth in 

core through active peat bog.

37.8141° N.107.7364° W.

734 Environmental Effects of Historical Mining, Animas River Watershed, Colorado
Ferricrete Classification, Morphology, Distribution, and Age Constraints 735

Field No. Lab No.1 δ13CPDB

o

/oo

2

14C age

years B.P.3

Calibrated ages4

Comments

Locality

(lat N.,

long W.) Intercept(s)5 1-sigma range6 2-sigma range6

South Fork Mineral Creek—Continued

Samples of peat from iron-rich bog sediments:97-ABS319 GX-27698 –24.1 1,480±40 A.D. 600 A.D. 541–639 A.D. 444–655 Peat from 8 in. depth in core through 

active iron bog forming in peat deposit.

37.8163° N.107.7298° W.

97ABS320C GX-27699 –25.4 1,920±190 A.D. 80 160 B.C.–A.D. 

340 

40 B.C.–A.D. 540 Peat from 12 in. depth in core 

through active iron bog forming in peat deposit.

37.8316° N.107.7298° W.

Organic carbon from ferricrete deposits:SDY9813A GX-26682-AMS –25.2 1,170±40 A.D. 887 A.D. 780–943 A.D. 729–980 Colluvial ferricrete matrix from 

bottom of stratigraphic sequence.

37.8215° N.107.7443° W.

SDY9813B GX-26683 –24.4 4,430±390 B.C. 3090–3030 B.C. 3640–2500 B.C. 3990–1980 Colluvial ferricrete matrix from top 

of stratigraphic sequence.

37.8215° N.107.7443° W.

Cement Creek basin

Organic carbon from casts in ferricrete deposits:99ABFC139B GX-26391 –23.3 5,000±190 B.C. 3780 B.C. 3980–3640 B.C. 4250–3370 Iron spring deposit, cast was not 

identifiable.

37.8546° N.107.6762° W.

99ABFC163C

Total

GX-27515 –22.4 6,010±70 B.C. 4900–4850 B.C. 4990–4800 B.C. 5190–4720 Log in colluvial ferricrete deposit 

that has been totally replaced by goethite (bulk sample).

37.8763° N.107.6441° W.

99ABFC163C

Fulvic

GX-27971-AMS –25.4 2,720±30 B.C. 887–834 B.C. 900–828 B.C. 965–807 Fulvic acid component extracted 

from sample.

99ABFC163C

Humic

GX-27972-AMS –27.0 2,080±40 B.C. 91–61 168 B.C.–A.D. 6 199 B.C.–A.D. 17 Humic acid component extracted 

from sample.

99ABFC163C

Residual 

GX-28131 –23.6 5,410±220 B.C. 4320–4252 B.C. 4458–3979 B.C. 4766–4759

B.C. 4716–3756B.C. 3744–3726B.C. 3725–3712

Residual cellular carbon remaining 

after extraction of organic acids.

99ABFC123C GX-26390 –23.3 7,680±200 B.C. 6470 B.C. 6690–6270 B.C. 7060–6090 Goethite replaced log in colluvial 

ferricrete.

37.8787° N.107.6685° W.

Organic carbon from ferricrete deposits, Topeka Gulch:SDY0017A GX-27517-AMS –22.7 24,020±100 Outside of calibration range Finely laminated matrix in bog 

iron deposit.

37.8496° N.107.6897° W.

SDY0017B GX-27518-AMS –26.3 3,950±40 B.C. 2466 B.C. 2545–2410 B.C. 2569–2309 Ferricrete matrix from colluvial 

deposit.

37.8494° N.107.6897° W.

Table 2. Radiocarbon data for organic carbon in ferricrete samples collected from the Mineral Creek, Cement Creek, and upper Animas River basins, and for wood and peat 

samples collected from the Mineral Creek basin.—Continued
Field No. Lab No.1 δ13CPDB

o

/oo

2

14C age

years B.P.3

Calibrated ages4

Comments

Locality

(lat N.,

long W.) Intercept(s)5 1-sigma range6 2-sigma range6

Upper Animas River basin

Organic carbon from manganocrete samples, California Gulch:SVMCS GX-27520-AMS –25.1 1,590±40 A.D. 433 A.D. 420–537 A.D. 393–595 Matrix from manganiferous colluvial 

ferricrete from talus fan.

37.9242° N.107.6089° W.

Organic carbon from manganocrete samples, Eureka Gulch:HPDY0016 GX-27516-AMS –22.8 8,000±40 B.C. 7055–6870 B.C. 7055–6828 B.C. 7062–6703 Matrix from manganiferous colluvial 

ferricrete from talus fan. 

37.9046° N.107.6751° W.

Animas River basin, downstream from Silverton 

Charcoal from Elk Park:01ABS126C1 GX-28384 –23.3 2,930±130 B.C. 1190–1130 B.C. 1370–1340 

B.C. 1320–970 

B.C. 1490–1480 B.C. 1440–820 

Charcoal from depth of 18 in. below 

base of tailings deposited during Gladstone flood (Oct. 5, 1911).

37.7275° N. 107.6559° W.

1Radiocarbon data have been obtained from two commercial laboratories, Beta Analytic Inc. (Lab. No. B-xxxxxx) and Geochron Laboratories (Lab. No. GX-xxxxx). Some of the analyses were done using 

accelerator mass spectrometry (AMS) where the amount of carbon was small. Some of the 13C/12C data for samples marked with an asterisk (*) were not measured and were estimated to be –25 o

/

oo (per mil) 

based on values typical of this type of material.

2Carbon isotopic composition relative to Peedee belemnite (PDB) in per mil (o

/

oo).

3Conventional radiocarbon age (13C corrected) based on the Libby half life (5,570 years) for 14C, as reported by the laboratory in years before A.D. 1950 (B.P.). The error is ± 1 sigma as judged by the analytical data alone. The sample was crushed if necessary and dispersed in water. The eluted clay/organic fraction was treated in hot dilute 1N HCl to remove any carbonates. It was then filtered, washed, dried, 

and combusted in oxygen to recover carbon dioxide for analysis.

4Ages calibrated using the CALIB4.3 program based on Stuiver and Reimer (1993) with data from Stuiver and Braziunas (1993) and Stuiver and others (1998). The 1998 atmospheric decadal data set and 

laboratory error multiplier K=1 were used in the calculations.

5Maximum and minimum values given if the radiocarbon age intercepted the calibration curve at multiple locations. Dates rounded to the nearest decade if the standard deviation in the radiocarbon date was 

≥50 years.

6Calibrated age range (or the maximum and minimum if there was more than one range) using the intercept method. Dates rounded to the nearest decade if the standard deviation in the radiocarbon date was 

≥50 years. 1 sigma = square root of (sample standard deviation2 + curve standard deviation2); 2 sigma = 2 × square root of (sample standard deviation2+ curve standard deviation2).

Table 2. Radiocarbon data for organic carbon in ferricrete samples collected from the Mineral Creek, Cement Creek, and upper Animas River basins, and for wood and peat 

samples collected from the Mineral Creek basin.—Continued

736 Environmental Effects of Historical Mining, Animas River Watershed, Colorado
Ferricrete Classification, Morphology, Distribution, and Age Constraints 737

14C Ages Determined in “Unidentified Organic 

Carbon”

Logs could not be found in alluvial ferricrete at some 

important localities—specifically, the 20–30 m thick ferricrete 

deposits along the valley walls of the lower portion of Middle 

Fork Mineral Creek, Mineral Creek downstream from the 

confluence with Middle Fork Mineral Creek, lower Cement 

Creek downstream from the confluence with Soda Gulch, and 

at the confluence of Blair Gulch with the Animas River (Yager 

and Bove, this volume, pl. 2). To determine the age of ferricrete formation in these thick deposits, unidentified organic 

carbon from iron-oxide casts of algal material was sampled 

from a block of ferricrete approximately 13 m above stream 

level in the Mineral Creek section (sample 99VMS33). The 

shape of the casts resembled the morphology of the filamentous algae Ulothrix observed in acidic seeps near the base 

of the outcrop. The 14C age of this sample was 2,760±40 yr 

B.P. (GX-26690-AMS, table 2) and could define the time of 

cementation. Other ages from unidentified organic carbon 

samples (table 2) appear enigmatic, and thus we tried to test 

the reliability of using dates from unidentified organic carbon 

samples.

In order to evaluate the reliability of ages obtained from 

“unidentified organic carbon,” we extracted different organic 

phases from a goethite-replaced log from site 99ABFC163C 

on South Fork Cement Creek (this volume, pl. 2). We determined four 14C ages on different organic carbon fractions and 

the residue. From these data we determined that the 14C system 

was behaving as an open system in cases where we dated 

“unidentified organic carbon.” The results of this experiment 

are in table 2; sample preparation and extraction procedures 

follow.

For the “bulk” 14C age, the iron-oxide matrix was 

leached with hot (100°–150°C) 1N HCl to remove any 

mineral (carbonate) carbon. Generally, mineral carbon in the 

iron-oxide samples did not exceed about 0.25 weight percent. The organic carbon remaining after the leach was then 

combusted for 14C analysis. The results include components 

of fulvic and humic acids trapped in the goethite as well as 

the cellular carbon preserved in the replaced log. The 14C 

age for the bulk sample 163C (replaced woody material) 

was 6,010±70 yr B.P. (GX-27515, table 2). A second aliquot 

of sample (99ABFC163C) was subjected to a humic-fulvic 

extraction, and these organic fractions were also dated. Humic 

and fulvic fractions represent the products of degradative 

and synthetic reactions involving organic substances and are 

major components of the pool of organic matter. The humic 

and fulvic fractions were first extracted using 0.1 N NaOH 

for 24 hours at room temperature with shaking. The solution was separated from the solids by centrifugation and 

0

1

2

0 2,000 4,000 6,000 8,000 10,000

0

1

2

3

4

5

6

7

8

0

1

2

COUNT

AGE, IN YEARS B.P.

Upper Animas River basin

Cement Creek basin

Mineral Creek basin

(n = 11)

(n = 44)

(n = 16)

AGE, IN YEARS B.P.

AGE, IN YEARS B.P.

0 2,000 4,000 6,000 8,000 10,000

0 2,000 4,000 6,000 8,000 10,000

A

B

C

COUNT COUNT

Figure 9. Distribution of new 14C ages by basin. A, upper Animas 

River basin; B, Cement Creek basin; C, Mineral Creek basin.
then acidified to pH 2 with 1N HCl to precipitate the humic 

fraction. After filtration and removal of the solid humic precipitate, fulvic fraction was precipitated by adjusting the pH 

upwards to >5 with 1 N NaOH. The 14C age from the humic 

fraction precipitate was 2,080±40 yr B.P. (GX-27972-AMS, 

table 2), and the 14C age from the fulvic fraction precipitate 

was 2,720±30 yr B.P. (GX27971-AMS). Analysis of the 

residual carbon remaining in the goethite sample following 

extraction of the humic and fulvic fractions gave an age of 

5,410±220 yr B.P. (GX-28131, table 2). If the carbon in the 

goethite-replaced log had behaved as a closed system, the 

residual age would have had to be older than the “bulk” age 

of 6,010±70 yr B.P. The fact that both the humic and fulvic 

fractions gave ages that were both different and younger than 

the 14C age from the “bulk” sample indicated open-system 

behavior and thus, the 14C age from the “bulk” sample age 

represents a minimum age for the wood carbon preserved in 

the goethite matrix of the replaced log.

The extraction method and the fact that 14C ages from 

the humic and fulvic fractions were similar suggest that a 

pool of mobile carbon is present in the iron-oxide matrix. Terrestrial humic and fulvic substances originate from the decay 

of plant materials. The δ13C composition of terrestrial plant 

material ranges from –24 to –34 per mil (o

/

oo; Faure, 1986). 

0

2,000

4,000

6,000

8,000

10,000

1 2 3

Ages from 

alluvial

ferricrete

deposits

NUMBER OF SAMPLES

0 0 1

Ages from 

colluvial

ferricrete

deposits

0 1

Ages from

spring or

iron bog 

deposits

0 246

Ages from

unconsolidated

gravel

0 1 2

Ages from

peat deposits

1 3 5

Wood

Peat (either wood or cellulose)

Unidentified organic carbon

(n = 21)

(n = 23) (n = 9) (n = 5)

(n = 13)

AGE, IN YEARS B.P.

Figure 10. Distribution of 14C by deposit type. Samples displayed by type of material dated and include 

wood, peat (twigs or cellulose), and unidentified organic carbon. Ages of organic material in clastic 

deposits date the time of deposition and thus are maximum ages of iron cementation.

738 Environmental Effects of Historical Mining, Animas River Watershed, Colorado
Ferricrete Classification, Morphology, Distribution, and Age Constraints 739

Values within this range were obtained for humic (–27.0 o

/

oo) 

and fulvic (–25.4 o

/

oo) fractions. The “bulk” sample was 

slightly outside of this range at –22.4 o

/

oo, indicating perhaps 

some dilution by lichen or algal materials (which have lower 

ranges of δ13C, –12 to –23 o

/

oo; Faure, 1986). Furniss and others (1999) reported ages of organic carbon from pine needles 

and leaf litter preserved in iron bog deposits in the New World 

mining district in south-central Montana and obtained 14C ages 

in reasonable agreement with the onset of deglaciation (late 

Pinedale) in northern Yellowstone National Park. However, 

ages of unidentified organic carbon in colluvial ferricretes 

could in part result from unidentified organic carbon transported through the system by ground water. Whereas we do 

not reject the “unidentified organic carbon” ages outright, we 

have little confidence in them except as minimum ages for the 

original carbon in the replaced sample.

Basin-Wide Implications

Results from 14C age determinations in this study 

are complementary to those from previous investigations. 

Andrews and others (1975) presented age determinations from 

wood fragments in peat bogs located in Eureka and Picayune 

Gulches (upper Animas River basin), Elias and others (1991) 

summarized previous 14C data obtained from former Lake 

Emma sediments, and Carrara and others (1991) published 

14C data from bogs in California Gulch and Eureka Gulch 

in the upper Animas River basin. Figure 11, a histogram 

of the ages from ninety-nine 14C ages from wood, summarizes results from our study and previous investigations. 

Excluded from this compilation are the 14C ages of about 

15,000 yr B.P. from moss fragments recovered from former 

Lake Emma sediments. Elias and others (1991) rejected these 

10,000

8,000

6,000

4,000

2,000

0

012 3 4 5 6 7 8 9

Ages, this study

0 1 2 3 4 5

Published ages

AGE, IN YEARS B.P.

NUMBER OF SAMPLES

Figure 11. Distribution of all 14C ages. Dates from wood and peat in Animas River watershed 

study area; age distribution spans entire age range of the Holocene. Published data from 

Elias and others (1991 and references therein).
older ages from moss because they were inconsistent with 

14C ages from insect remains and wood from the same stratigraphic horizons.

Carrara and others (1991) used ages from conifer wood 

fragments to constrain treeline changes in the Holocene, and 

Fall (1997) interpreted timberline variations in the region 

using pollen and plant macrofossil samples from a suite of 

bog samples in the southern Rocky Mountains of Colorado. At 

present, timberline (the upper altitude of upright trees) in the 

southern Rocky Mountains is between 3,535 and 3,600 m, and 

treeline (small, scattered, windswept trees) is at about 3,660 m 

(Carrara and others, 1991; Fall, 1997). Fall (1997) demonstrated that timberline was approximately 300 to 700 m below 

the present timberline at 11,000 yr B.P. and was approximately 

270 m higher than present between 9,000 and 4,000 yr B.P. 

Carrara and others (1991) documented that in the San Juan 

Mountains, treeline was at least 75 m higher than present from 

9,600 to 5,400 yr B.P. The thick alluvial ferricrete deposits 

that do not contain logs could have been deposited when 

timberline was lower (about 11,000 yr B.P). The volume of 

these deposits is consistent with a late glacial outwash setting, 

and the upper portions of these deposits crop out at an elevation of 3,075 m, which is above the early Holocene timberline 

(Carrara and others, 1991; Fall, 1997). Carrara and others 

(1991) postulated that timberline has moved down over the 

last 5,000 years to its present position. More recent work by 

Fall (1997) suggests that after 4,000 yr B.P., timberline began 

to retreat, and the modern climatic regime was established 

approximately 2,000 yr B.P. That peat bogs started forming 

before 2,000–5,000 yr B.P. and continue to form shows that 

the climate within the Animas River watershed study area has 

been relatively stable since the late Holocene.

The summary of 14C ages from wood fragments in 

figure 11 shows that the physiography of the Animas River 

watershed study area has been relatively stable through the 

Holocene. Studies of the distribution of ferricrete (this volume, 

pl. 2) and the stability of the Cement Creek stream channel over the last several thousand years (Vincent and others, 

this volume) are interpreted to indicate that the fundamental 

weathering and erosion processes active in the formation of 

the landscape today are representative of those active throughout the Holocene. Ferricrete deposits are the product of those 

weathering processes and span the last 10,000 years of surficial history in the study area.

Distribution of Ferricrete Deposits

Ferricrete deposits are not evenly distributed within the 

Animas River watershed study area but crop out in conjunction with specific geologic and geomorphologic features. In 

general, ferricrete and manganocrete form in surficial deposits 

on or down gradient from mineralized bedrock, fractures, or 

veins. In addition, most ferricrete deposits are found within or 

down gradient from parts of the watershed where hydrothermal alteration was most intense.

Alluvial ferricretes were mapped along most of the 

valley floor in Mineral Creek downstream of Middle Fork to 

the confluence with the Animas River. Similarly, in Cement 

Creek alluvial ferricretes were mapped in the valley floor 

from Gladstone to the town of Silverton. Only one outcrop 

of alluvial ferricrete was mapped in the Animas River valley 

upstream of the confluence with Cement Creek. However, 

manganocretes were mapped along the valley floors in Placer, 

California, and Eureka Gulches. Iron spring and bog deposits were mapped along many of the reaches in Cement and 

Mineral Creeks that contained alluvial ferricretes. Some of 

these deposits were located in the valley floor and others 

along lower parts of the valley walls. Colluvial ferricretes 

were mapped along tributaries to Mineral and Cement Creeks 

that drain peak 3,792 m, Anvil Mountain, Ohio Peak, and Red 

Mountain No. 3.

Ferricretes in the Mineral Creek basin provide good 

examples of how the distribution of ferricrete deposits is 

closely related to bedrock geology and alteration intensity. 

No ferricrete deposits were mapped in the Red Mountain 

district upstream of Chattanooga, although some of the richest 

silver ore in the watershed study area was mined in the Red 

Mountain district. The mineral deposits of the Red Mountain 

district consist of base-metal ores within breccia pipes that 

intruded along fractures in the Silverton Volcanics (Burbank 

and others, 1972). Much of the district is characterized by 

intense acid-sulfate alteration, but the western part of the 

district that drains into the Mineral Creek basin is propylitically altered with more intense alteration adjacent only to 

individual breccia pipes (Fisher and Leedy, 1973). The propylitically altered volcanic rocks contain pyrite but do not tend 

to produce iron-rich drainage (Mast and others, this volume, 

Chapter E7). Upstream of Chattanooga most of the springs are 

circumneutral (pH 6.54–7.69) and contain low iron concentrations, generally <0.040 mg/L (Mast, Evans, and others, 2000).

At Chattanooga, an iron-rich sedge bog lies in the valley floor, and paleo-iron spring deposits crop out along the 

west side of the valley. This locality marks the intersection of 

the structural margin of the Silverton caldera and a series of 

east-west-trending faults (Luedke, 1975); it is also within a 

more intensely altered part of the basin (Bove and others, this 

volume). Downstream of Chattanooga, no ferricrete deposits 

were observed until near the confluence with Middle Fork 

Mineral Creek.

The reach of Mineral Creek between its confluences 

with Middle and South Forks contains alluvial ferricretes and 

numerous acidic springs and seeps on both sides of the creek. 

The east side of this reach is dominated by Anvil Mountain, 

an acid-sulfate altered zone (Bove and others, this volume). 

The west side consists of peak 3,792 m, a low-grade coppermolybdenum porphyry deposit, characterized by a zoned alteration pattern with propylitically altered rocks near the periphery and quartz-sericite-pyrite alteration at the center (Ringrose, 

1982). In addition, this reach follows the structural margin of 

the Silverton caldera. Most of the non-mining-affected spring 

and stream water draining Anvil Mountain and peak 3,792 m 

740 Environmental Effects of Historical Mining, Animas River Watershed, Colorado
Ferricrete Classification, Morphology, Distribution, and Age Constraints 741

is acidic (pH 2.99–4.92); dissolved iron concentrations range 

from 0.4 to 120 mg/L (Mast, Evans, and others, 2000). Colluvial ferricretes were mapped along hillslopes or small tributaries in this reach (Yager and Bove, this volume, pl. 2). Note that 

some outcrops of colluvial ferricrete are too small to map at 

the 1:24,000 scale.

Similar to Mineral Creek, alluvial ferricretes along 

Cement Creek tend to form adjacent to or down gradient to 

bedrock where hydrothermally alteration was most intense. 

Colluvial ferricretes were primarily mapped within acidsulfate altered areas, including Anvil Mountain, Ohio Peak, 

and Red Mountain No. 3. In the upper Animas River basin 

upstream of the confluence of Cement Creek, few ferricretes 

were observed. The reach of the Animas River between the 

confluence of Cement Creek and Eureka townsite follows the 

structural margin of the Silverton caldera and is dominantly 

surrounded by propylitized volcanics. The scarcity of ferricrete deposits in this area may be interpreted to indicate that 

basin-scale structures are not solely responsible for the formation of ferricrete deposits.

Upstream from Eureka, numerous manganocrete 

deposits were mapped. Manganocrete crops out commonly 

as finer grained colluvial and alluvial deposits in drainage 

areas located downstream from weathered, manganese-rich, 

pyroxmangite- and rhodochrosite-bearing vein structures associated with the Eureka graben. These manganocrete deposits 

overlie the northwest-trending Sunnyside fault where sulfiderich polymetallic, base-metal veins formed along the north 

edge of the Eureka graben (Burbank and Luedke, 1969; Yager 

and Bove, this volume, pl. 1). California Gulch, Eureka Gulch, 

and Placer Gulch drain the central region of the Eureka graben 

near the northeast-trending Sunnyside fault. Alluvial fans and 

colluvium in the California, Eureka, and Placer Gulch valley 

floors provide porous and permeable pathways for transport 

and precipitation of the manganese-rich fluids draining from 

the veins.

The distribution of ferricrete and manganocrete in the 

study area highlights the link between pyritic host rocks, acid 

water, and ferricrete localities and manganese-rich mineral 

deposits and manganocrete localities. Detailed descriptions 

of the possible processes that lead to the formation of ferricrete and manganocrete are presented in Wirt and others (this 

volume); an overview of the scenarios that lead to ferricrete 

formation instead of manganocrete follows. To form ferricretes, a source of iron is needed. Elevated ferrous-iron concentrations are found in ground water associated with parts of 

the study area where hydrothermal alteration was intense; geochemical modeling supports the conclusion that the dissolved 

iron is derived from oxidation of pyrite (Mast, Verplanck, and 

others, 2000; Bove and others, 2000). In acid water, iron in the 

ferrous state is highly soluble, but with the addition of oxygen, 

ferrous iron oxidizes and transforms to ferric iron. Ferric iron 

is less soluble, which leads to precipitation of iron oxyhydroxides. Two processes that provide oxygen to ground water are 

(1) ground water emanates from springs and contacts atmospheric oxygen and (2) ground water mixes with oxygenated 

surface water. Dissolved oxygen in most surface water in the 

study area is at saturation (Mast, Evans, and others, 2000). 

Ferricrete forms where these iron precipitates accumulate on 

surfaces, which can include colluvial and alluvial deposits 

as well as land surfaces adjacent to springs. Evaporation and 

hardening of iron-rich crusts are likely an important part of a 

preservation process.

To form manganocrete, a source of manganese is essential. Polymetallic vein deposits were mined in much of the 

study area, but some veins, primarily in the upper Animas 

River basin upstream of the confluence with Cement Creek, 

contain abundant manganese minerals (Casadevall and 

Ohmoto, 1977; Nash, 1999). This type of polymetallic vein 

deposit has a much smaller alteration halo than do the porphyry and breccia types in Mineral Creek basin (Nash, 1999), 

and spring samples in this part of the study area tended to have 

higher pH values, higher manganese concentrations, and lower 

iron concentrations than did springs in the parts of the study 

area with more intense hydrothermal alteration (Mast, Evans, 

and others, 2000). In aqueous solutions manganese exists as 

the Mn2+ ion, but with the addition of oxygen, manganese can 

form insoluble oxides (MnO2

, Mn2O3, and Mn3O4) depending 

on pH and Eh conditions. Overall, the processes that lead to 

the formation of either ferricrete or manganocrete are similar; 

they entail the weathering of iron or manganese minerals, 

the transport of dissolved iron or manganese in a reduced 

state, and precipitation of iron or manganese in an oxidizing 

environment. Source water composition and redox variations 

appear to be the primary reasons why manganocrete or ferricrete is formed.

Conclusions

A classification scheme of ferricrete deposits based on 

physical properties has been developed in the Animas River 

watershed study area, and three types of deposits have been 

mapped and characterized. Spring and bog iron deposits 

contain essentially no clasts and mainly form along valley 

floors or near the base of the valley walls. Colluvial ferricretes 

contain predominantly angular clasts, usually of a single rock 

type, and most form on hillslopes. Alluvial ferricrete deposits 

are characterized by subangular to rounded clasts of several 

lithologies and may contain sedimentary structures. These 

deposits crop out along valley floors or form steep walls in 

some downcut stream reaches. Deposits that have a black 

matrix were mapped as manganocrete.

The chemical composition of the ferricrete matrix is 

dominantly iron, ranging from 5.7 to 77.2 wt. percent FeO, 

and comprises variable proportions of amorphous iron oxides, 

schwertmannite, and goethite. In addition, jarosite was identified in a few samples. Ferricrete deposits tend to form within 

or down gradient from rocks where hydrothermal alteration is 

most intense.
The matrix of the manganocrete samples contains 

3.1–7.7 wt. percent MnO and is amorphous. Manganocrete 

deposits primarily crop out along the upper Animas River 

and its tributaries upstream of the town of Eureka, where 

polymetallic veins containing primary manganese minerals 

are weathered and oxidized, releasing manganese to ground 

and surface water. A few other ferricrete matrix samples containing elevated manganese concentrations were located near 

manganese-rich veins or outcrops of a lithology associated 

with manganese-rich veins.

Carbon-14 ages from wood fragments within ferricrete 

deposits ranged in age from 9,150±50 yr B.P. to modern. Ages 

of logs encased in alluvial and colluvial ferricretes represent 

the age of clastic deposition and are thus a maximum age of 

cementation. Some of the outcrops are dry and above creek 

level, indicating that cementation did not occur recently. 

Carbon-14 ages from peat and wood from active bogs ranged 

from 5,000 yr B.P. to modern. Although our new age determinations span the Holocene, most of the ages are less than 

5,000 yr B.P., likely reflecting the difficulty in preservation 

of older deposits in the valley floor and the likelihood that the 

oldest deposits, outwash and old fans, did not contain wood 

because treeline was lower during this time. In the study area, 

ferricrete deposits are a product of weathering processes that 

span the last 10,000 years and are evidence that, in parts of the 

study area, iron-rich water flowed prior to the onset of historical mining late in the 19th century.

References Cited

Andrews, J.T., Carrara, P.E., King, F.B., and Stuckenrath, 

Robert, 1975, Holocene environmental changes in the 

Alpine Zone, northern San Juan Mountains, Colorado; 

evidence from bog stratigraphy and palynology: Quaternary 

Research, v. 5, p. 173–197.

Battiau-Queney, Yvonne, 1996, A tentative classification of 

paleoweathering formations based on geomorphological 

criteria, in Bouchard, Mireille, and Schmitt, J.-M., eds., 

Paleoweathering and paleolandforms: Amsterdam, Elsevier, 

p. 87–102.

Bigham, J.M., Schwertmann, Udo, and Carlson, Liisa, 1992, 

Mineralogy of precipitates formed by biogeochemical oxidation of Fe(II) in mine drainage, in Skinner, H.C.W., and 

Fitzpatrick, R.W., eds., Biomineralization processes of iron 

and manganese—Modern and ancient environments: Catena 

Supplement, Cremlingen-Destedt, v. 21, p. 219–232.

Bigham, J.M., Schwertmann, Udo, Traina, S.J., Winland, 

R.L., and Wolf, M., 1996, Schwertmannite and the chemical modeling of iron in acid sulfate waters: Geochimica et 

Cosmochimica Acta, v. 60, p. 2111–2121.

Bove, D.J., Mast, M.A., Wright, W.G., Verplanck, P.L., 

Meeker, G.P., and Yager, D.B., 2000, Geologic control on 

acidic and metal-rich waters in the southeast Red Mountains 

area, near Silverton, Colorado, in ICARD 2000; Proceedings of the Fifth International Conference on Acid Rock 

Drainage, Volume 1: Society for Mining, Metallurgy, and 

Exploration, Inc., p. 523–533.

Briggs, P.H., 1996, Forty elements by inductively coupled 

plasma–atomic emission spectroscopy, in Arbogast, B.F., 

ed., Analytical methods manual for the Mineral Resources 

Program, U.S. Geological Survey: U.S. Geological Survey 

Open-File Report 96–525, p. 77–94.

Burbank, W.S., and Luedke, R.G., 1969, Geology and ore 

deposits of the Eureka and adjoining districts San Juan 

Mountains, Colorado: U.S. Geological Survey Professional 

Paper 535, 73 p.

Burbank, W.S., Luedke, R.G., and Ward, F.N., 1972, Arsenic as an indicator element for mineralized volcanic pipes 

in the Red Mountains area, western San Juan Mountains, 

Colorado: U.S. Geological Survey Bulletin 1364, 31 p.

Carrara, P.E., Trimble, D.A., and Rubin, Meyer, 1991, 

Holocene treeline fluctuations in the northern San Juan 

Mountains, Colorado, U.S.A., as indicated by radiocarbondated conifer wood: Arctic and Alpine Research, v. 23, 

p. 233–246.

Casadevall, Thomas, and Ohmoto, Hiroshi, 1977, Sunnyside 

mine, Eureka mining district, San Juan County, Colorado—Geochemistry of gold and base metal ore deposition in a volcanic environment: Economic Geology, v. 72, 

p. 1285–1320.

Church, S.E., Fey, D.L., and Blair, Robert, 2000, Pre-mining 

bed sediment geochemical baseline in the Animas River 

watershed, southwestern Colorado, in ICARD 2000; 

Proceedings of the Fifth International Conference on Acid 

Rock Drainage, Volume 1: Society for Mining, Metallurgy, 

and Exploration, Inc., p. 499-512.

Comstock, T.B., 1883, Notes on the geology and mineralogy of San Juan County, Colorado: Transactions of the 

American Institute of Mining, Metallurgical and Petroleum 

Engineers, Inc., v. 11, p. 165–191.

Cross, C.W., Howe, Ernest, and Irving, J.D., 1905, Descriptions of the Silverton quadrangle: U.S. Geological Survey 

Atlas Folio 120, 20 p.

742 Environmental Effects of Historical Mining, Animas River Watershed, Colorado
Ferricrete Classification, Morphology, Distribution, and Age Constraints 743

Cross, C.W., and Purington, C.W., 1899, Descriptions of 

the Telluride quadrangle: U.S. Geological Survey Atlas 

Folio 153, 19 p.

Elias, S.A., Carrara, P.E., Toolin, L.J., and Jull, A.J.T., 1991, 

Revised age of deglaciation of Lake Emma based on new 

radiocarbon and macrofossil analyses: Quaternary Research, 

v. 36, p. 307–321.

Fall, P.L., 1997, Timberline fluctuations and late Quaternary paleoclimates in the Southern Rocky Mountains, 

Colorado: Geological Society of America Bulletin, v. 109, 

p. 1306–1320.

Faure, Gunter, 1986, Principles of isotope geology, Second 

Edition: New York, John Wiley, 589 p.

Fisher, F.S., and Leedy, W.P., 1973, Geochemical characteristics of mineralized breccia pipes in the Red Mountain mining district, San Juan Mountains, Colorado: U.S. Geological 

Survey Bulletin 1381, 43 p.

Furniss, George, and Hinman, N.W., 1998, Ferricrete provides 

record of natural acid drainage, New World District, Montana, in Arehart, G.B., and Hulston, J.R., eds., 9th International symposium on water-rock interaction: Rotterdam, 

Balkema, p. 973–976.

Furniss, George, Hinman, N.W., Doyle, G.A., and Runnells, 

D.D., 1999, Radiocarbon-dated ferricrete provides a record 

of natural acid rock drainage and paleoclimatic changes: 

Environmental Geology, v. 37, p. 102–106.

Hanshaw, Bruce, 1974, Geochemical evolution of a goethite 

deposit, in 1st International symposium on water-rock interactions: Rotterdam, Balkema, p. 70–75.

Harrer, C.M., and Tesch, W.J., Jr., 1959, Reconnaissance of 

iron occurrences in Colorado: Bureau of Mines Information 

Circular 7918, 82 p.

Kirkham, R.M., Lovekin, J.R., and Sares, M.A., 1995, Sources 

of acidity and heavy metals in the Alamosa River basin 

outside of the Summitville mining area, Colorado, in Posey, 

H.H., Pendleton, J.A., and Van Zyl, D.J.A., eds., Proceedings; Summitville Forum ‘95: Colorado Geological Survey, 

p. 42–57.

Lamplugh, G.W., 1902, Calcrete: Geologic Magazine, v. 9, 

p. 575.

Logsdon, M.J., Miller, Scott, Swanson, Eric, Goodhard, 

William, and Perino, Larry, 1996, Pre-mining water quality in historically mined areas; iron bogs and ferricretes in 

the San Juan Mountains, Colorado: Geological Society of 

America Abstracts with Programs, v. 28, p. 518.

Luedke, R.G., 1975, Preliminary geologic map of the Silverton 

Report 75–433, scale 1:20,000.

Luedke, R.G., and Burbank, W.S., 1987, Geologic map of the 

counties, Colorado: U.S. Geological Survey Quadrangle 

Map GQ–1595, scale 1:24,000.

Mason, R.J., Brink, A.B., and Knight, K., 1959, Pleistocene 

climatic significance of calcretes and ferricretes: Nature, 

v. 184, no. 4685, p. 568.

Mast, M.A., Evans, J.B., Leib, K.J., and Wright, W.G., 2000, 

Hydrologic and water-quality data at selected sites in the 

upper Animas River watershed, southwestern Colorado, 

1997–99: U.S. Geological Survey Open-File Report 00–53, 

20 p.

Mast, M.A., Verplanck, P.L., Yager, D.B., Wright, W.G., and 

Bove, D.J., 2000, Natural sources of metals to surface waters 

in the upper Animas River watershed, Colorado, in ICARD 

2000; Proceedings of the Fifth International Conference on 

Acid Rock Drainage, Volume 1: Society for Mining, Metallurgy, and Exploration, Inc., p. 513–522.

Miller, W.R., Bassett, R.L., McHugh, J.B., and Ficklin, W.H., 

1999, The behavior of trace metals in water during natural 

acid sulfate weathering in an alpine watershed, in Filipek, 

L.H., and Plumlee, G.S., eds., The environmental geochemistry of mineral deposits—Part B, Case studies and research 

topics: Society of Economic Geologists, Reviews in Economic Geology, v. 6B, p. 492–503.

Nahon, D.B., 1991, Self-organization in chemical lateritic 

weathering: Geoderma, v. 51, p. 5–13.

Nash, J.T., 1999, Geochemical investigations and interim 

recommendations for priority abandoned mine sites, 

BLM lands, upper Animas River watershed, San Juan 

County, Colorado: U.S. Geological Survey Open-File 

Report 99–0323, 43 p.

Plumlee, G.S., Gray, J.E., Roeber, M.M., Jr., Coolbaugh, 

Mark, Flohr, Marta, and Whitney, Gene, 1995, The 

importance of geology in understanding and remediating 

environmental problems at Summitville, in Posey, H.H., 

Pendleton, J.A., and Van Zyl, D.J.A., eds., Proceedings; 

Summitville Forum ‘95: Colorado Geological Survey, 

p. 13–22.

quadrangle, Colorado: U.S. Geological Survey Open-File 

Handies Peak quadrangle, San Juan, Hinsdale, and Ouray 
Ransome, F.L., 1901, A report on the economic geology of 

the Silverton quadrangle, Colorado: U.S. Geological Survey 

Bulletin 182, 265 p.

Ringrose, C.R., 1982, Geology, geochemistry, and stable 

isotopes of a porphyry-style hydrothermal system, west 

Silverton district, San Juan Mountains, Colorado: Aberdeen, 

Scotland, University of Aberdeen Ph. D. dissertation, 

256 p., 19 plates.

Stuiver, Minze, and Braziunas, T.F., 1993, Sun, ocean, climate 

and atmospheric 14CO2; An evaluation of causal and spectral 

relationships: The Holocene, v. 3, p. 289–305.

Stuiver, Minze, and Reimer, P.J., 1993, Extended 14C database and revised CALIB radiocarbon calibration program: 

Radiocarbon, v. 35, p. 215–230.

Stuiver, Minze, Reimer, P.J., Bard, E., Beck, J.W., Burr, G.S., 

Hughen, K.A., Kromer, B., McCormac, F.G., van der Plicht, 

J., and Spurk, M., 1998, INTCAL98 Radiocarbon 

age calibration 24,000–0 cal B.P.: Radiocarbon, v. 40, 

p. 1041–1083.

Vhay, J.S., 1962, Geology and mineral deposits of the area 

south of Telluride, Colorado: U.S. Geological Survey Bulletin 1112–G, p. 209–310.

Yager, D.B., Mast, M.A., Verplanck, P.L., Bove, D.J., Wright, 

W.G., and Hageman, P.L., 2000, Natural versus miningrelated water quality degradation to tributaries draining 

Mount Moly, Silverton, Colorado, in ICARD 2000; Proceedings of the Fifth International Conference on Acid 

Rock Drainage, Volume 1: Society for Mining, Metallurgy, 

and Exploration, Inc., p. 535–550.

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