U. S. DEPARTMENT OF THE INTERIOR

U.S. GEOLOGICAL SURVEY

ACID GENERATION AND METAL MOBILITY OF SOME METAL- MINING 

RELATED WASTES IN COLORADO

George Desborough1 , Reinhard Leinz 1 , Kathy Smith1 , Philip Hageman1 , David Fey1 , and Tom

Nash1

Open-File Report 99-322

This report is preliminary and has not been reviewed for conformity with U.S. Geological Survey editorial standards or with the North 

American Stratigraphic code. Any use of trade, product, or firm names is for descriptive purposes only and does not imply 

endorsement by the U.S. Government

Denver, Colorado

1999
CONTENTS

Page 

Abstract.......................................................................................... 3

Introduction..................................................................................... 3

Mineralogical, chemical, and leaching studies of four metal-mining related wastes.

Mineralogy............................................................................. 4

Passive leaching studies with smaller leachate volumes........................ 6

Passive leaching with larger leachate volumes................................... 9

Acid-generating capacities of some Fe-dominant secondary sulfates.................. 10

Significance of jarosite-member minerals of the alunite group in acid-generating 

metal-mining related wastes................................................................... 11

Summary........................................................................................ 13

Acknowledgments............................................................................. 13

References Cited............................................................................... 13

FIGURES 

Figure 1. Cations dissolved in each of the three leachates of Mayday <0.090 mm

fraction................................................................................ 15

Figure 2. Cations dissolved in each of the three leachates of Venir <0.090 mm

fraction............................................................................... 16

Figure 3. Cations dissolved in each of the three leachates of Yukon <0.090 mm

fraction................................................................................ 17

Figure 4. Cations dissolved in each of the three leachates of Sunday No. 2 <0.090

mm fraction........................................................................... 18

TABLES

Table 1. Minerals found in wastes of the May Day, Venir, Yukon, and Sunday No. 2.5

Table 2. Concentrations of selected elements in untreated, dry sized (<0.090 mm) and 

an acetone decanted solids fraction from <0.090 mm splits from four waste 

dumps...................................................................................... 7

Table 3. Concentrations of 12 elements dissolved in each of three serial leachates of

1-gram samples of the <0.090 mm fraction frorrru,ar' wasxe piles determined by 

ICP/MS and ICP/AES (of 0.45 micron filtered leachates)........................ 8

Table 4. Concentrations of selected elements dissolved in each of three serial leachates 

for 0.5 g of <0.090 mm fractions from four waste piles determined by ICP/MS 

(of 0.45 micron filtered leachates.................................................... 9

Table 5. Sum of selected metals dissolved in two sets of three passive 24 hour serial

leachates of the four mine-related waste dumps................................. 10

Table 6. Minimum measured acid-generating capacity of four soluble hydrated Fe 

sulfates determined by exposing each of them to 2 L of deionized water for 24 h.. 11

Table 7. Relations of X-ray diffraction detected jarosite in metal-mining related wastes 

and the final pH of their leachates after exposure of 100 grams of waste to 2 L 

of deionized water for 24 hours...................................................... 12
ABSTRACT

Four metal-mining related wastes, two from near Silverton, Colorado, and two from 

near Leadville, Colorado were studied in the laboratory to evaluate mineralogy, 

chemistry, and their reactions to multiple deionized water leachates in solid:leachate 

ratios ranging from 1:20 to 1:8,000. These wastes were weathered for more than 50 

years, but still contain pyrite and minor Cu, Zn, and Pb.

Mineralogical studies did not determine the acid-generating phase, but in two sets of 

three serial leaches, significant acid-generation was measured, showing that initial 

solid:leachate ratios of 1:20 or 1:4,000 do not completely remove acid-generating 

phases in 24 h. Leaching studies used a passive method in which deionized water was 

poured onto the samples and the mixture was stirred once for five seconds after one hour, 

but at rest otherswise for 24 h in glass beakers.

Three large volume serial leaches of 1:4,000, 1:8,000, 1:8,000 produced 44-63 times 

more acid than three small-volume 1:20, 1:200, 1:100 leaches conducted under identical 

laboratory conditions. In terms of dissolved metals that are potentially toxic to aquaticlife; Cu was two to six times higher in the large-volume leachates (than in the smallvolume leachates), Zn was more than two times higher in the large-volume leachates 

(than in the small-volume leachates), and Pb was two to seventeen times higher in the 

large-volume leachates (than in the small-volume leachates). Hydrolysis of Fe and the 

associated precipitation of amorphous orange Fe-oxide appears to be a source of acid 

generation in both the small-volume and large-volume leachates. We suspect that 

unidentified Fe sulfates were mainly responsible for generated acidity in leachates.

The study of large-volume leachate exposures of surficial metal-mine wastes to 

deionized water of pH about 5 was intended to simulate spring/summer snowmelt runoff; 

if this approach is appropriate, it shows that large amounts of acid generation and 

associated toxic-metal mobility may be relevant factors in water-quality degradation in 

terms of Class 1 Aquatic Life standards.

INTRODUCTION

This study was done for the mine waste characterization project that is focussed on the 

chemical, mineralogical, physical, biological^ and geopyhsical aspects of abandoned mine 

land (AML) wastes in the Rocky Mountain region. The effort here is directed toward the 

mineralogical and chemical characteristics of surfical wastes from four waste piles and 

the reactions of these waste samples with deionized water in the laboratory. The wastes 

we studied contain significant pyrite, and minor amounts of zinc and lead (originally in 

sphalerite and galena); and all of them have been exposed to natural weathering for more 

than 5 decades.

Surficial waste sampling was done in such a way as to minimize sampling error to 17 

percent or less by sampling the < 2 mm fraction in 30 equally-spaced cells on the surface 

of each waste pile and them compositing these into one homogeneous sample prior to 

further study.

The four samples studied here include the samples from waste piles at the May Day 

and Yukon mines along Cement Creek near Silverton, Colorado and the Venir and 

Sunday No.2 mines near Leadville, Colorado. The two waste piles near Silverton occur 

at elevations of about 9,880 feet above mean sea level; the two waste piles near Leadville
occur at elevations between 11,600 and 12,000 feet above mean sea level. Both areas 

receive between 40-50 inches of precipitation per year, and most of it is as snowfall.

MINERALOGICAL, CHEMICAL, AND LEACHING STUDIES OF FOUR 

METAL MINING RELATED WASTES

MINERALOGY

Splits of the < 2mm fraction, aggregating several kg each from the May Day and 

Yukon mines (both from near Silverton, CO) and from the Venir and Sunday No. 2 

mines, (both from near Leadville, CO) were studied in the laboratory. The mineralogy of 

each of these was studied by X-ray diffraction, non-destructive X-ray fluorescence 

analysis, microscopic methods, preparation of heavy-mineral concentrates, and 

separation of size fractions by dry sieving and by decantation of acetone slurries. X-raydiffraction (XRD) studies used Ni-filtered Cu Ka radiation in diffractometer mode. 

Minerals found by these methods are given for each of the four wastes in Table 1. 

Minerals listed for the <0.150 mm fraction, obtained by dry sieving, represent the major 

minerals in each sample. On a short-term basis (24-48 h.) most of these are relatively 

insoluble and none of them have a large acid-neutralizing or large acid-generating 

potential.

Also in Table 1 are minerals in the heavy-mineral concentrate of three other size 

fractions. These concentrates were obtained by mechanical panning on a Wilfley table 

(English and others, 1987) using deionized water (pH = 4.9-5.2) in a solid:liquid ratio of 

1:600 in order to minimize acid generation from the solids (maintain a pH above 4.0). 

These concentrates reveal that pyrite is present in all sized fractions. Anglesite crystals in 

the size range of about 0.010-0.020 mm were identified in the two smallest size fractions 

of the May Day and Venir wastes (table 1) by scanning electron microscopy (SEM) using 

an energy-dispersive X-ray fluorescence detector in semi-quantitative analysis mode. 

XRD analysis confirmed anglesite which is a product of oxidation of galena. Cerrusite 

was identified in the Sunday No. 2 heavy-mineral concentrates as small (<0.020 mm) 

crystals, and as crusts or coatings on galena.

The acetone decantation solid concentrates (table 1) were obtained using about 20 g of 

dry-sized (< 2 mm") solids in 500 ml of acetone, shaking the mixture in a circular motion 

befoie immediate. u^aiiaJon of the slurries for which the solids were retained on filter 

paper. The air-dried acetone decantations were studied by XRD and non-destructive 

XRF analysis. The minerals identified by XRD analysis are given at the bottom of Table 

1; the XRF analyses for Fe, Cu, Zn, and Pb are given in Table 2 for these acetone decant 

solids.
Table 1. Minerals found in wastes of the May Day, Venir, Yukon, and Sunday No. 2. 

[Minerals are listed in decreasing order of abundance]

Bulk <0.150 mm Samples 

May Day: quartz, muscovite, jarosite, plagioclase, K-feldspar

Venir: quartz, muscovite, kaolinite, jarosite, K-feldspar 

Yukon: quartz, plagioclase, muscovite, jarosite, pyrite 

Sunday No. 2: quartz, jarosite, muscovite, kaolinite

Sized Fractions of Heavy-Mineral Concentrates 

0.50-2.0 mm 0.090-0.50 mm <0.090 mm

May Day

pyrite, galena coated pyrite, ferberite, sphalerite, pyrite, ferberite, 

with anglesite O.050 mm anglesite <0.050 mm anglesite,

barite

Venir

pyrite, <0.050 mm pyrite, O.050 mm anglesite pyrite, anglesite, 

anglesite, ferberite and barke, ferberite, native Au barite, ferberite,

native Au 

Yukon 

pyrite, ferberite pyrite, ferberite pyrite, ferberite

Sunday No.2

galena coated with <0.050 mm cerrusite, pyrite, O.050 mm cerrusite, 

cerru.° 1'fe, pyrite, galena barite, pyrite 

sphaieute, <0.050 mm 

cerrusite

Acetone Decantation Solids from <0.090 mm Fraction

May Day: quartz, jarosite, muscovite, chlorite (clinochlore), kaolinite, anglesite 

Venir: quartz, muscovite, kaolinite, jarosite, chlorite (clinochlore), anglesite 

Yukon: quartz, muscovite, jarosite, gypsum, kaolinite, anglesite, pyrite 

Sunday No. 2: quartz, jarosite, muscovite, kaolinite, gypsum, anglesite (?)
Comparisons of the Fe, Cu, Zn, and Pb concentrations in the untreated <0.090 mm 

fractions and the acetone decant of the <0.090 mm fractions are interesting. Both Fe and 

Pb are enriched significantly in the acetone decant fractions, compared to the untreated 

<0.090 mm fractions (table 2). Jarosite is significantly enriched in the acetone decant 

fractions, compared to the untreated fraction. These observations indicate that an Fe-rich 

phase and a Pb-rich phase are present as extremely small particles that stayed in 

suspension in the decanted fraction. The increased concentrations of jarosite in the 

decanted fractions are not sufficient to account for the increased concentrations of Fe for 

the May Day and the Sunday No. 2 samples. The increased amounts of jarosite, as well 

as the concentration of chlorite in the May Day and Venir decants might be sufficient to 

account for the increased Fe in these fractions. The enrichment of pyrite in the acetone 

decant of the Yukon material may account for the increased iron content. There is no 

XRD detectable Fe mineral in the Sunday No. 2 decant that is present in concentrations 

sufficient to account for the iron enrichment. The increased amounts of Pb in the decants 

of the May Day, Venir, and Yukon are due to the concentrations of fine-grained 

anglesite. For the Sunday No. 2, the decant probably concentrated the fine-grained 

cerrusite identified in heavy-mineral concentrates. Because of X-ray peak interferences 

of kaolinite with cerrusite, the presence of cerrusite in this decant could not be confirmed.

PASSIVE LEACHING STUDIES WITH SMALLER LEACHATE VOLUMES

About 100-200 g from each < 2mm fraction of these was sizetfto obtain a <0.090 mm 

fraction of 30-60 g. Each of these was split with a microsplitter to obtain two 

approximately 12-15 g fractions. One of these was used as a reference sample for XRD 

and non-destructive chemical analysis. The other, aggregating 10 g was exposed to 

leachates sequentially using deionized water of pH = 5.0 in waste:water ratios of 1:20, 

1:200, and 1:100 for 24 hour periods each, in side-by-side tests. During each exposure the 

pH of the leachate was measured shortly after initial exposure and at the end (24 h) of the 

exposure of waste to leachate. At the end of each test, the leachate was sampled for 

cation analyses; 60 ml leachate samples were filtered (0.45 microrneic*), mid uie 

leachates for cation analysis were acidified with nitric acid. After sampling the leachate, 

the liquid was decanted prior to the following leach test. Samples were not allowed to dry 

between sequential leaches. The initial total concentrations of Fe, Cu, Zn, and Pb are 

given in Table 2 prior to any leach test. Table 3 gives the concentrations of 12 elements 

dissolved in the three separate leachates based on ICP/MS analysis. In each case, the 

concentrations of Cu, Zn, and Pb in the leachates were 5 mg L" 1 or less. The sums of the 

12 dissolved cations in each leachate, converted to mg g" 1 are also give in Table 3, as are 

the final pH of the three leachate solutions. Using the sums of the cations, the relative 

percentages of the sums in each leachate are given. Eighty nine to 97 percent of the 12 

dissolved cations (table 3) were released in the first (1:20) leach of the Venir, Yukon, and 

Sunday No.2 wastes; the May Day waste released only 61 percent during the first leach. 

Figures 1, 2, 3, and 4 show the amounts of dissolved Ca, Fe, Cu, Zn, Cd, and Pb in each 

of the three serial leaches for the May Day, Venir, Yukon, and Sunday No. 2, 

respectively. For the most part, the largest amounts of each dissolved cation was in the
first leach, partly because the pH of the first leachate was significantly lower than 

subsequent leaches. The exception to this is Fe for the May Day leach (fig. 1).

The concentrations of Ca in the first leaches of the Yukon and Sunday No. 2 (table 3 

and figs. 3 and 4) are due to the dissolution of gypsum, found in the acetone decantation 

solids (table 1).

The untreated <0.090 mm fraction splits show the highest Pb concentration to be in 

the Sunday No. 2 waste (table 2), and the most abundant Pb mineral in the <0.090 mm 

heavy-mineral concentrate of this waste is the Pb-carbonate, cermsite (table 1) which is 

present as O.050 mm crystals. One might expect that the fine crystals of cermsite would 

contribute significant Pb to the first (1:20) leachate, of pH = 2.67, as compared to the two 

other <0.090 mm wastes samples (May Day and Venir) which have anglesite as the major 

mineral in the <0.090 mm fraction (table 1). However, the dissolved Pb from the first 

(1:20) leachates of the May Day (pH = 3.14) and the Venir (pH= 2.58) significantly 

exceeded those of the Sunday No. 2. (table 3).

After the third leach, each of the remaining solids was air dried and pulverized in 

tungsten carbide (WC) lined mills with WC balls for 5 minutes. These pulverized solids 

were then exposed to deionized water (pH = 5.0) in a ratio of 1:100 for 24 hours. The pH 

of these leachates is listed at the bottom of Table 3. Except for that of the Sunday No. 2 

residue, the pH of the leachates is lower that the final pH of third 1:100 leach. This is 

believed to indicate that a water-soluble, acid-generating mineral or minerals were 

liberated or exposed by the reduction in grain size (pulverizing).

Table 2. Concentrations of selected elements in untreated, dry sized (O.090 mm) and an 

acetone decanted solids fraction from O.090 mm splits from four waste dumps. X-ray 

diffraction (XRD) intensities for jarosite are given for each fraction. Final pH of the 

1:200 leachate (24 h) for each fraction are also shown.

Fe Cu Zn Pb pHof XRD jarosite 

wt. % parts per million leachate peak intensity

Untreated 6.1 

Acetone decant 9.0 

solids

Untreated 5.6 

Acetone decant 6.6 

solids

Untreated 9.3 

Acetone decant 10.7 

solids

Untreated 5.5 

Acetone decant 10.0 

solids

MAYDAY

270 1500 6500

460 770 12000

VENIR

220 200 2200

310 250 3600

YUKON

560 1000 3000

300 470 4300

SUNDAY No. 2 

340 540 7900 

590 800 14600

3.85

3.79

3.25

3.30

3.48

3.36

3.34

3.28

20

27

11

22

12

32

21

31
Table 3. Concentrations of 12 elements dissolved in each of three serial leachates 

of 10-gram samples of the <0.090 mm fraction from four waste piles determined by 

ICP/MS and ICP/AES (of 0.45 micron filtered leachates).

leach Na MgXI Si K Ca Mn Fe Cu Zn Cd Pb 2 %of 

ratio milligrams L'1 mg g' 1 total

MAYDAY 

1:20 1.3 1.0 0.6 1.8 0.7 4.4 0.2 0.84 0.13 1.2 0.007 2.7 0.29 61

1:200 0.12 0.03 0.2 0.1 <0.001 0.07 0.005 0.11 0.004 0.03 0.0002 0.06 0.132 31 

1:100 0.03 0.02 0.01 0.1 O.OOK0.05 0.004 0.12 0.003 0.02 0.0001 0.05 0.036 8

VENIR 

1:20 1.1 8.7 16.1 1.3 0.2 14.7 1.2 44.2 5.1 1.0 0.017 0.83 1.89 97

1:200 <0.02 0.02 O.002 O.002 O.001 <0.05 0.002 0.12 0.01 0.003 0.00005 0.05 0.041 2 

1:100 O.02 0.01 O.002 O.002 O.001 O.05 0.001 0.08 0.006 0.003 0.00004 0.03 0.013 1

YUKON 

1:20 1.1 6.2 12.8 1.5 0.1 106.0 2.3 23.1 0.8 , 1.6 0.01 0.04 3.111 91

1:200 <0.02 0.04 0.1 0.14 O.001 0.42 0.12 0.24 0.007 0.02 0.00009 0.05 0.206 7 

1:100 <0.02 0.02 0.1 0.1 O.001 0.3 0.007 0.12 0.005 0.0070.00003 0.03 0.069 2

SUNDAY No. 2 

1:20 1.2 0.9 8.0 1.0 0.2 17.3 1.1 15.8 0.38 5.2 0.036 0.50 1.032 89

1:200 O.02 0.01 0.02 0.4 <0.001 <0.05 0.002 0.05 0.002 0.02 0.0001 0.05 0.111 10 

1:100 O.02 0.004 <0.002 0.1 O.001 O.05 0.002 0.02 0.001 0.01 0.0001 0.01 0.015 1

Leachate pH values after each of three leaches

f ratios are solid:liquid]

MAYDAY VENIR YUKON SUNDAYNo.2 

First 1:20 3.14 2.58 2.80 2.67 

Second 1:200 3.93 3.82 3.73 3.81 

Third 1:100 4.07 4.12 3.91 3.98

Leachate pH after the four samples, already leached three times, were pulverized for 5 minutes in a 

tungsten carbide ball mill using tungesten carbide balls and then leached with the pH 5.00 deionized water

for 24 hours 

1:100 3.82 3.89 3.81 4.03
PASSIVE LEACHING WITH LARGER LEACHATE VOLUMES

These experiments were done to simulate metal mobility and acid generation during 

spring freezing-thawing and associated snowmelt runoff. Diurnal thawing and runoff 

will result in exposure of relatively thin ( a few to several cm) surficial layers to very 

large natural leachate volumes.

Splits from the O.090 mm solid fractions were used to obtain 0.5 g samples from 

each of the four untreated waste samples that were used in the prior leaching 

experiments. These 0.5 g samples were exposed to deionized water, side-by-side in 

solid:liquid ratios of 1:4,000, 1:8000, and 1:8,000 in three serial 24 h periods each. 

Leachate samples (0.45 fim filtered) of 60 ml each were obtained for cation analysis 

(HNOs acidified) and sulfate analysis, respectively, from each of the 12 leachates. 

Analytical data for the ICP/MS analyses and the sulfate analyses as well as the final pH 

of each leachate are given in Table 4.

After the end of the second 24 h leach of the Venir waste, an orange precipitate was 

visible on the upper surface of the leached waste. This is believed to be an amorphous 

Fe-oxide, or Fe-oxyhydroxide precipitated from the leachate. No detectable Fe was 

present in any of the leachates with a pH higher than 4.48, even though significant acid 

was generated in all of the leachates (table 4). The Fe detection limit is < 0.2 ng L"1 .

Table 4. Concentrations of selected elements dissolved in each of three serial leachates for 0.5 g of <0.090 

mm fractions from four waste piles determined by ICP/MS on acidified 0.45 wm-filtered samples. [Na <20, 

and K <30 micrograms per liter, leach ratios refer to solids:leachate]

leach

ratio

1:4000

1:8000

1:8000

Mg

2.4

0.8

0.6

Al

<0.2

<0.2

<0.2

Si

67

46

47

Ca

<50

<50

<50

Mn

0.5

0.1

0.1

Fe Cu

crograms per li

MAYDAY

O.2 2.0

<0.2 0.3

<0.2 0.2

Zn

4.4

1.0

0.9

Cd

0.02

<0.01

<0.01

Pb

14.0

2.4

0.5

SO4

600

100

100

final

pH

4.48

4.80

4.90

VENIR

1:4000

1:8000

1:8000

38.5

1.7

0.6

37.4

<0.2

<0.2

43

36

<20

<50

<50

<50

4.3 120.0

0.2 <0.2

0.1 O.2

22.0

1.2

0.3

5.8

<0.5

0.5

0.04

O.01

0.01

53.0

4.7

2.1

2800

300

200

4.05

4.60

4.66

YUKON

1:4000

1:8000

1:8000

22.3

1.8

0.8

21.1

<0.2

<0.2

52

41

87

268

<50

<50

7.7

0.5

0.2

23.0

O.2

O.2

3.1

0.6

0.2

8.1 0.04

0.9 <0.01

0.8 <0.01

2.8

0.9

0.8

2600

400

300

4.19

4.55

4.66

SUNDAY No. 2

1:4000 4.6 9.0 24 71 5.0 14.0 2.1 28.0 0.14 31.0 1800 4.12 

1:8000 1.0 <0.2 61 <50 0.3 O.2 0.2 1.7 <0.10 3.8 300 4.67 

1:8000 0.8 <0.2 34 <50 0.2 <0.2 0.1 2.1 <0.10 2.0 200 4.82
Table 5 summarizes the comparison of some parameters of the smaller volume 

leachates versus those of the larger volume leachates. For Fe the smaller volume 

leachates left the most dissolved Fe, except for the Venir leachate; the much higher pH 

values of the large volume leachates caused initially dissolved Fe to be precipitatated as 

amorphous Fe-oxyhyrdoxide. For Cu, Zn, and Pb, the larger volume leachates produced 

twice as much, or more dissolved metals. In terms of acid generation, the large volume 

leachates produced 45-63 times more IT than the smaller volume leachates. One can 

infer from these comparisons that the greatest detrimental impact on drainage waters 

from waste piles (natural leaching) probably occurs during spring runoff from snowmelt 

in environments of thick snowfall accumulations such as Silverton (Animas River 

headwaters) and Leadville (Arkansas River headwaters) Colorado, and the Boulder River 

headwaters in northern Jefferson County, Montana.

The data presented in Table 5 also demonstrate that the potential for metal mobility 

(solubility) and acid generation, on a short term (leaching) basis (e.g. 24-72 h) for old 

(40-100 yr) metal-mining related wastes is best determined using large leachate volumes 

(1:4000) rather than small leachate volumes. This applies for climatic conditions where 

snowfall accumulations are more than about 100-200 cm y"1 and spring/summer thawing 

melts virtually all of the snow.

Table 5. Sum of selected metals dissolved in two sets of three passive 24 hour serial leachates of the four 

mine-related waste dumps. One set has a relatively low ratio of liquid to solid, the second has a relatively 

high ratio of liquid to solid. Relative amounts of acid generation are also given in terms of FT. The sum 

of dissolved metals is expressed in micrograms per gram of solid.

Ratios of solid to liquid 

in three serial leaches

Fe Cu Zn 

 micrograms gram"1

Pb pH of serial Moles of Ft" g"1 

leachates X 10"4

MAYDAY

1:20; 1:200; 1:100 

1:4000; 1:8000; 1:8000

1:20; 1:200; 1:100 

1:4000; 1:8000; 1:8000

1:20; 1:200, 1:100 

1:4000; 1:8000; 1:8000

51 

<8

916 

960

522 

184

4 

24

105 

200

18 

38

32 

66

VENIR

21

54

YUKON

37 

92

71 

158

30 

533

14 

50

3.14,3.93,4.07 

4.48, 4.80, 4.90

2.58,3.82,4.12 

4.05, 4.60, 4.66

2.80, 3.73, 3.91 

4.19,4.55,4.66

0.049 

2.2

0.104 

6.2

0.093 

5.9

SUNDAY NO. 2

1:20; 1:200; 1:100 328 8 110 21 2.67,3.81,3.98 0.095 

1:4000; 1:8000; 1:8000 112 22 268 341 4.12,4.67,4.82 4.5

10
ACID-GENERATING CAPACITIES OF SOME Fe-DOMINANT SECONDARY

SULFATES

Fe-rich hydrated sulfates originating from the oxidation of pyrite-rich metal-mining 

related wastes, or natural oxidation of pyrite-rich deposits may generate highly acidic 

conditions in many environments (Alpers and others, 1994). Four simple leaching 

experiments were done using natural copiapite [Fe+2Fe+3(SO4)6(OH)2 .20H2O] and natural 

coquimbite [Fe2+3 (SO4)3.9H2O] from the Vulcan mine near Gunnison, Colorado, and 

ferricopiapite [Fe 5(SO4)6O(OH).20H2O] obtained by evaporation of 2 L of laboratory 

leachate of the Sunday No. 2 waste, and a synthetic preparation of rhomboclase 

[HflFe+3 (SO4)2.4H2O]. Each was identified by XRD. In order to simulate what may 

happen when a metal-mining related waste aggregating 100 grams, but only containing 

0.2 g, or 0.2 weight percent of one of these four minerals, 0.2 grams of each of them was 

exposed to 2 L of deionized water of pH = 5.0. The results are given in Table 6 which 

shows that after 24 hours, large amounts of acid are generated, even though the ratio of 

hydrated Fe-sulfates to leachates is 1:10,000. Such small amounts of hydrated Fesulfates, in the range of 0.2 to even 0.5 weight percent in bulk (<2.0 mm) samples, will 

probably not be detected by conventional XRD analysis. The values given in Table 6 are 

those that were measured before precipitation of any visible Fe-oxyhydroxides.

Table 6. Minimum measured acid-generating capacity of four soluble hydrated Fe 

sulfates determined by exposing 0.2 g of each of them to 2 L of deionized water (pH= 5.0 

+0.3) for 24 h. [After about 24 h, precipitation of amorphous Fe-oxyhydroxides may 

occur and this will further reduce the pH of the leachates. e.g. for ferricopiapite the pH of 

the leachate was 3.30 at 24 h. After 48 h significant precipitation of highly visible orange 

colloidal or amorphous Fe-oxyhydroxides, the pH of the leachate was 3.10]________

Mineral Acid generation potential 

____________________HT in millimoles g'1___________________

rhomboclase 0.114

coquimbite 0.069

copiapite 0.060

ferricopiapite 0.056

SIGNIFICANCE OF JAROSITE-MEMBER MINERALS OF THE ALUNITE GROUP 

IN ACID-GENERATING METAL-MINING RELATED WASTES

From our studies of the minerals present in more than 100 separate waste piles, it is 

apparent that the jarosite members of the alunite group (Fleischer, 1993, p. 187) seem to 

be most frequently associated with the wastes that generate the most acidity, and thereby 

the most mobile (dissolved) heavy metals (Z > Fe). These sulfates of the jarosite 

members include: argentojarosite, beaverite, hydronium jarosite, jarosite, natrojarosite, 

osarizawaite, and plumbojarosite. They have the general formula of AB3(SO4)2W(OH)6, 

where A = Ag, K, Na, H, Pb; and B = Al, Cu, Fe+ , but many substitutions of other ions 

may occur (Scott, 1987). The jarosite members of the alunite group have Fe+3 in the B

11
site, rather than Al, which occupies the B site in alunite. The term jarosite, as used here, 

refers to any of the minerals of the jarosite family listed above which have the strongest 

XRD reflection (hid = 113) at 29.0+-0.2 ° 2 9 using Cu X-radiation.

Jarosite is the dominant sulfate detected by XRD analysis of splits of 111 metalmining related waste pile samples from the Animas River headwaters near Silverton, 

Colorado collected in 1997. Desborough and Fey (1997) reported jarosite, found by 

XRD analysis, to be the dominant sulfate in 15 of 19 metal-mining related waste piles in 

the headwaters of the Boulder River near Basin, Montana. Table 7 shows the relations of 

XRD detected jarosite in waste samples, and the final pH of 1:20 (waste:leachate) 

leachates of some mine wastes in Colorado and Montana. The significant differences in 

the mean pH of leachates of those samples with XRD detectable jarosite versus those 

without XRD detectable jarosite seems to indicate some association of the presence of 

jarosite with wastes that produce more acidic leachates.

Table 7. Relations of X-ray diffraction detected jarosite in metal-mining related wastes 

and the final pH of their leachates after exposure of 100 grams of waste to 2 L of 

deionized water for 24 hours._______________________________

111 Animas River headwater wastes near Silverton, Colorado 

70 samples with XRD detectable jarosite 41 samples without XRD detectable jarosite

Mean pH = 3.51 Mean pH = 4.72 

pH range = 2.65-6.65 pH range = 2:83-7.77

19 Boulder River headwater wastes near Basin, Montana 

15 samples with XRD detectable jarosite 4 samples without XRD detectable jarosite

Mean pH = 3.69 Mean pH = 5.12 

pH range = 2.75-5.93 pH range = 3.25-6.78

It is important to recognize that the association of jarosite minerals with acidgenerating capacities of these wastes does not prove that these minerals are responsible 

for acid generation. Other more soluble iron sulfates, possibly present in these wastes, 

but not detectable by XRD studies, may be responsible for significant amounts of the acid 

generated in leachates. However, the jarosite-group minerals probably have different 

solubilities, and mineral grain size will also affect their solubilities. Bigham and others 

(1994, 1996) show that in low temperature oxidizing (weathering) conditions jarositegroup minerals form chiefly at a pH in the range of 2.0-3.6 in Fe-rich and sulfate-rich 

waters. Therefore the identification of jarosite in metal-mining related wastes is

12
important in identifying those wastes that may have significant adverse water-quality 

degradation. The presence of jarosites in metal-mining related wastes can only be 

verified by XRD or detailed electron microscopy because these jarosites are extremely 

fine grained, shown by Bigham and others (1990, fig. If and 1996, fig. 3c) to be less than 

0.05 micrometers.

Presently, it is not clear whether or not jarosites in mine wastes that we have studied 

might contribute acidity or soluble metals to our water leachates. Baron and Palmer 

(1996) show that jarosite [KFe3(SO4)2(OH)6] is essentially insoluble. We conclude that 

the jarosite-group minerals present in our wastes are probably much less soluble than the 

hydrated Fe-rich sulfates that we infer are present in these wastes.

SUMMARY

We have been unable to identify the principal acid-generating mineral(s) or 

amorphous phase(s) present in the metal-mining related wastes studied here. We 

presume this is due to the fact that soluble Fe and/or Al sulfates generating acid are 

present in such low concentrations that our XRD methods did not detect these phases.

A single laboratory leach (24 h.) of weathered mine waste using deionized water, 

regardless of the ratio of solid waste: leachate in the range of 1:20 to 1:4000, probably 

will not reveal the total short-term acid-generating capacity and metal mobility that might 

be of concern for degradation of water quality for aquatic life.

Mineralogical and waste leaching studies of about 140 metal-mining related waste 

piles show that those wastes with XRD-detectable jarosite generally produce leachates 

with a lower pH than those wastes without XRD-detectable jarosite. This does not 

indicate jarosite contributes to acid generation, because it is virtually insoluble in 

deionized water.

ACKNOWLEDGMENTS

We thank Paul Briggs (USGS) for ICP/AES analysis of several leachates analyzed to 

test the reliability of ICP/MS analyses of the same leachates by ACTLABS, Inc. reported 

here.

REFERENCES CITED

Alpers, C.N., Blowes, D.W., Nordstrom, D.K., and Jambor, J.L., 1994, Secondary

minerals and acid mine-water chemistry: in Short course handbook on environmental

geochemistry of sulfide mine-wastes, Mineralogical Association of Canada, Waterloo,

Ontario, May, 1994, pp. 247-270. 

Baron, D., and Palmer, C.D., 1996, Solubility of jarosite at 4-35°C; Geochimica et

Cosmochimica Acta, v. 60, pp. 185-195. 

Bigham, J.M., Schwertmann, U., Carlson, L., and Murad, E., 1990, A poorly crystallized

oxyhydroxysulfate of iron formed by bacterial oxidation of Fe(H) in acid mine waters:

Geochimica et Cosmochimica Acta, v. 54, pp. 2743-2758. 

Bigham, J.M., Carlson, L., and Murad, E., 1994, Schwertmannite, a new iron

oxyhydroxy-sulphate from Pyhasalmi, Finland, and other localities: Mineralogical

Magazine, v. 58, pp. 641-648. 

Bigham, J.M., Schwertmann, U., and Pfao, G, 1996, Influence of pH on mineral

speciation in a bioreactor simulating acid mine drainage: Applied Geochemistry,

v. 11, pp. 845-849.

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Desborough, G.A., and Fey, DL., 1997, Preliminary characterization of acid-generating

potential and toxic metal solubility of some abandoned metal-mining related wastes in

the Boulder River headwaters, northern Jefferson County, Montana: U.S.Geological

Survey Open-File Report 97-478, 21 p. 

English, B.L., Desborough, G.A., and Raymond, W.H., 1987, A mechanical panning

technique for separation of fine-grained gold and other heavy minerals: U. S.

Geological Survey Open-File Report 87-364, 9 p. 

Fleischer, M., 1983, Glossary of mineral species: the Mineralogical Record, Inc., Tucson,

Arizona, 202 p. 

Scott, K.M., 1987, Solid solution in, and classification of, gossan-derived members of the

alunite-jarosite family, northwest Queensland, Australia: American Mineralogist, v.

72,pp.l78-187.

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Cations dissolved in each of three leaches of Mayday <0.090 mm

fraction

100

0.001

1:20 1:200 1:100

Figure 1. Cations dissolved in each of three leaches of Mayday <0.090 mm fraction.

15
Cations dissolved in each of three leaches of Venir <0.090 mm

fraction

1000

.c 

o

COa)

E

CO 

O)

o 

o

1

100

0.001

0.01 H

1:20

Figure 2. Cations dissolved in each of three leaches of Venir <0.090 mm fractions.

16
Cations dissolved in each of three leachates of Yukon <0.090 mm

fraction

10000

1000 -

100

JC

o

(0

2

0)

o 

o

£

0.001

0.01 -

1:20 1:200 1:100

Figure 3. Cations dissolved in each of three leachates of Yukon <0.090 mm fractions.

17
Cations dissolved in each of three leachates of Sunday No. 2, <0.090

mm fraction

1000

SI

o

ra 

o

(0

2

O)

o

100

0.001

0.01 -

1:20 1:200 1:100

Figure 4. Cations dissolved in each of three leachates of Sunday no. 2, <0.090 mm 

fractions.

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