U.S. DEPARTMENT OF THE INTERIOR 

U.S. GEOLOGICAL SURVEY

PRELIMINARY MINERALOGICAL AND CHEMICAL STUDIES RELATED TO THE 

POTENTIAL MOBILITY OF SELENIUM AND ASSOCIATED ELEMENTS IN 

PHOSPHORIA FORMATION STRATA, SOUTHEASTERN IDAHO

George Desborough1 , Ed DeWitt1 , Jeff Jones2, Alan Meier1 and Gregory Meeker1

Prepared in Cooperation with: 

U.S. Bureau of Land Management 

U.S. Forest Service 

Agrium U.S., Inc. 

FMC Corporation 

J.R. Simplot, Inc. 

Rhodia, Inc. 

Solutia, Inc.

Open-File Report 99-129

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.

'U.S. Geological Survey, Denver, Colorado; 2U.S. Forest Service, Soda Springs, Idaho

1999
CONTENTS

Page

Abstract........................................................................................................................................ 4

Introduction.................................................................................................................................. 4

Previous work.................................................................................................................. 4

This study........................................................................................................................ 5

Results.......................................................................................................................................... 5

Stratigraphic variability of selenium................................................................................. 5

Electron beam studies....................................................................................................... 7

Oxidizing thermal treatment............................................................................................. 8

Passive leaching studies ................................................................................................... 11

Summary...................................................................................................................................... 12

References cited............................................................................................................................ 13

FIGURES

Figure 1. Locations of phosphate mines and the Pritchard Creek locality in southeastern Idaho from

which samples were taken for this study........................................................................... 14

Figure 2. Selenium concentrations in one-foot intervals from samples of drill cuttings at the abandoned 

Gay mine on the Fort Hall Indian Reservation, southeastern Idaho.................................... 15

Figure 3. Selenium concentrations in Stratigraphic samples of each bed along Pritchard Creek in the

Caribou Range, southeastern Idaho................................................................................... 16

Figure 4. Plot of organic carbon versus weight loss for samples of the Phosphoria Formation,

southeastern Idaho............................................................................................................ 17

Figure 5. Plot of selenium in leachate versus sulfate in leachate for waste rock sample 220-1 from the 

Dry Valley mine, southeastern Idaho................................................................................ 18

Figure 6. Plot of selenium in leachate versus sulfate in leachate for wr te rock sample 220-2 from the 

Dry Valley mine, southeastern Idaho................................................................................ 19

Figure 7. Plot of selenium in leachate versus selenium in rock from samples from the Enoch Valley mine, 

southeastern Idaho............................................................................................................ 20
TABLES

Page

Table 1. Selenium concentrations in samples of Phosphona Formation at the abandoned Gay mine on 

the Fort Hall Indian Reservation and along Pritchard Creek in the Caribou Range, southeastern 

Idaho................................................................................................................................ 6

Table 2. Concentrations of selected elements in phosphate-bearing waste rock samples of the Phosphoria 

Formation, southeastern Idaho ........................................................................................ 7

Table 3. Relative abundance of minerals in phosphate-bearing waste rock samples of the Phosphoria 

Formation, southeastern Idaho ......................................................................................... 8

Table 4. Selenium loss during initial thermal treatments of fresh samples of phosphate-bearing waste 

rock samples of the Phosphoria Formation, soutneastern Idaho ........................................ 9

Table 5. Selenium loss during second thermal treatments of fresh samples of phosphate-bearing waste 

rock samples of the Phosphoria Formation, southeastern Idaho......................................... 10

Table 6. Selenium loss during thermal treatment at 300° C for 48-51 hours of fresh phosphate-bearing 

waste rock samples of the Phosphoria Formation, southeastern Idaho .............................. 10

Table 7. Selenium loss during thermal treatment at 340° C for 24 hours of fresh phosphate-bearing waste 

rock samples of the Phosphoria Formation from the Enoch Valley mine, southeastern Idaho 

........................................................................................................................................ 11

Table 8. Concentrations of selected elements in deionized water leachates of phosphate-bearing waste 

rock samples of the Phosphoria Formation from the Dry Valley mine, southeastern Idaho 12

Table 9. Concentrations of selected elements in deionized water leachates of fresh, phosphate-bearing

waste rock samples of the Phosphoria Formation from the Enoch Valley mine, southeastern Idaho 

........................................................................................................................................ 12
ABSTRACT

The Phosphoria Formation in southeastern Idaho is mined for commercial extraction of phosphate 

to be used either in fertilizer or as elemental phosphorus. Waste beds that are interbedded with the 

phosphate ore are placed on the surface for disposal. Selenium in these wastes could be leached by 

normal weathering action and could accumulate in water or vegetation in amounts that might be toxic to 

both vertebrates and invertebrates. This study (1) determined the residence of selenium in the waste and 

(2) conducted laboratory experiments applicable to determining the potential for natural weathering 

processes to leach selenium from the waste in a surficial oxidizing environment.

Concentrations of selenium in fresh waste rocks utilized in this study range from <10 parts per 

million (ppm) to about 200 ppm. Furnace-grade phosphate samples from the abandoned Gay mine on the 

Fort Hall Indian Reservation range from about 300 to 400 ppm selenium. Selenium is associated both 

with organic matter (kerogen) in carbon-rich rocks and with pyrite in rocks that have lower concentrations 

of organic carbon. The highest selenium concentrations are in carbon-rich rocks from the Gay mine.

Oxidizing thermal treatments above about 350° C for 24-hour exposure periods revealed virtually 

all selenium was volatilized from all samples. During these exposures, pyrite was oxidized. However, 

during thermal treatments at 340° C and below for 24-hour periods, pyrite was not oxidized, but most of 

the organic material was oxidized. Oxidation of the organic material, as determined by weight loss of the 

sample, resulted in volatilization and removal of significant selenium from many samples, especially 

those samples that had high original concentrations of selenium. These experiments indicate that both 

selenium associated with organic material and selenium associated with pyrite are easily mobilized during 

heating and oxidation. Whether or not these conditions are met in nature should be investigated by field 

studies.

Passive leaching experiments of fresh waste-rock from two mines indicate that selenium is 

mobile in the presence of deionized water having a pH of 5.0. Coarse-grained blocks (1.3 to 4.4 cm) of 

fresh waste that contained about 100 ppm Se were interacted with deionized water in a wasteiwater ratio 

of 1:20 for 48 hours. The leachates contained 15 jig/L of selenium. Fine-grained blocks (< 0.5 mm) of 

the same waste produced a leachate containing about 40 jig/L of selenium. Eleven samples of fresh 

waste-rock, sized to 0.5 to 2.0 mm, produced a leachate that ranged from 0.2 to 180 jig/L and averaged 78 

fig/L. Selenium in the leachate is positively correlated with Mg, Ca, and Fe in the leachate and with Se, 

Cd, and pyrite in the rock samples that were leached.

INTRODUCTION

Mammals and otiier vertebrates may be poisoned by abnormally high concentrations of selenium 

in surface waters. Southeastern Idaho has the potential for problems related to high levels of selenium 

due to naturally occurring deposits of phosphate minerals in sedimentary rocks of the Permian Phosphoria 

Formation. These strata are rich in organic matter and generally have higher selenium (Se) concentrations 

in fresh rocks than most other Paleozoic strata in the western U.S. (Gulbrandsen, 1960, 1975; 

Desborough, 1977; Desborough and Poole, 1983; Desborough and others, 1984). In addition, these strata 

have elevated concentrations of V, Cr, Ni, Zn, Mo, Cd, and Ag (Desborough and Poole, 1983) compared 

to other Paleozoic sedimentary rocks. Open-pit mining of the phosphate deposits results in the removal of 

large amounts of interbedded waste rock. This waste may contribute more selenium to the environment 

than the same beds do in undisturbed or unmined areas. Moving the waste beds results in disaggregation, 

which probably accelerates oxidation of the rocks and removal of selenium. 

Previous Work

Fresh rocks of the Phosphoria Formation, away from commercially developed phosphate mines. 

contain less than about 165 parts per million (ppm) selenium (Gulbrandsen, 1960, 1975; Desborough, 

1977). The principal residence of selenium in these rocks, as determined by microanalysis, is in pyrite. 

Four fresh samples from Coal Creek Canyon, which lies about 75 km southeast of the study area, are 

phosphate-poor, contain about 6.5 to 13 weight percent organic carbon, and have 100 to 150 ppm Se
(Gulbrandsen, 1960). Samples collected at the Maybe Canyon mine, except for the "carbon seam," were 

too oxidized to supply adequate material for microanalysis by electron microprobe (Desborough, 1977). 

Samples of the "carbon seam" studied by microanalysis, X-ray diffraction, and bulk chemical analysis, 

revealed no pyrite, but Se concentrations of 165 ppm and 32 weight percent organic carbon. Selenium in 

this rock is undoubtedly associated with the organic material.

Although minor or trace concentrations of pyrite are present in almost all of the unoxidized 

organic-rich strata of the Phosphoria Formation in the study area, the presence of carbonate minerals 

(dolomite and, or, calcite) tends to neutralize production of sulfuric acid from the oxidation of pyrite. 

Unfortunately, selenium is most mobile, or soluble, in high-pH aqueous environments. Natural oxidation 

due to weathering will eventually mobilize selenium that may be bound in either pyrite or organic-rich 

material (kerogen). The fresh, organic-rich waste rock is almost black, and therefore absorbs sunlight 

which generates high surface temperatures and promotes oxidation. Also, freezing and thawing 

accelerate oxidation due to an increase of surface area of the rock. 

This Study

This study focuses on determining the residences of selenium in waste rock associated with 

phosphate mines. If the mineralogical siting of selenium can be determined, efforts to ameliorate 

potential pollution that may be associated with oxidation and weathering of waste rock may be enhanced. 

Knowledge of the citing of selenium may aid in understanding selenium mobility and thereby permit 

storage of mine waste by techniques that would decelerate the rate of release of selenium to surface water. 

We also want to understand which climatic and weathering factors are important in the release of 

potentially harmful levels of selenium to the environment.

Although the average concentration of selenium in the waste rocks was relatively well known, the 

range and variation of selenium from bed to bed was not well known. We conducted a stratigraphic 

sampling of waste rock at three localities to better understand the variability of selenium within the waste 

rock.

Because most individual non-phosphate grains of the waste are less than about 0.030 mm across, 

and because they contain opaque, intergranular organic material (kerogen), petrographic study is of 

limited use in determining individual minerals and their spatial relations. We therefore studied the waste 

rocks with a combination of scanning electron microscopy (SEM) with an energy dispersive system 

(EDS) and an electron microprobe using wavelength X-ray emission analyzers.

Although bulk chemical analysis reveals the concentration of elements present, it does not show 

the residence of elements in mineral or organic material. Mineralogical, or organic, residence of elements 

is critical to understanding the potential mobility (solubility) of any metal species that may relate to their 

bioavailability. Therefore, we employed oxidizing thermal treatment and leaching techniques in order to 

better understand the citing of selenium in the waste rocks.

This study also attempted to design simple, inexpensive, and time-efficient methods to evaluate 

the residence(s) of selected elements, especially selenium, and how that residence may control potential 

short-term and long-term mobility under natural weathering conditions. For background information, a 

sample of the waste rock containing five weight percent of pyrite, which contains 0.2 weight percent of 

selenium and no selenium in the organic fraction, would contain 100 ppm of Se. A waste rock containing 

20 weight percent of organic carbon, which contains 0.1 weight percent of Se and no selenium in pyrite, 

would contain 200 ppm of Se.

RESULTS

Results of the four methods of investigation are reported below. Discussion of the results follows 

in this section. 

Stratigraphic Variability of Selenium

Samples were collected at one-foot intervals from one drill hole at the abandoned Gay mine on 

the Fort Hall Indian Reservation, and samples were collected from each bed along Pritchard Creek in the 

Caribou Range (fig. 1). A 34-foot stratigraphic section of rock at the Gay mine was sampled and a 31- 

foot stratigraphic section was sampled at Pritchard Creek (Table 1). The numbered beds do not correlate
from one location to the other. Selenium concentrations range from less than 10 to 139 ppm at the Gay 

mine and vary by as much as a factor of 7 from bed to bed (fig. 2). Selenium concentrations range from 

less than 10 to 280 ppm along Pritchard Creek and vary by as much as a factor of 6 from bed to bed 

(fig. 3). Due to the extreme variation from bed to bed, no average concentration of selenium is calculated 

for either section.

Fresh waste rock samples of phosphate-bearing strata of the Phosphoria Formation were selected from 

the Dry Valley, Enoch Valley, and Gay mines (Table 2). The three fresh samples 

Table 1. Selenium concentrations in samples of Phosphoria 

Formation at the abandoned Gay mine on the Fort Hall Indian 

Reservation and along Pritchard Creek in the Caribou Range, 

southeastern Idaho [Samples collected by Simplot Inc. in 1985; 

Selenium analyses by energy-dispersive X-ray fluorescence 

(EDXRF) by G.A. Desborough]

Gav mine. Fort Hall Indian Pritchard Creek. Caribou Range

Reservation 

Footage Se (ppm)

1

2

3

4

5

6

7

8

9

10

11

12

13

14

14

15

16

17

18

19

20

21

22

23

24

25

26

27

28

29

30

31

32

33

25

48

48

41

34

39

32

5

73

68

38

18

48

28

18

24

5

17

35

54

112

126

135

83

80

100

120

50

30

35

55

35

10

25

Bed number Se (ppm)

1

2

3

4

5

6

7

8

9

10

11

12

13

14

14

15

16

17

18

19

20

21

22

23

24

25

26

27

28

29

30

14

5

12

78

160

230

110

25

180

66

96

69

40

280

150

240

100

100

70

57

110

60

67

67

60

85

67

0

0

0

50
from the Gay mine were previously used in petroleum source-rock studies and have total organic carbon 

concentrations of 10.6 to 22.5 weight percent (Desborough and others, 1988). The fresh waste rocks 

range in Se concentration from < 20 to 360 ppm, have about a four-fold range in total organic carbon, and 

about a three-fold range in pyrite concentration, as determined by analyses of commercial laboratories 

and by analysis of panned concentrates (G.A. Desborough, unpub. data, 1998). 

Table 2. Concentrations of selected elements in phosphate-bearing waste rock samples of the Phosphoria 

Formation, southeastern Idaho [Analyses by energy-dispersive X-ray fluorescence (EDXRF) by G.A. 

Desborough; Four non-phosphatic samples (2, 20, 60, and 85 ppm Se) were provided by Steve Wilson of the U.S. 

Geological Survey for standards by which the EDXRF was calibrated for determination of Se; Pb and As in all 

samples are less than 50 ppm; all rocks are fresh unless otherwise indicated; samples from Dry Valley mine in the 

Soda Springs area collected in January, 1998; samples from Enoch Valley mine in the Soda Springs area collected 

in February, 1998; samples from Gay mine on the Fort Hall Indian Reservation collected in 1983]_________ 

Sample Fe Cu Zn Se Rb Sr Y Zr Mo Cd Ba 

Wt% ppm ppm ppm ppm ppm ppm Ppm ppm ppm ppm

Dry Vallev mine

220-1

220-2

20-3 (oxidized)

83FP211

83FP215

83FP216

1.8

2.0

2.3

0.9

0.8

1.0

<30

<30

370

120

230

180

1400

1100

450

1500

3500

2000

110

90

<20

360

290

300

90

85

120

60

50

55

100

160

600

Gay mine

540

560

440

30

55

240

230

110

140

300

300

130

160

100

140

55

30

< 10

60

420

270

1

< 1

4

16

50

29

250

330

520

130

140

170

Enoch Vallev mine

MW11

MW10

MW9

MW8

MW7

MW6

MW5

MW4

MW3

MW2

MW1

1.6

2.0

1.1

1.7

1.8

1.9

1.9

1.3

1.4

1.2

1.3

50

<30

70

160

75

130

<30

<30

90

35

100

2900

4200

2400

1400

1200

1300

200

1800

2000

1100

910

190

30

85

100

70

130

<20

40

<20

75

140

65

65

45

70

60

75

55

50

55

55

70

410

150

470

480

250

420

100

. 300

330

290

270

150

60

110

170

120

170

25

150

130

110

130

250

280

150

240

250

250

400

300

250

310

320

200

10

15

40

125

85

70

35

70

60

110

7

2

5

4

2

2

< i

1

1

< 1

1

190

200

130

210

200

240

180

250

160

240

250

The relative abundance of minerals in samples of waste rock of the Phosphoria Formation was 

determined by X-ray diffraction studies of back-packed sample mounts (Table 3). Relative peak 

intensities of the diffraction maxima (strongest X-ray peak) are given, excluding the quartz, orthoclase, 

and muscovite maxima because their strongest peaks are essentially coincident. For orthoclase, the 040, 

002 maxima was used; for quartz the 112 maxima was used. For mica, the 002 peak was used. Trace 

amounts of marcasite may be present in Enoch Valley mine samples MW 10 and MW 5, and siderite may 

be present in MW 5.

Electron Beam Studies

Two polished thin sections (sample numbers 220-1 and 220-2) of fresh waste rock from the Dry 

Valley mine were studied with the SEM. The average diameter of most mineral grains ranges from about 

0.005 to 0.040 mm. Major minerals, as determined by both X-ray diffraction and the energy-dispersive
Table 3. Relative abundance of minerals in phosphate-bearing waste rock samples of the Phosphoria 

Formation, southeastern Idaho. [Numerical values listed are intensities for selected peaks determined by 

X-ray diffraction; reflections (hkl) used for each mineral are noted. - not detected; all rocks are fresh 

unless otherwise indicated.

apatite orthoclase albite dolomite Calcite pyrite quartz mica 

hkl= 112 040,002 002 104 104 311 112 002 

Sample X-ray diffraction peak intensities

220-1

220-2

20-3 (oxidized)

83FP211

83FP215

83FP216

MW11

MW10

MW9

MW8

MW7

MW6

MW5

MW4

MW3

MW2

MW1

6

9

24

56

36

31

30

7

19

23

19

28

6

23

37

14

15

20

23

11

6

9

11

15

15

8

15

17

11

25

15

14

17

18

35

46

4

6

8

15

16

18

13

18

11

37

17

15

21

19

Dry Valley mine

20

7

0 5

Gay mine

5

4 5

Enoch Valley mine

4 

11

100

3 _

4

3 5

25

4

4

..

_

14

17

5

6

6

15

~

8

6

4

5

3

2

..

7

11

34

20

17

14

19

29

26

21

32

35

29

38

45

41

49

35

6

7

4

5

4

3

6

-

4

6

6

7

-

6

7

7

6

system on the SEM, are quartz, albite, and orthoclase. Minor minerals include apatite, pyrite, and a clay 

mineral which is similar to smectite. Traces of detrital zircon and monazite are present. An intergrowth 

of pyrite and TiC^is present and probably represents pyrite replacement of a detrital Fe-oxide containing 

ilmenite or rutile. The organic material (kerogen) interstitial to the minerals contains a few tenths to a 

few percent of sulfur. Authigenic minerals include albite, orthoclase, apatite, two kinds of pyrite, and the 

clay mineral. The two kinds of pyrite include framboids and angular pyrite. The feldspars, albite and 

orthoclase are high-purity end member^, as expected for authigenic feldspars!

SEM analysis of numerous pyrite grains of both types using the EDS did not detect selenium 

because the limit of detection for selenium may be as much as 0.5 weight percent, or higher, we cannot 

determine which phases in the sample contain selenium.

Electron microprobe wavelength dispersive analysis, with a Se detection limit of about 0.01 

weight percent (100 ppm), of sample 220-1 revealed that pyrite has Se concentrations in both framboidal 

and angular pyrite of from < 100 ppm to about 2000 ppm. The concentration varied widely among 

several grains. Ten kerogen masses analyzed under similar conditions showed no detectable Se; sulfur 

concentrations in kerogen ranged from about 1 to 3 weight percent. 

Oxidizing Thermal Treatment

Prior work showed that the weight percent of organic carbon of the waste rock was proportional 

to the weight loss after ashing at 500°C (Desborough, 1977, fig. 2). This technique is applicable because 

of the low concentration of carbonate or hydrous minerals in the rock. In addition, the technique is an 

inexpensive way to determine organic carbon concentration. If two data points from the present study are 

combined with the results from Desborough (1977), the correlation of the regression analysis for 

calculated organic carbon concentration versus weight loss for ashed samples has an R2 value of 0.985.
Clearly the correlation is quite good (fig. 4), and suggests that ashing at 500° C for 24 hours can be used 

to estimate the organic carbon concentration of these samples with an uncertainty of about + 1 weight 

percent for concentrations less than 15 weight percent.

A second reason for examining the effects of oxidizing thermal treatments was to determine if 

selenium associated with non-sulfide phases such as kerogen could be volatilized without affecting the 

selenium in pyrite. Experiments were conducted to quantify the percentage of pyrite using X-ray 

diffraction (XRD) and to measure the concentration of selenium before and after heating under controlled 

environments. For these experiments, numerous XRD and X-ray fluorescence (XRF) analyses were 

completed on both fresh and thermally treated samples in order to determine loss of mineral phases and 

elements. Pyrite was detected by XRD in most of the samples studied (table 3) used in these thermal 

experiments.

Virtually all of the selenium is lost in oxidizing thermal treatments of samples heated at 350° to 

500°C for 24 hours (Tables 4 and 5). Most of the organic matter and its associated selenium is removed 

by oxidizing treatments at 340° C for 19.5 to 72 hours, but such heating does not oxidize pyrite and its 

associated selenium. Significant differences in the amount of selenium removed from sample 83FP211 

(table 5) may be due to the presence of oxides of selenium or selenate minerals along fractures. Such 

compounds decompose at less than 350° C and selenium is volatilized.

Table 4. Selenium loss during initial thermal treatments of fresh samples of phosphate-bearing waste 

rock samples of the Phosphoria Formation, southeastern Idaho [Samples heated in beakers 45 mm in 

diameter and 70 mm tall]_________________________________________ 

Sample Grain size Organic C Sample wt. Temp. Heating time Weightless Seloss pyrite

mm wt. % grams hours percent percent lost

Dry Valley mine

220-2

220-2

220-2

220-2

220-2

< 0.044

< 0.044

< 0.044

< 0.044

< 0.044

5.0

5.0

5.0

5.0

5.0

3.0

2.0

2.0

2.0

2.0

500

450

400

300

340

17.0

1.5

4.3

17.0

23.0

9.4

12.4

7.8

1.8

7.9

100

0

100

0

0

yes

yes

yes

no

no

Gav mine

83FP211

83FP215

83FP215

< 0.044

0.063-0.090

< 0.044

10.6

22.5

22.5

3.0

3.0

3.0

340

500

340

24.0

3.0

19.5

15.9

30.7

32.1

54

100

46

no

yes

no
Table 5. Selenium loss during second thermal treatments of fresh samples of phosphate-bearing waste 

rock samples of the Phosphoria Formation, southeastern Idaho [Samples heated in petri dish 96 mm in 

diameter and 12 mm tall]

Sample Grain size 

mm

Se Sample wt. 

ppm grams

Temp °C Heating time 

hours

Weight loss 

percent

Se lost 

percent

pyrite 

lost

Dry Valley mine

220-2

220-2

220-2

83FP211

83FP211

83FP211

83FP211

83FP215

83FP215

83FP216

83FP216

83FP216

0.063-0.150

<0.50

0.044-0.150

0.063-0.150

0.090-0.150

0.090-0.150

0.090-0.150

0.063-0.150

0.150-0.250

0.063-0.150

0.063-0.150

< 0.044

60

80

75

315

295

220

220

290

335

215

255

340

3.0

3.0

3.0

3.0

3.0

3.0

3.0

3.0

3.0

3.0

3.0

3.0

340

340

340

340

340

340

340

340

340

340

340

340

26.0

25.5

27.0 

Gav mine

26.0

27.0

72.0

72.0

26.0

46.5

26.0

44.0

26.0

8.3

7.7

9.5

16.1

15.1

15.7

15.0

31.2

32.3

29.6

30.1

35.0

28

10

100

95

45

90

60

63

95

100

86

80

no

no

yes

no

no

no

no

no

no

no

no

no

Heating of fresh waste rock from the Dry Valley mine (sample 220-1) to 300° C for 48 to 51 

hours did not result in any loss of selenium (table 6). As determined by electron microprobe study, the 

pyrite from this rock contains < 100 to about 2000 ppm selenium. Kerogen from the rock contained no 

selenium. Because pyrite is not affected by heating at 300° C, selenium associated with the pyrite was 

not lost.

Thermal treatment of fresh waste rock from the Enoch Valley mine to 340° C for 24 hours 

showed variable loss of selenium, from none to more than 80 percent (table 7). Percent of selenium lost 

is not proportional to weight loss, estimated organic carbon concentration, or pyrite concentration. At this 

temperature, pyrite should not have been affected by the thermal treatment but kerogen was. The lack of 

correlation of any single factor with selenium loss suggests that selenium in the samples may be contained 

in multiple presently unknown phases.

Table 6. Selenium loss during thermal treatment at 300° C for 48-51 hours of fresh 

phosphate-bearing waste rock samples of the Phosphoria Formation, southeastern Idaho [*, 

minimum selenium lost; all samples < 0.150 mm diameter; pyrite remained in all samples, 

based on X-ray diffraction; organic carbon estimated from percent of weight lost; samples 

heated in petri dish 96 mm in diameter and 12 mm tall]____________________ 

Sample Weightless Initial Se Selost Pyrite XRD peak intensity Estimated organic C 

__________percent_____ppm ppm____________________wt. %____

220-1 6.6

Dry Valley mine 

95 0 14 3.0

220-2

220-2

83FP211

83FP215

83FP216

7.8

7.1

16.3

37.4

32.8

75

95

410

375

295

0

0

Gav mine

390*

350*

270*

12

10

5

5

5

3.5

3.0

10

23

20

10
Table 7. Selenium loss during thermal treatment at 340° C for 24 hours of fresh phosphatebearing waste rock samples of the Phosphoria Formation from the Enoch Valley mine, 

southeastern Idaho [pyrite remained in all samples based on X-ray diffraction; organic carbon 

estimated from the percent of weight lost; all samples < 0.150 mm in diameter]_________ 

Sample Weight loss Initial Se Se lost Pyrite XRD peak intensity Estimated organic C 

percent ppm ppm____________________wt. %

MW11

MW10

MW9

MW8

MW7

MW6

MW5

MW4

MW3

MW2

MW1

7.4

5.7

8.7

14.6

14.4

11.9

0.2

9.0

7.6

8.9

11.4

180

30

95

115

90

100

< 10

50

30

85

100

55

0

40

35

25

25

0

0

>20

40

40

8

0

3

4

5

5

0

3

4

3

6

3.5

2.5

5.0

8.0

8.0

7.0

0.0

5.0

3.5

5.0

7.0

Passive Leaching Experiments

Passive leaching experiments were conducted of waste-rock samples of the Phosphoria Formation 

in order to evaluate the solubility of selected elements. Deionized water having a pH of 5.0 in a 

waste:leachate ratio of 1:20 was interacted with the samples for 48 hours. The final pH of the leachate 

was measured for each sample. Acidified and filtered (0.45 micrometer) leachates were analyzed by 

inductive-coupled plasma/ mass spectrometry (ICP/MS). Leaching of waste samples utilized a passive (at 

rest with no stirring) method. Samples exposed to the leachates ranged in weight from 5 to 20 grams.

For the unoxidized samples, more selenium and sulfate are leached from the rock as grain size 

decreases (table 8). The concentration of selenium and sulfate in the leachate are positively correlated 

(figs. 5 and 6). The oxidized waste behaved in the same fashion as the fresh waste, but less selenium was 

concentrated in the leachate. The final pH of leachates increased with decreasing grain size, as did the 

concentrations of Mg, Ca, and Zn in the leachates. Leachates were decanted and the samples were 

allowed to dry. Fine white precipitates of gypsum (identified by XRD) developed on the dried samoles

11
Table 8. Concentrations of selected elements in deionized water leachates of phosphate-bearing waste rock 

samples of the Phosphoria Formation from the Dry Valley mine, southeastern Idaho [size fraction exposed to 

deionized water for 48 hours in a waste:leachate ratio of 1:20; analysis by inductive-coupled plasma mass 

spectrometry (ICP-MS); all metal concentrations in ug/L; all leachates had Cr < 15 iig/L and As < 2 ug/L]

Size 

mm

final 

pH

Mg Ca S04 Se Fe Zn Cu Mo Cd Pb

Sample 20-3, oxidized

12.7-44.4

4.2-12.7

1.65-4.2

0.5-1.65

<0.5

6.22

6.41

6.43

6.43

6.51

200

210

250

350

1800

820

850

970

1500

1800

<300

900

1000

1000

1300

0.4

0.7

0.8

1.0

1.0

90

90

110

120

120

3

8

2

3

3

7

14

9

11

11

1.6

1.1

1.1

<0.5

<0.5

0.3

0.3

0.3

0.4

0.3

0.4

0.4

0.4

Samole 220-1, unoxidized

12.7-44.4

4.2-12.7

1.65-4.2

0.5-1.65

<0.5

6.01

6.44

6.79

6.96

7.18

4200

5500

5400

5600

6500

3900

4700

13000

16000

20000

21000

28000

57000

73000

85000

13

17

31

39

44

15

17

33

38

45

460

250

510

510

220

<0.5

<0.5

<0.5

<0.5

<0.5

5.1

9.0

16.8

34.0

320

0.8

0.2

0.4

0.3

0.2

0.1

0.1

<0.1

0.2

0.1

Sample 220-2. unoxidized

12.7-44.4

4.2-12.7

1.65-4.2

0.5-1.65

<0.5

4.24

4.37

4.08

4.43

5.50

1700

3200

5300

6200

5500

4500

7000

19000

23000

24000

23000

32000

74000

88000

100000

6.30

15

25

34

36

3100

2300

6500

1600

46

1400

2100

5400

5600

7200

6

20

42

7

5

0.7

0.7

1.4

<0.5

<0.5

1.4

2.1

7.7

6.9

6.0

1.5

55.0

79.0

2.4

2.4

Fresh waste samples from the Enoch Valley mine were likewise used for leaching experiments. 

Ten grams of sample was used for each experiment. Sulfate concentrations were not determined for the 

leachates (table 9). For the eleven samples, selenium in the leach is positively correlated (R > 0.7) with 

Mg, Ca, and Fe in the leach and is strongly correlated (R > 0.8) with Se, Cd, and pyrite in the rock. Final 

pH does not correlate strongly with any other factor. In general, waste rock having the highest Se 

concentration (table 2) yielded the highest concentration of Se in the leachate (fig. 7).

Table 9. Concentrations of selected elements in deionized water leachates of fresh, phosphate-bearing 

waste rock samples of the Phosphoria Formation from the Enoch Valley mine, southeastern Idaho [all 

samples were 0.5-2.0 mm in diameter; samples exposed to deionized water for 48 hours in a waste:leachate 

ratio of 1:20; analysis by inductive-coupled plasma mass spectrometry (ICP-MS); all metal concentrations 

in ug/L; all leachates had Cr and As < 10 ng/L and Pb < 0.2 itg/L]

Sample

MW11

MW10

MW9

MW8

MW7

MW6

MW5

MW4

MW3

MW2

MW1

final pH

6.32

7.15

6.98

6.38

6.41

6.63

7.06

6.25

6.04

5.65

5.55

Mg

1860

940

1730

680

730

620

1300

280

260

135

2040

Ca

9030

4500

5000

3200

2800

2400

2730

1100

1540

1400

8540

Se

181

27

184

123

95

82

0.2

19

3

17

130

Fe

47

<0.2

21.0

<0.2

<0.2

<0.2

<0.2

<0.2

<0.2

<0.2

41

Zn

1570

40

110

60

46

34

20

50

87

84

500

Cu

12

1

2

1

1

2

3

3

2

0.6

0.7

Mo

290

23

37

26

46

54

320

50

68

75

85

Cd

37

0.3

3.5

1.1

0.9

0.5

0.6

0.2

0.3

0.5

11

SUMMARY

Samples of fresh waste rock from the Phosphoria Formation in southeastern Idaho contain from < 

20 to 360 ppm selenium. Concentrations of selenium vary by a factor of 6 or more for adjacent beds. 

Due to the extreme variation between beds, an average concentration for the investigated beds is not

12
calculated. The waste-rock beds consist of variable concentrations of extremely fine-grained (0.005 to 

0.040 mm diameter) quartz, albite, orthoclase, apatite, dolomite, a clay mineral similar to smectite, pyrite, 

and calcite in a matrix of organic-rich (kerogen) material. Selenium is associated both with pyrite in 

carbon-poor rocks and rocks of average composition, and with kerogen in carbon-rich rocks.

Oxidizing thermal treatment of the waste rock indicates that selenium associated with kerogen is 

readily volatilized at temperatures below 300° C. Selenium associated with pyrite is volatilized at 

temperature above 340° C. Total organic carbon concentration can be determined by ashing the sample at 

500° C and using a regression equation for samples of this and a prior study.

Passive leaching experiments indicate that much of the selenium in the rock is mobile in the 

presence of deionized water having a pH of 5.0. The highest concentrations of selenium and sulfate in the 

leachate are derived from samples crushed to the finest grain size. Selenium and sulfate in leachates from 

these samples are positively correlated. The final pH of these leachates increase with decreasing grain 

size.

Selenium in leachates interacted with eleven waste-rock samples is positively correlated (R > 0.7) 

with Mg, Ca, and Fe in the leachate and is strongly correlated (R > 0.8) with Se, Cd, and pyrite in the 

rock.

REFERENCES CITED

Desborough, G. A., 1977, Preliminary report on certain metals of potential economic interest in thin 

vanadium-rich zones in the Meade Peak Member of the Phosphoria Formation in western 

Wyoming and eastern Idaho: U.S. Geological Survey Open-File Report, 77-341, 27 p.

Desborough, G. A., and Poole, F. G.,1983, Metals in marine black shales of western United States: 

Society of Economic Geologists/American Institute of Mining and Engineering, Cameron Volume on 

Unconventional Mineral Resources, p. 99-110.

Desborough, G. A., Poole, F. G, Hose, R. K., and Green, G. N., 1984, Metalliferous oil shale in the

Upper Devonian Gibellini facies of the Woodruff Formation in the southern Fish Creek Range, 

Nevada: U.S. Geological Survey Bulletin 1694, p. 93-104.

Desborough, G. A., Poole, F. G., Harris, A. G., and Dav 3, Ted, 1988, Thermal history and organic

maturation of Paleozoic rocks, Pocatello area, southeastern Idaho: U.S. Geological Survey OpenFile Report 88-0289, 8 p.

Gulbrandsen, R. A., 1960, Petrology of the Meade Peak Phosphatic Shale Member of the Phosphoria

Formation at Coal Canyon, Wyoming, and a method of X-ray analysis for determining the ratio 

of calcite to dolomite in mineral mixtures: U.S. Geological Survey Bulletin 111 1-C, D, 152 p.

Gulbrandsen, R. A., 1975, Analytical data on the Phosphoria Formation western United States: 

Geochimica et Cosmochimica Acta, v. 30, p. 769-778.

13
112°00

Figure 1. Locations of phosphate mines and the Pritchard Creek Locality in southeastern Idaho from 

which samples were taken for the present study.

14
Selenium concentrations in 1-foot samples of drill cuttings, South 

40, Fort Hall Indian Reservation

33

31

29

27

25

0 20 40 60 80 100 120

Se, ppm

140 160

Figure 2. Selenium concentrations in one-foot intervals from samples of drill cuttings at the abandpned 

Gay mine on the Fort Hall Indian Reservation, southeastern Idaho.

15
Selenium concentrations in unoxidized Pritchard Creek section

M&&Ki%KiSi»»ai&»mZ8»8» J3J8JS?»3SiJ8Jg>8aaaS<!JS»B^^

50 100 150 200

Se, parts per million

250 300

Figure 3. Selenium concentrations in stratigraphic samples of each bed along Pritchard Creek in the 

Caribou Range, southeastern Idaho.

16
Organic C versus weight loss by ashing at 500 C

60

O

o 

o in

w" 

w 

o

C

o 

o

o

a

50

40

30 -

20 -

10 -

10 15 20 25

Organic carbon, weight percent

30 35

Figure 4. Plot of organic carbon versus weight loss for samples of the Phosphoria Formation, southeastern 

Idaho. Samples were ashed at 500°C for 24 hours.

17
Se versus Sulfate in leachate of sized fractions-Sample 220-1

90

80

70

60

I so

O)

40

30

20-

10 

<O.S mm

12.7-44.4 mm

10 15 20 25 30

Se, micrograms/L

35 40 45 50

Figure 5. Plot of selenium in leachate versus sulfate in leachate for waste-rock sample 220-1 from the Dry 

Valley mine, southeastern Idaho. Five sized fractions of the sample were interacted with deionized water 

(pH = 5.0) for a 48-hour period using a waste:leachate ratio of 1:20.

18
Se versus Sulfate in leachate of sized fractions-Sample 220-2

120

100

80

1 "

o*

3 

CO

20 ^

<0.5 mm

12.7-44.4 mm

10 15 20 25

Se, micrograms/L

30 35

Figure 6. Plot of selenium in leachate versus sulfate in leachate for waste-rock sample 220-2 from the Dry 

Valley mine, southeastern Idaho. Five sized fractions of the sample were interacted with deionized water 

(pH = 5.0) for a 48-hour period using a waste:leachate ratio of 1:20.

19
Se in Enoch Valley waste rock vs. Se in leachate

200

180

160

140

o * ! 

(0 

(0

^ 100 -

E 

a 

a

«* so

CO

60 -

40-

20 -

0 20 40 60 80 100 120 140 160 180 200

Se, ug/L in leachate

Figure 7. Plot of selenium in leachate versus selenium in rock from samples from the Enoch Valley mine, 

southeastern Idaho. Rocks were interacted with deionized water (pH = 5.0) for 48 hours.

20