c. 212

GEOLOGICAL SURVEY CIRCULAR 212

URANIUM-BEARING COAL AND

CARBONACEOUS ROCKS IN THE 

FALL CREEK AREA

BONNEVILLE COUNTY, IDAHO

By James D. Vine and George W. Moore
UNITED STATES DEPARTMENT OF THE INTERIOR 

Oscar L. Chapman, Secretary

GEOLOGICAL SURVEY 

W. E. Wrather, Director

GEOLOGICAL SURVEY CIRCULAR 212

URANIUM-BEARING COAL AND CARBONACEOUS ROCKS IN THE 

FALL CREEK AREA, BONNEVILLE COUNTY, IDAHO

By James D. Vine and George W. Moore

This report concerns work done on behalf of the 

U. S. Atomic Energy Commission and is published 

with the permission of the Commission.

Washington, D. C., 1952

Free on application to the Geological Survey, Washington 25, D. C.
URANIUM-BEARING COAL AND CARBONACEOUS ROCKS IN THE 

FALL CREEK AREA, BONNEVILLE COUNTY, IDAHO

CONTENTS

Page

Abstract ................................... 1

Introduction ............................... 1

Geography ................................. 2

Location.............................. 2

Topography............................ 2

Accessibility .......................... 2

General geology ............................. 3

Structure ............................. 3

Sedimentary rocks..................... 3

Igneous rocks ......................... 3

Page

Fall Creek coal prospect .................... 3

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

Structure............................. 4

Sedimentary rocks.................... 4

Uranium-bearing coal, 

carbonaceous shale, and 

carbonaceous limestone ............. 6

Mineralogy........................... 9

Origin ............................. . 10

References cited ......................... . 10

ILLUSTRATIONS

Page

Plate 1. 

Figure 1.

2.

3.

Graphs of relations at the Fall Creek coal prospect, Bonneville County, Idaho: A> Diagrammatic

longitudinal section; ,B_ Percent uranium and ash in samples ........................... In pocket

Index map of southeastern Idaho showing the areal distribution of coal-bearing rocks of Cretaceous age and radioactive silicic volcanic rocks ...................................... 2

Geologic sketch map of the Fall Creek area, Bonneville County ........................... 5

Stratigraphic section of the upper part of the Bear River formation and the lower part of

the Wayan formation as measured at the Fall Creek adit.....'......................... 6

Aj View of Fall Creek coal prospect showing position of entry and trace of coal bed with 

relation to road and surrounding hills;_B, Interior view of Fall Creek coal prospect, 

taken 50 ft from portal, showing Stratigraphic sequence of mineralized beds and percent 

uranium in the sample ............................................................. 7

TABLE

Table 1. Analyses of samples collected from the Fall Creek coal prospect,

Page

ABSTRACT

Uraniferous coal, carbonaceous shale, and 

carbonaceous limestone occur in the Bear River formation of Early Cretaceous age at the Fall Creek prospect, in the Fall Creek area, Bonneville County, Idaho. 

The uranium compounds are'believed to have been 

derived from mildly radioactive silicic volcanic rocks 

of Tertiary age that rest unconformably on all older 

rocks and once overlay the Bear River formation and 

its coal. Meteoric water, percolating downward through 

the silicic volcanic rocks and into the older rocks along 

joints and faults, is believed to have brought the uranium compounds into contact with the coal and carbonaceous rocks in which the uranium was absorbed.

INTRODUCTION

Uraniferous coal, lignite, and associated carbonaceous rocks were searched for in the summer of 1951 

as part of the Geological Survey's program for the

investigation of carbonaceous sedimentary rocks, in 

the western states, that might be potential sources of 

uranium. This work was done on behalf of the Division 

of Raw Materials of the U. S. Atomic Energy Commission. The occurrence of small quantities of uranium with lignite had been reported previously from 

other areas by Slaughter and others (1946), Staatz 

and others (1951), and'Wyant and others (1951). The 

uraniferous coal at the Fall Creek prospect was found 

as the result of reconnaissance radiometric examination of coals in Colorado, Wyoming, and Idaho closely 

associated with volcanic rocks, an association believed 

to be favorable for the occurrence of uranium in carbonaceous rocks (Denson, Bachman, and Zeller, 1950).

Radiometric examination was made chiefly with 

a Geiger-Muller counter. A scintillometer was available for a short time and it was used in the radiometric 

examination of the silicic volcanic rocks because this 

instrument permits the recognition of lower radioactivity anomalies than does the counter used. All
radioactive coals and associated carbonaceous rocks 

found were sampled for analysis and lithologic study. 

Mildly radioactive igneous rocks, considered to be 

possible source rocks for uranium, were sampled 

for mineralogic and petrographic study.

Louis S. Gardner visited the area and assisted 

with the interpretation of the structural and stratigraphic relations; he also made available his manuscript maps of the area. Norman M. Denson also 

visited the area and aided in the interpretation of 

factors contributing to the localization of the uranium 

compounds.

GEOGRAPHY 

Location

The Fall Creek prospect is in the NEi sec. 4, 

T. IS., R. 42 E., in the Caribou Mountains of 

southeastern Idaho (fig. 1). The adit is adjacent to 

the Fall Creek road in the Caribou National Forest 

and may be reached as follows: Take State Highway 

29 west from the town of Swan Valley about 3 miles; 

turn left onto a graveled road at the west side of the 

bridge which crosses the Snake River; proceed along 

this road for about 1 mile, then turn right at the 

junction with the Fall Creek road and continue along 

this road about 12 miles. The adit is marked by a 

small wooden structure about 1 mile beyond the Fall

Creek Guard station, on the south side of the road at 

a sharp curve in the road. The total driving distance 

from Swan Valley is about 16 miles.

Topography

The topography, as shown on the U. S. Geological Survey Hell Creek quadrangle sheet, ranges 

in elevation from about 5, 800 to 7, 600 ft above sea 

level, thus indicating a maximum relief of 1, 800 ft. 

The grassy, soil-covered slopes in the vicinity of 

the prospect opening are known as the Fall Creek basin 

and form the headwaters of Fall Creek, which flows 

northeast to join the Snake River opposite the town 

of Swan Valley. Except for a few rock ledges the 

area is covered with a dense growth of grass and 

stands of aspen and conifers.

Accessibility

The nearest town is Swan Valley, 16 miles distant, 13 miles of which is a graded dirt road and 

3 miles of which is paved. A railroad station is located 

on the Union Pacific Railroad at Victor, Idaho, a 

distance of about 21^ miles beyond Swan Valley to the 

northeast, mostly on graded dirt road. The railroad 

at Idaho Falls, to the northwest, may be nearer, but 

the distance was not measured and the condition of 

the road is not known.

EXPLANATION

Tertiary silicic volcanic racks

Cretaceous rocks

(T) Fall Creek coal prospect

@ Cloward entry 

(5) Brinson mine

0Croley mine 

(5) Horseshoe district

(5) Cottonwood district 

© Pine Creek district

Figure 1. Index map of southeastern Idaho showing the areal distribution of coal-bearing rocks of Cretaceous age

and radioactive silicic volcanic r.ocks.

2
GENERAL GEOLOGY

Structure

The Caribou Mountains are part of the system 

of parallel mountains which form an arcuate belt along 

the Idaho-Wyoming border. The Caribou Range lies 

at the northern end of the arc where the prominent 

structural features trend northwest to plunge beneath 

the lavas of the Snake River Plain. Tight folds and 

thrust sheets characterize the complex structure of the 

Caribou Mountains. The Snake River flows along a 

strike valley with the Caribou Range on the southwest 

side and the Snake River Range on the northeast side.

Also to the southwest lie other ridges belonging to the 

same system of parallel mountains; eventually they 

give way to the block faulted type of mountains which 

characterizes the Great Basin.

Sedimentary rocks

Mesozoic and Paleozoic strata are exposed in 

the Caribou Range. The following tabular description 

of the Cretaceous sedimentary rocks exposed in the 

vicinity of the Fall Creek prospect is taken chiefly 

from Kirkham (1924, pp. 23-29). The Bear River 

formation is described from the authors' observations 

in the field, following the usage of Louis S. Gardner.

Sedimentary strata exposed in the vicinity of the Fall Creek prospect

Cretaceous

Jurassic

Series

Upper 

Cretaceous.

Lower 

Cretaceous

Upper 

Jurassic

Thickness 

Formation (feet)

Wayan 8,800 

Bear River 150+

Gannett 

group

Tygee 

sandstone

Draney 

limestone

Bechler 

shale--

Peterson 

limestone

Ephraim

. conglomerate

Stump 

sandstone.

1,020

175

225

50

560

Description

Sandstones, grits, conglomerates, shales, 

limestones, and ash beds of fresh-water 

and continental origin.

Black shale of brackish and fresh-water 

origin, quartzite, and thin beds of coal 

and carbonaceous limestone.

Sandstone, reddish, yellowish, and gray 

and carbonaceous shale.

Limestone, very fine grained, light gray, 

also 25 feet of dark-colored coarsegrained limestone at the top.

Red shale, soft, weathers into a red 

soil.

Limestone, massive, fine grained, dark 

gray; contains calcite seams and dark 

chert nodules.

Conglomerate, massive; sandstone, reddish; 

shale, reddish; and limestone.

Kirkham (1924, p. 26) states that the Tygee sandstone 

unconformably underlies the Wayan formation and 

makes no mention of the Bear River formation, which 

Gardner (personal, communication) recognizes between 

the Tygee and the Wayan. It seems probable that 

Kirkham has included the Bear River formation in the 

Tygee sandstone.

Igneous rocks

Tertiary volcanic rocks of several types and ages 

lie unconformably on the steeply tilted Mesozoic and 

Paleozoic strata. Ross and Forrester (1947) show 

three classes or groups of volcanic rocks in this area:

Pleistocene and Recent--Snake River basalt 

(chiefly basaltic flows).

Pliocene(?)--Salt Lake formation and associated 

strata (rather poorly consolidated sand, silt, 

and gravel of lacustrine and fluviatile origin,

including fan deposits. Minor quantities of 

rhyolitic flows and welded tuffs and of basalts 

are included. Some of the sediments are 

tuffaceous, and fresh water limestone is 

locally present).

Miocene and Pliocene--Silicic volcanic rocks 

associated with the Snake River basalt (welded 

tuffs and flows of rhyolitic appearance).

Small remnants of the silicic volcanic rocks cap many 

of the hills in the vicinity of the Fall Creek area. 

Radiometric tests with a scintillometer indicate that 

these volcanic rocks are mildly radioactive.

FALL CREEK COAL PROSPECT 

Introduction

The Fall Creek coal prospect is an adit which 

extends about 83 ft down the dip of a coal bed in the
Bear River formation (fig. 4A). The prospect was 

dug about 30 years ago by J. H. Smith of Rigby, 

Idaho. He reports that he went down the dip of 

the coal bed for a distance of about 98 ft and 

ceased operations when he reached water. Good 

timbering and a solid roof have helped to preserve the opening, though the floor of the mine 

and the lower 15 ft of the opening are covered 

with rubble. Plate 1A is a diagrammatic longitudinal section of the adit.

Structure

The adit was driven in the coal bed, on 

the northeast limb of a faulted anticline (fig. 2). 

The beds dip about 33° NE., and the axes of 

the major structural features trend northwest. 

The anticline is faulted along the axis of the 

fold dropping the Wayan formation down on the

southwest against the Bear River formation. 

Erratic dips in the surrounding area may indicate 

that the structure is more complex than suggested 

here. The coal forms a thin zone of incompetent 

strata which has been sheared by differential movement between the' overlying and underlying competent 

beds, and is characterized by drag folds as shown 

on plate 1A. Clay and shale pods have been 

dragged into the coal so that the quality of the 

coal is too poor to make a commercial coal mine.

Sedimentary rocks

A stratigraphic section was measured beginning in the creek bottom below the adit and 

extending northeastward in the direction of dip 

through the coal horizon. This section is shown 

graphically in figure 3 and corresponds to the 

following description:

Stratigraphic section of the upper part of the Bear River formation .and the lower part of the Wayan formation, measured at the Fall Creek entry about 16 miles southwest of Swan Valley in Bonneville County, Idaho

Wayan formation:

Thickness 

Ft. In.

Unit 1. Sandstone, light gray, medium-grained, thin-to thick-bedded, cross-bedded; forms

prominent ledge............................................................... 23

2. Covered; probably contains red shale; forms slope ......... ........................ 106

3. Sandstone, bluish-gray, fine- to medium-grained .................................. 2

4. Covered; probably contains red shale; forms slope ................................. 34

5. Sandstone, bluish-gray, fine-grained ............................................. 5

6. Covered; probably contains red shale; forms slope ................................. 108

7. Sandstone, greenish-gray, fine-grained ............ . ..... ........................ 4

8. Covered; probably contains red shale; forms slope ................................. 13

9. Sandstone, light gray, fine- to medium-grained, friable; forms ledge locally........... 10

10. Sandstone, light gray, fine- to medium-grained, cross-bedded, thin-bedded, 

slightly friable; forms ledge; the lower 6-8. in. contains a siltstone pebble 

conglomerate.................................................................. 6

11. Covered with reddish soil; probably red shale; forms slope.......................... . 75

Total Wayan formation (measured)....................................... 386

Bear River formation:

Unit 12. Quartzite, greenish-gray to gray, fine-grained to dense ............................ 6

13. Covered; probably shale, green, not fissile ............. .......................... 20

14. Shale, greenish- to purplish-gray, not fissile; forms slope.......................... 20

15. Covered by road fill; probably shale, green, not fissile ............................. 25

16. Limestone, carbonaceous, black, weathers gray, dense to finely crystalline,

radioactive; contains fossil fragments including smooth gastropod shells............. 4 6

17. Shale, carbonaceous, dark greenish-gray.......................................... 0 4

18. Limestone, carbonaceous, fossiliferous; fresh fragments give off fetid odor;

radioactive...............................'..................................... 1 3

19. Clay gouge; radioactive ......................-.'.................................. 0 6

20. Coal, clay and thin limestone lenses; sheared; highly radioactive .................... 4

21. Limestone, carbonaceous, and shale, carbonaceous; radioactive ..................... 1

22. Quartzite, greenish-gray to brown, fine-grained; forms ledge ...................... 4

23. Covered slope-forming unit; probably gray, fissile shale .......................... 15

24. Limestone, impure, dark brown, weathers brown, dense to finely crystalline;

fresh fragments give off a fetid odor; contains fossil fragments including smooth

gastropod shells .............................................................. 0- 8

25. Covered, slope-forming unit; probably gray, fissile shale .......................... 15

26. Quartzite, greenish-gray to brown, fine-grained; cross-bedded in part; forms massive

ledge. The lower part of- the Bear River formation is not exposed................... 35____

Total Bear River formation exposed (measured)........................... 152
EXPLANATION

Coal bed

Fault

Prospect trench

Mine adit

Syncline

Stump sandstone J

,33

Strike and dip of beds

Strike and dip of overturned beds

R.42 E.

Geology after unpublished map 

by Louis S. Gardner

4000 4000

1 Mile

Figure 2. Geologic sketch map of the Fall Creek area, Bonneville County, Idaho.

5
EXPLANATION

Unit 

1

Quortzite

Coal

Concealed or poorly exposed

Figure 3. Stratigraphic section of the upper part of 

the Bear River formation and the lower part of the 

Wayan formation as measured at the Fall Creek adit

Carbonaceous beds in the upper part of the Bear 

River formation contain uranium (fig. 4B). Units 16,

17. and 18 may be considered together as a limestone 

unit with a thin shale parting which overlies the coal 

and forms the roof of the entry. The limestone is 

black and carbonaceous, contains fossil fragments 

including smooth gastropod shells, and gives off a 

fetid odor when crushed. Cavernous joints are visible 

in the roof of the entry, indicating solution by circulating 

ground water. Unit 19, immediately below the limestone and separating it from the underlying coal, is 

a unit of varying thickness and lithology. It consists 

of clay gouge, carbonaceous shale, lenses of carbonaceous limestone, and thin lenses of dense black brittle 

coal with a bright luster. The shear planes in this 

unit are parallel or nearly parallel to the bedding of 

the overlying limestone. The thickness of this unit 

ranges from about 4 in. to 2 ft. Directly underlying 

unit 19 is the coal bed, unit 20. The coal is highly 

distorted and is characterized by shear planes rather 

than bedding planes. As a result of this distortion 

the coal contains a considerable quantity of yellow and 

gray clay and carbonaceous lime stone, which has been 

dragged into the coal bed to form lenses and pods 

parallel to the shear planes. Directly below the coal 

horizon is unit 21, which is similar to unit 19 above 

the coal. Exposures of this unit were poor because 

of the quantity of rubble on the floor of the opening. 

It consists of carbonaceous shale, lenses of clay, 

lenses of dense black brittle coal,and, at the bottom, 

an impure bed of carbonaceous limestone.' The shear 

planes in this unit are wavy but tend to parallel the 

bedding.

Uranium-bearing coal, carbonaceous shale, 

a-fld carbonaceous limestone

Radioactivity was detected first at the mouth 

of the Fall Creek prospect. Further investigation 

showed that all the carbonaceous beds, units 16, 17, 18

18. 19, 20, and 21, were radioactive. Three preliminary 

samples, Nos. 1, 2, and 3 were collected for prompt 

analysis. Other samples for analysis were collected 

from a section normal to the dip of the strata at the 

mouth of the entry and from similiar sections at 10-ft 

intervals along the length of the opening. Forty-eight 

samples were collected from eight sections in order 

to obtain data representative of the mine. This information is summarized in table 1, which shows the 

variation of uranium content in samples collected from 

different places in the opening. The same information 

is shown graphically on plate 1A and B.

In general, mineralization is greatest at or near 

the top of the coal bed, unit 20. Mineralization diminishes downward through the coal and the carbonaceous 

shale below the coal. It also diminishes upward through 

the overlying carbonaceous shale and limestone. A 

relatively high concentration of uranium, 0.053 percent uranium in the coal and 0. 31 percent uranium in 

the ash in sample No. 2, was found in one of the lenses 

of dense black brittle coal in the zone of gouge immediately above the main coal horizon. Unfortunately 

there is only a small quantity of such material in the 

prospect. The average uranium content in the top 

1 foot of coal is about 0.045 percent in the coal and 

0.082 percent in the ash. It is interesting to note 

that the grade increases toward the lower portion of 

the opening, and that the average for the top of the 

coal in the last two sections is 0.110 percent
Figure 4 A. --View of Fall Creek coal prospect, showing 

position of entry and trace of coal bed with relation to 

road and surrounding hills.

Figure 4 B. --Interior view of 

Fall Creek coal prospect, 

taken f>0 ft from portal showing stratigraphic sequence of 

mineralized beds and percent 

uranium in the sample.
Table 1. Analyses of samples collected from the Fall Creek coal, prospect 

[U, uranium; eU, equivalent uranium]

Sample 

number Lab. No.

eU 

(percent)

Ash 

(percent)

U in Ash 

(percent)

U in sample 

(percent) Sample description

Preliminary samples, Nos. 1-3* collected about 35 ft from portal, Sept. 13, 1951

1. 

2. 

3.

52040 

52041 

52042

0.064 

.10 

.020

43-9 

17-3 

77-5

0.090 

.31 

.013

0.040 

-053 

.010

Grab sample sheared coal. 

Grab sample vitreous coal. 

Channel sample 4 ft sheared 

coal.

Comprehensive suite of samples, Nos. 4-51, collected from vertical sections stationed at 10-ft 

intervals from portal on Sept. 24, 1951

Samples collected from station at portal

4.

5-

6.

7-

8.

9-

10.

11.

66874

66873

66861

66862

66863

66864

66865

66866

0.002

.027

.019

.011

.013

.009

.007

.004

-___

____

84.8

78.8

74.6

77.5

 

____

____

0.018

.014

.025

.016

____

____

0.001

.024, .021

.015, .014

.011

.019, .023

.012, .013

.007

.003

3 in. shale above limestone

15 in. carbonaceous limestone above coal.

2 in. carbonaceous shale,

limestone, and coal.

Top !-£ ft sheared coal.

Next 1-j;- ft sheared coal.

Bottom 1-5- ft sheared coal.

6 in. carbonaceous limestone .

6 in. carbonaceous shale

and clay.

Samples collected from station 10 ft from portal

12.

13.

14.

15-

16.

17-

66867

66868

66869

66870

66871

66872

.0.021

.011

.017

.009

.005

.004

77.5 69.4 71.5

73.0

72.6

72.8

0.029

.018

.027

.022

.006

.007

0.022

.012

.019, .020

.016

.004

.005

4 in. carbonaceous shale.

Top 1 ft sheared coal.

Next 1 ft sheared coal.

Next 1 ft sheared coal . .

Bottom 1 ft sheared coal.

Carbonaceous shale and clay

Samples collected from station 20 ft from portal

18.

19.

20.

21.

22.

23-

66875

66876

66877

66878

66879

66880

0.012

.035

.006

.009

.005

.004

__ _

61.3

71.8

73.4

77-9

69.3

____

0.066

.011

.015

.007

.008

0.013

.040

.008

.011

.005

.006

Carbonaceous limestone and

shale .

Top 1 ft sheared coal.

Next 1 ft sheared coal.

Next 1 ft sheared coal.

1 ft clay and carbonaceous

shale .

1 ft clay, carbonaceous

shale, and limestone.

Samples collected from station 30 ft from portal

24.

25.

26.

27.

28.

66881

66882

66883

66884

66885

0.024

.008

.006

.007

.007

_ _

72.0

68.3

69-7

74.4

____

0.014

.011

.014

.009

0.025, -028

.010

.008

.010

.007

1 ft carbonaceous shale and

clay.

Top 1 1/3 ft sheared coal.

Next 1 1/3 ft sheared coal.

1 1/3 ft sheared coal.

15 in. sheared coal, shale,

and limestone.

Samples collected from station 40 ft from portal

29.

30.

66886

66887

0.026

.028

 -

70.8

__ _

0.044

0.029, .032

.031

6 in. carbonaceous shale

and clay.

10 in. carbonaceous limestone and vitreous coal.
Table 1. Analyses of samples collected from the Fall Creek coal prospect Continued

Sample 

number Lab. No.

ell 

(percent)

Ash 

(percent)

U in Ash 

(percent)

U in sample 

(percent) Sample description

Samples collected from station 40 ft from portal Continued

31. 

32. 

33. 

34.

66888 

66889 

66890 

66891

0.021 

.006 

.018 

.005

72.3 

74.6 

71.4 

78.2

0.033 

.010 

.030 

.009

0.024 

.007 

.021 

.007

Top 1 1/3 ft sheared coal. 

Next 1 1/3 ft sheared coal. 

Next 1 1/3 ft sheared coal. 

1 1/3 ft coal, carbonaceous 

shale, an4«^|inestone*

Samples collected from station 50 ft from portal !'""

35- 

36.

37- 

38. 

39. 

40.

66892 

66893

66894

66895 

66896 

67239

0.026 

.025

.033 

.007 

.005 

.006

60.4 

66.0 

70.8 

70.8

0.066 

.013 

.010 

.008

0.028, .030 

.027

.040 

.009 

.007 

.006

1 ft carbonaceous shale and 

limestone . 

8 in. carbonaceous limestone, shale, and vitreous 

coal . 

Top If ft sheared coal. 

Next l£ ft sheared coal. 

Next If ft sheared coal. 

1 ft coal and clay.

Samples collected from station 60 ft from portal

41.

42.

43.

44.

45.

46.

67240

67241

67242

67243

67244

67245

0.016

.018

.096

.025

.006

.007

____

 

43.7

66.2

75-9

77-4

__

 

0.30

.044

.010

.009

0.018, .020

.022

.131

.029

.008

* .007

8 in. carbonaceous shale.

15 in. carbonaceous limestone .

Top l£ ft sheared coal.

Next 1-5- ft sheared coal.

Next l£ ft sheared coal.

Bottom l£ ft sheared coal.

Samples collected from station 75 ft from portal

47-

48.

49.

50.

51.

67246

67247

67248

67249

67250

0.003

.010

.064

.007

.008

__ __

 

58.4

64.9

72.4

__

0.145

.009

.012

0.003

.011

.085, .089

.006

.009

4 in. carbonaceous shale.

17 in. carbonaceous limestone.

Top 1.1 ft sheared coal.

Next 1.1 ft sheared coal.

Next 1.1 ft sheared coal.

Sample from dump of abandoned prospect half a mile northwest of portal

52. 66858 0.066 37-0 0.22 0.08 Coal fragments.

uranium in the coal and 0. 222 percent uranium in the 

"ash.

No other exposures of the coal bed were found 

along its strike. The hills in this area are covered 

by thick soil and colluvium, and only the most resistant 

beds crop out (fig. 4A). Fragments of radioactive 

carbonaceous limestone were found within half a mile 

northwest of the adit. A trench 3-5 ft deep and 20 ft 

long was dug adjacent to a caved and abandoned coal 

prospect about half a mile northwest of the adit in an 

attempt to uncover an exposure of the coal, but bedrock was not found. Enough small chunks of coal, 

however, were gathered from the dump of the abandoned 

coal prospect to fill a quart container. These fragments analyzed 0.08 percent uranium in the sample, 

37 percent ash, and 0.22 percent uranium in the ash. 

This substantiates the belief that the uranium-bearing 

coal bed extends for at least half a mile along the 

strike northwest of the main adit.

Mineralogy

No uranium minerals have been identified by 

this field study of the coal,- carbonaceous shale,or 

carbpnaceous limestone even though the analyses 

indicate a comparatively large quantity of uranium. 

Tolmachev (1943) has shown in his experiments on the 

adsorption of uranyl nitrate by activated carbon and 

carbonaceous shales that after the adsorption had taken 

place the amount of nitrate in the solution remained the 

s£me, but the amount of uranyl ion diminished. This 

suggested £o him that the uranyl ion was adsorbed 

between t$e graphitic layers of carbonaceous material. 

A similar mechanism may explain why the uraniumbearing coal in-fee F,all Creek area contains no 

megascopic alls? detectable uranium minerals. That 

is, the uranium may be present in the ionic state, 

adsorbed by carbonaceous material.
Origin

It seems fairly evident that the carbonaceous 

material is in some way responsible for the localization of the ore, whether it be due to ionic adsorption 

or some other mechanism such as chemical reduction, 

because; the uranium is associated with beds of several 

different lithologies which have but one common factor, 

a high content of carbon. Carbonaceous beds near the 

top of the carbonaceous zone are the most intensely 

mineralized and below the top foot of coal the uranium 

content diminishes rapidly downward. This is interpreted as meaning that the mineralization was effected 

by downward percolation of solutions bearing uranium 

ions and that the ions were held by adsorpti'on by the 

first carbonaceous material which they encountered. 

The diminishing amount of uranium in the lower part 

of the coal bed, therefore, could indicate that few ions 

were able to penetrate so far before being absorbed.

Silicic volcanic rocks cap several of the hills 

in the vicinity of the Fall Creek coal prospect and are 

mildly radioactive. Analysis of a sample of the volcanic 

rock gave 0.004 percent equivalent uranium but less 

than 0.001 percent uranium. The areal pattern formed 

by the exposures of the silicic volcanics suggests that 

they once blanketed the entire surrounding area. Such 

a large mass of mildly radioactive rock can readily 

be imagined as the source for the uranium in the coal. 

Meteoric waters percolating through the source rock 

could take small quantities of uranium in solution and 

later deposit it under the proper physical and chemical 

environment such as was provided by the coal and 

carbonaceous material. A similar mechanism has been 

suggested for the source of the uranium in certain 

lignites in the Dakotas (Denson and others, 1950).

In trying to reconstruct the sequence of events 

which led to the mineralization of the coal in the 

Fall Creek area the fact that at least 650 ft of sediments 

separate the coal from the overlying silicic volcanics 

must be explained. These intervening sediments contain red and green shales, sandstones, and quartzites 

which are relatively impervious for the circulation 

of ground water. However, the fault which offsets 

the axis of the anticline and passes no more than several 

hundred feet south of the Fall Creek prospect may have 

provided a conduit which f acilitated the circulation of 

ground water. Ground water solutions may have left 

the fault to continue down the dip of the cavernous

limestone which overlies the coal and thus enter the 

coal horizon from above. The average of the analyses 

of samples from the various sections indicates higher 

uranium content for some sections than for adjacent 

sections. This more intense mineralization may be 

due to the solutions being introduced through an open 

joint in the limestone above that section.

Geologic features believed to be significant in 

the localization of uranium in the Fall Creek area are: 

(1) the mildly radioactive silicic volcanic rocks, which 

could have been a source of mineralizing meteoric 

waters; and (2) the coal, carbonaceous limestone, and 

carbonaceous shale below the volcanic rocks, which 

now hold the uranium that was introduced by percolating 

meteoric water.

REFERENCES CITED

Denson, N. M., Bachman, G. O., and Zeller, H. D.,

1950.. Summary of new information on uraniferous 

lignites in the Dakotas: U. S. Geol. Survey Trace 

Elements Mem. Rept. 175.

Gardner, L. S., Unpublished manuscript maps, Hell 

Creek quadrangle, Bonneville County, Idaho: U. S. 

Geol. Survey.

Kirkham, V. R. D., 1924, Geology and oil possibilities 

of Bingham, Bonneville, and Caribou Counties, Idaho: 

Idaho Bur. Mines and Geology Bull. 8.

Mansfield, G. R., 1920, Coal in eastern Idaho: U.S. 

Geol. Survey Bull. 716-F, pp. 123-154.

Ross, C. P., and Forrester, J. D., 1947, Geologic 

map of the State of Idaho: U. S. Geol. Survey and 

Idaho Bur. Mines and Geology.

Slaughter,. A. L., and Nelson, J. M., 1946, Trace 

Elements reconnaissance in South Dakota and Wyoming, 

preliminary report: U. S. Geol. Survey Trace Elements Inv. Rept. 20.

Staatz, M. H., and Bauer, H. L., 1951, Uraniumbearing lignite beds at the Gamma property, Churchhill County, Nevada: U. S. Geol. Survey Trace 

Elements Mem. Rept. 226.

Tolmachev, I. M., 1943, Adsorption of uranyl salts 

on solid adsorbents: U. S. S. R. Acad. Sci. Bull. 1, 

pp. 28-34.

Wyant, D. G., Sharp, W. N., and Sheridan, D. M.,

1951. Uranium deposits in the Red Desert of the 

Great Divide Basin, Sweetwater County, Wyoming: 

U. S. Geol. Survey Trace Elements Inv. Rept. 122.

10

INT.-CUP. SEC., WASH., D.C.
UNITED STATES DEPARTMENT OF THE INTERIOR 

GEOLOGICAL SURVEY CIRCULAR 212 PLATE 1

80

A- DIAGRAMMATIC LONGITUDINAL SECTION

Coal and carbonaceous shale in the sheared zone 

Coal lenses not in main sheared zone 

Carbonaceous limestone 

|- | Carbonaceous shale and coaly gouge

Clay gouge 

^ Shear planes in the coal 

 -"' Joint

I Slope distance from portoi indicated every 5 feet

xJ2\ The top figure indicates ttie number of sample collected for analysis 

'(007) Tne lower f'9ure ind'C ates rne percent of uranium in the sample

.^

The bracket indicates the thickness of the interval sampled 

le

Percent ash | Percent Percent ash Percent Ptrcent ash

At portal 10 feet from portal 20 feet from portal 30 feet from portal

Percent ash | Percent 

uranium

^_____ _ !

40 feet from portal

Percent ash | Percent 

uranium

50 feet from portal

Percent uranium

75 feet from portal

100 .90

Percent uranium

Grab samples and 4-foot channel sample 

35 feet from portal

EXPLANATION

Percent ash shown by stippled bar 

to left of zero line

Percent uranium in sample shown by open bar 

to right of zero line

Percent uranium in ash shown by end of solid bar 

to right of zero line

Vertical scale

B- PERCENT URANIUM AND ASH SAMPLES

GRAPHS OF RELATIONS AT THE FALL CREEK COAL PROSPECT, BONNEVILLE COUNTY, IDAHO

Horizontal and vertical scale