GEOLOGICAL SURVEY CIRCULAR 351

OCCURRENCES OF RADIOACTIVE 

MINERALS IN THE BALD MOUNTAIN 

GOLD-MINING AREA, NORTHERN 

BLACK HILLS, SOUTH DAKOTA

This report concerns work done on 

behalf of the U. S. Atomic Energy 

Commission and is published with 

the permission of the Commission.

UNITED STATES DEPARTMENT OF THE INTERIOR 

Douglas McKay, Secretary

GEOLOGICAL SURVEY 

W. E. Wrather, Director

GEOLOGICAL SURVEY CIRCULAR 351

OCCURRENCES OF RADIOACTIVE MINERALS IN THE BALD MOUNTAIN 

GOLD-MINING AREA, NORTHERN BLACK HILLS, SOUTH DAKOTA

By R. C. Vickers

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., 1954

Free on application to the Geological Survey, Washington 26, D. C.

OCCURRENCES OF RADIOACTIVE MINERALS EN THE BALD MOUNTAIN 

GOLD-MINING AREA, NORTHERN BLACK HILLS, SOUTH DAKOTA

By R. C. Vickers

CONTENTS

Page

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

Introduction.................................. 2

Location and accessibility ................ 2

History of reported uranium

occurrences .......................... 2

Geology................... . ................. . 3

General ................................ 3

Metamorphic rocks ...................... 3

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

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

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

Ore deposits ............................ 4

Page

Geology Continued

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

Radioactive occurrences ....................... 4

Annie Creek autunite occurrence .......... 4

Dakota mine ............................ 6

Marie and Mary claim ................... 6

Decorah mine ........................... 6

Mikado claim ............................ 6

Diatreme............................... 7

Dark Horse claim ....................... 7

Conclusion .................... .............. 7

Literature cited .............................. 8

ILLUSTRATIONS

Figure 1. Index map showing location of radioactive occurrences, Bald Mountain gold-mining area, 

Lawrence County, S. Dak .......................................................

2. Geologic map of the Annie Creek autunite occurrence ................................

Page

TABLES

Page

Table 1. Analyses of samples, Annie Creek autunite occurrence ........................................ . 6

2. Analyses of samples from the Bald Mountain gold-mining area ................................. . 7

ABSTRACT

Six radioactive occurrences were found in July 

1953 during a reconnaissance of the Bald Mountain 

gold-mining area, Lawrence County, S. Dak.

The rocks in the area consist mainly of the flatlying Deadwood formation of Cambrian age intruded 

by many sills, laccoliths, and dikes of Tertiary age. 

Associated with the intrusive rocks are gold-silver 

deposits which have been interpreted as being localized 

by solutions migrating up fractures and replacing 

favorable beds, mainly in the Deadwood formation. 

The radioactive occurrences are in fractured and 

altered siltstone of the Deadwood formation and the 

overlying Whitewood limestone and in altered intrusive

rocks. Samples contain as much as 0.19 percent 

equivalent uranium and also contain thorium and the 

rare earths. All the occurrences are :n the weathered 

zone, and differential leaching has probably disturbed 

the original thorium/uranium ratio. The increase in 

rare-earth content toward the center of the goldproducing area and the occurrence of uranium minerals 

(autunite and torbernite) on the fringe of the productive 

area are probably related to a zonal pattern.

Although none of the occurrences are large 

enough to constitute ore, uranium-bear ing hydrothermal solutions were probably present during 

Tertiary mineralization in the Bald Mountain area and 

may have given rise to the carnotite-type deposits in 

the sediments surrounding the Black Hills.
INTRODUCTION

Reconnaissance for radioactive materials in 

the northern Black Hills, South Dakota, was begun 

during the 1952 field season, at which time a 

deposit of autunite was found about 1.5 miles 

southwest of Trojan, Lawrence County, S. Dak. 

(Vickers, 1953). Many of the intrusive rocks of 

the northern Black Hills were abnormally radioactive (as much as 0.014 percent equivalent uranium). A subsequent study of these intrusive rocks 

indicated that most of the radioactivity was due 

to thorium.

Part of the 1953 field season was concerned 

with a detailed study and geologic mapping of the 

autunite deposit and detailed reconnaissance of 

mines and prospects in the Bald Mountain mining 

area. During this work, six additional radioactive 

occurrences were found.

Location and accessibility

The area of investigation includes parts of 

sees. 1, 2, and 3, T. 4 N., R. 2 E., and sees. 35 

and 36, T. 5 N., R. 2 E., Black Hills meridian, 

Lawrence County, S. Dak. (fig. 1).

The area is accessible by many secondary 

roads and abandoned railroad grades, many of 

which can be negotiated only by vehicles equipped 

with four-wheel drive.

History of reported uranium occurrences

According to the available literature, uranium 

was first reported in the northern Black Hills by 

Stillwell (1885, p. 82). He mentions the occurrence 

of pitchblende and uranium mica (probably autunite) 

on Bald Mountain.

Scott (1897) mentioned the occurrence of a 

mineral with the composition of zeunerite occurring 

on Anna Creek at Carbonate and also at the Poisoned 

Ox mine near Pactola. Smith (1898, p. 419) reported 

that one sample from the Ross-Hannibal mine showed 

crystals of torbernite and assayed 255.08 ounces of 

silver and 0.44 ounce of gold per ton. This occurrence of torbernite was again mentioned by Irving 

and Emmons (1904, p. 139-140) who stated that large 

quantities of a greenish uranium mica were reported 

in association with the ores of the Ross-Hannibal mine 

in Whitetail Gulch and that this deposit was similar 

to the occurrence of uranocircite in several localities 

in the productive mining region.

O'Harra (1902, p. 78) states:

Uranium minerals have "been observed in various 

places, particularly in the Poisoned Ox mine near 

Pactola and in the Davier mine on Anna Creek near Portland. Uranium is also reported as occurring near 

Carbonate. At the Poisoned Ox mine the mineral occurs 

with copper in slates. In the Anna Creek property it 

is intimately associated with porphyry. At the latter 

place effort has "been made to obtain the mineral in 

commercial quantities, but as yet success has not been 

attained.

DAKOTA MINE

(Location projected)

MARIE AND MARY CLAIM

Base from air photos. Section lines approximately located

Figure 1. Index map showing location of radioactive occurrences, Bald Mountain gold-mining area,

Lawrence County, South Dakota.
Ziegler (1914, p. 205-206) described the torbernite from the Ross-Hannibal mine as follows:

This mineral occurs in minute square tabular 

crystals of bright grass green color in vugs and small 

cavities in the blue siliceous gold ore of Lower 

Cambrian age, at the Ross Hannibal mine about one and 

one-half miles southeast of Terry.

None of the crystals attain a size larger than 

one-fourth of an inch square, while the majority are 

about one-eighth inch square. The top of these crystals has a fine pearly luster, while the sides are 

weakly metallic. The streak is pale green, the specific 

gravity about 3«5> and the hardness 2.5 the mineral 

just barely yielding a scratch by the finger nail. 

Torbernite possesses excellent basal cleavage, which 

yields an extremely thin almost micaceous, but brittle 

flake.

Some torbernite also occurs in very small micaceous aggregates, formed by the overlapping and intergrowth of a number of small square crystals. More 

commonly we find the torbernite arranged in beautiful 

radiating fibers pressed out flat in minute fissures 

in the blue siliceous ore, which yield fine almost 

circular rosettes up to three-fourths of an inch in 

diameter.

Ziegler (1914, p. 206-207) also mentions the 

occurrence of autunite in the porphyries (phonolite) 

on Annie Creek west of Portland. His description 

follows:

......crystals are very rare, and the autunite usually

occurs as a very thin micaceous coating or a series of 

thin irregularly shaped flakes or as a number of small 

partially radiating moss-like aggregates, coating 

minute fissures in the porphyries. The cleavage is 

excellent, yielding thin brittle laminae of a lemonyellow color which are weakly transparent. The luster 

varies from dull silky to adamantine, while an occasional square crystal shows pearly luster. The streak 

is deep yellow, the hardness about equal to the finger 

nail, and the specific gravity 3.07. It is associated 

with limonite, and a little fluorite.

Specimens of autunite and torbernite from the 

northern Black Hills can be found in museums at 

Deadwood and Rapid City, S. Dak., at Denver, Colo., 

and at Chicago, HI.

GEOLOGY 

General

The general geology of the Black Hills has been 

summarized by Connolly (1927, p. 11), who states:

In a broad way the general structure of the Hills 

is simple. The Black Hills uplift is an outlier of the 

Rocky Mountain uplift to the west........It has the

shape of an elliptical dome whose longer axis trends 

northwest and southeast. The central core, consisting 

of metamorphic and igneous rocks, is surrounded on all 

sides by concentric outcrops of Paleozoic and Mesozoic 

sediments. The pre-Cambrian nucleus and the outwardly 

dipping sediments are overlapped by sediments of 

Oligocene age. Intruded into the pre-Cambrian metamorphic rocks and the Paleozoic and Mesozoic sediments 

are many dikes, sills, and laccoliths of Tertiary age,...

The most complete general geologic description 

of the northern Black Hills has been written by Darton 

and Paige (1925).

Metamorphic rocks

Pre-Cambrian rocks, which consist of amphibolite, calcareous schist, garnetiferous schist, 

amphibole-garnet schist, quartzite, slate, and mica 

schist, crop out in the central part of the northern 

Black Hills. These rocks are complexly folded and 

generally strike parallel to the main trend of the 

uplift and are considered to be of pre-Cambrian age.

Sedimentary rocks

Sedimentary rocks cropping out in the central 

part of the northern Black Hills include the Deadwood 

formation of Cambrian age, Whitewood limestone of 

Ordovician age, and the Englewood and Pahasapa 

limestones of Mississippian age. Ore deposits are 

found only in the Deadwood, Whitewood, and Pahasapa 

formations. Only these three formations will be 

further described.

Deadwood formation. The Deadwood formation 

of Late Cambrian age lies unconformably on the preCambrian rocks and is divided into three parts. The 

basal member is a brownish-red sandstone or quartzite, usually containing lenticular beds of conglomerate 

at the base. The middle member is a thick series of 

alternating shales and dolomitic limestones, and some 

thin sandstone layers. The upper member is red to 

brown sandstone, in places quartzitic, and is commonly 

overlain by a few feet of shales. The Deadwood 

formation has a maximum thickness of about 500 feet 

in the northern Black Hills but thins rapidly southward.. 

Most of the siliceous gold ore of Tertiary age in this 

region is in this -formation.

Whitewood limestone. Paleontologic evidence 

indicates that the upper 70 feet of the Deadwood 

formation, at the type locality, consisting of green 

fissile shale and gray siltstone is of Ordovician age. 

Furnish, Barragy, and Miller (1936, p. 1329-1341) 

suggest that these beds should be included with the 

Whitewood limestone. The green shale contains 

phosphatic nodules which are similar to the phosphatic 

nodules found in the autunite-bearing siltstone at the 

Annie Creek autunite occurrence. Probably some 

of the gold ores of Tertiary age in the northern Black 

Hills are in rocks classified as part of the Deadwood 

formation which are now believed to be of Ordovician 

age. No ore deposits are known to occur in the Whitewood as originally described by Darton.

Pahasapa limestone. The Pahasapa limestone 

of Mississippian age is a massive-bedded gray to buff 

fine-grained limestone, generally dolomitic. It ranges 

in thickness from 300 to more than 600 feet. Only 

minor silver-lead-gold deposits are*found in this 

formation.

Igneous rocks

Pre-Cambrian igneous rocks are found in the 

northern Black Hills but are not directly related to the 

mineralization in the Bald Mountain area.

Extensive igneous activity took place in this 

region during early Tertiary time, and many dikes, 

sills, laccoliths, and irregular intrusive bodies were 

injected into the pre-Cambrian, Paleozoic, and
Mesozoic rocks. Paige (1924, p. 16-17) describes 

the following Tertiary igneous rocks from the northern 

Black Hills: rhyolite, rhyolite porphyry, porphyritic 

rhyolite, quartz monzonite porphyry, monzonite 

porphyry, grorudite, and phonolite.

These Tertiary igneous rocks are of prime 

importance from the standpoint of ore deposition in 

the northern Black Hills because of the hydrothermal 

activity associated with them. The known occurrences 

of uranium- and thorium-bearing minerals in the 

region are also directly associated with the Tertiary 

mineralization or with the Tertiary intrusive rocks.

Structure

The principal structural features of the northern 

Black Hills are the doming of the Paleozoic and 

Mesozoic formations, which occurred in Late Cretaceous time, and the deformation of the pre-Tertiary 

rocks caused by the igneous intrusions in Tertiary time.

Paleozoic and Mesozoic rocks generally dip away 

from the central part of the region at less than 20 degrees. This dip has been modified in places by 

intrusion of igneous masses into the sedimentary 

strata of the northern Black Hills in early Tertiary 

time. In the Bald Mountain mining area the sedimentary formations are nearly horizontal, and the dips are 

usually less than 10 degrees.

Many fractures and faults of small displacement 

have resulted from the intrusion of the larger igneous 

masses and have acted as channelways for the orebearing solutions.

Ore deposits

The gold-silver deposits of Tertiary age in the 

northern Black Hills have been interpreted by Connolly 

(1925) as hydrothermal replacement deposits that are 

genetically associated with extensive intrusive activity. They occur principally in the Deadwood formation. 

Most of the ore deposits have been formed by solutions 

migrating up fractures and replacing favorable beds 

along the upper and lower contacts of the Deadwood 

formation. Some of these channelways (locally termed 

"verticals") were sufficiently mineralized to constitute 

ore bodies extending for hundreds of feet vertically, 

but nearly all of the production has come from the 

replacement bodies. The ore bodies are in the form 

of elongate shoots, or irregular channellike bodies, 

roughly paralleling the strike of the major system of 

verticals. The largest ore bodies have been formed 

at the intersection of two sets of closely spaced verticals,

Mineralogy

Connolly (1927, p. 119) states that the mineralization is of a very simple type:

......The non-metallic constituents that have "been

introduced consist of silica, "both crystalline quartz 

and chalcedonic silica, simple carbonates, dolomite and 

calcite, sulphates, barite,. gypsum, and alunite, and 

fluorite. Silicates and heavy oxides are totally lacking. The metallic minerals are auriferous pyrite, 

argentiferous galena, wolframite and huebnerite, 

tetrahedrite, lollingite, arsenopyrite, a little chalcopyrite, stibnite, sylvanite, and calaverite.

Autunite, torbernite, uranocircite, zeunerite, and 

pitchblende have also been reported (see p. 2-3); but 

with the exception of autunite, none of these minerals 

have been mentioned in the literature on the area 

since 1914.

RADIOACTIVE OCCURRENCES

Abnormal radioactivity, that is, radioactivity 

significantly higher than that of the Tertiary igneous 

rocks of the area which commonly contain 0.01 percent equivalent uranium, has been found at seven 

localities in the Bald Mountain mining district of the 

northern Black Hills (fig. 1). With the exception of 

the autunite deposit, all the occurrences were found 

during the 1953 field season.

Chemical and spectrographic analyses indicate 

the presence of both uranium and thorium, and the 

occurrences range from those dominantly uranium - 

bearing to those dominantly thorium-bearing.

Because all the deposits are in the weathered 

zone, differential leaching probably has disturbed 

the original thorium/uranium ratio.

Annie Creek autunite occurrence

Autunite, as fracture coatings and disseminations in siltstone of the Whitewood limestone of 

Ordovician age, was found during the 1952 field season near Annie Creek in the NE-j- sec. 3, T. 4 N., 

R. 2E., Lawrence County, S. Dak. (Vickers, 1953).

The autunite-bearing siltstone is exposed in a 

partly caved adit that was driven just above and 

parallel to a nearly horizontal contact between siltstone and an underlying trachyte porphyry. A winze, 

now partly filled with debris, extends through the 

siltstone and into the porphyry (fig. 2).

During the 1953 field season the area immediately 

surrounding the deposit was mapped at a scale of 1 inch 

equals 50 feet and sampled. Further reconnaissance 

in the area showed abnormal radioactivity associated 

with an altered lamprophyre(?) dike about 200 feet 

west of the portal and at a small area covered with 

talus about 300 feet northwest of the portal. A 3-foot 

chip-channel sample of the dike rock contained 

0.034 percent equivalent uranium and 0.013 percent 

uranium. The sample was extensively weathered, 

which probably accounts for the difference between 

eU and U content. No appreciably radioactive rocks 

were found at the second anomalous area, but it is 

worthy of note that the anomaly lies on the projected 

contact between the siltstone and the underlying 

trachyte porphyry.

Sampling of the occurrence did not indicate material of ore grade. However, the siltstone is poorly 

exposed, and sample information is available only from 

the limited underground workings and old prospect 

pits. The sample data are tabulated in table 1.

Field mapping indicated a northward- and northeastward-trending system of fractures in the siltstone 

and to a lesser extent in the porphyry below the siltstone. The highest radioactivity was observed where
Contact, showing dip; dashed 

where opproximately located

Strike ond dip of beds

as 

Strike and dip of joints

90

Strike of vertical joints

SL-40-53; 0.018,0.007

-32-53 0.031, 0.028

Portal of odit

o

Prospect pit

Dump

SL-44-53-, 0.034, 

Somple location showing 

equivalent uranium and 

uranium content, in 

percent

X- SL-74 -52; 0.030,0.020 v > 

T >"

Geology and topography by 

Figure 2. Geologic map of the Annie Creek autunite occurrence, Lawrence County, South 
Table 1. Analyses of samples, Annie Creek autunite occurrence, Lawrence County, S. Dak.

[Analyses by U. S. Geological Survey ]

Field no. Percent

eU U

Remarks

SL-31-53 

SL-32-53 

SL-33-53

SL-34-53

SL-35-53 

SL-38-53 

SL-39-53 

SL-40-53

SL-44-53 

SL-50-53

0.013

.031

.043

.060

.013

.022

.012

.018

.034

.090

0.009

.028

.029

.057

.007

.014

.006

.007

.013

.064

4-foot chip-channel sample, across face, northward-trending adit.

4-foot chip-channel sample, across face, northeastward-trending adit.

16-inch channel sample, most radioactive part of face, northeastwardtrending adit.

1-foot channel sample, northeastward-trending adit, 1 foot above 

porphyry-siltstone contact.

Porphyry from winze below contact.

3.5-foot channel sample; pit 50 feet east of portal.

3-foot chip-channel sample, pit 30 feet east of portal.

Dump sample, pit northwest of adit. Also contains O.OX percent thorium 

as determined spectrographically.

3-foot chip channel sample, dike rock.

Grab sample of phosphatic siltstone 10 feet east of portal.

the fractures were more closely spaced in the siltstone. 

Most of the autunite occurred as fracture-fillings inboth 

sets of fractures. The writer believes that the uranium 

was introduced into the siltstone along this system of 

fractures and that the phosphatic and calcareous siltstone 

above the porphyry contact was a favorable environment 

for uranium deposition. The thorium content of sample 

SL-40- 53 (0. OX percent) indicates a genetic relationship 

of the Annie Creek autunite occurrence to the other thoriumuranium occurrences in the area.

Phosphatic nodules found about 10 feet east of the 

portal probably correlate stratigraphically with similar 

nodules which occur near the base of the Whitewood limestone at the type locality on Whitewood Creek. The age 

of the siltstone would then be Ordovician, instead of 

Cambrian as reported by Vicker s.

Because of the lack of good outcrops in the area, 

the limits of uranium occurrence are unknown. The known 

extent of uranium deposition, measured across the general 

strike of the fracture system, is about 220 feet. If uranium-bearing solutions moved up along the altered dike 

as evidenced by the abnormal uranium content of the dike 

rock, then a favorable area of deposition might be the 

siltstone adjacent to or above the dike. Shallow trenching 

would be necessary to confirm this. Shallow trenching 

may also be effective in finding additional mineralized 

siltstone between the underground workings and the dike.

Dakota mine

During a scintillation-counter survey of the 

Dakota mine, NW| sec. 2, T. 4 N., R. 2 E. (fig. 1), 

high scintillation counts were detected near the south 

edge of the 100-foot level. The radioactivity was 

associated with several narrow alteration zones in the 

Deadwood formation of Cambrian age. One 3-inch wide 

alteration zone was sampled and contained 0.19 percent 

equivalent uranium, 0.10 percent uranium, 0. X percent thorium, and 0. 23 ounce gold per ton.

The occurrence is of interest because a system 

of "verticals" at this locality strikes toward the Annie 

Creek autunite deposit, which is about 2, 700 feet to 

the southwest.

Although economic quantities of ore-grade material were not observed, only a small part of the underground workings were examined. A detailed scintillation-counter survey of the mine and underground 

mapping of the occurrences are planned for the 1954 

field season.

The Dakota mine is now being worked for gold 

by the Bald Mountain Mining Co.

Marie and Mary claim

An examination of the abandoned prospects between the Annie Creek autunite occurrence and the 

Dakota mine was made, and several specimens from a 

dump on the Marie and Mary claim were found to be 

abnormally radioactive (fig. 1). The workings consist of a caved adit which was probably about 60 feet 

long. A sample of silicified siltstone from the property contained 0.041 percent equivalent uranium, 

0.007 percent uranium, and 0. X~ percent thorium. 

This deposit is about 500 feet southwest from the surface projection of the occurrence in the Dakota mine.

Decorah mine

During a scintillation-counter survey of part of 

the Decorah mine, abnormal radioactivity (as much 

as 2.0 mr/hr) was detected near a rhyolite porphyry 

dike located about 200 feet from the incline portal. 

A grab sample of the dike contained 0.025 percent 

equivalent uranium, 0.008 percent uranium, and 

O.OX percent thorium. The sample consisted mainly 

of altered rhyolite porphyry containing abundant clay 

minerals and limonite.

Mikado claim

Local concentrations of radioactive material were 

found in siltstone near a sheared contact with a porphyry 

dike in a railroad cut on the Mikado claim, NE-^ sec. 1, 

T. 4 N., R. 2 E. A selected sample contained 

0.053 percent equivalent uranium, 0.009 percent 

uranium, and 0.16 percent thorium.
Diatreme

Specimens of radioactive altered diatreme material were found on an old dump about 4,000 feet.northeast of Trojan, S. Dak. Analyses of three samples 

are listed below.

CT OC C"3t

SL- 30-53 

SL-51-53 r 

Perc

eU

O.Qlt-5

.035

.032

ent

U

0.006

.010

.007

Th 1

0 X"

n.d.

.OX+

1 Determined spectrographically.

Thin sections of the diatreme material show that 

it is composed mainly of a breccia of pre-Cambrian 

rocks, such as graphitic schist and quartzite, cemented by a rhyolite porphyry. Small amounts of fluorite 

occur as cavity fillings in the sections.

No outcrops of the diatreme material were observed in the vicinity of the occurrence, but these outcrops about one-half mile to the north exhibited 

no anomalous radioactivity. The radioactive material was probably introduced after intrusion of the 

rock.

Dark Horse claim

At the Dark Horse claim, NE^ sec. 1, T. 4'N., 

R. 2 E., abnormal radioactivity was found associated 

with manganese-filled fractures in a shear zone 

which is partly exposed at the end of an adit 10 feet 

long. Selected samples of the |- to |-inch wide 

manganese fracture-fillings contained 0.026 percent 

equivalent uranium and 0.017 percent uranium. 

(See table 2.)

CONCLUSION

Although minable amounts of ore-grade radioactive material have not been found in the Bald Mountain gold-mining area, the occurrences are of interest 

because they extend the limits of known thoriumuranium mineralization within the Black Hills area.

The results of chemical and spectrographic 

analyses of eight samples are shown in table 2. The 

analytical data indicate a progressive increase in the 

raj-e^-earth content from the Annie Creek autunite 

deposit to the vicinity of Bald Mountain, which is near 

the center of the gold-producing area. Although conclusive evidence is lacking, the Annie Creek autunite 

deposit is believed to be on the fringe of the goldproducing area with uranium having been deposited 

slightly later than gold. The Ross-Hannibal mine 

(see p. 2-3) is also on the fringe of the productive 

area.

All the known radioactive occurrences are in 

the zone of weathering and have been found only in 

underground workings or on old mine dumps. Pyrite, 

or its weathering products, is present in all the 

samples, and differential leaching probably has disturbed the thorium/uranium ratios because of the 

greater mobility of uranium during weathering.

In the Bald Mountain gold-mining area most of 

the ore that has been mined and most of the mine 

workings have been in oxidized rock. Because the 

uranium is more readily transported during weathering in a pyrite gangue, minable quantities of uranium 

would probably occur only in relatively unoxidized 

ore. The torbernite at the Ross-Hannibal mine is 

reported to have occurred in large quantities in unoxidized ore (Irving, 1904, p. 139-140), but no 

mention is made of any positively identified primary 

uranium mineral.

Table 2. Analyses of samples from Bald Mountain gold-mining area, Lawrence County, S. Dak.

[Analyses by U. S-Geological Survey. Spectrographic notations, in percent: O.OX"= 0.01-0.03; O.OX = 0.04-0.06; O.OX+ =0.07-0.09]

Field no.

SL-34-53

SL-40-53 

SL-48-55

SL-46-53 

SL-47-53 

SL-51-53

SL-25-53 

SL-27-53

Location and typ$ of sample

Annie Creek:

Autunite-bearing slltstone 

Marie and Mary claim: 

Silicified siltstone 

Dakota mine:

A ~] i~oT*orl Q~f T "f'Q'HrM'-io

De corah mine: 

Altered porphyry 

Bald Mountain:

A "I "HoT»oH H "1 Q "HTV^ttio _ _

________ do 3 

Dark Horse claim:

Mo v^rffln^ CIO T7"O"? Tl~l O + Q

Percent

eU

0.060 

.018

.041 

.19 

.025

.052 

.045

.026

U

0.057 

.007

.007 

.10 

.008

.007 

.006

.017

Th. 1

O.OX 

,X~ 

.X 

.OX 

.OX4" .xTotal 

rare 

earths 2

O.OX .ox-

.OX 

.X~ 

.X~ 

.X~

.xX.0~

Determined spectrographically.

2 Sum of rare earths, which were determined spectrographically.

3Graphitic schist and quartzite breccia cemented with rhyolite porphyry.
The presence of uranium, thorium, and rare 

earths in these occurrences suggests a genetic relationship to the thorium-uranium-rare earth deposits 

of the Bear Lodge Mountains about 30 m'les to the 

northwest. These deposits were described (1953) in 

an unpublished report by Wilmarth and Johnson.

The radioactive occurrences in the Bald Mountain mining area are of special interest because of 

their possible relationship to the carnotite-type 

deposits .n the sediments surrounding the Black Hills. 

The small scattered occurrences of hydrothermal 

uranium in the Bald Mountain area may reflect local 

conditions which precipitated uranium. Most of the 

uranium may not have been depos'ted but remained 

in solution to be mixed with and distributed by ground 

water.

LITERATURE CITED

Oonnolly, J. P., 1925, Geology and mineralogy of the

Keystone district [Pennington County, S. Dak.]:

Black Hills Engineer, v. 13, no. 1. 

_______ 1927, The Tertiary mineralization of the

northern Black Hills: S. Dak. School of Mines

Bull. 15. 

Darton, N. H., and Paige, Sidney, 1925, Description

of the central Black HUIs: U. S. Geol. Survey Geol.

Atlas, folio 219.

Furnish, W. M., Barragy, E. J., and Miller, A. K., 

1936, Ordovician fossils from upper part of type 

section of Deadwood formation, South Dakota: 

Am. Assoc. Petroleum Geolog sts Bull., v. 20, 

no. 10, p. 1329-1341.

Irving, J. D., 1904, Economic resources of the 

northern Black Hills, with contributions by S. F. 

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INT.-DUP. SEC., WASH.,