GEOLOGICAL SURVEY CIRCULAR 286

AN OCCURRENCE OF AUTUNITE

- LAWRENCE COUNTY, SOUTH DAKOTA
UNITED STATES DEPARTMENT OF THE INTERIOR 

Douglas McKay, Secretary

GEOLOGICAL SURVEY 

W. E. Wrather, Director

GEOLOGICAL SURVEY CIRCULAR 286

AN OCCURRENCE OF AUTUNITE, LAWRENCE COUNTY, SOUTH DAKOTA

By R. C. Vickers

This report concerns work done partly on behalf of the

U. S. Atomic Energy Commission 

and is published with the permission of the Commission

Washington, D. C.,1953

Free on application to the Geological Survey, Washington 26, D. C.
AN OCCURRENCE OF AUTUNITE, LAWRENCE COUNTY, SOUTH DAKOTA

By R. C. Vickers

CONTENTS 

Page

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

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

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

History of the property.................... 2

General geology............................... 2

Uranium deposits.............................. 2

Page

Uranium deposits Continued

Mineralogy. ............................. 2

Ore bodies.............................. 5

Origin.................................. 5

Conclusions.................................. 5

Literature cited .............................. 5

ILLUSTRATIONS

Page

Figure 1. Index maps showing location of the autunite prospect, Lawrence County, S. Dak.................... 3

2. Geologic map and cross section of the autunite prospect ......................................... 4

TABLES

Table 1. Radiometric and chemical analyses of samples from the autunite occurrence,

Lawrence County, S. Dak................................................................... 2

2. Semiquantitative spectrographic analyses of siltstone samples from the autunite occurrence......... 5

ABSTRACT

In July 1952 an occurrence of autunite was found 

in the northern part of the Black Hills, South Dakota, 

during a reconnaissance for radioactive deposits.

The autunite occurs as fracture coatings and 

disseminations in siltstone of the Deadwood formation 

of Cambrian age and is concentrated mainly in the 

lower 2 feet of the siltstone at the contact with an intrusive rhyolite porphyry; the radioactive zone is 

exposed in two old workings, which are 90 feet apart. 

An 18-inch vertical channel sample of the autunitebearing siltstcne contained 0.048 percent uranium. 

The gangue minerals are fluorite and limonite.

The uranium is believed to have been introduced 

into the siltstone by solutions of magmatic origin that 

migrated along th^ L'wer contact of the siltstone after 

or during emplaceme-.t of the porphyry.

INTRODUCTION

Uranium minerals have been reported in the 

northern Black Hills by several authors. Uranium 

was iirst n.entloned by Scott (1897), who reported 

a uraniam rrineraL with the composition of z^unerite 

from the P> iscned Ox mine near Pactola, 

Pennington County, S. Dak., and along Annie Creek

at Carbonate, Lawrence County. O'Harra (1902, 

p. 78) states:

Uranium minerals have been observed in various places, 

particularly In the Poisoned Ox mine near Paccola and 

in the Davier mine on Anna Creek near Portland. Uranium 

is also reported as occurring near Carbonate. At the 

Poisoned Ox mine the minerals occur 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.

Ziegler (1914, p. 206) described autunite 

(Ca(UO 2)2(PC>4)2.10-12H 2O)from an occurrence in the 

phonolite porphyries on Annie Creek west of Portland, 

Lawrence County, S. Dak. His description follows:

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

as a very thin micaceous coating or a series of thin 

irregular 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 lemon-yellow 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.

Although specimens of autunite from the northern Black Hills may be seen in the museum at the
South Dakota School of Mines, Rapid City, S. Dak., 

no mention has been made of the Annie Creek ccurrence, 

so far as known, since Ziegler's report in lc-14.

Because of these reported occurrences of radioactive minerals, a radiometric reconnaissance of ..he 

mining districts of the northern Black Hills was conducted by the U. S, Geol gical Survey during July 1952 

on behalf of the Division of Raw Materials of the 

U. S. Atomic Energy Commission. During this Investigation, an occurre1 ce of autunite was f ound in the 

Annie Creek area near Trojan, Lawrence County, S. Dak.

The autunite observed by O'Harra (1902. p. '3), 

and by Zeigler (1914, p. 206) is described as occurring 

as thin coatings on f±. sures in porphyry, whereas the 

autunite described in this report is in siit?tone. It is 

the writer's belief, therefore, that although both occurrences are in the same general area, they probably 

are not the same deposit.

Location and accessibility

The autunite occurrence is 5 miles west of Lead 

in the Bald Mountain mining district, Lawrence 

County, S. Dak., in sec. 3, T. 4 N., R. 2 E., Black 

Hills Meridian (fig. 1). Th^ property may be reached 

by following the abandoned riyht-of-way of the Chicago, 

Burlington, & Quincy Railroa . north from Elmore, S. Dak., 

for about 2 miles to the end of the railroad fill that 

crosses Annie Creek, and tr.er by walking aiong the 

abandoned Annie Creek road for - distance of about 

two-thirds of a mile to the firs- gulch c- the right, northeast of Lost Camp Gulch. From there, proceed 

N. 70° E. up the bare ridge on the southeast s'de of 

Annie Creek, for a distance of 'bou o e-third of a mi e 

to the prospect site (fig. 1).

History of the prop*- rty

The property was located about 1890 for gold 

and silver. Fluorite and, to a lesser extent, uranium 

minerals are associated with some of the g' id ores of 

Tertiary age in the Bald Mountain mining district, and 

the presence of fluorite and autunite on the pr pert,, 

probably encouraged the owners to explore the prospect 

further. There is no reported production from the 

property, and the workings were probably aba- ' i ed 

soon after the underground work failed to sho.v my 

g! Id or silver.

The prospect is on patented ground believed to 

be owned by the Annie Creek Mining Company, "cut it

was r- located for uranium by L. J. Bisch,

R. B Sherman, O. B. Sherman, and F. S. Budzynski

on July - , 1952.

GENERAL GEOLOGY

The autunite occurrence is in a area of widespre^d Tertiary igneous activity in the northwestern 

part f the Black Hills uplift. The rocks in the vicinity 

of the i rop^rty are sandstones, siltstones, and shales 

of tne DeadwL od formation of Cambrian age, which have 

been intruded and displaced by several sills and 

laccoliths.

In tne immediate vicinity of the prospect a 

30-foot thick bed of nearly flat-lying siltstone of the 

Deadwc od formation is overlain by rhyolite and underlain by rhyclite porphyry (fig. 2). The siltstone is 

composed mainly of angular to subangular quartz grains, 

averaging 0. U5 mm inlength, and interstitial carbonate, 

which is distributed unevenly through the rock.

URANIUM DEPOSITS

\ radiometric traverse of the property with a 

Geiger " junter showed abnormal radioactivity on the 

dump of the anderground workings and at the two 

prospect pits northwest of the adit. Autunite was 

observed in th underground workings and on the large 

dump. The results of radiometric and chemical 

analyses of the collected samples are given in table 1. 

The sample localities are shown in figure 2.

The w Tkings consist of a partly caved adit about 

45 eet long, a 1 '-foot winze, and several prospect 

p ts. The adit wts driven about 1 foot above, and 

parallel to, the c ntactof the siltstone of theDeadwood 

formation witt he underlying porphyry. The winze, 

now partly fi.led w.th debris, extends through the 

siltstone and into the porphyry.

Mineralogy

Autunite, the only uranium mineral recognized 

a the property, is ap: _e-green and highly fluorescent; 

it forms finely crystaltine masses and well-crystall zed 

plates, as much as 5 mm in length. The gangue 

minerals are purple fluorite and limonite. Semiquan 

titative spectrographic analyses of four samples 

(table 2, see p. 5 indicat that minor quantities of 

cesium, cobalt, molybder , niobium lanthanum, and 

tin are concentrated in the mineralized siltstone

Table 1. Radiometric and chemical analyses of samples from the autunite occurrence, Lawrence

County, S. Dak.

[Analyses by U. S. Geological Survey Trace Elements Laboratory, Denver, Colorado]

Fi-ld 

number

si -74-52 

SL-75-52

SL-76-52 

SL-77-52 

SL-78-52

Equivalent 

uranium 

(percent)

0.030 

.059

.012

.037 

.008

Uranium 

(percent)

0.020 

.048

.004 

.016 

.004

Remarks

Composite dump sample 

l8-inch vertical channel sample of au unite-bearing 

siltstone. 

Rhyolite porphy y from winze. Grab sample. 

Grab sample of siltstone from dump of ^ro pect pit no. 1. 

Grab sample of siltstone, prospect pit o. 2
SOUTH

LAWRENCE 

COUNTY

i DAKOTA 14

LAWRENCE

Deadwood

i

Lead i

OF DETAILED

MAP

COUNTY

L.

Figure 1. Index maps showing location of the autunite prospect, Lawrence County, S. Dak.

3
SL-76-52 

0.012-0.004

SL-75-52 

0.059^0.048

UNDERGROUND 

WORKINGS 51^78-52 

0.008;0.004

PROSPECT PIT 2

^PORTAL OF ADIT

EXPLANATION

Deadwopd formation 

Si Its tone

o

Approximate contact

,5

Strike and dip af beds

IX ^

Head of winze

Prospect pit

SL-77-52 

0.037; 0,016

Sample location, 

showing percent 

equivalent uranium 

and percent uranium

Tape and compass map, July 1952 Geology by R.C. Vickers

SL-77-52 and Jacob Freedman 

PIT I ^/Q.037-,0.016 ^-inferred autunite-bearing zone

SL-75-52 , 0.059-,0.048

A ^^^' '-^ ^ ^- -"'"« / ^"""-W PIT 2 " -"^ // s, <\ -J-A-- _ . . ,-. :-.i.... ..-r ^^-^ _. __ _

SL-78-52 

_ °*QC>8; 0.004

30 90 Feet

Contour interval 20 feet

Datum assumed

Figure 2. Geologic map and cross section of the autunite prospect, Lawrence County, S. Dak.

4
Table 2. Semiquantitative spectrographic analyses of siltstone samples from the autunite occurrence,

Lawrence County, S. Dak.

[Analyses by U. S. Geological Survey Trace Elements Laboratory, Denver, Colorado]

Field 

number

SL-74-52

<3T 'TC CO

Ce

OY

Co

.OOOX

Mo

.OOX 

.OOX

Nb

.OOOX

OOOY

La

OY

Sn

.OOX 

.OOX

Chemical 

uranium 

(percent)

0.020 

.048

Field 

number

SL-77-52

QT TQ CO

Ce

 

Co

.OOOX

Mo

.OX

Nb

.OOOX

La

 

Sn

.OOX

Chemical 

uranium 

(percent)

0.016 

.004

relative to their concentrations in unmineralized 

siltstone.

Thin sections of the autunite-bear ing siltstone 

show that the quartz grains and the interstitial carbonate 

have been corroded and embayed by autunite and 

fluorite. The fluorite occurs mainly as small veinlets, 

as much as 3 mm thick, that cut the siltstone and as 

small irregular replacements with distinct crystal 

outlines.

The abundance of limonite associated with the 

fluorite and autunite suggests that pyrite is present in 

the unoxidized material and that the autunite has resulted 

similarly by the oxidation and alteration of a primary 

uranium mineral, possibly pitchblende.

Ore bodies

The autunite is concentrated mainly in the lower 

2 feet of the siltstone of the Deadwood exposed in the 

adit, and it is believed to form a tabular, essentially 

flat-lying deposit in the siltstone just above the porphyry 

contact (fig. 2). An 18-inch vertical channel sample 

(SL-75-52), cut just above the porphyry-siltstone contact, contained 0. 059 percent equivalent uranium and 

0. 048 percent uranium. A grab sample of porphyry 

from the winze (SL-76-52) contained 0.012 percent 

equivalent uranium and 0. 004 percent uranium, which 

is similar to the radioactive content of other similar 

porphyries in the northern Black Hills.

Prospect pit 1, 90 feet northwest of the portal, 

may be an old caved shaft that reached the autunite zone 

(cross section, fig. 2). A grab sample of siltstone 

from the dump of this prospect (SL-77-52) contained 

0. 037 percent equivalent uranium and 0. 016 percent 

uranium. No autunite was observed on this dump, but 

abundant limonite and fluorite were present.

Slight abnormal radioactivity was detected in 

siltstone at prospect pit no. 2, located 20 feet east of 

the portal, and a grab sample (SL-78-52) analyzed 

0. 008 percent equivalent uranium and 0. 004 percent 

uranium.

The difference between equivalent uranium and 

chemical uranium in the siltstone samples suggests 

that some oxidation and leaching have occurred and also 

suggests that the near-surface samples are probably of 

lower uranium content than the unoxidized material.

The extent of the autunite zone is not known 

accurately because of the lack of suitable exposures 

in the area. The zone probably extends continuously 

from the exposures in the underground workings to the 

mineralized siltstone encountered in pit no. 1.

The writer believes, because of the spatial relations of the uranium minerals to the porphyry sill, 

that the uranium was introduced into the'siltstone by 

hydrothermal solutions that moved along the contact 

between the siltstone and the underlying porphyry. The 

fractured siltstone near the contact with the lower 

porphyry, which may be the result of movement along 

the contact produced after or during the emplacement 

of the porphyry, formed a permeable zone through 

which the ore-bearing solutions could migrate. These 

solutions probably were related to a late stage of 

hydrothermal activity that accompanied the intrusion 

of the porphyries.

CONCLUSIONS

The occurrence of autunite associated with the 

Tertiary intrusive activity in the northern Black Hills 

suggests that the area should be considered as a 

possible source of uranium deposits. Although the 

deposit described in this report is not of ore grade, 

additional prospecting in the area might disclose higher 

grade deposits or suffiqient tonnages of low grade 

material to make mining profitable.

This occurrence of autunite confirms one of the 

uranium mineral localities mentioned in the literature, 

and additional prospecting could very well confirm 

other uranium-bearing localities that have been reported 

i'n the literature on the northern Black Hills mining 

area before 1915.

LITERATURE CITED

O'Harra, C. C., 1902, Mineral resources of South 

Dakota including mineral wealth of the Black Hills: 

South Dakota Geol. Survey Bull. 3.

Scott, S. E., 1897, Map of the Black Hills of South 

Dakota and Wyoming with full descriptions of 

mineral resources, Philadelphia, E. P. Noll Co.

Zeigler, Victor, 1914, The minerals of the Black Hills: 

South Dakota School of Mines Bull. 10.

IHT.-DUP. SEC.. WASH., D.C.