URANIUM ROLL-FRONT DEPOSITS IN THE SOUTHERN BLACK HILLS, SOUTH DAKOTA

N. SCOFIELD, S. FAIRCLOTH

Institute of Mineral Deposits,

South Dakota School of Mines and Technology, Rapid City, South Dakota

B. BLAKE

Earth, Water and Air Resources, Minneapolis, Minnesota

J.C. LAUL

Battelle Pacific Northwest Laboratories, Richland, Washington

United States of America

Abstract

In the southern Black H i l l s of South Dakota, uranium roll-type deposits are present in early Cretaceous Lakota Formation sandstone.

Analysis of a 45-foot d r i l l core, representing a vertical section through the nose of the roll-front, by scanning X-ray fluorescence

(XRF), standard XRF, X-ray diffraction (XRD), organic petrography, electron microprobe, and scanning electron microscopy (SEM) is underway.

The core is 1ithologically composed of an orthoquartzite with

varying amounts of chert and consists of an oxidized zone, four reduced zones, and three ore zones. The oxidized zone is characterized by medium- to fine-grained, well-sorted, well-rounded orthoquartzite with minor hematitic kaolinite weakly cementing and coating quartz grains.

Reduced zones have angular to rounded, medium- to fine-grained gray to black quartz grains; coarse-grained, angular, white to black chert; thin

laminae and small isolated fragments of carbonaceous material with

pyrite. Ore zones are composed of trough cross-bedded, poorly-cemented, well-sorted mixtures of rounded quartz grains and angular to rounded chert with yellow to olive-green uranium- and vanadium-rich clays, and carbonaceous material with pyrite.

The chemistry of the entire core was determined by scanning X-ray fluorescence for 46 elements. In the oxidized, reduced, and ore zones, uranium and vanadium averaged 40 ppm U and 3300 ppm V, 800 ppm U and 3000 ppm V, and 3800 ppm U and 5000 ppm V, respectively.

No uranium or vanadium minerals have been found by XRD or microprobe analysis. However, certain segments of the core are enriched in U and V, including those with high clay content. Preliminary SEM analyses have shown smectite, kaolinite, and possibly i l l i t e as coatings on quartz grains. U and V are present in some of the clay coatings, and probable carnotite and an authigenie vanadium mineral were found.

Carbonaceous laminae are present in the reduced and ore zones and commonly contain pyrite with associated uranium.

1.1. Introduction

Although the uranium roll-front deposits of the southern Black H i l l s of South Dakota were mined successfully for a number of years, some fundamental questions remain concerning 1) how and in what minerals or other constituents the uranium is present, 2) what the source(s) of the uranium is(are), and 3) what is the relationship of these deposits to the regional geology of the Black H i l l s . Uranium roll-front deposits in other parts of the United States (Rackley, 1976) have been subjected to detailed and thorough study, and their origin and development is well understood. However, those in South Dakota have been relatively

neglected. In a general way, they appear very much like other deposits, but in detail, there are significant differences in sedimentology, in age, in mineralization, and in relationship to the regional geologic setting. In the southern Black H i l l s , uranium roll-front deposits are

in braided channel sandstones of Cretaceous age with only a minor

feldspar component. Clay minerals appear to have been a favored site of uranium precipitation, and vanadium is a significant component.

1.2. Geologic background of area

The early Cretaceous Inyan Kara Group, which is divided into the older Lakota Formation and the younger Fall River Formation, is host to these deposits. The Lakota Formation makes up the lower two-thirds of the group and is 200-500 feet thick. The Lakota Formation is composed mainly of orthoquartzites and feldspathic orthoquartzites which were derived from preexisting sedimentary rocks. The F a l l River Formation contains a significantly higher proportion of angular detrital material from igneous and metamorphic rocks. The Inyan Kara Group was deposited in a variety of continental environments, with the dominant environment being a fluvial system characterized by northwesterly-flowing streams

(Gott and others, 1974; Chisholm, 1963).

During the late Cretaceous, the Black H i l l s area was uplifted into a domal structure and subsequently eroded to expose a Precambrian igneous and metamorphic core (Fig.l). As a result of this uplift, fractures developed and groundwater moved through them and into aquifers of several formations. According to Gott and others (1974), this moving groundwater dissolved evaporites in the Permian Minnelusa Formation, and breccia pipes resulted from subsidence within the Minnelusa Formation.

These breccia pipes extend upwards into the Inyan Kara Group and form part of a complex "plumbing" system through which fluids circulate.

These fluids carry low concentrations of uranium. Uranium precipitates under reducing conditions, and roll-front type deposits are present in channel sandstones of the Inyan Kara Group at the interface between oxidizing and reducing environments. This system of channel sandstones changed course along its length and through time and produced deposits of complex geometry.

1.3. Research to date

1.3.1. Core and sample selection

A 45.5-foot d r i l l core from the Lakota Formation (PT-152; Fig. 2) was selected for detailed study. This particular core was selected because 1) it is complete, 2) it exhibits a variety of faciès, and 3) it

Harney Peak Monocline: arrowhead Granite shows vergence

Precambrian Foliated and

Igneous Rocks Fault 1000 Structure contour Basal contact of l'ne^: base of C r e t a c e o u s r o c k s Cretaceous Fall

River Formation

\STUDY AREA

\ ^ v

^ \ \

Figure 1. Tectonic map of the Black H i l l s , South Dakota, Wyoming and Montana (from Rich, 1981).

is from the approximate roll-front. A diagram of the core is shown in Figure 3 and indicates the methods of analysis used at various core

intervals and lithofacies or zones within the core.

7581759

SCALE___

600 0 6001 ZO01800 feet

Figure 2. Location of uranium roll-front deposits, southern Black H i l l s , South Dakota.

1.3.2. Pétrographie description

The 45-foot long core is composed predominantly of orthoquartzitic sandstone that has undergone various stages of alteration. The sand-stone ?s underlain and capped by an Impermeable fine-grained unit at the base and top. Three principle lithofacies units have been recognized along the length of the core (Fig. 3). These have been classified as reduced, oxidized, and ore zones. Because of the complex geometry of the channel deposits and the fluctuations in the roll-front, the core repeatedly passes through the oxidized, mineralized, and reduced litho-facies or zones.

Figure 3. Diagram of 45-foot core, showing lithofacies zones, types of analyses, and location of samples analyzed.

1. Reduced zones

The reduced sandstones consist of over 95 percent fine- to medium-grained subrounded to subangular quartz with minor proportions of clay and chert as matrix. All the sandstone along the core is

clast-supported. Also present are minor amounts of coaliferous material plus epigenetic pyrite.

2. Oxidized zones

The oxidized sandstones are composed of barren orthoquartzite with no discernible organic material or pyrite. The oxidized core has a reddish tinge to it due to a hematitic coating on quartz grains. Less chert is present than in the other two zones.

3. Ore zones

The ore zones of the core, which contain most of the trace and minor elements, also consist of orthoquartzite but with a substantial and varied amount of clay, chert, pyrite and organic matter. The ore zones are easily recognizable by their yellow-green coloration due to uranium ox i dat i on.

1.3.3. X-ray fluorescence (XRF)

Core PT-152 was transported to Battelle Pacific Northwest

Laboratories, where the entire core was subjected to scanning X-ray fluorescence (XRF) for more than 40 elements. These continuous chemical analyses of the core were followed by standard XRF analyses of selected samples. Sample selection for standard XRF was based on the scanning XRF data.

Elemental concentrations are given in terms of ranges and means for selected elements from this core in Figure 4. Figure 5 is a schematic diagram of part of the core showing the correlation between uranium and vanadium. It can be seen that, in general, uranium and vanadium vary sympathetically although not uniformly.

U V

Ti As Fe Se

ox.

l ORE i RED.

RANGE MEAN

ORE RED.

1 10 100 1000 10 10*

CONCENTRATION (ppm)

Figure 4. Diagram of overall ranges and means of concentration of uranium and related elements, in ppm, for oxidized (OX), reduced (RED), and ore zones.

20

°

376 377 '385 386 387 388

D E P T H ( F T . )

389 390 391 392

Figure 5. Diagram of XRF scan of PT-152 for uranium and vanadium.

1.3.4. X-ray diffraction (XRD)

X-ray diffraction (XRD) shows interstitial material that consists predominantly of kaolinite plus montmori1lonite, i l l i t e , quartz, gypsum, pyrite, and orthoclase. Diffraction patterns from powdered samples of two ore zones show the principal minerals to be montmori1lonite,

kaolinite, quartz, pyrite, gypsum and, possibly, jarosite. Uranium minerals are conspicuous by their absence.

1.3.5. Organic petrography

Several polished pellets containing coaliferous sandstone were point-counted with a reflecting Zeiss photometer. The most abundant macérai was vitrinite with lesser amounts of semi-fusinite and

resins. This reflected light microscopy also illustrated the abundance of framboidal and matrix-forming epigenetic pyrite within the organic material. Electron microprobe analyses of pyrite showed significant concentrations of uranium and/or titanium.

1.3.6. Scanning electron microscopy (SEM)

Several fragments of sandstone from the ore zones were examined by SEM. Results showed:

A. A mineral between the layers of kaolinite that apparently contains uranium. The overall spectrum contains uranium, but its presence in the inter layer mineral could not be absolutely confirmed (Fig. 7-).

The kaolinite appears "chewed up", consistent with radiation damage.

B. The presence of clay films on detrital quartz grains. Figure 6 shows a quartz grain with multiple coatings representing the

paragenetic sequence of a smectite, partially covered by kaolinite, with remnants of possible i l l i t e on the kaolinite. Uranium and/or vanadium is concentrated in scattered sites, especially in the kaolinite.

Figure 6. SEM micrograph, bar scale = 10 microns. Quartz grain with coating of smectite (upper part), partially covered by kaolfnite (lower part).

C. The presence of uranium in carbonaceous bands in the sediments. A uranium distribution map from within a coaliferous band indicates an organic structure with the uranium concentrated preferentially in certain parts of the c e l l structure.

D. An authigenie vanadium mineral surrounded by authigenie potassium feldspar.

E. An authigenie U-V-Th-Ti mineral on a smectite coating on a quartz gra i n.

F. Some detrital clay with a major potassium and a minor calcium peak.

G. Considerable authigenic potassium feldspar in several samples.

2.1. Synthesis

Despite a wealth of chemical data, finding where and how the uranium and related elements are present in these deposits has been elusive.

This was due largely to the absence of uranium-bearing minerals in the XRD patterns, the difficulties in the preparation of polished thin

section which removed a good deal of the material interstitial to quartz grains, and the complex geometry of the roll front. That no uranium-bearing phases were detected by XRD suggests that uranium is present I)

in the carboniferous component as in other roll front deposits, 2) in amorphous or very fine or poorly crystalline phases, and/or 3) in or adsorbed onto some of the clay material that was detected. It is

important to note that only a small fraction of one percent of uranium-bearing mineral would be detectable by XRD due to the high absorption of uranium.

However, with the addition of the very small amount of SEM analysis performed, a picture of these deposits is beginning to emerge. The major cementing material in these sandstones is clay. Several clay minerals—smectite, kaolinite, and i l l i t e are present and may represent

Figure 7 A and 78. SEM micrographs, bar scales = 1 0 , 1 microns in A and B respectively. Uranium-vanadium mineral sandwiched between kaolinite

layers.

WS • •»•* M.I

Figure 1C. Overall EDS spectrum of area shown In 7A and 78.

different paragenetic intervals. The layers of different clay minerals are evidence of radically different pore waters. Clay phases were

identified by their morphology and composition on the SEM. Kaolinite, montmori1lonite (smectite), and i l l i t e were confirmed by XRD.

Uranium is associated with these clays. Because no uranium-bearing phases were found by XRD, uranium as an inter layer cation in clay is a possibility. However, the apparent preferential association of uranium with kaolinite (Fig. 6) makes it unlikely. The crystal structure of kaolinite, including the lack of an unbalanced site charge, probably cannot accommodate uranium as an inter layer cation. The morphology of the kaolinite, as observed in the secondary electron image (Fig. 7) is not typical of kaolinite and suggests interlayering with another

mineral. The overall spectrum supports the probability that this other mineral is uranium-bearing. If this is true, the latter mineral should be detectable in an XRD powder pattern. This needs to be investigated along with how the uranium affects the XRD pattern of the clay with which it is associated.

In many roll-type uranium deposits, carbonaceous material and/or pyrite have facilitated reduction and precipitation of uranium. In this deposit, pyrite was observed megascopically within pieces of charcoal in coarse-grained layers in the core. Uranium was consistently detected when analyzing authigenie pyrite by microprobe. The uranium

distribution map within a coaliferous lamina shows a preference for uranium to be deposited in certain parts of a cellular structure. The association of the uranium with the pyrite within the carbonaceous material suggests that the precipitation of uranium was associated with the su Ifidization of the coaliferous material.

Authigenie uranium and vanadium minerals were also detected by SEM analysis. They are very fine-grained, and the difficulty in finding them suggests that they are rare components of the ore zones. These factors could account for the lack of detection by XRD.

Drastic changes in the chemistry of the pore waters suggested by the different clay layers on quartz (Fig. 6 ) are also indicated by the presence of detrital potassium feldspar showing partial dissolution and by the presence of authigenie potassium feldspar.

In summary, these deposits have undergone a complex history which produced radical changes in pore waters, presumably in phase with the migration and fluctuation of the roll-front and its ore zone. Uranium was precipitated when and where conditions were favorable, and this apparently was during the deposition of kaolinite and the sulfidization of carbonaceous material. Authigenic uranium and vanadium minerals formed but make up only an extremely minor proportion of the existing ore zones.

ACKNOWLEDGEMENTS

From June through September, 1985, Bonnie Blake was on a fellowship with the Northwest College and University Association for Science

(NORCUS)/Department of Energy at Battelle Laboratories in R ich land, Washington. The Battelle facilities are state-of-the-art and have a unique capability in the area of scanning X-ray fluorescence. To J.C.

Laul and to Richard Arthur, she is particularly grateful for help in all aspects of this work.

Susan Faircloth would like to acknowledge the support received as a Mining and Minerals Research and Resources Institute (MMRRI) Fellow from July 1, 1986, through June 30, 1987.

REFERENCES

[1] CHISHOLM, W.A., "The Petrology of Upper Jurassic and Lower Cretaceous Strata of the Western Interior", Wyo. Geol. Assn. -B i l l i n g s Geol. Soc. Guidebook, Joint F i e l d Conference, (1963) 71.

[2] GOTT, G.B., et al., "Stratigraphy of the Inyan Kara Group and

Localization of Uranium Deposits, Southern Black H i l l s , South Dakota and Wyoming", U.S. Geol. Surv. Prof. Paper 763, (1974).

[3] Rackley, R.I., "Origin of Western-States Type Uranium

Mineralization", Handbook of Strata-Bound and Stratiform Ore

Deposits, II. Regional Studies and Specific Deposits, 7 (1976) 89.

[4] Rich, F.J., ed., "Geology of the Black H i l l s , South Dakota and Wyoming", Geol. Soc. America, Rocky Mountain Section, 1981 Annual Meeting, Field Trip Guidebook, 221 p.

HEMATITE-ENRICHED SANDSTONES AND CHROMIUM-RICH