A

Uranium Deposits in 

Oolitic Limestone Near 

Mayoworth, Johnson 

County, Wyoming

GEOLOGICAL SURVEY BULLETIN 1030-K

This report concerns work done on behalf 

of the U. S. Atomic Energy Commission 

and is published with the permission of the 

Commission.

Uranium Deposits in 

Oolitic Limestone Near 

Mayoworth, Johnson 

County, Wyoming

By R. R. GUILINGER and P. K. THEOBALD

CONTRIBUTIONS TO THE GEOLOGY OF URANIUM

GEOLOGICAL SURVEY BULLETIN 1030-K

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 GOVERNMENT PRINTING OFFICE, WASHINGTON : 1957
UNITED STATES DEPARTMENT OF THE INTERIOR 

Fred A. Seaton, Secretary

GEOLOGICAL SURVEY 

Thomas B. Nolan, Director

For sale by the Superintendent of Documents, U. S. Government Printing Office 

Washington 25, D. C. - Price 15 cents (paper cover)
CONTENTS

	Page 

Abstract________________________________________________________ 335

Introduction._____________________________________________________ 335

Stratigraphy._ ____________________________________________________ 335

Structure_________________________________________________________ 338

Uranium deposits_____-__-__-___-________-_-__-__-__-__-_-___-_-_ 340

Mineralogy and mode of emplacement ___________________________ 340

Economic potential-___________________________________________ 342

Literature cited_______________________________________________ ._ ... 342

ILLUSTRATIONS

FIGURE 62. Map showing distribution of radioactive localities in the Sundance formation near Mayo worth. ______-_____-_--_-____ 336

63. Photomicrograph of limestone showing radial and concentric

oolite growth___--__-___________-__-__-__-_-__----_-__ 337

64. Photomicrograph of limestone showing a metatyuyamunitefilled fracture_____-_--__-__-_--_-_____-__-_--____--_ 338

65. Map showing lithologic units and structure of part of the Sundance formation near Mayo worth- ______________________ 339

66. Photomicrograph of limestone showing opaque uranium (?)

mineral veining an oolite_______________________________ 341

m

CONTRIBUTIONS TO THE GEOLOGY OF URANIUM

URANIUM DEPOSITS IN OOLITIC LIMESTONE NEAR 

MAYOWORTH, JOHNSON COUNTY, WYOMING

By R. R. GUILINGER and P. K. THEOBALD

ABSTRACT

The uranium deposits of the Mayoworth area, Johnson County, Wyo., are in 

oolitic limestone at the base of the Sundance formation of Late Jurassic age. The 

uranium mineral has been identified as metatyuyamunite, a hydrous calcium 

uranium vanadate that coats joints and fractures and replaces calcite in both the 

cement and the oolites. In the largest area of measured radioactivity, the 

deposits are coatings on joint surfaces, but in the other areas, replacement of 

oolites and cement is most common. Some of the uranium may be syngenetic. 

Secondary metatyuyamunite has been concentrated along groundwater channels. 

The uranium content ranges from 0.017 to 0.32 percent and the VaOs content 

ranges from 0.06 to 0.17 percent. The known deposits of the area are of sub-ore 

grade and are not of economic importance at this time.

INTRODUCTION

The uranium deposits of the Mayoworth area are 2.2 miles south of 

the abandoned postoffice at Mayoworth, Johnson County, Wyo. 

The area is accessible by a dirt road from Mayoworth, which is reached 

by a paved secondary road from U. S. Highway 87 at Kaycee.

The stratigraphy and structure of the area surrounding the uranium 

deposits have been mapped and described by Richardson 1 and the 

general area to the north by Hose (1954). Love (1954) described the 

uranium deposits of the Mayoworth area.

To evaluate the economic potential and to study controls of deposition of the uranium, the writers examined the deposits in August 

1954 and mapped the area by plane table methods in September 1954. 

This work was done by the U. S. Geological Survey on behalf of the 

Division of Raw Materials of the U. S. Atomic Energy Commission.

STRATIGRAPHY

Sedimentary rocks of Triassic and Jurassic age are exposed in the 

immediate vicinity of the Mayoworth uranium deposits. The Chugwater formation of Triassic age is composed of massive red sandstones

1 Richardson, A. L., 1950, Geology of the Mayoworth region, Johnson County, Wyoming: Univ. of 

Wyoming, M. A. thesis, unpub.

405342^ 57 335
336 CONTRIBUTIONS TO THE GEOLOGY OF URANIUM

and shales. The Sundance formation of Late Jurassic age unconformably overlies the Chug water formation. In the mapped area 

(fig. 1), the Sundance formation is divided into three units. The 

basal unit, in which the uranium deposits occur, is a gray oolitic

EXPLANATION

Chugwater formation

Contact

Dashed where indefinite

J2

Strike and dip of beds

_/5

Strike and dip of joints

f 90

Strike of vertical joints

Radioactivity greater 

than 0.05 mr per hr

500 Feet

Contour interval 20 feet

Odtunt assumed

Geology and base by P. K. Theobald, 1954

FIGUKE 62. Map showing distribution of radioactive localities in the Sundance formation near Mayoworth,

Johnson County, Wyo.
URANIUM DEPOSITS IN OOLITIC LIMESTONE 337

limestone, 8 to 10 feet thick. Conformably overlying the limestone 

is 30 feet of soft gray fossiliferous shale, and directly overlying the 

shale is white to buff sandstone. Isolated patches of gravel of late 

Cenozoic age uncomformably overlie both the Triassic and Jurassic 

rocks.

The limestone of the Sundance formation is composed of oolites 

formed primarily around elongate cores of colorless calcite. The 

oolites are generally uniform in size, averaging about 0.5 mm in 

diameter but ranging from 0.1 mm to 2 mm in diameter. They tend 

to follow the form of the cores, with elongated shapes predominant. 

Radial and concentric growth is evident around the cores, and in 

many oolites both kinds occur (fig. 63). Where both forms of growth

FIGURE 63. Photomicrograph of limestone near Mayovvorth, Wyo., showing radial and concentric 

oolite growth. Crossed nicols. X100.

are present in a single oolite, the radial growth is on the inside, surrounded by the concentric rings and separated from the concentric 

rings by a thin layer of clay. This suggests two stages and two 

processes of oolite formation with a slight interruption between the 

stages. Another interruption in the oolite formation is suggested by 

some corrosion and clay accumulation on a concentric ring at about 

the same position in several oolites.

Though most of the cores consist of colorless calcite, there are other 

kinds. Of these, fossils are the most common. They include small 

pelecypod shells, Foraminifera(?), or shell fragments. Opaque black 

minerals form the cores of some oolites. Most of these black minerals 

are hematite or goethite, but some have not been positively identified. 

The cores of some oolites are quartz grains, but grains of this mineral
338 CONTRIBUTIONS TO THE GEOLOGY OF URANIUM

are generally larger and occur as clastic grains free of oolite forming 

calcite.

The oolites are imbedded in a cement of colorless coarsely crystalline 

calcite. At the time of cementation some solution of the oolites 

occurred, and their outer edges appear corroded and partly replaced 

by the cementing calcite. Later solutions, apparently moving along 

the interfaces between the cement and the oolites, introduced iron 

oxide that replaced both the cement and oolites. In some places the 

cement is completely replaced by iron oxide, arid an unidentified 

green mineral replaces the edges of some oolites. In places finely 

divided iron oxide gives a reddish color to the oolites and cement. 

Hematite and metatyuvamunite have filled fractures that cut both 

the cement and oolites (fig. 64).

FIGITKE 64. Photomicrograph of limestone near Mayoworth, "tt'yo., showing a irotatyuyamunite-filled

fracture. X100.

STRUCTURE

The Mayo worth area is on the southeast flank ol the Bighorn arch. 

The sedimentary rocks in the Mayoworth area are separated from 

Precambrian crystalline rocks to the north by a northwest-trending 

thrust fault about 6 miles north of the mapped area. South of this 

thrust fault, the east limb of the major arch is warped into smaller 

anticlines and synclines. The uranium-bearing limestone is exposed 

on the east limb ol an anticline that forms the eastern limit of exposure 

of the minor folds.

The structure in the vicinity of the uranium occurrences is shown 

on figure 65. The strike of the Chugwater formation and the sandstone of the Sundance formation is uniform. The Chugwater forma-
URANIUM DEPOSITS IN OOLITIC LIMESTONE 339

500 Feet

Geology and base by P. K. Theobald, 1954

FIGURE 65. Map showing lithclogic units and structure of part of the Sundance formation near Mayoworth,

Johnson County. "VVyo.

tion has an average strike of X. 23° W., and an average dip of 15° 

NE.; the sandstone unit of the Sundance formation has an average 

strike of N. 28° W., and an average dip of 14° NE. The structure 

of the oolitic limestone is complex; bedding planes are warped, and 

there is some crossbedding. The erratic strike and dip symbols on 

figure 63 reflect these features.
340 CONTRIBUTIONS TO THE GEOLOGY OF URANIUM

Three sets of joints are conspicuous in the limestone; one is essentially parallel to bedding and strikes from N. 15 to 20° W., and two 

are vertical with strikes of N. 70° E. and N. 30° W. (fig. 62). The 

joints probably were formed during the Laramide orogeny. No 

faults were observed in the area mapped.

URANIUM DEPOSITS

MINERALOGY AND MODE OF EMPLACEMENT

Metatyuyamunite, a hydrous calcium uranium vanadate, was 

identified by W. F. Outerbridge of the Geological Survey by X-ray 

methods in a selected sample of oolitic limestone. It occurs as 

coatings on joints and small fractures and as replacements of calcite 

in the cement and oolites. In zones of highest radioactivity, metatyuyamunite is prominent as joint coatings, particularly in the north 

half of the mapped area. The coatings are most conspicuous along 

joints striking N. 70° E. and N. 15° W.

In the less radioactive rocks, replacement was the most common 

mode of emplacement. Uranium-bearing solutions entered the 

limestone along small fractures, and replaced cement or oolites 

adjacent to the fractures. Where fractures are absent the solutions 

appear to have moved along the interfaces between crystals of the 

cement or more commonly between cement and oolites. In the latter, 

both cement- and oolite-forming calcite were replaced, but the ooliteforming calcite seems to be more susceptible to replacement. In 

several thin sections only the oolites or oolite cores were replaced 

but there is no evidence of channels through which replacing solutions 

could have entered. In one polished section an unidentified black 

mineral formed the core of the oolite. This black mineral has a 

dark-gray or black streak in contrast to the red and brown streak of 

the iron minerals. Immediately surrounding this black mineral is 

an orange alteration product, which is surrounded in turn by a 

concentration of a yellow uranium mineral. The black mineral may 

be a svngenetic primary uranium mineral, but it is too small to be 

positively identified. An autoradiograph of this polished section 

gave inconclusive results. The same polished section also showed a 

black mineral, with orange and yellow alteration halos, veining an 

oolite (fig. 66).

Association of the radioactive rocks with present water courses is 

evident from figure 62, suggesting some geomorphic control of the 

deposits. The relations of the Quaternary gravel shown at the north 

end of the map and similar occurrences at about the same altitude 

north of the mapped area suggest that the present trunk stream, 

which parallels the road shown at the north edge of figure 62, developed 

its course after the gravel was deposited. The small streams shown
URAKTUM DEPOSITS IX OOLITIC LIMESTONE 341

FIGURE 66. Photomicrograph of limestone near Mayo worth showing opaque uranium (?) mineral veining

an oolite. X132.

on figure 62 are entrenched along their lower reaches. Along most of 

the stream that flows parallel to the strike along the contact of the 

limestone and shale there is no flood plain though there is a welldeveloped flood plain near its head in the south half of the map.

Anomalous radioactivity is confined to areas as high or higher than 

the Quaternary gravel and the patch of alluvium at the head of the 

strike valley. This suggests that at least some of the uranium concentration took place along the courses of mature streams as they 

entered the limestone at the two gaps in the flatirons shown on the 

west side of figure 62. Channels of flow would be northeast along 

the bedding-plane joints and the vertical joints parallel to the stream 

flow, and not along vertical northwest-striking joints perpendicular to 

the flow. Thus greater uranium concentrations should occur along 

the joints striking N. 70° E. and N. 15° W., as they do.

In the discussion of the uranium mineralogy it is suggested that 

some of the uranium minerals may be syngenetic. If this is true, 

syngenetic uranium could have been leached from the limestone up 

dip and reconcentrated as secondary metatyuyamunite along ground 

water channels such as joints and fractures in the limestone. Love 

(1954) suggests that solutions bearing uranium could have been 

derived from tuffaceous rock in the White River formation of Oligocene 

age, which may have covered this area. Also, streams entering the 

Mayoworth area could have derived uranium from known bodies of 

radioactive rocks higher in the Bighorn Mountains (Hose, 1954, and 

Jones, 1952).
342

ECONOMIC POTENTIAL

About 20,000 square feet within the area mapped is more radioactive than 0.05 milliroentgens per hour (mr per hr). Maximum radioactivity is 2 mrperhr in the large mineralized zone in the north half 

of the area (fig. 62) where the background is 0.1 mrperhr. There are 

several areas of high radioactivity in the south half of the mapped 

area where a maximum radioactivity of 0.25 mrperhr was noted. The 

average background for the southern area is 0.025 mrperhr. Radioactive localities also occur in the limestone north and south of the 

area mapped.

Analyses of samples from the Mayoworth deposits indicate that 

the radioactivity ranges from 0.04 to 0.16 percent equivalent uranium 

(see table below). The uranium content ranges from 0.017 to 0.32 

percent. The V2OS content ranges from 0.06 to 0.17 percent. The 

CaCO3 content of sample 214833 is 96.3 percent.

Laboratory 

sample No.

D-97107 *

D-97108 l

RW-5132 2

214833 i

eU

(percent)

0. 16

.039

. 040

U

(percent)

0.21

. 052

.32

.017

V2 05 

(percent)

0. 17

.06

. 10

Description of sample

Selected sample of

oolitic limestone.

Grab sample of

oolitic limestone.

Selected sample of

oolitic limestone.

Grab sample of

oolitic limestone.

Location

Radioactive deposit 500 feet

south of road.

Do.

Do.

YZ mile south of

mapped area.

i Analysts: S. Furman, W. Mountjoy, J. Wilson, H. Lipp, J. Schuch, U. S. Geological Survey. 

Analyst: J. J. Warr, U. S. Geological Survey.

The known uranium deposits of the Mayoworth area are of sub-ore 

grade, and of small size. Vanadium content of the deposits is too 

small to provide a by-product. From the available information it is 

unlikely that large, high-grade deposits occur in the oolitic limestone.

LITERATURE CITED

Hose, R. K., 1954, Geology of the Crazy Woman Creek area, Johnson County.

Wyo.: U. S. Geol. Survey Oil and Gas Map OM 142. 

Jones, E. E., 1952, Airborne radiometric survey of the east flank of the Big Horn

Mountains, Wyoming and Montana: U. S. Atomic Energy Commission

RME-4006, Tech. Inf. Service, Oak Ridge, Tenn. 

Love, J. D., 1954, Uranium in the Mayoworth area, Johnson County, Wyo. A

preliminary report: U. S. Geol. Survey Circ. 358.

o