Contributions to Geology 8.2.1

Aspects of the geologic history of Wyoming related to the formation of uranium deposits

ROBERT S. HOUSTON Department of Geology, University of Wyoming, Laramie, Wyoming

Pages
65-66

Keywords
uranium, Wyoming, Eocene

Abstract
Uranium has been found in virtually every major time rock unit known in Wyoming from rocks of Precambrian age to rocks of Pleistocene age (Finnell and Parrish, 1958). The most important deposits economically, however, are in various formations of early Eocene age and we will therefore emphasize aspects of the geologic history of Wyoming related to the formation and subsequent history of these units.

Ground water as related to the origin and search for uranium deposits in sandstone

ARTHUR P. BUTLER, JR. Geologist, U. S. Geological Survey, Denver, Colorado

Pages
81-86

Keywords
ground water, uranium, sandstone, Wyoming, deposits, drainage

Abstract
Slightly alkaline bicarbonate sulfate ground water generally similar to the modern ground water is thought to have been the agent that formed the uranium deposits in fluviatile sandstone in Wyoming. Such water has the general characteristics of the aqueous solution which the investigations by Hostetler and Garrels (1962) indicate could transport uranium at low temperatures and from which it could be precipitated.

Lack of evidence for any conduits in underlying rocks and the absence of igneous rocks near deposits in some areas suggest that deposits were not formed by juvenile thermal water. The large volume of mildly oxidized rock to which the deposits are marginal suggests that oxygen-poor connate water was not the agent that formed the deposits.

Modern ground water moves downward from higher to lower elevations in the same general direction as associated surface drainage; the ancient water which formed the deposits undoubtedly moved in a similar way with respect to the paleodrainage.

In addition to being the agent for emplacement of the deposits, ground water may be a guide to general areas in which deposits occur, although its utility as a guide to unoxidized deposits below the water table is not clearly established.

Uranium deposits in the Lower Cretaceous of the Black Hills

A. R. RENFRO Sr. Geologist, Teton Exploration Drilling Company, Inc., Casper, Wyoming 82601

Pages
87-92

Keywords
uranium, Cretaceous, Black Hills, deposits, ore, roll fronts

Abstract
The Black Hills have produced approximately four million pounds of U3O8. All of the known uranium ore bodies are in continental and marginal marine sandstone of the Lower Cretaceous, Inyan Kara Group. The Inyan Kara Group is comprised of the Lakota and overlying Fall River Formations which represent the proximal and distal portions of a single, transgressive depositional system. The regional depositional environments that affected each formation indirectly affected the geometric complexity and extent of related uranium deposits.

Uranium deposits of the Inyan Kara Group are of the roll front type. They were deposited by down-plunge migrating geochemical cells which were initiated during the Laramide orogeny. Passage of geochemical cells through the host rocks caused physical and chemical changes that are excellent exploration guides. These changes include oxidation of massive, pore filling pyrite, destruction of disseminated carbon, and leaching of indigenous uranium.

The source of uranium in the Inyan Kara roll fronts is considered to be the altered host sandstone. This conclusion is supported by relative lack of uranium in altered ground as opposed to relative abundance of uranium in fresh ground.

Extensive low grade reserves are indicated or inferred in the northern Black Hills. Typical deposits contain approximately 250,000 pounds of U3O8 per mile of roll front. Individual roll fronts can be traced for tens of miles though they do not everywhere contain ore grade mineralization. Similar roll fronts are anticipated to extend, with interruption, around the Black Hills. Such deposits ultimately will be the main source of uranium in the Black Hills.

Uranium deposits of the Gas Hills

DON C. ANDERSON Utah Construction and Mining Company, Riverton, Wyoming

Pages
93-104

Keywords
deposits, uranium, Gas Hills, Wind River, Miocene, solution-front, source

Abstract
The Gas Hills Uranium District is located in central Wyoming along the southeastern margin of the Wind River Basin. The initial discovery was made by Neil E. McNeice in the fall of 1953.

Earliest development of the district's ore reserves was quite slow, but accelerated when the larger, more experienced mining firms became active in the area.

Surface drilling proved to be the most effective tool for finding and developing the uranium ore reserves. New drilling techniques were needed to obtain samples of the below water-table ores. Many methods were tried, but frozen core drilling and bucket augering proved to be the most reliable for obtaining accurate samples.

Since the initial discovery, the Gas Hills Uranium District has produced about 12% of the United States total.

The present land surface is characterized by barren, subdued, rolling hills. These are traversed locally by steeply dipping hogback ridges of older, more resistant rocks, which are the flanks of truncated, northward plunging folds formed by crustal disturbances prior to the deposition of the Wind River Formation. A steep erosional escarpment that rises abruptly above the north sloping basin floor bisects the region and divides the surface drainage between tributaries of the Wind River to the north and tributaries of the Sweetwater River to the south.

Volcanism occurred during late Eocene time, as evidenced by relic vents found at the southern end of the Rattlesnake Hills, and by local volcanic debris found in the middle and upper Eocene rocks.

Sedimentary rocks exposed in the Gas Hills Uranium District include sandstones, limestones, dolomites, shales, and tuffaceous sandstone, mudstones, and shales. They range in age from Cambrian to Miocene and have a composite thickness of over 14,000 feet.

The source beds for the uranium deposits are arkosic sandstones interstratified with lensing mudstones and shales. Two distinct types of sandstone are present in the Wind River Formation. The youngest is yellowish-orange to yellowish-gray arkose, derived primarily from Precambrian gneissoid and granitoid rocks; it contains little clay, abundant calcium carbonates, and limonite cement, and is host for all uranium deposits of the district.

The second type of sandstone is pale yellowish-gray to pale olive, derived from areas of schists of Precambrian age; it contains abundant clay matrix.

There are four types of uranium deposits found in the district, the most important being the solution-front deposits. They were formed along the margins of highly altered, tabular sand beds that are enclosed by overlying and underlying fine-grained siltstone, claystone, and carbonaceous mudstone beds. Solution fronts can be followed for long distances and individual ore bodies are found along them that may reach thousands of feet in length.

The solution fronts are ideally crescentic or "C" shaped when viewed in cross section, with thin mineralization forming the tips of the crescents. The uranium minerals occur as earthy brown to black coating on and interstitial fillings between the quartz sand grains. The primary uranium ore minerals are coffinite and uraninite.

The three other types of deposits include transitional bedded, oxidized, and residual remnant deposits. There have been several quite-large transitional bedded deposits mined, but the oxidized and residual remnant deposits are usually small and difficult to mine.

Ground waters trapped by the southward tilting of the Tertiary rocks during late Miocene time became stagnated. These waters dissolved uranium and other elements from the enclosing rocks,and after erosion had exposed the highest beds of the Wind River Formation, the mineral-rich solutions gained egress, from the enclosing sand aquifers, toward the north and the solution-front ore deposits began to form.

Uranium deposits in the Great Divide Basin-Crooks Gap area, Fremont and Sweetwater Counties, Wyoming

ROBERT V. BAILEY Mining Exploration Geologist, 1901 West 38th Street, Casper, Wyoming 82601

Pages
105-120

Keywords
uranium, Miocene, Battle Spring, Crooks Gap, Wyoming, Great Divide, geochemical, coal

Abstract
The Great Divide Basin - Crooks Gap area encompasses approximately 3,500 square miles in south-central Wyoming. Uranium mineralization has been found in three types of deposits in this area: (1) low-grade deposits associated with Eocene sub-bituminous coal and carbonaceous shale in the central and eastern part; (2) low grade caliche-type deposits of schroeckingerite in Eocene sediments in the north-central part; and (3) higher grade deposits, some of which are minable, in Eocene sandstone and conglomerate at Crooks Gap at the north edge of the basin.

It has been estimated that there are more than 60 million pounds of U3O8 associated with minable coal in the Great Divide Basin, and additional unestimated quantities in carbonaceous shales. The schroeckingerite deposits are believed to be small, and the mineral is water soluble. Production plus minable reserves at Crooks Gap are estimated at between 11 and 12 million pounds U308.

Evidence presented in previous studies indicates that the uranium associated with the coal was derived from overlying Miocene (?) sediments. A similar theory is postulated for the origin of uranium found in the schroeckingerite deposits. As a working hypothesis, it is suggested here that the uranium in the Battle Spring Formation at Crooks Gap also originated in younger volcanic-rich sediments, and that the uranium, subsequent to its release from the tuffaceous rocks, migrated in neutral to slightly alkaline, weakly oxidizing ground water into the highly permeable Battle Spring Formation where Eh and pH changes resulted in uranium concentration in zones along geochemical interfaces.

Stratigraphic control of sandstone uranium deposits in Wyoming

M. DEAN WEBB Kerr-McGee Corporation, Oklahoma City, Oklahoma

Pages
121-130

Keywords
compaction, uranium, Wyoming, microorganisms, oxidation, Tertiary

Abstract
A conceptual model is presented which proposes that the Wyoming uranium deposits in Tertiary sandstones are post depositional accretions largely related to facies changes developed in major paleo-drainages. These facies changes represent the optimum location of carbonaceous accumulation in coarse permeable sandstones along the paleostream margins.

Sediments were largely derived from granitic and metamorphic terrains adjacent to the basins of deposition. The uranium was derived from the source terrain and after mobilization was deposited and redistributed by connate, phreatic and vadose waters. Uranium concentration was initiated during the period of compaction by direct absorption in humic components and by a system of microorganisms living on the carbonaceous substrate. Later Tertiary uplift introduced extensive oxidation into the system creating major modification and reaccretion of uranium in the same geochemically favorable carbonaceous environment. These late oxidation phases are postulated to have been responsible for most of the economically exploitable deposits. In the uplift and oxidation cycles the physiological activities of at least two genera of microorganisms played a significant role in both the mobilization and in the accretion of uranium.

Uranium deposits of the Powder River Basin

JAMES F. DAVIS Division Geologist, Mining, Union Pacific Railroad Company, Natural Resources Division, Laramie, Wyoming 82070

Pages
131-142

Keywords
uranium, roll front, ore, Powder River, Wasatch, Fort Union, tuffs

Abstract
Uranium in the Powder River Basin is in the form of geochemical roll fronts associated with a decrease in permeability in arkosic sandstones of the Wasatch Formation and to a limited extent, the Fort Union Formation. Previously mined deposits were for the most part oxidized. Many of the recent discoveries are more extensive unoxidized ore bodies. The host sands are correlated over several miles. The origin of the uranium is postulated to be the Oligocene, Miocene, and Pliocene tuffs which once covered the area. Hydrolysis of the tuffs produced an alkaline ground water which dissolved the uranium and carried it as a uranium-tricarbonate ion. The solutions were carried by coarse, regionally transmissive sand units which were stained pinkish-red by hematite, formed from oxidation of pyrite by oxygen in the solutions. Cores of unaltered Wasatch arkosic sand contain less than two ppm U, whereas cores from well back in the altered rock contain 18 ppm U.

The ore deposits are usually multiple 'C'-shaped rolls distorted by variations in the gross lithology. The individual rolls range in thickness from two to 20 feet and may be several thousand feet in length. Low-grade (.05 - .10% U3O8) protore, up to 40 feet thick and several hundred feet wide, is commonly present on the unaltered side of the higher-grade roll front. The unoxidized ore bodies are protected from weathering by silt and claystone overburden. Important variations are noted in mass mean diameter of the sand grains, and in organic carbon, carbonate, manganese, selenium, sulphate, chromium, and vanadium amounts in the altered, unaltered, and mineralized zones. In the unoxidized ore deposits studied, the U/eU ratio is almost universally high. Comprehensive geological exploration practices have played an important part in many of the recent discoveries of the Powder River Basin.

Uranium deposits in Shirley Basin, Wyoming

ROBERT E. MELIN Geologist, American National Bank Bldg., Denver, Colorado 80202

Pages
143-149

Keywords
uranium, Shirley Basin, Wyoming, ore, deposits, ground water

Abstract
Uranium deposits in Shirley Basin occur in arkosic sandstone beds of the Eocene Wind River Formation. The Wind River ranges from a wedge-out to about 500 feet thick and consists of light gray fine-grained to conglomeratic sands mostly less than 100 feet thick interlayered with green clay-silt beds. In the mines area the beds dip about one degree north-northeast. Uranium deposits consist of disseminations and impregnations of uraninite, calcite, pyrite, and marcasite in arkosic sands. Much of the uranium is in crescentic rolls, but important amounts are in tabular bodies near the rolls. The larger ore bodies in the mines area are distributed along the down-dip and lateral periphery of tongues of altered ground. Obvious alteration effects include a color change from pale gray to light tan or yellowish-gray, and removal of pyrite, calcite, and carbonaceous material. The deposits apparently were formed by migrating reactive ground water solutions that collected uranium from the ground it altered, moved uranium downstream, and concentrated it in emplacements at interfaces between altered ground and unaltered ground.