Sedimentary rocks of the Wind River Formation of early Eocene age and its equivalents (Battle Springs and Wasatch Formations) are the host rock for at

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least 95% of the uranium reserves in Wyoming. Wyoming reserves were estimated by the AEC at 53,270 tons of U₃O₈ as of January 1, 1967. Distribution of reserves in tons of U₃O₈ together with production to January 1, 1967 are estimated as follows: Gas Hills, 19,560± tons reserve, 17,300 tons produced; Shirley basin, 27,000± tons reserve, 3,450 tons produced; Crooks Gap, 4,000± tons reserve, 2,400 tons produced; other Tertiary basins including Powder River, 1,000± tons reserve, 1,100 tons produced.

Wind River and equivalent rocks crop out in broad areas within the intermontane Tertiary basins of central Wyoming. The sediments that comprise these lower Eocene beds were derived from mountain ranges that were uplifted during Late Cretaceous through earliest Eocene times. Two facies predominate within the major basins. A coarse arkosic sandstone and conglomerate facies with interbedded siltstone dominates near the mountain fronts. Major uranium deposits occur in this facies. Farther out in the basins the coarse-grained sediments grade into or interfinger with a variegated fine-grained facies.

Economic concentrations of uranium occur near the margins of tongues of altered sandstone within the coarse-grained facies and are classified as roll-type deposits. Character of alteration differs from basin to basin but has been recognized to some degree in all basins. Alteration consists of the oxidation products produced by mineralized ground water passing through a transmissive sandstone bed. Geometrically the deposits are tongue-shape in plan and crescent-shape in vertical section with the concave side toward the altered sandstone.

Deposition of uranium occurred at the front of an advancing aqueous chemical system which moved through the host sandstone bed. The oxygenated water rich in uranium, selenium, and other trace elements moved along the hydrologic gradient and oxidized and leached various minerals, including uranium, as it progressed. Precipitation of the uranium and associated elements occurred at a point within the aquifer where the pH and Eh of the system dropped sharply. The change in chemical environment was the result of the presence of H₂S, probably produced by anaerobic bacteria.

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