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 uraniumtricarbonate 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, chromium, and vanadium amounts in the altered, unaltered, and mineralized zones. In the oxidized 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.