Abstract

Uranium deposits in the southern Powder River Basin of Wyoming are excellent examples of large roll-type ore bodies. The host rocks are Paleocene sandstones deposited as point bars by a meandering stream. The source of uranium is tuffaceous and arkosic debris indigenous to the sedimentary sequence containing the host rocks. The largest deposits of highest grade occur near the distal margins of permeable, slightly dipping sandstones where they grade laterally into organic-rich siltstones, claystones and lignites deposited in backswamp or flood basin environments. The deposits are epigenetic in origin, formed by precipitation of uranium from groundwater solutions that moved through the host rocks from a recharge area southwest of the deposits towards a discharge area northeast of the deposits. The deposits are large because the host rocks are extensive and the groundwater system remained relatively stable. Pyrite formed early in and around the host units through a biogenic process utilizing sulphate-reducing bacteria. This was important in establishing a permissive geo-chemical environment for later ore genesis. Oxidation of pyrite by incursing groundwater caused sulphite to form. Sulphite disproportionation into SO^2-₄ and HS^- developed the final reducing mechanism for uranium precipitation in the ore rolls. Deposit characteristics and associated alteration products are compatible with the chemical theory of Granger and Warren.