The Geology and Origin of the Sawyer Uranium Prospect, Live Oak County, Texas

The Sawyer uranium prospect is a subsurface uranium occurrence hosted within the basal Oligocene Catahoula Formation of the Texas coastal plain. The host rocks consist of tuff-ball conglomerate, tuffaceous sandstone and tuffaceous claystone whose geometry and lithological characteristics indicate that they are the products of a crevasse-splay depositional environment. Compositionally, these lithologies are feldspar-depleted litharenites, with the feldspar depletion due to the corrosive, ore-forming processes. These sediments display pedogenic to early diagenetic features including diffuse to discrete micrite nodules, clay cutans, fresh to partially argillized glass shards, clay booklets, authigenic zeolites and sulfides, paleosoil horizons and calcite cement. Uranium mineralization occurs throughout the crevasse-splay channel sediments, with the richest accumulations (0.1% to 0.94% U308) concentrated in the tuff-ball conglomerates at the base of the channel sequences. Uranium is correlatable with anomalous concentrations of Pb, As, Rb and Y within the orebodies. Organic carbon content (0.01% to 0.28%) is uniformly low throughout the ore zone and does not display a significant correlation with uranium mineralization. No uranium minerals were detected by X-ray diffraction techniques; however, SEM energy dispersion analysis shows that uranium occurs as scaly encrustions adsorbed onto the surfaces of favorable mineral grains. The uranium-mineralized lithologies also host an appreciable amount of iron-disulfide minerals which provide data useful in the interpretation of ore paragenesis. Textural relationships between framboidal pyrite and ore-stage marcasite overgrowths provide evidence that the host rocks were reduced prior to uranium mineralization. The presence of organic carbon and botryoidal clusters of pyrite frmnboids suggests that the pre-ore reduction was accomplished by sulfate-reducing bacteria. This pre-ore reduction also resulted in the alteration of detrital ilmenite to pyrite and anatase. Following the initial reduction, the host rocks were invaded by oxygenated, uranium-enriched ground waters creating an oxidation-reduction interface along which uranium and associated trace elements were precipitated. The oxidation of pre-ore pyrite in the alteration tongue released Fe2 + and unstable sulfur oxyanions which were then available for further reaction upon entering the redox interface. The Fe2 + and free sulfur recombined under acidic conditions to form ore-stage marcasite as fine to coarse aggregates and as overgrowths surrounding framboidal pyrite. δ34S values of bulk sulfide samples (-9.9 to +8.4 per mil) support the interpretation of biogenic sulfide precipitation. However, the origin of the reductant responsible for post-ore re-reduction of the alteration tongue is uncertain