Distribution of secondary uranium minerals in the W. Wilson deposit, Boulder Batholith, Montana

Abstract

Weathering of pitchblende in the reef deposit at the W. Wilson mine, near Clancy, Montana, has produced an assemblage of secondary uranium minerals, including meta-autunite, meta-uranocircite, meta-torbernite, meta-zeunerite, uranophane, beta-uranophane, phophu-ranylite, gummite, and an unidentified mineral, possibly a complex uranium silicate. The secondary minerals show a distinct zonation about primary uranium concentrations. Variation in composition of the groundwater solutions with depth, together with the differences in solubility of the secondary minerals, may be responsible for the zonation. Vitreous orange and yellow replaced pitchblende directly, and a fine-grained yellow mixture of oxides, silicates and phosphates of uranium (called gummite in the field) was deposited in the vein and wall rock breccias around the pitchblende. Meta-autunite and some meta-uranocircite were deposited farther out from pitchblende in the intergranular pore spaces of the wall rock. Meta-torbernite and meta-zeunerite tend to be concentrated along fractures at considerable depths below the surface, due, it is thought, to the probable higher concentration of copper in the solutions with increasing depth.The apparent deposition of the bulk of the uranium secondaries within a short distance from the primary source, pitchblende, appears to indicate that supergene enrichment in the oxidation zone is not important. The sparse distribution of sulfides in the siliceous veins and the apparent lack of other strongly reducing conditions below the water table would seem to render unlikely the possibility of a pitchblende enrichment by reduction of the U (super 6+) ion in meteoric water moving downward below the water table.