Uranium

Abstract

Uranium, though generall y mobile in the sexivalent state, can be relatively stabilized by precipitation as arsenate , phosphate , or vanadat e (McKelvey et al, 1 955), but these compounds commonl y prefer arid climates. In dry environments , uranyl silicates, carbonates , and sulfates form by desiccation and are fairly stable at the surface. Uranium reduction and precipitation of "primary" uranous silicate or uranous oxide require an external agent , most commonl y organi c material , of which coals of bituminous or lower rank are most efficient (Moore, 1954). Uranium is extracted from seawater most commonl y as a substitution for calcium in phosphorit e (Altschuler et al, 1958), for which some reduction mechanis m is required, and as an adsorption on carbonaceou s matter in black shales (Conant and Swanson, 1961). Uranium in both these environment s is very stable, as indicated by the uniformity in grade along outcrops throughout large areas and even regions. The uranium so fixed is removed from circulation and can reenter the mobilization cycle only by being erosivel y destroyed or fed into the anatecti c root of an orogene. Uranium migrating in continental groundwater has relatively littl e opportunity for fixation. Hven in the arid climate of the Colorado Plateau, the relatively insoluble sexivalent vanadates are never far from carbon trash concentrations , and these obviously have been oxidized from "primary" unoxidized pitchblende deposit s at shal low depths. Therefore, the original fixation of uranium on the Colorado Plateau was mostly through reduction by organi c matter. Reduction by iron, sulfur oxidation in limestone, and fixation by adsorption on clay are interpreted generall y to have been of minor importance in forming the presentl y known, "conventional " type of sandstone uranium impregnations , but they may have been more influential in other lower grade types of deposits. The exact mechanisms of reduction and fixation by organi c matter have been extensively investigated (Moore, 1954; Breger and Deul, 1956; Schmidt-Collerus , 1969), but still are not well understood. Gruner (1956) exerted maximum influence on the shift of interpretation toward laterogenesi of shallow uranium with his paper on the multiple migration-accretio n mechanism, based on the precipitation of pitchblende in the laboratory with HTS and organi c matter at surface temperatures . Since then, many have visualized unoxidized uranium concentrations in sandstone as being in slow but constant migration downdip, by means of constantl y repeated oxidation and reduction, in favorabl e aquifers and near the top of the saturated zone. The source of H2S was a problem for a while. Jensen (1958) and others have emphasized the role of anaerobi c bacteri a in reducing supergen e sulfate to H2S, the reoxidation of which is believed to reduce the uranium. Schmidt-Collerus (1969) failed to find anaerobi c bacteri a in any of the uranium deposit s he investigated; he believes that reduction by organi c complexing of still uncertai n character is more probabl y responsible for uranium fixation. This alternative is supported by the uranium impregnations in Westwater sandstone in the Grants district. The Westwater lacks abundant carbon trash and pyrite, but does