Fixation of uranium
Uranium, though generally mobile in the sexivalent state, can be relatively stabilized by precipitation as arsenate, phosphate, or vanadate (McKelvey et al, 1955), but these compounds commonly 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 commonly organic material, of which coals of bituminous or lower rank are most efficient (Moore, 1954).
Uranium is extracted from seawater most commonly as a substitution for calcium in phosphorite (Altschuler et al, 1958), for which some reduction mechanism is required, and as an adsorption on carbonaceous matter in black shales (Conant and Swanson, 1961). Uranium in both these environments 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 erosively destroyed or fed into the anatectic root of an orogene.
Uranium migrating in continental groundwater has relatively little opportunity for fixation. Even 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 deposits at shallow depths. Therefore, the original fixation of uranium on the Colorado Plateau was mostly through reduction by organic matter. Reduction by iron, sulfur oxidation in limestone, and fixation by adsorption on clay are interpreted generally to have been of minor importance in forming the presently known, “conventional” type of sandstone uranium impregnations, but they may have been more influential in other lower grade types of deposits.