Geochemical exploration models for sedimentary uranium deposits

Abstract

The geochemical detection of uranium deposits in sedimentary rocks, especially sandstone-type deposits, depends on the geochemical behavior of U and pathfinder elements. Uranium is extensively dispersed under oxidizing conditions but is immobile under reducing conditions. Adsorption on freshly precipitated Fe-oxides and certain types of organic matter also limits dispersion unless high concentrations of CO 3 2− or other complexers are present. Thorium accompanies U in most plutonic processes, but the two elements are separated under oxidizing conditions. Background values for U in water are increased by evaporation and transpiration in arid regions, and are low because of extensive leaching in humid regions. Possible pathfinder elements associated with U in sandstone-type deposits include S, V, Mo, Se, As and at some deposits Cu, Ag, Cr, Pb, Zn, Ni, Co, Re, Be, P, Mn and rare earths, plus He, Rn and other radioactive decay products. Exploration models must take into account channelling of dispersion and possible concealment of deposits by bedding in sedimentary rocks. Most uranium deposits in sedimentary rocks are associated with geochemical provinces enriched in U and Th or with U-rich intrusives or volcanic rocks. However, the deposits may be separated from these U-rich source rocks by tens of kilometers or more. Weak regional U and Th anomalies in sediments containing U deposits are found in the Catskill Fm. of eastern Pennsylvania and may be present around other districts. Zircons from the Catskill sediments are also enriched in U. Halos of anomalous U and small concentrations of U are present around many U deposits but have not been documented in detail. The paleoaquifer leading to a Wyoming-type roll deposit in Pennsylvania contains distinctly anomalous U. Anomalies in U, Se, Mo, V, As, He, Rn, and other pathfinder elements in rock and in ground and surface waters can furnish geochemical guides to ore, as can thermoluminescence, S and C isotopes, and textures of Fe and Ti oxides.