Clay minerals in two South Texas roll-type uranium deposits

Abstract

Semiquantitative X-ray diffraction techniques have been used to identify and analyze distribution patterns of clay minerals in the host rocks for two south Texas roll-type uranium deposits. In sandstone and mudstone of the Benavides deposit, emplaced in the Miocene Catahoula Tuff, raontmorillonite is the dominant clay mineral, and illite is present in small amounts. Claymineral distribution in the Lamprecht deposit, in the Miocene Oakville Sandstone, is controlled primarily by lithology: Montmorillonite is the dominant clay mineral in sandstone, whereas illite is the dominant clay in mudstone. Kaolinite is present in most samples from the Lamprecht deposit but commonly in small amounts (less than 10 percent of the total clays). Chlorite, in trace amounts, was identified in only two of 62 samples from the Lamprecht deposit. Clay-mineral distribution in the Benavides and Lamprecht deposits is not related to processes of mineralization. In contrast, clay minerals (primarily chlorite) have been concentrated in mineralized rock of some roll-type and tabular uranium deposits in Wyoming and New Mexico. Organic matter is present in host rocks for deposits having such concentrations and may have influenced clay-mineral authigenesis. The lack of claymineral zonation relative to ore zones of the Benavides and Lamprecht deposits may be due in part to the lack of organic matter in the host rocks.