Release of uranium from volcanic glass in sedimentary sequences; an analysis of two systems

Uranium can be released from volcanic glass shards by solution of the shards. Studies of the Tertiary Tascotal Formation of Trans-Pecos Texas suggest that total solution of shards during diagenesis in an open meteoric flow system does not release uranium for migration. Instead, uranium is only redistributed locally. In the Tertiary Catahoula Formation of the Texas Coastal Plain, uranium also was not mobilized for long-distance migration and ore formation during diagenesis of glass in the zone of saturation within an open meteoric flow system but was released during solution and argillation of glass by subaerial soil-forming processes and diagenesis in the vadose zone. Tuffaceous facies are depleted in proportion to the intensity of vadose and pedogenic processes. It is unclear why uranium mobility is limited within meteoric flow systems.Release for long-distance migration can be documented by chemical evidence for depletion of uranium in potential source beds. Soil processes impart typical vadose microscopic textures and cause formation of distinctive outcrop features. Diagenesis in open meteoric flow systems occurs below the water table producing characteristic isopachous rims of secondary minerals. Together such field, textural, and geochemical criteria allow recognition of functional sources of uranium, which in turn provides a rapid, inexpensive evaluation of regional resource and exploration potential.