Preliminary model of regional Mesozoic groundwater flow and uranium deposition in the Colorado Plateau.

Qualitative and numerical simulation of regional groundwater flow in the Colorado Plateau during Late Jurassic and Early Cretaceous time has led to a model of U deposition in areas of upwelling solutions. Palaeographic reconstruction shows that surface drainage and groundwater flow was generally to the NE and E. Groundwater flowing in these directions, principally through Triassic-Jurassic and Permian sandstones, encountered sediments of variable thickness due to buried uplifted or downdropped Precambrian blocks. The buried uplifted blocks caused upward movement of groundwater around them. These inferred zones of upwelling are closely associated with concentrations of Jurassic- and Cretaceous-age U deposits. The results are consistent with hypotheses of an upwelling brine mixing and reacting with descending meteoric water and causing U precipitation at the fluid interface. Whether the U came from above or below the interface is an unsolved problem.-A.P.