Effects of Paleoregolith and Fault Offset on the Formation of Unconformity-Type Uranium Deposits

Abstract

Regional paleoregolith is found to exist immediately below unconformities separating basin fills from basement rocks in sedimentary basins. However, the controlling role of paleoregolith on unconformity-type uranium mineralization has not been quantitatively addressed before. Coupled hydrothermal fluid flow and reactive mass transport modeling are therefore performed in this study by using the software TOUGHREACT. The modeling results reveal that preferential flow occurs in the regolith due to its relatively high permeability in comparison with that of the host rocks. The thicker the regolith is, the more concentrated the fluids in the footwall of a fault zone are, leading to more compact and higher-grade deposits therein, and vice versa. Also, displacement of the regolith caused by fault offset plays an important role, as it appears to control the shape of uranium deposits. When the displacement is less than 30 m, the deposits are characterized by a more compact shape. When the displacement is over 60 m, the deposits extend more laterally and even exhibit a ‘discrete’ shape due to the expelling effect of downslope flow that occurs at the fault offset site.