Effect of Fault Extension Relevant to Unconformity on Hydrothermal Fluid Flow, Mass Transport, and Uranium Deposition

In this study, a conceptual model is developed based on common features of typical unconformity-related uranium deposits in the Athabasca Basin, Canada. Three reactive flow modeling scenarios are designed to address the effect of fault extension on the formation of uranium deposits. Our results indicate that the location of the fault zone relevant to the unconformity is crucial to the fluid circulation in both the sandstone layer and the basement unit, the temperature distribution, the transport of aqueous components, and the uranium deposition. In particular, this research reveals that the circulating pattern of the basement brine is critical for the ore genesis. The reducing basal brine is capable of carrying aqueous uranium from depth to react with the shallow oxidizing fluid, being percolated to the basement from the overlain sandstone layer, for uranium precipitation. Scenarios 1 and 2, in which the fault zone is mainly in the basement, are in favor of focusing ore-forming hydrothermal fluids into the footwall area in the basement, leading to the formation of uranium deposits therein. Scenario 3, in which the fault zone is mainly in the sandstone layer with a limited extension below the unconformity, is unfavorable for the focusing of fluids, and hence no significant deposits can be formed, except for some minor uranium mineralization occurring in the footwall and other areas in the basement that are spatially associated with the upwelling flow zones in the sandstone layer.