Effect of graphite zone in the formation of unconformity-related uranium deposits: Insights from reactive mass transport modeling

Abstract

In this study, reactive mass transport modeling is conducted for evaluating the role of faulted graphite zones in the formation of unconformity-related uranium (URU) deposits. For this aim, two different reducing mechanisms are examined for the precipitation of uraninite in a typical URU deposit. In the first mechanism, precipitation of the uraninite involves methane as a reducing agent which is produced by the alteration of the graphite zone. The second reducing mechanism does not incorporate methane as a reducing agent, and oxygen is used for formulating the redox reaction of uraninite precipitation. Numerical simulations show the following results. Firstly, by employing either reducing mechanisms, uraninite can precipitate in the basement close to the unconformity interface. Secondly, uraninite can precipitate below the unconformity interface away from the faulted graphite zone even if methane is not involved as a reducing agent. Physiochemical parameters such as oxygen fugacity and temperature play a significant role in localization of uraninite. Localization of uraninite below the unconformity interface is related to the decrease of oxygen fugacity, generally resulting from the interaction of oxidized uranium-bearing fluids with reductants. Through the second reducing mechanism, uraninite cannot precipitate around the faulted graphite zone. In comparison with the precipitation by the first reducing mechanism, it takes longer time for uraninite to precipitate through the second mechanism, and the volume fraction of uraninite precipitated by the second mechanism is lower than that by the first mechanism. Finally, faulted graphite zone has a major role in providing the pathway for transporting the fluid. The uranium bearing brines flow into the faulted graphite zone and interact with the basement lithology. Also, the basal fluids use faulted graphite zone as a conduit to mix with the basinal fluids. Maximum fluid flow rate happens along the faulted graphite zone because of the high permeability of this zone in comparison with that of other stratigraphic units present in the model.