Factors Controlling the Formation of Unconformityrelated Uranium Deposits in Sedimentary Basins: Insights from Reactive Mass Transport Modeling

This thesis aims at investigating the formation and controlling factors of unconformity-related uranium (URU) deposits through fluid flow and reactive mass transport modeling.;The role of a graphite zone was first addressed using two different reducing mechanisms. In the first mechanism, uraninite precipitation involves methane as a reductant. In the second mechanism, oxygen is used for formulating the redox reaction. Results show that uraninite can precipitate below the unconformity and away from the graphite zone regardless of the reducing agent. However, methane leads to a higher grade in uranium mineralization. Fe-rich chlorite was next investigated. Results show that Fe2+, released by destruction of Fe-rich chlorite, reduces the oxidized uranium and therefore is a viable mechanism to precipitate uraninite. Both basinal sandstone and basement were evaluated as a uranium source using two separate models. For both models uraninite precipitates in the basement and either away from or along the fault, depending on the fault permeability. However, precipitated uraninite has a greater grade in the first model than that in the second model and exhibits both chlorite and muscovite alterations that are commonly present in most basement-hosted URU deposits. The second model exhibits only muscovite alteration.;The role of hydrodynamic factors was finally investigated by assigning different dip angles and directions to faults and various permeabilities to hydrostratigraphic units. Results show that these factors govern the fluid flow pattern, temperature distribution, and uranium mineralization. A vertical fault results in uranium mineralization at the bottom of the fault within the basement, while a dipping fault leads to uraninite precipitation below the unconformity either away from or along the fault. A more permeable fault causes uraninite precipitation along the fault, whereas a less permeable fault gives rise to the precipitation of uraninite away from it. No economic ore mineralization forms when either very low or very high permeabilities are assigned to the sandstone or basement.;Physicochemical parameters also exert an additional control on both location and grade of URU deposits. Uranium mineralization occurs in locales experiencing a reduction of oxygen fugacity and having a temperature of 160-180 °C and a pH of about 4-4.5.