Uranium mobilization from the Variscan Questembert syntectonic granite during fluid-rock interaction at depth

Leaching of uranium-fertile granites represents a major source of uranium, as uraninite is easily dissolved inoxygenated aqueous solutions. This phenomenon is well documented at surface conditions, but remains poorlydocumented for granites at depth. In this study, we propose that surface-derived oxidized hydrothermal fluidsleached uranium from uraninite in the Questembert peraluminous granite at temperatures greater than ∼70°to 160°C. This Variscan synkinematic granite is characterized by widespread and pervasive development of verticaland permeable C-S structures. These structures likely facilitated the infiltration of oxidized hydrothermalfluids from the surface, their circulation at depth, and the subsequent fluid-rock interaction in the granite. Publishedoxygen isotope data shows that it has undergone subsolidus fluid-rock interaction, dated between 312and 303 Ma by ⁴⁰Ar/ ³⁹Ar analyses on muscovite. These interactions were responsible for the concomitant decreaseof the feldspar δ¹⁸O values together with uranium leaching. Mass-balance calculations suggest that thishydrothermal event could have liberated several hundred thousand tonnes of uranium from the Questembertgranite. The liberated uranium may have been dispersed in Early Permian intramountainous basins and thereforedisseminated over large areas. This study emphasizes that the efficiency of uranium leaching may bedirectly related to the extent of subvertical structure development in granites emplaced along strike-slip shearzones, which allow for downward infiltration of oxidized surface-derived fluids. A specific and systematicsampling is required to better constrain the proposed model.