Tracing past migrations of uranium in Paleoproterozoic basins: New insights from radiation-induced defects in clay minerals

Radiation-induced defects were identified in kaolinite, illite, and sudoite by electron paramagnetic resonance spectroscopy of the alteration halo surrounding the uranium orebodies in the Athabasca Basin (Saskatchewan, Canada). Clay minerals are assumed to behave similarly under irradiation. In all samples, defects are similar in nature, but their concentrations can vary widely over several orders of magnitude. The maximum fluctuations in defect concentrations are observed along the regional Paleoproterozoic unconformity between the lower sandstones and the metamorphic basement rocks and close to crosscutting brittle structures, both of which appear to be the main vectors of uranium-bearing fluid transfer in the basin. In the basement, some Hudsonian faults connected to this unconformity also show high defect concentrations, attesting that uranium-bearing fluids may have circulated in the fracture network. The proximity of mineralization can be revealed through defects that record the past presence of uranium in altered rocks at significant distances from the mineralized bodies. The absence of correlation between defect concentrations and present dose rates indicates that migrations of uranium-bearing fluids took place after the formation of clay minerals.