Deciphering Fluid and Uranium Mobilities in Unconformity-Related Uranium Deposits Using Uranium Oxides: The Waterfound Case Study

Abstract The Athabasca basin (Saskatchewan, Canada) hosts high-grade unconformity-related uranium deposits occurring at the interface between Archean-Paleoproterozoic crystalline basement and the Paleozoic to Meso-proterozoic sedimentary basin, rooted above basement-hosted graphite-rich structures. Episodic fluid circulations (1.6–1.0 Ga) are considered to have produced extensive mineralization and hydrothermal haloes in this context. The succession of fluid episodes after the initial uranium deposition nevertheless obscures understanding of the timing and conditions of deposit formation. This study targets the Waterfound U prospect in the northeastern basin, where structural and mineralogical contexts linked to uranium oxides are preserved. A framework is developed and applied to uranium oxides to decipher the structural, physico-chemical, and geochronological evolution of the prospect. The results indicate that uranium oxides at Waterfound formed at 1360 ± 5 Ma in veins and fault cores, in a context of fluid overpressure, associated with clay minerals (first generation of illite and chlorite). These oxides subsequently experienced three main alteration phases: (1) localized brecciation circa 1263 Ma, associated with Ni-Co sulfarsenide precipitation; (2) alteration around 1050 Ma with partial UO2 dissolution, Pb loss, and first chemical trend substitutions, linked to formation of clay minerals (second generation of illite and chlorite); and (3) a near-recent (<100 Ma) alteration causing Pb loss, a second trend of chemical substitutions, and formation of new generation UO2 from oxidized meteoric water. The study highlights the necessity of integrating structural, mineralogical, and geochemical data to accurately reconstruct uranium deposit formation and postcrystallization alteration throughout the geologic history of the Athabasca basin and related Canadian Shield.