Fault reactivation and tectonic conditions for unconformity-related uranium deposit: A paleostress approach (Athabasca Basin, Canada)

High-grade unconformity-related uranium (URU) deposits in the Athabasca Basin are spatially associated with graphitic-rich shear zones rooted in the basement and their propagation as brittle faulting and damage zones in the overlying sandstone. Investigating the tectonic phases associated with URU formation is particularly challenging due to its large polyphased history. This study is based on geological observations from 19 exploration drill cores in the Waterfound project (NE Athabasca Basin) and combines structural characterization with paleostress joint-inversions. Three main tectonic regimes were identified: (1) a pre-Athabasca Basin deposition NNE–SSW shortening affecting only the basement, (2) a syn-lithification NW–SE extension, and (3) a post-Athabasca NW–SE shortening. This polyphased tectonic history led to an increase in both the density and variety of fractures in the basin above a low-friction ENE–WSW graphitic shear zone in the basement. The extensional phase (2) developed with low horizontal differential stress, low fluid pressure and potentially relates to post-orogenic collapse or burial stress conditions only. The latest shortening stage (3), associated with uranium mineralization, locally reactivated a pre-existing fracture network in the sandstone and developed above an inherited ENE–WSW-trending graphitic shear zone in the basement. It requires tectonic stresses and fluid pressures estimated at 45–65 MPa to explain the observed structures. These results highlight the interplay between the mechanical strength of inherited structures, stress states, and fluid pressure in governing uranium-bearing fluid flow and provide new insights for Athabasca URU deposits exploration.