Macro to micro fault zone characteristics, fracture systems and porosity in basement hosted unconformity-type uranium deposits of the Athabasca region (Saskatchewan, Canada)

The Athabasca region in western Canada is a metallogenic province hosting numerous uranium deposits that contain among the highest grades as known. The uranium mineralization is associated with the unconformity between the Archean basement and the overlying undeformed Athabasca sandstones. Some of the known economic deposits are located within basement faults that developed along inherited ductile shear zones. Development of these brittle fault zones is usually characterized by breccia, damage zones, and frequently by a vertical offset at the unconformity. Reactivated inherited basement structures allowed the development of structural traps in which the mineralization could be precipitated. The typical known model for these basement-hosted deposits, or 'Ingress deposits', implies that basin-derived fluids enriched in uranium flowing along basement faults. Interaction between these fluids and basement rocks leads to uranium precipitation in the structures. In a mineralizing system in which structural control is prominent, deformation zones may have been acted either as fluid conduits or barrier regarding the fluid circulations. Numerous studies mention such structures and their role played in uranium mineralization process in the basement, in the Athabasca region, in the Thelon and in the Mc Arthur basin (Australia), with however lack of knowledge about the exact mechanisms and the conditions related to the development of the fertile structures. Distinction is not made between structures that may act as a fluid conduit or as fluid screen, which strongly limits the knowledge of the role played by different episodes of deformation and alteration into the setup of a fertile porosity network. This thesis focuses on the study of basement structures, U-mineralization and their alteration halo at different scales and allows to highlight the lead role played by the preexisting ductile fabric in the development of a favorable system for uranium-bearing fluid flow. Basement shear zones testify about a long fluid history (hydrothermal alteration, silicification, dissolution), and while reactivated later under brittle conditions made favorable environment for fluid flow beneath the Athabasca basin. Reactivation in the brittle domain leads to the development of a fractured corridors that are characterized by the reworking of ductile fabric into open fractures, accompanied with development of tensile jogs, which are suitable for mineral precipitation. The mineralization was emplaced lately within such structures that form a favorable porosity network within these strongly deformed and altered intervals. The present work allowed to define a structural signature that is common for two different prospects located at two opposite sides of the Athabasca basin. This signature can be qualified as a 'structural footprint' is based on the overprinting of ductile shear zones by a network micro-fracture, in strongly deformed and altered basement rocks. The results also showed the importance of the structural approach at different scales (from project-scale, to drill core sample scale, and microstructures analysis) in the understanding of which mechanisms are critical in the emplacement of a favorable porosity network related with uranium mineralization. (author)