Polyphase hydrothermal breccias associated with unconformity-related uranium mineralization (Canada): from fractal analysis to structural significance

Abstract

Sandstone-hosted breccias surrounding unconformity-related uranium ore in the Athabasca Basin, northern Saskatchewan, were mapped and described using drill core. Four main breccia types were distinguished and classified chronologically based on the type of matrix: microcrystalline quartz, sudoite–dravite, Fe–chlorite and polyphase carbonate. Mapping of the breccias has shown that they are controlled by various sets of basement faults, including a major graphite-rich structure. Breccia fragment morphologies were quantified using a fractal dimension analysis technique. The matrix percentage and the shape of the fragment morphometry distribution curve allow determination of the degree of maturity and the nature of fluid–rock interactions for each breccia. Microcrystalline quartz breccias are tectonic and fluid-assisted. Sudoite–dravite, Fe–chlorite and polyphase carbonate breccias were formed by dissolution of quartz. Mature sudoite–dravite and Fe–chlorite breccias have collapse textures formed, respectively, by dissolution opening and fluid-assisted brecciation. Breccias are the expression of basement fault reactivation postdating sandstone deposition (circa 1.7Ga).Each seismic event created structural conduits between variably pressured compartments, inducing fluid circulation from the basement to the sandstone. Fluids flowed more slowly in the strongly fractured sandstones and dissolved quartz to form solution breccias. Interaction of these basement-derived fluids with basinal fluids may have led to unconformity-related uranium mineralization deposition. This may explain the good correlation between breccia bodies and sandstone-hosted uranium ore, especially for the sudoite alteration event.