Abstract

The McArthur River unconformity-related uranium deposit, located near the southeastern margin of the Proterozoic Athabasca basin in northern Saskatchewan, Canada, contains both basement-hosted and sandstone-hosted orebodies, one of which is the sandstone-hosted Zone 4 orebody and its alteration system. Early pre-ore quartz cementation forming immediately above Zone 4 produced a 200-m-thick silicified zone within the lower Manitou Falls Formation. Late pre-ore oxidizing basinal fluids (δ18O = 1.1–8.9‰, δ2H = −70 to −34‰) formed zones of dickite, magnesiofoitite, and sudoite-rich alteration in the Manitou Falls Formation above the deposit. These zones were overprinted by an outer halo of illite and illite-chlorite mixed-layer clay that coincided with illite and clinochlore alteration of the Wollaston Group basement rocks. Circulation of basinal fluids into the fault zone, promoted by fault reactivation, created a reducing fluid (δ18O = 3.9–7.8‰, δ2H= −62 to −25‰) through basement interaction, which formed a narrow sudoite alteration halo near the faulted unconformity. Mixing of these two contrasting fluids initiated uranium mineralization at ca. 1600 Ma. Post-ore alteration caused isotopic resetting of primary uraninite, broadly coincident with the intrusion of the Mackenzie dikes into the basin, followed by alteration of uraninite to coffinite, poorly crystalline uraninite, and gummite during the Neoproterozoic. Cool (ca. 10°C), oxidizing, meteoric waters (δ18O = −15.7 to −13.2‰, δ2H = −120 to −93‰) later infiltrated fractures, produced post-ore kaolinite, and caused partial oxidation of uraninite to uranophane.

The silicified zone at Zone 4 functioned as a physical barrier to fluid flow, creating favorable conditions for uranium mineralization by focusing basinal fluids into the fault zone, localizing reducing basement-modified basinal fluids, and enhancing ore preservation. However, the silicified zone limited the spatial extent of late pre-ore sudoite alteration and obstructed the post-ore mobilization of components such as radiogenic Pb and other pathfinder elements into the overlying strata.

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