World-class unconformity-related uranium deposits: Key factors for their genesis

Unconformity related uranium deposits from the Athabasca basin (Saskatchewan, Canada) represent the world largest high-grade U ore-bodies. They are located in the vicinity of an unconformity between an Archean to Paleoproterozoic basement and a Paleo- to Meso-proterozoic intracontinental clastic basin. Their exceptional grade and size results from the combined efficiency of a series of U-fractionation mechanisms, which will be reviewed here. U-enrichment of the basement is exceptional. U-extraction efficiency from source rocks is shown by the alteration of highly refractory minerals such as monazite and zircon in the sandstone, the regolith, and the basement by slightly acidic, hot (160 to 220°C, about 1kbar), oxidized and Na-Ca-Mg rich (up to 6 molar Cl) basinal diagenetic brines generated within continental, oxidized, organic free, clastic formations. Exceptional trapping conditions resulted from (i) the strong redox gradient developed between the oxidized Paleo- to Meso-Proterozoic sediments and the epicontinental Paleoproterozoic organic-rich metasediments of the basement and (ii) the creation of large openings by the combined effects of reverse faulting and intensive quartz dissolution in the sandstone. U-Pb geochronology and REE patterns of the U-oxides provide evidence for a succession of mineralization events rather than one unique ore-forming event.