The Millennium uranium deposit, Athabasca Basin, Saskatchewan, Canada: An atypical basement-hosted unconformity-related uranium deposit

The Millennium uranium basement-hosted unconformity-related deposit is one of the most recent discoveries of basement-hosted deposits, and may hold as much as 46.8 million pounds (18,000 metric tonnes U) at an average grade of 4.53 percent U₃O₈. The alteration zone associated with the deposit is atypical, as it consists predominantly of muscovite rather than the chlorite and muscovite commonly found in the other basement-hosted deposits of the Athabasca basin.

The preore stage consists of early minor clinochlore alteration at temperatures of about 275°C, interpreted to result from retrograde metamorphism of the basement rocks prior to the deposition of the Athabasca basin. This clinochlore event is followed by an extensive 1Mc muscovite alteration of the basement rocks by Na-K-Fe-rich basinal brines at temperatures of about 250°C, creating up to 25 percent void space in the basement rocks. Subsequently, a weak chamoisite alteration resulted from Fe-Mg-Ca-rich fluids with temperatures of about 300°C that evolved from reactions between the basinal brines and the basement rocks. Ore stage uraninite has an extrapolated chemical age of 1590 Ma and is contemporaneous with synore 1Mc muscovite and iron oxide deposited at temperatures near 250°C from fluids with similar δ¹⁸O but lower δD values to preore basinal fluids. The low δD values indicate postdepositional exchange of H isotopes with Cenozoic fluids. Postore alteration consists of dravite and sudoite precipitated from Mg-Ca rich fluids, from the same evolved basinal brines that precipitated preore chamoisite, at temperatures of about 185°C, filling voids created during the preore stage. Our results indicate that the pre- and synore minerals were formed dominantly by basinal fluids, with no contribution from basement fluids, which differ from some other basement-hosted deposits in the Athabasca basin where basement-derived fluid influx was more significant.

Several U-Pb and Ar-Ar resetting events recorded on preore muscovite and on uraninite are coincident with fluid flow induced by distal orogenies, remobilizing radiogenic Pb from the deposit. The radiogenic Pb remobilization events were not widespread as the presence of radiogenic Pb is restricted to basement rocks proximal to the deposit and along the unconformity.