Cloutier, and 4 co-authors, reported in 2009 that 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 pre-ore 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 1 Mc 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 (aka high saline groundwater) and the basement rocks. Ore stage uraninite has an extrapolated chemical age of 1590 Ma and is contemporaneous with syn-ore 1Mc muscovite and iron oxide deposited at temperatures near 250°C from fluids with similar δ18O but lower δD values to pre-ore basinal fluids (aka groundwater). The low δD values indicate post-depositional exchange of H isotopes with Cenozoic fluids. Post-ore alteration consists of dravite and sudoite precipitated from Mg-Ca rich fluids, from the same evolved basinal brines that precipitated pre-ore chamoisite, at temperatures of about 185°C, filling voids created during the pre-ore stage.
The authors found that the pre- and syn-ore minerals were formed dominantly by basinal fluids (aka groundwater), with no contribution from deep 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 pre-ore 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.
The authors concluded that The atypical alteration halo at Millennium is the result of pervasive muscovite alteration of the basement rocks by Na-K-Fe basinal brines (aka groundwater) during the pre-ore stage at ca. 250°C and the lack of predominant pre-ore clinochlore alteration. As alteration of the basement rocks progressed, the basinal brines acquired Ca, Fe, and Mg and precipitated chamoisite at ca. 300°C at a similar stratigraphic level as the mineralization. This chamoisite alteration might have been one of the reductants for the precipitation of uraninite during the syn-ore stage. Pre-ore muscovite and chamoisite alteration were important for the formation of the deposit as basement rocks altered to muscovite by basin-derived fluids created up to 20 percent voids in the basement rocks while chemically modified basinal fluids emplaced an Fe-rich chamoisite that demarcates a redox front during the syn-ore stage where uranium ore was precipitated.
Uranium mineralization occurred at ca. 1590 Ma, primarily from basin-derived fluids (groundwater). Post-ore dravite and sudoite precipitated from the residual Ca-Na-Mg fluids filling the voids created during the pre-ore stage and sealing the system. Timing of post-ore "sudoide" alteration was no more than 160 m.y. after the mineralizing event. Pb mobilized from the deposit by different resetting events did not infiltrate the sandstone, except along fractures, and affected the basement up to 100 m from the ore zone.
Managed & Built by I2M Consulting, LLC | © 2010-
