Abstracts Earth and Environmental Sciences

The Paragenesis and Chemical Variation of Alteration Minerals Associated with Basement Rocks of the P2 Fault and the McArthur River Uranium Deposit, Athabasca Basin, Northern Saskatchewan, Canada

by Erin Elizabeth Adlakha

The P2 reverse fault in the metasedimentary basement rocks of the eastern Athabasca Basin is the main structural control of the world-class McArthur River uranium deposit. The earliest preserved assemblage along the fault is oxy-dravite, rutile, quartz, pyrite and graphite. This assemblage formed at temperatures of up to 890 °C, during regional metamorphism or a thermal event at ~1.75 Ga. The exhumation and surface exposure of the rocks was accompanied by paleo-weathering, and the deposition of the Athabasca sandstones. Diagenetic fluids of the sandstones altered the basement rocks to form Sr-Ca-SO42- rich aluminum phosphate sulfate (APS) minerals + hematite ± kaolin. The onset of hydrothermal activity along the basement and the P2 fault is recorded by local anatase at 1569 ± 31 Ma. Uraniferous fluid formed an assemblage of sudoite illite, magnesio-foitite and LREE+P rich APS minerals (rims earlier diagenetic-type APS minerals) along the entire P2 fault. Magnesio-foitite exhibits a high X-site vacancy (0.70 – 0.85 apfu) and contains high Al in its Y-site (0.70 – 1.12 apfu), suggesting that magnesio-foitite likely replaced kaolin. The REE pattern of magnesio-foitite is similar to that of uraninite (CeN<YN), likely due to their co-crystallization with LREE-rich APS minerals. APS minerals show variably high S/P ratios (0.05 - 0.21) in proximity to the deposit and low ratios (0.11 - 0.13) far from the deposit along the P2 fault, indicating reducing conditions in the ore zone. Low values of δD (-41 to -98 ‰) and high values of δ11B (+13.1 to +23.2 ‰) for magnesio-foitite suggest that groundwater interacted with carbonates or evaporites and was progressively enriched in 11B through interaction with illite and kaolin minerals. This work demonstrates that i) the P2 fault was a site of extensive fluid-rock interaction, ii) mineralizing fluids travelled along the entire P2 fault in the basement, iii) the deposition of the McArthur River deposit was controlled by the availability of a reducing fluid through the P2 fault, and iv) mineral chemistry (tourmaline and APS minerals) may help identify fertile faults in exploration for uranium deposits.