The P2 fault, a 13 km-long steeply dipping reverse fault, is the main structural control of the McArthur River uranium deposit in the eastern Athabasca Basin, northern Saskatchewan, Canada. Three types of tourmaline were observed in the metasedimentary basement rocks along the P2 fault: early oxy-schörl [(Na0.470.37Ca0.16)(Fe2+1.30Al0.91Mg0.72Ti0.07)Al6(Si5.79Al0.21O18)(BO3)3OH3(O0.63OH0.29F0.08), where □ = vacancy] of metamorphic-anatectic origin, hydrothermal oxy-dravite [(Na0.57Ca0.230.18K0.02)(Mg1.93Fe2+0.62Al0.29Ti0.15)Al6(Si5.93Al0.07O18)(BO3)3OH3(O0.57OH0.23F0.20)], and magnesio-foitite [(□0.77Na0.20Ca0.02K0.01)(Mg1.99Al0.92Fe3+0.07)Al6(Si6O18)(BO3)3(OH3)(OH0.71O0.25F0.04)]. Oxy-schörl formed in granitic pegmatites, a partial melt product of the metasediments during peak metamorphism. Oxy-dravite formed from hydrothermal fluids after the peak metamorphism but before deposition of the Athabasca sandstones, whereas magnesio-foitite is a product of later, low-temperature hydrothermal activity. Both oxy-schörl and oxy-dravite are coarse-grained (from 500 μm up to 1 cm), whereas magnesio-foitite occurs as radial aggregates of fine, prismatic crystals (<15 μm in width). Magnesio-foitite crystallized together with sudoite, illite, and “APS” minerals (alunite supergroup LREE-rich aluminum phosphate-sulfate minerals) along the entire studied length (∼7 km) of the P2 fault and is abundant in proximity to the Zone 2 ore body of the McArthur River deposit. In the ore zone, the assemblage occurs with uraninite and is partially overprinted by late, remobilized uraninite and sudoite. Therefore, magnesio-foitite is likely contemporaneous with the main stage of uranium mineralization. It is characterized by a high vacancy at its X-site (0.70–0.85 apfu) and high Al at its Y-site (0.70–1.12 apfu), suggesting that magnesio-foitite likely replaced pre-existing high-Al phases, such as kaolin and sudoite. The occurrence of magnesio-foitite along the entire P2 fault, in both areas of mineralization and apparently barren areas, suggests chemically similar fluids travelled along the entire P2 fault, but only produced ore in localized areas.

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