Compositional Variation and Timing of Aluminum Phosphate-Sulfate Minerals in the Basement Rocks Along the P2 Fault and in Association with the McArthur River Uranium Deposit, Athabasca Basin, Saskatchewan, Canada - 2015

Uranium (Nuclear Minerals) / Exploration, Mining, Marketing, & Acquisitions

Adlakha and Hattori reported in 2015 that the Athabasca Basin hosts world class uranium deposits, such as the McArthur River deposit. The authors opined onm the occurrence of aluminum phosphate-sulfate (APS) minerals in the metasedimentary rocks along the P2 fault, the main ore-hosting fault of the McArthur River deposit. It compares the APS minerals along the P2 fault with those outside the fault, examined in this study, and those from other deposits of the Athabasca Basin and from other Paleo- to Mesoproterozoic basins worldwide.

APS minerals are common along the P2 fault but rare outside of the P2 fault zone in the basement and along the unconformity between the Athabasca sandstones and the basement. The APS minerals along the P2 fault occur with sudoite (± illite, magnesiofoitite) and are zoned with Sr-, Ca-, and S-rich cores (solid solution between svanbergite, crandallite, and goyazite) and LREE- and P-rich rims (close to lorencite composition). APS minerals in the Bleached Zone (altered rocks along the unconformity consisting predominantly of kaolin and illite) are Sr-, Ca-, and S-rich (high svanbergite component) and occur with kaolin.

APS minerals in the Red-Green Zone (mingled red hematitic and green chloritic basement rocks below the Bleached Zone) occur with sudoite and clinochlore. They contain relict cores of LREE- and As-rich arsenolorencite-(Ce) and rims of svanbergite-goyazite-crandallite solid solution. The occurrence of svanbergite-crandallite-goyazite along the unconformity suggests their formation by relatively oxidizing luids during diagenesis of the overlying sandstones. The relict cores of arsenolorencite-(Ce) in the Red-Green Zone are interpreted to be the product of paleo-weathering before the deposition of the Athabasca sandstones.

Florencitic APS minerals are found along the entire studied strike length (7 km) of the P2 fault, including the ore zone and non-mineralized areas, but are
absent outside the fault zone. The lorencitic APS minerals contain low SO₄^2– in the ore zone, suggesting relatively reducing conditions during their crystallization. Zoned APS minerals (with svanbergitic cores and lorencitic rims) proximal to ore contain elevated U (up to 16 ppm). These features suggest
that diagenetic, oxidizing, and uranium-bearing luids traveled along the P2 fault and became relatively reduced, especially within the ore zone. It also suggests lorencitic APS minerals are contemporaneous with uranium mineralization. The restricted occurrence of lorencitic APS mineral along the P2 fault in the basement suggests their use in identifying fertile basement structures associated with uranium mineralization.

The authors concluded that APS minerals are very small in size, but ubiquitous in the altered metasedimentary rocks along the P2 fault, which hosts the McArthur River uranium deposit. Previous researchers noted the spatial association of florencitic APS minerals and uranium deposits and suggested their use as an exploration vector (e.g., Gaboreau et al. 2007). The authors' study revealed that APS minerals record the timing and characteristics of geological events, from paleoweathering, diagenesis to uraniferous hydrothermal activity.

The further conclude that oxidizing diagenetic fluids traveled along P2 fault and formed svanbergite-crandallite-goyazite. The fluid evolved to become reducing through interaction with the basement rocks and formed florencite rims. Their study showed that APS minerals represent an excellent mineralogical tool to fingerprint fluid pathways.

APS minerals with similar chemistry (high florencite component) are found along a 7 km of strike length of the P2 fault, including areas barren of uranium mineralization. This suggests that uranium-bearing fluids likely passed along the entire P2 fault, but only produced uranium mineralization in localized areas. The authors confirmed that APS minerals are good indicators of alteration related to uranium mineralization, thus they may be useful to identify fertile structures associated with uranium deposits.