PYRITE ENRICHMENT IN THE P2 FAULT AT THE MCARTHUR RIVER URANIUM MINE, ATHABASCA BASIN, SASKATCHEWAN: POSSIBLE REDOX CONTROLS ON MINERALIZATION

The Proterozoic McArthur River mine is the largest high-grade uranium deposit in the world. Fault-controlled mineralization in this unconformity-related deposit occurs in two distinct settings. Nose or wedge ore is located within the Athabasca sandstone along the unconformity at the tip of the post-Athabasca reactivated P2 reverse fault wedge. Basement ore occurs in the P2 fault and the immediate footwall, as deep as 120 m below the unconformity.

Genetic models for McArthur River invoke oxidizing meteoric groundwater that initially leached uranium from the sedimentary basin rocks. Subsequent reduction of the groundwater flowing along the unconformity is thought to have resulted in uranium precipitation. The nature of the reductant is unknown, but may have included reducing basement rocks such as graphitic schists or basement-derived fluids migrating up along the P2 fault.

Drill core logging was conducted to study the nature of the P2 fault along strike, down dip of, and within the ore bodies. Most striking is the pyrite enrichment along the fault, which may include as much as 10 m of core containing up to 20 percent pyrite. Pyrite is most abundant within the P2 fault and decreases in abundance outward. Visible bleaching of the basement rocks occurs in proximity to the P2 fault. In addition to core logging, systematic sampling was performed to study the pyrite enrichment within the P2 fault and to constrain the alteration mineralogy of the basement rocks.

Thin section microscopy suggests that euhedral quartz crystals were the first hydrothermal minerals formed along the P2 fault. Fluid inclusion data suggests a progression from high- to low-salinity fluids during quartz growth, and a decrease in temperature from deeper in the basement up to the unconformity. The large amount of pyrite visible in drill core formed paragenetically after the euhedral quartz. The first uraninite generation replaced quartz and vein-hosted pyrite, and is rimmed by darker uraninite of the second generation. Marcasite formed between these uraninite generations. Chalcopyrite is intergrown with the second uranium generation and was replaced by the third uraninite generation. The study shows that the massive pyrite enrichment can be linked to fluid flow along the P2 fault, which has implications to current models for the formation of McArthur River.