Hydrothermal Rare Earth Element (Xenotime) Mineralization at Maw Zone, Athabasca Basin, Canada, and Its Relationship to Unconformity-Related Uranium Deposits

The Maw zone rare earth element (xenotime) deposit is hosted in brecciated sandstones in the Athabasca Basin and is associated with silicification, hematitization, and tourmalinization magnesiofoitite. Petrographic son-lies indicate that xenotime precipitated after significant compaction of the host rocks and was coeval with the main phase of tourmaline and drusy quartz. Sensitive high-resolution ion microprobe U-Pb dating of xenotime yielded a Pb-207/Pb-206 age of 1547 +/- 14Ma. In situ secondary ion mass spectrometry-oxygen isotope analysis of coeval tourmaline and quartz gives temperatures from 89 degrees to 385 degrees C (average 185 degrees C) and delta O-18,(VSMOW-Fluid) values from -6.2 to +10.8%(0) (average +1.6%(0). l'he coexistence of different types of fluid inclusions (liquid dominated biphase, vapor-dominated biphase, vapor only, and halite-bearing triphase) in individual fluid inclusion assemblages in drusy quartz suggests boiling and heterogeneous trapping. Excluding the heterogeneously entrapped find inclusions, the liquid-dominated biphase inclusions yielded homogenization temperatures from 63 degrees to 178 degrees C, ice-melting temperatures mainly from -37.4 degrees to -21.8 degrees C, and salinities mainly from 22.0 to 28.3 wt %. Raman spectroscopic and mass spectrometric analyses indicate low nonaqueous volatile concentrations (<1 mole %). In conjunction With regional stratigraphy indicating burial depth less than 2.5 km at the time of mineralization, these data suggest an epithermal mineralization environment The similarities in wall-rock alteration, paragenesis, mineralization age, oxygen isotopes, and fluid inclusion attributes between the Maw zone rare earth element deposit and unconformity-related uranium deposits in the region invokes a possible genetic link between them. The hydrothermal fluid at Maw zone probably represents one of the Wilds that carried reducing agents and caused uraninite precipitation in the unconformity-related uranium deposits.