Abstract

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 studies 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 207Pb/206Pb age of 1547 ± 14 Ma. In situ secondary ion mass spectrometry oxygen isotope analysis of coeval tourmaline and quartz gives temperatures from 89° to 385°C (average 185°C) and δ18Ovsmsow-Fluid values from −6.2 to +10.8‰ (average +1.6‰). The coexistence of different types of fluid inclusions (liquiddominated 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 fluid inclusions, the liquid-dominated biphase inclusions yielded homogenization temperatures from 63° to 178°C, ice-melting temperatures mainly from −37.4° to −21.8°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 fluids that carried reducing agents and caused uraninite precipitation in the unconformity-related uranium deposits.

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