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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. [ABSTRACT FROM AUTHOR]

The origin and nature of fluids responsible for U deposits of the Athabasca Basin have been in debate. This study presents the trace element abundances and stable isotope ratios (D/H and 11 B/ 10 B) of well characterized tourmaline samples determined by in situ analytical techniques, in order to evaluate the nature of fluids during the mineralization of the McArthur River U deposit, the world's largest high-grade U deposit. Magnesio-foitite, an alkali-deficient Mg-rich tourmaline, is abundant along the 13-km long P2 fault, a reverse structure that controls the location of the McArthur River deposit. Magnesio-foitite contains variable concentrations of rare earth elements (REEs, up to 46 ppm in total) and Y (up to 35 ppm). Chondrite normalized patterns indicate low light REEs (LREEs) relative to heavy REEs (HREEs) (Ce N < Y N , Y as a proxy for HREEs) and slight negative Eu anomalies, similar to those of uraninite from the deposit. The data combined with textural evidence suggest that magnesio-foitite and uraninite co-crystallized with LREE-rich aluminum phosphate sulfate minerals. Relatively high REE contents of magnesio-foitite in the ore zone indicates the incursion of a REE-rich basement fluid. Low values of δD (− 98 to − 41‰) for magnesio-foitite suggests that the mineralizing fluid originated from groundwater. Variably high values of δ 11 B (+ 13.1 to + 23.2‰) are explained by the dissolution of B from carbonate or evaporitic rocks and preferential removal of 10 B by the crystallization of illite and kaolinite. The results of this study support the ascent of a REE-rich basement fluid during mineralization and extensive modification of basinal fluids through the crystallization of clay minerals. [ABSTRACT FROM AUTHOR]