Characterizing fluids associated with the McArthur River U deposit, Canada, based on tourmaline trace element and stable (B, H) isotope compositions

Abstract

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 11B/10B) 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 46ppm in total) and Y (up to 35ppm). Chondrite normalized patterns indicate low light REEs (LREEs) relative to heavy REEs (HREEs) (CeN<YN, 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 δ11B (+13.1 to +23.2‰) are explained by the dissolution of B from carbonate or evaporitic rocks and preferential removal of 10B 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.