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Abstract

The timing of Cu-Mo-U mineralisation at the Nori/RA prospect in the Paleoproterozoic Great Bear magmatic zone has been investigated using Re-Os molybdenite and (40)Ar-(39)Ar biotite geochronology. The Re-Os molybdenite ages presented are the first robust sulphide mineralisation ages derived from the Great Bear magmatic zone. Cu-Mo-U mineralisation is hosted in early to syn-deformational hydrothermal veins consisting of quartz and K-feldspar or more commonly tourmaline-biotite-quartz-K-feldspar, with associated wall-rock alteration assemblages being predominantly biotite. Sulphide and oxide minerals consist of chalcopyrite, molybdenite and uraninite with lesser pyrite and magnetite. Elevated light rare earth elements and tungsten concentrations associated with the Cu-Mo-U mineralisation have also been reported at the prospect by previous workers. Molybdenite and uraninite occur intimately in dravitic tourmaline growth zones and at grain margins, attesting to their syngenetic nature (with respect to hydrothermal veining). Two molybdenite separates yield Re-Os model ages of 1,874.4 +/- 8.7 (2 sigma) and 1,872.4 +/- 8.8 Ma (2 sigma) with a weighted average model age of 1,873.4 +/- 6.1 Ma (2 sigma). Laser step heating of biotite from the marginal alteration of the wall-rock adjacent to the veins yields a (40)Ar-(39)Ar maximum cooling age of 1,875 +/- 8 Ma (MSWD=3.8; 2 sigma), indistinguishable from the Re-Os molybdenite model age and a previously dated 'syn-tectonic' aplitic dyke in the region. Dravitic tourmaline hosts abundant primary liquid-vapour-solid-bearing fluid inclusions. Analytical results indicate liquid-vapour homogenisation at >260 degrees C constraining the minimum temperature of mineralisation. The solids, which are possibly trapped, did not homogenise with the liquid-vapour by 400 degrees C. Salinities in the inclusions are variable. Raman spectra identify that at least some of the solids are calcite and anhydrite. Raman spectra also confirm the vapour phases contain some CO(2); whereas clathrates or CH(4) was not observed or detected. Quartz grains only host secondary fluid inclusions, which fluoresce under ultraviolet light, indicating trapped hydrocarbons. We speculate that these resulted from Phanerozoic fluid circulation through the Proterozoic basement. The collective interpretation of the age, hydrothermal character and associated metals, high temperature and variable salinity suggests that the Nori/RA Cu-Mo-U mineralisation can be linked with the earliest stages of plutonism in the Great Bear magmatic zone. From a regional perspective, the mineralisation may pre-date the extensive multi-element mineralisation now recognised as part of the iron oxide copper-gold (IOCG) spectrum of deposits. As IOCG provinces generally contain a variety of mineralisation styles, we interpret this as the earliest phase of the extensive mineralising system.