Origin, timing, and fluid characteristics associated with paleoproterozoic Jasper lode-gold deposit, Saskatchewan Canada
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The Jasper mesothermal gold deposit, in northern Saskatchewan, Canada, is situated within the Paleoproterozoic La Ronge domain of the Trans-Hudson orogen. The deposit occurs within a northeasterly striking, and steeply northwest-dipping, mylonitic shear zone which cross-cuts the Island Lake pluton having a zircon Pb-Pb age of 1855 +/- 8Ma. Gold mineralization, which is hosted by subvertical quartz veins that occupy the most strained portions of the shear zone, occurs in late fractures and as particles within recrystallized quartz. The gold mineralized zones are 2-3 metres wide and plunge steeply to the northeast. Mineralized veins have a complex paragenesis wherein recrystallized quartz, muscovite, sphalerite, galena, and chalcopyrite are paragenetically associated with gold. Obvious alteration of the granitic hostrock adjacent to the veins is lacking except for slight 18O-enrichments of wallrocks up to 20 metres from the veins. Quartz veins which occur peripheral to the most-strained regions of the shear zone are discontinuous bands with little to no subgrain development and do not contain gold. The Î´18O values of this barren quartz average 11.1 per mil, whereas the Î´18O values of quartz comprised of at least 50% subgrains, and associated with gold mineralization, average 12.9 per mil. Oxygen isotope fractionation between quartz and coexisting muscovite associated with gold indicate gold mineralization occurred at ca. 300Â°C. Calculated Î´18O and Î´D values for the ore-forming fluid are compatible with fluids derived from, or which have interacted with, igneous or metamorphic rocks at low water/rock ratios. The initial 87Sr/86Sr ratio of the auriferous fluid is 0.7028, which is similar to the surrounding igneous rocks, and implies that the strontium was likely derived by leaching of Paleoproterozoic rocks similar in age and strontium isotope composition to those presently exposed on surface. Two-phase secondary aqueous fluid inclusions outline and dominate healed fracture planes in both barren quartz veins and in the less deformed portions of mineralized quartz veins. The fluids in these inclusions have low salinities, generally not exceeding 7 wt% NaCl equivalent, and homogenization temperatures of 150Â°-200Â°C. Fluid inclusions within dynamically recrystallized quartz tend to concentrate at quartz subgrain boundaries along with gold, are vapour-rich, have complex gas and cation compositions, homogenization temperatures between 300Â°C and 375Â°C, and show characteristics indicative of fluid-phase immiscibility. The age of gold mineralization was close to 1720 Ma, as indicated by Ar-Ar systematics of gold-associated muscovite, and Rb-Sr ages of muscovite and tourmaline. The 1720 Ma age indicates gold mineralization occurred ca. 100 million years after peak regional metamorphism, initial development of the shear zone and quartz vein emplacement and the cessation, at ca. 1820 Ma, of collision in this section of the Trans-Hudson orogen. These data are consistent with a model wherein earlier quartz veins were reactivated and partially recrystallized during a post- tectonic, gold-depositing fluid event. Gold distribution was regulated by the development of fracture-controlled permeability and by H2O-CO2 phase separation of a low salinity, CO2-rich, metamorphic fluid at 300Â°C to 375Â°C, which was derived from a source similar to the igneous rocks spatially associated with the deposit.