Abstract

The Jasper mesothermal gold deposit, in northern Saskatchewan, Canada, is situated within the Proterozoic La Ronge domain of the Trans-Hudson orogen. The deposit occurs within a northeasterly striking, steeply northwest-dipping, mylonitic shear zone that crosscuts the Island Lake pluton, which has a zircon Pb-Pb age of 1855 + or - 8 Ma. 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 to 3 m 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 host rock adjacent to the veins is lacking except for slight 18 O enrichments of wall rocks up to 20 m 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 delta 18 O values of this barren quartz average 11.1 per mil, whereas the delta 18 O values of quartz comprised of at least 50 percent 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. 300 degrees C. Calculated delta 18 O and delta 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 87 Sr/ 86 Sr ratio of the auriferous fluid is 0.7028, which is similar to the surrounding igneous rocks, and implies that the strontium was most likely derived by leaching of Proterozoic 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 percent NaCl equiv, and homogenization temperatures of...

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