Stable isotopic composition of the hydrothermal fluids responsible for the Nanisivik ZnPb deposits, Northwest Territories, Canada

Direct measurements of °18O and δD of water in inclusion fluids in sulfide and carbonate gangue minerals from the Nanisivik deposit are used to place constraints on the origin of the fluids responsible for hydrothermal mineralization. Values of δD of inclusion fluids in sphalerite and dolomite are near −50%, whereas those of inclusion fluids in two galena samples are much lower at −72 ± 3‰; δ18O of inclusion fluids in sphalerite are −2.5 ± 3‰; dolomite inclusions are 2‰ lighter, perhaps due to post-trapping exchange with host dolomite. Dolomite-water fractionations calculated from the inclusion data are consistent with other estimates of temperature of deposition (70°C to 250°C).Previous isotopic studies by the same authors have shown that S in the ore was derived from coeval sea water SO4. However, water in the hydrothermal fluid cannot have been trapped, unaltered sea water (either fresh or evaporated), nor coeval meteoric water (which would have resembled sea water), ultrafiltered sea water, or meteoric water. Values of δ18O and δD of inclusions can best be explained by a scenario which includes: 1. infiltration or entrapment of either sea water, or meteoric water that has redissolved evaporitic SO4; 2. exchange of the water with shales of the Arctic Bay Formation to lower δD, with subsequent leaching of the metals; and 3. further H isotope exchange with fluid or solid organic matter prior to entrapment.