Iron and sulfur isotopes of sulfides from the Wocan hydrothermal field, on the Carlsberg Ridge, Indian Ocean

Abstract The basalt-hosted Wocan hydrothermal field (WHF) was discovered in 2013 during Chinese Dayang 28th cruise, which locates on the slow-spreading Carlsberg Ridge, northwest Indian Ocean. Few investigation has been carried out for the hydrothermal activities and associated sulfide deposit in this area. Here, we report Fe and S isotope compositions of sulfide-minerals including chalcopyrite, bornite, pyrite and sphalerite collected from the WHF. The results show that all sulfide minerals display small δ34S variations (2.44‰ to 5.37‰, mean 4.00‰), and show wide δ56Fe ranges (-2.05‰ to 0.10‰, mean -0.92‰). Sulfur and iron isotopes in paired pyrite and chalcopyrite minerals reflect that it did not reach the equilibrium fractionation during pyrite precipitation, and support that the FeS precipitates first during the pyrite formation. The δ56Fe values of chalcopyrite and concurrent bornite are characterized by a well-defined negative correlation, which may be related to the repeated dissolution-reprecipitation of these copper-bearing sulfides in the Fe-limited conditions. Most pyrite from the WHF display a positive correlation between δ56Fe and δ34S values, in accord with those from other hydrothermal fields, which can be explained by the temperature gradient and similar degrees of isotope exchange in the stable hydrothermal system. In constant, δ56Fe and δ34S values of chalcopyrite exhibit a slightly negative correlation, in which the temperature gradient can explain the negative correlation, but cannot cover the wide δ56Fe and δ34S ranges. More work on fractionation parameters and chalcopyrite formation mechanisms are still needed.