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 identified in 2013 during Chinese Dayang 28th cruise, which is located on the slow-spreading Carlsberg Ridge, northwest Indian Ocean. Few investigation has been carried out on the hydrothermal activities and associated sulfide deposit in this area. Herein, we report Fe and S isotope compositions of sulfide minerals including chalcopyrite, bornite, pyrite and sphalerite collected from the WHF. The results showed that all sulfide minerals display small δ34S variations (2.44‰ to 5.37‰, with an average of 4.00‰), and a wide δ56Fe range (−2.05‰ to 0.10‰, with an average of −0.92‰). Sulfur and iron isotopes in paired pyrite and chalcopyrite minerals reveal 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 under Fe-limited conditions. Most pyrites from the WHF demonstrate 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. Conversely, the 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 ranges of δ56Fe and δ34S values. More work on fractionation parameters and chalcopyrite formation mechanisms is still needed. In summary, the coupling of Fe and S isotopes is a useful tool in helping to delineate processes of sulfide mineralization in hydrothermal systems