Two episodes of sulfide mineralization at the Yuhuang-1 hydrothermal field on the Southwest Indian Ridge: Insight from Zn isotopes

Abstract

The newly discovered Yuhuang-1 hydrothermal field (YHF) on the Southwest Indian Ridge primarily contains two different sulfide distribution areas: the southwest sulfide (SWS) and the northeast sulfide (NES) distribution areas. In this study, Zn isotope compositions and element ratios of Zn, Fe, Cu, and Cd in the surface sulfides were analyzed to characterize Zn isotopic fractionation, ore-forming conditions, and mineralization processes of the YHF. The results indicate that sulfide-rich samples in the YHF exhibit intensive Zn isotopic fractionation, with δ66Zn values ranging from −0.43‰ to 1.24‰. These values are not only larger than those currently reported for seafloor sulfides but are also larger than the values reported for the deposits on land. Remarkably, the obtained δ66Zn value (−0.43‰) is the lowest one observed in seafloor sulfides (−0.26‰–1.17‰), currently. A study of Zn isotope systematics revealed that Zn isotope variations of sulfide-rich samples in the YHF were controlled by the sphalerite and pyrite content; the study also revealed that early minerals are rich in light Zn isotopes, but later minerals are rich in heavy Zn isotopes. A comparative study of previous data on Zn isotopes shows that Zn isotope fractionations in the YHF are likely attributed to Rayleigh fractionation, caused by mineral precipitation and multiple generations of pyrite. Further, Zn/Cd ratios of sulfide-rich samples indicate that the ore-forming temperature of the silicified sulfide-rich samples in the SWS probably formed under lower temperatures as compared with the silicified sulfide-rich chimney in the NES. Furthermore, sulfur and Zn isotope compositions in the SWS are significantly different from those in sulfide-rich samples of the NES, which suggests pervasively different metallogenic conditions, further indicating that they are probably products of separate mineralization episodes.