Bulk geochemistry, sulfur isotope characteristics of the Yuhuang-1 hydrothermal field on the ultraslow-spreading Southwest Indian Ridge

The Yuhuang-1 hydrothermal field (HF) was discovered recently on the ultraslow-spreading Southwest Indian Ridge (segment 29). This field comprises two main sulfide deposition areas, including the southwest sulfide area (SWS) and the northeast sulfide area (NES), all of which are about 500 m in diameter. Sulfide-rich samples collected on the seafloor consist of zinc-rich, iron-rich, and silicified samples. The Zn-rich samples contain up to 60% sphalerite and 5% chalcopyrite, while iron-rich samples primarily contain pyrite and marcasite, and silicified sulfide rich samples primarily consist of amorphous silicon (70%), pyrite (15%-20%), and minor sphalerite. Sulfide rich samples generally show mound features with similar mineral assemblages, except that the NES contains pyrrhotite, indicating precipitation under relatively reduced conditions. Bulk geochemistry of the SWS and NES massive sulfide samples display similar Au, Co, Ni, and Si contents and Ni/Co ratios, similar to mafic related HFs, while the Sn content and the Cd/Zn ratio are comparable to ultramafic-related HFs, which probably indicates a multi-stage origin. Sulfides rich samples from the SWS have delta S-34 values ranging from -1.37 parts per thousand to 6.02 parts per thousand with a median value of 0.83 parts per thousand. Currently, the delta S-34 value of -1.37 parts per thousand is the lowest sulfur isotope composition observed in HFs on ultraslow-spreading ridges. Bacteria-derived sulfur, which is calculated to be 10%-25% of the total sulfur component, is likely the cause of the negative sulfur isotope composition. Conversely, the delta S-34 values of the seafloor sulfides rich samples in the NES ranges from 3.75 parts per thousand to 8.73 parts per thousand with a median value of 4.74 parts per thousand, which is distinctly heavier than that of the SWS. This study suggests that the SWS and NES of the Yuhuang-1 HF probably formed during different mineralization stages.