Source origin and ore-controlling factors of hydrothermal sulfides from the Tianzuo hydrothermal field, Southwest Indian Ridge

Abstract A newly confirmed hydrothermal sulfide ore hosted by ultramafic rocks was identified in the Tianzuo field on the Southwest Indian Ridge. In this study, detailed mineralogy, geochemistry, and stable isotope analyses were carried out to assess the source origin and mineralization of the Tianzuo field sulfide deposits. The mineralogical examination indicates that the massive sulfides recovered from the seafloor in this area exhibit multi-stage deposition features with four distinct mineralization stages. In terms of geochemistry, the deposit is characterized by high Fe (31.57–44.59 wt%), Co (up to 2400 ppm), and Ba contents (up to 47.89 ppm), and relatively low Cu + Zn (0.17–7.25 wt%) and Ni contents (3.74–30.6 ppm). These results somewhat different from the features of massive sulfides of the ultramafic-hosted ridge in the Atlantic Ocean. The elevated Ba and Co contents presumably reflect a contribution of deep gabbroic intrusions that, together with serpentinization, drove the high-temperature (∼335 °C) hydrothermal vent activity in the Tianzuo field. The Co distribution is influenced by high-temperature and high-gradient conditions apart from the basement rock. In contrast, the depletion of Ni suggests that most of the Ni-rich sulfides are concentrated in the deep stockwork mineralization, whereas the low Cu + Zn content indicates a partial influence of the depleted mantle. With regard to isotopic composition, the sulfide in the Tianzuo field shows relatively heavy in situ S isotopic values, which implies that the widely developed transform faults and episodic hydrothermal events allowed cold seawater to repeatedly rinse the entire plumbing system. In comparison, the bulk Pb isotopic composition indicates that Pb in these sulfide deposits is mainly derived from the leaching of basalts and peridotites by seawater. By integrating all of the evidence, we demonstrate that, apart from the basement rock composition, the pervasive transform faults and multistage hydrothermal activity are important factors that affect the composition of the Tianzuo field sulfide.