Multi-source hydrothermal mineralisation in the ultramafic-hosted Mirae-2 vent field, Central Indian Ridge

Modern seafloor massive sulfide deposits distributed along mid-ocean ridges are typically classified as mid-ocean ridge basalt- and ultramafic-hosted types, based on mineralogical and geochemical characteristics that result from the different water–rock interactions between the two substrates. However, the Mirae-2 vent field (MVF-2) along Central Indian Ridge, which was newly discovered on the slope of an oceanic core complex, deviates from this common concept. Mineralogical and geochemical data indicate that the formation of chimney and mound samples was primarily controlled by changes in physicochemical fluid conditions (temperature, pH, ƒS₂, and ƒO₂) driven by varying degrees of fluid–seawater mixing. In particular, the prevalence of sulfide assemblages (pyrrhotite + isocubanite + Fe-rich sphalerite), the Cu–Au-rich mineralisation, and the enrichments of Co (average = 1109 ppm) and Sn (203 ppm) are similar to those of other ultramafic-hosted sulfide deposits, but the high amounts of barite and galena, and the enrichments of Ba (> 100,000 ppm) and Pb (up to 8.91 wt%) reflect the contribution of distinct metal sources other than ultramafic substrates. The extremely positive δ³⁴S values of pyrite (average = + 15.1 ± 1.7‰) and pyrrhotite (+ 6.37 ± 0.5‰) indicate that metals and S in the MVF-2 were likely derived from serpentinised ultramafic rocks with intense mixing of fluids with seawater, whereas the unusually radiogenic Pb isotope ratios of sphalerite (²⁰⁶Pb/²⁰⁴Pb = 18.531–18.559, ²⁰⁷Pb/²⁰⁴Pb = 15.540–15.564, and ²⁰⁸Pb/²⁰⁴Pb = 38.632–38.693) suggest that the enriched mid-ocean ridge basalts (i.e., MVF-2 basalts) near the ridge axis also had an important role in the supply of some metals (Pb and Ba) to the MVF-2 fluids. Our results indicate that the multi-stage fluid-rock reactions with basalt and subsequent ultramafic rocks produced the multi-source hydrothermal fluids, thereby resulting in the different mineralogy and geochemistry of the MVF-2 compared with other ultramafic-hosted sulfide deposits.