Sulfur sources for ultramafic‐hosted sulfide mineralization in the Tianzuo hydrothermal field, 63.5° E, South‐west Indian Ridge: Insights from sulfur and carbon isotopes

To date, discovered ultramafic‐hosted seafloor massive sulfide (UM–SMS) deposits are rare, remain poorly understood in terms of their genesis, and typically form along slow‐ and ultraslow‐spreading mid‐oceanic ridges. The Tianzuo hydrothermal field (THF) is the only UM–SMS deposit identified so far along the ultraslow‐spreading South‐west Indian Ridge (SWIR). This study presents new detailed mineralogical and S–C isotopic data for serpentinized peridotite, gabbro, serpentinite, and sulfide ore samples from the THF and to provide new insights into the processes that formed the deposit. Chromium‐reduced sulfur (CRS) and sulfate–sulfur (SS) in samples of serpentinized peridotite have positive δ34S values (up to 10.22‰ and 20.67‰, respectively). The CRS formed through thermochemical reduction of seawater sulfate during reactions between seawater and ultramafic rocks, whereas the SS most likely occurred as dissolved sulfate phases in seawater occupying the pore space of serpentinized peridotite after exposure at the seafloor. The serpentinite samples contain SS with δ34S values of 19.21‰–20.9‰, indicating infiltration by seawater that led to the deposition of seawater sulfate. The sulfide ores have acid‐volatile sulfur (AVS), CRS, and SS δ34S values of 10.99‰–12.07‰, 2.34‰–10.98‰, and −4.13‰ to 2.76‰, respectively, indicating that the AVS and CRS within the samples are dominated by sulfur reduced from seawater sulfate (e.g., anhydrite) and leached from the surrounding wall rocks, although the re‐oxidization of microbial‐reduced sulfur most likely generated the sulfate with low and negative δ34S values. The total carbon and total organic carbon δ13C values of the THF samples are negative and range from −21.99‰ to −0.34‰ and −26.7‰ to −21.02‰, respectively, further suggesting that the samples record microbial activity. Gabbro samples have strongly negative AVS (−10.69‰) and CRS (−7.2‰) δ34S values, indicating that microbial‐reduced sulfur derived from shallow levels in the crust was incorporated into the hydrothermal system that circulated through the deeper‐seated gabbroic units. Our results suggest that the ultramafic and gabbroic rocks, seawater, and microbial activity all provided sulfur to the THF, with well‐developed fractures and local magma supply probably being the key factors controlling the formation of this field and similar UM–SMS deposits elsewhere.