Hydrothermal fluid chemistry and implications for sulfide deposit formation at the ultramafic-hosted Semenov vent field, Mid-Atlantic Ridge (13°30′N)

Seafloor massive sulfide (SMS) deposits formed at ultramafic-hosted hydrothermal systems along slow- and ultraslow-spreading ridges are among the most metal-rich known on the seafloor, yet the processes governing metal transport and deposition in these environments remain poorly constrained. The Semenov hydrothermal field at 13°30′N on the Mid-Atlantic Ridge is one of the largest known ultramafic-hosted SMS systems, comprising multiple sulfide mounds developed on an oceanic core complex with a long-lived hydrothermal history (~124 kyr). This setting provides a valuable opportunity to link present-day hydrothermal fluid chemistry with the formation and preservation of extensive sulfide deposits.

Here we present the first detailed geochemical characterisation of hydrothermal fluids from the active Semenov-2 vent field, based on samples collected from three high-temperature vent sites (Ash Lighthouse, Phantom Urchin, and Yellow Submarine). Fluids were analysed for major elements, trace metals, volatiles, and isotopes, alongside mineralogical characterisation of associated chimney material. The chemical composition of end member vent fluids, calculated by extrapolation to zero magnesium, are similar across all three vents, consistent with a shared hydrothermal source. Relative to other ultramafic-hosted systems, Semenov fluids are characterised by elevated CO₂ concentrations but comparatively low metal and H₂S contents.

Chimney material recovered from the vent orifices were dominated by sulfate minerals (anhydrite-gypsum), with sulfide phases present only in minor amounts in the recovered chimney material. Together, the fluid and mineralogical data suggest that metal precipitation may occur predominantly beneath the seafloor, potentially driven by evolving pH-temperature conditions, redox state, and fluid-rock interaction associated with serpentinization. Alternatively, these signatures may reflect a waning or evolving hydrothermal system in which reduced or migrating heat input limits the transport of metals and reduced sulfur to the seafloor. These observations highlight the importance of subsurface processes in controlling metal fluxes and the development of SMS deposits in ultramafic-hosted hydrothermal systems.