Compositions of hydrothermal vent fluids as a guide to subseafloor mineralization processes

Studying active hydrothermal systems in the deep-sea provides unique opportunities for furthering our understanding of how polymetallic seafloor massive sulfide accumulations form. The possibility of sampling the ore-forming fluids that are emitted through sulfide-sulfate chimneys is particularly powerful. The use of gas-tight samplers in collecting hydrothermal vent fluids facilitates measurements of the contents of dissolved gases and metals and allows for accurate reconstructions of in situ pH and redox conditions. Metal transport in seafloor hydrothermal systems is affected by fluid-rock interactions, magma degassing, phase separation, and subseafloor mixing of the upwelling hydrothermal fluids with entrained seawater. The composition of basement hosting deep-sea hydrothermal vent systems, i.e. the type of rock involved in fluid-rock interactions, ranges from ultramafic to felsic. Geotectonic settings of vent systems vary from mid-ocean ridges to backarc spreading centers to island arc and intraplate volcanoes, which show strong contrasts in water depths and influx of magmatic fluids. Our recent compilation of vent fluid data (doi:10.1029/2020GC009385) allows a first complete assessment of how these differences affect the compositions of fluids in the root zones of hydrothermal systems. Beyond an examination of these general differences, valuable insights into processes in the discharge zone of hydrothermal systems can be obtained from detailed fluid sampling in individual vent fields. We present examples from selected arc/backarc hydrothermal vent sites in felsic crust for how vent fluid compositional data and thermodynamic computations can yield detailed insights into km-scale metal transport as well as smaller scale processes of zone refining.