Constraints on fluid evolution and growth processes of black smoker chimneys by pyrite geochemistry: A case study of the Tongguan hydrothermal field, South Mid-Atlantic Ridge

•The Tongguan hydrothermal field is the southernmost SMS discovered so far on the South Mid-Atlantic Ridge. Its discovery provides clues for prospecting mineral resources in the South Mid-Atlantic Ridge.•The black smoker chimney in our study is a mature chimney and perfectly preserved which provides a unique opportunity to supply evidence to exhibit the on-going ore-forming process.•Cobalt concentrations in pyrite from the modern black smoker chimneys located on mid-ocean ridges apparently vary systematically in response to salinity change of vent fluids.•Different degrees of fluid-seawater mixing and phase separation are the main controlling factors during the black smoker chimney formation.