Sulfur and copper isotopic composition of seafloor massive sulfides and fluid evolution in the 26 °S hydrothermal field, Southern Mid-Atlantic Ridge

In-situ sulfur isotopic compositions of morphologically different pyrites from the SMAR (southern Mid-Atlantic Ridge) 26 degrees S hydrothermal field, integrated with delta S-34 and delta Cu-65 values in associated chalcopyrite, reflect a recurring flux of magmatic-hydrothermal fluids and subsequent leaching by seawater. The delta S-34 values of sulfides (1.27 to +5.25 parts per thousand) indicate mixing of magmatic sulfur with seawater sulfate-derived sulfur. Compared to other Mid-Ocean Ridge (MOR) hydrothermal vent sites, a significant contribution of magmatic fluids is herein distinctive. Three concentric mineralogical zones around the conduit in an inactive black smoker sample include an inner chalcopyrite-dominated zone, an intermediate zone of idiomorphic and massive pyrite, and an outermost zone of porous and colloform pyrite with/without traces of chalcopyrite. The systematic increase in delta S-34 from the interior conduit (avg. 0.60 parts per thousand, n = 8) through the intermediate zone (avg. +2.36 parts per thousand, n = 14) to the exterior wall (avg. +3.02 parts per thousand, n = 27), and contributions of seawater sulfate (0 to 10.7%, and to 18.6%, respectively) are most likely related to interaction between hydrothermal fluid and previously precipitated sulfides. A significant contribution of seawater sulfate-derived sulfur occurred likely via permeability or porosity upsurge across the chimney. Chalcopyrite with consistently positive delta Cu-65 values (+0.17 to +0.48 parts per thousand), which are typical of mid-ocean ridge (MOR) hydrothermal vent fluids, is suggestive of reaction/mixing with hydrothermal fluids. Sulfur and Cu isotopic compositions of chalcopyrite further imitate the significant contributions of magmatic gases in the formation of metalliferous seafloor massive sulfides.