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°S hydrothermal field, integrated with δ34S and δ65Cu values in associated chalcopyrite, reflect a recurring flux of magmatic-hydrothermal fluids and subsequent leaching by seawater. The δ34S values of sulfides (−1.27 to +5.25‰) 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 δ34S from the interior conduit (avg. −0.60‰, n = 8) through the intermediate zone (avg. +2.36‰, n = 14) to the exterior wall (avg. +3.02‰, 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 δ65Cu values (+0.17 to +0.48‰), 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.