LAUSR

Abstract In-situ sulfur isotopic compositions of morphologically different pyrites from the SMAR (southern Mid-Atlantic Ridge) 26°S hydrothermal field integrated with S and Cu isotopic values in chalcopyrite reflect a recurring flux of magmatic-hydrothermal fluids. 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, significant contributions of magmatic fluids are herein distinctive. Three concentric circular mineralogical zones around the conduit in an inactive black smoker sample are include an inner chalcopyrite-dominated zone, an intermediate idiomorphic and massive pyrite zone, and an outermost zone of porous and colloform pyrite-dominated zone with chalcopyrite becoming trace mineral. The systematic increase in δ34S from the interior conduit (−0.60‰, n = 8) through the intermediate zone (+2.36‰, n = 14) to the exterior wall (+3.02‰, n = 27), and contributions from seawater sulfate (0 to 10.7%, and to 18.6%, respectively) are most likely related to interaction between previously precipitated sulfides and hydrothermal fluid. These results suggest a greater contribution of seawater sulfate sulfur via permeability or porosity upsurge across the chimney. The δ65Cu values of chalcopyrite vary from +0.17 to +0.48‰, which are typical of mid-ocean ridge (MOR) hydrothermal vent fluids. The δ65Cu variations are most likely influenced by reaction/mixing with later hydrothermal fluids. Sulfur and Cu isotopic composition of chalcopyrite further imitate the significant contribution of magmatic gases in metalliferous seafloor sulfides.