Sulphides to weathering products: metal evolution in seafloor massive sulphide deposits at the Semenov hydrothermal cluster, Mid-Atlantic Ridge

Abstract Seafloor massive sulphide (SMS) deposits are potential sources of Cu, Zn, Au and Ag, however, how seafloor weathering affects the content of these metals in the deposits remains poorly understood. Key sulphide weathering products, i.e. Fe-oxyhydroxide and atacamite, can provide insight into metal mobilisation during and after oxidation of sulphides. Ten Fe-oxyhydroxide and three massive sulphide samples from the Semenov hydrothermal cluster (13°30′N, Mid-Atlantic Ridge) were studied; five Fe-oxyhydroxides and all sulphides were analysed by LA-ICP-MS with one sulphide sample analysed by LA-TOF-ICP-MS mapping. Additionally, a sequential leaching study comprising nine Fe-oxyhydroxide samples was performed. Copper is mainly sequestered into the sulphide weathering products atacamite (~ 60%) and Fe-oxyhydroxide (~ 30%). Initially, Fe‑oxyhydroxide precipitates as ferrihydrite, inheriting the metal content of its sulphide precursor (e.g., pyrite or chalcopyrite). Concurrently, Au and Ag are displaced from the sulphide and become concentrated along the oxidation front at the sulphide rims rather than being incorporated into the nascent Fe‑oxyhydroxide. As oxidation proceeds, further breakdown of sulphides such as chalcopyrite, releases Cu that initially adsorb onto existing Fe‑oxyhydroxide. When sufficient mobilised Cu reacts with oxygenated seawater, atacamite precipitates, sequestering the Cu in a stable mineral form, serving as an exploration indicator for Cu‑rich Fe‑oxyhydroxide deposits and underlying massive sulphide deposits. Complete sulphide oxidation leads to the loss of Au and Ag from the solid system into seawater or their migration downwards toward a redox boundary within the SMS mound. Crystallisation of ferrihydrite into more stable goethite liberates substantial amounts of Cu (92%) and Zn (57%). However, the metal loss caused by this process may be limited if released metals re-adsorbed onto adjacent Fe‑oxyhydroxide. These results reveal that Fe‑oxyhydroxides can act as effective long‑term traps for Cu and Zn, suggesting that older, fully or partially oxidised off-axis SMS deposits may retain considerable base metals, expanding exploration areas.