Mineral assemblages and metal concentrations of seafloor massive sulfides strongly influence the fate of heavy metals released during deep-sea mining.

Seafloor massive sulfide (SMS) deposits are economically valuable, but their exploitation risks heavy metal release from oxidative dissolution of mining-derived sulfide particulates. This study investigated heavy metal mobility via membrane-confined in situ deep-sea incubation of seven settled sulfide particulates over 2-35 days at hydrothermal fields in the Indian Ocean. We find that oxidation rates are highly dependent on mineral assemblages. Chemically, most sulfides generally exhibited an initial rapid oxidation phase followed by a slower phase, while high-purity chalcopyrite resisted oxidation. Hydrous ferric oxides (HFOs) formed during sulfide oxidation can effectively adsorb and immobilize heavy metals. Heavy metal mobility is closely linked to their initial concentrations in sulfides. At low concentrations, HFOs sequester released metals and adsorb additional metals from the surrounding environment. At high concentrations, unretained metals are lost from the oxidation residues and may pose an environmental hazard. At the study sites, using local seawater metal concentrations as guidelines, metals from pyrite-rich sulfides required no or only ∼10-200 times dilution to approach background levels, whereas Zn, Cd, and Pb from sphalerite-rich and galena-rich sulfides required ∼1000-30,000 times dilution. We propose that chalcopyrite-rich, galena-poor SMS deposits with extensive HFO coatings merit more detailed pre-mining geochemical risk assessment.