The weathering process of seafloor massive sulphide (SMS) deposits can be considered analogous to weathering of terrestrial volcanogenic massive sulphides (VMS) deposits. However, in the context of SMS deposits, the process occurs in chemically buffered waters of near neutral pH, resulting in the formation of insoluble Fe oxy-hydroxide minerals including goethite and hematite as well as sulphates such as jarosite. As a result of this precipitation, it is commonly assumed that any SMS deposit is unlikely to exhibit a significant loss of metals (dissolution and release of heavy metals) into the water column. However, galvanic interactions have never been considered in this seafloor context, whilst they have already been shown to have the ability to increase dissolution significantly in terrestrial deposits. If heavy metal release is not temporally balanced by precipitation of oxide phases, there is the potential that these metal occurrences lose economic value. This is specifically significant if there is an SMS deposits Oxidation Galvanic Cells Metal release Economic potential Mining impact

With mining of seafloor massive sulfides (SMS) coming closer to reality, it is vital that we have a good understanding of the geochemistry of these occurrences and the potential toxicity impact associated with mining them. In this study, SMS samples from seven hydrothermal fields from various tectonic settings were investigated by in-situ microanalysis (electron microprobe (EMPA) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS)) to highlight the distribution of potentially-toxic trace elements (Cu, Zn, Pb, Mn, Cd, As, Sb, Co, Ni, Bi, Ag and Hg) within the deposits, their minerals and textures. We demonstrate that a combination of mineralogy, trace element composition and texture characterisation of SMS from various geotectonic settings, when considered along with our current knowledge of oxidation rates and galvanic coupling, can be used to predict potential toxicity of deposit types and individual samples and highlight which may be of environmental concern. Although we cannot quantify toxicity, we observe that arc-related sulfide deposits have a high potential toxicity when compared with deposits from other tectonic settings based on their genetic association of a wide range of potentially toxic metals (As, Sb, Pb, Hg, Ag and Bi) that are incorporated into more reactive sulfosalts, galena and Fe-rich sphalerite. Thus, deposits such as these require special care when considered as mining targets. In contrast, the exclusive concern of ultra-mafic deposits is Cu, present in abundant, albeit less reactive chalcopyrite, but largely barren of other metals such as As, Pb, Sb, Cd and Hg. Whilst geological setting does dictate metal endowment, ultimately mineralogy is the largest control of trace element distribution and subsequent potential toxicity. Deposits containing abundant pyrrhotite (high-temperature deposits) and Fe-rich sphalerite (ubiquitous to all SMS deposits) as well as deposits with abundant colloform textures also pose a higher risk. This type of study can be combined with “bulk lethal toxicity” assessments and used throughout the stages of a mining project to help guide prospecting and legislation, focus exploitation and minimise environmental impact.