Description: Sea-floor massive sulfide deposits represent a new type of base and precious metal resources that may be exploited by future deep-sea mining operations. These deposits occur in diverse tectonic environments and are mostly located along the global mid-ocean ridge system within international waters and arc-related settings within the exclusive economic zones of the world’s oceans. Much controversy is currently centered on the question whether sea-floor massive sulfide deposits represent a significant resource of metals that could be exploited to meet the metal demand of modern technology-based society. Chemical analysis of sulfide samples from sea-floor hydrothermal vent sites worldwide shows that sea-floor massive sulfides can be enriched in the minor elements Bi, Cd, Ga, Ge, Hg, In, Mo, Sb, Se, Te, and Tl, with concentrations ranging up to several tens or hundreds of parts per million. The minor element content of seafloor sulfides broadly varies with volcanic and tectonic setting. Massive sulfides on mid-ocean ridges commonly show high concentrations of Se, Mo, and Te, whereas arc-related sulfide deposits can be enriched in Cd, Hg, Sb, and Tl. Superposed on the volcanic and tectonic controls, the minor element content of sea-floor sulfides is strongly influenced by the temperature-dependent solubility of these elements. The high- to intermediatetemperature suite of minor elements, Bi, In, Mo, Se, and Te, is typically enriched in massive sulfides composed of chalcopyrite, while the low-temperature suite of minor elements, Cd, Ga, Ge, Hg, Sb, and Tl, is more typically associated with sphalerite-rich massive sulfides. Temperature-related minor element enrichment trends observed in modern sea-floor hydrothermal systems are broadly comparable to those encountered in fossil massive sulfide deposits. Although knowledge on the mineralogical sequestration of the minor elements in sea-floor massive sulfide deposits is limited, a significant proportion of the total amount of minor elements contained in massive sulfides appears to be incorporated into the crystal structure of the main sulfide minerals, including pyrite, pyrrhotite, chalcopyrite, sphalerite, wurtzite, and galena. In addition, the over 80 trace minerals recognized represent important hosts of minor elements in massive sulfides. As modern sea-floor sulfides have not been affected by metamorphic recrystallization and remobilization, the minor element distribution and geometallurgical properties of the massive sulfides may differ from those of ancient massive sulfide deposits. The compilation of geochemical data from samples collected from hydrothermal vent sites worldwide now permits a first-order evaluation of the global minor element endowment of sea-floor sulfide deposits. Based on an estimated 600 million metric tons (Mt) of massive sulfides in the neovolcanic zones of the world’s oceans, the amount of minor elements contained in sea-floor deposits is fairly small when compared to land-based mineral resources. Although some of the minor elements are potentially valuable commodities and could be recovered as co- or by-products from sulfide concentrates, sea-floor massive sulfide deposits clearly do not represent a significant or strategic future resource for these elements.