Abstract Despite recent commercial interest in possible mining of sea-floor massive sulfide (SMS) deposits, there has been a great reluctance to attempt any estimate of their global abundance, owing to the limited exploration of the oceans and the general lack of knowledge of the deposits. The need for such an assessment is now more urgent, as a number of leading companies and international consortia have begun to invest in intensive exploration campaigns for SMS, and governments and other organizations have begun to establish the legal framework for sea-floor exploration and exploitation of mineral resources in territorial and international waters. A growing database of global SMS occurrences is beginning to provide clues to the likely distribution, size, and grade of the deposits. More than 300 sites of sea-floor hydrothermal activity and associated mineralization are now known on the ocean floor; about 200 of these are sites of confirmed high-temperature venting (black smokers) and associated polymetallic sulfide deposits. They occur primarily at mid-ocean ridges (65%) but also in back-arc basins (22%) and on submarine volcanic arcs (12%). More than 3,800 samples have been collected from 95 of the best studied deposits, and preliminary estimates of the sizes of the deposits have been made at 62 sites. The total amount of massive sulfide contained in the known deposits is estimated to be ˜50 million tons (Mt); the top 10 percent of deposits (≥2 Mt) contain about 35 Mt of massive sulfide or about 70 percent of the total. The largest deposits, excluding the Atlantis II Deep in the Red Sea, are on the order of 10 Mt in size. However, the median deposit size is only about 70,000 t. The average concentrations of metals based on analyses of surface samples are 3.6 wt percent Cu, 7.9 wt percent Zn, 0.4 wt percent Pb, 1.7 g/t Au, and 115 g/t Ag, although comparisons with drill core indicate that grades can be significantly lower below the sea floor in many, but not all, deposits. A number of independent datasets, including global heat flow, circulation models for high-temperature vent fluids, geochemical budgets of the oceans, and the incidence of hydrothermal plumes, all arrive at similar estimates of ˜1,000 active vent sites along the mid-ocean ridges. As many as 500 additional vent sites may be located on submarine volcanic arcs and in back-arc basins, for a total of ˜1,500 sites. However, an analysis of the spatial distribution of known deposits, both on the mid-ocean ridges and in subduction-related environments, suggests that this is likely a maximum and that the total number of significant SMS occurrences along the neovolcanic zones of the world’s oceans is closer to ˜900. If the size distribution of the known deposits is representative of what remains to be discovered, then the total tonnage of SMS, excluding the Red Sea, is expected to be on the order of 600 Mt (˜1,000 deposits with a minimum size of 100 t and a maximum size of 10 Mt). The total contained metal would be about 30 Mt, based on a grade of 5 wt percent combined Zn + Cu + Pb. This estimate is similar to the total discovered metal in Cenozoic VMS deposits on land. However, it does not include long extinct deposits that may be located far off-axis. If present-day rates of massive sulfide formation on the mid-ocean ridges and back-arc spreading centers are extrapolated to older crust, then significant tonnages of massive sulfide may be expected beneath off-axis sediments. In contrast to land-based exploration, where larger deposits are commonly discovered early in the exploration history of a VMS district, exploration of the modern sea floor has discovered a high proportion of small, widely spaced SMS deposits. Large, inactive deposits are more difficult to identify by current exploration methods but may exist in isolated areas that have yet to be fully explored, such as in heavily sedimented back-arc rifts. This raises the possibility of a dramatically different resource future for SMS if one or more large deposits or “districts” are discovered that contain a high proportion of the total metal.

Abstract The discovery of metal-depositing hot springs on the sea floor , and especially their link to chemosynthetic life, was among the most compelling and significant scientific advances of the twentieth centuryMore than 300 sites of hydrothermal activity and sea-floor mineralization are known on the ocean floor About 100 of these are sites of high-temperature venting and polymetallic sulfide deposits. They occur at mid-ocean ridges (65%), in back-arc basins (22%), and on submarine volcanic arcs (12%). Although high-temperature, 350°C, black smoker vents are the most recognizable features of sea-floor hydrothermal activity , a wide range of different styles of mineralization has been found. Different volcanic substrates, including mid-ocean ridge basalt, ultramafic intrusive rocks, and more evolved volcanic suites in both oceanic and continental crust, as well as temperature-dependent solubility controls, account for the main geochemical associations found in the deposits. Although end-member hydrothermal fluids mainly originate in the deep volcanic basement, the presence of sediments and other substrates can have a large effect on the compositions of the vent fluids. In arc and backarc settings, vent fluid compositions are broadly similar to those at mid-ocean ridges, but the arc magmas also supply a number of components to the hydrothermal fluids. The majority of known black smoker vents occur on fast-spreading mid-ocean ridges, but the largest massive sulfide deposits are located at intermediate- and slow-spreading centers, at ridge-axis volcanoes, in deep backarc basins, and in sedimented rifts adjacent to continental margins. The range of deposit sizes in these settings is similar to that of ancient volcanic-associated massive sulfide (VMS) deposits. Detailed mapping, and in some cases drilling, indicates that a number of deposits contain 1 to 5 million tons (Mt) of massive sulfide (e.g., TAG hydrothermal field on the Mid-Atlantic Ridge, deposits of the Galapagos Rift, and at 13°N on the East Pacific Rise). Two sediment-hosted deposits, at Middle Valley on the Juan de Fuca Ridge and in the Atlantis II Deep of the Red Sea, are much larger (up to 15 and 90 Mt, respectively). In the western Pacific, high-temperature hydrothermal systems occur mainly at intraoceanic back-arc spreading centers (e.g., Lau basin, North Fiji basin, Mariana trough) and in arc-related rifts at continental margins (e.g., Okinawa trough). In contrast to the mid-ocean ridges, convergent margin settings are characterized by a range of different crustal thicknesses and compositions, variable heat flow regimes, and diverse magma types. These variations result in major differences in the compositions and isotopic systematics of the hydrothermal fluids and the mineralogy and bulk compositions of the associated mineral deposits. Intraoceanic back-arc basin spreading centers host black smoker vents that, for the most part, are very similar to those on the mid-ocean ridges. However, isotopic data from both the volcanic rocks and the sulfide deposits highlight the importance of subduction recycling in the origin of the magmas and hydrothermal fluids. Back-arc rifts in continental margin settings are typically sediment-filled basins, which derive their sediment load from the adjacent continental shelf. This has an insulating effect that enhances the high heat flow associated with rifting of the continental crust and also helps to preserve the contained sulfide deposits. Large hydrothermal systems have developed where initial rifting of continental crust or locally thickened arc crust has formed large calderalike sea-floor depressions, similar to those that contained major VMS-forming systems in the geologic record. Hydrothermal vents also occur in the summit calderas of submarine volcanoes at the volcanic fronts of arcs. However , this contrasts with the interpreted settings of most ancient VMS deposits, which are considered to have formed mainly during arc rifting. Hydrothermal vents associated with arc volcanoes show clear evidence of the direct input of magmatic volatiles, similar to magmatic-hydrothermal systems in subaerial volcanic arcs. Several compelling examples of submarine epithermal-style mineralization, including gold-base metal veins, have been found on submarine arc volcanoes, and this type of mineralization may be more common than is presently recognized. Mapping and sampling of the sea floor has dramatically improved geodynamic models of different submarine volcanic and tectonic settings and has helped to establish a framework for the characterization of many similar ancient terranes. Deposits forming at convergent margins are considered to be the closest analogs of ancient VMS. However, black smokers on the mid-ocean ridges continue to provide critically important information about metal transport and deposition in sea-floor hydrothermal systems of all types. Ongoing sea-floor exploration in other settings is providing clues to the diversity of mineral deposit types that occur in different environments and the conditions that are favorable for their formation.

Abstract Although generally considered a poor cousin of Au-rich deposits such as orogenic or epithermal deposits, a significant number of volcanic-hosted massive sulfide (VHMS) deposits are significant repositories of Au. Several of these deposits had original Au resources exceeding 8 Moz and in some recently discovered deposits Au, not base metals, is the primary economic metal. Although most Au in volcanic-hosted massive sulfide districts is hosted by massive sulfide lenses, recent discoveries, both on land and on the ocean floor, indicate that significant Au occurs outside of these lenses. In most deposits, Au has a metallogenic association with either Cu or Zn. When associated with Cu, Au is concentrated toward the base of the massive sulfide lens. Gold-rich deposits of this metallogenic assemblage commonly are associated with (metamorphosed) advanced argillic assemblages and are inferred to have formed from acidic, high-temperature (>300°C), oxidized fluids. These deposits have been equated to high-sulfidation epithermal deposits and may be detected using recently developed spectral techniques such as PIMA (Portable Infrared Mineral Analyzer) and airborne hyperspectral scanners. When associated with Zn, Au is concentrated near the top of massive sulfide lenses, in some cases in baritic zones. Gold-rich deposits of this metallogenic assemblage tend to be formed from low-temperature (200° ± 50°C) and/or near-neutral fluids as indicated by fluid inclusion studies or by alteration assemblages (e.g., K feldspar or carbonate). A small number of deposits cannot be classified into the Au-Zn or Au-Cu association. In these deposits, Au is concentrated in pyritic zones that contain relatively low amounts of base metals. Moreover, a consistent relationship with Zn or Cu is not present. Although this group is small, it includes deposits such as Horne. Mineralogically Au can occur in electrum or native gold, Au tellurides, or auriferous pyrite or arsenopyrite. In deposits of the Au-Cu association, Au tends to occur as native gold or tellurides, whereas electrum and auriferous pyrite and/or arsenopyrite is more common in the Au-Zn association. Metamorphic recrystallization tends to liberate Au held in auriferous pyrite or arsenopyrite, potentially enhancing metallurgical recoveries.

Abstract 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.