Description: The results of three decades of seafloor research provide the most reliable information on the importance of water depth in massive sulfide formation. Available data from over 130 occurrences show that water depths of seafloor vent sites vary with plate tectonic setting and the regional magmatic and volcanic environment. The shallowest hydrothermal systems in subduction-related settings are hosted by arc volcanoes. These shallow vent sites have a number of features in common with subaerial epithermal systems. Massive sulfide occurrences in arc-related rifts, the most likely setting for many ancient analogs, are generally restricted to water depths from ~700 to 2,000 m, with rifts developing within old arc crust at the deeper end of this range. Back-arc spreading centers proximal to arcs host massive sulfide deposits at depths of ~1,500 to 2,000 m. The deepest hydrothermal systems occur along mature back-arc spreading centers distal to volcanic arcs where water depths range from ~2,000 to 3,700 m. These deeper vent sites probably represent the best modern analogues of ophiolite-hosted massive sulfide deposits. Boiling of the hydrothermal fluids is common at volcanic arcs and in arc-related rifts. In these environments, elevated magmatic gas contents of the hydrothermal fluids can contribute to the widespread occurrence of phase separation and associated gas loss. By contrast, the high ambient pressures in deep marine hydrothermal systems along mature back-arc spreading centers prevent fluids from boiling during their ascent to the seafloor. Boiling controls the maximum temperature at which hydrothermal fluids discharge at the seafloor and, therefore, influences the metal content of seafloor sulfide deposits. Copper-rich massive sulfides typically occur at water depths exceeding ~1,000 m, whereas Zn- and Pb-rich occurrences may form at any water depth. Boiling can be an important control on Ag and Au grades but is not the only factor controlling precious metal enrichment in massive sulfides. Shallow marine hot spring deposits can be highly enriched in trace metals such as As, Hg, and Sb. Submarine volcanic arc and back-arc settings are geologically complex and significant variations in water depth can occur over short distances. Paleoenvironmental reconstruction of these environments in ancient volcanic terranes is hampered by the lack of unequivocal volcanological or sedimentological criteria that indicate water depth. The relationships established here using modern seafloor observations provide important constraints on the paleoenvironmental setting of ancient volcanic-hosted massive sulfide deposits.

Description: A subseafloor replacement-style barite and sulfide occurrence was drilled in shallow waters at the Palinuro volcanic complex, the northernmost Aeolian arc volcano in the Tyrrhenian Sea, Italy. Using a lander-type drilling device, 11 successful drill holes yielded a total of 13.5 m of core from a sediment-filled depression located at a water depth of 630 to 650 m. The longest continuous drill core recovered consists of 4.84 m of massive to semimassive barite and sulfides with abundant late, native sulfur overprint. Seafloor observations suggest that the hydrothermal system associated with the formation of the subseafloor barite and sulfide ore zone is still active, although black smoker activity does not occur on the seafloor. The recovered drill core shows that the subseafloor deposit is zoned with depth. The top of the mineralized zone is comprised of a variably silicified vuggy barite-sulfide facies that shows notable polymetallic metal enrichment, while the deeper portion of the mineralized zone is dominated by massive pyrite having distinctly lower base and precious metal grades. Metal zonation of the barite and sulfide deposit is related to the evolution of the hydrothermal fluids in space and time. The barite cap and the massive pyrite present in the deeper portion of the mineralized zone appear to have formed early in the paragenesis. During the main stage of the mineralization, the barite cap was brecciated and cemented by a polymetallic assemblage of barite and pyrite with minor chalcopyrite and tetrahedrite, trace famatinite, and rare cinnabar. Lower temperature precipitates formed during the main stage of mineralization include sphalerite, galena, pyrite, opal-A, and barite, which are associated with traces of Pb-Sb-As sulfosalts such as bournonite-seligmannite, or semseyite. A distinct mineral assemblage of fine-grained anhedral enargite, hypogene covellite, chalcopyrite, and galena is commonly associated with colloform sphalerite, galena, and pyrite as a late phase of this main stage. Colloform pyrite and marcasite are the last sulfides formed in the paragenetic sequence. The deposit is interpreted to have formed from fluids having an intermediate-sulfidation state, although excursions to high- and very high sulfidation states are indicated by the presence of abundant enargite and hypogene covellite. Laser ablation and conventional sulfur isotope analyses show that pyrite formed close to the seafloor within the zone of polymetallic metal enrichment has a variable sulfur isotope composition (δ34S = −39 to +3‰), whereas a more narrow range is observed in the massive pyrite at depth (δ34S = −10 to 0‰). Similar variations were also documented for the late native sulfur overprint. Overall, the negative sulfur isotope ratios at depth, the intermediate- to very high sulfidation conditions during mineralization, and the abundance of native sulfur suggest contributions of magmatic volatiles to the mineralizing fluids from a degassing magma chamber at depth. Biological processes are interpreted to have played a major role during late stages of ore formation. The combination of a subseafloor replacement deposit with a massive to semimassive barite cap rock overlying massive pyrite, the intermediate- to high-sulfidation characteristics, and the strong biological influence on the late stages of mineralization are distinct from other modern seafloor massive sulfide deposits and represents a style of submarine mineralization not previously recognized in a modern volcanic arc environment. The barite and sulfide occurrence at Palinuro shares many characteristics with porphyry-related base metal veins and intermediate-sulfidation epithermal deposits, suggesting that metallogenic processes associated with arc-related magmatic-hydrothermal systems are not restricted to the subaerial environment.

Description: The TAG hydrothermal field is a site of major active and inactive volcanic-hosted hydrothermal mineralization in the rift valley of the slow-spreading Mid-Atlantic Ridge at 26[degree]N. The axial high is the principal locus of present magmatic intrusions. The TAG field contains three main areas of present and past hydrothermal activity: (1) an actively venting high-temperature sulfide mound; (2) two former high-temperature vent areas; (3) a zone of low-temperature venting and precipitation of Fe and Mn oxide deposits. The volcanic centers occur at the intersections between ridge axis-parallel normal faults and projected axis-transverse transfer faults. The intersections of these active fault systems may act as conduits both for magmatic intrusions from sources beneath the axial high that build the volcanic centers and for hydrothermal upwelling that taps the heat sources. Radiometric dating of sulfide samples and manganese crusts in the hydrothermal zones and dating of sediments intercalated with pillow lava flows in the volcanic center adjacent to the active sulfide mound indicate multiple episodes of hydrothermal activity throughout the field driven by heat supplied by episodic intrusions over a period of at least 140 [times] 10[sup 3] yr. The sulfide deposits are built by juxtaposition and superposition during relatively long residence times near episodic axial heat sources counterbalanced by mass wasting in the tectonically active rift valley of the slow-spreading oceanic ridge. Hydrothermal reworking of a relict hydrothermal zone by high-temperature hydrothermal episodes has recrystallized sulfides and concentrated the first visible primary gold reported in a deposit at an oceanic ridge.

Description: Gold-rich, siliceous veins with disseminated polymetallic sulfides and pyritic stockwork mineralization have been recovered from the top of Conical seamount, a shallow (1,050-m water depth) submarine volcano located about 10 km south of Lihir island, Papua New Guinea. Grab samples from the summit of Conical seamount contain the highest concentration of gold yet reported from the modern sea floor (max 230 ppm Au; avg 26 ppm, n = 40). The gold occurs in sulfide-rich veins of black amorphous silica hosted by intensely altered, high K calc-alkaline basalts. Sulfides in the veins consist of sphalerite, galena, pyrite, chalcopyrite, marcasite, and a variety of Cu-Pb-As-Sb sulfosalts. The gold occurs as native gold and electrum in the amorphous silica and as inclusions in the sulfides. The highest gold concentrations are associated with high Ag, As, Sb, and Hg. Zoned alteration adjacent to the veins consists of illite, smectite, amorphous silica, K feldspar, secondary plagioclase, minor chlorite, and trace carbonate. The association of gold with illite, smectite, amorphous silica, and K feldspar indicates deposition from near neutral pH hydrothermal fluids. However, the auriferous polymetallic sulfide veins and the associated alteration are overprinted on stockwork pyrite mineralization that is associated with earlier acid alteration containing alunite, aluminum phosphate sulfates, kaolinite, and other clay minerals. The platy habit of the alunite in this assemblage, the presence of alumium posphate sulfate minerals, and the sulfur isotope ratios of the crystalline pyrite (–8.6 to –0.2‰ δ34S, n = 28) and alunite (7.5 and 6.4‰ δ34S) are consistent with a contribution of magmatic volatiles in the earliest stages of the hydrothermal system. Framboidal pyrite within and at the margins of the mineralized zone has δ34S values suggesting involvement of biogenic activity (–11.6 to –13.9‰ δ34S). The gold-rich veins at Conical seamount are distinct from sea-floor massive sulfide deposits and represent a new style of mineralization on the modern sea floor. The mineralogy, alteration, geochemistry, and texture of the veins resemble those of some subaerial epithermal gold deposits and indicate that features long considered to define a subaerial setting can also form in a submarine environment. The proximity of Conical seamount to the giant Ladolam epithermal gold deposit on nearby Lihir island also raises the possibility that both subaerial and submarine gold mineralization in the region may be related to the same district-scale magmatic events.

Description: 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 century. More 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.

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.

Description: Tinakula is the first seafloor massive sulfide deposit described in the Jean Charcot troughs and is the first such deposit described in the Solomon Islands—on land or the seabed. The deposit is hosted by mafic (basaltic-andesitic) volcaniclastic rocks within a series of cinder cones along a single eruptive fissure. Extensive mapping and sampling by remotely operated vehicle, together with shallow drilling, provide insights into deposit geology and especially hydrothermal processes operating in the shallow subsurface. On the seafloor, mostly inactive chimneys and mounds cover an area of ~77,000 m2 and are partially buried by volcaniclastic sand. Mineralization is characterized by abundant barite- and sulfide-rich chimneys that formed by low-temperature (〈250°C) venting over ~5,600 years. Barite-rich samples have high SiO2, Pb, and Hg contents; the sulfide chimneys are dominated by low-Fe sphalerite and are high in Cd, Ge, Sb, and Ag. Few high-temperature chimneys, including zoned chalcopyrite-sphalerite samples and rare massive chalcopyrite, are rich in As, Mo, In, and Au (up to 9.26 ppm), locally as visible gold. Below the seafloor, the mineralization includes buried intervals of sulfide-rich talus with disseminated sulfides in volcaniclastic rocks consisting mainly of lapillistone with minor tuffaceous beds and autobreccias. The volcaniclastic rocks are intensely altered and variably cemented by anhydrite with crosscutting sulfate (± minor sulfide) veins. Fluid inclusions in anhydrite and sphalerite from the footwall (to 19.3 m below seafloor; m b.s.f.) have trapping temperatures of up to 298°C with salinities close to, but slightly higher than, that of seawater (2.8–4.5 wt % NaCl equiv). These temperatures are 10° to 20°C lower than the minimum temperature of boiling at this depth (1,070–1,204 m below sea level; m b.s.l.), suggesting that the highest-temperature fluids boiled below the seafloor. The alteration is distributed in broadly conformable zones, expressed in order of increasing depth and temperature as (1) montmorillonite/nontronite, (2) nontronite + corrensite, (3) illite/smectite + pyrite, (4) illite/smectite + chamosite, and (5) chamosite + corrensite. Zones of argillic alteration are distinguished from chloritic alteration by large positive mass changes in K2O (enriched in illite/smectite), MgO (enriched in chamosite and corrensite), and Fe2O3 (enriched in pyrite associated with illite/smectite alteration). The δ18O and δD values of clay minerals confirm increasing temperature with depth, from 124° to 256°C, and interaction with seawater-dominated hydrothermal fluids at high water/rock ratios. Leaching of the volcanic host rocks and thermochemical reduction of seawater sulfate are the primary sources of sulfur, with δ34S values of sulfides, from –0.8 to 3.4‰, and those of sulfate minerals close to seawater sulfate, from 19.3 to 22.5‰.