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: Brothers caldera volcano is a submarine volcano of dacitic composition, located on the Kermadec arc, New Zealand. It hosts the NW caldera vent field perched on the steep slope of the caldera walls and includes numerous, active, high-temperature (max 302°C) chimneys and a greater amount of dead, sulfide-rich spires. Petrographic studies of these chimneys show that three main zones can occur within the chimneys: a chalcopyrite-rich core, surrounded by a sulfate-dominated zone, which is in turn mantled by an external rind of Fe oxides, calcite, and silicates. Four chimney types are identified based on the relative proportions of the chalcopyrite and sulfate layers and the presence or absence of anhydrite. Two are Cu rich, i.e., chalcopyrite-sulfate and chalcopyrite-bornite chimneys, and two are Zn rich, i.e., sphalerite-barite and sphalerite-chalcopyrite. Chimney growth begins with the formation of a sulfate wall upon which sulfides precipitate. Later, zone refining results in a chalcopyrite-rich core with pyrite/marcasite and sphalerite occurring predominantly near the outer margins. In chalcopyrite-bornite chimneys, the chalcopyrite core rapidly loses permeability and limits the thickness of the surrounding sulfate layer. In these chimneys, bornite, chalcocite, and covellite form along the outer margin of the chalcopyrite zone as a result of oxidation by seawater. Zinc-rich chimneys display a more vertical zonation and their growth involves an upward-advancing barite cap followed by chalcopyrite deposition (if present) nearer the base. The vertical zonation and lack of anhydrite in these chimneys also implies that larger chalcopyrite and anhydrite deposits may exist subsea floor. The different chimney types are related to subsea-floor permeability, the amount of fluid mixing that occurs prior to venting, and heterogeneous fluid compositions. The occurrence of specular hematite and Bi or Au tellurides associated with chalcopyrite are consistent with magmatic contributions to the NW caldera vent site. These tellurides are the first gold-bearing phase to be identified in these chimneys, and the Bi-Au association suggests that gold enrichment up to 91 ppm is due to scavenging by liquid bismuth. The presence of tellurides in Brothers chimneys have implications for other telluride-bearing deposits, such those in the Urals. Likewise, other aspects of the mineralogy (i.e., textures) and zonation, including the implied subsea-floor deposition, presented here from an active, undeformed environment can aid in understanding ancient volcanogenic massive sulfide (VMS) deposits that have undergone various degrees of metamorphism.

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 ( 34 S = –39 to +3), whereas a more narrow range is observed in the massive pyrite at depth ( 34 S = –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: SuSu Knolls comprises three steep-sided conical volcanic peaks, standing on a N-NW-trending ridge in the eastern Manus basin, a complex zone of convergence between the major Indo-Australian and Pacific plates. The knolls consist of three porphyritic andesite-to-dacite domes, each 1.0 to 1.5 km in diameter, with crests ranging from 1,150 to 1,520 m below sea level. An intense hydrothermal plume, with peak transmission anomalies in excess of 40%, originates from SuSu Knolls, and associated hydrothermal venting, with fluid temperatures exceeding 300°C and sulfide mineralization, has been detected on the crests of all three edifices. The most important of these is the 2.47 million metric ton (Mt) Solwara 1 copper-gold deposit on Suzette Knoll, slated for mining by Nautilus Minerals. The porphyritic dacite is commonly strongly altered by reaction with acidic fluids. Fragments of sulfide mineralization, collected as part of the seafloor talus surrounding the crest of North Su and South Su, are characterized by an assemblage of pyrite-(fukuchilite)-enargite ± covellite-chalcopyrite. By contrast, there are actively venting chimneys on the crest of Suzette that are typically zoned, with chalcopyrite-pyrite-tennantite inner zones and barite-dominated outer zones. The knolls are covered by black sulfidic sediments that contain up to 2.4 wt % Cu and 3.2 ppm Au. Primary feldspars have been obliterated by the hydrothermal activity that ranges from incipient to intense and which is characterized by natroalunite, alunite (North Su only), cristobalite, tridymite, and rare quartz and kaolinite. Most volcanic rocks exhibit a patchy surficial coating of native sulfur, which may also fill vesicles. No altered rocks were dredged from Suzette because of the thick sulfidic sediment cover. Mass-balance calculations for altered and unaltered rock from North and South Su show that major and trace lithophile elements are depleted in the altered rocks, except for Ti, Al, Si, and Zr (all near-immobile), in concert with the destruction of the primary minerals and removal of primary components by acidic fluids. Sulfur and chalcophile trace elements (i.e., As, Au, Ba, Cd, Cu, Mo, Pb, and Se), typically associated with magmatic Cu-Au mineralization, are enriched by orders-of-magnitude in both leached and mineralized rock. Sulfide and native sulfur 34 S values from all three domes range from –7.4 to +0.4, indicating a magmatic component for the sulfur. A small group of ore elements (i.e., Ag, Bi, In, Sb, and Zn) are strongly enriched in mineralized breccias and depleted in the altered dacites, suggesting redistribution during alteration. The presence of advanced argillic alteration, the paragenesis of the Fe-Cu-As sulfide assemblage and presence of native sulfur, together with the sulfur isotope evidence for magmatic input into the hydrothermal fluid at SuSu Knolls, are consistent with a submarine high sulfidation magmatic Cu-Au hydrothermal system. Furthermore, the sulfide assemblage pyrite-enargite (±covellite-chalcopyrite) observed at North and South Su, and orders-of-magnitude enrichment of chalcophile elements in both altered and mineralized volcanic rocks, relative to unaltered volcanic rock, are also characteristic of subaerial high sulfidation epithermal mineralization. The SuSu Knolls hydrothermal field is a good example of a modern, high sulfidation, Cu-Au submarine hydrothermal system.