ALBERT

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 distribution of platinum group elements (PGE) in Cu- and Zn-rich samples from the Roman Ruins and Satanic Mills vent sites in the PACMANUS hydrothermal field (Papua New Guinea) was studied and compared to that from selected ancient volcanogenic massive sulfide (VMS) deposits. Samples from the Satanic Mills site are enriched in Pd and Rh when compared to samples from Roman Ruins and reach highest values in active and inactive Cu-rich black smoker chimneys and chalcopyrite-cemented dacite breccias (up to 356 ppb Pd and up to 145 ppb Rh). A significant positive correlation was established between Cu and Pd and Rh in samples from both vent sites. Comparisons of chondrite normalized patterns and values of Pd/Pt and Pd/Ir ratios in Cu-rich sulfides and probable source rocks (felsic volcanic rocks/MORB) along with the evidence for a magmatic component in the PACMANUS hydrothermal system indicate that leaching of back-arc volcanic rocks together with addition of magmatic volatiles to the convecting hydrothermal system was the most important factor for PGE enrichment at PACMANUS and likely at some PGE-enriched ancient VMS deposits.

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Description: Polymetallic sulfide-sulfate mineralization enriched in Pb-Ag-As-Sb-Hg occurs in the Bransfield Strait, a late Tertiary-Quaternary marginal basin close to the Antarctic Peninsula. The mineralization is associated with bimodal volcanism and pelagic and volcaniclastic sediment in rifted continental crust. Hydrothermal precipitates have been recovered from two shallow (1,050–1,000 m water depth) submarine volcanoes (Hook Ridge and Three Sisters) in the Central Bransfield Strait. Mineralization at Hook Ridge consists of polymetallic sulfides, massive barite, and pyrite and marcasite crusts in semilithified pelagic and volcaniclastic sediment. Native sulfur commonly infills void space and cements the volcaniclastic sediment. The polymetallic sulfides are dominated by sphalerite with minor galena, enargite, tetrahedrite-tennantite, pyrite, chalcopyrite, and traces of orpiment cemented by barite and opal-A. The presence of enargite at Hook Ridge, the abundance of native sulfur, and the low Fe content of sphalerite indicate a high sulfur activity of the hydrothermal fluids responsible for mineralization. The sulfur isotopic composition of Hook Ridge precipitates documents the complexity of the sulfur sources in this hydrothermal system with variable influence of biological activity and possibly magmatic contributions. Homogenization temperatures and salinities of fluid inclusions in barite and opal-A suggest that boiling may have affected the hydrothermal fluids during their ascent. The discovery of massive barite-silica precipitates at another shallow marine volcano (Three Sisters volcano) attests to the potential for hydrothermal mineralization at other volcanic edifices in the area. The characteristics of the mineralization in the Bransfield Strait with rifting of continental crust, the presence of bimodal volcanism, including highly evolved felsic volcanic rocks, the association with sediments, and the Pb-Ag-As-Sb-Hg enrichment are similar to the setting of massive sulfide deposits in the Okinawa Trough, and distinct from those of sediment-dominated hydrothermal systems such as Escanaba Trough, Middle Valley, and Guaymas Basin. The geological setting of the Bransfield Strait is also broadly similar to that of some of the largest volcanogenic massive sulfide deposits in the ancient record, such as the Iberian Pyrite Belt.

Description: During the expeditions ANT-XV/2 with R/V Polarstern in 1997/98 and NBP 99-04 with R/V IB N.B. Palmer in 1999, the first samples of hydrothermally influenced sediments of Bransfield Strait were obtained at Hook Ridge, a volcanic edifice in the Central Basin of the Strait. The vent sites are characterized by white siliceous crusts on top of the sediment layer and temperatures measured immediately on deck are up to 48.5°C. The shallow depth of these vent sites (1050 m) particularly controls the chemistry of the pore fluids that are enriched in silica and sulfide and show low pH values. Chloride is depleted up to 20% and the calculated hydrothermal endmember concentration is in the range of 1–84 mM. Since other mechanisms for Cl depletion can be ruled out clearly, the composition of this fluid is attributed to phase separation. While the Cl-depleted fluid is emanating at Hook Ridge, a Cl-enriched fluid can be identified in the adjacent King George Basin. Using a p,x diagram the two corresponding endmember concentrations reveal that the phase separation takes place at subcritical conditions (total depth: ∼2500 m), probably along the whole volcanic edifice

Description: ODP drilling of the active TAG hydrothermal mound at 26°N on the Mid-Atlantic Ridge provided the first insights into the third dimension of a volcanic-hosted massive sulfide (VMS) deposit on a sediment-free mid-ocean ridge. Sulfide precipitation at this site started at least 20,000 years ago and resulted in the formation of a distinctly circular, 200-m diameter, 50-m-high pyritic mound and a silicified stockwork complex containing approximately 3.9 million tonnes of sulfide-bearing material with an average of 2.1 wt% Cu and 0.6 wt% Zn in 95 samples collected from 1–125 m below the seafloor. The periodic release of high-temperature hydrothermal fluids at the same location for several thousand years with intermittent periods of hydrothermal quiesence is the dominating process in the formation of the TAG hydrothermal mound. Distinct geochemical, mineralogical and isotopic zonation as well as a complex assemblage of sulfide-anhydrite-quartz bearing breccias can be related to this process. Geochemical depth profiles indicate extremely low base and trace element concentrations for the interior of the mound, which clearly contrasts with published analyses of samples collected from the surface of the TAG mound. This is explained by continued zone refining during which metals were mobilized from the interior of the mound by upwelling, hot (〉350 °C) hydrothermal fluids. Mixing of these fluids with infiltrating ambient seawater subsequently caused redeposition of metals close to the mound-seawater interface. The sulfur isotopic composition of bulk sulfides (+4.4 to +8.2‰δ34S; average +6.5‰) is unusually heavy when compared to other sediment-free mid-ocean ridge deposits and implies the introduction of heavy seawater sulfur to the hydrothermal fluid. The slight increase in sulfur isotope ratios with depth and distinct variations between early, disseminated sulfides related to wallrock alteration, and massive as well as late vein sulfides indicates widespread entrainment of seawater deep into the system. Fluid inclusion measurements in quartz and anhydrite reveal high formation temperatures throughout the TAG mound (up to 390 °C) at one time with an overall increase in trapping temperatures with depth. Lower formation temperatures close to the paleo-seafloor indicate local entrainment of seawater into the mound. Formation temperatures for a central anhydrite-bearing zone range from 340–360 °C and are slightly lower than the exit temperature of hydrothermal fluids presently venting at the Black Smoker Complex (360–369 °C). Fluid inclusions in quartz and anhydrite from the stockwork zone are characterized by formation temperatures higher than 375 °C, indicating conductive cooling of the hydrothermal fluids or mixing with ambient seawater prior to venting. Formation temperatures for quartz from an area of extremely low heat flow at the western side of the mound reach up to 390 °C, implying that this area was once part of a high-temperature hydrothermal upflow zone. The low heat flow and the absence of anhydrite within this part of the mound are strong indications that the recent pulse of high-temperature hydrothermal activity is not affecting this area and provides evidence for significant changes in the fluid flow regime underneath the deposit between hydrothermal cycles.