ALBERT

Description: Transects of the submersible Alvin across rock outcrops in the Oregon subduction zone have furnished information on the structural and stratigraphic framework of this accretionary complex. Communities of clams and tube worms, and authigenic carbonate mineral precipitates, are associated with venting sites of cool fluids located on a fault-bend anticline at a water depth of 2036 meters. The distribution of animals and carbonates suggests up-dip migration of fluids from both shallow and deep sources along permeable strata or fault zones within these clastic deposits. Methane is enriched in the water column over one vent site, and carbonate minerals and animal tissues are highly enriched in carbon-12. The animals use methane as an energy and food source in symbiosis with microorganisms. Oxidized methane is also the carbon source for the authigenic carbonates that cement the sediments of the accretionary complex. The animal communities and carbonates observed in the Oregon subduction zone occur in strata as old as 2.0 million years and provide criteria for identifying other localities where modern and ancient accreted deposits have vented methane, hydrocarbons, and other nutrient-bearing fluids.

Description: To provide an observational basis for the Intergovernmental Panel on Climate Change projections of a slowing Atlantic meridional overturning circulation (MOC) in the 21st century, the Overturning in the Subpolar North Atlantic Program (OSNAP) observing system was launched in the summer of 2014. The first 21-month record reveals a highly variable overturning circulation responsible for the majority of the heat and freshwater transport across the OSNAP line. In a departure from the prevailing view that changes in deep water formation in the Labrador Sea dominate MOC variability, these results suggest that the conversion of warm, salty, shallow Atlantic waters into colder, fresher, deep waters that move southward in the Irminger and Iceland basins is largely responsible for overturning and its variability in the subpolar basin.

Description: Hydrothermal fluids responsible for the formation of volcanogenic massive sulfides may transport gold in solution as Au(HS) 2 (super -) at low temperatures (〈350 degrees C), near neutral pH, and high concentrations of H 2 S (10 (super -3) -10 (super -2) m). To a lesser extent gold may be transported as AuCl 2 (super -) at higher temperatures (〉 or =350 degrees C), low pH, and elevated salinities (〉 or =1 m NaCl). The documented fluid chemistry of active hydrothermal vents on the modern seafloor confirms that gold transport is mainly due to Au(HS) 2 (super -) , and the calculated solubility of gold as Au(HS) 2 (super -) is highest in low-temperature (150 degrees -250 degrees C) vent fluids at elevated oxygen and sulfur activities. Sulfides which coprecipitate with gold commonly have properties which can be related to the temperature and sulfidation state of the hydrothermal fluids and therefore reflect conditions which were favorable for the transport or deposition of gold as Au(HS) 2 (super -) .A comparison of gold grades with sulfide mineral equilibria and the FeS contents in sphalerite from a variety of deposits indicates that gold enrichment is closely related to the temperature-a (sub S 2 ) conditions in the ore-forming fluids. Sea-floor sulfides range from sulfur-rich, pyrite-marcasite assemblages to relatively sulfur-deficient, pyrrhotite-isocubanite assemblages, with coexisting sphalerite containing 0 to 55 mole percent FeS. Significant gold enrichment occurs exclusively in low-temperature (〈300 degrees C) pyritic sulfides, together with sulfur-rich trace minerals and Fe-poor sphalerite. Bulk gold contents range from 〈0.5 ppm Au in sulfides with sphalerite containing 10 to 55 mole percent FeS up to 6.7 ppm Au in sulfides with sphalerite containing 〈10 mole percent FeS. Fluid inclusions in Fe-poor sphalerite (〈5 mole % FeS) from one gold-rich deposit (avg 4.9 ppm Au) have trapping temperatures of 235 degrees + or - 13 degrees C (Hannington and Scott, 1988). Low-temperature ores with high gold contents are also common in Phanerozoic Zn-Cu-Pb deposits in Japan (Kosaka, Furutobe, Shakanai) and similar deposits in British Columbia (HW mine, Lynx, Seneca). Gold-bearing polymetallic sulfides contain sulfur-rich mineral assemblages (e.g., bornite + pyrite) and recognizable trace mineral equilibria which indicate a high a (sub S 2 ) (e.g., argentite-electrum, tennantite-enargite, covellite-digenite). Zn-Cu-Pb ores formed at 200 degrees to 300 degrees C typically contain Fe-poor sphalerite (〈1 mole % FeS) and gold grades of i to 3 ppm Au, whereas Cu-rich ores formed at 〉300 degrees C contain Ferich sphalerite (1-5 mole % FeS) and gold grades 〈 1 ppm Au. Archean Cu-Zn deposits at Noranda, Quebec, consist mainly ofpyrite-pyrrhotite ores containing 0.5 to 1 ppm Au together with Fe-rich sphalerite (10-12 mole % FeS). However, pyritic ores which occur stratigraphically up-section commonly contain 1 to 3 ppm Au and relatively Fe-poor sphalerite (5 mole % FeS). Different sulfide mineral equilibria and FeS contents in sphalerite are interpreted to reflect the same physical and chemical conditions which influence gold grades and suggest that petrologic indicators of the sulfidation state may be useful guides to gold mineralization in volcanogenic massive sulfides.

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: The Monowai volcanic center is located at the midpoint along the ~2,530-km-long Tonga-Kermadec arc system. The Monowai volcanic center is comprised of a large elongate caldera (Monowai caldera area ~35 km2; depth to caldera floor 1,590 m), which has formed within an older caldera some 84 km2 in area. To the south of this nested caldera system is a large composite volcano, Monowai cone, which rises to within ~100 m of the sea surface and which has been volcanically active for the past several decades. Mafic volcanic rocks dominate the Monowai volcanic center; basalts are the most common rock type recovered from the cone, whereas basaltic andesites are common within the caldera. Hydrothermal plume mapping has shown at least three major hydrothermal systems associated with the caldera and cone: (1) the summit of the cone, (2) low-temperature venting (〈60°C; Mussel Ridge) on the southwestern wall of the caldera, and (3) a deeper caldera source with higher temperature venting that has yet to be observed. The cone summit plume shows large anomalies in pH (a shift of −2.00 pH units) and δ3He (≤358%), and noticeable H2S (up to 32 μm), and CH4 (up to 900 nm). The summit plume is also metal rich, with elevated total dissolvable Fe (TDFe up to 4,200 nm), TDMn (up to 412 nm), and TDFe/TDMn (up to 20.4). Particulate samples have elevated Fe, Si, Al, and Ti consistent with addition to the hydrothermal fluid from acidic water-rock reaction. Plumes extending from ~1,000- to 1,400-m depth provide evidence for a major hydrothermal vent system in the caldera. The caldera plume has lower values for TDFe and TDMn, although some samples show higher TDMn concentrations than the cone summit plume; caldera plume samples are also relatively gas poor (i.e., no H2S detected, pH shift of −0.06 pH units, CH4 concentrations up to 26 nm). The composition of the hydrothermal plumes in the caldera have higher metal contents than the sampled vent fluids along Mussel Ridge, requiring that the source of the caldera plumes is at greater depth and likely of higher temperature. Minor plumes detected as light scattering anomalies but with no 3He anomalies down the northern flank of the Monowai caldera most likely represent remobilization of volcanic debris from the volcano flanks. We believe the Monowai volcanic center is host to a robust magmatic-hydrothermal system, with significant differences in the style and composition of venting at the cone and caldera sites. At the cone, the large shifts in pH, very high δ3He% values, elevated TDFe and TDFe/TDMn, and the H2S- and CH4-rich nature of the plume fluids, together with elevated Ti, P, V, S, and Al in hydrothermal particulates, indicates significant magmatic volatile ± metal contributions in the hydrothermal system coupled with aggressive acidic water-rock interaction. By contrast, the caldera has low TDFe/TDMn in hydrothermal plumes; however, elevated Al and Ti contents in caldera particulate samples, combined with the presence of alunite, pyrophyllite, sulfide minerals, and native sulfur in samples from Mussel Ridge suggest past, and perhaps recent, acid volatile-rich venting and active Fe sulfide formation in the subsurface.

Description: Bulk chemical analyses of 48 samples of hydrothermal precipitates from seven polymetallic sulfide deposits at sea-floor spreading centers in the eastern Pacific reveal distinct patterns of gold enrichment. High gold contents are found in samples from two sulfide deposits: the Axial Seamount (45 degrees 57' N, 130 degrees 02' W), and the southern Explorer Ridge (49 degrees 45.6' N, 130 degrees 16.2' W). A large, 160-kg sample composed of silica, barite, and sphalerite from the top of the Axial Seamount deposit gives analyses up to 6,700 ppb Au, averaging 4,900 ppb Au. Similar material from the southern Explorer Ridge gives analyses up to 1,500 ppb Au, averaging 660 ppb Au. Detailed mineralogical studies together with bulk chemical analyses of a wide range of samples from these two sites reveal strong elemental associations and paragenetic controls on gold deposition. Gold at concentrations of about 200 ppb is associated with high Cu (〉1 wt %) and Mo (up to 470 ppm); at concentrations 〉 800 ppb, with high Zn (〉10 wt %), Ba (〉3 wt %), and SiO 2 (〉20 wt %); and at concentrations 〉 1,200 ppb, with high Pb (〉0.1 wt %), Ag (〉100 ppm), As (〉300 ppm), and Sb (50-100 ppm). Samples with the highest gold values from both locations contain late, sinterlike, low-temperature sulfosalts of Pb, As, Sb, Ag, and S in a matrix of amorphous silica. These sulfosalts are the probable repositories of the gold.Published analyses of Au-poor (〈200 ppb Au) samples from other basalt-hosted sea-floor deposits (21 degrees N, Galapagos rift, southern Juan de Fuca Ridge, Endeavour Ridge) are high in metals and sulfur but low in silica relative to samples from the Axial Seamount and the southern Explorer Ridge. Hydrothermal deposits in the Guaymas basin are underlain by sediments that are enriched in gold relative to midocean ridge basalts, but samples from these deposits do not reflect this enrichment and give average analyses 〈200 ppb Au. It is suggested that gold is preconcentrated to about 200 ppb in high-temperature (〉300 degrees C), Cu-Fe-rich sulfides and subsequently remobilized by late, sustained, low-temperature (〈250 degrees C) fluids and concentrated in SiO 2 -Ba-Zn-rich precipitates near the surface. Suitable fluid chemistry is required to mobilize the gold and a favorable precipitation mechanism is needed to concentrate it.These observations could explain the occurrence of high gold concentrations in selected ore types of some ancient massive sulfide deposits on land.

Description: The Jbel Tirremi fluorite-barite sulfide deposit in northeastern Morocco is hosted in a Jurassic-aged structurally high carbonate platform known as the Jbel Tirremi dome. The host rocks consist of unmetamorphosed, flat-lying early Jurassic dolomitized limestones, locally intruded by Eocene lamprophyre dikes. The orebodies consist mostly of fluorite and barite, and occur as open-space fillings and partial to massive replacement of the enclosing medium- to coarse-grained dolomitized limestones. The ore mineralogy is dominated by fluorite of different colors and habits, barite, and, to a lesser extent, sulfides. Rare earth element compositions along with fluid inclusion, halogen and isotopic data suggest that the fluorite barite mineralization and the spatially associated Eocene alkaline magmatism are petrogenetically unrelated, pointing instead to the regional circulation of hydrothermal basinal brines mixed to various degrees with meteoric water in a dominantly closed rock-buffered system at progressively higher temperatures and fluid/rock ratios. In this respect, fluid inclusion microthermometric measurements show that the ore-bearing hydrothermal system developed in two separate stages of fluorite-barite mineralization, as also revealed by isotopic data. Both stages precipitated from saline fluids at shallow crustal levels (i.e., 〈5 km), and were related, in varying degrees, to different stages of basin evolution and salt dome growth (salt mobilization and mineralization). During the first stage, the ore fluid was a highly saline aqueous brine with a total salinity up to 44.2 wt % NaCl + KCl equiv, at temperatures 〉= 82 degrees C and possibly up to 218 degrees C, whereas in the second stage the mineralizing fluid had a similar temperature range, but lower salinities (similar to 20-10 wt % NaCl equiv). The recorded high salinities are interpreted to represent the involvement of a mixture of halite dissolution water and evaporated seawater component. Oxygen (delta O-18 = 21.7 to 29.6%0 V-SMOW) and carbon (delta C-13 = -7.9 to 0.2%0 V-PDB) isotope data along with strontium (Sr-87/Sr-86 = 0.70300-070789) and lead (Pb-206/Pb-204 = 17.961-20.96, Pb-207/Pb-204, 15.511-15.697, Pb-208/Pb-204 = 37.784-39.993) isotope ratios suggest the involvement of a mixture of oil-bearing fluids, basinal brines, and meteoric fluids that interacted extensively with the early Jurassic host carbonates, the underlying Triassic salt-bearing diapir, associated siliciclastic rocks, and the highly fractionated and greisenized Hercynian granitic crystalline basement, resulting in the release of fluoride, metals, and other constituents to form the Jbel Tirremi deposit. Petroleum-bearing fluid, released from overpressured portions of the Guercif Basin at lithostatic pressures, and bittern brines dominated the first stage of mineralization. Mixing of saline, oxidized, CaCl2- and sulfate-rich bittern brine with oil-bearing fluid resulted in fluorite precipitation of stage I. Conversely, during the second stage of mineralization, the hydrothermal system was open to the influx of oxidized meteoric water as a consequence of the upward migration of the Triassic salt-bearing diapir and associated pressure decrease. The shift from stage I to stage II is associated with the evolution of the system from lithostatic to mostly hydrostatic pressure conditions. Stage I mineralization is thought to have occurred during the Late Miocene in response to rapid sedimentation and high subsidence rates and subsequent hydrocarbon migration associated with the outward migration of the Rif thrust front. Conversely, stage II mineralization occurred coevally with the uplift phase during Tortonian time.

Description: The LaRonde Penna Au-rich volcanogenic massive sulfide (VMS) deposit is the largest Au deposit currently mined in Canada (58.8 Mt at 4.31 g/t, containing 8.1 Moz of Au). It is part of the Doyon-Bousquet-LaRonde mining camp located in the eastern part of the Blake River Group of the Abitibi greenstone belt which is host to several of the world’s most important, present and past, Au-rich VMS deposits (e.g., Horne, Quemont, Bousquet, Bousquet 2-Dumagami). The LaRonde Penna deposit consists of massive to semimassive sulfide lenses (Au-Zn-Ag-Cu-Pb), stacked in the upper part of a steeply dipping, south-facing homoclinal volcanic sequence composed of extensive tholeiitic basaltic flows (Hébécourt Formation) overlain by tholeiitic to transitional, mafic to intermediate, effusive and volcaniclastic units at the base (lower member of the Bousquet Formation) and transitional to calc-alkaline, intermediate to felsic, effusive and intrusive rocks on top (upper member of the Bousquet Formation). The mafic to felsic volcanism of the Hébécourt Formation and of the lower member of the Bousquet Formation formed an extensive submarine basement or platform on which the intermediate to felsic rocks of the upper member of the Bousquet Formation were emplaced at restricted submarine eruptive centers or as shallow composite intrusive complexes. The submarine felsic volcanic rocks of the upper member of the Bousquet Formation are characterized by dacitic to rhyodacitic autoclastic (flow breccia) deposits that are cut and overlain by rhyodacitic and rhyolitic domes and/or partly extrusive cryptodomes and by intermediate to mafic sills and dikes. This volcanic architecture is thought to have been responsible for internal variations in ore and alteration styles, not only from one lens to another, but also along a single mineralized horizon or lens. In the upper part of the mine, the 20 North lens comprises a transposed pyrite-chalcopyrite (Au-Cu) stockwork (20N Au zone) overlain by a pyrite-sphalerite-galena-chalcopyrite-pyrrhotite (Zn-Ag-Pb) massive sulfide lens (20N Zn zone). The latter was formed, at least in part, by replacement of footwall rhyodacitic autoclastic deposits emplaced within a subbasin located between two rhyolite domes or cryptodomes. The 20N Zn zone tapers with depth in the mine and gives way to the 20N Au zone. At depth in the mine, the 20N Au zone consists of semimassive sulfides (Au-rich pyrite and chalcopyrite) enclosed by a large aluminous alteration halo on the margin of a large rhyolitic dome or cryptodome. U-Pb zircon geochronology gives ages of 2698.3 ± 0.8 and 2697.8 ± 1 Ma for the footwall and hanging-wall units of the 20 North lens, respectively. Thus, the formation of the 20 North lens was coeval with other VMS deposits in the Bousquet Formation and in the uppermost units of the Blake River Group. Although deformation and metamorphism have affected the primary mineral assemblages and the original geometry of the deposit, these events were not responsible for the different auriferous ore zones and alteration at LaRonde Penna. Studies of the LaRonde Penna deposit show that the hydrothermal system evolved in time and space from near-neutral seawater-dominated hydrothermal fluids, responsible for Au-Cu-Zn-Ag-Pb mineralization, to highly acidic fluids with possible direct magmatic contributions, responsible for Au ± Cu-rich ore and aluminous alteration. The different ore types and alteration reflect the evolving local volcanic setting described in this study.

Description: In his In Depth News story “Warming may not swamp islands” (1 August, p. 496), C. Pala argues that “coral reefs supporting sandy atoll islands will grow and rise in tandem with the sea,” based largely on studies that showed stable Pacific-island area over recent decades (1–4). He suggests that recent land losses are driven mostly by bad choices and that islanders are being affected “for the same reason as millions of people on the continents: because they live too close to shore.” We disagree with these conclusions.