Shallow submarine hydrothermal systems in the Aeolian Volcanic Arc, Italy

The majority of known high-temperature hydrothermal vents occur at mid-ocean ridges and back-arc spreading centers, typically at water depths from 2000 to 4000 meters. Compared with 30 years of hydrothermal research along spreading centers in the deep parts of the ocean, exploration of the approximately 700 submarine arc volcanoes is relatively recent [de Ronde et al., 2003]. At these submarine arc volcanoes, active hydrothermal vents are located at unexpectedly shallow water depth (95% at <1600-meter depth), which has important consequences for the style of venting, the nature of associated mineral deposits, and the local biological communities. As part of an ongoing multinational research effort to study shallow submarine volcanic arcs, two hydrothermal systems in the submerged part of the Aeolian arc have been investigated in detail during research cruises by R/V Poseidon (July 2006) and R/V Meteor (August 2007). Comprehensive seafloor video surveys were conducted using a remotely operated vehicle, and drilling to a depth of 5 meters was carried out using a lander-type submersible drill. This research has resulted in the first detailed, three-dimensional documentation of shallow submarine hydrothermal systems on arc volcanoe

Sea-floor tectonics and submarine hydrothermal systems

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

Drilling of shallow marine sulfide-sulfate mineralisation in south-eastern Tyrrhenian Sea, Italy; Seafloor sulfides, Tyrrhenian Sea, highsulfidation; hydrothermal systems, Palinuro

Semi-massive to massive sulfides with abundant late native sulfur were drilled in a shallowwater hydrothermal system in an island arc volcanic setting at the Palinuro volcanic complex in the Tyrrhenian Sea, Italy. Overall, 12.7 m of sulfide mineralisation were drilled in a sediment-filled depression at a water depth of 630 - 650 m using the lander-type Rockdrill I drill rig of the British Geological Survey. Polymetallic (Zn, Pb, Sb, As, Ag) sulfides overlie massive pyrite. The massive sulfide mineralisation contains a number of atypical minerals, including enargite-famatinite, tennantite-tetrahedrite, stibnite, bismuthinite, and Pb-,Sb-, and Ag-sulfosalts, that do not commonly occur in mid-ocean ridge massive sulfides. Analogous to subaerial epithermal deposits, the occurrence of these minerals and the presence of abundant native sulfur suggest an intermediate to high sulfidation and/or high oxididation state of the hydrothermal fluids in contrast to the near-neutral and reducing fluids from which base metal-rich massive sulfides along mid-ocean ridges typically form. Oxidised conditions during sulfide deposition are likely related to the presence of magmatic volatiles in the mineralising fluids that were derived from a degassing magma chamber below the Palinuro volcanic complex

Trace metal distributions in sulfide scales of the seawater-dominated Reykjanes geothermal system: Constraints on sub-seafloor hydrothermal mineralizing processes and metal fluxes

Highlights • Predictable trace element enrichments and depletions in the Reykjanes system. • Boiling exerts a major influence on the enrichment of metals. • High concentrations of Au and Ag and Pb indicate accumulation in reservoir fluids. • Three quarters of the metal budget is deposited at depth or in the upflow zone. Abstract Mineral precipitation in the seawater-dominated Reykjanes geothermal system on the Mid-Atlantic Ridge, Iceland is caused by abrupt, artificially induced, pressure and temperature changes as deep high-temperature liquids are drawn from reservoir rocks up through the geothermal wells. Sulfide scales within these wells represent a complete profile of mineral precipitation through a seafloor hydrothermal system, from the deep reservoir to the low-temperature silica-rich surface discharge. Mineral scales have formed under a range of conditions from high pressures and temperatures at depth (>2 km) to boiling conditions in the upflow zone and at the surface. Consistent trace element enrichments, similar to those in black smoker chimneys, are documented: Cu, Zn, Cd, Co, Te, V, Ni, Mo, W, Sn, Fe and S are enriched at higher pressures and temperatures in the deepest scales, Zn and Cu, Bi, Pb, Ag, As, Sb, Ga, Hg, Tl, U, and Th are enriched at lower temperatures and pressures nearer to the surface. A number of elements (e.g., Co, Se, Cd, Zn, Cu, and Au) are deposited in both high- and low-pressure scales, but are hosted by distinctly different minerals. Other trace elements, such as Pb, Ag, and Ga, are strongly partitioned into low-temperature minerals, such as galena (Pb, Ag) and clays (Ga). Boiling and destabilization of metal-bearing aqueous complexes are the dominant control on the deposition of most metals (particularly Au). Other metals (e.g., Cu and Se) may also have been transported in the vapor phase. Very large enrichments of Au, Ag and Pb in the scales (e.g., 948 ppm Au, 23,200 ppm Ag, and 18.8 wt.% Pb) versus average concentrations in black smoker chimneys likely reflect that some elements are preferentially deposited in boiling systems. A mass accumulation of 5.7 t/yr of massive sulfide was calculated for one high-temperature production well, equating to metal fluxes of 1.7 t/yr Zn, 0.3 t/yr Cu, 23 kg/yr Pb, 4.1 kg/yr Ag, and 0.5 kg/yr Au. At least three quarters of the major and trace element load is precipitated within the well before reaching the surface. We suggest that a similar proportion of metals may be deposited below the seafloor in submarine hydrothermal systems where significant boiling has occurred. Mass accumulation estimations over the lifetime of the Reykjanes system may indicate significant enrichment of Zn, Pb, Au, and Ag relative to both modern and ancient mafic-dominated seafloor massive sulfide deposits, and highlights the potential for metal enrichment and accumulation in the deep parts of geothermal systems

CTD data profiling to assess the natural hazard of active submarine vent fields: the case of Santorini Island

Almost three quarters of known volcanic activity on Earth occurs in underwater locations. The presence of active hydrothermal vent fields in such environments is a potential natural hazard for the environment, the society, and the economy. Despite its importance for risk assessment and risk mitigation, monitoring of the activity is impeded by the remoteness and the extreme conditions of underwater volcanoes. The large difference of population present on Santorini between the winter and summer seasons, all within a partially enclosed system, make the Santorini-Kolumbo volcanic field, an ideal place for detailed exploration. In 2017, GEOMAR in collaboration with the National and Kapodistrian University of Athens (mission: POS-510 ANYDROS), used an Autonomous Underwater Vehicle (AUV) to map the NE–trending Santorini–Kolumbo line, where it also collected CTD data. Here we present the preliminary results from the 15-hour survey held on the 25th March 2017, during the POS-510 expedition targeting the vent field which is located in the North Basin of Santorini Caldera. Detailed CTD 3D profiles have been reconstructed from the raw data of Santorini’s vent field. An anomaly emerges at the depth of 350 m in the Conductivity and Salinity depth profiles, as the CTD sensor is placed directly above the vent sources. Anomalies were evident in the 3D maps reconstructed, showing for the first time a rather weak, but underlying hydrothermal vent activity at various locations. As the present results are the first ones produced from this expedition, further investigation is required incorporating the full dataset. Based on those results, the impact of developing appropriate mechanisms and policies to avoid the associated natural hazard is expected to be immense

Geology, sulfide geochemistry and supercritical venting at the Beebe Hydrothermal Vent Field, Cayman Trough

The Beebe Vent Field (BVF) is the world's deepest known hydrothermal system, at 4960m below sea level. Located on the Mid-Cayman Spreading Centre, Caribbean, the BVF hosts high temperature (∼401°C) ‘black smoker' vents that build Cu, Zn and Au-rich sulphide mounds and chimneys. The BVF is highly gold-rich, with Au values up to 93 ppm and an average Au:Ag ratio of 0.15. Gold precipitation is directly associated with diffuse flow through ‘beehive' chimneys. Significant mass-wasting of sulphide material at the BVF, accompanied by changes in metal content, results in metaliferous talus and sediment deposits. Situated on very thin (2-3km thick) oceanic crust, at an ultraslow spreading centre, the hydrothermal system circulates fluids to a depth of ∼1.8km in a basement that is likely to include a mixture of both mafic and ultramafic lithologies. We suggest hydrothermal interaction with chalcophile-bearing sulphides in the mantle rocks, together with precipitation of Au in beehive chimney structures, has resulted in the formation of a Au-rich volcanogenic massive sulphide (VMS) deposit. With its spatial distribution of deposit materials and metal contents, the BVF represents a modern day analogue for basalt hosted, Au-rich VMS systems. This article is protected by copyright. All rights reserved

Four-Hundred-and-Ninety-Million-Year Record of Bacteriogenic Iron Oxide Precipitation at Sea-Floor Hydrothermal Vents

Fe oxide deposits are commonly found at hydrothermal vent sites at mid-ocean ridge and back-arc sea floor spreading centers, seamounts associated with these spreading centers, and intra-plate seamounts, and can cover extensive areas of the seafloor. These deposits can be attributed to several abiogenic processes and commonly contain micron-scale filamentous textures. Some filaments are cylindrical casts of Fe oxyhydroxides formed around bacterial cells and are thus unquestionably biogenic. The filaments have distinctive morphologies very like structures formed by neutrophilic Fe oxidizing bacteria. It is becoming increasingly apparent that Fe oxidizing bacteria have a significant role in the formation of Fe oxide deposits at marine hydrothermal vents. The presence of Fe oxide filaments in Fe oxides is thus of great potential as a biomarker for Fe oxidizing bacteria in modern and ancient marine hydrothermal vent deposits. The ancient analogues of modern deep-sea hydrothermal Fe oxide deposits are jaspers. A number of jaspers, ranging in age from the early Ordovician to late Eocene, contain abundant Fe oxide filamentous textures with a wide variety of morphologies. Some of these filaments are like structures formed by modern Fe oxidizing bacteria. Together with new data from the modern TAG site, we show that there is direct evidence for bacteriogenic Fe oxide precipitation at marine hydrothermal vent sites for at least the last 490 Ma of the Phanerozoic

Boiling-induced formation of colloidal gold in black smoker hydrothermal fluids

Gold colloids occur in black smoker fluids from the Niua South hydrothermal vent field, Lau Basin (South Pacific Ocean), confirming the long-standing hypothesis that gold may undergo colloidal transport in hydrothermal fluids. Six black smoker vents, varying in temperature from 250 °C to 325 °C, were sampled; the 325 °C vent was boiling at the time of sampling and the 250 °C fluids were diffusely venting. Native gold particles ranging from <50 nm to 2 μm were identified in 4 of the fluid samples and were also observed to precipitate on the sampler during collection from the boiling vent. Total gold concentrations (dissolved and particulate) in the fluid samples range from 1.6 to 5.4 nM in the high-temperature, focused flow vents. Although the gold concentrations in the focused flow fluids are relatively high, they are lower than potential solubilities prior to boiling and indicate that precipitation was boiling induced, with sulfide lost upon boiling to exsolution and metal sulfide formation. Gold concentrations reach 26.7 nM in the 250 °C diffuse flow sample, and abundant native gold particles were also found in the fluids and associated sulfide chimney and are interpreted to be a product of colloid accumulation and growth following initial precipitation upon boiling. These results indicate that colloid-driven precipitation as a result of boiling, the persistence of colloids after boiling, and the accumulation of colloids in diffuse flow fluids are important mechanisms for the enrichment of gold in seafloor hydrothermal systems

Constraints on the behavior of trace elements in the actively-forming TAG deposit, Mid-Atlantic Ridge, based on LA-ICP-MS analyses of pyrite

The distribution of trace ore elements in different paragenetic stages of pyrite has been documented for the first time in the sub-seafloor of the actively-forming TAG massive sulfide deposit. Trace element distributions have been determined by in-situ laser ablation inductively-coupled plasma mass spectrometry (LA-ICP-MS) of pyrite formed at different stages of mineralization, and at different temperatures constrained by previously published fluid inclusion analyses. The data reveal a strong dependence on paragenetic stage, with distinct low- and high-temperature enrichments. Porous pyrite (and marcasite) formed at low temperatures (350 °C) at the base of the hydrothermal mound and in the stockwork zone is enriched in Co, Se, Bi, Cu, Ni, and Sn. A number of different sub-types of pyrite also have characteristic trace element signatures; e.g., the earliest pyrite formed at the highest temperatures is always enriched in Co and Se compared to later stages. Ablation profiles for Co, Se, and Ni are smooth and indicate that these elements are present mainly in lattice substitutions rather than as inclusions of other sulfides. Profiles for As, Sb, Tl, and Cu can be either irregular or smooth, indicating both lattice substitutions and inclusions. Lead and Ag have mostly smooth profiles, but because Pb cannot substitute directly into the pyrite lattice, it is interpreted to be present as homogeneously distributed micro- or nano-scale particles. The behavior of the different trace elements mainly reflects their aqueous speciation in the hydrothermal fluids at different temperatures, and for some elements like Co and Se, strong partitioning into the pyrite lattice at elevated temperatures. Adsorption onto pyrite surfaces controls the distribution of a number of redox-sensitive elements (i.e., Mo, V, Ni, U), particularly in the upper part of the mound which is infiltrated by cold seawater. Where micro- or nano-scale inclusions of chalcopyrite, sphalerite, galena, or sulfosalts are present, there is still a strong temperature dependence on the inclusion population (e.g., more abundant chalcopyrite in the highest-temperature pyrite), suggesting that the inclusions were co-precipitated with pyrite rather than overgrown. However, at the deposit scale, the trace element distributions are also strongly controlled by remobilization and chemical zone refining, as previously documented in bulk geochemical profiles. The results show that pyrite chemistry is a remarkably good model of the chemistry of the entire hydrothermal system. For many trace elements, the concentrations in pyrite are highly predictive in terms of the conditions of mineral formation over a wide range of temperatures, from the stockwork zone to the cooler outer margins of the deposit. Calculated minimum concentrations of the trace elements in the fluids needed to account for the observed concentrations in pyrite show good agreement with measured vent fluid concentrations, particularly Pb, As, Mo, Ag, and Tl. However, significantly higher concentrations are indicated for Co (and Se) than have been measured in sampled fluids, confirming the strong partitioning of these elements into high-temperature pyrite