Seafloor Sulfide Deposits Research Articles

A review on theory, modeling, inversion, and application of self-potential in marine mineral exploration

The self-potential (SP) method is one of the passive geophysical exploration techniques, which employs measurements of naturally occurring electric potential differences on the ground surface and in boreholes or (sea-)water due to causative sources from electrochemical, electrokinetic, and thermoelectric mechanisms. SP signals are directly related to groundwater flow and (electro-)chemical gradient. There has been increasing interest in the application of SP measurements in mineral exploration, environmental surveys, and hydrogeophysical problems. This review focuses primarily on the theory concerning the causative source mechanism, numerical modeling, and inversion and interpretation of SP signals related to ore bodies and applications in mineral exploration. Three field examples focusing on seafloor sulfide deposits exploration are summarized to show the application effect of SP measurements in marine mineral exploration. This study would be helpful to investigating the use of SP surveys in ore prospecting, especially in marine environments.

Gravity Data Reveal New Evidence of an Axial Magma Chamber Beneath Segment 27 in the Southwest Indian Ridge

Hydrothermal systems are integral to mid-ocean ridge activity; they form massive seafloor sulfide (SMS) deposits rich in various metallic elements, which are potential mineral resources. Since 2007, many hydrothermal fields have been discovered along the ultraslow-spreading Southwest Indian Ridge (SWIR). The Duanqiao hydrothermal field is located at segment 27’s axis between the Indomed and Gallieni transform faults; tomography models reveal an obvious low-velocity anomaly beneath it, indicating a possible axial magma chamber (AMC). However, confirmation of an AMC’s existence requires further study and evidence. In this study, we first calculated the gravity effect to identify the heterogeneous distribution of crustal density beneath segment 27 and the surrounding area. Next, we used the gravity-inversion method to obtain the crustal density structure beneath the study area. The results indicate that a thickened crust and low-density crustal materials exist beneath segment 27. The low-density anomaly in the lower crust beneath the Duanqiao hydrothermal field suggests the existence of an AMC covered with a cold and dense upper crust. The density results identify several faults, which provide potential channels for magma migration. In addition, the melt migrates westward and redistributes laterally toward the segment’s western end. However, when migrating toward the segment’s eastern end, the melt is affected by a rapid cooling mechanism. Therefore, the segment’s ends present different density features and morphologies of nontransform discontinuities (NTDs).

Seafloor Hydrothermal Activity around a Large Non-Transform Discontinuity along Ultraslow-Spreading Southwest Indian Ridge (48.1–48.7° E)

Non-transform discontinuity (NTD) is one category of tectonic units along slow- and ultraslow-spreading ridges. Some NTD-related hydrothermal fields that may reflect different driving mechanisms have been documented along slow-spreading ridges, but the discrete survey strategy makes it hard to evaluate the incidence of hydrothermal activity. On ultraslow-spreading ridges, fewer NTD-related hydrothermal activities were reported. Factors contributing to the occurrence of hydrothermal activities at NTDs and whether they could be potential targets for hydrothermal exploration are poorly known. Combining turbidity and oxidation reduction potential (ORP) sensors with a near-bottom camera, Chinese Dayang cruises from 2014 to 2018 have conducted systematic towed surveys for hydrothermal activity around a large NTD along the ultraslow-spreading Southwest Indian Ridge (SWIR, 48.1–48.7° E). Five new potential hydrothermal anomaly sites (2 inferred and 3 suspected) of high or low temperature and the previously inferred Sudi hydrothermal field occurred in diverse morphotectonic settings along a 78 km long ridge axis. The calculated vent frequency (Fs, sites/100 km) was ~7.7 over the entire study area, higher than the modified value (Fs ≈ 6.5) between 48 and 52° E of SWIR. Even only for the 54 km long large NTD, three hydrothermal anomaly sites yielded an Fs of ~5.6, which is higher than that of most ridge sections and is comparable to some fast-spreading ridges with high-resolution surveys. This indicates that NTDs along ultraslow-spreading ridges could be promising areas in fertilizing hydrothermal activities. Moreover, the deeply penetrating faults on the rift valley walls and strain-focused areas may contribute to the formation of NTD-related hydrothermal circulations. We suggest that NTDs along ultraslow-spreading ridges may be potential targets for further exploration of hydrothermal activities and seafloor sulfide deposits.

Functional diversity of microbial communities in inactive seafloor sulfide deposits.

The seafloor sulfide structures of inactive vents are known to host abundant and diverse microorganisms potentially supported by mineralogy of sulfides. However, little is known about the diversity and distribution of microbial functions. Here, we used genome-resolved metagenomics to predict microbial metabolic functions and the contribution of horizontal gene transfer to the functionality of microorganisms inhabiting several hydrothermally inactive seafloor deposits among globally distributed deep-sea vent fields. Despite of geographically distant vent fields, similar microbial community patterns were observed with the dominance of Gammaproteobacteria, Bacteroidota and previously overlooked Candidatus Patescibacteria. Metabolically flexible Gammaproteobacteria are major potential primary producers utilizing mainly sulfur, iron and hydrogen as electron donors coupled with oxygen and nitrate respiration for chemolithoautotrophic growth. In addition to heterotrophic microorganisms like free-living Bacteroidota, Ca. Patescibacteria potentially perform fermentative recycling of organic carbon. Finally, we provided evidence that many functional genes that are central to energy metabolism have been laterally transferred among members within the community and largely within the same class. Taken together, these findings shed light on microbial ecology and evolution in inactive seafloor sulfide deposits after the cessation of hydrothermal activities.

Paleobathymetry of Submarine Lavas in the Samail and Troodos Ophiolites: Insights From Volatiles in Glasses and Implications for Hydrothermal Systems

AbstractHydrostatic pressure exerted by the ocean water column fundamentally influences magmatic and hydrothermal processes in submarine volcanic settings and is therefore an important parameter to know when investigating such processes. Currently, there are few reliable methods for reconstructing past ocean depths for ancient volcanic terranes. Here, we develop and test an empirically calibrated statistical approach for determining paleodepths of eruption from the concentrations of H2O and CO2 dissolved in volcanic glasses, utilizing the well‐defined pressure‐dependent solubility of these volatiles in silicate melts. By comparing newly determined and published glass compositions from the Samail and Troodos ophiolites with sedimentary and fluid inclusion evidence, we propose that the Samail lavas erupted at ocean depths of ∼3.4 km, and the Troodos lavas at ∼4.1 km. These depths are 1–2 km deeper than those assumed in most previous studies of hydrothermal activity in the two ophiolites. These high depths imply high hydrostatic pressures within the underlying oceanic crust. Such pressures may have allowed convecting hydrothermal fluids to attain significantly higher temperatures (e.g., >450°C) than in typical modern ocean ridge hydrothermal systems during metal leaching in the crust and metal precipitation in seafloor sulfide deposits.

Autonomous-underwater-vehicle-based marine multicomponent self-potential method: observation scheme and navigational correction

Abstract. Marine self-potential (SP) investigation is an effective method to study deep-sea hydrothermal vents and seafloor sulfide deposits. At present, one of the commonly used marine self-potential systems is a towed array of electrodes. Large noises are recorded when great changes in electrode distance and array attitude occur due to the complex seafloor topography. In this paper, a new multicomponent electrical field observation system based on an autonomous underwater vehicle (AUV) was introduced for the measurement of seafloor self-potential signals. The system was tested in a lake, and the multicomponent self-potential data were collected from there. Observed data involve the navigational information of the AUV, which could be corrected using a rotation transform. After navigational correction, measured data can recover the location of the artificial source using self-potential tomography. The experimental results showed that the new SP system can be applied to marine SP observations, providing an efficient and low-noise SP acquisition method for marine resources and environmental investigations.

Behavior of Trace Elements during Oxidation of Sphalerite of the Irinovskoe Hydrothermal Sulfide Field (13°20′ N, Mid-Atlantic Ridge)

The formation of present-day seafloor sulfide deposits is accompanied by their continuous oxidation and crystallization of insoluble Fe oxyhydroxides, which absorb metals (including heavy metals) (Fallon et al., 2017). Due to the high sorption ability of Fe oxyhydroxides, it is suggested that the removal of metals during submarine oxidation into ambient seawater is insignificant (Fallon et al., 2017). Quantitative data on the redistribution of trace elements between primary sulfides and their oxidation products, however, are insufficient. Recently, we revealed the enrichment of trace elements in covellite with respect to primary sphalerite, chalcopyrite, and isocubanite (Melekestseva et al., 2017). In this study, we estimate the behavior of trace elements during submarine oxidation of sphalerite from smokers at the Irinovskoe hydrothermal field (Atlantic Ocean), which is intensely replaced by Fe oxyhydroxides. As a result, it is established that Fe oxyhydroxides are enriched in many trace elements with respect to sphalerite and the mode of their occurrence is identified.

Hydrothermal vent fields discovered in the southern Gulf of California clarify role of habitat in augmenting regional diversity.

Hydrothermal vent communities are distributed along mid-ocean spreading ridges as isolated patches. While distance is a key factor influencing connectivity among sites, habitat characteristics are also critical. The Pescadero Basin (PB) and Alarcón Rise (AR) vent fields, recently discovered in the southern Gulf of California, are bounded by previously known vent localities (e.g. Guaymas Basin and 21° N East Pacific Rise); yet, the newly discovered vents differ markedly in substrata and vent fluid attributes. Out of 116 macrofaunal species observed or collected, only three species are shared among all four vent fields, while 73 occur at only one locality. Foundation species at basalt-hosted sulfide chimneys on the AR differ from the functional equivalents inhabiting sediment-hosted carbonate chimneys in the PB, only 75 km away. The dominant species of symbiont-hosting tubeworms and clams, and peripheral suspension-feeding taxa, differ between the sites. Notably, the PB vents host a limited and specialized fauna in which 17 of 26 species are unknown at other regional vents and many are new species. Rare sightings and captured larvae of the ‘missing’ species revealed that dispersal limitation is not responsible for differences in community composition at the neighbouring vent localities. Instead, larval recruitment-limiting habitat suitability probably favours species differentially. As scenarios develop to design conservation strategies around mining of seafloor sulfide deposits, these results illustrate that models encompassing habitat characteristics are needed to predict metacommunity structure.

Hydrothermal plume mapping as a prospecting tool for seafloor sulfide deposits: a case study at the Zouyu-1 and Zouyu-2 hydrothermal fields in the southern Mid-Atlantic Ridge

Seafloor hydrothermal polymetallic sulfide deposits are a new type of resource, with great potential economic value and good prospect development. This paper discusses turbidity, oxidation–reduction potential, and temperature anomalies of hydrothermal plumes from the Zouyu-1 and Zouyu-2 hydrothermal fields on the southern Mid-Atlantic Ridge. We use the known location of these vent fields and plume data collected in multiple years (2009, 2011, 2013) to demonstrate how real-time plume exploration can be used to locate active vent fields, and thus associated sulfide deposits. Turbidity anomalies can be detected 10 s of km from an active source, but the location precision is no better than a few kilometers because fine-grained particles are quasi-conservative over periods of many days. Temperature and oxidation–reduction potential anomalies provide location precision of a few hundred meters. Temperature anomalies are generally weak and difficult to reliably detect, except by chance encounters of a buoyant plume. Oxidation–reduction potential is highly sensitive (nmol concentrations of reduced hydrothermal chemicals) to discharges of all temperatures and responds immediately to a plume encounter. Real-time surveys using continuous tows of turbidity and oxidation–reduction potential sensors offer the most efficient and precise surface ship exploration presently possible.

Occurrences and distribution of “invisible” precious metals in sulfide deposits from the Edmond hydrothermal field, Central Indian Ridge

Here, we report the first documented occurrences of “invisible” gold and silver in seafloor sulfide deposits from an active hydrothermal system on the Central Indian Ridge. A detailed mineralogical and geochemical study of polymetallic sulfides from the Edmond vent field was conducted in order to identify controls on the distribution of precious metals. Bulk samples (N=18) contain up to 18.7ppm Au and 1450ppm Ag, with average concentrations of 2.3ppm Au and 218.9ppm Ag. Among them, several Zn-rich chimney fragments and anhydrite-dominated ore samples have higher contents of precious metals than Fe-Cu-rich massive sulfides and silica-rich hydrothermal precipitates. Native gold grains are mainly associated with sphalerite, anhydrite, barite and Fe-oxyhydroxides. Abundant submicroscopic Au-Ag alloys tend to occur along grain boundaries between Cu-Fe sulfides and tennantite, or close to the rims of Fe-poor sphalerite. In contrast to primary electrum with high Ag/Au ratios, the absence of detectable silver in high-purity gold indicates that secondary Au enrichment has probably occurred after a direct co-precipitation with Zn-rich mineral assemblages upon cooling and mixing of vent fluids with cold seawater. A suite of late-stage Ag-rich phases, including argentotennantite, pearceite and acanthite, occur as crack-filling veinlets and patches in low-temperature fahlores, or as tiny inclusions enclosed by pyrite, chalcopyrite and colloform sphalerite. By using HRTEM combined with HAADF-STEM imaging, we have found out that silver is also present in significant quantities as discrete colloidal nanoparticles in tennantite. Minor native copper is closely associated with altered chalcopyrite, sphalerite and covellite, exhibiting signs of dissolution, recrystallization and reprecipitation. Extensive hydrothermal reworking resulted from a long history of high-temperature venting in this field, together with post-depositional supergene replacement processes (involving oxidation, leaching or coupled dissolution-reprecipitation mechanisms facilitated by a permeable porosity generated in primary Cu-Fe sulfides) are considered to be important for the remobilization and local reconcentration of early-formed precious metals, and may have been responsible for the formation of relatively coarse-grained native gold or silver within recrystallized massive sulfides and chimney debris.

Factors affecting the rare earth element compositions in massive sulfides from deep‐sea hydrothermal systems

AbstractTo reconstruct the evolution of ore‐forming fluids and determine the physicochemical conditions of deposition associated with seafloor massive sulfides, we must better understand the sources of rare earth elements (REEs), the factors that affect the REE abundance in the sulfides, and the REE flux from hydrothermal fluids to the sulfides. Here we examine the REE profiles of 46 massive sulfide samples collected from seven seafloor hydrothermal systems. These profiles feature variable total REE concentrations (37.2–4092 ppb) and REE distribution patterns (LaCN/LuCN ratios = 2.00–73.8; (Eu/Eu*)CN ratios = 0.34–7.60). The majority of the REE distribution patterns in the sulfides are similar to those of vent fluids, with the sulfides also exhibiting light REE enrichment. We demonstrate that the variable REE concentrations, Eu anomalies, and fractionation between light REEs and heavy REEs in the sulfides exhibit a relationship with the REE properties of the sulfide‐forming fluids and the massive sulfide chemistry. Based on the sulfide REE data, we estimate that modern seafloor sulfide deposits contain approximately 280 t of REEs. According to the flux of hydrothermal fluids at mid‐ocean ridges (MORs) and an average REE concentration of 3 ng/g in these fluids, hydrothermal vents at MORs alone transport more REEs (>360 t) to the oceans over the course of just 2 years than the total quantity of REEs in seafloor sulfides. The excess REEs (i.e., the quantity not captured by massive sulfides) may be transported away from the systems and become bound in sulfate deposits and metalliferous sediments.

Noble gases in sulfide deposits of modern deep-sea hydrothermal systems: Implications for heat fluxes and hydrothermal fluid processes

Studies of noble gases in seafloor sulfide deposits are an important tool for understanding the origin of ore-forming fluids, both enabling the determination of noble gas sources and revealing the degree of fluid–rock interaction and mantle degassing. The noble gas concentrations and isotopic compositions of 27 sulfide, 3 sulfate, and 2 opal mineral aggregate samples have been studied in different hydrothermal fields from the East Pacific Rise, Mid-Atlantic Ridge, Central Indian Ridge, Southwest Indian Ridge, and North Fiji Back-Arc Basin. The helium concentrations and isotopic ratios in the sulfide aggregate samples are variable (4He 0.12 to 22×10−8cm3 STP/g; 3He/4He ~0.6 to 10.4 Ra), and most of the sulfide helium concentrations are higher than those in the opal mineral samples (4He 0.017 and 0.028×10−8cm3 STP/g), suggesting that the low-temperature fluids have lost their mantle helium during cooling. The helium of high (>7 Ra), medium (1–7 Ra), and low (≈1 Ra) 3He/4He ratio samples is derived mainly from the MORB or OIB mantle by magma degassing, from mixing between hydrothermal fluid and seawater during ore-forming processes, and from ambient seawater, respectively. The high 3He/4He ratios (>7 Ra) of sulfides imply that high-temperature sulfides retain the helium isotopic compositions of the primary hydrothermal fluid, whereas low-temperature sulfides, sulfates, and opal minerals do not.The neon, argon, krypton, and xenon concentrations in the sulfide aggregate samples are also variable; in most of the sulfide aggregates, they are significantly lower than in the sulfate and opal mineral samples. It is known that barite and opal minerals are characteristic of low-temperature (<200°C) paragenetic associations, indicating that heavier noble gases (Ne, Ar, Kr and Xe) are enriched under low-temperature conditions. Most of the sulfide, sulfate and opal mineral aggregate samples possess heavy noble gas elemental abundances similar to those of air-saturated seawater and Ne, Ar, and Xe isotopic compositions that span narrow ranges around atmospheric values, an observation which is most easily explained by the dominance of a seawater-derived component.In addition, based on the calculated helium/heat ratios, global helium and heat fluxes to high-temperature hydrothermal vents are approximately 0.05–6×104kg per year and 0.1–12×1012W, meaning that roughly 0.3% of ocean heat is supplied by seafloor high-temperature hydrothermal activity.

Constraints on Water Depth of Massive Sulfide Formation: Evidence from Modern Seafloor Hydrothermal Systems in Arc-Related Settings

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.

Rhenium–osmium abundance and isotopic compositions of massive sulfides from modern deep-sea hydrothermal systems: Implications for vent associated ore forming processes

Studies of rhenium (Re) and osmium (Os) concentrations and isotopic compositions in seafloor hydrothermal sulfides are an important tool for understanding the evolution of hydrothermal systems, allowing the determination of both metal sources and reconstructing the physicochemical conditions of their deposition. The Re–Os concentrations and isotopic compositions of 38 massive sulfide samples have been studied in different hydrothermal fields from the East Pacific Rise (EPR), Mid-Atlantic Ridge (MAR), Central Indian Ridge (CIR), Southwest Indian Ridge (SWIR), and Back-Arc Basin (BAB). The majority of the sulfides possess 187Os/188Os that span a narrow range (1.004 to 1.209), which is most easily explained as a seawater-derived component. This may suggest that those initial 187Os/188Os isotope compositions of ancient seafloor hydrothermal sulfide deposits which were formed by the mixing process between seawater and hydrothermal fluid, are possible for analysing ancient seawater Os components. Only two of samples (MAR05-TVG1-10-2 and MAR05-TVG1-21 from the Logatchev hydrothermal field (LHF), MAR) possess moderately less radiogenic 187Os/188Os (0.645 to 0.730), which may reflect the lesser extent of hydrothermal fluid–seawater mixing during hydrothermal ore-forming process.The Re and Os concentrations and 187Re/188Os ratios of pyrite and Fe–Cu sulfide mineral aggregates (avg Re 11.46 ppb; avg Os 17.76 ppt; avg 187Re/188Os 11,980.4; n=24) are usually higher than those of sphalerite or Zn-enriched sulfide mineral aggregate samples (avg Re 0.31 ppb; avg Os 7.09 ppt; avg 187Re/188Os 206.99; n=9), suggesting that Re and Os are more likely to be incorporated into Fe- and Fe–Cu sulfide mineral facies. A significant positive correlation is observed between Os/Re ratios and Pb concentrations in the sulfide samples from the Edmond hydrothermal field (EHF), whereas Pb-enriched sulfide is characteristic of low-temperature paragenetic association, suggesting that Os enriched under low-temperature (<200 °C) condition. In addition, our Re–Os data are used to estimate that modern seafloor sulfide deposits contain roughly 0.6–44 t (avg 4 t, n=38) of Re, and 1 to 48 kg (avg 8 kg, n=38) of Os. The Os flux of global low-temperature hydrothermal fluids to vents is about 11 kg/yr, and the excess Os (i.e. Os not incorporated in sulfides) may be carried and become bound in metalliferous sediment, Fe–Mn crusts and nodules distal to the hydrothermal vents.

Mineralogy Drives Bacterial Biogeography of Hydrothermally Inactive Seafloor Sulfide Deposits

Mid-ocean ridge hydrothermal venting creates sulfide deposits containing gradients in mineralogy, fluid chemistry, and temperature. Even when hydrothermal circulation ceases, sulfides are known to host microbial communities. The relationship between mineralogy and microbial community composition in low-temperature, rock-hosted systems has not been resolved at any spatial scale, local or global. To examine the hypothesis that geochemistry of seafloor deposits is a dominant parameter driving environmental pressure for bacterial communities at low-temperature, the shared community membership, richness, and structure was measured using 16S rRNA gene sequences. The focus of the study was on hydrothermally inactive seafloor deposits from multiple locations within one deposit (e.g., single extinct chimney), within one vent field (intra-vent field), and among globally distributed vent fields from three ocean basins (inter-vent field). Distinct mineral substrates, such as hydrothermally inactive sulfides versus basalts, host different communities at low temperature in spite of close geographic proximity and contact with the same hydrothermally influenced deep-sea water. Furthermore, bacterial communities inhabiting hydrothermally inactive sulfide deposits from geographically distant locations cluster together in community cladograms to the exclusion of other deep-sea substrates and settings. From this study, we conclude that at low temperature, mineralogy was a more important variable determining microbial community composition than geographic factors. Supplemental materials are available for this article. Go to the publisher's online edition of Geomicrobiology Journal to view the supplemental file.

日本のＥＥＺ（排他的経済水域）・大陸棚の深海底鉱物資源開発の可能性と必要性

Japan has a manganese nodule mining claim in the Clarion Clipperton Fracture Zones, the Kuroko-type massive seafloor sulfide deposits (SMS) and cobalt-rich manganese crusts (CMC) in Japan's exclusive economic zone (EEZ) and the continental shelves. Japan needs to use these deep-sea mineral resources as future strategic metal and rare earth element supply sources. Furthermore, the development technologies have wide variations in applying for the other food and energy supply targets in EEZ and continental shelves and the same-type resources of Pacific island nations'.Some current topics in deep-sea mineral resources development and the development technologies are introduced. Possibility and necessity of deep-sea mineral resources development for Japan are discussed.

Seafloor bioalteration of sulfide minerals: results from in situ incubation studies

We present results of incubation studies conducted at low temperatures (∼4°C) in the vicinity of a seafloor hydrothermal vent system. We reacted Fe-, S-, Cu-, and Zn-bearing minerals including pyrite, marcasite, chalcopyrite, sphalerite, elemental sulfur, and a portion of a natural chimney sulfide structure for 2 months at the Main Endeavour Segment of the Juan de Fuca Ridge in the Pacific Ocean. Our study utilizes Fluorescent In Situ Hybridizations (FISH), Scanning and Transmission Electron Microscopy (SEM, TEM), and light microscopic analysis. The surfaces of these minerals are solely colonized by Bacteria and not by Archaea. Colonization densities vary over an order of magnitude with the following sequence: elemental sulfur > chimney sulfide > marcasite > pyrite > sphalerite > chalcopyrite, and correspond well with the abiotic oxidation kinetics of these materials, excepting elemental sulfur, which is both the least reactive to oxidizing species and the most heavily colonized. Colonization densities also correspond with apparent degree of reaction (dissolution pitting + accumulation of secondary alteration products). Heavy accumulations of secondary Fe oxides on Fe-bearing minerals, most notably on the chimney sulfide, form in situ as the result of mineral dissolution and the activity of neutrophilic Fe-oxidizing bacteria. Results suggest that mineral-oxidizing bacteria play a prominent role in weathering of seafloor sulfide deposits, and that microbial utilization of mineral substrates contributes to biomass production in seafloor hydrothermal environments.

Constraints on the energy and chemical balances of the modern TAG and ancient Cyprus seafloor sulfide deposits

The size, chemical composition, energy flux, and fluid composition of the TAG hydrothermal sulfide deposit at the Mid‐Atlantic Ridge and the size, chemical composition and reaction zone characteristics of the Skouriotissa volcanogenic massive sulfide deposit of the Troodos ophiolite in Cyprus are used to examine the energy requirements and chemical balances associated with the generation of a large volcanogenic massive sulfide deposit. We conclude that formation of large sulfide deposits from oceanic hydrothermal systems is a geologically rapid process and occurs on timescales of hundreds of years, with episodes of activity as short as a few tens of years separated by thousands of years of inactivity. About 2×1019 J of energy supplied at high temperature is required to form a deposit the size of the TAG mound. Metals (up to 4 times the mass of any element present in the sulfide deposit) are leached out of relatively small reaction zones most likely at the base of the sheeted dikes. Chemical balance can be struck for all elements except sulfur with a reaction zone 1–2 km3 in volume from which a small proportion of iron and large proportions of copper, zinc, and manganese are removed. A sulfur balance requires that a significant fraction of sulfur be derived from reduction of seawater sulfate, as suggested by stable isotope analyses. We argue that the principal source of energy that drives hydrothermal circulation is latent heat of crystallization of magma close to the top of the plutonic section. Furthermore, we speculate that activity of the TAG hydrothermal system is related to periods of more rapid magma supply from the mantle at magma supply rates similar to those observed in volcanoes in Hawaii and Iceland.

Radioisotopic studies of submarine hydrothermal vents

The importance of mid‐ocean ridge hydrothermal systems has been recognized for their role in the regulation of ocean and sediment chemistry, as well as for providing a chemosynthetic source of carbon which drives a unique population of animals found at hydrothermal vents. Despite the importance of these systems the rates, length, and depth scale of submarine hydrothermal processes are not precisely known because they are, for the most part, inaccessible to observational tools. We must therefore rely on indirect methods to quantify these processes. One way of investigating the rates, or timescales, of processes in a hydrothermal (or any natural) system is through the study and modeling of naturally occurring radioisotopes. Disequilibria among the naturally occurring radioactive decay series in vent fluids, associated mineral deposits, and overlying effluent plume have provided geochemical tools to investigate the rates of various processes occurring in submarine hydrothermal systems. Because the half‐lives of the radioisotopes vary from days to many years, processes which encompass a wide range of spatial and temporal scales can be studied. This paper presents a review of methods that estimate the residence time of hydrothermal fluids in the ocean crust, establish the geochronology of seafloor sulfide deposits, investigate the rates of chemical reactions within hydrothermal effluent plumes, and derive the heat and mass flux from seafloor hydrothermal areas.

Hydrothermal sulfide and oxide deposits on seamounts near 21°N, East Pacific Rise

Hydrothermal deposits were sampled by submersible from the calderas and summit benches of two seamounts 20 and 30 km west of the East Pacific Rise at 21°N. The deposits document a wide variety of off-axis hydrothermal activity, including samples from the interiors of seafloor sulfide deposits. In contrast to most mid-ocean ridge deposits, sulfide deposits on Green Seamount are rich in Si, Fe, and Cu, are poor in Zn, and contain abundant quartz. Sulfide chimneys grew by quenching of hydrothermal fluids and deposition of walls of colloform pyrite, marcasite, Zn sulfide, and minor galena, whereas coarse-grained pyrite and chalcopyrite were deposited at higher temperatures (>250 °C) in chimney interiors. As chimney walls grew outward into sea water, increased temperatures in the inner walls resulted in recrystallization to pyrite and deposition of pyrite and chalcopyrite in pore space. Cooling in the outer portions of chimneys caused deposition of opal at temperatures of about 80–170 °C. As chimneys coalesced and opal-rich material was heated in the interiors of sulfide edifices, opal was recrystallized to chalcedony and quartz. Faulting has exposed the interiors of sulfide deposits, where further recrystallization and deposition of chalcopyrite and quartz occurred at temperatures of 230–320 °C. Clogging of chimneys, changes in flow paths, and cooling of the deposits led to the deposition of late mixed-layer illite-smectite, iron sulfides, Fe-poor sphalerite, opal, and barite. The sulfide deposits formed over a time span of at least 140,000 to 70,000 yr ago, probably coincident with caldera and pit crater formation. The ages of the sulfide deposits provide constraints on the last volcanic and tectonic activity, and they also suggest that it took a maximum of ∼260,000 yr for the seamount to reach its present height. Red Seamount is presently hydrothermally active, and deposits consisting predominantly of amorphous Fe-oxyhydroxide are forming on small pillow cones in the caldera at temperatures of 10–15 °C. Hydrothermal nontronite deposits capped by Mn-oxide crusts cover much of the southern summit bench of Red Seamount and formed at temperatures of around 30 °C. Iron-rich talc and sulfide grains in nontronite provide evidence of former high-temperature activity and suggest that a sulfide deposit is present nearby.