Articles de revues sur le sujet « Shallow hydrothermal sources »

Iron is an essential nutrient that regulates productivity in ~30% of the ocean. Compared with deep (>2000 meter) hydrothermal activity at mid-ocean ridges that provide iron to the ocean’s interior, shallow (<500 meter) hydrothermal fluids are likely to influence the surface’s ecosystem. However, their effect is unknown. In this work, we show that fluids emitted along the Tonga volcanic arc (South Pacific) have a substantial impact on iron concentrations in the photic layer through vertical diffusion. This enrichment stimulates biological activity, resulting in an extensive patch of chlorophyll (360,000 square kilometers). Diazotroph activity is two to eight times higher and carbon export fluxes are two to three times higher in iron-enriched waters than in adjacent unfertilized waters. Such findings reveal a previously undescribed mechanism of natural iron fertilization in the ocean that fuels regional hotspot sinks for atmospheric CO 2 .

The Mesoproterozoic North American Midcontinent Rift hosts the world’s largest accumulation of native copper in Michigan’s Keweenaw Peninsula. During a regional metamorphogenic-hydrothermal event, native copper was deposited along with spatially zoned main-stage minerals in a thermal high. This was followed by deposition of late-stage minerals including minor copper sulfide. Inferences from the oxygen and carbon isotopic composition of main-stage hydrothermal fluids, as calculated from 296 new and compiled isotopic measurements on calcite, are consistent with existing models that low-sulfur saline native copper ore-forming fluids were dominantly derived by burial metamorphic processes from the very low sulfur basalt-dominated rift fill at depth below the native copper deposits. Co-variation of oxygen and carbon isotopic compositions are consistent with mixing of metamorphic-derived fluids with two additional isotopically different fluids. One of these is proposed to be evolved seawater that provided an outside source of salinity. This fluid mixed at depth and participated in the formation of a well-mixed hybrid metamorphic-dominated ore-forming fluid. Secondary Ion Mass Spectrometry in-situ isotopic analyses of calcite demonstrate a high degree of variability within samples that is attributed to variable degrees of shallow mixing of the hybrid ore-forming fluid with sulfur-poor, reduced evolved meteoric water in the zone of precipitation. The oxygen and carbon isotopic compositions of 100 new and compiled measurements on late-stage calcite are mostly isotopically different than the main-stage hydrothermal fluids. The late-stage hydrothermal fluids are interpreted as various proportions of mixing of evolved meteoric water, main-stage hybrid ore-forming fluid, and shallow, evolved seawater in the relatively shallow zone of precipitation.

Hydrothermal vents are one of the important sources of major or trace elements in the ocean. The elemental fluxes, however, may be dynamic due to coastal processes and hydrothermal plumes, especially in shallow-water hydrothermal vents. We collected water samples by using the trace-metal clean technique inside and outside two shallow-water hydrothermal vents (white vent: low temperature, high pH; and yellow vent: high temperature, low pH) off Kueishantao Islet, Taiwan, China via SCUBA divers. We analyzed these samples for their hydro-chemical parameters and dissolved elements (Fe, Mn, Mg, V, Cu, and Mo) thereafter. Our results show that dissolved metals’ concentrations were significantly different between the two vents, with higher Mn and Fe in the White Vent than in the Yellow Vent, likely due to the decreased affinity of the dissolved metals for particles in the white vent. We estimated the plume fluxes of dissolved metals from the hydrothermal mouth by multiplying in situ hydrothermal discharge flowrates with metals’ concentrations inside the vents, which were: 1.09~7.02 × 104 kg Mg, 0.10~1.23 kg Fe, 0.08~28 kg Mn, 33.4~306 g V, 2.89~77.7 g Cu, and 54.3~664 g Mo, annually. The results further indicate that such plumes probably have impacted nearby seawater due to coastal currents and particle desorption during transport. Furthermore, the concentrations of biogenic elements could be further modified in seawater, and potentially impact nearby ecosystems on a larger scale. Our study provides information with which to further understand metal redeployment in submarine shallow nearby ecosystems.

We analyzed sulfur isotopes, trace elements and chalcophile elements (Se, Te, As, Sb, and Hg) in the native sulfur matrix from the Kueishantao hydrothermal system and conducted a systematic micro-analytical investigation. The sulfur matrix lacked all measured metals (e.g., Fe, Cu) and rare earth elements (REEs) while being significantly enriched in Te, As, Se (750–1500 ppm), Sb (around 100 ppm) and some Hg. The δ34S data (0.2–2.4‰) suggest a magmatic source leached from igneous rocks and a small contribution of seawater sulfates to the sulfur in hydrothermal deposits. Correlations between Te, As, Sb, and S (r2 = 0.30–0.61) indicate that these elements behave coherently in magmatic-hydrothermal processes. The enrichment factors and content ratios of these elements demonstrate their abundance in the sulfur matrix and minor fractionation after being partitioned into the metallic melt and forming a separate vapor phase to transport. Our study focuses on the native sulfur matrix in a shallow-water volcanic hydrothermal system, to which relatively little attention has previously been paid. This will expand our understanding of hydrothermal precipitates. The study of volatile chalcophile elements in the matrix will provide significant information about their sources, distributions and other geochemical behaviors in magmatic-hydrothermal processes and help to understand the Kueishantao hydrothermal circulation better.

To elucidate the sulphur sources for chemoautotrophy by the symbiotic bacteria of a vestimentiferan tubeworm, Lamellibrachia satsuma, living in Kagoshima Bay at depths of 80–100 m, we analysed the sulphur isotopic ratios of the animal tissues and compared them with environmental sulphur species collected in the field. Animals that had been maintained in an aquarium for over a year and supplied a known sulphur source were also investigated. The gas emitted from volcanic source in Kagoshima Bay contained rather heavy sulphide (+12·7 to +22·9‰ δ34S) compared with deep-sea hydrothermal vent systems (0 to +5‰). The tissue of the tubeworms contained very light sulphide (−21·5 to −25·9‰). It is inferred from the analysis of the aquarium-maintained specimens that the fractionation by the tubeworm or its symbiont was <1·5‰. The sulphur source assimilated by the tubeworms in the field is therefore inferred to have δ ratio −19·1 to −24·6‰. This means that only 9·7 to 25·0% of the sulphur in the worm tissues can be derived from the volcanic gas and the rest must come from other sources, such as microbial activity in the bottom sediment.

The energy reserves in hot dry rock and hydrothermal systems are abundant in China, however, the developed resources are far below the potential estimates due to immature technology of enhanced geothermal system (EGS) and scattered resources of hydrothermal systems. To circumvent these problems and reduce the thermal resistance of rocks, here a shallow depth enhanced geothermal system (SDEGS) is proposed, which can be implemented by fracturing the hydrothermal system. We find that, the service life for SDEGS is 14 years with heat output of 4521.1 kW. To extend service life, the hybrid SDEGS and solar energy heating system is proposed with 10,000 m2 solar collectors installed to store heat into geothermal reservoir. The service life of the hybrid heating system is 35 years with geothermal heat output of 4653.78 kW. The novelty of the present work is that the hybrid heating system can solve the unstable and discontinuous problems of solar energy without building additional back-up sources or seasonal storage equipment, and the geothermal thermal output can be adjusted easily to meet the demand of building thermal loads varying with outside temperature.

Geogenic and anthropogenic sources of atmospheric particulate and CO2 can lead to threats to human health in volcanic areas. Although the volcanic CO2 hazard is a topic frequently debated in the related scientific literature, space and time distribution of PM2.5 are poorly known. The results of combined CO2/PM2.5 surveys, carried out at Salina, Stromboli, and Vulcano islands (Aeolian archipelago, Italy) in the years 2020–2021, and integrated with investigations on bioaccumulation of metallic particulate matter by the mean of data on the magnetic properties of oleander leaves, are presented in this work. The retrieved results indicate that no significant anthropogenic sources for both CO2 and PM2.5 are active in these islands, at the net of a minor contribution due to vehicular traffic. Conversely, increments in volcanic activity, as the unrest experienced by Vulcano island since the second half of 2021, pose serious threats to human health, due to the near-ground accumulation of CO2, and the presence of suspended micro-droplets of condensed hydrothermal vapor, fostering the diffusion of atmophile viruses, such as the COVID-19. Gas hazard conditions can be generated, not only by volcanic vents or fumarolic fields, but also by unconventional sources, such as the outgassing from shallow hydrothermal aquifers through drilled or hand-carved wells.

Dolomite bodies in the Middle Permian of the central Sichuan Basin have been reported as favorable natural gas reservoirs. The Middle Permian dolomite consists of three types of recrystallized dolomite (Rd1, Rd2, and Rd3) and one type of dolomite cement (Sd). Rd1 might be formed as the primary mineral along the calcite in the original sea-water. Its δ13C value and 87Sr/86Sr ratio, consistent with those of marine limestone and Permian seawater, support that the dolomitizing fluid for Rd1 was Permian seawater preserved in the strata. Rd2 consists of fine to medium (50 μm to 250 μm) and planar to curved crystals. Geochemical indicators (slightly high 87Sr/86Sr ratio, similar rare earth element patterns, negative δ18O, slightly high salinity) confirm that the dolomitizing fluid of Rd2 was mainly Permian seawater during shallow burial, with a small number of hydrothermal fluids. Rd3 and Sd are featured by very large (>250 μm), curved crystals, and high-temperature, high-salinity, and obviously positive Eu anomalies, suggesting that their diagenetic fluids were mainly hydrothermal fluids from deep. Additionally, inherited carbon sources and the 87Sr/86Sr ratios of some samples fall within the range of Permian seawater distribution, confirming the contribution of Permian seawater. ELIP activity caused the formation of this dolomite through the mixing of seawater and hydrothermal fluids. The main fluid circulation channels were activated basement faults, epigenetic karst pores, and shallowly buried high-permeability strata. During the peak period of ELIP activity, the continuous upwelling of deep hydrothermal fluids led to the continuous formation of Rd2, Rd3, and Sd. The dolomitization fluid of Rd2 was mainly composed of seawater and featured a certain lateral extension, which was away from faults. Rd3 and Sd are mainly distributed along the fault system, and excessive dolomitization caused by the hydrothermal activity, to some extent, inhibited the lateral movement of hydrothermal fluids. This study provides a good example for exploring the genetic mechanism and distribution pattern of structurally controlled dolomites under a volcanic activity background.

Research subject.This research study was aimed at investigating metasomatic minerals and ores in the Tamunier Deposit, which is located in the Northern Urals, at the Eastern side of the Tagil megazone within the Auerbach volcano-plutonic belt.Materials and methods.Well core samples were investigated using a complex of research methods, including optical and electron microscopy, X-ray spectral microanalysis, mineral geothermometry, thermobarogeochemistry (microthermometry, gas chromatography, determination of the salt composition of fluid inclusions in minerals) and isotope geochemistry (isotopes C, O, S, Sr, Pb).Results.A genetic model describing the formation of the Tamunier deposit was developed using the data obtained on its geological structure, mineral composition of metasomatites and ores, fluid formation mode, sources of ore matter and ore-bearing fluid. In the proposed model, the magmatogenic sodium chloride fluid carrying ore components and S is separated from the Auerbach complex at the depth of intrusion. Penetrating to the surface, this fluid interacts with the rocks of volcanic-sedimentary strata, thereby extracting a number of components, including CO2, S and Sr.Conclusion.Despite the presence of sulphide mineralization of hydrothermal-sedimentary genesis in the volcanogenic-sedimentary rock mass, the data obtained has allowed us to refer the gold-sulphide ores under study to magmatogenic-hydrothermal formations. The estimated P-T conditions (t= 100–370ºС andP= 0.4–0.6 kbar) and the shallow depth of the Tamunier field have shown its correspondence to the sub-epithermal level in the model of the porphyry-epithermal ore-magmatic system.

Abstract Iron oxide copper-gold (IOCG) and iron oxide-apatite (IOA) deposits are major sources of Fe, Cu, and Au. Magnetite is the modally dominant and commodity mineral in IOA deposits, whereas magnetite and hematite are predominant in IOCG deposits, with copper sulfides being the primary commodity minerals. It is generally accepted that IOCG deposits formed by hydrothermal processes, but there is a lack of consensus for the source of the ore fluid(s). There are multiple competing hypotheses for the formation of IOA deposits, with models that range from purely magmatic to purely hydrothermal. In the Chilean iron belt, the spatial and temporal association of IOCG and IOA deposits has led to the hypothesis that IOA and IOCG deposits are genetically connected, where S-Cu-Au–poor magnetite-dominated IOA deposits represent the stratigraphically deeper levels of S-Cu-Au–rich magnetite- and hematite-dominated IOCG deposits. Here we report minor element and Fe and O stable isotope abundances for magnetite and H stable isotope abundances for actinolite from the Candelaria IOCG deposit and Quince IOA prospect in the Chilean iron belt. Backscattered electron imaging reveals textures of igneous and magmatic-hydrothermal affinities and the exsolution of Mn-rich ilmenite from magnetite in Quince and deep levels of Candelaria (&gt;500 m below the bottom of the open pit). Trace element concentrations in magnetite systematically increase with depth in both deposits and decrease from core to rim within magnetite grains in shallow samples from Candelaria. These results are consistent with a cooling trend for magnetite growth from deep to shallow levels in both systems. Iron isotope compositions of magnetite range from δ56Fe values of 0.11 ± 0.07 to 0.16 ± 0.05‰ for Quince and between 0.16 ± 0.03 and 0.42 ± 0.04‰ for Candelaria. Oxygen isotope compositions of magnetite range from δ18O values of 2.65 ± 0.07 to 3.33 ± 0.07‰ for Quince and between 1.16 ± 0.07 and 7.80 ± 0.07‰ for Candelaria. For cogenetic actinolite, δD values range from –41.7 ± 2.10 to –39.0 ± 2.10‰ for Quince and from –93.9 ± 2.10 to –54.0 ± 2.10‰ for Candelaria, and δ18O values range between 5.89 ± 0.23 and 6.02 ± 0.23‰ for Quince and between 7.50 ± 0.23 and 7.69 ± 0.23‰ for Candelaria. The paired Fe and O isotope compositions of magnetite and the H isotope signature of actinolite fingerprint a magmatic source reservoir for ore fluids at Candelaria and Quince. Temperature estimates from O isotope thermometry and Fe# of actinolite (Fe# = [molar Fe]/([molar Fe] + [molar Mg])) are consistent with high-temperature mineralization (600°–860°C). The reintegrated composition of primary Ti-rich magnetite is consistent with igneous magnetite and supports magmatic conditions for the formation of magnetite in the Quince prospect and the deep portion of the Candelaria deposit. The trace element variations and zonation in magnetite from shallower levels of Candelaria are consistent with magnetite growth from a cooling magmatic-hydrothermal fluid. The combined chemical and textural data are consistent with a combined igneous and magmatic-hydrothermal origin for Quince and Candelaria, where the deeper portion of Candelaria corresponds to a transitional phase between the shallower IOCG deposit and a deeper IOA system analogous to the Quince IOA prospect, providing evidence for a continuum between both deposit types.

A mesophilic, strictly microaerophilic, chemosynthetic bacterium, designated strain P2DT, was isolated from the sediment of an active shallow-water hydrothermal vent in Paleochori Bay, on the Greek island of Milos. The cells were Gram-staining-negative rods that measured approximately 0.8–1.3 µm in length and 0.4–0.5 µm in width. Strain P2DT grew at 20–50 °C (optimum 35 °C), with 1.0–5.0 % (w/v) NaCl (optimum 3.0 %), and at pH 4.5–8.0 (optimum pH 5.5). The generation time under optimal conditions was 1.1 h. Growth occurred under chemolithoautotrophic conditions with S 2 O 3 2 - and CO2 as the energy and carbon sources, respectively. Oxygen (5 %) was used as sole terminal electron acceptor. No growth was observed in the presence of acetate, formate, lactate, tryptone or peptone. Chemolithoheterotrophic growth occurred when d-glucose or sucrose were present as carbon sources. None of the organic compounds tested was used as an electron donor. The genomic DNA G+C content of the novel strain was 44.9 mol%. In a phylogenetic analysis based on 16S rRNA gene sequences, strain P2DT was found to be most closely related to Thiomicrospira psychrophila DSM 13453T (92.8% sequence similarity). Based on the phylogenetic, physiological and chemotaxonomic evidence, strain P2DT represents a novel species of a new genus within the class Gammaproteobacteria of the family Piscirickettsiaceae , for which the name Galenea microaerophila gen. nov., sp. nov. is proposed. The type strain of the type species is P2DT ( = DSM 24963T = JCM 17795T).

Abstract. Resurgent calderas are excellent targets for geothermal exploration, as they are associated with the shallow emplacement of magma, resulting in widespread and long-lasting hydrothermal activity. Resurgence is classically attributed to the uplift of a block or dome resulting from the inflation of the collapse-forming magma chamber due to the intrusion of new magma. The Los Humeros volcanic complex (LHVC; Mexico) consists of two nested calderas: the outer and older Los Humeros formed at 164 ka and the inner Los Potreros formed at 69 ka. The latter is resurgent and currently the site of an active and exploited geothermal field (63 MWe installed). Here we aim to better define the characteristics of the resurgence in Los Potreros by integrating fieldwork with analogue models and evaluating the spatio-temporal evolution of the deformation as well as the depth and extent of the intrusions responsible for the resurgence, which may also represent the local heat source(s). Structural field analysis and geological mapping show that the floor of the Los Potreros caldera is characterized by several lava domes and cryptodomes (with normal faulting at the top) that suggest multiple deformation sources localized in narrow areas. Analogue experiments are used to define the possible source of intrusion responsible for the observed surface deformation. We apply a tested relationship between the surface deformation structures and depth of elliptical sources to our experiments with sub-circular sources. We found that this relationship is independent of the source and surface dome eccentricity, and we suggest that the magmatic sources inducing the deformation in Los Potreros are located at very shallow depths (hundreds of metres), which is in agreement with the well data and field observations. We propose that the recent deformation at LHVC is not a classical resurgence associated with the bulk inflation of a deep magma reservoir; rather, it is related to the ascent of multiple magma bodies at shallow crustal conditions (<1 km depth). A similar multiple source model of the subsurface structure has also been proposed for other calderas with an active geothermal system (Usu volcano, Japan), suggesting that the model proposed may have wider applicability.

The complexity of the Krafla volcano and its geothermal system(s) has puzzled geoscientists for decades. New and old geoscientific studies are reviewed in order to shed some light on this complexity. The geological structure and history of the volcano is more complex than hitherto believed. The visible 110 ka caldera hosts, now buried, an 80 ka inner caldera. Both calderas are bisected by an ESE-WNW transverse low-density structure. Resistivity surveys show that geothermal activity has mainly been within the inner caldera but cut through by the ESE-WNW structure. The complexity of the geothermal system in the main drill field can be understood by considering the tectonic history. Isotope composition of the thermal fluids strongly suggests at least three different geothermal systems. Silicic magma encountered in wells K-39 and IDDP-1 indicates a hitherto overlooked heat transport mechanism in evolved volcanos. Basaltic intrusions into subsided hydrothermally altered basalt melt the hydrated parts, producing a buoyant silicic melt which migrates upwards forming sills at shallow crustal levels which are heat sources for the geothermal system above. This can explain the bimodal behavior of evolved volcanos like Krafla and Askja, with occasional silicic, often phreatic, eruptions but purely basaltic in-between. When substantial amounts of silicic intrusions/magma have accumulated, major basalt intrusion(s) may “ignite” them causing a silicic eruption.

Bransfield Strait, a narrow active rift with three submarine basins, separates the South Shetland Islands from northern Antarctic Peninsula. Volcanism in Bransfield Strait commenced prior to 0.75 Ma and continues, with recent subaerial eruptions at Deception, Bridgeman and Penguin islands, submarine hydrothermal activity and numerous young basaltic seamounts located along the rift axis. Gravity cores were collected from five locations within the central Bransfield basin. Diatomaceous mud interbedded with terrigenous detritus and discrete ash layers up to 10 cm thick are the commonest sediment types in all the cores. The major element compositions of glass shards within the ash layers are, apart from the uppermost layer, compositionally similar to pyroclastic units preserved on Deception Island, a young (<0.75 Ma) active stratovolcano. The uppermost ash layer cannot be closely matched compositionally to any known source in the Antarctic–Scotia Sea–southern South America region. Its presence indicates that a volcanic centre other than Deception Island contributed ash to the Bransfield basin. Based on the shallow stratigraphical position of the compositionally distinctive ash layer, only a few decimetres beneath the seafloor, its source volcano was probably active in historical times (<few hundred years).

Proximity to magma bodies is generally acknowledged as providing the energy source for hot hydrothermal reservoirs. Hence, it is appropriate to think of a “magma–hydrothermal system” as an entity, rather than as separate systems. Repeated coring of Kilauea Iki lava lake on Kilauea Volcano, Hawaii, has provided evidence of an impermeable, conductive layer, or magma–hydrothermal boundary (MHB), between a hydrothermal system and molten rock. Crystallization on the lower face of the MHB and cracking by cooling on the upper face drive the zone downward while maintaining constant thickness, a Stefan problem of moving thermal boundaries with a phase change. Use of the observed thermal gradient in MHB of 84 °C/m yields a heat flux of 130 W/m2. Equating this with the heat flux produced by crystallization and cooling of molten lava successfully predicts the growth rate of lava lake crust of 2 m/a, which is faster than simple conduction where crust thickens at t and heat flux declines with 1 / t . However, a lava lake is not a magma chamber. Compared to erupted and degassed lava, magma at depth contains a significant amount of dissolved water that influences the magma’s thermal, chemical, and mechanical behaviors. Also, a lava lake is rootless; it has no source of heat and mass, whereas there are probably few shallow, active magma bodies that are isolated from deeper sources. Drilling at Krafla Caldera, Iceland, showed the existence of a near-liquidus rhyolite magma body at 2.1 km depth capped by an MHB with a heat flux of ≥16 W/m2. This would predict a crystallization rate of 0.6 m/a, yet no evidence of crystallization and the development of a mush zone at the base of MHB is observed. Instead, the lower face of MHB is undergoing partial melting. The explanation would appear to lie in vigorous convection of the hot rhyolite magma, delivering both heat and H2O but not crystals to its ceiling. This challenges existing concepts of magma chambers and has important implications for use of magma as the ultimate geothermal power source. It also illuminates the possibility of directly monitoring magma beneath active volcanoes for eruption forecasting.

Potash plays an important role in agricultural production. The Qaidam Basin (QB) in western China is a typical inland evaporite basin, which contains an abundance of K-rich brines, including shallow brines (i.e., surface brines and intercrystalline brines) in salt lakes and deep brines (i.e., pore brines and oilfield brines) in the strata. Significant studies on these brines have been reported; however, the integrated studies on sources of K, its distribution, and the sedimentary pattern of the two brine types are still inadequate. In this study, the K+ concentrations of sixty-four intercrystalline brines from the Qarhan Salt Lake (QSL), the largest playa in the QB, are presented. After combining those results with the major ionic compositions of river waters and deep K-rich brines, and the K+ concentrations of shallow brines in the QB, we concluded that: (1) The K of brines in the QSL is mainly from the high–flux K input by rivers which gain K from silicate weathering, while the “ancient Qaidam Lake” contributed little K to the QSL; (2) the large K flux supplied by rivers, the appropriate concentration degree, and the mixing of river waters and spring waters, cooperatively account for the highest K concentrations of brines in the QSL in the QB. The different river K recharges in different sections and isolated depressions are responsible for uneven K+ concentrations of brines in the QSL. (3) The deep brines are mainly distributed in the western QB. The K source of pore brines is from the interaction of pore water with the overlying evaporite layer. While the K in oilfield brines may be meteoric water, salt dissolution, the mixing of hydrothermal fluids, and the conversion of clay minerals to K-feldspar may consume K in the oilfield brines.

Abstract Pyrite is ubiquitous in the world-class iron oxide copper-gold (IOCG) deposits of the Candelaria-Punta del Cobre district, documented from early to late stages of mineralization and observed in deep and shallow levels of mineralized bodies. Despite its abundance, the chemical and isotopic signature of pyrite from the Candelaria-Punta del Cobre district, and most IOCG deposits worldwide, remains poorly understood. We evaluated in situ chemical and isotopic variations at the grain scale in a set of pyrite-bearing samples collected throughout the district in order to characterize and further understand the nature of mineralization in this IOCG system. Our multianalytical approach integrated synchrotron micro-X-ray fluorescence (μ-XRF) mapping of pyrite grains with electron probe microanalysis and laser ablation-inductively coupled plasma-mass spectrometry data, and sulfur isotope determinations using secondary ion mass spectrometry (SIMS) complemented with bulk sulfur isotope analyses of coeval pyrite, chalcopyrite, and anhydrite. Synchrotron μ-XRF elemental concentration maps of individual pyrite grains reveal a strong zonation of Co, Ni, As, and Se. The observed relationships between Ni and Se are interpreted to reflect changes in temperature and redox conditions during ore formation and provide constraints on fluid evolution. Co and Ni concentrations and ratios suggest contributions from magmas of mafic-intermediate composition. Pyrite chemical concentrations reflect potential stratigraphic controls, where the sample from the upper part of the stratigraphy diverges from trends formed by the rest of the sample set from lower stratigraphic levels. The SIMS δ34S values of pyrite (and chalcopyrite) range between –2 up to 10‰, and bulk δ34S values of pyrite range between 4 up to 12‰. The majority of the δ34S analyses, falling between –1 and 2‰, indicate a magmatic source for sulfur and, by inference, for the hydrothermal ore fluid(s). Variation in the δ34S signature can be explained by changes in the redox conditions, fluid sources, and/or the temperature of the hydrothermal fluid. The Se/S ratio combined with δ34S values in pyrite is consistent with mixing between a magmatic-hydrothermal fluid and a fluid with a probable basinal signature. The results of this study are consistent with the hydrothermal fluids responsible for mineralization in the Candelaria-Punta del Cobre district being predominantly of magmatic origin, plausibly from mafic-intermediate magmas based on the Ni-Co content in pyrite. External fluid incursion, potentially from a basinal sedimentary source, occurred late in the evolution of the system, adding additional reduced sulfur as pyrite. There is no evidence to suggest that the late fluid added significant Cu-Au mineralization, but this cannot be ruled out. Finally, the data reveal that trace element ratios coupled with spatially resolved sulfur isotope data in pyrite are powerful proxies to track the magmatic-hydrothermal evolution of IOCG systems and help constrain the source of their contained metals.

Unconventional geothermal resource development can contribute to increase power generation from renewable energy sources in countries without conventional hydrothermal reservoirs, which are usually associated with magmatic activity and extensional faulting, as well as to expand the generation in those regions where conventional resources are already used. Three recent drilling experiences focused on the characterization of unconventional resources are described and compared: the Campi Flegrei Deep Drilling Project (CFDDP) in Italy, the United Downs Deep Geothermal Power (UDDGP) project in the United Kingdom, and the DEEP Earth Energy Production in Canada. The main aspects of each project are described (geology, drilling, data collection, communication strategies) and compared to discuss challenges encountered at the tree sites considered, including a scientific drilling project (CFDDP) and two industrial ones (UDDGP and DEEP). The first project, at the first stage of pilot hole, although not reaching deep supercritical targets, showed extremely high, very rare thermal gradients even at shallow depths. Although each project has its own history, as well as social and economic context, the lessons learned at each drilling site can be used to further facilitate geothermal energy development.

Bulgaria is rich in sources of mineral water of temperatures in the range of 25 °C–100 °C and various chemical compositions due to diverse geological and hydrogeological conditions. A large portion of the reservoirs of low-mineralisation water is suitable for bottling. Most of them are situated in Southern Bulgaria and related to fractured hydrothermal systems, while in Northern Bulgaria mineral waters of low mineralisation are formed in the relatively shallow levels of artesian aquifers. In the last 30 years, the utilisation of bottled mineral water has increased and, currently, there are at least 18 bottling facilities, while 12 others have closed down for various reasons. The purpose of the present study is to characterise the chemical composition of the Bulgarian mineral waters utilised for bottling, in terms of geological setting, quality, compliance with regulations and possible problems associated with mineral deposition and corrosion. The water type largely depends on the host rocks. In most cases, sodium-type waters are utilized and, in the case of anions, bicarbonate and sulphate. The only exceptions are the waters formed in limestone, dolomite and marble, which are calcium-bicarbonate and calcium-magnesium type.

Water-covered areas may lead to gaps in surface electromagnetic surveys, causing reduced resolution and, as a consequence, increased uncertainty in derived subsurface models. We have evaluated a boat-towed floating central loop time-domain electromagnetic technique that mitigates this problem. It facilitates obtaining data with a spatial sampling density, which is rarely possible with standard instrumentation on land, and it only requires moderate logistical effort. A unique field study on a shallow volcanic lake demonstrated that this method is feasible with only a minor loss of accuracy when compared to anchored and land soundings. We found that the noise sources arising from the moving instrument and the boat engine can be neglected. The field survey was performed on the Lagoa das Furnas (São Miguel, Azores Islands, Portugal), which is located within an active volcanic area and is characterized by fumarolic fields and [Formula: see text] degassing. Thus, the associated hydrothermal system is expected to extend below the lake. However, the character, geometry, and extent of this system are unknown because of the lack of boreholes and geophysical studies. In total, 600 soundings, combining towed profiles with anchored and land-based soundings, were acquired with an aim of imaging the hydrothermal system beneath the lake down to 200 m. The results from all three types of measurements compare well and thus led to consistent 1D inversion models. The inversion results delineate a highly conductive, smectite-rich cap layer dipping below the lake away from the main fumarole zone. Near this zone, the extent of the conductor agrees well with an area of intense dispersed [Formula: see text] degassing, which appears to be controlled by at least two electrically distinctive fault zones in which the conductor is found at greater depths.

Abstract. Hydrothermally active and altered fault/shear zones in crystalline rocks are of practical importance because of their potential similarities with petrothermal reservoirs and exploitable natural hydrothermal systems. The petrophysical and hydraulic characterization of such structures is therefore of significant interest. Here, we report the results of corresponding investigations on a prominent shear zone of this type located in the crystalline Aar massif of the central Swiss Alps. A shallow borehole was drilled, which acutely intersects the core of the shear zone and is entirely situated in its surrounding damage zone. The focus of this study is a detailed characterization of this damage zone based on geophysical borehole measurements. For this purpose, a comprehensive suite of borehole logs, comprising passive and active nuclear, full-waveform sonic, resistivity, self-potential, optical televiewer, and borehole radar data, was collected. The migrated images of the borehole radar reflection data together with the optical televiewer data reveal a complicated network of intersecting fractures in the damage zone. Consequently, the associated petrophysical properties, notably the sonic velocities and porosities, are distinctly different from intact granitic formations. Cluster analyses of the borehole logs in combination with the structural interpretations of the optical televiewer data illustrate that the variations in the petrophysical properties are predominantly governed by the intense brittle deformation. The imaged fracture network and the high-porosity zones associated with brittle deformation represent the main flow pathways. This interpretation is consistent with the available geophysical measurements as well as the analyses of the retrieved core material. Furthermore, the interpretation of the self-potential and fluid resistivity log data suggests a compartmentalized hydraulic behavior, as evidenced by inflows of water into the borehole from different sources, which is likely to be governed by the steeply dipping structures.

The Longquanzhan deposit is one of the largest gold deposits in the Yi-Shu fault zone (central section of the Tan-Lu fault zone) in Shandong Province, China. It is an altered-rock type gold deposit in which ore bodies mainly occur at the contact zone between the overlying Cretaceous rocks and the underlying Neoarchean gneissic monzogranite. Shi et al. reported that this deposit formed at 96 ± 2 Ma using pyrite Rb–Sr dating method and represents a new gold mineralization event in the Shandong Province in 2014. In this paper, we present new He–Ar–S isotopic compositions to further decipher the sources of fluids responsible for the Longquanzhan gold mineralization. The results show that the δ34S values of pyrites vary between 0.9‰ and 4.4‰ with an average of 2.3‰. Inclusion-trapped fluids in ore sulfides have 3He/4He and 40Ar/36Ar ratios of 0.14–0.78 Ra and 482–1811, respectively. These isotopic data indicate that the ore fluids are derived from a magmatic source, which is dominated by crustal components with minor mantle contribution. Air-saturated water may be also involved in the hydrothermal system during the magmatic fluids ascending or at the shallow deposit site. We suggest that the crust-mantle mixing signature of the Longquanzhan gold deposit is genetically related to the Late Cretaceous lithospheric thinning along the Tan-Lu fault zone, which triggers constantly uplifting of the asthenosphere surface and persistent ascending of the isotherm plane to form the gold mineralization-related crustal level magma sources. This genetic model can be applied, to some extent, to explain the ore genesis of other deposits near or within the Tan-Lu fault belt.

In September 2021, the La Fossa crater at Vulcano, in Italy, entered a new phase of unrest. We discuss a set of monitoring parameters included in the INGV surveillance network, which closely tracked the sequence of effects related to the crisis. The low-frequency local seismicity sharply increased, while the GPS and tiltmeter networks recorded the inflation of the cone, as an effect of fluid expansion in the hydrothermal system. Gravity variations were probably the effects of fast processes within shallow sources. The anomalies in soil CO2 flux, fumarole temperature, and in plume SO2 flux marked the strong increase in the vapor output from crater fumaroles. The signs of the impending crisis had been evident in the chemical and isotopic composition of fumarole gases since July 2021. These geochemical anomalies were clearly indicative of the enhanced input of gases from a magmatic source. In October, the massive degassing also influenced the areas at the base of the cone. In some areas, soil CO2 degassing and the thermal aquifer recorded strong anomalies. By early November, the crisis reached its acme. Afterward, the monitored parameters started a slow and discontinuous decreasing trend although remaining, some of them, sensibly above the background for several months. The multidisciplinary approach proved decisive for the interpretation of the underlying processes acting in the different phases of the unrest, thus allowing a consistent evaluation of the multiple hazards.

AbstractThe Ediacaran Doushantuo Formation at Weng'an, South China hosts well-preserved phosphatized microfossils known as the Weng'an biota. A laser ablation ICP-MS analysis of rare earth element (REE) characteristics of the fossil-bearing black phosphorite unit of the Doushantuo Formation at Weng'an was conducted, with the aim of unravelling the depositional conditions and diagenetic processes during formation of the phosphorites. Spherical phosphatized microfossils and phosphatic clasts were analysed, and the REE data display middle REE (MREE) -enriched shale-normalized REE patterns. The spherical phosphatized microfossils show an increase in total REE contents (∑REE) from core to rim. Negative correlations between ∑REE and the extent of MREE enrichment over the other REE (indicated by LaN/SmN, YbN/SmN) are observed for analysed spots within individual phosphatic grains, which may be due to complex diagenetic history of the phosphatic grains, with fluctuations in REE sources and chemical parameters in a high-energy shallow-water environment being additional factors. The LaN/YbNand LaN/SmNcharacteristics of the phosphatic grains suggest they were mostly influenced by early diagenetic alteration rather than late extensive recrystallization. The negative Ce anomalies in the samples suggest they formed under oxic-bottom-water conditions. Positive Eu anomalies are present in all samples, and are likely to reflect involvement of hydrothermal fluids rather than changes in redox conditions of porewater. Overall this study has major implications for phosphorites as important archives for the study of geochemical proxies, the Ediacaran period and also evolutionary changes.

Abstract Tinakula is the first seafloor massive sulfide deposit described in the Jean Charcot troughs and is the first such deposit described in the Solomon Islands—on land or the seabed. The deposit is hosted by mafic (basaltic-andesitic) volcaniclastic rocks within a series of cinder cones along a single eruptive fissure. Extensive mapping and sampling by remotely operated vehicle, together with shallow drilling, provide insights into deposit geology and especially hydrothermal processes operating in the shallow subsurface. On the seafloor, mostly inactive chimneys and mounds cover an area of ~77,000 m2 and are partially buried by volcaniclastic sand. Mineralization is characterized by abundant barite- and sulfide-rich chimneys that formed by low-temperature (<250°C) venting over ~5,600 years. Barite-rich samples have high SiO2, Pb, and Hg contents; the sulfide chimneys are dominated by low-Fe sphalerite and are high in Cd, Ge, Sb, and Ag. Few high-temperature chimneys, including zoned chalcopyrite-sphalerite samples and rare massive chalcopyrite, are rich in As, Mo, In, and Au (up to 9.26 ppm), locally as visible gold. Below the seafloor, the mineralization includes buried intervals of sulfide-rich talus with disseminated sulfides in volcaniclastic rocks consisting mainly of lapillistone with minor tuffaceous beds and autobreccias. The volcaniclastic rocks are intensely altered and variably cemented by anhydrite with crosscutting sulfate (± minor sulfide) veins. Fluid inclusions in anhydrite and sphalerite from the footwall (to 19.3 m below seafloor; m b.s.f.) have trapping temperatures of up to 298°C with salinities close to, but slightly higher than, that of seawater (2.8–4.5 wt % NaCl equiv). These temperatures are 10° to 20°C lower than the minimum temperature of boiling at this depth (1,070–1,204 m below sea level; m b.s.l.), suggesting that the highest-temperature fluids boiled below the seafloor. The alteration is distributed in broadly conformable zones, expressed in order of increasing depth and temperature as (1) montmorillonite/nontronite, (2) nontronite + corrensite, (3) illite/smectite + pyrite, (4) illite/smectite + chamosite, and (5) chamosite + corrensite. Zones of argillic alteration are distinguished from chloritic alteration by large positive mass changes in K2O (enriched in illite/smectite), MgO (enriched in chamosite and corrensite), and Fe2O3 (enriched in pyrite associated with illite/smectite alteration). The δ18O and δD values of clay minerals confirm increasing temperature with depth, from 124° to 256°C, and interaction with seawater-dominated hydrothermal fluids at high water/rock ratios. Leaching of the volcanic host rocks and thermochemical reduction of seawater sulfate are the primary sources of sulfur, with δ34S values of sulfides, from –0.8 to 3.4‰, and those of sulfate minerals close to seawater sulfate, from 19.3 to 22.5‰. The mineralization and alteration at Tinakula are typical of a class of ancient massive sulfide deposits hosted mainly by permeable volcaniclastic rocks with broad, semiconformable alteration zones. Processes by which these deposits form have never been documented in modern seafloor massive sulfide systems, because they mostly develop below the seafloor. Our study shows how hydrothermal fluids can become focused within permeable rocks by progressive, low-temperature fluid circulation, leading to a large area (>150,000 m2) of alteration with reduced permeability close to the seafloor. In our model, overpressuring and fracturing of the sulfate- and clay-cemented volcaniclastic rocks produced the pathways for higher-temperature fluids to reach the seafloor, present now as sulfate-sulfide veins within the footwall. In the geologic record, the sulfate (anhydrite) is not preserved, leaving a broad zone of intense alteration with disseminated and stringer sulfides typical of this class of deposits.

Abstract. Seismic arrays provide tools for the localization of events without clear phases or events outside the network, where the station coverage prohibits classical localization techniques. Beam forming allows the determination of the direction (back azimuth) and horizontal (apparent) velocity of an incoming wavefront. Here we combine multiple arrays to retrieve event epicentres from the area of intersecting beams without the need to specify a velocity model. The analysis is performed in the time domain, which allows selecting a relatively narrow time window around the phase of interest while preserving frequency bandwidth. This technique is applied to earthquakes and hybrid events in the region of Fogo and Brava, two islands of the southern chain of the Cape Verde archipelago. The results show that the earthquakes mainly originate near Brava, whereas the hybrid events are located on Fogo. By multiple-event beam stacking we are able to further constrain the epicentral locations of the hybrid events in the northwestern part of the collapse scar of Fogo. In previous studies, these events were attributed to shallow hydrothermal processes. However, we obtain relatively high apparent velocities at the arrays, pointing to either deeper sources or complex ray paths. For a better understanding of possible errors of the multi-array analysis, we also compare slowness values obtained from the array analysis with those derived from earthquake locations from classical (local network) localizations. In general, the results agree well. Nevertheless, some systematic deviations of the array-derived back-azimuth and slowness values occur that can be quantified for certain event locations.

Geological investigation of the rocks in the Elu Link has provided new information on the geodynamic origin of the Neoarchean (ca. 2716–2663 Ma) Hope Bay and Elu granite–greenstone belts. Stratigraphic and geochemical features of these rocks and those of the nearby Flake Lake area in the Hope Bay belt suggest that the two greenstone belts are contiguous, having similar mafic-dominated bimodal rocks comprising abundant basalts to andesites and less common dacites and rhyolites hosting gabbroic and trondhjemite–tonalite–granodiorite (TTG) intrusions. The corresponding parental magmas, whose evolution likely occurred via fractional crystallization and juvenile crustal contamination, formed from both deep and shallow mantle sources. The basalts, andesites, gabbros, and felsic volcanic rocks are variably tholeiitic to calc-alkaline. Chondrite- and primitive mantle-normalized profiles demonstrate (1) flat to slightly fractionated heavy rare-earth element (HREE) patterns with a weak negative Eu anomaly and (2) light rare-earth element (LREE) enriched and strongly fractionated HREE patterns with variable negative to positive Eu anomalies. In contrast, TTG rocks are calc-alkaline, with strong LREE enrichment, HREE depletion, and variable positive Eu anomalies. Altogether, the rocks exhibit Nb and Ti troughs, and variable Nb/Ta, La/Ta, and Zr/Hf ratios indicative of crustal contamination. Chalcophile elements and related ore deposits in the area are inferred to be formed from hydrothermal fluids mobilized during emplacement and after crystallization of their host rocks. An extensional, high-heat-flow back-arc tectonic environment is proposed to explain the stratigraphic and geochemical characteristics and the presence of large gold resources in these greenstone belts.

The hydrology and geology of the Xinzhou geothermal field in South China and its surrounding area have been well studied. Previous studies have indicated that mantle heat, granite radioactive heat, and partial melting are the likely key thermal sources. However, neither the geometry of the geothermal reservoir nor the groundwater circulation pattern has been fully characterized. Toward that aim, a grid of magnetotelluric (MT) data has been acquired to reveal the subsurface distribution of electrical conductivity and to detect the geothermal reservoir. An analysis of the phase tensors suggests that 3D MT inversion is essential for accurate interpretation of the data. We perform a 3D full-impedance MT inversion and observe pronounced low electrical conductivity zones in the inverse model. We interpret these materials as widely developed granites that form the bedrock, which is cut by a crisscross of faults. The cracks in the bedrock provide pathways for the vertical and lateral movement of the hydrothermal fluid. The geothermal reservoir appears as a prominent high electrical conductivity anomaly beneath the hot spring. A northwest-dipping zone of enhanced electrical conductivity reaching approximately 6–7 km in depth is found in the southeast part of the study area, and it is interpreted as the geothermal reservoir recharge channel. We develop an inferred underground fluid circulation model, informed by the electrical conductivity of the area, in combination with hydrologic data. We suggest that the water supply of the Xinzhou geothermal field mainly comes from meteoric water, which is mixed with the invading seawater and shallow cold groundwater as it rises along the fault. The results of our MT study confirm earlier inferences that the Xinzhou geothermal field belongs to an intraplate deep-circulation type originated from the mantle heat flow.

Arjuno-Welirang Volcanic Complex (AWVC) is one of geothermal fields whichlocated in East Java province, Indonesia. It belongs to a Quarternary volcanic arc and has potential for development of electricity. The field is situated in a steep volcanic terrain and there are only few geothermal manifestations, i.e., hot springs, fumaroles, solfataras, steaming ground and hydrothermal alteration. This study aims to classify the type and source of geothermal fluid and to estimate the reservoir condition of Arjuno- Welirang geothermal system. Data are obtained from collecting water samples including hot springs, cold springs, river waters and rain water, then they are analyzed using ICP-AES, titration and ion chromatography.All thermal waters have temperatures from 39.5–53°C and weakly acidic pH (5.2–6.5). Cangar and Padusanhot springs show bicarbonate water, formed by steam condensing or groundwater mixing. On the other hand, Songgoriti shows Cl-HCO3 type, formed by dilution of chloride fluid by either groundwater or bicarbonate water during lateral flow. All of the waters represent immature waters, indicating no strong outflow of neutral Cl-rich deep waters in AWVC. Cl/B ratios show that all water samples have a similar mixing ratio, showing they are from common fluid sources. However, Padusan and Songgoriti have higher Cl/B ratios than Cangar, suggesting that geothermal fluids possibly have reacted with sedimentary rocks before ascending to the surface. All waters were possibly mixed with shallow groundwater and they underwent rock-water reactions at depth before ascending to the surface. An estimated temperatures reservoir calculated using CO2 geothermometer yielded temperatures of 262–263 °C based on collecting of fumarole gas at Mt. Welirang crater. According to their characteristics, Cangar and Padusan are associated with AWVC, while Songgoriti is associated with Mt. Kawi.

Pockmarks, as depression morphology related to fluid escape on the seafloor, are revealed by 3D seismic data on the northwestern South China Sea (SCS) margin. The pockmarks can be classified into two groups based on their various shapes in plan view: the circular group and the elongating group. These pockmarks in the study area could be defined as mega-pockmarks because their maximum diameters can reach to 7.5 km. They commonly develop more than one crater, which are called the central crater and the secondary crater. The seismic data illuminated their complicated internal architectures in the subsurface, as well as their evolution periods, such as the initiation stage, mature stage, and abandonment stage. According to the buried structures and their genesis mechanism, mega-pockmarks could be classified into linear faults-associated pockmarks and volcano-associated pockmarks. The linear fault-associated pockmarks root on the top Middle Miocene, where the linear faults are distributed. The linear faults on the top of fluid reservoir in Middle Miocene act as conduits for fluid seepage. The fluid seepage is driven by the break of balance between the hydrostatic and pore pressure. When the fluid seepage initiates, they will migrate along the linear faults, making the linear feature of pockmarks on the seafloor. Thermogenic gas from deep intervals and biogenic gas from shallow intervals may be fluid sources for the genesis of pockmarks. However, the volcanic activities control the genesis and evolution of volcano-associated pockmarks. Volcano-associated pockmarks root on the craters of volcanoes. The volcanoes underneath the pockmarks provide volcanic hydrothermal solutions, such as phreatomagmatic eruptions through the volcanic craters. The confined fluid seepages make the pockmarks exhibit a more circular shape on the seafloor. Long-term, multiepisode fluid expulsions generate the complicated internal architecture that leads to multicratered mega-pockmarks on the northwestern margin of SCS.

Three-dimensional Mineral Prospectivity Mapping (3DMPM) is an innovative approach to mineral exploration that combines multiple geological data sources to create a three-dimensional (3D) model of a mineral deposit. It provides an accurate representation of the subsurface that can be used to identify areas with mineral potential. These 3D geological models are the typical data source for 3D prospective modeling. Geological data sets from multiple sources are used to construct 3D geological models. Since in practice there is a significant imbalance in the ratio of mineralized to non-mineralized classes, the classification results will be biased in favor of the more observed classes. Borderline-SMOTE (BLSMOTE) is an oversampling technique used to solve the problem of unbalanced datasets and works by generating synthetic data points along the boundary line between the minority and majority classes. This helps to create a more balanced dataset without introducing too much noise. Non-mineralized samples can be generated by randomly selecting non-mineralized locations, which means that uncertainties are generated. In this paper, we take the shallow-forming low-temperature hydrothermal deposit Guizhou Lannigou gold deposit as an example to extract the ore-controlling elements and establish a 3D geological model. A total of 50 training samples are generated using the sampling method described above, and 50 mineralization prospects are generated using Random Forests. A return–risk analysis was used to explore the uncertainties associated with synthetic positive samples and randomly selected negative samples, and to determine the final mineral potential values. Based on the evaluation metrics G-mean and F-value, the model using BLSMOTE outperforms the model without the synthetic algorithm and the models using SMOTE and KMeansSMOTE. The optimal model BLSMOTE18 has an AUC of 0.9288. The methodology also performs superiorly with different levels of class imbalance datasets. Excluding the predictions where the results highly overlap with known deposits, five target zones were circled for the targets using a P-A plot, all of which have obvious metallogenic geological features. Among them, Target1 and Target2 have good potential for mineralization, which is of great significance for future mineral exploration work.

Hydrogeochemical characterization studies are regarded as an important method for determining the origin of hot springs. The major elements, trace elements, and stable isotopes of four groups of hot spring water samples and two groups of gas samples collected from the intersection of the Altyn Tagh fault zone and the East Kunlun fault belt were investigated in this study. The hot spring water temperature ranged between 6 °C and 14 °C. The water chemistry types of the hot springs were Na·Mg-Cl, Mg·Na-Cl·HCO3, Na-Cl·SO4, and Na-Cl·HCO3. The δD values ranged from −50.00% to −68.60%, while the δ18O values ranged from −6.90% to −8.60%. The hot spring water was recharged mainly by infiltrating precipitation, with a recharge elevation of 3390~3676 m. The heat storage temperature ranged from 66.7 to 164.9 °C. The circulation depth was estimated to range between 1043 and 2679 m. The strontium isotopic composition of the water samples in response to the main weathering sources comprised carbonate and sulfate. CO2 was the main component in the hot spring gas in the study region, and its content was over 95%. The 3He/4He-R/Ra relationship diagram revealed that the mantle-sourced helium from the Yitunbulake spring was 3.06%. In comparison, that from the Aiken spring was 7.38%, which indicated an intrusion of mantle-source material mixed into the hot springs in the study region. The crustal marine limestone contributed significantly to the carbon inventory of the hot spring gas samples (>75%). The dissolution of the marl aquifer resulted in the release of CO2. Carbon was primarily obtained through metamorphism and hydrothermal reactions in the basement lithologies. The Yitunbulake and Aiken hot springs are found near the intersection of the Arjin and East Kunlun fractures, where the water–rock response is relatively strong and the depth of circulation and thermal storage temperature are both high. This causes relatively high ambient pressure to be released from the deep fluid, resulting in microseismic activity in this region. The continuous observation of Aiken spring water chemistry allows for the monitoring of fracture activity in the region. The results of the study could serve as a foundation for further exploration of the relationship between geothermal water and deep faults, shallow geological formations, hydrogeological conditions, and geothermal resource development in the region.

Giant impacts can generate transient hydrogen-rich atmospheres, reducing atmospheric carbon. The reduced carbon will form hazes that rain out onto the surface and can become incorporated into the crust. Once heated, a large fraction of the carbon is converted into graphite. The result is that local regions of the Hadean crust were plausibly saturated with graphite. We explore the consequences of such a crust for a prebiotic surface hydrothermal vent scenario. We model a surface vent fed by nitrogen-rich volcanic gas from high-temperature magmas passing through graphite-saturated crust. We consider this occurring at pressures of 1–1000bar and temperatures of 1500–1700 ∘C. The equilibrium with graphite purifies the leftover gas, resulting in substantial quantities of nitriles (0.1% HCN and 1ppm HC3N) and isonitriles (0.01% HNC) relevant for prebiotic chemistry. We use these results to predict gas-phase concentrations of methyl isocyanide of ∼1 ppm. Methyl isocyanide can participate in the non-enzymatic activation and ligation of the monomeric building blocks of life, and surface or shallow hydrothermal environments provide its only known equilibrium geochemical source.

In this work, we study the subsidence of Dallol, an explosive crater and hydrothermal area along the spreading Erta Ale ridge of Afar (Ethiopia). No volcanic products exist at the surface. However, a diking episode in 2004, accompanied by dike-induced faulting, indicates that Dallol is an active volcanic area. The 2004 diking episode was followed by quiescence until subsidence started in 2008. We use InSAR to measure the deformation, and inverse, thermoelastic and poroelastic modelling to understand the possible causes of the subsidence. Analysis of InSAR data from 2004–2010 shows that subsidence, centered at Dallol, initiated in October 2008, and continued at least until February 2010 at an approximately regular rate of up to 10 cm/year. The inversion of InSAR average velocities finds that the source causing the subsidence is shallow (depth between 0.5 and 1.5 km), located under Dallol and with a volume decrease between −0.63 and −0.26 × 106 km3/year. The most likely explanation for the subsidence of Dallol volcano is a combination of outgassing (depressurization), cooling and contraction of the roof of a shallow crustal magma chamber or of the hydrothermal system.

Abstract. The deep sea, the largest biome on Earth, has a series of characteristics that make this environment both distinct from other marine and land ecosystems and unique for the entire planet. This review describes these patterns and processes, from geological settings to biological processes, biodiversity and biogeographical patterns. It concludes with a brief discussion of current threats from anthropogenic activities to deep-sea habitats and their fauna. Investigations of deep-sea habitats and their fauna began in the late 19th Century. In the intervening years, technological developments and stimulating discoveries have promoted deep-sea research and changed our way of understanding life on the planet. Nevertheless, the deep sea is still mostly unknown and current discovery rates of both habitats and species remain high. The geological, physical and geochemical settings of the deep-sea floor and the water column form a series of different habitats with unique characteristics that support specific faunal communities. Since 1840, 27 new habitats/ecosystems have been discovered from the shelf break to the deep trenches and discoveries of new habitats are still happening in the early 21st Century. However, for most of these habitats, the global area covered is unknown or has been only very roughly estimated; an even smaller – indeed, minimal – proportion has actually been sampled and investigated. We currently perceive most of the deep-sea ecosystems as heterotrophic, depending ultimately on the flux on organic matter produced in the overlying surface ocean through photosynthesis. The resulting strong food limitation, thus, shapes deep-sea biota and communities, with exceptions only in reducing ecosystems such as inter alia hydrothermal vents or cold seeps, where chemoautolithotrophic bacteria play the role of primary producers fuelled by chemical energy sources rather than sunlight. Other ecosystems, such as seamounts, canyons or cold-water corals have an increased productivity through specific physical processes, such as topographic modification of currents and enhanced transport of particles and detrital matter. Because of its unique abiotic attributes, the deep sea hosts a specialized fauna. Although there are no phyla unique to deep waters, at lower taxonomic levels the composition of the fauna is distinct from that found in the upper ocean. Amongst other characteristic patterns, deep-sea species may exhibit either gigantism or dwarfism, related to the decrease in food availability with depth. Food limitation on the seafloor and water column is also reflected in the trophic structure of deep-sea communities, which are adapted to low energy availability. In most of the heterotrophic deep-sea settings, the dominant megafauna is composed of detritivores, while filter feeders are abundant in habitats with hard substrata (e.g. mid-ocean ridges, seamounts, canyon walls and coral reefs) and chemoautotrophy through symbiotic relationships is dominant in reducing habitats. Deep-sea biodiversity is among of the highest on the planet, mainly composed of macro and meiofauna, with high evenness. This is true for most of the continental margins and abyssal plains with hot spots of diversity such as seamounts or cold-water corals. However, in some ecosystems with particularly "extreme" physicochemical processes (e.g. hydrothermal vents), biodiversity is low but abundance and biomass are high and the communities are dominated by a few species. Two large-scale diversity patterns have been discussed for deep-sea benthic communities. First, a unimodal relationship between diversity and depth is observed, with a peak at intermediate depths (2000–3000 m), although this is not universal and particular abiotic processes can modify the trend. Secondly, a poleward trend of decreasing diversity has been discussed, but this remains controversial and studies with larger and more robust datasets are needed. Because of the paucity in our knowledge of habitat coverage and species composition, biogeographic studies are mostly based on regional data or on specific taxonomic groups. Recently, global biogeographic provinces for the pelagic and benthic deep ocean have been described, using environmental and, where data were available, taxonomic information. This classification described 30 pelagic provinces and 38 benthic provinces divided into 4 depth ranges, as well as 10 hydrothermal vent provinces. One of the major issues faced by deep-sea biodiversity and biogeographical studies is related to the high number of species new to science that are collected regularly, together with the slow description rates for these new species. Taxonomic coordination at the global scale is particularly difficult but is essential if we are to analyse large diversity and biogeographic trends. Because of their remoteness, anthropogenic impacts on deep-sea ecosystems have not been addressed very thoroughly until recently. The depletion of biological and mineral resources on land and in shallow waters, coupled with technological developments, is promoting the increased interest in services provided by deep-water resources. Although often largely unknown, evidence for the effects of human activities in deep-water ecosystems – such as deep-sea mining, hydrocarbon exploration and exploitation, fishing, dumping and littering – is already accumulating. Because of our limited knowledge of deep-sea biodiversity and ecosystem functioning and because of the specific life-history adaptations of many deep-sea species (e.g. slow growth and delayed maturity), it is essential that the scientific community works closely with industry, conservation organisations and policy makers to develop conservation and management options.

Interpretation of Aeromagnetic and Radiometric Data covering the basement region of Benue-Niger confluence was executed to delineate major structures and other geologic frame works of mineral interest. The study area which hosts the Benue-Niger confluence also encloses two major geologic units which are basement complex and sedimentary basins. The Aeromagnetic data set comprising sheet 227 (Koton-Karfe), 247 (Lokoja) and 267 (Idah) was enhanced to reveal geologic structures while radiometric data was analysed to map lithology and zones affected by hydrothermal alterations. A set of mathematical algorithms was used to enhance the data for interpretation. First Vertical derivatives, Analytical Signal and Euler deconvolution filters were applied to the Aeromagnetic data while Ratio and Ternary images of the three radiogenic elements were obtained for the radiometric data. Magnetic signatures from the TMI showed a mixture of high and low susceptibility below koton-karfe due to intrusion of oolitic iron ore within the sedimentary formation. Lokoja regions recorded highest susceptibility of 165 nT due to magnetic signatures emanating from exposed basement rocks. The southern Idah regions recorded relatively low susceptibility. Result of First Vertical Derivative revealed near surface mineral potent structures labelled F1 – F8, cringing surface features B1, B2 and B3. Analytical signal revealed high amplitudes range of 0.174 to 0.579 cycles for magnetic sources majorly at the basement regions, while low amplitude range of 0.021 to 0.157 cycles were recorded around the sedimentary regions. Euler depth analysis revealed shallower depth to sources in the basement and deeper depth to sources in the sedimentary regions due to thick overburden. Radiometric signatures from the K/Th ratio map revealed portions around Latitude 8°00’ NW and 7°30’ SW shaded in pink colour and having values above known threshold of 0.2 %/ppm to be hydrothermally altered. Mapping of lithology from Ternary map revealed K-Feldspar mineral bearing rocks dominated the NW and SW regions, while sandstones, ironstones, mudstones, shale, alluvium and other fluvial sedimentary lithologies dominated the sedimentary North-east and South-Eastern regions. The western regions (NW and SW) hosted the major structures in form of magnetic lineaments trending NE-SW and E-W which also coincided with regions delineated to be hydrothermally altered and apparently represents the most prospective regions of mineralisation in the study area.

The Ildeus mafic–ultramafic complex represents plutonic roots of a Triassic magmatic arc tectonically emplaced into the thickened uppermost crust beneath the Mesozoic Stanovoy collided margin. The mafic–ultramafic complex cumulates host Ni-Co-Cu-Pt-Ag-Au sulfide-native metal-alloy mineralization produced through magmatic differentiation of subduction-related primary mafic melt. This melt was sourced in the metal-rich sub-arc mantle wedge hybridized by reduced high-temperature H-S-Cl fluids and slab/sediment-derived siliceous melts carrying significant amounts of Pt, W, Au, Ag, Cu and Zn. Plutonic rocks experienced a pervasive later-stage metasomatic upgrade of the primary sulfide–native metal–alloy assemblage in the presence of oxidized hydrothermal fluid enriched in sulfate and chlorine. The new metasomatic assemblage formed in a shallow epithermal environment in the collided crust includes native gold, Ag-Au, Cu-Ag and Cu-Ag-Au alloys, heazlewoodite, digenite, chalcocite, cassiterite, galena, sphalerite, acanthite, composite Cu-Zn-Pb-Fe sulfides, Sb-As-Se sulfosalts and Pb-Ag tellurides. A two-stage model for magmatic–hydrothermal transport of some siderophile (W, Pt, Au) and chalcophile (Cu, Zn, Ag) metals in subduction–collision environments is proposed.

Abstract Paleosalinity is an important environmental feature but it is difficult to evaluate. In the present study, paleosalinity was assessed during the deposition of three sets of source rocks in the western Pearl River Mouth Basin, South China Sea, utilizing four different kinds of methods, i.e., saturated hydrocarbon biomarkers, strontium abundance, non-pollen microalgae assemblies, and carbon-sulfur relationships. Results show that the second member of the Eocene Wenchang Formation (E2w2) was deposited in a freshwater environment and the Oligocene Zhuhai Formation (E3z) was deposited in a shallow marine environment. The Oligocene Enping Formation (E3e), which was believed to be deposited in a freshwater environment, was actually deposited in a brackish water environment. Mechanisms of salinity increase during the non-marine E3e deposition were mainly deep hydrothermal fluid input through the south boundary fault and episodic marine transgressions, not evaporation. The effect of salinity on organic matter content and type was investigated. Results show that salinity has no significant influence on total organic carbon (TOC) and hydrogen index (HI) of the E2w2, which was caused by the balance between freshwater algae and euryhaline algae. TOC and HI decrease with increasing salinity for samples from the E3z, which is contrary to the conventional hypothesis that marine transgressions promote source rock deposition. The decrease of TOC with carbon/sulfur ratios for samples from the E3e actually reflect the influence of thermal maturity but thermal maturity only plays the second role in HI. The effect of salinity on HI during the E3e deposition can be divided into two stages. During the first stage, the increase of salinity was mainly caused by deep hydrothermal fluid input without an oxygen level increase. HI values, therefore, remained relatively stable. During the secondary stage, the increase of salinity was mainly caused by marine transgressions which increased the oxygen level and as a consequence, HI decreased sharply with increasing salinity. This study provides a long-term salinity evolution of the western Pearl River Mouth Basin and suggests that salinity is an important factor controlling source rock deposition. In addition, this study presents an example that goes against conventional wisdom that marine transgressions promote source rock deposition in a shallow marine environment. This study also suggested that marine transgressions had already begun at the end of the early Oligocene.

Cadmium (Cd) is a highly toxic heavy metal particularly susceptible to mobilization by anthropogenic and natural processes. The volcanic nature of oceanic islands in the Macaronesia geographical region such as the Azores archipelago, located near the Mid-Atlantic Ridge, is reflected in deep-sea and shallow-water hydrothermal activities that release heavy metals such as Cd to seawater, affecting marine organisms and integrating food webs. In this paper, a thorough systematic review of all studies performed on coastal marine species in Macaronesia focusing on Cd was conducted, specifically considering the Azorean geological setting and socioeconomic context. Present results are compared and discussed with data from the Mediterranean, a region with apparent strong anthropogenic pollution. The Azorean marine species seem to be particularly strong Cd accumulators, displaying high levels that should be closely monitored, reflecting an important local natural source that should not be underestimated; especially considering the high consumption rates of some of these organisms, which may lead to a potential seafood safety issue. In light of these findings, the potential effects, impacts, and future research challenges are discussed, from an ecological and public health perspective.

Mixed-layer illite-smectite (I-S) from a new set of Pennsylvanian-aged Illinois Basin underclays, identified as paleosols, are investigated to assess the impact of (1) regional diagenesis across the basin and (2) the extent to which ancient environments promoted illitization during episodes of soil formation. Interpretations from Reichweite Ordering and Δ° 2θ metrics applied to X-ray diffraction patterns suggest that most I-S in Illinois Basin paleosols are likely the product of burial diagenetic processes and not ancient soil formation processes. Acid leaching from abundant coal units and hydrothermal brines are likely diagenetic mechanisms that may have impacted I-S in Pennsylvanian paleosols. These findings also suggest that shallowly buried basins (<3 km) such as the Illinois Basin may still promote clay mineral alteration through illitization pathways if maximum burial occurred in the deep past and remained within the diagenetic window for extended periods of time. More importantly, since many pedogenic clay minerals may have been geochemically reset during illitization, sources of diagenetic alteration in the Illinois Basin should be better understood if Pennsylvanian paleosol minerals are to be utilized for paleoclimate reconstructions.

In this study, we analyze the 2009–2013 uplift phenomenon at Campi Flegrei (CF) caldera in terms of temporal and spatial variations in the stress/strain field due to the effect of an inflating source. We start by performing a 3D stationary finite element (FE) modeling of X-band COSMO-SkyMed DInSAR and GPS mean velocities to retrieve the geometry and location of the deformation source. The modeling results suggest that the best-fit source is a three-axis oblate spheroid ~3 km deep, which is mostly elongated in the NE–SW direction. Furthermore, we verify the reliability of model results by calculating the total horizontal derivative (THD) of the modeled vertical velocity component; the findings emphasize that the THD maxima overlap with the projection of source boundaries at the surface. Then, we generate a 3D time-dependent FE model, comparing the spatial and temporal distribution of the shear stress and volumetric strain with the seismic swarms beneath the caldera. We found that low values of shear stress are observed corresponding with the shallow hydrothermal system where low-magnitude earthquakes occur, whereas high values of shear stress are found at depths of about 3 km, where high-magnitude earthquakes nucleate. Finally, the volumetric strain analysis highlights that the seismicity occurs mainly at the border between compression and dilatation modeled regions, and some seismic events occur within compression regions.

Abstract The Plio-Pleistocene El Laco iron oxide-apatite (IOA) orebodies in northern Chile are some of the most enigmatic mineral deposits on Earth, interpreted to have formed as lava flows or by hydrothermal replacement, two radically different processes. Field observations provide some support for both processes, but ultimately fail to explain all observations. Previously proposed genetic models based on observations and study of outcrop samples include (1) magnetite crystallization from an erupting immiscible Fe- and P-rich (Si-poor) melt and (2) metasomatic replacement of andesitic lava flows by a hypogene hydrothermal fluid. A more recent investigation of outcrop and drill core samples at El Laco generated data that were used to develop a new genetic model that invokes shallow emplacement and surface venting of a magnetite-bearing magmatic-hydrothermal fluid suspension. This fluid, with rheological properties similar to basaltic lava, would have been mobilized by decompression-induced collapse of the volcanic edifice. In this study, we report oxygen, including 17O, hydrogen, and iron stable isotope ratios in magnetite and bulk iron oxide (magnetite with minor secondary hematite and minor goethite) from five of seven orebodies around the El Laco volcano, excluding San Vicente Bajo and the minor Laquito deposits. Calculated values of δ18O, Δ17O, δD, and δ56Fe fingerprint the source of the ore-forming fluid(s): Δ17Osample = δ17Osample – δ18Osample * 0.5305. Magnetite and bulk iron oxide (magnetite variably altered to goethite and hematite) from Laco Sur, Cristales Grandes, and San Vicente Alto yield δ18O values that range from 4.3 to 4.5‰ (n = 5), 3.0 to 3.9‰ (n = 5), and –8.5 to –0.5‰ (n = 5), respectively. Magnetite samples from Rodados Negros are the least altered samples and were also analyzed for 17O as well as conventional 16O and 18O, yielding calculated δ18O values that range from 2.6 to 3.8‰ (n = 9) and Δ17O values that range from –0.13 to –0.07‰ (n = 5). Bulk iron oxide from Laco Norte yielded δ18O values that range from –10.2 to +4.5‰ (avg = 0.8‰, n = 18). The δ2H values of magnetite and bulk iron oxide from all five orebodies range from –192.8 to –79.9‰ (n = 28); hydrogen is present in fluid inclusions in magnetite and iron oxide, and in minor goethite. Values of δ56Fe for magnetite and bulk iron oxide from all five orebodies range from 0.04 to 0.70‰ (avg = 0.29‰, σ = 0.15‰, n = 26). The iron and oxygen isotope data are consistent with a silicate magma source for iron and oxygen in magnetite from all sampled El Laco orebodies. Oxygen (δ18O Δ +4.4 to –10.2‰) and hydrogen (δ2H ≃ –79.9 to –192.8‰) stable isotope data for bulk iron oxide samples that contain minor goethite from Laco Norte and San Vicente Alto reveal that magnetite has been variably altered to meteoric values, consistent with goethite in equilibrium with local δ18O and δ2H meteoric values of ≃ –15.4 and –211‰, respectively. The H2O contents of iron oxide samples from Laco Norte and San Vicente Alto systematically increase with increasing abundance of goethite and decreasing values of δ18O and δ2H. The values of δ2H (≃ –88 to –140‰) and δ18O (3.0–4.5‰) for magnetite samples from Cristales Grandes, Laco Sur, and Rodados Negros are consistent with growth of magnetite from a degassing silicate melt and/or a boiling magmatic-hydrothermal fluid; the latter is also consistent with δ18O values for quartz, and salinities and homogenization temperatures for fluid inclusions trapped in apatite and clinopyroxene coeval with magnetite. The sum of the data unequivocally fingerprint a silicate magma as the source of the ore fluids responsible for mineralization at El Laco and are consistent with a model that explains mineralization as the synergistic result of common magmatic and magmatic-hydrothermal processes during the evolution of a caldera-related explosive volcanic system.

We used moored 75 kHz acoustic Doppler current profilers (ADCPs) to examine seasonal cycles in zooplankton deep scattering layers (DSLs) observed below 1300 m depth at Endeavour Ridge hydrothermal vents. DSLs are present year-round in the lower water column near vent plumes. Temporal variations suggest passive, flow-induced displacements superimposed on migratory movements. Although the strongest DSLs are shallower than the neutrally buoyant plumes (1900–2100 m), anomalies also occur at and below plume depth. Upward movement from plume depth in the main DSL is evident in late summer/fall, resulting in shallower DSLs in winter, consistent with the timing of adult diapause/reproduction in upper-ocean migratory copepods. Movement from the upper ocean to plume depth coincides with pre-adult migration to greater depths in spring. Synchronous 20–40 d cycles in DSLs may account for patchiness in space and time of above-plume zooplankton layers observed in summer during previous net-sampling surveys, and suggests lateral and vertical migratory movements to counter current drift away from plume-derived food sources. Persistent near-bottom DSLs move vertically between the spreading plume and seafloor. Historical net data suggests that these are deep, resident fauna. Unlike upper ocean fauna, they seem to be advected considerable distances from the ridge axis, where they are evident as remnant scattering layers.

Abstract The Saint-Gilles breccias, on the western flank of Piton des Neiges volcano, are clearly identified as debris avalanche deposits. A petrographic, textural and structural analysis of the breccias and inter-bedded autochthonous lava flows enables us to distinguish at least four successive flank slides. The oldest deposit sampled the hydrothermally-altered inner parts of the volcano, and has a large volume. Failure was favored by the presence of a deep intensely-weathered layer. The younger deposits are from superficial sources, as their products are rarely hydrothermalized and are more vesicular. The breccia formation, and especially the progressive breaking up occurring during the debris avalanche displacement, indicates the existence of high speed transport. In the Cap La Houssaye coastal area, abrasion and striation of the underlying lava formation, as well as the packing features observed in the breccia, are considered to be deceleration structures. Introduction Huge landslides of volcano flanks, whether or not initiated by magmatic intrusions, have been recognized as catastrophic events since the 1980 Mount St Helens eruption. On oceanic shield volcanoes, the contribution of failure to the edifice-building process was proposed by Moore [1964] and suggested elsewhere for Hawaii [Lipman et al., 1985 ; Moore et al., 1989], Reunion island [Lénat et al., 1989], Etna [McGuire et al., 1991], and Canarias [Carracedo, 1994, 1996 ; Marty et al., 1996]. This contribution is particularly obvious in island volcanoes showing a U-shaped caldera open to the ocean. Several mechanisms inherent to the causes of failure have been proposed, such as dyke intrusion [McGuire et al., 1990 ; Iverson, 1995 ; Voight and Elsworth, 1997], caldera collapse [Marty et al., 1997], or volcanic spreading [Borgia et al., 1992 ; van Wyk de Vries and Francis, 1997]. Invariably, other factors have been proposed as favorable to volcanic destabilization, such as the probable occurrence of deep low-cohesion layers due to the existence of pyroclastic or hyaloclastic layers [Duffield et al., 1982 ; Siebert, 1984] or an old basement. Gravity spreading models are now frequently proposed to explain the destruction of volcanic edifices [Borgia et al., 1992 ; Merle and Borgia, 1996 ; van Wyk de Vries and Borgia, 1996 ; van Wyk de Vries and Francis, 1997], most of them taking into account basal or intra-volcanic weakness zones. We propose that in such a scenario, density heterogeneity should be an important factor governing the slow evolution of the volcanic pile. Clague and Denlinger [1994] proposed a olivine-rich ductile basal layer that influences the stability of volcano flanks. On Reunion island, a large volcanic landslide has been proposed to explain the peculiar morphology of Piton de la Fournaise-Grand Brûlé [Vincent and Kieffer, 1978]. Bathymetric surveys [Bachèlery and Montagionni, 1983 ; Lénat et al., 1989, 1990 ; Cochonnat et al., 1990 ; Lénat and Labazuy, 1990 ; Labazuy, 1991 ; Bachèlery, 1995 ; Ollier et al., 1998] have confirmed the offshore occurrence of debris avalanche deposits. Similar deposits are also known to exist along the western, northern and southwestern submarine flanks of the Piton des Neiges volcano. Unlike other deposits showing inland prolongation, “Saint-Gilles breccias” displays a well-preserved and non-weathered texture and structure. Because of striking analogies between the “Saint-Gilles breccias” and, for example, the Cantal stratovolcano debris avalanche deposits [Cantagrel, 1995], we conclude that these formations are the products of repeated avalanches during the Piton des Neiges basaltic period [Bachèlery et al., 1996]. We propose an interpretation of their origin, emplacement mechanism and their role in the evolutionary process of the western flank of Piton des Neiges. Volcano-structural setting Mechanical instability of oceanic volcanic edifices generates huge flank landslides, with lateral and mainly submarine transport of sub-aerial materials. These landslides participate in the building of the lower submarine slopes of the volcano. Geophysical surveys have detected low cohesion materials in most offshore Reunion island areas [Malengrau et al., 1999 ; de Voogd et al., 1999 ; Lénat et al., 2001] showing that these materials have largely contributed to the construction of offshore Reunion Island. Such deposits are also found in the inner part (“Cirques”) of Piton des Neiges [Maillot, 1999]. On the other hand, electric and electromagnetic soundings have revealed a deep extending conductor within the Piton de la Fournaise volcanic pile [Courteaud et al., 1997 ; Lenat et al., 2000]. Interpretations about the nature and origin of this conductor depend on its location. In the central caldera zone, as revealed by SP positive anomalies [Malengrau et al., 1994 ; Zlotnicki et al., 1994], the hydrothermal and magmatic complex is probably responsible for the observed low resistivities. Along the flanks, such a hypothesis may not be realistic. Courteaud [1996] suggests the occurrence of a deep argilized layer of volcano-detritic origin. In any case, the hydrothermal complex with high fluid pressures and secondary minerals appears as a potential weak zone that may contribute to the volcano’s instability [Lopez and Williams, 1993 ; Frank, 1995]. Chronology and stratigraphy Extent of the debris avalanche deposits The various breccias found at the western end of Reunion island, on the Piton des Neiges volcano flank, cover a 16 km2 area between Cap Marianne and Saint-Gilles (fig. 1). They are overlain upwards (&gt; 250 to 300 m) by trachyandesitic (mugearite) lava flows of Piton des Neiges differentiated series [Billard, 1974]. Some restricted breccia outcrops in deep valleys from Bernica to the north up to l’Hermitage to the south indicate the existence of larger extension of the debris avalanche deposits. Furthermore, breccias with similar “Saint-Gilles” facies appear down the Maïdo cliff to Mafate “Cirque” at an altitude 1300 m, beneath 600 m of mugearite and some olivine basalt flows. Unpublished electromagnetic data (CSAMT soundings) confirm the inland continuity of the “Saint-Gilles breccias” up to the Maïdo along the Piton des Neiges western flank, hidden by mugearitic flows. Available bathymetric surveys offshore Saint Paul – Saint Gilles areas show the obvious underwater prolongation of “Saint-Gilles breccias” : a shallow depth (&lt; 100 m) plateau followed by a slope with hummocky surface down to 2 500 m depth [Bachèlery et al., 1996 and fig. 2]. From this data, the total surface of “Saint-Gilles” debris avalanche deposits is estimated as more than 500 km2. Chronology A coastal cliff, from Ravine Bernica to Boucan Canot, provides the best outcrop of the northern part of “Saint-Gilles breccias”, with a clear inter-bedding of breccia units and lava formations (photo 1and fig. 3). – The lower breccia unit (Br I), of unknown thickness, has a remarkable friable aspect and a grayish color. – The first autochthonous lava formation (L1) consists in thin pahoehoe olivine basalt flows filling large valleys dug into “Br I”. The top of this formation is striated by the overlying “Br II” unit (photo 2). – Breccia unit “Br II” is interbedded between L1 and L2 olivine basalts. More compact and massive, “Br II” is characterized by a reddish matrix and dark blocks, with many curved fracture surfaces. – On “Br II” or directly on L1, picritic basalt flows L2 are found, filling narrow valleys. – Breccia unit “Br III” lies on “Br II” with a striking sheared contact plane visible along the main road (photo 3). It is a typical debris avalanche deposit with large imbricate blocks within a fine-grained beige matrix. – Once again, basaltic flows of lava formation L3 fill a valley dug into “Br III” near Petite Anse river. – Breccia unit “Br IV” rests on L3 at Petite Anse, but its contact with “Br III” elsewhere is not clear. The facies of this unit is very similar to the “Br III”. All the breccia units are covered by basaltic and trachyandesitic flows from the end of the Piton des Neiges basaltic series, and differentiated series. In the Saint-Gilles river, two formations are superposed : picritic basalts (L4) have flowed on the “Br IV” breccia unit, latter aphyric trachy-andesitic (mugearite) flows (L6) overlapped L4 and the breccia landforms, reaching in places the coastal area. To the north, at Plateau Caillou, plagioclase-phyric basalt flows (L5) are found between mugearite and breccias. Elsewhere on Piton des Neiges, such flows are symptomatic of the transition from the basaltic series to the differentiated series [Billard, 1974]. The occurrence of autochthonous basaltic formations L1 to L3, inter-bedded with “Saint-Gilles breccias”, enables us to distinguish at least four superposed breccia units. Although the emplacement age of the lower “Br I” is not known precisely, it is overlain and therefore older than Cap Marianne pahoehoe lavas (L1) dated at 0.452 Ma [Mc Dougall, 1971]. On the other hand, the upper breccia units are younger than the pahoehoe olivine basalt at Cap la Houssaye dated at 0,435 Ma but older than L5 plagioclasic basalts dated at 0.35 Ma. Geological description of the “breccia sequence” In the synthetic lithologic log (fig. 4) of the Saint-Gilles area, autochthonous lava formations are clearly broken into four separate breccia units. Lava formations. – L1 formation consists of numerous thin pahoehoe olivine-rich to aphyric basaltic flows. Both L2 and L3 formations are characterized by a few thicker (decametric) olivine (frequently picritic) basalt flows. Breccia units. – All breccia units display common characteristics such as the universal association of two facies (photo 4) : (i) a matrix – sandy to silty – facies containing a non-sorted mixture of non-stratified heterogeneous materials ranging from granular size to blocky elements, (ii) coherent large blocks and large pieces (‘block’ facies) of various lithology such as lava flow, scorias, pyroclastics or other breccias ; blocks displaying frequent “jigsaw” features. The lower breccia unit “Br 1” (fig. 4) has a more compact but very heterogeneous aspect, with a chaotic distribution of blocks in a less-developed matrix. This unit is characterized by a deep hydrothermal alteration with a lot of zeolites, chlorite, clays, calcite and oxides. The upper breccia units, “Br II” to “Br IV” (fig. 4) are less heterogeneous than “Br I” because their matrix facies are more voluminous and because the matrix clearly separates the bigger blocks. In both facies, a great diversity of fresh lithologic types such as picritic basalt, olivine-phyric basalt, plagioclase-phyric basalt and aphyric more or less vesicular basalts, gabbro, dunite are found, with no or only few slightly zeolitised blocks. Plurimetric to metric blocks are severely fractured, disintegrated into millimetric to decimetric angular pieces. The frequent polygenic aspect is due to block juxtaposition or imbrication. The abundant matrix is composed of crushed rocks and mineral elements, fine-grained (&lt; mm), showing frequent fluidity and bedding marks (photo 5). The very heterogeneous composition of the matrix is confirmed at a microscopic scale. On the contrary, cores of blocks appear as jigsaw-puzzle-like monolithologic pieces of various basaltic rocks. At their edges, disintegration leads to progressive mixing with neighboring blocks that feed the matrix. Discussion Originality of “Saint-Gilles breccias” “Saint-Gilles breccias” constitute one of the few cases [see also Cantagrel et al., 1999] of debris avalanche deposit outcroppings on the sub-aerial part of an oceanic shield volcano. The main part of the deposit is suspected to be offshore. Their hummocky surface in delineating parallel ridges can be compared to the one described offshore the Grand Brûlé area, east of Piton de la Fournaise [Bachèlery et al., 1996]. “Saint-Gilles breccias” were deposited after several Piton des Neiges flank slide events that were separated by basaltic flows. Repeated debris avalanches have also been proposed to explain Piton de la Fournaise offshore deposits [Lenat et al., 1990 ; Labazuy, 1991]. The occurrence of autochthonous interbedded lava formations is essential to interpret the thick piling up of slide material along Reunion volcano flanks as deposits of repeated avalanches at the same place, instead of as being the products of a single huge event. Many structural and textural features noticed in the upper breccia units reveal crucial information on the emplacement mechanism of debris avalanches. For instance, brecciated blocks are typical of progressive break-up during transport processes. Blocks can simply be fractured, or they can be so severely disintegrated that stretching and mixing with other blocks and matrix formation are observed. The observation of such phenomena implies the existence of numerous percussive events between rocks, as well as internal vibrations in the debris avalanche and therefore the existence of high-speed transport. Lava formations L1 underlying upper breccia units are truncated and strongly striated in a seaward direction (photo 2), parallel to the breccia morphological ridges. In the same way, internal contact surfaces between upper breccia units are shear planes underlain by cataclastic layers and lenses (photo 3). Such structures are interpreted as due to drastic deceleration effects of avalanches reaching a topographic leveling out in the coastal area. This concords with the occurrence of sub-vertical contact areas between the blocks and the matrix. These injections of matrix between the blocks are generated bottom-up from the shear plane at the moment of the sudden deceleration of the avalanche. Other fracture planes that are in accordance with the morphology of ridges, are found in “Br III” unit (see fig. 5). They are interpreted as the result of packing effects. Origin of flank failures Although the source area of breccia formations has not yet been clearly identified, it has to be in the central part of Piton des Neiges as seen in the western cliff of “cirque de Mafate”. Furthermore, “Br I” deeply weathered materials evidently come from the hydrothermalized core of the volcano. Though the “Br I” thickness is not known, the volume involved may be considerable and a part of this volume must constitute the main body of Saint-Gilles offshore deposits. The upper breccias units “Br II” to “Br IV” display very similar textures and lithologies, with dominant non-altered basaltic rocks from the “Phase II” building stage of Piton des Neiges [Billard, 1974]. These units are very thin in the coastal area of Cap La Houssaye (see fig. 2) despite a proximal facies (meaning a deposit in the transport zone nearer than the main deposit zone). They obviously originate from shallow flank slides of restricted extent. We suggest that the upper Saint-Gilles deposits are due to repeated events that produced thin high-speed debris avalanches. Emplacement modalities The morphology of “Saint-Gilles breccias”, or submarine deposits offshore Grand Brûlé (east of Piton de la Fournaise volcano), are typical of sliding movements along shallow depth shear planes (several hundred meters up to two kilometers) within the volcanic pile. But several levels of decollement are suggested by seismic refraction and reflection profiles offshore La Reunion, the deepest corresponding to the top of the preexisting oceanic sediments [de Voogt et al., 1999]. Until now, in Reunion Island, only shallow failures affecting the upper parts of volcanic edifices, with deposits on the lower slopes, have been positively identified. Conditions that trigger giant flank landslides affecting oceanic shields remain poorly understood but we can reasonably speculate that weak hydrothermally-altered layers in the inner part of the volcano favor these gravity-driven processes related to repeated dike injections. The “Saint-Gilles breccia” sequence is considered as a multiphase lateral collapse structure whose first event (“Br I”) was apparently the most voluminous. The corresponding deposit displays frequent hydrothermally-altered material symptomatic of originating from the Piton des Neiges core. Within Piton des Neiges, the low cohesive weathered layer is quite extensive [Nativel, 1978 ; Rançon, 1982] possibly reaching down the volcano flanks [Courteaud et al., 1997]. The interpretative scheme that we propose (fig. 6) in our evaluation of the conditions for the emplacement of Saint-Gilles sequence, takes into account the existence of such a mechanical discontinuity within the volcanic pile. We propose that the massive landslide failure of the west flank of Piton des Neiges volcano that produced the “Br I” breccia, provided efficient channels for younger Piton des Neiges lavas to reach the western and southwestern coastline. Morphological features, as well as radiometric data [Mc Dougall, 1971 ; Gillot and Nativel, 1982] and magnetic surveys [Lénat et al., 2001], yield evidence for preferential accumulation of lava during the last 0.5 m.y. (corresponding mainly to the differentiated series) in this part of the volcano. The relative asymmetry of Piton des Neiges was acquired by rift migration in response to the first huge landslide that produced the “Br I” unit of “Saint-Gilles breccia”, in the manner described by Lipman et al. [1990] for Mauna Loa volcano in Hawaii. The later repetition of flank collapses is consistent with similar structures on other oceanic islands. Since the first lateral collapse, the Piton des Neiges edifice was probably characterized by the existence of an asymmetrical steeper western flank where the old zeolite-rich “Br I” deposits possibly act as a detachment surface for later successive landslides which may have occurred recurrently over a short time interval.

The Blackbush uranium prospect (~12,580 tonnes U at 85 ppm cut-off) is located on the Eyre Peninsula of South Australia. Blackbush was discovered in 2007 and is currently the single example of sediment-hosted uranium mineralisation investigated in any detail in the Gawler Craton. Uranium is hosted within Eocene sandstones of the Kanaka Beds and, subordinately, within a massive saprolite derived from the subjacent Hiltaba-aged (~1585 Ma) granites, affiliated with the Samphire Pluton. Uranium is mainly present as coffinite in different lithologies, mineralisation styles and mineral associations. In the sandstone and saprolite, coffinite occurs intergrown with framboidal Fe-sulphides and lignite, as well as coatings around, and filling fractures within, grains of quartz. Microprobe U–Pb dating of coffinite hosted in sedimentary units yielded a narrow age range, with a weighted average of 16.98 ± 0.16 Ma (343 individual analyses), strongly indicating a single coffinite-forming event at that time. Coffinite in subjacent saprolite generated a broader age range from 28 Ma to 20 Ma. Vein-hosted coffinite yielded similar ages (from 12 to 25 Ma), albeit with a greater range. Uraninite in the vein is distinctly older (42 to 38 Ma). The 17 ± 0.16 Ma age for sandstone-hosted mineralisation roughly coincides with tectonic movement as indicated by the presence of horst and graben structures in the Eocene sedimentary rocks hosting uranium mineralisation but not in stratigraphically younger sedimentary rocks. The new ages for hydrothermal minerals support a conceptual genetic model in which uranium was initially sourced from granite bedrock, then pre-concentrated into veins within that granite, and is subsequently dissolved and reprecipitated as coffinite in younger sediments as a result of low-temperature hydrothermal activity associated with tectonic events during the Tertiary. The ages obtained here for uranium minerals within the different lithologies in the Blackbush prospect support a conceptual genetic model in which tectonic movement along the reactivated Roopena Fault, which triggered the flow of U-rich fluids into the cover sequence. The timing of mineralisation provides information that can help optimise exploration programs for analogous uranium resources within shallow buried sediments across the region. The model presented here can be predicted to apply to sediment-hosted U-mineralisation in cratons elsewhere.

The eastern Pontide tectonic belt (EPTB) contains greater than 350 identified Kuroko type volcanogenic massive sulfide deposits/mineralization/occurrences (VMSD). The deposits are associated with Late Cretaceous felsic volcanics consisting mainly of dacitic and rhyolitic lavas and pyroclastics that outcrop within a narrow zone running parallel to the eastern Black Sea coast and represent the axial zone of a paleo-magmatic arc. The Cerattepe deposit is the second-largest and is a hybrid VMS system with some epithermal features. To date, no geochemical research constrains the origin and timing of mineralization in the Cerattepe VMS deposit. Here, we provide Cu, O, H and S, isotope analysis of ores and alteration minerals to understand the hydrothermal history of the deposit and date the massive ore with Re-Os geochronology. Secondary weathering mobilized and redistributed metals in the deposit. The copper isotope signatures of shallow ores in the gossan follow patterns resulting from oxidative weathering of copper minerals with gossan Fe oxides of δ65Cu = −2.59‰, enrichment zone copper sulfide of d65Cu = +2.23 and +1.73‰, and primary ores of δ65Cu = +0.71 and +0.41‰. At the boundary of the enrichment zone, further cycling and migration of enrichment zone copper are evidenced by two samples having larger ranges of the δ65Cu = +3.59‰, and −2.93‰. Evidence for a magmatic source for fluids and S are evidenced by the O and H isotope values from quartz veins (δ18O = +7.93‰ to +10.82‰, and δD = −78‰ and −68‰) and sulfides that possess δ34S ratios of –5 and 0‰ from drill core samples. 187Os/188Os–187Re/188Os ratios from drill core sulfide samples of Cerattepe VMS deposit yields a 62±3 Ma isochron age and a highly radiogenic Os initial ratio. This age is compatible with silicate alteration ages from a proximal deposit and clearly shows mineralization occurs at a much younger time than previously proposed for VMS mineralization in the eastern Pontides. The new Re-Os age and source of Os imply that mineralization in the area occurs at a distinctly younger interval in the back-arc basin and metals could be sourced from the surrounding host rocks.