Seafloor Hydrothermal Vents Research Articles

The sulfur isotope evolution of magmatic-hydrothermal fluids: insights into ore-forming processes

Metal-rich fluids that circulate in magmatic-hydrothermal environments form a wide array of economically significant ore deposits. Unravelling the origins and evolution of these fluids is crucial for understanding how Earth’s metal resources form and one of the most widely used tools for tracking these processes is sulfur isotopes. It is well established that S isotopes record valuable information about the source of the fluid, as well as its physical and chemical evolution (i.e. changing pH, redox and temperature), but it is often challenging to unravel which of these competing processes drives isotopic variability.Here we use thermodynamic models to predict S isotope fractionation for geologically realistic hydrothermal fluids and attempt to disentangle the effects of fluid sources, physico-chemical evolution and S mineral disequilibrium. By modelling a range of fluid compositions, we show that S isotope fingerprints are controlled by the ratio of oxidised to reduced S species (SO42−/H2S), and this is most strongly affected by changing temperature, fO2 and pH. We show that SO42−/H2S can change dramatically during cooling and our key insight is that S isotopes of individual sulfide or sulfate minerals can show large fractionations (up to 20‰) even when pH is constant and fO2 fixed to a specific mineral redox buffer. Importantly, while it is commonly assumed that SO42−/H2S is constant throughout fluid evolution, our analysis shows that this is unlikely to hold for most natural systems.We then compare our model predictions to S isotope data from porphyry and epithermal deposits, seafloor hydrothermal vents and alkaline igneous bodies. We find that our models accurately reproduce the S isotope evolution of porphyry and high sulfidation epithermal fluids, and that most require magmatic S sources between 0 and 5‰. The S isotopes of low sulfidation epithermal fluids and seafloor hydrothermal vents do not fit our model predictions and reflect disequilibrium between the reduced and oxidised S species and, for the latter, significant S input from seawater and biogenic sources. Alkaline igneous fluids match model predictions and confirm magmatic S sources and a wide range of temperature and redox conditions. Of all these different ore deposits, porphyry and alkaline igneous systems are particularly well-suited to S isotope investigation because they show relationships between redox, alteration and ore mineralogy that could be useful for exploration and prospecting. Ultimately, our examples demonstrate that S isotope forward models are powerful tools for identifying S sources, flagging disequilibrium processes, and validating hypotheses of magmatic fluid evolution.

Complete genome sequence of Exiguobacterium mexicanum A-EM, isolated from seafloor hydrothermal vents in Atlantic Ocean.

Exiguobacterium mexicanum A-EM was isolated from seafloor hydrothermal vents(Caifan field, 14.0S 14.4W) and was shown to degrade toxins and contaminants. Here, we present the complete genome sequence of A-EM, consisting of 2,412,492bp, with a GC content of 53.16%. A-EM sequence contains genes encoding enzymes that degrade toxins and contaminants. Complete genome sequence of the strain A-EM can further provide insights into microbial adaption to the seafloor hydrothermal system and the genomic basis for the biotechnological application of strain A-EM as an efficient agent to degrade environmental contaminants.

Geological, physical, and chemical characteristics of seafloor hydrothermal vent fields

Seafloor hydrothermal vent fields (SHVFs) are located in the mid-ocean ridge (MOR), backarc basin (BAB), island arc and hot-spot environments and hosted mainly by ultramafic, mafic, felsic rocks, and sediments. The hydrothermal vent fluids of SHVFs have low oxygen, abnormal pH and temperature, numerous toxic compounds, and inorganic energy sources, such as sulfuric compounds, methane, and hydrogen. The geological, physical, and chemical characteristics of SHVFs provide important clues to understanding the formation and evolution of seafloor hydrothermal systems, leading to the determination of metal sources and the reconstruction of the physicochemical conditions of metallogenesis. Over the past two decades, we studied the geological settings, volcanic rocks, and hydrothermal products of SHVFs and drawn new conclusions in these areas, including: 1) the hydrothermal plumes in the Okinawa Trough are affected by the Kuroshio current; 2) S and Pb in the hydrothermal sulfides from MOR are mainly derived from their host igneous rocks; 3) Re and Os of vent fluids are more likely to be incorporated into Fe- and Fe-Cu sulfide mineral facies, and Os is enriched under low-temperature (<200°C) hydrothermal conditions in global SHVFs; 4) compared with low-temperature hydrothermal sulfides, sulfates, and opal minerals, high-temperature hydrothermal sulfides maintain the helium (He) isotopic composition of the primary vent fluid; 5) relatively low temperature (<116°C), oxygenated, and acidic environment conditions are favorable for forming a native sulfur chimney, and a “glue pudding” growth model can be used to understand the origin of native sulfur balls in the Kueishantao hydrothermal field; and 6) boron isotope from hydrothermal plumes and fluids can be used to describe their diffusive processes. The monitoring and understanding of the physical structure, chemical composition, geological processes, and diverse organism of subseafloor hydrothermal systems will be a future hot spot and frontier of submarine hydrothermal geology.

Reversible scavenging traps hydrothermal iron in the deep ocean

Recent studies suggest that seafloor hydrothermal vents could be an important source of iron (Fe) to the surface ocean, stimulating plankton growth and biological carbon export. However, quantifying the supply of hydrothermal Fe to the surface ocean requires accurately modeling its stabilization and removal processes, which are poorly known. Here, we determine the physical speciation of dissolved Fe along an oceanographic transect following a coherent hydrothermal plume that emanates from the East Pacific Rise (EPR) and persists westward over 4,000 km in the Tropical South Pacific. Our observations show that the plume persists horizontally, but descends vertically, and consists primarily of very large Fe colloids. Guided by these observations, we develop a new size-resolved mechanistic model of hydrothermal Fe dispersion in this region, in which the stabilization of hydrothermal Fe is explained by a reversible particulate exchange process. This model accurately captures the lateral dispersion, downward settling and physical speciation of hydrothermal Fe along this transect. An alternate model that uses a hydrothermal source of Fe-binding ligands to facilitate Fe transport within the deep ocean can reproduce the long-range transport of hydrothermal Fe, but does not reproduce the vertical descent of the plume. Our model shows that hydrothermal Fe vented from the EPR is trapped in the deep ocean, and only 1% of this iron ever makes it to the surface where it can stimulate biological productivity. At the global scale, 3-5% of hydrothermal Fe makes it to the surface ocean, the vast majority of which originates from Southern Ocean vents and upwells in the Southern Ocean. Our best estimate of the global supply of hydrothermal Fe to the surface ocean, based on data-constrained estimates of ocean circulation, mantle 3He venting, and the hydrothermal Fe:3He ratio from the EPR, is 0.12±0.07 Gmolyr−1. This is about 60-70 times lower than the supply of Fe from aerosol dust deposition, but could be regionally important in the Antarctic zone of the Southern Ocean.

Underwater Image Enhancement based on Histogram Manipulation and Multiscale Fusion

Abstract The underwater images are a good source of information which explores idea about sea creatures, to study about seafloor hydrother-mal vents. Low contrast, color distortion and poor visual appearance are the major issues that an underwater image has to undergo. Such problems were caused by dispersion and refraction of light as they penetrate from rarer to denser media. The scattering of light reduces color contrast. The influence of water in underwater images is not only due to scattering but also due to the presence of underwater organisms. Here we introduce an improved method for underwater image enhancement based on the fusion method that is capable to restore accurately underwater images. The proposed work takes a single image as the input and a sequence of operations such as white balancing, gamma correction, sharpening, manipulating weight maps are performed on the input image. Finally multiscale image fusion of the inputs is done to obtain the resultant output. In the initial stage, color distorted input image is white balanced to remove the color casts maintaining a realistic subsea image. In the second stage, CLAHE is performed on the gamma corrected image. CLAHE plays a significant role in luminance enhancement of underwater images. At the same time, histogram equalization is performed on the sharpened image. The weight maps analyze image characteristics that properly specify the spatial pixel relationship. Finally in the last stage, multiscale pyramidal fusion of the inputs and weight maps are performed. The fusion performed here is the Pyramidal fusion. Result analysis depict the improvement of the underwater images using proposed method.

Trace element proxies of seafloor hydrothermal fluids based on secondary ion mass spectrometry (SIMS) of black smoker chimney linings

Sampling of paired black smoker chimney linings and seafloor hydrothermal vent fluids supports the development of trace element proxies for sulfide mineral deposition environments by facilitating analyses of trace element partitioning between mineral and fluid phases under well-constrained physiochemical conditions. Here, concentrations of Co, Ni, Ga, Ag, and In in chalcopyrite lining 22 black smoker chimneys (29 for Co, Ag, and In) are measured using secondary ion mass spectrometry (SIMS) calibrated against inductively coupled plasma mass spectrometry (ICP-MS) and NIST-traceable reference solutions. To provide additional data on the trace element concentrations of vent fluid pairs for 19 of the 29 black smoker chimney linings investigated, this paper also presents new ICP-MS data for 33 hydrothermal vent fluids collected from the Tahi Moana-1, ABE, Tu’i Malila, and Mariner vent fields on the Eastern Lau Spreading Center and Valu Fa Ridge.The chalcopyrite black smoker chimney linings investigated represent a variety of temperature (269–395 °C), chemical (e.g., pH (at 25 °C) = 2.3–4.4), and geologic conditions. Electron microprobe results indicate that mineral stoichiometry ranges from stoichiometric chalcopyrite to mol Cu : mol Fe = 0.65. Trace element concentrations obtained by SIMS are: Co (<2 ng/g–760 μg/g), Ni (<17 ng/g–454 μg/g), Ga (<0.9 ng/g–48 μg/g), Ag (60 μg/g–3800 μg/g), In (<0.5 ng/g–270 μg/g). Concentrations of Ag in chalcopyrite strongly correlate with the free ion activity ratio of {Ag+}:{Cu+} in paired vent fluids, with high Ag concentrations in chalcopyrite indicating formation from near neutral vent fluids containing low Cu concentrations or low-pH vent fluids with high Ag concentrations attributable to subsurface Ag remobilization. Chalcopyrite with low Ag precipitates from low-pH Cu-rich fluids unaffected by extensive Ag remobilization. Concentrations of Ga and In in chalcopyrite exhibit a negative trend with vent fluid pH, possibly reflecting the strength of Ga and In OH− complexes. Thus, Ga and In concentrations differentiate Ag-rich chalcopyrite formed from near-neutral Cu-poor vent fluids or that formed from Ag-rich low-pH vent fluids. In contrast, Co and Ni exhibit no trend with fluid data, but correlate with mineral Cu:Fe ratios, possibly reflecting the greater availability of Fe(II) lattice sites or paired substitution of 2+ ions.Overall, this study demonstrates the potential of paired vent fluid and black smoker chimney samples to provide insight into the partitioning of trace elements in sulfide mineral deposition environments and related proxies of important fluid parameters such as pH and metal concentrations. This study also demonstrates the utility of SIMS to precisely analyze trace elements in chalcopyrite at high spatial resolutions and low detection limits.

High definition large area mapping of geological samples using a Maia detector array in the Nuclear Microprobe

A Maia detector array has been adapted and installed on the CSIRO-MARC Nuclear Microprobe. Maia uses an annular array of 384 silicon detectors to provide a solid-angle of 1.3 sr to improve the collection of X-rays induced by Particle Induced X-ray Emission (PIXE) using a micron focussed MeV proton beam. It features event-by-event data acquisition with dead-time correction and pileup rejection and a total count-rate capacity above 10 M/s, versatile stage and beam scanning modes (coupled to the DAQ-36 system described previously) to cater for a variety of sample configuration and experimental needs, precise measurement of transit time and integration of beam fluence per pixel, and spectral deconvolution of event data in real-time using the Dynamic Analysis method to provide element images during data collection. Here we demonstrate the system on thin sections made from geological samples extracted from active seafloor hydrothermal vents in the PACMANUS hydrothermal field in Papua New Guinea.

A Novel Visual Apparatus for Laboratory Simulation of Seafloor Hydrothermal Venting

In this paper, a novel visual experimental apparatus for simulating seafloor hydrothermal venting is proposed. The instrument consists mainly of an acrylic pressure vessel and a hydrothermal fluid syringe pump, which provided a 360 deg view of the simulated hydrothermal venting and plumes. Theoretical calculation and finite element analysis (FEA) were conducted to demonstrate the appropriateness of material selection and structural design for the acrylic pressure vessel. The experimental apparatus was tested at elevated temperature and pressure of up to 300 °C and 12 MPa. Hydrothermal venting experiments were successfully carried out with this apparatus, and clear images of hydrothermal plumes were obtained.

The deep structure of the Duanqiao hydrothermal field at the Southwest Indian Ridge

Polymetalic sulfide is the main product of sea-floor hydrothermal venting, and has become an important sea-floor mineral resources for its rich in many kinds of precious metal elements. Since 2007, a number of investigations have been carried out by the China Ocean Mineral Resources Research and Development Association (COMRA ) cruises (CCCs) along the Southwest Indian Ridge (SWIR). In 2011, the COMRA signed an exploration contract of sea-floor polymetallic sulfides of 10 000 km2 on the SWIR with the International Seabed Authority. Based on the multibeam data and shipborne gravity data obtained in 2010 by the R/V Dayang Yihao during the leg 6 of CCCs 21, together with the global satellite surveys, the characteristics of gravity anomalies are analyzed in the Duanqiao hydrothermal field (37°39′S, 50°24′E). The “subarea calibration” terrain-correcting method is employed to calculate the Bouguer gravity anomaly, and the ocean bottom seismometer (OBS) profile is used to constrain the two-dimensional gravity anomaly simulation. The absent Moho in a previous seismic model is also calculated. The results show that the crustal thickness varies between 3 and 10 km along the profile, and the maximum crustal thickness reaches up to 10 km in the Duanqiao hydrothermal field with an average of 7.5 km. It is by far the most thicker crust discovered along the SWIR. The calculated crust thickness at the Longqi hydrothermal field is approximately 3 km, 1 km less than that indicated by seismic models, possibly due to the outcome of an oceanic core complex (OCC).

Sounding the Black Smoker Plumes

Imaging sonar, an emerging technique for monitoring heat from seafloor hydrothermal vents, gives scientists a new look at interacting systems off the coast of Canada.

Long-term, quantitative observations of seafloor hydrothermal venting using an imaging sonar

While connected to the NEPTUNE observatory operated by Ocean Networks Canada (ONC) at the Endeavour Segment of the Juan de Fuca Ridge (http://www.oceannetworks.ca/observatories/pacific), the Cabled Observatory Vent Imaging Sonar (COVIS) recorded an unprecedented long-term (&amp;gt;4 years) acoustic dataset capturing local hydrothermal venting. Processing of the acoustic backscatter data yields three-dimensional (3-D) images of plumes rising tens of meters from black smoker vents on a sulfide structure named Grotto. More importantly, analysis of the Doppler frequency shift in acoustic backscatter yields estimates of the flow rates of those plumes and their volume fluxes. Subsequent calculations based on the vertical variation in volume flux and a theoretical heat-to-volume-flux relationship for buoyancy-driven plumes give estimates of plume heat flux, which are essential for studying the temporal evolution of a hydrothermal system and its coupling with geological, oceanic, and biological processes. In addition to black-smoker plume observations, the ping-to-ping decorrelation of seafloor backscatter recorded by COVIS provides an acoustic indicator of diffuse-flow (i.e., low- temperature, clear hydrothermal discharge) distribution over Grotto and its surrounding areas. Furthermore, acoustic techniques for quantifying the temperature fluctuations, and ultimately, heat flux of diffuse-flow venting are currently under development. [Work supported by NSF.]

Progress in Deciphering the Controls on the Geochemistry of Fluids in Seafloor Hydrothermal Systems.

Over the last four decades, more than 500 sites of seafloor hydrothermal venting have been identified in a range of tectonic environments. These vents represent the seafloor manifestation of hydrothermal convection of seawater through the permeable oceanic basement that is driven by a subsurface heat source. Hydrothermal circulation has fundamental effects on the transfer of heat and mass from the lithosphere to the hydrosphere, the composition of seawater, the physical and chemical properties of the oceanic basement, and vent ecosystems at and below the seafloor. In this review, we compare and contrast the vent fluid chemistry from hydrothermal fields in a range of tectonic settings to assess the relative roles of fluid-mineral equilibria, phase separation, magmatic input, seawater entrainment, and sediment cover in producing the observed range of fluid compositions. We focus particularly on hydrothermal activity in those tectonic environments (e.g., mid-ocean ridge detachment faults, back-arc basins, and island arc volcanoes) where significant progress has been made in the last decade in documenting the variations in vent fluid composition.

地球とエウロパの海底熱水噴出孔（特集「太陽系におけるアストロバイオロジー」）

地球とエウロパの海底熱水噴出孔（特集「太陽系におけるアストロバイオロジー」）

B1-27 トカラ列島の活動的海底火山 : 海底熱水噴出系探査のための新たな道具(ラハール・水蒸気爆発・熱水系,口頭発表)

B1-27 トカラ列島の活動的海底火山 : 海底熱水噴出系探査のための新たな道具(ラハール・水蒸気爆発・熱水系,口頭発表)

Opposing authigenic controls on the isotopic signature of dissolved iron in hydrothermal plumes

Iron is a scarce but essential micronutrient in the oceans that limits primary productivity in many regions of the surface ocean. The mechanisms and rates of Fe supply to the ocean interior are still poorly understood and quantified. Iron isotope ratios of different Fe pools can potentially be used to trace sources and sinks of the global Fe biogeochemical cycle if these boundary fluxes have distinct signatures. Seafloor hydrothermal vents emit metal rich fluids from mid-ocean ridges into the deep ocean. Iron isotope ratios have the potential to be used to trace the input of hydrothermal dissolved iron to the oceans if the local controls on the fractionation of Fe isotopes during plume dispersal in the deep ocean are understood. In this study we assess the behaviour of Fe isotopes in a Southern Ocean hydrothermal plume using a sampling program of Total Dissolvable Fe (TDFe), and dissolved Fe (dFe). We demonstrate that δ56Fe values of dFe (δ56dFe) within the hydrothermal plume change dramatically during early plume dispersal, ranging from −2.39±0.05‰ to −0.13±0.06‰ (2 SD). The isotopic composition of TDFe (δ56TDFe) was consistently heavier than dFe values, ranging from −0.31±0.03‰ to 0.78±0.05‰, consistent with Fe oxyhydroxide precipitation as the plume samples age. The dFe present in the hydrothermal plume includes stabilised dFe species with potential to be transported to the deep ocean. We estimate that stable dFe exported from the plume will have a δ56Fe of −0.28±0.17‰. Further, we show that the proportion of authigenic iron-sulfide and iron-oxyhydroxide minerals precipitating in the buoyant plume exert opposing controls on the resultant isotope composition of dissolved Fe passed into the neutrally buoyant plume. We show that such controls yield variable dissolved Fe isotope signatures under the authigenic conditions reported from modern vent sites elsewhere, and so ought to be considered during iron isotope reconstructions of past hydrothermalism from ocean sediment records.

Kinetics of sulfide mineral oxidation in seawater: Implications for acid generation during in situ mining of seafloor hydrothermal vent deposits

Growth in global metal demand has fostered a new age of unconventional mining on the seafloor. In situ pulverization and extraction of seafloor massive sulfide (SMS) deposits is economically attractive due to minimal overburden and high ore grades. However, important environmental questions remain on the significance of localized acid generation via irreversible sulfide mineral oxidation. Data on the reaction kinetics are necessary to estimate anthropogenic acid production during seafloor mining.Laboratory experiments were performed to evaluate the effects of pH, temperature, dissolved oxygen, and surface area on the oxidation rate of pyrrhotite and chalcopyrite in seawater. These minerals were chosen to constrain the range of reaction rates because pyrrhotite oxidizes relatively quickly while chalcopyrite is kinetically slow. The rate laws for the abiotic oxidation of pyrrhotite and chalcopyrite in seawater at 22 °C are given in the form:Rsp=K(mO2)a(mH+)bwhere Rsp is the specific rate (moles m−2 sec−1), k is the rate constant, oxygen and proton concentrations are expressed in molalities (m), and their reaction orders as a and b, respectively. The specific rate laws obtained for each sulfide studied are:Rsp(pyrrhotite)=−10−7.27(mO2(aq))0.51±0.08(mH+)0.08±0.03Rsp(chalcopyrite)=−10−9.38(mO2(aq))1.16±0.03(mH+)0.36±0.09When used to quantitatively predict maximum acid generation rates, these rate laws indicate that acid production from in situ SMS mining is insufficient to exceed the buffering capacity of advecting seawater. We also calculated the residence times of crushed sulfides in seawater with low PO2 (0.10 atm, pH of 8, 23 °C) and find that, depending on grain size, mining waste may persist near the seafloor for years. The implications are positive in terms of slow acid production, but potentially problematic considering the potential ecological effects of an unnatural influx of particulates.

Ultra-high Curie temperature (>800 °C) low sintering temperature Bi2(1−x)La2xWO6 piezoelectric material for the applications of seafloor hydrothermal vents detection

Searching low sintering temperature material with ultra-high Curie temperature (>800 °C) is urgent to seafloor hydrothermal vents detection. Bi2WO6, as the simplest member of the Aurivillius family was improved to possess ultra-high Curie temperature and ultra-high depolarization temperature by experiment at first time. The crystal structure was determined by ab initio and Rietveld refinement calculations. The symmetry group of ultra-high Curie temperature Bi2WO6 is Aba2 (41). The Curie temperatures of Bi2(1−x)La2xWO6 (x = 0, 0.005, 0.01, 0.02, 0.04) with increasing x are 952 °C, 1008 °C, 905 °C, 853 °C, 822 °C, respectively, and depolarization temperatures of them are around 915 °C, 905 °C, 880 °C, 800 °C, and 725 °C, respectively. The typical properties are as follows: Curie temperature Tc = 905 °C, depolarization temperature Td = 880 °C, mechanical quality factor Qm = 621.8, d33 = 17 pC/N, K33 = 82.01, tanδ = 0.19 × 10−2 with x value of 0.01.

Crystal structure and thermal characteristics of Mn modified ultra-high curie temperature (>800 °C) Bi2WO6 piezoelectric ceramics

Searching low sintering temperature material with ultra-high Curie temperature (>800 °C) is urgent to seafloor hydrothermal vents detection. Bi2WO6 was first improved to possess ultra-high Curie temperature and ultra-high depolarization temperature by experiment. The MnCO3 was selected as dopant materials due to its excellent effect on improvement of sintering abilities and piezoelectric properties. The density test shows that Mn addition can obviously improve the density of Bi2WO6 ceramics from 93.6% to 97.76%, and the grains also changed from round to layered. The crystal structure change with Mn addition was tested by synchrotron radiation and calculated by ab initio and Rietveld refinement. The symmetry group of ultra-high Curie temperature Bi2WO6 is Aba2 (41). The XAFS results show that Mn atom more likely at the Bi-site in the crystal structure of Bi2W1-xMnxO6 and the Mn ion has the valence alternation. The Curie temperature of Bi2W1-xMnxO6 with increasing x are 997 °C, 915 °C, 890 °C, 729 °C, respectively, and the depolarization temperatures are around 940 °C, 895 °C, 825 °C, and 700 °C, respectively. The results show that Bi2W1-xMnxO6 (x ≤ 0.01) ceramics are suitable for the ultra-high temperature applications of sensors or transducers on seafloor hydrothermal vents detection.

A Review of Research Progress in the Genesis of Colloform Pyrite and Its Environmental Indications

AbstractColloform pyrite is a special form of nano‐micro polycrystalline aggregation growth, for which a suitable term is “aggregates of nano‐micro crystals”. This kind of colloform texture is observed in various geological bodies, such as ancient sedimentary rocks, modern marine and lake sediments, various types of ore deposits, and modern seafloor hydrothermal vents. This paper summarizes the latest developments and research into the definition, formation mechanisms, and environmental indications of colloform pyrite. There appears to be three main formation mechanisms of colloform pyrite: pseudomorphic replacement; biogenic precipitation; and inorganic precipitation. The morphology, particle size, trace element content and preferential growth orientations of colloform pyrite microcrystals can be important indicators for sedimentary environments, hydrothermal activity, and ore‐forming processes. We suggest that the microscopic features of nano‐micro crystals in colloform pyrite and their aggregation growth patterns need further investigation. The relationships between formation mechanisms of colloform pyrite, organic activity and depositional environments require further exploration. To reveal the nature of nano‐micro grain aggregation growth in colloform pyrite and analyse its growth environment and evolutionary history, it is supposed to apply nanoscientific and nanotechnological methods, further integrate consideration of macroscopic geological backgrounds and microscopic mineral growth phenomena, combine high–resolution imaging systems and in situ quantitative microanalysis methods and constitute a mergence of earth science, thermodynamics and kinetics, life science, material science, and chemistry in the study.

Fluid inclusion and stable isotopic constraints on fluid sources and evolution of the Luojiahe Cu deposit in the southern margin of the North China Craton

The Luojiahe Cu deposit in the Zhongtiaoshan region is located in the southern margin of the North China Craton. The orebodies are hosted in the mafic volcanic-sedimentary sequences of the metamorphosed (greenschist-facies) Neoarchean Songjiashan Group. The Luojiahe Cu mineralization can be divided into the primary volcanogenic massive sulfide (VMS) mineralization stage (Stage I, banded or stockwork ores) and the subsequent metamorphic remobilization stage (Stage II, coarse-vein ores).Three types of quartz selected for fluid inclusion (FI) studies were collected from the Stage I banded (Q1) and stockwork (Q2) ores and Stage II coarse-vein (Q3) ores. Four types of FIs were identified: (1) liquid-rich FIs (L-type), (2) pure vapor and vapor-rich FIs (V-type), (3) daughter mineral-bearing FIs (S-type), and (4) CH4-H2O FIs (C-type). Systematical microthermometric and H-O isotopic studies show that the Stage I ore-forming fluids consist predominantly of high salinity evolved seawater (125–220°C; 23.9–27.9wt.% NaCl equiv.) and some magmatic-hydrothermal fluids (249–339°C; 34.5–42.2wt.% NaCl equiv.). The two fluid end-members are represented by the L-type FIs in Q1 and the S- and V-type FIs in Q2. The temperature- and salinity variation trends of the L-type FIs in Q1 indicate a mixing process between the hot evolved seawater and cold seawater at Stage I. Furthermore, the V- and S-type FI coexistence in Q2 and their microthermometric data suggest that fluid unmixing has occurred in original magmatic fluids at Stage I. In contrast, the Stage II ore-forming fluids consist of CH4-rich metamorphic fluids (192–350°C; 10.6–43.2wt.% NaCl equiv.). Carbon isotopic analysis of the Stage II calcite (−4.58 to −10.83‰) and graphite (−32.01 to −39.16‰) in the ore-hosting chlorite schist indicates that the metamorphic ore-forming fluids had exchanged carbon isotope with graphite. The generation of CH4 may have resulted from the interaction between H2O (released by metamorphic devolatilization) and graphite. The continuous consumption of H2O in the hydrothermal fluid system may have increased the fluid salinity and triggered fluid unmixing in the CH4-NaCl-H2O system. In addition, the VMS metallogenic environment is generally favorable for microbial communities. It is considered that the graphite at Luojiahe may have been derived from sedimentary organic matter formed in seafloor hydrothermal vent systems, as also supported by carbon isotopic data.We propose that at Stage I, the main mineralization may have been resulted from 1) fluid mixing of hot evolved seawater and cold seawater in the near-surface environment; and 2) fluid unmixing caused by the percolation of magmatic fluids into syn-volcanic faults, forming the stockwork ores. At Stage II, the interaction between H2O and graphite may have resulted in the reduction of ore-forming fluids and Cu precipitation, and fluid unmixing in the CH4-NaCl-H2O system may have further promoted the Cu mineralization.