Sulfur and chlorine are the two most important ligands accounting for metal transport in the upper crust. In this study, four metal- and sulfur-saturated model fluids with varying salinities and ...redox states were simulated in the Fe-Cu-Pb-Zn-Au-S-C-H-O system, over a wide pressure-temperature (P-T) range (50–650°C, 0.8–5.0kbar), in order to compare the roles of chloride and bisulfide complexing for metal transport at the light of the latest available thermodynamic properties. The range in simulated Zn and Pb concentrations of the model fluids compares well with those of natural hydrothermal fluids, suggesting that the model can be used to evaluate hydrothermal ore-forming processes in Nature.
The modeling reveals two different modes of Cu, Pb and Zn complexing in sulfur-saturated hydrothermal solutions. At lower temperature, chloride complexes are the predominant Cu, Pb and Zn species in sulfide-saturated systems, as expected from previous studies. However, hydrosulfide Cu, Pb and Zn complexes predominate at higher temperature. The predominance of bisulfide complexing for base metals at high temperature in sulfur-saturated systems is related to the prograde dissolution of pyrite and/or pyrrhotite, which results in a rapid increase in sulfur solubility.
Metals transport as chloride or bisulfide complexes determines the modes of metal enrichment. In chloride-complexing dominated systems (e.g., Mississipi Valley Type deposits), low sulfide solubilities mean that the ore fluids cannot carry both reduced sulfur and metals, and ore precipitation is triggered when the ore fluid encounters reduced sulfur, e.g., via fluid mixing or via sulfate reduction. In contrast, in fluids where bisulfide complexing is predominant, cooling and desulfidation reactions are efficient mechanisms for base metal sulfide precipitation. Since both Au and base metals (Cu, Pb and Zn) are predicted to be transported as hydrosulfide complexes in high-temperature primary magmatic fluids in equilibrium with sulfide minerals, high-salinity is not a necessity for magmatic hydrothermal deposits such as porphyry- and skarn-style deposits.
•Metal complexing is coupled with the prograde dissolution of Fe-sulfides.•At lower temperature, chloride complexes are the predominant Cu, Pb and Zn species.•Hydrosulfide Cu, Pb and Zn complexes predominate at higher temperature.•Hydrosulfide complexes are predicted to be predominant in primary magmatic fluids.
Scavenging of gold during metamorphic devolatilization is a widely accepted model for fluid and metal sourcing in orogenic gold deposits. In order to further constrain this process and quantify the ...capacity of autogenous fluids to release metals from pelites, we investigated the behaviors of S, Au, As, Cu, Pb, and Zn during pelite metamorphic devolatilization using thermodynamic modeling within the Al–As–Au–Cl–Cu–Fe–H–K–Mg–Na–O–Pb–S–Si–Ti–Zn system over a P–T range of 350–650°C and 0.8–5kbar.
The model revealed that S, Au and base metals are predominantly released via partitioning into the fluid phase during reactions that liberate H2O: the dehydration of chlorite and muscovite, and to a lesser extent, the replacement of pyrite by pyrrhotite. Negligible sulfur is liberated during the pyrite–pyrrhotite transition, because the excess sulfur reacts with Fe in chlorite and muscovite to form pyrrhotite. The sulfidation of chlorite/muscovite releases water, so that a significant amount of Au can be liberated from S-rich pelites at the pyrite to pyrrhotite transition: up to 0.5ppb Au (as a proportion of bulk rock) can be stripped from a pelite containing 1wt.% sulfur, whereas only trace amounts of base metals can be mobilized under these conditions. Chlorite dehydration is the most important process in metal extraction; up to 2ppb Au, 1.5ppm Cu, 1ppm Pb and 2ppm Zn (as proportions of bulk rock) can be extracted from a pelite by autogenous fluids upon crossing the greenschist–amphibolite facies boundary. In comparison, an average pelite contains ∼3ppb Au, indicating that most Au within an average pelite can be scavenged as a result of the breakdown of chlorite.
Prograde metamorphism is an efficient mechanism for generating Au-bearing ore fluids: most Au can be extracted during chlorite dehydration from a source rock of average pelitic composition. In contrast, only a small portion of base metals can be released in autogenous fluids, and therefore only minor enrichment in base metals is expected within large orogenic gold deposits in metamorphic terrains, which is consistent with observations globally.
The contiguous region between Guangxi, Guizhou, and Yunnan, commonly referred to as the Golden Triangle region in SW China, hosts many Carlin-type gold deposits. Previously, the ages of the gold ...mineralization in this region have not been well constrained due to the lack of suitable minerals for radiometric dating. This paper reports the first SIMS U–Pb age of hydrothermal rutile crystals for the Zhesang Carlin-type gold deposit in the region. The hydrothermal U-bearing rutile associated with gold-bearing sulfides in the deposit yields an U-Pb age of 213.6 ± 5.4 Ma, which is within the range of the previously reported arsenopyrite Re–Os isochron ages (204 ± 19 to 235 ± 33 Ma) for three other Carlin-type gold deposits in the region. Our new and more precise rutile U-Pb age confirms that the gold mineralization was contemporaneous with the Triassic W–Sn mineralization and associated granitic magmatism in the surrounding regions. Based on the temporal correlation, we postulate that coeval granitic plutons may be present at greater depths in the Golden Triangle region and that the formation of the Carlin-type gold deposits is probably linked to the coeval granitic magmatism in the region. This study clearly demonstrates that in situ rutile U–Pb dating is a robust tool for the geochronogical study of hydrothermal deposits that contain hydrothermal rutile.
The geochemistry of basaltic rocks is widely used to investigate the tectonic setting of magmatism. The limitation of traditional two-dimensional tectonic discrimination diagrams is mainly risen from ...the fact that they can only simultaneously use the information of two (x-y plots) or three (ternary diagrams) elements (or element ratios) for discrimination. This obstacle can be overcome with the assistance of machine learning method, which shows great performances in classification of multidimensional datasets. In this study, we present a neural network-based model that uses whole rock major and trace elements to discriminate basaltic rocks (SiO2 45–55 wt%) from a wide range of tectonic settings, including continental arc basalt (CAB), island arc basalt (IAB), intra-oceanic arc basalt (IOAB), mid-ocean ridge basalt (MORB), oceanic plateau basalt (OPB), oceanic island basalt (OIB), continental flood basalt (CFB) and continental rift basalt (CRB). Using a modified method of cross validation, it is estimated that the model can discriminate the tectonic setting with an average accuracy of ~86%, and ~98% in discriminating the major tectonic regimes (arc, spreading center, or within-plate magmatism). This discrimination model was programed as a stand-alone Microsoft Excel spreadsheet that can be directly used by pasting the whole rock data into it. The discriminator was then applied to investigate the geodynamic background of the Paleoproterozoic (~1.75 Ga) Xiong'er volcanism in the southern margin of the North China Craton (NCC). It has long been debated whether this magmatism took place in a continental arc or within-plate rift environment. The discrimination result shows that both the Xiong'er Group volcanic rocks and coeval intrusive rocks have CFB affinities, indicating that they were products of mantle plume activity and defines a large igneous province (LIP) in a within-plate setting. This, along with previous studies, constrains the breakup of NCC, a part of the Columbia supercontinent, not later than ~1.79 Ga, and supports the idea that the fragmentation of the Columbia was triggered by mantle plume impingement.
•A tectonic discriminator for basaltic rocks was trained with neural network.•The discriminator is provided as an easy-to-use Excel spreadsheet.•The ~1.75 Ga Xiong'er volcanic rocks and related mafic dykes are classified as CFB.
Sulfate is traditionally considered to have retrograde solubility in aqueous solutions. However, our recent hydrothermal diamond-anvil cell (HDAC) experiments have shown that the solubility of Na2SO4 ...changes from retrograde to prograde in the presence of silica, leading to the formation of sulfate-rich solutions at high temperatures, in line with observations on natural geofluids. In this study, we use synthetic inclusions of fused silica capillary capsules containing saturated Na2SO4 solutions and Na2SO4 crystals to quantitatively investigate the solubility of Na2SO4 at different temperatures in the Na2SO4-SiO2-H2O system. Sulfate concentrations were measured using Raman spectroscopy and calibrated using Cs2SO4 solutions with known concentrations. The solubility of crystalline Na2SO4 dropped slightly when heated from 50 to 225°C and dramatically from 225 to 313°C. At 313°C, the Na2SO4 crystals began to melt, forming immiscible sulfate melt coexisting with the aqueous solution, with or without solid Na2SO4. With the formation of sulfate melt, the solubility of Na2SO4 was reversed to prograde (i.e., solubility increased considerably with increasing temperatures). The solubility of Na2SO4 in the measured solution was significantly higher than that predicted in the absence of SiO2 over the entire temperature range (except for temperatures around 313°C). This indicates that the presence of SiO2 greatly changes the dissolution behavior of Na2SO4, which may be caused by the formation of a sulfate-silicate intermediates such as Si(OH)4SO42-. Considering that most crustal fluids are silica-saturated, the solubility curve of Na2SO4 obtained in this study can better reflect the characteristics of geofluids when compared to that of Na2SO4-H2O binary system. At temperatures of 313-425°C, the solubility of Na2SO4 increases with temperature following the function Csulfate=-3173.7/T to +5.9301, where Csulfate and T represent the solubility of Na2SO4 in mol/kg H2O and temperature in Kelvin, respectively. As an application, this temperature-solubility relationship can be used to evaluate the sulfate contents in fluid inclusions that contain sulfate daughter minerals, based on the temperature of sulfate disappearance obtained from microthermometric analysis. The sulfate concentrations of the ore-forming fluids of the giant Maoniuping carbonatite-related rare earth element (REE) deposit (southwest China) were calculated to be 4.67-4.81 m (mol/kg H2O). These sulfate concentrations were then used as internal standards to calibrate the previously reported semi-quantitative results of laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) analysis of REE-forming stage fluid inclusions at this deposit. The calculated Ce concentrations in the REE-mineralizing fluid range from 0.42 to 0.49 wt%. The high fluid REE contents suggest that the sulfate-rich fluids are ideal solvents for REE transport. A mass-balance calculation was carried out to evaluate the minimal volume of carbonatite melt that was required for the formation of the giant Maoniuping REE deposit. The result indicates that the carbonatite dikes in the mining area are enough to provide the required fluids and metals, and thus a deep-seated magma chamber is not necessary for ore formation.
•In situ Raman spectra of tungstate-bearing solutions were collected at ≤400°C and ≤60Mpa.•CO32- and HCO3- do not associate with tungstate to form carbonic tungstate species.•Polymeric tungstate ...species are stable in CO2-bearing fluids at <300°C.•Monomeric tungstate species are responsible for hydrothermal transport of W at >300°C.•The presence of CO2 favors the extraction of Fe(II) from the host rock and subsequent W-mineralization.
Knowledge on hydrothermal tungsten (W) species is vital towards a better understanding of tungsten transport and mineralization mechanisms. In this study, in situ Raman spectra of a 0.005 – 0.1mol/kg (m) K2WO4 solution containing CO2, HCl, and NaHCO3 were collected at 50–400°C and 20–60MPa. The spectra for the symmetric stretching vibration mode of the WO bond, v1(WO), were analyzed to investigate the hydrothermal tungstate species. Results showed that carbonate/bicarbonate do not associate with tungstate to form carbonic tungstate species. Nevertheless, the presence of CO2 can increase the fluid acidity, which favors the formation of polymeric tungstate species at <300°C. Above about 300°C, monomeric tungstates (e.g., WO42-, HWO4-, H2WO4 and alkali tungstate ion pairs) are responsible for the hydrothermal transport of tungsten, and the v1(WO) modes of these species are centered at ∼930 cm-1 and 950cm-1. Based on the above observations, we simulated the mineralization process in the context of fluid-rock interactions using tungstate and alkali tungstate ion pairs as the only aqueous W species. The thermodynamic simulations showed that (a) the timing of mineralization mainly depends on the W concentration in the initial mineralizing fluid and the availability of Ca2+, Fe2+ and Mn2+, with higher W concentrations generally favoring higher temperature mineralization; (b) highly W-enriched fluid is not essential for W mineralization, while extremely low contents of Fe, Mn and Ca in the magma are useful to maintain the mobility of aqueous W until favorable host rocks are encountered; and (c) a “hydrogen reservoir” effect was identified for dissolved CO2. The presence of CO2 can promote the extraction of Fe(II) from the pelitic host rocks, thereby facilitating a high-grade vein-type W mineralization.
At <∼300°C, polytungstate species, whose v1(WO) modes are centered at ∼965 – 995 cm-1, are important hydrothermal W species along with monomeric tungstates. Therefore, polymeric tungstate species should be considered in future thermodynamic modeling of W transport and mineralization at <300°C. An increase in fluid pH induced by CO2-escape and/or fluid-rock interactions will destabilize the polymeric tungstates to form WO42- and other monomeric tungstate, which interacts with metal cations to form wolframite and/or scheelite.
The genesis of the giant Dongshengmiao in the northern margin of the North China Block has been debated since its discovery in the 1950 s,because it shows geological and geochemical characteristics ...with both syngenetic and epigenetic signatures.It has geological settings and sulfur and lead isotopic compositions that are similar with typical SEDEX（sedimentary exhalative） deposit,while the Zn-Pb-Cu mineralization was controlled by shear deformation and metamorphism,showing similarities with orogenic-type deposits.In this contribution,both the syngenetic and epigenetic features of the Dongshengmiao are envisaged,and accounted for in the context of a genetic model with two metallogenic periods.Massive pyrite at the Dongshengmiao was mostly recrystallized during metamorphism,but finegrained texture was locally preserved,indicating its syngenetic origin.On the contrary,all the Zn-Pb-Cu ores observed in this study show characteristics of epigenetic hydrothermal mineralization that controlled by metamorphism and accompanying shear deformation.The sulfur and lead isotopic compositions of sphalerite and galena indicate that they were in situ remobilized from a syngenetic stratabound source,and the oxygen and hydrogen isotopic ratios of ore-fluid indicate that the large-scale remobilization was assisted by metamorphic fluid.The thermodynamic modeling indicates that the orefluid during remobilization has a great potential of transporting Cu.This may account for the abnormally enriched Cu in the remobilized SEDEX deposit.The metamorphic fluid might strip Cu from the fluid source during devolatilization,and overprint it on the Zn-Pb orebodies during remobilization.A secondary flowthrough modeling reveals that Zn- and Cu-sulfides would be preferentially redistributed in Fe-rich carbonates during remobilization,as a result of fluid-rock interaction.Conclusively,a multistage genetic model is proposed.During the development of the Proterozoic rift,stratabound Zn-Pb mineralization took place in a SEDEX ore-forming system.The syngenetic sulfides subsequently underwent a large-scale fluidassisted remobilization during the early Cretaceous metamorphism and thrusting,forming the shear zone-controlled epigenetic orebodies.During the remobilization process,Cu was scavenged from the source of metamorphic fluid,and deposited accompanying remobilized Zn-Pb sulfides.Shear structures and Fe-rich carbonates are ideal sites for redistribution and re-deposition of remobilized sulfide.
Hydrothermal H2S is an important energy source for hydrothermal ecosystems. However, it is difficult to obtain accurate hydrogen sulfide concentrations in high‐temperature hydrothermal fluids because ...they are highly susceptible to oxidation and compositional variability with mixing. In this study, a new in situ approach for measuring H2S, HS− and pH in hydrothermal fluids was developed and applied to the detections of Okinawa Trough hydrothermal activities. The in situ total H2S concentrations in the Jade and Biwako fluids were determined to be 31.4 and 76.7 mmol/kg, respectively. The in situ measured pH of the Jade fluids was determined to be 6.3, which has exceeded that of a neutral fluid at a specific temperature and pressure, indicating that the pH of Jade fluids is weakly alkaline. The pH transition of hydrothermal fluids from alkaline to acidic may be attributed to the thermal decomposition of organic matter and sulfide precipitation.
Plain Language Summary
Hydrogen sulfide is one of the important gases released from hydrothermal systems, but it is difficult to measure its concentration accurately because it is highly susceptible to oxidation and compositional variability with mixing. The in situ pH of hydrothermal fluids is also a challenging parameter to obtain, as it is significantly affected by factors such as temperature variation and mineral precipitation. Traditional methods of sampling followed by laboratory analysis inevitably result in changes in hydrogen sulfide concentration and pH. Furthermore, conventional chemical sensors and pH electrodes cannot withstand the high‐temperature extreme environment of hydrothermal vents, making it impossible to directly obtain these parameters at the high‐temperature vent orifice. Here, we developed a new method for in situ measurement of the H2S‐HS− ionization equilibrium system based on Raman spectroscopy and used it to determine the H2S concentration and in situ pH of hydrothermal fluids in the Okinawa Trough.
Key Points
The first in situ measured pH of high‐temperature hydrothermal vent fluids at arc‐back arc basins was reported
A new approach to obtain in situ H2S/HS− concentration and in situ pH of high temperature hydrothermal vent fluids was established
The pH transition of hydrothermal fluids from alkaline to acidic should attributes to the precipitation of sulfide minerals
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