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Abstract 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.

Abstract The HighPGibbs program is designed to calculate thermodynamic equilibrium of fluid‐rock systems up to depths of lithospheric mantle. It uses the Gibbs free energy minimization function of the HCh package to calculate mineral‐fluid equilibrium. Chemical potentials of minerals are calculated using the equations of states included in HCh; free energy of aqueous species is calculated using the Deep Earth Water model; and activity coefficients of charged species are estimated using the Davies variant of the Debye‐Hückel equation. HighPGibbs was applied to calculate nitrogen speciation in eclogite‐buffered fluids from 400 to 790 °C and 30 to 54 kbar, to evaluate the mobility of nitrogen in subducting oceanic crust. Regardless of whether the protolith was altered (and oxidized) or not, N2(aq) or NH3(aq) is the predominant form of nitrogen in the slab fluids at subarc temperatures, especially in cases of moderate or hot geotherms. Given that molecular nitrogen is highly incompatible in silicate minerals, the simulation indicates that nitrogen (as NH4+) in silicate minerals can be liberated during metamorphic devolatilization. The majority of nitrogen in subducting crusts can be unlocked during slab devolatilization and eventually expelled to the atmosphere via degassing of arc magmas. Therefore, oceanic crusts recycled to deep Earth will be depleted in nitrogen compared to the newly formed crust at spreading centers. As a result of the long‐term mantle convection, large proportions of the bulk silicate Earth may have suffered nitrogen extraction via subduction, and this may account for the nitrogen enrichment in the Earth's atmosphere.

In situ Raman spectra of aqueous K2WO4-HCl, K2WO4-HCl-NaCl, and K2WO4-CO2-NaCl solutions were collected at elevated temperature (T, 100–400°C) and constant pressure (P) of 30 MPa. The stretching vibration band of the W=O bond (v1) was analyzed to reveal the species of tungsten (W) responsible for the hydrothermal transport of W during mineralization. Results showed that monomeric tungstates with v1(W=O) bands at ~930 and 950 cm-1 are the dominant W species in weakly alkaline (room temperature, pH=7.7) to near-neutral (room temperature, pH=7.2) solutions under the investigated T‐P conditions. Overall, the stability of polymeric tungstate species with v1(W=O) bands at ~965–995 cm-1 decreases with rising temperature and was not detected at ≥300°C in moderately acidic solution (room temperature, pH=4.9). However, increased fluid acidity and salinity obviously enlarged the temperature stability field of polymeric tungstate species. In highly acidic solution (room temperature, pH=1.4), polymeric tungstates are the only stable W species even at 400°C. In the presence of 1.9 mol/kg NaCl, polymeric tungstate(s) can persist to at least 350°C in moderately acidic solution. Considering that 300–350°C is the major W-mineralizing T range and W-mineralizing fluid is generally characterized by a moderately acidic nature, we propose that, in addition to monomeric tungstates, polymeric tungstates can also be important W species in some natural geological fluids that are responsible for the mineralization of W. Future studies of the W mineralization mechanism should take this issue into account.

Orogenic gold deposits contribute the largest proportion of the world's gold reserves, and the source of their ore-forming components has been recognized as the metamorphic devolatilization of metapelites or metabasites across the greenschist-to amphibolite-facies transition. However, hypozonal orogenic gold deposits represent an enigma in this context. Some of these apparently formed in higher-grade metamorphic rocks when temperatures were beyond the wet solidus of quartz-feldspar-bearing rocks; it is therefore puzzling how these fluids were generated in the source and migrated through the crust without causing partial melting. Here, we show that devolatilization of hydrated komatiites, a volumetrically significant lithological unit in Precambrian greenstone belts, is a viable model that can plausibly lead to gold mineralization at amphibolite-facies conditions. Our thermodynamic simulations indicate that subsolidus metamorphic devolatilization of komatiites at ~700 °C (upper amphibolite facies) can unlock significant amounts of gold via dehydration of talc and chlorite. This genetic model is supported by the geochemical characteristics of, and estimated pressure-temperature (P-T) formation conditions of, hypozonal gold deposits and the intimate spatiotemporal association between hypozonal deposits and komatiites in greenstone belts. This work expands the P-T range of the metamorphic devolatilization model and enhances its robustness in explaining gold mineralization in metamorphic terranes. [ABSTRACT FROM AUTHOR]

The article provides information on the transition from carbonatitic magmas to hydrothermal brines in carbonatite systems, which are important sources of rare earth elements (REEs). The study investigates the pressure-temperature-composition properties of the Na2CO3-H2O system and shows that under high-pressure and high-temperature conditions, Na2CO3 melts transition into hydrothermal brines, leading to the enrichment of REEs in the brine-melts and the formation of economic ore bodies.

The presence of sulfate-rich fluids in natural magmatic hydrothermal systems and some carbonatite-related rare earth element (REE) deposits is paradoxical, because sulfate salts are known for their retrograde solubility, implying that they should be insoluble in hightemperature geofluids. Here, we show that the presence of quartz can significantly change the dissolution behavior of Na2SO4, leading to the formation of extremely sulfate-rich fluids (at least 42.8 wt% Na2SO4) at temperatures >~330 °C. The elevated Na2SO4 solubility results from prograde dissolution of immiscible sulfate melt, the water-saturated solidus of which decreases from ≥~450 °C in the binary Na2SO4-H2O system to ~270 °C in the presence of silica. This implies that sulfate-rich fluids should be common in quartz-saturated crustal environments. Furthermore, we found that the sulfate-rich fluid is a highly effective medium for Nd mobilization. Thermodynamic modeling predicts that sulfate ions are more effective in complexing REE(III) than chloride ions. This reinforces the idea that REEs can be transported as sulfate complexes in sulfate-rich fluids, providing an alternative to the current REE transport paradigm, wherein chloride complexing accounts for REE solubility in ore fluids. [ABSTRACT FROM AUTHOR]

In hydrothermal ores, gold is commonly hosted by sulfide minerals due to its affinity for reduced sulfur. In contrast the occurrence of gold in oxide minerals remains poorly documented and incompletely understood. In this contribution, we demonstrate that magnetite can be a significant host mineral for gold. Skarn ores from the giant Beiya Au deposit (323 t Au at 2.47 g/t), southwest China, contain abundant nanometer-to micronsized gold hosted by magnetite. A range of bismuth minerals (e.g., native bismuth, maldonite, and bismuthinite) occur within the same assemblage. Textural and mineralogical studies as well as thermodynamic modeling indicate that Bi melts scavenged Au from hydrothermal fluids during magnetite growth. Fluetuating fo2(g) at the surfaces of crystallizing magnetite led to the precipitation of Bi-Au melt, ongoing gold refinement, and entrapment of gold and bismuth minerals within magnetite. Sulfidation of Bi, triggered by increased sulfur content and/or decreased temperature, further promoted gold precipitation. Bismuth melts are interpreted to have played a crucial role in Au enrichment from gold-undersaturated fluids at Beiya. The results imply that magnetite-rich systems are also potential targets for Au exploration and extraction, particularly in cases where coexisting Bi minerals are observed in abundance. [ABSTRACT FROM AUTHOR]