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Sulfate-bearing geofluids are widespread in many geological environments and play important roles in ore-forming processes. Recent experiments provide evidences that in silica-rich fluids, Na2SO4 solubility exhibits a complex temperature dependence, first decreasing from 25 to 313 °C and then increasing from 313 to 425 °C. The fluid-borne silica have critical implications for sulfate-bearing hydrothermal systems and formations of rare earth element deposits. However, whether other sulfate salts share the same characteristics remains unknown. In this study, fused silica capillary capsules (FSCCs) containing K2SO4-saturated solutions and K2SO4 crystals are used to quantitatively investigate the solubility of K2SO4 at different temperatures in the K2SO4-SiO2-H2O system. In the K2SO4-SiO2-H2O system, K2SO4 shows prograde solubility from 50 to 400 °C, and does not melt through the whole investigated temperature range. The relationship between temperature and the solubility of K2SO4 can be described by a simple linear function with R2 = 0.991: Csulfate = 4.1307 × 10−3 × T + 0.5532, where Csulfate and T represent the solubility of K2SO4 in m (mol/kg H2O) and temperature in °C, respectively. These results show that the solubility of K2SO4 can be enhanced by 3 to 5 times at 300–400 °C by the presence of silica, compared with that in silica-absent system, even without the formation of sulfate melt. Considering that quartz is ubiquitous in the Earth's crust, the solubility of sulfate in natural geofluids may reach a much higher level than previously expected. The temperature dependences of the solubilities of K2SO4 and Na2SO4 in silica-saturated fluids were used to constrain the compositions of the mineralizing fluids of the synmetamorphic hematite-quartz veins at Ouro Fino (SE Brazil) and the Maoniuping REE deposit (SW China), based on the previously reported microthermometric results of fluid inclusions that contain sulfate daughter minerals. For the hematite-quartz vein at Ouro Fino, the sulfate concentration was calculated to be 8.9–19.2 wt%, which is in line with previous laser ablation measurements (~15.3 wt%). For the Maoniuping REE deposit, the total sulfate concentration was determined to be 16.3–27.4 wt%, and the REE concentration of the ore-fluid was further estimated to be 0.30–0.44 wt%.

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, 0.8–5.0 kbar), 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.

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 – 0.1 mol/kg (m) K2WO4 solution containing CO2, HCl, and NaHCO3 were collected at 50–400 °C and 20–60 MPa. The spectra for the symmetric stretching vibration mode of the W O bond, v1(W O), 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 O) modes of these species are centered at ∼930 cm-1 and 950 cm-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 O) 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