In situ Raman spectroscopic investigation of the hydrothermal speciation of tungsten: Implications for the ore-forming process

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