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 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 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 <∼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.