Effects of pH and Salinity on the Hydrothermal Transport of Tungsten: Insights from In Situ Raman Spectroscopic Characterization of K2WO4-NaCl-HCl-CO2 Solutions at Temperatures up to 400°C.

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 ( v 1 ) was analyzed to reveal the species of tungsten (W) responsible for the hydrothermal transport of W during mineralization. Results showed that monomeric tungstates with v 1 (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 v 1 (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. [ABSTRACT FROM AUTHOR]