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

• Published in: Geofluids 2020, Vol.2020 (2020), p.1-12
• Main Authors: Li, Guanglai, Zhang, Rongqing, Chou, I-Ming, Qiu, Ye, Wang, Xiaolin, Zhong, Richen
• Format: Article
• Language: English
• Subjects: Acidity; Addition polymerization; Carbon dioxide; Fluids; High temperature; Hydrogen chloride; Hydrothermal solutions; Lasers; Mineralization; pH effects; Raman spectra; Raman spectroscopy; Room temperature; Salinity; Salinity effects; Sodium chloride; Species; Stability; Stainless steel; Temperature; Transport; Tungstates; Tungsten; Vibration; Vibration analysis
• ISSN: 1468-8115; 1468-8123
• DOI: 10.1155/2020/2978984

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.