Importance of tungstate species for the transport of tungsten in hydrothermal solution: Case studies on major Japanese tungsten deposits.

Abstract

Thermodynamic calculations on the stability relations among various tungsten species are performed in order to elucidate the possible form of tungsten during the transportation in hydrothermal solution, using the knowledge of the fluoride and chloride concentrations. Based on the fluorine contents in micas and topaz and the fluid inclusion data, it is calculated that HF/H2O and HC1/H2O fugacity ratios in the natural hydrothermal solutions responsible for major Japanese tungsten deposits at the Kaneuchi, Ohtani, Takatori, Fujigatani, Kiwada, and Kuga range from 10-3.15 to 10-5.91 and are less than 10-3.33, respectively. Pressure and temperature conditions for the formation of the studied deposits are summarized and calculated to be from 1 to 2 kbars, and from 300° to 600°C, respectively. Oxygen fugacities are assumed to be those defined by the C-CO-CO2 buffer as the most reducing condition encountered. Based on the pressure, temperature, f(O2), f(HF)/f(H2O), and f(HCl)/f(H2O) conditions evaluated for the studied deposits, thermodynamic calculations are performed to compare the stabilities of halogen-bearing species with those of tungstates. Calculated results show that H2WO4 may be a predominant species in the natural hydrothermal solutions responsible for the formation of major Japanese tungsten deposits.