Experimental Determination of Ferberite Solubility in the KCl–HCl–H2O System at 400–500 °C and 20–100 MPa

The solubility of ferberite, FeWO4 was studied at 400–500 °C, pressures of 20, 25, 40, 50 and 100 MPa, oxygen fugacity corresponding to the Ni–NiO, Fe3O4–Fe2O3 buffers, in 0.7 ÷ 8.9 mKCl solutions and acidity controlled by quartz-microcline-muscovite buffer assemblage. The parameters of the experiments cover both the field of homogeneous solutions and the region of immiscibility in the KCl–H2O system. The total W concentration depends upon mCl, T and fO2 in the system and range from 1 · 10−4–0.05 mol kg−1 in 0.7 mKCl to 0.01–0.15 in 8.9 mKCl. The results suggest a large bulk solubility in the dense, salt-rich phase of the two-phase fluid. Ferberite dissolution in KCl solutions under pH and fO2 buffered conditions at 400–500 °C proceeds congruently as well as incongruently with accompanying potassium tungsten bronzes formation, KxWO3, (x = 0.2–0.3). Thermodynamic calculations performed for a homogeneous solution at P = 100 MPa indicate that the predominant aqueous species of tungsten in KCl–HCl solutions at fO2 = fO2(Ni–NiO) may be W(V, VI) species: \({\text{WO}}_{3}^{ - }\), \({\text{HWO}}_{4}^{ - }\), \({\text{H}}_{2} {\text{W}}_{2} {\text{O}}_{7}^{ - }\) at 500 °C and \({\text{HWO}}_{4}^{ - }\), \({\text{W}}_{5} {\text{O}}_{16}^{3 - }\) at 400 °C. Application of the extended FeWO4 solubility model to natural systems suggest that deposition of tungsten from ore-bearing solutions is due to interaction with wall rocks containing feldspars, and iron oxides together with decreasing temperatures. In the magnetite bearing system, the equilibrium tungsten concentration does not exceed 2 · 10−5 mol kg−1 at temperatures of 400 °C.