The role of fluorine in granite-related hydrothermal tungsten ore genesis: Results of experiments and modeling

• Published in: Geochimica et cosmochimica acta Vol. 292; pp. 170 - 187
• Main Authors: Wang, Xin-Song, Williams-Jones, A.E., Hu, Rui-Zhong, Shang, Lin-Bo, Bi, Xian-Wu
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 01-01-2021
• Subjects: Fluoride; Hydrothermal experiments; Tungsten ore formation; Tungsten solubility and speciation; Tungsten solubility and speciation; Hydrothermal experiments; Tungsten ore formation; Fluoride
• ISSN: 0016-7037; 1872-9533
• DOI: 10.1016/j.gca.2020.09.032

Most of the World’s large tungsten deposits are genetically related to fluorine-rich granitic magmas, their ores contain fluorine minerals, such as fluorite and topaz, and their host rocks have commonly undergone fluorine metasomatism to form greisens. In view of this evidence that tungsten ore fluids are enriched in fluorine, and the potential for fluoride to form stable complexes with hard acids (e.g., W6+), we investigated the solubility and speciation of tungsten in fluoride-bearing fluids at hydrothermal conditions. Experiments were conducted to measure the solubility of tungsten trioxide solid in fluoride-bearing aqueous fluids at temperatures between 100 and 250 °C and vapour-saturated water pressure. Based on the results of these experiments, tungsten is dissolved predominantly as H3WO4F2− in solutions containing 0.05–0.30 m HF at pH(T) values between 1.5 and 3.4. Significantly, the concentration of H3WO4F2− was up to two orders of magnitude higher than that of tungstate species in these experimental solutions. This species formed via the reaction WO3cryst + H2O + 2HF = H3WO4F2− + H+, logarithms of the equilibrium constants for which, vary from −4.39 ± 0.18 at 100 °C to −5.10 ± 0.17 at 250 °C. In solutions containing <0.05 m HF, the dominant species is H2WO40. Fluid-rock interaction models employing these data predict that fluorine-bearing fluids will deposit ten times the mass of tungsten at a given fluid-rock ratio than fluorine-free hydrothermal fluids (the latter models consistently underestimate tungsten deposit grades). They also successfully explain why wolframite and not scheelite is the major tungsten mineral in vein- and greisen-type granite-hosted deposits, showing that calcium released to the fluid is consumed to form fluorite, thereby precluding saturation of the fluid with scheelite. The occurrence of fluorite in tungsten deposits is therefore not coincidental but instead a byproduct of the mineralizing process.