Occurence of topaz from silica and alunite deposits at the Ugusu mine and its implication for high fluoride concentration in fossil geothermal water

抄録

Topaz occurs in the central part of the altered zone of the Ugusu mine. Fluorine content of topaz was obrained by X-ray diffraction technique, with an average (±SD) of 19.7 (±0.5) wt%. Based on the assumption that the hydrothermal solution responsible for the formation of topaz has a temperature of 300°C, HF/H₂O activity ratio is estimated to be 10^-1.86. This activity ratio leads to 262ppm as fluoride concentration in hydrothermal solution. Comparison with other acid geothermal waters suggests that fluoride-rich solutions existed during the formation of the topaz. Such a high fluoride concentration may be ascribed to the condensation of volcanic gases.