ABSTRACT

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.