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 assump tion that the hydrothermal solution responsible for the formation of topaz has a temperature of 300°C, HF/H20 activity ratio is estimated to be 10-1.86. This activity ratio leads to 262ppm as fluoride con centration in hydrothermal solution. Comparison with other acid geothermal waters

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... s that fluoride rich solutions existed during the formation of the topaz. Such a high fluoride concentration, may be as cribed to the condensation of volcanic gases.