Detailed study of the calc-silicate mineralogy in the Cerro Prieto geothermal system was used to demonstrate examples for thermodynamic evaluation of phase relations among minerals of variable composition and to calculate the chemical characteristics of hydrothermal solutions compatible with the observed calc-silicate assemblages. In general there is a close correlation between calculated and observed fluid compositions. Calculated fugacities of O 2 at about 320 degrees C in the Cerro Prieto geothermal system are about five orders of magnitude less than those at the nearby Salton Sea geothermal system. This observation is consistent with the occurrence of Fe (super +3) -rich epidotes and andradite garnets in the latter system and the presence of prehnite at Cerro Prieto.--Modified journal abstract.

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