The study of hydrothermal processes in an active geothermal system provides an excellent opportunity to test the equilibrium between the hydrothermal mineral assemblage and present-day fluid. Simulations of selected hydrothermal processes in the Tongonan geothermal system were performed to ascertain the effectiveness of different depositional mechanisms for hydrothermal minerals. Hydrothermal processes, such as boiling, fluid mixing and water-rock interaction were simulated using the reaction path programme CHILLER. Adiabatic boiling was done from 300 to 100 degreesC. The steam-heated end-member used for the mixing was generated by titrating the vapor phase at 100 degreesC after boiling of the primary water and mixing with a sample of the groundwater. Water-rock interaction in the Tongonan geothermal field was simulated by titrating andesite into 1 kg of primary water at 300 degreesC. Adiabatic boiling resulted in the precipitation of a sequence containing paragenetically early traces of gold with quartz, followed by quartz, acanthite, chalcocite and late quartz and base metals. Mixing of the steam-heated water with the boiled primary water resulted in the formation of the ore minerals acanthite, bornite, chalcocite, covellite, galena, pyrite, and sphalerite. Gangue minerals include anhydrite, Mg-chlorite, alunite, kaolinite, muscovite, and quartz. The mineral assemblages predicted using the water-rock interaction calculations provide a close approximation to the hydrothermal alteration commonly observed for measured temperatures of 300 degreesC. Minerals predicted to precipitate include tremolite, actinolite, diopside, and epidote, together with anhydrite. These minerals are observed to occur in hot regions of the geothermal system. The results of the chemical modeling suggest that the combination of boiling, mixing and water-rock interaction could precipitate the hydrothermal minerals in a Tongonan-like geothermal system. However, the absence of some minerals predicted by the simulation (e.g. acanthite and calcite) suggests that physico-chemical conditions other than those assumed in the simulations are prevailing in the system. (C) 2003 CNR. Published by Elsevier Ltd. All rights reserved.