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Based on detailed petrological information on secondary mineral assemblages and the composition of fluids trapped in inclusions and discharged from five wells, the Alto Peak geothermal field was found to represent a combined vapor and liquid-dominated system. A central core or chimney, with a diameter of about 1 km, a height of some 3 km and occupied by a high gas vapor (1.1 to 5.6 molal CO{sub 2}), is surrounded by an envelope of intermediate salinity water (7,000 mg/kg Cl) with temperatures between 250 and 350 C. The transition from purely vapor-dominated to liquid-dominated zones takes place via two-phase zones occupied by fluid mixtures of highly variable compositions. Much of the lower temperature, mature neutral pH Cl water is likely to have formed during an earlier stage in the evolution of the system. High temperatures of > 300 C, and associated alteration, are limited to wells AP-1D and the lower parts of AP-2D and are ascribed to re-heating by recent magmatic intrusions. The isotopic composition of the well discharges suggests that they contain some 40 to 50% of magmatic water. Alto Peak is considered a typical example of hydrothermal systems associated with many dormant volcanoes.