The Tiwi geothermal field is related to young volcanic activity on the southern part of Luzon Island, Philippines. In 1992, nearly 1650 m of continuous core was obtained from the western part of the Matalibong sector where measured temperatures are close to 270?C.

Eight stages of alteration and vein mineralization have been documented. The earliest stage is characterized by phyllic alteration of the reservoir rocks (stage 1). Stages 2 through 6 produced veins characterized by quartz + epidote + pyrite (stage 2), anhydrite + calcite (stage 3 ), epidote and/or quartz and/or adularia (stage 4), calcite + anhydrite (stage S), wairakite and/or epi-dote + quartz (stage 6), calcite (stage 7), and illite *chlorite (stage 8). The maximum fluid-inclusion temperatures of stages 3, 4, and 5 vein minerals are close to the boiling point and typically exceed 300?C while minimum temperatures are as much as 35?C below the present measured temperatures. Together, these data lead to the conclusion that mineralization reflects: 1) early heating of the system (stage 1); 2) successive periods of boiling, pressure drawdown and the incursion of cooler fluids (stages 2-3; 4-5; 6-7); and 3 ) renewed heating and boiling (stage 8) in response to the emplacement of recent intrusions
beneath Mt. Malinao.

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