Mineralization and hydrothermal history of the Tiwi geothermal system, Philippines

Tiwi is one of several large geothermal fields in the Philippines. Nearly 1650 m of drill core is available, indicating a maximum T of 275 degrees C at 1829 m with intense hydrothermal alteration. Six alteration stages and vein mineralization have been established, the earliest stage being represented by the deposition of chalcedony and clays. Stage 2 is marked by the appearance of sericite deposited by steam-heated waters, and stage 3 reflects episodic cycles of fluid upwelling and boiling followed by the incursion of cooler fluids. Veins deposited by boiling fluids are filled with quartz + or - adularia + or - epidote + or - pyrite. Fluid inclusions suggest that mineral deposition occurred at 333 to < 270 degrees C. Stage 4 represents a second major episode of sericite deposition, followed by wairakite + or - epidote and then actinolite; stage 5 involved calcite. The modern (stage 6) fluids are again in equilibrium with sericite.