Mineralization and hydrothermal history of the Tiwi geothermal system, Philippines

Tiwi is one of several large geothermal fields in the Philippines. In 1992, the drilling of Matalibong-25 provided nearly 1,650 m of continuous core f rom the geothermal reservoir A maximum temperature of 275 degrees C at 1,82 9 m and intense hydrothermal alteration were encountered in this well. Six stages of alteration and Vein mineralization have been documented in the co red portion of the well. The earliest stage, which reflects the initial dev elopment of the system, is represented by the deposition of chalcedony and clays (stage 1). The transition to a high-temperature environment is marked by the appearance of sericite (stage 2) deposited by steam-heated waters. Stage 3 reflects episodic cycles of fluid upwelling and boiling followed by the incursion of cooler fluids. Veins deposited by boiling fluids are fill ed with quartz +/- adularia +/- epidote +/- pyrite and base-metal sulfides, whereas heating of the recharging fluids led to the deposition of calcite and/or anhydrite. Maximum temperatures of fluid inclusions trapped in two q uartz crystals deposited by the upwelling fluids range from 325 degrees to 332 degrees C. Temperatures varied widely during the recharge phase of thes e cycles. Fluid inclusions trapped in calcite and anhydrite suggest that mi neral deposition occurred at temperatures ranging from 333 degrees C to les s than 270 degrees C. Fluid-inclusion salinities indicate that seawater dominated below a depth o f 1,600 m, whereas fresh waters dominated at shallower depths. Gaseous spec ies trapped in these inclusions were released by crushing or thermal decrep itation and analyzed with a quadrupole mass spectrometer. CO2/CH4 and N-2/A r ratios indicate that the gaseous species trapped in calcite and anhydrite were derived primarily from meteoric and crustal sources. In contrast, gas eous species in quartz from Matalibong-25 and advanced argillic assemblages from other Tiwi wells have magmatic and crustal origins. Stage 4 represents a second major episode of sericite deposition. Subsequen t mineralization consisted of wairakite +/- epidote, followed by actinolite , and then calcite (stage 5). The presence of actinolite implies that tempe ratures exceeded 300 degrees C at the end of this stage, Thermochemical mod eling indicates that the modern (stage 6) fluids are again in equilibrium w ith sericite. Ar-40/Ar-39 spectrum dating of stage 3 adularia from three de pths has been combined with the mineral parageneses and fluid-inclusion hom ogenization temperatures to constrain the thermal history of the geothermal system. Taken together; these data record the deposition of adularia at si milar to 330 degrees C between 314 and 279 ka, minor cooling followed by re heating to produce stage 5 actinolite at similar to 200 to 220 ka, incursio n of marginal waters and cooling to 235 degrees C by similar to 190 ka, a l ong period of quiescence to similar to 50 ka, anti Finally, development of the modern thermal regime at 10 to 50 ka in response to a recent subvolcani c intrusion.