Characteristics of the Volcanic-Hydrothermal System in Mt. Labo, Philippines: Implications to Development

Abstract

Deep drilling results confirmed the presence of a hightemperature geothermal system beneath the southwestern slopes of Mt. Labo, an inactive andesitic stratovolcano in southeastern Luzon, Philippines. Numerous andesite and dacite domes exist around the central volcanic edifice. The most probable heat source of the present thermal activity is a cooling pluton related to the youngest deposits erupted by the central cone 27,000 years before present. Three of the seven completed wells produce low-pH, SO4-rich brine with temperatures of 260?-280?C. The low pH is due to the dissociation of HSO4 - upon flashing of the fluids during discharge. In the reservoir, all wells encounter near neutral-pH fluids, although the acid wells have much higher dissolved sulfate (400-1300 ppm) compared to neutral bores (<20 ppm). The increase in pH, decrease of SO4 concentrations, and decline in measured temperatures along the postulated major outflow direction signify fluid neutralization process through waterrock interaction and dilution by cooler fluids along the outflow paths and some permeable channels. Vertical Electrical Sounding (VES) and Magnetotelluric (MT) survey data define a 5 ohm-meter resistivity anomaly within a larger 10 ohm-meter enclosure. This suggests a possible minor upflow zone west of the acidic sector. Measured temperatures and chemical geothermometry suggest reservoir temperatures between 230? and 240?C in this sector. The occurrence of acid fluids in the hottest region, the temperature decline west of the acid sector and the incompetent formation in the north restrain development of the Mt. Labo prospect.