Stable isotope systematics of fluids and epidote in the Bacon-Manito Geothermal Field, Philippines: Indicators of fluid origin and evolution

The physiochemical evolution of active geothermal systems is important for assessing their long-term viability. Although discharge fluid chemistry provides information on geothermal well conditions, it only typically reflects the current state of the reservoir. Integration of fluid chemistry with rock and mineral chemistry can fill this gap by providing a longer-term record of fluctuations in geothermal conditions. This study examines the stable isotope systematics and the hydrogeological model of the Bacon-Manito geothermal field (Bacman) in the Philippines, a liquid-dominated, volcanic field-type geothermal system in a convergent setting. Geothermal well discharge geochemistry is supported by hydrogen and oxygen isotope data and elemental composition data from hydrothermal epidote and reservoir rocks. Chemical and isotope analysis of well discharges reveal short-term processes that are affecting the Bacman reservoir from the onset of fluid extraction due to field operations. Boiling within the Botong sector and the incursion of peripheral and injection fluids into the main reservoir of Palayang Bayan and Cawayan sectors was observed from the changes in isotope and chemical compositions of well discharges. Meanwhile, changes in epidote morphology and associated mineral assemblages, as well as epidote δ¹⁸O and major-element compositions, are related to reservoir rock composition, fluid chemistry, and temperature. These factors dictate the extent to which water-rock interaction and fluid mixing, the two most prominent processes in the isotopic evolution of epidote and fluids in Bacman, affect each particular sector in the reservoir.

We present an updated and expanded hydrogeological model of Bacman by incorporating the exploration sectors of Rangas and Kayabon. A heat source associated with carbonate reservoir rocks is postulated for the Rangas sector. Fluids in the Rangas sector originated in the main reservoir upflow in Botong and are isotopically enriched through interaction with carbonate reservoir rocks, suggesting that water-rock interaction is the dominant process. In contrast, the existence of fluids with variable isotopic compositions in the Palayan Bayan and Cawayan sectors indicates the dominant process is fluid mixing. The present-day fluids evolved through the continuous mixing of isotopically enriched parent waters with isotopically depleted peripheral waters. Furthermore, fluids and epidotes from Kayabon are found to be isotopically and petrologically distinct from those from the rest of the Bacman reservoir, supporting a relatively cooler and waning heat source beneath the sector.