Hydrothermal Alteration and Evolution of the Bulalo Geothermal Field, Philippines

Abstract

The Bulalo geothermal system, Philippines, is characterized by broadly distributed alteration related to multiple episodes of shallow igneous intrusion and hydrothermal fluid circulation. The most recent hydrothermal event is associated with the extrusion of dacite domes <22,000 years ago. At depth, high-temperature phyllic (sericite-quartz-pyrite) and propylitic (epidote-chlorite-quartz-pyrite) assemblages dominate. Anhydrite-bearing phyllic assemblages and advanced argillic alteration (pyrophyllite-type) are locally present in the contact region of intrusions emplaced deep on the W and SSW margins of the field. In the upper part of the system, smectite-and interlayered illite-smectite- and chlorite-smectite- bearing rocks form an impermeable cap that deepens and thickens outward from the region of upflow. Calcite is locally abundant within the clay cap and on the E and SE margin of the system, where noncondensable gas concentrations are highest. Vein filling sequences and fluid inclusion measurements highlight the effects of the initial heating, catastrophic decompression and boiling, and later inflow of cooler marginal fluids.