Magmatic Contributions to Philippine Hydrothermal Systems

Abstract

The geochemical characteristics of known volcanomagmatic hydrothermal systems in the Philippines (Pinatubo, Biliran) are studied. Similar characteristics include solfataric activity with bearing acid springs or mud pools at the summit or high elevations with deposits formed where the hot waters mix with diluting cold waters. Isotopic compositions show positive and shifts and a magmatic water (or volcanic gas) - meteoric water mixing line, indicative that these are just condensed magmatic vapors with meteoric waters. Steam discharges have gas with non-reactive gas compositions showing definite magmatic gas signatures with temperatures at depth greater than Wells drilled towards or beneath these features encounter high temperatures at depth and discharge acid 2-3 fluids with high gas contents. However, wells drilled away from the center of active acid gas input discharge neutral fluids with significant concentrations of Mg, F, Ca and SO4 due to neutralization of an initially acidic fluids by water-rock interaction as the fluid migrates. In all other geothermal fields, neutral fluids also show distinct and shifts, which can be accounted for by mixing with magmatic water of similar composition to volcanic gases. Geochemical models for these types of systems are introduced and development strategies are suggested.