SGP-TR-134 GEOCHEMICAL CHARACTERISTICS OF ACID FLUIDS IN MT PINATUBO, PHILIPPINES

The surface geochemical characteristics of Mt Pinatubo indicate widespread deep acid fluids as shown by the following: presence of solfataras or sulphur-depositing fumaroles near or at the summit, and recent or active volcanism; discharge of mixed fluids with significant chloride at middle elevations; significant amounts of sulphate found in spring discharges at almost all levels (except at lowest levels or near sea level) ; springs with neutral pH and have high amounts of chloride and significant amounts of magnesium and boron; low Cl/Mg values accompanied by significant amounts of Mg observed at various elevations; higher proportions of gases, especially CO2 and significant N2; widespread occurrence of iron hydroxide deposits at almost all elevations; and all springs discharge immature waters. A new mixing model is proposed, the Cl-B-Mg ternary diagram, to differentiate the effects on well and spring water chemistry of the following processes: seawater mixing, groundwater dilution and magmatic input. Seawater mixing is not indicated for Mt Pinatubo springs as shown by crossplots of chloride versus other chemical constituents and the Cl-B-Mg ternary diagram. Neutral alkali chloride waters are present in the area. This is supported by the chemical crossplots, and the C1-B-Mg plots. In the C1-B-Mg plot, springs that discharge low pH waters are shown to be manifestations of acid fluids that are localized within a shallow or deep fault zone