Geochemical Characteristics of Acid Fluids in Mt. Pinatubo, Philippines

Abstract

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.