Mineral Equilibrium Modeling and Geothermometry of the Pre-1991 Eruption Mount Pinatubo Exploratory Well Fluids

English Abstract:

The equilibrium state between the present-day geothermal fluids and hydrothermal mineral phases of the pre-1991 eruption Mount Pinatubo exploratory wells were tested using the program Geochemist’s Workbench 10.0.3. A technique employed by Reed and Spycher (1984) uses mineral saturation indices or log (Q/K) of different species (where Q is the ion activity product or reaction quotient and K is the equilibrium constant) over a range of temperature to determine mineral equilibria. Molecular ratios of both fluid and gases were also used to characterize the geothermal wells.

Well fluids range from immature to partially equilibrated (Giggenbach, 1988). Temperature of 240°C for PN-1 and 300-340°C for PN-2D and –3D were obtained using Na-K-Mg geothermometers. PIN-2D is the least mixed water (having uniform Cl/B) that is nearest the magmatic upflow characterized by low Cl/SO4, high Mg, low pH and being the nearest to the pre-eruption caldera. Magmatic input was confirmed using CO₂/N₂ gas ratio comparable with White Island and some Japanese volcanoes. Mineral saturation indices (log Q/K) were calculated from 20° to 300°C revealing a neutral-pH alteration assemblage consisting of pyrite, amorphous silica, chalcedony, cristobalite, quartz, tridymite, dolomite, talc (at >280° C for PIN-2D), calcite (for PIN-3D), huntite, pyrrhotite and pyrite. Only PIN-2D exhibited covergence at 245°C consistent with the molecular geothermometers. Undersaturation of some clay minerals and pyroxene and the absence of important clay geothermometers supported by the disconvergence of the mineral saturation plots for most wells suggests that the present day hydrothermal fluids is not in equilibrium with the hydrothermal mineral phases.