Hydrochemical Features of a Geothermal Test Well in a Volcanic Caldera, Mt. Pinatubo, Philippines

Mt. Pinatubo is one of several recent-age volcanoes along the west Luzon volcanic arc. A fumarole near the suminit emits gases with magmatic characteristics. Several thermal springs on the east and west flanks yield various fluid typos, including neutral chloride and bicarbonate. Three wellbores probed the Mt. Pinatubo caldera from elevations of +1230 through -1600 mRSL. Trajectories may be described i W central, crossing a boundary wall from the inside, and skirting a wall [probably] on the inside. Brine discharges indicate severe evapo-concentration effects accompanied by other phenomena. Severity of evapo-concentration indicates low fluid mobility near the wellbores. Large variations for ratios of component concentrations were observed, indicating negligible natural circulation (mixing). Implications about fluid movements and heat transfer processes are explored. Three components of steam can be quantified and all are significant: separate entry, adiabatic boiling, and boiling by rock heat. 2Philippine National Oil Company Geothermal Division EDC Fort Bonifacio, Metro Manila Republic of the Philippines SETTING: The West Luzon Volcanic Arc results from eastward subduction of the Manila Trench and includes a number of major stratovolcanoes of dominantly andesitic composition. Mt. Pinatubo is in western Luzon, north of Bataan peninsula, where the northwest Iba fracture (or graben) intersects the Arc. This region of Luzon has been built upon the Zambales Ophiolite Complex, part of the Eocene oceanic crust, along with overlying, unconformable marine, non-marine, and volcaniclastic sediments. These tilt eastward, forming the western margin of the Central Luzon Basin. Mt. Pinatubo (Figure 1) is locally the most prominent of several volcanic vents, mostly aligned along the northtrending volcanic axis. Initial volcanism began in late Pliocene and consists of andesitic to basaltic-andesitic lavas, tuffs, breccias, and lahars. Younger volcanics are dominantly dacitic ash-flow and ash-fall tuffs with minor andesitic-to-dacitic lavas and breccias. GEOLOGIC CROSS-SECTION THROUGH MT. PINATUBO