The Awibengkok Core Research Program, Pt. II - Stratigraphy, Volcanic Facies, and Hydrothermal Alteration

Abstract

More than 1 km of continuous core from well Awi 1-2, in the eastern part of the Awibengkok geothermal field, Indonesia, provides a clear record of the late Cenozoic volcanic succession as well as multiple episodes of fracturing, brecciation, and hydrothermal alteration. The core, spanning the depth range 762-1830 m, samples the main parts of two stratigraphic ìpackagesî present throughout much of the field. The upper package comprises principally dacitic-composition flow-dome rocks, lahars, and tuffs. The deeper, andesitic package is made up of thinly interstratified flow rocks, tuffs, and lahars. The latter deposits dominate the upper portions of the packages, a relationship that in both cases could reflect declining volcanism coupled with erosion of initially constructive volcanic edifices. The core samples the lower part of the geothermal systemís clay-rich caprock as well as underlying propylitically altered volcanics which reach their shallowest levels in this part of the field. Abundant and pervasive smectite, illite/smectite, and chlorite in the caprock seal the geothermal system from incursion by overlying groundwaters. Some massively clay-altered tuff layers in the cap, particularly at its base, form ìductile lidsî which appear to have resisted rupturing by flexing and shearing, thereby maintaining integrity of the hydrologic seal. Textural and mineralogical evidence indicates that much of the clay alteration affecting these tuffs occurred at and near the surface in response to weathering. Rocks of the propylitic zone are near-completely altered to various combinations of epidote, chlorite, illite, calcite, adularia, wairakite, titanite, and pyrite. The same minerals along with quartz and localized anhydrite and prehnite (1) abundantly fill and line dissolution vugs and high-angle fractures; and (2) tightly cement generally high-angle hydrothermal breccia bodies. The commonly spectacular breccias and fracture fillings/linings are not systematically related to modern thermal fluid entries. The single large and two minor hot water entries in the corehole are in recently fractured rocks, some mineralized and some not. The largest fluid entry is additionally distinctive in containing euhedral chalcopyrite crystals with abundant adularia, but without the otherwise ubiquitous vein epidote and chlorite.