| Abstract |
Three wells drilled from the same pad at the Ulumbu geothermal field, Flores Island, encountered temperatures up to 240?C. Quaternary andesite lavas and pyroclastic rocks occur down to about 840 m with Tertiary sediments, dominantly limestones below, to at least 1900 m. The volcanic rocks have been pervasively altered by neutral pH, alkali chloride waters to produce a suite of hydrothermal minerals which occur both in veins and as replacement products. These include quartz, albite, titanite, calcite and calc-silicate minerals whose distributions are thermally sensitive, ie. laumontite occurs at depths where the temperatures are between 160 and 23O?C, pumpellyite from 185 to 200?C, wairakite above 190?C, prehnite above 205?C and epidote above 200?C. There is a regular zoning of the clay minerals with depth whereby smectite is stable from ambient temperature to 1 20?C, smectite plus chlorite from 120 to 200?C, interlayered chlorite-smectite from 160 to 200?C and discrete chlorite plus illite above 200?C. The homogenisation temperatures of fluid inclusions mostly match both the measured well temperatures and those deduced from the distribution of the thermally sensitive minerals. Permeable zones occur at depths of 500 and 800 m and are characterised by euhedral quartz and adularia and the intense alteration of the host rocks. |