GEOTHERMAL STRUCTURE AND FEATURE OF SULFIDE MINERALS OF THE MATALOKO GEOTHERMAL FIELD, FLORES ISLAND, INDONESIA

The Bajawa geothermal area is located in the central part of Flores Island, one of the Nusa Tenggara Islands in eastern Indonesia. Many volcanoes are distributed on Flores and many hydrothermal alteration zones occur around the Bajawa area. The exploration wells MT-1 and MT-2 were drilled by the Japan-Indonesia Cooperation Research Program at the Mataloko geothermal field, one of the most active fields in the Bajawa area. The Mataloko area is located along the southeast margin of the Bajawa Depression and has a suitable geological structure for geothermal reservoirs. Cinder cones are arranged in the Bajawa area, which extend in NW-SE and NE-SW directions. The heat source of this geothermal system is considered to be residual magma under the young volcanic cone. The Mataloko alteration zone is characterized by strong argillization, where many high temperature hot springs are distributed. A very low-resistivity layer distributes widely in this area. It is also considered that the alteration cap rock developed in this zone. The hot spring water of acid SO4 type resulted from shallower ground water being heated by gases containing H2S and CO2 and the ground water recharged from the surrounding area. Recharged meteoric water flows down to the deep. It changes to geothermal brine flows up toward the southeast and results in a geothermal reservoir. According to the chemical composition, sulfur isotopic ratio of sulfide and previous explorations, the fluid system can be divided into two zones at about 160 - 180 m: the deeper fluid which comes from the steam-dominated reservoir is oxidized and rich in CO2 and H2S and has temperatures between 230q - 250�` . This fluid led to the precipitation of wairakite and As- and Co-rich pyrite.