Sulfide Minerals and Sulfur Isotope Compositions from Wells MT-1 and MT-2 in the Bajawa Geothermal Field, Flores Island, Indonesia

Abstract

Extensive geothermal activities occur in the Bajawa area, Flores Island, Indonesia. Chemical composition and sulfur isotopic ratio of sulfide were analyzed for cuttings from the exploration wells MT-1 and MT-2, which were drilled at the Mataloko geothermal field, one of the most active fields in the Bajawa area. Almost all of the sulfide minerals are pyrite with a little amount of sphalerite as small inclusions in pyrite. Pyrite contains As up to 2.5 wt.%, Co up to 0.8 wt.% and Mn up to 0.9 wt.%. High contents of As and Co are present in the core of pyrite crystals in the deeper samples, whereas Mn is enriched in the shallower samples. Sulfur isotopic ratios were analyzed for 17 samples. The δ³⁴S values of the pyrite show the following two characteristics: (1) the isotopic values gradually change from about -8‰ at the shallower level to about -2‰ at the deeper level, (2) the values abruptly change to about -10‰ at the deepest part. The δ³⁴S_py values concentrate around -5‰ which is not equilibrated with the δ³⁴S_H2S (-2.5 ‰) for the explosive gas of the well MT-1 at temperatures below 200°C. Combining these results with the previous results of geochemical and geophysical 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 of temperatures between 230°-250°C. This fluid led to the precipitation of wairakite and As- and Co-rich pyrite. The fluid of the shallower zone is made from the condensation of the deeper steam separated from the CO₂- and H₂S-rich fluids, and led to the re-equilibration of pyrite, and new alteration minerals of montmorillonite, kaolinite and alunite at temperatures less than 200°C. The deeper fluid retreated to deeper depth and the hydrothermal activity dropped below its peak. The present results of As, Co and sulfur isotopic anomalies of pyrite below 160 m can be possibly applied to detect an uppermost of the vapor-dominated geothermal reservoirs.