The Geology and Geochemistry of Mataloko-Nage-bobo Geothermal Areas, Central Flores, Indonesia

Abstract

The preliminary interpretation of geological and geochemical data from the Mataloko-Nage-Bobo areas indicates the presence of a geothermal prospect. The geothermal manifestations are situated in andesitic to basaltic volcanic terrain between 500-1400 m above sea level. K/Ar and 14C ages of the volcanics range from 2.4 - 0.01 Ma, consistent with a high temperature geothermal heat source at depth. The prospect is divided into three areas with different characteristics. The SE-NW trending alteration zone of Mataloko (900 m asl) is mainly characterized by strong argilitization, consisting of kaolinite, alpha-cristobalite, alunite and pyrite that are probably associated with a fault structure of Wai Luja. The extension of hydrothermal alteration to deeper levels is suggested by low resistivity soundings (<10 Ÿ-m) and strongly affected by sulfuric acid dissolution. The hot, sulphate water assumed to be caused by H2S oxidation in near-surface yields gas geothermometer temperatures of ~ 283oC. The low values for ‰ 34 S(SO4) and chloride suggest that the gases are not derived from a magma reservoir, but rather from a deep aquifer (reservoir). The NESW trending alteration zones of Nage (520 m asl) are characterized by silicification-argilitization (pyrophyllite, quartz, and gypsum), with an average alteration age less than 0.2 Ma. The sulphate-chloride hot water has high boron, fluorine, arsenic and bromium contents, probably due to volcanic gases mixing with shallow ground water. The relatively high values for ‰ 34S (SO4) are due to an increasing SO4 concentration, probably indicating a contribution of magmatic SO2 to the hot spring aquifer. The N-S trend of young volcanic cones (1400 m asl) in the Bobo area have mainly alunite, kaolinite and cristobalite clay alteration. The presence of volcanic gases in fumaroles, especially SO2, suggests high temperature gases and a young heat source. Gas geothermometry indicates an underground temperature of 287oC.