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This institution reserves the right to refuse to accept a copying order if, in its judgment, fulfillment of the order would involve violation of copyright law. Geothermal Resources Council Transactions, Vol. 24, September 24-27, 2000 Hydrothermal Alterations in Geothermal Prospects of Ngada District, Flores Indonesia Asnawir Nasution', I. Takashima*, H. Muraoka3, M. Takahashi3, H. Takahashi4, K. Matsuda5, H. Akasako5, F. Nanlohil, D. Kusnadil, Masao FutagoishP Volcanological Survey of Indonesia, Bandung, Indonesia 2Akita University, Akita, Japan 3Geological Survey of JapanTsukuba, Japan 4MMRC, Tokyo, Japan 5We~t-Je~,Fukuoka, Japan 6NED0, Tokyo, Japan ABSTRACT The Mataloko and Nage geothermal prospects are situated The values of 6D (H,O) and 6 '*O (H,O) of Mataloko hot on a volcanic terrain (500- 1400 m above sea level) that repre- springs indicate a meteoric origin. sents andesitic to basaltic rock compositions. The lineaments, The alteration zone of Nage (520 m) is characterized by faults and fracture systems are indicated by existing volcanic silicification and argilitization (pyrophyllite, quartz, and gyp- alignments, topographical differences and rivers, alteration, sum), with an average alteration age less than 0.2 Ma. The hot spring elongation. The WAr ages of the volcanic cones sulphate-chloride hot spring water has high boron, fluoride, ar- yield 1.1 to 0.01 Ma, consistent with geothermal heat sources senic and bromide contents, probably due to volcanic gases at depth. mixing with shallow ground water. The relatively high values The surface and shallow rock alterations of Mataloko of 6 34S (SO,) are due to a high SO, concentration, probably (900 m) are mainly characterized by strong argillitization, indicating a contribution of magmatic SO,. The values of 6 '*O consisting of montmorilonite, kaolinite, alpha cristobalite, of Nage hot springs indicate meteoric origin. alunite, illite and py- rite. The extension of hydrothermal alter- ation to the deeper level is supported by a low resistivity of Schlumberger and low frequency MT data. The alteration results from acid sulphate wa- ter resulting from H,S gas which passing through NE-SW and N-S fracture systems, oxidized then close to the surface. The low values of 634S (SO,) and chloride suggest that the gases are not derived from mag- matic sources, but from a deep aquifer (reservoir), having temperature - 283°C. Figure 1. Plate boundaries of Indonesia (from Katili, 1973). 265 Nasution, et. a/. The low pressure (3 bars) and boiling temperature (1 15°C) (relative to SMOW) for meteoric water has variable values, fluids found in the Mataloko shallow drilling survey (103 m depending on latitude and altitude. In geothermal systems, water depth) are acid fluids. The surface acid fluids and an interlayer has two trends: mixing between meteoric and magmatic illite-montmorilonite (?) at the shallow depth (66-70 m), prob- (volcanic vapor) water, and reaction of meteoric water and host ably indicate increasing temperature at deeper levels. Therefore, rocks (resulting in “0-shift”, Taylor, 1979). The 6 34s (SO,) Mataloko is a promising area for continued exploration. studies help to indicate high temperature acid gases from magma and mixing processes. This paper will give early exploration results (shallow rock Introduction alteration and geochemistry) and an interpretation of water-rock The Mataloko and Nage geothermal areas are located in the interaction at the Mataloko and Nage prospects. Ngada regency on the island of Flores between 120”55’-121’ 05’ E latitude 08’41.5’- 08’43.8’ longitude (Figure 2, previous Tectonic and Geologic Setting page). It has good accessibility and high rain fall (k 1750-2250 mdyear) . The Flores island on the Eastern Sunda Arc is part of a col- The study of secondary minerals in surface alteration and lision zone between the Indian-Australian Plate to the south and cuttings, using petrography and X-RD analysis, provide the Eurasian plate to the north (Figure l.,Katili, 1973; Hamilton, quantitative information, allow recognation and estimation of 1979). The subduction zone generates an east-west trending subsurfacetemperatures, thermal gradients, and assist in refining volcanic chain, including the Lewotobi, Egon, Kelimutu, Iya, hydrological models (Brown, 1993). Rock alteration of active Ebulobo, he-Rie, he-Lika and Anak Ranakah volcanoes. The geothermal fields with few discharge features have been used Mataloko and Nage prospects are located between three active to help interpret the field. K. Sumi (1 968) used alteration type volcanoes, lne-rie, he-lika and Ebulobo (Figure 2) and are as- and extent to define the size and thermal history of Matsukawa, sociated with fault and fracture trends passing through the Japan. volcanic complex (e.g. Wolo Pure, Sasa, Rhea, Bela, Hoge and Surface alteration assemblages reflect the fluid type. Alkali Belu, Bobo, and Bejawa volcanic cones). These young volca- chloride springs commonly deposit amorpoush silica. nic features in the area suggest an active magmatic heat source. Bicarbonate springs deposit calcite as C02is lost. Acid springs The geology of the prospect areas (Figure 2) comprises or acid condensate (pH 2-4) mostly occur in regions of high young Quaternary andesitic-basaltic volcanic cones (Qvc), the relief and deposit a variety of sulphate minerals, for example, Mataloko andesites (Qma), the Bejawa andesitic lavas and thin sulfur, gypsum (Brown, 1978). The hydrothermal alteration pyroclastics (Qba-b) and the Waebela basalt (Qvwb) which is produced by acid fluids in active geothermal fields have shown intercalated with volcanic tuff, weathered and shows columnar several typical minerals, for example, kaolinite, dickite, illite, jointing. These rocks are characterized by high relief, a rela- cristobalite,alunite which can be used to indicate the temperature tively high erosion rate and high topography (500- 1400m asl). of mineral formation and type of alteration (Brown, 1993). WAr dates of lavas and pyroclastics give ages from 2.4 to c0.1 Oxygen, hydrogen and sulfur isotopes of hot spring waters Ma (Takahashi, 1998, Muraoka et.al, 1999 and Nasution et.al, have been used for an early evaluation of geothermal fields, 1999). including origin of geothermal water, geothermometry and The geological structures associated with the southeast- physical processes. The relationship between 6 D and 6 180 northwest trending fault and fracture systems occupying regional structures of Central Corelation maD unit Flores (Figure 2) and are probably influenced by the subduction zone driv- ing from the south (Figure 1). Satellite imag- IF I ery of the survey area helps showing volcanic lineaments, ring structure forms, and geological structures (Figure 3). The SE-NW Wailuja normal fault (?) and fractures of Mataloko are major con- trol structure of thermal channel fluids of the Mataloko geothermal area, indicated by a trend of hot springs and alter- . ~~~~~ ation zone distributions. Figure 2. Geothermal geological map of Mataloko, Wolo Bobo and Nage areas. 266 Nasution, et. a/. The extent of alteration to the deeper level is suggested by low resistivity soundings,5-25 R m for AEV2 1.5-2000m (Nasution et.al., 1999). The Head On geophysical survey shows that the e a4u1 fault dipping is over 70" to the north, suggested that fluid dis- charges are much higher ascending to northern part of the LEGEND : Wailuja fault (Nasution et.al., 1999). Alunite zone The Northeast-Southwest fractures or fault (?) of Nage (520 a Kaolinite zone m asl) are characterized by hot springs and trending alteration Monmonilonite zone distributions; a density contras of gravity survey (Dendi et.al., & Hotspring Mudpool 1998) and low resistivity sounding (5-10 R m for AB/2 1.5- 6 1000 m). The thermal discharges in the prospect areas seem to & Fumarole be associated with structure or fracture systems oriented in NW- 0 Sampling point SE, SW-NE, N-S (Figure 2) and influence or replace original minerals of rocks to form hydrothermal alteration and clay min- erals. Hydrothermal Alteration Secondary minerals occur as a replacement of primary min- Figure 4. Alteration zones of Mataloko geothermal area. erals and as precipitates around the thermal springs. They seem to result from hydrothermal metasomatism, where cations of and high Hg soil air content (>3000ng). In lateral order, the original minerals are replaced by hydrogen ions that react with alteration is divided into alunite-illite, kaolinite and oxygen atoms in the original silicate minerals to form alteration monmorilonite zones (Figure 4). The alunite-illite zone is lo- minerals with (OH)- groups (Hemley,J.J, and Jones,W.R.,1964). cated in the inner part, probably affected by a strong sulfuric Secondary minerals may be identified by X-ray diffraction acid and high temperature solutions, indicated by alunite. The (XRD).In this study an X-ray difractometer analyze powder kaolinite zone is characterized by kaolinite, alpha-cristobalite, samples using three different beams on both bulk and clay monmorilonite and quartz, reflecting acidic to weak acidic so- samples, after crushing samples to 50-100 mess and powder lutions. The outer zone is characterized monmorilonite which dispersion of clay samples.