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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 alteration to the deeper level is supported by a low resistivity of Schlumberger and low frequency MT data. The alteration results from acid sulphate water 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 magmatic 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 discharges 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 minerals.

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. The analyses of Mataloko sur- may be partly derived from weathering as well. face alteration and shallow drilling samples (1 m and 103.3 Shallow drilling samples at Mataloko (Figure 5, overleaf) m depths) show kaolinite, monmorilonite, natro alunite to represents argilitic alteration minerals. They show kaolinite, alunite, alunogen, alpha cristobalite, illite and plagioclase. alunite, plagioclase, quartz to illite at shallow depth (- 70 m The alteration samples of Nage fumarolic and hot spring ar- depth). At deeper levels (70-103.3 m), the minerals are mostly eas show Pyrophilite, quartz, kaolinite, alpha- cristobalite quartz and monmorilonite, indicating a neutral pH fluid influ- and gypsum. ence. Interlayer illite-monmorilonite at the shallow depth (66-70 The Wailuja alteration zone at Mataloko is characterized by m) indicates an increasing temperature to the deeper level. The an SE-NW zone of strong argilitic to advanced argilic alter- presence of these minerals may be as a relict as well, formed at ation (natroalunite, alunite, alunogen, cristobalite and quartz) an early time. The well head temperature and pressure of a flow test are 1 15°C and 3 bars respectively. These data and high Hg soil air content suggest increasing temperature to the deeper level.

...... 0 ...... olnckrdCn(V-om) ...... 6-SBm MuedTMBmda 50 I

Figure 3. Satellite imagery of survey area. Figure 5. Lithologic log of Mataloko shallow drilling.

267 Nasution, et. a/.

Isotopic compositions of ground water and thermal CI discharges are presented in Table 1. The ground or surface cold waters of the Mataloko and Nage areas are close to a local meteoric trend line for this area. These demostrate

Figure 6a. Diagram Cf, S04, HC03 Figure 6b. Diagram Ha, K & Mg Figure 6c. Diagram Cl, Ll and B, ters are derived (Giggenbach, 1968)of Bajawa area. (Giggenbach, 1968) of Bajawa area. (Giggenbach, 1968) of Bajawa area. from meteoric water (Figure 7a). The Nage alteration zone is characterized by NE-SW hot The values of 6 D (H20) and 6 l80(H,O) of hot spring water of springs alignment and silicification-argilitizatjonminerals Mataloko are grater than those of the cold surface water. Some (Pyrophilite, kaolinite, quartz and gypsum) which reflect a thermal discharges also plot close to the meteoric water line strongly acidic solution. Pyrophilite and alpha cristobalite oc- (Figure 7a), indicating a meteoric origin. The positive shift is cur in one rock samples, indicating two periods of hydrothermal processes. Formerly, a high temperature acid fluid process yielded pyrophilite, followed then by a lower temperature acid ~uidprocess, producing alpha c~s~~balite. Figure 7a. The~oluminecen~dating of quartz from Wailuja and Nage Relatjonship alteration gives ages of 0.087 Ma and less than Ma, respec- between 6D 0.2 and iS80 (Hot tively. These probably indicate the thermal history of the and surface preliminary Wailuja and Nage fracture zones. Therefore, high water). subsurface temperatures of the hydrothermal system probably still exist.

Water C~e~istry

10 Chemical analyses of thermal discharges are listed in Table 1. 12~m u Generally, They show high sulfate and low chloride, sodium, and calcium contents, indicating a sulphate-type water (Figure 6a). The high sulphate suggests that volcanic gases, particularly H2S, Figure 7b. Relationship oxidize close to the surface, influencing the shallow ground wa- between CI and 634S(So4). ter composition. The water chemistry suggests immature water beneath the survey area (Figure 6b), and strong mixing with shal- 0 low ground water. Low CUB and BlCl ratios Figure 6c) are consistent with the system being hosted by andesitic rocks. -2 The chemical concentrations of Mataloko and Nage hot springs are different. The former has low chloride, boron, fluoride, arsenic and bromide contents, probably indicating a neutral pH water flowing through volcanic terrain and interacting with shallow ground water. The latter, however, is a sulphate-chloride water with high chloride, boron, fluoride, arsenic and bromide concentrations, presumably the result of volcanic gases Figure 7c. mixing with brine water and shallow ground water. B versus Cf diagram on hot spring water. isotopic Composition of Fluid Oxygen-18 and deuterium (D) contents are an indicator of I fluid origin and the degree water-rock interaction (Craig, 1963).

268 Nasution, et. a/.

considered to be the re- Table 1. Chemical Analyses of Hot Spring Water sult of steam loss. The Mambkoa Kdl.2 Nage hot springs shift MaWOk0-1 14 3.6 3.0 -7,4 slightly toward greater 248 49.4 189 108 values, probably indi- S1,O m aM 0.SW 1930 520 1240 841 cating a partial mixing 27 15,e 627 945 with magmatic water. 48m2 4.62 a7 47 6 34S (SO,) values OPZr3 OW2 40289 a- 59.3 91.3 127 and C1 concentrations 30,3 19,3 47,b 158 (Figure 7b) of the 45 248 am Mataloko hot springs 19 1,a q- 2.91 zm 4084 36.6 are relatively low (of - e28 2lO 680 158 1.6 to 2.5 O/oo ) and ex- n.d n.d nd 404 n.d n.d n.d nd tremely low (e3 mg/L) 0.271 0,129 0.74 0.158 respectively. The air am3 0,0247 0,518 am 0.00242 n.d oms11 0.m show an H2S smell and 296 179 154 Ids independent on SO4 7s 4-2 n.d n.d concentration. The low n.d n.d 0,146 411s -14 44 38 41 values of 6 34S (SO,) 1.5 47 a,o -7,2 and chloride suggest -2.5 -2.1 4.8 0 that the gases are not derived from magmatic sources, but are prob- Table 2. Chemical Composition of Surface Water. ably derived from a panmetan esibwa Mulrulblclr WaeRhha Matabld Mll$b(lD3 Wee RMh sola deeper geothermal aqui- 7 7~ 7 7,2 7 2,s 7,2 rain wit cowlprim ==ww PtRem a-tm msprina 8bWm fer. It is considered that 1220 1260 1390 loo0 1150 1050 110 this geothermal brine in 270 6 20 40 0 5 the deeper reservoir will 21,l 18.1 21.4 21,7 23,8 24 27.3 20 19,2 19,8 22,l 25,3 22.4 27,8 be a chloride type with 3,18 18,2 9,47 19,9 183 neutral pH of a general 6.2 383 -38.8 -30 -37.4 -34.3 -21,5 geothermal system (eg. -33 -7 -7.2 46 6,7 6.5 4 Hachobaru, Japan; Ulumbu and Salak, In- donesia). The up flow of the brine water to the shallower aquifer (Figure 7c, Pasternack and Varecamp, 1994). This indicates that will be prevented by a sealing zone above the reservoir. Judg- magmatic gases might contribute to the Nage geothermal sys- ing from the high temperatureof hot springs (nearly the boiling tem. The values of 6 34S (SO,) of the Nage hot spring are point), the reservoir probably has considerablepotential. relatively high, 9.9 to 11.1 O/oo (Figure 7a) and they increase The Nage hot springs are acid, show high C1 concentrations with increasing SO, concentration. These high values of 6 34s (Table 1) and are classified into sulfate- chloride type water (SO,), together with the high fluoride, bromide and arsen con- (Figure 6a). The SO&l ratios of Nage (about 1) are very close tents (Table 1) perhaps indicating a contribution magmatic SO, to those of the acid crater lake water from Kelimutu, Ende to the hot spring aquifer.

Elevation (ma.s.1) Elevation (m a.s.1)

Figure 8a. SD versus Elevation Figure 8b. Relationship between Figure 8c. Relationship between 6 l80and Diagram on Hot Spring Water. SD and elevation (surface water). elevation (Surface water) (From NEDO, 1998).

269 Nasution, et. a/.

Is0 tope Hydrology Discussion Meteoric water from the recharge area will penetrate into The geological, geochemical and shallow rock data of the geothermal reservoir, acquiring heat and dissolved solids. Mataloko suggest that shallow alteration is affected by gases Giggenbach (1 978) used stable isotope data from meteoric and from steam condensate, especially H2S which oxidized closed geothermal water to locate recharge areas. He contented that to the surface. These gases dissolve primary minerals, forming geothermal water and recharging meteoric water have similar clay minerals (eq.monmorilonite, kaolinite, illite, and alunite) 6D values, since rock contains little exchangeable hydrogen to and indicate that fluid pH gradually decreased from cause a D shift. Consequently, unchanged 6 D value define the monmorillonite through kaolinite to alunite zone. The ages of recharge areas of a geothermal system. argilitization zone are less than 0.2 Ma, probably correlated with Isotopic compositions of ground meteoric water (Table 2) the formation of young fractures.The geothermal heat source is are influenced by latitude, altitude, distance from the sea and presumably associated with several inactive young volcanic season (wet or dry). In the survey area, they are affected by cones (Qvc). However, crystallization magma beneath Mataloko altitude and steep topography. The isotope data of local surface area (for example Mataloko andesites, Qma) is a heat contribu- water is represented by the regression function: 6 D = 8.58 6 I8O tor of geothermal system as well. + 19.8. Mataloko geothermal brine is derived from meteoric water. The altitude effects on 6 D (H20) and 6 I8O (H,O) are It has low values of 6 34S (SO,), and 6 13C (CO,) - 3.6 and - shown on Figure 8a, b and Figure 8c respectively. 6D and 6 3.9 %O (NED0,2000), indicating that they are not derived from I8O values of cold surface water decrease at the rate of 98 "/ magmatic sources. They appear to be derived from deeper res- 00 and of 0.14%0 respectively, per 100 m ascending altitude. ervoir which has temperature f 242 - 283°C. A neutral pH The distribution of 6D in cold surface water is estimated as reservoir water (?) is indicated by the secondary monmorilonite shown in Figure 8b. It is likely that the origin of geothermal and quartz at the shallow level (70-103 m depth). Therefore, fluids is meteoric water falling to the ground at altitude over the Mataloko reservoir probably has high potential for a geo- 1400 m. The local faults fractures, assist penetration of mete- thermal development. oric water. Nage geothermal brine is derived from meteoric water as In addition, gas concentrations from Mataloko and Nage are well. Chemically, it has high chloride, boron, fluoride, arsenic shown on Table 3. They show high CO,/H,S, CO,/H, and N2/Ar and bromide concentrations. Values of 6 34S(SO,) are relatively ratios, consistent with high temperature fluids traveling rapidly high ( 9.9 to 11.1 %o) and they increase with increasing SO, from the source before condensing in the upper part of the sys- concentration, probably indicating a contribution of SO, and tem or shallow ground water. By using gas geothermometry, partial mixing of magmatic fluids into the hot water aquifer. both areas indicate high underground temperatures, the former These strongly acidic solution altered the surrounding rocks to being - 242-283°C while the Nage hot springs show - 204°C form secondary minerals at Nage, which are characterized by (Table 1). silicification-argilitization (pyrophyllite, quartz, and gypsum). The NE-SW trending alteration zones are associated with the NE-SW fractures or faults. In addition, average alteration ages (Thermoluminecence dating) are Table 3. Result of Chemical and Isotopic Analyses of Fumarolic Gas. less than 0.2 Ma, suggesting an early thermal history of geothermal area. No. 0-1 G2 63 AREA Matakko MOtalOkO Naw WE hWOkO.Gl Mat8kkoG2 NaiP DEscAlpTKIN mfflnggas Conclusion DATE 7EEstF 1eBame 7w DISCHAROE-TEMP. Oc gexI 95.8 71.B QAscUUmJl Vd 1.36 0.96 100 The data presented here suggest that the Mataloko GAscoMeosm cot Vd% 90.5 92.0 79.1 area is a significant geothermal prospect. Crystalliza- H2s Vd% 3.40 4.78 052 RddUdGRs Vd% 3.15 322 20.4 tion of andesitic to basaltic magma supply a heat source REslDuu GAS CoMmsrrIoN that heats deep meteoric water. The up-flows of sul- N1 Vd% =a 73.0 85.5 H1 Vd% 2.80 17.5 0.008 phate type water at the surface, indicate a high w Vd% 112 8.20 n.d. sub-surface fluid temperature (242-283°C). The geo- o? Vd% 0.05 nd. 3.00 k Vd% 0.36 0.31 1A7 thermal water has low values of 6 34 S (S04)and ).lr Vd% 0.061 1 0- 0.- ow031 0.- O.Wla7 chloride, indicating that it is derived from a deeper b :$ 6.k.M 6.W.W 4.9ao.04 reservoir of neutral pH, high temperature chloride mw % -3.6 9.8 4.5 ww % -27.7 -8.0 nd. fluid. The deep hydrothermal fluids interact with sur- a00 % 482 683 n.d. rounding rocks to form secondary minerals on the eo(w % -1 89 -182 nA. ~ysw) % -2.8 -2.7 -10.3 surface and at shallow level in the subsurface. % 97 80 % SA -105 The Nage hot springs are sulphate-chloride type 2Z2mabJ Men 028 water, deriving from meteoric water. Their recharge F in arndenslte MOn 4.05 n.d. :notdatannld due to lowumanmbn dCH4. H2 artd HZS (From Wo, 1999) area is located north of Nage and Bobo at an altitude

2 70 Nasution, et. a/. over 1400 rn. The high values of 6 34S(SO,) indicate a partial Hemley,J.J, and Jones,W.R., 1964. Chemical Aspects of Hydrothermal Al- teration with Emphasis on hydrogen metasomatism. Economic Geol- mixing of magmatic fluid into the hot water aquifer at tempera- ogy, 59,538-569. ture - 204°C. The Nage geothermal area represents argilitic and silicified rocks. Tbo periods of hydrothermal processes occurred Hedenquist, J.W. 1998. Hydrothermal System in Volcanic Arc, Origin of there, formerly, a high temperature acid fluid yielded Pyrophilite, and Exploration for Epithermal Gold Deposits. Lecture Notes, followed then by a lower temperature acid fluid, producing al- Bandung, Indonesia (Unpublished report). pha cristobalite. Katili, J.A., 973, Geochronologyof West Indonesia and its Implication on The interlayer of illite-monmorilonite at the shallow depth, Plate Tectonics, Tectonophysics, 19, 195-212. high values of Hg soil air concentrations and gas Koesoemadinata, S., Y. Noya and D. Kadarisman., 198 1. Preliminary Geo- geothermometry of Mataloko fumaroles indicate an increasing logical Map of Ruteng Quadrangle, Nusa Tenggara. Scale 1 :250,000. temperature to deeper level. The well head temperature and pres- Geological Research and Development Centre. Indonesia sure flow test of steam represent 115OC and 3 bars respectively Muchsin, M.C. 1975, Tnventarisasi dan penyelidikan pendahuluan terhadap support the existence of a viable prospect. Therefore, the gejala panasbumi di daerah Flores. Direktorat Geologi Bandung, In- Mataloko reservoir is probably to be consider to have as a higher donesia, unpublished report. potential for development of a small scale geothermal power Muraoka,H., Nasution, A., Urai, M. and Takahashi, M., 1998. A Start of plant to support rural electrification of the district. Geothermal the “Research Cooperation Project on Exploration of Small scale brine of the Nage deeper reservoir may be strongly acid with Geothermal Resources in Remote Islands in Indonesia”. Chishitsu significance magmatic contribution. (Geological ) News, No.52 I ,34-48 Muraoka, H., Nasution, A., Urai,M. and Takashima, I., 1999. Regional Acknowledgments Geothermal Geology of the Ngada Distric, Central Flores, Indonesia. In Muraoka, H. and Uchida, T (eds) 1998 Intrim Rept., “Research We are gratefully acknowledgethe supporting data from GSJ Cooperation Project on Exploration of Small scale Geothermal Re- and NED0 to complete the manuscripts for attending “the GRC sources in Eastern Part of Indonesia”, Geological Survey of Japan. p. 17-46. 2000 Annual Meeting,” September 24 - 27, Burlingame, Cali- fornia USA. Thanks for helping prepare this paper to Geothermal Nasution, A, D.aswin. 1996. Prospect of Flores Geothermal Field, East Division Scientists of VSI. Nusa Tenggara Viewed From Its Volcanism and Hotwater Geochemis- try. Proceeding of The 1st Indonesian Geothermal Association An- nual Convention. U.Sumortarto, T.Silitonga, .P. Atmojo and B. References Hutabarat (Eds.) p. 133- 148. Brown, P.R.L., 1978. Hydrothermal alteration in active geothermal fields. Nasution, A, H. Muraoka, 1. Takashima, M. Takahashi, Y. Okubo, H. Annual Reviews of Earth and Planetary Science, 6,229-250. Takahashi, T. Uchida, A. Andan, H. Akasako, K. Matsuda, Nanlohi, D. Kusnadi, B. Sulaiman, N.Zulkamain, 1999. Preliminary Survey of Brown, P.R.L., 1993. Hydrothermal Alteration and Geothermal Systems. Bajawa Geothermal Area, Ngada District, Flores, East Nusa Tenggara, Lecture Notes 86.102. Geothermal Tnsti tute, Auckland University (Un- Indonesia. Geothermal Resources Council Transactions, Vol. 23, Oc- published report). tober 17-20, 1999, p. 467- 472. D’amore, F. and Panichi, C., 1980. Evaluation of deep temperatures of Silver, E. and More, J.C. , 1981. The Molucca sea collision zone, in The hydrothermal systems by a new gas geothermometer. Geology and Tectonics of Eastern Indonesia, Geological Research Geochim.Cosmochim.Acta,44: 549-556. and Development Centre, Spec. Publ.,2. Giggenbach, W. F. 1988. Geothermal Solut Equilibria Deviation of Na-K- Mg-Ca Geoindicators, Geochim. et Cosmochim. Acta.52. p. 2749-2765. Sumi, K., 1968. Hydrothermal rock alteration of the Matsukawa geothermal area, Northeast Japan, Geological Survey of Japan Report 225. Giggenbach, W.F., 1992. Isotopic shifts in waters from geothermal and volcanic systems along convergent plate boundaries and their origin. Taylor, H.P. Jr. 1979. Oxygen and Hydrogen isotope relationships in hy- Earth and Planetary Sciences 113,495-510. drothermal mineral deposits. In Hedenquist, J.W. 1998. Hydrother- mal System in Volcanic Arc, Origin of and Exploration for Epithermal Hamilton, W., 1979, Tectonic of the Indonesian Region, United State Gold Deposits. Lecture Notes, Bandung, Indonesia (Unpublished re- Geol.Surv.Prof.Pap., 1078., 345p. port)

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