Proceedings World Geothermal Congress 2015 Melbourne, Australia, 19-25 April 2015 1 Hydrothermal Alteration and Evolution of the Lahendong Geothermal System, North Sulawesi P. Utami 1,3 , D.S.Widarto 2 , J.P. Atmojo 2 , Y. Kamah 2 , P.R.L. Browne 3 , I. W. Warmada 1 , G. Bignall 4 and I. Chambefort 4 1 Geothermal Research Centre and Geological Engineering Dept., Faculty of Engineering, GadjahMada University, Indonesia. 2 Upstream Technology Centre, PT Pertamina, Jakarta 10110, Indonesia 3 The University of Auckland, New Zealand 4 GNS Science, New Zealand p.utami@gadjahmada.edu Keywords:Lahendong, Indonesia, hydrothermal alteration, hydrothermal system evolution ABSTRACT The Lahendong geothermal system (North Sulawesi) is the first geothermal system in Eastern Indonesia to be developed for electricity generation. Itis also the first geothermal system developed in a tectonically-active arc-arc collision setting. The geothermal system is liquid-dominated and located in steep terrain, with thermal manifestations at about 750 m asl. The field is now penetrated by 28 wells drilled to depths ranging from 1500 to 2500 m, with typical measured temperatures of ~250 o C at about -250 mRSL. The reservoir rocks are mostly andesite and rhyolitethat have been radiometrically dated at 2.2 to 0.5 Ma. Typical hydrothermal (secondary) mineral assemblages at Lahendong at -250 mRSL were formed by near-neutral pH fluids, with occurrences of calc-silicates ± secondary feldspars ± chlorite ± illite ± quartz ± calcite ± hematite. Space-fill mineral paragenesis in drillcore and cuttings recovered from selected wells across the field point to at least five alteration stages (hydrothermal events). Chronologically, the earliest stage is characterized by mono-mineralic veins of chlorite, formed when the system was liquid- dominated. Stage 2 is characterized by calcite and chlorite, and Stage 3 by calcite ± quartz. Stage 4 is marked by calc-silicates ± alkali feldspar, and is inferred to record thepeak intensity of hydrothermal activity. Stage 5 is characterized by late-stage calcite, quartz and hematite, with anhydrite conspicuous in the central part of the system, where brecciated veins are common. Mineralogical evidence has helped resolve the geological evolution of the Lahendong area and highlights thermal and hydrological changes in the system, marked by episodic heating, boiling and cooling, and local fluid mixing in the presently active system (and in inactive/fossil parts of it in the past). Stable isotope analysis of hydrothermal minerals indicates an influx of magmatic fluid at some time into the central part of the system. The Lahendong geothermal system has experienced waxing/waning and relocation of thermal foci, due to sealing and rejuvenation of permeability (e.g. by fault-reactivation) in a long-lived system. These changes may have resulted from a migration of the heat source. 1. INTRODUCTION Lahendong is a water-dominated geothermal field located in North Sulawesi, Indonesia, approximately 30 km south of Manado, the Capital City of North Sulawesi Province. Two other geothermal fields in this province are Tompaso and Kotamobagu (Figure 1). Lahendong is the first geothermal field in eastern Indonesia to generate electricity and has been producing since 2001.Up to May 2014, 28 wells had been drilled to depths ranging from 1500 – 2500 m, and the field is currently producing 60 MWe. The steam field is managed by PT. Pertamina Geothermal Energy (PGE) and the electricity generation by PT. PLN (State Electricity Enterprise). This paper describes the subsurface hydrothermal alteration of the system, including its mineral parageneses,andreports its spatial and temporal hydrological changes. 2. GEOLOGIC SETTING 2.1 Field overview Lahendong is part of the Sangihe volcanic arc. It is the only example of an active geothermal system located within an arc-arc collision zone. Here the Sangihe arc over rides the Halmahera arc. The geothermal system is situated about 10 km west of Tondano, a lake which occupies the structure referred to by Newhall and Dzurisin (1998) as the Tondano caldera (20 X 10 km 2 ), of inferred Quaternary age. The active Lokon and Soputan volcanoes are 9 km northwest and 20 km southwest respectively of the system (in Figure 1).