Geology and Geothermal Setting of Tompaso Geothermal System, Indonesia, with Comparisons of Andesite Alteration Patterns with Wairakei, New Zealand By Kartika Palupi Savitri A thesis submitted to Victoria University of Wellington in partial fulfillment of requirements for the degree of Master of Science in Geology School of Geography, Environment and Earth Sciences Victoria University of Wellington 2016 ii Abstract Tompaso geothermal system is a typical volcanic arc geothermal system in North Sulawesi, Indonesia. Although situated close to the Tondano caldera, subsurface lithologies and structures do not show any evidence for caldera-related features and the system is inferred to be related to the andesitic Soputan volcano. The subsurface geology of Tompaso consists of Tuff B unit, Rhyolite unit, Andesite B unit, Pitchstone unit, Pyroclastic Breccia unit,Andesite A unit, Pumice unit, and Tuff A unit, respectively, from the oldest penetrated unit. The silicic Pitchstone and Rhyolite units are presumed to be sourced from the same magma chamber. Petrological and mineralogical observations using binocular and petrographic microscopy, short-wave infrared (SWIR) analysis, and back-scattered electron (BSE) imaging combined with energy dispersive X-ray spectroscopy (EDS) have been applied to cuttings and limited core material from three boreholes: LHD-26, LHD-27, and LHD-32. Age dating has not been undertaken and, thus, conclusions on correlations between subsurface geology inferred here with surface formation groupings from previous works cannot be drawn. Tompaso geothermal system is characterised primarily by variations in the fracturing within the reservoir. Secondary mineralogy and the structure of present-day temperature of the system suggest that the movement of hydrothermal fluids at Tompaso is controlled by faults: the Soputan, Tempang, and A-A’ faults, the last defined for the first time in this thesis. Soputan Fault controls the outflow of the system. On the other hand, the influence of Tempang and A-A’ faults is dominant only in the upper portion of the system. The A-A’ fault likely acts as a channel for cooler meteoric surface water, while the Tempang Fault is inferred to have experienced self-sealing and appears to be an impermeable structure in the system. The self-sealing process of the Tempang Fault and/or the introduction of meteoric water through the A-A’ fault may be related to the cooling of the northern and western part of the system. The challenges in identifying protoliths in active geothermal areas is addressed here through studies of the influence of andesite textures on the preferences of hydrothermal alteration processes. Wairakei andesites were chosen for comparison to Tompaso andesites, especially because of its different geological setting and geothermal reservoir structure. The results suggest that mineral composition and arrangement affect the preference of hydrothermal alteration on andesites. iii iv Acknowledgement Firstly, I am grateful for Geothermal Research Centre, Universitas Gadjah Mada (UGM), for encouraging and giving me a recommendation and full support for me to pursue my Master’s study. Thanks are also expressed to Faculty of Engineering (UGM), Geological Agency of Indonesia, Indonesian Society of Economic Geologists and GNS Science of New Zealand for giving me recommendations, without which I would not have received my scholarship. The Ministry of Foreign Affairs and Trade (MFAT) of New Zealand funded my study in Victoria University of Wellington (VUW) through a New Zealand ASEAN Scholars (NZAS) Award in 2013-2016. I would like to thank my primary supervisor, Prof. Colin Wilson. I am privileged to get a chance to be guided by and to have learned from you. You have taught me not only geology and volcanology, but also how to be a researcher and a teacher. It was a pleasure to work with you. I especially thank my co-supervisor Dr. Isabelle Chambefort for random questions and discussion about petrology, mineralogy, and geothermal systems. I also thank my co- supervisor Dr. Sarah Milicich for discussions about geology and geothermal systems and helping me with Leapfrog Geothermal software. Thanks also go to my third co-supervisor, Dr. Greg Bignall, for helping me during the initial phase of my research. I also thank you for flying to Indonesia to help me with permit application for the use of my samples. PT Petamina Geothermal Energy (PGE) has given me access to Tompaso Geothermal Field and permission to use its core, cuttings, and other relevant data. Messrs. Imam Rahardjo, Eben Siahaan, Imam Prasetyo, Yustin Kamah, and Sardiyanto and Ms. Vivi Nusantara have been the key persons of this substantial process. The Directorate General of New Energy and Renewable Energy Conservation as part of the Ministry of Energy and Mineral Resources of Indonesia has approved the usage of Tompaso samples and made this work possible. I would like to thank Contact Energy along with GNS Science for giving me access and permission to use cores and cuttings from Wairakei Geothermal Field. Thanks to Mark Simpson for teaching me the Short Wave Infrared (SWIR) analysis as well as GNS Science for giving access to the instrument as well as its related software. Thanks also go to Fiona Sanders for the helpful discussions on Wairakei. David Flynn through the School of Chemistry and Physical Sciences VUW gave me access to and trained me to use the Scanning Electron Microscopy and Energy Dispersive X-Ray (SEM-EDS) system: your help was invaluable. I thank Stewart Bush for undertaking the polished thin section preparation v and helping me polishing grain mounts. I would also like to thank Ian Schipper for undertaking probe analysis on the pitchstone samples. I would like to acknowledge the support from ARANZ Geo Ltd. for granting me a year licence to use Leapfrog Geothermal software which made this thesis work possible. Thanks also go to AusSpec International for supporting the TSG Viewer software for SWIR analysis. I appreciate all the Minahasan people at Tompaso for allowing me to access their land and giving me helpful insights during my fieldwork. I thank the Geothermal Research Centre for facilitating all the fieldwork instruments. Thanks also go to Mr. Tondo Wicaksono for your assistance during sample collection in Kamojang core shed. I thank Andrea Blair and Mauro Passarella for letting me stay in their lovely house during my lab work in Wairakei. I would also like to thank my ISO, Helena Cook, for always be my ear for the last two years. Thanks to Lorena, Sandra, Pete, and Ti for being such lovely office mates. Jill Fernandes, thank you for being a fantastic study-mate and flatmate. Thank you for all Indonesian friends in New Zealand for always providing a heart-warming place I could call home. Malia, Esti, Lina, Dika, Amalia, and many other friends living 7000 km away, thanks for always willing to pick up my calls and reply my messages even at the most ridiculous times, day and night; thank you for random questions and discussions and specially thank you for keeping me from falling when the road was not smooth. I am grateful for my mentor, Mrs. Pri Utami, for putting your trust in me and helping me gain my self-esteem back when I need it the most. Last but not least, to my mother, my father and my brother, thank you for your endless love, encouragement, and support. My father especially; thanks for always being my geology partner and accompanying me doing fieldwork. My dear partner Adityo; thanks for being a source of spirit boost, inspiration, and motivation. I could never have done it without you! vi Contents Abstract .......................................................................................................................... iii Acknowledgement ................................................................................................................... v Contents ......................................................................................................................... vii Figures .......................................................................................................................... ix Tables .......................................................................................................................... xi CHAPTER 1 INTRODUCTION ............................................................................................... 1 1.1. Background .............................................................................................................. 1 1.2. Study Locations ........................................................................................................ 2 1.3. Thesis Aims ............................................................................................................... 4 1.4. Thesis Roadmap ....................................................................................................... 4 1.5. Limitations to this research ...................................................................................... 6 1.6. Thesis Outline ........................................................................................................... 7 CHAPTER 2 GEOLOGICAL SETTING ................................................................................... 11 2.1. Tompaso Geothermal System ................................................................................ 11 2.1.1. Tectonic Framework .................................................................................. 11 2.1.2. Volcanic Framework of Tompaso