Conceptual Model of Outflow Zone in Mount Ungaran Geothermal System, Semarang, Central Java Using Geology and Geochemistry of Manifestation Analysis

Abstract

This research is conducted in the outflow zone of Mount Ungaran geothermal system, Semarang, Central Java, Indonesia, i.e. Kendalisodo and surrounding which is located in the eastern and southeastern part of Mount Ungaran. The purpose of the study is to build the conceptual model of the Kendalisodo geothermal system. Methods used comprise of literature study, geological traverse mapping, water sampling, and geochemical study for anions, cations, gas, and stable isotopes ?18O, ?2H, and ?13C analysis. Geothermal surface manifestation in the study area includes Kendalisodo, Diwak, Derekan, and Kaliulo warm springs; with travertine and salt deposits in some areas. Fumarole and hot springs appear in the upflow zone of Gedongsongo. In the study area, there are two geothermal systems, i.e. Kendalisodo and Diwak system. These two systems are separated by the Old Ungaran Caldera. Both systems are non-volcanic and medium enthalpy systems. The Kendalisodo reservoir has a temperature of 170 ± 10oC. The hydrothermal fluid of the Kendalisodo system originates from meteoric water catchment on the southeast slope of Mount Ungaran; the meteoric water is heated up by the remaining heat of Mount Kendalisodo intrusion. The fluid has experienced mixing with cold water before discharge in the upflow zone at the Kendalisodo area. The Kendalisodo geothermal system, geologically and geochemically, is different from the main geothermal system of Mount Ungaran (Gedongsongo). The Diwak geothermal system has a reservoir with a temperature of 160 ± 10oC. The hydrothermal fluid of the Diwak system is also from meteoric water, which is heated up by high heat-flow and/or overpressure system. The fluid then flows upward through the Derekan Fault and discharges in the upflow zone as the Diwak and Derekan warm springs. The fluid also flows laterally through the Derekan Fault and mixed with formation water to form Kaliulo warm spring in the outflow zone.