Geology and surface hydrothermal alteration of Malabar area, northern part of the Wayang Windu Geothermal Field, Indonesia

The Wayang Windu Geothermal Field is situated in the West Java province of Indonesia. Geological field surveys were conducted in the Malabar area of the field to characterize distribution of rock units, geological structures, hydrothermal alteration and geothermal manifestations. Geological features and surface hydrothermal manifestations could indicate an extension of geothermal potential in northward from the developed area. Rock samples were collected from surface and subjected to petrographic observations and X-ray diffraction analyses. The volcanic stratigraphy consists of four units, from oldest to youngest. There are: Pangalengan Volcanogenic Deposits Unit, Old Malabar Volcanic Unit, Young Malabar Volcanic Unit, and Andesite Intrusion Unit. Geologic structures developed in the Malabar area include brecciation, fractured zone, normal fault and dextral strike-slip faults trending NE and NW. Geothermal manifestations include fumaroles at Kawah Burung-1 and Kawah Burung-2, hotsprings at Kawah Burung-3, H ₂S gas emissions and altered ground. The hydrothermal alteration is spatially associated with faults and fractures. The alteration is characterized by the occurrence of secondary cristobalite, tridymite, quartz, alunite, natroalunite, montmorilonite, halloysite, kaolinite, pyrophyllite, muscovite, calcite, chlorite, epidote, zeolite, pyrite, and iron oxide.