Study of altered lithologies in Gunung Lawu, Central Java, Indonesia using petrographic analysis

Gunung Lawu is one of the areas that have geothermal potential in Indonesia. One way to find out if an area has a geothermal potential is to know the alteration that occurs in the rocks in that area. Alteration is a very complicated process that involves changes in mineralogy, chemistry, and texture caused by the interaction of hot fluids with the rock in its path. This research aims to identify mineral and intensity of alteration in the research area with petrographic analysis and hand specimen description. The petrographic analysis method is used to identify unaltered or altered minerals present in the sample rocks of the study area. In addition to using petrographic analysis, it is also seen through hand specimen descriptions and also outcrops to see the lithology of rocks in the research area. From the sample of this research, it can be classified by three categories. There are unaltered rocks, weak intensity alteration and strong intensity alteration. Unaltered rocks were found in SLM and BNA, weak intensity alteration was found in NGY 1 and strong intensity alteration was found in LWU. In NGY 1 sample, chlorite alteration mineral was found, while in LWU, goethite alteration mineral was found. The occurrence of alteration in the area may be due to the area being passed by faults which interact with hydrothermal fluid. These faults also become the fluid pathway for the geothermal reservoir. The heat from the reservoir is able to change the mineral composition of the rock. In LWU, a fault is passed which produces fumarole manifestations, and NGY 1 is passed by a fault that forms hot spring manifestations.