Gunung Endut located in Lebak, Banten is one of the prospected areas of geothermal energy resource on the island of Java, Indonesia. The presence of clay mineral in the surface can help to determine the hydrothermal fluid temperature circulating in Gunung Endut. Based on the collected samples, petrography analysis revealed the presence of clay minerals while diffraction method confirmed the presence of dickite, montmorillonite, kaolinite, pyrophyllite, chlorite, diaspore and illite. There are three different temperature range from each volcanic facies. It is predicted that clay minerals at their temperature range in Gunung Endut are formed based on the type of rocks and the topography of the research area. © 2021 Journal of Physics: Conference Series.