Characteristics of Geothermal manifestations in/around the Buyan-Bratan caldera, central Bali, Indonesia.

Abstract

Geothermal survey of hot springs and hydrothermal alteration were conducted in/around the Buyan-Bratan caldera, central part of Bali, Indonesia. The eight hot spring samples and three river water samples were collected mainly from the southern flank of the caldera, ranging from 320 m to 890 m in elevation. The measured temperature of hot springs is 37 to 52°C, and pH is almost neutral at 6.17 to 6.84. The hot springs are dominant in HCO3 ranging from 480 to 2250 mg/L. On the southern flanks, the chemical composition of hot springs is systematically changing with decreasing elevations from SO4-HCO3 and HCO3 to Cl-HCO3 types. These waters are classified as immature waters on a Na-K-Mg diagram. Isotopic data of hot springs and river waters are plotted along the meteoric water line.
Hydrothermal altered zone in Teratai Bang is only the site of gas discharge in/around the Buyan-Bratan caldera, but it is cold and, composed of amorphous silica and/or cristobalite. These rocks are also sometimes associated with native sulfur. This means that at Teratai Bang silicification occurred with very low pH solutions. Probably once volcanic fluid had acted, and resulted in silicified rocks. Here is the only remnant of past high temperature solfatara activities in the Buyan-Bratan caldera.