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The Chemistry Characteristics of Pamancalan Hot Springs, Lebak Regency, Banten Province, Java, Indonesia
The Pamancalan area is located at Lebak regency, Banten province and on a subduction pathway in the southern part of the magmatic arc of West Java in the tectonic setting of Western Indonesia. The geology of area is volcanic, intrution, and sedimentary rock mostly. The eruption sources consist of Bayah Tua, Pasir Reungit, G. Gede, G. Batu and Bantar Gadung products with andesite - basaltic types. The results of rock age dating in diorite breakthrough rocks are 1.2 ± 0.1 million years or during the Pleistocene. The heat source is estimated to be related to intrution activities in Diorite rocks around Mount Hanjawar (Plistocene). The Cipamancalan Fault (U-S) which is cut by the Warung Banten Fault (B-L) facilitates the emergence of Cipamancalan hot springs. Geothermal manifestations in the study area are grouped into 3 groups, which consist of Cipamancalan hot springs, Citando (outside the area) and altered rocks. The hot springs are having the temperature about 61-72oC, clustered into sulphate-bicarbonate type, immature water, as an indication of manifestations that appear to the surface in addition to being influenced by the interaction between fluids and rocks in a hot state, also mixes with surface water (meteoric water), and appears in a volcanic environment. The position of hot water on the isotope diagram is located away from the meteoric water line, indicating that Pamancalan hot water comes from a depth. Geothermal fluid with sulfate-bicarbonate type in Pamancalan is an outflow from the Pamancalan geothermal system. The estimated subsurface temperature associated with the geothermal reservoir is 160oC, based on calculations using Na-K geothermometry and is included in the medium enthalpy.