Water-Rock Interactions in the Metamorphic Complex of Geothermal Field: Case Study in Wahlua Complex, Buru Island

Abstract

In Indonesia, surface thermal manifestation is commonly found in a volcanic geological setting. However, hot springs and alteration minerals in study area occur on metamorphic rock. In order to study its correlation with the emergence of the thermal fluid, the objective of this study is to characterize each metamorphic zone based on rock chemistry and the appearance of index minerals. Consequently, the correlation with thermal fluid chemistry can be observed. Data for this study are surface rock samples and thermal manifestation data. Metamorphic zones were determined by integrating geomorphology, petrography, and X-Ray Fluorescence (XRF) analyses. Three metamorphic zones were identified, namely, Garnet Zone, Biotite Zone, and Chlorite Zone. Garnet Zone is characterized by high concentrations of CaO and Al2O3 due to the presence of garnet. The abundance of biotite and muscovite in Biotite Zone may contribute to the highest concentration of K2O than the other two zones. Meanwhile, Chlorite Zone is characterized by the concentrations of SiO2, Fe2O3, and MgO. However, the composition of MgO might be influenced by mineral-fluid reaction. There are two clusters of thermal manifestation, which are western and eastern cluster manifestations. Western cluster manifestations are in the Biotite Zone. Meanwhile, the eastern cluster manifestation is in the Chlorite Zone. However, both clusters’ fluid chemistry shows significant differences in some elements such as SiO2, Na, K, Fe, and Mg concentration. It might suggest that the enrichment and depletion of these constituents in thermal fluid might correlate with each metamorphic zone’s characteristics. An inconsistent pattern between thermal fluid composition and rock chemistry was found in Mg concentration. It is most likely that the Mg concentration was influenced by groundwater dilution rather than the mineral-fluid interaction. The study concludes that the appearance of index minerals shows consistencies with composition variation of some elements in the manifestation fluid, such as SiO2, Na, K, and Fe.