Origin and hydrogeochemical formation processes of geothermal fluids from the Kibiro area, Western Uganda

Abstract

The Kibiro geothermal field, located in the western branch of the East African Rift Valley System, has many geothermal surface manifestations. Several geochemical research has already been carried out in this area. However, the sources of dissolved ions in geothermal fluids have not been assessed so far. The hydrochemical and gas isotopic characteristics have been examined in this study to identify and determine the sources of the main dissolved components and the geochemical processes involved in the formation of the geothermal fluid. Methods applied in the analysis of the genesis and hydrogeochemical evolution of geothermal fluids include isotope tracers, major ion ratios, graphical methods (stability diagrams, durov diagrams, chloro-alkaline indices, piper diagrams, and HNC-plot), and hydrogeochemical simulations using PHREEQC software. The Kibiro hot springs are high salinity Na–Cl waters. Based on δ18O and δ2H values, the geothermal waters are found to be of meteoric origin. The assessments indicate that Na+ and Cl− are derived from halite dissolution. The dissolution of dolomite, calcite, gypsum, and anhydrite is responsible for the concentration of Ca2+, Mg2+, SO42−, and HCO3−. The main source of K+ is derived from the dissolution of silicate rocks (K-feldspar). The gas isotopes analysis indicates that 97.5% of He with isotope ratio of 0.217 Ra mainly originates from crustal sources and 2.47% is derived from the mantle. 51.39% of CO2 originates from marine limestone, 46.55% from sedimentary organic carbon and 2.06% from the mantle. This evidence further confirms that there are no direct inputs of volatiles from a magma chamber below the Kibiro geothermal field. The evolution of the fluids suggests that most of the common cations and anions found in the Kibiro hot spring waters originated from high-temperature water/rock interaction.