Origin and Hydrogeochemical formation processes of Geothermal fluids from the Kibiro area in Hoima District, Western Uganda.

Abstract

The Kibiro geothermal field, located in the western branch of the East African Rift System, has many geothermal surface manifestations. Geochemical studies on the classification of the geothermal water, evaluation of thermal mixing with cold groundwater, reservoir temperature assessments, and identification of high permeability zones have already been conducted in this area. However, origin of the geothermal fluids and the hydrogeochemical formation processes that control the fluid chemistry 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 gases, main dissolved components and the geochemical processes involved in the formation of the geothermal fluid. Methods applied in the analysis of the origin and hydrogeochemical formation processes of the 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. Based on δ18O and δ2H values, the geothermal waters are found to be of meteoric origin and may probably be mixed with paleo-waters. The gas isotopes analysis indicates that 97.5% of Helium with isotope ratio of 0.217 Ra mainly originates from crustal sources and 2.47% is derived from the mantle. CO2 mainly originates from marine limestone and sedimentary organic carbon and there is negligible input from the mantle. The low contribution from the mantle suggests that there are negligible inputs of volatiles from a magma chamber below the Kibiro geothermal field. The Kibiro hot springs are high salinity Na-Cl waters. The hydrogeochemical assessments indicate that Na+ and Cl- are derived from halite dissolution which occurs within the sedimentary rocks. 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 hydrogeochemical processes 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. The circulation depth of the geothermal water was calculated based on the estimated reservoir temperature, annual surface temperature, and the geothermal gradient. It was inferred at a depth of 2133 m to 3800 m below ground level.