Origin and hydrogeochemical characteristics of geothermal fluids from the Buranga Area, Western Uganda

Abstract

Buranga area is located in the western branch of the East African Rift System. The geophysical and geological studies conducted in the area suggest that Buranga area is the most suitable area for geothermal development. However, the origin and hydrogeochemical characteristics of the geothermal fluids are poorly understood. The hydrochemical and stable isotope characteristics have been examined in this study to identify and determine the source of the main dissolved components and the hydrogeochemical characteristics of the geothermal fluids. Methods applied in the analysis of the origin and hydrogeochemical characteristics include isotope tracers, major ion ratios, geothermometers and graphical methods as well as hydrogeochemical simulations. Based on the δ18O and δ2H values, geothermal waters were found to be meteoric mixed with palaeo-waters. The gas isotope analysis indicates that CO2 mainly originates from the mantle. Based on the total dissolved solids (TDS) values, the Buranga hot springs are high salinity Na-Cl waters. The hydrogeochemical assessments indicate that Na+ and Cl- are derived from silicate dissolution that occurs within the sedimentary rock formation. The silicate dissolution is responsible for the concentration of Ca2+, Mg2+ and HCO3- while SO42- concentration results from oxidation of inorganic sulphides of volcanic origin. Further, K+ originates from the dissolution of silicate rocks with potassium-bearing minerals such as K-feldspar. The hydrogeochemical processes of the fluids suggest that most of the common cations and anions found in the Buranga hot spring waters originate from high-temperature water/rock interaction processes. The geothermal fluids are in full equilibrium with host rock. Therefore, the estimated reservoir temperature range between 160.8 ℃ to 163.9 ℃ determined using Na-K (1988) geothermometer was considered the most reliable. The reservoir temperatures determined using Quartz-vapour loss range from 116.6 to 125.5 ℃, Quartz-no vapour loss ranges from 118.2 to 128.7 ℃, K-Mg (1988) ranges from 177.0 to 190.7 ℃, Na-K (1979) ranges from 143.1 to 146.5 ℃, Na-K (1988) ranges from 160.8 to 163.9 ℃ and Na-K (1983b) ranges from 145.3 to 148.4 ℃. The inferred estimated circulation depth of the geothermal water is at 2,812 to 2,876 m below ground level.