The chemistry of geothermal waters of SW-Uganda

Abstract

The SW-Uganda geothermal areas are located outside the active volcanic zones of the western arm of the East African Rift Valley with varying proximity. In this study, the Cl-SO4-HCO3, Li-Cl-B, and Na-K-Mg ternary diagrams, stable isotopes, silica and cation geothermometers, mixing models and saturation index plots were used to characterize the waters and estimate the subsurface temperature. The results show that most of the waters are in partial equilibrium with the rock but seem to be associated with volcanism. Waters of Ihimbo, Kitagata, and Rubaare are partially equilibrated; Kisiizi, Kagamba and Bubare are immature waters while the rest plot on the boundary between partially equilibrated and immature. It follows then, that the geoindictors can be applied to Ihimbo, Kitagata, and Rubaare with a fairly good degree of confidence. The geothermal waters from Kagamba, Birara, Bubare and Kisiizi are peripheral or mixed with cold groundwater while Rubaare, Ihimbo, Kanyinabarongo, and Kitagata waters are probably volcanic. The rest may be a combination of the two. The waters are meteoric and of local origin, with no apparent δ18O shift. This may indicate that hydrothermal alteration reactions have long ceased; aquifer temperatures are not high; and good permeability exists. From the plausible geothermometers the predicted subsurface temperatures are: 50-60°C for Kisiizi; 50-80°C for Kagamba, Bubare and Kanyinabarongo; 80-100°C for Kiruruma and Ihimbo; 100-120°C for Karungu; 100-130°C for Kitagata, Rubabo 1 and 2, and Minera; and 100-140°C for Rubaare. The SiO2 enthalpy model and solution- mineral equilibria also give a similar range of subsurface temperature predictions.