Isotope Hydrology in the Exploration of the Katwe-Kikorongo, Buranga and Kibiro Geothermal Systems, Uganda

Abstract

Several sampling campaigns have been carried out in three geothermal areas in western Uganda, namely Katwe-Kikorongo (Katwe),Buranga and Kibiro. 118 water samples from hot and cold springs, dug wells, rivers, lakes and 13 rock samples from surface outcrops have been collected and analyzed for chemistry and isotopes of hydrogen (82HH20, 3HH20), oxygen (8180H20, 180S04), c,a l· bo· n (U~ DC Dlt', 14C Die), su I p h ur (~u' 34'S" S04), an d strontI'u(8m7 /86Sr H20, 87786s rRock )' Th e resu It s suggest a meteoric origin of the geothermal water with minimal secondary alteration. Based on isotope data, Katwe and Buranga are recharged from the Rwenzori Mountains while Kibiro is rcdlargcd from high ground represented by the Mukihani-Waisembe Ridge in Kitoba Subcounty 20 km to the southeast. Oxygen isotope geothermometry based on aqueous sulphate and walcr cqui lihriunl fractionation, indicates a subsurface temperature of 200°C for Buranga, which is highcr than that inferred from chemical geothermometry (1 20-1 30°C),_ but lower temperatures (140-160 DC) for Katwe and Kibiro that are similar to the results of chemical geothermometry. Tritium concentrations indicate some involvement of modem cold water close to the surface at Kihiro but 110t at Buranga and Katwe, where hot springs discharge tritium-free waters. Sulphur isutopc ratios (83-lSsnd of hot water suggest magmatic contributions of sulphate in all the three areas, confirming results of earlier geochemical investigations, Strontium isotope ratios in water alllI rock' sump Ic s (:7/::(JS fH2(), X7/X6S rRod. ) a II ow a pre1 1m 1"na'ryd I e ntl' f'l catl. on 0 f roc k types t h at may have I nteracted with the thermal waters.