Hydrogeochemistry and Geothermometry of the Hot Springs Along the Magma-poor Western branch of the East African Rift System

We investigated the stable isotope and major ion chemistry and used silica and cation concentrations to reassess the geothermometry from published or reported hot spring data along the Western branch of the East African Rift System (EARS). The Western branch of the EARS is generally considered magma-poor with little evidence of magmatic activity. The presence of potential geothermal energy is inferred from elevated heat flow and the occurrence of hot springs. Our objectives were to (1) investigate spatial variations in the chemical characteristics of hot springs, (2) investigate spatial variations in the hot spring reservoir temperatures, and (3) identify processes that affect hot spring water chemistry during ascent from their geothermal reservoirs. Our results show that all the hot springs plotted along the Global Meteoric Water Line (GMWL); the most negative 18O and 2H hot springs are from Zambia, followed by Malawi, then Rwanda, with the most positive 18O and 2H belonging to hot springs from Uganda. The 18O and 2H differences indicate variations in the sources of the meteoric water that recharges the hot springs. The geochemistry data also demonstrated that the hot springs are mainly Na-K-HCO3, Na-Cl (SO4), and Ca-HCO3 water types controlled by the host rocks' mineralogy. Our findings also indicate that the hot springs undergo mixing with groundwater, water-rock interaction, steam loss, and CO2 outgassing, as the parent geothermal fluid ascends to the surface. The adiabatic quartz silica and Na/K geothermometer estimate reservoir temperatures range from 61-180°C and 93-253 °C, respectively. Based on the reservoir temperature, we conclude that the Western branch is of high to low enthalpy that can be used to generate electricity or for other direct uses.