Identification of water–rock interaction of surface thermal water in Songwe medium temperature geothermal area, Tanzania

Abstract The Songwe geothermal prospect is situated in western Tanzania the Rukwa Rift of branch East African System. Thermal springs discharge along NW–SE oriented fracture zones two separate areas: main graben (Iyola, Main springs, Rambo and Kaguri) eastern (Ikumbi). Lithologies forming filling are metamorphic gneiss shist as basement rocks, overlain by Karoo sandstones, Red both silt- sandstones with a carbonatic matrix. In some areas graben, volcanic rocks intruded these formations basalt outflows. temperatures between 37 85 °C Na-HCO 3 type fluids. Carbonate deposits surround most springs. Using previous geophysical, geological studies historical fluid geochemical data mineral data, system interpretation was updated, including new reservoir temperature, flow pathway water–rock interaction models. classical geothermometers K-Mg Na-K-Ca (Mg correction) were used to predict temperature show that emerging area reaches 125 148 °C. Reservoir characteristics reconstructed based on geothermometer calculation PHREEQC model which deep reacts certain lithologies. Minerals precipitating at surface depth calibrate models run calibrated minerals around suggest thermal fluids interact sandstone only, while Ikumbi spring water only interacts all lithologies (simplified referred as: sandstone). indicates rising also contact composition again influenced graben. Our conceptual summarizes showing meteoric origin fluids, travel through basement, Mbeya fault reaction lateral towards hot proposed reinforce idea carbonate dissolution from layers common interaction. supported deposition observed dominated HCO Na.