Circulation process of geothermal fluids and potential assessment of geothermal resources in the Songwe half-graben and Kiejo-Mbaka prospects in southwestern Tanzania: Insight from hydrochemistry and stable isotopes

Abstract

The Songwe and Kiejo-Mbaka geothermal prospects in Tanzania's East African Rift System are anticipated to be harnessed for geothermal power generation by 2030. This study examines the geothermal fluids' origin, evolution, and natural geothermal potential through geochemical analyses of stable isotopes, major ions, trace solutes, and fluid–mineral equilibria. Analysis samples were taken from hot, warm and cold springs, rivers, and rainwater from the prospects. The predominant Na and K in the samples suggest advanced chemical reactions composed primarily of ion exchange and calcite dissolution in the geothermal system. Four reservoirs, Songwe-Rambo and Kaguri (Songwe) and Ilwalilo and Kilambo (Kiejo-Mbaka), are identified as fluid upflow zones of geothermal systems. Mineral equilibria modeling reveals two key features: saturation with carbonate minerals like aragonite, dolomite, and calcite, linked to travertine development near discharge zones, and silica saturation, primarily quartz and chalcedony, across varying temperatures, suggesting potential silica deposits. Estimated reservoir temperatures range from 90 to 135 °C (Songwe) to 100–145 °C (Kiejo-Mbaka), based on geothermometry methods. Given flow rates of 0.05–10 L/s and temperature differences of 20.8–145 °C, the geothermal power potential is assessed at 4 MW_th for Songwe and 5 MW_th for Kiejo-Mbaka. The primary source of geothermal fluids is meteoric water, recharged at about 2100 m above sea level, which infiltrates to an average depth of ca. 2000 m, driven by heat sources, before re-emerging at the surface. A conceptual model of the geothermal system over these prospects is developed, enhancing the groundwork for evaluating geothermal resources and planning future exploratory drilling.