Medium to low enthalpy geothermal reservoirs estimated from geothermometry and mixing models of hot springs along the Malawi Rift Zone

Abstract

We investigated the major ion and stable isotope geochemistry and used silica and cation concentrations to assess the geothermometry of hot springs along the Malawi Rift Zone (MRZ). The MRZ is a magma-poor rift, where potential geothermal energy is postulated from elevated heat flow and the occurrence of hot springs. Our objectives were to (1) estimate hot spring reservoir temperatures, (2) identify processes that could affect hot spring water during the ascent from their geothermal reservoirs, and (3) identify the most promising areas for geothermal energy generation. New geochemistry data from 27 hot springs were analyzed and classified as Na-HCO3, Na-SO4, or Na-Cl (SO4) water types controlled mainly by the mineralogy of the host rocks. The hot springs in the MRZ are associated with the local meteoric waters that infiltrate at depth and are heated by an anomalously high geothermal gradient. Our findings indicate that a parent geothermal fluid is mixed and diluted with a proportion of more than ∼45 % of cold water from shallow aquifers. Additionally, other processes such as water-rock interaction, and CO2 outgassing take place during the ascent of the geothermal fluid to the surface. The quartz conductive silica geothermometer indicates reservoir temperatures that range from 60 °C to 130 °C, with enthalpies ranging from 250 kJ/kg to 537 kJ/kg, respectively. However, the Na-K cation geothermometer indicates reservoir temperatures from 92 °C to 190 °C with enthalpies from 388 kJ/kg to 808 kJ/kg. Three areas of potential geothermal energy generation were identified: (1) The Chiweta area, (2) Nkhotakota area, and (3) the Shire graben area. We conclude that the hot springs along the MRZ have geothermal energy capabilities of medium to low enthalpy that can be exploited for electricity generation or other direct uses.