Assessment of the Kiejo-Mbaka geothermal feld by three-dimensional geophysical modelling

High-resolution magnetotelluric and gravity data have been collected over the Kiejo-Mbaka geothermal feld, located along the NW-SE trending Mabka fault, in the Karonga Rift basin (East Africa Rift System). Such resolution allowed to reconstruct the feld structure with unprecedented detail. Resistivity modelling has been obtained by three-dimensional fnite-diferences inversion of MT data, while density modelling has been accomplished by surface-oriented inversion of gravity data. Geophysical modelling has identifed two sedimentary sub-basins separated by the Mbaka fault ridge, exposing the basement; these previously unknown sedimentary flls have a maximum thickness of ca. 1.5 km. The estimation of the clay cation exchange capacity (CEC) from magnetotellurics identifes a layer of low-temperature smectit alteration in the south-western sub-basin sediments, interpreted as a clay cap. The resulting updated conceptual model of the Kiejo-Mbaka geothermal system is therefore a fault-controlled system with lateral leakage into the sediments, expectably implying a larger reservoir volume than previously estimated.