Geophysical Exploration of the Kiejo-Mbaka Geothermal Field, Tanzania

The Kiejo-Mbaka geothermal prospect (Tanzania) lies along the eastern margin of the late Miocene - Pliocenic Karonga Rift Basin, and is part of the Rungwe volcanic province. The prospect is characterised by an uplifted and outcropping block of Precambrian Gneiss basement, limited toward SW by the NW-trending, SW-dipping Mbaka Fault. The geothermal manifestations mainly consist of hot springs, occurring along this fault. To get a detailed assessment of the prospect, an integrated geophysical survey has been carried out by TGDC (Tanzania Geothermal Development Company), with the supervision of ELC-Electroconsult (Italy). A dense station grid was adopted, comprising 76 Magnetotelluric (MT) and Transient Electromagnetic (TEM) soundings and 133 gravity measurements. The 2/3D gravity modelling depicted the structure of the basement, primarily consistent with a tilted and uplifted block of the Karonga half-graben. The high-density basement block (3000 kg/m3) is surrounded by lower density layers (2500 kg/m3), lying beneath the plain SW of the Mbaka fault and NE of it. Magnetotelluric dimensional analysis indicated strong 3D conditions and yx-component phases often beyond 90°. The MT 3D modelling identified a resistive body (>2000 Ohm m) deepening toward SE and representing the gneiss basement. Two conductive layers (<10 Ohm m) are located on the ridge NE of the Mbaka fault, and another one (<5 Ohm m) lies beneath the plain, showing a thickness of about 1 km. We found a strict correlation between the low-density and the high-conductivity layers. This leads to hypothesise the presence of syn/post-rift sediments possibly hosting a geothermal reservoir.