Geothermometry and quantifying of mixing and water-rock interactions in the Ngozi geothermal field, SW-Tanzania

Abstract

Ngozi geothermal field is located at the southern triple junction of the East African Rift System (EARS) where the Rukwa-Tanganyika basin, the Ruaha-Mtera-Usangu segment and the Karonga-Malawi rift branches meet. Being at the triple junction the area is characterized by complex tectonic forces leading to development of structures trending in the NW, NE and NS directions. The area is characterized by the Ngozi volcano, which is one of the four volcanoes in the Mporoto-Rungwe volcanic province. Surface geology is characterized by pyroclastic ash cover while outcrops of basalts, trachy-andesite, phonolitic trachyte, alluvial sediments and sandstones occur in some parts. Geothermal surface manifestations in the area include thermal springs and altered ground and gas vents. The surface water consists of rivers, cold and warm springs, hot springs and a lake. The waters are mostly dilute (<100 ppm Cl) with the exception of the lake water that is more saline (~1500 ppm Cl) and have discharge temperatures of 30-82°C. Based on geothermometry, the maximum reservoir temperatures for the springs are ~120-130°C. Mixing model reviles that the surface thermal waters are a mixture of non-thermal water (rivers), reservoir thermal fluid, and saline lake water. Some thermal and non-thermal water show elevated CO2 concentrations considered to originate from deep source(s). The chemistry of thermal waters shows that the concentration of main rock forming elements are controlled by water-rock interaction.