Chemical and isotopic composition of thermal waters in northern part of Malawi

Abstract

Geochemical research is an integral part of any geothermal exploration before subsurface investigations such as drilling are performed. The data obtained from geochemical studies is valuable as it gives information about the predicted subsurface temperature and the nature of the geothermal resource. Malawi is conducting the first detailed geothermal investigation in the country, with emphasis on geochemical surveys in the first and second stage. Results obtained during the field campaign which took place in September 2016 were used in this study to trace the origin of thermal waters and predict the subsurface temperature of the Northern Malawi geothermal prospects, as well as to look for favourable chemical characteristics that will enhance future developments. The geothermal prospects of interest include; Chiweta, Kanunkha, Kasitu and Kasanama which form a linear pattern along Lake Malawi. The prospects have similarities in the geological set-up as well as structural set-up. Stable isotopes were used to trace the origin of thermal waters. The thermal waters are meteoric in origin with a slight shift of oxygen for Chiweta samples, probably due to water-rock interaction. Karoo sediments influence the HCO3(SO4) composition of the thermal waters but the connection between Cl(SO4) composition of the Chiweta samples and rocks is yet to be identified. The thermal waters are partially equilibrated except for Chiweta which is considered mature geothermal water. Subsurface temperatures suggest that Chiweta system has the highest temperature among the prospects with a temperature above 100°C; the other prospects have temperatures above 70°C. Multiple mineral equilibria supplement the geothermometry results, indicating temperatures within the same range. Calcite and amorphous silica are undersaturated in the systems.