Evaluation of Hydrogeochemical Facies of the Barrier Volcanic Complex Geothermal Fluids in Developing a Conceptual Model

Abstract

The Barrier Volcanic Complex (BVC) geothermal prospect is situated on the floor of the northern sector of the Kenya rift, immediately south of Lake Turkana. It is composed of four distinct polygenetic volcanic edifices; Kaloleyang, Likaiu East, Likaiu West and Kakorinya, which are characterised by a wide spectrum of lava types including basanite, basalt, hawaiite, mugearite, benmorite, trachyte, phonolite and pyroclastics. Geothermal manifestations occur in the form of fumaroles, silica sinters, Logipi hot springs and hydrothermally altered rocks characterised by argillic alteration deposits of alunite, kaolinite, and other clay minerals found in the areas of fumarolic activity in the Kakorinya caldera and its summit. The caldera is the youngest volcanic centre and has the most promising geothermal potential. This project presents the evaluation of the hydrogeochemical characteristics of the sampled seven Logipi hot springs and six Kakorinya fumaroles in the BVC with the aim of developing a conceptual model. It is inherent that the key to successful exploration, development (including drilling) and subsequent sustainable utilization and optimization of a geothermal resource is to clearly define and understand the nature, components and characteristics of the geothermal system. This is best achieved by developing a conceptual model. The Logipi thermal springs discharge waters of Na-Cl-HCO3 facies suggesting peripheral waters, which indeed marks the outflow of the geothermal system as confirmed from the Na/K and HCO3/SO4 ratios as well as the δ18O and δ2H stable isotopes composition. The fumarole gas chemistry indicates a high atmospheric contamination, propelled by the interconnectivity of the various fissures, fractures and minor faults in the caldera or from dissolved air in shallow aquifers. Nevertheless, the discharge chemistry of fumaroles BF-01 and BF-04 suggest their proximity to the upflow zone. The study of the stable isotopes of δ18O and δ2H indicate that thermal waters in the BVC show a derivative recharge from Lake Turkana and meteoric waters from the rift flanks in varying proportions. The silica-enthalpy mixing model denotes a conductive cooling of the thermal waters before or after mixing that also lead to the low enthalpies (600kJ/kg) reflecting low temperature (142.5°C). The correlation plots of the conservative constituents and Na-HCO3 correlation plots display a linear relationship hence confirming mixing between Lake Turkana waters, Lake Logipi thermal springs waters and ground water of Parkati well. A high temperature geothermal resource exists in the BVC with an estimated equilibrium reservoir temperature above 280oC based on the gas geothermometers. Further geoscientific exploration methods should be carried out in order to affirm and validate the extent of the resource, thus refining the model.