Interpretation of exploration geochemical data for geothermal fluids from the geothermal field of the Rungwe volcanic area, SW-Tanzania

Abstract

The Rungwe volcanic field in the Mbeya region, SW-Tanzania is known for its hot springs, the common manifestations of a geothermal energy source. This study is a part of the search for a potential high-enthalpy reservoir in this region. Chemical analysis for solutes in water samples from the Rungwe volcanic hot springs and caldera water from Lake Ngozi were used for interpretation in this report. The chemical characteristics of the hot springs were evaluated using Cl-SO4-HCO3, Li-Cl-B and Na-K-Mg ternary diagrams. All waters have low B/Cl ratios, suggesting that they are derived from an old hydrothermal system. Three groups of waters can be distinguished on the basis of predictions of subsurface temperatures with geothermometers. The first group consists of cold springs (Shiwaga, Mulagara and Isebe) which are characterised as immature waters. For these, the use of geothermometers for calculating subsurface temperature is unrealistic. The second group of thermal springs (Kilambo, Mampulo B, Kandete and Kasimulo) is characterised by peripheral partially equilibrated waters with lower predicted Na-K subsurface temperatures, but with higher temperatures (calculated with silica geothermometers) than the third group (Songwe hot springs: Ilatile 1, River spring, Ilatile 4 and Main spring B). The quartz, chalcedony and Na-K temperatures were calculated by the WATCH computer program and the Na-K-Ca temperature by the SOLVEQ computer program. Log (Q/K) diagrams show the range and cluster of equilibrium temperatures which, in most cases, are close to the chalcedony temperature. With the exception of two springs from the second group (Kandete and Kasimulo), the chalcedony temperature estimates lie within the subsurface temperature range estimated from the log (Q/K) diagrams. The silica-enthalpy model predicts maximum subsurface temperatures for the most prominent springs of the second group.