GEOTHERMOMETRY OF HOT SPRINGS IN THE MALAWI RIFT

The Malawi rift is located in the Western Branch of the East African Rift System, which has an enormous potential of geothermal energy generation. Previous geothermal studies in Malawi focused on the geochemistry of hot springs and recognized that the hot fluids are not related to magmatic or mantle sources but rather to heat from elevated geothermal gradient and deep fault circulation. The Mesozoic shear zones intersecting with Quaternary rift faults are controlling the emplacement of the major hot spring cluster in the Malawi Rift Valley. Despite the occurrence of geothermal resources in Malawi, geothermal energy is undeveloped because of a dearth of exploration data to classify and quantify the resources.

This study focused on the estimation of silica and cation geothermometry of geothermal systems to better characterize the geothermal energy potential in Malawi. The Piper diagram shows that the 27 hot springs composition can be classified as 1) sodium bicarbonate or 2) sodium sulfate waters. The sodium bicarbonate water type is associated mainly with meteoric waters mixed from shallow aquifers and poor affected by the admixture of deeper thermal waters. The silica geothermometer used to estimate temperatures in shallow aquifers with immature rock-water equilibrium show quartz conductive temperatures from 55°C to 130°C. The sodium sulfate water type is related to deeper geothermal aquifers where partial rock-water equilibrium has occurred. Major anion analyses indicate that sodium sulfate waters could be related to groundwater heated by steam from deeper reservoirs. The Na-K geothermometer shows the highest reservoir temperatures of about 190°C in the Chiweta hot springs. These hot springs also show the highest chloride concentrations, which may indicate a high temperature geothermal system at depth. The isotopes of δD versus δ¹⁸O show that most of the hot springs are correlated with a meteoric water source that is infiltrated at depth through faults.

Our results indicate that some of the hot springs in Malawi have geothermal energy capabilities of medium enthalpy. The most promising area to continue exploration work is in the north region of the country. In addition, structural studies should be conducted to understand the hot fluid storage and transportation through faults.