| Title | Hydrogeochemistry of Chiweta Geothermal Prospect, Northern Malawi |
|---|---|
| Authors | Gift W. TSOKONOMBWE, Daði ÞORBJÖRNSSON, Sigurður G KRISTINSSON, Andri STEFÃNSSON, Yankho KALEBE |
| Year | 2020 |
| Conference | World Geothermal Congress |
| Keywords | geochemistry, geothermometry |
| Abstract | The geothermal and non-geothermal water chemistry associated with Chiweta geothermal prospect was studied. The main aim was to quantify chemical and physical characteristics of the geothermal water. Hydrogeological, geothermal mapping coupled with water chemistry was used to track water movement and quantification of related processes. Thermal water hosted by metamorphic and Karroo sedimentary rocks emerge along NW-SE fault lineaments as hot springs, thermally altered grounds and shallow hot water borehole. Surface temperatures of thermal springs are about 80°C. The discharged thermal water from the 32 m depth well registered a temperature of 46°C. The geothermal system of the Chiweta geothermal prospect is suggested to be a low temperature Neogene fault controlled geothermal system in the sedimentary environment. The recharge waters belong to Ca-Mg-HCO3 type with a temperature between 23°C to 33°C and pH of 5.7-7.5. The inflow waters attain the heat from elevated geothermal gradient at depth, an anomaly assumed to be associated with crustal thinning due to Malawi Rift spreading. All thermal waters belong to Na-Cl-SO4-HCO3 facies. Chemical geothermometers suggest the subsurface reservoir temperature of about 132°C - 157°C. Multiple mineral equilibria and mixing models are in good agreement with the solute geothermometers estimated subsurface temperature range. The elevated temperature is enough to drive dissolution of host rock and ion exchange reaction in the reservoir of the geothermal system that modifies the chemical composition of the reservoir and thermal springs water to Na-Cl-SO4-HCO3 facies from recharged Ca-Mg-HCO3 water type. No boiling is occurring during the ascent of the thermal waters but steaming at the surface causes minimal δD and δ18O isotopic fractionation. Reservoir pH is slightly lower than in situ pH in the liquid phase due to loss of acid gases, mainly CO2. Saturation of calcite and quartz is low in thermal spring waters indicating limited scaling potential. Reconstructed reservoir waters are saturated in talc, chrysotile, quartz and calcite indicating high chanced of scaling. Both thermal and non-thermal waters of the Chiweta system originate as precipitation in the western highlands as indicated by δD and δ18O stable isotopes. Heavily enriched δD and δ18O isotope lake water do not contribute to recharge of the geothermal system. |