| Title | Geochemistry of Extensional-Type Geothermal Systems in Uganda |
|---|---|
| Authors | T. Benjamin |
| Year | 2020 |
| Conference | New Zealand Geothermal Workshop |
| Keywords | geochemistry, deep circulation, amagmatic, extensional |
| Abstract | The purpose of this paper is to explain the geochemistry of Uganda’s amagmatic extensional-type geothermal systems, particularly the origin of the chemical properties and corresponding reservoir properties. This type of system is a fault bound deep circulation system unlike magmatic systems which rely on shallow magma chambers. The fault zones are tectonically active which ensures deep circulation and high permeability. These amagmatic systems have low to medium temperature sources with subsurface temperatures ranging between 100oC and 200oC. Deep circulation geothermal fluids are less chemically charged compared to those from magmatic systems due to low temperature sources. Geothermal fluids from most of Uganda’s geothermal areas are of neutral to slightly basic pH (6.0-9.0) and can be classified as fresh water to moderately saline based on their recorded TDS and conductivity values. Their TDS varies depending on the surrounding rocks through which the fluids flow from the reservoirs to the surface where they manifest as hot springs. Fluids from South-Western Uganda geothermal prospects have a relatively low Cl concentration and are mainly classified as HCO3 type or SO4 type. Fluids from Western Uganda geothermal prospects have a relatively high Cl concentration and are mainly classified as Cl-HCO3 type or intermediate type with moderately high salinity in excess of 10,000 TDS. Possible subsurface temperatures for Uganda’s hot springs range between 100oC and 250oC based on geothermometry and mixing models proving they are low to medium temperature sources. Soil-gas and gas-flux measurements along Kibiro, Buranga, Panyimur, Katwe and Ihimbo hot springs revealed concealed deep penetrating structures which control geothermal activity. The 3He/4He ratios of geothermal fluids from Kibiro fault-bounded geothermal system suggest no deep mantle signature. Stable isotopic studies of deuterium and oxygen-18 indicated high altitude fluid sources. Normally geothermal fluids contain <1ppm Mg content but most of Uganda’s hot spring waters contain 1-10ppm Mg suggesting mixing with surface cold waters. |