| Keywords |
stratovolcanoes, hydrogeology of geothermal systems, Western Branch, East African Rift System, Virunga Volcanic Province, Rungwe Volcanic Province, Karisimbi, Ngozi |
| Abstract |
High-temperature geothermal systems associated with stratovolcanoes (also known as composite cones) can be found at locations around the world. For example, in Indonesia, most successful geothermal developments have been completed on the lower slopes of (or adjacent to) stratovolcanoes where there are surface manifestations of a hydrothermal system. In the Western Branch of the East African Rift System, stratovolcanoes are located in two areas: the Virunga Volcanic Province (VVP) in Rwanda/DRC/Uganda, and the Rungwe Volcanic Province (RVP) in southwestern Tanzania. Extensive geothermal exploration surveys have been performed at the stratovolcano Karisimbi in the VVP (including a 3,000-m deep well), and the former stratovolcano Ngozi (now a caldera) in the RVP, based on the tacit assumption that they host high-temperature reservoirs. The results of the exploration work completed to date suggest that neither site appears to host a high-temperature geothermal system. Understanding the hydrogeology of a geothermal system provides insight into recharge and possible outflow directions. This paper will compare the data from recent studies at Karisimbi and Ngozi and evaluate the hydrological and geological characteristics similar to both sites including: (1) the absence of surface manifestations (hot springs, fumaroles, alteration), (2) the presence of CO2-rich cold or tepid springs, (3) the shallow depth to Proterozoic basement rock, (4) the likely deep source of heat and magmatic volatiles, and (5) the relatively strong topography-driven groundwater flow systems. Lessons learned from Karisimbi and Ngozi are applicable to possible future geothermal resource assessments at other stratovolcanoes in the Western Branch including, for example, at Kyejo (RVP) or Kinigi (on the SE flanks of Visoke in the VVP). |