Quaternary Volcanic System in Uganda and Its Associated Geothermal Significance
[MINISTRY OF ENERGY UGANDA, Uganda]
Geothermal resources are usually genetically related to areas of recentlyactive volcanic centers. Young volcanic activity and thermal anomalies indicate geothermal potential. Accordingly, such areas are considered viable geothermal exploration targets and are given priority. Uganda volcanism is of basaltic type(purely basic volcanic system). Mid-Miocene to Recent lavas in the Western Branch of the East African Rift are confined to four intrabasinal accommodation zones (Ebinger, 1989; Pasteels et al., 1989; Furman, 1995) and Pouclet et al. (1983) noted a trend towards increasing potassium and C02 coupled with increasing silica-undersaturation towards the north, where carbonatites also occur (Barker and Nixon, 1989; Stoppa et al., 2000). Quaternary volcanic fields occur to the east and south-east of the Ruwenzori massif along the border between Uganda and Congo. The volcanic edifices of this area are the type locality of kamafugite: pyroclastics dominate over lavas (Holmes and Harwood, 1932) due to the extremely volatile-rich, explosive nature of the volcanism. Magmatism was active in the Upper Pleistocene and continued intermittently until recent times (Holmes, 1950; Lloyd et al., 1991). According to K-Ar and Ar-Ar age determinations all the volcanics are younger than 50 ka (Boven et al., 1998). Generally, the basaltic terrain rarely form thermal anomalies of economic interest, whereas silicic volcanic system probably do if they are large enough. In oceanic systems there is admitted possibility of the existence of high level basic magma chambers (Reykanes and Kilauea Volcano Hawaii). Purely basic volcanic system of Uganda (Kalsilite-bearing lava and ejecta)are ofKamafugite series. Kamafugites represent primitive ultrapotassic rocks which are extremely rare but widespread. Reece (1955) noted that volcanic activity probably continued into historical times. Kamafugites are characterized by Si02 under-saturation, low AI, moderately high K but extremely high Ca content and as a consequence of this unusual major element composition modal kalsilite, melilite and perovskite frequently occur. Basaltic lava has low viscosity and can flow long distances. These basaltic magmas originate deep from upper mantle melt (30-50km) and most of them rises relatively rapidly to the earth’s surface and reacts very little with crustal rocks because of its low viscosity. Basaltic materials are more fluid than Rhyolites, andesites and dacites which contain more silica. The basalts rise rapidly from the mantle to the surface through pre-existing weakness zones in the crust. As a result their heat was dispersed rather than stored and does not provide useful geothermal concentration (Smith and Shaw, 1975). This is not the case with high silicic variety (which are lacking in Uganda), which are highly viscous and commonly associated with magma chambers at shallow levels (long lived heat source) in crust (Smith and Shaw, 1975). Uganda’s basaltic magma unlike silicic magma cannot sustain high temperature conventional systems for thousands of years. Due to basaltic nature of Uganda’s volcanism, the existence of high level magma storage chambers in the upper 10m of the crust is less likely. Many large geothermal systems appear to be associated with young silicic volcanism. The heat source of Uganda’s geothermal systems is related to conduction dominated environments ascribed to extension and thinning of the crust as opposed to high level magma chambers. High heat flow is related to thinned crust and high temperature mantle. The high level magma chambers are presumed not to be the ultimate heat source of Uganda geothermal systems in volcanic terrain. The volcanic rocks of Uganda are fresh with no visible exposure to high temperatures and thermal influence. It can be presumed that Uganda’s geothermal systems are extensional-type geothermal systems as opposed to magmatically heated geothermal system notwithstanding the existence of recent volcanism in tectonically active rift zone. This observation is consistent with conventional wisdom that suggests that basaltic provinces are poor targets for high temperature geothermal exploration because of low viscosity of basalts results in rapid flow to the surface along narrow conduits rather than forming shallow magma chambers that are large enough to support high temperature fluid convection. Uganda is presumed to host low to medium grade (lower temperature) geothermal resources which are fault-hosted largely due to its basaltic terrain.
|        Topic: Geology||Paper Number: 12010|