Proceedings of the 4th African Rift Geothermal Conference 2012 Nairobi, Kenya, 21-23 November 2012 Geology of Kibiro, Katwe and Buranga Geothermal Prospects of Uganda James Francis Natukunda Department of Geological Survey and Mines (DGSM), P.O. Box 9, Entebbe, Uganda. [email protected] Keywords: rifting, hot springs, fault permeability, geology The Buranga field is located at the NW end of the Rwenzori ABSTRACT massif near the base of Bwamba escarpment. The hot The three main Ugandan geothermal fields namely, Kibiro, springs emerge through Pleistocene sediments. Precambrian Katwe and Buranga are located in the western arm (the rocks underlie the sediments. The main rift fault strikes 450 Albertine Rift) of the East African Rift System (EARS). and dips 600-650. The three hot spring areas namely The general strike of the Albertine Rift is NE-SW. Mumbuga, Nyansimbe and Kagoro lie on a line striking 400 Geophysical surveys indicate 2500 to 3000m thickness of parallel to the main rift fault. Precambrian rocks form the Pleistocene sediments in the rift floor. northern half of the Rwenzori massif that strike 100-300 and consists of migmatites and gneisses. The sediments consist The Kibiro geothermal prospect, which is located at the of fine to medium-grained, poorly consolidated sands and Eastern escarpment of Albertine Rift is comprised of hot clays, some of which are coated with calcareous material. springs that emerge at the base of the escarpment at the The geothermal manifestations of Buranga field include intersection of three oblique faults. The escarpment forms a three hot spring areas with a maximum temperature of boundary between the old basement rocks (to the east) and 980C, travertine cones and some sulphur deposits at Kagoro the young sedimentary formation of the rift (to the west). spring area. The area is seismically active and the frequent The basement consists of Precambrian granites, granitic earthquakes in the region are reactivate and create new gneisses, gneisses and N-S striking basic amphibolite manifestations like new hot springs near Nyansimbe pool. intrusives. Mylonitic gneisses characterise the fault- controlled valleys. The NE striking faults in the area are 1.0 REGIONAL GEOLOGY oblique to the main rift fault and crosscut E-W striking The geology of Uganda consists of an exposed pre- faults. All rocks are heavily fractured with crosscutting Cambrian basement dissected by the western branch of the joints. The geothermal manifestations include; hot springs, East African Rift System in the western part of the country solfatara, extinct clayey alterations and secondary (Figure 1). The eastern branch, the Gregory Rift, passes hydrothermal minerals that include gypsum and calcite through the central part of Kenya. The Western branch, the filling joints in altered mylonitic gneisses on the escarpment Albertine Rift, starts to the north along the Sudan border, among others. The maximum temperature of the hot springs and then curves to the west and then southwest along the is 86.40C. border with the Democratic Republic of Congo, and south to Rwanda, Burundi and western Tanzania. Spreading The Katwe volcanic field is on the SE of the Rwenzori began at least 15 million years ago in Miocene time. The massif and consists of 78 randomly distributed craters of western Rift is considered to be at an early stage in the which seven have water. The volcanics are deposited on development, and is younger (late Miocene-Recent) than Pleistocene sediments. Precambrian rocks of Toro system the more mature eastern branch (Morley and Westcott, lie on the western side while on the eastern side; the 1999). The Albertine Rift is seismically active, sediments are underlain by Karagwe-Ankolean (K-A) characterized by deep-seated (27–40km) large earthquakes. metasediments. In the south-eastern side of Katwe field, The region of the Rift has a markedly higher heat flow than there is Bunyaruguru volcanic field that consists of tuffs the surrounding Pre-Cambrian terrain. Two different en mixed with K-A rock fragments. The Katwe volcanics are echelon strands are found in the Western Rift Valley, mainly phreatomagmatic pyloclastic deposits consisting of separated by the Rwenzori Mountains, which rise from a ash, tuff, lapilli, volcanic bombs and xenoliths of basement base of less than 1,000m in the Rift to over 5,000m rocks. Basaltic lava flows and ejected lava blocks occur elevation. Within the Rift Valley there are thick layers of around Kyemengo and Kitagata craters. The volcanic late Tertiary and Quaternary sediments, fresh water and material pile rises up to 420m above surrounding saline crater lakes, volcanic, and plutonic bodies have been sediments. NE-SW striking faults that characterize the field identified beneath L. Albert and L. Edward (EDICON, are parallel the strike of the main rift fault. Carbon dating 1984). The three main geothermal areas of Uganda are puts the age of volcanism as Pleistocene to Holocene. The Kibiro, Katwe, and Buranga. Katwe geothermal manifestations include travertine deposits that indicate extinct hot springs, warm springs (300C) at L. Katwe and hot springs (700C) at L. Kitagata. 59 3.0 THE GEOLOGY OF KIBIRO GEOTHERMAL FIELD 3.1 Introduction The Kibiro area is located on the eastern escarpment of the western branch of EARS. It covers the small peninsula below the escarpment where the villages of Kachuru and Kibiro are located, the escarpment itself as well as the land extending from the escarpment shoulder towards the east. The area’s altitude is from 620m above sea level (asl) at the rift floor and Lake Albert to 1,100m asl on the Rift shoulder. The area consists of deep and steep valleys (fault zones), especially closer to the escarpment. The temperature of the area is in the range of 32-38°C. Figure 1: The East African Rift System 3.2 Description of Rock Units The escarpment cuts through the field from SW to NE and 2.0 LOCATION OF KIBIRO, KATWE AND divides the study area in two entirely different geological BURANGA GEOTHERMAL AREAS environments. To the east, the geology consists of The Kibiro geothermal area is situated in Hoima District crystalline basement, characterized by granites and granitic (Figure 2), on the eastern shore of Lake Albert. Kibiro is gneisses, whereas in the rift there are thick sequences of located at the foot of the escarpment of the western branch sediments. The best rock outcrops are found in the of the EARS. The escarpment rises over 300 m above Lake escarpment face, but the rock is usually highly transformed Albert. by the fault movements. Good outcrops are also found along the numerous fault-lines in the block-faulted rock and The Katwe-Kikorongo (here after refered to as Katwe) on the eastern side, the outcrops are restricted to a few geothermal prospect is one of three geothermal areas, which outcrops on hills. These are the hardest rocks, and therefore have been studied by the Ministry of Energy and Mineral resistive to weathering. The softer rocks have weathered Development (MEMD), the other two being Kibiro and and formed depressions in the undulating landscape Buranga in Hoima and Bundibugyo Districts respectively (Gislason et al., 2004). (Figure 2). Katwe is in the Kasese District. It is within the Queen Elizabeth National Park, south of the Rwenzori 3.2.1 Granite and Gneiss Massif and north of Lake Edward. Katwe is a volcanic area These Precambrian rocks form the entire basement east of with many phreatic craters but very little lava at surface. the escarpment. They are fine to coarse-grained composed Some of the craters have crater lakes. The most pronounced of quartz, feldspars, amphiboles (mainly hornblende) and hot spring is on bottom of the crater lake in the Kitagata biotite. Their colour ranges from grey to light brown and in crater. some places pink hue depending on the type of feldspar present. Banding is common and is more pronounced closer Being in a national park, the Katwe area is mostly to the escarpment, and usually dipping very steeply (60- uninhabited, except for the Katwe village, south of the 90°), with the most common direction of the strike being saline Lake Katwe. Considerable salt mining takes place in close to N20°E although E-W direction is also common Lake Katwe. (Gislason et al., 2004). Close to the escarpment and in association with fault lines the granites are mylonitic or brecciated, typical of highly a faulted environment. The stage of weathering or alteration of the granites depends strongly on the structure and location of the rock. The massive, un-banded granites tend to be very fresh, and generally, the rock is less altered with distance from the escarpment. 3.2.2 Amphibolite Dykes Simmons (1921) was the first to describe dyke-forming intrusives within the present study area and referred to them as “pyroxenites” - a field term, but later authors (Kakenga et al., 1994) have referred to it as diorites. This formation is black, angular blocks on the surface, forming a small, elongated ridge in the landscape (Figure 3). These outcrops are usually 10-20m wide, but can be as long as a few km Figure 2: The geothermal areas of Uganda and are most common on the slopes and hilltops above Kibiro. Their strike varies from N-S to N20°E. In a hand specimen the rock is fine to medium-grained, dark grey in 60 colour and usually very fresh looking. No phenocrysts are granitic gneiss and mylonitic rocks. Several distinctive seen, but joints are common. Microscopic examination of strike directions of banding have been observed, where these intrusives in thin sections reveals that the rock is east-west and N20°E are the most prominent. A less composed of quartz, pyroxene (orthopyroxene and common strike direction is N130°E.