XIV.—The Kalsilite-bearing Lavas of Kabirenge and Lyakauli, South-West Uganda

Abstract

The Western or Toro-Ankole volcanic province of Uganda consists of several areas of tuffs, explosion craters, and rare lava flows situated to the east and south-east of Ruwenzori (text-fig. 1). From north to south these volcanic fields are distinguished under the names (a) Rusekere; (b) Fort Portal; (c) Ndale, formerly known as the “Kyatwa area”; (d) Katwe-Kikorongo; (e) Bunyaruguru, formerly known as the “Kichwamba area”; and (f) Katunga. The Bunyaruguru field extends southwards from Lake George in and adjacent to the north-eastern portion of the Lake Edward section of the western rift. The Kabirenge and Lyakauli lavas (text-fig. 2) occur on the eastern edge of the rift depression immediately north-west of Nyondo crater (E. 30° 04′ 25″; S. 0° 14′ 30″). As a group, they constitute one of the four lava occurrences now known in Bunyaruguru, the other three being those of Chamengo, Mafuru, and Kazimiro (text-fig. 2). Otherwise, lava occurs only as fragments and lumps in the tuffs and as bombs and ejected blocks. The Kabirenge-Lyakauli lavas are also of special interest because, as described later by Professor Holmes, they consist of various members of the mafurite series, characterised by the presence of the recently discovered mineral kalsilite, a polymorph of KAlSiO4.