Sangu-Ikola (Karema Depression)

Within the Karema Depression, which is probably a branch of the Rukwa Rift Valley, metamorphic rocks of the Ubendian system are overlain by various groups of sediments the oldest of which is the Bukoban. Within the Ubendian elongate bodies of carbonatite strike obliquely to the foliation. The carbonatite comprises three units having an overall length of 18 km and is referred to by Coetzee (1963) as the Sangu Carbonatite Group. The most easterly, and largest, body, with an overall lngth of 18 km, was named the Ikambwa carbonatite, the most westerly the Ikola carbonatite, while the small mass lying between them was referred to simply as the Middle Carbonatite. The greatest width attained is 1.5 km. Contacts between basement rocks and carbonatites are not exposed and there is little direct evidence of fenitization, but extensive areas of albitite and aegirine-albite rock, which are mainly within the carbonatites, are considered to be fenites. Coetzee (1963) mapped the carbonatites principally as white, dolomitic and red. The coarse-grained, equigranular dolomitic carbonatite comprises 80-95% dolomite, apatite, which is often in bands and commonly >10% modally, but in some quartz-bearing varieties is absent, and accessory opaque minerals and phlogopite. The white carbonatites consist of dolomite and calcite forming complex vermiform intergrowths, which are interpreted as exsolution textures; a textural type of large, rounded dolomite grains corroded by calcite is also common. Apatite averages 8.6% but may comprise >30% of the rock; opaque minerals, phlogopite and sodic amphibole are common accessories, but quartz is only present in apatite-free varieties. The red carbonatite is characterised by the presence of finely divided hematite, which appears to have been introduced into what was formerly dolomitic or white carbonatite. Magnetite, partly oxidized, may form masses up to a metre across; baddeleyite and pyrochlore have been identified. The albitites and aegirine-albite rocks are equigranular and may be foliated or streaked. Aegirine forms stubby prisms and may comprise as much as 50% of the rock, while albite may form as much as 80%. More than 60% quartz may be present but in most aegirine-albite rocks is very much less. Opaques and carbonate are sometimes abundant and accessories include sodic amphibole, orthoclase, apatite and phlogopite. Analytical data for Sr and Nb in carbonatites are available in Coetzee (1963) and REE for a sample of white carbonatite from the Ikambwa unit will be found in Schofield and Haskin (1964). Analyses, including trace element data, of four carbonatites are given by van Straaten (1989), who also gives a number of REE chondrite normalized plots. Coetzee (1963) lists and briefly describes four other occurrences of carbonate rocks in the Karema Depression, but concludes that they are of metasomatic origin.