stripes
This is an almost circular carbonatite complex 4 km in diameter around which is a collar of alkaline syenites. Although the carbonatite is reasonably well exposed the syenites are unexposed and information is based solely on drilling. The carbonatite comprises ring-dykes and cone sheets which are cut by a system of radial faults and dykes. They vary from sovites and alvikites to beforsites and ferruginous types. There are all gradations between calcite carbonatite and those with approximately equal proportions of calcite and dolomite; apatite is ubiquitous and may be abundant, magnetite is usually present, biotite is a common accessory and other accessories include pyrite, pyrochlore, of which an analysis is given by Davies (1956), zircon, baddeleyite, barite, knopite, pyrrhotite, chalcopyrite, galena and gold. The ferruginous carbonatites occur as veins and dykes with both alvikitic and beforsitic types present in which goethite and hematite form tiny inclusions. Areas are found within the carbonatite varying from pure apatite through to magnetite rocks, with no carbonate, but generally the apatite is the more abundant. Analyses of carbonates and apatite are given in Dawson et al. (1996). In two 3-5 m-wide phonolite dykes cutting the carbonatite nepheline forms small euhedral prisms set in a matrix of green pyroxene with occasional phenocrysts of orthoclase as well as small laths in the groundmass; a barkevikitic amphibole is rare and vesicles are filled with analcime and natrolite. Little is known of the syenites surrounding the carbonatites because of the lack of exposure, but drilling has indicated that they form only a narrow zone and are principally feldspathic rocks with concentrations of biotite (Davies, 1956). It is likely that these rocks are fenites but it is not clear from the description of Davies whether this is so. Zoned phlogopites in carbonatite have been studied by McCormick and Le Bas (1996). Bell and Powell (1970) give Sr isotope data and Rb and Sr abundances for carbonatite and Lancelot and Allegre (1974) U, Th and Pb isotopic data for a carbonatite.
BELL, K and BLENKINSOP, J. 1987. Nd and Sr isotopic compositions of East African carbonatites: implications for mantle heterogeneity. Geology, 15: 99-102.BELL, K. and POWELL, J.L. 1970. Strontium isotopic studies of alkalic rocks: the alkalic complexes of eastern Uganda. Bulletin of the Geological Society of America, 81: 3481-90.DAVIES, K.A. 1956. The geology of part of south-east Uganda. Memoir, Geological Survey of Uganda, 8: 1-76.DAWSON, J.B., STEELE, I.M., SMITH, J.V. and RIVERS, M.L. 1996. Minor and trace element chemistry of carbonates, apatites and magnetites in some African carbonatites. Mineralogical Magazine, 60: 415-25.DEANS, T. 1966. Economic mineralogy of African carbonatites. In O.F. Tuttle and J. Gittins (eds), Carbonatites. 385-413. John Wiley, New York.DENAEYER, M.M.E. 1970. Rapports isotopiques (O et (C et conditions d'affleurement des carbonatites de l'Afrique centrale. Comptes Rendus Hebdomadaires des Seances de l'Academie des Sciences, Paris, 270D: 2155-8.KURTANJEK, M.P. and TANDY, B. C. 1983. The potential development of phosphate rock resources in tropical Africa. In A.I. More (ed) Fertilizer 83, Proceedings of the Conference held in London 1: 363-76. The British Sulphur Corporation, London.LANCELOT, J.R. and ALLEGRE, C.J. 1974. Origin of carbonatite magma in the light of the Pb-U-Th isotope system. Earth and Planetary Science Letters, 22: 233-8.MCCORMICK, G.R. and LE BAS, M.J. 1996. Phlogopite crystallization in carbonatitic magmas from Uganda. The Canadian Mineralogist, 34: 469-78.NELSON, D.R., CHIVAS, A.R., CHAPPELL, B.W. and MCCULLOCH, M.T. 1988. Geochemical and isotopic systematics in carbonatites and implications for the evolution of ocean-island sources. Geochimica et Cosmochimica Acta, 52: 1-17.REEDMAN, J.H. 1984. Resources of phosphate, niobium, iron, and other elements in residual soils over the Sukulu carbonatite complex, southeastern Uganda. Economic Geology 79: 716-24.