Alkaline Rocks and Carbonatites of the World

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Coola (Chimbumbo-Coola)

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Occurrence number: 
005-00-018
Country: 
Angola
Location: 
Longitude: 15.27, Latitude: -12.52

This occurrence consists of three centres: in the north is an elongate body of breccias centred on Mount Chimbumbo several kilometres south of which is a hill on which are several areas of carbonatite, fenite and basement granite that are surrounded by alluvial and other superficial deposits which obscure the full extent of the complex. In the south a third hill is mostly occupied by ijolite with some nepheline syenite, syenite and breccia, but this centre also is much obscured by recent deposits. The ijolites are heterogeneous rocks of nepheline and abundant mafic minerals but Lapido-Loureiro (1973) thought he recognised replacement of nepheline by alkali feldspar leading to production of rocks he termed essexite and theralite. Nepheline syenite forms a single small intrusion within the ijolites and contains aegirine-augite and amphibole, while possible syenites have phenocrysts of alkali feldspar in a microcrystalline feldspathic matrix. Quartz was identified in some samples and nepheline in others. Feldspathic carbonatitic breccias are confined to the central group of outcrops where they are in contact with granites. They comprise altered fragments of granite and feldspathic clasts in a ferruginous carbonatitic matrix. This breccia contains abundant barite and some varieties with carbonatite clasts and matrix contain fluorite; parisite/synchysite and rare earth phosphates have been identified (Issa et al., 1991). Analyses indicate Ba>5000 ppm, Sr 4000 ppm and La 12500 ppm; Nb was not detected. Two types of fenite have been distinguished by Lapido-Loureiro (1973), the first of which outcrops on the eastern side of the central area and consists of microcrystalline feldspathic material and iron oxides. The second type of fenite is developed as a 1 km long arcuate zone on the western side of the central area, west of the carbonatitic breccias. It is a feldspathic rock which is heavily silicified and almost invariably contains fluorite and/or barite, sometimes in great abundance. These rocks are radioactive (counts up to 130 times those of the basement granite) and one sample gave >5000 ppm Ba and 2500 ppm Nb. The breccia which forms the most northerly part of the complex, and builds Mount Chimbumbo, consists of syenite clasts, which could be fenitized country rock granite, and in one type abundant xenoliths of carbonatite set in a feldspathic matrix. An analysis of magnesiocarbonatite, containing 3.5 wt % BaO, is given by Lapido-Loureiro (1973) and a further very full analysis of barite-dolomite carbonatite is in Alberti et al. (2000).

References: 
ALBERTI, A., CASTORINA, F., CENSI, P., COMIN-CHIARAMONTI, P. and GOMES, C.B. 2000. Geochemical characteristics of Cretaceous carbonatites from Angola. Journal of African Earth Sciences, 29: (in press).ISSA, A., DOS SANTOS, A.B.R.M.D., RIFFEL, B.F., LAPIDO-LOUREIRO, F.E.V. and MCREATH, I. 1991. Aspects of the geology, petrology and chemistry of some Angolan carbonatites. Journal of Geochemical Exploration, 40: 205-26.LAPIDO-LOUREIRO, F.E. 1973. Carbonatitos de Angola. Memórias e Trabalhos do Instituto de Investigação Científica de Angola, 11: 1-242.
Location: 
Scratchpads developed and conceived by (alphabetical): Ed Baker, Katherine Bouton Alice Heaton Dimitris Koureas, Laurence Livermore, Dave Roberts, Simon Rycroft, Ben Scott, Vince Smith