Alkaline Rocks and Carbonatites of the World

Setup during HiTech AlkCarb: an online database of alkaline rock and carbonatite occurrences

Longonjo (Mount Chibilundo)


Occurrence number: 
Longitude: 15.22, Latitude: -12.92

Longonjo is a carbonatite intrusion that forms a hill rising some 280 m above the surrounding peneplained Precambrian granite gneiss. The principal area of carbonatite is 3 km across but the outer margins are everywhere obscured by an apron of superficial deposits. The carbonatite could have a diameter of 6 km. No contemporaneous igneous silicate rocks have been identified within the intrusion. The central rocks were mapped by Lapido-Loureiro (1973) as 'feldspathic carbonatites and carbonatites' and these grade from carbonate-free feldspathic breccias to carbonatites. The feldspathic rocks are concentrated nearer the margins of the outcrop and are clearly fenites, but whether the feldspar is potassic or sodic is not described, although the former seems the more probable. The carbonatites are described by Lapido-Loureiro (1973) as principally calcitic with ankeritic veins, but three carbonatites analysed by Alberti et al. (2000) are dolomite carbonatites. Iron oxides, apatite, barite and quartz are usual in the carbonatites and strontianite and parisite/synchysite have also been identified, while rare earth-, Nb- and Th-bearing minerals have been reported from the central area. The carbonatite analyses reported by Alberti et al. (2000) include a full suite of trace elements and C, O, Sr and Nd isotope data.

The following values in soil and bedrock respectively have been obtained ThO2 1.03 and 0.83%; Nb2O5 0.24 and 0.25%; Ce2O3 0.87 and 0.62%; La2O3 0.11 and 0.09% (Lapido-Loureiro, 1973, Table 23). P2O5 values up to 8.5% were obtained from carbonatites (Machado, 1958).

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.MACHADO, F.J. de S. 1958. The volcanic belt of Angola and its carbonatites. Publication. Commission for Technical Cooperation in Africa South of the Sahara. Joint Meeting, Leopoldville, 44: 309-17.

Fig. 3_14 Longonjo (after Lapido-Loureiro, 1973, p. 134).
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