Alkaline Rocks and Carbonatites of the World

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Occurrence number: 
Longitude: 34.08, Latitude: -16.32

Muambe is a circular carbonatite plug which with its aureole of feldspathic rocks is 5 km in diameter. The feldspathic rocks form a circular ridge, rising to some 400 m above the surrounding plain, that envelopes an inner crater-like depression within which are craggy hills of carbonatite. Stalactites and stalagmites are developed in caves within the carbonatite (Dias, 1961, Figs 1 and 2). The carbonatitic rocks cover 8.7 km2. No petrographic descriptions of the carbonatites appear to be available but the chemical data in Dias (1961) indicate they are principally calcite carbonatites with some iron-rich types and specimens in the Natural History Museum, London, are essentially heterogeneous ferrocarbonatites. Cilek (1989) reports that there are a lot of silicate minerals in the carbonatite and that fluorite is abundant; he gives three carbonatite analyses. The feldspathic rocks, which are essentially potassic fenites, form a continuous ring around the carbonatite and in some places are heavily brecciated. They consist principally of turbid K-feldspar which may be fine-grained, form angular grains in a finely comminuted matrix, or angular rock fragments veined by finer grained feldspathic material. Dixey et al. (1937) describe some of these rocks as being like tuffs; in some varieties 'lapilli' of carbonate are also abundant. Three analyses of feldspathic fenites contain 12-13.7% K2O (Cilek, 1989). These rocks pass outwards into feldspathic Karoo sandstones. Dykes within the vent are of olivine nephelinite and phonolite. The former contain phenocrysts of augite and rather fewer of olivine in a pyroxene-rich matrix and a very fine-grained base that probably contains nepheline. The phonolites contain nepheline phenocrysts in a matric of nepheline and aegirine and subordinate feldspar; poikilitic plates in one sample described by Dixey et al. (1937) are possibly eudialyte.

There is extensive fluorite mineralization in the contact zone between carbonatite and fenite. In some trenches masses of fluorite up to 20 m thick were encountered. Blue and yellow varieties are present, the former being rich in Be (concentrations as high as 10,000 ppm), Sr, Y and La. Analyses and a full account are in Cilek (1989).
CILEK, V.G. 1989. Industrial minerals of Mozambique. Geological Survey, Prague. 326 pp.DIAS, B. 1961. Geologia do Monte Muambe. Boletim, Serviços de Geologia e Minas, Provincia de Moçambique, 27: 37-64.DIXEY, F., SMITH, W.C. and BISSET, C.B. 1937. The Chilwa Series of southern Nyasaland. Bulletin, Nyasaland Geological Survey, 5: 1-82. (reprinted 1955)
Fig. 3_185 Muambe (after Dias, 1961, 1:20,000 map).
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