Alkaline Rocks and Carbonatites of the World

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Mabounie

stripes

Occurrence number: 
057-00-001
Country: 
Gabon
Location: 
Longitude: 10.58, Latitude: -0.75

Located 40 km east-southeast of Lambarene, the Mabounie complex is emplaced in Precambrian gneisses, migmatites and mica schists, but has a faulted western margin where it is in contact with Cretaceous sediments. A central carbonatite is surrounded to the east by a semi-circular intrusion of syenite around which is a broad arc of fenites. The 1.2 km diameter carbonatite comprises sovite and beforsite which are interleaved with rocks rich in mica and/or magnetite. Xenoliths of phlogopitised basement rocks are common. Apart from major calcite, dolomite, apatite and tetraferriphlogopite, accessory minerals include pyrochlore, ilmenite, olivine, baddeleyite, pyrite and pyrrhotite. The syenites consist of a leucocratic facies of albite, hornblende, which is partly or wholly replaced by magnesio-riebeckite, aegirine and accessory pyrochlore and opaque phases. A melanocratic facies consists of aegirine-augite, microcline, partly replaced by albite, some magnesio-riebeckite and pyrochlore. Both syenite facies contain <0.3% K2O but >10% Na2O. The fenites are characterised by the development of riebeckite and/or arfvedsonite and aegirine. Dykes cutting the syenites and fenites and extending into the country rocks include trachytes, lamprophyres and quartz-baryte rocks. The trachytes contain biotite and an analysis (Laval et al., 1988) includes 13.9% K2O. The lamprophyres have carbonated phenocrysts of phlogopite, olivine and possible clinopyroxene in a matrix of carbonate, aegirine-augite and accessories. Analyses of all major rock types are given by Laval et al. (1988).

Economic: 
Weathering of the carbonatite has led to the formation of a residual soil averaging 40 m in thickness in the lower part of which phosphate (apatite) is concentrated and averages 24%. In one horizon Nb2O5 reaches 3%, and the deposit is now being evaluated for the production of pyrochlore. The economic geology is discussed in some detail by Laval et al. (1988).
Age: 
K-Ar on phlogopite from carbonatite gave 660(13 Ma (Laval et al., 1988).
References: 
LAVAL, M., JOHAN, V. and TOURLIÈRE, B. 1988. La carbonatite de Mabounié: exemple de formation d'un gîte résiduel à pyrochlore. Chronique de la Recherche Minière, 491: 125-36.
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