Bonga

Located 11 km northeast of Tchivira (005-00-029) Bonga forms a steep-sided hill rising 900 m from a peneplain; there is a central depression 200 m deep. It is clearly shown in photographs in Alves (1968). Bonga is a carbonatite plug which cuts Precambrian granites, migmatites and gneisses. Half a dozen small 'pipes' west and south of Bonga, and two 4 km to the north, consist of breccias with clasts of many rock types including trachytic and phonolitic varieties. Amongst xenocrysts garnet is abundant, which may suggest a kimberlitic association (Lapido-Loureiro, 1973), and aegirine-augite and a blue sodic amphibole are found in two pipes. Bonga consists predominantly of carbonatite with an outer zone of feldspathic carbonatite which is clearly defined on the map of Alves (1968). The carbonatites are normally white rocks sometimes showing a foliation defined by concentrations of magnetite, pyrite, pyrochlore and biotite. Calcite, ankerite/dolomite and what Lapido-Loureiro (1973) calls 'ferro-mangano-dolomitic carbonatites’, with more than one of these carbonate minerals often present in one rock, are predominant. Accessory minerals include apatite, with rock analyses giving 3-5 wt% P2O5, barite, pyrochlore, magnetite, rutile, parisite, synchysite, strontianite, amphibole, sometimes alkaline, quartz and iron oxides. Five whole rock analyses of carbonatite, which prove to include calcio-, magnesio- and ferro-carbonatites, and some qualitative trace element data are given in Lapido-Loureiro (1973). Chemical data on pyrochlore and an average chemical composition of nine carbonatites are given by Issa et al. (1991). Three carbonatite analyses will be found in Coltorti et al. (1993) and five in Alves (1968). The fenites which occur at Bonga are poorly exposed, and prove to be highly potassic (1.15 wt% Na2O; 9.83 wt% K2O) (Issa et al., 1991).