Ngualla

Intruded into Precambrian gneisses and quartzites Ngualla is a 3.5x2.5 km carbonatite-fenite complex of oval outline. Over a north-south-trending central ridge, and an area on the northwestern side of the complex, outcrops carbonatite which is surrounded by an area thickly capped by red soil. This in turn is surrounded by a ring of low hills that are dominantly composed of fenites. The fenite zone is up to one kilometre wide with a breccia zone adjacent to the carbonatite. Sodic pyroxene and/or sodic amphibole are present. The carbonatite is broadly of three types (James, 1954). The outer sovite is poorly banded and is magnetite-free; apatite, biotite, muscovite, quartz and chlorite are accessories. An intermediate zone sovite is well banded and contains euhedral magnetite; both the inner and intermediate zones commonly contain dolomite. A central zone comprises a poorly banded sovite containing fluorite, biotite, amphibole and parisite. Rare pyrochlore is found in this zone and James and McKie (1958) have described columbite which pseudomorphs pyrochlore. Dolomitic and ankeritic veins are widespread throughout the complex, as are calcite-quartz veins with minor galena, barite and chalcopyrite; monazite is also reported by James (1954). Barite veins several metres wide occur. Analyses, including some trace elements, of fenite, which is highly potassic, sovite, magnesiocarbonatite, ferrocarbonatite, alnoite and apatite-magnetite veins are given by van Straaten (1989).