Melkfontein

In the East Griqualand kimberlite field the Melkfontein carbonate tuff occurrence was formerly considered to be kimberlitic, but more recent work seems to indicate that a carbonatite association is more likely (Boctor et al., 1983). The tuff has a thickness of about 50 m and dips at a shallow angle. It consists of calcite with varying quantities of angular xenoliths of Karoo sandstone and shale, basement rocks and garnet granulite together with xenocrysts of garnet, clinopyroxene, amphibole and plagioclase; magnetite and apatite form disseminated crystals in the carbonate matrix. The last two minerals have similar chemistries to these minerals in carbonatites (Boctor et al., 1983) and a whole rock analysis of the tuff, with a range of trace elements, including REE (Nixon et al., 1984), also indicates a carbonatitic composition. Other mineral phases in the tuff appear to be derived from granulites.