Mkwisi

The Mkwisi limestone is described by Bailey (1961) as intruding a series of late Precambrian metasediments. It is associated with a breccia which consists of country rock fragments in a calcite cement. The limestone contains an abundance of included fragments that are essentially of the local country rocks. The limestone forms about eight major bodies, up to several hundred metres across, and a number of smaller ones, with the surrounding rocks intensely brecciated and veined with calcite. There is little or no evidence of flow structures. Feldspathisation on a small scale along joints and bedding planes of the country rocks and at the margins of xenoliths was noted by Bailey (1961). The limestones are of calcite but iron oxides and apatite are abundant accessories and monazite and rutile are generally present; niobian rutile was identified in soils. Analyses of soils gave low niobium values. Bailey (1961) discusses the similarities and differences of these rocks from typical carbonatites and concludes that they were indeed carbonatitic. See also the discussion at the end of the Bailey (1961) paper, and Keshya (No. 180-00-011). Cairney (1964) considered, however, that these rocks are not carbonatites but part of the Mampompo Limestone which underlies the Mkwisi area at shallow depth. Hamilton and Deans (1963) and Powell (1965) showed that the limestones have 86Sr/87Sr ratios higher than those of accepted carbonatites and similar to those of sedimentary limestones.