Alkaline Rocks and Carbonatites of the World

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Occurrence number: 
Longitude: 18.83, Latitude: -27.58

A series of diatremes, dykes and sills extends over an area of 3,000 km2 in the Great Karas Mountains (Verwoerd, 1993). About 100 occurrences have been identified of which some 41% are in the form of sills and 25% pipe-like bodies. Most dykes and diatremes are filled with coarse breccia in which there are also accretionary lapilli and other spherical bodies of 5-10 mm diameter, these structures and the groundmass consisting of ankerite and subordinate calcite, chlorite, biotite, albite and Fe-hydrates. Veinlets of a similar composition penetrate the country rocks. Most sills and dykes consist in part of a relatively homogeneous rock with 10-20% CO2 and about 25% SiO2 and phenocrysts of biotite, augite, kaersutitic hornblende and ilmenite in a matrix of ankerite. Carbonate lath-shaped pseudomorphs, probably after melilite or feldspar, are common. Most trace elements are low and show no obvious parallels with carbonatite or kimberlite but are similar to alnoite. However, one sill had >1000 ppm Sr supporting a carbonatitic affinity while fenitization at two localities, with the production of K-feldspar and blue amphiboles, certainly suggests a carbonatite association.

A Rb-Sr isochron age of 491(8 Ma was obtained from a whole rock and two biotite samples (Allsopp et al., 1979).
ALLSOPP, H.L., KOSTLIN, E.O., WELKE, H.J., BURGER, A.J., KRONER, A. and BLIGNAULT, H.J. 1979. Rb-Sr and U-Pb geochronology of late Precambrian - early Palaeozoic igneous activity in the Richtersveld (South Africa) and southern South West Africa. Transactions of the Geological Society of South Africa, 82: 185-204.VERWOERD, W.J. 1993. Update on carbonatites of South Africa and Namibia. South African Journal of Geology, 96: 75-96.
Scratchpads developed and conceived by (alphabetical): Ed Baker, Katherine Bouton Alice Heaton Dimitris Koureas, Laurence Livermore, Dave Roberts, Simon Rycroft, Ben Scott, Vince Smith