Alkaline Rocks and Carbonatites of the World

Setup during HiTech AlkCarb: an online database of alkaline rock and carbonatite occurrences

Melkfontein

stripes

Occurrence number: 
151-00-034
Country: 
South Africa
Location: 
Longitude: 29.08, Latitude: -30.5
Carbonatite: 
Yes

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.

Age: 
Davis (1978) obtained a zircon age of 63.4 Ma, which is much younger than the associated kimberlites.
References: 

BOCTOR, N.Z., NIXON, P.H., BUCKLEY, F. and BOYD, F.R. 1983. Petrology of carbonate tuff from Melkfontein, East Griqualand, Southern Africa. In J. Kornprobst (ed) Kimberlites and related rocks. 73-82. Elsevier, Amsterdam.DAVIS, G.L. 1978. Zircons from the mantle. Short Papers of the Fourth International Conference. Geochronology, Cosmochemistry and Isotope Geology. United States Geological Survey, Open File Report, 78-701: 86-8.NIXON, P.H., BOYD, F.R. and BOCTOR, N.Z. 1983. East Griqualand kimberlites. Transactions of the Geological Society of South Africa, 86: 221-36.

Scratchpads developed and conceived by (alphabetical): Ed Baker, Katherine Bouton Alice Heaton Dimitris Koureas, Laurence Livermore, Dave Roberts, Simon Rycroft, Ben Scott, Vince Smith