Paragenetic Trends of Oxide Minerals in Carbonate-rich Kimberlites, with New Analyses from the Benfontein Sill, South Africa

Abstract

The relationship among kimberlites, carbonate-rich bodies associated with them, and the carbonatites associated with alkalis rock complexes are reviewed. Particular attention is paid to the parageneses of oxide minerals in six carbonate-kimberlites: Peuyuk, Tunraq, Wesselton, Liqhobong, De Beers, and Benfontein. New analyses of spinel, limonite, and perovskite from the lower Benfontein Sill, are consistent with previous reports and can be divided into (1) early macrocrysts and cores of grains, and (2) late rims and groundmass grains. The evolution of a carbonate-rich residuum with progressive crystallization appears to be typical of carbonate-rich kimberlite magmas, and is texturally related to the two stages of oxide precipitation in these carbonate-kimberlites. Thus, early Mg-ilmenite and Cr-rich spinel are separated by reaction textures and carbonate from later Mg-Al-titanomagnetite, perovskite, and accessory utile and apatite. The spinels span a large range in composition from Mg-Al-chromite to Mg-Al-titanomagnetite, with an intermediate gap. This simplified paragenetic scheme, and in particular the spinel trend, is repeated in the five other carbonate-kimberlites reviewed. It may be representative of the hypabyssal kimberlites in general, and others where fluidization processes did not completely disrupt the crystallization sequence.