ORIGINS OF CHROMIAN AND ALUMINOUS SPINEL MACROCRYSTS FROM KIMBERLITES IN SOUTHERN AFRICA

Abstract

A comparison is made of the major- and minor-element compositions of spinel-group minerals that occur as single-crystal macrocrysts in kimberlites from southern Africa (approximately 1500 grains from 20 kimberlites on the Kaapvaal and Zimbabwe cratons) with those from a variety of mantle xenoliths from southern Africa and North America (approximately 150 xenoliths). Extensive compositional overlap between macrocryst and xenolith samples leads to the conclusion that most macrocrysts of spinel-group minerals are likely to be xenocrysts from garnet-bearing peridotite. None are considered to be igneous phenocrysts related to the host kimberlites. Chromian spinel with Cr/(Cr + Al) > 0.88 is derived from garnet-free assemblages that have bulk Cr/(Cr + Al) values too low to form garnet, or from assemblages in which aluminum is partitioned into a different phase, such as metasomatic phlogopite. Such Cr-rich macrocrysts are more common in Group-I kimberlites than in Group-II kimberlites. Moderate degrees of metasomatism produced spinel-group minerals enriched in Fe, Ti and Cr in many Group-I suites, but chromite xenocrysts from highly metasomatized assemblages ( e.g. , zircon-bearing) are uncommon. Unusually Fe- and Ni-rich macrocrysts unique to the Lace mine are from Fe-rich orthopyroxene-bearing peridotites or pyroxenites that may be the product of impact-induced mantle melting associated with the Vredefort structure. Regional variations in chemical characteristics of spinel-group minerals suggest that the mantle is more “fertile” [ i.e. , lower Cr/(Cr + Al) values] near craton margins ( e.g., Liqhobong on the southeastern margins of the Kaapvaal Craton). Though situated on the Kaapvaal Craton, the Jwaneng suite of “spinel” has chemical characteristics similar to Orapa and Letlhakane suites, which suggests closer affinity to the Zimbabwe cratonic mantle than the Kaapvaal mantle.