Geochemistry of magnesian ilmenites from kimberlites in South Africa and Lesotho

Abstract

Magnesium ilmenite from discrete nodules and lamellar intergrowths with pyroxene from the Kao, Sekameng, Frank Smith, and Monastery kimberlites has been analysed for Ti, Fe, Mg, Nb, Zr, Cr, Cu, Zn, Mn, Co, and Ni. Each kimberlite contains discrete ilmenites which exhibit a wide compositional range within the ilmenite-geikeilite series. Lamellar ilmenites from Frank Smith and Monastery differ in composition but both show a limited range in composition which lies within the compositional range shown by discrete ilmenites from these pipes. The ilmenites are enriched in Nb, Zr, Cr, Co, and Ni and depleted in Mn, Cu, and Zn relative to Mg-poor ilmenites from basic intrusions. Nb, Zr, and Ni correlate with Fe, Ti and Mg variations but the abundances of the other trace elements are independent of major element variation. R-mode factor analysis is interpreted to imply that the geochemistry cannot be interpreted in terms of a differentiation hypothesis in which trace elements are removed from or concentrated in residua. Factor scores and major element abundances indicate that each pipe is characterized by a particular suite of discrete ilmenite nodules, which are considered to be phenocrysts in a proto-kimberlite magma. Lamellar ilmenite-pyroxene intergrowths are unlikely to have had a eutectic origin, and show no simple relationship to the discrete ilmenites.