ASPECTS OF THE GEOCHEMISTRY OF KIMBERLITES FROM THE PREMIER MINE, AND OTHER SELECTED SOUTH AFRICAN OCCURRENCES WITH PARTICULAR REFERENCE TO THE RARE EARTH ELEMENTS

Comparative major element, as well as trace element data for REE, Sr, Ba, Th, U, Rb and Cs for three South African kimberlite groups: Premier, Koffyfontein—Ebenhaezer and Bellsbank kimberlites are presented. These kimberlites have different ages, modes of emplacement, degree and type of crustal contamination. Major element data indicates such contamination and requires correction if SiMg > 0.9. Loss of volatiles in the relative order Cs > Rb > K near wall rock contacts and intrusive dykes are evident in Premier kimberlites. High and variable ThU generally indicates loss of U by groundwater leaching in kimberlites. The refractory elements, Ti, Nb, Ta, Zr, Hf and the REE as well as P, are considered to be insignificantly affected by crustal contamination. It is suggested that a study of their geochemistry may elucidate the genetic relationship of upper mantle derived material. The close and linear association of some of these elements observed in kimberlites suggests the presence of a carbonated-phosphate phase, probably apatite, as a host mineral for LREE, Th, U, Sr and volatiles (e.g. F−, Cl−, H2O) in fertile upper mantle assemblages. REE in South African kimberlites show different degrees of enrichment and LREE/HREE fractionation previously only reported in carbonatites. South African kimberlites show varying degrees of Eu depletion, with anomalies most conspicuous in carbonate-rich micaceous kimberlites. More “primitive” Premier kimberlites exhibit weak negative Yb anomalies attributed to garnet fractionation. REE whole rock patterns in kimberlites are influenced by mineralogical effects. REE enrichment and fractionation in kimberlitic magmas is considered to be dominantly controlled by the selective formation and removal of mobile REE-carbonate complexes into carbonatitic fluids which separate from the silica-richer complementary “kimberlite” magma during emplacement. A close genetic relationship between kimberlites and carbonatites is implied.