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Platinum-group elements in peridotite xenoliths and kimberlite from the Premier kimberlite pipe, South Africa

Centre for Research on Magmatic Ore Deposits, Department of Geology, University of Pretoria, Pretoria 0002, South Africa, e-mail: wdmaier{at}scientia.up.ac.za

P. Peltonen and R. Juvonen

Geological Survey of Finland, P.O. Box 96, FIN-02151 Espoo, Finland, e-mail: petri.peltonen{at}gtk.fi

C. Pienaar

Centre for Research on Magmatic Ore Deposits, Department of Geology, University of Pretoria, Pretoria 0002, Private Bag X112, Silverton, Pretoria, 0001, South Africa. (present address: Council for Geoscience, Pretoria, South Africa), e-mail: cjoubert{at}geoscience.org.za

Peridotite mantle xenoliths from the Premier kimberlite pipe. South Africa, have mostly similar Os-Ir-Ru-Rh contents as primitive mantle, but they are relatively depleted in Pt and Pd. The concentrations of the IPGE and Rh remain largely constant with decreasing indices of melt extraction (e.g. Al₂O₃, CaO, Sc), but there is a positive correlation between Pt and Pd contents and the basaltic components. At large degrees of melt extraction, all PGE are highly depleted indicating complete consumption of the PGE host phases. These compositional patterns could suggest that Pt and Pd behave in an incompatible manner and the IPGE and Rh in a compatible manner during the initial stages of partial melting. In addition, there is some evidence that Pt and Pd may be mobile during mantle metasomatism. For example, there is a positive correlation between Pt and Pd contents and those of mobile elements such as Ba, Rb, K₂O and Sr. Furthermore, those xenoliths that equilibrated at the shallowest levels (the spinel harzburgites) are most depleted in Pt and Pd, in agreement with a model of refertilization of the deeper portions of the SCLM. It has been suggested by previous workers that the refertilization of the Premier xenoliths occurred during the Bushveld magmatic event. This could be tested by collecting more PGE data from other metasomatised Kaapvaal xenoliths and comparing them to the present data.