Controls on Pt/Pd ratios in Bushveld magmas and cumulates: a review complemented by new W isotope data

Abstract

The Bushveld Complex of South Africa is underlain by a fine-grained sill complex which most workers interpret to represent the quenched parent magmas to the intrusion. The sills have unusually high Pt contents (up to ~ 25 ppb) and Pt/Pd ratios (average 1.50) exceeding those in most other mantle magmas globally. Unusually high Pt/Pd is also found in many Bushveld cumulates. Understanding the origin of the high Pt/Pd is important for exploration, in view of the contrasting monetary value of the metals, but also for unravelling the petrogenesis of the intrusion. Here, we review existing platinum-group element (PGE) data and present the first radiogenic W isotope data on a Bushveld rock, to evaluate a range of potential models, including PGE fractionation prior to final magma emplacement and within the Bushveld magma chamber, magma derivation from the sub-continental lithospheric mantle (SCLM), contamination of Bushveld magma with Pt-rich continental crust, and a meteoritic component in the mantle source to the magmas or in the crust with which the magmas interacted. We identify three key processes causing fractionation of metals prior to final magma emplacement and within the Bushveld chamber, namely crystallisation of Pt alloys, partial melting of cumulus sulfides triggered by flux of volatiles followed by sulfide melt percolation, and mobilisation of PGE by percolation of volatiles through the cumulate pile. The currently available W and Ru isotope data are inconsistent with derivation of the Bushveld magmas from mantle or crustal sources containing an enhanced meteoritic component relative to normal post-Hadean mantle.