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.

Petrogenesis of Massive Sulfide Ores and Associated Globular Sulfide Texture in Raglan Horizon: Implications for Exploration of Concealed Orebodies at Depth

“…Park et al (2013) found a decrease in Pd/Pt ratios of Pual Ridge lavas and they suggested that sulfide melt segregation would cause Pd/Pt ratio to drop due to higher sulfide-silicate partition coefficient (D) Pd has than Pt. In addition, Maier et al (2022) also proposed that the variation in 𝐷 𝑃𝐺𝐸𝑠 𝑠𝑢𝑙𝑓𝑖𝑑𝑒 𝑚𝑒𝑙𝑡−𝑠𝑖𝑙𝑖𝑐𝑎𝑡𝑒 𝑚𝑒𝑙𝑡 might be responsible for the anomalous Pd/Pt ratio found at the Bushveld Complex, while also alluding to the possible presence of a Pt-rich primordial mantle domain. While there is a long-standing debate on the partition coefficients of PGEs, the experiments performed by Mungall and Brenan (2014) suggested that 𝐷 𝑃𝑡 and 𝐷 𝑃𝑑 are almost identical with no clear evidence such that one is consistently higher than the other; those authors proposed that low Pd/Pt in the Bushveld resulted from partial melting of a source mantle rich in residual Pt alloy but lacking in sulfide-hosted Pd.…”

“…While there is a long-standing debate on the partition coefficients of PGEs, the experiments performed by Mungall and Brenan (2014) suggested that 𝐷 𝑃𝑡 and 𝐷 𝑃𝑑 are almost identical with no clear evidence such that one is consistently higher than the other; those authors proposed that low Pd/Pt in the Bushveld resulted from partial melting of a source mantle rich in residual Pt alloy but lacking in sulfide-hosted Pd. Besides, for systems with relatively low R-factor such as the massive sulfide accumulations at Raglan (300-1000) and the basal mineralization in Flatreef (~1000), the PGE contents are insensitive to the potential difference in D (Maier et al, 2022). Taking Raglan as an example, the Pd/Pt ratio of 3 cannot be reproduced by manipulating model partition coefficients without assuming unrealistically low 𝐷 𝑃𝑡 (~500) which is more than three orders of magnitude smaller than assumed 𝐷 𝑃𝑑 .…”