Platinum-group element behavior in the lower part of the upper zone, eastern Bushveld Complex; implications for the formation of the main magnetite layer

Abstract

Analytical data for the platinum-group elements (PGE) and gold on 17 samples from sulfide-mineralized and -unmineralized rocks of the lower portion of the upper zone and two samples from the main zone of the eastern Bushveld Complex are presented. Almost all samples contain less than 100 ppb total PGE, indicating that the sulfide-enriched layers of the upper zone generally contain only negligable amounts of PGE. However, PGE enrichment previously reported from a sulfide-bearing anorthosite below the lower magnetite layer 2 has been confirmed by microscopic investigations and is interpreted as a zone locally enriched by hydrothermal fluids.Increasing PGE concentrations and Pd/Ir and Pt/Pd ratios with stratigraphic height indicate differences in the behavior of the PGE. Incompatibility follows the sequence Ir < Rh < Ru < Pd < Pt, with Pt being approximately eight times more incompatible than Ir. Rocks below and above the main magnetite layer are characterized by two different PGE patterns which are seen as the effect of a magma-mixing event which resulted either from a breakdown of density stratified liquid layers or an influx of a small volume of new magma at the level of the main magnetite layer.Rocks below the main magnetite layer display a chondrite-normalized PGE pattern similar to that of the Picket Pin Pt-Pd deposit in the Stillwater Complex.