Platinum-group elements in the Pyroxenite Marker, Bushveld Complex: implications for the formation of the Main Zone

Abstract

Concentrations of platinum-group elements and sulphides in the Pyroxenite Marker of the upper Main Zone are variable, but generally low (up to 100 ppb PGE and 0.2 weight % S). The metal patterns may mostly be explained by sulphide segregation from PGE depleted residual Upper Critical Zone magma, but they are inconsistent with sulphide segregation from a replenishing influx of undepleted Critical Zone magma. Instead, we favour a model whereby a relatively cool and dense Main Zone crystal mush intruded the Bushveld chamber during the later stages of the deposition of the Upper Critical Zone and displaced warmer and lighter residual magma depleted in chalcophile metals (Sharpe, 1985). Based on the metal contents and textural evidence such as the occurrence of ophitic textures, we hypothesize that the Pyroxenite Marker formed in response to localized supercooling and the suppression of plagioclase crystallization. The model implies that the layer represents a poor target for PGE mineralization in the upper portions of the Bushveld Complex.

In situ Sr Isotope Compositions of Plagioclase from a Complete Stratigraphic Profile of the Bushveld Complex, South Africa: Evidence for Extensive Magma Mixing and Percolation

“…Evidence for the injection of UZ magma on top of the MZ cumulate pile is provided by the undulating contact between the MZ anorthosite and the Pyroxenite Marker with typical flame-like protrusions of anorthosite into the overlying gabbroids, described in the eastern limb of the Bushveld Complex (Fig. 2, Maier et al, 2001). In the sedimentological research community, these structures are unequivocally interpreted to result from density contrasts between unlithified layers, where the underlying less dense layer pushes up into the overlying denser layer (e.g., Anketell et al, 1970).…”

The Merensky Reef, Bushveld Complex: Mixing of Minerals Not Mixing of Magmas

“…This process eroded a portion of the Critical zone, forming the unconformity at the base of the Merensky pyroxenite. Maier et al (2001) also suggested that that magma of the Main zone had a low temperature and high density. This injection of new magma elevated the Critical zone liquid, which continued to crystallize orthopyroxene and plagioclase.…”

Investigation of Mantle Dynamics from Platinum Group Elements and Rhenium-Osmium Isotope Systematics of Mantle Xenoliths from Oahu

“…The PGE concentrations of the pyroxenites show positive correlations with each other (Fig 5.4), for example, good correlation exists between Os, Ir, Ru (R (Bennett et al, 2000) and pyroxenites ( Maier et al, 2001;Luguet et al, 2008) and in these pyroxenitic sulfides , although the origin of the Pt enrichment in these samples is not currently understood.…”