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Geochemistry and mineralogy of the Platreef and “Critical Zone” of the northern lobe of the Bushveld Complex, South Africa: implications for Bushveld stratigraphy and the development of PGE mineralisation

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Abstract

The northern lobe of the Bushveld Complex is currently a highly active area for platinum-group element (PGE) exploration. This lobe hosts the Platreef, a 10–300-m thick package of PGE-rich pyroxenites and gabbros, that crops out along the base of the lobe to the north of Mokopane (formerly Potgietersrus) and is amenable to large-scale open pit mining along some portions of its strike. An early account of the geology of the deposit was produced by Percy Wagner where he suggested that the Platreef was an equivalent PGE-rich layer to the Merensky Reef that had already been traced throughout the eastern and western lobes of the Bushveld Complex. Wagner’s opinion remains widely held and is central to current orthodoxy on the stratigraphy of the northern lobe. This correlates the Platreef and an associated cumulate sequence that includes a chromitite layer—known as the Grasvally norite-pyroxenite-anorthosite (GNPA) member—directly with the sequence between the UG2 chromitite and the Merensky Reef as it is developed in the Upper Critical Zone of the eastern and western Bushveld. Implicit in this view of the magmatic stratigraphy is that similar Critical Zone magma was present in all three lobes prior to the development of the Merensky Reef and the Platreef. However, when this assumed correlation is examined in detail, it is obvious that there are significant differences in lithologies, mineral textures and chemistries (Mg# of orthopyroxene and olivine) and the geochemistry of both rare earth elements (REE) and PGE between the two sequences. This suggests that the prevailing interpretation of the stratigraphy of the northern lobe is not correct. The “Critical Zone” of the northern lobe cannot be correlated with the Critical Zone in the rest of the complex and the simplest explanation is that the GNPA-Platreef sequence formed from a separate magma, or mixture of magmas. Chilled margins of the GNPA member match the estimated initial composition of tholeiitic (Main Zone-type) magma rather than a Critical Zone magma composition. Where the GNPA member is developed over the ultramafic Lower Zone, hybrid rocks preserve evidence for mixing between new tholeiitic magma and existing ultramafic liquid. This style of interaction and the resulting rock sequences are unique to the northern lobe. The GNPA member contains at least seven sulphide-rich horizons with elevated PGE concentrations. Some of these are hosted by pyroxenites with similar mineralogy, crystallisation sequences and Pd-rich PGE signatures to the Platreef. Chill zones are preserved in the lowest Main Zone rocks above the GNPA member and the Platreef and this suggests that both units were terminated by a new influx of Main Zone magma. This opens the possibility that the Platreef and GNPA member merge laterally into one another and that both formed in a series of mixing/quenching events involving tholeiitic and ultramafic magmas, prior to the main influx of tholeiitic magma that formed the Main Zone.

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Acknowledgements

The authors would like to thank the staff at Anglo Platinum’s Sandsloot Mine, particularly Alan Bye and Alfred Sarila, for logistical support during the mapping and sampling. Eveline de Vos and Peter Fisher are thanked for their assistance with the ICP and SEM analyses at Cardiff and Terry Williams, John Spratt and Anton Kearsley are thanked for their assistance with analytical work by Paul Armitage at the Natural History Museum. Wes Gibbons and Jock Harmer are thanked for their advice and input. The paper benefited from constructive reviews by Chris Lee and Tony Naldrett. Tony Naldrett also kindly made available his unpublished PGE assays from Drenthe for use in Table 4 and this is greatly appreciated. This research makes new use of Larry Hulbert’s remarkable 1983 DSc thesis and re-interprets his work into the GNPA member in the light of new data. The authors would like to thank Wolf Maier for helping us to obtain a copy of this thesis. Key elements of this paper could not have been assembled without Dr Hulbert’s original work and we are extremely grateful to have had access to it. We accept full responsibility for our conclusions (and any errors in interpretation), particularly where they differ from the original. Paul Armitage’s PhD research was funded by the University of Greenwich. David Holwell’s PhD research is funded by the NERC and supported by Anglo Platinum through Industrial CASE project (NER/S/C/2003/11952). PGE analytical work at Cardiff is supported by the Leverhulme Trust (grant F/00407/K to IMcD) and the ICP laboratory is supported by the NERC, through Joint Infrastructure Fund award NER/H/S/2000/00862. Finally, the first author would like to thank Steph and Robbie for their patience and support throughout the completion of this work.

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  1. Department of Earth, Ocean and Planetary Sciences, Cardiff University, P.O. Box 914, Cardiff, CF10 3YE, UK

    Iain McDonald & David A. Holwell

  2. Department of Earth and Environmental Sciences, University of Greenwich, Chatham Maritime, Kent, ME4 4TB, UK

    Paul E. B. Armitage

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  1. Iain McDonald
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McDonald, I., Holwell, D.A. & Armitage, P.E.B. Geochemistry and mineralogy of the Platreef and “Critical Zone” of the northern lobe of the Bushveld Complex, South Africa: implications for Bushveld stratigraphy and the development of PGE mineralisation. Miner Deposita 40, 526–549 (2005). https://doi.org/10.1007/s00126-005-0018-6

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