- © 2011 Geological Society of South Africa
The four platiniferous dunite pipes in the eastern limb of the Bushveld Complex, South Africa, differ sufficiently from one another to warrant investigation as separate entities, but despite their differences they are similar enough to enable us to establish a holistic genetic hypothesis. In our previous contributions on the Driekop, Onverwacht, and Twyfelaar localities we noted that despite the pipes being aligned perpendicular to the Rustenburg Layered Suite they may include sheets and lenses that pseudomorph the cumulate layering. Moreover, wall rocks may be severely disrupted and downwarped. In this contribution we investigate Mooihoek, the site of the first discovery of economic platinum mineralization in South Africa. The pipe crosscuts a sequence of leucocratic cumulates in the lowermost part of the Upper Critical Zone, outcropping well into the footwall of the stratiform UG2 and Merensky reefs, and reveals a pronounced mineralogical and chemical zonation. PGE are concentrated within a small, carrot-shaped core-zone (almost entirely mined out) within a stock-like body of magnesian dunite which in turn is constrained within concentric envelopes of iron-rich wehrlite and iron-rich clinopyroxenite pegmatite. In the upper parts of the mine the core-zone was reported by Wagner (1929) to have consisted of a kernel of very richly mineralized and very coarse-grained hortonolite (iron-rich olivine) dunite pegmatite, enclosed by an annular body of somewhat less well-mineralized and less coarse-grained iron-rich wehrlite pegmatite. The composition of the magnesian dunite at Mooihoek is broadly similar to that reported from the other pipes (Fo85–82). The iron-rich ultramafic assemblages at Mooihoek define a relatively broad range of compositions (Fo53–44). The most evolved olivine occurs in the central kernel, in association with the richest mineralization, which in turn was reported to be spatially associated with metre-sized aggregates of phlogopite and hornblende. Lumps of chromitite (in the main stock) and metre-sized aggregates of Ti-magnetite and ilmenite (in the core-zone) are interpreted as xenoliths derived from dismembering of primary layers, the Fe-Ti oxides resulting from reaction of the chromite with iron-rich melts.
The spatial association of magnesian dunite and iron-rich ultramafic assemblages in the four platiniferous pipes is reconciled with complex, multi-stage processes. The magnesian dunite formed by a process of flowage differentiation of ultramafic magma, possibly of Lower Zone lineage, intruded through vertical conduits within the Rustenburg Layered Suite (Scoon and Mitchell, 2009). The moderately differentiated envelope of wehrlite and clinopyroxenite pegmatite (Fo65) around the Driekop pipe was ascribed to disequilibrium partial melting of noritic wall rocks (due to heat associated with emplacement of the magnesian dunite). Ferromagnesian melts percolated downward within the partially melted wall rocks. The somewhat more differentiated outer envelope at Mooihoek (Fo53) is ascribed to a similar process, albeit the locally-derived melts were blended with exotic melts associated with formation of the core-zone. Comparison of the unusual mineralogy and chemistry of the innermost part of the core-zone at Mooihoek (Fo44) with Replaced Merensky Reef at the Amandelbult mine (Fo44) i.e. reef replaced by discordant bodies of iron-rich ultramafic pegmatite, or IRUP, is revealing. The silicate and oxide mineralogy of both localities is broadly similar. The entire absence of plagioclase from these rocks is a diagnostic feature. Both localities reveal a diagnostic suite of base-metal sulphides (dominated by pyrrhotite that includes troilite, together with Co-rich pentlandite and cubanite) and PGM (dominated by PtFe alloy and sperrylite), quite different to that found in the typical Merensky Reef.
The discordant bodies of IRUP, including the Replaced Merensky Reef are interpreted to have crystallized from exotic (iron-rich silicate-oxide) melts, which drained downward as a result of disequilibrium partial melting of anorthositic layers. Melting was induced by episodic replenishment of the chamber. The preservation of relict layering of the Rustenburg Layered Suite in many of the IRUP bodies, including Replaced Merensky Reef, is indicative of a volume-for-volume process of magmatic replacement. The IRUP generally occur independently of magnesian dunite pipes, and it is only in the platiniferous pipes where earlier-formed magnesian dunite stocks, rather than layered cumulates, were replaced. The core-zones at Mooihoek and Onverwacht (Fo44) are interpreted as end-member compositions indicative of the iron-rich melt. The less differentiated (and poorly-formed) core-zones at Driekop and Twyfelaar (Fo73) reflect arrested stages of replacement. Under normal conditions, the IRUP only contain significant amounts of PGE where a primary layer such as the Merensky Reef has been replaced. Mitchell and Scoon (2007) ascribed PGE in the Merensky Reef to an orthomagmatic process of lateral mixing. A similar hypothesis, albeit of vertical mixing, is applied to the mineralized pipes. The absence of PGE from the iron-rich outer envelopes of the pipes supports our suggestion that the PGE were not introduced with the exotic melts. We suggest PGE were initially concentrated within the central (hottest) parts of conduits during fractionation of the magnesian dunite. The PGE were then redistributed and re-fractionated by fluids segregated from the iron-rich melts in situ, during the process of magmatic replacement. The spectacular grades in the pipes resulted from concentration of PGE into volumetrically small core-zones during the final part of the complex, multi-stage process of pipe formation.