An evaluation of element mobility in the Modderfontein ultramafic complex, Johannesburg: Origin as an Archaean ophiolite fragment or greenstone belt remnant?

Abstract

The Johannesburg Dome – a tonalite-trondhjemite-granodiorite (TTG)-dominated terrane in the central Kaapvaal Craton – contains a suite of ultramafic-mafic complexes that are concentrated largely along its southern rim. These >3.3 Ga ultramafic-mafic complexes have recently been re-interpreted as fragments of an Archaean ophiolite (Anhaeusser 2006a), challenging a longstanding hypothesis whereby the complexes represent the intruded remnants of an Archaean greenstone belt. As with similar interpretations of ultramafic-mafic units in other Archaean cratons, the ophiolite hypothesis is used as evidence in favour of Phanerozoic-style plate tectonic processes having operated in the Archaean, with this geodynamic regime the prevailing explanation for the rocks and structures displayed by the Kaapvaal Craton. Through detailed new geological mapping of the scarcely studied Modderfontein Complex, alongside petrography, bulk-rock geochemistry and mineral chemistry, we here assess the validity of both hypotheses. Moreover, having experienced amphibolite-facies metamorphism and substantial hydrothermal alteration, we assess the degree of element mobility experienced by the Modderfontein Complex and discuss the implications for subsequent geodynamic interpretations. The 1 km2 area mapped comprises separate northern and southern domains, with the former dominated by homogenous serpentinite that contains irregularly-shaped chromitite lenses, and the latter comprising coarsely-layered peridotite, pyroxenite, gabbro and amphibolite. The data indicate that the Modderfontein Complex has experienced significant mobility of Pd, the fluid-mobile lithophile elements (e.g., Ba, Rb and Cs) and potentially some elements generally considered immobile. Mobility of Pd is restricted to chromitite lenses, where Pd was originally hosted by sulphide mineral phases (e.g., pentlandite). This element was immobile in all other Modderfontein lithologies, where it is hosted by nano-scale PGM, demonstrating that PGE mobility is, in-part, controlled by the host phase(s). Moreover, based on a variety of petrographic and geochemical characteristics, including PGE mineralogy and spinel mineral chemistry, it is considered unlikely that the Modderfontein Complex represents an ophiolite fragment. Instead, the Complex is interpreted as the intrusive remnant of a greenstone belt that was subsequently intruded by TTG magmas.