Abstract
The 3·5 Ga mafic-ultramafic rocks of the Barberton greenstone belt, South Africa, form a pseudostratigraphy comparable to that of Phanerozoic ophiolites. This Archaean complex, referred to here as the Jamestown Ophiolite Complex, consists of high temperature tectonometamorphic peridotites overlain by an intrusive-extrusive igneous section, which in turn is capped by a chert-shale sequence. Elevated PGE concentrations with distinctly different chondrite normalized patterns have been found at three pseudostratigraphic levels of this ophiolite. From the top down, high PGE values are first encountered in an undeformed chromitite, interlayered with meta-pyroxenites. The PGE abundances and normalized patterns are similar to those reported from PGE mineralized zones in large mafic layered complexes. Chromitemagnetite pods occur stratigraphically below the chromitite. Their PGE patterns resemble those reported from chromite pods in Phanerozoic ophiolites and those of the overlying chromitite. The host rocks of the pods (deformed talc-carbonate rocks) are interpreted as a mixture of deformed high-temperature cumulates and depleted upper mantle residue, within a ductile shear zone. In all the above occurrences, the PGE were concentrated by igneous upper mantle-crustal fractionation processes of mantle derived liquids. The third type of PGE concentration is found in an unusual, sulphur-poor NiFe-silicate-spinel pod (called the Bon Accord-[BA]-deposit) and an associated hematite-magnetite-chromite (HMC) pod, both of which occur in coarse-grained meta-dunites at the lowest levels in the ophiolite. These host rocks are interpreted as high-temperature tectonites (strongly depleted in REE); they probably represent depleted upper mantle residue from which the overlying rocks were derived by partial melting. BA can be divided into two groups. Group A has pat, chondritelike PGE patterns. Group B has a lower concentration of all the PGE, but the relative depletion is most severe for Os and Ir (which are the most refractory); thus the patterns have a positive slope. Both BA groups have negative Pt anomalies in their PGE patterns (more pronounced for Group A), while the HMC-pod, which is depleted in the other PGE, relative to BA, has a strong positive Pt anomaly. It is difficult to explain these fractionations in terms of known upper mantle-crustal processes, especially in the absence of sulphur. The PGE differentiation of BA is interpreted as an indication of chemical fractionation which started in the lower mantle. We conclude that the body represents a metal-silicate sample derived from an almost chondrite-like lower mantle where it resided as a left-over from inefficient core formation. This siderophile-enriched mantle fractionated during ascent and some of its residue was incorporated into the lower portion of the lithosphere during formation of the Archaean oceanic crust. Tectonic and chemical data suggest that the thick and old ‘lithospheric keel’ beneath the South African craton has a large component of Archaean residual sima and is therefore likely to be relatively enriched in PGE.
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J. De Wit, M., Tredoux, M. (1988). PGE in the 3·5 Ga Jamestown Ophiolite Complex, Barberton Greenstone Belt, with Implications for PGE Distribution in Simatic Lithosphere. In: Prichard, H.M., Potts, P.J., Bowles, J.F.W., Cribb, S.J. (eds) Geo-Platinum 87. Springer, Dordrecht. https://doi.org/10.1007/978-94-009-1353-0_34
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