Mantle sources and formation processes of highly-siderophile element ore layers in magmatic, mafic-ultramafic intrusions: an alternative model

Layered mafic-ultramafic intrusions of magmatic origin are the major hosts for the valuable, highly-siderophile elements (HSE), which include Ru, Rh, Pd, Os, Ir and Pt (the so-called platinum-group elements; PGE), together with Re and Au. These elements are either incorporated in the crystal lattices of base metal sulfides and chalcogenides, or combined as metallic alloys, to be concentrated at ore levels in layers of only a few meters thick. These layers are otherwise characterized by the elevated modal abundances of orthopyroxene and/or chromite. Decades of research have allowed extensive studies at all scales of some of these ore layers, such as the Merensky Reef, UG2 chromitite and Platreef of the Paleoproterozoic Bushveld Complex (South Africa). However, a wide range of models has been proposed for the formation of the ore layers throughout the past fifty years, without any consensus. Here I will shortly review the existing models for some of the Bushveld Complex ore layers, according to two major criteria: (I) the parental magma source(s) and composition(s); and (II), the formation mechanism(s) of ore mineralization. With respect to criteria (I), recent radiogenic isotope and/or geochronological data suggest that some of the ore layers have formed from mantle sources of distinct compositions and have intruded a pre-existing cumulate pile (i.e. non-sequential injection). With respect to criteria (II), recent petrological and geochemical data have outlined the formation processes of sulfide- and alloy-rich orthopyroxenite dikes or sills through a unique combination of different mechanisms at high pressure. I will further summarize the lines of evidence from studies of those orthopyroxenite dikes or sills, for which I will present the results of new numerical calculations to propose an alternative model for the formation of the Bushveld Complex ore layers. These layers could have formed from harzburgite sources experiencing low-degree melting under oxidized conditions, either in the mantle asthenosphere or lithosphere, and triggered by plume activity. Ore layer formation from such melt involves HSE enrichment under sulfide-undersaturated conditions, both during mantle melting and melt fractionation, which is followed by S6+-Fe2+ redox reactions with the host intrusion and hydrothermal, metal-rich fluid exsolution during emplacement. The two latter processes collectively lead to the abrupt and localized precipitation of sulfides and alloys during layer formation. This model is in line with the compositional range of the cratonic mantle lithosphere beneath South Africa, as well as recent data on the Merensky Reef, UG2 and Platreef layers. These data involve modal, phase (e.g. sulfides and alloys) and inferred parental melt (e.g. chilled margins) compositions, including distinctive chondrite-normalized Pt/Pd>1, and geochronological constraints. Finally, I will provide evidence from PGE-rich, Phanerozoic magmas for the influence of harzburgitic mantle rocks in their petrogenesis, either through direct melting or melt/rock reaction processes under oxidized conditions, and during plume activity in intra-continental and intra-oceanic settings.