Petrogenesis of the Merensky Reef: The hydromagmatic (“Uppers”) model

Abstract This work presents a review of the evidence for volatile fluids and models that illustrate how such fluid can give rise to the Merensky Reef and associated stratigraphic features. The unusually Cl-rich nature of the rocks below the Merensky are consistent with experimental evidence of the importance of Cl in ore metal transport. The high background platinum-group elements (PGE) of the Lower and Critical zones imply they are a better source rock than the parent magma. The lower Merensky and UG2 ore grades where the underlying section is thin is consistent with metals have come from beneath the reefs. Furthermore, volatiles, particularly aqueous fluids and brines, have a strong effect on phase stability and marked changes in liquidus and solidus of partially molten systems. Constitutional zone refining, vapour refining, and chromatographic processes operating in the crystal pile can result in pronounced modal changes in a solidifying crystal pile, mimicking features conventionally interpreted to have formed directly from the crystallising magma. In the most complete quantitative model, chromite is precipitated at an upward-moving reaction front as a Cr-bearing norite protolith reacts with a Cl-rich fluid that becomes progressively undersaturated in pyroxene as it rises into hotter parts of the crystal pile, leaving an anorthositic residue. Coupled redox reaction results in the co-precipitation of an immiscible sulphide liquid (ISL) with the chromite: 6CrClOH0fluid+4FeCl2fluid+SO2fluid+4H2Ofluid=14HClfluid+3FeCr2O4chromite+FeSISL As the reaction front encounters an orthopyroxenite the model produces a first-order approximation for the Merensky Reef section of anorthosite → sulphide-bearing chromite → orthopyroxenite. It is evident that future advancement in understanding the formation of the Merensky Reef will require a paradigm shift from magma-centric to crystal pile-centric processes.