Reverse Compositional Zoning in the Uitkomst Chromitites as an Indication of Crystallization in a Magmatic Conduit

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Abstract

Well-preserved primary magmatic zonation is observed in the harzburgite-hosted chromitites in the central part of the Uitkomst Complex in South Africa. The complex represents a magmatic conduit system of Bushveld affinity and age. Stratigraphic changes in chromite composition suggest that the chromitites were formed from pulses of incrementally more primitive, Cr-rich melts that precipitated chromite with progressively higher Cr 2 O 3 (from 44 to 55% Cr 2 O 3 ) and MgO (from 9 to 12% MgO), and lower TiO 2 contents from the base upwards. This reverse compositional trend is believed to be part of a major reverse trend of increasing Mg# in both olivine and whole-rock compositions through the entire ultramafic section of the complex. The lower part of chromitites was formed from a contaminated melt that had chemically interacted with and replaced host-rock dolomites, resulting in crystallization of chromite with low Mg (as low as 0·5% MgO), and high Ti (up to 4% TiO 2 ) contents. The preservation of chemical trends suggests that the precipitation of chromite and olivine occurred ‘onstage’ in an open system; this is not consistent with either a cumulate concept or the supply of a magmatic slurry. AlphaMelts modelling shows that the melt needs to have a komatiitic composition to be capable of crystallizing olivine of Mg# 92, which forms the thick dunite lenses overlying the chromitite body. We propose a model for chromitite formation in magmatic conduits that has much in common with models conventionally proposed for the formation of podiform chromitites and komatiite-hosted chromitites, albeit that the Uitkomst chromite chemistry lies within the field of the most chemically primitive compositions found in layered intrusions such as the Lower Zone of the Bushveld Complex.

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