Abstract

Lateral and vertical variations of f (sub O 2 ) and carbon in whole rock and f (sub O 2 ) , Fe, Cr, Mg, and Al variations in separated chromites are employed to elucidate the chemical relationships between the Merensky Reef and associated structures termed "potholes." These chemical distributions are utilized to develop a model relating the origin of the potholes to the platiniferous Merensky Reef.Potholes are large, roughly circular, and/or rarely irregular depressions (up to hundreds of meters wide and tens of meters deep) in the Merensky Reef footwall filled with Merensky equivalent material. Pothole and adjacent Merensky Reef samples studied herein were the footwall anorthosite, the overlying basal chromitite, and the platinum-bearing gabbroic pegmatoidal pyroxenite.Three potholes were analyzed in this study. Intrinsic oxygen fugacity analyses demonstrate that pothole bottoms are more reduced than adjacent normal Merensky Reef (10 (super -13.9) - 10 (super -12.1) at 1,150 degrees C), and analyses show that noncarbonate inorganic carbon concentration is up to 14 times greater in the pothole bottom. Cation distributions in the basal chromitite also reflect the variation in f (sub O 2 ) . For example, the ratio FeO (super *) /(FeO (super *) + MgO), where FeO (super *) = total iron as FeO, increases from pothole bottom to adjacent normal Merensky Reef (0.78-0.84). The carbon and intrinsic oxygen fugacity gradients, in addition to the variations in spinel chemistry, are all consistant with more reduced conditions within the pothole as compared to the adjacent normal Merensky Reef.For the studied parameters, geochemical gradients not only exist along stratigraphically equivalent units (lateral gradients) but also exist between units (vertical gradients). Various hypotheses which address the genesis of pothole formation are reviewed in light of these discovered gradients.A model of potholes acting as plutonic fumaroles releasing reducing fluid rich in carbon and/or sulfur is discussed as a possible mechanism for Merensky Reef petrogenesis. The model integrates our geochemical data with published research from the literature to present an integration of ideas for platinum-group element enrichment in the Merensky Reef.

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