An Oxygen, Iron, Carbon Geochemical Study Of Selected Merensky Reef Potholes And Their Possible Role In Merensky Reef Petrogenesis

Abstract

Oxygen, iron, magnesium, and carbon are the principal geochemical parameters employed to elucidate the chemical relationship between the Bushveld Merensky Reef and associated structures termed "potholes". Potholes are large (hundreds of meters wide and tens of meters deep) depressions in the footwall filled with Merensky equivalent materials. Three potholes were analyzed in this study. The concentration of inorganic carbon was found to range from 1,835 ppm to 131 ppm. The fO2 analyses at 1150 C have a range from 10^-11.9 to 10^-13.9 atmospheres. In particular, stratigraphically equivalent basal chromites show a decrease from 10^-12.1 in the Normal Merensky Reef to 10^-13.9 in the pothole bottom. The ratios of Total Fe (as FeO/MgO + Total Fe (as FeO)) in these basal chromites show a range of 0.779 to 0.839 from pothole bottom to Normal Merensky Reef, respectively. Finally, Fe^+3 concentration in basal chromites, relative to Fe^+2 concentration, shows a depletion in the bottom of potholes. Parameter analyses indicate that geochemical gradients not only exist along stratigraphically equivalent horizons (lateral gradients) but also exist along stratigrpahic units (vertical gradients). Various hypotheses which address the genesis of pothole formation are reviewed in light of the above mentioned gradients. The author has found that the plutonic fumarole hypothesis as proposed in Ulmer et al. (1981) is best suited to explain the lateral and vertical geochemical gradients. Potholes acting as plutonic fumaroles releasing reducing carbonaceous and/or sulfurous gases are discussed as a possible mechanism for Merensky Reef petrogenesis. The author's geochemical data and published research from the literature are integrated into a discussion of new ideas for the platinum-reef petrogenesis in the Bushveld Complex.