Abstract

Potholes are locations where parts of the magmatic stratigraphy are missing in the foot-wall sequence of the Merensky reef. In general, potholes are circular to elliptical in plan and funnel shaped in cross section. Within narrow limits, most potholes reveal the same three-dimensional orientation to the strike and dip of the magmatic layering. The updip pothole flank is steep to vertical; the downdip flank is horizontal. The pothole bottom generally coincides with one of the marker horizons of the footwall stratigraphy and is conformable with layering.Where the Merensky reef fills potholes, it is more variable in thickness than it is at its normal stratigraphic level. In the potholes, it changes over short distances from an unusually thin chromite-rich contact phase of the reef, to thick unlayered lenses of harzburgite pegmatite in deep-seated parts of the potholes. When the Bastard reef is observed in potholes, it sometimes grades into massive graphite-rich transgressive gabbroic pegmatite. The marker horizons in the anorthositic footwall units around potholes are often intensely mineralized with stratiform and crosscutting sulfides. Such marginal footwall mineralization is absent in the undisturbed footwall sequence away from potholes.Potholes are traditionally considered to be resorption pits due to the influx of a new hot primitive magma at the Merensky reef level or to represent scars of a late magmatic fumarole activity. The present study suggests that cumulate resorption played a minor role only. Potholes are sites of nondeposition, where the missing rock units did not crystallize. They formed where locally high concentrations of dissolved C-H-O-S volatiles lowered the liquidus temperature of the magma and suppressed the crystallization of cumulus plagioclase. Potholes seem to be concentrated above anticlines of the metasedimentary basement rocks in the footwall of the Bushveld Complex. Volatiles derived by pyrometamorphism were focused upward into the overlying melt above such topographic highs and were accumulated at the level of the Merensky reef.

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