POTHOLES OF THE MERENSKY REEF AT BRAKSPRUIT SHAFT, RUSTENBURG PLATINUM MINES - PRIMARY DISTURBANCES IN THE MAGMATIC STRATIGRAPHY

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. 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 fuma-role 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. Additional aspects of the subject are discussed.