Platinum-group minerals in various rock types of the Merensky Reef; genetic implications

Abstract

In addition to what is usually termed Merensky reef, mining operations have revealed various other types of the Merensky reef, which are not only of geologic interest but also display specific mineralization characteristics. The platinum mineralogy of these reef conditions is described with regard to the following varieties: normal, wide, pothole, contact, rolling, lens, regional pothole, pegmatoid-replaced, and dunite-affected.Utilizing our current data on the platinum-group minerals, certain suites peculiar to normal and anomalous reef conditions have been recognized. A model for their genesis is proposed which invokes various kinds and periods of volatile activity.Potholes are characterized by a zonal distribution of platinum-group minerals, with Pt-Fe at their centers and Pt-Pd-Te at their edges. Similarly, dunite pegmatites contain Pt-Fe and Pt-Pd-Te as well as Pt-As.Monomineralic (anorthositic) footwall sintering releases volatiles that result in the overlying Merensky reef being Pt-Fe (and olivine) bearing, with large amounts of platinum-group elements in the base metal sulfides. Multiphase footwall cumulates do not generate excess volatiles, and in such localities the Merensky reef is characteristically Pt-Pd-S bearing with low platinum-group element contents in the base metal sulfides. Local deep-seated volatiles, perhaps associated with floor-rock upwarpings, are responsible for potholes of the Brakspruit type.Pothole and dunite platinum-group mineral characteristics are locally superimposed on the Merensky reef platinum-group minerals, which are in the first place characterized by footwall type and perhaps regionally by feeder proximity.