Geological interpretations from the PGE distribution in the Bushveld Merensky and UG2 chromitite reefs

It is well known that the platinum group element (PGE) mineralization in the Merensky and UG2 chromitite reefs is restricted to a very narrow vertical interval of rocks (Wagner, 1929). In the UG2 chromitite there is essentially no PGE above or below the chromitite layer that rarely exceeds 1 m thick. In the Merensky Reef a very large proportion of the PGE is confined within 1 m vertically in many cases, although there are some thicker intervals, and occasionally the mineralization becomes split with impoverished intervals between. Even within the relatively narrow reefs there is considerable variation in the exact location of the highest grades with respect to different host rock types. Variation in the silicate mineral rock types and PGE distribution has led to the definition of different reef facies (e.g. Wagner, 1929; Viljoen, 1999). This paper examines the published data on the distribution of PGE within the Merensky and UG2 reefs and uses this information to discuss models for the origin of the mineralization. There are geological and structural anomalies in the reefs, such as potholes, in which the mineralization may have a different distribution from the surrounding layered reefs, but these issues will not be discussed in detail here (see Carr et al., 1994).