Observations on the relationship between the Platreef and its hangingwall

Abstract

Observations on the nature of the contact between the Platreef and its hangingwall have revealed that not only were the hangingwall gabbronorites intruded after the Platreef igneous rocks and the development of platinum group element (PGE) mineralisation, but that there appears to have been a significant time-break separating the two intrusive events. The hangingwall gabbronorites truncate several features present within the Platreef pyroxenites but not in the hangingwall, such as shear zones and reef which has undergone alteration by Fe-rich fluids, implying that these features were formed prior to intrusion of the gabbronorites. A fine-grained leuconorite at the base of the hangingwall exhibits textures showing erosion of Platreef orthopyroxene by fine-grained cumulus plagioclase, suggesting intrusion of a hot magma over cooled Platreef. Xenoliths of reef pyroxenite are also found in the hangingwall. PGE mineralisation is present within basal zones of the hangingwall where the hangingwall overlies mineralised Platreef pyroxenite. We interpret the contact as a magmatic unconformity and, as the gabbronorites do not appear to be PGE-depleted, suggest that PGEs and S were scavenged or assimilated from the reef by the intruding magma, producing zones of orthomagmatic PGE mineralisation in topographic depressions at the base of the crystallising hangingwall. The presence of calc–silicate xenoliths in the hangingwall gabbronorites can be explained by footwall anticlines or diapirism which the relatively thin Platreef had not overtopped, allowing footwall dolomite to be exposed to the main influx of hangingwall magma. The identification of a time-break between Platreef and hangingwall intrusion, and the most likely source of basal hangingwall PGE mineralisation being the underlying Platreef, shows that the magma that formed the gabbronorites could not have been the source of PGE for the Platreef as previously thought.