FACIES MODEL FOR INTERVAL BETWEEN UG2 AND MERENSKY REEF, WESTERN BUSHVELD COMPLEX, SOUTH-AFRICA

A lithological and geochemical study of the interval between the UG2 chromitite and the Merensky Reef in the Upper Critical Zone of the Bushveld Complex, South Africa, revealed significant lateral variation over 170 km of strike in the Western Bushveld. Comprising ca 30 m at the Union Section, the interval reaches a total thickness of 220 m near Brits. Concurrently, the cumulates become more evolved and leucocratic with distance from the Union Section towards both the northeast (Amandelbult) and the southeast (Impala, Rustenburg and Brits areas). It is inferred that the reduced thickness of the interval is mainly the result of the non-deposition of felsic cumulates in the proximity of a putative feeder zone near the Union Section. The scale of non-deposition was governed by a thermal regime that was related to the heat flux from the feeder. A persistent heat flux delayed crystallization of successive phases in those parts of the chamber which were proximal to the feeder. New influxes were therefore deposited on a footwall of varying thickness and lithology in response to different degrees of crystallization and accumulation along strike. The development of cyclic units (chromitite-harzburgite-pyroxenite-norite-anorthosite) was interrupted at different stages in different parts of the chamber.