Abstract

Geological features of the Merensky Reef in the western Bushveld Complex are outlined. Regional variations in the nature of the reef are used to define four main subfacies for the Rustenburg facies south of the Pilanesberg and two main subfacies for the Swartklip facies north of the Pilanesberg. Three of the Rustenburg subfacies comprise varieties of narrow, pegmatoidal, and generally highly potholed, high-grade reef. The fourth facies is a wide, non-pegmatoidal and low-grade to sub-economic reef with few major potholes, extending from Western Platinum Mine to Brits. A normal and potholed facies and a regional pothole facies characterize the Swartklip facies. A number of distinct types of Merensky Reef occur within the regional pothole subfacies. Mineralogical features for the two main facies differ, the Rustenburg facies being dominated by PGM sulphides while that of the Swartklip facies is dominated by ferroplatinum. The ruthenium sulphide, laurite, dominates the PGM within the chromitite layers. Narrow pegmatoidal reef invariably contains higher PGE grades, and where the Merensky Reef widens, a top loading of mineralization is evident. A correlation between reef thickness and grade is demonstrated. Two major gravity highs are interpreted as representing magma feeders. Gravity data coupled with seismic profiles suggests a broad saucer shape for the layering. Although half concealed by the granite sheet, the western lobe is considered to have a structure akin to that of the Great Dyke. A relationship between Merensky Reef facies and the gravity highs is demonstrated. Narrow and highly potholed (including regional pothole) conditions correlate with gravity highs. The Merensky Reef is interpreted as representing an irregular, regional unconformity associated with a new injection of magma. The top chromitite contact represents the unconformity surface which has caused thermal erosion of leuconorite rocks and reconstitution of mafic to ultramafic footwall layers to form mineralized pegmatoid. Geochemical studies by other workers support the break at the Merensky Reef. This includes an elevated Sr isotopic initial ratio and an increased Cu to Pd ratio in silicates above the Merensky Reef. An overall model for the origin of the Merensky Reef is proposed. It involves the emplacement of a new dense mass of magma at feeder sites. This resulted in gravity sagging triggering pull-apart structures induced by monoclinal footwall hot listric faults. Such areas initiated pothole formation, which was accentuated by thermo-chemical erosion. This resulted in the observed pattern of potholing (greatest approaching the gravity highs) and the formation of differing reef types on different layered footwall lithologies. It also resulted in the formation of the regional unconformity surface represented by the upper chromitite contact at the base of a new magma pulse, which brought in the bulk of the mineralization. PGE values were enhanced where underlying layers were also enriched in PGE.

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