The Mooihoek Platinum Pipe, Eastern Bushveld Complex, South Africa – Geochemistry and Mineralization

Abstract

Abstract The Mooihoek pipe is one of several platiniferous dunite pipes in the eastern Bushveld Complex that were discovered in 1924 and mined until 1930. It has a diameter of about 180 by 400 m and consists of magnesian dunite that crosscuts a sequence of layered leucocratic wall rocks of the lower part of the upper Critical Zone of the Bushveld Complex. The Mg-dunite pipe at Mooihoek hosts a smaller, carrot-shaped inner pipe, 13–16 m in diameter and cylindrical in shape near surface, consisting of mainly iron-rich dunite/wehrlite pegmatite, comprising the platinum-bearing orebody. Specific characteristics of the mineralization are the near absence of sulfides and Pt being the predominant platinum-group element (PGE). The present work comprises whole rock analyses, ore microscopy, scanning electron microscopy, and electron probe microanalysis. The main findings are as follows: Chondrite-normalized distribution patterns of the PGE in the Mooihoek samples demonstrate that mainly Pt is strongly enriched relative to the other PGE and Au. The shapes of the patterns are subparallel to each other and also to those of the Onverwacht pipe, indicating a similar mode of origin. Olivine of the ore zone is iron-rich and is frequently accompanied by coarse Ti-magnetite and ilmenite, whereas chromite is rare. Two major generations of platinum-group minerals (PGM) are present at Mooihoek, namely: (1) Early inclusions of mainly ferroan platinum in (magmatic) olivine, Ti-magnetite, and ilmenite, and (2) a later, more complex PGM assemblage, commonly associated with thin veinlets of magnetite, mainly found along grain boundaries of silicates and oxides. This assemblage consists of various Pt-Fe alloys and PGE-antimonides, -sulfarsenides, -arsenides, and -sulfides. Overall, and in line with the geochemical data of the mineralization, Pt-dominated PGM predominate (~70%), followed by Rh, Pd, and Ru species. Pt-Fe alloys are the most frequent PGM (∼44% by number of grains). Our hypothesis on the genesis of the Mooihoek pipe and its mineralization largely corresponds to that proposed for the Onverwacht pipe (Oberthür et al. 2021): The magnesian dunite pipe was formed by upward penetration of magmas which replaced the existing cumulates. Fractional crystallization of olivine within a deeper magma chamber and/or during ascent of the melt resulted in the formation of a consanguineous residual, more iron-rich, and PGE-enriched melt. This melt also contained highly mobile, supercritical fluids and was continuously enriched in PGE and other incompatible elements. The PGE-enriched residual melts formed and sealed the inner ore pipe. Apparently, the PGE mineralization is not related to the well-established mechanism of sulfide collection. Instead, it was introduced in the form of nanoparticles and small droplets of PGM, which coagulated to form larger grains during evolution of the mineralizing system. Concomitant supercritical magmatic to hydrothermal fluids were continuously active and caused a certain redistribution of the initial PGE inventory and possibly added further quantities of PGE from lower parts of the mineralizing system. Reactions with ligands like Sb, As, and S led to the formation of composite PGM grains and complex intergrowths in a continuous process of mineralization. The present work underlines that sulfide-poor magmas may constitute fertile potential precursors of PGE deposits and, therefore, deserve more attention in future exploration.