Abstract

The Onverwacht pipe, situated on the leasehold of Eastern Chrome mines in the eastern limb of the Bushveld Complex, warrants preservation as a national monument, as it is the site of the oldest platinum mine in South Africa. The central part of the pipe consists of a relatively small, yet richly mineralized core-zone that has been mined out from a spectacular glory-hole. Onverwacht and the other three mineralized pipes that have been recognized in the Bushveld Complex do not only have their ultramafic composition in common but also reveal an unusual mineralogical-chemical zonation in which sharply defined magnesian and iron-rich ultramafic assemblages are juxtaposed. In the first in our series of contributions on the mineralized pipes, we ascribed formation of the Driekop pipe to orthomagmatic and partial melting processes (Scoon and Mitchell, 2009). Although we regard this as a universal principle, each locality is sufficiently distinct as to warrant separate explanations of their individual complexities. All of the mineralized pipes are aligned approximately perpendicular to the primary layering, but surface mapping has revealed Onverwacht to be more complex, far larger and far more irregular than previously reported. The three-dimensional shape of the unmineralized, magnesium-rich assemblage at Onverwacht is difficult to constrain. It is dominated by a stock-like body of magnesian dunite (approximately 100 m in diameter), surrounded by gently-dipping sheets, fingers, and lenses of magnesian dunite and harzburgite that splay outward for several hundreds of meters, ultimately merging imperceptibly into the wall rocks. The wall rocks, which belong to the uppermost part of the Lower Critical Zone, are dominated by a thick sequence of feldspathic orthopyroxenite, punctuated by prominent chromitite layers (LG5 through MG1). Palimpsest stratigraphy is defined by chromitite layers in the peripheral parts of the pipe. Structural disruptions have sterilized the LG6 chromitite layer, the principal orebody at Eastern Chrome mines, over a strike length >500 m, in part due to minor downwarping associated with the pipe. Irregular slabs of chromitite within the central parts of the pipe, interpreted as xenoliths of the LG6 layer by Wagner (1929) have been displaced downward over a few tens of metres. The mineralized core-zone originally formed a downward tapering (“carrot-shaped”) body enclosed within the unmineralized magnesian dunite. It was composed of anomalous iron-rich dunite (“hortonolite dunite”) and iron-rich wehrlite pegmatites (Wagner, 1929). Coarse crystals of hornblende and phlogopite, small grains of apatite, and meter-sized lumps of Ti-magnetite-ilmenite were reported as prominent accessories in the core-zone.

The existence of two principal assemblages in the pipe, i.e., magnesian ultramafics in the main stock and irregular outer envelope and iron-rich ultramafics in the core-zone, is confirmed by whole-rock and mineral compositions. The composition of the former is tightly constrained (olivine Fo83–80), whereas the latter exhibit a much broader range (olivine Fo64–44). The origin of the magnesian dunite comprising the main stock is ascribed to flowage differentiation of U-type magma injected upward through a vertical conduit, as discussed by Scoon and Mitchell (2009). The pipe-like form and occurrence of chromitite xenoliths confirm an intrusive origin. Heat associated with intrusion of the U-type magma triggered pervasive replacement of the orthopyroxenite wall rocks by magnesian olivine. The harzburgite is an arrested stage of this process. Chromitite layers are preserved in the periphery of the pipe due to their refractory nature, but those in the main stock and core-zone were dismembered. The iron-rich ultramafics in the core-zone reflect a complex genesis that will be addressed in conjunction with the origin of the mineralization in a subsequent contribution on the Mooihoek pipe.

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