Abstract

Structure and tectonism within the Kaapvaal Craton of the southern African subcontinent are controlled by a basic conjugate framework originating from the dawn of geological time. This pattern governs structure and tectonism on a gross regional scale down to the most local scale. Apart from that which is fundamental to the basic framework, local folding plays a very minor role in tectonic adjustment which is dominated by faulting, diapirism, and all magnitudes of point intrusion at favorable loci determined by the framework. Pipelike orebodies are thus characteristic of this environment.To understand the gross fractionation pattern within the Bushveld Complex requires comprehension of the details of the repetitive cyclic differentiation and its geochemistry. Detailed studies of stratigraphic geochemistry lead to the conclusion that, with the exception of sulfur, all the elements which constitute the nickel orebodies are available within the basal rocks of the Bushveld Igneous Complex.The area in which the nickel pipes occur is one of the few highly faulted areas of the Bushveld Complex. Consistent with the fundamental cratonic framework, it is found that faulting and conjugate dilatent fracturing exercise the major control of the distribution of the nickel pipes, combined with deep basining within the rocks of the complex. It is postulated that extensive basification, with resultant development of linear magnetic harzburgite zones, was caused by tectonic overpressure along major faults, thereby concentrating the metals required to form the nickel pipes. The faults provided a path needed either to distil sulfur or to add sulfur originating from the sediments by circulating waters to these metals. Renewed overpressure mobilized the sulfide ore fluid so formed into favorable structures created by the basic framework.Yet, since most of the faulting can be related to the intrusion of the Pilanesberg Alkaline Complex which postdates the Bushveld Complex by at least 500 m.y., the nickel pipes are considered to be epigenetically post-Bushveld although the rocks of that complex supplied the metals which make up the orebodies.

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