Abstract

The Driekop dunite pipe crosscuts part of the noritic layered sequence of the upper critical zone in the eastern Bushveld Complex. The central orebody of the pipe contains platinum-group minerals, base metal sulfides, magnetite, and iron-rich olivine. The country rock forms a collapse structure around the pipe, with the layered sequence becoming almost vertical near the contact with the pipe.It is proposed that a high-temperature chloride solution flowed through irregular channel-ways along a structural weakness near the axis of the pipe and reacted with the noritic host rocks, leading ultimately to the formation of dunite. Reaction fronts advanced at different rates outward from the channelways and produced a sequence of metasomatic zones. Characteristics of infiltration metasomatism that are evident in the pipe include the movement of material over large distances in predominantly one direction, the formation of a monomineralic rock, the occurrence of a marginal zone that is enriched by displaced components, and plateaus in the compositions of mineral solid solutions.The reactions responsible for the major changes in the bulk composition of the host rocks were desilication of orthopyroxene to produce olivine, and dissolution of plagioclase. The 67 percent volume loss accompanying these reactions can account for the structural collapse of the critical zone around the pipe. Some olivine in the pipe is too fayalitic to have been produced by simple desilication of host-rock orthopyroxene. The enrichment of olivine in iron can take place byMg 2 SiO 4 + 2FeCl (sub 2 (sub (aq)) ) <--> Fe 2 SiO 4 + 2MgCl (sub 2 (sub (aq)) ) (forsterite) (fayalite)Aqueous chloride complexes are also believed to have played an important role in the transport and precipitation of the platinum-group elements, the base metals, and other cations. Interdependent chemical reactions involving a chloride solution, olivine, magnetite, and the platinum-group elements can account for the preferential association of the ore with iron-rich olivine. Infiltration metasomatism by a high-temperature chloride solution may have an important bearing on the interpretation of other features of the Bushveld Complex.

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