The platiniferous dunite pipes in the eastern limb of the Bushveld Complex: Review and comparison with unmineralized discordant ultramafic bodies

Abstract

The platiniferous dunite pipes of the Bushveld Complex are subvertical, cylindrical bodies (diameters of 180–500m) that crosscut the layered cumulates in the central sector of the eastern limb more or less at right angles. They are restricted to four localities. The Driekop and Mooihoek pipes are emplaced in the Upper Critical zone, the Onverwacht and Twyfelaar pipes occur in the Lower Critical and Lower zones, respectively. The discovery of the Mooihoek pipe by Hans Merensky and his Lydenburg Platinum Syndicate on 15th August 1924, the first occurrence of economic platinum mineralization to be located in the Bushveld Complex, is recounted. We also discuss details of the history of the evaluation and mining of the Driekop, Mooihoek and Onverwacht pipes, which are among the oldest platinum mines in the World, and review the comprehensive geological literature, including the original and very detailed descriptions by Percy Wagner. Discordant bodies are a well known feature of layered tholeiitic intrusions, but the four platiniferous pipes are unusual in that they are semi-concentrically zoned. The principal component is magnesian dunite, a fine-to-medium-grained assemblage of forsteritic olivine (Fo 85–80 ) with accessory Cr-spinel. The magnesian dunite forms comparatively deep-rooted, stock-like bodies. At Onverwacht and Twyfelaar, the stocks are rimmed by irregular bodies of magnesian harzburgite that is gradational into the pyroxenitic wallrocks, but at the noritic-hosted Driekop and Mooihoek pipes the stocks are rimmed by envelopes consisting of a younger discordant assemblage that is dominated by iron-rich wehrlite and iron-rich clinopyroxenite pegmatites. Both the magnesian dunite stocks and pegmatitic envelopes are typically barren; the PGE mineralization is concentrated within cylindrical, parsnip-shaped core-zones (maximum diameter 24m) wholly enclosed within the magnesian dunite stocks. The core-zones at Mooihoek and Onverwacht consist of hortonolite dunite and hortonolite wehrlite pegmatites (hortonolite is a moderately iron-rich olivine; average composition is Fo 50–44 ). The hortonolite dunite-wehrlite pegmatites are unusual, entirely ultramafic rocks (plagioclase is notably absent) that locally include metre-sized crystals of hornblende, phlogopite, and Ti-magnetite-ilmenite, the latter being derived from the alteration and dismembering of chromitite layers in the cumulate wallrocks. The core-zone at the Driekop pipe differs somewhat in that it consists of discrete mineralized pods and veins of iron-rich dunite and iron-rich wehrlite (average composition is Fo 73–72 ) intermingled with barren magnesian dunite. The mineralization at Twyfelaar is also associated with pods and veins of iron-rich dunite-wehrlite, but these are sporadically distributed throughout the magnesian dunite stock and do not constitute a mineable core-zone. There is no consensus as to the origin of the platiniferous pipes, but we suggest the spatial association of primitive and evolved ultramafic rocks within semi-concentrically zoned discordant bodies has not been adequately addressed. Comparisons with unmineralized discordant ultramafic bodies in the Bushveld Complex, specifically the magnesian dunite pipes and the iron-rich ultramafic pegmatite, may resolve this dilemma. Our preliminary conclusions suggest a two-stage magmatic hypothesis is applicable. The magnesian dunite is ascribed to flowage differentiation of new injections of primitive ultramafic magmas within vertical conduits, a hypothesis presented by Scoon and Mitchell (2004) to explain unmineralized magnesian dunite pipes in the central sector of the eastern limb. The core-zones and pegmatitic envelopes possibly formed due to magmatic replacement of either earlier formed magnesian dunite or layered noritic-anorthositic wallrocks; in both cases replacement was triggered by disequilibrium with downward draining iron-rich silicate melts in accordance with the hypothesis of Scoon and Mitchell (1994) for the iron-rich ultramafic pegmatite. Compositional differences between the core-zones and pegmatitic envelopes are ascribed either to preservation of arrested stages of replacement or localized derivation of the iron-rich silicate melts. The PGE were scavenged from reef-style deposits by the iron-rich silicate melts. These melts are only fertile in localities where the earlier-formed magnesian dunite pipes have provided the necessary heat to selectively melt PGE-bearing phases from layered cumulates located higher in the succession, or where the Merensky reef, for example, has been replaced by iron-rich ultramafic pegmatite in situ.