The petrogenesis of a platiniferous dunite pipe in the Bushveld Complex; infiltration metasomatism by a chloride solution

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 platinumgroup 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 channelways 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 by Mg2SiO4 + 2FeC12•i • Fe2SiO4 + 2MgCI• . forsterite fayaliteAqueous 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.

Relationship Between Pge and PGM in the Bushveld Complex

“…It has been suggested that the pipes are feeders of mineralization into the stratiform reefs (Schiffries 1982, Stumpfl & Rucklidge 1982. Figure 13 shows that the PGE content and relative proportions do not appear to be influenced by proximity to a pipe, although the mineral assemblage was.…”

Genèse des dépôts de Fe-Ti-P associés aux intrusions litées (exemples l intrusion mafique de Sept-Iles au Québec complexe de Duluth aux États-Unis)

“…Il existe deux types de pegmatites ultramafiques riches en oxyde : i-pegmatite à olivine et clinopyroxène, et ii-pegmatite riche en oxydes (Scoon et Mitchell, 1994). Scoon et Mitchell (1994) La même origine métasomatique similaire à celle proposée par Schiffries (1982) a été suggérée par Ross (1985) pour les dunites à péridotites riches en oxydes et de granulométrie grossière à la base de la série litée de Duluth. Ross (1985) …”

Suspected Presence of Hibbingite in Olivine Pyroxenite Adjacent to the Ug2 Chromitite, Bushveld Complex, South Africa

“…Conversely, the samples of discordant wehrlite have low Cl contents and so appear not to have been affected. On the basis of the absence of alteration in these wehrlites and the low temperature at which the hibbingite was formed in the layered rocks, we conclude that these discordant bodies are not Cl-rich, as proposed by Schiffries (1982), nor likely to have contained this Cl-rich fluid.…”