The platiniferous dunite pipes of the eastern Bushveld

Abstract

The platinum-group mineralogy of the platinum pipes has been well documented in recent years. The same, however, cannot be said for their silicate and oxide constituents. A microscopic study of mineralized and unmineralized dunites, hortonolite dunites, and dunite-bronzitites from the Onverwacht, Mooihoek, and Driekop pipes revealed the presence of one-phase and polyphase spinels.The results of more than 300 microprobe analyses show that pipe deposit-type spinels do not plot within the field of stratiform deposit-type spinels from the chromitite layers of the Bushyeld Complex, because they are characterized by lower Cr-Fe and Mg and, occasionally, higher Ti. There are, however, certain similarities with spinels from the Merensky Reef. Polyphase spinels comprise up to four constituent phases, with increasing reflectance and Fe contents corresponding to decreasing Cr, Al, and Mg. They are interpreted as products of complex postmagmatic processes.Graphite occurs in dunties as globules, as intergranular films between olivine grains, and on fractures in Cr-Ti spinel. Small amounts of sulfides and platinum-group minerals are also found in these locations.Accelerator mass spectrometry of dunites and chromitite reveals extremely low Pt (0.03-38.0 ppb) values. The results suggest that the actual platinum-group element content of the silicates is low, with the platinum-group elements contained in sulfides or platinum-group minerals.Microprobe data and the presence of graphite in the dunites support the concept of metasomatic pipe formation from Fe-rich, late and postmagmatic solutions, with some of the platinum-group elements possibly transported as organometallic compounds. Pegmatoid features, spinel compositions, and graphite encountered in both the pipes and the Merensky Reef suggest a common genetic denominator.