- © 2009 Geological Society of South Africa
The occurrence of platiniferous dunite pipes is restricted to four localities in the eastern limb of the Bushveld Complex. They comprise wholly discordant bodies aligned perpendicular to the regional layering, and reveal a complex internal zonation. They remain poorly understood, despite a number of contributions dealing with the ore mineralogy, due mainly to the fact that field relationships and the silicate and oxide mineralogy have not been adequately addressed. The principal geological features of one such pipe, at Driekop, are revisited on the basis of detailed mapping and sampling of mine workings that are not generally accessible. The pipe forms a subcircular body (diameter 300 m) that is persistent to considerable depth. The principal unit is a stock-like body composed entirely of fine-to-medium grained magnesian dunite (Fo83.4). Chromite is a prominent accessory phase of the magnesian dunite. The chromite is locally altered to highly unusual, polyphase grains in which discrete Cr-rich, Al-rich, and Fe-rich phases are identified. The magnesian dunite is rimmed by a zoned outer envelope that consists of an inner ring of iron-rich wehrlite (Fo65) and an outer ring of iron-rich clinopyroxenite pegmatite.
Mineralisation is mostly contained in a near-cylindrical core-zone (diameter 18 to 24 m) characterised by segregations of iron-rich dunite (Fo73) and subordinate iron-rich wehrlite within the magnesian dunite. Mineralised veins of iron-rich dunite and wehrlite, as well as barren veins of iron-rich clinopyroxenite pegmatite, crosscut the magnesian dunite outside of the core-zone. All of the iron-rich ultramafics at Driekop are distinguished by the absence of a spinel phase. The pipe crosscuts a thick sequence of leucocratic cumulates (Merensky Footwall unit), that in turn are underlain by layers of chromitite and feldspathic orthopyroxenite (UG3 and UG2 units). The wall rocks are severely disrupted and form a km-wide downwarped feature. Layering proximal to the pipe on the southern margin dips at 75° (north-east) in comparison to the regional dip of 12° (west). The leucocratic wall rocks are mostly unaltered, apart from a thin zone of sausseritization, but include discrete satellite bodies of iron-rich clinopyroxenite pegmatite (Fo63). The underlying, more mafic UG3 and UG2 units, however, are anomalous. They are considerably thinner in comparison to the regional stratigraphy and some layers are entirely absent. The feldspathic orthopyroxenite in these units has been pervasively replaced by magnesian peridotite (Fo80-78). The downwarped UG3 and UG2 chromitite layers are partly dismembered and include spots of pipe-related magnesian olivine. Mineralogical and chemical similarities with dunite layers from the Lower Zone (Fo85.8) and Lower Critical Zone (Fo85.1) suggest that the magnesian dunite in the Driekop pipe is orthomagmatic. This hypothesis is supported by mapping of a meter-wide sill of magnesian dunite (Fo85), in the wall rocks at Driekop. Field relationships demonstrate this sill to have been intruded between layers of leuconorite and anorthosite. The magnesian dunite in the pipe formed as a consequence of “flowage differentiation” of an ultramafic magma through a vertical conduit. Magnesian olivine and Cr-spinel were the only liquidus phases during this phase of pipe formation. Heat associated with the intrusion resulted in formation of magnesian olivine spots within orthopyroxenitic wall rocks (albeit located some distance from the pipe); they are ascribed to the incongruent melting relationship between orthopyroxene and olivine. Within the leuconorite-anorthosite wall rocks, however, the increased temperature associated with the intrusion resulted in selective partial melting of ferromagnesian components (plagioclase was refractory). This produced a moderately iron-rich ferromagnesian melt that was relatively dense and drained downward into fractures propagated by the primary conduit. The downward draining of melt, together with the partial melting of more mafic layers, caused a catastrophic downwarping of the wall rocks. The ferromagnesian melt reacted with the leucocratic wall rocks to produce an iron-rich clinopyroxenite pegmatite (outermost part of the outer envelope and discrete satellites). Reaction with earlier-formed magnesian dunite resulted in either an iron-rich dunite (e.g. core-zone) or an iron-rich wehrlite (e.g. innermost part of the outer envelope). The occurrence of polyphase Cr-spinel in selective samples of magnesian dunite, as well as the entire absence of a spinel phase from the iron-rich ultramafics, suggests the pipe chromite was in disequilibrium with the downward-draining ferromagnesian melt. The origin of the mineralisation is not addressed here but the new hypothesis is consistent with the suggestion of Wagner (1929) that PGE were transported into the platiniferous dunite pipes by iron-rich melts derived by melting of earlier-formed layered reefs.