Iron-Rich Ultramafic Pegmatites of the Bushveld Complex, South Africa and Their Relationships with the UG2 Platiniferous Chromitite Layer

English Abstract:

Iron-rich ultramafic pegmatites (IRUP) of the Bushveld Complex are coarse-grained pyroxenites and wherlites, which form discordant bodies from several meters to several kilometers in size and randomly cut through the rocks of the Rustenburg Layered Suite in stratigraphic interval from the Upper Zone down to the Upper Critical Zone (Scoon and Mitchell 1994; Reid and Basson 2002). In the Upper Critical Zone, the IRUP preferentially replace anorthosites and norites but in fully developed cases pyroxenite and even chromitite layers are also affected. The replacement reactions result in (1) reverse zoning of plagioclase in the IRUP at contacts with norite or anorthosite, and almost complete disappearance of plagioclase from inner parts of the IRUP bodies; (2) the replacement of low-Ca pyroxene by ferro-augite and olivine; (3) the replacement of chromite by magnetite. The nature and the origin of the IRUP replacement bodies remain contentious.

Here we present a study of IRUP occurrences in the U2 open pit of the Lonmin Platinum Marikana mine in the Rustenburg area of Western Bushveld. IRUP outcrops in the mine provide excellent opportunity for mapping and sampling individual IRUP bodies over an area of several square kilometers and allow detailed studies of field relationships between IRUP, the UG2 chromitite layer and silicate cumulates right above and below the latter. In the UG2 open pit IRUPs form tube-like, sill-like and very complex irregular branching bodies from a few meters to hundreds of meters in size. The main rock-forming minerals are Fe-augite (Wo_34-46En_21-40Fs_18-40), olivine (Fo_38-70), magnetite and ilmenite. There are patches or pockets inside the IRUP bodies tens of centimeters in size, which contain significant amounts of interstitial, euhedral plagioclase An_88-97, Ca-Mg-Fe amphiboles and biotite. The plagioclase- and amphibole-bearing facies are locally enriched in sulfides and apatite. Contacts of IRUP bodies with layered cumulates of the Upper Critical Zone are often sharp but diffuse, gradational contacts are also common. The UG2 chromitite layer, when intersected by IRUP, is replaced by massive magnetite rock and looses the grade.

References.

Reid D.L., Basson I.J. (2002) Mineral. Mag. 66:895-914.

Scoon R.N., Mitchell A.A. (1994) J. Petrol. 35:881­917