Petrology and mineralisation of the southern Platreef: northern limb of the Bushveld Complex, South Africa |
| |
Authors: | Judith A Kinnaird D Hutchinson L Schurmann P A M Nex Renee de Lange |
| |
Institution: | (1) Economic Geology Research Institute, University of the Witwatersrand, Wits, 2050, South Africa;(2) Ivanhoe Nickel and Platinum Ltd, 2 First Avenue, Mokopane, 0600, South Africa |
| |
Abstract: | The Platreef, the putative local analogue of the Merensky Reef, forms the floor to the mafic succession in the northern limb
of the Bushveld Complex. We define the Platreef as ‘the lithologically variable unit, dominated by pyroxenite, which is irregularly
mineralised with PGE, Cu and Ni, between the Transvaal metasedimentary footwall or Archaean basement and the overlying Main
Zone gabbronorite’. We define the mineralisation around calcsilicate xenoliths within the Main Zone in the far north of the
limb as a ‘Platreef-style‘ mineralisation. The Platreef (ss) has a strike extent of ∼30 km, whereas Platreef-style mineralisation
occurs over a strike length of 110 km. The Platreef varies from 400 m thick in the S to <50 m in the N. The overall strike
is NW or N, with dips 40–45°W at surface, shallowing down dip, The overall geometry of the southern Platreef appears to have
been controlled by irregular floor topography. The maximum thickness of the southern Platreef occurs in two sub-basins on
the farms Macalacaskop and Turfspuit. Lithologically, the southern Platreef is heterogeneous and more variable than sectors
further north and, although predominantly pyroxenitic, includes dunites, peridotites and norite cycles with anorthosite in
the mid to upper portion. Zones of intense serpentinisation may occur throughout the package. Faults offset the strike of
the Platreef: a N–S, steeply dipping set is predominant with secondary ENE and ESE sets dipping 50–70°S. The fault architecture
was pre-Bushveld and also locally controlled thickening and thinning of the succession. Country rock xenoliths, <1500 m long,
are common. On Macalacaskop, these are typically quartzites and hornfelsed banded ironstones, shales, mudstones and siltstones
whereas on Turfspruit dolomitic or calcsilicate xenoliths also occur. Sulphides may reach >30 modal% in some intersections.
These are dominated by pyrrhotite, with lesser pentlandite and chalcopyrite, minor pyrite and traces of a wide compositional
range of sulphides. In the southern sector, mineralised zones have Cu grades of 0.1–0.25% and Ni 0.15–0.36%. Massive sulphides
are localised, commonly, but not exclusively towards the contact with footwall metasedimentary rocks. Magmatic sulphides are
disseminated or net-textured ranging from a few microns to 2 cm grains of pyrrhotite and pentlandite with chalcopyrite and
minor pyrite. Much of the sulphide is associated with intergranular plagioclase, or quartz-feldspar symplectites, along the
margins of rounded cumulus orthopyroxenes. The PGEs in the southern sector occur as tellurides, bismuthides, arsenides, antimonides,
bismuthoantimonides and complex bismuthotellurides. PGM are rarely included in the sulphides but occur as micron-sized satellite
grains around interstitial sulphides and within alteration assemblages in serpentinised zones. The Pt:Pd ratio ∼1 and PGE
grade may be decoupled from S and base metal abundance. |
| |
Keywords: | Platinum-group elements PGE Base metals Platreef Bushveld Complex |
本文献已被 SpringerLink 等数据库收录! |
|