An investigation of the mineralisation and associated structures within the Grasvally area, South Africa

Abstract

The mafic rocks of the Bushveld Complex differ to the north and south of the Ysterberg-Planknek Fault, part of the array that is the Thabazimbi-Murchison Lineament. To the north, there is sporadic Lower Zone, succeeded by a PGE-Cu-Ni-bearing Platreef, Main Zone and Upper Zone successions. To the south is Lower Zone, the Grasvally Norite-Pyroxenite-Anorthosite (GNPA) member and Main Zone. This investigation on the farm of Grasvally aims to provide a better understanding of the mafic rocks south of Mokopane, in order to provide an improved understanding of the relationship between the geology and mineralisation of this region. The farm Grasvally is situated approximately 20 km south of Mokopane. The geology of the Grasvally area comprises mainly mafic to ultramafic Bushveld rocks of Lower Zone to Main Zone affinity which were within sedimentary rocks of the Pretoria Group. The GNPA member was originally divided into two major sub-units but more recently it has been divided into the Lower Mafic Unit (LMF), the Lower Gabbronorite Unit (LGN) and the Mottled Anorthosite Unit (MANO). The GNPA member may be the equivalent to the Platreef to the north of the Ysterberg-Planknek Fault. The Lower Zone displays little to no variation in Mg# indicative of no fractionation and continuous replenishment of magma. Two well-developed chromitites ~60-90 cm in thickness are intersected within the Lower Zone (LZ).These chromitite layers display the highest Cr2O3within the Bushveld Complex and are not nearly as thick within the LZ anywhere else. The chromitites, disseminated chromite, pyrrhotite, chalcopyrite and minor pentlandite mineralisation are hosted within the harzburgites and pyroxenites of the LZ. The kink banding in olivine of the Lower Zone is indicative of high-pressure solid-state deformation. The Lower Zone also displays serpentinisation, which increases from east to west, possibly having occurred along fractures associated with this high-pressure high-temperature system. Three-dimensional modelling of the area with the use of borehole logs, existing surface mapping and structural data proves the existence of a 70°W dipping fault which has brought the Transvaal sedimentary rocks, the marginal member and Critical Zone to the east of the fault in contact with the LZ. To the west of the fault the Lower Zone, LMF, MANO, and Upper Zone form a conformable sequence, shallowly dipping to the west. The Thabazimbi-Murchison Lineament may have acted as a barrier to the flow of magma causing the GNPA member to be compartmentalised and to follow a different evolution from the Critical Zone magma in the eastern or western limbs or to the Platreef to the north. Core logging indicated that the GNPA member overlies the Magaliesberg quartzites of the Pretoria Group and the Lower Zone to the east and the west of the farm respectively. Previous work (Smith et al., 2014) suggests PGE mineralisation is associated with sulphides and is not restricted to one lithological unit within the GNPA member. Within the LGN blebs (<1 cm) of fine-grained cumulus magnetite within norite have not been previously documented. Additionally, below this unit, within the LMF are disseminated to well-developed layers of chromite, as well as isolated pods of chromitite within norite and gabbronorite which are not easily correlated with the known stratigraphy. The boreholes also intersect a pyroxenite unit within the LMF that has visual similarities to the mineralisation and the same hanging and footwall as the UG 2 chromitite layer. This pyroxenite comprising orthopyroxene, interstitial plagioclase and clinopyroxene, hosts disseminations and a well-developed chromitite layer with associated disseminated to blebby pyrrhotite and chalcopyrite. The Pt/Pd ratios associated with chromitiferous pyroxenites and chromitites within the pyroxenites are similar to that of the UG2whereasthe Pt/Pd variation within the GNPA member is indicative of multiple influxes of magma