The mineralogy and mineral associations of platinum-group elements and precious metals in the Aurora Cu-Ni-Au-PGE deposit, Northern Limb, Bushveld Complex
Aurora is a platinum-group element (PGE) prospect hosted in the Northern Limb of the Bushveld Complex, South Africa. It is one of only three deposits discovered in the Northern Limb so far to be hosted in the melanocratic-leucocratic gabbroic cumulates of the Main Zone of the Rustenberg Layered Suite (Aurora, Moorddrift and Waterberg T Zone deposits), rather than in predominantly ultramafic rocks (e.g. Platreef). The host cumulates at Aurora have been divided into three principal units and they intrude the dolomites of the lower Transvaal Supergroup. Base metal sulphide (BMS) mineralisation with PGE is present in the leucogabbronorites and gabbronorites of Unit 2, and in coarse grained gabbronorite veins which intrude the peridotites of Unit 1. These veins contain up to 50% interstitial pyrrhotite-pentlandite-chalcopyrite ± pyrite. Unit 2 contains 1-3% pentlandite-pyrrhotite-chalcopyrite assemblages, and 1-5% chalcopyrite ± pyrite/pyrrhotite associated with hydrothermal alteration. The PGE content of Aurora however is predominantly controlled by the presence of platinum-group minerals (PGM), not BMS. LA-ICP-MS analysis of sulphides shows the BMS in Aurora have lower PGE concentrations than other Bushveld magmatic sulphides, with pentlandite carrying much lower concentrations of Pd (average 23 ppm) than the Platreef or the Merensky Reef. SEM-EDS analysis of 26 sections characterised 995 platinum-group minerals (PGM) and precious metal-bearing minerals (PMM), with a total area of 27850 μm 2 and an average size of 28.2 μm 2 . Of the PGM and PMM identified in Aurora 85% (by area) are Pd-Te-Bi minerals, with 6% Pd-Te minerals, 4% electrum and 3% Ag-Te minerals, along with minor Pd-Bi, Pd-As, Pt-Te-Bi, Pt-As and Pt-S minerals that collectively comprise 2% of total area. Only 25% of the PGM and PMM in Aurora are BMS hosted, with the rest hosted in silicates. Of the total PGM and PMM area 22% are hosted in alteration-silicates (quartz, chlorite or actinolite) in an alteration halo around sulphides. Unusually, 52% of the PGM and PMM are spatially removed from BMS, instead hosted in alteration silicates and within cracks in primary silicates away from any BMS. This indicates a multi-stage ore genesis model, with hydrothermal remobilisation of PGE important for ore formation. The style and host rocks for mineralisation in the Aurora deposit are fundamentally different from other deposits in the Northern Limb of the Bushveld hosted in ultramafic rocks, such as the Platreef, GNPA member deposits and the F zone of the Waterberg deposit, all of which contain a greater diversity of PGM and BMS with higher precious metal contents. The mineralisation most similar to Aurora is the T Zone of the Waterberg deposit, located to the north of Aurora, which been suggested to be an along-strike equivalent of the Aurora Main Zone mineralisation. However, despite strong similarities in PGM mineralogy and S isotope signatures there are significant differences in BMS mineralisation and host lithology meaning it is unlikely they are directly linked stratigraphically. At present it seems more likely that Aurora and the Waterberg T Zone reflect similar fluid-influenced processes operating in different parts of the Main Zone, perhaps at different times and in different structural basins, rather than a continuous mineralised zone along strike.