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Hydrothermal transport of PGEs in porphyry systems – a fluid history of the Skouries Cu–Au (PGE) porphyry deposit

development of South African PGE mining will almost certainly be focused on the northern Bushveld Complex. The northern limb is intruded north of the Thabazimbi-Murchison lineament (TML); a 2.9 Ga suture zone that juxtaposed Archaean crust and subcontinental lithospheric mantle (SCLM) of the Pietersburg block against the crust and mantle of the Kaapvaal Shield. These blocks shared no common history prior to 2.9 Ga and the Bushveld thermal event probably involved melting of fundamentally different types of granite-greenstone crust and SCLM north and south of the TML. Mantle peridotite xenoliths north of the TML carry Co-rich base metal sulphides with different PGE and semi-metal concentrations to those found south of the TML. High-Co sulphides with low Pt/Pd ratios are recorded in the SCLM under southern Scotland; they are linked to older subduction events and are potential source of Co and PGE for post-Caledonian mafic magmas (Hughes et al. 2016). The contrasting nature of the SCLM and the greater proportion of Au-rich greenstone belt material exposed in the crust north of the TML are considered to contribute to a strong first-order geodynamic influence on why both chilled magmas and magmatic sulphide orebodies in the northern limb are consistently Pdand Au-rich and Pt-poor compared to their equivalents to the south (McDonald & Holwell 2011). The manner in which the Bushveld magmas intruded also played a role. The Northern Bushveld magma chamber had the potential to interact more extensively with metalliferous sediments (Black Reef and Duitschland formation) of the lower Transvaal Supergroup than magmas south of the TML which intruded into the upper Transvaal Supergroup. Cu/Pd ratios suggest the Main Zone magma south of the TML supplied metals to the PGE reefs of the Upper Critical Zone, whereas Main Zone magma that intruded north of the TML had an undepleted PGE budget that ultimately formed deposits such as Aurora and the T Zone at Waterberg (McDonald et al. 2017).