The Uitkomst intrusion and Nkomati Ni-Cu-Cr-PGE deposit, South Africa: trace element geochemistry, Nd isotopes and high-precision geochronology

Abstract

The Uitkomst intrusion is a tubular maficultramafic layered body that hosts one of South Africa's largest Ni-Cu-Cr-PGE deposits, Nkomati. The sulphide ore occurs in the form of massive lenses in the immediate quartzitic footwall and as disseminations within peridotite. The chromite ore forms an up to ∼10-m-thick layer in the lower portion of the intrusion. Uitkomst has generally been interpreted as a magma conduit, possibly related to the Bushveld event. Here, we present a new high-precision U-Pb zircon date of 2057.64 ± 0.69 Ma that overlaps with the age of the Merensky Reef of the Bushveld Complex and thus demonstrates a coeval relationship between the intrusions. Based on incompatible trace elements as well as O-and Nd isotope data (εNd −4.5 to −6.2), we show that the Uitkomst parent magmas were contaminated with up to 20% Archean upper crust prior to emplacement, and with up to 15% dolomitic country rock during emplacement. Ore formation at Nkomati was critically aided by substantial devolatisation and removal of dolomitic floor rocks leading to hydrodynamic concentration of sulphide and chromite during slumping of crystal mushes into the trough-like centre of the subsiding intrusion and its footwall.

Igneous Rock Associations 23. The Bushveld Complex, South Africa: New Insights and Paradigms

“…2000;Prevec et al 2005), suggesting on the order of up to 30 to 40 wt.% crustal contamination for parts of the Bushveld, although lesser quantities (< 30 wt.%) of more ancient (3.5-3.1 Ga) crust have also been proposed as alternatives (Prevec et al 2005;Roelofse and Ashwal 2012). This would also be consistent with the suggestion that an isotopically enriched source, such as specifically a metasomatized subcontinental lithospheric mantle (SCLM), interacting with a mantle plume, was the primary melt source, requiring lesser amounts of syn-emplacement crustal contamination subsequently Maier et al 2018).…”

“…See text for additional information. Molopo stratigraphy modified afterKaavera et al (2018) and Uitkomst afterMaier et al (2018).…”

A Process Mineralogical Evaluation of Chromite at the Nkomati Nickel Mine, Uitkomst Complex, South Africa

“…The Uitkomst Complex is a satellite intrusion of the Bushveld Complex [33,34]. It is a long tubular shaped (or chonolith, [34][35][36]) layered intrusion that hosts base-metal (Ni, Co, Cu and Cr) as well as platinum-group element (PGE) mineralization [37]. Nkomati Nickel mine, located on the Uitkomst Complex approximately 250 km due east of Pretoria, is currently the sole primary nickel producer in South Africa ( Figure 1C, [34]).…”

“…It is a long tubular shaped (or chonolith, [34][35][36]) layered intrusion that hosts base-metal (Ni, Co, Cu and Cr) as well as platinum-group element (PGE) mineralization [37]. Nkomati Nickel mine, located on the Uitkomst Complex approximately 250 km due east of Pretoria, is currently the sole primary nickel producer in South Africa ( Figure 1C, [34]). Although the Uitkomst Complex was described by Wagner [38], mining of the massive sulphide ore for its primary base-metal sulphide (BMS) and secondary PGE mineralization only began more than six decades later in 1996 [39].…”

“…The MSB has been mined out [34,40] with the BMZ currently being unexploited [40]. Since inception of the mining operation, previous studies of this deposit placed emphasis on the process mineralogy of the base-metal sulphide mineralization focusing on reagent and flotation optimization [41][42][43][44][45][46][47][48].…”