The Merensky Reef of the Bushveld Complex, South Africa: Mineral systems analysis of its emplacement and timing of the PGE-Ni-Cu mineralisation

Abstract Critical genetic aspects of platinum-group elements (PGE) mineralised layers in the 2.05 Ga Bushveld Complex are examined using Mineral Systems methodology, considering macro-, meso-, and micro-scale features, focussing on the Merensky Layer, and deriving an observation-controlled model of unit emplacement and timing/controls on mineralisation. Macro-scale relationships between the major units of the Bushveld Complex (Marginal, Lower, Critical, Main and Upper Zones) and its satellite intrusions (Uitkomst, Magaliesberg, Koster, Helvetia intrusions), constrain emplacement into multiple episodes, progressively building the complex upwards. In the Critical Zone, the parental magma of the Merensky Layer was emplaced over an erosional surface of the intrusion floor. The early-formed basal Merensky chromitite layer, sulphide, and PGE mineralisation were locally introduced into the floor, forming downward-penetrating pure anorthosite apophyses. The Merensky Layer is a composite unit formed by multiple magma influxes that locally eroded the earlier formed floor cumulates. This is indicated by a locally erosional base to the upper chromitite stringer and occasional intermediate PGE mineralised zones where the layer is thicker. Following emplacement, post genetic feldspathic pegmatoid veins form upwardly expanding plumes, initiating at the base, locally displacing the basal chromitite layer. The pegmatoid veins pond beneath and break the upper chromitite layer, forming liquid escape features at the top contact of the Merensky Layer. Later stage pegmatite veins composed of feldspar-quartz-phlogopite-tourmaline truncate all earlier features, usually terminating at the top contact. The Merensky Layer has a variably sharp to gradational upper contact, occasionally with rounded orthopyroxenite “boulders” coalescing downwards. The thickness of the overlying liquid resident magma column at the time of Merensky Layer intrusion is not constrained; however, no Main Zone magma was likely present in the proto-complex. The top of the Merensky Layer is also locally eroded by the overlying stratigraphy, fundamentally discounting models that propose its emplacement as a later sill into consolidated cumulates. The upward increase in the Pt/(Pt + Pd) ratio within the underlying Bushveld satellite intrusions and the Bushveld Complex precludes within-intrusion PGE accumulation models and suggests that the mineralised layers are repositories of magma that has undergone metal accumulation within deeper parts of the magma column underlying the Bushveld Complex.