Strontium and neodymium isotope systematics of the UG2 Chromitite and Merensky-Bastard Reef Interval at Winnaarshoek, Eastern Bushveld Complex: Implications for a Stacked Sill Complex

Abstract Whole-rock geochemistry and Sr–Nd isotope data are presented for a 500-m-thick section at Winnaarshoek in the Eastern Limb of South Africa’s Bushveld Complex. Extending from the Upper Critical Zone, including the UG2 and UG3 chromitites and the Merensky and Bastard Reefs, into the Lower Main Zone (LMZ), which has a bimodal lithostratigraphy. The regionally persistent anorthosites and associated norites are intercalated with subordinate pyroxenites. Lithological units are therefore descriptive rather than assigned a genetic origin. Across the UG2 to Merensky Footwall interval, feldspathic rocks show nearly constant initial Sr isotope ratios (SrR 0 ) over 400 m, broadly overlapping the more variable pyroxenite values. SrR 0 values are higher in the Merensky–Bastard Reef pyroxenites than in the UG2–UG3 pyroxenites, increase stepwise upward through the anorthosites capping the Merensky–Bastard units, and reach their highest levels in the LMZ. In contrast, Ɛ Nd remains tightly constrained throughout the profile. This decoupling of the Sr and Nd isotope systems, together with Sr-isotopic differences between anorthosites and pyroxenites, supports intrusion as thin magma sheets rather than differentiation within a high-level magma chamber. The pyroxenites and chromitites are interpreted as composite sills emplaced along contacts between pre-existing noritic and anorthositic strata. The magma sheets retained isotopic heterogeneity acquired as their parental magmas evolved in a deep-crustal staging chamber. Up-section isotope trends indicate increasing crustal contamination or local assimilation of an isotopically enriched contaminant. Interpreting these layered igneous rocks as a stacked sill complex directly links chromitites and PGE mineralisation to the staging chamber. Pyroxenitic sill magmas transported chromite slurries and PGE from depth. This model explains both the metre- to centimetre-scale alternation of mineralised chromitites and pyroxenitic reefs with compositionally similar, PGE-poor layers and the lateral persistence of richly mineralised chromite stringers.