Non-Sequential Injection of PGE-rich Ultramafic Sills in the Platreef Unit at Akanani, Northern Limb of the Bushveld Complex: Evidence from Sr and Nd Isotopic Systematics

Abstract

Abstract The Platreef Unit is a deceptively complex sequence of layered cumulates located in the northern limb of the 2·055 Ga-old Bushveld Complex. The unit contains the Platreef, a thick, richly mineralized stratabound PGE orebody which differs markedly from the comparatively thin, predominantly stratiform Merensky Reef found in the Upper Critical Zone of the eastern and western limbs. The Platreef Unit is interpreted as a localized facies of the Upper Critical Zone, despite layering being neither as systematic nor as clearly defined as in the equivalent stratigraphy found in the other limbs. The Platreef Unit in the Akanani project area includes well-defined layers of feldspathic harzburgite and norite, in addition to the ubiquitous feldspathic orthopyroxenite–melanorite that characterizes other sections. The paucity of floor-rock xenoliths is an additional feature. The relatively well-developed nature of the layering and paucity of xenoliths in the Platreef Unit at Akanani is explained by separation of the unit from the floor of the intrusion by a thick succession of ultramafics assigned to the Lower Critical Zone. We identify three lithological subgroups in the Platreef Unit at Akanani. They do not define an upward-younging stratigraphy. The primary stratigraphy, or PU1 subunit, is dominated by multiple layers of feldspathic orthopyroxenite, melanorite, and norite. This subunit built up from incremental addition of relatively small magma pulses. Repeated magma replenishment induced concomitant partial melting of earlier-formed layers. The PU1 subunit includes thin chromite stringers that contain Cr-spinels with unusual, amoeboidal textures consistent with several stages of growth and re-equilibration. The feldspathic harzburgite of the younger PU2 subunit was emplaced non-sequentially into the already complexly-layered PU1 subunit as a series of sinuous lenses or syn-intrusive sills. One of the PU2 sills contains the richest and most consistent of the mineralized sections at Akanani, i.e., the Main Mineralized Reef (MMR). The irregularly-developed pegmatoidal lithologies of the PU3 subunit are ascribed to recrystallization of earlier-formed cumulates (PU1 and PU2). Whole rock isotopic data for a section of the Platreef Unit, together with the overlying Lower Main Zone and underlying Lower Critical Zone, mostly from drill-hole ZF-1, demonstrate a complex pattern in both Sr87/Sr86 initial ratios and ϵNd values. These patterns are consistent with multiple lineages of parental magmas. The Lower Main Zone and the majority of the Platreef Unit are characterized by anomalously high Sr initial ratios (with a large degree of scatter) and low ϵNd values (relatively tightly constrained). Harzburgite layers from the Lower Critical Zone have a low Sr initial ratio and a relatively high ϵNd value. The new isotopic data suggest these sequences crystallized from multiple magma batches, broadly constrained within the U-type (ultramafic) and A-type (tholeiitic) lineages, derived from mantle sources and/or staging chambers which experienced varying degrees of crustal contamination. The MMR crystallized from a specific pulse of the U-type magma lineage characterized by a high Sr87/Sr86 initial ratio (average of 0·71113) and a markedly low ϵNd value (average of -11·35). The olivine-saturated magmas associated with the MMR were derived from a localized mantle source and yet underwent an unusually high degree of crustal contamination. Some layered PGE orebodies in the Bushveld Complex, including the Platreef and Merensky Reef, were emplaced as syn-magmatic sills which crystallized from anomalously PGE-rich parental magmas with an unique isotopic fingerprint.