THE PLATREEF OF THE BUSHVELD COMPLEX, SOUTH AFRICA: A NEW HYPOTHESIS OF MULTIPLE, NON-SEQUENTIAL MAGMA REPLENISHMENT BASED ON OBSERVATIONS AT THE AKANANI PROJECT, NORTH-WEST OF MOKOPANE

Detailed logging of relatively deep drill-core from the Akanani Project, in the northern limb of the Bushveld Complex, South Africa, has afforded new perspectives on the Platreef and its origins. In line with most recent studies, we believe the mineralization to be entirely orthomagmatic, rather than contamination-related. The primary orebody at Akanani is what we describe as the Main Mineralized Reef (MMR), which is situated within the Platreef Unit. Whereas the entire Platreef Unit may be as much as 250 m thick, the MMR is typically 4 to 30 m in thickness. In most previous descriptions of the Platreef, pyroxenite has been listed as an important constituent, but our investigations lead us to emphasize the importance of harzburgite (a lithology not previously described as a major component of the Platreef) as a critical component at Akanani. The MMR, despite being more complex than the seemingly layer-cake sequences associated with the Merensky Reef, is more or less laterally continuous (particularly in the southern or proximal facies at Akanani), and thus potentially more predictable than previously believed. Lithological relationships are significantly clearer than in the adjacent open pit operations, such as Sandsloot, where the sequence is disrupted by higher concentrations of dolomitic xenoliths and mineralization is located much closer to and even partially within the floor rocks to the intrusion. The principal component of the Platreef Unit at Akanani (sub-unit PU1) is a pervasive suite of complexly-layered medium to fine-grained gabbronorite-websterite lithologies, with minor feldspathic orthopyroxenite. These rocks are intercalated with numerous ultramafic layers (sub-unit PU2) that are dominated by harzburgite with minor chromitite. Coarse-grained (pegmatoidal) lithologies, including pyroxenite and gabbronorite, locally with minor olivine, constitute a third group (sub-unit PU3), typically associated spatially with the harzburgite. The MMR is a composite sequence of layers dominated by mineralized harzburgite. Pegmatoidal lithologies juxtaposed with the harzburgite are equally well mineralized, but rocks of the PU1 sub-unit incorporated within the reef-zone generally contain sparse PGE. We find that the three sub-units of the Platreef were emplaced subsequent to the lower few hundred meters of the overlying Main Zone. This is well illustrated in the northern (distal) part of Akanani where the poorly-mineralized PU1 component of the Platreef Unit and the most important of the harzburgite layers (part of the MMR) are intercalated with the Main Zone, into which they are interpreted to have intruded. The PU1 sub-unit crystallized from repeated additions of gabbroic magma and in situ differentiation prior to development of the ultramafic and pegmatoidal components of the Platreef Unit. The PU2 sub-unit formed from injections of ultramafic magma into the partially crystalline substrate, the latter being comprised of the PU1 sub-unit and the lowermost part of the Main Zone. The PU3 sub-unit developed from partial melting and recrystallization of the PU1 or Main Zone lithologies in response to intrusion of the ultramafic magmas. The latter are repeated at various depths in many drill-hole intersections, and the stratigraphic sequence of the PU1 and PU2/PU3 sub-units thus does not constitute a simple upward-younging sequence. This conflicts with the widely held view that the Platreef Unit is made up of three “reefs” emplaced more-or-less in stratigraphic sequence. Our categorization of the Platreef Unit into three sub-units is probably overly simplistic (and coincidental as regards the older A, B, C terminology) but has enabled us to cut to the chase and propose a new hypothesis directed at assisting the mining industry. Changes along strike are of fundamental significance, and in the northern part of Akanani, as well as the Drenthe property further north, the PU1 and PU2/PU3 sub-units increasingly bifurcate as they were intruded, lit-par-lit , into an increasingly greater thickness of Main Zone. The PGE at Akanani preferentially occur in association with the ultramafic lithologies. The gabbronorite-websterite lithologies are typically poorly mineralized, except where they are spatially associated with ultramafic lithologies. The location of the richest and most consistently mineralized ultramafic layers (i.e. the MMR) near the top of the Platreef Unit at Akanani is possibly significant, in that it mirrors the location of the Merensky Reef near the top of the Upper Critical Zone elsewhere in the Bushveld Complex. A key factor in the development of all mineralized reefs in the Bushveld Complex is, we believe, the non-sequential intrusion of discrete lineages of tholeiitic/gabbroic and ultramafic magmas (Mitchell and Scoon, 2007). Thus we suggest that both the Merensky Reef and the Platreef can be explained within our overarching hypothesis of lateral mixing, whereby thin sheets of PGE-rich ultramafic magma were intruded into a pre-existing sequence of mafic cumulates.