A review of the emplacement and formation of magmatic platinum-group elements-enriched deposits in large layered mafic/ultramafic intrusions, with special reference to the “big three” (Stillwater Complex, Great Dyke, and Bushveld Complex)

Abstract

Layered intrusions occur on every continent and in rocks ranging in age from Neoarchean to the Tertiary. The “big three” large layered mafic intrusions (Stillwater Complex, Great Dyke, and Bushveld Complex), belong to this select group not only because of their economic importance since they constitute ∼94% of the world's reserves of platinum-group elements (PGEs), but also because of their immense sizes and continuity of individual layers across dozens of kilometers. The “big three” were all emplaced in the Precambrian (Neoarchean to Paleoproterozoic) Era.PGE-enriched horizons in these intrusions are generally thought to have formed as a consequence mixing between an early ultramafic (U-type) parental magma and a later tholeiitic (T-type) parental magma. The PGEs are concentrated is thin stratiform layers termed reefs located in layers that occur close to the level at which the predominant crystallization is inferred to switch from U- to T-magma types. Mineral compositions, stable and radiogenic isotopes all vary from intrusion to intrusion across the PGE-mineralized zone. Although several models have been proposed to have been proposed for the concentration of the PGEs, it is concluded that several models likely contributed, to varying degrees depending on each intrusion, in the precipitation and/or concentration of the PGE mineralization.