Magmatic Stratigraphy and Platinum Group Element Mineralization at Tweefontein, Northern Bushveld Complex: Evidence of a Complex Intrusion History of Lower, Critical, and Main Zone Magmas

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Abstract The Platreef, northern limb of the Bushveld Complex, South Africa, forms one of the world’s largest resources of platinum group elements (PGEs), with additional Ni-Cu-Co mineralization. It is widely considered that the Platreef formed via the emplacement of a series of discrete magmatic units; however, the relationship between this magmatic stratigraphy and the distribution of Ni-Cu-Co-PGE mineralization remains poorly constrained. This study constitutes the first in-depth examination of the Platreef magmatic stratigraphy at Tweefontein 238 KR, located directly north of the Flatreef extension at Turfspruit. Petrology and whole-rock and mineral chemistry define three magmatic units: the Upper Platreef, Main zone finger, and Lower zone transition, each displaying distinct pyroxene Mg# contents (79.6, 71.2, and 88.6 respectively), mineral assemblages, and bulk geochemistries. Updip the sequence thins considerably from >600 to <350 m, and contamination signatures of elevated CaO and FeO increase. However, local contamination is seldom evident in the PGE-bearing Upper Platreef. The intrusion of the overlying Main zone is proposed to have eroded the Upper Platreef considerably in some locations, locally reducing the economic viability of this mineralized horizon. The presented stratigraphy indicates that at Tweefontein (1) the Lower and Critical zone magmas are not necessarily separate and evolve from Lower to Critical over a distinct transitional zone, (2) there is only one main Critical zone unit that is host to the PGE mineralization, and (3) the Main zone not only forms a magmatic uniformity at the top of the Critical zone but also intrudes the Critical zone.