Abstract

The exposed northern limb of the Bushveld Complex forms the eastern part of a much larger basin, ~160 X –125 km in size based on geophysical data. Within this basin, the recently discovered ultramafic-mafic succession that extends 24 km northeast from the Hout River shear zone has been shown to have a Bushveld intrusion age of 2.06 Ga. The succession is typically 1 km thick, dips west at 34° to 38°, and comprises a basal Ultramafic sequence, a troctolite-gabbronorite-anorthosite sequence, and an Upper zone. Overlying Waterberg red beds were deposited between ~2.05 and ~1.93 Ma. The basal Ultramafic sequence comprises a lower orthopyroxenite <60 m thick overlying Archean granite gneisses with an intervening agmatite, which is interpreted to be remobilized footwall gneiss interfingered with pyroxenite. The upper portion of the Ultramafic sequence is composed of serpentinized harzburgite. The troctolite-gabbronorite-anorthosite sequence is <850 m thick and is dominated by gabbronorites with minor anorthosites at the top and thick troctolite layers <50 m thick, which are locally pegmatoidal, toward the base. The boundary with the Upper zone is defined by increased magnetic susceptibility due to increased cumulus magnetite. The Upper zone of ferrogabbro and magnetite gabbronorite varies between 0 and 500 m in thickness. Disseminated magnetite is ubiquitous but magnetitite layers that are characteristic of the Upper zone elsewhere in the Bushveld Complex are lacking, except in one core from the extreme south of the project.

There are two PGE-Cu-Ni-Au mineralized intervals, a lower F zone and an upper T zone that vary from 3 to 60 m in thickness. The T zone is restricted to the southern portion of the project, whereas the F zone extends 17 km along strike. In both there is a correlation between visible chalcopyrite and pentlandite abundance and grade of PGE. The F zone of the Ultramafic sequence hosts disseminated sulfides with accessory chromite. Platinum group minerals (PGM), based on more than 1,000 grains recovered, are dominated by sperrylite with lesser Pt-Pd bismuthotellurides, Au-Ag alloys, Pd arsenides, and Pt-Rh sulfoarsenide with a grade of <7 g/t Pt + Pd + Au. The precious metal budget comprises 64% Pd, 30% Pt, 1% Rh, and 5% Au with 0.16% Ni and 0.07% Cu. Mineralization occurs over an interval of ~10 m although in the central portion of the northeasterly oriented body, the “Super F zone” thickens to 60 m, with grades of 3 g/t over this interval. The T zone occurs in a lithologically variable sequence close to the boundary between the troctolite-gabbronorite-anorthosite sequence and the Upper zone. In the south, the T zone is ~30 m thick but increases northward to >50 m thick. An upper T1 and lower T2 are separated by a barren middling that is sometimes absent. Grade in both T1 and T2 is indicated as 3.88 g/t Pt + Pd + Rh + Au, although locally with up to 14 g/t. PGM are less varied than in the F zone and are dominated by Pt-Pd bismuthotellurides. Both T1 and T2 zones have a consistent and unusually gold-enriched metal ratio of ~49% Pd, ~29% Pt, 1% Rh, and 21% Au with 0.08% Ni and 0.16% Cu. Neither the T nor F mineralized zones has direct correlatives in other limbs of the Bushveld Complex.

Although there are comparisons with the Bushveld succession elsewhere, significant differences in thickness and type of lithologies, geochemistry, and mineralization of this arcuate intrusion of the Waterberg segment, compared to the eastern, western, and northern limbs of the Bushveld Complex, suggest that the Waterberg ultramafic-mafic body may represent a structurally controlled separate compartment north of the Hout River shear zone.

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