The Waterberg platinum-group element deposit, South Africa
Date
2018
Authors
McCreesh, Matthew James Gerard
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Abstract
The Waterberg platinum-group element deposit of the Bushveld Complex is located within the Southern Marginal Zone of the Limpopo Mobile Belt, north of the Hout River Shear Zone. The Bushveld succession is approximately 1.2 km thick, and dips 34o to 38o towards the northwest. The succession is composed of basal Marginal sills, an Ultramafic Sequence, a troctolite-gabbronorite-anorthosite sequence, and an Upper Zone. The overlying Waterberg Group was deposited between ~2.05 to ~1.95 Ma, after extensive erosion of the roof rocks and the Upper Zone and before the intrusion of a dolerite sill associated with the ~1.1 Ga Umkondo Igneous Province.
The magmatic history of the Waterberg deposit stratigraphy is believed to have started with the intrusion of the basal Marginal sill composed of fine-grained pyroxenite and gabbronorite similar to the Marginal Zone lithologies elsewhere in the Bushveld Complex. These sill-like intrusions interacted with the granitic gneiss footwall of the Southern Marginal Zone to form a basal granofels unit. The overlying Ultramafic Sequence is the most primitive unit of the Waterberg stratigraphy, and hosts the lower F zone mineralisation. This sequence is dominated by a basal orthopyroxenite, overlain by serpentinised harzburgite. Chromite occurs as disseminated clusters with rare discontinuous chromitite stringers. The overlying troctolite-gabbronorite-anorthosite sequence forms a magmatic erosional contact with the underlying Ultramafic Sequence. The basal troctolite is interpreted to have formed from magma mixing between the residual melt of the harzburgite and the influx of a gabbroic melts. The troctolite grades upwards into gabbronorite and anorthosite with sporadic occurrences of inverted pigeonite, which are typical characteristics of the Main Zone elsewhere in the Bushveld Complex. The top contact of this sequence is associated with the upper T zone mineralisation and the occurrence of cumulus magnetite, an indicator mineral of the Upper Zone elsewhere in the Bushveld Complex. The Upper Zone is composed of disseminated cumulus magnetite, however it lacks the magnetitite layers seen in the rest of the Bushveld Complex.
The two PGE-Cu-Ni-Au mineralised intervals vary from 3 m to 60 m thick. The upper T zone is restricted to the southern portion of the project area, whereas the lower F zone extends for a known 17 km along strike. The lower F zone mineralisation is dominated by sperrylite and Pt-Pd bismuthotellurides and these platinum-group metals are mainly associated with primary magmatic base-metal sulphides, which have undergone alteration during serpentinisation followed by the formation of secondary sulphide assemblages. The sulphur isotopic signatures suggest that the F zone mineralisation is mantle-derived with no contamination involved in the formation of the mineralisation. The upper T zone mineralisation hosted by more felsic-rich lithologies is dominated by Pt-Pd bismuthotellurides. The mineralised zone contains relics of primary magmatic sulphides and is characterised by the development of a chalcopyrite-millerite-pyrite assemblage, associated with hydrothermal quartz and hydrous silicates. The fluid–induced style of platinum-group metal remobilisation, high Au/PGE ratios, and high proportion of native gold in the high-grade T zone mineralisation in cumulus magnetite-bearing lithologies is unique to the Bushveld Complex. The genesis of the T zone is interpreted to result from the prolonged fractionation of the troctolite-gabbronorite-anorthosite sequence with the residual melt interacting with the first influx of Upper Zone fertile melt. The sulphur isotopic results indicate that the mineralisation is mantle-derived and that no significant contamination is involved in the formation of the T zone ores.
Although there are similarities with the Bushveld succession south of the Hout River Shear Zone and with the rest of the Bushveld Complex, significant differences in the thickness and type of lithologies, mineral and whole-rock geochemistry and mineralisation of the arcuate intrusion of the Waterberg segment, compared with the eastern, western and northern limbs of the Bushveld Complex, suggests that the Waterberg Bushveld body may represent a structurally controlled separate compartment wedge between the Hout River Shear Zone to the south and the Palala Shear Zone to the north
Description
2018
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Citation
McCreesh, Matthew James Gerard, (2018) The Waterberg platinum-group element deposit, South Africa, University of the Witwatersrand, Johannesburg, https://hdl.handle.net/10539/27140.