Geology and mineralisation of the Sheba's Ridge area, Eastern Bushveld Complex, South Africa

Stevens, Fiona Jean
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Sheba’s Ridge forms an area of approximately 20 km2 in the far western extension of the eastern limb of the Bushveld Complex. It is peripheral to the main Complex and lies within a structurally complex setting to the south-west of the Mineral Range area. The ultramafic-mafic rocks strike approximately east-west and lie to the southeast of the Dennilton Dome and to the west of the Rietfontein Dome. In the northwestern sector of the area, igneous rocks onlap onto an irregular floor rock topography of Transvaal Supergroup metasediments. The ultramafic-mafic sequence dips to the south, in contrast to the normal dip of the main Bushveld Complex, which dips inwards towards the centre of the intrusion. The basal pyroxenite at Sheba’s Ridge is host to a contact style Ni, Cu, PGE deposit. Succeeding intrusive pulses interfingered with earlier layers and incorporated disorientated xenoliths of country rock and earlier mafics. These norites and pyroxenites have similar mineralogical and geochemical signatures to Critical Zone lithologies of the eastern and western limbs of the Bushveld Complex. The final intrusion appears to have been gabbronorites which have been equated with the Main Zone of the Bushveld Complex. The complex stratigraphy of Sheba’s Ridge has been subdivided on the basis of Mg#, An# and element ratios Sr/Al2O3, Cr/MgO, Sr/Ba, Cu/Zr, Cr/V, and SiO2/Al2O3. Two distinct geochemical groupings are recorded and infer that the sequence at Sheba’s Ridge comprises an interfingering of these two distinct packages. From a comparison with the main eastern and western lobes of the Bushveld Complex, these two packages have been correlated to the upper Critical Zone and Main Zone of the Bushveld Complex. Furthermore, element ratios show that peaks in mineralisation occur where these two packages have interacted. This mineralised unit shows considerable similarities to the Platreef in the northern limb of the Bushveld Complex. Both have a hanging wall of norite with inverted pigeonite, both have varying Pt:Pd ratios throughout the mineralised package unlike that of the Merensky Reef. However the Platreef is directly overlain by the Main Zone and does not possess any upper mineralised layers. A model for the emplacement of the igneous rocks at Sheba’s Ridge envisages an early ‘Marginal Zone’ magma intruded into cold Transvaal Supergroup sediments. This was followed by the successive intrusion of pyroxenitic layers which incorporated both country rock xenoliths and earlier ‘Marginal Zone’ intrusives. The next phase of magma influx was the noritic package, which disrupted the pyroxenite package and formed discontinuous interfingering sills. The final magma influx was the gabbronorite package which now forms Sheba’s Ridge. This phase may have dislodged and incorporated earlier mafic lithologies. This atypical sequence at Sheba’s Ridge may have formed from a separate feeder to the main limbs of the Bushveld Complex. Alternatively, the peripheral location of the basin and dome topography may have locally controlled and isolated the magma from the main Bushveld intrusion, such that the stratigraphic succession that crystallised not only differs somewhat from the Bushveld as a whole but also occurs as an interfingered package rather than a typical layered sequence.