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Location: 34 km SE from Gweru, Zimbabwe
55 Marshall Street MarshalltownJohannesburgSouth Africa2001
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Unki Mine is located in the Selukwe subchamber of the Great Dyke in Zimbabwe. In transverse section, the subchamber is synclinal in shape, with essentially the same lithological succession being exposed on both sides of the longitudinal axis. The general dip decreases from outcrops to the central area varying from 14° to 0°. Within the Special Mining Lease, the intrusion strikes north north-east/south south-west and extends about 26km. The Great Dyke in Zimbabwe occurs as a 2.5 billion-year-old mafic to predominantly ultramafic layered intrusion, is about 550km long and between 3km and 11km wide. The Great Dyke is trending in a north north-easterly direction cutting across the Archaean rocks of the Zimbabwe Craton, which are largely granite and greenstone belt rocks. Unki Mine is located in the Selukwe (Shurugwi) subchamber of the Great Dyke. The subchamber is 90km long, and up to about 7km wide. The shape of the Selukwe (Shurugwi) subchamber has to some extent been controlled by the proximity of the Selukwe greenstone belt, in that it has been deflected and constricted in places.The Great Dyke has been preserved within a narrow graben structure and is synclinal in shape, with essentially the same mafic and ultramafic lithological successions being exposed on both sides of the longitudinal axis. The dyke comprises an upper mafic unit of alternating sequences of pyroxenite and duniteharzburgites successions and a lower ultramafic zone of dunite. The layers are dipping and flattening towards the axis of the intrusion and the dip decreases from outcrops to the central near-axis area varying from 14° to 0°. The PGMs and associated base metal mineralisation are developed within the uppermost pyroxenite horizon, the Main Sulphide Zone (MSZ). The Main Sulphide Zone (MSZ) The mineralisation occurs at the same stratigraphic horizon throughout the Great Dyke within the uppermost pyroxenite layer, the P1 unit, and follows the igneous layering. The MSZ occurs some 10m below the mafic-ultramafic contact and is hosted in the P1 unit of the ultramafic sequence. The P1 unit consists of a thin upper unit of websterite and a lower thick unit of plagioclase pyroxenite where the MSZ occurs near the base of the websterite unit. Based on geochemistry, the MSZ has two distinguishable subzones – the base metal subzone (BMSZ), which is dominated by nickel and copper, and the PGMs subzone. The transition from the upper zone to lower zone is marked by a reduction in iron-nickel-copper sulphides disseminated in the pyroxenite. The base of the upper zone is an important stratigraphic horizon, referred to as the BMSZ.The base metals occur as disseminated inter-cumulus iron-nickel-copper sulphides. Although the mineralised zone is characterised by the absence of identifiable markers, this risk has been successfully mitigated by using handheld X-ray fluorescence (XRF) technology, as well as regular underground sampling of the mineralised horizon, enabling optimal extraction.The MSZ is structurally affected by faults, xenoliths and replacement pegmatites. The most prominent is the Footwall fault, which occurs at an average stratigraphic distance of 1.6m below the BMSZ, with this distance ranging from approximately 1m to 2.5m. This fault is localised over a small section of the eastern section of the mine.
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