Abstract
The about 2055-Ma-old mafic to ultramafic Uitkomst Complex in the Mpumalanga Province of South Africa hosts the low-grade-large-tonnage Ni-Cu-PGE deposit, Nkomati. The complex is regarded to represent a satellite to the Bushveld Complex and a feeder to an eroded magmatic reservoir in the southeast. Aeromagnetic surveys and previous drilling indicated an overall northwestern-downdip extension of the complex, but the question is to what extent and in which expression can the complete intrusion be found under cover in the northwest? Answering this, a mineralogical, geochemical and geochronological investigation of a borehole intersection of the whole complex at Little Mamre was carried out, using petrography, XRF, EPMA and LA-ICP-MS U–Pb analyses of zircons for age determination. Although the total thickness of the rock units is larger than to the southeast, emplacement, litho- and mineral chemistry trends, expression of alteration mineralogy and style of sulphide mineralisation are similar. The amount of sulphide mineralisation is on average less than in the southeast. The upper ultramafic unit contains, more frequently, pegmatoidal sections, and the Chromitiferous Harzburgite unit has less massive chromitite layers than the southeastern parts of the complex, whereas the gabbro(-norite) units contain more interstitial liquid with late-stage minerals. The findings confirm that the anvil-shaped intrusion in cross section continues with increased thickness towards northwest at a shallow dip; although approaching the deeper part of the igneous reservoir, mineral compositions are partially more evolved. The overall mineralogical consistency downdip supports a situation of multiple magma replenishment along a flat-lying, northwest–southeast trending conduit, resulting in an evolved cumulus mineral assemblage in the upper part.
Highlights
Introduction published maps and institutional affilThe Nkomati Nickel Mine in the mafic to ultramafic Nkomati Nickel lies in the northeastern part of South Africa near Machadodorp in the Mpumalanga Province
In line with earlier assumptions, the intrusion dips with a similar angle to the country rock (Figure 4), with an undulate floor contact on sheared Malmani quartzite on top of a thick Black Reef quartzite layer
Detailed petrography allowed to estimate the modal proportions of the entire profile of the Uitkomst Complex as intersected in Little Mamre (LM)-6, attributing the alteration mineral proportions to the respective primary igneous minerals (Figure 7)
Summary
The Nkomati Nickel Mine in the mafic to ultramafic Nkomati Nickel lies in the northeastern part of South Africa near Machadodorp in the Mpumalanga Province. It was the first primary Nickel-producer in South Africa (Figure 1a). Nkomati represents a large Nickel reserve in South Africa hosting measured and indicated mineral resources of 170.25 million tonnes (Mt) of ore grading at. There are measured and indicated Chromium resources of 0.51 Mt grading at about 24.7% Cr2 O3 , and proven Chromium reserves of. After the mining of massive sulphide lenses (MSB mine) on the farm Slaaihoek (SH) from 1995 until 2003, open-pit mining of the predominantly lowgrade-large-tonnage Ni-Cu-PGE disseminated sulphide ore body continued until early iations
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