Talc-carbonate-alteration of the Chromitiferous Harzburgite unit of the Uitkomst Complex, Nkomati Ni-Cu-Cr-PGE Mine, South Africa

The Nkomati Ni-Cu-Cr-PGE Mine of the Uitkomst Complex in the Mpumalanga Province was the main primary nickel producer in South Africa until 2021, when the open-pit operation was placed on care and maintenance. The nickel demand generated by the EV battery market and the long-term increase of the Ni price may be reason to consider further detailed study of the petrology and alteration processes of the complex.

The lower three rock units of the about 2057.6 ± 0.7 Ma old (Maier et al. 2018) mafic to ultramafic complex contain Ni-Cu-PGE sulphide and chromite mineralisation. From bottom to top they are intensely saussuritizised, amphibolitised, serpentinised, talcified, and carbonatised. The original igneous cumulate mineral assemblage determines the composition of the alteration minerals, which in turn challenge ore processing of the unit. However, a positive correlation between the degree of alteration and increasing grade and aggregate size of the ore is obvious.

The formation of talc-carbonate schist is regarded as the ultimate stage of ultramafic rock alteration under the influence of a H₂O- and CO₂-rich fluid phase derived from pelitic and dolomite country rocks. These were strongly contact metamorphosed and partially assimilated by the ultramafic magma. Carbonate devolatisation provided the CO₂ for continuous talc-carbonate-alteration, from a late-magmatic to a hydrothermal stage. Pseudosections of CO₂ fraction vs. temperature phase diagrams allow the estimation of a formation temperature of about 500°C for the main part of the alteration paragenesis. The formation of the talc-carbonate-pyrite schist is regarded as a retrograde alteration process of serpentinised harzburgites.