The nature and characteristics of sulphide mineralisation at the Kamoa - Kakula copper deposit of the Katanga basin, Central African Copperbelt, Lualaba, Democratic Republic of Congo

Abstract

The Kamoa-Kakula deposit is a world-class stratiform copper deposit located in the southern part of the Democratic Republic of Congo (DRC) approximately 25 km from Kolwezi town and close to the border of DRC and Zambia. The ore deposit is in the southern area of the western foreland domain of the Congolese Copperbelt (Central African Copperbelt). The structural setting of the deposit is characterised by the West scarp fault, binding Kamoa to the west whilst it is bounded by faults of the Kansoko Sud trend to the east. The mineral resource footprint of the deposit envelopes the Makulu dome and wraps around the southern edge of the Kamoa dome. The main sedimentary lithologies hosting copper mineralisation are diamictite and interbedded siltstones of the Grand Conglomérat Formation at the base of the Nguba Group overlying sandstones of the Mwashya Subgroup (Roan Group). Previous studies within DRC and Zambia showed that the glaciogenic diamictite (Grand Conglomérat Formation) is a laterally extensive regional marker. The geological interpretation and literature data reveals a distinct stratigraphic correlation of several copper deposits across the Central African Copperbelt confirming multiple occurrences at various stratigraphic levels. Furthermore, this analysis identifies the Fishtie deposit (Lusale basin, Zambia), which occurs on the eastern margin of the Katanga basin, as the closest geological analogue to Kamoa – Kakula. At Kamoa-Kakula, hydrothermal sulphide mineralisation occurs mainly towards the base of the Grand Conglomérat with the dominant sulphide mineral assemblage composed of pyrite – chalcopyrite – bornite - chalcocite, and minor covellite . Optical microscopy demonstrates the multi-stage crystallisation of few generations of pyrite including the earliest diagenetic framboidal pyrite. The widespread development of the symplectic texture linked to bornite and chalcocite intricate intergrowths and regular rimming of clasts by sulphide overgrowth, as well as the occurrences of framboidal chalcopyrite and bornite argue for the sulphide paragenesis being linked to sulphide replacement. This is consistent with the apparent mineral zonation progressing downwards from pyrite → chalcopyrite → bornite ± covellite → chalcocite. The S isotope variations suggest that a significant portion of S at Kamoa originated from early bacterial sulphate reduction, which resulted in precipitation of fine-grained framboidal and sooty pyrite with the negative δ34S values as low as -19.9 ‰. The similar, while narrower, range of δ 34S values for chalcopyrite is due to inheritance and homogenization of the S isotopic signature of diagenetic sulphides during ore stage replacement, whereas the highly negative δ 34S values for chalcocite (down to -35.1 ‰) indicate the subsequent extreme isotope fractionation under low-temperature conditions. The study includes a comprehensive overview of the regional stratigraphic and geological correlation, in conjunction with detailed mineralogical and isotopic observations, contributing to further understanding of Cu mineralisation at Kamoa-Kakula as well as on a scale of the entire Central African Copperbelt.