Geology of the Fishtie deposit, Central Province, Zambia: iron oxide and copper mineralization in Nguba Group metasedimentary rocks

The Fishtie copper deposit, located in the Central Province of Zambia, contains approximately 55 Mt of 1.04 % Cu at a 0.5 % Cu cut-off in oxide, sulfide, and mixed oxide–sulfide ores. The deposit is hosted in Neoproterozoic diamictites and siltstones of the Grand Conglomerat Formation and overlying Kakontwe Limestone Formation of the lower Nguba Group. The Grand Conglomerat Formation at Fishtie directly overlies basement schists and quartzites. Mineralized zones are located adjacent to high-angle normal faults that appear to control thickness variations in the Grand Conglomerat Formation suggesting synsedimentary fault movement. Iron-rich rocks consisting of nearly monomineralic bands of magnetite and ankerite occur within the Grand Conglomerat Formation. The absence of magnetite-rich clasts in overlying diamictites and the presence of disseminated magnetite, ankerite, and apatite in adjacent diamictites suggest this iron-rich rock formed by replacement of siltstone beds. These magnetite-rich rocks thicken towards normal faults suggesting the faults formed conduits for oxidized hydrothermal solutions. The magnetite–ankerite–quartz rock was overprinted by later hydrothermal alteration and sulfide mineralization. Copper sulfide precipitation was associated with growth of both muscovite and chlorite, together with weak silicification. Sulfides are zoned relative to normal faults with bornite more common in proximity to faults and ore stage pyrite most common in an outer zone with chalcopyrite. Copper sulfides display generally heavy sulfur isotopic values, suggesting sulfide derivation from thermochemical reduction of Neoproterozoic seawater sulfate. Copper mineralized zones in the Grand Conglomerat at Fishtie are megascopically similar to those observed in the newly discovered Kamoa deposit in the southern Democratic Republic of Congo. Alteration and mineralization at Fishtie display lateral zoning relative to normal faults unlike the broad vertical zonation observed at the giant Kamoa deposit. The small size of the known mineralized zones at the Fishtie deposit relative to Kamoa is probably due to the absence of a thick siliciclastic palaeoaquifer beneath the Grand Conglomerat Formation as is present at Kamoa.