Metallogenesis of the Nkana copper–cobalt South Orebody, Zambia

Abstract

The Central African Copperbelt is one of the largest and richest metallogenic provinces in the world. Despite the many studies, the genesis of the stratiform Cu–Co-mineralization remains a subject of intense discussion. A diagenetic, pre-folding origin is proposed for most ore deposits both in Zambia and the Democratic Republic of Congo. However, later mineralization and/or remobilization seem to be important in the enrichment of the ores. The geological mapping of the South Orebody mine at Nkana (Zambia) indicates a relation between the mineralization and the host rock but also with compressional deformation. The location of the rich ore bodies generally corresponds with the hinge zones of tight to isoclinal folds and with the contact between the sandstones and conglomerates of the Footwall Sandstone Formation and the overlying organic-rich shales of the Ore Formation. The circulation of the mineralizing/remobilizing fluids through the rocks was facilitated by fracturing, especially in the hinge zones of the folds resulting in a structural permeability. A petrographical study demonstrated that, in addition to disseminated sulphides, three successive vein generations occur at Nkana South Orebody, i.e. layer parallel veins, irregular, crosscutting veins and massive veins. These vein generations respectively formed during the initial phase of basin inversion, the main phase of deformation and a late phase of orogenesis or later extensional tensions. Early diagenetic disseminated framboidal pyrites were replaced by Cu–sulphides. The timing of this replacement could not be constrained. Silicification, K-feldspar alteration, albitization, carbonatization and replacement by anhydrite are the main alteration phases.