Supergene enrichment of the Kitumba IOCG deposit, Zambia

Abstract

Iron oxide copper-gold (IOCG) deposits host Cu and Au, as well as numerous other potential by-products such as U, Ag, Co, and rare earth elements. The Kitumba IOCG deposit in Zambia has undergone multiple stages of alteration and sulfide formation and subsequent supergene enrichment, with Cu reaching up to 30% in supergene zones down to depths of several hundred metres. Quantitative mineralogy characterised five styles of mineralisation. Stage 1 hypogene chalcopyrite and pyrite, is progressively replaced by Stage 2, a continuum of supergene stages: 2a chalcocite and minor covellite; 2b cuprite and native Cu, 2c malachite and 2d brochantite. In addition, a distinct zone of Au enrichment (up to 2 ppm) is found in Fe-oxide breccia near the upper parts of the deposit which appears to be hosted by a pocket of folded and heavily Fe-altered Katangan metasedimentary rocks. Replacement of hypogene sulfides by chalcocite occurred under acidic conditions (pH 5.5–6); with the precipitation of cuprite, native Cu under near neutral, more oxidising conditions (pH > 6) and precipitation of malachite under further oxidising conditions (pH 6.5–8), with localised zones of acidity (pH 5.9–7) that encouraged the precipitation of brochantite. Cobalt is notably depleted in the supergene zone relative to hypogene, whilst Ag is enriched. We present a genetic model for supergene mineralisation at the Kitumba IOCG deposit, and as such has implications for other supergene altered IOCG deposits with a similar supergene mineralogy. Importantly, the Cu mineralisation styles established each have distinctive geometallurgical characteristics, and as such, has implication for processing of ore.