Supergene enrichment of the Kitumba IOCG deposit, Zambia

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. </p

Mobilisation of deep crustal sulfide melts as a first order control on upper lithospheric metallogeny

Magmatic arcs are terrestrial environments where lithospheric cycling and recycling of metals and volatiles is enhanced. However, the first-order mechanism permitting the episodic fluxing of these elements from the mantle through to the outer Earth’s spheres has been elusive. To address this knowledge gap, we focus on the textural and minero-chemical characteristics of metal-rich magmatic sulfides hosted in amphibole-olivine-pyroxene cumulates in the lowermost crust. We show that in cumulates that were subject to increasing temperature due to prolonged mafic magmatism, which only occurs episodically during the complex evolution of any magmatic arc, Cu-Au-rich sulfide can exist as liquid while Ni-Fe rich sulfide occurs as a solid phase. This scenario occurs within a ‘Goldilocks’ temperature zone at ~1100–1200 °C, typical of the base of the crust in arcs, which permits episodic fractionation and mobilisation of Cu-Au-rich sulfide liquid into permeable melt networks that may ascend through the lithosphere providing metals for porphyry and epithermal ore deposits.</p