The nature and genesis of the Munali nickel sulfide deposit, Zambia

The Munali Ni-(Cu-Co-PGE) deposit is located within the Zambezi Belt in southern Zambia, and appears to represent a conduit-hosted sulfide ‘breccia’ deposit, situated along the Munali Fault. However, Munali displays many atypical features including abundant carbonate that question the applicability of traditional models of magmatic sulfide genesis. This work highlights how carbon, either as a primary mantle-sourced component, or a crustally-derived contaminant, can play an important role in the transport, generation and style of magmatic sulfides.

The Munali complex comprises a dynamic suite of mafic-ultramafic rocks emplaced at ~860 Ma, with sequential emplacement of magmas with increasing MgO and FeO contents that include gabbro and (Cr-poor apatite-bearing) clinopyroxenite, wehrlite and metasomatic ‘replacive’ dunite, emplaced during the initiation of Zambezi rifting. Thin S- and evaporite-rich limestone units adjacent to the Munali Fault provided favorable pathways for ascending mantle-derived magmas. Pervasive interaction and assimilation of S and CO2 were critical in promoting magnetite crystallisation and the formation of carbonate and sulfide melts.

Mineralisation is characterised by an unusually Ni-Fe-dominant massive sulfide, comprised of pyrrhotite>>pentlandite ± chalcopyrite ± pyrite, associated with abundant magmatic-hydrothermal carbonate-apatite-magnetite. Sulfide represents one of the youngest intrusive phases, present as a series of elongate lenses (0.5-30 m thick) that infiltrated the Munali Complex as a density-driven slurry, causing localised brecciation of the host mafic-ultramafic rocks. Sulfide displays extremely variable Ni/Cu ratios (0.1-71.5), moderate to high Pd/IrN ratios (~150-3500) and negative Cu and Au anomalies, primarily as a consequence of fractional crystallisation of the sulfide liquid during mobilisation, resulting in the segregation and crystallisation of Ni-Fe sulfide and the continued migration of a residual Cu-rich sulfide liquid.

The Munali Complex shows indications of significant post-emplacement metamorphism during the Pan African Orogeny, with deformation of host rocks and potential for late-stage remobilisation of Cu and Au. As such, Munali represents a complex igneous system resulting from a range of syn- and post-magmatic processes.