The ultramafic-intermediate Riwaka Complex, New Zealand: summary of the petrology, geochemistry and related Ni-Cu-PGE mineralisation

The Late Devonian ultramafic-intermediate Riwaka Complex represents a rare example of magmatic-related Ni-Cu-platinum group element (PGE) sulphide mineralisation within a convergent margin setting. We provide a comprehensive review of the petrography, geochemistry, geochronology and associated Ni-Cu-PGE sulphide mineralisation for the dominant lithologies of the Riwaka Complex. This study reveals that the c. 364 Ma Riwaka Complex was likely emplaced within an extensional intra-arc along the paleo-Pacific subduction margin of east Gondwana. Initial primary melts were derived by partial melting of a metasomatised heterogeneous mantle source, and potentially a young oxidised mafic underplate. Primary melts were tholeiitic, S-undersaturated, and enriched in Ni, Cu and PGE. Crystal accumulation (olivine, clinopyroxene, amphibole), fractionation, and minor magma mixing and crustal contamination account for most of the chemical diversity within the Riwaka Complex, and were the processes responsible for inducing S-saturation of the melt. Sulphide mineralisation occurs as disseminated pyrrhotite-pyrite-chalcopyrite-pentlandite in hornblende-bearing clinopyroxenite and gabbro cumulate lithologies, and is restricted to the southern portion of the complex, interpreted to represent the deeper-level conduit system. The northern portion of the complex represents a more evolved higher-level magma chamber.