Magnetite-hosted Cu-PGE and Fe-sulfide mineralization in 1078 Ma layered mafic intrusions in the west Musgraves region of Western Australia

Abstract

Three mafic-hosted, magnetite-related prospects called Jameson, Navigator and Latitude Hill occur in the most evolved, upper parts of the 1078 Ma layered intrusions belonging to the Giles Complex, Musgrave Province, Western Australia. The hosting layered intrusions are dominated by leucogabbronorite, gabbro and troctolite and each has a significant Fe-Ti oxide component comprising heavily disseminated, semi-massive and massive magnetite and ilmenite forming layers.Low grade Cu and PGE mineralization at Jameson and Navigator is strongly correlated with the presence of titaniferous magnetite. Copper, Au, Pt, Pd and S concentrations are low to very low below the first appearance of magnetite, but shift to significantly higher concentrations upon the first appearance of sulfide, which coincides with an increase in Fe2O3+TiO2+V2O5 concentrations. Copper grades up to 0.42wt.% and Pt+Pd+Au grades to 2ppm are associated with S concentrations that rarely exceed 0.4 wt.%. There is a very low sulfide abundance, typically <1 modal%, dominated by Cu-rich sulfides, bornite and chalcopyrite (Cu/S ratios are typically >1). Cobalt-rich pentlandite and millerite are trace sulfides in these prospects, but pyrrhotite is rare or absent. Some of the Pt and Pd peaks are stratigraphically off-set from Cu and Au peaks. Mineralization is further characterised by high Cu/Ni ratios and low S/Se ratios indicating Ni partitioning into silicates prior to sulfide formation and very low or no crustal contamination. The Cu/Pd ratios at Jameson and Navigator show a clear shift from low Cu/Pd ratios below the sulfide-bearing magnetite zone to high Cu/Pd ratios above the base of the mineralization, highlighting that reef-type mineralization has been intersected.Mantle-normalised PGE patterns for Jameson and Navigator are PPGE>IPGE, which is typical for magnetite-related, reef-type Cu-PGE deposits. The Jameson mineralization is related to a <4m thick magnetitite and has many characteristics that closely resemble reef-type mineralization in the Stella Intrusion, South Africa, whereas Navigator mineralization occurs in a >70m thick, magnetite-bearing gabbronorite and websterite sequence and has more in common with reef-type mineralization at Rio Jacaré in Brazil.The Latitude Hill prospect is characterised by a >200m thick zone of layered, semi-massive to massive magnetite, but it differs from Jameson and Navigator by having a sulfide assemblage dominated by pyrrhotite and very low whole rock Cu, Pt, Pd and Au grades. High Cu/Pd and S/Se ratios indicate that this intrusion experienced an earlier sulfide saturation event and that crustal contamination was an important factor in the formation of the sulfides at this prospect. A relatively flat mantle-normalised PGE pattern for samples from this prospect is similar to that from the Main Magnetite Layer from the Upper Zone of the Bushveld Complex.