Palladium, iridium, and gold in the ores and host rocks of nickel sulfide deposits in Western Australia

Abstract

Neutron activation analysis has been used to determine palladium, iridium, and gold and, occasionally, ruthenium and osmium in the ores and ultramafic host rocks of 14 nickel sulfide deposits in Western Australia. The deposits range from low-grade disseminated sulfides (typically pentlandite, pyrite, millerite, heazlewoodite) in intrusive ultramafic bodies to high-grade massive or matrix-type pyrrhotite and pentlandite at the base of volcanic-ultramafic units. The total precious metal tenor of typical Western Australian nickel sulfide ore is similar to that of average Sudbury ore although gold, ruthenium, iridium, and osmium are higher (and platinum may be lower) in the Western Australian ores. Both palladium and platinum are distinctly lower than in average ore from the Merensky Reef. Palladium ranges from 5,000 ppb in high-grade massive ores with 20 weight percent nickel to 0.2 ppb in unmineralized ultramafic rocks; iridium varies from 1,000 to 3 ppb in the same rocks. Palladium and iridium correlate strongly with the nickel sulfide content of the rocks. Deviations from perfect correlations are explained by fractionation of the metals during solidification of the parent sulfide melts or by redistribution during metamorphism or secondary alteration of the sulfides.The distribution of palladium and iridium strongly supports a magmatic origin for the nickel sulfide ores and permits assessment of the effect of exotic processes such as talc-carbonate alteration in the upgrading of low-grade nickel ores. The evidence in hand indicates that although such a process may produce local redistribution of nickel, it is not responsible for large-scale mobilization of nickel. However, it may release significant quantities of gold and sulfur.The ultramafic host rocks of the nickel sulfide ores have very low Pd/Ir ratios relative to the nickel ores and also to normal ultramafic rocks that are not known to be associated with major amounts of nickel sulfides. This relation may be a valuable exploration guide.