Magmatic history of the Eastern Creek Volcanics,Mt Isa,Australia: insights from trace-element and platinum-group-element geochemistry

The Paleoproterozoic basalts of the Eastern Creek Volcanics are a series of continental flood basalts that form a significant part of the Western Fold Belt of the Mt Isa Inlier, Queensland. New trace-element geochemical data, including the platinum-group elements (PGE), have allowed the delineation of the magmatic history of these volcanic rocks. The two members of the Eastern Creek Volcanics, the Cromwell and Pickwick Metabasalt Members, are formed from the same parental magma. The initial magma was contaminated by continental crust and erupted to form the lower Cromwell Metabasalt Member. The staging chamber was continuously replenished by parental material resulting in the gradual return of the magma composition to more primitive trends in the upper Cromwell Metabasalt Member, and finally the Pickwick Metabasalt Member formed from magma dominated by the parental melt. The Pickwick Metabasalt Member of the Eastern Creek Volcanics has elevated PGE concentrations (including up to 18 ppb Pd and 12 ppb Pt) with palladium behaving incompatibly during magmatic fractionation. This trend is the result of fractionation under sulfide-undersaturated conditions. Conversely, in the basal Cromwell Metabasalt Member the PGE display compatible behaviour during magmatic fractionation, which is interpreted to be the result of fractionation of a sulfide-saturated magma. However, Cu remains incompatible during fractionation, building up to high concentrations in the magma, which is found to be the result of the very small volume of magmatic sulfide formation (0.025%). Geochemical trends in the upper Cromwell Metabasalt Member represent mixing between the contaminated Cromwell Metabasalt magmas and the PGE-undepleted parental melt. Trace-element geochemical trends in both members of the Eastern Creek Volcanics can be explained by the partial melting of a subduction-modified mantle source. The generation of PGE- and copper-rich magmas is attributed to melting of a source in the subcontinental lithospheric mantle below the Mt Isa Inlier which had undergone previous melt extraction during an older subduction event. The previous melt extraction resulted in a sulfur-poor, metal-rich metasomatised mantle source which was subsequently remelted in the Eastern Creek Volcanic continental rift event. The proposed model accounts for the extreme copper enrichment in the Eastern Creek Volcanics, from which the copper has been mobilised by hydrothermal fluids to form the Mt Isa copper deposit. There is also the potential for a small volume of PGE-enriched magmatic sulfide in the plumbing system to the volcanic sequence.