The fractionation of copper isotopes in granitic and mafic intrusions: mafic enclaves in the Mannum granite, a case study

Abstract

The transfer of material between mantle-sourced magma and evolved magmatic rocks may provide a mechanism for the transportation of metals such as copper, Cu, to the crust. The Mannum granite provides a case study as the A-type granite is synchronously accompanied by mafic enclaves, providing a mafic-felsic interface with differing degrees of mixing/mingling. This mafic-felsic interface shows different mechanisms of transportation such as transport of xenocrysts, compositional rimming (rapakivi feldspars) and change in overall bulk chemistry. Samples of both the granite and mafic enclaves were taken, with a focus on Cu isotope analysis which characterises Cu behaviour between the mafic enclaves and granite and shows whether any copper has been assimilated from a highly fractionated source. This study used petrography, whole rock and mineral major and trace element chemistry and whole rock Cu isotope analysis. Processes such as fractional crystallisation and magma mixing/mingling provide mechanisms for the transfer of material between the granite and the intruding mafic enclaves. There is evidence of the transfer of material in the hybridised zones, particularly the formation of rapakivi feldspars and xenocrysts from both phases. Rare earth element signatures show mineral growth has occurred prior and post injection of mafic magma into the Mannum granite and that magmatic transfer is most applicable to lithophile elements. Cu isotope results show that δ65Cu values generally increase with decreasing copper concentrations and range from -0.12 to 2.34‰. The range of δ65Cu values is in line with those reported in mantle derived rocks and follow Rayleigh fractionation curves. Cu isotope analyses further our understanding of Cu isotope behaviour and can be applied to ore-forming environments. Other stable transition metal isotopes could be paired with Cu isotopes, such as Zn and Fe, to further examine the role of any redox reactions.