Copper isotope method development for determining the source of mineralised provinces

Abstract

Many isotope proxies have been applied to study the prosperous iron oxide copper gold (IOCG) province in the eastern Gawler Craton (E.G.C) and Au mineralised region of the cental Gawler Craton (C.G.C), in Southern Australia. Yet, copper isotope proxies- an indicator for low temperature fluid flow and sulfide mineralisation- have yet to be applied to the region. In this study, purification techniques using automatic column chromatography were demonstrated during separation of Cu from matrix elements. Cu isotopes – 65Cu & 63Cu – were used to understand the extent of mantle input and mantle metasomatism, potentially responsible for the Cu in IOCG mineralisation. Eleven samples were gathered. Three mafic enclaves and four intrusives from the Central Gawler Gold Province and four intrusives from the eastern Gawler IOCG province. Separation using automatic column chromatography proved challenging, with matrix elements abundant throughout the purified fractions (Co, Ti, Fe, Mg, Na), due to poor separation. Ti proved to a major interference during isotopic analysis using a Multicollector-ICP-MS, positively offsetting values. E.C.G samples showed the most positively fractionated δ65Cu values (+0.69 ± 0.024‰ to +1.422 ± 0.077‰). Enclaves from the C.G.C showed the most negatively fractionated δ65Cu values (-0.053 ± 0.023‰ to -0.897 ± 0.006‰), while intrusives from this region showed more positive δ65Cu values (+0.084±0.23‰ to +0.397±0.011‰). All samples showed a lack of hydrothermal alteration. Magmatic sulphide-containing E.G.C samples had the most positive δ65Cu values; which cannot be explained by current understanding of Cu isotope fractionation during sulfide saturation. This trend may instead be attributed to a heterogeneous sub-continental lithospheric mantle (SCLM) source. In contrast the negative δ65Cu values of mafic enclaves is possibly caused by assimilation of S-type granitic crust and/or possibly due to a heterogeneous SCLM source.