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Geochemical signatures of copper redistribution in IOCG-type mineralisation, Gawler Craton, South Australia

Abstract

The Emmie Bluff iron oxide, copper, gold (IOCG) prospect is located in the Olympic Dam district, South Australia, and hosts sub-economic 150-m-thick Cu–Au mineralisation associated with the hematite–chlorite–sericite alteration with chalcopyrite commonly replacing pre-existing pyrite at a depth of 800 m. With the use of cutting-edge synchrotron X-ray fluorescence microscopy and field emission gun-scanning electron microscopy, it is shown for the first time that sub-economic IOCG mineralisation in the Olympic Dam district was affected by a late fluid event, which resulted in partial dissolution of Cu mineralisation and transport of Cu in the form of chloride complexes. The porous chlorite–sericite matrix associated with the late alteration of chalcopyrite hosts a Cu–Cl–OH phase previously undescribed in IOCG rocks, which was identified as one of the polymorphs of the atacamite group of minerals, Cu2Cl(OH)3. Thermodynamic modelling shows that “atacamite” is produced during dissolution of chalcopyrite by an oxidised, Cl-bearing fluid. An acidic environment is produced within millimetres of the chalcopyrite grains during oxidation. This process drives chlorite recrystallisation that is recorded by compositional variation of chlorite proximal to chalcopyrite. The existence of the atacamite is discussed in the context of fluid evolution and interaction with IOCG-type mineralisation and its implications to ore preservation versus destruction and remobilisation.

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Acknowledgements

The work has been supported by the Deep Exploration Technologies Cooperative Research Centre whose activities are funded by the Australian Government’s Cooperative Research Centre Programme. This research is supported by the Science and Industry Endowment Fund. The authors are grateful to the staff at the Centre for Microscopy, Characterisation and Analysis, the University of Western Australia, for their help in acquiring electron microprobe data. We would also like to thank Dr. Nicholas Timms for his assistance with the TESCAN FEG SEM and Dr. Louise Fisher for her help with initial processing of the XFM data. This research was undertaken on the XFM beamline at the Australian Synchrotron, Victoria, Australia. The authors would like to thank staff at the XFM beamline of the Australian Synchrotron, namely Dr. David Patterson and Dr. Katherine Spiers, and funding from the Australian Synchrotron for the Proposal #4898. Adrian Fabris and GSSA are thanked for providing the samples and their multielement assay data for this research in the framework of the DET CRC Project 3.2 Hypogene Alteration. We also would like to thank Drs. Louise Fisher and Alistair White who reviewed the manuscript at early stages for thorough and helpful comments that improved the manuscript significantly.

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Correspondence to Yulia A. Uvarova.

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Appendix 1

Appendix 1

The trace element composition of the chalcopyrite and chlorite were obtained by integrating X-ray spectra from circular areas within the SRXF dataset to improve counting statistics and reduce detection limits for trace elements (Fisher et al. 2015). The circular areas selected for the chalcopyrite analysis were arbitrarily selected as the volume for mass balance calculations of trace elements between chalcopyrite and chlorite.

For a circle radius 266 μm, in a 30-μm-thick section, the total volume of material within the analysis cylinder is 6.67E6 μm3 or 6.6E−6 cm3. Using a density of 4200 gcm−3 for chalcopyrite, this results in a mass of 2.8E−2 g chalcopyrite being analysed. The Zn concentration in chalcopyrite is 750 ppm by mass; therefore, there is 2.1E−5 g Zn within the analysis volume.

Assuming all this Zn is incorporated into the same volume of chlorite with a density of 3200 gcm−3, this means that there is 2.1E−5 g Zn in 2.13E−2 g chlorite giving a Zn concentration of 984 ppm by mass.

This can be simplified as follows for non-specific volumes:

$$ M{\left(\mathrm{Zn}\right)}_{Cpy}= C{\left(\mathrm{Zn}\right)}_{Cpy}{\rho}_{Cpy}{V}_{Cpy}= C{\left(\mathrm{Zn}\right)}_{Chl}{\rho}_{Chl}{V}_{Chl} $$
$$ C{\left(\mathrm{Zn}\right)}_{C hl}=\frac{C{\left(\mathrm{Zn}\right)}_{C py}{\rho}_{C py}}{\rho_{C hl}} $$

Where C(X)y is the concentration of element y in mineral X, ρ is density and V y is the volume of mineral y.

Performing the same calculation for Mn where chalcopyrite contains ~1500 ppm, Mn predicts ~1970 ppm Mn in the equivalent chlorite volume, which is on the order of the background levels found in the chlorite away from the chalcopyrite margin.

Table 5 ᅟ

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Uvarova, Y.A., Pearce, M.A., Liu, W. et al. Geochemical signatures of copper redistribution in IOCG-type mineralisation, Gawler Craton, South Australia. Miner Deposita 53, 477–492 (2018). https://doi.org/10.1007/s00126-017-0749-1

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