Review of the application of isotopic studies to the genesis of Cu-Au mineralisation at Olympic Dam and Au mineralisation at Porgera, the Tennant Creek district and Yilgarn Craton

This paper reviews the application of radiogenic isotopes to the study of four Cu-U and Au deposits or deposit types in the Australasian region: Olympic Dam, Porgera, the Proterozoic Au deposits of the Tennant Creek district and the Archaean gold deposits of the Yilgarn Craton. In each case ii has been possible to date the mineralisation and to correlate ore formation with a specific igneous event or stage in crustal evolution, In three cases it was also possible to use radiogenic isotopes to trace the source of the metal(s) or to constrain the fluid pathway. The results illustrate the power of radiogenic isotopes in ore-genesis studies. At the Porgera gold deposit in Papua New Guinea K-Ar and Ar-39/Ar-40 dating has shown that both the mineralisation, and the spatially associated Porgera Igneous Complex, have an age of cc 6 Ma. Sr and Pb isotopic tracing of the ore fluids indicate that these elements were derived from a mixed sedimentary-igneous source, as might be expected for a large hydrothermal system developing around a hypabyssal igneous complex. SHRIMP U-Pb dating of zircons from the Olympic Dam Cu-U-Au deposit in South Australia has constrained the age of mineralisation to be ca 1590 Ma, the same as the age of the Gawler Range thermal event in South Australia. Nd isotopic tracing of the ore fluid has shown that it contains a significant mantle component. The most likely source of mantle Nd is a suite of hydrothermally altered alkali ultramafic dykes that cut, but are coeval with, the Olympic Dam breccias, A possible common link between Porgera and Olympic Dam is that both were derived from oxidised alkali mafic/ultramafic bodies. It is suggested that these magmas had unusually high f(O2) so that Cu and Au concentrated in these magmas as they fractionated to produce the Au-or Cu-Au-rich magmatic-hydrothermal fluids that gave rise to the mineralisation. SHRIMP U-Pb dating of zircons from fe(sic intrusives that cut, or are cut by, Au mineralisation constrain the age of most of the gold mineralisation in the Yilgarn Craton to lie between 2660 and 2630 Ma. Direct Ar-39/Ar-40 dating of hydrothermal minerals associated with the mineralisation gave ages of 2627 for Victory (Kambalda), 2629 for the Golden Mile (Kalgoorlie) and 2623 for Matilda M1 (Wiluna) that are consistent with the above constraint. An age of co 2630 Ma correlates with late felsic magmatism in the Yilgarn Craton and with metamorphism in the lower crust, Mt Charlotte (Kalgoorlie). Wiluna East Lode and Lady Bountiful give younger ages of 2602, 2565 and 2590 respectively. The young age at Mt Charlotte is consistent with the field relationships and is believed to be the age of mineralisation but the Wiluna and Lady Bountiful dates may have been reset by later thermal events. Isotopic tracing at Mt Charlotte, using Nd in scheelites, showed that most of that element came from a komatiltic source. Ar-39/Ar-40 dating of the gold mineralisation in the Tennant Creek Block place the age of this event at 1825 Ma, towards the end of the felsic volcanism of the Barramundi Orogeny. Therefore the gold mineralisation in both the Yilgarn Craton and Tennant Creek Block can be related to the waning stages of a major period of felsic magmatism. This can be interpreted to mean that gold originated from late felsic magmas or that it was derived from the supracrustal rocks, buried deep in the crust, during the peak metamorphism in the lower crust.