REVIEW OF THE APPLICATION OF ISOTOPIC STUDIES TO THE GENESIS OF CU-AUMINERALIZATION AT OLYMPIC DAM AND AU MINERALIZATION 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 reg ion: Olympic Dam, Porgera, the Proterozoic Au deposits of the Tennant Creek district and the Archaean gold deposits of the Yilgarn Craton. I n each case ii has been possible to date the mineralisation and to cor relate ore formation with a specific igneous event or stage in crustal evolution, In three cases it was also possible to use radiogenic isot opes to trace the source of the metal(s) or to constrain the fluid pat hway. The results illustrate the power of radiogenic isotopes in ore-g enesis studies. At the Porgera gold deposit in Papua New Guinea K-Ar a nd 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 eleme nts were derived from a mixed sedimentary-igneous source, as might be expected for a large hydrothermal system developing around a hypabyssa l igneous complex. SHRIMP U-Pb dating of zircons from the Olympic Dam Cu-U-Au deposit in South Australia has constrained the age of minerali sation to be ca 1590 Ma, the same as the age of the Gawler Range therm al 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 ultra mafic dykes that cut, but are coeval with, the Olympic Dam breccias, A possible common link between Porgera and Olympic Dam is that both wer e derived from oxidised alkali mafic/ultramafic bodies. It is suggeste d that these magmas had unusually high f(O2) so that Cu and Au concent rated 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 ar e cut by, Au mineralisation constrain the age of most of the gold mine ralisation in the Yilgarn Craton to lie between 2660 and 2630 Ma. Dire ct Ar-39/Ar-40 dating of hydrothermal minerals associated with the min eralisation gave ages of 2627 for Victory (Kambalda), 2629 for the Gol den Mile (Kalgoorlie) and 2623 for Matilda M1 (Wiluna) that are consis tent with the above constraint. An age of co 2630 Ma correlates with l ate felsic magmatism in the Yilgarn Craton and with metamorphism in th e lower crust, Mt Charlotte (Kalgoorlie). Wiluna East Lode and Lady Bo untiful give younger ages of 2602, 2565 and 2590 respectively. The you ng age at Mt Charlotte is consistent with the field relationships and is believed to be the age of mineralisation but the Wiluna and Lady Bo untiful dates may have been reset by later thermal events. Isotopic tr acing at Mt Charlotte, using Nd in scheelites, showed that most of tha t element came from a komatiltic source. Ar-39/Ar-40 dating of the gol d mineralisation in the Tennant Creek Block place the age of this even t at 1825 Ma, towards the end of the felsic volcanism of the Barramund i Orogeny. Therefore the gold mineralisation in both the Yilgarn Crato n and Tennant Creek Block can be related to the waning stages of a maj or period of felsic magmatism. This can be interpreted to mean that go ld originated from late felsic magmas or that it was derived from the supracrustal rocks, buried deep in the crust, during the peak metamorp hism in the lower crust.