Abstract

Isotopic data are presented for granitoids and felsic volcanics from the Stuart shelf of South Australia, some of which are spatially associated with the Olympic Dam Cu-U-Au deposit. Relatively unaltered quartz monzodiorite, more extensively altered quartz syenite, and felsic volcanics yield zircon U-Pb ages of 1,590 (super +5) (sub -4) , 1,613 (super +20) (sub -19) , and 1,576 (super +22) (sub -19) m.y., respectively. The zircons from this group of rocks, the Olympic Dam suite, have similar morphologies and similar 206 Pb/ 208 Pb ratios, suggesting an intimate genetic relationship. Their ages are comparable to the Gawler Range Volcanics cropping out approximately 130 km to the southwest in the Gawler orogenic domain.Close to the Olympic Dam deposit, the zircon U-Pb system of Olympic Dam suite quartz syenite was contaminated by barite precipitated from hydrothermal fluids associated with the mineralization. The barite is readily removed by air abrasion, which restores the reversely discordant and spuriously young zircons to more normally discordant positions on the concordia diagram. The abraded zircons then have U-Pb ages similar to uncontaminated Olympic Dam suite zircons. The barite lead must be enriched in 206 Pb which is concluded to be derived from radium extracted from U-bearing rocks and admitted to the Ba crystal lattice site.Deformed and altered basement granitoids on the Stuart shelf have complex zircons with discrete cores and overgrowths which yield complicated U-Pb isotope systematics. The cores are estimated to be about 1,800 m.y. old, and the overgrowths probably less than 1,730 m.y. old. These granitoids are temporally correlated with basement rocks of the Gawler orogenic domain.Comparable apatite U-Pb and biotite Rb-Sr isotope dates from all the granitoids and from thin veins in the Acropolis prospect suggest regional cooling through the respective blocking temperatures (about 350 degrees -400 degrees and 300 degrees C) between 1,602 + or - 7 and about 1,542 m.y. The Acropolis apatite date suggests at least some Stuart shelf mineralization was emplaced at, or before, about 1,602 m.y. A maximum age for the Olympic Dam mineralization is set by the Olympic Dam suite quartz syenite zircon age, 1,613 (super +20) (sub -19) m.y. Our data do not place minimum age limits on the mineralization.

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