The geochronology and radiogenic isotope systematics of the Olympic Dam copper-uranium-gold-silver deposit, South Australia

Abstract

The Olympic Dam Cu-U-Au-Ag deposit is a syngenetic orebody hosted by the Olympic Dam Breccia Complex, a high level, hematite-rich hydrothermal breccia system. The breccia complex occurs entirely within, and clearly post-dates, the Roxby Downs Granite. _Cu-Fe sulphide distribution within the deposit is zoned from pyrite-rich assemblages at depth, upwards to chalcopyrite-rich ores, ultimately to bornite-chalcocite ores. Mlnor proportions of paragenetically early magnetite are widely distributed. Felsic and mafic/ultramafic dykes are broadly coeval with ore deposition. The Roxby Downs Granite has an age of 1588 ± 4 Ma. Prior to this study the age of the breccia complex was unclear. SHRIMP U-Pb isotopic data for zircons from three igneous rock units constrain the minimum age for the bulk of the mineralisation. Twb autobrecciated felsic dykes that intrude hematite-rich sedimetitary rocks and·hydrothermal breccias have ages of 1592.± 8 Ma and 1584 ±- 20 Ma respectively. An ashfall tuff horizon from within a diatrerne that cross-cuts hematite-quartz breccias contains zircons with an age of 1597 ± 8 Ma. These three minimum age constraints are within error of the age of the host granite, meaning that the breccia complex has an age of -1590 Ma. This age determination allows confident correlation of ore deposition with a major regional magmatic episode, the Gawler Range/ Hiltaba-volcano-plutonic event. Pb isotopic measurements of hydrated U-Th-Y-REE~rich hydrothermal zircon-xenotime overgrowths on magmatic zircons yielded evidence of complex Pb mobility histories, and provided no strong evidence regarding the age of the deposit. SHRIMP analyses of Olympic Dam pitchblende indicate that U and Pb mobility has reset isotopic systematics heterogeneously, even on a microscopic scale. This open system behaviour is also apparent from the data of earlier workers. An earlier U-Pb concordia upper intercept "age" of 1400 Ma for Olympic Dam pitchblende, calculated by regression analysis of averaged isotopic analyses, should therefore be regarded with caution. Sm-Nd isotopic data indicate that the different hematite-sulphide assemblages share an initial ENd signature of -2.5. This suggests that these ore types are cogenetic at 1590 Ma. This signature also indicates that ore fluids received contributions of Nd from crustal rocks such as the host granite (£Nd -5) and from rocks or magmas derived from the mantle at 1590 Ma. In contrast, the volumetrically minor magnetite-rich assemblages have the same initial Nd signature as the host granite, suggesting that they are possibly cogenetic. The hematitic ores and breccia~ yield a fourteen point Sm-Nd isochron age of 1572 ± 99 Ma consistent with the age constraints provided by zircon geochronology. The least altered mafic/ultramafic dykes within the deposit have an initial isotopic signature of eNd +4 and are therefore a plausible source of the primitive Nd component in the hematitic rocks. With progressive alteration, these dykes have become depleted in several of the elements that are enriched in the Olympic bam ores and breccias, particularly Cu, Cr, Ni, V, Mn, Nb and Y. The trace element and Nd isotopic relationships strongly implicate the mafic/ultramafic dykes, or their plutonic or extrusive equivalents, as metal sources. Interpreted primary enrichments in incompatible elements suggest a strongly alkaline affinity for the mafic/ultraniafic rocks. Rb-Sr isotopic systematics of the Olympic Dam breccias and mafic/ultramafic dykes show evidence of open system behaviour after ore deposition. No firm conclusions regarding ore genesis are drawn from these data. In view of the data presented, plausible hypotheses of ore genesis include: a) fluid mixing involving a hypogene, granite-related fluid, and a saline meteoric fluid transporting metals of economic interest that have been leached from the Gawler Range volcanic pile, and b) the sequential activity of a granite-related fluid that produced magn-tite-rich mineralisation, and a fluid derived by late-stage exsolution of magmatic volatiles from an alkaline mafic or ultramafic pluton .