Re–Os geochronology and isotope systematics of the Tanami, Tennant Creek and Olympic Dam Cu–Au deposits B. I. A. MCINNES 1 *, R. R. KEAYS 2 , D. D. LAMBERT 3 , J. HELLSTROM 4 AND J. S. ALLWOOD 5 1 CSIRO Exploration & Mining, PO Box 1130, Bentley, WA 6102, Australia. 2 School of Geosciences, PO Box 28E, Monash University, Vic. 3800, Australia. 3 US National Science Foundation, Arlington, VA 22230, USA. 4 School of Earth Sciences, University of Melbourne, Vic. 3010, Australia. 5 Geomodelling Ltd, 56 Adelaide Street, Petone, Lower Hutt, New Zealand. The 187 Re– 187 Os system can determine directly the age of formation of ore deposits and can characterise the isotopic provenance of metals in ore systems. We report rhenium–osmium (Re–Os) isotope data from the Callie gold deposit of the Tanami district in the Northern Territory, the Gecko Cu– Au–Bi deposit of the Tennant Creek district in the Northern Territory and the giant Olympic Dam Cu–Au– U deposit in South Australia. Re–Os data for vein sulfides from the Callie deposit produce an isochron age of 1622+120 Ma, indicating that gold mineralisation occurred after emplacement of adjacent granitoid plutons in the Tanami region. Sulfide minerals from the Callie gold–quartz vein system have initial Os isotope ratios that overlap mantle values, indicating that the Os in the Callie gold deposit was derived from a non-radiogenic source with a distinctly different isotopic composition than the surrounding crustal rocks. Cu–Au–Bi sulfide ores from the Gecko 44 Deposit in the Tennant Creek camp produced a Re–Os age of 1665+66 Ma, confirming a post-Barramundi epigenetic formation age for the Cu-sulfide ores. The Re–Os age supports earlier hypotheses that late-stage granitoids (e.g. Warrego Granite) were a potential source of fluids and Cu, Au, Bi, U and S to the sulfide stage deposits of the Tennant Creek district. The initial Os isotope ratio of the Gecko Cu-sulfide deposit is non-radiogenic and suggests a mantle-like source for Os. Whole-rock ore samples and mineral separates from the chalcopyrite zone of the Olympic Dam Cu–Au–U deposit yielded a Re–Os age of 1258+28 Ma. Because the ages of both the associated Roxby Downs Granite and felsic dykes that cross-cut mineralisation are much older (ca1590 Ma), the ca 1258 Ma age is interpreted as the time when the Re– Os isotope system at Olympic Dam was reset by a secondary process, possibly related to thermal metamorphism or supergene alteration. KEY WORDS: copper, geochemistry, geochronology, gold, Olympic Dam, ore deposits, rhenium– osmium isotopes, Tanami, Tennant Creek. INTRODUCTION The rhenium–osmium (Re–Os) isotopic system has had many applications in the geosciences, including the geochronology of molybdenite-bearing ores (Stein et al. 1997; Selby & Creaser 2001), the geochronology of hydrocarbon formation (Selby & Creaser 2005), the determination of the source of metals in subduction- related porphyry Cu–Au deposits (McInnes et al. 1999; Mathur et al. 2005) and the timing of oxygenation of the Earth’s atmosphere (Bekker et al. 2004). Because Re and Os are strongly chalcophile (i.e. they are hosted by sulfides) and Os is also siderophile (i.e. it can occur as a native metal or alloy), the system can be used to directly date sulfide and oxide minerals, potentially including metals such as native gold (Kirk et al. 2002). This property makes the Re–Os isotope system an ideal method of directly determining the age of ore minerals and ore deposits. In addition, because of the very different geochemical partitioning properties of Re and Os during melting, the Re–Os isotope system can discriminate between crustal and mantle metal sources in ore systems. For these reasons, a research project—AMIRA P563— was initiated by Monash University and CSIRO re- searchers to test the application of Re–Os dating at Australasian ore deposits. Of particular interest was the investigation of various styles of hydrothermal miner- alisation that have been difficult to date directly, such as the Cu–Au ore systems in the Tanami and Tennant *Corresponding author: brent.mcinnes@csiro.au Australian Journal of Earth Sciences (2008) 55, (967 – 981) ISSN 0812-0099 print/ISSN 1440-0952 online Ó 2008 Geological Society of Australia DOI: 10.1080/08120090802097443