Re–Os geochronology and isotope systematics of the Tanami, Tennant Creek and Olympic Dam Cu–Au deposits

Abstract

The ¹⁸⁷Re–¹⁸⁷Os 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

Acknowledgements

This publication is an outcome of a multi-institutional project (AMIRA P563) principally funded by the Australian Mineral Industry Research Association (AMIRA) and the Australian Research Council (ARC). Project sponsors included Geoscience Australia, Anglogold, BHP Exploration, Billiton International Metals BV, Normandy Exploration Limited, Pasminco Exploration and WMC Resources Ltd. We thank Joe Cucuzza of AMIRA for coordinating the project. Field activities for the project were supported by the Northern Territory Geological Survey and Geoscience Australia through the North Australian Craton Project. Andrew Wygralak, David Huston and Terry Mernagh were instrumental in providing samples and advice during the collection and interpretation phase of this project. David Huston provided samples for the Tenant Creek project from his personal research collection. The Tanami Mine Joint Venture and Normandy NFM Ltd. provided additional logistical support, food and lodging. Industry geologists that provided time and expertise, including Stewart Eldridge, Kathy Ehrig, Simon Henderson, Mike Roache, Louise Mohamed, Chris Large, Helen Burgess, Dave Hammond, John Hammond and John Campbell, are all gratefully acknowledged. The assistance of Rachelle Pierson for Re–Os preparation, Jeff Davis for the provision of mineral separates and Travis Naughton for drafted figures is appreciated. We also thank Noreen Evans and Rob Hough for comments on an initial draft, and acknowledge the constructive journal reviews of Mike McWilliams and Roland Maas.