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.