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.