40Ar/39Ar evidence for the timing of Paleoproterozoic gold mineralisation at the Sandpiper Deposit, Tanami region, northern Australia

At the Sandpiper gold deposit in the Tanami region of northern Australia sericite is intimately intergrown with arsenopyrite in gold-bearing quartz veins and breccias, suggesting sericite crystallisation synchronous with gold-bearing fluid flow. This ore-stage sericite yields a 40Ar/39Ar plateau age of 1785 +/- 32 Ma (2 sigma including both analytical and systematic uncertainties). Recalculation using revised and more precise values for the K-40 decay constants and the age of the Fish Canyon Sanidine standard shifts the age to 1794 +/- 12 Ma (2 sigma including all known uncertainties). Given the possibility of post-mineralisation isotopic resetting this age can be conservatively interpreted as a minimum constraint on the timing of gold deposition although, given local geological relationships and estimates for the argon retentivity of white mica, we consider complete isotopic resetting to be unlikely. The preferred interpretation is, therefore, that the sericite 40Ar/39Ar age indicates the timing of gold mineralisation. The sericite age accords with a limited dataset of Pb-207/Pb-206 xenotime ages of ca 1800 Ma from other gold deposits in the Tanami region, interpreted as mineralisation ages. The agreement between independently derived ages from several gold deposits lends support for a widespread gold-mineralising event at ca 1800 Ma in the Tanami region.