Timing of Palaeoproterozoic Au–Cu–Bi and W-mineralization in the Tennant Creek region, northern Australia: Improved constraints via intercalibration of 40Ar/ 39Ar and U–Pb ages

Abstract

The magnitude of systematic uncertainties inherent in comparisons of Proterozoic 40Ar/ 39Ar- and U–Pb-based ages is illustrated via a consideration of age constraints on two contrasting styles of Palaeoproterozoic mineralization in northern Australia. Published 40Ar/ 39Ar ages for Au–Cu–Bi-mineralization in the Tennant Creek region range from 1825 to 1829 Ma, apparently younger than the timing of deformation bracketed by 207Pb/ 206Pb zircon ages of igneous rocks at ∼1850 Ma. Despite this apparent age offset, propagation of systematic uncertainties in the 40Ar/ 39Ar ages shows that they are indistinguishable from the ∼1850 Ma zircon ages. Recalculation using revised estimates of the 40K decay constant and age of the 40Ar/ 39Ar standard shifts the 40Ar/ 39Ar ages to 1847–1851 Ma, in excellent agreement with 207Pb/ 206Pb zircon ages from Tennant Supersuite igneous rocks and with local geological constraints. To the south of Tennant Creek, in the Davenport Ranges, numerous small W prospects have been interpreted to be coeval with post-tectonic granites of the Devils Suite at ∼1710–1720 Ma. New 40Ar/ 39Ar ages from muscovite selvages on wolframite-bearing veins range from 1697 to 1703 Ma. These ages shift to 1711 to 1717 Ma when recalculated using revised estimates for 40K decay constant and age of standard material, bringing them into agreement with existing 207Pb/ 206Pb zircon ages for Devils Suite granitoids, and consistent with field and geochemical interpretations. These examples highlight the importance of considering systematic sources of uncertainty when comparing radiometric ages determined via different isotopic methods, particularly in the Proterozoic and Archean parts of the timescale. Ongoing work to calibrate the 40Ar/ 39Ar and U–Pb-based timescales will further enhance the application of multiple isotopic methods to constrain geological processes.