Age of Cu‐Au mineralisation, Cloncurry district, eastern Mt Isa Inlier, Queensland, as determined by 40Ar/39Ar dating*

Abstract

The 40Ar/39Ar dating of alteration biotite, muscovite and amphibole from a number of post‐peak metamorphic Cu‐Au deposits and alteration systems in the Cloncurry district, north Queensland has determined the timing of mineralisation and hydrothermal activity. Alteration biotite from the Ernest Henry Cu‐Au, Starra Au‐Cu, and Mt Elliott Cu‐Au deposits, sericite associated with hematite breccias in the Wimberu Granite, muscovite from an albitite pipe that intrudes the Gilded Rose Breccia, and sericite from a granitoid near the Osborne Cu‐Au deposit, yield ages which are broadly contemporaneous with the late ca 1510–1485 Ma phases of the Williams and Naraku Batholiths. Hornblende and biotite alteration, which pre‐date Cu‐Au mineralisation at Osborne, give a maximum age of ca 1540 Ma for the deposit, which is also a probable minimum age for peak metamorphism. Metamorphic minerals from the vicinity of Osborne yield dates which are significantly older (ca 1590–1570 Ma) than those from the hydrothermal phases. Dating by the K‐Ar method of red, hematitic K‐feldspars which are regionally common in the Cloncurry district, was not effective, as the mineral yields ages up to 300 million years younger than coexisting alteration sericite. The 40Ar/39Ar age spectra obtained from most hydrothermal phases in the Eastern Fold Belt, Mt Isa Inlier, commonly contain flat parts which comprise a large proportion of the released gas, and are indicative of rapid cooling through the temperature of partial closure to Ar diffusion for the respective minerals.