Western Australia is well known by its Archean gold endowment, spanning across the Yilgarn and, in lesser extent, Pilbara Cratons. The renewed interest into the origin and age of gold mineralisation was recently fuelled by the finding of ‘conglomerate gold’ in Pilbara sedimentary sequences, with some arguing the similarity with the major Witwatersrand gold event in South Africa. Previous studies of gold mineralisation in WA were based on gold-associated minerals. In this study, we focused on the native gold itself, performing state-of-the-art analysis of Pb isotopes coupled with gold nugget morphology and inclusions. We report that the Pb isotope composition of native gold and its galena inclusions can provide valuable information about the timing of gold formation and subsequent history (e.g. Tessalina et al., 2023). In our study, we analysed one 2.9 g gold nugget collected from the informally named Egina Gravels unit, composed of unconsolidate conglomerate fragments bearing gravels, that are thought to be derived from the conglomerate unit in the 2780-2629 Ma Fortescue Group and overlying gold mineralised Mallina Formation (3015-2931 Ma) in the Central Pilbara. The nugget has been divided into several sub-samples and analysed for Pb isotopes by TIMS in JdLC using a Pb double spike. The 206Pb/204Pb and 207Pb/204Pb ratios span a range between 16.00-27.33 and 15.55-17.42, respectively. The least radiogenic group of sub-samples with 206Pb/204Pb ratios averaging 16.14+/-0.09 (n=5), is similar to the least radiogenic Pb isotope analyses from other well dated deposits located within the Capricorn Orogen: Magellan sediment-hosted Pb deposit (ca. 1.8 Ga), Labouchere and Nathans orogenic gold deposits (ca. 1.8 Ga), and less radiogenic DeGrussa VHMS deposit (ca. 2 Ga). All the least-radiogenic Pb isotope analyses from these deposits and the studied gold plot above all 206Pb-207Pb lithospheric growth curves, indicating a high U/Pb signature of an older Pb source, probably coming from the Archean basement. Our data also plot well below the 208Pb-206Pb lithospheric growth curve, indicating low Th/U in the Pb source of the studied conglomerate gold nugget. The studied nugget could be formed by two different mechanisms: (1) an older (ca. >2.9 Ga) detrital nugget with strong overprint and Pb addition during the ca. 1.8 Ga event related to the regional Capricorn orogenic event; or (2) nugget formation at ca. 1.8 Ga in-situ. First model is supported by previous evidence of post-deposition isotopic disturbance, based on Rb-Sr and Pb isotopes (e.g., Oversby, 1976). Further advances in understanding the source and timing of conglomerate gold in Pilbara await more comprehensive studies.

We acknowledge the Novo Resources Corporation for donating the nugget and background information.

Reference(s)

Oversby, VM 1976, Isotopic ages and geochemistry of Archean acid igneous rocks from the Pilbara, Western Australia: Geochimica et Cosmochimica Acta 40, 817-829. 

Tessalina, SG et al. 2023, Isotopic fingerprinting of native gold from Western Australia: Geological Survey of Western Australia, Report 229, 32p.