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Katherine Bassano, SRK Consulting (Australia)
Janet Hergt, University of Melbourne (Australia)
Roland Maas, University of Melbourne (Australia)
Jon Woodhead, University of Melbourne (Australia)
Maya Kamenetsky, ARC Centre of Excellence in Ore Deposits (Australia)
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Attempts to employ step-leaching Pb-Pb dating (PbSL) to directly date low-Pb sulphide and oxide minerals in iron-oxide-copper-gold (IOCG-) deposits in the Mesoproterozoic North Australian Craton (NAC) failed to produce primary mineral formation ages (1.7-1.5 Ga). Instead, age estimates obtained from imprecise Pb-Pb isotope arrays are in the range ∼1.2-1.0 Ga. Step-leaching produced multiple release peaks of radiogenic Pb, variably associated with REE, Ca, Ba and U. No correlation was observed between release of radiogenic Pb and dissolution of the host sulfide or oxide mineral. Laser ablation scans of chalcopyrite and magnetite revealed a pronounced nugget effect for radiogenic 206Pb at variable (10-1000m) length scales, with possible diffusive effects. Finally, high-resolution SEM mineral mapping (MLA) identified trace quantities of U oxide, LREE-rich Ca carbonate, barite, titanite, molybdenite, scheelite, feldspar, chlorite, calcite and other mineral impurities. In many cases, these minerals occur on grain boundaries of, or in fractures within, the host sulphide/oxide, implying a secondary origin. All of this evidence suggests that U, associated ingrown radiogenic Pb and other characteristic elements, are present in discrete impurities within the host sulphide/oxide. The textural and Pb-Pb isotopic data further indicate these impurities were incorporated long after host mineral formation, in a period dated imprecisely as 1.2-1.0 Ga. Post-mineralization introduction of U in these deposits may be related to hydrologic change associated with tectonic events during amalgamation of Australia-Antarctica and Laurentia to form the Neoproterozoic supercontinent Rodinia (Grenville Orogeny ∼1.14 Ga). Crustal fluid flow during this period may have been promoted by extensive continental basaltic magmatism (∼1.05-1.1 Ga Warakurna LIP, west-central Australia; 1.1. Ga mafic dyke swarms, Mt Isa region). The results of this work illustrate the potential to harness the U-Pb isotope record preserved in secondary minerals to detect multiple fluid flow events in the history of sedimentary basins, and continental crust more broadly. The work also highlights the complexities inherent in applying Pb isotope data to the study of low-Pb ore minerals. Open system behaviour must be taken into consideration when interpreting Pb isotope tracing and PbSL results of such minerals.
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