Extreme silver isotope variation in orogenic gold systems implies multistaged metal remobilization during ore genesis

Christopher R. Voisey, Roland Maas, Andrew G. Tomkins, Michael Brauns, Gerhard Brügmann

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We report the frst Ag isotope data for the Paleozoic orogenic Au deposits in the Victorian Goldfelds, southeast Australia, a world-class province with a historic production of 2,400 tonnes of Au. Despite their relatively uniform geology—similar host-rock types, age, mineralization style—deposits in Victoria show a wide range in 107Ag/109Ag ratios in native Au (e107Ag -6.6 to +8.3, relative to the NIST SRM 978a Ag standard), comparable to the entire previously known terrestrial range (-9.4 to +5.3). The data show no correlation with mineralization age or host-rock composition, and there is no obvious isotopic link to established "mantle" or "crustal" Ag isotope values, implying that source rock signatures are unlikely to be the main control on Ag isotope variations. Instead, it is suggested that the Ag isotope variation is primarily related to physicochemical processes, particularly Ag isotope fractionation during redox reactions such as conversion of Ag0 in native Au to Ag+ in dissolved Ag(HS)2- or sulfde-borne Ag. Repeated Ag 0 Ag + reactions along transport pathways and at sites of ore accumulation could generate a wide range in e107Ag, and evidence of this range is presented in the data here. Silver isotope fractionation via numerous deposition-dissolution cycles provides a different perspective into large-scale ore genesis that has not previously been recognized for orogenic gold systems; multistaged metal remobilization along fluid transport pathways is standard during their formation. Detailed Ag isotope studies have considerable potential for understanding the history of episodic metal addition and within-deposit redistribution.

Original languageEnglish
Pages (from-to)233-242
Number of pages10
JournalEconomic Geology
Issue number2
Publication statusPublished - 1 Mar 2019

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