Silver isotope fractionation in ore-forming hydrothermal systems

Narrowing provenance for ancient Greek silver coins using Ag isotopes and Sb contents of potential ores

“…The low ε 109 Ag of the cerussite sample from the ancient Sykia mine in Lavrion is not easily explained by the fractionation factors ofFujii and Albarède (2018); oxygen-rich compounds should be isotopically heavy. This is precisely where the Rayleigh distillation effects postulated byWang et al (2022) may play a role; the heavy 109 Ag isotope is depleted by early precipitation of carbonates or sulfates from low-temperature hydrothermal fluids. This sample can therefore be seen as having been formed from well-ventilated residual solutions circulating in the karst.…”

“…Europe, colonial Spanish Americas) is extremely narrow (ε 109 Ag = 1 to +1; Desaulty et al, 2011;Desaulty and Albarède, 2013;Albarède et al, 2016Albarède et al, , 2021; Hacksilber from the ancient southern Levant: ε 109 Ag = 2 to +1; Eshel et al, 2022), the range is much broader for silver ores (-8 to +9; Mathur et al, 2018;Arribas et al, 2020;Wang et al, 2022) and for Iberian galena samples (ε 109 Ag = 8 to +3; Milot et al, 2021a). This stark contrast offers the potential to differentiate Ag-bearing ores that actually may have produced metal used for coinage from other mines which did not contribute to coin production.…”

“…This stark contrast offers the potential to differentiate Ag-bearing ores that actually may have produced metal used for coinage from other mines which did not contribute to coin production. In addition, the high temperature of metallurgical processes applied to Agbearing ores does not cause significant isotopic fractionation (Albarède et al, 2020;Wang et al, 2022). Thus, while Pb isotopes in silver coinage and artefacts allow potential sources to be identified, which are often too many to be of practical use, Ag isotopes allow most of them to be eliminated.…”

“…The range of Ag isotopic variability in coinage (1⨉10 -4 to +1⨉10 -4 ) is extremely narrow regardless of its origin (Greece, Iberia, Americas) and calls for a high-temperature origin of the bullion because isotope fractionation among minerals and fluids vanishes at temperatures in excess of ~300°C(Fujii and Albarède 2018). With proper attention paid to the units used byWang et al (2022) (parts per 1000 instead of parts per 10,000 as used byFujii and Albarède [2018]), Rayleigh-type fractional crystallization further enhances isotope fractionation in low-to high-temperature environments and reinforces the necessity of a magmatic or, at the very least, a hypogene origin of the silver used as bullion. In contrast, the ~15 per mil range of  34 S for potential galena sources requiring sulfur isotope fractionation by low-to mid-temperature effects (<300°C) is in stark contrast to the narrow range of Ag isotope variability.…”

“…The dual character of the population of Ag isotope compositions cannot, at this stage, be uniquely explained. Multiple parameters such as a dual sources, temperature, redox conditions, and Rayleigh fractionation(Wang et al, 2022) may, in isolation or in combination, account for the observed dichotomy. Samples are too few and mineralogical and geochemical data on potential ores are too fragmentary to support further speculation at this point.The present work on Greek mines confirms the suggestion made about Iberian mines(Milot et al, 2021b) that measuring Ag isotopes in galena is important to corroborate the provenance of bullion sources.…”