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

Abstract

Variations of 109Ag/107Ag in silver coins and ores are particularly useful in assessing the provenance of silver bullion. Silver isotope variability results from the temperature-dependent thermodynamic fractionation of Ag isotopes among the solutions and minerals participating in ore formation. They differ from lead isotopic variations which result from the decay of uranium and thorium and reflect the geochemical properties and the tectonic age of the possible ore sources. A remarkable property of Ag isotopes is the very narrow range of isotopic variations in silver bullion used for coinage (±1×10−4) with respect to the range of ores (±1×10−3). To test the practical usefulness of the technique, we analyzed the Ag isotopic abundances of 29 ore samples from ancient mining districts in the Aegean with major and minor Ag-bearing mineralizations, and of 34 ancient Greek coins minted from the sixth to late fourth centuries BC. We distinguished two groups among the coins: a dominant population (93% of the samples) with 109Ag/107Ag consistent with literature data (ε109Ag = −1 to +1) and an isotopically lighter population (ε109Ag = −2 to −1) which we show originated from Ag-bearing mineralizations in Lavrion (Attica). We further found that sulfur (also analyzed in this study) and silver isotope compositions in Aegean ores do not correlate, a finding that we confirmed on a selection of Iberian galena samples. This shows that the genetic ore type (whether hypo, meso, or epithermal) and silver productivity are not related. Finally, we undertook chemical analysis of the Aegean ore samples and confirmed that Ag-rich ores are also Sb-rich in both Greece and Iberia.A remarkable outcome of the present Ag isotope studies of galena ores from Iberia and Greece is that silver isotope compositions can exclude, with a high degree of reliability, the majority of mines identified by lead isotope analysis as sources from which coinage silver could plausibly have been extracted and thus significantly narrow down the actual source(s). Silver isotope data on galena ores are thus a useful tool for deciding which Pb isotope data included in ore databases should be included in provenance assessment studies. Contrary to some earlier assessments, subtle silver isotope variations can occasionally help determine ore provenance within a single mining district such as Lavrion.