Silver isotope and volatile trace element systematics in galena samples from the Iberian Peninsula and the quest for silver sources of Roman coinage

Abstract

Silver played a key role in the progressive monetization of early Mediterranean civilizations. We combine Pb and Ag isotopes with volatile trace elements (Bi, Sb, and As) to assess whether, during the Roman occupation of Iberia, galena constituted a significant source of silver. We find that the Pb and Ag isotopic compositions of 47 samples of galena from eight different Iberian mining provinces, many of them exploited during Roman times, are uncorrelated. This indicates that their respective isotopic variabilities depend on different petrogenetic processes. Moreover, the range of Ag isotopic abundances is approximately six times wider than that displayed worldwide by silver coins in general and Roman silver coins in particular. Although galena from the Betics provides the best fit for Pb isotopes with Roman coins, their fit with Ag isotopic compositions is at best sporadic. We suggest that, together with Sb, Bi, and As, silver is primarily derived from fluids boiled off from differentiated mantle-derived magmas. These fluids, in turn, reacted with preexisting galena and functioned as a silver trap. Lead sulfides with ε109Ag of ~0 and unusually rich in Ag, Sb, Bi, and As were the most probable sources of ancient silver, whereas samples with ε109Ag departing significantly from ~0 reflect low-temperature isotopic fractionation processes in the upper crust.

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

“…The authors realize that the first-order rules for using Ag isotopes and Sb concentrations to narrow down acceptable sources of silver as advocated by Milot et al (2022) and the present work may have limits. At the present stage, coins with Ag isotope compositions falling significantly outside the range suggested by Fujii and Albarède (2018) have not been recorded.…”

“…Finally, galena samples with high Sb concentrations have ε 109 Ag in the same range as the worldwide population of silver coins (1 to +1) and, therefore, present the strongest potential as candidates for Ag sources for coinage (Milot et al, 2022). This correlation is observed in Greek ores as well and potential sources for silver coinage can be proposed as displayed in Fig.…”

“…No significant correlation is observed between the two isotope systems. Figure 4b shows Sb in Greek (Table 1) and Iberian (Milot et al, 2022) galena samples as a function of ε 109 Ag (Table 1 and Supplementary Material). The high Sb concentrations which are so conspicuous in Iberian galena samples with 1<ε 109 Ag<+1 (Milot et al, 2022) are also observed in the Greek ore samples of this study for the same ε 109 Ag interval (Fig.…”

“…The companion Pb isotopic compositions are published and discussed in Vaxevanopoulos et al (2022). We also tested whether sulfur isotopes provide a means for ore source discrimination or offer any other insight into the petrological processes of silver ore formation by measuring  34 S on a subset of the Aegean ores investigated by Vaxevanopoulos et al (2022) and the Iberian galena samples for which Ag and Pb isotopes are published by Milot et al (2021aMilot et al ( , 2022.…”

“…Hacksilber analysis byEshel et al (2022) are consistent with this range. The ε 109 Ag values of galena ores are from this workand Milot et al (2022).…”