Abstract
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.
Highlights
A more detailed description of Iberian galena deposits is given in Milot et al (2021)
Less Ag-rich than the south, the northern half of the Iberian Peninsula counts several noticeable deposits. This is the case of the Catalan Coastal Ranges where the Variscan granitic basement covered by Mesozoic sedimentary sequences hosts numerous mineralized veins including barite, fluorite, and base-metal sulfides (Canals and Cardellach, 1997)
In the West Asturian-Leonese Zone, located northwest of the Iberian Peninsula, noticeable Pb-Zn deposits outcrop at the contact of the calcareous Vegadeo Formation within the Lower Cambrian sedimentary succession (Tornos et al, 1996)
Summary
The protocols for galena sample preparation and Ag isotopic analysis are described in detail in Milot et al (2021). An aliquot of 100 μL was taken for each sample and diluted in 0.5M distilled HNO3. A multi-elementary standard solution (SCP Science, SCP33MS 140-130-321) was diluted in 0.5M distilled HNO3 into four separate solutions of 1, 2, 5, and 10 ppb for all elements. A scandium standard solution (Alfa Aesar, Specpure 35755 Scandium plasma standard solution) was added as an internal standard to each sample, blank, and 33MS solutions at a concentration of 2 ppb. The multi-elementary 33MS solutions were measured at the beginning of the analytical sequence for calibration, followed by the galena sample solutions. The blank was measured regularly in between samples to ensure cleanliness and check for potential cross-contamination.
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