Jean Milot
email: jean.milot@ens-lyon.fr

Mariano Ayarzagüena Sanz

Chloé Malod-Dognin

Francis Albarède

Philippe Telouk

Janne Blichert-Toft

Mariano Ayarzagüena Sanz

Philippe Télouk

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

Introduction

Considered a luxury commodity, silver has been massively used for coinage from the 7 th century BC onwards. It took monetized silver, which first appeared in the Aegean and Anatolian regions, centuries to reach the Occidental Mediterranean. Argentiferous galena (PbS) is considered by some as the main source of silver during ancient times (e.g. [START_REF] Domergue | Les Mines Antiques[END_REF][START_REF] Domergue | Les mines de la Péninsule Ibérique dans l'antiquité romaine[END_REF] with silver having been extracted from PbS by smelting and subsequent cupellation in the presence of metallic lead (e.g. [START_REF] Pernicka | Late Uruk silver production by cupellation at Habuba Kabira, Syria[END_REF][START_REF] Tylecote | A history of metallurgy[END_REF]. Others suggest alternative Ag-rich minerals such as plumbojarosite, tetrahedrite, or chlorargyrite (e.g. [START_REF] Anguilano | The importance of lead in the silver production at Riotinto (Spain): L'importance du plomb dans la production d'argent à Riotinto (Espagne)[END_REF]. In general, whichever the sources of silver for Roman coinage, they have since been used up and whatever is left is so dispersed that it is difficult to assess whether they ever constituted major sources of bullion or were barren PbS.

In contrast to the relative abundances of radiogenic Pb isotopes, which vary with time and the relative proportions of U, Th, and Pb in the ore source, Ag, Cu, and Zn isotopic abundances vary with the nature of fluids and mineral phases contributing to the ore, redox conditions, and the temperature of ore formation. Isotopic variations are related to mass-dependent fractionation and are typically in the range of a fraction of a percent per atomic mass unit for Cu and Zn and one per mil for Ag. Silver isotopic variations are most conveniently reported in epsilon units (ɛ 109 Ag; the deviation per 10,000 of a given sample relative to the NIST 978a Ag standard). For the entire body of antique coinage analyzed so far, this range is strikingly narrow within only ±1 ɛ unit [START_REF] Albarède | From commodity to money: the rise of silver coinage around the ancient Mediterranean (6 th -1 st century BCE)[END_REF][START_REF] Albarède | A glimpse into the Roman finances of the Second Punic War through silver isotopes[END_REF][START_REF] Desaulty | Isotopic Ag-Cu-Pb record of silver circulation through 16th-18th century Spain[END_REF][START_REF] Desaulty | Copper, lead, and silver isotopes solve a major economic conundrum of Tudor and early Stuart Europe[END_REF]. Since Ag, Cu, and Zn isotopic variations are controlled by thermodynamics rather than geology, they are of marginal interest for provenance studies and rather track ore genesis processes [START_REF] Arribas | The Isotopic Composition of Silver in Ore Minerals[END_REF].

A trademark of Ag isotopes particularly relevant to archeology is the contrast between the narrow range of ɛ 109 Ag (<2 ɛ units) in silver coins of different origins and age of minting (Ancient Greece and Rome; Medieval Europe; colonial South and Central Americas) [START_REF] Desaulty | Isotopic Ag-Cu-Pb record of silver circulation through 16th-18th century Spain[END_REF]Fujii and Albarède, 2018) and the 12 ɛ unit range measured in Ag and Ag-bearing base metal (Pb, Zn, Cu) ores [START_REF] Arribas | The Isotopic Composition of Silver in Ore Minerals[END_REF]. Iberia was the major source of silver for the Carthaginian and Roman worlds up until the 2 nd century AD and Pb isotope data on potential silver ores are extremely well documented (see García de Madinabeitia et al., 2021 and references therein). We therefore measured Ag isotopes and trace element abundances on the galena sample suite documented for Pb isotopes by Milot et al. (2021a) to identify which of these Iberian Pb ores may have actually been mined for use of their silver in minting Roman denarii.

Potential Iberian sources of Roman lead and silver

Geological information on the galena samples analyzed here is summarized in Milot et al. (2021a) and further details can be found in the IBERLID database (García de [START_REF] Garcia De Madinabeitia | IBERLID: A lead isotope database and tool for metal provenance and ore deposits research[END_REF]. Briefly, most deposits of economic importance are located in the southern part of the Iberian Peninsula (see Supplementary Material). Lead-zinc-rich deposits occur particularly in the Betic Cordillera (southeast Iberia), the Sierra Morena (south-central Iberia), which belongs to the Central Iberian Zone (CIZ), and the Ossa Morena Zone (OMZ, southwest of the Sierra Morena). The South Portuguese Zone (SPZ) counts many world-class giant and supergiant volcanogenic massive sulfide (VMS) deposits all belonging to the Iberian Pyrite Belt that covers the southwest corner of the Iberian Peninsula. Although less common, notable deposits occur in the northern part of the Iberian Peninsula, such as in the Catalonian Coastal Ranges, the Northern Iberian Massif, the Basque-Cantabrian Basin, the Cantabrian Zone, and the West Asturian Leonese Zone (Fig. 1). These mining regions exhibit archeological evidence of ancient exploitation from the Bronze Age to the Roman period.

Materials and methods

The Ag isotopic compositions and Ag, Sb, Bi, As, Pb, and Cu concentrations of 47 Iberian galena samples are listed in Table SM1. Table SM2 lists the Pb isotopic compositions for these same samples and samples from the literature. Samples were grouped into eleven distinct regions (Fig. 1), which correspond to the different tectonostratigraphic units that form the Iberian Peninsula (e.g. [START_REF] Vergés | An introduction to the Alpine cycle in Iberia[END_REF]. For better readability, the Central Iberian Zone to which the metallogenic province of the Sierra Morena belongs was divided into two distinct mining areas, those of the Linares-La Carolina and Alcudia Valley-Los Pedroches districts.

The Pb and Ag isotopic compositions of Roman silver coins discussed below are from the literature [START_REF] Albarède | From commodity to money: the rise of silver coinage around the ancient Mediterranean (6 th -1 st century BCE)[END_REF][START_REF] Albarède | A glimpse into the Roman finances of the Second Punic War through silver isotopes[END_REF][START_REF] Desaulty | Isotopic Ag-Cu-Pb record of silver circulation through 16th-18th century Spain[END_REF][START_REF] Westner | ROME'S Rise to Power. Geochemical Analysis of Silver Coinage from the Western Mediterranean (Fourth to Second Centuries BCE)[END_REF]. The type and age of these coins are reported in Table SM3 and date mainly from the 3 rd and 2 nd centuries BC, with a few exceptions dating to the early 1 st century BC. We established two groups corresponding to coins minted before and after the Roman 211 BC monetary reform.

The analytical protocols for Ag separation and isotopic measurement are described in Milot et al. (2021b), while the protocols used for measuring the concentrations of Ag, Sb, Bi, As, Pb, and Cu are given in the Supplementary Material.

Results

The Ag isotope compositions of the present galena samples span a range of more than 10 ɛ 109 Ag units (-8.05 to +2.27), consistent with the findings of [START_REF] Arribas | The Isotopic Composition of Silver in Ore Minerals[END_REF], and vary broadly within each region with significant overlap between regions. In stark contrast, ɛ 109 Ag of Roman silver coins fall within a narrow range (-1.05 to +0.37) except for a single outlier with heavier ɛ 109 Ag of 1.32±0.08. No noticeable differences in Ag isotopic composition are observed between Roman coins minted before and after the 211 BC monetary reform or for three Carthaginian coins. Iberian galena samples match only sporadically the Ag isotopic compositions of Roman coins: among the 47 samples analyzed, only six galena ores fall within the range of the coins (one from the Eastern Betics, one from the Linares-La Carolina district, two from the Alcudia Valley-Los Pedroches district, one from the Ossa Morena Zone, and one from the South Portuguese Zone).

The extent of Ag isotopic variability displayed by galena samples from the Iberian Peninsula is of the same order as the -4 to +4 ɛ 109 Ag range exhibited by hypogene native silver [START_REF] Arribas | The Isotopic Composition of Silver in Ore Minerals[END_REF][START_REF] Mathur | Fractionation of silver isotopes in native silver explained by redox reactions[END_REF] but tends to be overall more negative (Fig. 2). ɛ 109 Ag < -4 is common in Iberian galena samples, whereas they are rare in other primary or oxidized silver minerals.

With a few exceptions, the log-log (Ag+Cu)/Pb vs Sb/Pb diagram shows a positive correlation between these trace elements (Fig. 3a). Furthermore, both Sb vs ε 109 Ag (Fig. 3b) and Ag vs ε 109 Ag (Fig. 3c) show that samples with the highest Sb and Ag contents (>4000 and >3000 ppm, respectively) have ε 109 Ag values centered around about -1.5, only slightly lighter than the ε 109 Ag range for silver coins.

Discussion

Only few studies so far have investigated the mechanisms of Ag isotope fractionation [START_REF] Arribas | The Isotopic Composition of Silver in Ore Minerals[END_REF][START_REF] Fujii | 109Ag-107Ag fractionation in fluids with applications to ore deposits, archeometry, and cosmochemistry[END_REF][START_REF] Mathur | Fractionation of silver isotopes in native silver explained by redox reactions[END_REF]. Sulfides and sulfosalts play a major role in reduced environments. Pb 2 S 2 (galena) can form solid solutions with AgBiS 2 (matildite), AgSbS 2 (miargyrite), and AgAsS 2 (proustite) [START_REF] Hackbarth | A fractional crystallization model for the deposition of argentian tetrahedrite[END_REF][START_REF] Renock | A first principles study of coupled substitution in galena[END_REF] and a strong correlation between Ag and Sb+Bi contents is commonly observed in argentiferous galena [START_REF] George | Trace and minor elements in galena: A reconnaissance LA-ICP-MS study[END_REF]. These mutually soluble molecules can exsolve from galena upon cooling [START_REF] Chutas | A solution model for hightemperature PbS-AgSbS2-AgBiS2 galena[END_REF] or, alternatively, upon metamorphic reheating and melting, and recrystallize as interstitial minerals observable in thin sections [START_REF] Voudouris | Carbonate-replacement Pb-Zn-Ag±Au mineralization in the Kamariza area, Lavrion, Greece: Mineralogy and thermochemical conditions of formation[END_REF]. Other minerals may contain large concentrations of silver, notably chlorides (chlorargyrite). Under low-temperature, oxidized conditions, jarosite (KFe 3 (SO 4 ) 2 (OH) 6 ) and native Ag are important silver-bearing minerals. [START_REF] Arribas | The Isotopic Composition of Silver in Ore Minerals[END_REF] proposed that the oxidation of primary Ag-sulfides under supergene weathering conditions preferentially releases isotopically heavy Ag, which in turn reprecipitates beneath the water table as secondary Ag phases of equally heavy Ag isotopic composition.

As far as silver sources are concerned, the two most crucial questions to emerge from the present work are:

1. Which of the samples analyzed here may have been active sources of monetized Roman silver? 2. Why is the worldwide range of ɛ 109 Ag (-1 to +1; Fujii and Albarede, 2018), including Ancient Greece and Rome, Medieval Europe, and colonial South and Central Americas, so narrow in coins compared to the range of ɛ 109 Ag in potential Ag ores which is nearly two orders of magnitude wider [START_REF] Arribas | The Isotopic Composition of Silver in Ore Minerals[END_REF]this work)?

The answer to the first question is that, overall, only very few Iberian galena ores fit the Ag isotope compositions of Roman silver coins. Because the sources of Iberian Pb and Ag used in the coinage process may have been disconnected, Pb isotopes unfortunately are of little use to help identify potential silver sources with precision. Answering the second question is more complicated because of two factors: (i) Ag-O bonds tend to concentrate the heavy isotope 109 Ag with respect to Ag-S and Ag-Cl bonds; (ii) increasing temperature tends to reduce isotope fractionation with 1/T 2 , where T is the absolute temperature.

The narrow range of ɛ 109 Ag in Roman coins indicates that their silver source was primarily extracted at high temperatures, presumably from the mantle [START_REF] Fujii | 109Ag-107Ag fractionation in fluids with applications to ore deposits, archeometry, and cosmochemistry[END_REF]. High temperature will suppress any isotopic fractionation between magmas and crystallizing minerals. The similar ranges of ɛ 109 Ag in minted silver extracted by smelting (Greece, Rome) and the patio method (colonial Latin America; [START_REF] Desaulty | Isotopic Ag-Cu-Pb record of silver circulation through 16th-18th century Spain[END_REF] do not support the idea that the high temperatures of smelting reduced the range of ɛ 109 Ag. Petrologic evidence, however, is inconsistent with Ag-rich sulfides, such as argentiferous galena and sulfosalts, crystallizing at magmatic temperatures. Fluid inclusion studies (e.g. [START_REF] Arribas | Geology, geochronology, fluid inclusions, and isotope geochemistry of the Rodalquilar gold alunite deposit, Spain[END_REF][START_REF] Voudouris | Carbonate-replacement Pb-Zn-Ag±Au mineralization in the Kamariza area, Lavrion, Greece: Mineralogy and thermochemical conditions of formation[END_REF] point to crystallization and exsolution of Ag(Bi,Sb,As)S 2 minerals from galena solid solutions at temperatures of 150-400°C. For tetrahedrite, [START_REF] Hackbarth | A fractional crystallization model for the deposition of argentian tetrahedrite[END_REF] used strongly curved Cu, Ag, As, and Sb trajectories to suggest that these elements are enriched by fractional crystallization from hydrothermal solutions. These trajectories can, however, be alternatively interpreted as mixing relationships between Agpoor galena and an Ag(Sb,As)S 2 end-member: the Cu+Ag/As+Sb ratios used in [START_REF] Hackbarth | A fractional crystallization model for the deposition of argentian tetrahedrite[END_REF] study being highly variable along the trajectories, mixing such endmembers results, by definition, in hyperbolae with very strong curvature.

Given the extreme volatility of Ag, As, and Sb, mixing can therefore be used as a model to interpret the Ag isotope compositions in galena: interaction between Ag-poor sulfides, such as galena and sulfosalts, and Ag-Bi-Sb-As-rich fluids boiled off from magmas is expected to be particularly efficient at assimilating Ag from percolating fluids. Chutas et al.'s (2008) data on mineral stability in the system PbS-AgSbS 2 (their Fig. 7) shows that above 200°C, galena behaves as a 'silver sponge' with respect to Sb-rich fluids. George et al.'s (2015) strong correlations (their Table 3) between Ag and Bi in galena makes the extrapolation of this suggestion to Sb-and As-rich fluids relatively straightforward. Argentiferous galena would form in the relatively broad temperature range of PbS stability (150-450°C) upon Ag+Sb uptake from Ag-Sb-Bi-As-rich magmatic and hydrothermal fluids by pre-existing Pb ore deposits. Silver-rich galena could form from either an early phase of the same mineralization event or, alternatively, multi-stage mineralization. Modern magmatic environments with fluids enriched in Ag, As, Sb, and Bi, which can be used as analogs for silver ores, are known, notably from the PACMANUS hydrothermal field situated on Pual Ridge next to Papua New Guinea, and exsolved from felsic volcanic rocks [START_REF] Binns | Data Report: Geochemistry of Massive and Semimassive Sulfides from Site 1189, Ocean Drilling Program Leg 193[END_REF][START_REF] Wohlgemuth-Ueberwasser | Distribution and solubility limits of trace elements in hydrothermal black smoker sulfides: An in-situ LA-ICP-MS study[END_REF].

With few exceptions, the Ag and Sb contents of our samples are positively correlated (Fig. 3) (r=0.81), which argues for silver enrichment of galena by coupled substitution of 2 Pb 2+ by Cu/Ag + and As/Bi/Sb 3+ leading to the formation of a solid solution between Pb 2 S 2 and AgSbS 2 from which each phase may subsequently exsolve upon cooling [START_REF] Chutas | A solution model for hightemperature PbS-AgSbS2-AgBiS2 galena[END_REF][START_REF] George | Trace and minor elements in galena: A reconnaissance LA-ICP-MS study[END_REF][START_REF] Renock | A first principles study of coupled substitution in galena[END_REF] (Fig. 4). Moreover, samples with the highest Ag contents, which may have been of economic interest to Roman smelters, have a narrow range of ɛ 109 Ag centered at about -1.5 ɛ 109 Ag, which is included in the range of ore-forming fluid compositions determined by [START_REF] Arribas | The Isotopic Composition of Silver in Ore Minerals[END_REF] but slightly lighter than that of silver coins. The lighter ɛ 109 Ag values of Ag-rich galena may be explained by leaching of preexisting fractionated Ag-rich deposits by Bi-Sb-As-rich hydrothermal fluids boiled off from mantle-derived magma. Silver isotopic fractionation during partial exsolution of Ag sulfides or sulfosalts from galena solid solutions may also be an explanation. Even slightly fractionated from the mantle value of ɛ 109 Ag~0, the Ag isotopic composition of Ag-rich galena is likely to get averaged out in silver bullion during smelting. The Betics, where Neogene magmatism took place (e.g. [START_REF] Arribas | Isotopic composition of Pb in ore-deposits of the Betic Cordillera, Spain -Origin and relationship to other European deposits[END_REF], is a plausible source of such Ag-enriched ores as attested to by archeological evidence [START_REF] Baron | Le complexe d'ateliers du Cabezo del Pino (Sierra Minera de Cartagena-La Unión, Murcia) et l'organisation de l'activité minière à Carthago Noua à la fin de la République romaine[END_REF][START_REF] Domergue | Les Mines Antiques[END_REF][START_REF] Domergue | Les mines de la Péninsule Ibérique dans l'antiquité romaine[END_REF]. Similarly, in other provinces, hydrothermal fluids of magmatic origin most likely are responsible for the Ag enrichment of lead ores, which may have been important sources of silver for ancient miners. Since the Pb isotope compositions of the silver coins considered here range between those of two lead sources from the Eastern Betics (Fig. SM1), we suggest that Pb ore deposits from this province mostly were exploited for Pb in general and cupellation in particular.

In contrast, silver from Ag-poor samples is isotopically fractionated relative to a given reference value which, pending proper isotopic documentation, could be that of the mantle or the crust and hence rather reflects supergene Ag fractionation processes in upper crustal environments. Their ɛ 109 Ag values differ markedly from those of Roman silver coins. Circles: samples from this study (Table SM1); triangles: samples from the literature (Table SM2). SM3) and Iberian galena samples (Table SM1). Dotted lines delimit the range of Ag isotopic variation of silver coins, with the exception of one outlier. SM1.

Acknowledgments

Figure captions

  This work is a contribution of Advanced Grant 741454-SILVER-ERC-2016-ADG 'Silver Isotopes and the Rise of Money' awarded to FA by the European Research Council. We are grateful to those who contributed samples to the present study and spared time for their careful selection: Eloïse Gaillou (Mineralogy Museum, MINES ParisTech), Ramón Jiménez Martínez (Instituto Geologico y Minero de Espana), and Paul W. Pohwat and Elizabeth Cottrell (Smithsonian Institution). Insightful reviews by Sonia Garcia de Madinabeitia, Antonia Arribas, and an anonymous reviewer are gratefully acknowledged.

Figure 1 .

 1 Figure 1. Map of the Iberian mining provinces and galena samples analyzed in this study.

Figure 2 .

 2 Figure 2.  109 Ag of silver coins (TableSM3) and Iberian galena samples (TableSM1). Dotted

Figure 3 .

 3 Figure 3. (a) (Ag+Cu)/Pb vs Sb/Pb of Iberian galena samples in atomic concentration. The red line corresponds to a linear regression with a slope of 1. Note the logarithmic scale. (b) Sb vs  109 Ag and (c) Ag vs  109 Ag of Iberian galena samples. Dotted lines delimit the range of Ag isotopic variation of silver coins, with the exception of one outlier. The elemental concentrations are reported in TableSM1.

Figure 4 .

 4 Figure 4. Solubility relationships in the PbS-AgSbS 2 system (after Chutas et al., 2008). gn = galena; gn ss = galena solid solution; fre = freieslebenite; mia = miargyrite. Silver sulfoantimonides are remarkably soluble in galena over a large range of temperatures. Galena deposits represent an efficient trap for Ag-Sb rich fluids.

  Figure2