Geological and Mining Constraints on Historical Mine Production: The Case of Early Medieval Lead-Silver Mining at Melle, France

Abstract

Early Medieval silver production for the Melle Pb-Ag deposit in western France has been estimated up to 15 tonnes per year over hundreds of years (Téreygeol, 2013) which would place it amongst the top silver mines of all times prior to the New World discoveries. However, this deposit has geological and mining characteristics economically unsuitable for substantial production, because it is thin, sub-horizontal and comprised of discontinuous pods. Furthermore, it is a Mississippi Valley type (MVT) base metal deposit, which are typically not major silver producers. There is no geological evidence for primary or secondary enrichment to generate silver-rich ore. Inaddition, the fact that Melle remained unmined in all later historical periods is enigmatic compared to the almost ubiquitous reworking of significant ore deposits elsewhere.In this paper, I discuss the geological characteristics, mining parameters, documented historical mining rates and workforce considerations, all of which can be used to constrain production estimates for Melle. The largest uncertainty for production estimates is the workforce size, which reflects scarce information about Carolingian Melle and its surroundings. A model employing realistic mining parameters and a workforce appropriate for a small mining village (100 miners/fire-setters, within a workforce of 250-300, from a settlement of at least 400-500) yields 52 tonnes of lead metal and 150 kg of silver per year. Doubling the workforce would double this estimate. Conversely, it could be half or less if mining was a seasonal activity between agricultural priorities such as harvesting and seeding.The previously claimed production rates require improbable mining assumptions together with at least 500 dedicated full-time miners and a population in the thousands. Furthermore, it would yield per capita silver productivity more than four times higher than in well-documented Early Modern operations that were leading silver deposits of their time. This seems unlikely. However, even at the much lower production levels, favoured here, Melle still might have been a major factor in the Carolingian economy, with its lead production perhaps as important as its silver.

Byzantine plate and Frankish mines: the provenance of silver in north-west European coinage during the Long Eighth Century (c. 660–820)

“…2). Nevertheless, it is commonly agreed that, even at its height, Melle did not produce silver on a scale sufficient to supply all the mints of the Carolingian Empire (Sarah 2019: 196;Bettenay 2022): the data presented here show that the extent to which coins of central and eastern Francia were supplied with low-gold silver was modest before 793. For now, however, we propose that Melle, or Melle-like, silver permeated regional silver stocks to a greater (but still variable) extent after c. 750, being mixed with other (presumably relict, higher-gold) stocks of metal.…”

“…These data derive from eighth-and ninth-century galleries within the mine but, given that Melle is a low-temperature ore field (96-155°C) and carbonate-replacement deposit with only one ore-forming fluid and one host rock (Cathelineau et al 2012), it is unlikely that isotopic values of galena (lead sulphide also containing silver) from other, earlier contexts within the mine would differ. The absence of secondary ore enrichment (Bettenay 2022) and the basic ore formation conditions are unfavourable for the presence of gold, thus it is expected that silver produced from Melle galena would be poor in gold. Existing archaeometric evidence indicates that Melle silver, identifiable by its lead isotope ratios and low gold levels, was used to mint some of the silver coins in its vicinity (i.e.…”