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.