Potential use of Fe isotopes for ancient non-ferrous metals tracing through the example of a lead-silver production site (Imiter mine, Anti-Atlas, Morocco)

Determining the provenance of non-ferrous metals is a major issue in archaeology and history. The classical method using Pb isotopes shows several limitations, and new isotopic tracers need to be developed in archaeometallurgy. Recently, Fe isotopes were found to be promising for iron metal tracing. In this context, we evaluated the pertinence of Fe isotopes as a new tool for provenance studies of non-ferrous metals. We collected slag and furnace lining samples from the Imiter silver mine in Morocco (Anti-Atlas), where many archaeological remains attest ancient lead-silver production. Their mineralogical composition was investigated by optical microscopic observations, SEM-EDS and EPMA. Besides, we measured both Pb and Fe isotope compositions of these samples by high resolution MC-ICP-MS after wet sample decomposition and purification chemistry. The occurrence of Ag, Pb, Cu and Ag/Pb metallic beads in the slag samples and their high Fe content suggests that these metallurgical materials result from the exploitation of Ag-rich polymetallic ores associated with gossan formations. Isotopic analyses of these slags highlighted their heterogeneous Pb isotope compositions and homogeneous Fe isotope composition. This points to the exploitation of two distinct ore sources with different Pb but similar Fe isotope compositions, despite the Fe isotope variability often encountered in metallic ores worldwide. Hence, Fe isotopes provide a complementary approach to trace lead-silver products from Imiter. Overall, the combination of Pb and Fe isotope analyses constitutes a promising method for further provenance investigations of non-ferrous metals.