Chemical and isotopic analyses reconstruct multi-scalar copper circulation networks in West Africa: evidence from Akjoujt and Walaldé, 1st millenium BCE – 1st millenium CE

For the past 50 years, copper sourced from the Akjoujt region has been inferred primarily through the enrichment of arsenic in metal samples from archaeological sites in West Africa. However, arsenic is common to a variety of mineral deposit types and this signature was based on early low-resolution spectroscopic data. Here, we apply lead isotopic and bulk chemical analyses to copper metal and ore samples from the region of Akjoujt, in order to interrogate this signature and contribute more precise data. Our results confirm that arsenic is indeed a significantly enriched element for copper produced around at Akjoujt, but we broaden its chemical signature to include enrichment in Sn, Ni, and Co, and depletion in other chalcophile elements, but particularly depletion in Pb. Isotopically, we demonstrate that hydrothermal mineralization at Akjoujt included uranium and thorium, and produced a characteristic radiogenic lead isotopic signature dating to the late Archaean/early Proterozoic periods, in alignment with new U-Pb dating for Akjoujt. Using this signature, we analyze copper from new excavations at the Walaldé site in Senegal, which has previously been suggested as a destination for copper from Akjoujt. We show that the copper for six objects associated with a burial excavated in 2016 originates from across the Sahara and likely dates to the 1st millennium CE, as does one previously analyzed copper sample from Walaldé. We also identify three previously analyzed copper objects from the 1999 excavations at Walaldé which likely originate from Akjoujt and date to the later first millennium BCE.