Copper-based metallurgy from an Early Iron Age workshop in the Middle Guadiana basin (Portugal): first evidence of imports of Mediterranean copper ingots using Pb isotopes

Abstract

Cabeço Redondo is a fifth century BC archaeological site located on the left bank of the Guadiana river in the municipality of Moura (southern Portugal). The site was severely damaged by agricultural works in 1990, but among the preserved remains recorded by later archaeological surveys, a set of copper-based debris from a metallurgical workshop stands out. The set includes copper ingots, namely a large 6.4 kg fragment, a small plano-convex bronze ingot and numerous metal lumps and prills. Metal lumps and prills have a diverse elemental composition, characterised as pure copper, binary and ternary bronzes, very probably being intermediate or waste products from the manufacture of artefacts. In order to determine the provenance of the copper used by this workshop, nine samples including ingot fragments were selected for Pb isotope analysis by MC-ICP-MS, which results were complemented by minor and trace element contents determined by ICP-QMS. Crossing analytical data with archaeological evidence suggests that the majority of those metal items has an extra-peninsular provenance located on the Central Mediterranean. However, the small bronze ingot may have copper from the Iberian Peninsula, namely from the Torrubia mine (Los Pedroches Batholith complex, Central Iberian Zone). The combination of this evidence with other known imports from this period suggests an important trade originating in Central Mediterranean regions, whose products reach the inland regions of southwestern Iberian Peninsula via routes that are still uncertain.