A geological context for ancient copper production in the Niari basin (Republic of Congo)

Copper deposits of the Niari basin (Republic of Congo) are known from historical texts since the 16thC and the metal produced has been involved in long distance trade, particularly to provide copper to regional kingdoms. Highly appreciated and exchanged in ancient central African cultures, copper is hence essential to understand the economic networks in this vast part of Africa. Recent archaeological survey and excavation near the deposits in this area – at Mindouli, Boko-Songho, and Mfouati – allowed the unambiguous identification of copper production sites in the vicinity of all three ore deposits. Results indicate that the area of Mindouli, where the Cu-Pb-Zn deposit is widely exposed, has been actively exploited in the 13th-14thC AD, concomitantly with the rise of regional kingdoms. Copper production in this area was later continued, until the industrial exploitation of these polymetallic deposits began in the early 20thC, by the French colonists. Besides these first-hand data, late 19thC European texts record various manufacturing processes and give an overview of copper metallurgy in pre-colonial times.Joint archaeological and geological investigation led by the authors in the Summer 2015 (see also De Putter and Nikis, this volume) had as double objective to improve the understanding of the geological formation of these poorly known deposits, and to document the use of the copper ore in archaeological sites, with further view on copper sourcing analyses, throughout Central Africa. Fieldwork yet allows several preliminary interpretations. In most areas, the location and nature of archaeological sites suggest that copper was smelted from carbonate ore (mostly malachite), in the immediate vicinity of the deposits. Ore procurement strategy also seems to have changed with time, as some deposits have been exploited at precise periods, at the exclusion of others. Almost any ore showing/deposit with obvious evidence of copper minerals has been exploited in the past, irrespective of the variable quantity/grade of suitable ore for copper production. Variable exposure of supergene deposits in the Niari basin also accounts for different exploitation modes – in open trenches, pits or in galleries. These exploitation modes in turn influence the type of raw material (copper or lead ore, carbonates or sulfides) recovered and used in manufacturing processes, at different periods. For instance, the ore was crushed at smelting sites in 13th-14thC to select suitable minerals while in the 19th c. this ore sorting apparently took place in the mines.