Iron Age Botswana Used Copper From Across Africa but Tin From One Valley

About 700 years ago, residents of Bosutswe, a hilltop settlement in eastern Botswana, were using metal whose raw materials had traveled surprisingly different routes. Copper reached the community from mines scattered across southern Africa, while the tin used to make bronze appears to have come from one place. The finding comes from a study that analyzed the chemical and lead-isotope signatures of 30 metal objects from Bosutswe and the nearby Iron Age sites of Khubu la Dintša and Lose. These signatures can be compared with those of known ore deposits, allowing researchers to investigate where the metals originated. According to the study, Bosutswe’s copper came from several widely separated sources. The identified locations included the Copperbelt along the Congo-Zambia border, Copper Queen in Zimbabwe, and Musina, Phalaborwa and Rooiberg in South Africa. Some of the sources were more than 950 kilometers from the settlement. The pattern differed from that found at some other regional sites. Mapungubwe, for example, obtained most of its copper from Phalaborwa. Although Botswana had several known pre-colonial copper mines, only Selebi Phikwe and Dukwe—about 140 kilometers from the sites—were represented among the objects examined. The researchers said the evidence does not establish exactly how the metal moved or who transported it. Bosutswe developed from a cattle post in the seventh century into a trading center with links that included Asian glass beads, coastal cowrie shells and gold. Its location near river headwaters draining toward the Indian Ocean placed it along routes connecting eastern and western parts of the region. Tin produced a much more concentrated result. Every bronze object that the researchers were able to source pointed to the Rooiberg valley in South Africa, about 360 kilometers away. The valley is the only tin field in the region with evidence of pre-colonial mining, according to the study. Because the same tin source was combined with copper from multiple locations, the researchers concluded that bronze was probably produced at Bosutswe rather than arriving there as finished objects. The objects also contained varying amounts of tin, producing colors ranging from red and salmon pink to golden yellow. The study argues that this broad color range expanded the decorative choices available to people who wore the metal as bangles, beads and clips. Bronze appeared at Bosutswe by around the 13th century. The settlement continued for centuries before being abandoned in the 17th century, giving researchers evidence of a durable community connected to an extensive and complex trade network.