Recent study show that “prehistory” tin ingots/prills and slages as well as bronzes from South Africa and Zimbabwe also have highly radiogenic lead isotopes, which do plot on an isochron very similar to the Yin-Shang bronzes (Fig. 4)36. These tin and bronze samples from South Africa and Zimbabwe are located between the Archean Kaapvaal and Zimbabwe cratons (Fig. 5).

Figure 4 206Pb/204Pb versus 207Pb/204Pb diagram for ancient bronzes from China2,7,9 in comparison with tin ingots/prills, bronzes and slags from South Africa and Zimbabwe36 and from copper, lead and galena from the 18th Dynasty of Ancient Egypt42. Yin-Shang bronzes and Africa metals plot roughly along the same line. Metals from the 18th Dynasty have homogenous lead isotopes, probably due to repeated recycling. Nonetheless, metals from Ancient Egypt plot in the field defined by Yin-Shang bronzes. Full size image

Figure 5 Sketched geologic map of Africa showing the distributions of Archean cratons, all of which have provide highly radiogenic “old” lead (2.0–2.5 Ga). Also shown are the locations of prehistory sites of tin ingots/prills, slag and broznes36 with highly radiogenic lead isotopes. In addition to Kaapvaal and Zimbabwe cratons, Congo, Tanzania and Uganda cratons are close the Nile River and thus were more accessible to ancient people. In addition, there are micro Archean cratons in the Sahara desert, which was more accessible in ancient time. This map used part of the map “color_etopo1_ice_full.tif.zip” from NOAA maps at: http://www.ngdc.noaa.gov/mgg/global/relief/ETOPO1/image/. Image created by J. Varner and E. Lim, CIRES, University of Colorado at Boulder. Full size image

In a 206Pb/204Pb versus 207Pb/204Pb diagram, bronzes from Yin-Shang, Sanxingdui, South Africa and Zimbabwe are all seen to plot along similar, very old isochrons of ~2.0–2.5 Ga (Figs 1 and 4), implying that these lead came from occurrences within ancient cratons. Both Africa and North China have Archean cratons older than 2.5 billion years and thus are in principle potential candidates for supplying the highly radiogenic lead of the Yin-Shang bronzes.

The North China Craton suffered multiple plate subductions37,38,39 and was eventually destroyed40 in the Early Cretaceous, likely due to ridge subduction41,42. The destruction of the North China Craton resulted in major magmatism43 and mineralization41,44, which obliterated most of the original Archean lead isotope signatures of the old craton. However, the Tongkuangyu copper deposit was already formed 1.9 billion years ago and thus roughly preserved its older lead isotope characteristics. It is the only ore deposit so far reported in China with a lead isotopic composition and ischron age close to but slightly younger than those of early Yin-Shang bronzes. As discussed above, the Tongkuangyu copper deposit cannot be considered the single ore source for the early Yin-Shang bronzes because of its very low lead and tin contents. Neither does the Tongkuangyu deposit appear to have been the source of the systematically less radiogenic lead found in the bronzes at the Yuanqu smelting site near Tongkuangyu.

Tectonically, Tongkuangyu is located in the Paleo-Proterozoic Trans-North China Orogenic Belt formed through collision between the Eastern and Western Blocks of the North China Craton in the Paleo-Proterozoic era45. Convergent margins usually have very high oxygen fugacities, favorable for copper deposits27,46,47. This is supported by the high oxygen fugacity of Tongkuangyu. Tin deposits, on the other hand, are usually associated with reduced environments30 and thus the whole Trans-North China Orogenic Belt is unlikely to be favorable for tin deposits. In general, tin deposits are usually located several hundred kilometers away from convergent margins, e.g., as found in the South America and South China30,48,49. Furthermore, the North China Craton is a small block that is surrounded by subduction zones. In addition to the Paleo-Proterozoic Trans-North China Orogenic Belt, the Qinling-Dabie Orogenic Belt lies to the south50,51,52, the Central Asian Orogenic Belt lies to the north53 and the Pacific subduction zone lies to the east39,42,54. All these subductions face the North China Craton and probably have elevated the oxygen fugacities, making them unfavorable sites for tin mineralization.

When putting together all the observations, the most straightforward conclusion is that both the Yin-Shang and the Sanxingdui bronzes were obtained in Africa, bearing the highly radiogenic lead isotopic signatures of the Africa Archean cratons. Alternatively, some ancient people might have come to China from Africa, carrying tin and/or bronzes with them.

South Africa and Zimbabwe are known for abundant archaeological sites55,56. Most archaeologists, however, consider these places to have been too far away from China for people to have been in contact in the Bronze Age. Even their relationship with ancient Egypt before ~1400 BC is not clear. Interestingly, so far published lead isotopic compositions of ancient Egyptian bronzes from the late Bronze Age are mostly less radiogenic57 than the Yin-Shang and the South African bronzes (Fig. 4). Mention was once made of some Ancient Egyptian bronzes have highly radiogenic lead isotopes, but that report did not show supporting data9. Nevertheless, the mystery remains as to how the Yin-Shang people would have gotten bronzes from these places?

In any case the lead isotopic signature of the Yin-Shang bronzes suggests that the ore deposits supplying their lead were most likely located in Archean cratons. The Africa continent is made up of several large Archean cratons (Fig. 5).The Congo, Tanzania and Uganda Cratons are essentially of the same age as the Kaapvaal and Zimbabwe Cratons. They would also be expected to have lead ore deposits with Archean age lead signatures. These cratons lie much closer to the Nile and parts of them once even belonged to ancient Egypt during its early history. Might not the Yin-Shang people have gotten bronzes and/or raw materials from these places through trade or by other means at this time? During the late Dynasties, ancient Egypt lost access to these cratonic deposits, because of its shrinking territory, such that Egyptian bronzes in the 18th Dynasty or later have less radiogenic lead isotopes (Fig. 4). The ancient Egyptian people may have also gotten lead ore from Saharan deposits. In addition to the large Archean cratons, there are several small Archean cratons within the Saharan metacraton (Fig. 5). Ore deposits–later abandoned and are now buried in the desert and having lead with Archean isochron ages–could possibly have been associated with these Saharan micro-cratons.

Given that bronze is often recycled18, early bronzes in Egypt may have been re-smelted and later mixed with more normal lead, thus explaining the very homogenous, but still radiogenic lead isotopes of 18th Dynasty coppers (Fig. 4). In contrast, the original, highly radiogenic isotopic signature of Early Yin-Shang bronzes is well preserved in Yin-Shang tombs. More archaeological evidence is needed to confirm any of the above speculations. Until then, whether and how the ancient people in China obtained bronzes with highly radiogenic lead isotopes from Africa remains a puzzle.