An investigation of the trace element compositions of gold from Zimbabwe and South Africa: implications for tracing the source of archeological gold artefacts
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The early black farmers who settled in southern Africa were involved in trading and metal technology. The history of mining for metals like iron, copper, tin and gold in southern Africa spans at least the past 2000 years. The main aim of the research was to test the viability of using gold chemistry to compare the composition of gold ores in South Africa and Zimbabwe with those of the archaeological gold artefacts from Thulamela, Mapungubwe and Great Zimbabwe. Samples from the Archaean greenstone belts in South Africa and Zimbabwe, as well as samples from ores associated with the Witwatersrand Supergroup, were used in the study. Trace element signatures were determined by laser ablation inductively coupled plasma mass speetrometry (LA-ICP-MS), a technique whereby low concentrations (down to low ppb levels) can be detected. In addition, Ag concentrations (wt %) were determined using a scanning electron microprobe, so that Ag could be used as an internal standard during the LA-ICP-MS runs to give semi-quantitative data. The most commonly occurring isotopes in gold, namely, 56Fe, 59Co, 60Ni 63CU, 66zn, 75As, 1880S, 105pd, 195pt, 202Hg, 107.109Ag, and 204, 206,207,208Pb and 209Bi, were used to construct the signatures, using their intensities in the mass spectra in counts per second (cps). Isotopic ratios were used to compare the gold ores with each other. The results show some variations in the signatures of gold from the greenstone belts and the Witwatersrand Basin. The 107Ag and 202Hg concentrations in gold from the Witwatersrand Basin are high compared to the greenstone belts. These differences have implications for the various models of gold deposition in these environments, pointing to different geochemical histories. Multivariate correspondence analysis plots for the major gold deposits show the wide group of the Barberton samples with little or no distinctive characteristics, compared to the Zimbabwean gold samples. The Witwatersrand gold plotted differently to the Barberton Greenstone Belt but closely related to the Zimbabwean greenstone belts. The ratio plot of 56Fe/107 Ag versus 202HglI07 Ag shows that archaelogical gold artefacts differ completely from the natural gold, indicating that the gold could not merely have been cold-worked, as has been suggested. This suggests that gold from anyone archaeological site could not be related to any particular or even regional source. This could be associated with the possibility of mixing of gold from multiple sources, recycling, contamination in melting and trade in items.