An investigation of the trace element compositions of gold from Zimbabwe and South Africa: implications for tracing the source of archeological gold artefacts

[摘要] The early black farmers who settled in southern Africa were involved in trading andmetal technology. The history of mining for metals like iron, copper, tin and gold insouthern Africa spans at least the past 2000 years. The main aim of the research wasto test the viability of using gold chemistry to compare the composition of gold oresin South Africa and Zimbabwe with those of the archaeological gold artefacts fromThulamela, Mapungubwe and Great Zimbabwe. Samples from the Archaeangreenstone belts in South Africa and Zimbabwe, as well as samples from oresassociated with the Witwatersrand Supergroup, were used in the study. Trace elementsignatures were determined by laser ablation inductively coupled plasma massspeetrometry (LA-ICP-MS), a technique whereby low concentrations (down to lowppb levels) can be detected. In addition, Ag concentrations (wt %) were determinedusing a scanning electron microprobe, so that Ag could be used as an internal standardduring the LA-ICP-MS runs to give semi-quantitative data. The most commonlyoccurring 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 thesignatures, 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 showsome variations in the signatures of gold from the greenstone belts and theWitwatersrand Basin. The 107Ag and 202Hg concentrations in gold from theWitwatersrand Basin are high compared to the greenstone belts. These differenceshave implications for the various models of gold deposition in these environments,pointing to different geochemical histories. Multivariate correspondence analysisplots for the major gold deposits show the wide group of the Barberton samples withlittle or no distinctive characteristics, compared to the Zimbabwean gold samples. TheWitwatersrand gold plotted differently to the Barberton Greenstone Belt but closelyrelated to the Zimbabwean greenstone belts. The ratio plot of 56Fe/107 Ag versus202HglI07 Ag shows that archaelogical gold artefacts differ completely from the naturalgold, indicating that the gold could not merely have been cold-worked, as has beensuggested. This suggests that gold from anyone archaeological site could not berelated to any particular or even regional source. This could be associated with thepossibility of mixing of gold from multiple sources, recycling, contamination inmelting and trade in items.

[发布机构] University of the Free State