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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Date
2011-03
Authors
Netshitungulwana, Robert
Journal Title
Journal ISSN
Volume Title
Publisher
University of the Free State
Abstract
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.
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Keywords
Gold ores -- Zimbabwe, Gold ores -- South Africa, Gold ores -- Geology, Dissertation (M.Sc. (Geology))--University of the Free State, 2011