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Scientists have for the first time directly dated gold from South Africa's Witwatersrand gold deposits, source of more than 40% of all the gold mined on Earth.
Age of the Gold
An international team of geologists, led by the University of Arizona (UA), has discovered that the gold is around three billion years old - older than its surrounding conglomerate rock by a quarter of a billion years. Furthermore, their state-of-the-art dating technique shows that the gold deposits formed along with crustal rock directly from the mantle beneath South Africa. The event at this magnitude appears to be unique in Earth’s geological history.
Source of the Gold
Jason Kirk, Joaquin Ruiz and John Chesley of the UA, John Walshe of Australia’s Commonwealth Scientific and Industrial Research Organisation, and Gavin England of the University of Edinburgh report these findings in a recent issue of Science. The Witwatersrand gold is found in a sedimentary basin, but its age and origin have been hotly debated. One theory suggests that the gold was carried into the basin by a sedimentary processes. A conflicting theory holds that hydrothermal fluids - the equivalent of hot springs emplaced the gold from the upper continental crust.
How the Gold Deposit Formed
Deposition
The new results confirm that the Witwatersrand gold deposits are ‘placer’ deposits that millions of years ago, an ancient river carried gold particles, along with sand and silt, into the Witwatersrand basin then a great lake - possibly from granite mountains to the north and southwest.
Solidification
Over time and under pressure, the gold-bearing sediments solidified into rock, forming the rich gold-bearing reefs of South Africa’s ‘golden arc’, which have been mined since their discovery in 1886.
The UA scientists’ new findings confirm that the gold crust first formed in older rocks, rocks that formed when upwelling mantle formed a major piece of South African continental crust, called the Kaapvaal craton. Cratons are areas of Earth’s crust that have remained stable over time, with the Kaapvaal craton one of the oldest known. Later the gold was weathered and reconcentrated in the Witwatersrand paleolake sediments.
How the Gold is Dated
Kirk is studying the age and extent of the gold deposits around the world for a UA doctoral geosciences degree. He used a rhenium-osmium isotope gold-dating technique, developed by Ruiz at the university’s NTIMS laboratory. Ruiz, Dean of the UA College of Science and Professor of Geosciences, was instrumental in developing the Negative Thermal Ionization Mass Spectrometer (NTIMS) with a grant from the W M Keck Foundation. The laboratory is one of the few of its kind in the world.
‘This is precisely the kind of research that I envisioned when I was building the laboratory,’ Ruiz said. ‘The analytical capabilities of the W M Keck Laboratory are such that we will continue to discover aspects of how the earth worked, questions that previously we could only dream of.’
Using Radioactive Isotopes
Gold and other minerals contain a rare metallic chemical element called rhenium – rhenium-187 is its radioactive form. NTIMS directly dates minerals by counting the number of their rhenium-187 and osmium-187 atoms. Rhenium-187 has a half-life of 45 billion years, or about ten times the age of the solar system, decaying into osmium-187. So by determining the ratio of radioactive rhenium-187 to daughter osmium-187 atoms, scientists can directly calculate when the minerals formed.
Identifying if the Gold Came from The Earth’s Mantle or Crust
‘One of the reasons I think our results are so significant is that the rhenium-osmium system can be used directly on gold, and can tell us if the gold came from the mantle or the crust,’ Kirk said. There is relatively more rhenium than osmium in the Earth’s crust, but relatively more osmium than rhenium in the Earth’s mantle. ‘Witwatersrand has a clear mantle signature,’ Kirk said. ‘It’s possible that this mantle signature is so big because three billion years ago, Earth’s mantle might have been hotter and richer in gold at this particular spot, compared to more recent deposits.’
Summary
People are keenly interested in why South Africa is so rich in gold deposits - since 1886 the Witwatersrand gold fields have yielded a half-trillion dollars-worth. ‘Estimates are, that there’s another half-trillion dollars stilled to be mined, and that’s a lot of money,’ said Kirk.
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