Witwatersrand Basin & Gold Origins
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Questions and Answers

What geological process occurred due to the burial of sedimentary rocks in the Witwatersrand basin?

  • Formation of diamond deposits
  • Cooling of rocks at high temperatures
  • Compaction and cementation of sediments (correct)
  • Formation of volcanic rocks

    • What was the impact of the meteor that struck the Witwatersrand basin?

  • It triggered volcanic activity.
  • It lifted rocks from the lower crust and upper mantle. (correct)
  • It created a large oceanic trench.
  • It caused a mass extinction event.

    • What likely contributed to the preservation of gold-bearing sedimentary layers?

  • The tilting of these layers due to the meteor impact (correct)
  • The formation of new volcanic layers above them
  • The introduction of seawater during burial
  • The erosion caused by glacial movements

    • How did the burial fluids affect the originally sedimentary gold?

    <p>They dissolved some gold but left some untouched.</p> Signup and view all the answers

    How long after the emplacement of the Bushveld did the meteor impact occur?

    <p>About 30 million years</p> Signup and view all the answers

    What geological event contributed to the preservation of the gold-bearing sedimentary layers in the Vredefort dome?

    <p>Large meteor impact</p> Signup and view all the answers

    Which mineral is referred to as 'fool's gold'?

    <p>Pyrite</p> Signup and view all the answers

    How did gold and pyrite end up in the basin according to the placer model?

    <p>Carried by rivers</p> Signup and view all the answers

    What role do the peculiar geologic structures in the Witwatersrand basin play?

    <p>They explain the mineralogical riches in the region.</p> Signup and view all the answers

    What primarily transformed the deposits of gold and pyrite in the basins into conglomerates?

    <p>Density differences of materials</p> Signup and view all the answers

    What is the approximate age of the impact that created the Vredefort dome?

    <p>2 billion years ago</p> Signup and view all the answers

    Which material primarily fell out of suspension along with the gold and pyrite?

    <p>Quartz pebbles</p> Signup and view all the answers

    What geological feature is associated with the Witwatersrand basin?

    <p>Conglomerates</p> Signup and view all the answers

    What is the primary reason gold grains are difficult to date?

    <p>They need to be physically separated from their host rock.</p> Signup and view all the answers

    What isotope is primarily used to date gold grains?

    <p>Rhenium–187</p> Signup and view all the answers

    At what age does rhenium–187 decay into osmium–187?

    <p>3.01 billion years</p> Signup and view all the answers

    What process is used to separate rhenium and osmium from gold grains?

    <p>Dissolution in strong acids</p> Signup and view all the answers

    Why is the finding about the dating of Witwatersrand gold significant?

    <p>It supports the placer model of the gold’s origin.</p> Signup and view all the answers

    What is a characteristic of the host conglomerates compared to the gold grains?

    <p>They are older than the gold grains.</p> Signup and view all the answers

    What analytical technique is used to measure the amounts of rhenium and osmium?

    <p>Mass spectrometry</p> Signup and view all the answers

    What must happen to the gold grains after they are separated from the rock for accurate dating?

    <p>They must be dissolved in strong acids.</p> Signup and view all the answers

    What is the concentration of gold found in the Kraaipan and Murchison belts?

    <p>Slightly elevated compared to normal crust</p> Signup and view all the answers

    What primarily contributes to the evolution of crustal rocks from the mantle?

    <p>Partial melting of the mantle</p> Signup and view all the answers

    What geological features are associated with the low gold concentrations in the granite-greenstone belts?

    <p>Erosion or younger rocks covering the area</p> Signup and view all the answers

    What does the presence of slightly elevated osmium concentrations in the rocks suggest?

    <p>They cannot provide conclusive information about gold content</p> Signup and view all the answers

    How does rhenium behave during the melting process of crustal rocks?

    <p>It goes readily into the melt</p> Signup and view all the answers

    Which process is suggested to have concentrated the available gold in the Witwatersrand?

    <p>Wind and wave action during deposition</p> Signup and view all the answers

    What happens to the 187Os/188Os ratio of the crust compared to the mantle over time?

    <p>It diverges and increases</p> Signup and view all the answers

    What is the significance of the concentrations of gold in the Kraaipan and Murchison belts?

    <p>They are not rich enough for economic interest</p> Signup and view all the answers

    Which layer of the Earth is directly involved in forming crustal rocks?

    <p>The mantle</p> Signup and view all the answers

    What can be inferred about the relationship between mantle and crust over tens of millions of years?

    <p>They diverge rapidly in 187Os/188Os ratios</p> Signup and view all the answers

    What does the phrase 'gold-rich parts have already been eroded away' imply?

    <p>Previous geological processes removed valuable gold</p> Signup and view all the answers

    What characteristic of crustal rocks is associated with the result of partial melting and evolution?

    <p>Increased 187Os/188Os ratios</p> Signup and view all the answers

    Which of the following elements is mentioned in relation to the rock compositions of the belts?

    <p>Osmium</p> Signup and view all the answers

    What might account for younger rocks covering the gold-rich parts in these belts?

    <p>Depositional processes over time</p> Signup and view all the answers

    Which mineral is mentioned as being associated with gold grains in the discussed sample?

    <p>Pyrite</p> Signup and view all the answers

    How does stream transport affect the shape of pyrite grains?

    <p>They are shaped by abrasion</p> Signup and view all the answers

    Study Notes

    Witwatersrand Basin

    • The Witwatersrand Basin is located in South Africa.
    • The basin is a product of a large meteor impact that occurred about 2 billion years ago.
    • The impact created the Vredefort dome and tilted gold-bearing sedimentary layers.
    • The basin is host to a number of unique geological structures.
    • The tilting of the sedimentary layers helped to protect the rocks from erosion.
    • The basin is rich in gold and pyrite deposits.

    Gold Origins

    • The placer model explains the origin of gold in the Witwatersrand Basin.
    • The model suggests that rivers carried gold and pyrite into the basin.
    • Due to their high density, gold and pyrite settled out of the water along with larger quartz pebbles.
    • These deposits were eventually transformed into the conglomerates being mined today.

    Gold Dating

    • Gold grains are difficult to date directly.
    • Scientists use rhenium–187 (187Re) to date gold.
    • Rhenium-187 decays into osmium–187 (187Os) at a known rate.
    • The dating process involves separating gold grains from the conglomerate, dissolving the grains in strong acids, and measuring the relative amounts of rhenium–187 and osmium–187.
    • This method revealed that Witwatersrand gold is older than the basin sediments, supporting the placer model.
    • The gold was mineralized before the basin sediments were deposited.

    Hydrothermal Model

    • The hydrothermal model offers an alternative explanation for the gold.
    • It suggests that hot fluids transported gold from the mantle to the crust.
    • This model struggles to explain the presence of rounded pyrite grains associated with gold.

    Witwatersrand Basin Development

    • The basin underwent two significant events.
    • The first event involved the deposition of sedimentary rocks with gold-rich conglomerates.
    • The second event involved a meteor impact about 30 million years after the first event.
    • The impact was larger than the one that created the Chicxulub crater in Mexico.
    • The impact brought rocks from the lower crust and upper mantle to the surface.
    • The impact tilted the sedimentary layers of the basin.
    • The burial of the sediments caused pressure and heat, squeezing water out and solidifying the rock.
    • The released fluids may have dissolved and redistributed some of the gold.

    Gold Concentration

    • The Kraaipan and Murchison belts contain only slightly elevated concentrations of gold compared to normal crust.
    • The Witwatersrand basin has a high concentration of gold.
    • This difference suggests that the gold in the Kraaipan and Murchison belts may have been eroded away, covered by younger rocks, or not concentrated as effectively as in the Witwatersrand basin.

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    The Origin of Gold in South Africa (PDF)

    Description

    Explore the geological features and origins of gold in the Witwatersrand Basin of South Africa. This quiz covers the basin's formation due to a meteor impact, the placer model for gold deposition, and the methods used to date gold deposits. Test your knowledge on these unique aspects of geology and resource study.

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