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Questions and Answers
What geological process occurred due to the burial of sedimentary rocks in the Witwatersrand basin?
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?
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?
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?
How did the burial fluids affect the originally sedimentary gold?
How long after the emplacement of the Bushveld did the meteor impact occur?
How long after the emplacement of the Bushveld did the meteor impact occur?
What geological event contributed to the preservation of the gold-bearing sedimentary layers in the Vredefort dome?
What geological event contributed to the preservation of the gold-bearing sedimentary layers in the Vredefort dome?
Which mineral is referred to as 'fool's gold'?
Which mineral is referred to as 'fool's gold'?
How did gold and pyrite end up in the basin according to the placer model?
How did gold and pyrite end up in the basin according to the placer model?
What role do the peculiar geologic structures in the Witwatersrand basin play?
What role do the peculiar geologic structures in the Witwatersrand basin play?
What primarily transformed the deposits of gold and pyrite in the basins into conglomerates?
What primarily transformed the deposits of gold and pyrite in the basins into conglomerates?
What is the approximate age of the impact that created the Vredefort dome?
What is the approximate age of the impact that created the Vredefort dome?
Which material primarily fell out of suspension along with the gold and pyrite?
Which material primarily fell out of suspension along with the gold and pyrite?
What geological feature is associated with the Witwatersrand basin?
What geological feature is associated with the Witwatersrand basin?
What is the primary reason gold grains are difficult to date?
What is the primary reason gold grains are difficult to date?
What isotope is primarily used to date gold grains?
What isotope is primarily used to date gold grains?
At what age does rhenium–187 decay into osmium–187?
At what age does rhenium–187 decay into osmium–187?
What process is used to separate rhenium and osmium from gold grains?
What process is used to separate rhenium and osmium from gold grains?
Why is the finding about the dating of Witwatersrand gold significant?
Why is the finding about the dating of Witwatersrand gold significant?
What is a characteristic of the host conglomerates compared to the gold grains?
What is a characteristic of the host conglomerates compared to the gold grains?
What analytical technique is used to measure the amounts of rhenium and osmium?
What analytical technique is used to measure the amounts of rhenium and osmium?
What must happen to the gold grains after they are separated from the rock for accurate dating?
What must happen to the gold grains after they are separated from the rock for accurate dating?
What is the concentration of gold found in the Kraaipan and Murchison belts?
What is the concentration of gold found in the Kraaipan and Murchison belts?
What primarily contributes to the evolution of crustal rocks from the mantle?
What primarily contributes to the evolution of crustal rocks from the mantle?
What geological features are associated with the low gold concentrations in the granite-greenstone belts?
What geological features are associated with the low gold concentrations in the granite-greenstone belts?
What does the presence of slightly elevated osmium concentrations in the rocks suggest?
What does the presence of slightly elevated osmium concentrations in the rocks suggest?
How does rhenium behave during the melting process of crustal rocks?
How does rhenium behave during the melting process of crustal rocks?
Which process is suggested to have concentrated the available gold in the Witwatersrand?
Which process is suggested to have concentrated the available gold in the Witwatersrand?
What happens to the 187Os/188Os ratio of the crust compared to the mantle over time?
What happens to the 187Os/188Os ratio of the crust compared to the mantle over time?
What is the significance of the concentrations of gold in the Kraaipan and Murchison belts?
What is the significance of the concentrations of gold in the Kraaipan and Murchison belts?
Which layer of the Earth is directly involved in forming crustal rocks?
Which layer of the Earth is directly involved in forming crustal rocks?
What can be inferred about the relationship between mantle and crust over tens of millions of years?
What can be inferred about the relationship between mantle and crust over tens of millions of years?
What does the phrase 'gold-rich parts have already been eroded away' imply?
What does the phrase 'gold-rich parts have already been eroded away' imply?
What characteristic of crustal rocks is associated with the result of partial melting and evolution?
What characteristic of crustal rocks is associated with the result of partial melting and evolution?
Which of the following elements is mentioned in relation to the rock compositions of the belts?
Which of the following elements is mentioned in relation to the rock compositions of the belts?
What might account for younger rocks covering the gold-rich parts in these belts?
What might account for younger rocks covering the gold-rich parts in these belts?
Which mineral is mentioned as being associated with gold grains in the discussed sample?
Which mineral is mentioned as being associated with gold grains in the discussed sample?
How does stream transport affect the shape of pyrite grains?
How does stream transport affect the shape of pyrite grains?
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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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.
