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Questions and Answers
What does the placer model suggest about the origin of gold in the Witwatersrand basin?
What does the placer model suggest about the origin of gold in the Witwatersrand basin?
- Gold was formed from magma cooling in the Earth’s crust.
- Gold is a result of volcanic activity in the basin.
- Gold was formed in situ from hydrothermal processes.
- Gold was eroded from a pre-existing source and transported into the basin. (correct)
Which model posits that gold was introduced long after the sediment was consolidated into rock?
Which model posits that gold was introduced long after the sediment was consolidated into rock?
- Alluvial model
- Sedimentary model
- Placer model
- Hydrothermal model (correct)
What is referred to as a 'smoking gun' in the context of determining the origin of gold?
What is referred to as a 'smoking gun' in the context of determining the origin of gold?
- The temperature of the Earth's crust.
- The age of the host conglomerate rock.
- The determination of when the gold was mineralized. (correct)
- The presence of pyrite in gold deposits.
If gold grains are younger than their host rocks, which model does this support?
If gold grains are younger than their host rocks, which model does this support?
Which mineral is often used as a proxy for dating gold, although its ages can be misleading?
Which mineral is often used as a proxy for dating gold, although its ages can be misleading?
What geological feature is primarily responsible for the preservation of gold-bearing rocks in the Witwatersrand basin?
What geological feature is primarily responsible for the preservation of gold-bearing rocks in the Witwatersrand basin?
What difficulty has arisen in dating gold mineralization in the Witwatersrand basin?
What difficulty has arisen in dating gold mineralization in the Witwatersrand basin?
Which rock type contains most of the gold in the Witwatersrand basin?
Which rock type contains most of the gold in the Witwatersrand basin?
How does the age of gold found in the Witwatersrand basin compare to the age of its host conglomerates in some instances?
How does the age of gold found in the Witwatersrand basin compare to the age of its host conglomerates in some instances?
What is the approximate concentration of gold in the Kimberly Reef within the Evander goldfields?
What is the approximate concentration of gold in the Kimberly Reef within the Evander goldfields?
Which option correctly describes the condition needed for the placer model to be considered valid?
Which option correctly describes the condition needed for the placer model to be considered valid?
According to the placer model, where did the gold grains in the Witwatersrand basin originate?
According to the placer model, where did the gold grains in the Witwatersrand basin originate?
What do both the placer and hydrothermal models agree upon regarding the occurrence of gold?
What do both the placer and hydrothermal models agree upon regarding the occurrence of gold?
What is a major argument presented by the hydrothermal model concerning the source of gold?
What is a major argument presented by the hydrothermal model concerning the source of gold?
Which layer is specifically mentioned as being about 1.5 kilometers beneath the surface?
Which layer is specifically mentioned as being about 1.5 kilometers beneath the surface?
What does the term 'hydrothermal' refer to in the context of the gold occurrence in the Witwatersrand basin?
What does the term 'hydrothermal' refer to in the context of the gold occurrence in the Witwatersrand basin?
What primary sources did ancient river systems bring to the Witwatersrand basin?
What primary sources did ancient river systems bring to the Witwatersrand basin?
How does the presence of rhenium and osmium isotopes help in gold formation studies?
How does the presence of rhenium and osmium isotopes help in gold formation studies?
What ratio is referenced to measure the age of gold formation?
What ratio is referenced to measure the age of gold formation?
What is the relationship between the initial composition of osmium isotopes and the age of the rock?
What is the relationship between the initial composition of osmium isotopes and the age of the rock?
Which feature of the rounded grains challenges the hydrothermal model?
Which feature of the rounded grains challenges the hydrothermal model?
Why does the crust develop higher 187Os/188Os ratios over time compared to the mantle?
Why does the crust develop higher 187Os/188Os ratios over time compared to the mantle?
What method is used to determine the age of gold according to the content?
What method is used to determine the age of gold according to the content?
Which of the following is true regarding the extraction of gold from the Witwatersrand region?
Which of the following is true regarding the extraction of gold from the Witwatersrand region?
What happens to the 187Os/188Os ratios of the mantle compared to those of crustal rocks over millions of years?
What happens to the 187Os/188Os ratios of the mantle compared to those of crustal rocks over millions of years?
What was revealed by radioisotope-dating techniques regarding the Vaal Reef deposits?
What was revealed by radioisotope-dating techniques regarding the Vaal Reef deposits?
Which geologic event may have helped preserve the rocks in the Witwatersrand basin from erosion?
Which geologic event may have helped preserve the rocks in the Witwatersrand basin from erosion?
What does the osmium ratio of the 3-billion-year-old Witwatersrand gold suggest?
What does the osmium ratio of the 3-billion-year-old Witwatersrand gold suggest?
Which type of rocks are indicated to have lower concentrations of osmium compared to Witwatersrand gold?
Which type of rocks are indicated to have lower concentrations of osmium compared to Witwatersrand gold?
What does the evidence suggest about the origin of gold in the Witwatersrand basin?
What does the evidence suggest about the origin of gold in the Witwatersrand basin?
What geological features are suggested to have directed ancient river flow toward the Witwatersrand basin?
What geological features are suggested to have directed ancient river flow toward the Witwatersrand basin?
What is known about the creation of the South African continental crust?
What is known about the creation of the South African continental crust?
What is suggested by the presence of rounded disk-like gold grains and irregular dendritic gold grains?
What is suggested by the presence of rounded disk-like gold grains and irregular dendritic gold grains?
Why doesn't the rhenium-osmium clock get reset by local mobilization of gold?
Why doesn't the rhenium-osmium clock get reset by local mobilization of gold?
What was the significant geological event that occurred about 100 kilometers northeast of the Witwatersrand basin?
What was the significant geological event that occurred about 100 kilometers northeast of the Witwatersrand basin?
How did the collisional event affect the sedimentary layers of the Witwatersrand basin?
How did the collisional event affect the sedimentary layers of the Witwatersrand basin?
What is the implication of having high concentrations of rhenium and osmium in Witwatersrand gold compared to hydrothermal fluids?
What is the implication of having high concentrations of rhenium and osmium in Witwatersrand gold compared to hydrothermal fluids?
What was the effect of the two geological events following the deposition of gold?
What was the effect of the two geological events following the deposition of gold?
Approximately how many years after the emplacement of the Bushveld did the meteor impact occur?
Approximately how many years after the emplacement of the Bushveld did the meteor impact occur?
What feature characterizes the rhenium-osmium clock in relation to gold?
What feature characterizes the rhenium-osmium clock in relation to gold?
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Study Notes
The Witwatersrand Basin
- Located in South Africa, the Witwatersrand Basin holds nearly half the world’s gold.
- It formed approximately 2 billion years ago from a large meteor impact that created the Vredefort dome.
- The collision tilted gold-bearing sedimentary layers, protecting them from erosion.
Gold Formation Theories
- Two primary theories explain the gold concentration in the basin: the Placer Model and the Hydrothermal Model.
- Placer Model: Gold was eroded from an earlier source and transported into the Witwatersrand Basin, where it settled with other sediments forming conglomerates.
- Hydrothermal Model: Gold was introduced to the basin after sediments solidified into rock, carried by hot fluids from deep beneath the Earth's crust.
Evidence for Placer Model
- Osmium Isotopes: Gold samples in the Witwatersrand Basin exhibit osmium isotope fingerprints similar to those found in mantle rocks and komatiites—ancient volcanic rocks, evidence suggesting they were the original source of the gold.
- Sedimentary Features: Studying sedimentary features in the Witwatersrand Basin suggests ancient river systems carried gold and sediments from the north and west, mirroring locations rich in komatiites.
Supporting Evidence
- Gold Morphology: Presence of both rounded, sedimentary gold grains and irregular, hydrothermal gold grains indicate both placer and hydrothermal activity played roles in gold deposition.
- Osmium Concentrations: Osmium concentrations in Witwatersrand gold are similar to those found in komatiites and the mantle, contrasting with typically lower osmium concentrations observed in hydrothermal gold deposits.
Additional Factors on South African Mineral Riches
- Bushveld Igneous Complex: A large upwelling of magma from the mantle near the Witwatersrand Basin resulted in the Bushveld igneous complex, the world’s largest source of platinum and chromium. Fluid movement triggered by this event possibly contributed to short-distance gold redistribution.
Key Takeaways
- The gold in the Witwatersrand Basin was primarily sourced from earlier mantle rocks and komatiites.
- The osmium isotopic signature in the gold is consistent with this origin and supports the Placer Model.
- Other events, such as the Bushveld igneous complex and the Vredefort meteor impact, played important roles in shaping the landscape and mineral distribution of the region.
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