Witwatersrand Basin Gold Formation

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Questions and Answers

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?

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?

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?

The hydrothermal model that suggests gold was added after deposition. Signup and view all the answers

Which mineral is often used as a proxy for dating gold, although its ages can be misleading?

Mica Signup and view all the answers

What geological feature is primarily responsible for the preservation of gold-bearing rocks in the Witwatersrand basin?

Impact from a large meteor Signup and view all the answers

What difficulty has arisen in dating gold mineralization in the Witwatersrand basin?

Associated materials give inconsistent ages. Signup and view all the answers

Which rock type contains most of the gold in the Witwatersrand basin?

Conglomerate Signup and view all the answers

How does the age of gold found in the Witwatersrand basin compare to the age of its host conglomerates in some instances?

Gold can be both older and younger than its host conglomerates. Signup and view all the answers

What is the approximate concentration of gold in the Kimberly Reef within the Evander goldfields?

1 kilogram per metric ton Signup and view all the answers

Which option correctly describes the condition needed for the placer model to be considered valid?

The gold must be older than the sediments into which it was eroded. Signup and view all the answers

According to the placer model, where did the gold grains in the Witwatersrand basin originate?

From river transport of sediments Signup and view all the answers

What do both the placer and hydrothermal models agree upon regarding the occurrence of gold?

Gold is concentrated in specific geological structures. Signup and view all the answers

What is a major argument presented by the hydrothermal model concerning the source of gold?

Gold-rich fluids added gold after sediment consolidation. Signup and view all the answers

Which layer is specifically mentioned as being about 1.5 kilometers beneath the surface?

The Kimberly Reef Signup and view all the answers

What does the term 'hydrothermal' refer to in the context of the gold occurrence in the Witwatersrand basin?

It indicates the influence of hot mineral-rich fluids. Signup and view all the answers

What primary sources did ancient river systems bring to the Witwatersrand basin?

Gold and other minerals Signup and view all the answers

How does the presence of rhenium and osmium isotopes help in gold formation studies?

They help identify the source of the gold Signup and view all the answers

What ratio is referenced to measure the age of gold formation?

187Os/188Os Signup and view all the answers

What is the relationship between the initial composition of osmium isotopes and the age of the rock?

Older rocks have higher ratios Signup and view all the answers

Which feature of the rounded grains challenges the hydrothermal model?

Their shape caused by abrasion Signup and view all the answers

Why does the crust develop higher 187Os/188Os ratios over time compared to the mantle?

Rhenium goes more readily into the melt Signup and view all the answers

What method is used to determine the age of gold according to the content?

Analyzing the isotopic ratios of Rhenium and Osmium Signup and view all the answers

Which of the following is true regarding the extraction of gold from the Witwatersrand region?

Intensive exploration has not revealed the primary source Signup and view all the answers

What happens to the 187Os/188Os ratios of the mantle compared to those of crustal rocks over millions of years?

They remain nearly constant. Signup and view all the answers

What was revealed by radioisotope-dating techniques regarding the Vaal Reef deposits?

They are older than the conglomerate rocks. Signup and view all the answers

Which geologic event may have helped preserve the rocks in the Witwatersrand basin from erosion?

The Vredefort meteor impact. Signup and view all the answers

What does the osmium ratio of the 3-billion-year-old Witwatersrand gold suggest?

It may relate to the mantle and komatiites. Signup and view all the answers

Which type of rocks are indicated to have lower concentrations of osmium compared to Witwatersrand gold?

Hydrothermal gold deposits. Signup and view all the answers

What does the evidence suggest about the origin of gold in the Witwatersrand basin?

It came from highlands with komatiites and mantle rocks. Signup and view all the answers

What geological features are suggested to have directed ancient river flow toward the Witwatersrand basin?

Greenstone belts. Signup and view all the answers

What is known about the creation of the South African continental crust?

It was produced by volcanic eruptions and granitic intrusions. Signup and view all the answers

What is suggested by the presence of rounded disk-like gold grains and irregular dendritic gold grains?

The differing morphologies indicate different origins of gold. Signup and view all the answers

Why doesn't the rhenium-osmium clock get reset by local mobilization of gold?

Rhenium and osmium are generally very insoluble and remain with undissolved gold. Signup and view all the answers

What was the significant geological event that occurred about 100 kilometers northeast of the Witwatersrand basin?

Formation of the Bushveld igneous complex. Signup and view all the answers

How did the collisional event affect the sedimentary layers of the Witwatersrand basin?

It resulted in the tilting of the gold-bearing layers. Signup and view all the answers

What is the implication of having high concentrations of rhenium and osmium in Witwatersrand gold compared to hydrothermal fluids?

Most rhenium and osmium remains with undissolved gold. Signup and view all the answers

What was the effect of the two geological events following the deposition of gold?

They contributed to the preservation and movement of gold deposits. Signup and view all the answers

Approximately how many years after the emplacement of the Bushveld did the meteor impact occur?

30 million years。 Signup and view all the answers

What feature characterizes the rhenium-osmium clock in relation to gold?

It is unaffected by local gold mobilization due to concentration differences. Signup and view all the answers

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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Description

Explore the rich geological history of the Witwatersrand Basin in South Africa, a site that contributes nearly half of the world's gold. Learn about the formation theories, including the Placer and Hydrothermal Models, and the unique evidences supporting each theory. Dive into how meteor impacts shaped this remarkable natural resource.