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Gold Formation

 

Because gold is very stable over a wide range of conditions, it is very widespread in the earth’s crust. While its overall concentration is very low (about 5 milligrams per ton of rock), rich concentrations of gold, forming ore deposits, are known throughout the world. The well-known saying amongst prospectors that "gold is where you find it" suggests its occurrence is unpredictable, but it is now known that certain geological environments favor gold’s formation.

A popular misconception is that natural gold has cooled from a molten state. In fact, gold is transported though the Earth’s crust dissolved in warm to hot salty water. These fluids are generated in huge volumes deep in the Earth’s crust as water-bearing minerals dehydrate during metamorphism. Any gold present in the rocks being heated and squeezed is sweated out and goes into solution as complex ions. In this form, dissolved gold, along with other elements such as silicon, iron and sulphur, migrates wherever fractures in the rocks allow the fluids to pass. This direction is generally upwards, to cooler regions at lower pressures nearer the Earth’s surface. Under these conditions, the gold eventually becomes insoluble and begins to crystallise, most often enveloped by masses of white silicon dioxide, known as quartz. This association of gold and quartz forms one of the most common types of "primary gold deposits".

Veins and reefs of gold-bearing quartz can occur in many types of rock, for example around granites, in volcanic rocks or in regions of black slate, but in most cases these host rocks are not the immediate source of the gold.

Gold deposits have formed at many different times during Earth’s history. For example, those in Western Australia are believed to have formed about 2400 million years ago, during a period of intense metamorphism and intrusion of igneous rocks. The gold-bearing quartz reefs in Victoria are significantly younger, about 400 million years, but also owe their origin to a period of intense metamorphism in the region.

As chemical weathering and erosion gradually break down the host rocks and lower the land surface, the quartz and gold veins are eventually exposed to the atmosphere. The veins provide far more resistance to chemical attack than the surrounding rocks, so that mechanical weathering is required to fragment the quartz, thereby releasing the gold. Because they are relatively heavy, particles of gold are more difficult to move and so become naturally concentrated in the soil or in adjacent gullies or streambeds. These concentrations are known as alluvial or placer deposits and have yielded incredible riches on some goldfields, such as those in California and central Victoria.

Alluvial deposits take many forms, including sands and gravels in the beds of modern-day streams, in old river valleys buried under lava flows or perched on hilltops due to uplift of the land surface. The terms shallow and deep leads are used in Victoria for gold-bearing gravels covered by younger sedimentary layers or lava flows. These were especially important in the Ballarat district. Because of its resistance to chemical attack, gold can be recycled from one type of alluvial deposit to another.

Credits: By Dr Bill Birch, Senior Curator, Geosciences, Museum Victoria