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 |
