How does gold formed and find its way to the crust?
Gold is formed as the result of molten rock, called magma,
being intruded into solid rock. As the magma cools and
solidifies, water and other volatile substances separate
out from the magma under high pressure. The high pressure
of hot water and steam force open fissures in the
surrounding solid rock, through which these hydrothermal
solutions travel. When the hydrothermal solutions cool,
deposition of material occurs, especially quartz in the
form of quartz veins.
Because gold has a relatively low melting temperature, it
is sometimes carried by these hydrothermal solutions through
the fissures in the rock and solidifies inside the quartz
veins. Thus, the place to look for gold is usually in
quartz veins near the intrusion of a magma body. One example
is the so-called Mother Lode of the Sierra Nevada in
California. If these quartz veins are eroded, the gold
may be found in streams and rivers; this explains why
the 49-ers of the last century were able to pan for gold
in the foothills of the Sierra Nevada.
It needs to be pointed out that gold is not formed within the Earth, but was created by an exploding star that provided the materiasl for our current solar system.
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Update: June 2012