Page 38 - Curriculum Visions Dynamic Book

Gold at the surface
Much of the gold found during a gold rush was collected from river beds rather than from veins of gold dug from mines. Gold occurs in river beds as a result of natural landscape erosion. Where the gold veins are exposed at the land surface, erosion eventually breaks them down into small fragments and carries them away in streams and rivers.
However, because gold is such a heavy element, it is not easily moved and so it accumulates in the river bed. The fragments
of gold are very small, but they are not found among the tiny fragments of other kinds of rock. Rather, they settle out among gravels and larger pieces of rock. This is because sand and pebbles are made of silica, a mineral that is much less dense than gold. A fast-flowing stream will carry small, but denser, particles of heavy gold along with larger, but less dense gravel.
Sluice boxes and dredging
The sluice box, shown below,
is a much more efficient way
of collecting gold. The larger pebbles are screened away and the other fragments washed down a chute. Bars placed along the chute collect the heavy gold, while the rest is washed away. Washing river sediment over wool fleeces is another way to collect gold.
Most placer deposits, especially those worked in navigable rivers and in coastal areas, are now mined on a far larger scale using dredgers.
 A man using a sluice box in Canada at the turn of the century.
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