In gold-bearing country

In gold-bearing country, prospectors look for gold
where coarse sands and gravel have accumulated and
where “black sands” have concentrated and settled
with the gold. Magnetite is the most common mineral
in black sands, but other heavy minerals such as cassiterite,
monazite, ilmenite, chromite, platinum-group
metals, and some gem stones may be present.
Placer deposits have formed in the same manner
throughout the Earth’s history. The processes of
weathering and erosion create surface placer deposits
that may be buried under rock debris. Although these
“fossil” placers are subsequently cemented into hard

 Laboratory researchers develop new methods of analyzing rocks
for gold content.

 rocks, the shape and characteristics of old river channels
are still recognizable.
The content of recoverable free gold in placer
deposits is determined by the free gold assay method,
which involves amalgamation of gold-bearing
concentrate collected by dredging, hydraulic mining,
or other placer mining operations. In the period when
the price of gold was fixed, the common practice was
to report assay results as the value of gold (in cents or
dollars) contained in a cubic yard of material. Now

results are reported as grams per cubic yard or grams
per cubic meter.