Alluvial Placers
Alluvial placers are those formed in river or stream sediments. Typical locations for alluvial gold placer deposits are on the inside bends of rivers and creeks; in natural hollows; at the break of slope on a stream; the base of an escarpment, waterfall or other barrier.
Alluvial placers are formed by the deposition of dense particles at a site where water velocity remains below that required to transport them further.
To form a placer deposit, the particles desired must show a marked density contrast with the gangue material, which is able to be transported away from the trap site. Only if the deposit is winnowed in this way can the minerals be concentrated to economic levels.
Read more about this topic: Placer Deposit