GOLD MININGAlluvial or "placer" gold comes from eroded hard-rock (lode) sources. Rivers and glaciers flowing over gravels have washed and sorted them, concentrating the heavy metal gold in specific layers. This often makes placer gold deposits much richer than their hard-rock sources. While "hard rock" gold is usually mined by means of mining hardware, using explosives and "cyanidation", alluvial gold can be mined by the process of digging it up and washing with special equipment. Therefore, alluvial deposits usually have a lower mining and exploration costs than hard-rock deposits.
ALLUVIAL GOLDTypical locations for alluvial deposits are on the inside bends of rivers and creeks, in natural hollows, at the base of a waterfall, within sand dunes, beach profiles or in gravel beds. Alluvial "placers" are formed by the deposition of dense particles at sites where water velocity remains low than which is required to transport them further.
To form a placer deposit, the particles sought after must show a marked density contrast with the surrounding material, which is transported away from the trap site. Only if the deposit is winnowed in this way can the minerals be concentrated to economic levels.
All methods of placer deposit mining use gravity as the basic sorting force. The mining process uses a drum which is composed of a slightly-inclined rotating metal tube (the "scrubber section") with a screen at its discharge end. Lifter bars, sometimes in the form of bolted-in angle irons, are attached to the interior of the scrubber section. The mineral containing sand that passes through the screen is then further concentrated in smaller devices such as sluices and jigs. The larger pieces of sand that do not pass through the screen are carried to a dump by gravity flow over a discharging chute.
The sand is then further concentrated in smaller devices. The placer gold is finally obtained, containing gold and fine mineral granules. It is sent to the refining plant, where the gold is melted out by using high temperatures. .