DEFINITION OF PLACER

A surficial mineral deposit formed by the mechanical

concentration of heavy minerals eroded from weathered

rock masses by currents in streams, along beaches, or by

wind. Many types of valuable minerals including gold, platinum,

cassiterite, ilmenite, zircon, rutile, sapphire, ruby, and

diamond are considerably denser than the average sand or

sediment in an area. When the air or water current moves

these sediments, the heavy minerals tend to be concentrated

by several processes. Denser grains tend to become trapped in

riffles, cracks, and in areas of low flow velocity. Less dense

material also may be winnowed away and removed by the

current, concentrating the heavy minerals. For the placer

minerals to be concentrated, they must also be resistant to

chemical weathering, mechanical abrasion, and fragmentation

during transport. Famous alluvial placer deposits include

the California and Klondike alluvial gold deposits of California,

Alaska, and the Yukon, the sites of the famous gold rushes

of the mid and late 1800s. The largest gold placer deposit

in the world is an ancient, 2.6–2.7-billion-year-old paleoplacer

system found in the Witwatersrand basin of South Africa.

It has accounted for nearly half of the world’s production of

gold. Beach placers include the diamonds found in ancient

offshore beach deposits off southwest Africa, gold in beach

ridges near Nome, Alaska, and Ti-placers that have concentrated

ilmenite along the southeast shores of Madagascar. An

additional class of placers includes colluvial deposits, in

which weathered material accumulates on a slope but less

dense, more weatherable material is preferentially removed

downslope, so the soil profile tends to become concentrated

in the heavy weather-resistant minerals.

Placer mining is one of the oldest forms of mining and

involves the further concentration of economical minerals by

using running water to remove the less dense material. It has

been responsible for the production of much of the world’s

gold, tin, titanium, platinum, diamonds, rubies, emeralds,

and sapphires.

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