Placer deposits are collections of some mineral existing in discrete particles, mixed with sand, gravel, and other forms of eroded rock. Some of the minerals most commonly found in placer deposits are diamond, gold, platinum, magnetite, rutile, monazite, and cassiterite. These deposits are formed by the action of wind, water, and chemical changes on more massive beds of the mineral. Placer deposits can be classified according to the method by which they are produced. Some examples include stream placers, eolian placers (formed by the action of winds), beach placers, and moraine placers (formed by glacial action).
Placer deposits are important sources of some minerals that are more dense than the sand or gravel around them. For example, the gold in a placer deposit tends to settle out and accumulate in a stream as surrounding materials wash away.
With few exceptions, the mining of placer deposits is carried out by traditional surface (compared to underground) mining. Four general methods are in use. The oldest and perhaps most familiar to the general public is the hand method. Placerville, California, commemorates by its name the type of mining that was done during the Gold Rush of 1849. As the worker rocks his or her box or pans back and forth, clay and sand are washed away, and the heavier gold (or other minerals) settles to the bottom. This method is less used today since deposits with which it works best have been largely depleted.
Artificial streams of water are used in a second method of placer mining--hydraulic mining. In this process, intense streams of water are directed with large hoses at a placer deposit. The water washes away the overlying sand and clay, leaving behind the desired mineral.
A number of variations of this procedure exists. In some cases, a natural stream is diverted from its course and directed against a placer deposit. The action of the stream substitutes for that of a hose in the traditional hydraulic method. In another variation, a stream may be dammed rather than diverted. When the dam is broken, the pennedup water is released all at once with a force that can tear apart a placer deposit. This technique is known as "booming" a deposit and although it is no longer used in the United States, the method is used around the world.
An increasingly popular approach to placer mining makes use of familiar surface mining machinery used to extract coal and other minerals. This machinery is often very large and efficient. It removes overburden , extracts mineral-containing earths with streams of water and then separates and processes the extracted material by gravity methods.
A fourth method of mining is used with deposits underwater, as in a lake or along the seashore. In this process, a large machine somewhat similar to a surface mining excavator is placed on a barge. The machine then scoops materials off the lake, stream, or ocean bottom and carries them to a processing area on another part of the barge. This type of dredging machine might be thought of as a very large, mechanized version of the old prospector's gold pan and rocker box.
Placer mining poses two kinds of environmental problems. The first is one that is common to all types of surface mining, the disturbance of land. Federal law now requires that land disturbed by surface mining such as placer mining be restored to a condition that approximates its original state.
The extensive use of water during placer mining is a second source of concern. By its nature, placer mining produces huge flows of water that have become contaminated with mud, sand, and other suspended solids. As this water is dumped into natural waterways, these solids may damage aquatic plant and animal life and, as they settle out, create new navigational hazards. In the United States, the Clean Water Acts and other environmental laws carry provisions to reduce the potential harm posed by placer mining.
[David E. Newton ]
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