The word placer, as will be found throughout here, is derived from the same Spanish word which means "sandbank". It specifically refers to an alluvial deposit of detrital material, such as gravel, which contains particles of precious chemical elements.
The term "placer gold", therefore, refers to gold that has formed in rocks moved and placed on stream beds by some geological forces and by the action of water. Lode gold tends to erode from its source, distributing itself naturally among other rocks that have been subjected to similar geological forces. This results to the formation of a secondary deposit.
Thus the mining of alluvial deposits for gold and other precious metal deposits is called "placer mining". Placer mining may be done through a number of tunneling procedures into riverbeds. There also are the open-cast mining and hydraulic mining. In the former, placer mining is done by open-pit; in the latter, water pressure is used for excavation.
There are three placer mining methods used to mine placer gold:
This method, which involves the use of a pan, is the oldest and simplest way to extract gold from a placer deposit. In this method, mined ore is placed in a large pan (made either of plastic or metal) and poured with a liberal amount of water; it is then agitated. The gold particles, having higher density than the other materials (examples, mud, sand and gravel; also, gold is about nineteen times heavier than water), settle to the bottom of the pan, while the lighter materials are washed over the side.
This method uses the same principle as that in gold panning, only on a larger scale. In this method, a short sluice box is used. The box is constructed with barriers along its bottom, so that the gold particles are trapped as all materials are washed by water. The sluice box method is best suited for excavation in which certain implements, such as shovels, are used to feed ore into the box.
This method involves the use of a screened cylinder to separate materials by size (trommel is Dutch word for "drum"). A trommel specifically consists of a rotating metal tube that is slightly tilted, with a screen at the discharge end. Attached to the inside part of the metal tube are lifter bars. Ore is fed into the trommel through its elevated end. Pressurized water is supplied to the tube and the screen sections. Valuable minerals from the ore are separated by the combination of water and mechanical action. The small pieces of ore bearing the valuable minerals pass through the screen and are concentrated further in sluices. The larger ones (those that do not pass through the screen) are moved to a waste stack using a conveyor.
Today, placer mining goes on in many parts of the world as a source of gems and industrial metals and minerals. This is true in countries like Sri Lanka and Myanmar. Placer mining for placer gold continues in British Columbia, the Yukon, and especially in Alaska.
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