Placer mining is the mining of alluvial sediments for minerals. This may be through by open-pit (also known as open-cast mining) or by a variety of forms of tunneling into antique riverbeds. Excavation might be achieved using water pressure (hydraulic mining), layer digging out equipment or tunneling equipment.

The name comes from Spanish, placera, meaning "alluvial sand." It means that mining the precious metal deposits (mainly gold and gemstones) found in alluvial deposits sediments of sand and gravel in modern or very old stream beds. The metal or gemstones, having been moved by torrent flow from a unique source like a vein, is characteristically only a minuscule part of the total deposit. The containing material might be so loose to securely mine by tunneling. Where water under pressure is obtainable, water under pressure might be used to mine, move, and separate the valued material from the deposit.

History

Placers supplied most of the gold for a hefty portion of the prehistoric world. Hydraulic mining techniques like hushing were used broadly by the Romans athwart their empire, but particularly in the gold fields of northern Spain after its take-over by Augustus in 25 BC. One of the main sites was at Las Medulas, where seven 30 mile lengthy aqueduct were used to work the alluvial gold deposits during the first century AD. In North America, placer mining was well-known in the context of more than some gold rushes, chiefly the California Gold Rush, the Fraser Canyon Gold Rush and the Klondike Gold Rush. Placer mining carries on in a lot of parts of the world as a basis of diamonds, industrial minerals and metals, gems (in Myanmar and Sri Lanka), platinum, and of gold (in the Yukon, Alaska and British Columbia).

Methods

The simplest method to take out gold from placer ore is panning. In panning, few mined ore is placed in a big metal or plastic pan, combined with a generous quantity of water, and restless so that the gold materials, being of higher density than the other material, settle to the underneath of the pan. The lighter gangue material like sand, mud and gravel are then washed above the side of the pan, leaving the gold at the rear. Once a placer deposit is located by gold panning, the miner normally shifts to equipment that may treat volumes of sand and gravel more rapidly and proficiently.

A trommel is composed of a faintly leaning rotating metal tube (the 'scrubber section') with a screen at its release end. Lifter bars, at times in the form of bolted in angle iron, are affixed to the interior of the scrubber part. The ore is fed into the high end of the trommel. Water (frequently under pressure) is given to the scrubber and screen parts and the mixture of water and mechanical action frees the precious minerals from the ore. The mineral consisting ore that passes throughout the screen is then more concentrated in smaller devices like sluices and jigs. The bigger pieces of ore that do not pass throughout the screen can be carried to a waste stack by a conveyor.