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Placer mining (/[invalid input: 'icon']ˈplæsər/ or /ˈpleɪsər/)^[1] is the mining of alluvial deposits for minerals. This may be done by open-pit (also called open-cast mining) or by various surface excavating equipment or tunneling equipment.

The name derives from Spanish, placer, meaning "sandbank." It refers to mining the precious metal deposits (particularly gold and gemstones) found in alluvial deposits—deposits of sand and gravel in modern or ancient stream beds. The metal or gemstones, having been moved by stream flow from an original source such as a vein, is typically only a minuscule portion of the total deposit. Since gems and heavy metals like gold are considerably more dense than sand, they tend to accumulate at the base of placer deposits.

The containing material may be too loose to safely mine by tunneling. Where water under pressure is available, it may be used to mine, move, and separate the precious material from the deposit, a method known as hydraulic mining, hydraulic sluicing or hydraulicking.

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Methods

A number of methods are used to mine placer gold.

Panning

The simplest technique to extract gold from placer ore is panning. In panning, some mined ore is placed in a large metal or plastic pan, combined with a generous amount of water, and agitated so that the gold particles, being of higher density than the other material, settle to the bottom of the pan. The lighter gangue material such as sand, mud and gravel are then washed over the side of the pan, leaving the gold behind. Once a placer deposit is located by gold panning, the miner usually shifts to equipment that can treat volumes of sand and gravel more quickly and efficiently.

Sluice box

The same principle may be employed on a larger scale by constructing a short sluice box, with barriers along the bottom called riffles to trap the heavier gold particles as water washes them and the other material along the box. This method better suits excavation with shovels or similar implements to feed ore into the device. Sluice boxes can be as short as a few feet, or more than ten feet (a common term for one that is over six feet +/- is a "Long Tom"). Similar in principle to a sluice is a rocker, a cradle-like piece of equipment that could be rocked to sift sands through screens, which was introduced by Chinese miners in British Columbia and Australia, where the practice was referred to as "rocking the golden baby". Another Chinese technique was the use of blankets to filter sand and gravels, catching fine gold in the fabric's weave, then burning the blankets to smelt the gold. Chinese were noted for the thoroughness of their placer extraction techniques, which included hand-washing of individual rocks as well as the complete displacement of streambeds and advanced flume and ditching techniques which became copied by other miners.

Trommel

A trommel 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 iron, are attached to the interior of the scrubber section. The ore is fed into the elevated end of the trommel. Water, often under pressure, is provided to the scrubber and screen sections and the combination of water and mechanical action frees the valuable minerals from the ore. The mineral bearing ore that passes through the screen is then further concentrated in smaller devices such as sluices and jigs. The larger pieces of ore that do not pass through the screen can be carried to a waste stack by a conveyor.

Deposits

A well protected area from the flow of water is a great location to find gold (McCracken). Gold is very heavy and is often found in a stream bed (McCracken). Many different gold deposits are dealt with in different ways. Placer deposits attract many prospectors because their costs are very low (Lawler). There are many different places gold could be placed, such as a residual, alluvial, and a bench deposit.

Residual

Residual deposits are more common where there has been weathering on rocks and where there hasn’t been water (Lawler). They are deposits which have not been washed away yet or been moved. The residual usually lies at the site of the lode (McCracken). This type of deposit undergoes rock weathering ("Types of Placers," ).

Alluvial

Alluvial or eluvial deposits sometimes have the largest gold deposit and are very common (Lawler). This deposit is created when a force of nature moves or washes the gold away, but it doesn’t go into a stream bed (McCracken). It contains pieces of ore that have been washed away from the lode. Alluvial deposits are the most common type of placer gold (McCracken). This type of deposit occurs mostly in valleys ("Types of Placers,").

Bench

Bench deposits are created when gold reaches a stream bed (McCracken). Gold accumulations in an old stream bed that are high are called bench deposits. They can be found on higher slopes that drain into valleys (Lawler). Dry stream beds (benches) can be situated far from other water sources and can sometimes be found on mountain tops. Today, many miners focus their activities on bench deposits.

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References

Assembling California, by John McPhee, published 1993 by Farrar, Straus and Giroux, New Jersey

^ Oxford English Dictionary

External links

Historical Bathymetric Changes in Suisun Bay, 1867-1990. includes mapes of bottom depths for 1867, 1887, and the change between these dates. on line at: http://sfbay.wr.usgs.gov/sediment/suisunbay/1867to1887bathy.html

[1] Oxford English Dictionary

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 The containing material may be too loose to safely mine by tunneling. Where water under pressure is available, it may be used to mine, move, and separate the precious material from the deposit, a method known as [[hydraulic mining]], hydraulic [[Sluice#Placer_mining_applications|sluicing]] or hydraulicking.
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-==History==
-[[Image:Placermine.jpg|thumb|300px|left|A sluice box used in placer mining.]]
-Placers supplied most of the gold for a large part of the ancient world. Hydraulic mining methods such as [[hushing]] were used widely by the Romans across their empire, but especially in the gold fields of northern [[Spain]] after its conquest by [[Augustus]] in 25 BC. One of the largest sites was at [[Las Médulas]], where seven 30 mile long [[aqueduct]]s were used to work the [[alluvial]] gold deposits through the first century AD.  (Inclusions of [[platinum]]-group metals in a very large proportion of gold items indicate that the gold was largely derived from placer or alluvial deposits.  [[Platinum group]] metals are seldom found with gold in hardrock reef or vein deposits.)  In [[North America]], placer mining was famous in the context of several [[gold rush]]es, particularly the [[California Gold Rush]], the [[Fraser Canyon Gold Rush]] and the [[Klondike Gold Rush]].  Placer mining continues in many areas of the world as a source of diamonds, industrial minerals and metals, gems (in [[Myanmar]] and [[Sri Lanka]]), platinum, and of gold (in the [[Yukon]], [[Alaska]] and [[British Columbia]]).
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 ==Methods==