Placer mining
The metal or gemstones, having been moved by stream flow from an original source such as a vein, are typically only a minuscule portion of the total deposit.
Since gems and heavy metals like gold are considerably denser than sand, they tend to accumulate at the base of placer deposits.
[4] The word in Spanish is thus derived from placea and refers directly to an alluvial or glacial deposit of sand or gravel.
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 aqueducts were used to work the alluvial gold deposits through the first century AD.
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 Yukon, Alaska and British Columbia).
Deep leads are created when a former stream bed is covered over by later sediments or by igneous rock from a volcanic eruption.
[10] A number of methods are used to mine placer gold and gems, both in terms of extracting the minerals from the ground, and separating it from the non-gold or non-gems.
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.
[13] A rocker box (or "cradle") is capable of greater volume than a gold pan; however, its production is still limited when compared to other methods of placer mining.
The grizzly is a set of parallel bars placed at a 45-degree angle over the main sluice box, which filter out larger material.
[13] These smaller dredges commonly operate by sucking water and gravel up through long hoses using a pump, where the gold can then be separated using more traditional methods such as a sluice box.
[13] In areas where the ground is permanently frozen, such as in Siberia, Alaska, and the Yukon, placer deposits may be mined underground.
Although this procedure is not required, the process water may be continuously recycled and the ore from which the sought-after minerals have been extracted ("the tailings") can be reclaimed.
The remains of a Roman alluvial gold mine at Las Médulas, Spain are so spectacular as to justify the site being designated UNESCO World Heritage status.
The methods used by the Roman miners are described by Pliny the Elder in his work Naturalis Historia published in about 77 AD.
[21] Most modern placer mines use settling ponds, if only to ensure that they have sufficient water to run their sluicing operations.
In California, from 1853 to 1884, "hydraulicking" of placers removed an enormous amount of material from the gold fields, material that was carried downstream and raised the level of portions of the Central Valley by some seven feet in affected areas and settled in long bars up to 20 feet thick in parts of San Francisco Bay.
[22] Despite environmental concerns, placer mining is still important in areas such as the Yukon, and it has considerable support, with local businesses often displaying signs to that effect.
