Placer mining
From Wikipedia, the free encyclopedia
Placer mining (pronounced "plass-er") refers to the mining of alluvial deposits for minerals. This may be done by open-pit or open-cast mining or by various forms of tunneling into ancient riverbeds. Excavation may be accomplished using water pressure (hydraulic mining), surface excavating equipment or tunneling equipment.
The name derives from Spanish, placer, meaning "sand bank" and refers to 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. The containing material may be too loose to safely mine by tunneling. Where water under pressure is available, water under pressure may be used to mine, move, and separate the precious material from the deposit.
Placers supplied most of the gold for a large part of the ancient world. (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, if ever, found with gold in reef or vein deposits - in the modern era only in the Tulameen River in British Columbia, Canada and on the Amur River in the Russian Far East.) In North America, placer mining was famous in the context of several gold rushes, particularly the California Gold Rush, the Fraser Canyon Gold Rush and the Klondike Gold Rush. It is a source to this day of gems in Myanmar and Sri Lanka, and of gold in the Yukon, Alaska and British Columbia.
The simplest technique to extract gold from placer ore is panning. In panning, some mined ore is placed in a large metal 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 ore 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.
The same principle may be employed on a larger scale by constructing a short sluice box, with barriers along the bottom 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 sediment into the device. Similar in principle to a sluice is a rocker, a cradle-like piece of equipment that could be rocked like a cradle to sift sands through screens, which was introduced by Chinese miners in British Columbia . Another Chinese technique was the use of blankets to filter sand and gravels, catching all the 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.
Environmental activists describe the hydraulic mining form of placer mining as environmentally destructive because of the large amounts of silt that it adds to previously clear running streams. Most placer mines today 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 the Central Valley by some seven feet in some areas and settled in a huge layer at the bottom of San Francisco Bay. The process raised an opposition calling themselves the "Anti-Debris Association". In January of 1884, a United States District Court banned the flushing of debris into streams, and the hydraulic mining mania in California's gold country came to an end.
[edit] References
- Assembling California, by John McPhee, published 1993 by Farrar, Straus and Giroux, New Jersey
[edit] See also
| Mining techniques | |
|---|---|
| Surface mining: | Open-pit mining, Quarrying, Strip mining, Placer mining, Mountaintop removal |
| Sub-surface mining: | Drift mining, Slope mining, Shaft mining, Hard rock mining, Borehole mining |
