Placer mining
This article needs additional citations for verification. (September 2010) |
Placer mining (/ˈplæsər/)[1] is the mining of stream bed (alluvial) deposits for minerals.[2] This may be done by open-pit (also called open-cast mining) or by various surface excavating equipment or tunneling equipment.
Placer mining is frequently used for precious metal deposits (particularly gold) and gemstones, both of which are often found in alluvial deposits—deposits of sand and gravel in modern or ancient stream beds, or occasionally glacial deposits. 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.
Placer deposits can be as young as a few years old, such as the Canadian Queen Charlotte beach gold placer deposits, or billions of years old like the Elliot Lake uranium paleoplacer within the Huronian Supergroup in Canada.[3]
The containing material in an alluvial placer mine may be too loose to safely mine by tunnelling, though it is possible where the ground is
Etymology
The word placer derives from the
An alternative etymology derives the English word from American Spanish placer (placer, sandbank), from earlier placel, apparently from obsolete Portuguese placel (placer, sandbank).[5]
History
Placers supplied most of the gold for a large part of the ancient world. Hydraulic mining methods such as
In
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).
Deposits
An area well protected from the flow of water is a great location to find gold. Gold is very dense and is often found in a stream bed. Many different gold deposits are dealt with in different ways. Placer deposits attract many prospectors because their costs are very low. 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. They are deposits which have not been washed away yet or been moved. The residual usually lies at the site of the lode. This type of deposit undergoes rock weathering.
Alluvial
Alluvial or eluvial deposits are the most common type of placer gold, and are often the richest. They contain pieces of gold that have been washed away from the lode by the force of water, and have been deposited in sediment in or near watercourses or former watercourses. Therefore, they are mostly found in valleys or flood plains.
Bench
Bench deposits are created when gold reaches a stream bed. 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. Dry stream beds (benches) can be situated far from other water sources and can sometimes be found on mountaintops. Today, many miners focus their activities on bench deposits.
Deep leads
Deep leads are created when a former stream bed is covered over by later sediments or by igneous rock from a volcanic eruption. Examples existed in the goldfields of
Methods
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.
Panning
The simplest technique to extract gold from placer ore is
Rocker
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. It is only capable of processing about 3 or 4 yards of gravel a day.[13] It is more portable and requires less infrastructure than a sluice box, being fed not by a sluice but by hand. The box sits on rockers, which when rocked separates out the gold, and the practice was referred to as "rocking the golden baby". A typical rocker box is approximately 42 inches long, 16 inches wide and 12 inches deep with a removable tray towards the top, where gold is placed.[14] The rocker was commonly used throughout North America during the early gold rush, but its popularity diminished as other methods that could handle a larger volume became more common.[13]
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"). While they are capable of handling a larger volume of material than simpler methods such as the rocker box or gold panning, this can come at the cost of efficiency, since conventional sluice boxes have been found to recover only about 40% of the gold that they process.[15]
The sluice box was used extensively during the
Dry washing
Sluicing is only effective in areas where there is a sufficient water supply, and is impractical in arid areas. Alternative methods developed that used the blowing of air to separate out gold from sand. One of the more common methods of dry washing is the Mexican dry wash. This method involves placing gravel on a riffle board with a bellows placed underneath it. The bellows is then used to blow air through the board in order to remove the lighter material from the heavier gold.[17] The amount of gravel that can be processed using the Mexican dry wash technique varies from 1 1/2 to 4 cubic yards per day, and can be processed at a maximum efficiency of 80%.[18] Another form of dry washing is "winnowing". This method was most commonly used by Spanish miners in America, and only requires a blanket and a box with a screen on the bottom. The material is first filtered through the box so only the finer material is placed onto the blanket. The material on the blanket is then flung into the air so that any breeze can blow away the lighter material and leave the gold behind. While this method is extremely simple and requires very few materials, it is also slow and inefficient.[13]
Trommel
A
Gold dredge
Large-scale sifting of placer gold from large volumes of alluvial deposits can be done by use of mechanical dredges. These dredges were originally very large boats capable of processing massive amounts of material; however, as the gold has become increasingly depleted in the most easily accessible areas, smaller and more maneuverable dredges have become much more common.[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]
Underground mining
In areas where the ground is permanently frozen, such as in Siberia, Alaska, and the Yukon, placer deposits may be mined underground.[20] As the frozen ground is otherwise too hard and firm to mine by hand, historically fires were built so as to thaw the ground before digging it.[20] Later methods involve blasting jets of steam ("points") into the frozen deposits.
Deep leads are accessed by techniques similar to conventional underground mining.
Environmental effects
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. While these recycling and reclamation processes are more common in modern placer mining operations they are still not universally done.
In earlier times the process water was not generally recycled and the spent ore was not reclaimed. The remains of a
Environmental activists describe the
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
Examples
- The Witwatersrand Basin in South Africa is an example of a placer deposit, as it is a 3 billion-year-old, alluvial sedimentary basin containing at least 70 ore minerals.[23]
- The Fraser Canyon Gold Rush prompted in 1858 the incorporation of Colony of British Columbia (1858–1866).
- The alluvial placer mining deposit, which it soon became when 30,000 gold-seekers trekked the region.[24]
See also
References
- ^ "Placer mining (US)". American English Dictionary. Retrieved January 9, 2015.
- ^ "Placer mining | Techniques, Processes & Equipment | Britannica". www.britannica.com.
- ^ A.D. McCracken, E. Macey, J.M. Monro Gray, and G.S. Nowlan (March 1, 2018). "Placer Gold" (PDF). Geological Association of Canada.
{{cite journal}}
: CS1 maint: multiple names: authors list (link) - ^ "Placers etymology and definition". Thefreedictionary.com. Retrieved July 20, 2013.
- ^ "placer in Wiktionary". en.wiktionary.org. Retrieved August 18, 2019.
- ^ "Gulgong – Gold Rush Heritage". www.geomaps.com.au. Retrieved 2023-09-05.
- ^ Design, UBC Web. "Buried Rivers of Gold Heritage Trail | Business & Tourism Creswick Inc". www.creswick.net. Retrieved 2021-01-22.
- ^ "Buried Rivers of Gold | Creswick". Buried Rivers Gold. Retrieved 2023-11-01.
- ^ "The Forest Reefs Gold Field. II". Australian Town and Country Journal. 1900-04-21. p. 41. Retrieved 2023-05-15.
- ^ "The Dark Labyrinth". Buried Rivers Gold. Retrieved 2023-11-01.
- ^ Cohen Duncan, Lynn (1999). Roman Deep Vein Mining.
- ISBN 9780806109091.
- ^ a b c d e f g Basque, G. (1999). Methods of Placer Mining. Surrey, B.C.: Heritage House.
- ^ Basque, G. (1999). Methods of placer mining. Surrey, B.C: Heritage House. p. 41
- .
- ^ Park County Local History Digital Archive
- ^ Lynch, O. (2001). Finding Gold in the Desert: the Art of Dry-Washing.
- ^ Smith, Alfred Merrit (1932). Placer Mining in Nevada. Reno, Nevada: University of Nevada Bulletin.
- ^ Park County Local History Digital Archive
- ^ a b Placer Mining: A Hand-Book for Klondike and Other Miners and Prospectors. Scranton, Pa.: Colliery Engineering Co. 1897. pp. 66–69.
- ISBN 9781596291300.
- ^ "California's First Environmental Law". California Parks and Recreation Department. Retrieved 21 April 2019.
- ^ "Chamber of Mines of South Africa".
- ^ Skagway, Mailing Address: Klondike Gold Rush National Historical Park P. O. Box 517; Us, AK 99840 Phone: 907 983-9200 Contact. "What Was the Klondike Gold Rush? - Klondike Gold Rush National Historical Park (U.S. National Park Service)". www.nps.gov.
{{cite web}}
: CS1 maint: numeric names: authors list (link)
Notes
- platinum-groupmetals 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.
Further reading
- Assembling California, by John McPhee, published 1993 by Farrar, Straus and Giroux, New Jersey[ISBN missing]
- West, Robert C. Colonial Placer Mining in Colombia. Baton Rouge: Louisiana State University Press 1952. [ISBN missing]