Placer mining
Encyclopedia
Placer mining is the mining of alluvial
deposits for mineral
s. This may be done by open-pit (also called open-cast mining
) or by various surface excavating equipment or tunneling equipment.
The name is sometimes said to derive from Spanish
placer, meaning "to please," (pleasure) because it is easy mining as compared with others. However, the origin of the word is actually from Spanish
, placer meaning shoal or alluvial/sand deposit, from Catalan
placer, (shoal), from plassa, (place) from Medieval Latin
placea (place) the origin word for place and plaza in English. The word in Spanish is thus ultimately derived from placea and refers directly to an alluvial or glacial deposit of sand or gravel.
By extention, placer mining refers to mining the precious metal
deposits (particularly gold
and gemstone
s) found in alluvial
deposits—deposits of sand and gravel in modern or ancient stream bed
s, or occasionally glacial deposits. 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.
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
).
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.
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 shovel
s 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.
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.
In earlier times the process water was not generally recycled and the spent ore was not reclaimed. The remains of a Roman
alluvial gold mine at Las Médulas
are so spectacular as to justify the site being designated UNESCO World Heritage status. The methods used by the Roman miners are fully described by Pliny the Elder
in his work Naturalis Historia
published in about 77 AD. The author was a Procurator
in the region and so probably witnessed large-scale hydraulic mining of the placer deposits there. He also added that the local lake Curacado had been heavily silted by the mining methods.
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 (also known as the "Dahlonega Method"). 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 long bars up to 20 feet thick in parts of San Francisco Bay
. The process raised an opposition calling themselves the "Anti-Debris Association". In January 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.
Alluvium
Alluvium is loose, unconsolidated soil or sediments, eroded, deposited, and reshaped by water in some form in a non-marine setting. Alluvium is typically made up of a variety of materials, including fine particles of silt and clay and larger particles of sand and gravel...
deposits for mineral
Mineral
A mineral is a naturally occurring solid chemical substance formed through biogeochemical processes, having characteristic chemical composition, highly ordered atomic structure, and specific physical properties. By comparison, a rock is an aggregate of minerals and/or mineraloids and does not...
s. This may be done by open-pit (also called open-cast mining
Mining
Mining is the extraction of valuable minerals or other geological materials from the earth, from an ore body, vein or seam. The term also includes the removal of soil. Materials recovered by mining include base metals, precious metals, iron, uranium, coal, diamonds, limestone, oil shale, rock...
) or by various surface excavating equipment or tunneling equipment.
The name is sometimes said to derive from Spanish
Spanish language
Spanish , also known as Castilian , is a Romance language in the Ibero-Romance group that evolved from several languages and dialects in central-northern Iberia around the 9th century and gradually spread with the expansion of the Kingdom of Castile into central and southern Iberia during the...
placer, meaning "to please," (pleasure) because it is easy mining as compared with others. However, the origin of the word is actually from Spanish
Spanish
Spanish may refer to:* Of or related to Spain* Spanish language, the official language of Spain and most of Latin America* Spanish people * The Spanish Empire, the empire administrated by Spain from the 15th century* Spanish cuisine...
, placer meaning shoal or alluvial/sand deposit, from Catalan
Catalan
Catalan is an inhabitant of CataloniaCatalan can also refer to:*Catalan people*Catalan language*Països Catalans, territories where Catalan language is spokenPersons with the surname Catalan:*Eugène Charles Catalan, a mathematician...
placer, (shoal), from plassa, (place) from Medieval Latin
Medieval Latin
Medieval Latin was the form of Latin used in the Middle Ages, primarily as a medium of scholarly exchange and as the liturgical language of the medieval Roman Catholic Church, but also as a language of science, literature, law, and administration. Despite the clerical origin of many of its authors,...
placea (place) the origin word for place and plaza in English. The word in Spanish is thus ultimately derived from placea and refers directly to an alluvial or glacial deposit of sand or gravel.
By extention, placer mining refers to mining the precious metal
Precious metal
A precious metal is a rare, naturally occurring metallic chemical element of high economic value.Chemically, the precious metals are less reactive than most elements, have high lustre, are softer or more ductile, and have higher melting points than other metals...
deposits (particularly gold
Gold
Gold is a chemical element with the symbol Au and an atomic number of 79. Gold is a dense, soft, shiny, malleable and ductile metal. Pure gold has a bright yellow color and luster traditionally considered attractive, which it maintains without oxidizing in air or water. Chemically, gold is a...
and gemstone
Gemstone
A gemstone or gem is a piece of mineral, which, in cut and polished form, is used to make jewelry or other adornments...
s) found in alluvial
Alluvium
Alluvium is loose, unconsolidated soil or sediments, eroded, deposited, and reshaped by water in some form in a non-marine setting. Alluvium is typically made up of a variety of materials, including fine particles of silt and clay and larger particles of sand and gravel...
deposits—deposits of sand and gravel in modern or ancient stream bed
Stream bed
A stream bed is the channel bottom of a stream, river or creek; the physical confine of the normal water flow. The lateral confines or channel margins, during all but flood stage, are known as the stream banks or river banks. In fact, a flood occurs when a stream overflows its banks and flows onto...
s, or occasionally glacial deposits. 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
Sand
Sand is a naturally occurring granular material composed of finely divided rock and mineral particles.The composition of sand is highly variable, depending on the local rock sources and conditions, but the most common constituent of sand in inland continental settings and non-tropical coastal...
, 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 mining
Hydraulic mining, or hydraulicking, is a form of mining that uses high-pressure jets of water to dislodge rock material or move sediment. In the placer mining of gold or tin, the resulting water-sediment slurry is directed through sluice boxes to remove the gold.-Precursor - ground...
, hydraulic sluicing or hydraulicking.
History
Hushing
Hushing is an ancient and historic mining method using a flood or torrent of water to reveal mineral veins. The method was applied in several ways, both in prospecting for ores, and for their exploitation. Mineral veins are often hidden below soil and sub-soil, which must be stripped away to...
were used widely by the Romans across their empire, but especially in the gold fields of northern Spain
Spain
Spain , officially the Kingdom of Spain languages]] under the European Charter for Regional or Minority Languages. In each of these, Spain's official name is as follows:;;;;;;), is a country and member state of the European Union located in southwestern Europe on the Iberian Peninsula...
after its conquest by Augustus
Augustus
Augustus ;23 September 63 BC – 19 August AD 14) is considered the first emperor of the Roman Empire, which he ruled alone from 27 BC until his death in 14 AD.The dates of his rule are contemporary dates; Augustus lived under two calendars, the Roman Republican until 45 BC, and the Julian...
in 25 BC. One of the largest sites was at Las Médulas
Las Médulas
Las Médulas is a historical site near the town of Ponferrada in the region of El Bierzo , which used to be the most important gold mine in the Roman Empire...
, where seven 30 mile long aqueduct
Aqueduct
An aqueduct is a water supply or navigable channel constructed to convey water. In modern engineering, the term is used for any system of pipes, ditches, canals, tunnels, and other structures used for this purpose....
s were used to work the alluvial gold deposits through the first century AD. (Inclusions of platinum
Platinum
Platinum is a chemical element with the chemical symbol Pt and an atomic number of 78. Its name is derived from the Spanish term platina del Pinto, which is literally translated into "little silver of the Pinto River." It is a dense, malleable, ductile, precious, gray-white transition metal...
-group metals in a very large proportion of gold items indicate that the gold was largely derived from placer or alluvial deposits. Platinum group
Platinum group
The platinum group metals is a term used sometimes to collectively refer to six metallic elements clustered together in the periodic table.These elements are all transition metals, lying in the d-block .The six...
metals are seldom found with gold in hardrock reef or vein deposits.) In North America
North America
North America is a continent wholly within the Northern Hemisphere and almost wholly within the Western Hemisphere. It is also considered a northern subcontinent of the Americas...
, placer mining was famous in the context of several gold rush
Gold rush
A gold rush is a period of feverish migration of workers to an area that has had a dramatic discovery of gold. Major gold rushes took place in the 19th century in Australia, Brazil, Canada, South Africa, and the United States, while smaller gold rushes took place elsewhere.In the 19th and early...
es, particularly the California Gold Rush
California Gold Rush
The California Gold Rush began on January 24, 1848, when gold was found by James W. Marshall at Sutter's Mill in Coloma, California. The first to hear confirmed information of the gold rush were the people in Oregon, the Sandwich Islands , and Latin America, who were the first to start flocking to...
, the Fraser Canyon Gold Rush
Fraser Canyon Gold Rush
The Fraser Canyon Gold Rush, began in 1858 after gold was discovered on the Thompson River in British Columbia at its confluence with the Nicoamen River. This was a few miles upstream from the Thompson's confluence with the Fraser River at present-day Lytton...
and the Klondike Gold Rush
Klondike Gold Rush
The Klondike Gold Rush, also called the Yukon Gold Rush, the Alaska Gold Rush and the Last Great Gold Rush, was an attempt by an estimated 100,000 people to travel to the Klondike region the Yukon in north-western Canada between 1897 and 1899 in the hope of successfully prospecting for gold...
. Placer mining continues in many areas of the world as a source of diamonds, industrial minerals and metals, gems (in Myanmar
Myanmar
Burma , officially the Republic of the Union of Myanmar , is a country in Southeast Asia. Burma is bordered by China on the northeast, Laos on the east, Thailand on the southeast, Bangladesh on the west, India on the northwest, the Bay of Bengal to the southwest, and the Andaman Sea on the south....
and Sri Lanka
Sri Lanka
Sri Lanka, officially the Democratic Socialist Republic of Sri Lanka is a country off the southern coast of the Indian subcontinent. Known until 1972 as Ceylon , Sri Lanka is an island surrounded by the Indian Ocean, the Gulf of Mannar and the Palk Strait, and lies in the vicinity of India and the...
), platinum, and of gold (in the Yukon
Yukon
Yukon is the westernmost and smallest of Canada's three federal territories. It was named after the Yukon River. The word Yukon means "Great River" in Gwich’in....
, Alaska
Alaska
Alaska is the largest state in the United States by area. It is situated in the northwest extremity of the North American continent, with Canada to the east, the Arctic Ocean to the north, and the Pacific Ocean to the west and south, with Russia further west across the Bering Strait...
and British Columbia
British Columbia
British Columbia is the westernmost of Canada's provinces and is known for its natural beauty, as reflected in its Latin motto, Splendor sine occasu . Its name was chosen by Queen Victoria in 1858...
).
Panning
The simplest technique to extract gold from placer ore is panning
Gold panning
Gold panning, or simply panning, is a form of placer mining that extracts gold from a placer deposit using a pan. The process is one of the simplest ways to extract gold, and is popular with geology enthusiasts because of its cheap cost and the relatively simple and easy process involved. It is the...
. In panning, some mined ore is placed in a large metal or plastic pan, combined with a generous amount of water
Water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...
, and agitated so that the gold particles, being of higher density
Density
The mass density or density of a material is defined as its mass per unit volume. The symbol most often used for density is ρ . In some cases , density is also defined as its weight per unit volume; although, this quantity is more properly called specific weight...
than the other material, settle to the bottom of the pan. The lighter gangue
Gangue
In mining, gangue is the commercially worthless material that surrounds, or is closely mixed with, a wanted mineral in an ore deposit. The separation of mineral from gangue is known as mineral processing, mineral dressing or ore dressing and it is a necessary and often significant aspect of mining...
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
Sluice
A sluice is a water channel that is controlled at its head by a gate . For example, a millrace is a sluice that channels water toward a water mill...
, 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 shovel
Shovel
A shovel is a tool for digging, lifting, and moving bulk materials, such as soil, coal, gravel, snow, sand, or ore. Shovels are extremely common tools that are used extensively in agriculture, construction, and gardening....
s 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
Trommel
A trommel is a screened cylinder used to separate materials by size - for example, separating the biodegradable fraction of mixed municipal waste or separating different sizes of crushed stone....
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.Environmental effects
Although 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.Ancient Rome
Ancient Rome was a thriving civilization that grew on the Italian Peninsula as early as the 8th century BC. Located along the Mediterranean Sea and centered on the city of Rome, it expanded to one of the largest empires in the ancient world....
alluvial gold mine at Las Médulas
Las Médulas
Las Médulas is a historical site near the town of Ponferrada in the region of El Bierzo , which used to be the most important gold mine in the Roman Empire...
are so spectacular as to justify the site being designated UNESCO World Heritage status. The methods used by the Roman miners are fully described by Pliny the Elder
Pliny the Elder
Gaius Plinius Secundus , better known as Pliny the Elder, was a Roman author, naturalist, and natural philosopher, as well as naval and army commander of the early Roman Empire, and personal friend of the emperor Vespasian...
in his work Naturalis Historia
Naturalis Historia
The Natural History is an encyclopedia published circa AD 77–79 by Pliny the Elder. It is one of the largest single works to have survived from the Roman Empire to the modern day and purports to cover the entire field of ancient knowledge, based on the best authorities available to Pliny...
published in about 77 AD. The author was a Procurator
Procurator (Roman)
A procurator was the title of various officials of the Roman Empire, posts mostly filled by equites . A procurator Augusti was the governor of the smaller imperial provinces...
in the region and so probably witnessed large-scale hydraulic mining of the placer deposits there. He also added that the local lake Curacado had been heavily silted by the mining methods.
Environmental activists describe the hydraulic mining
Hydraulic mining
Hydraulic mining, or hydraulicking, is a form of mining that uses high-pressure jets of water to dislodge rock material or move sediment. In the placer mining of gold or tin, the resulting water-sediment slurry is directed through sluice boxes to remove the gold.-Precursor - ground...
form of placer mining as environmentally destructive because of the large amounts of silt that it adds to previously clear running streams (also known as the "Dahlonega Method"). Most placer mines today use settling ponds, if only to ensure that they have sufficient water to run their sluicing operations.
San Francisco Bay
San Francisco Bay is a shallow, productive estuary through which water draining from approximately forty percent of California, flowing in the Sacramento and San Joaquin rivers from the Sierra Nevada mountains, enters the Pacific Ocean...
. The process raised an opposition calling themselves the "Anti-Debris Association". In January 1884, a United States District Court
United States district court
The United States district courts are the general trial courts of the United States federal court system. Both civil and criminal cases are filed in the district court, which is a court of law, equity, and admiralty. There is a United States bankruptcy court associated with each United States...
banned the flushing of debris into streams, and the hydraulic mining mania in California's gold country came to an end.
External links
- Historical Bathymetric Changes in Suisun Bay, 1867-1990. includes maps of bottom depths for 1867, 1887, and the change between these dates. on line at: http://sfbay.wr.usgs.gov/sediment/suisunbay/1867to1887bathy.html