Gold mining is the process of mining of gold or gold ores from the ground. There are several techniques and processes by which gold may be extracted from the earth.
There are two main methods of gold mining: placer mining and hard rock mining.
Placer miningPlacer mining is the technique by which gold that has accumulated in a placer deposit is extracted. Placer deposits are composed of relatively loose material that makes tunneling difficult, and so most means of extracting it involve the use of water or dredging. There are a variety of placer mining methods, but the most common ones include:
PanningGold panning is mostly a manual technique of separating gold from other materials. Wide, shallow pans are filled with sand and gravel that may contain gold. The pan is submerged in water and shaken, sorting the gold from the gravel and other material. As gold is much denser than rock, it quickly settles to the bottom of the pan. The panning material is usually removed from stream beds, often at the inside turn in the stream, or from the bedrock shelf of the stream, where the density of gold allows it to concentrate, a type called placer deposits.
|Methods of Gold Mining. The Super Pit gold mine in Australia
Gold panning is the easiest and quickest technique for searching for gold, but is not commercially viable for extracting gold from large deposits, except where labor costs are very low or gold traces are substantial. Panning is often marketed as a tourist attraction on former gold fields. Before large production methods are used, a new source must be identified and panning is useful to identify placer gold deposits to be evaluated for commercial viability.
SluicingTaking gold out of a sluice box, western North America, 1900s Using a sluice box to extract gold from placer deposits has long been a very common practice in prospecting and small-scale mining. A sluice box is essentially a man made channel with riffles set in the bottom. The riffles are designed to create dead zones in the current to allow gold to drop out of suspension. The box is placed in the stream to channel water flow. Gold-bearing material is placed at the top of the box. The material is carried by the current through the volt where gold and other dense material settles out behind the riffles. Less dense material flows out of the box as tailings.
Larger commercial placer mining operations employ screening plants, or trommels, to remove the larger alluvial materials such as boulders and gravel, before concentrating the remainder in a sluice box or jig plant. These operations typically include diesel powered, earth moving equipment, including excavators, bulldozers, wheel loaders, and rock trucks.
DredgingAlthough this method has largely been replaced by modern methods, some dredging is done by small-scale miners using suction dredges. These are small machines that float on the water and are usually operated by one or two people. A suction dredge consists of a sluice box supported by pontoons, attached to a suction hose which is controlled by a miner working beneath the water.
State dredging permits in many of the United States gold dredging areas specify a seasonal time period and area closures to avoid conflicts between dredgers and the spawning time of fish populations. Some states, such as Montana, require an extensive permitting procedure, including permits from the U.S. Corps of Engineers, the Montana Department of Environmental Quality, and the local county water quality boards.
Some large suction dredges (100 horsepower (75 kW) & 250 mm (10 in)) are used in commercial production throughout the world. Small suction dredges are much more efficient at extracting smaller gold than the old bucket line. This has improved the chances of finding gold. Smaller dredges with 50-to-100-millimetre (2 to 4 in) suction tubes are used to sample areas behind boulders and along potential pay streaks, until "color" (gold) appears.
Other larger scale dredging operations take place on exposed river gravel bars at seasonal low water. These operations typically use a land based excavator to feed a gravel screening plant and sluice box floating in a temporary pond. The pond is excavated in the gravel bar and filled from the natural water table. "Pay" gravel is excavated from the front face of the pond and processed through the floating plant, with the gold trapped in the onboard sluice box and tailings stacked behind the plant, steadily filling in the back of the pond as the operation moves forward.
This type of gold mining is characterized by its low cost, as each rock is moved only once. It also has low environmental impact, as no stripping of vegetation or overburden is necessary, and all process water is fully recycled. Such operations are typical on New Zealand's South Island and in the Klondike region of Canada.
Rocker boxThe rocker box, also called a cradle, uses a riffles located in a high-walled box to trap gold in a similar manner to the sluice box. A rocker box uses less water than a sluice box and is thus well suited for areas where water is limited. A rocking motion provides the water movement needed for the gravity separation of gold in placer material.
Hard rock Gold MiningHard rock mining is the extraction of gold from solid rock formations rather than fragments in loose sediment. This type of mining is typically done on a large scale, using heavy machinery and equipment. Hard rock mining is a more complex and expensive process than placer mining, but it can produce more gold. Hard rock mining produces most of the world's gold.
There are two main types of hard rock mining: underground mining and open-pit mining.
Underground Gold MiningUnderground mining is the extraction of gold from underground ore deposits. This type of mining is dangerous and requires specialized equipment and training. Underground miners work in tunnels and shafts that are deep underground. They use a variety of tools and equipment to extract the ore, including explosives, drills, and loaders.
Open-pit Gold MiningOpen-pit mining: Open-pit mining is the extraction of gold from surface ore deposits. This type of mining is less dangerous than underground mining, but it can have a significant environmental impact. Open-pit mines are large excavations that are dug into the ground. The ore is extracted from the pit using heavy machinery, such as excavators and trucks.
Once the gold ore has been mined, it is processed to extract the gold. This process typically involves crushing the ore, grinding it into a fine powder, and then using chemicals to dissolve the gold. The gold is then precipitated out of the solution and collected.