Extraction of Rocks, Ores and Minerals

What an ore is, and when a deposit is worth working

An ore is rock containing minerals and metals. The mineral is the useful part and the rest of the rock is waste. Rock only counts as an ore if the metal in it can be extracted and sold for more than the extraction costs, so a rock that contains a metal is not automatically an ore. The ore grade is the proportion of useful mineral or metal in the rock: it is written as a percentage for metals such as copper and iron, and in grams per tonne for gold, because gold occurs in tiny amounts.

Most crustal rock contains a little of almost every metal. A deposit becomes worth mining only where geological processes have concentrated the metal many times above that background level. The unwanted minerals mixed with the ore are the gangue, and the rock and soil lying above a deposit are the overburden. Once the ore has been dug out it is crushed, ground and concentrated, and the metal is smelted or chemically extracted. The fine waste left after that process is called tailings, and it is stored in heaps or pumped as a slurry into a tailings dam.

Whether a body of rock is an ore is not fixed. Larger machines and better processing, and metals becoming scarcer and more valuable, turn deposits that were once rejected into resources. The grade at which a deposit just pays for itself is the cut-off grade. A mine can close when the rock left in it falls below the cut-off grade, and reopen years later if the metal price rises.

Surface extraction: opencast, open-pit, open-cut and strip mining

All four surface methods work on the same principle: the deposit is reached from above by removing whatever lies on top of it, so the mine is open to the sky. The soil and rock above the ore, the overburden, is stripped off and stored, the ore is then dug out in a series of steps called benches, and lorries, conveyors or trains carry it away. Because no roof has to be supported and no shaft has to be sunk, the machines used can be enormous.

The four names describe the same idea applied to different shapes of deposit. Opencast and open-cut mining are the usual terms for quarries and shallow coal workings, where a fairly flat deposit is worked by digging down in benches. Open-pit mining is used for steep-sided pits cut into hard rock to reach large metal orebodies such as copper and gold, so the pit becomes very deep and the walls must be cut back at an angle that will not collapse. Strip mining is used where a seam lies flat and close to the surface over a wide area: a long strip of overburden is removed, the ore is taken out, the strip is backfilled, and the machine moves on to work a strip parallel to the first one.

Surface extraction can recover a very high proportion of the ore in a deposit, costs less per tonne than underground working, and is much safer for the people employed there, because there is no roof to fall and no gas to collect in a tunnel. The land pays for this. The area disturbed can be enormous, the pit and spoil heaps dominate the view, blasting and haulage produce dust, noise and vibration, and the water table can be lowered for kilometres around while rain water has to be pumped out of the pit.

Subsurface extraction: deep mining and shaft mining

When the deposit lies deep, or lies under a town, a farm or a protected forest, the ore has to be reached from underground. A vertical shaft is sunk from the surface, usually with a winding tower or headgear above it, and horizontal tunnels called drifts or levels are driven out to the orebody. People, machinery, ore and fresh air all travel through the shafts and roadways, so an underground mine is really a small town with its own transport and ventilation system.

Deep mining means working at great depth, often several kilometres down. The rock becomes hotter as the depth increases, roughly 25 °C for every kilometre, so the air has to be cooled and circulated. The hazards are the ones that make underground work dangerous: rock falls and rock bursts as the stresses in the rock change, methane gas in coal measures, flooding when water pours in through faults, and dust that scars the lungs and can explode. Dust is controlled with water sprays and ventilation, roadways are supported with steel arches, roof bolts and pillars of ore left in place, and miners carry self-rescuers and work to strict escape rules.

Removing ore underground leaves a void, and the surface above it can sink. Subsidence cracks roads, drains and buildings, and it continues long after the ore has gone. Coal mines also have to pump water out continuously while they work; when the pumps stop the water rises, dissolving minerals and metals from the rocks, and it can reach the surface as acid mine drainage.

Underground working disturbs very little land at the surface and can follow a narrow, deep vein that no open pit could ever reach, but it costs far more per tonne, needs a higher ore grade to pay for itself, and puts the workforce at much greater risk. The table sets the two families of methods side by side.

Biological extraction: phytomining and bioleaching

Phytomining uses plants to mine. Certain species, called hyperaccumulators, take metals such as nickel, zinc, cadmium, copper or gold out of the soil through their roots and store them in their stems and leaves, and a few of them can build up several per cent of their dry mass as metal. The crop is harvested, dried and burned, and the metal is recovered from the ash. In effect a field is cropped, cut and processed like any other harvest, and the same ground can be cropped again.

Phytomining is used where the metal is spread so thinly through the ground that digging, crushing and processing it could never pay: mine spoil, old tailings, low-grade ore and contaminated soil. The plants also hold the surface together against erosion, take metals out of soil that is too polluted for farming or housing, and grow without blasting or heavy machinery, so they produce almost no dust and use very little fuel.

Bioleaching uses bacteria instead of plants. Species such as Acidithiobacillus ferrooxidans live on sulphide minerals and oxidise them, producing sulphuric acid and iron(III) ions that dissolve the metal out of the rock into a solution. The metal-bearing solution is drained off and the metal is recovered from it. The bacteria are given crushed low-grade ore in heaps, or the solution is trickled through the broken rock left in an old working, and air and water are supplied so that the bacteria can keep working. Heap and dump bioleaching of copper is now common in Chile and other copper-producing countries, and bacterial oxidation has been used to release gold from stubborn sulphide ores at mines in South Africa.

Both methods are slow and both need careful management. Bioleaching takes months or years rather than hours, needs warm, wet, aerated conditions and the right bacteria, and produces acid solutions that must be contained so that they cannot reach a stream or the groundwater. Phytomining needs a large area of land for a small yield of metal, only a few metals are taken up in useful amounts, and the crop has to be harvested, dried and burned before the metal can be recovered. Neither method can replace conventional mining of a large, rich deposit, but both turn waste into a resource and both leave the land far less damaged.

Deciding whether to extract: the factors that decide if a deposit becomes a mine

A company begins with exploration. Geologists map the rocks, study satellite and aerial images, measure how the ground responds to magnetic, gravity and seismic surveys, sample soils and stream water for traces of metal, and finally drill boreholes to bring up cores that show how thick and how rich the deposit is. Exploration is expensive and most of it finds nothing worth mining, so a company goes ahead only when the drilling shows a deposit large enough to repay that risk.

Geology decides which method could be used at all. A flat seam near the surface can be stripped by machine, a narrow vein a kilometre down can only be worked through a shaft, and a huge, low-grade body of metal scattered through the rock suits a deep open pit. The hardness of the rock affects blasting and crushing costs, faults and joints make a pit wall or a roadway unstable, and whether the ore is an oxide or a sulphide decides how it must be processed. Accessibility and terrain decide whether machinery, ore and fuel can be moved: a mine beside a railway and a port is far cheaper to work than one at the end of 200 km of mountain track, and a deposit under a city or a national park may never be touched. Ore grade and quantity decide whether the whole scheme pays: a large, rich deposit can carry the cost of roads, a power line and a new town, while a small or low-grade one may only be worth working when the metal price is high. Climate matters too - heavy rain means pumping, drainage and a constant threat to tailings dams, a desert means the mine competes with farms and towns for water, and ice and extreme cold shorten the working season at remote northern mines.

The last three factors are economic and legal. An environmental impact assessment (EIA) is carried out before permission is granted: the company must predict the effects on air, water, land, wildlife and people, explain how it will reduce them, and publish the findings so that the public can comment. A mine that cannot satisfy the regulators, or that would destroy a protected habitat, will not be allowed however rich the ore is. Supply and demand decides the price the ore will fetch: demand from construction, electronics and the shift to electric vehicles can lift a metal out of a slump, and a new source, a substitute material or a recession can collapse the price instead. Cost and profit bring all of this together - the cost of exploration, land, machinery, wages, energy, transport, taxes and restoration is set against the income from the metal, and if the cost per tonne of metal is above the price it will fetch, the deposit is left in the ground.

• Exploration: mapping, geophysics, sampling and drilling to prove that a deposit exists and to measure its size and grade.
• Geology: the shape, depth and hardness of the orebody, its faults and its mineral type, which together decide the mining method.
• Accessibility and terrain: roads, railways and ports, the slope of the land, and whether towns or protected areas lie above the ore.
• Quantity and quality of deposit (ore grade): how many tonnes of ore there are and how much metal each tonne contains.
• Climate: rainfall, drought, ice and heat, which affect water supply, drainage, tailings stability and the working season.
• Environmental impact assessment: the prediction of damage and the conditions attached to permission to mine.
• Supply and demand: the world market for the metal, which sets the price the ore will fetch.
• Cost and profit: total extraction, transport and restoration costs set against the income from the metal.

The impacts of extraction: environment, economy and society

The most obvious environmental impact is the loss of habitat and biodiversity. A surface mine removes the vegetation, soil and rock over an area that may be measured in square kilometres, and the spoil heaps, haul roads, processing plant and tailings dams take more land again. Animals are killed or driven out, and species that need a particular soil, slope or water table may disappear from the district altogether. Underground mining leaves the surface largely intact, but it still lowers the water table, leaks acid drainage and can cause subsidence. Mines are visually intrusive as well: a pit, a headgear and a mountain of waste rock turn a landscape into something industrial, which matters most where the area depends on farming or tourism.

Pollution travels by several routes. Dust from blasting, crushing and lorry traffic carries fine rock and silica into the air and into people's lungs, and the engines of machinery and lorries add nitrogen oxides and particulates. Blasting and crushing cause noise and vibration that crack plaster and unsettle livestock. Water pollution is the most serious: rain falling on spoil heaps and tailings dissolves metals and sulphur, and the acid drainage that results can kill fish and plants for kilometres downstream. Silt washed off the site smothers river beds, and a tailings dam failure releases a wave of toxic sludge that can destroy a whole valley. Waste management is part of the same problem: overburden, waste rock, tailings and used chemicals all have to be stored safely, and the heaps and dams have to be maintained and monitored for as long as they exist.

A mine also uses water. Drilling, dust suppression, ore processing and the separation of metal from gangue all need large volumes, and a mine in a dry region competes directly with farmers and towns for the same supply, which is why some mines pipe desalinated seawater hundreds of kilometres inland or pump groundwater and lower the water table for everyone else.

The economic and social effects cut both ways. A new mine creates employment, often in a district where there were few jobs, and the wages it pays support shops, transport and services across the area. It pays taxes and royalties to the government, earns foreign currency by exporting ore or metal, and its facilities and infrastructure - roads, a railway, a power line, a clinic or a school - are usually left behind for the community to use. Against that, many of the best-paid jobs are skilled and go to workers brought in from outside, the district may end up dependent on one employer and on the price of a single metal, and the settlement can grow faster than its housing, water supply and schools. When the ore runs out the mine closes, thousands of jobs can end at once, and the community is left with damaged land, empty buildings and the cost of cleaning up.

Managing damaged landscapes: land restoration and repurposing

Land restoration aims to put the land back into a productive or natural use. It starts before the mine closes, with the careful stripping and storage of topsoil and subsoil so that they can be returned rather than buried. The overburden and spoil are re-graded to stable slopes and the overburden is replaced over the waste. Stored soil loses its structure and its nutrients, and mine spoil is often acidic and short of organic matter, so the soil has to be improved: lime to cut the acidity, fertiliser or compost, and often sewage sludge or imported topsoil to give the new plants something to grow in. Drains are laid so that water leaves the site without carrying silt with it. The work is expensive and slow, and it has to be done in stages as each part of the mine is finished.

Bioremediation uses living things to clean the ground: bacteria, fungi and plants that break down or take up the metals and hydrocarbons left behind, including the pollution draining out of the spoil. Tree planting follows, with fast-growing species used first to hold the surface, add organic matter and shelter slower-growing native trees, and the finished site is fenced, drained and monitored for years. The aim is a self-supporting cover of vegetation that holds the soil, takes up water, provides habitat and reduces the acid drainage at its source.

Repurposing means giving the land a completely new use. Old quarries and pits make convenient landfill sites, because the hole is already there and the waste can be sealed and covered with clay and soil. Deep pits can be flooded to become lakes used for fishing, sailing or water storage. Worked-out ground close to a town is often levelled for recreation, with playing fields, cycle tracks and country parks. Large, quiet former mine areas are increasingly handed over as nature reserves, where the bare rock, cliffs, spoil slopes and pools left behind become habitat for rare plants, insects and birds that cannot survive on farmland.

Both approaches need a two-sided answer and the limitations carry the marks. Restoration is expensive, takes many years, and rarely produces the ecosystem that was there before: the soil is different, the shape of the land is different, and species that depended on the original bog or woodland may never return. Repurposing accepts that the old landscape has gone and gives the site a new value, but landfill brings heavy traffic, smell and leachate that must be collected, lakes can be acidic or contaminated, spoil slopes can slump, and recreation can conflict with the wildlife that has colonised the site. Either way the work has to be paid for and monitored long after the company has gone, which is why permission to mine is usually tied to a restoration bond or to a condition that the site is restored in stages as extraction proceeds. One benefit is easy to overlook: a well-planned rehabilitation programme keeps local people in work after the ore has gone.