How mineral and ore deposits form and are concentrated, methods of exploration and extraction, the concept of ore grade and reserves, and the environmental impacts of mining and ways to reduce them.

What this dot point is asking

AQA wants you to explain how mineral ore deposits form and become concentrated, define ore grade and reserves, describe exploration and extraction methods, and evaluate the environmental impacts of mining and the ways they can be reduced. Strong answers handle the economics (grade, price, reserves) quantitatively and link extraction method to specific impacts.

How mineral deposits form

The main concentrating processes are:

• Hydrothermal deposition: hot, mineral-rich water circulates through fractures in rock near magma bodies and precipitates metal sulphides (for example copper, lead, zinc and gold) as veins when it cools.
• Igneous (magmatic) segregation: dense minerals such as chromite and magnetite crystallise early and settle to the base of a cooling magma chamber, forming layered deposits.
• Sedimentary deposition: minerals are sorted by moving water and concentrated, as in placer deposits where dense grains of gold or tin collect in river beds, or banded iron formations laid down chemically in ancient oceans.
• Secondary enrichment (weathering): chemical weathering removes soluble material and leaves behind insoluble metal oxides, as in bauxite (aluminium ore) formed by intense tropical weathering of aluminium-rich rock.

Ore grade and reserves

The boundary between reserve and uneconomic resource is not fixed. A higher metal price, or cheaper extraction technology, moves lower-grade ore into the reserve category, so reserves can grow even when no new deposit is discovered. As high-grade ores are used up, miners turn to lower-grade ores, but these require more energy and water per tonne of metal and produce far more waste rock, raising both cost and environmental impact. This relationship between grade, energy use and waste is central to AQA's treatment of resource sustainability.

Exploration and extraction

Minerals are located by exploration using geological mapping, geophysical surveys (magnetic, gravity and seismic methods that detect dense or magnetic ore bodies), geochemical sampling of soil and water, and remote sensing. Promising targets are then drilled to confirm grade and tonnage before a mine is developed.

Extraction method depends on depth:

• Surface (open-cast or open-pit) mining is used for shallow, large deposits. It is cheaper, safer and recovers a high proportion of the ore, but causes large-scale landscape destruction, removes all overburden, and disturbs a wide surface area.
• Deep (shaft or underground) mining reaches deep, concentrated deposits. It disturbs much less surface area but is more expensive, more hazardous, recovers less of the ore, and can cause subsidence.

Environmental impacts and reducing them

Mining causes habitat destruction as land is cleared, landscape and visual damage, dust and noise, water pollution from acid mine drainage (sulphide minerals oxidising to sulphuric acid) and from toxic tailings, and very large volumes of spoil and waste rock, especially from low-grade ores. Processing and smelting add air pollution (sulphur dioxide) and energy demand.

Impacts are reduced by:

• Land restoration: re-contouring spoil, replacing stored topsoil and replanting after mining, sometimes leaving land for recreation or nature.
• Treating drainage and containing tailings to prevent acid and heavy-metal pollution of waterways.
• Recycling metals, which uses far less energy than primary extraction (recycling aluminium uses about 5 percent of the energy of smelting bauxite) and reduces demand for new ore.
• Efficient extraction and product design to make reserves last longer and reduce waste.

Try this

Q1. Explain the difference between a mineral resource and a reserve, and give one factor that can move resource into the reserve category. [3 marks]

• Cue. A resource is all the mineral present; a reserve is the part extractable economically now. A higher price or improved technology moves resource into reserves.

Q2. State two environmental impacts of open-cast mining and one method of reducing the impact of mining overall. [3 marks]

• Cue. Impacts: any two of habitat destruction, landscape damage, dust and noise, acid mine drainage, waste rock. Reduction: restoration, recycling, or efficient use.

Q3. An iron ore contains 4 000 000 tonnes of rock at 60 percent iron, with 95 percent recovery. Calculate the mass of iron recovered. [2 marks]

• Cue. $4\,000\,000 \times 0.60 = 2\,400\,000$ tonnes iron present; recovered $= 2\,400\,000 \times 0.95 = 2\,280\,000$ tonnes.