What is an ore, and how do useful metal deposits form and get extracted?
An ore is a rock from which a metal can be extracted economically; ore minerals (for example galena for lead, haematite for iron) are concentrated by geological processes such as hydrothermal veins, magmatic settling and weathering and deposition; whether a deposit is worked depends on its grade, size, depth and the metal price; extraction by surface or underground mining has environmental costs, so it is balanced against the need for the metal and is followed by site restoration.
A focused answer to the Eduqas GCSE Geology statement on mineral resources. Covers the definition of an ore, the named ore minerals, how ore deposits are concentrated (hydrothermal, magmatic, weathering), the economic factors that decide whether a deposit is mined, and the environmental costs of extraction.
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What this dot point is asking
Eduqas wants you to define an ore (a rock from which a metal can be extracted economically), to name the common ore minerals (galena for lead, haematite for iron), to explain how ore deposits are concentrated by geological processes (hydrothermal veins, magmatic settling, weathering and deposition), and to explain the economic factors (grade, size, depth and metal price) that decide whether a deposit is worked. You also need to discuss the environmental costs of mining and the idea that they are balanced against the need for the metal and followed by site restoration.
The answer
What an ore is
Common ore minerals named at GCSE include galena (lead sulphide, the main ore of lead), haematite (iron oxide, an ore of iron), and other sulphides and oxides. The unwanted minerals dug up with the ore (such as quartz and calcite) are called gangue.
How ore deposits are concentrated
Metals are spread thinly through most rocks. They only become worth mining where a geological process has concentrated them:
- Hydrothermal veins. Hot, mineral-rich fluids move through cracks and faults; as they cool, the dissolved metals crystallise on the fracture walls, filling the crack with ore minerals (this is how many galena and haematite veins form).
- Magmatic settling. In a cooling magma, dense metal-rich minerals can crystallise early and sink to the floor of the magma chamber, concentrating them into a layer.
- Weathering and deposition. Weathering can leave behind a concentrated residual ore (the soluble parts washed away), or transport and deposit heavy, resistant ore minerals into a placer deposit where a river slows.
The economics: when is a deposit worked?
Whether a deposit is actually mined is an economic decision, not just a geological one. It depends on:
- Grade. The percentage of metal in the rock; higher grade means more metal per tonne mined.
- Size. A large deposit can justify the cost of setting up a mine; a tiny one may not.
- Depth. A shallow deposit is cheap to reach (surface mining); a deep one needs expensive underground mining.
- Metal price. When the price is high, lower-grade or deeper deposits become worth working; when it falls, they do not.
Because the metal price and mining technology change over time, the same deposit can switch between being an ore and being uneconomic.
Extraction and its costs
Ore is extracted by surface (open-pit) mining for shallow deposits or underground mining for deep ones. Mining brings environmental costs:
- land disturbance, habitat loss and spoil heaps;
- water pollution from acidic or metal-rich drainage;
- dust, noise and visual scarring of the landscape.
These costs are balanced against the need for the metal (society needs metals for construction, electronics and energy), and once a site is worked out it is restored (landscaped, replanted, and pits filled or reused) to reduce the lasting impact.
Examples in context
Example 1. Cornish tin and copper. South-west England's hydrothermal veins, formed by hot fluids around cooling granite, were mined for tin and copper for centuries until cheaper deposits elsewhere made them uneconomic, a textbook case of price and cost deciding ore status.
Example 2. Placer gold. Heavy, resistant gold grains are carried by rivers and concentrated where the current slows, forming placer deposits that can be panned, an example of weathering and deposition concentrating an ore.
Try this
Q1. State what makes a rock an ore rather than just a rock containing a metal. [1 mark]
- Cue. The metal must be extractable economically (at a profit).
Q2. Name one process that concentrates metals into an ore deposit and describe how it works. [2 marks]
- Cue. For example hydrothermal veins: hot mineral-rich fluids cool in fractures and the dissolved metals crystallise on the walls, concentrating the ore.
Q3. Give one environmental cost of mining. [1 mark]
- Cue. Any one of: land disturbance and habitat loss; water pollution; dust and noise; visual scarring of the landscape.
Exam-style practice questions
Practice questions written in the style of WJEC Eduqas exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Eduqas 20204 marksExplain what is meant by an ore, and explain why a particular deposit might be mined at one time but left in the ground at another.Show worked answer →
Define an ore, stressing the economic element, then explain how the economics change.
What an ore is. An ore is a rock or mineral deposit from which a metal (or other valuable substance) can be extracted at a profit, that is, economically. The same metal in too low a concentration is not an ore, because extracting it would cost more than the metal is worth.
Why a deposit's status changes. Whether a deposit is worth mining depends on its grade (the percentage of metal), size, depth and the current metal price, balanced against the cost of extraction. If the metal price rises, or cheaper mining technology appears, a deposit that was uneconomic can become an ore and be mined. If the price falls, an ore can become uneconomic and be left.
Markers reward the idea that an ore is defined economically (extractable at a profit) and that a deposit's status changes when the metal price or the cost of extraction changes, so the same deposit can switch between ore and not-ore.
Eduqas 20186 marksDescribe two geological processes that can concentrate metals into an ore deposit, and discuss two environmental impacts of extracting the metal by mining.Show worked answer →
Give two concentration processes, then two environmental impacts, with brief explanation.
- Hydrothermal veins
- Hot, mineral-rich fluids move through cracks and faults, and as they cool the dissolved metals crystallise on the fracture walls, concentrating ore minerals such as galena and haematite into veins.
- Magmatic settling
- In a cooling magma, dense metal-rich minerals can crystallise early and sink to the floor of the magma chamber, concentrating them into a layer. (Weathering and deposition, which leaves heavy resistant minerals as a placer or concentrates residual ores, is also valid.)
- Environmental impacts (any two)
- Land disturbance and habitat loss from open-pit mining and spoil heaps; water pollution from acidic or metal-rich drainage; dust and noise; and the visual scarring of the landscape. These are balanced against the need for the metal, and sites are restored afterwards.
Markers reward two valid concentration processes (hydrothermal, magmatic, weathering) and two genuine environmental impacts, each briefly explained.
Related dot points
- Hydrocarbons (oil and gas) form from buried organic matter, then migrate from a source rock into a porous, permeable reservoir rock where an impermeable cap rock and a trap (for example an anticline or fault) hold them; groundwater is stored in a porous, permeable aquifer beneath the water table and supplied to wells; both depend on the rock properties porosity (storage) and permeability (flow), and both can be over-exploited or polluted.
A focused answer to the Eduqas GCSE Geology statement on hydrocarbons and groundwater. Covers how oil and gas form, migrate and are trapped by a reservoir, cap rock and structure, how groundwater is stored in aquifers below the water table, and the key rock properties porosity and permeability.
- Engineering geology assesses the ground before construction: foundations, tunnels, dams and reservoirs must suit the rock and soil present; geologists check the strength and stability of rock, the presence of faults, the slope stability, the permeability of the ground (for a reservoir to hold water or a tunnel to stay dry), and the hazards of weak, soluble or swelling materials; poor ground investigation can lead to subsidence, leakage, collapse or failure, so a site investigation is carried out first.
A focused answer to the Eduqas GCSE Geology statement on engineering geology. Covers why the ground must be assessed before construction, the factors checked (rock strength, faults, slope stability, permeability, weak or soluble materials), and how poor investigation leads to subsidence, leakage or collapse.
- Minerals are identified using physical properties: colour, crystal size, hardness (tested against fingernail, copper coin, steel and glass), cleavage and fracture, lustre, streak, and the reaction of carbonates with dilute hydrochloric acid; common minerals include quartz, feldspar, mica, calcite, halite, galena and haematite.
A focused answer to the Eduqas GCSE Geology statement on identifying minerals. Covers the physical properties used (colour, crystal size, hardness, cleavage and fracture, lustre, streak and the acid test) and the diagnostic features of quartz, feldspar, mica, calcite, halite, galena and haematite.
- Igneous rocks form by the crystallisation of magma or lava; cooling rate controls crystal size (slow cooling at depth gives coarse-grained intrusive rocks such as granite, fast cooling at the surface gives fine-grained extrusive rocks such as basalt); rocks are classified by crystal size and by silica content (felsic, intermediate, mafic); minerals also crystallise from hydrothermal fluids to form veins.
A focused answer to the Eduqas GCSE Geology statement on igneous rocks. Covers how magma and lava crystallise, how cooling rate controls crystal size (intrusive granite versus extrusive basalt), classification by silica content (felsic to mafic), and the crystallisation of minerals from hydrothermal fluids in veins.
- The rock cycle links igneous, sedimentary and metamorphic rocks through the processes of weathering, erosion, transport, deposition, burial and lithification, melting and crystallisation, and metamorphism; the cycle is driven by energy from the Sun (at the surface) and from the Earth's interior (at depth), and any rock can be changed into any other given time and the right conditions.
A focused answer to the Eduqas GCSE Geology statement on the rock cycle. Covers the three rock families and the processes that connect them (weathering, erosion, transport, deposition, lithification, melting, crystallisation and metamorphism), the two energy sources that drive the cycle, and how any rock can become any other.
Sources & how we know this
- WJEC Eduqas GCSE (9-1) Geology specification (teaching from 2017) — WJEC Eduqas (2017)