What makes a mineral deposit worth mining, and how is the metal content estimated?
Mining geology: the economic terms (ore grade, cut-off grade, reserves and resources) and the factors affecting whether a deposit is mined (grade, tonnage, depth, location, technology, price and environmental constraints); the calculation of contained metal from grade and tonnage; the extraction methods (open-pit and underground) and the geological and environmental issues of mining (waste, tailings and acid mine drainage).
A focused answer to the OCR H414 dot point on mining geology. Covers ore grade, cut-off grade, reserves versus resources, the factors affecting whether a deposit is mined, calculating contained metal from grade and tonnage, open-pit and underground extraction, and the environmental issues of mining (waste, tailings and acid mine drainage).
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What this dot point is asking
OCR wants you to define the economic terms (ore grade, cut-off grade, reserves and resources), to state the factors affecting whether a deposit is mined, to calculate contained metal from grade and tonnage, to describe open-pit and underground extraction, and to outline the environmental issues of mining.
The answer
The economic terms
- Ore grade. The concentration of the valuable metal in the ore, usually as a percentage by mass (or grams per tonne for precious metals).
- Cut-off grade. The lowest grade that can be mined at a profit under current costs and prices; material below it is not worth extracting.
- Resource. The total amount of a mineral known or estimated to be present, economic or not.
- Reserve. The part of the resource that can be extracted legally and economically with current technology and prices (a subset of the resource).
Factors affecting whether a deposit is mined
A deposit is mined only if the value of the metal exceeds the cost of extraction. The factors are:
- Grade and tonnage (how rich and how large).
- Depth and location (deep or remote deposits cost more to mine and transport).
- Technology available to extract and process.
- Metal price (sets the cut-off grade).
- Environmental, legal and social constraints (planning permission, protected land, opposition).
Calculating contained metal
The mass of metal in a deposit is the grade multiplied by the tonnage:
(with the grade as a fraction, so a grade is ).
Extraction and environmental issues
- Open-pit mining suits large, shallow, low-grade deposits; cheaper per tonne but moves huge volumes and leaves a large pit.
- Underground mining suits deep, high-grade or narrow deposits; more expensive and constrained by the strength of the rock and water inflow.
- Environmental issues include waste rock and tailings (fine processing waste), land disturbance, and acid mine drainage (sulfide minerals oxidising to produce acidic, metal-rich runoff that pollutes water), all requiring management and reclamation.
Examples in context
Example 1. A price rise reclassifying a deposit. When the copper price rose, many low-grade porphyry deposits that had been sub-economic crossed the cut-off and were reclassified from resource to reserve, illustrating how economics, not just geology, defines a reserve.
Example 2. Acid mine drainage. Where mining exposes sulfide minerals, they oxidise in air and water to form sulfuric acid and dissolve metals, producing acidic, metal-rich drainage that must be controlled to protect rivers, a key environmental issue of mining.
Try this
Q1. Define the cut-off grade. [1 mark]
- Cue. The lowest ore grade that can be mined at a profit under current costs and prices.
Q2. A body contains tonnes at zinc. Calculate the contained zinc. [2 marks]
- Cue. of zinc.
Q3. Explain how a rise in metal price can increase reserves. [2 marks]
- Cue. A higher price makes lower-grade material profitable, so it crosses the cut-off and moves from resource into reserve, increasing reserves.
Exam-style practice questions
Practice questions written in the style of OCR exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
OCR H414/01 20204 marksAn ore body contains 5 million tonnes of rock at a copper grade of 0.8 percent. Calculate the mass of copper metal contained, and state one reason the deposit might still not be mined despite containing this much copper.Show worked answer →
Calculate the contained metal, then give an economic or environmental reason.
Contained metal. Grade is the percentage of metal by mass, so the contained copper is the grade multiplied by the tonnage.
So the body contains tonnes of copper metal.
Reason it might not be mined (any one). The grade may be below the cut-off grade once costs are accounted for; the deposit may be too deep or remote (high extraction and transport costs); the copper price may be too low to make extraction profitable; or environmental, legal or social constraints may prevent mining.
Markers reward the correct calculation () and one valid reason the deposit may be uneconomic or unminable.
OCR H414/01 20184 marksExplain the difference between a mineral resource and a mineral reserve, and explain how a change in the metal price can move material between the two.Show worked answer →
Define each, then link price to the boundary.
- Resource
- A mineral resource is the total amount of a mineral known (or estimated) to be present, whether or not it can currently be extracted economically.
- Reserve
- A mineral reserve is the part of the resource that can be extracted legally and economically with current technology and prices. Reserves are therefore a subset of resources.
- Effect of price
- If the metal price rises, some lower-grade material that was previously too expensive to extract becomes profitable, so it moves from resource into reserve (reserves increase). If the price falls, marginal material becomes uneconomic and moves back from reserve into resource (reserves shrink). The cut-off grade shifts with price.
Markers reward resource (all known material) versus reserve (the economically extractable part) and a clear explanation that a price rise converts resource into reserve.
Related dot points
- Ore formation: the processes that concentrate metals into economic mineral deposits (hydrothermal vein and disseminated deposits, magmatic segregation, placer deposits and residual deposits); the conditions and host rocks typical of each; the distinction between ore and gangue and the idea that a deposit is economic only if the metal is concentrated well above its crustal average.
A focused answer to the OCR H414 dot point on ore formation. Covers hydrothermal vein and disseminated deposits, magmatic segregation, placer deposits and residual deposits, the conditions and host rocks of each, the distinction between ore and gangue, and the requirement that a metal be concentrated well above its crustal average to be economic.
- Hydrocarbons: the petroleum system (source rock, maturation, migration, reservoir rock, trap and seal); the formation of oil and gas from organic-rich source rocks by burial and heating; the properties needed in a reservoir (porosity and permeability) and a seal (low permeability); the types of trap (structural and stratigraphic); the formation of coal from plant material with increasing rank.
A focused answer to the OCR H414 dot point on hydrocarbons. Covers the petroleum system (source rock, maturation, migration, reservoir, trap and seal), the formation of oil and gas by burial and heating, the porosity and permeability needed in a reservoir, low-permeability seals, structural and stratigraphic traps, and the formation of coal with increasing rank.
- Groundwater: porosity and permeability and how they differ between rock types; aquifers, aquitards and the water table; confined and unconfined aquifers; the calculation of porosity from pore and total volumes; the use of a simple form of Darcy's law to relate groundwater discharge to hydraulic conductivity, hydraulic gradient and area; the issues of over-abstraction and contamination.
A focused answer to the OCR H414 dot point on groundwater. Covers porosity and permeability and how they vary between rock types, aquifers, aquitards and the water table, confined and unconfined aquifers, calculating porosity, using a simple form of Darcy's law for groundwater flow, and the issues of over-abstraction and contamination.
- Engineering geology: the engineering properties of rocks and soils (strength, jointing and discontinuities, weathering and the behaviour of clays, sands and gravels); the purpose and methods of site investigation (desk study, boreholes, trial pits and core logging); the ground conditions that cause problems for foundations (weak or compressible soils, swelling clays, solution cavities in limestone, made ground and high groundwater); the role of foundations and the ground model.
A focused answer to the OCR H414 dot point on engineering geology. Covers the engineering properties of rocks and soils, the purpose and methods of site investigation (desk study, boreholes, trial pits, core logging), the ground conditions that cause foundation problems (weak or swelling soils, solution cavities, made ground, groundwater), and the role of foundations and the ground model.
- Surface processes: mechanical weathering (freeze-thaw, exfoliation and abrasion) and chemical weathering (solution, hydrolysis and oxidation); the difference between weathering and erosion; transport by water, wind and ice and its effect on the rounding and sorting of sediment; how the maturity and texture of a sediment record its transport history.
A focused answer to the OCR H414 dot point on surface processes. Covers mechanical weathering (freeze-thaw, exfoliation, abrasion) and chemical weathering (solution, hydrolysis, oxidation), the difference between weathering and erosion, transport by water, wind and ice, and how rounding, sorting and maturity record a sediment's transport history.
Sources & how we know this
- OCR A Level Geology (H414) Specification — OCR (2017)