How do economically valuable mineral deposits form and become concentrated?
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.
Reviewed by: AI editorial process; not yet individually human-reviewed
Have a quick question? Jump to the Q&A page
Jump to a section
What this dot point is asking
OCR wants you to describe the processes that concentrate metals into economic mineral deposits (hydrothermal, magmatic segregation, placer and residual), to give the conditions and host rocks typical of each, to distinguish ore from gangue, and to explain why a metal must be concentrated well above its crustal average to be economic.
The answer
Ore, gangue and concentration
Most metals are present in ordinary rock at tiny concentrations, far too low to mine. A concentrating process must raise the metal many times above its crustal average before a deposit becomes economic.
Hydrothermal deposits
Hot, mineral-rich fluids (often water heated by a cooling intrusion) circulate through the crust and precipitate ore minerals when they cool, lose pressure or react with the wall rock:
- Veins. Fluids fill fractures, depositing minerals such as galena (lead), sphalerite (zinc) and gold along the fracture.
- Disseminated deposits. Fluids spread ore minerals finely through a large body of rock (for example copper in porphyry deposits).
Host: fractured rock around or above intrusions.
Magmatic segregation
Within a cooling magma, dense early-crystallising minerals settle to the floor of the chamber (linked to fractional crystallisation), concentrating heavy ore minerals such as chromite, magnetite and platinum-group metals in layers. Host: large basic and ultrabasic intrusions.
Placer deposits
Flowing water sorts sediment by density: where a current slows, dense, resistant minerals settle out while lighter grains are carried on, concentrating the heavy minerals. Examples: gold, cassiterite (tin), zircon and ilmenite in river gravels and beach sands. The minerals must be dense and resistant to survive transport and concentrate.
Residual deposits
Intense chemical weathering in warm, wet climates dissolves and removes soluble material, leaving insoluble valuable minerals concentrated in the weathered residue. Example: bauxite (aluminium ore) and laterites, formed by deep weathering. Host: deeply weathered profiles over suitable parent rock.
Examples in context
Example 1. Porphyry copper deposits. Hot fluids from a cooling intrusion disseminate low-grade copper through a large volume of rock; despite the low grade, the huge tonnage makes these the world's main source of copper, a classic hydrothermal example.
Example 2. Bauxite in the tropics. Deep, prolonged chemical weathering of aluminium-rich rocks in warm, wet climates removes soluble elements and leaves a residual concentration of aluminium hydroxides (bauxite), the chief aluminium ore.
Try this
Q1. Distinguish an ore from gangue. [2 marks]
- Cue. An ore is a deposit from which a metal can be extracted at a profit; gangue is the worthless minerals occurring with it that must be removed.
Q2. Explain how a placer deposit forms. [2 marks]
- Cue. Flowing water sorts grains by density; where the current slows, dense, resistant minerals (for example gold) settle out and concentrate while lighter grains are carried on.
Q3. Name the process that forms bauxite and the conditions it needs. [2 marks]
- Cue. Residual concentration by intense chemical weathering, in warm, wet (tropical) climates, which removes soluble material and leaves insoluble aluminium minerals.
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 20196 marksDescribe how hydrothermal, placer and residual processes each concentrate minerals into economic deposits, giving a typical example of a mineral concentrated by each.Show worked answer →
A level-of-response answer; describe each process and its product.
- Hydrothermal
- Hot, mineral-rich fluids (often heated by an intrusion) circulate through the crust and precipitate ore minerals when they cool or react with the wall rock, filling fractures as veins or spreading through the rock as disseminations. Example: galena (lead) or sphalerite (zinc) in hydrothermal veins; copper in disseminated porphyry deposits.
- Placer
- Flowing water (rivers, beaches) sorts sediment by density, concentrating dense, resistant minerals where the current slows, because they settle out while lighter grains are carried on. Example: gold, or cassiterite (tin), or zircon, concentrated in river gravels and beach sands.
- Residual
- Intense chemical weathering in warm, wet climates removes soluble material and leaves behind insoluble, valuable minerals concentrated in the weathered residue. Example: bauxite (aluminium ore) formed by deep weathering of aluminium-rich rocks; laterites.
Top-band answers correctly describe each concentrating process and give a valid mineral example for each (hydrothermal vein metal, placer dense mineral, residual weathering product).
OCR H414/01 20184 marksExplain what is meant by ore and gangue, and explain why a metal must be concentrated well above its average crustal abundance for a deposit to be economic.Show worked answer →
Define both terms, then reason about concentration and cost.
Ore and gangue. An ore is a rock or mineral deposit from which a metal (or mineral) can be extracted profitably. Gangue is the worthless (non-valuable) minerals occurring with the ore, which must be separated and discarded.
Why concentration matters. Most metals occur at very low average concentrations in ordinary crustal rock, far too low to extract profitably, because the cost of mining and processing huge volumes would exceed the value of the metal. A deposit is economic only where a geological process has concentrated the metal many times above its crustal average, so that enough metal can be recovered per tonne of rock to cover the costs and yield a profit. This concentration factor is what turns a mineral occurrence into an ore.
Markers reward ore (extractable profitably) versus gangue (worthless associated minerals), and the point that profitability requires concentration well above the crustal average.
Related dot points
- 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).
- 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.
- Igneous processes: Bowen's reaction series as the order in which silicate minerals crystallise from a cooling magma; the discontinuous (olivine to biotite) and continuous (calcium-rich to sodium-rich plagioclase) branches; the use of the series to explain fractional crystallisation, magma differentiation and the resistance of minerals to weathering.
A focused answer to the OCR H414 dot point on Bowen's reaction series. Covers the discontinuous and continuous branches, the crystallisation order of silicate minerals, how fractional crystallisation drives magma differentiation from basic to acid compositions, and how the series predicts weathering resistance.
- 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)