How do ore deposits form and how is a deposit evaluated?
The processes that concentrate metals into ore deposits (magmatic segregation, hydrothermal veins, secondary enrichment, placer and sedimentary processes), the meaning of grade, cut-off grade and reserves, and the principles of evaluating and exploiting a mineral deposit.
A focused WJEC and Eduqas A-Level Geology G4 answer on how metals are concentrated into ore deposits by magmatic segregation, hydrothermal veins, secondary enrichment, placer and sedimentary processes, the meaning of grade, cut-off grade and reserves, and the principles of evaluating and exploiting a mineral deposit.
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What this dot point is asking
This dot point covers metallic resources: how nature concentrates a metal from its low background level into a mineable deposit, and the economic vocabulary used to decide whether to mine. WJEC wants the main ore-forming processes and a clear grasp of grade, cut-off grade and reserves. It builds on igneous processes (G1) and weathering (F2).
The answer
Why ore deposits are special
Most metals are present in ordinary rock only at trace levels. An ore is a rock from which a metal can be extracted at a profit, so an ore deposit is a place where a natural process has concentrated the metal far above its average crustal abundance.
Processes that concentrate metals
Grade, cut-off grade and reserves
Because the cut-off grade depends on price and cost, reserves are not fixed: a higher metal price lowers the cut-off grade, so lower-grade rock becomes economic and reserves grow.
Evaluating and exploiting a deposit
Evaluation uses geological mapping, geophysical and geochemical surveys and drilling to define the size, shape and grade of the deposit, from which the tonnage and average grade (and so the reserve) are calculated. Extraction is by opencast (shallow, large, low-grade) or underground (deep, higher-grade) mining, balanced against environmental impact and restoration.
Examples in context
The Cornish tin and copper veins are classic hydrothermal deposits formed by fluids from the cooling granite batholith, mined for centuries. The Bushveld Complex in South Africa concentrates chromium and platinum by magmatic segregation in a layered intrusion. Placer gold in river gravels (as in the historic gold rushes) shows current concentration of a dense, resistant mineral, and banded iron formations are the world's main iron source, a sedimentary precipitate.
Try this
Q1. Define an ore and explain why an ore deposit is unusual. [2 marks]
- Cue. An ore is rock from which a metal can be profitably extracted; a deposit is where a process has concentrated the metal far above its average crustal level.
Q2. Describe how a hydrothermal vein deposit forms. [2 marks]
- Cue. Hot, mineral-rich fluids move through fractures and precipitate metal minerals as they cool or react, forming veins.
Q3. Explain why a rise in metal price can increase a deposit's reserves. [2 marks]
- Cue. A higher price lowers the cut-off grade, so lower-grade rock becomes economic to mine, increasing the reserve.
Exam-style practice questions
Practice questions written in the style of WJEC exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
WJEC Eduqas 20206 marksDescribe three processes by which metals can be concentrated into economic ore deposits.Show worked answer →
Name and explain each process, because three distinct mechanisms are required.
Hydrothermal processes: hot, mineral-rich watery fluids (driven off a cooling intrusion or circulating through the crust) move through fractures and precipitate metal minerals as veins when they cool or react, for example lead, zinc, copper and gold in veins.
Magmatic segregation: dense metal minerals (such as chromite or magnetite) crystallise early from a cooling magma and sink to settle in layers at the base of an intrusion, concentrating the metal.
Secondary enrichment: weathering of a low-grade sulphide deposit dissolves and carries metals downward, reprecipitating them below the water table as a richer, enriched zone (for example enriched copper ores).
Placer deposits (also acceptable): dense, resistant minerals (gold, cassiterite) are concentrated by flowing water where the current drops them, for example in river gravels.
Markers reward hydrothermal vein precipitation, magmatic segregation of dense early minerals, and secondary enrichment or placer concentration, each correctly described.
WJEC Eduqas 20185 marksExplain the terms grade and cut-off grade, and how they determine whether a mineral deposit is worth mining.Show worked answer →
Define the terms and link them to the economic decision, because both definition and application are needed.
Grade is the concentration of the valuable metal in the rock, usually given as a percentage or in grams per tonne. A high-grade deposit has more metal per tonne of rock.
Cut-off grade is the lowest grade at which the deposit can be mined and processed at a profit, given the metal price and the costs of extraction.
A deposit is worth mining where its grade is above the cut-off grade for enough tonnage to make a viable reserve. If the metal price rises or costs fall, the cut-off grade falls, so lower-grade rock becomes economic and reserves increase; if the price falls, the cut-off rises and some rock becomes uneconomic.
So the decision compares the grade and tonnage against the cut-off grade set by prices and costs, and reserves are the part of the resource that is economic to extract.
Markers reward grade as metal concentration, cut-off grade as the lowest profitable grade, and the link of the mining decision and reserves to prices and costs.
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Sources & how we know this
- WJEC Eduqas A-level Geology specification — WJEC Eduqas (2017)