What are the main ways economic mineral deposits form?

Four processes concentrate metals above their crustal average. Hydrothermal deposits precipitate from hot mineral-rich fluids as veins (galena, gold) or disseminations (porphyry copper). Magmatic segregation settles dense early crystals (chromite, platinum) in basic intrusions. Placer deposits concentrate dense, resistant minerals (gold, cassiterite) where flowing water slows. Residual deposits leave insoluble minerals (bauxite) after intense chemical weathering. Each is associated with characteristic host rocks and conditions.

What is the difference between a resource and a reserve?

A resource is the total amount of a mineral known or estimated to be present, whether or not it can be extracted economically. A reserve is the part of the resource that can be extracted legally and economically with current technology and prices, so it is a subset. A rise in the metal price (or a fall in costs) makes lower-grade material profitable, moving it across the cut-off grade from resource into reserve; a price fall does the reverse.

What is the difference between porosity and permeability?

Porosity is the proportion of a rock that is pore space, controlling how much fluid it can store. Permeability is how well those pores are connected, controlling how easily fluid can flow through. The two are not the same: clay has high porosity but very low permeability because its pores are tiny and poorly connected, so it stores water but does not transmit it. A good aquifer or reservoir, such as sandstone, needs both.

What is the single most common mistake in this module?

Confusing porosity with permeability, and confusing a resource with a reserve. Porosity is storage (how much pore space); permeability is flow (how connected the pores are). A resource is everything known; a reserve is only the economically extractable part now, and a price rise converts resource into reserve. In calculations, remember to express grade and porosity as fractions of the whole, and that Darcy's law is Q equals K times i times A with a dimensionless gradient.

EnglandGeology

OCR A-Level Geology Module 5 natural resources and economic geology overview

Q: What makes a working petroleum system?

Five elements correctly arranged in space and time. A source rock (organic-rich shale) matures by burial and heating through the oil and gas windows. The hydrocarbons, being less dense than pore water, migrate upwards. A reservoir rock that is both porous (to store) and permeable (to flow) holds them. A trap (structural, such as an anticline or fault, or stratigraphic, such as a pinch-out or unconformity) capped by a low-permeability seal stops them escaping. Crucially, the trap and seal must form before migration, or the oil is lost.

A deep-dive OCR A-Level Geology guide to natural resources and economic geology. Covers ore-forming processes, mining geology with ore-grade and contained-metal calculations, the petroleum system, and groundwater with porosity, permeability and Darcy's law, with the exam patterns OCR repeats.

Generated by Claude Opus 4.818 min readOCR-H414-Module-5Updated 2026-06-02

Reviewed by: AI editorial process; not yet individually human-reviewed

Jump to a section

What this topic actually demands
Mineral deposits and ore formation
Mining geology and mineral resources
Hydrocarbons and petroleum systems
Groundwater, aquifers and Darcy's law
How this topic is examined
Check your knowledge

What this topic actually demands

Natural resources and economic geology applies the science of rocks and fluids to the materials society depends on: metals, hydrocarbons and water. The topic runs from how ore deposits form, through the economics of mining, to the petroleum system, and finally to groundwater. The examiners test two linked skills: explaining the processes and the economics, and the quantitative skills of contained-metal, porosity and Darcy's-law calculations.

This guide walks through the four clusters in a sensible order, then sets out the exam patterns OCR repeats. Each cluster has a matching dot-point page with practice questions; this overview ties them together.

Mineral deposits and ore formation

Most metals are too dilute in ordinary rock to mine, so a concentrating process must raise them far above their crustal average. The four processes are hydrothermal (precipitation from hot mineral-rich fluids, as veins or disseminations), magmatic segregation (dense early crystals settling in basic intrusions), placer (dense, resistant minerals concentrated where flowing water slows) and residual (insoluble minerals left after intense chemical weathering). An ore is a deposit extractable at a profit; the gangue is the worthless rest. Each process is tied to characteristic host rocks and conditions.

Mining geology and mineral resources

Whether a deposit is mined is economics as much as geology. Ore grade is the metal concentration, the cut-off grade is the lowest profitable grade, resources are all known mineral and reserves are the economically extractable part now. A price rise moves material from resource into reserve. The factors that decide mining are grade, tonnage, depth, location, technology, price and environmental constraints. Contained metal equals grade times tonnage (grade as a fraction). Extraction is open-pit (large, shallow, low-grade) or underground (deep, high-grade), with environmental issues of waste, tailings and acid mine drainage.

Hydrocarbons and petroleum systems

A petroleum system needs five elements correctly arranged. A source rock (organic-rich shale) matures by burial and heating through the oil and gas windows. The hydrocarbons migrate upwards because they are less dense than pore water. A reservoir that is both porous (to store) and permeable (to flow) holds them, and a trap (structural such as an anticline or fault, or stratigraphic such as a pinch-out or unconformity) capped by a low-permeability seal stops them escaping. The trap and seal must form before migration. Coal forms from buried plant material, rising in rank (peat, lignite, bituminous, anthracite) with burial.

Groundwater, aquifers and Darcy's law

Groundwater storage and flow are controlled by porosity (pore space, storage) and permeability (connectivity, flow), which are not the same: clay has high porosity but low permeability. An aquifer stores and transmits water (sandstone), an aquitard restricts it (clay), and the water table is the top of the saturated zone; aquifers may be unconfined or confined (artesian). Porosity equals pore volume over total volume, and a simple form of Darcy's law, $Q = KiA$ , relates discharge to hydraulic conductivity, hydraulic gradient and area. Over-abstraction lowers the water table (causing subsidence or saltwater intrusion) and contamination spreads with the flow.

How this topic is examined

A typical OCR profile for natural resources and economic geology:

Process and economics questions (Paper 1). Describing ore-forming processes, distinguishing ore from gangue, resource from reserve, and explaining the petroleum system.
Calculation questions (Paper 1). Contained metal from grade and tonnage, porosity from volumes, and discharge from Darcy's law.
Interpretation questions (Papers 2 and 3). Assessing whether a structure will hold oil, and reading porosity or permeability data for different rock types.
Level-of-response extended answers (Papers 1 and 2). Describing the five elements of a petroleum system, and comparing ore-forming processes, are predictable extended questions.

Check your knowledge

A mix of recall and application questions covering the whole topic. Attempt them under timed conditions, then check against the solutions.

Describe how a hydrothermal vein deposit forms. (3 marks)
Explain how a placer deposit concentrates dense minerals. (2 marks)
Distinguish a mineral resource from a mineral reserve. (2 marks)
A body contains 6 million tonnes at 1.2 percent copper. Calculate the contained copper. (2 marks)
Name the five elements of a petroleum system. (3 marks)
Explain why a reservoir rock must be both porous and permeable. (2 marks)
A rock has a total volume of 400 cm3 and a pore volume of 60 cm3. Calculate its porosity. (2 marks)
Using Darcy's law, calculate the discharge for K = 4 m per day, i = 0.05 and A = 200 m2. (2 marks)

Solutions

Q1: Hot, mineral-rich fluids (often heated by an intrusion) circulate through the crust and precipitate ore minerals when they cool, lose pressure or react with the wall rock, filling fractures as veins (for example galena, sphalerite or gold).
Q2: 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: A resource is all the known mineral present, economic or not; a reserve is the part that can be extracted legally and economically now (a subset of the resource).
Q4: $\frac{1.2}{100} \times 6\,000\,000 = 72\,000\ \mathrm{t}$ of copper.
Q5: Source rock, maturation, migration, reservoir rock, and trap with seal.
Q6: Porosity lets the reservoir store hydrocarbons in its pore spaces; permeability (connected pores) lets the hydrocarbons migrate in and flow out to a well. A porous but impermeable rock holds fluid that cannot be produced, so both are needed.
Q7: $\frac{60}{400} \times 100 = 15\%$ .
Q8: $Q = KiA = 4 \times 0.05 \times 200 = 40\ \mathrm{m^3\,day^{-1}}$ .