Economic Geology

9 min read

Economic geology studies materials of economic value derived from the Earth — metals, industrial minerals, fossil fuels, gemstones and groundwater. Mineral resources have underpinned every civilisation from the Stone Age to the digital era; today, the energy transition is reshaping demand patterns, driving exploration for lithium, cobalt, copper and rare-earth elements.

Ore

A rock or mineral aggregate that can be extracted at a profit. An ore mineral is one carrying the economic element (e.g., chalcopyrite for copper); the gangue is the non-valuable accompanying material. A deposit becomes an ore only when grade × tonnage × extractability exceeds the cost threshold.

Classification of mineral deposits

Mineral deposits are usually classified by their genetic process:

Magmatic deposits

Form from cooling magma. Examples:

Chromite (Cr) and platinum-group elements (PGEs) in layered mafic intrusions (Bushveld, Stillwater).
Nickel-copper sulphides in komatiitic flows (Sudbury, Kambalda).
Diamonds in kimberlite pipes — fragments of mantle rock.

Hydrothermal deposits

Form from circulating hot fluids depositing minerals in fractures or pore spaces. Subtypes:

Porphyry copper-molybdenum deposits: low-grade, large tonnage, in association with intermediate intrusions (Chile, Peru).
Skarn deposits: replacement of limestone by Cu, Zn, Pb, W, Sn near intrusions.
Vein and disseminated gold deposits: classic mesothermal vein gold (Witwatersrand, Mother Lode).
Volcanogenic massive sulphide (VMS): Cu-Zn-Pb-Ag near submarine volcanoes (Cyprus, Kuroko deposits).
Sedimentary exhalative (SEDEX): stratiform Zn-Pb-Ag formed in anoxic basins (Mt Isa, Sullivan).
Mississippi Valley-Type (MVT): low-T Zn-Pb in carbonates, distal from intrusions.

Sedimentary deposits

Banded Iron Formations (BIFs): world's main source of iron ore. Deposited from anoxic Proterozoic oceans (Hamersley, Brazil).
Manganese nodules on deep-sea floor (commercial exploitation pending).
Phosphates (Florida, Morocco) for fertiliser.
Evaporites: halite, gypsum, sylvite (Khewra, Saskatchewan).
Placer deposits: heavy minerals concentrated by water or wind (alluvial gold, ilmenite-zircon sands).

Residual / weathering deposits

Bauxite (Al-rich laterite) — sole commercial aluminium ore.
Nickel laterites — important Ni source in tropical regions.
Supergene-enriched copper — leached and reprecipitated near the surface.

Key Points

Grade: concentration of valuable element (% Cu, g/t Au).
Reserve = economically extractable subset of a resource with current technology and price.
Cut-off grade separates ore from waste; rises if costs rise or price falls.
Mining methods: open-pit (low grade, near surface), underground (high grade, deep), in-situ leaching (uranium, copper), placer (alluvium).

Fossil fuels

Fuel	Origin	Use
Coal	Compaction of plant matter in swamps	Power generation, steel-making
Petroleum (oil)	Anaerobic decay of marine plankton in source rocks; migration to reservoirs	Transport fuels, petrochemicals
Natural gas	Same source rocks; gaseous hydrocarbons (mostly CH₄)	Heating, electricity, fertiliser feedstock
Oil shale	Kerogen-rich shale; not yet matured	Synthetic crude (energy-intensive extraction)
Tar sands	Heavy oil in unconsolidated sand (Athabasca)	Synthetic crude

Coal ranks from peat → lignite → bituminous → anthracite reflect increasing carbon content and energy density. A petroleum system requires: (1) source rock rich in organic matter; (2) maturation at "oil window" temperatures (~60–120 °C); (3) migration pathway; (4) reservoir rock with porosity and permeability; (5) trap (structural — anticline, fault; or stratigraphic — pinchout, reef); (6) seal (impermeable cap).

Pakistan's resources

Coal: Thar coalfield (Sindh) is among the world's largest lignite deposits (~175 billion tonnes).
Natural gas: Sui (Balochistan, discovered 1952) historically supplied much of the country.
Oil: limited reserves in the Potwar Plateau and Sindh.
Copper-gold: Reko Diq (Chagai, Balochistan) — world-class porphyry deposit, currently being developed.
Salt: Khewra Salt Mine (Punjab), world's second-largest salt mine.
Chromite: Muslim Bagh ophiolite (Balochistan).
Marble and granite: widespread in Khyber-Pakhtunkhwa and Balochistan.
Gemstones: emerald (Swat), ruby (Hunza), peridot (Kohistan), aquamarine, topaz.

Groundwater and hydrogeology

Groundwater is one of the most important geological resources, supplying potable water and irrigation in arid and semi-arid regions. Key concepts:

Aquifer: a permeable rock unit capable of storing and transmitting useful quantities of water.
Aquiclude / aquitard: impermeable or poorly permeable.
Water table: top of the saturated zone in an unconfined aquifer.
Confined aquifer: bounded above by an impermeable layer; water may be artesian.
Darcy's law: groundwater discharge Q = −K·A·(dh/dl), where K is hydraulic conductivity, A is cross-sectional area, and dh/dl is hydraulic gradient.

The Indus Basin Aquifer is one of the world's largest, but is being depleted faster than it recharges in many districts of Punjab and Sindh.

Environmental geology

Modern economic geology cannot ignore environmental costs:

Acid mine drainage — sulphide oxidation produces sulfuric acid, mobilising heavy metals.
Tailings dam failures — catastrophic releases (Brumadinho 2019, Mariana 2015).
Land subsidence — over-pumping aquifers and coal mining.
Mercury pollution in artisanal gold mining.
Carbon emissions from coal and oil drive climate change.

Sustainable mining now emphasises minimising waste, restoring landscapes, and recycling metals to reduce demand for new ores. The circular economy is an essential complement to mining.

For competitive exams, know:

Reko Diq (copper-gold, Balochistan).
Thar Coal (Sindh, lignite).
Sui (natural gas, Balochistan).
Khewra (salt, Punjab). These are the marquee Pakistani deposits routinely tested in current-affairs and geology questions.

Strategic and "critical" minerals

The energy transition is creating dependencies on a new set of strategic commodities:

Lithium (batteries) — chiefly from Australian spodumene and South American brines.
Cobalt (batteries) — DRC produces ~70 % of world output.
Copper (electrification) — Chile, Peru, USA, DRC.
Nickel (stainless steel, batteries) — Indonesia, Philippines, Russia.
Rare-earth elements (REE) — magnets in wind turbines and EVs; China dominates production.
Graphite (battery anodes).

Future economic geology will increasingly focus on these commodities, on recycling, and on the geopolitics of supply chains — making the field as relevant in the 21st century as it was during the industrial revolution.