Ore Deposits

Ore deposits are rare natural concentrations of metals. Learn the main deposit types — porphyry copper, VMS, epithermal and orogenic gold, magmatic nickel, banded iron formations, sediment-hosted, placers and brines — how they form, their typical size and grade, and how deposit type guides exploration and mining method.

Copper averages only ≈ 60 ppm in the Earth's crust, but ore needs ≈ 0.3–1 % — a 50–150× concentration. Geological processes (magmas, hot fluids, weathering, sedimentation) create these anomalies. Knowing the deposit model tells explorers where to look and engineers what shape and grade to expect.

An analogy

Ore-forming fluids are like a coffee filter in reverse: hot water dissolves metals over a huge volume and drops them in one small place when conditions change.

Magmatic–hydrothermal deposits

Porphyry Cu–Mo–Au: huge (0.5–10 Bt), low grade (0.3–1 % Cu), around intrusions — mined by open pit and block caving. Epithermal Au–Ag: shallow veins, high grade. VMS (volcanogenic massive sulphide): Cu–Zn–Pb lenses formed at ancient seafloor vents.

Other important types

Orogenic gold (quartz veins in shear zones), magmatic Ni–Cu–PGE (Norilsk, Sudbury), banded iron formations (Pilbara, Carajás — 60 %+ Fe), sediment-hosted copper (Central African Copperbelt), placers (gold, tin, diamonds in rivers), kimberlites (diamonds), lithium pegmatites and brines, bauxite (weathered aluminium).

Deposit model → mining

Geometry follows genesis: porphyries are massive and disseminated (bulk methods), veins are narrow and steep (selective stoping), stratiform deposits are tabular (room-and-pillar, longwall), placers are loose (dredging).

Mineral systems approach

Modern exploration targets the whole system — source, pathway, trap — using geophysics, geochemistry and machine learning on regional data. Deeper, covered deposits require new technologies as outcropping discoveries decline.

Formulas

• Concentration factor: CF = ore grade / crustal abundance
• Contained gold: oz = T × g / 31.1035
• In-situ value: Value = T × g × price

Worked example: Gold ounces

Given: 10 Mt at 2.5 g/t Au

Find: Contained gold.

• Au = 10 × 10⁶ × 2.5 = 25 × 10⁶ g
• oz = 25 × 10⁶/31.1035 = 803 800 oz

Answer: ≈ 0.8 Moz

Worked example: Concentration factor

Given: Porphyry at 0.6 % Cu; Crustal abundance 60 ppm

Find: CF.

• 0.6 % = 6000 ppm
• CF = 6000/60 = 100

Answer: 100× enrichment

Common mistakes

• Wrong: Treating in-situ value as profit. — Right: Costs, recovery and dilution consume most in-situ value.
• Wrong: Assuming all gold is visible. — Right: Refractory gold is locked in sulphides.

Applications

• Exploration targeting: Deposit models guide where to drill.
• Method selection: Deposit geometry determines mining approach.
• Supply forecasting: Critical-mineral supply depends on deposit types.

Key takeaways

• Ore = rare concentration of metal.
• Deposit types differ in size, grade and shape.
• Deposit model guides exploration and method.
• oz = T × g/31.1035.

Going further

• Mineral systems: Source–pathway–trap framework for targeting.
• Prospectivity mapping: Machine learning on geoscience datasets.
• Deep exploration: Seismic and magnetotellurics beneath cover.