Economic Geology
Domain focus
Economic Geology tests ore-forming systems, deposit classification, alteration and gangue assemblages, grade-tonnage calculations, and resource classification. The exam asks you to link a deposit style to tectonic setting and to calculate contained metal correctly.
Core rules and concepts
- Magmatic deposits include chromite, Ni–Cu sulfide, and PGE reefs; hydrothermal deposits include porphyry, epithermal, VMS, SEDEX, MVT, and orogenic gold.
- Porphyry Cu–Mo–Au systems show concentric alteration: potassic core, phyllic, argillic, and propylitic; mineralization is associated with multiple intrusive pulses.
- VMS deposits form at seafloor spreading centers; SEDEX and MVT are basinal brine systems; orogenic gold is structurally controlled in metamorphic terranes.
- Grade is metal concentration; tonnage is ore mass. Contained metal = tonnage × grade, with consistent units. Resources are not reserves until economic and technical factors are demonstrated.
Worked example
A deposit has 20 Mt at 1.5% Cu and 0.3 g/t Au. Cu metal = 20,000,000 t × 0.015 = 300,000 t Cu. Au = 20,000,000 t × 0.3 g/t = 6,000,000 g = 6 t Au. The reasoning is unit conversion: percent is parts per hundred, g/t is grams per tonne, and 1 t = 1,000 kg = 1,000,000 g.
Traps
Do not add grades from different elements as if they were one metal. Do not confuse resources with reserves. Do not ignore recovery and dilution in mineable estimates. Do not use a deposit model as proof of grade; it only predicts likely associations.