Module 1 · The metal and the rock

How copper deposits form

Level 1 · Foundation 10 min read 4 questions

Copper is common in the crust and almost never concentrated enough to mine. Three geological processes do the concentrating, and which one applies decides everything downstream.

Copper is one of the more abundant metals in the Earth's crust, and that abundance is useless. What makes a deposit is a process that gathered the metal from a large volume of rock into a much smaller one over millions of years. Three processes matter commercially, and a fourth - weathering - modifies the result of the first.

Porphyry: the one that matters most

A body of magma rises towards the surface and begins to cool. Water and other hot fluids are expelled, scavenge copper from the surrounding rock, travel along fractures in the cracked rock above the intrusion, and precipitate sulfide minerals - principally chalcopyrite, a compound of copper, iron and sulfur - as conditions change. The result is a very large, very low-grade body of mineralised rock, typically one to five kilometres across.

Porphyry deposits account for the great majority of the world's mined copper, and they explain the map. Chile, Peru and the American Southwest all sit on or near the ancient collision zone where the Pacific plate has driven under the Americas - exactly the setting that produces the right kind of magma in the right kind of crust.

Sediment-hosted, and magmatic sulfide

Sediment-hosted stratiform

Copper minerals laid down in ancient sedimentary basins, probably by metal-bearing brines circulating through porous rock. The Congolese and Zambian Copperbelts formed this way, and they tend to carry higher grades than porphyries, with bornite and chalcocite alongside chalcopyrite. This is why the DRC punches so far above its weight in the production tables.

Magmatic sulfide

Copper, nickel and platinum-group metals separated together from a cooling magma body and settled as sulfide. Norilsk and Sudbury are the classic examples, and they are usually mined as much for nickel or PGMs as for copper.

Supergene enrichment: nature's free upgrade

Near the surface, copper sulfide deposits are chemically altered by weathering. Rainwater carrying oxygen and weak acids reacts with sulfides, dissolves the copper, and redeposits it lower down as much higher-grade secondary minerals - chalcocite and bornite below the water table, and carbonates such as malachite in the oxide zone above it. Geologists call this supergene enrichment, and it is why many deposits have a rich cap sitting on a leaner primary sulfide body.

The minerals that actually carry the metal

Hold on to the gap between those two facts. Chalcopyrite is 34% copper; the ore is under 1% copper. The difference is how much of the rock is not chalcopyrite - and closing that gap is what the concentrator in lesson 5 exists to do.