If you used copper today, it almost certainly came from a porphyry. These are the giants of the metal world: not very rich, but truly huge.

How they form

A big body of magma rises and cools as an intrusion. As it crystallises it releases hot watery fluids. Those fluids shatter the surrounding rock into a stockwork of tiny veins and spread copper — with molybdenum and gold — as disseminated grains and vein coatings through an enormous volume of rock.

The signature is low grade but gigantic tonnage. A porphyry might be only ~0.5% copper, but hold billions of tonnes of rock, so the metal adds up. They're mined in vast open pits.

("Porphyry" really names the speckled texture of the host intrusion, but in mining it names the whole deposit.) Around the centre, the fluids alter the rock into concentric bullseye zones that explorers read like a treasure map pointing inward to the ore.

Where: above subduction zones, where these magmas are common. The Andes of Chile and Peru are the planet's greatest copper belt; the SW Pacific (Indonesia, PNG, Philippines) is rich in gold-bearing porphyries.