A few important deposits are built on a flood of iron-oxide minerals (magnetite or hematite) rather than on sulfides. Two related types matter most.

IOCG — Iron Oxide-Copper-Gold

These are large deposits where copper and gold ride along with abundant iron oxide, formed by hot, very salty (saline) fluids. Unusually, they're not clearly tied to a single intrusion the way a porphyry is. They often carry uranium and rare earths as bonuses, which makes some of them extraordinarily valuable.

The headliner is Olympic Dam in South Australia — a single giant that is at once a major copper, gold, uranium, and rare-earth deposit. There is nothing else quite like it on Earth.

Kiruna-type — iron oxide-apatite

A close cousin is almost pure iron: magnetite together with the phosphate mineral apatite, with little copper. These are major iron ore sources, named for Kiruna in Sweden — a mountain of magnetite mined for over a century.