Orogenic comes from orogeny — mountain-building, when plates collide and crumple the crust. This setting produces a huge share of the world's gold.

How it forms

As the crust is squeezed, heated, and metamorphosed during a collision, it releases hot fluids. Those fluids rise along big faults and shear zones (cracks where rock has slid) and deposit gold in quartz veins as they cool. (Recall faults and folds from Unit 1 — this is them at work.)

The signature: gold in quartz veins along deep faults, inside old, deformed (folded and faulted) metamorphic belts. This is often called lode gold — the hard-rock source that later erodes downstream to make placer gold (tying back to Unit 2). These systems can run kilometres deep, which is why some gold mines are the deepest in the world.

Where: the Golden Mile at Kalgoorlie (Australia), the Abitibi belt of Canada (Timmins, Kirkland Lake), the Mother Lode of California, Ashanti in Ghana, and the giant Muruntau deposit in Uzbekistan.