An ore deposit is simply an unusual concentration of valuable minerals — rich enough and big enough to mine at a profit. Almost all of the crust is barren rock; deposits are the rare spots where nature happened to pile the metal up. (From Unit 2: ore is "worth-it" rock, and grade and tonnage decide whether it pays.)

The good news for a beginner: nature has only a handful of engines for concentrating metal. Nearly every deposit type is one of these engines acting in a particular setting:

• Hot watery fluids — "hydrothermal" (hydro = water, thermal = hot). Hot fluids dissolve metal deep down, travel through cracks, and dump it where conditions change. This makes the biggest group of deposits.
• Cooling magma — metal separates straight out of a melt as it crystallises ("magmatic").
• Mountain-building — collisions heat and squeeze the crust, driving fluids that carry gold ("orogenic").
• Surface processes — weathering concentrates metal in soils, and rivers sort heavy grains.

Words you'll meet again and again

• Hydrothermal — involving hot, metal-carrying water.
• Intrusion — a body of magma that pushed into older rock and cooled there (Unit 1). A cooling intrusion is a powerful heat-and-fluid engine, so deposits often cluster around one.
• Host rock — the rock the ore actually sits in.
• Vein (ore filling a crack), disseminated (tiny ore grains sprinkled through the rock), stockwork (a dense mesh of small veins), massive (nearly solid ore).
• Metallogenic belt — a region where one deposit type repeats along a geological trend (like the Andean copper belt).