planet

Hydrothermal ore deposit

An ore body formed when hot water circulates through rock, dissolving metals in one place and precipitating them in another — and doing it persistently for millions of years, until the concentration runs thousands to tens of thousands of times above ordinary rock. Water is the essential agent, but the heat that keeps the circulation going usually comes from magma, and water-rich magma is concentrated at plate boundaries. That is why Earth's richest ore belts sit in predictable places, and why a world with an immobile crust struggles to make them.