Mineral Resources and Ore Deposits

Core Idea

An ore is rock that contains a useful mineral concentrated enough to be mined at a profit. Most metals are scarce in average crust, so an ore deposit forms only where a geologic process concentrates them many times above normal levels. The main processes are crystallization and settling in magma, precipitation from hot water (hydrothermal fluids), deposition by sedimentary processes, sorting of heavy minerals by moving water (placers), and deep weathering that leaves insoluble minerals behind. Because these deposits form slowly and are mined quickly, mineral resources are nonrenewable on human time scales, and mining has environmental costs.

How It's Best Learned

Start with a concentration problem: if average crust holds a metal at a few parts per million, ask how much rock would have to be processed without natural concentration. Then match each type of deposit to the process that formed it and to a tectonic or surface setting, using a world map of major deposits. End with a case study of a single mine that follows the ore from formation to extraction to waste and reclamation.

Common Misconceptions

• Whether a rock counts as ore depends only on its geology. (It also depends on metal prices, mining costs, and technology, so a deposit can become ore or stop being ore as prices change.)
• Metal deposits are spread evenly through the crust. (Ore deposits are rare and cluster in particular settings, such as above subduction zones or in old, deeply weathered tropical soils.)
• Running out of a mineral means the atoms are gone from Earth. (Reserves are the part that can be mined profitably today; the total amount in the crust is far larger but mostly too dilute to extract.)

Explainer

You know the common rock-forming minerals and how igneous, sedimentary, and hydrothermal processes work. This topic asks where the metals and other materials of modern life come from.

Most useful elements are rare in ordinary rock. Copper, for example, makes up less than one hundredth of one percent of average continental crust. Mining that rock would be hopeless. An ore deposit forms where a natural process concentrates a metal many times above its average level. Whether a deposit is ore also depends on price and technology. Geologists call the part that can be mined profitably today a reserve, and the larger known amount a resource.

Several processes make deposits. In magmatic deposits, dense minerals crystallize early in a cooling magma and settle into layers. The Bushveld Complex in South Africa holds layers rich in chromite and platinum-group metals formed this way. Hydrothermal deposits form when hot water dissolves metals and then deposits them as it cools or reacts with rock. Many copper, gold, silver, lead, and zinc deposits form like this, including the large porphyry copper deposits of the Andes, which sit above a subduction zone. On the seafloor, hot springs at spreading ridges deposit metal sulfides around vents.

Sedimentary processes also concentrate materials. Most of the world's iron ore comes from banded iron formations, laid down in ancient oceans. Evaporating seawater leaves salt, gypsum, and potash. Placer deposits form when moving water sorts grains by density. Gold, diamonds, and tin minerals collect in stream and beach sediments because they are dense and resist wear. Residual deposits form by weathering. In hot, wet climates, rain leaches away most elements and leaves aluminum-rich bauxite behind.

Not every resource is a metal. By volume, the largest mined materials are sand, gravel, and crushed stone for construction. Phosphate rock supplies fertilizer.

Mining has costs. Open pits and waste piles disturb land. When sulfide minerals such as pyrite are exposed to air and water, they oxidize and form sulfuric acid. This acid mine drainage can release metals into streams for decades. Demand is now shifting toward lithium, cobalt, and rare earth elements for batteries, motors, and wind turbines, which raises new questions about supply, recycling, and environmental care.