Igneous Ore Deposits
Igneous rocks form from the solidification of molten rock called magma. Magmas also contain dissolved gases, and partly solidified magmas contain mineral grains, some metalliferous. As magma solidifies, metallic elements usually remain widely dispersed, but igneous processes can cause their concentration. In rare cases, dense metallic minerals settle to form metal-rich layers at the bottom of the magma chamber. Metal may also separate from the magma if the sulfur content rises to the point where a sulfur-rich magma forms, separates, and sinks. Many metals are naturally attracted to sulfur, and they separate with the new magma. These processes are thought to have formed some chromium, nickel, and platinum-rich layers within igneous rock, but a related process forms a wider variety of ores.