Copper forms as molten rock with small amounts of copper fluid crystallizes. As the larger rock continues to solidify, the fluid rich in copper moves into cracks, eventually solidifying as well. Because the copper solidifies later than other minerals within the larger rock, the final deposits have a high concentration.
Copper appears most frequently in deposits in which lead, silver, gold and zinc are also present. Porphyry copper deposits contain the most significant finds of copper; porphyry deposits began as large concentrations of molten rock that slowly cooled, crystallizing inside Earth's crust. During the cooling process, large crystals developed, with smaller ones forming around them; the smaller ones are porphyries.
Initially, the amount of copper in the molten rock is fairly small. As the molten rock cools, the other minerals solidify first, so that the copper remaining in the fluid elevates in concentration. The process of crystallization makes the rock shrink, forming cracks, and the fluid that is left moves into those cracks. By this point, the fluid contains a great deal of copper, so by the time it solidifies, it is almost pure copper. The deposits appear when the rocks on top of them erode, exposing the copper to mining operations.