By far the largest amounts of copper are found in the crust in bodies known as porphyry copper deposits. These deposits were once large masses of molten rock that cooled and solidified deep in the Earth's crust. As they cooled, some large crystals grew, which were then surrounded by smaller crystals - geologists call these rocks porphyries.
At first, the molten rock contained a small amount of copper. As it cooled and crystals began to form the amount of fluid became smaller. The copper remained in the fluid, becoming more and more concentrated. When the rock was almost completely solid, it contracted and cracked and the remaining copper-rich fluid was squeezed into the cracks, where it too finally solidified.
Over many millions of years the rocks covering these deposits eroded away and the deposits eventually appeared at the surface.
These deposits can contain 2 billion tonnes of rock which, when processed, gives 30 million tonnes of copper metal.
Examples of porphyry deposits include Cadia Hill (NSW) and Cerro Colorado (Panama).
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