Porphyry deposits reportedly contribute up to half of the metal mined worldwide and account for two-thirds of the world's copper production. Typically, these deposits contain low to medium grade of 0.2% to 1% copper. Despite the low grades, porphyry copper deposits are economical due to their size, long mine life and high production rates. Secondary metals associated with porphyry deposits include gold, molybdenum, and silver.
Porphyry copper deposits are epigenetic in origin (mineral deposits formed later than the rocks enclosing it such as minerals found in veins), and are associated with porphyritic intrusive-related bodies. These deposits occur at convergent plate margins resulting in a subduction zone and are prevalent in continental & oceanic arc settings. Porphyry's are formed as hydrothermal solutions (hot, watery fluids derived from magma) are forced towards the surface as the less buoyant oceanic plate subducts below the continental crust. Initially, the hydrothermal solutions cools slowly deep in the crust then cools rapidly at a relatively shallow depth as it passes through fractured rock in and around an intrusion.
Mineralization associated with a porphyry system is typically characterized in one of the three following forms but may also be a combination of the following:
Disseminated - Mineralization occurs in a matrix and veinlets
Stockwork - Mineralization occurring in a network of larger veins
Breccia - Mineralization occurs in breccias
How Porphyry systems are formed:
Figure 1: Porphyry copper deposits are typically formed in a continental arc and island arc settings along the Pacific Ring of Fire - the west coast of North and South America to the Eastern portions of Asia. The subduction of oceanic tectonic plates results in strong generation and movement of hydrothermal solutions that eventually hardens near surface forming mineral deposits.