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Type of copper ore body

Porphyry copper deposits are copper ore bodies that are formed from hydrothermal fluids that originate from a voluminous magma chamber several kilometers below the deposit itself. Predating or associated with those fluids are vertical dikes of porphyritic intrusive rocks from which this deposit type derives its name. In later stages, circulating meteoric fluids may interact with the magmatic fluids. Successive envelopes of hydrothermal alteration typically enclose a core of disseminated ore minerals in often stockwork forming hairline fractures and veins. Because of their large volume, porphyry orebodies can be economic from copper concentrations as low as 0.15% copper and can have economic amounts of by products such as molybdenum, silver, and gold. In some mines, those metals are the main product. The first mining of low grade copper porphyry deposits from large open pits coincided roughly with the introduction of steam shovels, the construction of railroads, and a surge in market demand near the start of the 20th century. Some mines exploit porphyry deposits that contain sufficient gold or molybdenum, but little or no copper. Porphyry copper deposits are currently the largest source of copper ore. Most of the known porphyry deposits are concentrated in: western South and North America and Southeast Asia and Oceania – along the Pacific Ring of Fire; the Caribbean; southern central Europe and the area around eastern Turkey; scattered areas in China, the Mideast, Russia, and the CIS states; and eastern Australia. Only a few are identified in Africa, in Namibia and Zambia; none are known in Antarctica. The greatest concentration of the largest copper porphyry deposits is in northern Chile. Almost all mines exploiting large porphyry deposits produce from open pits.

Porphyry copper deposits represent an important resource and the dominant source of copper that is mined today to satisfy global demand. Via compilation of geological data, it has been found that the majority of porphyry deposits are Phanerozoic in age and were emplaced at depths of approximately 1 to 6 kilometres with vertical thicknesses on average of 2 kilometres. In general, porphyry deposits are characterized by low grades of ore mineralization, a porphyritic intrusive complex that is surrounded by a vein stockwork and hydrothermal breccias. Porphyry deposits are formed in arc related settings and are associated with subduction zone magmas. There also appear to be discrete time periods in which porphyry deposit formation was concentrated or preferred. For copper molybdenum porphyry deposits, formation is broadly concentrated in three time periods: Palaeocene Eocene, Eocene Oligocene, and middle Miocene Pliocene. The magmas responsible for porphyry formation are conventionally thought to be generated by the partial melting of the upper part of post subduction, stalled slabs that are altered by seawater. Shallow subduction of young, buoyant slabs can result in the production of adakitic lavas via partial melting. Porphyry deposits are commonly developed in regions that are zones of low angle (flat slab) subduction. Arc reversal occurs due to collision between an island arc and either another island arc, a continent, or an oceanic plateau. The presence of intra arc fault systems are beneficial, as they can localize porphyry development. At a given time differentiated magmas would burst violently out of these fault traps and head to shallower places in the crust where porphyry copper deposits would be formed. The deposits typically have an outer epidote – chlorite mineral alteration zone. A quartz – sericite alteration zone typically occurs closer to the center and may overprint. A central potassic zone of secondary biotite and orthoclase alteration is commonly associated with most of the ore.
Fractures are often filled or coated by sulfides, or by quartz veins with sulfides. Closely spaced fractures of several orientations are usually associated with the highest grade ore. The upper portions of porphyry copper deposits may be subjected to supergene enrichment. This involves the metals in the upper portion being dissolved and carried down to below the water table, where they precipitate. Porphyry copper deposits are typically mined by open pit methods.

Cadia Ridgeway Mine, New South Wales, copper gold deposit mined by open pit and block caving. Northparkes copper porphyry deposit, New South Wales, with 63 million tonnes at 1.1% Cu and 0.5 ppm Au.

Copper is not the only metal that occurs in porphyry deposits. There are also porphyry ore deposits mined primarily for molybdenum, many of which contain very little copper. Examples of porphyry molybdenum deposits are the Climax, Urad, Mt. Emmons, and Henderson deposits in central Colorado; the White Pine and Pine Grove deposits in Utah; the Questa deposit in northern New Mexico; and Endako in British Columbia. The US Geological Survey has classed the Chorolque and Catavi tin deposits in Bolivia as porphyry tin deposits. Some porphyry copper deposits in oceanic crust environments, such as those in the Philippines, Indonesia, and Papua New Guinea, are sufficiently rich in gold that they are called copper gold porphyry deposits.