porphyry copper deposit

porphyry copper deposit

A large body of igneous rock, having distinct crystals in a relatively fine-grained base, that contains chalcopyrite and other sulfide minerals. These deposits contain vast amounts of ore that averages a fraction of 1percnt copper by weight; although low-grade, the deposits are important because they can be worked on a large scale at low cost. Large porphyry copper deposits are worked in the southwestern U.S. (where molybdenum may be produced as a by-product), the Solomon Islands, Canada, Peru, Chile, Mexico, and elsewhere.

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Porphyry copper deposits are copper orebodies which are associated with porphyritic intrusive rocks. The ore occurs as disseminations along hairline fractures as well as within larger veins, which often form a stockwork. The orebodies typically contain between 0.4 and 1 % copper with smaller amounts of other metals such as molybdenum, silver and gold. They are formed when large quantities of hydrothermal solutions carrying small quantities of metals pass through fractured rock within and around the intrusive and deposit the metals.

Porphyry copper deposits are the largest source of copper, and are found in North and South America, Europe, Asia, and Pacific islands. None are documented in Africa. The largest examples are found in the Andes in South America.

Characteristics

Characteristics of porphyry copper deposits include:

  • The orebodies are associated with multiple intrusions and dikes of diorite to quartz monzonite composition with porphyritic textures.
  • Breccia zones with angular or locally rounded fragments are commonly associated with the intrusives. The sulfide mineralization typically occurs between or within fragments.
  • 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.

Porphyry copper deposits are typically mined by open-pit methods.

Examples of porphyry copper deposits

Chile

Peru

United States

Australia

Papua New Guinea

Mongolia

  • Oyu Tolgoi is one of the world's largest and richest Cu porphyry desposits

Porphyry-type ore deposits for metals other than copper

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, and Henderson deposits in central Colorado, and the Questa deposit in northern New Mexico.

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.

References

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