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
. 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 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
- Ajo, Arizona
- Bagdad, Arizona
- Lavender Pit, Bisbee, Arizona
- Morenci, Arizona
- Pebble Mine, Alaska
- Safford Mine, Safford, Arizona
- San Manuel, Arizona
- Sierrita, Arizona
- El Chino, Santa Rita, New Mexico
- Ely, Nevada
- Bingham Canyon Mine, Utah
- Ray Mine, Arizona
- Cadia-Ridgeway Mine, New South Wales, a major porphyry copper-gold deposit hosting a large open pit and underground block cave mining operation.
- Northparkes copper porphyry deposit, New South Wales, with 63 million tonnes at 1.1% Cu and 0.5 ppm Au.
- Boddington, Western Australia, one of the largest Archaean gold porphyry deposits in Australia
- Spinifex Ridge, Western Australia, a large molybdenum-copper-silver polymetallic porphyry deposit proposed as Australia's first molybdenum mine in 2009-10.
- Moonmera, Queensland.
- Callawa, Western Australia - location being confirmed as porphyry by drilling mid 2008 at lat -20.684398 lon 120.439596
Papua New Guinea
- 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.