Porphyry Copper Deposits

The word “porphyry” is a textural term [not a compositional one] referring to large-grained crystals (called phenocrysts) dispersed in a fine-grained (aphanitic) crystal groundmass. There is commonly a large difference in size between the tiny (can be microscopic) matrix material and the much larger crystals (>2mm). 

The above image is an example of a rock with porphyry texture: a rhyolite (volcanic igneous rock), 12 cm in high. This rock came from a river deposit of Mio-Pliocene age in southern California. This rock does not contain copper.

The next image is an example of pure native copper (50 mm maximum dimension). It was extracted from a porphyry copper rock at a copper mine near Morenci, Greenlee County, Arizona. 

Porphyry copper deposits (PCDs) account for the world’s largest source (about 60%) of copper. These deposits consist of disseminated copper (low to moderate grade [0.3 to 2 percent] copper and copper minerals in vein systems and in fractured rocks (breccias) with porphyry texture and containing copper formed from hydrothermal (heated) fluids that emanated from plug-like magma chambers in the upper crust (less than 5 to 10 km depth). These veins invade host rock, which can range from granitoid  porphyry intrusions (granites to diorites) and from adjacent volcanic wall rocks (rhyolites to basalts). Along with copper, other ores can be associated, like gold, silver, iron, molybdenum, lead, zinc, and tungsten.

The above two views are of the same hand-specimen sample of a copper porphyry from the Keweenaw Peninsula, Michigan, near Lake Superior. The native copper is disseminated in a conglomeratic sandstone/shale interbedded with rift-zone basalt (a volcanic rock). The maximum dimension of this hand specimen is 60 mm.

Porphyry copper deposits have formed throughout most of Earth’s history, but 90 percent of the known ones are Cenozoic or Mesozoic in age. They form mainly in dynamic tectonic areas: under big volcanoes and over suduction zones. They can form, furthermore, in continental magmatic areas along convergent tectonic plate margins or in island-arc environments. PCDs are found predominantly along the west coast of North America (from Alaska to Mexico), the Andes in South America, the Alps region in southern Europe, central Asia, the southwestern Pacific, and the east coast of Australia. 

 

An example of a continental magmatic area in southwestern North America is the one extending from Nevada to Mexico. An example of an island-arc environment is in Papua (province)  (shown below) is in the southwestern Pacific Ocean area. 

Porphyry copper deposits are currently the largest source of copper ores. Because the content of copper is very low, in many cases the ores are mined via huge open-pit from which massive amounts of rock are excavated and processed. One of the world’s largest and richest PCDs-mines is at Grasberg, Papau, southwest Pacific Ocean. The images below, which are from Google Earth (2020) show the location of this mine, at increasing magnification. In the second image, you can see a road spirally descending into the very large open-pit mine.

The main source of the information I used about PCDs is the very thorough paper by John, D.A., et al. 2010. Porphyry copper deposit model. United States Geological Survey Scientific Investigations Report 2010-5070-B. Chapter B. 169 pp. The pdf is available online for free.