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(phenocrysts) and then subsequently more rapid cooling due a sudden rise of the magma to a shallow
depth or even a volcanic eruption
Associated with the magma, deep below a volcano, hot water circulation creates low-grade
mineralization adjacent to the walls of the solidifying magma. This is why exploration geologists look for
characteristic alternation in rocks. The chemical alteration of the rocks surrounding a porphyry are a
hint of the presence of ore.
Over time, erosion removes the overlying volcanoes, covering the porphyry deposits. Weathering
leaches rainwater through the higher rocks, with mineralization precipitating out as it hits the water
table. This creates the higher-grade “cap” mineralization that found in the upper layers of many
porphyry copper mines.

Bougainville Copper Ltd, Truck Park 1986

Mineralization

Porphyry copper deposits are typically very-
low grade with between 0.4 and 1% copper.
The main mineral is chalcopyrite, which is
finely disseminated through the host
porphyritic rocks.
The only reason these ore bodies are
economic is because porphyries are huge,
with hundreds of millions of tonnes available in a fairly low-cost mining setting.
Many porphyry copper mines have significant economic amounts of secondary minerals including gold
(usually called a gold copper porphyry), molybdenite (in the south-western USA), and silver. Grades of
these secondary minerals include molybdenum 0.005-0.03%, silver 0.004-0.35 g/t, and gold 0.2-5g/t
Porphyry copper deposits can also be associated with epithermal gold deposits formed by the same
volcanic complex above the deeper-seated porphyry. If alternation occurs in carbonate-rich rocks, then
unusual skarn mineralization can also develop.

Where Are Porphyry Copper Deposits Found?
The majority of porphyry coppers are associated with Tertiary age volcanism around the circum-pacific
plate boundary. Deposits are frequently found in clusters. The biggest mines are in Chile, Utah and
Indonesia. The deposits along the Pacific coast of North and South America are associated with plate
subduction while the more gold-rich deposits in Indonesia and Papua New Guinea are associated with
island arc volcanism behind the subduction zone

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