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These are rocks that have been altered by heat and pressure so much so that they become another
rock type. Often geologists can recognize the original rock that has been metamorphosed and will refer
to the rock as a “meta-sedimentary” rock or meta-volcanic. Though sometimes it is not possible to
confidently determine the original rock type.

Metamorphic rock from an Archean greenstone belt.
Rocks in old terranes, such as the Archean greenstone belts of Canada,
are often highly metamorphosed, while those of younger terranes may
not be as altered depending on their geologic setting.
Metamorphic rocks may often be labelled as “high grade” or “low grade
metamorphic”. These “grades” are are a reference to the temperature and
pressure to which the rock was exposed based on it’s texture and
mineralogy and does not have any specific economic meaning.
However, metamorphic processes may help concentrate economic
minerals.

Examples:
• Red Lake Greenstone Belt: Gold
• Broken Hill, Australia: Lead, Zinc, Silver
While there many, many more ways to classify rocks and mineral deposits, understanding the basics of
rock classification and mineral associations can go a long way in helping one understand the processes
involved and the descriptions of economic deposits published by geologists.
Home / Mineral Deposits / Porphyry Copper Deposits

Bougainville Copper Ltd, Papua New
Guinea looking across the pit - 1986

Porphyry Copper Deposits

Most of the
world’s
copper is extracted from a type of mineral deposit called a “porphyry
copper”. If you’ve seen a picture of a great big open-pit mining
operation odds are that it was a porphyry copper deposit. Some of the
biggest man-made holes in the ground are porphyry copper mines:
Bingham Copper Mine, Utah, and Chuquicamata in Chile, and two of the
largest.

A porphyry is not a rock type, but rock with a particular texture. This porphyritic rock has large feldspar
crystals in a finer grained matrix.
Today a porphyry is, if it’s talked about at all, associated with large-scale mining. However, not all
porphyries are mineralised; it is a general term to describe an igneous rock that has some large-grained
crystals (“phenocrysts”) within a fine-grained matrix.

How Are Porphyry Copper Deposits Formed?
The magma, which cools to form porphyry copper deposits, is created when two tectonic plates collide.
One plate is forced under the other in what is called a subduction zone. The porphyry forms at depth
below an active volcano. The magma cools in two stages; at first slowly cooling to form the large crystals

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