This infographic demystifies copper prophyry deposits, how they formed, where, and why.
WHAT IS A PORPHYRY?
Porphyry deposits are very large, polymetallic systems that typically contain copper along with other important metals. Much of today’s mineral production depends on porphyries: 60% of copper, 95% of molbdenum, and 20% of gold comes from this deposit type.
WHERE AND HOW ARE PORPHYRIES FORMED?
Porphyries are most commonly found along the west coast of North and South America, as well as in the Southwest Pacific.
Porphyries are formed in tectonic plate convergent zones where oceanic crust has subducted beneath the continental crust, and in some cases the oceanic crust. As the plate subducts, the overlying upper mantle partially melts and the liquid magma rises to the surface. Hot fluids rise to the surface by flowing through cracks and fissures. Metals precipitate out of the solution as fluid cools and moves away from the heat source and pressure.
In porphyry mineralisation, there are many economic minerals that can be found: copper, gold, molybdenum, silver, lead, zinc, tin, and tungsten. There are also associated mineral deposits that can form that depend on the host rock and the distance from the heat source. These include skarn, epithermal, and breccia type deposits.
FOUR THINGS TO KNOW ON PORPHYRIES
- Polymetallic nature – Each porphyry is unique and holds different concentrations of minerals. Some deposits have such high concentrations of gold that they may be considered gold deposits rather than copper deposits. Others have barely any gold at all but may have plenty of molybdenum.
- Large Size, Low Grade – Porphyries typically have 100 million to 5 billion tonnes of ore with a lower grade (0.2% to >1% copper). It is the size of these deposits that allow for bulk mining and economies of scale.
- Long Mine Life – The size of porphyry systems usually mean that the life of the mine can be multi-decades long. This means that these deposits last through multiple market cycles, and are thus not as vulnerable to challenging market environments compared to other smaller mines.
- Infrastructure is Key – In order to process large amounts of ore, infrastructure can be a large part of initial capital expenditures (CAPEX). Access to power and water are key issues as large amounts of both are needed to process ore. The footprint of the mine and volume of tailings disposal can also make porphyries more challenging to permit.
Each year the Bingham Canyon Mine, located in Utah and owned by Rio Tinto and in production since 1906, produces approximately:
- 300,000 tons of copper
- 400,000 oz of gold
- 4,000,000 oz of silver
- 30,000,000 lbs of molybdenum
The value of the resources extracted to date from the Bingham Canyon Mine is greater than the Comstock Lode, Klondike, and California gold rush mining regions combined.