Porphyry Deposits Explained - El Salto TNR Gold Target

Porphyry Deposits Explained

Every investor in the mining industry has heard about porphyry deposits, yet few have a real understanding of what that term signifies. Many investors appreciate that porphyry deposits are often enormous in size and represent the most important source of copper and one of the leading sources of gold. In fact, the single largest supplier of gold in the world, with 2.5 million ounces of annual production, is the enormous Grasberg gold-copper mine in Indonesia, which is based on a porphyry deposit.In its simplest terms, porphyry deposits are formed when a mass of molten rock from deep in the crust works its way up to near the surface. Those magma bodies, often encompassing tens of cubic kilometers of hot molten rock, set off processes that "cook" the rocks for kilometers or even tens of kilometers around. That process can lead to the formation of ore deposits.Often, porphyry deposits encompass a district where the same process has been repeated a number of times over an area that sometimes extends for tens of kilometers.The most important part of the process of formation of a porphyry deposit is a hot water circulation system that can remain active for hundreds of thousands of years. In many cases, repeated pulses of magma keep the systems active for literally millions of years.The circulation of hot water serves to concentrate metals within the system. Water that is kilometers deep in the earth is heated by the molten rock to hundreds of degrees Celsius, and kept in the liquid state by the enormous pressure at that depth. When that superheated water is mixed with sulphur, chlorine and other reagents, it easily dissolves metals, including gold. Those fluids scavenge metals molecule by molecule from vast areas -- many tens of cubic kilometers of rock.The metal bearing fluids rise over the heat source. Where a process is present to extract the metals from solution, the metals are concentrated, occasionally to a sufficient extent to create an ore deposit.A porphyry deposit refers to the situation where the metals are deposited in a disseminated manner throughout a large volume of rock. Where larger conduits are present, such as in fracture zones, the metals are preferentially deposited along these structural corridors. Rocks tend to fracture over zones often tens of meters wide referred to as "fracture zones" or "shear zones".Porphyry deposits typically measure in the hundreds of millions of tonnes, with some deposits in excess of a billion tonnes. The most important economic metals are copper and gold.Porphyry deposits are frequently associated with other deposit types. For example, skarn deposits are formed when the metal bearing fluids encounter a limestone unit. The abrupt change in chemistry results in a sudden deposition of the metals.