Porphyry copper deposits are an essential resource that is the dominant source of copper worldwide. Porphyry is the mineralogical term for an igneous rock (volcanic rock) consisting of large-grained crystals of feldspars and quartz embedded within a fine-grained, non-visible, groundmass. 60% of United states copper production is located in Arizona, American is fourth largest in copper production in the world behind the Northern Chile, Peru, and China.
Copper deposits caused by hydrothermal precipitation of groundwater heated by magma intruding into crust. A plate subducting beneath the American west coast melts as it sinks below the crust into the mantle. Subterranean water, from the ocean which the plate once lied beneath, is also pulled into the Earth. Water, when subjected to the immense mantle temperatures, can decrease the melting point of rock. A process called dehydration melting. Melted subterranean rock is commonly known as magma. Magma has a lower density than the surrounding mantle and crust, given this density difference the magma rises above the “some-what” liquid mantle (the mantle is actually solid, counter to popular belief, the immense pressures within the earth make rock in to a malleable silly putty-like texture) and slowly sweeps its way through the crust (figure 1). Other sources of magma can be volcanic hot-spots, caused by plumes of magma originating at the Core-mantle boundary striking the bottom of the crust.
These magmatic bodies heat groundwater flowing through the crust since magma is brought from inside the Earth, where the overwhelming majority of all metals are located, the water the interacts with these magmatic bodies carry dissolved minerals and precipitate them in weak spots like pore, fissures, and faults.
If magma does not reach the surface, it will cool and crystallize below the surface, this is called an intrusive igneous or plutonic rock, these rocks are usually very crystalline and with large colorful crystals thanks to the extraordinarily long time it takes to cool allowing for crystal growth. The copper deposits are called porphyry copper deposit because of hydrothermal fluids in direct association with porphyritic (large crystal) igneous rocks which form fractures and veins of precipitated minerals.
If these magma bodies succeed in breaking through the crust, it results in a formation of a volcano. The western United States is fertile grounds for these magma bodies, famous Volcanoes like Mt. Saint Helene’s and Mount Rainier are few examples of the massive volcanic field ranging from British Columbia to California.
This process is nearly identical to the methods behind gold deposits in the Western United States, like the Nevada gold deposits copper is also is very fine grained or low-grade, and can be as low as 0.15% of ore bodies, but the bulk of the deposits make mining highly economical.
Commonly, the discovery of Copper is alongside silver, gold, and molybdenum; these metals are sometimes the mine product. There are currently 574 known porphyry copper deposits, of which, almost all are mined from open pits.
What it lacks in grade, it makes up for in size.
Deposits range from hundreds of millions to billions of tonnes of ore, and 0.2% to greater than 1% copper
Typical mines last for many decades, Bingham mine owned by Rio Tinto is a major operation in Utah, operations commenced in 1906 and is set to expire in 2019 if new expansion project to 2030’s is not approved.
Porphyries are very diverse in that every deposit has a unique concentration of minerals. Some mines mostly gold and silver mine with relatively low copper and some are overwhelming copper deposits with little precious metals.