Porphyry copper deposit
Successive envelopes of hydrothermal alteration typically enclose a core of disseminated ore minerals in often stockwork-forming hairline fractures and veins.
The first mining of low-grade copper porphyry deposits from large open pits coincided roughly with the introduction of steam shovels, the construction of railroads, and a surge in market demand near the start of the 20th century.
Porphyry copper deposits are currently the largest source of copper ore.[1] Most of the known porphyry deposits are concentrated in: western South and North America and Southeast Asia and Oceania – along the Pacific Ring of Fire; the Caribbean; southern central Europe and the area around eastern Turkey; scattered areas in China, the Mideast, Russia, and the CIS states; and eastern Australia.
[7] In general, porphyry deposits are characterized by low grades of ore mineralization, a porphyritic intrusive complex that is surrounded by a vein stockwork and hydrothermal breccias.
This relatively young age reflects the preservation potential of this type of deposit; as they are typically located in zones of highly active tectonic and geological processes, such as deformation, uplift, and erosion.
[8] It may be however, that the skewed distribution towards most deposits being less than 20 million years is at least partially an artifact of exploration methodology and model assumptions, as large examples are known in areas which were previously left only partially or under-explored partly due to their perceived older host rock ages, but which were then later found to contain large, world-class examples of much older porphyry copper deposits.
[10] The magmas responsible for porphyry formation are conventionally thought to be generated by the partial melting of the upper part of post-subduction, stalled slabs that are altered by seawater.
[7] After dehydration, solute-rich fluids are released from the slab and metasomatise the overlying mantle wedge of MORB-like asthenosphere, enriching it with volatiles and large ion lithophile elements (LILE).
[7] From this point forward in the evolution of a porphyry deposit, ideal tectonic and structural conditions are necessary to allow the transport of the magma and ensure its emplacement in upper-crustal levels.
[8] There are five key factors that can give rise to porphyry development: 1) compression impeding magma ascent through crust, 2) a resultant larger shallow magma chamber, 3) enhanced fractionation of the magma along with volatile saturation and generation of magmatic-hydrothermal fluids, 4) compression restricts offshoots from developing into the surrounding rock, thus concentrating the fluid into a single stock, and 5) rapid uplift and erosion promotes decompression and efficient, eventual deposition of ore.[12] Porphyry deposits are commonly developed in regions that are zones of low-angle (flat-slab) subduction.
[8] The subducting slab can be lifted by aseismic ridges, seamount chains, or oceanic plateaus – which can provide a favourable environment for the development of a porphyry deposit.
[14] It has been proposed that "misoriented" deep-seated faults that were inactive during magmatism are important zones where porphyry copper-forming magmas stagnate allowing them to achieve their typical igneous differentiation.
[15] At a given time differentiated magmas would burst violently out of these fault-traps and head to shallower places in the crust where porphyry copper deposits would be formed.
