Porphyry copper deposit

Geological background and economic significance Porphyry copper deposits represent an important resource and the dominant source of copper that is mined today to satisfy global demand. Via compilation of geological data, it has been found that the majority of porphyry deposits are Phanerozoic in age and were emplaced at depths of approximately 1 to 6 kilometres with vertical thicknesses on average of 2 kilometres. 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. Porphyry deposits are formed in arc-related settings and are associated with subduction zone magmas. There also appear to be discrete time periods in which porphyry deposit formation was concentrated or preferred. For copper-molybdenum porphyry deposits, formation is broadly concentrated in three time periods: Palaeocene-Eocene, Eocene-Oligocene, and middle Miocene-Pliocene. 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. Shallow subduction of young, buoyant slabs can result in the production of adakitic lavas via partial melting. Porphyry deposits are commonly developed in regions that are zones of low-angle (flat-slab) subduction. Arc reversal occurs due to collision between an island arc and either another island arc, a continent, or an oceanic plateau. The presence of intra-arc fault systems are beneficial, as they can localize porphyry development. 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. 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. ==Characteristics==

Characteristics of porphyry copper deposits include: • The orebodies are associated with multiple intrusions and dikes of diorite to quartz monzonite composition with porphyritic textures. • Breccia zones with angular or locally rounded fragments are commonly associated with the intrusives. The sulfide mineralization typically occurs between or within fragments. These breccia zones are typically hydrothermal in nature, and may be manifested as pebble dikes. • The deposits typically have an outer epidote – chlorite mineral alteration zone. • A quartz – sericite alteration zone typically occurs closer to the center and may overprint. • A central potassic zone of secondary biotite and orthoclase alteration is commonly associated with most of the ore. • Fractures are often filled or coated by sulfides, or by quartz veins with sulfides. Closely spaced fractures of several orientations are usually associated with the highest grade ore. • The upper portions of porphyry copper deposits may be subjected to supergene enrichment. This involves the metals in the upper portion being dissolved and carried down to below the water table, where they precipitate. Porphyry copper deposits are typically mined by open-pit methods. ==Notable examples==

Russia • Tominskoe, Ural • Mikheevskoe, Ural • Salavatskoe, Ural Mexico • Cananea • La Caridad • Santo Tomas Canada • Highland Valley • Gibraltar Mine Chile • Cerro Colorado • Resolution Copper, Superior, Arizona • El Chino, Santa Rita, New Mexico • Ely, Nevada • Ray Mine, Arizona Indonesia • Batu Hijau, Sumbawa • Grasberg, West Papua at >3 billion tonnes at 1 ppm Au, is one of the world's largest and richest porphyry deposits of any type • Tujuh Bukit, Java, still under exploration, but likely to be bigger than Batu Hijau • Sungai Mak and Cabang Kiri, Gorontalo, at 292 million tonnes at 0.50 ppm gold and 0.47% copper Australia • Cadia-Ridgeway Mine, New South Wales, copper-gold deposit mined by open pit and block caving. • Northparkes copper porphyry deposit, New South Wales, with 63 million tonnes at 1.1% Cu and 0.5 ppm Au. Papua New Guinea • Ok Tedi • Panguna/Bougainville Copper • Wafi-Golpu project/Wafi-Golpu mine Other • Coclesito, Panama • Majdanpek mine, Serbia ==Porphyry-type ore deposits for other metals==

Copper is not the only metal that occurs in porphyry deposits. There are also porphyry ore deposits mined primarily for molybdenum, many of which contain very little copper. Examples of porphyry molybdenum deposits are the Climax, Urad, Mt. Emmons, and Henderson deposits in central Colorado; the White Pine and Pine Grove deposits in Utah; the Questa deposit in northern New Mexico; and Endako in British Columbia. The US Geological Survey has classed the Chorolque and Catavi tin deposits in Bolivia as porphyry tin deposits. Some porphyry copper deposits in oceanic crust environments, such as those in the Philippines, Indonesia, and Papua New Guinea, are sufficiently rich in gold that they are called copper-gold porphyry deposits. ==References==