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porphyry deposit


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1 Porphyry copper occurs as large copper deposits centred around stocks of intermediate to acid, porphyritic, igneous rocks. Most occur in Mesozoic and Tertiary orogenic belts. They show concentric zones of minerals; for example at Bingham, Utah, there is an inner zone of Cu/Mo and an outer zone of Pb/Zn/Ag. The deposits are also characterized by extensive alteration halos. Most deposits are 3–8 km across and several kilometres deep. They consist of disseminated chalcopyrite and other sulphides mined on a large scale from open pits. The ore is low grade (less than 1% Cu) but of great economic importance. It was probably formed by a sudden release of volatiles near the surface, with shattering of the enclosing rocks.

2 A deposit of molybdenum-bearing ore, usually molybdenite, associated with rocks of porphyritic texture, and containing minor amounts of copper. It is formed in a similar manner to porphyry copper deposits, but the ore bodies are frequently shaped like inverted cups over the progenitor intrusive, e.g. those in the mineral belt of the USA.

3 A gold-enriched porphyry copper deposit frequently associated with island-arc environments. At the present time no true porphyry gold deposits are known, only those with co-product gold and copper.

Subjects: Earth Sciences and Geography.


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