Thick crusts (>40 km) increase porphyry Cu ore potential by producing voluminous and hydrous magmas in long-lived (≥2–3 Ma) mid to lower crustal magma reservoirs at ∼30–70 km depth, which can result in the formation of supergiant to giant porphyry Cu deposits if a combination of other ore-forming conditions is fulfilled. Prolonged injection of hydrous basaltic magmas and accumulation of andesitic magmas in the mid to lower crust are prerequisites to forming large porphyry deposits because these processes are required to maintain a long-lived magmatic system and associated hydrothermal activity in the shallow crust.Ĭrustal thickness influences the duration and volume of magma activity, timing of sulfide saturation, chalcophile element fertility and emplacement depth of porphyry intrusions. Future studies should aim to identify the predominant sulfide phases in felsic systems to determine their influence on the behaviour of chalcophile elements during magma differentiation. A shallow depth (<7 km) of fluid exsolution might play a role in increasing Au precipitation efficiency, as immiscible sulfide melts act as a transient storage of chalcophile metals and liberate them to ore fluids. For example, in thick crust, prolonged magma activity with hydrous and oxidized evolving magmas increases ore potential, whereas thin crust favours high chalcophile element fertility, owing to late sulfide saturation. Crustal thickness influences the longevity of lower crustal magma reservoirs and the sulfide saturation history. Chalcophile metal fertility of mantle-derived arc magmas is primarily modulated by the amount and nature of residual sulfide phases in the mantle wedge during partial melting. In this Review, we discuss advances in understanding crustal magmatic conditions that favour the formation of porphyry Cu deposits at subduction zones.
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However, the magmatic and redox processes required to form economic-grade porphyry deposits remain poorly understood. They are important as major sources of critical metals for industry and society, such as copper and gold.
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Porphyry deposits are large, low-grade metal ore bodies that are formed from hydrothermal fluids derived from an underlying magma reservoir.