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Diorite-type (andesitic) porphyry copper deposits can form in the forearc and back-arc zones of marginal-continental volcano-plutonic belts. The largest economic deposits are typically associated with forearc settings. Porphyry copper mineralization is in general related to orogenic volcanism. The typical sequence of magmatism involves initial andesitic volcanism, which is followed by the main phase of dioritic magmatism. This is in turn followed by porphyry copper mineralisation, which overlaps in time with late orogenic volcanic rocks and intrusions of alaskite-leucogranite-alkali granite compositions. The mineralizing intrusions are inferred to have been sourced by partial melting of late-oceanic crust. This can produce a magma chamber enriched in copper, rhenium and gold. The porphyry melt migrates through a series of intermediate intra-crustal magma chambers, eventually forming small, shallow-crustal porphyry intrusions.
Hydrothermal ore-formation occurs within and adjacent to the porphyry intrusions. Monzonite-type (shoshonitic) porphyry copper deposits can form in late-oceanic arc subduction-related settings, and also in marginal-continental rift zones. Small gabbroic to monzonitic intrusions and associated syenogranodiorite and granosyenite-porphyry intrusions form in late oceanic island arc settings, and are compositionally distinct from typical arc-related basalt-andesite-dacite-rhyolite and gabbro-diorite-granite magmatism. The small monzonite intrusions are separated temporally from the basalt-rhyolite and gabbro-granite suites in a given arc.
Monzonite porphyry mineralization occurs at the end of oceanic crust formation, probably during the post-oceanic tectonic reactivation, before the start of orogenic continental magmatism. Monzonite porphyry copper ores can be enriched in platinum group elements. They can contain high concentrations of rhenium and gold, and have elevated cobalt and nickel. The diorite and monzonite porphyry copper deposits form at different times and in different locations within oceanic island arcs under different geodynamic conditions.
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