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High Sr/Y magmas reflect arc maturity, high magmatic water content, and porphyry Cu ± Mo ± Au potential / Jeremy P. Richards in Economic geology, Vol. 107 N° 2 (Mars/Avril 2012) 

Public ISBD [article]  in Economic geology > Vol. 107 N° 2 (Mars/Avril 2012) . - pp. 295-332 Titre : High Sr/Y magmas reflect arc maturity, high magmatic water content, and porphyry Cu ± Mo ± Au potential : examples from the tethyan arcs of central and eastern Iran and western Pakistan Type de document : texte imprimé Auteurs : Jeremy P. Richards, Auteur ; Terry Spell, Auteur ; Esmaeil Rameh, Auteur Année de publication : 2012 Article en page(s) : pp. 295-332 Note générale : Economic geology Langues : Anglais (eng) Mots-clés : Tethayan  orogenic  belt  Arc  magmatism  Cu  ±  Mo  ±  Au  deposits  Prophyry-related  magmas  Iran-Pakistan Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : The temporal and geochemical evolution of arc magmatism that culminated in porphyry Cu ± Mo ± Au deposit formation has been studied in three separate Neo-Tethyan arc systems in central and eastern Iran, and western Pakistan. Porphyry Cu-Au deposits in the Lut block of eastern Iran formed in the middle Eocene at the end of a period of extensive Paleocene-Eocene volcanism; porphyry Cu-Mo deposits in the Kerman belt of central Iran formed in the middle Miocene at the end of a period of voluminous Eocene-Oligocene volcanism; and porphyry Cu-Au deposits in the Chagai belt of western Pakistan formed in four pulses during the Eocene, early Miocene, middle-late Miocene, and late Miocene-Pliocene, after a prolonged period of arc magmatism that began in the Late Cretaceous (and is still active). In each region, the late porphyry-related magmas are more geochemically evolved and more hydrous (as evidenced by the presence of hornblende phenocrysts) than the preceding volcanic rocks. We suggest that this reflects maturation of the arc magmatic system over a period of tens of millions of years, leading to the generation of more evolved, volatile-rich magmas at later stages of the arc’s history. High magmatic water contents are a prerequisite for the shallow crustal emplacement of arc magmas and the subsequent generation of potentially ore-forming subvolcanic magmatic-hydrothermal systems. It is thus suggested that the fertility of arc magmas within a given arc terrane can be assessed by observing the relative timing of plutonic suites (later suites are more prospective), noting the common presence of hornblende or biotite phenocrysts (indicating high magmatic water contents), and through lithogeochemical fingerprinting of magmatic fractionation processes (relatively high Sr/Y and La/Yb ratios, and Eun/Eu* ratios ≈1, indicating abundant early hornblende fractionation and suppression of plagioclase crystallization in hydrous magmas). DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/107/2/295.short [article] High Sr/Y magmas reflect arc maturity, high magmatic water content, and porphyry Cu ± Mo ± Au potential : examples from the tethyan arcs of central and eastern Iran and western Pakistan [texte imprimé] / Jeremy P. Richards, Auteur ; Terry Spell, Auteur ; Esmaeil Rameh, Auteur . - 2012 . - pp. 295-332. Economic geology Langues : Anglais (eng) in Economic geology > Vol. 107 N° 2 (Mars/Avril 2012) . - pp. 295-332 Mots-clés : Tethayan  orogenic  belt  Arc  magmatism  Cu  ±  Mo  ±  Au  deposits  Prophyry-related  magmas  Iran-Pakistan Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : The temporal and geochemical evolution of arc magmatism that culminated in porphyry Cu ± Mo ± Au deposit formation has been studied in three separate Neo-Tethyan arc systems in central and eastern Iran, and western Pakistan. Porphyry Cu-Au deposits in the Lut block of eastern Iran formed in the middle Eocene at the end of a period of extensive Paleocene-Eocene volcanism; porphyry Cu-Mo deposits in the Kerman belt of central Iran formed in the middle Miocene at the end of a period of voluminous Eocene-Oligocene volcanism; and porphyry Cu-Au deposits in the Chagai belt of western Pakistan formed in four pulses during the Eocene, early Miocene, middle-late Miocene, and late Miocene-Pliocene, after a prolonged period of arc magmatism that began in the Late Cretaceous (and is still active). In each region, the late porphyry-related magmas are more geochemically evolved and more hydrous (as evidenced by the presence of hornblende phenocrysts) than the preceding volcanic rocks. We suggest that this reflects maturation of the arc magmatic system over a period of tens of millions of years, leading to the generation of more evolved, volatile-rich magmas at later stages of the arc’s history. High magmatic water contents are a prerequisite for the shallow crustal emplacement of arc magmas and the subsequent generation of potentially ore-forming subvolcanic magmatic-hydrothermal systems. It is thus suggested that the fertility of arc magmas within a given arc terrane can be assessed by observing the relative timing of plutonic suites (later suites are more prospective), noting the common presence of hornblende or biotite phenocrysts (indicating high magmatic water contents), and through lithogeochemical fingerprinting of magmatic fractionation processes (relatively high Sr/Y and La/Yb ratios, and Eun/Eu* ratios ≈1, indicating abundant early hornblende fractionation and suppression of plagioclase crystallization in hydrous magmas). DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/107/2/295.short