Documents disponibles écrits par cet auteur

High Sr/Y arc magmas and porphyry Cu ± Mo ± Au deposits / Jeremy P. Richards in Economic geology, Vol. 106 N° 7 (Novembre 2011) 

Public ISBD [article]  in Economic geology > Vol. 106 N° 7 (Novembre 2011) . - pp. 1075-1081 Titre : High Sr/Y arc magmas and porphyry Cu ± Mo ± Au deposits : just add water Type de document : texte imprimé Auteurs : Jeremy P. Richards, Auteur Année de publication : 2011 Article en page(s) : pp. 1075-1081 Note générale : Economic geology Langues : Anglais (eng) Mots-clés : Arc  magmas  Au  deposits  Mineral  exploration Index. décimale : 001.424 Résumé : Porphyry Cu ± Mo ± Au deposits are commonly associated with intermediate composition arc-related igneous rocks with high Sr/Y (and La/Yb) ratios, and such rocks are equated by some researchers with melts derived from subducted oceanic crust undergoing eclogite facies metamorphism (“adakites”). However, similar geochemical characteristics are readily developed in normal asthenospheric mantle wedge-derived magmas by fractionation of amphibole and/or garnet, and/or by interaction with crustal materials during ascent through the upper plate lithosphere. While there is widespread evidence for such fractionation and contamination processes in porphyry magmas, there is little direct evidence for an origin by slab melting. The enhanced fertility of arc magmas relates primarily to their high water content, which is required for the formation of magmatic-hydrothermal systems upon shallow crustal emplacement. Magmatic water contents >4 wt percent H2O cause abundant amphibole (± garnet) fractionation but suppression of plagioclase crystallization at deep crustal levels, resulting in increasing Sr/Y and La/Yb ratios with differentiation, commonly into “adakitic” ranges, but not reflecting slab melting. Additional factors that affect arc magma fertility are relatively high oxidation states and sulfur content, but these are secondary to the requirement for sufficient water. Thus, arc magmatic suites with high Sr/Y ratios and evidence for the presence of hydrous phenocryst phases (hornblende and/or biotite) are indeed prospective for porphyry Cu ± Mo ± Au deposits, but only because these parameters indicate high magmatic water content. DEWEY : 001.424 ISSN : 0160-5682 En ligne : http://econgeol.geoscienceworld.org/content/106/7/1075.abstract [article] High Sr/Y arc magmas and porphyry Cu ± Mo ± Au deposits : just add water [texte imprimé] / Jeremy P. Richards, Auteur . - 2011 . - pp. 1075-1081. Economic geology Langues : Anglais (eng) in Economic geology > Vol. 106 N° 7 (Novembre 2011) . - pp. 1075-1081 Mots-clés : Arc  magmas  Au  deposits  Mineral  exploration Index. décimale : 001.424 Résumé : Porphyry Cu ± Mo ± Au deposits are commonly associated with intermediate composition arc-related igneous rocks with high Sr/Y (and La/Yb) ratios, and such rocks are equated by some researchers with melts derived from subducted oceanic crust undergoing eclogite facies metamorphism (“adakites”). However, similar geochemical characteristics are readily developed in normal asthenospheric mantle wedge-derived magmas by fractionation of amphibole and/or garnet, and/or by interaction with crustal materials during ascent through the upper plate lithosphere. While there is widespread evidence for such fractionation and contamination processes in porphyry magmas, there is little direct evidence for an origin by slab melting. The enhanced fertility of arc magmas relates primarily to their high water content, which is required for the formation of magmatic-hydrothermal systems upon shallow crustal emplacement. Magmatic water contents >4 wt percent H2O cause abundant amphibole (± garnet) fractionation but suppression of plagioclase crystallization at deep crustal levels, resulting in increasing Sr/Y and La/Yb ratios with differentiation, commonly into “adakitic” ranges, but not reflecting slab melting. Additional factors that affect arc magma fertility are relatively high oxidation states and sulfur content, but these are secondary to the requirement for sufficient water. Thus, arc magmatic suites with high Sr/Y ratios and evidence for the presence of hydrous phenocryst phases (hornblende and/or biotite) are indeed prospective for porphyry Cu ± Mo ± Au deposits, but only because these parameters indicate high magmatic water content. DEWEY : 001.424 ISSN : 0160-5682 En ligne : http://econgeol.geoscienceworld.org/content/106/7/1075.abstract

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

Mining, society and a sustainable world / Jeremy P. Richards in Economic geology, Vol. 105 N° 5 (Août 2010) 

Public ISBD [article]  in Economic geology > Vol. 105 N° 5 (Août 2010) . - p. 1039 Titre : Mining, society and a sustainable world Type de document : texte imprimé Auteurs : Jeremy P. Richards, Auteur Année de publication : 2011 Article en page(s) : p. 1039 Note générale : Economic geology Langues : Anglais (eng) Mots-clés : Mining  society Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/105/5/1039.extract [article] Mining, society and a sustainable world [texte imprimé] / Jeremy P. Richards, Auteur . - 2011 . - p. 1039. Economic geology Langues : Anglais (eng) in Economic geology > Vol. 105 N° 5 (Août 2010) . - p. 1039 Mots-clés : Mining  society Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/105/5/1039.extract