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Age and origin of orogenic gold mineralization in the otago schist belt, south island, New Zealand / James K. Mortensen in Economic geology, Vol. 105 N° 4 (Juin/Juillet 2010) 

Public ISBD [article]  in Economic geology > Vol. 105 N° 4 (Juin/Juillet 2010) . - pp. 777-793 Titre : Age and origin of orogenic gold mineralization in the otago schist belt, south island, New Zealand : constraints from lead isotope and 40Ar/39Ar dating studies Type de document : texte imprimé Auteurs : James K. Mortensen, Auteur ; David Craw, Auteur ; Douglas J. MacKenzie, Auteur Année de publication : 2011 Article en page(s) : pp. 777-793 Note générale : Economic geology Langues : Anglais (eng) Mots-clés : Orogenic  gold  Lead  isotope Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : The Otago Schist belt of southern New Zealand hosts numerous orogenic gold deposits that formed in a range of structural and lithological settings during and after Mesozoic metamorphism. Previous 40Ar/39Ar dating studies in the Otago Schist belt indicate formation ages of ~106 to 101 Ma for late, postmetamorphic, gold-bearing quartz veins and shear zones. Samples of hydrothermal muscovite from several mineralized quartz vein systems were dated using the 40Ar/39Ar method. Muscovite from a gold-bearing quartz stockwork vein at the Macraes mine, which is a relatively old mineralized system that formed during the latter stages of metamorphism, yields a well-defined plateau age of 135.1 ± 0.7 Ma. Fine-grained muscovite from a quartz shear vein at Macraes gives a rising age spectrum with a maximum age of 135.7 Ma. Five 40Ar/39Ar dates from the scheelite-bearing vein swarm in the Glenorchy district are interpreted to indicate that mineralization occurred between 142 and 134 Ma, and that relatively slow cooling and/or minor thermal overprints have disturbed the age spectra of some of the samples. Lead isotope compositions for a total of 42 sulfide samples from 17 separate mineralized zones in the Otago Schist belt are relatively radiogenic, consistent with an entirely upper crustal source for the Pb (and presumably Au). The Pb data define a less radiogenic and a more radiogenic cluster, with no overlap. Lead isotope compositions from the sulfides from the Macraes group of deposits form part of a less radiogenic cluster, together with Pb isotope compositions for sulfides from scheelite-rich veins in the Glenorchy camp. Lead isotope compositions for sulfides from younger, postmetamorphic quartz veins and shear zones fall exclusively in the more radiogenic cluster. The new 40Ar/39Ar dating results and Pb isotope studies, in conjunction with previously published work, demonstrate that most orogenic gold mineralization within the Otago Schist belt formed during two discrete mineralizing events—one at 142 to 135 Ma and a later one at approximately 106 to 101 Ma. We interpret the two pulses of gold mineralization to reflect sequential extraction of ore-forming components from the deeper part of the Otago Schist belt during short-lived thermal events. The younger event was probably associated with subduction of an actively spreading ridge, and similar ridge subduction may have been involved in the older event. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/105/4/777.abstract [article] Age and origin of orogenic gold mineralization in the otago schist belt, south island, New Zealand : constraints from lead isotope and 40Ar/39Ar dating studies [texte imprimé] / James K. Mortensen, Auteur ; David Craw, Auteur ; Douglas J. MacKenzie, Auteur . - 2011 . - pp. 777-793. Economic geology Langues : Anglais (eng) in Economic geology > Vol. 105 N° 4 (Juin/Juillet 2010) . - pp. 777-793 Mots-clés : Orogenic  gold  Lead  isotope Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : The Otago Schist belt of southern New Zealand hosts numerous orogenic gold deposits that formed in a range of structural and lithological settings during and after Mesozoic metamorphism. Previous 40Ar/39Ar dating studies in the Otago Schist belt indicate formation ages of ~106 to 101 Ma for late, postmetamorphic, gold-bearing quartz veins and shear zones. Samples of hydrothermal muscovite from several mineralized quartz vein systems were dated using the 40Ar/39Ar method. Muscovite from a gold-bearing quartz stockwork vein at the Macraes mine, which is a relatively old mineralized system that formed during the latter stages of metamorphism, yields a well-defined plateau age of 135.1 ± 0.7 Ma. Fine-grained muscovite from a quartz shear vein at Macraes gives a rising age spectrum with a maximum age of 135.7 Ma. Five 40Ar/39Ar dates from the scheelite-bearing vein swarm in the Glenorchy district are interpreted to indicate that mineralization occurred between 142 and 134 Ma, and that relatively slow cooling and/or minor thermal overprints have disturbed the age spectra of some of the samples. Lead isotope compositions for a total of 42 sulfide samples from 17 separate mineralized zones in the Otago Schist belt are relatively radiogenic, consistent with an entirely upper crustal source for the Pb (and presumably Au). The Pb data define a less radiogenic and a more radiogenic cluster, with no overlap. Lead isotope compositions from the sulfides from the Macraes group of deposits form part of a less radiogenic cluster, together with Pb isotope compositions for sulfides from scheelite-rich veins in the Glenorchy camp. Lead isotope compositions for sulfides from younger, postmetamorphic quartz veins and shear zones fall exclusively in the more radiogenic cluster. The new 40Ar/39Ar dating results and Pb isotope studies, in conjunction with previously published work, demonstrate that most orogenic gold mineralization within the Otago Schist belt formed during two discrete mineralizing events—one at 142 to 135 Ma and a later one at approximately 106 to 101 Ma. We interpret the two pulses of gold mineralization to reflect sequential extraction of ore-forming components from the deeper part of the Otago Schist belt during short-lived thermal events. The younger event was probably associated with subduction of an actively spreading ridge, and similar ridge subduction may have been involved in the older event. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/105/4/777.abstract

Longevity of porphyry copper formation at Quellaveco, Peru / Richard H. Sillitoe in Economic geology, Vol. 105 N° 6 (Septembre/Octobre 2010) 

Public ISBD [article]  in Economic geology > Vol. 105 N° 6 (Septembre/Octobre 2010) . - pp. 1157-1162 Titre : Longevity of porphyry copper formation at Quellaveco, Peru Type de document : texte imprimé Auteurs : Richard H. Sillitoe, Auteur ; James K. Mortensen, Auteur Année de publication : 2011 Article en page(s) : pp. 1157-1162 Note générale : Economic Geology Langues : Anglais (eng) Mots-clés : Porphyry  copper  Geochronology  Peru Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : Metal introduction at the late Paleocene to early Eocene Quellaveco porphyry copper-molybdenum deposit in southern Peru spans several phases of quartz monzonite porphyry emplacement and is bracketed by a precursor granodiorite pluton and a late-mineral porphyry body that postdates essentially all copper introduction. Together, the U-Pb ages of zircons from these intrusive rocks show that 1.08 ± 0.58 m.y. elapsed between the precursor pluton and initiation of stock emplacement; the porphyry system was active intermittently for at least 3.25 m.y. (4.07 ± 0.82 m.y.); and at least three-quarters of the copper inventory was deposited in a maximum of 3.12 m.y. (2.51 ± 0.61 m.y.). Recent U-Pb zircon dating of several other major central Andean porphyry copper deposits, in combination with other isotopic techniques, suggests that 2.5- to 4-m.y. life spans are commonplace. The longevity of porphyry copper systems implied by these studies appears to reflect the protracted time gaps between the multiple intrusions that intermittently replenished porphyry stocks. Other precise isotopic methods (Re-Os, 40Ar/39Ar) typically document shorter life spans because it is more difficult, if not impossible, to date the full sequence of events involved in porphyry copper formation. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/105/6/1157.abstract [article] Longevity of porphyry copper formation at Quellaveco, Peru [texte imprimé] / Richard H. Sillitoe, Auteur ; James K. Mortensen, Auteur . - 2011 . - pp. 1157-1162. Economic Geology Langues : Anglais (eng) in Economic geology > Vol. 105 N° 6 (Septembre/Octobre 2010) . - pp. 1157-1162 Mots-clés : Porphyry  copper  Geochronology  Peru Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : Metal introduction at the late Paleocene to early Eocene Quellaveco porphyry copper-molybdenum deposit in southern Peru spans several phases of quartz monzonite porphyry emplacement and is bracketed by a precursor granodiorite pluton and a late-mineral porphyry body that postdates essentially all copper introduction. Together, the U-Pb ages of zircons from these intrusive rocks show that 1.08 ± 0.58 m.y. elapsed between the precursor pluton and initiation of stock emplacement; the porphyry system was active intermittently for at least 3.25 m.y. (4.07 ± 0.82 m.y.); and at least three-quarters of the copper inventory was deposited in a maximum of 3.12 m.y. (2.51 ± 0.61 m.y.). Recent U-Pb zircon dating of several other major central Andean porphyry copper deposits, in combination with other isotopic techniques, suggests that 2.5- to 4-m.y. life spans are commonplace. The longevity of porphyry copper systems implied by these studies appears to reflect the protracted time gaps between the multiple intrusions that intermittently replenished porphyry stocks. Other precise isotopic methods (Re-Os, 40Ar/39Ar) typically document shorter life spans because it is more difficult, if not impossible, to date the full sequence of events involved in porphyry copper formation. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/105/6/1157.abstract

Volcanic stratigraphy and geochronology of the cretaceous lancones basin, northwestern Peru / Lawrence S. Winter in Economic geology, Vol. 105 N° 4 (Juin/Juillet 2010) 

Public ISBD [article]  in Economic geology > Vol. 105 N° 4 (Juin/Juillet 2010) . - pp. 713-742 Titre : Volcanic stratigraphy and geochronology of the cretaceous lancones basin, northwestern Peru : position and timing of giant VMS deposits Type de document : texte imprimé Auteurs : Lawrence S. Winter, Auteur ; Richard M. Tosdal, Auteur ; James K. Mortensen, Auteur Année de publication : 2011 Article en page(s) : pp. 713-742 Note générale : Economic geology Langues : Anglais (eng) Mots-clés : Volcanic  stratigraphy  VMS  deposits  Peru Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : A ~10-km-thick sequence of basaltic to rhyolitic volcanic rocks forms the arc component of the Cretaceous Lancones basin in northwestern Peru and underlies part of the Huancabamba deflection. The marine volcanic successions show markedly different compositional features and depositional facies consistent with a maturing arc within a shallowing marine basin. The earliest volcanism accompanying rifting was dominated by basaltic pillow lava and breccia with lesser aphyric to feldspar-quartz porphyritic felsic volcanic rocks. These volcanic successions filled the lowest exposed portion of the basin and were accompanied by volcanogenic massive sulfide (VMS) deposits, which are inferred to have formed in a localized but relatively deep marine setting. U-Pb zircon dating of felsic volcanic rocks associated with VMS deposits at Tambogrande indicates ages from 104.8 ± 1.3 to 100.2 ± 0.5 Ma for the ore-bearing volcanic sequence. The timing of onset of rift-related volcanism is not well constrained but is therefore of middle Albian age or older. Subsequent latest Albian to Turonian volcanism is composed of successions of relatively more felsic rich volcaniclastic rocks and yields U-Pb zircon ages of 99.3 ± 0.3 to 91.1 ± 1.0 Ma. These later volcanic successions are intercalated and overlain by siliciclastic and carbonate sedimentary sequences prevalent in the western forearc section of the Lancones basin. Finally, the basin was intruded by Late Cretaceous to Tertiary granitoids of the Coastal batholith. The genesis of the Cretaceous Lancones basin and other equivalent volcanic rift-related, marginal basins in western South America, including the western Peruvian trough, is related tectonically to the break-up of Gondwana. Early volcanism and associated VMS deposits formed in the Lancones basin during the Albian coincided with the initial rifting stage, prior to active oceanic spreading, between South America and Africa. During this time the relatively stationary western margin of continental South America was undergoing extension and rifting due to a westward and oceanward retreating arc, resembling a Mariana arc-type setting. The Mochica orogeny marks the termination of rifting, subsidence, and related volcanism along the western margin of South America. This orogenic event also broadly coincides with the onset of spreading of the South Atlantic and westward drift of the South American continent. Subsequent volcanism in the Lancones basin was more continental arclike under an Andean-type scenario. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/105/4/713.abstract [article] Volcanic stratigraphy and geochronology of the cretaceous lancones basin, northwestern Peru : position and timing of giant VMS deposits [texte imprimé] / Lawrence S. Winter, Auteur ; Richard M. Tosdal, Auteur ; James K. Mortensen, Auteur . - 2011 . - pp. 713-742. Economic geology Langues : Anglais (eng) in Economic geology > Vol. 105 N° 4 (Juin/Juillet 2010) . - pp. 713-742 Mots-clés : Volcanic  stratigraphy  VMS  deposits  Peru Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : A ~10-km-thick sequence of basaltic to rhyolitic volcanic rocks forms the arc component of the Cretaceous Lancones basin in northwestern Peru and underlies part of the Huancabamba deflection. The marine volcanic successions show markedly different compositional features and depositional facies consistent with a maturing arc within a shallowing marine basin. The earliest volcanism accompanying rifting was dominated by basaltic pillow lava and breccia with lesser aphyric to feldspar-quartz porphyritic felsic volcanic rocks. These volcanic successions filled the lowest exposed portion of the basin and were accompanied by volcanogenic massive sulfide (VMS) deposits, which are inferred to have formed in a localized but relatively deep marine setting. U-Pb zircon dating of felsic volcanic rocks associated with VMS deposits at Tambogrande indicates ages from 104.8 ± 1.3 to 100.2 ± 0.5 Ma for the ore-bearing volcanic sequence. The timing of onset of rift-related volcanism is not well constrained but is therefore of middle Albian age or older. Subsequent latest Albian to Turonian volcanism is composed of successions of relatively more felsic rich volcaniclastic rocks and yields U-Pb zircon ages of 99.3 ± 0.3 to 91.1 ± 1.0 Ma. These later volcanic successions are intercalated and overlain by siliciclastic and carbonate sedimentary sequences prevalent in the western forearc section of the Lancones basin. Finally, the basin was intruded by Late Cretaceous to Tertiary granitoids of the Coastal batholith. The genesis of the Cretaceous Lancones basin and other equivalent volcanic rift-related, marginal basins in western South America, including the western Peruvian trough, is related tectonically to the break-up of Gondwana. Early volcanism and associated VMS deposits formed in the Lancones basin during the Albian coincided with the initial rifting stage, prior to active oceanic spreading, between South America and Africa. During this time the relatively stationary western margin of continental South America was undergoing extension and rifting due to a westward and oceanward retreating arc, resembling a Mariana arc-type setting. The Mochica orogeny marks the termination of rifting, subsidence, and related volcanism along the western margin of South America. This orogenic event also broadly coincides with the onset of spreading of the South Atlantic and westward drift of the South American continent. Subsequent volcanism in the Lancones basin was more continental arclike under an Andean-type scenario. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/105/4/713.abstract