Geology of the Archean Intrusion-Hosted la-grande-sud Au-Cu prospect, la grande subprovince, James Bay region, Québec / Patrick Mercier-Langevin in Economic geology, Vol. 107 N° 5 (Août 2012)  

Public ISBD [article]  inEconomic geology > Vol. 107 N° 5 (Août 2012) . - pp. 935-962 Titre : Geology of the Archean Intrusion-Hosted la-grande-sud Au-Cu prospect, la grande subprovince, James Bay region, Québec Type de document : texte imprimé Auteurs : Patrick Mercier-Langevin, Auteur ; Réal Daigneault, Auteur ; Jean Goutier, Auteur Année de publication : 2012 Article en page(s) : pp. 935-962 Note générale : Economic geology Langues : Anglais (eng) Mots-clés : Au-Cu prospect  calc-alkaline intrusion  mineralization  archean intrusions  Quabec Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : The Archean La Grande and Eastmain domains of the La Grande subprovince in the James Bay region of northwestern Québec are the focus of renewed and extensive exploration as a result of recent major discoveries made in this region (e.g., ~8 Moz Au Roberto deposit). A number of significant Neoarchean Au ± base metal occurrences are present in the La Grande domain, including the La-Grande-Sud Au-Cu prospect (Zone 32, inferred resource of 4.2 million metric tons (Mt) at 2.1 g/t Au and 0.2 wt % Cu). The La-Grande-Sud Au-Cu prospect is hosted in the 2734 ± 2 Ma synvolcanic La-Grande-Sud Tonalite in the 2751 to 2732 Ma Yasinski Group volcanic rocks. The La-Grande-Sud Tonalite is a synvolcanic calc-alkaline intrusion emplaced in an arc-like tectonomagmatic setting. Timing relationships in mineralized zones as well as associated hydrothermal alteration zones support a pre- to early-D1 origin for at least a part of the mineralization in the La-Grande-Sud Tonalite. Deformation events were responsible for the overprinting of the early mineralization by auriferous shear zone-related systems associated with D2. Evidence for a pre-D2 and pre- to early-D1 alteration and mineralization system includes: (1) a broadly concentric pattern defined by the hydrothermal alteration assemblages within the La-Grande-Sud Tonalite, which suggests that the hydrothermal alteration was, at least in part, controlled by the geometry of the intrusion rather than by the D2 deformation; (2) the development of a weak biotite-bearing potassic assemblage (biotite-epidote-plagioclase-muscovite-calcite ± pyrite) that is gradually replaced outward from the center of the intrusion by a chlorite-bearing propylitic assemblage (chlorite-epidote-muscovite-plagioclase-calcite-pyrite), and locally overprinted by a sericitic alteration assemblage (muscovite-quartz-albite-chlorite-pyrite-carbonate), which represents the most intense alteration in the La-Grande-Sud Tonalite; (3) the presence of mineralized hydrothermal breccias along the margins of the intrusion; (4) deformed disseminated and stockwork-style Au-Cu sulfide mineralization consisting of pyrite, chalcopyrite, tennantite, arsenopyrite, pyrrhotite, Bi sulfosalts, and native Au ± Bi in the sericite, chlorite, and biotite facies alteration zones; (5) S1-parallel elongated tonalite clasts and sulfide veinlets in D2-folded and transposed hydrothermal biotite breccias; and (6) pyrite porphyroblasts that are elongated parallel to the S1 fabric (syn-D1 metamorphic recrystallization) and that were folded during D2. D2 was associated with the development of conjugate auriferous extensional quartz-tourmaline veins (orogenic-style veins) that were superimposed on the pre- to early-D1 intrusion-hosted mineralized system. Gold in these syn- to late-D2 veins could have been remobilized from preexisting mineralization (disseminated sulfides) or was related to a late (syn-D2) auriferous hydrothermal event. The intrusion-hosted Au mineralization of the La-Grande-Sud Au-Cu prospect illustrates that Archean synvolcanic intrusions, regardless of their size, can host a range of styles of alteration and mineralization, including both “early” and “late” mineralizing systems. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/107/5/935.short [article] Geology of the Archean Intrusion-Hosted la-grande-sud Au-Cu prospect, la grande subprovince, James Bay region, Québec [texte imprimé] / Patrick Mercier-Langevin, Auteur ; Réal Daigneault, Auteur ; Jean Goutier, Auteur . - 2012 . - pp. 935-962. Economic geology Langues : Anglais (eng) in Economic geology > Vol. 107 N° 5 (Août 2012) . - pp. 935-962 Mots-clés : Au-Cu prospect  calc-alkaline intrusion  mineralization  archean intrusions  Quabec Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : The Archean La Grande and Eastmain domains of the La Grande subprovince in the James Bay region of northwestern Québec are the focus of renewed and extensive exploration as a result of recent major discoveries made in this region (e.g., ~8 Moz Au Roberto deposit). A number of significant Neoarchean Au ± base metal occurrences are present in the La Grande domain, including the La-Grande-Sud Au-Cu prospect (Zone 32, inferred resource of 4.2 million metric tons (Mt) at 2.1 g/t Au and 0.2 wt % Cu). The La-Grande-Sud Au-Cu prospect is hosted in the 2734 ± 2 Ma synvolcanic La-Grande-Sud Tonalite in the 2751 to 2732 Ma Yasinski Group volcanic rocks. The La-Grande-Sud Tonalite is a synvolcanic calc-alkaline intrusion emplaced in an arc-like tectonomagmatic setting. Timing relationships in mineralized zones as well as associated hydrothermal alteration zones support a pre- to early-D1 origin for at least a part of the mineralization in the La-Grande-Sud Tonalite. Deformation events were responsible for the overprinting of the early mineralization by auriferous shear zone-related systems associated with D2. Evidence for a pre-D2 and pre- to early-D1 alteration and mineralization system includes: (1) a broadly concentric pattern defined by the hydrothermal alteration assemblages within the La-Grande-Sud Tonalite, which suggests that the hydrothermal alteration was, at least in part, controlled by the geometry of the intrusion rather than by the D2 deformation; (2) the development of a weak biotite-bearing potassic assemblage (biotite-epidote-plagioclase-muscovite-calcite ± pyrite) that is gradually replaced outward from the center of the intrusion by a chlorite-bearing propylitic assemblage (chlorite-epidote-muscovite-plagioclase-calcite-pyrite), and locally overprinted by a sericitic alteration assemblage (muscovite-quartz-albite-chlorite-pyrite-carbonate), which represents the most intense alteration in the La-Grande-Sud Tonalite; (3) the presence of mineralized hydrothermal breccias along the margins of the intrusion; (4) deformed disseminated and stockwork-style Au-Cu sulfide mineralization consisting of pyrite, chalcopyrite, tennantite, arsenopyrite, pyrrhotite, Bi sulfosalts, and native Au ± Bi in the sericite, chlorite, and biotite facies alteration zones; (5) S1-parallel elongated tonalite clasts and sulfide veinlets in D2-folded and transposed hydrothermal biotite breccias; and (6) pyrite porphyroblasts that are elongated parallel to the S1 fabric (syn-D1 metamorphic recrystallization) and that were folded during D2. D2 was associated with the development of conjugate auriferous extensional quartz-tourmaline veins (orogenic-style veins) that were superimposed on the pre- to early-D1 intrusion-hosted mineralized system. Gold in these syn- to late-D2 veins could have been remobilized from preexisting mineralization (disseminated sulfides) or was related to a late (syn-D2) auriferous hydrothermal event. The intrusion-hosted Au mineralization of the La-Grande-Sud Au-Cu prospect illustrates that Archean synvolcanic intrusions, regardless of their size, can host a range of styles of alteration and mineralization, including both “early” and “late” mineralizing systems. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/107/5/935.short

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