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Geology, alteration, and origin of archean Au-Ag-Cu mineralization associated with the synvolcanic chibougamau pluton / Olivier Côté-Mantha in Economic geology, Vol. 107 N° 5 (Août 2012) 

Public ISBD [article]  in Economic geology > Vol. 107 N° 5 (Août 2012) . - pp. 909-934 Titre : Geology, alteration, and origin of archean Au-Ag-Cu mineralization associated with the synvolcanic chibougamau pluton : the brosman prospect, abitibi greenstone belt, Canada Type de document : texte imprimé Auteurs : Olivier Côté-Mantha, Auteur ; Réal Daigneault, Auteur ; Damien Gaboury, Auteur Année de publication : 2012 Article en page(s) : pp. 909-934 Note générale : Economic geology Langues : Anglais (eng) Mots-clés : Au-Ag-Cu  Brosman  prospect;  geology;  mineralization;  Canada Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : The Au-Ag-Cu Brosman prospect is located in the Line Lake area on the northern limb of the Chibougamau anticline, in the northeast part of the Archean Abitibi greenstone belt. This prospect comprises a broad N-S, subvertical mineralized envelope formed in a synvolcanic fault-related environment. This envelope is at least 300 m long, up to 30 m wide, and is known to a depth of 950 m, where it remains open. It exhibits two contrasting mineralized zones that are representative of the varying styles of mineralization in the area. The Auriferous zone consists of both ubiquitous pyrite disseminations and Au-bearing silicate-sulfide veins in strongly fractured intrusive, effusive, and volcaniclastic rocks showing propylitic and chloritic alteration assemblages. The Polymetallic zone, on the other hand, is characterized by discrete, Cu-Ag-Au–rich magnetite-sulfide veins and lesser pyrite disseminations within distal propylitic and proximal potassic to sodic alteration zones. The Polymetallic zone is hosted by a chaotic assemblage of gabbro and tuff bodies interpreted as a talus megabreccia. Several lines of evidence, including the spatial distribution pattern and nature of the alteration and mineralization assemblages, metal associations, mass-balance calculations, pyrite composition, and crosscutting relationships are indicative of a preregional deformation, synvolcanic origin for the hydrothermal activity. Crosscutting relationships, alteration characteristics as well as whole-rock and LA-ICP-MS pyrite analyses reveal two stages of hydrothermal alteration and mineralization. Stage I consists of both ubiquitous disseminated pyrite, found in the Polymetallic and Auriferous zones, and contemporaneous magnetite-sulfide veins, restricted to the Polymetallic zone. Stage II is restricted to the Auriferous zone and corresponds to Au-bearing silicate-sulfide veins with characteristic vein-halo chloritic alteration overprinting Stage I alteration and mineralization. This hydrothermal activity is interpreted to have occurred concurrently with the development of the fertile, deeper magmatic-hydrothermal system related to the Chibougamau Pluton. Stage I hydrothermal activity, related to the early build-up of the volcanic sequence, formed a continuum of mineralization from discrete massive magnetite-sulfide veins to ubiquitous pyrite disseminations in a submarine talus megabreccia generated along synvolcanic faults. Afterward, a late Au-rich contribution to the hydrothermal fluids, possibly related to intermediate to felsic magmatism (the 2707.6 ± 1.4 Ma Line Lake stock), produced Stage II silicate-sulfide veins overprinting mineralization and alteration in favorably oriented and sustainably permeable fracture zones of the Auriferous zone. Overall, this study suggests that the Brosman prospect is a rare example of Archean polymetallic mineralization produced by a shallow subaqueous epithermal-style subsystem developed in a broader, fertile magmatic-hydrothermal arc setting. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/107/5/909.short [article] Geology, alteration, and origin of archean Au-Ag-Cu mineralization associated with the synvolcanic chibougamau pluton : the brosman prospect, abitibi greenstone belt, Canada [texte imprimé] / Olivier Côté-Mantha, Auteur ; Réal Daigneault, Auteur ; Damien Gaboury, Auteur . - 2012 . - pp. 909-934. Economic geology Langues : Anglais (eng) in Economic geology > Vol. 107 N° 5 (Août 2012) . - pp. 909-934 Mots-clés : Au-Ag-Cu  Brosman  prospect;  geology;  mineralization;  Canada Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : The Au-Ag-Cu Brosman prospect is located in the Line Lake area on the northern limb of the Chibougamau anticline, in the northeast part of the Archean Abitibi greenstone belt. This prospect comprises a broad N-S, subvertical mineralized envelope formed in a synvolcanic fault-related environment. This envelope is at least 300 m long, up to 30 m wide, and is known to a depth of 950 m, where it remains open. It exhibits two contrasting mineralized zones that are representative of the varying styles of mineralization in the area. The Auriferous zone consists of both ubiquitous pyrite disseminations and Au-bearing silicate-sulfide veins in strongly fractured intrusive, effusive, and volcaniclastic rocks showing propylitic and chloritic alteration assemblages. The Polymetallic zone, on the other hand, is characterized by discrete, Cu-Ag-Au–rich magnetite-sulfide veins and lesser pyrite disseminations within distal propylitic and proximal potassic to sodic alteration zones. The Polymetallic zone is hosted by a chaotic assemblage of gabbro and tuff bodies interpreted as a talus megabreccia. Several lines of evidence, including the spatial distribution pattern and nature of the alteration and mineralization assemblages, metal associations, mass-balance calculations, pyrite composition, and crosscutting relationships are indicative of a preregional deformation, synvolcanic origin for the hydrothermal activity. Crosscutting relationships, alteration characteristics as well as whole-rock and LA-ICP-MS pyrite analyses reveal two stages of hydrothermal alteration and mineralization. Stage I consists of both ubiquitous disseminated pyrite, found in the Polymetallic and Auriferous zones, and contemporaneous magnetite-sulfide veins, restricted to the Polymetallic zone. Stage II is restricted to the Auriferous zone and corresponds to Au-bearing silicate-sulfide veins with characteristic vein-halo chloritic alteration overprinting Stage I alteration and mineralization. This hydrothermal activity is interpreted to have occurred concurrently with the development of the fertile, deeper magmatic-hydrothermal system related to the Chibougamau Pluton. Stage I hydrothermal activity, related to the early build-up of the volcanic sequence, formed a continuum of mineralization from discrete massive magnetite-sulfide veins to ubiquitous pyrite disseminations in a submarine talus megabreccia generated along synvolcanic faults. Afterward, a late Au-rich contribution to the hydrothermal fluids, possibly related to intermediate to felsic magmatism (the 2707.6 ± 1.4 Ma Line Lake stock), produced Stage II silicate-sulfide veins overprinting mineralization and alteration in favorably oriented and sustainably permeable fracture zones of the Auriferous zone. Overall, this study suggests that the Brosman prospect is a rare example of Archean polymetallic mineralization produced by a shallow subaqueous epithermal-style subsystem developed in a broader, fertile magmatic-hydrothermal arc setting. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/content/107/5/909.short

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]  in Economic 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