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A carbonaceous sedimentary source-rock model for carlin-type and orogenic gold deposits / Ross R. Large in Economic geology, Vol. 106 N° 3 (Mai 2011) 

Public ISBD [article]  in Economic geology > Vol. 106 N° 3 (Mai 2011) . - pp. 331-358 Titre : A carbonaceous sedimentary source-rock model for carlin-type and orogenic gold deposits Type de document : texte imprimé Auteurs : Ross R. Large, Auteur ; Stuart W. Bull, Auteur ; Valeriy V. Maslennikov, Auteur Année de publication : 2011 Article en page(s) : pp. 331-358 Note générale : Géologie Economique Langues : Anglais (eng) Mots-clés : Carbonaceous  sedimentary  Genetic  model  Gold  deposits Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : This paper presents evidence and arguments that carbonaceous sedimentary rocks were a source for Au and As in sediment-hosted orogenic and Carlin-type gold deposits and develops a corresponding genetic model. In this two-stage basin-scale model, gold and arsenic are introduced early into black shale and turbidite basins during sedimentation and diagenesis (stage 1) and concentrated to ore grades by later hydrothermal, structural, or magmatic processes (stage 2). In reduced continental margin basin settings, organic matter, sedimented under anoxic to euxinic conditions, immobilizes and concentrates gold, arsenic, and a range of trace elements (particularly V, Ni, Se, Ag, Zn, Mo, Cu, U) present in marine bottom waters, into fine-grained black mudstone and siltstone of slope and basin facies. During early diagenesis, gold and certain other trace elements (Ni, Se, Te, Ag, Mo, Cu, ±PGE) are preferentially partitioned into arsenian pyrite that grows in the muds. These processes produce regionally extensive black shale and turbidite sequences enriched in syngenetic gold and arsenic, commonly from 5 to 100 ppb Au and 10 to 200 ppm As. Rare organic- and sulfide-rich metalliferous black shales may contain up to 1 to 2 ppm Au and over 1,000 ppm As, present as refractory gold in arsenian pyrite and nanoparticles of free gold. During late diagenesis and early metamorphism (stage 2) the diagenetic arsenian pyrite is recrystallized to form coarser grained pyrite generations, and the organic matter is cooked to bitumen. Under higher grade metamorphism (lower greenschist facies and above) arsenian pyrite in carbonaceous shales is converted to pyrrhotite. These processes release gold, arsenic, sulfur and other elements (Sb, Te, Cu, Zn, Mo, Bi, Tl, and Pb) from the source rocks to become concentrated by hydrothermal processes, locally to produce gold ores, in structural sites such as fold hinge zones, shear or breccia zones within or above the black shale sequence. LA-ICP-MS analyses of diagenetic pyrite in carbonaceous sediments, both associated and not associated with gold deposits, suggests that invisible gold contents of greater than 250 ppb in diagenetic pyrite, are indicative of carbonaceous shale source rocks with the potential to produce economic gold deposits. Application of this sedimentary source-rock model enables a systematic exploration approach for sediment-hosted gold deposits, based on the distribution, composition and structure of carbonaceous shale sequences and their contained diagenetic pyrite. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/cgi/content/abstract/106/3/331 [article] A carbonaceous sedimentary source-rock model for carlin-type and orogenic gold deposits [texte imprimé] / Ross R. Large, Auteur ; Stuart W. Bull, Auteur ; Valeriy V. Maslennikov, Auteur . - 2011 . - pp. 331-358. Géologie Economique Langues : Anglais (eng) in Economic geology > Vol. 106 N° 3 (Mai 2011) . - pp. 331-358 Mots-clés : Carbonaceous  sedimentary  Genetic  model  Gold  deposits Index. décimale : 553 Géologie économique. Minérographie. Minéraux. Formation et gisements de minerais Résumé : This paper presents evidence and arguments that carbonaceous sedimentary rocks were a source for Au and As in sediment-hosted orogenic and Carlin-type gold deposits and develops a corresponding genetic model. In this two-stage basin-scale model, gold and arsenic are introduced early into black shale and turbidite basins during sedimentation and diagenesis (stage 1) and concentrated to ore grades by later hydrothermal, structural, or magmatic processes (stage 2). In reduced continental margin basin settings, organic matter, sedimented under anoxic to euxinic conditions, immobilizes and concentrates gold, arsenic, and a range of trace elements (particularly V, Ni, Se, Ag, Zn, Mo, Cu, U) present in marine bottom waters, into fine-grained black mudstone and siltstone of slope and basin facies. During early diagenesis, gold and certain other trace elements (Ni, Se, Te, Ag, Mo, Cu, ±PGE) are preferentially partitioned into arsenian pyrite that grows in the muds. These processes produce regionally extensive black shale and turbidite sequences enriched in syngenetic gold and arsenic, commonly from 5 to 100 ppb Au and 10 to 200 ppm As. Rare organic- and sulfide-rich metalliferous black shales may contain up to 1 to 2 ppm Au and over 1,000 ppm As, present as refractory gold in arsenian pyrite and nanoparticles of free gold. During late diagenesis and early metamorphism (stage 2) the diagenetic arsenian pyrite is recrystallized to form coarser grained pyrite generations, and the organic matter is cooked to bitumen. Under higher grade metamorphism (lower greenschist facies and above) arsenian pyrite in carbonaceous shales is converted to pyrrhotite. These processes release gold, arsenic, sulfur and other elements (Sb, Te, Cu, Zn, Mo, Bi, Tl, and Pb) from the source rocks to become concentrated by hydrothermal processes, locally to produce gold ores, in structural sites such as fold hinge zones, shear or breccia zones within or above the black shale sequence. LA-ICP-MS analyses of diagenetic pyrite in carbonaceous sediments, both associated and not associated with gold deposits, suggests that invisible gold contents of greater than 250 ppb in diagenetic pyrite, are indicative of carbonaceous shale source rocks with the potential to produce economic gold deposits. Application of this sedimentary source-rock model enables a systematic exploration approach for sediment-hosted gold deposits, based on the distribution, composition and structure of carbonaceous shale sequences and their contained diagenetic pyrite. DEWEY : 553 ISSN : 0361-0128 En ligne : http://econgeol.geoscienceworld.org/cgi/content/abstract/106/3/331