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TitreGeology, alteration and mineralization of the Lemoine auriferous VMS deposit, Chibougamau camp, Abitibi greenstone belt, Quebec, Canada: Geologic evidence for a magmatic input
AuteurMercier-Langevin, P; Lafrance, B; Bécu, V; Dubé, B; Kjarsgaard, I; Guha, J; Ross, P -S
SourceAssociation géologique du Canada-Association minéralogique du Canada, Réunion annuelle, Programme et résumés vol. 36, 2013 p. 145
LiensOnline - En ligne
Séries alt.Secteur des sciences de la Terre, Contribution externe 20130546
Réunion GAC-MAC 2013; Joint annual meeting of Geological Association of Canada and Mineralogical Association of Canada; Winnipeg; CA; mai 22-24, 2013
Documentpublication en série
Mediapapier; en ligne; numérique
SNRC32G/09; 32G/10; 32G/15; 32G/16
Lat/Long OENS -75.0000 -74.0000 50.0000 49.5000
Sujetsgîtes volcanogènes; gîtes sulfureux; Archéen; métallogénie; altération; assemblages de minéraux; métamorphisme; faciès à schiste vert; déformation; chlorite; séricité; épidote; chloritoïde; cadre tectonique; or; minéralisation; altération hydrothermale; gisements minéraux hydrothermaux; lithologie; roches volcaniques; rhyolites; dacites; andésites; basaltes; gabbros; porphyres; analyses géochimiques; géochimie des éléments en trace; géochimie des éléments majeurs; Ceinture d'Abitibi Greenstone ; Formation de Waconichi ; minéraux métalliques; géologie structurale; pétrologie ignée et métamorphique; géochimie; Précambrien
ProgrammeÉtude des gîtes de sulfures massifs volcaniques, Initiative géoscientifique ciblée (IGC-4)
ProgrammeÉtude des gîtes d'or, Initiative géoscientifique ciblée (IGC-4)
Résumé(disponible en anglais seulement)
The Abitibi greenstone belt hosts many of the world´s best examples of Au-rich deposits and a few auriferous volcanogenic massive sulphide (VMS) deposits, including the ~2728 Ma Lemoine deposit (0.76 Mt at 4.6 g/t Au, 4.2 wt % Cu, and 9.5 wt % Zn) located in the Chibougamau camp. The deposit is hosted by a steeply dipping, south-facing homoclinal volcanic succession (~2729-2726 Ma Waconichi Formation, Lemoine Member) composed of effusive and intrusive tholeiitic rhyolites and andesites cut by comagmatic diorite and gabbro dikes and overlain by transitional to mildly calc-alkaline basalts, andesites, and rhyolites. Seven synvolcanic alteration assemblages, now intensely deformed and metamorphosed to upper greenschist facies, were defined based on their mineralogy and position relative to ore. Albite-quartz, sericite-carbonate, sericite-chlorite, sericite-chlorite-(sphalerite), and chlorite-sericite-epidote-carbonate assemblages define semiconcordant zones that are stacked from the paleosea-floor to the deep footwall. In contrast, chlorite and chlorite-sericite-chloritoid assemblages overprint the other alteration zones and form subconcordant to now transposed discordant zones in the deposit footwall. Whole-rock oxygen isotopes indicate that the temperatures of alteration ranged from ~100-150°C (sericite-carbonate assemblage) to ¡Ý350°C (sericite-chlorite, chloritesericite- chloritoid and chlorite assemblages). The chlorite-sericitechloritoid assemblage, and to some extent the sericite-chlorite assemblage, are associated with strong to near total depletion of light rare earth elements. Although speculative, the high-temperature (~300-400°C) and strongly light REE-depleted alteration zones present at depth in the Lemoine footwall are consistent with alteration by reaction with acidic, possibly CO2-rich and Cl-bearing solutions. Both CO2 and HCl are considered likely to be added to hydrothermal systems through magmatic degassing. It is proposed that hydrothermal alteration at Lemoine was controlled by both HCl and CO2 in a steep thermal gradient developed above the synvolcanic Dor¨¦ Lake Complex. The massive sulphides at Lemoine are particularly rich in Bi, which most probably came from a magmatic source. Even if all evidences indicate that Au is synvolcanic (e.g., metals distribution), it is not clear if Au was sourced in a magmatic chamber and provided through devolatilization. Moreover, the presence of Bi in the system may have helped optimize Au transport and precipitation within the ore zone by acting as a sink for Au, preventing it from being lost to the water column. The Au-rich nature of the Lemoine auriferous VMS deposit can be explained by a possible magmatic contribution to the hydrothermal system and by the combination of very efficient metal transport and highly effective capture of metals at or near the sea floor.