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TitreChapter 4: Physical volcanology of komatiites and Ni-Cu-(PGE) deposits of the southern Abitibi Greenstone Belt
AuteurHoulé, M G; Lesher, C M; Préfontaine, S
SourceRare Earth and Critical Elements in Ore Deposits ; Reviews in Economic Geology vol. 19, 2017 p. 103-132
LiensOnline - En ligne
Séries alt.Secteur des sciences de la Terre, Contribution externe 2016-0114
ÉditeurSociety of Economic Geologists
Mediapapier; en ligne; numérique
ProvinceQuébec; Ontario
SNRC42A; 32D
Lat/Long OENS -80.0000 -76.0000 50.0000 48.0000
Sujetskomatiites; ceintures de roche verte; roches volcaniques; ensembles volcaniques; minéralisation; assemblages de minéraux; lithofaciès; coulées de lave; Assemblage de Kidd-Munro ; Assemblage de Tisdale ; Assemblage de Pacaud ; Assemblage de Soughton-Roquemaure ; géologie régional; minéralogie; géologie économique
Illustrationstables; location maps; geological sketch maps; diagrams; graphs; photographs
ProgrammeInitiative géoscientifique ciblée (IGC-4), Étude des gîtes magmatiques de Ni-Cu-EPG
Résumé(disponible en anglais seulement)
Komatiitic rocks occur mainly in Archean greenstone belts, less commonly in Paleoprote-rozoic volcano-sedimentary belts, and only rarely in younger volcanic settings. As in most other greenstone belts worldwide, komatiitic rocks are locally abundant in the Abitibi greenstone belt but generally represent only a small proportion of the volcanic rocks in the volcanic succession. Although only locally exposed, glacially-sculpted exposures of only weakly metamorphosed and mildly deformed komatiites of mineralized and un-mineralized komatiites in the Abitibi greenstone belt are among the best in the world, characterized by excellent textural preservation and, in some cases, excellent mineralogi-cal preservation.
Komatiites in the Abitibi greenstone belt occur predominantly within the Pacaud (2750¿2735 Ma), Stoughton-Roquemaure (2723¿2720 Ma), Kidd-Munro (2720¿2710 Ma), and Tisdale (2710¿2704 Ma) assemblages, but have recently also been recognized in lesser abundances within the Deloro (2734-2724 Ma) and Porcupine (2690?2685 Ma) assemblages. Overall, the komatiitic rocks present in these assemblages are characterized by a wide variety of lithofacies (textural, compositional) and flow facies, however, a re-gional analysis of komatiite physical volcanology reveals some fundamental differences between each of the komatiite-bearing assemblages. The Kidd¿Munro and Tisdale ko-matiite-bearing assemblages contain the largest volumes of komatiitic rocks, in particular thick, highly magnesian cumulate lava channels and channelized sheet flows. This sug-gests that the magma discharge rates were higher for these assemblages and/or that they formed more proximal to the eruptive site. However, the recently discovered Grasset Ni-Cu-(PGE) deposit hosted within relatively high MgO cumulate rocks that are interpreted to occur within the Deloro assemblage highlights the possibility of the other komatiite-bearing assemblages to contain similarly prospective facies.
Geochemical studies indicate that regardless of age or petrogenetic affinity (Al-undepleted vs. Al-depleted vs. Ti-enriched vs. Fe-rich), almost all of the parental magmas were undersaturated in sulfide prior to emplacement and therefore represent favourable magma sources for Ni-Cu-(PGE) mineralization. Volcanological studies indicate that almost all komatiite-associated Ni-Cu-(PGE) deposits in the Abitibi greenstone belt ap-pear to be localized in lava channels or channelized sheet flows, which have the capacity to thermomechanically erode S-bearing country rocks and to efficiently transfer metals from the magma to sulfide xenomelts. Three type localities (Spinifex Ridge in La Motte Township, Pyke Hill in Munro Township, and Alexo in Dundonald Township) demon-strate how physical volcanology (lava channelization) and stratigraphic environment (S source) need to operate quasi-simultaneously to allow for the genesis of significant amounts of Ni-Cu-(PGE) sulfides within a komatiitic succession. As not all komatiite magma pathways are mineralized, one of the most important challenges is to be able to distinguish potentially mineralized successions from barren successions.
Résumé(Résumé en langage clair et simple, non publié)
Le but de cette contribution est de présenter les principales caractéristiques volcaniques des laves komatiitiques observé à trois sites classiques de komatiites (Spinifex Ridge, canton de La Motte; Potter Mine, canton de Munro; Alexo Mine, canton de Dundonald) dans la partie sud de la ceinture de Roches Vertes de l'Abitibi. Elle va également montrer l'importance de la volcanologie physique dans la genèse des gisements de Ni-Cu-ÉGP associés aux komatiites. De plus, cette contribution apporte également des mises à jour sur la distribution spatiale et temporelle des komatiites ainsi que la découvertes de nouveaux indices de nickel dans la ceinture de Roches Vertes de l'Abitibi.