Publication Type:
Journal ArticleSource:
Economic Geology and the Bulletin of the Society of Economic GeologistsEconomic Geology and the Bulletin of the Society of Economic Geologists, Economic Geology Publishing Company, Lancaster, PA, United States, Volume 100, Number 6, p.1169-1190 (2005)ISBN:
0361-0128Keywords:
Abitibi Belt, accuracy, alteration, Archean, assimilation, Canada, Canadian Shield, chalcophile elements, chemical composition, copper ores, country rocks, cumulates, detection, disseminated deposits, fractional crystallization, host rocks, igneous rocks, KOMATIITE, lava, magmas, major elements, massive deposits, melts, metal ores, metals, minor elements, nickel ores, North America, partial melting, platinum group, platinum ores, Precambrian, precision, sampling, stratabound deposits, stratiform deposits, superior province, Trace elements, variations, volcanic rocksAbstract:
Komatiitic rocks occur in four lithotectonic assemblages in the Abitibi greenstone belt of the Archaean Superior province: the 2750 to 2735 Ma Pacaud assemblage, the 2723 to 2720 Ma Stoughton-Roquemaure assemblage, the 2719 to 2710 Ma Kidd-Munro assemblage, and the 2710 to 2703 Ma Tisdale assemblage. There are significant volcanological and geochemical differences between the komatiitic rocks in these assemblages. Those in the older (Pacaud and Stoughton-Roquemaure) assemblages are primarily Ti- and Al-depleted-Ti-enriched komatiites, respectively, that appear to be uncontaminated by upper continental crustal rocks, form un-channelized lava flow facies, and are not underlain by sulphur-rich metasedimentary rocks. In contrast, those in the younger (Kidd-Munro and Tisdale) assemblages are primarily Al-undepleted komatiites, locally contaminated by upper continental crustal rocks, locally form lava channels and channelised sheet flow facies, and locally overlie sulphur-rich metasedimentary rocks. Sixty-four unmineralised samples from all four komatiite-bearing assemblages in the Abitibi greenstone belt have similar unfractionated platinum-group element (PGE) abundances at 0.5 to 5 times chondritic values. Minor variations in PGE abundances are attributable to variable (but uniformly high) degrees of partial melting, fractional crystallisation and/or accumulation of olivine, chromite, or clinopyroxene and, rarely, loss of sulphide or Ir-bearing alloys. Almost all of the komatiitic magmas and lavas represented by the samples, regardless of komatiite type (including Al-undepleted, Al-depleted-Ti-enriched, and Ti-depleted komatiites) and depth of melt extraction, appear to have been derived by high-degree partial melting of sources with essentially identical chalcophile element contents and to have remained undersaturated in sulphide during melt extraction, ascent, eruption, and emplacement. This is similar to most komatiites worldwide.
Notes:
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