Chalcophile Element Geochemistry of Komatiites in the Abitibi Greenstone Belt and Implications for their Source Region and Metallogenesis

CIM Montreal 2003
Mike Lesher, Michel Houlé, Reid Keays, John Ayer, Phillips Thurston,
Abstract U-Pb single zircon geochronology and stratigraphic mapping indicate that the komatiites in the Abitibi greenstone belt occur in four assemblages: 2750-2735 Ma Pacaud assemblage, 2725-2720 Ma Stoughton Roquemaure assemblage, 2318-2710 Ma Kidd-Munro assemblage, and 2710-2703 Ma Tisdale assemblage. There are important differences in the physical volcanology and geochemistry of the komatiites in the different assemblages: the Pacaud assemblage contains predominantly Ti-depleted (TDK) komatiites comprising spinifex-textured and pillowed facies with rare orthocumulate facies; the Stoughton-Roquemaure assemblage contains predominantly Al-depleted–Ti-enriched (ADK-TEK) komatiites with lesser ADK comprising spinifex-textured and pillow facies with rare ortho- to mesocumulate facies; the Kidd-Munro assemblage contains predominantly Al-undepleted komatiites (AUK) with lesser ADK-TEK comprising massive ortho-, meso-, and adcumulate, spinifex-textured, pillowed, and rare volcanoclastic facies with intercalated S-rich graphitic metasediments; and the Tisdale assemblage contains predominantly AUK comprising massive include ortho-, meso-, and adcumulate, spinifex-textured, massive, pillow (very rare), and volcanoclastic (rare) facies with intercalated S-rich iron formations. Komatiites in the Kidd-Munro and Tisdale assemblages also exhibit geological evidence of local thermomechanical erosion and geochemical evidence of local crustal contamination. Importantly, all of the komatiites, regardless of age and/or magma affinity, appear to have undepleted, slightly fractionated PGE contents similar to many other komatiites worldwide. This suggests that they were generated by high-degree partial melting of a relatively homogeneous mantle source, that all of the sulfides in the source were consumed, and that the magmas/lavas remained undersaturated in sulfide during ascent, eruption, and emplacement. Minor variations in PGE contents can be attributed to variable degrees of partial melting, fractionation or accumulation of olivine, chromite, and/or clinopyroxene, and, rarely, loss of sulfide or Ir-bearing alloys. The presence of komatiite-associated Ni-Cu-(PGE) mineralization only in the Kidd-Munro (e.g., Alexo, Dundonald, Marbridge) and Tisdale assemblages (e.g., Texmont, Sothman, Langmuir, Redstone) and the absence of mineralization in the other assemblages is consistent with a thermomechanical erosion model for ore genesis that requires a S-rich substrate. This suggests that further exploration in the Abitibi belt should be focussed in the Kidd-Munro and Tisdale assemblages, particularly in areas where lava channels locally transgress S-rich footwall rocks.
Keywords: Komatiite, Abitibi, Geochemistry, Nickel, Archean
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