Abstract
Archean shoshonitic lamprophyres are cotemporal and cospatial with gold mineralization in the Superior Province of Canada, both being emplaced along translithospheric structures that demark subprovince boundaries. By analogy with geochemically similar Phanerozoic counterparts, the dikes are a product of specific plate interactions rather than a deep asthenosphere plume-initiated event, and their onset in the late-Archean at ∼ 2.7 Ga signifies that Phanerozoic style plate-tectonics was operating at this time. Fresh shonshonitic dikes are characterized by normal background gold contents of 3.9 ± 8.1 ppb (lσ), close to the value of 3.0 ppb for the bulk continental crust, and average abundances of As, Sb, Bi, W, TI, B, Cu, Pb, Zn, and Mo are also close to their values in bulk continental crust. Thus, fresh lamprophyres are not intrinsically enriched either in Au or elements affiliated with gold in mesothermal deposits, and accordingly do not constitute a special source rock. Platinum group element contents (Ir = 0.4 ± 0.58 ppb; Pt = 5.9 ± 26.5, Pd = 5.5 ± 1.8), in conjunction with Cu, Au, and Ni abundances, define approximately flat patterns on primitive mantle-normalized diagrams, consistent with derivation of the alkaline magmas from a depleted mantle source variably enriched by incompatible elements. Comparable abundances and ratios of Pd/Au, Os/Ir, and Ru/Ir in Archean lamprophyres, Archean komatiites, and Gorgona komatiites signify that the Archean and Phanerozoic upper mantle had similar noble metal contents, such that the prolific greenstone belt Au-Ag vein deposits cannot be explained by secular variations in upper mantle Au abundance alone. The lack of covariation between Au and light rare earth elements in lamprophyres rules out mantle metasomatism as a process generating intrinsically Au-rich magmas. Emplacement of the lamprophyres was diachronous from north (2710 Ma) to south (2670 Ma) in the Superior Province, as was the gold mineralization. Both were related to late transpressional tectonics during successive accretions of individual subprovinces. Alkaline magmatism and gold mineralization are temporally and spatially related because they share a common geodynamic setting, but they are otherwise the products of distinct processes. Much of Archean time was devoid of shoshonites and mesothermal gold deposits. The first widespread inception of this duality at 2.71–2.65 Ga in the Superior and Slave Provinces, Canada, and in India and Australia, may reflect one of the first supercontinent aggregations involving accretionary, “Cordilleran style” tectonics. Giant mesothermal gold provinces and shoshonites recur through time in the Palaeozoic and Mesozoic in this geodynamic setting.
Published Version
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