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Abstract

Archean snosnonitic lampropnyres are cotemporal and cospatial with gold mineralization in the Superior province, both being emplaced along major translithospheric structures that demark subprovince boundaries. They occur internal to greenstone belts such as the Abitibi. Lamprophyres constitute a volumetrically minor but widespread component of late kinematic magma-tism, which is most prominently developed in fault graben or dilational jogs where the dikes are associated with alkali gabbros, trachytes, and molasse sediments. By analogy with geochemically similar Phanerozoic counterparts, the dikes are a product of specific plate interactions rather than a deep asthenosphere plume-initiated event.

Fresh shoshonitic 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. Average abundances of As (1.4 ± 0.5 ppm), Sb (0.25 ± 0.25), Bi (0.09 ± 0.02), W (1.9 ± 2.2), Tl (0.11), B (6.2 ± 3.3), Cu (71 ± 38), Pb (7 ± 5), Zn (93 ± 18), and Mo (1.4 ± 1.5) are also close to values of 1.0, 0.2, 0.06,1.0, 0.36, 10, 75, 8, 80, and 1.0 ppm, respectively, in bulk continental crust. 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 = 0.84 ± 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. Sporadically elevated Au abundances in lamprophyres proximal to gold deposits are interpreted as a secondary overprint in dispersion halos. Post-Archean ultramafic lamprophyres from the Superior province have suffered little crustal interaction and do not possess enhanced abundances either of Au (mean = 0.5 ppb) or most Au-associated elements.

Where anomalously rich gold contents occur in alkaline rocks, Au invariably defines a peak relative to Cu and neighboring platinum-group elements on normalized diagrams such that the anomaly is likely the result of a secondary overprint rather than an intrinsic feature of the noble metal budget of alkaline magmas. Comparable abundances and ratios of Pd/Au, Os/Ir, and Ru/Ir in Archean lamprophyres, Archean komatiites, and Gorgona komatiites (Brugman et al., 1987) 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 (2,705 Ma) to south (2,674 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; the alkaline magmas were derived from depths of 40 to 120 km in the continental mantle lithosphere and asthenosphere, whereas the gold-mineralizing systems were confined to the continental crust.

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