GEOLOGY, GEOCHEMISTRY, AND NEW ISOTOPIC AGES OF PGE-NI-CU BEARING, MAFIC/ULTRAMAFIC ROCKS OF THE FAREWELL AND SREDINNY TERRANES, ALASKA, USA, AND KAMCHATKA, RUSSIA

The Farewell and Sredinny terranes of Central Alaska, USA and Kamchatka, Russia contain tholeiitic, differentiated, peridotite, clinopyroxenite, and gabbro sills, norite-dominant, layered intrusions, and cogenetic, alkali olivine basalt flows that intrude and overlie Paleozoic and Mesozoic, continental margin deposits of accretionary origin. Significant PGE, nickel, cobalt and copper sulfide deposits are associated with mafic-ultramafic intrusions in both areas. In the Farewell area, the mafic and ultramafic sills and pillow basalts intrude and overlie: 1) silty limestone and shale of the Cambrian-Lower Ordovician Lyman Hills Formation and 2) calcareous sandstone and shale of the Permian-Pennsylvanian Sheep Creek Formation. The Farewell terrane mafic-ultramafic sills are enstatite rich, orthopyroxene poor, and contain Ti-enriched chromitites. Trace element data from the Farewell suite suggests a magma-mixing model with crustal contamination. Samples of the Sheep Creek sills yield plateau 40Ar/39Ar ages ranging from 225.6 to 233.7 Ma. In the Sredinny terrane of southern Kamchatka, more than 40 peridotite, pyroxenite, and noritic intrusions cut metamorphosed sandstone and siltstone of the Kamchatka and Kolpakova Groups. REE and trace element data and the widespread appearance of garnet in the gabbro-norites suggest that chrondritic magmas of the Sredinny terrane have been contaminated by crustal sources. 40Ar/39Ar plateau ages from six mafic sills and layered intrusions in the Sredinny terrane range from 49.8 to 53.0 Ma; a seventh sill yielded a 40Ar/39Ar age of 84.3 Ma. Overall geological and geochemical framework indicates that both belts of PGE-Ni-Cu bearing mafic-ultramafic intrusions formed in their host accretionary terranes during Late Triassic and early Tertiary rifting events. Mineralized mafic/ultramafic intrusions in the Farewell terrane can be compared to Late Triassic, tholeiitic sills and cogenetic basalts that intrude and overly the Wrangellia terrane of western North America, or the mafic intrusions and volcanic rocks that cut Paleozoic terrigenous deposits on the Taimyr Peninsula of eastern Siberia. The early Tertiary, mineralized intrusions in the Sredinny terrane are similar to rift related, mineralized, layered peridoitie-gabbro complexes of Tertiary age in the La Perouse-Glacier Bay region of southern Alaska.