OROGENESIS, HIGH-T THERMAL EVENTS, AND GOLD VEIN FORMATION WITHIN METAMORPHIC ROCKS OF THE ALASKAN CORDILLERA

Mesothermal, gold-bearing quartz veins are widespread within allochtho nous terranes of Alaska that are composed dominantly of greenschist-fa cies metasedimentary rocks. The most productive lode deposits are conc entrated in south-central and southeastern Alaska; small and generally nonproductive gold-bearing veins occur upstream from major placer dep osits in interior and northern Alaska. Ore-forming fluids in all areas are consistent with derivation from metamorphic devolatilisation reac tions, and a close temporal relationship exists between high-T tectoni c deformation, igneous activity, and gold mineralization. Ore fluids w ere of consistently low salinity, CO2-rich, and had deltaO-18 values o f 7 parts per thousand-12 parts per thousand and deltaD values between -15 parts per thousand and -35 parts per thousand. Upper-crustal temp eratures within the metamorphosed terranes reached at least 450-500-de grees-C before onset of significant gold-forming hydrothermal activity . Within interior and northern Alaska, latest Paleozoic through Early Cretaceous contractional deformation was characterised by obduction of oceanic crust, low-T/high-P metamorphism, and a lack of gold vein for mation. Mid-Cretaceous veining occurred some 50-100 m.y. later, during a subsequent high-T metamorphic/magmatic event, possibly related to e xtension and uplift. In southern Alaska, gold deposits formed during l atter stages of Tertiary, subduction-related, collisional orogenesis a nd were often temporally coeval with calc-alkaline magmatism.