Eastern Wrangellia ? A New Ni-Cu-PGE Metallogenic Terrane in North America

Abstract

Triassic ultramafic-mafic intrusive complexes along the eastern margin of ``Wrangellia' adjacent the Denali fault from east-central Alaska to northern British Columbia constitutes a newly recognized Ni-Cu-PGE metallogenic terrane that can be traced for at least 600 km. These sill- like intrusive centres acted as subvolcanic magma chambers that fed the extensive thick, overlying basalts and locally picritic pyroclastcs of the Nikolai Group in an oceanic plateau flood basalt setting. Confinement of the olivine-rich ultramafic sills, Ni-Cu-PGE mineralization, and more primitive coeval basalts and picrites exclusively to the eastern portion of Wrangellia is believed to be the product of melts forming in closer proximity to the hotter mantle plume tail or ``axial jet' relative to the cooler more distal portions. The recent discovery of proximal, picritic volcanics with some of the larger mineralized ultramafic bodies in Alaska finally concludes the past paradox oftholeiitic parental magmas and primitive Ni-Cu-PGE ores in this terrane. The unprecedented levels of platinum group elements, the vast extent of this mineralized terrane, the temporal association with Siberian Trap magmatism, and the accompanying Noril'sk mineralizing event, suggest that mantle plume activity during the Permo-Triassic was unique with respect to the Phanerozoic record.