GEOCHEMICAL, GEOCHRONOLOGICAL AND METALLOGENIC CHARACTERISTICS OF CRETACEOUS MAGMATISM AND MINERALIZATION IN EASTERN ALASKA: A NEW PIERCING ZONE ALONG THE TERTIARY TINTINA FAULT

Cretaceous magmatism is voluminous in eastern Alaska and western Yukon where it is associated with significant mineral deposits (Fort Knox, Manh Choh, Casino, Coffee, Dublin Gulch, etc.). Plutons have diverse characteristics indicative of varying source materials and resulting metallogeny. Geochronology and geochemistry from 138 samples in Alaska show affinities to magmatic belts across the Tintina fault in the Yukon. Results show compositionally and metallogenically diverse plutons emplaced episodically between114-67 Ma.

Plutons emplaced from 114-108 Ma are weakly to strongly peraluminous, have steep LREE and shallow MREE-HREE patterns with a negative Eu anomaly. Geochemical groupings are observed from weakly peraluminous plutons characterized by ilmenite fractionation (flat V/Sc vs. Sc slope) in the SE to strongly peraluminous plutons characterized by magnetite(±ilmentite) fractionation in the NW. No mineralization is associated with these plutons.

From 108-90 Ma metaluminous intrusives occur in the central upland with peraluminous intrusions to the north. Metaluminous plutons have shallower LREE and flat MREE-HREE slopes compared to peraluminous plutons. Both exhibit moderate negative Eu anomalies. Metaluminous intrusions are less alkaline with ilmenite fractionation. Peraluminous plutons trend toward magnetite fractionation (negative V/Sc vs. Sc slope). Au, Bi, Te, As, (W, Cu, Zn, Mo) mineralization occurs with these plutons.

Late Cretaceous magmatism occurred from 72-67 Ma, is sub-alkaline to alkaline and metaluminous to weakly peraluminous. Plutons have moderate LREE and shallow to flat MREE-HREE slopes. They exhibit hornblende fractionation (positive V/Sc vs. Sc slope) with associated porphyry deposits. Late Cretaceous Hf isotope ratios overlap with but are higher than the middle Cretaceous. All Cretaceous plutons become increasingly crustal to the NW.

Restoration of ~450 km of Tertiary separation across the Tintina fault connects major plutonic belts identified in the Yukon with temporally and geochemically similar belts in Alaska. This suggests a genetic link and evolving geologic setting with the continuation of the Tombstone (Au-Cu-Bi, U-Th-F), Mayo (Au-Bi-Te, W, As) and Tungsten (W, Cu-Zn-Mo) plutonic and metallogenic belts into eastern Alaska, revealing untapped exploration potential.