GEOCHEMICAL AND ISOTOPIC COMPOSITIONS OF THE MID-CRETACEOUS FORT KNOX AND ASSOCIATED PLUTONS, FAIRBANKS, ALASKA: IMPLICATIONS FOR INTRUSION-RELATED GOLD SYSTEMS

Whole-rock, major- and trace-element, and Sr and Nd isotopic compositions of the auriferous Fort Knox pluton, located ~ 20 km northeast of Fairbanks, AK, and of the weakly- to non-mineralized surrounding plutons were measured to evaluate relationships between magmatic processes and economic intrusion-related gold systems (IRGS). All plutons are ca. 90 Ma, metaluminous to peraluminous, and intrude metamorphic rocks of the Yukon-Tanana terrane. Fort Knox pluton is largely granite to granodiorite (based on modes), and hosts gold-bearing quartz and pegmatite sheeted veins, as well as quartz veins formed along late shear zones that cut the pluton. Fort Knox pluton is characterized by LREE enrichment (La_(n) = 100 – 300, and La/Sm ~ 5), flat M-HREE signatures, anomalous Nb and Ta depletion, 103 - 126 ppm Rb, 616 - 1190 ppm Sr, 6.3 – 14.80 ppm U, and initial ⁸⁷Sr/⁸⁶Sr = 0.7119 to 0.7144. The surrounding less widely mineralized plutons (Dolphin, Ester Dome) represent a wide range of granitic compositions, exhibit LREE enrichment (La_(n) = 70 – 300, La/Sm ~ 5), relative HREE depletion, Nb and Ta depletion, 82 – 90 ppm Rb, 407 – 568 ppm Sr, 4 – 9.7 ppm U, and initial ⁸⁷Sr/⁸⁶Sr = 0.7139 to 0.7146, εNd₉₀ of -10.7 to -11.6, and T_DM of 1.5 – 1.7 Ga. Non-mineralized granitic plutons (Gilmore Dome, Pedro Dome, Murphy Dome, Lincoln and Twin Creeks) have LREE enrichment and relatively flat M-HREE patterns that overlap those of Fort Knox, as well as Nb and Ta depletions, 76 – 243 ppm Rb, 142 – 648 ppm Sr, 1.57 – 4.05 ppm U, initial ⁸⁷Sr/⁸⁶Sr ratios, εNd₉₀ and T_DM of 0.7122 to 0.7169, -9.6 to -12.2, and 1.5 - 2.0 Ga. The Nb and Ta depletions suggest a subduction-related component, and negative εNd values reflect crustal involvement in magmatic processes. All T_DM ages suggest the parent magmas had long histories of separation from an underlying mantle source. Geochemical and isotopic results of this study are in good agreement with those of ca. 90 Ma plutons in western Yukon, (Mair et al., submitted 2005), which are also associated with IRGS. These data suggest that there are no discernable petrologic or isotopic distinctions, other than small differences in trace element concentrations, between auriferous Fort Knox and neighboring barren plutons.