TECTONOMAGMATIC CONTROLS ON LATE CRETACEOUS AU-CU MINERALIZATION IN EASTERN INTERIOR ALASKA

A compilation of new and existing radiometric U-Pb and ⁴⁰Ar/³⁹Ar radiometric ages have delineated a ~500 km arc affinity late-Cretaceous magmatic belt across interior Alaska and western Yukon north of the Denali Fault. The emplacement of these arc plutons is associated with Cu-Au mineralization.

The Road Metal prospect, controlled by Doyon Native Corporation, is a plutonic related Cu-Au system of late Cretaceous age. Au bearing greisen has been dated at 68.5 Ma (⁴⁰Ar/³⁹Ar, white mica) along with a biotite monzonite associated with Cu-Au mineralization dated at 71.2 Ma (⁴⁰Ar/³⁹Ar, biotite). These Cretaceous intrusive rocks at Road Metal have trace element compositions suggesting an arc magmatic origin. Intrusive rocks of the eastern Alaska Range, located 40-250 km west-northwest of Road Metal are also late Cretaceous in age. Mount Hayes (70 Ma Ma, U-Pb, zircon), Mount Balchen (72.1 Ma, U-Pb, zircon), Mount Hajdukovich (71 Ma, 40Ar/39Ar, biotite) and other intrusive rocks of late Cretaceous age form a latitudinal belt of plutonic bodies north of the Denali Fault. 150 km to the east of Road Metal are late Cretaceous arc related porphyry deposits (74-75 Ma) of the Casino porphyry belt. We propose that late Cretaceous arc magmatism in the Yukon extends west into interior Alaska towards the eastern Alaska Range.

This age and setting contrasts to 68-70 Ma plutonic rocks and prospects of the Forty-Mile mining district (~50 km to the north) which have been attributed to NE trending dextral fault systems and have trace element compositions suggesting an origin related to a collisional tectonic setting. Thus, the linear trend of late Cretaceous intrusions to coeval Cu-Au mineralization across eastern interior Alaska, near the Denali Fault, suggests arc magmatism was responsible for intrusion related mineralization and major plutonic formation from ~72-70 Ma. Following arc magmatism a change in tectonic setting emplaced plutons and prospects of the Forty-Mile and Sixty-Mile mining districts.