LATE CRETACEOUS TECTONIC EVOLUTION AND METALLOGENY OF SOUTHWESTERN ALASKA

During the Late Cretaceous, a major episode of epigenetic ore deposit formation took place in what is now southwestern Alaska when several large-scale tectonic factors led to epizonal hydrothermal activity. Older tectonostratigraphic terranes of the region had been assembled by mid-Cretaceous time and were soon overlapped by Upper Cretaceous turbidites of the Kuskokwim basin. Subsequently, oblique plate convergence and the resultant dextral strike-slip regime began to dominate the tectonics leading to both folding and faulting. Late Cretaceous dextral displacement in the region enhanced crustal dilation and fluid migration. Also during this period, extensive subduction-related arc magmatism, covering an area of perhaps 900 x 500 km, is recorded by intermediate to felsic epizonal plutons, dikes, and related volcanic rocks dating between 76 and 63 Ma. Both mantle and flysch-derived intrusive rocks are present. Two important types of mineral deposit formed around 70 Ma: 1) precious metal-bearing, B-rich, high-temperature intrusion-related deposits (Shotgun, Flat, Beaver Mountains), and 2) epizonal Hg-Sb (Red Devil, Cinnabar Creek) and Au (Donlin Creek) deposits, many of which are structurally associated with relatively competent sill and dike systems. Some of the intrusion-related deposits that lie along the main first-order faults (Fortyseven Creek, Nixon Fork) have structural features suggesting they were emplaced synkinematically during dextral strike-slip. On the other hand, deposits that lie between the first-order faults (Donlin Creek and Red Devil) suggest a roughly north-south principle compression direction, perpendicular to what would be expected in a dextral regime on the main strike-slip faults. The regional thermal gradient was high during this time as evidenced by elevated Hg across much of southwestern Alaska and by voluminous igneous activity that peaked at 70 Ma. The cause of this high gradient is problematic. Several factors could have contributed to it including flat-slab subduction or possibly arrival of a slab window related to subduction of the Kula-Resurrection ridge.