COMPLEX MESOZOIC DEFORMATION IN THE CENTRAL PART OF THE YUKON-TANANA UPLAND, ALASKA—IMPLICATIONS FOR GOLD DEPOSITION IN THE TINTINA GOLD PROVINCE

The Tintina gold province is an arcuate, 1,200-km-long gold province extending from northern British Columbia into southwestern Alaska. The province includes such large gold deposits as the Pogo, Fort Knox, True North, Donlin Creek, and Shotgun deposits and remains a prime area for gold exploration. The Yukon-Tanana Upland lies within the Tintina province in Alaska and consists of Paleozoic and older(?) supracrustal rocks intruded by Devonian to Mississippian granitoids. Paleozoic deformation (D1), potentially associated with plutonism, was obscured by intense Mesozoic deformation and metamorphism. Previous studies in the eastern Upland documented pre-late Triassic metamorphism, Jurassic contraction, and Cretaceous deformation interpreted to have accompanied regional extension and exhumation.

Recent geologic mapping and geochronologic investigations near Pogo, in the western part of the Upland, reveal complexities of the Mesozoic deformation. At least some of the rocks underwent tectonism (D2) during the Middle Jurassic (about 188 Ma), and subsequently, were deformed in an Early Cretaceous D3 event between ~130 to 116 Ma. New U-Pb SHRIMP data on zircons from biotite gneisses record inherited cores that range from 363 Ma up to about 2,130 Ma and have rims of euhedral Early Cretaceous metamorphic overgrowths (116 +/- 4 Ma), interpreted to record recrystallization during west-northwest-directed D3 thrusting and folding. U-Pb SHRIMP dating of monazite from a gneiss sample yields an age of 112 +/- 2 Ma; the monazite presumably grew during the waning stages of the intense regional ductile D3 deformation. The D3 ductile deformation was followed closely by cross-cutting granite diking (106 +/- 2 Ma) and gold mineralization during regional D4 uplift and extension. The main pulse of gold mineralization is temporally and spatially associated with the Cretaceous granitic dikes and plutons. These new data telescope the time window between the Early Cretaceous D3 ductile deformation (116 Ma) and the D4 extension, plutonism, and gold mineralization (106 Ma) to about 10 million years.