LATE CRETACEOUS ARC DEVELOPMENT AND IMPLICATIONS FOR INTRUSION-RELATED MINERALIZATION IN THE NORTHERN CORDILLERA

Early Late Cretaceous (79-74 Ma) magmatism and related mineralizing systems are well characterized in western Yukon (e.g. Casino; Sonora Gulch; Nucleus/Revenue); however, this age of magmatism was previously thought to be spatially and volumetrically restricted. Newly identified Late Cretaceous plutonic and volcanic rocks as well as newly dated rocks that were previously assigned either mid-Cretaceous or Paleogene ages in southwest Yukon significantly increase the areal extent of this magmatism and the potential for intrusion-related mineralizing systems.

Existing data suggests that a magmatic arc was built upon the Yukon-Tanana terrane in southwest Yukon and east-central Alaska. The arc was the response of east/northeast (today’s coordinates) dipping subduction between ca. 79-74 Ma and predated the more areally widespread late Late Cretaceous (73-67 Ma) Carmacks Group/Prospector Mountain magmatism. Much of the known early Late Cretaceous mineralization in west-central Yukon is considered to be intrusion-related (i.e., porphyry, epithermal). A limited amount of deformation and erosion of the arc has occurred since the Late Cretaceous, confirmed by the amount of volcanic rocks of this age exposed in the area and their relatively flat lying nature.

A strong structural control appears to be important as exhibited by northwest alignment of many of the mineral systems and their associated intrusions. Several large northwest striking structures appear to control the location of mineralization, including the Big Creek and Serpent Head Lake faults. These structures are seldom observed at surface but are clearly identified in new geophysical surveys and models.

The 79-74 Ma arc appears to follow the overall northwest Cordilleran trend into eastern Alaska where plutons and volcanic rocks of this age become scarce, likely due to extensive Quaternary cover in the Yukon-Tanana Uplands. Late Cretaceous ages once again appear in Alaska along a southwest trend west of Anchorage (e.g. ~76 Ma Whistler porphyry). Restoration of 400 km of dextral offset along the Denali fault aligns the Yukon trend and southwestern Alaska trend suggesting there is continuous Late Cretaceous magmatism that follows the Alaska oroclinal bend, greatly enhancing the mineral potential for intrusion related systems within Yukon and Alaska.