THE ROLE OF THE SIXTYMILE-PIKA FAULT SYSTEM ON LATE CRETACEOUS MAGMATISM AND HYDROTHERMAL MINERALIZATION, WESTERN YUKON AND EASTERN ALASKA

The Sixtymile-Pika fault system is a previously undescribed, orogen-normal (northeast-striking) structure in the Yukon-Alaska Cordilleran hinterland that has been interpreted from a range of geological, aeromagnetic, topographic, and gravity data. The fault system has at least 150 kilometers of strike length, and accommodated approximately 15 km of sinistral displacement, as documented by offset geological units and geomagnetic markers. Deviations in fault orientation (bends) or position (relays) account for the location of intermediate to felsic magmatism, porphyry Cu-(Au-Mo-W) mineralization, and intermediate-sulfidation epithermal Ag-Pb (-Zn-Cu-Au) mineralization. In the Sixtymile district of western Yukon, a relay in the fault system generated a major pull-apart basin. This feature controlled valley-filling Carmacks Group volcanic rocks, localized syn-tectonic porphyry and polymetallic vein mineralization, and resulted in the exposure and erosion of pre-existing orogenic gold mineralization to form rich placer gold deposits. In the Connaught area south of the Sixtymile district, and in the Pika area of Alaska’s southern Fortymile district, northeast-trending arrays of mutually cross-cutting faults, veins, alteration zones, breccias, and dikes suggest that faulting, magmatism, hydrothermal fluid activity, and mineralization were broadly synchronous processes. The Sixtymile-Pika fault system was particularly prolific for mineralization between 70 and 68 Ma, as indicated by U-Pb dating of zircon in syn- to post-mineralization intrusions, ⁴⁰Ar/³⁹Ar dating of hydrothermal mica in epithermal alteration zones, and ¹⁸⁷Re/¹⁸⁷Os dating of porphyry-style molybdenite.