NEW CONSTRAINTS ON THE ROLE OF VARIABLE FAULT ARCHITECTURE ON OROGENIC GOLD FORMATION IN THE CANADIAN CORDILLERA FROM INVESTIGATIONS IN THE KLONDIKE DISTRICT, YUKON, CANADA

The world-class placer gold fields of the Klondike District, Yukon, Canada have produced >20 Moz of gold, primarily from alluvium derived from 5-3 Ma White Channel gravel paleoplacer deposits that were themselves formed from erosion of quartz-carbonate orogenic gold vein systems. Thus, the endowment of the Klondike District reflects the influx of a remarkable volume of auriferous fluids into the presently exposed thrust nappes concomitant with progressive convergent margin tectonism in the Middle Jurassic to Early Cretaceous. Typically, orogenic gold deposits display a genetic and spatial association to regional, crustal-scale fault systems, which facilitate the concentration and transport of ore-forming metamorphic fluids from source domains near the greenschist-amphibolite facies transition to physical or chemical traps at higher crustal levels during fluid cycling events associated with tectonic uplift. However, in the Klondike District, no large-scale fault system has been identified that could have localized such fluid flow. Rather, like other gold-bearing districts in the Canadian Cordillera, such as Cassiar and Cariboo, the structural geometry of the Klondike may be dominated by a series of low-angle thrust sheets. To test this and to investigate why the Klondike has such an exceptional gold endowment in an atypically small area, we combine interpretations of regional structure from USGS-GSC geophysical compilations with local scale geophysical surveys conducted as part of bedrock gold exploration efforts. Interpretations of magnetic and electromagnetic data reveal the presence of lineaments forming anastomosing fault networks internal to the Yukon-Tanana Terrane. The lineaments display geometries that are consistent with dextral kinematics, including asymmetric step-over features that are inferred to represent accommodation faults secondary to the main transpressional corridor. Preliminary work indicates that the higher-order, secondary fault strands may have acted as the accommodation zones for the dispersion and emplacement of deeply sourced, gold-bearing orogenic fluids, which is similar other Phanerozoic, fault/shear-controlled deposits in the northern Canadian Cordillera that formed proximal to the ancestral continental margin.