A HYDROTHERMAL RECORD OF STRUCTURAL OVERPRINTING AND CRUSTAL EXHUMATION IN THE WHITE GOLD DISTRICT, YUKON

Middle to Late Jurassic orogenic gold in Yukon’s White Gold District formed during rapid unroofing of the Yukon-Tanana terrane, following crustal thickening and purported oroclinal terrane closure in the Early Jurassic. Gold mineralization is hosted in second-order faults associated with east-trending sinistral shear zones, which may locally have originated as lateral ramps to Early Jurassic thrust faults.

Rocks record a wide range of structural styles, vein textures, and mineral associations, which vary according to host rock lithology and rheology and inferred crustal depth. Single mineralized structures in the White Gold deposit (Kinross, discovered 2008) record: (1) an early stage of barren, milky quartz that formed ~163.5 Ma under mid-crustal, brittle-plastic conditions, and with no visible alteration; (2) a later stage of space-filling quartz that formed ~160-155 Ma along with gold, pyrite, and accessory molybdenite and tellurides, and with sericite-carbonate alteration; and (3) a final, yet-undated stage of chalcedony that precipitated with barite ± dickite ± galena. Late Jurassic gold mineralization ages, documented by ¹⁸⁷Re/¹⁸⁷Os ages of cogenetic molybdenite, are consistent with ⁴⁰Ar/³⁹Ar cooling ages of metamorphic mica regionally.

Overprinting vein types at White Gold represent progressively shallower crustal level, decreasing temperature, and increasing degrees of fluid-rock disequilibrium. These features are consistent with re-activation of the same zones of structural weakness over several million years of fault-fluid activity, and during several vertical kilometers of crustal exhumation.

Jurassic sinistral shear zones were variably reactivated as dextral strike-slip faults during northwestward extrusion of the Intermontane terranes in the Cretaceous. The Coffee gold deposits (Kaminak Gold, discovered 2010) record early brittle-ductile shear zones and barren mesozonal quartz veins (Jurassic?). These features are overprinted by Cretaceous brittle faults and breccias mineralized by auriferous arsenian pyrite. The related fault systems that governed disparate ages and mineralization styles at White Gold and Coffee are attractive exploration targets and provide unique opportunities to investigate the structural evolution of the Northern Cordillera.