Abstract A magmatic and metallogenic framework for the northern Yukon-Tanana terrane of west-central Yukon and eastern Alaska is proposed, which contextualizes syngenetic, intrusion-related, and orogenic styles of mineralization in the region. The framework applies to bedrock gold and base metal enrichments in the Dawson Range, White Gold, Klondike, Sixtymile, and Fortymile districts, which are historically known for their placer gold endowment, but which host few significant bedrock mineral resources. New field and geochronological (U-Pb, 40 Ar/ 39 Ar, 187 Re/ 187 Os) data, along with contributions from exploration companies, provide the key constraints on this framework. Sedimentary exhalative Pb-Zn mineralization and porphyry-style Cu-Au mineralization are associated with Late Devonian to Early Mississippian (365-342 Ma) rocks of the Finlayson assemblage and Simpson Range plutonic suite, respectively—both of which formed in a continental arc built on pre-Late Devonian continental margin sediments (Snowcap assemblage) along the ancient Pacific margin of North America. By the Late Permian, these assemblages had rifted away from North America, and W-dipping subduction of the intervening Slide Mountain Ocean was initiated. Volcanogenic massive sulfide-style Pb-Zn-Cu-(Ag-Au) mineralization formed in subvolcanic to volcanic rocks of the Late Permian (269-253 Ma) Klondike arc assemblage that was built on the Devono-Mississippian arc. Together these assemblages make up the Yukon-Tanana terrane. Gold mineralization formed sparsely with syn- to postmetamorphic Late Permian (253-250 Ma) anatectic melts. Five metallogenic events are recognized that coincide with magmatic episodes superimposed on the Yukon-Tanana terrane: (1) Cu-Au mineralization formed during an Early Jurassic (200-179 Ma) pulse of magmatism and was accompanied by rapid crustal exhumation (e.g., Minto); (2) Au-mineralized breccia complexes, skarns, intermediate-sulfidation epithermal systems, and polymetallic veins are associated with mid-Cretaceous (115-98 Ma) magnetite-series arc magmas in the Dawson Range, whereas age-equivalent Au deposits in the back-arc region to the north are associated with ilmenite-series magmas (e.g., Pogo); (3) variably Cu and Au rich porphyry systems formed within the mid-Cretaceous arc in the early Late Cretaceous (79-72 Ma) (e.g., Casino, Nucleus-Revenue); (4) porphyry Mo and Cu systems and Ag-rich polymetallic veins, carbonate-replacement, and skarn bodies are temporally and spatially associated with NE-trending, sinistral oblique-extensional fault systems in the latest Cretaceous (72-67 Ma); and (5) examples of disseminated U, Cu-Pb-Ag skarn, and Au-Ag epithermal systems are associated with dominantly felsic but locally bimodal Paleocene-Eocene (60-55 Ma) magmatism, emplaced into zones of extension during early activity on the Tintina fault zone. At least two distinct orogenic Au-mineralizing events are recognized. Within a Middle to Late Jurassic hiatus in magmatism, gold mineralization formed at 163 to 155 Ma in brittle-ductile to brittle structures within sinistral fault zones (e.g., White Gold), high-angle reverse faults, and kink folds. A subsequent episode of mid-Cretaceous (96-92 Ma) orogenic gold mineralization formed in structures cutting Paleozoic metamorphic rocks and mid-Cretaceous granitoids (e.g., Moosehorn, Boulevard). Weathering and surficial preservation in this unglaciated region since the Pliocene resulted in economic placer gold endowments in the Klondike, Sixtymile, Fortymile, White Gold, and Dawson Range districts. The framework we describe for the magmatism and metallogeny of west-central Yukon and eastern Alaska provides a testable platform for regional exploration targeting and property-scale exploration in a region with demonstrated mineral potential.

Transtensional deformation was concentrated in a zone adjacent to the Tintina strike-slip fault system in Alaska during the early Tertiary. The deformation occurred along the Victoria Creek fault, the trace of the Tintina system that connects it with the Kaltag fault; together the Tintina and Kaltag fault systems girdle Alaska from east to west. Over an area of ∼25 by 70 km between the Victoria Creek and Tozitna faults, bimodal volcanics erupted; lacustrine and fluvial rocks were deposited; plutons were emplaced and deformed; and metamorphic rocks cooled, all at about the same time. Plutonic and volcanic rocks in this zone yield U-Pb zircon ages of ca. 60 Ma; 40 Ar/ 39 Ar cooling ages from those plutons and adjacent metamorphic rocks are also ca. 60 Ma. Although early Tertiary magmatism occurred over a broad area in central Alaska, metamorphism and ductile deformation accompanied that magmatism in this one zone only. Within the zone of deformation, pluton aureoles and metamorphic rocks display consistent NE-SW–stretching lineations parallel to the Victoria Creek fault, suggesting that deformation processes involved subhorizontal elongation of the package. The most deeply buried metamorphic rocks, kyanite-bearing metapelites, occur as lenses adjacent to the fault, which cuts the crust to the Moho (Beaudoin et al., 1997). Geochronologic data and field relationships suggest that the amount of early Tertiary exhumation was greatest adjacent to the Victoria Creek fault. The early Tertiary crustal-scale events that may have operated to produce transtension in this area are (1) increased heat flux and related bimodal within-plate magmatism, (2) movement on a releasing stepover within the Tintina fault system or on a regional scale involving both the Tintina and the Kobuk fault systems, and (3) oroclinal bending of the Tintina-Kaltag fault system with counterclockwise rotation of western Alaska.

The Cordilleran Coast Plutonic Complex comprises the roots of a middle Cretaceous to Paleogene magmatic arc and orogenic belt that extends from the Yukon Territory to Washington State. Exceptional rock exposure and mineral preservation have made the Cascades crystalline core, the southernmost exposure of the Coast Plutonic Complex, a laboratory for understanding mid-crustal processes in contractional magmatic arcs. Perhaps surprisingly, after decades of study, fundamental tectonic models for the Late Cretaceous evolution of the core remain in question. This study evaluates tectonic models using phase equilibrium modeling, thermobarometry, and high-precision geochronology to constrain present crustal structure and the magnitudes, rates, and lateral preservation of Late Cretaceous regional metamorphism across the Nason terrane, Wenatchee block, Cascades crystalline core. Garnet Sm-Nd ages of 88–86 Ma restrict the last regional metamorphism to no less than 3 m.y. after emplacement of the 96–91 Ma Mount Stuart batholith. These ages and petrologic data reflecting only negligible to moderate pressure increases (0–3.6 kbar) during garnet growth indicate a time lag between a fairly rapid pressure increase (up to 0.5 kbar/m.y.) subsequent to low-pressure contact metamorphism associated with emplacement of the Mount Stuart batholith and temperature increases during initial garnet growth. This thermal relaxation signature supports a thrust-loading model for post–Mount Stuart regional metamorphism. The lateral extent and magnitude of regional metamorphism across the Nason terrane and Mount Stuart domain offer additional support for a tectonic loading model for regional metamorphism. Peak recorded pressures decrease from >8 kbar in the northeast of the terrane to 5 kbar southwest of the Mount Stuart batholith, compatible with loading by a tapered thrust sheet, followed by exhumation and shortening after peak metamorphism. The lack of structural evidence for thrusting and steepening of the paleobarometric gradient across the Nason terrane north of the batholith suggest that strain-partitioned folding was dominant at the exposed crustal level during and after the last regional metamorphism. Thus, a tectonic model compatible with metamorphic P-T data may include a decoupling of upper-crustal and mid-crustal shortening accommodation mechanisms during Late Cretaceous regional metamorphism.