ABSTRACT Superimposed porphyry systems are a subset of telescoped porphyry deposits, whereby significantly younger ore zones overprint older, nongenetically related systems. Recognition of superimposed features in porphyry systems is important for determining and assessing their prospectivity. The Mount Nansen gold corridor in the southern Dawson Range gold belt of Yukon, Canada, contains porphyry prospects and epithermal deposits with enigmatic genetic models. Geologic, petrologic, temporal (U-Pb zircon, Re-Os molybdenite), and geochemical (whole-rock) studies are used to demonstrate the presence of superimposed porphyry systems in this district. The arc-related episodic magmatism of the Mount Nansen gold corridor has been classified into four intrusive suites: (1) Late Triassic Minto, (2) mid-Cretaceous Whitehorse, (3) Late Cretaceous Casino (eLKc; 80–72 Ma), and (4) Late Cretaceous Prospector Mountain (lLKp; 72–65 Ma). Geochemical fingerprinting of these suites indicates intermediate to evolved, calc-alkaline compositions with a common lower crust melt source. The eLKc and lLKp suites lack an Eu anomaly and show increasing amounts of light rare earth element (LREE) enrichment and heavy rare earth element (HREE) depletion over time. These features suggest that garnet was stable in the melt source and oxidized magmas were generated in these Late Cretaceous suites. The mildly alkaline lLKp and associated Carmacks Group shoshonitic basalts reflect localized extension in an overall compressive arc setting in the Mount Nansen gold corridor, hence a setting conducive for Au-rich porphyry and epithermal systems. The ca. 79 to 72 Ma Casino suite is commonly interpreted as the causative magmatic event for most well-endowed porphyry deposits (76 to 74 Ma in age) in the Dawson Range gold belt. However, our detailed study of the Klaza setting shows that at this locality, intermediate-sulfidation epithermal veins are a distal expression of a Prospector Mountain-age (ca. 71 Ma) porphyry system, which overprints two Casino-age porphyry systems (ca. 77 and 80 Ma). The Mount Nansen gold corridor thus hosts at least two spatially and temporally overprinting Late Cretaceous magmatic-hydrothermal systems in the Dawson Range gold belt. Importantly, recognition of this feature at other porphyry deposit settings in the Dawson Range gold belt (e.g., Freegold Mountain district) is critical as it provides the potential for metal (Cu-Au-Mo)-enriched hypogene ore shells.

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.

Abstract The Boot Inlet Formation (Reynolds Point Group, Shaler Supergroup) is an early Neoproterozoic (< 1077 MA, >723 Ma) succession that crops out within the Minto Inlier on northern Victoria Island in the Canadian Arctic archipelago, and consists of strata that accumulated on a carbonate ramp. Inner-ramp facies comprise molar-tooth lime mudstone and current-bedded ooid grainstone (locally herringbone cross-laminated) with scalloped erosional surfaces. Ooid shoals ( 3–4 m thick) and sheets (0.5–1.0 m thick) are interbedded with 10–15 m thick stromatolite bioherms and biostromes forming complexes 0.5 to 5.0 km wide. The most common mid-ramp facies is parted to ribbon-bedded limestone with conspicuous ripples, gutter casts, hummocky cross-stratification, and intraformational breccias readily interprétable as storm deposits; these finegrained rocks form shallowing-upward, meter-scale cycles capped by oolitic limestone and small reefs. Outer-ramp facies comprise shale with large carbonate concretions. Reefs are most common in the lower half of the succession, where overall sea-level rise combined with higher-order transgressions to produce maximum accommodation space. A pronounced zonation of reef types occurs across the ramp. A current-oriented biostrome of Baicalia? is the only reef type on the inner ramp. Patch reefs and table reefs characterize the inner- to mid-ramp transition, and consist of stacked meter-scale bushes of Tungussia that pass upward into broad domal sheets of parallel, columnar stromatolites (Baicalia) oriented at a high angle to the sheets. Overall upward decrease in diversity of growth form is accompanied by evidence for increasing wave and current energy. Concentric-sheet bioherms up to 60 m in diameter and 15 m high, composed of sheets of closely spaced “pencil stromatolites” (Jurusania), grew in outer-ramp facies during rapid transgression. The Boot Inlet reefs are similar to other Prolerozoic reefs in being composed entirely of stromatolites, including some of the same forms as characterize other early Neoproterozoic patch reefs. Calcimicrobes are conspicuously absent, despite their abundance in coeval deeper-water reefs in the Mackenzie Mountains. The presence of kalyptra-like stromatolitic structures in the Boot Inlet reefs is similar to that of Early Cambrian calcimicrobe-archaeocyathan reefs, and lends support for the view that the Phanerozoic reef archetype originated during the Neoproterozoic.