New data from an Upper Triassic to Lower Cretaceous deep marine succession—the herein reinstated and restricted Gemuk Group—provide a vital piece of the puzzle for unraveling southwestern Alaska's tectonic history. First defined by Cady et al. in 1955 , the Gemuk Group soon became a regional catchall unit that ended up as part of at least four different terranes. In this paper we provide the first new data in nearly half a century from the Gemuk Group in the original type area in Taylor Mountains quadrangle and from contiguous rocks to the north in Sleetmute quadrangle. Discontinuous exposure, hints of complex structure, the reconnaissance level of our mapping, and spotty age constraints together permit definition of only a rough stratigraphy. The restricted Gemuk Group is at least 2250 m thick, and could easily be at least twice as thick. The age range of the restricted Gemuk Group is tightened on the basis of ten radiolarian ages, two new bivalve ages, one conodont age, two U-Pb zircon ages on tuff, and U-Pb ages of 110 detrital zircons from two sandstones. The Triassic part of the restricted Gemuk Group, which consists of intermediate pillow lavas interbedded with siltstone, chert, and rare limestone, produced radiolarians, bivalves, and conodonts of Carnian and Norian ages. The Jurassic part appears to be mostly siltstone and chert, and yielded radiolarians of Hettangian-Sinemurian, Pliensbachian-Toarcian, and Oxfordian ages. Two tuffs near the Jurassic-Cretaceous boundary record nearby arc volcanism: one at 146 Ma is interbedded with red and green siltstone, and a second at ca. 137 Ma is interbedded with graywacke turbidites. Graywacke appears to be the dominant rock type in the Lower Cretaceous part of the restricted Gemuk Group. Detrital zircon analyses were performed on two sandstone samples using SHRIMP. One sandstone yielded a dominant age cluster of 133–180 Ma; the oldest grain is only 316 Ma. The second sample is dominated by zircons of 130–154 Ma; the oldest grain is 292 Ma. The youngest zircons are probably not much older than the sandstone itself. Point counts of restricted Gemuk Group sandstones yield average ratios of 24/29/47 for Q/F/L, 15/83/2 for Ls/Lv/Lm, and 41/48/11 for Qm/P/K. In the field, sandstones of the restricted Gemuk Group are not easily distinguished from sandstones of the overlying Upper Cretaceous turbidite-dominated Kuskokwim Group. Petrographically, however, the restricted Gemuk Group has modal K-feldspar, whereas the Kuskokwim Group generally does not (average Qm/P/K of 64/36/0). Some K-feldspar-bearing graywacke that was previously mapped as Kuskokwim Group ( Cady et al., 1955 ) is here reassigned to the restricted Gemuk Group. Major- and trace-element geochemistry of shales from the restricted Gemuk Group and the Kuskokwim Group show distinct differences. The chemical index of alteration (CIA) is distinctly higherforshales of the Kuskokwim Group than forthose of the restricted Gemuk Group, suggesting more intense weathering during deposition of the Kuskokwim Group. The restricted Gemuk Group represents an estimated 90–100 m.y. of deep-water sedimentation, first accompanied by submarine volcanism and later by nearby explosive arc activity. Two hypotheses are presented for the tectonic setting. One model that needs additional testing is that the restricted Gemuk Group consists of imbricated oceanic plate stratigraphy. Based on available information, our preferred model is that it was deposited in a back-arc, intra-arc, or forearc basin that was subsequently deformed. The terrane affinity of the restricted Gemuk Group is uncertain. The rocks of this area were formerly assigned to the Hagemeister subterrane of the Togiak terrane—a Late Triassic to Early Cretaceous arc—but our data show this to be a poor match. None of the other possibilities (e.g., Nukluk and Tikchik subterranes of the Goodnews terrane) is viable; hence, the terrane subdivision and distribution in southwestern Alaska may need to be revisited. The geologic history revealed by our study of the restricted Gemuk Group gives us a solid toehold in unraveling the Mesozoic paleogeography of this part of the northern Cordillera.

Abstract The Tintina gold province comprises numerous (>15) individual gold belts and districts throughout interior Alaska and Yukon and has a recently defined lode gold resource of ~35 Moz and past placer-lode production of ~33 Moz Au. This recently defined province is underlain by a diverse geology comprising the following: (1) highly deformed and polymetamorphosed Neoproterozoic and Paleozoic schists and meta-igneous rocks of the pericratonic Yukon-Tanana terrane in eastern Alaska and western Yukon, (2) deformed continental margin Neoproterozoic and Paleozoic clastic rocks and limestones of the Selwyn basin in eastern Yukon, and (3) weakly deformed flysch of the Cretaceous Kuskokwim basin in western Alaska. Most gold lodes show a spatial and temporal association with mid-Cretaceous plutons in the former two areas and with Late Cretaceous plutons in the latter area. These calc-alkaline, reduced, radiogenic intrusions were emplaced within a wide variety of tectonic settings along the Cretaceous margin of North America. Significant lode gold resources, newly defined during the last decade, include those at Fort Knox (5.4 Moz), Donlin Creek (12.3 Moz), Pogo (5.8 Moz), True North (0.79 Moz), and Brewery Creek (0.85 Moz) Mineralization styles throughout the Tintina gold province are extremely varied. Most deposits are well defined as zones surrounding a central causative pluton. Sheeted quartz-feldspar veins are a distinctive ore style where ore is localized in the apical parts of plutons or in immediately adjacent hornfels, and they are typically characterized by a Te-W-Bi-Au geochemical signature. Polymetallic replacement bodies, auriferous breccias, and disseminated ores occur farther out within thermal aureoles; gold-rich skarns develop where reactive carbonate units are part of the surrounding country rock sequence. Other ore styles, although spatially and temporally associated with intrusions, continue to require more data before a definitive genetic link to magmatism can be assured. These styles include the epizonal fracture networks within carbonaceous sedimentary rocks and/or intruded dikes and sills, which have a consistent As-Au-Hg-Sb signature (e.g., Brewery Creek, Donlin Creek, True North). Also of uncertain origin are the shear-zone—related veins (e.g., Ryan Lode, Longline, Pogo) that have more abundant sulfide minerals (2—3 vol %) than other styles, common crack-seal textures, ductile and brittle features, and little metal zoning. Recognition in 1992 that an old prospect, known since 1913 at the site of the presently producing Fort Knox deposit, could be a world-class gold deposit changed precious metal exploration strategies in the northern North American Cordillera. Bulk tonnage orebodies with economic grades <1 g/t Au were for the first time seen as conventional milling or heap leaching targets. The resulting exploration boom throughout the 1990s in interior Alaska and Yukon led to four years of recent gold production at Brewery Creek, the present mining at True North, and the large defined resource at Donlin Creek. In addition, the Pogo deposit was discovered in the mid-1990s; it has ore grades (~18 g/t Au) about one order of magnitude greater than those of the other new resources in the Tintina gold province. Regional stream geochemistry followed by soil geochemistry has been most effective in discovering new orebodies in this vast region (e.g., Brewery Creek, Pogo), although the application of the intrusion-related deposit model to areas with known mineral deposits has also been successful (e.g., Fort Knox, True North, Donlin Creek). The notable characteristics of the Tintina gold province deposits pertinent to exploration (such as vast, remote under explored areas, unallocated regions with variable oxidation depths, and discontinuous permafrost), in combination with an evolving geologic model, create exploration challenges that can be overcome with the pragmatic application of selected exploration techniques and strategies.

Abstract Placer gold and precious metal-bearing lode deposits of southwestern Alaska lie within a region 550 by 350 km, herein referred to as the Kuskokwim mineral belt. This mineral belt has yielded 100,240 kg (3.22 Moz) of gold, 12,813 kg (412,000 oz) of silver, 1,377,412 kg (39,960 flasks) of mercury, and modest amounts of antimony and tungsten derived primalily from Late Cretaceous-early Tertiary igneous complexes of four major types: (1) alkali-calcic, comagmatic volcuuic-plutonic complexes and isolated plutons, (2) calc-alkaline, meta-aluminous reduced plutons, (3) peraluminous alaskite or granitc-porphyry sills and dike swarms, and (4) andesite-rhyolite subaerial volcanic rocks. About 80 perceut of the 77 to 52 Ma intrusive and volcanic rocks intrude or overlie the middle to Upper Cretaceous Kuskokwim Group sedimeutmy and volcanic rocks, as well as the Paleozoic-Mesozoic rocks of the Nixon Fork, lnnoko, Goodnews, and Ruby preaccretionury terranes. The major precious metal-bearing deposit types related to Late Cretaceous-early Tertiary igneous complexes of the Kuskokwim mineral belt are subdivided as follows: (1) plutonic-hosted copper-gold polymetallic stockwork, skarn, and vein deposits, (2) peraluminous granite-porphyry-hosted gold polymetallic deposits, (3) plutonic-related, boron-enriched silver-tin polymetallic breccia pipes and replacement deposits, (4) gold and silver mineralization in epithermal systems, and (5) gold polymetallic heavy mineral placer deposits. Ten deposits genetically related to Late Cretaceous-early Tertiary intrusions contain minimum, inferred reserves amounting to 162,572 kg (5.23 Moz) of gold, 201,015 kg (6.46 Moz) of silver, 12,160 metric tons (t) of tin, and 28,088 t of copper. The lodes occur in veins, stockworks, breccia pipes, and replacement deposits that formed in epithermal to Mesothernal temperature-pressure conditions. Fluid inclusion, isotopic age, mineral assemblage, alteration assemblage, and structural data indicate that many of the mineral deposits associated with Late Cretaceousearly Tertiary volcanic and plutonic rocks represent geologically and spatially related, vertically zoned hydrothermal systems uow exposed at several erosional levels. Polymetallic gold deposits of the Kuskokwim mineral belt are probably related to 77 to 52 Ma plutonism and volcanism associated with a period of rapid, north-directed subduction of the Kula plate. The geologic interpretation suggests that igneous complexes of the Kuskokwim mineral belt formed in an intracontinental back-are setting during a period of extensional, wrench fault tectonics. The Kuskokwim mineral belt has many geologic and metallogenic features similar to other precious metalbearing systems associated with arc-related igneous rocks such as the Late Cretaceous-early Tertiary Rocky Mountain alkalic province, the Jurassic Mount Milligan district of central British Columbia, the Andean orogen of South America, and the Okhotsk-Chukotka belt of northeast Asia.

Abstract Southwest Alaska lies between the Yukon-Koyukuk province to the north, and the Alaska Peninsula to the south (Wahrhaftig, this volume). It includes the southwestern Alaska Range, the Kuskokwim Mountains, the Ahklun Mountains, the Bristol Bay Lowland, and the Minchumina and Holitna basins. It is an area of approximately 175,000 km 2 , and, with the exception of the rugged southwestern Alaska Range and Ahklun Mountains, consists mostly of low rolling hills. The oldest rocks in the region are metamorphic rocks with Early Proterozoic protolith ages that occur as isolated exposures in the central Kuskokwim Mountains, and in fault contact with Mesozoic accretionary rocks of the Bristol Bay region. Precambrian metamorphic rocks also occur in the northern Kuskokwim Mountains and serve as depositional basement for Paleozoic shelf deposits. A nearly continuous sequence of Paleozoic continental margin rocks underlies much of the southwestern Alaska Range and northern Kuskokwim Mountains. The most extensive unit in southwest Alaska is the predominantly Upper Cretaceous Kuskokwim Group, which, in large part, rests unconformably on older rocks of the region. Volcanic rocks of Mesozoic age are common in the Bristol Bay region, and volcanic and plutonic rocks of latest Cretaceous and earliest Tertiary age are common throughout southwest Alaska. Two major northeast-trending faults are known to traverse southwest Alaska, the Denali-Farewell fault system to the south, and the Iditarod-Nixon Fork fault to the north. Latest Cretaceous and Tertiary right-lateral offsets of less than 150 km characterize both faults. The Susulatna lineament (or Poorman fault), north of the Iditarod-Nixon Fork

Abstract Alaska is commonly regarded as one of the frontiers of North America for the discovery of metalliferous mineral deposits. A recurring theme in the history of the state has been “rushes” or “stampedes” to sites of newly discovered deposits. Since about 1965, mining companies have undertaken much exploration for lode and placer mineral deposits. During the same period, because of the considerable interest in federal lands in Alaska and the establishment of new national parks, wildlife refuges, and native corporations, extensive studies of mineral deposits and of the mineral resource potential of Alaska have been conducted by the U.S. Geological Survey, the U.S. Bureau of Mines, and the Alaska Division of Geological and Geophysical Surveys. These studies have resulted in abundant new information on Alaskan mineral deposits. In the same period, substantial new geologic mapping has also been completed with the help of new logistical and technical tools. One result of the geologic mapping and associated geologic studies is the recognition of numerous faultbounded assemblages of rocks designated as tectonostratigraphic (lithotectonic) terranes. This concept indicates that most of Alaska consists of a collage of such terranes (Silberling and others, this volume, Plate 3). The purpose of this report is to summarize the local geology, geologic setting, and metallogenesis of the major metalliferous lode deposits and placer districts of Alaska. The term “major mineral deposit” is defined as a mine, mineral deposit with known reserve, prospect, or occurrence that the authors judged significant for any given geographic region. This report is