Abstract Cretaceous gold metallogenesis in southwestern Alaska comprises three distinct episodes related to the accretionary evolution of northwestern North America. The oldest mineralizing event is characterized by 112 Ma Cu-Au-Bi-Te porphyry-type(?) veining in the zoned Bonanza and adjacent plutons that intruded rocks of the Nyac terrane. Tectonic reconstructions and limited geological and geochemical data suggest that Cu-Au mineralization in the Nyac district may be related to terminal subduction during accretion of the Togiak-Koyukuk arc. The subsequent 100 to 89 Ma metallogenic event is a product of subduction-related magmatism immediately following accretion of the Peninsular-Alexander-Wrangellia superterrane and includes formation of the giant Pebble porphyry Cu-Au-Mo deposit. Pebble underwent a complex history of highly oxidized magmatism that is isotopically linked to enriched lithosphere or metasomatized mantle sources. Pebble and other porphyryrelated plutons were emplaced as a consequence of changes in plate motion and onset of dextral transpression along the continental margin to the southeast of their present-day locations. The final 75 to 65 Ma metallogenic event is regionally the most extensive. It followed a ~15-m.y.-long magmatic lull and is related to enigmatic subduction-related magmatism in the western part of the Alaska Range and the Kuskokwim basin. This event resulted in the formation of porphyry, reduced pluton-related, orogenic, and possible epithermal Au deposits. In the better-studied relatively low-relief areas of the Kuskokwim basin, many of the mineralized systems are spatially associated with ilmenite-series monzonite to quartz monzonite composite plutons that have isotopic signatures consistent with derivation from crustally contaminated mantle sources. These pluton-hosted Au deposits comprise low-sulfide stockworks, sheeted veins, and/or breccias in the cupolas of moderately differentiated intrusions that contain high Au; anomalous As, Bi, Sb, and/or Te; and have variable, but not uneconomic, Cu concentrations. In addition, Au, Sb, and/or Hg deposits hosted by competent NE-trending granite porphyry dike-sill complexes or along faults in flysch are widespread in the Kuskokwim basin. These deposits, including the giant Donlin Creek Au deposit, formed at shallow crustal levels and are classified here as epizonal orogenic Au deposits and related Hg and Sb lodes. Mesozonal orogenic Au deposits were formed along the margins of the uplifting Willow Creek pluton that is now exposed along the southern side of the Talkeetna Mountains batholith. Recent discoveries in the rugged Alaska Range comprise less well-studied porphyry Au-Cu (Whistler, Island Mountain), epithermal(?) Au (Terra), and reduced pluton-hosted Au deposits (Estelle). The cause of the broad latest Cretaceous magmatism associated with the final metallogenic event is enigmatic, but indicates widespread high heat flow possibly related to flat-slab subduction, lithospheric mantle delamination, or escape tectonics. Plutonism postdated regional folding and coincided with periods of movement along regional faults and formation of the NE-trending structural fabric. Magmatism resulted from initiation of transpressional faulting in more landward positions during oblique subduction of the Kula plate. Crustal contamination of mantle melts, either near their source or during ascent through the thick flysch of the Kuskokwim basin, produced low oxidation state magmas and controlled much of the metallogeny, particularly for those deposits that display similarities to both reduced porphyry Au-Cu deposits and, to a lesser degree, reduced intrusion-related Au systems. High heat flow also induced crustal melting and metamorphic devolatilization to form orogenic Au deposits. A transition to extension at ~60 Ma recorded elsewhere in Alaska temporally corresponds to termination of gold deposit formation in southwestern Alaska. The multiple periods of gold metallogenesis in southwestern Alaska offer a wide variety of targets for exploration within discrete parts of the region.

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.

Cretaceous through Paleogene dominantly calc-alkaline magmatism in the Lake Clark region, southwestern Alaska, can be divided into three main periods of activity based on previous work and new 40 Ar/ 39 Ar and U-Pb geochronology. These episodes follow a protracted history of arc magmatism related to the Talkeetna arc. The first episode is Albian based on new 40 Ar/ 39 Ar dates from hornblende in dikes cutting the Tlikakila complex of 101–97 Ma. These rocks have relatively flat REE patterns and Th/Nb values lower than typical arc rocks but higher than average mid-ocean ridge basalt (MORB). These data are consistent with rift-related magmatism in a region that had previously experienced arc magmatism. The second episode is represented by a granodiorite with a SHRIMP U-Pb zircon age of 64.5 ± 0.9 Ma and is similar to widespread 70–60 Ma volcanic rocks to the west of the study area. These calc-alkaline rocks are enriched in the light REE and have high Th/Nb values. They were generated during northward subduction of the Kula plate beneath Alaska. The Eocene episode is also calc-alkaline with enriched light rare earth element (REE) and high Th/Nb values. 40 Ar/ 39 Ar equilibria in 100 Ma dikes were reset at 44–42 Ma, and biotite from a rhyolite indicates an eruptive age of 43.0 ± 0.2 Ma. These 43 Ma rocks formed during initiation of Aleutian magmatism on the continent in response to north or northwest subduction of the Pacific plate and may be related to a change in the direction or velocity of subduction at this time.

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 Epithermal mineral deposits and occurrences of southwestern Alaska consist of Hg-Sb and gold- and sulfidebearing vein lodes. Numerous Hg-Sb lodes are located throughout a region measuring several tens of thousands of square kilometers in and surrounding the Kuskokwim River basin in southwestern Alaska. The Hg-Sb lodes are hosted in sedimentary rocks of the Cretaceous Kuskokwim Group. The Triassic to Cretaceous Gemuk Group, and the Paleozoic Holitna Group, as well as in Late Cretaceous and early Tertiary maflc to felsie intrusive rocks. Mineralized Hg-Sb vein and vein breccia lodes are found in the sedimentary or igneous rocks or at their contacts. The mineralogy of the Hg-Sb lodes is dominated by cinnabar and stibnite, with subordinate realgar, orpiment, native mercury, pyrite, gold, and hematite, as well as solid and liquid hydrocarbons; quarlz, carbonate, limonite, dickite, and sericite are alteration gangue minerals. The largest mercury mine in Alaska, Red Devil, produced about 36,000 flasks of mercury, bul the Hg-Sh lodes of southwestern Alaska gencntally consist of small, discontinuous veins that rerely exceed a few meters in width and a few tens of meters in strike length. The Hg-Sb lodes generally contain about 1 to 5 percent Hg and less that 1 percent Sb and As but are generally poor in base metals and precious metals. Anomalous concentrations of gold in some lodes, however, suggest that gold deposits may be present in higher temperature environments below some of the Hg-Sb lodes. The formation of the Hg-Sb lodes is closely correlated with igneous activity of a Late Cretaceous and early Tertiary magmatic are in southwestern Alaska. Geologic and geochemical characteristics of the Hg-Sb lodes suggest that are fluids were generated in local sedimentary rocks as they were intnuled by magmas. These intrusions provided the heat to initiate dehydration reactions and expel fluids from hrdrous minerals and formational waters in the sedimentary rocks, causing thermal convection and hydrothermal fluid flow along fractures and faults. Isotopic data from sulfide and alteration minerals of the Hg-Sb lodes indicate multiple sources for the ore fluids; most fluids appear to have originattd in local sedimentary rocks. Hydrothermal fluids with isotopically heavy oxygen but isotopically light hydrogen and sulfur compositions indicate derivation of these species from sedimentary rocks. Isotopically shifted, evolved meteoric water was a primary component in ore fluids from a few Hg-Sb lodes. Geochemical, isotopic, and fluid inclusion data also indicate that Hg. CO 2 , CH 4 , N 2 , and local hydrocarbons were derived from breakdown of organic matter in Sedimentary rocks when they were heated by intrusions. Radiometric 40 Ar/ 39 Ar ages of 70 ± 3 Ma from hydrothermal sericites in the Hg-Sb lodes indicate a temporal association of igneous activity and minemlization, which is consistent with the geologic characteristics. Most epithermal gold-bearing vein lodes on the Alaska Peninsula and Aleutian Islands are located in Eocene to Pleistocene volcanic-are rocks, commonly andesite and dacite. These vein and vein breccia lodes, such as the Alaska-Apollo and Shumagin deposits on Unga Island, tend to be aligned along regional, northcast-striking, steeply dipping faults and fractures. The Alaska-Apollo mine produced about 500,000 metric tons (t) of are that yielded an estimated 3,500 kg (130,000 oz) of gold from veins that were as much as 12 m wide and extended for 1,500 m laterally and 420 m vertically, Ore minerals include gold, galena, sphalerite, chalcopyrite, pyrite, marcasite, arsenopyrite, and native copper; gangue minerals are quartz, sericite, calcite, and chlorite and locally, barite, clay, rhodonite, and adularia. Ores gcnemlly have Au-Ag-Te-Pb-Zn-Mn-Cu geochemical signatures, wilh wide As-Hg aureoles around some veins. Geologic and mineralogical characteristics of these lodes are similar to adularia-sericitc volcanic-hosted epithermal deposits. The gold-bearing vein lodes may be related to are porphyry systems, but more data are required to verify this association.

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