Fort Knox

Site Name

Primary: Fort Knox

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Commodity

Primary: Gold

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Location

State: Alaska
District: Fairbanks

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Land Status

Not available

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Holdings

Not available

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Workings

Not available

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Ownership

Not available

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Production

Not available

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Deposit

Record Type: Site
Operation Category: Producer
Operation Type: Unknown
Years of Production:
Organization:
Significant:

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Physiography

Not available

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Mineral Deposit Model

Not available

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Orebody

Not available

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Structure

Not available

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Alterations

Alteration Type: L
Alteration Text: Alteration consists of vein-controlled phyllic, potassic, albitic, and argillic alteration (Bakke, 1992). A late, low-temperature, thermal alteration event has resulted in an assemblage of calcite, zeolite (stilbite), chalcedony and clay in breccia zones, on joint surfaces, and in fractures within the granite (Bakke, 1992).

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Rocks

Not available

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Analytical Data

Not available

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Materials

Ore: Gold

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Comments

Comment (Exploration): Status = Active

Comment (Production): Production Notes = The first gold bars were poured on December 20, 1996 (Swainbank and others, 1997, p. 12). From mid-November to the end of December 1996, Fairbanks Gold Mining, Inc., the operating subsidiary of Amax Gold, Inc., mined and milled 769,728 tons of ore and recovered 16,085 ounces of refined gold (Swainbank and others, 1997, p. 24). Gold production in 1999 was 351,120 ounces (Kinross Gold Corporation, press release, Feb. 3, 2000). The grade of gold produced in 1999 was 0.95 grams of gold per tonne (0.0277 ounce of gold per ton) (Kinross Gold Corporation, press release, Feb. 3, 2000). Total gold recovered as of January 31, 2000, was 1,113,137 ounces (J. Odden, oral commun., 2000).

Comment (Reserve-Resource): Reserves = Recent calculations indicate a resource of 186 million tons of ore with a grade of 0.027 ounce of gold per ton, or a total resource of 5.04 million ounces of gold (J. Odden, written commun., 2000).

Comment (Geology): Age = Stockwork white mica which is probably contemporaneous with the mineralization has been dated by the 40Ar/39Ar method at 86.2 to 89.3 Ma (McCoy and others, 1997, p. 207).

Comment (Reference): Primary Reference = Bakke, 1992

Comment (Geology): Geologic Description = Currently, the Fort Knox mine is the largest gold mine in Alaska, producing about 1,000 ounces of gold per day from 36,000 to 44,000 tons of ore (Szumigala and Swainbank, 1999). The most recent figures indicate a resource of 186 million tons of ore at a grade of 0.027 ounce of gold per ton, or a total resource of 5.04 million ounces of gold (J. Odden, written commun., 2000). The gold varies from 940-990 fine (Bakke, 1994). The Fort Knox gold deposit occurs in a granite body, now commonly referred to as the Fort Knox pluton (Bakke, 1992). This pluton has been dated at 92 Ma by the U/Pb method (Bakke, 1994). Gold occurs along margins of stockworks quartz veins and veinlets, on quartz-filled shear zones, and along fractures within the granite. In the ore zones, the sulfide content is low, less than 0.5 percent. There are only minor amounts of arsenopyrite (McCoy and others, 1997). The dominant sulfide is bismuthinite (J. Odden, oral commun., 2000). The pluton has been subdivided into three phases, primarily on on the basis of texture: (1) fine-grained, biotite- and hornblende-rich granite, (2) medium- to coarse-grained, seriate, porphyritic granite (the youngest phase), and (3) a hybrid, biotite- and hornblende-rich phase with relict texture similar to the porphyritic granite, formed by local contact metamorphism of impermeable country rocks (Bakke, 1992; J. Odden, written commun., 2000). The most significant gold mineralization is found within milky-white, quartz-stockwork veining and shear zones (Bakke, 1992). The stockwork veins dominantly strike E-W and have no consistent dip. The gold distribution within the stockwork veins is highly erratic; some ore grades as high as 7.0 ounces of gold per ton in grains ranging in size from less than 0.1 to 2 mm. In contrast, the quartz-filled shear zones contain evenly distributed, micron-size gold. White mica from the stockworks has been dated by the 40Ar/39Ar method at 86.2 to 89.3 Ma (McCoy and other, 1997, p. 207). Alteration consists of vein-controlled phyllic, potassic, albitic, and argillic alteration (Bakke, 1992). A late, low-temperature, thermal alteration event has produced an assemblage of calcite, zeolite (stilbite), chalcedony, and clay in breccia zones, on joint surfaces, and in fractures within the granite (Bakke, 1992).

Comment (Deposit): Model Name = Gold occurs along margins of stockwork quartz veins and veinlets, quartz-filled shear zones, and along fractures within granite; commonly called a Fort Knox type porphyry gold deposit.

Comment (Workings): Workings / Exploration = In 1913, a bismuth-bearing, gold-quartz vein was prospected on the ridge between Melba and Monte Cristo Creeks by Edward Voght (see FB116) (Chapin, 1914 [B 592-J, p. 330]). When visited in July of 1949, the workings were completely caved, and all that remained was almost completely disintegrated rock on dumps around an old filled shaft and the ruins of a small mill (Wedow and White, 1954). In 1987, a geologist walking behind a bulldozer on the Fort Knox claims picked up a piece of granite containing visible gold; this discovery was followed by an extensive exploration program. By 1989, Fairbanks Gold Ltd. and its joint venture partners had spent $4.5 million on exploration at the Fort Knox deposit. More than 72,000 feet of drilling and 5 miles of trenching outlined an area predicted to contain 4 million ounces of gold. In 1990, five drill rigs were operating 24 hours a day, drilling almost 60,000 feet in 104 drill holes, and a 170,000-ton bulk sample was taken. In 1990, the Fort Knox property was the largest exploration project outside of southeast Alaska (Swainbank and others, 1991, p. 11). Exploration consisted of 46,300 feet of diamond drilling and 62,600 feet of reverse circulation drilling. The 170,000-ton bulk sample was reduced to 45 tons; seven tons of this sample was split for metallurgical testing. In 1991, another 32,600 feet of reverse circulation drilling was completed (Bundtzen and others, 1991, p. 20). Geotechnical work consisted mainly of condemnation drilling to determine where to put the mill, tailings pond, and other mine support facilities (Bundtzen and others, 1991, p. 20). Late in 1991, Amax Gold Inc. announced its intention to purchase all assets of Fairbanks Gold Ltd. (Bundtzen and others, 1991, p. 20). By 1996, as many as 800 workers had completed a power line to the site, the freshwater dam and tailings dam, the primary crusher, a coarse-ore conveyor, apron feeders, and all components of the mill (Swainbank and others, 1997, p. 12). From mid-November to the end of December 1996, Fairbanks Gold Mining, Inc., the operating subsidiary of Amax Gold, Inc., mined and milled 769,728 tons of ore and recovered 16,085 ounces of gold from a large open pit (Swainbank and others, 1997, p. 24). The first gold bars were poured on December 20, 1996 (Swainbank and others, 1997, p. 12). In June 1998, Kinross Gold merged with Amax Gold, Inc., and added a 120-ton SAG mill to increase mill throughput by about 10 percent to 45,000 tons per day (Szumigala and Swainbank, 1999, p. 14). In the mine pit, 10,000 feet of reverse circulation drilling and 22,000 feet of core drilling were completed. Other projects in 1998 consisted of dewatering of tailings and increasing the height of the tailings dam. In 1999, resource drilling consisted of 2,503 feet of reverse circulation drilling and 5,768 feet of core drilling; the open pit was approximately 3,000 feet by 5,500 feet in area (J. Odden, oral commun., 2000).

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References

Reference (Deposit): Swainbank, R.C., Bundtzen, T. K., Clough A.H., and Henning, M.W., 1997, Alaska's mineral industry 1996: Alaska Division of Geological and Geophysical Surveys Special Report 51, 68 p.

Reference (Deposit): Bakke, Arne, 1992, Geology of the Fort Knox gold deposit, Fairbanks, Alaska [abs.]: Alaska Miners Association Annual Convention, Anchorage, Alaska, November 1992, Abstracts and Papers, 1 p.

Reference (Deposit): Wedow, Helmuth, Jr., and White, M.G., 1954, Reconnaissance for radioactive deposits in east-central Alaska, 1949: U.S. Geological Survey Circular 335, 22 p.

Reference (Deposit): Chapin, Theodore, 1914, Lode mining near Fairbanks, Alaska: U.S. Geological Survey Bulletin 592-J, p. 321-355.

Reference (Deposit): Bakke, Arne, 1994, The Fort Knox 'porphyry' gold deposit; structurallly controlled stockwork and shear quartz vein, sulfide-poor mineralization hosted by a Late Cretaceous pluton, east-central Alaska, in Schroeter, T. D., ed., Porphyry deposits of the northwestern Cordillera of North America: Canadian Institute of Mining, Metallurgy, and Petroleum Special Volume 46, p. 795-802.

Reference (Deposit): Swainbank, R.C., Bundtzen, T.K., and Wood, J.E., 1991, Alaska's Mineral Industry 1990: Alaska Division of Geological and Geophysical Surveys Special Report 45, 78 p.

Reference (Deposit): Szumigala, D.J., and Swainbank, R.C., 1999, Alaska's mineral industry 1998: Alaska Division of Geological and Geophysical Surveys Special Report SR 53, 71 p.

Reference (Deposit): Bundtzen, T. K., Swainbank, R. C., Wood, J. E., and Clough, A. H., 1991, Alaska's mineral industry 1991: Alaska Division of Geological and Geophysical Surveys Special Report 46, 89 p.

Reference (Deposit): McCoy, Dan, Newberry, R.J., Layer, Paul, DiMarchi, J.J., Bakke, Arne, Masterman, J.S., and Minehane, D.L., 1997, Plutonic-related gold deposits of interior Alaska, in R.J. Goldfarb, and L.D. Miller, eds., Mineral Deposits of Alaska: Economic Geology Monograph 9, p. 191-241.

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