Fort Knox

Mine, Active

Commodities and mineralogy

Main commodities Au
Other commodities As; Bi; Mo; Te; W
Ore minerals arsenopyrite; bismite; bismuth; bismuthinite; eulytite; gold; maldonite; molybdenite; scheelite; tellurobismuthite; tetradymite
Gangue minerals quartz

Geographic location

Quadrangle map, 1:250,000-scale FB
Quadrangle map, 1:63,360-scale D-1
Latitude 64.992
Longitude -147.3614
Nearby scientific data Find additional scientific data near this location
Location and accuracy As of the end of 2011, the Fort Knox Mine was a large operating open pit nearly a mile in diameter that covers much of the area between Melba Creek and Monte Cristo Creeks at the head of Fish Creek. The coordinates are at about the center of the pit. The mine occupies much of the northwest quarter of section 21 and the southwest quarter of section 16, T. 2 N., R. 2 E. The location is accurate.

Geologic setting

Geologic description

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). 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 caved shaft and the ruins of a small mill (Wedow and White, 1954).
In 1987, geologist Rodney A. Blakestad, graduate of the University of Alaska, Geology, 1973, was sampling and mapping a trench that was dug the previous day. It became apparent that close inspection of the two- to four-millimeter thick, sheeted quartz veins cutting through the granite contained specks of free gold approximately 0.5 mm in diameter. It took only an hour of observing the multitude of veinlets exposed over the entire length of the 200 foot long trench to decide that another trench might be instructive as to the magnitude of the 'discovery'.
The bulldozer was owned by Mr. Blakestad, so he fired it up and drove 200 feet up hill and dug a second trench parallel to the first. Similar quartz veinlets with similar free gold were encountered in the second trench. Without taking a sample, Blakestad drove back to Fairbanks and reported the news to the funding group and a second geologist, Dr. Benjamin I. Collins, was dispatched to verify the discovery (Rodney A. Blakestad, personal communication, 2015).
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). 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). 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). Late in 1991, Amax Gold Inc. announced its intention to purchase all assets of Fairbanks Gold Ltd. (Bundtzen and others, 1991). 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). In November, 1996, the first gold was produced from the mine and it has been in continual operation to the present (Feb. 2011) as a large open pit. In 1998, Kinross Gold Corp. acquired a 100 percent interest in the mine and operated it continuous to early 2012. As of early 2012, they have resources that they predict will allow operation until 2018 (Kinross Gold Corp., 2011).
The Fort Knox gold deposit is in a granite body, now commonly referred to as the Fort Knox pluton, which has been dated at 92 Ma by the U/Pb method (Bakke, 1992, 1994). Gold occurs along margins of stockwork quartz veins and veinlets, along 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) and the dominant sulfide is bismuthinite. There is strong geochemical association of the gold with bismuth and tellurium. The following bismuth and tellurium minerals have been identified in small amounts: native bismuth, maldonite, tellurobismuthite, bismite, tetradymite, and eulytite. Trace to minor molybdenite and scheelite also occur. 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 rock with relict texture similar to the porphyritic granite, and formed by local contact metamorphism of impermeable country rocks (Bakke, 1992; J. Odden, written communication, 2000). The most significant gold mineralization is in milky-white, quartz-stockworks veins and along shear zones (Bakke, 1992). The stockwork veins mostly strike east and have no consistent dip. The gold distribution in the stockwork veins is erratic; some ore contains as high as 7.0 ounces of gold per ton; the gold occurs in grains that vary 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 others, 1997). 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).
From 1996 to December 31, 2011, the Fort Knox Mine produced 5,246,631 ounces of gold (Quant, 29008; Szumigala and others, 2011, Kinross Gold, Corp., 2012). This included 289,794 ounces of gold that was produced in 2011. Most of this was processed through a carbon-in-pulp mill with a capacity of about 33 to 45 thousand tonnes per day. A large heap leach facility was constructed in 2009 that began processing a large stockpile of low grade ore; the company anticipates adding low-grade material to the heap leach pile from the pit, material that would not be economical to process through the mill. This heap-leach facility will extend the life of the mine to 2018 and and should increase gold production to an average 370,000 ounces of gold per year.
As of December 31, 2011, the Fort Knox Mine has proven and probable reserves of 314.669 million tonnes with an average grade of 0.43 grams of gold per tonne (or 4.303 million ounces of gold) (Kinross Gold Corp., 2012). There is an addition measured and indicated resource of 112.098 million tonnes with an average grade of 0.40 grams of gold per tonne (or 1.426 million ounces of gold).
Geologic map unit (-147.363839953929, 64.9915766974643)
Mineral deposit model Intrusion related gold.
Age of mineralization 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).
Alteration of deposit The deposit features 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 in the granite (Bakke, 1992).

Production and reserves

Workings or 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). 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). 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). 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). Late in 1991, Amax Gold Inc. announced its intention to purchase all assets of Fairbanks Gold Ltd. (Bundtzen and others, 1991). 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). In November, 1996, the first gold was produced from the mine and it has been in continual operation to February of 2011 as a large open pit. In 1998, Kinross Gold Corp. acquired a 100 percent interest in the mine and operated it continuous to early 2012. As of early 2012, they have resources that they predict will allow operation until 2018 (Kinross Gold Corp., 2011).
In 2016, Kinross Gold Corporation conducted exploration at Fort Knox mine and surrounding brownfield properties. Exploration drilling of the east and south walls of the existing open pit returned positive results. A total of 2,300 meters were drilled, and the 15-hole program tested the existing geologic-resource model for a potential mineral-resource addition, and refined the granite-schist contact. Positive intercepts were intersected in every hole drilled; highlights include 29 meters of 0.029 ounce of gold per ton, 13.7 meters of 0.054 ounce of gold per ton, 36.6 meters of 0.019 ounce of gold per ton, 35 meters of 0.029 ounce of gold per ton, and 24.4 meters of 0.058 ounce of gold per ton (Athey and Werdon, 2017).
Indication of production Yes; large
Reserve estimates
As of December 31, 2011, the Fort Knox Mine has proven and probable reserves of 314.669 million tonnes with an average grade of 0.43 grams of gold per tonne (or 4.303 million ounces of gold) (Kinross Gold Corp., 2012). There is an addition measured and indicated resource of 112.098 million tonnes with an average grade of 0.40 grams of gold per tonne (or 1.426 million ounces of gold).
As of December 31, 2016, Fort Knox contains proven and probable reserves of 115,399,732 short tons grading 0.014 ounce of gold per ton, for a contained metal content of 1,506,000 ounces of gold. Measured and indicated resources include 104,745,905 tons grading 0.016 ounce of gold per ton, for a contained metal content of 1,440,000 ounces of gold, and inferred resources include 14,369,713 tons grading 0.016 ounce of gold per ton, for a contained metal content of 193,000 ounces of gold (Kinross Gold Corporation, 2016).
Production notes
From 1996 to December 31, 2011, the Fort Knox Mine produced 5,246,631 ounces of gold (Quant, 2008; Szumigala and others, 2011, Kinross Gold, Corp., 2012). This included 289,794 ounces of gold that was produced in 2011. Most of this was processed through a carbon-in-pulp mill with a capacity of about 33 to 45 thousand tonnes per day. A large heap leach facility was constructed in 2009 that began processing a large stockpile of low grade ore; the company anticipates adding low-grade material to the heap leach pile from the pit, material that would not be economical to process through the mill. This heap-leach facility will extend the life of the mine to 2018 and and should increase gold production to an average 370,000 ounces of gold per year.
Fort Knox mine celebrated its 20th anniversary of operation and poured its 7-millionth ounce of gold in 2016; gold production totaled 409,844 ounces. Production increased slightly compared with 2015, mainly as a result of record production from the heap leach, but was partially offset by lower mill grades. Fort Knox mined 34,998,000 tons of material, processed 14,570,000 tons of ore through the mill, and processed 32,124,000 tons of ore on the heap leach pad. Mill grade averaged 0.022 ounce of gold per ton with an 83 percent recovery rate, and the heap-leach grade averaged 0.009 ounce of gold per ton. As of 2016, termination of open pit mining is scheduled for 2019. In May 2016, the U.S. Bureau of Land Management announced it is considering a proposal to relinquish 709 acres of National Oceanic and Atmospheric Administration land immediately west of the phase-8 expansion of Fort Knox mine to the State of Alaska, as this land is no longer needed to fulfill its mission. Access to this land could add to the mine’s life (Athey and Werdon, 2017).

References

MRDS Number 10307448

References

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.
Bakke, Arne, 1994, The Fort Knox 'porphyry' gold deposit; structurally 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.
Blum, J.D., 1982: The Petrology, geochemistry, and isotope geochronology of the Gilmore Dome and Pedro Dome plutons, Fairbanks District, Alaska: Unpublished M.S. Thesis, University of Alaska Fairbanks, 107 p.
Kinross Gold Corp., 2011, Mineral reserve and mineral resource statement: http://www.kinross.com/media/226595/2011%20mineral%20reserve%20and%20resource%20statement.pdf (as of December 31, 2011).
Kinross Gold Corp., 2012, Fort Knox, Alaska, USA: http://www.kinross.com/operations/operation-fort-knox-alaska-usa.aspx (as of May 2, 2012)
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 Goldfarb, R.J., and Miller, L.D., eds., Mineral Deposits of Alaska: Economic Geology Monograph 9, p. 191-241.
Quandt, David, Ekstrom, Chris, and Triebel, Klaus, 2008, Technical report for the Fort Knox Mine: Technical Report for the Kinross Gold Corporation, 79 p. (posted on www.sedar.com, March 31, 2008).
Reporters J.R. Guidetti Schaefer and C.J. Freeman (Avalon Development Corporation); D.J. Grybeck (Contractor, USGS); M.B. Werdon (DGGS)
Last report date 8/26/2017