Fort Knox is located 42 km by road northeast of the city of Fairbanks, in Alaska, United States of America. It is situated in the Fairbanks North Star Borough.
The Fort Knox area has been actively explored for gold placer deposits since 1902 when Felix Pedro discovered gold in Fish Creek, located downstream of the Fort Knox deposit. Since that initial discovery, the surrounding Fairbanks Mining district has produced in excess of 8 Moz of gold, predominantly from placer deposits, which actively continues.
Exploration for lode gold deposits was very limited during the early history of the region and focused on tracing the source of the placer deposits up headwaters and tributaries to Fish Creek. Alfred Brooks mapped the area while working for the United States Geological Survey (USGS) in the early 1900’s. Brooks’ mapping described a large granitic intrusive in the project area.
In 1913, H.A. Currier staked lode mining claims covering auriferous quartz veins on the Melba Creek-Monte Cristo Creek divide (covering part of what is now the Fort Knox gold deposit). A three-stamp mill was constructed on the property, but there is little evidence of any production from the claims. USGS geologists who examined the prospect noted the association of bismuthinite and gold in quartz veins and suggested a relationship between the observed mineralization and the large granitic intrusive located nearby.
Heiner and Wolff (1968) noted that the large amount of placer gold in the Fairbanks District led many workers to conclude that only the roots of the gold veins were left and that there was not enough economic incentive to test the theory by exploration.
After staking 19 state mining claims, Joe Taylor and George Johnson worked the placers of Monte Cristo Creek from 1980 to 1982 and recovered bismuthinite nuggets containing abundant gold. The demonstrated correlation between the gold and bismuth led Taylor and Johnson to prospect the slopes and divide between Melba and Monte Cristo Creeks. The prospecting operations involved panning and trenching, which suggested that Au mineralization was widespread and resulted in the prospectors staking an additional 34 mining claims.
In 1984, Rob Blakestead, a consulting geologist, noted the presence of visible gold in quartz veins hosted by granite in the Fort Knox area. This discovery led to increasing levels of exploration to locate the source of the gold.
FGMI initiated extensive exploration programs on the property including surface geochemical sampling, drilling and geophysics. Soil sampling proved the most useful exploration tool in delineating the ore body during initial exploration of the deposit. Later surface trenching and mapping of the anomalies developed by the soil geochemistry identified the favourable targets.
Ground magnetometer surveys performed in 1987, 1991 and 1992 were employed with limited success. 427 drillholes totaling 79,860 m had been completed on the property by late 1992. This work was followed by the completion of environmental and engineering studies examining the feasibility of commercial production from the deposit.
Construction of the Fort Knox mine and mill operations began in 1995 and was completed in 1997. The capital cost was approximately US$373 million, including US$28 million of capitalized interest. Commercial production at Fort Knox began in March 1997.
Fort Knox is mined as a conventional truck and shovel open pit mine and has operated continuously since start up, in seven progressive phases. The discovery and development of the True North deposit produced ore for the Fort Knox mill from 2001 to 2004. Fort Knox operated as a mill-only process until the price of gold and known quantities of low grade material enabled the addition of a heap leach process. Modifications were completed on the existing crushing circuit in 2008 to produce crushed material for the Walter Creek heap leach construction. Heap Leach construction is separated into a total of seven stages between 2009 and 2019. The facility includes a valley fill leach pad, solution pumping systems, and a carbon-in-column (CIC) plant. The original solution capacity was 30,000 L/m. The construction of a second CIC plant was completed in July 2013 and has increased the solution capacity to 70,000 L/m. Current flow rate to the CIC processing facility is 61,000 L/m.
Fort Knox has yielded approximately 469 Mt of ore with 7.5 Moz of produced gold since 1996 (data up to 2017 inclusive)
The Fairbanks mining district is located in the northwestern part of the Yukon-Tanana terrane (YTT), which consists of Paleoproterozoic, polymetamorphosed schist, of primarily sedimentary origin. The terrane is bounded on the north by the Tintina fault system and on the south by the Denali Fault system (Bundtzen, 1981). Hall (1985) suggests a more complex deformational history identifying four phases of penetrative tectonism. Northeast-trending faults, related to the Tintina and Denali fault systems, are the dominant structural trends. These regional fault systems have created different levels of crustal exposure, through oblique offsets that are primarily dip-slip, with a sinistral sense of shear (Robinson, 1990; Newberry, 1996). E-W to NW-directed faults and shears are present and are locally an important structural control to mineralization at most gold occurrences in the district.
The Fairbanks Schist, a unit of the Yukon-Tanana terrane, is host to much of the mineralization in the Fairbanks mining district (Figure 7-1). The dominant lithologies present include grey to brown, fine-grained micaceous-quartz schist and micaceous quartzite. Interlayered within the Fairbanks Schist is the Cleary sequence, a more varied assemblage of metamorphic lithologies. The Fairbanks Schist has undergone amphibolite facies metamorphism followed by a retrograde greenschist facies event.
The Fort Knox, Gilmore Dome, and Pedro Dome plutons are post-metamorphic, Late Cretaceous (~92 Ma) granitic complexes that intrude the metamorphic rocks in the eastern half of the Fairbanks mining district. A plutonic origin has been ascribed to much of the gold mineralization in the Fairbanks district. Fort Knox is hosted entirely within granite, whereas the other gold occurrences are in favourable metamorphic units or structures, near plutonic rocks.
The Fairbanks district hosts gold in a variety of geologic settings, including:
- the Fort Knox deposit (the largest lode deposit in the district), where gold is hosted in quartz, quartz-sericite, and quartz pegmatite veins, stockwork zones, and mineralized shear zones;
- the Gil project, where gold is hosted in skarns and quartz veins within the skarns;
- the True North deposit, where mineralization is hosted by deformed carbonaceous meta-sediments associated with quartz veins;
- the Ryan Lode deposit, where gold occurs in and adjacent to large-scale shear zones; and
- the Cleary Hill mine, where gold occurs in quartz veins within the Fairbanks Schist.
Fort Knox Deposit Geology and Mineralization
Gold mineralization at Fort Knox Mine is hosted entirely within the Late Cretaceous (~92 Ma), Fort Knox granite pluton. The contact with the Fairbanks Schist is abrupt. Drilling indicates that the pluton contacts plunge steeply to the north and south and moderately to the east and west. The surface exposure of the pluton is approximately 1,100 m (3,609 ft) in the east-west direction and 600 m (1,969 ft) north-south.
The Fort Knox Pluton has been subdivided into three phases based on grain-size and textural differences. Intrusion of a biotite-rich fine-grained granodiorite was followed by medium-grained porphyritic granite. The youngest intrusive phase is coarse-grained, porphyritic granite (Bakke, 1995). The texture is dominantly porphyritic, with megacrysts of quartz and k-feldspar, which become more sporadic in the fine-grained phase. The pluton is granite-granodiorite in composition, and the mineralogy of all phases is generally sub-equal amounts of quartz (30%), plagioclase (30-40%), and k-feldspar (20 to 30%) (Bakke, 1995; Blum, 1983).
The major structural trends controlling shear vein orientation and mineralization at the deposit-scale are defined by NW-trending, moderately to shallowly SW-dipping fault zones. The fault zones are typically filled with granulated white quartz, and range in thickness from 0.3 to 1.5 m. The zones have mixed groundmass alteration, with a range of phyllic and argillic alteration assemblages. In the vicinity of the fault zones, vein abundance increases and vein orientations are predominantly sub-parallel to the fault direction. The Monte Cristo Fault and Melba Fault zones are regional in extent and offset the Gilmore Dome Pluton south of Fort Knox and affect the orientation and geometry of the Fort Knox granite.
Gold occurs within and along the margins of pegmatite vein swarms and quartz veins and veinlets. Numerous SW-dipping fault zones influence the orientation of the vein swarms and the geometry of ore zones. Weak to moderate development of vein- and- fracture-controlled phyllic, potassic, albitic, and argillic alteration styles is present. Gold is closely associated with bismuth (Bakke, 1995; McCoy et al., 1997). Gold occurs attached to bismuth-minerals, sulfide, and non-sulfide gangue, and as complex intergrowth or solid solution/exsolution texture grains with native bismuth, maldonite, bismuthinite, and/or molybdenite.
The overall sulfide content of the orebody is <0.10%. The orebody is oxidized to the depths of the drilling primarily along NW-trending, SW-dipping fault zones which contain abundant iron oxide and clay gouge along the margins.
Property Geology at Fort Knox. Granitic intrusive is pink, all other rock is schist. Major ore controlling shears are shown in blue.
Vein-types and associated alteration styles by abundance and relative importance to gold mineralization found at Fort Knox are:
1. Stockwork quartz veins and veinlets, ranging in thickness from micro-scale to 15 cm. These veins possess thin albitic alteration halos. Phyllic alteration envelopes that range in thickness from centimetre to multi-metre scale occur where stockwork veining is abundant near NW fault zones.
2. Pegmatite veins and veinlets: ranging in thickness from micro-scale to 8 cm. Composed of clear to grey quartz, large K-spar megacrysts, and micaceous clots. Potassic alteration halos, rarely exceeding 1 cm thickness, consist of an assemblage of variable amounts of secondary biotite and K-spar overgrowths on primary K-spar within the granite matrix. Veins variably altered with phyllic (quartz-sericite-pyrite) assemblage.
3. Low temperature fracture coatings and chalcedonic veins and breccia composed of zeolite-calcite-clay-chalcedony. Pervasive throughout the deposit in the form of fracture coatings and breccia zones. Argillic alteration halos as much as 7 m in width are developed adjacent to the larger chalcedonic breccia zones. These zones have been largely mined out to-date.
The Fort Knox deposit is classified as an Intrusion-Hosted Mesozonal deposit in the Reduced Intrusion-Related Gold Deposit (RIRG) style (Hart, 2005). RIRG deposits typically occur associated with moderately reduced intrusions in reduced siliciclastic sequences, and have a common association with W-Sn±Mo metallogenic belts. The key characteristics of these deposits are a low sulfide content, association with reduced ore mineral assemblages, and metal assemblages of Au>Ag, Bi, As, W, and Mo (Robert, 2007). The reduced intrusion occurs as equigranular and multiphase granitic stocks and batholiths. Gold is hosted in sheeted veins, which are coeval with their causative intrusion (Robert, 2007). Although these deposits do not have a significant hydrothermal alteration footprint, there are often peripheral mineralization occurrences and proximal thermal alteration, which have a predictable distribution pattern, including secondary aluminosilicates, biotite, and tourmaline, skarns and polymetallic veins (Hart, 2005; Robert, 2007).
Mineral Resource & Reserve Estimate
Mineral Resources for Fort Knox are reported exclusive of Mineral Reserves within a US$1,400/oz gold price pit shell, but outside of the Life of Mine US$1,200/oz gold price pit. Mineral resources are reported at a cutoff grade of 0.10 g/t gold for Fort Knox. The Proven and Probable Mineral Reserves as of December 31, 2017 are based on a gold price of US$1,200/oz
Source: Fort Knox Mine Fairbanks North Star Borough, Alaska, USA National Instrument 43-101 Technical Report. Prepared by: John Sims, AIPG Certified Professional Geologist Effective Date: June 11, 2018.