Fort Knox

Alaska, USA

Main commodities: Au
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The Fort Knox porphyry gold deposit is located in the Fairbanks North Star Borough of Alaska, USA, some 22 km to the north-east of the town of Fairbanks.

This district had previously produced some 275 tonnes of gold since its discovery in 1902, almost entirely of placer origin. Placer and lode gold occurrences have been worked in the valleys and hills surrounding Fort Knox since the first discovery in the district. The Fort Knox orebody was discovered in the late 1980's and commenced production in November 1996.

The deposit occurs within the Fairbanks mining district, a SW-NE trending belt of lode and placer gold deposits, situated in the northwestern part of the Yukon-Tanana Uplands. The Yukon-Tanana terrane contains of a thick sequence of poly-metamorphic rocks ranging from Proterozoic to upper Palaeozoic, representing protoliths that were primarily sedimentary, volcanic, and volcanoclastic units, with only minor plutonic rocks. The region has been subjected to at least two periods of dynamo-thermal metamorphism, including an early prograde amphibolite event, and a subsequent retrograde, greenschist facies phase (Bundtzen, 1981).

The Proterozoic to lower Palaeozoic Fairbanks Schist is the dominant lithology, composed of quartz-muscovite schist, muscovite-feldspar-quartz schist, micaceous quartzites, meta-conglomerate, garnet-hornblende amphibolite and marble, indicating an emergent shelf environment. The Cleary Sequence composed of meta-basalt with actinolite schist, chlorite schist, graphite schist and impure marbles is intercalated with the Fairbanks Schist, suggesting the presence of immature rift basins within the shelf environment. Infaulted meta-rhyolite of the Devonian Muskox Sequence is in fault contact with the Fairbanks Schist and Cleary Sequence in parts of the district, while in the north, metamorphosed rocks of the middle Palaeozoic to Ordovician Chatanika terrane have been identified in fault contact with the Fairbanks Schist. This latter sequence includes type C eclogites, impure marbles, amphibolites, calc-muscovite schist, garnet-muscovite schist and muscovite schist, containing garnet, biotite, chlorite and graphite. The Chatanika unit may represent a telescoped, mature rift basin within the shelf environment.

The Fort Knox ore deposit is hosted entirely within the 1100 x 600 m, east-west elongated Fort Knox Pluton. Mineralisation has been identified to the 400 level, marking the lowest extent of available detailed information. The contact between the pluton and the Fairbanks schist is abrupt, and plunges steeply to the east and moderately to the north, south, and west.

The Fort Knox pluton has been subdivided into three texturally different phases, from oldest to youngest: i). biotite-rich fine-grained granite; ii). medium-grained porphyritic granite; iii). coarse-grained, seriate porphyritic granite. A fourth, volumetrically minor, biotite-hornblende rich phase (mapped as "mafic"), that commonly displays a medium-grained texture is locally present as pendants near the schist-granite contact.

Gold-bismuth-tellurium mineralisation is restricted to the Fort Knox pluton and is strongly structurally controlled. Gold occurs within, and along the margins of pegmatite veins, quartz veins and veinlets, within shear zones, and along fractures within the granite, with an overall very low (<0.1%) sulphide content. The orebody is oxidised to the depths of the current drilling.

The vein types and associated alteration styles at Fort Knox are as follows (after Bakke et al., 1998).
i). Pegmatite veins and veinlets, which range in thickness from micro-scale to 8 cm, and are composed of clear to grey quartz, large K feldspar megacrysts, and micaceous clots. Potassic alteration halos, rarely exceeding 1 cm in thickness, and consist of an assemblage of variable amounts of secondary biotite and K feldspar overgrowths on primary K feldspar within the granite matrix.
ii). Pegmatite veins similar to i), with alteration envelopes consisting of a variably developed phyllic (sericite + pyrite) assemblage.
iii). Quartz veins and veinlets (stockwork), that range in thickness from micro-scale to 15 cm. These veins possess thin albitic alteration halos
iv). Quartz veins and veinlets similar to iii) above, with phyllic alteration envelopes that range in thickness from 0.5 to 3 cm.
v). Low-temperature fracture coatings and chalcedonic veins and breccia, with low temperature assemblage of zeolite + calcite + clay + chalcedony, which are pervasive throughout the deposit in the form of fracture coatings and breccia zones. Argillic alteration halos as much as 7m in width are developed adjacent to the larger chalcedonic breccia zones.

Gold is found both within and along the margins of pegmatite vein swarms, and quartz veins and veinlets. The orientation of these veins and swarms, and the geometry of ore is influenced by numerous NW-SE trending shear zones.

Vein orientation and mineralisation is controlled by the major SE-NW trending, moderately to shallowly SW dipping shear zones, which are typically filled with granulated white quartz, and range from 0.3 to 1.5 m in thickness. They contain mixed phyllic and argillic alteration assemblages and carry abundant iron oxide clay gouge along their margins. Vein density increases in the vicinity of the shear zones, while vein orientations are predominantly parallel to the shear direction. Thin, subsidiary shears, are abundant between, and especially adjacent to, major shears.

Bakke (1995) and McCoy and others (1997) noted the strong geochemical correlation of Au with Bi and Te. Bi and Te mineral species that have been identified include: native bismuth, Bi; maldonite, AuBi; bismuthinite, Bi2S3; tellurobismutite, Bi2Te3; bismite, Bi2O3; tetradymite, Bi2Te2S; and eulytite, Bi4(SiO4)3. Other ore minerals include trace to minor amounts of molybdenite and scheelite.

Alteration takes the form of weak to moderately developed, and vein controlled phyllic, potassic, albitic and argillic assemblages, while gold is closely associated with trace amounts of bismuth and tellurium. The overall sulphide content of the ore zone is low, generally <0.1%, with oxidation to the limits drilled.

The mine is operated (2012) by Fairbanks Gold Mining Inc., a subsidiary of Kinross Gold Corporation.

The deposit had pre-production reserves in 1996 of 158.3 Mt @ 0.83 g/t Au with 0.39% Au cut-off (Bakke et al., 1998).

In December 31, 2011, reserves and resources were (Kinross Gold, 2012):
    Proven + probable mineral reserves - 314.67 Mt @ 0.43 g/t Au; plus
    Measured+indicated resource - 112.10 Mt @ 0.40 g/t Au;
    Inferred resource - 22.18 Mt @ 0.41 g/t Au;

This summary is largely based on a report to Kinross Gold Corporation and Fairbanks Gold Mining by Quandt, Ekstrom and Triebel, 2008

The most recent source geological information used to prepare this summary was dated: 2012.     Record last updated: 20/10/2012
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd.   Unauthorised copying, reproduction, storage or dissemination prohibited.

Fort Knox

  References & Additional Information
 References to this deposit in the PGC Literature Collection:
Baker T, Ebert S, Rombach C and Ryan C G,  2006 - Chemical Compositions of Fluid Inclusions in Intrusion-Related Gold Systems, Alaska and Yukon, Using PIXE Microanalysis: in    Econ. Geol.   v101 pp 311-327
Bakke A A, Morrell R P and Odden J C  1998 - The Fort Knox Porphyry Gold Deposit, Eastern-Central Alaska: An Overview and Update: in Porter T M (Ed.), 1998 Porphyry and Hydrothermal Copper and Gold Deposits - A Global Perspective PGC Publishing, Adelaide    pp. 89-98
Symons D T A and McCausland P J A,  2005 - Paleomagnetism of the Fort Knox Stock, Alaska, and rotation of the Yukon–Tanana terrane after 92.5 Ma : in    Tectonophysics   v419 pp 13-26

 References to this deposit in PGC Publications: Want any of our books ? Pricelist
Bakke A A, Morrell R, Odden J, 1998 - The Fort Knox porphyry gold deposit, Eastern-Central Alaska: An overview and update ,   in  Porter T M, (Ed.),  Porphyry and Hydrothermal Copper and Gold Deposits: A Global Perspective,    pp 89-98
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