Summary:
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). The Gil deposit is hosted in skarns and quartz veins within the skarns.
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.
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.
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.
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.
Gil Deposit Geology and Mineralization
Brittle deformation at Gil is related to contact metamorphism and the regional amphibolite/greenschist events. At least three penetrative phases of deformation are recognized. As a result, large-scale asymmetric folds and faults are present and trend northeast (35-55°), with variable dip (45-80° NW). The most laterally extensive of these faults are the regional-scale, oblique faults that occur throughout the YTT. The oldest fault surfaces are low-angle reverse faults, sub-parallel to the principal fold axes, and often form the contacts between lithologies.
Conjugate to the northeast-striking faults are numerous northwest-striking faults (300°-360°) that dip steeply to the southwest or vertically. These faults exhibit apparent strikeslip offsets of 15.2 to 45.7 m (50-150 ft), but the displacement is believed to be rotational, or scissor-like, with the footwall rotating counter-clockwise relative to the hanging wall. Distinct, closely-spaced joint sets crosscut nearly all lithologies, are predominantly northwest-striking (295-305°), and crosscut foliation in near-vertical configuration. Joints are commonly in-filled with quartz, quartz-carbonate, and quartz-sulphide (± iron oxide) and are universally important controls to mineralization.
Gold mineralization at Gil primarily occurs in quartz-sulphide and quartz-carbonate veins, clay-filled shear zones, and limonite-stained fractures, which crosscut nearly all lithologies. Gold mineralization is widespread, but grade and continuity are related to complex interactions among hydrothermal fluids, host rocks, and structure.
At Main Gil and South Sourdough, gold mineralization largely appears stratabound within calc-silicate units, however field-level observations support that gold mineralization is predominately localized within veins and joints in highly fractured rock. Veins are discrete, up to 30 cm wide, white quartz veins, and later thinner, discrete, quartz-calcite (±actinolite/pyroxene) veins. Both sets of veins tend to be steeply dipping and crosscut foliation. Alteration in the Main Gil zone is represented by an intense retrograde assemblage of calc-silicate minerals. Hydrothermal fluid interaction with a carbonate-rich protolith and calc-silicate minerals enhanced the precipitation of gold and sulphides.
At North Gil and North Sourdough, gold is almost exclusively associated with quartz veining. These veins occur within quartz-mica schist, feldspathic schist, and calcareous biotite-chlorite-quartz schist. The quartz veins are typically less than 5.1 cm in width and consist of milky-white quartz-arsenopyrite, quartz-calcite, and quartz-feldspar veins. Alteration is vein-controlled and consists of sericitic to potassic (secondary biotite) alteration along vein margins. Pyrite and/or arsenopyrite are the most common accessory minerals observed in the veins, typically in concentrations of <1%.