Metallogenesis and Tectonics of the Russian Far East, Alaska, and ...

Fairbanks Area
In the eastern and southern parts of the belt in the Fairbanks area, a wide variety of granitoid-related deposits (Table 4)
are spatially associated with mid-Cretaceous plutons. These deposits are: Sb-Au vein deposits at Dempsey Pup and Scrafford; a Sn
polymetallic vein deposit at Table Mountain; polymetallic vein deposits at Cleary Summit, Ester Dome (Ryan Lode), Democrat
(Mitchell Lode), Blue Lead, Tibbs Creek, and Gray Lead; a major granitoid-related Au deposit at Fort Knox; a high-grade,
granitoid-related Au quartz vein deposit at Pogo; and an Au-As vein deposit at Miller House. The deposits at Scrafford, Cleary
Summit, Gilrnore Dome, Ester Dome, and Democrat are in the Fairbanks district.
Fort Knox Granitoid-Related Au Deposit
The Fort Knox granitoid-related Au deposit (fig. 99) (Blum, 1985; Robinson and others, 1990; A.A. Bakke, written
commun., 1991; Bakke, 1995; Hollister, 1992, McCoy and others, 1997) occurs northeast of Fairbanks and consists of free gold,
bismuthinite, and minor to trace molybdenite and chalcopyrite which occur in a sulfide-poor, quartz vein stockwork in the Fort
Knox (porphyritic granodiorite) pluton. The stockwork is preferentially emplaced along a steeply dipping fracture system which
trends 290". The deposit is at least 1500 m long by 300 m wide, and 250 m deep, and contains an estimated reserve of 186.5
million tonnes grading 1.6 g/t Au (Bakke, 1995,2000). The gold is remarkably pure (greater than or equal to 980 fine). The Fort
Knox pluton is alkali-calcic and peraluminous. Mineralization is interpreted to have occurred either during the late stages of
emplacement of the nearby Gilmore Dome stock which has a K-Ar biotite age of 92 Ma (McCoy and others, 1997). However, a
younger, early Tertiary heating event that post-dates the older porphyry Mo-Bi-Cu-mineralization may have also introduced Au
(Bakke, 1995).
I
1
Medium-grained Late
granite Cretaceous
Devonian
or older
Shear zone showing s ' ericite halos
, - Pegmatitelaplite showing
- - mostly feldspar halos
Stockwork veining showing
feldspar-quartz andlor
white mica quartz halos
Figure 99. Fort Knox granitoid-related Au deposit, East-Central Alaska metallogenic belt, East-Central Alaska. Schematic cross
section looking west. Adapted from Bakke (1994).
Democrat (Mitchell Lode) Granitoid-Related
Au Deposit
The Democrat (Mitchell Lode) granitoid-related Au deposit (fig. 100) (Bundtzen and Reger, 1977; T.K. Bundtzen and
R.B. Forbes, written commun., 1990) occurs in the Richardson district south of Fairbanks, and consists of disseminated and
stockwork tetrahedrite, galena, acanthite, owyheeite, other Ag-sulfosalts, free gold, and quartz in a hydrothermally altered granite
porphyry. A strong sericite alteration halo surrounds the porphyry, which intrudes sillimanite-bearing metasedimentary schists of
the Yukon-Tanana terrane. The granite porphyry has a K-Ar biotite age of 89.1 Ma and is part of a 35-km-long sill complex which
intruded along the Richardson lineament. Free gold occurs as interlocking alloys of native silver and gold which average 67% Au
and 33% Ag. Silver sulfosalts are abundant locally and exhibit very high grades of up to 66,000 g/t Ag. In 1989, a pilot mill
produced 88,000 tonnes grading 2.2 g/t Au and 5.0 g/t Ag. The deposit is interpreted as forming during high level emplacement of
a Late Cretaceous granite porphyry along the Richardson lineament.