USGS Professional Paper 1697 - Alaska Resources Library

210 Metallogenesis and Tectonics of the Russian Far East, Alaska, and the Canadian Cordillera
parts, an older, mid-Cretaceous part, described and interpreted
herein, and a younger Late Cretaceous and early Tertiary part,
described in a subsequent section. The mid-Cretaceous part
contains a wide variety of granitoid-related Au, Sb-Au vein,
Pb-Ag-Zn-Au polymetallic vein, and W-Au skarn deposits
(table 4). The East-Central Alaska belt is hosted mainly by
mid Cretaceous granitoid rocks in the Yukon-Tanana Upland
(Miller, 1994; McCoy and others, 1997), which are part of the
Yukon-Tanana igneous belt (Moll-Stalcup, 1994). Recently,
the name was described in a series of papers in the volume
edited by Tucker and Smith (2000). However, their usage was
for both granitoid-related Au deposits and occurrences in the
East-Central Alaska metallogenic belt and in the correlative
Tombstone metallogenic belt that is described above.
Three major groups of deposits occur in the East-Central
Alaska metallogenic belt (table 4): (1) In the north-central part
of the belt, in the Manley and Livengood area, the significant
lode deposits are a Pb-Ag-Zn-Au polymetallic vein deposit at
Hot Springs Dome (mid-Cretaceous?), an Sb-Au vein deposit
at Sawtooth Mountain (mid-Cretaceous), and Sb-Au and Au
quartz vein deposits at Gertrude Creek, Griffen, and Ruth
Creek (table 4) (Nokleberg and others 1997a,b, 1998). (2) In
the central part of the belt are a variety of granitoid-related
deposits in the Fairbanks area. (3) In the southern part of the
belt are polymetallic and Sb-Au vein deposits in the Kantishna
area in the northern Alaska Range (Bundtzen, 1981; McCoy
and others, 1997). Described below are the major granitoidrelated
deposits at Fort Knox, Democrat, and Pogo, in the
Fairbanks area, and vein deposits in the Kantishna district. The
Fairbanks area is underlain by multiply metamorphosed and
penetratively deformed, middle Paleozoic and older quartzmetasedimentary,
sparse metavolcanic, and rare metagranitoid
rocks of the Yukon-Tanana continental-margin terrane (Nokleberg
and others, 1994c, 1997c). The characteristics and origin
of the mid-Cretaceous granitoid-related Au deposits in this
area are described by McCoy and others (1997).
S N
0 60 m
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 (1) Sb-Au vein deposits at Dempsey Pup and
Scrafford, (2) a Sn polymetallic vein deposit at Table Mountain,
(3) polymetallic vein deposits at Cleary Summit, Ester
Dome (Ryan Lode), Democrat (Mitchell Lode), Blue Lead,
Tibbs Creek, and Gray Lead, (4) a major granitoid-related Au
deposit at Fort Knox, (5) a high-grade, granitoid-related Au
quartz vein deposit at Pogo, and (6) an Au-As vein deposit
at Miller House. The deposits at Scrafford, Cleary Summit,
Gilmore 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
that 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
that trends 290°. The deposit is at least 1,500 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).
Coarse-grained
granite
Medium-grained
granite
Fine-grained
granite
Schist and
quartzite
Contact
Late
Cretaceous
Devonian
or older
Shear zone showing
sericite halos
Pegmatite/aplite showing
mostly feldspar halos
Stockwork veining showing
feldspar-quartz and/or
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). See figure 80 and table 4 for location.