USGS Professional Paper 1697 - Alaska Resources Library

232 Metallogenesis and Tectonics of the Russian Far East, Alaska, and the Canadian Cordillera
1992a,b; Bundtzen and Miller, 1997). The deposit consist of
vein stockworks, replacements, and tourmaline breccias with
anomalous Ag, Cu, Pb, W, Sn, Nb, and As and uncommonly Au.
The deposits occurs in both Late Cretaceous and early Tertiary
plutons and in overlying contact-metamorphic hornfels. The
significant lode prospects in the Beaver Mountains occur in a 15
km 2 area centered on the Cirque and Tolstoi prospects (Bundtzen
and Laird, 1982), which are called the South Quartz Zone
by Szumigala (1993). A K-Ar biotite age of 70.3 Ma (Bundtzen
and Laird, 1982) is obtained for quartz syenite near the northern
margin of the Beaver Mountains pluton. The Cirque and Tolstoi
deposits consist of a series of parallel tourmaline-axinite-sulfide
fracture fillings and tourmaline-syenite breccia pipes that occur
in the cupola of a quartz syenite phase of the Beaver Mountains
pluton. Chip-channel samples are as much as 21.0 percent Cu,
1,000 g/t Ag, 200 ppm Sn, and 1 g/t Au (Bundtzen and Laird,
1982; Miller and Bundtzen, 1994).
The Bismarck Creek deposit occurs in an east-west to
northeast-trending zone of tourmaline-axinite-cassiterite-sulfide
gossan in hornfels about 70 km southwest of McGrath (Bundtzen
and Miller, 1997). Secondary biotite is locally abundant and
occurs in fine network veins and replacement zones in breccia.
A 30-meter-wide zone extends for about 300 meters along
strike. On the basis of extensive surface sampling and geological
modeling, Bundtzen and Miller (1997) estimate that the
Bismarck Creek deposit contains about 500,000 tonnes grading
0.137 percent Sn, 47.8 grams/tonne Ag, 0.26 percent Zn, and
anomalous Cu, F, bi, Sb, and In. A nearby Sn granite intrusion
on Granite Mountain is radiometrically dated at 61 Ma.
The Win and Won deposits, each of that contain 5 or
more individual occurrences and prospects, are about 35 km
apart. Both deposits occur north of McGrath, and contain
some of the most important Sn-polymetallic resources in the
Kuskokwim Mineral Belt and perhaps in all of Alaska. Both
groups of deposits consist of polymetallic-sulfide and quartzcassiterite
assemblages in vein and breccia in quartz-tourmaline
hornfels (Burleigh, 1992a,b; Bundtzen and Miller, 1997;
Bundtzen, 1999). Although neither the Win nor Won deposits
are isotopically dated, both are spatially associated with
several subvolcanic plutons that have isotopic ages ranging
from 60 to 70 Ma (Moll and others, 1981). Sulfosalt-rich veins
and breccias at the Win deposit contain as much as 643 g/t Ag
and 6.97 percent Sn but very little Au. The better studied Won
deposit group is estimated to contain 1.94 millon tonnes grading
0.59 percent Sn and 42 g/t Ag. The Win and Won deposits
are geologically and morphologically similar to the Bismarck
Creek deposit described above.
Von Frank Mountain Porphyry Cu-Au Prospect
The Von Frank Mountain porphyry Cu-Au prospect
(J. DiMarchi, written commun., 1994; Nokleberg and others,
1995a; Bundtzen, 1999) occurs about 100 km northeast
of McGrath and consists of stockwork in a quartz diorite and
augite-biotite granodiorite. These rocks occur in a downdropped
structural block along the southern limit of a vol-
canic-plutonic complex exposed at Von Frank Mountain. The
stockwork consists of chalcopyrite and arsenopyrite, minor
molybdenite, and free gold in quartz-carbonate veins in a cupola
of the intrusive system. Alterations are sericite, silica, and
dolomite replacements that are similar to those in the Chicken
Mountain porphyry Au deposit (Bundtzen and others, 1992).
A K-Ar mineral isotopic age of 69.9 Ma was obtained from a
granitoid pluton north of the prospect (Moll and others, 1981).
Bundtzen (1999) indicates that Cu and Au have a correlation
coefficient of 0.92; coefficient values between Au and other
metals are much lower. Grades range from 0.5 to 19.1 g/t Au
and from 0.05 percent to 0.45 percent Cu. One drill hole intercepted
about 45 m grading 1.2 g/t Au and 0.08 percent Cu. The
Von Frank Mountain deposit is the most northeastern porphyry
deposit known in the Kuskokwim Mountains metallogenic belt.
Peraluminous Granite Porphyry Au Deposits, Kuskokwim
Mountains Metallogenic Belt
Peraluminous granite porphyry Au deposits are only
recently identified in the central Kuskokwim Mountains.
Placer Au deposits in the area have been mined for many
years (Mertie, 1936; Bundtzen and Laird, 1980; Bundtzen,
1986). The best examples of the porphyry deposits are at
Donlin Creek, the Independence Mine on Ganes Creek in the
Innoko District, and Vinasale Mountain, about 25 km south of
McGrath. Nine other occurrences are located in the Iditarod
Quadrangle (Bundtzen and others, 1985). K-Ar mineral ages
for the igneous rock hosting all the deposits range from 69
to 71 Ma (Bundtzen and Laird, 1982; Solie and others, 1991;
Miller and Bundtzen, 1994). A K-Ar sericite age of 68.1 Ma is
reported for the Vinasale deposit by DiMarchi (1993); this age
is within the analytical uncertainty of the age of the host pluton.
Donlin Creek Porphyry Au Deposit
The Donlin Creek porphyry Au deposit (fig. 110) is hosted
in a porphyritic rhyodacite, granite porphyry, and minor granodiorite
dike and sill complex that intruded a flysch of the Late
Cretaceous Kuskokwim Group (Ebert and others, 2000; Cady
and others, 1955). More than seven mineralized zones are known
(Bundtzen and Miller, 1997). K-Ar isotopic ages range from
65.1 to 70.9 Ma for granite porphyry sills at the Snow Gulch
portion of the property (Miller and Bundtzen, 1994); subsequent
40 39 Ar/ Ar and U-Pb ages are similar. The Au-As-Sb-Hg deposit
occurs in a 2 by 6 km area and is associated with narrow, steeply
dipping , irregular, and discontinuous quartz and quartz-carbonate
veins and veinlets (Ebert and others, 2000). Quartz vein
textures range from massive to fine grained and comb to bladed.
Ore minerals are As sulfosalts (rare), cinnabar (rare to uncommon),
stibnite (common), arsenopyrite (very common), and As
pyrite (very common). The Donlin Creek deposit is structurally
controlled along north-northeast-trending extensional fractures,
and the deposit is best developed where mineralizing fluids
intersected competent host lithologies, such as massive graywacke
and porphyry dikes and sills (Ebert and others, 2000). The
Donlin Creek deposit contains 127.4 million tonnes grading 2.89