USGS Professional Paper 1697 - Alaska Resources Library

172 Metallogenesis and Tectonics of the Russian Far East, Alaska, and the Canadian Cordillera
trace Au, which formed prior to movement on fault veins,
and a post-fault set of siderite-galena-sphalerite-pyrite-friebergite-pyrargyrite.
A K-Ar isotopic age of 90 Ma age, which
is interpreted as age of deposit, may be related to granitoid
intrusions of similar age to the north and south of Keno Hill.
Between 1921 and 1988, estimated production was 6,769
tonnes of Ag, half of that came from the Elsa, Keno No. 9,
Lucky Queen, Silver King, Sadie-Ladue, and Husky Mines.
Total production was 4.87 million tonnes of ore of average
grade 1,412 g/t Ag, 6.8 percent Pb, 4.6 percent Zn and 0.02
g/t Au (Yukon Minfile, 1992). More than 65 ore deposits
and prospects occur in the district. The Keno Hill-Galena
Hill district is the second largest silver producer in Canada.
Similar Ag polymetallic vein districts occur to the west and
northwest of Keno Hill at Nadaleen Mountain, Rusty Mountain,
Kathleen Lakes, and McKay Hill.
Brewery Creek Sb-Au Vein Deposit
The Sb-Au vein deposits at Brewery Creek (Loki Gold)
are hosted by sheared clastic rocks of the Earn Group and
adjacent porphyry sills (Bremner, 1990). Eight separate deposits
occur over a strike length of 5.5 km along a shear zone
between a sill of quartz monzonite, syenite, and latite of the
Tombstone suite and graphitic argillite, chert, sandstone, conglomerate,
and bedded barite. The deposits consist of gold that
occurs in fine-grained chalcedony-pyrite-arsenopyrite stockworks.
About 90 percent of the deposit is oxidized at depths
of 10 to 110 m. Narrow quartz-stibnite veins post-date the Au
veins. An open pit, heap-leach mine started at the deposit in
1995 with estimated preproduction reserves of 19.2 million
tonnes grading 1.53 g/t Au (Bremner, 1990; Loki Gold Corp.,
News Release, January 11, 1994). Other Au-Sb (W-Pb-Ag)
veins in the region are at Antimony Mountain, West Ridge,
and Spotted Fawn.
Eagle (Dublin Gulch) Porphyry Au-W Deposit
The Eagle (Dublin Gulch) porphyry Au-W deposit consists
of granitoid-related Au vein stockworks that are similar
to the Fort Knox deposits near Fairbanks, Alaska (Mortensen
and others, 1994; Hitchins and Orssich, 1995). The Late
Cretaceous Dublin Gulch biotite granodiorite stock, with a
K-Ar isotopic age of 95 to 87 Ma, contains (1) sheeted Au-
As-Cu-Bi-W quartz veins along the western end in the Eagle
Zone, (2) scheelite-quartz veins in the east-central part, and (3)
Au-sulfide quartz veins along the northern contact and to the
west. The associated Ray Gulch (Mar) W skarn deposit occurs
on the south side of the Dublin Gulch stock, and cassiterite
breccia deposits occur 2 km north of the stock. The Au quartz
veins in the Eagle Zone range from 1 to 2 cm wide, and are
associated with potassic and phyllic envelopes that coalesce to
form pervasive alteration containing the closely spaced quartz
veins. Vein mineral assemblages, in addition to free gold
grains as much as 1 mm in diameter, are arsenopyrite, pyrrhotite,
chalcopyrite, pyrite, bismuthinite, tetradymite, tellurobismuthinite,
native bismuth, and rare molybdenite and scheelite.
Estimated resources are 64.5 million tonnes grading 1.03 g/t
Au (Hitchins and Orssich, 1995).
Ray Gulch W Skarn Deposit
The Ray Gulch W skarn deposit consists of scheelite that
occurs as disseminations and tabular layers in sulfide-free
diopside-amphibole-epidote skarn. The deposit is hosted in
calcareous metasedimentary rocks and tuff of the Late Proterozoic
Hyland Group that is intruded by quartz monzonite
sills that dip gently northward towards the Potato Hills stock,
part of the mid-Cretaceous Tombstone Plutonic Suite. Eight
separate skarn zones form a resource of 5.44 million tonnes
of material grading 0.82 percent WO3 (Lennan, 1986). Other
W skarns are at Scheelite Dome, Lugdush, and Rhosgobel. Sn
skarns occur in the Keno Hill-McQuesten River region and
include Oliver Creek, Boulder Creek, East Ridge, and Barney
Ridge. Also occurring in the area are several vein and breccia
Sn-W deposits, which are hosted by Late Proterozoic to
Mississippian metasedimentary rocks and associated felsic
granitoid stocks of the Tombstone Plutonic Suite (Emond and
Lynch, 1992), Small Cu-Au skarns deposits occur at Marn,
Brenner, and Ida.
Origin of and Tectonic Controls for Tombstone
Metallogenic Belt
The Tombstone metallogenic belt is hosted in the mid-
Cretaceous Tombstone Plutonic Suite, which intrudes the
Proterozoic Hyland and Cambrian through Devonian Rocky
Mountain Assemblage and the Devonian and Mississippian
clastic wedge rocks of Earn Group of the North American
Craton Margin (fig. 62). The Tombstone Plutonic Suite, which
has isotopic ages of 95 to 89 Ma, consists mainly of alkaline
plutons that include syenite, hornblende-biotite granodiorite,
quartz monzonite, and quartz-feldspar porphyry. The plutonic
suite includes some felsic, peraluminous, two-mica granite
and quartz monzonite plutons that were previously included
in the slightly older Selwyn Plutonic Suite that has isotopic
ages of 97 to 112 Ma. The granitoid rocks of the Tombstone
Plutonic Suite, which are part of the Omineca-Selwyn plutonic
suite, are similar in age, geochemistry, and petrology to mid-
Cretaceous plutons that intrude the Yukon-Tanana terrane to
the west in Alaska across the Denali Fault, and the suites of
lode deposits associated with each suite of granitoid rocks are
similar. The Tombstone metallogenic belt contains Ag polymetallic
vein, granitoid-related Au, Sb-Au vein, W-Sn-Au skarn,
and Cu-Au skarn deposits. The older (mid-Cretaceous) part of
the East-Central Alaska metallogenic belt contains granitoidrelated
Au, Sb-Au vein, Pb-Ag-Zn-Au polymetallic vein, and
W-Au skarn deposits. In recent years, the name Tintina Gold
Belt (Tucker and Smith, 2000) has been used for granitoidrelated
Au deposits and occurrences that occur throughout the
Yukon Territory and in the East-Central Alaska metallogenic
belt (described below) of Nokleberg and others (1995a, 1996,
1997a), and in the correlative Tombstone metallogenic belt
of Nokleberg and others (1995a, 1996, 1997a), which occurs