USGS Professional Paper 1697 - Alaska Resources Library

174 Metallogenesis and Tectonics of the Russian Far East, Alaska, and the Canadian Cordillera
dikes exhibit a K-Ar isotopic age of 66 Ma. Estimated reserves
are 2.6 million tonnes grading 8.8 percent Zn, 6.4 percent Pb,
and 3258 g/t Ag according to drilling and underground exploration
(Bradford and Godwin, 1988; Mining Review, summer
2000; Peruvian Gold/Imperial Metals release, February 10,
2000). The deposit does not contain a calc-silicate gangue
typical of Zn skarn deposits and exhibits only minor amounts
of a silica and carbonate gangue adjacent to replacement
bodies, similar to the alteration in the large Ag-Zn-Pb manto
deposits of northern Mexico. At both the Midway and the
nearby YP manto deposits, a crude zonation in metal distribution
exists wherein Au is concentrated commonly in massive
sulfide zones that are rich in Fe, Cu, and Zn, rather than in
more distal Pb- and Ag-rich zones (Dawson, 1996a). An origin
similar to that of Sa Dena Hes Zn-Pb-Ag manto deposit near
Watson Lake, Yukon is proposed, where mantos are developed
distally to an intrusion inferred to underlie the deposit at depth
(Dawson, 1996a).
Ketza River Manto Au Deposit
The Ketza River manto Au deposit consists of (1) Auquartz–sulfide
veins and massive pyrrhotite-arsenopyritepyrite-chalcopyrite-Au
mantos that occur in a central part, and
(2) mantos and veins of similar mineralogy that contain lower
amounts of sulfide minerals that occur in a outer part, and (3)
Ag-Pb veins and mantos that occur in the periphery (Cathro,
1990). The deposit is hosted by Early Cambrian sedimentary
rocks of Cassiar continental-margin terrane. The district is
interpreted as underlain by a mid-Cretaceous pluton of the
Cassiar Plutonic Suite, that is interpreted as emplaced along
the Ketza-Seagull Arch, a major structural feature (Abbott,
1986a). Hornfels underlies the deposit and exhibits a K-Ar
whole-rock isotopic age of 101 Ma (K.M. Dawson, unpub.
data, 1986). The mine at the deposit operated between 1988
and 1990 with initial oxide ore reserves of 282,000 tonnes
averaging 13.45 g/t Au (Canamax Resources Inc., 1988
Annual Report).
JC Skarn Sn Deposit
The JC Sn skarn deposit consists of malayite, stannite,
stanniferous tetrahedrite, and cassiterite in hedenbergite-diopside
skarn that occurs along the contact between Devonian
and Mississippian carbonate rocks and porphyritic granite of
the mid-Cretaceous Seagull Batholith (Layne and Spooner,
1986; EMR Canada, 1989; Yukon Minfile, 1991). The Seagull
Batholith consists of a two-mica A-type granite, which is also
associated with the F-, Cl-, and B-rich, Sn skarn deposits at
Val A and Viola and with Zn-Pb-Ag skarn deposits at Atom,
Bom, and Bar (Dawson and Dick, 1978). At the JC deposit,
hedenbergite-diopside-andradite prograde skarn contains
elevated Sn in a distinctive green garnet skarn and is replaced
by a retrograde assemblage of cassiterite, stannite, tetrahedrite,
sphalerite, and malayite. Axinite and fluorite fill interstices
in a pipe-like breccia. Estimated resources are 1.25 million
tonnes grading 0.54 percent Sn (Layne and Spooner, 1986).
Origin of and Tectonic Controls for Cassiar Metallogenic Belt
The Cassiar metallogenic belt is hosted in the Cassiar
Plutonic Suite, which is lithologically similar to the Bayonne
Plutonic Suite, but the generally elongate plutons of the Cassiar
Suite exhibit post-emplacement deformation (Woodsworth and
others, 1991). The plutons and most of the associated deposits
are hosted by the Cassiar terrane, which is interpreted as a
displaced fragment of North American Craton Margin (Monger
and others, 1972, 1992; Monger and Nokleberg, 1996;
Nokleberg and others, 2000) and is composed of Proterozoic to
Carboniferous sedimentary rocks. The Cassiar Plutonic Suite
is part of the collisional mid-Cretaceous Omineca-Selwyn plutonic
belt, which extends from the southern part of the Canadian
Cordillera, across Interior Alaska, and northwestward into the
Russian northeast and consists chiefly of granodiorite, granite,
quartz syenite and minor syenite plutons of Early to mid-Cretaceous
age (110-90 Ma; Monger and Nokleberg, 1996; Nokleberg
and others, 1994c; 2000). The spatial location of the belt, about
200 km west of the eastern limit of Cordilleran deformation, and
chemistry suggests an anatectic origin of partial melting of cratonic
crust during thickening caused by Cretaceous contraction (Monger
and Nokleberg, 1996; Nokleberg and others, 2000) that was associated
with orthogonal convergence between the Farallon oceanic
plate and North America (Englebretson and others, 1985; 1992)
and subsequent regional extension (Pavlis and others, 1993).
Other metallogenic belts of granitic-magmatism-related deposits
hosted in the Omineca-Selwyn plutonic belt in the Canadian
Cordillera, Alaska, and the Russian Northeast include the Bayonne,
Selwyn, Tombstone, and Whitehorse belts (fig. 62; table 3).
Whitehorse Metallogenic Belt of Cu-Fe Skarn,
Porphyry Cu-Au-Ag, and Au-Ag Polymetallic Vein
Deposits (Belt WH), Southern Yukon Territory
The Whitehorse metallogenic belt of Cu-Fe skarn, porphyry
Cu-Au-Ag, and Au-Ag polymetallic vein deposits (fig. 62; tables
3, 4) occurs in the southern Yukon Territory and is hosted in the
Whitehorse Plutonic Suite. These plutonic rocks intrude a large
area of the northern Stikinia and Cache Creek terranes, and
Yukon-Tanana terrane in the northern Canadian Cordillera (fig.
62). The Whitehorse Plutonic Suite, which is part of the Omineca-Selwyn
plutonic belt, consists predominantly of mid-Cretaceous,
granodiorite plutons (Woodsworth and others, 1991).
The significant deposits in the belt are Cu skarn deposits near
Whitehorse (Little Chief, War Eagle), and Hopkins or Giltana
(table 4) (Nokleberg and others 1997a,b, 1998).
Whitehorse Copper Belt of Cu Skarn Deposits
The Whitehorse Copper Belt (fig. 78) consists of 32
Cu-Fe (Mo-Au-Ag) calc-silicate skarn deposits hosted by both
calcareous and dolomitic units of the upper Triassic Lewes
River Group of Stikinia terrane (Dawson and Kirkham, 1996).
The skarns contain bornite, magnetite, and chalcopyrite and
minor native copper, tetrahedrite, and molybdenite. The skarns