USGS Professional Paper 1697 - Alaska Resources Library

170 Metallogenesis and Tectonics of the Russian Far East, Alaska, and the Canadian Cordillera
(K-Ar age of 94.6 Ma) of the Selwyn Plutonic Suite. Prograde
garnet-pyroxene skarn is overprinted by a hydrous, retrograde,
pyrrhotite-andradite-amphibole-biotite-scheelite skarn. The
deposits include the Pit orebody, which produced 1.51 million
tonnes of ore yielding 40,087 tonnes of WO 3 between 1962
and 1986 and the E-Zone orebody with reserves of 4.2 million
tonnes grading 1.6 percent WO 3 and 0.23 percent Cu, with
associated Bi (Dawson, 1996c; Matheison and Clark, 1984). The
large, high-grade E-zone scheelite orebody contains estimated
combined reserves and resources of 7 million tonnes grading 1.5
percent WO 3 and 0.2 percent Cu. Production, which ceased in
1986, resumed in 2001 on mineable reserves of 630,00 tonnes
of 1.82 percent WO3.(North American Tungsten Corporation
news release, January 21, 2001).
Macmillan Pass (Mactung) Skarn W Deposit
The Macmillan Pass (Mactung) skarn W deposit consists of
scheelite, pyrrhotite, and minor chalcopyrite in several pyroxenegarnet
skarns that replace Cambrian to Ordovician limestone and
limestone breccia (Sinclair, 1986; EMR Canada, 1989; Dawson
and others, 1991; Mining Review, 1992). The host rocks are
part of a folded Cambrian and Devonian outer-shelf sequence of
carbonate and pelitic rocks of the North American Continental
Margin. The host rocks are flanked by and inferred to be underlain
by the Late Cretaceous quartz monzonite Mactung stock
(with a K-Ar isotopic age of 89 Ma) of the Selwyn Plutonic
Suite. Hydrothermal alteration has produced three distinct concentric
skarn zones—a peripheral zone of garnet-pyroxene skarn,
an intermediate zone of pyroxene skarn, and a central zone of
pyroxene-pyrrhotite skarn. Estimated combined reserves and
resources are 32 million tonnes grading 0.92 WO3 (Atkinson and
Baker, 1986; Dawson and others, 1991).
Sa Dena Hes, Quartz Lake, and Prairie Creek Skarn and
Manto Zn-Pb-Ag Deposits
Zn-Pb-Ag skarn deposits occur in several belts adjacent
to the Mount Billings batholith in southeastern Yukon Territory
and the Flat River and smaller stocks in southwestern Northwest
Territories. The associated granitoid plutonic rocks are part of
the Selwyn Plutonic Suite. The host rocks are mainly Early
Cambrian limestone; the associated granitoid plutons range
from high-silica leucogranite and topaz granite to syenite. Diorite
dikes also occur. In most cases, Zn skarns occur distally with
respect to associated igneous rocks (Dawson, 1996a).
The Sa Dena Hes deposit at Mount Hundere, near Watson
Lake, Yukon Territory consists of at least four tabular skarn
and manto ore bodies (Abbott, 1981; Bremner and Ouellette,
1991; Northern Miner, October 7, 1991). The ore bodies are
concordant to bedding in a deformed, Late Proterozoic and
Early Cambrian craton margin sequence of limestone and
phyllite. No nearby igneous rocks are exposed. The limestone
is replaced by coarse-grained actinolite-hedenbergite-grossularite
skarn, with pyrrhotite, magnetite, sphalerite, pyrite,
and galena, and retrograde quartz and fluorite. The deposit
has pre-production reserves of 4.8 million tonnes grading 12.7
percent Zn, 4 percent Pb, and 59 g/t Ag (Dawson and Dick,
1978; Dawson, 1996a).
The Quartz Lake (McMillan) deposit is a pyrite-bearing
Zn-Pb-Ag manto that is probably related to small, nearby Cretaceous
plutons. The deposit occurs in southeastern Yukon and
consists of a series of tabular and concordant bodies, lenses,
and disseminations that replace limy quartzite and argillite of
the Late Proterozoic to Early Cambrian Hyland Group of the
North American Craton Margin. The deposit consists pyrite,
galena, and sphalerite, with minor arsenopyrite, boulangerite,
tetrahedrite, and chalcopyrite. Galena-lead isotopes indicate
an age of about 100 Ma, similar to that of intrusives of the
Selwyn suite (Godwin and others, 1988). Estimated reserves
are 1.5 million tonnes grading 6.6 percent Zn, 5.5 percent Pb,
and 102 g/t Ag (Morin, 1981; Vaillancourt, 1982).
Recent exploration at the Prairie Creek (Cadillac) prospect,
which occurs near the South Nahanni River, Northwest Territories,
reveals a stratabound deposit with characteristics of a
Zn-Pb-Ag manto that occurs at depth beneath an extensive vein
zone. No plutons are associated with the deposit, which consists
of galena and sphalerite with minor tetrahedrite and chalcopyrite
hosted in a quartz-carbonate gangue. The deposit occurs along
a strike length of 10 km in 12 lenticular, stratabound vein zones
that are hosted in shale and dolomite of the Middle Devonian
Arnica Formation. Six deeper concordant ore lenses in the subjacent
Ordovician Whittaker Formation are more than 22 m thick.
An estimated resource of 6.2 million tonnes grades 13 percent
Zn, 12 percent Pb, and 180 g/t Ag (San Andreas Resources
Corp., News Releases, 1992, 1993, 1994, 1995; Dawson, 1996a).
Origin of and Tectonic Controls for Selwyn
Metallogenic Belt
The Selwyn metallogenic belt contains one of the world’s
largest reserves and resources of W skarn deposits. The associated
plutons, part of the Selwyn Plutonic Suite, are mainly
equant, high level bodies of granite, granodiorite and quartz
syenite with pronounced S-type characteristics that discordantly
intrude Late Proterozoic to early Paleozoic carbonate and
pelitic rocks of the North American Craton Margin (fig. 62).
The Selwyn Plutonic Suite is interpreted to have formed from
thickening and melting of the continental crust in the outer part of
a miogeoclinal sedimentary wedge during regional compression
(Woodsworth and others, 1991). Skarn W deposits commonly are
associated with marginal or satellite phases of two-mica granites
with aluminous accessory minerals (Anderson, 1983b). Radiometric
ages for the plutonic suite range from 88 to 114 Ma,but
most skarns are associated with early-phase plutons that range in
age between 112 and 100 Ma (Mortensen and others, 1994).
The Selwyn Plutonic Suite is part of the collisional mid-
Cretaceous Omineca-Selwyn plutonic belt that 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;