USGS Professional Paper 1697 - Alaska Resources Library

to the east in the central Yukon Territory. Because the term
Tintina Gold Belt includes deposits of several ages and differening
origins, the term is not used in this study. Bundtzen
and others (2000) introduced the term Tintina Gold Province
to denote Mesozoic and Tertiary plutonic-related gold deposits
that are related to those occurring throughout the Yukon Territory
and Interior Alaska.
As described above for the correlative East-Central
Alaska metallogenic belt (mid-Cretaceous), the polymetallic,
Sb-Au vein, and granitoid-related Au deposits of the Tombstone
metallogenic belt are generally hosted in or near mid-
Cretaceous granitoid plutons (Nokleberg and others, 1995a;
McCoy and others, 1997; Smith, 1999; Smith and others,
1999, 2000; Goldfarb and others, 2000). In both East-Central
Alaska and the Central Yukon Territory, these plutons are
interpreted as intruding during the waning stages of a major
mid-Cretaceous collision of the Wrangellia superterrane with
the previously accreted Yukon-Tanana terrane (Stanley and
others, 1990; Dusel Bacon and others, 1993; Pavlis and others,
1993; Nokleberg and others, 2000). The collision and associated
metamorphism and deformation is interpreted as forming
in two phases—(1) a relatively older period of collision and
thrusting, associated with high-temperature and high-pressure
metamorphism, and (2) a slightly younger period of extension
associated with lower greenschist facies metamorphism. The
mid-Cretaceous granitoid rocks and relatively younger quartz
veins intrude and crosscut both the relatively older, higher
grade, and relatively younger, lower grade metamorphic fabrics
in the region (Dusel Bacon and others, 1993; Pavlis and
others, 1993). These relations suggest that the the Tombstone
metallogenic belt formed in the waning stages of this complex
deformation event (Nokleberg, 1997e, 2000). 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, Cassiar,
Selwyn, and Whitehorse belts (fig. 62; table 3).
Cassiar Metallogenic Belt of Porphyry Mo-W; W
Skarn, Zn-Pb-Ag Manto, Sn Skarn, and Au Skarn
Deposits (Belt CA), Northern British Columbia
and Southern Yukon Territory
The Cassiar metallogenic belt of porphyry Mo-W; W skarn,
Zn-Pb-Ag manto, Sn skarn, and Au skarn deposits (fig. 62;
tables 3, 4) occurs in northern British Columbia and southern
Yukon Territory. The belt contains a variety of mineral deposit
types that are related to granitoid plutons of the mid-Cretaceous
Cassiar Plutonic Suite, which forms a narrow, linear
belt of dominantly biotite granite and granodiorite plutons
that is part of the Omineca-Selwyn plutonic belt (fig. 62). The
significant deposits in the belt are a porphyry W-Mo deposit
at Logtung (Logjam Creek), a Pb-Zn-Ag skarn and manto
deposit at Midway (Silver Tip), a W skarn deposit at Risby
(Cab), a Zn-Ag polymetallic vein deposit at Logan, and a Sn
skarn deposit at JC or Viola (table 4) (Nokleberg and others
Late Early Cretaceous Metallogenic Belts (120 to 100 Ma; figs. 61, 62) 173
1997a,b, 1998). The Logan polymetallic Zn-Ag vein deposit is
hosted by the Marker Lake Batholith (Dawson, 1996a).
Logtung Porphyry Mo-W Deposit
The large Logtung (Logjam Creek) porphyry Mo-W
deposit consists of disseminated scheelite, molybdenite and
powellite with minor associated fluorite and beryl in garnetdiopside
skarn, quartz vein stockwork and fractures (EMR
Canada, 1989; Dawson and others, 1991; Yukon Minfile,
1991; Noble and others, 1995). The deposit contains estimated
reserves of 230 million tonnes grading 0.104 percent WO3, 0.05 percent MoS2. The deposit is hosted in a large quartz
porphyry dike that is related to a nearby mid-Cretaceous
quartz monzonite stock with a K-Ar isotopic age of 109 Ma
that is part of the Cassiar Plutonic Suite. The Logtung deposit
is typical of a group of deposits (as at Stormy and Molly) that
contain a molybdenite-rich stockwork in a granitoid pluton
and a scheelite-rich garnet-diopside skarn assemblage in the
wall rocks (Dawson, 1996c). At the Logtung deposit, early
quartz-scheelite veins are related to a monzogranite stock,
a later stage of quartz-scheelite-molybdenite-pyrite-fluorite
veins is related to a felsic dyke complex, and a final stage of
polymetallic W-Mo veins that form a large zone is centered
on the dyke complex (Noble and others, 1984). Both mid-Cretaceous
intrusives are part of the Cassiar Plutonic Suite. The
wall rocks are Mississippian and Pennsylvanian chert, argillite,
and quartzite of the Cassiar continental-margin terrane.
Risby Skarn W Deposit
The significant, but undeveloped Risby (Cab) skarn W
deposit consists of two diopside-garnet skarns that occur in
Early Cambrian carbonates that are intruded by granitoid sills
of the Cassiar Plutonic Suite (EMR Canada, 1989; Yukon
Minfile, 1990; Mining Review, 1992). The No. 1 zone has a
high pyrrhotite and low chalcopyrite content, whereas the No.
2 zone, which is intruded by a sill, has higher WO3 mineralization
and low sulfide content (Sinclair, 1986). The deposit has
a drilled resource of 3.2 million tonnes grading 0.82 percent
WO3. The mineral assemblage is typical of W skarn deposits
associated with plutons of the Omineca-Selwyn plutonic belt.
Midway (Silvertip) Manto Pb-Zn-Ag Deposit
The Midway (Silvertip) manto Pb-Zn-Ag deposit consists
of a generally coarse-grained assemblage of sphalerite-galenapyrite
that contains elevated Ag and minor Au, Sb, and Bi
values that occur as irregular, pipe-like, open-space filling
and replacement bodies in limestone of the Middle Devonian
McDame Group of the Cassiar continental-margin terrane
(Bradford and Godwin, 1988; EMR Canada, 1989). The nearest
intrusive rock, interpreted as the source of mineralizing fluids,
is a group of quartz-feldspar porphyry dikes that are about
2 km from the deposit. The dikes intrude clastic sedimentary
rocks of the Earn Group that unconformably overlie the
McDame Group. The wall rocks exhibit sericite alteration. The