USGS Professional Paper 1697 - Alaska Resources Library

176 Metallogenesis and Tectonics of the Russian Far East, Alaska, and the Canadian Cordillera
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,
Cassiar, Selwyn, and Tombstone belts (fig. 62).
Bayonne Metallogenic Belt of Porphyry Mo and
Cu-Mo-W-Zn Skarn Deposits (Belt BA) ,Southern
British Columbia
The Bayonne metallogenic belt of porphyry Mo and
Cu-Mo-W-Zn skarn deposits (fig. 62; tables 3, 4) occurs in
southern British Columbia and is hosted in the mid-Cretaceous
Bayonne Plutonic Suite, which is the extreme, southern part
of the Omineca-Selwyn plutonic belt. The intrusions typically
are S-type, felsic, enriched in large-ion lithophile elements, and
have initial Sr ratios in the range 0.710 to 0.740 (Armstrong,
1988). Most of the suite forms roughly equant plutons and large
stocks of mainly granodiorite or granite; the stocks are strongly
discordant with the wall rocks of the Quesnellia and Kootenay
terranes and the North American Craton Margin. The significant
deposits in the belt are porphyry Mo deposits at Boss Mountain
and Trout Lake, a W skarn deposit at Emerald-Invincible, a Zn-
Pb skarn and manto deposit at Mineral King, a Mo skarn deposit
at Red Mountain Moly (Coxey, Novelty, Nevada), and a Cu-Au
skarn deposit at Phoenix-Greenwood (table 4) (Nokleberg and
others 1997a,b, 1998).
Boss Mountain Porphyry Mo Deposit
The Boss Mountain porphyry Mo deposit consists of
molybdenite in quartz veins, fracture zones and in collapse breccias
that are hosted by a granodiorite phase of the composite
Early Jurassic Takomkane batholith that is intruded by the mid-
Cretaceous Boss Mountain stock (Soregaroli and Nelson, 1976;
MacDonald and others, 1995). The emplacement of the stock
was accompanied by rhyolite dikes, brecciation, and multiple
stages of veining and Mo deposition. Molybdenite was mined
from a sheeted vein system, which describes a partial annulus
centered upon the apical region of the stock. Alteration assemblages
consists of garnet, hornblende, biotite, sericite, potassium
feldspar, chlorite, and talc. Pyrite forms a 1.5 km-wide halo.
Between 1965 and 1971, about 2.97 million tonnes were milled
with an average grade of 0.26 percent Mo. Between 1974 and
1980, 3.6 million tonnes were milled with an average grade of
0.188 percent Mo. Estimated remaining reserves are 3.84 million
tonnes grading 0.135 percent Mo (Soregaroli and Nelson,
1976; MacDonald and others, 1995).
Trout Lake Porphyry Mo Deposit
The Trout Lake porphyry Mo deposit consists of molybdenite
and pyrite in quartz veins and also scheelite, pyrrhotite,
and chalcopyrite, with lesser amounts of galena, sphalerite,
and tetrahedrite in peripheral skarns (Boyle and Leitch, 1983;
Linnen and others, 1995). The deposit is hosted mainly in
limestone, schist, and quartzite of the Paleozoic Lardeau
Group. A minor part of the deposit is hosted in altered granodiorite
and tonalite of the Late Cretaceous Trout Lake stock,
which forms part of the Bayonne Plutonic Suite. Skarn calcsilicate
alteration includes prograde clinopyroxene-garnet
and retrograde tremolite-clinozoisite with scheelite. Potassic
(biotite) alteration overprints retrograde skarn and forms envelopes
around quartz-albite Mo veins in skarn and hornfels,
whereas K-feldspar replaces plagioclase in the intrusion. The
highest Mo grades are associated with the later quartz-K-feldspar-muscovite
alteration. Estimated resources are 49 million
tonnes grading 0.19 percent Mo (Boyle and Leitch, 1983; Linnen
and others, 1995).
Red Mountain Mo Skarn Deposit
The Red Mountain Mo skarn deposit (Coxey, Novelty,
Giant) consists of molybdenite, pyrrhotite, chalcopyrite,
arsenopyrite, scheelite, pyrite, magnetite, bismuthinite, galena
and sphalerite that occur in veins, disseminations, and shears
within skarn and contact-metamorphosed siltstone and breccia
of the Pennsylvanian to Permian Mount Roberts Formation
(Ray and Webster, 1991; Ray and Dawson, 1998). The
small porphyritic intrusions of granite and granodiorite are
interpreted to be associated with a late phase of the Early to
Middle Jurassic subvolcanic, monzonite Rossland intrusion
that is associated with large Au-bearing pyrrhotite-chalcopyrite
vein deposits of the adjacent Rossland district (Hoy and
others, 1998). An alternative interpretation is that the intrusions
associated with the deposit are part of the mid-Cretaceous
Bayonne Plutonic Suite. At Coxey, molybdenite and
minor scheelite are associated with a prograde assemblage of
pyroxene-garnet-vesuvianite and a retrograde assemblage of
epidote-actinolite-chlorite. The Novelty and Giant Mo skarns
contain abundant arsenopyrite, and minor pyrrhotite, pyrite,
chalcopyrite, cobaltite, bismuthinite, native Bi, and Au. The
Novelty deposit also contains some uraninite. Estimated
production and reserves are 1.31 million tonnes grading 0.20
percent Mo (Ray and Webster, 1991; Ray and Dawson, 1998).
Emerald-Invincible W-Mo Skarn Deposit
The Emerald-Invincible W-Mo skarn deposit consists
of scheelite, wolframite, molybdenite, pyrrhotite, pyrite, and
chalcopyrite that generally occur as disseminations but locally
occur as massive lenses with pyrite and pyrrhotite with associated
gold (Ray and Webster, 1991; Dawson and others, 1991).
The deposit is hosted in the Early Cambrian Laib Formation
along the contact of the Reeves Member Limestone with the
Emerald Member argillite, as well as along the contact of the
limestone with the Cretaceous Emerald and Dodger Stocks.
The skarn consists of garnet, diopside, tourmaline, powellite,
calcite, biotite, and K-feldspar. Sericite alteration is predominant;
but kaolinite, tremolite, and silica alteration also occur.