USGS Professional Paper 1697 - Alaska Resources Library

96 Metallogenesis and Tectonics of the Russian Far East, Alaska, and the Canadian Cordillera
is hosted in calc-silicate skarn associated with contact metamorphism
of limestone of the eastern sedimentary facies of
the Late Triassic Nicola Assemblage adjacent to the Early
Jurassic Hedley diorite and gabbro intrusives. Production
1904 to 1991 was 8.43 million tonnes of ore, from which
was extracted 62.68 tonnes Au and 14.74 tonnes Ag from the
Nickel Plate, Mascot, French, Goodhope and Canty parts of
the deposit. Remaining reserves are 5.07 million tonnes grading
3.0 g/t Au, 2.5 g/t Ag, and 0.1 percent Cu. The ore bodies
are semiconformable, tabular sulfide zones developed near the
skarn-marble boundary, where alternating layers of garnet-rich
and diopside-hedenbergite-rich prograde skarn follow bedding.
Au together with anomalous amounts of Ag, Bi, Te, and
Co are concentrated with arsenopyrite, pyrrhotite, and pyrite
in the latest stage, a retrograde quartz-calcite-epidote-sulfide
assemblage (Ettlinger and others, 1992). The deposit age is
interpreted as Early Jurassic.
The intrusions associated with Au skarns in the Canadian
Cordillera constitute a distinctive suite of calc-alkaline to alkaline
plutons of synorogenic to late orogenic timing. At Hedley,
intrusions are enriched in Fe, depleted in total alkalies and
silica, and have low ferric/ferrous iron ratios, that is reduced
relative to other types of skarn deposits (Ray and Webster,
1991; Dawson, 1996b).
Origin of and Tectonic Controls for Guichon
Metallogenic Belt
The Guichon metallogenic belt of porphyry Cu-Mo deposits
is hosted in and adjacent to Late Triassic to Early Jurassic
plutonic rocks of the Quesnellia island arc (Monger and
Nokleberg, 1996; Nokleberg and others, 2000). The calc-alkaline
Guichon Creek batholith, which hosts the porphyry Cu-Mo
deposits in the Highland Valley district, was emplaced at about
210 Ma, approximately contemporaneous with the intrusion
of the composite alkaline Iron Mask and Copper Mountain
plutons, 40 and 120 km away to the northeast and southeast,
respectively (McMillan and others, 1995). About 100 km to the
south, the Alaskan-type, ultramafic-mafic Tulameen Complex
was emplaced. The Quesnellia island-arc is interpreted as forming
on the deformed continental-margin strata of Yukon-Tanana
terrane, as a rifted fragment of the North American Craton
Margin (Gehrels and others, 1990; Monger and Nokleberg,
1996; Nokleberg and others, 1994c, 1997c, 2000).
Texas Creek Metallogenic Belt of Porphyry Cu-
Mo-Au, Au-Ag Polymetallic Vein and Au Quartz
Vein Deposits (Belt TC), Northern British Columbia
The Texas Creek composite metallogenic belt of porphyry
Cu-Mo-Au, Au-Ag polymetallic vein, and Au quartz vein
deposits (fig. 32; tables 3, 4) occurs in northern British Columbia
and is hosted in Late Triassic to Middle Jurassic granitoid
plutons that intrude coeval marine and subaerial mafic to felsic
volcanic, volcaniclastic, and sedimentary rocks of the Stuhini
and Hazelton Groups in the Stikinia island-arc terrane. The
major porphyry Cu-Mo-Au deposits are at Schaft Creek (Liard
Copper), Kerr (Main Zone), and Sulphurets (Gold Zone) (table
4) (Nokleberg and others 1997a,b, 1998). The major polymetallic
vein deposits are at Brucejack Lake (West Zone, Shore
Zone), Snip (Shan), Red Mountain, Silbak-Premier (Premier
Gold), and Snowfields. Other significant deposits are the
Polaris-Taku (Whitewater) and Muddy Lake (Golden Bear,
Totem) Au quartz vein deposits, and the E & L gabbroic Ni-Cu
deposit at Snippaker Creek.
Texas Creek District Porphyry Cu-Mo-Au Deposits
The Texas Creek district contains significant porphyry
Cu-Mo and polymetallic vein deposits. The large Schaft Creek
(Liard Copper) porphyry Cu-Mo deposit is hosted mainly in
Triassic andesite that is intruded by diorite and granodiorite of
the Middle Jurassic Hickman batholith. The deposit consists
of a quartz-vein stockwork with potassic alteration in a lowgrade
core, an intermediate zone of bornite, chalcopyrite,
and molybdenite associated with chlorite-sericite alteration,
which contains the bulk of the ore, and epidote in the periphery
(EMR Canada, 1989; Spilsbury, 1995; MINFILE, 2002).
Estimated reserves are 971.5 million tonnes grading 0.298
percent Cu, 0.033 percent MoS 2, 1.20 g/t Ag, and 0.14 g/t Au.
The Hickman batholith has an isotopic age of 182 Ma.
The Kerr and Sulphurets porphyry Cu-Au deposits (Ditson
and others, 1995; Fowler and Wells, 1995; Kirkham and Margolis,
1995) occur in intermediate volcanic rocks, volcaniclastic,
and sedimentary rocks of the Early Jurassic Unuk River formation
of the Hazelton Assemblage. The Kerr deposit consists of
an elongate shear zone, about 2 km long by 900 m wide. The
deposit contains estimated reserves of 134.9 million tonnes
grading 0.76 percent Cu and 0.34 g/t Au (Ditson and others,
1995). The Sulphurets deposit consists of a 1.5 km northeasttrending
halo that surrounds the Kerr (Main Zone) deposit and
consists of a quartz-pyrite-sericite halo and associated stockwork
of chalcopyrite and bornite surrounding the main copper
deposit (Fowler and Wells, 1995). Estimated reserves are 18.2
million tonnes grading 0.82 g/t Au and 0.35 percent Cu.
The Snip (Au), Red Mountain (Au- Cu), Snowfields
Au-Ag, and Brucejack Lake Au-Ag polymetallic vein deposits
are hosted in clastic, volcaniclastic, and volcanic rocks of
the Early Jurassic Hazelton Assemblage and adjacent plutons
of the Texas Creek plutonic suite. The Snip deposit, the only
mine in the group, consists of a shear-vein system with high
Au values that crosscuts graywacke and siltstone adjacent to a
contact with a porphyritic quartz monzonite stock. Estimated
combined production and reserves are 1.9 million tonnes
grading 29.5 g/t Au (Rhys and Godwin, 1992; Rhys and others,
1995). The Red Mountain deposit (Brown and Kahlert,
1995) consists of a semitabular stockwork of pyrite-pyrrhotite
that contains high Au and Ag values (as much as 20 g/t Au),
disseminated sphalerite-pyrrhotite mineralization, and intense
sericite alteration. The deposit occurs above a quartz-molybdenite
stockwork at the top of a Early Jurassic monzodiorite