USGS Professional Paper 1697 - Alaska Resources Library

90 Metallogenesis and Tectonics of the Russian Far East, Alaska, and the Canadian Cordillera
Metallogenic Belts Formed in Middle Mesozoic
Stikinia-Quesnellia Island Arc
Galore Creek Metallogenic Belt of Porphyry Cu-
Au Deposits (Belt GL) Northern British Columbia
The Galore Creek metallogenic belt of porphyry Cu-Au
deposits (fig. 32; tables 3, 4) occurs in northern British Columbia
and is hosted in alkaline granitoid plutons that are coeval and
comagmatic with volcanic, volcaniclastic and subordinate sedimentary
rocks of the Late Triassic Stuhini Group of the northern
Stikinia terrane. The significant deposits are the Galore Creek,
Gnat Lake, and Red Chris porphyry Cu, porphyry Cu-Au, and Cu-
Au skarn deposits (table 4) (Nokleberg and others 1997a,b, 1998).
Galore Creek Alkalic Porphyry Cu-Au Deposit
The Galore Creek alkalic porphyry Cu-Au deposit consists
of chalcopyrite, pyrite, bornite and magnetite that occur as disseminations,
skarns, coarse replacements, and fracture fillings
in syenitic porphyry and breccias and the Late Triassic Stuhini
Group metasedimentary and metavolcanic rocks (Allen and
others, 1976; EMR Canada, 1989; Dawson and others, 1991;
Mining Review, 1992, J. Mortensen, written commun., 1993;
Enns and others, 1995). The deposit contains estimated reserves
of 125 million tonnes grading 1.06 percent Cu, 7.7 g/t Ag, and
0.4 g/t Au. Approximately 80 percent of the deposit consists of
altered skarn and replacements along contacts between syenite
intrusives and Triassic volcanic and sedimentary rock. A U-Pb
zircon isotopic age of 210 Ma is reported for the intramineral
syenite porphyry. The porphyry is typical of undersaturated,
feldspathoid-bearing subclass of alkalic porphyry deposits.
The Galore Creek deposit is hosted in volcanic rocks of
the Stuhini Formation. The rocks are dominantly augite and
plagioclase phyric basalt and andesite flows, fragmental rocks,
and less abundant feldspathoid-bearing flows. Prograde Cu
skarns, which occurs in calcareous pyroclastic, volcaniclastic,
and shoshonite volcanic rocks adjacent to contacts of pseudoleucite-phyric
syenite dikes, constitute about 80 percent of ore
reserves in the Central Zone. Potassic alteration assemblages
of orthoclase and biotite, which occur in or adjacent to potassic
host rocks, are replaced by a calcic skarn assemblage of
zoned andradite, diopside, Fe-rich epidote, and vesuvianite
along with chalcopyrite, bornite, pyrite, and magnetite, and
minor chalcocite, sphalerite, and galena. Retrograde assemblages
consist of anhydrite, chlorite, sericite, calcite, gypsum,
and fluorite (Dawson and Kirkham, 1996).
The Southwest Zone consists of disseminated chalcopyrite
in a late-stage, diatreme breccia and adjacent orthoclase porphyry.
The North Junction and other satellite deposits consist of disseminated
chalcopyrite and bornite in volcanic rocks. Reserves calculated
in 1992 are Central Zone—233.9 million tonnes grading
0.67 percent Cu, 0.35 g/t Au, and 7 g/t Ag; Southwest Zone—42.4
million tonnes grading 0.55 percent Cu, 1.03 g/t Au, and 7 g/t Ag;
and North Junction—7.7 million tonnes grading 1.5 percent Cu.
Red Chris Porphyry Cu-Au Deposit
The Red Chris alkalic porphyry Cu-Au deposit consists
of a stockwork and a set of sheeted-veins containing quartz,
pyrite, chalcopyrite, and increasing bornite at depth. The
deposit is hosted in an elongate porphyritic monzonite stock
emplaced within the Late Triassic alkaline volcanic and volcaniclastic
rocks of either the Stuhini, Group (EMR Canada,
1989; American Bullion Minerals Ltd., news release, Jan.
1995; Newell and Peatfield, 1995; Ash and others, 1997). The
monzonite exhibits an isotopic age of 203+1.3 Ma (Friedman
and Ash, 1997). An early K-Na alteration stage of orthoclasealbite-biotite
with variable quartz-sericite was succeeded by
pervasive quartz-ankerite-sericite-pyrite alteration. Pyrite
occurs as a halo to the steeply dipping deposit, which is both
controlled and offset by east-northeast trending subvertical
faults. The deposit contains estimated resources of 320 million
tonnes grading 0.38 percent Cu and 0.30 g/t Au (American
Bullion Minerals, news release, Jan., 1995). A minimum K-
Ar isotopic age of mineralization of 195 Ma (Early Jurassic)
is obtained from a post-mineral dike (Newell and Peatfield,
1995). Subvolcanic complexes similar to the Red Chris stock
occur at the Rose and Groat Creek porphyry Cu-Au prospects
that are located 10 km northwest, and 25 km southwest of Red
Chris, respectively (Newell and Peatfield, 1995). Porphyry
Cu-Au prospects at June and Stikine in the Gnat Lake area
are hosted in quartz monzonite and granodiorite phases of the
Hotailuh Batholith (Panteleyev, 1977).
Origin of and Tectonic Controls for Galore Creek
Metallogenic Belt
The Galore Creek metallogenic belt of porphyry Cu-Au
deposits is hosted in and adjacent to a Late Triassic (210 Ma)
center of alkaline volcanism, contemporaneous with multiphase
intrusion and magmatic-hydrothermal activity, and late
diatreme breccias that together probably contributed to the high
Cu and Au contents at Galore Creek relative to other alkaline
porphyry systems as at Cat Face in British Columbia (Enns and
others, 1995). The Galore Creek metallogenic belt is part of
the subduction related Stikinia island arc (Monger and Nokleberg,
1996; Nokleberg and others, 2000). Isotopic ages indicate
intrusion of host granitoid plutons and formation of associated
mineral deposits from the Late Triassic to the Early Jurassic.
This age represents the main and final part of subductionrelated
igneous building of the Stikinia island arc, just before
accretion of the Stikinia terrane, along the with tectonically
related Quesnellia island arc and Cache Creek subduction-zone
terranes, onto the North American Craton Margin (Monger and
Nokleberg, 1996; Nokleberg and others, 2000).
Sustut Metallogenic Belt of Basaltic Cu Deposits
(Belt SU), Northern British Columbia
The Sustut metallogenic belt of basaltic Cu deposits,
which occurs in northern British Columbia, is hosted in frag-