USGS Professional Paper 1697 - Alaska Resources Library

mental volcanic rocks of intermediate composition and interlayered
sedimentary rocks of the Late Triassic Takla Group
in the northeastern part of the Stikinia island-arc terrane (fig.
32; tables 3, 4) (Nokleberg and others, 1997b, 1998). A more
extensive but less coherent belt could be defined to the south
and west that would include minor volcanic redbed Cu occurrences
in the overlying Hazelton Assemblage and the Nicola
Assemblage in southern Quesnellia island-arc terrane. In each
case, the deposits are located within emergent, subaerial parts
of island-arc terranes. The significant deposit is at Sustut.
Sustut Basaltic Cu Deposit
The Sustut basaltic Cu deposit consists of a stratabound
assemblage of hematite, pyrite, chalcocite, bornite, chalcopyrite,
and native copper that occurs as disseminations and as blebs
and grains in the matrix of sandstone, conglomerate, tuff breccia
and lahar of the Late Triassic Takla Group (EMR Canada, 1989;
Dawson and others, 1991). The deposit is a large concordant
body that is strongly zoned inward from an outer zone of pyrite,
chalcopyrite, and bornite into a core of chalcocite, native copper
and hematite. The zonation is interpreted as reflecting the
migration of ore fluids along permeable aquifers. The host rocks
are sandstone, conglomerate, lahar, and red/green or grey tuff
breccia of subaerial origin. Estimated resources are 21 million
tonnes grading 1.11 percent Cu (Kirkham, 1996b; Harper,
1977; Mining Review, summer 2000). The grade increases in
finer grained units. Pyrite forms an incomplete envelope around
Cu-bearing lenses, and hematite is ubiquitous. The deposit age
is interpreted as Late Triassic. The Northstar deposit to the south
of Sustut is a faulted block of lower-grade, chalcocite-bearing
sedimentary rocks that are apparently interlayered within volcanic
flows of the Takla Group (Sutherland Brown, 1968).
Origin of and Tectonic Controls for Sustut Metallogenic Belt
The origin of basaltic Cu deposits hosted in volcanic rocks
is interpreted as analogous to that for diagenetic sedimentary
Cu deposits in sedimentary sequences. However, the common
presence of low-grade metamorphic minerals may also supports a
metamorphic origin (Kirkham, 1996b). In the Sustut metallogenic
belt, the deposits are interpreted as forming in the upper oxidized
parts of volcanic piles during shallow burial metamorphism and
diagenesis (Kirkham, 1996b) that was coeval with Late Triassic
island-arc volcanism in the Stikinia and Quesnellia terranes.
Copper Mountain (North) Metallogenic Belt of
Porphyry Cu-Au Deposits (Belt CMN), Northern
British Columbia
The Copper Mountain (North) metallogenic belt of porphyry
Cu-Mo-Au deposits (fig. 32; tables 3, 4) occurs in northern
British Columbia and is hosted in granitoid plutonic rocks
of the mainly in intermediate-composition granitoid plutons in
the Copper Mountain suite in the Quesnellia island-arc terrane.
Most plutons in the suite are small, equant stocks with diam-
Late Triassic Metallogenic Belts (230 to 208 Ma; fig. 32) 91
eters as much as a few kilometers. The significant deposits are
the Lorraine and Mount Mulligan porphyry Cu-Au deposits
(table 4) (Nokleberg and others 1997a,b, 1998).
Lorraine Porphyry Cu-Au Deposit
The Lorraine porphyry Cu-Au deposit consists of two
fault-bounded zones of chalcopyrite, bornite, and magnetite
that occur as disseminations in the 30- by 5-km-wide Middle
Jurassic Duckling Creek Syenite Complex, which is part of the
largest pluton in the Hogem Batholith of the alkaline Copper
Mountain Suite (EMR Canada, 1989; Dawson and others,
1991; Woodsworth and others, 1991; Bishop and others, 1995;
MINFILE, 2002). The sulfides are dominantly disseminated,
but also occur in veins. In the Lower Zone, sulfides occur in
mafic-rich lenses and are zoned from chalcopyrite and pyrite at
the rim through chalcopyrite with minor bornite to bornite with
minor chalcopyrite at the core. Magnetite is common in veinlets
and as an accessory mineral. The deposit contains an estimated
resource of 9.1 million tonnes grading 0.70 percent Cu and
0.27 g/t Au (MINFILE, 2002). An Upper Zone is similar but is
highly oxidized (Garnett, 1978). The Cu-Au deposit exhibits
characteristics of both hydrothermal and magmatic origins and
is related to orthomagmatic-hydrothermal fluid flow contemporaneous
with magmatism and development of migmatitic fabrics
(Bishop and others, 1995). Cu minerals are associated with
elevated intensity of biotite, chlorite, potassium feldspar, and
sericite alteration. A K-Ar isotopic age of 175+5 Ma (Middle
Jurassic) for the syenite at Lorrine is interpreted as a reset age; a
U-Pb zircon age is about 181 Ma (Bishop and others, 1995).
Mount Milligan Porphyry Cu-Au Deposit
The Mount Milligan Porphyry Cu-Au deposit consists
of pyrite, chalcopyrite, bornite, and magnetite that occur
as disseminations and in quartz veinlets (Delong and others,
1991; McMillan, 1991; Nelson and others, 1991; Barrie,
1993; Sketchley and others, 1995). The deposit has estimated
reserves of 298.4 million tonnes grading 0.22 percent Cu and
0.45 g/t Au. The deposit is hosted in augite porphyritic andesite
of the Witch Lake (informal) formation of the Late Triassic
to Early Jurassic Takla Group that is intruded by several small
brecciated diorite and monzonite porphyry dikes and stocks.
Cu-Au mineralization in the Main deposit accompanied the
emplacement of the MBX stock and Rainbow dyke; the
Southern Star deposit surrounds the stock of the same name.
A U-Pb zircon isotopic age of 183±1 Ma is obtained for the
Southern Star monzonite. Cu and Au minerals are associated
with moderate to intense potassic alteration around intrusive
contacts. Potassic alteration, which is ubiquitous in mineralized
stocks and surrounding volcanic rocks, is surrounded
by propylitic alteration that decreases in intensity outward
from intrusive. A well-developed mineral zoning consists of
a biotite-rich core in the potassic zone that contains most of
the Cu and Au. Numerous polymetallic veins are hosted by
the propyliyic alteration zone immediately beyond the limits
of the porphyry deposit.