USGS Professional Paper 1697 - Alaska Resources Library

250 Metallogenesis and Tectonics of the Russian Far East, Alaska, and the Canadian Cordillera
rane in southeastern Alaska, and are part of the more extensive
Late Cretaceous and early Tertiary Coast-North Cascade plutonic
belt. This belt extends along the western and central parts
of the Canadian Cordillera for several thousand km (Nokleberg
and others, 1994c, 1997c; Monger and Nokleberg, 1996). The
significant deposits in the central-southeastern Alaska metallogenic
belt are porphyry Cu-Mo deposits at Margerie Glacier,
Nunatak, and Quartz Hill, a porphyry Mo deposit at Burroughs
Bay, and a Fe skarn deposit at North Bradfield Canal (table 4)
(Nokleberg and others 1997a,b, 1998).
Margerie Glacier Porphyry Cu Deposit
The Margerie Glacier porphyry Cu deposit (MacKevett
and others, 1971; Brew and others, 1978; Berg and others,
1981; Berg, 1984) consists of chalcopyrite, pyrite, arsenopyrite,
sphalerite, molybdenite, and minor scheelite in shearzone-hosted
quartz veins, as massive sulfide veins, and as
disseminations in a propylitically altered, Tertiary(?) porphyritic
granite stock and adjacent hornfels. The granite stock is
part of the Glacier Bay belt of middle Tertiary granitoid rocks,
which have isotopic ages of about 25 to 30 Ma (Brew 1994).
The stock intruded Permian(?) metamorphosed pelitic and
volcanic rocks and sparse marble of the Alexander sequence
of the Wrangellia superterrane. The deposit contains estimated
resources of 145 million tonnes grading 0.02 percent Cu, 0.27
g/t Au, and 4.5 g/t Ag (MacKevett and others, 1971; Brew and
others, 1978). Parts of the deposit are higher grade.
Nunatak Porphyry Cu-Mo Deposit
The Nunatak porphyry Cu-Mo deposit (MacKevett and
others, 1971; Brew and others, 1978; Berg and others, 1981;
Berg, 1984) occurs in northern southeastern Alaska, and consists
of numerous, closely spaced molybdenite-bearing quartz
veins and stockwork and minor disseminated molybdenite in
hornfels, skarn, and a mineralized fault zone around a Tertiary
granite porphyry stock. The granite porphyry stock has not yet
been isotopically dated, but is part of the Glacier Bay magmatic
belt of middle Tertiary granitoid rocks, which range in age
from about 25 to 30 Ma (Brew and Morrell, 1983; Brew, 1988).
Disseminated sulfides within the granite porphyry stock consist
of varying amounts of pyrite, pyrrhotite, chalcopyrite, and
sparse tetrahedrite and bornite. The most mineralized part of the
stockwork contains a resource of 2.03 million tonnes grading
0.067 percent Mo and 0.16 percent Cu; the less mineralized part
of the stockwork contains 117.5 million tonnes grading 0.026
percent Mo and 0.18 percent Cu (MacKevett and others, 1971;
Brew and others, 1978). Similar tonnage and grade material
may exist below sea level. The granite porphyry stock intrudes
tightly folded Paleozoic metasedimentary rocks of the Alexander
sequence of the Wrangellia superterrane.
Quartz Hill Porphyry Mo Deposit
The Quartz Hill porphyry Mo deposit (fig. 119), which
contains one of the world’s largest concentrations of molybde-
num (Hudson and others, 1979; P.R. Smith and J.E. Stephens,
written commun., 1985; Wolfe, 1995; Ashleman and others,
1997), occurs in the southern part of eastern-southeastern
Alaska. The deposit consists of a flat lying, tabular stockwork
of molybdenite-bearing, randomly oriented quartz veins and
fractures and also disseminated molybdenite, all of which are
distributed throughout the multiply-altered hypabyssal Quartz
Hill composite quartz monzonite stock, which crops out over
an area of several square kilometers. The Quartz Hill stock
is roughly ovoid in outcrop, is approximately 5 km long by 3
km wide (Brew and Ford, 1984a,b), and is part of the Tkope-
Portland Peninsula volcanic-plutonic belt. The stock is cut by
a progressively younger sequence of plugs and dikes consisting
of porphyritic quartz latite, igneous breccia, quartz monzonite,
quartz feldspar porphyry, and latite, which are abundant in the
core of the Quartz Hill stock. A K-Ar isotopic age of about 27
Ma was obtain for the stock (Ashleman and others, 1997).
The host rocks are paragneiss and plutonic rocks of the Coast
Plutonic Complex. The deposit contains estimated reserves of
1,600 million tonnes grading 0.127 percent MoS 2, at an 0.08
percent MoS 2 cut-off and an estimated 210 million tonnes
grading 0.22 percent MoS 2 (Wolfe, 1995).
Origin of and Tectonic Controls for Central-Southeastern
Alaska Metallogenic Belt
The Glacier Bay magmatic belt and the Tkope-Portland
Peninsula volcanic-plutonic belt, which constitute some of
the youngest, extensive igneous units in southeastern Alaska
(Brew and others, 1992, 1993), hosts the central-southeastern
Alaska metallogenic belt.The Oligocene Glacier Bay magmatic
belt, which hosts the Margerie Glacier porphyry Cu and
polymetallic vein deposit and the Nunatak porphyry Cu-Mo
deposit, consists of calc-alkalic, biotite granite, and alkali
granite plutons that are dominantly unfoliated, metaluminous,
and moderately peraluminous. K-Ar isotopic ages range from
31 to 42 Ma. The Tkope-Portland Peninsula volcanic-plutonic
belt of Oligocene age hosts the Quartz Hill porphyry molybdenum
deposit (Brew and Morrell, 1983; Brew, 1988). The
Tkope-Portland Peninsula volcanic-plutonic belt consists of
calc-alkalic and alkalic, locally peraluminous hornblende-biotite
granite and granite porphyry and lesser syenite and gabbro
(Brew, 1988). K-Ar, Rb-Sr, and fission track isotopic ages
range from 19 to 35 Ma. The belt exhibits strongly fractionated
REE patterns, large Europium anomalies, and initial Sr
ratios of 0.747 to 0.705 (Arth and others, 1986).
The Glacier Bay magmatic and Tkope-Portland Peninsula
volcanic-plutonic belts are part of the extensive Late
Cretaceous and early Tertiary Coast-North Cascade plutonic
belt that extends the length of the Canadian Cordillera and
into East-Central Alaska (fig. 103) (Rubin and others, 1991;
Gehrels and others, 1990; Wheeler and McFeeley, 1991;
van der Heyden, 1992; Woodsworth and others, 1992;
Journeay and Friedman, 1993). The Coast-North Cascade
plutonic belt forms the major part of the Coast continentalmargin
arc in the region.