Metallogenesis and Tectonics of the Russian Far East, Alaska, and ...

Central-Southeastern Alaska Metallogenic Belt of
Porphyry Mo and Cu Deposits (Belt CSE)
Southeastern Alaska
The Central-Southeastern Alaska metallogenic belt of porphyry Mo and Cu deposits (fig. 103; tables 3,4) occurs along the
western coast of Southeastern Alaska and is hosted in (Nokleberg and others, 1995a): (1) a belt of late Oligocene and younger
Cenozoic granitoid stocks of the Glacier Bay magmatic belt; and (2) the Tkope-Portland Peninsula volcanic-plutonic belt. These
igneous belts intrude the Wrangellia superterrane 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, 1997~; 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, 197 1; Brew and others, 1978; Berg and others, 198 1;
Berg, 1984) consists of chalcopyrite, pyrite, arsenopyrite, sphalerite, molybdenite, and minor scheelite in shear-zone-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% 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, 197 1; Brew and others, 1978; Berg and others, 198 1;
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, skam. 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% MO and 0.16%
Cu; the less mineralized part of the stockwork contains 1 17.5 million tonnes grading 0.026% Mo and 0.18% 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. 1 19), which contains one of the world's largest concentrations of molybdenum
(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 eastem-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 Quark 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% MoS2, at an 0.08% MoSz cut-off, and an estimated 2 10 million tonnes grading 0.22% MoS,
(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-Southeastem 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 which are
dominantly unfoliated, metaluminous, and moderately peraluminous. K-Ar isotopic ages rangefrom 3 1 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