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

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GT Gatmet-n LDGT Cdocrase-gamel skarn
l:-:::::.::j . . Colluvium (Holocene)
Wrangell Lava (Quaternary)
E 7
N.abesna Stock (mid-Cretaceous}
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Argillite
Thin-bedded limestone
metamor hased
to cat-siicate homte~s
Massive limestme
metamorpkmd to marble
- Algae markex bed
GT-PYX &m4pyron3ne &an MT-SEW MaqwMewpentine skam
PYX P y r o m m MT f&lgndite Mes
SUL StMde ore
Massive dolostone
metamorphased to marble I
Late
- Triassk
Figure 56. Nabesna Fe-Au skarn mine, Eastern-Southern Alaska metallogenic belt, Southern Alaska. Schemattc cross section
showing suffide-magnetitwkam relations. Magnetite-rich ore replaces dolomite. Gold-rich ores form small marble fbnt
replacements. The chief ore mineral is chalcopyrite in garnet-pyroxene skarn. Adapted from Wayland (1953). Weglart (1991), and
Newberry and others (1997).
Pebble Copper Porphyry Au-Cu Deposit
The Pebble Copper porphyry Au-Cu deposit occurs in the western part of southern Alaska (Phil St. George, Mitten
comun., 1991; Bouley and others, 1995; Young and others, 1997). The deposit consists of disseminated chalcopyrite, pyrite, and
molybdenum, accompanied by minor ta trace galena, sphalerite, and arsenopyrite, in a stockwork vein system. The deposit
I contains an inferred reserve of 430 million tonnes grading 0.35% Cu, 0.4 g/t Au, and 0.015% Mo. Recent data indicates 1.0 billion
tonnes grading 0.6 1 % Cu equivalent, or 0.4% Cu and 0.30 g/t Au, 0.0 15% Mo (Northern Dynasty news release, September 25,
2003). The deposit is hosted in a mid-Cretaceous granodiorite porphyry and its adja~ent hornfels aureole. TBe sulfide mine&
! formed during a late-stage intense hydrofracruring which followed potassic, silicic, and sericitic alteration events. Tomaline
brsccias also exist locally. K-Ar ages for hydrothermal sericite and peous K-feldspar are 90 and 97 Ma, respectively. The
granodiorite hosting the deposit is part of a lager, composite 40 krn volcanic-plutonic complex wbich also includes pyroxenite,
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alkali gabbro, and granite, and overlying dacite volcanic rocks. The volcanic and plutonic rocks are alkalic-calcic &nd qua alkalic in composition. The pandorite porphyry intrudes the Late Jurassic and Early Cretaceous Kahilrna overlap ssambkge,
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and is overlain by Tertiary volcanic rocks.
9range Hill and Bond Cmek Porphyry
h$a E u-Mo Deposits
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I The Orange Hill a d Bond Creek porphyry Cu-Mo deposits (fig. 57) (Van Alstine and Black, 1946; Richter and ofiers,
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1975a, b; Nokleberg and others, 1995a) occur in the northern Wrangell Mountains. The deposits consist of pyrite, chalcopyrite,
and minor rnolybdenite which occur in quartz veins which contain K-feldspar and sericite, and as disseminations in the Cretaceous
Nabesna plutoa. The pluton, wbich has K-Ar isotopic ages of 1 12-1 14 Ma, forms a complex intrusion of pdorite wid quti.tTz
dorite intruded by granite porphyry. The deposits exhibit abundant biotite-quartz, quartz-sericite, and chlorite-sericite-epidote
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