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

elt occur over a distance of over 800 km. This belt is related to the epithermal and hydrothermal activity associated with the late-
magmatic stages of Tertiary and Quaternary hypabyssal plutonic and associated volcanic centers. These centers are along part of
the Aleutian arc, one of the classic continental-margin arcs along the rim of the Circum-Pacific rim.
Pyramid Porphyry Cu Deposit
The Pyramid porphyry Cu deposit (Armstong and others, 1976; Hollister, 1978; Wilson and Cox, 1983; G.L. Anderson,
written commun., 1984; R.L. Detterman, oral cornrnun., 1986) occurs in the southwestern Alaska Peninsula and consists of
disseminated molybdenite and chalcopyrite(?) in a Fe-stained dacite porphyry stock of late Tertiary age. A zonal alteration pattern
is defined by a core of secondary biotite, containing about 3 to 10% magnetite, which grades outward into an envelope of quartz-
sericite alteration. Fractures adjacent to the stock are filled with sericite. Local extensive oxidation and supergene enrichment by
chalcocite and covellite occur in a blanket as much as 100 m thick. The deposit is centered on a 3 km2 area within the stock, and
contains a resource of 110 million tonnes grading 0.4% Cu, 0.03% Mo, and trace of Au (G.L. Anderson, written commun., 1984).
The host stocks and dikes, and several smaller stocks which occur nearby all intrude the fine-grained clastic rocks of the Late
Cretaceous Hoodoo Formation, and the Paleocene or Eocene to Oligocene Stepovak(?) or Tolstoi(?) Formation. The sedimentary
rocks are contact metamorphosed adjacent to the stocks.
Bee Creek Porphyry Cu Deposit
The Bee Creek porphyry Cu deposit consists of chalcopyrite, pyrite, and traces of molybdenite in contact-metamorphosed
Jurassic and Cretaceous sedimentary rocks which are intruded by a small tonalite porphyry stock of Tertiary age (Cox and others,
198 1). The deposit, located on the Alaska Peninsula 25 km northeast of Chignik Lagoon, was discovered by Bear Creek Mining
Company in cooperation with Bristol Bay Native Corporation in the late 1970's (E.D. Fields, written cornrnun., 1977).The
contact-metamorphosed sedimentary rocks and the porphyry stock exhibit strong potassic (biotite) alteration, and quartz grains in
the porphyry contain abundant, high-salinity fluid-filled inclusions. The deposit is unexplored on the southwest and northeast
sides and probably contains a resource of between 5 and 30 million tonnes with average grade between 0.1 to 0.4 percent Cu. Five
drill holes totaling 2,359 m explore the deposit; one hole intercepted 150 m averaging 0.25% Cu.
Aleutian Arc
The Aleutian arc, which hosts the Alaska Peninsula and Aleutian lslands metallogenic belt, is composed of Oligocene
(post-30 Ma) to Holocene andesite to dacite flows, tuff, and intrusive and extrusive breccia; hypabyssal diode and quartz diorite
and small silicic stocks, dikes, and sills; and volcanic graywacke, shale, and lahars (Burk, 1965; Wilson and Cox, 1983; Wilson,
1985; Wilson and others, 1993; Vallier and others, 1994). To the northeast on the Alaska Peninsula, magmatism apparently started
in the late Miocene (about 15 Ma). An older Eocene and Oligocene succession of igneous rock that yields K-Ar ages ranging h m
40 to 30 Ma, is located in the northeastern Aleutian Islands and is defined as the Meshik arc by Wilson (1985).
Petrologic, chemical, and isotopic characteristics of the Aleutian arc (Miller and Richter, 1994) are as follow^. (1) The
arc consists predominantly of andesite and low-silica dacite; rhyolite occurs only in a few volcanic centers. (2) The axial part of
the central and northeastern parts of the arc is mostly calc-alkalic; SiOz contents typically range between 50 to 78 percent. (3)
Volcanic centers to the southwest are either tholeiitic or transitional to calc-alkalic. (4) The back arc contains alkalic volcanic
rocks which occur in widely separated centers. And (5) initial Sr ratios are relatively low, in the range of 0.70305 to 0.7046.
The lode deposits of Alaska Peninsula and Aleutian Islands metallogenic belt reflect the bedrock underlying tbe arc. The
northeastern part of the arc on the southwestern part of the Alaska Peninsula is underlain by Mesozoic bedrock of the Peninsular
terrane, part of the Wrangellia superterrane (Wilson and Cox, 1983; Nokleberg and others, 1994d). On the Alaska Peninsula, the
Peninsular terrane consists of a Late Triassic(?) and Early Jurassic sequence of volcaniclastic and volcanic rocks (Talkeetna
Formation), the Middle Jurassic part of the Alaska-Aleutian Range batholith (Reed and Lanphere, 1969, 1973), and younger
Cretaceous sedimentary rocks. In contrast, to the southwest, the Aleutian arc is interpreted to be underlain by oceanic crust
(Wilson and Cox, 1983; Vallier and others, 1994). The Pyramid Bee, Creek, Rex, and Warner Bay porphyry Cu deposits in the
central part of the metallogenic belt occupy a transitional zone between the parts of the magmatic arc underlain by oceanic crust to
the southwest and by continental crust to the northeast. Some of the deposits to the northeast are Mo-rich and contain anomalous
concentrations of Bi, Sn, and W which may be characteristic of continental-margin deposits (Wilson and Cox, 1983). K-Ar
isotopic studies indicate a variable time span of up to two million years between igneous activity and mineralization. Isotopic
studies also indicate sporadic occurrences of mineralization during a long period of igneous activity (Wilson and Cox, 1983).
Tectonic Setting for Alaska Peninsula and
Aleutian lslands Metallogenic Belt
The Aleutian arc, one of the classic igneous continental-margin arcs along the Circum-Pacific rim, occurs structurally
above the Aleutian megathrust, an active subduction zone which dips to the northwest and along which the Pacific Plerte is being
thrust under the southern margin of Alaska (fig. 126). The tectonic setting, and the field, petrologic, chemical, and isotopic data
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