USGS Professional Paper 1697 - Alaska Resources Library

278 Metallogenesis and Tectonics of the Russian Far East, Alaska, and the Canadian Cordillera
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 percent Cu.
Aleutian Arc
The Aleutian arc, which hosts the Alaska Peninsula and
Aleutian Islands metallogenic belt, is composed of Oligocene
(post 30 Ma) to Holocene andesite to dacite flows, tuff, and
intrusive and extrusive breccia; hypabyssal diorite 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 from 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 follows: (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 calcalkalic;
SiO 2 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 that occur in widely separated centers. (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 the
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 that may be characteristic of continental-margin
deposits (Wilson and Cox, 1983). K-Ar isotopic studies
indicate a variable time span of as much as 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
Islands Metallogenic Belt
The Aleutian arc, one of the classic igneous continentalmargin
arcs along the Circum-Pacific rim, occurs structurally
above the Aleutian megathrust, an active subduction zone that
dips to the northwest and along that the Pacific Plate is being
thrust under the southern margin of Alaska (fig. 126). The tectonic
setting and the field, petrologic, chemical, and isotopic
data summarized above are used by analogy to infer subduction-zone
settings for ancient belts of igneous rocks (Vallier
and others, 1994) and their associated granitoid-magmatism
metallogenic belts (Nokleberg and others, 1993).
Late Tertiary and Quaternary Metallogenic
Belts (4 to 0 Ma; figs. 125, 126)
Overview
The major late Tertiary and Quaternary metallogenic belts
in the Russian Northeast, Alaska, and the Canadian Cordillera
are summarized in table 3 and portrayed on figures 125
and 126. The major belts are as follows: (1) In the Russian
Southeast, the Sakhalin Island (SH) belt, which contains
Silica-carbonate, volcanic-hosted Hg deposits, is hosted in
shear zones and late Tertiary volcanic rock and is interpreted
as related to back-arc spreading behind the Kuril volcanic
arc. (2) In the Russian Northeast, the Kuril (KU) belt, which
contains granitic-magmatism-related deposits, is hosted in
the Kuril volcanic-plutonic belt and is interpreted as forming
during subduction-related granitic plutonism that formed the
Kuril Island part of Northeast Asia continental-margin arc. (3)
Also in the same region, several metallogenic belts continued
on from the early and middle Tertiary is the Olyutor (OT)
metallogenic belt, which is hosted in the East Kamchatka
Volcanic and Sedimentary Basin. (4) In southern Alaska, also
continuing on from the middle Tertiary, was the Alaska Peninsula
and Aleutian Islands (AP) belt, which contains graniticmagmatism-related
deposits and that is hosted in the Aleutian
volcanic belt. This metallogenic belt is interpreted as forming
during subduction-related granitic plutonism that formed the
Aleutian continental-margin arc. (5) In the southern Canadian
Cordillera, continuing on from the early and middle Tertiary,
was the Owl Creek (OC) belt, which contains granitic-magmatism-related
deposits and is hosted in the Cascade volcanicplutonic
belt. This metallogenic belt is interpreted during
subduction-related granitic plutonism that formed the Cascade
continental-margin arc. In the below descriptions of metallogenic
belts, a few of the noteable or signficant lode deposits
are described for each belt.