USGS Professional Paper 1697 - Alaska Resources Library

228 Metallogenesis and Tectonics of the Russian Far East, Alaska, and the Canadian Cordillera
The significant deposits in the belt (Nokleberg and others
1995, 1996, 1997a, 1998; Bundtzen and Miller, 1997)
include (1) the McLeod, and Molybdenum Mountain porphyry
Mo prospects, (2) the important Donlin Creek and Vinasale
Mountain porphyry Au-polymetallic deposits, (3) the Chicken
Mountain, Von Frank, and Golden Horn porphyry Cu-Au prospects,
(4) Au-polymetallic vein and replacement prospects at
Fortyseven Creek south of Sleetmute, (5) the Arnold prospect
near Marshall, and the Mission Creek Owhat and Headwall
prospects in the Russian Mountains, (6) Cu-Au-Bi skarn deposits
in the Nixon Fork area, (7) a small Fe skarn occurrence
at Medfra, (8) the Bismarck Creek, Win, and Won Sn-W-Ag
polymetallic, greisen, vein, and skarn deposits, (9) epithermal
Hg-Sb-Au vein and hots spring deposits at Red Devil, Kaiyah,
Kagati Lake, Kolmakof, Snow Gulch-Donlin, Cinnabar Creek,
DeCourcey, and Gemuk Mountain, and (10) felsic-plutonic U
prospects at Wolf Creek Mountain and Sichu Creek.
The lode deposits occur in veins, stockworks, breccia pipes,
skarns and replacement deposits, which formed in upper mesothermal
to epithermal environments (Bundtzen and Miller, 1991;
1997; Gray and others, 1997). A plausible metallogenic model
suggests that most of these deposits are similar vertically-zoned
hydrothermal systems that are exposed at various erosional
levels within the late Cretaceous and early Tertiary igneous
complexes and wall rocks. Selected deposits and environments
include (1) the Cirque, Tolstoi, Headwall, Mission Creek, Win,
Won, Bismarck Creek greisen Sn-Cu-Ag-As deposits (formed in
upper mesothermal environment), (2) the Golden Horn, Chicken
Mountain, and Von Frank porphyry Cu-Au deposits (formed
in lower mesothermal environment), and (3) the Red Devil,
DeCoursey, Donlin, and Mountain Top Hg-Sb (Au) deposits
(epithermal deposits). The important Donlin Creek and Vinasale
gold-polymetallic deposits resemble porphyry Au deposits
(Wilson and Keyser, 1988; Hollister, 1992) but lack metallic,
fluid inclusion and wallrock alteration more typical of porphyry
Au systems (Bundtzen and Miller, 1997; Ebert and others, 2000).
Unusual, U-rich, REE deposits occur in ultra-potassic felsic
igneous rocks at Sischu Mountain and Wolf Creek Mountain
(Patton and others, 1994; Bundtzen, 1998). The various Aupolymetallic
and other mineral deposits in the are interpreted as
forming in a distinctly higher structural level than the deeper,
mesothermal systems in the East-Central Alaska metallogenic
belt described below. Selected examples of important deposits in
the Kuskokwim Mineral Belt are described below.
The Southwestern Kuskokwim Mountians metallogenic
belt is hosted in the Kuskokwim Mountains sedimentary
and volcanic belt that consists of three major types of igneous
complexes—(1) calc-alkalic, metaaluminous granite and
quartz monzonite of Early and mid-Cretaceous age (about 109
to 98 Ma), (2) peraluminous to metaaluminous, alkali-calcic
to quartz alkalic, volcanic-plutonic complexes that contain
plutons ranging in composition from alkali gabbro to quartz
syenite and have an average age of about 70 Ma, and (3)
peraluminous granite porphyry sills and dikes that range in age
from 58 to 70 Ma. The various units of the Kuskokwim Mountains
sedimentary and volcanic belt intrude and overlie several
major bedrock units, including the Proterozoic to Paleozoic
Ruby terrane, the Paleozoic Nixon Fork terrane, the Late Paleozoic-Mesozoic
Innoko terrane, the Triassic to Early Cretaceous
Gemuk Group, the Ordovician to Jurassic Goodnews terrane,
the Late Cretaceous Kuskokwim Group, and other basement
rocks (Moll-Stalcup, 1994; Bundtzen and Miller, 1997).
Kuskokwim Mountains Sedimentary and Volcanic Belt
The Kuskokwim Mountains sedimentary and volcanic
belt, which hosts the Kuskokwim metallogenic belt, consists of
interlayered volcanic and sedimentary rocks that are intruded by
coeval plutonic rocks. Geochemical and isotopic studies of the
igneous rocks of the belt reveal two types of igneous complexes
(Bundtzen and Miller, 1997)—(1) peraluminous to metaluminous,
alkali-calcic to quartz alkalic and lesser calc-alkalic,
volcanic-plutonic complexes composed of plutons ranging in
composition from alkali gabbro, quartz diorite, granodiorite, monzonite,
to syenite, and (2) peraluminous granite porphyry sills and
dikes. These two suites exhibit K-Ar crystallization ages ranging
from 75 to 60 Ma (Late Cretaceous and early Tertiary).
The volcanic suite consists chiefly of rhyolite and dacite
domes, flows, and tuff, and dacite, andesite, and basalt flows that
exhibit K-Ar isotopic ages of 58 to 77 Ma (Bundtzen and Gilbert,
1983; Miller and Bundtzen, 1994; Moll-Stalcup, 1994; Moll-Stalcup
and others, 1994). They display moderate-K, calc-alkalic to
shoshonitic compositional trends with andesite and rhyolite being
most common. REE patterns are variable; initial Sr ratios vary
from 0.704 to 0.708; and trace element studies suggest assimilation
of small amounts of continental crust of the metamorphosed
continental-margin Ruby terrane (Miller, 1994; Moll-Stalcup,
1994; Moll-Stalcup and others, 1994). An unusually large range
of incompatible elements occur in the igneous rocks and their
significance is not well understood (Moll-Stalcup, 1994).
Sedimentary rocks underlying the Kuskokwim Mountains
igneous belt include the mid- and Late Cretaceous Kuskokwim
Group (Cady and others, 1955) and Cretaceous flysch of the
Yukon-Koykuk Basin (Patton and others, 1994). The Kuskokwim
Group consists mainly of conglomerate to coarse-grained
sandstone turbidites deposited in deep-marine conditions and
lesser sandstones and conglomerates deposited in shallow-marine
to nonmarine conditions. The Kuskokwim Mountains igneous
belt and the Kuskokwim Group are generally mildly folded and
faulted and, to the south, are interpreted to overlie the Kahiltna
sedimentary and volcanic assemblage (fig. 103). To the east, the
igneous belt locally overlies the Wrangellia superterrane. Bundtzen
and Miller (1997) and Patton and others (1994) provide more
detailed geologic frameworks for both of these areas.
Origin of and Tectonic Setting for Kuskokwim Mountains
Metallogenic Belt
The Kuskokwim Mountains metallogenic belt is hosted
by the Kuskokwim Mountains sedimentary and volcanic belt,
which is interpreted as the back-arc part of the extensive, subduction-related
Kluane igneous arc that occurred along the Late
Cretaceous and early Tertiary continental margin of southern