USGS Professional Paper 1697 - Alaska Resources Library

226 Metallogenesis and Tectonics of the Russian Far East, Alaska, and the Canadian Cordillera
Seward Peninsula are interpreted as part of the eastern edge
of the Okhotsk-Chukotka volcanic-plutonic belt that hosts the
Seward Peninsula metallogenic belt and extends for 3,000 km
along the western margin of Sea of Okhotsk (Nokleberg and
others, 2000). The Seward Peninsula metallogenic belt and the
nearby Northwestern Koyukuk Basin metallogenic belt of felsic
plutonic U deposits, described below, are interpreted herein
as the eastern extension of the Eastern-Asian-Arctic metallogenic
belt in the Russian Far East (figs. 102, 103). The Seward
Peninsula metallogenic belt with Sn granite and related
deposits, is correlated with the Chaun zone of the Eastern Asia
metallogenic belt, which contains similar deposits to the west
in the Russian Northeast (fig. 102).
Northwestern Koyukuk Basin Metallogenic Belt
of Felsic Plutonic U and Manto-Replacement
(Polymetallic Pn-Zn, Au) Deposits (Belt NWK),
West-Central Alaska
The Northwestern Koyukuk Basin metallogenic belt of felsic
plutonic U and manto-replacement (polymetallic Pn-Zn, Au) deposits
(fig. 103; tables 3, 4) occurs in the Purcell district and adjacent
area in the northwestern Koyukuk Basin in west-central Alaska.
The metallogenic belt is hosted in the part of the region underlain
by the Late Cretaceous Hogatza plutonic belt (Miller, 1994). The
significant felsic plutonic U deposits in the belt are at Clear Creek,
Wheeler Creek, and Zane Hills (table 4) (Miller and Elliott, 1969;
Miller, 1976; Jones, 1977; Nokleberg and others 1997a,b, 1998).
These felsic plutonic U deposits and host plutonic rocks are interpreted
as the extreme northeastern end of the Okhotsk-Chukotka
volcanic-plutonic belt of the Russian Far East (Nokleberg and
others, 1994c, 1997c, 2000). Also occurring in the metallogenic
belt are a polymetallic (Au-Pb-Zn) vein and manto replacements(?)
in the Illinois Creek area that produced 957 kg Au from
1997 to 1999 and are isotopically dated at 111 Ma, about the same
age as the nearby Khotol pluton (Flanigan, 1998).
Wheeler Creek, Clear Creek, and Zane Hills Felsic
Plutonic U Deposits
The felsic plutonic U deposits at Wheeler Creek, Clear
Creek, and Zane Hills (Eakin and Forbes, 1976; Miller, 1976;
Jones, 1977; Miller and Elliott, 1977) are of two main types—
(1) uranothorite and gummite in quartz-rich veinlets in altered
Late Cretaceous alaskite, uraniferous nepheline syenite, and
bostonite dikes that cut Early Cretaceous andesite, and (2)
uranothorite, betafite, uraninite, thorite, and allanite in veinlets
in a foliated monzonite border phase that locally grades to
syenite. Grab samples contain as much as 0.027 percent U
Illinois Creek Manto-Replacement (Polymetallic Pn-Zn,
Au) Deposit
A structurally controlled, plutonic-related epigentic
deposit occurs at Illinois creek about 70 Km south of Galena
(Flanigan, 1998; Bundtzen and others, 2000). The Illinois
Creek deposit is a supergene oxidized deposit that occurs in
an east-trending, moderately dipping shear zone. The deposit
has anomalous to ore-grade Au, Bi, AG, Cu, Pb, and Zn. The
deposit contains supergene pyrolucite, limonite, geothite,
and hemetite; sulfides are rare. Ar/Ar isotopic age, petrologic,
and microprobe studies indicate that the Illinois Creek
deposit is related to emplacement of the syncollisional, 111.3
Ma Khotol Mountain granite (Flanigan, 1998). The mine
produced 957 kg gold from 1.22 million tonnes of ore prior
to 1999 bankruptcy.
Origin of and Tectonic Controls for Northwestern Koyukuk
Basin Metallogenic Belt
The calc-alkaline granitoid rocks that host the Northwestern
Koyukuk Basin metallogenic belt extend about 300
km from Hughes on the Koyukuk River westward to near the
Seward Peninsula. The granitoid plutonic rocks are mainly
granodiorite and lesser tonalite and high-silica granite. The
granites intrude a sequence of andesitic flows, tuffs, breccia,
agglomerate, conglomerate, tuffaceous graywacke, and
mudstone containing local intercalations of Early Cretaceous
limestone that together form part of the Koyukuk island-arc
terrane (Patton and others, 1994). These plutons are interpreted
as forming in a subduction-related, continental-margin
arc on the basis of a calc-alkaline composition, high sodium
content, relatively and locally abundant mafic xenoliths, and
low initial Sr ratios (Miller, 1994). The Northwestern Koyukuk
Basin metallogenic belt and the nearby Seward Peninsula
metallogenic belt, described above, are herein interpreted as
the eastern extension of the Eastern-Asian-Arctic metallogenic
belt in the Russian Far East (figs. 102, 103). This major metallogenic
belt, as described above, extends west and southwest
for 3,000 km along western margin of Sea of Okhotsk.
West-Central Alaska Metallogenic Belt of
Porphyry Cu-Au Deposits (Belt WCA), West-
Central Alaska
The West-Central Alaska (Hogatza) metallogenic belt
of porphyry Cu-Au deposits (fig. 103; tables 3, 4) (Hollister,
1978; Nokleberg and others, 1995a) is hosted in a suite of Late
Cretaceous quartz monzonite plutons of the Hogatza plutonic
belt that intrude oceanic crust of the Yukon-Koyukuk islandarc
terrane and overlapping Cretaceous flysch (Nokleberg and
others, 1995a). Most of the older intrusions of the Hogatza
plutonic belt exhibit K-Ar ages generally from 100 to 120 Ma
and are undersaturated, alkalic igneous complexes that tend
to be barren of porphyry deposits. The significant deposits in
the belt are at Indian Mountain, Purcell Mountain, and Zane
Hills (table 4) (Nokleberg and others 1997a,b, 1998). All
these deposits are hosted in oval-shaped, epizonal, forcefully
injected plutons with K-Ar isotopic mineral ages of 80 to 82
Ma (Miller and others, 1966).