USGS Professional Paper 1697 - Alaska Resources Library

126 Metallogenesis and Tectonics of the Russian Far East, Alaska, and the Canadian Cordillera
1985; Nokleberg and others, 1994c, 1997c)—(1) a lower
ophiolite sequence at the southwestern end of the terrane that
consists of Late Triassic midocean-ridge pillow basalt, diabase,
gabbro, and ultramafic rocks; and (2) a coherent upper
sequence of Early Jurassic to Early Cretaceous marine volcaniclastic
sandstone, conglomerate, shale, tuffaceous chert, minor
argillaceous limestone, and marine to nonmarine andesite and
basalt flows, flow breccia, and tuff. The Togiak island arc is
interpreted as forming in the Jurassic and Early Cretaceous and
is tectonically linked to the Goodnews subduction-zone terrane
(Box and Patton, 1989; Decker and others, 1994; Plafker and
Berg, 1994; Nokleberg and others, 2000).
Yukon River Metallogenic Belt of Podiform Cr
Deposits (Belt YR), West-Central Alaska
The Yukon River metallogenic belt of podiform Cr
deposits (fig. 49; tables 3, 4) occurs along the southern flank of
the Yukon-Koyukuk Basin in west-central Alaska (Foley and
others, 1982, 1997). As in the Southern Brooks Range metallogenic
belt of podiform Cr deposits to the north, the metallogenic
belt is hosted in the upper structural level of the Angayucham
oceanic and subduction-zone terrane, which is interpreted as
the basal part of the Koyukuk island arc (Nokleberg and others,
1994c, 1997c, 2000; Patton and others, 1994). The Yukon
River metallogenic belt extends for several hundred kilometers.
The principal deposits in the northeastern part of this belt are
at Caribou Mountain, Lower Kanuti River, and Holonada, and
the significant deposits in the southwestern part of this belt at
Mount Hurst and Kaiyuh Hills are in the Tozitna and Innoko
areas (table 4) (Nokleberg and others 1997a,b, 1998).
Kaiyuh Hills Podiform Cr Deposit
The Kaiyuh Hills podiform Cr deposit (Loney and Himmelberg,
1984; Foley and others, 1984, 1997) consists of
banded and disseminated chromite from 1 cm to 1 m thick that
occur in fresh and serpentinized Jurassic(?) dunite of the Kaiyuh
Hills ultramafic belt. The dunite interlayered with harzburgite
tectonite. The largest deposit covers an area of 1 by 100 m and
consists of massive chromite containing an estimated 5,000
tonnes Cr2O3. Lesser occurrences consist of banded nodular
pods of chromite. Metallurgical grade chromite containing 46
percent Cr2O3 is present. The deposit contains an estimated
15,000 to 34,000 tonnes Cr2O3 in one deposit. Surface samples
from the largest deposit average 60 percent Cr2O3. Origin of and Tectonic Controls for Yukon River
Metallogenic Belt
Along the southern margin of the Brooks Range, the
Angayucham oceanic terrane occurs mainly in a major eastwest-striking,
south-dipping thrust sheet that extends for several
hundred km, and in sparse isolated klippen that forms the
upper structural level of the Angayucham subduction-zone terrane
in west-central Alaska (fig. 49; tables 3, 4). These thrust
sheets and klippen are thrust along north-dipping faults over
the highly deformed metamorphosed, middle Paleozoic and
older metasedimentary, metavolcanic, and lesser metagranitoid
rocks of the Ruby metamorphosed continental-margin terrane
to the south. To north is the Late Jurassic and Early Cretaceous
Koyukuk island-arc terrane (Moore and others, 1992;
Patton and others, 1994). The thrust slices of ultramafic rocks
in the highest structural level of the Angayucham terrane are
interpeted lower part of an ophiolite that consitutes the base of
the Koyukuk island arc (Loney and Himmelberg, 1989, Patton
and Box, 1989; Pattern and others, 1994). This interpretation
suggests that the Yukon River metallogenic belt of podiform
Cr deposits formed during subduction-related intrusion of
mafic-ultramafic plutons into the basal part of the Late Jurassic
Koyukuk island arc (Nokleberg and others, 1993; Goldfarb,
1997; Nokleberg and others, 2000).
Metallogenic Belts Formed in Late Mesozoic
Gravina Island Arc in Southern Alaska and
Canadian Cordillera
Eastern-Southern Alaska Metallogenic Belt of
Granitic Magmatism Deposits (Belt ESA),
Eastern-Southern Alaska
The major Eastern-Southern Alaska metallogenic belt
of granitic magmatism deposits (fig. 49; tables 3, 4) contains
porphyry Cu, Mo, and Au, polymetallic vein, and Fe-Au skarn
deposits (Nokleberg and others, 1995a). The metallogenic
belt is hosted in the northern part of the Wrangellia island-arc
superterrane, in and adjacent to the area underlain by the Late
Jurassic to mid-Cretaceous Gravina-Nutzotin belt and coeval
granitoid plutonic rocks (Nokleberg and others, 1994c, 1995a,
1997c). This igneous belt, designated as part of a volcanicplutonic
arc by Richter and others (1975), has been called the
Nutzotin-Chichagof belt by Hudson (1983), the Chisana arc
by Plafker and others (1989), and the Gravina arc by Stanley
and others (1990). This igneous belt extends for a few hundred
kilometers within and parallel to the northern margin of
the Wrangellia island-arc superterrane. The deposits of the
Eastern-Southern Alaska metallogenic belt are associated with
Early to mid-Cretaceous granitoid rocks, mainly granite and
granodiorite (Miller, 1994). Most of the granitoid rocks are
calc-alkaline and intermediate in composition. The significant
deposits are the Nabesna (fig 56) and Rambler Fe-Au skarn
deposits, the Pebble Copper porphyry Au-Cu deposit, the
Bond Creek-Orange Hill, and London and Cape, porphyry Cu
and Mo deposits, and the Midas Cu-Au skarn deposit (table 4)
(Nokleberg and others 1997a,b, 1998).
Pebble Copper Porphyry Au-Cu Deposit
The Pebble Copper porphyry Au-Cu deposit occurs in the
western part of southern Alaska (Phil St. George, written commun.,
1991; Bouley and others, 1995; Young and others, 1997).