USGS Professional Paper 1697 - Alaska Resources Library

49; tables 3, 4) (Foley and others, 1997). These plutons are part
of the Gravina arc and intrude mainly Late Jurassic to Early
Cretaceous flysch of the Gravina-Nutzotin-Gambier volcanicplutonic-sedimentary
belt, and adjacent units of the Wrangellia
island-arc superterrane (Plafker and Berg, 1994; Nokleberg
and others, 1994c, 1997c; Himmelberg and Loney, 1995). In
southeastern Alaska and the Canadian Cordillera, the Gravina-
Nutzotin-Gambier belt consists chiefly of intercalated volcanic
rocks and sedimentary rocks (Gravina, Dezadeash, and Gambier
units) that range in age from Late Jurassic (Oxfordian) through
Early Cretaceous (Albian) (Berg and others, 1972; Monger and
Berg, 1987; McClelland and others, 1992). Coarse clastic rocks
in the Gravina part of assemblage were largely derived from the
stratigraphically underlying Wrangellia superterrane, which lies
mainly to west, but may also have been derived in part from the
Stikinia and Yukon-Tanana terranes to the east.
The Gravina-Nutzotin-Gambier volcanic-plutonic-sedimentary
belt (Berg and others, 1972; Monger and Berg, 1987;
McClelland and others, 1992) forms a major Late Jurassic
and Early Cretaceous overlap assemblage. The assemblage is
generally interpreted as an island arc that was deposited on
and intruded into the Wrangellia island-arc superterrane in
southeastern Alaska. British Columbia, and Eastern-Southern
Alaska (Nokleberg and others, 1994c, 1997c). The zoned
mafic and ultramafic plutons and associated zoned maficultramafic
Fe-Cr-PGE deposits of the Klukwan-Duke metallogenic
belt are generally interpreted as the plutonic feeders to
the island arc (Plafker and Berg, 1994; Nokleberg and others,
2000; Monger and Nokleberg, 1996). Cessation of the island
arc is generally interpreted as coincident with accretion of the
Wrangellia island superterrane to the North American continental
margin in the mid-Cretaceous.
Metallogenic Belts Formed in Late Mesozoic
Collision and Overthrusting in Eastern Alaska
and Canadian Cordillera
Fortymile Metallogenic Belt of Serpentinite-Hosted
Asbestos Deposits (Belt ECA), East-Central Alaska
and Northwestern Canadian Cordillera
The Fortymile metallogenic belt of serpentinite-hosted
asbestos deposits (fig. 49; tables 3, 4) occurs in the Yukon-
Tanana Upland in east-central Alaska and in the northwestern
part of the Canadian Cordillera (Foley, 1982). The metallogenic
belt is hosted in the Seventymile oceanic and subduction-zone
terrane, which occurs in a discontinuous klippen
thrust over the eastern part of Yukon-Tanana metamorphosed
continental-margin terrane in the Yukon-Tanana Upland (Jones
and others, 1987; Foster and others, 1987; Nokleberg and
others, 1994c, 1997c). The significant deposits are a large
serpentinite-hosted asbestos deposit at Slate Creek, Alaska,
and at Clinton Creek, northwestern Canadian Cordillera (table
4) (Nokleberg and others 1997a,b, 1998).
Late Jurassic Metallogenic Belts (163 to 144 Ma; figs. 48, 49) 131
Slate Creek Serpentinite-Hosted Asbestos Deposit
The Slate Creek serpentinite-hosted asbestos deposit
(Foster and Keith, 1974; Robert K. Rogers, written commun.,
1984) consists of antigorite with minor clinochrysotile,
chrysotile, magnetite, brucite, and magnesite in serpentinized
harzburgite. The chrysotile asbestos occurs in zones of fracturing
near centers of thicker serpentinite, primarily as cross-fiber
asbestos in randomly oriented veins about 0.5 to 1 cm thick.
The veins contain alternating zones of chrysotile and magnetite,
and commonly exhibit magnetite selvages. Some chrysotile
is altered to antigorite. The harzburgite occurs as tabular
tectonic lenses, generally from 60 to 150 m thick and as much
as 800 m long. The deposit contains an estimated 58 million
tonnes grading 6.4 percent fiber.
Clinton Creek Serpentine-Hosted Asbestos Deposit
In the northwestern Yukon Territory, northwestern
Canadian Cordillera, the Fortymile metallogenic belt contains
a significant asbestos deposits at Clinton Creek (EMR
Canada, 1989). The asbestos occurs mainly in fractures zones
in fault-bounded lenses of serpentinite in sheared metasedimentary
rocks of the tectonically-imbricated Slide Mountain
and Yukon-Tanana (Nisling) terranes (Abbott, 1983). The
chrysotile occurs as cross-fiber asbestos veinlets. The deposit
has estimated reserves of 6.8 million tonnes grading 4.37
percent fibre. Approximately 0.94 million tonnes of fibre were
produced from 15.9 million tonnes of ore mined between 1967
and 1978. A maximum Permian age of chrysotile formation is
suggested by K-Ar and Rb-Sr isotopic ages, which are interpreted
as the age of metamorphism (Htoon, 1981).
Origin of and Tectonic Controls for Fortymile
Metallogenic Belt
In east-central Alaska, the Seventymile subductionzone
terrane, which contains the serpentinite-hosted asbestos
deposits of the Fortymile metallogenic belt, occurs as
discontinuous remnants of thrust sheets of ultramafic and
associated rocks that are structurally thrust onto the subjacent
Yukon-Tanana terrane. The Seventymile terrane contains
serpentinized harzburgite and associated ultramafic rocks,
gabbro, pillow basalt, chert, argillite, andesite, and graywacke
of Permian to Triassic age (Foster and others, 1987; Nokleberg
and others, 1994c, 1997c). The Seventymile terrane is
interpreted as a dismembered ophiolite that formed during a
Permian to Triassic period of sea-floor spreading (Foster and
others, 1987; Nokleberg and others, 1989a, 1994c, 1997c).
The Seventymile terrane is interpreted as a possible root to the
Stikinia(?) island-arc terrane in east-central Alaska (Nokleberg
and others, 2000). In the northern Canadian Cordillera, the Slide
Mountain accretionary-wedge terrane is also interpreted as part
of the root of the Jurassic Stikinia-Quesnellia island-arc terranes.
The serpentinite-hosted asbestos deposits in the Fortymile
metallogenic belt are the products of low-grade metamorphism
and alteration of ultramafic rock in both the Seventymile and