Metallogenesis and Tectonics of the Russian Far East, Alaska, and ...

tonnes of ore mined between 1967 and 1978. A maximum Pelmian 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 subduction zone 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 which 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, 1997~). The Seventymile terrane is interpreted as a dismembered ophiolite which
formed during a Permian to Triassic period of sea-floor spreading (Foster and others, 1987; Nokleberg and others, 1989a, 1994c,
1997~). 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 Stiki~ua-QuesneUia 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 Slide Mountain terranes. The metamorphism and alteration are herein
interpreted as occurring during either Jurassic thrusting of these terranes onto the North American Craton Margin, or more likely, during
younger hydrothermal activity associated with extensive Late Cretaceous granitic magmatism.
Cassiar Metallogenic Belt of
Serpentinite-Hosted Asbestos Deposits
Northern British Columbia (Belt CS)
The Cassiar metallogenic belt of serpentinite-hosted asbestos (and local jade) deposits occurs in northern British
Columbia in the east-central part of the Canadian Cordillera (fig. 49; tables 2, 3) (Nokleberg and others, 1997b, 1998). The
metallogenic belt is hosted in the altered ultramafic rocks of the Slide Mountain subduction-zone terrane. The two significant
deposits are at Cassiar and adjacent McDarne.
Cassiar (McDame) Serpentine-Hosted Asbestos Deposit
The Cassiar (McDame) serpentine-hosted asbestos deposit (fig. 59) (Burgoyne, 1986; Learning, 1978; Northern Miner,
December 12, 1987) consists of a chrysotile asbestos stockwork hosted in serpentinized alpine ultramafic intrusive rocks which
occur along the fault contact of the Slide Mountain terrane and structurally underlying shelf sedimentary rocks of the passive-
continental margin Cassiar terrane (Nokleberg and others, 1997b, 1998). The deposit consists of two-fibre vein type chrysotile
with magnetite which occurs in vein partings and in wall rocks, accompanied by some pyrite and jade. About 2.05 million tonnes
of fibre were produced between 1953 and 1984 from 23.3 million tonnes of ore mined between 1953-1984. The deposit age is
uncertain, but probably Cretaceous. The deposit is large and contains pre-production reserves of 55 million tonnes with high
quality chrysotile.
Origin of and Tectonic Controls for
Cassiar Metallogenic Belt
The chrysotile asbestos deposits of the Cassiar metallogenic belt occur in sheared and altered serpentine lenses and
bodies which occur along the contacts between alpine ultramafic intrusions of the Slide Mountain terrane and the structurally
underlying sedimentary rocks of the Cassiar passive continental-margin terrane (Nokleberg and others, 1997b, 1998, Sheet 3).
Chrysotile veinlets, along with lizardite, antigorite, magnetite, pyrite and nephrite, formed as infiltrational replacements of
serpentinite in and along shear zones (O'Hanley and Wicks, 1995). An incremental " ~ r - isotopic ~ ~ ~ age r of 94 Ma on phlogopite
in the orebody footwall (Nelson and Bradford, 1993) is interpreted as a minimum age of serpentinization. Phlogopite formation
apparently post-dates metamorphism related to emplacement of the Sylvester allochthon of the Slide Mountain terrane in the
Middle to Late Jurassic (Harms, 1986; Monger and others. 199 1).
The Slide Mountain terrane consists mainly of a fault-bounded oceanic assemblage of Devonian to Permian and locally
Late Triassic marine volcanic and sedimentary rocks, and local mafic and ultramafic plutonic rocks (Nokleberg and others,
1994~). The terrane occurs for about 2,000 krn along the length of the Canadian Cordillera (Nokleberg and others, 1997b, 1998,
Sheet 1). The Slide Mountain terrane is interpreted as a sequence of oceanic crustal rocks which formed adjacent to, and were
subducted under a late Paleozoic and early Mesozoic island arc now preserved as two fragments in the Quesnellia and Stikinia
terranes (Nokleberg and others, 1994~; Monger and Nokleberg, 1996; Nokleberg and others, 2000). The Slide Mountain terrane
occurs in large blocks to small, discontinuous remnants which are thrust over and (or) tectonically imbricated onto the Yukon-
Tanana, Kootenay, and Cassiar continental-margin terranes, and onto the North American craton margin (Nokleberg and others,
1997b, 1998, Sheet I). The Slide Mountain terrane is interpreted as being emplaced onto the North American craton and craton
margin, along with the Stikinia, Quesnellia, and Cache Creek terranes during a major period of Jurassic accretion (Monger and