USGS Professional Paper 1697 - Alaska Resources Library

86 Metallogenesis and Tectonics of the Russian Far East, Alaska, and the Canadian Cordillera
and Coleman, 1989; Foley and others, 1997). The assemblage
is a major belt of ultramafic tectonite, cumulate gabbro, and
norite that occurs along the southern, faulted margin of the
Peninsular sequence of the Wrangellia island arc superterrane
directly north of the Border Ranges Fault system (unit WR,
fig. 32) (MacKevett and Plafker, 1974; Burns, 1985; Plafker
and others, 1989; Nokleberg and others, 1994c, 1997c). In
this region, the ultramafic and mafic rocks are interpreted as
the deep-level root of the Late Triassic to Jurassic Peninsular
sequence (Talkeetna part of the Talkeetna-Bonanza island
arc) of the Wrangellia superterrane (Burns, 1985; Debari and
Coleman, 1989). This sequence consists of the Late Triassic(?)
and Early Jurassic marine andesite volcanic rocks of the
Talkeetna Formation and the Middle Jurassic plutonic rocks of
the Alaska-Aleutian Range batholith. The age of the Talkeetna
part of the Talkeetna-Bonanza island arc is interpreted as about
180 to 217 Ma (Newberry and others, 1986a; Roeske and
others, 1989). These data indicate that the Kodiak Island and
Border Ranges metallogenic belt are a deep-level suite of lode
deposits formed in the root of the Talkeetna-Bonanza island
arc along the margin of the Wrangellia superterrane (Nokleberg
and others, 1994d, 2000).
Eastern Alaska Range Metallogenic Belt of
Gabbroic Ni-Cu Deposits, Besshi Massive Sulfide,
and Related Deposits (Belt EAR), Southern
Alaska and Northwestern Canadian Cordillera
The Eastern Alaska Range metallogenic belt of gabbroic
Ni-Cu, Besshi massive sulfide, and related deposits (fig. 32;
tables 3, 4) occurs in the eastern Alaska Range and Wrangell
Mountains in southern Alaska and in the northwestern Canadian
Cordillera (Foley, 1982; Foley and others, 1997) and is
equivalent to the Kluane- Nikolai belt (Mortensen and Hulbert,
1991; Hulbert, 1995; Hulbert and Carne, 1995). Bundtzen and
others (2000) named this belt as the Kluane-Nikolai maficultramafic
belt. In Alaska, Barker (1987) first recognized that
the differentiated, Triassic sill-like plutons contained significant
PGE in addition to Cu and Ni. These mafic-ultramafic bodies
are the focus of intense exploration for PGE. The metallogenic
belt occurs in the Wrangellia sequence of the Wrangellia
superterrane. This sequence contains the areally extensive, Late
Triassic Nikolai Greenstone and coeval mafic and ultramfic
sills, dikes, and plutons (Nokleberg and others, 1994c, 1997c).
The significant deposits are at Denali, Fish Lake, and Wellgreen
(table 4) (Nokleberg and others 1997a,b, 1998). The
metallogenic belt is hosted in Late Triassic marine pillow basalt
and interlayered marine clastic metasedimentary rock of the
Wrangellia sequence of the Wrangellia superterrane.
Denali Cu-Ag Besshi(?) Massive Sulfide Deposit
The Denali Cu-Ag Besshi(?) massive sulfide deposit
(Stevens, 1971; Seraphim, 1975; Smith, 1981) contains at
least six stratiform bodies of very fine grained and rhythmi-
cally layered chalcopyrite and pyrite laminations in thin-bedded,
shaly, carbonaceous, and limy argillite enclosed in the
Late Triassic Nikolai Greenstone. The largest body is about
166 m long and 9 m wide and extends at least 212 m below
surface. The massive sulfide layers contain abundant Cu and
as much as 13 g/t Ag. The sulfide deposits and host rocks are
metamorphosed at lower greenschist facies and locally moderately
folded. The deposit contains several hundred meters
of underground workings that were developed from 1964 to
1969, but never put into production. The deposit is interpreted
as forming in a submarine volcanic environment of a reducing
or euxinic marine basin containing abundant organic matter
and sulfate reducing bacteria.
Fish Lake Gabbroic Ni-Cu Deposit
The Fish Lake gabbroic Ni-Cu deposit (Stout, 1976;
Nokleberg and others, 1984; I.M. Lange and W.J. Nokleberg,
written commun., 1985; Nokleberg and others, 1991) consists
disseminated and wispy-layered chromite, in serpentinized
olivine cumulate. The deposit occurs in a zone as much as 15
km long along strike, and is as much as 2 km wide. Isolated
grab samples contain greater than 0.5 percent Cr and as much
as 0.3 percent Ni, and local anomalous Cu and Ni in streamsediment
and rock samples. The gabbroic Ni-Cu deposit is
hosted in small- to moderate-size gabbro plutons and local
cumulate mafic and ultramafic rocks. The mafic and ultramafic
rocks intrude the Nikolai Greenstone and older rocks
and are interpreted as comagmatic with the mafic magmas
that formed the Middle and Late Triassic Nikolai Greenstone
(Nokleberg and others, 1994d, 2000).
Wellgreen Gabbroic Ni-Cu Deposit
The Wellgreen gabbroic Ni-Cu deposit (fig. 36) (Campbell,
1976; Hulbert and others, 1988; EMR Canada, 1989; Mining
Review, 1991) consists of massive pyrrhotite, pentlandite,
chalcopyrite, and magnetite lenses that are scattered along the
footwall contact of a steeply dipping fault zone in gabbroic rocks
of the Late Triassic Quill Creek Complex. In the Yukon Territory,
the belt includes the Canalask deposit at White River (Bremes,
1994) in which Cu-Ni sulfides generally occur as disseminations
in mafic dikes and peridotite. The deposit is medium size and has
estimated reserves of 50 million tonnes grading 0.36 percent Ni,
0.35 percent Cu, 0.51g/t Pt, 0.34 g/t Pd. The deposit occurs in a
130-km-long belt of Ni-Cu-Co-PGE occurrences that, along with
host gabbroic bodies, are interpreted as Late Triassic.
Origin of and Tectonic Controls for Eastern and Western
Alaska Range Metallogenic Belt
The gabbroic Ni-Cu and PGE deposits in the Eastern
Alaska Range metallogenic belt are hosted in gabbro and
ultramafic sills and plutons that are interpreted as coeval with,
and genetically related to the mainly Late Triassic Nikolai
Nikolai Greenstone (Nokleberg and others, 1994d). The Nikolai
Greenstone, which forms a major part of the Wrangellia