USGS Professional Paper 1697 - Alaska Resources Library

Canada Cordillera. The granitoid rocks in this arc are Early
to Middle Pennsylvanian plutons of granodiorite and granite
that were preceded by gabbro and diorite and succeeded by
shoshonite (Beard and Barker, 1989; Barker, 1994). U-Pb
zircon isotopic age studies reveal ages of 290 to 316 Ma (late
Paleozoic) for this suite of plutons and volcanic rocks (Richter
and others, 1975a; Barker and Stern, 1986; Aleinikoff and
others, 1987; Gardner and others, 1988; Plafker and others,
1989). Common Pb isotopic compositions for the granitoid
rocks yield low radiogenic Pb values and suggest derivation
from a mixture of oceanic mantle and pelagic sediment leads,
without an older continental component (Aleinikoff and others,
1987). Rb-Sr isotopic data and REE volcanic and plutonic
whole-rock chemical analyses suggest an intraoceanic island
arc origin (Barker and Stern, 1986; Beard and Barker, 1989;
Barker, 1994; Miller, 1994). A marine origin for the Skolai arc
is supported by submarine deposition of the volcanic flows,
tuff, and breccia, and associated volcanic graywacke and
argillite (Richter and Jones, 1973; Bond, 1973, 1976). The
principal data for an island arc origin are (1) the absence of
abundant continental crustal detritus in late Paleozoic stratified
rocks, (2) little or no quartz in the volcanic rocks and associated
shallow-intrusive bodies, (3) high-latitude fauna, and (4)
isotopic data summarized above.
Ketchikan Metallogenic Belt of Kuroko Massive
Sulfide Deposits (Belt KK), Southeastern Alaska
The Ketchikan Metallogenic Belt of kuroko massive sulfide
deposits occurs in southeastern Alaska (fig. 32; tables, 3, 4)
(Nokleberg and others, 1997b, 1998). The belt strikes northsouth,
is about 300 km long, and varies from 20 to 60 km
wide. The significant kuroko massive sulfide deposit at Moth
Bay consists of discontinuous lenses and layers of massive
pyrite and pyrrhotite, minor chalcopyrite and galena, and local
disseminated pyrite (Berg and others, 1978; Newberry and
others, 1997). The deposit contains an estimated 91,000 tonnes
grading 7.5 percent Zn and 1 percent Cu and an additional
181,000 tonnes grading 4.5 percent Zn and 0.75 percent Cu.
The host rocks are light brown-gray, upper Paleozoic or Mesozoic
muscovite-quartz-calcite schist, subordinate pelitic schist
and quartz-feldspar schist, and possibly metachert. Layers and
lenses of massive sulfides, as much as 1 m thick, occur parallel
to compositional layering in the schist.
The Moth Bay deposit is hosted in the former Taku terrane,
now designated as part of late Paleozoic sedimentary and
volcanic rocks of the Wrangellia sequence of the Wrangellia
superterrane that consists of a poorly understood sequence of
mainly Permian and Triassic marble, pelitic phyllite, and felsic
metavolcaniclastic and metavolcanic rocks that are overlain
by Late Jurassic to mid-Cretaceous units of the Gravina belt
(Gehrels and Berg, 1994). The Ketchikan metallogenic belt and
host rocks are herein interpreted as a fragment of a late Paleozoic
Skolai island arc, which formed early in the history of the
Wrangellia superterrane (Nokleberg and others, 1994c, 2000).
Late Triassic Metallogenic Belts (230 to 208 Ma; fig. 32) 81
Late Triassic Metallogenic Belts
(230 to 208 Ma; fig. 32)
Overview
The major Late Triassic metallogenic belts in the Alaska
and the Canadian Cordillera are summarized in table 3 and
portrayed on figure 32. No major Late Triassic metallogenic
belts exist in the Russian Far East. The major belts in Alaska
and the Canadian Cordillera are as follows: (1) In the same
region is the Farewell belt of gabbroic Ni-Cu-PGE deposits
that is hosted in the Dillinger, Mystic, and Nixon Fork passive
continental margin terranes. The tectonic origin of this belt is
uncertain. (2) In southern Alaska, three belts are interpreted
as forming in the middle Mesozoic Talkeetna-Bonzana island
arc in Wrangellia Superterrane. These belts are the (a) the
Kodiak Island and Border Ranges (KOD) belt of podiform Cr
deposits, which is interpreted as forming in the roots of the
Talkeetna-Bonzana arc, and (b) eastern and western Alaska
Range (EAR) belt of gabbroic Ni-Cu, Besshi massive sulfide
and related deposits and the Alexander (AX) belt of massive
sulfide and related deposits. Both belts are interpreted as
forming during back-arc rifting of the Talkeetna-Bonzana arc
preserved in the Wrangellia island-arc superterrane. (3) In the
Canadian Cordillera, the Copper Mountain (North; CMN),
Copper Mountain (South; CMS), Galore (GL), Guichon (GU),
and Texas Creek (TC) belts, which all contain granitoid magmatism-related
deposits, are interpreted as forming in the axial
parts of the Stikinia-Quesnellia island arcs. The Stikinia and
Quesnellia island-arc terranes are interpreted as forming on the
deformed continental-margin strata of Yukon-Tanana terrane,
which was previously rifted from the North American Craton
Margin (Gehrels and others, 1990; Monger and Nokleberg,
1996; Nokleberg and others, 1994c, 1997c, 2000). Mineralization
in all these belts continued into the Early Jurassic. (4)
Also in the Canadian Cordillera, the Sustut metallogenic belt
of basaltic Cu deposits, which is hosted in the Stikinia island
arc terrane, formed in the upper oxidized parts of an island arc
volcanic pile during shallow burial metamorphism and diagenesis.
In the below descriptions of metallogenic belts, a few of
the noteable signficant lode deposits (table 4) are described for
each belt.
Metallogenic-Tectonic Model for Late Triassic
(230 to 208 Ma; fig. 34)
During the Late Triassic (Carnian to Norian—230 to 208
Ma), the major metallogenic-tectonic events were (table 3) (1)
inception of continental-margin arcs and associated subduction
along the North Asian Craton Margin, (2) continued formation
of the Stikinia and Quesnellia island arcs and inception
of subduction-related Talkeetna and Bonanza island arc in the
Wrangellia superterrane, and associated metallogenic belts in
these island arc systems, and (3) beginning of sinistral-slip