USGS Professional Paper 1697 - Alaska Resources Library

Origin of and Tectonic Controls for Alaska Range and
Yukon-Tanana Upland Metallogenic Belt
The Alaska Range and Yukon-Tanana Upland metallogenic
belt of kuroko massive sulfide deposits is hosted in
Devonian metavolcanic and interlayered metasedimentary
rocks of the southern Yukon-Tanana terrane that is interpreted
as a fragment of metamorphosed Devonian and Mississippian
continental-margin arc (Lange and others, 1990, 1993;
Nokleberg and others, 1994c, 1997, 2000). The metavolcanic
rocks, which host most of the major base and precious
metal deposits, are derived from a volcanic suite that varies
in composition from spilite to andesite to keratophyre. Most
studies of kuroko massive sulfide deposits interpret a backarc
or arc-related rift origin for the deposits (Sawkins, 1990);
however, in the eastern Alaska Range, the lack of coeval mafic
plutonic or volcanic rocks appears to preclude a rift origin.
The Yukon-Tanana terrane in east-central Alaska, southeastern
Alaska, and the Canadian Cordillera, and the correlative Kootenay
terrane in southern British Columbia are interpreted to
be fragments of the herein, informally named, Kootenay arc, a
discontinuous Devonian continental-margin arc that extended
along the margin of the North American Cordillera from
Arctic Alaska to California (Grantz and others, 1991; Rubin
and others, 1991; Mortensen, 1992; Plafker and Berg, 1994;
Nokleberg and others, 1994c, 1997c; Monger and Nokleberg,
1996; Nokleberg and others, 2000). Fragments of the Kootenay
arc include several metallogenic belts, which host kuroko
massive sulfide and related deposits (Nokleberg and others,
1997a,b,c)—(1) The Arctic metallogenic belt in the Coldfoot
terrane in Arctic Alaska, (2) the Frances Lake metallogenic
belt (Murphy and Piercey, 1999) in the Yukon-Tanana terrane
in the southern Yukon Territory, (3) the Tracey metallogenic
belt in the Yukon-Tanana terrane in southeastern Alaska and
western British Columbia, and (4) the Kootenay-Shuswap belt
in the Kootenay terrane in the southern Canadian Cordillera.
Dawson Metallogenic Belt of Volcanogenic Pb-
Zn-Cu Massive Sulfide and SEDEX Pb-Cu-Zn-Ba
Deposits (Belt DA) Northwestern Yukon Territory
The Dawson metallogenic belt of volcanogenic Pb-Zn-
Cu massive sulfide and SEDEX Pb-Cu-Zn-Ba occurrences
is located in the Yukon-Tanana terrane in the southern Yukon
Territory (fig. 17; tables 2, 3) (Nokleberg and others, 1997b,
1998). Stratiform massive sulfide occurrences exist in three
sequences within the terrane. With more detailed study, each
of the three sequences and contained massive sulfide deposits
might be designated as separate metallogenic belts.
Besshi Cu-Zn-Pb massive sulfide occurrences, such as
Lucky Joe (Mortensen, 1992), are hosted in Devonian and Mississippian
mafic metavolcanic and carbonaceous metasedimentary
rocks. The occurrences are Kuroko and Besshi deposits that
are associated with calc-alkaline and tholeiitic volcanic rocks,
respectively. The host rocks are part of the Late Proterozoic to
early Paleozoic Nisling assemblage, which consists of continen-
Middle and Late Devonian Metallogenic Belts (387 to 360 Ma; figures 16, 17) 51
tal margin metasedimentary rocks. These deposits and host rocks
are correlated with similar, larger deposits of the Delta district in
Alaska Range and Yukon-Tanana Upland metallogenic belt of
Kuroko volcanogenic massive sulfide deposits described above.
These extensive middle Paleozoic volcanics and granitoids are
interpreted by and as part of a extensive continental-margin arc
in the Late Devonian and Early Mississippian that formed along
the margin of the North American Craton Margin (Lange and
others, 1985; Mortensen, 1992; Nokleberg and others, 2000).
Small pyrite-bearing Pb-Zn-Ba lens-shaped occurrences, as
at Mickey (Mortensen, 1992), are hosted in carbonaceous schist
and quartzite and Middle Mississippian felsic metatuff. These
deposits, which are interpreted as SEDEX Zn-Pb deposits, may be
correlated with similar-age SEDEX deposits of the Gataga metallogenic
belt 1,000 km to the southeast, across the Tintina Fault
(fig.17) (Johnston and Mortensen, 1994). However, the Dawson
metallogenic belt is hosted in the Yukon-Tanana terrane that is
interpreted as a rifted fragment of the North American Craton Margin
(Mortensen, 1992; Monger and Nokleberg, 1996; Nokleberg
and others, 2000), whereas these metallogenic belts to the east containing
SEDEX deposits are hosted in a Devonian-Mississippian
clastic wedge deposited on the North American Craton Margin
(Nokleberg and others, 1997b, 1998). The host rocks for this part
of the Dawson metallogenic belt are part of the Nasina assemblage
that consists of Late Devonian to Middle Mississippian mafic to
felsic metavolcanic rocks, quartzite, schist, and metaplutonic rocks.
The tectonic origin of this group of deposits is unclear.
Pyrite-bearing Kuroko Pb-Zn-Cu (Au-Ba) massive
sulfide occurrences, as at Lone Star (Mortensen, 1992), are
hosted in middle Permian felsic metavolcanic rocks. The host
rocks are part of the Klondike schist and associated units that
consist mainly of middle Permian felsic metavolcanic and
metaplutonic rocks (Mortensen, 1992). Structurally interleaved
with the metavolcanic rocks are blueschist and eclogite
that are interpreted as remnants of a former subduction zone
that was tectonically linked to the volcanic arc, which formed
the metavolcanic rocks and associated occurrences.
Frances Lake Metallogenic Belt of Volcanogenic
Zn-Cu-Pb Massive Sulfide Deposits (Belt FR)
Southern Yukon Territory
The Frances Lake metallogenic belt of volcanogenic
massive sulfide deposits (fig. 17; tables 3, 4) occurs in the
southeastern Yukon Territory and is hosted in Early Mississippian
felsic metavolcanic rocks and metavolcaniclastic units in
the Yukon-Tanana terrane. The kuroko massive sulfide deposits,
which occur northeast of the Tintina Fault and southwest
of Finlayson Lake, are interpreted as equivalent to (1) smaller
kuroko massive sulfide occurrences in the Dawson metallogenic
belt (as at Mickey) that occur southwest of the Tintina
Fault and are hosted in the Nasina Assemblage (Johnston and
Mortensen 1994), and (2) major kuroko volcanogenic massive
sulfide deposits, which occur across the Tintina Fault in the
Alaska Range and Yukon-Tanana Upland metallogenic belt