USGS Professional Paper 1697 - Alaska Resources Library

intrusion from about 402 to 366 Ma (Dillon and other, 1987;
Moore and others, 1994). Most of the gneissic granitoid plutons
contain a moderately to intensely developed, subhorizontal to
gently dipping schistosity that formed during lower greenschist
facies metamorphsim. K-Ar, and incremental Ar studies indicate
that mid-Cretaceous greenschist metamorphism was superposed
on older blueschist facies metamorphism (Dusel-Bacon and others,
1993; Moore and others, 1994).
These field, petrologic, chemical, and isotopic data indicate
that the Brooks Range metallogenic belt and associated
Devonian gneissic granitoid plutons formed along a Devonian
continental-margin arc that developed above a subduction zone
(Newberry and others, 1997a; Nelson and others, 1993; Miller,
1994; Moore and others, 1994; Nokleberg and others, 1995a,
2000). U-Pb zircon isotopic ages indicate that the Devonian
gneissic granitoid rocks intruded about 30 to 40 m.y. after the
eruption of the submarine volcanic rocks that host the kuroko
massive sulfide deposits to the west in the Arctic metallogenic
belt (Newberry and others, 1997a; Nokleberg and others, 1997a)
described above. Herein the Brooks Range metallogenic belt is
interpreted as the axial arc part of a continental-margin arc in
which the Arctic metallogenic belt of kuroko massive sulfide
and associated deposits formed in the back arc. Regional
tectonic analyses also suggest that the Devonian igneous of the
Brooks Range are part of a discontinuous Devonian continentalmargin
arc that extended along the margin of the North American
Cordillera (Rubin and others, 1991; Nokleberg and others,
1994c, 1997c, 2000; Plafker and Berg, 1994; Goldfarb, 1997).
An alternative interpretation by Goldfarb and others (1997,
1998) proposes that some of the deposits in the Arctic metallogenic
belt formed during subsequent rifting, as indicated by Pb
isotope data reported by Dillon and others (1987).
Alaska Range and Yukon-Tanana Upland Metallogenic
Belt of Kuroko Massive Sulfide Deposits
(Belt AKY) Central and East-Central Alaska
The Alaska Range and Yukon-Tanana Upland metallogenic
belt of kuroko massive sulfide deposits (fig. 17; tables 3, 4)
occurs in the central and eastern Alaska Range in the southern
part of the Yukon-Tanana metamorphosed continental margin terrane.
The massive sulfide deposits extend for 350 km along strike
on the northern flank of the Alaska Range and constitute one of
the longer belts of massive sulfide deposits in Alaska. The significant
deposits are WTF, Red Mountain, Sheep Creek, Liberty Bell,
Anderson Mountain, Miyaoka, Hayes Glacier, McGinnis Glacier,
and in several deposits in the Delta district (table 4) (Newberry
and others, 1997; Nokleberg and others 1997a,b, 1998).
Bonnifield District of Kuroko Massive Sulfide Deposits
In the Bonnifield district along the Wood River drainage,
the best-studied deposits are at Anderson Mountain, WTF, and
Red Mountain. Twenty sulfur isotopic analyses from seven
stratiform deposits in the Bonnifield district indicate enrichment
by heavier sulfur during deposition, typical of many
Middle and Late Devonian Metallogenic Belts (387 to 360 Ma; figures 16, 17) 49
volcanogenic massive sulfide deposits (Gilbert and Bundtzen,
1979; Newberry and others, 1997). One lead isotopic analysis
from the Anderson Mountain deposit yielded a single-stage
lead age of 370 Ma (Devonian).
Anderson Mountain Kuroko Massive Sulfide(?) Deposit
The Anderson Mountain kuroko massive sulfide(?) deposit
(Gilbert and Bundtzen, 1979; Curtis J. Freeman, written commun.,
1984; Newberry and others, 1997a) consists of massive
sulfide layers with pyrite, chalcopyrite, galena, sphalerite, enargite,
and arsenopyrite in gangue of quartz, sericite, chlorite, calcite,
barite and siderite. The deposit is hosted in metamorphosed
marine tuffaceous rhyolite and metamorphosed calcareous
clastic rocks that are correlated with the Moose Creek Member
of the Mississippian(?) Totatlanika Schist. Numerous high-angle
faults cut the deposit. The massive sulfide beds lie on an irregular
paleosurface in footwall with domal sulfide accumulations.
The absence of footwall alteration and stringer mineralization
suggests off-vent deposition. Grab samples contain as much as
19 percent Cu, as much as 5 percent Pb, 28 percent Zn, and 171
g/t Ag. High geochemical values of As, Sb, Hg, and W may be
derived from underlying schist.
WTF and Red Mountain Kuroko Massive Sulfide Deposit
The WTF and Red Mountain Kuroko massive sulfide
deposits (Gilbert and Bundtzen, 1979; David R. Gaard, written
commun., 1984) consist of massive pyrite, sphalerite, galena, and
chalcopyrite in a quartz-rich gangue. The sulfides are hosted in
in felsic metavolcanic rock derived from crystal and lapilli tuff,
minor flows, and metasedimentary rock. The stratiform massive
sulfide layers occur on both sides of a large, east-west trending
syncline. The massive sulfide layers at Red Mountain occur in a
proximal setting on the south limb of the anticline, and occur in
a sulfide-silica exhalite that is as much as 130 m thick. An older,
southern horizon contains sphalerite and coarse pyrite in black
chlorite schist. The WTF deposit occurs on the north limb of the
antiform and consists of a thin blanket of fine-grained sulfides
that are interpreted as having formed in a distal setting relative
to the vent. The WTF deposit contains an estimated 1.10 million
tonnes grading 0.15 percent Cu, 2.5 percent Pb, 7.9 percent
Zn, 270 g/t Ag, and 1.9 g/t Au. The deposits occur immediately
below the Sheep Creek Member and above the Mystic Creek
Member of the Mississippian(?) Totatlanika Schist.
Delta District of Kuroko Massive Sulfide Deposits
The best-known kuroko massive sulfide deposits of the
Alaska Range and Yukon-Tanana Upland metallogenic belt
are part of the Delta district in the eastern part of the Alaska
Range (fig. 23). The district and large massive sulfide deposits
have been described by several authors (Nauman and others,
1980; Lange and Nokleberg, 1984; C.R. Nauman and S.R.
Newkirk, written commun., 1984; Lange and others, 1990,
1993; Newberry and others, 1997a). The district comprises
an area of about 1,000 km 2 . The district contains about 26