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

area. Although not part of this metallogenic belt, a nearby porphyry Mo deposit in this area at Bear Mountain consists of
molybdenite- and wolframite-bearing Tertiary(?) granite porphyry stock (Barker and Swainbank, 1986). The stock intrudes the
Late Proterozoic(?) Neruokpuk(?) Quartzite, and the Tertiary(?) isotopic age for the stock may be a reset Devonian age. If so, the
Bear Mountain deposit would be part of the Brooks Range metallogenic belt.
Origin of and Tectonic Controls for
Brooks Range Metallogenic Belt
Field, chemical, and isotope data indicate the granitic magmatism deposits in the Brooks Range metallogenic belt formed
during intrusion of the Devonian gneissic granitoid rocks (Dillon and others, 1987; Nokleberg and others, 1995a). High initial Sr
ratios (about 0.715), and Pb and Sm-Nd isotopic studies indicate the presence of an older, inherited crustal component (about
1,000 to 800 Ma) and involvement of Proterozoic or older continental crust in the genesis of the plutons (Dillon and others, 1987;
Nelson and others, 1993; Miller, 1994; Moore and others, 1994). U-Pb zircon and Rb-Sr isotopic studies indicate 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 which formed at lower greenschist facies. K-Ar,
and incremental Ar studies indicate which 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 which developed above a subduction zone
(Newberry and others, I997a; 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 which host the kuroko massive sulfide deposits to the west in the Arctic metallogenic
belt (Newbeny 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 continental-margin arc which extended along the margin of the North American Cordillera
(Rubin and others, 1991 ; Nokleberg and others, 1994~. 1997c, 2000; Plafker and Berg, 1994; Goldfarb, 1997). An alternative
interpretation by Goldfarb and others (1997, 1998) proposes which 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 krn along st~ike 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).
BonnMeld District of Kuroko Massive Sulfide Deposits
In the Bonnifield district along the Wood River drainage, the best-studied deposits are at Anderson Mountain and 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 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 which 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 up to 19% Cu, up to 5% Pb, 28% Zn, and 17 1 g/t Ag. High
geochemical values of As, Sb, Hg, and W may be derived from underlying schist.