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

ocks and serpentinite which occur along the tectonic boundary between the Koyukuk island-arc terrane to the east and the Seward
metamorphosed cont~nental margin terrane to the west (Nokleberg and others, 1997b, 1998, Sheet 3). In the Marshall (Andreatsky
River) area, the unit consists of pillow basalt, chert, diorite, gabbro, serpentinite, and hanbwgite. In both areas, the ophiolite and
related rocks are interpreted as small, isolated klippen or faulted slivers of the upper part of the Angayucham oceanic and
subduction-zone terrane. The thrust slices of ultramafic rocks in the highest structural level of the Angayucham tern are
interpeted lower part of an opbiolite which consitutes the base of the Koyukuk island arc (L-y and Himuelberg, 1989, Patton
and Box, 1989). This interpretation suggests that the Egstern S uwd Peninsula and Marshall metallogenic belt afpodifom Cr
deposits formed during subduction-related intrusion of mfic-ultrenafic plutons Into tbe basal part ofthe Late Jwic Koyukuk
island arc (Nokleberg and others, 1993; Goldfarb, 1997; Nokleberg vlnd 0t.b~~. 2000).
Kobuk Metallogenic Belt of
Podiform Cr Deposits (Belt KB)
Northern Alaska
The Kobuk metallogenic belt of podifonn Cr and associated PGE deposits, ad one serpentine-hosted asbestos deposit
(fig. 49; tables 3,4), occurs for several hundred kilometers along tbc southern flank of the Broakskmge: in norhemi Ahska. The
metallogenic belt IS hosted in the upper structural ofthe Angayuch oceanic and subduction-zone (Jwes mdcrthers,
1987; Nokleberg and others, 1994c, 1997c) which is inkpreted as the basal part of the Koyukuk island arc (Patton and others,
1994). The major podiform Cr deposits are at Iyikrok Mountain, A~irtn, Misheguk M~tain, and Sinikbmeyak Mountain (table
4) (Nokleberg and others 1997% b, 1998). The serpentine-hosd asbestos depoat is at Asbestos Mountain,
Misheguk Mountain Podifonn Cr Deposit
The Misheguk Mountain Podifom Cr deposit (Roedtr and Mull, 1978; DegMhart and ofhers, 1978; Zirnmernraa and
Soustek, 1979; Foley and otbers. 1984, 1982, 1997) consists of disseminated he- to medium-grained chroznife in IYrasslc or
older, locally serpentinized dunite and peridotite tectonite. The &mite and hanburgitc layers are intensely defamed mto minor
folds. Grab samples contain up to 27.5% Cr and 0.3 1 g/t PGE, and the deposit conhim an estimated 1 10,W to 320,QOO toms
chromite. Chromite-bearing zones have surface dimensions of 3 1 by 107 m. The deposit is interpreted as part of a dismembered
ophiolite.
Avan Podiform Cr Deposit
The Avan podiform Cr deposit (Roeder and Mull, 1978; Degenlrart and others, 1978; Zimmm md Sowtek, 1979;
Mayfield and others, 1983; Folty and others, 1985, 1982) consists of disseminated fine- to rnedhmpahed chromite En Jumsic
or older dunite and hanburgitc tectonitb which is locally altered to sqxntinitc. Zones of chromite in &mite are up to a f&v meters
wide and a few hundred meters long; the host dunite a . harzburgite laps exhibit intense minor Eolds. Grab samples contain up
to 43% Cr and 0.48 g/t PGE. The deposit contains ao estimated 290,W to 500,000 toan- chromite.
The dunite and harzburgite tectonite host rocks at Avaa and elsewhere are hosted by which occu as fhult-bounded slab
in the Misheguk igneous sequence which aiso coataios pillow basalt, gabbro, chert, and minor limestwe ( Roe and Mull, 1978;
Zimrnerman and Souslek, 1979; Nelson and Nelson, 1982; Foley and others, 1982). The age of he ukm& rocks hosting the
podiform Cr deposits is probably Jurassic (Patton and othen, 1994). This sequ~lce is part of tbe Angayucham subduction zone
terrane (Patton and others, 1994). The ultramafic rocks hosting the Asbestos Mountain deposit also occur as fault-bounded slabs in
the upper part of the Angayucham lenrane.
Origin of and Tectonic Controls tbr
Kobuk Metallogenic Beti
The Kobuk metallogenie belt occurs in highly-defod mafic and ultxamafic rock h t famns the upper smchual kvd of
the Angayucham subduction zone terrane along the southern margin of the Brooks Range (fig. 49; tables 3,4). The belt occurs
mainly in a major east-wes&s&g, south-dipping tbrust ahwt which extends for several hundred km, md in sparse isohted
klippen. These Angayucham tmane occurs along south-dipping faults over the highly deformed metamorph~ed, middle
Paleozoic and older metssedimcntary, m&vo~canic, and lesser metagranitoid mks of the Coldfbot rnetaimorpW continental
margin terrane to the no&, and in turn are overthrust by the mainly island-arc, Late Jurassic and Edy Crdacdous Ihyukuk
island-arc terrane to the wutb (Moore and others, 1992; Patton and others, 1984). The tinst alioee of u l W rocks in the
highest structural kvel of the hgayucham terrane are inteqwbd lower part of an ophiolite which consitutes the b of the
Koyukuk island arc (Loney and Himmelberg, 1989, Patton and Box, 1989; Patton and others, 1994). This interpretation suggests
that the Kobuk metellogenic belt of podiform Cr and related deposits formed during subduction-related bmiw of mafcultramafic
plutons into the basal part of the Late Jurassic Koyukuk island arc (Nokleberg and others, 1993; GoM, 1997;
Nokleberg and others, 2000).