USGS Professional Paper 1697 - Alaska Resources Library

80 Metallogenesis and Tectonics of the Russian Far East, Alaska, and the Canadian Cordillera
occurs in layers as much as 1,300 m long and 400 m wide in
the dunite of the Ust-Belaya alpine-type ultramafic body. The
chromite occurrences extend northward for 13 km along a belt
more than 2 km wide. The chromite is of economic, low- and
medium-Cr metallurgical grade (Silkin, 1983). Associated
PGE placer deposits are dominated by Os, Ir, and Ru minerals,
which are typical of dunites and harzburgites, particularly in
the Koryak Highlands (Dmitrenko and others, 1990).
Origin of and Tectonic Controls for Ust-Belaya
Metallogenic Belt
The Ust-Belaya metallogenic belt is hosted in ophiolite
that is a tectonic fragment in the Ust’belaya accretionary
wedge or subduction zone subterrane that forms the northern
part of the Penzhina-Anadyr’ terrane (fig. 31). This Ust-Belaya
subterrane consists mainly of a large early Paleozoic ophiolite
with an areal extent exceeding 1,000 km 2
. Extensive zones of
chromite deposits are confined to dunites that occur together
with peridotite, metagabbro, amphibolite, and gabbro. The
subterrane consists of the following tectonic sheets, which
are distinguished by contrasting lithologies (Nokleberg and
others, 1994c, 1997c): (1) The Otrozhnaya sheet is composed
of an ophiolite that contains metamorphosed ultramafic rocks,
gabbro, diabase, basalt, and volcanic breccia and an overlying
sequence of chert, calcareous sandstone, tuff, and limestone
that yield Middle and Late Devonian and Early Carboniferous
faunas. The Otrozhnaya sheet is intruded by diabase,
plagiogranite, and diorite dikes that yield K-Ar ages of 180
to 304 Ma. (2) An unnamed sheet is composed of serpentinite
mélange. (3) The Mavrina sheet is composed of shallowmarine
sandstone and siltstone and interlayered conglomerate
and limestone that yield a Middle Jurassic fauna. And (4)
an uppermost sheet is composed of interlayered sandstone,
siltstone, and mudstone that yield an Late Jurassic to Early
Cretaceous fauna. The Penzhina-Anadyr’ terrane is interpreted
as accretionary wedge or subduction zone unit that contains
fragments of oceanic lithosphere, now preserved as ophiolites.
The Penzhina-Anadyr’ subduction zone terrane is tectonically
linked to the Late Jurassic part of the Kony-Murgal island-arc
terrane (Nokleberg and others, 2000).
Metallogenic Belts Formed in Late Paleozoic
Skolai Island Arc in Wrangellia Superterrane
Alaska Range-Wrangell Mountains Metallogenic
Belt of Granitic Magmatism Deposits (Belt ARW),
Central and Eastern- Southern Alaska
The Alaska Range-Wrangell Mountains metallogenic belt
of granitic magmatism deposits (fig. 32; tables 3, 4) (mainly
porphyry, polymetallic vein, and skarn deposits) occurs in the
Alaska Range and the Nutzotin and Wrangell Mountains in
central and eastern-southern Alaska (Nokleberg and others,
1995a). The metallogenic belt is hosted in the late Paleozoic
part of the Wrangellia sequence of the Wrangellia island-arc
terrane that contains late Paleozoic volcanic and granitoid rocks
(Nokleberg and others, 1994c, 1997c). The significant deposits
are the Rainy Creek Cu-Ag skarn deposit, and the Chistochina
deposits, and smaller occurrences or prospects as the Rainbow
Mountain and Slate Creek porphyry Cu deposits (table 4)
(Nokleberg and others 1997a,b, 1998). Farther to the southeast
in the Nutzotin and Wrangellia Mountains, similar small,
subvolcanic intrusions occur in the Permian and Pennsylvanian
Slana Spur, Hazen Creek, and Station Creek Formations and in
the Tetelna Volcanics (Richter, 1975; MacKevett, 1978).
Rainy Creek Cu-Ag Skarn District
The Rainy Creek Cu-Ag skarn deposit (Rose, 1966; Lange
and others, 1981; Nokleberg and others, 1984, 1991) comprises
a zone about 10 km long and as much as 5 km wide that contains
scattered garnet-pyroxene skarn bodies with disseminated
to small masses of chalcopyrite and bornite, minor sphalerite,
galena, magnetite, secondary Cu-minerals, and sparse gold. The
deposits occur in faulted lenses of marble of the Pennsylvanian
and Permian Slana Spur Formation adjacent to late Paleozoic(?)
metagabbro, metadiabase, and hypabyssal meta-andesite intrusive
rocks. Local disseminated sulfides also occur in metaandesite.
The sulfide-bearing bodies and adjacent wall rocks are
locally intensely faulted. Grab samples contain as much as 5.6
percent Cu, 300 g/t Ag, 1.2 g/t Au, 0.07 percent Zn.
Chistochina District
The Chistochina porphyry Cu and polymetallic vein
deposit (Richter, 1966; Rainier J. Newberry, written commun.,
1985) contains several small areas containing galena, pyrite,
chalcopyrite, tetrahedrite, and gold in quartz veins, small
masses, and disseminations in margins of the Pennsylvanian
and Permian Ahtell quartz diorite pluton and in adjacent volcanic
and sedimentary rocks of the Pennsylvanian and Permian
Slana Spur Formation. The quartz veins are as much as 10
m wide, locally contain massive barite, calcite, and cerussite,
and occur over an area about 5 km long and 3 km wide. The
district also contains small Cu-Au and Pb-Zn skarns. Grab
samples contain as much as 20 percent Pb, 1.4 percent Cu, 21
g/t Ag, 1.4 g/t Au.
Origin of and Tectonic Controls for Alaska
Range-Wrangell Mountains Metallogenic Belt
The Alaska Range-Wrangell Mountains metallogenic belt
is hosted by granitoid plutons and associated volcanic rocks of
the Pennsylvanian and Early Permian Skolai arc (Nokleberg
and others, 1984, 1985; 1995a; Nokleberg and Lange, 1985,
1994d; Plafker and others, 1989). The Skolai arc forms a
lithologically variable suite of volcanic and plutonic rocks that
is discontinuously exposed in the Wrangellia and Alexander
sequences of the Wrangellia superterrane that extends from
eastern-southern Alaska into adjacent parts of the western