USGS Professional Paper 1697 - Alaska Resources Library

These ore bodies are as much as 1 m wide and several meters
long. Ferruginous chromite occurs with as much as 48 percent
Cr 2O 3. Titanomagnetite and Cu sulfides occur in the peripheral
pyroxenites. PGE minerals occur in association with chromite
and form in chrome-spinel as small idiomorphic crystals and
as xenomorphic inclusions in interstices. Fe and Pt alloys are
predominant and contain inclusions of native Os. The chromespinel
interstices are dominated by sperrylite and tetraferroplatinum.
PGE grains are as much as 1 mm in diameter. PGE
minerals are similar in composition to those in podiform Cr
deposits in southeastern Alaska and in the Urals Mountains.
Galmeononsky-Seinavsky PGE Occurrences
The Galmeonsky-Seinavsky PGE (Alaskan PGE) occurrences
are located in zoned mafic-ultramafic complexes in
the geographic center of the Koryak Highlands metallogenic
belt (locally in Russia called the Koryak-Kamchatka platinum
belt) (Bundtzen and others, 2003a,b). The surface area of the
Galmeonsky pluton is about 45 km2 , and the pluton is about 16
km long and 3 to 4 km wide. An 40Ar/ 39Ar isotopic age of 60 to
73.9 Ma has been obtained for the pluton (Bundtzen and others,
2003b). The pluton contains a dunite core that constitutes
about 70 percent of the body and in the periphery is dunite that
is successively rimmed by wehrlite, olivine-magnetite pyroxenite,
and gabbro. Chromite-bearing dunite zones contain as
much as 100 g/t Pt (Kozin and others, 1999).
The Seinavsky pluton occurs 7 km northeast of the
Galmeononsky pluton, and covers an area of about 40 km2 .
Dunite constitutes about 20 percent of pluton and the rest is
mainly wehrlite and pyroxenite (Melkomukov and Zaitsev,
1999). A 40Ar/ 39Ar isotopic age of 60 to 73.9 Ma has been
obtained for the pluton (Bundtzen and others, 2003b). Chromite-rich
dunite grades is as much as 12 g/t Pt, and pyroxenite
contains as much as1 g/t Pd (Kozin and others, 1999). The
PGE mineral the pluton and associated placer deposits is
isoferroplatinum. Occurring in the region are native osmium,
iridium, ruthenium, and platosmiridium, which are a few percent
of the total PGE. About 4 percent gold occurs in concentrates
along with PGE arsenides and alloys (Melkomukov and
Zaitsev, 1999; Bundtzen and Sidorov, 1998).
Since 1994, rich PGE placers have been mined in six
streams that radially drain both complexes. Approximately
18.1 tonnes PGE were mined from 1994 to 1998 (Kozin and
others, 1999). Production in 2000 was approximately 3.4
tonnes PGE (A. Koslov, written commun., 2000). The production
from the Galmeononsky-Seinavsky district is a considerable
percentage of total Russian PGE production.
Origin of and Tectonic Controls for Koryak Highlands
Metallogenic Belt
The zoned mafic-ultramafic plutons that host the Koryak
Highlands metallogenic belt intrude the Late Cretaceous
volcanogenic-sedimentary rocks of the Olyutorka subterrane
of the Olyutorka-Kamchatka terrane (Nokleberg and others,
1994c, 1997c). The Olyutorka subterrane consists of a major
Early Late Cretaceous Metallogenic Belts (100 to 84 Ma; figs. 79, 80) 195
sequence of late Mesozoic and early Cenozoic island-arc volcanic
and sedimentary rocks and occurs in a large nappe that is
obducted onto the Ukelayat subterrane of the West Kamchatka
turbidite basin terrane (Nokleberg and others, 1994c, 1997c).
The Olyutorka subterrane consists of (1) a lower part composed
of volcanic and siliceous oceanic rocks (Albian to Campanian
Vatyn series), and (2) an upper part of Maastrichtian
to Paleocene volcanic and clastic island-arc deposits (Achayvayam
and Ivtiginskaya Formations). The subterrane is locally
intruded by zoned intrusives ranging from dunite to clinopyroxenite
to gabbro. The succession from pluton cores to
margins is generally dunite grading into pyroxenite that grades
into gabbro and gabbro-diabase. Intrusion occurred in the
latest Cretaceous and the plutons are interpreted as the deeplevel,
magmatic roots of an island arc (Bogdanov and others,
1987). The available geologic and petrologic and geochemical
data indicate that the zoned PGE-bearing plutons formed
in a marginal oceanic basin and frontal island arc during
subduction of an oceanic plate (Bogdanov and others, 1987).
Recent isotopic studies suggest intrusion of the zoned maficultramafic
plutons in the Late Cretaceous as young as 71 Ma
(Kepezhinskas and others, 1993; Sidorov and others, 1997;
Bundtzen and Sidorov, 1998). The Olyutorka-Kamchatka terrane
is interpreted as an island-arc rock sequence underlain by
oceanic crust. The Olyutorka subterrane is intricately faulted
with and thrust over the nearly coeval Late Cretaceous and
Paleogene Ukelayat turbidite basin subterrane of the West
Kamchatka turbidite basin terrane to the northwest (fig. 79).
The Late Cretaceous to early Tertiary Olyutorka-Kamchatka
island-arc terrane is was tectonically linked to the Vetlovskiy
accretionary-wedge terrane (Nokleberg and others, 2000).
Kepezhinskas and others (1993) interpret that the
zoned mafic-ultramafic complexes near Epilchak Lake at the
northern end of metallogenic belt were emplaced as part of
calc-alkaline magmas related to Late Cretaceous subduction
during a short period of crustal extension. They interpret that
the mafic Late Cretaceous lavas that crop out near Epilchak
Lake and in the Galmeononsky-Seinavsky areas (part of the
Olyutorka-Kamchatka island arc terrane) may be comagmatic
with the zoned mafic-ultramafic plutons. Preliminary Ar-Ar
isotopic ages from the Epilchak Lake and Galmeononsky
zoned plutonic bodies range from 69 to 71 Ma (P.W. Layer,
written commun., 1998).
Vatyn Metallogenic Belt of Volcanogenic Mn and
Fe Deposits (Belt VT), Southeastern Part of
Russian Northeast
The Vatyn metallogenic belt of volcanogenic Mn and Fe
deposits (fig. 79; tables 3, 4) occurs in the southeastern part of
the Russian Northeast. The belt occurs in several fragments,
strikes east-west, is as much as 680 km long, and ranges from
5 to 100 km wide. The belt is hosted mainly in the oceaniccrustal
and ophiolite rocks of the Late Carboniferous through
Early Jurassic and Cretaceous Olyutorka-Kamchatka island-