USGS Professional Paper 1697 - Alaska Resources Library

icite-biotite-quartz bodies in fracture zones and are conformable
to and crosscut bedding. The epithermal deposits may
be related to areas of higher carbon contents in thin-bedded
siltstone and argillite. Associated Au placer deposits occur in
adjacent parts of China. The deposit is small. Average grades
are 1.92 g/t Au and 49 to 52 g/t Ag.
Origin of and Tectonic Controls for Laoelin-Grodekovsk
Metallogenic Belt
The Laoelin-Grodekovsk metallogenic belt of porphyry
Cu-Mo and Au-Ag epithermal vein deposits is hosted by the
Laoelin-Grodekovsk island-arc terrane, which consists chiefly
of two units (Nokleberg and others, 1994c, 1997c): (1) A lower
tectonic mélange unit is composed of fragments of Early Silurian
granite-pebble-bearing conglomerate, sandstone, siliceous
mudstone and lesser interbedded basalt, andesite, rhyolite, and
tuff. The sedimentary rocks are locally intensely deformed and
metamorphosed to middle amphibolite facies. And (2) an upper
unit is composed of Permian basalt, andesite, rhyolite, conglomerate,
sandstone, mudstone, and shale and lesser interbedded
limestone lenses that contain Late Permian Tethyan fusulinids.
The structural thickness is about several thousand meters. The
Permian rocks are intruded by zoned dunite-clinopyroxenitegabbro
intrusions that form Alaskan-Uralian zoned mafic-ultramafic
plutons, and local tonalite and plagiogranite. The zoned
dunite-clinopyroxenite-gabbro intrusions may be favorable
for undiscovered zoned mafic-ultramafic Cr-PGE deposits
(V.V. Ratkin, this study). The Permian igneous rocks that host
Laoelin-Grodekovsk metallogenic belt are part of the younger
Permian sequence of the Laoelin-Grodekovsk terrane that is
interpreted a Permian island arc (Nokleberg and others, 2000).
Metallogenic Belts Formed In Late Paleozoic
Oceanic Lithosphere Preserved in Subduction
Zones Terranes in Russian Northeast
Aluchin Metallogenic Belt of Podiform Cr Deposits
(Belt AC) Central Part of Russian Northeast
The Aluchin metallogenic belt of podiform Cr deposits
(fig. 31; tables 3, 4) occurs in the Big Anui River Basin in the
central part of the Russian Northeast. The belt is hosted in the
Aluchin ophiolite terrane of the Kolyma-Omolon superterrane
(fig. 31) (Nokleberg and others, 1994c, 1997c). The podiform
Cr deposits, as at Teleneut, are hosted by dunite and serpentinite
and occur in the north-eastern and southern portions of the
metallogenic belt in a large, linear mass of dunite and harzburgite
that extends for more than 100 km (Nokleberg and others
1997a,b, 1998).
Teleneut Podiform Cr Deposit
The Teleneut podiform Cr deposit (Aksenova and others,
1970) occurs in serpentinite rocks at the southern end of the
Pennsylvanian Metallogenic Belts (320 to 286 Ma; figs. 31, 32) 79
Aluchin alpine-type ultramafic body, where ultramafic rocks
crop out on the surface as a fault-bounded lens that is 7 km
long by 2.5 km wide with a north-south trend. The ultramafic
rocks consist of serpentinite (70 percent) and subordinate
little-altered dunite and pyroxenite. The Teleneut podiform Cr
occurrence occurs in the central part of the lens that contains
mostly serpentinite. The chromite occurrence is 1,500 m long
and 700 m wide along a north-south trend. The occurrence
contains rare disseminations and massive accumulations
with as much as 70 percent chromite. The structure is usually
banded, lensoidal, or rarely massive. The main ore minerals
are chromite, magnetite, and spinel that are associated with
Ni, Fe, and Cu sulfides, and pentlandite, millerite, bravoite,
violarite, pyrrhotite and chalcopyrite. Sulfides occur both in
high-chromite ores and in low-chromite ores and listvenites.
Sulfide grains usually occur between chromite grains and
serpentinite-altered silicates and are xenomorphic.
Origin of and Tectonic Controls for Aluchin Metallogenic Belt
The ultramafic rocks hosting the deposits of the Aluchin
metallogenic belt are part of a faulted succession of the
Aluchin subduction-zone terrane (AC) that consists chiefly of
(Lychagin and others, 1989; Byalobzhesky and others, 1990;
Seslavinskiy and Ged’ko, 1990) of (1) dismembered ophiolites
of presumed middle Paleozoic age, including harzburgite,
pyroxenite, dunite, lherzolite, gabbro, plagiogranite, a mafic
dike suite, basalt, and local glaucophane schist, (2) tectonic
lenses of Middle Carboniferous to Early Permian island-arc
clastic-tuffaceous deposits, basalt, and andesitic basalt that
are intruded by diorite and tonalite, and (3) unconformably
overlying Late Triassic (Norian) shallow-marine volcanic and
sedimentary rocks, and Early Jurassic clastic deposits, which
contain pebbles of the underlying diorite and tonalite. In the
inner part of the Kolyma-Omolon superterrane, the subduction
zone linked to the Alazeya arc can be traced beneath Cenozoic
deposits along a horseshoe-like magnetic high (Parfenov,
1995c). The Aluchin subduction-zone terrane is interpreted as
a small part of an elongate subduction zone that was tectonically
linked to the mainly Late Triassic and Early Jurassic
Alazeya island arc (Nokleberg and others, 2000).
Ust-Belaya Metallogenic Belt of Podiform Cr
Deposits (Belt UB) Northeastern Part of Russian
Northeast
The Ust-Belaya metallogenic belt of podiform Cr
deposits (fig. 3; tables 3, 4) occurs in the Ust-Belaya duniteharzburgite
subterrane of the Penzhina Anadyr accretionary
wedge-oceanic terrane in the northeastern part of the Russian
Northeast (Nokleberg and others, 1994c, 1997a,b,c, 1998).
The one significant deposit at Ust-Belaya consists of zones of
closely spaced, banded chromite (10 to 30 percent chromite)
that occur as lenses, schlieren, and vein-like bodies of disseminated
and massive chromite (Silkin, 1983). The chromite