USGS Professional Paper 1697 - Alaska Resources Library

204 Metallogenesis and Tectonics of the Russian Far East, Alaska, and the Canadian Cordillera
in the region (Indolev and others, 1980). Alternatively, the
clastic-sediment-hosted Au-Sb deposits may be part of a suite
of quartz vein deposits and are part of the Yana-Kolyma metallogenic
belt (Berger, 1978, 1993). The epithermal and polymetallic
vein deposits of the Adycha-Taryn metallogenic zone are
herein interpreted as forming in the rear and transverse part of
the Cretaceous to early Tertiary Okhotsk-Chukotka volcanicplutonic
belt (fig. 79), which forms a major, postaccretionary
continental-margin arcs in the Russian Northeast (Nokleberg
and others, 1994c, 1997c, 2000).
Sentachan Clastic-Sediment-Hosted Au-Sb Deposit
The Sentachan clastic-sediment-hosted Au-Sb deposit
(Shur, 1985; Indolev and others, 1980; V.V. Maslennikov, written
commun., 1985) consists of two rod-like veins that vary
from 85 to 200 m long and are as much as 3.1 m thick that
occur in northwest-striking shear zones in northwest-trending
faults that are part of the Adycha-Taryn Fault zone. The veins
are hosted in Late Triassic (Norian and Rhaetian) clastic rocks
that are deformed northwest trending gently plunging folds
parallel to the fault. The main minerals are stibnite and quartz;
subordinate minerals are ankerite, muscovite, pyrite, arsenopyrite,
dickite, and hydromica. Rare minerals are sphalerite,
gold, chalcostibnite, berthierite, tetrahedrite, zinkenite, jamesonite,
aurostibnite, and chalcopyrite. The wallrocks exhibit
silicic, carbonate, hydromica, and dickite alteration. The grade
varies from 3.2 to 40.3 percent Sb. Mined and proven reserves
total 100,000 tonnes Sb, making the Sentachan Au-Sb deposit
one of Asia’s largest Sb deposits.
Ak-Altyn Au-Ag Epithermal Vein Deposit
The small Ak-Altyn Au-Ag epithermal vein deposit (Yu.A.
Vladimirtseva, written commun., 1985) consists of quartz and
quartz-carbonate veins, as much as 2 to 3 m thick, and stringers
that occur in a zone 10 to 30 m wide and 150 m long. The
deposit is hosted in gently dipping Middle Triassic (Ladinian)
terrigenous rocks that are intruded by Early Cretaceous diorite
porphyritic dikes. The ore is dominated by fine-grained quartz
(chalcedony) with sparse sulfides (about 1 percent), including
galena, sphalerite, chalcopyrite, arsenopyrite, and pyrite. Gold
fineness is low. Average grades are 0.2 to 60.4 g/t Au; and 0.1 to
1 percent combined Ag, Hg, Pb, Sb, Zn, As, and Cu.
Eastern Asia-Arctic Metallogenic Belt:
Omsukchan Zone of Sn Polymetallic Vein, Sn
Silicate-Sulfide, Porphyry Sn, Au-Ag Epithermal
Vein, Porphyry Mo-Cu, and Associated
Deposits (Belt EAOM), Southeastern Part of
Russian Northeast
The Omsukchan metallogenic zone (fig. 79; tables 3, 4)
occurs in the southeastern part of the Russian Northeast and
forms an extensive transverse (orthogonal) branch of the East
Asian-Arctic metallogenic belt. The zone occurs in and near
the Okhotsk-Chukotka volcanic-plutonic belt (Nokleberg
and others, 1994c, 1997c), is more 150 km long, and ranges
from 10 to as much as 70 km wide. The significant deposits
in the zone are Sn polymetallic vein deposits at Maly Ken and
Trood; Sn silicate-sulfide vein deposits at Egorlyk, Galimoe,
Khataren-Industrial, and Okhotnichie; Au-Ag epithermal
vein deposits at Arylakh, Dukat, and Rogovik; porphyry Sn
deposits at Ircha, Nevskoe, and Novy Djagyn; and Sb-Au
and polymetallic vein deposits at Elombal, Mechta, Tidit, and
Yakor (table 4) (Nokleberg and others 1997a,b, 1998). Mainly
Sn and Ag polymetallic vein, and W, Au, and Co vein deposits
occur in the northern and middle parts of the belt, whereas
mainly Au-Ag epithermal vein and porphyry Mo-Cu deposits
occur in the southern part.
The Omsukchan metallogenic belt occurs in a unique,
local extensional tectonic environment in continental crust as
much as 52 km thick, which is herein interpreted as forming
in the back-arc portion of the Okhotsk-Chukotka volcanicplutonic
belt. The rift-trough is filled by Early Cretaceous volcanic
and sedimentary sequences of continental coal-bearing
molasse that are unconformably overlain by Albian through
Cenomanian andesite, rhyolite, and ignimbrite. The total thickness
is greater than 3,000 m. The lower part of the molasse
includes the Aptian Ascoldin Formation, which is composed of
high-siliceous, ultra-potassic rhyolite. The plutonic rocks associated
with the metallogenic belt are dominated by the potassic
biotite granite of the Omsukchan Complex that has a K-Ar
isotopic age of approximately 90 Ma, and a Rb-Sr whole-rock
isochron age of 80 Ma (Goryachev, 1998).
The significant Sn and Sn-Ag deposits, mainly Sn
polymetallic vein, Sn silicate-sulfide, and porphyry Sn deposit
types, are at Nevskoe, Galimoe, Ircha, Khataren-Industrial,
Trood, Novy Djagyn, and Maly Ken. Some of these deposits
were recently discovered. Most of the numerous, small but
rich and quickly exploited Sn deposits were exhausted in the
1940’s. Various types of Sn deposits are associated with the
Omsukchan granite and comagmatic extrusive-subvolcanic
complexes. At depths, these Sn deposits are associated mainly
with granitoid plutonic rocks. At intermediate levels, the Sn
deposits are associated with volcanic and plutonic rocks. Near
the surface, the Sn deposits are associated with volcanic rocks
and consist mainly of Sn-Ag deposits, and associated Agpolymetallic
and Au-Ag epithermal vein deposits. The significant
Ag-polymetallic vein deposits are at Mechta and Tidit.
Nevskoe Porphyry Sn Deposit
The Nevskoe porphyry Sn deposit (Lugov, Makeev,
and Potapova, 1972; Lugov, 1986) consists of fine-grained,
complexly intergrown pyrophyllite, topaz, quartz, muscovite,
and cassiterite. Also widespread are tourmaline, chlorite,
wolframite, arsenopyrite, chalcopyrite, galena, sphalerite,
pyrite, pyrrhotite, marcasite, tetrahedrite-tennantite, stannite,
rutile, and scheelite. Semseyite, guanajuatite, laitakarite,
silver, zunyite, apatite, fluorite and other minerals are rare. Sn