USGS Professional Paper 1697 - Alaska Resources Library

44 Metallogenesis and Tectonics of the Russian Far East, Alaska, and the Canadian Cordillera
shale along the granite contacts, and in rare limestone inclusions
within the granite. The deposit is of medium size. The average
grade is 0.52 percent Sn. The deposit was mined from the
1950’s through the 1970’s. More than forty occurrences of and
vein deposits are known in the Yaroslavka metallogenic belt.
Origin of and Tectonic Controls for Yaroslavka
Metallogenic Belt
The leucogranites hosting the Yaroslavka metallogenic
belt of fluorite and Sn greisen deposits are lithium-fluorine-
REE enriched (Ryazantseva, 1998). The extensive deposits
occur in the apical parts of plutons, altered to quartz-mica-fluorite-REE
greisen, which intruded Early Cambrian limestone
of the Voznesenka passive continental-margin terrane. The
leucogranites are interpreted as forming during anatectic melting
of older granitic gneisses and Cambrian sedimentary rocks
(Khetchikov and others, 1992) presumably during collision of
the Voznesenka and Kabarga terranes in the early Paleozoic
(Nokleberg and others, 1994c, 1997c; Khanchuk and others,
1996, 1998). The Voznesenka terrane hosting the Yaroslavka
metallogenic belt consists of four major units: (1) Cambrian
sandstone, pelitic schist, rhyolite, felsic tuff, and limestone and
dolomite form a section as much as several thousand meters
thick, with rhyolite that yields a Rb-Sr whole-rock isotopic age
of 512 Ma, (2) Ordovician to Early Silurian conglomerate and
sandstone that contains a questionable flora, (3) Early Devonian
rhyolite and felsic tuff, Middle to Late Devonian rhyolite,
felsic tuff, and rare basalt, and (4) Late Permian basalt,
andesite, rhyolite, sandstone. The stratified Cambrian rocks are
intensely deformed and intruded by collision-related Devonian
granitoid rocks with isotopic ages of 440 to 396 Ma (Ryazantseva
and others, 1994. The Cambrian sedimentary and volcanic
units of the Voznesenka terrane are interpreted as a fragment
of a Late Proterozoic to early Paleozoic carbonate-rich sedimentary
rock sequence that formed on a passive continental
margin. Archaeocyathid in Cambrian limestone is related to the
Australia paleogeographic province. The Voznesenka terrane is
interpreted as a fragment of the passive continental margin of
Gondwanaland (Khanchuk and others, 1998).
Metallogenic Belts Formed in a Middle
Paleozoic Continental Arc Along North Asian
and North American Craton Margins
Kedon Metallogenic Belt of Au-Ag Epithermal
Vein, Porphyry Mo, Fe Skarn, and Associated
Deposits (Belt KE) Central Part of Russian
Northeast
The Kedon metallogenic belt of Au-Ag epithermal vein,
porphyry Mo, Fe skarn and associated deposits (fig. 16; tables
3, 4) occurs in the central part of the Russian Northeast. The
belt is hosted in early and middle Paleozoic granite, and
coeval rhyolite, andesite, trachyandesite, silicic tuff, and associated
sedimentary rocks of the Omolon cratonal terrane of the
Kolyma-Omolon superterrane (Nokleberg and others, 1994c,
1997c). The areal extent of the Kedon metallogenic belt is
approximately 40,000 km 2
.
The Au-Ag epithermal vein deposits occur in subaerial
extrusive rocks and subvolcanic equivalents and in tuff of
Middle Devonian through Early Carboniferous age. The significant
deposits are at Olcha, Kubaka, and Zet (table 4) (Nokleberg
and others 1997a,b, 1998). These deposits occur in trachyandesite-trachydacite
volcanic rocks of Early Carboniferous age
(Igor N. Kotlyar, written commun., 1995). Small deposits, such
as at Tumannaya, Obyknovennoe, and Yolochka, occur in felsic
volcanic rocks of Late Devonian age. Some epithermal vein
deposits, as at Grisha, also occur in early(?) Paleozoic syenite.
Porphyry Mo-Cu deposits, as at Vechernee and elsewhere, occur
in middle Paleozoic, potassic granitoid rocks and subvolcanic
rhyolites. Fe skarn deposits, as at Skarnovoe and elsewhere,
occur in early Paleozoic granite that intrudes an Archean iron
formation, which provided Fe for the Fe skarn deposit (Fadeev,
1975). The available field, isotopic, and paleoflora data indicate
that the magmatic rocks of the Kedon metallogenic belt formed
mainly in the Middle Devonian through the Early Carboniferous
(Lychagin and others, 1989).
Kubaka Au-Ag Epithermal Vein Deposit
The Kubaka Au-Ag epithermal vein deposit (fig. 21)
(Savva and Vortsepnev, 1990; Stepanov and others, 1991; V.A.
Banin, oral commun., 1993; I.N. Kotlar, written commun.,
1986; Layer and others, 1994) consists of veins and zones of
adularia-quartz and adularia-chalcedony-hydromica-quartz
veinlets that contain fluorite, barite, and carbonate. The veins
occur in a northwest-trending elongate caldera 4 km in diameter.
The caldera lies transverse to the northeast trend of the main
regional structural trend. The caldera is rimmed by Middle to
Late Devonian volcanic rocks and volcanogenic sediments and
is filled with Late Devonian to Early Carboniferous volcanic
rocks. The Au-bearing veins occur within the caldea and are
localized in subvolcanic trachydacite in a stratified Middle to
Late Devonian volcaniclastic sequence composed of ignimbrite,
pumiceous rhyolite to dacite, trachyandesite and rhyolite-dacite
sills, and tephra and agglomerate tuff of various compositions.
The veins die out in the overlying Early Carboniferous carbonaceous
shale and siltstone. The most intensely mineralized
veins trend about east-west and west-northwest. The host rocks
are intensely silicified, adularized, and sericitized, with the
development of much hydromica. Initial stage of mineralization
was marked by a gold-chalcedony association with colloidal
gold (with electrum and küstelite). A later adularia-quartz stage
contains coarser, recrystallized native gold and scattered, disseminated
pyrite, arsenopyrite, galena, freibergite, acanthite,
aguilarite, naumannite, argentopyrite, and Au-Ag sulfides
in fine-grained aggregates. Native gold predominates markedly
over sulfide-bound gold. The Au:Ag ratio is 1:1 to 1:2.
The deposit is medium size with proven reserves of about 100