USGS Professional Paper 1697 - Alaska Resources Library

156 Metallogenesis and Tectonics of the Russian Far East, Alaska, and the Canadian Cordillera
alteration to bersite. The deposit is judged to be exhausted
and was small with an average grade of 22 to 95 g/t Au. The
deposit occurs in the hinge of a brachyanticline at its periclinal
closure and is hostedby Late Triassic (Carnian) silty shale and
sandstone.
Utin Au Quartz Vein Deposit
The Utin Au quartz vein deposit (I.R. Yakushev, written
commun., 1950; P.I. Skornyakov, written commun., 1953;
Goryachev, 1995, 1998, 2003) occurs in a Late Jurassic suite
of ore-bearing dikes that extend for about 35 km. The dikes
cuts a Middle Jurassic sedimentary sequence at an acute angle
to bedding. The host sedimentary rocks are isoclinally folded
into west-northwest trending structures. The main ore body
extends 12 km and occurs in a steeply dipping dike, 0.4 to 1.3
m thick, which consists of hydrothermally altered andesite
porphyry. The dike is intensely crushed and deformed. The Au
quartz veins form complicated, often diagonally cross-cutting
systems within the dike. Some quartz veins also cut the dikes
obliquely and extend into the surrounding sedimentary rocks.
Arsenopyrite, pyrite, and pyrrhotite constitute as much as
several percent of the veins. Also occurring are gold, galena,
sphalerite, chalcopyrite, jamesonite, Bi-boulangerite, tetrahedrite,
scheelite, marcasite, and stibnite. The gold distribution is
quite irregular; individual ore shoots extend from 5 to 30 m in
strike and are as much as several hundreds of meters in width.
The small deposit was discovered in 1929 and produced 12
tonnes Au with grade ranging from 0.1 to 3,923 g/t Au. The
grade of ore shoots ranges from 5 to 3,923 g/t Au.
Alyaskitovoe Sn-W Greisen Deposit
The Alyaskitovoe Sn-W greisen deposit (Shur, 1985; Yu.
A. Vladimirtseva and V.M. Vladimirtsev, written commun.,
1987) consists of veins and greisen that occur inside and adjacent
to a stock of two-mica granite that intrudes Late Triassic
sandstone and shale. The veins are greisen, variable in thickness
and length and occur in fissures that strike north-northeast
and dip northwest at 75-85°. The veins have complex morphology
with enechelon lenses which alternate with thin stringers.
The main ore minerals are wolframite, cassiterite, and arsenopyrite.
The gangue minerals are mainly quartz, muscovite,
tourmaline, and apatite. A complex combination of sulfosalts
of lead, silver, and bismuth occur in the veins. The wallrocks
exhibit quartz-muscovite, muscovite-apatite, and tourmaline
greisen alteration. The deposit is small and has average grades
of 0.45 to 1.33 percent WO3 and as much as 0.38 percent Sn.
Origin and Tectonic Controls for Yana-Kolyma
Metallogenic belt
The Au quartz vein and associated deposits of the Yana-
Kolyma metallogenic belt occur as linear bands and clusters
that are controlled by two major sets of strike-slip fault zones
that trend northwest for more than 600 km (Goryachev, 1995,
1998, 2003). Secondary controlling structures are less exten-
sive, diagonal and transverse fault zones that are bounded by
northwest-trending strike-slip fault. The larger Au quartz vein
deposits generally occur in the northwest-trending zones. The
main types of the Au deposits occur in (1) mineralized shear
zones (linear stockworks) with low-grade ores that are not
as widespread as the other types of ore, but contain approximately
half of the Au reserves in the region, (2) quartz veins
that commonly contain high-grade ores and coarse gold that
are most favorable for milling, but contain smaller reserves,
(3) Au-bearing, quartz stockworks in altered dikes, with lowgrade
ores, but they contain high-grade ore shoots and medium
reserves, and (4) granitoid-related Au quartz veins and greisens
with low-grade Au and arsenic enriched ores
The host rocks for the Au quartz vein deposits of the
Yana-Kolyma metallogenic belt are Permian to Jurassic black
shale and tuffaceous sedimentary rocks that form part of the
Kular-Nera accretionary-wedge terrane. The shales locally
contain anomalous syngenetic carbonaceous matter. Au
mineralization is associated with burial and low-grade greenschist
facies metamorphism that consists of both contact and
regional, penetrative metamorphism and hydrothermal activity
(Gelman, 1976). The Au quartz vein deposits of the Yana-
Kolyma metallogenic belt are similar to other deposits of this
type throughout the world. The main features of this deposit
type are (1) predominance of quartz, (2) minor amounts of
subordinate sulfide minerals, mainly arsenopyrite and pyrite,
but telluride minerals are absent, (3) less than 3 to 5 percent
sericite, albite, carbonate minerals, and chlorite, and (4) in
many places, a considerable down-dip extension of ore with
little or no vertical zonation.
Most of the Au quartz vein deposits are associated with
the low-grade greenschist facies regional metamorphism. They
formed during a collision event after small dikes and stocks
of diorite-granodiorite intrusive suites, and before intrusion
of the granitoid plutons granodiorite-granite suites, and were
subsequently thermally metamorphosed. Consequently, the
Yana-Kolyma Au quartz vein deposits are interpreted as being
genetically or paragenetically related to the coeval, I-type,
diorite dikes and small granitoid plutons. The Au is interpreted
as forming in two stage (1) from the hydrothermal fluid generated
during metamorphism of sediment-hosted Au that in turn
was derived from an older period of multistage redeposition of
Au from auriferous zones of pyrrhotite and pyrite in SEDEX
deposits, and (2) from hydrothermal fluid generated during
intrusion of the I-type granitoids. The granitoid-related Au
quartz vein deposits are strongly genetically connected to the
Late Jurassic and Early Cretaceous granodiorite-granite and
granite-leucogranite plutons (Goryachev, 1998, 2003).
The Yana-Kolyma metallogenic belt is hosted in rocks
upper Paleozoic through middle Mesozoic age of the Yana-
Kolyma fold belt in the Kular-Nera accretionary-wedge
terrane of the Kolyma-Omolon superterrane (Nokleberg and
others, 1994c, 1997c; Goryachev, 1998, 2003). These units
are weakly metamorphosed to lower greenschist facies with
development of metamorphic chlorite and biotite. The Au
quartz vein, granitoid-related Au, and associated deposits of