USGS Professional Paper 1697 - Alaska Resources Library

The belt is hosted in the Late Jurassic and Early Cretaceous
rocks of the Pekul’ney subduction-zone terrane (Nokleberg and
others, 1994c, 1997c). The significant deposit is at Skalistaya.
Skalistaya Basaltic Cu Deposit
The Skalistaya deposit (Shkursky and Matveenko, 1973)
consists of a network of prehnite-pumpellyite-silica-carbonate
and epidote-carbonate veinlets that vary from 2 to 20 cm
thick and that contain disseminated native copper. The veinlets
occur in basalt and consist mostly of prehnite and low-Fe
pumpellyite. The secondary minerals consist of laumontite,
calcite, dolomite, chlorite, quartz, epidote, and adularia. Native
copper intergrowths, ranging from 0.5 to 8 mm in diameter,
occur in prehnite and pumpellyite masses and in wall rocks.
Cu content of the ore is about 1 to 2 percent, and the native
copper contains as much as 100 g/t Ag. The ore bodies occur
in amygdaloidal basalt and associated tuff in a Late Jurassic
to Early Cretaceous volcaniclastic sequence that extends over
an area of about 1.0 by 0.6 km. Similar occurrences of native
copper are known along a zone that is as much as 18 km long.
The deposit is small with Cu grading about 1-2 percent.
Origin of and Tectonic Controls for Northwestern
Pekulney Metallogenic Belt
The Pekul’ney subduction-zone terrane, which hosts the
Pekulney metallogenic belt, is divided into western and eastern
units (Nokleberg and others, 1994c, 1997c). The western unit
consists of (1) a basal serpentinite matrix melange that contains
fragments of metamorphic rocks, including greenschist, glaucophane
schist, and picritic basalt, (2) a metamorphic complex that
is composed of amphibolite and schist, which are derived from
dunite, spinel peridotite, clinopyroxenite that yields Pb-Pb isotopic
ages of 1,600 to 1,800 Ma, and eclogite inclusions that yield
isotopic ages of 2,400-1,900 Ma, and (3) the Late Jurassic and
Early Cretaceous Pekulneyveem Formation, which is composed
of basalt, tuff, hyaloclastite, radiolarian chert, siltstone, and sandstone.
The basalt flows range as much as 60 to 80 m thick and are
interbedded with tuff, and cherty shale, all with abundant hematite
(Shkursky and Matveenko, 1973). The eastern Televeem unit
consists of a thick flysch sequence of Early Cretaceous (Aptian to
Albian) and Late Cretaceous (Cenomanian to Turonian) age. The
basaltic Cu deposits occur in the Pekulneyveem Formation and
are interpreted as forming in a primitive island arc and neighboring
sea-floor environment with subsequent incorporation of the
host rocks and deposits into a subduction zone, now preserved
in the Pekul’ney subduction-zone terrane that was tectonically
linked to the Pekul’ney island arc.
Tamvatney-Mainits Metallogenic Belt of
Podiform Cr Deposits (Belt TAM), East-Central
Part of the Russian Northeast
The Tamvatney-Mainits metallogenic belt of podiform Cr
deposits (fig. 48; tables 3, 4) occurs in the Tamvatney ophio-
Late Jurassic Metallogenic Belts (163 to 144 Ma; figs. 48, 49) 121
lite and other similar units in the east-central part of the Russian
Northeast. The Tamvatney ophiolite is tectonically interlayered
with other units in the Mainitskiy island-arc terrane
(Nokleberg and others, 1994c, 1997c). The deposits consist
of sparse localities of massive chrome spinel with accessory
Os-Ru-Ir minerals in dunite, pyroxenite, and associated rocks
(Dmitrenko and others, 1987, 1990). The significant deposits
at Krasnaya and Chirynai occur in dunites and layered complexes
of gabbro, dunite, and peridotite (table 4) (Nokleberg
and others 1997a,b, 1998).
Krasnaya Podiform Cr Deposit
The Krasnaya podiform Cr deposit (fig. 54) (Dmitrenko
and Mochalov, 1986; Dmitrenko and others, 1987) consists of
two horizons with numerous chromite bodies that occur within
the Krassnaya Gora alpine-type ultramafic body. An upper
horizon occurs at the contact of dunite and an overlying intergrown
pyroxenite-dunite-harzburgite assemblage. Chromite
occurs in dunite bands. Podiform and schlieren occurrences
of nearly massive to massive chromite extend for 35 top 70 m
with a thickness of as much as several meters. Several large
podiform chromite bodies at the base of dunite layers contain
massive and concentrated chromite for 60 to 100 m along
strike and are more than 1 m thick. Zones of disseminated
chromite as much as 22 m thick also occur. PGE associated
with chromite occur as solid solution in the sulfides with Os,
Ir, and Ru in hexagonal sites and Ir, Os, Pt, Ru, and Rh in
cubic sites. Some secondary, rare, platinum, rhodium, and palladium
arsenides and sulfoarsenides are also identified.
Origin of and Tectonic Controls for Tamvatney-Mainits
Metallogenic Belt
The Tamvatney ophiolite that hosts the Tamvatney-
Mainits metallogenic belt consists of a large, steeply dipping
tectonic block composed of an older assemblage of mainly
serpentinite mélange with peridotite and lherzolite and a
younger assemblage of Late Jurassic to Early Cretaceous
(Neocomian) basalt, andesite, and mafic volcaniclastic rocks
(Dmitrenko and others, 1990). The serpentinite mélange has
a complex structure and consists of ultramafic rocks, gabbro,
chert, Paleozoic and Early Mesozoic limestone, amphibolite,
green and glaucophane schist, and eclogite. The younger
assemblage consists of jasper, shale, basalt, plagiorhyolite,
siltstone, and sandstone (Tilman and others, 1982; Markov and
others, 1982). Lower structural assemblage is an accretionary
prism dominated by former oceanic lithosphere, whereas
the upper assemblage is interpreted as the base of the Late
Jurassic and Early Cretaceous Mainitskiy island arc (Palandzhyan
and Dmitrenko, 1990). The podiform Cr deposits in
the metallogenic belt (Krassnaya Gora and other deposits) are
hosted in the older assemblage, whereas the minor Cyprus
massive sulfide deposits are hosted in the younger assemblage.
The Mainitskiy terrane is tectonically linked to the Alkatvaam
accretionary-wedge terrane (Nokleberg and others, 2000).