USGS Professional Paper 1697 - Alaska Resources Library

assemblages, which may have been mostly thrust under the
margin of what is now North America.
(10) The Gravina arc continued to form. Associated
with the Gravina arc was subduction of part of the Farallon
oceanic plate to form the Chugach (CG), Bridge River (BR),
Easton (EA), and Baker (BA) terranes. The arc is preserved
in the Kahiltna (kh) and Gravina-Nutzotin-Gambier (gg)
assemblages, which occur only on the Wrangellia superterrane.
Forming in the Gravina arc was the western-southeastern
Alaska (WSE) metallogenic belt, which contains granitic-magmatism-related
deposits.
(11) The Wrangellia superterrane (WRA) started to
accrete at about 60° paleolatitude with the continentward part
of the Wrangellia superterrane impinging onto a collage of
terranes previously accreted to the North American Craton
Margin (NAM). During accretion, the arc was extended to the
south during the formation of two major overlap assemblages,
the coeval Tahtsa-Three Sisters magmatic assemblage (tt) and
the Spences Bridge volcanic-plutonic belt (sb).
Metallogenic Belts Formed Along Late
Mesozoic Continental-Margin Transform Faults
in Russian Southeast
Samarka Metallogenic Belt of W Skarn, and
Porphyry Cu-Mo Deposits (Belt SA), West-
Central Part of Russian Southeast
The Samarka metallogenic belt of W and Sn skarn and
porphyry Cu-Mo deposits (fig. 61; tables 3, 4) occurs in aluminous,
mainly S-type granitoid rocks of Early to mid-Cretaceous
age that intrude the Samarka accretionary-wedge terrane
in the west-central part of the Russian Southeast. The belt
contains major W skarn deposits at Benevskoe and Vostok-2,
small porphyry Cu-Mo deposits at Khvoshchovoe, Kafen,
and Malakhitovoe, and a porphyry Mo deposit at Skalistoe
(table 4) (Nokleberg and others 1997a,b, 1998). The deposits
are hosted in Early to mid-Cretaceous granodiorite porphyry,
granite, and gabbro-diorite. The southern-most part of the
Samarka metallogenic belt occurs in a displaced fragment of
the Samarka terrane, which is displaced along a left-lateral,
north-east-trending strike-slip fault (fig. 61). The small W
skarn deposit at Benevskoe occurs in this displaced fragment.
Also possibly part of the same Samarka metallogenic belt is
a younger, Paleocene porphyry Cu-Mo deposit at Skalistoe
(fig. 61). This deposit consists of veinlet molybdenum ore and
wolframite that occur in a subvolcanic granite porphyry. Alternatively,
the Skalistoe deposit may be part of postaccretionary
Luzhkinsky metallogenic belt described below.
Vostok-2 W Skarn Deposit
The major productive Vostok-2 deposit (fig. 64) (Stepanov,
1977; Rostovsky and others, 1987) consists of vein
Early Cretaceous Metallogenic Belts (144 to 120 Ma; figs. 61, 62) 141
and sheet skarn that formed in several stages. An older stage
consists dominantly of pyroxene, plagioclase, amphibole, and
garnet. A subsequent stage consists of greisen alteration of
skarn and granitoid rocks with formation of quartz, feldspar,
and muscovite, along with minor chlorite and biotite that contains
scheelite and apatite, and minor arsenopyrite, pyrrhotite,
and chalcopyrite. A late stage consists of scheelite and quartz
followed by crystallization of low temperature scheelite and
arsenopyrite. The deposit occurs along the flat to steeply dipping
contacts of granitoid plutons that intrude olistostromes of
large, Carboniferous-Permian limestone and calcareous-shale.
Successive skarn and greisen alternation of limestone preceded
the deposition of scheelite and other minerals, including
gold and apatite, locally as much as a few tens of percent.
Plagiogranite with an approximate K-Ar age of 110 ma is
interpreted as forming with the deposit. The deposit is large
with average grades of 0.65 percent W 2O 3 and 1.64 percent
Cu. The deposit has been mined since the 1980’s.
The olistostromes that host the Samarka belt of Sn and
W skarn deposits are derived from the caps of guyots that
are enclosed in a matrix host of highly deformed Jurassic
sedimentary rocks in the accretionary-wedge complex of the
Samarka terrane. The skarns are hosted in limestone layers
and occur along the contacts of calcareous and alumosilicate
clastic rock.
Benevskoe W Skarn Deposit
The small Benevskoe W skarn deposit (V.D. Shlemchenko
and others, written commun., 1983) occurs along the
margin of an Early Cretaceous biotite, peraluminous granite
that intrudes olistoliths of Permian sedimentary shales and
interbedded with limestone. The skarn occurs in altered
limestone. Various mineral assemblages are magnetite, garnet,
pyroxene-garnet, garnet-epidote, and garnet-orthoclase.
Late-stage quartz-feldspar and quartz-amphibole overgrowths
replace the skarns and locally contain disseminated scheelite.
Late-stage quartz-sericite and zeolite alterations also occur.
The major ore minerals are scheelite with minor magnetite,
arsenopyrite, pyrite, and rare cassiterite. Gangue minerals are
quartz, feldspar, amphibole, epidote, biotite, and tourmaline.
Easily concentrated apatite also exists. The deposit is small
with average grades of 0.44 to 3.15 percent W2O3. As to the
north, small porphyry Cu-Mo occurrences also occur in this
part of the metallogenic belt.
Origin of and Tectonic Controls for Samarka
Metallogenic Belt
The W skarn deposits of the Samarka metallogenic belt
occur near mainly Early Cretaceous, S-type granitoid rocks
that intrude the Samarka accretionary-wedge terrane. The host
granitoid rocks and associated deposits are herein interpreted
as forming in underthrusting of the Kula oceanic ridge and
resultant genesis of bimodal igneous activity along the transform
continental margin of the Russian Southeast (A.I. Khanchuk,
written commun., 1997; Khanchuk and others, 1998).