Metallogenesis and Tectonics of the Russian Far East, Alaska, and ...

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 which occur in a subvolcanic
granite porphyry. Alternatively, the Skalistoe deposit may be part of post-accretionary Luzhkinsky rnetallogenic 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 and sheet
skarn which formed in several stages. An older stage consists dominantly of pyroxene, plagioclase, amphibole, and garnet. A
subsequent stage consists of greisen alteration of skam and granitoid rocks with formation of quartz, feldspar, and muscovite,
along with minor chlorite and biotite which contains scheelite and apatite, and minor arsenopyrite, pyrrhotite, and chalcopyrite. A
late stage consists of scheelite and quartz with followed by crystallization of low temperature scheelite and arsenopyrite. The
deposit occurs along the flat to steeply-dipping contacts of granitoid plutons which 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 up to a few tens of percent. Plagiogranite with an approximate K-
Ar age of 110 ma is interpreted as forming with the deposlt. The deposit is large with average grades of 0.65% WzO3 and 1.64%
Cu. The deposit has been mined since the 1980's.
The olistosh-omes which host the Samarka belt of Sn and W skarn deposits are derived from the caps of guyots which 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 cornmun., 1983) occurs along the margin
of an Early Cretaceous biotite, peraluminous granite which intrudes olistoliths of Permian sedimentary shales and interbedded
with limestone. The skarn occurs in altered limestone. Various mineral assemblages are magnetite, gamel, pyroxene-garnet,
garnet-epidote, and garnet-orthoclase. Late-stage quartz-feldspar and quartz-amphibole overgrowths which 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% Wz03.
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 which
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 cornmun., 1997; Khanchuk and others, 1998). K-Ar isotopic
studies indicate inh-usive ages of 1 I0 to 115 Ma for the granitoid rocks hosting the deposits (Rostovsky, 1987; Stepanov, 1977).
Algama Metallogenic Belt of Stratiform Zr Deposits
(Belt AL) Northern Part of Russian Southeast
The Algama metallogenic belt of stratiform Zr deposits (fig. 61; tables 2. 3) occurs to the north of the Kondyor massif at
Algama in the southeastern part of the Stanovoy block of the North Asian Craton (unit NSS; Nokleberg and others, 1997b, 1998).
in the western part of the Khabarovsk province of the Russian Southeast. The deposits are hosted in Vendian carbonate rock. The
belt contains many small occurrences and a significant deposit at Algaminskoe which has been drilled and explored by
underground workings (Nekrasov and Korzhinsky, 1991). The Algaminskoe deposit consists of hydrozircon and baddeleyite
which occurs with a colloform texture. The ore occurs in long lenses, conformable to a layer of cavernous dolomite marble which
ranges up to about 40 m thick, and in zones which crosscut metamorphosed Vendian metasedimentary rocks. The baddeleyite
deposits are interpreted as forming during hypogene and supergene alteration which was associated with karst formation. The U-
Pb zircon isotopic age of baddeleyite is approximately 100 Ma (J.N. Aleinikoff, written commun., 1992). The origin of the
stratiform Zr deposits is interpreted as related to alkalic igneous rocks, possibly including carbonatite, which were associated with
the late stage intrusion of the zoned mafic-ultramafic rocks, possibly part of the same belt of Late Jurassic to midCretaceous
intrusives at Kondyor. With this interpretation. the Algama belt formed during intrusion of alkali igneous rock associated with