Vostok 2

Primorskiy Kray, Russia

Main commodities: W Au Cu
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The Vostok-2 tungsten, base metal and gold deposit is located ~230 km SSW of Khabarovsk and 500 km NNE of Vladivostok within the Primorsky Kray of far eastern Russia (#Location: 46° 30' 3"N, 135° 54' 22"E)

The deposit was discovered in 1961, and from 1969 was mined by the Primorsky Mining and Concentrating Combine, first by open pit, and subsequently from an underground operation. The mine produced two main products in 2010, a 53 to 56 wt.% WO3 and a gold-sulphide concentrate (Soloviev and Krivoshchekov, 2011).

Vostok-2 is located in the Sikhote-Alin Belt of the Pacific Orogen and lies within a metallogenic province that includes numerous tungsten deposits and occurrences of differing combinations of metals, including base metal-tungsten, tin-tungsten, rare metal-tin-tungsten and style, including skarn, greisen and stockwork (Ivanov, 1974; Stepanov, 1977; Gvozdev, 2007; Soloviev, 2008). Most are localised in the wide and extended belt adjoining the major Sikhote-Alin Fault.

The Vostok-2 deposit is related to the 80 km
2 ilmenite series Late Cretaceous multiphase monzodiorite-granodiorite-granite Dal'nensky pluton intruded into a Jurassic to Cretaceous carbonaceous clastic-volcanic-carbonate sequence. It is also spatially associated with the minor, 2.5 km2 granodiorite porphyry Central (or Vostok-2) Stock, which has petrological features transitional to those of intrusive rocks accompanying Au-W and Au deposits elsewhere in the metallogenic province.

The Dal'nensky Pluton has been dated at 128 ±16 Ma (Rub et al., 1982) and is somewhat enriched in alumina, and characterised by a low degree of iron oxidation, which with the predominance of ilmenite among accessory minerals indicates that the granitic rocks belong to the ilmenite series (Ishihara, 1981). It is also enriched in F, P, Cr, Co, Ni, V, Ba and Sr with relatively low concentrations of Rb, Li, Be, Nb, Mo and Sn; and elevated K/Rb and low Rb/Sr ratios which are typical of derivatives of deep seated magmas (Kovalenko et al., 1988).

The Central Stock has been dated at 111 to 112 ±4 Ma (Rub et al., 1982) and is distinguished by a higher Na
2O content, with granodiiorites from the pluton and stock having Na2O:K2O ratios averaging 0.98 and 0.76 respectively (Rub et al., 1982). The Central Stock is also enriched in Ca and Mg, reflected by its more magnesian biotite (Stepanov, 1977). The stock also has a lower content of magnetite and ilmenite, with only a few ppm of the two minerals in total, compared to 45.5 ppm ilmenite and 5 ppm magnetite, on average, in granodiorite of the pluton. Other contrasts include 250 ppm zircon and 14.5 ppm titanite in the Da'nensky Pluton and 46 and 34.5 ppm of the same minerals, respectively in the Central stock. The smaller dimensions of the stock, absence of amphibole in granodiorite, and its younger age are interpreted to indicate it is a more evolved (porphyry) intrusive fraction of the Early Cretaceous igneous complex. However, Soloviev and Krivoshchekov (2011) suggest advanced fractionation does not necessarily explain the compositional differences between the two intrusions, and that the possibility the stock belongs to a separate complex of minor intrusions cannot be excluded.

Within the Sikhote-Alin Belt, suites of granite associated with gold mineralisation are generally magnetic sodic granitoids, distinguished by a high degree of iron oxidation (ƒ = Fe
2O3/[Fe2O3 + FeO] > 0.3), whilst 'tin-tungsten granites' are generally potassic and nonmagnetic with ƒ = <2 (Moissenko and Eirish, 1996). The Central Stock are enriched in sodium and have intermediate ƒ = 0.12 to 0.39, whilst other geochemical features either correspond to Au-bearing intrusions or are transitional between the granitoids productive for Au and W mineralisation.

Biotite granodiorite of the Central Stock is medium- to coarse-grained and porphyritic at shallower levels, with a zonation of plagioclase phenocrysts from andesine An
45-50 in the core to oligoclase An15-20 at the margins. Quartz and microcline dominate in the groundmass, with lesser plagioclase An15. It contains 10 to 12 vol.% biotite, although more melanocratic varieties approaching diorite are occasionally found on the stock margins. Internally, the stock grades from granodiorite into 'plagiogranite' that predominates in its southeastern part and immediately adjacent to the orebodies. This phase has slightly increased quartz and biotite contents with zonal plagioclase predominating over microcline in the groundmass. It differs from granodiorite and granite in the prevalence of sodium over potassium and in an enrichment in calcium relative to granite with the same silica content. A small, funnel-shaped breccia body is found in the centre of the stock, decreasing in diameter downward to pinch out at a depth of 90 to 110 m below the surface. Breccia clasts, the largest of which are composed of granodiorite, diorite, biotite hornfels, quartzite and other rocks are cemented by cataclastic biotite granodiorite.

Numerous basic, intermediate and siliceous dykes cut the deposit area. Large dolerite, porphyritic pyroxene-plagioclase and pyroxene-amphibole diorite dykes are developed within transverse NW trending faults and are commonly regarded as the youngest Cretaceous igneous rocks in the district

Mineralisation occurs within a NE striking thrust-fault zone that is a splay of the Sikhote-Alin Fault and cuts the sedimentary country rock sequence. The fault/tectonic zone ranges from 60 to 110, averaging 70 to 85 m in thickness. It encloses tectonic slabs, fragments and boudins of carbonate rock set in fine-grained silicified cherty material, and includes several tabular mineralised skarn bodies. The largest tabular skarn body is ~30 to 35 m thick (18 m at surface), thinning upwards and downwards, and extending for 650 to 700 m along strike and 700 to 800 m down-dip, dipping at 60 to 80°NW. Intense biotite hornsfels, which predates the skarn alteration, extends for 2.5 to 3.0 km, elongated in a NE direction, and is more extensive than the skarn, surrounding it. Scheelite and sulphide mineralisation replaces skarn altered masses, inheriting the structure of the original carbonate slab. As a consequence, the Main orebody corresponding to the largest skarn altered mass, which is confined to the central part of the ore bearing structure. A number of smaller, near parallel orebodies are largely localised in the hanging wall of the Main orebody, which is nearly conformable with the ore bearing zone and bedding of the host rocks. Small relict lenses of carbonate, biotite hornfels cherty rocks and skarns remain within the orebody.

Alteration and mineralisation took place through the following stages (after Soloviev and Krivoshchekov, 2011):
Early pre-skarn hornfels metamorphism, grading to a semi-regional propylitic alteration, that accompanied emplacement of the large Dal'nensky Pluton at a depth. This metamorphism extends along the bedding of host rocks, and is dominantly composed of biotite with associated quartz, calcite, K feldspar, amphibole, chlorite, plagioclase, titanite and pyrrhotite, largely occurring as a quartz-feldspar-biotite hornfels which locally grades into a epidote-chlorite variety. These rocks are truncated by granodiorite of the Central stock and are replaced by calc-skarn and postskarn metasomatic rocks of the stages described below;
Prograde Calc-skarn, which occurs as several kinds:
 -  Banded skarn, which is often conformable with lithologic contacts, with plagioclase-pyroxene and garnet-pyroxene skarns from centimetres to a few metres in thickness following alternations of aluminosilicate and carbonate metasedimentary rocks such as biotite hornfels, quartzite and marble. Generally biotite hornfels is converted via amphibole-quartz-plagioclase-titanite to pyroxene&plusn;plagioclase and titanite bands, whilst marble is transformed to garnet and pyroxene, often accompanied by vesuvianite. The calcareous exoskarns assemblages grade outward to from pyroxene-garnet to wollastonite and marble distal to the intrusion;
 -  Stockwork skarn - thin, typically up to 1 to 5 cm thick branching veinlets that crosscut host rock banding and occur as small, near conformable veined skarn bodies that are largely preserved on the flanks of the deposit. They are composed of pyroxene, occasionally with wollastonite and/or pyroxene-calcite and are usually in the more distal skarn, locally mainly developed within marble.
  Observed contacts between the Central granodiorite stock and prograde skarn are sharp and crosscutting, whilst skarn xenoliths are found in the granodiorite cemented eruptive breccia (Rub et al., 1982; Stepanov, 1997). The replacement of granodiorite with mineral assemblages that could be classified as prograde skarn has not been observed (Soloviev and Krivoshchekov, 2011).
Retrograde Quartz-Feldspar Metasomatic rocks, comprising garnet-pyroxene-quartz and pyroxene-quartz assemblages with amphibole, plagiocalse, sulphides and scheelite. These assemblage replace skarn alteration, whilst the substantially plagioclase or amphibole-quartz-plagioclase assemblages replace the adjacent aluminosilicate rocks. This stage involves silicification of the prograde skarn, replacement of pyroxene and garnet, and often recrystallisation of these minerals forming euhedral crystals of a different composition, particularly pyroxene which is transformed to the most ferroan variety with up to 80 to 95 mol.% hedenbergite. Euhedral crystals of scheelite, apatite, amphibole and sulphides (pyrrhotite, chalcopyrite and less frequent arsenopyrite) occur in the centre of zones of silicification. sulphides are associated with amphibole (ferroactinolite), forming interstitial impregnations, or replacing pyroxene. To a large extent, the scheelite-bearing metasomatic rocks of this stage correspond to the scheelite-skarn type of ores.
Propylitic alteration, a further retrograde phase that is widespread, and often intense, completely replace skarn alteration. Low Fe amphibole is dominant, occurring as quartz-amphibole, and less frequently, biotite-amphibole and amphibole-epidote-chlorite, accompanied by varying amounts of calcite, oligoclase-albite, epidote-zoisite, chlorite, titanite and apatite. It consistently contains pyrrhotite, scheelite and chalcopyrite, often in substantial amounts, contributing to the scheelite-sulphide ores. Cummingtonite occurs in the biotite rich facies replacing low-Ca rocks, whilst actinolite develops in amphibole rich phases, trending toward actinolite-tremolite in epidote-chlorite varieties.
Quartz-sericite alteration, the final retrograde and main mineralising phase. This event occurred after emplacement of the Central Stock, replacing its upper parts, including the eruptive breccia, and along fault zones into adjoining country rocks. The process commences with the formation of dark micas with a biotite to phlogopite composition, giving way to muscovite/sericite, quartz and chlorite (Apel'tsin et al., 1983). Soloviev and Krivoshchekov (2011) observed a zonation reflecting this from distal to proximal relative to to the intrusion and within veins, of chlorite → albite → sericite → quartz-carbonate-sericite → quartz-sericite → quartz. Quartz and sulphide with lesser apatite and scheelite dominate in the intermediate zone, whereas aggregate of albite An
5-10, adularia, carbonate, scheelite and apatite with subordinate quartz, muscovite-sericite and sulphides occur in the proximal zone. As such, scheelite is most abundant in the intermediate to proximal zone, accompanied by carbonate, albite and abundant (up to 20 vol.%) apatite. This phase contains variable amounts of sulphides from low-sulphide varieties corresponding to the scheelite-quartz, to high-sulphide corresponding to the scheelite-sulphide ore. The sulphides assemblages include arsenopyrite, chalcopyrite, chalcopyrite-pyrrhotite, arsenopyrite-pyrrhotite-chalcopyrite and sphalerite, all of which are associated with scheelite. Arsenopyrite is the most proximal and sphalerite the most distal. A wide set of sulphosalts, tellurides and gold minerals are also associated with the sulphides (Gvozdev and Tsepin, 2005). Stepanov (1977) demonstrated a gradual decrease in homogenisation temperature of fluid inclusions in the series scheelite-quartz (370 to 285°С, occasionally 420 to 400°С) → scheelite-arsenopyrite (310 to 270°С, rarely 340 to 330 and 250°C) → scheelite-chalcopyrite-pyrrhotite (310 to 240°C) → sphalerite-pyrrhotite (240 to 180°С).

The descriptions above show that scheelite was involved in the mineral assemblages of all four post-skarn mineralising stages, with the development of mineralization from scheelite via sulphide-scheelite with pyrrhotite and chalcopyrite to the gold-base-metal-scheelite assemblage with arsenopyrite, Bi-Sb-Te-Pb-Zn sulphides and sulphosalts.

Four main scheelite types are recognised as follows:
Scheelite 1 is associated with the retrograde quartz-feldspar alteration assemblage, particularly with pyroxene and quartz. It has a pale yellow fluorescence in ultraviolet (UV) light and a moderate molybdenum admixture of 0.05 to 0.5 wt.% MoO
3. Rocks containing only scheelite 1 typically grade <0.5 wt.% WO3.
Scheelite 2 was developed during the propylitic alteration stage, crystallised in association with amphibole, pyrrhotite and chalcopyrite which corroded and replaced it. It has a white fluorescence in UV light, with an Mo content of 0.01 to 0.05 wt.% MoO
3. The scheelite-molybdenite mineralisation of the propylitic stage contains up to 1 to 2 wt.% WO3.
Scheelite 3 was emplaced in two stages during the quartz-sericite alteration event.
 -  Scheelite 3/1 is related to the quartz-arsenopyrite assemblage of this stage, most intensely developed proximal to contacts with granodiorite. It has a white-blue fluorescence in ultraviolet (UV) light and Mo content of 0.005 to 0.01 wt.% MoO
3. In the quartz-scheelite zone the WO3 grade reaches a few percent.
 -  Scheelite 3/2 is related to the later quartz-apatite-sulphide assemblage. It has a blue fluorescence in ultraviolet (UV) light and is free of molybdenum. This zone may contain as much as 20 to 30 vol.% large (up to 3 to 5 cm) scheelite 3/2 crystals with the highest grade mineralisation locally reaching 20 to 30 wt.% WO

Pyrrhotite, chalcopyrite and arsenopyrite are the most abundant sulphides,occurring as two generations related to the propylitic and quartz–sericite stages respectively. Pyrrhotite 1 locally occurs as 80 to 90 vol.% of the propylitic zone, especially in the cores of orebodies. It is associated with scheelite 2. Pyrrhotite 2 was precipitate during the quartz–sericite stage. Chalcopyrite 1 is less abundant than pyrrhotite 1, which it replace, although chalcopyrite 2 associated with the quartz–sericite stage exceeds pyrrhotite 2. Rare arsenopyrite 1 occurs locally in the propylitic zone, but in contrast arsenopyrite 2 in the quartz–sericite may occupy up to 50% of the altered rock volume. Its broken crystals are cemented by pyrrhotite, chalcopyrite, sphalerite and other minerals (Stepanov, 1977).

Sphalerite (marmatite) is mainly developed on the flanks of the deposit, where it replaces pyrrhotite, scheelite, arsenopyrite and chalcopyrite, but also often contains pyrrhotite, chalcopyrite and stannite inclusions. The latter mineral also forms lattices and banded exsolution microstructures densely saturating sphalerite grains or substituting them at the margins.

Significant intervals of gold mineralisation e.g., 1 to 3 g/t Au over intervals of 10 to 30 m, are related to high or moderate sulphide developments associated with the quartz-sericite alteration stage. Gold predominantly occurs in that part of the Main Orebody composed of altered scheelite bearing skarn with superimposed quartz-sericite metasomatic rocks. The Au grade appears to increases toward the flanks of the Main orebody and in the small satellite tungsten orebodies. Free native gold, electrum and other gold minerals occur within sulphides and quartz, as well as gold finely dispersed between sulphides (Stepanov and Gvozdev, 1987; Gvozdev and Tsepin, 2005). Native gold with a fineness is 770 to 780 forms relatively large, up to 0.4 mm grains in the upper levels; whilst in the lower levels it is not so coarse (<0.1 mm). Low fineness gold and electrum (48% Ag) are associated with native bismuth; medium fineness gold is associated with Pb-Sb-Bi sulphosalts and hedleyite; and that of high fineness and 16 wt.% Ag occurs as micro inclusions in chalcopyrite. Finely dispersed gold is concentrated in arsenopyrite containing from 0.58 to 32 ppm Au (Vinogradova et al., 1996).

Reserves and Resources

The Vostok-2 deposit contained pre-mining 'reserves' of >180 000 t of contained WO
3 averaging 1 to 2 wt.% WO3 (Gvozdev and Tsepin, 2005), equating to ~12 Mt of ore, assuming a grade of 1.5% WO3.

In 2010, after 40 years of mining, the remaining identified reserves in categories A+B+С1+С2 at deep levels are ~28 000 t WO
3 averaging ~2.7 WO3.
According to Gvozdev (2006), the ore also contains 0.64% Cu. Before 1999, >43.5 kt Cu, 2.3 kt Bi, 23 t Ag and 8.5 t Au had been recovered.

This summary is almost exclusively drawn from Soloviev and Krivoshchekov (2011).

The most recent source geological information used to prepare this summary was dated: 2011.    
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd.   Unauthorised copying, reproduction, storage or dissemination prohibited.

  References & Additional Information
   Selected References:
Soloviev, S.G. and Krivoshchekov, N.N.,  2011 - Vostok-2 Gold-Base-Metal-Tungsten Skarn Deposit, Central Sikhote-Alin, Russia: in    Geology of Ore Deposits (Pleiades Publishing),   v.53, pp. 478-500.

Porter GeoConsultancy Pty Ltd (PorterGeo) provides access to this database at no charge.   It is largely based on scientific papers and reports in the public domain, and was current when the sources consulted were published.   While PorterGeo endeavour to ensure the information was accurate at the time of compilation and subsequent updating, PorterGeo, its employees and servants:   i). do not warrant, or make any representation regarding the use, or results of the use of the information contained herein as to its correctness, accuracy, currency, or otherwise; and   ii). expressly disclaim all liability or responsibility to any person using the information or conclusions contained herein.

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