INTRODUCTION

The older placer areas are sufficiently well known in Northeast Russia that it became obvious many years ago that rapid growth in gold mining should primarily be expected from exploration to detect epithermal Au–Ag deposits in volcano-plutonic belts (the OChVB and others). Over 140 tons of gold and 2000 tons of silver have been extracted in the short timespan of 10 years in the unique Kupol Au–Ag epithermal deposit.

The OChVB is the longest among all volcano-plutonic belts in Northeast Russia (over 3000 km) (Fig. 1). Most of the known OChVB Au–Ag epithermal deposits are being mined today. Several deposits have been exhausted: Karamken, Vetvistoe (Magadan Region), Sopka Rudnaya, and Severo-Vostok (Chukotka). A number of deposits (Moroshka, Sentyabr’skoe, Zhil’noe, and Gornoe) are being developed. Most OChVB deposits are of the low-sulfidation type (Kupol, Dvoinoe, Dzhul’etta, and others); the second most numerous group of deposits have intermediate (medium) sulfidation (Dukat, Lunnoe, Gol’tsovoe, and others), while only two deposits (Svetloe and Perekatnoe) can be classified as belonging to the high-sulfidation subtype.

Fig. 1.
figure 1

A map showing the regionalization of the OChVB and the location of the known epithermal Au–Ag deposits. (1) a generalized boundary of the OChVB igneous formations: Albian–Santonian: on land (a) and in sea (b); (2) boundary between the outer and inner zones of the OChVB; (3) boundary of the subzones of depression grabens and of that of magmatogenic uplifts; (4) inferred boundaries in water area; (5) boundaries of sectors in the outer zone and in flank zones; (6) volcanic rocks in outer zone; (7) OChVB magmatogenic uplifts; (8–11) OChVB outer zone, sectors: (8) Okhotsk, (9) Penzhina, (10) Anadyr, (11) Central Chukchi; (12, 13) flank zones: (12) West Okhotsk, (13) East Chukchi); (14) Late orogenic (late Barremian to early Albian) Mesozoic structures: O Omsukchan graben, P Verkhne-Penzhina, U Umkuveem, A Ainakhkurgen, T Tytyl’veem basins, KH Khurchan–Ortukan TMA zone; (15) gold–silver deposits: (1) Nyavlenga, (2) Dzhul’etta, (3) Tikhoe, (4) Dukat, (5) Agan, (6) Oira, (7) Dal’nee, (8) Kupol, (9) Moroshka, (10) Televeem, (10) Dvoinoe, (11) Sentyabr’skoe, (13) Pepenveem, (14) Pechal’noe.

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The OChVB was found to contain several tens of promising ore occurrences, over 2000 mineralization sites, and several thousand geochemical anomalies. The mining companies reported extracting 30 tons of Au and over 960 tons Ag from the OChVB epithermal deposits in 2017.

The main goal of the present study is to sum the data, both that available in the literature and new data obtained by these authors, in order to identify geochemical and thermal, pressure, and geochemical features in the Mesozoic OChVB Au–Ag epithermal mineralization, to derive new information concerning the conditions of mineralization, to use the sum of this knowledge in order to design new criteria and refine known ones for assessment of industrial significance and prediction of deposits. The method used in the analytical studies was described in detail in our previous paper (Volkov et al., 2017b).

THE MESOZOIC VOLCANISM AND EPITHERMAL Au–Ag MINERALIZATION IN THE OCHVB

The OChVB is 3000 km long; its average width is 200 km. The OChVB was forming during 25 myr (Belyi, 1994) at the boundary between the continental Verkhoyansk–Chukchi and Koryak–Kamchatka terrane (see Fig. 1). The OChVB evolution proceeded in two phases: (1) the earlier (Albian–Cenomanian) phase in which approximately 90% of all volcanic rocks were produced, and (2) the later (Turonian–Santonian) phase when the Koryak–Kamchatka structures evolved. The formation of the OChVB was probably caused by the Pacific plate being subducted under the collage of the terranes accreted to the Siberian continent. The main sources of acid magma were in the continental crust during this volcanic episode; the crust seems to have been Neoproterozoic (Tikhomirov et al., 2008).

The OChVB consists of three elements (see Fig. 1): the main arcuate part that extends from the lower reaches of the Ul’ya River to the coast of the East Siberian Sea (approximately 2000 km); this part is called the Tauisk–Chaun arcocline, with the other elements being the western Sea-of-Okhotsk and East Chukchi flank zones (Belyi, 1994). In fact, these elements are independent structures that are joined together by a left lateral end-part juncture that were formed during the same span of geologic time.

The outer and flank zones of the OChVB typically contain various isometric negative volcanic structures: subsidence structures with a caldera form (depressions and subsidences) and calderas; circular intrusive–effusive edifices; and volcanic grabens and semi-grabens that have inherited the fault directions in the basement. Dome-like intrusive–effusive and crypto-intrusive structures are abundant in all zones, but their role is relatively small, although the mineralization location is not infrequently controlled by these structures.

One important element in the OChVB structure consists in magmatogenic uplifts where the principal parts of the larger intrusive bodies are concentrated. In the inner zone, most of these structures were evolving inheriting the analogous uplifts in the Koni–P’yagin zone of the Taigonos volcanic arc. These were forming in the outer zone during the earlier phase in the OChVB evolution and their relative area is considerably below that in the inner zone. Some of these are elongate parallel to the overall OChVB trend, while others are transverse. Magmatogenic uplifts are infrequent in the flank zones.

The Verkhoyansk–Chukchi Mesozoic features experienced the greatest activation in the late Cretaceous to early Paleogene; the activation seems to have been synchronous with a phase in the OChVB evolution (Sidorov et al., 2013). The area of influence exerted by the Late Mesozoic tectono-magmatic activation (TMA) is identified as a perivolcanic zone of a hazy boundary (as wide as 500 km) that covers the southern part of the Yana–Kolyma system, the Oloi zone, and the Chukchi system. One feature that is noticeable in the deep structure of the perivolcanic zone is the presence of large low-density locations in the lithosphere. These locations are frequently adjacent to the OChVB volcanogenic depressions and basins that show some inheritance of the Mesozoic structural plan. The linear TMA (tectono-magmatic activation) zones are treated here as branches away from the OChVB; chains of volcanic fields and intrusive bodies are also frequently located on extensions of tongues of the belt blanket formations of varying lengths. At the same time, they are different from the belt formations in having different tectono-magmatic features, which are largely controlled by isolated local occurrences of magmatism and by metallogenic features.

All the OChVB structures and the feather TMA zones described above have ore potential. However, the ore potential of volcanogenic structures and of the feather TMA zones are known fragmentarily for the southern part of the Okhotsk sector of the OChVB, for the outer part of the Central Chukchi sector, and very poorly for the inner part of the Anadyr sector.

The complex structure of the basement terranes gave rise to a great diversity of ore assemblages in the OChVB (Sidorov et al., 2009). The inner zone of the belt more frequently contains deposits of the Cu–Mo-porphyry formation containing Au, Ag, and platinoids. The outer and perivolcanic TMA zones typically contain Au–Ag, Ag–base-metal and Sn–Ag mineralization (Sidorov et al., 2011). Epithermal Au–Ag deposits proper (with Au/Ag 1/1–1/10) are more abundant in the outer zone. Silver-dominated deposits (Au/Ag = 1/10–1/1000) are confined to the Omsukchan riftogenic volcanic depression, which introduces complications into the OChVB between the Yana–Kolyma and Omolon terranes (Sidorov et al., 2009). It should also be remarked that the OChVB does not contain any significant Au–Ag–Те deposits and occurrences, which were thought by Nolan (1933) to be related to small volcanic bodies in the Cenozoic volcanic belts of the western United States.

The generation of epithermal deposits of dominantly Ag and Au–Ag types in the volcanic belts is largely due to the fact that they are underlain by potentially gold- and silver-bearing rock sequences of the Verkhoyansk and Chukchi terrigenous units, as well as by older metamorphic units of median massifs (cratons), which were extra sources of metals for volcanogenic–plutonogenic deposits (Volkov et al., 2006).

Comparison among different OChVB sectors by the intensity of epithermal mineralization shows that the well-known Okhotsk sector (the Karamken, Nyavlenga, Dzhul’etta, and Dukat deposits, the Evensk group, etc.) is inferior to the poorly known Anadyr sector in terms of proved Au reserves in epithermal deposits. The Anadyr deposits are Kupol, Moroshka, Sentyabr’skoe, Valunistyi, and Dvoinoi.

This fact seems to be due to the rather complex geological structure of the Okhotsk sector. In this sector one finds abundant rejuvenated epithermal deposits (Goncharov and Sidorov, 1979). In addition, the Okhotsk sector is dominated by acidic igneous units of the Ag–Sn–Mo type (Sidorov et al., 2009). At the same time, one discerns a pronounced relationship between epithermal Au–Ag mineralization and the Cu-porphyric ore-forming system on the Kupol deposit in the Anadyr sector (Volkov et al., 2012).

The geochemical features of Au–Ag ores in epithermal deposits that will be discussed below show many similarities to the ore assemblage patterns pointed out above.

THE GEOCHEMICAL FEATURES OF THE OCHVB EPITHERMAL ORES

The ores sampled from the Mesozoic epithermal Au–Ag deposits are dominated by SiO2 (67.6–92.8%) along with noticeable concentrations of Al2O3 (3.2–14.0%), K2O (0.44–5.5), and Fe2O3 (0.48–5.1) (Table 1). The ores of the Tikhoe deposit are found to contain noticeable concentrations of Na2O (2.27%) and CaO (4.83%) (see Table 1), which is caused by mineralization in an argillite zone (Volkov et al., 2015). The Dukat ores typically show high concentrations of MnO (3.9) and the Nyavlenga ores have high concentrations of MgO (3.46) (see Table 1), which is due to the special type of mineralization in these deposits (Volkov et al., 2014). The metasomatic ores of the Agan deposit (Volkov et al., 2015) show the highest total Na2O + K2O (see Table 1). Judging from the data in this table, the concentration of sulfides in the ore samples studied here varies in a wide range (Stotal is between 0.3% and 3.2%).

Table 1.   The silicate composition of ore samples (wt %) from OChVB epithermal Au–Ag deposits
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The results of trace-element analyses for the OChVB ores of volcanogenic epithermal Au–Ag deposits and host rocks are presented in Table 2 and in Fig. 2 where the values have been normalized by the means for the upper crust (Taylor and McLennan, 1985). It follows from Table 2 and Fig. 2 that the ores are clearly enriched in a wide range of elements (Li, P, Rb, V, Cr, Sc, Ba, Mn, Au, Ag, As, Sb, Te, Cu, Mo, Zn, Pb, In, Cd, Ga, Co, Mo, Bi, Tl, Cs, Be, Se, W, and U) compared with the upper crustal means (Tayler and McLennan, 1985). The enrichment ratios vary between a few times (Li, P, Sc, Rb, Cs, Cr, V, Tl, Ba, Se, W, and U) and a few tens (Mo, Mn, and Sn), hundreds (Pb, Cu, Zn, Be, and Te), and thousands (As, Sb, In, Cd, and Bi), reaching a few tens or hundreds of thousands of times (Au and Ag), thus providing evidence of a geochemical affinity of trace elements and their synchronous participation in the mineralization.

Table 2. The element composition (g/t) of ore samples from the OChVB epithermal Au–Ag deposits
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Fig. 2.
figure 2

Trace elements in the ores of the OChVB epithermal Au–Ag deposits as normalized by the mean upper crustal values (Taylor and McLennan, 1985).

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Comparative analysis of mean trace-element concentrations in the ores of the OChVB epithermal Au–Ag deposits shows great similarities in composition and distribution (see Table 2, Fig. 2). This shows that the mineralization of these deposits occurred under similar conditions. At the same time, the Dukat ores are characterized by an appreciable synchronous enrichment in Be and Mn (see Fig. 2), which is due to a wide abundance in the ores of minerals that contain these admixtures (Konstantinov et al., 1998). The Pechal’noe ores show an obvious synchronous enrichment in Li, Be, Rb, Cs, Мо, W, and Tl, which corresponds with the composition of the host volcanic rocks (Egorov et al., 2005; Volkov et al., 2017b). The epithermal ores in the Agan deposit have a distinguishing feature, viz., a high enrichment in Sn (see Fig. 2), which classifies the ores as belonging to the tin-porphyritic mineralizing system (Volkov et al., 2015). Similar distributions but different intensities of individual elements occur in the Kupol, Moroshka, Dvoinoi, Sentyabr’skoe, Televeem, Dzhul’etta, Oira, Dal’nee, Nyavlenga, and Tikhoe deposits (see Fig. 2), which may have been because they belong to the Cu–Mo-porphyritic mineralizing system (Volkov et al., 2012, 2014). The low intensity of elements in the distribution of the Nyavlenga deposit compared with the ores of the other known deposits seems to be due to thermal metamorphism of the ores (Savva et al., 2007). The noticeable enrichment in Bi and W observed in the Pepenveem ores provides evidence that this deposit belongs to a mineralizing system related to granitoid intrusions (Lang et al., 2000).

Comparative analysis of the trace-element distributions in the ores over gold concentration class (Fig. 3) emphasizes a synchronous enrichment of the ores in a similar set of trace elements from low (1–5 g/t) to high class (>100 g/t).

Fig. 3.
figure 3

The trace-element distribution over classes of Au concentration in the ores of the OChVB epithermal Au–Ag deposits as normalized by the mean upper crustal values (Taylor and McLennan, 1985). Classes of Au concentration (g/t): >100 (a), 10–20 (b), and 1–5 (c).

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The OChVB host volcanic rocks are also enriched in a wide range of elements (Li, Sc, Ti, V, Mn, Sr, Co, Ba, REEs, etc.) compared with the upper crustal values (Taylor and McLennan, 1985), but the enrichment ratios for most of them are low (2 to 5 times). The highest enrichment ratios ranging between 5 and 10 or higher (As, Ag, Pb, Sb, and Мо) were found for host rocks within the epithermal ore fields (Sakhno et al., 2015; Kravtsova, 2010).

According to V.A. Zharikov et al. (1999), light REEs pass into aqueous fluid as the pressure increases, while heavy ones are retained in the magma, so that one can treat the former as “hydrophile” elements and the latter as “magmaphile” ones. Three REE sets can be identified (Mineev, 1974): the cerium set (La, Се, Pr, and Nd), the yttrium set (Sm, Eu, Gd, Dy, and Но), and the scandium set (Er, Yb, and Lu). Judging from the data in Table 3, the REE distributions of the ores are dominated by light “hydrophile” lanthanoids of the “cerium” set. A similar REE composition is also typical of the OChVB host volcanic rocks (Kravtsova et al., 2005; Kravtsova and Almaz, 2006; Kravtsova, 2010; Tikhomirov et al., 2008, 2016; Sakhno et al., 2015).

Table 3.   The indicator parameters of ore sampled from the OChVB epithermal Au–Ag deposits
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We know that Cl-bearing hydrothermal fluids can effectively concentrate light REEs, but are depleted in heavy REEs (Oreskes and Einaudi, 1990); in that case the Hf/Sm, Nb/La, and Th/La ratios in the ores are generally below 1, while F-rich fluids synchronously concentrate light and heavy REEs, since the Hf/Sm, Nb/La, and Th/La ratios are usually greater than 1 (Oreskes and Einaudi, 1990). The ores in most of the OChVB epithermal Au–Ag deposits are evidently enriched in light REEs and depleted in heavy REEs, and have Hf/Sm, Nb/La, and Th/La ratios below 1 (see Table 3); one exception is the Sentyabr’skoe deposit where Hf/Sm = 1.13. It follows that the ore-forming fluids of the deposits belonged to a NaCl–H2O hydrothermal system that was enriched in Cl relative to F; this agrees with studies of fluid inclusions in ore quartz (Volkov et al., 2012; Kolova et al., 2015; Prokof’ev et al., 2015, among others).

The U/Th ratio for an ore reflects the redox conditions of the host rocks (Jones and Manning, 1994): U/Th ≤ 0.75 occurs in an oxidizing environment; U/Th 0/75–1.25 is characteristic of an environment without oxygen, and U/Th > 1.25 for a reducing environment. Judging from the data in Table 3, the U/Th ratios in the ores of the deposits studied here are below 0.75 (they vary between 0.24 and 0.71), providing evidence of an oxidizing environment during their mineralization (Jones and Manning, 1994). The high values of that ratio (1.5) in the Agan ores (see Table 3) indicate a reducing environment of mineralization that is characteristic for the shallow level of the epithermal system where the deposit was formed (Volkov et al., 2015).

The Co/Ni ratios in the Dvoinoe, Pechal’noe, Tikhoe, Pepenveem, Televeem, and Agan ores (see Table 3) are considerably below 1.0 (they vary between 0.06 and 0.88), which is characteristic for medium- and low-temperature hydrothermal fluids of meteoric origin (Kun et al., 2014). Values above 1.5 in the Dzhul’etta, Dukat, Kupol, Moroshka and other ores (see Table 3) provide evidence of an obvious role played by a magmatic fluid in the mineralization (Kun et al., 2014). The high Co/Ni ratio in the ores shows the superposition of a later magmatic fluid on the earlier mineralization and is a consequence of a rejuvenated mineralization (Goncharov and Sidorov, 1979).

Effective use of the Y/Ho ratio for assessing the origin of mineralizing fluids was shown in (Bau, 1991; Jones and Manning, 1994; Monecke et al., 2002). According to Table 3, the Y/Ho ratios in the OChVB epithermal Au–Ag ores studied here vary between 23.53 and 42.48, which corresponds with the range of the ratios that are characteristic of the present-day hydrothermal fluids in backarc basins (Bau, 1991; Jones and Manning, 1994; Monecke et al., 2002).

The REE composition in the OChVB epithermal Au–Ag ores studied here is shown in Table 2, while the chondrite-normalized REE distributions are plotted in Fig. 4. Abnormally low values of ΣREE (between 4.3 and 18.95 g/t) were found in the Dvoinoe, Dzhul’etta, Dukat, Kupol, Moroshka, and Televeem ores (see Table 3). Lower values of ΣREE (24.49–39.99 g/t) are characteristic for typical epithermal ores in the Oira, Pechal’noe, Tikhoe, Pepenveem, and Sentyabr’skoe deposits. Higher values (43.25–62.66) of ΣREE were found in the Agan, Nyavlenga, and Dal’nee ores. Low total REE concentrations, the same as in the OChVB deposits, were found in productive quartz in the ores of the epithermal deposits on the Kuramin Range, Uzbekistan and of the Banská Štiavnica area, Slovakia (Vinokurov et al., 1999). The total REE concentration in the epithermal Au–Ag ores of the deposits studied here (see Table 3) is appreciably lower than those in the OChVB igneous and volcanic rocks and in island-arc andesites (Kravtsova et al., 2005; Kravtsova and Almaz, 2006; Kravtsova, 2010; Tikhomirov et al., 2008, 2016).

Fig. 4.
figure 4

The distribution of chondrite-normalized mean REE values ((a) over deposits, (b) over classes of Au concentration) in the ores of epithermal Au–Ag deposits and host rocks (c) in the OChVB. Numerals in circles (1–14) refer to known deposits: (1) Nyavlenga, (2) Dal’nee, (3) Agan, (4) Pechal’noe, (5) Oira, (6) Sentyabr’skoe, (7) Dukat, (8) Tikhoe, (9) Kupol, (10) Moroshka, (11) Pepenveem, (12) Televeem, (13) Dvoinoe, (14) Dzhul’etta. OChVB rocks: (1) rhyolite lavas; (2) rhyolite ignimbrites; (3) andesites; (4) dacites; (5) dacitic tuffs; (6) rhyolite dikes (Tikhomirov et al., 2008; Sakhno et al., 2015).

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The chondrite-normalized REEs of the ores studied here make slightly dipping near-chondrite distributions (see Fig. 4a) that exhibit many similarities in their configurations to those of the REE distributions for the OChVB rocks (see Fig. 4c), as well as for island-arc andesites (Kravtsova et al., 2005; Kravtsova and Almaz, 2006; Kravtsova, 2010, among others).

The REE distributions clearly distinguish three sets of ore (see Fig. 4a). The distributions of the first set (Dzhul’etta, Moroshka, Tikhoe, and Kupol) show well-pronounced Eu maxima (see Figs. 4a, 5). Pronounced Eu minima were found for the distributions of the Dvoinoe, Dukat, Pepenveem, and Agan ores (see Figs. 4a, 5). The slightly dipping near-chondrite distributions that involve no pronounced Eu maxima and minima are characteristic for the ores of the third set (Nyavlenga, Televeem, Pechal’noe, Oira, and Sentyabr’skoe). Differently directed distributions were found for the Kupol ores (see Fig. 5), which supports the identification of two types, viz., high- and low-sulfidation epithermal mineralization, on this deposit (Volkov et al., 2012).

Fig. 5.
figure 5

The chondrite-normalized REE distribution in the ores of the OChVB epithermal Au–Ag deposits. Ag.37 etc. are sample numbers.

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The REE distributions clearly separate the OChVB host volcanic rocks into two sets. Acidic rocks typically show distributions with pronounced Eu minima, while intermediate rocks make slightly dipping near-chondrite distributions. We note that the epithermal Au–Ag Cenozoic deposits of Kamchatka that are localized in intermediate (andesitic to dacitic) rocks were found to have near-chondrite REE distribution configurations (Takahashi et al., 2007; Andreeva, 2013).

To summarize, we can state that the different ratios in the ore-containing section that is composed of volcanic rocks with different compositions seems to control the REE distribution configurations in the ores. These results suggest that the OChVB host rocks, as well as the host rocks in the other volcano-plutonic belts in Northeast Russia (Volkov et al., 2017a) might be the source of the REEs and possibly of other trace elements for the ore-forming fluids.

According to S.V. Vinokurov et al. (1999), positive Eu anomalies constitute a characteristic feature of productive quartz in epithermal Au–Ag deposits, while oreless quartz in such deposits exhibits negative Eu anomalies (Vinokurov et al., 1999). This inference is also supported by our results (see Fig. 4b). Judging from this figure, the distributions of rich ores differ from those of poor ores in having pronounced Eu maxima. It may be hypothesized that the Eu maxima in the ores of the first set (see Fig. 4a) are due to their richness.

Eu and Cе anomalies are commonly treated as marking the redox potential of a mineralization environment (Bortnikov et al., 2007; Goryachev et al., 2008; Jones and Manning, 1994). The ores sampled from the OChVB deposits (see Table 3) show δCe and δEu varying between negative or weakly negative and moderately positive values (δCe = between 0.90 and 1.15) and (δEu = between 0.48 and 2.63). This combination of δCe and δEu indicates low-oxidation and oxidation conditions that existed during the deposition of the OChVB epithermal ores. The low Eu/Sm ratios (<1) in the ores studied here (see Table 3) suggest the inference that the OChVB mineralization occurred in the upper crust under similar physical and chemical conditions (Vinokurov, 1996).

Comparison between the data obtained here and known published examples (Vinokurov et al., 1999; Kravtsova, 2010, among others) indicates that the patterns identified here, viz., the depletion of ores in rare-earth elements, the prevalence of light lanthanoids over heavy ones, low Eu/Sm ratios (<1), and positive Eu anomalies, all constitute, in all probability, typical features for the epithermal mineralization system.

THE DISTINGUISHING FEATURES OF THE ORE-FORMING FLUIDS

Many results have been reported concerning physico-chemical parameters and composition of ore-forming fluids for the OChVB Au–Ag deposits (Table 4, Fig. 6). Papers exist concerning the conditions of mineralization for the Dukat (Goncharov and Sidorov, 1979; Berman et al., 1993; Kravtsova et al., 2003; Konstantinov et al., 1998, among others), Dal’nee (Kravtsova et al., 2003; Kravtsova, 2010), Kupol (Volkov et al., 2006, 2012; Sidorov et al., 2007; Prokof’ev et al., 2012), Dvoinoe (Volkov et al., 2012; Prokof’ev et al., 2012; Nikolaev et al., 2013; Kolova et al., 2018), Arykvaam (Volkov et al., 2012), Sentyabr’skoe (Nikolaev et al., 2013; Savva et al., 2016), Nyavlenga (Volkov et al., 2014), Dzhul’etta (Prokof’ev et al., 2015), and Tikhoe deposits (Kolova et al., 2015).

Table 4.   The physico-chemical parameters of mineral-forming fluids at the OChVB Au–Ag deposits
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Fig. 6.
figure 6

The temperature–salinity diagram for mineral-forming fluids in the OChVB epithermal gold–silver deposits. (1) Kupol, (2) Dvoinoe, (3) Dzhul’etta, (4) Nyavlenga, (5) Tikhoe, (6) Sentyabr’skoe.

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The ore-forming fluids of most deposits had temperatures of 105–359°С and salinities of 0–9.2 wt %-equi NaCl. Higher temperatures (up to 435°С) and salinities (up to 37.0 wt %-equi NaCl) have only been recorded for fluids of the late mineralization phase on the Dukat deposit (Goncharov and Sidorov, 1979; Berman et al., 1993; Kravtsova, 2010). This phase is thought to have been related to the influence of a granitoid massif that was emplaced near the deposit. Some deposits were found to involve the action of heterogeneous fluids (Dukat and Sentyabr’skoe); this enables us to estimate the pressure that existed during the mineralization to be in the range 80–570 bars. These pressures are relevant to shallow and medium (subvolcanic) depths of mineralization.

Bulk analysis of the composition of fluid inclusions in quartz monofractions furnished data on fluid compositions for ten OChVB Au–Ag deposits (Table 5, Fig. 7): Dvoinoe, Kupol, Nyavlenga, Pepenveem, Pechal’noe, Dukat, Moroshka, Dal’nee, Dzhul’etta, and Tikhoe. The main components in the fluid include (g/kg H2O): carbon dioxide (1.3–58.1), methane (0.02–3.18), chlorine (<0.1–5.15), sulfate ion (<0.3–21.65), hydrocarbonate ion (0–116.51), Na (0.79–23.90), K (0.16–18.79), Ca (0–4.70), and Mg (0–7.02). The following trace elements have been determined (mg/kg H2O): Br (0–972.8), As (6.52–1921.4), Li (2.39–184.42), Be (0–0.19), B (38.85–7603.0), Rb (0–61.98), Cs (0–26.42), Sr (0–144.36), Mo (0–41.18), Ag (0–15.98), Sb (0.88–903.82), Cu (0–641.75), Zn (0–654.57), Cd (0–0.66), Pb (0–122.51), Bi (0–0.08), Th (0–0.20), U (0–0.10), Ga (0–0.59), Ge (0–1.01), Ti (0–12.74), Mn (0–1599.9), Fe (0–1772.2), Co (0–10.15), Ni (0–5.46), V (0–3.84), Cr (0–5.46), Y (0–4.50), Zr (0–1.20), Sn (0–3.09), Ba (0–33.65), W (0–184.64), Au (0–0.669), Hg (0–1.35), Tl (0–0.85) and REE (0.003–0.49). The ratios of the most typical components vary as follows: Na/K from 0.2 to 15.4, CO2/CH4 from 3.2 to 216.0, and K/Rb from 122.8 to 2725.5.

Table 5.   The composition of solutions of the fluid inclusions in quartz sampled from ore veins from the OChVB Au–Ag deposits
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Fig. 7.
figure 7

The compositions of mineral-forming fluids in the OChVB gold–silver deposits. (a) Dvoinoe, (b) Kupol, (c) Nyavlenga, (d) Pepenveem, (e) Pechal’noe, (f) Dukat, (g) Moroshka, (h) Dal’nee, (i) Dzhul’etta, (j) Tikhoe.

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The analyses for the composition of mineral-forming fluids at the OChVB deposits are consistent with the results of cryometric studies: the fluids have low salinity and low concentrations of chlorine, carbon dioxide, and methane. The anions are dominated by the hydrocarbonate ion and, occasionally, by the sulfate ion, while the cations are dominated by sodium and potassium. Among the trace elements, one notes higher concentrations of Br, As, Li, B, Sb, Zn, and W.

The concentrations of cations and anions show a tendency of increasing K+ from earlier oreless quartz to productive quartz with increasing depth, as well as a slow decrease in the percentage of Na+, Ca++, and Cl. One observes a direct Ag–K correlation and an inverse Ag–Na correlation for productive quartz. The K/Rb ratio for solutions is generally high (occasionally reaching 2725), which may be evidence of an interaction between the ore-forming fluid and the host rocks. One occasionally encounters values below 250, which must most likely have resulted from a magmatic source of the fluid.

The most-pronounced feature that distinguishes the deposits studied here consists in sulfate concentrations in the fluid. One notes deposits with appreciable sulfate concentrations in the fluid (Dvoinoe, Kupol, Nyavlenga, Pepenveem, and Pechal’noe) and deposits with low sulfate concentrations (Dukat, Moroshka, Dal’nee, Dzhul’etta, and Tikhoe).

CONCLUSIONS

We conclude by noting that the ores of the OChVB epithermal Au–Ag deposits are characterized by an obvious enrichment in a wide range of elements compared with the mean upper crustal values.

Comparative analysis of the trace-element distributions in the ores studied here over classes of gold concentration shows a synchronous enrichment of the ores in similar sets of trace elements and allows indicator elements to be used to determine the type of ore-forming fluid.

The patterns identified here, viz., the prevalence of light lanthanoids over heavy ones and positive Eu anomalies, seem to be typical of the epithermal ore-forming system. The presence of Eu maxima in the REE distributions of epithermal ores is a characteristic feature of ore columns. The REE distributions of the ores studied here, as well as of the host rocks, are dominated by light “hydrophile” lanthanoids of the “cerium” set.

The epithermal ores studied here were mostly formed by fluids coming from depth (in the lower crust), as well as by solutions that were produced by deep fluids mixed with shallow infiltration waters. The fluid salinity increased toward later, low-temperature phases during the mineralization process. The ore-forming solutions are of the hydrocarbonate potassium type, having 60–80% hydrocarbonate and potassium of the total amount of cations and anions. The concentration of anions and cations shows a tendency of the potassium percentage to increase from the earlier oreless quartz toward productive quartz with increasing depth, as well as a slow decrease in the percentages of Na+, Ca++, and Cl–. One observes a direct Ag–K correlation and an inverse Ag–Na correlation for productive quartz. The most-pronounced feature of the deposits studied here consists in sulfate concentrations in the fluid.

The results indicate magma chambers of andesite magmas and meteoric waters as the most likely sources of the fluids that went to form the epithermal Au–Ag ores in the OChVB deposits.