INTRODUCTION

The Paleozoic epithermal Au–Ag deposits of northeastern Russia are concentrated in the pre-accretion Kedon volcanic belt (KVB) within the Omolon cratonic terrane (Fig. 1). The low sulfidation distinguishes the ores of these deposits from their Late Mesozoic equivalents in the Okhotsk–Chukotka volcanic belt (OCVB) (Volkov et al., 2011).

Fig. 1.
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Locations of gold deposits and occurrences studied within KVB in the Omolon cratonic terrane. Tectonic sketch map modified after (Gagieva, 2014). (1) Pre-Riphean basement highs; (2–9) platform cover: (2–3) lower structural unit: (2) sedimentary deposits (Riphean–Ordovician), (3) volcanosedimentary deposits (Cambrian, Vizual’naya, and Sezam formations), (4–7) middle structural unit (Devonian): (4) Namyndykan–Molandzha structural–facies zone, volcanosedimentary deposits; (5–7) Yukagir structural–facies zone, subaerial volcanics (Kedon Group): (5) trachybasaltic andesites–trachyrhyolites, (6) predominantly rhyolitic, trachyrhyolitic, and dacitic lavas, ignimbrites, and tuffs, (7) rhyolitic and trachyrhyolitic lavas, ignimbrites, and tuffs; andesites, and trachyandesites; (8–9) upper structural unit: (8) sedimentary deposits (Lower Carboniferous–Middle Jurassic), (9) sedimentary and volcanic deposits (Upper Jurassic–Lower Cretaceous); (10) structures of the folded surroundings of the cratonic terrane; (11) Cretaceous volcanics of the Okhotsk–Chukotka volcanic belt; (12–13) intrusive complexes: (12) Paleozoic, (13) Early Cretaceous; (14) geological boundaries; (15) faults; (16) epithermal Au–Ag deposits and occurrences studied. Circled numbers: (1–5) volcanic areas of KVB: (1) Tokur–Yuryakh; (2) Rassokha; (3) Abkit; (4) Kedon; (5) Ol’dyani–Koargychan; (6) Anmandykan volcanic zone.

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The Burgali deposit is located in the Severo-Evensk district of Magadan oblast, in the drainage area of the Burgali River, a left tributary of the Omolon River. During 1990–1994, the Severo-Evensk Geological Exploration Expedition delineated the Burgali ore field area and covered it with surface geophysical and geochemical surveys and mining works as part of the geological prospecting program, which enabled estimating the P2-category undiscovered resources. Further exploration activities (starting from 2009) were carried out by ZAO Omolon Gold Mining Company (OGMC), a subsidiary of PAO Polymetal Company. The financial interest in the Burgali deposit is explained by its relative proximity to the large mined Birkachan and Kubaka epithermal Au–Ag deposits (Fig. 1).

The reserves of the deposit, according to the OGMC data (2019), are 4 t gold, contained in orebodies with the average Au grades of 7.2 to 11.9 g/t, and 15 t silver (15–30 g/t). The gold resources of the deposit, estimated according to the JORC Standard, are 300–400 000 gold ounces (9–12 t gold), contained in 2–3 million tons of ores with the average Au grades of 3.5 to 5.0 g/t gold. The deposit has been mined since late 2018. The position of an orebody of the Burgali deposit (Northern zone) in the field is shown in Fig. 2.

Fig. 2.
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Burgali (Northern zone) orebody position in the field. Photograph by B.I. Ishkov.

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Ore mineralogy and geochemistry were studied at the Institute of Geology of Ore Deposits, Petrography, Mineralogy, and Geochemistry, Russian Academy of Sciences (IGEM RAS) and the Northeastern Interdisciplinary Scientific Research Institute, Far East Branch, Russian Academy of Sciences (SVKNII FEB RAS) under the support of the Russian Science Foundation (project no. 14–17–00170) in 2014–2016 and a state task for IGEM RAS, starting from 2018. This paper summarizes the results of these studies.

MATERIALS AND METHODS

The section “Geological Overview” was prepared at IGEM RAS based on the archival and published data integrated with the original findings. Ore petrology and mineralogy were studied at SVKNII FEB RAS (examination of numerous polished sections under an Axioplan Imaging microscope). Ore mineral compositions were determined with a Camebax X-ray electron-probe microanalyzer equipped with an Oxford Instruments INCA add-on device (analysts E.M. Goryacheva and T.V. Subbotnikova at SVKNII FEB RAS, Magadan, and I.A. Bryzgalov at Moscow State University).

Rock-forming and some foreign element concentrations in the ores were determined by X-ray fluorescent analysis at the analytical laboratory of IGEM RAS with a PANalytical’s Axios mAX sequential (wavelength–dispersive) vacuum spectrometer. The spectrometer was calibrated against industry-specific and state standard samples of the chemical compositions of rocks. The analysis was performed according to the 439-RS NSAM VIMS method, which ensures compliance of the results with the OST RF 41-08-205-04 industrial standard (analyst A.I. Yakushev). Trace element concentrations were measured with an X‑Series II ICP-MS mass spectrometer with inductively coupled plasma as the ionization source (analyst Ya.V. Bychkova). Element detection limits ranged from 0.1 ng/g for heavy and intermediate elements to 1 ng, for light elements. The uncertainty of the analysis was 1–3% RPD. Gold grades of samples were determined by atomic absorption spectrometry with electrothermal atomization using a Spectr AA 22OZ spectrometer (analyst V.A. Sychkova). Geochemical indicator values were determined to study the ore formation conditions. The obtained values were tabulated and used to plot the rare-earth element (REE) and other trace element distribution curves for the ores studied.

GEOLOGICAL OVERVIEW

The Middle Paleozoic Kedon volcanic belt overlaps the Archean–Early Proterozoic basement and the Phanerozoic (pre-Devonian) sedimentary cover of the Omolon cratonic terrane (Fig. 1). The KVB consists of numerous vast fields of the Early–Middle Paleozoic felsic igneous rocks. The area covered by the Middle Paleozoic volcanics totals around 40 000 km2 (40% of the territory of the Omolon terrane). A characteristic feature of the KVB is the absence of large batholith-like intrusions (Egorov, 2004). The Kedon Complex includes the subaerial sheeted volcanic rocks of the Kedon Group and the synchronous subvolcanic and extrusive massifs; its age range spans the Devonian and the basal Early Carboniferous (Gagieva, 2014).

According to Egorov (2004), the KVB is a large (400 by 80–130 km) fragment of the Circum-Siberian marginal continental belt. However, the paleoreconstructions made by Shpikerman (1998) demonstrate that the KVB could be formed on the margin of the Omolon–Okhotsk microcontinent, which was split off from the Siberian plate. This is confirmed by the fragments of the belt which were preserved not only in the Omolon but also in the Okhotsk cratonic terrane and in the Shantar Islands (Shpikerman, 1998).

The total thickness of the volcanic sheets within the largest Kedon segment of the KVB is up to 1500–2000 m in the central part and thins down to 500–1200 m on the margins. These volcanics rest on the underlying sequences with abrupt angular unconformity and compose homoclines with dip angles not higher than 5°–15°. The age of the volcanic rocks of the Kedon Complex was determined by the radiometric methods (Rb–Sr age is 334–377 Ma) and based on the comparatively rare fossil finds—the Givetian brachiopods and conodonts and the Famennian and Early Carboniferous conodonts and flora (Egorov and Sherstobitov, 2000).

The Kedon Group is generally characterized by the significant predominance of rhyolitic and trachyrhyolitic rocks (65–80 vol %) with subordinate dacites, trachyandesites, and trachytes (around 35%) and scarce (not more than 15%) trachybasalts and basalts (Gagieva, 2014).

In terms of petrogenic element concentrations, the basalts and trachybasalts of the Kedon Group are substantially similar to the tholeiitic basalts in the zones of rifting and intraplate magmatism, and the basaltic andesites and andesites, with the calc-alkaline basaltoids of the present-day island arcs and the Andean-type and Californian-type active continental margins (Gagieva, 2014).

The Burgali orefield consists of the stratified complex of the Middle–Late Devonian and Early Carboniferous sedimentary and volcanic rocks and the Quaternary sediments. The Middle–Late Devonian volcanic and volcanosedimentary rocks of the Kedon Group, represented by three contrasting formations, Ochakchan, Lednik, and Zakharenko (Fig. 3a), are predominant.

Fig. 3.
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Geological (a) and geochemical (b) maps of Burgali deposit after (Ishkov, 2014 г.) (1–2) Quaternary sedimentary deposits: (1) channel fill, (2) terrace deposits; (3) Carboniferous Period. Early Epoch. Tournaisian Age. Pushok Formation: limestones, siltstones, ash tuffs; (4) Devonian and Carboniferous periods. Late and Early epochs. Burgali Formation: tuffstones, tuffaceous siltstones; (5) Devonian Period. Middle and Late epochs. Zakharenko Formation: trachydacites, andesites, basaltic andesites, dacites, and ignimbrites; (6) Devonian Period. Middle and Late epochs. Lednik Formation: ignimbrites, rhyodacites, tuffstones; (7) Devonian Period. Middle and Late epochs. Ochakchan Formation: rhyolite and rhyodacite tuffs, tuffstones; (8) Bebekan complex of Late Jurassic subvolcanic intrusions: minor intrusions and dikes of porhyry syenites; (9) Bulun complex of Early Carboniferous hypabyssal intrusions: porphyry diorite laccoliths and dikes; (10) gold stockwork.

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The Ochakchan Formation is composed of rhyolitic, trachyrhyolitic, rhyodacitic, and dacitic ignimbrites and tuffs with tuffaceous gravelstone and tuffstone interbeds. The thickness of this formation is up to 500 m.

The Lednik Formation is composed of trachyrhyodacitic and rhyodacitic ignimbrites and tuffs with tuffstone and tuffaceous gravelstone interbeds. The thickess is 0–450 m.

The Zakharenko Formation is composed of trachyandesitic, andesitic, basaltic-andesitic, basaltic, trachydacitic, and dacitic lavas, tuffs, and ignimbrites; the tuffs contain tuffaceous conglomerate and tuffstone interbeds. The thickness varies from 10 to 400 m.

The Upper Devonian–Lower Carboniferous sequence (Fig. 3a) is composed of tuffaceous clastic and molassoid rocks of the Burgali Formation (tuffaceous conglomerates, tuffaceous gravelstones, tuffstones, and tuffaceous siltstones). The thickness is estimated at 340–350 m.

Above them are the rocks of the Carboniferous System. These rocks are represented by marine carbonate and clastic sediments of the Pushok and Vazhnyi formations. Composition: conglomerates, sandstones, siltstones, marls, limestones, silty limestones, ash tuffs, and tuffites. The total thickness is 150–720 m.

The part of the sediments of the Permian Period is subordinate within the orefield. The thickness of the Quaternary eluvial and colluvial slope wash, consisting of bedrock detritus, is 1–3 m.

The intrusive and subvolcanic rocks are represented by complexes of various ages and compositions: Kedon, Middle–Late Devonian subvolcanic rocks; Bulun, Early Carboniferous intrusions (rhyolitic); Bebekan, Late Jurassic intrusions; Omolon, Early Cretaceous intrusions; and Victoria, Late Cretaceous intrusions. According to the geophysical data, the orefield is localized in the supraintrusion zone of the Early Carboniferous intrusions of the Bulun Complex, which make up a dome.

The orefield has a block structure. It is bounded by northeast trending (20°–35°) thrusts to the north and south and northwest trending (350°) high-dipping normal faults, to the east and west (Fig. 4). Inside these boundaries, there is a system of lower order cross faults (NE 40°–45° and NE 60°–65°). The gold-bearing stockwork is controlled by this fault system and the subvolcanic rhyodacites of the Kedon Group and consists of parallel en-echelon zones of extensive quartz and carbonate–quartz veining. The stockwork was confirmed over a distance of 3700 m north-northeastward (~25°) along the strike (Fig. 3b). Some veins are as thick as 1.5–2 m and as long as 300–500 m.

Fig. 4.
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Cross section along line 350 and across strike of Burgali ore zone, after (Ishkov, 2014). (1) Devonian (D2–3) volcanic host rocks: andesites, dacites, andesitic and dacitic tuffs; (2) Devonian (D2–3) subvolcanic rhyolites; (3) gold-bearing quartz–carbonate and quartz veins; (4) quartz–carbonate and quartz veinlet zones; (5) faults; (6) core holes: number, thickness by intersection (apparent)/horizontal thickness (m), Au and Ag grades (g/t).

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Three separate ore zones have been recognized within the stockwork, Burgali North, Burgali Central, and Burgali South (Fig. 3b). The lengths of the veins in the zones are 300–500 m; they vary in thickness and length and are discontinuous. Au concentrations in veins vary from 5 to 50 g/t; Ag, 5–200 g/t; and the Au/Ag ratio is around 1 : 4. The veins dip predominantly southeastward at 50°–70° and the vertical span of mineralization, according to the exploration data, is around 250 m (Fig. 4). The strike of the veins in the Southern Zone gradually turns due north, and the veins flatten downdip.

METASOMATIC ALTERATION OF HOST ROCKS

Practically all the host rocks underwent regional propylitic alteration in low-temperature carbonate–chlorite and, to a lesser extent, medium-temperature epidote–chlorite facies (Fig. 5). The sericitization and silicification zone includes numerous quartz and carbonate–quartz veins and veinlet zones that make up the northeast-trending (~25°) linear stockwork, in which ore zones and orebodies were recognized (Fig. 3b).

Fig. 5.
figure 5

Metasomatic alteration of host rocks: (a–c) silicification and late quartz veinlets in early quartz (a); (d) illitization; (e, f) carbonate veinlets (e) and carbonate–chlorite alteration (f) in non-polarized light; (g) sporadic epidotization; (h) K-feldspathization; (i) coarsely and finely disseminated ore mineralization.

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The deposit also contains kaolinite–quartz–illite argillizites. They occur locally in fault zones and wallrock intervals. The intensity and extent of argillization are insignificant. They usually occur in the form of lenticular formations with a thickness of several tens of centimeters (up to one meter) and a length of up to ten meters. The argillizite type wallrock metasomatites, associated with ore veinlets, always contain 1–5% pyrite, which is a good ore guide. An interesting feature of this pyrite is its zoning, which accentuates the crystal habit variations in the course of crystal growth: the early pyrite is cuboctahedral, whereas the late pyrite is cubic. The lower horizons of the deposit contain widespread siderite.

The studies of the wallrock metasomatites demonstrated that the erosional truncation of the deposit is close to minimal, as indicated by the argillizites preserved on the surface.

ORE GEOCHEMISTRY

The ores of the Burgali deposit are composed predominantly of SiO2 (89.43% on average) with appreciable concentrations of Al2O3 (5.87), K2O (1.39), and Fe2O3 (0.84), as well as CaO (0.45) and MgO (0.24) (Table 1). Consequently, orebodies consist predominantly of quartz with subordinate adularia, carbonate, and illite. The ores are characterized by low and very low Na2O, TiO2, P2O5, and MnO concentrations (Table 1). The sulfide content in the studied ores is also very low (0.52% Stotal) and corresponds to the very low-sulfidation character of the established mineralization (see below), which is typical of the low-sulfidation epithermal Au–Ag ores of the KVB (Volkov et al., 2016).

Table 1.   Chemical composition of ores (in wt %) at Burgali deposit
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The elemental composition analysis data for the ores of the Northern, Central, and Southern zones and host rocks of the Burgali deposit are given in Table 2 and the plot (Fig. 6), in which they are normalized to the average upper crustal values (Teilor and Mak-Lennan, 1988). The REE spectra, normalized to the average values for chondrites (Anders, 1989), are shown in Fig. 7.

Table 2.   Elemental composition (g/t) of analyzed epithermal ore and host rock samples from Burgali deposit
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Fig. 6.
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Distribution of most important trace elements in epithermal ores (a) and host rocks (b) of Burgali Au–Ag deposit, normalized to average upper-crustal values (Teilor and Mak-Lennan, 1988).

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Fig. 7.
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Chondrite normalized (Anders, 1989) REE distribution in epithermal ores of Burgali Au–Ag deposit (a–c) and distribution of average REE values in ores and host rocks (d). See Table 2 for sample indices.

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The ores of the Burgali deposit are enriched in Au, Ag, As, Sb, Te, W, Mo, and Bi (Table 2, Fig. 6) as compared with their average concentrations in the upper crust (Teilor and Mak-Lennan, 1988). The enrichment factors vary from several-fold (Bi, Te) to tenfold (As, W, Mo), hundred-fold (Sb), thousand-fold (Ag), and ten-thousand-fold (Au) (Table 2, Fig. 6), providing evidence for the geochemical affinity of some trace elements and their synchronous participation in ore formation.

The REE spectra of epithermal ores are mostly chondrite-like with shallow slopes (Figs. 7a–7c), largely similar in configuration with the REE spectra of the host rocks (Fig. 7d) of the Burgali deposit. The host rocks are insignificantly enriched in a fairly large number of elements (Fig. 7d).

The ores of the Central zone are characterized by chondrite-like spectral profile unlike the Southern and Northern zones (Fig. 7b). Such spectral profile is characteristic of the host andesites (Fig. 7d). A few ore samples from the Southern and Central zones display Eu peaks (Fig. 7b, 7c), also characteristic of rhyolitic tuffs (Fig. 7d).

The ores of the Northern and Southern zones displayed fairly low ΣREE values (6.78 and 13.91 g/t on average, respectively), whereas the Central zone displayed much higher values, 45.68 g/t (Table 2). The ΣREE values in ores are appreciably lower than in the host rocks (Table 2).

The Eu/Eu* values in the ores of the Burgali deposit are mostly positive and slightly exceed 1 on the average, whereas the Ce/Ce* values vary from negative to slightly positive values (from 0.06 to 1.14). Similar Eu/Eu* and Ce/Ce* combinations were also established for the host rocks (see Table 2).

ORE STRUCTURES

The veined mineralization of the Burgali deposit displays all the signatures of the near-surface genesis, which is expressed by the wide development of structures typical of epithermal deposits, namely, the colloform-banded structures consisting of quartz, chalcedony, and adularia, and less frequent brecciated, cockade, and framboidal-platy structures (Fig. 8). The illite and chalcedony bands in colloform-banded aggregates contain fine dusty ore mineral dissemination, which imparts a dark gray color to these bands (Figs. 9a, 9b). Thin section microscopy demonstrated that the late quartz veins of the deposit contain Lower Carboniferous coaly siltstone fragments (Figs. 9c–9e), filled with the timber remains (Figs. 9f–9h).

Fig. 8.
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Structures of ore veins at Burgali deposit (core samples from boreholes CP300-276W and CP925-105W): banded and colloform-banded, patchy and streaky with dark gray finely cavernous ore-bearing quartz.

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Fig. 9.
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Ore structures at Burgali deposit: (a) typical colloform-banded structure with occasional adularia (white) in dark concentrated ore mineralization bands; (b) veinlet: ore-rich early quartz is cross cut by late gray chalcedonic quartz veinlet; (c) brecciated structure: coaly siltstone fragments are cemented by quartz; (d, e) veinlet and brecciated structures: large coaly siltstone fragments in barren veinlets; (f–h) fragments display silicified timber texture in transmitted light images.

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Three main groups of structures can be recognized: (1) replacement structures, which form as a result of the metasomatic replacement of the rocks and ores; (2) deposition structures, which form as a result of the various growth conditions of the mineral aggregates, free or restricted, as well as various fracture filling patterns, or veinlet formation by chalcedony deposition from viscous colloidal solutions; and (3) cataclasis structures (insignificantly presented at the deposit), which reflect the results of destruction of the early mineral aggregates with subsequent cementation of the fragments.

ORE MINERALOGY

In terms of mineral composition, the Burgali deposit can be classified as a low-sulfidation (not more than 2% sulfides in ores) epithermal Au–Ag deposit. The ores are characterized by colloform-banded structures, finely disseminated ore mineralization, and the presence of adularia and chalcedony (Figs. 8, 9). The outlook of the most typical ore at the Burgali deposit is shown in Fig. 9a. The mineral composition of the ores is given below (Table 3).

Table 3.   Mineral composition of ores at Burgali deposit broken down by abundance group
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Quartz is the most abundant gangue mineral. Two generations of this mineral have been recognized. Quartz-I is represented by the pale gray variety, which is intergrown with adularia and illite, making up rhythmically banded aggregates. In the course of vein formation, it was deposited first as chalcedony, which was later crystallized into the microcrystalline phase first and into columnar quartz aggregates afterwards. Quartz-II crosscuts the early quartz–adularia–illite aggregate as fine chalcedonic quartz veinlets (Fig. 5a).

Adularia occurs as feathery and patchy aggregates in quartz bands, has a pale cream tint, and is almost entirely replaced by clay minerals, because the samples were picked in surface mining works. Adularia concentrations in ore veins are not higher than 5%.

Illite replaces feldspar and forms independent banded aggregates in colloform structures. Due to its elevated porosity, the illite bands often contain the bulk of ore mineralization.

Clay minerals occur as pseudomorphs after adularia.

Pyrite occurs in the amount of around 3% in some parts of the veins but it was not found at all in most samples. Pyrite occurs as disseminated euhedral crystals with pentagonal and hexagonal dodecahedral habits; crystal intergrowths are less frequent. The segregations are up to 1.5 mm across. Pyrite was found to contain relics of an ovoid texture, which is typical of the transitional product of the pyrrhotite → pyrite replacement process and is referred to as the “bird’s eye.”

Native gold is more frequent than other minerals. It is characterized by fineness within the range of 338 to 719‰ with the average fineness of 596‰; the standard deviation is 107; the dispersion is 11471; and most values are in the range of 600 to 650‰ (Fig. 10). The fineness distribution is polymodal. The low fineness gold from chalcedonic quartz veinlets, which is scarce, falls within a separate fineness range of 300 to 350‰.

Fig. 10.
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Native gold fineness at Burgali deposit: occurrence frequency (%) as a function of fineness (‰).

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Low fineness native gold is emplaced as free gold directly in quartz and the quartz–illite aggregate (Fig. 11). The segregations are anhedral, interstitial, bounded by the faces of the matrix’s mineral crystals. The segregations can be subdivided into two size classes, fine to dusty (5–50 µm), which accounts for ~70%, and the relatively coarser material (100–300 µm), which accounts for 30%. Occasional segregations are as large as 800 µm.

Fig. 11.
figure 11

Morphology of low fineness native gold segregations at Burgali deposit: (a) coarse interstitial segregation; (b) low fineness native gold intergrowth with stephanite; (c) fine dust-like segregations; (d) typical low fineness native gold segregation filling interstice between quartz crystals.

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Native gold with various fineness values occurs predominantly as intergrowths with polybasite and is often surrounded by a scalloped rim composed of supergene acanthite, petrovskaite, and uytenbogaardtite (Figs. 12a–12c). In addition, the Burgali deposit is characterized by gold deposition as inclusions in pyrite crystals (Fig. 12b). A certain part of the gold is deposited directly in quartz and the quartz–illite aggregate. Structural etching revealed heterogeneity within each native gold segregation. The low fineness gold segregations are surrounded by thin lower fineness rims, which provide evidence for the weakly manifested thermometamorphism during the late quartz deposition.

Fig. 12.
figure 12

Typical intergrowths of ore minerals at Burgali deposit: (a) uytenbogaardtite rim on native gold segregation; (b) pyrite intergrowth with native gold; (c) petrovskaite formation on periphery of native gold segregation; (d) gold-bearing pearceite intergrowth with polybasite and supergene cerargyrite; (e, f) gold-bearing pearceite–polybasite.

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Native silver occurs episodically and is probably supergene, being deposited at the boundary between quartz and manganese carbonate, which is oxidized into pyrolusite. According to (Shilo et al., 1992), pyrolusite is a native silver precipitator. Native silver segregations are not larger than 200 µm.

Gold-bearing pearceite (Au,Ag)10(As,Sb)S6 is the most abundant silver mineral in the ores (Table 4, Fig. 12d–f). It occurs as anhedral segregations and close intergrowths with native gold. The segregations are 20–500 µm in size.

Table 4.   Gold and tellurium bearing pearceite (Ag,Au)10(As,Sb) (S,Se,Te)6 of Burgali deposit
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Among the most important mineralogical features of the Burgali deposit is the wide development of Au- and Te-bearing pearceite–polybasite, in addition to the common polybasite (Fig. 13). This mineral is close to polybasite–pearceite in its optical properties, except for the slightly creamy tint. A chlorargyrite rim sometimes separates it from the low fineness gold. An intergrowth of the Au- and Te-bearing phase and a Bi mineral, the non-stoichiometric pavonite, was reported in one case. The axial length of the latter mineral segregations is 20–30 µm. During recalculation to the formula coefficients, the mineral displays a high deficiency in S, which is not compensated by Se.

Fig. 13.
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Even Au, Ag, and Te distribution in gold-bearing pearceite (images in characteristic X-rays).

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Selenous uytenbogaardtite and petrovskaite form in the oxidized zone of the deposit (Table 5, Fig. 12). Uytenbogaardtite is widespread in the ores; it makes up rims up to 0.7 mm in thickness on native gold (Fig. 12a), sometimes together with chlorargyrite. Segregation morphology (rims, sometimes porous) confirms its supergene origin. The mineral is non-stoichiometric in terms of composition; some analyses correspond rather to petrovskaite (Table 5).

Table 5.   Compositions of non-stoichiometric uytenbogaardtite and petrovskaite of Burgali deposit
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Stephanite occurs predominantly as independent segregations, often confined to illite clusters, and occasionally as intergrowths with native gold. Its segregations are anhedral and do not exceed 200 µm.

Acanthite occurs both as intergrowths with electrum in overgrowth rims and as independent segregations in quartz–illite vein filling material. Acanthite segregations are not larger than 100 µm.

Selenous argyrodite can also be considered as a rare find in the Burgali ores (the Se concentration is 1.01–3.09 wt %, Table 6). Earlier argyrodite was encountered only at the Mesozoic Nyavlenga deposit (Savva, 2019), where the Au–Ag ores are combined with porphyry Mo mineralization. Other sulfides occur in very small amounts. Among them are pyrite, acanthite, galena, sphalerite, chalcopyrite, and fahlore.

Table 6.   Compositions of ore minerals of Burgali deposit, wt %
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RESULTS AND DISCUSSION

The problem of the age of the Au–Ag epithermal mineralization is of high metallogenic importance in the KVB. The adherents of the Paleozoic age (Stepanov and Shishakova, 1994; Kotlyar et al., 2001) believe that the Kubaka and Birkachan epithermal deposits are genetically related to the KVB and that they were formed at the Devonian–Carboniferous boundary (360–330 Ma). Other researchers believe that these deposits are Jurassic–Cretaceous (170–100 Ma) and their formation time coincides with the Yana–Kolyma orogeny (Natalenko et al., 2002). During this period the folded structures of the clastic–carbonate Verkhoyansk Complex (C–J) were thrust onto (overrode) the structures of the Omolon fragment of the KVB.

According to (Akinin et al., 2020), ore mineralization in the Tsokol’naya zone of the Kubaka deposit is sandwiched (in terms of formation time) between the eruptions of the ore-bearing Late Devonian volcanics of the Kedon Group (U–Pb zircon age of ca. 370 ± 2.5 Ma, SHRIMP) and the post-ore crosscutting Early–Middle Jurassic trachybasalt and picrobasalt dike of the Omolon Complex (with a 40Ar/39Ar plateau age of 179 ± 8 Ma), which does not penetrate the Carboniferous Korbin Formation. Data obtained by these authors indicate the Carboniferous upper age limit of the epithermal gold mineralization with a high degree of certainty (Akinin et al., 2020).

Fragments of the Lower Carboniferous coaly siltstones were for the first time recognized in the late quartz veinlets at the Burgali deposit (Figs. 9c–9e). They represent fragments of coalified timber with the distinctly visible cellular structure, cemented with quartz (Figs. 9f–9h). This fact allows us to estimate the age of mineralization at the Burgali deposit as Carboniferous and does not contradict the latest data on the age of the Kubaka deposit (Akinin et al., 2020).

Comparative analysis (Table 7) demonstrated a great similarity in the geological structure, orebody morphology, and ore composition between the Burgali deposit and other epithermal low-sulfidation Au–Ag deposits in the KVB.

Table 7.   Comparative geological–mineralogical characteristics of Burgali, Birkachan, and Kubaka deposits
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As a result of the increase in pressure, light REE migrate into the aquatic fluid, whereas heavy REE are retained in the magma, which allows us to classify the former as “hydrophile” and the latter as “magmaphile” elements (Zharikov et al., 1999). In addition, REE were divided into three groups: ceric (La, Ce, Pr, Nd), yttric (Sm, Eu, Gd, Dy, Но), and scandic (Er, Yb, Lu) (Mineev, 1974). Therefore, Table 2 demonstrates that the REE spectra of the ores and host rocks studied are dominated by light hydrophile lanthanides of the ceric group.

The Eu and Ce anomalies are usually considered as the markers of the oxidation–reduction potential of the ore formation environment (Bortnikov et al., 2007; Goryachev et al., 2008; Jones and Manning, 1994). The ores of the Burgali deposit are characterized by mostly positive Eu/Eu* values, whereas the Ce/Ce* values vary from negative to slightly positive values (see Table 2). This combination of Eu/Eu* and Ce/Ce* ratios indicates the existence of oxidizing conditions during the deposition of the epithermal ores of the deposit (Jones and Manning, 1994).

The distribution pattern and the profiles of the REE spectra (Fig. 7) as well as the combination of the Eu/Eu* and Ce/Ce* ratios (see Table 2) of the studied ores are largely similar to those of the host rocks of the deposit. These facts enable us to conclude that the host volcanics and andesitic magmas are the most probable sources of ore material for the ore-forming fluids.

The comparative analysis of the average concentrations of trace elements and REE in the ores of the Kubaka, Birkachan, and Burgali Au–Ag deposits in the KVB demonstrates an almost complete analogy in the composition and distribution spectra, which indicates the similarity in the ore formation conditions between these ore deposits (Volkov et al., 2016). We note the appreciable difference in the trace element compositions of the ores in the Northern, Central, and Southern vein zones of the Burgali deposit (see Fig. 6), which is attributable to their different erosional truncation level.

The comparison of the obtained data with the known published examples (Vinokurov et al., 1999; Kravtsova, 2010; Volkov et al., 2018) suggests that the established ore characteristics, i.e., the depletion of rare earth elements, prevalence of light lanthanides over heavy lanthanides, and the positive europium anomalies, are typical of the epithermal ore formation system.

Our study demonstrates that the ores of the Burgali deposit can be classified as very low-sulfidation (0.5–2%), which is typical of the Kubaka and Birkachan epithermal low-sulfidation Au–Ag deposits (Table 7). The low sulfidation makes the ores of the deposit easy for concentration, including heap leaching.

Among the main mineralogical features of the ores of the Burgali deposit is the wide occurrence of the Au- and Te-bearing pearceite–polybasite (Ag,Cu)16(As,Sb)2(S,Se)n. The compositions of the minerals of this series are characterized by isomorphous substitutions of Sb by As, Ag by Cu, and S by Se. The tellurium concentration in polybasite at the Burgali deposit varies from 0.00 to 2.62 wt %, and the Au concentration, from 2.38 to 10.00 wt % (most frequently, 6–7 wt %); the mineral is essentially arsenical, whereas Cu is absent altogether (Table 4). During the recalculation in the formula coefficients, polybasite displays a high deficiency in S, which is not compensated by Se (Table 4). Among the epithermal deposits of northeastern Russia, the Au-bearing polybasite was encountered in the ores of the Mesozoic Kupol, Dal’nee, and Sopka Kvartsevaya deposits (Savva, 2019). It was also encountered in the ores of the Pallancata epithermal Au–Ag deposit (Peru) (Jorge et al., 2013).

It should be noted that the polybasite–pearceite group is characterized by a wide isomorphous miscibility between Sb and As. The group includes not only polybasite and pearceite but also stephanite and billingsleyite. Moreover, the monoclinic polytypes Тас, 2Тас, and Т2а2с were also described for the natural phases with compositions close to those of polybasite–pearceite (Yushko-Zakharova et al., 1986). Based on the results of the thermodynamic analysis of the phase relations, Nekrasov (1991) demonstrated the possibility of the existence of intermediate aurostibite in the Au–Ag–Sb–S system and petzite, in the Au–Ag–Te system with their subsequent integration into complex compounds for the ores of epithermal silver sulfosalt deposits.

The presence of Se and Te in polybasites was reported earlier. For instance, Oreshin (1979) calculated the formula (Ag, Cu)15 (Sb, As)2 (S, Se, Te)15 for the selenous polybasite and referred Se and Te to S. The gold-bearing phase, which we termed as Au- and Te-bearing pearceite, is adequately calculated in 17 formula units and takes the form (Ag,Au)10 (As, Sb) (S, Se, Te)6 as a phase intermediate between polybasite and billingsleyite (based on S) with stable Au concentrations of 6 to 7 wt %. This complex mineral phase is presumably formed under nonequilibrium, steep-gradient conditions. The presence of polytypes in the polybasite–pearceite group complicates identifying such compositions from the X-ray structural analysis data.

Also unusual is the presence of selenous argyrodite with 15.82–15.86 wt % S, 1.01–3.09 wt % Se, 5.53–6.89 wt % Ge, and 73.92–76.23 wt % Ag in the ores (Table 6). Earlier, argyrodite was encountered only in the Mesozoic deposit Nyavlenga, where epithermal Au–Ag ore is combined with porphyry Mo mineralization (Savva, 2019). According to the ICP-MS data, the ores of the Burgali deposit are also characterized by elevated Mo concentrations (Fig. 6).

CONCLUSIONS

Based on the results of the comparative analysis, a similarity in geological structure, orebody morphology, and ore composition was established between the Burgali deposit and other epithermal low-sulfidation Au–Ag deposits in the KVB.

The ores of the deposit have very low-sulfidation with finely disseminated mineralization and widespread chalcedony. The structures are predominantly colloform-banded and often combined with the brecciated structures. The late quartz veins contain fragments of coaly siltstones (C1), filled with remains of timber fragments, which indicate the Carboniferous age of the ore mineralization. Among the most important mineralogical features, in our opinion, is the broad development of the Au- and Te-bearing pearceite–polybasite in the ores.

The ores’ geochemistry agrees well with the mineral composition. The ores are enriched in a fairly narrow range of elements (Au, Ag, As, Sb, Te, W, Mo, and Bi) and characterized by a small sum of REEs. The presence of positive Eu-anomalies was noted. The REE spectra are dominated by light hydrophile lanthanides of the ceric group. The ores are abnormally poor in REEs; in addition, the light lanthanides prevail over the heavy ones. The established combinations of the Eu/Eu* and Ce/Ce* ratios indicate the oxidizing ore formation environment.

The outlook for Au and Ag reserve additions at the Burgali deposit depends on the further studies and exploration of the ore-bearing stockwork.

The data given in this paper are practically important for regional metallogenic forecasts and for the prospecting and evaluation of epithermal Au–Ag deposits.