On the regional scale, the Badran gold deposit is located in the rear of the Verkhoyansk fold-and-thrust belt southeast of the Yana-Kolyma orogenic belt (Figure 1). It belongs to the orogenic-type deposits characteristic of one of the world’s largest Late Jurassic-Early Neocomian Yana-Kolyma metallogenic belts, where the total amount of gold produced and recorded in resources is ∼8320 tons (Goryachev and Pirajno, 2014). Here, along with large orogenic gold deposits (Natalka, Degdekan, Pavlik, Drazhnoe, and others), intrusion-related Au deposits of smaller size are also known (Chepak, Teutedjak, Chistoe, Ergelyakh, and others), along with related Quaternary placers (Fridovsky, 2002; Goldfarb et al., 2014; Goryachev and Pirajno, 2014; Fridovsky, 2018; Vikent’eva et al., 2018). There are also Au-Sb, Mo-W, and Sn-W deposits in this territory (Figure 1).

The Badran deposit was discovered in 1961, explored in 1970–1980, and in 1956–1976 by trenching and drilling, and underground mining. In 1984 underground Au mining began at the field, which continues to the present day. The total volume of gold production is about 34 tons. The gold reserves of the deposit are estimated at ∼ 9.3 t with an average content of 7.8 ppm. The deposit has been explored by surface and underground workings at a distance of more than 5 km; a downdip of more than 1.2 km makes it possible to study the structure of the deposit, the mineral composition of ores and the conditions of ore formation at various deep levels. The results of studying various aspects of geology, mineralogy, and conditions of ore formation of the Badran deposit are considered in a number of scientific papers (Anisimova et al., 1998; Fridovsky, 1999; Fridovsky, 2002; Fridovsky, 2018; Neustroev, 2003; Amuzinsky, 2005; Anisimova et al., 2008; Konstantinov, 2010; Obolensky et al., 2011).

Despite many years of study, the origin of the gold deposits of the Yana-Kolyma metallogenic belt and the Badran deposit is controversial. Based on the example of a large Natalka deposit, water entry from a magmatic source is proven (Goryachev et al., 2008). Gamyanin et al. (2018) explain the formation of the OGD region by the involvement of metamorphic fluids. A study by Obolensky et al. (2011) suggested that the Badran deposit was formed during the evolution of a metamorphic hydrothermal system with an important contribution to mantle fluids. For this reason, the main purpose of this study was to obtain new information for a better understanding of the Badran deposit genesis.

In this paper, based on detailed fieldwork in underground mine workings, we present a brief summary of data on the geology of the deposit and new results of studying the material composition of disseminated and vein mineralization, geochemical, stable S and O isotopes, and the fluid inclusion microthermometry data for understanding ore-forming events and reinterpretation of the source of gold and the hydrothermal fluid-flow processes that formed orogenic Au Badran deposit in the context of the evolution of the surrounding accretion-collision structures of the Verkhoyansk-Kolyma orogen.

The Badran Au deposit is located in the rear of the Verkhoyansk fold-and-thrust belt to the southeast of the Yana-Kolyma orogen (Figure 1А). The Verkhoyansk fold-and-thrust belt lies on the Archean-Proterozoic basement and is composed of the Mesoproterozoic–Devonian carbonate-clastic and carbonate rocks and the Carboniferous–Middle Jurassic clastic rocks of the passive continental margin of the Siberian сraton. The Verkhoyansk fold-and-thrust belt is separated from the Yana-Kolyma orogen by the regional Adycha-Taryn fault with an NW orientation and a length of about 2.000 km. The Yana-Kolyma orogen includes the Late Jurassic terranes of various origins (Khanchuk, 2006) (Figure 1А). Deformation processes and magmatism in the orogen in the Late Jurassic–Valanginian is related to the closure of the small Oymyakon paleoocean and the subsequent collision of the Kolyma-Omolon superterrane and the eastern margin of the Siberian craton (Parfenov and Kuzmin, 2001; Goryachev and Pirajno, 2014).

The Yana-Kolyma metallogenic belt (YKMB) is identified mainly in the structures of the Kular-Nera and Polousno-Debin terranes of the Yana-Kolyma orogen and the rear zone of the Verkhoyansk fold-and-thrust belt. The belt stretches in the NW direction for more than 1.000 km with a width of up to 200 km to the SW from the Main Kolyma batholith belt (154–144 Ma, zircon, U-Pb SHRIMP-RG (Akinin et al., 2009), SHRIMP-II (Gertseva et al., 2021)), and Uyandina-Yasachnaya volcanic arc (152–153 Ma, zircon, U-Pb SHRIMP (Toro et al., 2016). Metallogenic specialization of the Badran deposit area is determined by gold (Badran, Drazhnoye, Malo-Tarynskoe, Basovskoe, Talalakh, etc.) and Au-Sb (Sarylakh, Maltan, Kim, Vzbros-2, etc.) deposits and less than W-Sn (Alyaskitovoe), Mo-W (Bekkem) deposits. Quite often, the orogenic gold deposits of the region show the overlap of late Au-Sb and Ag-Sb mineralization associated with the development of the Cretaceous Paleo-Pacific active continental margin.

The main structures of the Badran deposit area are determined by systems of extended NW thrusts and linear folds. They are separated by vast fields with gentle and near horizontal occurrence of rocks. Transverse NE and NS faults (oblique ramps), mainly of strike-slip and normal-strike-slip fault kinematics, dividing the thrusts into separate segments, are also identified. Thrusts are the main ore-controlling structures. The longest (about 20 km) is the Badran-Egelyakh thrust of the Mugurdakh-Selerikan fault system. The faults are expressed by thick (up to the first tens of meters) zones of brecciation of clastic rocks and are accompanied by folds with horizontal and inclined (up to vertical) hinges. Differently directed movements are identified along the faults—early thrust and late sinistral- and dextral-strike-slip faults, studied in detail at the gold deposits of the Kular-Nera terrane (Fridovsky et al., 2014; Fridovsky et al., 2015; Fridovsky et al., 2017; Fridovsky, 2018).

Magmatic formations do not occur at the Badran deposit. To the northeast of the deposit, there are the Late Jurassic intrusive rocks and volcanic rocks of the intermediate and felsic composition of the Tas-Kystabyt belt (151–148 Ma, zircon, U-Pb SHRIMP-II (Prokopiev et al., 2018; Gertseva et al., 2021) and the Late Cretaceous granitoids of the Yus-Kuel complex (92–88 Ma, zircon, U-Pb SHRIMP-II (Protopopov et al., 2019)). Orogenic Au mineralization often involves the Late Jurassic-Early Cretaceous granitoids of small massifs and dikes from the mafic to the felsic composition of the complex of small intrusions (151–143 Ma, zircon, U-Pb SHRIMP II (Fridovsky et al., 2020a; Fridovsky et al., 2020b; Fridovsky et al., 2022)). These rocks were formed from a mixed source with the involvement of the mantle (OIB- and E-MORB type), lower-crust and subduction components with the Mesoproterozoic-Paleoproterozoic Sm-Nd model estimates of the age of their magmatic sources (Fridovsky et al., 2020a; Fridovsky et al., 2020b; Fridovsky et al., 2022).

There is usually no spatial connection between gold deposits and magmatism and metamorphism. The deposits were formed after basic and granitoid magmatism (about 150 million years), superimposed on regional metamorphic changes in clastic rocks of the greenschist facies. The metallogenic time of formation of the gold mineralization is synchronous to the late collisional event of the end of the Tithonian-Valanginian during crustal thickening before the orogen collapse. Such conditions are characteristic of orogenic gold mineralization events in accretion-collision orogens (Goldfarb and Groves, 2015; Yang et al., 2021).

Dating of micas (⁴⁰Ar/³⁹Ar) and native gold (Re-Os) from ore veins of orogenic gold deposits in the western part of the YKMB demonstrate a wide range of tectonothermal events that occurred from 148 to 126 million years (Fridovsky et al., 2015; Fridovsky et al., 2021; Fridovsky et al., 2022; Prokopiev et al., 2018). These events correlate with various tectonic processes on the eastern periphery of the Siberian craton in the Late Jurassic-Early Cretaceous. Isotopic Re-Os estimates of the age of gold (148 and 137 million years) (Fridovsky et al., 2021) coincide with the time of formation of dikes (151–145 million years) and cooling of granitoids of the small intrusions complex (138–137 million years) (Fridovsky et al., 2022) and reflect two stages of the formation of orogenic gold mineralization of the western part of the YKMB in connection with accretion-collision events on the eastern margin of the Siberian craton. According to the new ⁴⁰Ar/³⁹Ar geochronological data, for sericite from alterations, as well as feldspars and plagioclase from dikes of gold ore and gold-antimony deposits of this belt, the interval of 132–123 million years reflects the young tectonothermal processes of the final stage of the formation of gold mineralization (Fridovsky et al., 2022). These data are fit to the available age estimates for sericites from veins [126–127 Ma, ⁴⁰Ar/³⁹Ar (Fridovsky and Zaitsev, 2018; Prokopiev et al., 2018)] and granitoids [120–128 Ma, Rb-Sr and K-Ar (Zaitsev et al., 2018)] of various ore objects of the western part of the Yana-Kolyma metallogenic belt. The Hauterivian-Aptian event was probably a reflection of superimposed tectonic thermal processes occurring in the rear of the Uda-Murgal volcanic-plutonic belt. Thus, the features of the structure and metallogeny of the area of the Badran and YKMB deposits are associated with a complex interaction in the Late Jurassic to Early Cretaceous of subsynchronous geodynamic events on the eastern margin of the Siberian craton (subduction of the Oymyakon Ocean plate, accretion and collision of the Kolyma-Omolon superterrane) and Northern Asia (subduction of the Paleo-Pacific plate accretion (Parfenov and Kuzmin, 2001).

The Badran deposit is confined to the Nadvigovaya zone, the most ore-bearing section of the Badran-Egelyakh fault. The thickness of the thrust zone is up to 30 m. The displacement amplitude is estimated from 600 to 1,300 m (Anisimova et al., 1998; Neustroev, 2003). Several segments of the zone of different occurrence have been identified (Figure 1С). The north-western segment (survey profiles 0–250) occurs in the valley of the Bezvodny Creek. The Nadvigovaya zone here dips to C (5–20) at an angle of 30–50. The central segment (survey profiles 250–420) is the most ore-bearing, the zone dips gently to the NE (40–65), and the angle of dip is 24–30. The occurrence of the southeastern segment (east of the 420-survey profile) is similar to the NW segment—dipping to N (355–20), and the angle of dip is 30–50. Analysis of fractured structures and reconstruction of local tectonic stress fields showed changes in their orientation and kinematics in various segments of the Nadvigovaya zone (Fridovsky, 1999). There are thrust displacements for the central segment: NE-SW orientation of the compression axis and a near-vertical or steeply inclined (65–70) plunge of the extensional axis to the NE or SW. The NW and SE segments are characterized by strike-slip reverse fault movements similar to the central segment by the position of the compression axis, a gently inclined (20–28) plunge of the extensional axis.

The Nadvigovaya zone is composed of breccias of host rocks with vein-veinlet and disseminated gold-bearing pyrite-arsenopyrite mineralization (Figure 2). The folded structures of the deposit area are represented by the Mugurdakh syncline, Talnakh, and Selerikan anticlines of the NW strike, extending up to 120–200 km (Neustroev, 2003). The southwestern wing of the Talnakh anticline is thrust over the Badran-Egelyakh fault onto the structures of the Mugurdakh syncline. The latter has an asymmetric structure, the southwest wing is characterized by gentle (20–25) rock dips, and the northeast—moderately steep (30–45) ones. In the zone of the Badran-Egelyakh thrust, the near-fault compressed folds of the NW and NE strikes are developed (Fridovsky, 2018). Here, the angle of dip of the rocks on the wings of the folds increases to 80°, and overturned occurrences are manifested.

The host rocks are the Norian clastic deposits (sandstones, silty sandstones, and siltstones) of the Upper Triassic with a total thickness of more than 3.800 m. Vein-veinlet mineralization is most intensively developed in sandstone beds, and their contacts with siltstones determine the position and morphology of sheet ore bodies. Ore-bearing rocks are metamorphosed to the level of carbonate-sericite-chlorite and chlorite-carbonate subfacies of the greenschist facies. Proximal alterations of sericite-chlorite-carbonate-quartz composition with highly gold-bearing disseminated pyrite and arsenopyrite are widely manifested in clastic rocks.

Three main morphological types of ore bodies are identified at the deposit: vein, vein-veinlet, and vein-disseminated (Figure 2). Quartz veins of a tabular shape are the leading type of ore bodies (Figure 2A). Their thickness is up to several meters, strike extent is tens of meters, and dip extent—hundreds of meters. Veins usually lie in the axial part of the Nadvigovaya zone, less often in its footwall or hanging contacts. In the upswells of the Nadvigovaya zone, branching and several parallel veins are observed. In the outcrops of veins and their flanks, the number of quartz veinlets increases (vein-veinlet type), and massive and coarse-banded structures of veins are replaced by veinlet and breccia ores (Figure 2B, C). The veinlet-disseminated type is represented by breccias and alterations with veinlet silification (from 5% to 40%) and impregnations of pyrite and arsenopyrite (Figures 2D–F). The content of “invisible” gold in pyrite and arsenopyrite is up to the first hundred grams per ton. In quartz veins and veinlets, gold is free, its content ranges from the firsts to hundreds of ppm and is associated with galena, sphalerite, and chalcopyrite (Figures 2G, H).

The ores of the Badran deposit are characterized by an exceptionally diverse mineral composition (Anisimova et al., 2008). About 50 hypogene and hypergene minerals are described here. Eight different stages of mineral formation were macroscopically and microscopic studies described in the Badran gold deposit, as illustrated in Figure 3. There are four ore and four post-ore stages of mineral formation.

Metasomatic pyrite-arsenopyrite association in proximal sericite-chlorite-carbonate-quartz alterations is widespread. The typomorphic minerals of the association are gold-bearing pyrite3 (Au up to 120 ppm) and arsenopyrite1 (Au up to 205 ppm), which form dispersed dissemination of metacrystals in ore zones and in host rocks (Figures 2D–F; Figures 4C, D). This gold1 is in an “invisible” form (Fridovsky et al., 2022). Arsenopyrite1 is found everywhere, but less often than pyrite3. Arsenopyrite1 forms dispersed dissemination of idiomorphic prismatic crystals in quartz-bearing siltstones (Figures 2E, F). Radial growths of small crystals of arsenopyrite are characteristic. Often crystals are cataclastic (Figure 4D). Quartz1, carbonate, and sericite are associated with pyrite3 and arsenopyrite1 (Figures 2D–F). Pyrite1 and Pyrite2 are also found in altered rocks. Pyrite1 is represented by dust-like and fine-grained spherical (framboidal) particles and nodular aggregates ranging in size from 10 to 100 microns (Figures 4A, B). The location of Py1 follows the primary sedimentary texture of rocks. Metamorphic pyrite2 represented by crystals with subhedral and euhedral shapes, ranging in size from 2 to 3 microns to 2–4 mm, which form dissemination intergrowths, nests, and thin veinlets (Figures 4A, B) (Fridovsky et al., 2022).

Arsenopyrite-pyrite-carbonate-quartz vein association is the most common in ore bodies. Native gold of this association is usually represented by finely dispersed idiomorphic crystals and lumpy varieties of gold (Figure 2H). Quartz2 of the main producing association has an inequigranular and hypidiomorphic structure. Color is milky-white, grayish-white, and, rarely, white. In the selvages of veins and veinlets, as well as near the xenoliths of the host rocks, this quartz often has a crustified texture, and in the central part, a massive and knotted texture. The structure of the aggregate is hypidiomorphic-granular. Vein pyrite4 has, basically, an irregular shape (Figures 4E, F). It is characterized by a high gold content, sometimes reaching hundreds of ppm, but on average tens of ppm, the silver content of pyrite is next lower order. A typomorphic feature of pyrite4 is its increased arsenic content. Arsenopyrite2 is developed in quartz veins and is often found with Py4 (Figures 4E, F). Apy2 is surrounded by late minerals—chalcopyrite and fahlore often associate native gold with arsenopyrite (Figure 4G). The composition of arsenopyrite2 is characterized by high sulfur content and low arsenic content. The mineral is characterized by high gold content, reaching hundreds of ppm. Native gold2 is common in minerals of the early arsenopyrite-pyrite-carbonate-quartz association of vein filling (Figures 5A, B). A significant part of it is also in a finely dispersed form.

Polymetal-carbonate-albite-quartz mineral association is mainly distributed on the flanks of the upper horizons of the Nadvigovaya zone. At deep horizons, it is found in the central part of ore columns. Quartz3 of a sugar-like habit composes separate veins and veinlets. The proportion of this quartz in the vein filling ranges from 15% to 30%. Typomorphic minerals are chalcopyrite, galena, sphalerite, gold, dolomite, and albite (Figure 2G; Figure 4H). Galena is a minor mineral. The frequency of its occurrence increases with depth. It is associated with sphalerite, chalcopyrite, bournonite, fahlore, and native gold (Figures 4G, H; Figure 5C). Galena in the deposit contains bismuth and silver in the form of an impurity, the concentration of which disappears with depth. Sphalerite occurs in quartz in the form of irregularly shaped particles and scattered single crystals of yellowish-brown color, which is explained by low ferruginousity. It is associated with galena, chalcopyrite, bournonite, fahlore, and native gold (Figures 4G, 5C–E). Chalcopyrite is observed in quartz in the form of small irregularly shaped particles and emulsion dissemination of chalcopyrite in sphalerite and fahlore. The frequency of occurrence of chalcopyrite in association with native gold increases with depth (Figures 5C, D). Gold3 (fineness from 800‰ to 899‰) is associated with the main minerals, chalcopyrite and galena. In this case, chalcopyrite always acts as the host mineral, and galena forms close growths with gold (Figures 5C, D).

Sulfostibnite-sericite is a quartz mineral association with an admixture of rare sulfostibnites, characterized by an increase in the potential of antimony and sulfur. It occurs in the central most ore-bearing interval of the Nadvigovaya zone. Quartz4 is an ore-free milky white that mainly associates with carbonate and chlorite. On the contacts of the veins, such quartz usually has a comb structure. Tetrahedrite, gold, and sericite are typomorphic minerals. Tetrahedrite forms xenomorphic inclusions, veinlets, and granular aggregates in quartz in associations with native gold, bournonite, galena, and chalcopyrite (Figures 4I–K; Figure 5F). The silver content in tetrahedrite varies widely from 0% to 9.72%. Tetrahedrite, as a rule, is oxidized with the formation of thin rims, falsers of malachite, azurite, chalcosine, and covellite. The native gold (Au4) of this association is the most high-grade (900‰–998‰), it is predominantly interstitial, lenticular, wedge-shaped, ribbon-shaped, and lump-shaped (Figures 5E, F). The mineral associations described above are the most economically important. They formed the ore columns of the Badran deposit. The formation of pyrite five to six completes orogenic ore formation at the deposit. Pyrite five to six forms a rare euhedral and subhedral dissemination and is related to polymetallic-carbonate-albite-quartz and sulfostibnite-sericite-quartz mineral associations, respectively.

Silver-quartz and cinnabar-stibnite-carbonate-quartz mineral associations are widespread locally. Freibergite, acanthite, argentotetrahedrite, and low-grade mercury gold (400‰–700‰) are the typomorphic minerals of the silver-quartz association. The dark gray—black quartz5-6 of this association cements fragments of early white quartz (Figure 2I). The fahlore of the Badran deposit have an abnormally high silver content (>30%). It does not form large accumulations and occurs in growth with argentite, electrum, native gold, freibergite, and tetrahedrite. Sporadic veinlets of a typomorphic mineral of another association, stibnite with idiomorphic water-transparent quartz and dolomite nests, occupy a crosscutting position in relation to early paragenesis. Signs of cinnabar are found on the near-surface horizons of the deposit. Freibergite (Ag—10%–29%) occurs at the upper horizons, it forms close growths with tetrahedrite. Acanthite and low-grade native gold are often associated with it. Acanthite forms granular aggregates and sometimes myrmekitic growths with freibergite. In association with acanthite, bournonite and low-grade native gold (Au5) are also found in the ore (Figure 5G). Acanthite contains impurities of Fe, Sb, and Cu. It is likely that such a set of elements can be explained by the close association of acanthite with fahlore. Native gold has not been identified in this association. Pyrite 7–8 forms cogenetic growths with the minerals of silver-quartz and cinnabar-stibnite-carbonate-quartz associations.

In the process of hypergenesis, there was a change in ores and the formation of two mineral associations of sulfur-sulfate and oxide compositions. The typomorphic minerals of the first association are gypsum, jarosite, anglesite, covellite, and chalcosine; those of the secondare goethite, hydrogoethite, malachite, azurite, cervantite, tripuhyite, polianite, and uytenbogaardtite. They develop when pyrite, arsenopyrite, chalcopyrite, iron-magnesian dolomite, and mica minerals are replaced. At deep horizons, gypsum and iron hydroxides and oxides are noted in hypergenic minerals. Gold and silver sulfides in the ores of the Badran deposit were found at the horizon of 940–960 m of ore column I in association with high-silver fahlore, acanthite, pyrargyrite, bournonite, galena, various mercurous native gold, anglesite and close growth with hypergene minerals. Considering chemical composition, the analyzed compounds cover the entire interval between AgAuS and Ag₃AuS₂, but differences predominate, tending to the silver analog uytenbogaardtite. Au and Ag sulfides of the Badran deposit, in our opinion, were formed due to native gold and hypergene changes in high-silver fahlores, as evidenced by the ubiquitous joint presence of uytenbogaardtite with oxides of Sb and Fe with the admixture of Ag, Cu, and As. Native gold of the sixth generation (Au6) occurs with minerals of the hypergene stage, mainly with iron hydroxides, scorodite, and jarosite, and sometimes has a spongy habit (Figure 5H). This gold often forms thread-like veinlets in iron hydroxides with arsenopyrite relics.

1) The Badran polychronous gold deposit contains typically orogenic disseminated mineralization with arsenian pyrite and arsenopyrite ores with invisible gold, quartz veins with native gold and sulfides Fe, Pb, Zn, Cu, and sulfosalts, as well as locally post-ore minerals Ag, Sb, and Hg epithermal features. It was formed in six stages of mineralization: (I) metasomatic pyrite-arsenopyrite, (II) arsenopyrite-pyrite-carbonate-quartz, (III) polymetal-carbonate-albite-quartz, (IV) sulfostibnite-sericite-quartz, (V) silver-quartz, (VI) cinnabar-stibnite-carbonate-quartz. The sericite-chlorite-carbonate-quartz proximal alteration contains the disseminated arsenian pyrite and arsenopyrite with high contents of invisible structurally bound Au⁺. Disseminated and vein ores are controlled by thrust structures, which are transit routes for trans-crustal fluids.