INTRODUCTION

The Baikal–Vilyui Paleoproterozoic belt of the crystalline basement of the Siberian Platform and Baikal–Patom fold belt extends northeast from Lake Baikal to the eastern boundary of the platform (Kushev, 1985). In successive regionalization schemes of the crystalline basement of the Siberian Platform, the Baikal–Vilyui belt bears different names: Vitim–Vilyui (Mishnin et al., 1987), Akitkan (Rosen, 2003) (Fig. 1), and Nyurba (Smelov et al., 2001). Within the Siberian Platform, the belt is overlain by the sedimentary cover of the Vilyui Syneclise, which is more than 10 km thick. Early Precambrian metamorphic rocks of the belt outcrop within the Sarma–Tonod zone of highs in the Baikal–Patom fold belt and have been exposed at a depth of 368 m by the Suntar reference borehole under Lower Jurassic deposits of the Vilyui Syneclise.

Fig. 1.
figure 1

Sketch map of crystalline basement of eastern part of Siberian Platform and Baikal–Patom fold belt after (Rosen, 2003), simplified. (1) areas of development of Early Precambrian metamorphic rocks; (2) Paleozoic–Mesozoic foreland fold belts; (3) major fault zones within platform (sutures).

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The sedimentary deposits of the Vilyui Syneclise, which overlie the Baikal–Vilyui belt of the crystalline basement, contain gold placers, which are combined in the Vilyui gold-bearing region (Timofeev et al., 1970) or in the promising Nahar–Kepta gold placer zone of the East Siberian metallogenic province (Mineragenicheskaya …, 2008). The Vilyui Syneclise has also been evaluated as promising for Precambrian gold-bearing conglomerates that formed during breakup of the Suntar uplift (Chersky, 1971; Trushkov et al., 1975), and high-grade flake gold in conglomerates of the basal part of the Lower Jurassic Ukugut Formation and Vilyui-type sandstones (Timofeev, 1965).

Z.S. Nikiforova et al. (2013, 2018), based on the mineralogical and geochemical features of gold, demonstrated a relationship between gold of the Vilyui Syneclise and primary sources of gold–platinoid, low-sulfidation-gold-quartz, and gold–silver types. It has also been established that gold is represented by two types: Precambrian and Mesozoic. Precambrian gold is characterized by a fine fraction (0.1–0.25 mm) and high fineness (900–999‰).

The confinement of Vilyui Syneclise gold placers to the Suntar uplift in the Baikal–Vilyui Paleoproterozoic belt steered the objective of the article: to assess the relationship between the gold content of the sedimentary cover and tectonic elements of the crystalline basement. The Baikal–Vilyui belt is one of the largest regional linear structures of the basement of the Siberian Platform; therefore, the study of its structure is an integral problem. The contours of the belt in (Kushev, 1985; Rosen, 2003; Smelov et al., 2001) have been delineated in the zone with a reduced anomalous magnetic field level. In this study, the boundaries and tectonic structure of the Baikal–Vilyui Paleoproterozoic belt are substantiated from an analysis of the gravitational field structure.

ANALYSIS OF GEOLOGICAL AND GEOPHYSICAL MATERIALS

In order to substantiate the tectonic structure of the Baikal–Vilyui belt of the basement of the Siberian Platform, the authors analyzed geological and geophysical materials in an area bounded by 108° and 140° E and 60° and 68° N (Fig. 2a). For effective analysis of gravimetric (1 : 200 000–1 : 1 000 000) and magnetic (1 : 25 000–1 : 50 000, 1 : 200 000) maps, digital models were subjected to spectral-statistical processing with separation into high-, mid-, and low-frequency components corresponding to the upper and lower levels of the Earth’s crust and lithosphere. The geological and geophysical materials were interpreted according to the concept of divisibility of the crust into blocks (geoblock, megablock, and first- and second-order blocks).

Fig. 2.
figure 2

Sketch map of studied area (a) and Baikal–Verkhoyansk anomalous zone in the gravitational field structure (EGM2008 model, Bouguer anomaly) (b). (1) Eastern boundary of Siberian Platform; (2) contours of Baikal–Verkhoyansk anomalous zone; (3) axes of linear maxima of anomalous magnetic field. Anomalies of gravitational field are indicated: maxima (U, Urin; S, Suntar; T, Tyukyan; Ch, Chybyda; Kh, Khapchagai; P, Tompo); minima (N, Nyuya; K, Kempendyai; A, Lower Aldan). Inset: BP, Baikal–Patom fold belt.

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The gravitational and magnetic fields of the Siberian Platform are, to varying degrees, spatiotemporally informative about geological processes (Stogny, G.A. and Stogny, V.V., 2005). The gravitational field mainly reflects the tectonic structure of the crust and can be used as an indicator in developing geodynamic models. The magnetic field largely characterizes the material composition of Early Precambrian rocks and their metamorphic conditions.

Gravitational Field

The structure of the gravitational field of the studied area is considered at the following hierarchical levels: anomalous area, anomalous zone, and regional and local maxima and minima. Anomalous areas are large (more than 1000 km in diameter) sections of the gravitational field with a mosaic-block structure, which, depending on the adopted tectonic concept, correspond to geoblocks, terranes, or granite-greenstone and granulite-gneiss areas. Anomalous gravitational zones are extended (over 1000 km long and 100–300 km wide) field elements, including linear negative and positive anomalies with a common strike. The zones correspond to intergeoblock faults or granulite-gneiss belts of the basement. According to this criterion, the regional Baikal–Verkhoyansk anomalous zone has been delineated in the gravitational field of the Siberian Platform (Fig. 2b). The southwestern part of the Baikal–Verkhoyansk anomalous gravitational zone within the Baikal–Patom fold belt is considered in (Stogny, G.A. and Stogny, V.V., 2016).

Within the boundaries of the Baikal–Verkhoyansk anomalous zone of the Siberian Platform, the following gravity anomalies with a common northeastern strike are combined: the Urin, Suntar, Tyukyan, Chybyda, Khapchagai, and Tompo maxima and the Nyuya, Kempendyai, and Lower Aldan minima. The Urin gravity maximum, with an amplitude of more than 30 mGal in plan view, corresponds to the Predpatom foredeep. From the west, the Urin maximm is conjuage with the Nyuya minimum with an amplitude more than 25 mGal and size 25–150 km. In the contours of the maximum, Paleozoic sediments of the Predpatom trough and Early Precambrian metamorphic rocks of the Sarma–Tonod zone are widespread.

The Suntar, Chybyda, and Khapchagai maxima and Kempendyai and Lower Aldan minima are localized within the southern part of the Vilyui Syneclise. The Suntar maximum, with an amplitude of more than 65 mGal, reflects the uplifted block of the same name in the crystalline basement, the Early Precambrian metamorphic rocks of which were exposed at a depth of 368 m by a borehole under Cretaceous deposits of the Vilyui Syneclise (Mokshantsev et al., 1964). In (Isakov et al., 2012), the Suntar gravity maximum is explained by the massif of mafic rocks of the Chinei Complex. The Kempendyai gravity minimum, with an amplitude of more than 60 mGal, is confined to the basin of the same name up to 10 km deep, consisting mainly of Mesozoic and Paleozoic terrigenous–carbonate deposits (Gaiduk, 1988). The Tyukyan and Chybyda gravity maxima form a NW-trending transverse zone, which corresponds to the uplift of the Vilyui Syneclise. The Tompo maximum (with an amplitude of up to 50 mGal) of the Verkhoyansk–Kolyma fold area closes the Baikal–Verkhoyansk anomalous zone of local maxima and minima. The local gravity anomalies of the Baikal–Verkhoyansk zone reflect the total effect of density inhomogeneities in the crystalline basement and its relief. The basement relief in this area is fairly well studied (Gaiduk, 1988; Tektonika …, 2001).

The Baikal–Verkhoyansk anomalous zone in the low-frequency component of the gravitational field corresponds to the boundary of the Lena and Yakutsk anomalous regions (Fig. 3), which reflect the level of the upper mantle and are identified with lithospheric segments of the Siberian Platform up to 250 km thick: the Lena and Yakutsk lithospheric roots.

Fig. 3.
figure 3

Position of Baikal–Verkhoyansk anomalous zone on diagram of low-frequency component of gravitational field of Siberian Platform. (1) Early Precambrian Shield of Siberian Platform; (2) boundary of Siberian Platform; (3) contour of Baikal–Verkhoyansk anomalous gravitational zone (BVZ); (4) isoanomalies of gravitational field. Anomalous areas of low-frequency component of gravitational field: positive (Ln, Lena; Yak, Yakutsk), negative (Al, Aldan; Vr, Verkhoyansk).

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Magnetic Field

Regionally, the structure of the anomalous magnetic field in the eastern part of the Siberian Platform is represented by a system of linear maxima up to 900 km long and more than 30 km wide (Fig. 4). The linear magnetic maxima of the eastern part of the Siberian Platform are predominantly oriented to the northwest, orthogonal to the Baikal–Verkhoyansk gravitational zone. In the northeast of the Anabar–Zhigan, Olenek–Lena, and Magan–Nepsky platforms, the linear maxima of the Anabar Anteclise converge within the Anabar Shield into a complexly differentiated anomalous region created by subvertical bodies of magnetite-bearing rocks up to 2 km thick (Stogniy and Stogniy, 2006). The Yakutsk linear maximum is expressed as a deep-seated fault in the Aldan Anteclise (Istomin and Mishnin, 2003).

Fig. 4.
figure 4

Anomalous magnetic field of studied area after (Karta …, 1995), simplified. (1) Isodynamics of magnetic field; (2) eastern boundary of Siberian Platform; (3) boundaries of Baikal–Verkhoyansk gravitational anomalous zone (see Fig. 2). Letters indicate magnetic field anomalies of various rank: regional linear maxima (An, Anabar–Zhigansk, Ol, Olenek-Lena; Mg, Magan–Nepsky; Yak, Yakutsk); large anomalous areas of mosaic structure of positive values (Тn, Tyung; Bp, Bappagai); large anomalous areas of mosaic structure of negative values (Ln, Linden; Km, Kempendyai); local maxima (Uk, Ukugut; Tg, Tongo; Ks, Kysylsyr).

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The Baikal–Verkhoyansk gravitational zone corresponds to the area of the mosaic magnetic field (see Fig. 4). In the west and east, it is crosscut by the Olenek–Lena and Yakutsk linear maxima, respectively. The central part of the Baikal–Verkhoyansk gravitational zone is represented by the Kempendyai region of negative magnetic field values, to the north of which the Linden region of negative values is located.

The Ukugut, Tongo, and Kysylsyr local magnetic maxima form a chain along the northern boundary of the Baikal–Verkhoyansk gravitational zone, which separates the Kempendyai and Linden regions of negative values. The Ukugut and Tongo magnetic maxima, localized within the contours of the Suntar block of the crystalline basement, reflect blocks of magnetite-bearing Early Precambrian rocks. It is possible that the Kysylsyr magnetic maximum and Bappagai region of positive magnetic field values have a similar nature.

The boundaries of the Akitkan (see Fig. 1) and Nyurba (Smelov et al., 2001) belts of the basement have been delineated along the zone of low anomalous magnetic field values, which includes the Kempendyai and Linden regions of negative values. The Linden area of negative magnetic field values, corresponding in plan view to the northern limb of the Vilyui Syneclise, is located north of the Baikal–Verkhoyansk anomalous gravitational zone, the contours of which determine the position of the Baikal–Vilyui Paleoproterozoic belt in the structure of the basement of the Siberian Platform.

Thus, the Baikal–Vilyui belt of the basement in the structure of the gravitational field of the Siberian Platform is represented by a chain of local NE-trending anomalies, which are combined into the Baikal–Verkhoyansk anomalous zone. Crystalline basement rocks outcrop within the contours of the Suntar gravity maximum. The gravity maxima of the Baikal–Verkhoyansk anomalous zone are comparable in size and amplitude to those of the Stanovoy megablock of the Aldan–Stanovoy shield, reflecting blocks (Zverev, Upper Timpton, Sutam, Dambuka, and Chogar) of high-grade granulites (Stogny, G.A. and Stogny, V.V., 2005), which makes it possible to interpret the Kempendyai, Suntar, Tyukyan, Chybyda, and Khapchagai maxima as high-pressure granulites of the Early Precambrian crust.

BAIKAL–VILYUI PALEOPROTEROZOIC BELT: GOLD-CONTROLLING STRUCTURE OF THE VILYUI SYNECLISE

At the regional level, the structure of the Baikal–Vilyui Paleoproterozoic belt is represented by a system of crystalline basement blocks with a common northeastern strike (Fig. 5), which forms horsts (of the Suntar type) and grabens (of the Kempendyai type). The deep structure of the crust of the Baikal–Vilyui belt along the Berezovo–Ust’-Maya geotraverse, which crosscuts the Tyukyan and Khapchagai blocks in the southeastern direction, is described in (Egorkin et al., 1988; Egorov, 2004). The thickness of the crust of the belt, in comparison to the Anabar and Aldan anteclises, decreases to 35 km, which the thickness of the lithosphere, to 75 km. A.S. Egorov (2004) represented the structure of the earth’s crust along the geotraverse in this section as a rift structure bounded by deep-seated faults to the level of the Moho.

Fig. 5.
figure 5

Sketch map of tectonic structure of Baikal–Vilyui ore-controlling structure of crystalline basement of Siberian Platform based on analysis of geological and geophysical materials. (1) Eastern boundary of Siberian Platform; (2–3) faults: (2) intergeoblock; (3) interblock; (4) granulite blocks forming horsts; (5) tectonic faults identified within contours of linear magnetic maxima; (6) magnetite-bearing objects of crystalline basement; (7) gold alluvial zones and areas after (Mineragenicheskaya …, 2008): NPZ, Nakhar–Kepta prospective zone; KhPR, Khoryntsynsky prospective ore district; (8) gold–platinum-bearing placers after (Okrugin; 2005): (1) Vilyuy River; (2) Kempendyai River; (3–5) headwaters of Namana–Tongo–Chybyda rivers; (6) Kenkeme River; (9) formational types of primary gold sources of after (Nikiforova, 2018): a, gold–platinoid; b, low-sulfidation gold–quartz; c, gold–silver. Letters indicate blocks of Baikal–Vilyui belt of crystalline basement: Ns, Nyuya; Ur, Urin; Sn, Suntar; Km, Kempendyai; Tk, Tyukyan; Chb, Chybyda; Khp, Khapchagai; Nzh, Lower Aldan; Tm, Tompo.

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The Baikal–Vilyui belt, in the form of a regional suture zone dividing the Anabar and Aldan–Stanovoy geoblocks of the crystalline basement, is expressed in the structure of the low-frequency component of the gravitational field of the Siberian Platform (see Fig. 3). The features of the belt formation regime as an intergeoblock structure are also indicated by its strike, which is orthogonal to that of regional tectonic faults in the basement of the Anabar and Aldan–Stanovoy geoblocks (see Fig. 5). O.M. Rosen et al. (2006) consider the belt a Paleoproterozoic island arc, suturing together terranes of the south- and northeastern Siberian Platform.

The formation of the Baikal–Vilyui belt in the Paleoproterozoic, in our opinion, governed intraplate rifting processes, as a result of which granulite blocks were drawn from lower levels of the crust in extension–compression phases. Subsequently, the Baikal–Vilyui Paleoproterozoic belt controlled the sedimentation of the Vilyui Syneclise. Paleozoic–Mesozoic rifting (Kiselev et al., 2002; Levashov, 1975; Masaitis et al., 1975) contributed to expansion and deepening of the Vilyui sedimentary basin. O.P. Polyansky et al. (2013) classify the Suntar, Tyukyan, and Chybyda blocks as stable and less subsided during rifting in the Devonian, or that they occurred during the extension stage.

Gold-bearing placers of the Vilyui Syneclise are confined mainly to the Suntar, Tyukyan, and Chybyda blocks (see Fig. 5), which allows us to attribute Early Precambrian metamorphic and igneous rocks of the Baikal–Vilyui belt to a primary source. In metamorphic rocks of the Suntar block, the gold content was determined at 0.1 g/t (Timofeev et al., 1970). It has been suggested that the granulite blocks of the Baikal–Vilyui belt, as they were pushed out from lower levels of the crust, underwent gold redistribution with the formation of gold–quartz and gold–pyrite–pyrrhotite veins, and the suture zone of the Baikal–Vilyui belt was well permeable to deep fluids, responsible for both gold migration and local metasomatism of sedimentary rocks with the formation of quartz veins bearing ore gold. V.A. Mikhailov (2000) associates ore-bearing fluids with fluid-explosive structures of the Vilyui Syneclise sedimentary cover.

The destruction of metamorphic rocks of the uplifted blocks apparently began with the formation of the Baikal–Vilyui belt in the Paleoproterozoic. Subsequently, gold accumulated in sedimentary deposits with the formation of ancient buried placers in intermediate sources, mainly in conglomerate–sandy sediments of the Upper Triassic Tabysin Formation, basal horizons of the Lower Jurassic Ukugut Formation, and Cretaceous sandstones (Timofeev et al., 1970). Primary sources of gold–platinoid, low-sulfidation-gold–quartz, and gold–silver types, substantiated by Z.S. Nikiforova (2018) based on mineralogical and geochemical characteristics (see Fig. 5), reflect the evolution of Early Precambrian gold during sedimentation in the Vilyui Syneclise. An insignificant amount of placer gold was transported with clastic material from breakup of highs of the Aldan–Stanovoy shield and the Yakutsk uplift.

The metabasite complexes of granulite blocks of the Baikal–Vilyui belt were also a platinum source. In gold-bearing placers, platinum is present from a few to several hundred mg/m3 or more (Tektonika …, 2001). There are known gold–platinum-bearing placers on the Vilyui, Kempendyai, Namana, Tongo, and Chybyda rivers (see Fig. 5). It is considered that the primary source of platinum comprised Permian–Triassic trap gabbro–dolerites (Dodin et al., 1994), ultramafic massifs of basement highs (Okrugin, 2005), and the untapped Tyung intrusive located to the north within the Anabar geoblock (Istomin and Mishnin, 2003).

The Baikal–Vilyui belt of the crystalline basement in its geological and geophysical characteristics (length, width, chains of gravity maxima reflecting outcrops of Early Precambrian granulites) is similar to the Pristanovoy high-grade granulite belt of the Aldan– Stanovoy megablock described in (Glebovitskiy et al., 2009). Granulite blocks of Stanovoy megablock control the large Dambuka, Upper Timpon, Sutam, Preddzhugdzhur, and Chogar–Udykha gold placer districts. According to (Kogen, 1976), the main source of gold from placers in the Sutam district are quartz–pyrite–pyrrhotite and quartz–carbonate–polysulfide veins in granulites, and the source of placers in the Dambuka region is gold-enriched Early Precambrian rocks of the Kamrai Formation (Alektorova and Shmuraeva, 1997).

The areas controlled by the Suntar, Kempendyai, Tyukyan, Chybyda, and Khapchagai blocks of the Baikal–Vilyui Paleoproterozoic belt of the crystalline basement of the Siberian Platform should be classified as potentially promising for buried gold placers in the Vilyui Syneclise. Gold ore objects in the metamorphic rocks of these blocks may also exist. The thickness (370–500 m) of Jurassic deposits overlying the high of the Suntar block will make it possible to prospect for ore gold.

CONCLUSIONS

The placer gold content of the Vilyui Syneclise is controlled by the regional structure of the crystalline basement of the Siberian Platform— the Baikal–Vilyui Paleoproterozoic belt. The belt is overlain by the sedimentary cover of the Vilyui Syneclise, 0.4‒10 km thick (Early Precambrian metamorphic rocks have been exposed at a depth of 368 m by the Suntar borehole under Lower Jurassic deposits). The boundaries and tectonics of the Baikal–Vilyui belt have been substantiated by an analysis of the structure of the gravitational field. The belt includes a system of basement blocks with a common northeastern strike, which form horsts (of the Suntar type) and grabens (of the Kempendyai type). The Baikal–Vilyui belt was formed during intraplate rifting in the Paleoproterozoic: under extension–compression conditions, granulite blocks were brought up from lower levels of the crust.

The gold-bearing placers of the Vilyui Syneclise are mainly confined to the Suntar, Tyukyan and Chybyda granulite blocks, the metamorphic and igneous rocks of which were the primary gold source in the sedimentary cover. The suture zone of the Baikal–Vilyui belt was very permeable to deep fluids, responsible for both gold migration and formation of quartz veins bearing ore gold in the sedimentary cover. The types of possible primary sources of gold (gold–platinoid, low-sulfidation gold–quartz, and gold–silver) reflect the features of the evolution of Early Precambrian gold during sedimentation in the Vilyui Syneclise. The areas controlled by the Suntar, Kempendyai, Tyukyan, Chybyda, and Khapchagai blocks of Early Precambrian rocks, which may contain gold objects, should be considered promising for buried gold placers in the Vilyui Syneclise.