*Article* **Typomorphism of Native Gold (Geological-Industrial Types of Gold Deposits in the North-East of Russia)**

**Natalia E. Savva <sup>1</sup> , Raisa G. Kravtsova <sup>2</sup> , Galina S. Anisimova 3,\* and Galina A. Palyanova <sup>4</sup>**


**Abstract:** This study presents the typomorphic features of native gold grains from three different geological-industrial types (GIT) of gold deposits in the North-East of Russia: (1) gold–arsenic-sulfide in black shale strata (Natalka, Degdekan, Karalveem, Maldyak deposits), (2) gold–quartz veins in granitoids (Dorozhnoye, Butarnoye, Shkolnoye, Maltan deposits), and (3) gold–silver adularia in volcanogenic strata (Kupol, Olcha, Kubaka, Burgali, Primorskoe, Dalnee deposits). The reliability of the geological interpretation is directly related to mineral associations, fineness variations, its internal structure and the content of microimpurities. Native gold is a reliable indicator for identifying various GIT of gold deposits at the early geological-prospecting stages of studying gold-bearing areas. Typomorphic features of native gold for each of the considered GIT are stable and do not depend on the age and scale of mineralization. It is shown that using an integrated approach obtains genetic information about a particular ore object, which makes it possible to predict the vertical range of mineralization and outline the technology for processing ores. The information obtained can also be effectively used in the search for placer deposits in nearby watercourses. Identification of typomorphic features of ore and placer native gold opens up wide opportunities for delineating the distribution areas of placer deposits.

**Keywords:** geological-industrial types; ore formations; typomorphic features of native gold; gold deposits in the north-east of Russia

### **1. Introduction**

Successful exploration for gold is impossible without scientifically based forecasts, the reliability of which increases significantly due to the improvement of methods for integrated study of gold ores and the formation of their industrial grouping on this basis. Fundamentals of the geological and industrial grouping of mineral deposits were laid down by [1], who defined the geological-industrial type (GIT) as "a group of geologically similar deposits that have proven themselves in world and domestic practice as a real supplier of this type of mineral raw material". It was V.M. Kreiter who proposed to classify as industrial such natural geological and mineralogical types of deposits that provide more than 1% of the world production of a certain mineral type. It should be noted that, due to the fact that each deposit has some specific features, the unified technological schemes of concentrating plants at large mines are set up to process ores of one bulk geological and industrial type with all the range of its mineral varieties [2].

V.M. Kreiter's followers continued working in this direction [3–8]. They showed that the geological and industrial grouping of deposits is determined by their geological

**Citation:** Savva, N.E.; Kravtsova, R.G.; Anisimova, G.S.; Palyanova, G.A. Typomorphism of Native Gold (Geological-Industrial Types of Gold Deposits in the North-East of Russia). *Minerals* **2022**, *12*, 561. https:// doi.org/10.3390/min12050561

Academic Editor: Liqiang Yang

Received: 28 March 2022 Accepted: 26 April 2022 Published: 29 April 2022

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homogeneity and belonging to certain gold formations—groups of deposits with a similar material composition of ores and genesis. In Northeast Asia, gold deposits are most widely developed, which are grouped into the following GIT types: gold–arsenic-sulfide in black shale strata, gold–quartz veins in granitoids, gold–silver adularia in volcanogenic strata [3]. When studying a particular deposit and its geological and industrial typification, it often turns out that each mineral variety of ores may well be mistaken by researchers for a new or unconventional GIT. This often occurs on poorly studied fragments of gold deposits, ore fields and nodes. In this regard, at an early stage of prospecting and exploration, when predicting the industrial type, an important role belongs to the establishment of a set of typomorphic features of native gold obtained during its study. To be precise, these features give contrasting differences for each group of deposits. The methodical manual for the study of native gold in [9] allows researchers to develop a scientifically based forecast.

The description of the typomorphic features of native gold for each particular gold deposit makes it possible to obtain genetic information about a particular object. Native gold, due to chemical resistance, is capable of such information. To date, plenty of factual material has been obtained on the indicator properties of native gold in the study of ore and placer deposits for various regions of the world [10–23]. The beginning of a comprehensive study of this mineral was laid by outstanding works [24,25].

The concept of the typomorphism of minerals was formulated by A.E. Fersman [26] and developed in the form of the teaching of N.P. Yushkin [27,28]. It is that the mineral composition and properties contains information about its genetic nature, indicating conditions for the formation of the entire deposit. The authors of the doctrine emphasize that over the course of geological time, such information may be erased. In this regard, native gold can be attributed to one of the most stable minerals, preserving genetic information for a long time due to the ability to resist chemical weathering and the absence of brittle deformations. Thus, the complexity of constitutional features of native gold (composition, structure, mineral intergrowths) has excellent indicator qualities.

This study presents a solid dataset of using typomorphism of native gold grains from various types of gold deposits in northeastern Russia as examples to show the importance of applying such a concept. This will allow, with a limited amount of material, at an early prospecting and exploration stage of geological research, to determine the type of deposit, to predict the vertical range of mineralization and a possible method of processing ores.

#### **2. Regional Geological Setting and GITs of Gold Deposits**

#### *2.1. Geological and Structural Position of Northeast Asia*

In geological and structural terms, the north-east of Russia is a complex composition of the Kolyma Chukchi and Primorsky terranes of passive continental margins, island-arc and oceanic terranes, as well as middle cratons. According to A.I. Khanchuk [29], all layers of the earth's interior—from the Precambrian strata to the modern majestic volcanoes of Kamchatka—are exposed in this region and are available for the most detailed studies. Large gold deposits in the region are characterized by different tectonomagmatic stages of their development. The GITs of the deposits considered by us are confined to large structural elements, including the following: the passive margin of the Siberian craton (deposits of the Verkhoyansk complex); collisional structures in the zone of the Tenka deep fault with large zones of tectonomagmatic activation, Omolon cratonic terrane, Kedon—Late Paleozoic and Okhotsk-Chukotka—Mesozoic marginal-continental volcanic belts (Figure 1).

The article will consider three, contrastingly different, GITs of gold deposits in the north-east of Russia: (1) gold–arsenic-sulfide in black shale strata (Natalkinskoye, Degdekan, Maldyak, Karalveem); (2) gold–quartz veins in granitoids (Shkolnoye, Butarnoye, Dorozhnoye, Maltan), (3) gold–silver adularia in volcanogenic strata (Kubaka, Kupol, Olcha, Burgali, Primorskoe, Dalnee). Further in the text and captions, the abbreviated names of these GITs will be used—gold–arsenic-sulfide, gold–quartz veins and gold–silver adularia.

**Figure 1.** Location of research objects on a schematic geological and structural map of the north-east of Russia. The image was constructed by the authors of the present paper using data from [30]. The legend contains the following: 1—Siberian craton; 2—terranes with continental crust of the Siberian Craton (a—Omolon, b—others); 3, 4—deposits of the passive margin of the Siberian craton: 3—Paleozoic-Mesozoic deposits, 4—Mesozoic deposits; 5—folded cover of the Chukotka; 6—Paleozoic-Mesozoic deposits of the Verkhoyansk complex; 7, Late Mesozoic collisional sutures; 8—Kedon Late Paleozoic continental marginal volcanic belt; 9—Late Jurassic-Early Cretaceous volcanic belts; 10—Okhotsk-Chukotka Late Cretaceous volcanic belt; 11–13, Koryak–Kamchatka accretionary belt: 11—Late Jurassic–Early Cretaceous island-arc systems, 12—Late Mesozoic volcanic belts, 13—Cenozoic volcanic belts; 14—deep faults; 15—the main GITs of gold deposits: a—gold– arsenic-sulfide in black shale strata (1—Natalka, 2—Degdekan, 3—Maldyak, 4—Karalveem), b—gold– quartz veins in granitoids (5—Shkolnoye, 6—Butarnoye, 7—Dorozhnoye, 8—Maltan), c—gold–silver adularia in volcanogenic strata (9—Kubaka, 10—Kupol, 11—Olcha, 12—Burgali, 13—Primorskoe, 14—Dalnee).

#### *2.2. GITs of Gold Deposits*

#### 2.2.1. Gold–Arsenic-Sulfide in Black Shale GIT

In genetic terms, these are hydrothermal-metamorphogenic deposits, localized mainly in the Permian-Triassic sedimentary strata. In the north-east of Russia, deposits of this type are mainly concentrated in the area of the Kularo-Nera terrane and are controlled by the Tenkinsky deep fault zone, which forms all gold deposits into a single Yano-Kolyma gold-bearing belt. In Chukotka, a similar type of deposits is also noted in the area of the Chukchi terrane. All the deposits of this GIT described the similarity of tectonomagmatic conditions of formation, paragenetic associations, as well as similar conditions for the development of the ore process, can be attributed to the gold–quartz geological formation of the medium-deep type.

#### Natalka Deposit

The Natalka deposit is one of the largest in Russia (the giant deposit) and is located in the Tenkinsky district of the Magadan region. Structurally speaking, the deposit is confined to the zone of the Tenkinsky deep fault and is associated with the collisional stage of development of the Yano-Kolyma gold-bearing belt as part of the Verkhoyansk-Chukotka folded system. The age of the host rocks is Late Paleozoic (P3-T1), and the ores are presumably Mesozoic (J3-K1). The NW bearing of the ore deposit (320–340◦ ) in hydrothermally altered, predominantly silicified Permian-Triassic sedimentary strata is a mineralized zone penetrated by a network of quartz veins, lenses, brecciation areas, thin branching or parallel veinlets, with areas of massive silicification and arsenopyritization of varying intensity. The most common ore minerals are arsenopyrite and pyrite, less common are galena, chalcopyrite, sphalerite, pyrrhotite, rutile, and native gold. The size of gold grains is from 0.01 to 2 mm. A detailed description is given in [31].

#### Maldyak Deposit

The Maldyak deposit is located in the basin of the Berelekh river (upper reaches of the Kolyma river). It occurs in the Kularo-Nera terrane in the influence zone of the Tenka deep fault. The deposit was explored in 1938–1949, partially exploited in 1945–1948, and on a small scale in 1994–1998. It is estimated as large. At individual intersections defined by drilling, the concentration of Au in ores reaches 20.1 g/t [32]. Currently, prospecting and exploration work has been resumed at the Maldyak field. The ore field area of about 20 km<sup>2</sup> is composed of Middle Jurassic marine terrigenous deposits (J2): shale, siltstone, sandstone. Intrusive rocks are represented by felsic and intermediate dikes belonging to the Ner-Bokhapcha complex (γαJ<sup>3</sup> nr-bhr). The rocks form a complex linear folded structure with a general northwest bearing of 340◦ . Mineralization is developed both in sedimentary rocks and in dikes. Hydrothermal transformations of sedimentary rocks silicification, in dikes—propylitization and beresitization. Ore bodies are represented by differently oriented veins, veinlets, and veined and veinlet-disseminated ore folds with a thickness of 0.5 to 3 m. Their mineral composition is dominated by quartz, accompanied by calcite, dolomite, sericite, chlorite, carbonaceous matter, and among ore minerals there is arsenopyrite, pyrite and native gold [10]. The size of gold grains is 0.01–3.50 mm.

#### Karalveem Deposit

The Karalveem deposit is located in the Bilibinsky district of the Chukotka Autonomous Okrug. Structurally, it is confined to the Anyui subterrane of the Chukchi terrane, which is considered as a fragment of the Late Paleozoic–Early Mesozoic passive margin of the continent [30]. The mineralization is located among Triassic gabbro-diabase sills, which, together with the enclosing sand-shale deposits of the same age, were turned into folds. The ore field, 15 km by 3 km in size, is elongated in a northwesterly direction along folded structures and is surrounded by outcrops of granitoids (K1) that intersect diabases. Sedimentary rocks have intense contact changes and are transformed in a halo of about 1 km into cordierite-andalusite-biotite hornfelses. The mineralization is located among the Triassic gabbro-diabase sills, which, together with the enclosing sandy-shale deposits of the Triassic age, are crumpled into folds. The age of mineralization dated by K-Ar method is 130 Ma (SVKNII FEB RAS). The ores are partially transformed by Early Cretaceous granitoids (according to U-Pb determinations, 112 Ma). The deposit belongs to the vein type, where NW-bearing quartz vein bodies have a thickness of 0.5 to 2 m. The main vein minerals are quartz, ankerite, scheelite, ore minerals are arsenopyrite, galena, and native gold. The bonanza distribution of gold in veins with grades from traces to 190 g/t is typical [32].

#### Degdekan Deposit

The Degdekan deposit is located in the Tenkinsky district of the Magadan region (60 km north of the Natalka deposit). The ore-bearing area is confined to the OmchakNelkoba metallogenic zone, which was revealed based on the interpretation of gravimetric data. Structural features of the Degdekan ore field are due to different scale manifestations of dynamometamorphism in Permian carbonaceous shales (P2–3). Sedimentary rocks are intruded by Upper Jurassic (J3) diorite porphyry dikes and Late Cretaceous (K1) rhyodacites and dolerites of the Nera-Bokhapcha complex. Metasomatic transformations are represented by intense silicification, to a lesser extent by Fe-Mg carbonatization and albitization with cuts in carbonaceous matter. The ore bodies are represented by gold-bearing quartz-vein zones and quartz veins of sublatitudinal bearing (340–350◦ ) up to 2 m thick. The main vein minerals are quartz, carbonates and ore minerals such as pyrite, pyrrhotite, marcasite, and chalcopyrite. The size of gold grains is from 0.1 to 1.5 mm. The age of gold mineralization dating is estimated at 133–137 Ma [33], i.e., the ore formation refers to the upper Jurassic—the beginning of the Early Cretaceous (J3-K1).

### 2.2.2. Gold–Quartz Veins in Granitoids GIT

Deposits of this type are predominantly concentrated in the Kularo-Nera terrane, along which the thick Tenkinskaya fault zone extends, cutting through the Permo-Triassic sedimentary sequences. A zone of tectonic-magmatic activation is oriented along this zone, marked by a series of Mesozoic granitoid intrusions (Figure 1). It is assumed that the main contribution of gold to intrusive magmatic systems was provided by gold-bearing Permo-Triassic sedimentary strata. Deposits of gold–quartz veins in granitoids or porphyry gold [5,7] of the GIT are associated with granitoid intrusions that by according to the level of formation, deep and medium–deep.

#### Shkolnoye Deposit

The Shkolnoye deposit is located in the Tenkinsky district of the Magadan region. Its structural position is determined by its confinement to the southeastern flank of the Yano-Kolyma gold-bearing belt, its Ayan-Yuryakh segment (Duskaninsky ore cluster). Fissure sublatitudinal fault tectonics had a great influence on the formation of ore bodies. The mineralization is confined to the Burgaginsky stock of granitoids with an area of about 2.6 km<sup>2</sup> . The stock is composed of diorites, gabbro-diorites, tonalites, granodiorites, and biotite granites. The age of granitoids according to Rb/Sr determination by the North-East Common Use Center of the SVKNII FEB RAS is 127–152 Ma (J3-K1). Gold mineralization is localized in granodiorites and adamelites. Metasomatic changes are represented by silicification, muscovitization, carbonatization. The ore bodies are quartz veins up to 5 m thick and have a sublatitudinal bearing. They are confined to zones of cracking and increased fracturing of rocks. Gold is predominantly free in the ore. Gold mineralization is accompanied by the following: arsenopyrite, fahlore, silver lead sulfosalts; Au:Ag ratio—1:12 [34].

#### Butarnoye Deposit

The Butarnoye deposit is located in the Khasyn district of the Magadan region. It is confined to the central part of the Khurchan-Orotukan zone of tectonic-magmatic activation, traced in the submeridional direction for 150 km, with a width of 20–50 km. It is localized in a slightly eroded Late Jurassic-Early Cretaceous granitoid stock (J3-K1). The Butarny stockwork with an area of 4.6 km<sup>2</sup> is elongated in the submeridional direction for 2.9 km with a width of about 1.5 km. Granitoids of the Butarny stockwork belong to the Late Jurassic complex (J3). Judging by the K–Ar dates obtained at the North-East Common Use Center of the SVKNII FEB RAS, the age of the granitoids is 142 ± 5 Ma. This age was also confirmed by U-Pb zircon dating at 150 ± 3 Ma [35]. The deposit is dominated by subvertical quartz veins and veinlets, NE-bearing (30–35◦ ) with dip angles from 75◦ to vertical. The gold content in veins and veinlets reaches tens of grams per ton (up to 93.8 g/t). The ore bodies are represented by quartz veins up to 2 m thick and veinlets feathering them with poor nested-disseminated sulfide mineralization. Arsenopyrite predominates among ore minerals, rare finds of galena, galenobismuthite, Bi sulfotellurides, and maldonite are noted [36].

#### Dorozhnoye Deposit

The Dorozhnoye deposit is located on the right bank of the middle course of the Dorozhny brook in the Magadan region. It is localized in the Sylgytar granitoid intrusion. The territory is confined to the central part of the Burustakh synclinorium, which is part of the Inyali-Debinsk megasynclinorium of the Yano-Kolyma belt. It is composed of miogeosynclinal terrigenous sandy-silty flyschoid deposits of the Verkhoyansk complex (J1–2). Ore bodies are gold-bearing quartz veins. They are gently dipping plates, 0.1–2.0 m thick, up to 800 m long, lying vertically inside the granitoid stock at intervals of 100–120 m. Presumably, these are contraction (concentric) and radial cracks that arose during the cooling of granitoids. Their bearing is as follows: northeast, dip northwest 10–15◦ . When leaving the granites in the hornfelses, the veins branch and wedge out. The average content of Au ranges from 8 to 17 g/t, and Ag from 10 to 350 g/t. The Au/Ag ratio varies from 1:1 to 1:20. At the Nadezhda site, the veins are steeply dipping (radial) cracks and veining zones, the prevailing thickness is 10–30 cm, the zones are up to 2 m, their bearing is as follows: sublatitudinal and northeast, dip to the northwest at an angle of 60–80◦ , length up to 125 m. Au content is from 6 to 25 g/t and Ag up to 300 g/t. The deposit is described in detail in a number of publications [37].

#### Maltan Deposit

The Maltan deposit is located in the Tenkinsky district of the Magadan region. The deposit is located on the northwestern flank of the Taryn ore-placer cluster. The ore field is limited by the adjacent Malo- and Bolshetarynskaya branches of the Adycha-Tarynsky fault, which separates the zone of the Verkhoyansk fold-thrust belt from the Kular-Nersky shale belt. It is localized in echeloned backstage (fractured bodies) of biotite gabbro, quartz diorite, granodiorite-porphyry and Cretaceous porphyritic granite cutting sedimentary rocks of the Middle and Upper Triassic (T2–T3). The length of the veins is 2.5–3.0 km. Mineralization is controlled by NE-bearing fractures, which are transverse with respect to the elongation of fractured intrusions. There are quartz, quartz-carbonate, and sulfidequartz veins with a length of a few hundred meters, up to 50 cm thick. Together with gold, ore veins contain Bi (0.001–0.5%), As (0.1–1%) and W up to 1.0%; in rare cases, Mo is noted. The vein bodies are composed of quartz with large cluster-disseminated ore mineralization of arsenopyrite, native gold and bismuth, maldonite, bismuth tellurides, scheelite, and rarely molybdenite. The concentration of gold in ores is from 0.5 to 20 g/t. Gold mineralization is closely associated with bismuth and tellurium minerals.
