*2.3. Gold–Silver Adularia GIT*

Gold–silver adularia GIT refers to the epithermal-volcanogenic, the most developed in the north-east of Russia. The deposits are confined to various volcanogenic belts. The considered objects are located in the Okhotsk-Chukotka and Kedonsky volcanic belts (Figure 1). According to the conditions of formation, the deposits are classified as shallow and near-surface. Most of them are composed of low-sulfidation, enriched ores.

#### 2.3.1. Kubaka Deposit

The Kubaka deposit belongs to the class of large deposits. It is located in the basin of the Pravaya Aulandzha River in the North-Evensky district of the Magadan region. The ore field is composed of volcanics of the Kedon series (D2–3-C<sup>1</sup> kd), represented by tuff sandstones, tuff siltstones, tuffs, and ignimbrites of intermediate and felsic composition. The structure of the ore field is a collapse caldera that underwent resurgent dome formation during intrusion of subvolcanic bodies and was influenced by Jurassic and Early Cretaceous magmatism (C3-K1). The ore bodies are composed veins and stockwork-type zones. The subscripts of the veins are sublatitudinal, and the thickness is unsteady along the strike with bulges up to 20 m and constrictions up to 10 cm, averaging 1–10 m. A series of ore veins stretches for 2 km [31,36]. Main minerals are quartz and adularia, and ore minerals in early veins are native gold, and in later veins include native gold, pyrite, arsenopyrite,

acanthite, Sb-Ag and As-Ag sulfosalts and Ag selenides. The average grades of Au and Ag in the ore bodies of the Central zone are in the range 11–33 g/t and in the late ones the range is 12–23 g/t, with Au/Ag ratios of 1:1 and 1:100, respectively [31].

#### 2.3.2. Kupol Deposit

The Kupol deposit is large in terms of reserves, located in the Anadyr region of the Chukotka Autonomous Okrug in the northwestern part of the Anadyr Highlands. The Kupolsky ore cluster is confined to the Upper Yablonsky metallogenic zone of the Central Chukotka sector of the Okhotsk-Chukotka volcanogenic belt. Currently, its first stage has already been put into operation. The ore field is composed of Late Cretaceous andesite lavas (K1), less often basaltic andesites with interlayers of ash tuffs and Late Cretaceous tuffites. The ore body is a single thick (25–30 m) vein over 3 km long, presumably the mouth of a fissure volcano. Mineralization has been traced to a depth of more than 400 m. The mineral composition of the veins is quartz-adular, ore minerals are native gold, acanthite, Sb-Ag and As-Ag sulfosalts and Ag selenides. The concentration of Au in ores is from 0.1 to 230 g/t. Au/Ag ratio 1:10–1:100. Currently, Kinros LLC continues its operational work. The deposit is described in [31,38,39].

#### 2.3.3. Olcha Deposit

Olcha deposit, average in reserves. It is located on the watershed part of the Khebikendzha mountains and the Yukagir plateau. The ore field is composed of sedimentary-volcanogenic strata attributed to the Kedon series of the Middle-Upper Devonian–Carboniferous (D2–3-C1), which are underlain by Archean gneisses, granite-gneisses, and other various Cambrian and Ordovician rocks. The recent relationships between Devonian volcanics and basement strata of different ages are tectonic. In the central part, the ore field is intersected by an extended and thick post-ore granodiorite-porphyry dike of northeastern orientation and rare thin submeridional dikes of andesitic porphyrites. The ore bodies of the deposit are multidirectional quartz, carbonate-quartz and adularia-quartz veins and vein zones of a simple lenticular and bead-like shape. The host rocks are predominantly rhyodacite tuffs. Ore bodies are a series of echelon-shaped quartz-adularia veins with a thickness of 0.4 to 8 m. More than 40 mineral species were identified in the ores of the Olcha deposit, but quartz, adularia, native gold, acanthite, selenides and Ag sulfoselenides predominate [40–42]. The ores are characterized by low sulfide content (no more than 2%, and in bonanza the amount of ore minerals reaches 70%). The distribution of useful components in ores is uneven—on average 10–15 g/t, Ag 70–120 g/t, and the Au content in bonanza is up to 14 kg/t, Ag is up to 10 kg/t; Au/Ag ratio in ores 1:1–1:100.

### 2.3.4. Burgali Deposit

The Burgali deposit is located in the Severo-Evensky district of the Magadan region. The deposit is present in the Kedon volcanic belt. The ore field is composed mainly of rhyodacite tuffs of the Kedon volcanic series (D3-C<sup>1</sup> kd). The structure of the ore field is blocky. From the north and south, it is controlled by northeast-trending thrusts (20–35◦ ), from the west and east—by steeply dipping northwest-trending faults (350◦ ). Within these limits, a system of intersecting faults of a lower order (NE 40–45◦ and NE 60–65◦ ) is developed. The gold-bearing stockwork is controlled by this fault system and consists of parallel enchelon zones, characterized by the most intense veining of quartz and carbonatequartz composition. The stockwork is traced along the strike for 3700 m. The thickness of individual veins is 1.5–2 m, the length is 300–500 m [43]. Au in veins vary from 5 to 50 g/t, Ag—5–200 g/t, Au/Ag ratio—1:4. The vertical range of mineralization, according to exploration data, is about 250 m.

### 2.3.5. Primorskoye Deposit

The Primorskoye deposit is located in the south of the Omsukchansky district of the Magadan region. The territory is located at the junction of the Omsukchan (Balygychano-

Sugoi) tectonomagmatic zone with the Okhotsk volcanic zone of the Okhotsk-Chukotka volcanogenic belt, which developed on the folded base of the Sugoi marginal trough. The ore field is confined to an intrusive-dome structure and is localized in a gently sloping sequence of Late Cretaceous rhyolite ignimbrites (K1–2) more than 700 m thick, which is intruded by dykes of intermediate and basic composition. The array of leucocratic granites (K2), according to drilling data, is located under the deposit at a depth of 400–500 m. Metasomatic changes are largely due to the intruded intrusion and are represented by skarnoid associations of garnet–epidote–rhodonite composition. The veins and vein zones are 250–600 m long and 1–3 m thick, with a vertical span of 150 m [44]. The distribution of useful components is uneven. In the veins, Au content reaches 10–15 g/t, Ag reaches 100–150 g/t, and in ore columns values are 100 and 17,000 g/t, respectively. The Au/Ag ratio averages 1:10.

#### 2.3.6. Dalnee Deposit

The Dalnee deposit, average reserves. It is located on the territory of the Severo-Evensky district of the Magadan region. The structure of the ore field is determined by its location in the southwestern part of the Gizhigin trough, with the structures of the Chukotka branch of the Okhotsk-Chukotka volcanic belt (Even volcanic zone) superimposed on it. In terms of Au reserves, the deposit belongs to the class of small deposits. A series of veins was found at the deposit, confined to faults of the northwestern and northeastern directions with a steep and vertical dip. These veins, together with subparallel smaller veins, form a veined and veinlet-disseminated system about 1300 m long and 150–300 m wide. The host rocks are represented by Late Cretaceous andesites (K1). Isotopic determinations for host rocks by K-Ar are 92 ± 2 to 81 ± 2 Ma, and the age of Rb-Sr mineralization is 80 ± 5 Ma [45]. The ore bodies are represented by steeply dipping quartz, adularia-quartz veins and zones of explosive breccias with poor sulfide mineralization. Metasomatic alterations of host rocks are kaolinite-quartz-sericite. Vein minerals are mainly quartz and adularia, and ore minerals are predominantly pyrite, to a lesser extent native gold and silver, acanthite, pyrargyrite, and polybasite. Gold in ores is unevenly distributed and, on average, its content is 5.2–12.9 g/t, and in bonanza it is up to 500 g/t. The Au/Ag ratio ranges from 1:20 to 1:50. A detailed description of the deposit is given in the works [46,47].

#### **3. Research Methods and Objects**

#### *3.1. Research Objects*

The research objects for the study were ore samples and samples with visible gold from deposits of various GITs (see Sections 2.2.1 and 2.2.2), selected by the authors of this article in geological prospecting routes at a scale of 1:10,000 in different years of work, from 1978 to 2015. We also used technological samples provided to us for mineralogical study in different years by geologists of industrial organizations.

To date, the ore fields have been studied with varying degrees of details, since some of them are already in operation (Natalka, Butarnoye, Shkolnoye, Kubaka, Kupol, Primoskoye, Dalnee), while others (Maldyak, Degdekan, Maltan, Olcha, Burgali) are at the stage of detailed exploration.

#### *3.2. Research Methods*

From ore samples containing visible gold, silicates were dissolved with hydrofluoric acid and grains of native gold were isolated, which were then transferred to spectral quantitative analysis, and also placed in a compound for the preparation of polished sections.

The determination of microimpurities in native gold was carried out by a quantitative spectral method on a spectrograph from a micro-sample according to the method [48]. The advantage of this method is that before analysis, gold undergoes pre-treatment by rolling gold particles into the thinnest plate, followed by acid treatment to release it from mechanical mineral impurities.

The mineragraphic study of polished sections was carried out using optic microscopy (OM) on a Carl Zeiss AXIOPLAN Imagin reflected light microscope (Oberkochen, Germany). In these sections, minerals intergrown with native gold were studied and photographed using the MC-LCD visualization complex with the MMC software (LOMO, Petersburg, Russia).

The internal structure of native gold was revealed by etching the polished surfaces of the grains with standard reagents: for high-grade gold (HCl + CrO<sup>3</sup> of various concentrations) and for medium- and low-grade gold (HCl + 4HNO3). Interpretation of the nature of internal structures was carried out in accordance with the recommendations from [9]. Photographing of mineral intergrowths and internal structures of native gold was carried out using a microphotographic attachment on a reflected light microscope.

The determination of the native gold fineness was carried out in polished sections on a modernized POOS-1 microspectrophotometer with an internal standard and a high degree of stabilization of the illuminator and PMT power source according to the method of L. N. Vialsova [49], fineness in each grain was measured by 3–5 points. In each ore sample, measurements were carried out by the authors on 30–50 grains of native gold. Processing of measurement results by methods of mathematical statistics was carried out in the GOLD program developed by S.V. Preis. A significant part of the gold was analyzed by X-ray spectral electron probe microanalysis (XSMA) on the CAMEBAX device, for four main elements—Au, Ag, Cu, Hg—by the operator E.M. Goryacheva (North-East Common Use Center of the SVKNII FEB RAS, Magadan); on the same device, equipped with the mounting attachment "OXFORD INSTRUMENT", photographing of native gold in reflected electrons was performed.

### **4. Results**

*4.1. Typomorphic Features of Native Gold*

4.1.1. Gold–Arsenic-Sulfide in Black Shale Strata GIT

Fineness of Native Gold

This geological and industrial type is characterized by native gold of medium and high purity with maxima of this value in the areas of 750‰ and 950‰ (Figure 2). Trial histograms are unimodal with a relatively low dispersion of this indicator (from 600‰ to 1000‰).

#### Trace Elements

It has been established that, in addition to Ag, native gold grains constantly contain microimpurities of As, less often Sb, Pb, and Fe, and in rare cases Cu, Bi, Hg, and Sn. As concentrations range from 0.5 to 120 g/t, all other impurities do not exceed 50 g/t (Table 1). Sn microimpurities are usually associated with the influence of granitoids on the geochemistry of gold mineralization.


**Table 1.** Concentration of microimpurities in native gold from ores of gold–arsenic-sulfide GIT deposits.

Notes: —–below detection limit.

#### Minerals in Intergrowth

In most cases, the study of mineral parageneses reveals an association of native gold with arsenopyrite, less often with galena and sphalerite. Gold mineralization is associated with late stages of mineralization evidenced by superimposing and filling cracks in previously deposited minerals and intergranular space (Figure 3a,b). A significant part of

native gold is deposited directly in quartz. This type of deposit is favorable for enrichment of ores and is called "free gold".

**Figure 2.** Histograms showing fineness of native gold for deposits of gold–arsenic-sulfide GIT on the abscissa axis—frequency of occurrence, %; along the *x*-axis, fineness intervals, ‰; in the numerator—fineness, ‰, in the denominator—the number of determinations.

**Figure 3.** Typical mineral intergrowths of native gold in ores of gold–arsenic-sulfide GIT deposits: (**a**) intergrowth with arsenopyrite (Natalka); (**b**) development of native gold along cracks in arsenopyrite (Karalveem); (**c**) small inclusions of native gold in a single crystal of arsenopyrite (Degdekan); (**d**) overlay of native gold on an intergrowth of arsenopyrite crystals (Maldyak).

Internal Structures

Structural etching revealed a homogeneous polygonal-grain structure of native gold with simple twins (Figure 4), which indicates the medium-deep formation of this GIT and relatively stable conditions for gold crystallization. We associate the occurrence of twins with the formation of gold segregations in a turbulent tectonic setting, which is often evidenced by arsenopyrite cataclasis.

**Figure 4.** Polygonal-granular structure of native gold with simple twins in ores of various deposits of gold–arsenic-sulfide GIT: (**a**) Natalka, (**b**) Degdekan, (**c**) Maldyak.
