Parameters for the Formation of the Dobroe Gold Deposit (Yenisei Ridge, Russia): Evidence from Fluid Inclusions and S–C Isotopes

Abstract

The Dobroe deposit with 10 t gold reserves is one of the gold mines located within the Yenisei Ridge Orogenic Belt. The ore-forming conditions of orogenic gold deposits are have recently been widely discussed. A comprehensive study of fluid inclusions revealed that the Dobroe gold deposit was formed by water–carbon dioxide and carbon dioxide–hydrocarbon fluids within a temperature range of 180 to 360 °C, a pressure range of 0.8 to 1.3 kbar, and a salinity range of 1.5 to 15.0 wt.% (NaCl-equiv.). Gas chromatography–mass spectrometry showed that ore-forming fluids consisted of H₂O, CO₂, hydrocarbons, nitrogenated, sulfonated, and chlorinated compounds. The distribution patterns of δ¹³C in fluid inclusions (−11.3‰–−3.6‰) and δ³⁴S in sulfides (1.9‰–17‰) of the Dobroe deposit indicate a crustal source for ore-bearing fluids.

1. Introduction

At present, primary deposits are the main source of gold. The Yenisei Ridge is a strategically important region of Russia with regard to gold mining [1]. The gold ore deposits in the Yenisei Ridge are of the orogenic type [2]. There are likely more than 3000 t in reserves of ore fold. [3,4]. Within the gold-bearing orogenic belt, deposits with predicted gold resources of more than 100 t are known of and are being mined, including Olimpiada, Blagodatnoye, and Sovetskoe [4,5,6]. The deposits are located in shear zones and confined to fault zones. The ore bodies are represented by gold-sulfide quartz veins in the host metamorphic strata [7]. More than 100 primary gold ore occurrences with predicted resources of less than 10 t have also been identified on the Yenisei Ridge [4,8,9].

The Dobroe gold deposit, which has a predicted gold resource of about 10 t, is located 12 km from the Sovetskoe deposit, where over 100 t of gold has already been mined [4].

A huge number of factors affect the scale of a gold deposit [10]. There are several points of view regarding the genetic reasons for the localization of gold mineralization in deposits of different scales in terms of Au reserves. Some researchers [11,12] assume that the primary gold enrichment of ore-bearing rocks is associated with submarine hydrothermal activity near the junctions between basement faults, which are repeatedly melted and control the placement of gold-bearing hydrothermal mineralization. Other researchers [7,13,14,15,16,17,18,19,20,21,22,23,24,25,26] believe that the deposits are formed as a result of hydrothermal solutions rising from the deep horizons of the Earth’s crust. In hydrothermal models, ore-forming fluids of the Yenisei gold deposits play a major role in gold ore genesis [19,20,21,22,23,24,25,26,27,28,29,30]. In recent years, a new genetic model for the formation of gold-sulfide mineralization associated with mantle plume magmatism has been proposed within the tectonic evolution of the Yenisei Ridge [31,32].

Despite the variety of traditional fluid inclusion and isotope studies that have been conducted, the genesis of orogenic gold deposits still remains a debated topic [33,34,35]. Currently, there are two variations of the most viable metamorphic model: the slab devolatilization hypothesis, and the devolatilization of deeper supracrustal sequences underlying the host rocks of the deposit [33].

Improvement on the traditional thermobarogeochemical and the involvement of new methods and the latest generation of instruments [26,36] has allowed researchers to reveal the physicochemical parameters of the fluids involved in the formation of different scale gold deposits. These fluid properties can be used for clarifying the model of orogenic gold deposit formation, as well as for practical purposes in the ongoing search for, exploration, and evaluation of new gold occurrences.

The aim of this study was to reveal the ore-forming conditions of quartz-sulfide veins from the Dobroe gold deposit.

2. Geology and Mineralogy of the Deposit

The Dobroe deposit is a part of the Sovetsky ore cluster and is located 12 km west of the Sovetsky deposit, one of the largest in the Yenisei Ridge (Figure 1). The deposit was discovered during a geological survey carried out between 1976 and 1980. Gold mining at the deposit has been carried out via open-pit mining since 1992. Ores from the Dobroe deposit are processed at the Sovetskaya gold processing plant.

The Sovetsky ore cluster is confined to the junction zone of two large Proterozoic structures of the Yenisei Ridge: the Central Uplift and the eastern synclinal zone [37]. The boundary between the two structures is the Ishimba deep fault, which dictates the location of most gold-quartz deposits, including the Sovetskoe, Aleksandro-Ageevskoe, Polyarnaya Zvezda, Zayavka 14, Eldorado, and Dobroe deposits [7,38,39].

The major structure of the ore field is the Sergievskaya syncline, composed of deposits of the Corda and Gorbiloksk formations, metamorphosed under the conditions of the greenschist facies of regional metamorphism. Faults are established within the ore field, which are distinguished by zones of crushing and mylonitization with a thickness of 1–2 or 10–30 m. Ore bodies are concentrated in these zones, represented by hydrothermally altered schists containing quartz veins and veinlets with gold-sulfide mineralization.

The outcrops of granitoids from the Tatar-Ayakhta complex (RF₃ta) on the present surface are located 5 km southwest of the Dobroe deposit.

The deposit is in a separate tectonic block; bounded in the southwest by the West Ishimba deep fault, in the northeast by the Serafimovsky fault, and in the northwest and southeast, it is limited by sublatitudinal faults.

The deposit is a lenticular-shaped quartz-vein zone with a length of up to 2700 m, a thickness of up to 27.5 m, and a vertical span of mineralization of up to 540 m [7]. The ore bodies comprise thin quartz veins, lenses, vein zones, and metasomatically altered host quartz-sericite-chlorite schists, which are not constant regarding strike and dip. Ore bodies do not have natural geological boundaries; their contours are determined by sampling data. The internal structure of the ore bodies is complex, with intermittent mineralization, and barren “windows” are noted. The permanent minerals of the ore bodies are quartz and sulfides, which are associated with gold mineralization. Over the course of previous prospecting and appraisal work at the Dobroe deposit, eight ore bodies were identified and territorially united into ore blocks, including Dobry, Alexandriisky, and Tatyaninsky. Samples for the study of fluid inclusions in quartz and sulfides were taken from ore body No. 5.

Quartz is a constant mineral of the Dobroe deposit. There are two varieties of quartz: Quartz 1 and Quartz 2 (Figure 2a). Microscopically, Quartz 1 grains are transparent, equidimensional polygons with rectilinear outlines. These grains form a granoblastic honeycomb texture. The grain sizes range from 0.05 to 0.3 mm.

Quartz 2 forms aggregates of >2–3 mm irregular grains with sinuous outlines and fills the space in between Quartz 1 aggregates. The color of these grains varies from light gray to gray. In most cases, large individual grains or vein aggregates of this quartz are located in the central parts of veins and lenses, the selvedges of which are composed of metasomatic honeycomb quartz. Quartz 2 occurs more often in the ore zone of the deposit than in the barren zone.

The ores contain calcite, siderite, and chlorite, as well as ore minerals which form nested dissemination and fissure segregations. The content of ore minerals does not exceed 5%, and the distribution of those in ores is extremely uneven. The early association of sulfides consists of pyrrhotite, arsenopyrite, and pyrite. Late association (chalcopyrite, galena, ulmanite, and Bi-Te minerals) in a subordinate amount forms microscopic segregations in quartz veins. Ore minerals are also found in the host schists as layered, disseminated aggregates, thin veinlets, and dispersed, fine, crystalline, granular dissemination.

Microscopic (0.X–0.00X mm) and larger (>1 mm) visible native gold form primarily in quartz veins as fractured, lamellar, and vein-like segregations (Figure 2b,c). Gold resulting from early association presents in the form of intergrowths with sulfides (pyrrhotite, arsenopyrite), as well as fracture segregations with subisometric inclusions in them (Figure 3a–c). Late gold has been found to be associated with galena and bismuth minerals (native bismuth and hedleyite) (Figure 3e,f). Regularly, late sulfides and gold form rims at the boundaries of pyrrhotite, arsenopyrite, and quartz. Similar intergrowths with early sulfides make up galena-bismuth-ulmanite aggregates which lack visible gold (Figure 3d).

The fineness of native gold varies from 838 to 913‰, with an average value of ~900. The main impurity element is silver. The silver contents of gold of the galena-bismuth and pyrrhotite-arsenopyrite associations do not differ significantly; however, a decrease in the silver content (from ~10 wt.% to ~8 wt.%) of gold following contact with galena was observed.

Gold reserves at the Dobroe deposit do not exceed 10 t; at the Tatyaninsky site, they are less than 1 t [3,4].

3. Samples and Methods

We studied a collection of samples taken from across ore body No. 5 (RL 4.3) from horizons 380–390 (Figure 1) of the Tatyaninsky site of the Dobroe deposit. Six samples were collected from the barren zone, and the gold content in them did not exceed 0.1 g/t. From within the ore zone, where a gold content of more than 1.4 g/t was recorded, 15 samples were selected and studied. All samples were quartz-sulfide aggregates enclosed in a schist rim. The schist’s xenoliths and sulfide inclusions were visible in the quartz. Sulfides often form nests and veinlets in the form of arsenopyrite, pyrite, pyrrhotite, and chalcopyrite. We used half of the sample to make doubly polished thin sections for studying individual fluid inclusions (FIs). The second half was crushed and forced through a sieve. Pure quartz and sulfides without visible impurities were picked using a binocular magnifying glass.

Electron microscopy studies were performed on the SEM Tescan Vega III SBH with EDX Oxford X–Act at the R&D Nornickel Center, SibFU, Krasnoyarsk.

For individual fluid inclusions in quartz, the temperatures of general homogenization were measured, and the type of homogenization was observed—into the liquid or vapor phase. In the aqueous phase of the inclusions, the eutectic and ice melting temperatures were measured. In the gas phase of the inclusions, the temperatures of partial homogenization and the melting of liquefied gases were measured, and the type of homogenization was also noted—into the liquid or vapor phase. The measurements were carried out in a TH–MSG–600 microthermal chamber by Linkam with a measurement range of −196 to +600 °C. The standard instrumental measurement accuracy was ±0.1 °C in the negative and ±5 °C in the positive temperature range.

Fluid pressure in the hydrothermal system was calculated from syngenetic inclusions (type A (L_H2O+V(L)_CO2±CH4±N2 and type B (V(L)_CO2±CH4±N2)) according to the methods given in [40,41,42,43].

The salinity of the aqueous phase of fluid inclusions was estimated from the melting temperature of ice using a two-component water–salt system NaCl–H₂O [44] through an equivalent of NaCl, wt.%. The compositions of the aqueous phases of individual inclusions were determined based on the measured eutectic temperatures, which characterize the water–salt system [45].

The compositions of the gas phases of individual fluid inclusions in quartz were analyzed via a Raman spectroscopy on a single-channel Horiba J.Y. LabRAM HR800 Raman spectrometer equipped with an argon laser, which was 1.5 μm in diameter and had 0.75 W of power, according to the methods described in [46,47,48].

The bulk compositions of fluids in quartz and sulfides were determined via a pyrolysis-free coupled gas chromatography–mass spectrometry (GC–MS) using a Focus GS/DSQ II Series Single Quadrupole MS analyzer (Thermo Scientific, Waltham, MA, USA). When preparing samples for analysis, no acids, solvents, or organic substances were used which could distort the initial composition of the extracted fluid. The sample preparation procedure for analysis excluded its contact with any solvents or other possible contaminants. Input of the mixture extracted from the sample during the shock crushing was carried out online in the He flow without concentration, including cryofocus. This method does not pyrolyze the sample but heats it, only in order to convert any water within the sample into the gas phase. In this case, it is a gas mixture that is analyzed in situ rather than pyrolyzate, which contains more oxidized compounds (H₂O, CO, CO₂, etc.) due to the reactions between the gas mixture compounds, between the gas mixture and accumulator surface, and between the gas phase compounds and the sample. Blank online analyses were carried out before and after the “working” analysis. The previous analysis made it possible to control the release of gases absorbed by the sample surface, including atmospheric components, and to record the system blank at the end of the process. The degree and completeness of hydrocarbon and polycyclic aromatic hydrocarbon elution from the analytical column during temperature programming in a chromatograph thermostat were determined using the results of subsequent analysis. If necessary, the analytical column was thermoconditioned to achieve the required blank. The collected spectra were interpreted both manually and using the AMDIS 2.73 (Automated Mass Spectral Deconvolution and Identification System) software, with background correction against spectra from the NIST 2020 and Wiley Registry 12th Edition Mass Spectral libraries (NIST MS Search 2.4). Peak areas in chromatograms were determined using the ICIS algorithm Xcalibur (1.4SR1 Qual Browser). This method is suitable for the detection of trace volatile concentrations exceeding tens of femtograms. The relative concentrations (rel. %) of volatile components in the studied mixture were obtained by normalizing the areas of individual chromatographic peaks to the total area of all peaks. The GC–MS analysis technique is described in detail by [36,49].

The sulfur isotopes (δ³⁴S) of sulfides were measured in the SO₂ gas obtained during the interaction of sulfides with CuO at 1000 C and normalized to the isotope composition of troilite from the Canyon Diablo meteorite. The reproducibility of δ³⁴S values, including sample preparation, was 0.1% [50].

The isotope composition (δ¹³C) of carbon dioxide in fluid inclusions from quartz was determined in the gas extracted from 1000 mg samples by decrepitation at 600 °C. CO₂ was bound at the temperature of liquid nitrogen, after which the cryo trap was isolated from the vacuum line. The ampules with CO₂ were analyzed using a ThermoFinnigan Delta Plus–XP mass spectrometer equipped with the double inlet system. The results were normalized using the VPDB (Pee Dee Belemnite) standard.

Microthermometric, Raman, and GC–MS analyzes were performed at the Laboratory of Thermobarogeochemistry, IGM SB RAS. Isotopic analyzes were carried out at the Analytical Center for Multi-Elemental and Isotope Research SB RAS.

4. Results

4.1. Fluid Inclusion Types

Individual fluid inclusions were studied in Quartz 1 and Quartz 2 (Figure 4) from lenses, veinlets, and veins hosted by green schists of the barren and ore zones. According to the phase and chemical composition, two types (A and B) of fluid inclusions in quartz were distinguished. Type A is represented by two-phase water–carbon dioxide (L_H2O+V(L)_CO2±CH4±N2) inclusions (Figure 4a) with varying ratios of water and gas. The forms of vacuoles of this type of inclusions are the most diverse; their sizes do not exceed 10–15 µm. Type B are single-phase vapor or liquid carbon dioxide–hydrocarbon inclusions (V(L)_CO2±CH4±N2) (Figure 4b). In transmitted light under a microscope, these inclusions are dark (sometimes black), have the most diverse but more often elongated shape of vacuoles, and are two to three times larger than type A inclusions.

In Quartz 1, both from the barren and ore zones, primary and pseudosecondary generations of inclusions were practically absent. It contained mainly secondary generations of fluid inclusions confined to microcracks, sometimes mutually intersecting, crossing the grain boundaries of granoblastic Quartz 1 (Figure 4d).

In some cracks, there were inclusions of type A (water–carbon dioxide); in others, these were of type B (carbon dioxide–hydrocarbon) (Figure 4d).

In Quartz 2, fluid inclusions were represented by primary, pseudosecondary, and secondary generations. Primary and pseudosecondary generations of inclusions are located inside quartz grains as separate, isolated formations or groups of five to ten inclusions not associated with cracks. Secondary generations are inclusions associated with healed microcracks that cross the boundaries of quartz grains (Figure 4).

Quartz 2 contained both types (A and B) of fluid inclusions (Figure 4c, which differ in phase and chemical composition. The ratio between water–carbon dioxide (type A) and carbon dioxide–hydrocarbon (type B) types of fluid inclusions in Quartz 2 was varied. In one group of primary inclusions filled with water–carbon dioxide fluid, there were also single-phase carbon dioxide–hydrocarbon inclusions (type B). More frequently, type B inclusions were confined to mutually intersecting microcracks both inside and outside the quartz grains (Figure 4d).

4.2. Homogenization Temperatures, Salinity, and Pressure of Fluid

The results of microthermometric studies of fluid inclusions in quartz of the Dobroe deposit are presented in

Table 1. Summary of fluid inclusion microthermometricdata from the Dobroegold deposit.

FI Type	Th Total, °C	Type of Homogenization	Teut, °C	Tm, °C	Salinity, wt.%, NaCl-eq.	Tm of CO2±CH4±N2, °C	Partial Th, °C	Type of Homogenization	P, kbar
Gold-bearing zone, Au > 1.4 g/t
P, PS LH2O + V	$\frac{240–360}{320}$ (44)	L, V	$\frac{-23.6 ÷ -32.0}{-30.8}$ (9)	−2.0 ÷ −10.0 (8)	4.5–15.0	−63.8 ± −80.4	−3.1 ± −28.4	L, V	0.8–1.3
PS, S L(V)CO2±CH4±N2	–	–	–	–	–	−69.8 ± −89.6	−8.2 ± −93.6	L, V
S LH2O + V	$\frac{110–230}{200}$ (16)	L	$\frac{-19.3 ÷ -24.0}{-21.4}$ (5)	−0.5 ÷ −1.5 (5)	1.5–2.5	–	–	–
Barren zone, Au < 0.1 g/t
S LH2O + V	$\frac{180–320}{280}$ (15)	L, V	$\frac{-20.7 ÷ -29.5}{-24.8}$ (7)	−0.7 ÷ −5.0 (5)	1.5–9.0	−57.6 ± −71.2	−1.5 ± −8.3	L, V	0.2–0.8
S L(V)CO2±CH4±N2	–	–	–	–	–	−72.6 ± −83.0	−10.5 ± 140.6	L, V
S LH2O + V	$\frac{120–210}{190}$ (11)	L	$\frac{-17.5 ÷ -19.8}{-18.0}$ (5)	−0.3 ÷ −1.5 (5)	0.5–3.0	–	–	–

Note: FI types: P—primary, PS—pseudosecondary, S—secondary; T_h—homogenization temperature; types of homogenization: L—to liquid, V—to vapor; T_eut—eutectic temperature, T_m—melting temperature, P—pressure. In parenthesis—number of measurements.

. Since there were practically no primary fluid inclusions in Quartz 1, Table 1 shows the results of thermometric measurements in inclusions of secondary generations. The homogenization temperature of these inclusions from the barren zone (Au < 0.1 g/t) ranged from 180 to 320 °C with homogenization into the liquid or gas phase. These inclusions trapped fluid with a salinity of 1.5–9.0 wt.%, NaCl-eq. The composition of the aqueous phase is determined by Mg and Na chlorides. This is indicated by the eutectic temperatures, which ranged from −23.6 to −36.5 °C. In the secondary generations of inclusions, where the water phase prevailed over the gas phase, the temperature of general homogenization did not exceed 210 °C when homogenized into a liquid, and the salinity of the fluid did not exceed 3.0 wt.%, NaCl-equiv. In single-phase (type B) secondary inclusions in Quartz 1, the melting temperature of the phase that formed during cooling ranged from −72.0 to −83.0 °C with a temperature of partial homogenization into vapor and liquid ranging from −10.5 to −140.6 °C. The fluid pressure ranged from 0.2 to 0.8 kbar.

In Quartz 2 of the gold-bearing zone (Au = 1.4 g/t), the temperature of the general homogenization of primary and pseudosecondary water–carbon dioxide (type A) inclusions ranged from 240 to 360 °C for homogenization into both liquid and vapor. The fluid salinity reached 15.0 wt.%, NaCl-equiv. Fluid pressure varied between 0.8 and 1.3 kbar. When the temperature dropped below 0 °C, a phase where melting temperatures ranged from −63.8 to −80.4 °C occurred in the gas bubble of inclusions (type A), at a temperature of partial homogenization ranging from −3.1 to −28.4 °C, homogenization occurred into gas and into liquid. In pseudosecondary and secondary generations of single-phase carbon dioxide–hydrocarbon fluid inclusions (type B), fluid is trapped, with a melting temperature of the phase formed during cooling ranging from −69.8 to −89.6 °C and a partial homogenization temperature ranging from −8.2 up to −93.6 °C. In most type B inclusions, homogenization occurred into the vapor phase.

Secondary vapor–liquid inclusions, where the liquid water phase prevailed over the gas one (L_H2O >> V), homogenized only into the liquid phase within the temperature range of 110 to 230 °C, while salinity varied from 1.5 to 2.5 wt.% NaCl eq.

4.3. Composition of Fluid Inclusions in Quartz and Sulfides

The composition of the gas phase of 48 individual inclusions in quartz was determined using a Raman spectroscopy (

Table 2. Microthermometric characteristics and compositions of the gas phases of individual primary and pseudosecondary fluid inclusions in quartz of the Dobroe gold deposit (Raman spectroscopy data).

No.	Tm, °C	Th, °C	Type of Homogenization	Content, mol.%			CO2/CH4
				CO2	CH4	N2
FI type A (LH2O+L(V)CO2±CH4±N2)
D–1–1/18	−71.3	−28.4	V	82.0	17.0	1.0	48.0
D–1–1/20	−70.8	−5.0	L	53.0	10.0	37.0	5.3
D–1–1/21	−69.3	−6.1	V	50.0	10.0	40.0	5.0
D–1–1/22	−74.3	−6.9	V	47.0	11.0	42.0	4.3
D–6/1	−75.4	−7.8	L	47.0	7.6	45.4	6.2
D–6/2	−80.4	−9.3	V	94.2	1.3	4.5	72.5
D–6/3	−69.8	−7.4	V	62.6	22.6	14.8	2.8
D–6/4	−58.4	−9.3	L	80.0	1.2	18.8	66.7
D–6/5	−63.8	−3.1	V	60.7	3.7	35.6	16.4
D–2–5–a	–	–	–	39.6	7.3	53.1	5.4
D–2–5–f	–	–	–	22.8	1.4	75.8	16.3
D–6–d	–	–	–	98.8	0.3	0.9	329.3
D–1–3/2	−62.5	−3.6	L	71.5	22.9	5.6	3.1
D–1–3/3	−59.8	−8.3	V	54.1	14.1	31.8	3.8
D–1–3/4	−57.6	−11	V	91.4	8.6	–	10.3
D–1–3/7	−66.3	−4.2	L	78.0	20.0	2.0	3.9
D–1–3/9	−71.2	−1.5	L	94.0	4.0	2.0	23.5
D–1–3/10	−63.6	−4.5	V	76.4	23.2	0.4	3.3
D–1–5/1	−59.9	−3.8	L	81.7	6.7	11.6	12.2
D–1–5/2	−65.8	−5.1	V	91.3	7.3	1.4	12.5
D–1–5/3	−63.4	−4.9	V	93.0	5.9	1.1	15.8
D–1–5/4	–	–	–	93.8	2.3	3.9	40.8
D–1–5/6	−61.3	−6.1	L	98	0.7	1.3	140
D–1–5/7	–	−7.3	V	45.9	6.0	48.1	7.6
D–1–5/11	−64.1	−4.3	V	41.0	24.0	35.0	1.7
D–1–5/13	−66.5	−6.3	L	43.8	16.2	40.0	2.7
D–1–5/14	−69.8	−5.1	V	72.0	14.0	14.0	5.1
D–1–5/15	−68.3	−6.5	V	75.9	23.0	1.1	3.3
FI type B (L(V)CO2±CH4±N2)
D–1–9/4	–	−93.6	V	3.6	5.7	90.7	0.6
D–1–8/19	−89.6	−8.2	V	9.0	89.0	2.0	0.1
D–1–8/29	–	−122.1	V	15.0	16.0	69.0	0.9
D–2–5/3	−69.8	−14.8	L	35.8	17.5	32.1	2.0
D–6/7	−72.3	−15	L	43.6	36.8	19.6	1.2
D–6/8	–	−89.3	V	16.3	8.1	75.6	2.0
D–6/9	–	−91.4	L	15.2	8.7	76.1	1.7
D–2–5–b	–	–	–	17.7	2.8	79.5	6.3
D–2–5–e	–	–	–	13.1	2.6	84.3	5.0
D–6–b	–	–	–	39.7	1.9	58.4	20.9
D–6–c	–	–	–	54.9	1.8	43.3	30.5
D–1–3/1	–	−123.5	V	4.0	15.0	81.0	0.3
D–1–3/2	–	−140.6	V	13.0	15.0	72.0	0.9
D–1–3/4	–	−135.2	L	12.0	18.0	70.0	0.7
D–1–3/5	–	−93.0	V	15.0	29.0	56.0	0.5
D–1–3/8	–	−89.0	V	4.0	90.0	6.0	0.04
D–1–3/16	−83.0	−13.0	L	35.0	65.0	–	0.5
D–1–3/17	−82.6	−10.5	L	37.0	63.0	–	0.6

T_h—homogenization temperature; types of homogenization: L—to liquid, V—to vapor; T_m—melting temperature.

, Figure 5). The composition of the gas phase of the fluid revealed the presence of three main components: CO₂, CH₄, and N₂, the content of which varied over a wide range.

Water–carbon dioxide (A) inclusions from the barren and ore zones were dominated by CO₂, while carbon dioxide–hydrocarbon type (B) inclusions were dominated by CH₄ and N₂ (Figure 6). In inclusions (type A) where the water phase was constantly present, the CO₂/CH₄ ratio averaged 31.0 (n = 28). In inclusions filled with carbon dioxide–hydrocarbon fluid (type B), the CO₂/CH₄ ratio was lower and averaged 4.2 (n = 18).

The bulk compositions of volatiles in fluid inclusions were determined via a GC–MS of fluid inclusions from 12 samples of quartz and sulfides (

Table 3. Content (rel.%) and number (in parentheses) of volatiles extracted via mechanical shock destruction of minerals from the Dobroe deposit (according to a GC–MS).

Component	Gold-Bearing Zone, Au > 1.4 g/t					Barren Zone, Au < 0.1 g/t
	D–10			D–2–12		D–1–2	D–1–3
	Arsenopyrite	Quartz 2	Quartz 1	Arsenopyrite	Quartz 1	Quartz 2	Quartz 1
Aliphatic hydrocarbons
Paraffins (alkanes)	6.31 (13)	0.30 (11)	2.03 (16)	2.31 (13)	1.49 (13)	1.57 (19)	4.40 (16)
Olefins (alkenes)	0.20 (16)	0.07 (15)	0.55 (14)	0.27 (20)	0.36 (17)	1.28 (31)	0.12 (24)
Cyclic hydrocarbons
Cycloalkanes, cycloalkenes, arenes, PAH	0.43 (12)	0.06 (10)	0.57 (9)	0.25 (15)	0.42 (12)	0.23 (21)	0.15 (19)
Oxygenated hydrocarbons
Alcohols, ethers, and esters	0.51 (16)	0.81 (18)	1.82 (18)	1.49 (14)	2.90 (20)	0.27 (24)	2.12 (26)
Aldehydes	0.79 (20)	0.18 (22)	4.62 (23)	0.30 (2)	0.99 (23)	0.43 (22)	0.65 (23)
Ketones	0.75 (17)	0.26 (16)	2.21 (17)	0.30 (16)	1.09 (18)	0.29 (21)	0.43 (20)
Carboxylic acids	0.17 (13)	0.78 (11)	1.71 (9)	1.44 (19)	3.18 (12)	0.62 (13)	0.56 (13)
Heterocyclic compounds
Dioxanes, furans	0.14 (6)	0.01 (8)	0.18 (11)	0.09 (8)	0.11 (11)	0.02 (10)	0.01 (7)
Nitrogenated compounds
N2, ammonia, nitriles	0.96 (9)	12.31 (10)	13.48 (12)	0.89 (10)	46.9 (13)	37.58 (22)	34.07 (23)
Sulfonated compounds
H2S, SO2, CS2, COS, thiophenes	2.19 (9)	0.03 (8)	2.30 (10)	0.08 (9)	0.34 (8)	0.75 (16)	0.71 (15)
Inorganic compounds
CO2	52.34	58.13	32.85	26.36	38.80	10.75	45.77
H2O	34.16	27.09	37.68	66.23	3.38	47.21	10.96
Ar	–	–	–	–	–	0.02	0.03
Number of components	133	131	141	128	149	202	188
Alkanes/alkenes	31.5	3.0	3.3	7.7	3.8	1.2	36.7
CO2/(CO2 + H2O)	0.6	0.7	0.5	0.3	0.9	0.2	0.8
Σ(C5–C17)/Σ(C1–C4)	0.02	0.28	1.24	0.04	0.34	1.27	0.03
H/(H + O)	0.58	0.44	0.60	0.63	0.44	0.65	0.50

Note: The relative concentrations (rel.%) of volatile components in the mixture under study were obtained by normalizing the areas of individual chromatographic peaks to the total area of all peaks.

, Supplementary Tables S1–S12). Table 3 shows the most representative results of GC–MS analysis of volatiles of samples from the gold-bearing and barren zones and analyses of all samples were used for Figure 7. Water, carbon dioxide, and a wide range of hydrocarbons and their derivatives, as well as nitrogen- and sulfur-chlorinated compounds, were found in the composition of volatiles in the fluid (Figure 8, Figure 9 and Figure 10). The total number of detected components in sulfides ranged from 133 to 148, and in quartz it reached 202.

The compositions of volatiles from fluid inclusions in Quartz 1 and Quartz 2 were dominated by CO₂, which made up 60–70 rel.% with fluctuations ranging from 10.7 to 73.8 rel.%. The proportion of water in these inclusions was lower and averaged 21.9 rel.% (n = 8). The content of hydrocarbons represented by aliphatic, cyclic, and oxygenated compounds averaged 16.3 rel. % (n = 8), and that of sulfides was 7.4 rel.% (n = 5).

Of particular note was the high proportion (up to 47 rel.%) of nitrogenated compounds in fluid inclusions from quartz (Table 3, samples D–2–12; D–1–2; D–1–3). In these samples, 23 chemical compounds were identified among nitrogenated compounds, which is almost two times more than the number found in sulfides (Table 3). In nitrogenated compounds, molecular nitrogen (N₂) predominated, the proportion of which was 80%–90%.

4.4. Carbon Isotopic Composition (δ¹³C) of Carbon Dioxide in Fluid Inclusions

Carbon isotopes (δ¹³C) of carbonic acid were measured for fluid inclusions in quartz from the gold-bearing (n = 8) and barren (n = 3) zones (

Table 4. Carbon isotope composition (δ¹³C) of carbon dioxide in fluid inclusions in quartz from the Dobroe gold deposit.

Sample No.	δ13CCO2, ‰ (VPDB)
Gold-bearing zone
D–1	−6.7
D–2	−7.2
D–4	−6.4
D–10	−4.9
D–2–3	−8.3
D–2–12	−7.4
D–2–13	−7.7
D–2–17	−7.1
Barren zone
D–1–2	−7.5
D–1–3	−11.3
D–1–17	−3.6

Note: The carbon isotopic compositions (δ¹³C) of carbon dioxide in fluid inclusions were determined at the Analytical Center for Multi-Elemental and Isotope Research SB RAS, Novosibirsk. Analysts: V.N. Reutsky, M.N. Kolbasova, O.P. Izokh.

, Figure 11). The values of δ¹³C_CO2 in fluids of the gold-bearing zone varied from −4.9 to −8.3‰, showing a rather narrow range of fluctuations. A lighter carbon isotope (δ¹³C_CO2 = −11.3‰) was determined in the fluids of the barren zone, and the range of values expanded to a range of −3.6 to −11.3‰ (Figure 5).

4.5. Sulfur Isotopic Composition of Sulfides

Sulfur isotopes (δ³⁴S) were measured in 10 sulfide monofractions, including arsenopyrite-3, pyrite-4, and pyrrhotite-3 (

Table 5. Sulfur isotopic composition of sulfides from the Dobroe gold deposit.

Sample No.	Mineral	δ34S, ‰
Gold-bearing zone
D–10	Arsenopyrite	1.9
D–4	Arsenopyrite	3.8
D–2–12	Arsenopyrite	4.2
D–2	Pyrrhotite	6.0
D–2–13	Pyrrhotite	9.0
D–1	Pyrite	3.2
Barren zone
D–1–2	Pyrite	10.5
D–1–17	Pyrite	15.2
D–1–12	Pyrite	17.0
D–1–3	Pyrrhotite	8.8

Note: The sulfur isotopic composition of sulfides was determined at the Analytical Center for Multi-Elemental and Isotope Research SB RAS, Novosibirsk. Analysts: V.N. Reutsky, M.N. Kolbasov.

, Figure 12). In arsenopyrite, the δ³⁴S values had a rather narrow range of 1.9 to 4.2‰, and the same narrow range was present in pyrrhotite, whose values ranged from 6.0 to 9.0‰. In pyrite, the δ³⁴S fluctuated within a wider range from 3.2 to 17.0‰ (Figure 12).

5. Discussion

The data obtained in this study of fluid inclusions in the quartz and sulfides of the Dobroe gold deposit allow us to conclude that two types of fluid took part simultaneously or sequentially in the formation of the deposit: water–carbon dioxide, and carbon dioxide–hydrocarbon. The water–carbon dioxide type of fluid is indicated by the data obtained via Raman spectroscopy of individual vapor–liquid inclusions (type A) in quartz containing a water phase and a CO₂-dominated vapor phase (Table 2, Figure 3). This is also indicated by the GC–MS data (Table 3, Figure 4). The fluid consistently contains CO₂ and H₂O in various proportions. The carbon dioxide–hydrocarbon composition of the fluid present in the inclusions (type B) is also confirmed by the data obtained via both Raman spectroscopy (Table 2, Figure 3) and GC–MS (Table 3, Figure 4). These distinguished types of fluid differ in redox properties. The carbon dioxide–hydrocarbon type of fluid is more reduced than the water–carbon dioxide type, as evidenced by the CO₂/CH₄ ratio. According to a number of researchers [17,52,53,54,55,56,57,58], the /CH₄ ratio of a fluid is an indicator of hydrothermal system redox conditions.

According to the Raman spectroscopy analysis, in water–carbon dioxide (type A) inclusions, the CO₂/CH₄ ratio ranges from 1.7 to 329.3 (Table 2), averaging 31.0 (n = 28). In the carbon dioxide–hydrocarbon (type B) inclusions, the CO₂/CH₄ value varies from 0.04 to 30.5, averaging 4.2 (n = 18) (Table 2, Figure 3). From this, it follows that in the process of ore deposition, the redox properties of the fluids changed. This is also indicated by the data on the gross composition of volatiles obtained via GC–MS (Table 3). The H/(H + O) ratio calculated using the GC–MS data was 0.44–0.65. Alkanes/alkenes ratios were also used to reconstruct the redox potential of fluids, as suggested in [59,60]. According to Table 3, in the arsenopyrite of the Dobroe deposit, co-crystallizing with gold, the value of the ratio of alkanes/alkenes was on average 28.3 (n = 3), and in quartz it was 7.5 (n = 8). Therefore, we can infer that the fluids transporting ore elements were reduced.

The fluids that formed the Dobroe gold deposit are represented by complex chemical compounds. Up to 200 chemical compounds were found in the compositions of the fluids (Table 3). The main components included CO₂, H₂O, N₂, and a large group of hydrocarbon compounds. The proportion of hydrocarbons in the fluids of the Dobroe deposit was up to seven times lower than in the fluids of the native gold of the Sovetskoe deposit [36], and up to eleven times lower than in the fluids of the gold-bearing acicular arsenopyrite of the Olimpiada deposit [22]. Consequently, the gold potential of mineralization may depend on the amount of reduced carbon dioxide–hydrocarbon compounds in the ore-forming fluid. This is reflected in the saturation degree of quartz and sulfides with inclusions filled with reduced carbon dioxide–hydrocarbon fluid. Another difference between the fluids of the large (>100 t Au) Sovetskoe, Blagodatnoe, and Olimpiada deposits and the small (<10 t) Dobroe deposit has been revealed. The gold-bearing fluids of large deposits were found to contain a reduced amount of water: <0.3 rel.% in native gold of the Sovetskoe deposit [36], 7.3 rel.% in native gold of the Blagodatnoe deposit [61], and <13.7 rel.% in gold-bearing acicular arsenopyrite from the Olimpiada deposit [22]. Meanwhile, in the fluids of arsenopyrite from the Dobroe deposit, the water content reaches 74.5 rel.% (Figure 13). Water-depleted mineral-forming fluids of the large Perron gold deposit (Abitibi Belt, Canada) were reported by [62]. The authors concluded that gold was not transported by water complexes, but was transported mainly by CO₂ and hydrocarbons, forming organometallic complexes with them. The same results were obtained by us when studying the fluids of a number of gold deposits of the Yenisei Ridge [14,20,21,22,25,36].

Another unique feature of fluids from the Dobroe deposit is that quartz fluids contain high (up to 47 rel.%) contents of nitrogenated compounds (Table 3), among which the proportion of molecular nitrogen (N₂) makes up more than 90%. An abundance of N₂ (up to 90 mol.%) was also noted in the gas phase of individual fluid inclusions via Raman spectroscopy (Table 2). Elevated concentrations of molecular nitrogen in fluids were determined in quartz taken from quartz veins which were 2–5 cm thick hosted by the carbonaceous schist of the Dobroe deposit. The selvages of these veins also contained carbonaceous schists, while the vein itself contained xenoliths of carbonaceous schists.

According to [63], molecular nitrogen is a chemically inactive substance. Its inertness is due to the greater strength within the molecular bond, the breaking of which requires high energy. However, chemical compounds containing nitrogen in the form of the CN–ligand can enter into complex formation reactions with gold and transport it to the deposit sites [64]. The appearance of N₂ in the fluid in considerable amounts may be associated with chemical reactions between heated fluids and the ammonium-bearing silicates of ore-bearing rocks [30,65,66]. The presence of N₂ in the fluid may also be associated with the organic matter decomposition of the carbonaceous schists present in the field. Decomposition proceeds most intensively in zones of maximal hydrothermal–metasomatic development of host rocks, where the role of ammonium nitrogen decreases and N₂ becomes the predominant component [67,68]. A point of view has been expressed regarding the supply of N₂ during degassing from the deep zones of the Earth’s crust and even, in some cases, from the mantle [69,70].

Hydrothermal activity at the Dobroe deposit began with the formation of early quartz veins, veinlets, and lenses involving the metamorphogenic rearrangement of silica in host rocks. In subsequent geological periods, in zones of tectonic disturbance, early quartz formations underwent a local pressure gradient. This resulted in the rearrangement (granulation) of quartz. In these places, quartz acquired a granoblastic structural pattern (Figure 2). Then, vein quartz (Quartz 2) crystallized as large grains and veinlets, intersecting granoblastic quartz (Quartz 1) (Figure 2) with the new portion of fluid inflow at a temperature of 300–360 °C. In the grains of granoblastic quartz, many mutually intersecting microcracks appeared, filling mainly with gaseous secondary inclusions (Figure 4c).

The Dobroe deposit was formed by water—carbon dioxide—hydrocarbon fluids in the temperature range of 180 to 360 °C, with pressure ranging from 0.8 to 1.3 kbar and salinity from 1.5 to 15.0 wt.%, NaCl-equiv. (Table 1, Figure 14). Wide variations in temperature, pressure, and salinity are characteristic of many gold deposits on the Yenisei Ridge [19,20,21,22,23,25,26,27]. They reflect the complex and lengthy process of the ore formation.

The thermobarogeochemical characteristics (T, P, composition) of fluids trapped by primary inclusions in Quartz 2 and secondary inclusions in Quartz 1 are similar (Table 1, Table 2 and Table 3) (Figure 4). This indicates successive crystallization of first granoblastic and then veined quartz. Moreover, this points to the high penetrating ability of gaseous carbon dioxide–hydrocarbon fluids, which move through numerous microcracks in granoblastic quartz.

Carbon isotopic studies (δ¹³C) of carbon dioxide in fluid inclusions from gold-bearing and barren quartz of the Dobroe deposit showed close values ranging from −3.6 to −11.3‰ (Table 4, Figure 11). Carbons with such values are much heavier than organic carbon of terrigenous layers, for which δ¹³C ranges from −22.4 to −28.7‰ [71,72,73,74]. The increase in δ¹³C_CO2 in the quartz fluids of the Dobroe deposit could be caused by the influx of CH₄ into the fluid system in the temperature range of 250–350 °C [51]. The Dobroe deposit was formed at these same temperatures (Table 1), and the fluids consistently contain CH₄ and other hydrocarbons and their derivatives which are heavier than methane (Table 2 and Table 3). The most plausible source of δ¹³C found in the fluid of the Dobroe deposit is intracrustal fluids.

The isotopic composition of sulfur was determined in arsenopyrite, pyrrhotite, and pyrite (Table 5, Figure 12). The δ³⁴S values of arsenopyrite and pyrrhotite are found to remain within rather narrow ranges, from +1.9 to +4.2‰ and from +6.0 to +9.0‰, respectively. The range of δ³⁴S variations in pyrite significantly expands—from +3.2 to +17‰. The wide range of sulfur isotope values in pyrite can be explained by variations in redox conditions, as confirmed by the results of the Raman spectroscopy and the GC-MS. On the basis of isotopic data, sulfur in the sulfides of the deposit (from +1.9 to +17‰) can be obtained by “averaging” the sulfur of host rocks, and these values indicate a crustal nature [18,51]. At the same time, as δ³⁴S values approach the “mantle” level, sulfur in the sulfides of the Dobroe deposit could be partially brought forth by fluids of a deeper nature.

6. Conclusions

The Dobroe gold deposit, with probable gold reserves of less than 10 tonnes, was formed by oxidized water–carbon dioxide and reduced carbon dioxide–hydrocarbon fluids in temperatures ranging from 180 to 360 °C, pressures ranging from 0.8 to 1.3 kbar, and salinities ranging from 1.5 to 15.0 wt.%, NaCl-equiv.

Ore-forming fluids consist of H₂O; CO₂; N-, S-, and Cl-compounds; and hydrocarbons, which are potentially capable of transporting ore elements.

The gold contents of quartz-vein formations increase when reduced gaseous carbon dioxide–hydrocarbon fluids are superimposed on early barren parageneses.

The elevated hydrocarbon content of the ore-forming fluid may indicate the gold potential of quartz-vein zones. Presumably, the fluid supply occurred due to the sedimentary rocks’ devolatization during the tectonic activation of the Yenisei Ridge. Further, the fluid formed metalliferous quartz veins in the fault zones.