The Features of Native Gold in Ore-Bearing Breccias with Realgar-Orpiment Cement of the Vorontsovskoe Deposit (Northern Urals, Russia)

This paper describes native gold in ore-bearing breccias with realgar-orpiment cement from the Vorontsovskoe gold deposit (Northern Urals, Russia). Particular attention is paid to the morphological features of native gold and its relation to other minerals. The latter include both common (orpiment, barite, pyrite, prehnite, realgar) and rare species (Tl and Hg sulfosalts, such as boscardinite, dalnegroite, écrinsite, gillulyite, parapierrotite, routhierite, sicherite, vrbaite, etc.). The general geological and geochemical patterns of the Turyinsk-Auerbakh metallogenic province, including the presence of small non-economic copper porphyry deposits and general trend in change of the composition of native gold (an increase in the fineness of gold from high-temperature skarns to low-temperature realgar-orpiment breccias) confirm that the Vorontsovskoe deposit is an integral part of a large ore-magmatic system genetically associated with the formation of the Auerbakh intrusion.

The Vorontsovsko-Peshchanskaya hydrothermal system is located at the southwestern outer contact of the Auerbakh intrusion [1]. The Vorontsovskoe deposit is located in the southern part of this system, at a distance of 400-500 m from the southwestern exocontact of the Auerbakh massif ( Figure 1b) [7]. The volcanic-sedimentary rocks hosting the deposit form a monocline that gently dips to the west [6]. The sedimentary sequence includes limestones and layers of tuffites and siltstones with a thickness of approximately 1 km. Usually, the limestone is metamorphosed to marbles. This sequence is conformably overlain subsequently by volcanic-sedimentary and volcanic rocks, such as tuffaceous siltstone, tuffite and tuff. Coarse clastic breccias with tuffaceous cement are common at the contact of all these rocks with limestones within the entire Turyinsk-Auerbakh metallogenic province [14].

Local Geological Setting
A quarry in the Vorontsovskoe deposit has exposed a wedge body of volcano-sedimentary rocks with a predominance of tuffs of medium composition and tuffstones (Figure 2a). The western part of this body is bounded by a large tectonic fault (Figure 2b). The bulk of ore-bearing breccia, including realgar-orpiment cement, is located at the contact of the body of volcano-sedimentary rocks with limestones. The gold ore body has the form of a torch that expands outward the top [15]. Within this body, gold mineralization is localized mainly in breccias. Part of the gold mineralization is associated with metasomatically sericite-altered volcanic-sedimentary rocks.
Two stages of ore breccia formation were previously identified earlier [9]. The breccias of the first stage are quite widespread. Limestone fragments predominate in this type of breccias. These fragments are embedded in a matrix consisting of small fragments of volcanic-sedimentary rocks dominated by andesite [16,17]. Pyrite grains with an average size of 0.2 mm are widely distributed in the cement. The breccias of the second stage form a pod-like body of irregular shape (Figure 2c). Limestones, volcanogenic sedimentary rocks and siltstones are found as fragments in breccias of the second stage. First-stage breccias are also found in the wreckage. Realgar and orpiment are widely distributed in breccia cement of the second stage. Barite, quartz and calcite make up a significant part of the cement. Native gold and rare Hg and Tl sulfosalts are common in breccia cement of the second stage. The highest content of realgar and orpiment is typical for the central parts of pod-like bodies of the second stage breccias. a pod-like body of irregular shape (Figure 2c). Limestones, volcanogenic sedimentary rocks and siltstones are found as fragments in breccias of the second stage. First-stage breccias are also found in the wreckage. Realgar and orpiment are widely distributed in breccia cement of the second stage. Barite, quartz and calcite make up a significant part of the cement. Gold and rare Hg and Tl sulfosalts are common in breccia cement of the second stage. The highest content of realgar and orpiment is typical for the central parts of pod-like bodies of the second stage breccias.  [15][16][17]; (b) the photograph of the northern wall of the quarry with a tectonic contact between marmorized limestone and a layer of volcanic-sedimentary rocks (red dotted line); (c) a small body of fluid-explosive breccias with realgar-orpiment cement (sample 2016/2), exposed in one of the quarry layers. The white dotted line shows the boundary of the body. 1-Neogene-Quaternary cover deposits; 2-karst formations; Devonian formations: 3-limestones; 4-tuff aleurolites, tuffstones, tuff-conglomerates; 5-andesites, tuffs and lava breccias; 6-breccia of the 1st stage of breccia formation; 7-lamprophyre dikes; 8-dikes of diorite porphyrites. Metasomatites: 9-quartz-sericite, 10-quartz-sericite-albite, 11-berezite-listvenites, chlorite-sericites. 12-ore bodies with run-of-mine grades of gold content; 13-enriched ore pillars; 14-manifestation areas of realgar-orpiment mineralization.

Sample Collection and Preparation
A total of 173 ore samples were taken within the ore body to study the distribution of gold. Of these, 58 specimens were from ore breccias with realgar-orpiment cement. These samples were collected in 2016-2019 at the Northern quarry of the Vorontsovskoe deposit at horizons from -40 to +35 (meters above sea level) from 2016 to 2019 ( Figure 3). The weight of ore samples ranged from 0.2 to 0.4 kg.
To obtain native gold concentrates, two large samples weighing 35 kg were taken from areas richest in realgar and orpiment breccia fragments. The samples were crushed to a fraction of less than 1 mm and enriched with a KR-400 centrifugal concentrator. Grains and crystals of native gold were manually selected using a binocular microscope. Their surface structure details were studied by scanning electron microscopy. Then, the grains of native gold were mounted in polished sections in order to study their internal structure and determine their chemical composition.  To obtain native gold concentrates, two large samples weighing 35 kg were taken from areas richest in realgar and orpiment breccia fragments. The samples were crushed to a fraction of less than 1 mm and enriched with a KR-400 centrifugal concentrator. Grains and crystals of native gold were manually selected using a binocular microscope. Their surface structure details were studied by scanning electron microscopy. Then, the grains of native gold were mounted in polished sections in order to study their internal structure and determine their chemical composition.

Analytical Methods
Polished and thin sections were made for microscopic examination. After a preliminary description using an optical microscope under transmitted and reflected light, detailed studies of thin sections with native gold using a scanning electron microscope were carried out. The chemical compositions of minerals were identified by an electron microprobe using both energy and wavelength dispersive spectrometers.
A preliminary semi-quantitative analysis of the chemical composition was performed in the Fersman Mineralogical Museum of the Russian Academy of Sciences (Moscow, Russia) using a CamScan 4D scanning electron microscope (CamScan Electron Optics, Ltd, Cambridge, UK) and in the Institute of Experimental Mineralogy RAS (Chernogolovka, Russia) using a CamScan MV2300 scanning electron microscope. In both cases, INCA Energy 350 energy dispersive spectrometers (Tescan, Brno, Czech Republic) were used under the following operational conditions: accelerating voltage-20 kV, probe current-5 nA on metallic cobalt, working distance 25 mm, spectra accumulation time-70 s, spot size 5 µm.
Further study of the mineral chemical composition was carried out in the joint laboratory of electron microscopy and microanalysis of the Department of Geological Sciences of the Masaryk University and the Czech Geological Survey (Brno, Czech Republic) using a Cameca SX 100 wave dispersive electron probe microanalyzer (Cameca, Paris, France) and in the laboratory of the Department of Mineralogy of the Geological Faculty of Moscow State University (Moscow, Russia) using the Camebax SX 50 microanalyzer (Cameca, Paris, France). In the first case, the following operational conditions were applied: accelerating voltage-25 kV, probe current-20 nA, probe diameter-1 µm; reference materials (natural): Fe-FeS 2 ; Cu-Cu metal; Zn-ZnS; Ag-Ag metal; Hg-HgTe; Tl-Tl (Br, I); Pb and Se-PbSe; As-pararammelsbergite; Sb-Sb; S-chalcopyrite. In the second case, the operational conditions were as follows: accelerating voltage-20 kV, probe current-30 nA, probe diameter-1 µm. The following reference materials were used (natural): Zn-ZnS, As-CoAsS, S, Fe-FeS (troilite), Ag-Ag 2 Te, Cu, Sb-CuSbS 2 , Hg-HgTe, Tl-TlSbSe 2 , Pb-PbS.

Mineral Composition of the Breccias
The gold-bearing breccias of the second brecciation event consist of marmorized limestone, tuffstones, tuffites and andesite tuffs fragments and contain fragments of individual grains of minerals from these rocks embedded in the hydrothermal cement (manganoan calcite, prehnite, orthoclase and other). Rock fragments have an angular shape ( Figure 4) and vary in size from one millimeter to several centimeters. The quantitative ratio of the matrix and lithoclasts in breccias is not constant and varies from 15 to 75%. The breccias are altered to varying degrees. In the breccia matrix, thin (less than 1 mm) fragments of the main rock-forming minerals are widespread: chlorite (clinochlore and chamosite), amphibole (magnesio-ferri-hornblende, tremolite, pargasite), scapolite, quartz, feldspars (orthoclase, microcline, albite) and calcite. However, newly formed minerals predominate, forming the cement of ore breccias.

Morphological Features of Native Gold in Breccias
Native gold in the breccias occurs both in cement ( Figure 6a) and in the limestone's fragments. The analysis of ore concentrates obtained during the gravitational concentration of gold breccias with realgar-orpiment cement has made it possible to establish that the grains of 0.1-0.25 mm prevail among the gravitationally enriched aggregate of gold. However, the study of thin sections has revealed the predominance of gold grains less than 0.1 mm in size. Thus, most of the native gold grains (75% of all grains) are 20-80 µm in size. Large grains of native gold (0.2 to 0.8 mm) are quite rare. These grains occur as complex intergrowths of gold with realgar, orpiment and calcite (Figure 6b,c).
The study of thin sections from breccias allowed us to confirm the conclusion that most of the native gold is located directly in the breccias' cement. Gold grains intergrow with orpiment and realgar (Figure 7a,b). Gold grains with prismatic cross-sections are extremely rare and are located directly in the fragments of marmorized limestones ( Figure 7c). Gold crystals are relatively rare among the gold grains from the ore concentrates too (Figure 8a). They are characterized by a complex faceting with a combination of octahedron, pentagonal dodecahedron and cubic faces, which leads to the occurrence of spherical shape of the crystals.

Morphological Features of Native Gold in Breccias
Native gold in the breccias occurs both in cement (Figure 6a) and in the limestone's fragments. The analysis of ore concentrates obtained during the gravitational concentration of gold breccias with realgar-orpiment cement has made it possible to establish that the grains of 0.1-0.25 mm prevail among the gravitationally enriched aggregate of gold. However, the study of thin sections has revealed the predominance of gold grains less than 0.1 mm in size. Thus, most of the native gold grains (75% of all grains) are 20-80 μm in size. Large grains of native gold (0.2 to 0.8 mm) are quite rare. These grains occur as complex intergrowths of gold with realgar, orpiment and calcite (Figure 6b,c). The study of thin sections from breccias allowed us to confirm the conclusion that most of the native gold is located directly in the breccias' cement. Gold grains intergrow with orpiment and realgar (Figure 7a,b). Gold grains with prismatic cross-sections are extremely rare and are located directly in the fragments of marmorized limestones ( Figure  7c). Gold crystals are relatively rare among the gold grains from the ore concentrates too (Figure 8a). They are characterized by a complex faceting with a combination of octahedron, pentagonal dodecahedron and cubic faces, which leads to the occurrence of spherical shape of the crystals.  Gold grains cementing other minerals are most widely distributed (Figure 8b-e). Often, native gold in such aggregates intergrows with realgar, pyrite and calcite. Gold grains cementing other minerals are most widely distributed (Figure 8b-e). Often, native gold in such aggregates intergrows with realgar, pyrite and calcite.
Native gold contains numerous mineral inclusions. Small crystals of pyrite, prehnite, quartz and calcite are directly enclosed in the native gold. Polymineral inclusions, consisting of calcite, barite and dalnegroite, are found in gold aggregates as well (Figure 9a). Polymineral inclusions formed by prehnite, barite and routhierite are also detected (Figure 9b). Hyalophane grains are present as inclusions in gold in a very limited quantity. The abovelisted minerals can be found not only in the form of inclusions in gold but also as in-tergrowths with gold. Among the rare minerals intergrown with gold, we also note coloradoite, arsenolite ( Figure 9c) and parapierrotite (Figure 9d). Arsenolite is the most recent mineral formed as a result of supergene processes. Gold grains cementing other minerals are most widely distributed (Figure 8b-e). Often, native gold in such aggregates intergrows with realgar, pyrite and calcite. Native gold contains numerous mineral inclusions. Small crystals of pyrite, prehnite, quartz and calcite are directly enclosed in the native gold. Polymineral inclusions, consisting of calcite, barite and dalnegroite, are found in gold aggregates as well (Figure 9a). Polymineral inclusions formed by prehnite, barite and routhierite are also detected (Figure 9b). Hyalophane grains are present as inclusions in gold in a very limited quantity. The above-listed minerals can be found not only in the form of inclusions in gold but also as intergrowths with gold. Among the rare minerals intergrown with gold, we also note coloradoite, arsenolite ( Figure 9c) and parapierrotite (Figure 9d). Arsenolite is the most recent mineral formed as a result of supergene processes.  Notes: the points of analyses are given in Figure 9, 1 the total includes 1.12 wt.% of Zn; 2 the total includes 1.09 wt.% of Zn; 3 the total includes 39.95 wt.% of Te. Dash is element content below detection limits. Notes: the points of analyses of native gold No. 9-16 are given in Figure 9, No. 1-8, 17-20 are given in Figure 10. Dash is element content below detection limits.
Minerals 2021, 11, x FOR PEER REVIEW 11 of 17 Figure 9. BSE-photo of the minerals intergrown with gold from realgar-orpiment cement breccia. Analyses corresponding to 1-8 point numbers are given in Table 2; the analyses of native gold No. 9-16 are given in Table 3. In general, the combination of minerals enclosed by or intergrown with the native gold (Table 2) is similar to the mineral composition of ore-bearing breccia cement. The nature of the relationship of native gold with other minerals indicates that its formation was simultaneous with that of most of the sulfide and sulfosalt grains. The presence of calcite, prehnite, hyalophane and other minerals as inclusions in both gold and sulfides Figure 9. BSE-photo of the minerals intergrown with gold from realgar-orpiment cement breccia. Analyses corresponding to 1-8 point numbers are given in Table 2; the analyses of native gold No. 9-16 are given in Table 3 In general, the combination of minerals enclosed by or intergrown with the native gold (Table 2) is similar to the mineral composition of ore-bearing breccia cement. The nature of the relationship of native gold with other minerals indicates that its formation was simultaneous with that of most of the sulfide and sulfosalt grains. The presence of calcite, prehnite, hyalophane and other minerals as inclusions in both gold and sulfides and sulfosalts indicates their earlier formation. Thus, the native gold and most of the sulfides and sulfosalts in the breccias' cement should be attributed to the same assemblage, that has formed at the final stage of the formation of the realgar-orpiment breccia cement.

Chemical Composition of Native Gold
Native gold from the ore-bearing breccia cement of the Vorontsovskoe deposit is characterized by a homogeneous internal structure ( Figure 10). It is divided into two groups by its chemical composition: 1) high-purity gold with Au content from 95 to 100 wt. % and 2) silver-containing gold with Au content from 80 to 95 wt. % (Table 3; Figure 11). Elevated silver concentrations in native gold are accompanied by elevated concentrations of Hg and Cu. The maximum content of mercury in gold reaches 1.85 wt. %. At the same time, copper is most characteristic of the high-purity gold and can reach 0.24 wt. %. However, trace amounts of copper are sometimes found in silver-containing gold as well. Parapierrotite Notes: the points of analyses are given in Figure 9, 1 the total includes 1.12 wt.% of Zn; 2 the total includes 1.09 wt.% of Zn; 3 the total includes 39.95 wt.% of Te. Dash is element content below detection limits.

Chemical Composition of Native Gold
Native gold from the ore-bearing breccia cement of the Vorontsovskoe deposit is characterized by a homogeneous internal structure ( Figure 10). It is divided into two groups by its chemical composition: 1) high-purity gold with Au content from 95 to 100 wt. % and 2) silver-containing gold with Au content from 80 to 95 wt. % (Table 3; Figure  11). Elevated silver concentrations in native gold are accompanied by elevated concentrations of Hg and Cu. The maximum content of mercury in gold reaches 1.85 wt. %. At the same time, copper is most characteristic of the high-purity gold and can reach 0.24 wt. %. However, trace amounts of copper are sometimes found in silver-containing gold as well.   Figure 11. The Au-Ag × 10-Cu × 100 ternary diagram [19] for the native gold composition from ore-bearing breccias of the Vorontsovskoe deposit.  Figure 9, No. 1-8, 17-20 are given in Figure 10. Dash is element content below detection limits.

Chemical Composition of Native Gold-A Comparison
Admixtures of Ag, Cu and Hg are found in native gold from breccias with realgarorpiment cement. The chemical composition of the native gold is similar to that obtained previously by other researchers [6,20]. Compared to the native gold from other ore types Figure 11. The Au-Ag × 10-Cu × 100 ternary diagram [19] for the native gold composition from ore-bearing breccias of the Vorontsovskoe deposit.

Chemical Composition of Native Gold-A Comparison
Admixtures of Ag, Cu and Hg are found in native gold from breccias with realgarorpiment cement. The chemical composition of the native gold is similar to that obtained previously by other researchers [6,20]. Compared to the native gold from other ore types of the Vorontsovskoe deposit [6], the studied gold is featured by its higher fineness (>860) (Figure 12a). This characteristic, along with a uniform internal structure of the grains, distinguishes the native gold in breccias with realgar-orpiment cement from the gold from skarns and other type of gold mineralization found within the Vorontsovskoe deposit (see Figure 12a). The fineness of gold increases in the range from the most high-temperature ore associations (skarns) to the lowest-temperature breccias with realgar-orpiment cement. The last stages of development of the granite-related hydrothermal ore system associated with the Auerbakh intrusion are characterized by the formation of high-purity gold in a single paragenesis with Tl and Hg sulfosalts. It should also be noted that most of the gold compositional data in the classification diagram after [19] fall into the field of epithermal deposits.
High fineness of gold from realgar-orpiment breccias of the Vorontsovskoe deposit is a distinguishing feature in contrast to the native gold from other deposits in the Turyinsk-Auerbakh metallogenic province (Figure 12b), including copper-skarn and iron-skarn sub-economic deposits with low gold concentrations. Thus, the gold in the copper-skarn ores of the Bashmakovskoe and Bogoslovskoe deposits [21], which are located northwest of Auerbakh intrusive, has the lowest fineness due to the significant concentration of silver. A similar composition of native gold is also characteristic of the gold ore skarns in the Dorozhnoe ore occurrence found in 2016 [9], which is located between the Vorontsovskoe deposit and the Auerbakh intrusion. Low-fineness silver-containing gold is also characteristic of sub-economic ore deposits associated with quartz-sericite-altered tuffs within the Turyinsk-Auerbakh metallogenic province. Some similarities in the chemical composition of the studied native gold from the ore breccias of the Vorontsovskoe deposit have been established for a part of the analyses of gold from the iron ore skarns of the Yuzhno-Peshchanskoe iron-skarn deposit. This similarity is likely a consequence of the unified ore formation process within the Vorontsovsko-Peshchanskaya ore-magmatic system [1]. tion of silver. A similar composition of native gold is also characteristic of the gold ore skarns in the Dorozhnoe ore occurrence found in 2016 [9], which is located between the Vorontsovskoe deposit and the Auerbakh intrusion. Low-fineness silver-containing gold is also characteristic of sub-economic ore deposits associated with quartz-sericite-altered tuffs within the Turyinsk-Auerbakh metallogenic province. Some similarities in the chemical composition of the studied native gold from the ore breccias of the Vorontsovskoe deposit have been established for a part of the analyses of gold from the iron ore skarns of the Yuzhno-Peshchanskoe iron-skarn deposit. This similarity is likely a consequence of the unified ore formation process within the Vorontsovsko-Peshchanskaya ore-magmatic system [1]. Figure 12. Ternary diagrams [19] for the native gold from different ore types in the Vorontsovskoe deposit (a) and other deposits in the Turyinsk-Auerbakh metallogenic province (b): 1-jasperoids [6]); 2-skarns [6]; 3-medium-temperature metasomatites developed on tuffs and tuffstones [6]; 4-gold from realgar-orpiment cement of ore breccia (this data); 5gold from ore breccia [6]; 6-the skarns of the Dorozhnoe ore occurrence [9]; 7-the bornite-chalcopyrite exo-and endoskarns of the Bashmakovskoe deposit (19); 8-medium-temperature gold ore metasomatites on tuffstones (authors' data); 9-the chalcopyrite-pyrrhotite ores of the Bogoslovskoe deposit [21]; 10-the pyrite mineralization of the iron-skarn Yuzhno-Peshchanskoe deposit [21].  [19] for the native gold from different ore types in the Vorontsovskoe deposit (a) and other deposits in the Turyinsk-Auerbakh metallogenic province (b): 1-jasperoids [6]); 2-skarns [6]; 3-medium-temperature metasomatites developed on tuffs and tuffstones [6]; 4-gold from realgar-orpiment cement of ore breccia (this data); 5-gold from ore breccia [6]; 6-the skarns of the Dorozhnoe ore occurrence [9]; 7-the bornite-chalcopyrite exo-and endoskarns of the Bashmakovskoe deposit (19); 8-medium-temperature gold ore metasomatites on tuffstones (authors' data); 9-the chalcopyrite-pyrrhotite ores of the Bogoslovskoe deposit [21]; 10-the pyrite mineralization of the iron-skarn Yuzhno-Peshchanskoe deposit [21].

Discussion
The regular change in the composition of native gold from the most silver-enriched varieties to practically pure native gold in different ore types suggests an evolution of the ore-forming system associated with the Auerbakh polyphase intrusion As a result of the comparative analysis of the gold composition from different rocks of the Vorontsovskoye deposit-trend of changes in the composition of gold has been established. A similar trend with a decrease of silver in native gold depending on the temperature of metasomatite formation, has been established for the ore cluster as a whole [1]. The general geological and geochemical patterns of the Turyinsk-Auerbakh metallogenic province [1], including the presence of small non-economic porphyry copper deposits, suggest that the Vorontsovskoe deposit is an integral part of a large ore-magmatic system genetically associated with the formation of the Auerbakh intrusion. The composition of native gold from the ore-bearing breccias of the Vorontsovskoe deposit corresponds to epithermal deposits [18].
The significant amounts of Tl-containing ore minerals sharing the same paragenesis as native gold from breccias with realgar-orpiment cement in the Vorontsovskoe deposit also indicate a unique mineral gold ore assemblage that has no analogues among the deposits of the Urals. The presence of Tl-As-Hg-Sb-(Te) geochemical signature, including those belonging to the same paragenesis as gold, is also characteristic for Carlin-style gold deposits [22,23]. However, thallium minerals are also widely distributed in deposits associated with the metamorphosed carbonate strata composed of limestone and dolomite [24][25][26] with the formation of mineralized bodies according to the principle of alpine-type veins. However, in these deposits, the thallium mineralization is not accompanied by gold mineralization. Often, thallium minerals are also found in a distal disseminated Carlin-style deposit, such as the Allchar deposit [27][28][29] located in southern part of North Macedonia or in deposits with intermediate position between epithermal deposits and sediment-hosted gold deposits (for example-the Jas Roux deposit, French Alps [30]).
In general, the presence of thallium mineralization in ore bodies of different genetic types can be explained by actively developed models of the magmatic formation of Carlinstyle deposits [22][23][24][25][26][27][28][29][30][31][32][33]. Within these models, thallium mineralization can be localized at a maximum distance from the magmatic source in the zone of intermediate argillic alteration and the distance from the magmatic source can reach more than 5 km.
Native gold in the ore breccias of the Vorontsovskoe deposit is concentrated in the form of both small grains ranging in size from 0.05 to 0.15 mm and fairly large grains up to 0.5 mm. In accordance with the technological classification of size [41], the gold of the Vorontsovskoe deposit belongs to the large type. The aggregates of native gold, considering their substantial distribution in the ore breccia cement, represent the bulk of the gold contained in the ores.
The high fineness of native gold from ore breccias distinguishes it from native gold found in other associations of the Vorontsovskoe deposit and other deposits of Turyinsk-Auerbakh metallogenic province. According to the composition, the native gold from the ore breccia cement in the Vorontsovskoe deposit differs from the native gold of other deposit ore types and from the other ore bodies in the Turyinsk-Auerbakh metallogenic province. The formation of gold in breccias occurred during the final stages of the formation of the Vorontsovsko-Peshchanskaya ore-magmatic system, together with barite, pyrite and Tl and Hg sulfosalts.