4.1. 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.
The most common hydrothermal gangue minerals of breccia cement are manganoan calcite, prehnite, orthoclase var. hyalophane, fluorapatite and barite. Prehnite forms small, idiomorphic prismatic crystals or granular masses cemented by ore minerals—pyrite, realgar, stibnite, aktashite, boscardinite, parapierrotite, routhierite and chabournéite (Figure 5
a,b,d,e). Orthoclase var. hyalophane forms intergrowths with prehnite granular aggregates (Figure 5
b), or, more rarely, single prismatic crystals. Fluorapatite is found as rare prismatic crystals included in the aggregates of other minerals (Figure 5
b). Barite and manganoan calcite form only fine-grained masses (Figure 5
c). Gangue minerals are intergrown mainly with realgar (Figure 5
b,d,e) and less often with orpiment.
Stibnite (Figure 5
d), orpiment, realgar (Figure 5
b,d,e) and pyrite (Figure 5
a,b,d,e) prevail among the ore minerals in the breccia cement. Pyrite forms idiomorphic pentagonal dodecahedra and often contains up to 4 wt.% As in its structure. Rare Tl- and Hg-bearing sulfosalts, sulfides and tellurides are found among the ore cement minerals and include aktashite, bernardite, boscardinite, weissbergite, vrbaite, gillulyite, dalnegroite, sicherite, imhofite, coloradoite, christite, laffittite, lorándite, parapierrotite, picotpaulite, rebulite, routhierite, philrothite, hutchinsonite, chabournéite, écrinsite, etc. (full list of minerals identified at Vorontsovskoe deposit including rare Tl-Hg-bearing ones is given by [16
]). These ore minerals were formed later than most gangue minerals. Thus, aktashite and boscardinite cement prehnite crystals (Figure 5
a,b), parapierrotite replaces stibnite (Figure 5
d), routhierite cements individual prehnite and diopside crystals (Figure 5
d) and chabournéite fills veinlets intersecting silicates and realgar (Figure 5
c,e). The chemical composition of selected rare Tl and Hg minerals mentioned above is given in Table 1
4.2. Morphological Features of Native Gold in Breccias
Native gold in the breccias occurs both in cement (Figure 6
a) 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 6
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 7
a,b). Gold grains with prismatic cross-sections are extremely rare and are located directly in the fragments of marmorized limestones (Figure 7
c). Gold crystals are relatively rare among the gold grains from the ore concentrates too (Figure 8
a). 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 8
b–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 9
a). Polymineral inclusions formed by prehnite, barite and routhierite are also detected (Figure 9
b). 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 9
c) and parapierrotite (Figure 9
d). Arsenolite is the most recent mineral formed as a result of supergene processes.
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.
4.4. Chemical Composition of Native Gold–A Comparison
Admixtures of Ag, Cu and Hg are found in native gold from breccias with realgar-orpiment cement. The chemical composition of the native gold is similar to that obtained previously by other researchers [6
]. 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 12
a). 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 12
a). 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 12
b), 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