Ullmannite and Heyrovskyite veins of the Genevskaya Darasunskoye gold deposit in Eastern Transbaikalia (Russia)

. The first data on the mineral composition of the vein of the Geneva Darasunskoye gold deposit in Transbaikalia are given. Particular attention is paid to the rare mineral ulmannite NiSbS, which has not been described before, and Heyrovskyite, which is previously unknown in the ores of this deposit, which belongs to rare minerals of the lead-bismuth sulfosal salt group. Both mineral species are found in propylites containing a gold-bearing vein. Among the ulmannites, arsenic and non-arsenic varieties were found. The arsenic content is in the range of 4.9 – 7.37 wt. %. Two of the analyzed samples contained copper contents of 0.83 – 0.87 wt. %. All measured individuals contain iron ranging from 0.57 to 1.75 wt. % and there is a sulfur deficiency. For the first time, a lead-silver-bismuth sulfosalt belonging to heyrovskite was identified in the near-salband part of the Geneva gold-bearing vein of the Darasun gold deposit. Its composition is not constant. Kheyrovskite of the Darasun deposit differs in chemical composition from its other known occurrences in its relatively high antimony content up to 2.12%, as well as a high bismuth content and sulfur deficiency. Both minerals belong to the early stage of the formation of the Geneva gold vein.


Introduction
The Darasunskoye gold deposit in the Trans-Baikal Territory is located in the urban-type settlement of Vershino-Darasunsky in the Tungokochensky District of the Trans-Baikal Territory (Fig. 1).
It is part of the large Darasunsky ore field and is an integral part of the Darasun-Mogochinsky structural and formational zone of the gold-molybdenum belt of Transbaikalia, covers an area of about 60 km2 and includes the Darasunskoye, Talatuiskoye gold ore, Teremkinskoye gold-silver, Ust-Teremkinskoye silver deposits and a number of ore occurrences.The alluvial gold content of the basin of the Darasun, Zharcha and other rivers has been known since the middle of the XIX century.References to indigenous gold mineralization in the Darasun ore region have been appearing since 1889 and the further history of discoveries, study and development of veins with indigenous gold is given in works [1 -4] and is not considered here.The ore field is located in the zone of influence of the deep Mongolian-Okhotsk belt, at the intersection of the Darasuno-Voskhodninsky and Baleysko-Darasunsky faults [2,3]
The Geneva-based vein was discovered by one of the authors on September 2, 1985 in a 1-bis drift along the Epovskaya vein, which wedged out [4].A thin exit of it was seen in the wall of the drift.It had a thickness of 2 -3 cm and a calcite-quartz-pyrite composition.The pyrite content in the form of cubic and cuboctahedral crystals in it was about 70%.The enclosing rock in this place is represented by intensively pyritized gabbro-amphibolite, in which pyrite is predominantly cubic habit up to 0.2 -1.0 mm in size with a content of up to 15%.Here, at the contact of intensively pyritized and chloritized fine-grained microdiorite and gabbro-amphibolite, there was a thin vein of ankerite-pyrite-arsenopyrite composition.There are practically no publications on the mineralogy of this vein, although it is characterized by high gold content with a thickness of up to 0.6 m.At present, most of it has been worked out.The vein yielded about 3.5 tons of gold.In the process of studying it in 2023, new data were obtained not only on the mineralogy of this vein, but also on the deposit as a whole.This article is devoted to the description of two previously unexplored ullmannitе and heyrovskyite.

Material and methods of research
Sampling was carried out along cross-sectional lines located approximately 50 m apart, depending on the nature of the variability of the thickness of the vein, the enclosing rocks and the intensity of the near-vein changes.Their position in space is known.In each crosssection, in the direction from the hanging or lying side of the vein to its axial part, samples were taken in the enclosing rocks at intervals of up to 1 m, then within 1 meter from the vein after 0.5 m.The next half a meter after 25 cm, then the entire interval of the nearcontact part of the rock with a width of 10 cm was selected.The vein material was selected entirely.Each sample was numbered and an arrow was drawn on it, indicating the direction of selection and its position in the core.As they left the vein, the distances between the samples changed in reverse order.In fact, space-oriented samples of vein material were selected.Samples were prepared from the selected samples to study the chemical composition by quantitative spectral, assay and X-ray spectral analysis.The study of the mineral composition of the vein material and the enclosing rocks was carried out in thin sections and thin sections by optical and electron microscopy.28 sections were made, studied using an optical polarizing microscope AXIO ScopeAI.Of these, 11 thin sections E3S Web of Conferences 462, 03023 (2023) AFE-2023 https://doi.org/10.1051/e3sconf/202346203023were studied in order to confirm the results by the electron-probe method on a scanning electron microscope LEO 1430 VP (GIN SB RAS, Ulan-Ude, analysts E.V. Khodyreva and E.A. Khromova, head of the laboratory, Ph.D. S.V. Kanakin).Samples of native gold and silver of known composition were used as standards.The gold content was determined by spectrogoldmetry and assay analysis.

Research results and discussion
The structural features and composition of the vein fragments were studied using samples from the thickest part of the vein, where rare ore minerals are present.Here the vein is composed of a coarse-grained aggregate of quartz-sulfide composition.Sulfides are represented by pyrite, arsenopyrite and chalcopyrite.At the same time, the chalcopyrite content reaches up to 30-80% in some areas.Massive quartz of two generations.One, the early one, is cut by sulfides, and the other penetrates into them along thin cracks.A detailed study of the mineral composition of the ores of the Geneva vein of the Darasun deposit provided new data on the occurrence, mineral association and composition of ullmannite (NiSbS) and heyrovskyite (Pb6Bi2S9).Both minerals were found not in the vein itself, but in propylite along granosyenite, 0.5 m from its hanging side.It contains barium potassium feldspar, albite-oligoclase, quartz and chlorite, which is similar in composition to chamosite.Accessory minerals are represented by zircon, ilmenite, apatite, and rutile.Ullmannite was found in the marginal part of chalcopyrite grains at the contact with quartz and dolomite.Figure 3 shows a fragment of such a polished section under an electron microscope in backscattered electrons.In other parts of the same polished section, ullmannite is intergrown with siderite and quartz.Ullmannite, as can be seen in Fig. 3, forms an almost euhedral segregation up to 5 μm in size, as well as thin (up to 2 μm) rims at the contact of chalcopyrite with quartz and dolomite up to 10 -15 μm in length.Figure 4 shows its deposits of irregular shape and in the form of small lenses up to 5 μm long in an ankerite-quartz aggregate near the contact with chalcopyrite.Ullmannite belongs to the antimony generation of the formation of quartz-sulfide veins.The source of nickel here may be early amphiboles replaced by chlorite.The chemical composition of ullmannite is presented in Table 1, from which it is clear that it is variable.In total, 7 individuals of ullmannite were analyzed, for four of them, due to their small sizes, the analyzes are characterized by amounts in the range of 80.17 -95.15 wt.%.Therefore, the formula coefficients of these samples do not correspond to the stoichiometric ratios of the contents of chemical elements typical of ullmannite.Among the ullmannites, arsenic and non-arsenic ones were found.The arsenic content is in the range of 4.9 -7.37 wt.%.Two of the analyzed samples contained copper contents of 0.83-0.87wt.%.All measured individuals contain iron ranging from 0.57 to 1.75 wt.%.In all measured individuals, a deficiency of sulfur is observed, which is probably due to its consumption for the formation of chalcopyrite and pyrite, which precede the release of ullmannite.The nickel content is also less than theoretical and is compensated in the cationic part by the presence of iron and copper.Note: 1-weight %; 2 -normalized to 100%; 3 -formula coefficients Heyrovskyite was found both in association with and without ullmannite in propylite along gabbro-amphibolite.It is in paragenesis with pyrite in the form of inclusions in it with a magnitude of 5 -10 μm, chalcopyrite, galena, sphalerite in quartz-epidote-chloriteplagioclase aggregate, which also contains magnetite, titanite, apatite in the form of accessory minerals (sample 594/30).In the near-salband part of the vein (sample 579a/30), heyrovskite is found in an association containing pyrite, chalcopyrite, arsenopyrite, galena, and pyrrhotite.Vein and accessory minerals are represented by quartz, plagioclase, chlorite,  Silver in the amount of 1.84% is also contained in Heyrovskite.The size of heyrovskite individuals is in the range of 5 -30μm.They are xenomorphic.

E3S
Features of the chemical composition of heyrovskite are presented in table .2. It provides data on six reliable analyzes in two samples.Currently, there is no single formula for heyrovskite due to its variable composition.The formula Pb6Bi2S9 is given in [10].In M. Fleischer's Dictionary of Mineral Species 1987 [11] its expanded analogue Pb10AgBi5S18 is given, and in MINCRIST (card 1966, specification [1] Ag, Bi) the formula Pb3.67 Cd0.05 Ag0.15 Bi3.13 S9.19 is given., corresponding to the theoretical formula Pb4 Ag Bi3 S9. .Formulaic Heyrovskite coefficients of the Geneva vein, as can be seen from Table 2, are more consistent with the formula of this mineral species first described from the Hurky deposit in the Czech Republic, the formula of which Pb5.19Ag0.46Cu0.02Bi2.72S9.00[13]    However, for all measured individuals of heyrovskite, even in the case of an almost ideal analysis sum of 594/30 10.1_1, equal to 99.98%, an excess of Bi with a deficiency of Pb and S is observed.Kheyrovskite of the Darasun deposit is constantly found in inclusions in pyrite.Unlike ulmannite, it is formed in the selvage part of the vein simultaneously with the minerals of the main part of the ore process or slightly earlier.It is replaced and captured by second generation pyrite.

Conclusion
1. Ulmannite was first identified in wall-ore propylites accompanying the formation of gold mineralization in the South-Western section of the Darasun deposit.Among the ulmannites, arsenic and non-arsenic ones were found.The arsenic content is in the range of 4.9 -7.37 wt.%.Two of the analyzed samples contained copper contents of 0.83-0.87wt.%.All measured individuals contain iron ranging from 0.57 to 1.75 wt.%. and there is a sulfur deficiency.
2. For the first time, a lead-silver-bismuth sulfosalt, related to heyrovskite, the composition of which is variable, was identified in the near-salband part of the Geneva gold vein of the Darasun gold deposit.Kheyrovskite of the Darasun deposit differs in chemical composition from its other known occurrences in its relatively high antimony content up to 2.12%, as well as a high bismuth content and sulfur deficiency.
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Table 1 .
The chemical composition of ullmannite

Table 2 .
Chemical composition of heyrovskyite