Research subject . Minerals and mineral assemblages of noble elements in chromitites of the Alapaevsk massif. Aim . A systematic mineralogical study of high-chromium (Cr) and alumina-rich (Al) chromitites with the development of a sequence scheme for mineral formation, including platinum-group minerals (PGMs) and gold alloys. Materials and Methods. Samples of high-Cr and Al chromitites from chromite deposits in various parts of the Alapaevsk massif. Scanning electron microscopy (Tescan VEGAII XMU and JSM-6390LV Jeol with EDX INCA Energy 450 X-Max 80 spectrometers) and electron microprobe analysis (Cameca SX 100 with five wave spectrometers) were used. Results . A diagram showing the sequence of mineral formation in chromitites was designed; primary and secondary mineral assemblages were distinguished, with the latter assemblage being divided into early and late mineral associations. Primary assemblages of high-Cr (Cr 2 O 3 > 50 wt %) and Al (Cr 2 O 3 < 50 wt %) ores are represented by similar minerals, including chromespinel, clinopyroxene, and olivine, with characteristically distinct compositions of these minerals in each type of ores. Minerals of the primary assemblage are synchronous with chrome-spinel and are represented by pentlandite, Cu-bearing pentlandite, chalcopyrite, pyrrhotite, bornite, as well as PGMs (laurite RuS 2 , erlichmanite OsS 2 , native osmium) and Cu-rich gold. Minerals of the secondary early association occur in the form of polyphase inclusions within chrome-spinel. Polyphase inclusions are composed of Cr-bearing chlorite, amphibole, garnet, sulfides (millerite, heazlewoodite) and minerals of native elements, including (Ni, Cr)-bearing copper, nickel-bearing copper, (Cu, Fe, Cr)-bearing nickel, awaruite. Noble metal minerals from the secondary early association were found only in Al chromitites and are represented by laurite, Pt- and Pd arsenides and stibnides, Ru-bearing pentlandite, and high-grade native gold. The secondary late mineral association consists of native copper and awaruite, which are intergrown with serpentine in high-Cr ore. The temperature conditions for the formation of secondary assemblages were estimated using a chlorite geothermometer. The formation temperatures of the studied chlorites from chromitites fall within the range of 250–284°C. Conclusions . Noble metal minerals from secondary associations were formed at temperatures below 350°C together with garnet, amphibole, chlorite, and nickel sulfides. Grains of primary Os–Ir–Ru alloys during epigenetic processes underwent sulfurization with the formation of a fine-grained porous mixture of native and sulfide (sometimes with As) phases, and replacement by Ru-pentlandite. The presence of awaruite and native Cu and Ni in both primary and secondary assemblages of chromotites indicates the reducing conditions for the formation of noble metal minerals. The limited occurrence of high-Cr ores, along with the manifested processes of sulfurization for primary grains of Os–Ir–Ru alloys, led to the scarcity of detrital PGM in the area of the Alapaevsk massif.

Research subject. Carbonate rocks in the frame of the Suunduk granite-gneiss massif of the East Ural uplift (Southern Urals). Aim. Determination of possible petrological and geochemical zonation in carbonate rocks of the metamorphic frame of the granite-gneiss massif. Identification of structural regularities of distribution and the character of relationships between carbonate rocks of different types. Materials and Methods. Expedition works were carried out using the methods of field structural geology and mineralogy. Samples of carbonate rocks were selected along the profile transverse to the strike of rocks. The contents of petrogenic elements were determined by X-ray fluorescence analysis using CPM-35 and EDX-8000 spectrometers; rare and trace elements were determined by ICP-MS using an inductively coupled plasma ELAN 9000 mass spectrometer. Results. Carbonate rocks are represented by calcite marbles, Mg-calcite marbles, dolomite marbles, calcite-dolomite marbles, and organogenic marmorized limestones (C1–2). Zonality in the carbonate framing of the southern part of the massif was identified, apart from metamorphic zonality: marbles and marmorized limestones in the distance. At the early collision stage, a terrigenous-carbonate sequence was formed. At the end of C2, due to regional metamorphism and formation of a granite-gneiss dome structure, in the areas of movement of metamorphogenic fluids, limestones underwent Mg-metasomatosis with the formation of pre-metamorphic dolomites with relics of fauna of C1–2 age. As a result of progressive dynamothermal metamorphism, under stress conditions, metasomatic dolomites in places experienced dedolomitization, with the formation of calcite marble with a polygonal-grained structure; this marble was also formed on limestones. Under stress, calcite marble was squeezed out along weakened zones in the direction of pressure drop, i.e. upward. This process was associated with plastic and quasi-plastic deformations, metamorphic recrystallization, metasomatic alterations. Mg-calcite marble with accessory red corundum, phlogopite, Cr-tourmaline, and Cr-muscovite was formed in the areas of movement of metamorphogenic fluids. At the early regressive stage, Mg-metasomatosis reappeared in the fluid movement areas with the formation of dolomite-calcite marble with accessory graphite, tremolite, Cr-tourmaline, and pyrite. The lack of a pronounced zonality of marbles in the frame of the granite-gneiss massif can be explained by the duration and variety of marmorization processes. Conclusion. The granite-gneiss massif was the center of zonal metamorphism. At the progressive and regressive stages of metamorphism, the earlier- middle-carboniferous limestones in the frame of the massif underwent metamorphic and metasomatic transformations with the formation of marbles of various compositions; there were plastic and quasi-plastic deformations, rheomorphic displacements of calcite marble. There is no obvious zonality in the marbles.

The results of studying the sulfur isotope system in platinum-group minerals (PGM) are rare and generally limited to S-isotope data for Ru-Os sulfides from dunite-harzburgite massifs. To partially fill this gap, we for the first time characterized features of the S-isotopic composition of kuvaevite (Ir5Ni10S16) and tolovkite (IrSbS) from the Verkh-Neivinsk dunite-harzburgite massif, a typical representative of the ophiolitic association at the Middle Urals. The study employed a number of analytical techniques, including scanning electron microscopy, electron microprobe analysis and a femtosecond laser ablation with a gas source isotope ratio mass spectrometry. The primary PGM assemblage is formed by osmium and iridium minerals, laurite, kuvaevite and Pt–Fe alloys, which are replaced by As-bearing laurite, irarsite, tolovkite and other PGM of secondary origin. Kuvaevite is characterized by a predominance of Ni over Fe, Cu, and Co (Ni/(Ni + Fe + Cu + Co from 0.56 to 0.58), as well as Ir over other platinum-group elements (PGE) (i.e., Ir/(Ir + Rh + Os + Ru + Pt + Pd) = 1.00); tolovkite is characterized by trace amounts of Pt (0.51–2.86 wt

Research subject. Vein carbonate-magnetite-serpentine rocks with nests of gem garnet (demantoid) among antigorite serpentinites of the Korkodinsky massif in the Middle Urals. Aim. To characterize the composition of native gold and physicochemical conditions of its formation in a previously undescribed type of gold mineralization in ultrabasic rocks. Materials and methods. Grains of native gold from carbonate (calcite)-serpentine (clinochrysotile) veins with demantoid and host serpentinites were examined by scanning electron microscopy (JSM-6390LV by Jeol) and X-ray microanalysis (Cameca SX 100 with five-wave spectrometer). Results. Significant differences in the morphology and chemical composition of native gold grains from serpentinite and carbonate-serpentine veins were revealed. In serpentinite, these differences are represented by Au–Cu intermetallides, i.e., AuCu3 auricupride and AuCu tetraauricupride. The intermetallide compositions are close to stoichiometric with a Cu excess of up to 0.1 p.u. Rare inclusions of electrum (37–44 wt % Ag, sample grade 555–617‰) are present in tetraauricupride; copper and nickel sulfides are present in auricupride. Sulfide inclusions are represented by accretions of digenite (Cu/S = 1.88) with pentlandite, as well as digenite with flame-like phenocrysts of bornite-digenite solid solution. In the peripheral parts of auricupride grains, hypergene rims of up to 100 µm in thickness were detected. Hypergene gold in reflected light is brown in color, having a porous, collomorphic or monolithic-cracked structure. This gold is characterized by low Cu contents (1.4–10.6 wt %), relative to the primary auricupride composition (50.7–52.3 wt % Cu), a deficit of total components ranging from 1.5 to 20.0 wt %, and the presence of an oxygen line on EDX spectra. The grains of native gold from the vein carbonate-serpentine mass are composed of Au–Ag solid solution particles, belonging to high-grade gold (913–961‰, 4–10 wt % Ag), less frequently to lower-grade gold (808–866‰, 13.0–19.4 wt % Ag). Mineral inclusions in silver gold were not found. Conclusions. Differences in the composition of native gold from vein mass and serpentinite reflect the variability of physical and chemical conditions of its deposition. The deposition of Au–Cu intermetallides occured from fluids with low oxygen and sulfur fugitivities: log fS2 = –8…–20, log fO2 = –26…–39, and Au-Ag solid solutions under higher values of sulfur fugitivities and more oxidized conditions. It is assumed that cuprous gold is associated with antigorite serpentinization, when the hyperbasite body was in the depth. During its rise to the surface and decompression, reducing conditions were replaced by oxidizing conditions. As a result, the fluid became carbonic acidic. In such conditions, carbonate-serpentine veins with demantoid and native silver gold were formed. The presence of auricupride hypergene replacement products of collomorphic, porous and fractured structure, as well as the presence of oxygen line in EDX spectra testify both to the processes of redistribution of micropores formed during copper removal and a possible formation of oxide or hydroxide forms of gold and copper.

Herein, we describe the design, implementation and operation principles of an astronomical camera system, based on a large-format CCD261-84 detector with an extremely thick 200 μm substrate. The DINACON-V controller was used with the CCD to achieve high performance and low noise. The CCD system photometric characteristics are presented. A spatial autocorrelation analysis of flat-field images was performed to reveal the dependence of substrate voltage on the lateral charge spreading. The investigation of the dispersion index for the optimal choice of exposure time is discussed. Studies of the patterns of fringes were carried out in comparison with previous detectors. The amplitude of fringes with CCD261-84 was significantly lower, compared to previous-generation detectors. The results of using a new camera for imaging and spectral observations at the Russian 6 m telescope with the SCORPIO-2 multimode focal reducer are considered. The developed CCD camera system makes it possible to significantly increase the sensitivity in the 800–1000 spectral range.

This paper reviews and summarizes the available information on the composition of palladian gold with various contents and sets of isomorphic impurities (Ag, Cu, Hg) at 50 deposits and ore occurrences with Au-Pd mineralization. It is revealed that Palladian gold is represented by the systems Au–Pd, Au–Pd–Hg, Au–Pd–Cu, and Au–Pd–Ag–Hg, but more frequently corresponds to Au–Pd–Ag, Au–Pd–Ag–Cu, and Au–Pd–Ag–Cu–Hg. Objects with palladian gold belong to different types of gold deposits and to the deposits at which the main components of ores are PGE, Cr, Cu, Ni, V, and Ti. We propose a classification of the types of deposits with palladian gold: (1) PGE ore deposits related to mafic–ultramafic magmatic complexes (two subtypes—(a) low-sulfide-grade (less than 2%–5% sulfides) Alaskan, and (b) high-sulfide-grade (more than 5% sulfides) Norilsk); (2) orogenic gold deposits (OG); (3) epithermal (porphyry) gold–copper deposits (EPGC); (4) iron oxide copper gold deposits (IOCG); (5) ferruginous quartzite deposits; (6) volcanic exhalation; and (7) gold-PGE placers of five subtypes corresponding to the types of 1–5 primary sources. Physicochemical conditions of the formation of palladian gold at some deposits of type 1 cover two areas—magmatic high-temperature and hydrothermal low-temperature. At the majority of deposits of types 2–4, its formation proceeds with the participation of hydrothermal fluids (300–60 °C) of various salinities (0.2–30 wt.% NaCl eq.). Palladian gold is mainly high-fineness (910‰–990‰), is less frequently medium-fineness, and contains Ag and Cu, but does not contain Hg at the deposits of types 1, 3, and 4. The only exception is the Au-Pd-Hg Itchayvayam ore occurrence (Kamchatka, Russia), for which two varieties of Pd,Hg-bearing native gold (fineness 816‰–960‰ and 580‰–660‰) are determined. Low-fineness palladian gold with the major content of Ag is typical of OGD deposits. Medium-fineness palladian gold occurs at ferruginous quartzite deposits and in volcanic exhalations. Hg, Ag, Cu-bearing high-fineness palladian gold is present mainly in placer deposits (type 7). The most common minerals in association with palladian gold are arsenides, stibioarsenides, sulfides, stannides, bismuthides, tellurides, and selenides of Pd and Pt. These are typical of deposit types 1 and 7. The minerals of Au, Ag, and Cu (tetra-auricupride, aurostibite, chalcopyrite, bornite, chalcocite, eucairite, etc.) are in association with palladian gold at OG, EPGC, and IOCG deposits. Hg minerals (cinnabar, tiemannite, coloradoite, potarite) are at some deposits (types 1, 2, 7-1, 7-4). Cu, Fe, and Pd oxides (tenorite, hematite, magnetite, PdO, (Pd,Cu)O) and Fe and Pd hydroxides (goethite, (Fe,Pd)OOH) occur at the deposits of the 3, 4, and 7 groups and indicate the highly oxidizing conditions of ore formation. The most common minerals among host minerals are quartz and muscovite, including fuchsite (Cr-Ms), chlorite, albite, K-feldspar, hornblende, and carbonates (calcite, siderite, etc.). The fineness, content, and set of impurities in palladian gold and minerals in association with it reflect the mineralogy of Au-Pd ores and allow them to be used as indicators for the deposit types.

Polymineral aggregates of rounded shapes (“nodules”) composed of native and sulfide minerals of Cu, Ni, Fe, Ag, and other elements from vein magnetite–calcite–chrysotile rocks with jewelry demantoid in the Korkodinskoe hypermafic massif are described. A common feature of the six identified types of native sulfide nodules, composed of native copper, heazlewoodite, pentlandite, cuprite, and other native sulfide minerals, is their spheroidal shape, which makes them similar to individual grains of other gangue minerals (calcite, magnetite, etc.). In heazlewoodite–pentlandite nodules, specific symplectites of mercuric silver and nickel copper in heazlewoodite, as well as awaruite in Co–pentlandite, were found. The matching set of ore minerals in the host serpentinite vein mass (native copper, mercuric silver, heazlewoodite, pentlandite, awaruite) and nodules from the vein material indicates their genetic connection and the conjugation of demantoid mineralization with the evolving processes of serpentinization. It was established that the nodules formed at temperatures below 380°C under reducing conditions at very low sulfur fugacity values (10 –17 –10 –27 bar) and oxygen (10 –30 bar at 200°C to 10 –21 bar at 350°C). For heazlewoodite–pentlandite nodules, such conditions persisted throughout the entire time of their formation, while, for other nodules, the reducing conditions of early parageneses were replaced by oxidative conditions in late parageneses, which is recorded by the replacement of native copper with cuprite. It is assumed that the features of the morphology and structure of native sulfide nodules and the presence of symplectite intergrowths of ore minerals in them are associated with specific conditions created during the decompression of the crust-mantle mixture rising to the surface in the fault zone. The source of the metals was a deep, high-temperature fluid interacting with mafic and ultramafic rocks under reducing conditions at a low water-to-rock ratio.

During studying chloritolites from copper ore occurrences near the Muldakaevo village (Uchalinsky district, Bashkortostan, Russia), very exotic scandium-vanadium garnets were identified, corresponding in composition to the series of eringaite - goldmanite - andradite. Previously, similar garnets (but represented by Sc-Zr varieties) were found only in rodingite-like rocks on the Vilyui River, Yakutia. The ore occurrences are located 35 km southeast of the city of Miass in the eastern foothills of the Southern Urals and are localized in metasomatites of the Late Devonian-Permian age.

We studied the mineralization and sulfur isotopic composition of sulfides of gold–palladium ores in olivine clinopyroxenites from the Dzelyatyshor massif made up of a continuous layered series of rocks: olivine-free clinopyroxenite–olivine clinopyroxenite–wehrlite. The primary igneous layering of rocks, manifested as different quantitative ratios of clinopyroxene and olivine in them, controls the local trends of variability in the chemistry of mineral-forming medium and the concentrations of ore components, including noble metals, and sulfur in each separate layer during its cooling. The replacement of primary rock-forming minerals by secondary minerals, when the temperature decreases, is a characteristic trend for pyroxenites: (a) olivine → serpentine, secondary magnetite, and (b) clinopyroxene → amphibole, secondary magnetite → chlorite. The deposition of native gold in parageneses with PGM and sulfides at the Ozernoe occurrence took place during the replacement of earlier rock-forming minerals by chlorite. This process completed mineral formation at the deposit and took place at temperatures 150–250 °C and at the high activity of S, Te, Sb, and As of fluid. The variability of mineral formation conditions during chloritization is reflected in the change of native-sulfide forms of Pd by arsenide-antimonide forms and the sulfur isotopic composition of sulfides. The Pd content in native gold increases in the series—Au-Ag solid solution (<1.5 wt.% Pd)—Au-Cu intermetallides (to 6 wt.% Pd)—Cu-Au-Pd solid solutions (16.2–16.9 wt.% Pd). The sulfur isotopic composition of pyrite, chalcopyrite, and bornite varies from −2.1 to −2.9‰. It is assumed that a deep-seated magmatic basic melt was the source of fluid, ore components, and sulfur.

Composition of native gold and minerals in intergrowth of the Chudnoe Au-Pd-REE deposit (Subpolar Urals, Russia) was studied using optical microscopy, scanning electron microscopy, and electron microprobe analysis. Five varieties of native gold have been identified, based on the set of impurity elements and their quantities, and on intergrown minerals. Native gold in rhyolites from the Ludnaya ore zone is homogeneous and contains only Ag (fineness 720‰, type I). It is in intergrowth with fuchsite or allanite and mertieite-II. In rhyolites from the Slavnaya ore zone, native gold is heterogeneous, has a higher fineness, different sets and contents of elements: Ag, Cu, 840–860‰ (type II); Ag, Cu, Pd, 830–890‰ (III); Ag, Pd, Cu, Hg, 840–870‰ (IV). It occurs in intergrowth with fuchsite, albite, and mertieite-II (type II), or albite, quartz, and atheneite (III), or quartz, albite, K-feldspar, and mertieite-II (IV). High-fineness gold (930–1000‰, type V) with low contents of Ag, Cu, and Pd or their absence occurs in the form as microveins, fringes and microinclusions in native gold II–IV. Tetra-auricupride (AuCu) is presented as isometric inclusions in native gold II and platelets in the decay structures in native gold III and IV. The preliminary data of a fluid inclusions study showed that gold mineralization at the Chudnoe deposit could have been formed by chloride fluids of low and medium salinity at temperatures from 105 to 230 °C and pressures from 5 to 115 MPa. The formation of native gold I is probably related to fuchsitization and allanitization of rhyolites. The formation of native gold II-V is also associated with the same processes, but it is more complicated and occurred later with a significant role of Na-, Si-, and K-metasomatism. The presence of Pd and Cu in the ores and Cr in fuchsite indicates the important role of mafic-ultramafic magmatism.

This study presents new compositional and S-Os isotope data for primary Ru-Os sulfides within a platinum-group mineral (PGM) assemblage from placer deposits associated with the Verkh-Neivinsk massif, which is part of the mantle ophiolite association of Middle Urals (Russia). The primary nature of Ru-Os sulfides represented by laurite (RuS2)–erlichmanite (OsS2) series is supported by occurrence of euhedral inclusions of high-Mg olivine (Fo92–94) that fall within the compositional range of mantle (primitive) olivine (Fo 88–93). The sulfur isotope signatures of Ru-Os sulfides reveal a range of δ34S values from 0.3 to 3.3‰, with a mean of 2.05‰ and a standard deviation of 0.86‰ (n = 18), implying that the sulfur derived from a subchondritic source. A range of sub-chondritic initial 187Os/188Os values defined for Ru-Os sulfides (0.1173–0.1278) are clearly indicative of derivation from a sub-chondritic source. Re-depletion (TRD) ages of the Verkh-Neivinsk Ru-Os sulfides are consistent with prolonged melt-extraction processes and likely multi-stage evolution of highly siderophile elements (HSE) within the upper mantle. A single radiogenic 187Os/188Os value of 0.13459 ± 0.00002 determined in the erlichmanite is indicative of a supra-chondritic source of HSE. This feature can be interpreted as evidence of a radiogenic crustal component associated with a subduction event or as an indication of an enriched mantle source. The mineralogical and Os-isotope data point to a high-temperature origin of the studied PGM and two contrasting sources for HSE in Ru-Os sulfides of the Verkh-Neivinsk massif.

Research subject. The mineral compositions of titanomagnetitic (apatite, titanomagnetite) and copper-titanomagnetitic (bornite, chalcopyrite, apatite, titanomagnetite) ores of the Volkovskoe Cu-Fe-Ti-V deposit (Middle Urals, Russia).Methods. The research was carried out using a Jeol JSM-6390LV scanning electron microscope and X-ray spectral microanalyzers JXA-5 (Jeol) at the Geoanalitik Collective Use Center of the IGG UB RAS. Results and conclusions.Native gold (with ≤ 0.3 wt % Pd, 0.2–0.4 wt % Cu; fneness 800–914 ‰), tellurides of Pd, Au and Ag (merenskyite, keithconnite, sylvanite, hessite) and Pt arsenide (sperrylite) were found in the copper-titanomagnetitic ores. For the frst time, two generations of native gold (fneness 1000 and 850–860 ‰) and palladium telluride (keithconnite Pd3-xTe) were detected in titanomagnetitic ores. The sequence of ore mineral formation and the features of their genesis were revealed. Native gold (fneness 1000‰) in the form of microinclusions in titanomagnetite was attributed to the magmatic stage. Noble metal minerals, intergrown with copper sulfdes (bornite, chalcopyrite, digenite) and associated with late hydroxyl-bearing minerals (amphibole, epidote, chlorite), are superimposed in relation to the magmatic minerals (pyroxene, plagioclase, hornblende, apatite, titanomagnetite, ilmenite, etc.) of these ores. Merenskyite, sperrylite, high fneness gold (800–914 ‰), as well as carrolite, cobaltite, copper-cobalt telluride and bismuth tellurium-selenide kawazulite Вi2Te2Se are syngenetic with copper sulfdes. The Au-Ag tellurides were deposited later than these minerals. It is shown that the high fugacity of tellurium, which binds Pd, Au, and Ag into tellurides, prevents the occurrence of native gold containing high concentrations of palladium and silver.

Research subject. The results of studying the geology and mineralogy of the Poldnevskoye deposit of demantoid are presented. Material and methods. The factual material was collected during feld research. Demantoid samples were pro vided by mine owners. Rock and vein minerals were studied in hand specimens, polished specimens and thin sections; the chemical composition was determined by X-ray fluorescence spectroscopy, scanning electron microscopy, electron probe X-ray spectral microanalysis, inductively coupled plasma mass spectrometry with laser sampling (LA-ICP-MS).Results. The Korkodinsky ultrabasic massif containing the deposit experienced strong decompression and syndecompression mineral formation during the ascent. At an early stage, veins of clinopyroxenite formed in dunite, which also experienced decompression cracking. Then the rocks underwent antigoritization and the appearance of vein antigorite, which was replaced by veins of clinochrysotile (+ magnetite ± carbonate ± demantoid). This was followed by the formation of lizardite (+ magnetite ± carbonate ± demantoid), which also developed along the earlier vein serpentines. Demantoid is represented by rounded grains and rounded grain aggregates with the signs of growth under the conditions of allround extension. The predominant color is brownish-green, yellow-green, rarely green and dark green. Green and dark green demantoids contain Cr2O3 0.52–2.3 wt %. In the central part of some grains and demantoid aggregates, a brown color is observed, which binds to TiO2, the content of which reaches 1 wt %. The demantoids of the Poldnevskoye deposit bear typomorphic features of the Ural-type demantoids (inclusions of the “horse’s tail” type). The distribution of demantoid is nested. The length of the veins with demantoid is frst meters. Their pillar-like shape is assumed. Conclusion.Demantoids from nests in serpentinized massive or brecciform dunites are more intact; demantoid nests in strongly serpentinized rocks were tectonic crushed and contain little gemstone.

The term "demantoid", first proposed in 1856 by the famous Finnish mineralogist Nils von Nordensheld, refers to a highly dispersed yellow-green mineral from the Central Urals placers. In 1874, it was found to be a gem variety of andradite garnet. "Horsetail" inclusions are considered a sign of the Ural type demantoid. Although these inclusions are large (visible to the naked eye), their diagnostics remains debatable: some researchers attribute them to byssolite (amphibole-asbestos), others consider them chrysotile. We investigated the horsetail inclusions in the Ural demantoids through various methods: optical microscopy, scanning electron microscopy (SEM), Raman spectrometry, X-ray powder diffraction, and thermal analysis. In most cases, "horsetail" inclusions in the Ural demantoid were represented by hollow channels and only the outcrops, on the demantoid surface, were occasionally filled with serpentine (established by SEM); in one case, magnetite was observed. Hollow canals were usually collected not in bundles, such as a "horsetail", but in fans, sometimes curved into cones. The structure of the grains was spheroidal, sectorial, and sometimes had induction surfaces, which, to the periphery of the grain, were replaced by tubular channels assembled in a fan. The specifics of the growth of the "horsetail" inclusions of the demantoid grains can be explained by the decompression conditions that arose when the ultrabasites (a crust-mantle mixture) were squeezed upwards during collision.

The Lipovka and Alabashka ruby occurrences in the Central Urals hosted in marble of the Murzinka–Adui Anticlinorium are considered. The field works were carried out; the chemical compositions of rocks and minerals and oxygen isotopic composition of corundum and silicates were determined; and micas were dated by the K–Ar method. These occurrences are similar in geological environment and mineral assemblages to the ruby occurrences in the South Urals and deposits in Southeast Asia. At the Alabashka occurrence, ruby was also found from brecciated mineralized granite and amphibolite. The mineralized breccia and ruby-bearing marble are enriched in K- and Cr-bearing minerals. Ruby characterized by a narrow range of the oxygen isotopic composition (δ18О = 14.2–17.8‰) corresponding to that of metamorphic and metasomatic corundum. The hydrogen isotopic composition of fuchsite ranges from –78.5 to –89.6‰, while the δD value calculated for fluid equilibrated with fuchsite varies from –48.8 to –79.7‰ that is consistent with the high-temperature equilibrium with magmatic rocks. Aluminum and Cr were mobile elements and could be inflowed by fluids during ruby formation. Corundum mineralization at both occurrenceswas dated at 280–260 Ma, which is consistent with the beginning of post-collisional relaxation. The Murzinka–Adui Anticlinorium is the most prospective area in the Urals for the high-quality ruby deposits.

The limited literature data on gold-bearing albite–pyroxene rodingites are summarized for the Agardag ultramafic massif in southern Tuva. These data are supplemented by new mineralogical, geochemical, thermobarogeochemical, and isotopic–geochemical results in order to reveal the physicochemical mineral formation conditions and sources of ore matter and fluids on gold deposition in rodingites. Rodingites and associated schistose nephritoids are near-fault metasomatites and are confined to a latitudinal tectonic zone in antigorite serpentinites. They formed in two stages. Stage I minerals (pyroxene, albite, etc.) are rodingites and stage II minerals (Na-bearing actinolite, albite, etc.) are nephritoids and veinlets that intersect rodingites. Disseminated sulfides of the Cu–S series (chalcocite, digenite, etc.) and Au minerals (tetra-auricupride and electrum) were deposited during both stages. The temperature regime (500–250°C) and low amount of CO2 in fluid ($${{{\text{X}}}_{{{\text{C}}{{{\text{O}}}_{{\text{2}}}}}}}$$ = 0.017–0.025) correspond to the formation conditions of typical bimetasomatic rodingites. The degree of oxidation of gas components in fluids CO2/(CO2 + Σreduced gases) increases from rodingites (0.189) to nephritoids (0.299) and antigorite serpentinites (0.738). The O isotopic composition of silicates and calculated O isotopic composition of the fluid during antigorite serpentinization (5.8 to 7.6‰ δ18Оfl and –66 to –69‰ δDfl) correspond to juvenile and magmatic water in contrast to metamorphic water during nephritization and rodingitization (6 to 9.9‰ δ18Оfl and –39 to –46‰ δDfl) with involvement of heavy oxygen that was subjected to the sedimentary cycle. It is suggested that the magmatic ore-bearing fluid (7.3–7.6 wt % NaCl-equiv) separated from gabbroic melts. The oxidized fluid was modified to a reduced fluid during interaction with ultramafic rocks. The mafic and ultramafic rocks were the source of Na, REE, Au, Ag, Cu, and Ni. Deformations with the formation of veins and filling of veinlets are favorable for a high local gold concentration.

The chlorite rocks (chloritolites) exposed in a bedrock of the Mokhovoe boloto (Moss swamp) gold placer (East Uralian Megazone, South Urals), which occurs on ultramafc rocks, are studied. The trace element composition of chloritolites is characterized by elevated contents of Mn, Ti, V (hundreds of ppm), Cu, zn, Ni, Co, Cr, zr, Li, Sc (tens of ppm), w, zr, Y and REE. Chloritolites contain up to 3 vol. % of disseminated magnetite, ilmenite and accessory minerals (rutile, xenotime, monazite, zircon, apatite, scheelite, U-bearing thorite) from a mineral assemblage, which cocrystallize with the main volume of chlorite. The mineralogical and geochemical features of the Mokhovoe boloto chloritolites and gold-bearing chloritolites of the Karabash massif in the Main Uralian Fault zone are slightly similar. The elevated Ti and P contents of the studied chloritolites, the level of REE contents corresponding to mafc rocks, and the lack of relict chromite indicate their possible metasomatic formation after dolerite dikes known within Chebarkul-Kazbai ultramafc complex. Figures 8. Tables 7. References 14.

Limited data on the isotopic composition of sulfur of platinum-group minerals (PGM) are reported in the literature. The LA-ICP-MS method was used to study the S-isotopic composition of Ru-Os-Ir sulfides and sulfoarsenides forming secondary corrosion and superimposed PGM assemblages from placer deposit of the East Shishim river occurring within the Verkh-Neivinsky dunite-harzburgite massif. Sulfides from corrosion PGM assemblage (laurite, As-containing laurite) replace the grains of primary native Os and Ru minerals, forming rims on them. PGM from superimposed assemblage (laurite, irarsite, tolovkite, etc.) originated on the surface of laurite rims that form part of corrosion PGM assemblage. Variations in 34S values of secondary PGM (from –4.6 to 7.6 ‰) are more pronounced than those for laurite and erlichmanite from primary PGM assemblage (i. e., 0.2—2.3 ‰). The results are consistent with a model of the ultramafic massif ascending to the surface and subsequent water metamorphism during the tectonic flow and crust-mantle interaction. At the initial stage of metamorphism, mantle sulfur is present, isotopically lightened under oxidation conditions and temperature reduction. The final stage is marked by sulfur of the metamorphic fluid formed by the involvement of isotopically-heavy sulfur derived from the host sedimentary rocks.