Gold and silver accessory minerals in ultramafites of the Kyzyr-Burlyuksky ultramafic massif (Western Sayan)

The study focuses on gold and silver accessory minerals (native silver, cuprous gold, luanheite (Ag3Hg), unspecified mineral phase (Cu,Ag,Hg), first diagnosed in dunites and apodunite serpentinites of the Kyzyr-Burlyuksky ultramafic massif, which is part of the Kurtushibin ophiolite belt of Western Sayan. The revealed ore minerals are mainly observed in the form of single hypidiomorphic, irregular microscopic precipitates (0.5– 3.0 μm) mainly inside magnetite, much less often in grains of avaruite. Typomorphic and chemical features of ore minerals, their natural setting in rock-forming silicate matrix are characterized. Formation and concentration of these accessory minerals is associated with superimposed low-temperature transformation (hydration) processes affecting original ultramafic rocks. At the same time, the presence of luanheite and an unnamed phase (Cu,Ag,Hg), along with the previously identified potarite (PdHg), is probably evidence of low-temperature conditions of mineral formation during the manifestation of epigenetic processes of serpentinization (lowgrade metamorphism) due to solutions enriched in mercury. The source of such solutions could be gabbro intrusions that penetrated later into the main ultramafic body.

ACCESSORY SULPHIDES AND ARSENIDES FROM CHROMYTITES OF KHARCHERUZ ULTRAMAFIC MASSIF, THE POLAR URALS

The study is focused on accessory sulfide mineralization revealed in massive chromitites of Kharcheruz ultramafic massif, which is part of the Khadatinsky ophiolite belt closing ophiolite complexes of the Polar Urals in the north. Three paragenetic associations of accessory sulfides associated with various processes of host chromitites formation and transformation are identified. Typomorphic and chemical features of ore minerals are characterized.

High Temperature Reduced Granulite-Facies Nature of Garnetites in the Khabarny Mafic–Ultramafic Massif, Southern Urals: Evidence from Fluid and Mineral Analyses

High-grade metamorphic rocks underlying the intrusive layered dunite–pyroxenite–gabbronorite East-Khabarny Complex (EKC) are integrated in the complex Khabarny mafic–ultramafic Massif in the Sakmara Allochthon zone in the Southern Urals. These rocks are associated with high-temperature shear zones. Garnetites from the upper part of the metamorphic unit close to the contact with EKC gabbronorite are chemically and texturally analysed to estimate their formation conditions and fluid regime. Fluids provide crucial information of formation conditions and evolution of these garnetites during high-grade metamorphism, and are preserved in channel positions within Si6O1812- rings of cordierite, and in fluid inclusions in quartz and garnet. Minerals and fluid inclusions of the garnetites are studied with X-ray fluorescence spectrometry, electron microprobe analyses, Raman spectroscopy, and microthermometry. The garnetites mainly consist of garnet (up to 80 vol. %), cordierite and quartz. Accessory minerals are rutile, ilmenite, graphite, magnetite and cristobalite. Granulite-facies metamorphic conditions of the garnetites are estimated with the garnet–cordierite–sillimanite–quartz geothermobarometer: temperatures of 740 to 830 ˚C and pressures of 770–845 MPa. The average garnet composition in end-member concentrations is 48·5 mole % almandine (±3·9), 34·7 mole % pyrope (±3·3), 10·3 mole % spessartine (±1·1), 1·8 mole % grossular (±1·5), and 1·5 mole % andradite (±1·5). The cordierite electron microprobe analyses reveal an average Mg2+ fraction of 0·79 ± 0·01 in the octahedral site. Relicts of a strong positive temperature anomaly (up to 1000 ˚C) are evidenced by the preservation of cristobalite crystals in garnet and the high titanium content of quartz (0·031 ± 0·008 mass % TiO2) and garnet (0·31 ± 0·16 mole % end-member Schorlomite-Al). The fluid components H2O, CO2, N2 and H2S are detected in cordierite, which correspond to a relatively oxidized fluid environment that is common in granulites. In contrast, a highly reduced fluid environment is preserved in fluid inclusions in quartz nodules, which are mono-fluid phase at room temperature and composed of CH4 (>96 mole %) with locally minor amounts of C2H6, N2, H2S and graphite. The fluid inclusions occur in homogeneous assemblages with a density of 0·349 to 0·367 g·cm-3. The CH4-rich fluid may represent peak-temperature metamorphic conditions, and is consistent with temperature estimation (∼1000 ˚C) from Ti-in-garnet and Ti-in-quartz geothermometry. Tiny CH4-rich fluid inclusions (diameter 0·5 to 2 µm) are also detected by careful optical analyses in garnet and at the surface of quartz crystals that are included in garnet grains. Graphite in fluid inclusions precipitated at retrograde metamorphic conditions around 300–310 ± 27 ˚C. Aragonite was trapped simultaneously with CH4-rich fluids and is assumed to have crystallized at metastable conditions. The initial granulite facies conditions that led to the formation of a cordierite and garnet mineral assemblage must have occurred in a relative oxidized environment (QFM-buffered) with H2O–CO2-rich fluids. Abundant intrusions or tectonic emplacement of mafic to ultramafic melts from the upper mantle that were internally buffered at a WI-buffered (wüstite–iron) level must have released abundant hot CH4-rich fluids that flooded and subsequently dominated the system. The origin of the granulite-facies conditions is similar to peak-metamorphic conditions in the Salda complex (Central Urals) and the Ivrea–Verbano zone (Italian Alps) as a result of magmatic underplating that provided an appearance of a positive thermal anomaly, and further joint emplacement (magmatic and metamorphic rocks together) into upper crustal level as a high temperature plastic body (diapir).