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Nickeloan tourmaline (up to 0.531 apfu Ni) associated with nickeloan talc (0.511-0.582 apfu Ni) or chromian muscovite is developed in magnesite-quartz veins hosted in ultramafic talc + magnesite ± hematite ± chromian muscovite hydrothermal alteration located in Uspenskaya Gorka, eastern flank of the Berezovskoe gold deposit, Middle Urals, Russia. The unusual Nirich composition of the silicates reflects the geochemical environment of formation. Tourmalines are represented by light and dark green needles in aggregates up to 2 cm in length. Crystals are characterized by a strong pleochroism, from colorless to green. Keywords: nickeloan tourmaline, nickeloan talc, chromian muscovite, ultramafic talc-carbonate alteration, Berezovskoe gold deposit, Urals, Russia. SOMMAIRELa tourmaline nickelifère (jusqu'à 0.531 apfu Ni) associée au talc nickelifère (0.511-0.582 apfu Ni) ou à la muscovite chromifère s'est développée dans des veines à magnésite + quartz recoupant des zones d'altération hydrothermales ultramafiques à talc + magnésite ± hématite ± muscovite chromifère situées à Uspenskaya Gorka, sur le flanc oriental du gisement aurifère de Berezovskoe, dans les Ourales moyennes, en Russie. La composition inhabituellement enrichie en Ni des silicates découle du milieu géochimique de formation. Mots-clés: tourmaline nickelifère, talc nickelifère, muscovite chromifère, altération ultramafique à talc-carbonate, gisement aurifère de Berezovskoe, Ourales, Russie. §

Although putatively possessing hexagonal R3m symmetry, reports of optically anomalous tourmaline are common, and recently an occurrence of triclinic tourmaline was reported with dissymmetrization that resulted from non-equivalency of the occupants of the Y sites. We report the atomic arrangement of Ni-bearing dravite from the Berezovskoe gold deposit, Middle Urals, Russia, in the non-conventional triclinic space-group R1 (R = 4.41%) to facilitate comparison with the conventional tourmaline R3m cell. The dissymmetrization occurs as a result of inequalities among both the hexagonally equivalent Y and hexagonally equivalent Z tourmaline sites. The atomic arrangement of this triclinic dravite demonstrates that the atomic arrangement of tourmaline is robust, and is capable of incorporating various substituents by modifying the putative hexagonal structure in lower symmetries, suggesting that further exploration of tourmaline's role in trace-element variation is warranted. Optical studies demonstrate the heterogeneous biaxial character of the crystals. Domains of different optical orientation and 2V correspond directly to trigonal prism |100|, |010| and pedion |001| sectors, indicating optical sectoral zoning. Compositional sectoral and concentric zoning are also observed within the crystals. Spectroscopic studies show the optical absorption spectrum of the Berezovskoe tourmaline has strong absorptions in the 400, 600-700, and 1100 nm regions, in addition to OH features near 1450, 2300, and 2700 nm. We conclude that the color in the E ⊥ c polarization comes dominantly from Fe mixed-oxidation-state couples on the Y sites, and from Cr 3+. Contributions to the color from the nickel are believed to be minor and will fall in the regions of strong Cr and Fe absorption. The ordered arrangement of cations on the Y and Z sites and the correlation of optical orientation with specific sectors indicate that dissymmetrization occurs during growth by differential incorporation at structurally different atomic sites at the surface of the crystal, which in the bulk are symmetrically equivalent.

The mineral composition of the Talatui gold deposit has been studied with modern methods. Previously unknown minerals (ilmenite, siegenite, glaucodot, wittichenite, matildite, hessite, pilsenite, zircon, tremolite, cummingtonite, hercynite, and goethite) have been identified in the ore. A high Re content has been detected in molybdenite. The spatiotemporal separation of Au and Ag is caused by different mineral species of these elements and their diachronous precipitation during the ore-forming process. Gold crystallized along with early mineral assemblages, beginning from virtually pure gold (the fineness is 996). Silver precipitated largely at the end of the process as hessite ( Ag 2 Te ) and matildite ( AgBiS 2 ). The temperature of ore deposition varied from 610 to 145 ° C, the pressure was 3370-110 bar, and the salt concentration ranged from 56.3 to 0.4 wt % NaCl equiv. The heterogeneous state (boiling) of fluid at the early stages has been documented. The chemical and isotopic compositions of the fluid testify to its magmatic nature and the participation of meteoric water at late stages in the ore-forming process. Thermodynamic modeling reproduces the main specific features of ore formation, including separation of Au and Ag. A physicochemical model of the gold mineralization in the Darasun ore district has been proposed. On the basis of several attributes, the Talatui deposit has been referred to the porphyry gold-copper economic type.

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