Platinum group elements in gold-sulfide and base-metal ores of the Sayan-Baikal Fold Region and possible platinum and palladium speciation in sulfides

The concentration levels and distribution features of the platinum group elements (PGE) in quartz-sulfide and base-metal ores in deposits of the Sayan-Baikal Fold Region (SBFR) are discussed. Microfire assay neutron activation analysis (MF-NAA), which enables one to work on a nondestructive basis and allows one to avoid inaccuracies related to chemical sample preparation, was used as the main analytical technique. Three types of hydrothermal mineralization with elevated grades of PGE (especially Pt, Pd, and Ru) have been identified: (1) pyrite-pyrrhotite (massive sulfide) mineralization hosted in black shales of the Il’chir Sequence; (2) gold-sulfide ores of the Zun-Kholba, Tainsky, Kamenny, and some other gold deposits; and (3) silver-basemetal ores of the Dzhida-Vitim Zone. The PGE contents significantly vary, from global average values to tens of grams per ton. An absence of PGE minerals implies that these elements are finely dispersed in sulfide minerals and native gold. Taking into account difficulties in conversion of PGE into analytical forms, their nonuniform distribution in sulfide minerals, their high affinity to coordination compounds, and experimental results, cluster species of Pt and Pd in major minerals are suggested for the gold-sulfide and silver-base-metal ores in deposits, which are related to suprasubduction ophiolites and island-arc and intraplate settings in the SBFR.     

Wall-rock metasomatism of carbonaceous terrigenous rocks in the Lena gold district

The Lena gold district is situated in the fold-and-shear belt of the southern framework of the Siberian Platform. The gold deposits are hosted in the Riphean-Vendian Khomolkho and Aunakit formations, revealing the strict control of ore mineralization by folding and shearing. The microstructure of metasomatically altered ore-bearing carbonaceous sedimentary rocks at the Sukhoi Log, Golets Vysochaishy, and Verninsky deposits (the latter includes the Pervenets vein zone) testifies to parallelism in the development of shearing, foliation, and ore-forming metasomatism. The local pressure gradients are marked by removal of silica from pressured zones into opened cleavage fractures and pockets. Two metasomatic stages are recognized: (1) early sodic metasomatism, which is characterized by the assemblage of magnesian siderite and paragonite, and (2) late potassic metasomatism, with formation of muscovite in association with sideroplesite and ankerite. The rocks altered at the early stage are distinguished by elevated Ni, Cr, and probably PGE contents. The second stage, close in age to the emplacement of Hercynian granitic plutons, was accompanied by the gain of chalcophile metals and deposition of the bulk of gold. In mineral composition, the metasomatic rocks are close to beresites, but the alteration differed in somewhat elevated alkalinity, so that microveinlets of albite and potassium feldspar occur in the ore zone together with muscovite. The ratio of modal muscovite to paragonite contents in orebodies is substantially higher than in the surrounding metasomatized rocks. This ratio directly depends on the degree of rock permeability and the intensity of the flow of ore-forming solutions. Carbonaceous matter (CM) in the ore zone underwent reworking and redeposition. CM is graphitized to a lesser extent than in the rocks affected by regional metamorphism. The spatial distribution of CM containing nitro and amino groups indicates more oxidizing conditions in the zone of ore deposition than at a distance from this zone. The temperature of metasomatic processes estimated from the muscovite, muscovite-paragonite, and chlorite mineral thermometers and fluid inclusions in quartz was 300–350°C at a pressure of about 1 kbar. The S, O, and C isotopic compositions of ore-forming fluids that pertain to the second stage of metasomatism (δ³⁴S= +8.5‰, δ¹⁸O = +10‰, and δ¹³C= ?11 to ?18‰) indicate their crustal origin. The generally similar conditions and products of the ore-forming metasomatic process at the giant Sukhoi Log deposit and at the small Golets Vysochaishy deposit are combined with some differences. The formation of the described deposits was related to the deep convection of fluids along shear zones followed by more local flows of postmagmatic solutions derived from the emplaced granitic magma.     

Low-temperature hydrothermal deposits in the rift zone of the Mid-Atlantic Ridge

New data on the low-temperature hydrothermal deposits of the Snake Pit, TAG, Broken Spur, and Lucky Strike fields in the rift valley of the Mid-Atlantic Ridge (MAR) obtained during the 47th cruise of the R/V Akademik Mstislav Keldysh are reported. These deposits are related to focused and diffuse flows of hydrothermal solutions and the material precipitated from hydrothermal plumes. Electron microscopy study allowed us to identify protoferrihydrite, ferrihydrite, goethite, hematite, Mn-feroxyhyte, and Fe-vernadite. Fe-free vernadite, akaganeite, opal, nontronite, lepidocrocite, and jarosite occur in subordinate amounts. Bacteria-like forms made up of protoferrihydrite particles replaced with ferrihydrite, nontronite, and goethite along with particles of Fe-free vernadite and opal were found. The contents of major, minor, and noble metals were determined in samples with AAS, INAA, and the direct chromatographic method. Elevated PGE contents were established in the low-temperature hydrothermal deposits of the MAR for the first time. The highest PGE contents were detected in the inner zone of Fe-Mn crusts on the surface of a hydrothermal chimney in the Snake Pit field ((ppm) 0.13 Pd, 0.12 Rh, and 0.03 Pt, along with 16.5 Au, 642 Ag, and 355 Se) and in the Au-depleted crust on the surface of a sulfide ore fragment in the Lucky Strike field (0.12 ppm Pd and 0.04 ppm Rh along with 0.03 ppm Au, 683 ppm Se, and 1.5 wt % Ba).     

Typomorphism and parasteresis of Yushkinite, the Pai-Khoi anticlinorium

Yushkinite found in quartz-calcite hydrothermal veins in the Pai-Khoi Anticlunorium (the middle reaches of the Silova-Yakha River) is associated with fluorite, sphalerite, and sulvanite and occurs as fine-lamellar aggregates. The mineral is pinkish purple, with perfect cleavage parallel to (0001). The Moh hardness is lower than 1. In reflected light, yushkinite is anisotropic, with strong bireflectance. It is uniaxial and optically negative. Yushkinite was discovered and approved by the Commission on New Minerals and Mineral Names, International Mineralogical Association, in 1983. Twenty years later, reexamination of yushkinite and associated minerals gave rise to the discovery of a new carbonate phase and specification of the physical properties and chemical composition of yushkinite.     

Dynamical evolution of stellar orbits in close binary systems with conservative mass transfer

We have found that the well-known rule for the distance between the stars to increase or decrease during conservative mass transfer in close binary systems is not valid. We use the equations of Mestschersky to study the evolution of the relative orbits of the stars.