Vol 55, No 2 (2017)

The contents and speciation of nitrogen, carbon, and hydrogen were determined in basalt–basaltic andesite melts in equilibrium with liquid Fe alloys at 1.5 Gpa, 1400°C, and oxygen fugacity (fO₂) 1.4–1.9 log units below that of the Fe–FeO buffer (ΔlogfO₂(IW) =–1.4 …–1.9). Experiments were carried out on a piston- cylinder type apparatus using welded Pt capsules in the presence of excess С (graphite). Starting mixture consisted of natural ferrobasaltic glass and silicon nitride (Si₃N₄) as nitrogen source in the system. Experimental quench products representing glasses with spherical inclusions of iron alloy were analyzed using electron microprobe, Raman, and IR spectroscopy. With increase of Si₃N₄ in the starting mixture and, respectively, decrease of fO₂, silicate melt forming during experiments became depleted in FeO and enriched in SiO₂. It was established that the nitrogen content in the glasses increases from 0.13 to 0.44 wt % with decrease of ΔlogfO₂(IW) from–1.4 to–1.9, whereas C content in the first approximation remains constant within 1.18–1.13 wt %, while the total water content (ОН^– + Н₂О) determined by IR spectroscopy decreases from 4.91 to 1.20 wt %. The N (0.13–0.48 wt %) and C (0.75–2.26 wt %) contents determined in the Fe alloy show no clear correlation with fO₂. The IR and Raman spectroscopic study of the glasses indicates the formation of molecules and complexes with bonds N–H (NH₃, NH₂⁻, NH₂⁺, NH₄⁺), Н–О (Н₂О, OH^–), С–Н (СН₄) as well as N₂ and Н₂ molecules in silicate melts. IR spectra also reveal the presence of complexes with С=О, С–N bonds and СО₂ molecules. Obtained data are compared with results of previous studies on the solubility and speciation of N, С, and Н in the model FeO–Na₂O–SiO₂–Al₂O₃ melts in equilibrium with liquid iron alloys at 1.5 GPa (1400°C) and 4 GPa (1550°C) (Kadik et al., 2011, 2015).

This study provides geochemical, mineralogical, and isotope data for rocks and ores from Lower Proterozoic black shale formations of the Kodar–Udokan structural and formational zone, which host the Khadatkanda gold—uranium deposit. The results indicate that the uranium and gold mineralizations were formed at different times in relation to different geodynamic settings. The gold mineralization is associated with the inception of the Syulban fault and has a juvenile source. The later Th–U mineralization originated during tectonic rejuvenation of the Syulban fault zone, while the sources of radioactive elements were presumably the underlying sediments of the Kodar Group, which are widespread throughout the area of the Baikal mountain region (BMR). Based on the above results, the Au–U mineralization in the study area can be recognized as unconformity-type deposits, analogous to the well-known deposits of Australia and Canada. In this connection, the Baikal mountain region has a good potential for the discovery of Au—U deposits.

The analysis of published data and our experimental results showed that the existing estimates of gypsum solubility in water (C_m^↑) at 25°C range from 0.0147 to 0.0182 M. Such a scatter (more than 20%) is the result of a combined influence of experimental conditions and ability of gypsum to form supersaturated solutions. This influence appears most noticeable during gypsum dissolution in a dispersed state. Under such conditions, equilibrium is determined with a precision of no better than ±4.5%; if a flat surface similar to natural one is dissolved, the (C_m^↑) value is characterized by the minimum scatter (±1%). According to experimental data (25°C), the solubility of gypsum particles in water is inversely proportional to their size at r < 1 μm and is r-invariant for larger grains. The invariance of (C_m^↑) at r > 1–5 μm is supported by the data of analytical calculations using the approximate Ostwald–Freundlich equation. It is supposed that the difference of the concentrations of dissolved gypsum at the boundaries of the field of metastable state of gypsum-saturated groundwater can be 1–5%. The results of our study can be used for the description of gypsum dissolution in the models of mass transfer between gypsum-bearing rocks and groundwater.

The paper reports data on concentrations of lanthanides in rocks, fluorite, and wolframite from the Spokoininskoe greisen deposit in eastern Transbaikalia. Lanthanide concentrations in the ore-forming fluid are calculated using mineral/fluid distribution coefficients. The data on REE are consistent under the assumption of a single, but evolving, genetic source. The REE concentrations are similar for rocks variably affected by greisenization and are controlled by the solubility of monazite contained in the granite. The younger quartz–albite–muscovite veins and segregations contain elevated concentrations of LREE, which is explained by an increase in the monazite solubility in alkaline solutions during the late evolution of the fluid.

Using the method of direct synthesis calorimetry, we determined the standard enthalpy of formation of PtSb (stumpflite), Δ_fH°_298.15 (PtSb, cr) =–105.16 ± 0.84 kJ/mol and PdSb₂ (geversite), Δ_fH°_298.15 (PtSb₂,cr) =–160.92 ± 0.84 kJ/mol. Isothermal (298.15 K, p = 1 bar) phase diagrams were computed for the Pt–Sb–S and Pt–Sb–O ternary systems in the coordinates composition of the Pt–Sb binary system versus fugacity of a gaseous volatile component (O₂, S₂).