Volume 66, Nº 6 (2024)

A thermodynamic description of gold-bearing pyrite as a substitution solid solution is proposed depending on external conditions and gold content in the fluid. The model is based on experimental data of the synthetic pyrite grown in hydrothermal chloride solutions in the presence of metallic gold. The proposed model makes it possible to estimate the upper limit of gold content in pyrite. The thermodynamic model of gold-bearing pyrite was verified by modeling the process of formation of mineral associations of gold ores of the Darasun and Talatui deposits by chloride aqueous fluids in an integrated fluid-magmatic system of the Darasun ore field. Previously it was revealed that the average gold content in pyrite from the Darasun and Talatui deposits, intergrown with native gold, does not exceed 1 ppm, while synthetic pyrite grown in equilibrium with metallic gold under similar conditions contains 10–150 ppm of gold. This contradiction does not allow explaining the formation of gold ores of the Darasun ore field by its direct crystallization from hydrothermal fluid. A possible explanation is the presence of an additional stage of ore transformation, in which recrystallization of sulfides occurred with gold undersaturated solutions. It was shown that a decrease in gold concentration in the mineral-forming fluid below the saturation limit should lead to a synchronous decrease in the gold concentration in the resulting pyrite. The calculated values of gold concentration in model pyrite make it possible to estimate gold concentrations in mineral-forming chloride fluids at different stages of the formation of the Darasun ore field deposits at a known temperature. It has been shown that natural gold-bearing pyrite from various gold deposits was formed mainly from chloride fluids undersaturated to gold.

The Voimakan deposit of dolomite type nephrite has been investigated in order to clarify the features of its formation. 12 samples of nephrite and 5 samples of host rocks were studied. A binocular stereomicroscope, a gemological flashlight and a polarizing petrographic microscope were used. The contents of macro- and micro-components, the isotopic composition of oxygen were determined. Nephrite is light salad, salad, gray-salad and brown (honey). It forms separations in calcite-tremolite skarn bodies at the contact of dolomite marble and amphibolite transformed into epidote-tremolite skarn. The value of δ ¹⁸ O of nephrite is –18.5 ÷ –18.8%; calcite-tremolite skarn –17.4%; epidote-tremolite scarn –4.4, 2.6%; dolomite 26.1%. Nephrite meets the requirements for gemstone raw materials. Diopsidite with nephrite lenses and interlayers can be used for carving multicolored products or inlays. The green shade of nephrite increases with an increase in the Fe ²⁺ content. The brown color of nephrite is determined by Fe ³⁺ in the tremolite structure. The dolomite type of nephrite is confirmed by the ratio of Mg and Fe, a reduced content of Cr, Ni, Co, an increased content of F and the ratio of Sr to Ba, and the nature of the REE distribution. The distribution of REE in nephrite is determined by the composition of the initial dolomite under the influence of epidote-tremolite scarn. The source of abnormally isotopically light oxygen of nephrite is a meteoric fluid depleted in ¹⁸ O as a result of dolomite decarbonation. Granite only provides regional heating, activating the fluid. Both metasomatic and metamorphic processes were involved in the nephrite formation and transformation. The formation of nephrite is associated with the formation of calcite-tremolite and epidote-tremolite skarns. Tectonic stresses caused the crushing of rocks, facilitating the penetration of fluid, provided the formation of a nephrite cryptocrystalline tangled fibrous structure. But further regressive metamorphism led to the development of chlorite and talc, which worsened the nephrite quality.

A detailed description of the new noble metal (Pt-Au-Pd) Vasilinovskoe occurrence discovered near the Kharp town of the Yamalo–Nenets Autonomous Okrug is given. It is associated with amphibolized gabbroids and clinopyroxenites. Mineralization zones with an apparent thickness from 0.5 to 50 m (sulfides 1–3 vol.%, occasionally more) are developed in these rocks. In areas with scattered or finely nested sulfide inclusions, feldspar-quartz and epidote veinlets are often present. The first expedition to study the platinum-bearing area of the Rai-Iz mountains was organized by Professor A.N. Zavaritsky 100 years ago, in 1925 (A.G. Betekhtin was the head of the рrospecting party), but the expected placer platinum deposits were not found. Communication 1 includes general geological and detailed mineralogical characteristics of the ore occurrence and brief information on the bulk geochemistry of rocks and ores of the object. The mineralized gabbro-amphibolite ore macrocomponents, which are often found in bulk samples, can be noted (wt.%) V up to 0.2, Co up to 0.06 and Ni up to 0.02. According to assay data, in bulk 0.5–1 kg samples with sulfide inclusions, the Pd content reaches 1.4 g/t, Au – 0.8 g/t, and Pt – 0.2 g/t. The PGE minerals are represented by abundant impregnation of micron–sized palladium minerals: tellurides (merenskiite, temagamite, kotulskite, sopcheite), antimonides (stibiopalladinite, sadberite), arsenoantimonides (arsenic stibiopalladinite, isomertiite), as well as other noble metal compounds – moncheite, native osmium and others. In addition, the magnetite–chalcopyrite–pyrite assemblage contains microinclusions of native silver, bismuth and tin. In the later polysulfide–feldspar–carbonate–quartz assemblage, Au and Ag tellurides, native gold (including Hg-bearing), Se-containing argentite, and greenockite are found. In the zones of sulfide impregnation of the Podgornensky site (1.5 km to the south), occurring in the diorites of the Sob’ complex and closely associated with quartz veins, the amount of sulfides is higher, the copper profile is enhanced, and concentrations of Co, Ni, and especially Ti, V, Pd and Pt are falling. According to the LA-ICP-MS analyses of pyrite, chalcopyrite, pyrrhotite of the Vasilinovskoe occurrence, the profiling trace element for them is cobalt – up to 1.2 wt.% Co in pyrite of the early assemblage. The Ni impurity is also high (400-800 ppm, up to 0.2 wt.%) in the early pyrite and decreases to 16–90 ppm in the late pyrite. The impurity of Se, on the contrary, increases in pyrite of the late assemblage (up to 207 ppm). Chalcopyrite commonly contains As and Se (~100–300 ppm). In contrast to the Vasilinovskoe occurrence, at the Podgornensky site, an admixture of Mo (up to microinclusions of molybdenum), Te (up to 35 ppm), noticeable impurities Tl (up to 25 ppm) and Re (0.3 ppm) are present in pyrite. Impurities are often found in chalcopyrite: Ag up to 65 ppm, Sn up to 65 ppm, Cd up to 35 ppm and Bi up to 11 ppm. Significant impurities of Co and Ni (up to 0.n wt%) are typical here only for minor pyrrhotite. According to the mineral composition and geochemical spectrum of Pt-Au-Pd-Co ±Ni-Cu-V-Ti, the low-sulfide platinoid mineralization of the Vasilinovskoe occurrence contrasts quite strongly with the zones of low-sulfide mineralization (+chalcedony quartz) with the specialization Fe-Cu-Au-Ag (±W, Bi, Sn, Mo, Re) of the Podgornensky site, which probably belong to the skarn-porphyry gold-bearing system. The conclusion is made about the prospects of expanding the contours of Pd mineralization to the west and east, where the halos of Cu, Co and Ni, as well as magnetic anomalies, occur in the rocks of the basite-ultrabasite association.