


Vol 67, No 2 (2025)
Articles
Vasilinovskoe platinum-palladium occurrence – a new type of mineralization in the ophiolites of the Polar Urals. Communication 2. Metamorphism, PTX parameters and sources of matter
Abstract
In this paper, ore-containing amphibolized gabbroids from the new low–sulfide ore occurrence of noble metals (Pt–Au-Pd) Vasilinovskoe, located near the Kharp town in the Yamalo-Nenets Autonomous okrug, are studied in detail. These rocks presumably belong to the Kershorsky complex, which is dated mainly to the Late Ordovician. Due to metamorphism, up to the appearance of amphibolites, a number of generations of amphiboles and plagioclases occurred; then epidote, chlorite, and some other minerals crystallized. Peak PT parameters probably reached ~6 kbar and ~700 °C; for later paragenesis, ~4 kbar and ~650 °C, decreasing to 1 kbar and ~550 °C, thus decompression occurred. For the late low-temperature transformations, chlorite geothermometer estimations reveal t = 275–100 °C. Mineralization zones are developed in these rocks (from the first cm to 50 m, sulfides 1–3 vol.%). The platinum group elements in them are represented by micron–sized palladium minerals – tellurides (merenskiite, temagamite, kotulskite, sopcheite), antimonides (stibiopalladinite, sadberite) and arsenoantimonides (arsenic stibiopalladinite, isomerthite), as well as others – moncheite, native osmium and others. The formation of noble-metal parageneses was associated with late-magmatic processes, as well as with subsequent redistribution by magmatogenic hydrothermal fluids, at temperature decreasing down to ~250 °C; pressure decreased from ~0.9–1.3 to ~0.4–0.5 kbar. The sulfur isotopic composition δ34S (‰) in pyrite varies from -4.2 to +6.3, in chalcopyrite from –1.6 to +4.2. In pyrite of southern flank of the occurrence, δ34S = –2.02 ... +2.72 %; in chalcopyrite, δ34S = –1.74 ... +0.29. According to the Pb-Th-U isotopic parameters and the sulfur isotopic composition of sulfides, the sources of the studied low-sulfide mineralization are similar to mantle-type sources.



“Invisible” gold and other impurity elements in pyrite and arsenopyrite from the Mayskoye deposit (Chukotka)
Abstract
The gold-bearing sulphides (pyrite and arsenopyrite) from disseminated refractory ores of the Mayskoe gold deposit (Central Chukotka) were studied using modern precision methods (electron microprobe analysis and laser ablation-inductively coupled plasma-mass spectrometry). The distribution patterns of macro elements (As, Fe, S), as well as trace elements (Ni, Zn, Sb, Co, Cu, Ag), in pyrite and arsenopyrite, including the content of “invisible” Au and its correlation with other elements were studied. Based on recieved data, the sequence of crystallization and the relationships of gold-bearing sulphides at the main and most productive gold–sulphide stage of the deposite formation were established.



Distribution and speciation features of finely dispersed and “Invisible” gold in arsenopyrite and pyrite of the Natalka Deposit (Northeastern Russia)
Abstract
The distribution and speciation features of finely dispersed and “invisible” gold in the arsenopyrites and pyrites of the Natalka gold deposit (Northeastern Russia) were examined in detail using a “phase” chemical analysis based on atomic absorption spectrometry (PCA-AAS), light microscopy (LM), electron probe microanalysis (EPMA), and atomic absorption spectrometry with analytical data selections for single crystals (AAS-ADSSC). According to the LM and EPMA data, the finely dispersed and invisible forms (< 0.01 mm) amount to 20% of total gold quantity, and mostly enclosed in sulfide minerals, mainly arsenopyrite and pyrite. The main composition (fineness) of finely dispersed inclusions often differs from the composition of coarse gold by a greater fineness: 750–990‰ and 550–850‰, respectively. The PCA-AAS showed that arsenopyrites and pyrites of the Natalka deposit are the gold concentrators with the highest concentrations in the monofractions of arsenopyrite – up to 1383 ppm, lesser in the monofractions of pyrite – up to 158.2 ppm. The AAS-ADSSC method revealed two forms of uniformly distributed “invisible” Au corresponding to the chemically bound element in the structure of the mineral, and in the superficial non-autonomous phase (NAP). The superficially bound form dominates over the structural form and presumably exists in a very thin surface layer of the crystal (~100–500 nm). The occurrences of micromineral forms of native gold in sulfide crystals and on their surfaces give evidence of the transformation of NAP resulting in the formation and oriented aggregation of nano-to-micron-sized Au particles. This can point the way to “invisible” Au extraction under ore processing which allows enhancing the quality and value of primary products.



Composition and formation conditions of sulfide globules in pyroxenites of the Shigir Hills, Middle Urals
Abstract
The Ni-Cu sulfide droplets (globules) firstly has been established in the Precambrian ultramafites of the Shigir Hills in the Western Slope of the Middle Urals. The ultramafites are represented by wehrlites and pyroxenites related with the crystallization of ankaramite melt, which was produced during the rifting in the margin of the East European Platform terms of petrogeochemical signatures, the Shigir pyroxenites are similar to high-Ca ultramafites from Ural-Alaskan-type complexes, which form the Ural Platinum Belt. Sulfide globules consist of a polymineral aggregate of troilite, pyrrhotite, pentlandite, chalcopyrite grains; bornite, cubanite, pyrite, galena, and molybdenite are less common. The compositions of sulfides reflect low-temperature equilibrium below 300 °C. The whole composition of globules corresponds to monosulfide solid solution. It contains about 48–60% of iron, 1–12% nickel, 1–8% copper and less than 1% of cobalt. The formation of sulfide globules reflects the sulfur saturation of melt took place after crystallization of the most volume of olivine and clinopyroxene. Close association of sulfide globules with enstatite, kaersutite, plagioclase and Ti-rich Cr-spinel indicates their formation at temperatures above 900–1000 °C and a pressure of 5 kbar. The values -0.4 – +0.3‰ of δ34S in sulfides reflects the mantle source of sulfur. The presence of sulfide droplets in pyroxenites can be considered as one of the search signs for magmatic sulfide-platinum metal mineralization in the ultramafites of the Shigir complex and expands the potential productivity of Ural-Alaskan-type intrusions to magmatic sulfide mineralization.


