Volume 61, Nº 5 (2019)

Integrated structural geological, minerogenic, and metallogenic studies with allowance for previous topical surveying, geological mapping and prospecting, and mineral exploration data has revealed that Upper Paleozoic and Early Mesozoic tectonomagmatic structures are widespread in the Selenga ore district. They are associated with the evolution of the transregional Upper Paleozoic Selenga–Vitim rift-related volcanoplutonic belt and the formation of the Early Mesozoic West Transbaikalian region of intraplate magmatism. Late Paleozoic–Mesozoic igneous activity accounts for the bulk of mineable mineral resources in the Selenga ore district concentrated inside and outside the ore clusters (Kunalei, Kizhinga, Cheremshana–Oshurkovo, Tashir, etc.). It is demonstrated that the main mineable metals in the district are molybdenum and beryllium, which determine the minerogenic specificity of the ore district. New compositional characteristics of the Upper Paleozoic and Early Mesozoic intraplate magmatic complexes and the associated mineral deposits (Mo, Be, Ti, quartz, fluorite, and apatite ores), as well as other promising gold, uranium, and REE–Ba–Sr ore occurrences were obtained. The geodynamic settings of their formation and the ages of the main ore-forming processes have been established; the viability of the mining industry in the Selenga ore district and the feasibility of involving its ore potential in the district’s economic modernization program have been assessed.

“Invisible” gold in hydrothermal ores is frequently scattered in the most abundant minerals of the Fe–As–S system. It is assumed that “invisible” gold does not incorporate into the mineral structure (nanoscale inclusions of the metal or its compounds) or is chemically bound (isomorphous substitution). The aim of this study is to determine the concentration range of “invisible” gold, species of its occurrence in arsenopyrite, and conditions facilitating the formation of gold-bearing arsenopyrite using synthetic crystals and natural samples from the Vorontsovka Carlin-type deposit in the Northern Urals. Arsenopyrite crystals have been synthesized using the ampoule method in a eutectic melt of alkali metal chlorides and Al at a stationary thermal gradient and 400–500°C at the cold ampoule end. The chemical composition of arsenopyrite has been measured by electron probe microanalysis. The chemical composition of synthesized arsenopyrite is, at %: 32.6–34.4 Fe, 30–36.5 As, and 29.4–36.0 As. The gold concentration ranges from below the detection limit (<45 ppm) to 3 wt %. The obtained chemical data for synthetic crystals are compared with theoretical trends calculated for various gold species. It has been established that the slope of the trends of the average arsenopyrite compositions is very close to that of the theoretical line of isomorphous substitution Au ↔ Fe. It allows the assumption that the isomorphous solid solution in which Au occupies the Fe site formed during experiment. In general, all our data on synthetic and natural arsenopyrite show a strong negative correlation between Au and Fe, which supports the formation of the solid solution with Au at the cation site. In addition, a weak positive correlation between Au and As is observed: the higher As concentration is characteristic of As-rich (As/S > 1 at %) arsenopyrite and is close to stoichiometry, whereas in the S-rich variety, the Au content is as low as 0.25 wt %. This dependence is not only within individual grains, but also at the deposit in general: later As-rich arsenopyrite formed at lower temperature and sulfur fugacity (T = 250–370°C, log f  S₂ = –12 to –17) is enriched in Au compared to early arsenopyrite (T = 270–400°C, log f  S₂ = –7 to –9). Comparison of our data with the literature shows that an increasing Au content with increasing As concentration and decreasing Fe content in arsenopyrite is a common feature of Carlin-type deposits. We believe that in contrast to the negative correlation between Au and Fe, the correlation between Au and As is not obvious and may be caused by external factors, different composition of hydrothermal fluids, and sulfur fugacity.

The Chernogorskoe gem-quality scapolite deposit, which is attributed to a new genetic type, is described. It is located in the Central Pamirs (Tajikistan), in the Sary-Dzhilga Formation of the Muzkol Group (PR₁). Host rocks include metamorphosed mafic–ultramafic formations of the Kukurt complex, among which harzburgite was the first discovered. Albitite lenses host gem-quality scapolite mineralization. They were formed by the metasomatic replacement of nepheline syenite. This reaction was accompanied by a decrease in the volume of solid phases by 14% and the appearance of voids. Albitite bodies form a lenticular nesting structure, have crosscutting contacts, and make up amphibolite, which metasomatically replaces melanocratic gabbroid and partially altered harzburgite. Gem-quality scapolite is localized in 28 albitite lenses; the walls of their voids are encrusted with columnar purple scapolite. The sequence of processes at the studied deposit is as follows: formation of melanocratic gabbroids with small nepheline syenite bodies → metasomatic development of amphibolite after the former and albitite with voids after the latter → growth of columnar scapolite on voids walls → crystallization of gem-quality scapolite in voids.