Vol CLIII, No 3 (2024)
ARTICLES
Composition of micas from rocks and melt inclusions in quartz of Li-F granites of the Orlovka massif in Eastern Transbaikalia as a genetic indicator
Abstract
Based on the study of the chemical composition of rock-forming micas and micas from melt inclusions in quartz of a full range of differentiates of the Li-F granite Orlovka massif in Eastern Transbaikalia, possible mechanisms of formation of the massif are considered. An early stage with a mica evolution trend in rocks (biotite — Li-rich aluminous annite — Li-rich phengite-muscovite), manifested in the synchronous accumulation of Li and F in the melt, mica from the rock and melt inclusions (an ongonite trend of the melt evolution), culminated in the formation of specific porphyroblast microcline-albite granites with Li-rich phengite-muscovite and snow-ball quartz. It was this melt that served as the basis for all its subsequent transformations (repeated manifestation of silicate-salt liquid immiscibility, post-magmatic metasomatism), which determined the development of the “apogranite process”. The crystallization of exclusively lithium-free high-alumininous muscovite in the melt inclusions of microcline-albite granites and the subsequent series of amazonite-bearing rocks with a high content of Li and F in the homogenized glass of these inclusions allows us to assume the crystallization of this mineral from the depleted melt, coexisting with the isolated Li-F hydrosalt phase. The obtained results indicate the convergence of the mechanism of formation of Li-Fe micas, which allows the probability of their crystallization from a fluid-saturated melt and as a result of metasomatic reworking at the different stages of formation of the Orlovka massif.
3-28
REE geochemistry and Sr isotopic composition of silicate-carbonate rocks from the Zelentsovskaya mine (Kusa-Kopan intrusive complex, Southern Urals)
Abstract
The object of this study is silicate-carbonate rocks located in gabbroid of the Kusinsko-Kopan intrusive complex within the Zelentsovskaya mine in the South Urals. Low Mn and Fe contents and low 87Sr/86Sr ratio in calcite (0.7045–0.7048) indicate that the source of carbonate matter for these rocks was recrystallized limestones of the Lower Riphean Satka Formation. Isotopic-geochemical data (87Sr/86Sr ratio and flat REE distribution pattern) indicate the influence of gabbroid melt on their composition. It is suggested that the silicate-carbonate rocks were formed as a result of the geological process closest to the skarn formation.
29-41
Problems of dating the Au-Pd mineralization of the Chudnoye deposit (the Subpolar Urals)
Abstract
The ages values of the minerals that make up the ores of the Chudnoe deposit, established using isotope geochronology methods, do not allow us to correctly estimate the time of formation of Au-Pd mineralization due to the lack of reliable signs of their syngeneticity with gold. Based on linking isotope-geochronological data with the history of the geological development of the Subpolar Urals and the entire complex of endogenous and exogenous processes which determined the features of its geological structure and minerageny, the Early Paleozoic age of mineralization is substantiated. The similar age of the metarhyolites hosting the mineralization suggests not only a spatial, but also a paragenetic relations between ore mineralization and felsic volcanites. When studying the mineral composition and structural and textural features of ores, it is necessary to take into account the imposition of post-ore processes, especially the most powerful metamorphism manifested in the Subpolar Urals, the peak of which occurred in the Upper Paleozoic (~250 Ma).
42-63
Moganite in agates from shungite rocks of the Paleoproterozoic Onega basin (Karelia)
Abstract
The monocline phase of silica – moganite for the first time has been found in carbon-rich sedimentary rocks (shungites) of Zaonega formation within the Paleoproterozoic Onega basin. Moganite forms intergrowths with fibrous chalcedony in concentric layers of agates, which are presumably associated with pseudomorphic replacement of carbonate nodules. According to local Raman spectroscopy analyses, content of moganite within individual chalcedony bands is not homogenous and varies in the range of 22–50%. The estimated values of moganite content are very high for agates formed in the Paleoproterozoic rocks aged 2050 Ma and indicate a large temporary gap between formation of the sedimentary sequence and the hydrothermal process that initiated agate mineralization.
64-75
Variations in chemical compositions of titanite group minerals from ore skarnes in the Ladoga Lake Region (South Karelia, Russia)
Abstract
Titanite, aluminium- and fluorine-enriched titanite, tin-bearing titanite and malayaite from ore skarns in the Ladoga Lake region were studied. Composition of these minerals from skarns with W-Zn-Pb-Bi (Latvasyrja, Jokiranta) and Sn-Zn-Cu-Fe-In (Pitkäranta Mining District) mineralization, related genetically to S-type and А-type granites, was analyzed. For the first time for ore deposits and occurrences in Karelia, there was detected titanite enriched in aluminum (Al2O3 5—7 wt%) and fluorine (~3 %). Isomorphic substitutions in titanite from skarns with different metallogenic specialization were considered. It is shown that the following isomorphic schemes are realized for studied titanite: (Al, Fe)3+ + F– ↔ Ti4+ + O2–; (Al, Fe)3+ + (OH)– ↔ Ti4+ + O2–, where Al ≥ Fe (skarns with W-Zn-Pb-Bi mineralization); and Sn4+ ↔ Ti4+ (skarns with Sn-Cu-Fe-Zn-In mineralization). The Sn-bearing titanite from Sn-bearing skarns nearly in all cases contains Fe, what it seems due to the high Fe# in rapakivi granites (containing biotite and other mafic minerals with Fe# >0.9) and the associated post-magmatic mineralization (columbite-(Fe), synchysite-(Fe), marmatite). The formation of titanite enriched in aluminum and fluorine was controlled by protolith and fluid compositions rather than temperature and pressure (≤500 ◦C, ≤5 kbar). Crystallization of this titanite in Jokiranta ore occurrences took place during a post-ore-forming process, potentially capable to the remobilization of base-metals ores.
76-96
Varieties of fossil resins of South Sakhalin and their molecular composition
Abstract
This article provides the first up-to-date overview of the history of fossil resin finds in the southern part of Sakhalin Island. On the base of little-known facts and casual references scattered in geological literature, main locations of the amber-like fossil resins are reviewed and described. Peculiarities of their molecular composition were studied, and, for the first time, it has been done for resins from Moneron Island. The assumption of S. S. Savkevich about mineralogical non-identity of resins from the coast of the Sea of Okhotsk and the Naiba river basin was analytically confirmed. It is shown that the resins are represented not only by a ductile variety — rumenite, but also by brittle ones — gedanite and retinite, which can’t be used as jewelry raw materials.
97-112