Vol 8, No 4 (2022)
Minerals and mineral assemblages
To the ontogenesis of aeschynite in feldspar vein no. 31 of the Vishnevye Gory, South Urals
Abstract
Small grains of accessory aeschynite in feldspar vein no. 31 of Vishnevye Mountains (South Urals) co-crystallize with late growth zones of feldspar crystals, as well as with ilmenite, titanomagnetite, muscovite, zircon, monazite, thorite, rutile, columbite and other minerals. Crystallization of aeschynite was followed by its metamict decay and formation of a nanocrystalline structure and further repeated dissolution and metasomatosis along microfractures of the mineral aggregate. Metasomatosis of aeschynite was accompanied by the formation of microporosity in muscovite-ilmenite aggregates at the contact with aeschynite grains.



Mineralogy of chromitites of Mount Poklonnaya of the Karabash massif, South Urals
Abstract
Mineral composition of chromitites and host serpentinites from a quarry at Mt. Poklonnaya of the Karabash serpentinite massif is studied using optical and electron microscopy. In addition to previously known native osmium and laurite, eight minerals of platinum group elements (PGE) are found: isoferroplatinum, irarsite, iridium, naldrettite, cuproiridisite, sperrilite, tolovkite and erlichmannite. It is found that magnesioalumochromite is the early magmatic Cr-spinel and magnesiochromite and ferrichromite are late magmatic. The earliest native iridium and native osmium are replaced by PGE sulfides, arsenides and stibnides. Magnesiochromite is associated with native gold, Ni chalcohenides (gersdorffite, millerite, pentlandite, heaslewoodite) and chalcopyrite. The formation of Cr-magnetite, magnetite, native iron, native nickel, galena and barite is related to serpentinization. Carbonates (calcite and dolomite), brucite, andradite, sepiolite and an unidentified Ca-silicate formed at the latest stage of serpentinization. Secondary Ni minerals (gaspeite, nepuite, «garnierite») are most likely the products of the latest mineral-forming process.



Magnetite-ilmenite-spinel intergrowths in amphibole gabbro of the Serebryansky Kamen intrusion, Central Urals
Abstract
Ontogenic analysis of magnetite-ilmenite-spinel intergrowths of ore-bearing amphibole gabbro of the Serebryansky Kamen intrusion (Central Urals) and their relationships with rock-forming silicates, apatite and Cu sulfides allowed us to reveal a number of features indicating their nearly synchronous formation. Morphological features of minerals of ore-bearing amphibole gabbro suggest their possible crystallization from silicate, iron oxide and sulfide melts under close-to-eutectic conditions. It is found that regularly oriented ilmenite lamellas in magnetite, which are typically interpreted as exsolution structures, often intersect the boundaries of variously oriented magnetite grains, which is inconsistent with traditional interpretation of their origin. The ontogenetic features of minerals should be taken into account in petrological reconstructions.



Garnet mineralization associated with late magmatic and hydrothermal-metasomatic processes in dunites of the Nizhny Tagil clinopyroxenite-dunite massif, Central Urals
Abstract
The miarolitic dunites of the Nizhny Tagil massif (Central Urals), the structure of miaroles and the principles of co-occurrence of minerals are studied. Garnets from miaroles are characterized in details and are compared with those from chromitites and metasomatites after dunites. The structure of miarolitic dunites and miaroles is described using ontogenic approaches. The structure and composition of garnets are studied by SEM and LA-ICP-MS. Three assemblages of garnets are established in rocks of the Nizhny Tagil massif: Cr-andradite in miarolitic dunites, uvarovite in massive veined chromitites and demantoid in metasomatites after dunites. Garnets from all assemblages belong to the ugrandite group and are characterized by specific features of chemical composition including higher Cr2O3 content. Garnets are also characterized by contrasting concentrations of trace elements, such as V, Mn and Ti, as well as differences in REE pattern. A paragenetic sequence of formation of major rock-forming minerals is suggested for dunites, chromites and metasomatites after dunites of the Nizhny Tagil massif.



Noble metal mineralization in gabbroids of the Kumba intrusive, the Uralian platinum belt (North Urals)
Abstract
The paper presents the first data on the distribution and composition of copper-noble metal mineralization in gabbroids of the Kumba intrusive (North Urals). The noble metal mineralization is associated with digenite-bornite, bornite-chalcopyrite, and pyrite-chalcopyrite ores and mainly occur minerals of precious metals. Nine noble metal mineral species are found for the first time in amphibole and amphibole-olivine gabbro of the Kumba intrusive: native gold, Au-Ag alloys, Au, Ag, and Pd tellurides (hessite, merenskiite), Bi tellurides (kotulskite), antimonide-arsenides (isomertieite), arsenides (arsenopalladinite, sperrilite), and stannides (atokite) of Pt and Pd. Noble metal minerals from all sulfide assemblages in heavy concentrates are often accompanied by antimonides (stibnite) and Bi mineralization represented mainly by native bismuth and bismuthinite and less common sulfotellurides (baksanite) and tellurides (tsumoite). Our results make it possible to estimate the prospects of the discovery of new copper-noble metal deposits hosted in gabbro of the Uralian Platinum Belt. Taking into account the principles of occurrence of noble metal and copper mineralization, most gabbro intrusives of the Uralian Platinum Belt can be considered the promising objects for large-tonnage copper deposits with associated ore Au and Pd grades.



Gold potential of the upper devonian basal conglomerates of the Kanin Peninsula (Nenets autonomous district, Russia)
Abstract
Native gold and garnets are described for the first time in heavy concentrates from coarse-grained sediments of the Upper Devonian Tayaokuyakha Formation, which occurs on the eroded Upper Riphean schists with angular and azimuthal unconformity (middle reaches of the Murseyakha River, Kanin Peninsula, Nenets autonomous district, Russia). Particular attention was paid to indicators of primary source of native gold for the Upper Devonian conglomerates. The composition of weakly rounded almandine-spessartine garnets indicates their metamorphic origin and allows us to relate the primary source of native gold in conglomerates (D3tk) with underlying metamorphosed Riphean strata or intrusive rocks in them. Native gold is mainly characterized by a heterogeneous composition and contains Ag, Cu and Hg. The poorly rounded native gold grains, the absence of supergene rims and preservation of mineral inclusions in the margins indicate its proximal primary source. The presence of Cu suggests a link of native gold with mafic/ultramafic rocks: metamorphosed dikes of dolerites of the Nekhaiteyakha metagabbroid complex (mνβRF3) or dolerite dikes of the Kanin-Timan complex (βD3kt), which intrude the Precambrian metamorphic schists. It is established that the Devonian conglomerates are intermediate reservoirs for the possible present-day placer formation.


