Six minor alluvial chromite placers (Kolkhoznyi Prud, Verkhne-Yaushevo, Sukhoy Izyak, Bazilevo, Novomikhaylovka, Kiryushkino) and one major littoral placer (Sabantuy) were found in sandy sediments of the Kazanian stage of the Permian System (Late Roadian and Wordian Stages) in the Southern Pre-Urals. It is shown that the morphological features of chromian spinels are diverse, which is not evidence of the heterogeneity of the source. The bulk chemical composition of chromian spinels from all placers is similar and generally correlates with compositions of chromian spinels from the Kraka ophiolitic complex in the Southern Urals. The morphological diversity of grains, varied chemical composition and presence of melt inclusions in Ti-high octahedral grains of chromian spinels comply with the ophiolitic nature of the source. Thus, there is no need to refer to other sources for chromite ores but ophiolitic. The new placers expand the dissemination area of chromite-bearing deposits on the east edge of the East-European Platform and offer a prospect to discover new placers.

—In the section of the sedimentary cover of the East European Platform, the new Sabantuy chromite ore occurrence was revealed. It is localized in Upper Kazanian sandstones of the erosional terrace above the floodplain of Malaya Berkutla Creek in the south-west of the Republic of Bashkortostan. The article presents the first results of geological, lithological, and mineralogical–geochemical studies of the new ore occurrence with signatures of an ancient coastal–marine placer. The Cr₂O₃ content in chromite sandstones (chromitolites) is 15.9–17.2 wt %. The bulk of the heavy fraction (67%) is composed of sub-angular and sub-rounded Cr-spinel grains 0.15–0.25 mm in size. Their mineralogical features indicate proximity to Cr-spinels from ultramafic rocks of ophiolite complexes.

A chromite occurrence called the Sabantuy paleoplacer was discovered in the Southern Pre-Ural region, at the east edge of the East-European Platform in the transitional zone to the Ural Foredeep. A ca. 1 m-thick chromite-bearing horizon is traced at a depth of 0.7–1.5 m from the earth’s surface for the area of ca. 15,000 m². The chromspinel content in sandstones reaches 30–35%, maximum values of Cr₂O₃ are 16–17 wt.%. The grain size of detrital chromspinel ranges from 0.15 to 0.25 mm. Subangular octahedral crystals dominate; rounded grains and debris are rare. The composition of detrital chromspinel varies widely and is constrained by the substitution of Al³⁺ and Cr³⁺, Fe²⁺ and Mg²⁺ cations. Chemically, low-Al (Al₂O₃ = 12 wt.%) and high-Cr (Cr₂O₃ = 52–56 wt.%) chromspinel prevail. The compositional analysis using discrimination diagrams showed that most chromites correspond to mantle peridotites of subduction settings. Volcanic rocks could be an additional source for detrital chromites. It is confirmed by compositions of monomineralic, polymineralic and melt inclusions in chromspinels. The presented data indicates that ophiolite peridotites and related chromite ore associated with oceanic and island-arc volcanic rocks, widespread in the Ural orogen, could be the main sources of the detrital chromspinel of the Sabantuy paleoplacer.

Subject research. Morphology and chemical composition of amphiboles from ultramafic-mafic rocks of the Khudolaz complex. Methods. Morphological studies were carried out using optical (Carl Zeiss Axioskop 40A) and electron (Tescan Vega Compact) microscopes. The chemical composition of minerals was determined using CAMECA SX 100 and JEOL JXA-8230 electron probe microanalyzers. Results. The rocks under study are dominated by xenomorphic brown titanium hornblende crystals, formed mainly due to the reaction of clinopyroxene with a residual water-saturated melt at 920–1040°C. Euhedral brown hornblende, which crystallized directly from the residual water-saturated melt in the same temperature range, was found in small amounts. Green hornblende originated along the edges and cracks in brown hornblende crystals during the late magmatic stage and early hydrothermal stage (670–830°C) at the subsolidus transformation. At the hydrothermal stage (620–650°C and below), brown and green hornblende were partially replaced by actinolite and cummingtonite. Conclusions. The nature of changes in the composition of brown hornblende indicates similar petrogenesis conditions at the late magmatic stage in all intrusions of the Khudolaz complex. The process of transition from brown hornblende to green hornblende occurred with a gradual increase in oxygen fugacity (ΔNNO from –0.2…+0.4 to +0.9…+2.5), and was accompanied by decrease of Ti, Fe²⁺, Na and the increase of Si, Al^VI, Mg, and K in the mineral structure. The low concentrations of F and Cl in hornblende crystals indicate their formation after migration of halogens from the melt.

We provide results of a comprehensive mineralogical and microstructural study of relict lherzolites of the main ore field and fresh rocks from a deep structural borehole drilled in the south-eastern part of the Kempirsay massif. Olivine and orthopyroxene from lherzolites contain numerous pieces of evidence of material redistribution at different scales caused mainly by solid-state processes, such as plastic flow of mantle, syntectonic recrystallization, and annealing. The results of deformation-induced processes at the submicron scale are recorded by optical and electronic microscopy. In olivine, the plastic deformation caused segregation of impurities at structural defects. As a result, abundant tiny rods of newly formed Cr-spinels occurred inside its grains. Moreover, in enstatite, deformation caused partial or complete chemical decomposition with exsolution of diopside, pargasite and spinel lamellae up to the formation of a “fibrous” structure. In other cases, it provided partial or complete recrystallization to form new phases of enstatite-2, forsterite, diopside, pargasite, and spinel. Petrographic observations are validated by geochemical data, i.e., regularly decreasing concentrations of minor elements in neoblasts compared to large grains (porphyroclasts). Further redistribution of spinel grains with the formation of chromitite bodies is witnessed by their permanent association with the most mobile phase of the upper mantle, i.e., olivine, which is the only mineral that remains stable under the intense plastic flow. An increased concentration of Cr-spinel grains during formation of massive chromitites could appear under conditions close to pressure sintering, as evidenced by stressed textures of ores and an increased grain size compared to disseminated chromitites. The formation of unique chromitite deposits is associated with integration of numerous disparate podiform bodies into “ore bunches” due to the tectonic impact in the shear-compression regime. This was most likely associated with transition of the rifting (spreading) regime to that of the upper mantle of the fore-arc basin.