The Syum-Keu massif is the northernmost ophiolite complex of the Ural mobile belt. It differs from other massifs of the Polar Urals due to the prominent distribution of lherzolites in the upper mantle section. This feature aligns it more closely to some massifs in the southern part of the belt (Kraka). Thus, a comparison of the ultramafic rock compositions in these massifs is highly relevant. Thus, comparing the compositions of ultramafic rocks from these massifs is highly relevant and is one of the primary objectives of this study. Our second objective is to study the microstructural features of ultramafic rocks from the upper mantle, as they can indicate modes of subsolidus processes that played a key role in the formation of this massif. Our study utilizes optical microscopy, assessments of bulk rock composition using X-ray fluorescence and ICP-MS, as well as mineralogical methods, such as scanning electron microscopy with energy dispersive spectroscopy and electron backscattered diffraction, for the microstructural analysis of peridotites. In addition to ultramafic rocks from the upper mantle section, the composition and mineralogy of mafic rocks from the crustal section were studied. The microstructural analysis of ultramafic rocks indicates their two-stage evolution. The first is associated with plastic flow under the upper mantle conditions dominated by the olivine slip along the {0kl}[100] system, while the second reflects formation in the lower crust, with lower-temperature deformation along the {110}[001] slip system. Comparing the mineralogy of the Syum-Keu peridotites to lherzolite massifs in the Southern Urals reveals a significant difference in accessory Cr-spinel composition; the former show elevated iron content (Fe trend), indicating intense crustal metamorphism. Similarly, amphiboles in Syum-Keu ultramafic rocks exhibit a significant crustal (metamorphic) component, while the same minerals in the Kraka massif suggest a mantle (magmatic) origin. Mafic rocks in the Syum-Keu massif also typically display a high degree of metamorphism. The obtained results generally corroborate prior findings on a longer evolution of the upper mantle ultramafic rocks of the Syum-Keu massif compared to those of the Kraka massif. Our results are also consistent with the suprasubduction nature of these ultramafic rocks. Our findings can be utilized in further studies of the microstructure and composition of ophiolites from the Polar Urals to provide a more detailed characterization of the partial melting conditions of the mantle source, the plastic flow of peridotites, and their interaction with melts and fluids.
The article discusses the petrographic, mineralogical and petrogeochemical features of alkaline granites of the Balbuk complex in three massifs - Aushkul, Kamatal and Shartym. The rocks of the massifs are similar in structural and textural features and mineral composition: they are composed of acidic plagioclase, potassium feldspar, quartz and muscovite. In the Aushkul massif, phenocrysts of potassium feldspar, plagioclase, quartz and primary oligoclase are found in the groundmass, while in the Kamatal and Shartym massifs only phenocrysts of altered plagioclase and quartz are developed, and secondary albite is present in the groundmass. The existing differences in the geochemistry of the rocks of the Aushkul massif in comparison with Kamatal and Shartym (depleted in barium and enriched in niobium) may be associated with minor differences in the composition of magmas and with the peculiarities of their fractionation in intermediate chambers (fractionation of alkali feldspar and Nb-containing phases).
In the section of the Biryan subformation in the Inzer synclinorium on the western slope of the Zilmerdak ridge, Cr, Ni, Si, Zn, Pb and elevated concentrations of As, typical for sandstones of this stratigraphic interval, were found, which are on average 2–13 times higher than clark. It was found that the main minerals concentrators of As are arsenopyrite and goethite. Goethite mainly replaces leached arsenopyrite and/or arsenical pyrite, and also develops along the boundaries of quartz grains, occurs in the form of fringes around quartz pebbles and is observed along the cleavage cracks of potassium feldspar. The zonal distribution of As with the highest concentrations up to 207 g/t in one of the layers of the section was established. This is explained by the lithological control and the fact that As is confined to competent rocks (gravelites with interlayers of arkose quartzite-like sandstones), which are underlain by less competent ones (siltstones). It is assumed that siltstones acted as a fluid barrier during the layered movement of metasomatic fluids. It is shown that the rocks of the section have lithological, geochemical and mineralogical search signs for gold mineralization. The area in the eastern continuation of the section, in which deposits of the Biryan subformation are intruded by numerous dikes of the Inzer gabbrodolerite complex, is assumed to be promising. The enrichment of the rocks of the studied section in As may have been due to the circulation of postmagmatic fluids associated with the dikes of this complex. В разрезе бирьянской подсвиты в Инзерском синклинории на западном склоне хребта Зильмердак установлены типичные для песчаников этого стратиграфического уровня содержания Cr, Ni, Сu, Zn, Pb и повышенные концентрации As, которые в среднем в 2–13 раз превышают кларк. Обнаружено, что главными минералами-концентраторами As являются арсенопирит и гетит. Гетит преимущественно замещает выщелоченный арсенопирит и/или мышьяковистый пирит, а также развивается по границам зерен кварца, встречается в виде оторочек вокруг кварцевых галек и наблюдается по трещинам спайности калиевого полевого шпата. Установлено зональное распределение As с наибольшими концентрациями до 207 г/т в одном из слоев разреза. Это объясняется литологическим контролем и приуроченностью As к компетентным породам (гравелиты с прослоями аркозовых кварцитовидных песчаников), которые подстилаются менее компетентными (алевролиты). Предполагается, что алевролиты выступали в роли флюидоупора при послойном движении метасоматических флюидов. Показано, что породы разреза обладают литологическими, геохимическими и минералогическими поисковыми признаками на золотое оруденение. Перспективным предполагается участок в восточном продолжении разреза, в котором отложения бирьянской подсвиты прорывают многочисленные дайки инзерского габбродолеритового комплекса. Обогащение пород изученного разреза As, возможно, было обусловлено циркуляцией постмагматических флюидов, связанных с дайками этого комплекса.
The paper presents new data on the structure and composition of ultramafic rocks in the eastern part of the South Kraka massif. It is shown that the studied area is composed predominantly of spinel peridotites, among which the main role belongs to lherzolites with a low content of clinopyroxene (2–5%) and harzburgites, among which lens-shaped and bandshaped inclusions of monomineral olivine rocks – dunites – are quite often observed. The leading role in the structure of the rocks belongs to high-Mg olivine (Fo 87-94 ), a secondary role (2016). Nature of the lithospheric mantle beneath the Arabian Shield and genesis of Al-spinel micropods: Evidence from the mantle xenoliths of Harrat is played by high-Mg orthopyroxene (enstatite) and Ca-Mg clinopyroxene diopside. Rare minerals of the mantle stage are amphibole and plagioclase. A typical accessory mineral of ultramafic rocks is spinel, the composition of which varies from high-Al in lherzolites (Cr# = 0.16–0.3), to moderate-Al in harzburgites (Cr# = 0.3–0.55) and to high-Cr in dunites (Cr# = 0.6–0.85). Of the rare accessory minerals in the rocks, the following were identified: native copper, pentlandite, awaruite and PGM (laurite, irarsite, Os-Ir-Ru-alloys). The section of mantle ultramafic rocks of the Sargan Range completed its high-temperature evolution to the levels of the plagioclase peridotite facies, partly in the transition zone from spinel to plagioclase facies. An assessment of oxygen fugacity allows us to speak about more reducing conditions for the formation of rocks of the studied area compared to other similar formations of upper mantle origin and allows us to classify the studied ultramafic rocks as moderately depleted restites of the upper mantle under the riftogenic structure of the Paleo-Ural basin. The geochemical specialization of PGM also indicates a restite origin of ultramafic rocks. At the stage of cooling and tectonic transformations in the crust, ultramafic rocks underwent serpentinization.
The manuscript provides a petrochemical and mineralogical description of the gabbro-dolerite body of the Suran fluorite deposit. The investigated body has structural and microelement heterogeneity. For the first time, intrusive rocks of this deposite were studied using scanning electron microscopy. A high content of cobalt and nickel in pyrite was revealed. Three types of pyrite and two types of manganilmenite have been identified. The conditions for the formation of clinopyroxene correlate with the geodynamic position of intrusive rocks.
A comparative analysis of the composition of amphiboles from ultramafic rocks of mantle sections of two ophiolite complexes of the Urals — Kraka and Syum-Keu, which are often referred to as «lherzolite-type massifs» is carried out. Three morphological types of amphibole segregations in peridotites are recognized: 1) anhedral grains (up to 100–200 μm in size), often in association with clinopyroxene; 2) small prismatic grains (25–50 μm) in the peripheral parts or close to orthopyroxene porphyroclasts, also in association with small Opx, Cpx or Ol grains; 3) lamellae inside deformed grains of orthopyroxene, less often — clinopyroxene. The fourth amphibole type is represented by submicron inclusions in ore-forming chromite. The vast majority of the studied grains in the Kraka peridotites are represented by essentially calcium amphiboles, among which the most frequently diagnosed are pargasite, magnesian hornblende and tschermakite; tremolite and edenite are very rare. In the ultramafic rocks of the Syum-Keu massif, on the contrary, the leading role belongs to edenite, in isolated cases hornblende is noted. The compositions of the amphiboles of the Syum-Keu massif are quite densely concentrated in the field of the «crust-mantle source», whereas for the compositions of the amphiboles of the Kraka massifs, a more significant range of values with a predominance of the «mantle component» is characteristic. The noted differences in the composition of amphiboles correlate with the estimated PT conditions of rock formation: higher temperatures and pressures are characteristic of the Kraka lherzolites (the boundary of the spinel and plagioclase facies of the upper mantle source), the Syum-Keu peridotites were probably recrystallized under the conditions of the lower part of the earth’s crust. Проведен сравнительный анализ состава амфиболов, которые присутствуют в ультрамафитах мантийных разрезов двух офиолитовых комплексов Урала — Крака и Сыум-Кеу, которые часто относят к «массивам лерцолитового типа». Установлено три морфологических типа выделений амфиболов в перидотитах: 1) относительно крупные ксеноморфные зерна (размером до 100–200 мкм), часто в ассоциации с клинопироксеном; 2) мелкие (25–50 мкм) призматические зерна в периферических частях, либо вблизи порфирокластов ортопироксена, также в ассоциации с мелкими зернами клино- и ортопироксенов и оливина; 3) ламели внутри деформированных зерен ортопироксена, реже — клинопироксена. Четвертый тип выделений представлен субми-кронными включениями в рудообразующем хромите. Подавляющее большинство изученных зерен в перидотитах массивов Крака представлено существенно кальциевыми амфиболами, среди которых наиболее часто диагностированы паргасит, магнезиальная роговая обманка и чермакит, очень редко встречается тремолит и эденит. В ультрамафитах массива Сыум-Кеу, напротив, ведущая роль принадлежит эдениту, в единичных случаях отмечается роговая обманка. Составы амфиболов массива Сыум-Кеу довольно компактно концентрируются в поле «корово-мантийного источника», тогда как для составов амфиболов массивов Крака характерен более широкий диапазон значений с преобладанием «мантийной составляющей». Отмеченные различия в составе амфиболов коррелируют с оцененными РТ-условиями образования пород: более высокие температуры и давления характерны для лерцолитов Крака (граница шпинелевой и плагиоклазовой фации верхнемантийного источника), перидотиты Сыум-Кеу, вероятно, были перекристаллизованы в условиях нижней части земной коры.
Lode gold deposits are widespread in orogenic belts of various ages and are a valuable gold source, but their genesis remains debatable. The close relationship between native gold and quartz was considered a reason to search for acid-magmatic sources of heat and fluids (i.e., granite intrusions), while small gabbro bodies were often ignored. Six minor gold deposits associated with NE-strike faults were studied in the Khudolaz area of the South Urals (Tukan, Bilyan-Tau, Fazly-Tau, Muildy-Tamak, Alasiya-II and Isyanbet-I). It was established, for the first time, that all of the studied deposits are similar geologically but differ in mineralogical diversity of ore-bearing quartz veins, which is due to the different composition of host rocks and ore-bearing intrusions of the Khudolaz (325–329 Ma, U-Pb) and the Ulugurtau (321 ± 15 Ma, Sm-Nd) ultramafic-mafic complexes. Results of the geochemical study of quartz veins (ICP MS) and their fluid inclusions (microthermometry, gas chromatography) showed that native gold was mostly precipitated at temperatures of 230–330 °C from a low- to moderate-saline (8–12 wt.% NaCl-eq.) H2O–CO2–CH4-bearing fluid, when weakly oxidized or near-neutral conditions, were replaced by reducing ones. No significant differences between barren milky white and ore-bearing brownish quartz veins were defined, which indicates their common formation settings and an impulse pattern of vein injection. The stable pattern of the fluid salinity, along with low hydrocarbon and N2 contents, as well as a narrow range of δ18O values, indicate a prevailing magmatogenic source with a certain influence of host rocks but without the influence of meteoric waters. Based on the presented data, the studied deposits were attributed to the epizonal orogenic type. This study shows the formation of lode gold deposits is possible without the participation of granite massifs.
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.
For the first time, late hypergene mineralization is described in the outcrop of the Zigaza-Komarovo Formation of the Middle Riphean of the Bashkir meganticlinorium near the Kagarmanovo village in the Beloretsk region of the Republic of Bashkortostan. The section reveals tectonized rocks: sandstones, siltstones, clayey and low-carbon shales. Geological observations have revealed the widespread development of supergene sulfate minerals in black shales, which form thin yellow-brown layers, light gray, white and yellow growths and deposits with a square of several tens of dm2. Data from scanning electron microscopy, X-ray fluorescence analysis and X-ray powder diffractometry confirmed the presence of minerals of the copiapite (copiapite, ferricopiapite) and halotrichite (pickeringite) groups. The most intensive formation of hypergene mineralization has occurred under modern conditions over the past 15 years after the opening of rocks during the construction of the Beloretsk – Starosubkhangulovo highway. The formation of sulfate minerals was most influenced by the dislocation of rocks, the presence of black shales containing abundant sulfide mineralization and Corg, and the influence of infiltration and, possibly, meteoric waters. The formation of minerals occurred at a geochemical barrier, and is an intermediate stage in the process of migration and distribution of elements under the conditions of the hypergenesis zone. The results of the study must be taken into account when conducting geoecological monitoring of environmental pollution processes and are of interest for future detailed mineralogical studies. Впервые в обнажении зигазино-комаровской свиты среднего рифея Башкирского мегантиклинория около д. Кагарманово в Белорецком районе Республики Башкортостан описывается поздняя гипергенная минерализация. В разрезе вскрываются тектонизированные породы: песчаники, алевролиты, глинистые и низкоуглеродистые сланцы. Геологическими наблюдениями выявлено широкое развитие по черным сланцам гипергенных сульфатных минералов, которые образуют в них маломощные желто-коричневые прослои, светло-серые, белые и желтые наросты и натеки площадью несколько десятков дм2. Данными растровой электронной микроскопии, рентгенофлуоресцентного анализа и рентгенофазовой порошковой дифрактометрии подтверждено присутствие минералов групп копиапита (копиапит, феррикопиапит) и галотрихита (пиккерингит). Наиболее интенсивно образование гипергенной минерализации происходило в современных условиях в течение последних 15 лет после вскрытия пород при строительстве автодороги Белорецк – Старосубхангулово. На формирование сульфатных минералов наибольшее влияние оказали дислоцированность пород, присутствие черных сланцев, содержащих обильную сульфидную минерализацию и Сорг, воздействие инфильтрационных и, возможно, метеорных вод. Образование минералов происходило на геохимическом барьере, и является промежуточным этапом в процессе миграции и распределении элементов в условиях зоны гипергенеза. Результаты проведенного изучения необходимо учитывать при проведении геоэкологического мониторинга процессов загрязнения окружающей среды и представляют интерес для будущих детальных минералогических исследований.
