Precambrian paleosols provide important evidence for deciphering ancient atmospheric conditions; however, reconstruction using the Archean -Early Proterozoic weathering crusts is challenging due to reworking by later superimposed events. In this paper, we describe 2.6 Ga paleoweathering crusts developed on ultramafic rocks recognized in the Eastern Fennoscandian Shield of northwest Karelia, Russia. Mafic index of alteration shows an increasing trend from parent rock to the paleosol and indicates moderate weathering. Geochemical and mineralogical data provide evidence for paleosol formation under anoxic acidic conditions. The results allow reconstruction of palaeoatmospheric p O 2 and p CO 2 levels estimated to be 1 x 10 -4 PAL and 22 PAL, respectively. Analysis of the paleosols in the Eastern Fennoscandian Shield reveals that the anoxic weathering seems to have prevailed over Archean -Early Proterozoic interval. The study contributes to understanding of evolution of atmospheric oxygen levels and paleoenvironmental conditions prior to the Great Oxidation Event.

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The Ural type magmatic copper deposits in gabbro are assigned to a specific type of copper deposits which exploitation substantially depends on the oxidation degree of the ores, that is often not unambiguous. At the Volkovskoe Cu-Fe-V deposit (Middle Urals, Russia), three technological grades of the copper ores are known, the primary, oxidized, and mixed ones, that are distinguished based on the relative content of Cu2+ in the total copper balance. This paper presents petrological, petrochemical, geochemical, and mineralogical characteristics of natural varieties of the copper ores of the Northwestern zone of the Volkovskoe deposit. It is demonstrated that the criterion for distinguishing the technological grades of the copper ores by the relative Cu2+ content does not reflect the real degree of the supergene oxidation for the selected technological varieties. The results of our research suggest a necessity of studying the copper sulfides in more detail to clarify peculiarities of the redox conditions of the ore formation, to correct the indicators of the primary and supergene copper oxidation degree, which may provide new criteria to determine the boundaries between the technological grades of copper ores, using the Volkovskoe ore deposit as an example.

In this contribution, we present the results of mineralogical investigation of the agates in Paleoproterozoic organic carbon-rich sedimentary rocks within the Onega Basin (Fennoscandian shield, Russia) aimed at reconstructing the agate-forming processes. Optical and scanning electron microscopy, EDS microanalysis, thermal analysis, X-ray powder diffraction, Raman spectroscopy, and carbon isotope analysis were used for the study. Three main varieties of agates differing in morphology and texture were identified, including concentrically zoned nodules, fine-banded, and carbon-rich moss agates. Mineralogical evidence indicates the participation of hydrothermal fluids in agate formation. Concentrically zoned nodules could be formed due to the dissolution of carbonate concretions in the organic carbon-rich siltstones and their silicification as a result of late hydrothermal processes. Fine-banded vein agates occur in stockworks crosscutting organic carbon-rich rocks and are widely accompanied by sulfides, selenides, carbonates, sulfates, and iron oxides. Carbonaceous matter in moss agates is present as poorly ordered carbon and is characterized by a low δ13Corg value (−25.64‰), suggesting a biogenic origin. Raman spectroscopy data showed an elevated amount of moganite besides alpha quartz in the concentrically zoned nodules compared to other agate varieties, indicating different ages of the mineralization processes. We suggest that the revealed varieties of agates were formed at different stages of long-term hydrothermal processes occurring in the Onega Basin.

The paper considers a previously unstudied hydrothermal tourmaline from the Paleoproterozoic basalts of the Girvas paleovolcano (Onega basin, Karelian Craton). Tourmaline in association with quartz and carbonate forms large nests in lavas, as well as nested and disseminated mineralization in the contact zones between lava flows and the eruptive center. Three morphological types of black-brown tourmalines were identified: large (up to 20 cm) prismatic crystals in quartz veins; fine-needle tourmaline crystals in nest-shaped clusters in association with quartz, albite, epidote, chlorite, and sulfides; radiated aggregates of fine-needle crystals on sub-vertical planes of tectonic dislocations of basalts − «tourmaline suns». According to their chemical composition tourmalines belongs to the alkaline series and corresponds to the isomorphic schorl-dravite group. Under electron microscope in cross-sections of tourmaline, needles zoning was revealed. Microscopic zoning is consistent with the chemical variation of Fe, Mg, Al, Na and vacancy content in individual zones. The zoning of tourmaline crystals provide evidence for multi-stage process of their formation, which proceeded during several impulses of hydrothermal activity, differing in the physicochemical parameters of the mineral formation environment.

Chemical composition, texture and zoning of pyroxenes are frequently used as indicators of magmatic system evolution. This paper presents trace element data for zoned clinopyroxene phenocrysts hosted in the variolitic lavas of the Suisaari Formation, Karelian Craton, Eastern Fennoscandian Shield. Clinopyroxene phenocrysts from variolitic lavas are characterized by various zoning patterns, mainly manifested in Fe и Mg variations. V/Sc and V/Ga ratios determined for normal and reverse zoned clinopyroxene phenocrysts were used to estimate the variations of redox conditions during the clinopyroxene crystallization. The obtained data suggest that the variations in redox conditions during clinopyroxene crystallization were negligeable. The obtained results support the model that crystallization of zoned clinopyroxenes from basalts of the Yalguba Ridge involves magma mixing and crystal convection.

The amorphous carbonaceous material (CM) in mafic igneous rocks is important for deciphering the relationships between igneous and hydrocarbon systems in Earth interior. Deciphering of CM in Precambrian rocks it is a challenge because of reworking by later superimposed events but could give insights into evolution of these systems in time. We report the results of combined field and petrological studies, Raman spectroscopy and carbon isotope analysis of well-preserved ca. 2.0 Ga pillow lavas in the Onega Basin, Karelian craton, Eastern Fennoscandian Shield. In these rocks CM is present in both pillow and inter-pillow areas as poorly ordered carbon. In the MORB-like basalts of the Zaonega Formation, CM occurs in post-magmatic veinlets formed via migration of hydrothermal fluids. In the OIB-type basalts of the Suissary Formation, CM is found in amygdales, inclusions in feldspar, and in devitrified volcanic glass indicating a CM deposition via a rapid cooling of C-rich magma. Despite morphological diversity carbon isotopic composition indicates a biogenic origin of CM in both the MORB- and OIB-like basalts. The most likely source was the Paleoproterozoic oilfield in the Zaonega Formation, commonly referred to as the Shunga event.

Upper Paleozoic (D3–P1) carbonaceous shales and siltstones in the surrounding of the Kara astrobleme are studied. The material composition of the target rocks is analyzed by methods of multidimensional statistics—correlation, factorial, and hierarchical cluster methods. As a result, we revealed the geochemical specificity of the components, generally indicating the contamination of the target rocks of the Kara astrobleme by some ore and rare earth elements by the post-impact hydrothermal activity, mobilization, redeposition, and concentration of the substance. The analysis of the contents of Ni, Cr, and Co indicates partial enrichment of the target rocks by cosmic matter in the area of intense impact crushing, disintegration, and hydrothermal mineralization.

Agate gemstones occurring in the Mesoproterozoic volcanic rocks of the Priozersk Formation (PrF) within the Pasha–Ladoga Basin (Fennoscandian Shield, NW Russia) were investigated to characterize the mineral and geochemical composition of the agates and provide new information concerning their origin. Optical and scanning electron microscopy, EDS microanalysis, X-ray powder diffraction, X-ray fluorescence spectrometry, Raman spectroscopy, inductively coupled plasma mass spectrometry (ICP-MS), and C-O isotope analysis were used for the study. Agate mineralization appears mostly as an infill of fissures, cavities, gas vesicles in massive and vesicular basalts, lava-breccias. The mineral composition of agates is dominated by alpha-quartz (fibrous chalcedony, microcrystalline and macrocrystalline quartz), but it also displays abundances of calcite. The characteristic red-brownish agate’s coloration is caused by multiple hematite inclusions distributed in an agate matrix. The study revealed the two phases of agate formation in the PrF volcanics, which are most likely controlled by two distinctly different fluids and/or their mixture. At first, agates appeared due to post-magmatic iron-rich fluids. The late hydrothermal activity was probably triggered by intrusion of gabbro-dolerite sill and resulted in the second phase of agate formation. We suggest that the late hydrothermal fluids remobilized the iron compounds from the crust of weathering underlying the PrF volcanics, which led to additional formation of vein agates and filling of gas vesicles with hematite-rich calcite/silica matter.

The Yalguba Ridge volcanic rocks form part of the Middle Paleoproterozoic (ca. 1.97 Ga) volcano-sedimentary sequence within the Karelian Craton in the Fennoscandian Shield. Yalguba variolitic textures are known worldwide and have been previously considered to originate from liquid immiscibility. The present study reveals two new variolite types recognized in the Yalguba sequence: (1) Variolites with unzoned varioles have distinct chemical and mineralogical compositions of varioles and matrix that support an origin by liquid immiscibility. They were recognized in quenched zones of pillows, so it might be assumed that melt separation caused by liquid immiscibility occurred before magma emplacement. The difference from the previously described variolites lies in the variole microtexture and might be caused by the various cooling conditions. (2) Spherulitic variolites have varioles composed of andesine–oligoclase spherulites embedded in the cryptocrystalline matrix with oligoclase–anorthoclase composition, thus the variole and matrix have similar chemical and mineralogical composition. The mineralogical and textural features of these variolites suggest that the spherulites have a primary magmatic origin due to the rapid cooling of superheated magma. The variety of variolitic textures in the Yalguba section might be caused by the different H2O saturation of parental magma and cooling conditions.

A tectonic map is a cartographic image of models showing the structure (architecture) of the earth crust/lithosphere and the geodynamic settings of formation of its constituent mineralogical complexes. A new tectonic map of Karelia has been compiled 1) to delineate areas differing in formation of Earth crust and 2) to analyze the Archean to Paleozoic evolution of geodynamic settings upon the formation of the region's earth crust.

A combined geochemical and Sm–Nd isotopic study on the Palaeoproterozoic (1,845 Ma) granites of the Lesser Himalayan Crystalline Sequence (LHCS) in the Garhwal region of NW India has been done in the present study. These granite samples are characterized by high silica, alumina, and potash and belong to a peraluminous to strongly peraluminous series, having molar A/CNK values of 1.01 to 2.4. The low P2O5 contents and its negative correlation with SiO2 presiding out that the granites have S‐type affinity, also supported by various classification diagrams (ACF; SiO2 vs. P2O5, Na2O + K2O‐CaO, and Th). The concentration of trace elements Ba, Sr, Nb, and Ti are low, and Rb, Th, U, and Pb are found to be high. The granites have low total rare earth elements contents of 56.19–229.16 ppm with enrichment in Light rare earth elements (LREE) ([La/Yb]N = 1.61–15.08) and negative europium anomaly (Eu/Eu* = 0.12–0.31). Sm–Nd isotope studies were also performed for three granite samples. Estimated model ages as 2.5–2.7 Ga, indicates the contribution of the Archean crustal substrate as their protolith source. Therefore, we assume that the melting of metasedimentary rocks with Archean protolith can form these peraluminous granites in an accretional–collisional event, during the Palaeoproterozoic on the western flank of the Columbia supercontinent.

In the present work, we studied zircons from the less foliated granites of the Chail Group, which form a thrust sheet of the Lesser Himalayan Sequences, Garhwal region. Compositionally, these granites are S–type, formed in a collisional tectonic setting. Zircons possess an internal structure, mineral inclusions, and geochemical characteristics typical of magmatic origin. The U–Th–Pb geochronology and geochemistry were assessed using the laser ablation multi–collector inductively coupled plasma spectrometry (LA–ICP–MS) technique. U–Th–Pb isotope dating of zircons from two different samples revealed their age, estimated from the upper intersection of the discordia, to be 1845 ± 19 Ma. Zircons from one sample contained inherited cores belonging to three age groups: Paleoarchean (3.52 Ga), Neoarchean (2.78 Ga and 2.62 Ga), and Paleoproterozoic (2.1 Ga). Zircons with ages of 3.52, 2.62, and 2.1 Ga were interpreted as magmatic based on their geochemical characteristics. The 2.78 Ga core was interpreted as metamorphic. The observed inheritance is consistent with the melting of sedimentary rocks. The inherited zircons could have originated from Aravalli and Bundelkhand Craton and Paleoproterozoic Aravalli Fold Belt rocks. This confirms that the studied granites are S–type and could have been formed in a collisional environment at 1.85 Ga on the western flank of the Columbia Supercontinent.

Marcial waters are the most famous deposit of ferruginous waters in north-western Russia. It is where Russia's oldest resort, which opened in 1719, is situated. Four water boreholes (Bh) with different water mineralization and iron content are in use today. Marcial waters have been studied for 300 years in terms of their chemical composition, mainly the content of major elements. However, precise investigations of trace elements have not been performed previously. Recently, five-year monitoring (2015-2019) of the distribution of trace elements in water boreholes and other water bodies located in the resort's area was accomplished. Also, the chemical composition (trace elements) of host rocks and the mineral phases forming them were analyzed to assess their possible contribution to the mineralization of the studied water. It has been shown that Marcial waters contain a wide range of trace elements with concentrations from 0.001 to 260 mu g kg(-1), and displayed chemical stability for most elements during the 5 years. High concentrations of some trace elements in Marcial waters can be explained by their transfer from silicate, accessory minerals and sulfides of the host rocks. The investigations showed that waters are formed as a result of infiltration waters interacting with ore and silicate minerals, which controls the specific chemical composition. REE distribution indicates different mechanisms of their accumulation, which justifies the distinction of two contrasting water types formed as a result of mixing high-ferruginous waters of Bh 4 with infiltration water.

Data on the concentrations of trace elements in coals and coal ashes from productive beds I and II of the Mugunsk deposit (Irkutsk coal basin) are presented. The mineral composition of the test samples is given. It was found that the concentrations of Sc, Ti, and Sr reached minimal commercial contents. Mineral carriers of the trace elements are proposed.

We studied Upper Paleozoic (P1pt-ng) carbonaceous shales and siltstones from the area of the Ust'-Kara astrobleme (PayKhoy). We analyzed mineralogical and geochemical features of carbonaceous rocks of the target in the vicinity of the Ust'-Kara astrobleme event using a complex of modern methods to identify possible mobilization, redeposition and concentration of ore substance under intensive post-impact hydrothermal activity. Geochemical features of carbon deposits, altered by post-impact hydrothermal processes in the vicinity of the Ust'-Kara impact structure, have been determined. We found anomalous contents of Ti, Mn, Cr, Zr, Ni, Li, Co, Sc and REE. Inherent rare metal and rare earth minerals (monazite, florensite), sulfides (pyrite, chalcopyrite, marcasite, sphalerite), apatite, barite, anatase, chrome spinels were diagnosed.