Hematite Inclusions Research Articles

Iolite from Tanzania with Large Hematite Inclusions

Iolite from Tanzania with Large Hematite Inclusions

Xiongnu ancient nomad inlaid buckles (2nd-1st centuries BC): Multi-analytical research

The recently excavated burial grounds of Ala-Tey of the 2nd-1st centuries BCE, located in the basin of the Ulug-Khem (Yenisei) River, at the entrance to the Sayan Canyon of the Yenisei, belong to the archaeological monuments on the territory of the Republic of Tuva (Central Asia) and are associated with the Xiongnu culture. The part of preserved inlays and the base of buckles were investigated using Raman and FTIR spectroscopy. By Raman spectroscopy in the buckle base, amorphous carbon was identified. The investigation also revealed the mineral contribution in the FTIR spectra of buckles plates such as kaolinite (Al2O3·2SiO2·2H2O) and quartz (SiO2) in addition to the absorbance bands of some organic components. Buckles inlays mineral composition was identified via Raman spectroscopy. Two different types of blue-green minerals were found: turquoise (CuAl6(PO4)4(OH)8·4H2O) and muscovite (KAl2(Si3Al)O10(OH)2). According to the Raman spectroscopy data all red inlays contain α-quartz and inclusions of hematite. The red inlay transparency is associated with the presence of moganite (polymorphic monoclinic modification of SiO2). The white inlay material consists of aragonite (CaCO3). The results of the study are relevant for better understanding of Xiongnu belt buckles materials and choosing appropriate strategies of its conservation.

Superparamagnetism of Artificial Glasses Based on Rocks: Experimental Data and Theoretical Modeling

Artificial glasses containing nanoscale inclusions of iron oxides, including magnetite and hematite, were obtained via the method of the high-temperature melting of rocks. The main factors influencing the magnetic properties of glasses are the composition of the initial charge and the conditions of cooling of the melt. The data of magnetic granulometry and frequency-field dependencies of magnetic susceptibility showed the presence of a sufficiently large superparamagnetic fraction in the samples. Coordinated theoretical modeling using two independent models that take into consideration possible the chemical inhomogeneity of particles and magnetostatic interaction between them made it possible to calculate hysteresis characteristics corresponding to the experiment and to estimate ferrimagnetic concentrations in the samples, including the superparamagnetic fraction.

Evaluation of the Gemological Properties of Datolites from the Campotrera Deposit in the Northern Apennines (Italy)

This work investigates the gemological properties of the datolite from the famous field of Campotrera near Reggio Emilia in Italy, for a possible commercial use in the market. This mineral occurs in widespread multi-centimeter veins, together with calcite and prehnite, within polygenic breccias in basaltic ophiolites. The most common form for this datolite is the double wedge with a prism (110) and a pinacoid (001). The gems obtained are mixed or carré cut, colorless or salmon pink, transparent, with a vitreous luster and weight between 1 to 5 carats. They have high brilliance, transparency and birefringence, glassy luster, absence of cleavage. The major chromophore is probably Fe, which occurs as inclusion of hematite and ilmenite. Raman investigations highlighted different fluid inclusions. The primary are randomly distributed or, in some cases, follow the growth zones, while the secondary form aligned tracks along the microcracks. Fluid inclusions can be biphasic and made up by liquid + gas (L + G), generally >10 mm in size, and more rarely, monophasic, composed only by liquid (L) generally <10 mm. The gems extracted from the rough sample are very valuable but their delicacy requires attention in the cutting and preparation of the jewels.

Influence of Fe<sub>3</sub>O<sub>4</sub> on Physicochemical and Photocatalytic Properties of Nanosized Barium Titanate

A procedure has been proposed for the synthesis of a nanocomposite based on barium titanate modified by adding nanodispersed magnetite using the sol–gel method in an acetic acid medium followed by annealing at 800°C. The physicochemical analysis of the products has shown that the matrix phase after annealing is barium titanate with an admixture of barium carbonate, and, in addition to magnetite, there are minor inclusions of hematite and wustite. The elemental composition of nanosized samples has been determined using energy-dispersive X-ray spectroscopy. It has been demonstrated that the concentration of introduced Fe3O4 affects the morphological and phase composition of the composites. The specific surface area and type of porosity of calcined samples have been determined by the low-temperature nitrogen adsorption/desorption method. The effect of BaTiO3, BaTiO3/Fe3O4-1%, and BaTiO3/Fe3O4-10% powders on the adsorption capacity and photocatalytic activity in the process of decolorization of the dye rhodamine B from an aqueous solution in the dark and under the action of ultraviolet light has been studied. The kinetics of adsorption in the dark and photocatalytic decomposition of rhodamine B under the action of ultraviolet radiation in an aqueous suspension of the obtained composites have been analyzed using pseudo-first and pseudo-second order kinetic models.

Agates from Mesoproterozoic Volcanics (Pasha–Ladoga Basin, NW Russia): Characteristics and Proposed Origin

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.

Study on the Color Genesis of South Red Agate and the Geological Characteristics of the Siliceous Gravel Layer

The aim of the study was to explore the gem mineralogical characteristics of southern red agate and then analyze its structural characteristics and color causes. The mineral composition, structure, and chromogenic minerals of Nanhong agate were tested by a conventional gemmological test and observed under the microscope, infrared spectrum, and microlaser Raman spectrum. It is found that the color of southern red agate samples is mainly dark red or orange red, and colorless, white or yellow bands can be seen in some samples; the relative density is in the range of 2.59∼2.65. The matrix structure of the sample is mainly long fiber and short fiber, and some of them are fine-grained and crystalline plasmid. There are concentric ring-banded structures, and some horizontal banded and vein structures can also be seen. The analysis and test results show that the south red agate has a typical radial fiber structure, and the main chemical component is SiO2; Under the orthogonal polarizing microscope, a large number of round reddish brown mineral inclusions with banded distribution can be seen, the spot-colored mineral is iron oxide hematite, and calcite of the late hydrothermal origin can be seen in the cracks. In addition, the study shows that the formation of south red agate is multistage and the formation of hematite inclusions is also multistage.

Microbial formation and stabilisation of soil organic carbon is regulated by carbon substrate identity and mineral composition

The view that soil organic C (SOC) is formed mainly from non-metabolised and recalcitrant organic residues is being challenged by an emerging view that metabolic by-products form more stable associations with soil minerals. However, the effects of C substrate identity and soil mineral composition (and interactions) on microbial physiology and SOC formation are still not well understood. We added contrasting substrates (glucose, alanine and a mixture of glucose, alanine, and oxalic acid) into artificial soils of varying mineral composition (montmorillonite, kaolinite, and kaolinite plus goethite and hematite) for 12 weeks. We found that glucose led to 1.45 and 1.75 times more SOC formation than alanine and the mixed substrate, respectively. Montmorillonite-based soils gained approximately 1.3 times more SOC compared to the other two soils. Compared with kaolinite-only soils, the inclusion of goethite and hematite had a positive effect on total SOC, extracellular C and biologically stable C when amended with alanine, but a negative effect on these SOC fractions when amended with glucose. Soils with greater SOC formation were associated with high microbial C use efficiency (CUE) and extracellular C, suggesting that spatial allocation by the microbial biomass is pivotal for creating stable SOC. Fungi-dominated soils typically had a higher CUE, which was positively correlated with the formation of new SOC. These results suggest that the identity of plant inputs will have a strong bearing on the formation of SOC via interactions with the soil microbial community and soil mineralogy.

Neobvyklá asociace minerálů s willemitem z prostoru centrální části příbramského uran - polymetalického revíru (Česká republika)

The unusual mineralization with willemite, rare Cu arsenides (koutekite, kutinaite) and balkanite was found at a sample originating from the Jerusalém deposit (shaft No. 9), Příbram uranium and base-metal ore district (Czech Republic). The gangue is formed by earlier siderite and quartz intensively replaced by abundant willemite and younger dolomite group minerals with hematite inclusions and the youngest calcite. The ore mineralization represented by sphalerite, djurleite, anilite, covellite, stromeyerite, balkanite, koutekite, kutinaite, primary anglesite and Hg-rich silver is accompanied by Zn-rich hisingerite, baileychlore and Fe-oxides with variable but significant Si contents. The detailed description and quantitative chemical data for individual mineral phases are given. The studied mineralization was formed in significantly changing redox conditions (from reducing to highly oxidizing) with a distinct decrease of temperature (down to below 70(3) °C).

Inclusions in calcite phantom crystals suggest role of clay minerals in dolomite formation

AbstractMicro- and nano-inclusions embedded in calcite phantom crystals from Gemerská Ves, Slovak Republic, have been characterized by a combination of Raman spectroscopy, scanning and transmission electron microscopy, X-ray powder diffraction, and C and O isotope analysis. Whereas the outer, colorless part of the phantom crystal is relatively homogeneous and cavity and inclusion-free, the inner terracotta-colored part contains abundant cavities, dolomite, hematite, goethite, titanite, phyllosilicates (mainly kaolinite and illite), and apatite inclusions and nanostructures that have formed on the walls of cavities. The nanostructures comprise hematite and goethite particles sandwiched between either two phyllosilicate crystals or a phyllosilicate and a carbonate (calcite or dolomite) crystal. Our observations suggest that all inclusions in the terracotta calcite originate from the terra rossa (a common soil type in karstic areas) and limestone outcropping adjacent to the calcite crystals. While the micrometer-sized phyllosilicate and hematite particles were likely transported from the terra rossa and attached to the surface of growing calcite, the presence of phyllosilicates that are only a few atomic layers thick and of euhedral hematite, goethite, and dolomite crystals suggests that these particles precipitated along with the phantom calcite in situ, from an aqueous solution carrying terra rossa-derived and limestone-derived solutes. The compositional differences between the terra rossa (e.g., smectite as the only major Mg-rich phase) and terracotta calcite inclusions (e.g., dolomite as the only major Mg-rich phase and the presence of only Mg-free clays) hint that a smectite-illite conversion provides the Mg necessary for the precipitation of dolomite and possibly the Fe associated with the iron oxyhydroxide nanostructures. Phyllosilicate nucleation on calcite and dolomite nucleation on phyllosilicates, as inferred from nanoscale mineralogical associations, suggest that carbonates and phyllosilicates may mutually enhance nucleation and growth. This enhancement may result in the formation of large-scale clay-carbonate successions in aqueous settings, including the enigmatic, pink-colored cap dolostones succeeding late Neoproterozoic “Snowball Earth” deposits. The distribution of inclusions in the terracotta calcite and the preferred nucleation of hematite and goethite on phyllosilicate, rather than on carbonate surfaces, indicates that phyllosilicates have a potential to not only disrupt crystal growth and trigger the formation of cavities in the structure of the calcite host, but also to provide surfaces for the precipitation of different phases in the cavities and to uniformly distribute otherwise incompatible materials in a calcite host crystal. This calls for further exploration of the potential application of phyllosilicates in composite structure development.

Impact of hematite on dust absorption at wavelengths ranging from 0.2 to 1.0 µm: an evaluation of literature data using the T-matrix method.

Hematite is the absorbing mineral component of dust aerosols in the shortwave spectral region. However, dust shortwave absorption related to hematite suffers from significant uncertainties. In this study, we evaluated available hematite complex refractive index data in the literature on determining the dust effective refractive index at wavelengths ranging from 0.2 to 1.0 µm using rigorous T-matrix methods. Both spherical and super-spheroidal dust with hematite inclusions were examined to compute the dust optical properties and associated effective refractive indices. We found that the imaginary part of the effective refractive index retrieved from all available hematite complex refractive index data is larger than the measured effective values from Di Biagio et al. [Atmos. Chem. Phys.19, 15503, (2019)10.5194/acp-19-15503-2019]. The result obtained using the hematite refractive index from Hsu and Matijevic [Appl. Opt.241623 (1985)10.1364/AO.24.001623] is closest to but approximately two times larger than Di Biagio et al. [Atmos. Chem. Phys.19, 15503, (2019)10.5194/acp-19-15503-2019]. Our results emphasize the importance of accurate measurements of mineral refractive indices to clarify the dust absorption enigma.

Fluid inclusions in amethyst quartz of different geological environments from Brazil

Abstract Fluid-inclusion studies were conducted on amethyst quartz from three different geological environments: basalt cavities; hydrothermal veins; and granitic pegmatites of Eastern Brazil, to understand the conditions of amethyst crystallisation in each of these environments. In samples from basalt cavities, fluid inclusions are exclusively one-phase aqueous types suggesting a low-temperature formation environment. Crystals from the two other environments show that fluid inclusions can be either one-phase aqueous, two-phase aqueous, three-phase aqueous carbonic, or three-phase aqueous with the presence of precipitated solid halite. The carbonic composition of the system H2O–CO2–NaCl was confirmed by Raman spectroscopic analysis and suggests a metamorphic or magmatic fluid source. Fluid inclusions trapped in samples from hydrothermal veins reveal at least two different fluid generations based on homogenisation temperatures. The first generation has minimum trapping temperatures between 249°C and 391°C. The second generation of lower temperature fluids has minimum trapping temperatures varying from 82°C to 203°C. Fluid inclusions of this group record eutectic temperatures that indicate the presence of Ca and Fe cations in addition to Na. Fluid inclusions trapped in amethyst from a pegmatite body have moderate salinity values between 15 and 25 eq. wt.% NaCl, thus reflecting the elevated salt content in pegmatite-forming fluids. In this sample, the first fluid generation is represented by aqueous fluid inclusions with minimum trapping temperatures ranging from 268°C to 375°C. The estimated eutectic temperatures, generally below –50°C, indicate the presence of Ca cations in addition to Na. Minimum trapping temperatures correspond to temperatures of late-to-post-pegmatitic hydrothermal stages. The second generation records minimum trapping temperatures between 125°C and 247°C. Amethyst from both hydrothermal veins and pegmatite environments contain solid inclusions of hematite, an indication that the mineralising fluid was Fe rich and thus, possibly magmatic in origin.

Transformation from oxidized to reduced alkaline magmas in the northern North China Craton

The oxygen fugacity of a magma controlling element distribution and mobility is influenced by multiple factors including source composition and magmatic processes (e.g., assimilation and degassing). Here, whole-rock and in situ mineral analyses of the alkaline complex from the northern North China Craton are reported to investigate the influence of source composition and magmatic processes on the evolution of the melt oxidation state. The alkaline complex comprises orthoclase pyroxenites, pyroxene syenites, and syenites with similar mineral assemblages and trace element patterns. Consistent zircon and titanite U-Pb age of ~234 Ma is presented for the alkaline rocks. Their high large ion lithophile elements content, negative high field strength element anomalies, and high 87Sr/86Sr ratio suggest a metasomatized mantle origin by recycled crustal material. Magnetite and hematite inclusions were found in the early-crystallized minerals including clinopyroxene and apatite. Fluorapatites are characterized by high S (SO3 up to 0.64 wt%) and low Mn contents, with estimated ΔFMQ higher than +3. These features together indicate a primary oxidized magma inherited from the metasomatized mantle.However, inclusion-rich fluorapatites with decreasing S and increasing Mn content from the core to the rim were found in some pyroxene syenites. Moreover, the S content of inclusion-rich fluorapatite from these pyroxene syenites is lower than that of other rock types in the complex, which may correspond to a decrease in oxygen fugacity of ~2.5 log units. These features suggest that the oxidized magma was later transformed into reduced magma. Oriented Cu-rich sulfide in inclusion-rich fluorapatite formed during the reducing event. δ65Cu of the pyroxene syenite with inclusion-rich fluorapatite (−0.06 to −0.24‰) and sulfides in fluorapatite (−0.26‰) are lighter than those of other rock types (+0.30 to +0.43‰), suggesting that the sulfides were derived from extraneous materials via magma assimilation. Variable and high 87Sr/86Sr ratios (0.7071–0.7085) in titanites and high-207Pb/206Pb rims of zoned K-feldspar in the pyroxene syenite imply that the reduced extraneous materials were derived from ancient crust. Overall, in situ mineral analyses and Cu isotopes indicate a direct link between the transformation of alkaline magma from an oxidized to a reduced state with source composition and magmatic processes, and improve our understanding of element mobility and distribution.

Мінералогічні особливості відновлених окатишів Північного гірничозбагачувального комбінату (Криворізький басейн)

The advantages of reduced iron ore pellets over oxidized ones have been considered. The chemical and physical parameters of the primary material have been given: iron ore (magnetite) concentrate (iron content is 67.30 mass %; particle size distribution is 97.0 % of 0.056 mm fraction), bentonite (montmorilonit content is 70.43 mass %; particle size distribution is 94.5% of 0.056 mm fraction), ground coke. The course of experiments has been described. The zonal structure of the obtained pellets has been characterized: the central zone is composed of hematite with relics of magnetite; intermediate one contains magnetite particles with inclusions of hematite; peripheral zone includes subidiomorphic crystals of magnetite and their dendritic aggregates. The bentonite component in the central and intermediate zones retained a crypto-crystalline structure, in the peripheral one it had been transformed into a crystalline aggregate of garnet and pyroxene. Emphasis is placed on the need to verify the impact of various indicators of temperature, firing time, quantitative ratios of charge components, mineral and chemical composition of the cementing material on mineralogical transformations.

Jewelry and ornamental ammonites from the deposits of the Hauterivian stage of the Ulyanovsk Region: Mineral composition, gemological characteristics

Significant volumes of products from ammonites of jewelry and ornamental quality of the Ulyanovsk region are entering the Russian and world markets. Ammonites are associated with the Lower Cretaceous Hauterivian deposits and consist mainly of aragonite, calcite and pyrite. The walls and partitions of the ammonite shells retained the original aragonite composition. Also established inclusions of apatite, algodonite, pyrolusite, hematite, goethite, quartz, ferrihydrite, magnetite, uranium oxide, vernadite, todorokite, monazite, halloysite, plagioclase, kaolinite, glauconite, dolomite, graphite and organic matter. An important role in the formation of new mineral species was played by bacteria. Mineralization of undamaged ammonite chambers was carried out by the sequential formation of calcite and pyrite layers without metasomatic substitutions. The color of calcite layers is determined by the contents of Mn and Fe. Ammonites are characterized by high decorativeness and good technological characteristics; they can be mined in the coastal cliffs and the beach area of the river Volga without mining, without violating the environment.

Genesis of baddeleyite and high δ18O zircon in impure marble from the Tongbai orogen, Central China: insights from petrochronology and Hf–O isotope compositions

Coexistence of baddeleyite and zircon is rarely documented in metacarbonate rocks and poorly understood. In this study, metamorphic baddeleyite and zircon were recognized in marbles from the Tongbai orogen, central China. We carried out high spatial resolution analysis of trace element geochemistry and U‒Pb‒Lu‒Hf‒O isotope systematics for the zircon and baddeleyite. SIMS (Secondary Ion Mass Spectroscopy) U‒Pb analysis yields weighted average 207Pb/206Pb ages of 1848.0 ± 3.3 Ma (MSDW = 1.2, n = 12) for baddeleyite and 1840 ± 13 Ma (MSWD = 1.7, n = 13) for zircon. The baddeleyite grains have relatively low REE (Rare Earth Element) contents (ΣREE = 1.30–47.3 ppm) with flat chondrite-normalized patterns and moderate contents of HFSE (High Field Strength Element), which are ascribed to the nature of its host metacarbonate rock and mineral assemblage. The zircon grains have low Ti contents (1.63–3.56 ppm), positive Ce anomalies (Ce/Ce* = 1.91–6.96), and mineral inclusions of hematite, calcite, dolomite, lizardite and H2O. These characteristics indicate zircon formation in a relatively low-temperature setting under oxidizing and water-rich conditions. The baddeleyite and zircon have similar ranges of eHf(t) values of – 7.7 to – 10.5 and – 7.8 to – 11.6, respectively, which suggest their derivation from a relatively uniform Hf source. The high δ18O values of these zircon grains (20.0–21.0‰) preclude any percolation of external meteoric fluids during their crystallization. Therefore, a “detrital-zircon-dissolution” reaction is preferred to provide the Zr for the metamorphic zircon and baddeleyite in this study. The O isotope disequilibrium between whole-rock (δ18O = 16.2–16.3‰) and metamorphic zircon is most likely caused by metamorphic devolatilization or interaction with an H2O-dominant fluid during later metamorphism. On the other hand, the baddeleyite grains yield δ18O values ranging from 4.5 to 8.7‰, which is in disequilibrium with metamorphic zircon at the temperature of “detrital-zircon-dissolution” reaction (ca. 660–710 °C; Zircon + Dolomite → Baddeleyite + Forsterite + Calcite + CO2). The genesis of metamorphic zircon can be interpreted by a baddeleyite-to-zircon reaction of ZrO2 + SiO2 → ZrSiO4 at a retrograde stage, consistent with the low Ti-in-Zircon temperatures. The results highlight the importance of integrating trace element and multiple isotopic compositions in studying the genesis of metamorphic baddeleyite and zircon. Additionally, the U–Pb geochronology data also indicate that the protoliths of the marble were deposited in the early Paleoproterozoic and experienced a metamorphism at 1.85 Ga. The ca. 1.85 Ga high-grade metamorphic event might have been induced by the coeval magmatism in an extensional setting.

Fe and C isotopes constrain the pathways of hematite and Fe–rich carbonates formation in the Late Neoproterozoic Dahongliutan BIF, NW China

Unlike most Neoproterozoic BIFs in which Fe occurs almost exclusively in hematite, the Late Neoproterozoic Dahongliutan BIF uniquely contains Fe oxides (hematite), Fe–rich carbonates (siderite and ankerite) and Ca–Mg carbonates (calcite and dolomite). The diversification of minerals presents a unique opportunity to constrain the biotic and/or abiotic pathways for the precipitation of Fe–bearing minerals involved in Neoproterozoic BIF genesis. Here, we present integrated Fe, Nd and C compositions of hematite, Fe–rich carbonates, bulk–rock various facies BIFs, silicified calcic marbles and schists from the Dahongliutan BIF. These data suggest that the aqueous Fe2+ with light δ56Fe (~−1.60‰) at Dahongliutan was sourced from mixed continentally derived Fe (δ56Fe ~ −2‰) that was mobilized by microbial dissimilatory Fe reduction (DIR) and hydrothermally sourced Fe (δ56Fe ~ 0‰) from MORB systems. Lower than 65% of this dual–source mixed Fe occurred oxidation and formation of Fe(OH)3 via limited atmospheric O2, and/or anaerobic photosynthetic Fe2+ oxidation. The majority of these poorly crystalline Fe(OH)3 precipitates undergo dehydration and recrystallization to eventually bury as hematite with light δ56Fe (averaging −0.15‰). δ56Fe (~−0.11‰) and δ13CV-PDB (−4.00‰ to −6.60‰) of the siderite suggest that conversion of a small amount of Fe(OH)3 into siderite occurred via near–complete reduction by DIR. This DIR process is coupled to the oxidation of organic C, in which the ratio of HCO3− derived from oxidation of CH2O (HCO3−org) to total HCO3− (HCO3−total) = 1/4. Petrographic evidence (the common occurrence of hematite inclusions in the siderite and ankerite), coupled with mineral chemistry, support a model that some siderite transforminto ankerite with consistent δ56Fe (~−0.11‰) and δ13CV-PDB values (−4.20‰ to −5.40‰). In summary, our results demonstrate that DIR play an important role in Neoproterozoic BIFs.

Agates from Western Atlas (Morocco)—Constraints from Mineralogical and Microtextural Characteristics

Agate samples collected from the vicinity of Asni and Agouim (Western Atlas, Morocco) were investigated using microscopic observations supported by Raman micro-spectroscopy. The agates are marked by the presence of various microtextures typical of epithermal vein deposits, including jigsaw-puzzle, feathery, and lattice-bladed. The first two indicate that the formation of agates was likely marked by recrystallization of metastable silica phases (i.e., opaline silica or massive chalcedony). The presence of lattice-bladed (after barite and calcite) quartz may be, in turn, ascribed to the boiling-related conditions that could have triggered the formation of abundant copper and iron sulfides found within silica matrix. Additionally, the local occurrence of growth lines (so-called Bambauer quartz) and intergrowth of length-slow and length-fast chalcedony are linked to the variations of physico-chemical conditions during rock formation (alkaline-acidic). According to Raman spectroscopy, silica matrix of the agates is made of α-quartz with a local admixture of moganite (from 0.0 up to 78 wt.%), but also contains numerous solid inclusions of hematite, celadonite, as well as poorly-organized carbonaceous material and rutile. These phases were likely emplaced during low-temperature hydrothermal activity of SiO2-bearing fluids that originated from post-magmatic hydrothermal activity developed within host rocks and/or meteoric waters.

Agate mineralization in spilitized Permian volcanics from “Borówno” quarry (Lower Silesia, Poland) – microtextural, mineralogical, and geochemical constraints

Two forms of silica mineralization, i.e.: (1) vein agates and (2) moss agates, were encountered for the first time within spilitized (albite-rich) alkali-basalts from the upper parts of Borówno quarry (Intra-Sudetic Basin). Vein agates consist predominately of length-fast chalcedony evolving into strongly-zoned prismatic quartz with a distinctive blue cathodoluminescence. They exhibit the abundance of microtextures typical of epithermal vein systems (i.e. pseudo-bladed, flamboyant/feathery, jigsaw puzzle/mosaic, and Bambauer quartz), but also comprise numerous solid inclusions such as sulphides (pyrite, marcasite, chalcopyrite, and chalcocite), sulphates (barite), carbonates (calcite, dolomite), carbonaceous material, and hematite. The formation of vein agates was marked by strong variations in physicochemical conditions (pH, silica content) of SiO2-bearing fluids and recrystallization of metastable silica phases (i.e. moganite), coupled with the local replacement of pre-existing sulphates and/or carbonates by silica during boiling-related conditions. Their LREE-enriched rare-earth patterns with positive Eu anomaly invoke the close relationship between host rocks alterations (spilitization) and origin of SiO2-rich fluids. Conversely, moss agates have formed as a result of a spherical growth of siliceous phases within vesicles and contain numerous mineral inclusions of hematite, goethite, anatase, as well as ore-related phases (mottramite and dickite). The examination of their surface micro-morphology showed a strong age-related maturation marked by the development of stacked silica plates (plate-edge-like structure). The botryoidal-shaped, white-coloured, and Al-enriched outermost layer of these agates possibly reflects a rapid deposition of the initial silica gel onto the cavity walls. The HREE-enriched rare-earth patterns of the moss agates, combined with the presence of positive Eu and Ce anomalies, suggest their parental SiO2-rich fluids could be derived from multiple sources (e.g. volcanics mesostasis and/or meteoric waters). The fluids responsible for the formation of agate mineralization within alkali-basalts in Borówno could be derived from syn- or post- alterations of their host rocks.

The Carboniferous Shikebutai Iron Deposit in Western Tianshan, Northwestern China: Petrology, Fe-O-C-Si Isotopes, and Implications for Iron Pathways

Abstract Submarine volcanic-hosted iron deposits are important sources of iron ore in northwestern China. Here we present the petrological characteristics and coupled Fe-O, C, and Si isotope data of iron ores from the Shikebutai submarine volcanic-hosted hematite deposit in the Western Tianshan region. Several stratiform and lenticular hematite-dominated orebodies occur in Carboniferous submarine volcano-sedimentary sequences in this region. The ores are mainly composed of hematite, quartz, and minor siderite with distinct alternating iron-rich and silica-rich bands. The hematite shows δ56Fe and δ18O values in the range of –0.31 to 0.80‰ and 2.2 to 7.0‰, respectively, and the jasper yields δ30Si values of –1.90 to –1.20‰. Iron and Si were both derived from hydrothermal fluids related to submarine magmatism/volcanism. The Fe-bearing minerals in the Shikebutai deposit define distinct formation pathways. Hematite is the primary dehydrated Fe(III) oxyhydroxide, and the Fe isotope data indicate fractionation resulting from the partial oxidation of Fe(II). The O isotope data reflect inheritance from the submarine hydrothermal fluids source. Jasper was produced during coprecipitation of silica adsorbed onto the Fe(III) oxyhydroxides. The siderite-rich iron ore/volcaniclastic rock samples with a high and variable total organic carbon content (0.14–5.57%) show negative δ13C values (–3.0 to –1.1‰) and light δ56Fe values (–1.11 to –0.84‰). Our isotope data, together with the common occurrence of hematite inclusions in siderite, suggest that siderite was mainly produced by microbial dissimilatory iron reduction during diagenesis. The geologic, petrological, and isotopic data suggest that the Carboniferous Shikebutai deposit was precipitated through chemical and biogenic processes.