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A second study of ferriallanite-(Ce) from Nya Bastnäs, Sweden, extends current data by using electron probe micro-analysis (EPMA), laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) analysis and brings new insights about its crystal chemistry obtained by Raman spectroscopy. The study presents the first Raman spectra for ferriallanite-(Ce) member of the allanite group (not considering the rather low-quality spectra published in preceding papers). The material does not show significant radiation damage, which is rare as allanite-group minerals often have undergone metamictisation due to significant amounts of incorporated radionuclides (U, Th). Some interior regions show pronounced zoning that correlates with variations in Raman-band positions. In spite of its significant REE content, the material is virtually non-luminescent. New additional data for allanite-(Ce) from Oßling, Germany and Domanínek, Czech Republic are also presented, which were used for comparison. [ABSTRACT FROM AUTHOR]

The long-known presence of a sharp OH absorption band in the tetragonal fluoride mineral sellaite, MgF2, inspired us to conduct a detailed study of the OH incorporation modes into this IR-transparent (where IR stands for Infrared) material as well as to search for hydrogen traces in two other IR-translucent halides—villiaumite (NaF) and fluorite (CaF2). Among these three phases, sellaite is the only one to incorporate 'intrinsic' OH groups, most commonly as O–H∙∙∙F defects oriented nearly perpendicular to the c-axis along the shortest edge of the constituent MgF6 polyhedra, in analogy with the isostructural mineral rutile, TiO2. Another defect type, seen only scarcely in untreated natural material, develops when subjecting sellaite to temperatures above 900 °C. It involves an O–H∙∙∙O cluster along the 2.802 Å edge of the original MgF6 dipyramid, as fluorine atoms are progressively expelled from the structure, being replaced by O2- anions. This is corroborated by the appearance of spectral absorption features typical for brucite (Mg(OH)2) and ultimately periclase (MgO), the presence of which could be proven via powder diffraction of the heat-treated material. Except for a 'dubious' peak most probably caused by included phases, neither villiaumite (NaF) nor fluorite (CaF2) showed any presence of 'intrinsic' OH defects. They do however decompose along a similar route into the respective oxide and hydroxide phases at high temperature. This thermal decomposition of the studied halide phases is accompanied by the emission of gaseous (HF)n species at temperatures well below their established melting point - a subject which seems to be quite overlooked. [ABSTRACT FROM AUTHOR]

Floods in 1997 and 2010 exposed the Frýdek and Frýdlant formations of the Subsilesian Unit in the Ostravice River bed near Frýdek-Místek. In the sedimentary sequence of upper Campanian to Maastrichtian marls and paraconglomerates, clasts of strongly altered basic volcanic rock were found, accompanied by carbonate concretions and layers. Rare apatite, biotite, and a Cr-rich spinel subgroup mineral are the only relatively well-preserved primary minerals in the clasts. The matrix contains buddingtonite, albite, sanidine, kaolinite, illite-muscovite, a mineral of the smectite group, and possibly also a mixed structure mineral of the chlorite-smectite type. Laths of buddingtonite, identified by powder X-ray diffraction and wavelength-dispersive X-ray spectrometry, are not homogenous. Their compositions range from Bd41 to Bd59 molar component, with Kfs ranging between 26 and 35 mol%, Nafs between 5 and 27 mol%, and Ca-feldspar between 1 and 4 mol%. The matrix is irregularly dolomitized. Carbonates are also present in pseudomorphs after idiomorphic olivine and in fill of amygdaloidal cavities. These carbonates reveal complicated alteration rock history, having cores of magnesite passing into almost pure siderite outer parts. Calcite is always the youngest and most homogenous carbonate, probably connected with a different geological event. Accompanying carbonate concretions are composed of three dolomitic phases with quartz, calcite, and muscovite. We can conclude that buddingtonite originates in alteration of primary feldspar and/or volcanic glass during the catagenetic breakdown of kerogen in the sediment, surrounded by clayey sediments rich in decomposing organic matter. Volcanic clasts have similar texture and supposed pre-alteration phase composition as the rocks of teschenite association, namely monchiquites to picrites. However, the source of volcanic clast within the sediments remains unclear. [ABSTRACT FROM AUTHOR]

A multi-methodological study was conducted in order to provide further insight into the structural and compositional complexity of rare earth element (REE) fluorcarbonates, with particular attention to their correct assignment to a mineral species. Polycrystals from La Pita Mine, Municipality de Maripí, Boyacá Department, Colombia, show syntaxic intergrowth of parisite–(Ce) with röntgenite–(Ce) and a phase which is assigned to B3S4 (i.e., bastnäsite-3–synchisite-4; still unnamed) fluorcarbonate. Transmission electron microscope (TEM) images reveal well-ordered stacking patterns of two monoclinic polytypes of parisite–(Ce) as well as heavily disordered layer sequences with varying lattice fringe spacings. The crystal structure refinement from single crystal X-ray diffraction data – impeded by twinning, complex stacking patterns, sequential and compositional faults – indicates that the dominant parisite–(Ce) polytype M1 has space group Cc. Parisite–(Ce), the B3S4 phase and röntgenite–(Ce) show different BSE intensities from high to low. Raman spectroscopic analyses of parisite–(Ce), the B3S4 phase and röntgenite–(Ce) reveal different intensity ratios of the three symmetric CO3 stretching bands at around 1100 cm−1. We propose to non-destructively differentiate parisite–(Ce) and röntgenite–(Ce) by their 1092 cm−1 / 1081 cm−1 ν1(CO3) band height ratio. [ABSTRACT FROM AUTHOR]

The onset of thermally induced, heterogeneous structural reorganization of highly radiation-damaged allanite-(Ce) begins at temperatures below 700 K. Three strongly disordered allanite samples (S74 20414: ~ 0.55 wt% ThO2, 22.1 wt% REE oxides, and maximum radiation dose 3.5 × 1018 α-decay/g; LB-1: ~1.18 wt% ThO2, 19.4 wt% REE oxides, and maximum radiation dose 2.0 × 1019 α-decay/g; R1: ~ 1.6 wt% ThO2, 19.7 wt% REE oxides, and maximum radiation dose 2.6 × 1018 α-decay/g) were step-wise annealed to 1000 K in air. Using orientation-dependent nanoindentation, synchrotron single-crystal X-ray diffraction (synchrotron XRD), X-ray powder diffraction (powder XRD), differential scanning calorimetry and thermogravimetric analysis (DSC/TG), mass spectrometry (MS), 57Fe Mössbauer spectroscopy and high-resolution transmission electron microscopy (HRTEM), a comprehensive understanding of the structural processes involved in the annealing was obtained. As a result of the overall increasing structural order, a general increase of hardness (pristine samples: 8.2–9.3 GPa, after annealing at 1000 K: 10.2–12 GPa) and elastic modulus (pristine samples: 115–127 GPa, after annealing at 1000 K: 126–137 GPa) occurred. The initially heterogeneous recrystallization process is accompanied by oxidation of iron, the related loss of hydrogen and induced stress fields in the bulk material, which cause internal and surface cracking after step-wise annealing from 800 to 1000 K. HRTEM imaging of the pristine material shows preserved nanometer-sized crystalline domains embedded in the amorphous matrix, despite the high degree of structural damage. The results show that hardness and elastic modulus are sensitive indicators for the structural reorganization process. [ABSTRACT FROM AUTHOR]

The Mössbauer, infrared and Raman spectra of iron-rich oxyphlogopite from a new locality, the Bartoy occurrence, Transbaikalia, Russia were obtained and its crystal structure was solved. The mineral is characterized by the absence of OH groups and ordered distribution of Fe2+ and Fe3+ between sites having octahedral coordination. Unlike oxyphlogopite holotype sample which is monoclinic (1M polytype), iron-rich oxyphlogopite belongs to the 3T polytype (space group P3112) with the unit-cell parameters a = 5.3248(2) Å, c = 29.788(3) Å, V = 731.44(8) Å3. Its refined crystal-chemical formula is (Z = 3): A(K0.9Na0.1) [M1(Fe2+0.6Mg0.4) M2(Fe3+0.4Ti0.4Mg0.2) M3(Mg0.4Fe3+0.3Ti0.2Al0.1)] [T1,2(Si0.7Al0.3)2O5]2X(O0.9F0.1)2. [ABSTRACT FROM AUTHOR]

An assemblage of tantalite-(Mn), tantalian rutile, tapiolite-(Fe), titanowodginite, ferrotitanowodginite, and hydroxycalciomicrolite occurs in the Nancy granitic pegmatite, San Luis range, Argentina. The Nancy beryl-type, beryl–columbite–phosphate subtype of LCT (Li-Cs-Ta) rare-element pegmatite was emplaced in the Paleozoic Conlara pegmatitic field. The assemblage occurs at the core margin of the pegmatite, forming an irregularly shaped, 18 by 6 cm nodule. The chemical composition of tantalite-(Mn) shows median Ta# [= (Ta/(Ta + Nb) apfu (atoms per formula unit)] and Mn# [= (Mn/(Mn + FeT) apfu] values of 0.57 and 0.64, respectively; Ti, U and Zr show maximum and [median] contents of: 3.37 [1.25] wt.% TiO2, 0.58 [0.24] wt.% UO2, and 0.72 [0.50] wt.% ZrO2. The unit-cell parameters indicate a moderately ordered structure. Tantalian rutile occurs as anhedral grains replacing tantalite-(Mn), associated with hydroxycalciomicrolite. Its chemical composition shows moderate to high Ti contents, with a maximum and [median] of 64.77 [38.67] wt.% TiO2. The proportion of Ta is very high, with 49.67 [39.59] wt.% Ta2O5. Tapiolite-(Fe), with 82.49 [81.86] wt.% Ta2O5, 2.51 [2.33] wt.% Nb2O5, 0.94 [0.79] wt.% TiO2, and 13.31 [13.18] wt.% FeO, has uniform Ta# and Mn# values, 0.95 and 0.09, respectively. Titanowodginite shows Ta# values ranging from 0.82 to 0.88, whereas in ferrotitanowodginite it ranges from 0.88 to 0.94. The Mn# value is similar in titanowodginite (0.51–0.64), and decreases in the ferrotitanowodginite (0.04 to 0.41). These minerals form a replacement sequence of tantalite-(Mn). Hydroxycalciomicrolite occurs in two generations: I and II. The dominant A cation is Ca, with a median value of 14.39 wt.% CaO. The MnO content, with a median of 1.16 wt.% MnO, is relatively constant. The amount of UO2 is usually below 3 wt.%, but locally attains 6.9 wt.%, and exceptionally 43.6 wt.%, in irregular rims that show a low analytical total, giving compositions that depart from the expected stoichiometry; it is clearly a subsolidus phase. In the more plausible explanation for the evolution of this assemblage, the magmatic crystallization of tantalite-(Mn) was followed during the early subsolidus stage by its partial replacement by tantalian rutile + tapiolite-(Fe) + titanowodginite + ferrotitanowodginte, associated with hydroxycalciomicrolite I, and later, by hydroxycalciomicrolite II produced by the influx of a late Fe–Ti–Ca-bearing fluid phase likely entering the pegmatite from the wall rocks. [ABSTRACT FROM AUTHOR]

The Kongsberg silver district has been investigated by microscopy and electron microprobe analysis, focusing primarily on the Ag-Hg-Sb mineralization within the context of the updated mineral paragenesis. The earliest mineralization stage is represented by sulfides, including acanthite, and sulfosalts. Native silver formed initially through breakdown of early Ag-bearing phases and later through influx of additional Ag-bearing fluids and silver remobilization. The first two generations of native silver were separated in time by the formation of Ni-Co-Fe sulfarsenides and the monoarsenide niccolite along rims of silver crystals. The presence of As-free sulfosalts and the absence of di- and tri-arsenides suggest a lower arsenic/sulfur activity ratio for the Kongsberg deposits compared to other five-element deposits. Native silver shows binary Ag-Hg and Ag-Sb solid solutions, in contrast to the ternary Ag-Hg-Sb compositions typical for other deposits of similar type. Antimonial silver together with allargentum, dyscrasite, and pyrargyrite was documented exclusively from the northern area of the district. Elsewhere, the only Sb-bearing minerals are polybasite and tetrahedrite/freibergite. Hg-rich silver (up to 21 wt% Hg) has been documented only in the central-western area. Myrmekite of freibergite and chalcopyrite reflects exsolution from an original Ag-poor tetrahedrite upon cooling, while myrmekite of pyrite and silver, forming through breakdown of low-temperature phases (argentopyrite or lenaite) upon heating, characterizes the Kongsberg silver district. Based on the stabilities of minerals and mineral assemblages, the formation of the silver mineralization can be constrained to temperatures between 180 and 250 °C. [ABSTRACT FROM AUTHOR]

Hafnium contents and Zr/Hf ratios were studied in zircons and their parent rocks from three magmatic suites associated with the Teplice caldera, Eastern Erzgebirge: rhyolite and dacite from the peraluminous Schönfeld Unit, relatively younger A-type Teplice rhyolite, and post-caldera A-type biotite and zinnwaldite granite and greisen. New data suggest that zircon crystallizing from a geochemically less evolved volatile- and water-poor melt is, compared to the host rock, relatively Hf-depleted, while zircon crystallizing from an evolved volatile- and water-rich melt has a Zr/Hf value approximately identical to that of the parental melt. Zr/Hf values in zircon did not change substantially either during greisenization, or during low-temperature alteration after metamictization. Zr/Hf values in the whole rock may serve as a sensitive indicator of magmatic fractionation of evolved granitic melts, as they are only negligibly influenced by the following hydrothermal processes. Zr/Hf values in individual cogenetic zircon grains are scattered but their general evolution trend in the rock series is consistent with the evolution of the whole-rock Zr/Hf values. [ABSTRACT FROM AUTHOR]

Nine beryl samples from Western Carpathians, Slovakia, were investigated by infrared and Raman spectroscopy and differential thermal analysis. Two types of water HO I and HO II were detected. Infrared spectroscopy proved the presence of water type I and II in the presence of alkali cations with several bands: (1) symmetric stretching vibration-ν; (2) antisymmetric stretching mode-ν; (3) bending vibration-ν. The presence of singly and doubly coordinated type II water (IIs and IId) was confirmed by single-crystal IR spectroscopy. From Raman spectra a band at 3606 cm was assigned to ν of water type I and the range of 3597-3600 cm to water type II. The presence of doubly coordinating water indicates a relatively highly hydrated environment with the presence of alkali ions including Na as the dominant cation coordinated by HO II. CO bands were detected only by single-crystal IR spectroscopy. Thermal analysis proved total water loss in the range of 1.4-2.0 wt% and three main dehydration events. Based on the study of bond-topological arrangements two molecules of HO IId are each bound with two H···O1 bonds and one Na-O bond with an angular distortion, and by releasing one HO molecule more stable HO IIs is produced. The HO I molecule is bound only by two equivalent hydrogen bonds. The HO IIs molecule with a Na-O bond strength of 0.28 vu and two H···O1 bonds of 0.14 vu without any forced angular distortion is the most stable of all. [ABSTRACT FROM AUTHOR]

Optical and crystal-chemical changes in two beryl varieties after the heat treatment were determined using a wide spectrum of analytical methods. Studied aquamarines are generally more enriched in Fe (up to 0.25 apfu) and alkali (up to 0.08 apfu) than yellow beryls (up to 0.07 apfu Fe, up to 0.04 apfu alkali). The determined c/ a ratio of 0.997-0.998 classified all our studied beryls as 'normal' beryls. While no crystal structure changes were observed in samples heated to 700 °C, those heated to 900 and 1,100 °C exhibited cracks and fissures. Reduced Fe occurred in samples heated between 300 and 700 °C, and subsequent oxidation from 900 to 1,100 °C induced changes in their colour and clarity. The Fe-bearing beryl colour is controlled by the position of the absorption edge and the presence of a broad band attributed to Fe in the NIR region. Blue colour results from the absorption edge located deeper in the UV region and the presence of broad band in the NIR region. Shift of absorption edge to the visible region at the presence of the broad band gives a yellow colour. Although our studied beryls are enriched in HO I molecule due to their low alkali content, the HO II molecule is also present. The following two dehydration processes were observed: (1) release of one double-coordinating HO II molecule at 300-500 °C and (2) total dehydration at 900-1,100 °C. The observed cracks and fissures likely resulted from channel water release in large beryl crystals. [ABSTRACT FROM AUTHOR]

In this paper, possibilities and limits of the application of REE luminescence (especially the NdF → I emission) as structural probe are evaluated. Important factors controlling the Nd luminescence signal are discussed, including effects of the crystal-field, crystal orientation, structural state, and temperature. Particular attention was paid to the study of the accessory minerals zircon (ZrSiO), xenotime-(Y) (YPO), monazite-(Ce) (CePO) and their synthetic analogues. Based on these examples we review in short that (1) REE luminescence can be used as non-destructive phase identification method, (2) the intensities of certain luminescence bands are strongly influenced by crystal orientation effects, and (3) increased widths of REE-related emission bands are a strong indicator for structural disorder. We discuss the potential of luminescence spectroscopy, complementary to Raman spectroscopy, for the quantitative estimation of chemical (and potentially also radiation-induced) disorder. For the latter, emissions of Nd-related centres are found to be promising candidates. [ABSTRACT FROM AUTHOR]

The crystal structure of natural magnesiozippeite, Mg[(UO)O(SO)](HO), from East Saddle mine, San Juan County, U.S.A., has been determined by single-crystal X-ray diffraction analysis and refined to R = 0.0348. The mineral is monoclinic, crystallizing in the space group C2/ m, with unit cell parameters a = 8.7005(5), b = 14.2541(6), c = 8.8433(5)Å, β = 104.408(5)°, V = 1062.24(9) Å and Z = 4. The structure consists of the structural sheets of zippeite topology, represented by a structure unit [(UO)O(SO)], and an interlayer where two Mg Φ octahedra are located. The crystal structure of natural magnesiozippeite shows considerable differences to the structure of previously reported synthetic analogue, namely in coordination of Mg in the interlayer. [ABSTRACT FROM AUTHOR]

This study addresses whether Raman spectra can be used to estimate the degree of accumulated radiation damage in monazite-(Ce) samples whose chemical composition was previously determined. Our results indicate that the degree of disorder in monazite-(Ce), as observed from increasing Raman band broadening, generally depends on both the structural state (i.e., radiation damage) and the chemical composition (i.e., incorporation of non-formula elements). The chemical effects were studied on synthetic orthophosphates grown using the Li-Mo flux method, and non radiation-damaged analogues of the naturally radiation-damaged monazite-(Ce) samples, produced by dry annealing. We found that the 'chemical' Raman-band broadening of natural monazite-(Ce) can be predicted by the empirical formula,where, FWHM = full width at half maximum of the main Raman band of monazite-(Ce) (i.e., the symmetric PO stretching near 970 cm), and (Th+U+Ca+Pb) = sum of the four elements in apfu (atoms per formula unit). Provided the chemical composition of a natural monazite-(Ce) is known, this 'chemical band broadening' can be used to estimate the degree of structural radiation damage from the observed FWHM of the ν(PO) band of that particular sample using Raman spectroscopy. Our annealing studies on a wide range of monazite-(Ce) reference materials and other monazite-(Ce) samples confirmed that this mineral virtually never becomes highly radiation damaged. Potential advantages and the practical use of the proposed method in the Earth sciences are discussed. [ABSTRACT FROM AUTHOR]

Nb-Ta-Ti-bearing oxide minerals (Nb-Ta-bearing rutile, columbite-group minerals) represent the most common Nb-Ta host in topaz-albite granites and related rocks from the Krásno-Horní Slavkov ore district. Tungsten-bearing columbite-(Fe), W-bearing ixiolite, wodginite and tapiolite-(Fe) are extremely rare in these rocks. Rutile contains significant levels of Ta (up to 37 wt.% TaO) and Nb (up to 24 wt.% NbO), with Ta/(Ta + Nb) ratio ranging from 0.04 to 0.61. Columbite-group minerals are represented mostly by columbite-(Fe) and rarely by columbite-(Mn), with Mn/(Mn + Fe) ratio ranging from 0.23 to 0.94. The exceptionally rare Fe-rich, W-bearing ixiolite occurs only as inclusions in Nb-Ta-bearing rutile from quartz-free alkali-feldspar syenites (Vysoký Kámen stock). Wodginite was found only in the topaz-albite microgranite of gneissic breccia matrix that occurs in the upper most part of the Hub topaz-albite granite stock. In wodginite, the Mn/(Mn + Fe) ratio is 0.42-0.51, whereas the coexisting tapiolite-(Fe) has a distinctly lower Mn/(Mn + Fe) ratio close to 0.06. [ABSTRACT FROM AUTHOR]

A complex assemblage of Nb-Ta-(Sn) oxide minerals occur in a relatively narrow (~1−2 m thick) extensively albitized, Hercynian granitic pegmatite dike intruding biotite granodiorites near Bratislava, SW Slovakia. The dike shows enrichment in beryl (locally Cs-rich) but absence of Li- and B-rich phases. Compositions and textural relationships indicate complex evolutions of Nb-Ta oxide phases with several generations presenting distinct textural and compositional features. The first generation of the Nb-Ta minerals from the quartz-microcline-muscovite zone show Ta,Fe-rich compositions with Ta# [Ta/(Ta + Nb)] = 0.52−0.70 (Ct I columbite-tantalite), 0.88−0.90 (Tap I ferrotapiolite) and 0.73−0.86 (Fw I ferrowodginite); Mn# [Mn/(Mn + Fe)] = 0.32−0.49 (Ct I), 0.06−0.10 (Tap I) and 0.33−0.41 (Fw I). The 2nd generation is represented by ferrocolumbite to ferrotantalite (Ct II) in saccharoidal albite zone, replacement zones of Ct II in Ct I, and irregular overgrowths of ferrotapiolite (Tap II) and ferrowodginite (Fw II) on Tap I grains. The minerals of the 2nd generation show decreasing of Ta# in comparison to the 1st group: 0.10−0.60 (Ct II), 0.85−0.87 (Tap II) and 0.73−0.77 (Fw II); Mn# attains 0.30−0.45 (Ct II), 0.06−0.09 (Tap II) and 0.26−0.37 (Fw II). The 3rd generation includes fissure fillings, overgrowths and replacement zones of manganocolumbite and manganotantalite (Ct III), ferrotapiolite (Tap III) and ferrowodginite (Fw III) on the older Nb-Ta phases (Ct I, Tap I, Fw I, Fw II), in the coarse-grained unit. The 3rd population displays distinct Mn# increasing (Ct III: 0.51−0.69, Tap III: 0.11−0.24, Fw III: 0.40−0.41), Ta# values reach 0.16−0.79 (Ct III), 0.88−0.92 (Tap III) and 0.80−0.81 (Fw III). The latest, 4th generation of the Nb-Ta phases represents irregular veinlets and patches of fluorcalciomicrolite, replacing Ct I, Tap I, Fw I, Ct II and Tap III. Decrease of Ta/(Ta + Nb) values in Ct II from the saccharoidal albite unit can be explained by crystallization from the albite-rich melt, which was significantly impoverished in Ta with respect to Nb, after crystallization of Ta-rich phases from the 1st generation (ferrotapiolite I, ferrowodginite I). [ABSTRACT FROM AUTHOR]

The actinide-containing mineral monazite–(Ce) is a common accessory rock component that bears petrogenetic information, is widely used in geochronology and thermochronology, and is considered as potential host material for immobilisation of radioactive waste. Natural samples of this mineral show merely moderate degrees of radiation damage, despite having sustained high self-irradiation induced by the decay of Th and U (for the sample studied herein 8.9 ± 0.3 × 1019 α/g). This is assigned to low damage-annealing temperature of monazite–(Ce) and "alpha-particle-assisted reconstitution". Here we show that the response of monazite–(Ce) to alpha radiation changes dramatically, depending on the damage state. Only in radiation-damaged monazite–(Ce), 4He ions cause gradual structural restoration. In contrast, its high-temperature annealed (i.e. well crystalline) analogue and synthetic CePO4 experience He-irradiation damage. Alpha-assisted annealing contributes to preventing irradiation-induced amorphisation ("metamictisation") of monazite–(Ce); however, this process is only significant above a certain damage level. [ABSTRACT FROM AUTHOR]

Sulfoselenides [Ag2(S,Se)] and Se-bearing polybasite have been discovered at the Kongsberg silver district. The selenium-bearing minerals occur in two samples from the northern part of the district, forming either single or polyphase inclusions together with chalcopyrite within native silver. The Ag-sulfoselenides show large chemical variations, covering nearly the complete compositional range between acanthite (Ag2S) and naumannite (Ag2Se). For the data presented here, there is no local maximum at the composition Ag4SSe attributed to the distinct phase called aguilarite, suggesting that this composition can be considered as one of many possible along the monoclinic Ag2S-Ag2S0.4Se0.6 solid solution series rather than a specific mineral phase. We present a model explaining the variations in the Se-content of Ag2(S,Se) as a result of gradual de-sulfidization of the rock under oxidizing conditions. During this process, sulfur from the Ag2S-component of Ag2(S,Se) oxidized and dissolved in the fluid phase as SO42−, resulting in the formation of native silver. The activity ratio aS2-/aSe2- of the system gradually decreased due to the removal of SO42−, which resulted in the stabilization of a sulfoselenide with higher selenium content. As a result of reaction progress, grains of Ag2(S,Se) became gradually enclosed in newly formed native silver, and therefore isolated from further reactions with the grain-boundary fluid. Grains isolated early during the process show low content of Se reflecting high aS2-/aSe2- of the equilibrium fluid, while grains showing high Se reflect the composition of late low aS2-/aSe2- fluids. Analyses of Se-bearing polybasite show that selenium is preferentially partitioned into Ag2(S,Se) compared to polybasite. The model presented here demonstrates how oxidation of sulfoselenides leads to fractionation of sulfur and selenium. [ABSTRACT FROM AUTHOR]

The Bohemian Massif hosts significant hydrothermal U-deposits associated with shear zones in the high-grade metamorphic basement. But there is a lack of evidence of a genetic link between mineralization and U-fertile igneous rocks. This contribution provides constraints on the major U source of the vein-type U-deposits, the timing of ore formation and the metallogenetic model. The anomalous trace element signatures of the low-temperature hydrothermal deposits (high Zr, Y, Nb, Ti, ∑REE) and their close spatial relation with ultrapotassic rocks of the durbachite series point to a HFSE and REE enriched source rock. The durbachites have high U content (13.4–21.5 ppm) mainly stored in magmatic uraninite and other refractory minerals (e.g., thorite, zircon, allanite) that became metamict over a time interval sufficient to release U from their crystal structure, as suggested by the time gap between emplacement of the durbachites (EMP uraninite U–Pb age ~ 338 Ma) and hydrothermal activity (SIMS uranium ore U–Pb age ~ 270 Ma). Airborne radiometric data show highly variable Th/U ratios (1.5–6.0), likely reflecting a combination between (1) crystallization of magmatic uraninite, (2) hydrothermal alteration, and (3) leaching and mobilization of U along NW–SE-trending fault zones, manifested by elevated Th/U values in the radiometric map. The presence of rare magmatic uraninite in durbachites suggests almost complete uraninite dissolution; EMP imaging coupled with LA-ICP-MS analyses of refractory accessory phases revealed extensive mobilization of U together with HFSE and REE, providing direct evidence for metal leaching via fluid-driven alteration of radiation-damaged U-rich minerals. The large-scale HFSE and REE mobilization, demonstrated by the unusual trace element signatures of the U-deposits, was likely caused by low-temperature (270–300 °C), highly alkaline aqueous solutions containing F-, P-, and K-dominated complexing ligands. The first SIMS U–Pb age of 270.8 ± 7.5 Ma obtained so far for U-mineralization from the Bohemian Massif revealed a main Permian U mineralizing event, related to crustal extension, exhumation of the crystalline basement, and basin formation, as recorded by U–Pb apatite dates (280–290 Ma) and AFT thermal history models of the durbachites. The Permo-Carboniferous sedimentary cover probably represented a source of oxidized basinal brines infiltrating the basement-hosted durbachite plutons and triggering massive metal leaching. The interaction between basin-derived brines and durbachites resulted in significant modification of the chemical composition of the hydrothermal system (K and F release during biotite chloritization, P liberation through monazite alteration), leading to the formation of ore-bearing fluids responsible for the metallogenesis of the basement-hosted unconformity-related U-deposits in shear zones in the Bohemian Massif. [ABSTRACT FROM AUTHOR]