Accessory minerals of the Cínovec (Zinnwald) granite cupola, Czech Republic: indicators of petrogenetic evolution

Summary A fully cored drillhole was drilled to 1596m by the Czech Geological Survey in 1961–1963 in the central part of the Cínovec (Zinnwald) granite cupola. Two types of granite were intersected: zinnwaldite granite (ZG), observed down to a depth of 730m, and protolithionite granite (PG), occurring to the end of the hole. The core was used to study the distribution and chemistry of: zircon, thorite, xenotime, monazite, bastnäsite, synchysite, REE oxyfluorides and hydroxyfluorides. Zircon occurs throughout the drillcore; it is strongly hydrated and fluorinated with about 18.5wt.% H₂O content in the apical part of the cupola. Its F-content reaches 2.41wt.%. Within the PG, the F concentration in zircon is low. Zircon is poor in Th and U and its HfO₂ contents vary from 1.01 to 5.24wt.%. Thorite is common in the PG, becoming rare in the ZG. It is strongly hydrated (up to 14wt.% H₂O) and fluorinated (up to 2.04wt.% F). Extensive solid solution between ThSiO₄ and YPO₄ was observed. Xenotime is strongly hydrated (up to 16wt.% H₂O), but its F content is low (<0.31wt.%). Two types of monazite were identified: Th-rich (up to 9.3wt.% ThO₂) in the ZG, and Th-poor (<2.5wt.% ThO₂) in the PG. Monazite remained stable during the hydration and fluorination process. Its REE chondrite-normalized distribution patterns show negative anomalies for La and Nd and a pronounced negative anomaly for Eu. Chemical compositions of several REE oxyfluorides and hydroxyfluorides were studied. REE fluorocarbonates are represented by bastnäsite and synchysite. Bastnäsite is abundant in the ZG. Its chondrite-normalized REE patterns are characterized by an important negative Eu anomaly and downward kinks at La and Nd. Synchysite-(Ce) and synchysite-(Y) are particularly well developed in the deeper parts of the cupola, and exhibit REE distribution patterns characterized by a weak negative Eu anomaly (synchysite-(Ce)), or a weak positive Eu anomaly (synchysite-(Y)).The distribution of accessory minerals reveals five major evolution stages: (1) Early magmatic crystallization of albite and orthoclase. (2) A late magmatic stage comprising protolithionite, quartz, accessory zircon, thorite, xenotime and monazite. (3) Interaction of this magmatic association with a fluid phase rich in F, CO₂ and H₂O, leading to the transformation protolithionite zinnwaldite and to the remobilisation of Nb, Ta, Ti, W, Sn. Accessory minerals formed during stage (2) were hydrated and fluorinated, except monazite. (4) The transfer of volatiles into the apical part of the cupola followed by the opening of the magmatic system generated microgranites and hydrolysis-type reactions leading to the appearance of REE oxyfluorides and hydroxyfluorides. (5) A late CO₂- and F-rich fluid phase was responsible for the deposition of REE fluorocarbonates. Monazite and xenotime became unstable in the apical part of the cupola. An influx of fluids with high Ca-activity occurred late during stage (5) and led to the formation of synchysite, and finally to the extensive precipitation of fluorite.     

Accessory minerals of the Cínovec (Zinnwald) granite cupola,Czech Republic Part 1: Nb-, Ta- and Ti-bearing oxides

Summary In 1961–63 the Czechoslovakian Geological Survey drilled a 1596 m deep borehole in the Sn-W-mineralized Cinovec (Zinnwald) granite cupola. The hole traversed zinnwaldite granite (ZG) to 730 m, then protolithionite granite (PG). The boundary between the two granites is a transition zone (TZ) about 10 m thick. The oxides of Nb, Ta and Ti, present in accessory amounts, are columbite, ilmenorutile, rutile and pyrochlore. The columbite occurs in both granites, but in the PG only below 1558 m depth. Its crystals are strongly zoned, the zoning representing variations in Nb/(Nb + Ta) on the one hand, and non-uniform distribution of W on the other. The columbite in the TZ is strongly enriched in W, up to 32.6 wt% WO₃. The columbites with W < M⁴⁺ show the substitutions W⁶⁺ + M⁴⁺ 2(Nb, Ta)⁵⁺, where (M⁴⁺ = Ti, Sn, Th, U, Zr) and 6M⁴⁺ + 3M³⁺ 4Fe²⁺ 5(Nb, Ta)⁵⁺, where (M³⁺ = Sc, Y). In columbites with W > M⁴⁺, tungsten is introduced by the substitution W⁶⁺ + M⁴⁺ 2(Nb, Ta)⁵⁺, but also through the appearance of Fe³⁺ in the B site according to the replacement 2W⁶⁺ + Fe³⁺ 3(Nb, Ta)⁵⁺. The ratio Fe/(Fe + Mn + Ca) increases with depth, and Nb/(Nb + Ta) is higher in the PG.The ZG is characterized by the presence of ilmenorutile, which does not occur in the PG, where rutile contains at most only 4 wt% Nb₂O₅. Two types of substitution have been found in the ilmenorutile: Fe³⁺ + (Nb, Ta)⁵⁺ 2Ti⁴⁺; (Fe, Mn)²⁺ + 2(Nb, Ta)⁵⁺ 3Ti^4-. For the ilmenorutiles studied, the ratio [Fe³⁺/(Fe, Mn)²⁺]at is near 1.AU- and Nb-rich phase, containing up to 36.2 wt% UO₂, included in protolithionite, and missing from the ZG, has the composition of a defect pyrochlore, A²⁺ ₂ ⁵⁺(O₆), and forms a solid solution with U⁴⁺B₂ ⁴⁺(O₆]), where B^4-= Ti, Si, Zr, Sn. Electron microprobe analyses indicate that this phase is strongly hydrated.The crystal chemistry of Nb-, Ta- and Ti-oxides in the Cinovec cupola reflects the complex geochemistry of its component granites and the interaction of the minerals with an F- and CO₂-rich fluid phase. Among the thermodynamic parameters, fO₂ plays a predominant role in the early evolutionary stages.

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Résumé Un sondage profond (jusqu'à -1596m), a été réalisé en 1961–63 par le Service géologique tchécoslovaque, dans la coupole granitique, minéralisée en Sn-W, de Cinovec (Zinnwald), République tchèque. Ce sondage a recoupé un granite à zinnwaldite (ZG), relayé en profondeur (–730 m) par un granite à protolithionite (PG). Le contact entre ces deux granites est matérialisé par une zone de transition (TZ) puissante de 10 m environ. Les oxydes de Nb, Ta et Ti, présents en quantité accessoire, sont représentés par: columbite, ilménorutile, rutile et pyrochlore.La columbite apparaît tant dans ZG que dans PG, mais dans ce dernier uniquement dans la zone profonde (-1558.0 m). Ses cristaux sont fortement zonés. Le zonage reflète des variations du rapport Nb/(Nb + Ta) d'une part et une distribution hétérogène de W, d'autre part. La columbite de la zone de transition ZG-PG est très enrichie en W (jusqu'à 32.6 wt.~/ 0 W03). Les coiumbites à W < SM4+ présentent des substitutions W⁶⁺ + M⁴⁺ 2(Nb, Ta)⁵⁺, où (M4⁴⁺ = Ti, Sn, Th, U, Zr) et 6M⁴⁺ + 3M³⁺ 4Fe²⁺ + 5(Nb, Ta)^5-, où (M³⁺ = Se, Y). Dans celles à W > EM⁴⁺, outre la substitution W⁶⁺ + M⁴⁺ 2(Nb, T)⁵⁺, le tungstène est introduit grâce à l'apparition de Fe 3+ sur le site B suivant le schéma: 2W⁶⁺ + Fe³⁺ 3(Nb, Ta)⁵⁺. Le rapport Fe/(Fe + Mn + Ca) croit avec profondeur; celui Nb/(Nb + Ta) augmente dans PG.Le ZG est caractérisé par la présence de l'ilménorutile; par contre, celui-ci est absent dans PG, oú le rutile ne contient que 4 wt.% Nb₂0₅ au maximum. Deux types de substitution sont mis en évidence dans l'ilménorutile: Fe³⁺ + (Nb, Ta)⁵⁺ 2Ti4⁺; (Fe, Mn)²⁺ + 2(Nb, Ta)⁵⁺ 3Ti⁴⁺. Pour les ilménorutiles étudiés, le rapport [Fe3+/(Fe, Mn)2+]à, est proche de 1.Une phase riche en U (jusqu'à 36.2 wt.% UO₂) et Nb, incluse dans la protolithionite et absente dans ZG, a composition d'un pyrochlore lacunaire A²⁺[B₂ ⁵⁺(06), formant une solution solide avec U4+E:B₂4+(O₆), où B⁴⁺ = Ti, Si, Zr, Sn. Les analyses à la microsonde électronique indiquent que cette phase est fortement hydratée.La cristallochimie des oxydes de Nb, Ta et Ti dans la coupole de Cinovec reflète tant la complexité géochimique des granites qui la composent que l'interaction des minéraux avec une phase fluide riche en F et CO,. Parmi les paramètres thermodynamiques, fO₂ joue un râle prépondérant lors des stades d'évolution précoces.

     

Quartz chemistry – A step to understanding magmatic-hydrothermal processes in ore-bearing granites: Cínovec/Zinnwald Sn-W-Li deposit,Central Europe

Quartz from granites, greisens and quartz veins from a 1596 m long vertical section through the Cínovec/Zinnwald Li-Sn-W deposit (Czech Republic) was studied using cathodoluminescence (CL) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP MS). The trace contents of Al, Ti, Li and the Ge/Ti and Al/Ti values in quartz reflect the degree of fractionation of parental melt from which primary quartz crystallized. From the biotite granite to the younger zinnwaldite granite, quartz is characterized by increasing contents of Al (from 136–176 to 240–280 ppm) and decreasing Ti (from 16–54 to 6–14 ppm), while the contents of Li and Ge are similar (15–36 and 0.8–1.7 ppm, respectively). Quartz of the greisen stage and vein stage is poor in all measured elements (26–59 ppm Al, 0.5–1.6 ppm Ti, 2–13 ppm Li, 0.8–1.6 ppm Ge). The youngest low-temperature quartz forming thin coatings in vugs in greisen and veins differs in its extreme enrichment in Al (>1000 ppm) and Li (∼100 ppm) and very low Ti (<1 ppm). Within the greisen, remnants of primary magmatic quartz should be distinguished from metasomatic greisen-stage quartz in their higher intensity of CL and relatively higher Ti contents. A part of primary magmatic quartz may by secondarily purified via infiltration of hydrothermal fluids and dissolution–reprecipitation processes. Such quartz parallels newly formed greisen-stage quartz in its chemical and CL properties; the share of greisen-stage quartz may by therefore overestimated.     

Tourmalinites and Sn-Li mineralization in the Valdeflores area (Cáceres,Spain)

Summary Tourmalinites containing quartz, Li- and Cs-micas, and small amounts of arsenopyrite occur near Cáceres, Spain. The tourmalinites are developed within Ordovician pelites near the eastern margin of the Cabeza de Araya batholith and close to several small bodies of tectonized granite. Field and textural relationships document an epigenetic character for the tourmalinites. The tourmaline is fine to very fine-grained and its composition is intermediate within the schorl-dravite solid-solution series. Hydrothermal veins containing quartz, muscovite, amblygonite-montebrasite, apatite, fluorite, and/or cassiterite appear spatially related to the tourmalinites. The mineralized area is located in a shear zone that was active during D₃ Hercynian deformation and the emplacement of granitic bodies. Tourmalinites and veins derived from B- and Li-rich magmatic-hydrothermal fluids are believed to have been associated with the intrusion of a hidden granite dome, the tourmalinites being produced by extensive metasomatism of Ordovician metasedimentary rocks.

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Turmalinite and Sn-Li-Vererzungen im Valdeflores Gebiet (Cáceres, Spanien)

Zusammenfassung Turmalinite, die Quarz, Li- und Cs-Glimmer, sowie geringe Mengen an Arsenkies führen, kommen in der Ne von Cáceres, Spanien, vor. Sie sind an ordovizische Metapelite in der Nähe des Cabeza de Araya Batholiths und kleinere tektonisierte Granitkörper gebunden. Eine epigenetische Genese der Turmalinite ist auf Grund von Feldbeziehungen und texturellen Beobachtungen belegt. Die Zusammensetzung der sehr feinkörnigen Turmaline entspricht intermediären Gliedern der Schörl-Dravit- Reihe. Hydrothermale Gänge mit Quarz, Muskowit, Amblygonit-Montebrasit, Apatit, Fluorit und/oder Zinnstein treten benachbart zu den Turmaliniten auf. Der mineralisierte Bereich ist an eine Scherzone gebunden, die während der hercynischen D3 Deformation und der Platznahme der Granite aktiv war. Turmalinite und Gänge lassen sich genetisch von magmatisch-hydrothermalen Fluiden ableiten, die mit der Intrusion eines nicht aufgeschlossenen Granitdomes zusammenhängen. Die Turmalinite sind das Produkt einer weiträumigen Metasomatose der ordovizischen Metasedimente.

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With 8 Figures     

Glass at endocontacts of granite massifs in Volcanic–Plutonic belts (Exemplified by granite of Central Kazakhstan)

Glass formation takes place under conditions of rapid cooling of a melt at a great temperature difference between the melt and the host rock, which must have high thermal conductivity. The most favorable conditions for glass formation exist when the melt intrudes in the form of thin apophyses. Glass has been found in granite massifs related to the volcanic–plutonic association.     

Some regional indicators of the Tertiary–Quaternary geodynamics in the paleocoastal part of the Bengal basin (India)

Sedimentary processes in the paleocoastal part of the Bengal basin that occured in the Tertiary and Quaternary have been addressed. Three indicators were used: sedimentary bedding forms, microstructure of the sediment, and trace fossils. Various forms of sedimentary structures developed under the influence of dynamic geomorphic processes in the study area in the Quaternary. The microstructure analysis of the sedimentary materials was made by two methods: microphotography and Digital Color Analysis (DCA). The microstructure analysis shows that the geomorphic process remained very dynamic in the Quaternary, influencing the form, thickness, and mineral composition of the sediment strata. The enrichment of the sediments in heavy minerals evidences either oscillating or combined flow sedimentation, while under stable conditions light-mineral deposition took place. The digital data of microfabric study by the DCA method also show that larger particles deposited in the oscillating or high-flow environment and evidence a greater amount of heavy minerals like ferruginous materials. Trace fossils found in the sediments of this area also strongly support the concept that the environment remained dynamic during the Tertiary and Quaternary. The Late Tertiary deposition shows that, during these periods, the sediments were transported from tide-dominated marine coast with low flow energy, which is typical of hot and humid conditions. From Late Tertiary to Early Quaternary, the macrotidal coast became mesotidal (wave-dominated). The second phase is the Middle Pleistocene, when the environment was stable, favoring the continuous deposition of finer particles under low- to medium-flow energy conditions. The third phase, the Recent, is marked by the shoreline shift and modification of the environment. In the Early–Middle Holocene, the shoreline started to shift, which modified the geomorphic conditions of this place from coastal to estuarine and, finally, inland fluvial.     

Magmatic and hydrothermal behavior of uranium in syntectonic leucogranites: The uranium mineralization associated with the Hercynian Guérande granite (Armorican Massif,France)

Most of the hydrothermal uranium (U) deposits from the European Hercynian belt (EHB) are spatially associated with Carboniferous peraluminous leucogranites. In the southern part of the Armorican Massif (French part of the EHB), the Guérande peraluminous leucogranite was emplaced in an extensional deformation zone at ca. 310 Ma and is spatially associated with several U deposits and occurrences. The apical zone of the intrusion is structurally located below the Pen Ar Ran U deposit, a perigranitic vein-type deposit where mineralization occurs at the contact between black shales and Ordovician acid metavolcanics. In the Métairie-Neuve intragranitic deposit, uranium oxide-quartz veins crosscut the granite and a metasedimentary enclave.Airborne radiometric data and published trace element analyses on the Guérande leucogranite suggest significant uranium leaching at the apical zone of the intrusion. The primary U enrichment in the apical zone of the granite likely occurred during both fractional crystallization and the interaction with magmatic fluids. The low Th/U values (< 2) measured on the Guérande leucogranite likely favored the crystallization of magmatic uranium oxides. The oxygen isotope compositions of the Guérande leucogranite (δ¹⁸O_{whole rock} = 9.7–11.6‰ for deformed samples and δ¹⁸O_{whole rock} = 12.2–13.6‰ for other samples) indicate that the deformed facies of the apical zone underwent sub-solidus alteration at depth with oxidizing meteoric fluids. Fluid inclusion analyses on a quartz comb from a uranium oxide-quartz vein of the Pen Ar Ran deposit show evidence of low-salinity fluids (1–6 wt.% NaCl eq.), in good agreement with the contribution of meteoric fluids. Fluid trapping temperatures in the range of 250–350 °C suggest an elevated geothermal gradient, probably related to regional extension and the occurrence of magmatic activity in the environment close to the deposit at the time of its formation. U-Pb dating on uranium oxides from the Pen Ar Ran and Métairie-Neuve deposits reveals three different mineralizing events. The first event at 296.6 ± 2.6 Ma (Pen Ar Ran) is sub-synchronous with hydrothermal circulations and the emplacement of late leucogranitic dykes in the Guérande leucogranite. The two last mineralizing events occur at 286.6 ± 1.0 Ma (Métairie-Neuve) and 274.6 ± 0.9 Ma (Pen Ar Ran), respectively. Backscattered uranium oxide imaging combined with major elements and REE geochemistry suggest similar conditions of mineralization during the two Pen Ar Ran mineralizing events at ca. 300 Ma and ca. 275 Ma, arguing for different hydrothermal circulation phases in the granite and deposits. Apatite fission track dating reveals that the Guérande granite was still at depth and above 120 °C when these mineralizing events occurred, in agreement with the results obtained on fluid inclusions at Pen Ar Ran.Based on this comprehensive data set, we propose that the Guérande leucogranite is the main source for uranium in the Pen Ar Ran and Métairie-Neuve deposits. Sub-solidus alteration via surface-derived low-salinity oxidizing fluids likely promoted uranium leaching from magmatic uranium oxides within the leucogranite. The leached out uranium may then have been precipitated in the reducing environment represented by the surrounding black shales or graphitic quartzites. As similar mineralizing events occurred subsequently until ca. 275 Ma, meteoric oxidizing fluids likely percolated during the time when the Guérande leucogranite was still at depth. The age of the U mineralizing events in the Guérande region (300–275 Ma) is consistent with that obtained on other U deposits in the EHB and could suggest a similar mineralization condition, with long-term upper to middle crustal infiltration of meteoric fluids likely to have mobilized U from fertile peraluminous leucogranites during the Late Carboniferous to Permian crustal extension events.     

Scattering of magnetic fabrics in the Cambrian alkaline granite of Meruoca (Ceará state,northeastern Brazil)

Anisotropy of magnetic susceptibility (AMS) applied to an alkaline granite from Meruoca (NE Brazil) recorded weak anisotropies, typically below 4%, and a considerable dispersion of the AMS axes. Red-clouded feldspars and clots of metasomatic minerals enclosed in magmatic crystals indicate that hydrothermal fluids altered the granite. U–Pb isotopic data show high-common Pb on zircons but allowed the calculation of a mean SHRIMP age of 523 ± 9 Ma attributed to the magmatic crystallization. Growth of fine oxides by late fluid–rock interactions was responsible for the scattering of AMS. Rock magnetic data indicate they consist mainly of an oxidized magnetite and (titano)hematite. Shape preferred orientation of mafic aggregates measured in granite quarries shows that the pluton preserves a gently dipping magmatic foliation. AMS in some quarries with a well-defined magmatic fabric, however, remains highly dispersed. When AMS mimics the mafic shape fabric, only magnetic foliations share a common orientation. Locally, AMS grounded in coarse Ti-poor magnetite associated with titanite develops a consistent subhorizontal oblate fabric that agrees with tectonic models suggesting that the cupola of the pluton has been exposed by erosion.     

Thin-skinned thrust mineralization in the Brasília fold belt: the example of the old Luziânia gold deposit

The Luziânia gold deposits in southern Goiás lie within the Late Proterozoic Brasília fold belt. The rocks that host the gold mineralization are a monotonous series of hydrothermally altered phyllites that have been subject to low grade regional metamorphism. The major controls on the gold mineralization are northeast trending and gently northwest dipping ductile-brittle, dextral-reverse shear zones associated with regional thin-skinned thrusting of the Canastra Group. From a preliminary fluid inclusion study it is deduced that low salinity, 7 eq. wt% NaCl, moderately dense, H₂O-CO₂ ± CH₄ ore fluids deposited gold at temperatures of 300 ± 75°C and pressures of 1.5 to 3 kb in the filling stage of the vein formation. Post-filling stage gold deposition probably occured by mixing of fluids at higher crustal levels (1.5–2 kb). During thrusting, prograde metamorphism released pore water which penetrated along thrust planes that acted as high permeability zones for the ponding and release, by hydraulic fracturing, of overpressured fluids. Later in the tectonic evolution and at shallower crustal levels, there was likely an incursion of near suface water into the fault zone.     

Soil carbon stock of the Central-East Asia in the climate warming

Water samples from the Wujiang River, a typical karst river system, were analyzed for major ion concentrations and δ^34S values of dissolved sulfate in order to identify the sources of sulfate, quantify the sulfate export flux and understand the role of sulfur cycling in chemical weathering rate of carbonate. Spatial variations in sulfate concentration and sulfur isotopic composition of tributaries over the catchment area are obvious, allowing to decipher S sources between rocks and atmosphere. According to the variations in sulfate concentration and isotopic composition, it is inferred that sulfate ions in the upper-reach river waters may have three sources, rain water, sulfate resultant from oxidation of pyrite in coal, and sulfate from sulfide deposits. In the lower reaches, the S isotopic composition of the samples lies mainly on a mixing trend between evaporite sulfate and rainwater sulfate, the contribution of sulfate from oxidation of pyrite being lesser. A pronounced seasonal variation in both content and isotopic composition of sulfate characterizes the Wujiang River. The average sulfate concentration of the waters is 0.65 mmol/L in winter, 0.17 mmol/L higher than that in summer. River water δ^34S values range from -15.7‰ to 18.9‰ in winter, while the δ^34S values of river waters in summer vary to a lesser extent than in winter, from -11.5‰ to 8.3‰. The δ^34S values of the main stream range from -6.7‰ to -3.9‰ in summer, averaging 3‰ lower than in winter. This indicates that in summer, when the discharge increases, the contribution of a source enriched in light isotopes to the atmosphere or the oxidation of pyrite in coal is more important.     

Geomorphological processes and landforms in the southern part of the Polická vrchovina Highland (Czech Republic)

The paper examines relations among geomorphological processes and the landforms of the Polická vrchovina Highland in the Bohemian Massif. Geologically, the Polická vrchovina Highland is a part of the Intra-Sudetic Basin on the boundary between Bohemia and Poland. The basin structure of the Highland developed during the neotectonic period. In the outer parts of the Highland, Mesozoic rocks dip gently to the centre of the basin to form cuestas. In the central part, layers are horizontal or subhorizontal. Mesas (Hejda and Osta Mesa) provide evidence of the erosion of the Mesozoic deposits. The relative uplift of the Highland resulted in the incision of the Metuje River and its tributaries. Unloading of rocks and erosion initiated typical geomorphological processes such as deep- seated creep, cambering, rockfalls, landslides and pseudokarst processes. These processes produced some typical forms controlled by rock properties, jointing, relative differences in height and climatic changes in the Tertiary and Quaternary.     

Natural background of hydrocarbon gases (C1–C5) in the waters of the Kara Sea

The distributions of methane and hydrocarbons (HC) C2+ throughout the vertical section of the water mass and sediments of the Kara Sea and the estuaries of the Ob and Yenisei rivers were examine during expeditions aboard the R/V “Akademik Boris Petrov” in 1997–2002. Data obtained during the expeditions and extensive information provided by the complex study of the carbon cycle in the aquatic area were used to analyze the genesis of hydrocarbon gases in marine water. The example of particulate matter was used to demonstrate that hydrocarbon gases of the composition C2–C5 contained in seawater are genetically related to aquatic organic matter (OM), regardless of seawater salinity. The hydrocarbon complex is dominated (80%) by higher C4–C5 homologues. The concentration of C4–C5 hydrocarbons in the estuarine fresh waters is comparable with the high methane concentration (2–3 μl/l), drastically decreases in the zone of water mixing, and then rapidly increases to values several times higher than the methane concentration in seawater outside the outer boundary of the sedimentation depocenter of riverine particulate matter. A direct correlation was established for the concentration of the C4 + C5 homologues with the OM of the water mass, and the leading role was demonstrated to be played by labile OM of the autochthonous biomass (marine phytoplankton) in the genesis of higher hydrocarbon gases (alkenes, alkanes, and isoalkanes) during the early diagenetic stage of OM transformations in water. Along with the biomass of phytoplankton, a significant role in the genesis of C2–C3 hydrocarbons is played by destruction products of terrigenous OM. The destruction of OM and generation of gaseous hydrocarbons, including methane, are restricted to the pycnocline and the water-sediment interface. The absolute predominance of the sum of the C4 + C5 homologues in the hydrocarbon C2+ gases and the presence of unsaturated C2–C4 hydrocarbons are typical of the natural background of hydrocarbons of the Kara Sea water mass.     

Evolution of mariupolite (nepheline syenite) in the alkaline Oktiabrski Massif (Ukraine) as the host of potential Nb–Zr–REE mineralization

Mariupolite, aegirine-albite nepheline syenite, outcropping only in the Oktiabrski massif in south-eastern Ukraine, is a potential resource of Nb, Zr and REE for future exploration and development. Some types of this rock can be also used in ceramics, glass and building industry and jewellery. Mariupolite is composed of (1) magmatic and (2) subsolidus and hydrothermal components. The magmatic assemblage includes zircon, aegirine, nepheline, albite, K-feldspar, pyrochlore, fluorapatite, fluorbritholite-(Ce) and magnetite. Alkaline-carbonate-chloride-rich fluids exsolved very early in the history of the rock, in a late stage of, or directly after, its consolidation, induced intensive high-temperature alteration of the primary mariupolite components resulted in formation of cancrinite, calcite, fluorite, REE-bearing minerals such as monazite, parasite-(Ce), bastnäsite-(Ce), as well as sodalite, natrolite and hematite. The genesis of this peculiar mineralization seems to be associated with multistage magmatic and tectonic activity of the Ukrainian Shield and fluids mediated metasomatic processes.     

Magmatic–hydrothermal rare-element mineralization in the Songshugang granite (northeastern Jiangxi,China): Insights from an electron-microprobe study of Nb–Ta–Zr minerals

The Songshugang granite, hidden in the Sinian metasedimentary stratum, is a highly evolved rare-element granite in northeastern Jiangxi province, South China. The samples were systematically taken from the CK-102 drill hole at the depth of 171–423 m. Four types of rocks were divided from the bottom upwards: topaz albite granite as the main body, greisen nodules, topaz K-feldspar granite and pegmatite layer. Electron-microprobe study reveals that the rare-element minerals of the Songshugang granite are very different from those of other rare-element granites. Mn^# [Mn/(Fe + Mn)] and Ta^# [Ta/(Nb + Ta)] of columbite-group minerals and Hf^# [Hf/(Zr + Hf)] of zircon are nearly constant within each type of rocks. However, back-scattered electron imaging revealed that Nb–Ta oxides and zircon of the Songshugang granite, especially those of topaz albite granite, topaz K-feldspar granite and greisen, are commonly characterized by a specific two-stage texture on the crystal scale. The early-stage Nb–Ta oxide is simply subhedral-shaped columbite-(Fe) (CGM-I) with low Mn^# (0.16–0.37) and Ta^# (0.05–0.29). Columbite-(Fe) is penetrated by the later-stage tantalite veinlets (CGM-II) or surrounded by complex Nb–Ta–Sn–W mineral assemblages, including tantalite-(Fe), wodginite (sl), cassiterite, and ferberite. Tantalite has wide range of Mn^# values (0.15–0.88) from Fe-dominance to Mn-dominance. Wodginite with Ta>Nb has large variable concentrations of W, Sn and Ti. Cassiterite and ferberite are all enriched in Nb and Ta (Nb₂O₅ + Ta₂O₅ up to 20.12 wt.% and 31.42 wt.%, respectively), with high Ta^# (>0.5). Similar to Nb–Ta oxides and Nb–Ta–Sn–W mineral assemblages, the early-stage zircon is commonly included by the later-stage zircon with sharply boundary. They have contrasting Hf contents, and HfO₂ of the later-stage zircon is up to 28.13 wt.%. Petrographic features indicate that the early-stage of columbite and zircon were formed in magmatic environment. However, the later-stage of rare-element minerals were influenced by fluxes-enriched fluids. Tantalite, together with wodginite, cassiterite, and ferberite implies a Ta-dominant media. An interstitial fluid-rich melt enriched in Ta and flux at the magmatic–hydrothermal transitional stage is currently a favored model for explaining the later-stage of rare-element mineralization.     

Geochronology and thermochronology of gold mineralization in the Turmalina deposit,NE of the Quadrilátero Ferrífero Region,Brazil

The Turmalina gold deposit comprises three epigenetic domains whose development is related to the propagation/reactivation events of the NW–SE Pitangui Shear Zone (PSZ). The lodes are hosted in a Late Archaean sedimentary sequence on top of a strongly deformed (mafic-dominated) metavolcanic pile metamorphosed under 3.5–4 kbar and 540–610 °C; the association forms the upper part of a lithostratigraphic succession (Pitangui Group) that overlies an older TTG gneissic basement. According to field evidence and petrographic observations, the ore-forming process is polyphasic, starting at the time when the PSZ crossed the metamorphic quartz + staurolite + biotite + almadine + hornblende isograde; the main evolving stages, however, mostly took place throughout the metamorphic retrogression path. Fluid inclusion microthermometry also shows that metamorphogenic aqueous–carbonic solutions (initially with ≈ 16–20 eq. wt.% NaCl and circulating at approximately 4 kbar and 550 °C) were subjected to repeated boiling and mixing with cooler aqueous solutions at approximately 1–2 kb and 300–350 °C. These boiling events, which were triggered by depressurization, were contemporaneous with gold (and later sulfide) deposition, preceding a late stage of hydrothermal activity under lower PT conditions (< 1 kbar and ≈ 130–230 °C). To constrain both the mineralization age and the source/pathways of the ore-forming fluids in the Turmalina deposit, a multi-system isotope (Pb–Pb, Rb–Sr and Sm–Nd) study was carried out using various whole-rock and mineral samples. The main results are as follows: (i) the onset of the ore-forming process took place at ca. 2.2–2.1 Ga; (ii) the critical timing for gold formation was confined to ca. 2–1.9 Ga; and (iii) the late hydrothermal influxes occurred after 1.75 Ga. Therefore, the ore-forming process can be envisaged as a result of successive physical–chemical processes that took place during two major, long-lasting (≈ 250 Ma) periods under initial cooling rates of approximately 1 °C/Ma and, after ca. 2–1.9 Ga, approximately 2.5 °C/Ma. Moreover, the rejuvenation episodes at ca. 2–1.9 Ga and ca. 1.75 Ga probably indicate reactivation events in the PSZ, which triggered new fluid inflows into the system and revitalized the ore-forming process. That was the case for fluids that circulated deeply through both the older basement rocks and the mafic volcanic pile, allowing either metal enrichment from multistage leaching processes of various reservoirs or possible U/Th decoupling during ore remobilization. The Turmalina ore-system lifetime is consequently confined to the Rhyacian period (Paleoproterozoic), which is compatible with the age constraints presented by other studies regarding the fold-thrust belt's development (ca. 2.125 Ga) and orogenic collapse (ca. 2.095 Ga).     

The first finds of paragenetic Th–REE mineralization in precambrian rocks of the Shatak complex (Southern Urals)

Detailed characterization of Th–REE mineralization confined to terrigenous deposits of the Shatak complex is presented. For the first time the veinlet xenotime mineralization, unknown in the scientific literature, as well as diverse unidentified Th–REE compounds of variable composition were described in terrigenous deposits. It was concluded that permanent Th and REE admixtures and rare-earth minerals and thorite, respectively, as well as the occurrence of paragenetic intergrowths of Th and REE minerals of variable composition, are evidence that Th–REE mineralization crystallized as a result of the influence of the fluid phase on the sedimentary substrate upon magmatic melt intrusion into the frame rocks. The superimposed dynamothermal greenschist-facies metamorphism led to decay of metastable Th–REE phases without affecting significantly the redistribution of thorium and rare earth elements.     

Biogenic sedimentation factors of mineralization in the Neoproterozoic strata of the Baikal–Patom region

The formation environments of stratiform ore deposits in the Neoproterozoic Baikal–Patom region (BPR) have been considered. A model for the formation of the Sukhoi Log gold ore deposit in the Bodaibo zone has been put forward. The first stage is gold concentration by a chemolithotrophic bacterial community. Independently established facts suggest that bacterial communities may also have contributed to initial metal accumulation in the sediments of the Kholodnaya Pb–Zn deposit. The ore beds occur in the high-carbon sediments of the side and trough of a back-arc basin. Sedimentation (Dal’nyaya Taiga and Zhuya regional horizons) took place during the “back-arc basin–foreland basin” transition. This transition is characterized by increased sediment bioproductivity, which is clearly evidenced from the increased biophile-element content and taxonomic diversity of organic remains. Hundreds of microfossil sites in the Neoproterozoic BPR host littoral benthos (cyanobacteria and brown algae) and plankton (green algae). Most microfossils in the outer shelf, on the basin side, and in its trough belong to chemolithotrophic bacteria. These bacteria are assumed to have accumulated metals in the vent field of the back-arc basin. Studies showed the ability of microorganisms (bacteria, algae, fungi, etc.) to accumulate Fe, Mn, Au, Pb, Zn, and other metals. Bacterial communities are particularly important for metal accumulation in the vent fields of rift zones and areas of arc volcanism. All these conditions were observed in the Neoproterozoic BPR.