Sorption of uranium(VI) at the clay mineral–water interface

Batch experiments were conducted to study the sorption of uranium on selected clay minerals (KGa-1b and KGa-2 reference kaolinite, SWy-2 and STx-1b reference montmorillonite, and IBECO natural bentonite) as a function of pH (4–9) and 0.001, 0.01, and 0.025 M NaCl in equilibrium with the CO₂ partial pressure of the atmosphere. Uranium concentrations were kept below 100 μg L⁻¹ to avoid precipitation of amorphous Uranium-hydroxides. Solely PTFE containers and materials were used, because experiments showed significant sorption at higher pH on glass ware. All batch experiments were performed over a period of 24 h, since kinetic experiments proved that the common 10 or 15 min are in many cases by far not sufficient to reach equilibrium. Kaolinite showed much greater uranium sorption than the other clay minerals due to the more aluminol sites available. Sorption on the poorly crystallized KGa-2 was higher than on the well-crystallized KGa-1b. Uranium sorption on STx-1b and IBECO exhibited parabolic behavior with a sorption maximum around pH 6.5. Sorption of uranium on montmorillonites showed a distinct dependency on sodium concentrations because of the effective competition between uranyl and sodium ions, whereas less significant differences in sorption were found for kaolinite. The presence of anatase as impurity in kaolinite enhanced the binding of uranyl-carbonate complexes with surface sites. The kinetic of uranium sorption behavior was primarily dependent on the clay minerals and pH. A multisite surface complexation model without assuming exchange is based on the binding of the most dominant uranium species to aluminol and silanol edge sites of montmorillonite, respectively to aluminol and titanol surface sites of kaolinite. For eight surface species, the log_k was determined from the experimental data using the parameter estimation code PEST together with PHREEQC.     

Refertilization of serpentinites in the Sulu UHP terrane,Eastern China: constraints from geochronology,mineral composition,geochemistry, and Re–Os isotopes

ABSTRACT

Serpentinites from Junan (JN), Rizhao (RZ), and Rongcheng (RC) in the Sulu ultra-high-pressure (UHP) terrane, China, were analysed for U–Pb zircon geochronology, mineral chemistry, whole-rock major and trace element chemistry (including rare-earth elements (REEs) and platinum-group elements (PGEs)), and Re–Os isotopes, in order to better constrain their petrogenesis and geodynamic process. The serpentinite zircons yield two age groups: 731 ± 10 to 780 ± 10 Ma for relic magmatic zircon cores, which may indicate early crystallization and emplacement of the peridotite in the Yangtze crust, and 209 ± 2 to 218 ± 3 Ma for metamorphic zircon, which coincides with Triassic UHP metamorphism. The spinels in the serpentinites exhibit significant Cr# variation (0.6–0.91) and have undergone multi-stage metamorphism. The serpentinites are characterized by enrichment in incompatible trace elements, low Ni and IPGE concentrations, and high Pd/Ir ratios, and the bulk-rock major elements plot in the ultramafic cumulate region. Their Re and Os concentrations are similar to those of typical orogenic peridotite, but they have high ¹⁸⁷Os/¹⁸⁸Os ratios (0.12433–0.14423). We believe that the serpentinite’s protolith consisted of cumulates from an asthenosphere-derived melt that intruded into the continental crust of the Yangtze craton in the Neoproterozoic. These cumulates were later subducted and metamorphosed during the subduction of the Yangtze craton in the Triassic. The serpentinites underwent melt–rock interactions and fluid enrichment, both prior to and during serpentinization.     

HP–UHP metamorphism as an indicator of slab dip variations in the Alpine arc

HP/UHP and LT metamorphic units that commonly occur in the inner parts of mountain belts result from the subduction of continental and oceanic material, most often exhumed prior to continental collision. The prograde pressure–temperature history of HP–UHP rocks strongly depends on the convergence rate and on the subduction zone geometry. The maximum pressure recorded provides a proxy for the depth of shearing off and stacking of HP metamorphic nappes. A 2-D thermal model of continental subduction at lithospheric scale is used to compute the length and pressure peak of detached HP metamorphic units as a function of the slab dip angle and the convergence rate. Model results are applied to the metamorphic nappe pile of the inner Alps. A mean convergence rate of 1 cm/year during the subduction of the Briançonnais terrane is indicated by the paleogeographic reconstructions between 46 and 38 Ma. On this basis, the available petrological data and lengths of metamorphic units are used to compute the variations of the slab dip angle. The slab dip angle is shown to increase, from the northeast to the southwest, along the Alpine arc with estimated values of 20° for Suretta, 30–45° for Monte Rosa and Gran Paradiso, and 60° for Dora Maira. From Eocene to Oligocene times, the increase in slab dip angle is controlled by changes of buoyancy, due to the spatial configuration of the Valaisan trough and the incoming of crustal material within the subduction zone.     

The mineral–geochemical evidence of contact transformation of ores of the Dzhusinskoe pyrite–polymetallic deposit (Southern Urals)

The Dzhusinskoe pyrite–polymetallic deposit is characterized by an abundant concentration of dykes of basic and intermediate rocks. Thermal metamorphism of ore-host rocks and the recrystallization of ore minerals are associated with the intrusion of post-ore dykes. A regular increase in the homogenization temperature from 156° at a distance from a dyke to 287–305°C in the contact zone was established. Highly saline (6.4–15.7 wt % NaCl eq.) CO₂–H₂O–NaCl fluids under high pressure (up to 1500 bar) can be associated with the processes of contact and regional metamorphism.     

Influence of the halokinesis on the clay mineral repartition of upper Maastrichtian–Ypresian in the Meknassy-Mezzouna basin,centerwestern Tunisia

The Meknassy-Mezzouna basin is affected by a fault system, assembling two main directions, northsouth and eastwest. The Triassic outcrops are widely noticeable at Jebel Jebbes El Meheri and the Mezzouna link. During the late Maastrichtian–Ypresian, the sedimentation in the basin is influenced by halokinetic events, which are clearly manifested either by the thickness of El Haria formation along the Triassic outcrops (rim syncline) or by an alteration surface at the top of the Abiod formation. Such events also confirm the emersion of the basin from the late Maastrichtian to the early Lutetian. However, the present work tries to highlight the effects of halokinetic uplift on the clay mineralogical variations at that area. In harmony with this halokinetic activity, the clay minerals of this time interval (during the late Maastrichtian–Ypresian) show a trend of variation which corresponds to the evolution from illite and kaolinite, indicating a strongly hydrolytic marine environment, to smectite, sepiolite, and palygorskite, reflecting a very rapid evolution from this marine environment to a lagoon environment, then to a more confined continental environment. In fact, the appearance of sepiolite and palygorskite on the top of El Haria formation and the Paleocene–Eocene transition can be explained by a transformation or neoformation mechanism in an alkaline environment, rich in silica and magnesium, under arid to semi-arid climatic conditions. Moreover, the palygorskite can be formed in continental deposits as well as in close marine environment, which displays a limited communication with the open sea. Under these conditions, the evaporation leads to high ionic concentration of alkaline pH, which is favorable to the formation and stability of this mineral.     

A comparative study of uranium–thorium accumulation at the western edge of the Arabian Peninsula and mineral deposits worldwide

Jordan, located at the western edge of the Arabian Plate, stands out from the remaining part of the Arabian Peninsula by its abundance in radioactive elements, mainly uranium, in a way so far not found elsewhere on the Arabian Peninsula. Uranium (U) and thorium in Jordan are concentrated in eight different types of ore mineralization: (1) intrusive-related (intramagmatic), (2) vein-type, (3) superficial, (4) sandstone-hosted (5) limestone-hosted, (6) U-Th-REE placer-type, (7) black shales, and (8) phosphorites. The major concentration of radioactive elements are synsedimentary and diagenetic in nature, mainly in near-shore marine depositional environments where uranium contents are abnormally high in the late Cretaceous to Paleogene phosphorites and increasing towards the mobile shelf of the Tethys ocean. These uraniferous phosphorites form the source of uranium that was redeposited within terrigenous chemical residues of lacustrine-fluvial depositional systems in Central Jordan (calcretes). Faultbound radiometric anomalies are caused by hot brines being vented along with the Jordan-Dead-Sea rifting. Presumably, low-grade U accumulation in (hot) black shales and marls of Silurian age are responsible for these radiometric anomalies. In the present paper, the Jordanian uranium concentrations are compared with reference types of uranium deposits elsewhere in the world to get an idea if the geological, chemical, and mineralogical features of analogue uranium mineralization in Jordan are indicative of economic targets. The uranium concentration in Jordanian phosphorites has been tracked beyond the border into Syria, Iraq, Israel, and Saudi Arabia. The uranium potential in neighboring countries is assessed based on the current geological data available for the Mediterranean Phosphorite Belt which is poised to become a another string to the bow with respect to energy supply on the Arabian Peninsula.     

Genesis of the mineral groundwaters of the Razdol’nenskii occurrence in Primorye

Original isotopic and chemical data are reported on the groundwater and gases from the unique occurrence of mineral water in the coastal zone of southern Primorye. Results of the δ¹⁸O and δ²H analysis of the underground and surface water of the area integrated with their δ¹³C composition made it possible to solve the problem of the genesis and evolution of groundwater and gases in the coastal part of the Sea of Japan. It was established that meteoric waters penetrate into the Mesozoic terrigenous rocks and changed their chemical composition under the influence of transformation of organic matter from the host rocks. CO₂ released owing to reactions provides multiple enrichment of the water in HCO₃ and stimulates Na influx via dissolution of aluminosilicates.     

Polyphase Deformation of the Weihai-Rongcheng UHP Unit Rocks, NE Sulu： Insights into the Tectonic Evolution of the Dabie-Sulu UHP and HP Belts, China

Different scales of structural data reveal a complex deformation history of ultrahigh- pressure （UHP） rocks exposed in the Weihai-Rongcbeng area, NE Sulu （northern Jiangsu-eastern Shandong）, eastern China. Excluding pre-UHP deformations, at least five major sequential deformational stages （D1-Ds） are recognized. The first deformation （DO produced a weak foliation and lineation in massive eclogites. The foliated eclogite with a dominant foliation containing a stretching and mineral lineation was developed during the I）2 deformation. Both the D1 and D2 deformations occurred under UHP metamorphic conditions, and are well preserved in the eclogite bodies. D3 structures which developed shortly after the formation of granulite/amphibolite facies symplectites are characterized by imbricated associations marked by a regional, steeply dipping foliation, compositional layering, eclogite boudinage, isoclinal folds and reverse ductile shear zones. The D3 deformation was accompanied by decompressional partial melting. A regional, gently dipping amphibolite facies foliation and stretching lineation, low-angle detachments, and dome- and arc-shaped structures formed during the D4 deformation stage dominate to some degree the map pattern of the Weihai-Rongcbeng UHP domain. The last stage of deformation （Ds） gave rise to the final exhumation of the UHP rocks. Ds is characterized by development of brittle-dominated high-angle faulting associated with emplacement of large volmnes of undeformed granite plutons and dykes dated at 134-100 Ma. The deformational and metamorphic sequence followed by the UHP rocks in the Weihai-Rongcheng area is similar to that studied in the entire Dabie-Sulu UHP and HP metamorphic belts from microscopic to mapping scale. Based on structural data, combined with available petrographic, metamorphic and geochronological data, a speculative tectonic evolutionary model for the Dabie-Sulu UHP and IIP belts is proposed, involving continental subduction/collision between the Sino-Korean and Yangtze cratons and subsequent polyphase exhumation histories of the UHP and IIP metamorphic rocks.     

Intermittent generation of mafic enclaves in the 1991–1995 dacite of Unzen Volcano recorded in mineral chemistry

Mafic enclaves in the 1991–1995 dacite of Unzen volcano show chemical and textural variability, such as bulk SiO₂ contents ranging from 52 to 62 wt% and fine- to coarse-grained microlite textures. In this paper, we investigated the mineral chemistry of plagioclase and hornblende microlites and distinguished three enclave types. Type-I mafic enclaves contain high-Mg plagioclase and low-Cl hornblende as microlites, whereas type-III enclaves include low-Mg plagioclase and high-Cl hornblende. Type-II enclaves have an intermediate mineral chemistry. Type-I mafic enclaves tend to show a finer-grained matrix, have slightly higher bulk rock SiO₂ contents (56–60 wt%) when compared with the type-III mafic enclaves (SiO₂?=?53–59 wt%), but the overall bulk enclave compositions are within the trend of the basalt–dacite eruptive products of Quaternary monogenetic volcanoes around Unzen volcano. The origin of the variation of mineral chemistry in mafic enclaves is interpreted to reflect different degree of diffusion-controlled re-equilibration of minerals in a low-temperature mushy dacitic magma reservoir. Mafic enclaves with a long residence time in the dacitic magma reservoir, whose constituent minerals were annealed at low-temperature to be in equililbrium with the rhyolitic melt, represent type-III enclaves. In contrast, type-I mafic enclaves result from recent mafic injections with a mineral assemblage that still retains the high-temperature mineral chemistry. Taking temperature, Ca/(Ca?+?Na) ratio of plagioclase, and water activity of the hydrous Unzen magma into account, the Mg contents of plagioclase indicate that plagioclase microlites in type-III enclaves initially crystallized at high temperature and were subsequently re-equilibrated at low-temperature conditions. Compositional profiles of Mg in plagioclase suggest that older mafic enclaves (Type-III) had a residence time of ~100 years at 800 °C in a stagnant magma reservoir before their incorporation into the mixed dacite of the 1991–1995 Unzen eruption. Presence of different types of mafic enclaves suggests that the 1991–1995 dacite of Unzen volcano tapped mushy magma reservoir intermittently replenished by high-temperature mafic magmas.     

Microbial control of mineral–groundwater equilibria:Macroscale to microscale

macroscale processes that perturb general groundwater chemistry and therefore mineral–water equilibria; and microscale interactions, where attached organisms locally perturb mineral–water equilibria, potentially releasing limiting trace nutrients from the dissolving mineral. In the contaminated unconfined glacio-fluvial aquifer near Bemidji, Minnesota, USA, carbonate chemistry is influenced primarily at the macroscale. Under oxic conditions, respiration by native aerobic heterotrophs produces excess carbon dioxide that promotes calcite and dolomite dissolution. Aerobic microorganisms do not colonize dolomite surfaces and few occur on calcite. Within the anoxic groundwater, calcite overgrowths form on uncolonized calcite cleavage surfaces, possibly due to the consumption of acidity by dissimilatory iron-reducing bacteria. As molecular oxygen concentration increases downgradient of the oil pool, aerobes again dominate and residual hydrocarbons and ferrous iron are oxidized, resulting in macroscale carbonate-mineral dissolution and iron precipitation. Feldspars, in contrast, weather exclusively at the microscale near attached microorganisms, principally in the anoxic region of the plume. Native organisms preferentially colonize feldspars that contain trace phosphorus as apatite inclusions, apparently as a consequence of the low P concentration in the groundwater. These feldspars weather rapidly, whereas nearby feldspars without trace P are uncolonized and unweathered. Feldspar dissolution is accompanied by the precipitation of secondary minerals, sometimes on the bacterial cell wall itself. These observations suggest a tightly linked biogeochemical system whereby microbial processes control mineral diagenesis at many scales of interaction, and the mineralogy and mineral chemistry influence microbial ecology. Only the macroscale interaction, however, is easily observable by standard geochemical methods, and documentation of the microscale interactions requires microscopic examination of microorganisms on mineral surfaces and the locally intense diagenetic reactions that result. Received, May 1999/Revised, October 1999/Accepted, October 1999     

P–T paths and modeling the evolution of metapelitic mineral assemblages in the MnNCKFMASH system: A case of Northern Ladoga area

Phase equilibria in medium-grade metapelites of the Northern Ladoga area are calculated with the pseudosection method and Theriak/Domono software on the basis of zoning in garnets. It is shown that garnet in the staurolite-bearing parageneses crystallized at 520–600°C against the background of a pressure release from 7.0 to 3.5–4.0 kbar. The evolution of the mineral composition of rocks in the course of porphyroblast formation is discussed, placing special emphasis on the comparative analysis of P–T paths of garnet growth under decompression conditions.     

Re–Os dating of chalcopyrite from selected mineral deposits in the Kalatag district in the eastern Tianshan Orogen,China

The Kalatag Cu–Zn–Au district contains a number of economically important Cu deposits in eastern Tianshan in Xinjiang, NW China. Due to the lack of precise mineralization ages, the metallogenesis of this area has long been a matter of debate. In this study, chalcopyrite Re–Os isotope methods are used to date the South Meiling Cu–Zn and Hongshi Cu deposits in the eastern part of Kalatag area.The South Meiling Cu–Zn deposit is hosted in volcanic-sedimentary rocks of the Late Ordovician to Early Silurian Daliugou Formation. The deposit consists of two parts: a concordant massive sulfide ores and discordant vein-type ores located in the footwall strata. The principal ore minerals are pyrite, chalcopyrite, sphalerite, minor tetrahedrite, galena and pyrrhotite. Gangue minerals include quartz, sericite and barite, and minor chlorite, plagioclase and carbonate minerals. The Hongshi Cu deposit represents a hydrothermal vein system hosted in the mafic volcanic rocks of Daliugou Formation. The orebodies are associated with quartz veins and controlled by subsidiary faults of the Kalatag fault. The ore-forming process can be divided into the early, middle and late stages and is characterized by quartz–pyrite, quartz–chalcopyrite–pyrite and quartz–carbonate–gypsum veins, respectively.Re–Os analyses of chalcopyrite from the South Meiling Cu–Zn deposit yield an isochron age of 434.2 ± 3.9 Ma and initial ¹⁸⁷Os/¹⁸⁸Os ratio of 0.647 ± 0.098 (MSWD = 0.59). Re–Os analyses of chalcopyrite from the Hongshi Cu deposit yield an isochron age of 431.8 ± 2.7 Ma and initial ¹⁸⁷Os/¹⁸⁸Os ratio of − 0.165 ± 0.075 (MSWD = 0.77). Since chalcopyrite is the primary copper mineral, we interpret these isochron ages as the timing of Cu mineralization, based on field geology and petrographic evidence. These results suggest that the Re–Os ages presented here provide, for the first time, a direct constraint on an early Paleozoic Cu mineralization event of the eastern Tianshan Orogen. The high initial ¹⁸⁷Os/¹⁸⁸Os ratios (0.647 ± 0.098) ratio of ~ 434 Ma chalcopyrite from the South Meiling deposit suggest that the metal was sourced from a two end-member mixing of crust and mantle materials. Moreover, we propose that the VMS mineral system and hydrothermal vein system of the Kalatag district were related to the south-dipping subduction of the Kalamaili oceanic plate during the Late Ordovician–Silurian.     

Abiotic Methane Reservoirs in the Western Tianshan HP–UHP Metamorphic Belt,China

Natural gas, consisting primarily of methane(CH₄), has become a major source of clean energy in modern society in many parts of the globe. Recent experimental observations and discoveries of deep-sourced abiotic CH₄ in cold subduction zones indicate the important ability of cold subducted slabs to generate natural gas reservoirs. However, most CH₄ flux and reservoirs remain unknown and their potential is overlooked in global carbon flux estimations. Massive abiot...     

Application of the content–area (C–A) fractal model and prediction–area (P–A) plot for mineral prospectivity modeling in the Luchun area of Yunnan Province,China

This method of assigning weights based on expert opinion introduces bias when we are evaluating the relative importance of evidence values. In this paper, we used a prediction–area (P–A) plot method and content–area (C–A) fractal model to estimate the weight of each evidence map. In this paper, we used the content region (C–A) fractal model to divide the evidence maps to the threshold of the corresponding dimensions. The P–A plot approach is an objective data-driven approach for evaluating map weights. Using geochemical layer and remote sensing, hydroxyl layers as weight evidence maps are the highlights of this study. We use the P–A method from which we can evaluate the predictive ability of each evidence map with respect to the known ore occurrences. We used the P–A plot for weighting each evidence map and choosing the appropriate threshold for predictor maps in the Luchun area of Yunnan Province, China. The method adopted in this paper can improve the prediction efficiency of ore prospecting.     

Modeling the Seismicity of the Altai–Sayan–Baikal Region

Doklady Earth Sciences - Numerical modeling of the dynamics of the block structure and the resulting seismicity of the Altai–Sayan–Baikal region has been carried out. The earthquake...     

Nb–Ta–(Ti–Sn) oxide mineral chemistry as tracer of rare-element granitic pegmatite fractionation in the Borborema Province,Northeastern Brazil

The Borborema Pegmatitic Province (BPP), northeastern Brazil, is historically important for tantalum mining and also famous for top-quality specimens of exotic Nb–Ta oxides and, more recently, for the production of gem quality, turquoise blue, ‘Paraíba Elbaite.’ With more than 750 registered mineralized rare-element granitic pegmatites, the BPP extends over an area of about 75 by 150 km in the eastern part of the Neoproterozoic Seridó Belt. The Late Cambrian pegmatites are mostly hosted by a sequence of Neoproterozoic cordierite–sillimanite biotite schists of the Seridó Formation and quartzites and metaconglomerates of the Equador Formation. The trace-element ratios in feldspar and micas allow to classify most pegmatites as belonging to the beryl–columbite phosphate subtype. Electron microprobe analyses (EMPA) of columbite, tapiolite, niobian–tantalian rutile, ixiolite and wodginite group minerals from 28 pegmatites in the BPP are used to evaluate the effectiveness of Nb–Ta oxide chemistry as a possible exploration tool, to trace the degree of pegmatite fractionation and to classify the pegmatites. The columbite group mineral composition allows to establish a compositional trend from manganoan ferrocolumbite to manganocolumbite and on to manganotantalite. This trend is typical of complex spodumene- and/or lepidolite-subtype pegmatites. It clearly contrasts with another trend, from ferrocolumbite through ferrotantalite to ferrowodginite and ferrotapiolite compositions, typical of pegmatites of the beryl–columbite phosphate subtype. Large scatter and anomalous trends in zoned crystals partially overlap and conceal the two main evolution patterns. This indicates that a large representative data set of heavy mineral concentrate samples, collected systematically along cross-sections, would be necessary to predict the metallogenetic potential of individual pegmatites. Other mineral species, e.g. garnets and/or tourmaline, with a more regular distribution than Nb–Ta oxides, would be more appropriate and less expensive for routine exploration purposes. The currently available Nb–Ta oxide chemistry data suggest the potential for highly fractionated Ta–Li–Cs pegmatites in the BPP, so far undiscovered, and encourages further, more detailed research.     

Unique СО2–saturated mineral waters of the Mukhen deposit (Khabarovsk Krai): Composition and genesis

Doklady Earth Sciences - The results of studies of the ion-salt, gas, and isotopic compositions of unique СО2–saturated cold mineral waters from the Mukhen deposit, as well as the...     

Minerals of the Fe–Ni–Co–Cu–S System in Picrite Intrusions of the Southern Urals: Signatures of Liquation and Differentiation of the Sulfide Melt

Doklady Earth Sciences - New data on the minerals of the Fe–Ni–Co–Cu–S system in the differentiated intrusions of the Southern Urals are presented. Based on a detailed study...