Remobilization Of Rare Earth Elements Research Articles

Early diagenetic mobilization of rare earth elements and implications for the Ce anomaly as a redox proxy

The Rare Earth Elements (REE) archived in carbonate rocks retain a wealth of information on paleo-seawater chemistry and local-regional redox conditions. However, interpretations are often ambiguous due to the potential for REE remobilization in marine environments. In this regard, many carbonate rocks that retain primary seawater isotopic signatures exhibit non-seawater-like REE patterns, implying either unusual REE behaviour in seawater or diagenetic overprinting of otherwise well-preserved rock samples. Here, we apply sequential leaching to constrain the possible effects of different REE-bearing phases on measured carbonate REE patterns in order to address this quandary. Our results show that the exchangeable phase contains negligible REE, but could be an important host phase for other elements such as Sr, Ba and K. The acidified hydroxylamine hydrochloride leach is shown here not only to dissolve Fe–Mn oxides but also phosphate minerals, which induces middle REE enrichment in corresponding leachates of dolostone samples in our study. The demonstrable Fe–Mn oxide phase in limestone samples is characterized by middle and/or light REE enrichment and positive Eu anomalies, which are attributed to hydrothermal processes and continuing exchange with marine fluids after initial precipitation. The non-seawater-like REE patterns observed in the carbonate phase of otherwise well-preserved limestones resemble those of the co-existing Fe–Mn oxide fraction, and are interpreted to reflect the dissolution of REE carrier phases during early diagenesis. This view is supported by fluid-rock interaction modelling, which shows how REE can be mobilized at relatively low fluid/rock ratios in shallow porewaters. Non-seawater-like REE patterns may therefore be caused by the incorporation of REE from shallow porewaters before final lithification due to an elevated flux of particulate REE carrier phases, e.g. Fe–Mn oxides and organic matter, to the seafloor. In spite of the sensitivity of carbonate REE patterns to early diagenetic exchange, the co-occurrence of non-seawater-like patterns with primary strontium and carbon isotope values suggests that REE should not be viewed as a general indicator for the preservation of other geochemical proxies. Importantly, Ce anomalies of the carbonate phase will also be affected by porewaters, masking primary seawater values. Our results favour a reevaluation of redox interpretations by taking into account REE patterns as a whole.

Hydrothermal alteration and the remobilization of rare earth elements during reprecipitation of nano-scale apatite in phosphorites

Sedimentary phosphorites enriched in rare earth elements (i.e., REE) are an important reservoir of these critical metals and may have the potential to resolve the global REE supply risk in the future, which is widely distributed around world. However, the occurrence and utilizability of REE in phosphorites remain contentious. In this study, multiple-scale analysis techniques were used to investigate the mineralogical and geochemical characteristics of typical REE-enriched phosphorites from Zhijin, SW China. The mineralization was divided into two stages: early fluorapatite (Fap1) that formed during the sedimentary–diagenetic stage, and late fluorapatite (Fap2) that formed during the hydrothermal alteration stage. The in situ REE geochemical data (i.e., quantitative data and mapping analysis) and scanning electron microscopy images show that Fap1 is the main REE host phase, because it is abundant and high REE concentration. Data obtained by both laser-ablation–inductively coupled plasma–mass spectrometry and high-resolution transmission electron microscopy reveal that most REE are resident in the lattice of nano-scale fluorapatite, which could be remobilized by hydrothermal fluids during the process of dissolution of nano-scale Fap1 and reprecipitation of micro-scale Fap2. In addition, the formation of synchysite-(Y) in the altered phosphorites involved reprecipitation of the mobilized REE. We suggest that F− and SO42− in fluids play a major role in the mobilization of REE in fluorapatite and that CO32− sourced from dolomite alteration contributes to the precipitation of REE. Our results indicate that REE in fluorapatite can be remobilized and further enriched by hydrothermal alteration to REE fluorocarbonates (the current predominant economic source of REE minerals). Overall, our results reveal that REE occur mainly in the lattices of fluorapatite and synchysite-(Y) in phosphorites, and provide new insights into the mineralization mechanism and industrial availability of REE-enriched phosphorites.

Two discrete stages of fenitization in the Lizhuang REE deposit, SW China: Implications for REE mineralization

Fenitization is an alkali metasomatism that widely occurs in and around the alkaline-carbonatite complex, and is closely associated with rare earth elements (REE) mineralization. However, the origin and nature of the fenitizing fluids and its role in REE mineralization are poor understood. The Lizhuang REE deposit in the Panxi region, SW China, is hosted in a syenite-carbonatite complex with low- and high-grade fenitization. In this study, we conducted detailed petrographic, compositional and isotopic studies on the fresh/unaltered syenite and low- and high-grade fenites to reveal the origin and nature of the fenitizing fluids and their effects on REE mineralization. Our results show that the whole-rock Na/K ratios gradually increase from the fresh syenite, low-grade fenites to high-grade fenites, as well as mineral replacements, such as K-feldspar is replaced by albite and Ca-rich clinopyroxene is replaced by Na-rich clinopyroxene, indicating that sodic fenitization occurred in the Lizhuang syenite system. In situ U-Pb dating of the unaltered and altered apatite shows that the hydrothermal alteration processes are nearly simultaneous with the emplacement of syenite and carbonatite during 27.4–25.3 Ma, maybe lasting for ~2 Ma. In addition, the altered regions of feldspar, clinopyroxene and apatite have in situ 87Sr/86Sr ratios ranging from 0.70580 to 0.70674, which is consistent in error with those of the unaltered regions and host rock syenite-carbonatite complex (0.70566–0.70666). It is suggested that the fenitizing fluids were exsolved from the syenite-carbonatite complex in a relatively closed system. As the increase of alteration degree, the proportion of hydrothermal minerals (e.g., barite, calcite, fluorite and bastnäsite) in fenite increased gradually, indicating that the fenitizing fluids are enriched in SO42−, CO32− and F−. This is also supported by the elevated F and SO3 contents from the unaltered to altered apatite. The increasing REE concentrations, from the fresh syenite, low-grade fenite to high-grade fenite, indicate that REE minerals such as bastnäsite were precipitated from the fenitizing fluids during fenitization processes. In-situ analyses on the altered clinopyroxene and apatite show variable REE losses compared to the unaltered precursors, which indicates the successive remobilization and transferring of REE by fenitization fluids. Therefore, two discrete stages of fenitization have been identified in the Lizhuang REE deposit, resulted from fluids exsolved from the syenite and carbonatite, respectively. Meanwhile, such fenitization fluids are effective extracting agents of REE in post-magmatic stage, which is crucial to REE remobilization and precipitation to form economic deposit.

Geochemical and isotopic study of metasomatic apatite: Implications for gold mineralization in Xindigou, northern China

The 0.7 Moz Xindigou gold deposit lies on the north margin of North China Craton. The orebodies are controlled by NWW-trending shear zones and hosted by an Archean metavolcanosedimentary sequence. These host schist, amphibolite and marble rocks were intruded by late Paleozoic granites and mafic dikes. Two independent magmatic-hydrothermal assemblages were identified based on petrographic evidence: a pre-ore magmatic-hydrothermal magnetite – apatite ± pyrite ± zircon assemblage and an ore-related potassium feldspar – muscovite – sulfide ± minor monazite assemblage.Apatite that has undergone later alteration shows significant intracrystalline spatial variability with respect to the concentrations of rare earth elements (REEs), Si, and Sr. Scanning electron microscope backscattered images and statistical interrogation of the compositional data acquired by LA-ICP-MS indicate that apatite grains in the pre-ore magnetite – apatite assemblage are homogeneous and enriched in REEs and Si (Ap1). Where sulfide-related alteration of mineralization stage was overlapped, these apatite grains display a distinctive texture of homogeneous cores and turbid rims. This dissolution-reprecipitation process preferentially transported elements from the original apatite. REEs and Si gradually decrease from the core (Ap1) to the altered rim (Ap2 & Ap3) whereas Sr increases. Localized leaching of REEs (especially LREEs) from apatite was followed by precipitation of secondary monazite grains in the metasomatic apatite rims and the muscovite – sulfide – quartz – phosphate sequence. In situ Sr isotope analyses of the apatite show the 87Sr/86Sr ratios varying from the lowest value, 0.707874 in unaltered Ap1, to the highest, 0.714160 in altered Ap3. Zircon grains from migmatite display a peak metamorphic age at 2512 ± 9 Ma, whereas zircons from the early magnetite-apatite association formed near 1776 ± 12 Ma, and late hydrothermal monazite yields a Th-Pb age of 301.0 ± 1.7 Ma.These geochronology results show that the mineralization occurred during the late Paleozoic orogenesis, long after Neoarchean regional metamorphism and late Paleoproterozoic rift-related magmatism. The typical hydrothermal features of orogenic gold deposit could be inferred from the fluorapatite. The elemental patterns of fluorapatite grains indicate that a carbon and Sr enriched, acid, slightly oxidized and low salinity hydrothermal system was responsible for apatite metasomatism and gold mineralization. Strontium isotopes further reveal that the fluid related to gold deposition was derived from mafic lower crust, with extensive radiogenic Sr accumulation during fluid-rock interaction. This study demonstrates the potential application of apatite in exploration for orogenic gold deposits and its use in tracking hydrothermal REE re-mobilization during metasomatism.

Coral-inferred monsoon and biologically driven fractionation of offshore seawater rare earth elements in Beibu Gulf, northern South China Sea

We present a monthly dataset (AD 2002–2005) of rare earth elements (REEs) recorded in Porites coral, which were obtained from the Weizhou Island (WZI), Beibu Gulf, northwest of South China Sea (SCS). This offshore coral shows a strong seasonal cycle in REE/Ca ratios, with enriched REEs (total REEs, 100~140 ppb) in winter and depleted REEs (40~60 ppb) in summer. Since the influence of the river discharge is limited to the river mouth, its contribution to the dissolved REEs is negligible for the offshore area of WZI. Given the similar seasonal pattern of Ba/Ca, we suggest that the remobilization of REEs from river-transported sediments on the shelf of SCS through winter monsoon-driven mixing is the major source of REEs to WZI. Moreover, the peak time of the Nd/Yb ratio (occurred in spring) is not coupled with the occurrence of the maximum REEs, which could mainly be attributed to the degradation of biogenic particles that causes more light REE released in comparison to heavy REE. The Ce anomaly also displays a distinct seasonality (i.e., enhanced anomaly in winter and summer and diminished anomaly in spring), likely reflecting the microbial oxidation activities driven by seasonal temperature and nutrient cycles. Large Gd anomaly (Gd/Gd* up to 1.7) recorded in coral skeletons is unlikely originated from the natural sources, but reflects anthropogenic activities through the recent excessive use of Gd complexes by magnetic resonance imaging of medical examination in this region.

Formation of monazite-(Ce, La) by fluid-apatite interaction: the Floresta Azul Alkaline Complex, Bahia, Brazil

Monazite is a common accessory mineral in the Floresta Azul Alkaline Complex, occurring in three different rock types, which form this batholith: nepheline syenite, granite and fenite. Two compositional types of monazite can be found: monazite-(Ce) and monazite-(La), the latter being found only in syenites. Monazite occurs in close association with apatite as anhedral crystals ranging in size from 0.1–100 μm, and textures indicate different genetic processes. In nepheline syenite, monazite is closely related to ancylite and apatite, as a late crystallizing phase. Monazite occurs in granites as pore fillings in apatite and was formed by the remobilization of rare earth elements (REE) from apatite by late CO 2 — rich fluids. In fenite, monazite appears as acicular crystals, parallel to apatite’s crystallographic c-axis formed by exsolution from apatite by metamorphism. The data show that the monazite genesis was closely related to the activity of fluids associated either with the final stages of crystallization of nepheline syenite and granite or with fenitization of country rock.

Geochemistry and Mineralogy of Rare Earth Elements (REE) in Bauxitic Ores of the Catalan Coastal Range, NE Spain

Karst bauxite deposits are currently investigated as a new resource for rare earth elements (REE) in order to avoid present and future supply shortfalls of these critical metals. The present work focuses on the geochemistry and mineralogy of the REE in karst bauxite deposits of the Catalan Coastal Range (CCR), NE-Spain. It is revealed that the studied bauxitic ores have a dominant breccia and local ooido-pisoidic and pelitomorphic texture. The bauxitic ores are mostly composed of kaolinite and hematite, as well as of lesser amounts of boehmite, diaspore, rutile and calcite. The mineralogy and major element composition indicate incomplete bauxitization of an argillaceous precursor material possibly derived from the erosion of the Mesozoic Ebro massif paleo-high. The studied bauxites are characterized by ∑REE (including Sc, Y) between 286 and 820 ppm (av. 483 ppm) and light REE to heavy REE (LREE/HREE) ratios up to 10.6. REE are mainly concentrated in phosphate minerals, identified as monazite-(Ce) and xenotime-(Y) of detrital origin and unidentified REE-phosphates of a possible authigenic origin. REE remobilization presumably took place under acidic conditions, whereas REE entrapment in the form of precipitation of authigenic rare earth minerals from percolating solutions was related to neutral to slightly alkaline conditions. During the bauxitization process no significant REE fractionation took place and the REE distribution pattern of the bauxitic ores was governed by the REE budget of the precursor material. Finally, adsorption as a main REE scavenging mechanism in the studied CCR bauxite deposits should not be considered, since the presented data did not reveal significant REE contents in Fe-and Mn-oxyhydroxides and clay minerals.

Geochemical Characterization of Bauxite Deposits from the Abruzzi Mining District (Italy)

The Abruzzi bauxite district includes the deposits located on the Campo Felice plateau and those of the Monti d’Ocre, which had been mined in the first part of the 20th century. Bauxite is of the karst type, with textures ranging between oolitic and oolitic-conglomeratic, the latter suggesting a partial reworking of evolved lateritic soils. The high contents of Al2O3 and Fe2O3 (average values 53.76 and 21.76 wt %, respectively) are associated with the presence of boehmite, hematite, and minor goethite. SiO2 and TiO2 have average values of 7.79 and 2.75 wt %, corresponding to the presence of kaolinite, anatase and rutile. Among the minor so-called “bauxitophile” elements V, Co, Ni, Cr and Zr, the most enriched is Cr, with an average value of 0.07 wt %. Nickel has an average value of 210.83 ppm. Vanadium shows an average value of 266.57 ppm, whereas the average Co concentration is 35.89 ppm. The total rare earth elements (REE) concentration in the sampled bauxite sites is variable between ca. 700 and 550 ppm. Among REEs, the most abundant element is Ce, with Ce anomalies commonly associated with authigenic REE-fluoro-carbonates, probably produced after the REEs remobilization from primary detrital minerals and their precipitation in neo-formed phases during the bauxitization process. Scandium and Ga occur in small amounts (57 and 60 ppm, respectively), but geochemical proxies of their remobilization and uptake in neo-formed minerals (Fe- and Al-(hydr)oxides, respectively) have been observed. The mean Eu/Eu* and Al2O3/TiO2 ratios and the Ni-Cr contents of the Abruzzi bauxites suggest that the parent rock of these deposits was a material of acid affinity, likely corresponding to volcanic tephra or eolic loess-type sands.

The characteristics of zircon as the evidence for post-magmatic remobilization of REE and HFSE in the northern Motzfeldt alkaline igneous complex, southern Greenland

The Motzfeldt intrusions in the Gardar Province, southern Greenland, split into syenitic plutonic and hypabyssal rocks, in which the latter include ring dykes and sheet intrusions. Sheet intrusions, considered as the source for rare earth elements (REE) and high field strength elements (HFSE), comprise sheets of peralkaline microsyenite (SPM), syenitic pegmatite (SP), and peralkaline microsyenite (PM). SP exhibits extremely high concentrations of REE and HFSE, which are positively correlated with increasing alkalinity from early towards late intrusion, caused by magmatic processes. In contrast, some of the SPM and PM are also significantly enriched in REE and HFSE, caused by post-magmatic fluids. The REE-and HFSE-rich phases in SP consist mainly of zircon and allanite with smaller amounts of pyrochlore in pseudomorph from the inferred eudialyte, whereas some of the PM and SPM consist of pyrochlore, REE-carbonate, and zircon in the matrix. The zircon grains in the Motzfeldt So Formation (MSF) syenite occur in interstitial spaces, exhibiting an association with magnetite and a bipyramidal form in texture. They are characterized by a highly fractured and embayed rim. Zircons from PM and SP are clearly enriched in Fe, Al, Ca, Na, Y, P, Hf, Y, P, Nb, Ta, and REE, and are depleted in Zr and Si in comparison with magmatic zircon. They also show a clear trend of higher LREE/HREE and Eu/Eu* ratios, and lower Ce/Ce* ratios, which define them as typical hydrothermal zircons. In contrast, zircons from the MSF syenite show a relatively lower LREE/HREE ratio and Eu and Ce anomalies of a similar magnitude compared with those from SP and PM. The occurrence and mineral composition of the zircon suggest that post-magmatic fluids have played an important role in the remobilization of REE and HFSE as well as the primary concentration of REE and HFSE, caused by magmatic processes.

On the Origin of Bastnaesite-(La,Nd,Y) in the Nissi (Patitira) Bauxite Laterite Deposit, Lokris, Greece

A detailed geochemical study and a thorough mineralogical description of the rare-earth elements (REE)-minerals and associated minerals were carried out in two vertical profiles of approximately 4 m length, from the Nissi (Patitira) bauxite laterite deposit, Lokris, Greece, characterized by the presence of goethite in small sizes resembling bacterial cell coated by goethite and a significant REE enrichment. The enrichment of the REE concentrated in bastnaesite-group minerals, the intergrowths between REE-minerals and Al–Ni–silicates with significant sulfur contents and their association with goethite microtextures interpreted as bacteriomorphic, indicate REE remobilization along with iron bio-leaching and re-precipitation on karstified limestone. In addition to the previous-reported hydroxylbastnaesites, a (La,Nd,Y)(CO3)F member of the bastnaesite-group associated with Al–Ni–silicates were identified, the stability of which may reflect the dependence on the source rocks and the local variations of pH-Eh. Interaction between downward percolating water and carbonate rocks seems to be a very effective mechanism for REE fluorocarbonates deposition under alkaline and reducing conditions.

Zirconosilicates in the kakortokites of the Ilímaussaq complex, South Greenland: Implications for fluid evolution and high-field-strength and rare-earth element mineralization in agpaitic systems

AbstractThe layered agpaitic nepheline syenites (kakortokites) of the Ilímaussaq complex, South Greenland, host voluminous accumulations of eudialyte-group minerals (EGM). These complex Na-Ca-zirconosilicates contain economically attractive levels of Zr, Nb and rare-earth elements (REE), but have commonly undergone extensive autometasomatic/hydrothermal alteration to a variety of secondary mineral assemblages. Three EGM alteration assemblages are recognized, characterized by the secondary zirconosilicates catapleiite, zircon and gittinsite. Theoretical petrogenetic grid models are constructed to assess mineral stabilities in terms of component activities in the late-stage melts and fluids. Widespread alteration of EGM to catapleiite records an overall increase in water activity, and reflects interaction of EGM with late-magmatic Na-, Cl- and F-rich aqueous fluids at the final stages of kakortokite crystallization. Localized alteration of EGM and catapleiite to the rare Ca-Zr silicate gittinsite, previously unidentified at Ilímaussaq, requires an increase in CaO activity and suggests post-magmatic interaction with Ca-Sr bearing aqueous fluids. The pseudomorphic replacement of EGM in the kakortokites was not found to be associated with significant remobilization of the primary Zr, Nb and REE mineralization, regardless of the high concentrations of potential transporting ligands such as F and Cl. We infer that the immobile behaviour essentially reflects the neutral to basic character of the late-magmatic fluids, in which REE-F compounds are insoluble and remobilization of REE as Cl complexes is inhibited by precipitation of nacareniobsite-(Ce) and various Ca-REE silicates. A subsequent decrease in F– activity would furthermore restrict the mobility of Zr as hydroxyl-fluoride complexes, and promote precipitation of the secondary zirconosilicates within the confines of the replaced EGM domains.

REE and HFSE mineralization in peralkaline granites of the Ambohimirahavavy alkaline complex, Ampasindava peninsula, Madagascar

The Cenozoic Ambohimirahavavy alkaline complex in the northwest of Madagascar is characterized, in its outer flanks, by a network of discontinuous granitic dykes mineralized in rare-earth elements (REE) and high-field-strength elements (HFSE). These granitic dykes contain alkali amphibole and pyroxene, are strongly ferrian, have a peralkaline affinity [molar (Na2O+K2O)/Al2O3>1], but differ in textures and mineral assemblages. Based on the latter parameters, three varieties of granite could be distinguished: coarse-grained, pegmatitic and eudialyte-bearing granite. Detailed study of the REE- and HFSE-bearing minerals revealed the presence of both magmatic and hydrothermal phases. In coarse-grained granite (GR-I) prevails a magmatic miaskitic assemblage, rich in zircon. In the pegmatitic granite (GR-II), the rare metals are contained in secondary minerals, essentially amoeboid zircon and rare-earth fluorocarbonates, forming pseudomorphs after eudialyte. The eudialyte-bearing granite (GR-III), the richest in REE and HFSE, is characterized by a magmatic agpaitic assemblage, dominated by eudialyte. A skarn develops at the contact of GR-I with limestone. Mineralization in this zone is in the form of pseudomorphs of rare-metal-bearing minerals after aegirine–augite.Presence of a magmatic miaskitic vs. an agpaitic assemblage in the coarse-grained and eudialyte-bearing granites, respectively, is interpreted to result from crystallization of differently evolved melts that probably originated from a common basaltic parental magma. Origin from the melting of a metasomatized lower crust cannot be excluded. In the eudialyte-bearing granite, very high contents of Na and volatiles maintained the rare metals in the melt to the very last stages of crystallization, whereas in the coarse-grained granite, Na and volatiles were probably lost to an exsolving fluid, causing the precipitation of a miaskitic assemblage. Subsolidus remobilization of REE and HFSE took place via an alkali-, silica- and F-rich orthomagmatic fluid exsolved from the crystallizing peralkaline granitic melt.

Rare earth element mobility in high-alumina altered metavolcanic deposits, South Carolina, USA

Mineralogical and geochemical studies of Neoproterozoic metavolcanic rocks in the Carolina terrane, USA, document processes leading to remobilization of rare earth elements (REE) during residual weathering. The Haile kaolin deposits near Kershaw, South Carolina, occur predominantly in crystal metatuff and contain reserves of >650,000metric tons of specialty clay having ~60 to 450ppm REE. The kaolins are temporally and spatially related to epithermal deposits of Fe, Au, and other metals that comprise the Haile gold mine property. Chemical alteration of the host volcanic rocks has strongly modified the original bulk rock composition producing a high-quality white clay resource composed of ~35% kaolinite, ~20% sericite ~45% quartz. The kaolin deposits are proposed to have formed by a combination of hydrothermal alteration and surficial weathering and the REE signature reflects the effects of volcanism, epithermal alteration and mineralization, metamorphism, and intense saprolitization.The metavolcanic rocks span from 64.9 to 76.8wt.% SiO2 with average contents of ~14.43wt.% Al2O3, and ~3.98wt.% K2O. They have REE from 58 to 113ppm (ave. ~90ppm), and are light REE-enriched (~60–200×, ave. 100× chondritic abundance). Silicified, altered and Au-pyrite mineralized volcanic rocks as a group have SiO2 contents (49–99wt.%) that are inversely correlated with Al2O3 (~1.3–27.16wt.%). Mineralized and altered volcanic rocks range from 2 to 203ppm REE (av. ~77); subvolcanic rhyolite porphyry has 2 to 144ppm REE (av. ~74). The kaolin deposits have SiO2 contents (51–85wt.%) that are inversely correlated with Al2O3 (10.1–33.4wt.%).Chemical weathering of the metavolcanic rocks remobilized REE and caused variations in whole rock patterns that reflect differential weathering of primary titanite, allanite, and apatite and precipitation of secondary minerals. Newly formed minerals in the saprolitized volcanics rocks include Ba-Al phosphates, poorly crystalline and hydrated REE-phosphates, gibbsite, diaspore, goethite, lepidocrocite, rutile and siderite. The REE distributions in the kaolin deposits reflect the presence of monazite-(La), monazite-(Nd), and lesser amounts of poorly crystalline xenotime (Y, Dy, Gd). The minerals and chemical processes producing the weathered metavolcanic and subvolcanic rocks at the Haile mines are analogous to those leading to the formation of REE ion adsorption clay deposits in altered granitic rocks of China and Southeast Asia.

Generation of Ce anomalies in SW Sardinian Mn ores

Sedimentary (pedogenic) and hydrothermal Mn ores are hosted by a volcanic succession in south-western Sardinia. The sedimentary Mn ores are characterized by chondrite-normalized Ce anomalies (Ce/Ce*) of 1.08–3.40, whereas the hydrothermal Mn ores have Ce anomalies of 0.35–3.15.The presence of both negative and positive Ce anomalies in the hydrothermal Mn ores is related to the remobilization of rare earth elements (REEs) within the hydrothermal system, which occurred via the dissolution and redistribution of REE-bearing accessory minerals by mineralizing fluids. The REEs were removed from wall-rocks and redeposited within the Mn ores as apatite, monazite, and cerianite, which have textures characteristic of growth into a Mn-oxide matrix during mineralization. In addition, the redistribution of cerium led to a progressive increase in the magnitude of the Ce anomaly from E to W, suggesting that mineralizing fluids moved along an E–W-striking fault close to the Mn ore.In the sedimentary Mn ores, the spatial variability of Ce/Ce* values probably relates to changes in the deposition environment, from organic-rich to organic-poor. Positive Ce anomalies with Ce/Ce* values of 1.08–1.75 are associated with samples from thick sedimentary layers within a paleosoil or samples from veins related to paleosoils, and potentially formed in a environment where organic speciation of the REEs acted to limit cerianite precipitation. Conversely, spheroidal concretions of Mn-oxides within pumice and ash layers have larger positive Ce anomalies (Ce/Ce*>2.5) and likely formed in an organic-free environment matter, with REE speciation in circulating fluids dominated by inorganic species.Finally, given the large range in Ce/Ce* ratios of Mn ores, we stress the need to carefully consider sampling methodologies, to ensure the accurate interpretation and determination of ore deposit models, the identification of mechanisms involved in Ce distribution, and the relationships between Ce and Mn phases.

Hydrothermal remobilization of rare earth elements and its effect on Nd isotopes in rhyolite and granite

Although several studies have shown that the Sm Nd radiometric system is not remobilized during the hydrothermal alteration of felsic rocks, it is doubtful that the rare earth element (REE) mobility noticed under certain hydrothermal conditions always leaves the Sm Nd systematics unperturbed. This contribution describes two cases of such isotopic disturbances of hydrothermalised felsic rocks from two different geological contexts: in rhyolites from the Estérel (southeastern France) and in the red granite of Porto (northwest Corsica, southeastern France). Although these two igneous bodies were initially very homogeneous as regards their Nd isotopes, some samples display very contrasting Nd isotopic signatures (i.e. comparatively low initial 143Nd/ 144Nd ratios) that are anomalous from a magmatic point of view. Moreover, these isotopically perturbed rocks are invariably associated with peculiar REE patterns that are also difficult to explain in terms of differentiation or contamination at the magmatic stage. It is thus inferred that the Sm Nd system in these samples was perturbed by hydrothermal alteration. In the two examples studied, the perturbation of the Sm Nd radiometric system cannot be solely explained by a modification of theSm/Nd ratio significantly after the emplacement of the igneous bodies. Although the net result of the hydrothermal alteration is leaching of the REEs from the rocks (i.e., mainly the middle and heavy REE for the granite and the light REE for the rhyolites), the isotopic compositions of the samples were modified by a component introduced from a fluid having contrasting 143Nd/ 144Nd ratios. This inference, together with structural observations, indicates that REEs were transported over distances exceeding several tens to hundreds of metres, even in the case of the granite studied in this work where mineralised fractures are absent. This is far greater than previous estimates and this conclusion is of great importance for environmental and nuclear waste disposal site studies where light-REE are often taken as natural analogues for artificial radionuclides.

Late alteration in titanite (CaTiSiO5): Redistribution and remobilization of rare earth elements and implications for U/Pb and Th/Pb geochronology and nuclear waste disposal

Abstract Translucent, dark-coloured titanites from a wide variety of geological environments are commonly partly replaced by very fine-grained aggregates of REE-rich minerals (including allanite, monazite, and bastnasite), rutile, quartz, calcite, apatite, and chlorite along grain fractures and boundaries. The aggregates of REE-rich minerals are bordered by an irregular zone of altered titanite, which, by secondary ion mass spectrometry (SIMS), is considerably lower in REEs, U, and Th than the adjacent unaltered matrix. Light REEs and Th are differentially depleted in the altered zone and correspondingly enriched in secondary minerals. Additionally, HREEs and U have been lost to the external system, the loss of HREEs being in proportion to the increase in atomic number. Natural radiation damage was not an important factor in promoting the late alteration of titanite. Titanite which has been subjected to late hydrothermal activity would not remain a closed system in respect to either U or Th decay series and therefore cannot be used for U/Pb and Th/Pb geochronology. The observed leaching and partial dissolution of titanite under hydrothermal conditions are generally consistent with laboratory experimentation, indicating that proper isolation from any communication with hydrothermal fluids is essential in the application of titanite-based ceramics for the immobilization of radioactive waste.

Introdução à geoquímica dos elementos terras raras

The rare earth elements (REE) have been widely used in petrogenetic studies since they are excelent indicators of geologic processes. In igneous rocks the partial melting and fractional crystalization processes lead to a relative enrichment of the light rare earth elements (LREE) in the liquid phase of the crystal-liquid silicate systems. This conduets to a LREE enrichment in crustal rocks, particularly in the more felsic ones. The minerais taking part in these processes have characteristic effects in the REE patterns of the melts allowing their identification. The sedimentary rocks show little variation in their REE patterns pointing out to the homogenization capability of the sedimentary processes. The REE content of elastie sediments reflects the composition of the source áreas subject to erosion and the variation of their REE patterns during the geological time indicates the compositional changes of the upper continental crust. The REE have been considered as immobile during metamorphism and generally they provide excellent information about the premetamorphic precursors. However, REE remobilization has been observed in several cases where there is a high fluid/rock ratio, such as during spüitization, high-temperature metasomatism, lowgrade hydrous metamorphism, weathering, etc. The REE have been also used as a monitor of the chemical changes involved in hydrothermal ore deposits