HREE Enrichment Research Articles

Geochemical composition of fluvial sediments in the Milo River basin (Guinea): is there any impact of artisanal mining and of a big African city, Kankan?

For the first time, the bed sediments of the Milo River and its tributaries were sampled and analyzed to characterize their geochemical signature in terms of major, trace and rare earth elements and to determine the potential impacts of artisanal mining and of the anthropogenic activities of Kankan city. 24 sites have been sampled along the Milo river from upstream to downstream and on its tributaries during one sampling campaign to assess the spatial variations of the geochemical signature. During one year, the bed sediments have been sampled monthly on the Milo river upstream and downstream Kankan to determine the seasonal variations of their chemical composition and to investigate the potential impacts of Kankan on the fluvial sediment quality. The geochemical signatures of the sediments are mainly controlled by the geological substratum and the lateritic soil cover, showing that there is no impact of the artisanal mining and very few impacts of Kankan, except for As and Sb on two small tributaries draining the urban agglomeration. All the sediments are more concentrated and naturally enriched in Zr, Hf, U and Th compared to PAAS, due to the regional geochemical background. REE distribution patterns show LREE and HREE enrichments and positive Ce and Eu anomalies. The La/Yb ratios are positively correlated to Al and Fe-oxides and negatively to silica. The non-residual fractions (labile) extracted with EDTA represent less than 20% of the total concentration, showing that the threats for aquatic ecosystems and human populations are limited. This study can represent a reference with regard to the potential development of industrial mining and increase of population due the abundant mineral resources within this catchment.

The spessartine–almandine garnet from Val Codera pegmatite, Central Alps, Italy: a new insight on the crystallochemistry and a 3D image analysis of its inclusions

This paper is a new crystallographic and chemical insight including REE and other trace elements of garnets from the “garnet dike”, located in Val Codera (Central Alps, Italy). The crystals may reach up to 2 cm with trapezohedral shape and show a homogeneous dark red colour; however, there are also present many eye-visible inclusions and cracks. Therefore, synchrotron X-ray computed micro-tomography was carried out on selected samples to visualise the 3D distribution and shape of those inclusions, as well as the distribution of voids and cracks within the crystals. To notice here, that because of those latter features a workability of this mineral could result difficult even though these crystals would otherwise may be interesting as gem. The analysed garnets are intermediate members of the almandine–spessartine solid-solution series, varying in composition from Mn-rich core (up to Sps64 mol%) to Fe-rich rim (up to Alm54 mol%). The samples exhibit a depleted LREE and an enriched HREE patterns with a strong negative Eu anomaly. The increase of total content of REE as well as of Y from core to rim suggests that their incorporation in the [X] site, mainly occupied by Mn and Fe2+, is controlled by the variation of both these cations. In the same way, the cell parameter and the bond lengths, measured both by powder and single crystal X-ray diffraction methods, result between those of pure spessartine and almandine, consistent with the chemistry and related to the variation of the ratio Mn/Fe. The chemical zoning of garnet represents its growth history and makes it a useful indicator of fractional trend of pegmatite-forming melt. Our results suggest that the spessartine-rich garnet crystallisation took place from a Mn-rich melt in presence of volatiles, which prevented the incorporation of Y and REE in the structure favouring the crystallisation of Y and REE-rich accessory minerals.

Discriminating groundwater salinization processes in coastal aquifers of southeastern India: geophysical, hydrogeochemical and numerical modeling approach

Groundwater salinization in coastal aquifers is mainly due to severe groundwater extraction, global sea level rise, usage of agricultural fertilizers and pesticides, waste disposal and influences of industrial and domestic effluents. The study area is the Nagapattinam and Karaikal coastal aquifers, noticeable with quick growth in population, severe industrial and urban activities resulting in groundwater abstraction ensuing deeper water level and salinization. Attempt has been made to discriminate sources of groundwater salinization using geophysical, hydrochemical and modeling techniques. The electrical resistivity survey isolated resistivity ranges between 0.5 and 1.5 Ω m as seawater intruded. Groundwater samples were collected for two different seasons and analyzed for major, minor and rare earth elements. The results signify higher conductivity (12,430.0 μS/cm), chloride (5060.0 mg/L) and sodium (1330.0 mg/L), indicating the saline nature of groundwater. The normalized REE patterns in groundwater exhibit enrichment of HREEs than LREEs due to higher mobility. The SEAWAT code predicted seawater intrusion along the eastern portion of the study area.

Anorthosites from an Archean continental arc in the Dharwar Craton, southern India: Implications for terrane assembly and cratonization

Anorthositic rocks have attracted attention in terms of their possible role as the primordial crust of our planet, and also as markers of Archean-Paleoproterozoic plate tectonic regimes. Here we investigate the Konkanhundi gabbro-anorthosite suite from Peninsular India, adjacent to the major collisional suture between the Western and Eastern Dharwar cratonic blocks. The anorthosites display high Al2O3 contents with negative correlation against MgO, high CaO and Sr contents, and positive Eu anomalies indicating plagioclase flotation in the magma chamber. The pyroxenite and gabbroic rocks of the suite show high Ni, Cr, Co contents and geochemical features typical of pyroxene cumulation. LA-ICPMS U-Pb analyses of magmatic zircon grains yield weighted mean 207Pb/206Pb ages of 2601 ± 12 Ma for pyroxenite, 2616 ± 12 Ma for gabbro, 2615 ± 13 Ma and 2627 ± 14 Ma for anorthositic gabbro, 2594 ± 16 Ma for anorthosite, and 2605 ± 27 Ma for microgabbro. The age data from the different rock types in the anorthosite suite are broadly consistent, marking the emplacement time as ca. 2.6 Ga, and providing insights on one of the oldest anorthosite complexes in Peninsular India. Zircon grains in the surrounding TTG gneisses into which the gabbro-anorthosite suite was emplaced show weighted mean 207Pb/206Pb age of 3321 ± 11 Ma suggesting Mesoarchean basement. The zircon grains display high Th/U values and REE patterns typical of magmatic crystallization with enriched HREE, positive Ce and Sm anomalies, and negative Pr, Nd, Eu anomalies. Zircon Lu-Hf analysis yield negative εHf(t) values of −4.9–−0.7 with TDMC (two stage model ages) of 3123–3376 Ma for pyroxenite, −4.5–−1.7 and 3192–3366 Ma for gabbro, −5.3–0.9 and 3034–3410 Ma for anorthositic gabbro, and −3.9–-2.4 and 3221–3311 Ma respectively, suggesting the incorporation of Mesoarchean components within the Neoarchean magmatic suite. However, zircon grains in the TTG gneiss possess more ‘juvenile’ εHf(t) values in the range of −0.2–1.3 with TDMC in the range of 3554–3646 Ma. The zircon Hf isotopes and trace element data, together with the whole rock geochemical features suggest that the parent magma of the Konkanhundi gabbro-anorthosite suite was derived from subduction-related depleted mantle source that also incorporated continental crustal components. The time of emplacement of the gabbro-anorthosite complex broadly correlates with widespread arc magmatism and crust production as well recycling in other domains of the Dharwar Craton. We envisage multiple convergence of microblocks during craton assembly at the end of Archean in Dharwar, with the greenstone belts marking zones of paleo-ocean closure.

Environmental geochemistry of rare earth elements in Cu-porphyry mine tailings in the semiarid climate conditions of Sarcheshmeh mine in southeastern Iran

Sarcheshmeh mine is one of the largest Oligo-Miocene porphyry Cu deposits in the world. It is located in the Kerman Cenozoic magmatic arc in southeastern Iran. This paper discusses the geo-environmental behavior of rare earth elements (REEs) in the Sarcheshmeh dammed tailing pond, situated in semiarid climate. For this purpose, solid and water samples were collected to consider: (1) weathering/oxidation reactions and upward migration of soluble elements; (2) super-saturation and formation/dissolution of the secondary evaporative phases; (3) hydro-geochemical changes of the surface and groundwater resources; (4) flotation of the Cu-sulfide ore and tailing management strategies. The mean values of the ∑REEs for fresh and weathered tailings and the evaporative phases were 113.0, 103.7 and 74.6mg/kg, respectively. When compared with the fresh and weathered tailings, the evaporative phases showed significant enrichment of HREEs and a well-depleted pattern of LREEs (except for that of Eu). This geochemical pattern suggests a higher solubility and mobility potential of HREEs and Eu compared with LREEs, resulting in good fractionation and enrichment of these elements in the surface evaporative layer formed on top of the weathered tailings. The water samples showed a range of hydro-geochemical features under acidic (pH of 2.5 to 4.7), near neutral (pH of 6.4 to 7.9), alkaline (pH of 8 to 10) and highly alkaline (pH>10) conditions. The maximum concentration of REEs was observed in the acidic waters produced by the weathered tailings after rainfall (∑REEs of 489 to 1903μg/L). Although ∑REEs decreased as the pH value increased, the general trend was compatible with the highest concentration of HREEs rather than LREEs. The speciation calculations indicate that LnSO4+, Ln(SO4)2− and Ln3+ are the primary forms of dissolved REEs at acidic and near neutral pH and the dominant species was LnSO4+. LnCO3+ (40.78%), Ln(CO3)2− (31.68%) and LnSO4+ (31.07%) were the dominant species under alkaline conditions. At high alkaline pH, the speciation pattern changed to LnOH2+ (53.15%), Ln(CO3)2− (44.88%) and LnCO3+ (2.61%), where Ln denotes lanthanide. Based on the results and considering previous works, it appears that ionic potential plays an important role in the geo-environmental behavior of REEs of adsorption tendency, mobility potential and ionic substitution.

Weathering of the Madeira world-class Sn-Nb-Ta (Cryolite, REE, U, Th) deposit, Pitinga Mine (Amazon, Brazil)

The paper deals on the laterization of the Madeira deposit associated with the albite-enriched granite facies of the A-type Madeira granite (~1.82Ga). The Madeira deposit is located in the Amazon rain forest, where chemical weathering is intensive and lateritization is a major process of ore deposit formation. This deposit represents a particular case, where the parent rock is an ore deposit; thus primary mineralization and lateritic mineralization occur in the same profile. The parent rock has an unusual mineral association, which includes quartz, albite, k-feldspar, zircon, cryolite (Na3AlF6), fluorite, polylithionite, Zn-rich riebeckite, Zn-F-rich annite, thorite, cassiterite, pyrochlore, columbite, xenotime, gagarinite-(Y), fluocerite-(Ce), and genthelvite. An important feature of the rock is the F richness (2 to 6wt%) mainly in the form of cryolite or fluorspar in the matrix. We first investigated the micromorphological changes of these minerals throughout the soil profile and then focused the geochemical studies in selected profiles. The chemical data were converted into volumetric proportions to quantify the variations in element contents in samples with different degrees of lateritization, and we performed mass balance calculations with Al as the reference element. In this way, we obtain many new constraints on the processes that formed the weathering profile from the Madeira deposit. The parental rock was a clearly aluminous system with lower amounts of Fe. The total loss of alkalis and partial loss of SiO2 created kaolinitic clay minerals. The SiO2/Al2O3 molar ratio ≈2 was suitable for generating aluminous clay minerals with 1:1 structures, such as kaolinite. Greater losses of SiO2 occurred and gibbsite formed at the top of the weathering profiles. The leaching of alkaline elements led to relative enrichment in some economically important elements, such as Sn, Nb, and REEs, in the lateritic profiles. However, the distribution of some of the metals, such as Pb, Zn, and REEs, in the weathering profile is very unusual and may be explained by some special characteristics of the paragenesis and the richness of F in the solutions, which greatly influenced the weathering processes in two different ways. The intense corrosion of even very resistant minerals due to the presence of HF in the system and relative enrichment of HREE in the profile due to the formation of complexes with F− and the downward movement of these elements.

Origin and petrogenetic implications of garnet from Rb‐rich pegmatites in North Qinling Orogen, China

A group of Rb‐rich granitic pegmatites near Ziyugou, Danfeng, in the North Qinling Orogen lack typical lithological zoning and have a simple mineralogy dominated by quartz + microcline + albite with garnet in minor quantities. Biotite is rare, and muscovite was mostly formed during greisenization. Graphic texture can be observed within the pegmatites near the contact with the country rock. The pegmatites have localized garnet‐rich zones that contain 5–15 vol.% garnet and accessory zircon, columbite, xenotime, and thorite. The pegmatites are peraluminous (A/CNK = ~1.02–1.23) and contain low Ca but high alkali and elevated Rb (~600–1300 ppm). In REE patterns and spider diagrams, the pegmatites exhibit a pronounced negative Eu anomaly, strong Ti, P, Sr, and Ba depletion, and enrichment of Rb, HFSE, and HREE. In addition, the garnet‐rich zones contain lower K and Rb but higher Na, Mn, Fe, Zr, Hf, Nb, Y, Th, and U concentrations than the part with graphic texture and accessory garnet. Two types of garnets occur in the pegmatites. Type 1 garnet is well‐zoned and closely associated with the above‐mentioned HFSE minerals in the garnet‐rich zones, whereas Type 2 garnet included in either feldspar or quartz defining graphic texture has no associated HFSE minerals. Both types of garnets are almandine‐spessartine solid solution and contain trace to minor amounts of pyrope, grossular, and uvarolite. Based on garnet chemistry, Type 1 garnet can be subdivided into two subtypes, namely, Type 1a with higher Alm content and Type 1b with higher Sps and Prp contents. Unlike garnets from LCT‐type pegmatites but similar to those from NYF‐type pegmatites, the garnets from the Ziyugou pegmatites contain up to 1.12 wt.% Y2O3 and 1.16 wt.% Yb2O3, which makes garnet the main carrier of Y and HREE. This explains why the garnet‐rich part contains higher Y and HREE. Type 1 garnet has higher Mn/Fe ratio than Type 2 garnet, indicating that the garnet‐rich part of the pegmatites likely represents a more evolved melt. Although the pegmatites somehow show S‐type granite affinity (i.e., elevated Rb concentration and strong depletion in Sr and Ba), the HFSE and HREE enrichment of the pegmatites and garnet chemistry suggest that the pegmatites may have had a hybridized source.

Geochemistry of ∼2.5 Ga granitoids at the northern margin of the Yinshan Block: Implications for the crustal evolution of the North China Craton

In order to further constrain the crustal development and tectonic evolution of the North China Craton (NCC) during late Neoarchean to early Paleoproterozoic, we performed petrological and geochemical analyses on forty-six granitoid samples from the northern margin of the craton at Siziwangqi in central Inner Mongolia. The granitoids commonly consist of a tonalite-trondhjemite-granodiorite (TTG) suite and alkali feldspar (rich in potassium) granites. The geochemistry and isotopic characteristics suggest that they formed in a subduction-related arc setting. An Andean-type magmatic arc model is proposed to explain several unique geochemical features of the Siziwangqi granitoids, notably that the rocks are metaluminous to peraluminous and belong to the calc-alkaline (TTG) and subalkaline to alkaline (alkali feldspar granite) series. The TTG granitoids are characterized by light LREE enrichment, a weak positive Eu anomaly, and flat heavy HREE profiles. The alkali granite is also enriched in the LREE but has a strong positive chondrite-normalized Eu anomaly and displays weak HREE enrichment. Petrogenetic studies reveal that most samples are of I-type granitoids. The data further suggest that the subduction-induced magmatism and slab rollback processes may have triggered reworking of both juvenile arc crust and minor older continental crust material along the north margin of Yinshan Block, leading to the formation of granitoids from S- to I-type in the study area. Overall, the NCC experienced a tectonic regime that changed from compressional to extensional during the early Paleoproterozoic at ∼2.4Ga, after the ca. 2.5Ga cratonization event.

Trace-element and Nd-isotope systematics in detrital apatite of the Po river catchment: Implications for provenance discrimination and the lag-time approach to detrital thermochronology

Detrital thermochronology is often employed to assess the evolutionary stage of an entire orogenic belt using the lag-time approach, i.e., the difference between the cooling and depositional ages of detrital mineral grains preserved in a stratigraphic succession. The impact of different eroding sources to the final sediment sink is controlled by several factors, including the short-term erosion rate and the mineral fertility of eroded bedrock. Here, we use apatite fertility data and cosmogenic-derived erosion rates in the Po river catchment (Alps–Apennines) to calculate the expected percentage of apatite grains supplied to the modern Po delta from the major Alpine and Apenninic eroding sources. We test these predictions by using a cutting-edge dataset of trace-element and Nd-isotope signatures on 871 apatite grains from 14 modern sand samples, and we use apatite fission-track data to validate our geochemical approach to provenance discrimination. We found that apatite grains shed from different sources are geochemically distinct. Apatites from the Lepontine dome in the Central Alps show relative HREE enrichment, lower concentrations in Ce and U, and higher 147Sm/144Nd ratios compared to apatites derived from the External Massifs. Derived provenance budgets point to a dominant apatite contribution to the Po delta from the high-fertility Lepontine dome, consistent with the range independently predicted from cosmonuclide and mineral-fertility data. Our results demonstrate that the single-mineral record in the final sediment sink can be largely determined by high-fertility source rocks exposed in rapidly eroding areas within the drainage. This implies that the detrital thermochronology record may reflect processes affecting relatively small parts of the orogenic system under consideration. A reliable approach to lag-time analysis would thus benefit from an independent provenance discrimination of dated mineral grains, which may allow to proficiently reconsider many previous interpretations of detrital thermochronology datasets in terms of orogenic-wide steady state.

Age and P–T conditions of the Gridino‐type eclogite in the Belomorian Province, Russia

ABSTRACTAlthough eclogites in the Belomorian Province have been regarded as Archean in age and among the oldest in the world, there are also multiple studies that have proposed a Paleoproterozoic age. Here, we present new data for the Gridino‐type eclogites, which occur as boudins and metamorphosed dykes within tonalite–trondhjemite–granodiorite gneisses. Zircon from these eclogites has core and rim structures. The cores display high Th/U ratios (0.18–0.45), negative Eu anomalies and strong enrichment in HREE, and have Neoarchean U–Pb ages of c. 2.70 Ga; they are interpreted to be magmatic in origin. Zircon cores have δ18O of 5.64–6.07‰ suggesting the possibility of crystallization from evolved mantle‐derived magmas. In contrast, the rims, which include the eclogite facies minerals omphacite and garnet, are characterized by low Th/U ratios (<0.035) and flat HREE patterns, and yield U–Pb ages of c. 1.90 Ga; they are interpreted to be metamorphic in origin. Zircon rims have elevated δ18O of 6.23–6.80‰, which was acquired during eclogite facies metamorphism. Based on petrography and phase equilibria modelling, we recognize a prograde epidote amphibolite facies mineral assemblage, the peak eclogite facies mineral assemblage and a retrograde high‐P amphibolite facies mineral assemblage. The peak metamorphic conditions of 695–755°C at >18 kbar for the Gridino‐type eclogites suggest an apparent thermal gradient of <39–42°C/kbar for the Lapland–Kola collisional orogeny.

The Proterozoic Guanhães banded iron formations, Southeastern border of the São Francisco Craton, Brazil: evidence of detrital contamination

The Guanhães banded iron formation (BIF) bearing succession occurs as tectonic slices, juxtaposed to Archean TTG granite-gneissic basement rock, developed during the Neoproterozoic-Cambrian Brasiliano collage. The succession has a maximum depositional age of ~2.18 Ga, from detrital zircons in quartzite, and consists of quartzites, schists, BIFs, gneiss and amphibolite, all metamorphosed under amphibolite facies conditions. The Guanhães BIF shows HREE enrichment and consistent positive Eu anomaly (PAAS-normalized REE+Y). Two types of contamination were observed in the samples. The first is contamination by an exotic detrital component, which resulted in low Y/Ho (< 30) and Pr/Yb (SN) ratios. Evidence of such contamination, combined with inferred stratigraphic stacking data, indicates that the Guanhães BIFs were deposited on a shallow marine environment. The second type of contamination resulted in higher Eu-anomalies, positive Ce-anomalies, and higher REE+Y concentrations, possibly due to the interaction between later magmatic fluids and the Guanhães BIF. A strong Cambrian event is recorded in zircon age data. The uncontaminated samples display REE+Y distribution similar to other Precambrian BIFs, particularly those from the Morro-Escuro Sequence and the Serra da Serpentina Group, without true Ce-anomalies and Y/Ho close to seawater values (45). Geochronological and geochemical data presented in this paper strongly suggest a correlation between the Guanhães supracrustal succession and the Serra da Serpentina and Serra de São José Groups.

The amphiboles of the REE-rich A-type peralkaline Strange Lake pluton – fingerprints of magma evolution

Major and trace element compositions of amphibole in igneous environments commonly reflect evolving magma compositions. In this study, we use the amphibole-group minerals from the Strange Lake, REE-enriched peralkaline granitic pluton to gain insights into the evolution of the magma. This 1240Ma old pluton consists of two main intrusive facies, an early hypersolvus granite, which occurs as separate northern and southern intrusions, and a more evolved transsolvus granite. In the hypersolvus granite the amphibole is a late interstitial phase, whereas in the transsolvus granite, it is present as phenocrysts. The amphibole compositions vary from calcic–sodic (ferro–ferri-katophorite) in the southern hypersolvus granite to sodic (arfvedsonite, ferro–ferri-leakeite) in the other, more evolved granitic units. High Na, Si, Li, and low Al and Ca concentrations in the amphibole phenocrysts of the transsolvus granite indicate formation from a more evolved magma compared to the hypersolvus granite, despite the fact that these crystals formed early. We interpret the increasing Fe3+/Fe2+ ratios in the amphibole of the hypersolvus granite to reflect crystal chemical effects (Na/Ca-ratio) and increasingly oxidizing conditions in the magma, whereas in the phenocrysts of the transsolvus granite, the increasing ratio was the product of increasing proportions of F− and OH− in the melt. The amphiboles of all the granite units have elevated Nb, Zr, Hf and REE concentrations compared to the bulk rock, suggesting that these elements are compatible in amphibole. By contrast the much lower Ti concentration was due to saturation of the magma in sodium-titanosilicates. The amphibole REE concentrations vary greatly among the granite units. Amphibole of the southern and northern hypersolvus granite contains 0.16 and 0.07wt.% ∑REE+Y, on average, respectively, and in the transsolvus granite, the average ∑REE content is only 0.01wt.%, despite the more evolved nature of its host transsolvus granite. We intrepret this compositional difference to be due to the fact that the latter represents phenocrysts, which crystallized early, whereas the hypersolvus arfvedsonite is a late interstitial phase. Chondrite-normalized REE profiles emphasise the wide range in LREE-, and the narrow range in HREE-concentrations of the amphiboles. The variations in the LREE-profiles reflect the variable crystallization of primary LREE-bearing phases, including monazite-(Ce), pyrochlore group minerals and gagarinite-(Ce), prior to or contemporaneous with the amphibole, as well as the exsolution of a LREE-rich fluoride melt. The LREE are incompatible in the amphibole structure (apparent D<0.01) and are preferably accommodated by the octahedral C-site, whereas the HREE occupy the B-site. The chondrite-normalized HREE profiles are steep and display an increasing relative enrichment that culminates in compatible behavior for Yb and Lu (apparent D>1). Owing to their small ionic radius and their compatibility with the amphibole structure, HREE concentrations were more controlled by partitioning (crystal chemical effects) than by the concentrations in the corresponding magma. Large proportions of the bulk HREE content (up to 70%) reside in the amphibole, and their later release through hydrothermal replacement helps to explain the extreme and unusual HREE enrichment of the Strange Lake pluton.

Ductile–brittle deformation effects on crystal-chemistry and U–Pb ages of magmatic and metasomatic zircons from a dyke of the Finero Mafic Complex (Ivrea–Verbano Zone, Italian Alps)

A detailed microstructural, geochemical and geochronological study was performed on zircon grains from plagioclase-rich dioritic dykes discordantly intruded within meta-diorites/gabbros forming the External Gabbro unit of the Finero Mafic Complex (Italian Alps). This unit is exposed as part of a near complete crustal section spanning from mantle rocks to upper crustal metasediments (Val Cannobina, Ivrea–Verbano Zone, Italy). The leucocratic dykes consist mainly of plagioclase (An18–24Ab79–82Or0.3–0.7) with subordinate amounts of biotite and spinel defining melanocratic layers. Zircon and corundum are common accessory phases. Both the dykes and the surrounding meta-diorites/gabbros show evidence of ductile deformation under amphibolite-facies conditions.Zircon grains/fragments (up to 2mm in length) occur as porphyroclasts surrounded by fine-grained plagioclase within the leucocratic domains and may occur within the melanocratic layers completely or partially surrounded by biotite and spinels. Fractures are common within zircon, define networks and have associated displacements occasionally and/or they can be filled by secondary minerals.Cathodoluminescence (CL) images reveal that zircon grains from the leucocratic layers show relicts of primary magmatic (i.e. oscillatory and or sector) zoning generally related with the crystal shapes or crystallographic orientation, whereas those from the melanocratic domains do not. In both cases, zircon shows secondary CL features, i.e. mosaic-like textures, due to deformation. EBSD maps confirmed a profuse mosaic texture, resulting in an internal misorientation of ca. 10°, generally associated with fractures. Locally, zircon shows clear evidence of crystal-plastic deformation at the edges, with a gradual misorientations of up to 12°, suggesting an origin prior fragmentation.Trace elements and U–Pb analyses were carried out by LA-ICP-MS directly on petrographic thin sections. Such in situ investigations revealed a strong correlation between internal zircon structures, chemistry, U–Pb isotope ratios and mylonitic fabric.U–Pb data return highly discordant and variable ages. The 206Pb/238U ages may range from ca. 297 to 198Ma within the same zircon grain from the leucocratic layers, whereas 206Pb/238U younger than 250Ma were systematically obtained from zircon within the melanocratic layers. The 206Pb/238U data younger than ca. 240Ma from zircon grains within the leucocratic layers were obtained from narrow axial stripes observed in CL images and oriented parallel or at low-angle with respect to the foliation planes. These stripes are characterized by an overall HREE, Y, U and Th enrichment possibly reflecting deformation of the grain in presence of interstitial fluid phases. Combining U–Pb data, microstructure and zircon CL features, we suggest a multistage evolution of the dykes whereby the melanocratic layers are the result of a Late Permian metasomatic event promoting modification of the pre-existing mineral assemblage of the (Late Carboniferous–Early Permian?) dykes and deformation and partial resetting of zircon porphyroclasts. This has important implications in the geology of the Ivrea–Verbano Zone because, having the dyke intruded the External Gabbro unit before Triassic (probably during Carboniferous–Permian), the latter cannot be considered a Triassic intrusion. At least part of the External Gabbro unit is Carboniferous–Permian as the other mafic bodies of the Ivrea–Verbano Zone.

Post-Late Glacial calcareous tufas from the Kurai fault zone (Southeastern Gorny Altai, Russia)

Calcareous tufa deposits have been discovered in the Chibitka River valley near Lake Cheybek-Kohl, at the junction of the Kurai and Teletsk-Kurai large active faults in the southeastern Gorny Altai, Russia, at an altitude of 1800–2000m. Fossil tufa is composed of calcite and cements Holocene grey colluvium and glacial till deposited by the Late Glacial Chibitka Glacier. Current tufa precipitation has been observed from a low-flow spring with cold (10°C) HCO3-SO4–Ca-Mg water, рН=6.86. The stable isotope composition of spring water is −5.8‰ VPDB δ13C of dissolved inorganic carbon and −14.5‰ VSMOW δ18O. Modern tufa consists of thin laminated Mg-calcite and Sr-aragonite crusts, with abundant algae and biofilms on their surfaces. Both modern and fossil tufas are depleted in REE (a total of 0.40–16.4ppm and 0.40–3.80ppm, respectively) and share similar PAAS-normalised REE+Y spectra with HREE enrichment and slight progressive LREE depletion. The modern tufas show positive δ13C values of 0.1‰ to 0.9‰ VPDB while the fossil ones have an isotopically lighter composition of δ13C=−4.1‰ to −1.9‰ VPDB; the δ18O range is very narrow (−13.0 to −13.8‰ VPDB). Both stable isotope and trace-element signatures (including REE patterns) of the tufas indicate precipitation from cold groundwaters subjected to prolonged interaction with a carbonate aquifer (the Baratal Group of limestone and dolostone) in a cold continental climate similar to the present conditions. Tufa deposition in the Lake Cheybek-Kohl area began with the onset of post-Late Glacial global warming and permafrost degradation. Unlike the fossil tufa formation, current precipitation of freshwater carbonates has been microbially mediated. The discovered tufa deposits provide new palaeoclimatic and active tectonic proxies in the southeastern Gorny Altai.

Changes in depositional environment for the past 35 years in the Thane Creek, central west coast of India: inferences from REEs, metals and magnetic properties

The role of diagenetic processes in influencing the behaviour of metals (Fe, Mn, Cu, Ni, Zn), rare earth elements (REEs) and environmental magnetic parameters in two sediment cores from a polluted creek environment (the Thane Creek, Mumbai) was investigated. The sediment age was determined using 210Pb analysis. Silty clays were dominant sediments in both the cores. Mineralogy showed the presence of quartz, calcite and plagioclase feldspar in bulk sediments, while smectite was the abundant clay mineral in the finer fraction. The mean ΣREE in both the cores was lower than in post-Archaean average Australian shale (PAAS). The PAAS-normalized REE patterns showed MREE and HREE enrichment with strong positive Eu and Y anomalies. The source rocks in the hinterland played an important role on the REE composition. Magnetic mineralogy showed dominance of magnetite. Both the cores showed low magnetic concentration of Al, Ti, Fe and Mn coinciding with high magnetic grain size at the middle portions. The early diagenetic changes at the middle portion of the cores were masked by significant abundance of clay content at the creek head and with OC and fine magnetite at the lower creek region. Metals (Cu, Ni, Zn; enrichment factor >1.5) and magnetic concentration in sediments were enriched in both the cores suggesting of an anthropogenic source.

Geochemistry of rare earth elements in groundwater from different aquifers in the Gboko area, central Benue Trough, Nigeria

Water samples from different aquifers in Gboko area, Lower Benue Trough, Nigeria, were evaluated to determine the quality, hydrochemical facies and the concentrations and fractionation patterns of rare earth elements (REEs) in the groundwater. The results revealed that the groundwater is contaminated with factors emanating from the concentrations of NO3, Cl, Mn, and the TDS (total dissolved solids). Five hydrochemical facies were identified, namely Na + KSO4Cl, Na + KHCO3, MgSO4Cl, and two forms of CaMgHCO3 (viz., Na + KHCO3 and Na + KSO4Cl). Ion exchange leads to the formation of Na + KHCO3 from CaMgHCO3, while reversed ion exchange leads to the formation of Na + KSO4Cl from CaMgHCO3, usually in the presence of saline water conditions. Also, simple dissolution or linear mixing gives rise to CaMgSO4Cl from CaMgHCO3. The results also showed that the contents of REE in the samples range from 1 to 445 ppb, with significant contributions from La, Ce and Nd. The shallow groundwater is predominantly LREE-enriched NASC-normalized patterns. The LREE enrichment is attributed to weathering of host rock minerals, such as phosphate monazite, florencite, and allanite, in an oxic environment, while the HREE enrichment is attributed to processes such as complexation reactions, desorption processes, re-adsorption processes, and chemical weathering in a reducing condition. The groundwater shows positive anomalies for both Ce and Eu. The Ce is controlled by oxidation of Ce3+ to Ce4+, incorporation of Ce into Mn and Fe oxyhydroxides phases, and Eu anomalies by weathering of zircon-rich minerals.

Rare Earth Element Determination in Olivine by Laser Ablation‐Quadrupole‐ICP‐MS: An Analytical Strategy and Applications

Olivine offers huge, largely untapped, potential for improving our understanding of magmatic and metasomatic processes. In particular, a wealth of information is contained in rare earth element (REE) mass fractions, which are well studied in other minerals. However, REE data for olivine are scarce, reflecting the difficulty associated with determining mass fractions in the low ng g−1 range and with controlling the effects of LREE contamination. We report an analytical procedure for measuring REEs in olivine using laser ablation quadrupole‐ICP‐MS that achieved limits of determination (LOD) at sub‐ng g−1 levels and biases of ~ 5–10%. Empirical partition coefficients (D values) calculated using the new olivine compositions agree with experimental values, indicating that the measured REEs are structurally bound in the olivine crystal lattice, rather than residing in micro‐inclusions. We conducted an initial survey of REE contents of olivine from mantle, metamorphic, magmatic and meteorite samples. REE mass fractions vary from 0.1 to double‐digit ng g−1 levels. Heavy REEs vary from low mass fractions in meteoritic samples, through variably enriched peridotitic olivine to high mass fractions in magmatic olivines, with fayalitic olivines showing the highest levels. The variable enrichment in HREEs demonstrates that olivine REE patterns have petrological utility.

Ore-forming mechanism of quartz-vein-type W-Sn deposits of the Xitian district in SE China: Implications from the trace element analysis of wolframite and investigation of fluid inclusions

The Xitian W–Sn district is located in eastern Hunan Province of SE China and comprises the Heshuxia, Goudalan, and Longshang W–Sn deposits. Our study on the occurrence, mineral assemblage, and geochemistry of the orebodies shows that quartz-vein orebodies are dominant mode of mineralization and can be divided into three stages: stage I, quartz–molybdenite–wolframite (I); stage II, quartz–wolframite (II)–polymetallic sulfides; and stage III, quartz–pyrite–fluorite. The elements Nb, Ta, Sc, Sn, Zn, Cr, V, Mo, Cu, and Zr are abundant in both wolframite (I) and wolframite (II). Wolframite (I) contains higher concentrations of Nb (1408–9245ppm), Ta (241–2407ppm), and Sc (123–521ppm) than wolframite (II) (Nb 144–1080ppm, Ta 3.76–315ppm, and Sc 1.86–11.44ppm). These results indicate that wolframite (I) was formed closer to the host rock and at a greater ore-forming depth than wolframite (II). The total REE concentrations of wolframite (I) and (II) range from 34.96 to 133.36ppm and 21.30 to 43.73ppm, respectively. Both wolframite (I) and wolframite (II) have low LREE concentrations (0.00–0.05 and 0.08–8.18ppm, respectively) and high HREE concentrations (34.96–133.31 and 21.23–41.87ppm, respectively). The enrichment of HREEs in wolframite may be due to the fact that the sizes of the HREEs3+ (0.94–1.02Å) in combination with Nb5+ or Ta5+ (0.72Å) are closer to that of W6+ (0.68Å) in combination with Ca2+ (1.08Å) or Mg2+ (0.80Å) than that of the LREEs3+ (1.03–1.13Å), making the coupled substitution easier. A study of fluid inclusions in the coexisting gangue minerals shows that the homogenization temperatures of the fluid inclusions decrease from stage I to stage III (stage I, 187–382°C, average 275°C; stage II, 122–278°C, average 175°C; stage III, 92–172°C, average 139°C), whereas the salinities of the fluid inclusions increase from stages I to II and decrease from stages II to III (stage I, 1.8–18.2wt% NaCleqv, average 10.3wt% NaCleqv; stage II, 7.3–24.4wt% NaCleqv, average 14.9wt% NaCleqv; stage III, 0.2–5.3wt% NaCleqv, average 1.5wt% NaCleqv). Raman spectroscopy reveals that the fluid inclusions mainly contain H2O; CO2 is minor and H2S, CH4, and N2 are rare. We conclude that the formation of wolframite was driven by post-magmatic thermodynamic processes. The ore-forming fluid flowed through a multi-fractured low-pressure zone in the fault system in the district, and a temperature–pressure decrease led to fluid immiscibility characterized by CO2 escaping in a low-pH and high-Eh environment in stage I, crust–mantle mixed fluid and meteoric water mixing in stage II, and natural cooling of the fluid system in stage III.

Petrogenesis and geochemical characteristics of plagiogranites from Naga Ophiolite Belt, northeast India: Fractional crystallization of MORB-type magma

The Naga Ophiolite Belt is a part of the Naga-Arakan-Yoma flysch trough that occurs along the Indo-Myanmar border. It is represented by peridotites, mafic-ultramafic cumulates, mafic volcanics, mafic dykes, plagiogranites, pelagic sediments and minor felsic to intermediate intrusives. Minor plagiogranites, gabbros and thin serpentinite bands occur juxtaposed near Luthur, with the slate-phyllite-metagreywacke sequence (Phokpur Formation) adjacent to the contact. The development of tonalites, trondhjemites and diorites in the oceanic crust, which is grouped as plagiogranites, offers an opportunity to study the process of formation of silicic melts from mafic crust. Plagiogranites from Naga Ophiolite Belt contains moderate SiO2 (51.81–56.71wt.%), low K2O (0.08–1.65wt.%) and high Na2O (4.3–5.03wt.%). The Naga Ophiolite Belt plagiogranites like ocean-ridge granites contain low K2O, high Na2O and CaO. The rocks investigated from Naga Ophiolite Belt contain TiO2 concentrations above the lower limit for fractionated Mid Oceanic Ridge Basalt which is above 1wt% of TiO2 and the ternary plots of A (Na2O+K2O) F(FeOT) M(MgO) and TiO2-K2O-SiO2/50 indicate that the plagiogranite are tholeiitic in character and gabbro samples are calc-alkaline in nature. The plagiogranites are enriched in Rb, Ba, Th, U, Nb and Sm against chondrite with negative anomalies on Sr and Zr whereas Y and Yb are depleted to Mid Oceanic Ridge Basalt. The chondrite normalized REE patterns of the plagiogranite display enrichments in LREE (LaN/SmN: 2.37–3.62) and flat HREE (Eu/Eu*: 0.90–1.06). The Mid Oceanic Ridge Basalt normalization of gabbro is characterized by strong enrichment of LILE like Ba and Th. The REE pattern is about 50–100 times chondrite with slight enrichment of LREE (LaN/SmN=2.21–3.13) and flat HREE (Eu/Eu*: 0.94–1.19). The major-element and trace element data of the NOB plagiogranites and their intrusive nature with host gabbroic rock suggest that the plagiogranites were produced by fractional crystallization of basaltic parental magmas at Mid Oceanic Ridge.

Paleofluids circulation associated with the Gerês late-orogenic granitic massif, northern Portugal

The Hercynian late-orogenic granites from the Gerês massif, northern Portugal, underwent intense hydrothermal activity along tectonic structures striking N-S and NE-SW. The first hydrothermal stage is characterized by the albitization of feldspars (primary K-feldspar and plagioclase) followed by the dissolution of magmatic quartz, the chloritization of biotite, and the muscovitization of magmatic biotite and feldspars and occasionally of authigenic albite. Whole-rock geochemistry shows a decrease of SiO2, K2O and Rb and an increase of Na2O, Al2O3 and Sr amounts during the albitization. The second hydrothermal stage is characterized by a mineral assemblage consisting of secondary quartz, albite, chlorite, hematite, apatite, muscovite, epidote, sphene and carbonates, which infilled the cavities produced by the early quartz leaching.Mass balance calculations demonstrate a noticeable element mobilization during the granite alteration. The average ∑REE of the Gerês granite is nearly constant at 154ppm, but ranges from 91 to 163ppm in the altered rocks. A relative LREE depletion and a slight HREE enrichment associated with a negative Eu anomaly characterize the feldspathization process of Gerês granite. The average ∑REE of the Carris granite is at 159ppm but in the altered epidote+chlorite+hematite assemblage it reaches about 201ppm with a slight HREE increase.Two different fluids were involved in the hydrothermal alteration. A first aqueous fluid (Lw1), with a low to intermediate salinity (<10wt.% eq. NaCl) circulated along the main structures (N-S, NE-SW and NW-SE), characterized by entrapment temperature lower than 350°C and maximum pressure of 115MPa followed by a later colder and more saline fluid (Lw2) under a temperature of 220°C and a pressure of about 27MPa.Oxygen isotope data obtained on magmatic quartz indicate δ18Oquartz of+9.3 and+11.0‰, pointing to an interaction of magmatic fluids with others of meteoric origin. For the secondary quartz, δ18Owater of +0.5 and +0.7‰ were calculated for a temperature of 250°C, suggesting a meteoric fluid with a possible seawater signature.The K-Ar data of K-feldspar from the altered rocks confirm an age of 273.6±11.7 Ma, attributed to the first alteration process. Younger K-Ar ages between 155.8±6.7 Ma and 124±5.3 Ma were also obtained in the feldspathized rocks, confirming the late hydrothermal activity.Albitization and quartz dissolution of granitic rocks from the Gerês massif occurred at depths shallower than 5km, induced by the circulation of fluids along brittle structures, during the orogenic uplift and extensional tectonics which affected the Iberian Massif in the Early Permian. A second major event is attributed to late hydrothermal circulations of aqueous brines until a depth of about 3km, presumably derived from interaction with sub-surface evaporites throughout Late Jurassic to Early Cretaceous. These late hydrothermal events probably reflect the rifting episodes and the rising of geothermal gradient, associated with the opening of the Atlantic Ocean and Gulf of Biscay, respectively.