Rare Earth Element Variations Research Articles

Metamorphic P-T conditions and variation of REE between two garnet generations from granulites in the Sør-Rondane mountains, East Antarctica

In this paper, we describe the metamorphic conditions of Fe-rich granulite and variations in rare earth elements (REE) between peak garnet porphyroblasts and secondary garnet coronae. The Fe-rich granulites were collected from Vesthaugen, Sor-Rondane Mountains, East Antarctica, and consist mainly of cordierite, garnet, spinel, perthite, K-feldspar, plagioclase, and orthopyroxene or sillimanite. Temperatures estimated from perthitc–mesoperthitic feldspar compositions, experimentally calibrated geothermobarometers and the modeling of P-T pseudosections suggest that the rocks experienced peak ultrahigh-temperature (UHT) metamorphic conditions of 900–950 °C and 5.0 ± 0.5 kbar. Spinel contains quartz inclusions that also provide evidence for UHT metamorphism. Evidence of partial melting is characterized by the presence of leucocratic bands. The second generation of garnet occurs as coronae around spinel, formed during isobaric cooling following the peak conditions of UHT metamorphism. Garnet coronae and garnet porphyroblasts have distinct trace element patterns. Textural evidence and REE geochemistry suggest that the development of garnet coronae was controlled by (1) the REE composition of reactant phases and melt and/or (2) the crystallization of HREE-rich accessory phases (e.g., zircon and monazite) during secondary garnet growth.

Water-soluble rare earth elements in atmospheric deposition at Shihua Cave, Beijing, north China

Atmospheric deposition is an important source of trace elements in speleothems. In a pilot study, atmospheric deposition was collected monthly from Shihua Cave in Beijing, North China, from November 2012 to October 2013, to tentatively investigate the annual fluxes and seasonal variations of the water-soluble rare earth elements (REEs) in atmospheric deposition. The study led to the following results: (1) the annual fluxes of all the soluble REEs were in the descending sequence Ce, 67.9 μg m−2 yr−1; La, 42.1 μg m−2 yr−1; Nd, 23.8 μg m−2 yr−1; Pr, 5.94 μg m−2 yr−1; Sm, 4.68 μg m−2 yr−1; Gd, 4.26 μg m−2 yr−1; Dy, 3.53 μg m−2 yr−1; Er, 2.10 μg m−2 yr−1; Yb, 2.00 μg m−2 yr−1; Eu, 0.90 μg m−2 yr−1; Ho, 0.77 μg m−2 yr−1; Tb, 0.61 μg m−2 yr−1; Lu, 0.32 μg m−2 yr−1; and Tm, 0.31 μg m−2 yr−1. (2) The seasonal deposition fluxes of all the REEs displayed two peaks, one in the spring and the other in the autumn. Lower deposition fluxes were observed in the summer and winter. Changes in the rainfall amount and in turn soil moisture content, snow coverage of the earth surface, and atmospheric dust activity were responsible for the seasonal changes in the REE deposition fluxes. (3) Natural materials, especially wind-blown dust, were the dominant source of water-soluble REEs in atmospheric deposition. The relative contribution from anthropogenic materials, such as automobile emissions, increased significantly in the winter when the snow coverage was large and the atmospheric dust activity limited.

Investigation of major, trace and rare earth elements variations in Esfezar bentonite deposit, east of Birjand, South Khorasan Province

Investigation of major, trace and rare earth elements variations in Esfezar bentonite deposit, east of Birjand, South Khorasan Province

Deducing flow path mixing by storm-induced bulk chemistry and REE variations in two karst springs: With trends like these who needs anomalies?

Karst aquifers are dynamic hydrologic systems which can be sensitive to short-term recharge events (storms) and heterogeneous recharge characteristics (point recharge at sinks, irregular soil thicknesses). In this study, two adjacent karst springs, Tippery Spring and Near Tippery Spring, were monitored to better understand flow and source mixing characteristics in response to two storms in May and June 2017. Monitoring techniques included high-resolution discharge and temperature logging and collection of spring water using automatic samplers to analyze chemical parameters such as stable water isotopes and Mg/Ca ratios along with Ca/Zr ratios and rare earth elements (REEs) which have had limited testing in karst systems. The two springs exhibited behaviors which were both unique to each spring and shared depending on which parameter is considered. Spring-specific behaviors included discharge and isotopic response, with Tippery Spring having a flashier and faster conduit flow behavior relative to Near Tippery Spring’s delayed storm response. Absolute Mg/Ca values were spring-specific, owing to the relative fraction of limestone and dolostone in their respective recharge areas, although both springs showed similar Mg/Ca ratio shifts after storms. While Ca/Zr ratios changed in timing and intensity with storm intensity, both springs exhibited a decline in Ca/Zr ratios as calcium-rich carbonate matrix water was displaced by zirconium-rich storm recharge water from sinking streams off the clastic upland ridges. Flushing of recharged storm water resulted in an increase in total REE concentrations at both springs (from <0.15 ppb to >1.0 ppb), with the timing and magnitude of concentration increases determined by the degree of surface connectivity intrinsic to each spring and the intensity of the recharge event. REEs and Ca/Zr ratios provided additional and complementary tracer techniques to better understand transient recharge and flow behaviors in karst springs.

Quaternary history of the Lake Magadi Basin, southern Kenya Rift: Tectonic and climatic controls

Sediments from the Magadi Basin (south Kenya Rift) preserve a one-million-year palaeoenvironmental record that reflects interactions between climatic, volcanic and tectonic controls. Climate changes that impacted sedimentation include wet-dry cycles on variable timescales and an overall progressive trend towards greater aridity. Volcanic influences involved inputs of tephra to the basin, significant inflow of geothermal fluids, and the effects of weathering, erosion and transportation of clastics from trachyte and basalt terrains. Tectonic controls, which were often step-like, reflect the influence of faults that provided pathways for fluids and which controlled accommodation space and drainage directions.Intensified aridity and evaporative concentration resulted in salinity and pH increasing with time, which led to a change from calcite deposition in mildly saline lakes before 380 ka to the later formation of zeolites from reactions of volcaniclastic debris with highly alkaline lake and pore water. After 105 ka, hyperalkaline conditions led to trona accumulation and increasingly variable rare earth elements (REEs). The presence of mixed saline and freshwater diatom taxa between 545 and 16 ka indicates climate variability and episodic inputs of fresh water to saline lakes. Calcrete formed in lake marginal settings during semi-arid periods.Tectonic controls operated independently of climate, but they interacted together to determine environmental conditions. Aquatic deposition was maintained during periods of increasing aridity because fault-controlled ambient and geothermal springs continued to flow lakewards. This recharge, in turn, limited pedogenesis: palaeosols are common in other rift floor sequences. Trona formed when aridity and evapoconcentration increased, but its precipitation also reflects increased magmatic CO2 that ascended along faults. Basin fragmentation and north-south fractures caused loss of cross-rift (east-west) drainage from rift-marginal basalts, resulting in reduced transition metals after 545 ka. The Magadi Basin demonstrates how a careful reconstruction of these complex tectono-climatic interactions is essential for accurate palaeoenvironmental reconstruction in continental rifts and in other tectonic settings.

Zircon Macrocrysts from the Drybones Bay Kimberlite Pipe (Northwest Territories, Canada): A High-Resolution Trace Element and Geochronological Study

Zircon macrocrysts in (sub)volcanic silica-undersaturated rocks are an important source of information about mantle processes and their relative timing with respect to magmatism. The present work describes variations in trace element (Sc, Ti, Y, Nb, lanthanides, Hf, Ta, Pb, Th, and U) and isotopic (U-Pb) composition of zircon from the Drybones Bay kimberlite, Northwest Territories, Canada. These data were acquired at a spatial resolution of ≤100 µm and correlated to the internal characteristics of macrocrysts (imaged using cathodoluminescence, CL). Six types of zircon were distinguished on the basis of its luminescence characteristics, with the majority of grains exhibiting more than one type of CL response. The oscillatory-zoned core and growth sectors of Drybones Bay zircon show consistent variations in rare-earth elements (REE), Hf, Th, and U. Their chondrite-normalized REE patterns are typical of macrocrystic zircon and exhibit extreme enrichment in heavy lanthanides and a positive Ce anomaly. Their Ti content decreases slightly from the core into growth sectors, but the Ti-in-zircon thermometry gives overlapping average crystallization temperatures (820 ± 26 °C to 781 ± 19 °C, respectively). There is no trace element or CL evidence for Pb loss or other forms of chemical re-equilibration. All distinct zircon types are concordant and give a U-Pb age of 445.6 ± 0.8 Ma. We interpret the examined macrocrysts as products of interaction between a shallow (&lt;100 km) mantle source and transient kimberlitic melt.

Geology and Mineralogy of Rare Earth Elements Deposits and Occurrences in Finland

Rare earth elements (REE) have critical importance in the manufacturing of many electronic products in the high-tech and green-tech industries. Currently, mining and processing of REE is strongly concentrated in China. A substantial growth in global exploration for REE deposits has taken place in the recent years and has resulted in considerable advances in defining new resources. This study provides an overview of the mineralogical and petrological peculiarities of the most important REE prospects and metallogeny of REE in Finland. There is a particularly good potential for future discoveries of carbonatite hosted REE deposits in the Paleozoic Sokli carbonatite complex, as well as in the Paleoproterozoic Korsnäs and Kortejärvi Laivajoki areas. This review also provides information about the highest known REE concentration in the alkaline intrusions of Finland in the Tana Belt and other alkaline rock hosted occurrences (e.g., Otanmäki and Katajakangas). Significant REE enrichments in hydrothermal alteration zones are also known in the Kuusamo Belt (Uuniniemi and Honkilehto), and occurrences of REE-rich mineralisation are also present in granite pegmatite bodies and greisens in central and southern Finland (Kovela monazite granite and the Rapakivi Granite batholith at Vyborg, respectively). REE minerals in all of the localities listed above were identified and analyzed by scanning electron microscopy (SEM) and electron microprobes (EMPs). In localities of northern and central Finland, both primary rock forming and epigenetic-hydrothermal REE minerals were found, namely phosphates (monazite-Ce, xenotime-Y), fluorcarbonates (bastnäsite-Ce, synchysite), and hydrated carbonates (ancylite-Ce), hydrated aluminium silicates (allanite-Ce, Fe-allanite, cerite, chevkinite), oxides (fergusonite, euxenite) and U-Pb rich minerals. The chondrite normalized REE concentrations, the La/Nd ratios and the REE vs. major element contents in several types of REE bearing minerals from prospects in Finland can be used to identify and define variable REE fractionation processes (carbonatites), as well as to discriminate deposits of different origins.

Seasonal and spatial variations in rare earth elements and yttrium of dissolved load in the middle, lower reaches and estuary of the Minjiang River, southeastern China

With the aim of elucidating the spatial and seasonal behaviors of rare earth elements (REEs), we investigated the dissolved REE concentrations of surface water collected during four seasons from middle, lower reaches and estuary of the Minjiang River, southeastern China. The results display that the REE abundances in Minjiang River, ranging from 3.3–785.9 ng/L, were higher than those of many of the major global rivers. The total REE concentrations (ΣREE) were seasonally variable, averaging in 5 937.30, 863.79, 825.65 and 1 065.75 ng/L during second highest flow (SHF), normal flow (NF), low flow (LF) and high flow (HF) season, respectively. The R(L/M) and R(H/M) ratios reveal the spatial and temporal variations of REE patterns, and particularly vary apparently in the maximum turbidity zone and estuary. REE patterns of dissolved loads are characterized by progressing weaker LREEs-enrichment and stronger HREEsenrichment downstream from middle reaches to estuary during all four seasons. Comparing with NF and LF seasons, in which REE patterns are relatively flat, samples of SHF season have more LREE-enriched and HREE-depleted patterns that close to parent rocks, while samples of HF season are more LREEs-depleted and HREE-enriched. REE fractionations from the middle to lower reaches are stronger in the SHF and HF seasons than those in NF and LF seasons. Generally, spatial and seasonal variations in REE abundance and pattern are presumably due to several factors, such as chemical weathering, mixture with rainfall and groundwater, estuarine mixing, runoff, biological production and mountain river characters, such as strong hydrodynamic forces and steep slopes. The highest Gd/Gd * always occurs at north ports during all four seasons, where most of the large hospitals are located. This suggests Gd anomalies are depended on the density of modern medical facilities. Y/Ho ratios fluctuate and positively correlate to salinity in estuary, probably because of the geochemical behavior differences between Y and Ho.

The Influence of Basaltic Islands on the Oceanic REE Distribution: A Case Study From the Tropical South Pacific

The Rare Earth Elements (REEs) have been widely used to investigate marine biogeochemical processes as well as the sources and mixing of water masses. However, there are still important uncertainties about the global aqueous REE cycle with respect to the contributions of highly reactive basaltic minerals originating from volcanic islands and the role of Submarine Groundwater Discharge (SGD). Here we present dissolved REE concentrations obtained from waters at the island-ocean interface (including SGD, river, lagoon and coastal waters) from the island of Tahiti and from three detailed open ocean profiles on the Manihiki Plateau (including neodymium (Nd) isotope compositions), which are located in ocean currents downstream of Tahiti. Tahitian fresh waters have highly variable REE concentrations that likely result from variable water–rock interaction and removal by secondary minerals. In contrast to studies on other islands, the SGD samples do not exhibit elevated REE concentrations but have distinctive REE distributions and Y/Ho ratios. The basaltic Tahitian rocks impart a REE pattern to the waters characterized by a middle REE enrichment, with a peak at europium similar to groundwaters and coastal waters of other volcanic islands in the Pacific. However, the basaltic island REE characteristics (with the exception of elevated Y/Ho ratios) are lost during transport to the Manihiki Plateau within surface waters that also exhibit highly radiogenic Nd isotope signatures. Our new data demonstrate that REE concentrations are enriched in Tahitian coastal water, but without multidimensional sampling, basaltic island Nd flux estimates range over orders of magnitude from relatively small to globally significant. Antarctic Intermediate Water (AAIW) loses its characteristic Nd isotopic signature (-6 to-9) around the Manihiki Plateau as a consequence of mixing with South Equatorial Pacific Intermediate Water (SEqPIW), which shows more positive values (-1 to -2). However, an additional Nd input/exchange along the pathway of AAIW, eventually originating from the volcanic Society, Tuamotu and Tubuai Islands (including Tahiti), is indicated by an offset from the mixing array of AAIW and SEqPIW to more radiogenic Nd isotope compositions.

U–Pb geochronology and trace element composition of zircon from the Horseshoe Lake greenstone belt, Superior Province, Canada: implications for the tectonic and metamorphic history

Depth profile techniques for U–Pb geochronology and rare earth element (REE) geochemistry were conducted on unpolished, Archean zircon with metasomatic rims from metasedimentary rocks within the Horseshoe Lake greenstone belt, western Superior Province, Canada. These zircon crystals are shown to have isotopically distinct rims (typically &lt;5 μm thick) compared with the interiors of the crystal. Secondary ion mass spectrometry (SIMS) U–Pb depth profile analyses of the rims define two different 207Pb/206Pb age populations at ca. 2920 and 2869 Ma, which are &gt;100 million years younger than the cores. The 207Pb/206Pb rim ages can be temporally correlated with regional magmatism and with a later, potentially Au-bearing, hydrothermal event synchronous with greenschist-facies metamorphism and regional deformation. Notably, the zircons do not record evidence of local ca. 2741–2715 Ma magmatism manifested by the emplacement of a quartz–feldspar porphyry dike swarm. Laser ablation – inductively coupled plasma – mass spectrometry (LA–ICP–MS) REE depth profile analyses on the same unpolished zircon show that the rims are commonly characterized by low Th/U ratios, elevated Hf, and variable REE concentrations in comparison with the interior of the grains. The variations in rim and core chemistry suggest that these elements, along with common Pb, were mobilized by fluids and interacted with zircon in the metasediments, resulting in the production of the metasomatic rims. In summary, this paper shows that depth profiling techniques applied to unpolished zircon can be useful to elucidate the tectonic, and potentially metallogenic, history of a complex Archean terrane.

Lithologically-related rare earth element variations the Mio-Pliocene Poznań Formation (Poland)

Concentrations of rare earth elements (REEs) were determined in 129 clay samples collected from the Mio-Pliocene Poznan Formation in Poland. The REEs occur in a relatively wide range from 58.2 to 1,709 mg/kg. Low North American Shale Composite (NASC)-normalized total REE contents with heavy rare earth element (HREE) depletion were noted in most of the samples analysed. Small variations were detected in the REE contents in the lithologies distinguished. Red clays are characterised by slightly lower REE concentrations relative to green and grey clays. All the Poznan Formation clays exhibit a negative Eu anomaly when normalized to the average of chondrites. Values increase according to the following sequence: grey clays grey clays > red clays. The variability of the REE contents of the Poznan Formation decreased from west to east.

Variation of Rare Earth Elements (REEs) in the Sulu and Celebes Seas of Malaysian waters during PMSE 09’ expedition using analytical and chemometric techniques

This study provides a dataset of rare earth elements (REEs) in the seawater of Sulu and Celebes Seas of Malaysian waters during the PMSE 09’ expedition, which was conducted to define the pattern of REEs in both seas. Samples were collected, filtered and pre-concentrated on-board ship then analyzed by using ICP-MS. Generally, the spatial distribution pointed out a high concentration of REEs in vicinities with high number of local settlements and Borneo Island riverine flush-outs. The chemometric analysis has showed the competition of binding sites between REEs and salt ions. The REEs normalized against NPDW evidence the influence of South China Sea waters lessen with the increase in water depth. Additionally, the studied area shows that both seas are affected by local river drainage from Borneo Island, Indonesia and Philippine archipelagos.Keywords: dissolved REEs; particulate REEs; Sulu Sea; Celebes Sea; environmetric analysis

Major and trace elements redistribution in weathered claystones from the Corumbataí Formation, Paraná Sedimentary Basin, São Paulo, Brazil

ABSTRACT: We studied 30 samples from 10 mining fronts of clayish rocks of the Corumbataí Formation, in the region of the Santa Gertrudes Ceramic Pole (São Paulo, Brazil), in order to evaluate the variation of rare earth elements (REE), mineralogy, weathering effects and provenance. The profiles show diagnostic geochemical signatures that are uniform across the Corumbataí Formation samples in the studied region and can be correlated from mine to mine, i.e.: 1) the constant MgO/K2O ratio and the variation of other oxides, which allow the identification of three main groups of samples; 2) similar groups of chemical index of weathering (CIW) and chemical index of alteration (CIA); 3) the immobile character of REE, Th, and Sc. The erosion/denudation processes were apparently the same among the studied profiles, allowing the identification of a similar weathering pattern among the mines, despite its location within the stratigraphic column. Mineralogical and geochemical data here reported suggest a dominant felsic source, but intermediate or mixed sources cannot be discarded. The chemical and mineralogical aspects observed are practically uniform along the studied profiles and mines, which allow them to be used as raw material by the industries of the Santa Gertrudes Ceramic Pole (SGCP) region.

In-situ trace element and Sr isotopic compositions of mantle xenoliths constrain two-stage metasomatism beneath the northern North China Craton

Subduction and collision are the key processes triggering geochemical refertilization of the lithospheric mantle beneath cratons. However, the way that the subducted plate influences the cratonic lithospheric mantle remains unclear. Here, in-situ major and trace-element and Sr isotopic compositions of peridotite and pyroxenite xenoliths carried by the Dongbahao Cenozoic basalts, located close to the northern margin of North China Craton (NCC), were examined to investigate the effects of the subducted Paleo-Asian oceanic plate on the lithospheric mantle of the NCC. Based on petrographic and geochemical features, peridotites were subdivided into two types recording two-stage metasomatism. Clinopyroxene (Cpx) in both types of peridotites show chemical zoning. In those peridotites we refer to as Type 1 peridotites, Cpx exhibit uniform convex-upward rare earth element (REE) patterns but core-rim variations in 87Sr/86Sr ratios (0.7065–0.7082 in the cores and 0.7043–0.7059 in the spongy rims), and have high (La/Yb)N ratios (>1.12) (N means normalized to chondrite), relatively low Ti/Eu ratios (<3756) and negative high field strength element (HFSE) (Nb, Ta, Zr, Hf and Ti) anomalies in the cores, indicating early-stage metasomatism by carbonatitic melts derived from the subducted sedimentary carbonate rocks. Cpx in the Type 2 peridotites have highly variable REE patterns (from light rare earth element (LREE)-depleted to LREE-enriched) and feature zoned Sr isotopic compositions contrasting to those in Type 1, i.e., increasing 87Sr/86Sr ratios from the cores (0.7020–0.7031) to the spongy rims (0.7035–0.7041). Accompanying variations of 87Sr/86Sr ratios, Cpx in both types of peridotites display increasing Nb/La ratios from the cores to the spongy rims. In addition, Cpx in the Type 2 peridotites show remarkably increased (La/Yb)N, Ca/Al, Sm/Hf and Zr/Hf ratios but decreased Ti/Eu and Ti/Nb ratios from the cores to the spongy rims. These features imply a later-stage metasomatism by CO2-rich silicate melts derived from carbonated eclogites. Pyroxenites were also classified into two types. Both types of pyroxenites show higher Ni content in Cpx and orthopyroxene than peridotites at the same Mg# (=100∗Mg/(Mg+Fe), atomic number) level. Their Cpx show high Ti/Eu, Ti/Sr ratios and similar 87Sr/86Sr ratios (0.7039–0.7055) to the Cpx spongy rims in peridotites, suggesting that pyroxenites originated from silicate melt-peridotite reactions in the later-stage metasomatism.These observations collectively indicate that the lithospheric mantle beneath the northern NCC presents evidence for two distinct mantle metasomatic events. We propose that both were caused by the subduction of the Paleo-Asian oceanic plate, which could have contributed significantly to the transformation of the lithospheric mantle beneath the northern NCC.

Assessment of species-specific and temporal variations of major, trace and rare earth elements in vineyard ambient using moss bags

Since the methodological parameters of moss bag biomonitoring have rarely been investigated for the application in agricultural areas, two mosses, Sphagnum girgensohnii (a species of the most recommended biomonitoring genus) and Hypnum cupressiforme (commonly available), were verified in a vineyard ambient. The moss bags were exposed along transects in six vineyard parcels during the grapevine season (March‒September 2015). To select an appropriate period for the reliable ‘signal’ of the element enrichment in the mosses, the bags were simultaneously exposed during five periods (3 × 2 months, 1 × 4 months, and 1 × 6 months). Assuming that vineyard is susceptible to contamination originated from different agricultural treatments, a wide range of elements (41) were determined in the moss and topsoil samples. The mosses were significantly enriched by the elements during the 2-month bag exposure which gradually increasing up to 6 months, but Cu and Ni exhibited the noticeable fluctuations during the grapevine season. However, the 6-month exposure of moss bags could be recommended for comparative studies among different vineyards because it reflects the ambient pollution comprising unpredictable treatments of grapevine applied during the whole season. Although higher element concentrations were determined in S. girgensohnii than H. cupressiforme, both species reflected the spatio-temporal changes in the ambient element content. Moreover, the significant correlation of the element (Cr, Cu, Sb, and Ti) concentrations between the mosses, and the same pairs of the elements correlated within the species, imply the comparable use of S. girgensohnii and H. cupressiforme in the vineyard (agricultural) ambient. Finally, both the moss bags and the soil analyses suggest that vineyard represents a dominant diffuse pollution source of As, Cr, Cu, Ni, Fe, and V.

The structural roles of Sc and Y in aluminosilicate glasses probed by molecular dynamics simulations

The incorporation of rare-earth (RE) elements into an aluminosilicate (AS) glass generally enhances its optical and mechanical properties. Atomistic molecular dynamics (MD) simulations were employed for probing the coordinations of Y3+ and Sc3+ in Y2O3–Al2O3–SiO2 and Sc2O3–Al2O3–SiO2 glasses of variable Si, Al, and RE contents. YO6 and ScO5 polyhedra are the most abundant RE species. We explore the trends in the distributions of the various (non)bridging oxygen species and Si/Al/RE cations in the first and second RE coordination spheres, respectively. The lowest RE[4] and RE[5] coordinations of both Y and Sc exhibit a strong preference for coordinating non-bridging O species, which gradually relaxes for increasing coordination number p. Clear deviations from a statistical Si/Al/RE distribution around the REOp polyhedra is observed, with preferences for RE–RE contacts relative to RE–Al and (notably) RE–Si. The extents of RE–RE associations are similar for Y and Sc and grow slightly for increasing Si content and/or glass network polymerization. The propensity for RE–Al contacts becomes emphasized in the Sc AS glasses, mainly at the expense of Sc–Si. The YOp and ScOp polyhedra connect to their Si/Al/RE neighbors primarily by sharing corners, but a significant extent of edge-sharing is also observed, which increases both with the RE[p] coordination number, and along the series Si <Al[4] <Al[5]/Al[6] ≈ RE of neighbors.

Geochronology and geochemistry of the Fe ore-bearing Zhonggu intrusions of the Ningwu Basin: Implications for tectonic setting and contemporaneous Cu-Au mineralization in the Middle–Lower Yangzte Metallogenic Belt

The Zhonggu ore field is located in the southern Ningwu volcanic basin, the largest iron ore district within the Middle–Lower Yangtze River Metallogenic Belt (MLYRMB) of eastern China. This area has produced more than 2000Mt of iron ore although comparatively little research has been done on the ore-bearing intrusions in this region despite the significance of the mineralization in this area. This in turn means that the petrogenesis and the links between these intrusions and the metallogenesis in this area remain unclear. This study focuses on the six intrusions that are associated with almost all of the iron deposits within the Zhonggu ore field, namely the Gushan porphyritic gabbrodiorite, the Longshan gabbrodiorite, the Hemushan diorite, the Baixingshan diorite, the Zhongjiu diorite and the Taipingshan monzonite. All of these rocks are either high- to medium-K and calc-alkaline or are shoshonitic, although the former dominates over the latter. Zircon laser ablation–inductively coupled plasma–mass spectrometry (LA–ICP–MS) U–Pb dating yielded ages of 132.6±1.6 to 131.6±1.6, 130.7±2.2 to 129.4±1.9, 131±2.0, and 132.3±2.1Ma for intrusions associated with the Longshan, Taipingshan, Baixiangshan and Zhongjiu deposits, respectively, indicating that all of these intrusions were emplaced in the Early Cretaceous (132.6–129.4Ma). This magmatism and mineralization was contemporaneous with the second stage (135–127Ma) of magmatic activity that has been documented elsewhere within the MLYRMB. The mineralized intrusions associated with the Zhonggu iron ore field have similar chondrite-normalized rare earth element (REE) and primitive mantle-normalized multi-element variation patterns that are characterized by light REE (LREE) and light ion lithophile element (LILE; Rb, Ba, K, and Sr) enrichments and heavy REE (HREE) and high field strength element (HFSE; Nb, Ta, Zr, and Hf) depletions. These intrusions also have total REE (ΣREE) concentrations that are consistent with the evolution of the magma from mafic-intermediate to felsic. The magmas that formed these intrusions were derived from an enriched region of the mantle and underwent contamination by the assimilation of continental crustal or lithospheric material. The magmatism within the Zhonggu iron ore field formed during an Early Cretaceous period of lithospheric thinning and extension. In addition, the arc affinities of the intrusions in this region indicative of a relationship to the subduction of the Paleo-Pacific Plate, a process that was most likely the main factor in the generation of the mineralization within the MLYRMB. The Fe mineralization-related intrusions in the study area are also distinctly different from intrusions in this region associated with Cu–Au mineralization, most likely as a result of the heterogeneous distribution of slab-derived fluids in this period over time. This research also highlights the exploration potential of this region, indicating that the 132–129Ma monzonites and diorites in this area and beyond should be considered prime targets for Fe exploration but are also unlikely to be related to Cu–Au mineralization.

Combining Nd isotopes in monazite and Hf isotopes in zircon to understand complex open-system processes in granitic magmas

Mapping the age and trace element and Sm-Nd isotope compositions of monazite grains from a peraluminous Cretaceous granite using laser ablation–split stream analysis reveals a wide range in Nd isotope and rare earth element (REE) compositions within and between single grains. These data corroborate isotopic variability indicated by Hf isotope analysis of zircon in the same granite sample. The REE variations indicate that monazite grew during fractional crystallization. Hf and Nd isotopes indicate that the granitic magma was generated from at least two distinct Proterozoic sources of approximately the same age: one component that had highly radiogenic initial 176Hf/177Hf and 143Nd/144Nd and a second component that was notably less radiogenic. This study highlights the utility of in situ REE and Sm-Nd isotope data in monazite in magmatic systems. Further, it refines the zircon-based constraints on magmatic processes because of sensitivity of light REEs to fractional crystallization, lower probability of complications owing to inheritance, and smaller analytical volumes required.

Temporal Variations of REE in Several AMD Sources of the Odiel River (SW Spain)

Waters affected by acid mine drainage (AMD) can contain high concentrations of rare earth elements (REE) which have a great interest not only by its application as tracers in hydrogeochemical processes but also as a potential economic resource. The Iberian Pyrite Belt (IPB) is rich in sulfide deposits which have been exploited intensively since ancient times, generating numerous AMD discharges. This work studies the REE content in five different AMD sources located in the upper reach of the Odiel River (SW Spain). The concentration of REE in these sources ranged from 113 to 7208 ppb. Differences in NASC-normalized REE patterns were observed among AMD sources that remained constant along the year despite the variability in REE concentrations, being lower during the rainy season due to dilution processes.

Geochemistry, Nd-Pb Isotopes, and Pb-Pb Ages of the Mesoproterozoic Pea Ridge Iron Oxide-Apatite–Rare Earth Element Deposit, Southeast Missouri

Iron oxide-apatite and iron oxide-copper-gold deposits occur within ~1.48 to 1.47 Ga volcanic rocks of the St. Francois Mountains terrane near a regional boundary separating crustal blocks having contrasting depleted-mantle Sm-Nd model ages (TDM). Major and trace element analyses and Nd and Pb isotope data were obtained to characterize the Pea Ridge deposit, improve identification of exploration targets, and better understand the regional distribution of mineralization with respect to crustal blocks. The Pea Ridge deposit is spatially associated with felsic volcanic rocks and plutons. Mafic to intermediate-composition rocks are volumetrically minor. Data for major element variations are commonly scattered and strongly suggest element mobility. Ratios of relatively immobile elements indicate that the felsic rocks are evolved subalkaline dacite and rhyolite; the mafic rocks are basalt to basaltic andesite. Granites and rhyolites display geochemical features typical of rocks produced by subduction. Rare earth element (REE) variations for the rhyolites are diagnostic of rocks affected by hydrothermal alteration and associated REE mineralization. The magnetite-rich rocks and REE-rich breccias show similar REE and mantle-normalized trace element patterns. Nd isotope compositions (age corrected) show that: (1) host rhyolites have ɛ Nd from 3.44 to 4.25 and TDM from 1.51 to 1.59 Ga; (2) magnetite ore and specular hematite rocks display ɛ Nd from 3.04 to 4.21 and TDM from 1.6 to 1.51 Ga, and ɛ Nd from 2.23 to 2.81, respectively; (3) REE-rich breccias have ɛ Nd from 3.04 to 4.11 and TDM from 1.6 to 1.51 Ga; and (4) mafic to intermediate-composition rocks range in ɛ Nd from 2.35 to 3.66 and in TDM from 1.66 to 1.56. The ɛ Nd values of the magnetite and specular hematite samples show that the REE mineralization is magmatic; no evidence exists for major overprinting by younger, crustal meteoric fluids, or by externally derived Nd. Host rocks, breccias, and magnetite ore shared a common origin from a similar source. Lead isotope ratios are diverse: (1) host rhyolite has 206Pb/204Pb from 24.261 to 50.091; (2) Pea Ridge and regional galenas have 206Pb/204Pb from 16.030 to 33.548; (3) REE-rich breccia, magnetite ore, and specular hematite rock are more radiogenic than galena; (4) REE-rich breccias have high 206Pb/204Pb (38.122–1277.61) compared to host rhyolites; and (5) REE-rich breccias are more radiogenic than magnetite ore and specular-hematite rock, having 206Pb/204Pb up to 230.65. Radiogenic 207Pb/206Pb age estimates suggest the following: (1) rhyolitic host rocks have ages of ~1.50 Ga, (2) magnetite ore is ~1.44 Ga, and (3) REE-rich breccias are ~1.48 Ga. These estimates are broadly consistent and genetically link the host rhyolite, REE-rich breccia, and magnetite ore as being contemporaneous. Alteration style and mineralogical or textural distinctions among the magnetite-rich rocks and REE-rich breccias do not correlate with different isotopic sources. In our model, magmatic fluids leached metals from the coeval felsic rocks (rhyolites), which provided the metal source reflected in the compositions of the REE-rich breccias and mineralized rocks. This model allows for the likelihood of contributions from other genetically related felsic and intermediate to more mafic rocks stored deeper in the crust. The deposit thus records an origin as a magmatic-hydrothermal system that was not affected by Nd and Pb remobilization processes, particularly if these processes also triggered mixing with externally sourced metal-bearing fluids. The Pea Ridge deposit was part of a single, widespread, homogeneous mixing system that produced a uniform isotopic composition, thus representing an excellent example of an igneous-dominated system that generated coeval magmatism and REE mineralization. Geochemical features suggest that components in the Pea Ridge deposit originated from sources in an orogenic margin. Basaltic magmatism produced by mantle decompression melting provided heat for extracting melts from the middle or lower crust. Continual addition of mafic magmas to the base of the subcontinental lithosphere, in a back-arc setting, remelted calc-alkaline rocks enriched in metals that were stored in the crust. The St. Francois Mountains terrane is adjacent to the regional TDM line (defined at a value of 1.55 Ga) that separates ~1600 Ma basement to the west, from younger basements to the east. Data for Pea Ridge straddle the TDM values proposed for the line. The Sm-Nd isotope system has been closed since formation of the deposit and the original igneous signatures have not been affected by cycles of alteration or superimposed mineralizing events. No evidence exists for externally derived Nd or Sm. The source region for metals within the Pea Ridge deposit had a moderate compositional variation and the REE-rich breccias and mineralized rocks are generally isotopically homogeneous. The Pea Ridge deposit thus constitutes a distinctive isotopic target for use as a model in identifying other mineralized systems that may share the same metal source in the St. Francois Mountains terrane and elsewhere in the eastern Granite-Rhyolite province.