REE Enrichment Research Articles

Rare earth element enrichment in the ion-adsorption deposits associated granites at Mesozoic extensional tectonic setting in South China

Ion-adsorption type rare earth element mineralization occurs in weathered profile of three Mesozoic granites in the Bachi area, Guangdong Province, South China. In this study, we investigate the REE enrichment processes in these granites using geochemical and geochronological data. The three granites yield zircon U-Pb ages of ca. 189 Ma, 153 Ma and 94 Ma, respectively, and could be divided into Early Jurassic, Late Jurassic and Cretaceous granites. The Early Jurassic and Cretaceous granites show similar geochemical characteristics, i.e. high K2O and low CaO, Fe2O3, and MgO contents, low K2O/Na2O ratios, and depletion of Eu, Sr, Ba, Ti, and P. They are characterized by high HFSE (high field strength elements) contents and enriched isotopic compositions with εNd(t) values ranging from −5.4 to-2.7, and εHf(t) values ranging from −4.5 to 1.7. The Late Jurassic granite also shows Eu, Sr, Ba, Ti, and P depletion and high HFSE contents. It has, however, higher CaO, Fe2O3, MgO and lower K2O contents than the Early Jurassic and Cretaceous granites. The Late Jurassic granite also shows more enriched isotopic compositions with εNd(t) values ranging from −9.0 to −8.9, and εHf(t) values ranging from −11.1 to −6.1. In addition, the Late Jurassic granite comprises, in contrast to the Early Jurassic and Cretaceous granites, abundant inherited/captured zircons cores and accessory REE-rich minerals of allanite, titanite, apatite and fluorite. These Mesozoic granites have the geochemical affinity of A-type granite. The Zr saturation temperature is >820 °C for the Early Jurassic and Cretaceous granites, and ~900 °C for the Late Jurassic granite. The Early Jurassic and Cretaceous granites were derived from partial melting of felsic igneous rocks under high-temperature, low-pressure, and anhydrous conditions. The felsic igneous source could provide relatively high REE contents for the melts, and the high-temperature, low-pressure, and anhydrous conditions resulted in residual plagioclase and melting of REE-rich minerals in the source area, which favour to REE enrichment in the melt. The Late Jurassic granite, on the other hand, was derived from a more mafic source and associate with fast crustal magma generation as indicated by its abundant presence of inherited/captured zircon cores in the high-temperature condition. The fast magma generation of the Late Jurassic granite prevented the separation of REE-rich minerals from the melt resulting in REE enrichment. Further, the high-temperature, low-pressure, and anhydrous physicochemical conditions and fast magma generation, which favour to REE enrichment in the granites, are tightly associated with large-scale Mesozoic extension in South China.

Element mobility as a proxy for delineating shear zone boundaries in orogenic belts: A case study of Iwaraja shear zone, southwestern Nigeria

Shear zones are localised regions of intense deformation and are important archives of element mobility history during crustal deformation. The boundary is determined by examining shear deformation fabrics in affected rocks. However, the "mixture" of relicts of undeformed rocks coupled with newly formed shear fabrics affect the determination of the internal boundaries of the shear zone and its boundary with the unsheared protolith, which in turn limits the evaluation of the shear zone's width. A new approach involving element mobility across the shear zone is proposed as a proxy to address this problem and corroborate existing methods. The petrographic and geochemical analyses of granitic gneiss across the Iwaraja shear zone and its adjacent rock were carried out. The element mobility patterns and volume of chemical changes were evaluated using isocon and Cartesian element-distance plots to interpret the chemical changes in the granitic gneiss across the shear zone. The result indicates a very high enrichment of TiO2, Fe2O3, MnO, MgO, P2O5, and REEs with relatively similar mobility patterns and a high depletion of Na2O, Rb, and Th in the shear zone. Al2O3 is not significantly mobilized but the mobility pattern of K2O is erratic. Based on the chemical changes in the granitic gneiss, the ductile shear zone is a CaO–MgO–Fe2O3-REE-enriched and Na2O–Rb–Th-depleted system. Under greenschist facies condition, the chemical mobility of TiO2, MnO, P2O5, Th, Rb, Zn, Ba, Zr, Th, Hf, and REEs is sensitive to the changes in the deformation intensity between the strongly- and weakly-deformed zones whereas REE [Lu, Yb, Gd, Eu, Ho, Tb] is useful to determine the boundary between the weakly-deformed zone and the unsheared rock. These index minerals are useful in other similar ductile shear zones in any orogenic belt. Based on the element mobility patterns, the width of the affected granite gneiss in the shear zone is ~680 m.

Bulk composition of a zoned rare-earth minerals-bearing pegmatite in the Pikes Peak granite batholith near Wellington Lake, central Colorado, U.S.A.

ABSTRACT A previously undescribed small lenticular (~5 × 5 × 5 m) pegmatite, located near Wellington Lake in the NW part of the 1.08 Ga ‘A-type’ (anorogenic) ferroan Pikes Peak granite batholith, ~15 km SW of the South Platte pegmatite district in central Colorado, is concentrically zoned around a mostly monomineralic quartz core with interconnected miarolitic cavities. Major constituents of the Wellington Lake pegmatite are quartz, perthitic microcline, albite (variety cleavelandite), hematite, and biotite. Accessory minerals include fluocerite, bastnäsite, columbite, zircon (var. ‘cyrtolite’), thorite, and secondary U phases. Fluorite is conspicuously absent, although it is a common phase in the South Platte district NYF-type pegmatites, which are rich in niobium (Nb), yttrium (Y), fluorine (F), and heavy rare-earth elements (HREE). Notable for the Wellington Lake pegmatite are a small quantity of well-developed tabular crystals of fluocerite that reach up to 4 cm in diameter, with sub-mm epitaxial bastnäsite overgrowths, suggesting formation from F- and CO2-bearing solutions rich in light rare-earth elements (LREE), with decreasing a(F-)/a(CO32-) during the last crystallization phase. An Nd-isotope value of εNd1.08Ga = -1.6 for the fluocerite is within the range of εNd1.08Ga = -0.2 to -2.7 of the host coarse-grained, pink K-series Pikes Peak Granite (PPG), indicating that REE and other pegmatite constituents derived from the parental PPG magma. A calculation of total pegmatite composition based on whole-rock chemistry and volume estimates of the different pegmatite zones reveals an overall composition similar to the PPG with respect to Si, Al, Na, and K. Yet the pegmatite is depleted in Fe, Mg, Ca, Ti, Mn, and P, the high-field-strength elements (HFSE; Zr, Hf, Nb, Y, Th), and, most significantly, total REE compared to the PPG. Despite containing the LREE minerals fluocerite and bastnäsite, the lack of a net overall REE enrichment of the pegmatite compared to the PPG reflects the large amount of REE-poor silicate minerals forming the wall, intermediate, and core zones of the pegmatite. The calculated total pegmatite composition suggests that the pegmatite formed by the separation from the PPG magma of an F-poor H2O-saturated silicate melt depleted in REE and HFSE compared to the F-rich melts, which formed the NYF-type HREE-rich (LaN/YbN < 1) pegmatites in the South Platte district. Homogenization temperatures of < 500°C for possibly primary fluid inclusions in large quartz crystals from the core of the Wellington Lake pegmatite are consistent with recent models of pegmatite petrogenesis leading to nucleation controlled mega-crystal growth resulting from supercooling.

Geology, geochemistry, and petrogenesis of Jebal Ruro A-type granite, Blue Nile State, Sudan: implications for origin of HFSE mineralization

The Jebal Ruro granites occur west of the late Proterozoic Ingessana-Kurmuk suture of the Arabian-Nubian Shield located in the southeastern region of the Blue Nile State of the Sudan. A-type post-collisional granitoids from Jebal Ruro region were here studied through new field, petrographical and whole-rock data. These granitic intrusions comprise alkali granites consisting of aegirine granite, aegirine-arfvedsonite granite, and arfvedsonite granite. Geochemically, they are characterized by high Na2O, K2O, Fe/Mg, Ga/Al, Zr, Y, Nb, and REE (except Eu), and low abundances of CaO, MgO, TiO2, P2O5, and molar (Na2O+ K2O)/Al2O3 ratio ≥ 1 represent their peralkaline affinity. The Jebal Ruro granites show enrichment in Zr, Hf, Y, Nb, Ta, U, Th, Zn, and REE particularly in the aegirine granite and aegirine-arfvedsonite granite. Ruro granites show relatively high ratios of Nb/Ta and Zr/Hf close to that estimated for mantle magma source as well as Th/Ta, Th/U, Zr/Nb, La/Nb, and Rb/Nb. Furthermore, Ta+Yb vs. Gd/Yb, Sm/Yb, La/Yb, and Ta/Yb plots show that Ruro granites have distinguishable characteristics among arc and slab wedge-derived A-type magmas. In this study, we concluded that Jebal Ruro granites are A-type peralkaline to transitional metaluminous granites generated mainly by fractional crystallization from a mantle-derived OIB-like parent magma, emplaced in within-plate post-collisional tectonic environment.

Genetic model of carbonatite hosted rare earth elements mineralization from Ambadongar Carbonatite Complex, Deccan Volcanic Province, India

Carbonatites and associated alkaline rocks are the primary sources for REE mineralization. The Ambadongar Carbonatite Complex (ADCC) from NW Deccan Volcanic Province (DVP) constitutes the largest Carbonatite Associated REE Deposits (CARD) in India. ADCC belongs to the final stages of the Late Cretaceous alkaline-carbonatite magmatism associated with main Deccan basalt volcanic activity. The ADCC is an envisioned diatreme structure in which four carbonatitic phases are recognized, mainly calcio-carbonatites and ferro-carbonatites. Each successive carbonatite phase shows higher REE enrichment. The primary REE mineralization with bastnäsite as the dominant REE phase is hosted by pervasive hydrothermally altered ferro-carbonatite plugs. The secondary mineralogy formed with barites in the main orebody during late- to post-magmatic hydrothermal fluid alteration is fluorite, quartz, ankerite, and other REE-bearing minerals like bastnäsite, parisite, synchysite, strontianite, florencite, monazite and columbite.Carbonatite samples contain 18.61% to 52.42% of CaO, and the LOI varies from 5.28% to 38.79%. Most can be classified as calcio-carbonatites. Since all the samples also contain an appreciable amount of Fe2O3 (4.13% to 20.20%) and MnO (0.07% to 5.46%), some may be classified as ferro-carbonatites. Total REE content varies from 0.6 to 4%, with a high Ce concentration and LREE/HREE ratio. The highest values for La, Ce, Pr, and Nd are 1.95%, 1.56%, 0.16%, and 0.45%, respectively.Metasomatism of SCLM from asthenospheric melts followed by the low degree partial melting of the SCLM region is responsible for fertile carbonatite generation in ADCC. The multiphase liquid immiscibility of carbonatite melts from carbonate-silicate magma followed by immiscibility of REE rich carbonatite melt and REE deficient fluoride-rich aqueous fluids explain the higher level of REE enrichment in each successive phases of carbonatites in ADCC.The mineralizing fluids were probably the result of residual magmatic volatiles that brought mainly REE and later SiO2 into the overprinted rocks. Ambadongar carbonatites' stable isotopic compositions agree with a magmatic origin (δ13C = −4.1 ± 1.9‰ [PDB] and δl8O = 10.3 ± 1.7‰ [SMOW]). The C–O stable isotopic modeling indicates re-equilibration under hydrothermal conditions between 180 °C and 70 °C. Significant amounts of REE fluorocarbonate minerals, relatively Sr- and Th-rich, were deposited during re-equilibration. The REE fluorocarbonate bastnäsite-(Ce) occurs as late individual crystals, overgrown on the synchysite and parisite polycrystals. Textural and chemical reactions between the REE fluorocarbonates provide insights into rare-earth elements' mobility during fluid-rock interaction. Early crystallization of synchysite/parisite indicates the high activity of Ca2+, OH−, (SO4)2−, Al and Si in the fluid. Later, the fluid was characterized by increased activity of F−, (SO4)2−, REE and Si, and decreased activity of Ca2+ as reflected in the association of barite, fluorite, quartz, and bastnäsite typical of strongly overprinted ferro-carbonatites.Re-equilibration and recrystallization of the primary minerals in the presence of OH−, (SO4)2−, F−, REE, Al, and Si carried in solution by the hydrothermal fluid is the leading cause behind the refixing of REE in the form of REE fluorocarbonate in REE rich ferro-carbonatites.

Signatures of Heavy Metal concentration over Laterites identified through Regional Stream Sediment Geochemical Survey in parts of Katni, Panna and Satna Districts of Madhya Pradesh, India

Abstract As a part of National Geochemical Mapping (NGCM) programme of GSI, regional geochemical mapping was carried out for an area of 705 sq km in parts of Katni, Panna and Satna districts of Madhya Pradesh covering toposheet number 64A/05 bounded by latitudes N23°45′00″ to N24°00′00″ and longitudes E80°15′00″ to E80°30′00″. Chemical analyses of 182 composite stream/slope sediments samples were carried out and baseline geochemical maps of oxides and elements were prepared. Data analyses has led to significant conclusions on environment hazard due to metal contamination and possible mineral potential due to REE enrichment. Study of the spatial distribution of individual variables showed Cr and Co have values higher than the permissible level in dry soils in some parts of the area which requires investigation by concerned agencies. Some samples over the laterites show higher than threshold values of REEs which are much higher than their crustal abundance. Factor analyses on the geochemical data with 46 oxides and elements has led to the identification of five factors accounting for 84% of the total variance of the data set and describing five physico-chemical processes controlling the spatial geochemical variation. The analyses points to the enrichment of the REEs in the lateritisation process. The laterites covering 171 sq km of the area need to be considered for investigation of REE potential.

Geochronology and geochemistry of an early Silurian plagiogranite from Xiarihamu area, East Kunlun, northwest China: implications for Proto-Tethyan evolution

Oceanic plagiogranites are directly or indirectly related to ophiolites. A small plagiogranite vein has been discovered in the No. III mafic–ultramafic complex of the Xiarihamu ophiolite from the eastern margin of Qiman Tagh, East Kunlun Orogen, northwest China. This plagiogranite yielded a weighted mean zircon U–Pb age of 435.7 ± 4.3 Ma (MSWD = 0.073) and is characterized by high SiO2 (73.08–76.24 wt.%) and Na2O (6.42–8.34 wt.%) contents and high Na2O/K2O ratios (3.06–12.36) with metaluminous and calc-alkaline affinity. It is enriched in Th and U, depleted in Nb, Sr, Ti, and Ba, and has low ΣREE contents (7.64–11.35 ppm), negative Eu anomalies (δEu = 0.31–0.71), and light REE enrichment. These features are typical of oceanic plagiogranites. Whole-rock Sr–Nd isotopic data for the plagiogranite show obvious EMII mantle characteristics. The petrographic, geochemical, and Sr–Nd isotopic characteristics suggest the plagiogranite is part of an ophiolite formed by partial melting of oceanic crust in the presence of hydrothermal fluids. Given this, other parts of the Xiarihamu ophiolite and other ophiolites in Qiman Tagh area, we propose the plagiogranite formed in a back-arc basin setting in the early Silurian, probably the youngest known intrusion related to the Proto-Tethys Ocean.

The trace and rare earth element contributions to the understanding of Chouichia iron-copper deposits in Northern Tunisia: metal sources interrelated with magmatism and metamorphism

The Chouichia ore deposits are one of the most important polymetallic Cu-Fe-rich mineralizations located in the front of the thrust zone of the Tunisian Tellian belt. This mineral occurrence is carried by veins and breccias hosted by the Upper Cretaceous to Oligocene-Lower Miocene sediments in the Fej Et Tamer tectonic corridor. Geochemical work carried out on host rock and mineral samples shows an enrichment and fractionation of REEs with Eu negative anomalies due to interactions of hydrothermal fluids with Eu-depleted magmatic rocks presumably lying in the subsurface. Co/Ni ratios help distinguish three pyrite generations. Pyrite I disseminated in the host rock is sedimentary-syngenetic, formed at low temperature (Co/Ni 1 and confirmed by varying Y/Ho ratios (range 19.625 to 36.5). Trace element amounts normalized to the average contents of trace metals in primitive mantle testify to a metal source from the upper crust and upper mantle with high enrichments of ore bodies in As, Sb, Cu, Pb, Ag, Au, W, and Mo. Marked depletions in Hf, Zr, and Sc indicate their high fractionation degree in both mineralizing and magma fluids and/or igneous rock/fluid interaction effects in the subsurface. Positive anomalies of magmaphile trace element markers such as Mo and W support high-temperature mineralizing fluids driven by magmatism. These geochemical characters easily allow the classification of Chouichia veins as a hydrothermal affiliation deposit type where fluids are imprinted by magmatism and metamorphism. Deep-rooted faults in the Fej Et Tamer tectonic corridor have had an essential role in metal transfer, fluid conveying to the surface, and mineral formation.

Combined In Situ Chemical and Sr Isotopic Compositions and U–Pb Ages of the Mushgai Khudag Alkaline Complex: Implications of Immiscibility, Fractionation, and Alteration

The Mushgai Khudag complex consists of numerous silicate volcanic-plutonic rocks including melanephelinites, theralites, trachytes, shonkinites, and syenites and also hosts numerous dykes and stocks of magnetite-apatite-enriched rocks and carbonatites. It hosts the second largest REE–Fe–P–F–Sr–Ba deposit in Mongolia, with REE mineralization associated with magnetite-apatite-enriched rocks and carbonatites. The bulk rock REE content of these two rock types varies from 21,929 to 70,852 ppm, which is much higher than that of syenites (716 ± 241 ppm). Among these, the altered magnetite-apatite-enriched rocks are characterized by the greatest level of REE enrichment (58,036 ± 13,313 ppm). Magmatic apatite from magnetite-apatite-enriched rocks is commonly euhedral with purple luminescence, and altered apatite displays variable purple to blue luminescence and shows fissures and hollows with deposition of fine-grained monazite aggregates. Most magmatic apatite within syenite is prismatic and displays oscillatory zoning with variable purple to yellow luminescence. Both magmatic and altered apatite from magnetite-apatite-enriched rocks were dated using in situ U–Pb dating and found to have ages of 139.7 ± 2.6 and 138.0 ± 1.3 Ma, respectively, which supports the presence of late Mesozoic alkaline magmatism. In situ 87Sr/86Sr ratios obtained for all types of apatite and calcite within carbonatite show limited variation (0.70572–0.70648), which indicates derivation from a common mantle source. All apatite displays steeply fractionated chondrite-normalized REE trends with significant LREE enrichment (46,066 ± 71,391 ppm) and high (La/Yb)N ratios ranging from 72.7 to 256. REE contents and (La/Yb)N values are highly variable among different apatite groups, even within the same apatite grains. The variable REE contents and patterns recorded by magmatic apatite from the core to the rim can be explained by the occurrence of melt differentiation and accompanying fractional crystallization. The Y/Ho ratios of altered apatite deviate from the chondritic values, which reflects alteration by hydrothermal fluids. Altered apatite contains a high level of REE (63,912 ± 31,785 ppm), which are coupled with increased sulfur and/or silica contents, suggesting that sulfate contributes to the mobility and incorporation of REEs into apatite during alteration. Moreover, altered apatite is characterized by higher Zr/Hf, Nb/Ta, and (La/Yb)N ratios (179 ± 48, 19.4 ± 10.3, 241 ± 40, respectively) and a lack of negative Eu anomalies compared with magmatic apatite. The distinct chemical features combined with consistent Sr isotopes and ages for magmatic and altered apatite suggest that pervasive hydrothermal alterations at Mushgai Khudag are most probably being induced by carbonatite-evolved fluids almost simultaneously after the alkaline magmatism.

Magmatic-hydrothermal evolution of an unusual Mo-rich carbonatite: a case study using LA-ICP-MS fluid inclusion microanalysis and He–Ar isotopes from the Huangshui’an deposit, Qinling, China

The Huangshui’an deposit located in East Qinling (China) is an unusual case of a Si-rich carbonatite hosting economic Mo and minor Pb and REE mineralization. The role of mantle-sourced carbonatite melts and fluids in the formation of the Mo mineralization remains poorly understood. Our integrated study based on field geology, petrography, microthermometry, and LA-ICP-MS analysis of single fluid inclusions, and noble gas isotopes of pyrite permits to reconstruct the source characteristics, the magmatic-hydrothermal evolution of the carbonatitic fluids, and their controls on Mo mineralization. Fluid inclusions hosted in calcite in the carbonatite dikes have the highest concentrations of Mo (9.9–62 ppm), Ce (820–9700 ppm), Pb (1800–19500 ppm), and Zn (570–5800 ppm) and represent the least modified hydrothermal fluid derived from the carbonatite melt. Fluid inclusions hosted in calcite (Cal) and quartz (Qz2 and Qz3) of the stage I carbonatite dikes have different metal concentrations, suggesting that they formed from two distinct end member fluids. The FIA in calcite represent fluid A evolved from carbonatite melt with relatively high-ore metal concentrations, and those in quartz characterize fluid B having a crustal signature due to metasomatic reactions with the wall rocks. The FIA in quartz (Qz1) within the altered wall rock have overlapping elemental concentrations with those of massive quartz (Qz2) and vuggy quartz (Qz3) in carbonatite. This suggests that the volumetrically significant quartz in the Huangshui’an carbonatite has been formed by the introduction of Si by fluid B. The positive correlations between Rb, B, Al, Cl, and Sr in stage II fluid inclusions in late fluorite + quartz + calcite veins indicate that this late mineralization formed from the mixing of primary hydrothermal fluid B with meteoric water. The He–Ar isotope data, in combination with available C–O–Sr–Nd–Pb isotope data, constrain the carbonatite source as an enriched mantle source modified by contributions from crustal material which was probably the fertile lower crust in the region. This distinct source facilitated the enrichment in Mo, REE, and Pb in the primary carbonatite magma. The carbonatite magmatism and Mo mineralization at 209.5–207 Ma occurred in the regional-scale extensional setting at the postcollision stage of the Qinling Orogenic Belt.

Histology and Geochemistry of Allosaurus (Dinosauria: Theropoda) From the Cleveland-Lloyd Dinosaur Quarry (Late Jurassic, Utah): Paleobiological Implications

The Late JurassicAllosaurusis one of the better-studied dinosaurs. A histological and geochemical study of a tibia and a femur ofA. fragilisrecovered in the Upper Jurassic Cleveland-Lloyd Dinosaur Quarry, Utah, United States has been done in order to address growth characteristics of this species. The two bones, probably belonging to separate individuals, are among the largest known for this species, which make them suitable to address such issues. The inclusion of our data on femur growth markings in the previously published data reflects a range of growth variability rather than two distinct growth strategies. The tibia has a well-developed external fundamental system indicating somatic maturity achievement. Using a quantitative method of superimposition to retrocalculate missing lines of arrested growth, the tibia appears to correspond to an individual that reached its skeletal maturity at 22 years and died at approximately 26 years. In the tibia, the concentration of zinc, a potential biomarker associated with bone formation, displays a higher concentration in zones of rapid growth compared to annuli. There is no direct relationship between the values of δ18Opand the lines of arrested growth distribution. The absence of relations between the histological organization and an enrichment in REE of the bone, indicates that the variations of δ18Oplikely represent a diagenetic process rather than a primordial, biologic composition. However, the geochemical composition of the bones is not homogeneous along the sections, indicating that the signal variations have not been completely erased by diagenesis.

Geochemistry and origin of high-Sr carbonatite from the Nuba Mountains, Arabian-Nubian Shield, Sudan

Abstract Carbonatite from the Arabian-Nubian Shield of Sudan occurs as dykes in the Nuba Mountains. It is composed of calcite with some feldspars, quartz and fluorite. CaO is the major constituent in this carbonatite and accordingly, it is classified as calico-carbonatite. The studied carbonatite shows exceptionally high concentrations of SrO (4.4 to 5.9 wt%). Ba, Pb and Y occur in relatively higher concentrations compared to other trace elements. Concentration of rare earth elements (ΣREEs) is relatively low (average 1550 ppm) compared to many primary igneous carbonatites. The chondrite-normalized REE patterns display higher light rare earth elements (LREEs) compared to heavy rare earth elements (HREEs) with slight negative Ce/Ce* and Eu/Eu* anomalies. The δ18OV-SMOW values range between 7.48 and 10.05‰, while δ13CV-PDB values vary from −6.24 to −7.38‰, which is close to the primary carbonatites values. Occurrence of carbonatite as dykes with cumulate and triple junction textures, plot of the carbonatite in the true carbonatite fields of the Ba-Sr and Ba + Sr-REE + Y diagrams, igneous-derived δ13CV-PDB and δ18OV-SMOW values and high (La/Yb)N ratios indicate its primary igneous origin. The strong positive correlation between REEs and Sr suggests the occurrence of these elements as secondary strontianite, which was confirmed by SEM and EDX analyses. This might indicate that the enrichment of REEs and Sr in the studied carbonatite is not from the primary magma and most probably took place during a sub-solidus metasomatic process after the carbonatite emplacement.

Subduction to post-collisional volcanism in the Northern Arabian-Nubian Shield: Genesis of Cryogenian/Ediacaran intermediate-felsic magmas and the lifespan of a Neoproterozoic mature island arc

The Neoproterozoic intermediate-felsic volcanic rocks in the Wadi Ambagy area, Eastern Desert of Egypt, comprise three units: Dokhan intermediate-felsic volcanics, post-Dokhan felsic volcanics and felsites. The Dokhan lavas are generally porphyritic and composed of andesite and dacite with subordinate basalt, whereas the post-Dokhan felsic volcanics are represented by phyric and aphyric rhyolites. The medium-K calc-alkaline nature, LILE enrichment, Ta-Nb anomalies and LREE-enriched patterns of the Dokhan volcanic rocks are consistent with development in a subduction setting. Arc crustal thickness estimation using average (La/Yb)n is ~ 32 km, indicating that these rocks were generated in a Neoproterozoic island arc with thick crust. The subduction magmas were generated by hydrous mantle melting and they evolved by fractional crystallization. In contrast, the post-Dokhan felsic volcanics and felsites have medium- to high-K and shoshonitic affinities, respectively. They are highly silicic with weak to absent Ta-Nb anomalies and significant Nb, Zr, Hf and REE enrichment, which increases from the post-Dokhan felsic volcanics to the felsites. These geochemical features suggest generation in a post-collisional to within-plate tectonic setting by partial melting of mafic or intermediate arc crustal sources. SHRIMP U-Pb zircon dating indicates a crystallization age of ~ 660 Ma for the subduction-related Dokhan andesite and dacite, reflecting a magmatic arc lifespan longer than previously considered. The change of setting from subduction to post-collisional was accompanied by an increased role of crustal sources in the generation of the felsic magmas, resulting in differentiation of the accreted arc crust.

Depositional and Environmental Constraints on the Late Neoarchean Dagushan Deposit (Anshan-Benxi Area, North China Craton): An Algoma-Type Banded Iron Formation

Abstract The late Neoarchean, ~2.53 to 2.51 Ga Dagushan banded iron formation (BIF), is a typical Algoma-type BIF located in the northeast part of the North China craton. Despite having undergone upper greenschist to lower amphibolite facies metamorphism, the Dagushan BIF retains evidence of varied depositional facies, making it an ideal archive to evaluate the paleomarine environment and the paragenesis of the ore minerals. A transition from oxide to silicate to carbonate facies BIF is evident in a northward direction. The mineralogical composition shifts from magnetite and quartz in the south through a magnetite-quartz-cummingtonite/stilpnomelane assemblage in the transition zone to magnetite-siderite in the north. Such a distinct distribution of mineralogical facies correlates well with the depositional environment of the BIF. The carbonate facies BIFs formed in a near-shore, proximal environment, whereas the oxide and silicate facies BIF assemblages formed in deeper waters, distal to the paleoshoreline. The BIF samples display characteristic seawater-like rare earth element + yttrium (REE + Y) profiles with positive La and Y anomalies and heavy REE enrichment relative to the light REEs when normalized to post-Archean Australian shale. Positive Eu anomalies suggest a high-temperature hydrothermal contribution to the BIF. The absence of a negative Ce anomaly in nearly all samples, coupled with positive δ56Fe in magnetite in all mineralogical facies, indicates a dominantly anoxic water column contemporaneous with deposition of the BIF. At ~2.53 Ga in the Anshan area, seawater was mostly anoxic and rich in ferrous iron. Dissolved ferrous iron in upwelling hydrothermal fluids was oxidized and precipitated as Fe(III) oxyhydroxides in the photic zone leading to BIF formation. Proximal to hydrothermal vents, magnetite formed via the reaction of Fe(III) oxyhydroxides and aqueous Fe(II) supplied from the hydrothermal fluids and microbial dissimilatory iron reduction (DIR) coupled to organic carbon oxidation. Proximal to a paleoshoreline, siderite formed through DIR, as evidenced by the depleted δ13C values and the presence of graphite. Silicates, such as stilpnomelane and cummingtonite, are considered to be the metamorphic products of early diagenetic silicates (e.g., nontronite) that formed in the water column from admixtures of Fe(III) oxyhydroxides and amorphous silica.

Episodic hydrofracturing and large-scale flushing along deep subduction interfaces: Implications for fluid transfer and carbon recycling (Zagros Orogen, southeastern Iran)

We investigate the late Cretaceous blueschist-facies (480 °C-1.8 GPa) segment of the Zagros suture zone, a well-preserved block-in-matrix paleo-subduction channel. We aim to determine the relative chronology, conditions of deformation, and potential fluid sources and processes associated with the widespread occurrence of lawsonite + clinopyroxene + glaucophane veins and aragonite-bearing hydraulic breccias. We use a multi-scale approach methodology to provide new insights into deep fluid flow mechanisms as well as to constrain possible sinks of CO2-bearing fluids in the subducting slab. Petrological analyses suggest that silicate-rich vein systems began precipitating during early burial and evolved with ongoing burial and shearing-related deformation in the blueschist-facies, while most carbonate-rich veins and hydrofractures formed at near-peak P-T conditions. In situ LA-ICP-MS trace element analyses reveal that: (i) individual silicate host-vein pairs have similar REE signatures, reflecting local-scale fluid-mediated element redistribution, (ii) carbonate-bearing veins and metasediments also have similar trace element signatures and (iii) lawsonite in blueschist-hosted veins exhibit REE enrichments along their rims, suggesting an increasing contribution of metasedimentary-derived fluids upon approaching peak P-T. Carbonate OC isotope compositions of the veins and metasedimentary rocks range from +13.6 to +17.9‰ (δ18OVSMOW) and − 1.0 to +3.1‰ (δ13CVPDB), demonstrating metasedimentary-derived fluid sources related to large-scale H2O homogenization with far-traveled mafic- ultramafic-derived fluids. SrNd isotopic ratios in carbonate veins and the adjacent host resemble their host composition indicating that host rock-buffered isotopic homogenization occurred between the infiltrating fluids and the rock matrix, possibly during episodic porous flow. Thermodynamic modeling predicts that decarbonation via fluid-assisted reactions is inefficient at blueschist-facies and that carbon release likely occurs deeper along the subduction interface (i.e., at eclogite-facies). We propose that deeply produced H2O-rich fluids interacted with the carbonate-bearing lithologies along the subduction interface facilitating fluid-mediated decarbonation and further fluid transport as hydraulic pulses (e.g., porosity waves) that traveled at the kilometer-scale parallel to the subduction interface, (i) contributing to the isotopic homogenization herein observed and (ii) triggering episodic hydrofracturing in the lawsonite-blueschist-facies (≈50-60 km depth). Veinsets in exhumed subducted rocks hence provide a unique opportunity to understand fluid-rock interaction processes in the region at which episodic tremor and slow slip events phenomena occur.

Neoarchean-early Paleoproterozoic granitoids, the geothermal gradient and geodynamic evolution in the Hengshan Terrane, North China Craton

The Hengshan Terrane (HST) is a typical Archean terrane in the central North China Craton (NCC) and features Neoarchean-early Paleoproterozoic granitoid rocks and associated supracrustal rocks. Five geological events are identified in the HST, including (1) ~2.71–2.67 Ga tonalites-trondhjemites-granodiorites (TTGs) and monzogranites, (2) ~2.54–2.48 Ga diorites, TTGs and volcanics, (3) ~2.44–2.43 Ga monzogranites, (4) ~2.14–2.04 Ga monzogranites and (5) ~1.92–1.84 Ga granulite- to high-grade amphibolite- facies metamorphism. The ~2.5 Ga diorites exhibit low SiO2 concentrations (55.67–62.61 wt%), high MgO content of 1.39–4.89 wt% with Mg# values (45–62) and positive ɛHf(t2) values of +3.2−+7.8, and the magma originated from a mantle source that had been metasomatized by dehydration fluids and melts from subduction-related sediments and slabs. The ~2.5 Ga TTGs are characterized by variable MgO contents (0.56–2.32 wt%) and Mg# values (44–67), (La/Yb)N values (13.76–98.19) and positive ɛHf(t2) values of +2.8 to +6.5, and derived from the partial melting of tholeiitic basalts with slightly enriched REE patterns, similar to those of back-arc basin basalts (BABBs). And these granitoid melts were contaminated by mantle materials. The ~2.4 Ga monzogranites show high SiO2 (69.09–73.46 wt%) and K2O (3.31–7.52 wt%) contents and low MgO (0.11–1.23 wt%) contents, and their parent magmas originated from the partial melting of metamorphic graywackes.The mantle material that generated the dioritic magmas was enriched by the important metasomatic agent of sediment-derived melts and fluids prior to magmatism. These observations, in conjunction with the occurrence of widespread coeval calc-alkaline volcanic rocks in the adjacent Wutai region, suggest that a slab subduction was the most likely geodynamic regime leading to the late Neoarchean magmatism in the HST. In view of the BABB sources of the TTGs, a Neoarchean back-arc basin slab subduction model may be appropriate, which is supported by several Phanerozoic analogs. The partial melting P-T conditions of the ~2.5 Ga slab-derived TTG magmas are estimated to have been 1.5 ± 0.2 GPa and 823 ± 32 °C on the basis of thermodynamic and trace element simulations, indicating a geothermal gradient of ~17 ± 2 °C/km and a possible Neoarchean hot subduction process in the HST.

Geochemistry of apatite in Late Permian coals, Bowen Basin, Australia

Apatite may be a common accessory mineral in coal seams, but interpretation of its origins can vary from syndepositional mineral detritus, early precipitation during diagenesis, and post coalification precipitation and replacement from hydrothermal or other fluids. It is hypothesised that paragenesis is reflected in the modes of occurrence in different stratigraphic units of the Late Permian in the Bowen Basin. From samples analysed in this study, apatite in the youngest unit, the Rangal Coal Measures, most commonly occurs as pore-apatites (i.e. apatites infilling preserved plant cell cavities of fusinite and semifusinite macerals and commonly associated with kaolinite) and less commonly as fracture-apatites (i.e. apatites infilling micro-fractures) and detrital-apatites intermixed with the organic coal layers (Detrital-B). In the tuffaceous Fort Cooper Coal Measures and equivalents, apatite occurs more commonly as detrital-apatites intermixed with the lithic layers (Detrital-A) (Detrital. Within the basal unit, the Moranbah Coal Measures and its equivalents, the mode is commonly encrusting-apatites (i.e. small apatite crystallites encrusting silicate minerals). Almost all apatites in the coal samples tested by electron microprobe microanalysis (EPMA), regardless of stratigraphic location and mode, are fluorapatite. Although there are some locality-driven enrichments, the minor and trace-element chemistry vary more so by mode of occurrence. For comparison, apatites from dykes intersecting the coal measures and from tuffs were also added to the study. These had lower fluorine (F) but measurable chlorine (Cl) contents and are enriched in light rare earth elements (LREE), similar to the Durango apatite that is of magmatic origins. Most of the detrital-A apatites are also enriched in LREE similar to the tuff-apatites, but with variable enrichment in the rare earth and yttrium elements. Detrital-B apatites have minimal Cl but also show the magmatic LREE trend. The pore- and fracture-apatites exhibited two trends. In the samples from tuffaceous coals (e.g., Fort Cooper Coal Measures), the pore- and fracture apatites tended to follow the magmatic LREE trend. In the Rangal Coal Measures, the pore- and fracture-apatites have elevated F contents, minimal Cl contents and depleted LREE contents (or Middle REE enrichment). Depletion of LREE could occur through leaching in an acidic environment, which is suggested by the abundance of kaolinite. Apatite precipitates under more neutral conditions although pore-apatites are commonly considered “early” and fracture-apatites “late” or post coalification. Their geochemical similarity suggests they have similar fluid origins and/or timing, with geothermal fluids moving through the porosity afforded by the structured inertinite group macerals and fractures. Verification of that geothermal source still requires further work and a technique that can analyse the isotopic composition—e.g., carbon (13C/12C), oxygen (18O/16O) and strontium (87Sr/86Sr)—of micron-sized crystals in situ.

Calcium transfer and mass balance associated with soil carbonate in a semi‐arid silicate watershed (North Cameroon): An overlooked geochemical cascade?

AbstractCalcium is a key element of the Earth system and closely coupled to the carbon cycle. Weathering of silicate releases Ca, which is exported and sequestered in oceans. However, pedogenic calcium carbonate constitutes a second Ca‐trapping pathway that has received less attention. Large accumulations of pedogenic calcium carbonate nodules, associated with palaeo‐Vertisols, are widespread in North Cameroon, despite a carbonate‐free watershed. A previous study suggested that a significant proportion of Ca released during weathering was trapped in palaeo‐Vertisols but the pathways involved in the transfer of Ca from sources (the granite and the Saharan dust) to a temporary sink (the carbonate nodules) remain unclear. This study aims to compare the distribution of elements in carbonate nodules and their associated past and present compartments for Ca in the landscape. These compartments are all characterised by a distinctive geochemical composition, resulting from specific processes. Three end members have been defined based on geochemical data: (a) the granite and its residual products, dominated by K2O and Na2O, Ti and Zr, HREE, and a positive Ce anomaly; (b) the soil parental material and the Saharan dust, dominated by Al2O3, Fe2O3 and MgO, V, HREE, and a positive Ce anomaly; and finally (c) the carbonate nodules, which are dominated by CaO, a depletion in V, Ti and Zr, and an enrichment in REE with a negative Ce anomaly. Mass balance calculations in soil profiles demonstrated that the accumulation of Ca in carbonate nodules exceeds the Ca released by chemical weathering of the parental material, because of a continuous accumulation and contribution from lateral transfers. Consequently, at the landscape scale, carbonate nodules associated with palaeo‐Vertisols constitute a temporary sink for Ca. Such a spatial relationship between sources and transient compartments opens an avenue to the new concept of ‘geochemical cascade’, similar in terms of geochemistry, to the concept of ‘sediment cascade’ developed by continental sedimentologists.

Provenance identification of soil at the confluence of Thar desert, Aravalli hills, and alluvial plain based on trace and rare earth elements geochemistry

Khetri Copper Belt (KBC), India, lies in the semi-arid region at the confluence of Aravalli ranges and alluvial soil. A study is designed to understand the influence of aeolian processes on soil geochemistry (major oxides, trace elements and REE) at the edge of desert. Mines of copper, quartzite and granite are active in the region and for the study the region is divided into three zones such as copper mine zone (CMZ), quartzite mine zone (QMZ) and granite mine zone (GMZ). CMZ is located in the windward of Aravalli hills (considering Thar desert as a reference point) and significantly influenced by strong winds from Thar desert. Low values of CIA (60) at CMZ compared to PAAS (CIA = 70) are attributed to significant impact of aeolian processes. QMZ is quartzite enriched zone and low average value of CIA (60) is due to less weathering of parent quartzite rocks. Cu and Zn are major nutrient for plants and a positive correlation of Cu and Zn with N and C at QMZ indicates high fertility of soil (alluvial soil) and negligible influence of aeolian processes is attributed to its leeward location from Aravalli hills, the physical barrier. Eu anomaly and variations in trace elements (La/Th, Th/Sc and La/Sc) confirm the felsic source of soil. The UCC normalised plot for ∑REE shows the enrichment of REE at QMZ and GMZ as compared to CMZ is due to presence of heavy minerals. Thus, less impact of aeolian processes on QMZ lies in the leeward of Aravalli ranges indicating that the Aravalli ranges shield the transportation of aeolian sand.

Geochemical characteristics of magnesiocarbonatites from Muostalaah apatite occurrence and Birikeen apatite deposit (Aldan-Stanovoy shield, South Yakutia, Russia)

Apatite-bearing ore occurrence Muostalaah and phosphate deposit Birikeen are magnesiocarbonatite complexes of the Seligdar type, located within the Nimnyrskaya submeridional metallogenic domain in the Aldan-Stanovoy shield in South Yakutia, Russia. This study presents geochemical characteristics of carbonatites and adjacent apatite-bearing silicate rocks of the Birikeen apatite deposit and Muostalaah apatite occurrence. Comparison of these complexes with the Seligdar deposit and other magnesiocarbonatite complexes of the world was made. Based on the chemical analysis of the petrogenic components of the Muostalaah and Biriken carbonatites, it was concluded that these rocks belong to magnesiocarbonatites, similar to the magnesiocarbonatites of the Seligdar deposit. The chondrite-normalized REE spectra of magnesiocarbonatites and apatite rocks are characterized by LREE enrichment relative to HREE with average La/Yb(cn) ratios of 45 and 100 for the Muostalaah and Birikeen rocks, respectively. Most of the REE patterns are within the field of the world magnesiocarbonatites. The trace-elements abundances of carbonatites and apatite rocks record negative Nb, Pb, Sr, Zr, Hf and Ti anomalies and Th, U and REE enrichment. Сoncentrations of Th, U, Zr, and Hf are characterized by a wide spread of values. The trace-element patterns of the Birikeen and Muostalaah carbonatites are generally similar to the patterns of the world magnesiocarbonatites, including Seligdar rocks. Based on the results of isotopic studies of the Sm/Nd and Rb/Sr systems, it was concluded that the primary melts for the Muostalah and Birikeen carbonatites were formed from a long-lived enriched mantle source that separated from the depleted mantle in the Late Archean.