Pronounced Positive Eu Anomalies Research Articles

Fluctuations of trace elements and Sr isotopes of the multi-stage scheelite in the reduced polymetallic Yupo tungsten skarn deposit, South China: Implications for ore-forming process

The Yupo scheelite (Sch)-bearing deposit is hosted in the external contact zones between the Yupo granodiorite and Cambrian Huangdongkou Formation of South China. The stratiform ore bodies contain hedenbergitic pyroxene (Hd23–69) and andradite-poor garnet (Ad0–22) assemblages, indicating that Yupo is a typical reduced W skarn deposit. The garnet compositions are zoned in a deposit scale inwardly from the periphery of the grossular-rich skarn reaction front to more subcalcic compositions. Scheelite occurs mainly as disseminated grains in the skarn coeval with retrograde alteration (Sch I) and in quartz veins crosscutting skarns (Sch II-1, -2, and -3). Sch II-1 occurs as cores rimmed by Sch II-2, and Sch II-3 as veinlet cutting both of them. The Sch I and Sch II-3 show weak Eu anomalies different from the pronounced positive Eu anomalies of Sch II-1 and -2. All the scheelite contains low Mo contents (<0.1 wt%), together with the subordinate precipitation of molybdenite, indicating a reduced ore-forming fluid system. The Mo content and Eu anomaly jointly reveal that the reducing condition enhanced from Sch I to Sch II-1 and alleviated from Sch II-1 to Sch II-3. The four scheelite generations show gradually increased initial 87Sr/86Sr ratios from 0.7144 to 0.7242, which are higher than that of the magmatic apatite (0.7082–0.7127) and the carbonate rocks (0.7104–0.7109). The Cambrian carbonaceous shale is a high (87Sr/86Sr)i endmember that interacted with the initial ore-forming fluid; continuing interaction was deemed to enhance the reduction of the ore-forming fluid from Sch I to Sch II-1. Weakening of the intensity of fluid-rock interaction with carbonate rocks during vein-type mineralization caused the (87Sr/86Sr)i values of the scheelite to increase gradually. The input of meteoric water decreased the fluid reduction capability during the formation of Sch II-1 to Sch II-3. Fluid-rock interaction dominated the scheelite mineralization of the reduced W skarn deposit.

Paleoproterozoic positive δ13C excursion recorded in the carbonatite of southeast Tarim Craton, northwest China

The Paleoproterozoic Aketashitage carbonatite of northwest China is one of few high-δ13C carbonatites worldwide. Its absence of associated alkaline silicate rocks positions it as an intriguing case for probing the complex formation process of carbonatites. To provide further constraints on the genesis of these carbonatites, we have executed an exhaustive study encompassing petrography, geochemistry, geochronology, and radiogenic-stable isotope analysis. The Aketashitage carbonatites exhibit complex intrusive relationships with surrounding wall-rocks and house a diverse array of xenoliths, unmistakably signifying their igneous origin. Their rare earth elements (REE) diagrams are characterized by a nearly flat pattern and pronounced positive Eu anomalies, indicating a significant impact from hydrothermal fluids. These carbonatites are notably deficient in REEs, Ba, Sr, U, Th, Nb, and Ta. Their 143Nd/144Nd ratios and εNd (t) values range from 0.5109 to 0.5111 and −8.13 to −6.11, respectively. Remarkably, their δ13CV-PDB values (V-PDB, Vienna Pee Dee Belemnite) are exceptionally elevated, ranging from +13.31 to +15.48 ‰, thereby setting them apart from typical mantle-derived carbonatites, which exhibit δ13C values below 0 ‰. These comprehensive geochemical traits diverge significantly from those of mantle-derived carbonatites but share striking resemblances with crustal-origin carbonate rocks. Moreover, petrographic and mineralogical analyses reveal indicators of regional metamorphism within these carbonatites, proposing their possible for formation through the anatexis of high-grade marbles during the peak metamorphic event of the Paleoproterozoic era. Coupled with determined formation ages, these results imply a potential connection of Aketashitage carbonatites formation to the collision and amalgamation of the supercontinent Columbia. Moreover, the notable δ13C enrichment observed in these carbonatites presents substantial evidence for the impactful reach of the global Lomagundi-Jatuli event on the Tarim Craton.

Trace and Rare Earth Element Partitioning in Organic Fractions of Mudstones during the Oil Formation

In order to investigate trace and rare earth element partitioning in organic fractions of mudstones, this study isolated organic fractions of mudstones, including insoluble organic fraction (kerogen), soluble organic fraction (extract), or expelled hydrocarbon (reservoir solid bitumen), and the isolated organic fractions and their corresponding whole rocks for trace and rare earth element compositions were measured. Analysis of trace and rare earth element compositions in organic fractions of lacustrine and marine mudstones revealed that mudstone kerogens were more enriched in rare earth elements (REE) and redox-sensitive trace elements (e.g., U, Mo, and Ni) relative to corresponding whole rocks. During the oil generation, middle rare earth elements (Sm-Ho), especially Eu, migrated from kerogen to extract more easily than the rest REE. The Eu was easily transferred to soluble hydrocarbon under the acidic and reducing environments formed by oil generation, resulting in the higher concentrations of Eu relative to its neighboring REE (Sm and Gd) and the pronounced positive Eu anomalies. Transition metal elements (e.g., Mo, V, Cr, Co, Ni, Cu, and Zn) also more easily released from kerogen than the rest elements, especially V and Ni. The enrichment and mobilization of trace elements in organic fractions of mudstones, such as Mo, U, V, Ni, and Ba, are closely related with their geochemical behaviors during the depositional and early digenetic processes, providing the potential information for predicting the distribution characteristics of trace elements in the expelled hydrocarbons of mudstones (e.g., crude oil and solid bitumen) and fingerprinting of oil to source.

Petrogenesis of Garnet Clinopyroxenite and Associated Dunite in Hujialin, Sulu Orogenic Belt, Eastern China

The origin of ultramafic rocks, especially those in suture zones, has been a focus because they are not only important mantle sources of magma, but also provide substantial information on metamorphism and melt/fluid–peridotite interaction. Ultramafic rocks in Hujialin, in the central part of the Sulu orogen, include peridotite and pyroxenite. Although many papers on their origin and tectonic evolution have been published in the past few decades, these questions are still highly debated. Here, we present mineralogy, mineral composition, and bulk-rocks of these ultramafic rocks to evaluate their origin and tectonic evolution. The garnet clinopyroxenite is low in heavy rare-earth elements (HREE, 5.97–10.6 ppm) and has convex spoon-shaped chondrite-normalized REE patterns, suggesting the garnet formed later, and its precursor is clinopyroxenite. It is high in incompatible elements (i.e., Cs, Rb, Ba) and shows negative to positive U, Nb, and Ta anomalies, without pronounced positive Sr or Eu anomalies. Clinopyroxene in garnet clinopyroxenite contains high MgO (Mg# 0.90–0.97). The mineral chemistry and bulk-rock compositions are similar to those of reactive clinopyroxenite, suggesting that it originally formed via peridotite–melt interaction, and that such silicic and calcic melt might derive from the subducted Yangtze continent (YZC). Dunite contains olivine with high Fo (93.0–94.1), low NiO (0.11–0.29 wt.%) and MnO (≤0.1 wt.%), chromite with high Cr# (0.75–0.96), TiO2 (up to 0.88 wt.%), and Na2O (0.01–0.10 wt.%). It has negatively sloped chondrite-normalized REE patterns. Mineral chemistry and bulk rocks suggest dunite likely represent residual ancient lithosperic mantle peridotite beneath the North China Craton (NCC) that was overprinted by aqueous fluids. The lack of prograde and retrograde metamorphic minerals in dunite and irregular shaped mineral inclusions in chromite suggest dunite did not subduct to deep levels. Dunite mingled with garnet clinopyroxenite during exhumation of the latter at shallow depths. These ultramafic rocks, especially hydrated peridotite, may be important sources of Au for the Jiaodong gold province in the NCC.

Magmatic-hydrothermal processes of the Laojunshan metamorphic massif in Southeastern Asia: Evidence from chemical and B-isotopic variations of deformed tourmalines

Multi-stage fluid influx has led to the volumetrically important occurrence of granite-related tourmalines at the Laojunshan metamorphic massif in Southeast Asia, which exhibits variable deformed structures and decomposition textures. These include disseminated tourmaline porphyroclasts (type-I), crosscutting tourmaline-quartz veins (type-II) and tourmaline veinlets (type-III). The chemical variations of these three types of tourmalines reveal a source transition from early boron-rich melt to late hydrothermal fluid, accompanying with variable fluid-rock interaction. Hydrothermal tourmalines are characterized by higher Mg/(Mg + Fe) ratios, lower Na/(Na + Ca) ratios, more pronounced positive Eu anomalies, higher Sr and heavy rare earth element (HREE) concentrations, and lower Li, Nb, Zr, Hf, and light rare earth element (LREE) concentrations compared to magmatic tourmalines. The tourmalines exhibit δ11B values ranging from −13 to −7.9‰ for type-I, from −15.5 to −7.5‰ for type-II, and from −18.6 to −11.6‰ for type-III. This variation is mainly due to multiple fluid exsolution, Rayleigh fractionation and the mixing of two isotopically distinct sources. UPb dating results of magmatic zircons from two tourmaline-bearing gneisses reveal the ages of magma emplacement and crystallization from 445 to 420 Ma. Magmatic-hydrothermal fluids released from Silurian peraluminous granites are responsible for the formation of the disseminated type-I tourmalines. Further hydraulic fracturing during exhumation led to the formation of the tourmaline-quartz veins (type-II) and the tourmaline veinlets (type-III), possibly from Cretaceous granitic magma-derived fluids that interacted with metamorphic rocks. The decreasing grain size resulted from decomposition and cataclasis coupled with the overpressured fluids weakening the host rocks along fractures and eventually promoting the deformation during exhumation of the metamorphic massif. The chemical and isotopic compositions of tourmaline suggest that the oxidized conditions possibly through fluid boiling and fluid-rock interaction may promote the regional Sn-mineralization.

Chemostratigraphy of the Carajás banded iron formation, Brazil: A record of Neoarchean Ocean chemistry

One of the most important occurrences of banded iron formation (BIF) worldwide is situated in the Carajás Mineral Province, southeastern Amazonian Craton. The BIFs are jaspilites and are hosted in the Neoarchean (∼2.74 Ga) volcano-sedimentary sequence of the Grão-Pará Group. They are mostly composed of cm-thick intercalations of hematite, jasper, and chert. Their primary textures and structures are still preserved, which make them an ideal archive to evaluate the paleomarine environment. Low abundance of Al2O3 (<1.0 wt%) and HFSE (<1 ppm) for most BIF samples indicate an essentially detritus-free depositional environment. Overall, the rare earth elements and yttrium (REY) patterns show a weak positive lanthanum (La) anomaly, and a pronounced positive europium (Eu) anomaly (Eu/EuPAAS = 1.86 – 5.05), although the presence of true cerium (Ce) anomaly is not evident. Stratigraphic variations in iron isotope compositions, up to 0.80‰ (δ56Fe = +1.10 to +1.90‰) over tens to hundreds of meters of stratigraphic section, point to relative changes in the iron isotope composition of Carajás seawater over periods of a few million years. The jaspilites show heterogeneous distribution of Nd isotopic signature throughout the BIF sequence, and rocks from near the basaltic flows/jaspilite contact (type-II) have negative εNd (t) values (–4.97 to –0.90). In contrast, predominantly positive εNd(t) values (–0.84 to +5.40) are common in the remaining samples (type-I). The strongly positive δ56Fe values indicate a low degree of partial oxidation of Fe(II), which, combined with the pronounced positive Eu anomalies and the absence of Ce anomalies, hint towards that the deposition occurred mainly on a deep-sea environment with intense hydrothermal activity under anoxic and suboxic conditions, distal to continental landmasses. Locally, considerable oxygen was probably present in the ancient ocean's water masses, which led to the precipitation of BIFs.

The geochronology of the Haobugao skarn Zn-Pb deposit (NE China) using garnet LA-ICP-MS U-Pb dating

The Haobugao skarn Zn-Pb ore deposit (reserves of 0.29 Mt @ 4.24% Zn, 0.15 Mt @ 2.25% Pb) is located in the southern Great Xing’an Range (SXGR) Cu-Mo-Ag-Au-Pb-Zn-Fe metallogenic province in northeastern China. The ore bodies mainly occur near the contact zones between the early Cretaceous granite and the lower Permian carbonates and sometimes in distal locations. Two types of garnet are identified, i.e., green and fine-grained garnet (Grt A), and brown and coarse-grained garnet (Grt B). The Grt A contains a wider compositional range (Adr71.46-95.07Grs0.16-24.10), whereas the Grt B contains a relatively narrow compositional range (Adr87.65-97.52Grs0.07-10.11). The Grt A displays chondrite-normalized HREE-enriched and LREE-depleted patterns with negative Eu anomalies, suggesting that the Grt A was likely crystallized from a nearly neutral fluid. In contrast, the Grt B is LREE-enriched, HREE-depleted with pronounced positive Eu anomalies, indicating that the Grt B was probably formed from a mildly acidic fluid. The Grt A contains lower U contents than the Grt B, implying that the Grt A was likely formed under a relatively oxidized condition, while the Grt B crystallized from a more reduced condition.Skarn mineralization at Haobugao (139.10 ± 5.40 and 140.70 ± 1.89 Ma, garnet LA-ICP-MS U-Pb ages) and Zn-Pb mineralization (138.27 ± 0.14/0.69/0.81 and 138.82 ± 0.07/0.68/0.80 Ma, molybdenite ID-N-TIMS Re-Os ages) were associated with the emplacement of the granitoids dated from 143.49 ± 0.76 to 140.85 ± 0.75 Ma (zircon LA-ICP-MS U-Pb ages). The integrated geochronological data suggest that the ore-related granitoids, skarn, and Zn-Pb mineralization in the Haobugao deposit all formed in the early Cretaceous, which occurred in an extensional tectonic setting associated with regional Paleo-Pacific slab roll-back. This study highlights the reliability and viability of garnet U-Pb dating for constraining the timing of skarn formation, and utilizing its age with textural and trace element concentrations could well reconstruct the ore-forming process of skarn deposits.

Archean (about 2500 Ma) anatexis in eastern North China Block

Two Neoarchean alkaline feldspar-rich granites sourced from partially melted granulite-facies granodioritic orthogneiss have been here recognised in the eastern part of the North China Block (NCB). These poorly foliated granites have previously been assumed to be Mesozoic in age and never dated, and so their significance has not been recognised until now. The first granite (AG1) is a porphyritic syenogranite with megacrystic K-feldspar, and the second (AG2) is a quartz syenite with perthitic megacryst. Zircons from the granites yield LA-ICP-MS U-Pb ages of 2499 ± 10 Ma (AG1), and 2492 ± 28 Ma (AG2), which are slightly younger than the granodioritic orthogneiss that they intrude with a crystallisation U-Pb age of 2537 ± 34 Ma. The younger granites have higher assays for SiO2 (71.91% for AG1 and 73.22% for AG2) and K2O (7.52% for AG1 and 8.37% for AG2), and much lower assays for their other major element than the granodioritic orthogneiss. All of the granodioritic orthogneiss and granite samples have similar trace element patterns, with depletion in Th, U, Nb, and Ti and enrichment in Rb, Ba, K, La, Ce, and P. This indicates that the granites are derived from the orthogneiss as partial melts. Although they exhibit a similar REE pattern, the granites have much lower total REE contents (30.97×10−6 for AG1, and 25.93×10−6 for AG2), but pronounced positive Eu anomalies (Eu/Eu* = 8.57 for AG1 and 27.04 for AG2). The granodioritic orthogneiss has an initial 87Sr/86Sr ratio of 0.70144, εNd(t) value of 3.5, and εHf(t) values ranging from −3.2 to +2.9. The orthogneiss is a product of fractional crystallisation from a dioritic magma, which was derived from a mantle source contaminated by melts derived from a felsic slab. By contrast, the AG1 sample has an initial 87Sr/86Sr ratio of 0.6926 that is considered too low in value, εNd(t) value of 0.3, and εHf(t) values between +0.57 and +3.82; whereas the AG2 sample has an initial 87Sr/86Sr ratio of 0.70152, εNd(t) value of 1.3, and εHf(t) values between +0.5 and +14.08. These assays indicate that a Sr-Nd-Hf isotopic disequilibrium exists between the granite and granodioritic orthogneiss. The elevated εHf(t) values of the granites can be explained by the involvement of Hf-bearing minerals, such as orthopyroxene, amphibole, and biotite, in anatectic reactions in the granodioritic orthogneiss. Based on the transitional relationship between the granites and granodioritic orthogneiss and the geochemical characteristics mentioned above, it is concluded that the granites are the product of rapid partial-melting of the granodioritic orthogneiss after granulite-facies metamorphism, and their crystallisation age of about 2500 Ma provides the minimum age of the metamorphism. This about 2500 Ma tectonic-metamorphic event in NCB is similar to the other cratons in India, Antarctica, northern and southern Australia, indicating a possible connection between these cratons during the Neoarchean.

Rare earth elements and stable sulphur (δ34s) isotope of baryte mineralization in Liji Area, Northern Benue trough, northeastern Nigeria

The Liji area lithologically consists of inliers of granite and pegmatite members of the Pan-African granitoids surrounded by Cretaceous sedimentary deposits of Bima, Yolde, Pindiga and Gombe Formations. Epigenetic fracture-filling baryte mineralization hosted by granite, pegmatite and Bima Sandstone were delineated, sampled and analyzed for rare-earth elements (REEs) and stable sulphur isotope geochemistry. The REEs of the distal (unaltered) rocks indicated normal values (26.15-36.81 ppm) before mineralization was marked by enrichment of light rare-earth elements (LREEs) (27.94 ppm) relative to the heavy rare-earth elements (HREEs) (5.34 ppm) and negative Eu anomalies typical of calc-alkaline granites of Pan-African age. The baryte separates were marked by enriched LREEs and depleted HREEs with pronounced positive Eu anomalies indicating the invasion and consequent deposition of baryte-rich hydrothermal fluid under oxidizing conditions in the N-S and E-W striking fractures. Stable sulphur isotope of the baryte gives values that ranged from 18.3 - 19.8o/oo CDT indicating that the source of sulphur is from ocean water and not from magmatic, fresh water and connate water sources from the nearby granite, pegmatite and sandstone.&#x0D; Keywords: Baryte, Mineralization, Hydrothermal, Liji, REE, Sulphur-Isotope.

The Vesuvianite Gems of the Val d’Ala (Piedmont, Italy)

In Val d’Ala (Western Alps in Piedmont, Italy), the most interesting rocks for mineralogical research are represented by rodingite (rich in mineralized veins and fractures) associated with serpentinite in the eclogitized oceanic crust of Piedmont Zone, south of Gran Paradiso Massif. Among the vein-filling minerals, vesuvianite is well appreciated for its potential as gem-quality materials, even though it has never been characterized in detail. This study provides a gemological characterization of eleven vesuvianite crystals from different localities of the Val d’Ala. The refractive index (1.717–1.708) and density (1.705–1.709) values of our vesuvianite are in the range of those in the literature. Scanning electron microscopy with energy-dispersive spectroscopy (SEM/EDS) and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) established that the samples are pretty compositionally homogeneous in terms of major elements, while trace and rare earth elements (REE) contents are more variable. All REE patterns are characterized by pronounced positive Eu anomalies. The variations in color (from olive green to dark green with chocolate (reddish-brown color shades and polychrome bands) are due to the relevant presence of Fe and, to a lesser extent, Ti and Cr. The X-ray powder diffraction (XRPD) analyses and SEM/EDS quantitative study indicate that the other phases associated with vesuvianite are represented by diopside, garnet, clinochlore.

Geochemical and isotopic compositions of Late Permian metavolcano–sedimentary sequence in the southern Songpan–Ganzi Orogen, eastern Tibetan Plateau

AbstractThe Permian metavolcano–sedimentary sequence of the Jianglang and Taka domes in the southern Songpan–Ganzi Orogen is composed of bedded marble and sandwiched metabasalt. This study presents geochemistry, C–O–Nd isotope systematics and zircon U–Pb geochronology data to explore the formation mechanism and tectonic setting of its protolith. The marble samples have high δ13CV-PDB (4.0 ‰ to −0.1 ‰) and δ18OV-SMOW (16.3 ‰ to 13.6 ‰) values, with similar εNd(t) values (−5.3 to −7.2) to Late Permian conodonts. They display seawater-like features, e.g. superchondritic Y/Ho ratios, negative Ce and positive La, Gd and Y anomalies, indicating a marine carbonate protolith in oxygen-rich palaeoseawater. Their pronounced positive Eu anomalies suggest a contribution (5 % to 1 %) of submarine high-T hydrothermal fluids. The metabasalt samples show low SiO2, high Fe2O3T and MgO contents. They have low La/Sm, (Th/Ta)PM and (La/Nb)PM ratios, without correlations between MgO and other major/trace elements. These signatures could be attributed to insignificant fractional crystallization and crustal contamination. Their rare earth elements (REE) patterns, trace element profiles and trace element ratios are comparable to those of mid-ocean ridge basalt (MORB), suggesting a depleted MORB-type mantle reservoir. Melt modelling illustrates low degrees (&lt;10 %) of partial melting in the spinel stability field at a depth of 30–60 km. Moreover, the metabasalt contains 2453.7–258.9 Ma detrital zircons significantly related to the Palaeoproterozoic crystalline basement, Rodinia supercontinent, Caledonian events and c. 260 Ma Emeishan large igneous province. Combined with previous studies, we advocate that the investigated metavolcano–sedimentary suquence was most likely formed during the Late Permian in a mature back-arc basin of the Palaeo-Tethys ocean.

In-situ elemental and boron isotopic variations of tourmaline from the Maogongdong deposit in the Dahutang W-Cu ore field of northern Jiangxi Province, South China: Insights into magmatic-hydrothermal evolution

In this paper, we present a systematic investigation of in-situ major, trace elements and boron isotopic variations of tourmaline from the Maogongdong deposit in the Dahutang W-Cu ore field of northern Jiangxi Province, South China, which demonstrates both the magmatic and hydrothermal origin for the formation of the tourmalines. The tourmaline occurs mainly in granites, pegmatites and quartz veins. Five types of tourmaline are identified: (1) disseminated tourmaline in the pegmatites (Tur-P type); (2) isolated and disseminated tourmaline clots in the granites (Tur-R1 type); (3) typical anhedral quartz-tourmaline nodulars in the granites (Tur-R2 type); (4) tourmaline clots in quartz-tungsten-sulfide veins (Tur-Q1 type); (5) euhedral needle-columnar tourmaline crystals in quartz or quartz-sulfide veins (Tur-Q2 type). These tourmalines mostly fall into the alkali group representing schorl-dravite solid solution series. Petrography and chemical discrimination diagrams suggest that the Tur-P, Tur-R1 and Tur-R2 are of magmatic origin and the Tur-Q1 and Tur-Q2 are of hydrothermal origin. Hydrothermal tourmalines have higher Mg/(Mg + Fe) ratios, higher Li, Be, Sn, Rb, Co, Sr, V, Pb, Zn, Ni contents and lower Na/(Na + Ca) ratios, lower Nb, Ta contents than the magmatic tourmalines. The Tur-P show negative Eu anomalies and their REE patterns are equivalent to the muscovite granite. The Eu anomalies in Tur-R1 and Tur-R2 change from negative to positive. Compared with magmatic tourmalines, Tur-Q1 and Tur-Q2 exhibit more pronounced positive Eu anomalies, and a majority of the Tur-Q1 and Tur-Q2 have higher ∑REE contents. Tourmalines from the Maogongdong deposit have a B isotopic composition range from −17.9‰ to −12.3‰. The average δ11B values of the Tur-P, Tur-R1 and Tur-R2 are −13.8‰, −14.8‰ and −14.5‰, respectively. Tourmalines in early hydrothermal veins have similar δ11B values (average −14.6‰), while the tourmalines in later hydrothermal veins have slightly lower δ11B values (average −16.5‰). Boron isotopic variations of tourmaline are controlled by temperature, Rayleigh fractionation and external fluid.

Major, trace, and rare earth element geochemistry of the Ayder and İkizdere (Rize, NE Turkey) geothermal waters: Constraints for water–rock interactions

This study aims to determine the sources of rare earth elements (REEs) in the geothermal and cold waters of the Ayder and İkizdere (Rize) geothermal fields (NE Turkey). The differentiation stages of REEs in water–rock interactions as well as the migration processes of aqueous complexes of rare earth elements (REEs) in the waters were evaluated. Based on results for major ions, the İkizdere geothermal water with a temperature of 61 °C can be classified as Na-Ca-HCO3 type water, whereas the Ayder geothermal water with a temperature of 54 °C can be classified as Na-Ca-CO3−SO4 type water. ∑REE concentrations were between 0.64–2.13 μg/L in the İkizdere geothermal field and between 0.01–0.32 μg/L in the Ayder geothermal field. As well as lower pH values, the waters in the İkizdere geothermal field were found to have higher REE concentrations than those of the Ayder geothermal field. Geochemical characteristics of REEs from the host rocks and water samples indicate that the REEs are derived from granite and granodiorite host rocks. Normalized REE plots for the host rocks and waters showed similar patterns, and in particular, the waters interacting with granitic rocks display a pronounced positive Eu anomaly and a less clear Ce anomaly. The positive Eu anomaly in the altered surrounding rocks is associated with argillization of feldspars in the granitic host rocks that the waters have passed through. Application of a speciation model showed that amorphous iron oxy-hydroxide complexes of REEs were significant for a pH value of nearly 9 in the Ayder geothermal waters whereas fluoride complexes are dominant in the İkizdere geothermal waters whose pH is just below 7.

Revisiting the Lushan-Taihua Complex: New perspectives on the Late Mesoarchean-Early Neoarchean crustal evolution of the southern North China Craton

The Taihua Complex in the Lushan area was divided into gneisses series and supracrustal rocks. The former is mainly composed of TTG gneisses and amphibolites, and records the Late Mesoarchean-Early Neoarchean crustal evolution of the southern North China Craton (NCC). Late Mesoarchean amphibolites (>2.84 Ga) are enriched in light rare-earth elements (LREEs) and large-ion-lithophile elements (LILEs) with negative Nb-Ta and Ti anomalies, derived from partial melting of a metasomatic mantle source. Late Mesoarchean TTG gneisses (>2.81 Ga) have high La/Yb and Sr/Y ratios and low MgO, Cr and Ni contents, and are interpreted as resulting from partial melting of a thickened lower crust with residual garnet and amphibole in the source. Early Neoarchean amphibolites (2.80–2.73 Ga) show flat REE patterns but slightly negative Nb-Ta and Ti anomalies due to the fractionation of Ti-rich minerals and have depleted Hf-Nd isotopes, and were derived from large degree partial melting of a depleted mantle source. Most of the Early Neoarchean TTG gneisses show pronounced positive Eu anomalies and have relatively lower trace element concentrations and more depleted Nd-Hf isotopes than the Late Mesoarchean TTG gneisses, resulted from the partial melting of thickened juvenile lower crust with residual garnet and amphibole in the source and subsequent plagioclase accumulation. In addition, some of the Early Neoarchean TTG gneisses have high MgO, Cr and Ni contents, which were likely derived from partial melting of delaminated lower crust or a thickened lower crust interacting with mafic melt.Late Mesoarchean amphibolites and TTG gneisses are restricted in the center of Lushan area, denoting an ancient continental nucleus. In contrast, Early Neoarchean amphibolites and TTG gneisses outcrop in peripheral of the ancient nucleus, exhibiting as an elongated belt along the Dangzehe River, which is evidence for the subduction. Overall, the Lushan amphibolites and TTG gneisses show slightly negative to positive whole-rock εNd(t) and εHf(t) values and zircon εHf(t) values, indicating a dominant crustal accretion with subordinate crustal reworking. Therefore, the continental growth recorded in the Lushan-Taihua Complex was caused by the subduction-accretion process, suggesting a convergent margin in the southern NCC during the Late Mesoarchean and Early Neoarchean.

In-situ elemental and boron isotopic variations of tourmaline from the Sanfang granite, South China: Insights into magmatic-hydrothermal evolution

Tourmaline often occurs in boron-rich granites and its genesis is still in dispute either formed from residual boron-rich silicate melt or from magmatic hydrothermal fluids with or without external fluid involvement. Here, we present a systematic investigation of in-situ major, trace elemental and boron isotopic variations of tourmaline from the Sanfang granite in Guangxi Province of South China, which demonstrate both the magmatic and hydrothermal origin for the formation of the tourmalines. The tourmaline occurs mainly in quartz-tourmaline nodules and tourmaline pegmatitic segregations within the Sanfang granite. Four types of tourmaline are identified: (1) isolated and disseminated tourmaline (Tur-D type) distributed in granite; (2) euhedral and subhedral tourmaline in quartz-tourmaline nodules (Tur-N type); (3) the earlier stage tourmaline (Tur-PE type) in pegmatitic segregations and (4) the later stage tourmaline (Tur-PL type) which replaced the Tur-PE type tourmaline in pegmatitic segregations. All the tourmalines belong to the alkali group representing dravite-schorl solid solution series with the former three types being schorl and the Tur-PL type being dravite. Petrography, chemical discrimination diagrams and Al occupations in the Y-site suggest that the Tur-D, Tur-N and Tur-PE type tourmalines are of magmatic origin and the Tur-PL type tourmalines are of hydrothermal fluid origin. Elemental and boron isotopic composition variations of tourmalines reflect the compositional and environmental evolution from late boron-rich magma to exsolved hydrothermal fluids. Chemical variations from the Tur-D to the Tur-N tourmalines are controlled by magma differentiation. The increase of Mg/(Mg + Fe) and Ca/(Ca + Na) ratios from the Tur-N to the Tur-PE tourmalines suggests the contamination of surrounding strata (e.g. the Sibao Group). Hydrothermal tourmalines have higher Mg/(Mg + Fe) and lower Na/(Na + Ca) ratios than the magmatic tourmalines. There is clear correlation at least in some trace elements (e.g. Sr, Pb) with major elements for all tourmalines, indicating the potential crystal chemical effects on their incorporation. Other trace element incorporation in tourmaline is largely controlled by melt and fluid composition. Hydrothermal tourmalines have high and stable V, Co and Ni contents, indicating relatively constant partition coefficient between hydrothermal tourmalines and the fluid. Hydrothermal tourmalines exhibit lower total REE contents but more pronounced positive Eu anomalies than the magmatic tourmalines. The δ11B values of all tourmalines from the Sanfang granite vary from −14.4‰ to −9.3‰, which indicate the boron in the Sanfang granite was mainly derived from partial melting of the metasedimentary source rocks. Boron isotopic variations of tourmaline are controlled by fractionation between melt-fluid and Rayleigh fractionation.

Genesis of the Pb−Zn deposits of the Qingchengzi ore field, eastern Liaoning, China: Constraints from carbonate LA−ICPMS trace element analysis and C–O–S–Pb isotopes

The Qingchengzi ore field is an important Pb–Zn polymetallic ore district located in northeastern China, where more than ten Pb–Zn deposits and several Au-Ag deposits occur. The Pb–Zn mineralization is mainly hosted in the marble of the Dashiqiao Formation. Three distinct mineralization styles (strata-bound, open-space-filling and transitional type) are recognized. Two genetic models have been proposed to explain the Pb–Zn mineralization in this district: the sedimentary-exhalative model and the superimposed multi-stage model, in which syn-sedimentary deposition is overprinted by metamorphic and magmatic hydrothermal modification. Based on these three mineralization styles, this study provides new C–O–S–Pb isotopic data and LA–ICPMS trace element analyses of carbonates in order to place more constraints on the genesis of the Pb–Zn deposits.The δ13CPDB values of the syn-ore carbonates range from −5.11‰ to −1.06‰, and their δ18OSMOW values range from 3.30‰ to 13.2‰; these values are distinctively lower than those of barren meta-sedimentary rocks (δ13C=−1.98‰–−0.55‰, δ18O=19.69‰−22.76‰). The depleted C–O isotopic values of syn-ore carbonates are inferred to result from fluid-rock interactions combined with the impact of graphite. In situ LA–ICPMS trace element analyses show that syn-ore carbonates record enriched REE contents, pronounced positive Eu anomalies and lower Y/Ho ratios compared to barren carbonates. The depleted C–O isotopic signatures and distinctive REE characteristics of syn-ore carbonates indicate that the hydrothermal fluid is likely magmatic in origin and has mixed with low δ18O fluids, such as meteoric or formation water. Sulfide minerals have δ34S values ranging from 3.16‰ to 9.14‰ and display a gradually increasing trend from open-space-filling to transitional type to strata-bound mineralization styles, which reflect different mixing proportions of the two end-members of magmatic sulfur and sedimentary sulfur. The Pb isotopic compositions of sulfide minerals are relatively uniform (206Pb/204Pb=17.511–17.883, 207Pb/204Pb=15.549–15.64 and 208Pb/204Pb=37.670–38.178) for different mineralization styles; however, the open-space-filling mineralization records relatively lower lead isotope values. The linear correlation of ore lead isotopes between schist and Triassic intrusions indicates that lead may have originated from a mixture of magmatic and sedimentary end-members. The relatively lower Pb isotopes of the open-space-filling mineralization suggest the presence of higher proportions from a magmatic source. Mineralization styles respond to diverse ore-controlling structures and their proximity to the mineralizing source: open-space-filling mineralization shows the least stratigraphic control, with more pronounced “magmatic marks” (i.e., lower S–Pb isotopic values and more depleted C–O values), compared to strata-bound mineralization.These mineralization styles, combined with C–O–S–Pb isotopic geochemistry, consistently preclude a SEDEX origin for the Pb–Zn deposits in the Qingchengzi district, but suggest that they represent a distal hydrothermal mineralization type related to Triassic magmatic activity.

Geochemical and stable isotope signatures of Proterozoic stromatolitic carbonates from the Vempalle and Tadpatri Formations, Cuddapah Supergroup, India: Implications on paleoenvironment and depositional conditions

Spectacular stromatolitic dolomites with varied morphological features are preserved in the Vempalle and Tadpatri Formations of the Proterozoic Cuddapah basin, eastern Dharwar Craton. They exhibit stratifera, columnar, conical, domal and spheroidal/lobate morphologies in which columns are branched at some places (in Tadpatri Formation). Based on their trace elements such as REY and stable isotope compositions (carbon and oxygen) the Vempalle stromatolites have been classified as three types. Vempalle I stromatolites exhibit pronounced positive Eu anomalies with small scale positive Ce anomalies whereas Vempalle II counterparts have slight negative to positive Eu, feeble positive Ce and positive Gd anomalies. Vemaplle III stromatolites display positive Ce, Eu and Gd anomalies. The Tadpatri Formation stromatolites have pronounced Gd anomalies with low order positive Ce and Eu anomalies. Significant depletion in δ18O is observed in Vempalle ranging from – 10.70‰ to −7.07‰ and 20.73‰ to −11.87‰ in Tadpatri Formations. The Tadpatri stromatolites display negative δ13C varying from −4. 56‰ to −1.26‰. Contrarily, the Vempalle stromatolites recorded both positive and negative δ13C ranging from −0.24‰ to 2.00‰ thereby reflecting their deviation from the popular belief of the boring billion concept of displaying Midproterozoic δ13C stasis. The flat REE patterns along with Ce, Eu and Gd anomalies, chondritic to superchondritic Y/Ho ratios collectively reflect on marine water conditions for their deposition with variable inputs of siliciclastic/terrigenous materials and hydrothermal signatures. These cherty dolomitic stromatolites were deposited in subtidal-intertidal-supratidal zones of a shallow shelf of Proterozoic open ocean under warm, alkaline and anoxic conditions.

Fluid inclusion and geochemistry studies of calcite veins in Shizhu synclinorium, central China: Record of origin of fluids and diagenetic conditions

Calcite veins in carbonate fracture have been investigated by petrographic, fluid inclusion, geochemical analyses and coupled with basin modeling techniques to provide useful insights into fluid activity and deformation conditions of the Cambrian to Triassic Shizhu synclinorium from the western region of Mid-Yangtze, central China. The results of the fluid inclusion microthermometry show a wide range of homogenization temperatures (78.6-215.5 °C) and salinities (0.18-23.11 wt.% NaCl equivalent), indicating the formation under diverse fluid conditions. All the calcite veins have negative Ce anomalies, which are the typical characteristic of marine carbonate sediments; it is therefore plausible that calcite veins were precipitated from the marine basin fluid. The stable carbon iso-topic compositions of calcites (δ 13CV-PDB=-2.5‰-4.26‰) and host limestones (δ 13CV-PDB=-3.56‰-5.80‰) are very similar with a correlation coefficient of 0.86, however, four calcites from the Lower Permian and Lower Triassic show lower δ 13C values relative to the host limestones, and they are depleted in total REE concentrations (ΣREE ratio varies from 0.74 to 2.06), suggesting the derivation of dissolved carbon from marine carbonates hosting the calcite veins and, less commonly, from the degradation of organic matter. Calculated δ 18O of the fluids-precipitating calcites (δ 18OV-SMOW=-0.41‰-14.42‰), 87Sr/86Sr ratios varying in the range of coeval seawater and the distinct REE pattern simultaneously suggest calcite-forming fluids in each stratigraphic unit could have formed from the involvement of fluids that originated from coeval seawater and evolved through different degrees of water rock interaction. However, the presence of more radiogenic 87Sr/86Sr ratios than coeval seawater and pronounced positive Eu anomalies in calcites of Lower to Middle Ordovician rocks indicate that terrestrial input from upper strata mudstone and siliciclastic rocks could be involved in the precipitation of the Ordovician calcite. Fluid-inclusion data combined with burial and thermal history modeling indicate there was large-scale flow of evolved basinal fluids through the carbonate formation fractures spanning a time frame from 135 to 50 Ma (Early Cretaceous-Eocene). Therefore, the geochemical characteristics of calcite veins can provide the basis for deformation events in Late Yanshanian and Early Himalayan orogeny.

Kyanite/corundum eclogites from the Kaapvaal Craton: subducted troctolites and layered gabbros from the Mid- to Early Archean

An oceanic crustal origin is the commonly accepted paradigm for mantle-derived eclogites. However, the significance of the aluminous members of the eclogite suite, containing kyanite and corundum, has long been underrated and their role neglected in genetic models of cratonic evolution. Here, we present a geochemical and petrological study of a suite of kyanite- and corundum-bearing eclogites from the Bellsbank kimberlite, S. Africa, which originate from depths between 150 and 200 km. Although clearly of high-pressure provenance, these rocks had a low-pressure cumulative origin with plagioclase and olivine as major cumulate phases. This is shown by the very pronounced positive Eu anomalies, low REE abundances, and δ 18O values lower than the Earth’s mantle. Many chemical features are identical to modern-day troctolitic cumulates including a light REE depletion akin to MORB, but there are also distinguishing features in that the eclogites are richer in Na, Fe, and Ni. Two of the eclogites have a minimum age of ~3.2 Ga, defined by the extremely unradiogenic 87Sr/86Sr (0.7007) in clinopyroxene. Phase equilibria indicate that the parent melts were formed by partial melting below an Archean volcanic center that generated (alkali-)picritic to high-alumina tholeiitic melts from a mantle whose oxygen fugacity was lower than today. Fractional crystallization produced troctolites with immiscible sulfide melt droplets within the mafic crust. Instability of the mafic crust led to deep subduction and re-equilibration at 4–6 GPa. Phase relationships plus the presence of a sample with appreciable modal corundum but no Eu anomaly suggest that kyanite- and corundum-bearing eclogites may also originate as plagioclase-free, higher pressure cumulates of highly aluminous clinopyroxene, spinel, and olivine. This is consistent with the crystallizing phase assemblage from an olivine tholeiitic to picritic magma deeper in the Archean oceanic crust or uppermost mantle. We postulate that the magmatic and subduction processes driving modern plate tectonics already existed in the Meso- to Early Archean.

Subduction-related origin of eclogite xenoliths from the Wajrakarur kimberlite field, Eastern Dharwar craton, Southern India: Constraints from petrology and geochemistry

Major and trace elements as well as the first oxygen isotopes are reported on eclogite xenoliths from the Mesoproterozoic KL2 and P3 kimberlite pipes of the Wajrakarur kimberlite field, Eastern Dharwar craton, Southern India. Garnets in kyanite-bearing samples are rich in grossular, whereas they are predominantly pyrope-almandines in the bimineralic (kyanite-free) samples. The kyanite eclogite from the P3 pipe is more Mg-rich than those from KL2 pipe. Equilibration temperatures indicate derivation from 4.5 to 5.3GPa and 1060 to 1220°C for the KL2 samples and 3.6GPa, 918°C for the P3 sample.Garnet rare earth element patterns show two characteristic types, one with relatively low and flat heavy rare earth element patterns: Wajrakarur Group 1 and a second with lower light to heavy rare earth element ratios: Wajrakarur Group 2. Most samples in Wajrakarur Group 1 show pronounced positive Eu anomalies in garnet and positive Eu and Sr anomalies in the reconstructed whole rock trace element patterns; these are among the strongest anomalies in eclogite xenoliths worldwide. In contrast, Wajrakarur Group 2 samples show only subtle positive Eu anomalies.Oxygen isotopic ratios of garnets range between +5.3‰ and +7.8‰ δ18O. This range extends significantly beyond the range for unchanged mantle. Similar to many other eclogite suites worldwide, the Wajrakarur Group 1 and Group 2 eclogite suites shows evidence for an origin as crustal gabbroic material, likely once part of the oceanic crust, which was subducted and imbricated under the Eastern Dharwar craton. Their surface origin therefore lends support to geodynamic models that favor amalgamation of the Dharwar craton by subduction.