Low REE Contents Research Articles

Genesis of the oxidized Sn ores in the Gejiu district, Yunnan Province, SW China

The Gejiu Sn ore district, one of the largest Sn producing area (300Mt @1 wt% Sn) in the world, is located in the western margin of the Cathaysia Block, SW China. There are five types of Sn ores in the Gejiu district, including greisen, skarn, veined tourmaline, massive sulfide and oxidized. Among these types of Sn ores, the genesis of the oxidized Sn ores is most controversial, such as magmatic hydrothermal origin and sedimentary exhalative (SEDEX) or hot water deposition (Red Sea type). This paper reports U-Pb ages, fluid inclusions, trace elements and H-O isotopes of cassiterite from the oxidized Sn ores, aiming to solve this issue. LA-MC-ICPMS cassiterite U-Pb dating yielded an age of 86.7 ± 2.8 Ma, which is accordant with the formation ages (~86 Ma) of granitic intrusions in the Gejiu district. Two types of fluid inclusions in cassiterite from the oxidized Sn ores were observed, one type is CO2-rich inclusions (Type 1) and the other type is aqueous inclusions (Type 2). The homogenization temperatures range from 220 to 366 °C and the salinities range from 10.9 to 21 wt% NaCl equiv. for the two types of fluid inclusions. Furthermore, fluid inclusions in cassiterite from the oxidized Sn ores located at 300 m far away from granitic intrusions have higher temperatures (357–366 °C) than those located at 500 m (273–330 °C). Hence, the ore-forming temperatures tend to decrease with the distance far away from the magmatic hydrothermal center. The vapor components for the two types of fluid inclusions are CO2 and H2O, respectively, and the liquid phase compositions of them are all H2O. The CO2 in the ore-forming fluids may source from marble by decarbonization. This indicates that the precipitation of cassiterite is resulted from fluid immiscible. The δ18O values of fluid inclusions in cassiterite range from +6.1 to +7.7‰, and the δD values range from −135 to −153‰, indicating a mixture of magmatic and meteoric waters. Trace elements show that cassiterite is rich in In, Rb and U, but Ba, Nb and Ti are depleted. In addition, the lower Sr (0.4–2.1 ppm) and higher Sc (35.9–53.7 ppm) contents reflect that the formation of cassiterite is related to F-rich intermediate-acid magma. Cassiterite has a relatively low total REE contents (2.7–9.2 ppm) with negative Eu (δEu = 0.14–0.76) and Ce (δCe = 0.1–0.6) anomalies, indicating that the ore-forming environment is oxidized. This is consistent with the cassiterite precipitation conditions. Hence, all the evidence supports that the oxidized Sn ores are genetically associated with the Gejiu granitic magmatism.

Generation of Jurassic high-Mg diorite and plagiogranite intrusions of the Asa area, Tibet: Products of intra-oceanic subduction of the Meso-Tethys Ocean

Intra-oceanic subduction is a fundamental process on Earth, the study of which can improve understanding of plate tectonic processes and the history of continental growth. Several studies of the intra-oceanic subduction of the Meso-Tethys Ocean have been published in recent years, particularly relating to the Bangong–Nujiang suture zone (BNSZ) and the Shiquanhe–Nam Tso mélange zone (SNMZ) in central Tibet. Here, we report newly discovered occurrences of Jurassic diorite (161.2–160.6 Ma) and plagiogranite (161.0 Ma) from the western part of the central SNMZ. The diorite samples have moderate SiO2 and high MgO contents and Mg# values; high Ba/Th and Ba/La ratios; low K2O, TiO2, and REE contents; low (La/Yb)N, (La/Sm)N, and Th/Yb ratios; and positive εHf(t) values, similar to high-Mg diorite formed by partial melting of a subduction-modified mantle wedge. The plagiogranite intrudes gabbro of the ophiolite suite and has high SiO2 and Na2O contents; low Al2O3, K2O, P2O5, TiO2, and total REE contents; and clear negative Eu anomalies, similar to shear-related oceanic plagiogranite, which can be formed by partial melting process of gabbro in the high temperature shear zone near the expansion center. The formation processes of Asa high-Mg diorite and shear-related plagiogranite are consistent with initial subduction within the Meso-Tethys ocean. Based on these data and those for sedimentary and Jurassic magmatic rocks of the region, we propose that, during subduction, the Northern Lhasa terrane was formed from a collage of oceanic island arcs, oceanic plateaus, and accretionary complexes.

The Zhangjiatun igneous complex in the southeastern margin of the Central Asian Orogenic Belt, NE China: Evidence for an Early Paleozoic intra-oceanic arc

The subduction-accretionary evolution of the southeastern margin of the Central Asian Orogenic Belt (CAOB), consisting of the northern North China Craton, Bainaimiao arc, and Ondor Sum accretionary complex from south to north, is still not well understood. In order to clarify the Early Paleozoic subduction process in the southeastern margin of the CAOB, we conducted geochronological, geochemical, and Sr-Nd-Hf isotope analyses of the Zhangjiatun igneous complex rocks. The Zhangjiatun complex is composed of hornblende gabbro, quartz diorite, and tonalite. Zircon U-Pb dating of the complex yields ages ranging from 486 Ma to 440 Ma. We interpret this result as indicating Early Paleozoic arc magmatism along the southeastern margin of the CAOB. Late Cambrian to Early Ordovician mafic-intermediate intrusions have low-K tholeiitic compositions characterized by low (La/Yb)N (0.4–1.2) and initial 87Sr/86Sr (0.70439–0.70447) ratios, positive εNd(t) (+6.5 to +7.6), and εHf(t) (+13.5 to +19.0) values. The geochemical values indicate the rocks to be of N-MORB affinities, likely generated by melt differentiation from a depleted mantle. An Early Silurian tonalite exhibits oceanic plagiogranite signatures, expressed by high Na2O, and low K2O, Rb, and total REE contents. The tonalite also yields high (La/Yb)N values, low Sr/Y and initial 87Sr/86Sr (0.70345) ratios, and positive εNd(t) (+3.3) and εHf(t) (+8.0 to +12.5) values. Geochemical modelling suggests that the tonalite may have originated from hydrous melting of hornblende gabbro with a minor contribution from melting of oceanic sediments. The Late Cambrian to Early Ordovician mafic-intermediate intrusions with MORB characteristics likely resulted from initial intra-oceanic subduction along the southern margin of the Paleo-Asian Ocean. We interpret the Early Silurian tonalite to have been generated as the arc became fully developed. We further suggest that the southeastern margin of the CAOB has involved two subduction systems during the Early Paleozoic: a northern south-dipping intra-oceanic arc and a southern north-dipping continental arc.

Rinkite-(Ce) in the nepheline syenite pegmatite from the Saima alkaline complex, northeastern China: Its occurrence, alteration, and implications for REE mineralization

AbstractThe nepheline syenite pegmatite in the Saima alkaline complex in northeastern China is characterized by REE mineralization, mainly rinkite-(Ce) and associated alteration minerals. As the most abundant REE-bearing mineral in the pegmatite, rinkite-(Ce) closely coexists with microcline, nepheline, natrolite, and calcite. Some rinkite-(Ce) grains show compositional sector-zonation, in which the inner core displays relatively high Ti, Ca, and Sr concentrations, but low Zr, REE, and Na contents. Primary rinkite-(Ce) has undergone multiple episodes of fluid interactions, and accordingly, from weak to strong, three different mineral assemblages of hydrothermal alteration can be summarized: (1) rinkite-(Ce) + secondary natrolite ± K-feldspar ± minor fluorbritholite-(Ce); (2) rinkite-(Ce) relics + secondary natrolite + K-feldspar + fluorbritholite-(Ce) + unidentified Ca-Ti silicate mineral + fluorite and calcite; and (3) pseudomorphs after rinkite-(Ce). The pseudomorphs can be divided into two groups characterized by distinct mineral associations: (1) Ca-bearing strontianite + fluorbritholite-(Ce) + natrolite + fluorite + calcite coexisting with silicate minerals; and (2) calcite + fluorite + fluorbritholite-(Ce) + rinkite-(Ce) relics ± Ca-bearing strontianite ± ancylite-(Ce) associated with a calcite matrix. These alteration mineral assemblages are evidence of magmatic-derived alkali metasomatism due to an alkali-CO2-F-rich fluid and Ca-metasomatism due to a different, externally derived Sr- and Ca-rich fluid. The metasomatic events acted as the potential driving force for the rinkite-(Ce) dissolution and pseudomorph-forming process. The high concentration of rinkite-(Ce) in the nepheline syenite pegmatite results from the fractional crystallization of the Saima CO2-rich alkaline silicate magma, and the successive alterations of rinkite-(Ce) attest to the important role played by hydrothermal fluids in controlling the remobilization of REE and the crystallization of secondary rare earth minerals.

Sapphirine-bearing pyroxenite xenoliths in Cenozoic alkali basalt from Jabel El Arab (Syria): Insights into the nature and composition of the lithosphere beneath the southern Syrian Rift, northern part of the Arabian Plate

Pyroxenites and composite xenoliths occur in the Cenozoic alkali basalts of Jabel El Arab (the southern extremity of the Syrian Rift). The anhydrous xenolith suite consists mainly of spinel garnet-sapphirine–bearing websterites, spinel-olivine–bearing websterite, as well as rare spinel plagioclase-garnet–bearing websterites and composite garnet-bearing websterite/anorthosite xenoliths. We report for the first time sapphirine coronas around spinel in spinel-garnet–bearing websterite xenoliths from two outcrops in this region. Their compositional and microstructural features are consistent with a reaction from spinel + orthopyroxene + anorthite to clinopyroxene + garnet + sapphirine. The full pyroxenite suite is divided into two groups based on mineralogical characteristics and geochemical composition: (A) tholeiitic to transitional mantle segregates having spinel ± garnet ± sapphirine characterized by a high Al2O3 and CaO content, low REE content, LREE depletion, and a highly variable Sr-Nd isotopic ratio; (B) alkaline segregates that include spinel-olivine–bearing websterite and composite xenoliths characterized by a high TiO2 and alkali content, relatively high REE, and low Ni and Cr. Slight LREE enrichments in clinopyroxenes from meta-igneous rocks of tholeiitic to alkaline affinity indicate that they were later metasomatized by small volumes of percolating silicate melts in the upper lithospheric mantle. The xenoliths have been re-equilibrated within the garnet-pyroxenite stability field. Combining xenolith-derived P-T estimates with mineral and chemical composition and seismic data, we show that the websterite xenoliths formed in the shallow lithospheric mantle near the Moho discontinuity at 28–44 km in depth. Nd-model ages and the geodynamic history of the Arabian Plate favor the scenario of tholeiitic-transitional and alkaline magmas being underplated below or intruded at the base of a thinned Syrian crust. This occurred around 200–100 Ma in response to rifting and magmatism beneath the Arabian Plate during the early assembly of the eastern Mediterranean region.

Геохимические особенности микробиолитов миоцена (Юго-Западный Крым) по результатам ICP-MS исследований

Background. In this paper, we provide new data on the morphology and geochemistry of heraclites, which are represented by specific products formed by degradation of the carbonate paleostructures of prokaryotes around the degassing zones of the Miocene age. The heraclites exhibiting siltstone and sandstone structures are referred to as micrometanolites. An analysis of literature data showed that heraclites had formed in the upper sedimentary strata under the conditions of increased methane levels in the affinity of the stream exits. Methods. The geochemical examination of heraclites of a different colour and morphology was carried out using ICP-MS at the facilities of the Geoanalytik centre, UB RAS Institute of Geology and Geochemistry, Ekaterinburg. Results. According to lithological characteristics and morphology, the following heraclite types were distinguished: slag-like, banded, angular and сementation slabs. Morphologically, the heraclites under study were characterized by high concentrations of Sr, Ba, Co, Ni, Bi, Ce, Nd and Yb, which significantly exceed those in carbonate rocks. The concentrations of Rb, Cs, Nb, Th, Cu, Tl, As, Sc, Sb, Eu, Gd, Tb, Dy and Er in some morphological differences were above the Clarke values, while others demonstrated their deficit. The samples under study showed low concentrations of some heavy REE. A high content of lithophilic, chalcophile, siderophilic and REE elements indicate paleo-fluids of a deep mantle-derived character. The data on the oxide environment during construction material formation was obtained, which is confirmed by low concentrations of U (0.579 to 2.096 ppm) and Bi (0.014-0.084 ppm), as well as the ratios of U/Th (0.4-2.1), V/Cr (0.3-0.9) and Mo/Mn (less than 0.0014). Such conditions existed near or within the cellular organism of prokaryotes. Low concentrations of U, Ti, Mn and Zr confirm a significant growth rate of heraclite carbonate substances. Minor differences in the concentrations of chemical elements in heraclites of a different morphology are believed to have resulted from the physic-geographical and chemical conditions of their formation across the areas of regional faults with an active tectonic regime. Conclusions. Our results have confirmed the assumption that the carbonate material of heraclites formed by processing of deep fluids. In the South-Western Crimea, heraclites constitute a geological phenomenon of the Miocene hydrocarbon paleo-degassing, which was associated with the neotectonic stage of the uplift of the Crimean Mountains. The presence of methane, ethane, propane and petroleum products in the composition of heraclites allows an assumption to be made about the presence of oil and gas deposits in the Sevastopol region.

Mineralogical and geochemical characterization of the unconsolidated sands from the Mefou River terrace, Yaoundé area, Southern Cameroon

Nine sediment core samples from the right terrace of the Mefou River in the Yaoundé area (southern Cameroon) were used to investigate the provenance and evolution of sediments. The mineralogical and geochemical data have been reported using XRD, XRF and ICP-MS instruments. The sediment core is 87 cm thick and is subdivided from the bottom to the top into sandy, clayey sandy and, silty sandy layers. The sandy layer is 70 cm thick with a decreasing grain size from the bottom to the top. The nine sediment core samples are mainly composed of quartz, and accessory feldspars, kaolinite, goethite, rutile and muscovite/illite. This mineral assemblage can be explained by the combined effect of metamorphic source rocks and surrounding soils. The sandy texture as well as the abundance of quartz were confirmed by the high SiO2 contents particularly at the bottom of the sequence (71–85 wt%). The high SiO2/Al2O3, low alkali (Na2O + K2O) values, low REE contents (~107–389 ppm; avg. = 247 ppm) and the negative correlation between SiO2 with most of major and trace elements confirm the siliciclastic nature of sediments. The SiO2/Al2O3 ratio justifies the high proportion of quartz relative to clay minerals and the coarse grain size of the sediments. The Mefou alluvial sediments are classified as Fe-shale and Fe-sand. The LREE/HREE ratio and (La/Yb)N values exhibit the light REE-abundance linked to the nature of REE mineral-bearers. The spider diagrams reveal the high depletion of K and Sr due to the high feldspar alteration and the quartz-abundance. The REE-normalization relative to chondrite portrays slightly positive Ce anomalies (Ce/Ce*~1.11–1.15) and negative Eu anomalies (Eu/Eu*~0.43–0.63).The mineralogical and geochemical features are in concordance with the textural parameters and portray the siliciclastic nature of sediments inherited from the gneiss basement. The predominance of sands and the low trace element contents in alluvial sequence from the Mefou River are linked to the intensity of the hydrodynamic forces. The rock sources have undergone high degree of weathering and there is no evidence of significant sediment recycling.

Sr, Nd, Pb and trace element systematics of the New Caledonia harzburgites: Tracking source depletion and contamination processes in a SSZ setting

The New Caledonia ophiolite (Peridotite Nappe) consists primarily of harzburgites, locally overlain by mafic-ultramafic cumulates, and minor spinel and plagioclase lherzolites. In this study, a comprehensive geochemical data set (major and trace element, Sr-Nd-Pb isotopes) has been obtained on a new set of fresh harzburgites in order to track the processes recorded by this mantle section and its evolution.The studied harzburgites are low-strain tectonites showing porphyroclastic textures, locally grading into protomylonitic textures. They exhibit a refractory nature, as attested by the notable absence of primary clinopyroxene, very high Fo content of olivine (91–93 mol.%), high Mg# of orthopyroxene (0.91–0.93) and high Cr# of spinel (0.44–0.71). The harzburgites are characterised by remarkably low REE concentrations (<0.1 chondritic values) and display "U-shaped" profiles, with steeply sloping HREE (DyN/YbN = 0.07–0.16) and fractionated LREE-MREE segments (LaN/SmN = 2.1–8.3), in the range of modern fore-arc peridotites. Geochemical modelling shows that the HREE composition of the harzburgites can be reproduced by multi-stage melting including a first phase of melt depletion in dry conditions (15% fractional melting), followed by hydrous melting in a subduction zone setting (up to 15%–18%). However, melting models fail to explain the enrichments observed for some FME (i.e. Ba, Sr, Pb), LREE-MREE and Zr–Hf. These enrichments, coupled with the frequent occurrence of thin, undeformed films of Al2O3, and CaO-poor orthopyroxene (Al2O3 = 0.88–1.53 wt.%, CaO = 0.31–0.56 wt.%) and clinopyroxene with low Na2O (0.03–0.16 wt.%), Al2O3 (0.66–1.35 wt.%) and TiO2 (0.04–0.10 wt.%) contents, point to FME addition during fluid-assisted melting followed by late stage metasomatism most likely operated by subduction-related melts with a depleted trace element signature.Nd isotopic ratios range from unradiogenic to radiogenic (−0.80≤εNdi≤+13.32) and negatively correlate with Sr isotopes (0.70257≤87Sr/86Sr ≤ 0.70770). Pb isotopes cover a wide range, trending from DMM toward enriched, sediment-like, compositions. We interpret the geochemical signature displayed by the New Caledonia harzburgites as reflecting the evolution of a highly depleted fore-arc mantle wedge variably modified by different fluid and melt inputs during Eocene subduction.

Ultramafic–mafic and granitoids supra‐subduction magmatism in the southern Ashanti volcanic belt, Ghana: Evidence from geochemistry and Nd isotopes

Geochemical and Nd isotope data are presented for Palaeoproterozoic ultramafic to mafic rocks and granitoids which are associated with volcanic rocks in the southern part of the Ashanti greenstone belts of Ghana. The Prince's Town granitoids display subduction‐zone geochemical features, with some showing signatures similar to high‐SiO2 adakites (HSA). Their initial εNd (2.1 Ga) values range from −1.01 to +2.92, and they have TDM2 of 2.17–2.51 Ga. The gabbros show slightly LREE‐depleted and ‐enriched patterns, Th–U troughs, negative Nb, Zr–Hf and Ti anomalies, spikes in Sr and Pb, have εNd (2.1 Ga) values ranging from −1.23 to +5.23, and TDM2 values from 2.08 to 2.57 Ga. The Ahama ultramafic body is characterized by two groups of pyroxenites. Both groups show LREE enrichment patterns, display Th–U and Nb–Ta troughs, minor or pronounced negative Zr–Hf and Ti anomalies. However, the Group I pyroxenites exhibit minor Ce anomalies and lower total REE contents (28.1–32.8 ppm) whereas Group II pyroxenites show significant negative Ce anomalies and relatively higher total REE contents of 57.1–124 ppm. The pyroxenites show negative initial εNd (2.1 Ga) values (−0.58 to −5.68) and have TDM2 values of 2.52 to 2.75 Ga. The pyroxenitic and gabbroic rocks were most likely originated from a subduction‐related lithospheric mantle. The Aketakyi ultramafic–mafic complex is made up of mainly cumulate dunite, harzburgite, pyroxenites, and gabbro. The rocks display LREE‐depleted to flat patterns with low total REE contents (1.71 to 22.3 ppm) and demonstrate a supra‐subduction affinity. They show high positive initial εNd (2.1 Ga) values (+3.69 to +4.93), and TDM2 values of 1.99 to 2.04 Ga, which suggest that they were derived from depleted mantle magmas and were juvenile at their time of formation. The Nd isotopic data provides evidence for a possible contamination of the juvenile Birimian crust of the southern Ashanti belt by some amount of a pre‐Birimian (or Archaean?) crustal material. The trace element and isotopic signatures as well as the field relations of the studied granitoids and mafic–ultramafic rocks show they were derived through supra‐subduction‐related magmatism during the Palaeoproterozoic.

Petrogenesis and metallogenic significance of multistage granites in Shimensi tungsten polymetallic deposit, Dahutang giant ore field, South China

The Shimensi tungsten polymetallic deposit, situated in the Dahutang ore field, South China, is one of the largest tungsten deposits in the world, with an estimated WO3 reserve of 0.74 million tons. Coarse-grained porphyritic biotite granite (CPBG), fine-grained porphyritic biotite granite (FPBG), fine-grained biotite granite (FBG) and biotite granite porphyry (BGP) are all ore-related, but their diagenetic relationships and contributions to W-Cu-Mo mineralization are still in dispute. LA-ICP-MS monazite U-Pb dating of the CPBG, FPBG, FBG and BGP yield emplacement ages of 147.9 ± 1.1 Ma, 146.4 ± 1.1 Ma, 138.6 ± 0.98 Ma and 142.8 ± 1.7 Ma, respectively. Whole-rock geochemical results indicate that the four granites should be classified as S-type granites, but BGP has distinct features transitional between S- and I-type granites. They were possibly generated by partial melting of upper crustal pelites and basic volcanic rocks with different proportion from the Neoproterozoic Shuangqiaoshan Group in the source. Proportional variation in the magmatic source (clay and basic basalts) induces the change of geochemical compositions of the Shimensi granites. Geochemical characteristics suggest that they were derived from two magma chambers (the CPBG, FPBG and FBG vs. the BGP) and experienced different evolutionary processes and different degree of magmatic differentiation during magmatic evolution. Chondrite-normalized REE patterns for the four granites display low total REE contents, variable and strongly enriched LREE relative to HREE and medium-strong negative Eu anomalies. They are enriched in Rb, Th, U, Ta and depleted in Ba, Nb, Sr, P, Ti. Biotites are iron-rich and aluminum-poor, and can be classified as ferro-biotite (CPBG, FPBG and FBG) and siderophyllite (BGP). The partial melting of tungsten-rich metasediments of the Shuangqiaoshan Group and high degree of fractional crystallization led to enrichment in tungsten in the magma suites. Oxygen fugacities of the CPBG and FPBG declined from early (most above the NNO buffers) to late stages of fractional crystallization (between the NNO and QFM buffers) because of the higher degree of magmatic differentiation in the late stages. In the early stages of fractionation, tungsten accumulated in the residual melts rather than partitioning into accessory minerals. In the late stages, lower oxygen fugacities and high fluorine contents promoted the removal of tungsten from the residual magma into reduced hydrothermal fluids. On the other hand, the FBG and BGP remained constant (above the NNO buffers) over the entire process of crystallization owning to the stable degree of magmatic differentiation, promoting retention of tungsten in the melt and resulting in low grade tungsten mineralization. Tungsten mineralization in the Shimensi deposit is greatly controlled by the redox states of the associated magma. The two porphyritic granites (the CPBG and FPBG) are most likely the main contributors of tungsten, while the FBG and BGP are mainly responsible for copper and molybdenum in the Shimensi deposit. Prolonged multiphase magmatism and prolonged W-Cu-Mo mineralization play important roles in the formation of Shimensi large tungsten polymetallic deposit.

Crustal versus mantle origin of carbonate xenoliths from Kimberley region kimberlites using C-O-Sr-Nd-Pb isotopes and trace element abundances

Carbonate-bearing assemblages in the mantle have been interpreted to be the source for Si-undersaturated, CO2-rich magmas, including kimberlites. However, direct evidence for carbonate in the mantle is rare in the contemporary literature. Here we present petrography, trace element, and C-O-Sr-Nd-Pb isotope composition for a suite of carbonate xenoliths from the Kimberley region kimberlites to ascertain their mantle or crustal origin and gain insight to the potential for the occurrence of carbonate in the mantle. Carbonate xenoliths were found in large kimberlite blocks from the Bultfontein kimberlite and Big Hole region. The xenoliths are characterised by pale green alteration margins made of fine-grained microlites of an unknown mineral as well as spherules surrounded by glassy material. They are generally 1–4 cm in size, coarse-grained (1–2 mm), and comprised entirely of calcite.Carbonate xenoliths from the Bultfontein kimberlite have low total REE concentrations (0.2–4.9 ppm), constant 87Sr/86Sri (0.7047–0.7049) combined with variable ƐNdi (−0.1 to −26.2) and 206Pb/204Pbi, 207Pb/204Pbi, and 208Pb/204Pbi of 16.7–18.8, 15.3–15.6, 36.5–38.4, respectively. Xenoliths from the Big Hole sample have higher 87Sr/86Sri (0.7088–0.7095), lower ƐNdi (−24.5 to −3.8), and 206Pb/204Pbi, 207Pb/204Pbi, and 208Pb/204Pbi of 18.9–19.9, 15.7–15.8, 38.4–38.8, respectively. The δ13C values for both Bultfontein (−5.7 to −6.6‰) and Big Hole (−4.7 to −5.4‰) carbonates are within the typical range expected for mantle-derived carbonate. The δ18O values (15.5–17.5‰) are higher than those of mantle silicate rocks, indicative of late-stage low-temperature interaction with fluids; a common feature of groundmass calcite in the Kimberley kimberlites.The Sr- and C- isotope composition of the Bultfontein xenoliths indicates a mantle origin whereas the Big Hole xenolith Sr- and C-isotopes are more ambiguous. Isotope mixing models are inconsistent with interaction between the host kimberlite and carbonate xenoliths. Correlation between ƐNdi and δ18O values for the Bultfontein xenoliths indicates late-stage interaction with low-temperature fluids, which may also be responsible for the large range in ƐNdi. This in turn indicates that the highest ƐNdi of −0.1 represents the primary carbonate xenolith signature, and this value overlaps typical Group I kimberlites.We discuss two possible origins for the carbonate xenoliths. (1) Carbonate xenoliths from the sub-continental lithospheric mantle (SCLM), where quenched margins and the large range of ƐNdi are related to formation in the mantle. (2) Carbonate xenoliths from an earlier phase of carbonatite magmatism. The similarity of isotope signatures of the Bultfontein carbonates to Group I kimberlite may further suggest a link between kimberlite and carbonatite volcanism such as observed elsewhere in the world.

Genesis of the Huangshaping W–Mo–Cu–Pb–Zn deposit, South China: Role of magmatic water, metasomatized fluids, and basinal brines during intra‐continental extension

The Huangshaping world‐class W–Mo–Cu–Pb–Zn deposit formed during Jurassic intra‐continental extension in the central Nanling region (South Hunan), South China. In order to assess the role of fluids in ore genesis and the origin of polymetallic mineralization, three types of mineralized porphyries (quartz porphyry, granophyre, and granite porphyry) were studied. The multi‐method approach included whole‐rock Sr–Nd isotopic geochemistry and determination of the trace element content and gaseous and aqueous composition of ore minerals from the three stages of intrusion and ore emplacement. Geochemical results show that the Sr–Nd isotopes in these altered porphyries were strongly affected by fluid metasomatism. The 87Sr/86Sr and 147Sm/144Nd ratios for the quartz porphyry and granophyre range from 0.72894 to 0.83093 and 0.1524 to 0.2080, respectively, whereas higher and considerably more variable ratios were determined for the granite porphyry (0.90396 to 1.51943 and 0.2391 to 0.2914). These observations imply that granite porphyry‐associated W–Mo mineralization underwent more intensive fluid–rock interaction. The relatively low REE concentrations and high abundances of CH4 and H2 in the Cu ores associated with the quartz porphyry unambiguously suggest a deep‐sourced magmatic fluid for the Cu mineralization. The high REE concentrations, pronounced negative Eu anomalies, M‐type tetrad effects, and high CO2, H2O, and Ca2+ contents of the W–Mo ores associated with the granite porphyry imply that the W–Mo polymetallic mineralization was formed in a relatively oxidizing, high‐temperature environment with strong metasomatism. In contrast, the slightly negative Eu anomalies, W‐type tetrad effects, low CH4, and high F− and Cl− concentrations of the Pb–Zn ores indicate involvement of a sediment‐derived basinal brine and reducing mineralization conditions. Collectively, a three‐stage genetic model is proposed for the Huangshaping polymetallic deposit. Corresponding to initiation, development, and cessation of Jurassic intra‐continental extension, magmatic water, metasomatized fluids, and basinal brines played key sequential roles in Cu, W–Mo, and Pb–Zn mineralization.

Агаты в палеопротерозойских вулканитах Онежской структуры (Центральная Карелия)

В статье приводятся результаты детального минералого-геохимического изучения агатов из палеопротерозойских подушечных лав Онежской структуры Центральной Карелии, относимых к&nbsp;заонежской и суйсарской свитам (возраст 2.05&mdash;1.95&nbsp;млрд лет). Установлено, что минералогия агатов своеобразна и представлена силикатами, окислами и гидроокислами, карбонатами, фосфатами и сульфидами. Показано, что наличие контрастных ритмов внутри агатовых миндалин маркируется изменением состава минеральных фаз и их морфологией, отражая химическую гетерогенность питающего раствора. Рассмотрена геохимическая характеристика агатов и вмещающих их вулканитов.

Primorskoe Epithermal Ag-Au Deposit (Northeastern Russia): Geological Setting, Mineralogy, Geochemistry, and Ore Formation Conditions

The potentially large Primorskoe epithermal Au-Ag deposit is represented by three areas: the Kholodnyi, Spiridonych, and Teplyi. It is located in the Omsukchan district of Magadan oblast, where mining is carried out at the largely similar Dukat, Lunnoe, Gol’tsovoe, Arylakh, Tidit, Pereval’noe, and other deposits. The deposit under review is located in the Kalalaga volcano-tectonic depression, where ore has been emplaced in a gently dipping sequence of Late Cretaceous ignimbrites and rhyolites more than 700 m in thickness crosscut by numerous intermediate and mafic dykes. According to drilling data, there is a leucocratic granite massif 400–500 m beneath the deposit, which is exposed on the surface in the northeastern part of the ore field. The presence of Bi-bearing galena and matildite, as well as medium- to high-temperature metasomatic facies (epidote and actinolite) and the specific physicochemical conditions of epithermal Ag-Au ore emplacement, attests to the above-intrusion position and the role of granitoids as high-temperature magmatic fluid generators responsible for supplying Bi and heating the host rock. The ore chemistry is quite consistent with its mineral composition. High Mn and Ag; elevated Au; low Cu, Pb, Zn, Sb, As, Bi, and Te; and low total REE concentrations were established, along with negative Eu and positive Ce anomalies. The high Te/Se, Sr/Ba, Y/Ho, and U/Th ratios in the ores are due to their location in the area influenced by the granitoid pluton. The physicochemical parameters of ore emplacement in the Teplyi area are unusual: high temperatures, low salt concentrations, and fluid densities typical of a “dry vapor” environment. The obtained data allow the Primorskoe to be classified as an intermediate sulfidation epithermal deposit. The data discussed below are of practical use for regional metallogenic forecasting, exploration, and economic assessment of epithermal Ag-Au deposits.

Heterogeneous Oceanic Arc Volcanic Rocks in the South Qilian Accretionary Belt (Qilian Orogen, NW China)

Primitive arc magmas in oceanic island arcs are probes of sub-arc magmatic processes and are crucial for understanding oceanic subduction. We report data for an Early Paleozoic oceanic arc volcanic complex in the Lajishan–Yongjing terrane, South Qilian Accretionary Belt (SQAB), Qilian Orogen, including zircon U–Pb dating and Hf–O isotopes, mineral and whole-rock geochemistry, and Sr–Nd isotope compositions. New zircon ages of ∼455–440 Ma constrain the timing of arc volcanism and the subduction of the Qilian Ocean. Based on petrography and bulk-rock composition, five lithological types have been identified: ankaramite; high-Mg basaltic andesite; high-Al andesite; boninite; sanukite. The volcanic sequence thus is one of the few island arcs where three types of near-primitive arc rocks including boninite, ankaramite and sanukite have been simultaneously produced. All these rocks have variably enriched Sr–Nd isotopic compositions, positive to slight negative zircon εHf(t) values and elevated zircon δ18O values. Boninites, ankaramites and sanukites are interpreted as contemporary near-primitive melts generated from different sources and conditions within an island arc setting. Boninites are characterized by low Ti and REE concentrations and high Cr# chrome spinel, and are interpreted as melts of refractory, Cpx-poor, spinel lherzolite or harzburgite at >25% partial melting. Anomalous zircon δ18O values of 6·57–7·61‰ and Sr–Nd mixing calculations suggest less than 2% incorporation of subducted oceanic sediments into the mantle source of the magmas. The ankaramites are characterized by low SiO2 and high MgO (Mg#), Cr, Ni and La/Yb ratios, and have similar isotopic ratios to tectonically adjacent ocean island basalt (OIB) lavas. The ankaramite lavas are likely to have derived from mantle sources similar to those of OIB; that is, pyroxenite-bearing garnet peridotite enriched in incompatible elements. High-Mg basaltic andesites and high-Al andesites may be derived from parental ankaramite magmas. Sr–Nd–Hf isotopic mixing modeling constrains the amount of silicic melt to ∼1–4% for ankaramite magma. Sanukites are of andesitic–dacitic composition with high Mg#, Cr and Ni, and enriched large ion lithophile elements and high La/Yb ratios. They are interpreted as having been generated by reaction of mantle peridotite with a silicic melt, itself derived from subducted sediments. Enriched Sr–Nd–Hf isotopic compositions constrain the amount of silicic melt to ∼10–15% for sanukite. Large compositional variations among the volcanic rocks from the same arc reflect heterogeneous mantle sources and variable degrees of mantle metasomatism by sediment-derived hydrous fluids or silicic melts, accompanied by secondary assimilation–fractional crystallization processes during magma ascent to the surface. The generation of the island arc volcanic sequence in the Lajishan–Yongjing Terrane is a response to the collision between the Lajishan–Yongjing Oceanic Plateau (recorded by the Lajishan–Yongjing Ophiolite) and the pre-existing trench–continental margin. Evolution from a continental margin in the North Qilian Accretionary Belt to an oceanic island arc in the SQAB records subduction advance and retreat in the history of the Qilian Ocean.

Genesis of ore‐bearing volcanic rocks in the Derbur lead–zinc mining area of the Erguna Massif, western slope of the Great Xing'an Range, NE China: Geochemistry, Sr–Nd–Pb isotopes, and zircon U–Pb geochronology

The Derbur lead–zinc deposit is located in the Derbugan metallogenic belt in the north‐western portion of the Mesozoic Hailaer‐Genhe volcanic basin in the northern Great Xing'an Range. This deposit occurs in the Middle Jurassic intermediate–mafic volcanic rocks of the Tamulangou Formation and is spatially and temporally associated with acidic pyroclastic rocks. The host rocks are bedded trachyandesite and rhyolitic lithic‐crystal tuffs, and basaltic‐andesitic aplite veins are associated with the ore body. Because the deposit is a hypabyssal low‐temperature hydrothermal Pb–Zn deposit associated with volcanism, to determine the precise petrogenesis of the volcanic rocks, this study analysed the zircon U–Pb ages of the trachyandesite and rhyolitic lithic‐crystal tuffs, the Sr–Nd–Pb isotopic geochemistry of the trachyandesite and basaltic‐andesitic aplite, and the whole‐rock geochemistry of all three rock types. The results showed that the trachyandesite and basaltic‐andesitic aplite belong to the shoshonitic series and have similar whole‐rock and isotopic geochemical characteristics, including enrichments in large‐ion lithophile elements and light rare earth elements and depletions in high‐field‐strength elements. They also have low initial 87Sr/86Sr values of 0.705007–0.705240 and εNd(t) values of +0.6 to +1.7, with model ages (TDM) of 699–883 Ma. The ratios of 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb vary from 18.438 to 18.476, 15.570 to 15.577, and 38.254 to 38.320, respectively, with μ values of 9.40–9.41. In contrast, the rhyolitic lithic‐crystal tuffs are characterized by high SiO2 contents (73.44–79.48 wt.%); low Al2O3 (11.36–12.53 wt.%), TiO2 (0.14–0.18 wt.%), and K2O + Na2O (3.48–3.89 wt.%, Na2O ≤ K2O) contents; and low Mg# values (0.17–0.41), indicating that they belong to the calc–alkaline series. Additionally, they have relatively low REE contents and strong Sr depletions. LA‐ICP‐MS zircon U–Pb dating of the trachyandesite and rhyolitic lithic‐crystal tuffs indicates that their ages are 167.0 ± 2.0 Ma and 164.8 ± 1.6 Ma, respectively. We conclude that the trachyandesite and the basaltic‐andesitic aplite were derived from the partial melting of lower crustal material assimilated by a depleted lithospheric mantle that was subsequently metasomatized by subducted slab‐derived fluids. The rhyolitic lithic‐crystal tuffs likely originated from the partial melting of accreted lower crust. In summary, because the acidic volcanism was accompanied by increasingly felsic volcanism, bimodal volcanic rocks were produced. Among volcanogenic deposits, bimodal volcanic rocks are the most favourable ore‐hosting rocks, and ore‐forming materials were contributed by the magmatic system. These processes created conditions conducive to the formation of large‐scale Ag, Pb, and Zn mineralization in this area.

Phanerozoic lower crustal growth from heterogeneous mantle beneath the North China Craton: Insights from the diverse Hannuoba pyroxenite xenoliths

Though many models of continental crustal growth rely on the assumption that juvenile crust is derived from depleted mantle of fairly uniform composition, mafic magmas derived from the mantle – potential candidates for newly formed continental crust – often bear heterogeneous radiogenic isotope compositions. Mafic granulite and some pyroxenite xenoliths from Cenozoic basalts at Hannuoba, in the northern margin of the North China Craton, have younger crystallization ages than the exposed basement rocks, and are generally interpreted to represent juvenile additions to the crust. Here we report mineralogical and geochemical data for the websterite xenoliths from the basalts with the aim of constraining the chemical and isotopic compositions of juvenile continental crust. Two-pyroxene geothermometry suggests that the studied websterite xenoliths originated in the lower crust and last equilibrated at temperatures in the range of 830 to 930 °C. The cumulate texture, together with the geochemical and geochronologic data, indicate that they formed as high-pressure cumulates through crystal fractionation from Phanerozoic mafic magmas. These xenoliths can be subdivided into two groups. Group 1 websterites have low Mg# of 66.1–79.2 and high REE concentrations, and are characterized by extremely evolved radiogenic Sr-Nd-Hf isotopic compositions and high radiogenic Pb ratios. Group 2 websterites have high Mg# of 89.4–89.5, very low REE concentrations, and MORB-like depleted Sr-Nd-Hf-Pb isotopic compositions. We interpret these websterites as additions to the crust formed via magma underplating. The parental magmas for these two websterite groups formed by partial melting of mantle materials with both isotopically enriched (Group 1) and depleted (Group 2) components. The data provide evidence that components from both mantle sources contributed to the underplating magmas and to lower crustal growth beneath the craton.

Early Paleoproterozoic (ca. 2.36 Ga) post-collisional granitoids in Yunnan, SW China: Implications for linkage between Yangtze and Laurentia in the Columbia supercontinent

The early Paleoproterozoic is an essential period for understanding the early tectonic evolution of the Yangtze Block and its reconstruction in the Columbia supercontinent, whereas these issues are largely unknown due to the scarce preservation of early Paleoproterozoic geological records. This study presents rarely reported early Paleoproterozoic granitoids from the southwestern Yangtze Block, with integrated studies of zircon U-Pb ages, Lu-Hf isotopic compositions and whole-rock geochemistry. Magmatic zircon U-Pb dating for two monzogranite samples yields weighted mean 207Pb/206Pb ages of 2359 ± 16 Ma and 2363 ± 16 Ma, providing unambiguous evidence for the presence of early Paleoproterozoic magmatism in the Yangtze Block. All the samples have high A/CNK values of 1.30–1.61, typical of S-type granite. These rocks have low total REE contents of 72.5–139.0 ppm with marked enrichment in LREEs ((La/Yb)N = 10.73–23.83) and conspicuously negative Eu anomalies (Eu/Eu* = 0.46–0.55). The rocks exhibit uniform primitive mantle-normalized trace element patterns with enrichment in Rb, Th, U and Pb and depletion in Sr, Nb, Hf and Ti. These dated zircon grains have consistent negative εHf(t) values of −6.2 to −2.8 and TDM2 ages of ca. 2.97–3.16 Ga, indicative of Mesoarchean crustal materials as their magma source. Considering the existence of the ca. 2.36 Ga regional high-grade tectono-metamorphic event, we propose that the ca. 2.36 Ga monzogranites were most likely generated in the early stage of post-collisional extension. Therefore, the southwestern Yangtze Block likely experienced a collision event during ca. 2.40–2.36 Ga, indicative of the involvement of the Yangtze Block in the ca. 2.5–2.3 Ga Arrowsmith orogeny of Laurentia. In addition, a series of accretionary events might also have occurred in the western Yangtze Block before the final assembly of the Columbia supercontinent, implying a tight spatial connection between the Yangtze Block and northwestern Laurentia within this global-scale supercontinent.

Geochemical evidence for K-metasomatism related to uranium enrichment in Daejeon granitic rocks near the central Ogcheon Metamorphic Belt, Korea

A new type of uranium occurrence in Korea was identified in pegmatitic and hydrothermally altered granite in the Daejeon area. The U-bearing parts typically include muscovite, pink-feldspar and sericite as alteration minerals. In this study, the geochemical characteristics and alteration age of the granitic rocks were examined to provide evidence for hydrothermally-enriched uranium. The K-Ar ages of muscovite coexisting with U-bearing minerals were determined as 123 and 128 Ma. The U-bearing rocks have relatively low (CaO + Na2O), high K2O contents, and high alteration index values by major element geochemistry. The trace element geochemistry shows that the uraniferous rocks have significantly low Th/U ratios and strongly differentiated features. The rare earth element patterns indicate that the uraniferous rocks have a low total REE and LREE contents with depletion of Eu. Considering the geochemical variation of the granitic rock major, trace and rare earth elements, it can be concluded that uranium enrichment in pegmatites and altered granite should be genetically related to post-magmatic hydrothermal alteration of K-metasomatism after emplacement of the two-mica granite. This is the first report for geochemical characteristics of Mesozoic granite-related U-occurrences in South Korea. This study will help further research for uranium deposits with similarities in geological setting, mineralogy and age data between South China and Korea, and can also be expected to help solve the source problems related to high uranium concentrations in some groundwater occurring in the granitic terrane.

Petrogenesis of Triassic granite from the Jintan pluton in Central Jiangxi Province, South China: implication for uranium enrichment

Numerous Triassic biotite granites and two-mica granites crop out in the interior of South China, and some of them possess high U contents, which have been regarded as the sources for later hydrothermal mineralization. Their petrogenesis is therefore crucial for constraining the possible origins of the U enrichment. Here we report new LA-ICPMS zircon UPb ages, mineral geochemistry of biotite and muscovite, whole rock geochemical results and SrNd and zircon Hf isotope data from the Jintan pluton in Central Jiangxi Province, South China. LA-ICPMS zircon UPb dating indicates that both biotite granite (BG) and two-mica granite (TMG) in the Jintan pluton crystallized at ~220 Ma. The TMG have higher U contents (7.85 to 48.90 ppm, average18.44 ppm) than theBG (4.99 to 17.72 ppm, average 8.64 ppm). Both BG and TMG show negative whole-rock εNd(t)and zircon εHf(t)values and contain some inherited zircons. The TMG are strongly peraluminous (A/CNK = 1.13–1.33), contain abundant primary muscovite, and display S-type affinity on plots of Yvs Rb and Th vs Rb, suggesting that they are S-type granites. The BG also display S-type granite affinities on plots of Yvs Rb and Th vs Rb. The suites display similar SrNd isotope compositions (BG initial 87Sr/86Sr values = 0.711389 to 0.714225 and εNd(t) = −9.91 to −9.16, TMG initial 87Sr/86Sr values = 0.711832 and εNd(t) = −10.02) and are spatially associated, suggesting that the BG should also be classified as S-type granites. The TMG have higher zircon εHf(t)values (−6.4 to −1.1) than the BG (−8.7 to −3.7), indicating the TMG and BG might be derived from similar sediments but possibly with some distinct characteristics in their sources. The BG exhibit linear covariations in chemical compositions with relatively high total REE and light REE contents and MgO contents, while the TMG displays broader compositional variations but with relatively low total REE, light REE and MgO contents. Biotite geochemistry indicates the TMG formed in a more reduced magmatic system than the BG. The temperatures estimated by zircon saturation thermometry indicate the BG had distinctly higher magmatic temperatures than the TMG. The TMG display relatively high Al2O3/TiO2 ratios and low CaO/Na2O ratios than the BG but have higher Sr/Y and La/Yb ratios. The geochemical and petrological data suggest the BG were derived from clay-poor psammite sources at deeper levels with higher temperatures and higher oxygen fugacity, and underwent an extensive fractional crystallization, while the TMG was derived from clay-rich pelitic sources at higher levels and lower temperatures and oxygen fugacity with only limited fractional crystallization. We conclude that the combination of U-rich sources, physical-chemical conditions such as low partial melting temperature or low degrees of partial melting, a reduced environment and low REE and LREE contents of magmas controlled the U enrichment in TMG.