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Abstract The Dunde iron–zinc deposit is a typical submarine volcanogenic iron oxide deposit in the eastern Awulale Metallogenic Belt of the western Tianshan. We report new petrography, rare earth elements (REE), and fluid inclusion microthermometry and C–O–Sr isotopes of the calcite from the skarn and the magnetite ores from the Dunde deposit. Calcite from both the skarn and magnetite ores displays high LREE/HREE (44–172), implying significant REE fractionation. We infer that the complexation of Cl (± F) during the fluid circulation and mineralogical control of calcite at the depositional sites were important for the LREE enrichment in the Dunde calcite. The restricted Y/Ho (26.7–32.1) suggests that calcite in the skarn and magnetite ores was co-genetic and the superchondritic Y/Ho may have resulted from the preferential incorporation of Y over Ho in calcite. Positive Eu anomaly (1.23–1.63) of the skarn calcite may have been inherited from the parental fluids, while the negative or slightly positive anomaly (0.65–1.04) of the magnetite ore calcite may have been resulted from the high Sr2 + content that suppressed Eu2 + in calcite. Fluid inclusion microthermometric results exhibit a wide range of homogenization temperatures (106 °C–364 °C) and salinities (0.2 wt.%–41.5 wt.% NaCl equiv.). Three kinds of ore-forming fluids can be distinguished, i.e., high-salinity (> 35 wt.% NaCl equiv.), medium-salinity (10 wt.%–35 wt.% NaCl equiv.) and low-salinity (< 10 wt.% NaCl equiv.) fluids. The high-salinity fluid was likely to be magma-derived and occurred in both the skarn- and magnetite ore calcite. The low-salinity fluid, possibly seawater-derived, was found only in the skarn calcite. The medium-salinity fluid, mainly of CaCl 2 –H 2 O aqueous- and brine-featured fluids, may have been the product of the fluid–rock interactions as the magmatic fluids and seawater circulated through the strata. The Dunde calcite has δ13C comparable to marine carbonates, but δ18O substantially lower, suggesting possibly a result of limestone dissolution. Calculated δ18O of the fluids in equilibrium with calcite vary from − 0.78‰ to 5.46‰ (− 0.78‰–1.85‰ and 2.15‰–5.46‰ for skarn calcite and magnetite ore calcite, respectively), implying magmatic fluids and seawater mixing. The Dunde calcite 87Sr/86Sr (0.7071 to 0.7078) correlate positively with 1/Sr, implying that two sources, i.e., basaltic/andesitic volcanic wall rocks (87Sr/86Sr: 0.7056–0.7066) and limestones/seawater (87Sr/86Sr: 0.7075–0.7089), may have been involved in the mineralization. Mass-balance calculation between fresh basaltic tuff and its neighboring skarn showed that extra Ca, Mg and Fe were added to the basaltic tuff during the hydrothermal alteration. The limestone dissolution during the fluid–rock interactions may have added Ca to the ore-forming fluids, while Fe and Mg may have been introduced from the deep-seated magmatic system via the heated Cl-rich fluid circulation. Graphical abstract Highlights • The Cl (± F) complexation and mineralogical control account for REE fractionation. • Magmatic fluid, seawater and brine-featured fluid were involved in ore-forming process. • Deep magmatic system instead of basaltic wall rocks provided iron for mineralization. [ABSTRACT FROM AUTHOR]

[Display omitted] • Trachyte developed due to protracted magmatic differentiation of mafic magmas. • Rare metals initially enriched through magmatic differentiation. • Hydrothermal processes cause rare metal mineralization and precipitation. South Qinling hosts abundant alkaline igneous rocks characterized by rare metal mineralization, which, however, is not well understood, particularly for those extrusive alkaline magmatic systems. This study focuses on the Tianbao trachyte Nb(-REE) deposit located in the North Daba Mountains (South Qinling) and investigates textures and geochemistry of ore-bearing trachytes and Nb-REE minerals to reveal relative contributions of magmatic and hydrothermal processes to rare metal enrichment. Low K 2 O/Na 2 O ratios (0.26–1.64), depletions of Ba, Sr, and enrichments of Nb, Ta, Zr, Hf in the Tianbao trachytic complex resemble geochemical signatures of rifting-related alkaline igneous rocks, highlighting an intraplate rifting origin. Positive εNd(t) values (3.1 – 3.6) and constrained La/Yb (21.2 – 35.8) and Nb/Zr (3.7 – 4.8) ratios indicate that they might have originated from mixing of the asthenospheric mantle and metasomatized lithospheric mantle and formed due to protracted magmatic differentiation of coeval mafic magmas. Niobium and REEs in the Tianbao trachytic complex primarily originated from an enriched mantle source, and then upgraded to initial enrichment due to intense differentiation of the trachytic magmas. Niobium and REEs in the trachytic magmas were firstly incorporated into magmatic mineral phases (e.g., biotite, niobian titanite, niobian ilmenite and apatite) under relatively reduced (below the Ni-NiO oxygen fugacity buffer) conditions. With the differentiation of trachytic magmas, niobium, REEs and anion ligands (F-, CO 3 2– and PO 4 3-) got saturated in the late-stage alkaline melts and then entered the exsolved fluids. Ca-rich components should have been leached from the magmatic minerals during the hydrothermal stage and reacted with these ligands to develop Ca-rich hydrothermal mineral phases, including fluorite and calcite veins, allanite and apatite overgrowth under relatively oxidized (between the Ni-NiO and hematite-magnetite oxygen fugacity buffer) conditions. The remaining Nb and REEs finally precipitated as the economic minerals (e.g., allanite-(Ce), loparite-(Ce) and niobian Y-rich zircon) along fractures. Mass balance calculations indicate that these hydrothermal economic mineral phases account for the majority of Nb-REE reserve in the trachytic complex, and thus, the hydrothermal process might have played the key role in leading to Nb and REE concentrating to ore grade. [ABSTRACT FROM AUTHOR]

[Display omitted] • Maoniuping syenite and carbonatite are formed by liquid immiscibility. • Syenite and carbonatite underwent independent magmatic–hydrothermal evolution. • Mica can be used as an indicator for the genesis of alkaline–carbonatite complex. Mica, as a long-lasting mafic mineral in the magmatic–hydrothermal evolution of alkaline–carbonatite complexes, is an ideal monitor of petrogenesis and mineralization processes. Here, the compositions of mica in the bulk syenite, carbonatite and their related hydrothermal veins from the Maoniuping rare earth elements (REE) deposit (SW China) were analyzed in detail, in order to reveal the petrogenesis and REE mineralization processes of the Maoniuping syenite–carbonatite complex. Micas in the bulk syenite and carbonatite are common phlogopite with indistinguishable compositions, while micas in the hydrothermal veins of syenite and carbonatite are quite different from that in the bulk syenite and carbonatite and compositionally close to ideal tetraferriphlogopite. The common phlogopite is interpreted as orthomagmatic xenocryst crystallizing from parent magma system, whereas tetraferriphlogopite precipitates in the REE-enriched hydrothermal fluids. The compositional variations of mica show that Fe and F contents increase, whereas Al and Ti contents and Nb/Ta ratios decrease evolving from common phlogopite to tetraferriphlogopite. Compared to common phlogopite, tetraferriphlogopite has higher levels of REE (up to 77 ppm), Ba (up to 5000 ppm), Sr (up to 700 ppm), Li (up to 14000 ppm), Rb (up to 1700 ppm), and Na (up to 1200 ppm). The overlapped chemical compositions of common phlogopite in the bulk syenite and carbonatite support that the liquid immiscibility of syenitic and carbonatitic melts from a common parent magma, which may happen due to moderate crystal fractionation induced by decreasing temperature/pressure in the crustal magma chamber. Meanwhile, the presentation of tetraferriphlogopite in the hydrothermal veins of syenite and carbonatite indicates that syenitic and carbonatitic melts subsequently experienced independent magmatic–hydrothermal evolution path after separation, leading to REE mineralization of syenite and carbonatite respectively. The tetraferriphlogopite in the hydrothermal veins of carbonatite has higher oxygen fugacity and higher contents of incompatible elements (e.g., Li, Rb, Na, F, Sr, Ba, and REE) than that in the hydrothermal veins of syenite, which indicates that the carbonatite-derived fluids have higher REE mineralization potential than the syenite-derived fluids. Therefore, we conclude that liquid immiscibility and separated magmatic–hydrothermal evolution may be the key processes controlling the formation of the giant Maoniuping REE deposit. [ABSTRACT FROM AUTHOR]

The Baerzhe alkaline granite pluton hosts one of the largest rare metal (Zr, rare earth elements, and Nb) deposits in Asia. It contains a geological resource of about 100 Mt at 1.84 % ZrO, 0.30 % CeO, and 0.26 % NbO. Zirconium, rare earth elements (REE), and Nb are primarily hosted by zircon, yttroceberysite, fergusonite, ferrocolumbite, and pyrochlore. Three types of zircon can be identified in the deposit: magmatic, metamict, and hydrothermal. Primary magmatic zircon grains occur in the barren hypersolvus granite and are commonly prismatic, with oscillatory zones and abundant melt and mineral inclusions. The occurrence of aegirine and fluorite in the recrystallized melt inclusions hosted in the magmatic zircon indicates that the parental magma of the Baerzhe pluton is alkali- and F-rich. Metamict zircon grains occur in the mineralized subsolvus granite and are commonly prismatic and murky with cracks, pores, and mineral inclusions. They commonly show dissolution textures, indicating a magmatic origin with later metamictization due to deuteric hydrothermal alteration. Hydrothermal zircon grains occur in mineralized subsolvus granite and are dipyramidal with quartz inclusions, with murky CL images. They have 608 to 2,502 ppm light REE and 787 to 2,521 ppm Nb, much higher than magmatic zircon. The texture and composition of the three types of zircon indicate that they experienced remobilization and recrystallization during the transition from a magmatic to a hydrothermal system. Large amounts of Zr, REE, and Nb were enriched and precipitated during the transitional period to form the giant low-grade Baerzhe Zr-REE-Nb deposit. [ABSTRACT FROM AUTHOR]

[Display omitted] • Alkaline mafic dikes were coetaneous with the REE mineralization. • Alkaline mafic magma has been influenced by subducted materials. • The REE-rich carbonatitic melts mix with the alkaline mafic magma. • The REE-rich carbonatitic melts formed by melting of subducted sediments. Carbonatite-associated rare earth element (REE) deposits are the most important source of REEs worldwide. Recycled subducted sediments are critical for REE mineralization, but it remains unclear how the subducted sediments contribute to the REE mineralization. This study presents new whole-rock and apatite geochemical data for alkaline mafic dikes from the Lizhuang REE deposit, southwest China. These alkaline mafic dikes were emplaced at ca. 28.4 Ma (monazite U-Pb age), broadly coeval with the carbonatite–syenite complex and REE mineralization in the Lizhuang REE deposit. The alkaline mafic dikes have enriched Sr-Nd-Pb isotopic compositions, and variable Ba/Th and La/Sm ratios, demonstrate they were derived from an enriched mantle source, which had been modified by both slab-derived fluids and sediment melts. The apatite grains in the alkaline mafic dikes have distinctive geochemical features in their irregular cores (apatite A) and rim overgrowths (apatite B). Apatite A has low REE contents (18.6–161 ppm) and apatite B has high REE contents (1869–4288 ppm). The different REE contents in these apatites could have been caused by the addition of REE-rich carbonatitic melts at lower crustal depths. We propose that the REE-rich carbonatitic melts could be formed by melting of a sediment-rich mantle source, and that collision between India–Asia triggered the melting. [ABSTRACT FROM AUTHOR]

The Baerzhe Cretaceous (123.7 ± 0.9 Ma) peralkaline pluton underwent extensive fractional crystallization and extreme enrichment in incompatible elements including rare earth elements (REE) and the high field strength elements (HFSE). It is one of the largest resources of rare metals in China, containing approximately 100 million tons of ore with an average grade of 1.84 wt% ZrO 2 , 1.00 wt% REE 2 O 3 (34% heavy rare-earth oxides), and 0.26 wt% Nb 2 O 5. Zr, REE and Nb are mainly hosted by hydrothermal minerals, such as zircon, hingganite-(Y), monazite-(Ce), polycrase, pyrochlore, fergusonite and columbite. An integrated investigation of field geology, mineral textures and compositions of minerals and host granites was carried out to examine the evolution of the Baerzhe pluton and the roles of magmatic and hydrothermal processes in concentrating REE and HFSE. Most minerals in the intensively altered subsolvus granite show secondary textures or replaced pseudomorphs, such as the replacement of arfvedsonite by aegirine, zircon dissolution and reprecipitation, and the replacement of monazite-(Ce) and polycrase-(Y) by hingganite-(Y). Compositions of the key economic minerals and changes with respect to these alteration stages reveal evidence that hydrothermal alteration played a role in the mineralization of the pluton. In-situ analyses and element mappings suggest that large volume of metals were remobilized and redistributed during hydrothermal replacement, such as the replacing of monazite-(Ce) and polycrase by hingganite-(Y). It is suggested that subsolidus re-equilibration and hydrothermal alteration, in addition to magmatic fractionation, is critical for further concentrating REE and HSFE in peralkaline granitic systems. • Enrichment of REE and HFSE in the Cretaceous Baerzhe peralkaline pluton. • Rare metals concentrated in the upper subsolvus granite with intensive alteration. • Remobilization and redistribution of rare metals during hydrothermal processes. • Subsolidus re-equilibration and alteration was critical to mineralization. [ABSTRACT FROM AUTHOR]

[Display omitted] • Allanite is the main LREE contributor to the Bachi ion-adsorption LREE deposit. • F−-rich, medium–low-temperature, and oxidizing fluids interacted with the allanite. • LREE is migrated from allanite to synchysite-(Ce) during hydrothermal alteration. • Altered allanite show less stable mineral structure and is more easily weathered. Most ion adsorption rare earth element (REE) deposits (>85%) are light REE (LREE) enriched, but it is unclear whether the magmatic and hydrothermal activity have a control to the LREE mineralization, and why only some granitoids generate ion adsorption LREE deposits despite similar weathering conditions and REE contents. In this study, we investigated the main LREE-bearing mineral (i.e., allanite) in the bedrocks of the large Bachi ion adsorption LREE deposit (REE oxide reserves of 42,589 t) in South China. The allanite hosts at least 80% of the LREE in the bedrocks of the Bachi deposit. The allanite comprises primary (Aln-1) and altered (Aln-2) allanite. Aln-2 shows larger lattice parameters, smaller diagnostic Raman spectroscopy peaks, with extensive micro-pores and -fractures infilled by synchysite-(Ce), indicating a lower crystallinity and less stable mineral structure. In addition, Aln-2 has lower LREE contents, higher Sr, Ba, and Th contents, more negative Eu anomalies, and less positive Ce anomalies than Aln-1, but both types of allanite have similar Nd isotopic compositions with those of the whole-rock samples. These morphological and geochemical features suggest that F−, CO 2 , and Th4+-rich, REE-undersaturated, medium–low-temperature, and oxidizing fluids interacted with the allanite, leached LREE from the allanite, and then formed synchysite-(Ce). The LREE leached from the allanite and precipitated as synchysite-(Ce) on a micron scale, and external materials carried in the fluid did not make a significant contribution to the REE budget and Sm–Nd isotopic compositions of the allanite. The structural and geochemical changes from Aln-1 to Aln-2 made it easier for the allanite to be weathered, and synchysite-(Ce) was also easily weathered, which released LREE3+ ions and generated the ion adsorption LREE deposits. This study highlights that the LREE could be migrated from allanite to synchysite-(Ce) during hydrothermal alteration, which contributed to the generation of ion adsorption REE deposits. The LREE mobilization during hydrothermal alteration may explain why only some granitoids generated ion adsorption LREE mineralization in South China. [ABSTRACT FROM AUTHOR]

Understanding the enrichment of heavy rare earth elements (HREE) and high HREE/LREE (light REE) ratios in the granites is critical to understand the ore-genesis of exclusive ion-adsorption HREE deposits in South China. In this study, we conducted geochronological, whole-rock and apatite geochemical, and isotopic studies on the Xinfeng pluton, of which only parts show high HREE/LREE ratios and are ore-related. Both the ore-related granite and ore-unrelated granite from the Xinfeng pluton are characterized by high SiO 2 contents, the same Late Jurassic age (ca. 155 Ma), and enriched Nd isotopic compositions. However, the ore-related granite has lower TiO 2 , CaO, P 2 O 5 , Fe 2 O 3, Ba, Sr, Eu, and LREE contents, lower La/Yb, Zr/Hf and Th/U ratios, higher Rb, Cs, Nb, Ta, Pb, and HREE contents, and higher Rb/Sr ratios than the ore-unrelated granite. These features indicate that the ore-unrelated and ore-related granite are cogenetic and their geochemical differences can be explained by magmatic differentiation. The primary igneous apatite from both the ore-unrelated granite and ore-related granite shows negligible hydrothermal alteration. Fractional crystallization of feldspar (∼20%) and allanite (< 0.3%) led to higher HREE contents and HREE/LREE ratios, lower Sr contents, and a more pronounced negative Eu anomaly in the apatite from the ore-related granite. This study highlights that the magma evolution via mineral fractional crystallization could achieve the relatively high HREE contents and HREE/LREE ratios in the granites, and favors the formation of exclusive ion-adsorption HREE deposits in South China. • The ore-unrelated and ore-related granites are cogenetic Jurassic granites. • ∼20% feldspar and < 0.3% allanite fractionation crystallization occurred. • The igneous apatite without hydrothermal alteration records the HREE enrichment. • High HREE/LREE ratios achieved by magmatic differentiation. [ABSTRACT FROM AUTHOR]

• Garnet texture and composition record the transition process of granitic magma from volatile undersaturated to oversaturated. • High-HREE garnet have high potential for forming ion-absorption HREE deposit. • Garnet should be regarded as a reliable indicator for ion-absorption HREE deposits. Ion-absorption rare earth element (REE) deposits in South China are the world's most important source of heavy REEs (HREEs). These deposits were formed by the weathering of granitic rocks whose formation involved primary HREE enrichment. Previous studies have identified the key role of late-stage magmatic evolution, especially the magmatic–hydrothermal transition stage played in HREE enrichment, but the detailed processes need further investigation. Garnet is a common HREE carrier in parent rocks and also a main contributor of these elements in formation of ion-absorption HREE deposits. Here, we investigate textural and compositional variations in garnets from parent rock (muscovite granite) of the Dabu ion-absorption HREE deposit to constrain the primary HREE enrichment of the parent rock during late-stage magmatic evolution. Mass-balance calculations reveal that garnet accounts for ∼67 % of the Y and 64 % of the REEs in the Dabu muscovite granite. The garnets can be classified into three types: i) magmatic garnets (Grt-1A) are intergrown with plagioclase, K-feldspar, and quartz, host both melt and mineral inclusions, and have high REE + Y contents (6488–19,215 ppm); ii) magmatic–hydrothermal garnets (Grt-1B) occur as overgrowths on Grt-1A, host both melt and fluid inclusions, and have intermediate REE + Y contents (2681–8683 ppm); and iii) hydrothermal garnets (Grt-2) are intergranular with quartz and altered biotite, host primary fluid inclusions, and have the lowest REE + Y contents (476–1247 ppm). The texture and composition of the three types of garnet indicate that the magma have undergone a transition from a volatile-undersaturated to a volatile-oversaturated aqueous system. The fluid, from which some REE minerals precipitated, present in the magma system was derived from the magma itself rather than from an external source, as evidenced by the similarity in Nd isotopic composition between the REE minerals and the whole-rock samples. During this transition, the presence of high-HREE garnet prevents the HREE partitioning into refractory minerals (e.g., zircon, REE-bearing phosphate) or extracting from the magma system by the fluid. Our findings show that granites containing high-HREE garnet have high potential for forming ion-absorption HREE deposits and that garnet can reliably record their magmatic evolution. [ABSTRACT FROM AUTHOR]

[Display omitted] • The Huanglongpu carbonatite is a primary carbonatite. • The mantle source was modified the recycled slab sediments. • HREE and Mo sourced from addition of eclogite facies sediments into the mantle. Carbonatites are the most important sources of global niobium (Nb) and rare earth element (REE) resources, particularly the light REEs. The Huanglongpu carbonatite in central China is unique for the enrichment of molybdenum (Mo) and heavy REEs. Till now, the source nature of the Huanglongpu carbonatites is still poorly understood. In this contribution, we present elemental and C-O-Sr-Nd-Pb isotopic data of calcite to constrain the source signatures of the Huanglongpu carbonatite. Elemental geochemistry and C-O isotopes of the calcite vein-networks in the carbonatite reveal that the Huanglongpu carbonatites is of igneous genesis and originated from mantle-derived carbonatitic melts. The negative εNd(t) values (−8.03–−5.05) and elevated initial 87Sr/86Sr ratios (0.7049–0.7056) indicate considerable contribution of recycled crustal components in the mantle source. Combination of O-Sr-Nd-Pb isotopes suggests that over 10 wt% of recycled sediments might have been involved into the depleted mantle, accounting for the source of the Huanglongpu carbonatite. Considering the enrichment of heavy REE, REE distribution simulation indicates that the Huanglongpu carbonatite might be from the 30% partial melting of eclogite facies sediments. Accordingly, we infer that the Huanglongpu carbonatite might have been derived from the partial melting of a hybridized mantle source modified by recycled sediments of eclogite facies. The enrichment of Mo and heavy REEs plausibly resulted from the contribution of garnet/pyroxene from the eclogitic sediments within the subducted slab. [ABSTRACT FROM AUTHOR]

[Display omitted] • Two episodes of peralkaline magmatism occurred at Tudiling deposit. • Protracted fractional crystallization initially controlled rare metal enrichment. • Effects of hydrothermal alteration on upgrading rare metal depend on rock structure. Relative contribution of magmatic and hydrothermal processes to rare metal enrichment has been a key issue for the alkaline-associated rare metal deposits. Till now, our understanding on the rare metal concentration of extrusive peralkaline magmatic systems is rather limited. We try to address this issue by studying the Tudiling trachyte Nb-Ta-Zr-REE deposit in the South Qinling (SQ), which is well endowed with abundant alkaline-associated rare metal deposits. Four lithofacies (i.e., trachyte, trachytic tuff, trachytic phyllite and trachytic ignimbrite) with rare metal mineralization were thoroughly investigated on their emplacement ages, whole-rock geochemistry, and Nb-REE mineralogy to decipher the evolution of the system and the concentration mechanisms for the rare metals. Zircon U-Pb dating indicates that the former three lithofacies developed during the Early Silurian (445–442 Ma) and are coeval with their intrusive counterparts, i.e., the Miaoya and Shaxiongdong syenites in the SQ. They share similar element distribution patterns (Nb-Ta, Zr-Hf, LREE enrichments, and Sr, P, Ti depletions) and moderately depleted Nd isotopes (εNd(t) values: 2.3–3.1), indicating a common mantle source. In contrast, the trachytic ignimbrite erupted during the late Jurassic (157 ± 1.9 Ma) and has enriched Nd isotope compositions (εNd(t) values: −2.9–−2.8), implying an enriched mantle source, which had probably been modified by the residuals of the Mianlue oceanic sediments. Their bulk-rock rare metal concentrations indicate that the former three lithofacies are characterized by Nb-Ta-Zr mineralization/enrichment with the phyllite containing the highest grades, while the latter one has REE mineralization as well as Nb-Ta-Zr mineralization/enrichment. Our geochemical and mineralogical results suggested that rare metal enrichment in these trachytic rocks was initially attributed to the partial melting of an enriched mantle source and subsequent protracted fractional crystallization of alkaline basaltic magmas. Effects of late-stage hydrothermal alteration on rare metal upgrading are highly dependent on rock structures and distinct on the intensively altered phyllite and ignimbrite. [ABSTRACT FROM AUTHOR]

[Display omitted] • Primary bastnäsite can be altered to parisite and synchysite. • Bastnäsite U-Pb age of 141.8 ± 4.3 Ma represents the timing of REE mineralization. • Synchysite U-Pb age of 53.3 ± 4.4 Ma indicates a late tectono-thermal event. • REE fluorocarbonates U-Pb geochronology is a powerful tool for dating geological processes. The Gansha Obo deposit is a recently discovered rare earth element (REE) deposit in northwestern China, with REE 2 O 3 reserves of 0.6 Mt at 1.39–1.65 wt%. It is hosted in the Early Cretaceous Gansha Obo alkaline–carbonatite igneous complex. In this study, we present a detailed investigation of the carbonate mineral assemblage in the economic carbonatite veins in the Gansha Obo REE deposit, including the mineral textures, geochemical compositions, and U–Pb ages. The mineral textures and compositions show that bastnäsite is the primary ore mineral, while parisite and synchysite are both secondary. In situ U–Pb dating of bastnäsite and synchysite yielded ages of 141.8 ± 4.3 Ma (n = 41; MSWD = 0.53) and 53.3 ± 4.4 Ma (n = 85; MSWD = 1.7), respectively. This significant interval of ages cannot be explained by continuous magmatic–hydrothermal activity. The bastnäsite U–Pb age reflects the timing of REE mineralization in the deposit. In contrast, the synchysite U–Pb age records a later tectono-thermal event that might have been related to Eocene collision between the Indian and Eurasian plates. Our results show that REE carbonate U–Pb dating is a powerful tool for understanding the precipitation of REE minerals and, potentially, for identifying regional tectono-thermal events. [ABSTRACT FROM AUTHOR]

[Display omitted] • Maoniuping parental magma is composed of exotic hybrid partial melts. • Subducted marine sediments contributed to the enrichment of REE in deep mantle. • Boron isotopes can be used a tracer of subduction zone processes. The Maoniuping giant rare earth element deposit is genetically related to a Cenozoic alkaline syenite–carbonatite complex. However, the origin of the alkaline complex and the genesis of the Maoniuping deposit are still unclear. In this study, the origin of the Maoniuping complex was constrained through B–Sr–Nd–Pb isotopic data for fresh/unaltered alkaline syenites. Our results indicate that the Maoniuping alkaline syenite has arc-like trace element patterns with elevated initial 87Sr/86Sr (0.706040–0.706541), 206Pb/204Pb (18.00–18.32), 207Pb/204Pb (15.53–15.60), and 208Pb/204Pb (38.25–38.78) ratios, and relatively low ε Nd (t) values (–4.92 to –2.66), suggesting derivation by partial melting of fertile mantle source. It also has higher B contents (1.06–7.65 ppm) and heavier δ11B values (−5.38‰ to − 2.46‰) than the upper mantle, indicating modification of the mantle source by interaction with carbonatitic melts/fluids derived from marine sediments and altered oceanic crust within the subducting Indian slab. Our results, together with recently acquired seismic tomographic data, indicate that hybridisation in the mantle source was likely induced by subduction input of marine sediments to the mantle transition zone. Boron isotopes provide a novel indicator of the deep recycling of subducted crustal materials in the sources of alkaline magmas. [ABSTRACT FROM AUTHOR]

[Display omitted] • Two discrete stages of fenitization have been identified in the Lizhuang deposit. • The nature of fenitizing fluids is distinguishable on geochemistry. • Intensive alteration by fenitizing fluids are crucial to REE minerlization. Fenitization is an alkali metasomatism that widely occurs in and around the alkaline-carbonatite complex, and is closely associated with rare earth elements (REE) mineralization. However, the origin and nature of the fenitizing fluids and its role in REE mineralization are poor understood. The Lizhuang REE deposit in the Panxi region, SW China, is hosted in a syenite-carbonatite complex with low- and high-grade fenitization. In this study, we conducted detailed petrographic, compositional and isotopic studies on the fresh/unaltered syenite and low- and high-grade fenites to reveal the origin and nature of the fenitizing fluids and their effects on REE mineralization. Our results show that the whole-rock Na/K ratios gradually increase from the fresh syenite, low-grade fenites to high-grade fenites, as well as mineral replacements, such as K-feldspar is replaced by albite and Ca-rich clinopyroxene is replaced by Na-rich clinopyroxene, indicating that sodic fenitization occurred in the Lizhuang syenite system. In situ U-Pb dating of the unaltered and altered apatite shows that the hydrothermal alteration processes are nearly simultaneous with the emplacement of syenite and carbonatite during 27.4–25.3 Ma, maybe lasting for ~2 Ma. In addition, the altered regions of feldspar, clinopyroxene and apatite have in situ 87Sr/86Sr ratios ranging from 0.70580 to 0.70674, which is consistent in error with those of the unaltered regions and host rock syenite-carbonatite complex (0.70566–0.70666). It is suggested that the fenitizing fluids were exsolved from the syenite-carbonatite complex in a relatively closed system. As the increase of alteration degree, the proportion of hydrothermal minerals (e.g., barite, calcite, fluorite and bastnäsite) in fenite increased gradually, indicating that the fenitizing fluids are enriched in SO 4 2−, CO 3 2− and F−. This is also supported by the elevated F and SO 3 contents from the unaltered to altered apatite. The increasing REE concentrations, from the fresh syenite, low-grade fenite to high-grade fenite, indicate that REE minerals such as bastnäsite were precipitated from the fenitizing fluids during fenitization processes. In-situ analyses on the altered clinopyroxene and apatite show variable REE losses compared to the unaltered precursors, which indicates the successive remobilization and transferring of REE by fenitization fluids. Therefore, two discrete stages of fenitization have been identified in the Lizhuang REE deposit, resulted from fluids exsolved from the syenite and carbonatite, respectively. Meanwhile, such fenitization fluids are effective extracting agents of REE in post-magmatic stage, which is crucial to REE remobilization and precipitation to form economic deposit. [ABSTRACT FROM AUTHOR]

[Display omitted] • Hydrothermal zircon crystals contain primary fluid inclusion assemblages. • Hydrothermal zircon crystallized from boiling H 2 O-CO 2 -bearing hydrothermal fluid. • Hydrothermal zircon has low contents of HfO 2 (<1%), Y (<400 ppm), Ti (<4ppm) Zircon is a principal source of zirconium and hafnium, as well as a key mineral both for petrogenetic modeling and age dating. Although zircon occurrences have been reported from hydrothermal mineral systems, detailed studies on zircon that crystallized entirely from saturated hydrothermal fluid are rare. Here we report on hydrothermal zircon crystals dated at about 1911 Ma that crystallized from boiling high-salinity H 2 O-CO 2 hydrothermal fluid with fluid inclusion homogenization temperatures of 295 to 325 °C and the corresponding pressure of 71 bar. These zircon crystals were then overprinted by two stages of hydrothermal zircon veins at 1864 Ma and 528 Ma. The zircon veins crystallized from boiling hydrothermal fluids of moderate to low salinities with fluid inclusion homogenization temperatures and the corresponding pressures of 302 to 329 °C and 75 bar and 290 to 320 °C and 70 bar, respectively. The zircon crystals in our study show sharp and regular cathodoluminescent oscillatory zonation and low and consistent trace element concentrations and Th/U ratios that decrease with fluid salinity. The Th/U ratios of these zircons lie between the magmatic zircon values (>0.5) and metamorphic zircon values (≤0.1). The rare earth element concentrations and trace element spidergram patterns of the zircon samples do not record their hydrothermal origin. Importantly, the low concentrations of HfO 2 (<1%), Y (<400 ppm) and Ti (<4 ppm) can discriminate hydrothermal zircon from igneous and metamorphic zircons. Hydrothermal zircon is considered as an effective tool for the dating of mineral deposits and also represents one of the main sources of critical elements such as zirconium and hafnium. [ABSTRACT FROM AUTHOR]

This study presents major, trace element and Li-B-Sr–Nd–Pb-Hf isotope compositions of post-collisional (Permian) shoshonitic volcanic rocks (SVRs) from West Tianshan, NW China. Zircon U Pb dating indicates that the West Tianshan SVRs were mainly formed in the late Early Permian (272–279 Ma). They include absarokites, shoshonites and banakites, which have variable SiO 2 (46–63 wt%) and K 2 O (2.12–8.12 wt%) contents. They display enrichment in light rare earth elements (LREE) and depletion in heavy rare earth elements. They are enriched in large ion lithophile elements (e.g., La, Rb and Ba) and depleted in high field strength elements (e.g., Nb, Ta and Ti) with slightly negative to negligible Eu anomalies, pronounced positive Pb anomalies and low Nb/U (9.32 ± 6.54), Ce/Pb (0.14–10.0, average 5.03 ± 3.02), relatively high Nb/Ta (16.74 ± 2.72) and Zr/Hf (39.56 ± 6.06) ratios. They have relatively high Li contents (26.0–93.0 ppm) and B contents (6.19–84.7 ppm), and low but variable δ7Li (−4.0‰ to +2.4‰). They are characterized by low (87Sr/86Sr) i (0.7045–0.7068), positive ε Nd (t) values (+0.31 − +5.53), relatively low 206Pb/204Pb (18.16–18.65), 207Pb/204Pb (15.51–15.63) and 208Pb/204Pb (37.97–39.22) and positive ε Hf (t) (+5.4 − +12.5). These features consistently suggest a metasomatized enriched mantle source having affinities with an EM I-like reservoir for the SVRs. Source enrichment was generated dominantly by subducted slab related aqueous fluid and altered oceanic crust (AOC) melts. Dehydration modeling of Li and B isotopes and Li, Y, B, Nb trace element fractionation collectively show that the flat, deeply subducted and stagnated, eclogitic slab suffered progressive dehydration(~5–12%)and was distinctly 7Li-depleted. Based on these compositional features and model calculations, we suggest that West Tianshan SVRs had an enriched mantle source metasomatised by aqueous fluid and hydrous slab melt, and their parental magmas were derived from variable mixing of spinel and garnet peridotite (60Gt:40Sp − 80Gt:20Sp). Taking into account petrogenetic and regional tectonic data, we suggest that the West Tianshan SVRs developed in an intraplate setting during the late Early Permian. It is most probable that the Early Permian Tarim mantle plume, located in the south side of the study area, promoted Early Permian asthenospheric upwelling that resulted in melting of a fossil subduction-enriched source to produce shoshonitic volcanic rocks in the West Tianshan. • Lower δ7Li and wider range are evidently lighter than most volcanic suites world-wide. • A deeply subducted and stagnated eclogitic slab is one end-member of enriched source. • Parental magmas were from mixing of garnet and spinel peridotite (60/40–80/20). • SVRs formed in post-collision transition from subduction-related to intraplate setting. [ABSTRACT FROM AUTHOR]

[Display omitted] • Two type zircons and volatile-rich HREE mineral are distributed in the granites for all HREEs ion-adsorption deposits. • Highly fractionated granite to a magmatic-hydrothermal stage is vital to the generation of HREEs ion-adsorption deposits. • Long-term extensional setting in South China during Mesozoic favors to magmatic differentiation. • Zircons could uncover the HREEs ion-adsorption mineralization potential of granites. The key point for further prospecting of heavy rare earth elements (HREE) ion-adsorption deposit is to figure out the granites that could generate HREE ion-adsorption mineralization in weathering processes. In this study, we present a detailed study of zircons from granites associated with Zudong, Dabu, and Xinfeng HREE ion-adsorption deposits in South China. The zircons were studied with regards to their texture, crystallinity, U-Pb dating, and geochemistry. The zircons from these granites all can be subdivided into two types. The type-1 zircons show oscillatory zonation and have Th/U and Zr/Hf mass ratios of 0.4–1.0 and 30–50, respectively. These textural and geochemical features indicate crystallization in a fractionated magma. The type-2 zircons are unzoned, occasionally porous, and have a low crystallinity. They occasionally rim type-1 zircons. The type-2 zircons show significantly higher F, P, Hf, Th, U, and REE contents, but display lower ZrO 2 and SiO 2 contents and lower Th/U, Zr/Hf, La/Yb ratios than those of the type-1 zircons. These geochemical features are consistent with zircon formation in a volatile-HREE-rich magmatic-hydrothermal transition stage. Under these conditions, the HREEs were also hosted in volatile-rich REE mineral phases including synchysite-(Y), aeschynite-(Y), calcybeborosilite-(Y), and atelisite-(Y), which have been observed in these HREE deposits. These volatile-rich REE-mineral phases can easily be dissolved during weathering and release HREE3+ to generate ion-adsorption HREE deposits. Therefore, we conclude that the granitic magma progression to a volatile-HREE-rich magmatic-hydrothermal system is vital for the generation of HREE ion-adsorption deposits. The long-term Mesozoic extension of the South China favors the generation of highly fractionated granites and is thus important for the generation of HREE ion-adsorption deposits. Furthermore, the zircons generated in a volatile-rich environment could be used to determine the HREE ion-adsorption mineralization potential of granites. [ABSTRACT FROM AUTHOR]

[Display omitted] • Mesozoic A-type granites in the Bachi area are sourced from melting of felsic and mafic igneous rocks, respectively. • High-temperature, low-pressure and anhydrous source conditions favour REE enrichment in the granite. • Fast crustal magma generation prevents the separation of REE-rich accessory minerals from melts. • The generation of Mesozoic REE-rich granites in South China relates to long-term extension. Ion-adsorption type rare earth element mineralization occurs in weathered profile of three Mesozoic granites in the Bachi area, Guangdong Province, South China. In this study, we investigate the REE enrichment processes in these granites using geochemical and geochronological data. The three granites yield zircon U-Pb ages of ca. 189 Ma, 153 Ma and 94 Ma, respectively, and could be divided into Early Jurassic, Late Jurassic and Cretaceous granites. The Early Jurassic and Cretaceous granites show similar geochemical characteristics, i.e. high K 2 O and low CaO, Fe 2 O 3 , and MgO contents, low K 2 O/Na 2 O ratios, and depletion of Eu, Sr, Ba, Ti, and P. They are characterized by high HFSE (high field strength elements) contents and enriched isotopic compositions with ε Nd (t) values ranging from −5.4 to-2.7, and ε Hf (t) values ranging from −4.5 to 1.7. The Late Jurassic granite also shows Eu, Sr, Ba, Ti, and P depletion and high HFSE contents. It has, however, higher CaO, Fe 2 O 3 , MgO and lower K 2 O contents than the Early Jurassic and Cretaceous granites. The Late Jurassic granite also shows more enriched isotopic compositions with ε Nd (t) values ranging from −9.0 to −8.9, and ε Hf (t) values ranging from −11.1 to −6.1. In addition, the Late Jurassic granite comprises, in contrast to the Early Jurassic and Cretaceous granites, abundant inherited/captured zircons cores and accessory REE-rich minerals of allanite, titanite, apatite and fluorite. These Mesozoic granites have the geochemical affinity of A-type granite. The Zr saturation temperature is >820 °C for the Early Jurassic and Cretaceous granites, and ~900 °C for the Late Jurassic granite. The Early Jurassic and Cretaceous granites were derived from partial melting of felsic igneous rocks under high-temperature, low-pressure, and anhydrous conditions. The felsic igneous source could provide relatively high REE contents for the melts, and the high-temperature, low-pressure, and anhydrous conditions resulted in residual plagioclase and melting of REE-rich minerals in the source area, which favour to REE enrichment in the melt. The Late Jurassic granite, on the other hand, was derived from a more mafic source and associate with fast crustal magma generation as indicated by its abundant presence of inherited/captured zircon cores in the high-temperature condition. The fast magma generation of the Late Jurassic granite prevented the separation of REE-rich minerals from the melt resulting in REE enrichment. Further, the high-temperature, low-pressure, and anhydrous physicochemical conditions and fast magma generation, which favour to REE enrichment in the granites, are tightly associated with large-scale Mesozoic extension in South China. [ABSTRACT FROM AUTHOR]