Search Results (4)

Fluorite, a key accessory mineral associated with rare earth element (REE) deposits, exerts a significant influence on REE migration and precipitation through complexation, adsorption, and lattice substitution within fluorine-bearing fluid systems. It therefore provides a valuable archive for constraining REE enrichment processes. The Weishan alkaline–carbonatite-related REE deposit, the third-largest LREE deposit in China, is formed through a multistage magmatic–hydrothermal evolution of the carbonatite system. However, limited mineralogical constraints on REE enrichment and precipitation have hindered a comprehensive understanding of its metallogenic processes and exploration potential. Here, cathodoluminescence imaging and LA-ICP-MS trace element analyses were conducted on fluorite of multiple generations from the Weishan deposit to constrain the physicochemical conditions of mobility and precipitation mechanisms of this REE deposit. Four generations of fluorite are recognized, recording progressive evolution of the ore-forming fluids. Type I fluorite, which coexists with bastnäsite and calcite, is LREE-enriched and exhibits negative Eu anomalies, indicating precipitation from high-temperature, weakly acidic, and reducing fluids. Type II fluorite occurs as overgrowths on Type I, while Type III fluorite replaces Type II fluorite, with both displaying LREE depletion and MREE-Y enrichment, consistent with cooling during continued hydrothermal evolution. Type IV fluorite, which is interstitial between calcite grains and associated with mica, is formed under low-temperature, oxidizing conditions, reflecting REE exhaustion and the terminal stage of fluorite precipitation. Systematic shifts in REE patterns among the four generations track progressive cooling of the system. The decreasing trend in La/Ho and Tb/La further suggests that these fluorites record dissolution–reprecipitation events and associated element remobilization during fluid evolution. Full article

The alkaline complex in the southwest region of Luxi Terrane of the North China Craton is spatially correlated with the newly discovered Longbaoshan REE deposit. Its petrogenesis, however, remains ambiguous. In this study, we present an integrated petrology, whole-rock geochemistry, sphene U-Pb and rare earth element data from the Longbaoshan alkaline complex to investigate the petrogenesis, magma source and tectonic evolution. The Longbaoshan alkaline complex consists of mafic to intermediate rocks of hornblende diorite and alkaline hornblende syenite porphyry, biotite monzonite porphyry and aegirine diorite porphyrite. The hornblende diorites show a composition of low SiO₂, high MgO, Fe₂O₃ and moderate Na₂O, CaO and are metaluminous and medium-to-high-K calc-alkaline. The hornblende syenite porphyries, biotite monzonites and argirine diorite porphyrites display a relatively higher content of SiO₂, Na₂O, K₂O and Al₂O₃ and lower contents of MgO, Fe₂O₃ and CaO and are metaluminous, peralkaline, high-K calcic-alkaline and shoshonite. The sphene U-Pb data shows that the parent magma of the hornblende diorite was emplaced at ca. 120 Ma. All these samples show a common depletion in Th, Nb-Ta and Zr-Hf and enrichment in large ion lithophile elements (e.g., Pb, Ba, Sr) and Light Rare Earth Elements. The magma may have experienced fractionation of pyroxene, amphibole, sphene, apatite and zircon during its evolution. The variable La content, La/Sm, Rb/Sr and (Ta/Th) _N ratios indicate that the parent magma may produce by partial melting of a mantle source that was interacted with sediment-derived melts in a subduction setting. Therefore, we propose that the parent magma of the Longbaoshan alkaline complex was derived from a lithospheric mantle which was metasomatized by sediment-derived melt in a prior subduction process. The enriched magma was emplaced through an extension process and experienced subsequent fractionation and assimilation with the continental crust during the rollback of the Paleo Pacific Ocean plate. Full article

The Weishan REE deposit is located in the southwest of the Luxi Terrane of the North China Craton (NCC), where a large number of lamprophyre dikes are spatially exposed with the deposit. Here, we report petrology, geochemistry and zircon U-Pb geochronology data for the lamprophyre of the Weishan REE deposit in order to develop constraints for the determination of the petrogenesis, magma source and evolution of the lamprophyre and the tectonic environment. LA-LCP-MS zircon U-Pb dating shows that the crystallization age of the lamprophyre is 125 ± 0.86 Ma. The geochemical data suggest that these lamprophyres have high levels of Al₂O₃, K₂O, MgO and alkalis, moderate level of Na₂O and low levels of SiO₂, Fe₂O₃ and TiO₂, and that they are enriched with LREEs (La, Ce) and LILEs (Rb, Ba) and depleted with regard to HREEs and HFSEs (Nb, Ta, Ti). They displayed negative εHf(t) values of −14.98 to −9.03, T_DM1 ages of 1.1–1.4 Ga and T_DM2 ages of 1.7–2.1 Ga, which suggest that the magma source originates from an enriched mantle. Low Rb/Sr and high Dy/Yb ratios suggest that the enriched mantle source was partially melted at the amphibole-bearing lherzolite garnet-facies. The high Ba/Th and Sr/Th ratios indicate that the enriched source was derived from subduction dehydration fluids of the oceanic crust. We propose that the mafic dike intrusions are consistent with an Early Cretaceous alkaline magma emplacement in an extensional setting, in which the magma was not contaminated by crustal material during its emplacement. Full article

The Luxi Terrane (eastern China) exposes widespread Early Cretaceous alkaline rocks, whereas their petrogenesis remains controversial, including fractional crystallization, partial melting and crustal contamination regime. Here, we present petrology, geochemistry, sphene U-Pb geochronology and trace element data from the syenogranite, quartz syenite and quartz monzonite of the Guandimiao alkaline complex rocks to investigate their petrogenesis. Geochemical data suggest that these alkaline rocks show alkalic and peralkaline characters, and high Ga/Al ratios, SiO₂, light rare-earth element (LREE), Zr and Nb, and low MgO, CaO, Eu contents, corresponding to A-type granites. Sphene trace elements in syenogranite and quartz monzonite show obvious fractionation between LREE and heavy rare-earth element (HREE) and high Th/U ratios, indicating a magmatic origin. They yield U-Pb lower intercept ages of 128 ± 2.3 Ma and 127 ± 1.3 Ma, representing the crystallization ages of these alkaline rocks. The negative correlations between CaO, Fe₂O₃ (Total), MgO, P₂O₅, TiO₂, MnO and the pronounced depletion in Nb, Ta and Ti suggest that the alkaline rocks were formed by fractional crystallization. Additionally, the positive correlation between La/Hf and La, Th and Th/V, Ce/Yb and K₂O, and Tb/Yb and Yb suggest that the alkaline melts are generated by partial melting. Such high Rb/Nb, (Th/Nb)_N and Nb/Th ratios indicate crustal contamination during the magma emplacement. We, therefore, propose the magma source of the alkaline rocks in the Guandimiao complex originated by partial melting of lithospheric mantle, which experienced fractional crystallization and crustal contamination processes during its emplacement. Such complex alkaline rocks were probably formed in an extensional back-arc setting induced by the retreat of the subducting Izanagi plate. Full article