四川冕宁牦牛坪稀土矿床成岩成矿年代学及其找矿意义OA

The Maoniuping deposit in Mianning,Sichuan,is one of the most significant rare earth element(REE)deposits,situated within the Mianning-Dechang Himalayan REE metallogenic belt in southwestern China.Its REE mineralization is closely associated with syenite-carbonatite complexes;however,uncertainties persist regarding the mineralization age and the petrogenetic evolution of these complexes.To clarify the magmatic evolution and mineralization mechanisms,this study carried out zircon U-Pb geochronology and whole-rock major and trace element analyses on alkali-feldspar granite porphyry and syenite from the deposit.The results yielded zircon ages of 25.4±0.3 Ma(MSWD=0.8,n=12)for the alkali-feldspar granite porphyry and 26.1±0.4 Ma(MSWD=1.4,n=16)for the syenite,suggesting that they were emplaced contemporaneously during the Himalayan orogeny.Geochemical analyses reveal that the alkali-feldspar granite porphyry is characterized by high SiOz(71.67％～73.67％);low MgO,CaO,and P2O5;no significant-negative Eu anomalies;and relative depletion in Ba,Sr,Ti,Nb,and Ta.Its REE distribution pattern mirrors that of the syenite.The results confirm both silicate and carbonate magmatic evolution pathways within the mining area.The 10000× Ga/Al ratio(2.6～3.5)and Zr+Nb+Ce+Y contents(179×10-6～960 × 10-6)classify the alkali-feldspar granite porphyry as an A1-type granite formed during intraplate extension within a collisional orogenic belt,marking the regional transition of tectonic stress from compression to lithospheric extension.LA-ICP-MS U-Pb dating of bastnäsite from ore veins yields ages concentrated between 32.7 and 27.6 Ma,which slightly predate the emplacement ages of the alkali-feldspar granite porphyry and syenite.This temporal relationship indicates that the mineralization is associated with early magmatic activity prior to the emplacement of the alkaline complex,suggesting that ore formation is a phased product of prolonged magmatic evolution.The late-stage alkali-feldspar granite porphyry,characterized by high silica,low volatiles,and rapid crystallization,did not undergo significant mineralization but serves as an indicator for the terminal phase of regional large-scale REE mineralization.This study broadens the known rock types within the deposit's complexes and deepens the understanding of mantle-derived alkaline magmatic evolution and mineralization patterns in collisional orogenic settings,offering insights for regional REE exploration.