Magmatic Evolution and Nb-Ta Enrichment of Early Jurassic Granitic Porphyry from the Shangxiahu Nb-Ta Deposit of the Nanling Range

The Shangxiahu Nb-Ta deposit is located in the Yongding region of Fujian Province, the south-eastern section of the Nanling Range, South China. A series of Nb-Ta deposits, associated with granitic porphyries, are present in the Yongding region. Nevertheless, the genesis of the Nb-Ta mineralisation remains practically ambiguous, and further study of the latest Nb-Ta granitic porphyries in the Nanling Range may prove beneficial in understanding the mineralisation mechanism. This paper presents a detailed analysis of the petrographic, geochemical, chronological and isotopic characteristics of the Shangxiahu granite porphyry. Shangxiahu granite porphyry is high-potassium subalkaline granite. The Rittman index (δ) is < 3.3, indicating a calc-alkaline signature. The aluminium saturation index (A/CNK) is >1.1, indicating a strong peraluminous nature. Furthermore, the rare earth elements diagram exhibits Eu-negative anomalies and an M-type ‘tetrad effect’, while the multi-element distribution patterns diagram shows an overall right-leaning trend. The SIMS zircon U-Pb age of the Shangxiahu granite porphyry is 183.2 ± 3.2 Ma, revealing the emplacement of the granite porphyry in the Early Jurassic. The high Zr + Nb + Ce + Y content and Ga/Al ratios, along with the zircon saturation temperature TZr value of 1033 °C, indicate that the Shangxiahu granite porphyry exhibits the characteristics of a highly differentiated A-type granite. The Sr-Nd isotope signatures indicate that the Nb-Ta-bearing magma was formed by the mixing of crust–mantle material. The evolution and mineralisation process of granite porphyry in the Shangxiahu Nb-Ta deposit can be divided into four principal stages: (1) the upwelling of Asthenosphere material due to the extensional background induced partial melting of the crust; (2) the mixing of mantle-derived magma and crustal-derived magma; (3) magmatic fractional crystallisation and (4) magma–hydrothermal interaction. Combined with the reported data of Yongding Daping niobium-tantalum mining area, it can be postulated that the enrichment of Nb-Ta ores in Shangxiahu is primarily the result of the upwelling of Asthenosphere material, which caused the partial melting of the niobium-rich Mesoproterozoic crust with high Nb/Ta value. The enrichment of Nb-Ta ore bodies in Daping is primarily attributed to the fractional crystallisation of the magma and the hydrothermal alteration, which is postulated to be associated with the presence of a crust exhibiting elevated Nb/Ta ratios in Yongding. Additionally, it is postulated that ore may be found in area with high crustal source components in the crust–mantle mixed zone. There may be high volatile-rich Nb-Ta orebodies and Sn-rich and W-rich orebodies at depth or in the surrounding area.