Geology and mineralization of the Yangla polymetallic orefield in northwestern Yunnan, SW China: Age, petrogenesis and rare-metal metallogenic implications of aplite dykes

Abstract

Aplite dykes commonly represent more evolved magma and are closely associated with rare metal mineralization. However, the multiphase magmatism and rare metal mineralization of aplite dykes in the Yangla orefield have not been well-constrained. Here, we present new data on the Linong aplite dykes and the associated Jiangbian granodiorite in the Yangla orefield, including data of zircon U-Pb age and Hf isotopes, and lithogeochemistry and Pb-Sr-Nd isotopes, in order to elucidate their emplacement age, petrogenesis, and rare metal mineralization potential. The three aplite dykes studied were emplaced at 214.3 ± 5.1 Ma, 209.2 ± 5.2 Ma, and 205.5 ± 3.4 Ma. These Late Triassic aplite dykes have high SiO₂ (75.81–76.94 wt%) and K₂O (avg. 5.60 wt%) and are peralkaline–metaluminous (A/CNK = 0.86–0.93). They have high whole-rock (⁸⁷Sr/⁸⁶Sr)_i (0.6947–0.7094), ε_Nd(t) (−6.26 to − 5.77), (²⁰⁶Pb/²⁰⁴Pb)_t = 18.365–18.804, (²⁰⁷Pb/²⁰⁴Pb)_t = 15.688–15.715, (²⁰⁸Pb/²⁰⁴Pb)_t = 38.436–38.681, and zircon ε_Hf(t) =  − 8.10 to + 1.62. The Triassic (208–228 Ma) Jiangbian granodiorite is high-K calc-alkaline, metaluminous/weakly peraluminous (A/CNK = 0.52–1.19) with high Al₂O₃ (12.49–16.74 wt%). They have high (⁸⁷Sr/⁸⁶Sr)_i (0.7075–0.7102), whole-rock ε_Nd(t) (−6.12 to − 5.88), (²⁰⁶Pb/²⁰⁴Pb)_t = 18.155–18.384, (²⁰⁷Pb/²⁰⁴Pb)_t = 15.665–15.698, (²⁰⁸Pb/²⁰⁴Pb)_t = 38.579–38.719, and zircon ε_Hf(t) =  − 4.20 to + 2.40. These geochemical features imply that both the aplite dykes and granodiorite have their primary magma derived from partial melting of the Proterozoic basement, with minor mantle input. The aplite samples have an average estimated crystallization temperature of 712 ℃, and relatively low magmatic oxygen fugacity ([log(fo₂)] = − 21.98 to – 5.48, avg. = –17.05), which are slightly higher than that of most Triassic granitoids in the Yangla orefield. The result shows that the crystallization temperature and oxygen fugacity gradually increase, while the ¹⁷⁶Hf/¹⁷⁷Hf and ε_Hf(t) gradually decrease. Our new and published geochemical data from the Yangla granitoids reveal that the aplite dykes and granodiorite were formed in a post-collisional setting. Whole-rock Nb/Ta, Zr/Hf, Rb/Sr, Ba/Rb, and zircon REE features of the aplite dykes highly mimic those of typical Sn-W related granites, suggesting certain Sn-W rare metals mineralization potential in the Yangla orefield.