Petrogeochemistry, chronology, Sr-Nd-Pb isotopic compositions, and geological significance of the ore-causative intrusion in the Langdu skarn deposit, northwest Yunnan

The Langdu intrusion in northwestern Yunnan is located in the Zhongdian arc of the southern Yidun terrane and it belongs to the ore-causative intrusion of Langdu skarn-type copper deposit, spatially. This study focuses on the quartz-diorite porphyry within the Langdu ore-bearing intrusion and investigates its zircon SIMS U-Pb dating, rock geochemistry, and Sr-Nd-Pb isotopic composition. The zircon U-Pb dating results reveal a crystallization age of 219.2±1.8 Ma (2σ, MSWD=1.3, n=13) for the Langdu quartz-diorite porphyry, which is consistent with the late Triassic intrusive rocks in the Zhongdian arc. The Langdu quartz-diorite porphyry exhibits medium SiO₂ (60.42%–66.44%), high Al₂O₃ (14.56%–16%), and high K₂O (3.49%–4.49%) contents. It is characterized by enrichment of LREE and large ion lithophile elements such as Rb, Ba, and Sr, and relative depletion of HREE and high field strength elements such as Ta, Nb, and Ti, belonging to the high-K calc-alkaline series. Additionally, the initial ⁸⁷Sr/⁸⁶Sr ratios of the Langdu quartz-diorite porphyry range from 0.7054 to 0.7061, and ε_Nd(t) values range from －3.5 to －2.5. All samples are plotted near the orogenic evolution line on the Pb isotope evolutionary diagram. Combining the petrological, geochemical, and isotopic characteristics of the Langdu quartz-diorite porphyry, this study suggests that the Langdu ore-bearing intrusion formed in relation to the westward subduction and accretion of late Triassic Garez-Litang oceanic crust. It was a product of partial melting of the metasomatized mantle, mixed with lower crustal material during its ascent, and experienced intense fractional crystallization. The high Sr/Y and high V/Sc ratios of the Langdu quartz-diorite porphyry indicate the characteristics of high-water content and relatively high oxygen fugacity in the magma, which are probably key factors controlling the formation of the porphyry-skarn copper deposit in this region.