S and Pb Isotopic Constraints on the Relationship between the Linong Granodiorite and the Yangla Copper Deposit,Yunnan, China

The Yangla copper deposit, located in western Yunnan Province, China, is a typical giant, newly started mining copper deposit with an estimated Cu reserves of about 1,200,000 tons. The deposit is spatially and temporally associated with the Linong granodiorite, which is rich in SiO₂ (SiO₂=58.25 wt%–69.84 wt%) and alkalis (Na₂O+K₂O=5.98 wt%–8.34 wt%), indicating an association with shoshonitic series to high‐K calc‐alkaline series granites, and shows low contents of TiO₂ (0.35 wt%–0.48 wt%), MgO (1.51 wt%–1.72 wt%), and Al₂O₃ (13.38 wt%–19.75 wt%). The δ³⁴S values of sulfides of the main ore stage from copper ores vary range from −4.2‰ to −0.9‰, indicating a much greater contribution from the mantle to the ore‐forming fluids. The δ³⁴S values of the late ore stage is −9.8‰, indicating enrichment of biogenic sulfur which may derive from the crustal hydrothermal fluid. The ²⁰⁸pb/²⁰⁴pbj ²⁰⁷pb/²⁰⁴pb and ²⁰⁶pb/²⁰⁴pb of sulfides of the main ore stage from copper ores range within 38.66–38.73, 15.71–15.74 and 18.35–19.04, respectively, implying that the Pb was derived from the mantle, with the crustal component, probably representing mixtures of mantle lead and crustal lead. Sulfide of the late ore stage in their Pb isotopic composition, ²⁰⁸Pb/²⁰⁴Pb= 38.69, ²⁰⁷Pb/²⁰⁴Pb=15.70, ²⁰⁶Pb/²⁰⁴Pb=18.35, implying that the Pb was derived from the crust. The Linong granodiorite is syncollisional, produced by partial melting of thickened lower crust, which was triggered by the westward subduction of the Jinshajiang Oceanic plate. During a transition in geodynamic setting from collision‐related compression to extension, gently dipping ductile shear zones (related to subduction) were transformed to brittle shear zones, consisting of a series of thrust faults in the Jinshajiang tectonic belt. The tensional thrust faults would have been a favorable environment for ore‐forming fluids. The ascending magma provided a channel for the ore‐forming fluid from the mantle wedge. After the magma arrived at the base of the early‐stage Linong granodiorite, the platy granodiorite at the base of the body would have shielded the late‐stage magma from the fluid. The magma would have cooled slowly, and some of the ore‐forming fluid in the magma would have entered the gently dipping thrust faults near the Linong granodiorite, resulting in mineralization.