Isotope geochemistry and its implications in the origin of Yangla copper deposit, western Yunnan, China

The Yang la copper deposit is located in western Yunnan Province, China, with an estimated Cu reserve of approximately 1.2 million tons. It is a typical giant copper deposit, and its mining started only recently. The delta C-13(V-PDB) values of the calcites studied vary from -5.1 parts per thousand to 1.0 parts per thousand, implying that the hydrothermal fluids from which the calcites precipitated were derived from the granitic magma. The delta O-18(SMOW(H2O)) and delta D-SMOW values of quartz fluid inclusions range from 0.11 parts per thousand to 2.50 parts per thousand and from -120 parts per thousand to -100 parts per thousand, respectively. These data may suggest the following: (1) mixing between meteoric and magmatic fluids, or (2) the evolution of meteoric fluid by its interaction with igneous or metamorphic rocks. The delta S-34 values of sulfides range from -4.20 parts per thousand to 1.85 parts per thousand (average: -0.85 parts per thousand), which is consistent with the magmatic origin. Based on the He-3/He-4 ratios of fluid inclusions trapped in sulfides of the deposit (0.14-0.17 Ra) and Ar-40/Ar-36 ratios of 301-1053, it can be inferred that the ore-forming fluids of the deposit were derived primarily from the crust with a minor mantle component during the metallogenic processes. Based on C, H, O, and S isotopic compositions, and the Yangla copper deposit is bordered primarily by gently dipping thrust faults near the Linong granodiorite. Moreover, the Re-187-Os-187 isochron age of molybdenite puts the time of metallogenesis at 233.3 +/- 3 Ma, which is virtually coeval with the emplacement of the Linong granodiorite (235.6-234.1 Ma) and highlights the genetic link between the Yangla copper deposit and the Linong granodiorite. It is likely that the ore-forming fluids exsolved from the Linong granodiorite, which was formed by crustal melting induced by the intrusion of mantle-derived magma. During the late Early Permian, the Jinshajiang oceanic plate was subducted to the west, resulting in the formation of a series of gently dipping thrust faults in the Jinshajiang tectonic belt. Subsequently, the thrust faults was tensional during the early Late Triassic, which was a time of transition from collision-related compression to extension in the Jinshajiang tectonic belt; such conditions produced an environment favorable for the formation of ore fluids. This extension, in turn, induced the upwelling of hot asthenosphere, triggering intense melting in the lithospheric mantle and producing voluminous basaltic magma. Subsequently, the mantle-derived magma likely ascended along the fractures and faults to underplate the lower crust, which underwent partial melting to generate voluminous granitic magma. After the magma reached the base of the early-stage Yangla granodiorite, the platy granodiorite at the base of the Yangla body shielded the late-stage magma. Then, this magma cooled slowly, releasing some of its ore-forming fluids into the gently dipping thrust faults near the Yangla granodiorite and producing mineralization.