Fluid inclusion and isotope geochemistry of the Yangla copper deposit, Yunnan, China

The Yangla copper deposit, with Cu reserves of 1.2 Mt, is located between a series of thrust faults in the Jinshajiang–Lancangjiang–Nujiang region, Yunnan, China, and has been mined since 2007. Fluid inclusion trapping conditions ranged from 1.32 to 2.10 kbar at 373–409 °C. Laser Raman spectroscopy confirms that the vapour phase in these inclusions consists of CO2, CH4, N2 and H2O. The gas phases in the inclusions are H2O and CO2, with minor amounts of N2, O2, CO, CH4, C2H2, C2H4, and C2H6. Within the liquid phase, the main cations are Ca2+ and Na+ while the main anions are SO4 2− and Cl−. The oxygen and hydrogen isotope compositions of the ore-forming fluids (−3.05‰ ≤ δ18OH2O ≤ 2.5‰; −100‰ ≤ δD ≤ −120‰) indicate that they were derived from magma and evolved by mixing with local meteoric water. The δ34S values of sulfides range from −4.20‰ to 1.85‰(average on −0.85‰), supporting a magmatic origin. Five molybdenite samples taken from the copper deposit yield a well-constrained 187Re–187Os isochron age of 232.8 ± 2.4 Ma. Given that the Yangla granodiorite formed between 235.6 ± 1.2 Ma and 234.1 ± 1.2 Ma, the Cu metallogenesis is slightly younger than the crystallization age of the parent magma. A tectonic model that combines hydrothermal fluid flow and isotope compositions is proposed to explain the formation of the Yangla copper deposit. At first, westward subduction of the Jinshajiang Oceanic Plate in the Early Permian resulted in the development of a series of thrust faults. This was accompanied by fractional melting beneath the overriding plate, triggering magma ascent and extensive volcanism. The thrust faults, which were then placed under tension during a change in tectonic mode from compression to extension in the Late Triassic, formed favorable pathways for the magmatic ore-forming fluids. These fluids precipitated copper-sulfides to form the Yangla deposit.