Genesis of the Yandong porphyry Cu deposit in eastern Tianshan, NW China: Evidence from geology, fluid inclusions and isotope systematics

The Yandong porphyry Cu deposit (372 Mt at 0.58% Cu) is located in the center of the eastern Tianshan, Xinjiang, NW China. Multiple-stage hydrothermal activities have resulted in potassic, chlorite-sericite, phyllic, and propylitic alteration in this deposit. The Yandong deposit formed in four stages, i.e., an early quartz ± magnetite ± pyrite stage (I), a quartz + pyrite ± chalcopyrite stage (II), a quartz + chalcopyrite ± pyrite ± molybdenite stage (III), and a late quartz + carbonate stage (IV). Three evolutionary episodes were observed by a detailed fluid inclusion study: (1) Early ore stage (I) fluids were trapped under two-phase conditions, as evidenced by the coexistence of vapor-rich (V-type) inclusions (homogenization temperatures = 410–494 °C, average salinity = 2.2 wt% NaCl equiv), liquid-rich (L-type) inclusions (homogenization temperatures = 311–430 °C, average salinity = 9.5 wt% NaCl equiv) and hypersaline (H-type) inclusions (trapped at ∼300 bars, or a depth of approximately 1 km assuming lithostatic pressure conditions). (2) Main ore stage (II and III) fluid inclusions in quartz were also trapped under two-phase conditions (boiling), as identified by the coexistence of V- and L-type fluid inclusions; L-type inclusions homogenized between 161 and 390 °C (average = 270 °C), with salinities of 1.6–9.9 wt% NaCl equiv (average = 4.9 wt% NaCl equiv) and a depth of ∼1 km (∼100 bars, hydrostatic pressure conditions). (3) Late ore stage (IV) fluids are represented by inclusions in barren quartz–carbonate veinlets, characterized by homogenization temperatures ranging from 123 to 261 °C (average = 179 °C) and salinities between 0.7 and 4.7 wt% NaCl equiv (average = 2.0 wt% NaCl equiv). The initial hydrothermal fluids are characterized by high temperature, high salinity and high oxygen fugacity, belonging to a H 2 O–NaCl system. Hydrogen and oxygen isotope data indicate that the hydrothermal fluids have a dominantly magmatic signature and mixed with abundant meteoric waters in the quartz-carbonate stage. Sulfur stable isotope suggests that the ore-forming metals came primarily from the tonalite magma that sourced from the subducted oceanic slab involving some mantle peridotites during ascent. The Cu mineralization of the Yandong porphyry deposit occurred at a shallow crustal level (approximately 1 km), and the fluid boiling or phase separation, decrease of temperature, as well as fluid-rock interaction were likely important factors responsible for metal accumulation and deposition in the magmatic-hydrothermal system.