Fluid evolution and metallogenesis of the Meiling Cu-polymetallic deposit in the East Tianshan, NW China: Constraints from ore geology, fluid inclusions, stable isotope, and U-Pb geochronology

The Meiling Cu-polymetallic deposit is located in the Kalatag Cu metallogenic belt of the East Tianshan, Xinjiang, NW China. Multiple-stage hydrothermal activities have resulted in the silicification, chlorite alteration and sericite alteration in this deposit. At least three mineralization stages are recorded: (a) a phase comprising quartz-pyrite veining, (b) a quartz-chalcopyrite-pyrite-sphalerite phase and (c) a quartz-calcite phase, respectively. LA-ICP-MS zircon U-Pb dating yielded 302 +/- 2.1 Ma for an ore-bearing quartz porphyry and 296 +/- 3.2 Ma for a post-mineralization diorite porphyry dyke, respectively, indicating that the copper mineralization took place during the Late Carboniferous period (302-296 Ma). Fluid evolution during the three stages of mineralization was determined by a detailed fluid inclusion study: (a) Stage I fluids were trapped under two-phase conditions, as evidenced by the coexistence of vapour-rich (type II) inclusions (T-h = 183-242 degrees C, average salinity = 4.8 wt% NaCl equiv.), liquid-rich (type I) inclusions (T-h = 171-259 degrees C, average salinity = 5.3 wt% NaCl equiv.), and daughter-bearing (type III) inclusions (T-h = 192-208 degrees C, average salinity = 8.6 wt% NaCl equiv.). (b) Stage II 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 T-h between 136 and 218 degrees C (average = 189 degrees C), with salinities of 2.2-7.6 wt% NaCl equiv. (average = 3.8 wt% NaCl equiv.). V-type inclusions homogenized temperatures between 153 and 226 degrees C (average = 192 degrees C), with salinities of 2.4-6.7 wt% NaCl equiv. (average = 3.5 wt% NaCl equiv.). (c) Stage III fluids are represented by inclusions in barren quartz-carbonate veinlets, characterized by homogenization temperatures ranging from 98 to 177 degrees C (average = 135 degrees C) and salinities between 0.5 and 3.2 wt% NaCl equiv. (average = 1.9 wt% NaCl equiv.). The initial hydrothermal fluids are characterized by low-intermediate temperature, low-salinity and near-neutral pH condition, belonging to a H2O-NaCl bearing hydrothermal system. The mineralization of the Meiling deposit occurred at a shallow crustal level (similar to 0.5 km), and the decrease of temperature was likely an important factor responsible for metal accumulation and deposition in the subvolcanic hydrothermal system.