Nature and evolution of hydrothermal fluids in the formation of the Tuwu porphyry copper deposit in the Eastern Tianshan Mountains, NW China.

Graphical abstract Highlights • Ore-forming fluids are moderate-high temperature and salinity, and relatively oxidized, and they evolved from magmatic to meteoric in origin. • Ore-forming materials were mainly derived from magma. • Temperature, redox, pH, and fluid-rock interaction are important controls on the transportation and precipitation of sulfides. Abstract The Tuwu porphyry copper deposit is located in the Eastern Tianshan Mountains, Xinjiang, China. Lentiform-like ore bodies are hosted in the Early Carboniferous Qi'eshan group, and Cu mineralization shows a close association with tonalite porphyry. Porphyry and country rocks have undergone extensive potassic, chlorite-sericite, phyllic and propylitic alteration. The mineralized veins in the Tuwu deposit can be divided into: earlier quartz–biotite ± magnetite ± chalcopyrite veins (stage I), quartz–chalcopyrite ± bornite ± pyrite veins (stage II) and quartz ± molybdenite ± chalcopyrite ± pyrite ± calcite veins (stage III), and later calcite ± quartz veins (stage IV). The Stage II and III are the main Cu mineralization stages. At the Tuwu deposit, three types of fluid inclusions have been distinguished: liquid-rich (L-type), vapor-rich (V-type) and halite-bearing (S-type), respectively. Laser Raman spectra identified magnetite, hematite, chalcopyrite, and pyrite at S-type inclusions. The quartz veins in stage I and II contain all inclusion types. L-type and V-type inclusions are recognized in stage III quartz minerals, but only L-type inclusions are present in stage IV quartz minerals. Fluid inclusions in quartz of stage I, II, III and IV are characterized by homogenization temperature vary of 275–492 °C, 232–477 °C, 199–287 °C and 133–227 °C, with corresponding NaCl equiv. salinities of 2.2–58.7 wt% NaCl equiv. , 1.4–58.3 wt% NaCl equiv. , 1.1–6.0 wt% NaCl equiv. , and 1.1–4.2 wt% NaCl equiv. , respectively. Initial ore-forming fluids of the Tuwu deposit belong to relatively oxidized H 2 O-NaCl system with moderate-high temperature and salinity. Sulfur isotope data from chalcopyrite and pyrite suggest a magmatic source. Two types of chlorite (type 1 and type 2) are recognized based on the relationship with metal sulfides. Electron microprobe analysis shows that chlorites are plotted in ripidolite and pycnochlorite. Decreasing Fe/(Fe + Mg) ratios of chlorites indicate that the meteoric water was involved in the magmatic water, combined with the positive correlation with AlIV, implying that the chlorite formed in a relatively reduced and acidic environment. The hydrothermal fluids in type 1 chlorite bring large quantities of reduced metallic minerals compared to the barren fluids in type 2 chlorite. The formation temperatures of chlorite are roughly coincident with the results that obtained from fluid inclusion studies, which show a decrease trend. Changes in temperature, redox, pH, and fluid-rock interaction likely play an important role in the transportation and precipitation of metal sulfides of the Tuwu deposit. [ABSTRACT FROM AUTHOR]