Multistage hydrothermal quartz veins record the ore-forming fluid evolution in the Meiling Cu–Zn (Au) deposit, NW China

Abstract

The Meiling Cu–Zn (Au) deposit, located in the center of the southeastern Kalatag district in the eastern Tianshan, NW China, is characterized by multistage ore-bearing quartz veins. The fluid inclusion, textures, and trace element compositions of the quartz associated with three main mineralization stages—Stage I: quartz (Qtz1) + pyrite ± chalcopyrite ± sphalerite veins; Stage II: quartz (Qtz2) + pyrite + chalcopyrite + sphalerite veins; and Stage III: quartz (Qtz3) + pyrite + chalcopyrite + covellite veins—are investigated to reveal the ore-forming fluid evolution. Stockwork and breccia Qtz1 (245 °C to 348 °C) with impure surfaces have high concentrations of the trace elements Li, Ga, Ge, Sb, and Sn, indicating that Qtz1 crystallized during a fluid boiling event under relatively hydrofracturing conditions. The fluid composition, temperature, and fluid flow rate largely influenced the trace elements in Qtz2-1 (wide temperature range of 111 °C to 279 °C) and Qtz2-2 (121 °C to 218 °C), which are characterized by smoky gray and milky quartz veins, respectively. Decreasing temperature and pressure and enhanced fluid-rock reactions may have been the factors causing the precipitation of chalcopyrite and sphalerite in Stage II. Qtz3 (110 °C to 202 °C) + chalcopyrite + covellite veins in Stage III mostly show comb-like and porous textures, and Qtz3 has relatively low Li, Al, Ti, Ga, Ge, and B contents and Li/Al ratios, indicating that Qtz3 crystallized under open and oxidized conditions. The locally higher Li, Al, and Ti contents in the rims than in the cores of Qtz2-2 and Qtz3 can be attributed to rapid growth rates. The Ti contents and fluid inclusion of quartz indicate a decreasing trend of temperature and salinity during the evolution of ore-forming fluid from the early to late stages. Furthermore, the low Ti contents (<10 ppm) of quartz in the main mineralization stages generally reveal that the mineralization temperatures were less than 350 °C, as evidenced by the fluid inclusion microthermometry data (150 °C to 230 °C). The quartz data indicate that the Meiling deposit probably shows an affinity with epithermal deposits. Nevertheless, the medium- to high- temperature and salinity fluid in Stage I and high-Ti and high-Al field of epithermal deposits, as well as the complex “C”-shaped trend in the Ti vs. Al discrimination diagram, indicate that the Meiling deposit underwent potentially magmatic-hydrothermal fluid evolution and metallogenesis.