Geochronology and fluid evolution of the Machangqing Cu-Mo polymetallic deposit, western Yunnan, SW China

The Machangqing porphyry-skarn Cu-Mo polymetallic deposit is located to the east of the Jinshajiang-Ailaoshan alkaline porphyry metallogenic belt in western Yunnan, SW China. Three major paragenetic stages have been identified in the Machangqing skarn mineralization, namely the (I) prograde skarn stage dominated by anhydrous minerals (e.g., garnet and pyroxene); (II) iron oxide stage dominated by magnetite and quartz, and coexisting hydrous minerals (e.g., epidote, biotite, allanite and chlorite) and trace sulfides (e.g., chalcopyrite, pyrite and molybdenite); (III) the main sulfide stage, dominated by quartz, chalcopyrite, pyrite and minor molybdenite. Zircon U-Pb dating of the ore-related granite porphyry yielded an age of 36.1 ± 0.2 Ma (n = 21; MSWD = 1.13), consistent with the reported skarn-related hydrothermal titanite U-Pb dating of 34.3 ± 1.2 Ma, indicating that the granite porphyry and skarn mineralization are closely related. Trace element compositions of these zircon grains demonstrate a positive Ce4+/Ce3+ (110816, avg. = 401) vs. Ce/Nd (13.9101, avg. = 36.0) correlation. The high zircon Ce4+/Ce3+ and Ce/Nd ratios point to an oxidized magma that produced hydrothermal fluids enriched in sulfophilic elements. Stage I garnet contains aqueous (L-type), halite-bearing (SH-type) and undissolved mineral-bearing (SU-type) inclusions, which homogenized at >590 °C with 47.4 wt% NaCl eqv. (SH-type) salinity. The hematite daughter mineral in SU-type indicates that the fluid in this stage is oxidized. Stage II quartz contains multiphase vapor-rich CO2-bearing C-type (4.811.1 wt% NaCl eqv.), SH-type (38.8 wt% NaCl eqv.) inclusions and coexisting vapor-rich (V-type) and L-type inclusions, which homogenized at around 350 °C. Stage III quartz contains L-, V-, C-, SH- and SU-type inclusions, which homogenized between 340 and 400 °C, yielding salinities from 2.7 to 20.3 and 29.2 to 53.3 wt% NaCl eqv. The coexistence of low-density (C-/V-type) and high-density (SH-type) inclusions with similar homogenization temperatures suggests that fluid boiling took place in stages II and III. It is concluded that the ore-forming fluids are characterized by high temperature, high salinity, high oxygen fugacity and high CO2 content. Fluid-rock interaction leads to magnetite crystallization and reduces the fluids, while fluid boiling causes escapes of CO2, increasing pH and sulfide precipitation. © 2020 Elsevier B.V.