Origin and Evolution of Ore-Forming Fluid and Metallogenic Mechanism of the Baoshan Cu-Pb-Zn Deposit, South China: Constraints of Fluid Inclusion and C-H-O Isotopes.

The Southern Hunan area is located in the superposition of the Qin-Hang Cu-Pb-Zn polymetallic ore belt and the Nanling W-Sn-Mo polymetallic ore belt, which is an important window to study the mineralization of W-Sn-Mo and Cu-Pb-Zn polymetallic deposits. The Baoshan deposit is a large Cu-Pb-Zn polymetallic deposit in Southern Hunan Province with obvious zones of Cu mineralization and Pb-Zn mineralization: the central part of the Baoshan deposit demonstrates contact metasomatic (skarn) Cu mineralization, while the western, northern and eastern parts demonstrate hydrothermal vein Pb-Zn mineralization. However, the origin and evolution of the ore-forming fluid and mechanism of Cu and Pb-Zn mineral precipitation are still unclear. The metallogenic process of the Baoshan Cu-Pb-Zn deposit can be divided into four stages: (1) the early skarn stage (S1); (2) the late skarn stage (S2); (3) the Cu-Fe sulfide stage (S3); and (4) the Pb-Zn sulfide stage (S4). The results of microtemperature measurements and a Raman spectrometric analysis of fluid inclusions show that the ore-forming fluid was the H2O-NaCl (-CO2 ± N2 ± C2H6) system in the skarn stages (S1 + S2) and changed into the H2O-NaCl-CO2 (±N2 ± C2H6) system in the sulfide stages (S3 + S4). The temperature (S1: 436.6~548.2 °C; S2: 344.1~435.1 °C; S3: 134.1~413.1 °C; S4: 183.9~261.0 °C) and salinity (S1: 17.4~51.2 wt.%NaClequiv; S2: 13.6~41.7 wt.%NaClequiv; S3: 1.2~32.3 wt.%NaClequiv; S4: 1.8~9.6 wt.%NaClequiv) showed a downward trend from the early to late stages. From the skarn stages (S1 + S2) to the sulfide stages (S3 + S4), the ore-forming pressure results from the static rock pressure and the hydrostatic pressure, and the ore-forming depth is estimated to be about three to six km. The C-H-O isotopic compositions of hydrothermal minerals such as quartz and calcite indicate that the ore-forming fluid is predominately magmatic fluid, but a significant amount of meteoric water is added in the Pb-Zn sulfide stage (S4). The formation of the mineralization zonation of the Baoshan deposit is the result of many factors (e.g., stratigraphy, structure and metal precipitation mechanism): the Cu mineralization is controlled by the contact zone, and the Pb-Zn mineralization is controlled by the fault. In addition, the precipitation of Cu is mainly controlled by fluid boiling, while the precipitation of Pb and Zn is mainly controlled by the mixing of magmatic fluid and meteoric water. [ABSTRACT FROM AUTHOR]