Baiyangping Pb-Zn-Cu-Ag Polymetallic Deposit in Lanping Basin: A Discussion on Characteristics and Source of Ore-Forming Fluids and Source of Metallogenic Materials

Located in Mesozoic strata of northern Lanping basin, Northwest of Yunnan Province, Baiyangping deposit is a sediment-hosted Pb-Zn-Cu-Ag polymetallic deposit controlled by fracture. In this paper, a study is carried out on the fluid inclusions in ore-forming stage calcite, quartz, sphalerite and C, O isotope of calcite and S, Pb isotope of sulfur-bearing minerals, on basis of which the character and source of ore-forming fluids and the source of ore-forming materials are discussed. The fluid inclusions of Baiyangping deposit are generally less than 10 μm, main gas-liquid phase. The ore-forming fluid system was Ca²⁺-Na⁺-K⁺-Mg²⁺-Cl^--F^--NO₃^- brine system. The freezing temperature of fluid inclusions in mineral deposits ranges from -26.4 to -0.2 ℃, with an average of -14.6 ℃; the homogenization temperature is concentrated in 120-180 ℃, and the salinity varies from 0.35% to 24.73% (NaCl_eq), with an average of 16.9% (NaCl_eq). ore-forming fluid density is between 0.84 g/cm³ and 1.11 g/cm³, with an average of 1.04 g/cm³, mineralization pressure is between 28.0 MPa and 46.9 MPa, with an average of 37.6 MPa; the corresponding mineralization depths is between 1 058 m and 2 452 m, with an average of 1 555 m, concentrated in 1 200 m to 1 800 m; Carbon and oxygen isotope data indicate a homogeneous carbon source, and the carbon in hydrothermal calcite is derived from the dissolution of carbonate rock strata. The ore-forming fluid belongs to the basin brine fluid system, which is mixed with the precipitate water; S isotope data show that sulfur in sulfides and sulphosalts is derived from thermochemical sulfate reduction, or the thermal decomposition of sulfur-bearing organic matter; the metal mineralization material is from sedimentary strata and basement.