Concentration Mechanism of Ore-Forming Fluid in Huize Lead-Zinc Deposits, Yunnan Province

The Huize Pb-Zn deposits of Yunnan Province,located in the south-central part of the Sichuan-Yunnan-Guizhou (SYG) Pb-Zn multimetal mineralization district (MMD),are strictly controlled by fault zones. The sources of ore-forming fluid in the deposits have been debated for a long time. Calcite,a gangue mineral,has uniform C and O isotopes. The δ¹³C_PDB and δ¹⁸O_SMOW values vary respectively from -2.1×10^-3 to -3.5×10^-3 (mean -2.8×10^-3) and 16.7×10^-3-18.6×10^-3 (mean 17.7×10^-3). No obvious difference can be found in C and O isotopes among occurrences and elevations and even ore-bodies. Types of inclusions include those of pure liquid (L),liquid-rich gas-liquid (L+V),and three-phase ones containing a daughter mineral (S+L+V) and immiscible CO₂ with three-phases (V_CO2+L_CO2+L_H2O). Their homogenization temperatures vary from 110 to 400 ℃,and two peaks are shown. (⁸⁷Sr/⁸⁶Sr)₀ ratios of calcite in the deposits are higher than those in the mantle and Emeishan basalts,and slightly higher than those in the Baizuo Formation,which the Huize lead-zinc deposits are found in. All of the (⁸⁷Sr/⁸⁶Sr)₀ are low relative to those in the basement rocks. Fractionation of Sr isotope did not occur in the ore-forming fluid during the precipitation of minerals. The results indicate that the ore-forming fluid is homogeneous and derived from the mixing of different fluids. Gas-liquid inclusions can be separated into two groups in 300-400 ℃ with a salinity of 5%-6% and 12%-16% NaCl respectively. However,the salinities of inclusions vary from 7% to 23% NaCl in 100-300 ℃,especially in 150-250 ℃. The formation pressures of faulted zones are (50-320)×10⁵ Pa. The estimated pressures of the overlying rocks on the ore bodies are 574×10⁵-640×10⁵ Pa. The pressures of ore-forming processes would be 145×10⁵ to 754×10⁵ Pa. Therefore,pressure sharply reduced and boiling occurred when the ore-forming fluid flew into the fault zones. As a result,the ore-forming fluid was highly concentrated,and metallic minerals began to precipitate from the fluid on a great scale. The high-grade lead-zinc deposits were formed when the fluid was under saturation or over-saturation conditions.