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

摘要

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 δ13CPDB and δ18OSMOW 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 （VCO2+LCO2+LH2O）. 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.