Fluid inclusion characteristics and mineralization of Dadonggou Pb-Zn ore deposit in Altay,Xinjiang

The Dadonggou Pb-Zn ore deposit is located in the Kelan volcanic-sedimentary basin, the biggest basin on the southern margin of the Altay Mountains, with the attitude of its ore bodies the same as that of the strata. The ore-bearing wall rock is mainly volcanic-sedimentary rock of the Lower Devonian Kangbutiebao Formation, and the ores occur in bedded, disseminated and veinlet forms. Ore minerals consist simply of galena, sphalerite and pyrite. In this paper, the authors carried out systemic measurements of homogeneous and freezing temperatures and salinities of fluid inclusions in quartz, calcite and sphalerite formed at different ore-forming stages in the Dadonggou Pb-Zn deposit. Chromatographic analysis of quartz samples and laser Raman spectroscopic analysis of fluid inclusions in quartz and sphalerite were also conducted. Three main types of fluid inclusions, namely NaCl-H2O inclusions, CO2-H2O±CH4 inclusions and CO2-H20-NaCl inclusions were identified. The homogenization temperatures of the fluid inclusions vary remarkably from 97.3℃ to 480℃, mostly in the range of 140～300℃. The salinities w(NaCleq) of fluids range from 0.2% to 57.1 %, mostly between 3.2 % and 14.8%. The gas components of fluid inclusions are mainly CO2 and H2O, with minor amounts of such components as CH4, N2 and H2. The liquid components of fluid inclusions are mainly Na+, Ca2+, F-and Cl-, subordinately K+ and SO42-, and small amounts of Mg2+, Br-and NO-3, with the calculated ion concentration ranging from 36.6% to 58.0%. According to previously analyses of the isotopic composition of C, H, O and S of the ores, the δ13CV-PDB values of calcite range from -4.2‰ to 0.4‰, the δDV-SMOW of the inclusions in quartz range from -89‰ to -127‰ , and the δ18OH2O values of quartz and calcite range from -11.4‰ to 7.6‰. It is suggested that the ore-forming fluid was mainly derived from the mixed meteoric water and magmatic water. The carbon in the ore fluid was mainly derived from marine carbonates. The change of physicochemical conditions and the immiscibility of CO2-H2O exerted the main effect on the ore-forming process of the Dadonggou Pb-Zn ore deposit.