Study On The Ore-forming Fluid Of The Baiyangping Poly-metallic Deposit In The Lanping Basin, Northwestern Yunnan, China

Baiyangping poly-metallic deposit, located in the central,slightly north of the Lanping Mesozoic-Cenozoic sedimentary basins, is one of important deposits in the famous Sanjiang metallogenic belt. It has received much concerned and been studied deeply since it was discovered, however, considerable differences still exist between genetic type and ore-forming mechanism. Therefore, the understanding of the deposit is in urgent need to be improved. As the inclusions in the minerals are the representative samples of ore-forming fluid, as well as the signal of determining the genetic type. In this paper, the method of staining of carbonate minerals was used to determine the associate mineral assemblages, and then microthermometry and Laser Raman spectroscopy were used to conduct the study of fluid inclusions. Finally, combining these with the study of isotope, trace elements and REE, the genetic characteristics of the deposit were tried to be restricted.Based on the petrology and mineragraphy study by the method of staining of carbonate minerals, associate mineral assemblages in the main ore-forming stage of Baiyangping poly-metallic deposit are sphalerite-galena-dolomite-quartz, as well as Ag-bearing minerals and Cu-bearing minerals, and calcite is mainly a product of mineralization in late stage. According to the analysis of the microthermometry and Laser Raman spectroscopy, the system of the ore-forming fluids is H2O-NaCl-CaCl2, peak value of salinity w(NaCleq)is in the range of 22.00wt%～25.00wt%, ore-forming temperature concentrated between 110.0℃～180.0℃, and its density over 1.00g/cm3. Therefore, Baiyangping is a low-temperature, high-salinity, and low-density Pb-Zn poly-metallic deposit. The ore-forming pressure and depth are between 28.02MPa～70.64MPa and 1.04km～2.62km,respectively.The study H, C, O, S and Pb isotope shows that ore-forming fluid was hot basin brine originated from palaeo-precipitate water, C from marine sedimentary carbonate melt by basin fluids, S from gypsum-salt in basin sedimentary strata, and Pb and other metal elements are mainly derived from the supracrust rocks.Ore-forming fluid is always in relatively reducing environment, and metal ions together with reducing sulfur start precipitation to form deposits under the changes of the physical and chemical conditions, even though they coexist stably in the same fluid system privately. The factors of geologic background, ore-forming sources and ore-controlling structure lead to the conclusion that Baiyangping poly-metallic deposit has the characteristics of Mississippi Valley type.