Study on the fluid inclusions of Baiyangping poly-metallic deposit in 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 the important deposits in the famous Sanjiang metallogenic belt. Based on the petrology and mineragraphy studies by the method of staining of carbonate minerals, the mineral assemblages 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. The fluid inclusion in the sphalerite as well as the associate gangue minerals, which occurred predominately in terms of isolated primary inclusions, is mainly rice-shaped, oval, negative crystal and irregular. 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 is in the range of 22.00% similar to 25.00% NaCleqv, ore-forming temperature concentrated between 110.0 similar to 180.0 degrees C, and its density over 1.00g . cm(-3). Therefore, Baiyangping is a low-temperature, high-salinity, and low-density poly-metallic deposit. The ore-forming pressure and depth are between 28.02 similar to 70.64 MPa and 1.04 similar to 2.62km,respectively. 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.