Study On Metallogenic Mechanism Of Fluid In Huize Lead-Zinc Deposit, Yunnan Province, China

Huize lead-zinc deposit which is famous for its large scale and high grade, is belonged to the Sichuan-Yunnan-Guizhou metallogenic region, and the district is located the southwest edge of the Yangzi plateform. Based on fully summarizing existing researches and field investigation, this thesis focuses on the geological characteristics of the deposit, the characteristics of fluid inclusions, isotopic, rare-earth elements and sphalerite composition. The evolution of ore-forming fluid and the mechanism of fluid mineralization in the Huize lead-zinc deposit are discussed on the basis of the study of the characteristics and origin of ore-forming fluids and the source of ore-forming materials. Finally, a mineralizing model of Huize lead-zinc deposit is therefore established. The results yielded as follow:The ores are evidently dominated by different orders of structure. The ore bodies are ore-bodies are hosted in the broken belts of interlayer, and the transformation of tectonic stress environment in turn by tension and compression, and then torsion. The characteristic of formation of Huize lead-zinc deposit is of multistage. On the basis of the field work and microscopic observations, four stages of mineralization situation have been recognized:the Fine pyrite stage (Ⅰ), the Complex sulfide stage (Ⅱ), the Pyrite-Galena stage (Ⅲ), and the Carbonate stage (Ⅳ). The mineralization mainly occurred in phase Ⅰ is controlled by tension stress which is characterized by coarse to megacrystals and obvious rock fragmentation, is the product of a tension environment.Fluid inclusion (FI) petrographic study shows that there are mainly four types of FIs, including aqueous two-phase type (L+V)-type, liquid (L)-type, gas (V)-type and daughter mineral-bearing (S)-type, and the aqueous two-phase type (L+V)-type is the most development. Boiling inclusions and some bearing halite daughter minerals FLs are discovered in phase Ⅱ. Microthermometric data show that there is a decreasing trend of mineralization temperature and salinity along with the mineralization, the show. At stage Ⅰ, homogenization temperatures of FIs range from 280 to 340 ℃ with salinities of 1.57-19.84 wt.% NaCl equiv, and the FIs could be divided into two groups according to salinities. The average density of FIs is 0.825 g/cm3 shows stage Ⅰ in-high temperature and low-density fluid. At stage Ⅱ, homogenization temperatures of FIs range from 220 to 280 ℃ with salinities of 0.71-21.82 wt.% NaCl equiv, and the average density of FIs is 0.876 g/cm3. The salinities of two bearin g halite daughter minerals FLs are obtained (34.13wt.% and 34.87wt.%) according to the disappearance temperature of gas bubbles(233.6℃ and 241.7℃) and halite daughter crystal(245.4℃ and 256.8℃). The homogenization temperature of boiling FIs is 278.3 ℃ with salinities of 7.6-11.4wt.% NaCl equiv. Accordingly, The main ore body was formed by a mesothermal and low-density fluid-system conditions. The microthermometric data at stage III show low temperature and low-density that the FIs have totally homogenized concentrating at 160-200℃ with average density of 0.836g/cm3 and salinities of 0.53-15.17 wt.% NaCl equiv. The homogenization temperature of the Carbonate stage range from 160 to 180 ℃ with salinities of 9.08-21.33 wt.% NaCl equiv.The mineralization pressures is estimated to be 27.5-33.7 MPa by using the disappearance temperature of gas bubbles and halite daughter crystal in bearing halite daughter minerals FLs. Trapping pressure and the depths for the formation of FIs are obtained to be 7MPa and 0.68km with boiling fluid inclusions. According to the empirical formula, we got a trend of decreasing gradually that the averages of ore-forming pressure from early to later were 29.87Mpa,23.21Mpa,16.79Mpa and 19.05Mpa.Laser Raman spectrometer is applied to the identification of single FI. The samples come from the Complex sulfide stage (Ⅱ) to test. Liquid ingredients of V+L type is H2O, moreover, gas is CO2. By analyzing the data of group FI, gas are H2O, CO2, CO, CH4 and H2. Furthermore, K+, Na+, Ca2+, Mg2+, F- and Cl- are existence in liquid. The ore-forming fluid is a type of NaCl-CaCl2-CO2-H2O rich in metals.The migration of elements is verified by the lateral numerical comparison of the ore-forming elements in the mining area and the periphery of the mining area. Sulfide in deposit is characterized by rich in heavy sulfur isotope. The values of 834S are concentrated in the range of 14‰ to 16‰ and with high homogenization indicate that sulfur originates from the TSR effect of seawater sulfate in the Carboniferous. Pb isotope of ore deposit has obvious characteristics of crust with homogeneous isotope composition and enrichment of radioactivity. Compared with the Pb isotope of the surrounding stratum, basement strata and Mount Emei basalt, the result shows that the mineralizing material is origin from the carbonate stratum and the basement of the fold.The carbon and oxygen isotopic composition of the hydrothermal calcite in the deposit is close to but slightly lower than the dissolution of the carbonate rock, indicating that the carbon and oxygen in fluid are mainly origin from the surrounding carbonate stratums, and may have the addition of organic fluid. Hydrogen and oxygen isotope fall into the region between metamorphic fluids and basin brine with composition of homogenization, but also close to the region of basin brine with reaction of wall rock and organic matter, suggesting that ore-forming fluid may mix of metamorphic fluid and basin brine, and reacts with wall rock and organic matter. Stable strontium isotope of calcite in ore-bodies, it is higher than that of wall rock in mining area, but significantly lower than the basement rocks, suggesting strontium isotope of fluid may be jointly provided by basement rocks and carbonate stratums. In the temperature and salinity diagram of fluid, the ore-forming fluid of the Huize lead zinc deposit falls into the brine of the basin, the metamorphic fluid and the the region between the two. All of the above results indicate that the ore-forming fluid of the deposit may be a mixed fluid of the brine and metamorphic fluid in the basin.The REE characteristics of the clay minerals vein associated with ore bodies is very similar to formation shale in the mining area, and it suggest that once the ore-forming fluid leaching Baizuo formation to cause the water-rock reaction. Wall rock is enriched in LREE and relatively depleted in HREE. The wall rock is more close to the ore body, the higher the total REE. This characteristic shows the diffusion of REE-rich hydrothermal to wall rock. There is a pronounced positive Eu anomaly; there is no pronounced Ce anomaly. This indicates the ore-forming fluid is a reducing fluid, which provides the conditions for the migration of the reduced sulfur and metal components. Characteristics of REE in gangue calcite in the section of ore-body without obvious difference, which display the ore-bodies are the products of pulsing injection of multi-stage metallogenic fluid.The content of Cd and Ge in sphalerite suggests Huize lead-zinc deposit belongs to the mesothermal hydrothermal type. Chemical composition of sphalerite shows characteristics of Mayuan lead-zinc deposit and the MVT type lead-zinc deposit in Sichuan, Yunnan and Guizhou, indicates that it may belong to MVT type lead-zinc deposit related to basin brine activities. The dispersed elements are mainly enriched in sphalerite, and Cd mainly concentrated in the light sphalerite. The enrichment mechanism of Cd is to substitute Fe during high temperature then substitute Zn during the low temperature enter into the sphalerite lattice. The Ga substitute Zn as isomorphism entered into the sphalerite lattice, and the Ge may substitute Fe entered into the sphalerite.On the basis of the above results, a model about metallogenic mechanism and evolution of fluid in Huize lead-zinc deposit is suggested:During the period of sediment diagenesis, a thick ore source layer is formed in the region. Dolomitization and Mount eruption of Emei basalt provide favorable conditions for fluid migration. Large scale migration of brine in the basin driven by regional extrusion in Indo-Chinese epoch, and the metal components of the surrounding rock were extracted to form a fluid with low temperature, high salinity, metal enriched and sulfate radical. On the other hand, the metamorphic fluid rich in CO2 and materials from fold basement transports along the deep fault zone from bottom to top and mixes with brine with lateral migration to form a homogeneous hydrothermal. During the process of fluid migration, Thermal degradation of organic materials resulted in the generation of CH4 and other reductive components, and then the sulfate chemical reduction (TSR) was initiated to generate a large amount of reductive sulfur. Metal and reductive sulfur in reducing fluid together migrate to the vicinity of the mining area. With the increasing of stratum deformation, the broken belts of interlayer in the mining area are formed. Due to the changes in physical and chemical environment, especially the ore bearing fracture changed from compressive to tensile, the fracture space moment expansion. A large amount of polymetallic sulfide precipitates for decompression of fluid, and then the rich ore-bodies are formed.