Research On Fluid Inclusions Of The Gejiu Tin-Copper Polymetallic Deposit

Gejiu Sn-Cu ore deposit,located at the western end of south-eastern Yunnan and western Guangxi Sn-W ore-forming province,is one of the superlarge polymetallic tin ore deposit in the world.This thesis investigates the fluid inclusions of Kafang,Songshujiao,Laochang mine fields and Tangziwa mining area in the Gejiu ore district.Based on the petrography,microthermometry,and raman spectroscopy method we get the fluid inclusions' homogenization temperature,freezing temperatures and gas compositions.Furthermore,the thesis calculates the thermodynamic parameters of fluid inclusions,such as the salinity,density,pressure and depth,then researches the ore-forming fluid evolution characteristics,metallogenic mechanism and mineralization process.The main conclusions are listed as follow:1.Based on the types of fluid inclusions,during the Skarn Stage,Oxide Stage and the Sulfide Stage(stage I-II-III),the characteristic of boiling fluid inclusions has been found.The aqueous-carbonic(type C)fluid inclusions found in the Sulfide Stage(stage III),indicates that the CO2 of the ore-bearing fluid has increased.The hydrothermal activity isn't intense since the Carbonate Stage because the gas-liquid ratios become smaller compared to the main mineralization stage.2.The components of the fluid inclusions,trapped by each ore field,are roughly same.The liquid phase mainly has H2 O,and contains a small amount of CO32-.The vapor phase is mainly composed of CO2,CH4,has a minor amount of N2.From mineralization stage I to stage III,the content of CH4 decreases while the CO2 increases.The content of CH4 probably comes from the deep source fluid.3.Fluid inclusions' microthermometry results show that: from the Skarn Stage,Oxide Stage,Sulfide Stage then to the Carbonate Stage,the fluid temperature(the mean value from 463.2? to 360.7?,then to 256.8? and decreased to 182.2?)has significantly decreased,the fluid salinity(the mean value from 14.32% to 18.49%,then to 11.98% and decreased to 3.27%)decreases gradually overall,the fluid density's range(the peak value from 0.5~0.8g/cm3 to 0.64~0.88g/cm3,then to 0.71~1.02 g/cm3 and increased to 0.80~0.97g/cm3)becomes narrow and increases slowly.From the Skarn Stage to the Sulfide Stage,the mineralization pressure ranges is 19.89 ~ 37.89 MPa,the depth of mineralization concentrated within <500m ~ 1.5km.4.By comparison with the typical deposit,we find that the characteristics of the fluid inclusions of the Gejiu ore district are more in accordance with the fluid inclusions of the magmatic hydrothermal type deposits.5.In the Skarn Stage and the Oxide Stage,the ore fluid boiling and water(gas)-rock reaction are the main types of mineralization mechanism.When got to the Sulfide Stage,the fluid mixing mechanism plays the main role.And ore-forming fluid boiling has the characteristic of multi-epoches,which are concerned with convection cirrulation between the heating of the rock mass in the bottom and the low temperature and low salinity infiltration atmospheric water.6.The ore-forming fluids of the Gejiu ore district may come from the deep source fluid,after the mixing with the bottom granite magma fluid it becomes the main ore-bearing fluid which has high temperature and high salinity.The fluid upwells along the structural fissures,making water(gas)reactions with the surrounding rocks.The decreasing of pressure caused by structure,the lowering of temperature and the mixing with atmospheric precipitation,makes the ore-bearing fluid boiled,then the fluid gradually develops to low temperature and low salinity fluid.Finally,the fluid forms Gejiu tin-copper polymetallic deposits with the Skarn type,Greisenization type,Layer and Hydrothermal Metasomatic type and Tourmaline-Sulfide veinlet-like type mineralization.