Characteristics And Evolution Of Ore-Forming Fluids In Dongzigou Pb-Zn Deposit,Xishui In Guizhou Province

Dongzigou Pb-Zn deposit is located in the transitional position from Guizhou uplift belt to Sichuan basin.The anticline and fault of Sangmuchang are the main ore-controlling structures,and the most important ore-bearing strata are the strata of Dengying formation of Sinian system.The morphology of ore body is mainly veined and lenticular.The boundary line between ore body and country rock is distinct.Pyrite,sphalerite and galena are the main minerals,gangue minerals are mainly quartz,fluorite,dolomite and calcite,etc.The main wall rock alteration is silicification and followed by dolomitization,pyritization,calcitization and so on.Dongzigou Pb-Zn deposit is divided into 2 phase and 4 stages.Metallogenic hydrothermal phase is divided into quartz-pyrite stage(?stages),sphalerite-galena stage(?stage),sphalerite-barite-bitumen stage(?stages).The next phase is supergene phase.The types of fluid inclusions are H₂O inclusions,pure gas CO₂ inclusions and CO₂-H₂O inclusions.The variation range of homogenization temperature of fluid inclusions is concentrated in 120?290?,range of salinity is 2.1?11.8 wt%Na Cl;The homogenization temperature of I stage is concentrated in 230?290?,range of salinity is 3.4?11.8 wt%Na Cl;The homogenization temperature of?stage is concentrated in130?190?,range of salinity is 3.5?10.0 wt%Na Cl;The homogenization temperature of?stage is concentrated in 120?180?,range of salinity is 2.1?6.5 wt%Na Cl.The density of ore-forming fluid is 0.75?1.0 g/cm³,metallogenic pressure are 93.9?312.1bar,and the ore-forming depth of sphalerite is about 550 m.Generally,the ore-forming fluids of ore deposit shows the characteristics of middle and low temperature,low salinity and low density,and the ore-forming pressure and depth are also low.With the evolution of mineralization,the change trend is decreasing gradually.Analyse hydrogen and oxygen isotopic composition,mean of?¹³C_V-PDB from ore-bearing surrounding dolostone is 2.9‰,mean of?¹⁸O_-SMOW is-8.0‰;Mean of?¹³C_V-PDB from hydrothermal dolomite is 2.3‰,mean of?¹⁸O_-SMOW is 21.9‰.Bring out hydrothermal fluid comes from formation water in the layers with mineralization process,later,the continuous mixing of atmospheric precipitation.Diagram of C-O isotope shows that the formation of hydrothermal dolomite are due to combination of reasons,the fluid-rock interaction between surrounding rock and hydrothermal fluid and the gradually decreasing temperature.The C of hydrothermal fluid should be derived from surrounding rock strata under carbonate dissolution.The(⁸⁷Sr/⁸⁶Sr)_i of sphalerite,barite,dolostone and quartzis is between 0.7140?0.7143.but the?⁸⁷Sr/⁸⁶Sr of dolostone between barite and sphalerite is about 0.0005.It is infered that the ore-forming material originated from the source region rich in Sr isotopes than Dengying formation,or the ore-forming fluid has flowed through the formation rich in Sr isotopes.It is inferred that the metallogenic hydrothermal fluid has the characteristics of"double source"due to overlying clastic sediments of Dengying formation are not sufficient to provide Sr isotopes,as a result part of the hydrothermal fluid is derived from siliceous clastic deposition on the Dengying formation,the other part comes from the base or brine hydrothermal fluid.The ore-forming process of the deposit is supposed to be that the hydrothermal fluid flowing through the overlying strata of the Sr-rich than Dengying formation and the brine hydrothermal fluid rising from the basement,and then the hydrothermal fluid mainly formed in the formation water during the migration process of carbonate rock strata.With the decrease of temperature,density and pressure,the hydrothermal fluid rich in useful metals is precipitated after dissolution by carbonatein in faults,interlaminar fracture zones,structural fissures,and further enriched and mineralized with the infiltration of atmospheric precipitation in the later stage.