The Geological-geochemical Features And Its Formation Mechanism Of The Middle-Upper Ordovician Gypsum-salts In Yuele Lead-zinc Ore Field,northeastern Yunnan

As the important research field,the relationship among the gypsum-salt rocks,salt tectonics and petroleum accumulation is attentioned by many geologists.And a amount of researching results have been made.Till to now,the characteristics of the gypsum-salt rocks in the metal mineral deposits is still relatively scarce,it is difficult to establish the relationship between gypsum-salt rocks and metal mineralization.The exploration of Yuele lead-zinc ore field in Daguan,Zhaotong found that,there widely developed gypsum-salt rocks in the Middle-Late Ordovician strata,and show closely genesis relationship with the copper-lead-zinc multi-metal.Therefore,carrying out the developmental characteristics and forming pattern of Yuele lead-zinc ore field,ascertaining its distribution,material composition,structure characteristics,is not only important for guiding the prospection exploration work,but also is supplement for understanding the gypsum-salt rocks in our country,especially gypsum-salt rock hosted Ordovician in upper yantze craton.This time,the gypsum-salt rocks in the Middle-Late Ordovician strata of Yuele lead-zinc ore field Daguan Zhaotong is the research object.Centering on gypsum-salt rocks distribution characteristics.Through studying on 24 holes,2 measured sections and 1 exploration hang road system to clarify the basic characteristics,lithological association characteristics,spatial distribution law of gypsum-salt rocks.Then clarify its Formation mechanism and control factors,establish its formation model,provide evidence for contacting gypsum rock with metal mineralization.There are the mainly achievements and recognize:(1)The gypsum-salt rocks are mainly located in Middle Ordovician Shizipu group and Middle-Upper Ordovician Daqing group,which are located in the secondary anticline of mine.The gypsum-salt rocks mainly store in siltstone,silty mudstone,gray dolomite,argillaceous limestone.(2)According the output location,structural features and basic components characteristics of gypsum-salt rocks,it can be divided into sedimentary type,epigenetic hydrothermal type,and later tectonic deformation type three types.The Sedimentary gypsum-salt rocks can be divided into two subclasses:①The sedimentary type gypsum-salt rocks of Shizipu group are mainly dolomitic gysum rock,gypseous dolostone.This type mostly thin-bedded,lump output.②The gypsum-salt rock of Daqing group of type Ⅰ are mainly gypseous sandstone,gypseous mudstone,little of argillaceous gysum rocks.This type mostly thin-bedded output.The type of epigenetic hydrothermal:The gypsum-salt rocks of Daqing group of type Ⅱ generally coexist with the type Ⅰ.This type mostly veins,stockwork output.(3)The lithology,form characteristics and geochemical featuresb of Middle-Upper Ordovician strata reveal that the sedimentary environment of this time is complex and changing.The gypsum-salt rocks are forming under the changeable climate and sea level.It was controlled by the changes of land and sea in the Yangtze.The MgO/Al2O3、V/Ni、V/Ni of rock sample indicate that Shizipu period was coastal-tableland and Daqing period was coastal-lagoon.The CaO/MgO、Sr/Cu、Ni/Co of rock sample indicate that the weather conditions were hot and dry with strong oxidizing,strong evaporation characteristics.(4)The Sr/Ba of gypsum-salt rocks sample indicate that it form in saltwater environment.87Sr/86Sr isotopic composition of gypsum indicates that its material source is the sea and a little of terrigenous.(5)The sedimentary type gypsum-salt rocks of Shizipu group formed in coastal sabkha.The gypsum-salt rock of Daqing group of type Ⅰ formed in lagoon.The brine formed the gypsum-salt rocks of Daqing group of type Ⅱ by filling fissure.The original gypsum-salt rocks occur varying degrees of retrograde metamorphism,forming gypsum breccias.Some gypsum-salt rocks re-flow under the tectonic stress,experiencing wrinkled bottom wall puncture,forming different forms of gypsum-salt structure.