Metallogenesis Of The Pingdian Barite Deposit In Guangxi

The Pingdian barite deposit,Guangxi Province,is located in the southern margin of the Youjiang fold-belt,north-west wing of the Silingcheng syncline in the northwestern side of the Daming mountain uplift.Previous studies on the deposit mainly focused on production investigation,the composition,properties and source of ore-forming fluid of Pingdian barite deposit are not clear,and the genesis of the deposit is still controversial,which seriously restricts the next prospecting work..Based on the detailed field geological survey,through the preliminary analysis of the geological characteristics of the deposit,this study systematically collected relevant samples.The ore-forming fluid properties and material sources are discussed by the method of rare earth elements analysis of barite,laboratory measurement and analyses of fluid inclusions and stable isotope,and the genesis model of the deposit is established finally,providing guidance to the regional ore-prospecting work.Through three years of work,the conclusions are including as following:1.Orebodies of the Pingdian deposit are mainly hosted in fracture zones between the Middle Devonian limestone and lower Devonian mudstone,argillaceous siltstone.The ore bodies are strictly controlled by the fault,as the vein of output,and the ore-controlling structure is near East-West F3 fault,and According to the cutting relationship,the metallogenic period could be divided into 4 periods,as including:the first period of grey-disseminated pyrite barite,the second period of white block barite,the third for yellow stripes bedded barite period,and the fourth period forming milk white barite vein.2.All barites in the deposit are rich in fluid inclusions,mainly liquid inclusions.There are also inclusions of gas phase,mineral three-phase and liquid CO₂ three-phase inclusions.And inclusions in barite in the first stage and the fourth stage are less development.The temperature measurement results show that the homogenization temperature of fluid inclusions in Pingdian barite ranges from 36.8℃to 373.0℃,with an average of 187.7℃,mainly concentrated in the range of 100℃to 275℃,indicating that the ore-forming fluid of the deposit is medium-low temperature hydrothermal fluid;The salinity ranges from1.45 to 18.40 wt%Na Cl_eqv,with an average of 8.43 wt%Na Cl_eqv,mainly concentrated in7-9 wt%Na Cl_eqv,indicating that the ore-forming fluid of the deposit is medium-low salinity fluid.The laser Raman results show that the first stage of the ore-forming fluid inclusions are rich in Ba and Sr and SO₄^2-and CO₃^2-.The second stage,the inclusions are rich in H₂O,CO₂,and a few amount of O₂,H₂S and olefin.The third stage they are rich in H₂O,CO₂,and a small amount of H₂S and butane.The fourth stage they are rich in H₂O,CO₂,H₂S and few of O₂,propane,methane and N₂,suggesting that the different ore-forming stages have different nature of ore-forming fluid.3.Analyses results of the trace and rare earth in barite show that the total rare earth is not the same during different stages,suggesting the complexity of ore-forming material sources.The Sr content in most barite is higher,with obvious Eu negative anomaly and negative Ce anomaly,suggesting that the barite probably form in the oxygen enriched environment.The value of Y/Ho ratio in different stage barite changes a lot and the ratio of outward from Luowei rock increases gradually,suggesting the deep mineralization may also be influenced by Luowei magmatic fluid.4.Analyses of stable isotope S and O in barite and pyrites show that the Pingdian deposit generally have higher O isotopic composition,and the value of oxygen isotope in barite is relatively concentrated,far higher than that of modern water,and the S isotope variation range is larger,but significantly lower than that of modern water,with a lower value ofδ³⁴S/δ¹⁸O.Therefor,it is suggested that the sulfur and oxygen of the barite in the Pingdian deposit should not all come from the seawater,and the formation of the first stage of barite probably come from the evaporation and concentration of primary water or seawater.The formation of the second phase of the massive barite experienced organic matter degradation.The third stage of barite is likely related to the marine transgression and regression,and also to the release of methane,and the overlying karst caves in limestone or dolomite could provide ore-forming spaces.The fourth stage of the vein barite has similar characteristics with the second stage of the barite.5.Together with the characters of ore-controlling structure,ore-forming fluid properties and its source of materials,the metallogenic model of the Pingdian barite deposit is preliminarily established,suggesting that in the Devonian time,the sedimentary origin of barite ore（or ore-source）layer were deposited,forming a symbiotic with pyrite gray barite lump barite（S1）and white block barite（S2）.During the later time of tectonic movement stages,such as the Indosinian and Yanshanian tectonic reworking stages,early formed orebodies or Ba-rich host-rocks are deformed,leading the extraction of ore-forming material and forming layered barite（S3）showing as the hydrothermal types,and the vein-type barite ore body（S4）should form during the latest stages.As a result,the ore deposit should belong to a type of sedimentary and hydrothermal-modified deposits.