| Pangushan tungsten deposit is one of the most important quartz-vein type of W-polymetallic deposit in Southern Jiangxi Province,China. After90years of development,it is high time to find substitute resources for the resources of the mine is nearly exhausted. Although exploration and development history in this deposit is very long, the research is not high particularly insufficient in the field ore-forming fluids.Due to lack of cogniotion in ore-forming conditions and ore-controlling factors,further prospecting and exploration work has been restricted. In this paper,ore-forming fluids is as an entry point,fluid inclusions are the main study object. We got some valuable progress and understanding from carried out systematic study of deposit physical and chemical conditions,sources of ore-forming fluids and metallogenic age on the basis of fluid inclusions investigative work.Three types of fluid inclusions:H2O-NaCl (Ⅰ type)、H2O-NaCl-CO2(Ⅱ type)、pure liquid phase CO2(IIItype) have been recognized in samples from the wolframite-bearing quartz of the major mineralizing stage. Then,type I can also be divede into two forms:rich liquid L+V two-phase H2O-NaCl(Ⅰa type)、single aqueous phase H2O-NaCl(Ⅰb type); type Ⅱ can also split into three phase H2O-NaCl-CO2(Ⅱa type) and two phase H2O-NaCl-CO2which lack of liquid CO2(Ⅱb type).Type I fluid inclusions homogenization temperatures(Without the pressure correction) are widely from100℃to370℃with multi-peaks distribution, mainly in110～210℃、230～270℃and270～370℃. It indicates this deposite has a complicated mineralization process and many times metallogenic,ore-forming fluids shows characteristic of low salinity,generally<10w(NaCl)%. The type II fluid inclusions homogenization temperatures(Without the pressure correction) are range in three scopes:220～250℃,260～350℃and290～320℃, while mainly in290～320℃,with low salinity,generally<8w(NaCl)%. The homogenization temperatures of type III are-6.3～26.4℃.In order to find out how many periods of ore-forming fluid in Pangushan tungsten deposit,we analyse type I fluid inclusions by the means of "fluid inclusion assemblage",and choose5groups clustering distribution FIA,4groups directional distribution FIA,3groups distributed along the fracture FIA. After petro-graphic observation and temperature measuring statistics,we know primary FIA are clustering and directional distributed,which homogenization temperatures can fall into three stages:220～260℃、250～290℃as well as310～320℃; secondary FIA are distributed along the fracture, its homogenization temperatures can also divede into two stages:150～180℃、170～200℃.Thus, we can infer that in Pangushan tungsten deposit, there are existing at least five periods ore-forming fluids,of which former three stages are metallogenic period hydrothermal activity,produced primary inclusions;while followed two peroids are hydrothermal activived after diagenesis, formed secondary inclusions.After statistical analysis of type Ⅰ、Ⅱ fluid inclusions homogenization temperatures acording to rock ore and levels in this deposit,we found different levels analyze results in both of the whole mining area and single ore body are reflect the following basic features:1)in every levels,the fluid inclusions homogenization temperatures are widely in range;multi-peaks,to be divided into three temperature sections,which is consistent with the inclusions overall distribution characteristics.2)From bottom to up,the homogenization temperatures are getting high trend,that is,in one ore wall,the high-temperature data in upper levels are exceeding in bottom,which shows the reverse zoning characteristics,this may be related to CO2escape characteristics which existing in most of tungsten polymetallic ore deposit. The ore-forming fluids containing CO2escape to upper ore body through developed mineralization cracks in this deposit,which makes temperatures of reverse zonation.This paper adopted two means to get fluid inclusions compositions,one is single inclusions testing,the other is groups of inclusions testing.As single inclusions using laser Raman test, results showed that inclusions detected a broad envolope of liquid H2O peak,except CO2,the gas composition of fluid inclusions contain a small amount of CH4and N2,have typical CO2,N2and CH4peaks in the Raman spectra,which is consistent with the micorothermemetry.Groups of inclusions using the thermal explosion-ultrasonic extraction,the results show inclusions univalengt cation ΣMe+>>divalent cations ΣMe2+.In divalent cation,Na+occupies absolutely superiority,and in divalent ion,Ca2+>Mg2+.Dominant anions is Cl-,followed by SO42-,while F" is minimal. As to its vapor component,CO2amounts for the top,other component such as CO、CH4and H2content is minimal,and caters to Raman results.Thus, we can conclud that in Pangushan tungsten deposit,CO2is the main vapor of tungsten mineralization fluid and plays a big role in the mineralization process,but other violatile are not so important.Using fluid inclusions to estimate the ore-forming pressure has been very widely applied. As CO2-bearing inclusions is a typical sign of the high-pressure phase,through CO2homogenization temperatures and final homogenization temperatures of type Ⅱ fluid inclusions, this paper calculated its ore-forming fluid trapping pressures,which is 36.3～97.8Mpa, an average of72.2Mpa, converted into ore-forming depth of1.4～3.76km by lithostatic pressure,an average of2.78km.,belongs to the range of tungsten deposite metallogenic depth in south China.Homogenization temperature is measured in the atomospheric conditions, and do not represent the capture temperature. If we want to get capture temperature by homogenization temperature,we should considering the influence of pressure: Tt=Th+ΔT.As the mean pressure is72.2Mpa in Pangushan tungsten deposit,we get the type I fluid inclusions trapping temperature are55～75℃higgher than homogenization temperature by pressure and salinity diagram of ore-forming fluid. So,its capture temperature ranges between160～450℃,of which,primary fluid inclusions assemblage capture temperature are respectively:280～320℃、310～350℃and370～420℃,In sum,this deposite is a high-temperature hydrothermal deposite.Combining Rb-Sr isochron dating data of this paper and Re-Os isochron dating material from our program team,Pangushan tungsten deposit can be seperate into three metallogenic ages:182±2Ma,158±1Ma and147±2Ma.It is in line with fluid inclusions reasearch result.Inclusions parameters such as mean value of fo2:-28.16、pH:5.02and Eh:0.16V are indicate that ore-forming fluid shows the features of strong oxidability and weak aciddulous in Pangushan tungsten deposit. At the same time, it is caters to the tungsten deposit mineral assemblage trait of quartz as gangue mineral,wolframite as major ore mineral.The hydrogen and oxygeon of fluid influsions aqueous isδ18OH2O=-1.5～+5.3‰;8D=-64.5～-49.3(‰).It manifests that ore fluid are mainly composed by magmatic water and rainfall sweaked into it at later stage. |
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