Characteristic Of Fluid Inclusions And Genesis Of The Xishanwanyangchang Ag Polymetallic Deposit, Inner Mongolia

Xishanwanyangchang silver polymetallic deposit is located in the east section of Langshan-Zha’ertaishan Mesoproterozoic rift zone of the northern margin of north china plate that is a good metallogenic belt. On the basic collection of datas, geological survey in the field, analysising of the fluid inclusions, the metallogenic fluid characteristics and genesis are researched.Four types of inclusions in quartz of this deposit are distinguished including pure liquid inclusions, pure gas inclusions, gas-liquid two-phase aqueous inclusions and daughter mineral-bearing inclusions. The two-phase aqueous inclusions can be divided into liquid-rich and gas-rich inclusions according to the ratio of liquid to gas. And the daughter mineral-bearing inclusions can be divided into daughter halite-bearing and daughter polymetallic-sulfide-bearing inclusions according to the different mineral. Microthermometric study shows that the homogenization temperature values of the fluid inclusions mainly peak at 250~310℃, the salinity values peak at 0.35~10.0wt% NaCl and 28.76~32.66wt% NaCl, and density values peak at 0.434~0.929 g/cm3, which indicates the ore-forming fluid of the deposit is characteristiced by moderate Th, low salinity and low density. The high-salinity daughter mineral-bearing inclusions came into being in the mainly ore-forming period, because high temperature hypotherm met meteoric water or other low temperature fluids, which made the fluid boiling, and then uniformed fluid separated out the daughter mineral-bearing multiple-phase inclusions. It is estimated the average ore-forming depth is 2.01 km, which shows Xishanwanyangchang silver polymetallic deposit is plutonic with slight hydrostatic pressure.Laser Raman micro-spectroscopy of the fluid inclusions indicates the ingredients of the gas phase in the fluid of Xishanwanyangchang silver polymetallic deposit are mainly H2, CH4, N2 and H2 S, in which the content of H2, N2 are relatively more, CH4 is less, and H2 S whose content is less can be found occasionally. So the ore-forming fluid is a multicomponent system: H2—CH4—N2—H2S, CH4, H2 S and other reducing gases show that the ore-forming environment is reductive. The liquid phase composition is mainly H2 O, generally mixed with a small amount of CO2 and CH4. Furthermore, pyrite is the daughter mineral of a group of measured daughter mineral-bearing inclusions.According to the geological background,the mineral composition,orebody characteristics, the wall rock candle and the ore-forming fluid,we can summarize the genesis of the deposit. Hercynian movement and Indosinian movement squeezed the intrusive rocks and neutral- acidic magma invaded. The volcanic hydrothermal fluid invaded through the ductile shear zone, leached mineral substances of the layer, formed the ore-forming fluid.In the middle and the last period it was mixed with rainfall,that the physical and chemical characteristics of the fluid was changed. Silver complex separated and formed the spiderweb or vein silver ore body.Above all Xishanwanyangchang silver polymetallic deposit is a mesothermal and middle depth type deposit.