Abstract: Using the X-ray diffraction to analyse the fault gouge from the fault in deep of DongSheng-miao polymetallic pyrite deposit.The phase results show that the pyrite,sphalerite,pyrrhotite,kusuite,nantokite and some xanthoconite are contained in fault gouge.Then minerals such as quartz,calcite,gypsum,dolomite and phlogopite reflect some properties of the surrounding rock.In addition,the distribution of XRD diffraction peak from mineral crystals can indicate the mineral crystallization degree,which corresponding with the activities of deep-seated faults.A fierce activity in fault would lead to a higher degree of mineral crystallization,and vice versa.Furthermore,according to the Scherrer's formula,we estimated the average size of mineral particles each sample by selecting the FWHM (β) and the diffraction angle (θ) in strong peaks.The results show that the size distribution of most mineral crystals in fault gouge is nanoscale.Combining with the geogas theories,thought that such nano-sized mineral particles can be carried to the surface by the ascending gas flow from interior of the earth.Moreover,it can provide some basis for study of deposit and prospecting.