The far-infrared reflectance and transmittance spectra of natural FeS2(Pyrite) at room and low temperatures are reported in this artical. The spectral structure and the physical origin are discussed in detail. Through our research we found that the plasmon reflectance edge at low frequency of room temperature reflectance spectra of FeS2(Pyrite) samples have a close relation with the natural impurity: PbS(Galena) in samples. We use the reflectance spectra at room temperature of FeS2(Pyrite) and PbS (Galena) mineral single crystals as basic spectra to reproduce the experimental spectra of different samples in various proportions. the x values we obstained form it not only reflect the relative quantities of PbS (Galena) impurity, but also have positive correlation with the noble metal: Au(Gold) indirectly. This may have some applications in mineral formation research.