The Shilongtou gold deposit in Kaihua of western Zhejiang lies in the Anhui-Zhejiang-Jiangxi fault-fold belt, with its orebodies obviously controlled by regional faults. In order to understand the mineralogy of pyrite and the indicator of gold mineralization, the authors studied the pyrite by using X-ray diffraction (XRD) analysis, electron microprobe analysis and sulfur isotopic composition analysis. The results show that major metal minerals are pyrite and arsenopyrite, with minor limonite and pyrrhotite, whereas gangue minerals are dominated by quartz, sericite, illite and dolomite. Electron microprobe analyses of auriferous pyrite show that the (Fe)/(S+As) ratios are between 0.777 and 0.886, indicating that it formed at the middle to shallow depth. Co content is relatively low (64×10^-6～111×10^-6), and Au/Ag ratios vary from 3.6 to 17.8, suggesting that it formed in a low temperature environment. Co/Ni ratios are between 1.95 and 4.47, with an average of 3.59, indicating its hydrothermal genesis. The high As content and the As-Co-Ni ternary diagram show that the hydrothermal fluid was related to primary magmatic activity, with a nature of underground thermal brine. Sulfur isotope values vary from 6.14‰ to 8.27‰, similar to those of ultramafic rocks from deep crust, with the values affected by crustal sulfur at the later stage.