The Rb-Sr isotopic dating of pyrite mineral from ore deposits can directly provide mineralization age. However, many geological factors may affect the Rb-Sr isotopic system, which baffles application of this method. Employing ultra-low procedural blank Rb-Sr method, we have dated pyrites separated from the No. 4 breccia pipe of the Qiyugou gold deposit, western Henan Province. Single grains of euhedral pyrite crystal with few micro-cracks yield an isochron age of 126 ± 11 Ma, which represents time of the main mineralization stage of the deposit. Pyrite grains of cataclastic type show nevertheless scattered Rb-Sr isotopic composition and no reasonable isochron can be defined. Crystal morphology and mineral inclusion studies reveal that Rb and Sr of pyrite mineral probably are preserved mainly in biotite, K-feldspar, and sericite mineral inclusions. The dating results likely suggest that cataclastic pyrite is not suitable for the Rb-Sr dating due to modification of the Rb-Sr isotopic system by later hydrothermal activity of fluid.pyrite, Rb-Sr dating, single grain, ultra-low procedural blank, QiyugouAs a common sulphide in gold deposits, pyrite mineral is an important gold host usually forming simultaneously with mineralization. Therefore, isotopic dating on pyrite mineral can directly provide ore-forming ages because of its strong crystalline capacity, rapid crystalline rate and thereby potential to keep isotopic closure [1,2] . The Rb-Sr isochron method has been applied to pyrite in recent years and whereas some successful examples have been reported [3][4][5] , in many cases this method failed to give reliable ages. In order to improve application of this method effectively, it is crucial to reveal geological factors influencing the Rb-Sr dating of pyrite. Yang and Zhou [3,4] firstly dated pyrite successfully using the Rb-Sr isochron method in combination with a "sub-sample" technique (pyrites from different parts of a sample). But this method needs a large quantity of pyrite samples (about 500 mg) and in most cases, it is difficult to identify and separate crystals of syngenetic and synchronous pyrite grains. Recently, a combination of ultra-low procedural blank and high precision measurement of Rb-Sr isotopic ratios has been achieved for analysis of micro-samples [6,7] , which leads to attempting dating single-grain or micro-sample of pyrite [5] . In this study, euhedral and cataclastic pyrite minerals from a breccia pipe of the Qiyugou gold deposit were dated by using the single-grain Rb-Sr method. The euhedral pyrite grains yield a good isochron age, while the cataclastic pyrites cannot define reasonable isochron probably due to modification of the Rb-Sr sys-