The Xigou Pb-Zn-Ag deposit in Luanchuan, Henan Province is located north of the Luanchuan fault which occurs along the southern margin of the North China Craton. This deposit is of stratabound and hosted in Neoproterozoic carbonate strata and was considered as Neoproterozoic SEDEX-type deposit. The paragenesis, from early to late ore stage, includes: fine-grained pyrite, coarse-grained pyrite-sphalerite-dolomite.-quartz assemblage, polymetallic sulfides-dolomite-quartz assemblage, and pyrite-quartz-carbonate assemblage, respectively. In this study, the single-grain Rb-Sr analytical technique is employed on sulfides and biotite to date the Pb-Zn-Ag mineralization of the deposit. Five biotite grains from a host-rock calcic two-mica schist sample yield a Rb-Sr isochron age of 366.0 +/- 10 Ma, which is interpreted as metamorphic time of the host rocks. Field observations show that mineralization most probably postdated the metamorphism of the host rocks, as pyrite-bearing carbonates or sphalerite veinlets crosscut the schistosity or band of the metamorphic rocks and the orebodies did not undergo regional metamorphism. Considering that the tectonic deformation, massive granitic magmatism and mineralizations of Mo, Ph, Zn and Ag cluster in the period of 156 similar to 134 Ma with a peak at 138 Ma in the adjacent areas, it is reasonable to assume that the Xigou Pb-Zn-Ag deposit formed at ca. 138 Ma. Fine-grained pyrite of the early ore stage, coarse-grained pyrite and sphalerite of the main ore stage fail to give reasonable Rb-Sr isochron ages, probably due to overprint of later hydrothermal processes. Five fine-grained pyrite grains of the early ore stage give low ISr-138Ma values (the initial Sr-87/Sr-86 values at 138Ma) of 0.7100 similar to 0.7151, with an average of 0.7127, slightly higher than those of late Jurassic to early cretaceous intermediate-felsic porphyries and granite plutons, significantly lower than those of the Archean Taihua Group (metamorphic basement rocks), the Mesoproterozoic Xiong' er Group (andesitic volcanic rocks), and overlying schists of Meso-Neoproterozoic Luanchuan and Guandaokou groups, but close to those of hosting marble of the Luanchuan Group, indicating that metamorphic devolatilization of carbonate rocks and magmatic fluids are possible sources of ore fluids. In contrast, sulfides in main ore stages show higher and variable radiogenic Sr isotope contents: fourteen coarse-grained pyrite grains show ISr-138Ma values of 0.7152 similar to 0.7344, with an average of 0.7247. Thirteen sphalerite grains give ISr-138Ma values between 0.7108 similar to 0.7398, with an average of 0.7283. These ISr-138Ma values are significantly higher than those of late Jurassic to early Cretaceous intermediate-felsic porphyries and granite plutons but close to or lower than those of the Taihua Group, the Xiong' er Group, and the Luanchuan and Guandaokou groups. Thus, it can be deduced that the early ore stage was probably dominated by metamorphic or magmatic fluids, and in the main ore stage, involvement of the overlying strata (the Luanchuan Group) was significant, through water-rock interaction and/or input of meteoric or formation water.
