Sr-Nd-Pb isotope geochemistry is widely used in the study of ore deposits, mainly including: (1) tracing the source of ore-forming materials (Kesler et al.,1988; Schneider et al., 2002; Xu Dongqing et al., 2008c); (2) Isotopic evolution tracing of ore-forming fluids (Ruiz et al.,1988); (3) Inhibition of hydrothermal fluid activity and fluid mixing (Andrew et al.,1984; Ruiz et al.,1988; Schneider et al., 2002). Sucha deposit and Aobaotu fluorite deposit are mainly located in the outer contact zone of early Cretaceous granite, while the fluorite mineralization point in Guiletai and the fluorite mineralization in fine-grained granite veins are all located in granite. The Sr, Nd and Pb isotopic geochemical characteristics of fluorite from different deposits undoubtedly record the evolution of ore-forming fluids and the main information of ore-forming materials during fluorite mineralization. Many previous works have proved that the ore-forming fluids and material sources of fluorite are mixed (Deans et al.,1965; Ruiz et al, 1985,1988; Barbieri et al.,1987; Kannar et al.,1993; Galindo et al, 1994,1997; Simonetti et al.,1995; Menuge et al., 1997). The strontium isotopic composition of fluorite in Sucha fluorite deposit ranges from early Cretaceous granite to lower Permian volcanic rocks of Dashizhai Formation. Therefore, the strontium isotopic composition of fluorite can not be provided by volcanic rocks of Dashizhai Formation, but mainly comes from early Cretaceous granite. The initial Nd isotope ratio of fluorite (143Nd/ 144Nd)i is larger than that of volcanic rocks and marble of Dashizhai Formation (143Nd/ 144Nd)i, but smaller than that of early Cretaceous granite (143nd//kloc-) From the Dashizhai Formation of the Early Permian to the early Cretaceous granite, the Nd isotope evolution of magmatic rocks in this area shows a trend of increasing εNd(t) with time, which reflects that mantle-derived materials are more involved in the diagenesis and mineralization process with the evolution of magma from the old to the new. Geochemical research shows that fluorine-rich ore-forming fluids can be distinguished during the evolution of alkali-rich magma (Simonetti et al., 1995), neodymium isotopes can be used to trace the source of fluorine in rocks (Ronchi et al., 1995), and the sources of calcium and strontium in the ore-forming process may mainly come from volcanic rocks and marble lenses of Dashizhai Formation. It is not uncommon that the sources of fluorine and calcium in this ore-forming fluid are inconsistent. The experimental results of Eills( 1979) also show that the fluid rich in fluorine is not necessarily rich in calcium. Therefore, the formation of fluorite ore-forming fluid generally exists the mixing process of fluorine-rich fluid and calcium-rich surrounding rock. In the early Cretaceous, fluorine-rich ore-forming fluid was formed by acid and alkali-rich magmatism, and rose along the structural weak plane of volcanic-sedimentary rocks in Dashizhai Formation (interlayer fracture zone between different lithologic sections), and reacted with rhyolite, rhyolitic tuff and halite lens in Dashizhai Formation, resulting in the oversaturation of CaF2 _ 2 in ore-forming fluid and large-scale precipitation, forming fluorite deposits. In this process, ore-forming fluids are mixed by different geological units and inherit different isotopic marks, thus showing the characteristics of mixed source in fluorite isotopic composition. Kaolinization, which is widely developed around fluorite veins, is a record of this major geological event. The K-Ar ages of altered minerals sericite and illite are highly coupled with the magmatic crystallization age indicated by the SHRIMP U-Pb age of granite, indicating that the acidic and alkali-rich magmatism in the early Cretaceous provided the main ore-forming fluid and material source for fluorite mineralization.

The two-stage model age T2DM of all fluorite minerals is between 1153 ~1599 Ma, and the two-stage model age of only1sample is 2068 Ma, and most samples are in153 ~/kloc-0.

Sucha area is located in the Late Paleozoic orogenic belt in Central Asia, which experienced island arc magmatism on the 200 Ma active continental margin (Chen et al., 2008) and Permian acid volcanic eruption, and then in the Late Permian and Early Triassic (about 234 Ma) (Chen et al., 2000, 2008; Xiao et al., 2003) The paleo-Asian Ocean subducted and closed, and the North China plate and the Siberian plate joined together to form a unified continent.

Since the late Mesozoic, this area has undergone the transformation of the tectonic system. From the late Jurassic to the early Cretaceous, this area turned into the extensional tectonic stage, and the crust gradually extended from south to north, forming a series of extensional fault basins and alkaline granites. Combined with geophysical research, it is considered that the lower crust subsidence occurred in this area, resulting in the thinning of the lithosphere (crust thickness (29 ~ 37 km), Wu Fuyuan et al.,1999; Zhai Ming et al., 2002; Xu Xuan et al., 2004). The discovery of potassium salt (11210 ′ e, 4200 ′ n) in the north of Siziwangqi (108 ~128ma, Xu Yao et al., 2004) confirmed the transformation of this tectonic system. Zhang Shuangtao et al., 2005; Li Yi et al., 2007). The emplacement age of the early Cretaceous Aobaotu granite exposed in Sumochagan area is highly consistent with the K-Ar age of altered minerals sericite and illite in fluorite veins of Sucha deposit and the emplacement age of fine-grained granite veins containing fluorite mineralization, indicating that Yanshanian acid magmatism is closely related to fluorite mineralization in time and space. The petrochemistry of granite is characterized by high silicon and alkali, high neodymium isotope εNd(t), and lead isotope distributed near the lead evolution line of orogenic belt, showing the mixed source characteristics of crust and mantle. Therefore, the formation process of early Cretaceous granite may be formed under the background of rising asthenosphere material and thinning lithosphere. In this process, the rising asthenosphere material underlay the overlying crustal material, and some Precambrian blocks or volcanic-sedimentary rocks of Dashizhai Formation of Lower Permian were deeply melted, and after a certain degree of crystallization differentiation, they ascended and emplaced to form granite distributed in a large area. At the same time, the alkali-rich granitic magma precipitated ore-bearing fluid with high fluorine content during its magmatic activity, which mixed with the ascending mantle fluid, and reacted with volcanic rocks and carbonate rocks in the early Permian not only in the granite body, but also along the reactivated regional large faults and interlayer fracture zones, resulting in the supersaturated precipitation of CaF2 _ 2 in the ore-forming fluid, forming super-large Sucha deposit, Aobaotu deposit and scattered fluorite mineralization points.