The relationship between ductile shear zone and mineralization in the Jiufeng Sn deposit, northern Guangxi, South China: Evidence from structural analysis and cassiterite U-Pb dating

Abstract

The Jiufeng Sn deposit is located in the outer contact zone southwest of the Neoproterozoic Motianling granitic pluton, in northern Guangxi, South China. The deposit is characterized by a NNE-striking, 30–200-m-wide ductile shear zone, named the Jiufeng ductile shear zone. Majority of the Sn orebodies, including four large ones, are hosted by the shear zone. Field and microstructural observations, electron backscatter diffraction measurements, and magnetic fabric analyses show that the shear zone is characterized by sinistral thrust shearing. The shear zone has also slightly modified the Sn orebodies. Quartz in the shear zone exhibits core-mantle structure and undulose extinction. Quartz has experienced significant sub-grain rotation and grain-boundary migration recrystallization. Lattice preferred orientation analysis reveals primarily basal <a>, rhomb <a>, and prism <a> slips. Cassiterite grains in the shear zone, however, demonstrate brittle deformation. Therefore, the deformation temperature during the ductile shearing is estimated lower than 650 °C and below U-Pb closure temperature of cassiterite. As such, the U-Pb isotopic composition of the cassiterite was not disturbed by the shearing and the U-Pb age of the cassiterite could be used for the age of the Sn mineralization. LA-ICP-MS U-Pb dating of cassiterite grains selected from a cassiterite-bearing tourmalinite sample performed in this study yielded a Tera–Wasserburg U-Pb lower intercept age of 832.3 ± 5.3 Ma. This age is consistent within error with that of the neighboring Neoproterozoic Motianling granitic pluton, which suggests a close genetic relationship between the Sn mineralization and the granitic magmatism. Studies show that the Jiufeng ductile shear zone was produced by a widespread Caledonian (or Kwangsian) tectonic event in South China, indicating that the shearing was much later than the Sn mineralization and has no generic relationship to the Sn deposit.