Petrogenesis of the Neoproterozoic Fanjingshan leucogranite associated with W-Sn mineralization: Insights from U-Pb ages and geochemistry

The Jiangnan Orogen in China host large volumes of Neoproterozoic igneous rocks and is a well-preserved Neoproterozoic orogenic belt in the South China Block. A series of W-Sn mineralization occurrences and ore deposits are spatially related to Neoproterozoic granites in the orogen. However, the geochronology and petrogenesis of the host Neoproterozoic granites and associated W-Sn mineralization in the western Jiangnan Orogen remain poorly understood. In this study, the U-Pb ages of zircon, cassiterite, and apatite, and the geochemistry of minerals and whole rock of the Fanjingshan (FJS) leucogranites, which are located along the western margin of the Jiangnan Orogen, were used to re-evaluate the Neoproterozoic magmatism that occurred in response to the orogeny and the mechanism of W-Sn mineralization. The FJS leucogranites consist mainly of strongly peraluminous tourmaline-muscovite granites. The leucogranites have a characteristic magmatic and hydrothermal mineral assemblage, including cassiterite, tourmaline and fluorite. Zircon U-Pb dating indicates the leucogranites were emplaced at ca. 812 Ma, which is consistent with apatite U-Pb age. Cassiterite U-Pb dating yields an age of ca. 810 Ma, suggesting the mineralization was coeval with the crystallization of the FJS leucogranites. In multi-element variation diagrams, all samples display sea-gull-shaped rare earth element patterns with significant negative Eu anomalies. The whole-rock geochemical data suggest the leucogranites originated by muscovite dehydration melting of a metasedimentary source. Extensive fractional crystallization and magmatic-hydrothermal interactions, as inferred from extremely low rare earth element contents, low Nb/Ta and Zr/Hf ratios, and a large tetrad effect, lead to the W-Sn enrichment and crystallization of ore minerals, such as cassiterite and scheelite. The compositions of muscovite, apatite, cassiterite and tourmaline, along with the occurrence of quartz veins, also provide information on the nature of the magmatic-hydrothermal processes and mineralization. Our data and those for other S-type granites (840-800 Ma) in the western Jiangnan Orogen, suggest that magmatic-hydrothermal processes had a key role in W-Sn mineralization. The Neoproterozoic FJS leucogranites likely formed in an extensional setting during the final stage of orogenesis.