Petrogenesis of granites in Shimensi in northwestern Jiangxi Province and its implications for tungsten deposits

The Late Mesozoic Dahutang tungsten deposit, as one of the largest tungsten deposits in the world, is a recently discovered deposit in Jiangxi Province. It is situated in border area of Wuning, Xiushui and Jing'an counties, and also located in the eastern part of the Neoproterozoic Jiangnan orogen. This area belongs to Mesozoic W-Cu-Mo polymetallic ore-forming belt in South China Block with widespread Jinningian diorites and Yanshanian granites. In this study, the authors investigated the petrography, mineralogy, zircon and cassiterite U-Pb geochronology and trace element concentrations of the Neoproterozoic and Yanshannian granitic intrusions in the Shimenshi ore district. Zircon U-Pb dating results of the Neoproterozoic biotite granodiorites and Yanshannian biotite granite porphyry, greisenized fine-grained granites and greisenized biotite granite porphyry are (829.9±4.7) Ma, (145.5±3.6) Ma, (152.6±2.0) Ma and (154.2±1.7) Ma, respectively. In particular, the crystallization ages of the Yanshannian granites are consistent with the new cassiterite U-Pb age results obtained by the authors. The tungsten content in cassiterite from the ore-related fine-grained biotite granites is obviously higher then tungsten content of other Yanshannian intrusions. This implies a higher tungsten concentration in the fine-grained biotite granitic magmas. Apparent temperatures for zircon crystallization calculated by Ti-in-zircon thermometer suggest lower temperatures of Yanshannian magmas with weighted average temperature ranging from 734℃ to 788℃. Trace element concentrations of zircon exhibit that fine-grained biotite granites were formed under low oxygen fugacity condition, which was favorable for the formation of tungsten ores.