The Zonal Distribution of W- and Sn-Mineralized Granites in the Nanling Metallogenic Belt, South China: The Role of Magma Sources and Differentiation

The Nanling metallogenic belt of South China hosts numerous world-class late Mesozoic W deposits in the eastern domain and Sn deposits in the western domain. However, the cause of this zonal distribution remains enigmatic. Geochemical discrepancies between Late Jurassic Sn- and W-mineralized granites may account for the metallogenic zonation. The W granites have lower CaO/Na2O ratios as well as MgO, TiO2, and F contents, and higher Rb/Sr and Rb/Ba ratios and Mg# than the Sn granites, suggesting that the magmas were derived from metapelite-dominated and metagraywacke-dominated sources, respectively. Phase equilibrium modeling reveals that anatexis of pelitic basement rocks may produce melts with W contents of up to 20 ppm, five times higher than the source rock. In contrast, high-temperature partial melting of psammitic basement rocks would generate Sn-rich melts with up to 18 ppm Sn, four times higher than the source rock. The modeling results suggest that two episodes of melt extraction are not a prerequisite for the generation of Sn-rich melts. The genetic relationship of pelitic protoliths with W deposits and psammitic protoliths with Sn deposits is also applicable to many W and Sn deposits globally. The W granites experienced higher degrees of differentiation, with lower Zr/Hf (<18), Nb/Ta (<5), and Th/U (<2) ratios than the Sn granites. The weaker differentiation of the Sn granites was related to their higher cooling rates, which reflects their shallow emplacement. Tungsten and Sn in fluids have distinctly different geochemical behavior, with strongly evolved magmas being more favorable for W enrichment than Sn, leading to the decoupling of W-Sn and the observed zonal distribution of W-Sn deposits.