Neoproterozoic tin mineralization in South China: geology and cassiterite U–Pb age of the Baotan tin deposit in northern Guangxi

Abstract

The Baotan tin deposit (23 Mt @ 0.43% Sn) is located in the Jiuwandashan–Yuanbaoshan area, South China. It is hosted in Neoproterozoic mafic/metasedimentary rocks and apical portions of the Pinying granite pluton. Six alteration and mineralization stages have been identified: pre-ore alteration, cassiterite greisen, cassiterite–tourmaline–quartz vein, cassiterite–quartz vein, cassiterite–sulfide vein, and post-ore quartz/calcite–quartz vein stages. Tin mineralization is mainly in the cassiterite greisen, cassiterite–tourmaline–quartz, and cassiterite–quartz vein stages. The deposit is characterized by widespread tourmalinization. Both pre-ore and ore-stage tourmaline is schorl. Tourmaline from pre-ore tourmaline–quartz nodules has elevated Al2O3 and F contents and Fe/(Fe + Mg) and Na/(Na + Ca) ratios, which are probably controlled by the initial magmatic fluid. Ore-stage tourmaline shows low Al2O3 and F contents and Fe/(Fe + Mg) and Na/(Na + Ca) ratios, which are likely influenced by the surrounding mafic rocks. LA–ICP–MS U–Pb dating on two cassiterite samples from disseminated cassiterite–tourmaline–quartz ore and cassiterite–quartz vein yields 206Pb/238U weighted mean ages of 832 ± 5 Ma and 834 ± 4 Ma (2 σ), respectively. These two dates are consistent with the previously reported zircon U–Pb ages of 834–835 Ma for the Pingying granite, which indicates that tin mineralization is related to the granite. The granite has low magnetic susceptibility and zircon Ce4+/Ce3+ ratios, which are similar to those of Sn-bearing ilmenite-series granites. Our study confirms the Neoproterozoic tin mineralization event in South China and indicates that the Neoproterozoic highly fractionated S-type granites in the southeastern margin of Yangtze Block have a great potential for tin mineralization.