In situ analysis of mica from the Daguanshan granites in Western Yunnan: Implication for Sn enrichment and mineralization

Abstract

The potential of tin mineralization associated with the Triassic Lincang granite batholith in Western Yunnan, China, has been poorly understood, despite the batholith being considered the northern extension of the Southeast Asian Tin Belt. To address this issue and gain insights into the mechanism of tin enrichment, we investigate the major and trace element compositions of biotite and muscovite from four granite units in Daguanshan, including hornblende-biotite granite, biotite granite, two-mica granite, and muscovite granite in the Hongmaoling Sn deposit. From the hornblende-biotite granite to two-mica granite, the biotite composition varies from Mg-biotite to Fe-biotite with increasing MnO and decreasing K/Rb. The Fe3+-Fe2+-Mg2+ diagram and decreasing Mg/(Fe + Mg) ratios of the biotite indicate reducing conditions. From two-mica granite to muscovite granite, the muscovite Li, F, Rb and Cs concentrations generally increase with decreasing K/Rb, suggesting a magmatic fractionation trend. The variations in zoned muscovite composition reflect a high degree of magmatic fractionation and a lack of fluid interaction. Biotite tin concentrations increase from ∼ 18 ppm in the hornblende-biotite granite to ∼ 36 ppm in the biotite granite and further to ∼ 62 ppm in the two-mica granite. In addition, tin concentrations increase from ∼ 180 ppm in the core to ∼ 330 ppm in the rim of muscovite in the muscovite granite. These data indicate that magmatic fractionation is critical to Sn enrichment in the Hongmaoling deposit. The Nb/Ta ratio and tin concentration in mica from tin mineralized granites are generally higher than those from barren granites, indicating they could be used as reliable indicators for assessing tin metallogenic potential.