Mineralogical chemical characteristics of biotites in granite (porphyry) of the Dulong Tin-polymetallic mine in the southeastern Yunnan and their geological significances

The late Yanshanian Laojunshan granite complex in the Dulong mine in the southeastern Yunnan is closely related to the W and Sn mineralization. The in-depth comparative study of the compositional characteristics of minerals in different types of granites is of great significance for understanding the ore-forming process and evaluating the ore-forming potential of different types of granites. In this paper, we have carried out mineral chemical study of biotites in the late Yanshanian granite and granite-porphyry in the Dulong mine by using the EPMA and LA-ICP-MS analytical methods. The results show that the Fe/(Fe+Mg) ratios of biotites are relatively high with ratios mainly concentrated in a range from 0.60 to 0.70. Combined with data of previous studies, it is believed that biotites of the Laojunshan granite and granite-porphyry belong to siderophyllite and ferrous biotite. By using the Ti saturation thermometer and Al manometer of biotite, we have estimated the crystallization temperatures of biotites in granite and granite-porphyry in the Dulong mine. The calculated temperatures vary from 569℃ to 676℃ for biotites in granite and from 544 ℃ to 642 ℃ for biotites in granite-porphyry, and the calculated pressures vary from 419Mpa to 460Mpa for biotites in granite and 440Mpa to 494Mpa for biotites in granite-porphyry, respectively. In the diagram of Fe3+-Fe2+-Mg2+, data of the biotite samples are mainly plotted in fields between the Ni-NiO and Fe2SiO4-SiO2-Fe3O4 buffer lines, indicating that biotite crystals in granite and granite-porphyry of the Dulong mine were mainly formed under the relatively low oxygen fugacity condition. Biotite is the main carrier mineral of Sn, In and other ore-forming elements in granite and granite-porphyry of the Dulong mine, and the contents of Sn, In and other ore-forming elements in biotites can be used as indicators to evaluate their ore-forming potentials of granite.