Biotite mineral chemistry and their implications for petrogenesis and mineralization in the porphyry copper deposit belt in northwestern Yunnan, China

The Zhongdian island arc is located at the south end of the Yidun island arc within the Sanjiang tectono-magmatic belt. Influenced by subduction of the Ganzi-Litang Ocean, Indosinian magmatism is closely related to the Pulang porphyry and Langdu porphyry-skarn type copper deposits and Songnuo porphyry copper spot. Majority of researches thus far have been focused on the petrological, chronological and geological features of Pulang and Langdu mineralized and Songnuo barren quartz monzonite porphyries. In this paper, the major and trace element concentrations of biotites, measured by EPMA and LA-ICP-MS techniques, are used to investigate the petrogenesis and mineralization of biotites. The EPMA analyses of major elements demonstrate that biotites from Pulang are distinguished from Langdu or Songnuo biotites by higher MgO, TiO2 and lower FeO, Al2O3 abundances but little differences in SiO2, Na2O, K2O and CaO contents. Analyses of REE suggest that ∑REE of biotites are less than 5%(mass fraction) of the whole rock in all three regions. Trace elements analyses illustrate that biotite is an important carrier of Rb, Ba, K, Ti, Nb, Ta, but it has less ability in hosting Th, U, Pb, Sr, Zr, Hf and Y in comparison with host rocks; nevertheless, ∑REE and Rb, Hf and Cr contents in biotites from Pulang are higher than that from Langdu and Songnuo. The mineral chemistry of biotites show that, the quartz monzonite porphyry from Pulang formed under condition of higher temperature and oxygen fugacity, and it has a higher degree of differential evolution compared with porphyry copper deposit from Langdu and Songnuo. Moreover, the results show that Cu concentration is higher in mineralized plutons from Pulang or Langdu than in barren plutons from Songnuo. © 2017, Editorial Office of Earth Science Frontiers. All right reserved.