Characteristics of Micas from Sericitolite of the Gongpoquan and Baishantang Deposits, Beishan Area by Scanning Electron Microscope, X-ray Diffraction and Electron Microprobe Analyses

The phyllic alteration zone is regarded as one of the most important near-surface markers in mineral exploration of porphyry deposits. The Gongpoquan and Baishantang copper deposits are two typical porphyry deposits in the Beishan metallogenic belt. A comprehensive structural and chemical study for white micas from sericitolites of these two deposits, utilizing polarized microscope, Scanning Electron Microscope (SEM), powder X-ray Diffractometer (XRD) and Electron Microprobe Analyzer (EMPA) is reported in this paper. It was discovered that both 1M polytype common sericites/muscovites and 2M1 polytype phengites co-exist in the samples. The micas occur as irregular petals, kinked platelets or straight platelets during SEM observation. According to XRD analyses, both 1M and 2M1 polytypes co-exist with different b0 values in micas, indicating their different forming conditions. The SiⅣ atoms in micas range from 3.04 to 3.37 and from 3.03 to 3.35 for Gongpoquan and Baishantang deposits, respectively, indicating the co-existence of sericites/muscovites and hydrothermal phengites. The co-existence of two types of white micas in porphyry copper deposits of the Beishan area indicate the rapid changes of ore-forming fluids from higher temperature and acidity to lower temperature and less acidity during mineralization.