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 Si^Ⅳ 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.