Application of SWIR spectrometry to the determination of biotite compositions in hydrothermally altered units of the Yangyang iron-oxide-apatite (IOA) deposit, South Korea

Abstract The Yangyang iron oxide-apatite deposit in South Korea has gone through multiple events for igneous activity, structural evolution, hydrothermal alteration, and mineralization. Trioctahedral mica is indicative of potassic alteration in the Yangyang IOA deposit and its mineral chemistry is quite variable. The variability can be described in terms of biotite colors and short wavelength infrared (SWIR) reflectance spectra. Brown to greenish brown biotite displays lower values of Mg# (0.48–0.58) than green biotite (0.62–0.78), whereas colorless phlogopite related to Fe–P mineralization has the highest values of Mg# (0.85–0.95). To compare Mg# values with the positions and intensities of absorption in the trioctahedral micas, we used three types of spectrum: reflectance, hull quotient, and Gaussian. Brown biotite produces a short Mg–OH peak near 2340 nm, green biotite has a peak near 2330 nm, and colorless phlogopite displays an absorption peak near 2310 nm. These positional variations are consistent with compositional changes. A mica alteration index (MAI) represents the ratio of Fe–OH and Mg–OH peak absorption intensities for trioctahedral micas. From brown to green biotite and colorless phlogopite, the MAI values gradually increase with decreasing Fe–OH intensity. Correlation coefficients for plots of Mg–OH absorption values vs. Mg# vary from 0.89 for the reflectance method, to 0.93 for the hull quotient method, and 0.93 for the Gaussian method, and for plots of MAI vs. Mg# they vary from 0.91 for the reflectance method, to 0.92 for the hull quotient method, and 0.92 for the Gaussian method. Analyses using our new SWIR spectroscopic methods confirm that the abundance of indicator minerals and the Fe:Mg ratios in trioctahedral micas vary systematically as a function of hydrothermal fluid chemistry and mineral composition within the altered lithologies. SWIR spectroscopy of trioctahedral micas could provide a useful tool for mapping vectors of alteration in the Yangyang IOA system.