Mineral Mapping Using Spectroscopy - From Field Measurements to Airborne Satellite-Based Imaging Spectrometry
Mineral physics dictates the appearance of rocks and soils across the electromagnetic spectrum. In the Visible/Near-Infrared (VNIR) and Short Wave Infrared (SWIR), many materials absorb radiation at specific wavelengths, allowing their identification by the position and character of absorption features. Electronic processes at wavelengths less ~1.0 micrometers allow identification of minerals containing Fe+3. Molecular vibrational features at wavelengths between ~1.0 and 2.5 micrometers are diagnostic of minerals containing anion groups such as Al-OH, Mg-OH, Fe-OH, Si-OH, CO3, NH4, and SO4. Small differences in absorption band position and shape are correlated with mineral compositional differences and variability. Imaging spectrometry has been used since the early 1980s to perform 2-dimensional mapping of mineral distribution based on spectroscopic characteristics. Geologic applications include areas such as lithologic mapping; exploration for precious and base metals; and oil, gas, and geothermal energy exploration. Field spectroscopy plays a critical role in the calibration, analysis, and validation of imaging spectrometer data. Spectral libraries have been measured for a variety of minerals. Imaging spectrometer datasets have been acquired around the world using airborne platforms and recent satellite systems provide spectral measurements for selected areas. Case histories will be presented demonstrating the link between laboratory, field, and imaging spectrometer data. Selected physics-based analysis methodologies will be discussed and the level of information available from the data will be demonstrated. Examples will include mineral identification and mapping in the context of hydrothermal alteration associated with active and fossil hot springs and mineral deposits. Specific mineral mapping examples will include hematite, goethite, and jarosite in the VNIR and identification and separation of calcite, dolomite, and calc-silicates along with phyllosilicates and sulfate minerals such as alunite and jaorsite using SWIR spectroscopy. Spectral variability caused by processes such as anion substitution in illite/muscovite, crystallinity in kaolinite-group minerals, and spectral mixing will also be discussed.