Reflectance Spectroscopy Applied to Exploration for Mineral Deposits and Geothermal Systems

The Great Basin of the Western United States has thin continental crust and geologic structures that have controlled the emplacement of significant precious and base metal mineralization. Many of the mineral deposits in the Great Basin were emplaced by hydrothermal processes that altered the mineralogy of their host rock assemblages and facilitated concentration of metals. In the 1960’s and 1970’s research with laboratory spectrometers determined that some alteration minerals associated with mineral deposits have discrete spectral signatures that should permit their identification and mapping in the field. Research in the 1970’s found that clays and iron oxides, associated with mineralized systems, could be detected in multiband image data and mapped using their broad spectral signatures. Beginning in the 1980’s, research with prototype airborne imaging spectrometer data and ground-based spectrometers identified suites of alteration minerals that are key indicators of mineralized systems. Recent airborne and satellite systems have demonstrated that detailed mineral mapping is possible from aerospace measurements, as verified by ground-based spectrometer measurements. These spectral measurements document the zonation of low and high temperature mineral assemblages. Minerals like chlorite, epidote, calcite were found as propylitic alteration farther from igneous centers while quartz, quartz-alunite, and alunite were identified nearer the centers. Clay mineral assemblages were located between the two alteration zones, including: kaolinite, dickite, illite and montmorillinite. Iron oxide and iron sulfate minerals (hematite, goethite and jarosite respectively) were also found associated with alteration zones. There are now several reference spectral libraries and airborne/spaceborne case histories available that illustrate the unique spectral character of these distinctive mineral spectra and our ability to identify and map them from imagery. The technology and methods developed are used in a variety of related areas; mineral and geothermal exploration, detection of acid mine drainage, and the assessment of the effectiveness of mined land reclamation.


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