Spectral, multispectral, hyperspectral: What works, what could work, and what will not work
Across the earth sciences, spectroscopy is the main tool for identifying and characterizing the composition of minerals, rocks, glasses, liquids, and gases. Infrared spectroscopy is one amongst many spectral methods that were developed in a non-geological discipline, adapted to planetary exploration, and subsequently adopted by exploration geologists as a powerful tool to identify and map alteration zones associated with hydrothermal mineral deposits.
Satellite borne remote sensing infrared spectral systems include multispectral and hyperspectral sensors that measure surface reflectance and involve minimal to null ground penetration. Therefore, they have limited applicability in areas that are covered by vegetation, lakes, glacial till, or other unconsolidated deposits. Satellite borne systems are also subject to illumination effects that limit their applicability in northerly latitudes, and to atmospheric effects that mask the central portion of their spectra.
Hyperspectral field portable infrared spectrometers are mainstream in drilling programs, where they are used to obtain rapid routine identification of the most frequently encountered alteration minerals. Less commonly, infrared spectroscopy data is used to define clay and sericite compositional zones that are indicative of specific hydrothermal fluid acidities and temperatures, allowing for the indirect identification of structural corridors that channeled high temperature hydrothermal fluids, as well as of redox boundaries.
Recent advances in infrared spectroscopy have resulted in the development of portable instruments that operate in higher wavelength ranges than those currently utilized in mineral exploration. These instruments allow for the identification and in some cases for the compositional characterization of many primary and alteration silicate mineral species.
Unconventional uses of infrared spectroscopy techniques applicable to mineral exploration include remote sensing mapping of modified vegetation reflectance patterns produced in areas of anomalous metal content, and laboratory-based mineral and major oxide quantification through multivariate calibration methods using whole rock geochemistry.
Potential infrared spectroscopy applications that warrant further research include the identification of radioactive damage in minerals through the use of crystallinity indices obtained from portable hyperspectral instruments, and in vegetation through the use of remote sensing techniques, and the quantification of silicification and de-silicification.
Anna Fonseca, MSc, PGeo, SRK Consulting (Canada)
Anna Fonseca earned a bachelor’s degree in geology and Russian language from the University of Alaska Fairbanks in 1993 and a master’s of science degree in economic geology from the University of British Columbia in 1998. During the first decade of her career, she conducted regional bedrock and structural mapping in Canada, Russia, US, and Mexico for government surveys and the mineral exploration industry. During her second field decade, she shifted her focus to alteration mapping and expanded her geographical range to include the Andes, Europe and Turkey.
In 2012 she joined SRK’s structural geology group in Toronto and has since focused on combining structural and alteration geology for exploration vectoring and geotechnical applications. She teaches applied structural-alteration geology courses in four and a half languages to industry, universities, geological surveys, and mineral exploration conferences, both in the field and in the classroom.