Swayze, G.A. R.N. Clark, S. Sutley, and A Gallagher, Ground-Truthing AVIRIS Mineral Mapping at Cuprite, Nevada, Summaries of the Third Annual JPL Airborne Geosciences Workshop, Volume 1: AVIRIS Workshop. JPL Publication 92-14, 47-49, 1992.
Gregg Swayze, Roger N. Clark, Fred Kruse(*), Steve Sutley, and Andrea Gallagher
Mineral abundance maps of 18 minerals were made of the Cuprite Mining District using 1990 AVIRIS data and the Multiple Spectral Feature Mapping Algorithm (MSFMA) as discussed in Clark et al. (1991). This technique uses least-squares fitting between a scaled laboratory reference spectrum and ground calibrated AVIRIS data for each pixel. Multiple spectral features can be fitted for each mineral and an unlimited number of minerals can be mapped simultaneously. Quality of fit and depth from continuum numbers for each mineral are calculated for each pixel and the results displayed as a multicolor image.
Cuprite consists of two acid-sulfate hydrothermal alteration centers straddling highway 95 in southwestern Nevada, with alteration involving Tertiary volcanic host rocks in the eastern center and Cambrian metasedimentary host rocks in the western center. Cuprite spectral mineral abundance maps of the area show two elliptical zonation patterns. The western center is zoned progressively inward from sericite, to halloysite-dickite, kaolinite, Na-alunite, through K-alunite with halloysite in the interior. While the eastern center is zoned inward from halloysite to kaolinite, intermediate alunite, through K-alunite, with a central core of siliceous sinter.
Many of these minerals had never been mapped before or in such detail. We have subdivided kaolinite group minerals into grades of crystallinity, alunites and montmorillonites into solid solution endmembers, and have differentiated between jarosite, goethite, hematite, and ammonium minerals. We collected 60 field samples using the mineral abundance maps as guides to locate areas with the strongest mineral signatures. The PIMA II field spectrometer(1) was used to do preliminary mineral identification in the field. Natural weathered surfaces were marked and were measured, along with rock chips from sample interiors, in reflectance on a laboratory Nicolet(1) 740 Fourier transform IR spectrometer. In all cases the mineralogy of the interior matched that of the weathered surface indicating that we are mapping the rock units themselves. An example of a library reference and field sample spectra for Dickite at lab and AVIRIS spectral resolution is shown along with an AVIRIS spectrum (avg= 8 pixels) extracted from the collection location of the field sample (Figure 1). This 2.2-microns feature was used to identify Dickite with the MSFMA. Twenty-eight field samples of 17 mineral types were submitted for X-Ray Diffraction (XRD) analysis to verify or disprove our spectral identifications (Table 1).
Figure 1: 18K GIF.
Refectance spectra of a reference dickite and field sample,
from Cuprite, identified as dickite from analysis of AVIRIS data. Lab
spectra (dash-dot and dash-dash curves) collected on an Fourier
transform IR spectrometer at 4 cm-1 resolution. Dotted curve is the
field sample spectrum convolved (cv) to AVIRIS spectral resolution.
Solid curve is an average of eight pixel spectra extracted from AVIRIS
data corresponding in location to the field sample collection locality.
Spectra have been vertically offset for clarity.
At the time of writing 12 spectrally identified minerals were confirmed by XRD. Since spectroscopy is more sensitive for identifying small quantities of Fe-oxides and clays than XRD, we expect that nontronite is present along with Fe-Chlorite. Chemical analysis will be used to determine the cation content of K-alunite and Na-montmorillonite samples. The wavelength position of the 2.2-micron band in muscovite appears to be a function of its Al/Fe ratio (Jim Post, pers. comm.) and may explain why we mapped two different muscovites as Na-montmorillonite and paragonite at some locations in the western center. More details on this will be presented. We have also realized that opal and hydroxyl-bearing chalcedony both have Si-OH bands and that we identified chalcedony instead of opal at Cuprite. Overall, our mineral identifications have proven very accurate in those cases where we have adequate reference spectra. Our difficulties arise from subtle wavelength shifts in spectral features of solid solution series minerals, because the MSFMA is sensitive to small band shifts. Such sensitivity to band position coupled with great sensitivity to band shape indicates that the MSFMA may open up a new frontier in remotely sensing subtle chemical changes in surface materials.
(1) Any use of trade names is for descriptive purposes only and does not imply endorsement by the U.S. Geological Survey.
Clark, R.N., G.A. Swayze, A. Gallagher, N. Gorelick, and F. Kruse, Mapping with Imaging Spectrometer Data Using the Complete Band Shape Least-Squares Algorithm Simultaneously Fit to Multiple Spectral Features from Multiple Materials, Proceedings of the Third Airborne Visible/Infrared Imaging Spectrometer (AVIRIS) Workshop, JPL Publication 91-28, p. 2-3, 1991.
Table 1: Minerals detected by AVIRIS and identified with XRD ------------------------------------------------------------ AVIRIS Identified X-Ray Diffraction ------------------------------------------------------------ K-Alunite Na-Alunite (submitted for K/Na analysis) Na-Alunite Na-Alunite Buddingtonite Buddingtonite Calcite Not Submitted (occurs as limestone) Fe-Chlorite Probably Fe-Chlorite Dickite Dickite Goethite Goethite Halloysite Halloysite (moderately crystallized) Hematite Hematite NH4 Ill/Smectite Montmorillonite + tr. Illite Jarosite K-jarosite Kaolinite (wxl) Kaolinite (well crystallized) Kaolinite (pxl) Kaolinite (still resolving crystallinity) Na-Montmorillonite Ca-Montmorillonite (submitted for anal.) Ca-Montmorillonite tr. Smectite and muscovite Nontronite Possibly Nontronite Opal Quartz (SEM: Qtz + chalcedony(#) Paragonite K-Muscovite (submitted for analysis) ------------------------------------------------------------ (#) Opal and chalcedony are both OH-bearing quartz.
U.S. Geological Survey,
a bureau of the U.S. Department of the Interior
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