Visible and near-infrared (0.35 to 2.5 •m) bidirectional reflection spectra were recorded for a suite of well-characterized hydrothermally altered rock samples. The spectra typically display well-defined bands caused by both electronic and vibrational processes in the individual mineral constituents. Electronic transitions in the iron-bearing constituent minerals produce diagnostic minima near 0.43, 0.65, 0.85, and 0.93 •m. Vibrational transitions in clay and waterbearing mineral constituents typically produce characteristic single or nmltiple features over limited spectral ranges near 1.4, 1.75, 1.9, 2.2, and 2.35 /•m. The most abundant feature-producing minerals present in these rocks are hematite, goethite, and alunite, while others frequently present are jarosite, kaolinite, potassium micas, pyrophyllite, montmorillonite, diaspore. and gypsum.This study shows that visible-near infrared sl)ectrometry is a reliable and rapid technique for detecting and identifying clay minerals and alunite in rocks. Because these minerals are important constituents of altered rocks, the feasibility of using the visible and near infrared for detecting altered rocks by remote-sensing techniques is indicated. The spectral region near 2.2 t•m is particularly important for this purpose.
Mineral maps based on Airborne Visible/Infrared Imaging Spectrometer (AVIRIS) data were used to study late Miocene advanced argillic alteration at Cuprite, Nevada. Distributions of Fe-bearing minerals, clays, micas, sulfates, and carbonates were mapped using the Tetracorder spectral-shape matching system. The Al-content of white micas increases toward altered areas and near intrusive rocks. Alunite composition varies from pure K to intimate mixtures of Na-K endmembers with subpixel occurrences of huangite, the Ca-analogue of alunite. Intimately mixed Na-K alunite marks areas of relatively lower alteration temperature, whereas co-occurring Na-alunite and dickite may delineate relict hydrothermal conduits. The presence of dickite, halloysite, and well-ordered kaolinite but absence of disordered kaolinite is consistent with acidic conditions during hydrothermal alteration. Partial lichen cover on opal spectrally mimics chalcedony, limiting its detection to lichen-free areas. Pods of buddingtonite are remnants of initial quartz-adularia-smectite alteration. Thus, spectral maps provide a synoptic view of the surface mineralogy, and define a previously unrecognized early steam-heated hydrothermal event. Faulting and episodes of hydrothermal alteration at Cuprite were intimately linked to upper plate movements above the Silver Peak-Lone Mountain detachment and growth, collapse, and resurgence of the nearby Stonewall Mountain volcanic complex between 8 and 5 Ma. Isotopic dating indicates that hydrothermal activity started at least by 7.61 Ma and ended by about 6.2 Ma. Spectral and stable isotope data suggests that Cuprite is a late Miocene lowsulfidation adulariasericite type hot spring deposit overprinted by late-stage steam-heated advanced argillic alteration formed along the margin of the Stonewall Mountain caldera.
Mineralogical differences between altered rocks and most unaltered rocks in south‐central Nevada cause visible and near‐infrared (0.45 to 2.4 μm) spectral‐reflectance differences which can be used to discriminate them broad categories of rocks in multispectral images. The most important mineralogical differences are the increased abundance of goethite, hematite, and jarosite, and the presence of alunite, montmorillonite, and kaolinite in the altered rocks. Analysis of reflectance spectra recorded in the field showed that the altered rock spectra are characterized by broad absorption bands in the 0.45–0.50 μm and 0.85–0.95 μm regions which are due to electronic processes in the iron ions, and a band near 2.2 μm which is due to vibrational processes in the OH ions. These features are absent or weak in most of the unaltered rock spectra. Therefore, the shapes of the 0.45–2.4 μm spectra for these altered and unaltered rocks are conspicuously different. However, because of the wavelength positions and widths of the Landsat Multispectral Scanner (MSS) bands, these spectral differences are not apparent in individual or color‐infrared composite MSS images. The technique developed to enhance these subtle spectral differences combines ratioing of the MSS bands and contrast stretching. The stretched ratio values are used to produce black‐and‐white images which depict materials according to spectral reflectance; ratioing minimizes the influence of topography and overall albedo on the grouping of spectrally similar materials. Color compositing of two or more stretched ratio images to form color‐ratio composites provides additional enhancement. The most effective color‐ratio composite for discriminating between the altered and unaltered areas, as well as among many of the unaltered rocks in south‐central Nevada, was prepared using the following diazo color and stretched ratio image combinations: blue for MSS 4/5, yellow for MSS 5/6, and magenta for MSS 6/7. Altered areas appear green and brown in this combination. Field evaluation of this color‐ratio composite shows that excluding alluvial areas, approximately 80 percent of the green and brown color patterns are related to hydrothermal alteration. The remaining 20 percent consists mainly of pink hematitic crystallized tuff, a result of vapor‐phase crystallization, and of tan and red ferruginous shale and siltstone. Discrimination of this unaltered tuff from the altered rocks may be possible in the 2.2 μm region because this absorption band is absent in the tuff spectra. However, because the shale and siltstone are mineralogically and spectrally similar to the altered rocks, there appears to be little prospect of distinguishing these rocks from altered rocks in visible and near‐infrared multispectral images.
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