Unaltered Areas Research Articles

DISCRIMINATION OF HYDROTHERMALLY ALTERED AND UNALTERED ROCKS IN VISIBLE AND NEAR INFRARED MULTISPECTRAL IMAGES

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.

Alteration of metamict, rare-earth, AB 2O 6-type NbTaTi oxides

Primary and secondary alteration affect composition, physical properties and the results of annealing studies of metamict, rare-earth, AB 2O 6-type Nb-Ta-Ti oxides. Primary alteration (e.g. hydrothermal) causes a consistent increase in calcium content, generally a decrease in the uranium and thorium content, a decrease in the absolute rare earth concentrations, (with only one exception, the REE distributions remain constant), a slight decrease in B-site cations, and an increase in structural and absorbed water. Secondary alteration (e.g. weathering) is similar but produces a decrease in Ca content, an increased leaching of A-site cations and relative increase in B-site cations. R.I., S.G., and reflectance (546 nm) decrease with both types of alteration, but VHN 50 remains approximately constant in altered and unaltered areas of the same specimen. The deviation from stoichiometric proportions of A- and B-site cations with increased alteration results in complex recrystallization phase assemblages during annealing studies (500–1000°C at 1 bar). The effect of alteration on systems of nomenclature and identification procedures is discussed.

Fish Population Structure in Altered and Unaltered Areas of a Small Kansas Stream

Collection of invertebrates and fish were made from channelized and unchannelized stream sections. A species list of invertebrates is presented, but no quantitative estimates were made. Numbers and weights of fish were used to calculate diversity indices, and equitability measure, and extrapolated to produce section values in Ibs./acre and numben/acre. Index values indicated less species diversity in the altered section than in the unaltered sections when numbers of fish were used, but there was no signihcant difference in diversity when biomass units were used. The equitabilib component did not indicate a significant trend in sectional differences. Both biomass and numbers were less in the channelized section than in the unchanneEzed sections. It was concluded that the lower biomass and numbers of fish, and lower diversity index values in the altered area, were attributable to the effects of channelization. Trans. Kans. Acad. Sci., 77 (1), 1974.

Fish Populations of a Trout Stream in Relation to Major Habitat Zones and Channel Alterations

Abstract The relationship of fish populations to major habitat zones and channel alterations was studied in Little Prickly Pear Creek, Montana, during the summers of 1965 and 1966. Five major zones were defined as follows: headwater, meadow, mountain, lower meadow, and Wolf Creek Canyon, with at least one representative study section in each. Approximately 23% (6 of 30 miles) of the stream has been altered. Field measurements showed no pool-riffle periodicity in the altered mountain sections, while successive riffles were spaced at intervals of 5.7 widths, in the unaltered areas. Altered sections of the mountain zone consisted, by area, of 87% shallow-fast with no deep-slow water, compared to 49 and 14%, respectively, for the unaltered sections. Amount of cover per acre of stream was about 80% greater in the unaltered mountain section than in the altered. Rock deflectors in the altered section of Wolf Creek Canyon rendered the physical characters of the stream nearly comparable to the unaltered sections. ...