Geology topics
L. C. Rowan
Publications and source records attributed to L. C. Rowan.
Mapping contact metamorphic aureoles in Extremadura, Spain, using Landsat thematic mapper images
In the Extremadura region of western Spain, Ag, Pb, Zn, and Sn deposits occur in the pieces of late Hercynian granitic plutons and near the pluton contacts in late Proterozoic slate and metagraywacke that have been regionally metamorphosed to the green schist facies. The plutons generally are well exposed and have distinctive geomorphological expression and vegetation; poor exposures of the metasedimentary host rocks and extensive cultivation, however, make delineation of the contact aureoles difficult. Landsat Thematic Mapper (TM) images have been used to distinguish soil developed on the contact metamorphic rocks from soil formed on the stratigraphically equivalent slate-metagraywacke sequence. The mineral constituents of these soils are similar, except that muscovite is more common in the contact metamorphic soil; carbonaceous material is common in both soils. Contact metamorphic soil have lower reflectance, especially in the 1.6-micrometers wavelength region (TM 5), and weaker Al-OH, Mg-OH, and Fe 3+ absorption features than do spectra of the slate-metagraywacke soil. The low-reflectance and subdued absorption features exhibited by the contact metamorphic soil spectra are attributed to the high absorption coefficient f the carbonaceous material caused by heating during emplacement of the granitic plutons. These spectral differences are evident in a TM 4/3, 4/5, 3/1 color-composite image. Initially, this image was used to outline the contact aureoles, but digital classification of the TM data was necessary for generating internally consistent maps of the distribution of the exposed contact metamorphic soil. In an August 1984, TM scene of the Caceras area, the plowed, vegetation-free fields were identified by their low TM 4/3 values. Then, ranges of TM 4/5 and 3/1 values were determine for selected plower fields within and outside the contact aureoles; TM 5 produced results similar to TM 4/5. Field evaluation, supported by X-ray diffraction and petrographic studies, confirmed the presence of more extensive aureoles than shown in published geologic maps; few misclassified areas were noted. Additional plowed fields consisting of exposed contact metamorphic soil were mapped digitally in an August 1985 TM scene. Subsequently, this approach was used to map two 1-km-wide linear zones of contact metamorphosed rock and oil in the San Nicolas-Sn-W Mine area, which is located approximated 125 km southeast of the Caceras study area. Exposures of granite in the San Nicolas area are limited to a few unaltered granitic dikes in the mine and a small exposure of unaltered pegmatite-bearing granite in a quarry about 1.5 km west of the mine. The present of coarsely crystalline biotite and beryl in the granite in the quarry and of contact metamorphosed slate up to 2.5 km from the nearest granite exposure suggest that only the apical part of a pluton is exposed in the quarry and that a larger, shallowly buried body is probably present. These results indicate that potential application of TM image analysis to mineral exploration in lithologically similar areas that are cultivated in spite of poor rock exposures.
New techniques for the quantification and modeling of remotely sensed alteration and linear features in mineral resource assessment studies
Linear structural features and hydrothermally altered rocks that were interpreted from Landsat data have been used by the U.S. Geological Survey (USGS) in regional mineral resource appraisals for more than a decade. In the past, linear features and alterations have been incorporated into models for assessing mineral resources potential by manually overlaying these and other data sets. Recently, USGS research into computer-based geographic information systems (GIS) for mineral resources assessment programs has produced several new techniques for data analysis, quantification, and integration to meet assessment objectives.
Spectral reflectance of carbonatites and related alkalic igneous rocks: Selected samples from four North American localities
Laboratory spectral reflectance measurements were made in the 0.4- to 2.5-mu m wavelength range for samples collected from four North American carbonatite-alkalic igneous rock complexes. Alkalic rock spectra produced few absorption features; however, the carbonatites typically showed conspicuous carbonate, ferric and ferrous iron, and in some cases rare earth element absorption bands. Trivalent neodymium was the most abundant rare earth element producing absorption features and could be detected in amounts as low as 300 ppm. Ferrous iron was present in most dolomitic carbonatite samples and caused more intense ferrous absorption bands than commonly is seen in carbonate rocks of sedimentary origin. High carbonatite iron content also resulted in pervasive limonitic staining and related ferric iron absorption features. These absorption features sometimes masked other absorption bands.Because reflectance measurements can be carried out rapidly, with minimal sample preparation, reflectance spectroscopy may provide a useful field and laboratory tool for analyzing economic rare earth concentrations. In addition, the distinctive spectral characteristics of carbonatite material, in conjunction with the discordant spatial relationships between associated alkalic and carbonatite rocks, may provide a basis for remote-sensing detection of these complexes.
Map showing distribution of altered rocks, faults, and linear features in the Walker Lake 1° x 2° quadrangle, Nevada and California
No abstract available.
Map showing analysis of linear features in the Wallace 1 degree by 2 degrees Quadrangle, Idaho-Montana
No abstract available.
Mineral resource potential map of the Anaconda-Pintlar Wilderness and contiguous roadless area, Granite, Deer Lodge, Beaverhead, and Ravalli Counties, Montana
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Map of the Walker Lake 1° by 2° quadrangle, California and Nevada, showing the regional distribution of linear features
No abstract available.
Map of the Walker Lake 1° by 2° quadrangle, California and Nevada showing the regional distribution of hydrothermally altered rocks
No abstract available.
Mineral identification from orbit: Initial results from the shuttle multispectral infrared radiometer
A shuttle-borne radiometer containing ten channels in the reflective infrared has demonstrated that direct identification of carbonates and hydroxyl-bearing minerals is possible by remote measurement from Earth orbit.
Objectives, accomplishments, and future plans of IGCP project 143, remote sensing and mineral exploration
The International Geological Correlation Programme (IGCP) is a worldwide cooperative research programme that began in 1974 under the auspices of the International Union of Geological Sciences. Because of the global availability of Earth resources data collected by satellites and the great interest among geologists in taking advantage of these new sources of information, a project was begun in 1976 to improve the rate of technology transfer in the field of remote-sensing exploration for energy and mineral resources. Conducting joint workshops in cooperation with COSPAR has been an important part of this project. It is to be hoped the project will improve our capability to explore, identify, and develop new resources to meet the burgeoning demands of society. ?? 1981.
Geologic remote sensing
Remote-sensing techniques are now being used routinely in geologic interpretation for mineral and energy exploration, plant siting, waste disposal, and the development of models for regional and continental tectonics. New spaceborne methods and associated technologies are being developed to produce data from which geologic information about large areas can be derived much more rapidly than by conventional techniques.
Detection and mapping of hydrothermally altered rocks in the vicinity of the Comstock Lode, Virginia Range, Nevada, using enhanced Landsat images
The Virginia Range, immediately southeast of Reno, Nev., consists mainly of flows, breccias, and turfs of Miocene age. Most of these volcanic rocks are of intermediate composition; rhyodacite is the most common rock type. Basalt, rhyolite and rhyolite tuff, and tuffaceous sedimentary rocks of Miocene and Pliocene age also cover substantial areas in the range. Pre-Tertiary metasedimentary, metavolcanic, and granitic rocks are exposed in scattered inliers, mostly along the southern and eastern margins of the range. Several large areas and many small areas within the volcanic pile were subjected to hydrothermal alteration during and after the period of intermediate volcanic activity. Economic precious metal mineralization is spatially and temporally associated with the hydrothermal alteration in several areas. The most important deposit is the Comstock Lode, which produced 192 million troy ounces of silver and 8.3 million troy ounces of gold from epithermal veins (Bonham, 1969). The hydrothermally altered rocks include silicified, advanced argillic, montmorillonite-bearing argillic, and propylitic types. The first three types typically contain pyrite, and some propylitic rocks contain pyrite as well. Supergene oxidation of these pyritic rocks produces limonitic bleached rocks. The term 'limonite,' as used here, refers to any combination of the minerals hematite, goethite, and Jarosite. Where vegetation cover is sparse to moderate, these limonitic rocks are readily identified on Landsat images enhanced by the color-ratio composite technique developed by Rowan and others (1974), so the altered areas can be mapped. About 30 percent tree cover (here mainly pinyon pine) is sufficient to change the spectral signature of individual picture elements (pixels) enough so that limonitic materials can no longer be uniquely identified. As in all other areas where this technique has been applied, limonitic unaltered rocks with intermediate to high albedos have the same appearance on the color-ratio composite as limonitic altered rocks. This problem represents the most important limitation to the use of enhanced Landsat images for detection and mapping of hydrothermally altered rocks. Reflectance spectra of altered and unaltered rocks taken in the field in the Virginia Range show that most altered rocks have a conspicuous absorption band near 2.2 ?m produced by clay minerals or alunite, whereas unaltered rocks have no features in this spectral region. Thus spectral information for selected bands in the 1.1-2.5 ?m region may allow discrimination between limonitic altered and limonitic unaltered rocks (Rowan and others, 1977; Abrams and others, 1977; Rowan and Abrams, 1978). Another potential limitation is loss of spectral information on slopes with low effective sun angle. Although a minor problem in the Virginia Range, loss of information sufficient to preclude identification of limonitic altered rocks occurs with effective sun angle lower than 20-25 degrees. Thus, even at moderate latitudes substantial parts of areas with high topographic relief may be lost to observation.
Mapping of hydrothermal alteration in the Cuprite mining district, Nevada, using aircraft scanner images for the spectral region 0.46 to 2.36µm
Color composites of Landsat Multispectral Scanner ratio images that display variations in the intensity of ferric-iron absorption bands are highly effective for mapping limonitic altered rocks but are ineffective for mapping nonlimonitic altered rocks. Analysis of 0.45- to 2.5-µm field and laboratory spectra shows that iron-deficient opalized rocks in the Cuprite mining district, Nevada, have an intense OH-absorption band near 2.2 µm, owing to their content of clay minerals and alunite, and that this spectral feature is absent or weak in adjacent unaltered tuff and basalt. Altered rocks in the district can be discriminated from unaltered rocks with few ambiguities by use of color-ratio composite images derived from multispectral (0.46 to 2.36 µm) aircraft data. In addition, some effects of mineralogical zoning can be discriminated within the altered area. Only variations in amounts of limonite can be discerned in shorter wavelength aircraft data, Landsat Multispectral Scanner bands, and color aerial photographs.
Discrimination of hydrothermal 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 mu m) spectral-reflectance differences which can be used to discriminate these 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 mu m and 0.85-0.95 mu m regions which are due to electronic processes in the iron ions, and a band near 2.2 mu m 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 mu m 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 rationing 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 mu m 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.
Lineament map of parts of Virginia, North Carolina, and South Carolina
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Use of imaging in the 0.46-2.36 [micrometers] spectral region for alteration mapping in the Cuprite mining district, Nevada
Color composites of Landsat MSS ratio images that display variations in the intensity of ferric-iron absorption bands are highly effective for mapping limonitic altered rocks, but ineffective for mapping nonlimonitic altered rocks. Analysis of 0.45-2.5 ?m field and laboratory spectra shows that iron-deficient opalites in the Cuprite mining district, Nevada, have an intense OH-absorption band near 2.2 ?m owing to their clay mineral and alunite contents and that this spectral feature is absent or weak in adjacent unaltered tuff and basalt. To evaluate the usefulness of this spectral feature for discriminating between altered and unaltered rocks, we generated color-ratio composite images from multispectral (0.46-2.36 ?m) aircraft data. The altered rocks in the district can be discriminated from unaltered rocks with few ambiguities; in addition, some effects of mineralogical zoning can be discriminated within the altered area. Only variations in amounts of limonite can be discerned in shorter wavelength aircraft data, Landsat MSS bands, and color aerial photographs.