Lawrence C. Rowan

Lithologic mapping in the Mountain Pass, California area using Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) data

Abstract Evaluation of an Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) image of the Mountain Pass, California area indicates that several important lithologic groups can be mapped in areas with good exposure by using spectral-matching techniques. The three visible and six near-infrared bands, which have 15-m and 30-m resolution, respectively, were calibrated by using in situ measurements of spectral reflectance. Calcitic rocks were distinguished from dolomitic rocks by using matched-filter processing in which image spectra were used as references for selected spectral categories. Skarn deposits and associated bright coarse marble were mapped in contact metamorphic zones related to intrusion of Mesozoic and Tertiary granodioritic rocks. Fe-muscovite, which is common in these intrusive rocks, was distinguished from Al-muscovite present in granitic gneisses and Mesozoic granite. Quartzose rocks were readily discriminated, and carbonate rocks were mapped as a single broad unit through analysis of the 90-m resolution, five-band surface emissivity data, which is produced as a standard product at the EROS Data Center. Three additional classes resulting from spectral-angle mapper processing ranged from (1) a broad granitic rock class (2) to predominately granodioritic rocks and (3) a more mafic class consisting mainly of mafic gneiss, amphibolite and variable mixtures of carbonate rocks and silicate rocks.

Regional mapping of phyllic- and argillic-altered rocks in the Zagros magmatic arc, Iran, using Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) data and logical operator algorithms

A method for regional mapping of phyllic and argillic hydrothermally altered rocks using Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) data was developed and tested at the Cuprite, Nevada, calibration and validation site, and then extensively used in the Zagros magmatic arc in Iran, which consists of the High Zagros and Jebal Barez Mountains, and the Bazman volcanic area. Logical operator algorithms were developed to perform multiple band ratio and threshold value calculations, which can be applied to a scene using a single algorithm, thus eliminating separate production and application of vegetation and dark pixel masks. Argillic and phyllic band-ratio logical operators use band ratios that define the 2.17 µm and 2.20 µm absorption features to map kaolinite and alunite, which are typical in argillic-altered rocks, and muscovite, which is a common mineral in phyllic-altered rocks. Regional mapping of the Zagros magmatic arc using the logical operators illustrates distinctive patterns of argillic and phyllic rocks that can be associated with regional structural features and tectonic processes, and that can be used in regional mineral assessments. Semicircular patterns, 1–5 km in diameter, of mapped phyllic- and argillic-altered rocks are typically associated with Eocene to Miocene intrusive igneous rocks, some of which host known porphyry copper deposits, such as at Meiduk and Sar Cheshmeh. Linear phyllic-altered rock patterns associated with extensive faults and fractures indicate potential epithermal or polymetallic vein deposits. On the basis of argillic and phyllic alteration patterns, ∼50 potential porphyry copper deposits were mapped northwest of the Zagros-Makran transform zone in an eroded, exhumed, and dormant part of the magmatic arc, whereas only 11 potential porphyry copper deposits were mapped to the southeast of the transform, in the volcanically active part of the magmatic arc. The Zagros-Makran transform zone, which separates the volcanically dormant and active parts of the Zagros magmatic arc, exhibits extensive linear patterns of phyllic-altered rocks that indicate the potential for polymetallic-epithermal vein deposits.

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 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 o...

Airborne imaging spectrometer data of the Ruby Mountains, Montana: Mineral discrimination using relative absorption band-depth images

Abstract Airborne imaging spectrometer data collected in the near-infrared (1.2–2.4 μm) wavelength range were used to study the spectral expression of metamorphic minerals and rocks in the Ruby Mountains of southwestern Montana. The data were analyzed by using a new data enhancement procedure—the construction of relative absorption band-depth (RBD) images. RBD images, like bandratio images, are designed to detect diagnostic mineral absorption features, while minimizing reflectance variations related to topographic slope and albedo differences. To produce an RBD image, several data channels near an absorption band shoulder are summed and then divided by the sum of several channels located near the band minimum. RBD images are both highly specific and sensitive to the presence of particular mineral absorption features. Further, the technique does not distort or subdue spectral features as sometimes occurs when using other data normalization methods. By using RBD images, a number of rock and soil units were distinguished in the Ruby Mountains including weathered quartz - feldspar pegmatites, marbles of several compositions, and soils developed over poorly exposed mica schists. The RBD technique is especially well suited for detecting weak near-infrared spectral features produced by soils, which may permit improved mapping of subtle lithologic and structural details in semiarid terrains. The observation of soils rich in talc, an important industrial commodity in the study area, also indicates that RBD images may be useful for mineral exploration.

Evaluation of multispectral middle infrared aircraft images for lithologic mapping in the East Tintic Mountains, Utah

Six channels of multispectral middle infrared (8 to 14 micron) aircraft scanner data were acquired over the East Tintic mining district, Utah. This area has high relief and moderate vegetation and consists mainly of Tertiary silicic igneous rocks and Paleozoic quartzite and carbonate rocks that have been locally hydrothermally altered. These digital-image data were computer processed to create a color-composite image based on principal component transformations. Color differences in this image are related to the spectral differences in the surface material and allow discrimination of several rock types, depending primarily on their silica content. When combined with a visible and near infrared color-composite image from a previous flight, with limited field checking, it is possible to discriminate quartzite, carbonate rocks, quartz latitic and quartz monzonitic rocks, latitic and monzonitic rocks, silicified altered rocks, argillized altered rocks, and vegetation.

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.

Thermal-infrared spectra and chemical analyses of twenty-six igneous rock samples

Abstract Emittance spectra in the 7.5 μm to 14 μm wavelength region and chemical compositions of 26 igneous rocks are reported. Experimental measurements on the rocks were made under simulated daytime field conditions. Some surface silicate contaminants, such as clayey silt, significantly altered the spectral emittance of a fresh sample, whereas, for these samples, hydrous and anhydrous ferric oxide weathering products did not mask important silicate spectral information. In the 11.75 μm to 13.75 μm wavelength region, the mean emittance of all the silicate samples was0.956 ± 0.008, except for periodtite, which had an average emittance of 0.895. This region of uniform emittance should be useful in remote sensing experiments for the separation of the effects of temperature and chemical composition on the spectral emittance of silicate rocks.

ASTER spectral analysis and lithologic mapping of the Khanneshin carbonatite volcano, Afghanistan

Advanced Spaceborne Thermal and Reflection Radiometer (ASTER) data of the early Quaternary Khanneshin carbonatite volcano located in southern Afghanistan were used to identify carbonate rocks within the volcano and to distinguish them from Neogene ferruginous polymict sandstone and argillite. The carbonatitic rocks are characterized by diagnostic CO3 absorption near 11.2 μm and 2.31–2.33 μm, whereas the sandstone, argillite, and adjacent alluvial deposits exhibit intense Si-O absorption near 8.7 μm caused mainly by quartz and Al-OH absorption near 2.20 μm due to muscovite and illite. Calcitic carbonatite was distinguished from ankeritic carbonatite in the short wave infrared (SWIR) region of the ASTER data due to a slight shift of the CO3 absorption feature toward 2.26 μm (ASTER band 7) in the ankeritic carbonatite spectra. Spectral assessment using ASTER SWIR data suggests that the area is covered by extensive carbonatite flows that contain calcite, ankerite, and muscovite, though some areas mapped as ankeritic carbonatite on a preexisting geologic map were not identified in the ASTER data. A contact aureole shown on the geologic map was defined using an ASTER false color composite image (R = 6, G = 3, B = 1) and a logical operator byte image. The contact aureole rocks exhibit Fe2+, Al-OH, and Fe, Mg-OH spectral absorption features at 1.65, 2.2, and 2.33 μm, respectively, which suggest that the contact aureole rocks contain muscovite, epidote, and chlorite. The contact aureole rocks were mapped using an Interactive Data Language (IDL) logical operator. A visible through short wave infrared (VNIR-SWIR) mineral and rock-type map based on matched filter, band ratio, and logical operator analysis illustrates: (1) laterally extensive calcitic carbonatite that covers most of the crater and areas northeast of the crater; (2) ankeritic carbonatite located southeast and north of the crater and some small deposits located within the crater; (3) agglomerate that primarily covers the inside rim of the crater and a small area west of the crater; (4) a crater rim that consists mostly of epidote-chlorite-muscovite–rich metamorphosed argillite and sandstone; and (5) iron (Fe3+) and muscovite-illite–rich rocks and iron-rich eolian sands surrounding the western part of the volcano. The thermal infrared (TIR) rock-type map illustrates laterally extensive carbonatitic and mafic rocks surrounded by quartz-rich eolian and fluvial reworked sediments. In addition, the combination of VNIR, SWIR, and TIR data complement one another in that the TIR data illustrate more laterally extensive rock types and the VNIR-SWIR data distinguish more specific varieties of rocks and mineral mixtures.

Relation Between Regional Lineament Systems and Structural Zones in Nevada

Analysis of diverse geologic, geophysical, and geochemical data shows that six of eight extensive lineament systems delineated in Landsat images of Nevada are morphologic and tonal expressions of three broad structural zones. The lineament systems defined in this report are the Walker Lane, Rye Patch, Midas, Pancake Range, Warm Springs, Timpahute, Pahranagat, and Northern Nevada Rift. The three structural zones are the Walker Lane structural zone, the Humboldt structural zone, and the Southern Nevada structural zone. The northwest-trending Walker Lane structural zone, which includes the Walker Lane lineament system, is 150 to 200 km wide and is marked by northwest-oriented faults, a broad belt of high-frequency aeromagnetic anomalies, and concentrations of volcanic rocks 34 to 6 m.y. old. The northwest-trending Rye Patch lineament system may mark the northeast boundary of this structural zone, and is characterized by stratigraphic discontinuities, alignment of aeromagnetic anomalies, and contrasting aeromagnetic anomaly patterns on opposing sides. Northern Nevada is dominated by the northeast-trending, 150 to 200-km wide Humboldt structural zone which is marked by the Midas lineament system, northeast-oriented faults, broad gravity anomalies, and the Battle Mountain heat-flow high; this zone appears to extend into central Montana. The Humboldt structural zone is transected by the Northern Nevada Rift lineament system, which is partly defined by a relatively narrow belt of north-northwest-trending basaltic dikes that produce a series of pronounced aeromagnetic highs. The east-trending, 150 to 175-km wide Southern Nevada structural zone includes the Pancake Range, Warm Springs, and Timpahute lineament systems; this zone is marked by disruption and termination of ranges, stratigraphic discontinuities, east- to east-northeast-trending faults, and an easterly grain in the aeromagnetic and gravity patterns. The other lineament system in southern Nevada, the northeast-trending Pahranagat, may lie within a poorly defined southward extension of a broad structural zone defined in Utah by a belt of volcanic rocks 34 to 0 m.y. old, stratigraphic discontinuities, coincidence with three extensive gravity lineaments, and a belt of aeromagnetic anomalies. This paper proposes that the lineament systems, except for the Northern Nevada Rift lineament system, are conjugate shears formed during and after middle Miocene extension of the Great Basin; these lineament systems probably reflect only the most recent movement along broad crustal zones, which may be of pre-Miocene age. The three documented structural zones and possibly one including the Pahranagat lineament system may reflect transition zones between the central Great Basin, where 30% extension has been documented, and the adjacent regions that responded differently to spreading since middle Miocene time. The Humboldt structural zone appears to be especially important for geothermal and mineral exploration in Nevada because of the presence of the Battle Mountain heat-flow high and a high concentration of metal districts within this zone in Nevada.