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J.K. Crowley

Publications and source records attributed to J.K. Crowley.

23 records · Page 2Linked to original sources
USGS1989

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

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.

Montana↗
USGS1986

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.

Economic Geology↗