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USGS1996

Lithologic analysis from multispectral thermal infrared data of the alkalic rock complex at Iron Hill, Colorado

Airborne thermal-infrared multispectral scanner (TIMS) data of the Iron Hill carbonatite-alkalic igneous rock complex in south-central Colorado are analyzed using a new spectral emissivity ratio algorithm and confirmed by field examination using existing 1:24 000-scale geologic maps and petrographic studies. Color composite images show that the alkalic rocks could be clearly identified and that differences existed among alkalic rocks in several parts of the complex. An unsupervised classification algorithm defines four alkalic rock classes within the complex: biotitic pyroxenite, uncompahgrite, augitic pyroxenite, and fenite + nepheline syenite. Felsic rock classes defined in the surrounding country rock are an extensive class consisting of tuff, granite, and felsite, a less extensive class of granite and felsite, and quartzite. The general composition of the classes can be determined from comparisons of the TIMS spectra with laboratory spectra. Carbonatite rocks are not classified, and we attribute that to the fact that dolomite, the predominant carbonate mineral in the complex, has a spectral feature that falls between TIMS channels 5 and 6. Mineralogical variability in the fenitized granite contributed to the nonuniform pattern of the fenite-nepheline syenite class. The biotitic pyroxenite, which resulted from alteration of the pyroxenite, is spatially associated and appears to be related to narrow carbonatite dikes and sills. Results from a linear unmixing algorithm suggest that the detected spatial extent of the two mixed felsic rock classes was sensitive to the amount of vegetation cover. These results illustrate that spectral thermal infrared data can be processed to yield compositional information that can be a cost-effective tool to target mineral exploration, particularly in igneous terranes.

Geophysics↗

USGS1987

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.

Extremadura↗