ANALYSIS OF AIRBORNE VISIBLE INFRARED IMAGING SPECTROMETER (AVIRIS) DATA OF THE IRON-HILL, COLORADO, CARBONATITE ALKALIC IGNEOUS COMPLEX

The Iron Hill, Colorado, carbonatite-alkalic igneous rock complex cons ists mainly of pyroxenite, uncompahgrite, fenite and nepheline syenite , and carbonatite. The study area has been explored intermittently for several decades for thorium, niobium, rare earth elements, titanium, and vermiculite. Airborne visible-infrared imaging spectrometer (AVIRI S) data were acquired for the Iron Hill Complex on August 28, 1992, un der clear sky conditions. The data were calibrated using a laboratory reflectance spectrum of a tuff-sagebrush mixture to represent the spec tral response of a topographically flat, homogeneous image area within the ash-flow tuff. Analysis of the data was conducted using the spect ral angle mapper (SAM) and linear spectral unmixing algorithms in the spectral image processing system (SIPS). Image spectra in the 2.1- to 2.45-mu m wavelength region were used in the SAM method to map dolomit e in the Iron Hill carbonatite (rauhaugite) stock and in several small er bodies, calcite in a small carbonatite (sovite) plug and in travert ine spring deposits, illite mainly in sericitized felsite and granite, and kaolinite in hydrothermally altered ash-flow tuff. The general di stribution of the main alkalic igneous rocks (pyroxenite, uncompahgrit e, and ijolite) was mapped using a full AVIRIS wavelength range (0.40- 2.45 mu m) image spectrum of pyroxenite, excluding channels affected b y strong atmospheric absorption. However, some areas consisting of fel site and fenite were not spectrally distinguished from the alkalic roc ks. Linear spectral unmixing, using full wavelength image spectra, was used to improve the compositional mapping achieved using the SAM algo rithm. The fractional abundance images obtained for dolomite and calci te were similar to the SAM results, but the distinction between illite and kaolinite was significantly improved, especially where scattered coniferous trees are resent. Hematite and goethite, which showed widel y overlapping patterns in the SAM similarity images, were distinguishe d much better in the fractional abundance images. Biotite was used ins tead of pyroxenite as a reference end member in the unmixing analysis, and the results were better than those obtained in the SAM analysis. The results of the unmixing analysis show the potential for using imag ing spectrometer data for identifying and mapping carbonate minerals w ithin carbonatite bodies and hot spring deposits and biotite within py roxenite. In addition, the most intense Al-OH absorption features occu r in sericitized granite and felsite associated with thorium veins and in kaolinite in hydrothermally altered tuff. Concentrations of kaolin ite, travertine, and silicified carbonatite along the trace of the Cim arron fault indicate that the fault was reactivated after deposition o f the Tertiary tuff and provided a conduit for fluids.