2. Methods
The original data were acquired in 1995 by NASA's Airborne Visible and Infrared Imaging Spectrometer (AVIRIS). In this simulated mineral mapping, the original AVIRIS data (224 bands from 0.4 mm to 2.5 mm; 20 m pixel) were re-sampled according to the proposed ARIES-1 specifications (105 bands from 0.4 mm to 2.5 mm; 30 m pixel, signal to noise ratio between 1000 and 400.). the reduced data were then processed to remove atmospheric effects (ATREM, Gao et. al. 1997) and instrument noise (EFFORT, Boardman 1998) to derive the apparent ground reflectance. The 32 short-wave infrared bands of ARIES from 2.0 mm to 2.5 mm were further processed to derive mineral end-members present in the scene. The ENVI software package was used for the spectral processing and mapping work.
Relative abundance of the key alteration minerals such as lillite, chlorite, kaolin clays and alunite was determined based on the digital number of the respective end-member minerals unmixed from the scene.
For ground validation, field spectral work was carried out with an ISPL PIMA-II portable infrared spectrometer. The PIMA-II instrument measures the hemispherical reflectance in 601 channels in the wavelengths region from 1.4 to 2.5 mm.
3. Results
In the Hyperspectral mineral mapping, advanced argillic, argillic, propylitic, and phyllic alteration zones were identified by the occurrence of alunite, kaolin clays, chlorite, and illite as the dominant alteration minerals, respectively. The spectra of these minerals, derived from unmixing of the airborne hyper spectral reflectance data, are displayed in Figure 2.
Figure 2 Stacked reflectance spectra of end-member minerals unmixed from the simulated hyper spectral data.
1.-chlorite,
2.alunite,
3-kaolite,
4-dickite,
5-long Al-OH wavelength illite,
6-medium Al-OH wavelength illite,
7-short Al-OH wavelength illite,
The absorption minima of the Al-OH band for the three illities are approximately at 2212 nm, 2204 nm and 2194 nm respectively.
The mapping results show that two kaolin clays, dickite and kaolinite, commonly coexist in the bleached zones (Figure 3). The most significant enrichment of dickite was identified around Gold Hill in the central part, and near Cedar Hill in the northern part of the mapped area. As expected, alunite is spatially associated with kaolin, although the mapped alunite outcrops are less extensive simply because alunite is less abundant than kaolin in the bleached rocks.
Illite enrichment was mapped in large outcrops in the central and south-central parts of the district (Figure 3d-f). Three types of illite with long (2212 nm), medium (2204 nm) and short (2194 nm) Al-OH absorption features, respectively, were distinguished.
Figure 3. Area distribution of kaolinite (a), alunite (b), dickite (c), illite with short (d), medium (e) and long (f) Al-OH wavelengths and chlorite (g) in the Comstock mining district. Relative mineral abundance corresponding to digital numbers from 0.1 and 0.6 is stretched from black (undetectable) to white (most abundant). Major faults are marked. The images cover the same area as shown in Figure 1. see Figure 2 for the spectra of the end-member minerals.
The illite with short Al-OH absorption wavelengths is located dominantly in the bleached volcanic rocks. This illite occurs mainly along or east of the Comstock and Silver City Faults (Figure 3d). It commonly coexists with kaolin clays and alunite and so belongs to the advanced argillic assemblage.
The illite with long Al-OH band wavelengths occurs mainly in the unbleached Alta and Kate Peak Formations west of the Comstock and Silver City Faults and in the Mesozoic rocks in the southern part of the district (Figure 3f). This type of illite forms part if the propyllic or phyllic assemblages.
The medium Al-OH absorption wavelengths illite was mapped mainly in the central part of the district, close to the Comstock Fault between Virginia City and Gold Hill and around the Flowery mine (Figure 3e).
Propylite alteration, as indicated by chlorite enrichment, has extensively affected rocks of the mapped area, especially in the central and southern parts (Figure 3g). In contrast to the kaolin clays, chlorite tends to develop west of the Comstock fault in the central part, or away from the fault in the southern part of the district.
The mapped area distribution of the various alteration assemblages were confirmed by ground validation with field spectral measurement using the PIMA-II portable infrared spectrometer.
