Laguna, 1981). This part of the massif constitutes 
the study area, which is approximately 2 km wide 
by 3 km long (Figure 1). In this area, leaching 
processes have concentrated insoluble elements 
(silica and iron oxides) in the surface and 
developed extensive lateritic crusts, that induce 
the predominance of herbaceous plants. The areas 
of alteration materials are recognized in the field 
by the presence of abundant quartz, microcline, 
albite, and muscovite, covered by thin grasses and 
sparse tiny shrubs. 
3. FIELD DATA ACQUISITION 
Eighty-five field spectra obtained from altered 
and non-altered rock/soils under variable grass 
cover, were acquired between May 5th to 12th, 
from 9:00 to 11:00 am local time, roughly in 
accordance with the date and day-time of the 
Landsat Thematic Mapper (TM) images (path 
221, row 69), which were obtained on May 10, 
1984, under a solar elevation angle of 42°. 
Determinations were made with a portable 
radiometer (Barringer's Hand Held Ratioing 
Radiometer - HHRR) fitted with Landsat-TM 
equivalent bandpasses. Measurements were 
obtained with nadir viewing at a distance of about 
1.3 meters from the surface. Field photographs of 
the scene under the field of view of the sensor 
system were obtained, in order to estimate the 
percentage of the different surface constituents 
that contributed to compose each spectra. 
The in situ spectra provided a unique insight on 
the spectral behavior of the different terrain 
features, since the measurements were obtained 
with the surface materials in their natural 
conditions. The major concerns were to define the 
best Landsat-TM bands to discriminate 
hydrothermally altered materials and to 
determine the effect of the vegetation cover 
(especially herbaceous plants), in the 
discrimination of different rock-soil types. 
4. DATA PROCESSING AND ANALYSIS 
Figure 2 shows field spectra obtained from 
different rock-soil-vegetation associations in the 
study area. Two main conclusions may be 
deduced from these data. The first one indicates 
that the equivalent bands TM-1, TM-2 and TM-7 
show the best spectral separability between bare 
soils derived from hydrothermally altered 
8 
granitoids, and bare lateritic soils developed over 
non-altered granitoids (biotite-granites). In the 
visible region (TM-1 and TM-2), the separability 
is due to the presence of broad absorption bands 
associated with iron-oxide minerals from the 
lateritic soils, which are absent in the 
hydrothermally altered areas. In the infrared band 
(TM-7), the spectral separability between both 
soils is related to prominent absorptions bands 
around 2200 nm due to hydroxide-bearing 
minerals associated with the hydrothermal 
activities. As a second conclusion, the spectra 
data showed that even under vegetation cover of 
up to 60%, hydrothermally altered areas could be 
discriminated from the surrounding non-altered 
areas, particularly, areas of lateritic covers. 
However, the presence of the green vegetation 
tends to lower the overall spectra as the 
percentage of vegetation cover is increased. On 
the other hand, the decrease on the overall spectra 
depends on the soil background. So, areas of 
lateritic soils derived from biotite-granites with 
40% of vegetation cover, show lower overall 
spectra than areas of hydrothermally-derived soils 
with 60% of vegetation cover. This conclusion is 
in accordance with the results obtained by Siegal 
& Goetz (1977). 
Based on the field spectra data, the first attempt 
to discriminate hydrothermally altered areas was 
to combine the TM-1, TM-2, and TM-7 bands to 
produce different color composites of the study 
area. However, the high radiometric signals from 
the bare soils in the TM-7 band masked the subtle 
spectral contrast among the different soil covers 
in the color composites. For this reason, TM-7 
band was discarded. Best results were obtained 
with a natural color composite combining TM-1, 
TM-2, and TM-3 bands with blue, green, and red 
filters, respectively. According to Davis and 
Grolier (1984), the main advantage of this 
product is to reproduce the scene in natural 
colors, as observed by human eyes, a fact which 
facilitates geologic interpretation. In this color 
composite, areas of hydrothermally altered 
materials appear in light tones due to the high 
spectral response in the three TM bands. Lateritic 
soils derived from biotite-granites appear in 
yellow and red-brownish colors due to the 
response of ubiquitous ferric oxides in the yellow- 
red TM-3 band. Heavily vegetated areas appear in 
dark greenish tones because of the green 
vegetation peak in the TM-2 band. 
To obtain a better definition of the study area, 
Landsat-TM bands were combined with a high- 
resolution panchromatic aerial photograph (pixel 
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B7. Vienna 1996