393 
ances of the 
/ the values 
: variations 
nponent, it 
; difference 
processing, 
th maximal 
; very dark 
tracks was 
le extreme 
ice of the 
1 from the 
influenced 
Dup. 
al absolute 
it has to be 
put bands, 
■esence of 
the fourth 
1, 5, and 7, 
nted out in 
1 with the 
i limonite 
:d for each 
I. Thus, a 
:nts can be 
al fourth 
iges were 
hydroxiles 
y, a new 
1 between 
lit will be 
limonites, 
Vith this 
>nent was 
were the 
;hows the 
where the 
s the the 
ig to the 
fom CP2, 
>xile and 
, and low 
concentrations imply darker colors for both 
minerals. 
Table 4: Covariances of the eigenvector for 
the principal components of hydoxides and 
limonite 
PCI 
PC2 
Fe 
-0.039 
0.999 
H 
0.999 
-0.039 
It can be concluded, that three virtual images 
were created, one formed by the component H, 
other by the F component and the third one by a 
combination of the former ones. To illustrate 
them graphically, the RGB technique constitute 
an efficient technique to create maps, using the 
following channel assignation: 
Red = component H 
Green = Second principal component of the 
components H and F 
Blue = component F 
As a result, a map was obtained, where the 
abundance of clay minerals such as caolinite, 
illite and other hydroxide silicates appears in 
redish tones. 
The principal result was the reconnaissance of 
all hydrothermal alteration zones with evidences 
in the field. On the other hand, many clayey 
zones got included without hydrothermal origin, 
especially recently reworked agricultural soils. 
A segmentation of the image was realized to 
distinguish between hydrothermal and not 
hydrothermal clays, using exclusively pixels 
recognized as “alteration zones”. The result is a 
notable reduction of the detected alteration 
zones; the majority of them are accommodated 
along faults and along the caldera borders, 
which occurs in the majority of the geothermal 
fields with this structural type. 
CONCLUSIONS 
1. The Acoculco Caldera forms part of a 
hydrothermal activity stage, causing zones 
of surface alteration. 
2. The presence of quartz, calcite, limonite, 
illite, smectite and caolinite stands out in the 
mineralogy of the surface alteration. 
3. The analysis of remote sensing images was 
proved as a numerical tool for the 
reconnaissance and mapping of mineral 
groups, such as iron oxides and aluminium 
hydroxides. These two groups form part of 
the typical hydrothermal alteration 
mineralogy of the Acoculco Caldera. 
4. The principal component technique was 
shown to be appropriate to recognize iron 
oxides (limonite) and aluminium 
hydroxides (clays). 
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