Full text: International cooperation and technology transfer

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Recently, the Institute de Investigaciones 
Eléctricas, Cuemacava, Mexico, is realizing 
environmental studies in the region to make sure 
the development of the geothermal field without 
casusing damages to the environment. The 
investigation includes the mapping of the zones 
of hydrothermal alteration, the chemical and 
isotopic characterization of the fluids, including 
heavy metals, and the elaboration of shallow and 
deep aquifer flow models. This paper presents 
the results of the mapping of hydrothemal 
alteration zones applying the technique of 
remote sensing with one Landsat image from 
Tectonically, the caldera is located within the 
Transmexican Volcanic Belt (CVT) and forms 
part of the basement of the Sierra Madre 
Oriental (SMOr). Due to its volcanological and 
structural characteristics, analogies consist with 
the caldera of Los Humeros, located 80 km to 
the NE. 
The basement of the zone is formed by a thick 
sequence of mesozoic limestones with anticlines 
directed NW-SE and an intense folding towards 
the NE. They are affected by inverse faults in its 
eastern flank. 
In the early Tertiary, igneous rocks were 
emplaced. Their base is formed by andesites and 
a sequence of regional rhyolites. Above the 
latter one, a resurgent caldera system was 
The Acoculco Caldera is a caldera with a normal 
acid evolution and strongly differentiated. 
During its principal explosion, large quantities 
of ignimbrites and tuffs were produced. The 
borders of the principal collapse were sealed by 
basalts (cinder cones and lava flows) and domes 
and mesas of dacitic and rhyodacitic lava during 
the final stage of the caldera. Recently, the 
caldera is found in its hydrothermal stage with 
local discharges of gases, distal discharges of 
thermal water and steam explosions. 
At the surface, various zones of hydrothermal 
alteration were recognized. Most of them are 
related with discharges of gases such as in the 
case of the alteration zones of Los Azufres and 
La Alcaparrosa. The mineralogy of the 
hydrothermal alteration is formed by calcite, 
quartz, clay minerals, zeolites, epidote, chlorite 
and biotite. Sulphides were recognized in form 
of pyrite, chalcopyrite, areSnopyrite, marcasite, 
bomite, pyrotite and idaite. Limonite, hematite 
and ilmenite are the principal oxidation 
minerals. The most commun paragenesis is 
calcite + quartz + pyrite. The group of clay 
minerals is formed by various paragenesis. One 
of the identified one is chlorite + sodic smectite 
+ illite and smectite + chlorite. 
The use of the remote perception methods is 
based principally on the reflection or emission 
of a certain intensity of light for each type of 
objects in different regions of the wave length 
within the electromagnetic spectrum. This 
depends on the chemical, mineralogical and 
topological composition and the physical 
characters itics of the object. Using the 
information of one or several ranges of wave 
lengths, it is possible to distinguish between the 
different types of objects or surfaces of the earth 
and to obtain distribution maps. 
At the level of the contained information in a 
satélite image, each observed point on the 
terrestrial surface represents a combination of 
electromagnetic components, whose result is the 
measured radiation by the sensor. At the 
moment of the image taking, the contribution of 
each component within the measured global 
value is unknown and the response variables are 
just radiance values for each band. For this 
study, the analytical technique of principal 
components of images was applied to extract 
from them information about the mineralogy of 
hydrothermal alteration. One Landsat image 
from 1996 was used for this application. 

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