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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
1996.
2. GEOLOGICAL FRAME
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
evolved.
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.
4. REMOTE SENSING
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.
3. HYDROTHERMAL ALTERATION
5. MAPPING OF HYDROTHERMAL
ALTERATION BY REMOTE SENSING
