Mas et
ZEOLITE ZONING IN DRILL HOLES OF THE COPAHUE GEOTHERMAL FIELD, NEUQUEN, ARGENTINA
Graciela
Luis
Leandro
Departamento de Geologia. Universidad Nacional del Sur. 8000-Bahia Blanca. Argentina
Ente Provincial de Energia del Neuquen.
Argentina.
Key Words: stilbite, laumontite, wairakite, Copahue Geothermal Field
ABSTRACT
The objective of this work is the study of the zeolite zoning in
the Copahue Geothermal Field, and the relation between the
zoning with temperature, pressure, composition, etc.
The most common zeolites in Copahue are stilbite,
laumontite, and wairakite. Depth zonation of secondary
minerals were delineated for Ca zeolites as:
Zeolite is an important group of authigenic minerals found in
the drill hole samples from the active geothermal field of
Copahue. X- ray diffraction, petrographic microscopy, and
SEM studies of the zeolitic minerals in drilling-cores and
cuttings from the wells COP-2 and COP-3 were carried out.
The two former occur at temperatures below
whereas
the latter, accompanied by epidote, prehnite, garnet, chlorite
and tremolite, occurs within a range of temperatures from
to greater than
In addition to these common
zeolites; mordenite, clinoptilolite, heulandite and others, were
also identified. The Occurrence of wairakite in the reservoir
other Ca-AI silicates, indicates that
zone coexisting
thermal waters at depth are hot and neutral to slightly alkaline
is rather low. It also strongly suggests a good
and that
path for the vapor.
STUDY AREA
The Copahue Geothermal field is located at latitude
and
71
some 1170
WSW of Buenos Aires
City, and adjoining the border with Chile. The area is
connected to the city of Neuquen by national and provincial
through Zapala,
Lajas and
routes, covering nearly 360
This geothermal field is on the east side
Loncopue (figure
Andes,
the ridge which forms the watershed
of
separating the river basins of the Pacific and Atlantic sides, as
typified by Volcan Copahue and Paso Copahue in the western
part of the area. This area rises to about 2000 m above sea
level.
1. INTRODUCTION
Zeolites occur in almost all of the geothermal systems of the
world. The more frequent species are heulandite, laumontite
and wairakite, but stilbite, chabazite, thompsonite, scolecite,
mordenite, yugawaralite and others have also been mentioned.
All the Ca-zeolites, except wairakite, occur in the lower
of the fields. Wairakite is almost
temperature zones
always constrained to active or fossil geothermal systems,
to areas of high geothermal gradients.
In wide view, several composite volcanos, similar to each
other in terms of type and period of activity, range for about
The volcanos unconformable cover pre-Pliocene
220
sedimentary and volcanic rocks and form stratovolcanos with
a gentle dip. The volcanic activity which is considered, up to
now, to have started in Pliocene time is located around the
tensional stress field which was formed in the North-South
tectonic relief, behind a subduction zone. The volcanic
activity is characterized by the calc alkaline series and the
shoshonitic series (Pesce, 1985).
The Copahue Caviahue composite volcano, in which this
Figure I : Location Map
1077
Mas et
geothermal field is located. stands nearly at the center of the
mentioned volcanic row, forming a distinguished ring shaped
the basin are 2200 to
topography. The mountains
m above sea level. In this area there are five active geothermal
manifestations, which mainly consist of fumaroles, hot springs
and mud pots. Four of these manifestations are placed in
Argentina: Termas de Copahue,
Las
Maquinitas and Anfiteatro, and the
on the Chilean side:
Chancho Co. All of them are on the horst northeast of Vn
Copahue, and the overall area comprises aproximately 20
Three exploration wells had been drilled here, in a sector NW
of the circular basin and about 6
northeast of Vn Copahue.
The well COP-1 is located 2005 m above sea level on the
south shore of
Mellizas lower pond and reached a depth of
1414 COP-2 located about 2120 m above sea level on the
north of
Mellizas and is 1241 m in total depth; and COP-3
is located about 201 1 m above sea level, at 30 m from the east
shore of Las Mellizas upper pond, and is 1065 m deep. (Figure
1, inset). This latter was drilled in the course of the "Northern
Neuquen Geothermal Development Project" carried out by
EPEN (Ente Provincial de
del
and JICA
(Japan International Cooperation Agency). These exploration
wells confirm the Occurrence of a vapor dominated reservoir
below a depth of 800 m. The three wells form a triangle with
km side length, placed over a predominant fault WNW-ESE
which links the Chancho Co and
manifestations.
A demostration binary cycle geothermal power plant was
installed in April 1988 in COP-I, and produces 670 Kw. It is
the first geothermal power plant in South America.
Secondary minerals in core samples were recorded in the
JICA-EPEN (1992) report and have been studied in detail by
Mas (1993) for the three drill holes. This study consisted of
hand specimen, petrographic, X-ray diffraction and SEM
observations.
Conditions of temperature, pressure, and the chemical
characteristics of the hydrothermal fluid associated to these
minerals were obtained from drill hole measurements (JICAEPEN, 1992) and fluid inclusions studies (JICA-EPEN, op.
Mas et 1993).
3. RESULTS
The drill-cores and cuttings show that the degree of alteration
changes from surface to the bottom of the wells. The
alteration is rather weak above 600 m of depth on the whole,
and
mainly
consists of
comparatively
strong
montmorillonitization and chloritization.Alteration is stronger
below 600 m, with chloritization and epidote filling cracks.
Below 800 m of depth three types of alteration can be
distinguished:
-
-
wide chloritization, which affects almost the whole rock;
Ca-silicate minerals, such as epidote, prehnite, garnet and
tremolite, that occur specially in cracks, although they can
be found also as a replacement
Wairakite, crystallized on veins lined by quartz andor
prehnite and epidote.
The sequence penetrated by drilling is dominated by basaltic
andesite to andesite lavas, with minor pyroclastics rocks. The
host rock at the reservoir depth is a strongly altered andesite
whose original phenocrystals mainly comprised
and
matrix now altered to a fine-grained
augite, in a
assemblage of quartz, albite, chlorite, and minor illite, pyrite
and leucoxene. Primary plagioclase was replaced by epidote,
albite and K-feldspar. Filled veins of wairakite, quartz,
prehnite, epidote and some garnet cut the rock, and coarsely
crystalline wairakite (up to 3 mm in diameter) coats surfaces
of what were once cracks and open veins filled with fluid.
Zeolite minerals are well developed in the drill-core and
cutting samples from the Copahue Geothermal Field. Some
Ca-zeolites are rather abundant; such common zeolites
include stilbite, laumontite and wairakite. In addition to these
common zeolites, mordenite, heulandite, thompsonite and
natrolite were also identified. All of them, except mordenite
and heulandite, are very minor in quantity and occur
sporadically in some samples between depths of 150 and 600
m. All the zeolitic minerals were determined by a
of
SEM and optical microscopy.
Stilbite occurs at shallow depths of the drill-core samples. It is
found at depths of less than 300 m, and most commonly
between depths of 70 210 m. Stilbite occurs predominantely
as the recrystallization product of the siliceous minerals
cristobalite) which are formed under acid-sulfatic conditions
in the upper section of the field or from the glassy siliceous
matrix of the volcanic tuffaceous rocks. It also occurs in some
surface samples far away from the areas of intense acidsulfate alteration. This mineral is intimately associated to
crystobalite and it is presumed to has been formed from the
recrystallization of siliceous material poorly crystalline.
Figure 2 shows the scanning electron microscopy image of a
sample from 110 m of depth in the COP 3 well. It is
Figure 2. SEM photograph showing stilbite fibers in fissures and voids. COP-3,
1078
m.
Mas et
possible to see fine
to fibrous crystals of stilbite
merging from the walls of voids and fissures in the siliceous
mass. The stilbite is accompanied by quartz and plagioclase,
with subordinate mordenite and crystobalite. The main
peaks of this mineral are 9.18,
8.98,
4.068,
4.048,
and others. The least square
adjusted unit cell constants of stilbite are as follow: a,
13,6124 8,
18,2112
11,2602
8,
52‘853
Volume 2203.217
(i3.5010). (Appleman
Evans, modified by J.
1991).
Laumontite is characteristically found at depths greater than
200 m and shallower than nearly 650 m, commonly between
400 and 650 m, although it was found in some samples at
about 800 m. It occurs overlapping the stilbite zone above and
the wairakite zone below, and appears as a filling of small
veins and amigdules or replacing plagioclase phenocrysts and
glassy groundmass, commonly as sub-parallel lath aggregates.
It is the least common of the three main zeolites and could not
be concentrated. It was determined by means of optical
by its characteristic reflexions at:
microscopy and by
9.58,
6.848,
4.168,
3.518,
and others.
Wairakite is the most abundant zeolitic mineral in the Codrill-cores. It has been identified in samples beginning
Figure 3: COP-3,
In the well COP-2 it was identified in some samples as
shallow as 480 m of depth. In the upper levels it appears as a
and to a lesser extent
fine filling of pores and
replacing plagioclase and fine grained matrix. At greater
depths the crystals are bigger, commonly up to 0.8 mm to
mm of diameter, and they are found filling the cores of veins.
Microscopically it occurs as irregular masses and
grains. It is colorless, with low relief, refraction index about
and very low
The more conspicuous optic
feature of this mineral is the presence of cross-hatched
twinning, besides patches irregularly arranged.
In the reservoir levels, between 800-890 m and 1002-1034 m
at COP-3 and 853-1065 m at COP-2, there are commonly
veins of up to 5 mm of thickness filled with wairakite
intimately associated to prehnite, quartz and tiny crystals of
epidote. The wairakite typically forms euhedral crystals, with
didodecahedral or trapezohedra1 shapes or
combinations with a subordinated cube. The
electron-microscopy photograph of figure 3, shows the
morphology and development of the wairakite crystals from
COP-3 well.
m. SEM imagen of a druse of wairakite crystals lining a vein.
Figure 4 shows the scanning-electronic-microscopy
photograph of some prehnite crystals associated with
wairakite and quartz from a vein of 4-5 mm of thickness from
1015 m, COP-3. It is possible to see the tabular orthorhombic
habit of this mineral, with development of three
intergrowing with the pseudocubic trapezohedron of
wairakite.
pattern of this mineral shows characteristics
The
wairakite peaks at 6.858,
5.578,
4.848,
and others. The least square
are as follow: a,
adjusted constants (Benoit,
13,6824088,
13,6989858,
c,
13,5497728,
90
Volume
2539,5815 (14,3835).
4. DISCUSSION
at depths of 600 m, and extending to the bottom of the
holes, though it is more abundant in the zones with cracks,
fissures and open spaces.
When the distribution of the zeolitic minerals is examined as a
function of depth in the drill holes, one pattern is apparent:
their zonal distribution with increasing depth (hence
temperatures) varies from stilbite to laumontite and wairakite,
with minor presence of mordenite, clinoptilolite and
heulandite. The occurrence of laumontite is much less
compared to those of the other two zeolites. In a general way,
the three determined zeolites show a zonation trend with
depth. In some samples two zeolites coexist simultaneously as
there are some overlapping of zones in their depth
distribution. In no place do all three zeolites occur together.
diagram where it can be seen the
Figure 5 shows a
for the
equilibrium constraints determined for Liou
pairs
laumontite-stilbite,
and
laumontite-wairakite.
The experimentally determined reactions are:
(1) Stilbite
= Laumontite + 3 Quartz + 3
(2) Laumontite Wairakite + 2
(3) Laumontite
2 Quartz 2
under conditions of
=
1079
Mas et al.
Figure 4:
COP-3, 1015 m. SEM photograph of a wairakite trapezohedron in a groundmass of tabular crystals of prehnite.
This graph shows that
- The sequence of zeolites reflects a progresive
dehydratation
increase in temperature, and
- the lack of lawsonite in the
of Copahue
indicates that the pressures of formation of these minerals
were moderate to low, the higher limit of pressure for
laumontite
respect to the lawsonite is 3 0 Kb
to slightly greater than
The
temperatures of
lower temperature occurrence of prehnite and epidote overlap
with wairakite.
Filling temperatures of fluid inclusions in the quartz
coexisting with wairakite range widely from
to 280°C
(Maset 1993). This wide range of filling temperatures and
the coexistence of vapor- and liquid-rich fluid inclusions in
the same quartz crystal suggests that boiling took place when
the zeolite was precipitated.
The filling temperature of the quartz in the laumontite zone
ranges from -210" to 320°C and in the stilbite zone between
and
These wider ranges indicate that the liquidvapor separation occurred in these stages, leading to the
trapping of fluids with different phase ratios. On the other
hand, the static temperature for the bottom hole is about
for COP-2 and
for COP-3.
239°C for
i
onit e
(1985) stated some stability conditions for the
zeolitic minerals in the Seigoshi Gold Silver Mining District,
Japan. In summarizing the fluid inclusion data, and taking into
account these criteria, the most likely temperature for the
stilbite, laumontite and wairakite zones are:
and
respectively. However the presence of garnet
in some quartz veins of the wairakite zone may raise the upper
limit up to
Liou
stated that the temperature range necessary for
the formation of laumontite is aproximately 150' to 250°C at
bars
where aqueous fluid pressure equal to total
pressure, but in natural hot springs where
may be about
of
and the fluid phase has contaminants such as
and S in significant amounts this range would tend to shift
toward lower temperatures.
After
Figure
(1971)
-
diagram
Liou, 1971
Wairakite shows a strong relation with the lost circulation
zone and it is crystallized along cracks at reservoir depth.
The calcium zeolites, exclusive wairakite, are generally
restricted to temperatures
wairakite occurs from
1080
Liou et al. (1985) pointed out that the paragenetic depth
sequence of Ca-zeolites in a geothermal system is highly
dependent on the imposed thermal gradient, on the
ratio, and on other factors like solution
composition.
Seki et
experimentally determined the stability
relations between laumontite, yugawaralite and wairakite at
y 0.3.
absence of yugawaralite in the
Mas etal.
Copahue cores may be due to the narrow stability range of this
The zeolite zonation
mineral at low
laumontite-wairakite, as found in Copahue, may be. stable in a
system with relatively low geothermal gradient and low
Seki et
stated that the most characteristic Casilicates in geothermal systems are stable at
values of
less than 0.1 mol. The
content of the fluid phase controls
also the appearence of the calcite-clay assemblage at the
expense of the Ca-AI silicates, and the pH-value of the
hydrothermal solution. Liou et
1985) confirmed the
generally accepted idea that the
silicates are stable at a
low activity of CO,, hence in slightly alkaline solution.
Bird (1984) found that the Ca zeolites laumontite and
wairakite can coexist only with an aqueous solution in which
is aproximately equal to or greater than that
for saturation with
On the other hand, wairakite,
prehnite and epidote grew together in equilibrium from quartz
saturated water.
The ubiquitous occurrence of pyrite and the absence of
hematite suggest that the Copahue Field had
condition within the pyrite stability field.
5. CONCLUSIONS
When the distribution of zeolitic minerals is examined as a
function of depth in the drill holes of the Copahue Geothermal
field is apparent that there is a zonation.
The following points can be made regarding their distribution:
There are some overlapping of zones.
-Not all three zeolites occur in any one core.
The occurrence of laumontite is much less compared to
those of the other two zeolites.
The distribution of secondary minerals in the Copahue Field is
primeraly controlled by changes in temperature. Depthdependent zonation patterns are observed, among other
minerals, for the Ca-zeolites.
This zonation is as follow:
of Ca-zeolites, besides calcite in some levels of the
wells, indicates that thermal water at depth are neutral to
slightly alkaline and
is low enough to stabilize
silicates. The environment in which the crystals grew was
open space filled with liquid. The presence of wairakite in the
reservoir level suggest a good path for vapor.
The distinct sequence of Ca-zeolites found, reflects
progressive dehydratation with increasing temperatures. This
alteration resulted from interaction of the host rocks with
ascending near neutral fluids.
In summarizing the fluid inclusions data, and taking into
account the alteration mineralogy present, the most likely
temperature for the stilbite, laumontite and wairakite zones
are:
and 240" - 280°C This upper. limit
may higher (up to -300'1330°C) taking into account the
ACKNOWLEDGEMENTS
The authors would like to thank to the Ente Provincial de
Energia del Neuquen for providing us with the samples,
facilities at field and drills information; to the Consejo
Nacional de Investigaciones Cientificas y Tecnicas for the
funds for financing this project and to the Universidad
Nacional del Sur for the laboratory and equipment facilities.
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