A new type of hydrothermal alteration at the Kizildere geothermal field in the rift zone of the
Biiyiik Menderes, western Anatolia, Turkey
N. Özgür, M. Vogel & A.Pekdeger
Freie Universität Berlin, FR Rohstoff- und Umweltgeologie, Germany
ABSTRACT: In the geothermal field of Kizildere, high temperature acid water infiltrates the
sedimentary rocks and dissolves CaCO3 The fluids aim still to get in an equilibrium
constantly, but does not reach suitable parameters of the corresponding environment
Consequently, the fluids lose T, P, and CO2 on ist way to the surface, for which reason pH
increases up to 9.2 Therefore, the fluid is unable to keep CaCO3 in solution, but it precipitates
and occurs in sedimentary rocks near the surface The Kolonkaya and Tosunlar formations in
Kizildere show typical color of alteration which is considered as a new alteration type of
carbonatization.
1 INTRODUCTION
Turkey is a country with a big potential for the exploitation of geothermal energy. Therefore,
we have also investigated hydrogeochemical and isotope geochemical features of geothermal
waters in Kizildere and its environs.
The subduction of the Africa plate under the Eurasian plate causes compressional tectonic
features, which result in the lifting of the Menderes Massif. From the Middle Miocene to
Pleistocene, the continental rift zone of Büyük Menderes was formed because of extensional
tectonic features, which are represented by a great number of hot springs The thermal waters
are related to faults which strike preferentially in NW-SE and NE-SW directions, located
diagonal to the general strike of the rift zone of Büyük Menderes, and are generated by
compressional tectonic stress revealing the deformation of uplift between two extentional rift
zones (Özgür et al., 1997)
We have selected to investigate the Kizildere thermal field with a geothermal power plant of
20 NW. For comparison, the other hot springs in the environs have been studied. The aim of
this paper to present a new type of hydrothermal alteration of carbonatization in the
geothermal field of Kizildere and hydrogeochemical features of the thermal waters in
combination with the origin and evolution of the thermal waters.
A research scheme was carried out in 1993/1995, divided into two main fields:
(i) geological and geochemical investigations based on detailed geological mapping and rock
sampling and (ii) comprehensive hydrogeological and hydrogeochemical investigations with
sampling of groundwater and thermal waters.
2 GEOLOGIC SETTING
The Kizildere geothermal field is located in the northern part of the Büyük Menderes rift
zone, which represents an important tectonic structure of Quaternary age within the Menderes
Massif (Fig. 1). The Massif is one of the oldest basements in Turkey and consists of (i) a
gneiss-core surrounded by a schist and marble envelope, (ii) a less metamorphosed cover
series and (iii) an intensely deformed volcano sedimentary sequence with incipient HP/LT
metamorphism. The rift zones within the massif are the results of an extension which is
believed to be closely related to the northward movement of the Arabian plate in the east
pushing Anatolia westwards through the North Anatolian and East Anatolian Faults. The
southerly bending of the North Anatolian Fault in the northern Aegean and Greece prevents
the escape of the Anatolian plate further westwards placing the system in a locking geometry
(Dewey & Sengör, 1979). This creates an E-W compression in the Menderes Massif which is
relieved by N-S extension. The driving force of extension in the Aegean is believed to be the
subduction along the Hellenic Trench (McKenzie, 1978). For the timing of initiation of
extension an age within a range from 26 Ma (Spakman, 1989) about 12 Ma (Le Pichon &
Angelier, 1979) to 5 Ma (Patton, 1992) is given. In the thermal field of Kizildere, the
basement is comprised by the Paleozoic metamorphics which consist of gneiss, schist and the
igdecik formation with intercalations of quartzites, mica schists, and marbles (Simsek, 1985;
Vogel, 1997).
The basement rocks are overlain by Pliocene sediments These sediments show fluvial and
lacustrine characters and consist of (i) the 200-m-thick Kizilburun formation, (ii) the Sazak
formation with a thickness from 100 to 250 m, (iii) the Kolonkaya formation in a range of
thickness from 350 to 500 m, and (iv) the 500-m-thick Tosunlar formation. The Kizilburun
formation is an intercalation of red and brown conglomerates, sandstones, shales, and
lignites, the Sazak formation is composed of intercalated grey limestones, marls, and
siltstones. The Kolonkaya formation contains yellowish green marls, siltstones, and
sandstones, the Tosunlar formation is composed of an alternation of conglomerates,
sandstones and mudstones with fossiliferous clay units. Quaternary alluvium overlies all of
the units and reaches a maximum thickness of some hundred meters.
Fig. 1 Geologic setting of the Menderes Massif and location map of the geothermal field of
Kizildere (open triangle the geothermal field of Kizildere; closed triangles other geothermal
fields).
The thermal field is regionally controlled by E-W trending faults. Locally, NW-SE or NE-SW
trending faults are active in the field. The development of these trending faults lead to a
compression, which was generated by the extension during the formation of the rift zone of
Büyük Menderes (Özgür et al., 1997). The northern and southern flanks of the metamorphic
basement are affected by uplifting and dissected into a great number of step faults.
3 HYDROTHERMAL ALTERATION
In the geothermal field of Kizildere, the metamorphic and sedimentary rocks are distinguished
by intensely hydrothermal alteration. Besides phyllic, argillic and silicic alteration
±haematitization, a new alteration type of carbonatization is recognized in sedimentary rocks
especially. Silicification ±haematitization is an additional overprinting type of alteration and
does not depend upon the rock chemistry. A noticeable result of silicification is the increase
of density and hardness. Hydrothermal fluids will be able to precipitate quartz with haematite
in veins, and/or hollows in all kind of rocks.
The precondition for the formation of phyllic and argillic alteration is the presence of feldspar
rocks. The metamorphic rocks show potassium metasomatism and plagioclase. Over a wide
area, white clay minerals, such as montmorillonite and kaolinite and fine-grainec micas as
sericite and illite, displace K-feldspar and biotite by hydrolysis. In these metasomatic
reactions hydrogen ions will be consumed while pot will be released. The macroscopic effect
is a powdery texture of the involved rocks, which loses its texture.
The Pliocene sedimentary rocks of the Kizildere geothermal field show already from distance
intense coloured zones. Near the surface, the marls and limestones of the Kolonkaya and
Tosunlar formations vary from pale white to yellow, orange and brown to dark red colored
shapes. Responsible for the colors are iron mineralizations, but the fading is a result of the
phenomena of carbonatization (fig. 2). By the process of hydrothermal alteration Ca2+ is
usually heavily depleted except by the type carbonatization (Meyer & Hemley, 1967).
Fig. 2 Soil carbonatization by the infiltration of high temperature acid water in marls and
limestones of the Kolonkaya formation in Kizildere (Scale: 1:100).
In this process, the carbonic acid in geothermal waters determines whether these waters will
dissolve carbonates (limestones and marls) or precipitate them as follows:
ersion
CaCO3 + H2CO3 ↔ Ca2+ + 2HCO3Solid
solution deposition solution solutior
In contrast to other types of hydrothermal alteration, it is a formation of carbonate assemblage
by introduction of C032- which is able to fix metal ions such as Mg2+, Fe3+, Ca2+ and Mn2+.
Therefore, many different kind of carbonatization can be discriminated.. Dolomitization of
limestones by magnesium metasomatism is a simple base-exchange process and one of them.
Two or more different types of carbonates can occur in the same alteration zone depending on
the chemical composition of the rocks and the solutions. In the investigated area of Kizildere,
it seems to be obvious that the carbonate is dissolved in one place and precipitated at another
by hydrothermal solutions. This can be explained by the changes of physicochemical
parameters of hydrothermal fluids.
In general, strongly altered rocks occur along the tectonicaly weakened zones because of the
better possibility of intense hydrothermal water rock interaction. In spite of this the domain of
hydrothermal carbonatization shows the greatest horizontal extension in the investigated area.
For the formation of an alteration type of carbonatization, an immediate contact with high
temperature thermal waters is not necessary.
4 GEOCHEMISTRY
Within the present study, the outflow of 10 thermal springs, 8 drill holes, I groundwater
spring were sampled in different seasons from 1992 to 1995. For hydrogeochemical and
isotopic comparison, we have taken a rain water sample from the geothermal area of
Kizildere. Additionally, 120 rock samples and 48 precipitations were collected in the
investigated area. At the surface, the metamorphic and sedimentary rocks are distinguished by
an enrichment of the metals of Hg, Sb, As, Tl, and Ag in connection with the intensity of
hydrothermal alteration which geochemically can be compared with the fields of epithermal
ore deposits (Özgür et al., 1997).
Generally, Hg values are high in the less altered metamorphic and sedimentary rocks. The
rocks of Igdecik, Sazak and Kolonkaya formations show extremely high Hg values, between
800 and 865 ppm, which might be attributed to the geothermal activity in the investigated
area. The Tosunlar formation and travertine have Hg values ranging from 1.0 to 1.3 ppm. The
less altered metamorphic and sedimentary rocks show Sb values from 0.2 to 1.0 ppm. Besides
the Sb values up to 39 ppm in travertine, the Sazak formation, as a reservoir rock, shows the
highest Sb concentrations up to 106 ppm which might be also attributed to hydrothermal
activity. The less altered metamorphic rocks are distinguished by As contents of 1.0 ppm. The
rocks of Kizilburun, Sazak and Tosunlar formations show As concentrations of about 100
ppm. The Kolonkaya formation reaches As values of 265 ppm. Travertine has As contents up
to 420 ppm. The high As concentrations in the altered sedimentary rocks are linked to the
hydrothermal activity. High boron contents, up to 900 ppm, are found in gneiss which can be
led to the mineral phases of tourmaline and biotite. In the sedimentary formations, there are B
contents ranging from 2 to 20 ppm, but up to 680 ppm in the Kolonkaya formation and 2850
ppm in travertine. Moreover, the high concentrations of B might be attributed to a B deposit at
depth in sedimentary rocks. Fluorine shows values up to 1000 ppm in gneiss and up to 2125
in the sedimentary rocks and travertine which may be attributed to a hydrothermal activity.
The surface temperatures of the Kizildere thermal waters in drill holes differ from 95 to 100
°C whereas the outflows of 10 thermal springs in the surrounding area show temperatures
from 40 to 73 °C. In the thermal field of Kizildere, the geochemical thermometers indicate a
reservoir temperature of 220-230 °C (Na-K), 230-260 °C (Na-K-Ca), and 260 °C (SiO2). The
Kizildere thermal waters are distinguished by pH values of 9.2, an Eh value of -150 mV, a
mean value of conductivity of 5300 µS/cm, and total dissolved solids (TDS) of 4500 mg/l. In
comparison, the waters in the vicinity have pH values from 6.1 to 7.1, Eh values in a range
from -137 to 311 mV, conductivity values from 2360 to 3660 µS/cm, and TDS from 1780 to
3838 mg/l.
Hydrogeochemically, the thermal waters of Kizildere and its environs can be classified as Na(SO4)-HCO3-type, during the waters of Pamukkale and Karahayit show Ca-Na-(SO4)-HCO3type (Fig. 3). This can be explained by mixing of a thermal sodium bicarbonate component, a
cold calcium sulfate component and a cold calcium bicarbonate component in different
proportions (Guidi et al., 1990); alternatively, the sodium bicarbonate thermal water could
dissolve calcium sulfate from the Neogene sediments and mix with cold calcium bicarbonate
water to originate these intermediate waters. The shallow waters, located near the village of
Kizildere, exhibit sodium magnesium sulfate compositions and TDS of 244 mg/1.
Fig. 3 Thermal waters of Kizildere and its environs on a PIPER diagram.
The genetic mechanism of these waters implies the infiltration of sodium bicarbonate thermal
waters into shallow environments where leaching of Neogene sedimentary rocks and calcium
carbonate precipitation takes place. Trace elements which indicate intensive high temperature
water-rock interaction such as B (35 mg/l) and F (35 mg/l) are found in high concentrations in
these waters (Özgür et al., 1997). As heavy metals, there are high concentrations of As (1
mg/l) and Sb (0.1 mg/l) in the waters of Kizildere. The contents of Au and Ag together with
base metals (Cu, Pb, Zn) lie under the detection limits, which could be probably precipitated
in deep environments under suitable pH, Eh and temperature conditions.
5 CONCLUSIONS
Due to high temperature, pressure, and high content of CO2 in the thermal waters of Kizildere,
high temperature acid water infiltrates the soil formations and dissolves CaCO3. The solution
aims to get in an equilibrium constantly, but does not reach the suitable parameters of the
corresponding environment. Thus, the solution loses temperature, pressure and (CO2) gas on
its way to the surface, for which reason the pH values increase up to pH 9,2 Consequently, the
geothermal fluid is unable to keep CaCO3 in solution, but it precipitates and occurs in the
sedimentary rocks near the surface Especially in the S and SE of the Kizildere geothermal
field, the marls and limestones of the Pliocene Kolonkaya and Tosunlar formations show
typical values in color and structure which can be considered as an alteration type of
carbonatization.
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