A new type of hydrothermal alteration at the Kizildere geothermal field in the rift zone of the Biiyiik Menderes, western Anatolia, Turkey

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1 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 CaCO 3 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 CO 2 on ist way to the surface, for which reason ph increases up to 9.2 Therefore, the fluid is unable to keep CaCO 3 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

2 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

3 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

4 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 CaCO 3 + H 2 CO 3 Ca HCO 3 Solid solution deposition solution solutior In contrast to other types of hydrothermal alteration, it is a formation of carbonate assemblage by introduction of C0 2-3 which is able to fix metal ions such as Mg 2+, Fe 3+, Ca 2+ and Mn 2+. 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

5 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 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

6 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 C (Na-K), C (Na-K-Ca), and 260 C (SiO 2 ). 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- (SO 4 )-HCO 3 -type, during the waters of Pamukkale and Karahayit show Ca-Na-(SO 4 )-HCO 3 - type (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.

7 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 CO 2 in the thermal waters of Kizildere, high temperature acid water infiltrates the soil formations and dissolves CaCO 3. 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 (CO 2 ) 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 CaCO 3 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. 6 REFERENCES Dewey, J.F. & A.M.C. Sengör, Aegean and surrounding regions: complex multiplate and continuum tectonics in a convergent zone. Geol. Soc. Am. Bull. Part I, 90: Guidi, M., L. Marini, & C. Principe, 990. Hydrogeochemistry of Kizildere geothermal system and nearby region. Geothermal Resources Council Transactions, Vol. 14, Part II; Le Pichon, X. & J. Angelier, The Hellenic arc and trench system: a key to the neotectonic evolution of the eastern Mediterranean. Tectonophysics 60: Mckenzie, D. P., Active tectonics of the Alpin-Himalayan belt: the Aegean and surrounding regions: geophys. J. R. A.str. Soc. 55:

8 Meyer, C. & J.J. HEMLEY, Wall-rock alteration. In: Geochemistry of hydrothermal ore deposits, H. L. Barnes (ed.): p Özgür, N., P. Halbach, A. Pekdeger, N. Sönmez, O. Ö. Dora, D.-S. Ma, M. WOLF, & W. Stichler, 1997, Epithermal antimony, mercury, and gold deposits in the rift zone of the Kücük Menderes, Western Anatolia, Turkey: preliminary studies: in: Mineral Deposits: Research and Exploration, Where do They Meet?, H Papunen (ed.): 4th Biennnial SGA Meeting, Turku, Finland, August 11-13, 1997, p Patton, S., The relationship between extension and volcanism in western Turkey, the Aegean Sea and Central Greece. Ph.D. Thesis, University of Cambridge, 300 p. Simsek, S Geothermal model of Denizli, Sararyköy-Buldan area: Geothermics 14: Spakman, W., Tomographie mapping of the upper mantle structure beneath the Alpine collision belt. In: Crust-mantle recycling at convergence zones, S.R. Hart & L. Gülen (eds.): NATO ASI series, C258, Kluwer, Dodrecht. Vogel, M Zur Geologie und Hydrogeochemie des Kizildere Geothermalfeldes und seiner Umgebung in der Riftzone des Büyük Menderes, W-Anatolien/Türkei: M.Sc. Thesis, Freie Universität Berlin (in prep.).