Oxygen, carbon and sulphur isotope studies in the

Journal of African Earth Sciences 59 (2011) 341–348

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Oxygen, carbon and sulphur isotope studies in the Keban Pb–Zn deposits, eastern Turkey: An approach on the origin of hydrothermal fluids Leyla Kalender ⇑ Firat Univetsity, Department of Geology Engineering, 23119 Elazig, Turkey

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Article history: Received 1 December 2009 Received in revised form 22 November 2010 Accepted 2 January 2011 Available online 9 January 2011 Keywords: Stable isotopes Contact pyrometasomatic type East Anatolian Keban Turkey

a b s t r a c t Pb–Zn deposits are widespread and common in various parts of the Taurus Belt. Most of the deposits are of pyrometasomatic and hydrothermal origin. The Keban Pb–Zn deposits are located along the intrusive contact between the Paleozoic – Lower Triassic Keban Metamorphic Formation and the syenite porphyry of the Upper Cretaceous Keban igneous rocks. Various studies have already been carried out; using fluid inclusion studies on fluorite, calcite and quartz on the pyrite–chalcopyrite bearing Keban ore deposits. This study focuses on the interpretation of stable isotope compositions in connexion with fluid inclusion data. Sulphur isotope values (d34S) of pyrite are within the range of 0.59 to +0.17‰V-CDT (n = 10). Thus, the source of sulphur is considered to be magmatic, as evidenced by associated igneous rocks and d34S values around zero‘‘0’’. Oxygen isotope values d18O of quartz vary between +10.5 and +19.9‰(SMOW). However, d18O and d13C values of calcite related to re-crystallized limestone (Keban Metamorphic Formation) reach up to +27.3‰(SMOW) and +1.6‰(PDB), respectively. The d34S, d13C and d18O values demonstrate that skarn-type Pb–Zn deposits formed within syeno-monzonitic rocks and calc-schist contacts could have developed at low temperatures, by mixing metamorphic and meteoric waters in the final stages of magmatism. Ó 2011 Elsevier Ltd. All rights reserved.

1. Introduction The Keban mining district in eastern Turkey has been a major metal province with Pb–Zn–Ag and Cu–Mn–Mo ores. Mining activity operated from 2000 BC up to the recent (Seeliger et al., 1985). The mine was operated by Roman, Seljuk and Ottoman Empires and the Republic of Turkey until it was abandoned in 1992. Pyrometasomatic ores are variably thick and contain 6.34% Pb; 4.95% Zn and 256 ppm Ag. Wastes produced by abandoned works comprise 58 sites near the Derebaca gallery. Their volumes range between 1.5 m3 and 468 m3 and their weight is approximately 8000 tons. Wastes contain 2.28 ppm Au, 55.86 ppm Ag, 2.17% Pb, and 0.81% Zn (Kalender and Hanelçi, 2001). The Keban Pb–Zn deposit is located along the contact of calcschists of the Keban Metamorphic Formation and Upper Cretaceous syeno-monzonitic rocks. Major sulphides occurring are galena, sphalerite, chalcopyrite, pyrite, arsenopyrite, enargite, lollingite, and their secondary minerals seruzite, smithsonite, azurite, malachite, covellite and chalcocite. In addition, skarn minerals diopside, grossularite, epidote and scheelite were found. The Keban Pb–Zn skarn deposit occurs within the Eastern Taurus Belt. Several investigations including fluid inclusion studies ⇑ Tel.: +90 4242478610; fax: +90 4242415525. E-mail addresses: leylakalender@firat.edu.tr, [email protected] 1464-343X/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.jafrearsci.2011.01.001

were conducted on mineralizations within the Taurus Belt in order to determine the ore genesis (Kalender et al., 2009). The Asßvan skarn-type Fe and Kizildag˘ vein-type Pb–Zn mineralizations are some of them (Kalender et al., 2009, 2010). Most of the previous researches in the Keban Pb–Zn skarn deposit area were concerned almost exclusively with large contact zones related to Pb–Zn mineralizations (Arni, 1937; Ziserman, 1969; Köksoy, 1972; Çag˘layan, 1984). Recent studies of contact metasomatism indicate two different types of ore forming hydrothermal fluids. The early stage is represented by quartz-chalcopyrite-pyrite bearing fluids, with homogenization temperatures of 463–510 °C and salinity of 23.64–30.68 wt.% equivalent NaCl. The second stage is marked by pyrite-fluorite bearing fluids, with homogenization temperatures of 105–114 °C and salinity of 12.5–15.6 wt.% equivalent NaCl. The late stage is represented by pyrite–calcite bearing fluids, with homogenization temperatures of 93.7–134.2 °C and salinity of 12.5–15.65 wt.% equivalent NaCl. Microscopic investigations of ore-textures, petrologic and petrographical features and fluid inclusion studies suggest that sulphides were emplaced at the final stage of regional metamorphism within the orogenic belt (Kalender, 2000; Kalender and Hanelçi, 2002, 2004). In order to unravel the ore genesis, sulphur, oxygen and carbon isotope studies were conducted. Sulphur-bearing minerals analyzed come from quartz–pyrite–chalcopyrite zones bordered by calc-schists. Samples were selected from a diamond-drill core

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L. Kalender / Journal of African Earth Sciences 59 (2011) 341–348

Fig. 1. Location and geology maps of the study area, modified from Kalender (2000).


obtained by EIEI in 1972 as a part of a deep exploration program at Keban.

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passes from bottom to top to sandy limestone, multi-fossil massive knob limestone and marls. Asterigerina rotula, Discocyclina sp., Assilina sp., Globoratalia sp., fosills are detected and the unit was given a Sub-Miocene age (Perinçek, 1979).

2. Geology The geological units of the study area are the Paleozoic – Lower Triassic Keban Metamorphic Formations, the Upper Cretaceous Keban igneous complexes, the Mid-Eocene to Oligocene Kırkgeçit Formation and the Early Miocene Alibonca Formation (Fig. 1). Keban Metamorphic Formations were first examined by Moucher (1938) and Kovenko (1941). In later studies, Kipman (1976) classified the unit as re-crystallized limestone, calc-schist, meta-conglomerate and calc-phyllite from bottom to top. After that Hanelçi (1991) detected presence of five different rock formations in the Keban Metamorphic Formations, which are, calc-schist, sericite–chlorite–schist, bedded limestone, laminar limestone and dolomite limestone from bottom to top. Said calc-schists are seen in Southern Keban, along the Firat (Euphrates) Valley. Calc-schists, surrounding around the Siftil Hill and are cut by sub-volcanic rock in sill and dike forms. Kink bands have developed on the taken samples as a result of lepidoblastic texture and multistage metamorphism. Keban igneous complexes consist of alkali syenite, syenite, quartz syenite and quartz monzonite, monzonite compound subvolcanic rocks and their surface rocks. This intrusive mass changes between a few meters and a few ten-meters while sill thicknesses reach 100–150 m. Twenty meter deep dikes with 1–1.5 m diameters are seen around Keban Creek. These igneous complexes (syeno-monzonitic) not containing the same particle size and colour in the study area may be linked to sudden changes in cooling, magnitude of the metamorphism or the compound of the igneous. Due to its relation to mineralization, some researchers (Kinesß, 1969; Köksoy, 1972; Kipman, 1976, 1981, 1982; Kalender, 2000) state that the Keban igneous complexes contain syeno-monzonitic compouns, such as alkali-rich magma, and that it was subject to formation of potassium metasomatizm during alkali feldspar phenocrystals crystallization period. Çelebi et al. (1997) states in his the Keban igneous complexes petrology and geochemistry study that in sanidine crystals that contain 1.97–6.46% anorthite, 19.24–20.86% albite and 74.13–77.71% orthoclase and K2O, and BaO and Rb amounts increase directly. Researcher also states that towards the end of magmatic separations, Na increases in comparison to K and this indicates that transitions to albite, and Ca attached to the plagioclases may decrease in comparison to Sr. in sanidines inside the micro-syenites cutting the Keban Metamorphic Formations (K2O ratio, 3–98%). In age tests made using the 40 Ar/40Ar method, it was determined that cooling ages of the Keban igneous complex range between 76 ± 2.5 and 78.5 ± 2.5 my (Upper Cretaceous–Cenomanian) (Asutay, 1988). Kusßçu and Erler (1999) and Kalender (2000) stated that in granat-pyroxene skarns diopside presence indicate the existence of an alkali-rich system and Cu and W richness was observed as well as presence of Ag, Pb and Zn around Siftil Hill where pyroxenes are seen as diopsides. Another unit in the study area is the Middle Eocene Kırkgeçit Formation and it mainly consists of conglomerates, carbonates and flysch facieses rocks. The Unit contains rock parts and blocks from the Keban Metamorphic Formations around Keban. Fossils are found in the samples collected from the lowest sections of the unit: Europertia magna, Nümmilites sp., Discocycline sp., and Spaerogypsina sp. (Asutay, 1988). Alibonca Formation is observed on the northwest of unit Keban town. This formation asymmetrically overlies the Keban Metamorphic Formations. It starts with conglomerates that contain blocks and pebbles of the Keban Metamorphic Formations, and then

3. Analytical methods In Fig. 1, 10 pyrites and four calcites are separated from the core sample taken from 50 m deep of the drill SKY-1. Isotope analysis were made in pyrite samples d34S, and in calcite samples d13C and d18O. d18O isotope analysis was made taking five quartzes from only two of the quartz lodes (Q1 and Q2) in the examination area (Fig. 1). Five-milligrams pyrite was reacted with 100 mg V2O5 and SiO2 (1:1) at 950 °C in 7 min in a quartz tube to release SO2 (Ueda and Krouse, 1986). The analyses were carried out by the EA–IRMS (Elemental Analysis–Isotope Ratio Mass Spectrometry) in the Activation Laboratory, Canada. Sulphur-isotope composition is expressed by using the delta per mil notation with respect to Vienna Canyon Diablo Troilite (V-CDT):

d34 Sð‰Þ ¼ ½ð34 S=32 SÞsample =ð34 S=32 SÞV-CDTsd 1 1000: Carbon and oxygen analysis were carried out (according to Degans and Epstein, 1964; Mc Crea, 1950; Epstein et al., 1963) in Geochron Laboratories (USA). The calcite sample separates were converted to CO2 with a reaction containing 100% phosphoric acid at 50 °C. d13C and d18O of the CO2 were determined by SIRA, with a suitable correction applied to the oxygen ratio to adjust oxygen isotopic fractionation. Carbonate mineral (calcite) separates were crushed to 200 mesh ( metamorphic rocks > magmatic rocks in Fig. 6. d18O‰SMOW values show presence of metamorphic water area due to calc-schist in the study area (Fig. 6). According to Ohmoto and Rye (1979), d18O isotope compositions of metamorphic sedimentary and magmatic rocks change in range of +15‰ to +35‰. However, continental contamination may change d18O (increase) and d32S (decrease) values. In the skarn-type Pb–Zn mineralizations which are formed in intrusive contacts in Keban, d18O isotope values may increase depending on fault zones due to active tectonism, between wall rock and meteoric/metamorphic reaction and formation heat connected to all these. Besides, processes such as multiple boiling events (as evidenced by fluid inclusion data, Kalender, 2000) represent the early stage (463–510 °C and 23.64–30.68 wt.% equivalent NaCl in primary fluid inclusions) in quartz-chalcopyrite and pyrite bearing and late stage change between 302–352 °C and 20.69–21.75 wt.% equivalent NaCl in primary fluid inclusions) in quartzes from quartz veins at Keban, fluid inclusion values support contact metasomatic mineralization type (high temperature and medium salty). At medium temperature environments (from 352 °C down to 302 °C) related to Zn–Pb–Ag mineralization may be inferred not only from the oxygen isotope ratios of quartz, but also from the fluid inclusion data of later quartzes.

Fig. 6. d18O values as ‰ (permil); Average dD values taken from Shepherd (1976) and Goldfarb et al. (1991) (see above-mentioned reference for further information.).

5. Conclusions The Keban Pb–Zn skarn orebodies at the contact zone between siyeno-monzonitic intrusion and calc-schist country rocks are composed of galena, sphalerite, chalcopyrite, pyrite, arsenopyrite, enargite, lollingite, diopside, grossularite, epidote, scheelite, quartz and calcite. The sulphur-isotope composition has been analyzed in sulphide mineral (pyrite). The d34S values of pyrite samples change in range of 0.59‰ to 0.17‰. It shows that skarn was provided by hydrothermal fluids. The carbon and oxygen isotopes composition

Fig. 5. d18O values of important geological reservoirs (Hoefs, 1997).


of the major deposits and host geological units has been analyzed in calcites. d13C values change in range of 3‰ to 1.6‰ and d18O values range from 12.9‰ to 27.3‰. It is believed that d18O values could be increased because of their circulation in the shallow sea sediments in hydrothermal solution compounds and possibly this occurs because of their mixture with metamorphic originated waters. Decrease of d13C ratio in one sample to 3‰, supports this theory. According to micro-thermometric measurement results (93.7 and 134.2 °C and 12.5–15.65 wt.% equivalent NaCl) in liquid enclaves in calcite samples, it is effective in sea sediment enclaves’ salt concentrations, and meteoric waters decrease formation temperatures and form late calcites. In this study, d18O values in quartzes in the quartz lodes change range 10.5–19.9‰ and it is believed that solutions mix with metamorphic waters. This causes a decrease in the temperature and salt values (302–352 °C, 20.69–21.75 wt.% equivalent NaCl) of quartzes formed by hydrothermal solutions in the early stages (463–510 °C and 23.64 with 30.68 wt.% equivalent NaCl). Different isotope composition variations and fluid inclusions data of ore-forming fluids should be considered: (a) mixing between primary magmatic water and meteoric waters; (b) oxygen isotope exchange between syeno-monzonitic rocks and meta-sedimentary rocks leading to formation of shifted magmatic water; and (c) carbon isotope exchange reactions between meteoric water and meta-sedimentary rocks leading to formation of shifted meteoric water. According to magmatic processes, late-stage magmatic water derived from deep igneous activity have migrated upwards and interacted with cooling early crystallized surrounding syenomonzonit and calc-schist. When the proportion of meteoric water in the ore-forming fluid increased, late-stage mineralisations (such as W, and Au mineralization) may be indicated passing from Cu– Pb–Zn–Ag mineralizations in the Keban district.

Acknowledgement I thank Mr. Prof. Dr. Bernard Bonin for reading and his constructive advise to improve the manuscript. I would like to thank the editor for improved significantly the content and clarify of the paper.

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