AN IMPORTANT CHABASITE OCCURRENCE IN CENTRAL ANATOLIA AND ITS MINERALOGICAL FEATURES

Mineral Res. Expl. Bull., 135, 31-44, 2007 AN IMPORTANT CHABASITE OCCURRENCE IN CENTRAL ANATOLIA AND ITS MINERALOGICAL FEATURES M. Bahadýr ÞAHÝN* ABSTRACT.- The Lower - Middle Miocene Aktepe Formation cropping out to the south of Ankara (Central Anatolia) is comprised of volcano - sedimentary lacustrine deposits. In the formation mainly three zeolitic tuffaceous layers including dominant chabasite were determined. Based on the data obtained from XRD and SEM investigations, it was determined that the authigenic zeolite minerals are chabasite, erionite and clinoptilolite, and minor smectite in some samples. Where the clinoptilolite is dominant, at a location, it has been observed that Kfeldspar enters into the mineralogical composition. The main mineral assemblages in zeolitic tuffs are determined as "chabasite + erionite + clinoptilolite" and "chabasite + erionite + clinoptilolite + K-feldspar".The first data obtained points out that the Aktepe Formation has deposited in a saline - alkaline lacustrine environment. It can be said that, the zeolitic tuff occurrences in which chabasite is abundantly present, has economical potential. Key words: Chabasite, zeolite, Central Anatolia, Lower - Middle Miocene, volcano sedimentary rocks. INTRODUCTION In Central Anatolia, according to the studies carried out by Temel and Gündoðdu (1996) around Cappadocia, chabasite mineral is found in zeolitic tuffs which are known to be widespread in this region. The first chabasite occurrence that have economical potential, however, has been found in Lower - Middle Miocene volcanosedimentary - lacustrine deposits to the 80 km south of Ankara (Figure 1) in the scope of a project realized by MTA in Central Anatolia. The zeolite occurrences located in the study area corresponds to the "closed basin zeolites" proposed by Sheppard and Simandi (1999). In this region, during the previous geological investigations on the basis of the formations (Aktepe Formation), it has ben stated that the lacustrine deposits contained volcanic intercalations and various tuff layers (Uðuz et al., 1999). However, no information on the mineralogical compositions of the tuffs and zeolite occurrences was given in this study (Uðuz et al., 1999). During the studies explained in this paper, pre-sence of zeolitic occurrences and their mine- ralogical compositions have been revealed for the first time. The field work carried out on the Ak-tepe formation cropping out near Akvirançarsak Village, which is comprised of a volcanosedimentary lacustrine sequence including three main zeolitic tuff layers. The distance between the tuff layers, of which thickness is about 20 cm - 1.5 m, varies between 1.5 - 4 m. The dips of the beds vary (25°-90°) based on tectonic effects. The samples collected from these beds have been studied by using the XRD and SEM methods and the mineralogical and textural features of the zeolite minerals (chabasite, erionite, clinoptilolite) have been revealed. TECTONIC SETTING The study area is located in about 80 km south of Ankara on a subduction zone separating the deposits of active and passive continental margin, called Sakarya continent and Central Anatolian Metamorphic Massifs (Görür et al., 1984; Dellaloðlu et al., 2001; Figure 2). The basement rocks cropping out in Central Anatolia, in general, are defined as a micro-plate surrounded by branches of Neo-Tethys during Mesosoic - Paleocene period (Þengör et al., 1984). * Maden Tetkik ve Arama Genel Müdürlüðü, Maden Etüt ve Arama Dairesi, 06520 Ankara 32 M. Bahadýr ÞAHÝN Figure 1- Location and simplified geological map of the study area (after Uðuz et al., 1999). CHABASITE OCCURRENCES IN CENTRAL ANATOLIA 33 Figure 2- Simplified geological map of the Lake Tuz Basin and the sur rounding regions (Görür et al., 1984). As a result of the continental collision (Kýrþehir Block - Sakarya Continent) during Late Maastrictian - Late Paleocene, Haymana and Salt Lake basins have formed, on the other hand, the ophiolitic melange known as Ankara Melange has emplaced. This ophiolitic melange, which separates the Haymana and Salt Lake basins with a structural rise namely Samsam rise (Görür et al., 1984) on which the study area has been developed (Figure 2). lange, in certain regions have volcanic products as well. The volcanic series which are comprised of tuff and lavas in general, are laterally transitive to lacustrine deposits mainly dominated by carbonates and clastics. The ophiolitic melange of the Samsam Rise and the overlying volcanic and / or volcanoclastics are widespreadly observed, and at the basement, marine units of the Haymana Basin take place. GEOLOGICAL CHARACTERISTICS While the lacustrine deposits are comprised of Neogene clastics and evaporites overlying the Eocene marine deposits and the ophiolitic me- The formation in which zeolitic tuff occurrences are included in the study area is the Lo- 34 M. Bahadýr ÞAHÝN wer - Middle Miocene Aktepe formation (Uðuz et al., 1999). This is a widespread formation in Central Anatolia and typically crops out around Çankýrý where it is called Hançýlý Formation by Akyürek et al. (1984). Aktepe formation is stratigraphically located on the Ankara Melange, however, it is observed that Ankara Melange tectonically overlies Aktepe Formation in the study area. The Upper Miocene -Lower Pliocene terrestrial deposits of the Kömüþini Formation (Uðuz et al., 1999) unconformably overlies the Aktepe formation (Figure 1). Aktepe formation is a volcano - sedimentary sequence made up of siltstone, mudstone, bituminous shale, zeolitic siltstone and zeolites. These series corresponds to a lacustrine depositional environment that has developed during Lower - Middle Miocene and is observed in relation to the Karacadað Volcanics comprising basaltic - andesitic lava, tuffs and agglomerates (Uðuz et al., 1999). In the study area, mainly three different zeolitic tuff layers were differentiated: layer 1: average thickness= 40 cm, layer 2: average thickness= 65 cm and layer 3: average thickness= 80 cm (Figure 3). Chabasite bearing zeolitic tuff lenses of 10 - 20 cm thick in average accompanying these layers also exist in the area. METHODOLOGY For mineralogical studies, samples were collected from three tuff layers as well as the other lithological units. The samples selected are outcrop, trench and core samples. On these samples XRD and SEM analyses were carried out and the results were compared. The mineralogical compositions of the samples were first determined at the XRD Laboratory (Philips, PW 1830: Cu-K , 30 k V, 40mA, Figure 3- Zeolitic tuff layers bearing chabasite (from older to younger: Layer 3, Layer 2 and Layer 1). 2 0 :2.5°-70°, 6°/dk.) of the Mineralogy - Petrography Laboratory of MTA, later on, the same samples were investigated at the SEM Laboratory of the Hacettepe University (Zeiss, EVO 50-EP: scanning conditions are shown on the photographs). During the SEM investigations especially the textural features o (...truncated)