Granitlerde mineraloji kaynaklı jeolojik sorunların araştırılması : Aksaray (İç Anadolu Bölgesi-Türkiye)

Abstract

Granit, dış etkenlere karşı dayanıklılığı, duraylılığı, estetik görünümü ile zengin renk ve doku çeşitliliğine sahip olmasından dolayı, plaka, fayans ve blok şeklinde iç ve dış mekânlarda kaplama, döşeme ve parke-bordür taşı olarak kullanımı en çok tercih edilen sert doğal taş türüdür. Türkiye'de üretilen blok taş granitler, genellikle tektonik ve petrolojik süreçlerin sonuçlarıyla ilgili olarak farklı mineralojik bileşimlerde oluşmuştur. Bunlar çoğunlukla, Kretase ve Paleojen (Oligosen) dönemindeki magmatik faaliyetlerin ürünleridir. Türkiye'de üretilen blok taş granit rezervleri başlıca; batı, orta ve kuzeydoğu Anadolu bölgelerinde yer almaktadır. Bu granit rezervleri, başta, magmatizma, tektonizma, ayrışma ve alterasyon etkisiyle meydana gelen paslanma, süreksizlikler (faylar, çatlaklar), anklav oluşukları (MMA, ksenolit, şiliren, otolit vd.), aplit damarları-daykları, pegmatit cepleri (yamaları), kseno-kristaller, miyarolitik boşluklar, mineral segregasyonları (ayrımlanmaları) ile renk ve dokusal değişiklikleri içeren jeolojik sorunlardan zarar görmektedirler. Bu jeolojik sorunların, boyutlandırılmış taş olarak kullanılan granitlerin kalitesi üzerinde beklenmedik olumsuz etkileri bulunmaktadır. Paslanma, ticari granitlerde karşılaşılan en yaygın sorunlardan biridir. Boyutlandırılmış taş yüzeylerindeki paslanmanın esas tipi, taşın bileşimde bulunan ve demir içeren mineral fazları olarak tanımlanabilmektedir. Bu mineral fazları başlıca; demir sülfürler (pirit, markazit ve pirotit gibi), demir karbonatlar (siderit gibi) ve demir-magnezyum içeren silikatlar (biyotit, hornblend ve garnet gibi)'dan oluşmaktadır. Granitlerdeki paslanma, bu minerallerin bileşiminde bulunan ferrüs demirin (Fe+2) ayrışma sonucunda serbest kalarak atmosferik koşullarda oksitlenmesi ve granit plaka ve fayans yüzeylerinde sarı-kahverengi renkli demir oksi-hidroksitler (limonit ve götit gibi) şeklinde çökelmesi ile oluşmaktadır. Jeolojik olarak, çalışma alanı; Orta Anadolu Kristalen Karmaşığı (Kırşehir Masifi) içinde tanımlanan Ağaçören İntrüzif Takımının alt birliği olan Ekecikdağ Plütonu içinde yer almaktadır. KB-GD yönelimindeki plütonik kayalardan oluşan bu birlik, litolojik olarak gabrodan başlayarak granite kadar değişim göstermektedir. Çalışma alanındaki bu kaya birimleri bu çalışmada; "Ozancık Monzograniti" ve "Keleşdağ Gabro-diyoriti" olarak iki ana gruba ayrılmış ve adlandırılmıştır. Jeokimyasal analizlere göre; Ozancık Monzograniti, kabuk ve manto bileşimindeki magmaların homojen ve heterojen karışım süreçleri sonrasında hibrit bir magmadan türeyen yüksek potasyumlu (K2O: > % 4.00), kalk-alkalen (KCG) ve aşırı fraksiyonlaşmış (SiO2: > % 75) hafif peraluminus (A/CNK: 1.04-1.05 > 1) karakterli "I-tipi" granit olarak tanımlanmıştır. Tektonik oluşum ortamı açısından, iz element (örneğin, Y, Nb, ve Rb) verilerine göre bu granit, "çarpışma sonrası granitleri (COLG)" olarak sınıflandırılmıştır. Tez çalışması kapsamında araştırılan granit blok taş ocağı; Ozancık Monzograniti içinde ve Keleşdağ Gabro-diyorit plütonik biriminin kontağında yer almaktadır. Ocakta, üç farklı renkte (pembe, açık pembe ve gri) granit blok taş olarak üretilmektedir. Özellikle gri renkli ve bazı açık pembe renkli granit türleri üretimi takiben paslanma sorunundan etkilenmekte olup, taş yüzeyinde hızlı şekilde gelişen pas lekeleri oluşmaktadır. Bu çalışmada, esas olarak, granitlerdeki mineraloji kaynaklı "paslanma" sorununun ana nedenlerinin ve çalışma bölgesinde işletilen üç farklı ticari granitin "radyoaktivite" potansiyellerinin ve oluşturabilecekleri radyolojik risklerin araştırılması üzerine odaklanılmıştır. Paslanma sorununun ve doğal radyoaktivite potansiyellerinin araştırıldığı granitlerde, paslanma sorununun belirlenmesi amacıyla ileri mineralojik, jeokimyasal ve petrojenetik özelliklerin karakterize edilebilmesi için tüm kayaç üzerinde; polarizan ve cevher mikroskobisi çalışmaları, XRD (X-ışını Difraktometresi), XRF (X-ışını Floresans) ve ICP-MS (İndüktif olarak eşleştirilmiş plazma – Kütle Spektrometresi), biyotitler ve piritler üzerinde EPMA (Elektron Mikroprop), piritlerde kükürt izotopu (SI), ocak yerindeki fay zonlarından alınan su örneklerinde İyon Kromatografisi (IC) ve biyotitler, piritler ile pas fazlarının özelliklerinin belirlenmesi amacıyla Raman Spektroskopisi (RS) analizleri ve doğal radyoaktivite potansiyellerini belirlemek için ise Gamma Spektroskopisi (GS) analizleri gibi bazı analitik teknikler kullanılmıştır. Ayrıca, hızlandırılmış paslanma sürecinin ilgili granit örnekleri üzerinde paslanma riskini belirlemek ve gözlemlemek amacıyla da termal şok ve alkali çözelti yöntemleri kullanılarak laboratuvar deneyleri yapılmıştır. Sonuç olarak, araştırma kapsamında yapılan bu analizler ve laboratuvar deneylerinden elde edilen bulgulara göre, incelenen granitlerdeki paslanma sorununun ana nedenlerinin; magma karışım (magma mixing/mingling) süreçlerinde oluşan "hidrojenetik biyotit" (yüksek Fe+2 içeren lepidomelan bileşimli) ve hidrotermal evrede biyotitlerin alterasyon süreçlerinde oluşan "hidrotermal pirit" (önemli miktarda arsenik içeren) minerallerinin atmosferik koşullar altında duraylı olmamasından kaynaklandığı tespit edilmiştir. Bu mineraller hızlı bir şekilde oksitlenerek taş yüzeylerinde pas lekeleri oluşturabilmektedirler. Bu bağlamda, taş yüzeyinde oluşan bu pas lekelerinin temizlenmesi (örneğin oksalik asit çözeltisi ve diyatomit toprağı kullanarak) ve önlenmesi için (örneğin su itici silan/siloksan bileşimli) kimyasallar kullanarak bazı çözüm önerilerinde bulunulmuştur. Ayrıca, çalışma bölgesinde işletilen üç farklı ticari granitin (Crema Lal, Rosalin/Aksaray Pink ve Aksaray Yaylak) "radyoaktivite" potansiyelleri belirlenerek, yapılarda özellikle iç mekanda kullanılmalarının insan sağlığı açısından radyolojik risk oluşturup oluşturmadığı değerlendirilmiştir. Bu bağlamda, elde edilen sonuçlara göre "Rosalin/ Aksaray Pink ve "Crema Lal" ticari isimli granitlerin yapıların iç mekanlarında kullanılmaması önerilmiştir.

Granite, due to its resistance to external factors, its durability, being rich in color and texture with aesthetic appearance, is the most preferred hard natural stone for internal and external uses as cladding, flooring and paving in plate, tile and border forms. The dimension stone granites of Turkey generally occur in different mineralogical compositions resulted from the consequences related to the tectonic and petrological processes. These are mostly products of Cretaceous and Paleogene (Oligocene) magmatic activities. The important granite reserves for dimension stone production are mainly located in western, central and northeastern regions of Turkey. These granite reserves suffered from tectonism, weathering and alteration leading mainly to staining and they also comprise discontinuities (faults, joints), enclaves (MME, xenoliths, schlieren, autolith etc.), aplite veins-dykes, pegmatite patches, xeno-crystals, miarolitic cavities, mineral segregations, chromatic and textural varieties which are altogether known as the geological flaws. These geological flaws have unexpected effects on the quality of granites used as dimensional stone utilizations. Staining is one of the most common problems encountered in the commercial granites. This type of discoloration certainly does not only affect the aesthetic appearance of the stone, it can also result in physical damages through volume expansion. The major type of staining can be identified on dimension stone surfaces generated by the oxidation of iron-bearing mineral phases present in the stones. These minerals are mainly iron sulfides (e.g., pyrite, marcasite, and pyrrhotite), iron carbonates (e.g., siderite), and ferro-magnesian silicates (e.g., biotite, hornblende, and garnet). They most probably occur through hydrothermal alteration and magma mixing processes as dispersed and/or as disseminated and also as concentrated fillings along veins and joints in the granites. Staining forms when ferrous iron (Fe2+) is released by weathering and exposed to the atmospheric conditions on the surfaces of the granite slabs and tiles where it is re-precipitated as yellow-brown colored iron oxy-hydroxides (e.g., limonite and goethite) known as rust. In this study, a granite dimension stone quarry, which is located in Aksaray (Ozancık) province in the Central Anatolia region (Turkey), has been investigated where the production suffered from geological flaws especially in the form of extensive staining problem due to biotite and pyrite oxidation. Geographically, the study area and the dimension stone granite quarry are located in the east of Tuzgölü (Salt Lake), the north of Aksaray province and the south of Ortaköy district near the Ozancık town and its environment. Geologically, the study area is located in the Ekecikdağ Pluton, which is a sub-suite of the Ağaçören Intrusive Suite in the Central Anatolian Crystalline Complex (Kırşehir Massif), which is consisted of plutonic rock suites striking NW-SE and lithologically ranging from gabbros to granites. The rock units exposed in the study area are divided into two major units, The Ozancık Monzogranite and Keleşdağ Gabbro-diorite. These plutonic rocks formed by the interaction between mafic and felsic magmas in a dynamic, silicic magma chamber replenished by magmatic influx from the mantle. These two plutons derived from coeval (Upper Cretaceous-Paleogene aged) magmas, therefore, gabbros are not roof-pendants but intrusive bodies continuing at depth. Contact between the two plutonic units is the gradual transitional in places, sharp and sinusoidal as observed during the field studies. The Ozancık Monzogranite is pink and gray in color, fine-to-medium grained and contains mafic microgranular enclaves (MME), which are diorite and quartz diorite in composition and formed as a result of magma-mixing and mingling processes. These enclaves represent the products of early stages of interaction between mafic and felsic magmas when the former has a low mass fraction, whereas the Keleşdağ Gabbro-diorite represents a relatively late stage intrusion, during the cooling and crystallization of the Ozancık monzogranite resulting from a large volume of mafic magma injection into the felsic magma chamber. Low relief flattened areas are mostly covered by granitic soil (saprolite), which consists of disintegrated quartz, clayey feldspar, and biotite grains derived as a result of weathering of the Ozancık monzogranite and they were mapped as "weathered granite". The Keleşdağ Gabbro-diorite is very hard, resistant to weathering and crops out as irregular, small-sized and sharp-edged blocks at the summits of the highest hills (e.g., Keleşdağ) and in their vicinities in the study area. There is a gradual transition from gabbro to dioritic composition towards monzogranite contact in the field. The Keleşdağ Gabbro-diorite is dark green to black in color and medium-to-coarse grained and also cut by felsic and mafic dykes. The Upper Miocene-Pliocene Peçenek Formation, which is lithologically consisted of conglomerate and sandstone, unconformably overlies the plutonic rocks. This formation is conformably overlain by the Upper Miocene-Pliocene Kızılkaya Ignimbrite. Minerallogically, the Ozancık Monzogranite has holocrystalline granular texture and is mainly consisted of quartz, perthitic alkali feldspar (orthoclase), plagioclase (albite and oligoclase), and biotite. Zircon, apatite, magnetite, pyrite and chalcopyrite are also present as the accessory minerals. Chloritization is particullary observed in association with biotites, serisitization with plagioclases and kaolinization with K-feldspars as the common alterations. Chlorite, sericite, kaolinite, and muscovite account for the secondary minerals. This rock is lithologically defined as "monzogranite" using the QAP triangle diagram. The Keleşdağ Gabbro-diorite has holocrystalline granular texture and is mainly made up of two different lithologies, gabbro and diorite. Gabbro is relatively coarse and fine grained and has sub-ophitic texture consisting mainly of plagioclase (labradorite), clinopyroxene (augite and titano-augite) and amphibole (hornblende and fibrous tremolite-actinolite). Apatite, zircon, magnetite and ilmenite are present as the accessory minerals. Chloritization and epidotization observed in hornblendes, uralitization in pyroxenes, and saussuritization in plagioclases as the common alterations. Chlorite, epidote and calcite are present as the secondary minerals. This rock is lithologically defined as "gabbro" and "pyroxene-hornblende gabbro" using the "Plag-Px-Hbl" triangle diagram. Diorite is coarse-grained and has poikilitic texture that consists mainly of plagioclase (andesine), amphibole (hornblende) and quartz varying between 2-10 vol %. Apatite, zircon, magnetite and ilmenite are present as the accessory minerals. Chloritization in hornblendes, and saussuritization in plagioclases are the common alterations. Chlorite and epidote are present as the secondary mineral. This rock is lithologically defined as "diorite" and "quartz diorite" using the QAP triangle diagram. According to the geochemical analyses; the Ozancık Monzogranite is defined as high-K (K2O: > 4.00 %) calc-alkaline (KCG) and highly fractionated (SiO2: > 75 %) I-type, slightly peraluminous (A/CNK: 1.04-1.05 > 1) granite derived from a hybrid magma containing both crustal and mantle components through magma mixing and mingling processes. This rock contains high amounts of Y, Ta, and Nb and radioactive elements such as uranium (U) and thorium (Th). Tectonically, this granite can be classified as post-collosional granite (COLG) according to the trace elements data (e.g., Y, Nb, and Rb). The investigated dimension stone granite quarry in this study is located in the Ozancık Monzogranite plutonic unit at the contact of the Keleşdağ Gabbro-diorite plutonic unit. The quarry has been active since 2009. There are three different colored granites (pink, light pink, and gray) that have been exploited as dimension stone. Especially, gray color and some of light pink color types have been suffering from staining problems in the form of rust spots that rapidly developing on the stone surfaces following the production. This study is mainly focused on the main causes of staining problem and natural radioactivity of the investigated regional granites. In order to characterize staining problem, a number of analytical techniques were employed including the polarizing optical and ore microscopy studies, XRD (X-Ray Diffraction), XRF (X-Ray Flourescence) and ICP-MS (Inductively Coupled Plasma – Mass Spectrometer) on whole rock, EPMA (Electron Microprobe) of biotites and pyrites, SI (Sulfur Isotopes) of pyrites, IC (Ion Chromatography) of water samples taken from the fault zones in the quarry site and RS (Raman Spectroscopy) of biotites, pyrites and rust analysis methods for further mineralogical, geochemical and petrogenetic speciations. Thermal shock and alkali solution laboratory tests were also carried out for accelerating the staining process on the subject granite samples to determine and observe staining risk of them. Also, GS (Gamma Spectroscopy) analysis method is applied in order to determine the natural radioactivity of subject granite samples exploited in the study region. Consequently, based on the results of these analyses and laboratory tests, the main causes of the staining of investigated granites are defined as "hydrogenetic biotite" (lepidomelane in composition with high ferrous iron content) formation through magma mixing/mingling processes and "hydrothermal pyrite" (with significant arsenic content) formation in hydrothermal stage and biotite alteration processes where the both minerals are not stable under atmospheric conditions. They can rapidly stain and produce rust spots on the stone surfaces. Finally, several suggestions were proposed for both removing method of rust spots (e.g., use of oxalic acid solution) and preventive applications (e.g., use of water repellent silanes/siloxanes-based chemicals). Also, the natural radioactivity levels (i.e., activity concentrations of radionuclides; 226Ra, 232Th, and 40K) of three subject granite samples were determined by measuring high resolution gamma-ray spectrometry. Using these activities, radiological hazard dose rates (absorbed and annual effective) and gamma-activity indice were calculated by standard equations accepted by the public health, taking relevant international reports and guidelines into account. One of the granites (Rosalin/ Aksaray Pink) exploited in the Aksaray-Ortaköy-Kalebalta district was found to be high in the radium-equivalent activities and annual effective doses due to radiogenic minerals bearing the radionuclides including zircon, apatite, xenotime, K-feldspars, and biotite. Based on the available data, it is suggested that "Rosalin/Aksaray Pink" and "Crema Lal" granites should not be used particularly as counter and vanity tops in kitchen and bathroom also as cladding and flooring in the indoor applications.