Genesis of sedimentary- and vein-type magnesite deposits at Kop Mountain, NE Turkey

Sedimentary- and vein-type magnesites were deposited within and on ultramafic rocks of the Kop Mountain region in Bayburt province. In the field, magnesites are exposed along NE-SW trending normal faults and in fractures in the ultramafic rocks. Petrographic studies reveal that magnesite is predominantly micrite, but also occurs as microsparite formed by recrystallization of micrite.

Turkish Journal of Earth Sciences Turkish J Earth Sci (2013) 22: 98-114 © TÜBİTAK doi:10.3906/yer-1101-22 http://journals.tubitak.gov.tr/earth/ Research Article Genesis of sedimentary- and vein-type magnesite deposits at Kop Mountain, NE Turkey 1, Selahattin KADİR *, Hasan KOLAYLI , Muhsin EREN Eskisehir Osmangazi University, Department of Geological Engineering, TR–26480, Eskisehir, Turkey Karadeniz Technical University, Department of Geological Engineering, TR–61080, Trabzon, Turkey Mersin University, Department of Geological Engineering, TR–33343, Mersin, Turkey Received: 24.01.2011 Accepted: 11.08.2011 Published Online: 04.01.2013 Printed: 25.01.2013 Abstract: Sedimentary- and vein-type magnesites were deposited within and on ultramafic rocks of the Kop Mountain region in Bayburt province In the field, magnesites are exposed along NE-SW trending normal faults and in fractures in the ultramafic rocks Petrographic studies reveal that magnesite is predominantly micrite, but also occurs as microsparite formed by recrystallization of micrite The ultramafic rocks hosting the magnesites consist of serpentinized olivine, hypersthene and diopside Ni, Co and Ti contents of magnesites suggest precipitation from percolating water through the serpentinized ultramafic rocks The sedimentary- and vein-type magnesites have different d18O and d13C values, characterizing formation under different conditions Temperature estimates using the average d18O values reveal precipitation from water at ~24.5°C for sedimentary magnesite and ~37.0°C for vein-type magnesite The d13C values of vein-type magnesites are distinctly more negative than those of sedimentary magnesites, indicating carbon isotopes derived from predominantly decarboxylation of organic sediments in shales and carbonate dissolution Less negative d13C values in the sedimentary magnesite are mainly due to outgassing of mineralizing water Our data suggest a petrogenetic model in which the surface water percolates through the ultramafic and sedimentary rocks becoming heated by volcanics at depth and enriched in Mg+2 and light carbon isotopes, followed by migration upward to form magnesite near the surface in ultramafic rocks as fracture-fill and as sediment at the surface Key Words: Magnesite, sediment, vein, ultramafic rocks, Kop Mountain, mineralogy, and geochemistry Introduction Magnesite (MgCO3) is a scarcer carbonate mineral than calcite and dolomite in the world and is mainly used in the manufacture of refractory materials It occurs in two major geological settings: as sediment in marine and lacustrine environments, or within or near ultramafic complexes (Abu-Jaber & Kimberley 1992) Both these types of magnesite occurrence are reported in the literature from Turkey and worldwide (e.g., Zedef et al 2000, Melezkih et al 2001 for marine and lake setting; Fallick et al 1991, Gartzos 1990, Bashir et al 2009, Yılmaz & Kuşcu 2007, Horkel et al 2009, Zedef et al 2000 for ultramafic setting) In the Kop Mountain region, magnesite widely occurs in ultramafic rocks forming part of the İzmir-AnkaraErzincan suture zone This study investigates genesis of magnesite deposits in the Kop Mountain region and provides information on their geological, mineralogical and geochemical characteristics because of their economic importance In the region, magnesite deposits, having a probable reserve of ~ 1,000,000 tons are mined for the manufacture of refractory materials * Correspondence: skadir_esogu@yahoo.com 98 Geological setting The eastern Pontides geographically corresponding to the eastern Black Sea Region of Turkey, is a part of the Alpine orogenic belt In the eastern Pontides, ultramafic rocks occur in the Kop Mountain range between Bayburt, Erzincan and Erzurum cities These ultramafic rocks are termed the Kop ophiolite complex The magnesite deposits in the Kop area are hosted within ultramafic rocks consisting of serpentinized harzburgite and dunite cut in places by pyroxenite dykes (Figures 1, 2) The Kop ophiolite is a part of the İzmir-Ankara-Erzincan suture zone which records the closure of the northern branch of Neotethys as a result of the convergence between the Eurasia and Gondwana plates (Şengör & Yılmaz 1981) It is widely accepted that the Neotethys began to close in the Late Cretaceous (Yalỗn & Bozkaya 2004), with subsequent Early Tertiary continental collision occurring along the suture zone following northward subduction of Tethyan oceanic lithosphere (Okay & Tüysüz 1999) The age of emplacement of ophiolites in the Pontides is assumed to be middle Eocene (Yılmaz et al 1997) KADİR et al / Turkish J Earth Sci Figure Geological map of the Kop Mountain area (a), sedimentary magnesite (b) and vein magnesite (c) in ultramafic units 99 KADİR et al / Turkish J Earth Sci Figure Simplified general stratigraphic column section of the study area (modified from Kolaylı 1996) 100 KADİR et al / Turkish J Earth Sci In the study area, the basement consists of Palaeozoic Pulur metamorphic units including gneiss, amphibolite and schist (Figure 2) These metamorphics are unconformably overlain by Liassic volcano-sedimentary rocks of the Hamurkesen Formation which comprises intercalations of sandstone, claystone, marl and basalt The Malm - Lower Cretaceous Hozbirikyayla Formation conformably ovelies the Hamurkesen Formation, and comprises predominantly limestone and sandstone The ultramafic rocks were thrust over the Mesozoic and Palaeozoic rocks during Upper Cretaceous to Middle Eocene time The ultramafic rocks are unconformably overlain by the Kaplankaya Formation (Upper Cretaceous), consisting of limestone interbedded with sandstone The Tertiary units are represented by conglomerate of the Sığırcık Formation (Eocene) and limestone, sandstone, claystone and marl of the Göller Formation (Miocene) Methods Thin-sections were prepared from the samples and then examined using an optical microscope (Nikon Pol 400) Mineralogical characteristics of the samples were determined by X-ray powder diffractometry (XRD) (Rigaku Geigerflex) and scanning-electron microscopy (SEM-EDX) (JEOL JSM 84A-EDX), differential thermal analysis-thermal gravimetry (PerkinElmer - Diamond TG/DTA thermal analyzer) and infrared spectrometry (FT-IR) (PerkinElmer 100 FT-IR spectrometer) Representative magnesite, dolomite and hydromagnesite-dominated bulk samples were prepared for SEM-EDX analysis by adhering the fresh, broken surface of each sample onto an aluminium sample holder with double-sided tape and coating with a thin (350 Å) gold coating using a Giko ion coater DTA-TG curves were obtained using a 10 mg sample of powdered claysized fraction (300°C, volcanogenic sources, deep-seated sources, or combinations of all of these In this study, the vein-type magnesites have d13C values predominantly ranging from –9.00 to –11.23‰ PDB with two exceptions These d13C values differ from KADİR et al / Turkish J Earth Sci Table Chemical compositions of magnesite and associated rocks Sedimentary Magnesite Magnesitic dolomite Claystone Silicate Oxide (%) SM-1 SM-4 SM-5 SM-6 SM-7 SM-10 SM-12 SM/D-9 SM/D-13 SM-3 SM-11 SiO2 Al2O3 ΣFe2O3 MgO CaO Na2O K2O TiO2 MnO LOI TOTAL   ppm Ba Co Cs Ga Hf Nb Rb Sn Sr Ta Th U V W Zr Y Pb Zn Ni Mo Cu As Sb La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu  2.06 0.16 0.54 44.63 0.91 0.08