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SPECTROSCOPIC CHARACTERIZATION OF MANGANESE OCCURRENCES ALONG CRACK SURFACES OF THE RHYOLITIC IGNIMBRITE (INNER WEST ANATOLIA-TURKEY)

Tamer Koralay

Abstract

Manganese occurrences (eg. dendrite and/or surface coating) are highly usual products of many geologic environments. Morphology, spectroscopic and chemical properties of the manganese occurrences may vary depending on mode of environments. This study is concern with manganese occurrences along the minor cracks of Miocene ignimbrite unit, which has a wide range of exposure in the Afyonkarahisar region of the inner western part of Anatolia. The host rock has light gray, yellowish white in color and characterized by a matrix-supported structure. It is poorly sorted, with lithic and pumice clasts of varying size scattered in the ash matrix. The host rock has assemblage of quartz + sanidine + plagioclase + biotite and Fe-Ti oxide (magnetite, ilmenite) mineral composition showing hypohyaline porphryric and rarely trachytic, spherulitic textures under the microscope. The chemical composition of host rock is characterized by high SiO2 (wt. % 72.14-77.69), K2O (wt. % 4.48-6.10) with remarkably low MgO (wt. %0.02-0.31), Fe2O3 (wt. % 0.74-4.30), CaO (wt. % 0.84-1.17) and MnO (wt. % 0.03-0.08) contents. It has calc-alkaline in character and A/CNK ratios are between 1.29-1.50. Bright to pale black and blackish gray colored manganese formations have been found as dendritic and/or radial shaped surface coatings within the cracks in rhyolitic ignimbrite lithology. According to the intensity values, manganese samples have six chief Raman groups of bands centered at 618-638, 574-583, 505-512, 363-373, 269-292 and 181-182 cm-1. The most intense peaks are observed in the 574-583 and 181-182 cm-1 region. Manganese occurrences are essentially potassium-bearing manganese oxide (Cryptomelane - K(Mn4+,Mn2+)8O16) phases in EPMA results. These results are supported by CRS (Confocal Raman Spectroscopy) and XRD (X-Ray Diffractometer) studies. Manganese dendrites and coatings have limited major elements compositional ranges that vary from MnO (wt. % 62.75-67.57), SiO2 (wt. % 4.53-14.92), CaO (wt. %0.28-0.46) and Fe2O3 (as total iron) (wt. % 0.0004-0.284). The Continental Crust (CC) normalized elemental pattern of manganese samples reveal enrichment in Ba, U, Ta, Nb, Y and Pb elements and depletion in Ti and Zr elements. All spectroscopic and geochemical data indicate that relatively low temperatures vapor-phase solutions originated from the late stage of felsic magma may have played play an important role during the formation of manganese dendrites and coatings.

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DOI resolver: doi.org/10.5593/sgem2018/1.1/s01.061

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