Geology, mineralogy, geochemistry and genesis of gevrekseydi (Kutahya) and its vicinity's clay deposits

Western Turkey host to many kaolin and halloysite deposits which were formed from hydrothermal alteration of Miocene volcanic rocks. The determination of clay minerals in these deposits is essential in terms of their physical-chemical properties and potential industrial uses. In this study; the kaolin deposits of Gevrekseydi village and its vicinity (Kutahya, Turkey) formed as a result of hydrothermal alteration of the similar volcanic rocks were studied in terms of their geology, mineralogy and geochemistry in order to determine the types of the clay minerals as well as their distribitions and formations. For mineralisation and characterisation studies, X-ray diffraction (XRD), scanning electron microscope (SEM), Fourier transform infrared spectroscopy (FTIR), differential thermal and thermogravimetric analysis (DT/TGA) and elemental analysis methods were used. The clay minerals detected in the XRD studies were kaolinite, halloysite and smectite, illite and vermiculite. As non-clay minerals; silica group (quartz, cristobalite, tridymite), iron group (hematite, magnetite, goethite), sulphate group (alunite, natro-alunite, minamide), silicate group (muscovite, biotite, pyroxene, sanidine, albite, chabazite), carbonate group (calcite, dolomite, magnesite, ankerite) minerals and titanium minerals (rutile, anatase) were determined. Scanning electron microscopy studies revealed that kaolinite crystals have hexagonal leaf morphology; halloysites have tubular shaped, stubby shaped and elongate shaped morphology; smectites have rose-like morphology. In SEM images, it was determined that clay minerals were formed as a result of the dissolution of volcanic glass and feldspar minerals in the host rock, halloysite was also formed by the transformation of kaolinite crystals. In DT/TGA analysis, the main endothermic reaction peak which shows the separation of crystal water of kaolinite and halloysite was detected between 530-590 oC and 3690,52 cm-1 and 3619,91 cm-1 bands of kaolinite were detected in FTIR analyses. According to isotopic δ18O and δD data, the formation temperature of kaolin is between 57-108 °C and the formation temperature of smectites is between 50 and 97 °C. Major oxide, trace element, and rare earth element analyses showed that elemental mobility occurred in clay deposits during the hydrothermal alteration process. The results of the chemical alteration index (CIA) showed that highly argillic alteration occurred at the region and Ce + Y + La versus Ba + Sr and δ18O versus δD diagrams revealed that the deposits in the region are hypogenous in origin. Mass change diagram showed that compared with host rock SiO2 and Al2O3 were enriched in both kaolinitic and smectitic zone, Fe2O3, CaO and Na2O were leached in the kaolinitic zone, MgO and K2O were enriched in the smectitic zone. The trace elements Ba+2 and Sr+2 were significantly enriched in the alunitic and kaolinitic zones compared to the host rock. In the smectitic zone, the transition metals enriched by Ni 93 fold, Zn 51 fold, Co 19 fold. In the spider diagram, it was found that rare earth elements decreased in the kaolinitic and smectitic zones compared to the parent rock, while the decrease in the kaolinitic zone was higher which alteration degree was also higher. A genetic model is proposed showing that the clay deposits in Gevrekseydi village and its vicinities are formed in an acidic, high sulfidation epithermal system. In this system, mineralisation is provided by hydrothermal fluids controlled by faults, and the main mineralisation in the beds is formed from top to bottom as opal silica cap, kaolinite + alunite + halloysite, quartz + kaolinite + alunite + smectite and alunite + kaolinite + halloysite + quartz.