Origin of Kaolin Deposits: Evidence From the Hisarcık (Emet-Kütahya) Deposits, Western Turkey

Authors: ALİ SAYIN

Abstract: Kaolin deposits, situated approximately 20 km south of Hisarcık, have been formed by the alteration of dacite and dacitic tuffs related to the Miocene volcanism associated with extensional tectonics. The Hisarcık kaolin deposits occur in the Kızılcukur, Ulaşlar and Kurtdere areas. Kaolinite is the only clay mineral associated with a-quartz, K-feldspar, plagioclase, alunite, natroalunite and hematite in some kaolins, whereas, other kaolinite accompanies smectite, which represents a moderate kaolinization. Low-cristobalite is the dominant silica mineral in these kaolins. In spite of strong kaolinization, the kaolins contain a high amount of finely-disseminated a-quartz in places, resulting in low Al_2O_3 values (13.80 wt% at the lowest). Variation in the thermal regime of the palaeohydrothermal system may affect the solubility of silica. In rainy seasons, due to a temperature drop, dissolved silica may be precipitated with the clays. The mineralogical zonations reveal that hydrothermal alteration is the main cause for the development of the kaolin deposits in the region. Hydrothermal silicification becomes more intense upwards. It results from dissolved silica moving in that direction, replacing and silicifying surrounding rocks, forming a silica zone (silica gossan) above the kaolin deposits. Basically, these silica gossans are the striking features on the exploration of the hydrothermal kaolin deposits. In places, the kaolin deposits also include thin silica veins and veinlets. Trace-element distribution data may not conclusively help to clarify the processes through which the kaolin formed. On the other hand, Pb and Sr enrichment within the deposits is supportive of the magmatic origin of hydrothermal solution. This is attributed to extensive Miocene volcanic activities in western Turkey. However, the data show that corrosive solutions, which may have arisen from the magma, have played a role in the kaolinization process together with hot meteoric waters. SEM studies show that there is a phase transition from montmorillonite to kaolinite.

Keywords: Alunite, Hisarcık, hydrothermal alteration, kaolinite, Kütahya, Natroalunite, Smectite, silica zone (silica gossan)

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