Comparison of Epithermal Kaolin Deposits from the Etili Area (Çanakkale, Turkey): Mineralogical, Geochemical, and Isotopic Characteristics

Abstract

Hydrothermal solutions related to magmatic intrusions that occurred during the Oligo-Miocene resulted in advanced multi-stage alterations and, to varying degrees, kaolinization over a wide area of the Biga Peninsula. The most important formations in these kaolin deposits occurred along the NE–SW-trending Çan-Etili-Bayramiç fault zone. The Bahadırlı, Duman, and Çaltıkara quarries are well preserved kaolin deposits throughout these fault zones located within the Çan Volcanics. Mineralogical, geochemical, and isotopic analyses were performed to identify the environments of formation of kaolins and the origin of the hydrothermal solution that led to the formation of these deposits.The mineralogical assemblages of each of the quarries differ from each other. The Bahadırlı kaolin quarry comprises kaolinite + alunite ± quartz ± smectite ± plagioclase ± K-feldspar. The Çaltıkara kaolin deposits consist of kaolinite + quartz + alunite ± iron-oxide and ore minerals. The Duman kaolin quarry was considered in two different slopes as a hanging wall block and a footwall block, and the blocks contain kaolinite + quartz + plagioclase ± smectite and kaolinite ± alunite ± smectite ± quartz ± plagioclase ± K-Feldspar ± gypsum, respectively. During petrographic investigations, it was observed that kaolinization occurred generally in K-feldspar and plagioclase phenocrysts and partially in the matrix. Mineralogical and micromorphological investigations revealed that the kaolin group includes the dioctahedral minerals kaolinite and halloysite. According to the trace element contents of the kaolinites, the Çaltıkara and Bahadırlı deposits had a hypogene origin, while the Duman deposit had a supergene origin. δ18O isotopic values of kaolinites ranged from +10.3 to +18.3‰. δ34S isotopic values ranged from –17.2 to +20.2‰ of alunite, galena, and pyrite minerals, which indicate different formation environments for the kaolin quarries. 40Ar/39Ar dating of alunite revealed that the timing of acid-sulfate alteration was compatible with the magmatic intrusions. All these data revealed the mineralogical, chemical, and isotopic differences caused by the effect of different hydrothermal processes in three kaolin deposits located on the same fault zone, close to each other and similar in age.

Global Occurrence, Geology and Characteristics of Hydrothermal-Origin Kaolin Deposits

“…Chemical silica deposits which are observed in the form of beds or mounds in the field are also quite resistant to erosion. In this way, silica caps protect underlying kaolin deposits against erosion and they can have topographical highs and small hills [126,157,[166][167][168]. These sinter deposits develop near the surface and above the water table in subaerial volcanic terrains where the heat input of volcanic activity and subterranean magmas is sufficient to cause hydrothermal circulation of groundwater from deep reservoirs with minor magmatic input [126,169,170].…”

“…In this process, firstly, blankets of opal begin to invert to chalcedony over time below the vadose zone [76,169], and finally, in time, chalcedony inverts to quartz [109]. A laterally developed silicification zone has been observed around the epithermal veins in the bedrock, which extend radially over large distances (hundreds of meters) [9] due to the widespread presence of high-porosity and -permeability bedrock (i.e., tuffs, andesites and volcano-clastics), and these porous rocks are easily silicified in their shallow levels [168]. Silicified rocks occur in the form of mounds in a very wide area due to their resistance to erosion [87].…”