Characteristics and evolution of the Etili silica sinter epithermal deposits, Çanakkale – Turkey: Relation to alkali chloride vs acid-sulfate fluids

Abstract

The Oligo-Miocene Etili epithermal deposits are a well-preserved fossil geothermal system in the Çanakkale Region and is one of the largest fossil siliceous hot spring deposits of Turkey. Main E-W and NE-SW trending faults systems occur with minor NW-SE fracture systems perpendicular to the main faults. Silica-rich deposits are commonly observed in or on the calc-alkaline tuffs, ashes and pyroclastic rocks that were the products of the magmatism controlled by the extensional tectonic regime.The Etili epithermal system was examined at two representative locations including the Hamamtepe and Muratlar sites. Morphologies of the proximal apron were defined by lithofacies that included silica infiltrate, spring conduits, nodular and finely laminated geyserite, sinter clast breccia, silicified volcanic rocks, and epithermal veins. Microfossils were detected in mat structures developed in the proximal apron. The silica deposits have low abundances of altered mineral assemblages (e.g., kaolinite, halloysite, and alunite), which are otherwise commonly observed in the region. The origin of the Etili Fossil Silica Sinter Region (EFSSR) was constrained by using geochemical and isotopic data. δ18O and δD isotopic values of kaolinites ranged +9.4/+9.6 ‰; to 84/−74 ‰ respectively. δ18O isotopic values from siliceous and silicified samples ranged from 8.3 to 18.4 ‰, which shows two different formation types for the Etili Fossil Silica Sinter Region. We hypothesized hypogene origins for the kaolins and a mixed hypo/supergene origin for siliceous and silicified samples. The calculated model formation temperature from the δ18O values of the silica and siliceous samples is in the range of 63° to 140 °C. δ34S isotopic values of alunite ranged from −19.6 to +16.6 ‰, which also indicated two different sources that include magmatic hydrothermal and meteoric waters affected by bacterial activity. The timing of acid-sulphate alteration can be grouped into three periods by 40Ar/39Ar dating of alunites. These ages are: (A) 32.4 ± 1.2 to 22.6 ± 0.22 Ma in the Muratlar silica site, (B) 12.3 ± 0.3 to 15.2 ± 0.3 Ma in the northern part of the Hamamtepe site and (C) 5 ± 0.18 to 7 ± 0.3 Ma in the southern part of the Hamamtepe silica site. These consistent findings show that the emplacement periods of the Evciler pluton and the ages of alteration deposits are similar to each other. Silica sinters and geyser mounds represent very late stage near-neutral pH alkali chloride geothermal activities. The epithermal deposits in the EFSSR comprises a complex history of three different hydrothermal processes, each of which have different compositional and temporal emplacement periods related to the Evciler pluton.