Browsing by Author "Schroeder, Paul A."

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Citation - WoS: 7
Citation - Scopus: 6
Characteristics and Evolution of the Etili Silica Sinter Epithermal Deposits, Canakkale - Turkey: Relation To Alkali Chloride Vs Acid-Sulfate Fluids
(Elsevier, 2022) Ercan, Hatice Ünal; Ece, O. Işık; Schroeder, Paul A.; Gülmez, Fatma
The Oligo-Miocene Etili epithermal deposits are a well-preserved fossil geothermal system in the Canakkale 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. delta O-18 and dD isotopic values of kaolinites ranged +9.4/+9.6 parts per thousand; to 84/74 parts per thousand respectively. delta O-18 isotopic values from siliceous and silicified samples ranged from 8.3 to 18.4 parts per thousand, 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 delta O-18 values of the silica and siliceous samples is in the range of 63 degrees to 140 degrees C. delta S-34 isotopic values of alunite ranged from -19.6 to +16.6 parts per thousand, 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 Ar-40/Ar-39 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.
Multidisciplinary Insights into Zeolite and Clay Formation in Hydrothermally Altered Volcanic Rocks of Cubuk, Turkey
(Cambridge University Press, 2025) Unal Ercan, Hatice; Schroeder, Paul A.; Gencoglu Korkmaz, Gulin; Ece, Omer Isik; Ciftci, Emin
The present study addressed the influence of lithological variability on hydrothermal alteration processes in the Cubuk region of Ankara, recognized globally for its agate occurrences. The objective was to clarify how differing host rocks, tuff and ignimbrite in Karadana, and perlitic units in Yukariemirler, affect secondary mineral formation and alteration pathways. A combination of mineralogical (X-ray diffraction, Fourier-transform infrared spectroscopy, field-emission scanning electron microscopy) and geochemical (energy-dispersive X-ray spectroscopy, whole-rock geochemistry, delta O-18-delta D) methods was utilized to characterize alteration assemblages and fluid conditions. The alteration sequence progressed through five stages, beginning with unaltered volcanic rocks. In the second stage, smectite-group clays, particularly montmorillonite, were formed under mildly alkaline and low-temperature conditions (pH 7.5-9.0; T 43-50 degrees C). This initial clay formation was followed by zeolitization along two distinct pathways: clinoptilolite crystallized (K-rich, Si/Al similar to 4.2) in the Karadana tuff-ignimbrite units, while heulandite formed (Ca-rich, Si/Al similar to 2.7) in the perlitic host rocks of Yukariemirler. These variations are attributed to differences in host-rock composition, permeability, and hydrothermal fluid chemistry. In more advanced stages, mordenite and chabazite precipitated under progressively higher pH and temperature conditions (pH 9.5-10.0; T 70-80 degrees C). In the final stage, opal-quartz formed due to silica supersaturation triggered by a drop in pH, despite rising temperatures (pH similar to 8.5-9.0; T similar to 250 degrees C). The paragenetic sequence confirms that clay mineral formation preceded zeolitization in both zones. Variations in zeolite types reflect strong lithological and hydrochemical controls, as well as the origin of the hydrothermal fluids; clinoptilolite formation in Karadana is associated with Na- and K-rich supergene fluids, while Ca-rich hypogene fluids promoted heulandite precipitation in Yukariemirler. This study presents the first detailed paragenetic model for zeolite-clay alteration in the Cubuk volcanic system and offers new insights into post-caldera hydrothermal evolution in Central Anatolia.