Epigenetic Cu-Zn Mineralization in the Yığılca Formation Tuffs of Kuzuluk (Sakarya), Western Pontides, Turkey: Insights from Sulfur Isotope Analysis and Hydrothermal Processes

The Cu-Zn mineralization in the Kuzuluk (Sakarya) region has hosted within the tuffaceous rocks of the Eocene Yığılca formation along the fault zones in the Western Pontides. The study area is made up of the Permian-Triassic Sultaniye Metamorphites, Upper Cretaceous Abant formation, Early-Middle Eocene Çaycuma and Yığılca formations, and Pliocene Örencik formation. It is characterized by significant tectonic activity represented by dip-slip fault zones, particularly within the tuffaceous rocks of the Yığılca formation. The ore zone occurred within the fracture zone in the tuffaceous rocks of the Yığılca formation represented by an epigenetic mineralized vein-type structure including pyrite, chalcopyrite, sphalerite, bornite, quartz, and calcite. The geochemical studies indicated that this ore zone contains approximately 2620 ppm Cu and 1440 ppm Zn concentrated within the fracture zone. Carbonatization is the main hydrothermal alteration in the study area. To assess the sulfur origin in the mineralization, ten sulfide samples from pyrite and chalcopyrite minerals were analyzed for sulfur isotopes. Their δ³⁴S data vary from +28.56 to +29.52 ‰, which shows that the enrichment is due to the vigorous interaction between hydrothermal fluids and sedimentary sulfate reserves. Additionally, this reflects the impact of hydrothermal fluid and organic matter dissolution in the area, in contrast to magmatic sulfur sources. Therefore, circulation of the hydrothermal fluids along fault zones played a crucial role in the formation of the ore zone, facilitating the precipitation of Cu-Zn minerals and gangue minerals (quartz and calcite). These findings suggest that the geological processes that lead to the formation of the Cu-Zn Kuzuluk mineralization contribute to clarifying hydrothermal mineralization within Western Pontides fault zones.