Low-sulfidation type Au–Ag mineralization at Bergama, Izmir, Turkey

Bergama, the center of Bergama County, is located in western Turkey and includes the villages of Ovacik, Narlica and Saganci. The Ovacik epithermal gold–silver deposit is located in the Western Anatolian Volcanic and Extensional Province, adjacent to the ENE-trending Bergama graben, some 100 km north of the city of Izmir. Gold of economic grades at the Ovacik deposit (reserves 4.19 Mt at 7.6 g/t) occurs in epithermal quartz veins which display low-temperature epithermal textures, including crustiform banding, quartz pseudomorphs after bladed calcite, and multiphase hydrothermal breccias. Alteration minerals at both Ovacik and Narlica are dominated by smectite, mixed-layer illite/smectite, chalcedonic quartz and adularia, whereas major kaolinite and minor mixed-layer smectite/illite (> 13.4 Å) occur at Saganci. The total sulfide content at Ovacik is low (< 2%) and is dominated by pyrite with traces of electrum, chalcopyrite, arsenopyrite, acanthite, tetrahedrite, pyrargyrite, stibnite, galena, chalcocite, bornite, covellite and sphalerite, occurring mainly within breccia clasts. Pyrite and marcasite appear to be the most common opaque minerals at Narlica and form dark sulfide-rich bands along with traces of electrum, native silver and chalcopyrite; pyrite is the only sulfide identified at Saganci. 40Ar/39Ar dating of adularia from gold-bearing quartz veins indicates an age of mineralization of about 18.2 ± 0.2 Ma. Fluid inclusion studies at Ovacik reveal that main-stage quartz contains predominantly liquid-rich inclusions with homogenization temperatures (Th) ranging from 150 to 305 °C, with the majority of Th varying between 165 to 205 °C: ice-melting temperatures (Tm) ranging from − 0.4 to − 1.2 °C (salinity < 2 wt.% NaCl equiv.) are dominant. Higher Th (220 to 248 °C) at the Narlica deposit may be attributed to the deeper level of exposure.Geochemical variations in altered wall rocks at Ovacik and Narlica are generally characterized by two-fold enrichments in K, Rb, Cs and 25 to 93% depletions in Sr, Ca, Mg, Na and even higher depletions (96 to 99% decrease) in the quartz–adularia vein zone. Lanthanum, Ce, Pr, Hf, Zr, Sm, Eu, Gd, Tb and Ho also exhibit up to 50% depletions in the wall rock and even more so (up to 90% depletion for La and Nd) in the vein structure. Wall rock enrichment in Au, Ag, As, Hg and Sc is by factors of 60, 150, 88, 8 and 3, respectively. The ranges of REE in both the altered volcanic rocks and quartz–adularia veins are wide and may reflect either significant mobilization of REE during alteration and mineralization or their dilution by the metasomatic processes from fresh volcanic rocks through montmorillonite–illite–adularia-altered wallrock to quartz–adularia veins. High Rb/Sr ratios in adularia–illite-altered areas are closely related to the presence of K-rich alteration minerals. Low Rb/Sr ratios and corresponding low K values at Saganci are due to acid leaching of volcanic rocks. Positive correlation coefficients between Au and Ag, Pb, Zn, Cd, Cu and Sb in epithermal quartz veins, all of which are > 0.51, are significant. Silver and Sb are remarkably enriched at higher levels of the deposit. No correlation occurs between Au and As or Ag and As.The Ovacik quartz in Au–Ag-bearing veins has δ18O values ranging from + 9.5 to + 15.7‰, whereas δD values of fluid inclusions in quartz range from − 89 to − 125‰. However, the Narlica quartz in Au–Ag bearing veins returned δ18O values ranging from + 5.9‰ to + 8.3, whereas the δD values of fluid inclusion in quartz range from − 82 to − 99‰. δ18O results indicate that ore-forming hydrothermal fluids at Ovacik and Narlica had δ18OH2O values ranging from − 2.9 to + 3.5‰ (average: − 0.24‰) and from − 2.96 to − 0.6 (average − 1.6), respectively. They are thus 18O-enriched in comparison with present-day meteoric and hydrothermal meteoric waters (− 5.4‰). δ18O, δ18OH2O and δD values suggest that mineralizing solutions were a mixture of meteoric and magmatic waters. The δ34SΣsulfide data at Ovacik and Narlica range from − 2.1 to 5.3‰ (average: + 1.7) and from − 4.6 to + 2.7‰ (average: − 0.36), respectively. These δ34SΣsulfide values are consistent with a magmatic source for S.