The Koru deposit is a typical intermediate sulfidation base-metal (+/- Au) example of volcanic-volcaniclastic hosted mineralization in the Biga Peninsula and northwestern Turkey. Ore deposition was associated with the collisional and post-collisional tectonics related to the closure of the Tethys Ocean. Galena, baryte and quartz are main minerals, accompanied by minor amounts of sphalerite, pyrite, chalcopyrite, covellite and marcasite. The homogenization temperature of fluid inclusions indicates two distinct fluid pulses, one at a temperature commensurate with epithermal mineralization and boiling/near boiling conditions at c. 350 degrees C, with the second approximately 150-200 degrees C lower. Salinity in both instances was from 11.0 to 0.2 wt% NaCl. The dD and d18O values of water in equilibrium with early quartz and fluid inclusions plot close to the magmatic water box indicating the source of the high temperature fluid was magmatic. delta D and delta O-18 values from early and late baryte trend towards the meteoric water line (MWL), but this is not due to mixing with meteoric water, rather equilibration with alteration assemblages at decreasing temperature. LA-ICP-MS analyses of fluid inclusions reveal high Cu-Zn-Pb concentrations in the fluids, despite their low salinity, transported as chloride complexes. The range of temperatures within the early quartz and sphalerite mineralization can be explained by pressure variations during vein and fracture opening.