Evolution of magmatic-hydrothermal systems below boiling conditions: evidence from the Çöpler Au-Cu deposit, east central Turkey

Abstract

The 7.5 Moz Çöpler Au-(Cu) deposit, located in the Tauride mountain range of east central Turkey, represents a sub-economic porphyry-style vein stockwork system that is overprinted by an intermediate-sulfidation state vein zone. The deposit is hosted by the middle Eocene Çöpler–Kabataş intrusive complex that is hosted by late Paleozoic-Mesozoic basement rocks. To unravel fluid evolution at the deposit, detailed paragenetic studies were conducted integrating optical petrography, fluid inclusion petrography, optical cathodoluminescence microscopy, and scanning electron microscopy. These investigations showed that repeated reopening and reactivation of veins occurred throughout the entire evolution of the magmatic-hydrothermal system. The earliest quartz generation present in the porphyry-style veins formed under lithostatic and super-lithostatic pressure conditions at temperatures above 400°C. The second generation of quartz present in the porphyry-style veins records a pressure transition from lithostatic to hydrostatic conditions at a minimum paleodepth of 1.4 km. Both quartz types are associated with potassic alteration. Coarse chalcopyrite and pyrite were the first ore minerals that formed at Çöpler and are associated with chlorite and sericite alteration, respectively. Coarse chalcopyrite, accounting for the bulk of the secondary copper resource, precipitated at ca. 400°C under hydrostatic pressure conditions. Fluid inclusions and mineral stabilities record a consistent cooling trend of lower-salinity liquids. Base metal Zn-Cu-Pb-Sb mineralization at Çöpler occurred between 300−250°C, and Au mineralization below 250°C. Backscattered electron imaging and trace element analysis of pyrite showed that Au enrichment at Çöpler can be related to the hydrothermal alteration of earlier formed pyrite and the formation of small Au-bearing euhedral pyrite crystals late in the paragenesis. No evidence for boiling was identified in the veins. It is proposed here that the Çöpler deposit formed from a single cooling magmatic-hydrothermal system. The deposit represents a rare example of the transition from an early porphyry system to a porphyry-related intermediate-sulfidation state hydrothermal system. The results of the study provide exploration vectors that could be used in the search for peripheral deposits around Çöpler, and generally for intermediate-sulfidation state hydrothermal vein deposits.