Evolution of mineralizing fluid in the porphyry copper system of the Skouries Deposit, Northeast Chalkidiki (Greece); evidence from combined Pb-Sr and stable isotope data

The porphyritic syenite of the Skouries deposit intruded the metamorphic basement of the Serbomacedonian Massif in the northeast Chalkidiki peninsula at 19 Ma. The final eraplacement was controlled by dominantly northeast-southwest-trending steep faults that created pathways for the deep-seated magma to rise to shallow crustal levels. Following its eraplacement (stage I), the porphyritic syenite suffered a pervasive potassic alteration (stage II, approximately 590 degrees -640 degrees C). Two later veining stages with propylitic characteristics (stage IIIA, approximately 480 degrees C; stage IIIB, approximately 380 degrees C) brought in most of the Cu, which was deposited in the form of chalcopyrite. Quartz sulfide-(calcite) veins transect the porphyritic syenite as well as the country rocks in a narrow (<50 m wide) halo around the stock only. The range of delta ¹⁸ O values from 3.5 to 7.6 per rail for the fluids in equilibrium with vein quartz is within the typical range of early ore-forming fluids associated with porphyries, representing magmatic fluids that may, to some extent, have interacted with the igneous wall rock. A subsequent late propylitic alteration (stage IV, approximately 210 degrees C), more localized and patchy, affected the country rocks and the porphyritic syenite and deposited disseminated pyrite and chalcopyrite. The delta ³⁴ S values from vein and disseminated sulfides range from -2.1 to +0.3 per rail, indicative of a magmatic origin for the sulfur.The ²⁰⁷ Pb/ ²⁰⁴ Pb ratios from pyrite and chalcopyrite deposited in stage IIIA (early) and stage IIIB (late) veins (15.683--15.687) are markedly lower than those from disseminated sulfides related to stage IV (15.696--15.734) and lie within the range of 15.672 to 15.696 defined by feldspars from the porphyritic syenite. The Pb isotopes therefore indicate an evolution of the fluid composition from one characterized by magmatic components to one dominated by the country rocks, whereas ore hosted in veinlets (stage III) contains lead from both sources. Furthermore they suggest that the lead in the ores developed in continental crustal environments with high mu ₂ and W ₂ values and evolved in a reservoir with high Th/Pb and low U/Pb ratios.Age-corrected ⁸⁷ Sr/ ⁸⁶ Sr ratios from inclusion fluids in vein quartz range from 0.7086 to 0.7099 for veins within the country rock and from 0.7061 to 0.7080 in veins transecting the porphyritic syenite. Values for porphyry feldspars range from 0.7079 to 0.7081 and country rock from 0.7135 to 0.7342. These data confirm the Pb isotope trends and indicate that Sr contamination took place where the veins crosscut the country rocks. Fluids in veins within the porphyry could ascend rapidly from deeper levels without reacting with the porphyry and thus show an unaffected magmatic Sr isotope signature.Pb and Sr isotopes in inclusion fluids are shown to be extremely sensitive tracers for elucidating the hypogene leaching processes in the Skouries porphyry system.